CN1221680C - Thin steel sheet for automobile excellent in notch fatigue strength and method for production thereof - Google Patents

Abstract

The present invention provides a thin steel sheet, for automobile use, excellent in notch-fatigue strength, and a method for producing said steel sheet. Specifically, the present invention is a thin steel sheet for automobile use excellent in notch-fatigue strength, said steel sheet containing, in mass, 0.01 to 0.3% C, 0.01 to 2% Si, 0.05 to 3% Mn, 0.1% or less P, 0.01% or less S and 0.005 to 1% Al, with the balance consisting of Fe and unavoidable impurities, characterized in that, on a plane at an arbitrary depth within 0.5 mm from the surface of said steel sheet in the thickness direction thereof, the average of the ratios of the X-ray diffraction strength in the orientation component group of 100U<011> to 223U<110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of 554U<225>, 111U<112> and 111U<110> to random X-ray diffraction strength is 4 or less and that the thickness of said steel sheet is in the range from 0.5 to 12 mm, and a method for producing said steel sheet by subjecting a steel slab containing aforementioned chemical components to rolling at a total reduction ratio of 25% or more in a temperature range of the Ar3 transformation temperature + 100 DEG C or lower.

Description

Automotive thin steel sheet excellent in notch fatigue strength and manufacturing method thereof

technical field

The present invention relates to a thin steel sheet for automobiles excellent in notch fatigue strength and a method for producing the same, and in particular to parts suitable for automobile running parts, etc., where the growth of fatigue cracks is a problem in stress-concentrated parts such as blanking (punching) processing parts and welding areas. A thin steel sheet for automobiles which is a raw material and excellent in notch fatigue strength and a method for producing the same.

Background technique

In recent years, light metals such as aluminum alloys and high-strength steel sheets have been applied to automotive components for the purpose of weight reduction due to the increase in fuel costs of automobiles. However, although light metals such as aluminum alloys have the advantage of high specific strength, they are remarkably expensive compared with steel, so their application is limited to special applications. Therefore, in order to reduce the weight of automobiles in a wider range, it is urgently required to apply inexpensive high-strength steel sheets.

In response to such high-strength requirements, until now, in the field of cold-rolled steel sheets used in car bodies and cladding panels that account for about 1/4 of the car body weight, both strength and drawability (deep drawability) have been carried out. The development of high-quality steel plates and bake-hardenable steel plates contributes to the weight reduction of car bodies. However, at present, the object of weight reduction has shifted to structural members and running part members that account for about 20% of the weight of the vehicle body, and the development of high-strength thin steel plates used for these members has become a top priority.

However, high strength generally degrades material properties such as formability (workability), so how to achieve high strength without deteriorating material properties is an important factor in the development of high-strength steel sheets. In particular, properties required for steel plates for structural members and running part members, not to mention elongation, shearing and blanking workability, deburring workability, fatigue durability, and corrosion resistance are important. It is important to balance high strength with these properties in high dimensions.

For example, components such as the positioning arm of the suspension system are punched and punched through shearing and blanking (punching), and pressed into shape after punching, and further welded to form parts according to different components. For such components, there are many cases where cracks propagate from the sheared end face and near the weld zone so that fatigue failure occurs. That is, the sheared end face and the weld zone become a stress concentration portion like a notch, from which fatigue cracks propagate.

On the other hand, generally the fatigue limit of a material decreases when the notch is sharp. However, when the notch is sharpened to a certain extent, there occurs a phenomenon that the fatigue limit does not decrease any more. This is because the fatigue limit migrates from the crack initiation boundary to the crack growth boundary. When the strength of the material is increased, the crack initiation boundary increases, but the crack growth boundary does not increase. Therefore, the point where the fatigue limit shifts from the crack initiation boundary to the crack growth boundary moves to the sharp notch side. Therefore, even if the strength of the material is increased, the decrease in the fatigue limit due to the notch becomes significant, and the fatigue limit in the case of a sharp notch cannot be an index of high strength. That is, increasing the strength increases the sensitivity to notches.

At present, steel sheets of 340-440 MPa class are used as thin steel plates for these running parts of automobiles, but the required strength class of steel plates for these components is 590-780 MPa class, and further high strength is being pursued. Therefore, in order to meet these demands, it is indispensable to develop a steel plate that can obtain an indicator of high strength even when there are sharp notches.

There are roughly two methods for improving the fatigue strength in the presence of blanking and sheared end faces. One is to eliminate sharp notches such as burrs produced on the end face of blanking and shearing, and the other is to increase the resistance to crack propagation even if such notches exist.

As an invention belonging to the former, for example, Japanese Unexamined Patent Publication No. 5-51695 discloses a technique of reducing the amount of Si added and reducing the elongation at break due to precipitates of Ti, Nb, and V, thereby suppressing the occurrence of burrs. , Improve the fatigue strength in the state of blanking and shearing. In addition, Japanese Patent Laid-Open No. 5-179346 discloses a technology for improving the fatigue strength in blanking and shearing states by limiting the upper limit of the volume fraction of bainite by specifying the upper limit of rolling processing temperature. Japanese Unexamined Patent Publication No. 8-13033 discloses a technique of increasing the fatigue strength in blanking and shearing states by suppressing the formation of martensite by specifying the cooling rate after rolling.

Moreover, JP-A-8-302446 discloses the following technology: for steel with a composite structure, the hardness of the second phase is specified to be 1.3 times or more of the ferrite, reducing the strain energy during blanking and shearing, and improving Fatigue strength under blanking and shearing conditions. In addition, JP-A-9-170048 discloses a technique of specifying the length of grain boundary cementite, reducing burrs during blanking and shearing, and improving fatigue strength under blanking and shearing. Furthermore, Japanese Unexamined Patent Publication No. 9-202940 discloses a technique for improving blanking properties and improving fatigue strength in a blanking state by specifying parameters organized by the addition amounts of Ti, Nb, and Cr.

On the other hand, as an invention belonging to the latter, Japanese Unexamined Patent Publication No. 6-88161 discloses a technique in which the (100) plane strength of the structure parallel to the rolling surface in the surface layer is set to 1.5 or more to allow fatigue cracks to propagate. Speed reduction techniques. In addition, JP-A-8-199286 and JP-A-10-147846 disclose the technique of setting the (200) diffraction intensity ratio in the plate thickness direction measured by X-rays to 2.0-15.0, and making recovered or recrystallized iron The area ratio of the element body is 15-40%, so that the fatigue crack growth rate is reduced.

However, the above-mentioned publications such as JP-A-5-51695, JP-A-5-179346, JP-8-13033, JP-8-302446, JP-9-170048 and JP-9-202940, It is a technology to reduce notches such as burrs that occur on the end face of blanking and shearing. The degree of burrs that occur varies greatly depending on the clearance (clearance) during blanking and shearing. Therefore, it is not A technology that can be applied under any conditions is, of course, insufficient as a steel plate excellent in notch fatigue strength.

On the other hand, the technologies disclosed in JP-A-6-88161, 8-199286, and 10-147846 to improve resistance to crack propagation by controlling structures are mainly used in large structures such as construction machinery, ships, and bridges. The invention aimed at the steel used in the present invention is not aimed at the thin steel sheet for automobiles like the present invention.

In addition, the above-mentioned technology is a technology to control the crack propagation speed in the PARIS region, which is called in the destructive mechanics of the fatigue crack that mainly propagates from the welded seam. The technique in the case of the crack growth zone in the PARIS zone is insufficient.

In addition, the invention of evaluating the notch fatigue characteristics using the test piece shown in Fig. 1(b) by the planar fatigue test method used for thin steel plate applications has not been found so far.

Contents of the invention

Therefore, the present invention relates to the following technology: For thin steel sheets for automobiles, the structure is controlled regardless of the conditions such as blanking and shearing during blanking and shearing, and the resistance to crack propagation is improved, thereby improving the performance from blanking and shearing. A fatigue crack develops from such a notch. That is, an object of the present invention is to provide a thin steel sheet for automobiles excellent in notch fatigue strength, and a method for manufacturing the steel sheet at low cost and stably.

The inventors of the present invention have conducted intensive studies to improve the notch fatigue strength of steel sheets for automobiles under the concept of a manufacturing process for thin steel sheets produced on an industrial scale using conventional manufacturing facilities. As a result, it was newly found that the X-ray random intensity ratio (random intensity) of the {100}<011> to {223}<110> orientation group (orientation group) of the plate surface at any depth from the outermost surface to 0.5mm along the thickness direction ratio) is more than 2, and the average value of the X-ray random intensity ratio of the three orientations {554}<225>, {111}<112>, and {111}<110> is 4 or less. It is very effective to improve the notch fatigue strength by being 0.5 mm or more and 12 mm or less, and completed the present invention.

That is, the gist of the present invention is as follows:

(1) A thin steel sheet for automobiles excellent in notched fatigue strength, characterized in that {100}<011> to {223}<110> orientation groups of the sheet surface at any depth from the outermost surface to 0.5 mm in the thickness direction The average value of random intensity ratio of X-rays is more than 2, and the average value of random intensity ratio of X-rays of {554}<225>, {111}<112> and {111}<110> is 4 Below, the plate thickness is not less than 0.5 mm and not more than 12 mm.

(2) The thin steel sheet for automobiles excellent in notch fatigue strength described in (1) above, wherein the microstructure of the steel sheet is such that the phase with the largest volume fraction is bainite, or ferrite and bainite composite organization.

(3) The thin steel sheet for automobiles excellent in notch fatigue strength described in (1) above, wherein the microstructure of the steel sheet contains retained austenite at a volume fraction of 5% to 25% and the remainder is mainly Composite structure composed of ferrite and bainite.

(4) The thin steel sheet for automobiles excellent in notch fatigue strength described in (1) above, wherein the microstructure of the steel sheet is a composite in which the phase with the largest volume fraction is ferrite and the second phase is martensite organize.

(5) A thin steel sheet for automobiles excellent in notch fatigue strength, characterized by containing C: 0.01-0.3%, Si: 0.01-2%, Mn: 0.05-3%, and P: ≤ 0.1% in mass % %, S: ≤0.01%, Al: 0.005-1%, and the remainder is composed of Fe and unavoidable impurities, {100}<011> of the plate surface at any depth from the outermost surface to 0.5mm along the thickness direction The average value of the X-ray random intensity ratio of the {223}<110> orientation group is 2 or more, and the X-rays of the three orientations of {554}<225>, {111}<112>, and {111}<110> The average value of the random strength ratio is 4 or less, and the plate thickness is 0.5 mm to 12 mm.

(6) The thin steel sheet for automobiles excellent in notch fatigue strength described in (5) above, further comprising Cu: 0.2-2%, B: 0.0002-0.002%, Ni: 0.1-1% by mass % %, Ca: 0.0005-0.002%, REM: 0.0005-0.02%, Ti: 0.05-0.5%, Nb: 0.01-0.5%, Mo: 0.05-1%, V: 0.02-0.2%, Cr: 0.01-1% , Zr: 0.02-0.2% of one or more types.

