COLD-ROLLED STEEL SHEET AND MANUFACTURING METHOD THEREOF

Abstract

This cold-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure of a t/4 portion, which is at a ? position of a sheet thickness t from a surface of the cold-rolled steel sheet in a sheet thickness direction has a predetermined structure, and in both an edge portion, which is at a position 50 mm away from an end portion of the cold-rolled steel sheet in a width direction, and a center portion of the cold-rolled steel sheet in the width direction, a metallographic structure of a 20 ?m portion, which is at a position 20 ?m away from the surface in the sheet thickness direction, includes, by volume percentage, ferrite and bainite: 75.0% or more and 100.0% or less in total, and martensite and tempered martensite: 0.0% or more and 25.0% or less in total, an average grain size of the martensite and the tempered martensite in the metallographic structure of the 20 ?m portion is 5.0 ?m or less, and a metallographic structure of a 75 ?m portion, which is at a position 75 ?m away from the surface in the sheet thickness direction, includes, by volume percentage, ferrite and bainite: 0.0% or more and 15.0% or less in total.

Claims

1-9. (canceled)

10. A cold-rolled steel sheet comprising, as a chemical composition, by mass %: C: 0.180% or more and 0.350% or less; Mn: 2.00% or more and 4.00% or less; P: 0% or more and 0.100% or less; S: 0% or more and 0.010% or less; Al: 0% or more and 0.100% or less; N: 0% or more and 0.0100% or less; Si: 0% or more and 1.00% or less; Ti: 0% or more and 0.050% or less; Nb: 0% or more and 0.050% or less; V: 0% or more and 0.50% or less; Cu: 0% or more and 1.00% or less; Ni: 0% or more and 1.00% or less; Cr: 0% or more and 1.00% or less; Mo: 0% or more and 0.50% or less; B: 0% or more and 0.0100% or less; Ca: 0% or more and 0.010% or less; Mg: 0% or more and 0.0100% or less; REM: 0% or more and 0.0500% or less; Bi: 0% or more and 0.050% or less; and a remainder of Fe and impurities, wherein a metallographic structure of a t/4 portion, which is at a ? position of a sheet thickness t from a surface of the cold-rolled steel sheet in a sheet thickness direction, includes, by volume percentage, retained austenite: 2.0% or more and 8.0% or less, tempered martensite: 80.0% or more and 98.0% or less, ferrite and bainite: 0.0% or more and 15.0% or less in total, and martensite: 0.0% or more and 5.0% or less, and in both an edge portion, which is at a position 50 mm away from an end portion of the cold-rolled steel sheet in a width direction, and a center portion of the cold-rolled steel sheet in the width direction, a metallographic structure of a 20 ?m portion, which is at a position 20 ?m away from the surface in the sheet thickness direction, includes, by volume percentage, ferrite and bainite: 75.0% or more and 100.0% or less in total, and martensite and tempered martensite: 0.0% or more and 25.0% or less in total, an average grain size of the martensite and the tempered martensite in the metallographic structure of the 20 ?m portion is 5.0 ?m or less, and a metallographic structure of a 75 ?m portion, which is at a position 75 ?m away from the surface in the sheet thickness direction, includes, by volume percentage, ferrite and bainite: 0.0% or more and 15.0% or less in total.

11. The cold-rolled steel sheet according to claim 10, comprising, as the chemical composition, by mass %, one or more of: Si: 0.005% or more and 1.00% or less; Ti: 0.001% or more and 0.050% or less; Nb: 0.001% or more and 0.050% or less; V: 0.01% or more and 0.50% or less; Cu: 0.01% or more and 1.00% or less; Ni: 0.01% or more and 1.00% or less; Cr: 0.01% or more and 1.00% or less; Mo: 0.01% or more and 0.50% or less; B: 0.0001% or more and 0.0100% or less; Ca: 0.0001% or more and 0.010% or less; Mg: 0.0001% or more and 0.0100% or less; REM: 0.0005% or more and 0.0500% or less; and Bi: 0.0005% or more and 0.050% or less.

12. The cold-rolled steel sheet according to claim 10, wherein a tensile strength is 1,400 MPa or more, a uniform elongation is 5.0% or more, and R/t, which is a value obtained by dividing a limit bend radius R in 90? V-bending by the sheet thickness t, is 5.0 or less.

13. The cold-rolled steel sheet according to claim 11, wherein a tensile strength is 1,400 MPa or more, a uniform elongation is 5.0% or more, and R/t, which is a value obtained by dividing a limit bend radius R in 90? V-bending by the sheet thickness t, is 5.0 or less.

