JP2002115025A - Steel sheet having high stretch-flanging property and excellent shape freezability and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin steel sheet worked mainly by bending in which the amount of spring back and the amount of wall camber are small, and which has excellent shape freezability and hole expandability. SOLUTION: This hot rolled steel sheet and cold rolled steel sheet having high stretch-flanging properties and excellent shape freezability has a composition containing, by mass, 0.0001% to 0.3% C, 0.001 to 3.5% Si, 0.05 to 3% Mn, <=0.2% P, <=0.03% S, 0.01 to 3% Al, <=0.01% N and <=0.01% O, and the balance iron with inevitable impurities and has a composite structure formed of ferrite or bainite as the main phase by area ratio, and the total area ratio of pearlite, martensite and retained austenite is <=5%, also, the average value of the X-ray random intensity ratios in the orientation groups of [100]<;011>; to [223]<;110>; of the sheet plane in the sheet thickness of at least 1/2 is >=3.0, besides, the average value of the X-ray random intensity ratios in the three crystal orientations of [554]<;225>;, [111]<;112>; and [111]<;110>; is <=3.5, and further, at least one of the (r) value in the rolling direction and the (r) value in a direction orthogonal to the rolling direction is <=0.7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high elongation flangeable steel sheet excellent in shape freezing property and a method for producing the same, and the steel sheet is mainly used for automobile parts and the like. The steel sheet of the present invention contains both a hot-rolled steel sheet and a cold-rolled steel sheet.

[0002]

2. Description of the Related Art In order to reduce the amount of carbon dioxide gas emitted from automobiles, the use of high-strength steel sheets to reduce the weight of automobile bodies has been promoted. In addition, in order to ensure the safety of passengers, 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.

However, when bending deformation is applied to a high-strength steel sheet, the spring-back phenomenon, in which the shape after processing tends to return from the shape of the processing jig to the shape before processing because of its high strength, Elastic recovery from bending-unbending causes "wall warp phenomenon" in which the plane of the side wall becomes a surface with curvature, resulting in poor dimensional accuracy such that the shape of the target processed part cannot be obtained. Occurs.

[0004] Therefore, in the conventional automobile body, mainly high-strength steel sheets of 440 MPa or less have been used. In other words, despite the necessity of using a high-strength steel plate of 490 MPa or more to reduce the weight of the car body, spring back and wall warpage hardly occur and the dimensional accuracy is good, that is, In fact, there is no high-strength steel sheet having good shape freezing properties.

[0005] Needless to add, it is also extremely important to enhance the shape freezing property of a high-strength steel sheet or a mild steel sheet having a pressure of 440 MPa or less after processing, such as an automobile or a home electric appliance. Under such circumstances, Japanese Unexamined Patent Application Publication No. 10-72644 discloses that the degree of accumulation of {200} texture in a plane parallel to the rolling plane is 1.5 or more, and the amount of springback is small. An austenitic stainless cold rolled steel sheet is disclosed. However, there is no description about a ferritic steel sheet.

On the other hand, the stretch flangeability is also an indispensable property when a steel sheet is processed into automobile parts and the like. The application range of the high strength steel sheet becomes wider.

[0007]

When bending a mild steel sheet or a high-strength steel sheet, a shape defect such as a large spring back or a wall warp occurs depending on the strength of the steel sheet. At present, the shape freezeability of parts is poor. Stretch flangeability is an indispensable property when processing steel sheets, and in order to apply a high-strength steel sheet to automobile parts, etc., it is necessary to have excellent shape freezing properties and stretch flangeability. desired.

The present invention drastically solves this problem,
An object of the present invention is to provide a high elongation flangeable steel sheet excellent in shape freezing property and a method for producing the same.

[0009]

According to the conventional knowledge, as a measure for suppressing a shape defect such as a spring back or a wall warp, it is considered as important for the time being to reduce the deformation stress of the steel sheet. I was Then, in order to reduce the deformation stress, a steel plate having a low tensile strength had to be used. However, this alone is not a fundamental solution for improving the bending workability of the steel sheet and keeping the spring back amount low.

[0010] In view of the above, the present inventors newly investigated 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 wall warpage. Focusing on this, the effects and effects were investigated and studied in detail. Then, the present inventors have found a steel sheet excellent in bending workability. That is, {100} <011> to $ 22
3} <110> orientation group intensity, {554} <225
>, {111} <112>, {111} <110>, and controlling at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction as low as possible. It was clarified that by setting the value, the bending workability was dramatically improved.

