JP4740099B2 - High-strength cold-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet excellent in shape freezing property, ductility, and paint bake hardenability, which is suitable for automotive parts, and a method for producing the same.

In recent years, in order to suppress the discharge amount of carbon dioxide from automobiles, the weight reduction of automobile bodies has been promoted by using high-strength steel sheets. In addition to weight reduction, in order to ensure the safety of passengers, especially from the viewpoint of safety at the time of collision, high strength steel sheets are increasingly used for automobile bodies in addition to mild steel sheets. Yes. Furthermore, in order to further reduce the weight of automobile bodies, new demands to increase the use strength level of high-strength steel sheets more than ever are increasing.
However, since the formability of the steel sheet decreases as the strength increases, the application of the high-strength steel sheet to automobile members is practically limited.

In the background as described above, one of the important issues imposed on the formability of high-strength steel sheets is the improvement of bend formability. Specifically, shape freezing properties (springback resistance) ) And improvement of cracking limit due to ductile deterioration. Such an improvement is particularly important when a steel plate having a TS of 440 MPa or more is used for an automobile structural member or chassis member.
As a method of solving such a problem, the present applicant has proposed a steel plate and a manufacturing method thereof in which the texture is controlled and the shape freezing property is improved (for example, Patent Documents 1 to 4).

Patent Document 1 discloses a high-strength cold-rolled steel sheet having a developed {100} <011> orientation and excellent shape freezing property, and a method for producing the same. However, in the steel sheet described in Patent Document 1, only Ti or Nb is used as an essential element, and Mo and B are used as selective elements, and {100} <011> to {223 of the plate surface at 1/2 plate thickness. } The average value of the X-ray random intensity ratio of the <110> orientation group is 3.0 or more, and among these orientation groups, the X-ray random intensity ratio of the {112} <110> or {100} <011> orientation Satisfies the maximum value of 4.0 or more.
The steel sheet of Patent Document 1 has a problem that, although excellent shape freezing property is obtained by the above configuration, a forming margin at the time of bending is not sufficient.

  Patent Document 2 discloses a {100} <011> to {223} <110> orientation of a plate surface at 1/2 plate thickness in a cold-rolled steel plate containing Ti or Nb as essential elements and Mo and B as selective elements. A steel sheet having an average X-ray random intensity ratio of 3.5 or more and satisfying a maximum X-ray random intensity ratio of {100} <011> orientation of 4.5 or more in these orientation groups It is disclosed.

  Further, Patent Document 3 discloses a {100} <011> to {223} <110> orientation group of plate surfaces at 1/2 plate thickness in a hot-rolled steel plate that does not require any of Ti, Nb, Mo, and B. The average value of the X-ray random intensity ratio is 2.5 or more, and among these orientation groups, the X-ray random intensity ratio of the {100} <011> orientation is {211} <011> the X-ray random intensity ratio or more A steel sheet that satisfies the requirements is disclosed.

  Patent Document 4 discloses a {100} <011> to {223} <110> orientation group of plate surfaces at 1/2 plate thickness in a hot-rolled steel plate that does not require any of Ti, Nb, Mo, and B. The average value of the X-ray random intensity ratio is 2.5 or more, and among these orientation groups, the X-ray random intensity ratio of the {100} <011> orientation is 2.5 or more and {100} <011> A steel sheet is disclosed in which the X-ray random intensity ratio of the orientation satisfies the {211} <011> X-ray random intensity ratio or more.

  However, although the steel plates described in Patent Documents 2 to 4 have excellent shape freezing properties as in Patent Document 1, there is a problem that the forming margin at the time of bending is not sufficient.

  On the other hand, increasing the yield strength is effective for improving the impact resistance. However, since it is effective to reduce the yield strength to improve the shape freezing property, the increase in the yield strength of the member by the coating baking process after bending is extremely important for improving the impact resistance. For this reason, in order to achieve both bend formability and impact resistance, it is required to improve the paint bake hardenability (also referred to as Bake Hardness: BH property) of the steel sheet.

In Patent Document 5, in a cold-rolled steel sheet having Ti or Nb as an essential element and Mo and B as selective elements, the {100} <011> to {223} <110> orientations of the plate surface at 1/2 plate thickness A steel sheet satisfying an average X-ray random intensity ratio of the group of 4.0 or more is disclosed.
However, although the steel sheet described in Patent Document 5 has both shape freezing property and paint bake hardenability, it is said that, as with the steel sheets described in Patent Documents 1 to 4, the forming margin during bending is not sufficient. There was a problem.

Furthermore, in Patent Document 6, the average value of the X-ray random intensity ratio of {001} <110> to {223} <110> orientation groups on the plate surface at 1/2 plate thickness is 6.0 or more, and Among these orientation groups, a compound particle having an X-ray random intensity ratio of at least one of the {112} <110> orientation and the {001} <110> orientation of 8.0 or more and a diameter of 15 nm or less A steel sheet is disclosed in which the number of particles is 60% or more of the total number of compound particles.
However, although the steel sheet described in Patent Document 6 has good shape freezing properties and stretch flangeability, it does not take into account the above-described paint bake hardenability, and thus achieves both bendability and impact resistance. There was a problem that was difficult.
JP 2002-363893 A JP 2005-120453 A JP-A-2005-15854 Japanese Patent Laid-Open No. 2005-272988 JP 2005-12059 A JP 2006-22349 A

  The present invention has been made in view of the above circumstances, and particularly has a shape freezing property, ductility, and paint bakeability having a TS of over 440 MPa suitable for automobile structural members and chassis members to be bent. Another object is to provide a high-strength cold-rolled steel sheet and a method for producing the same.

