HOT-ROLLED STEEL SHEET AND METHOD OF MANUFACTURING SAME

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, in which a microstructure contains, by area %, bainite: 80.0% or more, ferrite: 10.0% or less, and a remainder in the microstructure: 10.0% or less, a total density of a length L.sub.7 of a grain boundary having a crystal orientation difference of 7° and a length L.sub.68 of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 μm/μm.sup.2, and a tensile strength is 780 MPa or more.

Claims

1. A hot-rolled steel sheet comprising, as a chemical composition, by mass %: C: 0.030% to 0.200%; Si: 0.05% to 2.50%; Mn: 1.00% to 4.00%; sol. Al: 0.001% to 2.000%; Ti: 0.030% to 0.200%, P: 0.020% or less; S: 0.020% or less; N: 0.010% or less; Nb: 0% to 0.200%; B: 0% to 0.010%; V: 0% to 1.00%; Mo: 0% to 1.00%; Cu: 0% to 1.00%; W: 0% to 1.00%; Cr: 0% to 1.00%; Ni: 0% to 1.00%; Co: 0% to 1.00%; Ca: 0% to 0.010%; Mg: 0% to 0.010%; REM: 0% to 0.010%; Zr: 0% to 0.010%; and a remainder consisting of iron and impurities, wherein a microstructure contains, by area %, bainite: 80.0% or more, ferrite: 10.0% or less, and a remainder in the microstructure: 10.0% or less, a total density of a length L.sub.7 of a grain boundary having a crystal orientation difference of 7° and a length L.sub.68 of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 μm/μm.sup.2, and a tensile strength is 780 MPa or more.

2. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet includes, as a chemical composition, by mass %, one or more selected from the group of: Nb: 0.005% to 0.200%; B: 0.001% to 0.010%; V: 0.005% to 1.00%; Mo: 0.005% to 1.00%; Cu: 0.005% to 1.00%; W: 0.005% to 1.00%; Cr: 0.005% to 1.00%; Ni: 0.005% to 1.00%; Co: 0.005% to 1.00%; Ca: 0.0005% to 0.010%; Mg: 0.0005% to 0.010%; REM: 0.0005% to 0.010%; and Zr: 0.0005% to 0.010%.

3. The hot-rolled steel sheet according to claim 1, wherein in the microstructure, an average grain size of prior austenite grains is 10 to 30 and a ratio I.sub.d/S.sub.d between a long axis I.sub.d and a short axis S.sub.d of the prior austenite grains is 2.0 or less.

4. A method of manufacturing the hot-rolled steel sheet according to claim 1, comprising: a heating step of retaining a slab having the chemical composition according to claim 1, at a heating temperature of 1200° C. or higher for 1.0 hour or longer; a hot rolling step of performing rough rolling so that a rough rolling completion temperature is 1000° C. or higher and a total rolling reduction is more than 65%, and performing finish rolling so that a finish rolling completion temperature is 860° C. to 980° C.; and a cooling step of performing cooling to a temperature range of 570° C. to 620° C. at an average cooling rate of 20° C./s or higher and performing winding, then, performing retaining at a temperature range of 500° C. to 580° C. for 2.0 to 12.0 hours, and then performing cooling to a room temperature.

5. The method of manufacturing the hot-rolled steel sheet according to claim 4, wherein in the hot rolling step, the total rolling reduction in the rough rolling is set to 70% or more, and the finish rolling is performed so that all rolling reductions of final three stages of the finish rolling are less than 25%.

6. The hot-rolled steel sheet according to claim 2, wherein in the microstructure, an average grain size of prior austenite grains is 10 to 30 μm, and a ratio I.sub.d/S.sub.d between a long axis I.sub.d and a short axis S.sub.d of the prior austenite grains is 2.0 or less.

7. A hot-rolled steel sheet comprising, as a chemical composition, by mass %: C: 0.030% to 0.200%; Si: 0.05% to 2.50%; Mn: 1.00% to 4.00%; sol. Al: 0.001% to 2.000%; Ti: 0.030% to 0.200%, P: 0.020% or less; S: 0.020% or less; N: 0.010% or less; Nb: 0% to 0.200%; B: 0% to 0.010%; V: 0% to 1.00%; Mo: 0% to 1.00%; Cu: 0% to 1.00%; W: 0% to 1.00%; Cr: 0% to 1.00%; Ni: 0% to 1.00%; Co: 0% to 1.00%; Ca: 0% to 0.010%; Mg: 0% to 0.010%; REM: 0% to 0.010%; Zr: 0% to 0.010%; and a remainder comprising iron and impurities, wherein a microstructure contains, by area %, bainite: 80.0% or more, ferrite: 10.0% or less, and a remainder in the microstructure: 10.0% or less, a total density of a length L.sub.7 of a grain boundary having a crystal orientation difference of 7° and a length L.sub.68 of a grain boundary having a crystal orientation difference of 68° about a <110> direction in the bainite is 0.35 to 0.60 μm/μm.sup.2, and a tensile strength is 780 MPa or more.

