JP2009263718A - Hot-rolled steel plate superior in hole expandability and manufacturing method therefor - Google Patents

Abstract

A high-strength hot-rolled steel sheet having small in-plane anisotropy and excellent hole expansibility and a method for producing the same are provided.
SOLUTION: In mass%, C: 0.005 to 0.150%, Si: 2.50% or less, Mn: 0.10 to 3.00%, P: 0.150% or less, S: 0.00. 0150% or less, Al: 0.150% or less, N: 0.0100% or less, Nb: 0.005 to 0.07%, the balance is made of Fe and inevitable impurities, and the structure is ferrite or ferrite And an average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups of the plate surface at 1/2 plate thickness is 4 or less. 0.0 or less, the average value of the X-ray random intensity ratio of the {554} <225> azimuth, {111} <112> azimuth, and {111} <110> azimuth is 4.5 or less. Hot rolled steel sheet with excellent properties.
[Selection] Figure 1

Description

  The present invention relates to a hot-rolled steel sheet excellent in hole expansibility, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet subjected to surface treatment, and methods for producing them.

  Since parts such as suspension arms of automobiles are subjected to burring and the like, the hot-rolled steel sheet as a material is required to have a hole expanding property. Since the hole expanding process is a process for uniformly expanding the diameter of the punched hole, a hot-rolled steel sheet with improved in-plane anisotropy and improved hole expanding property has been proposed (for example, Patent Document 1). ). However, since these hot-rolled steel sheets have low strength, they cannot cope with the increase in strength of hot-rolled steel sheets aimed at reducing the weight of automobiles.

  On the other hand, a hot-rolled steel sheet is proposed in which the strength is increased and the local elongation, which is considered to have a correlation with the hole expansion property, is controlled (for example, Patent Documents 2 to 4). However, these are hot-rolled steel sheets that have been subjected to hot rolling while suppressing recrystallization and controlled to a texture in which a specific crystal orientation has been developed to improve shape freezeability. Therefore, there is a problem that the r value (Rankford value) in the rolling direction (L direction) and the width direction (C direction) is low, and the r value in the 45 ° direction therebetween is high.

In addition, a hot-rolled steel sheet has been proposed in which the rolling reduction of the final stand is reduced, the development of the working texture of the austenite structure due to rolling is suppressed, the in-plane anisotropy is reduced, and the stretch flangeability is improved (patent) Reference 5). However, in this method, the texture is not sufficiently randomized, so that the hole expandability when evaluated under severe test conditions is insufficient.
JP-A-5-171270 JP 2004-183057 A JP 2004-250743 A JP-A-2005-15854 JP 2000-297349 A

  An object of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 490 MPa or more, excellent workability, particularly hole expansibility, and reduced in-plane anisotropy, and a method for producing the same. Is.

  In the present invention, the amount of Nb that delays recrystallization is set to an appropriate range, the finishing temperature of hot rolling, the rolling reduction of finishing rolling, and the temperature range are controlled, and more preferably by controlling the air cooling time, It is a hot-rolled steel sheet that promotes recrystallization, suppresses the development of rolling texture, and randomizes the crystal orientation, thereby improving strength, ductility, and hole expandability, and reducing in-plane anisotropy. The gist of the present invention is as follows.

(1) By mass%, C: 0.005 to 0.150%, Mn: 0.10 to 3.00%, Nb: 0.005 to 0.07%, Si: 2.50% or less , P: 0.150% or less, S: 0.0150% or less, Al: 0.150% or less, N: 0.0100% or less, the balance is made of Fe and inevitable impurities, and the area ratio of ferrite Is 5% or more, the area ratio of pearlite is limited to 20% or less, the total area ratio of one or both of martensite and retained austenite is limited to 5% or less, and the balance has a metal structure composed of bainite. The average grain size of the ferrite is 30 μm or less, and the average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups on the plate surface at 1/2 plate thickness is 4. 0 or less, {554} <225> orientation, 111} <112> orientation and {111} <110> excellent hot rolled steel sheet hole expansion to wherein the average value of the azimuth X-ray random intensity ratio of 4.5 or less.
(2) The heat excellent in hole expansibility according to the above (1), containing 1 type or 2 types of Ti: 0.05% or less and B: 0.0015% or less in mass% Rolled steel sheet.
(3) By mass%, Mo: 0.01-2.00%, Cr: 0.01-2.00%, W: 0.01-2.00%, Cu: 0.01-2.00% Ni: 0.01-2.00% of 1 type or 2 types or more, The hot rolled steel sheet excellent in hole expansibility as described in said (1) or (2) characterized by the above-mentioned.
(4) By mass%, one or more of Ca: 0.0005 to 0.1000%, Rem: 0.0005 to 0.1000%, V: 0.001 to 0.100% should be contained. The hot rolled steel sheet excellent in hole expansibility according to any one of (1) to (3) above.
(5) The Rankford value in the rolling direction, the Rankford value in a direction perpendicular to the rolling direction, and the Rankford value in a direction 45 ° from the rolling direction are more than 0.7 and 1.2 or less ( A hot rolled steel sheet excellent in hole expansibility according to any one of 1) to (4).
(6) A hot-dip galvanized steel sheet excellent in hole expansibility, wherein the hot-rolled steel sheet according to any one of (1) to (5) is subjected to hot-dip galvanization.
(7) Alloyed hot dip galvanized excellent in hole expansibility, characterized in that the hot rolled steel sheet according to any one of the above (1) to (5) is alloyed hot dip galvanized. steel sheet.
(8) The hot-rolled steel sheet according to any one of (1) to (5), the hot-dip galvanized steel sheet according to (6), or the galvannealed steel sheet according to (7) is optional. A steel pipe with excellent hole expandability characterized by being wound in the direction.

