JP4126007B2 - Cold-rolled steel sheet excellent in shape freezing property and bake hardenability and method for producing the same - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet (hereinafter, also simply referred to as a steel sheet or a thin steel sheet) having excellent shape freezing properties, mainly bending, and is mainly used for automobile parts.

  In order to reduce carbon dioxide emissions from automobiles, the weight of automobile bodies is being reduced using high-strength steel sheets. In addition, in order to ensure the safety of passengers, high strength steel plates are often used in automobile bodies in addition to mild steel plates. Furthermore, in order to further reduce the weight of automobile bodies in the future, new demands for increasing the strength level of use of high-strength steel sheets are increasing.

  However, when bending deformation is applied to a high-strength steel plate, the shape after processing is high-strength, so it is easy to return to the direction of the shape before processing away from the shape of the processing jig, Due to the elastic recovery from the bending back, a wall warp phenomenon occurs in which the flat surface of the side wall becomes a curved surface, resulting in a poor dimensional accuracy that the shape of the target processed part cannot be obtained.

  Therefore, the conventional automobile body 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, despite the need to reduce the weight of the body using high-strength steel sheets of 490 MPa or higher. This is the actual situation.

  Needless to say, increasing the shape freezing property after processing of a high strength steel plate or mild steel plate of 440 MPa or less is extremely important for improving the shape accuracy of products such as automobiles and home appliances.

  In recent years, there has been a great demand for imparting bake hardening (BH) ability to high-strength steel sheets. Conventionally, it is the bake hardenability imparted to improve the dent resistance of the mild steel sheet for body outer plates, but from the viewpoint of improving the impact absorbing ability, it is expected to be stably imparted to the high strength steel sheet.

  In the patent document 1, the present inventors have disclosed a method for producing 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. 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 must be directed to low-temperature + large-pressure hot-rolling, which places a heavy load on the hot-rolling process. Call. In addition, since the crystal orientation that deteriorates the shape freezeability tends to develop during cold rolling, it is necessary to reduce the rolling rate.

  On the other hand, some of the present inventors in Japanese Patent Application Laid-Open No. H11-228561 have disclosed a cold-rolled steel sheet in which the reflected X-ray intensity ratio of the {100} plane parallel to the plate surface is 3 or more as a technique for reducing the amount of springback. Although disclosed, the present invention is characterized by the definition of the X-ray intensity ratio at the outermost surface of the plate thickness, and is completely different from the present invention that defines the X-ray intensity ratio at 1/2 plate thickness.

  In addition, none of the techniques regulates the bake hardenability at all, and makes no mention of the compatibility between the shape freezeability and the bake hardenability.

JP 2001-303175 A JP 2001-64750 A

  When a mild steel plate or a high strength steel plate is bent, a large spring back is generated depending on the strength of the steel plate, and the shape freezing property of the processed molded part is poor. The present invention fundamentally solves this problem by texture control and solid solution C control, and provides a cold-rolled steel sheet excellent in shape freezing property and bake hardenability, and a method for producing the same.

  According to the conventional knowledge, it has been considered to be important for the time being to lower the yield point of the steel sheet as a measure for suppressing the spring back. In order to lower the yield point, a steel plate having a low tensile strength has to be used. However, this alone is not the fundamental solution for improving the bending workability of the steel sheet and keeping the amount of spring back low.

  Therefore, in order to improve the bending workability and fundamentally solve the occurrence of spring back, the present inventors have newly paid attention to the influence on the bending workability of the texture of the steel sheet, and its action. The effect was investigated and studied in detail. And the steel plate excellent in bending workability was found.

  As a result, controlling the intensity of each of {100} <011> to {223} <110> orientation group and {554} <225>, {111} <112>, {111} <110>, Has shown that the bending workability is dramatically improved 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 as low as possible.

  As a result of further earnest research, in order to further enhance the crystal orientation superior in shape freezing property in cold-rolled steel sheets, it is extremely important to leave solute C in the hot-rolled sheet as a cold-rolled base material. I found something new.

  In addition, by allowing solid solution C to remain in the state of the product plate by appropriate cold rolling and annealing, it is possible to further improve the shape freezing property and at the same time bake hardenability, remarkably improve the shock absorption capacity. Newly found.

  The present invention is configured based on the above-mentioned knowledge, and the main points thereof are as follows.