As a result of investigating the reasons why it was difficult to achieve both shape freezing and ductility in the prior art, the present inventors found that the steel fraction excellent in shape freezing obtained by the prior art has a non-recrystallized ferrite fraction. Therefore, it was found that excellent ductility was not obtained.
Furthermore, the present inventors examined a method for achieving both the recrystallization of ferrite and the development of a texture preferable for shape freezing. As a result, Ti, Nb and B and one or two of Mo and W were added in combination, and after developing the texture of the hot-rolled sheet by controlling the hot-rolling conditions, Ti, It has been found that by appropriately controlling the recrystallization behavior by optimizing the Nb amount and the annealing conditions, excellent ductility can be obtained without degrading the shape freezing property as compared with the conventional steel.
Furthermore, in order to improve the BH property in addition to the shape freezing property and ductility, by suppressing the addition amount of Al as much as possible, precipitation of AlN was suppressed and the solid solution N amount was increased. As a result, it has been found that an excellent BH property can be obtained without deteriorating the shape freezing property and ductility.
The present invention is configured based on the above-described knowledge, and the main points thereof are as follows.

(1) By mass%, C: 0.005% to 0.25%, Si: 0.001% to 2.5%, Mn: 0.01% to 2.5%, P: 0.00. 15% or less, S: 0.03% or less, Al: 2.0% or less, N: 0.01% or less, O: 0.01% or less, B: 0.0003% or more and 0.007% or less Further, Ti: 0.005% or more and 0.04% or less, Nb: 0.005% or more and 0.04% or less in total in an amount of 0.01% or more and 0.04% or less. Furthermore, the total of any one or two of Mo and W is contained in an amount of 0.001% or more and 1.0% or less, and the balance is composed of iron and inevitable impurities, and the area fraction is 1 % Or more of pearlite, bainite or martensite of 25% or less, with the remainder being 80% or less of the recrystallization rate It is a composite structure steel composed of the above ferrite, and the average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups on the plate surface at 1/2 plate thickness is 3.0 or more. And the X-ray random intensity ratio of {100} <011> orientation is maximum and satisfies 5.0 or more among these orientation groups, and {554} <225>, {111} <112> and { A high-strength cold-rolled steel sheet having an average X-ray random strength ratio of 111} <110> crystal orientation of 6.0 or less.
(2) The high-strength cold-rolled steel sheet according to (1), wherein the Al content is 0.0005% to 0.02% by mass.
(3) Further, in mass%, V: 0.20% or less, Cr: 1.5% or less, Cu: 2.0% or less, Ni: 1.0% or less, Sn: 0.20% or less The high-strength cold-rolled steel sheet having excellent shape freezing property and ductility according to (1) or (2), comprising at least one kind or two or more kinds.
(4) The high-strength cold-rolled steel sheet according to any one of (1) to (3), wherein the Al content and the N content satisfy the following formula Al / N = 2 or less.
(5) The product TS × EL [MPa ·%] of the tensile strength TS [MPa] and the total elongation EL [%] is 17000 [MPa ·%] or more (1) to (4) A high-strength cold-rolled steel sheet excellent in shape freezing property and ductility according to any one of the above.
(6) A high-strength cold-rolled steel sheet excellent in shape freezeability and ductility, wherein the high-strength cold-rolled steel sheet according to any one of (1) to (5) is plated.

(7) The slab having the chemical component according to any one of (1) to (4) above, the total rolling reduction in the temperature range of Ar ₃ transformation temperature to 900 ° C. is 25% or more, and the finish rolling temperature is Hot-rolled at an Ar ₃ transformation temperature or higher, then wound up at a temperature of 700 ° C. or lower, pickled, cold-rolled at 2 to 80%, and further heated at a rate of 15 ° C./sec or lower from 700 ° C. A method for producing a high-strength cold-rolled steel sheet excellent in shape freezing property and ductility, characterized by heating to a temperature range of 900 ° C. and then cooling after holding at that temperature for 3 seconds or more.
(8) High strength excellent in shape freezing property and ductility, characterized by subjecting the high strength cold-rolled steel sheet produced by the production method described in (7) above to skin pass rolling of 0.1% to 5%. A method for producing a cold-rolled steel sheet.

  According to the high strength cold-rolled steel sheet excellent in shape freezing property, ductility and BH property and its manufacturing method of the present invention, the shape freezing has a tensile strength of 440 MPa or more, a small amount of springback, and a high shock absorption capacity. Can provide a high-strength steel sheet having excellent properties, toughness, and BH properties. This makes it possible to use a high-strength steel sheet even for parts that are mainly formed by bending, such as an automobile body, where it has been difficult to apply a high-strength steel sheet due to problems of shape defects and cracks. Furthermore, it is possible to achieve both bending workability and impact resistance, which are difficult to achieve because of conflicting characteristics. Accordingly, it is possible to further reduce the weight of the automobile body, and the industrial contribution is extremely remarkable.

Hereinafter, embodiments of a high-strength cold-rolled steel sheet excellent in shape freezing property, ductility, and paint bake hardenability and a manufacturing method thereof will be described.
In addition, since this embodiment is described in detail for better understanding of the gist of the invention, the present invention is not limited unless otherwise specified.

