KR102498134B1 - Ultra-thick steel plate having excellent low-temperature impact toughness and method for manufacturing thereof - Google Patents
Ultra-thick steel plate having excellent low-temperature impact toughness and method for manufacturing thereof Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 91
- 239000010959 steel Substances 0.000 title claims abstract description 91
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 18
- 238000000034 method Methods 0.000 title description 19
- 238000005096 rolling process Methods 0.000 claims description 65
- 238000010438 heat treatment Methods 0.000 claims description 24
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims description 23
- 239000011651 chromium Substances 0.000 claims description 21
- 239000011572 manganese Substances 0.000 claims description 21
- 239000010936 titanium Substances 0.000 claims description 20
- 239000010955 niobium Substances 0.000 claims description 19
- 238000001816 cooling Methods 0.000 claims description 17
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 16
- 229910000859 α-Fe Inorganic materials 0.000 claims description 10
- 229910052799 carbon Inorganic materials 0.000 claims description 9
- 229910052804 chromium Inorganic materials 0.000 claims description 9
- 229910052748 manganese Inorganic materials 0.000 claims description 9
- 229910052759 nickel Inorganic materials 0.000 claims description 9
- 229910001562 pearlite Inorganic materials 0.000 claims description 9
- 229910052782 aluminium Inorganic materials 0.000 claims description 8
- 238000005098 hot rolling Methods 0.000 claims description 8
- 229910052758 niobium Inorganic materials 0.000 claims description 8
- 229910052757 nitrogen Inorganic materials 0.000 claims description 8
- 238000001953 recrystallisation Methods 0.000 claims description 8
- 229910052719 titanium Inorganic materials 0.000 claims description 8
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 7
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 7
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims description 6
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims description 6
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 claims description 6
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 6
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 claims description 6
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 claims description 6
- 239000012535 impurity Substances 0.000 claims description 6
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 claims description 6
- 229910052698 phosphorus Inorganic materials 0.000 claims description 6
- 239000011574 phosphorus Substances 0.000 claims description 6
- 229910052710 silicon Inorganic materials 0.000 claims description 6
- 239000010703 silicon Substances 0.000 claims description 6
- 229910052717 sulfur Inorganic materials 0.000 claims description 6
- 239000011593 sulfur Substances 0.000 claims description 6
- 239000000463 material Substances 0.000 abstract description 24
- 238000010276 construction Methods 0.000 abstract description 4
- 229910000746 Structural steel Inorganic materials 0.000 abstract description 3
- 230000000694 effects Effects 0.000 description 18
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- 239000000203 mixture Substances 0.000 description 12
- 230000000052 comparative effect Effects 0.000 description 9
- 239000002244 precipitate Substances 0.000 description 5
- 229910001566 austenite Inorganic materials 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 4
- 239000006104 solid solution Substances 0.000 description 4
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 3
- 230000000704 physical effect Effects 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- 230000002411 adverse Effects 0.000 description 2
- 239000002131 composite material Substances 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 238000009434 installation Methods 0.000 description 2
- 230000002441 reversible effect Effects 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- AFCARXCZXQIEQB-UHFFFAOYSA-N N-[3-oxo-3-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)propyl]-2-[[3-(trifluoromethoxy)phenyl]methylamino]pyrimidine-5-carboxamide Chemical compound O=C(CCNC(=O)C=1C=NC(=NC=1)NCC1=CC(=CC=C1)OC(F)(F)F)N1CC2=C(CC1)NN=N2 AFCARXCZXQIEQB-UHFFFAOYSA-N 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 239000003795 chemical substances by application Substances 0.000 description 1
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- 238000009792 diffusion process Methods 0.000 description 1
- 238000009826 distribution Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000009863 impact test Methods 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 230000002035 prolonged effect Effects 0.000 description 1
- 238000010791 quenching Methods 0.000 description 1
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- 239000002994 raw material Substances 0.000 description 1
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- 229920006395 saturated elastomer Polymers 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
- 230000000930 thermomechanical effect Effects 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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Abstract
본 발명은 해양, 교량, 건설 등의 소재로서 적용할 수 있는 구조용 강재에 관한 것으로서, 보다 상세하게는 저온 충격인성이 우수한 극후물 강판 및 이의 제조방법에 관한 것이다.The present invention relates to structural steel that can be applied as a material for marine, bridge, construction, etc., and more particularly, to an ultra-thick steel sheet having excellent low-temperature impact toughness and a manufacturing method thereof.
Description
본 발명은 해양, 교량, 건설 등의 소재로서 적용할 수 있는 구조용 강재에 관한 것으로서, 보다 상세하게는 저온 충격인성이 우수한 극후물 강판 및 이의 제조방법에 관한 것이다.The present invention relates to structural steel that can be applied as a material for marine, bridge, construction, etc., and more particularly, to an ultra-thick steel sheet having excellent low-temperature impact toughness and a manufacturing method thereof.
일정 두께 이상의 극후물재는 후판공정을 통해 제조할 수 있으며, 이때의 압연 방법은 일반 압연, 노멀라이징 압연, TMCP(Thermo mechanical controlled rolling) 등으로 구분할 수 있다. 이외에 압연 후 열처리 공정을 거칠 수 있는데, 이때의 열처리 공정은 노멀라이징(normalizing), 켄칭(quenching), 켄칭-템퍼링(tempering) 열처리 공정 등이 있다.Ultra-thick materials with a certain thickness or more can be manufactured through the thick plate process, and the rolling method at this time can be divided into general rolling, normalizing rolling, and TMCP (Thermo mechanical controlled rolling). In addition, a heat treatment process may be performed after rolling. In this case, the heat treatment process includes normalizing, quenching, quenching-tempering heat treatment, and the like.
전술한 압연 공정 중 일반 압연은 압연온도의 제어없이 압연하는 방법으로서, 주로 충격인성을 요구하지 않는 일반강에 적용할 수 있다. Among the above-mentioned rolling processes, general rolling is a method of rolling without controlling the rolling temperature, and can be mainly applied to general steel that does not require impact toughness.
이와 달리, TMCP는 온도제어를 통해 재결정역 압연, 미재결졍역 압연을 행하며, 필요에 따라 냉각을 통해 강도 및 충격인성의 확보가 가능하다. 하지만, 이러한 TMCP 공정을 통해 극후물재를 제조하는 경우 압연온도를 맞추기 위한 대기 시간이 오래 소요되어 심각한 생산성 저하를 초래하는 문제가 있다.Unlike this, TMCP performs recrystallization reverse rolling and non-recrystallization reverse rolling through temperature control, and it is possible to secure strength and impact toughness through cooling as necessary. However, when an ultra-thick material is manufactured through such a TMCP process, a long waiting time is required to adjust the rolling temperature, resulting in a serious decrease in productivity.
노멀라이징 압연은 비교적 높은 온도에서 압연이 종료되므로, 공냉 중 결정립 성장에 의해 강도와 인성의 하락을 가져올 수 있다.Since normalizing rolling is completed at a relatively high temperature, strength and toughness may decrease due to grain growth during air cooling.