(1) By mass%, C: 0.001% 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, and
O: 0.01% or less
Further containing Nb,
Ti and Nb in total 0.01% or more and 0.40% or less
(1) {100} <011> to {223} <of the plate surface at least ½ plate thickness , and the balance of (Formula 1) is satisfied and the balance is made of iron and inevitable impurities. 110> The average value (A) of the X-ray random intensity ratio of the orientation group is 4.0 or more,
(2) The average value (B) of X-ray random intensity ratios of crystal orientations of {554} <225>, {111} <112> and {111} <110> is 6.0 or less,
(3) (A) / (B) ≧ 1.2
Further, at least one of the r values in the rolling direction and the direction perpendicular to the rolling direction is 0.7 or less, and the BH amount is 50 MPa or more. .
C%-{(Ti% -48 / 32S% -48 / 14N%) + 48 / 93Nb%} / 4
> 0.001 (Formula 1)

( 2 ) Furthermore, in mass%, V: 0.20% or less,
Cr: 1.5% or less and
B: A cold-rolled steel sheet excellent in shape freezing property and bake hardenability as described in (1 ), containing one or more of 0.007% or less.

( 3 ) Furthermore, in mass%, Mo ≦ 1%,
Cu ≦ 2%,
Ni ≦ 1%, and
Sn ≦ 0.2%
The manufacturing method of the cold-rolled steel plate excellent in the shape freezing property and bake hardenability as described in (1) or (2) characterized by containing 1 type (s) or 2 or more types.

( 4 ) The slab having the chemical component according to any one of (1) to ( 3 ) is reheated to 1150 to 1350 ° C., and the total rolling reduction in the temperature range of Ar 3 transformation temperature to (Ar 3 transformation temperature +100) ° C. Perform hot rolling to 25% or more, finish hot rolling at the Ar3 transformation temperature or higher, cool so that the average cooling rate until winding is 20 ° C / s or higher, wind up at 600 ° C or lower, After pickling, cold rolling at 25% or more and 80% or less is performed, and further heating is performed at a temperature range of 3O <0> C to (Ac3 + 100) [deg.] C. at 3 to 100 [deg.] C. The manufacturing method of the cold-rolled steel plate excellent in the shape freezing property and bake hardenability characterized by cooling by.

( 5 ) The cold-rolled steel sheet produced by the method for producing a cold-rolled steel sheet having excellent shape freezing property and bake hardenability described in ( 4) is subjected to an overaging treatment at an annealing temperature of 550 ° C. or less after cold rolling. A method for producing a cold-rolled steel sheet having excellent shape freezeability and bake hardenability.

( 6 ) The pearlite volume fraction in the hot-rolled steel sheet obtained by hot rolling is 5% or less, and is excellent in shape freezing property and bake hardenability as described in ( 4 ) or ( 5) A method for manufacturing cold rolled steel sheets.

( 7 ) Aging index A. of hot-rolled steel sheet obtained by hot rolling. I. The manufacturing method of the cold-rolled steel plate excellent in the shape freezing property and bake hardenability in any one of ( 4 )-( 6 ) characterized by the above-mentioned.

( 8 ) The texture and hot rolling reduction ratio of the hot-rolled steel sheet obtained by the hot rolling satisfy the following formulas ( Formula 2) and ( Formula 3): ( 4 ) to ( 7 ) The manufacturing method of the cold-rolled steel plate excellent in the shape freezing property and bake hardenability in any one.

(A) + 0.02 × CR ≧ 3.5 ( Expression 2)
(B) + 0.04 × exp (0.05 × CR) ≦ 5.5 ( Formula 3)
Here, (a): average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group on the plate surface at at least 1/2 of the thickness of the hot rolled plate (b): hot rolled Average value CR of cold-rolling reduction ratio of X-ray random intensity ratio of crystal orientations of {554} <225>, {111} <112> and {111} <112> and {111} <110> on at least half the plate thickness (%)
( 9 ) The cold-rolled steel sheet having excellent shape freezing property and bake hardenability according to any one of ( 4 ) to ( 8 ), wherein the sheet passing temperature during cold rolling is 50 to 400 ° C. Manufacturing method.

( 10 ) Shape freezing property, characterized in that the cold rolled steel plate produced by the method for producing a cold rolled steel plate having excellent shape freezing property and bake hardenability according to any one of ( 4 ) to ( 9 ) is plated. And cold rolled steel sheet manufacturing method with excellent bake hardenability.

