JP2004131771A - Method for manufacturing cold rolled steel sheet having excellent shape fixability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for easily manufacturing a cold rolled steel sheet having excellent shape fixability. <P>SOLUTION: The hot rolled steel sheet which contains, by mass, 0.0001 to 0.25% C, and 0.01 to 0.40% in total one or two of Ti and Nb, and in which the mean value (A) of the X-ray random intensity ratios of ä100}<011> to ä223}<110> orientation groups of the plate faces at at half the plate thickness is ≤3.0 and the mean value (B) of the X-ray random intensity ratios of the three crystallographic orientations ä554}<225>, ä111}<112>, and ä111}<110> satisfies ≤6.0 is subjected to cold rolling of 30 to 75% after pickling. The cold rolled steel sheet which satisfies (A)≥3.0 and (A)/(B)≥1.0 and is ≤0.7 in at least one of the r values in the rolling direction and the direction perpendicular thereto is manufactured by further heating the steel sheet to 600 to (Ac3+150)°C at 4 to 50°C/s, then cooling the steel sheet at 1 to 250°C/s. <P>COPYRIGHT: (C)2004,JPO

Description

【００７６】
ところで図２及び図３に示した様に、スプリングバック量や壁そり量はＢＨＦ（しわ押さえ力）によっても変化する。本発明の効果は、いずれのＢＨＦで評価を行ってもその傾向は変わらないが、実機で実部品をプレスする際には、設備上の制約からあまり高いＢＨＦはかけられないため、今回は、ＢＨＦ２９ｋＮで各鋼種のハット曲げ試験を行った。なお、曲げはｒ値の低い方向と垂直に折れ線が入るように行った。
【００７７】
表２には、各鋼板の製造条件が本発明の範囲内にあるか否かを示している。「熱延板集合組織」の（ａ）欄には、Ｘ線で測定した熱延板の板厚の７／１６厚の位置での｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値、（ｂ）欄には、｛５５４｝＜２２５＞、｛１１１｝＜１１２＞及び｛１１１｝＜１１０＞の３つの結晶方位のＸ線ランダム強度比の平均値を示す。
【００７８】
いずれの鋼板も、焼鈍温度から３〜３０℃／ｓの冷却速度で冷却し、スキンパス圧延を０．５〜１．５％の範囲で施した。
【００７９】
【表２】

【００８０】
表３には、前記の方法によって製造された１．２ｍｍ厚の冷延鋼板の機械的特性値、焼鈍板の集合組織、及び、形状凍結性の指標が示されている。
【００８１】
「焼鈍板集合組織」の（Ａ）欄には、Ｘ線で測定した焼鈍板の板厚の７／１６厚の位置での｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値、（Ｂ）欄には、｛５５４｝＜２２５＞、｛１１１｝＜１１２＞及び｛１１１｝＜１１０＞の３つの結晶方位のＸ線ランダム強度比の平均値、そして、（Ａ）／（Ｂ）欄には、｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値と｛５５４｝＜２２５＞、｛１１１｝＜１１２＞及び｛１１１｝＜１１０＞の３つの結晶方位のＸ線ランダム強度比の平均値の比が示してある。
【００８２】
本発明で限定された条件を満たしているものは、いずれも良好な形状凍結性が達成されている。
【００８３】
各結晶方位のＸ線ランダム強度比やｒ値が形状凍結性に重要であることの機構については、現在のところ必ずしも明らかとはなっていない。おそらく、曲げ変形時にすべり変形の進行を容易にすることで、結果的に曲げ変形時のスプリング・バック量、壁反り量が小さくなっているものと理解される。
【００８４】
【表３】

【００８５】
【発明の効果】
薄鋼板の集合組織とｒ値を制御すると、その曲げ加工性は著しく向上することを以上に詳述した。本発明によって、スプリング・バック量が少なく、曲げ加工を主体とする形状凍結性に優れた薄鋼板が提供できるようになった。特に、従来は形状不良の問題から高強度鋼板の適用が難しかった部品にも高強度鋼板が使用できるようになる。
【００８６】
自動車の軽量化を推進するためには、高強度鋼板の使用は是非とも必要である。スプリング・バック量が少なく、形状凍結性に優れた高強度鋼板が適用できるようになると、自動車車体の軽量化をより一層推進することができる。従って、本発明は、工業的に極めて高い価値のある発明である。
【図面の簡単な説明】
【図１】ハット曲げ試験に用いた試験片の断面図である。
【図２】スプリングバック量に及ぼすＢＨＦ（しわ押え力）の関係を示す図である。
【図３】壁そり量とＢＨＦ（しわ押え力）の関係を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a ferritic thin steel sheet (hereinafter also simply referred to as a steel sheet or a thin steel sheet) having excellent shape freezing property mainly by bending, and is mainly used for automobile parts and the like.
