High-strength steel having excellent low-yield-ratio characteristics, and manufacturing method therefor

Abstract

A high-strength steel having excellent low-yield-ratio characteristics, according to one embodiment of the present invention, comprises, by wt %, 0.06-0.12% of C, 0.2-0.5% of Si, 1.5-2.0% of Mn, 0.003-0.05% of Al, 0.01% or less of N, 0.02% or less of P, 0.003% or less of S, 0.05-0.5% of Cr, 0.05-0.5% of Mo, 0.01-0.05% of Nb, 0.0005-0.005% of Ca and the balance of Fe and other inevitable impurities, and comprises polygonal ferrite as a microstructure, wherein the area fraction of the polygonal ferrite is 10-30% and the average hardness of the polygonal ferrite can be 180 Hv or less.

Claims

1. A steel material comprising, by wt %: 0.06 to 0.12% of C, 0.2 to 0.5% of Si, 1.5 to 2.0% of Mn, 0.003 to 0.05% of Al, 0.01% or less of N, 0.02% or less of P, 0.003% of less of S, 0.05 to 0.5% of Cr, 0.05 to 0.5% of Mo, 0.01 to 0.05% of Nb, and 0.0005 to 0.005% of Ca, with a balance of Fe and other unavoidable impurities, and wherein the polygonal ferrite has an area fraction of 10 to 30% and an average hardness of 180 Hv or less, wherein the residual structure comprises acicular ferrite, bainite, pearlite, and martensite, wherein the residual structure as an average hardness of 200 Hv or more, wherein a sum of area fractions of the pearlite and the martensite is 10% or less, excluding 0%, relative to a total area.

2. The steel material of claim 1, further comprising, by wt %, one or two or more of: 0.05 to 0.3% of Ni, 0.05 to 0.3% of Cu, 0.005 to 0.02% of Ti, and 0.0005 to 0.0015% of B.

3. The steel material of claim 1, wherein the steel material has a Charpy impact energy at −30° C. of 200 J or more.

4. The steel material of claim 1, wherein the steel material has a yield ratio of 90% or less.

5. The steel material of claim 1, wherein the steel material has a tensile strength of 500 MPa or more.

Description

EXAMPLES

(1) Steel slabs satisfying the component system described in the following Table 1 were manufactured, and steel materials for each slab were manufactured under the process conditions of Table 2.

(2) TABLE-US-00001 TABLE 1 Steel type C Si Mn P S Al Cr Mo Nb N Ca Ni Cu Ti B 1 0.06 0.3 1.6 0.015 0.0017 0.015 0.15 0.4 0.03 0.0045 0.0012 0.3 2 0.06 0.3 1.9 0.018 0.0015 0.003 0.3 0.1 0.04 0.004 0.001 0.2 0.1 3 0.06 0.5 1.6 0.016 0.0009 0.02 0.4 0.1 0.035 0.005 0.0015 0.1 0.012 4 0.07 0.3 1.5 0.01 0.0011 0.015 0.2 0.2 0.04 0.004 0.001 0.015 0.001 5 0.07 0.3 1.6 0.015 0.001 0.02 0.1 0.3 0.02 0.004 0.0008 0.2 6 0.07 0.4 1.8 0.012 0.0015 0.02 0.3 0.2 0.03 0.005 0.0013 0.1 0.1 0.015 7 0.08 0.3 1.5 0.009 0.0023 0.035 0.2 0.1 0.015 0.0055 0.0012 0.001 8 0.08 0.5 1.6 0.012 0.0024 0.03 0.2 0.2 0.025 0.0035 0.002 0.1 0.015 9 0.09 0.2 1.5 0.015 0.0018 0.035 0.1 0.05 0.025 0.004 0.0018 0.1 10 0.04 0.25 1.4 0.008 0.0008 0.03 0.1 0.055 0.005 0.0019 0.1 0.012 11 0.06 0.25 1.25 0.015 0.002 0.025 0.3 0.2 0.004 0.001 0.2 12 0.06 0.3 1.5 0.025 0.0015 0.055 0.2 0.3 0.02 0.005 13 0.08 0.15 1.6 0.018 0.0014 0.035 0.1 0.004 0.0012 14 0.08 0.3 1.3 0.023 0.0031 0.03 0.1 0.1 0.03 0.005 15 0.12 0.25 1.1 0.015 0.002 0.025 0.03 0.004 0.0013 0.01

