Cold-rolled steel sheet and its manufacturing method

Abstract

Provided is a cold-rolled steel sheet having excellent processability. This cold-rolled steel sheet has a chemical composition containing 0.0050% or less of C, etc., in mass%, and has a metal structure in which the volume percentage of ferrite is 95% or more and the average crystalline particle diameter of ferrite is 25 μm or more. The value of mean-r represented by formula (2) is 1.8 or more, and the absolute value of Δr represented by formula (3) is 0.4 or less. The area percentage of crystal grains having a crystal orientation in which [1 1 1] is within 15° from the normal line direction of the plate surface and in which [0 1 -1] is within 15° from the rolling direction is 20% or more. The area percentage of crystal grains having a crystal orientation in which [1 1 1] is within 15° from the normal line direction of the plate surface and in which [1 1 -2] is within 15° from the rolling direction is 20% or more.　(2): mean - r = (RL+2×RD+RC) / 4 (3): Δr = (RL-2×RD+RC) / 2

Description

[Technical field]

[0001] The present invention relates to a cold-rolled steel sheet and a method for producing the same. [Background technology]

[0002] Cold rolled steel sheets used for automobile exterior panels and the like are required to have excellent formability.

[0003] Japanese Patent Laid-Open Publication No. 10-287928 discloses a method for producing a cold-rolled steel sheet for deep drawing, in which, when performing continuous annealing on a cold-rolled steel sheet, the average heating rate is set to 5-30°C / sec up to a predetermined temperature of 650-750°C, and from that temperature, a rapid heating device is subsequently used to raise the temperature to a predetermined temperature of 750-910°C at an average heating rate of 60-500°C / sec, and then the average cooling rate to 750°C is set to 50°C / sec or less.

[0004] In Japanese Patent No. 6631762, a steel having a specified chemical composition and a steel structure in which the average aspect ratio of ferrite grains is 2.0 or less is described, and the r value is the average value when the r value is measured at three points spaced 200 mm apart in the sheet width direction. ave The value is 1.20 or more, and the absolute value of the in-plane anisotropy of the r value, |Δr|, is the average value of measurements taken at three points 200 mm apart in the sheet width direction. ave | is 0.40 or less, the difference between the maximum and minimum values ​​of |Δr| at the three locations is 0.15 or less, and the difference between the maximum and minimum values ​​of yield strength (ΔYP) at the three locations is 13 MPa or less.

[0005] The publication also discloses a method for producing a cold-rolled steel sheet, which includes an annealing step in which the cold-rolled steel sheet obtained in the cold rolling step is heated in a range of 350 to 650°C at an average heating rate of 35°C / second or more in a continuous annealing facility equipped with a preheating zone, a heating zone, a soaking zone, and a cooling zone, soaked at a soaking temperature of 700 to 900°C for a soaking time of 1 to 200 seconds, and bent and unbent at the soaking temperature a total of four times or more using rolls with a radius of 100 mm or more.

[0006] Japanese Patent No. 2660639 discloses a method for producing a galvannealed cold rolled steel sheet with excellent deep drawability, which comprises cold rolling by a conventional method, followed by recrystallization annealing at a heating rate of 300°C / sec or more, heating to a temperature range of 750 to 950°C, and then cooling without holding, immediately followed by hot-dip galvanizing, further alloying, cooling, and further temper rolling.

[0007] Japanese Patent Laid-Open Publication No. 7-97635 discloses an efficient method for producing cold-rolled steel sheets, in which coils having different materials and cross-sectional shapes are welded and continuously annealed, in which an induction or electric heating device is installed in a part of a continuous annealing furnace which heats by radiation heat, and the annealing temperature is partially increased at a heating rate of 100° C. / sec or more, thereby changing the annealing temperature to a predetermined annealing temperature in a short time. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 10-287928 [Patent Document 2] Patent No. 6631762 [Patent Document 3] Patent No. 2660639 [Patent Document 4] Japanese Patent Application Publication No. 7-97635 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a cold-rolled steel sheet having excellent workability. [Means for solving the problem]

[0010] The cold rolled steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.0050% or less, Si: 1.00% or less, Mn: 0.01 to 2.00%, P: 0.100% or less, S: 0.020% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, V: 0 to 0.50%, Mo: 0 to 0.20%, W: 0 to 0.20%, B: 0 to 0.0050%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, and REM: 0 to 0.0100%. and further contains at least one of Ti and Nb so as to satisfy the following formula (1), with the remainder being Fe and impurities; the metal structure has a volume fraction of ferrite of 95% or more and an average crystal grain size of the ferrite of 25 μm or more; the mean-r represented by the following formula (2) is 1.8 or more; the absolute value of Δr represented by the following formula (3) is 0.4 or less; the area fraction of crystal grains having a crystal orientation of