  The high-strength cold-rolled steel sheet (hereinafter sometimes abbreviated as “cold-rolled steel sheet”) of the present invention is mass%, C: 0.005% to 0.25%, Si: 0.001% to 2.5 %: Mn: 0.01% to 2.5%, P: 0.15% or less, S: 0.03% or less, Al: 2.0% or less, N: 0.01% or less, O: 0.01% or less, B: 0.0003% or more and 0.007% or less, respectively, Ti: 0.005% or more and 0.04% or less, Nb: 0.005% or more and 0.04% or less In a total amount of 0.01% or more and 0.04% or less, and further, the total of either one or two of Mo and W is 0.001% or more and 1.0% or less. Perlite, bainite, or martensite with an area fraction of 1% to 25%, with the balance being iron and inevitable impurities Any one or more of the above, and the balance is a composite structure steel composed of ferrite with a recrystallization rate of 80% or more, and {100} <011> to {223} of the plate surface at 1/2 plate thickness The average value of the X-ray random intensity ratio of the <110> azimuth group is 3.0 or more, and among these azimuth groups, the X-ray random intensity ratio of the {100} <011> azimuth is maximum and 5.0 or more. Is satisfied, and the average value of the X-ray random intensity ratios of the crystal orientations of {554} <225>, {111} <112> and {111} <110> is generally set to 6.0 or less. The preferable Al content is Al: 0.0005% or more and 0.02% or less, and the Al content and the N content preferably satisfy the following formula Al / N = 2 or less.

“Average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group of plate surface at 1/2 plate thickness”
In the present invention, the average value of the X-ray random intensity ratio is a particularly important characteristic value.
The average value of the {100} <011> to {223} <110> orientation groups when the X-ray diffraction of the plate surface at the center of the plate thickness is performed to determine the intensity ratio of each orientation to the random sample is 3 Must be greater than or equal to 0.0. When this average value is less than 3.0, the shape freezing property becomes poor. From this viewpoint, the average value of the X-ray random intensity ratio is more preferably 3.5 or more, and still more preferably 4.0 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 in each direction is a three-dimensional texture calculated by the vector method based on the {110} pole figure, or a plurality of pole figures among {110}, {100}, {211}, {310} pole figures. What is necessary is just to obtain | require from the three-dimensional texture calculated by the series expansion method using (preferably three or more). The pole figure can be obtained by the X-ray diffraction method.

  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) [1-10] may be used as they are. 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.

“X-ray random intensity ratio of {100} <011> orientation of plate surface at 1/2 plate thickness”
In the present invention, the X-ray random intensity ratio is a particularly important characteristic value.
This orientation is the orientation that is most effective in improving the shape freezing property. Therefore, {100} <011> needs to be 5.0 or more. If this is less than 5.0, it is difficult to ensure high shape freezing property. From this viewpoint, the X-ray random intensity ratio in the {100} <011> orientation is preferably 5.0 or more. More preferably, it is 6.0 or more.
Note that the {100} <011> orientation described here permits 12 ° as a range of orientations having the same effect, with a direction perpendicular to the rolling direction (Transverse Direction) as the rotation axis. The angle is preferably 6 °.

“Average value of X-ray random intensity ratio of {554} <225>, {111} <112> and {111} <110> crystal orientations of the plate surface at 1/2 plate thickness”
The average value of the X-ray random intensity ratio of the crystal orientations {554} <225>, {111} <112> and {111} <110> on the plate surface at 1/2 plate thickness needs to be 6.0 or less. is there. If this exceeds 6.0, it will be difficult to obtain good shape freezing properties.
The above-mentioned X-ray intensity can be measured by an X-ray diffraction method, and if the X-ray intensity satisfying these limitations is obtained not only for the position of 1/2 the plate thickness but also for as many thicknesses as possible, The shape freezing property is further improved.

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

Samples to be subjected to X-ray diffraction are mechanically polished to reduce the thickness of the steel sheet to a predetermined thickness, and then the distortion is removed by chemical polishing, electrolytic polishing, etc., and at the same time, the thickness 1/2 surface becomes the measurement surface. Make it. When there is a segregation zone or a defect in the thickness center layer of the steel sheet, which causes inconvenience in measurement, the above-described surface is set 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.
If the average value of the X-ray random intensity ratio on at least two measurement surfaces satisfies the above value, excellent shape freezing property can be obtained.

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>.
The crystal orientation index defined in the present invention is applicable to all cold-rolled steel sheets ranging from mild steel sheets with low tensile strength levels to high-strength steel sheets, and bending processing of cold-rolled steel sheets if the above limitation is satisfied. Sexually improves. In other words, it can be said that it is a basic material index regarding bending deformation exceeding the constraints of the mechanical strength level of the cold-rolled steel sheet.

“Chemical composition of cold-rolled steel sheet”
Below, the limiting conditions of the chemical component composition of the high-strength cold-rolled steel sheet of the present invention will be described in detail.

(C: C: 0.005% or more and 0.25% or less)
When the C content is less than 0.005%, it is difficult to disperse pearlite, bainite or martensite in an area fraction of 1% or more in the steel, while when the C content exceeds 0.25%, ductility Deteriorates. For this reason, the appropriate range of the C content is limited to a range of 0.005% to 0.25%. A more preferable addition amount for stably obtaining a tensile strength exceeding 440 MPa is 0.01% or more.

