STEEL SHEET AND MANUFACTURING METHOD THEREOF

Abstract

This steel sheet has a predetermined chemical composition, the metallurgical structure at a thickness ¼ portion is, by area ratio, martensite: 40% to 97%, ferrite+bainite: 50% or less, residual austenite: 3% to 20% and a remainder in microstructure: 5% or less, the aspect ratio of residual austenite having an aspect ratio of 3 or more is 80% or more with respect to the total area of residual austenite, and the number of carbides having a grain diameter of 8 to 40 nm per square micrometer is five or more in the residual austenite.

Claims

1. A steel sheet, wherein a chemical composition contains, by mass %: C: 0.20% to 0.40%, Si: 0.5% to 2.0%, Al: 0.001% to 1.0%, Mn: 0.1% to 4.0%, V: 0.150% or less, Ti: 0.10% or less, Nb: 0.10% or less, P: 0.0200% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0200% or less, Ni: 0% to 1.00%, Mo: 0% to 1.00%, Cr: 0% to 2.000%, B: 0% to 0.0100%, Cu: 0% to 0.500%, W: 0% to 0.10%, Ta: 0% to 0.10%, Sn: 0% to 0.050%, Co: 0% to 0.50%, Sb: 0% to 0.050%, As: 0% to 0.050%, Mg: 0% to 0.050%, Ca: 0% to 0.040%, Y: 0% to 0.050%, Zr: 0% to 0.050%, and La: 0% to 0.050% with a remainder consisting of iron and impurities, a total amount of V, Ti and Nb is 0.030% to 0.150%, the metallurgical structure at a thickness ¼ portion is, by volume percentage, martensite: 40% to 97%, ferrite and bainite: 50% or less, residual austenite: 3% to 20%, and a remainder in microstructure: 5% or less, an area ratio of residual austenite having an aspect ratio of 3 or more is 80% or more with respect to a total area of the residual austenite, and the number of carbides having a grain diameter of 8 to 40 nm per square micrometer is five or more in the residual austenite.

2. The steel sheet according to claim 1, wherein the chemical composition contains, by mass %, one or more of Ni: 0.01% to 1.00%, Mo: 0.01% to 1.00%, Cr: 0.001% to 2.000%, B: 0.0001% to 0.0100%, Cu: 0.001% to 0.500%, W: 0.001% to 0.10%, Ta: 0.001% to 0.10%, Sn: 0.001% to 0.050%, Co: 0.001% to 0.50%, Sb: 0.001% to 0.050%, As: 0.001% to 0.050%, Mg: 0.0001% to 0.050%, Ca: 0.001% to 0.040%, Y: 0.001% to 0.050%, Zr: 0.001% to 0.050%, and La: 0.001% to 0.050%.

3. The steel sheet according to claim 1, further comprising: a hot-dip galvanized layer on a surface.

4. The steel sheet according to claim 1, further comprising: a hot-dip galvannealed layer on a surface.

5. A manufacturing method of a steel sheet, comprising: a hot rolling step of heating a slab having the chemical composition according to claim 1 at 1150° C. or higher for one hour or longer and hot-rolling the slab to produce a hot-rolled steel sheet in which prior austenite grain diameters are less than 30 μm; a first cooling step of cooling the hot-rolled steel sheet to a temperature range of 800° C. or lower within three seconds from an end of the hot rolling step; a coiling step of cooling the hot-rolled steel sheet after the first cooling step to a temperature range of 300° C. or lower at an average cooling rate of 30° C./s or faster and coiling the hot-rolled steel sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step at a rolling reduction of 0.1% to 30% to produce a cold-rolled steel sheet; an annealing step of heating the cold-rolled steel sheet in a temperature range of 480° C. to Ac1 at an average heating rate of 0.5 to 1.5° C./s and soaking the cold-rolled steel sheet in a temperature range of Ac1 to Ac3, a second cooling step of cooling the cold-rolled steel sheet after the annealing step at an average cooling rate of 4° C./s or faster; and a temperature retention step of retaining the cold-rolled steel sheet after the second cooling step at 300° C. to 480° C. for 10 seconds or longer.

6. The manufacturing method of a steel sheet according to claim 5, wherein the hot rolling step has a finish rolling step of continuously passing the slab through a plurality of rolling stands to perform rolling, in the finish rolling step: a rolling start temperature in the rolling stand third from a final of the rolling stands is 850° C. to 1000° C.; in each of the three last rolling stands in the finish rolling, the slab is rolled at a rolling reduction of larger than 10%; an interpass time between the individual rolling stands in the three last rolling stands in the finish rolling is three seconds or shorter; and (Tn−Tn+1) that is a difference between an exit temperature Tn of the nth rolling stand and an entrance temperature Tn+1 of the (n+1)th rolling stand on the downstream side of the four last rolling stands in the finish rolling is 10° C. or more.

7. The manufacturing method of a steel sheet according to claim 5, wherein the cold-rolled steel sheet after the annealing step is controlled to be in a temperature range of (zinc plating bath temperature−40°) C. to (zinc plating bath temperature+50°) C. and immersed in a hot-dip galvanizing bath, thereby forming hot-dip galvanized layer.

8. The manufacturing method of a steel sheet according to claim 7, wherein the hot-dip galvanized layer is alloyed in a temperature range of 300° C. to 500° C.

