HOT-ROLLED STEEL SHEET

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure at a ¼ position of a sheet thickness in a sheet thickness direction from a surface, a primary phase is bainite, a secondary phase is martensite or a martensite-austenite mixed phase, an average grain size of the secondary phase is 1.5 μm or less, an average grain size of particles having grain diameters that are largest 10% or less out of all particles in the secondary phase is 2.5 μm or less, a pole density in a (110)<112> orientation is 3.0 or less, and, in a microstructure from the surface to a 1/16 position of the sheet thickness in the sheet thickness direction from the surface, a pole density in a (110)<1-11> orientation is 3.0 or less.

Claims

1. A hot-rolled steel sheet comprising, as a chemical composition, by mass %: C: 0.040% to 0.150%; Si: 0.50% to 1.50%; Mn: 1.00% to 2.50%; P: 0.100% or less; S: 0.010% or less; Al: 0.01% to 0.10%; N: 0.0100% or less; Ti: 0.005% to 0.150%; B: 0.0005% to 0.0050%; Cr: 0.10% to 1.00%; Nb: 0% to 0.06%; V: 0% to 0.50%; Mo: 0% to 0.50%; Cu: 0% to 0.50%; Ni: 0% to 0.50%; Sb: 0% to 0.020%; Ca: 0% to 0.010%; REM: 0% to 0.010%; Mg: 0% to 0.010%; and a remainder including iron and impurities, wherein, in a microstructure at a ¼ position of a sheet thickness in a sheet thickness direction from a surface, by area ratios, a primary phase is 90.0% to 98.0% of bainite, a secondary phase is 2.0% to 10.0% of martensite or a martensite-austenite mixed phase, an average grain size of the secondary phase is 1.5 μm or less, an average grain size of particles having grain diameters that are largest 10% or less out of all particles in the secondary phase is 2.5 μm or less, a pole density in a (110)<112> orientation is 3.0 or less, and in a microstructure from the surface to a 1/16 position of the sheet thickness in the sheet thickness direction from the surface, a pole density in a (110)<1-11> orientation is 3.0 or less.

2. The hot-rolled steel sheet according to claim 1, wherein, in the microstructure at the ¼ position of the sheet thickness in the sheet thickness direction from the surface, an average interval between MC carbide grains having a diameter of 20 nm or less is 50 nm or more.

3. The hot-rolled steel sheet according to claim 1, comprising, as the chemical composition, by mass %, one or more selected from the group of: Nb: 0.005% to 0.06%; V: 0.05% to 0.50%; Mo: 0.05% to 0.50%; Cu: 0.01% to 0.50%; Ni: 0.01% to 0.50%; Sb: 0.0002% to 0.020%; Ca: 0.0002% to 0.010%; REM: 0.0002% to 0.010%; and Mg: 0.0002% to 0.010%.

4. The hot-rolled steel sheet according to claim 2, comprising, as the chemical composition, by mass %, one or more selected from the group of: Nb: 0.005% to 0.06%; V: 0.05% to 0.50%; Mo: 0.05% to 0.50%; Cu: 0.01% to 0.50%; Ni: 0.01% to 0.50%; Sb: 0.0002% to 0.020%; Ca: 0.0002% to 0.010%; REM: 0.0002% to 0.010%; and Mg: 0.0002% to 0.010%.

Description

EXAMPLES

[0212] Next, examples of the present invention will be described. Conditions in the examples are examples of the conditions adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to these examples of the conditions. The present invention is capable of adopting a variety of conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

[0213] Steels having a chemical composition shown for Steel Nos. 1 to 42 in Tables 1 and 2 were made from melting, and slabs having a thickness of 240 to 300 mm were manufactured by continuous casting. Hot-rolled steel sheets were obtained under manufacturing conditions shown in Tables 3 and 4 using the obtained slabs. The “average cooling rate between FT and CT” in Tables 3 and 4 indicates the average cooling rate from the start of cooling after hot rolling to coiling (stop of cooling). In addition, before finish rolling, descaling was performed by a normal method (the collision pressure of water to be sprayed was less than 3.0 MPa). Only for No. 41, descaling was performed such that the collision pressure of water to be sprayed became 3.5 MPa.