[0002]
[Prior art]
2. Description of the Related Art In order to suppress the amount of carbon dioxide gas emitted from automobiles, the use of high-strength steel sheets has led to a reduction in weight of automobile bodies. In addition, in order to ensure the safety of occupants, high-strength steel sheets are increasingly used in automobile bodies in addition to mild steel sheets. Further, in order to further reduce the weight of automobile bodies in the future, new demands for increasing the use strength level of high-strength steel sheets more than ever have been increasing.
[0003]
However, when bending deformation is applied to a high-strength steel sheet, the shape after processing tends to return to the shape before processing away from the shape of the processing jig due to its high strength. Due to the elastic recovery from the bending back, a wall warpage phenomenon occurs in which the plane of the side wall portion becomes a surface having a curvature, and a dimensional accuracy defect in which a shape of a target processed component cannot be obtained occurs.
[0004]
Therefore, in the body of a conventional automobile, it has been mainly used only for high-strength steel sheets of 440 MPa or less. For automobile bodies, there is no high-strength steel sheet with low spring back and wall warpage and good shape freezing, although it is necessary to use a high-strength steel sheet of 490 MPa or more to reduce the body weight. This is the actual situation.
[0005]
Needless to add, it is extremely important to improve the shape freezing property of a high-strength steel sheet or a mild steel sheet of 440 MPa or less after processing, such as an automobile or a home appliance, in order to enhance the shape accuracy of the product.
[0006]
The present inventors have disclosed a method of manufacturing a hot-rolled steel sheet and a cold-rolled steel sheet having excellent shape freezing properties by controlling the texture at the center of the sheet thickness (for example, see Patent Document 1). However, in order to manufacture a cold-rolled steel sheet by this method, in order to accumulate the texture at the stage of the hot-rolled steel sheet, it is necessary to direct the hot-rolling under low temperature and large pressure, and a large load is applied to the hot-rolling process. .
[0007]
In addition, some of the present inventors have disclosed a cold-rolled steel sheet in which the reflected X-ray intensity ratio of a {100} plane parallel to the plate surface is 3 or more as a technique for reducing the amount of springback (for example, see Patent Reference 2). However, the present invention is characterized by the definition of the X-ray intensity ratio at the outermost surface of the plate thickness, which is completely different from the present invention which specifies the X-ray intensity ratio at the half plate thickness.
[0008]
[Patent Document 1]
JP 2001-303175 A [Patent Document 2]
JP 2001-64750 A
[Problems to be solved by the invention]
When bending a mild steel sheet or a high-strength steel sheet, a large spring back is generated depending on the strength of the steel sheet, and the shape-freezing property of the machined part is poor at present. The present invention is to solve this problem drastically and to provide a method for easily producing a cold-rolled steel sheet having excellent shape freezing property.
[0010]
[Means for Solving the Problems]
According to the conventional knowledge, as a measure for suppressing spring back, it is considered as important for the time being to lower the yield point of the steel sheet. Then, in order to lower the yield point, a steel sheet having a low tensile strength had to be used. However, this alone does not provide a fundamental solution for improving the bending workability of the steel sheet and keeping the amount of spring back low.
[0011]
Therefore, the present inventors newly focused on the influence of the texture of the steel sheet on the bending workability in order to improve the bending workability and fundamentally solve the occurrence of spring back, and studied the effect thereof. The effects were investigated and studied in detail. Then, a steel sheet excellent in bending workability was found.
[0012]
As a result, controlling the intensity of the {100} <011> to {223} <110> orientation groups and the orientations of {554} <225>, {111} <112>, and {111} <110>, and Has revealed that by setting at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction to a value as low as possible, the bending workability is dramatically improved.
