ULTRA-THICK STEEL MATERIAL HAVING EXCELLENT SURFACE PART NRL-DWT PROPERTIES AND METHOD FOR MANUFACTURING SAME

Abstract

Disclosed are a high-strength ultra-thick steel material and a method for manufacturing same. The high-strength ultra-thick steel material comprises in weight % 0.04-0.1% of C, 1.2-2.0% of Mn, 0.2-0.9% of Ni, 0.005-0.04% of Nb, 0.005-0.03% of Ti and 0.1-0.4% of Cu, 100 ppm or less of P and 40 ppm or less of S with a balance of Fe, and inevitable impurities, and comprises, in a subsurface area up to t/10 (t hereafter being referred to as the thickness of the steel material) , polygonal ferrite of 50 area % or greater (including 100 area %) and bainite of 50 area % or less (including 0 area %) as microstructures.

Claims

1. An ultra-thick high strength steel material, comprising: by weight %, 0.04 to 0.1% of C, 1.2 to 2.0% of Mn, 0.2 to 0. 9% of Ni, 0.005 to 0.04% of Nb, 0.005 to 0.03% of Ti, 0.1 to 0.4% of Cu, 100 ppm or less of P, 40 ppm or less of S, and a balance of Fe and inevitable impurities, wherein the ultra-thick high strength steel material comprises polygonal ferrite of 50 area % or higher, including 100 area %, and bainite of 50 area % or less, including 0 area %, as a microstructure in a region up to a t/10 position in a subsurface area, where t is a thickness of the steel material.

2. The ultra-thick high strength steel material of claim 1, further comprising: bainite of 50 area % or less, including 0 area %, in a region from a t/10 position to a t/5 position in a subsurface area.

3. The ultra-thick high strength steel material of claim 1, further comprising: a complex structure of acicular ferrite and bainite of 90 area % or higher, including 100 area %, and polygonal ferrite of 10 area % or less, including 0 area %, as a microstructure in a region from a t/5 position to a t/2 position in a subsurface area.

4. The ultra-thick high strength steel material of claim 1, wherein a nil-ductility transition temperature, an NDT temperature, based on a naval research laboratory drop-weight test, a NRL-DWT, prescribed in ASTM 208-06, is 60 C. or less in a sample obtained from a surface.

5. The ultra-thick high strength steel material of claim 1, wherein an impact transition temperature is 40 C. or less in a sample obtained from a t/4 position in a subsurface area.

6. The ultra-thick high strength steel material of claim 1, wherein a sheet thickness is 50 to 100 mm, and yield strength is 390 MPa or higher.

7. A method of manufacturing an ultra-thick high strength steel material, comprising: reheating a slab comprising, by weight %, 0.04 to 0.1% of C, 1.2 to 2.0% of Mn, 0.2 to 0.9% of Ni, 0.005 to 0.04% of Nb, 0.005 to 0.03% of Ti, 0.1 to 0.4% of Cu, 100 ppm or less of P, 40 ppm or less of S, and a balance of Fe and inevitable impurities; obtaining a hot-rolled steel sheet by rough-rolling the reheated slab and finish-rolling the rough-rolled slab under conditions of a temperature less than Ar3 C. on a slab surface during a final pass rolling and a temperature of Ar3 C. or higher and Ar3+50 C. or lower at a t/4 position from the slab surface; and water-cooling the hot-rolled steel sheet after a temperature of a surface of the hot-rolled steel sheet reaches Ar3-50 C.

8. The method of claim 7, wherein a temperature of the reheating the slab is 1000 to 1150 C.

9. The method of claim 8, wherein a temperature of the rough-rolling is 900 to 1150 C.

10. The method of claim 7, wherein an accumulated reduction ratio during the rough-rolling is 40% or higher.

11. The method of claim 7, wherein a cooling speed during the water-cooling is 3 C./sec or higher.

12. The method of claim 7, wherein a cooling terminating temperature of the water-cooling is 600 C. or less.

Description

MODE FOR INVENTION

[0051] In the description below, an example embodiment of the present disclosure will be described in greater detail. It should be noted that the exemplary embodiments are provided to describe the present disclosure in greater detail, and to not limit the scope of rights of the present disclosure. The scope of rights of the present disclosure maybe determined on the basis of the subject matters recited in the claims and the matters reasonably inferred from the subject matters.