(9) A steel slab having the chemical component according to any one of the above (1) to (4) is heated to 1050 to 1300 ° C., and the total rolling reduction at 1000 ° C. or less is 50% or more, finishing temperature FT [° C.] is subjected to hot rolling at 930 ° C. or _higher, from the Ae 3 -100 ° C., cooling the average cooling rate to higher of 650 ° C. or coiling temperature of less than 15.0 ° C. / s, A method for producing a hot-rolled steel sheet excellent in hole expansibility, characterized by winding at over 350 ° C to 700 ° C.
(10) The air cooling time after the end of hot rolling is air cooled as X [s] or more and 100 s or less obtained from the finishing temperature FT [° C.] according to the following (Equation 1). A method for producing hot-rolled steel sheets with excellent hole expandability.
X = 10 ^{(18.4−0.0194 × FT)} (Formula 1)
(11) A hot dip galvanized steel sheet having excellent hole expansibility, characterized by subjecting the hot rolled steel sheet having excellent hole expansibility manufactured by the method described in (9) or (10) to hot dip galvanization. Production method.
(12) The method for producing a hot-dip galvanized steel sheet having excellent hole expansibility described in (11) above, wherein hot-dip galvanizing is performed in a continuous line.
(13) Alloying with excellent hole-expandability, characterized by performing a heat treatment for 5 seconds or more in a temperature range from 450 to 600 ° C. after performing the hot dip galvanizing described in (11) or (12) above. Manufacturing method of hot dip galvanized steel sheet.
(14) A hot-rolled steel sheet obtained by the production method described in (9) or (10), a hot-dip galvanized steel sheet obtained by the production method described in (11) or (12), or (13 A method for producing a steel pipe excellent in hole expansibility, characterized in that the alloyed hot-dip galvanized steel sheet obtained by the production method described in 1) is formed as a base material.

  According to the present invention, it is possible to obtain a high-strength hot-rolled steel sheet having excellent hole expandability and reduced in-plane anisotropy, and the industrial contribution is extremely remarkable.

  The hole expandability is evaluated by the hole expansion rate when the entire circumference of the punched hole is uniformly expanded and a crack first occurs. That is, since deterioration of characteristics in a specific direction becomes a problem, there is a possibility that it is preferable to suppress the accumulation of specific crystal orientations and reduce characteristic anisotropy when hot rolling a steel sheet. is there. Then, the present inventors examined the method of improving the hole expansibility of a high strength steel plate by suppressing the development of a specific texture after hot rolling. Specifically, after completion of hot rolling in a temperature range where the metal structure is only an austenite phase (referred to as a γ range), the austenite phase is recrystallized before phase transformation starts to improve the hole expansion property. I planned.

  Nb is necessary for precipitation strengthening, but is also an element that suppresses recrystallization. Therefore, it is considered that when Nb is added excessively, a rolling texture develops during hot rolling, which hinders improvement in hole expansibility. Therefore, the present inventors first examined the amount of Nb. As a result, Nb needs to have a lower limit of 0.005% or more in order to increase the tensile strength to 490 MPa or more, and an upper limit of 0.07% or less in order to promote recrystallization. I found out.

  Next, the influence of the temperature of finish rolling on the recrystallization behavior of austenite was examined. In the laboratory, the present inventors performed a processing for master test simulating hot rolling by changing the processing temperature using steel having an Nb amount within an appropriate range. The processing for master test is performed using an apparatus that heats a cylindrical sample having a diameter of 10 mm, performs uniaxial compression processing while controlling the temperature, and cools the sample. Processing was performed at various processing temperatures with a processing rate of 50%, quenched, and ferrite transformation was suppressed, and the recrystallization rate of austenite was measured. The recrystallization rate of austenite was obtained by observing the structure with an optical microscope. As a result, as shown in FIG. 1, it was found that when the processing temperature (finishing temperature FT) was set to 930 ° C. or higher, the recrystallization rate was 90% or higher.

