HIGH STRENGTH HOT ROLLED STEEL SHEET HAVING EXCELLENT ELONGATED AND METHOD FOR MANUFACTURING THE SAME

Abstract

The present invention provides a high strength hot rolled steel sheet and a method for manufacturing same, the steel sheet containing, in weight percentage, 0.11-0.14% of C, 0.20-0.50% of Si, 1.8-2.0% of Mn, 0.03% or less of P, 0.02% or less of S, 0.01-0.04% of Nb, 0.5-0.8% of Cr, 0.01-0.03% of Ti, 0.2-0.4% of Cu, 0.1-0.4% of Ni, 0.2-0.4% of Mo, 0.007% or less of N, 0.001-0.006% of Ca, 0.01-0.05% of Al, a balance of Fe and inevitable impurities, wherein relational expressions 1 to 3 below are satisfied, and a microstructure includes, by area percentage, 88% or more of bainite, 10% or less of ferrite, 2% or less of pearlite, and 0.8% or less of island martensite. [Relational Expression 1] 7≤(Mo/93)/(P/31)≤16. [Relational Expression 2] 1.6≤Cr+3Mo+2Ni≤2. [Relational Expression 3] 6≤(3C/12+Mn/55)×100≤7.

Claims

1. A high strength hot rolled steel sheet having excellent elongation, comprising: by wt %, 0.11 to 0.14% of C, 0.20 to 0.50% of Si, 1.8 to 2.0% of Mn, 0.03% or less of P, 0.02% or less of S, 0.01 to 0.04% of Nb, 0.5 to 0.8% of Cr, 0.01 to 0.03% of Ti, 0.2 to 0.4% of Cu, 0.1 to 0.4% of Ni, 0.2 to 0.4% of Mo, 0.007% or less of N, 0.001 to 0.006% of Ca, 0.01 to 0.05% of Al, a balance of Fe, and inevitable impurities, wherein relational expressions 1 to 3 below are satisfied, and a microstructure comprises, by area %, 88% or more of bainite (excluding 100%), 10% or less of ferrite (excluding 0%), 2% or less of pearlite (excluding 0%), and 0.8% or less of martensite-austenite constituent (comprising 0%),
7≤(Mo/93)/(P/31)≤16  [Relational Expression 1]
1.6≤Cr+3Mo+2Ni≤2  [Relational Expression 2]
6≥(3C/12+Mn/55)≤100≤7  [Relational Expression 3] where, in relational expressions 1 to 3, the contents of alloying elements are based on wt %.

2. The high strength hot rolled steel sheet of claim 1, wherein an average grain size of the bainite is 8 μm or less.

3. The high strength hot rolled steel sheet of claim 1, wherein an average grain size of the ferrite is 10 μm or less.

4. The high strength hot rolled steel sheet of claim 1, wherein an average grain size of the pearlite is 4 μm or less.

5. The high strength hot rolled steel sheet of claim 1, wherein an average grain size of the martensite-austenite constituent is 1 μm or less.

6. The high strength hot rolled steel sheet of claim 1, wherein the hot rolled steel sheet has a yield strength of 850 MPa or more at room temperature, a tensile strength of 900 MPa or more at room temperature, and a total elongation of 13% or more.

7. A method for manufacturing a high strength hot rolled steel sheet having excellent elongation, comprising: reheating a steel slab satisfying conditions of relational expressions 1 to 3 below at 1100 to 1180° C., the steel slab comprising, by wt %, 0.11 to 0.14% of C, 0.20 to 0.50% of Si, 1.8 to 2.0% of Mn, 0.03% or less of P, 0.02% or less of S, 0.01 to 0.04% of Nb, 0.5 to 0.8% of Cr, 0.01 to 0.03% of Ti, 0.2 to 0.4% of Cu, 0.1 to 0.4% of Ni, 0.2 to 0.4% of Mo, 0.007% or less of N, 0.001 to 0.006% of Ca, 0.01 to 0.05% of Al, a balance of Fe, and inevitable impurities; extracting the reheated steel slab after maintaining the reheated steel slab at 1150° C. or higher for 45 minutes or longer; primarily rolling the extracted steel slab at 850 to 930° C. to obtain steel; secondarily rolling the steel at 740 to 795° C.; water-cooling the secondarily rolled steel at a cooling rate of 10 to 50° C./s; and coiling the water-cooled steel at 440 to 530° C.

