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 steel material, comprising: a composition consisting of: by weight %, 0.04 to 0.09% 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 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, wherein the steel material comprises acicular ferrite and bainite of 90 area % or higher, including 100 area %, and polygonal ferrite of 10 area % or less, including 0 area %, in a region from a t/5 position to a t/2 position in a subsurface area, where t is a thickness of the steel material, wherein the steel material is in the form of a steel sheet having a thickness of 50 to 100 mm.

2. The ultra-thick 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 steel material of claim 1, wherein a nil-ductility transition temperature, an NDT temperature, according to 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.

4. The ultra-thick 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.

5. The ultra-thick steel material of claim 1, wherein the steel sheet has a yield strength is 390 MPa or higher.

6. An ultra-thick steel material, comprising: by weight %, 0.04 to 0.09% 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 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, wherein the steel material comprises acicular ferrite and bainite of 90 area % or higher, including 100 area %, and polygonal ferrite of 10 area % or less, including 0 area %, in a region from a t/5 position to a t/2 position in a subsurface area, where t is a thickness of the steel material, and 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, wherein the steel material is in the form of a steel sheet having a thickness of 50 to 100 mm, and wherein a nil-ductility transition temperature, an NDT temperature, according to 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.

Description

MODE FOR INVENTION

(1) 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 may be determined on the basis of the subject matters recited in the claims and the matters reasonably inferred from the subject matters.

Embodiment

(2) 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.

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

(4) 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

(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

(6) 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.

(7) 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.

(8) 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 ¼t 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.

(9) 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.

(10) 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.

(11) 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.

(12) 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.

(13) 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.

(14) 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.

(15) 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.