The present invention has been made based on the above findings, and its gist is as follows. (1) Ferrite or bainite is a maximum phase in area ratio, is a composite structure steel in which the area ratio of pearlite, martensite, and retained austenite is 5% or less in total, and is at least a half of the sheet surface at a sheet thickness. $ 10
The average value of the X-ray random intensity ratio of the 0 <011> to {223} <110> azimuth group is 3.0 or more and {55}
4 {225}, {111} <112> and {11
The average value of the X-ray random intensity ratio of the three crystal orientations of 1 <110> is 3.5 or less, and at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction is 0.1. A high elongation flangeable steel sheet having an excellent shape freezing property of not more than 7.

(2) In mass%, C: 0.0001% or more and 0.3% or less, Si: 0.001% or more and 3.5%
Below, Mn: 0.05% or more, 3% or less, P: 0.2%
Below, S: 0.03% or less, Al: 0.01% or more, 3
%, N: 0.01% or less, O: 0.01% or less, and the balance is composed of iron and unavoidable impurities. Flange steel sheet.

(3) Further, in mass%, Ti: 1% or less,
The shape according to (2), wherein one or more of Nb: 1% or less, V: 1% or less, Cr: 3% or less, and B: 0.01% or less are contained. High stretch flangeable steel sheet with excellent freezing properties. (4) Further, by mass%, Mo: 1% or less, Cu: 3% or less, Ni: 3% or less, and Sn: 0.2% or less.
The high-stretch flangeable steel sheet according to the above (2) or (3), wherein the steel sheet comprises at least one of a kind or two or more kinds.

(5) Further, in mass%, Ca: 0.000
5% or more, 0.005% or less, and Rem: 0.0
The high-stretch flangeable steel sheet according to any one of the above (2) to (4), which contains one or more of 01% or more and 0.02% or less. (6) A steel sheet according to any one of (1) to (5), wherein the steel sheet is plated with a high-stretch flangeable steel sheet having excellent shape freezing properties.

(7) Any one of the above (1) to (6)
In manufacturing the steel sheet of Ar _ThreeTransformation temperature ~ (Ar_Three+
100) The total reduction in the temperature range of ° C is 25% or less.
Is controlled to be above, and Ar_ThreeHot rolling above the transformation temperature
And the critical temperature determined by the chemical composition of the steel shown in equation (1)
Winding at a temperature below To and at a temperature above 350 ° C
High-stretch flangeable steel sheet with excellent shape freezing characteristics
Production method.

To = −650.4 × C% + B (1) Here, B is obtained from the steel component expressed in mass%. B = -50.6 × Mneq + 894.3 Mneq = Mn% + 0.24 × Ni% + 0.13 × Si% + 0.38 × Mo% + 0.55 × Cr% +
0.16 × Cu% −0.50 × Al% −0.45 × Co% + 0.90 × V% (8) The total reduction ratio in the temperature range from the Ar ₃ transformation temperature to (Ar ₃ +100) ° C. is 25% or more, and ( Ar ₃
(7) In the hot rolling at a temperature of + 100 ° C. or lower, the hot rolling is controlled so that the friction coefficient is 0.2 or less in at least one pass or more. Manufacturing method of stretch flangeable steel sheet.

(9) Any one of the above (1) to (6)
In manufacturing the steel sheet of Ar _ThreeReduction below transformation temperature
Controlling the hot rolling so that the sum of the rates is at least 25%,
After finishing hot rolling, 350 ~ Ac_ThreeWinding at transformation temperature
Or after cooling once, 500-Ac_Three1 for transformation temperature
Excellent shape freezing characteristic characterized by heating for 0 to 120 minutes
Manufacturing method for high-strength flanged steel sheet.

(10) The hot rolling at a temperature not higher than the Ar ₃ transformation temperature is 25% or more, and at least one pass is rolled in hot rolling at an Ar ₃ or lower so that the friction coefficient is 0.2 or lower. (9) The method for producing a high-stretch flangeable steel sheet having excellent shape freezing property according to the above (9). (11) After the steel sheet according to any of (7) to (10) is pickled and subjected to cold rolling at a rolling reduction of less than 80%,
A method for producing a high elongation flangeable steel sheet having excellent shape freezing characteristics, wherein the steel sheet is heated to a temperature range of 00 ° C. to (Ac ₃ +100) ° C. and cooled.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The contents of the present invention will be described below in detail. {100} <011 of the plate surface at 1/2 plate thickness
> To {223} <110> orientation group average X-ray random intensity ratio, and {554} <225>, {111}
Average value of X-ray random intensity ratio of three crystal orientations of <112> and {111} <110>: The average value is a particularly important characteristic value in the present invention. {100} <011> to {223} <when X-ray diffraction of the plate surface at the plate thickness center position is performed and the intensity ratio of each direction to the random sample is obtained.
110> The average value of the orientation group must be 3.0 or more. If it is less than 3.0, the shape freezing property is inferior.