14. The cold-rolled steel sheet according to claim 10, wherein a hot-dip galvanized layer is formed on the surface.

15. The cold-rolled steel sheet according to claim 11, wherein a hot-dip galvanized layer is formed on the surface.

16. The cold-rolled steel sheet according to claim 12, wherein a hot-dip galvanized layer is formed on the surface.

17. The cold-rolled steel sheet according to claim 13, wherein a hot-dip galvanized layer is formed on the surface.

18. The cold-rolled steel sheet according to claim 14, wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.

19. The cold-rolled steel sheet according to claim 15, wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.

20. The cold-rolled steel sheet according to claim 16, wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.

21. The cold-rolled steel sheet according to claim 17, wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.

22. A manufacturing method of a cold-rolled steel sheet, the method comprising: a hot rolling process of heating, as necessary, a cast slab containing, as a chemical composition, by mass %, C: 0.180% or more and 0.350% or less, Mn: 2.00% or more and 4.00% or less, P: 0% or more and 0.100% or less, S: 0% or more and 0.010% or less, Al: 0% or more and 0.100% or less, N: 0% or more and 0.0100% or less, Si: 0% or more and 1.00% or less, Ti: 0% or more and 0.050% or less, Nb: 0% or more and 0.050% or less, V: 0% or more and 0.50% or less, Cu: 0% or more and 1.00% or less, Ni: 0% or more and 1.00% or less, Cr: 0% or more and 1.00% or less, Mo: 0% or more and 0.50% or less, B: 0% or more and 0.0100% or less, Ca: 0% or more and 0.010% or less, Mg: 0% or more and 0.0100% or less, REM: 0% or more and 0.0500% or less, Bi: 0% or more and 0.050% or less, and a remainder of Fe and impurities, and performing hot rolling on the cast slab to obtain a hot-rolled steel sheet; a coiling process of cooling the hot-rolled steel sheet to a coiling temperature of 550? C. or lower and coiling the hot-rolled steel sheet at the coiling temperature; a cold rolling process of pickling the hot-rolled steel sheet and performing cold rolling on the hot-rolled steel sheet to obtain a cold-rolled steel sheet; an annealing process of heating the cold-rolled steel sheet after the cold rolling process to a soaking temperature of 820? C. or higher in a nitrogen-hydrogen mixed atmosphere having a dew point of ?20? C. or higher and 20? C. or lower and containing 1.0 volume % or more and 20 volume % or less of hydrogen as an in-furnace atmosphere during heating so that an average heating rate from 700? C. to the soaking temperature is slower than 10.0? C./sec, and annealing the cold-rolled steel sheet at the soaking temperature for 30 seconds or longer and 200 seconds or shorter; a first cooling process of cooling the cold-rolled steel sheet after the annealing process to a temperature range of higher than 425? C. and lower than 600? C.; a holding process of allowing the cold-rolled steel sheet to stay in the temperature range of higher than 425? C. and lower than 600? C. for 200 seconds or longer and 750 seconds or shorter after the first cooling process; a second cooling process of cooling the cold-rolled steel sheet to a temperature of 50? C. or higher and 250? C. or lower after the holding process; a tempering process of tempering the cold-rolled steel sheet at a temperature of 200? C. or higher and 350? C. or lower for 1 second or longer after the second cooling process; a third cooling process of cooling the cold-rolled steel sheet to a temperature at which skin pass rolling is possible after the tempering process; and a skin pass process of performing skin pass rolling on the cold-rolled steel sheet after the third cooling process, wherein a temperature of the hot-rolled steel sheet is caused to reach 500? C. or lower within 10 hours from completion of the hot rolling process.

23. The manufacturing method of a cold-rolled steel sheet according to claim 22, wherein the cast slab contains, as the chemical composition, by mass %, one or more of Si: 0.005% or more and 1.00% or less, Ti: 0.001% or more and 0.050% or less, Nb: 0.001% or more and 0.050% or less, V: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Cr: 0.01% or more and 1.00% or less, Mo: 0.01% or more and 0.50% or less, B: 0.0001% or more and 0.0100% or less, Ca: 0.0001% or more and 0.010% or less, Mg: 0.0001% or more and 0.0100% or less, REM: 0.0005% or more and 0.0500% or less, and Bi: 0.0005% or more and 0.050% or less.

24. The manufacturing method of a cold-rolled steel sheet according to claim 22, wherein, in the holding process, the cold-rolled steel sheet is immersed in a plating bath in a state where a temperature of the cold-rolled steel sheet is higher than 425? C. and lower than 600? C. to form a hot-dip galvanized layer on a surface of the cold-rolled steel sheet.