[0111] within 15° from the normal direction to the sheet surface and [0 1 -1] within 15° from the rolling direction is 20% or more; and the area fraction of crystal grains having a crystal orientation of

[0111] within 15° from the normal direction to the sheet surface and [1 1 -2] within 15° from the rolling direction is 20% or more. 0.8×(C / 12+N / 14) <Ti / 48+Nb / 93+0.0001···(1) mean-r=(R L +2×R D +R C ) / 4···(2) Δr = (R L -2×R D +R C ) / 2···(3) The element symbols in formula (1) are substituted with the contents of the corresponding elements in mass %. R in formula (2) and formula (3) L , R D , and R C are the r-values ​​in a direction parallel to the rolling direction of the cold-rolled steel sheet, a direction at an angle of 45° from the rolling direction, and a direction at an angle of 90° from the rolling direction, respectively.

[0011] A manufacturing method according to one embodiment of the present invention is a method for manufacturing the above-mentioned cold-rolled steel sheet, and includes the steps of cold-rolling the steel sheet so that the sheet thickness reduction rate is 30 to 80%, heating the cold-rolled steel sheet to a holding temperature of 750 to 900°C, holding the heated steel sheet at the holding temperature for 40 seconds or more, and cooling the steel sheet held at the holding temperature, wherein in the heating step, the average heating rate in the temperature range from 500°C to 600°C is 8°C / sec or less, and the average heating rate in the temperature range from 720°C to the holding temperature is 100°C / sec or more. Effect of the Invention

[0012] According to the present invention, a cold-rolled steel sheet having excellent workability can be obtained. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing a schematic diagram of the relationship between the external shape and the structure of a cold-rolled steel sheet. [Diagram 2] FIG. 2 is a schematic diagram showing various crystal orientations rotated around the

[0111] axis while satisfying the

[0111] / / ND relationship. [Diagram 3] FIG. 3 is a schematic diagram showing a structure in which the ratio of crystal grains whose crystal orientation is (1 1 1) [0 1 -1] to crystal grains whose crystal orientation is (1 1 1) [1 1 -2] is 50:50. [Figure 4] FIG. 4 is a schematic diagram showing a structure in which the ratio of crystal grains whose crystal orientation is (1 1 1) [0 1 -1] to crystal grains whose crystal orientation is (1 1 1) [1 1 -2] is 30:70. [Diagram 5] FIG. 5 is a flow diagram showing a method for producing a cold-rolled steel sheet according to an embodiment of the present invention. [Figure 6] FIG. 6 is a diagram showing a schematic diagram of the generation of crystal grains in the heating step. [Figure 7] Figure 7 shows an example of an EBSD analysis in which the proportion of crystal grains with a (1 1 1) [0 1 -1] crystal orientation and the proportion of crystal grains with a (1 1 1) [1 1 -2] crystal orientation are approximately the same. [Figure 8] Figure 8 shows an example of an analysis using EBSD in which there is a bias in the ratio of crystal grains whose crystal orientation is (1 1 1) [0 1 -1] and crystal grains whose crystal orientation is (1 1 1) [1 1 -2]. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The present inventors have conducted various studies on the workability, particularly the deep drawability, of cold-rolled steel sheets and have obtained the following findings.

[0015] In steel of a given chemical composition, the deep drawability can be improved by increasing the average grain size of ferrite. More specifically, by increasing the average grain size of ferrite, the average r-value can be increased and the in-plane anisotropy of the r-value can be reduced.

[0016] It has been known that the more crystal grains with the

[0111] crystal orientation parallel to the normal direction (ND) of the plate surface (i.e., the {1 1 1} plane parallel to the plate surface), the higher the r-value. On the other hand, since the crystal orientation has three-dimensional degrees of freedom, the crystal orientation can take various directions even if the

[0111] / / ND relationship is satisfied. Specifically, even after one axis of a cube is determined, various crystal orientations can be taken by rotating around that axis. Usually, among the various crystal orientations rotated around the

[0111] axis, crystal grains with a specific crystal orientation develop preferentially.

[0017]

[0111] / / By satisfying the N-D relationship and equalizing the proportion of crystal grains with various crystal orientations rotated around the

[0111] axis, it is possible to increase the average r-value while reducing the in-plane anisotropy of the r-value.

[0018] When annealing steel sheets, they are slowly heated in the temperature range of 500°C to 600°C, rapidly heated in the temperature range from 720°C to the holding temperature, and then cooled after being held at the holding temperature for a specified time. This results in a structure with an average ferrite grain size of 25μm or more, satisfying the

[0111] / / ND relationship, and with crystal grains of various crystal orientations rotated around the

[0111] axis.

