[0001]The present invention relates to a hot-rolled steel sheet and a weld joint, and methods for producing the hot-rolled steel sheet and the weld joint. BACKGROUND ART [0002]From the viewpoint of improving safety and reducing weight, a steel sheet to be used for the body structure of an automobile is required to have enhanced strength and high press workability. In response to such requirements, a high-strength steel sheet that is excellent in hole expandability which is better than the conventional technology has been proposed. [0003] Conventionally, as such kinds of high-strength hot-rolled steel sheet for working, hot-rolled steel sheets having a mixed structure composed of a ferritic and martensitic structure or a ferritic and bainitic structure, and hot-rolled steel sheets having a substantially single-phase structure mainly composed of bainite or ferrite are widely known. [0004] For example, Patent Document 1 discloses a hot-rolled steel sheet that has a tensile strength of 780 MPa or more and has both high hole expandability and bake hardenability, and a method for producing the hot-rolled steel sheet. LIST OF PRIOR ART DOCUMENTS PATENT DOCUMENT 2 [0005] SUMMARY OF INVENTION TECHNICAL PROBLEM [0006] In this connection, due to reasons such as durability and impact properties with respect to impacts received from step heights and the like, a steel sheet to be used for a suspension system member of an automobile is required to have toughness as a member in addition to strength and workability during press forming. However, Patent Document 1 does not give sufficient consideration to toughness, and room for improvement remains. [0007] An objective of the present invention, which has been made to solve the problem described above, is to provide a hot-rolled steel sheet having excellent toughness in addition to high strength and hole expandability, and a weld joint that includes the hotrolled steel sheet, as well as methods for producing the hot-rolled steel sheet and the weld joint. SOLUTION TO PROBLEM [0008] The present invention has been made to solve the problem described above, and the gist of the present invention is a hot-rolled steel sheet and a weld joint as well as methods for producing the hot-rolled steel sheet and the weld joint which are described hereunder. [0009] (1) A hot-rolled steel sheet having a chemical composition including, in mass%: C: 0.02 to 0.20%, Si: 0.01 to 1.50%, Mn: 0.10 to 3.00%, 3 P: 0.10% or less, S: 0.010% or less, Al: 0.005 to 0.100%, Ti: 0.02 to 0.20%, N: 0.001 to 0.010%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 1.00%, Mo: 0 to 0.40%, Nb: 0 to 0.060%, V: 0 to 1.00%, B: 0 to 0.0100%, Ca: 0 to 0.0050%, O: 0.0100% or less, and the balance: Fe and impurities, wherein: a steel micro-structure includes, in area%: ferrite: 60 to 80%, and a total of ferrite and bainite: 90% or more; an average of a crystal grain size of ferrite and bainite is 7.0 m or less, and a standard deviation of the crystal grain size is 2.0 m or less; and a standard deviation of a diameter of Ti carbo-nitrides is 10 nm or less. [0010] (2) The hot-rolled steel sheet according to (1) above, wherein: the chemical composition contains at least one of, in mass%: Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Cr: 0.01 to 1.00%, Mo: 0.005 to 0.40%, Nb: 0.001 to 0.060%, 4 V: 0.01 to 1.00%, B: 0.0005 to 0.0100%, and Ca: 0.0005 to 0.0050%. [0011] (3) A method for producing a hot-rolled steel sheet, that includes performing, in order: (a) a process of casting a slab having a chemical composition according to (1) or (2) above; (b) a slabbing process of, after casting, without a temperature of the slab decreasing to less than 800C, performing a rough rolling process described hereunder, or inserting the slab into a slab heating furnace and heating the slab to within a range of 1100 to 1250C; (c) a rough rolling process of performing hot rolling of the slab in which a start temperature is within a range of 950 to 1200C, an end temperature is within a range of 800 to 1050C, and a total rolling