(7) The thin steel sheet for automobiles excellent in notch fatigue strength described in (5) or (6) above, wherein the microstructure of the steel sheet is any one of the following: 1) the phase with the largest volume fraction It is bainite, or a composite structure of ferrite and bainite; 2) It contains retained austenite with a volume fraction of not less than 5% and not more than 25%, and the rest is mainly composed of ferrite and bainite Composite structure; 3) The phase with the largest volume fraction is ferrite, and the second phase is martensite.

(8) A thin steel sheet for automobiles excellent in notch fatigue strength, wherein the thin steel sheet for automobiles according to any one of (1) to (7) is galvanized.

(9) A method of producing a thin steel sheet for automobiles excellent in notch fatigue strength, characterized in that C: 0.01-0.3%, Si: 0.01-2%, Mn: 0.05-3%, P : ≤0.1%, S: ≤0.01%, Al: 0.005-1%, and the remaining part is composed of Fe and unavoidable impurities. When hot rolling after rough rolling, the Ar ₃ transformation point temperature + 100 ℃ Finish rolling with a total reduction rate of 25% or more in the temperature zone of the steel plate, and {100}<011>～{223}<110 along the thickness direction from the outermost surface of the steel plate to any depth of 0.5mm on the plate surface > The average value of the X-ray random intensity ratio of the orientation group is 2 or more, and the average of the X-ray random intensity ratios of the three orientations {554}<225>, {111}<112>, and {111}<110> The value is 4 or less, and the plate thickness is 0.5 mm to 12 mm.

(10) The method of manufacturing a thin steel sheet for automobiles excellent in notch fatigue strength described in the above (9), characterized in that after the above-mentioned finish rolling, cooling is performed at a cooling rate of 20°C/s or higher, and the coiling temperature is 450°C or higher. Winding down.

(11) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength according to (9) above, wherein after the finish rolling, the steel sheet is rolled in a temperature range between the Ar ₁ transformation point temperature and the Ar ₃ transformation point temperature. Stay for 1-20 seconds, then cool at a cooling rate above 20°C/s, and wind at a winding temperature in the temperature range of greater than 350°C and less than 450°C.

(12) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength according to the above (9), characterized in that after the cooling, the steel sheet is coiled at a coiling temperature of 350°C or lower.

(13) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength according to any one of (9) to (12) above, wherein lubricated rolling is performed during the hot rolling.

(14) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength according to any one of (9) to (13) above, wherein descaling is performed after rough rolling is completed during the hot rolling.

(15) A method of producing a thin steel sheet for automobiles excellent in notch fatigue strength, characterized in that C: 0.01-0.3%, Si: 0.01-2%, Mn: 0.05-3%, P : ≤0.1%, S: ≤0.01%, Al: 0.005-1%, and the remaining part is composed of Fe and unavoidable impurities. When hot rolling after rough rolling, the Ar ₃ transformation point temperature + 100 ℃ In the temperature zone, finish rolling with a total reduction rate of 25% or more of the steel plate thickness, followed by pickling, and then cold rolling with a reduction rate of less than 80% of the steel plate thickness, is carried out at a temperature above the recovery temperature and Ac ₃ transformation point. The recovery or recrystallization annealing of the temperature zone below +100°C for 5-150 seconds, cooling process, {100}<011> of the plate surface from the outermost surface of the steel plate to any depth of 0.5mm along the thickness direction The average value of the X-ray random intensity ratio of the {223}<110> orientation group is 2 or more, and the X-rays of the three orientations of {554}<225>, {111}<112>, and {111}<110> The average value of the random strength ratio is 4 or less, and the plate thickness is 0.5 mm to 12 mm.

(16) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength as described in (15) above, characterized in that after the above-mentioned cold rolling, the rolling is performed at a temperature above the Ac ₁ transformation point and at a temperature above the Ac ₃ transformation point + 100° C. The following temperature zone is maintained for 5-150 seconds, and then the heat treatment of the cooling process is performed.

(17) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength described in the above (15), characterized in that, after holding in the above temperature range for 5 to 150 seconds, cooling is performed at a cooling rate of 20° C./s or more to Heat treatment in the temperature zone of greater than 350°C and less than 450°C, followed by maintaining in this temperature zone for 5-600 seconds and cooling to a temperature zone below 200°C at a cooling rate of 5°C/s or more.

(18) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength described in the above (15), characterized in that, after holding in the above temperature range for 5 to 150 seconds, cooling is performed at a cooling rate of 20° C./s or more to Heat treatment for processes in the temperature range below 350°C.

(19) A method of producing a thin steel sheet for automobiles excellent in notch fatigue strength, characterized in that the steel sheet described in any one of (11) to (18) further contains Cu: 0.2 in mass % -2%, B: 0.0002-0.002%, Ni: 0.1-1%, Ca: 0.0005-0.002%, REM: 0.0005-0.02%, Ti: 0.05-0.5%, Nb: 0.01-0.5%, Mo: 0.05- One or more of 1%, V: 0.02-0.2%, Cr: 0.01-1%, and Zr: 0.02-0.2%.

(20) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength according to (10) or (16) above, wherein the microstructure of the steel sheet is such that the phase with the largest volume fraction is bainite, or is Composite structure of ferrite and bainite.

(21) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength according to the above (11) or (17), wherein the microstructure of the steel sheet contains residual steel at a volume fraction of 5% to 25%. Composite structure composed of austenite and the rest mainly composed of ferrite and bainite.

(22) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength described in the above (12) or (18), wherein the microstructure of the steel sheet is such that the phase with the largest volume fraction is ferrite, and the second phase is ferrite. The phase is a composite structure of martensite.

(23) A method of producing a thin steel sheet for automobiles excellent in notch fatigue strength, characterized in that, after producing the hot-rolled steel sheet or recovery or recrystallization annealed sheet according to any one of (9)-(22), Further, the steel sheet is dipped in a galvanizing bath to galvanize the surface of the steel sheet.

(24) The method for producing a thin steel sheet for automobiles excellent in notch fatigue strength according to (23) above, wherein the alloying treatment is performed after the galvanizing.

Description of drawings

Fig. 1 is a diagram illustrating the shape of a fatigue test piece, wherein (a) shows a smooth fatigue test piece; (b) shows a notched fatigue test piece.

Figure 2 is the average value of the X-ray random intensity ratio in the {100}<011>～{223}<110> orientation group, and {554}<225>, {111}<112> and {111}<110 >A graph showing the results of the preliminary experiments of the present invention on the relationship between the average value of the X-ray random intensity ratios in these three orientations and the notch fatigue strength.

has implementation

First, the basic research results of the present invention will be described below.

Generally fatigue cracks originate from the surface. This is no exception when there is a stress concentration portion such as a notch. In addition, even when blanking or sheared end faces exist, fatigue cracks are often observed to grow from the edge of the steel plate surface under cyclic loading including a load mode in the out-of-plane direction. Therefore, it can be seen that even in such a case, an increase in the crack growth resistance at the outermost surface of the steel sheet or at a depth of about several grains is effective in improving the notch fatigue strength. In addition, even if the resistance to crack growth is increased in the central part of the plate thickness, it is already difficult to keep the cracks. Therefore, in the present invention, the range of the structure effective for improving the fatigue strength is limited to from the outermost surface up to 0.5 mm in the thickness direction. Preferably it is to 0.1 mm.

The average value of the X-ray random intensity ratio of the {100}<011> to {223}<110> orientation group and the {554}<225 >, {111}<112> and {111}<110> the average value of the X-ray random intensity ratio of the three orientations on the notch fatigue strength. The test materials used for this purpose were prepared as follows. That is, the hot finish rolling of the slab melted by adjusting the composition to 0.08%C-0.9%Si-1.2%Mn-0.01%P-0.001%S-0.03%Al is completed at any temperature above the Ar ₃ transformation point temperature , so that the plate thickness reaches 3.5mm, and then coiled.

In order to obtain the average value of the X-ray random intensity ratio of the {100}<011> to {223}<110> orientation group of the steel plate obtained in the thickness direction from the outermost surface to an arbitrary depth of 0.5 mm, And the average value of the X-ray random intensity ratio of {554}<225>, {111}<112> and {111}<110> three orientations, cut from the position of 1/4W or 3/4W of the plate width into φ30mm The test piece is ground from the outermost surface to a depth of about 0.05 mm, and then removed by chemical polishing or electrolytic polishing.

Furthermore, the crystal orientation represented by {hkl}<uvw> means that the normal direction of the sheet surface is parallel to {hkl}, and the rolling direction is parallel to <uvw>. The measurement of crystal orientation by X-rays is performed, for example, according to the method described on pages 274-296 of "New Edition X-ray Diffraction Essentials" (published in 1986, translated by Gentaro Matsumura, Agne Co., Ltd.).

Here, the mean value of the X-ray random intensity ratio of the so-called {100}<011>～{223}<110> orientation group is the 3-dimensional structure calculated by the vector method based on the {110} pole figure, or the {110 }, {100}, {211}, {310} pole diagrams using the 3-dimensional structure calculated by the series expansion method using multiple pole diagrams (preferably more than 3), to obtain the main Orientation, {100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>, {335}<110>, and {223}<110> X-ray diffraction intensity.

For example, the X-ray random intensity ratio of each crystal orientation in the latter method uses the (001)[1-10], (116)[1-10], (116)[1- 10], (114)[1-10], (113)[1-10], (112)[1-10], (335)[1-10], (223)[1-10] strength is Can. However, the average value of the X-ray random intensity ratio of the orientation group {100}<011>-{223}<110> is the summed average value of the above-mentioned orientations.

When the strength of all the above orientations cannot be obtained, {100}<011>, {116}<110>, {114}<110>, {112}<110>, {223}<110> can also be used The additive mean of each orientation is substituted.

Next, the average value of X-ray random intensity ratios of the three orientations {554}<225>, {111}<112>, and {111}<110> is obtained from the three-dimensional structure calculated in the same way as above. Just go out.

Next, in order to investigate the notch fatigue strength of the above-mentioned steel plate, a fatigue test piece having a shape shown in FIG. Here, the fatigue test piece shown in FIG. 1(a) is a general smooth test piece for obtaining the fatigue strength of the base material, and the fatigue test piece shown in FIG. 1(b) is a notched test piece made to obtain the notch fatigue strength. test piece. However, the fatigue test piece was ground from the outermost surface to a depth of about 0.05 mm by three-mount processing. The fatigue test uses an electric hydraulic servo fatigue testing machine, and the test method is based on JIS Z 2273-1978 and JIS Z 2275-1978.

Investigated the average X-ray random intensity ratio of {100}<011>～{223}<110> orientation group, and {554}<225>, {111}<112> and {111}<110>3 The effect of the average value of the X-ray random intensity ratio of each orientation on the notch fatigue strength is shown in Fig. 2. Here, the number in ○ is the fatigue limit (time strength under ¹⁰⁷ cycles) obtained from the fatigue test using the notch fatigue test piece of the shape shown in FIG.