(Si: 0.001% to 2.5%)
Si is an element effective for increasing the mechanical strength of the steel sheet. However, if it exceeds 2.5%, the chemical conversion property deteriorates or surface flaws occur, so this was made the upper limit. On the other hand, since it is difficult to make Si less than 0.001% with practical steel, this was made the lower limit.

(Mn: 0.01% to 2.5%)
Mn, like Si, is an element effective for increasing the mechanical strength of the steel sheet. However, if it exceeds 2.5%, the workability deteriorates, so this was made the upper limit. On the other hand, since it is difficult to make Mn less than 0.01% in practical steel, this was made the lower limit. In order to increase the strength, it is preferable to contain 0.5% or more of Mn.
In addition to Mn, when the element which suppresses the generation | occurrence | production of the hot crack by S is not fully added, it is preferable to add the amount of Mn which becomes Mn / S> = 20 by mass%.

(P: 0.15% or less, S: 0.03% or less)
P and S are impurities, and are 0.15% or less and 0.03% or less, respectively. This is to prevent secondary workability deterioration and cracking during hot rolling or cold rolling.

(Al: 2.0% or less, or 0.0005% or more and 0.02% or less)
Al is a very important element in the present invention for the following two reasons.

  The first reason why Al is important is the action as a deoxidizer, and it is preferable to add 0.01% or more in order to obtain this effect. However, if the addition amount is too large, the workability deteriorates and the surface properties become poor, so the upper limit is made 2.0%.

  The second reason why Al is important is the formation of AlN. In the present invention, it is preferable to improve the BH property by securing the solid solution N amount. Therefore, it is preferable to suppress the addition amount of Al as much as possible and suppress the formation of AlN. From the viewpoint of improving the BH property, the upper limit of Al content is preferably 0.02%. On the other hand, from the viewpoint of steelmaking cost, it is preferable that the lower limit of the Al amount is 0.0005% or more.

(N: 0.01% or less)
N is an impurity, and the upper limit is made 0.01% so as not to deteriorate the workability. On the other hand, N is an element that increases the yield strength by the coating baking process and improves the collision resistance of the member. Therefore, in order to ensure the solid solution N amount and improve the BH property, it is desirable to contain N 0.001% or more.

(Al (mass%) / N (mass%) = 2 or less)
In the present invention, in order to ensure the solid solution N amount, it is preferable to suppress the generation of AlN as much as possible as described above. Therefore, Al / N is preferably 2 or less in order to further improve the BH property.

(O: 0.01% or less)
O is an impurity, and the upper limit is made 0.01% in order to prevent deterioration of workability.

(B: 0.0003% or more and 0.007% or less)
B is very important in the present invention. B is not only effective for strengthening grain boundaries and increasing the strength of steel materials, but in the present invention, it is presumed that a texture of hot-rolled sheets advantageous for shape freezing properties is developed, and 0.0003% It is preferable to add the above. In addition, since the said effect will not only be saturated when the addition amount exceeds 0.007% but ductility is reduced, the upper limit was made 0.007%.

(Ti: 0.005% to 0.04%, Nb: 0.005% to 0.04%)
These elements are very important in the present invention. Conventionally, the main purpose of adding these elements has been to suppress recrystallization and grain growth during annealing after cold rolling, and to preserve the texture that is advantageous for shape freezing property without being destroyed. In the present invention, the upper limit of the addition amount of Ti and Nb is limited, recrystallization is completed, thereby obtaining an excellent shape freezing property and improving the ductility as compared with the prior art.
Therefore, excessive addition of Ti and Nb suppresses recrystallization and grain growth and impairs ductility. Therefore, the upper limit of the addition amount of Ti and Nb is 0.04% or less and 0.04% or less, respectively, and the total of both. Was set to 0.04% or less. Further, if the addition amount of Ti and Nb is extremely small, the {100} <011> orientation advantageous for shape freezing property does not develop, so the lower limit of the addition amount of Ti and Nb is 0.005% and 0.005, respectively. %, And the total of both was set to 0.01% or more. From the viewpoint of further improving the ductility by further increasing the fraction of recrystallized ferrite, the more preferable upper limit of the total of Ti and Nb is 0.03% or less.

(Mo, W: the total of any one or two of 0.001% to 1.0%)
Mo and W are very important in the present invention, and are essential elements for developing a texture that is advantageous for shape freezing property after annealing. If the total amount of any one or two of the two is less than 0.001%, the above effect is not exhibited, and the total of any one or two of both exceeds 1.0%. On the contrary, addition deteriorates workability, so the range was limited to 0.001% to 1.0%.

  The high-strength cold-rolled steel sheet excellent in shape freezing property and ductility of the present invention further contains at least one or more of V, Cr, Cu, Ni, and Sn as necessary. It is good also as a chemical component composition.

(V: 0.20% or less, Cr: 1.5% or less)
Since V and Cr improve the material through mechanisms such as C and N fixation, precipitation strengthening, structure control, and fine grain strengthening, it is preferable to add 0.0001% or more of each. However, even if added excessively, there is no remarkable effect, but rather the workability and surface properties are deteriorated, so the upper limit is preferably 0.20% and 1.5%, respectively.