[0013]
Further, as a result of intensive studies, in order to manufacture a cold-rolled steel sheet having such properties without applying a load to the hot rolling process, Nb or Ti is added to delay recrystallization during annealing, It has been newly found that it is very effective to optimize the elongation and the heating rate and cooling rate during annealing.
[0014]
The present invention is configured based on the above-mentioned findings, and the gist thereof is as follows.
[0015]
(1) In mass%,
C: 0.0001% or more, 0.25% or less,
Si: 0.001% or more, 2.5% or less,
Mn: 0.01% or more, 2.5% or less,
P: 0.15% or less,
S: 0.03% or less,
Al: 0.01% or more, 2.0% or less,
N: 0.01% or less,
O: 0.01% or less Further, one or two kinds of Ti and Nb are contained in a total of 0.01% or more and 0.40% or less, the balance being iron and unavoidable impurities, and at least 1/2 sheet thickness. The average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group of the plate surface is 3.0 or less, and {554} <225>, {111} <112> Hot-rolled steel sheet satisfying an average of the X-ray random intensity ratios of the three crystal orientations of {111} and <110> satisfying 6.0 or less is subjected to cold rolling of 30% or more and 75% or less after pickling. Further, after heating to a temperature range of 600 ° C. to (Ac3 + 150) ° C. at 3 ° C./s to 100 ° C./s, cooling at a rate of 1 ° C./s to 250 ° C./s, the plate surface at least １ ／ sheet thickness -Ray random intensity of {100} <011> to {223} <110> orientation group Of (554) <225>, {111} <112> and {111} <110> of the plate surface at least half the plate thickness with (A) of (A) is 3.0 or more. (A) / (B) which is the ratio of the average value (B) of the X-ray random intensity ratios of the three crystal orientations satisfies 1.0 or more, and at least one of the r values in the rolling direction and the direction perpendicular thereto. A method for producing a cold-rolled steel sheet having excellent shape freezing properties, wherein one steel sheet is 0.7 or less.
[0016]
(2) Further, in mass%,
V: 0.20% or less,
Cr: 1.5% or less, and
B: The method for producing a cold-rolled steel sheet excellent in shape freezing property according to (1), containing one or more kinds of 0.007% or less.
[0017]
(3) Further, in mass%,
Mo ≦ 1%,
Cu ≦ 2%,
Ni ≦ 1%, and
Sn ≦ 0.2%
The method for producing a cold-rolled steel sheet having excellent shape freezing properties according to (1) or (2), comprising one or more of the following.
[0018]
(4) A method for producing a cold-rolled steel sheet having excellent shape freezing properties, wherein the cold-rolled steel sheet having excellent shape freezing property according to any one of (1) to (3) is plated.
[0019]
(5) The shape freezing property characterized by subjecting the cold-rolled steel sheet excellent in shape freezing property according to any one of (1) to (4) to skin pass rolling of 0.4% or more and 5% or less. Excellent cold rolled steel sheet manufacturing method.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the contents of the present invention will be described in detail. First, the limiting conditions for the component composition will be described. In addition,% means mass%.
[0021]
The reason why the lower limit of C is set to 0.0001% is that the lower limit obtained in practical steel is used. If the upper limit is more than 0.25%, the workability deteriorates. Therefore, the upper limit is set to this value.
[0022]
Si is an element effective for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, the workability is deteriorated or surface flaws are generated, so the upper limit is 2.5%. . On the other hand, since it is difficult to make Si less than 0.001% in practical steel, 0.001% is made the lower limit.
[0023]
Mn is also an effective element for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, the workability deteriorates, so the upper limit is 2.5%. On the other hand, since it is difficult to make Mn less than 0.01% in practical steel, the lower limit is made 0.01%.
[0024]
When an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added in addition to Mn, it is desirable to add an Mn amount that satisfies Mn / S ≧ 20 in mass%.
[0025]
P and S are impurities and are set to 0.15% or less and 0.03% or less, respectively. This is to prevent deterioration in workability and cracks during hot rolling or cold rolling.