이에, TMCP 공정, 노멀라이징 압연 공정 또는 압연 후 열처리 공정을 통해서 극후물재를 제조하는 경우, 강도 확보를 위해 0.12% 이상의 C를 함유하는 고탄소 성분계의 적용이 요구되나, 인성의 저하가 심하여 상온, 0℃에서 충격인성의 보증이 가능한 수준이며, 열처리에 의한 비용이 증가하는 문제가 있다. Accordingly, when manufacturing ultra-thick materials through the TMCP process, normalizing rolling process, or post-rolling heat treatment process, the application of a high-carbon component system containing 0.12% or more of C is required to secure strength, but the toughness is severely reduced, so at room temperature, 0 It is a level at which impact toughness can be guaranteed at ° C, and there is a problem in that the cost of heat treatment increases.
한편, 극후물 강재는 선박, 해양구조물의 각종 프레임, 교량, 건설 등의 인프라산업용, 풍력하부구조용 등 다방면의 구조용 산업에 적용 가능하다.On the other hand, ultra-thick steel materials can be applied to various structural industries such as various frames of ships and offshore structures, bridges, infrastructure industries such as construction, and wind power substructures.
최근, 대부분의 인프라 산업, 에너지용 산업 등의 분야에서는 설치 비용의 최소화, 설치 환경의 열악화 등에 의해 구조물이 대형화되는 추세에 있으며, 이러한 구조물 대형화 변화에 맞춰 여러 산업 분야에 적용되는 구조용 강재 중 두께 100mm 이상의 극후물재의 요구가 증가할 것으로 예측된다.Recently, in most fields such as the infrastructure industry and the energy industry, structures are becoming larger due to the minimization of installation costs and the deterioration of the installation environment. It is expected that the demand for ultra-thick materials will increase.
그런데, 극후물 강재의 금속학적 단점은 압연량의 저하, 냉각 공정의 한계로 강도 구현 및 인성의 확보가 어렵다는 점이다.However, the metallurgical disadvantage of ultra-thick steel is that it is difficult to realize strength and secure toughness due to a decrease in rolling amount and limitations in the cooling process.
이러한 극후물 강재의 제조시 압연 및 냉각 공정의 한계로 인하여, 강재의 강도 구현을 위해 합금성분을 과도하게 첨가하는 경향이 있으며, 이 때문에 원가 상승의 문제뿐만 아니라 강재의 인성이 급격히 열위하게 되는 문제가 있다.Due to the limitations of the rolling and cooling processes when manufacturing such ultra-thick steel materials, there is a tendency to excessively add alloy components to realize the strength of the steel materials, which causes a problem of cost increase as well as a rapidly inferior toughness of the steel materials. there is
또한, 극후물 강재의 인성 확보를 위하여 인성에 악영향을 미치는 합금성분들을 제거하는 경우에는, 강도 하락의 원인이 된다.In addition, in the case of removing alloy components that adversely affect toughness in order to secure the toughness of ultra-thick steel, it causes a decrease in strength.
따라서, 극후물 강재의 강도와 인성을 양립할 수 있는 기술의 개발이 요구되는 실정이다.Therefore, the development of a technology that can achieve both the strength and toughness of ultra-thick steel is required.
본 발명의 일 측면은, 기존 극후물 강재의 금속학적 단점을 극복하여, 강도는 물론이고, 저온 충격인성이 우수한 극후물 강판 및 그 제조방법을 제공하고자 하는 것이다.One aspect of the present invention is to overcome the metallurgical disadvantages of existing ultra-thick steels to provide an ultra-thick steel sheet excellent in strength and low-temperature impact toughness and a manufacturing method thereof.
본 발명의 과제는 상술한 내용에 한정하지 않는다. 본 발명의 과제는 본 명세서의 내용 전반으로부터 이해될 수 있을 것이며, 본 발명이 속하는 기술분야에서 통상의 지식을 가지는 자라면 본 발명의 부가적인 과제를 이해하는데 아무런 어려움이 없을 것이다.The object of the present invention is not limited to the above. The subject of the present invention will be understood from the entire contents of this specification, and those skilled in the art will have no difficulty in understanding the additional subject of the present invention.
본 발명의 일 측면은, 중량%로, 탄소(C): 0.06~0.1%, 실리콘(Si): 0.3~0.5%, 망간(Mn): 1.35~1.65%, 알루미늄(Sol.Al): 0.015~0.04%, 니오븀(Nb): 0.015~0.04%, 티타늄(Ti): 0.005~0.02%, 크롬(Cr): 0.15~0.4%, 니켈(Ni): 0.3~0.5%, 질소(N): 0.002~0.008%, 인(P): 0.01% 이하(0%는 제외), 황(S): 0.003% 이하(0%는 제외), 잔부 Fe 및 기타 불가피한 불순물을 포함하고, 하기 관계식 1을 만족하며,One aspect of the present invention, in weight%, carbon (C): 0.06 ~ 0.1%, silicon (Si): 0.3 ~ 0.5%, manganese (Mn): 1.35 ~ 1.65%, aluminum (Sol.Al): 0.015 ~ 0.04%, Niobium (Nb): 0.015~0.04%, Titanium (Ti): 0.005~0.02%, Chromium (Cr): 0.15~0.4%, Nickel (Ni): 0.3~0.5%, Nitrogen (N): 0.002~ 0.008%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.003% or less (excluding 0%), the balance including Fe and other unavoidable impurities, and satisfying the following relational expression 1,
미세조직으로 면적분율 80~90%의 페라이트 및 잔부 펄라이트를 포함하는 저온 충격인성이 우수한 극후물 강판을 제공한다.Provided is an ultra-thick steel sheet having excellent low-temperature impact toughness including ferrite and remainder pearlite having an area fraction of 80 to 90% in microstructure.
[관계식 1][Relationship 1]
Mn + 5(Ni+Cr) ≥ 3.6Mn+5(Ni+Cr) ≥ 3.6
(여기서, 각 원소는 중량 함량을 의미한다.)(Here, each element means a weight content.)
본 발명의 다른 일 측면은, 상술한 합금조성과 관계식 1을 만족하는 강 슬라브를 준비하는 단계; 상기 강 슬라브를 1020~1150℃에서 가열하는 단계; 상기 가열된 강 슬라브를 1000℃ 이상에서 조압연하는 단계; 상기 조압연 후 Tnr(No-Recrystallization Temperature) 직상 또는 Tnr~A3 온도 범위에서 마무리 열간압연하는 단계; 및 상기 마무리 열간압연 후 공냉하는 단계를 포함하는 저온 충격인성이 우수한 극후물 강판의 제조방법을 제공한다.Another aspect of the present invention, preparing a steel slab that satisfies the above-described alloy composition and relational expression 1; heating the steel slab at 1020-1150 °C; Roughly rolling the heated steel slab at 1000° C. or higher; After the rough rolling, finishing hot rolling directly above Tnr (No-Recrystallization Temperature) or in the temperature range of Tnr to A3; and air-cooling after the finish hot rolling.