[0019] The present invention has been completed based on the above findings. Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals and their description will not be repeated. The dimensional ratios between the components shown in each drawing do not necessarily represent the actual dimensional ratios.

[0020] [Cold rolled steel plate] [Chemical composition] The cold-rolled steel sheet according to an embodiment of the present invention has a chemical composition as described below. In the following description, "%" for the content of an element means mass%.

[0021] C: 0.0050% or less Carbon (C) dissolves in steel and reduces the formability of the steel. C also has an adverse effect on the control of crystal orientation and reduces the r-value. Therefore, the C content is 0.0050% or less. The upper limit of the C content is preferably 0.0040%, and more preferably 0.0035%. From the viewpoint of performance, the lower the C content, the better, but excessive reduction in the C content increases the manufacturing cost. The lower limit of the C content is preferably 0.0005%, and more preferably 0.0010%.

[0022] Si: 1.00% or less Silicon (Si) dissolves in steel and reduces the formability of the steel. Si also reduces the weldability of the steel. Therefore, the Si content is 1.00% or less. The upper limit of the Si content is preferably 0.50%, and more preferably 0.35%. The lower limit of the Si content is preferably 0.01%.

[0023] Mn: 0.01 to 2.00% Manganese (Mn) dissolves in steel and reduces the formability of the steel. On the other hand, if the Mn content is excessively reduced, hot brittle cracking may occur during hot rolling. Therefore, the Mn content is 0.01 to 2.00%. The lower limit of the Mn content is preferably 0.05%. The upper limit of the Mn content is preferably 1.50%, more preferably 1.00%, and even more preferably 0.80%.

[0024] P:0.100% or less Phosphorus (P) is an impurity. P segregates at grain boundaries and reduces the formability of steel. Therefore, the P content is 0.100% or less. The P content is preferably 0.050% or less, and more preferably 0.030% or less.

[0025] S: 0.020% or less Sulfur (S) is an impurity. S reduces the hot workability of steel. Therefore, the S content is 0.020% or less. The S content is preferably 0.015% or less, and more preferably 0.010% or less.

[0026] Al: 0.010 to 0.100% Aluminum (Al) is contained as a deoxidizing agent. On the other hand, if the Al content is too high, inclusions are generated and the formability of the steel is reduced. Therefore, the Al content is 0.010 to 0.100%. The lower limit of the Al content is preferably 0.015%, and more preferably 0.020%. The upper limit of the Al content is preferably 0.080%, and more preferably 0.060%.

[0027] N: 0.0100% or less Nitrogen (N) dissolves in steel and reduces the formability of the steel. Therefore, the N content is 0.0100% or less. The upper limit of the N content is preferably 0.0060%, and more preferably 0.0040%. From the viewpoint of performance, the lower the N content, the better, but excessive reduction in the N content increases the manufacturing cost. The lower limit of the N content is preferably 0.0005%, and more preferably 0.0010%.

[0028] At least one of Ti and Nb: an amount satisfying the following formula (1) 0.8×(C / 12+N / 14) <Ti / 48+Nb / 93+0.0001···(1) Titanium (Ti) and Nb (niobium) combine with C and N in steel to reduce the amount of dissolved C and N, thereby improving the formability of steel. Ti and Nb are contained in amounts according to the amounts of C and N. Specifically, at least one of Ti and Nb is contained in an amount that satisfies the above formula (1). The element symbols in formula (1) are substituted with the contents of the corresponding elements in mass%. On the other hand, even if Ti and Nb are contained in excess, the effect is saturated. The upper limit of the total content of Ti and Nb is preferably 0.080%, more preferably 0.060%.

[0029] The chemical composition of the cold-rolled steel sheet according to the present embodiment may contain V, Mo, W, B, Ca, Mg, and REM. All of these elements are optional. That is, the chemical composition of the cold-rolled steel sheet according to the present embodiment may not contain part or all of V, Mo, W, B, Ca, Mg, and REM.

[0030] V: 0~0.50% Vanadium (V), like Ti and Nb, fixes C and N in steel and improves the formability of steel. This effect can be obtained even if even a small amount of V is contained. On the other hand, if the V content is excessive, the steel becomes hard and the formability decreases. Therefore, the V content is 0 to 0.50%. The lower limit of the V content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the V content is preferably 0.40%, and even more preferably 0.30%.

[0031] Mo: 0 to 0.20% W: 0~0.20% Both molybdenum (Mo) and tungsten (W) improve the corrosion resistance of steel. This effect can be obtained even if even a small amount of Mo and W is contained. On the other hand, if the content of Mo and W is excessive, the steel becomes hard and the formability decreases. Therefore, the content of each of Mo and W is 0 to 0.20%. The lower limit of the content of Mo and W is preferably 0.01%, more preferably 0.03%, and further preferably 0.05%. The upper limit of the content of Mo and W is preferably 0.18%.

