Technical field
[0001]
The present invention relates to steel sheets.
Background technology
[0002]
Due to the growing need for weight reduction against the background of energy problems, the application of high-strength steel sheets that can reduce the thickness of private cars and trucks is progressing in a wide range of applications. It is reaching up to
[0003]
　Since many of these automobile body parts are formed by press working, the high-strength steel sheets used are required to have excellent formability. In particular, many of the lid members include draw-forming processing elements in corners (corner ends) or embossed portions of door handles, etc. Therefore, the material steel plate is required to have high strength and a high r-value.
[0004]
　Many of the conventional r-value improvement technologies for thin steel sheets have been established for mild steel, which is substantially a ferrite single-phase structure. To summarize these technologies, solute carbon and/or solute nitrogen are reduced as much as possible before cold rolling, discontinuous recrystallization is caused in the cold rolling process and annealing process, and the crystal orientation of the material is changed to the rolled sheet surface. It is to control the texture so that the {111} planes are aligned to each other, that is, to increase the γ-fiber. As a representative steel type, there is IF (Interstitial Free) steel in which Ti and/or Nb is added to ultra-low carbon steel, and it has been studied to increase strength by adding solid solution strengthening elements to this.
[0005]
For example, Patent Document 1 discloses a cold-rolled steel sheet in which the contents of Al and Nb are controlled in relation to the contents of N and C, respectively, and P, Si and Mn are added. Patent Literature 2 proposes a high-strength steel sheet composed of a ferrite phase and a hard secondary phase, which are excellent in r-value and hole expansibility.
[0006]
On the other hand, Patent Document 3 discloses a high-strength steel sheet in which the ratio of the accumulated strength of γ-fiber and the accumulated strength of α-fiber is set to 1 or more. Furthermore, Patent Document 4 discloses a high-strength cold-rolled steel sheet with excellent rigidity, deep-drawability and hole-expandability, in which crystal orientations are concentrated in {332}<113>.
prior art documents
patent literature
[0007]
Patent document 1: JP-A-56-139654
Patent Document 2: JP-A-2005-264323
Patent document 3: JP 2016-141859 A
Patent Document 4: JP-A-2009-114523
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0008]
However, the strength (TS) of the steel sheet obtained by the technique of Patent Document 1 remains below 500 MPa. Further, in the technique of Patent Document 2, it is necessary to reduce the carbon content in order to develop γ-fiber, and the obtained strength remains at about 650 MPa.
[0009]
Patent Document 3 discloses a high-strength steel sheet of 780 MPa class. However, in order to obtain the steel sheet disclosed in this technique, it is essential to carry out two annealings to create a texture and a high-strength structure respectively after cold rolling, which raises the problem of increased costs. Further, Patent Document 4 discloses a steel sheet having a tensile strength of 890 MPa or more, a high average r-value and a high Young's modulus, and excellent hole expansibility. However, in order to obtain the desired texture, it is necessary to contain expensive Mo and W as essential elements, so there is a problem that the alloy cost and hot rolling load increase, and there is still room for improvement. .
[0010]
In addition, many of the lid members generally have a shape similar to a large rectangle. From the viewpoint of ensuring the material yield, it is common to extract members from a material steel plate so that the longitudinal direction of the member coincides with the rolling direction or width direction of the material steel plate. And, as described above, many of the lid members include processing elements for draw forming in the corners, so the r value in the direction of 45° to the rolling direction (hereinafter also referred to as “r 45 value”) should be increased. is particularly important.
[0011]
In addition, in recent years, there has been a demand for high design quality in parts such as hoods and doors among lidding parts, and locally heavily processed parts called character lines are provided. Therefore, in addition to the r45 value, it is also important for the lid member to have a high ultimate deformability.
[0012]
As described above, in order to obtain a member that is lightweight and has high designability, it is necessary to have a steel plate that not only has high strength but also has a high r45 value and ultimate deformability.
[0013]
The object of the present invention is to solve the above problems and to provide a steel sheet with high tensile strength, r45 value and ultimate deformability.
Means to solve problems
[0014]
The present invention has been made to solve the above problems, and the gist thereof is the following steel plate.
[0015]
(1) The chemical composition is mass%,
C: 0.03-0.25%,
Si: 0.1 to 2.0%,
Mn: 1.0 to 3.0%,
P: 0.200% or less,
S: 0.0500% or less,
Al: 0.01 to 1.00%,
N: 0.0100% or less,
Ti: 0.01 to 0.25%,
The balance: Fe and impurities,
The metal structure, in area%,
　Ferrite: Contains 50 to 85%,
the balance is one or more selected from martensite, bainite and retained austenite,
　The integrated intensity of γ-fiber exceeds 4.0 times as a random intensity ratio, and the average KAM value in crystal grains having a crystal orientation within 10° from the γ-fiber is 1.30° or less.
Steel plate.
[0016]
(2) the chemical composition is replaced by part of the Fe, in mass%,
Cr: 0.50% or less,
Ni: 0.50% or less, and
Cu: 0.50% or less,
containing one or more selected from
The steel plate according to (1) above.
[0017]
(3) The chemical composition is replaced by part of the Fe, in mass%,
Nb: 0.050% or less,
V: 0.15% or less,
Zr: 0.15% or less,
Mo: 0.15% or less, and
W: 0.15% or less,
containing one or more selected from
The steel plate according to (1) or (2) above.
[0018]
(4) The chemical composition is, in mass%, instead of part of the Fe,
Contains 0.100% or less in total of one or more selected from Sn, Sb and Te,
The steel plate according to any one of (1) to (3) above.
[0019]
(5) The chemical composition is, in mass%, instead of part of the Fe,
containing 0.0050% or less in total of one or more selected from Ca, Mg and REM,
The steel plate according to any one of (1) to (4) above.
[0020]
(6) The chemical composition is, in mass%, instead of part of the Fe,
B: 0.0050% or less,
containing
The steel plate according to any one of (1) to (5) above.
[0021]
(7) having a decarburized layer with a thickness of 4.0 μm or more in the depth direction from the surface,
The steel plate according to any one of (1) to (6) above.
The invention's effect
[0022]
According to the present invention, it is possible to obtain a steel sheet with excellent formability, such as a tensile strength of 700 MPa or more, an r45 value of 1.20 or more, and an ultimate deformability of 0.80 or more.
Brief description of the drawing
[0023]
