High strength hot rolled steel sheet and manufacturing method thereof

Abstract

This high strength hot rolled steel sheet has a predetermined chemical composition, in which the structure of the high strength hot rolled steel sheet contains martensite in an area ratio of 20% or more and 60% or less and ferrite in an area ratio of 40% or more, and the total area ratio of the martensite and the ferrite is 90% or more, the average grain size of the martensite is 5.0 m or more and 50 m or less, the ratio of the hardness of the martensite to the hardness of the ferrite is 0.6 or more and 1.6 or less, and the tensile strength of the high strength hot rolled steel sheet is 980 MPa or more.

Claims

1. A high strength hot rolled steel sheet comprising, by mass %: C: 0.02% or more and 0.30% or less; Si: 0.20% or more and 2.0% or less; Mn: 0.5% or more and 3.0% or less; P: 0.10% or less; S: 0.010% or less; Al: 0.10% or more and 1.0% or less; N: 0.010% or less; Ti: 0.06% or more and 0.20% or less; Nb: 0% or more and 0.10% or less; Ca: 0% or more and 0.0060% or less; Mo: 0% or more and 0.50% or less; Cr: 0% or more and 1.0% or less; and a remainder of Fe and impurities, wherein a structure of the high strength hot rolled steel sheet contains a martensite in an area ratio of 20% or more and 60% or less and a ferrite in an area ratio of 40% or more, and a total area ratio of the martensite and the ferrite is 90% or more, an average grain size of the martensite is 5.0 m or more and 50 m or less, a ratio of a hardness of the martensite to a hardness of the ferrite is 0.6 or more and 1.6 or less, and a tensile strength of the high strength hot rolled steel sheet is 980 MPa or more.

2. The hot rolled steel sheet according to claim 1, wherein the hot rolled steel sheet comprises one or more of, by mass %: Nb: 0.01% or more and 0.10% or less; Ca: 0.0005% or more and 0.0060% or less; Mo: 0.02% or more and 0.50% or less; and Cr: 0.02% or more and 1.0% or less.

Description

EXAMPLES

(1) Hereinafter, the high strength hot rolled steel sheet of the present invention will be described in detail with reference to examples. However, conditions in the examples are examples of conditions employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to the following examples. It is possible to carry out the present invention in appropriate modifications thereof within a range that conforms to the gist as long as the object of the present invention can be achieved without departing from the gist of the present invention. Therefore, the present invention can employ various conditions, all of which are included in the technical features of the present invention.

(2) Steel having the chemical composition shown in Table 1 was melted in a converter and was continuously cast into a slab having a thickness of 230 mm. Thereafter, the slab was heated to a temperature of 1200 C. to 1250 C., and was subjected to rough rolling, and finish rolling, primary cooling, intermediate air cooling, secondary cooling, and winding was performed thereon under the conditions shown in Table 2, thereby manufacturing a hot rolled steel sheet. The cooling rate of the intermediate air cooling was 3 to 8 C./s.

(3) Table 2 shows kinds of steel used, finish rolling conditions, and the sheet thicknesses of steel sheets. In Table 2, latter stage rolling reduction ratio is the ratio of the total rolling reduction of the final stand and the preceding stand to the sum of the rolling reductions of stands of continuous finish rolling stands, F5 rolling reduction is the rolling reduction in the stand in the stage preceding the final stand, FT5 is the rolling temperature of the stand in the stage preceding the final stand, F6 rolling reduction is the rolling reduction of the final stand, FT6 is the rolling temperature of the final stand, rolling reduction ratio is the ratio of the rolling reduction of the final stand to the rolling reduction of the preceding stand, cooling start is the time from the end of the finish rolling to the start of the primary cooling, primary cooling is the average cooling rate between the end of the finish rolling and the intermediate air cooling start temperature, air cooling temperature is the temperature at which the primary cooling is stopped and the intermediate air cooling is started, air cooling time is the intermediate air cooling time, secondary cooling is the average cooling rate during the secondary cooling until the winding after the intermediate air cooling, and winding temperature is the winding temperature after the end of the secondary cooling.