9. The steel sheet according to claim 2, further comprising: a hot-dip galvanized layer on a surface.

10. The steel sheet according to claim 2, further comprising: a hot-dip galvannealed layer on a surface.

11. A manufacturing method of a steel sheet, comprising: a hot rolling step of heating a slab having the chemical composition according to claim 2 at 1150° C. or higher for one hour or longer and hot-rolling the slab to produce a hot-rolled steel sheet in which prior austenite grain diameters are less than 30 μm; a first cooling step of cooling the hot-rolled steel sheet to a temperature range of 800° C. or lower within three seconds from an end of the hot rolling step; a coiling step of cooling the hot-rolled steel sheet after the first cooling step to a temperature range of 300° C. or lower at an average cooling rate of 30° C./s or faster and coiling the hot-rolled steel sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step at a rolling reduction of 0.1% to 30% to produce a cold-rolled steel sheet; an annealing step of heating the cold-rolled steel sheet in a temperature range of 480° C. to Ac1 at an average heating rate of 0.5 to 1.5° C./s and soaking the cold-rolled steel sheet in a temperature range of Ac1 to Ac3, a second cooling step of cooling the cold-rolled steel sheet after the annealing step at an average cooling rate of 4° C./s or faster; and a temperature retention step of retaining the cold-rolled steel sheet after the second cooling step at 300° C. to 480° C. for 10 seconds or longer.

12. The manufacturing method of a steel sheet according to claim 6, wherein the cold-rolled steel sheet after the annealing step is controlled to be in a temperature range of (zinc plating bath temperature−40°) C. to (zinc plating bath temperature+50°) C. and immersed in a hot-dip galvanizing bath, thereby forming hot-dip galvanized layer.

13. A steel sheet, wherein a chemical composition contains, by mass %: C: 0.20% to 0.40%, Si: 0.5% to 2.0%, Al: 0.001% to 1.0%, Mn: 0.1% to 4.0%, V: 0.150% or less, Ti: 0.10% or less, Nb: 0.10% or less, P: 0.0200% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0200% or less, Ni: 0% to 1.00%, Mo: 0% to 1.00%, Cr: 0% to 2.000%, B: 0% to 0.0100%, Cu: 0% to 0.500%, W: 0% to 0.10%, Ta: 0% to 0.10%, Sn: 0% to 0.050%, Co: 0% to 0.50%, Sb: 0% to 0.050%, As: 0% to 0.050%, Mg: 0% to 0.050%, Ca: 0% to 0.040%, Y: 0% to 0.050%, Zr: 0% to 0.050%, and La: 0% to 0.050% with a remainder comprising iron and impurities, a total amount of V, Ti and Nb is 0.030% to 0.150%, the metallurgical structure at a thickness ¼ portion is, by volume percentage, martensite: 40% to 97%, ferrite and bainite: 50% or less, residual austenite: 3% to 20%, and a remainder in microstructure: 5% or less, an area ratio of residual austenite having an aspect ratio of 3 or more is 80% or more with respect to a total area of the residual austenite, and the number of carbides having a grain diameter of 8 to 40 nm per square micrometer is five or more in the residual austenite.

Description

EXAMPLES

[0244] The present invention will be described more specifically with reference to examples.

<Manufacturing Method>

[0245] Slabs having a chemical composition shown in Table 1 were cast. A hot rolling step was performed on the cast slabs under conditions shown in Table 2 up to a sheet thickness of 2.8 mm. After hot rolling, a first cooling step, a coiling step, a cold rolling step, an annealing step, a second cooling step and a temperature retention step were performed on the hot-rolled steel sheets under conditions shown in Table 3. After that, in order to stably obtain desired metallurgical structures, the cold-rolled steel sheets were cooled to the Ms point or lower at an average cooling rate of 1° C./s or faster, thereby obtaining final steel sheets.

