High-strength steel sheet

Abstract

What is provided is a high-strength steel sheet including, by mass %: C: 0.05% to 0.15%; Si: 1.5% or less; Mn: 2.00% to 5.00%; P: 0.100% or less; S: 0.010% or less; Al: 0.001% to 2.000%; N: 0.010% or less; and a remainder of Fe and impurities, in which Ceq defined by Ceq=C+Si/90+Mn/100+1.5P+3S is less than 0.21, the high-strength steel sheet contains martensite in an area ratio of 98% or more, and a residual structure is in an area ratio of 2% or less, a two-dimensional homogeneous dispersion ratio S defined by S=Sy.sup.2/Sx.sup.2 (Sx.sup.2 is a dispersion value of Mn concentration profile data in a sheet width direction, and Sy.sup.2 is a dispersion value of the Mn concentration profile data in a sheet thickness direction) is 0.85 or more and 1.20 or less, and a tensile strength is 1200 MPa or more.

Claims

1. A high-strength steel sheet comprising, by mass %: C: 0.05% to 0.15%; Si: 1.5% or less; Mn: 2.00% to 5.00%; P: 0.100% or less; S: 0.010% or less; Al: 0.001% to 2.000%; N: 0.010% or less; and a remainder of Fe and impurities, wherein Ceq defined by Formula (1) is less than 0.21, the high-strength steel sheet contains martensite in an area ratio of 98% or more, and a residual structure is in an area ratio of 2% or less, a two-dimensional homogeneous dispersion ratio S defined by Formula (2) is 0.85 or more and 1.20 or less, and a tensile strength is 1200 MPa or more,
Ceq=C+Si/90+(Mn+Cr)/100+1.5P+3S  Formula (1)
S=Sy.sup.2/Sx.sup.2  Formula (2) wherein an amount, in mass %, of each element is substituted into a symbol corresponding to the element in Formula (1), 0 is substituted thereinto in a case where the element is not included, Sx.sup.2 in Formula (2) is a dispersion value of Mn concentration profile data in a sheet width direction and is represented by Sx.sup.2=( 1/200)×Σ(A−A.sub.i).sup.2, wherein A is an average value of Mn concentrations at 200 points in the sheet width direction, and Ai represents an i-th Mn concentration in the sheet width direction, where i=1 to 200, and Sy.sup.2 is a dispersion value of Mn concentration profile data in a sheet thickness direction and is represented by Sy.sup.2=( 1/200)×Σ(B−B.sub.i).sup.2, wherein B is an average value of Mn concentrations at 200 points in the sheet thickness direction, and Bi represents an i-th Mn concentration in the sheet thickness direction, where i=1 to 200.

2. The high-strength steel sheet according to claim 1, wherein, in a case where the residual structure is present, the residual structure is formed of retained austenite.

3. The high-strength steel sheet according to claim 1, further comprising, by mass %, one or two of: Ti: 0.100% or less; and Nb: 0.100% or less, in a total amount of 0.100% or less.

4. The high-strength steel sheet according to claim 1, further comprising, by mass %, one or two of: Cu: 1.000% or less; and Ni: 1.000% or less, in a total amount of 1.000% or less.

5. The high-strength steel sheet according to claim 1, further comprising, by mass %, one or two or more of: W: 0.005% or less; Ca: 0.005% or less; Mg: 0.005% or less; and a rare earth metal (REM): 0.010% or less, in a total amount of 0.010% or less.

6. The high-strength steel sheet according to claim 1, further comprising, by mass %: B: 0.0030% or less.

7. The high-strength steel sheet according to claim 1, further comprising, by mass %: Cr: 1.000% or less.

Description

EXAMPLES

(1) Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(2) A slab having the chemical composition shown in Table 1 was manufactured, and the slab was heated to 1300° C. for one hour, and then subjected to rough rolling and finish rolling under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was pickled and cold-rolled at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet. Subsequently, annealing and skin pass rolling were performed under the conditions shown in Table 2. In addition, each temperature shown in Table 2 is a surface temperature of the steel sheet. Furthermore, in Table 2, “difference in rolling reduction between passes in one reciprocation (return path−forward path)” indicates a difference in rolling reduction between two passes included in one reciprocating rolling in reverse rolling. In any of the examples, reverse rolling including a plurality of reciprocating passes was performed, and the difference in rolling reduction between the reciprocating passes was the same in all the reciprocating passes. For example, it is shown in the table that in Example No. 1, the “number of rough rolling passes” was 8, and the “difference in rolling reduction between passes in one reciprocation (return path−forward path)” was 5%. This means that in Example No. 1, four reciprocations of reverse rolling were performed, and the rolling reduction in the return path was larger than the rolling reduction in the forward path by 5% in all of the four reciprocations.