(4) TABLE-US-00001 TABLE 1 Composition (mass %) remainder: Fe and impurities Ar3 Kind of steel C Si Mn P S Al N Ti Nb Ca Mo Cr ( C.) A 0.04 1.20 1.0 0.015 0.0030 0.12 0.004 0.11 845 B 0.10 1.20 1.0 0.014 0.0042 0.25 0.004 0.08 0.0011 830 C 0.21 0.30 1.2 0.014 0.0030 0.25 0.003 0.12 0.0008 0.35 746 D 0.12 1.30 1.4 0.015 0.0010 0.15 0.004 0.12 0.015 786 E 0.08 1.20 2.0 0.015 0.013 0.15 0.003 0.13 0.20 741 F 0.15 0.80 2.0 0.014 0.0030 0.40 0.004 0.07 0.035 0.3 715 G 0.11 1.00 2.0 0.013 0.0060 0.30 0.003 0.11 0.0018 730 H 0.12 1.00 0.5 0.015 0.0050 0.05 0.004 0.11 855 I 0.15 0.40 2.0 0.015 0.0030 0.38 0.004 0.04 0.0021 0.05 701

(5) TABLE-US-00002 TABLE 2 latter stage F5 F6 Rolling Air rolling rolling rolling reduc- cooling Air Sec- Winding Sheet Kind reduction reduc- reduc- tion Cooling Primary tem- cooling ondary tem- thick- Test of ratio tion FT5 tion FT6 ratio start cooling perature time cooling perature ness No. steel % C. % C. sec C./sec C. sec C./sec C. mm 1 A 0.15 35 869 26 868 0.75 0.7 0 720 6 109 100 2.9 2 A 0.18 36 936 13 894 0.36 0.4 108 674 4 62 250 2.9 3 A 0.18 36 881 33 868 0.92 0.4 0.20 731 5 111 250 1.8 4 A 0.22 38 921 20 884 0.53 0.7 109 615 5 140 250 1.8 5 A 0.08 50 977 47 962 0.93 0.7 76 696 8 85 100 2.0 6 B 0.21 51 954 36 913 0.71 0.7 80 657 5 114 100 2.0 7 B 0.15 45 935 28 876 0.63 0.8 0 673 9 98 100 1.6 8 B 0.26 48 919 29 870 0.60 0.6 88 772 5 89 260 1.6 9 B 0.26 46 952 40 874 0.87 0.8 97 711 8 80 260 2.0 10 C 0.14 46 876 36 872 0.78 0.7 93 736 3 79 100 3.2 11 C 0.13 44 920 25 892 0.57 0.3 98 636 3 104 100 2.0 12 C 0.21 51 933 37 887 0.73 1.8 92 687 7 122 100 3.2 13 C 0.24 48 886 30 867 0.62 0.8 109 702 2 99 100 3.2 14 C 0.13 48 957 31 869 0.65 0.5 104 630 6 118 120 2.6 15 D 0.22 48 890 43 882 0.89 0.8 61 730 4 131 120 4.5 16 D 0.14 43 926 33 919 0.77 0.7 33 642 8 97 120 4.5 17 D 0.15 37 908 42 883 1.14 0.6 99 645 6 137 120 4.5 18 D 0.19 35 874 18 861 0.51 0.4 78 693 8 84 100 2.3 19 E 0.24 43 891 39 884 0.90 0.7 101 735 4 134 100 2.3 20 E 0.25 34 928 19 907 0.57 0.4 99 685 15 109 100 2.3 21 E 0.24 36 871 33 864 0.92 0.8 77 666 7 125 100 2.9 22 E 0.19 30 898 24 877 0.78 0.5 95 530 4 126 100 2.9 23 F 0.24 34 886 25 869 0.73 0.5 87 662 9 102 100 2.9 24 F 0.16 22 903 13 872 0.61 0.4 82 691 5 107 400 1.6 25 F 0.17 46 909 39 880 0.85 0.6 88 662 5 99 100 1.6 26 F 0.20 48 942 42 904 0.88 0.8 97 701 8 90 100 1.6 27 G 0.42 32 886 28 872 0.88 0.7 93 636 3 89 30 2.0 28 G 0.16 38 917 27 892 0.71 0.3 69 627 4 113 30 1.8 29 G 0.13 30 881 26 877 0.88 0.7 74 697 7 76 30 1.8 30 G 0.27 35 937 28 869 0.80 0.5 68 600 5 125 30 3.6 31 G 0.17 32 912 25 892 0.78 0.6 121 630 1 165 100 3.6 32 G 0.16 45 892 38 873 0.84 0.7 95 635 9 67 100 2.9 33 H 0.23 39 894 23 865 0.61 0.7 103 656 3 84 100 3.6 34 I 0.17 34 949 19 938 0.57 0.6 77 612 6 62 100 3.6

(6) Regarding the steel sheet obtained as described above, visual fields are randomly selected at a thickness position of the steel sheet, the structure fractions of ferrite and martensite and the hardness ratio between the martensite and the ferrite were examined in at least five visual fields using an optical microscope.