reduction is 20% or more, to form a sheet bar; (d) a sheet bar heating process of heating the sheet bar for 60 seconds or more to a temperature range of 1100 to 1250C at an average heating rate of 100C/min or more; (e) a finish rolling process of, within 20 seconds after the sheet bar heating process ends, subjecting the sheet bar to hot rolling with a start temperature within a range of 900 to 1250C and an end temperature within a range of an Ar3 point or more to less than 950C, and with a total rolling reduction of 50% or more, to form a steel sheet; (f) a cooling process of subjecting the steel sheet to primary cooling to a temperature range of 600 to 750C at an average cooling rate of 60C/s or more, and thereafter conducting slow cooling at an average cooling rate of 0 to 10C/s for a period of 0 to 10 seconds, and additionally thereafter conducting secondary cooling at an average cooling rate of 60C/s or more to a temperature which is equal to or less than a temperature 15C or more lower than an end temperature of the slow cooling and which is within a temperature range of 350 to 700C; and (g) a coiling process of coiling the steel sheet. [0012] 5 (4) A weld joint, including a first base metal portion, a second base metal portion and a weld metal portion, wherein: the weld metal portion is formed so as to extend at least in a first direction along an end portion of the first base metal portion; the first base metal portion is a hot-rolled steel sheet according to (1) or (2) above; a chemical composition of the weld metal portion includes, in mass%: C: 0.02 to 0.15%, Si: 0.01 to 1.50%, Mn: 0.10 to 1.50%, P: 0.10% or less, S: 0.010% or less, Al: 0.005 to 0.300%, Ti: 0.02 to 0.20%, N: 0.010% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 1.00%, Mo: 0 to 0.60%, Nb: 0 to 0.060%, V: 0 to 1.00%, B: 0 to 0.0100%, Ca: 0 to 0.0050%, O: 0.0010 to 0.0500%, and the balance: Fe and impurities; and when a direction perpendicular to the first direction as viewed from a thickness direction of the first base metal portion is taken as a second direction, and a thickness of the first base metal portion is represented by "t", at a cross-section which is parallel to a surface on one side in the thickness direction of the first base metal portion and which is at a position at a distance equivalent 6 to 1/8 t in the thickness direction from the surface, m0, m1, m2, m3, m4, and m5 that are area fractions (%) of bainite measured in order at a pitch of 50 m from a boundary between the first base metal portion and the weld metal portion toward the first base metal portion side in the second direction satisfy formula (i) below: 0 Coolirll Cooling End Coolirll Stop temp. temp. temp. temp. time time temp. temp. rate temp. rate time temp rate temp. reating ("C) ("C) ("C) reduction rate ("C) (s) (s) ("C) ("C) reduction ("Cis) ("C) ("Cis) (s) ("C) ("Cis) ("C) ("C) (%) ("C/min) (%) AI 826 989 875 64 148 11 13 96 17 11 12 837 98 190 742 5 5 717 264 596 - A2 826 844 809 63 182 1187 125 20 1186 866 98 72 652 2 7 638 269 472 - A A3 826 1021 944 61 183 1157 70 16 1156 861 98 213 709 4 7 68 1 141 430 A4 826 1139 1026 81 129 1206 84 13 1205 870 96 101 694 0 0 694 296 546 -* 786 1072 973 63 112 1164 102 6 1164 848 98 214 693 2 8 677 299 280 B 786 1101 1006 69 170 1228 78 4 1228 846 97 133 701 6 I 695 226 464 - 83 786 1080 1016 88 126 1157 67 16 1157 936 90 144 667 6 3 649 166 615 Cl 857 861 811 85 197 1212 122 II 1211 934 91 123 670 10 6 610 66 465 C2 c 857 1062 949 83 128 1156 97 18 1156 886 93 86 608 2 7 594 242 477 - C3 857 1029 998 69 105 1206 119 8 1206 903 98 67 701 0 0 701 242 437 Dl 904 1110 1022 77 120 1198 88 10 1198 937 95 !53 685 0 2 685 127 473 - 02 D 904 915 874 83 112 1179 163 I I 1178 909 93 128 698 0 7 698 112 457 - D3 904 1041 923 62 140 1155 99 10 1155 909 97 75 626 7 8 570 288 458 El 766 963 880 61 196 1119 73 18 1119 917 97 126 646 8 3 622 88 459 - E2 E 766 922 814 90 144 1137 135 13 1137 893 88 71 638 9 3 611 221 594 - E3 766 929 760 71 116 1106 179 14 1105 934 96 !53 669 4 8 637 245 419 Fl 769 1085 1001 