The main azimuth included in this azimuth group is $ 10.
0 {011>, {116} <110>, {114} <
110>, {113} <110>, {112} <110
>, {335} <110>, and {223} <11
0>. The X-ray random intensity ratio in each of these directions is
3 calculated by vector method based on {110} pole figure
Dimensional textures, {110}, {100}, {211},
It may be obtained from a three-dimensional texture calculated by a series expansion method using a plurality of pole figures (preferably three or more) of the {310} pole figures.

For example, in the latter method, the X-ray random intensity ratio of each of the crystal orientations described above is expressed by φ2 of the three-dimensional texture.
(001) [1-10], (11)
6) [1-10], (114) [1-10], (11)
3) [1-10], (112) [1-10], (33)
5) The strengths of [1-10] and (223) [1-10] may be used as they are.

{100} <011> to {223} <11
The average value of the X-ray random intensity ratios in the group of 0> azimuths is the arithmetic average of the intensity ratios in the respective directions. If the intensity ratios in all the above directions cannot be obtained, {100} <0
11>, {116} <110>, {114} <110
>, {112} <110>, and {223} <110> may be substituted by the arithmetic mean of the intensity ratios of the respective directions.

Further, the thickness of the plate surface at the half plate thickness is
4 {225}, {111} <112> and {11
The average value of the X-ray random intensity ratio of three crystal orientations of 1｝ <110> must be 3.5 or less. This is 3.
If it exceeds 5, {100} <011> to {223} <1
10> Even if the intensity ratio of the orientation group is appropriate, it is difficult to obtain good shape freezing properties. {554} <225>,
X of {111} <112> and {111} <110>
The line random intensity ratio may also be obtained from the three-dimensional texture calculated according to the above method.

More preferably, {100} <011>-
The average value of the X-ray random intensity ratio of the {223} <110> orientation group is 4.0 or more, {554} <225>, {111}
The arithmetic mean of the X-ray random intensity ratio of <112> and {111} <110> is less than 2.5. The reason why the X-ray intensity of the crystal orientation described above is important for shape freezing during bending is not necessarily clear, but it is presumed to be related to the slip behavior of the crystal during bending deformation. .

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. It is manufactured so that When a segregation zone or a defect is present in the thickness center layer of the steel sheet and inconvenience occurs in the measurement, the above-described method is performed so that an appropriate surface becomes the measurement surface in a range of ／ to ／ of the thickness. A sample may be prepared according to the method and measured. 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 近 傍 but also in as many thicknesses as possible, whereby the shape freezing property is further improved.

The crystal orientation represented by {hkl} <uvw> indicates that the normal direction of the sheet surface is parallel to <hkl> and the rolling direction is parallel to <uvw>. R value in the rolling direction (rL) and r value in the direction perpendicular to the rolling direction (rC): These r values are 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 intensity ratios of 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 ratio, it is essential that at least one of rL and rC is 0.7 or less. More preferably, it is 0.55 or less.

The effects of the present invention can be obtained without any particular lower limits for rL and rC. r value is JIS5
It is evaluated by a tensile test using a No. tensile test specimen. 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. Note that 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 perform the bending process in the direction in which the r value is small in a direction perpendicular or nearly perpendicular.

Although it is generally known that there is a correlation between the texture and the r-value, in the present invention, the above-described limitation on the X-ray intensity ratio of the crystal orientation and the limitation on the r-value are mutually different. It is not synonymous, and good shape freezing properties cannot be obtained unless both limitations are met at the same time. Structure: From the viewpoint of hole expandability, the structure has ferrite or bainite as a maximum phase in area ratio. However, comparing the respective textures of ferrite and bainite, {100} <011> to {22} which are advantageous for shape freezing in the bainite portion
A texture with a 3｝ <110> orientation is likely to develop. Although the reason for this is not clear, it is considered that the bainite structure is likely to inherit an austenite texture that is formed during hot rolling and is superior in shape freezing properties.

Therefore, it is more desirable that the space factor of bainite is large. From this viewpoint, the area ratio of bainite is desirably more than 35%. The area ratio of ferrite or bainite is determined by the optical microscope at the center of the plate thickness.
Observe at least 5 visual fields at 100 to 500 times, and determine from the average value. Further, as-processed ferrite or bainite significantly impairs the formability, and is not included in the area ratio described here.