(3) In Table 2, Ac.sub.3 was calculated by the following formula. Into an element symbol in the following formula, the mass % of the corresponding element was substituted. 0 mass % is substituted into the elements not contained.
Ac.sub.3=881−335×C+22×Si−24×Mn−17×Ni−1×Cr−27×Cu

(4) TABLE-US-00001 TABLE 1 Chemical composition (mass %) Steel type C Si Mn P S Al N Ti Nb Cu Cr Ni W Ca Mg REM B Ceq A 0.13 1.0 3.00 0.012 0.004 0.020 0.003 0.20 B 0.15 1.0 2.40 0.009 0.002 0.020 0.003 0.030 0.20 C 0.12 0.9 2.20 0.013 0.015 0.020 0.003 0.22 D 0.09 1.2 2.80 0.012 0.004 0.020 0.003 0.010 0.16 E 0.09 1.0 2.60 0.015 0.004 0.020 0.003 0.005 0.005 0.16 F 0.07 1.0 2.90 0.012 0.007 0.020 0.003 0.003 0.15 G 0.02 0.6 2.00 0.009 0.004 0.020 0.003 0.005 0.07 H 0.10 1.5 2.50 0.120 0.004 0.020 0.003 0.33 I 0.10 1.4 2.50 0.010 0.003 0.020 0.003 0.010 0.16 J 0.12 1.1 2.20 0.011 0.004 0.020 0.003 0.004 0.009 0.18 K 0.11 1.5 0.05 0.009 0.008 0.020 0.003 0.005 0.16 L 0.10 1.0 2.90 0.010 0.006 0.020 0.003 0.17 M 0.24 1.0 2.50 0.013 0.004 0.020 0.003 0.31 N 0.14 1.0 2.50 0.011 0.004 0.020 0.003 0.0019 0.20 O 0.13 0.8 2.20 0.020 0.004 0.020 0.003 0.005 0.20 P 0.10 1.5 2.50 0.010 0.006 0.020 0.003 0.17 Q 0.13 0.8 2.60 0.010 0.004 3.000 0.003 0.19 R 0.13 0.8 2.50 0.010 0.004 0.020 0.015 0.19 S 0.13 1.0 3.50 0.020 0.008 0.020 0.003 0.23 Bold and underlined value is out of scope of the present invention. Blank cell in the table indicates that the chemical composition corresponding thereto is not intendedly added.

(5) TABLE-US-00002 TABLE 2-1 Manufacturing conditions Rough rolling Difference in rolling reduction between Duration Rough passes between Finish rolling Number rolling in one Rough rough Finish Rolling Finish of Maximum start reciprocation rolling rolling and Hot rolling reduction rolling Coiling rough reduction temper- (return path − finish finish rolling start of finish temper- Steel rolling in rough ature forward temperature rolling stands temperature first temperature ature No. type passes rolling (%) (° C.) path) (%) (° C.) (second) (times) (° C.) stand (%) (° C.) (° C.) 1 A 8 25 1200 5 1050 7 4 1000 20 900 240 2 A 8 30 1200 5 1050 7 4 1000 20 900 230 3 B 8 30 1200 5 1050 7 4 1000 20 900 250 4 C 8 30 1200 5 1050 7 4 1000 20 900 270 5 D 8 30 1200 5 1050 7 4 1000 20 900 230 6 E 8 25 1200 5 1050 7 4 1000 20 850 250 7 E 8 25 1200 5 1050 7 4 1000 20 900 300 8 E 8 30 1200 5 1050 7 4 1000 20 900 200 9 F 8 30 1200 5 1050 7 4 1000 20 900 200 10 F 8 30 1200 5 1050 7 4 1000 20 900 200 11 G 8 30 1200 5 1050 7 4 1000 20 900 250 12 H 8 30 1200 5 1050 7 4 1000 20 850 200 13 I 8 25 1200 5 1050 7 4 1000 20 900 230 14 I 8 30 1200 25  1050 7 4 1000 20 900 230 15 I 8 25 1200 −10  1050 7 4 1000 20 900 230 16 J 8 30 1200 5 1050 7 4 1000 20 900 250 17 J 13  30 1200 5 1050 7 4 1000 20 900 220 18 J 8 30 1200 5 1050 7 4 1000 20 900 240 19 K 8 30 1200 5 1050 7 4 1000 20 900 180 20 L 8 30 1200 5 1050 7 4 1000 20 900 200 21 L 8 45 1200 5 1050 7 4 1000 20 900 200 22 L 8 30 1200 5 1050 2 4 1000 20 900 200 23 M 8 30 1200 5 1050 7 4 1000 20 900 180 24 N 8 30 1200 5 1050 7 4 1000 20 850 190 25 N 8 30 1200 5 1050 7 2 1000 20 850 190 26 N 8 30 1200 5 1050 7 4 1000 20 850 190 27 O 8 30 1200 5 1050 7 4 1000 20 900 210 28 P 8 30 1200 5 1050 7 4 1000 20 900 200 29 P 8 30 1200 5 1050 7 4 1000 10 900 200 30 P 8 30 1200 5 1150 12  4 1150 20 900 200 31 Q 8 25 1200 5 1050 7 4 1000 20 900 210 32 R 8 30 1200 5 1050 7 4 1000 20 900 270 33 S 8 30 1200 5 1050 7 4 1000 20 900 270 Bold and underlined value is out of the preferred range.