TABLE-US-00001 TABLE 1 Steel Chemical composition, mass % (remainder: Fe and impurities) No. C Si Mn P S Al N Ti B Cr Nb V 1 0.082 1.30 1.92 0.082 0.003 0.03 0.0023 0.110 0.0017 0.61 2 0.071 0.81 1.65 0.048 0.009 0.03 0.0032 0.110 0.0012 0.63 0.02 3 0.069 0.71 1.75 0.059 0.008 0.03 0.0039 0.110 0.0014 0.68 0.05 4 0.065 0.77 1.82 0.055 0.007 0.03 0.0024 0.110 0.0015 0.98 0.02 0.05 5 0.091 0.62 1.23 0.051 0.002 0.03 0.0034 0.110 0.0014 0.66 6 0.097 0.67 1.79 0.048 0.002 0.03 0.0036 0.120 0.0016 0.52 7 0.112 1.11 1.65 0.063 0.008 0.03 0.0015 0.042 0.0017 0.40 8 0.142 1.19 2.30 0.034 0.002 0.03 0.0027 0.022 0.0021 0.71 9 0.137 0.75 1.80 0.066 0.007 0.03 0.0039 0.032 0.0023 0.62 10 0.144 0.70 2.31 0.063 0.010 0.03 0.0029 0.023 0.0020 0.30 11 0.080 0.75 1.80 0.081 0.001 0.03 0.0040 0.071 0.0012 0.62 0.02 12 0.061 0.75 1.70 0.060 0.009 0.03 0.0025 0.087 0.0013 0.62 0.05 13 0.077 0.81 1.72 0.025 0.009 0.03 0.0019 0.098 0.0021 0.65 14 0.071 0.71 1.85 0.078 0.003 0.03 0.0034 0.112 0.0015 0.58 15 0.067 0.75 1.90 0.063 0.006 0.03 0.0022 0.127 0.0025 0.62 16 0.064 0.75 1.70 0.065 0.004 0.03 0.0036 0.110 0.0015 0.72 0.02 17 0.064 0.75 1.70 0.038 0.009 0.03 0.0019 0.110 0.0015 0.71 0.02 18 0.160 0.75 1.70 0.027 0.002 0.03 0.0017 0.110 0.0015 0.62 19 0.035 0.75 1.70 0.094 0.004 0.03 0.0022 0.110 0.0015 0.62 20 0.110 0.20 1.48 0.054 0.008 0.03 0.0034 0.053 0.0014 0.88 Steel Chemical composition, mass % (remainder: Fe and impurities) No. Mo Cu Ni Sb Ca REM Mg Note: 1 Present Invention Steel 2 Present Invention Steel 3 Present Invention Steel 4 Present Invention Steel 5 0.10 Present Invention Steel 6 0.10 0.05 Present Invention Steel 7 0.008 Present Invention Steel 8 0.002 Present Invention Steel 9 0.004 Present Invention Steel 10 0.003 Present Invention Steel 11 0.10 Present Invention Steel 12 0.10 0.10 Present Invention Steel 13 Present Invention Steel 14 Present Invention Steel 15 Present Invention Steel 16 Present Invention Steel 17 0.10 Present Invention Steel 18 Comparative Steel 19 Comparative Steel 20 Comparative Steel Underlines indicate that values are outside the scope of the present invention.