TABLE-US-00001 TABLE 1 Chemical composition (mass %)/ remainder is Fe and impurities Kind V + of Ti + Act1 Act3 steel C Si Al Mn V Ti Nb Nb P S N O Point point Others A 0.25 1.4 0.2 2.4 0.060 0.04 0.00 0.100 0.0016 0.0166 0.0053 0.0017 738 846 B 0.21 1.7 0.4 1.1 0.110 0.01 0.01 0.130 0.0011 0.0013 0.0087 0.0026 761 925 C 0.30 0.6 0.2 2.4 0.070 0.00 0.06 0.130 0.0010 0.0017 0.0023 0.0010 715 782 D 0.26 1.3 0.03 2.8 0.000 0.04 0.06 0.100 0.0032 0.0023 0.0035 0.0018 731 802 E 0.32 1.6 0.8 0.2 0.000 0.00 0.05 0.050 0.0024 0.0015 0.0136 0.0026 767 953 F 0.35 0.7 0.4 3.9 0.000 0.07 0.00 0.070 0.0021 0.0015 0.0018 0.0022 702 783 G 0.23 1.2 0.2 0.4 0.140 0.00 0.00 0.140 0.0081 0.0009 0.0020 0.0009 754 895 H 0.29 0.9 0.3 3.6 0.040 0.09 0.00 0.130 0.0011 0.0016 0.0153 0.0020 711 813 I 0.33 2.0 0.1 1.6 0.020 0.00 0.09 0.110 0.0037 0.0082 0.0027 0.0171 764 853 K 0.31 1.0 0.2 3.2 0.010 0.01 0.02 0.040 0.0165 0.0128 0.0015 0.0011 718 783 L 0.22 1.1 0.5 0.7 0.050 0.07 0.02 0.140 0.0018 0.0020 0.0013 0.0128 748 937 M 0.25 1.4 0.6 0.3 0.000 0.05 0.04 0.090 0.0017 0.0023 0.0019 0.0091 761 959 0.002B N 0.34 0.8 0.8 1.5 0.020 0.00 0.04 0.060 0.0049 0.0028 0.0017 0.0017 730 885 0.052Mo + 0.001B.sup.  O 0.28 1.5 0.3 0.9 0.050 0.03 0.00 0.080 0.0127 0.0013 0.0171 0.0019 758 901 0.055Cr + 0.001B P 0.21 0.7 0.5 1.2 0.050 0.07 0.00 0.120 0.0018 0.0017 0.0013 0.0151 731 902 0.061Mo + 0.052Cr Q 0.25 1.5 0.1 2.5 0.100 0.00 0.03 0.130 0.0017 0.0054 0.0019 0.0011 739 816 0.043Ni + 0.11Cu + 0.010W + 0.081Ta + 0.009Sn + 0.034Co R 0.37 1.9 0.7 3.2 0.100 0.01 0.01 0.120 0.0012 0.0020 0.0018 0.0032 746 877  0.003Sb + 0.006As S 0.27 1.8 0.9 0.2 0.000 0.02 0.08 0.100 0.0151 0.0032 0.0028 0.0026 773 1006 0.007Mg + 0.013Ca T 0.28 1.2 0.2 0.6 0.050 0.00 0.02 0.070 0.0025 0.0152 0.0013 0.0054 752 874 0.003Y + 0.0052Zr + 0.004La U 0.38 1.8 0.9 2.4 0.160 0.00 0.00 0.160 0.0015 0.0023 0.0052 0.0060 750 916 V 0.30 1.0 0.1 1.0 0.000 0.11 0.02 0.130 0.0025 0.0018 0.0012 0.0011 741 877 W 0.30 1.2 0.2 1.4 0.020 0.00 0.11 0.130 0.0029 0.0017 0.0157 0.0028 743 842 X 0.34 1.3 0.5 0.8 0.020 0.00 0.00 0.020 0.0165 0.0016 0.0020 0.0018 752 894 Y 0.23 1.1 0.7 2.1 0.070 0.05 0.04 0.160 0.0014 0.0052 0.0020 0.0013 733 910

TABLE-US-00002 TABLE 2 Hot rolling step Entrance temper- Rolling Rolling Rolling ature of reduction reduction reduction Slab third in third in second in first Maximum heating Slab rolling rolling rolling rolling value of Kind temper- retention stand stand stand stand interpass T.sub.n − T.sub.n+1 − T.sub.n+2 − Example of ature time from back from back from back from back time T.sub.n+1 T.sub.n+2 T.sub.n+3 No. steel (° C.) (minutes) (° C.) (%) (%) (%) (seconds) (° C.) (° C.) (° C.) Note 1 A 1200 90 900 25 25 25 0.2 14 12 14 Example 2 B 1200 60 980 25 25 25 0.1 16 23 13 Example 3 C 1200 60 940 25 25 25 0.2 23 14 26 Example 4 D 1200 90 960 25 25 25 0.1 15 11 27 Example 5 E 1200 90 920 20 20 20 0.1 19 16 11 Example 6 F 1250 60 920 20 20 20 0.2 15 11 20 Example 7 G 1250 90 860 20 20 20 0.1 10 11 19 Example 8 H 1250 60 970 20 20 20 0.1 12 15 11 Example 9 I 1250 60 950 25 25 25 0.1 23 12 21 Example 10 K 1200 60 900 25 20 20 0.1 10 15 17 Example 11 L 1250 60 870 20 30 30 0.2 14 26 25 Example 12 M 1200 90 880 25 25 25 0.1 18 18 13 Example 13 N 1200 90 950 25 25 25 0.1 21 17 18 Example 14 0 1200 60 970 20 20 20 0.1 13 18 10 Example 15 P 1250 60 900 25 25 25 0.1 19 14 18 Example 16 Q 1250 60 920 20 20 20 0.1 20 13 13 Example 17 R 1250 60 940 20 20 20 0.1 10 24 24 Example 18 S 1250 60 960 20 20 20 0.1 13 11 18 Example 19 T 1250 60 980 20 20 20 0.1 14 22 11 Example 20 A 1100 90 900 25 25 25 0.1 15 12 10 Comparative Example 21 A 1200 50 950 25 25 25 0.1 20 15 14 Comparative Example 22 A 1200 60 840 25 25 25 0.1 14 17 20 Comparative Example 23 A 1200 60 1010  25 25 25 0.1 14 20 11 Comparative Example 24 A 1250 60 880 9 25 25 0.2 12 21 14 Comparative Example 25 A 1250 60 900 25  9 25 0.1 12 15 23 Comparative Example 26 A 1250 60 980 25 25 9 0.1 10 22 23 Comparative Example 27 A 1250 60 940 25 25 25 4.0 24 10 12 Comparative Example 28 A 1250 60 960 25 25 25 0.1  7 16 22 Comparative Example 29 A 1250 60 960 25 25 25 0.1 16  7 18 Comparative Example 30 A 1250 60 960 25 25 25 0.1 23 18  7 Comparative Example 31 A 1250 60 920 25 25 25 0.1 17 24 10 Comparative Example 32 A 1250 60 860 25 25 25 0.1 13 13 23 Comparative Example 33 A 1250 60 970 25 25 25 0.1 25 16 22 Comparative Example 34 A 1200 60 950 25 25 25 0.2 14 19 13 Comparative Example 35 A 1200 60 900 25 25 25 0.1 12 13 18 Comparative Example 36 A 1200 60 950 25 25 25 0.1 26 14 15 Comparative Example 37 A 1200 60 970 25 25 25 0.1 17 17 17 Comparative Example 38 A 1200 60 900 25 25 25 0.1 16 12 16 Comparative Example 39 A 1250 60 920 25 25 25 0.1 10 11 11 Comparative Example 40 A 1250 60 940 25 25 25 0.1 27 26 14 Comparative Example 41 A 1250 60 900 25 25 25 0.1 13 12 18 Comparative Example 42 A 1250 60 950 25 25 25 0.2 18 21 26 Comparative Example 43 A 1250 60 900 25 25 25 0.1 19 11 23 Comparative Example 44 U 1200 60 880 25 25 25 0.1 20 26 23 Comparative Example 45 V 1200 60 950 25 25 25 0.1 11 19 12 Comparative Example 46 W 1200 60 920 25 25 25 0.1 16 12 21 Comparative Example 47 X 1250 60 900 25 25 25 0.1 19 14 23 Comparative Example 48 Y 1250 60 970 25 25 25 0.1 26 15 13 Comparative Example 49 A 1250 60 970 25 25 25 0.1 25 16 22 Comparative Example