[0026]
Al is added in an amount of 0.01% or more for deoxidation. Further, Al significantly increases the γ → α transformation point, and is an effective element particularly when directing hot rolling at an Ar point of 3 or less. However, if the content is too large, the workability is reduced and the surface properties are deteriorated, so the upper limit is made 2.0%.
[0027]
N and O are impurities and are set to 0.01% or less and 0.01% or less, respectively, so as not to deteriorate workability.
[0028]
Ti and Nb are very important elements in the present invention. That is, by adding these elements, recrystallization and grain growth during annealing after cold rolling are suppressed, and a texture advantageous for shape freezing is preserved without being destroyed.
[0029]
In addition, since these elements improve the material through mechanisms such as fixation of carbon and nitrogen, precipitation strengthening, and fine grain strengthening, one or two of these elements are added in a total amount of 0.01% or more depending on the purpose. .
[0030]
Since excessive addition deteriorates workability, the upper limit is set to 0.40% in total of one or two kinds.
[0031]
V, Cr, and B improve the material through mechanisms such as fixation of carbon and nitrogen, precipitation strengthening, structure control, and fine grain strengthening. Therefore, it is desirable to add each of 0.0001% or more as necessary. However, excessive addition has no remarkable effect, but rather degrades workability and surface properties. Therefore, the upper limits of 0.2%, 1.5%, and 0.007% are set respectively.
[0032]
Since Mo, Cu, Ni, and Sn have the effect of increasing mechanical strength and improving the material, it is desirable to add 0.001% or more to each component as necessary. Since the workability is deteriorated, the upper limits are set to 1%, 2%, 1%, and 0.2%, respectively.
[0033]
Although not particularly limited in the present invention, an appropriate amount of Ca, Mg, or Ce may be added for the purpose of deoxidation or morphological control of sulfide.
[0034]
Average values of X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups and {554} <225>, {111} <112 of the sheet surface at 1/2 sheet thickness of the hot-rolled sheet. > And {111} <110> mean values of X-ray random intensity ratios of three crystal orientations:
The average value of the {100} <011> to {223} <110> orientation group when X-ray diffraction of the plate surface at the plate thickness center position was performed and the intensity ratio of each orientation with respect to the random sample was obtained was 3. Must be 0 or less. In order to make this more than 3.0, it is necessary to apply a large load to the hot rolling process, and it also causes a variation in the material.
[0035]
Main orientations included in this orientation group are {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} <110> and {223} <110>.
[0036]
The X-ray random intensity ratio in each of these directions can be calculated from a three-dimensional texture calculated by the vector method based on the {110} pole figure or a plurality of {110}, {100}, {211}, and {310} pole figures. It may be obtained from a three-dimensional texture calculated by a series expansion method using a pole figure (preferably three or more).
[0037]
For example, in the latter method, the X-ray random intensity ratio of each crystal orientation is (001) [1-10], (116) [1-10], (114) in the φ2 = 45 ° cross section of the three-dimensional texture. ) [1-10], (113) [1-10], (112) [1-10], (335) [1-10], (223) [1-10] may be used as they are. .
[0038]
The average value of the {100} <011> to {223} <110> azimuth group is the arithmetic average of each azimuth described above. If it is not possible to obtain the intensities of all the above directions, {100} <011>, {116} <110>, {114} <110>, {112} <110>, {223} <110> May be substituted by the arithmetic mean of each direction. More preferably, the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group is 2.5 or less, more preferably 2.0 or less.
[0039]
Furthermore, the average value of the X-ray random intensity ratio of the three crystal orientations of {554} <225>, {111} <112>, and {111} <110> of the sheet surface at a half sheet thickness of the hot-rolled sheet ( B) must be less than or equal to 6.0. If it exceeds 6.0, a texture satisfying the shape freezing property of the cold rolled sheet cannot be obtained even if the subsequent cold rolling conditions and annealing conditions are optimized.
[0040]
Further, when the value of (B) exceeds 6.0 in the hot-rolled sheet, only a load is applied to the hot-rolling step, and no merit in characteristics can be obtained. The lower limit of (B) is not particularly specified, but is preferably 3.5 or more from the viewpoint of workability.
[0041]
The X-ray random intensity ratios of {554} <225>, {111} <112>, and {111} <110> may be determined from the three-dimensional texture calculated according to the above method.