TABLE-US-00002 TABLE 2 Steel Chemical composition, mass % (remainder: Fe and impurities) No. C Si Mn P S Al N Ti B Cr Nb V Mo Cu Ni Sb Ca REM Mg Note 21 0.095 1.70 1.73 0.023 0.002 0.03 0.0037 0.115 0.0005 0.46 Comparative Steel 22 0.092 0.53 0.80 0.058 0.004 0.03 0.0030 0.096 0.0042 0.90 Comparative Steel 23 0 065 1.09 2.60 0.055 0.002 0.03 0.0021 0.145 0.0037 0 30 Comparative Steel 24 0.092 0.52 1.57 0.074 0.003 0.03 0.0021 0.000 0.0050 0.99 Comparative Steel 25 0.103 1.49 2.13 0.028 0.002 0.03 0.0038 0.200 0.0033 0.55 Comparative Steel 26 0.061 0.73 1.50 0.021 0.007 0.03 0.0032 0.073 0.0000 0.25 Comparative Steel 27 0.040 0.60 2.40 0.076 0.009 0.03 0.0027 0.107 0.0021 0.05 Comparative Steel 28 0.109 1.40 1.97 0.051 0.002 0.03 0.0032 0.140 0.0018 1.20 Comparative Steel 29 0.100 1.46 2.38 0.029 0.007 0.03 0.0016 0.097 0.0031 0.16 Present Invention Steel 30 0.103 1.45 1.19 0.045 0.005 0.03 0.0026 0.059 0.0021 0.98 Present Invention Steel 31 0.041 0.61 1.82 0.085 0.003 0.03 0.0015 0.023 0.0016 0.63 Present Invention Steel 32 0.045 0.63 1.92 0.036 0.009 0.03 0.0024 0.04.2 0.0018 0.61 Present Invention Steel 33 0.130 0.54 1.80 0.066 0.002 0.03 0.0037 0.031 0.0013 0.73 Present Invention Steel 34 0.055 0.91 1.73 0.057 0.010 0.03 0.0020 0.020 0.0012 0.33 Present Invention Steel 35 0.048 0.81 1.65 0.084 0.003 0.03 0.0018 0.121 0.0019 0.32 Present Invention Steel 36 0.071 0.52 1.84 0.063 0.002 0.03 0.0026 0.101 0.0023 0.67 Present Invention Steel 37 0.082 0.56 1.82 0.065 0.005 0.03 0.0025 0.091 0.0021 0.27 Present Invention Steel 38 0.091 0.78 1.54 0.035 0.002 0.03 0.0024 0.076 0.0017 0.91 Present Invention Steel 39 0.063 0.99 2.12 0.098 0.005 0.03 0.0029 0.081 0.0023 0.87 Present Invention Steel 40 0.067 0.88 2.23 0.061 0.001 0.03 0.0031 0.081 0.0013 0.43 Present Invention Steel 41 0.071 0.71 1.82 0.051 0.002 0.03 0.0036 0.042 0.0021 0.71 Present Invention Steel 42 0.055 1.20 1.85 0.007 0.005 0.03 0.0021 0.120 0.0015 0.65 Present Invention Steel Underlines indicate that values are outside the scope of the present invention.

TABLE-US-00003 TABLE 3 Finish Average Rolling rolling Cooling cooling rate Coil Heating start completion start between FT Coiling cooling Test Steel temperature temperature temperature time and CT temperature rate No. No. ° C. ° C. ° C. Seconds ° C./sec ° C. ° C./hour Note 1  1 1264 1137 955 0.6 61 481 33 Present Invention Example 2  2 1295 1113 965 0.7 80 421 37 Present Invention Example 3  3 1250 1186 962 0.8 77 432 28 Present Invention Example 4  4 1287 1108 971 0.6 56 441 36 Present Invention Example 5  5 1285 1130 983 0.5 59 451 35 Present Invention Example 6  6 1277 1160 985 0.7 57 462 33 Present Invention Example 7  7 1264 1122 988 0.6 53 471 31 Present Invention Example 8  8 1291 1186 992 0.7 61 489 38 Present Invention Example 9  9 1253 1101 972 0.8 46 495 32 Present Invention Example 10 10 1292 1186 981 0.9 76 435 36 Present Invention Example 11 11 1300 1133 981 0.1 81 475 31 Present Invention Example 12 12 1288 1104 999 0.1 83 422 32 Present Invention Example 13 13 1279 1188 982 0.2 91 432 34 Present Invention Example 14 14 1287 1143 975 0.1 102 441 31 Present Invention Example 15 15 1273 1164 961 0.1 122 459 32 Present Invention Example 16 16 1265 1176 965 0.2 111 427 33 Present Invention Example 17 17 1275 1136 981 0.1 95 479 31 Present Invention Example 18 18 1275 1166 972 0.4 98 405 29 Comparative Example 19 12 1261 1129 972 0.2 98 450 28 Comparative Example 20 20 1295 1157 972 0.7 95 463 35 Comparative Example Underlines indicate that values are outside the scope of the present invention.