[Fig. 1] Fig. 1 is a schematic diagram for explaining a method for measuring the shortest distance from the surface of a steel sheet to a hard phase.
MODE FOR CARRYING OUT THE INVENTION
[0024]
The inventors of the present invention conducted studies and experiments on methods for improving the r45 value and ultimate deformability of high-strength steel sheets with a tensile strength of 700 MPa or more, and found the following findings.
[0025]
When performing the hot rolling process on steel with a predetermined chemical composition, the rolling conditions for the final three steps including the final pass are controlled to flatten the austenite grains before transformation. Subsequently, it is cooled and coiled at a low temperature for bainite transformation or martensite transformation. As a result, a hot-rolled sheet with developed {223}<252> orientation can be obtained. Then, by subjecting the obtained hot-rolled sheet to a cold-rolling process under appropriate conditions, a strong accumulation of γ-fibers can be produced.
[0026]
Here, strain is imparted to the metal structure by cold rolling, but if excessive cold rolling strain remains in the final structure, the ultimate deformability will deteriorate. However, in the subsequent annealing process, if the annealing temperature and the subsequent cooling rate are appropriately controlled, randomization of the texture can be avoided, γ-fibers can be accumulated extremely strongly, and cold-rolling strain can be reduced. can. Then, by precipitating the low temperature transformation phase after that, it is possible to obtain a steel sheet having both high ultimate deformability and r45 value and high strength.
[0027]
The present invention is made based on the above findings. Each requirement of the present invention will be described in detail below.
[0028]
(A) Chemical composition
The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %." Further, in the present invention, the chemical composition of the steel sheet means the average chemical composition in the region excluding the decarburized layer described later.
[0029]
C: 0.03-0.25%
　C is an element necessary to ensure strength. If the C content is less than 0.03%, a tensile strength of 700 MPa or more cannot be obtained. On the other hand, if the C content exceeds 0.25%, the martensite is excessively hardened, degrading toughness and impairing weldability. Therefore, the C content should be 0.03 to 0.25%. The C content is preferably 0.05% or more, preferably 0.18% or less, and more preferably 0.15% or less.
[0030]
Si: 0.1-2.0%
　Si is an element that contributes to strength improvement. On the other hand, when it is contained excessively, the productivity is lowered due to poor descaling during hot rolling. Therefore, the Si content should be 0.1 to 2.0%. The Si content is preferably 0.5% or more in order to stabilize austenite during annealing and promote the formation of a low temperature transformation phase during cooling to contribute to high strength. Moreover, when generating retained austenite to improve ductility, the total content of Si and Al, which will be described later, is preferably 1.0% or more.
[0031]
　Mn: 1.0 to 3.0%
Mn has the effect of stabilizing austenite, facilitating the formation of a low-temperature transformation phase, and contributing to ensuring strength. On the other hand, if it is contained excessively, the volume fraction of ferrite decreases and the ductility deteriorates. Therefore, the Mn content is set to 1.0 to 3.0%. The Mn content is preferably 2.1% or more and preferably 2.8% or less.
[0032]
P: 0.200% or less
Since P has the effect of increasing strength, it may be positively included. However, if it is contained excessively, embrittlement occurs due to grain boundary segregation. The P content is preferably 0.100% or less, more preferably 0.050% or less. There is no need to set a lower limit on the P content, and it may be 0%. However, excessive reduction causes an increase in manufacturing costs, so the P content is preferably 0.001% or more. In addition, in the steelmaking stage, about 0.010% of impurity level is usually mixed.
[0033]
S: 0.0500% or less
Since S forms sulfide-based inclusions and reduces ductility, the content is suppressed to 0.0500% or less. To ensure excellent ductility, the S content is preferably 0.0080% or less, more preferably 0.0030% or less.
[0034]
Al: 0.01-1.00%
Al is an element used for deoxidation. However, an excessive content makes stable continuous casting difficult. Therefore, the Al content is set to 0.01 to 1.00%. In addition, when the Al content is high, austenite at high temperatures becomes unstable, and it becomes necessary to excessively raise the finish rolling temperature in hot rolling, so the content should be 0.60% or less. is preferred. In addition, in this invention, Al content means content of acid-soluble Al (sol.Al). Generates retained austenite to increase ductilityFor improvement, the total content of Al and Si is preferably 1.0% or more.
[0035]
N: 0.0100% or less
Since N is an element that lowers the strength-ductility balance, its content should be 0.0100% or less. The N content is preferably 0.0060% or less. There is no need to set a lower limit on the N content, and it may be at the impurity level. Generally, about 0.0020% is mixed in at the steelmaking stage.
[0036]
Ti: 0.01-0.25%
Ti precipitates as a carbide in the structure of the hot-rolled steel sheet, and has the effect of reducing solute carbon and making it easier to obtain γ-fiber in the cold-rolled steel sheet. In addition, it has the effect of suppressing recrystallization and coarsening of austenite, promoting flattening of austenite in the hot rolling process, and making it easier to obtain the {223}<252> orientation of the hot rolled sheet. On the other hand, an excessive content forms coarse carbides or nitrides during furnace heating before hot rolling, impairing strength-ductility balance. Therefore, the Ti content should be 0.01 to 0.25%. The Ti content is preferably 0.02% or more, more preferably 0.03% or more, and preferably 0.20% or less.
[0037]
In the steel sheet of the present invention, in addition to the above elements, one selected from Cr, Ni, Cu, Nb, V, Zr, Mo, W, Sn, Sb, Te, Ca, Mg, REM and B The above elements may be contained.
[0038]
Cr: 0.50% or less
Ni: 0.50% or less
Cu: 0.50% or less
Cr, Ni and Cu have the effect of increasing hardenability and effectively forming martensite and/or bainite, so they may be contained as necessary. However, since an excessive content suppresses the formation of ferrite, the content of each of these elements is set to 0.50% or less. To obtain the above effects, it is preferable to contain at least one element selected from the above elements in an amount of 0.10% or more.
[0039]
Nb: 0.050% or less
Nb precipitates as carbides or nitrides, suppresses recrystallization and coarsening of austenite, promotes flattening of austenite in the hot rolling process, and has the effect of making it easier to obtain the {223}<252> orientation of the hot-rolled sheet. have In addition, it has the effect of suppressing recrystallization during annealing and suppressing randomization of the texture. Therefore, it may be contained as necessary. However, if the Nb content is excessive, a large amount of coarse carbides are generated during heating before hot rolling, impairing the balance between strength and ductility. The Nb content is preferably 0.030% or less. To obtain the above effects, the Nb content is preferably 0.010% or more.