(Cu: 2.0% or less, Ni: 1.0% or less, Sn: 0.20% or less)
Since Cu, Ni, and Sn have the effect of increasing mechanical strength or improving the material, it is preferable to add 0.001% or more for each component, and excessive addition conversely deteriorates workability. Therefore, the upper limit is preferably set to 2.0%, 1.0%, and 0.20%, respectively.

  In the present invention, although not particularly limited, 0.005% or less of Ca, Mg, or Ce in total may be added for one purpose or two or more kinds for the purpose of deoxidation and sulfide shape control.

"Microstructure of cold-rolled steel sheet"
The microstructure of the steel sheet is a structure containing any one or two of pearlite, bainite, and martensite with ferrite as the largest phase in area ratio.
If the total area fraction of one or more of pearlite and bainite or martensite is less than 1%, it is difficult to obtain a TS exceeding 440 MPa, and if it exceeds 25%, the strength is more than necessary. As ductility increases, ductility decreases significantly. For this reason, the total amount of any one or more of pearlite, bainite, and martensite is limited to a range of 1 to 25% in terms of area fraction.
The microstructure may be obtained by taking a sample with the cross section of the plate thickness parallel to the rolling direction as the observation surface, polishing the observation surface, performing nital etching, and if necessary, repeller etching, and observing with an optical microscope. By analyzing an image of a microstructure photograph obtained by an optical microscope, it is possible to obtain a total amount of one or more area fractions of pearlite, bainite or martensite.

  On the other hand, the ferrite phase is composed of ferrite recrystallized by annealing and non-recrystallized ferrite not recrystallized. Although non-recrystallized ferrite is effective for improving the strength, it also causes deterioration of ductility. Among the ferrite phases, if the area fraction of the recrystallized ferrite phase is less than 80%, good ductility cannot be obtained, so the fraction was limited to 80% or more. More preferably, it is 90% or more. If strength is not required, the recrystallized ferrite fraction may be 100%.

The recrystallized ferrite shown here is defined by analyzing crystal orientation measurement data of Electron Back Scattering Pattern (referred to as EBSP) by the Kernel Average Misoration method (KAM method).
The crystal orientation measurement of EBSP is performed at a measurement interval of 1/10 of the average crystal grain size of the sample after annealing in a range of 100 × 100 μm at a 1/4 thickness position in the plate thickness direction of any plate cross section, Measurement points are output as pixels. A sample to be used for EBSP crystal orientation measurement is prepared such that the steel plate is reduced to a predetermined plate thickness by mechanical polishing or the like, and then the strain is removed by electrolytic polishing or the like, and at the same time, the plate thickness ¼ surface becomes the measurement surface. . In a crystal grain immediately after cold rolling (unrecrystallized grain), a relatively continuous crystal orientation change caused by plastic deformation exists in the grain, whereas a crystal orientation change in the grain after recrystallization is completed. Is extremely small. In the KAM method, the crystal orientation difference between adjacent pixels (measurement points) can be quantitatively shown, so here, the region where the average crystal orientation difference between adjacent measurement points is within 1 ° is defined as recrystallized ferrite. To do.

The area ratio of the recrystallized ferrite phase is a percentage obtained by dividing the total area of the recrystallized ferrite phase by the total area of the ferrite phase.
The total area of the ferrite phase and the total area of the recrystallized ferrite phase can be obtained as follows. The total area of the ferrite phase was obtained by performing nital etching on the sample used for measuring the crystal orientation of EBSP, taking an optical microscope photograph of the field of view where the measurement was performed at the same magnification, and analyzing the resulting structure photograph. Just ask. Furthermore, the total area of the recrystallized ferrite can be obtained by collating the optical micrograph with the crystal orientation measurement of EBSP and analyzing the image.
The total area of the ferrite phase is the sum of the total area of the recrystallized ferrite phase and the total area of the non-recrystallized ferrite phase.

“Tensile strength TS (MPa) × Total elongation EL (%) = 17000 (MPa ·%) or more”
The cold-rolled steel sheet of the present invention satisfies the crystal orientation and structure as described above, and is not only excellent in shape freezing properties but also extremely excellent in ductility. Furthermore, from the viewpoint of formability, the product of tensile strength TS (MPa), which is an index of ductility, and total elongation × EL (%), that is, TS × EL (MPa ·%) is 17000 (MPa ·%) or more. It is preferable. Further, TS × EL is more preferably 17500 (MPa ·%) or more from the viewpoint of moldability.

"Method for producing high-strength cold-rolled steel sheets with excellent shape freezing and ductility"
Below, the manufacturing method of the high intensity | strength cold-rolled steel plate excellent in the shape freezing property and ductility of this invention is explained in full detail.

The manufacturing method of the high-strength cold-rolled steel sheet according to the present invention is such that the slab having the above chemical components has a total rolling reduction in the temperature range of Ar ₃ transformation temperature to 900 ° C. of 25% or more, and the finish rolling temperature is Ar ₃ transformation temperature or more. And then rolled up at a temperature of 700 ° C. or lower, pickled, cold-rolled at 2 to 80%, and further at a temperature increase rate of 15 ° C./sec or lower at a temperature range of 700 ° C. to 900 ° C. After the heating, the method is generally constituted by a method of cooling after holding at the temperature for 3 seconds or more.

The production method preceding hot rolling (hereinafter also referred to as hot rolling) is not particularly limited. That is, various secondary smelting may be performed following smelting in a blast furnace, electric furnace, etc., and then cast by a method such as thin continuous slab casting in addition to normal continuous casting and casting by 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.
In addition, you may use a scrap for a raw material.