[0032] B: 0 to 0.0050% Boron (B) strengthens grain boundaries and improves the formability of steel. This effect can be obtained even if even a small amount of B is contained. On the other hand, if the B content is excessive, the steel becomes hard and the formability decreases. Therefore, the B content is 0 to 0.0050%. The lower limit of the B content is preferably 0.0003%. The upper limit of the B content is preferably 0.0040%, and more preferably 0.0030%.

[0033] Ca: 0 to 0.0100% Magnesium: 0 to 0.0100% REM: 0~0.0100% Calcium (Ca), magnesium (Mg), and rare earth metals (REM) all improve the hot workability of steel. This effect can be obtained even if even a small amount of Ca, Mg, and REM is contained. On the other hand, if the contents of Ca, Mg, and REM are excessive, inclusions are generated and the formability of steel is reduced. Therefore, the contents of Ca, Mg, and REM are each 0 to 0.0100%. The lower limit of the contents of Ca, Mg, and REM is preferably 0.0001%. The upper limit of the contents of Ca, Mg, and REM is preferably 0.0040%, and more preferably 0.0030%. Note that REM is a collective term for a total of 17 elements, including Sc, Y, and lanthanoids, and the content of REM means the total amount of the above elements.

[0034] The balance of the chemical composition of the cold-rolled steel sheet according to the present embodiment is Fe and impurities. The impurities referred to here are elements that are mixed in from ores or scraps used as raw materials for steel, or elements that are mixed in from the environment during the manufacturing process. The impurities are not limited to these, but examples thereof include Cu, Ni, Cr, and O.

[0035] [Metal structure] The metal structure of the cold rolled steel sheet according to the present embodiment is mainly composed of ferrite, and the volume fraction of ferrite is 95% or more, and preferably 98% or more.

[0036] In the metal structure of the cold-rolled steel sheet according to the present embodiment, the average grain size of ferrite is 25 μm or more. By increasing the average grain size of ferrite, the average r-value can be increased and the in-plane anisotropy of the r-value can be reduced. The average grain size of ferrite is preferably 26 μm or more, more preferably 30 μm or more.

[0037] The metal structure of the cold-rolled steel sheet according to this embodiment has an area ratio of crystal grains whose crystal orientation

[0111] is within 15° from the normal direction of the sheet surface and whose [0 1 -1] is within 15° from the rolling direction of 20% or more, and an area ratio of crystal grains whose crystal orientation

[0111] is within 15° from the normal direction of the sheet surface and whose [1 1 -2] is within 15° from the rolling direction of 20% or more.

[0038] Fig. 1 is a schematic diagram showing the relationship between the external shape and the structure of a cold-rolled steel sheet. It has been known that the more crystal grains with the crystal orientation

[0111] parallel to the normal direction (ND) of the sheet surface (i.e., the {1 1 1} plane parallel to the sheet surface), the higher the r-value.

[0039] On the other hand, since the crystal orientation has three-dimensional degrees of freedom, the crystal orientation can take various orientations even if the

[0111] / / ND relationship is satisfied. Figure 2 is a schematic diagram showing various crystal orientations rotated around the

[0111] axis while satisfying the

[0111] / / ND relationship. Usually, among the various crystal orientations rotated around the

[0111] axis, crystal grains of a specific crystal orientation develop preferentially.

[0040] By equalizing the ratio of crystal grains with various crystal orientations rotated around the

[0111] axis while satisfying the relationship of

[0111] / / ND, the average r-value can be increased while reducing the in-plane anisotropy of the r-value. In this embodiment, the area ratio of crystal grains with a crystal orientation of (1 1 1) [0 1 -1] and the area ratio of crystal grains with a crystal orientation of (1 1 1) [1 1 -2] are used as indicators of whether the ratio of such crystal grains is uniform. Here, the crystal orientation parallel to the normal direction (ND) of the sheet surface is represented as (hkl), and the crystal orientation parallel to the rolling direction (RD) is represented as [uvw].

[0041] Figures 3 and 4 are schematic diagrams showing the structure of a cold-rolled steel sheet. Figure 3 shows a case where the ratio of crystal grains with a crystal orientation of (1 1 1) [0 1 -1] to crystal grains with a crystal orientation of (1 1 1) [1 1 -2] is 50:50, while Figure 4 shows a case where this ratio is 30:70. In the structure of Figure 3, the conditions are the same regardless of the direction in which the steel sheet is deformed, so the in-plane anisotropy of the r-value is smaller than that of the structure of Figure 4.