(6) TABLE-US-00003 TABLE 2-2 Manufacturing conditions Annealing Cold Cooling stop Skin pass rolling Average Annealing Average rolling rolling Rolling heating rate temperature Annealing cooling rate temperature Rolling No. reduction (%) Ac3(° C.) (° C./s) (° C.) time (Second) (° C./s) (° C.) reduction (%) 1 35 799 20 900 300 100 45 0.5 2 30 799 20 850 200 100 40 None 3 30 795 20 850 200 100 50 0.5 4 30 808 20 900 200 100 45 0.5 5 40 810 20 900 200 100 45 0.5 6 35 810 20 900 200 100 45 0.5 7 30 810 20 750 200 100 50 0.5 8 30 810 20 850  2 100 50 0.5 9 30 810 20 900 200  50 45 0.5 10 40 810 20 900 200  1 40 0.5 11 45 840 20 900 200  50 50 0.5 12 30 827 20 900 200  50 45 0.5 13 40 827 20 900 200  50 50 0.5 14 40 827 20 900 200  50 45 0.5 15 40 827 20 900 200  50 50 0.5 16 40 812 20 900 200  70 50 0.5 17 40 812 20 900 200  50 45 0.5 18 40 812 20 850 200  50 350  0.5 19 40 876 20 880 200 100 45 0.5 20 40 800 20 900 300  50 40 0.5 21 40 800 20 900 300 100 45 0.5 22 35 800 20 900 300  50 40 0.5 23 40 763 20 900 300  50 45 0.5 24 45 796 20 850 200  50 45 0.5 25 45 796 20 850 200  50 45 0.5 26 65 796 20 850 200  50 45 0.5 27 40 802 20 900 200 200 40 0.5 28 45 832 20 880 200 100 50 0.5 29 45 832 20 880 200 100 50 0.5 30 45 832 20 880 200 100 50 0.5 31 40 793 20 900 200 100 55 0.5 32 35 795 20 850 200  50 45 0.5 33 40 775 20 850 200  50 45 0.5 Bold and underlined value is out of the preferred range

(7) The area ratios of martensite and retained austenite were obtained for the obtained cold-rolled steel sheet by SEM-EBSD and an X-ray diffraction method.

(8) In particular, the area ratio of martensite was determined as follows. First, a sample was taken with a sheet thickness cross section perpendicular to the rolling direction of the steel sheet as an observed section, the observed section was polished, the structure thereof at a thickness ¼ position of the steel sheet was observed with SEM-EBSD at a magnification of 5,000-fold, the resultant was subjected to image analysis in a visual field of 100 μm×100 μm to measure the area ratio of martensite, and the average of values measured at any five visual fields was determined as the area ratio of martensite. The area ratio of retained austenite was determined by an X-ray diffraction measurement. Specifically, a portion from the surface of the steel sheet to the thickness ¼ position of the steel sheet was removed by mechanical polishing and chemical polishing, and the X-ray diffraction intensity at a depth ¼ position from the surface of the steel sheet was measured using MoKα radiation as a characteristic X-ray. Then, from the integrated intensity ratios between the diffraction peaks of (200) and (211) of a body-centered cubic lattice (bcc) phase and (200), (220), and (311) of a face-centered cubic lattice (fcc) phase, the area ratio of retained austenite was calculated by using the following formula.
Sγ=(I.sub.200f+I.sub.220f+I.sub.311f)/(I.sub.200b+I.sub.211b)×100

(9) In the above formula, Sγ represents the area ratio of retained austenite, I.sub.200f, I.sub.220f, and I.sub.311f respectively represent the intensities of the diffraction peaks of (200), (220), and (311) of the fcc phase, and I.sub.200b and I.sub.211b respectively represent the intensities of the diffraction peaks of (200) and (211) of the bcc phase.