( 11 ) Skin pass rolling at 0.4% or more and 5% or less on a cold-rolled steel sheet produced by the method for producing a cold-rolled steel sheet having excellent shape freezing property and bake hardenability according to any one of ( 4 ) to ( 10 ). A method for producing a cold-rolled steel sheet having excellent shape freezing property and bake hardenability, characterized by applying the above.

  According to the present invention, it is possible to provide a thin steel sheet having a small amount of spring back, an excellent shape freezing property mainly composed of bending, and a high BH amount. In particular, high strength steel plates can be used for parts that have conventionally been difficult to apply to high strength steel plates due to shape problems.

  The contents of the present invention will be described in detail below.

Average value (A) of X-ray random intensity ratio of {100} <011> to {223} <110> orientation groups on the plate surface at 1/2 plate thickness:
This is a particularly important characteristic value in the present invention. The average value of {100} <011> to {223} <110> orientation groups when the X-ray diffraction of the plate surface at the plate thickness center position and the intensity ratio of each orientation with respect to the random sample is obtained is 4. Must be greater than or equal to zero.

  If this is less than 4.0, the shape freezing property is poor. In this respect, the average value of the X-ray random intensity ratio is more preferably 4.5 or more, and further preferably 5.0 or more.

  The main orientations included in this orientation group are {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} < 110> and {223} <110>.

  The X-ray random intensity ratio of each of these orientations can be calculated by using a three-dimensional texture calculated by the vector method based on the {110} pole figure, and a plurality of {110}, {100}, {211}, {310} pole figures. What is necessary is just to obtain | require from the three-dimensional texture calculated | required by the series expansion method using the pole figure (preferably 3 or more).

  For example, the X-ray random intensity ratio of each crystal orientation in the latter method includes (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) The intensity of [1-10] may be used as it is.

  The average value of {100} <011> to {223} <110> orientation group is an arithmetic average of each of the above-mentioned orientations. When the strengths of all the above directions cannot be obtained, {100} <011>, {116} <110>, {114} <110>, {112} <110>, {223} <110> Alternatively, an arithmetic average of each direction may be substituted.

Average value (B) of X-ray random intensity ratios of crystal orientations of {554} <225>, {111} <112> and {111} <110> on the plate surface at 1/2 plate thickness:
The average value of the X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> crystal orientations of the plate surface at 1/2 plate thickness is not less than 6.0. Must not. If this is over 6.0, it is difficult to obtain good shape freezing property even if the strength of the {100} <011> to {223} <110> orientation groups is appropriate.

  The X-ray random intensity ratio of {554} <225>, {111} <112>, and {111} <110> may be obtained from the three-dimensional texture calculated according to the above method. More preferably, the arithmetic average value of the X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> is less than 5.0, more preferably less than 3.5. .

(A) / (B):
The ratio of the average values of the above two crystal orientation groups must be 1.2 or more. If this ratio is less than 1.2, the shape freezing property cannot be ensured even if each of the values (A) and (B) is within an appropriate range.

  On the other hand, as the ratio increases, the shape freezing property is improved, so the upper limit of the ratio is not specified.

  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 ½ but also as much as possible, so that the shape freezing property is further improved.

  The reason why the X-ray intensity of the cold-rolled sheet described above is important for the shape freezing property at the time of bending is not necessarily clear, but is presumed to be related to the sliding behavior of the crystal at the time of bending deformation.

  The sample to be subjected to X-ray diffraction is thinned to a predetermined plate thickness by mechanical polishing or the like, and then the distortion is removed by chemical polishing or electrolytic polishing, and at the same time, the 1/2 plate thickness becomes the measurement surface. To make.

  When there is a segregation zone or a defect in the thickness center layer of the steel plate, which causes inconvenience in measurement, the above-described method is adopted so that an appropriate surface becomes the measurement surface in the range of 3/8 to 5/8 of the plate thickness. The sample may be adjusted according to the above method.

  Naturally, the above-mentioned limitation of the X-ray intensity is satisfied not only in the vicinity of the plate thickness ½ but also as much as possible (especially the outermost layer to ¼ of the plate thickness), thereby further increasing the shape. Freezing property is improved.

  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>.

R value (rL) in the rolling direction and r value (rC) in the direction perpendicular to the rolling direction:
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 of the various crystal orientations described above are appropriate, good shape freezing properties cannot always be obtained. At the same time as the above 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. The lower limit of rL and rC is not particularly defined, and the effects of the present invention can be obtained.

  The r value is evaluated by a tensile test using a JIS No. 5 tensile test piece. The tensile strain is usually 15%, but if the uniform elongation is less than 15%, the strain may be evaluated as close to 15% as possible within the range of uniform elongation.