Description

EXAMPLES

[0207] Next, the effects of one aspect of the present invention will be described more specifically by way of examples, but the conditions in the examples are condition examples adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to these condition examples. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

[0208] Steels having chemical compositions shown in Steel Nos. A to AM in Table 1 were melted and continuously cast to manufacture slabs having a thickness of 240 to 300 mm. The obtained slabs were used to obtain hot-rolled steel sheets shown in Tables 5 to 7 under the manufacturing conditions shown in Tables 2 to 4. In, addition, “F1”, “F2”, and “F3” in Tables 2 to 4 represent a rolling reduction of the final pass of finish rolling, a rolling reduction of the second pass from the final pass, and a roiling reduction of the third pass from the final pass, respectively. In addition, a test material No. 63 in Table 4 was reheated after the stop of cooling, and then retained in the temperature range of 500° C. to 580° C.

[0209] With respect to the obtained hot-rolled steel sheet, the microstructural fraction, the total density of L.sub.7 and L.sub.68, the average grain size of the prior austenite grains, and the ratio I.sub.d/S.sub.d between the long axis I.sub.d and the short, axis S.sub.d of the prior austenite grains were determined. The results obtained are shown in Tables 5 to 7.

[0210] Evaluation Method of Properties of Hot-Rolled Steel Sheet

[0211] Tensile Strength (TS) and Total Elongation (EI)

[0212] Among the mechanical properties of the obtained hot-rolled steel sheet, the tensile strength (TS) and total elongation (EI) were measured by using a test piece No 5 of JIS Z 2241: 2011, in accordance with JIS Z 2241: 2011. A sampling position of the tensile test piece was set to the center position, in the sheet width direction, and the direction perpendicular to the rolling direction was set to the longitudinal direction. The cross-head speed was set to 3 mm/min.

[0213] In a case where the tensile strength (TS) was 780 MPa or more, the strength was determined excellent which was pass, and in a case where the tensile strength was less than 780 MPa, the strength was determined poor which was fail. In addition, in a case where the total elongation (EI) was 14.0% or more, the ductility was determined excellent which was pass, and in a case where the total elongation was less than 14.0, the ductility was determined poor which was fail.

[0214] Hole Expansion Rate (λ)

[0215] The hole expansion rate (λ) was evaluated by performing a hole expanding test in accordance with JIS Z 2256: 2010.

[0216] In a case where the hole expansion rate (λ) was 50% or more, the hole expansibility as determined excellent which was pass, and in a case where hole expansion rate (λ) was less than 50%, the hole expansibility was determined, poor which was fail.

[0217] Impact Value (vE.sub.40)

[0218] The toughness was evaluated by performing a Charpy impact test at −40° C. and determining the impact value. A sub-sized Charpy impact test piece was taken from a predetermined position of the hot-rolled steel sheet, and the impact value at −40° C. was determined in accordance with a test method described in JIS Z 2242: 2005 to evaluate the toughness.

[0219] In a case where the impact value (vE.sub.40) was 60 J/cm.sup.2 or more, the toughness was determined excellent which was pass, and in a case where the impact value (vE.sub.40) was less than 60 J/cm.sup.2, the toughness was determined poor which was fail.

[0220] Punching Properties

[0221] The punching properties were evaluated by performing a punching test and observing the properties of the punched end surface. First, a punched hole was prepared with a hole diameter of 10 mm, a clearance of 12.5%, and a punching speed of 80 mm/s. Next, a cross section of the punched hole perpendicular to the direction was embedded in a resin, and, the punched end surface was imaged with a scanning electron microscope. In a case where the obtained observation photographs were observed and end surface roughness was not observed, “E (Excellent)” was noted in Tables 5 to 7 as having particularly good punching properties. In addition, in a case where a small elopement of less than 100 um was observed, “G (Good)” is noted in Tables 5 to 7 as having good punching properties, and in a case where a large elopement of 100 μm or more is observed “B (Bad)” is noted in Tables 5 to 7 as having poor punching properties.

[0222] When referring to Tables 5 to 7, it can be seen that Invention Examples have high strength and excellent ductility, hole expansibility, and toughness. In addition, it can be, seen that Invention Examples in which the average grain size of the prior austenite grains is 10 to 30 μm, and the, ratio I.sub.d/S.sub.d between the long axis I.sub.d and the short axis S.sub.d of the prior austenite grains is 2.0 or less have particularly good punching properties.

[0223] On the other hand, it can be seen that Comparative Example is poor in any one or more of the strength, the ductility, the hole expansibility and toughness.