8. The method of claim 7, wherein a cumulative reduction ratio during the secondary rolling is 85% or more.

Description

MODE FOR INVENTION

[0057] Hereinafter, the present disclosure will be described in more detail through Inventive Examples. It should be noted that the following examples are for describing exemplary examples of the present disclosure, and the scope of the present disclosure is not limited by the following examples. This is because the scope of the present disclosure is determined by matters described in the claims and matters able to be reasonably inferred therefrom.

Inventive Example

[0058] After the molten steel having the alloy composition shown in Tables 1 and 2 below was manufactured as a steel slab by a continuous casting method, the steel slab was heated at 1100 to 1180° C., and then reheated, extracted, rolled, coiled, and cooled under the conditions shown in Table 3 below, thereby manufacturing the hot-rolled steel sheet having a thickness of 5 mm. The type and fraction of the microstructure, the average grain size, and mechanical properties of the hot rolled steel sheet thus manufactured were measured, and then were shown in Table 4 below.

TABLE-US-00001 TABLE 1 Steel Alloy Composition (wt %) type No. C Si Mn P S Nb Cr Ti Cu Inventive 0.136 0.338 1.98 0.008 0.001 0.038 0.60 0.014 0.270 Steel No. 1 Inventive 0.136 0.339 1.92 0.007 0.0013 0.015 0.61 0.015 0.275 Steel No. 2 Inventive 0.136 0.324 1.80 0.0067 0.0017 0.015 0.60 0.014 0.274 Steel No. 3 Inventive 0.138 0.372 1.92 0.0098 0.0013 0.037 0.62 0.017 0.285 Steel No. 4 Inventive 0.127 0.320 1.84 0.0107 0.0015 0.037 0.0 0.012 0.270 Steel No. 5 Comparative 0.16 0.35 1.98 0.018 0.001 0.02 0.55 0.015 0.270 Steel No. 1 Comparative 0.13 0.33 2.10 0.012 0.0013 0.03 0.54 0.02 0.272 Steel No. 2 Comparative 0.14 0.35 1.98 0.013 0.0017 0.02 0.53 0.018 0.279 Steel No. 3 Comparative 0.13 0.34 2.10 0.0124 0.0013 0.022 0.52 0.019 0.262 Steel No. 4 Comparative 0.08 0.35 1.80 0.0107 0.0015 0.021 0.54 0.011 0.274 Steel No. 5

TABLE-US-00002 TABLE 2 Steel Alloy Composition (wt %) Relational Relational Relational type No. Ni Mo N Ca Al Expression 1 Expression 2 Expression 3 Inventive 0.168 0.365 0.005 0.0021 0.032 15.2 2.0 7.0 Steel No. 1 Inventive 0.167 0.309 0.004 0.0025 0.0038 14.7 1.9 6.9 Steel No. 2 Inventive 0.169 0.315 0.003 0.0028 0.034 15.7 1.9 6.7 Steel No. 3 Inventive 0.172 0.255 0.004 0.0025 0.034 8.7 1.7 6.9 Steel No. 4 Inventive 0.169 0.241 0.005 0.0029 0.035 7.5 1.7 6.5 Steel No. 5 Comparative 0.150 0.320 0.005 0.0021 0.0032 5.9 1.8 7.6 Steel No. 1 Comparative 0.140 0.220 0.004 0.0025 0.038 6.1 15 7.1 Steel No. 2 Comparative 0.142 0.150 0.003 0.0028 0.034 3.8 1.3 7.1 Steel No. 3 Comparative 0.148 0.210 0.004 0.0025 0.034 5.6 1.4 7.1 Steel No. 4 Comparative 0.141 0.180 0.005 0.0029 0.035 5.6 1.4 5.3 Steel No. 5 [Relational Expression 1] (Mo/93)/(P/31) [Relational Expression 2] Cr + 3Mo + 2Ni [Relational Expression 3] (3C/12 + Mn/55) × 100