[0042]
In the sample to be subjected to X-ray diffraction, the steel sheet is reduced to a predetermined thickness by mechanical polishing or the like, and then the distortion is removed by chemical polishing or electrolytic polishing, etc., and at the same time, the 1/2 surface of the plate becomes the measurement surface To be manufactured. If segregation zones or defects are present in the thickness center layer of the steel sheet and inconvenience occurs in the measurement, the above-mentioned method is used so that an appropriate surface becomes the measurement surface in the range of ／ to ／ of the thickness. The sample may be adjusted and measured according to the method described in (1).
[0043]
The crystal orientation represented by {hkl} <uvw> indicates that the normal direction of the plate surface is parallel to <hkl> and the rolling direction is parallel to <uvw>.
[0044]
The method for producing a hot-rolled sheet having such a texture is not particularly limited. As-cast, or once cooled, heated again to a temperature in the range of 1000 to 1300 ° C., and (Ar 3−150) ° C. to (Ar 3 + 100) It is desirable to control so that the total reduction ratio in the temperature range of (° C.) is 25% or less, and then, after finishing the hot rolling at the Ar3 transformation temperature or higher, cool and wind.
[0045]
Also, if the coefficient of friction between the hot-rolled roll and the steel sheet during hot rolling is high, a crystal orientation mainly composed of {110} planes develops in the sheet surface near the surface of the hot-rolled steel sheet. It is desirable that the coefficient of friction between the steel sheet and the steel plate be at least one pass or more and 0.2 or less.
[0046]
The production method prior to hot rolling is not particularly limited. That is, various secondary smelting may be performed following smelting using a blast furnace, an electric furnace, or the like, and then casting may be performed by a method such as thin slab casting in addition to ordinary continuous casting, casting by an ingot method. In the case of continuous casting, after once cooling to a low temperature, it may be heated again and then hot-rolled, or the cast slab may be continuously hot-rolled. Scrap may be used as a raw material.
[0047]
When the hot-rolled steel sheet thus obtained is cold-rolled and annealed to obtain a final thin steel sheet, if the total reduction ratio of the cold-rolling exceeds 80%, general cold-rolling- The {111} plane and {554} plane components of the X-ray diffraction integrated plane intensity ratio of the crystal plane parallel to the plate plane, which is the recrystallized texture, are increased, and the crystal orientation defined in (1) which is a feature of the present invention. Is not satisfied, the upper limit of the rolling reduction of the cold rolling is set to 80%.
[0048]
In order to enhance the shape freezing property, it is desirable to limit the cold rolling reduction to 70% or less. On the other hand, if the cold rolling reduction is less than 25%, a texture advantageous for shape freezing properties does not sufficiently develop, so the lower limit of the rolling reduction of the cold rolling is set to 25%. From this viewpoint, the rolling reduction of the cold rolling is desirably limited to 30% or more.
[0049]
When the cold-rolled steel sheet cold-worked in such a range is annealed, if the heating rate is less than 3 ° C./s, recrystallization proceeds during heating and the texture is destroyed. Was set to 3 ° C./s. From this viewpoint, it is desirable to limit the temperature to 10 ° C./s or more. More desirably, it is at least 20 ° C./s.
[0050]
On the other hand, setting the heating rate to more than 100 ° C./s requires excessive capital investment, so 100 ° C./s was set as the upper limit of the heating rate.
[0051]
If the annealing temperature is lower than 600 ° C., the lower limit of the annealing temperature is set to 600 ° C., since the processed structure remains and significantly deteriorates the formability. On the other hand, if the annealing temperature is excessively high, the texture of ferrite generated by recrystallization is transformed into austenite, and then randomized by austenite grain growth, and the texture of the finally obtained ferrite is also randomized. You.
[0052]
In particular, when the annealing temperature exceeds (Ac3 + 150) ° C., such a tendency becomes remarkable. Therefore, the annealing temperature is set to (Ac3 + 150) ° C. or less. From this viewpoint, it is desirable that the annealing temperature be equal to or lower than the Ac3 transformation temperature.