본 발명에 의하면, 두께 100~200mm의 극후물재에 대해 강도와 함께 저온 충격인성이 우수한 극후물 강판을 제공할 수 있다.According to the present invention, it is possible to provide an ultra-thick steel sheet excellent in strength and low-temperature impact toughness for an ultra-thick material having a thickness of 100 to 200 mm.
이러한 본 발명의 극후물 강판은 구조용 소재로서 선박, 해양구조물의 각종 프레임, 교량, 건설 등의 인프라산업용, 풍력 하부구조용 등의 다방면으로 적용 가능한 효과가 있다.As a structural material, the ultra-thick steel sheet of the present invention has effects that can be applied in various fields, such as various frames of ships and offshore structures, bridges, and infrastructure industries such as construction and wind power substructures.
도 1은 본 발명의 일 실시예에 따른 극후물 강판의 미세조직을 관찰한 사진을 나타낸 것이다.1 shows a photograph of the microstructure of an ultra-thick steel sheet according to an embodiment of the present invention.
본 발명의 발명자들은 구조용 강재로서 적합한 두께 100mm 이상(100~200mm)의 극후물 강재를 제공함에 있어서, 강도와 더불어 저온 충격인성을 우수하게 확보할 수 있는 방안을 깊이 연구하였다.The inventors of the present invention have studied in depth ways to secure excellent low-temperature impact toughness as well as strength in providing an ultra-thick steel material having a thickness of 100 mm or more (100 to 200 mm) suitable as a structural steel material.
그 결과, 극후물 강재의 합금 성분계와 압연 공정을 최적화함에 의하여, 목표로 하는 물성을 가지는 극후물 강판을 제공할 수 있음을 확인하고, 본 발명을 완성하기에 이르렀다.As a result, it was confirmed that an ultra-thick steel sheet having target physical properties can be provided by optimizing the alloy composition system and the rolling process of the ultra-thick steel, and the present invention was completed.
특별히, 본 발명에 의할 경우, 기존 TMCP 강재의 생산성 문제, 일반 압연재와 열처리재의 물성 확보 문제, 나아가 열처리재의 비용 문제 등의 해결이 가능함에 기술적 의의가 있다.In particular, in the case of the present invention, there is technical significance in that it is possible to solve the productivity problem of the existing TMCP steel, the problem of securing physical properties of the general rolled material and the heat treatment material, and the cost problem of the heat treatment material.
이하, 본 발명에 대하여 상세히 설명한다.Hereinafter, the present invention will be described in detail.
본 발명의 일 측면에 따른 저온 충격인성이 우수한 극후물 강판은 중량%로, 탄소(C): 0.06~0.1%, 실리콘(Si): 0.3~0.5%, 망간(Mn): 1.35~1.65%, 알루미늄(Sol.Al): 0.015~0.04%, 니오븀(Nb): 0.015~0.04%, 티타늄(Ti): 0.005~0.02%, 크롬(Cr): 0.15~0.4%, 니켈(Ni): 0.3~0.5%, 질소(N): 0.002~0.008%, 인(P): 0.01% 이하(0%는 제외), 황(S): 0.003% 이하(0%는 제외)를 포함할 수 있다.The ultra-thick steel sheet having excellent low-temperature impact toughness according to one aspect of the present invention contains, by weight, carbon (C): 0.06-0.1%, silicon (Si): 0.3-0.5%, manganese (Mn): 1.35-1.65%, Aluminum (Sol.Al): 0.015 to 0.04%, Niobium (Nb): 0.015 to 0.04%, Titanium (Ti): 0.005 to 0.02%, Chromium (Cr): 0.15 to 0.4%, Nickel (Ni): 0.3 to 0.5 %, nitrogen (N): 0.002 to 0.008%, phosphorus (P): 0.01% or less (excluding 0%), sulfur (S): 0.003% or less (excluding 0%).
이하에서는, 본 발명에서 제공하는 강판의 합금조성을 위와 같이 제한하는 이유에 대하여 상세히 설명한다. Hereinafter, the reason for limiting the alloy composition of the steel sheet provided in the present invention as above will be described in detail.
한편, 본 발명에서 특별히 언급하지 않는 한 각 원소의 함량은 중량을 기준으로 하며, 조직의 비율은 면적을 기준으로 한다.Meanwhile, in the present invention, unless otherwise specified, the content of each element is based on weight, and the ratio of tissue is based on area.
탄소(C): 0.06~0.1%Carbon (C): 0.06~0.1%
탄소(C)는 고용강화를 일으키고, 강 중 Nb 등과 결합하여 탄질화물을 형성함으로써 강도 확보에 유리한 원소이다. Carbon (C) is an element that is advantageous for securing strength by causing solid solution strengthening and forming carbonitrides by combining with Nb in steel.
이러한 C에 의한 강도 효과를 충분히 얻기 위해서는 0.06% 이상으로 C를 포함할 수 있으나, 그 함량이 0.1%를 초과하게 되면 미세조직으로 펄라이트 상이 과도하게 형성되어 저온에서의 충격 및 피로 특성이 열화하는 문제가 있다. 또한, 고용 C의 함량이 증가함에 따라 충격 특성이 저하된다.In order to sufficiently obtain the strength effect of C, it may contain C at 0.06% or more, but when the content exceeds 0.1%, the pearlite phase is excessively formed in the microstructure, resulting in deterioration of impact and fatigue properties at low temperatures. there is In addition, as the content of solid solution C increases, the impact properties decrease.
따라서, 상기 C는 0.06~0.1%로 포함할 수 있으며, 보다 유리하게는 0.07% 이상, 0.09% 이하로 포함할 수 있다.Therefore, the C may be included in 0.06 to 0.1%, more advantageously, 0.07% or more and 0.09% or less.
실리콘(Si): 0.3~0.5%Silicon (Si): 0.3 to 0.5%
실리콘(Si)은 알루미늄(Al)과 함께 용강을 탈산시키는 역할을 한다. 이러한 Si은 강도 향상에도 영향을 미치지만, 그 함량이 과도할 경우 저온에서의 충격 및 피로 특성을 저해하므로 적절히 첨가할 필요가 있다.Silicon (Si) serves to deoxidize molten steel together with aluminum (Al). Si has an effect on improving strength, but when its content is excessive, it deteriorates impact and fatigue properties at low temperatures, so it is necessary to properly add Si.
상기 Si의 함량이 0.3% 미만이면 충분한 강도를 확보할 수 없으며, 반면 그 함량이 0.5%를 초과하게 되면 C의 확산을 방해하여 MA 상(마르텐사이트-오스테나이트 혼합조직)의 형성을 조장하는 문제가 있다.If the Si content is less than 0.3%, sufficient strength cannot be secured, whereas if the Si content exceeds 0.5%, the diffusion of C is hindered to promote the formation of the MA phase (martensite-austenite mixed structure). there is
따라서, 상기 Si은 0.3~0.5%로 포함할 수 있다.Accordingly, the Si may be included in an amount of 0.3 to 0.5%.