85 197 1191 58 18 1191 868 92 170 659 10 4 619 209 425 - F2 F 769 883 850 83 129 1164 146 6 1163 937 93 121 609 5 6 579 128 417 8 769 1107 989 80 !50 1176 75 15 1176 866 94 95 731 6 9 677 108 529 Gl 751 1035 961 76 187 1280 102 4 1280 817 95 220 697 10 6 637 183 446 -G2 G 751 1055 952 89 131 1220 123 7 1220 751 89 220 698 8 4 666 42 475 Ci3 751 1052 1018 73 108 1217 I l l I I 1217 781 97 114 670 2 15 640 296 478 HI 865 949 904 66 85 1197 207 15 1195 877 97 Il l 736 7 6 694 291 430 H2 H 865 1027 989 80 196 1149 49 9 1149 1030 95 186 660 I 7 653 69 423 - H3 865 869 830 61 114 1201 195 7 1199 920 95 89 728 0 4 728 79 659 II 769 944 824 71 !50 1105 112 12 1105 850 97 195 651 I 7 644 293 453 lz I 769 1067 1019 70 173 1030 1 I 1030 786 97 162 716 0 0 716 219 697 - 13 769 1013 960 74 144 1164 85 25 1164 885 98 202 717 9 7 658 130 507 Jl 825 1057 10 11 82 125 1189 85 6 1189 931 95 186 713 10 9 623 165 434 - 12 J 825 856 820 75 106 ! I ll 165 17 ! I ll 946 98 87 667 9 9 586 196 480 - J3 825 1000 650 75 161 1180 198 17 1180 882 96 142 695 3 3 683 126 645 Kl 825 937 887 74 132 1108 100 5 1107 876 97 93 697 7 8 641 268 554 - K2 K 825 1120 1023 65 135 1164 63 2 1164 829 97 78 739 0 4 739 164 - ill K3 825 1029 955 70 853 97 86 718 4 6 690 216 635 Ll 824 943 839 69 191 1153 99 16 1153 863 97 !56 683 6 4 659 91 643 - L L2 824 949 893 74 176 1214 109 2 1214 871 96 121 581 I 9 572 142 508 Ml 81 1 969 908 66 163 1138 85 3 1138 839 96 218 720 7 4 692 95 579 - M M2 811 958 906 78 196 1229 99 I 1229 909 96 12 664 5 I 659 271 505 Nl 814 969 850 90 122 1123 134 20 1122 909 91 78 689 4 10 649 225 458 N2 N 814 1057 941 81 131 1207 122 18 1207 900 95 128 707 0 2 707 161 646 ~ 853 1072 1029 88 108 1186 87 3 1186 872 95 201 762 0 7 762 92 622 0 853 1120 1026 87 106 1135 62 12 1135 927 95 97 726 9 3 699 262 572 PI 865 lOll 966 90 121 1178 105 13 1178 918 93 149 605 6 I 599 240 481 - P2 865 934 900 65 109 1108 114 5 1108 880 98 161 669 7 2 655 291 385 P3 p 865 1029 811 77 102 1241 252 7 1239 876 97 186 667 9 3 586 69 554 - P4 865 963 1068 88 53 1197 146 II 1196 939 95 87 685 I 4 659 271 481 Ql Q 777 960 891 74 146 1112 91 I 1112 852 97 149 698 I 4 696 97 603 Rl R 779 1023 951 80 149 1108 63 12 1108 856 93 181 693 4 6 66 1 126 528 Sl 891 1201 839 993 950 69 135 1110 71 14 1110 888 99 145 699 6 5 642 63 537 - S2 s 839 1020 953 14 122 1123 84 8 1123 909 99 78 689 4 10 649 225 458 S3 839 954 951 66 190 1112 51 15 1112 884 98 161 649 5 5 633 191 456 Tl T 736 924 867 89 110 1207 185 7 1207 945 94 199 658 3 7 637 243 414 Ul u 589 878 838 78 110 1185 189 9 1185 944 97 108 694 9 I 685 141 466 Observation of the steel micro-structure of the obtained test materials was performed, and the average and the standard deviation of the crystal grain sizes of ferrite and bainite as well as the average and the standard deviation of the diameter of Ti carbonitrides were respectively determined by the procedures described above. Note that, it was confirmed from the results of analysis by an EBSD mounted on the SEM that retained austenite was not observed in any of the steel sheets. [0126] [Mechanical Properties] Among the mechanical properties, tensile strength properties (tensile strength (TS), and total elongation (EL)) were evaluated in conformity with JIS Z 2241:2011 using a No. 5 test coupon specified in JIS Z 2241:2011 which, when the sheet width is represented by "W", was taken from a position at a distance equivalent to 1/4 W or 3/4 W from one end of the sheet in the sheet width direction, with a direction (width direction) perpendicular to the rolling direction being taken as the longitudinal direction. [0127] The hole expansion ratio was evaluated in conformity with a test method described in JIS Z 2256:2010 using a test specimen taken from a similar position to the position where the tensile test specimen was taken. Further, the toughness was evaluated by performing a C-direction-notch Charpy impact test at -40C using a 2.5 mm subsize V-notch test specimen defined in JIS Z 2242:2018. Further, for test specimens for which the final thickness of the steel sheet was less than 2.5 mm, the overall thickness was measured. [0128] A summary of the observation results for the steel micro-structures and the measurement results for the mechanical properties is shown in Table 3. [0129] [Table 3] 37 [0130] As is clear from Table 3, it is found that example embodiments of the present 38 Table 3 Steel micro-structure Mechanical properties Sheet Ferrite Ti carbo-nitrides Impact Test Steel Steel thickness FeiTite+ +bainite TS x/._o.