Furthermore, the inventors changed the processing temperature and the retention time after processing by the processing for master test, and the time until the recrystallization rate of austenite becomes 90% or more (referred to as recrystallization time). It was measured. As a result, the graph of FIG. 2 showing the relationship between the processing temperature and the recrystallization time was obtained. The straight line in FIG. 2 is an approximate expression of the processing temperature and the recrystallization time. If the processing temperature is FT [° C.] and the recrystallization time is X [s], it can be expressed by the following (Expression 1).
X = 10 ^{(18.4−0.0194 × FT)} (Formula 1)

  In addition, as shown in FIG. 2, although the straight line of (Formula 1) and an experimental result correspond substantially at 930 degreeC or more, the tendency for a difference to spread a little wide was seen below 930 degreeC. Therefore, in hot rolling, it is considered preferable to set the finishing temperature FT [° C.] to 930 ° C. or higher and further air-cool for a time equal to or longer than X [s] obtained by (Equation 1). By air cooling after hot rolling, recrystallization further proceeds and the in-plane anisotropy of the steel sheet can be significantly reduced.

  Based on the above examination results, the present inventors further examined the temperature and reduction rate of hot rolling. As a result, it was found that in hot rolling, fine recrystallized austenite grains can be obtained by increasing the rolling reduction at a lower temperature and accumulating dislocations as driving force for recrystallization. On the other hand, it was also found that when the rolling reduction at a higher temperature was increased, recrystallization occurred during rolling and the austenite grains became coarse. Specifically, in the hot rolling, it is necessary to set the rolling reduction at 1000 ° C. or less to 50% or more. Thereby, the austenite grains after hot rolling are recrystallized, and the development of the rolling texture is suppressed. The reduction ratio at 1000 ° C. or less of the present invention is defined as a numerical value expressed as a percentage by dividing the difference between the plate thickness at 1000 ° C. and the plate thickness after finish rolling by the plate thickness at 1000 ° C. Is done.

  Next, the present inventors perform hot rolling at a reduction rate of 50% or more at 1000 ° C. or less and a finishing temperature of 930 ° C. or more in a laboratory, and control the cooling rate so as to generate ferrite. A steel plate was produced. In addition, in order to simulate winding after hot rolling, after hot rolling and cooling, it hold | maintained in the furnace of predetermined temperature for 1 hour. A test piece was collected from the obtained steel sheet, and microscopic observation, tensile test, hole expansion test, and texture measurement were performed using an optical microscope.

  The tensile test was performed according to JIS Z 2241, and the Rankford value was measured according to JIS Z 2254. The hole expansion test was performed according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996. Texture was measured by X-ray diffraction.

  As a result, a hot-rolled steel sheet having excellent hole expansibility and small in-plane anisotropy has a texture close to random, and {100} <011> to {223} <110> of the sheet surface at 1/2 sheet thickness. The average value of the X-ray random intensity ratio of the orientation group is 4.0 or less, and the X-ray random intensity ratio of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> It was found that the average value was 4.5 or less. These orientations are textures developed by rolling. Moreover, as a result of observation of the microstructure of the steel sheet, it was found that the hot rolled steel sheet having good hole expansibility has a ferrite grain size of 30 μm or less. Moreover, when the Rankford value in the rolling direction, the Rankford value in the direction perpendicular to the rolling direction, and the Rankford value in the direction of 45 ° from the rolling direction of these steel plates are measured, they are 0.7 to 1.2 or less. It was found that the in-plane anisotropy was small.

As a result of the above investigation, in hot rolling, the rolling reduction at a relatively low temperature is increased, and the hot rolling is completed at 930 ° C. or higher to promote recrystallization. It was confirmed that the development of the structure was suppressed, the ferrite was refined, the hole expandability was improved, and the in-plane anisotropy was reduced.
Moreover, in order to obtain the above hot rolled steel sheet with good hole expansibility by the above examination, hot rolling is performed at a reduction rate of 50% or higher at 1000 ° C. or lower and a finishing temperature of 930 ° C. or higher, and Ae ₃ − It was confirmed that the average cooling rate from 100 ° C. to 650 ° C. or the higher one of the coiling temperatures should be less than 15.0 ° C./s, and the coil should be wound at 350 to 700 ° C.

Hereinafter, the present invention will be described in detail.
Nb is an important element that forms NbC that combines with C and contributes to precipitation strengthening. In order to ensure the strength, it is necessary to add 0.005% or more of Nb. However, since Nb is an element that suppresses recrystallization of austenite, the upper limit is made 0.07%. In order to promote the recrystallization of austenite, the upper limit of Nb is preferably 0.05% or less, and more preferably 0.04% or less.

  C is an element that increases the strength, and needs to be added in an amount of 0.005% or more. If the C content exceeds 0.150%, the weldability may be impaired, or the workability may be extremely deteriorated due to an increase in the hard structure, so the upper limit is made 0.150%. Further, if the C content exceeds 0.100%, the moldability deteriorates, so the C content is preferably 0.100% or less.