망간(Mn): 1.35~1.65%Manganese (Mn): 1.35 to 1.65%
망간(Mn)은 고용강화에 의한 강도 향상 효과가 큰 원소로서, 1.35% 이상으로 포함할 수 있다. 다만, 그 함량이 과도할 경우 MnS 개재물의 형성 및 중심부 편석에 의해 인성이 저하될 우려가 있으므로, 이를 고려하여 1.65% 이하로 포함할 수 있다.Manganese (Mn) is an element having a high effect of improving strength by solid solution strengthening, and may be included in an amount of 1.35% or more. However, if the content is excessive, since there is a concern that toughness may be deteriorated due to formation of MnS inclusions and central segregation, it may be included to 1.65% or less in consideration of this.
알루미늄(Sol.Al): 0.015~0.04%Aluminum (Sol.Al): 0.015 to 0.04%
알루미늄(Sol.Al)은 강의 주요한 탈산제이며, 강 중 질소(N)를 고정시키는데 유리하다. 이를 위해서는 Al을 0.015% 이상으로 포함하는 것이 유리한 반면, 그 함량이 0.04%를 초과하게 되면 Al2O3 개재물의 분율 및 크기의 증가로 저온 인성을 저해하는 원인이 된다. 또한, 상기 Si과 유사하게 모재 및 용접열영향부에서 MA 상의 생성을 촉진하여 저온 인성 및 저온 피로 특성을 저해하는 문제가 있다.Aluminum (Sol.Al) is a major deoxidizing agent for steel and is advantageous for fixing nitrogen (N) in steel. To this end, while it is advantageous to include 0.015% or more of Al, when the content exceeds 0.04%, the fraction and size of Al 2 O 3 inclusions increase, thereby impairing low-temperature toughness. In addition, similar to Si, there is a problem in that low-temperature toughness and low-temperature fatigue characteristics are deteriorated by accelerating the formation of MA phase in the base material and the weld heat-affected zone.
따라서, 상기 Al은 0.015~0.04%로 포함할 수 있다.Therefore, the Al may be included in 0.015 to 0.04%.
니오븀(Nb): 0.015~0.04%Niobium (Nb): 0.015 to 0.04%
니오븀(Nb)은 고용강화 효과가 있고, 탄질화물을 형성함으로써 압연 또는 냉각 중 재결정을 억제하여 조직을 미세하게 형성함으로써 강도를 향상시키는데 유리하다. Niobium (Nb) has a solid solution strengthening effect, and is advantageous in improving strength by forming a fine structure by forming a carbonitride to suppress recrystallization during rolling or cooling.
상술한 효과를 충분히 얻기 위해서는 0.015% 이상으로 Nb을 함유할 수 있다. 반면, 그 함량이 과도할 경우 C 친화력에 의해 C 집중이 발생하여 MA 상의 형성을 조장하여 저온에서의 인성과 파괴 특성을 저해하는 문제가 있으므로, 이를 고려하여 0.04% 이하로 제한할 수 있다.In order to sufficiently obtain the above effects, Nb may be contained in an amount of 0.015% or more. On the other hand, if the content is excessive, C concentration occurs due to C affinity, which promotes the formation of the MA phase, thereby impairing toughness and fracture properties at low temperatures.
따라서, 상기 Nb은 0.015~0.04%로 포함할 수 있으며, 보다 유리하게는 0.02% 이상으로 포함할 수 있다.Therefore, the Nb may be included in an amount of 0.015 to 0.04%, more advantageously, it may be included in an amount of 0.02% or more.
티타늄(Ti): 0.005~0.02%Titanium (Ti): 0.005 to 0.02%
티타늄(Ti)은 강의 충격 특성과 표면 품질을 열화시킬 수 있는 질소(N)와 결합하여 Ti계 질화물(TiN)을 형성하고, 고용 N의 함량을 감소시키는 역할을 한다. 상기 Ti계 석출물은 조직의 조대화를 억제하여 미세화에 기여하고, 인성을 향상시키는데 유용하다.Titanium (Ti) combines with nitrogen (N), which can deteriorate the impact properties and surface quality of steel, to form Ti-based nitride (TiN) and serves to reduce the content of dissolved N. The Ti-based precipitate contributes to miniaturization by suppressing the coarsening of the structure and is useful for improving toughness.
상술한 효과를 충분히 얻기 위해서는 0.005% 이상으로 Ti을 함유할 수 있으나, 그 함량이 0.02%를 초과하게 되면 석출물의 조대화에 의해 파괴의 원인이 되며, N과 결합하고 남은 고용 Ti이 Ti계 탄화물(TiC)을 형성함으로써 모재 및 용접부의 인성을 저해하는 문제가 있다.In order to sufficiently obtain the above-mentioned effect, Ti may be contained in an amount of 0.005% or more. However, if the content exceeds 0.02%, the coarsening of the precipitate causes destruction, and the dissolved Ti remaining after bonding with N is a Ti-based carbide. By forming (TiC), there is a problem of impairing the toughness of the base material and the welded part.
따라서, 상기 Ti은 0.005~0.02%로 포함할 수 있으며, 보다 유리하게는 0.01% 이상으로 포함할 수 있다.Therefore, the Ti may be included in an amount of 0.005 to 0.02%, and more advantageously, it may be included in an amount of 0.01% or more.
크롬(Cr): 0.15~0.4%Chromium (Cr): 0.15 to 0.4%
크롬(Cr)은 강의 소입성을 증가시켜 강도 향상에 유리한 원소이다.Chromium (Cr) is an element that is advantageous for improving strength by increasing hardenability of steel.
상술한 효과를 충분히 얻기 위해서는 0.15% 이상으로 Cr을 포함할 수 있으나, 그 함량이 0.4%를 초과하게 되면 용접성이 저하될 뿐만 아니라, 고가의 원소로서 제조비용의 상승을 초래하는 문제가 있다.In order to sufficiently obtain the above-mentioned effect, Cr may be included in an amount of 0.15% or more, but when the content exceeds 0.4%, not only the weldability is deteriorated, but also there is a problem of causing an increase in manufacturing cost as an expensive element.
따라서, 상기 Cr은 0.15~0.4%로 포함할 수 있다.Accordingly, the Cr may be included in an amount of 0.15 to 0.4%.
니켈(Ni): 0.3~0.5%Nickel (Ni): 0.3 to 0.5%
니켈(Ni)은 강의 강도와 인성을 동시에 향상시킬 수 있는 원소이다. Nickel (Ni) is an element that can simultaneously improve the strength and toughness of steel.
특히, 본 발명에 따른 압연 공정에서 강도 및 인성 향상 효과를 충분히 얻기 위해서는 0.3% 이상으로 Ni을 함유할 필요가 있다. 다만, 그 함량이 0.5%를 초과하게 되면 상술한 효과는 포화되는 반면, 제조비용이 상승하는 문제가 있다.In particular, in order to sufficiently obtain strength and toughness improvement effects in the rolling process according to the present invention, it is necessary to contain Ni at 0.3% or more. However, when the content exceeds 0.5%, the above-described effect is saturated, but there is a problem in that manufacturing cost increases.
