Hot-rolled steel sheet and production method therefor

Abstract

In a hot-rolled steel sheet having a predetermined chemical composition and having a metallographic structure including 90 vol % or greater of martensite and 0 vol % to 10 vol % of a residual structure, the residual structure includes one or both of bainite and ferrite, the average prior austenite grain size in an L-section parallel to a rolling direction and an average prior austenite grain size in a C-section parallel to a direction orthogonal to the rolling direction are 1.0 μm to 10.0 μm the aspect ratio associated with the prior austenite grain size is 1.8 or less, the average grain size of the residual structure in the L-section and the average grain size of the residual structure in the C-section are 5.0 μm or less, and the aspect ratio associated with the average grain size of the residual structure is 2.0 or less.

Claims

1. A hot-rolled steel sheet comprising, as a chemical composition, by mass %: C: 0.010% to 0.200%; Si: 1.00% or less; Mn: 3.0% or less; P: 0.040% or less; S: 0.004% or less; Al: 0.10% or less; N: 0.004% or less; Nb: 0% to 0.20%; Ti: 0% to 0.15%; Mo: 0% to 1.00%; Cu: 0% to 0.50%; Ni: 0% to 0.50%; and a remainder of Fe and impurities, wherein a metallographic structure includes 90 vol % or greater of martensite and 0 vol % to 10 vol % of a residual structure, the residual structure including one or both of bainite and ferrite, an average prior austenite grain size in an L-section parallel to a rolling direction and an average prior austenite grain size in a C-section parallel to a direction orthogonal to the rolling direction are 1.0 μm to 10.0 μm, an aspect ratio of the prior austenite grain size, which is a ratio of the average prior austenite grain size in the L-section and the average prior austenite grain size in the C-section is, 1.8 or less, an average grain size of the residual structure in the L-section and an average grain size of the residual structure in the C-section are 5.0 μm or less, an aspect ratio of the residual structure, which is a ratio of the average grain size of the residual structure in the L-section and the average grain size of the residual structure in the C-section is, 2.0 or less, in a case where the larger one of the average prior austenite grain size in the L-section and the average prior austenite grain size in the C-section is represented by Dpγ (L) and the smaller one is represented by Dpγ (S), a value obtained by Dpγ (L)/Dpγ (S) is defined as the aspect ratio of the average prior austenite grain size, and in a case where the larger one of the average grain size of the residual structure in the L-section and the average grain size of the residual structure in the C-section is represented by Dr (L) and the smaller one is represented by Dr (S), a value obtained by Dr (L)/Dr (S) is defined as the aspect ratio of the residual structure.

2. The hot-rolled steel sheet according to claim 1, comprising, as the chemical composition, by mass %, one or more of: Nb: 0.01% to 0.20%; Ti: 0.01% to 0.15%; Mo: 0.01% to 1.00%; Cu: 0.01% to 0.50%; and Ni: 0.01% to 0.50%.

3. A manufacturing method of a hot-rolled steel sheet according to claim 1, the method comprising: a hot rolling process in which a steel having the chemical composition according to claim 1 is heated to 1,100° C. to 1,350° C., and then subjected to plural passes of reduction to perform rough rolling and finish rolling, and thus a hot-rolled steel sheet is obtained; a cooling process in which after completion of the hot rolling process, cooling is started on the hot-rolled steel sheet within 5 seconds and performed to a temperature range of 300° C. or lower at an average cooling rate of 30° C./sec or greater; and a coiling process in which the hot-rolled steel sheet after the cooling process is coiled in the temperature range of 300° C. or lower, wherein the rough rolling is performed under the following condition (I), and the finish rolling is performed under the following condition (II), (I) a temperature T of the steel after a final rolling pass in the rough rolling is in a range of 1,000° C. to 1,300° C., a reduction of the final rolling pass is 105-0.05×T or greater by unit %, and cooling is started within 5 seconds after the steel pass through the final rolling pass and performed to a temperature of Ar.sub.3+30° C. to Ar.sub.3+300° C. at an average cooling rate of 20° C./sec or greater, (II) a temperature of the steel after a final rolling pass in the finish rolling is Ar.sub.3 or higher, and a reduction amount of the final pass in the finish rolling is in a range of 12% to 45%, where the Ar.sub.3 is a temperature determined by the following (Formula 1),
Ar.sub.3(° C.)=910−310×C −80×Mn −20×Cu −55×Ni −80×Mo  (Formula 1) in the Formula 1, C, Mn, Cu, Ni, and Mo each represent an amount of a corresponding element by mass %, each of which is substituted by zero in a case where the corresponding element is not contained.