TABLE-US-00003 TABLE 3 Second cooling First Cold Annealing step step cooling Coiling step rolling Average Average Temperature step Average step heating cooling retention step Kind Time taken cooling Coiling Rolling rate in Soaking rate Retention Retention Example of for cooling rate temperature reduction 480° C. to Ac1 temperature (° C./ temperature time No. steel (seconds) (° C./s) (° C.) (%) (° C./second) (° C.) second) (° C.) (seconds) Note 1 A 0.1 55 100 10 1.0 830 15 400 360 Example 2 B 0.1 55 100 10 1.0 880 15 400 360 Example 3 C 0.1 55 100 10 1.0 760 15 400 360 Example 4 D 0.1 55 150 10 1.0 800 10 400 360 Example 5 E 0.1 55 150 15 0.8 910 10 400 360 Example 6 F 0.1 55 200 15 0.8 760 10 400 360 Example 7 G 1.0 60 200 15 0.8 880 15 400 360 Example 8 H 0.1 60 200 15 0.8 805 15 400 360 Example 9 I 0.1 60  50 20 1.2 830 20 400 360 Example 10 K 0.1 55  50 20 1.2 780 20 400 360 Example 11 L 0.1 55  50 20 1.2 880 30 400 360 Example 12 M 0.1 55 100 10 1.0 910 15 400 360 Example 13 N 0.1 55 100 10 1.0 880 15 400 360 Example 14 0 0.1 55 150 10 1.0 870 15 400 360 Example 15 P 0.1 55 150 15 0.8 840 15 400 360 Example 16 Q 0.1 55 200 15 0.8 810 15 400 360 Example 17 R 0.1 55 200 15 0.8 850 15 400 360 Example 18 S 0.1 55  50 20 1.2 980 15 400 360 Example 19 T 0.1 55  50 20 1.2 830 15 400 360 Example 20 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 21 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 22 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 23 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 24 A 1.0 55 100 10 1.0 830 15 400 360 Comparative Example 25 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 26 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 27 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 28 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 29 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 30 A 0.1 55 100 10 1.0 830 15 400 360 Comparative Example 31 A 5.0 55 100 10 1.0 830 15 400 360 Comparative Example 32 A 0.1 20 100 10 1.0 830 15 400 360 Comparative Example 33 A 0.1 55 310 10 1.0 830 15 400 360 Comparative Example 34 A 0.1 55 100  0 1.0 830 15 400 360 Comparative Example 35 A 0.1 55 100 40 1.0 830 15 400 360 Comparative Example 36 A 0.1 55 100 10 0.4 830 15 400 360 Comparative Example 37 A 0.1 55 100 10 1.6 830 15 400 360 Comparative Example 38 A 0.1 55 100 10 1.0 720 15 400 360 Comparative Example 39 A 0.1 55 100 10 1.0 850 15 400 360 Comparative Example 40 A 0.1 55 100 10 1.0 830  1 400 360 Comparative Example 41 A 0.1 55 100 10 1.0 830 15 280 360 Comparative Example 42 A 0.1 55 100 10 1.0 830 15 500 360 Comparative Example 43 A 0.1 55 100 10 1.0 830 15 400  9 Comparative Example 44 U 0.1 55 100 10 1.0 860 15 400 360 Comparative Example 45 V 0.1 55 100 10 1.0 820 15 400 360 Comparative Example 46 W 0.1 55 100 10 1.0 810 15 400 360 Comparative Example 47 X 0.1 55 100 10 1.0 840 15 400 360 Comparative Example 48 Y 0.1 55 100 10 1.0 890 15 400 360 Comparative Example 49 A 0.1 37 505 10 1.0 830 15 400 360 Comparative Example

<Measurement of Metallurgical Structure>

[0246] Test pieces for scanning electron microscopic (SEM) observation were collected from the obtained final steel sheets (annealed steel sheets), and longitudinal sections parallel to the rolling direction were polished. After that, the metallurgical structures at the ¼ positions of the sheet thicknesses were observed, and the area ratio of each structure was measured by image processing. The volume percentage of each structure was shown in Table 4. “Remainder in microstructure” shown in Table 4 refers to structures other than ferrite, martensite, bainite and residual austenite. In addition, the metallurgical structures of the present examples were confirmed by a method in which a field emission-type scanning electron microscope was used, and the remainder in microstructure was found out to be formed of pearlite.

