JP2012012669A - Cold-rolled steel sheet - Google Patents

Abstract

A cold-rolled steel sheet having excellent in-plane rigidity is provided.
On the surfaces of both surfaces of a steel plate, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in terms of a random intensity ratio. And since a steel plate having such a texture increases strain accumulation, a convex portion having a height of 10 to 500 μm is formed on the surface of the steel plate when a strain having a strain amount of 5% or less is applied in a uniaxial direction. It will be. As a result, the in-plane rigidity is improved. For example, in the case of an automobile outer panel, when such a convex portion is formed on the processed steel sheet, the shape can be easily formed when an external force is applied after molding. It will not collapse. In order to achieve such a texture distribution, solute Ti is involved, Ti: 0.01 to 0.1%, and Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) When it is −4 × (C%), it is preferably contained in a range satisfying Ti *> 0.007. However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively.
[Selection figure] None

Description

  The present invention relates to a cold-rolled steel sheet having excellent in-plane rigidity, which is used for automobile steel sheets, home appliance steel sheets, building members, and the like.

  In recent years, steel sheets used for automobiles and the like have been reduced in thickness of members and increased in strength from the viewpoint of weight reduction of the vehicle body and collision safety. In particular, panel members such as doors and hoods are required to have excellent formability and in-plane rigidity (difficulty to dent) after forming as the strength of the steel plate increases. In addition, building materials and steel plates for home appliances are also required to have a certain in-plane rigidity from a viewpoint of weight reduction and simplification of the construction method as well as excellent workability.

  On the other hand, Patent Document 1 discloses a paint bake-hardening cold-rolled steel sheet having a low yield strength at the time of press forming and a high yield strength after the subsequent baking coating process and excellent in workability. Techniques for manufacturing the are disclosed. Patent Document 1 utilizes a strain aging phenomenon of solute C in steel formed when a steel sheet is annealed at a high temperature close to 900 ° C.

  On the other hand, Patent Document 2 discloses a building unit that has a certain in-plane rigidity as a building application, is easy to construct during building, and can be easily changed to any size according to the building purpose. Yes. The technique of Patent Document 2 is a corrugated thin plate in which a plurality of convex strips are formed in one direction by bending a steel plate having a thickness of about 1 mm. By providing such ridges, high strength resistance can be compensated for the load from the surface direction and the load from the direction in which the ridges are formed, and it can be used as a building flooring.

  However, in Patent Document 1, a heat treatment (generally about 20 minutes at 170 ° C.) is indispensable after press molding. Although it is possible to manufacture a member for automobiles also as a paint baking process, a member for an application such as a building material requires a new process, which is disadvantageous in terms of cost. Moreover, in order to obtain high in-plane rigidity, it is necessary to increase the yield strength before press molding, and the moldability is significantly reduced. In addition, in order to control the amount of solute C in the steel, it is necessary to manage the manufacturing conditions in a narrow range, and there are problems such as a significant decrease in productivity such as yield reduction and material variation.

  In the technique of Patent Document 2, it is possible to stably obtain high in-plane rigidity. However, since a convex strip is provided on the steel sheet by bending, cost is required. Moreover, although it is suitable for flat members such as floor materials and wall materials for building materials, there is a problem that a member having a curved surface by press molding or the like is extremely difficult to process and the formability cannot be secured.

Japanese Patent Publication No. 60-46166 JP 2006-342515 A

  The present invention has been made in view of such circumstances, and an object thereof is to propose a cold-rolled steel sheet having excellent in-plane rigidity and workability.

