HIGHLY THICK STEEL MATERIAL HAVING EXCELLENT LOW-TEMPERATURE IMPACT TOUGHNESS AND MANUFACTURING METHOD THEREFOR

Abstract

The present invention relates to a highly thick steel material and a manufacturing method therefor and, more specifically, to a highly thick steel material that exhibits excellent low-temperature impact toughness after long-term PWHT although the steel sheet is thick, and a manufacturing method therefor.

Claims

1. A steel material comprising: in weight %, carbon (C): 0.10 to 0.25%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.0 to 2.0%, aluminum (Al): 0.005 to 0.1%, phosphorus (P): 0.010% or less, sulfur (S): 0.0015% or less, niobium (Nb): 0.001 to 0.03%, vanadium (V): 0.001 to 0.03%, titanium (Ti): 0.001 to 0.03%, chromium (Cr): 0.01 to 0.20%, molybdenum (Mo): 0.01 to 0.15%, copper (Cu): 0.01 to 0.50%, nickel (Ni): 0.05 to 0.50%, calcium (Ca): 0.0005 to 0.0040%, with a balance Fe and unavoidable impurities, wherein a microstructure in a center in a range of t/4 to t/2 (where t indicates ae thickness of a steel plate) consists of 35 to 40% of ferrite and a remainder of bainite composite structure in area %, a packet size of the bainite is 10 m or less, and a porosity of the center is 0.1 mm.sup.3/g or less, a depth of a surface crack is 0.5 mm or less, and a center section hardness is 200 HB or less.

2. The steel material of claim 1, wherein a prior austenite average grain size of the steel material is 20 m or less.

3. The steel material of claim 1, wherein a thickness of the steel material is 133 to 250 mm.

4. The steel material of claim 1, wherein the steel material has a tensile strength of 450 to 650 MPa after PWHT and a center low-temperature impact toughness of 80 J or more at 60 C.

5. A method of manufacturing a steel material, comprising: primarily heating a steel slab having a thickness of 650 to 750 mm at a temperature ranging from 1100 to 1300 C., and then performing primary forging at a cumulative reduction of 3 to 15% and a strain rate of 1 to 4/s, and obtaining a primary intermediate material, the steel slab containing, in weight %, carbon (C): 0.10 to 0.25%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.0 to 2.0%, aluminum (Al): 0.005 to 0.1%, phosphorus (P): 0.010% or less, sulfur (S): 0.0015% or less, niobium (Nb): 0.001 to 0.03%, vanadium (V): 0.001 to 0.03%, titanium (Ti): 0.001 to 0.03%, chromium (Cr): 0.01 to 0.20%, molybdenum (Mo): 0.01 to 0.15%, copper (Cu): 0.01 to 0.50%, nickel (Ni): 0.05 to 0.50%, calcium (Ca): 0.0005 to 0.0040%, with a balance Fe and unavoidable impurities; after secondary heating of the primary intermediate material at a temperature ranging from 1000 to 1500 C., performing secondary forging processing at a cumulative reduction of 3 to 30% and a strain rate of 1 to 4/s, and obtaining a secondary intermediate material; a tertiary heating operation of heating the secondary intermediate material to a temperature range of 1000 to 1200 C.; obtaining a hot-rolled material by hot-rolling the tertiary heated secondary intermediate material at a finish hot rolling temperature of 900 to 1100 C.; cooling the hot-rolled material; a quenching operation of heating the cooled hot-rolled material at a temperature ranging from 820 to 900 C., maintaining for 10 to 40 minutes, and then cooling at a cooling rate of 5 C./s or more; and a tempering operation of holding the quenched steel at 600 to 680 C. for 10 to 40 minutes.

6. The method of manufacturing the steel material of claim 5, wherein in the cooling, the hot-rolled material is cooled at a cooling rate of 3 C./s or more to a temperature range of Bs+20 to Ar1+20 C.

7. The method of manufacturing the steel material of claim 5, further comprising an operation of cooling the hot-rolled material to a cooling end temperature and then air-cooling to room temperature.

8. The method of manufacturing the steel material of claim 5, wherein a thickness of the primary intermediate material is 450 to 550 mm.

9. The method of manufacturing the steel material of claim 5, wherein a thickness of the secondary intermediate material is 300 to 340 mm.

10. The method of manufacturing the steel material of claim 5, wherein a thickness of the hot-rolled material is 133 to 250 mm.