Embodiment

[0052] A steel slab having a thickness of 400 mm and having a composition as in Table 1 was reheated at 1015 C., and then was rough-rolled at 1015 C., thereby manufacturing a bar. An accumulated reduction ratio during the rough-rolling was 50% in all samples, and a thickness of the rough-rolled bar was 200 mm in all samples. After the rough-rolling, the rough-rolled bar was finish-rolled under conditions as in Table 2, thereby obtaining a hot-rolled steel sheet. The hot-rolled steel sheet was water-cooled to 300 to 500 C. at a cooling speed indicated in Table 2, thereby manufacturing an ultra-thick steel material.

[0053] Thereafter, a microstructure of the manufactured ultra-thick steel material was analyzed, tensile properties was examined, and the results were listed in Table 3.

TABLE-US-00001 TABLE 1 Steel Alloy Composition (weight %) Type C Mn Ni Cu Ti Nb P (ppm) S (ppm) Inventive 0.089 1.36 0.62 0.29 0.018 0.019 81 9 Steel 1 Inventive 0.066 1.65 0.27 0.15 0.021 0.021 46 28 Steel 2 Inventive 0.043 1.93 0.52 0.21 0.013 0.018 49 12 Steel 3 Inventive 0.075 1.53 0.51 0.22 0.019 0.023 78 13 Steel 4 Inventive 0.066 1.82 0.34 0.17 0.017 0.028 59 11 Steel 5 Compar- 0.13 2.01 0.42 0.31 0.023 0.019 65 19 ative Steel 1 Compar- 0.065 2.12 0.55 0.19 0.012 0.012 78 17 ative Steel 2 Compar- 0.031 1.15 0.45 0.18 0.016 0.018 51 23 ative Steel 3 Compar- 0.082 1.93 1.17 0.38 0.021 0.015 48 16 ative Steel 4 Compar- 0.079 1.68 0.32 0.22 0.044 0.048 57 13 ative Steel 5

TABLE-US-00002 TABLE 2 Surface Temperature at Surface Hot-rolled Steel Temperature t/4 Position Temperature When Steel Sheet Thickness During Final Pass During Final Pass Cooling Starts Cooling Speed Type (mm) Rolling ( C.) Rolling ( C.) ( C.) ( C./sec) Note Inventive Steel 1 95 Ar3 31 Ar3 + 15 Ar3 81 3.8 Embodiment 1 95 Ar3 68 Ar3 23 Ar3 117 3.9 Comparative Example 1 Inventive Steel 2 80 Ar3 17 Ar3 + 23 Ar3 79 4.8 Embodiment 2 80 Ar3 + 48 Ar3 + 78 Ar3 3 4.9 Comparative Example 2 Inventive 95 Ar3 27 Ar3 + 7 Ar3 81 3.9 Embodiment 3 Steel 3 95 Ar3 + 69 Ar3 + 95 Ar3 + 3 3.8 Comparative Example 3 Inventive Steel 4 100 Ar3 8 Ar3 + 36 Ar3 62 3.5 Embodiment 4 100 Ar3 71 Ar3 35 Ar3 113 3.6 Comparative Example 4 Inventive 80 Ar3 18 Ar3 + 12 Ar3 71 5.0 Embodiment 5 Steel 5 Comparative 80 Ar3 21 Ar3 + 14 Ar3 86 4.7 Comparative Steel 1 Example 5 Comparative 85 Ar3 9 Ar3 + 32 Ar3 62 4.5 Comparative Steel 2 Example 6 Comparative 90 Ar3 10 Ar3 + 27 Ar3 61 4.3 Comparative Steel 3 Example 7 Comparative 90 Ar3 12 Ar3 + 19 Ar3 64 4.2 Comparative Steel 4 Example 8 Comparative 95 Ar3 5 Ar3 + 44 Ar3 56 3.9 Comparative Steel 5 Example 9