In order to solve the above problems, the present inventors have conducted intensive research. As a result, the following knowledge was obtained.
After sufficiently accumulating non-recrystallized grains with a specific crystal orientation in the surface layer part, a convex part is formed on the surface of the steel sheet by applying strain, and the in-plane rigidity is improved by forming this convex part. It was.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.0005 to 0.01%, Si: 0.2% or less, Mn: 0.3% or less, P: 0.03% or less, S: 0.003 to 0.03%, Ti: 0.01 to 0.1%, Al: 0.01 to When 0.05%, N: 0.005% or less and Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%), Ti *> 0.007 Contained in the range to satisfy, the remainder has a composition composed of Fe and inevitable impurities, and the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in terms of random intensity ratio on both surfaces of the steel plate A cold-rolled steel sheet having a convex portion having a height of 10 μm or more and 500 μm or less on the surface of the steel sheet when a strain having an equivalent strain amount of 5% or less is applied.
However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively.

  In the present specification, “%” indicating the component of steel is “% by mass”.

According to the present invention, a cold-rolled steel sheet having good formability (workability) and excellent in-plane rigidity can be obtained.
In addition, the cold-rolled steel sheet of the present invention does not require any special process at the time of steel sheet production and press forming, is advantageous in terms of cost, and has extremely good productivity.
And the cold-rolled steel plate of this invention becomes a suitable material as a member for motor vehicles, a steel plate for household appliances, a building use material, etc.

It is a figure which shows the evaluation method of in-plane rigidity.

  The cold-rolled steel sheet of the present invention has a strain with a {100} plane X-ray intensity in a direction parallel to the plate surface of 2.5 or more in a random strength ratio and an equivalent strain amount of 5% or less on both surfaces of the steel plate. When added, the steel sheet has a convex portion having a height of 10 μm or more and 500 μm or less on the surface of the steel plate. This is the most important requirement in the present invention. In addition, the component composition at that time is C: 0.0005 to 0.01%, Si: 0.2% or less, Mn: 0.3% or less, P: 0.03% or less, S: 0.003 to 0.03%, Ti: 0.01 to 0.1%, Al: 0.01 to When 0.05%, N: 0.005% or less and Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%), Ti *> 0.007 It is contained within the range to satisfy, and the balance is Fe and inevitable impurities. However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively. Thus, the cold-rolled steel plate excellent in in-plane rigidity and workability can be obtained by prescribing | regulating the state of the steel plate surface.

  In the present invention, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in a random intensity ratio, and the structure has undergone only a recovery phenomenon without causing α → γ transformation or recrystallization. This shows that the dislocation density is higher than that of the recrystallized grains, and the unrecrystallized grains in the form extending in the rolling direction are accumulated on the {100} plane parallel to the plate surface. Therefore, it is different from a structure in which recrystallized grains generated through α → γ → α transformation obtained by annealing performed at a temperature equal to or higher than the transformation point are accumulated.

Thus, in the present invention, in the accumulation of {100} planes in the direction parallel to the plate surface, there is a feature in the morphology and dislocation density of the crystal grains accumulated in the {100} plane orientation, and the recrystallized grains that are usually obtained The composition is different from that of a structure composed of {100} plane orientations formed through the γ → α transformation. Therefore, in the present invention, the random strength ratio is measured to clarify the difference between the non-recrystallized grains and the recrystallized grains, and the structure of the cold-rolled steel sheet is shown using the random strength ratio.
If the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in terms of the random intensity ratio, most of the {100} surface in the direction parallel to the plate surface will be unrecrystallized grains, and the dislocation density will be high. Since the strain accumulation increases, a convex portion of 10 μm or more is formed on the surface when a predetermined strain is applied to the steel sheet, and the in-plane rigidity is improved. When the random intensity ratio is less than 2.5, {100} planes are not sufficiently accumulated, distortion accumulation is small, and convex portions having a height of 10 μm or more are not formed.

  Hereinafter, the present invention will be described in detail.