[0040]
　V: 0.15% or less
Zr: 0.15% or less
Mo: 0.15% or less
W: 0.15% or less
V, Zr, Mo and W have the effect of suppressing recrystallization and coarsening of austenite, promoting flattening, and making it easier to obtain the {223}<252> orientation of the hot-rolled sheet. may be included. However, if it is contained excessively, coarse carbides are formed and not only the strength-ductility balance is disturbed, but also the cost of the alloy increases. The content of each of these elements should be 0.15% or less, preferably 0.12% or less. To obtain the above effect, it is preferable to contain at least one element selected from the above elements in an amount of 0.01% or more.
[0041]
One or more selected from Sn, Sb and Te: 0.100% or less in total
Sn, Sb, and Te segregate on the steel surface, suppress decarburization of the surface layer of the steel sheet, and have the effect of suppressing a decrease in strength during the annealing process. Moreover, even if it is desired to positively form a decarburized layer on the surface layer of the steel sheet, the inclusion of these elements can prevent excessive decarburization due to abnormal oxidation. Therefore, one or more selected from Sn, Sb and Te may be contained as necessary. However, if they are contained excessively, they will segregate at grain boundaries and lower the toughness. In order to obtain the above effect, the total content of these elements is preferably 0.005% or more.
[0042]
　One or more selected from Ca, Mg and REM: 0.0050% or less in total
Ca, Mg and REM (rare earth metals) have the effect of refining the oxides and nitrides that precipitate during solidification to maintain the soundness of the slab, so they may be contained as necessary. However, since these elements are all expensive, their total content is made 0.0050% or less. In order to obtain the above effect, it is preferable that the total content of these elements is 0.0005% or more.
[0043]
Here, REM refers to 17 elements of Sc, Y and lanthanides. The REM content means the total content of these elements. REMs are industrially added in the form of misch metals.
[0044]
B: 0.0050% or less
B has the effect of suppressing recrystallization and coarsening of austenite, promoting flattening, and making it easier to obtain the {223}<252> orientation of the hot-rolled sheet, so it may be contained as necessary. In addition, since it has the effect of increasing the recrystallization temperature during annealing and suppressing the randomization of the texture, it may be added as necessary. However, an excessive content causes cracks on the surface of the steel material during casting, impeding productivity, so the upper limit is made 0.0050% or less. The B content is preferably 0.0040% or less, more preferably 0.0020% or less. To obtain the above effect, it is preferable to contain 0.0005% or more.
[0045]
In the chemical composition of the steel plate and molded member of the present invention, the balance is Fe and impurities. The term "impurities" refers to components that are mixed in with raw materials such as ores, scraps, etc., and various factors in the manufacturing process when steel is manufactured industrially. means something
[0046]
(B) Metal structure
The metal structure of the steel sheet according to the present invention will be explained below. In addition, "%" about the area ratio in the following description means "area%."
[0047]
　Ferrite: 50-85%
　Ferrite is a tissue necessary to develop an r45 value and ductility. On the other hand, if the ferrite area ratio is excessive, a tensile strength of 700 MPa or more cannot be obtained. Therefore, the area ratio of ferrite is set to 50 to 85%. In order to develop better ductility, the area ratio of ferrite is preferably 55% or more, more preferably 60% or more. On the other hand, from the viewpoint of strength improvement, the area ratio of ferrite is preferably 80% or less.
[0048]
In the present invention, ferrite includes not only polygonal ferrite but also granular bainitic ferrite and acicular ferrite that precipitate at low temperatures.
[0049]
In the metal structure of the steel sheet according to the present invention, the remainder other than ferrite is one or more selected from martensite, bainite and retained austenite. A tensile strength of 700 MPa or more can be obtained by making the remaining structure one or more selected from martensite, bainite, and retained austenite, which are hard phases. Martensite also includes MA (martensite-austenite constituent).
[0050]
In the present invention, martensite and bainite include as-quenched martensite and bainite, as well as tempered martensite and tempered bainite, respectively.
[0051]
The area ratio of ferrite and the metal structure that constitutes the material are obtained by structure observation with a scanning electron microscope (SEM). After the cross section of the steel sheet is mirror-polished, the microstructure is revealed with 3% nital (3% nitric acid-ethanol solution). Then, using an SEM at a magnification of 3000, observe the microstructure in the range of 40 μm (length in the thickness direction) × 40 μm (length in the rolling direction) at a depth of 1/2 the thickness from the surface of the steel plate. and the area ratio of each tissue can be measured.
[0052]
In addition, the existence of retained austenite can be confirmed by the X-ray diffraction method. First, a test piece having a width of 25 mm (length in the rolling direction), a length of 25 mm (length in the direction perpendicular to the rolling direction), and a thickness equal to the thickness of the steel plate is cut out from the steel plate. Then, this test piece is subjected to chemical polishing to reduce the thickness to a position half the plate thickness, thereby obtaining a test piece having a chemically polished surface. The surface of the test piece is subjected to X-ray diffraction analysis using a Co tube and a measurement range 2θ of 45 to 105°.
[0053]
The presence or absence of retained austenite can be confirmed by the presence or absence of part or all of the (111), (200), and (220) diffraction peaks.
[0054]
(C) Texture
　γ-fiber accumulation intensity: more than 4.0 times the random intensity ratio
The accumulated strength of γ-fiber means the accumulated strength of the {111} plane oriented in the normal direction of the rolled surface. To achieve a high r 45 value, the γ-fiber integrated intensity should be greater than 4.0 times the random intensity ratio. The integrated intensity of γ-fiber is preferably 6.0 times or more in terms of random intensity ratio.
[0055]
　The integrated intensity of γ-fiber is measured by the following procedure. First, a cross section parallel to the rolling direction (RD) and thickness direction (ND) of the steel plate was developed, and a 500 μm × 200 μm area at a depth of half the thickness was measured by electron beam backscatter diffraction (SEM-EBSD). The crystal orientation of the region is measured at 1.00 μm intervals. Next, based on the obtained crystal orientation data, an inverse pole figure with the ND direction as a reference is calculated by the spherical harmonic expansion method, and the integrated intensity of γ-fiber is obtained from the intensity of the (111) pole. When obtaining an inverse pole figure by the spherical harmonic function expansion method, the expansion order of the series expansion is set to 22, and the calculation is performed without applying additional smoothing such as Gaussian distribution.