4. The manufacturing method of a hot-rolled steel sheet according to claim 3, wherein by the rough rolling, a metallographic structure of the steel sheet before the finish rolling is controlled such that an average austenite grain size in an L-section parallel to a rolling direction of the rough rolling and an average austenite grain size in a C-section parallel to a direction orthogonal to the rolling direction are 100 μm or less, and an aspect ratio of the residual structure, which is a ratio of the average austenite grain size in the L-section and the average austenite grain size in the C-section is, 2.0 or less.

Description

EXAMPLES

(1) Hereinafter, the present invention will be described in greater detail with examples, but is not limited to these examples.

(2) Molten steels having chemical components shown in Table 1 were melted in a converter and made into slabs (steels) by a continuous casting method, respectively. Next, the steels were made into hot-rolled steel sheets having a sheet thickness of 3.0 mm by hot rolling, cooling, and coiling conditions shown in Table 2. Ar.sub.3 (° C.) in Tables 1 and 2 was calculated by the following formula.
Ar.sub.3(° C.)=910−310×C−80×Mn−20×Cu−55×Ni−80×Mo  (Formula 1)

(3) In Formula 1. C, Mn, Cu, Ni, and Mo each represent the amount (mass %) of a corresponding element, each of which is substituted by zero in a case where the corresponding element is not contained.

(4) TABLE-US-00001 TABLE 1 Steel Chemical Components (mass %) Remainder: Fe and Impurities Ar.sub.3 No. C Si Mn P S Al N Ti Nb Mo Cu Ni ° C. A 0.050 0.06 2.0 0.002 0.002 0.03 0.004 735 B 0.070 0.08 2.1 0.003 0.002 0.03 0.004 0.11 720 C 0.100 0.10 1.5 0.003 0.002 0.02 0.003 0.02 759 D 0.080 0.60 2.1 0.002 0.001 0.07 0.004 0.12 0.02 717 E 0.100 0.20 2.8 0.010 0.003 0.07 0.004 0.10 0.03 653 F 0.150 0.70 1.4 0.022 0.004 0.06 0.003 0.20 0.40 726 G 0.110 0.03 1.6 0.007 0.003 0.09 0.003 0.20 0.02 731 H 0.008 0.60 1.4 0.003 0.004 0.05 0.004 0.04 0.03 796 I 0.110 0.90 3.3 0.010 0.003 0.04 0.004 612 The underline represents that the underlined value is out of the scope of the present invention. The blank represents that the corresponding element is not positively contained.