[0247] The volume percentage of residual austenite (residue γ) was calculated by measuring diffraction intensities using X-rays. In the measurement using X-rays, a portion from the sheet surface of a sample to a depth ¼ position was removed by mechanical polishing and chemical polishing, and the integrated intensity ratio of the diffraction peaks of (200) and (211) of a bcc phase and (200), (220) and (311) of an fcc phase was obtained using MoKα rays at a sheet thickness ¼ position. The microstructural fraction of residual austenite was calculated from the obtained intensity ratio. At that time, a five-peak method, which is a general calculation method, was used.

[0248] The area ratio of fresh martensite was obtained by the following procedure. An observed section of the sample was etched with a LePera solution, and a secondary electron image of a 100 μm×100 μm region in the sheet thickness ⅛ to ⅜ range, in which the sheet thickness ¼ was centered, obtained with a field emission scanning electron microscope (FE-SEM) was observed at a magnification of 3000 times. Since fresh martensite and residual austenite are not corroded by LePera corrosion, the area ratio of uncorroded regions is the total area ratio of fresh martensite and residual austenite. The area ratio of fresh martensite was calculated by subtracting the volume percentage of residual austenite measured with X-rays from the area ratio of the uncorroded regions.

[0249] The area ratios of ferrite, bainite and tempered martensite were determined from a secondary electron image obtained by observing the ⅛ to ⅜ sheet thickness range (that is, the sheet thickness range in which the ¼ sheet thickness position was centered) with FE-SEM. In this FE-SEM observation, the sample was collected such that a sheet thickness cross section of the steel sheet parallel to the rolling direction became the observed section, and polishing and Nital etching were performed on this observed section. The same region as the region observed by the LePera corrosion was confirmed by leaving a plurality of indentations around the region observed by the LePera corrosion. Ferrite is a structure in which the insides of grain boundaries appear in uniform contrast. Bainite is a collection of lath-shaped crystal grains and is a structure in which iron-based carbides having a major axis of 20 nm or more are not contained or a structure in which iron-based carbides having a major axis of 20 nm or more are contained and the carbides belong to a single variant, that is, a group of iron-based carbides elongated in the same direction. Here, the group of iron-based carbides elongated in the same direction refers to a group in which the difference in the elongation direction in the group of iron-based carbides was 5° or less. Tempered martensite was distinguished from bainite due to the fact that cementite in the structure has a plurality of variants.

[0250] The area ratio of martensite was obtained by combining the area ratio of fresh martensite and the area ratio of tempered martensite specified by the above-described method.

[0251] The proportion of residual austenite having an aspect ratio of 3 or more in all residual austenite was obtained by an EBSD analysis method in which FE-SEM is used. Specifically, a test piece in which a sheet thickness cross section of the steel sheet parallel to the rolling direction was used as an observed section was collected, the observed section of the test piece was polished, then, a strain-influenced layer was removed by electrolytic polishing, and EBSD analysis was performed on an area of 2.0×10-8 m.sup.2 or more in total at one or more visual fields in a region of ⅛t (t: sheet thickness) to ⅜t (t: sheet thickness) from the surface of the sheet thickness at measurement steps set to 0.2 μm.

[0252] A residual austenite map was produced from measured data, residual austenite having an aspect ratio of 3 or more was extracted, and the area fraction of residual austenite having an aspect ratio of 3 or more was obtained.

[0253] The number density of the carbides having a grain diameter of 8 to 40 nm (fine carbides) in residual austenite (residue γ) was measured as described below.

[0254] First, an extraction replica sample of a circular region having a diameter of 3.0 mm at a ¼ position from the surface of the steel sheet was observed at three visual fields using a transmission electron microscope (TEM) at a magnification of 100000 times, a precipitate from which a corresponding alloy carbide-forming element was detected by the energy-dispersive X-ray spectroscopy (EDX) in each visual field was regarded as a carbide, the area of each precipitate was obtained using an image analysis apparatus and converted into a circle-converted diameter. Next, a value obtained by calculating the number of carbides having a circle-converted diameter of 8 nm or more and 40 nm or less and dividing this by the area of the observed visual field was regarded as the number density of carbides in each visual field, this was performed on the three visual fields, and the obtained arithmetic mean was determined as the number density of the carbides having a circle-converted diameter of 8 to 40 nm.

<Measurement of Characteristics>

(Tensile Strength)

[0255] The tensile strength TS was measured by collecting a JIS No. 5 tensile test piece from the steel sheet in a direction perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z 2241: 2011. A case where the tensile strength TS (unit: MPa) was 980 MPa or more was regarded as pass.

[0256] The measurement results of the tensile strength were shown in Table 5.

(Elongation)

[0257] The elongation was measured by collecting a JIS No. 5 tensile test piece described in JIS Z 2201 from the steel sheet in a direction perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z 2241: 2011. A case where the elongation (%) was a larger value than (49−0.03×TS) was regarded as pass. The measurement results of the elongation and the values of (49−0.03×TS) were shown in Table 5.

(Hole Expansion Rate λ)

[0258] The hole expansion rate λ of the steel sheet was evaluated according to the hole expanding test method described in JIS Z 2256: 2010. A sheet thickness t (mm) necessary to calculate a numerical value (criterion value) that served as an evaluation criterion of the hole expansion rate λ was obtained by the following method.

[0259] First, the sheet thickness (mm) was measured with a caliper at each of three points (a ¼ position, a ½ position and a ¾ position) in the width direction of the steel sheet, and the arithmetic mean thereof was regarded as the sheet thickness t (mm).