Description

MODE FOR INVENTION

[0121] A cast steel having a thickness of 700 mm and having the alloy components illustrated in Table 1 was manufactured. Primary forging, secondary forging, hot rolling, cooling and QT heat treatment were performed according to the process conditions in Table 2. At this time, the primary heating temperature of 1200 C., the secondary heating temperature of 1100 C., and the tertiary heating temperature of 1050 C. were commonly applied, and the quenching and tempering time was commonly applied for 30 minutes. For the thickness of the primary intermediate material, the condition of 550 mm was applied, and for the thickness of the secondary intermediate material, the condition of 400 mm was applied. In addition, the cooling end temperature after hot rolling and the cooling rate during quenching, which are not disclosed in Table 2, were applied under conditions satisfying the range of the present disclosure.

TABLE-US-00001 TABLE 1 Alloy Component (wt %) Steel Grade C Si Mn Al P* S* Nb V Ti Cr Mo Cu Ni Ca* A 0.12 0.27 1.18 0.03 80 10 0.013 0.015 0.011 0.02 0.10 0.20 0.25 25 B 0.15 0.3 1.35 0.03 80 10 0.015 0.015 0.013 0.05 0.10 0.08 0.20 25 C 0.13 0.35 1.24 0.03 85 12 0.013 0.017 0.012 0.014 0.08 0.02 0.23 22 D 0.17 0.31 1.29 0.02 81 10 0.015 0.02 0.012 0.019 0.06 0.04 0.18 21 E 0.14 0.28 1.35 0.03 83 11 0.016 0.018 0.015 0.15 0.11 0.15 0.31 20 F 0.31 0.3 1.41 0.02 82 13 0.018 0.015 0.001 0.13 0.08 0.12 0.28 22 G 0.18 0.35 0.7 0.02 85 12 0.018 0.013 0.010 0.11 0.11 0.20 0.19 23 *Unit is ppm

TABLE-US-00002 TABLE 2 Quenching and Tempering Primary Secondary Hot Rolling Heating Forging Forging Finish Cooling temperature Tempering Cumulative Cumulative hot rolling Cooling when heating Specimen Steel Reduction Strain Reduction Strain temperature Thickness Rate quenching temperature Number Grade (%) Rate (%) Rate ( C.) (mm) ( C./s) ( C.) ( C.) 1 A 10.2 2.4 17.5 2.5 905 163 3.8 890 621 2 B 12 1.8 18.2 2.1 923 157 3.3 880 635 3 C 13 1.9 16.9 3.1 951 203 3.6 881 641 4 D 10.5 2.5 20.1 3.5 937 187 4.5 891 640 5 E 13.7 3.1 27.3 2.9 940 167 5.3 899 629 6 A 24.4 2.1 21.2 2.8 938 135 4.7 890 640 7 B 12.5 6.7 20.5 3.1 943 171 4.3 890 662 8 C 8.9 1.8 24.5 0.7 945 173 5.3 851 619 9 D 10.5 1.9 23.5 3.1 1118 181 5.1 860 627 10 E 9.4 1.7 26.1 1.8 962 166 3.5 765 631 11 E 8.6 2.6 26.9 2.9 944 181 4.1 861 532 12 F 10.5 2.5 25.3 2.5 956 171 3.9 843 667 13 G 8.4 2.7 26.4 3.0 958 162 4.3 867 643

[0122] The microstructure and mechanical properties of the prepared steel were measured. The fraction of the microstructure was measured through a scanning electron microscope, and after Lepera etching the tissue specimen, an optical image was captured, and then, the tissue fraction was measured using an automatic image analyzer. At this time, the microstructure and porosity of the center in the range of t/4 to t/2 (where t means the thickness of the steel plate) were measured. The uniform elongation of the surface layer of the slab represents the value of the elongation measured at the maximum tensile stress portion after performing a tensile test on a tensile specimen prepared with the surface of the slab in the primary forging temperature range. In the size of the bainite packet, the grain size was determined centering on the high-tilt angle grain boundary of 15 by EBSD, and the cross-sectional surface hardness was measured using a Brinell hardness tester based on the cross-sectional hardness at the center of the specimen.

[0123] In addition, in Table 4 below, the mechanical properties are illustrated by measuring the tensile strength after PWHT and the low-temperature impact toughness at 60 C. After visually observing the surface of the steel material, grinding was performed at the point where the surface crack was formed, and the grinding depth until the crack disappeared was measured as the depth of the surface crack.