TABLE-US-00004 TABLE 4 Finish Average Rolling rolling Cooling cooling rate Coil Heating start completion start between FT Coiling cooling Test Steel temperature temperature temperature time and CT temperature rate No. No. ° C. ° C. ° C. Seconds ° C./sec ° C. ° C./hour Note 21 21 1287 1159 972 0.4  99 485 29 Comparative Example 22 22 1273 1143 972 0.7 101 426 31 Comparative Example 23 23 1267 1102 972 0.7 101 437 33 Comparative Example 24 24 1290 1101 951 0.9  99 451 41 Comparative Example 25 25 1264 1139 961 0.9 101 419 50 Comparative Example 26 26 1286 1185 963 0.8 101 492 80 Comparative Example 27 27 1265 1110 983 0.8  99 434 95 Comparative Example 28 28 1256 1174 972 0.8 101 463 100  Comparative Example 29 29 1277 1100 870 0.6 101 451 100  Comparative Example 30 30 1287 1196 1061  0.3  99 442 77 Comparative Example 31 31 1273 1175 1030  1.2 101 432 87 Comparative Example 32 32 1271 1123 1020  1.6 101 441 35 Comparative Example 33 33 1278 1175 983 0.9  10 475 41 Comparative Example 34 34 1297 1123 972 0.5 160 494 55 Comparative Example 35 35 1273 1175 980 0.4  98 385 51 Comparative Example 36 36 1291 1123 951 0.5  99 350 26 Comparative Example 37 37 1282 1175 971 0.6  97 465 10 Comparative Example 38 38 1256 1123 982 0.8 102 426 20 Comparative Example 39 39 1287 1175 911 0.7 101 438 31 Comparative Example 40 40 1277 1123 982 0.9  25 454 32 Comparative Example 41 41 1287 1130 985 0.7  51 442 33 Comparative Example 42 42 1273 1176 950 0.4  83 451 73 Comparative Example Underlines indicate that values are outside the scope of the present invention.

[0214] For the obtained hot-rolled steel sheets, the microstructural fraction at the ¼ position of the sheet thickness in the sheet thickness direction from the surface, the average grain size of the secondary phase, the average grain size of the particles having grain diameters that are largest 10% or less out of all of the particles in the secondary phase, the pole density in the (110)<112> orientation, the average interval between precipitates having a diameter of 20 nm or less, and the pole density in the (110)<1-11> orientation in the microstructure from the surface to the 1/16 position of the sheet thickness in the sheet thickness direction from the surface were obtained by the above-described methods. In Test Nos. 18, 33, 35, and 36, the secondary phase particles were connected, and it was not possible to measure the grain diameters as particles.

[0215] The obtained results are shown in Tables 5 and 6. In examples where the total of the area ratios of bainite and the secondary phase did not reach 100%, the remainder of the microstructure was ferrite. In addition, in Test No. 24, no precipitates having a diameter of 20 nm or less were observed.