TABLE-US-00001 TABLE 1 Kind of Chemical composition (unit: mass %, remainder consisting of Fe and impurities) steel C Si Mn sol. Al Ti P S N Others Remarks A 0.070 0.90 2.20 0.050 0.120 0.010 0.001 0.003 Present Invention Steel B 0.210 0.50 2.50 0.030 0.100 0.010 0.001 0.005 Comparative Steel C 0.028 1.20 2.20 0.050 0.090 0.010 0.002 0.003 Comparative Steel D 0.090 2.80 2.20 0.035 0.110 0.010 0.001 0.003 Comparative Steel E 0.063 1.40 0.85 0.100 0.110 0.020 0.001 0.003 Comparative Steel F 0.070 1.00 4.50 0.030 0.060 0.010 0.002 0.003 Comparative Steel G 0.041 1.50 1.50 2.200 0.110 0.020 0.001 0.002 Comparative Steel H 0.040 1.01 1.50 0.030 0.020 0.010 0.001 0.003 Comparative Steel I 0.102 1.20 1.90 0.030 0.250 0.010 0.001 0.003 Comparative Steel J 0.178 0.55 1.65 0.090 0.110 0.009 0.002 0.003 Present Invention Steel K 0.033 1.67 2.43 0.030 0.130 0.010 0.001 0.003 Present Invention Steel L 0.055 1.55 1.85 0.040 0.100 0.010 0.002 0.003 Nb: 0.020 Present Invention Steel M 0.089 1.25 2.00 0.100 0.090 0.010 0.002 0.003 Nb: 0.230 Comparative Steel N 0.065 1.20 2.10 0.020 0.110 0.005 0.001 0.002 Nb: 0.012, Cr: 0.50 Present Invention Steel O 0.060 1.13 2.41 0.025 0.112 0.010 0.001 0.002 Nb: 0.020, B: 0.002 Present Invention Steel P 0.049 1.20 1.80 0.060 0.090 0.010 0.003 0.003 B: 0.001 Present Invention Steel Q 0.057 0.90 1.60 0.020 0.110 0.009 0.003 0.003 Cr: 0.70, B: 0.002 Present Invention Steel R 0.060 0.53 1.95 0.050 0.150 0.010 0.002 0.002 V: 0.01 Present Invention Steel S 0.101 0.30 1.67 0.020 0.120 0.007 0.001 0.003 Mo: 0.02 Present Invention Steel T 0.070 1.20 1.50 0.030 0.100 0.010 0.002 0.003 Mo: 0.03, V: 0.03 Present Invention Steel U 0.062 1.10 2.20 0.013 0.080 0.010 0.002 0.001 Cr: 0.61, Mo: 0.02, Present Invention Steel V: 0.10 V 0.090 0.33 1.60 0.120 0.110 0.010 0.001 0.002 Cr: 0.45, Mo: 0.25, Present Invention Steel V: 0.18, Ca: 0.003 W 0.081 0.63 1.55 0.130 0.099 0.011 0.003 0.003 Cr: 0.61, Mo: 0.20, Present Invention Steel V: 0.24, B: 0.002 X 0.055 0.95 2.50 0.122 0.110 0.009 0.003 0.002 Mo: 1.30 Comparative Steel Y 0.071 0.10 2.80 0.500 0.130 0.008 0.002 0.003 Cu: 0.05 Present Invention Steel Z 0.053 1.00 1.50 0.050 0.130 0.002 0.003 0.003 Ni: 0.80 Present Invention Steel AA 0.068 1.30 2.00 0.030 0.060 0.002 0.002 0.003 Ni: 1.30 Comparative Steel AB 0.100 1.12 1.66 0.020 0.110 0.003 0.002 0.003 Co: 0.51 Present Invention Steel AC 0.055 1.03 1.78 0.030 0.100 0.004 0.002 0.003 Ca: 0.040 Present Invention Steel AD 0.081 0.90 2.03 0.030 0.120 0.010 0.001 0.003 Mg: 0.008 Present Invention Steel AE 0.056 1.50 1.91 0.022 0.070 0.010 0.001 0.003 REM: 0.005 Present Invention Steel AF 0.068 1.87 1.59 0.050 0.090 0.009 0.001 0.003 Zr: 0.003 Present Invention Steel AG 0.071 1.00 1.72 0.030 0.110 0.008 0.001 0.002 Cr: 0.64 Present Invention Steel AH 0.070 0.98 2.01 0.031 0.150 0.015 0.002 0.003 Nb: 0.050 Present Invention Steel AI 0.180 1.71 2.58 0.016 0.091 0.015 0.002 0.003 B: 0.001 Present Invention Steel AJ 0.071 1.20 2.10 0.040 0.090 0.007 0.002 0.003 W: 0.050 Present Invention Steel AK 0.080 0.08 2.20 0.020 0.100 0.003 0.002 0.003 Present Invention Steel AL 0.065 2.20 2.00 0.001 0.080 0.002 0.001 0.003 Present Invention Steel AM 0.072 1.40 1.20 0.070 0.130 0.002 0.001 0.003 Present Invention Steel An underline indicates that the value is outside a range of the present invention.