  Si is a deoxidizing element, and if the added amount exceeds 2.50%, the press formability deteriorates, so 2.50% is made the upper limit. Moreover, since chemical conversion processability will fall if there is much Si amount, it is preferable to set it as 1.20% or less, and also in order to make the surface pattern resulting from Si scale inconspicuous, it shall be less than 0.50%. It is preferable. In addition, when hot dip galvanizing is performed, problems such as a decrease in plating adhesion and a decrease in productivity due to a delay in the alloying reaction may occur. Therefore, the Si content is preferably 1.00% or less. . The lower limit is not specified, but if it is less than 0.001%, the manufacturing cost becomes high. Si is an element that increases the strength by solid solution strengthening. Therefore, 0.01% or more may be added according to the target strength level.

  Mn is an element contributing to solid solution strengthening and needs to be added in an amount of 0.10% or more. In order to improve the strength, the preferable lower limit is 0.5% or more. However, in order to make a segregation zone in steel by segregation and deteriorate stretch flange formability, the upper limit was made 3.0%. Further, when the strength is increased by the addition of Mn, the fatigue characteristics may be impaired, so the preferable upper limit is 2.0% or less.

  P is an impurity, and if the content exceeds 0.150%, the fatigue strength after spot welding deteriorates, the yield strength increases, and a surface shape defect is caused during pressing. Therefore, the upper limit of the P content is 0.150%. Moreover, since P is an element that delays the alloying reaction during continuous hot dip galvanization, the upper limit is preferably made 0.100% or less. Furthermore, since the secondary workability is improved by reducing P, the content is preferably made 0.050% or less. When it is necessary to increase the strength, 0.005% or more may be contained.

S is an impurity, and if it exceeds 0.0150%, it causes hot cracking or deteriorates workability, so 0.0150% is made the upper limit.
Al is a deoxidizing element, and if added in excess, weldability deteriorates, so the upper limit is made 0.150%. Although a minimum is not specifically limited, From a viewpoint of deoxidation, it is preferable to add 0.010% or more of Al.

  N is an impurity, and when coarse Nb nitride precipitates, the amount of NbC that contributes to an increase in strength decreases, so it is limited to 0.0100% or less. From this viewpoint, it is preferably 0.0050% or less, and more preferably 0.0020% or less. The lower limit is not particularly set, but the cost is increased to make it lower than 0.0005%.

Furthermore, Ti, B, Mo, Cr, W, Cu, Ni, Ca, Rem, and V may be added as necessary.
Ti is an element that combines with C to generate TiC to improve strength, and is added as necessary. Since Ti is also an element that suppresses recrystallization of austenite, the upper limit is preferably 0.05%. From this viewpoint, it is preferably 0.03% or less, more preferably 0.01% or less.

  B is an element that promotes the formation of bainite to increase the strength, and is added as necessary. Since B is also an element that suppresses recrystallization of austenite, the upper limit is preferably made 0.0015%. From this viewpoint, it is preferably 0.0010% or less, more preferably 0.0005% or less.

  Mo, Cr, W, Cu, and Ni are elements that contribute to the improvement of strength and material, and it is preferable to add one or more of each at 0.01% or more. On the other hand, when the addition amount of each element exceeds 2.00%, pickling property, weldability, hot workability and the like may be deteriorated, so a preferable upper limit is 2.00%.

  Ca, Rem, and V are elements that contribute to strength improvement and material improvement, and it is preferable to add one or more of them. If the addition amount of Ca and Rem is less than 0.0005% and the addition amount of V is less than 0.001%, sufficient effects may not be obtained. On the other hand, when Ca and Rem are added so that the addition amount exceeds 0.1000% and the addition amount of V exceeds 0.100%, ductility may be impaired. Therefore, Ca, Rem and V are preferably added in the range of 0.0005 to 0.1000%, 0.0005 to 0.1000% and 0.001 to 0.100%, respectively.

Next, the metal structure of the hot rolled steel sheet according to the present invention will be described.
The metal structure of the hot-rolled steel sheet of the present invention is ferrite or a mixed structure of ferrite and bainite. Since ferrite has a structure rich in ductility, the area ratio of ferrite needs to be 5% or more in order to ensure sufficient ductility. Ferrite is precipitation strengthened and may be a single phase, but even a composite structure composed of both ferrite and bainite can improve the strength without impairing the hole expanding property.

  Depending on the component composition and production conditions, pearlite may be generated, but if the upper limit is limited to 20%, strength and workability will not be impaired. In addition, if there is martensite or residual austenite (referred to as residual γ), the hole expandability deteriorates. Therefore, in the hot-rolled steel sheet of the present invention, the area ratio of one or both of martensite and residual γ is 5% or less. Restrict. The cause of the decrease in hole expansibility due to martensite and residual γ is that the interface between the main phase and the harder structure becomes the starting point of cracking. Therefore, it is preferable to limit martensite which is a hard structure and retained austenite which is transformed into martensite by punching.

  In addition, the smaller the average particle diameter of ferrite, the higher the strength can be achieved by making finer. Therefore, in the hot-rolled steel sheet of the present invention, the average grain size of ferrite is set to 30 μm or less. Moreover, since ductility improves by refinement | miniaturization, Preferably it is 20 micrometers or less, More preferably, it is 15 micrometers or less.