25. The manufacturing method of a cold-rolled steel sheet according to claim 23, wherein, in the holding process, the cold-rolled steel sheet is immersed in a plating bath in a state where a temperature of the cold-rolled steel sheet is higher than 425? C. and lower than 600? C. to form a hot-dip galvanized layer on a surface of the cold-rolled steel sheet.

26. The manufacturing method of a cold-rolled steel sheet according to claim 24, wherein an alloying treatment for alloying the hot-dip galvanized layer is performed in the holding process.

27. The manufacturing method of a cold-rolled steel sheet according to claim 25, wherein an alloying treatment for alloying the hot-dip galvanized layer is performed in the holding process.

Description

EXAMPLES

[0174] The present invention will be described more specifically with reference to examples.

[0175] Slabs having the chemical compositions shown in Table 1 were cast. The slab after the casting was heated to 1,100? C. or higher, hot-rolled to 2.8 mm so that the finish rolling outlet side temperature was the Ar3 transformation point or higher, coiled at the coiling temperature shown in Tables 2A and 2B, and then cooled to room temperature. Here, the time from the completion of the hot rolling until the temperature of the steel sheet reached 500? C. or lower and the time until the temperature reached 450? C. or lower were as shown in Tables 2A and 2B.

[0176] Thereafter, descaling was performed by pickling. cold rolling to 1.4 mm was performed, and then annealing was performed under the conditions shown in Tables 2A and 2B. A holding time at the soaking temperature was 120 seconds. In addition, the in-furnace atmosphere was set to the nitrogen-hydrogen mixed atmosphere having a dew point of-20? C. or higher and 20? C. or lower and containing 1.0 volume % or more and 20 volume % or less of hydrogen.

[0177] After the annealing, the steel sheet was cooled to the holding temperature shown in Tables 2A and 2B at 10? C./sec (first cooling) and then allowed to stay at the temperature for the time shown in Tables 2A and 2B.

[0178] In some of the examples, hot-dip galvanizing and alloying were performed during holding. In Table 6. CR indicates a cold-rolled steel sheet that has not been galvanized, GI indicates a hot-dip galvanized steel sheet, and GA indicates a hot-dip galvannealed steel sheet. The hot-dip galvanized steel sheet was subjected to hot-dip galvanizing at about 35 to 65 g/m.sup.2. The hot-dip galvannealed steel sheet was subjected to hot-dip galvanizing of about 35 to 65 g/m.sup.2 and then alloying at a temperature of lower than 600?. In the examples, although the temperature during the staying time at higher than 425? C. and lower than 600? C. was set to be constant, the temperature during the staying time may be changed as described above as long as the temperature was within the temperature range.

[0179] In addition, after the holding, a heat treatment was performed in which cooling (second cooling) to a cooling stop temperature of 50? C. or higher and 250? C. or lower at 10.0? C./sec or faster was performed and thereafter tempering at a tempering temperature of 250? C. or higher and 350? C. or lower for 1 second or longer was performed. In a case where the cooling stop temperature was lower than the tempering temperature, tempering was performed by heating to the tempering temperature and holding at the temperature, and in a case where the cooling stop temperature was the same as the tempering temperature, tempering was performed by cooling and then holding at the temperature.

[0180] Thereafter, cooling (third cooling) to 50? C. was performed, and skin pass rolling with an elongation ratio of 0.1% to 1.0% was performed.

[0181] A test piece for SEM observation was collected from the obtained annealed steel sheet (cold-rolled steel sheet) as described above, a longitudinal section parallel to the rolling direction was polished, metallographic structures of the 20 ?m portion, the 60 ?m portion, the 75 ?m portion, and the t/4 portion were observed, and the volume percentage of each microstructure was measured in the above-described manner. In addition, the average grain sizes of martensite and tempered martensite in the 20 ?m portion were also obtained.

[0182] In addition, a test piece for X-ray diffraction was collected, and from a surface chemically polished to depth positions of 20 ?m, 75 ?m. and ? of the sheet thickness. the volume percentage of retained austenite was measured by X-ray diffraction.

[0183] The volume percentage of each microstructure in the t/4 portion was obtained at the center portion in the width direction. On the other hand. the volume percentage of each microstructure in the 20 ?m portion, the 60 ?m portion, and the 75 ?m portion. and the average grain sizes of martensite and tempered martensite in the 20 ?m portion were obtained at each of the edge portion, which was at a position 50 mm away from the end portion in the width direction of the steel sheet, and the center portion in the width direction.

[0184] The results are shown in Tables 3, 4A, 4B, 5A, and 5B.

[0185] The tensile strength (TS) and the uniform elongation (uEl) were obtained by collecting a JIS No. 5 tensile test piece from the center portion in the width direction of the obtained cold-rolled steel sheet in a direction perpendicular to the rolling direction, and conducting a tensile test according to JIS Z 2241 (2011). The results are shown in Table 6.