(5) TABLE-US-00002 TABLE 2 Rough Rolling Final Pass Cooling Heating Temper- 105- Reduction Time Until Stop Temper- ature 0.05 Amount of Start of Cooling Ar.sub.3 + Ar.sub.3 + Temper- Test Components ature (T) T Final Pass Cooling Rate Ar.sub.3 30 300 ature No. of Steel ° C. ° C. % % sec ° C./sec ° C. ° C. ° C. ° C. 1 A 1250 1173 46 55 2 37 735 765 1035 914 2 A 1250 1192 45 55 1 26 735 765 1035 999 3 A 1200 1113 49 55 5 38 735 765 1035 1020 4 A 1300 1184 46 55 9 29 735 765 1035 849 5 A 1150 1089 51 55 4 20 735 765 1035 990 6 B 1150 1016 54 50 2 29 720 750 1020 894 7 B 1200 1110 50 50 2 35 720 750 1020 826 8 B 1250 1110 50 50 1 38 720 750 1020 915 9 B 1300 1167 47 50 1 29 720 750 1020 979 10 B 1200 1056 52 55 5 39 720 750 1020 928 11 C 1250 1132 48 55 4 44 759 789 1059 889 12 C 1250 1159 47 55 1 46 759 789 1059 901 13 C 1250 1195 45 55 1 47 759 789 1059 943 14 C 1250 1159 47 50 2 21 759 789 1059 1017 15 C 1200 1186 46 50 5 23 759 789 1059 895 16 D 1300 1095 50 55 1 29 717 747 1017 995 17 D 1300 1125 49 55 3 40 717 747 1017 990 18 D 1250 1199 45 50 2 35 717 747 1017 942 19 E 1300 1192 45 50 4 35 653 683 953 711 20 E 1200 1142 48 50 3 20 653 683 953 801 21 E 1200 1095 50 50 5 44 653 683 953 808 22 F 1250 1155 47 50 5 34 726 756 1026 875 23 F 1250 1100 50 55 4 42 726 756 1026 827 24 F 1250 1096 50 55 1 28 726 756 1026 832 25 G 1150 1182 46 55 3 25 731 761 1031 1097 26 G 1250 1145 48 55 2 49 731 761 1031 955 27 G 1250 1032 53 55 1 47 731 761 1031 889 28 G 1300 1200 45 55 2 10 731 761 1031 959 29 H 1200 1166 47 50 4 43 796 826 1096 998 30 I 1250 1200 45 50 1 24 612 642 912 910 31 A 1200 1100 50 40 — — 735 765 1035 1000 32 A 1250 1200 45 50 — — 735 765 1035 950 33 A 1050 1000 55 55 4 40 735 765 1035 900 34 A 1200 1100 50 40 — — 735 765 1035 1000 Finish Rolling Cooling Stop Temper- Final ature Reduction Rolling Time Until (coiling Amount of Temper- Start of Cooling temper- Test Final Pass ature Cooling Rate ature) No. % ° C. sec ° C./sec ° C. Remarks 1 13 888 4 192  234 Example 2 16 976 2 83 107 Example 3 28 997 1 60 245 Example 4 22 828 1 110  119 Comparative Example 5 38 962 3 52 150 Example 6 26 875 1 199  253 Comparative Example 7 11 778 4 25 43 Comparative Example 8 21 887 8 147  34 Comparative Example 9 19 945 3 96 108 Example 10 16 880 3 113  198 Example 11 10 846 5 93 115 Comparative Example 12 14 864 4 194  118 Example 13 26 926 2 124  161 Example 14 17 984 4 49 341 Comparative Example 15 36 871 1 34 289 Example 16 21 949 2 153  33 Example 17 33 980 4 77 131 Example 18 23 929 1 197  174 Example 19 17 640 3 156  259 Comparative Example 20 27 778 3 174  267 Example 21 32 770 4 53 47 Example 22 38 856 3 95 219 Example 23 13 791 2 100  33 Example 24 21 783 3 65 116 Example 25 14 1066  4 106  124 Comparative Example 26 29 911 4 102  98 Example 27 33 869 1 77 84 Example 28 42 939 3 143  190 Comparative Example 29 48 978 3 175  74 Comparative Example 30 30 887 3 78 171 Comparative Example 31 25 900 1 50 150 Comparative Example 32 20 900 3 100  200 Comparative Example 33 24 870 0.3 40 250 Comparative Example 34 25 900 1 50 450 Comparative Example The underline represents that the underlined value is out of the scope of the present invention.

(6) The “heating temperature” in Table 2 is a heating temperature of the slab. The final pass temperature in the rough rolling is a temperature of the steel sheet immediately after the steel sheet passes the final pass of the rolling mill in the rough rolling. The time until the start of cooling is a time from after the final pass in the rough rolling to the start of the injection of a cooling medium. The cooling rate during cooling is represented by an average rate obtained by dividing a temperature drop width of the steel sheet from when the steel sheet is introduced into cooling equipment (when cooling water is applied) to when the steel sheet is ejected from the water cooling equipment by a time required for the steel sheet to pass through the water cooling equipment. The cooling stop temperature is the temperature after the steel sheet is ejected from the water cooling equipment.