As another structure, when the area ratio of martensite, retained austenite, and pearlite exceeds 5%, the stretch flangeability deteriorates. Therefore, the total area ratio of these tissues is 5% or less. Next, the limiting conditions relating to the component composition will be described. First, the limiting conditions relating to the component composition of the above (2) will be described.

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 C exceeds 0.3%, workability and weldability deteriorate, so the upper limit is made 0.3%. Si is an element effective in increasing the mechanical strength of a steel sheet, but when it exceeds 3.5%,
Since workability deteriorates and surface flaws occur,
The upper limit is 3.5%. On the other hand, in practical steel, 0.00
Since it is difficult to make the content less than 1%, the lower limit is made 0.001%.

Mn is also an effective element for increasing the mechanical strength of the steel sheet, but if it exceeds 3%, the workability deteriorates, so the upper limit is 3%. On the other hand, Mn was 0.05
If it is less than%, the cost is high and there is no merit in material, so the lower limit is 0.05%. When an element such as Ti that suppresses hot cracking due to S is not sufficiently added in addition to Mn, Mn / S ≧ 20 in mass%.
It is desirable to add an amount of Mn that becomes

P and S are 0.2% or less and 0.0%, respectively.
3% or less. This is to prevent deterioration of workability and cracks during hot rolling or cold rolling. Al is added at 0.01% or more for deoxidation. However, if the content is too large, the processability is deteriorated and the surface properties are deteriorated.
%. N and O are impurities, and both are set to 0.01% or less so as not to deteriorate workability.

In the above invention (3), Ti, Nb,
V, Cr, and B improve the material through mechanisms such as fixation of carbon and nitrogen, precipitation strengthening, structure control, and fine grain strengthening.
0.005% or more and 0.00, respectively, as necessary
1% or more, 0.001% or more, 0.01% or more, 0.0
It is desirable to add 001% or more. However, excessive addition has no remarkable effect, but rather deteriorates workability and surface properties. Therefore, the upper limits of Ti, Nb, V, Cr, and B are each set to 1%. , 1%, 1%, 3% and 0.01
%.

In the above invention (4), Mo, Cu,
Ni and Sn have the effect of increasing the mechanical strength and improving the quality of the material.
It is desirable to add the above, but excessive addition adversely deteriorates the workability, so that Mo, Cu, Ni, and Sn are added.
Are 1%, 3%, 3%, and 0.2%, respectively.

In the above invention (5), Ca and Rem control the form of sulfide to improve stretch flangeability.
Are 0.0005% and 0.001%, respectively.
It is desirable to add above. However, excessive addition has no remarkable effect and increases the cost. Therefore, the upper limits of Ca and Rem are set to 0.005% and 0.02%, respectively.
%.

Although not particularly limited in the present invention, Mg may be added for the purpose of deoxidation or for controlling the form of sulfide. Plating: 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, vapor deposition plating, and the like.

Next, the manufacturing method will be described. The production method prior to hot rolling is not particularly limited. That is, following the smelting by a blast furnace, a converter, an electric furnace, etc., various secondary smelting may be performed, 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, the steel sheet may be once cooled to a low temperature, heated again, and then hot-rolled, or the cast slab may be continuously hot-rolled. Scrap may be used as a raw material. The high elongation flangeable steel sheet excellent in shape freezing property of the present invention is obtained by casting a steel having the above-mentioned composition, then cooling after hot rolling, heat treatment after hot rolling, cooling after hot rolling, and pickling and cold rolling. After that, it can also be obtained by plating on a hot-rolled steel sheet or a cold-rolled steel sheet, or while performing a heat treatment in a hot-dip galvanizing line, and further subjecting these steel sheets to a separate surface treatment.

As described in the invention of (7), in the latter half of the hot rolling, the temperature is higher than the Ar ₃ transformation temperature (Ar ₃ +100).
If the rolling of 25% or more is not performed below ℃,
Since the texture of the rolled austenite is not sufficiently developed, the predetermined X-ray intensity described in the above (1) is applied to the surface of the finally obtained hot-rolled steel sheet regardless of any cooling. Each level of crystal orientation cannot be obtained. Therefore,
The lower limit of the total reduction ratio at (Ar ₃ +100) ° C. or lower was 25%.

Since the formation of a sharper texture can be expected as the total rolling reduction at (Ar ₃ +100) ° C. or lower and the Ar ₃ transformation temperature or higher is higher, the total rolling reduction is preferably 35% or more. . However, the sum of the rolling reduction is 9
If it exceeds 7.5%, it is necessary to excessively increase the rigidity of the rolling mill, which causes economical disadvantages. Therefore, the total rolling reduction is desirably 97.5% or less.