  The direction in which the bending process is performed differs depending on the processed part, and is not particularly limited. However, it is preferable that the bending process is mainly performed in a direction perpendicular to or close to the perpendicular to the direction in which the r value is small.

  Incidentally, it is generally known that there is a correlation between the texture and the r value, but 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 not synonymous with each other. Unless both limitations are satisfied at the same time, good shape freezing property and workability cannot be obtained.

  The present invention can be applied to all cold-rolled steel sheets ranging from mild steel sheets having low tensile strength levels to high-strength steel sheets, and the bending workability of cold-rolled steel sheets can be dramatically improved if the above-mentioned limitations are satisfied. . In other words, it is a basic material index related to bending deformation that exceeds the constraints of the mechanical strength level of the cold-rolled steel sheet.

BH amount:
If the amount of BH is less than 50 MPa, neither an increase in impact absorption energy due to the provision of bake hardening ability nor an improvement in shape freezing property is sufficient. Therefore, the lower limit of the BH amount is 50 MPa. Desirably, the amount of BH is 60 MPa or more. Although it is not clear why the bake hardenability is imparted, that is, the remaining solid solution C improves the shape freezing property, the presence of the solid solution C is considered to have some influence on the slip during bending deformation. .

  The amount of BH described in the present invention means that after applying a pre-strain of 2% to a JIS No. 5 tensile specimen cut in the direction perpendicular to the rolling, the load is once unloaded and subjected to heat treatment at 170 ° C. for 20 minutes. The value obtained by subtracting the pre-strain load from the yield load after aging when the tensile test is performed again is divided by the cross-sectional area of the parallel part of the test piece before the pre-strain.

  Next, the component limiting conditions will be described. Note that. % Means mass%.

C:
The reason why the lower limit of C is set to 0.001% is that if it is less than 0.001%, it is difficult to leave the solid solution C, and both the texture and the amount of BH deteriorate. If C exceeds 0.25%, workability deteriorates, so 0.25% is made the upper limit.

Si:
Si is an effective element for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, workability deteriorates or surface flaws occur, so 2.5% is made the upper limit. 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.

Mn:
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 2.5% is made the upper limit. On the other hand, since it is difficult to make Mn less than 0.01% in practical steel, 0.01% is made the lower limit. In addition to Mn, when an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added, it is desirable to add Mn that satisfies Mn / S ≧ 20 by mass%.

P, S:
These are impurities, and are 0.15% or less and 0.03% or less, respectively. This is for preventing deterioration of workability and cracking during hot rolling or cold rolling.

Al:
Al is added in an amount of 0.01% or more for deoxidation. However, if the amount is too large, the workability deteriorates or the surface properties become poor, so the upper limit is made 2.0%.

  N and O are impurities, and both are made 0.01% or less so as not to deteriorate the workability.

Ti, Nb:
These elements are very important in the present invention. That is, by adding Nb alone or in combination with Nb and Ti, recrystallization and grain growth during annealing are suppressed, and the texture advantageous to shape freezing formed during hot rolling and cold rolling is destroyed. It is saved without.

  Therefore, 0.01% or more of 1 type or 2 types is added in total. However, excessive addition not only deteriorates the workability but also prevents the formation of texture by forming carbides and reducing the solid solution C, so the upper limit is set to 0. Set to 40%.

In addition, C, Ti, Nb, S, and N must satisfy the relationship of (Formula 1). When this relationship cannot be satisfied, the solid solution C cannot remain in the hot-rolled sheet and the product sheet. As a result, the texture is deteriorated and the shape freezing property is not improved. Moreover, bake hardening ability cannot be provided.

C%-{(Ti% -48 / 32S% -48 / 14N%) + 48 / 93Nb%} / 4
> 0.001 ( Formula 1)
V, Cr, B:
Since the material is improved through mechanisms such as carbon and nitrogen fixation, precipitation strengthening, structure control, and fine grain strengthening, it is desirable to add 0.0001% or more of each component as necessary. However, even if added excessively, there is no remarkable effect, but rather the workability and surface properties are deteriorated, so 0.20%, 1.5%, and 0.007% were set as upper limits, respectively.

Mo, Cu, Ni, Sn:
Since these elements have the effect of increasing the mechanical strength and improving the material, it is desirable to add 0.001% or more of each component as necessary. However, excessive addition conversely degrades the workability, so the upper limit is made 1%, 2%, 1%, and 0.2%, respectively.