TABLE-US-00002 TABLE 2 Manufacture conditions Heating step Rough rolling step Finish rolling step Sample Heating Retention Rolling completion Total rolling Rolling reduction Rolling start Rolling completion material Kind of temperature time temperature reduction (%) temperature temperature No. steel (° C.) (h) (° C.) (%) F1 F2 F3 (° C.) (° C.) 1 N 1244 2.3 1146 82 28 25 9 1060 890 2 A 1250 3.0 1156 81 26 26 11 1055 899 3 U 1263 3.0 1145 81 26 26 11 1054 901 4 A 1255 2.6 1152 80 25 25 14 1063 879 5 L 1222 3.0 1159 83 25 26 13 1051 878 6 B 1254 3.1 1154 82 28 18 12 1055 904 7 C 1260 3.2 1163 81 30 13 12 1066 900 8 D 1222 3.5 1133 82 28 15 9 1079 878 9 E 1248 2.8 1121 80 20 25 25 1025 881 10 F 1253 2.5 1145 83 28 15 9 1034 894 11 G 1255 3.3 1146 82 30 11 9 1028 900 12 H 1247 2.5 1143 81 27 22 10 1053 892 13 I 1231 4.5 1090 80 28 20 10 1045 888 14 J 1244 3.1 1162 81 26 26 11 1049 901 15 K 1261 3.6 1141 84 26 18 10 1054 887 16 L 1268 3.5 1151 82 28 20 10 1061 893 17 M 1255 3.1 1100 85 26 20 8 1045 906 18 N 1262 2.1 1133 78 26 20 12 1080 910 19 O 1251 3.0 1143 80 30 11 9 1064 897 20 P 1231 1.7 1131 76 26 18 10 1045 899 21 Q 1222 1.5 1108 81 27 18 8 1056 901 22 R 1258 2.3 1121 80 30 13 12 1054 911 23 S 1255 3.2 1151 79 28 18 10 1056 904 24 T 1252 2.2 1109 80 26 26 11 1033 874 25 U 1266 3.1 1143 80 26 22 12 1043 889 Manufacture conditions Cooling step Average cooling Sample rate after Cooling stop Retention start Retention end Retention Cooling material finish rolling temperature temperature temperature time method after No. (° C./s) (° C.) (° C.) (° C.) (h) winding Remarks 1 100 592 580 500 1.7 Air cooling Comparative Example 2 80 596 580 500 3.1 Air cooling Invention Example 3 87 589 580 500 5.3 Air cooling Invention Example 4 100 596 580 500 11.1  Air cooling Invention Example 5 100 599 580 500 14.2  Air cooling Comparative Example 6 122 582 580 500 3.5 Air cooling Comparative Example 7 125 592 580 500 4.3 Air cooling Comparative Example 8 100 584 580 500 4.2 Air cooling Comparative Example 9 45 593 580 500 2.2 Air cooling Comparative Example 10 41 581 580 500 8.0 Air cooling Comparative Example 11 67 595 580 500 4.6 Air cooling Comparative Example 12 98 583 580 500 2.6 Air cooling Comparative Example 13 80 585 580 500 3.5 Air cooling Comparative Example 14 85 589 580 500 4.1 Air cooling Invention Example 15 151 581 580 500 5.2 Air cooling Invention Example 16 120 594 580 500 3.4 Air cooling Invention Example 17 150 586 580 500 6.2 Air cooling Comparative Example 18 120 589 580 500 4.0 Air cooling Invention Example 19 130 588 580 500 7.1 Air cooling Invention Example 20 75 583 580 500 2.6 Air cooling Invention Example 21 89 598 580 500 4.5 Air cooling Invention Example 22 110 595 580 500 6.5 Air cooling Invention Example 23 120 593 580 500 4.7 Air cooling Invention Example 24 44 590 580 500 7.4 Air cooling Invention Example 25 130 591 580 500 5.1 Air cooling Invention Example An underline indicates that the value is outside a range of the present invention.