Conventionally, it is known that the texture of an automobile panel outer plate has many {111} surfaces formed in a direction parallel to the plate surface. When the present inventors repeatedly conducted experiments under various production conditions, it was found that a steel sheet having many {111} faces but many {100} faces in the surface layer was obtained. It was. In-plane rigidity when the {100} plane accumulation is not the normal {100} plane obtained by annealing above the transformation point, but the {100} plane of unrecrystallized grains with a high dislocation density. Has also found that it is far superior.
The {100} plane of the non-recrystallized grains has a higher hardness than the {111} plane that is the recrystallized grains because it contains a lot of strain in the crystal plane. Therefore, since a steel sheet having such a texture distribution has a high hardness distribution on the surface, when a strain of a certain amount of strain or less that does not break the steel sheet is applied in a uniaxial direction, a convex portion is formed on the steel sheet surface. Will be formed. The in-plane rigidity is improved by forming the convex portion. For example, in the case of an automobile outer panel, when such a convex portion is formed on a processed steel sheet, the shape does not easily collapse when an external force is applied after molding.

Based on the above examination results, in the present invention, in order to improve workability and obtain excellent in-plane rigidity, the {100} plane X-ray intensity in the direction parallel to the plate surface is random on both surfaces of the steel plate. The strength ratio is 2.5 or more, and a convex portion having a height of 10 μm or more and 500 μm or less is provided on the surface of the steel sheet when a strain having an equivalent strain of 5% or less is applied.
The {100} plane X-ray intensity in the direction parallel to the plate surface can be measured by the inverse pole figure method. The {100} plane X-ray intensity on the surface is measured after cleaning and drying the test piece, while the {100} plane X-ray intensity in the direction parallel to the plate surface at the center of the plate thickness is measured on one side of the test piece. Each is measured after chemical polishing with oxalic acid to expose the central portion of the plate thickness on the surface. White X-rays can be used as the X-ray source, and a Ge semiconductor detector can be used to detect {100} plane X-rays. At the same time, the {100} plane X-ray intensity (random intensity) of a random sample with no selective orientation and an irregular crystal orientation distribution is measured. The random intensity ratio is calculated by the ratio of the {100} plane X-ray intensity of the actual test piece to the {100} plane X-ray intensity of the random sample.
Moreover, the thickness from the outermost layer to the plate thickness center direction in the region where many non-recrystallized grains exist can be measured by observing a cross section in the rolling direction of the steel plate with an optical microscope. The form of the cross section in the rolling direction of the non-recrystallized grains can be easily distinguished because it has a smaller thickness than the recrystallized grains and extends in the rolling direction. And, in order to obtain the effect of improving the in-plane rigidity by forming convex portions on the steel plate surface, the {100} plane of unrecrystallized grains is generally from the outermost layer to the region of 5 μm in the plate thickness center direction. It is preferable that many exist. More preferably, it is from the outermost layer to the region of 10 μm in the plate thickness center direction.
Further, a steel plate having a {100} plane X-ray intensity in a direction parallel to the plate surface on the surface as described above can be obtained by controlling the production conditions of the hot rolling step. Specifically, for example, the coiling temperature in hot rolling is set to 630 ° C. or less, the hydrogen concentration in the atmosphere in the annealing heating process, particularly the atmosphere (mixed gas of nitrogen and hydrogen) is 5 vol% or more, and the dew point is − By setting the temperature to 40 ° C. or less, the {100} plane X-ray intensity in the direction parallel to the plate surface becomes 2.5 or more in terms of the random intensity ratio.

  As described above, in the present invention, for a steel sheet having a texture distribution in which non-recrystallized grains are accumulated in the {100} plane in a direction parallel to the plate surface, the steel sheet does not break and a strain below a certain strain amount. Is applied in a uniaxial direction, the in-plane rigidity is improved by forming a convex portion on the surface of the steel sheet. At this time, as a certain amount of strain in which the steel sheet does not break, in the present invention, the amount of equivalent strain is set to 5% or less. When the equivalent strain amount exceeds 5%, the portion of the unrecrystallized grains where the strain is accumulated is also deformed, so that a convex portion of 10 μm or more is not formed. The height of the convex portion formed on the surface of the steel sheet when the equivalent strain amount is 5% or less is set to 10 μm or more and 500 μm or less. If it is less than 10 μm, the difference between the original surface irregularities of the steel sheet is small and there is no effect of improving the in-plane rigidity. On the other hand, if it exceeds 500 μm, the flatness of the surface of the steel sheet is remarkably lowered, and it cannot be used for products.