  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 the control of sulfide morphology.

  Next, the manufacturing method will be described in detail.

  The production method preceding hot rolling is not particularly limited. That is, various secondary smelting may be performed following the smelting by a blast furnace or an electric furnace, and then the casting may be performed by a method such as a thin slab casting in addition to a normal continuous casting and an ingot method. In the case of continuous casting, after cooling to a low temperature once, 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.

Slab reheating temperature:
When the slab is reheated and hot rolling is performed, the reheating temperature is limited to a range of 1150 to 1350 ° C. When the slab reheating temperature is less than 1150 ° C., the re-solution of Nb and Ti is insufficient, and the texture does not develop during hot rolling. From this viewpoint, the lower limit of the slab reheating temperature is desirably 1200 ° C., and more desirably 1250 ° C.

  Further, even if the slab reheating temperature exceeds 1350 ° C., not only a special effect is obtained, but also the load on the equipment becomes high, so 1350 ° C. is made the upper limit.

Hot rolling reduction ratio:
The total rolling reduction in the temperature range of Ar3 transformation temperature C to (Ar3 transformation temperature +100) C during hot rolling is desirably 20% or more. If it is less than 20%, the development of the texture during hot rolling is insufficient and the shape freezing property is difficult to improve. From this viewpoint, the total rolling reduction is desirably 25% or more, and more desirably 30% or more.

  The upper limit of the total rolling reduction is not particularly specified, but in order to increase it to over 90%, it is indispensable to reinforce the equipment, and a special effect cannot be obtained.

Hot rolling end temperature:
When the end temperature of hot rolling becomes less than the Ar3 transformation temperature, an orientation unfavorable for shape freezing property may develop during hot rolling, so the hot rolling end temperature is set to the Ar3 transformation temperature or higher. The upper limit of the hot rolling end temperature is not particularly defined, but is preferably (Ar3 transformation temperature + 100) ° C. or lower from the viewpoint of texture development.

Average cooling rate:
The average cooling rate from the end of hot rolling to winding is 20 ° C./s or more. When the average cooling rate is less than 20 ° C./s, the solid solution C amount is reduced during cooling, and the solid solution C amount of the hot-rolled sheet cannot be secured, so that the shape freezing property is deteriorated. From this viewpoint, it is desirable to cool at 30 ° C./s or more. More desirably, it is 50 ° C./s or more.

  The higher the cooling rate, the more solid C can be secured, so the upper limit is not specified. However, even if it is cooled at over 250 ° C./s, no significant effect is obtained, and new cooling equipment investment is required. The upper limit is preferably 250 ° C./s.

Winding temperature:
When the coiling temperature exceeds 600 ° C., pearlite transformation occurs during winding and the amount of dissolved C decreases, and the subsequent cold rolling / annealing texture development becomes insufficient. Therefore, the upper limit of the coiling temperature is 600 ° C. And From this viewpoint, the upper limit is preferably 580 ° C., and more preferably 550 ° C.

  The lower limit is not particularly defined, but it is desirable to wind up at room temperature or higher because there is no particular merit in setting it to room temperature or lower.

Cold reduction rate:
When the hot-rolled steel sheet thus obtained is cold-rolled and annealed to obtain a final thin steel sheet, if the total rolling reduction of the cold-rolling exceeds 80%, a general cold-rolling- Since the {111} plane and {554} plane components of the X-ray diffraction integration plane intensity ratio of the crystal plane parallel to the plate surface, which is the recrystallized texture, become high, and the crystal orientation characteristic that is the feature of the present invention is not satisfied. The upper limit of the cold rolling reduction was 80%.

  In order to increase the shape freezing property, it is desirable to limit the cold rolling reduction to 70% or less. More desirably, the upper limit of the cold rolling rate is 60%.

  On the other hand, in order to reduce the cold rolling rate to less than 25%, it is necessary to reduce the thickness of the hot-rolled sheet, which places a heavy load on the hot-rolling process. Therefore, the lower limit of the cold rolling reduction is 25%. From this viewpoint, it is desirable to limit to 30% or more. More desirably, it is 35% or more.

Heating rate:
When annealing a cold-rolled steel sheet cold-worked in the above range, if the heating rate is less than 3 ° C / s, recrystallization proceeds during heating and the texture is destroyed, so the lower limit of the heating rate is set. The temperature was 3 ° C / s. From this viewpoint, it is desirable to limit to 5 ° C./s or more. More desirably, it is 10 ° C./s or more.