The average value of X-ray random intensity ratio of {100}<011>～{223}<110> orientation group, and {554}<225>, {111}<112> and {111}<110> three orientations There is a strong correlation between the average value of the X-ray random intensity ratio and the notch fatigue strength, and each average value is 2 or more and 4 or less, showing that the notch fatigue strength is significantly improved.

As a result of detailed studies of these experimental results, the present inventors have found the following new discovery: In order to improve the notch fatigue strength, {100}<011>～{223}<110 > The average value of the X-ray random intensity ratio of the orientation group is 2 or more, and the average of the X-ray random intensity ratios of the three orientations {554}<225>, {111}<112>, and {111}<110> Values below 4 are very important.

However, not only the notch, but also the {100}<011>～{223}<110> orientation group of the plate surface at any depth from the outermost surface to 0.5mm along the thickness direction in order to increase the fatigue crack occurrence resistance under smoothness The average value of the X-ray random intensity ratio is 4 or more, and the average value of the X-ray random intensity ratio of the three orientations of {554}<225>, {111}<112>, and {111}<110> is 2.5 or less is preferred.

This mechanism is not necessarily clear, but it is presumed as follows.

Generally, the fatigue limit in the presence of a sharp notch is determined by the limit of crack growth, that is, the magnitude of the resistance to crack growth that stops a crack. The growth of fatigue cracks is the repetition of small-scale plastic deformation at the bottom of the notch or the place where the stress is concentrated, but the crack length is relatively short, and when the plastic deformation is caused in the range of the size of the crystal grain, it can be estimated that the crystallographic slip plane and the slip direction are greatly affected. Therefore, if the proportion of crystals having a slip plane and a slip direction with high resistance to crack growth is large with respect to the crack growth direction and the crack plane, the growth of fatigue cracks is suppressed.

Next, the reasons for limiting the thickness of the steel sheet of the present invention will be described.

When the plate thickness is less than 0.5 mm, regardless of the degree of stress concentration, the small-scale yield condition cannot be satisfied, so there is a risk of ductile failure. In addition, from the viewpoint of crack retention, sufficient plastic restraint is required, so in order to ensure a plane strain state, it is desirable to have a plate thickness of at least 1.2 mm or more.

On the other hand, when the plate thickness exceeds 12 mm, the decrease in fatigue strength due to the plate thickness effect (size effect) becomes remarkable. When the plate thickness exceeds 8 mm, in order to realize hot rolling or cold rolling conditions to obtain a structure effective for improving the notch fatigue strength, excessive load may be applied to the equipment, so it is desirably 8 mm or less. Therefore, in the present invention, the plate thickness is limited to not less than 0.5 mm and not more than 12 mm. Preferably, it is not less than 1.2 mm and not more than 8 mm.

Next, the microstructure of the steel sheet of the present invention will be described.

In the present invention, for the purpose of improving its notch fatigue strength, it is not necessary to specifically limit the microstructure of the steel plate. In the ferrite, bainite, pearlite, and martensite structures that common steel presents, if it can Obtaining the structure within the range of the present invention (X-ray random intensity ratio within the range of the present invention) can obtain the effect of improving the notch fatigue strength of the present invention. Therefore, it is better to define the microstructure according to other necessary characteristics. However, for a specific microstructure, for example, a composite structure containing retained austenite with a volume fraction of 5% to 25% and the rest mainly composed of ferrite and bainite, or a phase with the largest volume fraction. This effect can be further enhanced by a composite structure in which ferrite is ferrite and the second phase is mainly martensite.

Furthermore, the bainite mentioned here also includes bainitic ferrite (bainitic ferrite) and acicular ferrite (acicular ferrite) structures. However, when the crystal structure such as retained austenite in the composite structure of more than two phases is not a bcc structure, if the X-ray random intensity ratio in terms of volume fraction of other structures is within the scope of the present invention, then also can. In addition, pearlite containing coarse carbides may become the site of fatigue cracking and extremely reduce the fatigue strength. Therefore, the volume fraction of pearlite containing coarse carbides is desirably 15% or less. Furthermore, in order to ensure good fatigue properties, the volume fraction of pearlite containing coarse carbides is desirably 5% or less.

Here, the volume fractions of ferrite, bainite, pearlite, martensite, and retained austenite are defined by the following area fractions: Or the sample cut at the 3/4W position is ground on the cross-section in the rolling direction, etched with nital reagent and/or the reagent disclosed in JP-A-5-163590, and observed with an optical microscope at a magnification of 200-500 times The area fraction of the microstructure of the thick 1/4t. However, corrosion of retained austenite by the above-mentioned reagents may not be easily discriminated, so the volume fraction can also be calculated by the following method.

That is, since austenite has a different crystal structure from ferrite, it can be easily identified crystallographically. Therefore, the volume fraction of retained austenite can also be obtained experimentally by the X-ray diffraction method. That is, it is a method of simply obtaining the volume fraction of austenite and ferrite from the difference in reflective surface strength of austenite and ferrite using Kα rays of Mo using the following formula.

Vγ=(2/3){100/(0.7×α(211)/γ(220)+1)}+(1/3){100/(0.78×α(211)/γ(311)+1) }

Among them, α(211), γ(220) and γ(311) are the X-ray reflective surface intensities of ferrite (α) and austenite (γ), respectively.

In the present invention, in addition to improving the notch fatigue strength, in order to impart good deburring workability, the microstructure is such that the phase with the largest volume fraction is bainite, or a composite structure of ferrite and bainite. However, unavoidable martensite, retained austenite and pearlite are allowed. In order to obtain good deburring workability (hole expansion value), the volume fraction of total hard retained austenite and martensite is desirably less than 5%. In addition, the volume fraction of bainite is desirably 30% or more. Furthermore, in order to obtain good ductility, the volume fraction of bainite is desirably 70% or less.

In addition, in the present invention, in addition to improving the notch fatigue strength, in order to impart good ductility, the microstructure contains retained austenite at a volume fraction of 5% to 25% and the remainder is mainly composed of ferrite. , Composite organization composed of bainite. However, it is allowed to contain less than 5% of unavoidable martensite and pearlite.

Furthermore, in the present invention, in addition to the improvement of the notch fatigue strength, in order to impart a low yield ratio to obtain good shape freezing properties, the microstructure is such that the phase with the largest volume fraction is ferrite, and the second phase is martensitic. composite tissue of the body. However, it is allowed to contain less than 5% of the total unavoidable bainite, retained austenite and pearlite. Furthermore, in order to secure a low yield ratio of 70% or less, the volume fraction of ferrite is desirably 50% or more.

Next, reasons for limiting the chemical components of the present invention will be described.

C is an element necessary for obtaining a desired microstructure. However, if the content exceeds 0.3%, the workability will deteriorate, so it is made 0.3% or less. In addition, when the content exceeds 0.2%, the weldability tends to deteriorate, so it is preferably 0.2% or less. On the other hand, if it is less than 0.01%, the strength will decrease, so it is made 0.01% or more. In addition, in order to stably obtain a sufficient amount of retained austenite for obtaining good ductility, it is preferably 0.05% or more.

Si is effective for improving the strength as a solid solution strengthening element. In order to obtain the desired strength, it is necessary to contain 0.01% or more. However, when the content exceeds 2%, workability deteriorates. Therefore, the content of Si is set at 0.01-2%.

Mn is effective as a solid solution strengthening element for improving strength. In order to obtain the desired strength, it is necessary to contain 0.05% or more. In addition to Mn, when an element such as Ti that suppresses the occurrence of hot cracks caused by S is not sufficiently added, it is desirable to add Mn in an amount so that Mn/S≧20 in mass %. Furthermore, Mn is an austenite stabilizing element, and in order to stably obtain a sufficient amount of retained austenite for obtaining good ductility, its addition amount is desirably 0.1% or more. On the other hand, if added over 3%, slab cracks will occur, so it is made 3% or less.

P is an impurity, and the lower the content, the better. If the content exceeds 0.1%, it will have a bad influence on the workability and weldability, and the fatigue properties will also decrease, so it is set at 0.1% or less.

S is an impurity, and the lower the content, the better. If it is too much, A-type inclusions that deteriorate local ductility and deburring workability will be formed. Therefore, it should be reduced as much as possible, but it is an acceptable range if it is 0.01% or less.

Al must be added in an amount of 0.005% or more for deoxidation of molten steel, but the upper limit is limited to 1.0% because it leads to an increase in cost. Also, if added in a large amount, the non-metallic inclusions will increase and the elongation will be deteriorated, so it is desirably 0.5% or less.

Cu has the effect of improving fatigue properties in a solid solution state, so it is added as necessary. However, when the content is less than 0.2%, the effect is small, and even if the content exceeds 2%, the effect is saturated. Therefore, the content of Cu is set in the range of 0.2-2%. However, when the winding temperature is 450° C. or higher, if the content exceeds 1.2%, it may precipitate after winding and significantly deteriorate the workability, so it is desirable to make it 1.2% or less.

B has the effect of raising the fatigue limit by adding it in combination with Cu, so it is added as necessary. However, when it is less than 0.0002%, the effect obtained is insufficient, and when it is added in excess of 0.002%, cracks in the slab are caused. Therefore, the addition of B is set at 0.0002-0.002%.

Ni is added as needed in order to prevent hot embrittlement caused by containing Cu. However, if it is less than 0.1%, the effect is small, and even if it is added in excess of 1%, the effect is saturated, so it is made 0.1-1%.

Ca and REM are elements that change the form of non-metallic inclusions that become the origin of fracture and degrade workability to make them harmless. However, adding less than 0.0005% has no effect, and adding more than 0.002% of Ca and more than 0.02% of REM will saturate the effect, so it is desirable to add Ca: 0.0005-0.002% and REM: 0.0005-0.02%.

Furthermore, in order to impart strength, one or two or more elements for precipitation strengthening or solid solution strengthening of Ti, Nb, Mo, V, Cr, and Zr may be added. However, when it is less than 0.05%, 0.01%, 0.05%, 0.02%, 0.01%, and 0.02%, respectively, the effect cannot be obtained. Moreover, even if it adds more than 0.5%, 0.5%, 1%, 0.2%, 1%, and 0.2%, respectively, the effect is saturated.

In addition, you may contain 1% or less of Sn, Co, Zn, W, and Mg in total in the steel which mainly consists of these. However, Sn may cause defects during hot rolling, so it is desirably 0.05% or less.

Next, the reasons for limiting the production method of the present invention will be described in detail below.

The present invention can be obtained by the following methods: after casting, in a state of cooling after hot rolling or cooling after hot rolling, pickling and annealing after cold rolling, or by subjecting hot-rolled steel sheets or cold-rolled steel sheets to heat treatment on a hot-dip coating line , and then additionally implement surface treatment on these steel plates.