(7) The final rolling temperature in the finish rolling is a temperature of the steel sheet immediately after the steel sheet passes the final pass of the rolling mill in the finish rolling. The time until the start of cooling is a time from when the steel sheet passes the final pass in the finish rolling to when the injection of a cooling medium is started. The cooling rate during cooling is represented by an average rate obtained by dividing a temperature drop width of the steel sheet from when the steel sheet is introduced into water cooling equipment (when cooling water is applied) to when the steel sheet is ejected from the water cooling equipment by a time required for the steel sheet to pass through the water cooling equipment.

(8) A test piece was collected from the obtained hot-rolled steel sheet, and structure observation (scanning electron microscope and EBSD), a tensile test, and a Charpy test were performed thereon. The structure observation was performed at an analysis speed of 200 to 300 points/sec using a device including a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (HIKARI detector manufactured by TSL). The average orientation difference within the same grain was calculated using software (OIM Analysis™) attached to the EBSD analyzer.

(9) In the tensile test, a JIS No. 5 test piece was collected from the hot-rolled steel sheet such that a tensile direction was parallel (L-direction) and orthogonal (C-section) to the rolling direction to perform the tensile test based on the provisions of JIS Z 2241:2011, and a tensile strength (TS) was obtained. In the present invention, the expression excellent in isotropy in tensile strength means that a value obtained by |TS (L)−TS (C)|, where TS (L) is a tensile strength in the L-direction and TS (C) is a tensile strength in the C-direction is less than 100 MPa. Accordingly, in a case where the tensile strengths in the L-direction and in the C-direction were 980 MPa or greater, and |TS (L)−TS (C)| was less than 100 MPa, it was judged that the steel sheet had a high strength and was excellent in isotropy in tensile strength.

(10) In the Charpy test, a sub-size test piece (V-notch) having a thickness of 2.5 mm was collected from the hot-rolled steel sheet such that a longitudinal direction of the test piece was parallel (L-direction) and orthogonal (C-section) to the rolling direction to perform the Charpy impact test at a temperature of room temperature to −198° C. based on the provisions of JIS Z 2242:2005, and a ductile-brittle transition temperature was obtained to evaluate toughness. Here, the test piece was prepared so as to have a sheet thickness of 2.5 mm by subjecting the hot-rolled steel sheet to double-side grinding. In the present invention, the expression excellent toughness means that the ductile-brittle transition temperature is −60° C. or lower, and the expression excellent in isotropy in toughness means that a value obtained by |vTrs (L) −vTrs (C)|, where vTrs (L) is a ductile-brittle transition temperature in the L-direction and vTrs(C) is a ductile-brittle transition temperature in the C-direction, obtained by the Charpy test, is less than 15° C. Accordingly, in a case where the ductile-brittle transition temperatures in the L-direction and in the C-direction were −60° C. or lower, and |vTrs (L) −vTrs (C)| was less than 15° C., it was judged that the steel sheet had excellent toughness and was excellent in isotropy in toughness.

(11) The shape evaluation was performed with a value calculated by Δt/tave, where tave was defined as an average of sheet thicknesses, and Δt was defined as a difference between the maximum value and the minimum value, when the sheet thickness was measured at 30 points at a ratio of 1 point per 2,500 mm.sup.2 of the steel sheet surface. The shape was evaluated to be excellent in a case where Δt/tave was less than 0.125. However, in a case where the tensile strength and its isotropy and the ductile-brittle transition temperature and its isotropy are at acceptable levels, the object of the steel sheet according to this embodiment can be achieved even in a case where Δt/tave is less than 0.125.

(12) The hot-rolled steel sheets of the examples have a desired tensile strength (TS: 980 MPa or greater in both the L-direction and the C-direction) and desired toughness (−60° C. or less in both the L-direction and the C-direction) with regard to both the tensile strength and the toughness in the L-direction and the C-direction. In addition, the hot-rolled steel sheets of the examples are excellent in isotropy in tensile strength and toughness (ITS (L) −TS (C)| is less than 100 MPa, and |vTrs (L) −vTrs (C)| is less than 15° C.). Furthermore, some hot-rolled steel sheets had an excellent product shape. A hot-rolled steel sheet including a residual structure included one or both of ferrite and bainite as the residual structure.