  On the other hand, since increasing the heating rate to 100 ° C./s or more requires excessive capital investment, 100 ° C./s was set as the upper limit of the heating rate.

Annealing temperature:
When the annealing temperature is less than 700 ° C., the work structure remains, so that workability deteriorates. Therefore, the lower limit of the annealing temperature is set to 700 ° C. On the other hand, when the annealing temperature is excessively high, the ferrite texture formed by recrystallization is randomized by austenite grain growth after transformation into austenite, and the finally obtained ferrite texture is also randomized. The

  In particular, when the annealing temperature exceeds (Ac3 + 100) ° C., such a tendency becomes remarkable. Therefore, the annealing temperature is set to (Ac3 + 100) ° C. or lower. From this viewpoint, it is desirable that the upper limit of the annealing temperature is not more than the Ac3 transformation temperature.

Cooling rate:
When cooling after annealing, if the average cooling rate is less than 3 ° C / s, the texture of the cold-rolled steel sheet finally obtained is not sufficiently developed, and good shape freezing property cannot be obtained. The lower limit of the cooling rate was 3 ° C./s. From this viewpoint, it is desirable that the lower limit of the cooling rate is 5 ° C./s. More desirably, it is 10 ° C./s or more.

  In addition, setting the cooling rate to more than 250 ° C./s requires excessive capital investment, so 250 ° C./s was set as the upper limit of the cooling rate.

  The cooling rate described here is an 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.

Reheating temperature (overaging treatment):
After annealing / cooling, adopt a continuous annealing process, slow cooling or isothermal holding corresponding to the temperature history in the continuous hot dip galvanizing process, or reheating process in the alloying process of the continuous hot dip plating process. May be. However, the average annealing temperature in that case shall be 550 degrees C or less. When the average annealing temperature exceeds 550 ° C., carbide precipitates and the solid solution C decreases, so that the shape freezing property and the bake hardenability decrease.

  On the other hand, the lower limit of the average annealing temperature is not particularly specified, but it is desirable to set the lower limit to 200 ° C. because it does not bring about a special effect. In addition, the annealing temperature described here is an average annealing temperature, and the maximum temperature reached may exceed 550 ° C.

  Next, conditions for hot-rolled sheets will be described.

Aging index of hot-rolled sheet I. :
Aging index of hot-rolled sheet used as cold-rolled base material I. (Aging Index) is preferably 20 MPa or more. This is because if it is less than 20 MPa, {111} and {554}, which are crystal orientations that are not good in shape freezing property, tend to develop during cold rolling. From this viewpoint, A.I. I. The lower limit is preferably 30 MPa. More desirably, it is 40 MPa or more.

  In addition, A. I. Is preferable from the viewpoint of shape freezing property and bake hardenability. I. There is no specific upper limit.

  A. in the present invention. I. Means that after applying 10% pre-strain to a JIS No. 5 tensile specimen cut in the direction perpendicular to the rolling direction, it was unloaded once, subjected to a heat treatment at 100 ° C. for 60 minutes, and then subjected to a tensile test again. The value obtained by subtracting the prestrain load from the yield load after aging is divided by the cross-sectional area of the parallel part of the specimen before prestrain.

  When the uniform elongation is less than 10%, the strain may be evaluated as close to 10% as possible within the uniform elongation range.

Hot rolled sheet microstructure:
The hot-rolled sheet serving as the cold-rolled base material preferably has ferrite or bainite as the phase with the largest volume fraction, and the pearlite volume fraction is preferably 5% or less. When the pearlite volume fraction exceeds 5%, the amount of dissolved C in the hot-rolled sheet decreases, and the {111} and {554} orientations that are not good for shape freezing during cold rolling and annealing develop. The upper limit was 5%. In addition to these phases, 20% or less of martensite and austenite may be included.

Relationship between cold rolling rate and hot rolled sheet texture:
From the viewpoint of improving the shape freezing property, it is desirable that the texture of the hot-rolled sheet and the rolling reduction of the subsequent cold rolling satisfy the following expressions ( Expression 2 ) and ( Expression 3 ).
(A) + 0.02 × CR ≧ 3.5 ( Expression 2 )
(B) + 0.04 × exp (0.05 × CR) ≦ 5.5 ( Expression 3 )
Here, (a): average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group on the plate surface at at least 1/2 of the thickness of the hot rolled plate (b): hot rolled Average value CR of X-ray random intensity ratios of three crystal orientations of {554} <225>, {111} <112> and {111} <110> on the plate surface at at least half the plate thickness CR: cold rolling Reduction ratio (%)

Cold rolling temperature:
When performing cold rolling, if the sheet passing temperature is controlled to 50 ° C. to 400 ° C., strain aging by solid solution C is promoted, and formation of a texture advantageous to shape freezing progresses during rolling. It is desirable to control the temperature range. Even if the sheet passing temperature is less than 50 ° C., there is no difference from rolling at room temperature from the viewpoint of texture formation, so 50 ° C. was set as the lower limit.