Furthermore, as a result of further investigation, in the annealing process of the steel sheet, Ti does not contribute to precipitation by forming a Ti compound with C, N, and S as Ti, that is, there is Ti dissolved in the steel. It has also been found that the use of a small steel component contributes to the accumulation of many {100} plane orientations of unrecrystallized grains in the surface layer as described above. The exact mechanism by which the presence of solid solution Ti accumulates many {100} plane orientations of unrecrystallized grains on the surface layer is not clear, but during annealing after cold rolling, Ti reacts with N present in the atmosphere. It is presumed that the nitride formed in the vicinity of the surface layer affects the recrystallization and inhibits the formation of the {111} plane that is originally formed in large numbers.
From the above, as the component composition for making the solid solution Ti exist in this way, Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%), Ti *> Contains in a range that satisfies 0.007. However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively. Detailed description will be given later.

Next, the reasons for limiting the component elements will be described.
C: 0.0005-0.01%
C is a solid solution strengthening element, contributes to an increase in yield strength, and is advantageous for improving the in-plane rigidity. However, excessive addition causes deterioration of workability and aging properties. In addition, if C is contained in a large amount, the amount of Ti carbide in the steel increases, the amount of solute Ti in the steel decreases, and the formation of the {100} plane parallel to the plate surface in the surface layer is obstructed. Is done. Based on the above, the upper limit is 0.01%. On the other hand, if it is less than 0.0005%, the crystal grain size becomes extremely coarse and the yield strength is greatly reduced, so that the in-plane rigidity is lowered and defects such as hip breakage are likely to occur. Moreover, the decarburization cost increases. Therefore, the content is 0.0005% or more and 0.01% or less.

Si: 0.2% or less
In addition to acting as a deoxidizer, Si is a useful element that strengthens steel by solid solution strengthening. On the other hand, it has the effect of suppressing the formation of carbides and the effect of promoting the formation of Ti carbides. Moreover, when it contains excessively, workability will be inhibited. Therefore, it is 0.2% or less.

Mn: 0.3% or less
In addition to acting as a deoxidizer, Mn strengthens steel by solid solution strengthening, increases yield strength, and is effective for in-plane rigidity. However, Mn sulfide acts as a precipitation site for Ti precipitates, reducing the amount of solid solution Ti, and excessive addition impairs workability. Therefore, it is 0.3% or less.

P: 0.03% or less
P is a solid solution strengthening element and is effective for strengthening and yield strength of steel. Moreover, it is advantageous for in-plane rigidity. On the other hand, it is an element that easily segregates at the grain boundaries, causing hot and cold cracking, and secondary workability is significantly inhibited. Therefore, it is set to 0.03% or less.

S: 0.003-0.03%
S is present in the steel as an unavoidable impurity, but if it exceeds 0.03%, hot cracking is likely to occur during the production of the steel sheet, and inclusions are formed in the steel, thereby significantly reducing workability. Excessive addition also promotes the formation of Ti sulfide and leads to a decrease in solid solution Ti. Therefore, the upper limit is 0.03%. On the other hand, it is preferable that the amount of S is small. However, if it is less than 0.003%, the desulfurization cost increases, so 0.003% is set as the lower limit.

Al: 0.01-0.05%
Al is an element added as a deoxidizer. Further, Al forms nitrides with N, but if the content is small, excess N may form Ti and nitrides, and the amount of solid solution Ti may decrease. Therefore, the amount of Al needs to be 0.01% or more. However, even if added in a large amount, a further deoxidizing effect cannot be obtained. Therefore, the upper limit is made 0.05%.

N: 0.005% or less
The smaller N, the better the workability, so the smaller N is desirable. Moreover, when it exceeds 0.005% and it adds excessively, it will lead to the remarkable fall of a moldability and the fall of solid solution Ti amount. Therefore, the upper limit is made 0.005%.