Here, as described in the invention of the above (8),
(Ar ₃ +100) ° C. or less When the friction coefficient between the hot-rolled roll and the steel sheet during hot rolling at the Ar ₃ transformation temperature or more exceeds 0.2, the sheet surface near the steel sheet surface has a ｛11
The crystal orientation mainly including the 0 ° plane develops, and the shape freezing property deteriorates. In order to improve the shape freezing property, the coefficient of friction between the hot-rolled roll and the steel sheet is reduced to 0 in at least one pass during hot rolling at (Ar ₃ +100) ° C. or lower and Ar ₃ transformation temperature or higher. .2 or less. The lower the coefficient of friction, the more preferable. If a better shape freezing property is required, (Ar ₃ +
The coefficient of friction is desirably set to 0.15 or less for all hot rolling passes at 100) ° C or lower and the Ar ₃ transformation temperature or higher.

The finishing temperature of the hot rolling must be equal to or higher than the Ar ₃ transformation temperature from the viewpoint of formability. The upper limit of the finishing temperature is not particularly limited, but is preferably (Ar ₃ +50) ° C. or lower in order to sharpen the texture having excellent shape freezing properties. In order to pass on the austenite texture formed in this way to the final hot-rolled steel sheet, it is necessary to wind it at a temperature not higher than the To temperature shown in the following equation (1). Therefore, To determined by the composition of the steel was set as the upper limit of the winding temperature.

The To temperature is thermodynamically defined as the temperature at which austenite and ferrite of the same component as austenite have the same free energy.
In consideration of the influence of components other than the above, the calculation can be simply performed using the equation (1). The effect of the other components specified in the present invention on the To temperature was not so great and was ignored here.

To = −650.4 × C% + B (1) Here, B is determined by the steel component expressed in mass% and can be expressed as follows. B = -50.6 × Mneq + 894.3 Mneq = Mn% + 0.24 × Ni% + 0.13 × Si% + 0.38 × Mo% + 0.55 × Cr% +
0.16 × Cu% -0.50 × Al% -0.45 × Co% + 0.90 × V% Also, if the hot rolling is below Ar ₃ transformation temperature,
The ferrite generated before processing is processed, and a strong rolled texture is formed. As described in the invention of the above (9),
In order to finally make such a texture a texture advantageous for shape freezing properties, the ferrite processed at a high temperature is wound at a transformation temperature of 350 to Ac _{3 during} cooling, or once cooled. it is necessary to recover and recrystallization by heating again 500～Ac ₃ transformation temperature.

The total rolling reduction below the Ar ₃ transformation temperature is 25.
%, The coil may be wound at a temperature equal to or higher than the recrystallization temperature or may be cooled and reheated to perform the recovery / recrystallization treatment. Since each crystal orientation could not be obtained, 25% was set as the lower limit of the total rolling reduction below the Ar ₃ transformation temperature. 35% is a more desirable lower limit.

In the case of heating once after cooling, the workability is deteriorated if the heating temperature is lower than 500 ° C., and the shape freezing property is deteriorated if the heating temperature is higher than the Ac ₃ transformation temperature. , 500 to Ac ₃ transformation temperature. Although the hot rolling end temperature is not particularly specified, it is 30
If the temperature is lower than 0 ° C., the load on the rolling mill increases.
It is desirable that the temperature be set to 00 ° C. or higher.

Here, as described in the invention of (10) above, when the friction coefficient between the hot-rolling roll and the steel sheet during hot rolling exceeds 0.2, the sheet surface near the steel sheet surface is In addition, the crystal orientation mainly including the {110} plane develops, and the shape freezing property deteriorates. Therefore, in order to achieve a better shape freezing property, it is preferable that the friction coefficient between the roll and the steel sheet is 0.2 or less for at least one pass in the hot rolling at Ar ₃ or less.

The lower the friction coefficient is, the more desirable it is. Particularly, when severe shape freezing property is required, Ar
_The coefficient of friction was set to 0.1 for all hot rolling passes of ₃ or less.
It is desirably 5 or less. In the hot rolling, the sheet bar may be joined after the rough rolling, and the finish rolling may be continuously performed. At this time, the coarse bar may be temporarily wound into a coil shape, and if necessary, stored in a cover having a heat retaining function, and may be rewound again before joining. The hot-rolled steel sheet may be subjected to skin pass rolling as necessary. It goes without saying that skin pass rolling has the effect of preventing stretcher strain generated at the time of working and correcting the shape.