In the present invention, the production method prior to hot rolling is not particularly limited. That is, after smelting in a blast furnace, electric furnace, etc., various secondary smelting is used to adjust the composition so that the target composition content is achieved, and then, in addition to the usual continuous casting and ingot casting, methods such as thin slab casting are used Just cast. It is also possible to use waste materials as raw materials. In the case of a slab obtained by continuous casting, it may be directly fed to a hot rolling mill in the state of a high-temperature cast slab, or it may be cooled to room temperature, reheated in a heating furnace, and then hot-rolled.

The reheating temperature is not particularly limited, but if it is 1400°C or higher, the amount of scale off will increase and the effective utilization rate will decrease. Therefore, the reheating temperature is preferably less than 1400°C. In addition, heating of less than 1000°C significantly impairs the operating efficiency on a procedural basis, so the reheating temperature is desirably 1000°C or higher.

In the hot rolling process, finish rolling is carried out after rough rolling, but when descaling is carried out after rough rolling, it is desirable to satisfy the impact pressure P (MPa) × flow rate L (liter/cm ² ) of high-pressure water on the surface of the steel plate ≥0.0025 condition.

The impact pressure P of high-pressure water on the steel plate surface is described as follows (see "Iron and Steel" 1991, vol.77, No.9, p1450).

P(MPa)＝5.64×P ₀ ×V/H ²

Among them, P ₀ : hydraulic pressure

V (L/min): nozzle flow volume

H(cm): the distance between the surface of the steel plate and the nozzle

The flow rate is described as follows.

L(L/cm ² )＝V/(W×v)

Among them, V (L/min): nozzle flow volume

W (cm): the width of each nozzle jet impacting the surface of the steel plate

V (cm/min): plate passing speed

The upper limit of the impact pressure P×flow rate L is not necessarily determined in order to obtain the effect of the present invention, but when the flow rate of the nozzle increases, there will be disadvantages such as increased wear of the nozzle, so it is preferably 0.02 or less.

Furthermore, the maximum height Ry of the steel sheet after finish rolling is desirably 15 μm (15 μm Ry, 12.5 mm, In 12.5 mm) or less. This can be seen from the following, that is, for example, according to "Handbook for Fatigue Design of Metallic Materials", edited by the Japan Society for Materials Science and Technology, page 84, the fatigue strength of a hot-rolled or pickled steel plate is related to the maximum height Ry of the steel plate surface. sex. In addition, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent scale formation after descaling.

In addition, after the rough rolling or subsequent descaling, the thin slabs may be joined and the finish rolling may be carried out continuously. At this time, the thick strips may be rolled into rolls first, stored in a cover having a heat-retaining function if necessary, and rewound again before joining.

For finish rolling, in the case of making the final product as a hot-rolled steel sheet, it is necessary to carry out rolling with a total reduction rate of 25% or more in the temperature range below the Ar ₃ transformation point temperature + 100°C in the second half of the finish rolling. system. Here, the so-called Ar ₃ transformation point temperature is simply expressed in relation to steel components by, for example, the following calculation formula. Right now,

Ar ₃ =910-310×%C+25×%Si-80×%Mn

When the total reduction rate in the temperature region below the Ar ₃ transformation point temperature + 100°C is less than 25%, the rolled austenite structure is not sufficiently developed. The effect of the present invention cannot be obtained. In order to obtain a sharper structure, it is desirable that the total reduction ratio in the temperature region of the Ar ₃ transformation temperature + 100° C. or lower be 35% or more.

In addition, the lower limit of the temperature range in which rolling is performed with a total rolling reduction of 25% or more is not particularly limited, but if it is less than the Ar ₃ transformation point temperature, the ferrite precipitated during rolling remains the worked structure, and the Therefore, the lower limit of the temperature range in which rolling is performed with a total rolling reduction of 25% or more is desirably set at or above the Ar ₃ transformation point temperature. However, even if the temperature is lower than the Ar ₃ transformation point temperature, recovery or recrystallization proceeds to a certain extent through the subsequent winding treatment or heat treatment after the winding treatment, and it is not limited thereto.

In the present invention, the upper limit of the total reduction ratio in the temperature region below the Ar ₃ transformation point temperature + 100°C is not particularly limited, but when the reduction ratio always exceeds 97.5%, the rolling load increases and must be excessively Increasing the rigidity of the rolling mill as much as possible is economically disadvantageous, so it is desirably 97.5% or less.

Here, when the friction between the hot roll and the steel sheet is large during hot rolling in the temperature range of the Ar ₃ transformation point temperature + 100°C, the {110} plane is the main crystal orientation on the sheet surface near the steel sheet surface. As it progresses, the notch fatigue strength deteriorates, so lubrication is performed as necessary in order to reduce the friction between the hot rolling roll and the steel plate.

In the present invention, the upper limit of the friction coefficient between the hot rolling roll and the steel plate is not particularly limited, but when it exceeds 0.2, the development of the crystal orientation mainly _in the {110} plane becomes remarkable, and the notch fatigue strength deteriorates. In at least one pass during hot rolling in the temperature range of change point temperature + 100°C or lower, it is desirable that the coefficient of friction between the hot rolling roll and the steel sheet is 0.2 or lower. More preferably, the coefficient of friction between the hot rolling roll and the steel sheet is 0.15 or less for all the hot rolling passes in the temperature range of the Ar ₃ transformation point temperature + 100°C or lower.

Here, the coefficient of friction between the hot rolling roll and the steel plate is a value obtained by calculation based on the rolling theory from values such as advance rate, rolling load, and rolling torque.

The final rolling pass temperature (FT) of the finish rolling is not particularly limited, but it is desirable to complete the finish rolling when the final rolling pass temperature (FT) of the finish rolling is equal to or higher than the Ar ₃ transformation point temperature. This is because, in hot rolling, when the rolling temperature is lower than the Ar ₃ transformation point temperature, the worked structure remains in the ferrite precipitated before or during rolling, the ductility is lowered, and the workability is deteriorated. However, even if the final rolling pass temperature (FT) of finish rolling is lower than the Ar ₃ transformation point temperature, in the subsequent coiling treatment or after the coiling treatment, heat treatment for recovery or recrystallization is performed. Not subject to this restriction.

On the other hand, the upper limit of the finish rolling temperature is not particularly set, but when it exceeds the Ar ₃ transformation point temperature + 100°C, the total rolling reduction is 25% in the temperature range below the Ar ₃ transformation point temperature + 100°C. The above rolling is practically impossible, and therefore, the upper limit of the finish rolling temperature is desirably set at Ar ₃ transformation point temperature + 100°C or less.

In the present invention, it is not necessary to specifically limit the microstructure of the steel plate for the purpose of improving its notch fatigue strength. Therefore, after finishing rolling, the cooling process until coiling at a prescribed coiling temperature is not particularly determined. However, cooling is performed as necessary for coiling at a predetermined coiling temperature or for controlling the microstructure. The upper limit of the cooling rate is not particularly limited, but the cooling rate is desirably 300° C./s or less in view of fear of plate warpage due to thermal strain. Furthermore, if the cooling rate is too fast, the cooling end temperature cannot be controlled, and the coil may overshoot and may be overcooled below the predetermined winding temperature. Therefore, the cooling rate here is desirably 150°C/s or less. In addition, the lower limit of the cooling rate is not particularly determined, but the air cooling rate when no cooling is performed is 5° C./s or more.

In the present invention, in addition to improving the notch fatigue strength, the phase with the largest volume fraction of the microstructure is bainite, or ferrite and bainite for the purpose of imparting good deburring workability. Composite structure, so after finishing rolling, there is no special regulation except for the cooling rate during the process until coiling at a specified coiling temperature, but the deburring property is not deteriorated like that, and the goal is to coexist with ductility In some cases, it is also possible to stay in the temperature region from the Ar ₃ transformation point to the Ar ₁ transformation point (two-phase region of ferrite and austenite) for 1-20 seconds. The stay here is to promote the ferrite transformation in the two-phase region, but when it is less than 1 second, the ferrite transformation in the two-phase region is not sufficient, so sufficient ductility cannot be obtained. At 20 seconds, pearlite is formed, and the expected microstructure with the largest volume ratio cannot obtain bainite, or is a composite structure of ferrite and bainite.

In addition, the temperature range in which to stay for 1 to 20 seconds is desirably not less than the Ar ₁ transformation point and not more than 800° C. in order to facilitate the ferrite transformation. Furthermore, the dwell time of 1-20 seconds is preferably 1-10 seconds so as not to reduce productivity extremely. Also, in order to satisfy these conditions, it is necessary to quickly reach this temperature range at a cooling rate of 20°C/s or more after finish rolling.

The upper limit of the cooling rate is not particularly limited, but 300° C./s or less is an appropriate cooling rate in view of the capacity of the cooling equipment. Moreover, if the cooling rate is too fast, the cooling end temperature cannot be controlled, and overshooting may cause the temperature to be overcooled below the Ar ₁ transformation point, and the effect of improving ductility will be lost. Therefore, the cooling rate here is preferably 150°C. /s below.

Secondly, from this temperature zone to the coiling temperature (CT), it is cooled at a cooling rate of 20 °C/s or more, but pearlite or carbide-containing bainite is formed at a cooling rate of less than 20 °C/s. The microstructure with the largest volume ratio cannot obtain bainite, or a composite structure of ferrite and bainite. The upper limit of the cooling rate up to the coiling temperature is not particularly limited as long as the effect of the present invention can be obtained, but it is desirably 300°C/s or less for fear of plate warpage due to thermal strain.

In addition, in the present invention, in addition to improving the notch fatigue strength, the microstructure contains retained austenite at a volume fraction of 5% to 25%, and the remainder is mainly composed of iron for the purpose of imparting good ductility. The composite structure composed of ferrite and bainite, so after completing the finishing rolling process, firstly, in the temperature zone from the Ar ₃ transformation point temperature to the Ar ₁ transformation point temperature (two-phase region of ferrite and austenite) ) for 1-20 seconds. The stay here is to promote the ferrite transformation in the two-phase region, but when it is less than 1 second, the ferrite transformation in the two-phase region is not sufficient, so sufficient ductility cannot be obtained. When it exceeds 20 In seconds, pearlite is formed, and the expected microstructure containing retained austenite with a volume fraction of 5% to 25% and the rest mainly composed of ferrite and bainite cannot be obtained.

In addition, the temperature range in which to stay for 1 to 20 seconds is desirably in the Ar ₁ transformation point temperature or higher and 800° C. or lower in order to facilitate the ferrite transformation. Furthermore, the dwell time of 1-20 seconds is preferably 1-10 seconds so as not to reduce productivity extremely. Also, in order to satisfy these conditions, it is necessary to quickly reach this temperature range at a cooling rate of 20°C/s or more after finish rolling. The upper limit of the cooling rate is not particularly limited, but 300° C./s or less is an appropriate cooling rate in view of the capacity of the cooling equipment. Moreover, when the cooling rate is too fast, the cooling end temperature cannot be controlled, and overshooting may result in overcooling below the Ar ₁ transformation point temperature. Therefore, the cooling rate here is desirably 150°C/s or less.