Ti: 0.01-0.1%
Ti is one of the most important elements in the present invention. Ti has an effect of improving workability by fixing C, N, and S in steel as precipitates. Further, in the present invention, by adding Ti in excess of the amount necessary to form the precipitate, a nitride with N in the atmosphere is formed at the time of manufacturing, and the unrecrystallized grains of the surface layer { 100} Increase the plane orientation. If it is less than 0.01%, such an effect cannot be obtained. On the other hand, adding more than 0.1% of Ti not only can not be expected, but also promotes the formation of an abnormal structure inside the plate and lowers the workability. From the above, it is set to 0.01% or more and 0.1% or less.
Furthermore, as described above, Ti in steel forms precipitates with C, N, and S in steel. Therefore, Ti is added in excess of the equivalent to these C, N, and S components. It is important in the present invention to accumulate {100} planes of non-recrystallized grains on the surface layer by causing excessive solid solution Ti to exist. Therefore, in addition to the above specification of 0.01% to 0.1%, the following relational expression should be satisfied.
Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%), Ti *> 0.007
However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively.
When Ti * exceeds 0.007, nitrogen in the atmosphere that penetrates into the steel during annealing and solute Ti form very fine nitrides, preventing the movement of grain boundaries and suppressing recrystallization. As a result, unrecrystallized grains having a high hardness distribution are likely to remain, and the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in terms of the random intensity ratio on both surfaces of the steel plate.

  The remainder other than the above consists of Fe and inevitable impurities. As an unavoidable impurity, for example, O forms non-metallic inclusions and adversely affects quality, so it is desirable to reduce it to 0.003% or less. Further, in the present invention, Cu, Cr, Ni, W is contained in a range of 0.1% or less and V, Zr, Sn, Sb is contained in a range of 0.01% or less as trace elements that do not impair the effects of the present invention. May be.

Next, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated.
The cold-rolled steel sheet of the present invention is preferably obtained by roughly rolling a steel adjusted to the above chemical composition range, finish rolling at a desired finishing temperature, then cooling under desired cooling conditions, winding, pickling Thereafter, it is obtained by cold rolling and continuous annealing. Among them, when the surface of the steel sheet, which is a feature of the present invention, {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in a random intensity ratio, and a strain with an equivalent strain of 5% or less is applied. In order to form a convex portion having a height of 10 μm or more and 500 μm or less on the surface of the steel plate, the winding temperature is preferably 630 ° C. or less. By setting the temperature to 630 ° C or less, the precipitate containing Ti becomes finer, and re-dissolves during subsequent annealing to increase the solid solution Ti. On both surfaces of the steel sheet, unrecrystallized in the direction parallel to the plate surface The {100} faces of the grains can be accumulated.
In addition, the atmosphere in the heating process during annealing is a mixed gas of hydrogen and nitrogen containing 5 vol% or more of hydrogen, and the dew point is −40 ° C. or less. It is possible to accumulate {100} planes of non-recrystallized grains in a direction parallel to a high plate surface. The reason for this is not always clear, but the higher the hydrogen concentration and the lower the dew point, the more nitrogen can penetrate into the steel and form more fine nitrides with Ti in the steel. This is presumed to be because the recrystallization suppression effect is enhanced. More preferably, the hydrogen concentration is 8 vol% or more and the dew point is −45 ° C. or less.

The molten steel consisting of the components shown in Table 1 is vacuum degassed, then slabd by continuous casting, reheated to 1180 ° C, hot rolled to a thickness of 3.5mm at a finishing temperature of 900 ° C, and wound up as shown in Table 2. The coil was wound up at temperature.
Next, the steel sheet after winding is pickled, cold-rolled to a thickness of 0.65 mm, annealed at 820 ° C. in a H ₂ —N ₂ mixed atmosphere in a continuous annealing line, and adjusted to an elongation rate of 0.8%. Quality rolling was performed.