When the hot-rolled steel sheet thus obtained is cold-rolled and annealed to form a final thin steel sheet, if the total rolling reduction of the cold-rolled steel sheet is 80% or more, a general method is used. Cold rolling-X of the crystal plane parallel to the plate surface which is a recrystallization texture
The {111} plane and {554} plane components of the X-ray diffraction integrated plane intensity ratio are increased, and the crystal orientation of the invention (1), which is a feature of the present invention, is not satisfied. The upper limit is set to 80%.

In order to enhance the shape freezing property, it is desirable to limit the cold rolling reduction to 70% or less. The effect of the present invention can be obtained without any particular limitation on the lower limit of the cold reduction ratio. However, in order to control the strength of the crystal orientation in an appropriate range, the cold reduction ratio should be 3% or more. preferable. When the cold-rolled steel sheet cold-worked in such a range is annealed, if the annealing temperature is lower than 600 ° C, the work structure remains and the formability is significantly deteriorated. And

On the other hand, if the annealing temperature is excessively high, the texture of the ferrite formed by recrystallization is transformed into austenite, and then randomized by austenite grain growth, and the texture of the finally obtained ferrite is also changed. Be randomized. In particular, when the annealing temperature is (Ac ₃ +10
When the temperature exceeds 0) ° C., such a tendency becomes remarkable.
Therefore, the annealing temperature is set to (Ac ₃ +100) ° C. or less.
The cold-rolled steel sheet may be subjected to skin pass rolling as necessary.

The steel sheet according to the present invention can be applied not only to bending but also to composite forming mainly based on bending, such as bending, overhang, drawing, and the like. With examples of the present invention,
The technical contents of the present invention will be described.

[0054]

EXAMPLES (Examples) The results of investigations using steels A to Q having the component compositions shown in Table 1 will be described.
These steels, after casting, as they are, or once cooled to room temperature, are reheated to a temperature range of 900 ° C. to 1300 ° C., then hot-rolled, and finally,
It was a hot rolled steel sheet having a thickness of 1.4 mm, 3 mm or 8.0 mm. The hot-rolled steel sheets having a thickness of 3.0 mm and 8.0 mm are cold-rolled to have a thickness of 1.4 mm,
Thereafter, annealing was performed in a continuous annealing step. These 1.
A test piece having a width of 50 mm and a length of 270 mm was prepared from a steel plate having a thickness of 4 mm, and a hat bending test was performed using a die having a punch width of 78 mm, a punch shoulder R5, and a die shoulder R5. The test piece subjected to the bending test was measured for the shape at the center of the plate width by a three-dimensional shape measuring device, and as shown in FIG. 1, points (1) and (2) were obtained.
The average value on the left and right of the value obtained by subtracting 90 ° from the angle of the intersection of the tangent of the point (3) and the point (4) with the point (3) and the point (4) is defined as the amount of spring back, and the average between the points (3) and (5) is obtained. The value obtained by averaging the reciprocal of the curvature on the left and right was defined as the wall warpage amount, and the value obtained by subtracting the punch width from the length between the left and right points (5) was defined as the dimensional accuracy, and the shape freezing property was evaluated. The bending was performed so that a polygonal line was perpendicular to the direction in which the r value was low.

As shown in FIGS. 2 and 3, the amount of springback and the amount of wall warpage are BHF (wrinkle holding force).
It also changes by. The effect of the present invention is
Although the tendency does not change even if the evaluation is carried out, since high BHF cannot be applied when an actual part is pressed with an actual machine, a hat bending test of each steel type was performed with BHF 29 kN this time.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

Tables 2 and 3 (continuation of Table 2) show the manufacturing conditions of each steel sheet and whether the manufacturing conditions are within the scope of the present invention. "Hot rolling temperature 1", when the hot rolling is completed at Ar ₃ transformation temperature or more, when it is (Ar ₃ +100) ° C. or less total reduction ratio in the Ar ₃ transformation temperature or higher 25% or higher " ○ ”, and“ × ”when less than 25%. “Hot rolling temperature 2” is “○” when hot rolling is at or below the Ar ₃ transformation temperature, and “○” when the total reduction ratio at or below Ar ₃ temperature is at least 25%, and “×” when it is less than 25%. "

In any of the above cases, the coefficient of friction for at least one or more passes in each temperature range is not less than 0.1.
In the case of 2 or less, “○” was entered in the “lubrication” column, and when the friction coefficient in all passes exceeded 0.2, “△” was entered.
The hot rolling was all performed at a temperature equal to or lower than the To temperature determined by the equation (1). When such a hot-rolled steel sheet is cold-rolled to a thickness of 1.4 mm,
When the cold rolling reduction was 80% or more, "cold rolling reduction" was set to "x", and when it was "less than 80%", "o" was set.