[0056]
　Average KAM value in crystal grains with a crystal orientation within 10° from γ-fiber: 1.30° or less
In the present invention, since the steel contains Ti, if the annealing conditions are inappropriate, cold rolling strain will remain excessively and the ultimate deformability will decrease. Therefore, it is necessary to reduce the cold rolling strain in the final structure.
[0057]
The degree of residual cold rolling strain can be determined by local orientation analysis using the SEM-EBSD method. In SEM-EBSD, the sample is irradiated with an electron beam at certain intervals, and the pseudo-Kikuchi pattern is analyzed to identify the crystal orientation of the measurement point. When cold-rolling strain remains, there is orientation fluctuation in the crystal grains, which can be detected as a change in adjacent orientation measured by the SEM-EBSD method.
[0058]
　There is a KAM (Kernel Average Misorientation) value as an index that indicates the degree of variation in crystal orientation between a certain measurement point in such a crystal grain and its surroundings. In order to obtain the KAM value, a measurement area, a measurement interval, a size of the area to be compared, and an angle threshold that guarantees comparison within the same crystal grain are required. In the present invention, an area of 100 μm×200 μm is measured at intervals of 0.05 μm to 0.10 μm, and the KAM value is measured using an area within a circumference radius of 0.20 μm. Note that the angle threshold is set to 5°.
[0059]
As a result of such experiments and analyses, it was found that the average KAM value of crystal grains belonging to γ-fiber is important for enhancing the ultimate deformability while developing a high r45 value. Specifically, when the texture is controlled so that the average KAM value in crystals having a crystal orientation within 10° from the γ-fiber is within 1.30°, an excellent ultimate deformability of 0.80 or more can be obtained. .
[0060]
(D) Decarburized layer
The steel sheet according to the present invention may have a decarburized layer on its surface. By having a soft decarburized layer on the surface layer, it is possible to further improve the bending properties. In particular, by forming a decarburized layer with a thickness of 4.0 μm or more in the depth direction from the surface of the steel plate, the bending radius (Rp) and the steel plate thicknessEven under severe forming conditions such that the ratio (Rp/t) to (t) is 0.5, it is possible to obtain excellent bending properties without cracks on the bending ridge line. The thickness of the decarburized layer is preferably 5.0 μm or more, more preferably 6.0 μm or more.
[0061]
The decarburized layer may be formed only on one surface layer in the thickness direction, or may be formed on both surface layers. However, if the decarburized layer becomes too thick, it becomes difficult to ensure the strength of the steel sheet as a whole. Therefore, the total thickness of the decarburized layer is preferably 20% or less of the total thickness of the steel sheet as the sum of the surface layers on both sides. When the strength of the steel sheet is to be emphasized, the thickness of the decarburized layer is preferably 20 μm or less, more preferably 15 μm or less per side.
[0062]
Furthermore, if the strength of the steel sheet is to be emphasized, it is preferable not to form the decarburized layer. Even if the steel sheet according to the present invention does not have a decarburized layer, if the forming conditions are such that Rp / t is 1.0, it is possible to obtain excellent bending properties that do not cause cracks on the bending ridge line. can be done.
[0063]
Here, in the present invention, the thickness of the decarburized layer refers to the average value of the shortest distance from the steel plate surface to the hard phase when the metal structure is identified in the depth direction from the steel plate surface. Specifically, the thickness of the decarburized layer is measured according to the following procedure. First, a cross-section parallel to the rolling direction and thickness direction of the steel plate is cut out, mirror-polished, and then nital-corroded to expose the metal structure. Subsequently, the structure is observed by SEM, and a 1000-fold SEM image of the cross-sectional structure near the surface layer is acquired.
[0064]
Fig. 1 is a schematic diagram for explaining the method of measuring the shortest distance from the surface of the steel sheet to the hard phase. As shown in FIG. 1, in the obtained SEM image, five lines extending in the thickness direction are drawn at intervals of 20 μm in the rolling direction, and the shortest distance from the steel sheet surface to the hard phase is measured on each line. Then, the average value of the obtained five measured values is taken as the thickness of the decarburized layer. In the present invention, the hard phase is martensite, tempered martensite, bainite and retained austenite.
[0065]
(E) Thickness
The thickness of the steel plate according to the present invention is not particularly limited. It is more preferably 0.2 to 1.5 mm, even more preferably 0.3 to 1.0 mm.
[0066]
(F) Steel plate manufacturing method
In general, according to the knowledge of ultra-low carbon steel such as IF steel, in order to strongly develop γ-fiber, which is advantageous for the r value, the cold rolling rate is increased to develop γ-fiber, Increases the accumulation in γ-fibers with crystals.
[0067]
However, in high-strength steels containing a relatively large amount of solute carbon, when the rolling reduction is high, shear banding occurs during cold rolling, and as a result, texture randomization or an orientation unfavorable to the r-value develops during annealing. However, the accumulation on the γ-fiber formed by cold rolling is impaired.
[0068]
In the present invention, in order to suppress the occurrence of shear bands, cold rolling is performed at a low rolling rate, and a preferable texture is formed in the hot-rolled sheet so that γ-fibers are accumulated even at a low rolling rate. . Specifically, the {223}<252> orientation is developed at the stage of the hot-rolled sheet, and the texture of the hot-rolled sheet is controlled so that γ-fibers are generated even at a low rolling rate.
[0069]
It is necessary to control the cold rolling conditions and the annealing conditions so that the texture accumulated in the γ-fiber by cold rolling is not damaged by annealing as much as possible, preferably so as to increase the accumulation.
[0070]
An example of the steel sheet manufacturing method according to the present invention will be described in detail below. A steel sheet according to the present invention can be obtained, for example, by a manufacturing method including the steps shown below.
[0071]