(7) Regarding the structure fractions and grain sizes of the ferrite and the martensite of the steel sheet, five visual fields of 500 m500 m were randomly photographed using the optical microscope after Nital etching, and the average area ratio and the average grain size of the five visual fields were obtained using image analysis.

(8) Regarding the hardnesses of the martensite and the ferrite, a micro Vickers test was conducted on each structure, the Vickers hardnesses (Hv) of 100 or more points in each of the structures of the martensite and the ferrite were measured, and the average thereof was obtained.

(9) Regarding a tensile test of the steel sheet, a JIS No. 5 test piece was taken in the rolling width direction (C direction) of the steel sheet, and according to JIS Z 2241, yield strength: YP (MPa), tensile strength: TS (MPa), and elongation: EL (%) were evaluated.

(10) Hole expansibility (%) was evaluated according to the method defined in JIS Z 2256.

(11) Table 3 shows the evaluation results of the obtained structure and material. In Table 3, area ratio of each structure is the area ratio of each of the ferrite, martensite, and other structures, M diameter is the average grain size of the martensite, and hardness ratio is the hardness ratio obtained by (hardness of martensite/hardness of ferrite).

(12) TABLE-US-00003 TABLE 3 Proportion of martensite having grain size Hard- Hole Area ratio of each of 10 to 30 M ness Yield Tensile Elon- expan- Test structure (%) m diameter ratio strength strength gation sibility No. Ferrite Martensite Others % m MPa MPa % % Note 1 55 45 0 42 13.3 1.0 827 1028 18 70 Example of Present Invention 2 36 64 0 15 62.5 0.4 843 1016 9 61 Comparative Example 3 53 47 0 44 30.1 0.6 810 997 19 64 Example of Present Invention 4 52 48 0 53 24.3 0.7 870 1015 19 71 Example of Present Invention 5 21 79 0 53 27.6 0.7 1053 1212 8 34 Comparative Example 6 54 46 0 44 18.6 0.8 854 1024 18 73 Example of Present Invention 7 50 50 0 52 25.1 0.7 821 997 18 83 Example of Present Invention 8 18 82 0 54 39.5 0.3 899 1025 8 43 Comparative Example 9 65 35 0 48 21.2 0.8 802 1001 18 79 Example of Present Invention 10 63 37 0 47 19.6 0.8 911 1028 18 74 Example of Present Invention 11 59 41 0 49 29.8 0.6 816 1015 19 70 Example of Present Invention 12 61 39 0 18 51.4 0.3 874 1023 9 41 Comparative Example 13 59 41 0 48 20.2 0.8 858 1000 18 80 Example of Present Invention 14 61 39 0 46 16.7 0.9 852 995 19 74 Example of Present Invention 15 59 41 0 40 31.9 0.6 848 987 19 76 Example of Present Invention 16 49 43 8 49 19.5 2.2 903 1030 17 32 Comparative Example 17 81 19 0 37 3.7 3.5 827 1009 19 39 Comparative Example 18 48 52 0 58 10.5 0.9 849 992 19 69 Example of Present Invention 19 50 50 0 47 13.6 0.9 860 1011 18 67 Example of Present Invention 20 49 12 39 31 13.1 1.2 838 984 8 82 Comparative Example 21 61 39 0 39 29.1 0.7 854 1012 19 69 Example of Present Invention 22 33 60 7 56 11.9 1.2 882 981 9 79 Comparative Example 23 62 38 0 50 12.3 1.1 855 995 19 75 Example of Present Invention 24 22 43 35 43 13.3 0.9 892 985 7 65 Comparative Example 25 58 42 0 44 10.7 1.2 999 1023 18 80 Example of Present Invention 26 45 55 0 50 20.0 0.9 902 1010 17 79 Example of Present Invention 27 53 47 0 3 59.2 0.2 921 1038 8 74 Comparative Example 28 75 25 0 34 19.0 0.9 895 1003 19 72 Example of Present Invention 29 54 46 0 49 15.5 0.8 831 1002 18 87 Example of Present Invention 30 63 37 0 40 19.1 0.7 827 998 19 79 Example of Present Invention 31 25 75 0 69 31.2 0.3 923 1185 8 42 Comparative Example 32 55 45 0 45 15.6 1.0 825 998 18 77 Example of Present Invention 33 35 42 23 43 14.0 0.8 831 984 7 83 Comparative Example 34 47 53 0 48 10.5 1.0 783 853 19 70 Comparative Example