[0042] 3 and 4, for the sake of simplicity, a cold-rolled steel sheet is described as being composed of crystal grains with two types of crystal orientations, but in an actual cold-rolled steel sheet, crystal grains with other crystal orientations are also present. If the area ratio of crystal grains with a crystal orientation of (1 1 1) [0 1 -1] and the area ratio of crystal grains with a crystal orientation of (1 1 1) [1 1 -2] are each 20% or more, it can be considered that crystal grains with various crystal orientations that satisfy the relationship of

[0111] / / ND are more evenly present than when they are not.

[0043] [Mechanical properties] The cold-rolled steel sheet according to this embodiment has an average r-value (mean-r) represented by the following formula (2) of 1.8 or more, and an absolute value of the in-plane anisotropy Δr of the r-value represented by the following formula (3) of 0.4 or less. mean-r=(R L +2×R D +R C ) / 4···(2) Δr = (R L -2×R D +R C ) / 2···(3) R in formula (2) and formula (3) L , R D , and R C are the r-values ​​in the direction parallel to the rolling direction of the cold-rolled steel sheet, the direction at an angle of 45° from the rolling direction, and the direction at an angle of 90° from the rolling direction, respectively.

[0044] The r-value (Lankford r-value) is a general indicator of the workability of thin plates, and materials with a high r-value have excellent deep drawability. The r-value is measured as follows:

[0045] Tensile test pieces are taken from the cold-rolled steel plate with the longitudinal direction parallel to the rolling direction (L direction), at 45° from the rolling direction (D direction), and at 90° from the rolling direction (C direction, i.e., plate width direction). The tensile test is interrupted (i.e., the load is removed) when these test pieces are pulled to EL = 15%. The grade distance and plate width of the pulled test pieces are measured. If the grade distance before pulling is L and the plate width is W, and the grade distance after pulling is l and the plate width is w, the r-value can be calculated by the following formula. r=-ln(w / W) / (ln(l / L)+ln(w / W)) The r values ​​measured on the specimens in the L, D, and C directions are R L , R D , and R C It is.

[0046] The average r value (mean-r) is preferably 1.9 or more, and more preferably 2.0 or more. The absolute value of the in-plane anisotropy Δr of the r value is preferably 0.35 or less, and more preferably 0.30 or less.

[0047] The cold rolled steel sheet according to the present embodiment preferably has a tensile strength of 270 to 340 MPa. The upper limit of the tensile strength is more preferably 300 MPa.

[0048] The cold rolled steel sheet according to the present embodiment preferably has a uniform elongation of 30% or more and a breaking elongation of 50% or more.

[0049] The cold-rolled steel sheet according to the present embodiment preferably has a work-hardening rate (n value) at EL=10% of 0.30 or more, more preferably 0.32 or more, and further preferably 0.35 or more.

[0050] [Method of manufacturing cold-rolled steel sheets] Next, an example of a method for producing the above-mentioned cold-rolled steel sheet will be described. The manufacturing method described below is merely an example, and does not limit the above-mentioned method for producing the cold-rolled steel sheet.

[0051] 5 is a flow diagram showing a method for producing a cold-rolled steel sheet according to an embodiment of the present invention. This method includes a step of preparing a hot-rolled steel sheet (step S1), a step of pickling the hot-rolled steel sheet (step S2), a step of cold-rolling the pickled steel sheet (step S3), and a step of annealing the cold-rolled steel sheet (step S4).

[0052] A hot-rolled steel sheet is prepared (Step S1). The hot-rolled steel sheet can be produced, for example, by hot-rolling a slab having the above-mentioned chemical composition by a normal method and coiling it. The hot-rolling reduction ratio, the plate thickness after rolling, the cooling method to room temperature, the coiling conditions, etc. are not particularly limited.

[0053] The hot-rolled steel sheet is pickled under normal conditions (step S2).

[0054] The pickled steel sheet is cold-rolled (step S3). The sheet thickness reduction rate in cold rolling (hereinafter referred to as "cold rolling rate") is 30 to 80%. If the cold rolling rate is less than 30%, the recrystallization of ferrite does not proceed in the annealing step (step S4), and a desired structure cannot be obtained. If the cold rolling rate exceeds 80%, the manufacturing cost increases. The lower limit of the cold rolling rate is preferably 40%, and more preferably 60%.

[0055] The cold-rolled steel sheet is annealed (step S4). The annealing step (step S4) includes a step of heating the cold-rolled steel sheet to a predetermined holding temperature (step S4-1), a step of holding the heated steel sheet at the holding temperature for a predetermined holding time (step S4-2), and a step of cooling the steel sheet held at the holding temperature (step S4-3).