When the annealing temperature is 600 ° C. or more (Ac ₃ +
100 ° C. or lower, the “annealing temperature” was “○”, and the other cases were “×”. Items that are not relevant as manufacturing conditions are marked "-". Skin pass rolling was performed on both the hot-rolled steel sheet and the cold-rolled steel sheet in a range of 0.5 to 1.5%. For the measurement of X-ray, a sample parallel to the plate surface was prepared at a position of 7 / 16th of the plate thickness as a representative value of the steel plate,
Carried out.

In the hole expansion test, a punched hole having a diameter of 10 mm was formed in the center of a test piece having a side of 100 mm, and the initial hole was pushed and expanded with a conical punch having a vertex angle of 60 °. Was evaluated by the hole expansion ratio λ (the following formula) with respect to the initial hole diameter of 10 mm. λ = {(d-10) / 10} × 100 (%) Tables 4 and 5 show the mechanical property values and holes of the hot-rolled steel sheet and the cold-rolled steel sheet having a thickness of 1.4 mm manufactured by the above method. The expansion ratio, spring back amount, wall warpage amount, and dimensional accuracy are shown. In all steel types except steel H in Table 4,
Examples of numbers "-2" and "-3" of each steel type correspond to the present invention, and examples of numbers "-1" and "-3" are out of the invention.

The microstructure of each of the microstructures except steel H was such that the area ratio of martensite, retained austenite, and pearlite was less than 5%, and ferrite or bainite was the largest phase in area ratio. However, E-1, H, I-1, O-
In No. 1, processed grains remained at an area ratio of 50 to 100%. The number of "-2" and "-3" of the present invention is smaller in the amount of spring back and the amount of wall warp than the number of "-1" and "-4" other than the invention. It turns out that the dimensional accuracy is improved as a result. Also,
In any of the cases of the present invention, stretch flangeability is also good.

That is, a high-stretch flangeable steel sheet having good shape freezing properties can be produced for the first time by satisfying the X-ray random intensity ratio, r value, and structure of each crystal orientation defined in the present invention. It is.

[0065]

[Table 4]

[0066]

[Table 5]

Next, R to T having the component compositions shown in Table 6
The result of the study using the steel No. will be described. Hot rolling is
In each case, the reaction was completed at the Ar ₃ transformation temperature or higher, and (Ar ₃
(+100) ° C. or less The total rolling reduction at an Ar ₃ temperature or more is 3
Rolling was performed to 5%. For steel R, the finishing temperature was changed by two levels within a range satisfying this condition.
No lubrication was performed in any case. Skin pass rolling was performed at a rolling reduction of 1%.

Table 7 shows the structural fraction, mechanical properties, and the like of the hot-rolled steel sheet having a thickness of 1.4 mm manufactured by the above method.
Hole expansion rate, spring back amount, wall warpage amount, and
The dimensional accuracy is shown. In any case, the steel R has a small amount of spring back and a small amount of wall warpage, has good shape freezing properties, and has good stretch flangeability. Also,
As the amount of bainite increases, the shape freezing property is further improved. On the other hand, steels S and T have a good shape freezing property but a deteriorated stretch flangeability because the total area ratio of martensite, retained austenite and pearlite exceeds 5%.

The mechanism by which the X-ray random intensity ratio and the r value of each crystal orientation are important for shape freezing has not always been clarified at present. Possibly, by facilitating the progress of slip deformation during bending, the amount of spring back during bending,
And the amount of wall warpage is reduced, resulting in dimensional accuracy,
That is, it is understood that the shape freezing property is improved.

[0070]

[Table 6]

[0071]

[Table 7]

[0072]

By controlling the texture and r-value of a thin steel sheet according to the present invention, the bending workability is remarkably improved, and by controlling the structure and carbide, both hole expandability and bending workability are achieved. Yes, spring back amount, and
It has become possible to provide a thin steel sheet having a small amount of wall warpage, an excellent shape freezing property mainly by bending, and an excellent hole expanding property.

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. The use of high-strength steel sheets is absolutely necessary in order to promote weight reduction of automobiles. When a high-strength steel sheet having a small amount of spring back and a small amount of wall warpage and excellent in shape freezing property and hole expandability can be applied, weight reduction of an automobile body can be further promoted.
Therefore, the present invention is an industrially extremely valuable invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cross section of a test piece used for a hat bending test.

FIG. 2 Amount of springback and BHF (wrinkle holding force)
FIG.

FIG. 3 is a diagram illustrating a relationship between a wall warpage amount and a BHF (wrinkle holding force).