The steel slabs to be hot-rolled can be manufactured by the usual method. That is, a slab obtained by continuous casting or casting/blooming, or a steel plate obtained by strip casting can be used.
[0072]

Hot rolling is performed on the billet. In order to develop the {223}<252> orientation in the hot-rolled sheet, it is necessary to specify the conditions in the hot-rolling process together with the coiling process described later. Specifically, it is important to develop a rolling texture in the austenite before transformation and make the shape flat. The conditions in the hot rolling process are described in detail below.
[0073]
　Heating temperature: 1050-1300℃
The heating temperature before hot rolling is set to 1050°C or higher in order to dissolve Ti in the steel. On the other hand, considering the durability of the heating furnace, the heating temperature is preferably 1300° C. or lower.
[0074]
　Total reduction amount: The total reduction amount in the final 3 stages is 40% or more in thickness reduction rate
In order to develop the texture of the hot-rolled sheet, it is preferable that the total rolling amount in the last three consecutive stages including the final finishing rolling stand is 40% or more in terms of sheet thickness reduction rate. Further, the final three-stage rolling including at least the final stand is performed within a range of 100° C. from the finish rolling temperature.
[0075]
　Amount of reduction in each pass: Effective rolling index 1.2 or more
If austenite recrystallization progresses excessively between passes during hot rolling, the accumulation of the texture weakens and the crystal grains become equiaxed, making it impossible to obtain a hot-rolled sheet with the desired texture. That is, in order to develop the {223}<252> orientation in the hot-rolled sheet, it is preferable to develop an austenite texture.
[0076]
However, even if rolling is performed as described above, if the amount of reduction in a single stage is large, or if the amount of accumulated strain during multi-stage rolling is sufficient to cause recrystallization, the austenite becomes equiaxed. The texture is also weakened, and as a result, the texture of the ferrite after cooling is also weakened. As a result of further investigation, it was found that the combination for flattening the crystal grains while increasing the texture in the final three-stage rolling process can be determined by the following method.
[0077]
First, of the stands of the final three stages, focusing on the stand (F1) that performs rolling first, the relationship between the Ti content (mass%) contained in the steel, the rolling strain in F1, and the finish rolling temperature FT (° C.) From the relationship, dT eff-p is determined, which is a value related to the time for 50% of austenite to recrystallize after F1 rolling.
[0078]
[Number 1]