[0056] The cold-rolled steel sheet is heated to a holding temperature T1 (step S4-1). The holding temperature T1 is 750 to 900°C. If the holding temperature T1 is less than 750°C, the recrystallization of ferrite does not proceed, and a desired structure cannot be obtained. If the holding temperature T1 exceeds 900°C, there is a possibility that the steel will transform into austenite during annealing, and the crystal orientation advantageous for the r-value decreases. The lower limit of the holding temperature T1 is preferably 760°C, and more preferably 770°C. The upper limit of the holding temperature T1 is preferably 880°C, and more preferably 860°C.

[0057] In the heating step (step S4-1), the average heating rate in the temperature range from 500° C. to 600° C. is set to 8° C. / sec or less, and the average heating rate in the temperature range from 720° C. to the holding temperature T1 is set to 100° C. / sec or more.

[0058] Figure 6 is a diagram showing the generation of crystal grains in the heating process (step S4-1). By slowly heating the steel sheet in the temperature range of 500°C to 600°C, dislocations contained in the steel sheet are restored, and a large number of latent nuclei for recrystallization can be generated in parts of the structure. Here, latent nuclei are tiny grains of submicron size with few dislocations, and many of them have a crystal orientation of

[0111] / / ND.

[0059] By rapidly heating the temperature range from 720°C, when the latent nuclei of recrystallization have fully grown, to the holding temperature T1, the latent nuclei grow rapidly. This results in coarse grains with a crystal orientation of

[0111] / / ND. Furthermore, these grains have various crystal orientations rotated around the

[0111] axis while still satisfying the

[0111] / / ND relationship. This makes it possible to reduce the in-plane anisotropy.

[0060] In contrast, when the heating rate in the temperature range from 720°C to the holding temperature T1 is small, latent nuclei with crystal orientations other than

[0111] / / ND are generated in this temperature range, and the number of crystal grains favorable for the r value decreases.

[0061] The average heating rate in the temperature range from 500° C. to 600° C. is preferably 6° C. / sec or less, more preferably 5° C. / sec or less. The average heating rate in the temperature range from 720° C. to the holding temperature T1 is preferably 300° C. / sec or more, more preferably 400° C. / sec or more.

[0062] The heating rate in the temperature range from room temperature to 500° C. and the heating rate in the temperature range from 600° C. to 720° C. are arbitrary. The heating rate in these temperature ranges may be, for example, 0.1° C. / sec to 1000° C. / sec, and the heating rate may be switched midway.

[0063] As a specific method for implementing the above heating, for example, the heating method may be switched at a predetermined temperature Tc between 600° C. and 720° C. In other words, the steel sheet may be heated by a heating method with a low heating rate (e.g., radiation heating using a radiant tube) in the temperature range from room temperature to temperature Tc, and the steel sheet may be heated by a heating method with a high heating rate (e.g., high-frequency induction heating) in the temperature range from temperature Tc to holding temperature T1.

[0064] After being heated to the holding temperature T1, the plate is held at the holding temperature T1 for a predetermined time (step S4-2). Specifically, the plate may be passed through a soaking furnace (e.g., a high-temperature furnace using a radiant tube) at the holding temperature T1 and allowed to dwell at the holding temperature T1 for a predetermined time.

[0065] The retention time (residence time) is 40 seconds or more. If the retention time is too short, the crystal grains may not grow sufficiently. The retention time (residence time) is preferably 45 seconds or more, more preferably 50 seconds or more. The upper limit of the retention time is not particularly limited, but is, for example, 300 seconds.

[0066] After the holding step (step S4-2), the steel sheet is cooled (step S4-3). The cooling conditions after holding are arbitrary. The cooling rate is, but is not limited to, for example, 1 to 70°C / sec. In addition, during cooling, the steel sheet may be held at 300 to 450°C for 100 to 500 seconds (overaging heat treatment).

[0067] The cold-rolled steel sheet and the manufacturing method thereof according to one embodiment of the present invention have been described above. According to this embodiment, a cold-rolled steel sheet having excellent workability can be obtained. EXAMPLES

[0068] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0069] A 180 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and hot forged into a 30 mm thick slab. The obtained slab was hot rolled using a hot rolling tester to produce a hot-rolled steel sheet having a thickness of 1.25 to 3.3 mm. The hot rolling was completed at the final rolling temperature shown in Table 2, and after 3 to 10 seconds, the steel sheet was cooled to the coiling temperature shown in the same table, and then cooled to 200°C or less at a cooling rate of 20°C / sec to simulate the coiling of the steel sheet. After cooling, cold rolling was performed at the cold rolling rate (sheet thickness reduction rate) shown in Table 2 to produce a cold-rolled steel sheet having a thickness of 1.0 mm, which was used as a steel material.