Next, the reason for limiting the texture will be described. In the present invention, it is important to randomize the texture and particularly suppress the development of the texture due to rolling.
Average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group: These orientation groups are orientation groups developed by rolling, and increase the in-plane anisotropy. The X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group is 4.0 or less. In order to reduce the in-plane anisotropy, the smaller the better, the more preferably 3.5 or less, and still more preferably 3.0 or less.

  Note that, among the {100} <011> to {223} <110> orientation groups, the X-ray random intensity ratio of the {001} <110> orientation is characterized by becoming higher as recrystallization proceeds. In the hot-rolled steel sheet of the present invention, the {001} <110> orientation tends to become the maximum among the {100} <011> to {223} <110> orientation groups. Further, when the X-ray random intensity ratio in the {001} <110> orientation increases, the in-plane anisotropy tends to increase, and is preferably 5.0 or less. From this viewpoint, it is more preferably 4.5 or less, and still more preferably 4.0 or less.

  The three crystal orientations of {554} <225>, {111} <112>, and {111} <110> are orientations developed by rolling, like the {100} <011> to {223} <110> orientation groups. In order to increase the in-plane anisotropy, it is preferable to reduce the X-ray random intensity ratio as much as possible. Therefore, the average value of the X-ray random intensity ratios of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> is set to 4.5 or less. From this viewpoint, it is preferably 4.0 or less, more preferably 3.5 or less.

  {100} <011> to {223} <110> orientation group, {001} <110> orientation, {554} <225> orientation, {111} <112> orientation and {111} <110> orientation X-rays The random intensity ratio is a three-dimensional texture calculated by a series expansion method based on a plurality of pole figures among {110}, {100}, {211}, {310} pole figures measured by X-ray diffraction. What is necessary is just to obtain | require from the crystal orientation distribution function (Orientation Distribution Function, ODF) to represent. Note that the X-ray random intensity ratio means that the X-ray intensity of a standard sample that does not accumulate in a specific orientation and the test material is measured under the same conditions by the X-ray diffraction method or the like. It is a numerical value obtained by dividing the line intensity by the X-ray intensity of the standard sample.

  The sample for X-ray diffraction is manufactured as follows. The steel plate is polished to a specified position in the thickness direction by mechanical polishing or chemical polishing, and finished to a mirror surface by buffing, and then the distortion is removed by electrolytic polishing or chemical polishing, and at the same time, the 1/2 plate thickness part is the measurement surface. Adjust so that Since it is difficult to accurately set the measurement surface to ½ plate thickness, the sample should be prepared so that the measurement surface is within the range of 3% of the plate thickness with the target position as the center. That's fine. When measurement by X-ray diffraction is difficult, a statistically sufficient number of measurements may be performed by an EBSP (Electron Back Scattering Pattern) method or an ECP (Electron Channeling Pattern) method.

Next, the reason for limiting the manufacturing conditions will be described.
Steel is melted and cast by a conventional method to obtain a steel piece to be subjected to hot rolling. Although this steel slab may be a forged or rolled steel ingot, it is preferable to manufacture the steel slab by continuous casting from the viewpoint of productivity. Moreover, you may manufacture with a thin slab caster.
Usually, the steel slab is cooled after casting, so when performing hot rolling, it is heated again. Also, a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting the molten steel may be employed.

  In the present invention, it is necessary to set the rolling reduction at 1000 ° C. or lower to 50% or more and the hot rolling finishing temperature to be 930 ° C. or higher. Therefore, the heating temperature of the steel slab is set to 1050 ° C. or higher. Moreover, in order to sufficiently dissolve alloy carbides such as TiC and NbC and to heat the steel slab efficiently and uniformly, the heating temperature is preferably 1100 ° C. or higher, more preferably 1150 ° C. or higher. Although the upper limit of the heating temperature is not specified, when the steel slab is heated to over 1300 ° C., the crystal grain size of the steel sheet becomes coarse and the workability may be impaired.

  In hot rolling, it is necessary to increase the rolling reduction at a lower temperature. At temperatures exceeding 1000 ° C., it is difficult to refine the crystal grain size even if the rolling reduction is increased. Therefore, the rolling reduction at 1000 ° C. or less becomes important. The rolling reduction at 1000 ° C. or less is set to 50% or more in order to sufficiently promote recrystallization after finish rolling. From this viewpoint, the rolling reduction at 1000 ° C. or less is preferably 60% or more, and more preferably 70% or more.

  The finish temperature FT [° C.] of hot hot rolling is extremely important in the present invention. When finish rolling is performed at a temperature lower than 930 ° C., recrystallization does not proceed thereafter, and the in-plane anisotropy is increased. Spreadability is reduced. Therefore, the lower limit of the finishing temperature is 930 ° C. In this respect, 940 ° C. or higher is preferable, and 950 ° C. or higher is more preferable.