(3) TABLE-US-00002 TABLE 2 Rough Finish rolling Finish Finish First Reheating rolling end effective rolling start rolling end cooling start Steel temperature temperature reduction temperature temperature temperature type Conditions (° C.) (° C.) ratio (%) (° C.) (° C.) (° C.) 1 1-1 1140 1080 65 880 830 730 1-2 1140 1080 65 910 850 740 1-3 1120 1050 75 880 830 780 2 2-1 1120 1050 70 910 820 740 2-2 1120 1060 70 930 820 730 2-3 1140 1080 55 830 790 740 3 3-1 1150 1100 65 900 830 770 3-2 1150 1090 65 880 800 760 3-3 1140 1080 70 850 810 775 4 4-1 1160 1100 75 950 880 770 4-2 1140 1080 75 980 880 780 4-3 1080 1000 78 850 770 710 5 5-1 1120 1060 65 850 800 750 5-2 1140 1080 65 880 820 740 6 6-1 1100 1050 70 900 830 750 6-2 1120 1060 70 880 810 740 7 7 1140 1070 65 880 830 780 8 8 1150 1090 65 850 800 770 9 9 1120 1060 70 930 850 780 10 10-1  1160 1080 80 1000 880 780 10-2  1180 1120 75 900 810 750 11 11-1  1120 1080 60 800 760 730 11-2  1180 1120 60 840 790 730 12 12-1  1080 1040 65 880 810 760 12-2  1100 1040 60 850 780 770 13 13-1  1140 1080 65 880 810 780 13-2  1160 1100 70 880 790 740 14 14  1120 1060 70 910 820 780 15 15  1140 1070 70 950 860 810 First Second Second cooling end First cooling end cooling Tnr Ar3 Steel temperature cooling rate temperature end rate temperature temperature type (° C.) (° C./s) (° C.) (° C./s) (° C.) (° C.) 1 650 10 400 30 894.99 718.6 650 15 380 32 894.99 718.6 — — 560 28 894.99 718.6 2 660 10 400 30 937.83 719.85 660 15 350 35 937.83 719.85 550 8 400 15 937.83 719.85 3 690 18 400 35 859.37 749.85 700 12 380 42 859.37 749.85 685 16 380 40 859.37 749.85 4 700 15 400 40 960.18 755.25 690 17 350 45 960.18 755.25 630 8 400 20 960.18 755.25 5 675 15 380 42 857.46 729.75 660 13 350 35 857.46 729.75 6 660 12 380 32 879.10 722.25 680 13 400 32 879.10 722.25 7 700 18 380 35 847.53 760.15 8 675 10 350 40 829.16 742.15 9 700 16 400 45 929.36 757.05 10 680 20 480 30 1041.32 784.55 700 10 500 25 1041.32 784.55 11 680 15 400 30 834.67 772.85 700 8 400 32 834.67 772.85 12 700 12 430 28 865.53 750.35 650 18 450 35 865.53 750.35 13 730 10 500 30 883.28 755.15 600 21 480 25 883.28 755.15 14 730 12 500 30 909.72 777.65 15 650 25 450 25 953.21 790.75

(4) A tensile test was performed for a tensile specimen obtained by cutting each steel material manufactured by Table 2 along a length direction to evaluate a yield strength, a tensile strength, and a Charpy impact energy at −30° C., and the results are shown in the following Table 3. In addition, for each steel material manufactured by Table 2, the microstructure after etching was observed, the hardness of each structure was measured, and the fraction and the hardness of polygonal ferrite of each steel material and the fraction and the hardness of the phases (pearlite+martensite) other than polygonal ferrite are as shown in the following Table 4.

(5) TABLE-US-00003 TABLE 3 Charpy impact Energy Yield strength Tensile strength Yield ratio at −30° C. Steel type Conditions (MPa) (MPa) (%) (J) Remarks 1 1-1 485 557 87 420 Inventive Example 1 1-2 501 583 86 415 Inventive Example 2 1-3 547 589 92 187 Comparative Example 1 2 2-1 473 544 87 397 Inventive Example 3 2-2 496 598 83 389 Inventive Example 4 2-3 442 480 92 167 Comparative Example 2 3 3-1 492 579 85 371 Inventive Example 5 3-2 503 599 84 375 Inventive Example 6 3-3 510 593 86 370 Inventive Example 7 4 4-1 504 600 84 358 Inventive Example 8 4-2 498 600 83 361 Inventive Example 9 4-3 467 513 91 185 Comparative Example 3 5 5-1 524 616 85 387 Inventive Example 10 5-2 535 652 82 364 Inventive Example 11 6 6-1 541 644 84 340 Inventive Example 12 6-2 545 626 87 349 Inventive Example 13 7 7 527 635 83 308 Inventive Example 14 8 8 552 681 81 311 Inventive Example 15 9 9 524 639 82 287 Inventive Example 16 10 10-1  475 528 90 198 Comparative Example 4 10-2  458 503 91 172 Comparative Example 5 11 11-1  486 540 90 187 Comparative Example 6 11-2  492 547 90 113 Comparative Example 7 12 12-1  502 558 90 175 Comparative Example 8 12-2  483 531 91 175 Comparative Example 9 13 13-1  511 568 90 198 Comparative Example 10 13-2  472 519 91 142 Comparative Example 11 14 14  535 582 92 167 Comparative Example 12 15 15  508 558 91 118 Comparative Example 13