[0186] The limit bend radius (R/t) was obtained by obtaining a minimum bend radius R at which no cracking occurs in the center portion in the width direction of the obtained cold-rolled steel sheet when a 90? V-bending die was used and a radius R was changed at a pitch of 0.5 mm, and dividing the minimum bend radius by the sheet thickness (1.4 mm). The results are shown in Table 6.

[0187] In addition, the following test was conducted to evaluate the hydrogen embrittlement resistance. That is, a test piece having a mechanically ground end surface was bent into a U shape by a press bending method to prepare a U-bending test piece having a radius of 5R, the U-bending test piece was tightened with bolts to be elastically deformed so that non-bent portions were parallel to each other, and thereafter a delayed fracture acceleration test in which hydrogen was allowed to penetrate into the steel sheet was conducted by immersing the U-bending test piece in hydrochloric acid having a pH of 1. Those in which cracking did not occur even when an immersion time was 100 hours were evaluated as steel sheets having a good (OK) delayed fracture resistance property, and those in which cracking had occurred were evaluated as defective (NG). In order to remove an influence of plating, regarding a plating material, the plating layer was removed with hydrochloric acid containing an inhibitor before the test, and thereafter the hydrogen embrittlement resistance was evaluated.

[0188] The results are shown in Table 6.

[0189] The component proof stress was obtained by the following method.

[0190] The obtained cold-rolled steel sheet was subjected to press bending at R5 to be formed into a hat shape having a height of 50 mm. an upper side of 70 mm, a lower side of 120 mm, and a length of 900 mm, a steel sheet having the same size was matched to the lower side, and a flange portion was fixed by spot welding, whereby a model component was produced. In addition, as the cold-rolled steel sheet, a steel sheet in which a metallographic structure of a surface layer (that is, a 20 ?m portion) was the same as that of a t/4 portion was used, and a comparative component was produced in the same manner as in the model component. For each of the model component and the comparative component, a maximum load when the center portion was pressed against a circular indenter and bent was defined as the component proof stress. A case where the component proof stress of the model component satisfied 95% or more of the component proof stress of the comparative component was evaluated as OK.

[0191] However, a test for the component proof stress was conducted only on cold-rolled steel sheets having a tensile strength of 1,400 MPa or more and a limit bend radius (R/t) of 5.0 or less. The results are shown in Table 6.

TABLE-US-00001 TABLE 1 Chemical composition (mass %) (remainder: Fe and impurities) Steel C Si Mn P S Al N Others A 0.175 0.41 2.21 0.009 0.001 0.029 0.0031 B 0.223 0.01 2.75 0.009 0.001 0.031 0.0036 C 0.237 0.98 1.95 0.009 0.001 0.034 0.0032 D 0.356 0.42 3.85 0.009 0.001 0.033 0.0028 E 0.221 1.06 2.32 0.010 0.001 0.034 0.0032 F 0.321 0.82 4.05 0.009 0.001 0.028 0.0027 G 0.225 0.76 2.21 0.009 0.001 0.118 0.0032 H 0.240 0.73 2.63 0.008 0.001 0.035 0.0031 I 0.244 0.72 2.54 0.010 0.001 0.036 0.0032 J 0.225 0.71 2.65 0.009 0.001 0.031 0.0033 K 0.191 0.76 3.24 0.010 0.001 0.033 0.0035 L 0.223 0.74 2.44 0.009 0.001 0.029 0.0030 V: 0.10 M 0.244 0.75 2.53 0.009 0.001 0.040 0.0031 Ti: 0.024 Nb: 0.008 B: 0.0017 N 0.341 0.76 2.23 0.010 0.001 0.032 0.0033 Mo: 0.08 Cr: 0.28 O 0.233 0.77 2.57 0.009 0.001 0.036 0.0029 Bi: 0.007 REM: 0.0009 P 0.231 0.75 2.45 0.008 0.001 0.027 0.0037 Ca: 0.009 Mg: 0.0011 Q 0.232 0.76 2.44 0.009 0.001 0.029 0.0031 Ni: 0.10 Cu: 0.12