[0079]
[Number 2]

[0080]
[Number 3]

[0081]
Here, t ini is the plate thickness (mm) on the F1 entry side, and t F1 is the plate thickness (mm) after F1 rolling. Moreover, W Ti is the Ti content (mass %) contained in the steel.
[0082]
When dT eff-p is greater than 2.0, it is determined that recrystallization did not occur in F1, and from the accumulated rolling strain up to F2, it is a value related to the time for 50% austenite to recrystallize after F2 rolling. dT eff−s−a is obtained by the following formula (iv).
[0083]
[Number 4]

[0084]
Here, tF2 is the plate thickness (mm) after F2 rolling.
[0085]
On the other hand, when dT eff-p is 2.0 or less, it is determined that recrystallization has occurred in F1, and the value corresponding to dT eff-sa is obtained from the rolling strain of F2 alone by the following formula (v). Let eff-sb.
[0086]
[Number 5]

[0087]
Based on the value of dT eff-s−a or dT eff-s−b obtained above, it is determined whether or not recrystallization occurs after F2, and the effective rolling index is obtained as follows. That is, when dT eff-s-a is greater than 2.0, it is determined that recrystallization has not occurred in both F1 and F2, and the effective rolling index C eff-t-a is calculated from the cumulative rolling strain from F1 to F3 as follows. (vi) Obtained from the formula.
[0088]
[Number 6]

[0089]
where tF3 is the sheet thickness (mm) after F3 rolling.
[0090]
On the other hand, when dT eff−s−a is 2.0 or less, it is determined that recrystallization has occurred between F2 and F3 due to the accumulated strain up to F2, and the effective rolling index C eff−t from the rolling strain of F3 alone -b is obtained from the following formula (vii).
[0091]
[Number 7]

[0092]
Furthermore, when dT eff-sb is greater than 2.0, the effective rolling index C eff-tc is obtained from the cumulative rolling strains of F2 and F3 using the following formula (viii).
[0093]
[Number 8]

[0094]
On the other hand, when dT eff-sb is 2.0 or less, it is determined that recrystallization has occurred after F2 rolling, and the effective rolling index C eff-tb is calculated by the above formula (vii).
[0095]
When the steel contains one or more selected from Ti, Nb, V, Mo, W and Zr, the K value is determined by the following formula (ix) instead of the above formula (ii).
[0096]
[Number 9]

[0097]
Here, W Mo, W V, W W, W Zr, and W Nb are the contents (% by mass) of Mo, V, W, Zr, and Nb contained in the steel, and are 0 when not contained. substitute.
[0098]
By controlling the hot rolling so that the effective rolling index obtained by the above procedure is 1.2 or more, it is possible to effectively obtain flat austenite grains with a sufficiently developed texture in the rolling direction. It is preferable to control hot rolling conditions so that the effective rolling index is 10.0 or more.
[0099]
Finish rolling temperature: 800-1000°C
　If the finish rolling temperature exceeds 1000°C, the surface quality may deteriorate due to scale defects. Therefore, the finish rolling temperature of hot rolling is set to 1000° C. or lower, preferably 980° C. or lower. On the other hand, if the finish rolling temperature is less than 800°C, the productivity may be impaired due to an increase in rolling load. Therefore, the finish rolling temperature of hot rolling is set to 800° C. or higher, preferably 850° C. or higher.
[0100]
Cooling start time: more than 0.5s and 2.0s or less
After rolling is completed, cooling is performed before the recrystallization of austenite is completed. Therefore, the time from the end of final rolling to the start of cooling is set to 2.0 seconds or less. On the other hand, if the cooling start time is excessively short, ferrite is generated with the shear bands formed in austenite by hot rolling as nuclei, so that the metal structure of the obtained hot-rolled sheet is mainly ferrite and mainly bainite. I can't do it. Therefore, the cooling start time should be longer than 0.5 s. Here, cooling means accelerated cooling by water cooling or the like.
[0101]
Cooling rate: 15°C/s or more
In order to develop {223}<252> orientation, it is important to undergo bainite transformation or martensite transformation. Therefore, the cooling rate after rolling should be 15° C./s or more, preferably 30° C./s or more. The cooling rate after rolling means the average cooling rate obtained by dividing the difference between the temperature at the start of cooling after the end of the final rolling and the following coiling temperature by the time required during that time.
[0102]