Secondly, from this temperature zone to the coiling temperature (CT), cooling at a cooling rate of 20°C/s or more, but pearlite or carbide-containing bainite is formed at a cooling rate of less than 20°C/s, which cannot Sufficient retained austenite is obtained, but the expected microstructure containing retained austenite at a volume fraction of 5% to 25% and the remainder mainly composed of ferrite and bainite cannot be obtained. The upper limit of the cooling rate up to the coiling temperature is not particularly limited as long as the effect of the present invention can be obtained, but it is desirably 300°C/s or less for fear of plate warpage due to thermal strain.

Furthermore, in the present invention, in addition to improving the notch fatigue strength, for the purpose of imparting a low yield ratio for obtaining good shape freezing properties, the phase that forms the largest volume fraction of the microstructure is ferrite, the second The phase is mainly a composite structure of martensite, so the process after finishing rolling is completed, first in the temperature zone from the Ar ₃ transformation point temperature to the Ar ₁ transformation point temperature (two-phase region of ferrite and austenite) Stay for 1-20 seconds. The stay here is to promote the ferrite transformation in the two-phase region, but when it is less than 1 second, the ferrite transformation in the two-phase region is not sufficient, so sufficient ductility cannot be obtained. At 20 seconds, pearlite was formed, and the expected composite structure in which the phase with the largest volume fraction was ferrite and the second phase was mainly martensite could not be obtained.

In addition, the temperature range in which to stay for 1 to 20 seconds is desirably in the Ar ₁ transformation point temperature or higher and 800° C. or lower in order to facilitate the ferrite transformation. Furthermore, the dwell time of 1-20 seconds is desirably 1-10 seconds so as not to reduce the productivity extremely. Also, in order to satisfy these conditions, it is necessary to quickly reach this temperature range at a cooling rate of 20°C/s or more after finish rolling. The upper limit of the cooling rate is not particularly limited, but 300° C./s or less is an appropriate cooling rate in view of the capacity of the cooling equipment. Moreover, when the cooling rate is too fast, the cooling end temperature cannot be controlled, and overshooting may result in overcooling below the Ar ₁ transformation point temperature. Therefore, the cooling rate here is desirably 150°C/s or less.

Secondly, from this temperature range to the coiling temperature (CT), cooling is performed at a cooling rate of 20°C/s or more, but pearlite or bainite is formed at a cooling rate of less than 20°C/s, and sufficient martinite cannot be obtained. Tensite, the expected microstructure in which ferrite is the phase with the largest volume fraction and martensite is the second phase cannot be obtained.

The upper limit of the cooling rate up to the coiling temperature is not particularly limited as long as the effect of the present invention can be obtained, but it is desirably 300°C/s or less for fear of plate warpage due to thermal strain.

In the present invention, for the purpose of improving the notch fatigue strength, there is no need to specifically limit the microstructure of the steel plate. Therefore, the upper limit of the coiling temperature is not particularly determined, but in order to make the temperature at the Ar ₃ transformation point +100 In the temperature range below ℃, the austenite structure obtained by rolling with a total reduction ratio of 25% or more is inherited, and it is desirable to coil at or below the coiling temperature T ₀ shown below. However, T ₀ does not need to be below room temperature. This T ₀ is a thermodynamically defined temperature at which austenite and ferrite having the same composition as austenite have the same free energy, and can be easily calculated using the following formula in consideration of the influence of components other than C.

T ₀ =-650.4×%C+B

Here, B is determined as follows.

B=-50.6×Mneq+894.3

Here, Mneq is determined by the mass % of contained elements shown below.

Mneq=%Mn+0.24×%Ni+0.13×%Si+0.38×%Mo+0.55×%Cr+0.16×%Cu-0.50×%Al-0.45×%Co+0.90×%V

In addition, since mass % of the components other than the above specified in this invention has little influence on _T0 , it can be ignored here.

In addition, the lower limit of the coiling temperature is just for the purpose of improving its notch fatigue strength, and there is no need to specifically limit the microstructure of the steel plate, so it does not need to be specifically limited, but when the coil is in a state of water immersion for a long time , because there is concern about poor appearance due to rust, it is preferably 50° C. or higher.

In the present invention, in addition to improving the notch fatigue strength, for the purpose of imparting good deburring workability, the phase that forms the largest volume fraction of the microstructure is bainite, or ferrite and bainite. Tenite composite structure, when the coiling temperature is lower than 450°C, there is a possibility that a large amount of retained austenite or martensite, which is considered to be detrimental to deburring performance, may be formed, and the expected significant increase in volume ratio cannot be obtained. The microstructure is bainite, or a composite structure composed of ferrite and bainite, so the winding temperature is limited to 450°C or higher.

In addition, the cooling rate after winding is not particularly limited, but when adding 1.2% or more of Cu, Cu precipitates after winding, and not only the workability deteriorates, but also Cu in a solid solution state effective for improving fatigue strength may be lost. Therefore, it is desirable to set the cooling rate after winding to 30°C/s up to 200°C.

In addition, in the present invention, in addition to improving the notch fatigue strength, also for the purpose of imparting good ductility, the microstructure is made to contain retained austenite at a volume fraction of 5% to 25%, and the remainder is mainly composed of Composite structure composed of ferrite and bainite, when the coiling temperature is above 450°C, carbide-containing bainite is formed, and sufficient retained austenite cannot be obtained, and the expected volume fraction of 5 % to 25% retained austenite, and the remainder is mainly composed of ferrite and bainite, so the coiling temperature is limited to less than 450°C. In addition, when the coiling temperature is 350° C. or lower, a large amount of martensite is formed, sufficient retained austenite cannot be obtained, and the expected retained austenite containing 5% to 25% by volume cannot be obtained. The microstructure is mainly composed of ferrite and bainite, so the coiling temperature is limited to more than 350°C.

In addition, the cooling rate after winding is not particularly limited, but when adding 1% or more of Cu, Cu precipitates after winding, and not only the workability deteriorates, but also Cu in a solid solution state effective for improving fatigue strength may be lost. Therefore, the cooling rate after winding is desirably set to 30°C/s or more up to 200°C.

Furthermore, in the present invention, in addition to improving the notch fatigue strength, for the purpose of imparting a low yield ratio for obtaining good shape freezing properties, in order to make the phase with the largest volume fraction forming the microstructure be ferrite, The second phase is mainly a composite structure of martensite. When the coiling temperature exceeds 350 ° C, bainite is formed, and sufficient martensite cannot be obtained, and the expected phase with the largest volume fraction of ferrite cannot be obtained. Since martensite is the microstructure of the second phase, the coiling temperature is limited to 350° C. or lower. In addition, the lower limit of the winding temperature does not need to be particularly limited, but when the coil is soaked in water for a long time, there is concern about poor appearance due to rust, so it is preferably 50°C or higher.

After completion of the hot rolling process, pickling may be carried out if necessary, and then skin skinning at a reduction ratio of 10% or less or cold rolling to a reduction ratio of about 40% may be performed on-line or off-line.

Next, in the case of making the final product as a cold-rolled steel sheet, the hot finish rolling conditions are not particularly limited. However, in order to obtain better notch fatigue strength, it is desirable that the total reduction ratio in the temperature range of Ar ₃ transformation temperature + 100° C. or lower be 25% or more. In addition, the final rolling pass temperature (FT) of the finish rolling may be completed at a temperature lower than the Ar ₃ transformation point temperature. In this case, the worked structure remains in ferrite precipitated before rolling or during rolling, so it is desirable to restore and recrystallize it by subsequent coiling treatment or heat treatment.

The total rolling reduction after the subsequent pickling was less than 80%. This is because, when the total reduction ratio of cold rolling is 80% or more, the X-ray diffraction integral surface intensity of the {111} plane or {554} plane of the crystal plane parallel to the general cold rolling-recrystallized structure plate plane Becomes high. In addition, it is desirable to be 70% or less. The lower limit of the cold rolling ratio is not particularly limited as long as the effect of the present invention can be obtained, but it is preferably 3% or more in order to control the strength of the crystal orientation within an appropriate range.

The heat treatment of such a cold-rolled steel sheet presupposes a continuous annealing process.

First, carry out for 5-150 seconds in the temperature range below the Ac ₃ transformation point temperature +100°C. When the upper limit of the heat treatment temperature exceeds the Ac ₃ transformation point temperature + 100 ° C, the ferrite formed by recrystallization will transform into austenite, and the structure caused by the grain growth of austenite will become random, and the finally obtained iron The structure of the element body is also randomized, so the upper limit temperature of the heat treatment is Ac ₃ transformation point temperature + 100°C or less.

Here, the Ac ₁ transformation point temperature and the Ac ₃ transformation point temperature are, for example, the calculations described on page 273 of "Resuri Iron and Steel Materials Science" (published in 1985, translated by Hiroshi Kumai and Tatsuhiko Noda, Maruzen Co., Ltd.) The formula is shown in relation to steel composition.

On the other hand, the lower limit of the heat treatment temperature does not need to specifically limit the microstructure of the steel plate in order to improve the notch fatigue strength, so it may be above the recovery temperature, but if it is lower than the recovery temperature, the processed structure remains and the forming Therefore, the lower limit temperature of heat treatment is set to be above the recovery temperature. In addition, when the holding time in this temperature range is less than 5 seconds, it is not sufficient for complete solid solution of cementite. On the other hand, even if the heat treatment is performed for more than 150 seconds, not only the effect is saturated, but also the production Sexuality is reduced, so the hold time is set at 5-150 seconds.

The subsequent cooling conditions are not particularly limited, but the following cooling or holding and cooling at an arbitrary temperature may be performed as necessary in order to control the microstructure.

In the present invention, in addition to improving the notch fatigue strength, for the purpose of imparting good deburring properties, the phase forming the largest volume fraction of the microstructure is bainite, or ferrite and bainite Therefore, the lower limit temperature of the heat treatment temperature is set above the Ac ₁ transformation point temperature. If the lower limit temperature is lower than the Ac ₁ transformation point temperature, the expected phase with the largest volume fraction is bainite, or a composite structure of ferrite and bainite cannot be obtained. Here, in the case of taking ductility into consideration without degrading the deburring property so much, in order to increase the volume fraction of ferrite, the temperature range is set to be above the Ac ₁ transformation point temperature and below the Ac ₃ transformation point temperature ( The temperature range of the two-phase region of ferrite and austenite). In addition, in order to obtain better deburring property, since the volume fraction of bainite is increased, it is desirable to be in the temperature range of the Ac ₃ transformation point temperature or more and the Ac ₃ transformation point temperature + 100° C. or less.