[0053]
When cooling after annealing, if the cooling rate is less than 1 ° C./s, the texture of the finally obtained cold rolled steel sheet is not sufficiently developed, and good shape freezing properties cannot be obtained. 1 ° C./s was set as the lower limit of the cooling rate. From this viewpoint, the lower limit of the cooling rate is desirably 10 ° C./s. Further, setting the cooling rate to more than 250 ° C./s requires an excessive capital investment, so the upper limit of the cooling rate was set to 250 ° C./s.
[0054]
The cooling rate described here is the average cooling rate up to the temporary cooling stop temperature, and the above-described cooling rate may be achieved by a combination of cooling at a low cooling rate and cooling at a high cooling rate.
[0055]
In addition, after annealing / cooling, the continuous annealing process, the cooling or isothermal holding corresponding to the temperature history in the continuous hot-dip galvanizing process, or the reheating process in the alloying process of the continuous hot-dip galvanizing process. May be adopted.
[0056]
The cold-rolled steel sheet manufactured by the above method may be subjected to skin pass rolling. When skin pass rolling is performed, not only the shape of the steel sheet is improved, but also the impact energy absorbing ability of the steel sheet is increased. At this time, if the rolling reduction in skin pass rolling is less than 0.4%, this effect is small, so 0.4% is made the lower limit.
[0057]
Further, when the rolling reduction exceeds 5%, it is usually necessary to remodel the skin pass rolling mill, which causes economical disadvantages and significantly deteriorates the workability of the steel sheet. Therefore, the upper limit of the rolling reduction is 5%.
[0058]
The structure obtained by the present invention is mainly composed of ferrite. However, as a metal structure other than ferrite, compounds such as pearlite, bainite, martensite, austenite, and carbonitride may be contained. In particular, since the crystal structures of martensite and bainite are equivalent or similar to that of ferrite, these structures may be mainly used instead of ferrite.
[0059]
The cold rolled steel sheet thus obtained satisfies the following characteristics.
[0060]
Average value (A) of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group of the plate surface at 1/2 plate thickness:
This is a particularly important characteristic value in the present invention. The average value of the {100} <011> to {223} <110> orientation groups when the X-ray diffraction of the plate surface at the plate thickness center position was performed and the intensity ratio of each orientation to the random sample was obtained (hereinafter referred to as ( A)) must be greater than or equal to 3.0. If this is less than 3.0, the shape freezing property will be poor. From this viewpoint, more preferably, the average value of the X-ray random intensity ratio of (A) is 4.5 or more, and further preferably 5.0 or more.
[0061]
Further, the average value of the X-ray random intensity ratios of the three crystal orientations {554} <225>, {111} <112>, and {111} <110> of the plate surface at (A) and 1/2 plate thickness ( B) must satisfy (A) / (B) ≧ 1. When this value is less than 1, the shape freezing property is deteriorated. From this viewpoint, it is more preferable that (A) / (B) ≧ 2.0. As described above, the X-ray random intensity ratio of the cold-rolled sheet may be determined according to the above-described method of measuring the X-ray random intensity ratio of the hot-rolled sheet.
[0062]
As a matter of course, the above-mentioned limitation of the X-ray intensity is satisfied not only in the vicinity of the plate thickness of about 1/2, but also as much as possible, so that the shape freezing property is further improved.
[0063]
Although the reason why the X-ray intensity of the cold-rolled sheet is important for the shape freezing property at the time of bending is not necessarily clear, it is presumed to be related to the slip behavior of the crystal at the time of bending deformation.
[0064]
R value in the rolling direction (rL) and r value in the direction perpendicular to the rolling direction (rC):
It is important in the present invention. That is, as a result of intensive studies by the present inventors, it has been found that even if the X-ray intensities in the various crystal orientations described above are appropriate, good shape freezing properties cannot always be obtained. At the same time as the X-ray intensity, it is essential that at least one of rL and rC is 0.7 or less. More preferably, it is 0.55 or less.
[0065]
Note that the effects of the present invention can be obtained without particularly setting the lower limits of rL and rC.
[0066]
The r value is evaluated by a tensile test using a JIS No. 5 tensile test piece. The tensile strain is usually 15%, but when the uniform elongation is less than 15%, the strain may be evaluated as close to 15% as possible within the uniform elongation range.
[0067]
The direction in which the bending process is performed is not particularly limited because it differs depending on the processed component, but it is preferable to mainly bend in a direction perpendicular or nearly perpendicular to the direction in which the r value is small.