TABLE-US-00005 TABLE 5 Pole density Average Average grain Pole density in (110)<1-11> interval Average size of particles in (110)<112> orientation from between grain having grain orientation at surface to sheet precipitates size of diameters that sheet thickness thickness 1/16 having Secondary secondary are largest ¼ position position diameter of 20 Test Steel Bainite phase phase 10% or less from Surface from surface nm or less No. No. Area % Area % μm μm — — nm Note  1  1 97.9 2.1 1.4 2.0 2.3 2.2 45 Present Invention Example  2  2 96.8 22 1.3 2.0 1.9 1.8 42 Present Invention Example  3  3 93.9 6.1 1.4 2.2 2.3 2.5 43 Present Invention Example  4  4 91.1 8.9 1.3 2.2 1.8 1.8 38 Present Invention Example  5  5 95.4 4.6 1.4 2.2 2.2 2.4 42 Present Invention Example  6  6 91.7 8.3 1.4 2.1 2.2 2.3 45 Present Invention Example  7  7 96.4 3.6 1.4 2.1 1.8 1.8 31 Present Invention Example  8  8 90.7 9.3 1.3 2.4 2.0 1.9 35 Present Invention Example  9  9 97.1 2.9 1.4 2.4 2.1 1.8 37 Present Invention Example 10 10 98.0 2.0 1.3 2.0 2.5 2.2 46 Present Invention Example 11 11 94.7 5.3 1.3 2.0 2.1 2.5 49 Present Invention Example 12 12 94.8 5.2 1.3 2.1 2.5 2.2 45 Present Invention Example 13 13 95.8 4.2 1.3 2.1 2.1 1.9 111 Present Invention Example 14 14 91.0 9.0 1.4 2.1 1.8 2.1 152 Present Invention Example 15 15 90.9 9.1 1.3 2.2 2.1 2.0 98 Present Invention Example 16 16 94.1 5.9 1.4 2.0 2.2 1.9 85 Present Invention Example 17 17 97.8 2.2 1.4 2.3 2.4 1.9 201 Present Invention Example 18 18 12.0 88.0  — — 2.4 2.2 35 Comparative Example 19 12 99.0 1.0 1.4 2.3 2.2 2.0 37 Comparative Example 20 20 92.0 8.0 1.4 2.0 2.3 2.4 21 Comparative Example Underlines indicate that values are outside the scope of the present invention.

TABLE-US-00006 TABLE 6 Pole density Average Average grain Pole density in (110)<I-11> interval Average size of particles in (110)<112> orientation from between grain having grain orientation at surface to sheet precipitates size of diameters that sheet thickness thickness 1/16 having Secondary secondary are largest ¼ position position diameter of 20 Test Steel Bainite phase phase 10% or less from surface from surface nm or less No. No. Area % Area % μm μm — — nm Note 21 21 95.2 4.8 1.3 2.2 2.4 2.0 22 Comparative Example 22 22 32.0 68.0  1.3 2.1 2.5 2.4 30 Comparative Example 22 23 86.9 13.1  1.4 2.1 2.2 1.9 33 Comparative Example 24 24 93.8 6.2 1.3 2.4 2.1 2.4 — Comparative Example 25 22 97 8 2.2 1.4 2.1 3.4 4.3 29 Comparative Example 26 26 29.8 0.2 1.3 2.0 1.9 2.2 34 Comparative Example 27 27 83.0 3.1 1.4 2.3 1.8 2.1 23 Comparative Example 28 28 87.8 12.2  1.3 2.4 2.0 2.2 35 Comparative Example 29 29 93.4 6.6 1.4 2.1 3.1 4.2 38 Comparative Example 30 30 94.2 5.8 1.6 2.2 2.5 1.6 41 Comparative Example 21 31 93.2 6.8 1.4 2.6 1.8 2.5 49 Comparative Example 32 32 97.2 2.8 1.4 2.8 2.4 1.8 48 Comparative Example 22 33 34.2 6.2 — — 1.8 2.2 35 Comparative Example 34 34 89.9 10.1  1.3 2.0 2.8 3.1 36 Comparative Example 25 35 15.0 85.0  — — 2.2 2.1 35 Comparative Example 36 36  0.0 100.0  — — 2.2 1.9 49 Comparative Example 37 37 98.5 1.5 1.3 2.3 2.4 2.1 48 Comparative Example 38 38 98.1 1.9 1.3 2.1 2.0 2.0 35 Comparative Example 39 39 92.3 7.7 1.3 2.2 2.5 3.2 40 Comparative Example 40 40 87.3 4.4 1.4 2.2 2.5 2.1 15 Comparative Example 41 41 91.2 8.8 1.4 2.0 1.9 3.5 38 Comparative Example 42 42 93.1 6.9 1.3 2.1 3.2 3.4 35 Comparative Example Underlines indicate that values are outside the scope of the present invention.