Next, the cooling process is not particularly limited in the present invention, but when the above-mentioned heat treatment temperature is above the Ac ₁ transformation point temperature and below the Ac ₃ transformation point temperature, it is desirable to cool at a cooling rate of 20° C./s or more to more than Temperature zone below 350°C and T ₀ temperature. This is because, if the cooling rate is less than 20° C./s, there is a possibility of being bound by bainite or pearlite transformation containing a large amount of carbides. Also, when the cooling end temperature is 350°C or lower, there is a possibility that a large amount of martensite, which is considered to be harmful to deburring properties, may be formed, and the expected microstructure of bainite or ferrite with the largest volume ratio may not be obtained. It is a composite structure composed of bainite, so it is expected to exceed 350°C. Furthermore, in order to inherit the structure obtained before the previous step, _T0 or less is desirable.

Finally, when the cooling rate until the final temperature in the cooling process is 20°C/s or more, there is a possibility that a large amount of martensite, which is considered to be harmful to the deburring property, may be formed during cooling, and the expected volume ratio may not be obtained. The largest microstructure is bainite, or a composite structure composed of ferrite and bainite, so it is desirable to be less than 20°C/s. Moreover, when the finish temperature of a cooling process exceeds 200 degreeC, since there exists a possibility that aging may deteriorate, it is desirable to be 200 degreeC or less. Also, the lower limit is desirably 50° C. or higher because there is concern about poor appearance due to rust when the coil is soaked in water for a long time in the case of water cooling or mist cooling.

On the other hand, when the above-mentioned heat treatment temperature is higher than the Ac ₃ transformation point temperature, but in the case of the Ac ₃ transformation point temperature + 100°C or less, the temperature at which the heat treatment is cooled to 200°C or less at a cooling rate of 20°C/s or more is the preferred temperature. hopefully. This is because, when the temperature is 20° C./s or more, there is a possibility of being bound by bainite or pearlite transformation containing a large amount of carbides. Moreover, when the cooling end temperature exceeds 200°C, there is a possibility that the aging property may be deteriorated, so it is desirably 200°C or less. Regarding the lower limit, in the case of water cooling or mist cooling, when the coil is soaked in water for a long time, there is a concern of poor appearance due to rust, so it is preferably 50°C or higher.

In addition, in the present invention, in addition to improving the notch fatigue strength, also for the purpose of imparting good ductility, the microstructure is made to contain retained austenite at a volume fraction of 5% to 25%, and the remainder is mainly composed of The composite structure composed of ferrite and bainite is the same as the above, when the coiling temperature is above 450 ° C, bainite containing carbides is formed, and sufficient retained austenite cannot be obtained, and the expected The microstructure contains retained austenite with a volume fraction of 5% to 25%, and the rest is mainly composed of _{ferrite and} bainite. The temperature zone below ℃ is carried out for 5-150 seconds. At this time, even in this temperature range, if the temperature is too low, in the case of cementite precipitation in the hot-rolled sheet stage, it will take too much time for the cementite to re-dissolve; if the temperature is too high, the volume of austenite will be reduced. If the rate is too high, the C concentration in the austenite will decrease, and it will easily fall into the shackles of bainite or pearlite transformation containing a large amount of carbides (nose); therefore, heating at 780°C to 850°C is preferred. If the cooling rate after holding is less than 20°C/s, there is a possibility of being trapped in the transformation of bainite or pearlite containing a large amount of carbides, so the cooling rate is set at 20°C/s or more.

Secondly, it is the process of promoting the bainite transformation to stabilize the necessary amount of retained austenite, but when the cooling end temperature is above 450°C, the retained austenite decomposes into bainite or pearlite containing a large amount of carbides, resulting in Unexpected microstructure containing retained austenite at a volume fraction of not less than 5% and not more than 25%, with the remainder mainly composed of ferrite and bainite. In addition, when the temperature is lower than 350°C, there is a possibility that a large amount of martensite is formed, and sufficient retained austenite cannot be obtained, and the expected retained austenite containing 5% to 25% by volume cannot be obtained. The remaining part is mainly composed of ferrite and bainite microstructure, so it is cooled to a temperature range exceeding 350°C.

Furthermore, as the holding time in this temperature range, if it is less than 5 seconds, the bainite transformation for stabilizing the retained austenite is insufficient, and the unstable retained austenite has There is a risk of martensitic transformation, and the expected microstructure containing retained austenite at a volume fraction of 5% to 25% and the remainder mainly consisting of ferrite and bainite cannot be obtained. In addition, when the time exceeds 600 seconds, the bainite transformation is too accelerated, the necessary amount of stable retained austenite cannot be obtained, and the expected retained austenite and residual austenite containing 5% to 25% by volume fraction cannot be obtained. Part of the microstructure is mainly composed of ferrite and bainite. Therefore, the retention time in this temperature range is set to not less than 5 seconds and not more than 600 seconds.

Finally, when the cooling rate until the end of cooling is less than 5°C/s, there is a possibility that the bainite transformation is too accelerated during cooling, and the necessary amount of stable retained austenite cannot be obtained, and the expected inclusion body cannot be obtained. A microstructure in which the integral ratio is 5% to 25% of retained austenite, and the remainder is mainly composed of ferrite and bainite. Therefore, it is set at 5°C/s or more.

In addition, when the cooling end temperature exceeds 200° C., there is a possibility that the aging property may be deteriorated, so it is made 200° C. or less. The lower limit of the cooling end temperature is not particularly limited, but in the case of water cooling or mist cooling, when the coil is immersed in water for a long time, there is concern about poor appearance due to rust, so it is preferably 50°C or higher.

Furthermore, in the present invention, in addition to improving the notch fatigue strength, for the purpose of imparting a low yield ratio for obtaining good shape freezing properties, in order to make the phase with the largest volume fraction forming the microstructure be ferrite, The second phase is mainly a composite structure of martensite. Therefore, as above, it is carried out for 5-150 seconds in a temperature range above the Ac ₁ transformation point temperature and above the Ac ₃ transformation point temperature +100°C. At this time, even in this temperature range, if the temperature is too low, in the case of cementite precipitation in the hot-rolled sheet stage, it will take too much time for the cementite to re-dissolve; if the temperature is too high, the volume of austenite will be reduced. If the rate is too high, the C concentration in the austenite will decrease, and it will be easily trapped in the transformation of bainite or pearlite containing a large amount of carbides. Therefore, heating at 780°C or higher and 850°C or lower is preferable.

If the cooling rate after holding is less than 20°C/s, there is a possibility of being trapped in the transformation of bainite or pearlite containing a large amount of carbides, so the cooling rate is set at 20°C/s or more. When the cooling end temperature exceeds 200°C, there is a possibility that the aging property may be deteriorated, so it is made 200°C or less. When the cooling end temperature exceeds 350°C, the expected microstructure in which ferrite is the phase with the largest volume fraction and martensite is the second phase cannot be obtained, so cooling is performed to a temperature range below 350°C. The lower limit of the end temperature of the cooling process is not particularly limited, but in the case of water cooling or mist cooling, when the coil is immersed in water for a long time, there is concern about poor appearance due to rust, so it is preferably 50°C or higher .

And, thereafter, skin-pass rolling may be performed as necessary.

In order to galvanize the hot-rolled steel sheet after pickling, or the cold-rolled steel sheet after the above-mentioned recrystallization annealing, immerse in a galvanizing bath, and alloying treatment may be performed as necessary.

Example

(Example 1)

The present invention is further illustrated by Example 1 below.

A-L steels having the chemical compositions shown in Table 1 were smelted in a converter, cast, reheated, rough rolled, and then finished rolled to a thickness of 1.2-5.5 mm, and coiled. The indications about the chemical compositions in the tables are % by mass.

Next, Table 2 shows details of the manufacturing conditions. Here, "SRT" represents the heating temperature of the slab; "FT" represents the finish rolling temperature of the final rolling pass, and the so-called "rolling rate" represents the reduction rate in the temperature range below the Ar ₃ transformation point temperature + 100°C total amount. However, when rolling is carried out in the cold rolling process later, since there is no such restriction, it is described as "-". In addition, "lubrication" means the presence or absence of lubrication in the temperature range of the Ar ₃ transformation point temperature + 100°C or lower.

In addition, in the term "coiling", if the winding temperature (CT) is T ₀ or less, "◯" is indicated, and when it exceeds T ₀ , it is indicated as "×". However, in the case of a cold-rolled steel sheet, since there is no need to specifically limit the production conditions, "-" is indicated.

Next, with respect to a part, pickling, cold rolling, and annealing were performed after hot rolling. The plate thickness is 0.7-2.3mm. Here, the so-called "cold rolling rate" is the total cold rolling rate; "time" is the annealing time; the so-called "annealing", if the annealing temperature is included in the temperature range above the recovery temperature and below the Ar ₃ transformation point temperature + 100 ° C Record as "○", if not in the temperature range, record as "×". In addition, regarding steel L, after rough rolling, descaling was carried out under conditions of a shock pressure of 2.7 MPa and a flow rate of 0.001 liter/cm ² . On the other hand, among the above-mentioned steel sheets, steel G and steel F-5 were galvanized.

The tensile test of the hot-rolled sheet obtained in this way is first processed into the No. 5 test piece described in JIS Z 2201, and is carried out in accordance with the test method described in JIS Z 2241. Table 2 collectively shows yield strength (σY), tensile strength (σB), and elongation at break (E1).

In addition, a test piece of φ30mm cut from the position of 1/4W or 3/4W of the plate width is ground from the outermost layer to a depth of about 0.05mm for three-mounted processing, and then chemical polishing or electrolytic polishing is used to remove strain. The X-ray diffraction intensity was measured according to the method described on pages 274 to 296 of "Keynotes of X-ray Diffraction, New Edition" (published in 1986, translated by Gentaro Matsumura, Agne Co., Ltd.).

Here, the mean value of the X-ray random intensity ratio of the so-called {100}<011>～{223}<110> orientation group is the 3-dimensional structure calculated by the vector method based on the {110} pole figure, or the {110 }, {100}, {211}, {310} pole diagrams using the 3-dimensional structure calculated by the series expansion method using multiple pole diagrams (preferably more than 3), to obtain the main Orientation, {100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>, {335}<110>, and {223}<110> X-ray diffraction intensity.

For example, the X-ray random intensity ratio of each crystal orientation in the latter method uses the (001)[1-10], (116)[1-10], (116)[1- 10], (114)[1-10], (113)[1-10], (112)[1-10], (335)[1-10], (223)[1-10] strength is Can. However, the average value of the X-ray random intensity ratio of the orientation group {100}<011>-{223}<110> is the summed average value of the above-mentioned orientations.

When the strength of all the above orientations cannot be obtained, {100}<011>, {116}<110>, {114}<110>, {112}<110>, {223}<110> can also be used The summed average of each orientation is substituted.

Next, the average value of X-ray random intensity ratios of the three orientations {554}<225>, {111}<112>, and {111}<110> is obtained from the three-dimensional structure calculated in the same way as above. Just go out.