~ al::sorbed No. sheet Ferrite Bainite TS El (mm) bainite Average Stan:lard deviatim Average Standard deviation energy (area%) (area%) (area%) grain size of grain size diameter of diameter (MPa) (%) (MPa ·%o.s) at -4()'C (~m) (f1!11) (run) (run) (J/cm2 ) I AI 1.2 77 13 90 6.2 1.2 25 5.6 875 18 7162 295 2 A2 1.2 66 31 97 5.1 1.1 7 1.7 1048 13 10739 238 Inventive A 3 A3 1.2 80 20 100 1.4 0.3 26 5.7 11 23 13 10654 227 example 4 A4 1.2 74 21 95 1.7 0.4 6 1.4 1053 14 11 439 255 5 Bl 1.2 .22 25 1§ 1.5 0.3 15 2.8 1012 15 8159 125 Cornp. ex. 6 B2 B 1.2 74 19 93 2.5 0.5 24 6.7 l11 7 12 9865 291 7 B3 2.9 71 19 90 6.2 1.3 16 3.6 893 18 9451 247 8 Cl 2.9 71 23 94 5.0 1.1 15 2.7 914 20 7312 253 9 C2 c 2.9 74 17 91 4.2 0.9 21 3.8 1063 13 9082 267 10 C3 2.9 71 29 100 6.3 1.5 19 4.9 1053 14 11 487 272 Inventive 11 D1 29 78 14 92 57 1 2 18 45 1086 12 11J90 215 example 12 D2 D 2.9 76 18 94 2.8 0.8 24 4.9 %7 15 7675 162 13 D3 2.9 71 22 93 4.4 1.0 28 5.3 11 33 12 9135 227 14 E l 2.9 70 20 90 2.4 0.6 9 1.7 950 19 10009 !53 15 E2 E 2.9 71 24 95 3.5 0.8 19 4.0 939 19 8240 161 16 E3 2.9 70 25 95 6.1 4.2 35 11.2 877 20 5822 Ill Comp. ex. 17 F l 2.9 75 18 93 5.1 1.0 21 4. 1 1045 14 9859 233 18 F2 F 2.9 71 22 93 3.7 0.7 19 3.7 987 16 9%8 166 Inventive example 19 F3 2.9 80 11 91 6.8 1.3 21 4.7 893 21 8795 253 20 Gl 2.9 79 13 92 lU £l 19 3.9 %0 16 10340 132 21 G2 G 2.9 43 19 62 4.6 1.2 14 3.2 823 21 5738 236 22 G3 2.3 95 5 100 2.7 0.7 16 4.0 %0 17 8587 142 Canparative example 23 HI 2.3 76 20 % 9.5 4.1 16 4.0 11 36 13 11%9 134 24 H2 H 2.3 67 24 91 8.7 3.8 24 13.2 778 28 5733 142 25 H3 2.3 74 24 98 3.6 0.9 14 3.0 840 23 8770 240 Imentive 26 II 2.3 74 16 90 3.2 0.7 22 5.6 11 04 12 11204 205 example 27 12 I 2.3 78 14 92 3.2 :u_ 42 12.3 837 23 5158 144 Ccmparative 28 13 2.3 73 23 % 8.3 2.2 35 15.2 992 17 5381 137 example 29 Jl 2.3 78 20 98 6.0 1.3 16 3.3 831 25 9065 205 Inventive 30 J2 J 2.3 70 27 97 4.0 1.0 17 3.3 983 16 8341 229 example 31 J3 2.3 63 14 77 6.0 2.2 51 162 974 16 4723 125 Comp. ex. 32 Kl 2.3 75 25 100 6.7 1.8 28 5.1 814 24 6663 285 Inv. ex 33 K2 K 2.3 88 0 88 2.9 0.8 9 1.8 849 20 7641 122 Canparative 34 K3 2.6 79 19 98 7.3 2.5 27 13.2 832 22 5320 131 example 35 L l 2.3 76 19 95 4.3 0.9 26 5.9 11 06 13 12014 276 Im. ex. L 36 L2 2.3 42 23 65 5.6 1.0 25 5.7 924 18 4823 2% Comp. ex. 37 Ml 2.3 80 12 92 2.4 0.5 19 5.4 1092 12 9005 223 Inv. ex. M 38 M2 2.3 71 27 98 9.3 4.3 I I 2.1 885 19 7509 127 Comp. ex. 39 N1 2.3 79 I I 90 4.9 1.3 15 3.6 821 23 6516 190 Inventive N 40 N2 1.6 75 17 92 4.9 1.4 23 6. 1 787 27 7079 236 example 41 01 1.6 55 45 100 4.1 0.8 22 4.1 866 16 5736 236 Comp. ex. 0 42 0 2 1.6 76 23 99 1.9 0.4 14 2.6 917 17 7450 292 43 P I 1.6 67 29 % 4.8 1.3 20 4.3 893 20 8704 170 Inventive 44 P2 1.6 73 27 100 5.9 1.1 18 4.7 1022 16 8047 171 example p 45 P3 1.6 74 24 98 5.1 0.8 60 9.0 837 25 9%8 229 46 P4 1.6 75 17 92 6.3 2.8 35 15.0 924 12 4620 131 Comp. ex 47 Ql Q 1.8 73 26 99 4.4 0.9 28 5.2 917 19 8542 216 Inventive 48 R l R 1.8 72 28 100 5.5 1.5 33 8.2 919 18 7835 175 example 49 Sl 1.8 74 15 89 5.4 1.4 35 8.5 925 18 7794 187 50 S2 s 1.8 80 13 93 10.2 3.4 35 11.2 957 18 5315 138 51 S3 1.6 73 24 97 2Ji :u 22 ill 836 23 7294 135 Comparative 52 T1 T 1.6 67 31 98 6.5 1.9 27 5.5 1037 15 4982 279 example 53 U1 u 1.6 73 18 91 1.5 0.4 14 2.5 797 24 4532 138 invention that satisfied all the requirements of the present invention have high strength and hole expandability as well as excellent toughness. In contrast to these example embodiments of the present invention, the results for the comparative examples showed that at least one of hole