TABLE-US-00003 TABLE 3 Prior- sustenite Slab Steel after QT heat treatment average surface Bainite grain uniform packet Fresh Section Specimen Steel size elongation Ferrite Bainite size martensite Porosity Hardness Number Grade (m) (%) (area %) (area %) (m) (area %) (mm.sup.3/g) (HB) Division 1 A 18.2 16.2 35.3 64.7 8.3 0 0.07 192 Inventive Example 1 2 B 16.9 15.4 35.8 64.2 9.4 0 0.06 198 Inventive Example 2 3 C 17.5 16.3 37.2 62.8 8.5 0 0.05 194 Inventive Example 3 4 D 18.3 15.8 38.3 61.7 7.9 0 0.03 197 Inventive Example 4 5 E 17.6 15.9 36.2 63.8 6.9 0 0.04 198 Inventive Example 5 6 A 18.3 16.4 35.9 64.1 8.3 0 0.06 193 Comparative Example 1 7 B 19.1 7.3 37.6 62.4 9.0 0 0.08 192 Comparative Example 2 8 C 15.7 16.9 38.1 61.9 9.2 0 0.27 188 Comparative Example 3 9 D 30.6 15.9 38.2 61.8 14.7 0 0.04 180 Comparative Example 4 10 E 18.2 14.7 39.1 13.9 7.9 47 0.05 300 Comparative Example 5 11 E 18.9 15.0 39.2 60.8 9.1 0 0.04 275 Comparative Example 6 12 F 17.3 15.8 0 100 8.3 0 0.04 189 Comparative Example 7 13 G 18.6 16.7 91.5 8.5 8.5 0 0.03 190 Comparative Example 8 F: ferrite, B: bainite, FM: fresh martensite

TABLE-US-00004 TABLE 4 Steel after PWHT LOW temperature Surface Tensile impact crack Specimen Steel Strength toughness depth Number Grade (MPa) (60 C., J) (mm) Division 1 A 493 189 0 Inventive Example 1 2 B 486 215 0 Inventive Example 2 3 C 504 210 0 Inventive Example 3 4 D 515 215 0 Inventive Example 4 5 E 490 231 0 Inventive Example 5 6 A 530 207 11.4 Comparative Example 1 7 B 507 215 8.7 Comparative Example 2 8 C 533 17 0 Comparative Example 3 9 D 547 21 0 Comparative Example 4 10 E 645 33 0 Comparative Example 5 11 E 630 18 0 Comparative Example 6 12 F 684 13 10.5 Comparative Example 7 13 G 427 385 0 Comparative Example 8

[0124] As illustrated in Table 3, it can be confirmed that the examples of the invention satisfying the alloy composition and manufacturing method proposed in the present disclosure satisfy all mechanical properties aimed at in the present disclosure.

[0125] On the other hand, Comparative Examples 1 and 2 are cases in which the cumulative reduction and strain rate in the primary forging exceed the range of the present disclosure, and since the uniform elongation of the slab surface layer in the forging temperature range did not satisfy the range of the present disclosure, cracks occurred on the surface of the steel.

[0126] In Comparative Example 3, during the secondary forging, the strain rate was less than the scope of the present disclosure, and the low-temperature impact toughness did not meet the range proposed in the present disclosure due to excessive voids in the center of the steel.

[0127] In Comparative Example 4, the finish hot rolling temperature exceeded the range of the present disclosure, the average prior austenite grain size was excessive, and the bainite packet size became coarse after quenching and tempering, resulting in poor low-temperature impact toughness.

[0128] In Comparative Examples 5 and 6, the heating temperature during quenching and tempering, respectively, fell short of the range of the present disclosure. In the case of Comparative Example 5, fresh martensite was formed and the hardness was excessive. In the case of Comparative Example 6, the hardness of bainite was excessive, and the hardness of the center section was excessively increased.

[0129] In the case of Comparative Example 7, the content of C exceeded the range of the present disclosure, and bainite was excessively formed, and as a result, the tensile strength was excessively increased, the low-temperature impact toughness was lowered, and cracks were also generated.

[0130] In the case of Comparative Example 8, Mn did not satisfy the range of the present disclosure, and ferrite was excessively formed, and thus tensile strength was not sufficiently secured.

[0131] Although the present disclosure has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.