Winding temperature: 300°C or higher and less than 600°C
　In order to develop the {223}<252> orientation, it is necessary to undergo bainite transformation or martensite transformation, so the coil is coiled at a temperature of less than 600°C. On the other hand, if the coiling temperature is too low, the cold rolling load increases, which may hinder productivity, so the lower limit is made 300°C. Considering the temperature controllability of cooling after hot rolling, the temperature is preferably 480° C. or higher. From the viewpoint of reducing the load during cold rolling, the temperature is more preferably 500° C. or higher.
[0103]
In the texture of the hot-rolled sheet thus obtained, the integrated intensity of the {223}<252> orientation is 5.0 times or more in terms of the random intensity ratio. Under preferable conditions, it is possible to obtain an integrated intensity of 6.0 times or more in terms of random intensity ratio.
[0104]
In addition, the metal structure of the hot-rolled sheet is mainly bainite. Martensite and/or retained austenite may be mixed in part of the metallographic structure, but if the total area ratio is 30% or less, the cold rolling property is not greatly impaired, and therefore it is allowed.
[0105]

　Hot-rolled sheet annealing temperature: 600℃ or less
For the purpose of reducing the cold rolling load due to equipment restrictions, the hot rolled sheet may be heat treated. If the α-γ transformation does not occur, the texture of the hot-rolled sheet is not significantly destroyed, but in view of the increase in annealing cost, the hot-rolled sheet annealing temperature is preferably 600° C. or less.
[0106]

　Cold rolling rate: 40-85%
By adding cold rolling after hot rolling, the accumulation in γ-fiber, which is advantageous for improving the r45 value, is increased. That is, cold rolling is performed at a cold rolling rate of 40% or more in order to rotate the {223}<252> orientation of the hot-rolled sheet to γ-fiber. The cold rolling rate is preferably 50% or more. On the other hand, if the cold rolling reduction is excessively high, discontinuous recrystallization becomes active and randomization of texture occurs due to shear bands, so the cold rolling reduction is set to 85% or less. Considering the decrease in productivity due to the increased load during cold rolling, the cold rolling rate is preferably 80% or less. More preferably, it is 75% or less.
[0107]

As mentioned above, when discontinuous recrystallization occurs, randomization of the texture tends to occur. Therefore, the annealing conditions are controlled so that discontinuous recrystallization does not occur excessively, continuous recrystallization is promoted, and the γ-fiber formed by cold rolling is taken over after annealing, and phase transformation is used to accumulate it on the γ-fiber. increase Each condition is described in detail below.
[0108]
　Heating rate: 2°C/s or more
If the heating rate is slow, discontinuous recrystallization of ferrite will proceed remarkably during heating, making it difficult for it to accumulate in γ-fiber. Therefore, the heating rate is set to 2° C./s or more. Although the upper limit of the heating rate is not particularly specified, it is preferably 30° C./s or less in view of temperature controllability. The above heating rate means an average heating rate obtained by dividing the difference between the temperature (room temperature) at the start of heating and the annealing temperature described below by the time required during that time.
[0109]
Annealing temperature: conditions that satisfy the following formula (x)
The annealing temperature (RHT) is a condition that satisfies the following formula (x). Here, Ac 1 is the temperature at which the transformation from ferrite to austenite (α-γ transformation) starts, and Ac 3 is the temperature at which the α-γ transformation is completed and becomes an austenite single phase. It is calculated by the formula and formula (xii).
[0110]
[Number 10]

[0111]
[Number 11]

[0112]
[number 12]