TABLE-US-00002 TABLE 2A Annealing Holding Average heating Staying time Hot rolling rate from 700? at the holding Coiling Time until Time until C. to soaking Soaking Holding temperature Test temperature 500? C. is 450? C. is temperature temperature temperature listed at left No. Steel (? C.) reached (hr) reached (hr) (? C./sec) (? C.) (? C.) (sec) 1 A 520 4 6 2.5 825 480 450 2 B 520 4 6 2.5 835 490 300 3 C 525 5 7 2.5 830 500 450 4 D 510 3 4 2.5 825 505 450 5 E 520 4 6 2.5 820 430 450 6 E 510 3 4 2.5 850 520 420 7 F 525 5 7 2.5 820 500 470 8 G 525 5 7 2.5 825 550 470 9 H 520 5 7 2.5 845 500 470 10 H 560 9 13 2.5 845 505 465 11 H 545 11 14 2.5 845 495 475 12 H 525 5 7 2.5 810 520 420 13 H 520 5 7 2.5 850 415 210 14 H 510 3 4 2.5 845 610 230 15 H 510 4 6 2.5 845 505 60 16 H 525 5 7 2.5 845 510 195 17 H 510 3 4 2.5 850 495 780 18 H 520 4 6 2.5 855 500 250 19 H 520 4 6 9.0 835 550 400

TABLE-US-00003 TABLE 2B Annealing Holding Average heating Staying time Hot rolling rate from 700? at the holding Coiling Time until Time until C. to soaking Soaking Holding temperature Test temperature 500? C. is 450? C. is temperature temperature temperature listed at left No. Steel (? C.) reached (hr) reached (hr) (? C./sec) (? C.) (? C.) (sec) 20 H 525 5 7 2.5 820 560 400 21 H 520 4 6 2.5 825 505 400 22 H 525 5 7 2.5 825 520 650 23 H 520 4 6 2.5 845 545 350 24 H 525 5 7 2.5 845 510 550 25 H 510 3 4 2.5 850 460 360 26 H 520 4 6 2.5 845 470 350 27 H 525 5 7 2.5 820 580 420 28 I 510 3 4 2.5 845 495 450 29 J 510 4 6 2.5 845 520 370 30 K 520 5 7 2.5 840 490 350 31 L 525 5 7 2.5 835 490 450 32 M 510 4 6 2.5 840 500 450 33 M 525 5 7 2.5 845 505 415 34 N 510 3 4 2.5 825 580 710 35 O 510 4 6 2.5 820 510 420 36 O 510 4 6 2.5 855 500 470 37 P 525 5 7 2.5 850 500 450 38 Q 530 5 7 2.5 855 500 470

TABLE-US-00004 TABLE 3 Metallographic structure of cold-rolled steel sheet (% indicates volume percentage) t/4 portion Ferrite and Test bainite in Retained Tempered No. Ferrite (%) Bainite (%) total (%) austenite (%) Martensite (%) martensite (%) Remainder (%) 1 4.2 10.9 15.1 3.2 0.0 81.7 0.0 2 0.0 5.6 5.6 2.1 0.0 92.3 0.0 3 3.1 25.2 28.3 5.2 0.0 66.5 0.0 4 0.0 0.0 0.0 7.6 5.4 87.0 0.0 5 15.4 35.4 50.8 10.3 6.4 32.5 0.0 6 0.0 14.6 14.6 5.1 0.0 80.3 0.0 7 0.0 0.0 0.0 6.1 5.2 88.7 0.0 8 25.4 0.0 25.4 6.4 0.0 68.2 0.0 9 0.0 9.8 9.8 5.0 0.0 85.2 0.0 10 0.0 9.5 9.5 4.9 0.0 85.6 0.0 11 0.0 10.2 10.2 5.1 0.0 84.7 0.0 12 18.6 3.6 22.2 6.0 1.4 70.4 0.0 13 0.0 20.4 20.4 6.2 1.5 71.9 0.0 14 22.4 0.0 22.4 5.9 1.4 64.9 5.4 15 0.0 1.2 1.2 5.3 0.0 93.5 0.0 16 0.0 3.1 3.1 5.2 0.0 91.7 0.0 17 0.0 25.2 25.2 5.1 0.0 76.1 0.0 18 0.0 3.9 3.9 5.0 0.0 91.1 0.0 19 8.6 0.0 8.6 5.2 0.0 85.4 0.8 20 10.5 0.0 10.5 5.1 0.0 83.1 1.3 21 3.8 4.8 8.6 5.4 0.0 85.1 0.9 22 3.8 10.1 13.9 5.5 0.0 80.6 0.0 23 0.0 0.0 0.0 5.1 0.0 94.9 0.0 24 0.0 11.5 11.5 5.9 0.0 82.6 0.0 25 0.0 14.7 14.7 5.2 0.0 80.1 0.0 26 0.0 10.8 10.8 5.3 0.0 83.9 0.0 27 12.2 0.0 12.2 5.3 0.0 80.4 2.1 28 0.0 9.2 9.2 5.5 0.0 85.3 0.0 29 0.0 0.0 0.0 5.3 0.0 94.7 0.0 30 0.0 3.1 3.1 5.1 0.0 91.8 0.0 31 0.0 9.8 9.8 5.6 0.0 84.6 0.0 32 0.0 10.1 10.1 5.6 0.0 84.3 0.0 33 0.0 9.3 9.3 5.5 0.0 85.2 0.0 34 8.7 0.0 8.7 7.4 2.8 80.2 0.9 35 3.7 6.5 10.2 5.4 0.0 84.4 0.0 36 0.0 8.5 8.5 5.5 0.0 86.0 0.0 37 0.0 9.7 9.7 5.6 0.0 84.7 0.0 38 0.0 9.6 9.6 5.5 0.0 84.9 0.0