expandability and toughness deteriorated. [0131] Specifically, in Test No. 5, the cooling stop temperature in the secondary cooling process was low and martensite was formed, and consequently the area fraction of ferrite and bainite was less than 90% and the toughness deteriorated. [0132] In Test No. 16, because the end temperature in the rough rolling process was too low, Ti carbo-nitrides completely precipitated and could not be redissolved thereafter, and consequently variations in the grain size of the Ti carbo-nitrides increased, and in accompaniment therewith, variations in the grain size of ferrite and bainite also increased, and as a result the hole expandability and the toughness both deteriorated. [0133] In Test No. 20, the heating temperature in the heating process after rough rolling was too high, and consequently ferrite and bainite coarsened and the toughness deteriorated. In Test No. 21, the cooling rate in the secondary cooling process was too low, and consequently pearlite, cementite or the like was excessively formed and the hole expandability deteriorated. [0134] In Test No. 22, the cooling time period was too long in the slow cooling process after primary cooling, and consequently the area fraction of ferrite was excessive, which resulted in the toughness deteriorating. In Test No. 23, the heating rate in the heating process after rough rolling was low, and consequently ferrite and bainite coarsened and the toughness deteriorated. [0135] In Test No. 24, because the finishing temperature in the finish rolling was too high, differences in the timings of precipitation of Ti carbo-nitrides were large. Further, variations in the grain size of the Ti carbo-nitrides increased, and in accompaniment 39 therewith, variations in the grain size of ferrite and bainite also became large, and consequently hole expandability and toughness both deteriorated. [0136] In Test No. 27, the heating temperature in the heating process after rough rolling was low and redissolution of Ti carbo-nitrides was insufficient, and consequently variations in the grain size of the Ti carbo-nitrides increased, and in accompaniment therewith, variations in the grain size of ferrite and bainite also became large, and as a result the hole expandability and the toughness both deteriorated. In Test No. 28, the time period from the end of the sheet bar heating process to the start of the finish rolling process was more than 20 seconds, and consequently the micro-structure coarsened and variations in the diameter of the precipitates became large. [0137] In Test No. 31, because the end temperature in the rough rolling process was too low, Ti carbo-nitrides completely precipitated and could not be redissolved thereafter, and consequently variations in the grain size of the Ti carbo-nitrides increased, and in accompaniment therewith, variations in the grain size of ferrite and bainite also increased, and as a result the hole expandability and the toughness both deteriorated. [0138] In Test No. 33, the cooling stop temperature in the secondary cooling process was high, and consequently the area fraction of ferrite was excessive, which resulted in the toughness deteriorating. In Test No. 34, because a sheet bar heating process was not performed, redissolution of Ti carbo-nitrides was insufficient, and consequently variations in the grain size of the Ti carbo-nitrides increased, and in accompaniment therewith, variations in the grain size of ferrite and bainite also increased, and as a result the hole expandability and the toughness both deteriorated. [0139] In Test No. 36, the cooling stop temperature in the primary cooling process was low and consequently pearlite was formed, and therefore the hole expandability deteriorated. In Test No. 38, because the cooling rate in the primary cooling process was low, variations in the grain size of ferrite and bainite were large, and therefore the 40 toughness deteriorated. In Test No. 41, because the cooling stop temperature in the primary cooling process was high, ferrite was not sufficiently