  Further, the temperature dependence of strain aging is maximum in the vicinity of 300 to 400 ° C., and even when heated to a higher temperature, the texture deteriorates conversely, so 400 ° C. is set as the upper limit. From this viewpoint, it is more desirable to set the temperature to 300 ° C. or lower.

  The heating method is not particularly defined, and the same effect can be obtained by any of current heating, atmospheric furnace heating, and heat generation due to friction between the roll and the plate.

Plating:
When the cold-rolled steel sheet produced according to the present invention is plated, the type of plating is not particularly limited, and the effects of the present invention can be obtained by any of electroplating, hot dipping, vapor deposition plating, and the like.

Skin pass:
Skin pass rolling may be applied to the cold-rolled steel sheet produced by the above method. When the 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.

  Further, if the rolling reduction exceeds 5%, it is usually necessary to modify the skin pass rolling mill, resulting in economic demerits and significant deterioration of the workability of the steel sheet, so the upper limit is 5%.

  The microstructure of the cold-rolled steel sheet produced by the above method is mainly composed of ferrite, but may contain precipitates such as bainite, austenite, martensite, pearlite, and carbide depending on the purpose.

  The technical contents of the present invention will be described with reference to examples of the present invention.

  As an Example, the result examined using the steel from A to L which has the component composition shown in Table 1 is demonstrated. These steels are either as they are after casting or once cooled to room temperature and then reheated, heated to a temperature range of 1200 ° C. to 1330 ° C., and then hot rolled under the conditions shown in Table 2 to obtain various thicknesses. The hot-rolled steel sheet.

  Thereafter, the hot-rolled steel sheet was subjected to cold rolling at a cold rolling rate shown in Table 2 to obtain a thickness of 1.0 mm, and thereafter annealed in a continuous annealing process.

  Test pieces having a width of 45 mm and a length of 270 mm were prepared from these 1.0 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 75 mm.

  The test piece subjected to the bending test was measured for the shape of the central part of the plate width with a three-dimensional shape measuring device, and as shown in FIG. 1, the tangent line of point (i) and point (ii) and point (iii) The average value on the left and right of the value obtained by subtracting 90 ° from the angle of the intersection of the tangent line with point (iv) and the inverse of the curvature between point (iii) and point (v) were averaged on the left and right A value obtained by multiplying the value by 1000 was evaluated by measuring the shape freezing property using the value obtained by subtracting the punch width from the length between the left and right points (v) as the dimensional accuracy. The bending was performed so that a polygonal line entered perpendicular to the direction of low r value.

  By the way, the amount of springback and the amount of wall warp also change depending on BHF (wrinkle pressing force). The effect of the present invention does not change its tendency even if it is evaluated with any BHF, but when pressing an actual part with an actual machine, a very high BHF cannot be applied due to equipment limitations. A hat bending test of each steel type was performed at BHF 28 kN. The bending was performed so that a polygonal line was inserted perpendicular to the direction of low r value.

  Tables 2 and 3 (continuation of Table 2) show whether or not the manufacturing conditions of each steel sheet are within the scope of the present invention.

  Table 4 shows the r-value, texture, BH content, and shape freezing index of the cold-rolled steel sheet having a thickness of 1.0 mm manufactured by the above method. As is apparent from the above, any one satisfying the conditions limited in the present invention has both good shape freezing property and bake hardenability.

  In order to promote the weight reduction of automobiles, the use of high-strength steel sheets is absolutely necessary. According to the present invention, when a high-strength steel sheet having a small amount of spring-back, excellent shape freezing property, and bake hardenability can be applied, the weight reduction of an automobile body can be further promoted. Therefore, the present invention is industrially extremely valuable.

It is a figure which shows the cross section of the test piece used for the hat bending test.