TABLE-US-00005 TABLE 4A Metallographic structure of cold-rolled steel sheet (% indicates volume percentage) Position 60 ?m away from Position 20 ?m away from surface (center portion in width direction) Position 75 ?m away surface (center Average from surface (center portion in grain size portion in width direction) width direction) Martensite of martensite Ferrite and Ferrite and Ferrite and and tempered and tempered Test Ferrite Bainite bainite in bainite in Ferrite Bainite bainite in martensite martensite No. (%) (%) total (%) total (%) (%) (%) total (%) in total (%) (?m) 1 4.3 11.2 15.5 18.7 74.6 10.8 85.4 15 3.5 2 0.0 5.7 5.7 7.6 25.2 51.5 76.7 23 4.6 3 3.2 25.3 28.5 33.1 20.8 55.4 76.2 24 4.3 4 0.0 0.0 0.0 1.4 20.1 22.4 42.5 58 9.3 5 50.2 30.1 80.3 80.7 50.6 31.4 82.0 18 3.8 6 10.6 25.4 36.0 38.7 48.6 35.4 84.0 16 3.7 7 0.0 0.0 0.0 1.4 22.4 25.1 47.5 53 7.7 8 25.6 0.0 25.6 28.4 56.4 28.1 84.5 16 3.6 9 0.0 10.0 10.0 11.5 28.0 54.0 82.0 18 3.5 10 4.5 10.8 15.3 17.9 29.2 53.1 82.3 18 3.5 11 0.0 10.2 10.2 11.6 27.6 56.2 83.8 18 3.5 12 25.4 20.1 45.5 54.2 59.4 27.6 87.0 13 3.6 13 0.0 20.5 20.5 23.3 3.7 71.4 75.1 25 4.5 14 22.5 2.1 24.6 29.0 60.9 3.5 64.4 36 6.2 15 0.0 1.3 1.3 2.4 19.5 37.2 56.7 43 7.3 16 0.0 3.2 3.2 5.6 22.9 45.7 68.6 31 5.3 17 0.0 25.4 25.4 29.3 30.8 53.7 84.5 13 3.7 18 0.0 4.1 4.1 6.0 26.8 48.9 75.7 24 4.7 19 8.4 0.0 8.4 11.2 47.2 30.0 77.2 23 4.5

TABLE-US-00006 TABLE 4B Metallographic structure of cold-rolled steel sheet (% indicates volume percentage) Position 60 ?m away from Position 20 ?m away from surface (center portion in width direction) Position 75 ?m away surface (center Average from surface (center portion in grain size portion in width direction) width direction) Martensite of martensite Ferrite and Ferrite and Ferrite and and tempered and tempered Test Ferrite Bainite bainite in bainite in Ferrite Bainite bainite in martensite martensite No. (%) (%) total (%) total (%) (%) (%) total (%) in total (%) (?m) 20 10.6 0.0 10.6 12.0 47.5 30.2 77.7 22 4.5 21 3.9 4.7 8.6 9.7 32.1 51.8 83.9 16 4.0 22 3.9 10.2 14.1 14.6 36.1 49.1 85.2 15 3.7 23 0.0 0.0 0.0 2.4 44.2 34.2 78.4 22 4.1 24 0.0 11.4 11.4 13.5 32.0 53.9 85.9 14 3.7 25 0.0 14.8 14.8 15.0 7.5 77.9 85.4 15 3.6 26 0.0 11.2 11.2 12.4 11.1 72.9 84.0 16 3.7 27 12.3 0.0 12.3 13.6 50.9 25.0 75.9 24 4.7 28 0.0 9.3 9.3 10.4 32.4 53.8 86.2 14 3.7 29 0.0 0.0 0.0 2.1 40.8 41.3 82.1 18 3.9 30 0.0 3.2 3.2 4.3 14.6 62.4 77.0 23 4.8 31 0.0 10.0 10.0 11.7 30.3 53.6 83.9 16 3.7 32 0.0 10.0 10.0 11.4 32.1 53.4 85.5 15 3.7 33 0.0 9.5 9.5 10.5 31.2 53.3 84.5 16 3.7 34 8.6 0.0 8.6 9.7 56.9 26.1 83.0 17 3.7 35 3.8 6.4 10.2 11.6 30.1 54.1 84.2 16 3.8 36 0.0 8.4 8.4 10.1 31.2 52.6 83.8 16 3.7 37 0.0 9.8 9.8 10.8 31.6 53.0 84.6 15 3.6 38 0.0 9.7 9.7 10.6 31.5 52.7 84.2 16 3.6