[0113]
When the RHT is low and the value of (RHT-Ac 1)/(Ac 3-Ac 1) is below the left-hand side value of equation (x), the amount of austenite to reverse transform is not sufficient, resulting in high strain around carbides or hard phases. It is difficult to obtain the effect of austenite erosion of discontinuous recrystallized ferrite with random orientation that inevitably occurs in the region. In addition, the rolling strain introduced by cold rolling remains in the structure and impairs ultimate deformability and strength-ductility balance.
[0114]
Also, when the RHT is high and the value of (RHT-Ac 1)/(Ac 3-Ac 1) exceeds the value on the right side of the formula (x), the amount of reverse-transformed austenite becomes excessive and new ferrite nucleation occurs during the cooling process. It becomes prominent, the texture is randomized, and the accumulation in γ-fibers gradually decreases. Therefore, the value of (RHT-Ac 1)/(Ac 3-Ac 1) is determined to be 0.40 to 0.85.
[0115]
If you want to further strengthen the γ-fiber and obtain a particularly good r 45 value, the value of (RHT-Ac 1)/(Ac 3-Ac 1) can be in the range of 0.50 to 0.80. Preferably, it is more preferably in the range of 0.60 to 0.75.
[0116]
Annealing atmosphere
Annealing is preferably carried out in a reducing atmosphere with a hydrogen concentration of 2% or more in volume fraction and a dew point of less than -30°C. If the hydrogen concentration is less than 2%, the surface oxide film of the material steel sheet cannot be sufficiently reduced, impairing the wettability of the hot-dip galvanizing process. The annealing furnace includes a heating zone where the steel sheet is heated to a predetermined annealing temperature and a soaking zone where the steel sheet is held at that annealing temperature.
[0117]
When it is desired to form the above-mentioned decarburized layer on the surface layer of the steel sheet, the atmosphere in the heating zone in the heating step should be such that the hydrogen concentration is 20% or less in volume fraction and the dew point is -30°C or higher and 20°C or lower. If the dew point is less than −30° C., the thickness of the decarburized layer is less than 5 μm, and a sufficient effect of improving bending properties cannot be obtained. On the other hand, if the dew point exceeds 20° C., dew condensation will occur in the equipment, impairing productivity. Further, when the hydrogen concentration exceeds 20% by volume, it becomes difficult to maintain the dew point at 20° C. or less, causing dew condensation in the equipment as described above, which impairs productivity.
[0118]
Annealing retention time: 30s or more
If the annealing holding time is short, the α-γ transformation does not proceed sufficiently, and cold-rolling strain remains in the ferrite, impairing the strength-ductility balance. Therefore, the annealing holding time should be 30 seconds or more, preferably 60 seconds or more. On the other hand, although the upper limit of the annealing holding time is not particularly specified, it is preferably 1000 seconds or less, more preferably 800 seconds or less, in view of productivity.
[0119]
　Cooling conditions after annealing
In cooling after annealing, primary cooling is performed to adjust the ferrite area ratio and texture, and secondary cooling is performed to form a hard phase. Furthermore, in the primary cooling, the ferrite grains having the γ-fiber orientation inherited from the cold-rolled sheet by continuous recrystallization can be transformed and grown toward the adjacent austenite side to increase the accumulation of γ-fibers in the entire structure.
[0120]
　Primary cooling rate: 3 to 30°C/s
　Ferrite is precipitated by primary cooling. If the average cooling rate of primary cooling (primary cooling rate) is too low, austenite is transformed into pearlite, resulting in a loss of strength. Therefore, the primary cooling rate is set to 3° C./s or more, preferably 5° C./s or more. On the other hand, if the primary cooling rate is too high, transformation accompanied by new nucleation occurs, resulting in insufficient accumulation in the γ-fiber due to grain growth. Therefore, the primary cooling rate is set to 30° C./s or less.
[0121]
Although the primary cooling end temperature is not particularly limited, if it is too low, ferrite will be excessive, the ratio of the hard phase will decrease, and the strength will decrease.
[0122]
　Secondary cooling rate: 30°C/s or more
Secondary cooling is performed following primary cooling. If the average cooling rate of the secondary cooling (secondary cooling rate) is too low, the untransformed austenite undergoes pearlite transformation, the hard phase decreases, and the strength decreases. Therefore, the secondary cooling rate is set to 30° C./s or higher. The upper limit of the secondary cooling rate is not particularly limited, but since it is difficult to achieve a cooling rate exceeding 250°C/s with ordinary equipment, it is preferably 250°C/s or less.

WE CLAIM:

The scope of the claims
[Claim 1]
The chemical composition, in mass%,
C: 0.03-0.25%,
Si: 0.1 to 2.0%,
Mn: 1.0 to 3.0%,
P: 0.200% or less,
S: 0.0500% or less,
Al: 0.01 to 1.00%,
N: 0.0100% or less,
Ti: 0.01 to 0.25%,
The balance: Fe and impurities,
The metal structure, in area%,
　Ferrite: Contains 50 to 85%,
the balance is one or more selected from martensite, bainite and retained austenite,
　The integrated intensity of γ-fiber exceeds 4.0 times as a random intensity ratio, and the average KAM value in crystal grains having a crystal orientation within 10° from the γ-fiber is 1.30° or less.
Steel plate.
[Claim 2]
　The chemical composition is replaced by a part of the Fe, in mass%,
Cr: 0.50% or less,
Ni: 0.50% or less, and
Cu: 0.50% or less,
containing one or more selected from
The steel plate according to claim 1.
[Claim 3]
　The chemical composition is replaced by a part of the Fe, in mass%,
Nb: 0.050% or less,
V: 0.15% or less,
Zr: 0.15% or less,
Mo: 0.15% or less, and
W: 0.15% or less,
containing one or more selected from
The steel plate according to claim 1 or claim 2.
[Claim 4]
　The chemical composition is replaced by a part of the Fe, in mass%,
Contains 0.100% or less in total of one or more selected from Sn, Sb and Te,
The steel plate according to any one of claims 1 to 3.
[Claim 5]
　The chemical composition is replaced by a part of the Fe, in mass%,
containing 0.0050% or less in total of one or more selected from Ca, Mg and REM,
The steel plate according to any one of claims 1 to 4.
[Claim 6]
　The chemical composition is replaced by a part of the Fe, in mass%,
B: 0.0050% or less,
containing
The steel plate according to any one of claims 1 to 5.
[Claim 7]
　It has a decarburized layer with a thickness of 4.0 μm or more in the depth direction from the surface,
The steel plate according to any one of claims 1 to 6.