따라서, 상기 Ni은 0.3~0.5%로 포함할 수 있다.Therefore, the Ni may be included in 0.3 to 0.5%.
질소(N): 0.002~0.008%Nitrogen (N): 0.002 to 0.008%
질소(N)는 강 중 Ti, Nb, Al 등과 결합하여 석출물을 형성하며, 이 석출물들은 재가열시 오스테나이트 조직을 미세하게 형성함으로써 강도 및 인성 향상에 유효하다. Nitrogen (N) combines with Ti, Nb, Al, etc. in steel to form precipitates, and these precipitates are effective in improving strength and toughness by forming a fine austenite structure during reheating.
상술한 효과를 충분히 얻기 위해서는 0.002% 이상 N를 첨가하는 것이 유리하나, 그 함량이 0.008%를 초과하게 되면 고온에서 표면 크랙을 유발하고, 석출물을 형성하고 남은 N이 원자 상태로 존재하여 강의 인성을 저해하는 원인이 된다.In order to sufficiently obtain the above-mentioned effect, it is advantageous to add 0.002% or more of N, but when the content exceeds 0.008%, surface cracks are caused at high temperatures, and the N remaining after forming precipitates exists in an atomic state, improving the toughness of the steel. cause obstruction.
따라서, 상기 N은 0.002~0.008%로 포함할 수 있다.Therefore, the N may be included in 0.002 ~ 0.008%.
인(P): 0.01% 이하(0%는 제외)Phosphorus (P): 0.01% or less (excluding 0%)
인(P)은 입계 편석을 일으키는 원소로서, 강을 취하시키는 원인이 될 수 있다. 따라서, P은 그 함량을 가능한 낮게 제어하여야 한다.Phosphorus (P) is an element that causes grain boundary segregation, and may cause steel to take off. Therefore, the content of P should be controlled as low as possible.
본 발명에서 상기 P은 최대 0.01%로 함유하더라도 의도하는 물성 확보에는 무리가 없는 바, 상기 P의 함량을 0.01% 이하로 제한할 수 있다. 다만, 불가피하게 첨가되는 수준을 고려하여 0%는 제외할 수 있다.In the present invention, even if the P is contained in a maximum of 0.01%, there is no problem in securing the intended physical properties, and the P content may be limited to 0.01% or less. However, 0% can be excluded considering the level that is unavoidably added.
황(S): 0.003% 이하(0%는 제외)Sulfur (S): 0.003% or less (excluding 0%)
황(S)은 주로 강 중의 Mn과 결합하여 MnS 개재물을 형성하며, 이는 저온 인성을 저해하는 요인이 된다. Sulfur (S) is mainly combined with Mn in steel to form MnS inclusions, which is a factor that inhibits low-temperature toughness.
따라서, 본 발명에서 목표로 하는 저온 인성과 저온 피로 특성을 확보하기 위해서는 상기 S의 함량을 가능한 낮게 제어하여야 하며, 바람직하게 0.003% 이하로 제한할 수 있다. 다만, 불가피하게 첨가되는 수준을 고려하여 0%는 제외할 수 있다.Therefore, in order to secure the low-temperature toughness and low-temperature fatigue characteristics targeted in the present invention, the content of S should be controlled as low as possible, and may be preferably limited to 0.003% or less. However, 0% can be excluded considering the level that is unavoidably added.
본 발명의 나머지 성분은 철(Fe)이다. 다만, 통상의 제조과정에서는 원료 또는 주위 환경으로부터 의도되지 않는 불순물들이 불가피하게 혼입될 수 있으므로, 이를 배제할 수는 없다. 이들 불순물들은 통상의 제조과정의 기술자라면 누구라도 알 수 있는 것이기 때문에 그 모든 내용을 특별히 본 명세서에서 언급하지는 않는다.The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.
상술한 합금조성을 만족하는 본 발명의 강재는 강 중 Mn, Ni 및 Cr의 관계가 하기 관계식 1을 만족하는 것이 바람직하다.In the steel material of the present invention that satisfies the above-described alloy composition, the relationship between Mn, Ni, and Cr in the steel preferably satisfies the following relational expression 1.
[관계식 1][Relationship 1]
Mn + 5(Ni+Cr) ≥ 3.6Mn+5(Ni+Cr) ≥ 3.6
(여기서, 각 원소는 중량 함량을 의미한다.)(Here, each element means a weight content.)
본 발명은 두께 100mm 이상, 바람직하게는 100~200mm의 두께를 가지는 극후물 강판의 저온 인성을 향상시키기 위하여, 강 중 C의 함량을 0.10% 이하로 제한한다. 본 발명은 상대적으로 C 함량을 낮추더라도 강도 확보에 불리한 영향이 없도록, 강 중 Mn, Ni 및 Cr의 관계를 관계식 1로 제어하는 특징이 있다.In the present invention, in order to improve low-temperature toughness of an ultra-thick steel sheet having a thickness of 100 mm or more, preferably 100 to 200 mm, the content of C in the steel is limited to 0.10% or less. The present invention is characterized in that the relationship between Mn, Ni, and Cr in the steel is controlled by relational expression 1 so that the strength is not adversely affected even when the C content is relatively lowered.
구체적으로, 본 발명에서 제안하는 합금조성 내에서 상기 Mn, Ni 및 Cr의 함량 관계가 상기 관계식 1을 만족하지 못할 경우, 즉 관계식 1의 값이 3.6 미만이면 최대 두께 200mm의 극후물 강판의 강도를 확보할 수 없게 된다.Specifically, when the content relationship of Mn, Ni, and Cr in the alloy composition proposed in the present invention does not satisfy the above relational expression 1, that is, when the value of relational expression 1 is less than 3.6, the strength of an ultra-thick steel sheet having a maximum thickness of 200mm is increased. will not be able to obtain
상술한 합금조성과 관계식 1을 만족하는 본 발명의 극후물 강판은 미세조직이 페라이트 및 펄라이트 복합조직으로 구성될 수 있다.The ultra-thick steel sheet of the present invention that satisfies the above-described alloy composition and relational expression 1 may have a microstructure composed of ferrite and pearlite composite structures.
구체적으로, 본 발명의 극후물 강판은 면적분율 80~90%의 페라이트 및 잔부 펄라이트를 포함하는 것이 바람직하다.Specifically, the ultra-thick steel sheet of the present invention preferably contains ferrite and the balance of pearlite in an area fraction of 80 to 90%.
상기 페라이트의 분율이 80% 미만이면 극후물 강판의 저온 인성의 확보가 어려워지며, 반면 그 분율이 90%를 초과하게 되면 펄라이트의 분율이 불충분해져 목표 수준의 강도를 확보할 수 없다.If the fraction of the ferrite is less than 80%, it becomes difficult to secure the low-temperature toughness of the ultra-thick steel sheet, whereas if the fraction exceeds 90%, the fraction of pearlite is insufficient, making it impossible to secure the target level of strength.
또한, 본 발명의 극후물 강판은 상기 페라이트의 평균 결정립 크기가 50㎛ 이하로서 조직이 미세한 특징을 갖는다.In addition, the ultra-thick steel sheet of the present invention has a fine structure as the average grain size of the ferrite is 50 μm or less.