TABLE-US-00007 TABLE 5A Metallographic structure of cold-rolled steel sheet (% indicates volume percentage) Position 60 ?m away from Position 20 ?m away from surface (edge portion in width direction) Position 75 ?m away surface (edge Average from surface (edge portion in grain size portion in width direction) width direction) Martensite of martensite Ferrite and Ferrite and Ferrite and and tempered and tempered Test Ferrite Bainite bainite in bainite in Ferrite Bainite bainite in martensite martensite No. (%) (%) total (%) total (%) (%) (%) total (%) in total (%) (?m) 1 4.2 11.3 15.5 18.6 74.5 10.7 85.2 15 3.5 2 0.0 5.6 5.6 7.4 25.3 51.6 76.9 23 4.5 3 3.1 25.2 28.3 32.8 20.7 55.3 76.0 24 4.4 4 0.0 0.0 0.0 1.2 20.2 22.3 42.5 58 9.2 5 50.2 29.9 80.1 80.6 50.5 31.2 81.7 18 3.7 6 10.5 25.3 35.8 38.4 48.5 35.3 83.8 16 3.8 7 0.0 0.0 0.0 1.3 22.3 25.0 47.3 53 7.6 8 25.4 0.0 25.4 28.1 56.2 28.2 84.4 16 3.7 9 0.0 10.1 10.1 11.4 27.9 53.6 81.5 19 3.6 10 4.4 10.7 15.1 17.4 29.1 53.0 82.1 18 3.5 11 0.0 10.1 10.1 11.4 16.0 38.5 54.5 46 6.8 12 25.3 20.0 45.3 55.6 59.3 27.4 86.7 13 3.7 13 0.0 20.4 20.4 23.4 3.6 71.3 74.9 25 4.4 14 22.4 2.0 24.4 29.2 60.8 3.6 64.4 36 6.3 15 0.0 1.2 1.2 2.5 19.6 37.1 56.7 43 7.2 16 0.0 3.3 3.3 5.5 22.8 45.6 68.4 32 5.4 17 0.0 25.5 25.5 29.4 30.7 53.6 84.3 16 3.8 18 0.0 3.9 3.9 5.6 26.7 48.8 75.5 25 4.6 19 8.2 0.0 8.2 11.4 47.5 30.4 77.9 24 4.6

TABLE-US-00008 TABLE 5B Metallographic structure of cold-rolled steel sheet (% indicates volume percentage) Position 60 ?m away from Position 20 ?m away from surface (edge portion in width direction) Position 75 ?m away surface (edge Average from surface (edge portion in grain size portion in width direction) width direction) Martensite of martensite Ferrite and Ferrite and Ferrite and and tempered and tempered Test Ferrite Bainite bainite in bainite in Ferrite Bainite bainite in martensite martensite No. (%) (%) total (%) total (%) (%) (%) total (%) in total (%) (?m) 20 10.6 0.0 10.6 11.8 47.6 30.1 77.7 22 4.5 21 4.0 4.6 8.6 10.0 32.1 51.7 83.8 16 4.1 22 3.8 10.1 13.9 14.7 36.2 48.8 85.0 15 3.6 23 0.0 0.0 0.0 2.1 44.1 34.1 78.2 22 4.2 24 0.0 11.3 11.3 13.4 31.9 53.8 85.7 14 3.6 25 0.0 14.6 14.6 15.0 7.3 77.8 85.1 15 3.7 26 0.0 11.1 11.1 12.1 10.9 73.0 83.9 16 3.6 27 12.1 0.0 12.1 13.4 50.7 25.1 75.8 24 4.8 28 0.0 9.2 9.2 10.5 32.3 53.6 85.9 14 3.8 29 0.0 0.0 0.0 2.4 40.7 41.2 81.9 18 3.8 30 0.0 3.1 3.1 4.6 14.5 62.3 76.8 23 4.7 31 0.0 9.9 9.9 11.5 30.2 53.5 83.7 16 3.6 32 0.0 10.1 10.1 11.6 32.0 53.3 85.3 15 3.8 33 0.0 9.4 9.4 10.3 31.0 53.1 84.1 16 3.6 34 8.5 0.0 8.5 9.8 56.8 26.2 83.0 17 3.8 35 3.8 6.3 10.1 11.4 30.1 54.2 84.1 16 3.7 36 0.0 8.3 8.3 10.3 31.1 52.5 83.6 16 3.8 37 0.0 9.7 9.7 10.9 31.5 52.8 84.3 16 3.6 38 0.0 9.6 9.6 10.7 31.2 52.5 83.7 16 3.7