(6) TABLE-US-00004 TABLE 4 Average Average hardness of Polygonal hardness of phases other Pearlite + Steel ferrite polygonal than polygonal martensite Type Conditions fraction (%) ferrite (Hv) ferrite (Hv) fraction (%) Remarks 1 1-1 27 167 234 7 Inventive Example 1 1-2 30 170 239 6 Inventive Example 2 1-3 7 165 210 12 Comparative Example 1 2 2-1 27 15 229 6 Inventive Example 3 2-2 25 177 237 7 Inventive Example 4 2-3 72 179 207 22 Comparative Example 2 3 3-1 27 177 235 6 Inventive Example 5 3-2 25 174 247 8 Inventive Example 6 3-3 27 174 241 6 Inventive Example 7 4 4-1 24 173 237 7 Inventive Example 8 4-2 25 177 249 8 Inventive Example 9 4-3 64 168 223 19 Comparative Example 3 5 5-1 27 170 239 6 Inventive Example 10 5-2 24 172 247 8 Inventive Example 11 6 6-1 29 168 241 7 Inventive Example 12 6-2 28 172 240 8 Inventive Example 13 7 7 18 178 253 9 Inventive Example 14 8 8 30 166 254 9 Inventive Example 15 9 9 24 173 251 8 Inventive Example 16 10 10-1  8 181 220 12 Comparative Example 4 10-2  6 188 209 11 Comparative Example 5 11 11-1  40 172 208 12 Comparative Example 6 11-2  33 164 248 11 Comparative Example 7 12 12-1  6 182 220 11 Comparative Example 8 12-2  7 188 207 11 Comparative Example 9 13 13-1  36 164 205 12 Comparative Example 10 13-2  7 189 238 13 Comparative Example 11 14 14  40 172 199 15 Comparative Example 12 15 15  2 191 268 14 Comparative Example 13

(7) Since Inventive Examples 1 to 16 satisfied all of the steel composition, the microstructure, and the process condition of the present disclosure, it was confirmed that a Charpy impact energy at −30° C. of 200 J or more, a tensile strength of 500 MPa or more, and a yield ratio of 90% or less were all satisfied. In particular, it was confirmed that Inventive Examples 1 to 16 all had a yield ratio of less than 90%.

(8) Comparative Example 1 satisfied the composition content of the present disclosure, but since cooling was performed only once without dividing into the first cooling and the second cooling and the first cooling start temperature was out of the range of the present disclosure, it was confirmed that the microstructure conditions of the present disclosure were not satisfied. Therefore, it was confirmed that the desired low-temperature toughness of the present disclosure was not secured in Comparative Example 1.

(9) Comparative Example 2 satisfied the composition content of the present disclosure, but since the finish rolling effective reduction ratio and the first cooling rate did not satisfy the range of the present disclosure, it was confirmed that the microstructure of the present disclosure was not satisfied. Therefore, it was confirmed that the desired tensile strength and low-temperature toughness of the present disclosure were not secured.

(10) Comparative Example 3 satisfied the composition content of the present disclosure, but since the reheating temperature, the rough rolling end temperature, the finish rolling end temperature, and the first cooling start and end temperature did not satisfy the range of the present disclosure, it was confirmed that the microstructure conditions of the present disclosure were not satisfied. Therefore, it was confirmed that the desired low-temperature toughness of the present disclosure was not secured in Comparative Example 3.

(11) In addition, in Comparative Examples 4 to 13, the steel composition, the microstructure, and the process conditions of the present disclosure were all not satisfied, and thus, it was confirmed that the desired physical properties of the present disclosure were not secured.

(12) Accordingly, a high-strength steel material having an excellent low yield ratio characteristic and a method of manufacturing the same according to an exemplary embodiment of the present disclosure satisfy all of a low yield ratio characteristic and a high strength property, thereby providing a pipe steel material allowing stability and pipe making ability to be secured and a method of manufacturing the same.

(13) Hereinabove, the present disclosure has been described in detail by the exemplary embodiments, but other exemplary embodiments having different forms are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited by the exemplary embodiments.