[0070] [Table 1]

[0071] [Table 2]

[0072] Test pieces measuring 30 mm in width and 200 mm in length were taken from the obtained steel material. Each test piece was annealed under the conditions shown in Table 2. In the annealing process, the test pieces were heated in the temperature range from 500°C to 600°C at an average heating rate R1 in Table 2, and then heated in the temperature range from 720°C to the holding temperature at an average heating rate R2 in Table 2, and held at the holding temperature for a predetermined time, and then cooled to room temperature. Each test piece after annealing was subjected to a structure observation and a tensile test.

[0073] To measure tensile strength and elongation, two JIS No. 5 test pieces were taken with the longitudinal direction parallel to the rolling direction (L direction) and tested at a tensile speed of 10 mm / min. From the results, the tensile strength (TS), uniform elongation (uEL), elongation at break (tEL), and work hardening rate (n value) at EL = 10% were calculated.

[0074] To measure the r-value, two pieces of 1 / 2 ASTM size (GL = 10 mm) were taken from each of the directions parallel to the rolling direction (L direction), 45° from the rolling direction (D direction), and 90° from the rolling direction (C direction, i.e., the sheet width direction). The test pieces were unloaded when stretched to EL = 15%, and the sheet width was measured to determine the r-values ​​in the L, D, and C directions. The average r-value (mean-r) and the in-plane anisotropy Δr of the r-value were calculated from the above-mentioned formulas (2) and (3).

[0075] The metal structure of the annealed test piece was measured by the following method.

[0076] First, from each annealed test piece, an observation sample was taken so that the cross section parallel to the rolling direction and the normal direction of the sheet surface was the observation surface. The measurement area was 2000 μm in the rolling direction × 200 μm in the normal direction of the sheet surface, and the scan step was 1 μm. The average ferrite grain size was determined by assuming that the area surrounded by grain boundaries with a crystal orientation difference of 15° or more in the EBSD measurement is a BCC grain, and calculating the average circle equivalent diameter d from the following formula.

[0077]

number

[0078] Furthermore, using the same EBSD analysis data, the rolling direction of the observation sample and the normal direction to the sheet surface were adjusted so that they were parallel to the measurement system coordinates of the EBSD analysis, and then (1) the area ratio of crystal grains whose crystal orientation

[0111] is within 15° from the normal direction to the sheet surface and whose [0 1 -1] is within 15° from the rolling direction, and (2) the area ratio of crystal grains whose crystal orientation

[0111] is within 15° from the normal direction to the sheet surface and whose [1 1 -2] is within 15° from the rolling direction were determined.

[0079] Examples of EBSD analysis are shown in Figures 7 and 8. Figure 7 shows an example where the ratio of crystal grains with a crystal orientation of (1 1 1) [0 1 -1] and the ratio of crystal grains with a crystal orientation of (1 1 1) [1 1 -2] are approximately the same. Figure 8 shows an example where there is a bias in the ratio of crystal grains with a crystal orientation of (1 1 1) [0 1 -1] and the ratio of crystal grains with a crystal orientation of (1 1 1) [1 1 -2].

[0080] [Table 3]

[0081] [Table 4]

[0082] As shown in Tables 1 to 4, the test pieces of test numbers 1 to 15 had a ferrite volume fraction of 95% or more and an average grain size of ferrite of 25 μm or more. In these test pieces, the average r-value r-meam was 1.8 or more, and the absolute value of the in-plane anisotropy Δr of the r-value was 0.4 or less. Furthermore, in these test pieces, (1) the area fraction of crystal grains in which the crystal orientation

[0111] is within 15° from the normal direction of the sheet surface and [0 1 -1] is within 15° from the rolling direction, and (2) the area fraction of crystal grains in which the crystal orientation

[0111] is within 15° from the normal direction of the sheet surface and [1 1 -2] is within 15° from the rolling direction were both 20% or more.

[0083] In contrast, the average grain size of ferrite was less than 25 μm in the test pieces of test numbers 16 to 28. This is considered to be due to the fact that the manufacturing conditions of these test pieces were inappropriate.

[0084] Specifically, in the test pieces of test numbers 16, 18, and 26, the average heating rate in the temperature range from 500°C to 600°C was too large, so that latent nuclei with a crystal orientation favorable for the r-value were not sufficiently generated during heating.

[0085] In the test pieces of test numbers 17, 19, 22, and 28, the average heating rate in the temperature range from 720°C to the holding temperature was too small, so that latent nuclei other than those with crystal orientations favorable for the r-value were generated during heating, and the ferrite with the crystal orientation that improves the r-value did not coarsen sufficiently.

[0086] The test piece of test number 20 was held at an excessively high temperature, and thus transformed into austenite during annealing, resulting in a decrease in the crystal orientation favorable for the r-value.