Further, it is more preferable to control the air cooling time after the hot rolling is completed in order to promote recrystallization of austenite. In order to reduce the in-plane anisotropy by recrystallization after finishing rolling, the air cooling time should be set to X [s] or more determined by the following (Formula 1) by the finishing temperature FT [° C.]. Is preferred.
X = 10 ^{(18.4−0.0194 × FT [° C.])} (Formula 1)
On the other hand, when the air cooling time is long, grain growth proceeds and the austenite grain size may become coarse. Therefore, it is preferable that the upper limit of the air cooling time after the hot rolling is 100 s. More preferably, it is within 10 s, and even more preferably within 5 s.

In order to sufficiently produce a ferrite structure rich in ductility, control of the cooling rate is important. In particular, during cooling, it is important to slow down the cooling rate to Ae ₃ -100 ° C. or lower and 650 ° C., which is the temperature range near the nose of the ferrite transformation. Incidentally, in the present invention, when the winding in the range of 650 ° C. greater than 700 ° C. or _less, which controls the cooling rate from the Ae 3 -100 ° C. to a coiling temperature, wind at 650 ° C. or less, Ae ₃ Control the cooling rate from -100 ° C to 650 ° C. When the cooling rate is high, the ferrite transformation does not proceed sufficiently, so the temperature is made less than 15.0 ° C./sec. From this viewpoint, a more preferable cooling rate is 10.0 ° C./s or less.

Ae ₃ [° C.] is calculated by the following (formula 2) depending on the content [mass%] of C, Mn, Si, Cu, Ni, Cr, and Mo. In addition, when not containing a selective element, it calculates as 0.
Ae ₃ = 911-239C-36Mn + 40Si-28Cu-20Ni-12Cr + 63Mo (Formula 2)

  In the present invention, the coiling temperature is also important, and it is necessary to set the temperature above 350 ° C. to 700 ° C. When it winds up above 700 degreeC, a grain growth will advance and it will become a coarse structure. In order to prevent the grain growth from proceeding, it is preferable to set the temperature to 650 ° C. or lower. The reason why the lower limit is set to over 350 ° C. is that martensite increases at 350 ° C. or lower, and the hole expandability deteriorates. In order to suppress the formation of martensite, the lower limit is preferably 400 ° C. or higher.

  The hot-rolled steel sheet may be subjected to a skin pass having a reduction rate of 10% or less by pickling, in-line or off-line as necessary.

  The hot-rolled steel sheet may be hot dip galvanized or alloyed hot dip galvanized. When the steel sheet is annealed, it may be subjected to hot dip galvanization as it is in a continuous hot dip galvanizing line after cooling. The composition of the galvanizing is not particularly limited, and besides zinc, Fe, Al, Mn, Cr, Mg, Pb, Sn, Ni, etc. may be added as necessary.

  The alloying heat treatment is performed within a range of 450 to 600 ° C. after hot dip galvanization. If it is less than 450 ° C, alloying does not proceed sufficiently, and if it exceeds 600 ° C, alloying proceeds excessively and the plated layer becomes brittle, which causes problems such as peeling of the plating due to processing such as pressing. . The alloying time is 5 s or longer. If it is less than 5 s, alloying does not proceed sufficiently. Although the upper limit is not particularly defined, it is preferably about 10 s in consideration of plating adhesion.

  The hot-rolled steel sheet may be subjected to Al-based plating or various electroplating. Further, the hot-rolled steel sheet and various plated steel sheets can be subjected to surface treatment such as organic coating, inorganic coating, and various paints according to the purpose.

  Moreover, there is no problem even if a steel pipe is manufactured using the above hot-rolled steel sheet and plated steel sheet as a raw material. The pipe making method may be any method such as UO pipe, electric resistance welding, spiral, or the like.

Steel pieces having the composition shown in Table 1 were melted to produce steel pieces. The steel slab was heated, followed by hot rough rolling and finish rolling under the conditions shown in Table 2. The rolling speed was about 300 to 1000 mpm. Moreover, among these steel plates, when hot dip galvanizing was performed after the hot rolling was completed, “melting” and when alloying hot dip galvanizing at 520 ° C. for 15 seconds was performed, “alloy” was indicated.
Ae ₃ [° C.] is a transformation temperature calculated by the following (formula 2) depending on the content [mass%] of C, Mn, Si, Cu, Ni, Cr, and Mo. In addition, when not containing the selective element, it calculated as 0.
Ae ₃ = 911-239C-36Mn + 40Si-28Cu-20Ni-12Cr + 63Mo (Formula 2)

Furthermore, in Table 2, SRT [° C.] is the heating temperature of the steel slab, FT [° C.] is after the final pass of rolling, that is, finishing temperature, t _AC [s] is the air cooling time after hot rolling is completed, and CR [° C. / S] is the average cooling rate from Ae ₃ -100 ° C. to 650 ° C. or the higher one of the coiling temperatures, and CT [° C.] is the coiling temperature. The rolling reduction is a value obtained by dividing the difference between the plate thickness at 1000 ° C. and the finished plate thickness by the plate thickness at 1000 ° C. According to (Formula 1), X [s] was calculated from the finishing temperature FT [° C.] and listed in Table 2.
X = 10 ^{(18.4−0.0194 × FT)} Expression (1)