(13) In contrast, the hot-rolled steel sheets of the comparative examples, which are out of the scope of the present invention, cannot secure a desired strength and desired toughness, or isotropy thereof. The residual structure thereof included one or both of ferrite and bainite.

(14) In No. 4. since the time from the completion of rough rolling to the start of cooling was long, the grains grew, and the austenite grain size before the finish rolling coarsened. Therefore, it was not possible to cause recrystallization during the finish rolling, and the prior austenite grain size was not sufficiently refined. In addition, since the aspect ratio associated with the austenite grain size before the finish rolling deteriorated, the aspect ratio of the prior austenite grains in the structure after the finish rolling also deteriorated. As a result, the tensile strength, toughness, and isotropy thereof deteriorated.

(15) In No. 6, the reduction amount of the final pass in the rough rolling was small, and recrystallization did not occur during the rough rolling. Accordingly, the austenite grain size before the finish rolling coarsened, and it was not possible to cause recrystallization during the finish rolling. In addition, since the prior austenite grain size was not sufficiently refined and the residual structure also coarsened, the tensile strength in the L-direction deteriorated, and the toughness in the L-direction and in the C-direction deteriorated. In addition, since the aspect ratio associated with the austenite grain size before the finish rolling deteriorated, the aspect ratio of the prior austenite grains in the structure after the finish rolling also deteriorated. As a result, isotropy in tensile strength and toughness deteriorated.

(16) In No. 7, the cooling rate after the finish rolling was low, ferrite was generated during the cooling, and the ferrite grain size coarsened. As a result, the tensile strength in the L-direction and in the C-direction deteriorated.

(17) In No. 8, since the time from after the finish rolling to the start of cooling was long and the grains grew after the finish rolling, the prior austenite grains coarsened. As a result, the toughness in the L-direction and in the C-direction deteriorated.

(18) In No. 11, the reduction amount of the final pass in the finish rolling was small. Therefore, recrystallization did not sufficiently proceed in the finish rolling, and the aspect ratio of the prior austenite grains after the finish rolling also deteriorated. As a result, anisotropy occurred in toughness.

(19) In No. 14. the cooling stop temperature (coiling temperature) after the finish rolling was high, bainite was generated, and the bainite grain size coarsened. As a result, the tensile strength in the L-direction deteriorated.

(20) In No. 19, the rolling temperature in the finish rolling was low, and ferrite was generated during the rolling. Accordingly, the tensile strength in the L-direction and in the C-direction deteriorated. In addition, the aspect ratio of ferrite (residual structure) deteriorated. As a result, the isotropy in toughness deteriorated.

(21) In No. 25, since the cooling stop temperature after the rough rolling was high, the grains grew, and the austenite grain size before the finish rolling coarsened. Accordingly, it was not possible to cause recrystallization during the finish rolling, and the prior austenite grain size was not sufficiently refined. As a result, the tensile strength in the L-direction deteriorated. The toughness in the L-direction and in the C-direction also deteriorated. In addition, since the aspect ratio associated with the austenite grain size before the finish rolling deteriorated, the aspect ratio of the prior austenite grains in the structure after the finish rolling also deteriorated. As a result, the isotropy in tensile strength and toughness deteriorated.

(22) In No. 28, since the cooling rate after the rough rolling was low, the grains grew, and the austenite grain size before the finish rolling coarsened. Accordingly, it was not possible to cause recrystallization during the finish rolling, and thus the prior austenite grain size was not sufficiently refined. As a result, the tensile strength and the toughness in the L-direction and in the C-direction deteriorated.

(23) In No. 29. the C content was low, and it was not possible to sufficiently generate martensite. As a result, the tensile strength in the L-direction and in the C-direction deteriorated. In addition, since the reduction amount of the final pass in the finish rolling was large, the shape was inferior.

(24) In No. 30, the rough rolling conditions and the finish rolling conditions were satisfied. However, since the Mn content was large and a band-like structure was formed, anisotropy occurred in tensile strength and toughness, and the toughness in the L-direction deteriorated.