(10) In addition, the two-dimensional homogeneous dispersion ratio represented by S was obtained by an EMPA device.

(11) Furthermore, the tensile strength TS, fracture elongation EL, amount of bake hardening BH, and tensile strength BHTS after bake hardening of the obtained cold-rolled steel sheet were measured. In the measurement of the tensile strength TS, fracture elongation EL, amount of bake hardening BH, and tensile strength BHTS after bake hardening, JIS No. 5 tensile test pieces whose longitudinal direction was perpendicular to the rolling direction were taken, and a tensile test was conducted according to JIS Z 2241. The amount of bake hardening BH is a value obtained by subtracting the stress at the time of application of 2% prestrain from the stress when a test piece subjected to a heat treatment at 170° C. for 20 minutes is re-tensioned after the application of 2% prestrain. The tensile strength BHTS after bake hardening is the stress when the test piece subjected to the heat treatment at 170° C. for 20 minutes after the application of 2% prestrain is re-tensioned. In order to satisfy the demand for a reduction in the weight of a vehicle body, the tensile strength is 1200 MPa or more, preferably 1300 MPa or more, and more preferably 1400 MPa or more. Furthermore, the elongation is preferably 5% or more for facilitating forming. In addition, regarding BH, with a BH of less than 130 MPa, it is difficult to perform forming and the strength after forming becomes low. Therefore, a BH of 130 MPa or more is required to provide excellent bake hardenability. The BH is more preferably 150 MPa or more. Regarding BHTS, a BHTS of 1350 MPa or more is required to improve collision performance by bake hardening. The BHTS is more preferably 1400 MPa or more.

(12) As an evaluation of weldability, a test piece was taken according to JIS Z 3137, the same steel sheets were spot-welded, and a cross tensile test was conducted. Specifically, when a cross tensile test was conducted on a welding material under the condition that a nugget diameter became 6 mm by changing a current value with an electrode DR6 mm-40R, a welding time of 15 cycles/60 Hz, and a welding pressure of 400 kgf, a case in which fracture occurred at the base metal was determined as good, and a case in which fracture occurred at the nugget was determined as bad.

(13) TABLE-US-00004 TABLE 3 Mechanical property and structure Steel structure Two- Area ratio Area ratio dimensional Mechanical property values of of homogeneous Example TS BH BHTS martensite residual dispersion No. (MPa) EL (%) (MPa) (MPa) Weldability (%) γ (%) ratio S Remarks 1 1351 7.3 153 1449 GOOD 99 1 1.06 Example 2 1346 7.5 120 1401 GOOD 97 3 0.98 Comparative Example 3 1259 9.0 145 1376 GOOD 99 1 1.01 Example 4 1345 7.4 161 1443 BAD 100  0 0.97 Comparative Example 5 1299 8.6 151 1366 GOOD 100  0 1.02 Example 6 1279 8.8 148 1352 GOOD 99 1 0.88 Example 7  441 30.2   64  451 GOOD 45 0 0.95 Comparative Example 8 1040 18.5   95 1089 GOOD 65 0 0.95 Comparative Example 9 1254 8.8 159 1389 GOOD 100  0 1.04 Example 10  989 11.1   92 1072 GOOD 64 0 1.03 Comparative Example 11  781 13.4   68  825 GOOD 100  0 0.98 Comparative Example 12 1304 7.9 147 1496 BAD 100  0 0.98 Comparative Example 13 1376 7.6 156 1489 GOOD 99 1 1.02 Example 14 1372 7.5 121 1444 BAD 99 1 0.76 Comparative Example 15 1372 7.7 122 1477 BAD 99 1 0.75 Comparative Example 16 1322 7.8 155 1442 GOOD 98 2 1.00 Example 17 1319 7.8 101 1420 BAD 99 1 0.72 Comparative Example 18 1251 9.1 111 1378 GOOD 97 3 1.03 Comparative Example 19  590 20.1   71  601 GOOD 11 0 1.02 Comparative Example 20 1363 7.3 156 1501 GOOD 99 1 1.09 Example 21 1352 7.3 121 1471 BAD 99 1 0.75 Comparative Example 22 1359 7.5 120 1481 BAD 99 1 0.71 Comparative Example 23 1557 4.0 111 1670 BAD 92 8 0.99 Comparative Example 24 1289 8.4 149 1358 GOOD 100  0 0.98 Example 25 1287 8.3 125 1331 BAD 100  0 0.71 Comparative Example 26 1281 8.6 122 1328 BAD 100  0 0.72 Comparative Example 27 1354 7.7 151 1477 GOOD 100  0 0.92 Example 28 1326 7.8 148 1489 GOOD 100  0 1.07 Example 29 1325 7.9 122 1422 BAD 100  0 0.74 Comparative Example 30 1330 7.9 124 1423 BAD 100  0 0.72 Comparative Example 31 1311 7.9 152 1441 BAD 100  0 1.06 Comparative Example 32 1299 8.2 148 1427 BAD 99 1 0.98 Comparative Example 33 1269 8.4 149 1374 BAD 99 1 1.05 Comparative Example Bold and underlined value is out of scope of the present invention, or the preferred range.