여기서, 평균 결정립 크기는 원 상당 직경을 기준으로 함을 밝혀둔다.Here, it should be noted that the average grain size is based on the equivalent circle diameter.
이와 같이, 본 발명은 극후물 강판의 조직을 미세하게 구현함으로써 목표로 하는 강도 및 저온 인성을 양립하여 우수하게 확보할 수 있는 효과가 있다.As described above, the present invention has an effect of ensuring both target strength and low-temperature toughness excellently by finely implementing the structure of the ultra-thick steel sheet.
구체적으로, 본 발명의 극후물 강판은 300MPa 이상의 항복강도와 더불어, -20℃에서 충격 인성이 200J 이상으로 고강도와 더불어 저온 인성이 우수한 효과가 있다.Specifically, the ultra-thick steel sheet of the present invention has a yield strength of 300 MPa or more and an impact toughness of 200 J or more at -20 ° C.
이하, 본 발명의 다른 일 측면에 따른 저온 충격인성이 우수한 극후물 강판을 제조하는 방법에 대하여 상세히 설명한다.Hereinafter, a method for manufacturing an ultra-thick steel sheet having excellent low-temperature impact toughness according to another aspect of the present invention will be described in detail.
간략히 설명하면, 본 발명에서 제안하는 합금조성 및 관계식 1을 만족하는 강 슬라브를 준비한 후, 이를 [가열 - 압연 - 공냉]의 공정을 거쳐 제조할 수 있다. 특히, 본 발명에서는 압연 공정을 완료한 후 별도의 열처리를 행하지 아니하며, 압연 공정으로서 노멀라이징(normalizing) 열처리 영역에서 압연 공정을 행함에 기술적 의의가 있다.Briefly, after preparing a steel slab that satisfies the alloy composition and relational expression 1 proposed in the present invention, it can be manufactured through a process of [heating - rolling - air cooling]. In particular, in the present invention, there is technical significance in that a rolling process is performed in a normalizing heat treatment area as a rolling process without performing a separate heat treatment after completing the rolling process.
각 공정 조건에 대해서는 하기에 상세히 설명한다.Each process condition is explained in detail below.
[강 슬라브 가열][Heating of steel slabs]
본 발명에서는 압연 공정을 행하기에 앞서 강 슬라브를 가열하여 균질화 처리하는 공정을 거치는 것이 바람직하며, 이때 1020~1150℃의 온도 범위에서 가열 공정을 행할 수 있다.In the present invention, it is preferable to go through a process of heating and homogenizing the steel slab prior to performing the rolling process, and at this time, the heating process may be performed in a temperature range of 1020 to 1150 ° C.
상기 강 슬라브의 가열 온도가 1020℃ 미만이면 Ti, Nb 등이 충분히 고용되지 못하여 강도 하락을 초래할 우려가 있으며, 반면 그 온도가 1150℃를 초과하게 되면 오스테나이트의 결정립이 조대화되어 강의 인성이 저하될 우려가 있다.If the heating temperature of the steel slab is less than 1020 ° C, Ti, Nb, etc. may not be sufficiently dissolved, resulting in a decrease in strength. There is a risk of becoming
상기 강 슬라브는 후속하는 압연 공정에 의해 최대 200mm의 두께를 가지면서, 강도 및 인성의 확보를 위하여 충분한 압연량을 확보할 수 있도록 400mm 이하의 두께를 가지는 것일 수 있다.The steel slab may have a thickness of 400 mm or less to secure a sufficient amount of rolling to secure strength and toughness while having a thickness of up to 200 mm by a subsequent rolling process.
[압연 공정][Rolling process]
상기에 따라 가열된 강 슬라브를 열간압연하여 열연강판을 제조할 수 있다.A hot-rolled steel sheet may be manufactured by hot-rolling the steel slab heated according to the above.
본 발명에서 상기 열간압연은 [재결정역 압연(조압연) - 미재결정역 압연(마무리 압연)] 단계로 행하는 것이 바람직하다.In the present invention, the hot rolling is preferably performed in a step of [recrystallization station rolling (rough rolling) - non-recrystallization station rolling (finish rolling)].
상기 조압연은 1000℃ 이상에서 행함함으로써 오스테나이트를 완전히 재결정할 수 있다.Austenite can be completely recrystallized by carrying out the said rough rolling at 1000 degreeC or more.
이후, Tnr(No-Recrystallization Temperature) 직상 또는 Tnr~A3 온도 범위 내의 오스테나이트 단상 영역에서 마무리 압연을 행할 수 있다. 이때, 결정립 미세화 효과를 더욱 도모하기 위해서는 A3 온도에 근접하여 마무리 압연을 수행하는 것이 유리하지만, 노멀라이징 효과를 얻기 위해서는 Tnr 온도 직상에서 행하는 것이 유리하다. 상기 Tnr 온도 직상은 Tnr 초과~Tnr+50℃의 온도범위로 나타낼 수 있다. Thereafter, finish rolling may be performed in an austenite single phase region directly above Tnr (No-Recrystallization Temperature) or within a temperature range of Tnr to A3. At this time, it is advantageous to perform finish rolling close to the A3 temperature in order to further promote the grain refinement effect, but it is advantageous to perform the finish rolling immediately above the Tnr temperature in order to obtain the normalizing effect. The temperature directly above Tnr can be expressed as a temperature range of greater than Tnr to Tnr +50°C.
상기 Tnr과 A3 온도는 다음의 식에 의해 구할 수 있으며, 하기 식에서 각 원소는 중량 함량을 의미한다.The Tnr and A3 temperatures can be obtained by the following equations, where each element means a weight content.
Tnr = 887 + 464C + (6445Nb - 644√Nb) + (732V - 230√V) + 890Ti + 363Al - 357SiTnr = 887 + 464C + (6445Nb - 644√Nb) + (732V - 230√V) + 890Ti + 363Al - 357Si
A3 = 910 - 203√C - 15.2Ni + 44.7Si + 104V + 31.5Mo - 30Mn + 11Cr + 20Cu - 700P - 400Al - 400TiA3 = 910 - 203√C - 15.2Ni + 44.7Si + 104V + 31.5Mo - 30Mn + 11Cr + 20Cu - 700P - 400Al - 400Ti
상기 마무리 압연시 온도가 A3 미만이면 이상역(two-phase region) 압연이 되어 노멀라이징 효과가 미비해져, 추가적인 열처리 공정이 요구될 우려가 있다.If the temperature during the finish rolling is less than A3, the two-phase region rolling is performed, and the normalizing effect is insufficient, so there is a concern that an additional heat treatment process may be required.
보다 바람직하게, 상기 마무리 압연은 820~900℃의 온도범위에서 종료할 수 있다.More preferably, the finish rolling may be finished in a temperature range of 820 ~ 900 ℃.