(Hot rolling reduction ratio)
The starting temperature of hot rolling is not particularly limited, but the total hot rolling reduction in the temperature range of Ar ₃ transformation temperature to 900 ° C. during hot rolling needs to be 25% or more. This is because when the sum of the hot rolling reductions in the temperature range is less than 25%, the texture of rolled austenite is not sufficiently developed, and after that, whatever cooling is applied, {100} This is because the average value of the <011> to {223} <110> orientation groups decreases, and the effects of the present invention may not be obtained. The upper limit of the total hot rolling reduction in the temperature range of Ar ₃ transformation temperature to 900 ° C. is preferably as high as possible, and may be determined according to the pass schedule.
Incidentally, the hot rolling reduction rate sum and the in a temperature range from Ar ₃ transformation temperature to 900 ° C., the difference between the thickness in the sheet thickness and the Ar ₃ transformation temperature at 900 ° C., the value obtained by dividing the thickness at 900 ° C. It is expressed as a percentage. In the production method of the present invention, the plate thickness at the Ar ₃ transformation temperature is the same as the plate thickness after finish rolling.

Here, the Ar ₃ transformation temperature is calculated by the following equation (1).
Ar ₃ = 901-325 × C% + 33 × Si% + 287 × P% + 40 × Al% −92 (Mn% + Mo% + Cu%) − 46 × (Cr% + Ni%) (1)

(Finish rolling temperature)
The temperature at which the hot rolling is finished, that is, the finish rolling temperature (FT) is set to the Ar ₃ transformation temperature or higher. This is because when the finish rolling temperature is lower than the Ar ₃ transformation temperature, the {100} <011> orientation after annealing does not develop, and the other {100} <011> to {223} <110> orientation groups The X-ray random intensity ratio is lower than the orientation, or the X-ray random intensity ratio of the {100} <011> orientation is less than 5.0, the shape freezing property is deteriorated, and the processed structure is formed in the ferrite precipitated during rolling. This is because the remaining ductility may decrease. On the other hand, when the finish rolling temperature of hot rolling exceeds 900 ° C., the effect of the present invention cannot be obtained, and the surface quality is also lowered by the oxide layer generated on the surface of the steel sheet.
In addition, what is necessary is just to perform the rolling of 1 pass which makes rolling reduction 25% or more at 900 degreeC, when finishing hot rolling at 900 degreeC.

(Winding temperature)
The coiling temperature (CT) is 700 ° C. or less. When the coiling temperature exceeds 700 ° C., the texture of {100} <011> to {223} <110> orientation groups advantageous for shape freezing properties after annealing does not develop, so 700 ° C. is the upper limit of the coiling temperature. Value. Further, in order to obtain both excellent shape freezing property and ductility, the temperature is preferably 630 ° C. or lower.
The lower limit of the coiling temperature is not particularly specified, and the effects of the present invention can be obtained. However, the workability of the steel sheet may be deteriorated, so that the temperature is preferably set to 300 ° C. or higher.

(Cold reduction ratio)
Cold rolling of the hot-rolled steel sheet obtained above is performed in a range of a reduction rate of 2 to 80%. If the rolling reduction (cold rolling ratio) of cold rolling is less than 2%, the {100} <011> orientation effective for shape freezing properties does not develop strongly, so 2% was made the lower limit. The lower limit is more preferably 5% or more from the viewpoint of securing the plate thickness accuracy. On the other hand, when the cold rolling reduction exceeds 80%, the {111} orientation rapidly develops, and the X-ray randomness of crystal orientations of {554} <225>, {111} <112> and {111} <110> Since the average value of the strength ratio exceeds 6.0 and the shape freezing property deteriorates, the upper limit is set to 80%. In order to improve the shape freezing property, it is preferable to limit the cold rolling reduction to 70% or less. More preferably, it is 60% or less.
After cold rolling, the cold-rolled steel sheet is subjected to annealing for heating and holding as described below and cooled.

(Temperature increase rate)
If the heating rate in annealing after cold rolling is more than 15 ° C./s, it is heated to the two-phase region before the recrystallization of the ferrite sufficiently proceeds, so the fraction of recrystallized ferrite is less than 80%. Become. For this reason, it is preferable to heat as slowly as possible. Therefore, the upper limit of the heating rate is set to 15 ° C./sec. Furthermore, in order to make the fraction of recrystallized ferrite 90% or more, the upper limit of the heating rate is more preferably 10 ° C./sec.

(Heating temperature)
When the heating temperature is less than 700 ° C., ductility deteriorates because an unrecrystallized ferrite structure remains. Therefore, in order to make the fraction of recrystallized ferrite 80% or more, the lower limit of the heating temperature is set to 700 ° C. In order to make the fraction of recrystallized ferrite 90% or more, the preferable lower limit of the heating temperature is 750 ° C. On the other hand, when the heating temperature is excessively high, the ferrite texture formed by recrystallization is randomized by austenite grain growth after transformation to austenite, and the finally obtained ferrite texture is also randomized. The In particular, when the heating temperature exceeds 900 ° C., such a tendency becomes remarkable. Therefore, the heating temperature is 900 ° C. or less. Further, from the viewpoint of ensuring better shape freezeability, the upper limit of the heating temperature is more preferably 860 ° C.