(25) In No. 31, the reduction amount of the final pass in the rough rolling was small. and recrystallization did not occur during the rough rolling. In addition, since cooling was not performed after the rough rolling, the austenite grain size before the finish rolling coarsened. Therefore, the prior austenite grain size after the finish rolling coarsened, and the aspect ratio also deteriorated. As a result, toughness deteriorated, and isotropy in toughness and tensile strength also deteriorated.

(26) In No. 32, since cooling was not performed after the rough rolling, the austenite grain size before the finish rolling coarsened. Therefore, the prior austenite grain size after the finish rolling coarsened. As a result, toughness deteriorated, and isotropy in toughness and tensile strength also deteriorated.

(27) In No. 33, since the slab heating temperature was low, solutionizing or elimination of segregation of elements did not sufficiently occur, and thus segregation remained, and the aspect ratio associated with the austenite grain size after the rough rolling coarsened. As a result, anisotropy occurred in tensile strength and toughness.

(28) In No. 34, the reduction amount of the final pass in the rough rolling was small, and recrystallization did not occur during the rough rolling. In addition, since cooling was not performed after the rough rolling, the austenite grain size before the finish rolling coarsened. Therefore, the prior austenite grain size after the finish rolling coarsened, and the aspect ratio also deteriorated. In addition, since the coiling temperature was high, the volume percentage of martensite was lowered. As a result, the tensile strength in the L-direction and in the C-direction deteriorated.