본 발명은 상술한 압연 공정을 거침으로써 최대 두께 200mm의 극후물 강판을 얻고자 하는 바, 상기 압연 공정에서 조압연 및 마무리 압연 시의 압하율 배분을 고려할 필요가 있다.Since the present invention intends to obtain an ultra-thick steel sheet having a maximum thickness of 200 mm by going through the above-described rolling process, it is necessary to consider the distribution of reduction rates during rough rolling and finish rolling in the rolling process.
본 발명에서는 조압연을 완료한 직후의 잔압하량을 25~35%로 제어하는 것이 바람직하다. 상기 잔압하량이 25% 미만이면 조압연 공정이 길어져 생산성이 저하되는 문제가 있으며, 반면 35%를 초과하게 되면 조압연 후 마무리 압연시 압연기 부하의 발생으로 건전한 압연이 이루어지지 못할 우려가 있다.In the present invention, it is preferable to control the residual pressure reduction immediately after completion of the rough rolling to 25 to 35%. If the residual pressure reduction is less than 25%, there is a problem that the rough rolling process is prolonged and productivity is lowered. On the other hand, if it exceeds 35%, sound rolling may not be achieved due to the generation of load on the rolling mill during finish rolling after rough rolling.
여기서, 잔압하량이라 하면 조압연 후, 목표 두께까지의 남은 마무리 압연량을 의미함을 밝혀둔다.Here, the residual pressure reduction means the amount of finishing rolling remaining to the target thickness after rough rolling.
[공냉][air cooling]
상기에 따른 압연 공정을 완료하여 얻은 열연강판에 대해 냉각을 행할 수 있으며, 이때 노멀라이징 효과를 구현하기 위하여 공냉을 행하는 것이 바람직하다.Cooling may be performed on the hot-rolled steel sheet obtained by completing the rolling process according to the above, and at this time, it is preferable to perform air cooling in order to realize the normalizing effect.
본 발명에 따른 압연 공정을 완료한 후 공냉을 행함으로써 결정립 미세화 효과를 달성할 수 있을 뿐만 아니라, 후속 열처리 공정을 행하지 않고서도 강도 및 인성이 우수한 극후물 강판을 얻는 효과가 있다.By performing air cooling after completing the rolling process according to the present invention, not only can grain refinement be achieved, but also an effect of obtaining an ultra-thick steel sheet having excellent strength and toughness without performing a subsequent heat treatment process.
보다 구체적으로, 본 발명의 극후물 강판은 의도하는 미세조직이 형성됨에 따라, 두께 100~200mm의 극후물 강에 대해 강도 및 인성 특성을 양립하여 우수하게 확보할 수 있다.More specifically, as the intended microstructure is formed in the ultra-thick steel sheet of the present invention, both strength and toughness characteristics can be excellently secured for ultra-thick steel having a thickness of 100 to 200 mm.
기존 노멀라이징 열처리에 의해 제조되는 강재는 강도 확보를 위하여, 제어압연+냉각에 의해 제조되는 TMCP 강재 대비 탄소 함량이 높기 때문에 열처리 후에도 충격 인성이 열위하는 경향이 있다. 또한, 열처리 온도가 지나치게 높거나, 그 시간이 길어지면 결정립 성장에 의해 열처리 전 압연 상태의 강재 대비 강도가 하락하는 경우도 발생한다.Steels manufactured by conventional normalizing heat treatment tend to have inferior impact toughness even after heat treatment because the carbon content is higher than that of TMCP steels manufactured by controlled rolling + cooling in order to secure strength. In addition, if the heat treatment temperature is too high or the time is too long, the strength of the steel material in the rolled state before heat treatment may decrease due to grain growth.
TMCP 공정에 의하여 극후물 강재를 제조하는 경우에는 온도 제어로 인해 수 분의 공냉 대기 시간이 필요하므로, 생산성이 저하되고 수(water) 처리에 의한 비용이 요구되는 바 경제적으로 불리하다.In the case of manufacturing ultra-thick steel materials by the TMCP process, since an air-cooling waiting time of several minutes is required due to temperature control, productivity is lowered and costs due to water treatment are required, which is economically disadvantageous.
본 발명은 상술한 공정에 의해 제조되는 극후물재의 단점을 극복할 수 있는 제조방법을 제안하는 것이며, 특정 합금 성분계를 가지는 슬라브에 대해 압연 및 냉각 조건을 최적화함으로써 강도 및 저온 인성 특성이 우수한 극후물 강재를 제공하는 효과가 있는 것이다.The present invention proposes a manufacturing method capable of overcoming the disadvantages of the ultra-thick material produced by the above-described process, and by optimizing rolling and cooling conditions for a slab having a specific alloy component system, an ultra-thick material with excellent strength and low-temperature toughness characteristics It has the effect of providing steel.
이하, 실시예를 통하여 본 발명을 보다 구체적으로 설명하고자 한다. 다만, 하기의 실시예는 본 발명을 예시하여 보다 상세하게 설명하기 위한 것일 뿐, 본 발명의 권리범위를 한정하기 위한 것이 아니라는 점에 유의할 필요가 있다. 본 발명의 권리범위는 특허청구범위에 기재된 사항과 이로부터 합리적으로 유추되는 사항에 의해 결정되는 것이기 때문이다.Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.
(실시예)(Example)
하기 표 1의 합금조성을 가지는 강 슬라브를 준비하였다. 이때, 상기 합금조성의 함량은 중량%이며, 나머지는 Fe와 불가피한 불순물을 포함한다.A steel slab having the alloy composition shown in Table 1 was prepared. At this time, the content of the alloy composition is % by weight, and the remainder includes Fe and unavoidable impurities.
상기 준비된 강 슬라브를 하기 표 2에 나타낸 조건으로 가열, 열간압연(조압연 및 마무리 압연) 및 냉각(공냉)하여 각각의 열연강판을 제조하였다. 이때, 조압연은 1000℃ 이상에서 행하였다.Each hot-rolled steel sheet was manufactured by heating, hot-rolling (rough rolling and finish rolling), and cooling (air-cooling) the prepared steel slabs under the conditions shown in Table 2 below. At this time, rough rolling was performed at 1000°C or higher.
식1relationship
formula 1
(℃)heating temperature
(℃)
상기에 따라 제조된 각각의 강판의 대해 미세조직과 기계적 물성을 측정하고, 그 결과를 하기 표 3에 나타내었다.Microstructure and mechanical properties were measured for each of the steel sheets prepared according to the above, and the results are shown in Table 3 below.
각 열연 강재의 미세조직은 두께 1/4t(여기서, t는 두께(mm)를 의미함) 지점에서 채취된 시편을 광학현미경(OM)으로 관찰하였으며, 동일한 시편에 대해 -20℃에서 샤르피 충격시험을 실시하여 충격인성을 평가하였다.The microstructure of each hot-rolled steel was observed with an optical microscope (OM) on specimens taken at the point of thickness 1/4t (here, t means thickness (mm)), and a Charpy impact test at -20 ° C was performed on the same specimen. was carried out to evaluate the impact toughness.
또한, JIS 5호 규격에 의거하여 채취된 시험편에 대해 만능인장시험기를 이용하여 인장강도, 항복강도, 연신율을 측정하였다.In addition, tensile strength, yield strength, and elongation were measured using a universal tensile tester for the specimens collected in accordance with the JIS No. 5 standard.