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/58 C22C 38/58 (72) Inventor Manabu Takahashi 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation (72) Inventor Toru Yoshida 20-1 Shintomi, Futtsu-shi, Chiba F-term (reference) in Nippon Steel Corporation Technology Development Division 4K037 EA00 EA01 EA02 EA04 EA05 EA06 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA22 EA23 EA25 EA27 EA28 EA31 EA32 EB02 EB06 EB07 EB08 EB09 EB11 FB03 FB07

Claims (11)

[Claims] 1. A steel sheet having a composite structure in which ferrite or bainite has a maximum area ratio of pearlite, martensite, and retained austenite in a total area ratio of 5% or less, and has a thickness of at least １ ／. The average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation group of the plane is 3.0 or more, and
The average value of the X-ray random intensity ratio of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> is 3.5 or less, and further, the r value in the rolling direction and A high elongation flangeable steel sheet excellent in shape freezing property, wherein at least one of r values in a direction perpendicular to the rolling direction is 0.7 or less. 2. In mass%, C: 0.0001% or more and 0.3% or less, Si: 0.001% or more and 3.5% or less, Mn: 0.05% or more and 3% or less, P : 0.2% or less, S: 0.03% or less, Al: 0.01% or more and 3% or less, N: 0.01% or less, O: 0.01% or less, with the balance being iron and The high elongation flangeable steel sheet having excellent shape freezing properties according to claim 1, comprising an unavoidable impurity. 3. One or two of the following by mass%: Ti: 1% or less, Nb: 1% or less, V: 1% or less, Cr: 3% or less, and B: 0.01% or less. The high elongation flangeable steel sheet having excellent shape freezing properties according to claim 2, comprising: 4. The composition according to claim 1, further comprising at least one of Mo: 1% or less, Cu: 3% or less, Ni: 3% or less, and Sn: 0.2% or less. The high elongation flangeable steel sheet having excellent shape freezing properties according to claim 2 or 3. 5. The composition according to claim 1, further comprising at least one of Ca: 0.0005% or more and 0.005% or less, and Rem: 0.001% or more and 0.02% or less. 3. The method according to claim 2, wherein
5. A high elongation flangeable steel sheet excellent in shape freezing property according to any one of items 4 to 4. 6. A high elongation flangeable steel sheet excellent in shape freezing property, wherein the steel sheet according to any one of claims 1 to 5 is plated. 7. In producing the steel sheet according to any one of claims 1 to 6, an Ar ₃ transformation temperature to (Ar ₃ +10
0) The total reduction in the temperature range of 0 ° C. is controlled to be 25% or more, the hot rolling is completed at the Ar ₃ transformation temperature or more, and the critical temperature T determined by the chemical composition of the steel shown in the equation (1).
A method for producing a high-stretch flangeable steel sheet having excellent shape freezing properties, wherein the high-strength flangeable steel sheet is wound at a temperature of not more than o and at least 350 ° C. To = −650.4 × C% + B (1) Here, B is obtained from the steel component expressed in mass%. B = -50.6 × Mneq + 894.3 Mneq = Mn% + 0.24 × Ni% + 0.13 × Si% + 0.38 × Mo% + 0.55 × Cr% +
0.16 × Cu% -0.50 × Al% -0.45 × Co% + 0.90 × V% 8. The total reduction in the temperature range from the Ar ₃ transformation temperature to (Ar ₃ +100) ° C. is 25% or more, and
8. The shape freezing property according to claim 7, wherein the hot rolling is controlled so that the friction coefficient becomes 0.2 or less in at least one pass in hot rolling at (Ar ₃ +100) ° C. or less. Manufacturing method of high stretch flangeable steel sheet. 9. In producing the steel sheet according to any one of claims 1 to 6, hot rolling is controlled so that the sum of the rolling reductions below the Ar ₃ transformation temperature is 25% or more. after completion between rolling or winding in 350～Ac ₃ transformation temperature, or, once cooled, the 500～Ac ₃ transformation temperature 10 to 1
A method for producing a high elongation flangeable steel sheet having excellent shape freezing properties, characterized by heating for 20 minutes. 10. Rolling such that the total rolling reduction at or below the Ar ₃ transformation temperature is 25% or more, and at least one pass or more in hot rolling at or below the Ar ₃ transformation temperature has a friction coefficient of 0.2 or less. The method for producing a high-stretch flangeable steel sheet having excellent shape freezing properties according to claim 9. 11. The temperature range of 600 ° C. to (Ac ₃ +100) ° C. after pickling the steel sheet according to claim 7 and subjecting the steel sheet to cold rolling at a rolling reduction of less than 80%. A method for producing a high-stretch flangeable steel sheet having excellent shape freezing properties, characterized by heating and cooling.