(Retention time)
When the holding time in the temperature range of 700 to 900 ° C. is less than 3 seconds, the structure is likely to be non-uniform, which leads to variations in shape freezing property and ductility in the longitudinal direction or width direction of the steel sheet. For this reason, the lower limit of the holding time is 3 sec.
The cooling conditions after annealing may be appropriately determined according to the annealing process, but are usually air cooling.
In addition, the annealing process may use any process of a continuous annealing process, a BAF process, and a continuous annealing-plating (alloying) process. What is necessary is just to select the cooling conditions after a continuous annealing process, and overaging conditions according to the required TS level.
Moreover, it is not necessary to specifically limit the conditions for hot dipping or plating alloying, and it may follow a conventional method. The type of plating is not particularly limited.

(Skin pass)
Skin pass rolling may be applied to the cold-rolled steel sheet produced by the above method. If the rolling reduction in skin pass rolling is less than 0.1%, the shape control effect of the steel sheet is small, so 0.1% is preferably set as the lower limit. Further, if the rolling reduction exceeds 5.0%, the ductility of the steel sheet is remarkably deteriorated, so 5.0% is preferable as the upper limit.

  The high-strength cold-rolled steel sheet having excellent shape freezing property, ductility and paint bake hardenability according to the present invention has a tensile strength of 440 MPa or more, a small amount of springback, and a high shock absorption capacity due to the above-described configuration. Excellent shape freezing and ductility. This makes it difficult to apply high-strength steel sheets due to problems of shape defects and cracks, and high-strength steel sheets can be used for parts that are mainly molded by bending, such as automobile bodies. It is possible to achieve both bending workability and anti-collision properties, which are difficult to achieve because of conflicting characteristics, and thus it is possible to further reduce the weight of the automobile body.

  Hereinafter, examples of the high-strength cold-rolled steel sheet according to the present invention will be given and the present invention will be described more specifically, but the present invention is not originally limited to the following examples, and the purpose described above and below It is also possible to carry out with appropriate modifications within a range that can be adapted, and these are all included in the technical scope of the present invention.

[Sample preparation]
Using steels of symbols A to N having the chemical composition shown in Table 1 below, after casting or after cooling to room temperature and reheating, hot rolling was performed under the conditions shown in Table 2 below. . It was set as the hot-rolled steel plate of various thickness as shown to 1-29.
And this hot-rolled steel sheet is subjected to cold rolling at a cold rolling rate shown in Table 2 below to a thickness of 1.4 mm, and then a continuous annealing line (No. 1, 3 to 20) and a BAF annealing step (No. In 2) and continuous annealing-zinc plating-alloying line (Nos. 21 to 29), annealing was performed at the heating rate, heating temperature, and holding time shown in Table 2 below, followed by cooling.
A test piece having a width of 25 mm and a length of 130 mm was prepared from each steel plate having a thickness of 1.4 mm, and various wrinkles were made using a die having a punch width of 40 mm, a punch shoulder R5, a die width of 43.4 mm, and a die shoulder R3. After forming into a hat shape with a pressing pressure of 40 mm, the amount of warpage of the wall was measured as the curvature radius ρ (mm), and the reciprocal number 1000 / ρ was determined. Here, the smaller the 1000 / ρ, the better the shape freezing property.
Thereafter, various properties were investigated in the evaluation test described below.

[Evaluation test]
Evaluation of the microstructure, texture, mechanical property value, and shape freezing property of the cold-rolled steel sheet having a thickness of 1.4 mm manufactured by the above method was carried out by the following methods, and the results are shown in Table 3 below.
The microstructure was observed with an optical microscope, and the total area ratio of pearlite, bainite, and martensite was obtained by image analysis of a structure photograph taken with an optical microscope. Further, the ferrite recrystallization rate was calculated by comparing the crystal orientation measurement of EBSP with an optical microscope structure photograph, and obtaining the total area of the ferrite phase and the recrystallized ferrite phase by image analysis. The texture was measured by X-ray diffraction.
In addition, the tensile test was performed in accordance with JIS Z 2201, and a No. 5 test piece was collected so that the longitudinal direction was parallel to the rolling direction, and in accordance with JIS Z 2241. TS (MPa) shown in Table 3 is the tensile strength, EL (%) is the total elongation, and YR is the yield strength YS (MPa) to tensile strength ratio YS / TS.

In Table 3 below, the remaining structure of pearlite, bainite, and martensite is ferrite. Moreover, as for the value of the texture of the steel plate after annealing, the average value of the values measured on the plate thickness 7/16 plane and 1/4 plane was used. In the column of “Evaluation of shape freezing property” in Table 3 below, “○” indicates that 1000 / ρ ≦ 0.017 × TS (MPa) −4.13, and “×” indicates other cases. It showed in.
The chemical composition of each steel of this example (Example 1) is shown in Table 1, the production conditions of the steel sheet are shown in Table 2, and the evaluation results are shown in Table 3.

  As is apparent from the results shown in Tables 1 to 3, when steel sheets having chemical components within the range specified by the present invention were used and the steel sheet was manufactured under the manufacturing conditions within the range specified by the present invention, good ductility was achieved. It can also be seen that a high-strength cold-rolled steel sheet having a very good shape freezing property can be obtained.