(29) TABLE-US-00003 TABLE 3 Structure After Finish Rolling Structure Before Prior γ Residual Structure Finish Rolling M Phase Grain Size Average Grain Size of γ Volume of Prior γ Volume Grain Size Test Components L C Aspect Percentage L C Aspect Percentage L C Aspect No. of Steel μm μm Ratio % μm μm Ratio % μm μm Ratio 1 A 44 37 1.2 90 7.4 5.0 1.5 10 3.5 2.7 1.3 2 A 33 21 1.6 92 7.5 6.8 1.1 8 4.5 2.6 1.7 3 A 49 73 1.5 95 8.1 6.6 1.2 5 3.3 2.3 1.4 4 A 134 55 2.4 91 36.0  17.0  2.1 9 4.2 2.5 1.7 5 A 79 65 1.2 91 9.0 9.9 1.1 9 4.3 3.3 1.3 6 B 154 58 2.7 94 22.0  10.0  2.2 6 8.4 5.4 1.6 7 B 38 59 1.6 71 10.0  7.4 1.4 29 15.0  8.9 1.7 8 B 77 70 1.1 96 15.0  15.0  1.0 4 4.0 2.3 1.7 9 B 16 22 1.4 100  4.1 7.1 1.7 0 3.0 1.9 1.6 10 B 28 50 1.8 96 4.8 3.1 1.5 4 2.3 4.4 1.9 11 C 23 20 1.2 91 13.0  5.5 2.4 9 6.1 3.1 2.0 12 C 57 67 1.2 99 2.1 3.7 1.8 1 2.2 1.1 2.0 13 C 60 59 1.0 96 8.8 7.1 1.2 4 3.7 2.1 1.8 14 C 37 68 1.8 75 1.9 2.5 1.3 25 8.9 5.1 1.7 15 C 44 66 1.5 95 4.7 5.7 1.2 5 4.9 2.9 1.7 16 D 66 44 1.5 99 10.0  9.4 1.1 1 2.4 2.5 1.0 17 D 28 37 1.3 100  1.3 1.7 1.3 0 3.3 4.6 1.4 18 D 34 26 1.3 91 9.1 8.1 1.1 9 4.6 2.7 1.7 19 E 30 47 1.6 59 5.2 4.0 1.3 41 4.7 1.2 3.9 20 E 49 77 1.6 96 8.8 6.8 1.3 4 3.9 4.2 1.1 21 E 69 45 1.5 97 7.1 5.4 1.3 3 3.9 3.5 1.1 22 F 20 34 1.7 95 8.5 8.0 1.1 5 3.5 3.2 1.1 23 F 60 48 1.3 90 4.1 6.7 1.6 10 2.8 2.9 1.0 24 F 18 15 1.2 90 7.4 4.3 1.7 10 3.8 4.9 1.3 25 G 131 61 2.1 94 34.0  15.0  2.3 6 3.4 2.0 1.7 26 G 70 48 1.5 94 5.9 9.9 1.7 6 2.4 3.3 1.4 27 G 42 73 1.7 94 8.5 6.3 1.3 6 4.8 5.0 1.0 28 G 127 89 1.4 92 29.0  19.0  1.5 8 3.0 3.6 1.2 29 H 62 41 1.5 19 7.9 7.4 1.1 81 3.1 3.8 1.2 30 I 36 57 1.6 98 7.4 5.2 1.4 2 3.3 2.2 1.5 31 A 150 65 2.3 96 40.0  17.5  2.3 4 3.9 2.0 2.0 32 A 130 65 2.0 96 30.0  17.0  1.8 4 3.9 2.0 2.0 33 A 140 60 2.3 90 35.0  15.5  2.3 10 3.9 2.0 2.0 34 A 150 65 2.3 15 40.0  17.5  2.3 85 2.5 1.3 2.0 Characteristics Toughness (transition Tensile Strength temperature) Test L C |L − C| L C |L − C| Shape No. MPa MPa MPa ° C. ° C. ° C. Evaluation Remarks 1 1045 1117 72 −100 −86 14 0.032 Example 2 1152 1204 52 −134 −121 13 0.080 Example 3 1220 1205 15 −105 −94 11 0.082 Example 4 874 977 103 −20 −50 30 0.064 Comparative Example 5 1063 1161 98 −110 −100 10 0.088 Example 6 880 992 112 −21 −59 38 0.074 Comparative Example 7 791 811 20 −98 −109 11 0.036 Comparative Example 8 1076 1055 21 −49 −57 8 0.076 Comparative Example 9 1212 1273 61 −63 −70 7 0.072 Example 10 1052 1099 47 −111 −112 1 0.062 Example 11 1001 1123 122 −67 −98 31 0.064 Comparative Example 12 1179 1184 5 −93 −80 13 0.052 Example 13 1070 1110 40 −70 −77 7 0.102 Example 14 911 990 79 −107 −109 2 0.044 Comparative Example 15 1279 1209 70 −98 −89 9 0.078 Example 16 1070 1048 22 −64 −77 13 0.064 Example 17 1262 1168 94 −91 −98 7 0.072 Example 18 999 1032 33 −96 −110 14 0.054 Example 19 741 824 83 −61 −114 53 0.082 Comparative Example 20 1201 1213 12 −140 −131 9 0.086 Example 21 1111 1124 13 −88 −88 0 0.081 Example 22 1044 1073 29 −91 −83 8 0.084 Example 23 1200 1186 14 −85 −100 12 0.048 Example 24 1024 1103 79 −95 −81 14 0.078 Example 25 877 999 122 −39 −58 19 0.036 Comparative Example 26 1067 1092 25 −74 −68 6 0.078 Example 27 1055 1140 85 −66 −80 14 0.112 Example 28 846 917 71 −37 −50 13 0.114 Comparative Example 29 577 590 13 −77 −67 10 0.135 Comparative Example 30 1016 1341 325 −17 −81 64 0.082 Comparative Example 31 1000 1150 150 −45 −12 33 0.130 Comparative Example 32 1000 1100 100 −52 −16 36 0.130 Comparative Example 33 990 1140 150 −43 −14 29 0.130 Comparative Example 34 675 725 50 −140 −123 17 0.130 Comparative Example The underline represents that the underlined value is out of the scope of the present invention.

INDUSTRIAL APPLICABILITY

(30) According to the present invention, it is possible to provide a hot-rolled steel sheet which is excellent in isotropy in tensile strength and toughness and has a tensile strength of 980 MPa or greater. According to the aspects of the present invention, it is possible to manufacture a hot-rolled steel sheet which has a high strength and is excellent in isotropy in tensile strength and toughness without an increase in load on a rolling mill. A hot-rolled steel sheet according to the present invention is suitable as a material for a structural component or a skeleton of a vehicle or a truck frame. By applying the hot-rolled steel sheet according to the present invention to a structural component of a vehicle or the like, it is possible to reduce a vehicle body weight while securing safety of the vehicle, and the environmental load can be reduced. Therefore, the present invention has high industrial applicability.