TABLE-US-00003 TABLE 3 Holding Non- Primary Secondary Time at recrystallized Rolling Rolling Reheating 1150° C. Average End End Cooling Coiling Steel Temperature or higher Reduction Ratio Temperature Temperature Rate Temperature Division type No. (° C.) (Minute) (%) (° C.) (° C.) (° C./s) (° C.) Inventive Inventive 1156 66 91 880 785 18 501 Example 1 Steel No. 1 Inventive Inventive 1176 67 86 893 781 21 512 Example 2 Steel No. 2 Inventive Inventive 1156 62 89 915 776 22 598 Example 3 Steel No. 3 Inventive Inventive 1162 67 92 905 780 32 493 Example 4 Steel No. 4 Inventive Inventive 1172 62 90 923 764 27 502 Example 5 Steel No. 5 Comparative Comparative 1277 78 88 944 798 21 503 Example 1 Steel No. 1 Comparative Comparative 1182 62 92 968 819 19 515 Example 2 Steel No. 2 Comparative Comparative 1178 63 88 932 822 23 520 Example 3 Steel No. 3 Comparative Comparative 1167 68 87 923 861 24 545 Example 4 Steel No. 4 Comparative Comparative 1181 71 91 943 862 19 515 Exampl e 5 Steel No. 5 Comparative Inventive 1165 58 89 948 833 20 563 Example 6 Steel No. 1 Comparative Inventive 1124 53 90 937 867 19 583 Example 7 Steel No. 2

TABLE-US-00004 TABLE 4 Martensite-austenite Ferrite Pearlite Bainite constituent Yield Tensile Total Fraction Size Fraction Size Fraction Size Fraction Size Strength Strength Elongation Division (area %) (μm) (area %) (μm) (area %) (μm) (area %) (μm) (MPa) (MPa) (%) Inventive 7.2 6 1 2 91 6 0.8 1 1010 1120 15.2 Example 1 Inventive 9.4 6 1 3 89 7 0.6 1 952 1110 14.5 Example 2 Inventive 10 7 2 3 88 4 0 — 904 970 15.4 Example 3 Inventive 5.5 6 1 3 93 5 0.5 1 907 970 14.5 Example 4 Inventive 9 8 2 2 89 6 0 — 908 976 15.6 Example 5 Comparative 8 5 1 2 88 6 3 2 1230 1150 10.2 Example 1 Comparative 10 6 1 2 87 6 2 1 1014 1135 11 Example 2 Comparative 5 7 2 3 91 5 2 1 958 1011 12 Example 3 Comparative 13 13 4 3 83 10 0 — 881 943 14.3 Example 4 Comparative 8 9 5 2 87 9 0 — 654 872 21 Example 5 Comparative 14 15 7 4 79 14 0 — 876 832 18 Example 6 Comparative 16 18 12 5 72 16 0 — 758 893 19.2 Example 7

[0059] As may be seen from Tables 1 to 4, in the case of Inventive Examples 1 to 5 satisfying the alloy composition, the component relational expressions, and the manufacturing conditions proposed by the present disclosure, the microstructure having the fine grain size of the appropriate fraction is included in an appropriate fraction, so it may be seen that the excellent yield strength, tensile strength and elongation are secured.

[0060] However, in the case of Comparative Examples 1 to 5 that do not satisfy the alloy composition, the component relational expressions, and the manufacturing conditions proposed by the present disclosure, it was found that the yield strength, the tensile strength, or the elongation was low as the microstructure of the present disclosure was not secured.

[0061] Comparative Examples 6 and 7 are cases in which the alloy composition and the component relational expression proposed by the present disclosed are satisfied, but it may be seen that the manufacturing conditions are not satisfied, and the yield strength, the tensile strength, or the elongation is at a low level as the microstructure of the present disclosure is not secured.