[0260] In the present examples, the criterion value that served as the evaluation criterion of the hole expansion rate λ was determined as (41−10×sheet thickness t). That is, a case where the hole expansion rate λ was a value larger than (41−10×sheet thickness t) was regarded as pass. The hole expansion rate λ (%) and the value of (41−10×sheet thickness t) were shown in Table 5.

(Shape after Hole Expansion: Anisotropy of Distortability)

[0261] The shape after the above-described hole expanding test was performed was evaluated by the following method. First, a test piece after hole expansion was captured from immediately above.

[0262] From the captured image, an area A0 of the shape of a hole after hole expansion was measured with image analysis processing software. Furthermore, an area A1 of the circumscribed circle of the shape after hole expansion was obtained, and a value (A0/A1) was obtained by dividing A0 by A1. A case where A0/A1 was 0.80 or more was regarded as pass as a steel sheet being excellent in terms of the anisotropy of distortability. The values of A0/A1 were shown in Table 5. “Circumscribed circle” mentioned herein means a circle having the major axis of the shape of a hole as the diameter, where the major axis is a straight line having the maximum length among line segments that are straight lines passing through the center of the hole and are composed of two points intersecting with the brink of the hole shape.

TABLE-US-00004 TABLE 4 Proportion of residual Number density of Metallurgical structure (volume percentage (%)) γ having aspect fine carbides in Example Kind of Ferrite + Remainder in ratio of 3 or more residual γ No. steel Martensite bainite Residual γ microstructure [%] [carbides/μm.sup.2] Note 1 A 62 31 7 0 87 10  Example 2 B 58 35 4 3 99 8 Example 3 C 41 46 11  2 89 7 Example 4 D 76 16 5 3 97 9 Example 5 E 57 34 8 1 81 7 Example 6 F 43 42 14  1 85 8 Example 7 G 72 22 4 2 91 10  Example 8 H 72 20 6 2 81 6 Example 9 I 57 35 7 1 82 6 Example 10 K 65 23 9 3 95 9 Example 11 L 52 43 5 0 81 10  Example 12 M 60 33 5 2 90 11  Example 13 N 80 13 5 2 91 9 Example 14 O 46 42 11  1 81 9 Example 15 P 47 47 5 1 97 7 Example 16 Q 70 23 5 2 84 11  Example 17 R 54 32 11  3 88 9 Example 18 S 68 24 6 2 98 9 Example 19 T 42 48 9 1 99 6 Example 20 A 62 29 7 2 91 0 Comparative Example 21 A 63 28 6 3 98 0 Comparative Example 22 A 63 30 6 1 82 4 Comparative Example 23 A 63 29 6 2 72 7 Comparative Example 24 A 63 31 6 0 78 7 Comparative Example 25 A 63 30 6 1 78 8 Comparative Example 26 A 63 30 6 1 75 7 Comparative Example 27 A 63 29 6 2 75 9 Comparative Example 28 A 63 29 6 2 75 9 Comparative Example 29 A 63 29 6 2 72 9 Comparative Example 30 A 63 29 6 2 78 6 Comparative Example 31 A 62 28 7 3 82 2 Comparative Example 32 A 63 28 6 3 21 3 Comparative Example 33 A 63 28 6 3 71 4 Comparative Example 34 A 63 28 6 3 90 3 Comparative Example 35 A 63 29 6 2 96 4 Comparative Example 36 A 63 28 6 3 97 2 Comparative Example 37 A 63 29 6 2 87 3 Comparative Example 38 A  0 100  0 0  0 0 Comparative Example 39 A 82 15 2 1  0 6 Comparative Example 40 A 36 47 2 15  75 7 Comparative Example 41 A 79 17 2 2  0 4 Comparative Example 42 A 64 25 2 9  0 0 Comparative Example 43 A 77 19 2 2 98 5 Comparative Example 44 U 46 41 11  2 98 4 Comparative Example 45 V 41 49 8 2 98 1 Comparative Example 46 W 58 35 4 3 85 2 Comparative Example 47 X 42 46 11  1 92 1 Comparative Example 48 Y 56 34 8 2 91 2 Comparative Example 49 A 58 33 5 4 52 3 Comparative Example