The structure of the obtained steel sheet was observed by corroding the mirror-polished steel sheet with a nital solution, and using an optical microscope, and determining the ferrite area ratio, bainite area ratio, pearlite area ratio, and average ferrite particle diameter by image analysis. For steel sheets whose structure was difficult to discern by optical microscope observation, the ferrite area ratio, bainite area ratio, and pearlite area ratio were determined by the Image Quality map obtained by the SEM-EBSP method. The total area ratio of retained austenite and martensite was determined by corroding a mirror-polished steel sheet with a repeller corrosive solution and observing with an optical microscope and image analysis.
In Table 3, V _α [%] is the ferrite area ratio, V _B [%] is the bainite area ratio, V _p [%] is the pearlite area ratio, and V _{γ + M} [%] is the retained austenite and martensite. Is the sum of the area ratios.

  Further, three {100} <011> to {223} <110> orientation groups, {554} <225>, {111} <112>, and {111} <110> in the ½ plate thickness portion of the steel plate The X-ray random intensity ratio of crystal orientation was measured as follows. First, after mechanically polishing and buffing the steel plate, X-ray diffraction was performed using a sample that was further electropolished to remove strain and adjusted so that the 1/2 plate thickness portion became the measurement surface. Note that X-ray diffraction of a standard sample having no accumulation in a specific orientation was performed under the same conditions. Next, ODF was obtained by the series expansion method based on {110}, {100}, {211}, {310} pole figures obtained by X-ray diffraction. From this ODF, X-ray random intensities of three crystal orientations of {100} <011> to {223} <110> orientation group, {554} <225>, {111} <112> and {111} <110> The ratio was determined.

Furthermore, the tensile test piece based on JISZ2201 was extract | collected from the steel plate, the tensile test was performed based on JISZ2241, and tensile strength (TS) and elongation (breaking elongation, EL) were measured.
The Rankford value (r value) is a Rankford value (rL) in the rolling direction according to JIS Z 2254, using a No. 13 B tensile test piece of JIS Z 2201, and Rankford in the direction of 45 ° from the rolling direction. The value (rD) and the Rankford value (rC) in the direction perpendicular to the rolling direction were measured. The amount of strain was set to 5% in consideration of the uniform elongation of the steel sheet having low ductility.

  The hole expansion test was performed according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996, and the initial hole was punched with a die diameter so that the clearance would be 12.5% according to the punch of φ10 and the plate thickness. The test was performed under severe conditions of the shoulder R2 (conditions in which the λ value becomes low), and the hole expansion value λ was evaluated.

  The results are shown in Table 3. In Tables 1 to 3, the underline means outside the scope of the present invention or outside the preferred range.

  As is apparent from Table 3, when steel having the chemical composition of the present invention is hot-rolled under appropriate conditions, a high strength (TS) -hole expansion value (λ) -elongation (EL) balance must be satisfied. I was able to. Hot rolling No. No. 6, although the air cooling time after finish rolling is slightly shorter than X and recrystallization does not proceed sufficiently, the anisotropy is slightly stronger, but shows better characteristics than the comparative example shown below. . Note that (TS), (λ), and (EL) are contradictory characteristics. In general, the higher the value of TS, the smaller the values of EL and λ. Therefore, a high value of (TS) × (λ) × (EL) indicates that the three contradictory characteristics of (TS), (λ), and (EL) are good regardless of the strength (TS) level. Show. As shown in Table 3, the invention steel has a better value of (TS) × (λ) × (EL) than the comparative steel, except for those with TS outside the range.

  On the other hand, hot rolling No. Nos. 25 to 27 are steel Nos. Whose chemical components are outside the scope of the present invention. It is a comparative example using P to R. Hot rolling No. No. 25, Nb is outside the scope of the present invention, and by excessive addition, recrystallization of austenite is suppressed, texture develops, and X-rays of {100} <011> to {223} <110> orientation group Since the X-ray random intensity ratio of the three crystal orientations of the random intensity ratio, {554} <225>, {111} <112> and {111} <110> exceeds the upper limit, the in-plane anisotropy is Large, the hole expansion value has deteriorated. Hot rolling No. In No. 26, C is out of the scope of the present invention, and the martensite structure, which is a hard phase, is 5% or more. Hot rolling No. In No. 27, Nb is not added, the amount of Nb added is lower than the lower limit of the present invention, and the strength is lowered.

  Hot rolling No. No. 3 has a low FT. No. 4 is an example in which SRT is low, FT is low, and recrystallization does not proceed. These have a developed texture, strong anisotropy, a low r value in the rolling direction, and a low hole expansion value. Hot rolling No. No. 8 has high CT, and hot rolling No. 8 No. 11 has a small rolling reduction at 1000 ° C. or lower, so that the ferrite grain size becomes large, and the elongation and hole expansion values are low.