produced, and therefore the hole expandability deteriorated. [0140] In Test No. 46, because the end temperature in the rough rolling process was too high, it was difficult for Ti carbo-nitrides to efficiently redissolve, and therefore variations in the grain size of Ti carbo-nitrides became large. Further, in accompaniment therewith, variations in the grain size of ferrite and bainite also became large, and as a result the hole expandability and the toughness both deteriorated. [0141] In Test No. 50, the total rolling reduction in the rough rolling process was low, while in Test No. 51 the heating time period in the sheet bar heating process was insufficient. Therefore, in both of these cases, variations in the grain size of Ti carbonitrides became large, and in accompaniment therewith, variations in the grain size of ferrite and bainite also became large, and as a result the hole expandability and the toughness both deteriorated. [0142] In Test No. 52, the content of C was excessive, while in Test No. 53 the content of Mn was excessive, and therefore the hole expandability decreased. EXAMPLE 2 [0143] Next, solid wires having the chemical compositions shown in Table 4 were prepared, and were adopted as welding material. The welding materials shown in Table 5 were then used to perform bead-on-plate welding with respect to the surface of the test materials described above. That is, in the present example, the first base metal portion and the second base metal portion were made of the same steel material. Welding was performed by gas-shielded arc welding, and the welding conditions were as follows: current value: 190 A, voltage: 23 V, welding speed: 100 cm/min, shielding gas: Ar+20%CO2. [0144] 41 [Table 4] [0145] The chemical composition of the weld metal portion of each obtained weld joint was measured. Specifically, machined chips were collected from the weld metal portion in a manner so that the base metal portion did not get mixed therein. Analysis was then performed by inductively coupled plasma optical emission spectrometry and a high frequency combustion using the collected machined chips. The results of measuring the chemical composition of the respective weld metal portions are shown in Table 5. [0146] [Table 5] 42 [0147] Thereafter, each obtained weld joint was cut out so that a cross-section which was parallel to the surface of the test material on which the welding was performed and which was at a position that was at a distance equivalent to 1/8 t in the thickness direction from the surface of the test material became the observation surface. Then, as described 43 above, regions of 50 m in the Y-direction and 500 m in the X-direction were selected at a pitch of 50 m from one of the boundaries between the steel material and the weld metal portion toward the steel material side in the Y-direction. The area fractions (m0 to m5: %) of bainite in the respective regions were measured. [0148] In addition, a V-notch Charpy impact test specimen was prepared in the manner illustrated in Figure 6 from each of the weld joints. With regard to the weld joint made by bead-on-plate welding having the shape illustrated in Figure 6(a), first, as illustrated in Figure 6(b), a portion of the weld metal portion protruding from the surface of the test material was ground to flatten the surface. [0149] Next, as illustrated in Figure 6(c), a V-notch test specimen having a length of 55 mm, a width of 10 mm, and a thickness of 2.5 mm defined in JIS Z 2242 (2018) was cut out so that the extending direction of the weld metal portion became the longitudinal direction. At such time, the surface of the steel material on which the welding was performed is made to be the surface of the V-notch test specimen. Further, in a case where the thickness of the steel material is 2.5 mm or less, the thickness of the steel material is taken as being the thickness of the V-notch test specimen. [0150] Figures 7(a) and 7(b) are views for describing the position for cutting out the Vnotch. Figure 7(a) is a view of the V-notch test