TABLE-US-00003 TABLE 3 Manufacture conditions Heating step Rough rolling step Finish rolling step Sample Heating Retention Rolling completion Total rolling Rolling reduction Rolling start Rolling completion material Kind of temperature time temperature reduction (%) temperature temperature No. steel (° C.) (h) (° C.) (%) F1 F2 F3 (° C.) (° C.) 26 V 1265 2.8 1154 78 26 18 10 1055 901 27 W 1255 3.1 1143 81 28 18 15 1066 920 28 X 1241 1.6 1151 80 26 15 12 1065 895 29 Y 1255 2.5 1163 82 28 15 10 1054 901 30 Z 1266 1.8 1128 77 26 17 12 1061 899 31 AA 1263 2.3 1139 80 26 18 10 1045 879 32 AB 1231 1.9 1154 74 30 11 9 1053 931 33 AC 1261 3.7 1163 82 28 12 10 1045 904 34 AD 1255 3.2 1155 81 30 11 9 1051 879 35 AE 1224 3.1 1148 73 26 18 10 1059 911 36 AF 1245 2.3 1147 81 28 14 12 1061 893 37 AG 1248 1.7 1161 82 30 22 11 1056 897 38 A 1238 3.1 1158 81 22 22 18 1061 920 39 L 1255 2.1 1167 81 18 18 15 1053 910 40 P 1245 3.4 1151 82 20 20 22 1054 903 41 Q 1248 3.1 1133 81 19 19 12 1071 921 42 S 1247 2.7 1181 82 15 12 10 1061 910 43 T 1243 1.6 1136 81 18 18 12 1056 899 44 U 1251 2.6 1152 80 20 18 10 1051 900 45 V 1255 2.5 1148 83 22 16 12 1065 911 46 W 1260 3.5 1139 82 21 16 12 1055 915 47 AC 1248 3.1 1157 84 18 18 14 1042 909 48 AD 1251 3.0 1141 81 22 15 9 1050 912 49 R 1255 3.1 1152 80 20 15 11 1060 906 50 AG 1248 2.9 1144 82 18 18 17 1061 913 Manufacture conditions Cooling step Average cooling Sample rate after Cooling stop Retention start Retention end Retention Cooling material finish rolling temperature temperature temperature time method after No. (° C./s) (° C.) (° C.) (° C.) (h) winding Remarks 26 100 591 580 500 4.3 Air cooling Invention Example 27 150 585 580 500 2.8 Air cooling Invention Example 28 120 598 580 500 3.4 Air cooling Comparative Example 29 85 588 580 500 8.5 Air cooling Invention Example 30 95 593 580 500 4.2 Air cooling Invention Example 31 105 595 580 500 5.1 Air cooling Comparative Example 32 65 588 580 500 6.2 Air cooling Invention Example 33 80 588 580 500 4.6 Air cooling Invention Example 34 180 582 580 500 3.9 Air cooling Invention Example 35 49 595 580 500 9.8 Air cooling Invention Example 36 120 588 580 500 7.3 Air cooling Invention Example 37 80 598 580 500 3.2 Air cooling Invention Example 38 141 588 580 500 5.5 Air cooling Invention Example 39 108 587 580 500 3.1 Air cooling Invention Example 40 72 590 580 500 2.9 Air cooling Invention Example 41 54 588 580 500 5.2 Air cooling Invention Example 42 100 591 580 500 3.9 Air cooling Invention Example 43 70 593 580 500 4.5 Air cooling Invention Example 44 150 595 580 500 2.9 Air cooling Invention Example 45 120 589 580 500 3.5 Air cooling Invention Example 46 110 593 580 500 3.1 Air cooling Invention Example 47 100 587 580 500 5.1 Air cooling Invention Example 48 150 598 580 500 7.9 Air cooling Invention Example 49 112 591 580 500 4.5 Air cooling Invention Example 50 121 589 580 500 2.9 Air cooling Invention Example An underline indicates that the value is outside a range of the present invention.