(13) As shown in Table 3, in the examples of the present invention, the tensile strength was 980 MPa or more, the structure fraction of the ferrite was 40% or more, the structure fraction of the martensite was 20% or more and 60% or less, and the hardness ratio of the martensite to the ferrite was 0.6 or more and 1.6 or less. Furthermore, as a result, in the examples of the present invention, the elongation was 10% or more, the hole expansibility was 50% or more, and thus the balance between the elongation and the hole expansibility was excellent.

(14) Contrary to this, in Test No. 2, a target structure fraction (area ratio of each structure) was not obtained. It is considered that this was caused by a low ratio (F6/F5) between the rolling reductions of F5 and F6 and delayed ferritic transformation. In addition, in Test No. 2, the grain size of the austenite was coarsened, the average grain size of the martensite grains was coarsened, the martensite was softened, and the hardness ratio decreased. As a result, the elongation was inferior.

(15) In Test No. 5, a target structure fraction was not obtained, and the elongation and the hole expansibility were inferior. It is considered that this was because the latter stage rolling reduction ratio was low, the finish rolling temperature was high, and the ferritic transformation was delayed.

(16) In Test No. 8, a target structure fraction was not obtained, and the elongation and the hole expansibility were inferior. It is considered that this was because the air cooling temperature was high and the ferritic transformation during the air cooling was delayed.

(17) In Test No. 12, the average grain size was the martensite grains was coarsened, the hardness ratio was less than 0.6, and thus the elongation and the hole expansibility were inferior. It is considered that this was because the cooling start time after the rolling was long and the austenite grains were coarsened.

(18) In Test No. 16, the hardness ratio was more than 1.6, and the hole expansibility was inferior. It is considered that this was because the primary cooling was slow, C enrichment in the austenite had proceeded, and thus the martensite was hardened.

(19) In Test No. 17, the hardness ratio was more than 1.6, and the hole expansibility was inferior. It is considered that this was because since the ratio of F6 to the rolling reductions of F5 was 1.0 or more, ferritic transformation had excessively proceeded, C enrichment was promoted, and thus the martensite was excessively hardened.

(20) In Test No. 20, the area ratio was the martensite was low, and the elongation was inferior. It is considered that this was because the air cooling time was 15 seconds, and bainitic transformation had proceeded during the air cooling.

(21) In Test No. 22, the area ratio was the ferrite was low, and the elongation was inferior. It is considered that this was because the air cooling temperature was low and the ferritic transformation had not sufficiently proceeded.

(22) In Test No. 24, a target structure was not obtained, and the elongation and the hole expansibility were inferior. It is considered that this was because the winding temperature was high.

(23) In Test No. 27, coarse martensite was formed, the hardness ratio between the structures was low, and the elongation was inferior. It is considered that this was because the rolling reduction in the latter stage was high, rolling in the former stage was insufficiently performed, and thus the austenitic structure was coarsened.

(24) In Test No. 31, a target structure was not obtained, and the elongation and the hole expansibility were inferior. It is considered that this was because the air cooling time was short.

(25) In Test No. 33, since the Al content was insufficient, a target area ratio of the ferrite was not obtained, and the elongation was inferior.

(26) In Test No. 34, since the Ti content was insufficient, the amount of precipitation strengthening caused by Ti was insufficient, and a tensile strength of 980 MPa was not obtained.

INDUSTRIAL APPLICABILITY

(27) According to the present invention, a high strength hot rolled steel sheet which is suitable for press components requiring high workability and is excellent in elongation and hole expansibility can be provided. With the high strength steel sheet, a reduction in the weight of the vehicle body of a vehicle or the like, integral forming of components, and a reduction in the number of working processes are possible, and the improvement of fuel efficiency and a reduction in manufacturing costs can be achieved. Therefore, the present invention has a high industrial value.