[0216] For the obtained hot-rolled steel sheets, the tensile strengths TS, the total elongations El, the hole expansion rates λ, the limit bend radii R, and the ductile brittle, transition temperatures vTrs were obtained by the following methods.

[0217] Tensile Strength TS and Total Elongation El

[0218] The tensile strength TS and the total elongation El were obtained by performing a tensile test using a JIS No. 5 test piece in accordance with JIS Z 2241: 2011. The cross-head speed was set to 10 mm/min. Cases where the tensile strength TS was 980 MPa or more were regarded as being excellent in terms of strength and determined as pass, and cases where the tensile strength was less than 980 MPa were regarded as being poor in strength and determined as fail. Cases where the total elongation El was 13.0% or more were regarded as being excellent in terms of ductility and determined as pass, and cases where the total elongation El was less than 13.0% were regarded as being poor in ductility and determined as fail.

[0219] Hole Expansion Rate 7

[0220] The hole expansibility was evaluated with the hole expansion rate λ that was obtained by punching a circular hole with a diameter of 10 mm using a 60° conical punch under a condition where the clearance became 12.5% and performing a hole expansion test such that burrs were formed on the die side. For each test number, a hole expansion test was performed five times, and the average value thereof was calculated, thereby obtaining the hole expansion rate λ. Cases where the hole expansion rate was 60% or more were regarded as being excellent in terms of hole expansibility and determined as pass, and cases where the hole expansion rate was less than 60% were regarded as being poor in hole expansibility and determined as fail.

[0221] Limit Bend Radius R

[0222] The bendability was evaluated with the limit bend radius R that was obtained by performing a V-bending test. The limit bend radius R was obtained by performing a V-bending test using a No. 1 test piece in accordance with JIS Z 2248: 2014 such that a direction perpendicular to a rolling direction became the longitudinal direction (the bend ridge line coincided with the rolling direction).

[0223] The V-bending test was performed by setting the angle between a die and a punch to 60° and changing the tip radii of the punches in 0.1 mm increments, and the maximum value of the tip radii of the punches that could be bent without cracking was obtained. The maximum value of the tip radii of the punches that could be bent without crack was regarded as the limit bend radius R. In a case where a value (R/t) obtained by dividing the limit bend radius R by the sheet thickness t of the test piece was 1.0 or less, the bendability was regarded as being excellent, determined as pass, and expressed as “Good” in Tables 7 and 8. On the other hand, in a case where a value (R/t) obtained by dividing the limit bend radius R by the sheet thickness t of the test piece was more than 1.0, the bendability was regarded as being poor, determined as fail, and expressed as “Bad” in Tables 7 and 8.

[0224] Ductile Brittle Transition Temperature vTrs

[0225] For the ductile brittle transition temperature vTrs, a Charpy impact test was performed using a V-notch test piece having a subsize of 2.5 mm regulated in JIS Z 2242: 2018. A temperature at which the brittle fracture surface ratio became 50% was obtained, and this was regarded as the ductile brittle transition temperature vTrs. In a case where the ductile brittle transition temperature vTrs was −40° C. or lower (−40° C. was included, negative values from −40° C.), the low temperature toughness was regarded as being excellent and determined as pass, and, in a case where the ductile brittle transition temperature vTrs was higher than −40° C. (−40° C. was not included, positive values from −40° C.), the low temperature toughness was regarded as being poor and determined as fail. In addition, in a case where the ductile brittle transition temperature vTrs was −70° C. or lower, the low temperature toughness was determined as more excellent.