Claims (11)

% By mass, C: 0.001% 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, and
O: 0.01% or less
Further containing Nb,
Ti and Nb in total 0.01% or more and 0.40% or less
(1) {100} <011> to {223} <of the plate surface at least ½ plate thickness , and the balance of (Formula 1) is satisfied and the balance is made of iron and inevitable impurities. 110> The average value (A) of the X-ray random intensity ratio of the orientation group is 4.0 or more,
(2) The average value (B) of X-ray random intensity ratios of crystal orientations of {554} <225>, {111} <112> and {111} <110> is 6.0 or less,
(3) (A) / (B) ≧ 1.2
Further, at least one of the r values in the rolling direction and the direction perpendicular to the rolling direction is 0.7 or less and the BH amount is 50 MPa or more, and the cold-rolled steel sheet having excellent shape freezing property and bake hardenability .
C%-{(Ti% -48 / 32S% -48 / 14N%) + 48 / 93Nb%} / 4
> 0.001 (Formula 1) Furthermore, in mass%, V: 0.20% or less,
Cr: 1.5% or less, and
B: One or more of 0.007% or less is contained, The cold-rolled steel sheet having excellent shape freezing property and bake hardenability according to claim 1 . Furthermore, by mass%, Mo ≦ 1%,
Cu ≦ 2%,
Ni ≦ 1%, and
Sn ≦ 0.2%
The cold-rolled steel sheet excellent in shape freezing property and bake hardenability according to claim 1 or 2, characterized by containing one or more of the following. The slab having the chemical component according to any one of claims 1 to 3 is reheated to 1150 to 1350 ° C, and a total rolling reduction in the temperature range of Ar3 transformation temperature to (Ar3 transformation temperature +100) ° C is 25% or more. subjected to hot rolling to, and terminating the hot rolling at Ar3 transformation temperature or more, cooling so the average cooling rate until the winding is equal to or higher than 20 ° C. / s, and wound at 600 ° C. or less, pickling Thereafter, cold rolling at 25% or more and 80% or less is performed, and further heating at a temperature range of 700 ° C. to (Ac 3 +100) ° C. at 3 to 100 ° C./s, followed by cooling at an average cooling rate of 3 to 250 ° C./s. A method for producing a cold-rolled steel sheet having excellent shape freezing property and bake hardenability. A cold-rolled steel sheet produced by the method for producing a cold-rolled steel sheet having excellent shape freezing property and bake hardenability according to claim 4 is subjected to an overaging treatment at an annealing temperature of 550 ° C. or lower after cold rolling. A method for producing a cold-rolled steel sheet having excellent shape freezing properties and bake hardenability. The cold rolling excellent in shape freezing property and bake hardenability according to claim 4 or 5, wherein a volume fraction of pearlite in the hot rolled steel sheet obtained by hot rolling is 5% or less. A method of manufacturing a steel sheet. Aging index A. of hot-rolled steel sheet obtained by hot rolling. I. It claims 4-6 or method of manufacturing a superior cold-rolled steel sheet shape fixability and bake hardenability according to one of, characterized in that but not less than 20 MPa. The texture and the cold rolling reduction ratio of the hot-rolled steel sheet obtained by the hot rolling satisfy the following formulas ( Formula 2) and ( Formula 3), according to any one of claims 4 to 7 : The manufacturing method of the cold-rolled steel plate excellent in the shape freezing property and bake hardenability of description.
(A) + 0.02 × CR ≧ 3.5 ( Expression 2)
(B) + 0.04 × exp (0.05 × CR) ≦ 5.5 ( Formula 3)
Here, (a): average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group on the plate surface at at least 1/2 of the thickness of the hot rolled plate (b): hot rolled Average value CR of cold-rolling reduction ratio of X-ray random intensity ratio of crystal orientations of {554} <225>, {111} <112> and {111} <112> and {111} <110> on at least half the plate thickness (%) The method for producing a cold-rolled steel sheet having excellent shape freezing property and bake hardenability according to any one of claims 4 to 8 , wherein a sheet passing temperature during the cold rolling is set to 50 to 400 ° C. . Shape fixability and bake hardenability, characterized in that plating the cold rolled steel sheet manufactured by the method for producing a cold-rolled steel sheet excellent in shape fixability and bake hardenability according to any one of claims 4-9 A method for producing cold-rolled steel sheets with excellent properties. Applying skin pass rolling of 0.4% or more and 5% or less to a cold-rolled steel sheet produced by the method for producing a cold-rolled steel sheet having excellent shape freezing property and bake hardenability according to any one of claims 4 to 10. A method for producing a cold-rolled steel sheet having excellent shape freezeability and bake hardenability.

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