TABLE-US-00005 TABLE 5 Hole expansion Example Kind of TS Elongation 49 − rate λ 41 − 10 × No. steel [MPa] [%] 0.03 × TS (%) (sheet thickness t) A0/A1 Note 1 A 1315 13 10 34 27 0.96 Example 2 B 1073 18 17 32 27 0.84 Example 3 C 1136 16 15 35 27 0.94 Example 4 D 1405 11 7 36 27 0.84 Example 5 E 1166 16 14 34 27 0.84 Example 6 F 1234 13 12 36 27 0.90 Example 7 G 1210 14 13 34 27 0.88 Example 8 H 1399 11 7 32 27 0.81 Example 9 I 1208 15 13 33 27 0.90 Example 10 K 1355 12 8 30 27 0.90 Example 11 L 1036 19 18 32 27 0.83 Example 12 M 1139 16 15 30 27 0.91 Example 13 N 1396 12 7 36 27 0.84 Example 14 O 1124 16 15 35 27 0.83 Example 15 P  998 21 19 32 27 0.87 Example 16 Q 1398 12 7 28 27 0.86 Example 17 R 1321 14 9 35 27 0.85 Example 18 S 1038 19 18 34 27 0.92 Example 19 T 1043 20 18 34 27 0.83 Example 20 A 1241 13 12 24 27 0.68 Comparative Example 21 A 1183 15 14 26 27 0.79 Comparative Example 22 A 1230 13 12 24 27 0.76 Comparative Example 23 A 1182 15 14 24 27 0.76 Comparative Example 24 A 1304 13 10 22 27 0.70 Comparative Example 25 A 1318 13 9 23 27 0.78 Comparative Example 26 A 1318 13 9 23 27 0.70 Comparative Example 27 A 1321 13 9 22 27 0.79 Comparative Example 28 A 1321 13 9 23 27 0.70 Comparative Example 29 A 1321 13 9 23 27 0.78 Comparative Example 30 A 1321 13 9 23 27 0.71 Comparative Example 31 A 1324 13 9 24 27 0.76 Comparative Example 32 A 1323 13 9 23 27 0.76 Comparative Example 33 A 1323 13 9 24 27 0.73 Comparative Example 34 A 1323 13 9 24 27 0.67 Comparative Example 35 A 1321 13 9 22 27 0.75 Comparative Example 36 A 1323 13 9 23 27 0.74 Comparative Example 37 A 1321 13 9 24 27 0.77 Comparative Example 38 A  751 28 26 45 27 0.81 Comparative Example 39 A 1490  7 4 24 27 0.78 Comparative Example 40 A 1029 14 18 18 27 0.78 Comparative Example 41 A 1661  7 −1 26 27 0.67 Comparative Example 42 A 1065 18 17 26 27 0.75 Comparative Example 43 A 1435 11 6 25 27 0.78 Comparative Example 44 U 1290  9 10 19 27 0.78 Comparative Example 45 V 1062 15 17 17 27 0.79 Comparative Example 46 W 1243 11 12 22 27 0.79 Comparative Example 47 X 1144 14 15 17 27 0.69 Comparative Example 48 Y 1022 16 18 17 27 0.67 Comparative Example 49 A 1196 12 13 26 27 0.68 Comparative Example

TABLE-US-00006 TABLE 6 Steel sheet temperature during Temperature Alloying Hole immersion in of plating treatment expansion 41 − 10 × Example plating bath bath temperature TS Elongation 49 − rate λ (sheet Kind of No. (° C.) (° C.) (° C.) [MPa] [%] 0.03 × TS (%) thickness t) A0/A1 plating Note 50 447 458 — 1135 18 15.0 28 27 0.83 GI Example 51 455 458 — 1124 18 15.3 29 27 0.86 GI Example 52 462 459 472 1113 19 15.6 29 27 0.86 GA Example 53 468 460 491 1086 19 16.4 29 27 0.88 GA Example 54 471 462 499 1076 19 16.7 30 27 0.89 GA Example

[0263] As shown in Table 5, in the present examples according to the present invention, the elongation (%) was values larger than (49−0.03×TS (unit: MPa)), the hole expansion rate λ (%) was values larger than (41−10×sheet thickness (unit: mm)), and the shapes after hole expansion were nearly circular.

[0264] In addition, a variety of characteristics were evaluated regarding a case where a plating treatment was performed under conditions shown in Table 6 on a part of a cold-rolled coil (cold-rolled steel sheet) after the cold rolling step produced in the example with Example No. 1. Specifically, first, an annealing step was performed under the same conditions as for Example No. 1, and a temperature retention step was performed at 400° C. for 380 seconds. After that, steel sheets were reheated to be controlled to steel sheet temperatures shown in Table 6, and the steel sheets were immersed in a zinc plating bath. After immersion, hot-dip galvanized steel sheets (GI) or galvannealed steel sheets (GA) were produced by cooling the steel sheets to room temperature in Example Nos. 50 and 51 and by performing an alloying treatment and then cooling the steel sheets to room temperature in Example Nos. 52 to 54. As shown in Table 6, in plated steel sheets on which a hot-dip galvanizing treatment or a hot-dip galvanizing treatment and an alloying treatment were performed as well, the elongation (%) was values larger than (49−0.03×TS (unit: MPa)), the hole expansion rate λ (%) was values larger than (41−10×sheet thickness t (unit: mm)), and the shapes after hole expansion were nearly circular.

[0265] On the other hand, in Comparative Example 20 where the slab heating temperature during hot rolling was low, it was not possible to sufficiently form the solid solutions of carbides or the like containing V, Ti and Nb formed in the casting stage, and the carbides were precipitated during hot rolling and became coarse. Therefore, the number densities of carbides having a grain diameter of 8 to 40 nm in residual austenite were not sufficient, and the hole expansibility and the anisotropy of distortability were poor.

[0266] In Comparative Example 21 where the retention time at the slab heating temperature during hot rolling was short, the solid solutions of carbides containing V, Ti and Nb formed in the casting stage were not sufficiently formed. Therefore, the number densities of carbides having a grain diameter of 8 to 40 nm in residual austenite were not sufficient, and the hole expansibility and the anisotropy of distortability were poor.