[0068]
By the way, it is generally known that there is a correlation between the texture and the r value. However, in the present invention, the limitation on the X-ray intensity ratio of the crystal orientation and the limitation on the r value are not synonymous with each other. If both limits are not satisfied at the same time, good shape freezing and workability cannot be obtained.
[0069]
The present invention can be applied to all cold-rolled steel sheets from low-tensile-strength mild steel sheets to high-strength steel sheets. If the above-mentioned limitations are satisfied, the bending workability of the cold-rolled steel sheets is dramatically improved. In other words, it is a basic material index relating to bending deformation beyond the restriction of the mechanical strength level of the cold-rolled steel sheet.
[0070]
When plating the cold-rolled steel sheet manufactured according to the present invention, the type of plating is not particularly limited, and the effects of the present invention can be obtained by any of electroplating, hot-dip plating, and vapor deposition plating.
[0071]
【Example】
The technical contents of the present invention will be described with reference to examples of the present invention.
[0072]
(Example)
As an example, a description will be given of the results of studies using steels A to M having the component compositions shown in Table 1. These steels are directly heated after casting, or once cooled to room temperature, reheated, heated to a temperature range of 1000 ° C. to 1300 ° C., then hot-rolled, and hot-rolled steel sheets of various thicknesses. did.
[0073]
Thereafter, the steel sheet was subjected to cold rolling at a reduction rate shown in Table 2 to have a thickness of 1.2 mm, and thereafter, was annealed in a continuous annealing step. A test piece having a width of 45 mm and a length of 270 mm was prepared from these 1.2 mm thick steel plates, and a hat bending test was performed using a die having a punch width of 78 mm, a punch shoulder R5, a die width of 81 mm, and a die shoulder R4. The molding height was 70 mm.
[0074]
For the test piece subjected to the bending test, the shape of the central portion of the plate width was measured by a three-dimensional shape measuring device, and as shown in FIG. 1, the tangent line between point (v) and point (w) and point (x) The average value on the left and right of the value obtained by subtracting 90 ° from the angle of the intersection of the tangent to the point (y) is averaged on the left and right, and the reciprocal of the curvature between the points (x) and (z) is averaged on the left and right. The value obtained by multiplying the value by 1000 was evaluated as the amount of wall warpage, and the value obtained by subtracting the punch width from the length between the right and left points (z) was used as the dimensional accuracy to evaluate the shape freezing property. The bending was performed such that a polygonal line was perpendicular to the direction in which the r value was low.
[0075]
[Table 1]

[0076]
By the way, as shown in FIGS. 2 and 3, the amount of springback and the amount of wall warpage also change depending on BHF (wrinkle holding force). The effect of the present invention does not change regardless of which BHF is evaluated. However, when pressing an actual part with an actual machine, a very high BHF cannot be applied due to facility restrictions. A hat bending test of each steel type was performed with BHF of 29 kN. The bending was performed so that a polygonal line was perpendicular to the direction in which the r value was low.
[0077]
Table 2 shows whether or not the manufacturing conditions of each steel sheet are within the scope of the present invention. Column (a) of “Hot rolled sheet texture” includes {100} <011> to {223} <110> orientation groups at a position 7/16 of the thickness of the hot rolled sheet measured by X-ray. The average value of the X-ray random intensity ratio of the three crystal orientations {554} <225>, {111} <112>, and {111} <110> is shown in the column (b). Indicates a value.
[0078]
Each steel sheet was cooled from the annealing temperature at a cooling rate of 3 to 30 ° C./s, and skin pass rolling was performed in a range of 0.5 to 1.5%.
[0079]
[Table 2]

[0080]
Table 3 shows the mechanical property values, the texture of the annealed sheet, and the index of shape freezing property of the cold-rolled steel sheet having a thickness of 1.2 mm manufactured by the above method.
[0081]
Column (A) of “Annealed plate texture” shows the X of {100} <011> to {223} <110> orientation group at the position of 7 / 16th of the plate thickness of the annealed plate measured by X-ray. The average value of the line random intensity ratio, the column (B) shows the average value of the X-ray random intensity ratio of three crystal orientations of {554} <225>, {111} <112> and {111} <110>, In the (A) / (B) columns, mean values of the X-ray random intensity ratios of the {100} <011> to {223} <110> orientation groups and {554} <225>, {111} <112 are shown. > And {111} <110> indicate the average ratio of the X-ray random intensity ratios of the three crystal orientations.