TABLE-US-00003 TABLE 3 Microstructure Tensile Properties AF and B Surface Up to t/10 in B Fraction Fractions Portion Impact Subsurface from from Yield NDT Transition Steel Area t/10 to t/5 t/5 to t/2 Strength Temperature Temperature Type (area %) (area %) (area %) (MPa) ( C.) ( C.) Note Inventive 78PF + 18 91 403 75 57 Embodiment 1 Steel 1 32B 89PF + 29 56 375 55 36 Comparative 11B Example 1 Inventive Steel 2 68PF + 29 95 456 70 63 Embodiment 2 32B 100B 65 97 544 50 21 Comparative Example 2 Inventive Steel 3 72PF + 41 96 468 65 61 Embodiment 3 28B 100B 59 98 559 55 18 Comparative Example 3 Inventive Steel 4 67PF + 38 97 448 70 59 Embodiment 4 33B 91PF + 33 77 381 50 31 Comparative 9B Example 4 Inventive 72PF + 29 96 487 75 73 Embodiment 5 Steel 5 28B Comparative 68PF + 72 98 556 45 72 Comparative Steel 1 32B Example 5 Comparative 72PF + 63 97 521 50 49 Comparative Steel 2 38B Example 6 Comparative 81PF + 15 52 312 70 64 Comparative Steel 3 19P Example 7 Comparative 71PF + 52 97 549 55 59 Comparative Steel 4 29B Example 8 Comparative 54PF + 47 96 519 50 29 Comparative Steel 5 46B Example 9 In the microstructure, PF refers to polygonal ferrite, AF refers to acicular ferrite, B refers to bainite, and P refers to pearlite. In all steel types, residual structures other than B were PF and AF in a region from t/10 to t/5, and a residual structure other than AF and B in a region from t/5 to t/2 was PF.

[0054] As indicated in Table 3, as for embodiments 1 to 5 which satisfied overall conditions suggested in the present disclosure, yield strength was 390 MPa or higher, a surface portion impact transition temperature was 40 C. or less, and a nil-ductility transition temperature (NDTT) value obtained in the NRL-DWT test based on a ASTM E208 standard was 60 C. or less.

[0055] As for comparative examples 1 to 4, as the temperature at the t/4 position during the final pass rolling in the finish-rolling was less than Ar3 C., a large amount of air-cooled ferrite was formed in a surface portion and up to the t portion before and in the middle of the rolling process. Accordingly, yield strength was 390 MPa or less. Also, a two-phase rolling was performed due to a low rolling temperature, and strength of a surface portion increased because of a large amount of ferrite in the surface portion such that a surface portion impact transition temperature exceeded 40 C., and an NDTT exceeded 60 C.

[0056] Also, in comparative examples 2 and 3, as the temperature at the t/4 position during the final pass rolling in the finish-rolling exceeds Ar3+50 C., air-cooled ferrite was not formed before water-cooling such that a microstructure in a region up to the t/10 in a subsurface area was formed of a single phase structure of bainite. Also, as a microstructure in a region from a t/10 position to a t/5 position in a subsurface area had bainite of 50% or higher, a surface portion impact transition temperature exceeded 40 C., and an NDT temperature exceeded 60 C.

[0057] As for comparative example 5, a value of a content of C was higher than an upper limit content of C suggested in the present disclosure. Accordingly, a large amount of bainite single phase structure was formed in a region from a t/10 position to a t/5 position in a subsurface area due to excessive hardenability, and accordingly, an NDTT exceeded 60 C.

[0058] As for comparative example 6, a value of content of Mn was higher than an upper limit content of Mn suggested in the present disclosure. Accordingly, a large amount of bainite single phase structure was formed in a region from a t/10 position to a t/5 position in a subsurface area due to excessive hardenability, and accordingly, an NDTT exceeded 60 C.

[0059] As for comparative example 7, values of contents of C and Mn were lower than lower limit contents of C and Mn suggested in the present disclosure. Accordingly, hardenability was insufficient such that a large amount of polygonal ferrite and pearlite structures were generated, and accordingly, yield strength was 300 MPa or less.

[0060] As for comparative example 8, as a value of a content of Ni was higher than an upper limit content of Ni suggested in the present disclosure. Accordingly, a large amount of bainite single phase structure was formed in a region from a t/10 position to a t/5 position in a subsurface area due to excessive hardenability, and accordingly, an NDTT exceeded 60 C.

[0061] As for comparative example 9, value of contents of Ti and Nb were higher than upper limit contents of Ti and Nb suggested in the present disclosure. Accordingly, strength increased due to excessive hardenability, and a central portion impact transition temperature exceeded 40 C. due to degradation of toughness caused by strengthened precipitation, and an NDTT exceeded 60 C.

[0062] While exemplary embodiments have been shown and described above, the scope of the present disclosure is not limited thereto, and it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.