specimen from the thickness direction, and Figure 7(b) is a cross-sectional view illustrating an AA portion in Figure 7(a). As illustrated in Figures 7(a) and 7(b), the tip of the V-notch is cut out so as to pass through the boundary between the weld metal portion and the HAZ. Subsequently, a Charpy test was performed at -40C using the obtained V-notch test specimen, and the value of the absorbed energy was evaluated. In the present example, a value of absorbed energy of 50 J/cm2 or more was regarded as acceptable. [0151] A summary of the results is shown in Table 6. [0152] 44 [Table 6] 45 Table 6 Test Steel Welding Bainite area fraction in HAZ (area%) Changes in Impact absorbed No. sheet Steel material micro-structure energy at -40"C mo m, m, m, m., m, (J/cm2 ) I AI \V 62 57 46 27 17 13 0 163 Inventive 2 A2 X 91 82 67 49 34 31 0 lSI example A 3 A3 y 86 76 60 39 24 20 X 42 4 A4 z 64 62 53 31 24 21 X 24 Comparative example s Bl \V 74 49 28 25 25 25 X 31 6 B2 B \V 63 58 48 35 22 19 0 83 7 B3 X 67 62 51 36 22 19 0 163 8 Cl \V 67 64 so 36 26 23 0 168 9 C2 c X 69 65 49 36 21 17 0 73 10 C3 X 80 73 60 46 32 29 0 180 Inventive 11 Dl \V 81 71 ss 36 18 14 0 Ill example 12 D2 D \V 68 61 51 34 21 18 0 121 13 D3 X 72 65 55 38 25 22 0 78 14 El X 70 66 so 36 23 20 0 168 IS E2 E X 79 75 61 45 28 24 0 133 16 E3 \V 77 77 63 28 25 25 X 45 Comp. ex. 17 Fl \V 73 67 57 38 21 18 0 150 18 F2 F \V 63 57 47 34 25 22 0 157 Inventive example 19 F3 X 72 66 52 33 16 11 0 !56 20 Gl X 74 74 58 16 13 13 X 24 21 G2 G \V 89 89 70 22 19 19 X 29 22 G3 X 61 61 46 8 5 5 X 42 Comparative example 23 HI X 71 71 57 23 20 20 X 37 24 H2 H X 81 81 66 27 24 24 X 31 25 H3 \V 93 81 65 49 32 24 0 124 Inventive 26 II \V 77 68 51 34 21 16 0 141 example 27 12 I \V 65 65 51 17 14 14 X 27 Comparative 28 13 \V 81 79 72 45 28 23 X 56 example 29 Jl X 93 81 62 43 27 20 0 168 Inventive 30 J2 J X 85 76 62 48 30 27 0 liS example 31 J3 X 79 78 75 57 21 14 X 43 Comp. ex. 32 Kl \V 67 61 53 40 28 25 0 107 Inv. ex. 33 K2 K X 67 67 49 3 0 0 X 44 Comparative 34 K3 X 71 70 65 32 21 19 X 43 example 35 Ll \V 62 59 50 31 22 19 0 % Inv. ex. L 36 L2 X 68 68 56 25 23 23 X 42 Comp. ex. 37 Ml \V 81 71 55 36 19 12 0 82 Inv. ex. M 38 M2 X 72 72 60 29 27 27 X 34 Comp. ex. 39 Nl \V 71 64 51 35 17 11 0 167 Inventive N 40 N2 X 73 65 52 35 21 17 0 138 example 41 0 1 \V 95 95 82 47 45 45 X 30 Comp. ex. 0 42 02 X 72 68 53 41 26 23 0 140 Inventive 43 PI X 94 87 72 53 33 29 0 140 example 44 P2 y 72 72 60 29 27 27 X 47 p Comparative 45 P3 z 92 92 74 27 24 24 X 40 example 46 P4 X 67 62 35 23 20 17 X 45 47 Ql Q \V 72 65 52 36 28 26 0 138 48 Rl R \V 68 66 55 42 32 28 0 125 Inventive example 49 Sl \V 65 62 42 28 20 15 0 143 50 S2 s \V 81 78 72 45 15 11 X 41 51 S3 \V 79 77 43 31 26 24 X 43 Comparative 52 Tl I X 92 92 76 34 31 31 X 45 example 53 U! !I X 91 91 71 22 18 18 X 32 [0153] As is also clear from Table 6, it is found that example embodiments of the present invention that satisfy all the requirements of the present invention have excellent lowtemperature toughness. In contrast to these example embodiments of the present invention, for the comparative examples, the results showed that because changes in the micro-structure in a HAZ were abrupt, the low-temperature toughness was poor. INDUSTRIAL APPLICABILITY [0154] According to the present invention, it is possible to obtain a hot-rolled steel sheet having excellent toughness in addition to high strength and hole expandability. Further, a weld joint made using the hot-rolled steel sheet has excellent low-temperature toughness in a weld zone. REFERENCE SIGNS LIST [0155] 1. Weld Joint 10. First Base Metal Portion 10a. End Portion 10b. Surface 20. Second Base Metal Portion 30. Weld Metal Portion 100, 200. HAZ 46 mass%: Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 1.00%, Mo: 0 to 0.40%, Nb: 0 to 0.060%, V: 0 to 1.00%, B: 0 to 0.0100%, Ca: 0 to 0.0050%, O: 0.0100% or less, and the balance: Fe and impurities, wherein: a steel micro-structure includes, in area%: ferrite: 60 to 80%, and a total of ferrite and bainite: 90% or more; an