[0087] In the test pieces of test numbers 23 and 27, the holding temperature was too low, so the recrystallization of ferrite did not progress.

[0088] In the test pieces of test numbers 21 and 24, the holding time was too short, so that the ferrite grains did not grow sufficiently.

[0089] In the test piece of test number 25, the sheet thickness reduction rate in cold rolling was too small, so recrystallization did not progress during annealing (only dislocation recovery occurred in ferrite).

[0090] As a result, these specimens did not have sufficient crystal orientation favorable for the r-value, so the average r-value, r-meam, was less than 1.8, and the absolute value of the in-plane anisotropy Δr of the r-value was greater than 0.4.

[0091] The test piece with test number 29 had an excessive C content. Since steel with this chemical composition has a high amount of solute C, when ferrite recrystallizes during the heating process, recrystallized ferrite grains with random crystal orientations tend to form. Therefore, crystal grains with crystal orientations favorable for the r-value did not develop. As a result, the average r-value r-meam was less than 1.8, and the absolute value of the in-plane anisotropy Δr of the r-value was greater than 0.4.

[0092] In the test piece of test number 30, the contents of Ti and Nb did not satisfy formula (1). Therefore, the action of fixing the solute C in the steel as TiC or NbC precipitates was weak, and a large amount of solute C was contained in the steel. As a result, as in the case of test number 29, crystal grains with a crystal orientation favorable for the r-value did not develop. As a result, the average r-value r-meam was less than 1.8, and the absolute value of the in-plane anisotropy Δr of the r-value was greater than 0.4.

[0093] Although the embodiment of the present invention has been described above, the above-mentioned embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-mentioned embodiment, and the above-mentioned embodiment can be appropriately modified and carried out within the scope of the invention.

Claims

1. The chemical composition, in mass%, is C: 0.0050% or less, Si: 1.00% or less, Mn: 0.01 to 2.00%, P: 0.100% or less, S: 0.020% or less, Al: 0.010-0.100%, N: 0.0100% or less, V: 0 to 0.50%, Mo: 0-0.20%, W: 0 to 0.20%, B: 0 to 0.0050%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, and REM: 0-0.0100%, Including, Further, at least one of Ti and Nb is contained so as to satisfy the following formula (1), The balance is Fe and impurities. The metal structure has a ferrite volume fraction of 95% or more and an average crystal grain size of the ferrite of 25 μm or more; The mean-r represented by the following formula (2) is 1.8 or more, The absolute value of Δr represented by the following formula (3) is 0.4 or less, The area ratio of crystal grains in which the crystal orientation [1 1 1] is within 15° from the normal direction of the sheet surface and the [0 1 -1] is within 15° from the rolling direction is 20% or more; A cold-rolled steel sheet, in which the area ratio of crystal grains whose crystal orientation [1 1 1] is within 15° from the normal direction of the sheet surface and whose crystal orientation [1 1 -2] is within 15° from the rolling direction is 20% or more. 0.8×(C / 12+N / 14)<Ti / 48+Nb / 93+0.0001...(1) mean-r=(R Ｌ +2×R Ｄ +R Ｃ ) / 4・・・(2) Δr=(R Ｌ -2×R Ｄ +R Ｃ ) / 2・・・(3) The element symbols in formula (1) are substituted with the contents of the corresponding elements in mass %. R in formula (2) and formula (3) Ｌ , R Ｄ , and R Ｃ are the r values ​​in a direction parallel to the rolling direction of the cold-rolled steel sheet, a direction at 45° from the rolling direction, and a direction at 90° from the rolling direction, respectively.

2. The cold-rolled steel sheet according to claim 1, The chemical composition, in mass%, V: 0.01-0.50%, Mo: 0.01-0.20%, W: 0.01 to 0.20%, and B: 0.0003 to 0.0050%, A cold-rolled steel sheet comprising one or more selected from the group consisting of:

3. The cold-rolled steel sheet according to claim 1, The chemical composition, in mass%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, and REM: 0.0001-0.0100%, A cold-rolled steel sheet comprising one or more selected from the group consisting of:

4. A method for producing the cold-rolled steel sheet according to any one of claims 1 to 3, A step of cold rolling the steel sheet so that the sheet thickness reduction rate is 30 to 80%; Heating the cold-rolled steel sheet to a holding temperature of 750 to 900°C; holding the heated steel sheet at the holding temperature for 40 seconds or more; and cooling the steel plate held at the holding temperature. In the heating step, the average heating rate in the temperature range from 500° C. to 600° C. is 8° C. / sec or less, and the average heating rate in the temperature range from 720° C. to the holding temperature is 100° C. / sec or more.

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