TABLE-US-00004 TABLE 4 Manufacture conditions Heating step Rough rolling step Finish rolling step Sample Heating Retention Rolling completion Total rolling Rolling reduction Rolling start Rolling completion material Kind of temperature time temperature reduction (%) temperature temperature No. steel (° C.) (h) (° C.) (%) F1 F2 F3 (° C.) (° C.) 51 Y 1251 3.5 1147 80 24 20 18 1065 911 52 Z 1241 2.5 1161 74 20 20 20 1053 889 53 AB 1245 2.5 1138 79 22 18 20 1061 908 54 AE 1235 4.2 1157 81 20 20 16 1058 893 55 AF 1246 3.1 1148 77 21 20 18 1067 908 56 A 1138 1.5 1085 81 30 18 15 1023 899 58 A 1261 2.5 1153 81 31 16 10 1033 840 59 J 1256 2.7 1152 81 30 20 15 1091 995 60 K 1283 3.1 1148 80 28 21 13 1064 905 61 K 1260 2.8 1162 82 28 20 14 1071 894 62 K 1253 3.1 1183 81 30 22 11 1055 903 63 K 1265 2.5 1165 80 28 19 13 1068 901 64 J 1241 2.8 1156 85 30 17 12 1067 911 65 J 1263 2.2 1147 84 28 25 12 1076 899 66 AH 1259 1.8 1151 76 40 40 40 1058 910 67 AH 1261 2.1 1139 81 22 20 15 1056 915 68 AI 1251 3.5 1145 76 55 50 45 1095 950 69 AI 1261 4.0 1153 68 24 24 18 1065 901 70 AJ 1258 3.2 1161 81 22 20 19 1081 892 71 AJ 1249 2.9 1150 76 28 26 20 1045 883 72 A 1250 3.5 1154 62 30 18 14 1051 900 73 AH 1249 3.1 1152 83 30 22 11 1042 903 74 AK 1249 2.6 1161 72 24 15 13 1048 890 75 AL 1260 1.8 1158 75 30 18 15 1035 895 76 AM 1250 2.4 1155 74 28 20 16 1052 915 Manufacture conditions Cooling step Average cooling Sample rate after Cooling stop Retention start Retention end Retention Cooling material finish rolling temperature temperature temperature time method after No. (° C./s) (° C.) (° C.) (° C.) (h) winding Remarks 51 102 591 580 500 6.1 Air cooling Invention Example 52 100 596 580 500 3.5 Air cooling Invention Example 53 120 587 580 500 4.6 Air cooling Invention Example 54  80 590 580 500 4.8 Air cooling Invention Example 55 101 587 580 500 6.2 Air cooling Invention Example 56 130 586 580 500 5.1 Air cooling Comparative Example 58 130 580 580 500 3.3 Air cooling Comparative Example 59  89 598 580 500 4.2 Air cooling Comparative Example 60  15 595 580 500 6.0 Air cooling Comparative Example 61  35 596 580 500 3.7 Air cooling Invention Example 62  80 630 580 500 7.2 Air cooling Comparative Example 63  78  381* 568 500 3.5 Air cooling Comparative Example 64 120 573 573 500 3.7 Air cooling Invention Example 65 100 577 577 513 2.3 Water cooling Invention Example 66  80 520 520 500 0.7 Air cooling Comparative Example 67 150 589 580 500 4.1 Air cooling Invention Example 68 120 550 550 500 1.4 Air cooling Comparative Example 69 130 581 580 500 5.6 Air cooling Invention Example 70 150 589 580 500 6.8 Air cooling Invention Example 71 120 579 579 500 4.2 Air cooling Invention Example 72 105 583 580 500 3.5 Air cooling Comparative Example 73 110 560 560 500 2.1 Air cooling Comparative Example 74  99 584 580 500 3.1 Air cooling Invention Example 75 105 591 580 500 3.5 Air cooling Invention Example 76 108 578 578 500 2.8 Air cooling Invention Example An underline indicates that the value is outside a range of the present invention. *Reheating after stop of cooling

TABLE-US-00005 TABLE 5 Microstructure “Martensite + Average tempered martensite” grain size Sample Remainder in in remainder in of prior l.sub.d/S.sub.d material Kind of Bainite Ferrite microstructure microstructure L.sub.7 + L.sub.68 γ grains of prior No. steel (area %) (area %) (area %) (area %) (μm/μm.sup.2) (μm) γ grains  1 N 83.2 3.1 13.7  9.0 0.64 18 3.1  2 A 92.5 4.0 3.5 2.0 0.57 26 2.7  3 U 91.3 6.5 2.2 1.0 0.51 29 2.8  4 A 94.1 3.2 2.7 2.5 0.41 20 3.0  5 L 87.4 9.3 3.3 2.6 0.30 24 3.4  6 B 51.2 0.4 48.4  45.2 0.55 22 3.4  7 C 35.0 45.0  20.0  3.0 0.41 18 3.1  8 D 76.3 1.2 22.5  11.0 0.51 15 3.2  9 E 76.2 19.3  4.5 1.2 0.52 18 3.5 10 F 31.0 0.2 68.8  61.0 0.42 14 2.5 11 G 61.3 32.1  6.6 3.2 0.44 18 2.2 12 H 85.6 9.1 5.3 4.1 0.56 24 2.4 13 I 86.3 9.8 3.9 3.2 0.57 14 4.1 14 J 88.9 1.6 9.5 8.6 0.51 20 2.3 15 K 92.1 4.5 3.4 2.1 0.47 21 2.4 16 L 91.2 3.1 5.7 5.2 0.53 17 2.8 17 M 91.3 6.3 2.4 2.1 0.43 18 3.6 18 N 93.5 2.6 3.9 3.5 0.55 23 2.1 19 O 91.4 1.5 7.1 6.3 0.47 21 2.2 20 P 86.1 8.3 5.6 4.2 0.54 20 2.3 21 Q 94.5 1.5 4.0 2.1 0.51 22 2.1 22 R 87.3 8.2 4.5 3.1 0.47 20 2.7 23 S 91.2 1.2 7.6 6.7 0.53 16 2.8 24 T 91.0 4.3 4.7 4.2 0.45 17 2.6 25 U 90.5 2.1 7.4 5.1 0.54 19 2.5 Mechanical properties Punching property Sample Strength Workability Toughness Property of material TS El λ vE.sub.40 punched end No. (MPa) (%) (%) (J/cm.sup.2) surface Remarks  1 1046  12.6 44 103  G Comparative Example  2 999 14.6 70 89 G Invention Example  3 990 16.0 73 68 G Invention Example  4 983 17.0 74 67 G Invention Example  5 901 19.0 72 35 B Comparative Example  6 1251  10.0 35 65 G Comparative Example  7 701 23.0 72 31 B Comparative Example  8 1065  17.2 24 44 G Comparative Example  9 756 21.3 65 41 B Comparative Example 10 1089  10.2 48 81 G Comparative Example 11 776 23.0 62 51 G Comparative Example 12 738 25.0 71 40 G Comparative Example 13 981 14.2 31 52 B Comparative Example 14 996 15.2 61 121  G Invention Example 15 803 20.8 72 78 G Invention Example 16 993 15.2 61 89 G Invention Example 17 1001  14.3 28 71 G Comparative Example 18 1021  15.4 66 108  G Invention Example 19 983 16.2 60 98 G Invention Example 20 956 15.1 67 83 G Invention Example 21 1035  16.2 67 72 G Invention Example 22 1021  16.3 71 99 G Invention Example 23 1098  14.8 54 102  G Invention Example 24 1033  15.3 66 82 G Invention Example 25 1100  15.4 61 120  G Invention Example An underline indicates that the value is outside a range of the present invention or that the property is not preferable.