average of a crystal grain size of ferrite and bainite is 7.0 m or less, and a standard deviation of the crystal grain size is 2.0 m or less; and a standard deviation of a diameter of Ti carbo-nitrides is 10 nm or less. WE CLAIMS 1. A hot-rolled steel sheet having a chemical composition comprising, in C: 0.02 to 0.20%, Si: 0.01 to 1.50%, Mn: 0.10 to 3.00%, P: 0.10% or less, S: 0.010% or less, Al: 0.005 to 0.100%, Ti: 0.02 to 0.20%, N: 0.001 to 0.010%, 47 2. The hot-rolled steel sheet according to claim 1, wherein: the chemical composition contains at least one of, in mass%: Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Cr: 0.01 to 1.00%, Mo: 0.005 to 0.40%, Nb: 0.001 to 0.060%, V: 0.01 to 1.00%, B: 0.0005 to 0.0100%, and Ca: 0.0005 to 0.0050%. 3. A method for producing a hot-rolled steel sheet, that comprises performing, in order: (a) a process of casting a slab having a chemical composition according to claim 1 or claim 2; (b) a slabbing process of, after casting, without a temperature of the slab decreasing to less than 800C, performing a rough rolling process described hereunder, or inserting the slab into a slab heating furnace and heating the slab to within a range of 1100 to 1250C; (c) a rough rolling process of performing hot rolling of the slab in which a start temperature is within a range of 950 to 1200C, an end temperature is within a range of 800 to 1050C, and a total rolling reduction is 20% or more, to form a sheet bar; (d) a sheet bar heating process of heating the sheet bar for 60 seconds or more to a temperature range of 1100 to 1250C at an average heating rate of 100C/min or more; (e) a finish rolling process of, within 20 seconds after the sheet bar heating process ends, subjecting the sheet bar to hot rolling with a start temperature within a range of 900 to 1250C and an end temperature within a range of an Ar3 point or more to less than 950C, and with a total rolling reduction of 50% or more, to form a steel sheet; (f) a cooling process of subjecting the steel sheet to primary cooling to a temperature range of 600 to 750C at an average cooling rate of 60C/s or more, and 48 thereafter conducting slow cooling at an average cooling rate of 0 to 10C/s for a period of 0 to 10 seconds, and additionally thereafter conducting secondary cooling at an average cooling rate of 60C/s or more to a temperature which is equal to or less than a temperature 15C or more lower than an end temperature of the slow cooling and which is within a temperature range of 350 to 700C; and (g) a coiling process of coiling the steel sheet. 4. A weld joint, comprising a first base metal portion, a second base metal portion and a weld metal portion, wherein: the weld metal portion is formed so as to extend at least in a first direction along an end portion of the first base metal portion; the first base metal portion is a hot-rolled steel sheet according to claim 1 or claim 2; a chemical composition of the weld metal portion comprises, in mass%: C: 0.02 to 0.15%, Si: 0.01 to 1.50%, Mn: 0.10 to 1.50%, P: 0.10% or less, S: 0.010% or less, Al: 0.005 to 0.300%, Ti: 0.02 to 0.20%, N: 0.010% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 1.00%, Mo: 0 to 0.60%, Nb: 0 to 0.060%, V: 0 to 1.00%, B: 0 to 0.0100%, Ca: 0 to 0.0050%, 49 O: 0.0010 to 0.0500%, and the balance: Fe and impurities; and when a direction perpendicular to the first direction as viewed from a thickness direction of the first base metal portion is taken as a second direction, and a thickness of the first base metal portion is represented by "t", at a cross-section which is parallel to a surface on one side in the thickness direction of the first base metal portion and which is at a position at a distance equivalent to 1/8 t in the thickness direction from the surface, m0, m1, m2, m3, m4, and m5 that are area fractions (%) of bainite measured in order at a pitch of 50 m from a boundary between the first base metal portion and the weld metal portion toward the first base metal portion side in the second direction satisfy formula (i) below: 0