[0082]
Any of those satisfying the conditions limited by the present invention achieves good shape freezing properties.
[0083]
The mechanism that the X-ray random intensity ratio and r value of each crystal orientation are important for shape freezing has not always been clarified at present. It is probably understood that by facilitating the progress of the slip deformation during the bending deformation, the spring back amount and the wall warpage amount during the bending deformation are reduced as a result.
[0084]
[Table 3]

[0085]
【The invention's effect】
Controlling the texture and r-value of a thin steel sheet significantly improves its bendability, as described in detail above. According to the present invention, it is possible to provide a thin steel sheet having a small amount of spring back and excellent in shape freezing property mainly by bending. In particular, high-strength steel sheets can be used even in parts where high-strength steel sheets have conventionally been difficult to apply due to the problem of poor shape.
[0086]
The use of high-strength steel sheets is absolutely necessary to promote the reduction in the weight of automobiles. If a high-strength steel sheet having a small amount of spring back and excellent shape freezing properties can be applied, it is possible to further reduce the weight of an automobile body. Therefore, the present invention is an industrially extremely valuable invention.
[Brief description of the drawings]
FIG. 1 is a sectional view of a test piece used for a hat bending test.
FIG. 2 is a diagram illustrating a relationship between a springback amount and a BHF (wrinkle holding force).
FIG. 3 is a diagram illustrating a relationship between a wall warpage amount and BHF (wrinkle holding force).

Claims (5)

In mass%,
C: 0.0001% or more, 0.25% or less,
Si: 0.001% or more, 2.5% or less,
Mn: 0.01% or more, 2.5% or less,
P: 0.15% or less,
S: 0.03% or less,
Al: 0.01% or more, 2.0% or less,
N: 0.01% or less,
O: 0.01% or less,
Furthermore, one or two of Ti and Nb are contained in a total of 0.01% or more and 0.40% or less, and the balance consists of iron and inevitable impurities. The average value of the X-ray random intensity ratio of the <011> to {223} <110> orientation group is 3.0 or less, and {554} <225>, {111} <112>, and {111} <110>. After pickling, a hot-rolled steel sheet satisfying an average value of the X-ray random strength ratios of the three crystal orientations of 6.0 or less is subjected to cold rolling at 25% or more and 80% or less, and further 3 ° C./s. After heating to a temperature range of 600 ° C. to (Ac3 + 150) ° C. at １００100 ° C./s, it is cooled at 1 ° C./s to 250 ° C./s, and {100} <011> of the plate surface in at least 板 plate thickness. The average value (A) of the X-ray random intensity ratios of the group of orientations from {223} <110> is 0.0 or more, and (A) X-ray random of three crystal orientations of {554} <225>, {111} <112> and {111} <110> at least 1/2 plate thickness A steel sheet in which the ratio (A) / (B) of the average value (B) of the strength ratios satisfies 1.0 or more and at least one of the r values in the rolling direction and the direction perpendicular thereto is 0.7 or less. A method for producing a cold-rolled steel sheet having excellent shape freezing characteristics, characterized by being obtained. Furthermore, in mass%,
V: 0.20% or less,
Cr: 1.5% or less, and
B: The method for producing a cold-rolled steel sheet excellent in shape freezing property according to claim 1, containing one or more kinds of 0.007% or less. Furthermore, in mass%,
Mo ≦ 1%,
Cu ≦ 2%,
Ni ≦ 1%, and
Sn ≦ 0.2%
The method for producing a cold-rolled steel sheet having excellent shape freezing properties according to claim 1 or 2, comprising one or more of the following. A method for producing a cold-rolled steel sheet having excellent shape freezing properties, comprising plating the cold-rolled steel sheet having excellent shape freezing properties according to any one of claims 1 to 3. The cold-rolled steel sheet excellent in shape freezing property, wherein the cold-rolled steel sheet excellent in shape freezing property according to any one of claims 1 to 4 is subjected to skin pass rolling of 0.4% or more and 5% or less. Manufacturing method.

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