  Hot rolling No. No. 19 has a low CT and a martensite structure fraction of 5% or more, so the hole expansion value is low. Hot rolling No. No. 24 has a high CR and a ferrite structure fraction of less than 5%, so its elongation is slightly lower than that of the steel of the present invention.

It is a figure which shows the relationship between processing temperature and a recrystallization rate. It is a figure which shows the relationship between processing temperature and recrystallization time.

Claims (14)

% By mass
C: 0.005-0.150%,
Mn: 0.10 to 3.00%,
Nb: 0.005 to 0.07%
Containing
Si: 2.50% or less,
P: 0.150% or less,
S: 0.0150% or less,
Al: 0.150% or less,
N: limited to 0.0100% or less, the balance being Fe and inevitable impurities,
The area ratio of ferrite is 5% or more, the area ratio of pearlite is limited to 20% or less, the total area ratio of one or both of martensite and retained austenite is limited to 5% or less, and the balance is bainite. Has a metallographic structure,
The ferrite has an average particle size of 30 μm or less,
The average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups on the plate surface at 1/2 plate thickness is 4.0 or less, {554} <225> orientation, {111} A hot rolled steel sheet excellent in hole expansibility, wherein an average value of X-ray random intensity ratios in the <112> orientation and the {111} <110> orientation is 4.5 or less. % By mass
Ti: 0.05% or less,
B: The hot rolled steel sheet excellent in hole expansibility of Claim 1 characterized by containing 1 type or 2 types of 0.0015% or less. % By mass
Mo: 0.01-2.00%,
Cr: 0.01 to 2.00%
W: 0.01 to 2.00%
Cu: 0.01-2.00%,
Ni: 0.01-2.00%
1 or 2 types or more of these are included, The hot-rolled steel plate excellent in the hole expansibility of Claim 1 or 2 characterized by the above-mentioned. % By mass
Ca: 0.0005 to 0.1000%,
Rem: 0.0005 to 0.1000%,
V: 0.001 to 0.100%
The hot-rolled steel sheet excellent in hole expansibility according to any one of claims 1 to 3, characterized by containing one or more of the following.   The Rankford value in the rolling direction, the Rankford value in a direction perpendicular to the rolling direction, and the Rankford value in a direction 45 ° from the rolling direction are more than 0.7 and 1.2 or less. A hot-rolled steel sheet excellent in hole expansibility according to any one of the above.   A hot-dip galvanized steel sheet excellent in hole expansibility, wherein the hot-rolled steel sheet according to any one of claims 1 to 5 is hot-dip galvanized.   An alloyed hot-dip galvanized steel sheet excellent in hole expansibility, wherein the hot-rolled steel sheet according to any one of claims 1 to 5 is subjected to alloyed hot-dip galvanizing.   The hot-rolled steel sheet according to any one of claims 1 to 5, the hot-dip galvanized steel sheet according to claim 6, or the galvannealed steel sheet according to claim 7 is wound in an arbitrary direction. This is a steel pipe with excellent hole expandability. A steel slab having the chemical composition according to any one of claims 1 to 4 is heated to 1050 to 1300 ° C, the total rolling reduction at 1000 ° C or less is 50% or more, and the finishing temperature FT [° C] is 930. ° C. performed in hot rolling is _higher, the Ae 3 -100 ° C., cooling the average cooling rate to higher of 650 ° C. or coiling temperature of less than 15.0 ℃ / s, 350 ℃ super to 700 ° C. A method for producing a hot-rolled steel sheet having excellent hole-expandability, which is characterized by being wound up by a roll. The hole-expanding property according to claim 9, wherein the air-cooling time after the end of hot rolling is air-cooled as X [s] or more and 100 s or less obtained from the finishing temperature FT [° C] according to the following (Equation 1). The manufacturing method of the hot-rolled steel plate excellent in.
X = 10 ^{(18.4−0.0194 × FT)} (Formula 1)   A method for producing a hot-dip galvanized steel sheet having excellent hole expansibility, wherein hot-rolled steel sheet having excellent hole expansibility produced by the method according to claim 9 or 10 is subjected to hot dip galvanization.   The method for producing a hot dip galvanized steel sheet having excellent hole expanding property according to claim 11, wherein hot dip galvanizing is performed in a continuous line.   A hot-dip galvanized steel sheet excellent in hole expansibility, which is subjected to a heat treatment for 5 seconds or more in a temperature range from 450 to 600 ° C after the hot dip galvanizing according to claim 11 or 12. Method. A hot-rolled steel sheet obtained by the production method according to claim 9 or 10, a hot-dip galvanized steel sheet obtained by the production method according to claim 11 or 12, or a production method according to claim 13. A method for producing a steel pipe excellent in hole expansibility, characterized in that pipe making is performed using a galvannealed steel sheet as a base material.

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