(14) [Evaluation Results]

(15) As shown in Table 3, in Examples 1, 3, 5, 6, 9, 13, 16, 20, 24, 27 and 28, excellent tensile strength, bake hardenability, and weldability could be obtained. In all the cases, the tensile strength was 1200 MPa or more, the BH was 130 MPa or more, the BHTS was 1350 MPa or more, and the base metal had fractured in the cross tensile test, so that it was shown that the strength was high, the bake hardenability was excellent, and the weldability was also excellent.

(16) On the other hand, in Comparative Example 2, since there was no skin pass rolling, retained austenite remained and the BH was low. In Comparative Example 4, since the S content was too high, and the Ceq was high and weldability was poor. In Comparative Example 7, since the annealing temperature was too low, a ferrite structure appeared, a sufficient martensite structure was not obtained, and as a result, the TS, BH, and BHTS were low. In Comparative Example 8, since the annealing time was too short, the martensite structure was formed not over the entire area, and the TS, BH, and BHTS were similarly low. In Comparative Example 10, since the average cooling rate in the annealing was too slow, the martensite structure was formed not over the entire area, and the TS, BH, and BHTS were low. In Comparative Example 11, since the C content was too small, the amount of solute carbon decreased, and the TS, BH, and BHTS were low. In Comparative Example 12, since the P content was too large, the weldability was poor. In Comparative Example 14, since the difference in the rolling reduction between the two passes during one reciprocation in the rough rolling was large, a structure with a uniform Mn concentration distribution was not formed, the BH was low, and the weldability was poor. In Comparative Example 15, since the rolling reduction in the even-numbered pass during one reciprocation in the rough rolling was smaller than the rolling reduction in the odd-numbered pass, a structure with a uniform Mn concentration distribution was not formed, the BH was low, and the weldability was poor. In Comparative Example 17, since the number of passes of reverse rolling in the rough rolling was an odd number, a structure with a uniform Mn concentration distribution was not formed, the BH was low, and the weldability was poor.

(17) In Comparative Example 18, since the cooling stop temperature in the annealing was high, a structure other than martensite appeared, and furthermore, iron carbide was precipitated, resulting in a reduction in the amount of solute carbon. Therefore, the BH was low. In Comparative Example 19, since the Mn content was too low, the TS, BH, and BHTS were low. In Comparative Example 21, since the rolling reduction of the reverse rolling in the rough rolling was high, a structure with a uniform Mn concentration distribution was not formed, the BH was low, and the weldability was poor. In Comparative Example 22, the time from the rough rolling to the finish rolling was too short, the Mn concentration distribution became flat, the BH was low, and the weldability was poor. In Comparative Example 23, since the C content was too high, the area ratio of retained austenite (y) was high, the BH was low, the Ceq was high, and the weldability was poor. In Comparative Example 25, since the number of rolling stands for the finish rolling was small, the Mn concentration distribution became flat, the BH and BHTS were low, and the weldability was poor. In Comparative Example 26, the cold rolling ratio was high, the Mn concentration distribution was elongated in the direction perpendicular to the sheet thickness and became flat, BH and BHTS were low, and the weldability was poor. In Comparative Example 29, the rolling reduction of the first stand in the finish rolling was small, the Mn concentration distribution became flat, the BH was low, and the weldability was poor. In Comparative Example 30, since the finish rolling temperature (finish rolling start temperature in Table 2) was too high, the Mn concentration distribution became flat, the BH was low, and the weldability was poor. In Comparative Example 31, since the Al content was too large, the weldability was poor. In Comparative Example 32, since the N content was too large, the weldability was poor. In Comparative Example 33, since the Ceq was too high, the weldability was poor.

INDUSTRIAL APPLICABILITY

(18) The high-strength steel sheet having excellent bake hardenability and weldability according to the present invention can be used as an original sheet of a structural material for a vehicle, particularly in an automotive industry field.