TABLE-US-00006 TABLE 6 Microstructure “Martensite + Average tempered martensite” grain size Sample Remainder in in remainder in of prior l.sub.d/S.sub.d material Kind of Bainite Ferrite microstructure microstructure L.sub.7 + L.sub.68 γ grains of prior No. steel (area %) (area %) (area %) (area %) (μm/μm.sup.2) (μm) γ grains 26 V 89.4 4.3 6.3 5.2 0.51 20 2.1 27 W 92.8 2.1 5.1 4.6 0.56 24 2.3 28 X 73.2 2.4 24.4  21.5 0.56 12 3.8 29 Y 83.1 8.1 8.8 6.2 0.38 19 2.3 30 Z 88.2 3.5 8.3 7.1 0.54 21 2.2 31 AA 72.5 0.6 26.9  24.1 0.49 17 2.7 32 AB 93.1 1.3 5.6 4.5 0.44 24 2.1 33 AC 88.4 4.1 7.5 5.6 0.51 22 2.4 34 AD 89.3 3.0 7.7 4.2 0.55 16 2.7 35 AE 90.5 6.1 3.4 3.1 0.48 21 2.2 36 AF 93.2 2.1 4.7 4.5 0.41 17 2.4 37 AG 92.5 4.2 3.3 3.1 0.52 19 2.3 38 A 88.3 2.6 9.1 6.3 0.54 25 1.8 39 L 93.2 2.4 4.4 4.1 0.57 25 1.6 40 P 88.5 5.3 6.2 4.2 0.58 23 1.5 41 Q 92.3 5.2 2.5 1.5 0.47 24 1.7 42 S 90.2 1.8 8.0 7.1 0.55 21 1.6 43 T 85.6 5.1 9.3 6.1 0.48 22 1.7 44 U 90.4 1.8 7.8 6.3 0.57 23 1.4 45 V 92.1 3.1 4.8 4.1 0.54 22 1.8 46 W 92.4 1.1 6.5 4.8 0.54 25 1.4 47 AC 88.3 6.2 5.5 3.9 0.49 24 1.5 48 AD 86.7 8.1 5.2 3.2 0.41 22 1.7 49 R 90.2 6.2 3.6 2.6 0.50 21 1.6 50 AG 93.1 2.1 4.8 3.1 0.56 24 1.7 Mechanical properties Punching property Sample Strength Workability Toughness Property of material TS El λ vE.sub.40 punched end No. (MPa) (%) (%) (J/cm.sup.2) surface Remarks 26 1035 15.8 54 151 G Invention Example 27 1065 15.1 67 115 G Invention Example 28 1108 12.3 44 65 B Comparative Example 29 1021 16.3 71 68 G Invention Example 30 1002 14.8 55 102 G Invention Example 31 1103 11.5 42 151 G Comparative Example 32 1098 15.1 66 95 G Invention Example 33 999 14.5 64 140 G Invention Example 34 1054 16.9 68 100 G Invention Example 35 1011 16.2 83 110 G Invention Example 36 1045 15.8 66 100 G Invention Example 37 993 15.1 71 81 G Invention Example 38 998 16.2 63 70 E Invention Example 39 981 16.3 72 85 E Invention Example 40 941 17.3 65 78 E Invention Example 41 975 16.7 79 81 E Invention Example 42 999 15.3 62 94 E Invention Example 43 1003 15.8 64 75 E Invention Example 44 1081 16.1 59 100 E Invention Example 45 1011 14.9 68 120 E Invention Example 46 996 16.8 75 98 E Invention Example 47 984 15.2 75 100 E Invention Example 48 989 16.7 67 105 E Invention Example 49 1007 15.6 75 100 E Invention Example 50 1001 14.9 71 90 E Invention Example An underline indicates that the value is outside a range of the present invention or that the property is not preferable.