TABLE-US-00009 TABLE 6 Presence or absence of Mechanical properties Hydrogen Test plating.sup.2) Uniform Limit bend embrittlement Component No. CR/GA/GI TS (MPa) elongation (%) radius.sup.1) (R/t) resistance proof stress.sup.3) Note 1 CR 1295 5.2 5.0 OK Comparative Example 2 CR 1565 5.1 4.6 OK OK Invention Example 3 CR 1361 7.4 5.0 OK Comparative Example 4 CR 1972 6.4 6.4 NG Comparative Example 5 CR 926 16.4 5.0 OK Comparative Example 6 CR 1420 6.8 5.0 OK NG Comparative Example 7 GA 1620 6.4 6.8 NG Comparative Example 8 CR 1295 8.1 3.9 OK Comparative Example 9 CR 1486 6.2 2.5 OK OK Example 10 CR 1482 6.3 2.5 OK NG Comparative Example 11 CR 1488 6.2 2.5 OK OK Comparative Example 12 CR 1322 7.7 5.4 OK Comparative Example 13 CR 1295 8.0 5.4 OK Comparative Example 14 GA 1250 8.2 5.4 OK Comparative Example 15 CR 1585 6.1 5.4 NG Comparative Example 16 CR 1565 6.0 5.4 NG Comparative Example 17 CR 1309 6.9 5.4 OK Comparative Example 18 CR 1565 6.0 2.5 OK OK Invention Example 19 GA 1471 6.3 2.9 OK OK Invention Example 20 GA 1452 6.3 2.9 OK OK Invention Example 21 CR 1490 6.4 2.5 OK OK Invention Example 22 GA 1401 5.9 2.9 OK OK Invention Example 23 CR 1588 5.7 2.1 OK OK Invention Example 24 CR 1449 7.0 2.9 OK OK Invention Example 25 GA 1419 6.8 2.9 OK OK Invention Example 26 CR 1463 6.7 2.5 OK OK Invention Example 27 CR 1415 6.7 2.9 OK OK Invention Example 28 CR 1480 6.8 2.5 OK OK Invention Example 29 GA 1580 6.1 2.1 OK OK Invention Example 30 CR 1402 7.2 2.1 OK OK Invention Example 31 CR 1475 6.7 2.5 OK OK Invention Example 32 GA 1477 6.4 2.5 OK OK Invention Example 33 CR 1485 6.4 2.5 OK OK Invention Example 34 CR 1844 6.5 4.6 OK OK Invention Example 35 GA 1471 6.4 2.5 OK OK Invention Example 36 GI 1493 6.3 2.5 OK OK Invention Example 37 GA 1475 6.6 2.5 OK OK Invention Example 38 GA 1479 6.7 2.5 OK OK Invention Example .sup.1)Limit bend radius is obtained by dividing a bend radius R at which cracking had occurred in 90? V-bending by the sheet thickness t .sup.2)CR indicates a non-plated material, GA indicates hot-dip galvannealing, and GI indicates hot-dip galvanizing .sup.3)A case where a component strength of a surface layer is 95% or more of a component strength when the same material as a center portion microstructure material is used was evaluated as OK

[0192] As can be seen from Tables 1 to 6, in invention examples in which the chemical composition and the metallographic structures of the t/4 portion, the 20 ?m portion, and the 75 ?m portion were within the ranges of the present invention, high strength and excellent bendability were exhibited, and the component proof stress was also sufficient.

[0193] Contrary to this, in comparative examples in which one or more of the chemical composition and the metallographic structures of the t/4 portion, the 20 ?m portion, and the 75 ?m portion were outside of the ranges of the present invention, any one or more of strength, bendability, and component proof stress did not satisfy the target.

[0194] In Test No. 11 (comparative example) in which Ferrite and bainite in total (% ), Martensite and tempered martensite in total (% ), and Average grain size of martensite and tempered martensite (?m) at the position 20 ?m away from the surface at the edge portion in the width direction were outside of the ranges of the present invention, although not shown in the tables, R/t (a value obtained by dividing the limit bend radius R in 90? V-bending by the sheet thickness t) measured at the edge portion in the width direction (position 50 mm away from the end portion in the width direction of the steel sheet) was as high as more than 5.0, and an overall quality of the steel sheet was not satisfied. As a result, a yield was significantly reduced.