In Table 2, among the X-ray random intensity ratios, the so-called "intensity ratio 1" is the average value of the X-ray random intensity ratios of {100}<011>～{223}<110> orientation groups, and the so-called "intensity The ratio 2" is the average value of the X-ray random intensity ratios of the three orientations of {554}<225>, {111}<112> and {111}<110>.

Next, in order to investigate the notch fatigue strength of the above-mentioned steel plate, the fatigue test piece of the shape shown in Fig. For testing. Among them, the fatigue test piece was ground from the outermost layer to a depth of about 0.05 mm by three-mount processing. The fatigue test uses an electric hydraulic servo fatigue testing machine, and the test method is based on JIS Z 2273-1978 and JIS Z 2275-1978. Table 2 also shows the notch fatigue limit (σWk) and the notch fatigue limit ratio (σWk/σB).

According to the present invention, steels A, E, F-1, F-2, F-5, G, H, I, J, K, and L are 11 types of steel, and thin steel sheets for automobiles with excellent notch fatigue strength can be obtained. The steel plate is characterized by: containing a specified amount of steel components, X-ray random intensity of {100}<011>～{223}<110> orientation groups along the thickness direction from the outermost surface to any depth of 0.5mm The average value of the ratio is more than 2, and the average value of the X-ray random intensity ratio of the three orientations {554}<225>, {111}<112>, and {111}<110> is 4 or less, and the plate thickness is 0.5 More than mm and less than 12mm. Therefore, the fatigue limit ratio of the conventional steel evaluated by the method described in the present invention exceeds 0.2-0.3.

Steels other than the above are out of the scope of the present invention for the following reasons.

That is, the C content of steel B is outside the range of the present invention, so sufficient strength (σB) cannot be obtained. The P content of Steel C is out of the range of the present invention, so sufficient notch fatigue strength (σWk/σB) cannot be obtained. The S content of steel D was out of the range of the present invention, so sufficient elongation (E1) could not be obtained. In steel F-3, the total reduction rate in the temperature range of Ar ₃ transformation temperature + 100°C is outside the range of the present invention, so the structure targeted by the present invention cannot be obtained, and sufficient notch fatigue strength cannot be obtained. (σWk/σB).

The finishing temperature (FT) of steel F-4 is outside the range of the present invention, and the coiling temperature is also outside the range of the present invention, so the structure targeted by the present invention cannot be obtained, and sufficient notch fatigue strength cannot be obtained. (σWk/σB). The cold rolling rate of steel F-6 was out of the range of the present invention, so the structure of the present invention could not be obtained, and sufficient notch fatigue strength (σWk/σB) could not be obtained. The annealing temperature of steel F-7 was outside the range of the present invention, so the structure targeted by the present invention could not be obtained, and sufficient notch fatigue strength (σWk/σB) could not be obtained. The annealing time of steel F-8 was outside the range of the present invention, so the structure of the present invention could not be obtained, and sufficient notch fatigue strength (σWk/σB) could not be obtained.

(Example 2)

Reheat two kinds of steels G and H having the chemical composition shown in Table 1 at the heating temperature shown in Table 3, after rough rolling, and then finish rolling to reach a plate thickness of 1.2-5.5mm, coiling . In addition, as shown in Table 3, several steels were descaled after rough rolling under conditions of impact pressure 2.7 MPa and flow rate 0.001 liter/cm ² .

Table 3 shows details of the production conditions. Here, "SRT" represents the heating temperature of the slab; "FT" represents the finish rolling temperature of the final rolling pass, and the so-called "rolling rate" represents the reduction rate in the temperature range below the Ar ₃ transformation point temperature + 100°C total amount. However, when rolling is carried out in the cold rolling process later, since there is no such restriction, it is described as "-". In addition, "lubrication" means the presence or absence of lubrication in the temperature range of the Ar ₃ transformation point temperature + 100°C or lower. In addition, "CT" means a winding temperature. However, in the case of a cold-rolled steel sheet, since the production conditions do not need to be particularly limited, "-" is indicated. Next, pickling, cold rolling, and heat treatment are performed on a part after hot rolling. The plate thickness is 0.7-2.3mm. The so-called "cold rolling rate" is the total cold rolling rate; the so-called "ST" is the heat treatment temperature; "time" is the heat treatment time. In addition, some of the above-mentioned steel sheets were galvanized.

Tensile tests of the hot-rolled sheets and cold-rolled sheets thus obtained were carried out in the same manner as above.

Table 4 shows yield strength (σY), tensile strength (σB), elongation at break (E1 ), and yield ratio (YR, strength-ductility balance (σB×E1 )). On the other hand, deburring workability (hole expansion property) was evaluated according to the hole expansion test method described in Japan Iron and Steel Federation Standard JFST1001-1996. Table 4 shows the pore expansion ratio (λ).

In addition, the microstructure is also shown in Table 4. Here, "others" means structures other than pearlite and/or ferrite, bainite, retained austenite, and martensite individually shown in Table 4. In the microstructure of the steel plate, the so-called volume fractions of ferrite, bainite, retained austenite, pearlite, and martensite are defined by the following area fractions: The sample cut at the 4W or 3/4W position is ground on the cross-section in the rolling direction, etched using a nital reagent and/or the reagent disclosed in JP-A-5-163590, and observed with an optical microscope at a magnification of 200-500 times The area fraction of the microstructure of 1/4t of the plate thickness.

On the other hand, since austenite has a different crystal structure from ferrite, it can be easily identified crystallographically. Therefore, the volume fraction of retained austenite can also be obtained experimentally by the X-ray diffraction method. That is, a method of simply obtaining the volume fraction of austenite and ferrite from the difference in reflective surface strength between austenite and ferrite via Kα rays of Mo is adopted using the following formula.

Vγ=(2/3){100/(0.7×α(211)/γ(220)+1)}+(1/3){100/(0.78×α(211)/γ(311)+1) }

Among them, α(211), γ(220) and γ(311) are the X-ray reflective surface intensities of ferrite (α) and austenite (γ), respectively. The volume fraction of retained austenite is substantially the same value by any method of optical microscope observation and X-ray diffraction method, so any measured value can be used.

Furthermore, the measurement of the X-ray diffraction intensity and the fatigue test were carried out in the same manner as above.

In addition, the fatigue test was performed by the same method as above. Table 4 shows the notch fatigue limit (σWk) and the notch fatigue limit ratio (σWk/σB).

Following the present invention are steels g-1, g-2, g-3, g-5, g-6, g-7, h-1, h-2, h-3, which can obtain notch fatigue A thin steel sheet for automobiles with excellent strength, which is characterized by: {100}<011>～{223}<110> of the sheet surface along the thickness direction from the outermost surface to any depth of 0.5 mm, containing a predetermined amount of steel components The average value of the X-ray random intensity ratio of the orientation group is more than 2, and the average value of the X-ray random intensity ratio of the three orientations {554}<225>, {111}<112> and {111}<110> 4 or less, and the plate thickness is 0.5 mm to 12 mm, and the phase with the largest volume fraction is bainite, or a composite structure of ferrite and bainite, or contains a volume fraction of 5% or more 25 % or less retained austenite, the remainder is mainly composed of ferrite and bainite, or a composite structure in which the phase with the largest volume fraction is ferrite and the second phase is martensite. Therefore, a significant difference can be seen from the fatigue limit ratio of 20-30% of the conventional steel evaluated by the method of the present invention.

Steels other than the above are out of the scope of the present invention for the following reasons.

That is, the finishing temperature (FT) of steel g-4 and the total rolling reduction in the temperature region below the Ar ₃ transformation point temperature + 100°C are outside the scope of the present invention, so the objective of the present invention cannot be obtained. structure, insufficient notch fatigue strength (σWk/σB) could not be obtained. The cold rolling rate of steel g-8 was outside the range of the present invention, so the structure targeted by the present invention could not be obtained, and sufficient notch fatigue strength (σWk/σB) could not be obtained. The finishing temperature (FT) of steel h-4 and the total rolling reduction in the temperature region below the Ar ₃ transformation temperature + 100°C are outside the scope of the present invention, so the structure that is the object of the present invention cannot be obtained. Sufficient notch fatigue strength (σWk/σB) could not be obtained.

Table 1

Table 2 manufacturing conditions X-ray Random Intensity Ratio mechanical properties fatigue properties hot rolling process Cold rolling and annealing process steel distinguish SRT(°C) FT(°C) Rolling ratio (%) lubricating wind up Cold rolling rate (%) annealing time(s) Intensity ratio 1 Intensity ratio 2 σY(MPa) σB(MPa) E1(%) σWk(MPa) σWk/σB(%) Remark A hot rolled 1250 880 42 none ○ - - - 5.8 0.7 221 311 47 100 32 this invention B hot rolled 1250 890 30 have ○ - - - 1.3 6.1 161 281 56 75 27 compare steel C hot rolled 1200 880 30 none ○ - - - 0.8 1.3 220 369 42 90 twenty four compare steel D. hot rolled 1200 880 30 none ○ - - - 1.2 0.9 195 306 44 75 25 compare steel E. hot rolled 1150 870 42 none ○ - - - 8.1 1.8 422 637 29 230 36 this invention F-1 hot rolled 1200 870 42 none ○ - - - 7.2 2.1 438 668 28 230 34 this invention F-2 hot rolled 1200 870 42 have ○ - - - 8.3 1.4 423 655 29 240 37 this invention F-3 hot rolled 1300 950 0 none ○ - - - 1.8 1.5 431 660 28 150 twenty three compare steel F-4 hot rolled 1300 970 0 none x - - - 1.8 1.7 400 622 32 150 twenty four compare steel F-5 cold rolled 1200 860 - have - 65 ○ 90 4.2 2.3 418 671 28 240 36 this invention F-6 cold rolling 1200 860 - have - 80 ○ 90 2.8 4.2 433 667 28 150 twenty two compare steel F-7 cold rolling 1200 860 - have - 65 x 90 1.7 2.6 552 721 20 150 twenty one compare steel F-8 cold rolled 1200 860 - have - 65 ○ 2 1.8 2.2 570 710 twenty one 150 twenty one compare steel G hot rolled 1150 870 71 none ○ - - - 8.5 0.8 441 661 30 235 36 this invention h hot rolled 1250 870 30 have ○ - - - 8.7 0.9 776 986 16 340 34 this invention I hot rolled 1200 870 42 none ○ - - - 6.7 2.0 404 638 27 220 34 this invention J hot rolled 1200 870 71 none ○ - - - 5.9 2.1 431 623 26 220 35 this invention K hot rolled 1200 870 71 none ○ - - - 7.8 1.0 425 627 30 220 35 this invention L hot rolled 1150 790 71 none ○ - - - 11.0 1.4 401 588 25 210 36 this invention

table 3 manufacturing conditions X-ray Random Intensity Ratio hot rolling process Cold rolling and heat treatment process steel distinguish SRT(°C) FT(°C) Ar ₃ +100(°C) Rolling ratio (%) lubricating CT(°C) T ₀ (℃) Rolling ratio (%) Ac ₁ (°C) ST(°C) Ac ₃ +100(°C) time 1(s) OA (°C) time 2(s) CR(°C/s) Intensity ratio 1 Intensity ratio 2 g-1 hot rolled 1150 870 916 71 have 50 782 - - - - - - - - 8.2 1.1 g-2 hot rolled 1150 870 916 71 have 400 782 - - - - - - - - 8.0 1.0 g-3 hot rolled 1150 890 916 42 have 600 782 - - - - - - - - 8.4 0.9 g-4 hot rolled 1250 930 916 0 none 600 782 - - - - - - - - 1.8 1.5 g-5 cold rolled 1150 870 - - have - - 65 730 800 982 90 - - 5 4.4 2.2 g-6 cold rolling 1150 870 - - have - - 65 730 800 982 - 400 180 30 4.6 2.4 g-7 cold rolling 1150 870 - - have - - 65 730 800 982 90 - - 30 4.8 2.6 g-8 cold rolled 1150 870 - - none - - 80 730 800 982 90 - - 5 2.6 4.3 h-1 hot rolled 1230 860 879 30 have 50 727 - - - - - - - - 8.6 1.2 h-2 hot rolled 1230 860 879 30 have 400 727 - - - - - - - - 8.5 0.9 h-3 hot rolled 1230 860 879 30 have 600 727 - - - - - - - - 8.4 1.3 h-4 hot rolled 1230 930 879 0 none 730 727 - - - - - - - - 1.8 2.1