TABLE-US-00007 TABLE 7 Microstructure “Martensite + Average tempered martensite” grain size Sample Remainder in in remainder in of prior l.sub.d/S.sub.d material Kind of Bainite Ferrite microstructure microstructure L.sub.7 + L.sub.68 γ grains of prior No. steel (area %) (area %) (area %) (area %) (μm/μm.sup.2) (μm) γ grains 51 Y 84.1 6.4 9.5 4.5 0.42 25 1.7 52 Z 84.6 6.1 9.3 7.5 0.55 24 1.6 53 AB 91.2 3.5 5.3 2.8 0.48 25 1.6 54 AE 90.2 4.2 5.6 5.1 0.55 25 1.7 55 AF 89.2 5.4 5.4 3.2 0.39 24 1.4 56 A 83.7 5.1 11.2  2.5 0.46 16 2.5 58 A 78.1 12.5  9.4 7.5 0.55 25 3.8 59 J 78.2 1.3 20.5  12.3 0.50 33 2.5 60 K 76.2 15.9  7.9 5.9 0.38 22 2.1 61 K 84.2 7.8 8.0 4.5 0.50 20 2.2 62 K 52.0 41.0  7.0 5.6 0.38 22 2.1 63 K 65.4 1.3 33.3  32.1 0.55 21 2.3 64 J 93.1 1.5 5.4 4.5 0.56 19 2.4 65 J 89.2 1.2 9.6 8.3 0.59 20 2.3 66 AH 85.1 8.1 6.8 4.0 0.71 21 2.6 67 AH 91.0 5.1 3.9 3.2 0.51 23 1.5 68 AI 77.5 14.1  8.4 6.2 0.63 22 3.2 69 AI 91.1 6.3 2.6 2.1 0.45 29 2.8 70 AJ 92.2 6.1 1.7 1.4 0.41 26 1.8 71 AJ 88.2 9.4 2.4 1.5 0.54 24 2.6 72 A 87.2 6.1 6.7 5.2 0.28 26 3.0 73 AH 91.0 2.1 6.9 2.8 0.25 26 2.6 74 AK 85.3 6.3 8.4 4.2 0.42 28 1.8 75 AL 86.2 6.1 7.7 3.4 0.43 25 2.6 76 AM 84.5 9.1 6.4 2.6 0.38 27 2.4 Mechanical properties Punching property Sample Strength Workability Toughness Property of material TS El λ vE.sub.40 punched end No. (MPa) (%) (%) (J/cm.sup.2) surface Remarks 51  996 15.3 75 80 E Invention Example 52  996 14.9 62 88 E Invention Example 53 1055 16.2 71 86 E Invention Example 54 1106 15.9 64 105  E Invention Example 55 1061 16.2 75 93 E Invention Example 56  771 23.0 45 70 G Comparative Example 58 1023 14.2 35 75 B Comparative Example 59 1013 14.5 56 35 B Comparative Example 60  763 22.5 80 75 G Comparative Example 61  791 23.0 77 65 G Invention Example 62  705 25.0 90 64 G Comparative Example 63 1105 10.5 50 120  G Comparative Example 64 1101 14.5 68 132  G Invention Example 65 1188 14.2 58 165  G Invention Example 66  986 18.0 45 132  G Comparative Example 67  902 21.0 71 95 E Invention Example 68  956 15.2 48 85 B Comparative Example 69 1021 15.3 71 82 G Invention Example 70 1028 16.3 79 76 E Invention Example 71  997 16.8 66 66 G Invention Example 72 1015 14.1 55 45 B Comparative Example 73 1084 14.6 51 52 G Comparative Example 74 1045 15.3 62 105  E Invention Example 75 1162 17.1 54 67 G Invention Example 76  998 16.7 62 89 G Invention Example An underline indicates that the value is outside a range of the present invention or that the property is not preferable.

INDUSTRIAL APPLICABILITY

[0224] According to the present invention, it is possible to provide a hot-rolled steel sheet having high strength, and excellent ductility, hole expansibility, and toughness, and a method of manufacturing the same. According to the above preferred aspect according to the present invention, it is possible to provide a hot-rolled steel sheet having excellent punching properties in addition to the above-mentioned properties and a method of manufacturing the same.