[Sample preparation and evaluation test]
Using steel from the symbols AA to AN having the chemical composition shown in Table 4 below, after casting, or once cooled to room temperature and reheated, it was hot-rolled under the conditions shown in Table 5 below. . Hot rolled steel sheets having various thicknesses as shown in 100 to 133 were used. In Table 5, the rolling reduction is the total rolling reduction at Ar _{3 to} 900 ° C. When the finish rolling temperature is Ar ₃ or higher, the rolling reduction was obtained from the plate thickness at 900 ° C and the thickness after finish rolling. In addition, it showed with "-" in the table | surface that the rolling reduction temperature exceeds 900 degreeC that this reduction rate cannot be calculated | required.
And this hot-rolled steel sheet is cold rolled at a cold rolling rate shown in Table 5 below to obtain a thickness of 1.4 mm, and then a continuous annealing line (No. 101 to 103, 105 to 120, 127 to 133), In the BAF annealing step (No. 104) and continuous annealing-zinc plating-alloying line (No. 121-126), after annealing at the heating rate, heating temperature, and holding time shown in Table 5 below, it was cooled. .

A test piece was prepared from each of these steel plates in the same manner as in Example 1 and formed into a hat shape. Then, the amount of warpage of the wall portion was obtained by reciprocal 1000 / ρ of the radius of curvature ρ (mm), and the microstructure and texture The mechanical property values were investigated.
Furthermore, the amount of BH was evaluated in accordance with the paint bake hardening test method described in the appendix of JIS G 3135. In Table 6, TS (MPa) is tensile strength, EL (%) is total elongation, YR is the yield strength YS (MPa) to tensile strength ratio YS / TS, and UBH (MPa) is the amount of BH. .

In Table 6 below, the remaining structure of pearlite, bainite, and martensite is ferrite. Moreover, as for the value of the texture of the steel plate after annealing, the average value of the values measured on the plate thickness 7/16 plane and 1/4 plane was used. In the column of “Evaluation of shape freezing property” in Table 6 below, “O” indicates that 1000 / ρ ≦ 0.017 × TS (MPa) −4.13, and “X” indicates other cases. It showed in.
The chemical composition of each steel of this example (Example 2) is shown in Table 4, the production conditions of the steel sheet are shown in Table 5, and the evaluation results are shown in Table 6.

As is apparent from the results shown in Tables 4 to 6, when a steel sheet having a chemical composition within the range specified by the present invention was manufactured under the manufacturing conditions within the range specified by the present invention, the shape freezing property was obtained. It can be seen that a high-strength cold-rolled steel sheet excellent in ductility and paint bake hardenability can be obtained.

Claims (8)

% By mass
C: 0.005% or more and 0.25% or less,
Si: 0.001% to 2.5%,
Mn: 0.01% or more and 2.5% or less,
P: 0.15% or less,
S: 0.03% or less,
Al: 2.0% or less,
N: 0.01% or less,
O: 0.01% or less,
B: 0.0003% or more and 0.007% or less, respectively,
Ti: 0.005% or more and 0.04% or less,
Nb: 0.005% or more and 0.04% or less in total in an amount of 0.01% or more and 0.04% or less, and further one or two of Mo and W in total In an amount of 0.001% to 1.0%,
The balance consists of iron and inevitable impurities,
1% or more and 25% or less of pearlite, bainite or martensite in an area fraction, or a composite structure steel composed of ferrite having a recrystallization rate of 80% or more.
The average value of the X-ray random intensity ratio of {100} <011> to {223} <110> orientation groups on the plate surface at ½ thickness is 3.0 or more, and among these orientation groups, { 100} <011> orientation X-ray random intensity ratio is maximum and satisfies 5.0 or more, and X-rays of crystal orientation of {554} <225>, {111} <112> and {111} <110> A high-strength cold-rolled steel sheet having an average random strength ratio of 6.0 or less.   The high-strength cold-rolled steel sheet according to claim 1, wherein the Al content is 0.0005% or more and 0.02% or less by mass%. Furthermore, in mass%,
V: 0.20% or less,
Cr: 1.5% or less,
Cu: 2.0% or less,
Ni: 1.0% or less,
The high-strength cold-rolled steel sheet according to claim 1 or 2, characterized by containing at least one or more of Sn: 0.20% or less. The high-strength cold-rolled steel sheet according to any one of claims 1 to 3, wherein the Al content and the N content satisfy the following formula: Al / N = 2 or less.   5. The product TS × EL [MPa ·%] of the tensile strength TS [MPa] and the total elongation EL [%] is 17000 [MPa ·%] or more. The high-strength cold-rolled steel sheet as described in 1.   A high-strength cold-rolled steel sheet obtained by plating the high-strength cold-rolled steel sheet according to any one of claims 1 to 5. The slab having the chemical component according to any one of claims 1 to 4, wherein the total rolling reduction in the temperature range of Ar ₃ transformation temperature to 900 ° C is 25% or more, and the finish rolling temperature is Ar ₃ transformation temperature or more. Hot-rolled, then wound up at a temperature of 700 ° C. or lower, pickled, cold-rolled at 2 to 80%, and further brought to a temperature range of 700 ° C. to 900 ° C. at a heating rate of 15 ° C./sec or lower. A method for producing a high-strength cold-rolled steel sheet, wherein the steel sheet is cooled after being heated for 3 seconds or more after being heated. A method for producing a high-strength cold-rolled steel sheet, comprising subjecting the high-strength cold-rolled steel sheet produced by the production method according to claim 7 to skin pass rolling of 0.1% to 5%.