[0267] In Comparative Example 22 where the rolling start temperature in the third rolling stand from the finish rolling stand (the entrance temperature of the third rolling stand from the back) was low, it was not possible to suppress the precipitation of carbides in austenite during rolling. Therefore, the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0268] In Comparative Example 23 where the rolling start temperature in the third rolling stand from the finish rolling stand (the entrance temperature of the third rolling stand from the back) was high, it was not possible to sufficiently suppress the coarsening of prior austenite grains, and thus the proportion of residual austenite having an aspect ratio of 3 or more was low. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0269] In Comparative Example 24 where the rolling reduction in the third rolling stand from the final stand (the third rolling stand from the back) among the three last rolling stands for finish rolling was low, Comparative Example 25 where the rolling reduction in the second rolling stand from the back was low, and Comparative Example 26 where the rolling reduction in the first rolling stand from the back (the finish rolling stand) was low, it was not possible to introduce sufficient rolling strain, and it was not possible to sufficiently refine austenite grains. Therefore, the proportion of residual austenite having an aspect ratio of 3 or more was low. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0270] In Comparative Example 27 where, among the interpass times between the individual rolling stands in the three last rolling stands in the finish rolling step, the longest interpass time (maximum value) was outside the scope of the invention, recovery and recrystallization between passes were not suppressed, and it was not possible to sufficiently accumulate strain. Therefore, the proportion of residual austenite having an aspect ratio of 3 or more was low. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0271] In Comparative Examples 28 to 30 where (T.sub.n−T.sub.n+1) that was the difference between the exit temperature T.sub.n of the n.sup.th rolling stand and the entrance temperature T.sub.n+1 of the (n+1).sup.th rolling stand on the downstream side of the four last rolling stands in finish rolling was less than 10° C., since recovery and recrystallization between passes were not suppressed, and it was not possible to sufficiently accumulate strain, the proportion of residual austenite having an aspect ratio of 3 or more was low. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0272] In Comparative Example 31 where the time taken to cool the hot-rolled steel sheet to a temperature range of 800° C. or lower from the end of the hot rolling step (time taken for cooling) was long, it was not possible to suppress the formation of carbides. Therefore, the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0273] In Comparative Example 32 where the average cooling rate in the coiling step was slow and Comparative Example 33 and Comparative Example 49 where the coiling temperature in the coiling step was higher than 300° C., it was not possible to suppress the precipitation of carbides. As a result, it was not possible to suppress ferritic transformation or pearlitic transformation, and a preferable full hard structure, which acts as the origin of a needle-like structure, could not be obtained. Therefore, the proportion of residual austenite having an aspect ratio of 3 or more was low, and the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0274] In Comparative Example 34 where the cold rolling step was not performed, strain was not imparted, and the precipitation sites of carbides were not increased. Therefore, the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0275] In Comparative Example 35 where the rolling reduction in the cold rolling step was high, recrystallization proceeded during heating for annealing, and imparted strain disappeared. Therefore, the number density of carbides having a grain diameter of 8 to nm in residual austenite was not sufficient. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0276] In Comparative Example 36 where the average heating rate in the annealing step was slow and Comparative Example 37 where the average heating rate was fast, the amount of carbides precipitated was not sufficient, and the carbides did not grow to preferable sizes for suppressing strain-induced transformation. Therefore, the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0277] In Comparative Example 38 where the soaking temperature in the annealing step was low, since residual austenite was not formed, residual austenite having an aspect ratio of 3 or more was not present, and carbides were also not present in residual austenite. Therefore, in Comparative Example 38, the tensile strength was not sufficient.

[0278] In Comparative Example 39 where the soaking temperature in the annealing step was high, since needle-like austenite was not formed along laths of tempered martensite, a desired metallurgical structure could not be obtained. Therefore, in Comparative Example 39, the proportion of residual austenite having an aspect ratio of 3 or more was low. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0279] In Comparative Example 40 where the average cooling rate in the cooling step after annealing (second cooling step) was low, since it was not possible to suppress ferritic transformation during cooling, it was not possible to obtain a desired metallurgical structure in the final structure. Therefore, the elongation, the hole expansibility and the anisotropy of distortability were poor.

[0280] In Comparative Example 41 where the retention temperature in the temperature retention step was low, the volume percentage of residual austenite was low, the proportion of residual austenite having an aspect ratio of 3 or more was low, and the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0281] In Comparative Example 42 where the retention temperature in the temperature retention step was high, the volume percentage of residual austenite was low, and the volume percentage of the remainder in microstructure was high. Therefore, the proportion of residual austenite having an aspect ratio of 3 or more was low, and the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the hole expansibility and the anisotropy of distortability were poor.

[0282] In Comparative Example 43 where the retention time at the retention temperature in the temperature retention step was short, since carbon did not sufficiently concentrate in untransformed austenite, the volume percentage of residual austenite was low.

[0283] In Comparative Example 44 where the V content and V+Ti+Nb were excessive, a number of coarse V carbides were precipitated, and accordingly, the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the elongation, the hole expansibility and the anisotropy of distortability were poor.

[0284] In Comparative Example 45 where the Ti content was excessive, coarse Ti oxides and TiN were formed, in addition, Ti carbides were precipitated in the hot rolling step, these Ti carbides became coarse in the following steps, and accordingly, the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the elongation, the hole expansibility and the anisotropy of distortability were poor.

[0285] In Comparative Example 46 where the Nb content was excessive, Nb carbides were precipitated in the hot rolling step, these Nb carbides became coarse in the following steps, and thus the number density of carbides having a grain diameter of 8 to nm in residual austenite was not sufficient. Therefore, the elongation, the hole expansibility and the anisotropy of distortability were poor.

[0286] In Comparative Example 47 where the total amount of V+Ti+Nb was small and Comparative Example 48 where the total amount was large, complex carbides were precipitated during hot rolling, these complex carbides became coarse in the following steps, and accordingly, the number density of carbides having a grain diameter of 8 to 40 nm in residual austenite was not sufficient. Therefore, the elongation, the hole expansibility and the anisotropy of distortability were poor.