  For the cold-rolled steel sheet obtained as described above, the {100} plane X-ray random intensity ratio, mechanical properties, convex height when strain is applied, in-plane rigidity (residual dent amount) ) Was measured and evaluated. The results obtained are shown in Table 2.

{100} plane X-ray random intensity The {100} plane X-ray intensity in the direction parallel to the specific plate surface was measured by a reverse pole figure method. The {100} plane X-ray intensity on the surface is measured after the specimen is cleaned and dried, while the {100} plane X-ray intensity in the direction parallel to the plane at the center of the thickness is Measurement was performed after chemically polishing one surface of the substrate with oxalic acid to expose the central portion of the plate thickness on the surface. White X-rays were used as the X-ray source, and a Ge semiconductor detector was used to detect {100} plane X-rays. At the same time, the {100} plane X-ray intensity (random intensity) of a random sample with no selective orientation and an irregular distribution of crystal orientation was measured. The random intensity ratio was calculated by the ratio of the {100} plane X-ray intensity of the actual test piece to the {100} plane X-ray intensity of the random sample.

Mechanical properties Formability was evaluated by tensile properties and r-value mechanical properties. Tensile properties were measured according to the test method described in JISZ 2241 after being processed into a No. 5 test piece described in JISZ 2201. The average r value was measured by a three-point method after applying a tensile pre-strain of 15%, and the average r value in the 90 ° direction, 45 ° direction, and 0 ° direction with respect to one direction of the steel sheet = It was determined as (r (0 °) + 2 × r (45 °) + r (90 °)) / 4.

Convex height when strain is applied and in-plane rigidity (residual dent amount)
The height of the convex part is the same as the height of the convex part to be observed by observing a 100 mm area at a magnification of 100 to 400 times with a laser microscope after applying strain to the test piece with a tensile tester. The average value was evaluated. The equivalent strain ε was calculated by ε = ln (1 + e) with respect to the strain (conventional strain) e applied in the tensile test.
As shown in FIG. 1, the in-plane rigidity was evaluated by measuring the amount of residual dent caused by a static load of 245 N (25 kgf) at the center of a scallop-shaped panel stretched and formed with a molding strain of 2%.

In the present invention, many {100} planes in the direction parallel to the plate surface were accumulated on the surface layer, and a convex portion having a height of 10 μm or more and 500 μm or less was formed when a strain of up to 5% was applied. As a result, it can be seen that the moldability is good, the residual dent amount is small, and the in-plane rigidity is excellent.
Further, as a result of observing the cross section in the rolling direction of the steel sheet with an optical microscope, it was confirmed that the thickness of the region where many non-recrystallized grains existed from the outermost layer to the thickness center direction was 5 μm or more.
When the amount of strain applied is 10%, the strain amount exceeds 5%, so the effect of the presence of many {100} surfaces on the surface layer does not appear, and the convex portion does not appear below 10 μm, and the in-plane rigidity Does not improve.
In the comparative example, the {100} plane in the direction parallel to the plate surface is not sufficiently accumulated on the surface layer, and the convex portion is not formed. As a result, the in-plane rigidity is low and inferior.

  Since the cold-rolled steel sheet of the present invention is excellent in in-plane rigidity and workability, it can be used in various applications, mainly for steel sheets for home appliances, steel sheets for automobiles, and building members.

Claims (1)

In mass%, C: 0.0005 to 0.01%, Si: 0.2% or less, Mn: 0.3% or less, P: 0.03% or less, S: 0.003 to 0.03%, Ti: 0.01 to 0.1%, Al: 0.01 to 0.05%, N: Within 0.005% or less and Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%) Containing, the remainder has a component composition consisting of Fe and inevitable impurities, the surface of both surfaces of the steel plate, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in random intensity ratio, and A cold-rolled steel sheet characterized by having a convex portion having a height of 10 μm or more and 500 μm or less on the surface of the steel sheet when a strain having an equivalent strain amount of 5% or less is applied.
However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively.

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