[0226] The above-described test results are shown in Tables 7 and 8.

TABLE-US-00007 TABLE 7 Tensile Total Hole Ductile brittle strength elongation expansion transition Test Steel TS EI rate λ temperature vTrs No. No. MPa % % Bendability ° C. Note  1  1 1028 13.2 63 Good −45 Present Invention Example  2  2 1035 13.1 69 Good −52 Present Invention Example  3  3 1020 13.1 69 Good −55 Present Invention Example  4  4  991 13.2 66 Good −65 Present Invention Example  5  5 1057 13.3 60 Good −46 Present Invention Example  6  6 1032 13.1 63 Good −41 Present Invention Example  7  7 1079 13.2 60 Good −47 Present Invention Example  8  8 1015 13.5 67 Good −54 Present Invention Example  9  9 1004 13.4 62 Good −60 Present Invention Example 10 10 1066 13.1 64 Good −58 Present Invention Example 11 11 1006 13.6 67 Good −49 Present Invention Example 12 12  987 13.5 69 Good −45 Present Invention Example 13 13 1034 13.3 60 Good −82 Present Invention Example 14 14 1021 13.2 66 Good −84 Present Invention Example 15 15 1012 13.4 65 Good −77 Present Invention Example 16 16 1015 13.1 66 Good −79 Present Invention Example 17 17  998 13.2 64 Good −81 Present Invention Example 18 18 1210 10.8 62 Good −53 Comparative Example 19 19  905 14.5 67 Good −49 Comparative Example 20 20  965 13.3 63 Good −42 Comparative Example Underlines indicate that values are outside the scope of the present invention or are not preferable characteristics.

TABLE-US-00008 TABLE 8 Tensile Total Hole Ductile brittle strength elongation expansion transition Test Steel TS EI rate λ temperature vTrs No. No. MPa % % Bendability ° C. Note 21 21 1021 13.5 60 Good −30 Comparative Example 22 22 1021 11.5 61 Good −43 Comparative Example 23 23 1074 13.4 45 Good −30 Comparative Example 24 24  971 13.5 61 Good −47 Comparative Example 25 25 1077 13.2 55 Bad −20 Comparative Example 26 26  712 19.0 69 Good −52 Comparative Example 27 27  870 17.0 62 Good −64 Comparative Example 28 28 1043 11.2 67 Good −54 Comparative Example 29 29 1025 13.1 45 Bad −51 Comparative Example 30 30 1034 13.1 61 Good −21 Comparative Example 31 31 1025 13.3 47 Good −10 Comparative Example 32 32 1055 13.7 52 Good  −5 Comparative Example 33 33  782 18.0 30 Good −68 Comparative Example 34 34 1031 12.8 68 Bad −69 Comparative Example 35 35 1020 10.0 64 Good −65 Comparative Example 36 36 1050  9.8 70 Good −48 Comparative Example 37 37  982 14.2 40 Good −41 Comparative Example 38 38 1049 13.1 58 Good −68 Comparative Example 39 39  992 13.1 61 Bad −69 Comparative Example 40 40  920 13.2 60 Good  10 Comparative Example 41 41 1022 13.5 65 Bad −55 Comparative Example 42 42 1002 14.1 51 Bad −65 Comparative Example Underlines indicate that values are outside the scope of the present invention or are not preferable characteristics.

[0227] From Tables 5 to 8, it is found that the present invention examples are excellent in terms of strength, ductility, bendability, hole expansibility, and low temperature toughness. In addition, it is found that the present invention examples in which the average interval between precipitates having a diameter of 20 nm or less was 50 nm or more have more excellent low temperature toughness.

[0228] On the other hand, it is found that the comparative examples are poor in one or more characteristics of strength, ductility, bendability and hole expansibility.

INDUSTRIAL APPLICABILITY

[0229] According to the aspect of the present invention, it is possible to provide a hot-rolled steel sheet being excellent in terms of strength, ductility, bendability, hole expansibility, and low temperature toughness and a manufacturing method thereof.