Table 4 Microstructure mechanical properties fatigue properties Ferrite (%) Bainite (%) Martensite (%) Retained austenite (%) other(%) σY(MPa) σB(MPa) E1(%) lambda (%) σB×E1(MPa*%) YR(%) σWk(MPa) σWk/σB(%) Remark 85 0 13 2 0 470 772 26 52 20072 61 2g0 36 this invention 80 8 0 12 0 512 646 37 67 23902 79 220 34 this invention 67 30 0 0 3 478 576 27 130 15552 83 190 33 this invention 65 35 0 0 0 502 588 28 141 16464 85 120 20 compare steel 70 28 0 0 2 482 584 25 87 14600 83 190 33 this invention 79 10 0 11 0 480 660 36 50 23760 73 230 35 this invention 87 0 10 3 0 444 731 26 42 19006 61 270 37 this invention 50 45 0 2 3 495 591 25 76 14775 84 135 twenty three compare steel 67 5 twenty one 4 3 613 991 twenty one 18 20811 62 330 33 this invention 63 15 3 17 2 694 902 27 26 24354 77 285 32 this invention 35 55 3 4 3 670 823 18 76 14814 81 255 31 this invention 30 63 0 3 4 673 796 20 70 15920 85 180 twenty three compare steel

As described in detail above, the present invention relates to thin steel sheets for automobiles excellent in notch fatigue strength and a method of manufacturing the same. By using these thin steel sheets, it is expected that fatigue cracks will grow from stress-concentrated parts such as blanking parts and welded areas. The problem is that the notch fatigue strength, which is one of the important characteristics of durability, is required to be greatly improved, such as the chassis parts of automobiles. Therefore, the present invention is an invention with high industrial value.

Claims (20)

1. the thin steel sheet for automobile of an excellent in notch fatigue strength, this steel plate contains C:0.01-0.3% in quality %, Si:0.01-2%, Mn:0.05-3%, P :≤0.1%, S :≤0.01%, Al:0.005-1%, remainder is made of Fe and unavoidable impurities, it is characterized in that, plate face along thickness direction from the most surperficial any degree of depth up to 0.5mm 100}＜011 〉～223}＜110〉orientation group's the mean value of the random strength ratio of X ray is more than 2, and 554}＜225 〉, 111}＜112〉and 111}＜110〉mean value of the random strength ratio of X ray of 3 orientations is below 4, thickness of slab is below the above 12mm of 0.5mm.

2. according to the thin steel sheet for automobile of the excellent in notch fatigue strength of claim 1 record, it is characterized in that, in quality %, also further contain Cu:0.2-2%, B:0.0002-0.002%, Ni:0.1-1%, Ca:0.0005-0.002%, REM:0.0005-0.02%, Ti:0.05-0.5%, Nb:0.01-0.5%, Mo:0.05-1%, V:0.02-0.2%, Cr:0.01-1%, more than a kind or 2 kinds of Zr:0.02-0.2%.

3. according to the thin steel sheet for automobile of excellent in notch fatigue strength of claim 1 or 2 records, it is characterized in that the microstructure of above-mentioned steel plate is any following tissue: 1) the volume fraction maximum mutually for bainite or be the complex tissue of ferrite and bainite; 2) containing volume fraction is the complex tissue that the residual austenite below 25% and remainder mainly are made of ferrite, bainite more than 5%; With 3) the volume fraction maximum be that ferrite, second is mutually for martensitic complex tissue mutually.

4. the thin steel sheet for automobile of an excellent in notch fatigue strength is characterized in that, is to carry out zinc-plated making by each thin steel sheet for automobile put down in writing to claim 1-3.

5. a method of making the thin steel sheet for automobile of the described excellent in notch fatigue strength of claim 1 is characterized in that, after the steel disc roughing that will be made of the described component of claim 1, when carrying out hot rolling, at Ar ₃It is finish rolling more than 25% that the thick total draft of steel plate is carried out in humidity province below phase point temperature+100 ℃, the plate face of the most surperficial any degree of depth up to 0.5mm along thickness direction from this steel plate 100}＜011 〉～223}＜110〉orientation group's the mean value of the random strength ratio of X ray is more than 2, and 554}＜225 〉, 111}＜112〉and 111}＜110〉mean value of the random strength ratio of X ray of 3 orientations is below 4, thickness of slab is below the above 12mm of 0.5mm.

6. according to the method for the thin steel sheet for automobile of the manufacturing excellent in notch fatigue strength of claim 5 record, it is characterized in that, after the above-mentioned finish rolling,, under the coiling temperature more than 450 ℃, reel with the above speed of cooling cooling of 20 ℃/s.

7. according to the method for the thin steel sheet for automobile of the manufacturing excellent in notch fatigue strength of claim 5 record, it is characterized in that, after the above-mentioned finish rolling, at Ar ₁Phase point temperature is above, Ar ₃The following humidity province of phase point temperature stops 1-20 second, thereafter again with the above speed of cooling cooling of 20 ℃/s, greater than 350 ℃ and be no more than under 450 ℃ the coiling temperature of humidity province and reel.

8. according to the method for the thin steel sheet for automobile of the manufacturing excellent in notch fatigue strength of claim 5 record, it is characterized in that, after the above-mentioned cooling, under the coiling temperature below 350 ℃, reel.

9. according to the method for the thin steel sheet for automobile of each manufacturing excellent in notch fatigue strength of putting down in writing of claim 5-8, it is characterized in that, when above-mentioned hot rolling, be lubricated rolling.

10. according to the method for the thin steel sheet for automobile of each manufacturing excellent in notch fatigue strength of putting down in writing of claim 5-8, it is characterized in that, when above-mentioned hot rolling, carry out oxide skin after roughing ends and remove.

11. a method of making the thin steel sheet for automobile of the described excellent in notch fatigue strength of claim 1 is characterized in that, will be by after the described steel disc roughing of claim 1, at Ar ₃It is finish rolling more than 25% that the thick total draft of steel plate is carried out in humidity province below phase point temperature+100 ℃, then pickling, again the thick draft of steel plate less than 80% cold rolling after, carry out more than recovery temperature, Ac ₃Humidity province below phase point temperature+100 ℃ keeps the answer or the recrystallization annealing of 5-150 second, refrigerative operation, the plate face of the most surperficial any degree of depth up to 0.5mm along thickness direction from this steel plate 100}＜011 〉～223}＜110〉orientation group's the mean value of the random strength ratio of X ray is more than 2, and 554}＜225 〉, 111}＜112〉and 111}＜110〉mean value of the random strength ratio of X ray of 3 orientations is below 4, thickness of slab is below the above 12mm of 0.5mm.

12. the method according to the thin steel sheet for automobile of the manufacturing excellent in notch fatigue strength of claim 11 record is characterized in that, above-mentioned cold rolling after, carry out at Ac ₁Phase point temperature is above, Ac ₃Humidity province below phase point temperature+100 ℃ keeps 5-150 second, carries out the thermal treatment of refrigerative operation thereafter.

13. method according to the thin steel sheet for automobile of the manufacturing excellent in notch fatigue strength of claim 11 record, it is characterized in that, keep 5-150 after second in the said temperature district, carry out being cooled to greater than 350 ℃ and less than 450 ℃ humidity province with the above speed of cooling of 20 ℃/s, thereafter, keep in this humidity province again 5-600 second, be cooled to the thermal treatment of the operation of the humidity province below 200 ℃ with the 5 ℃/speed of cooling more than the s.

14. method according to the thin steel sheet for automobile of the manufacturing excellent in notch fatigue strength of claim 11 record, it is characterized in that, keep 5-150 after second in the said temperature district, carry out being cooled to the thermal treatment of the operation of the humidity province below 350 ℃ with the above speed of cooling of 20 ℃/s.

15. method of making the thin steel sheet for automobile of excellent in notch fatigue strength, it is characterized in that, in claim 5 or 11 steel plates of being put down in writing, also further contain Cu:0.2-2%, B:0.0002-0.002%, Ni:0.1-1%, Ca:0.0005-0.002%, REM:0.0005-0.02%, Ti:0.05-0.5%, Nb:0.01-0.5%, Mo:0.05-1%, V:0.02-0.2%, Cr:0.01-1%, more than a kind or 2 kinds of Zr:0.02-0.2% in quality %.

16. the method for thin steel sheet for automobile according to the manufacturing excellent in notch fatigue strength of claim 6 or 12 records is characterized in that, the microstructure of above-mentioned steel plate be the volume fraction maximum mutually for bainite or be the complex tissue of ferrite and bainite.

17. the method for thin steel sheet for automobile according to the manufacturing excellent in notch fatigue strength of claim 7 or 13 records, it is characterized in that the microstructure of above-mentioned steel plate is that to contain volume fraction be the complex tissue that the residual austenite below 25% and remainder mainly are made of ferrite, bainite more than 5%.

18. according to Claim 8 or the method for thin steel sheet for automobile of the manufacturing excellent in notch fatigue strength of 14 records, it is characterized in that, the microstructure of above-mentioned steel plate be the volume fraction maximum be martensitic complex tissue mutually for ferrite, second mutually.

19. method of making the thin steel sheet for automobile of excellent in notch fatigue strength, it is characterized in that, after making claim 5 or 11 hot-rolled steel sheets of being put down in writing or answer or recrystallization annealing plate, this steel plate of dipping in zinc-plated bath is implemented zinc-plated to surface of steel plate again.

20. the method according to the thin steel sheet for automobile of the manufacturing excellent in notch fatigue strength of claim 19 record is characterized in that, above-mentioned zinc-plated after, carry out Alloying Treatment again.

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