(mm)thickness
(mm)
(%)F*fraction
(%)
(㎛)F size*
(μm)
(%)P* fraction
(%)
(MPa)yield strength
(MPa)
(MPa)tensile strength
(MPa)
(%)elongation rate
(%)
(J. -20℃)impact toughness
(J. -20℃)
F 크기*는 원 상당 직경을 기준으로, 평균 결정립 크기를 의미한다.F* means ferrite and P* means pearlite.
F size* refers to the average grain size based on the equivalent circle diameter.
상기 표 1 내지 3에 나타낸 바와 같이, 본 발명에서 제안하는 합금조성, 관계식 1 및 제조조건을 모두 만족하는 발명예 1 내지 3은 항복강도가 300MPa 이상이며, -20℃에서 충격인성이 200J 이상으로 고강도를 가지면서 저온 충격 특성이 우수함을 확인할 수 있다.As shown in Tables 1 to 3, Inventive Examples 1 to 3 satisfying all of the alloy composition, relational expression 1, and manufacturing conditions proposed in the present invention have a yield strength of 300 MPa or more, and an impact toughness of 200 J or more at -20 ° C. It can be confirmed that the low-temperature impact properties are excellent while having high strength.
반면, 본 발명에서 제안하는 합금 성분계를 만족하지만 마무리 압연시 종료온도가 과도하게 높은 비교예 1의 경우, 조대한 페라이트가 형성되어 강도 및 인성이 모두 열위하였다.On the other hand, in the case of Comparative Example 1, which satisfies the alloy composition system proposed in the present invention but had an excessively high end temperature during finish rolling, coarse ferrite was formed, resulting in inferior strength and toughness.
또한, 강 중 C 함량이 과도한 비교예 2는 펄라이트가 과도하게 형성되어 강도의 확보는 가능한 반면, 인성이 크게 열위하였다.In Comparative Example 2, in which the C content in the steel was excessive, pearlite was excessively formed, and strength was secured, but toughness was greatly inferior.
그리고, 본 발명에서 제안하는 관계식 1을 벗어나는 비교예 3은 미세조직이 본 발명에서 목표하는 바로 형성됨에도 불구하고, 강도가 저하된 것을 확인할 수 있다. 이는, 강 중 경화능 원소들의 함량이 본 발명의 관계식 1로 최적화되지 못할 경우, 목표로 하는 강도의 확보가 어려움을 증명하는 것이다.And, in Comparative Example 3, which deviate from the relational expression 1 proposed in the present invention, it can be confirmed that the strength is lowered even though the microstructure is formed as desired in the present invention. This proves that it is difficult to secure the target strength when the content of the hardenable elements in the steel is not optimized according to the relational expression 1 of the present invention.
도 1은 발명예 3의 미세조직을 관찰한 사진으로서, 미세한 페라이트 상을 주상으로 하여 펄라이트와의 복합조직이 형성된 것을 확인할 수 있다.1 is a photograph of the microstructure of Inventive Example 3, and it can be confirmed that a composite structure with pearlite is formed with a fine ferrite phase as the main phase.
Claims (8)
미세조직으로 면적분율 80~90%의 페라이트 및 잔부 펄라이트를 포함하는 저온 충격인성이 우수한 극후물 강판.
[관계식 1]
Mn + 5(Ni+Cr) ≥ 3.6
(여기서, 각 원소는 중량 함량을 의미한다.)
By weight %, carbon (C): 0.06-0.1%, silicon (Si): 0.3-0.5%, manganese (Mn): 1.35-1.65%, aluminum (Sol.Al): 0.015-0.04%, niobium (Nb) : 0.015~0.04%, Titanium (Ti): 0.005~0.02%, Chromium (Cr): 0.15~0.4%, Nickel (Ni): 0.3~0.5%, Nitrogen (N): 0.002~0.008%, Phosphorus (P) : 0.01% or less (excluding 0%), Sulfur (S): 0.003% or less (excluding 0%), the balance including Fe and other unavoidable impurities, and satisfying the following relational expression 1,
Ultra-thick steel sheet with excellent low-temperature impact toughness, containing 80-90% area fraction of ferrite and remainder pearlite in microstructure.
[Relationship 1]
Mn+5(Ni+Cr) ≥ 3.6
(Here, each element means a weight content.)
상기 페라이트의 평균 결정립 크기가 50㎛ 이하인 저온 충격인성이 우수한 극후물 강판.
According to claim 1,
An ultra-thick steel sheet having excellent low-temperature impact toughness in which the average grain size of the ferrite is 50 μm or less.
상기 강판은 항복강도 300MPa 이상, -20℃에서 충격인성이 200J 이상인 저온 충격인성이 우수한 극후물 강판.
According to claim 1,
The steel sheet is an ultra-thick steel sheet having a yield strength of 300 MPa or more and an impact toughness of 200 J or more at -20 ° C.
상기 강판은 100~200mm의 두께를 가지는 저온 충격인성이 우수한 극후물 강판.
According to claim 1,
The steel sheet is an ultra-thick steel sheet having excellent low-temperature impact toughness having a thickness of 100 to 200 mm.
상기 강 슬라브를 1020~1150℃에서 가열하는 단계;
상기 가열된 강 슬라브를 1000℃ 이상에서 조압연하는 단계;
상기 조압연 후 Tnr(No-Recrystallization Temperature) 직상 또는 Tnr~A3 온도 범위에서 마무리 열간압연하는 단계; 및
상기 마무리 열간압연 후 공냉하는 단계를 포함하는 저온 충격인성이 우수한 극후물 강판의 제조방법.
[관계식 1]
Mn + 5(Ni+Cr) ≥ 3.6
(여기서, 각 원소는 중량 함량을 의미한다.)
By weight %, carbon (C): 0.06-0.1%, silicon (Si): 0.3-0.5%, manganese (Mn): 1.35-1.65%, aluminum (Sol.Al): 0.015-0.04%, niobium (Nb) : 0.015~0.04%, Titanium (Ti): 0.005~0.02%, Chromium (Cr): 0.15~0.4%, Nickel (Ni): 0.3~0.5%, Nitrogen (N): 0.002~0.008%, Phosphorus (P) : 0.01% or less (excluding 0%), sulfur (S): 0.003% or less (excluding 0%), the balance including Fe and other unavoidable impurities, and preparing a steel slab that satisfies the following relational expression 1;
heating the steel slab at 1020-1150 °C;
Roughly rolling the heated steel slab at 1000° C. or higher;
After the rough rolling, finishing hot rolling directly above Tnr (No-Recrystallization Temperature) or in the temperature range of Tnr to A3; and
A method for producing an ultra-thick steel sheet having excellent low-temperature impact toughness comprising the step of air-cooling after the finish hot rolling.
[Relationship 1]
Mn+5(Ni+Cr) ≥ 3.6
(Here, each element means a weight content.)
상기 마무리 열간압연은 820~900℃의 온도범위에서 종료하는 것인 저온 충격인성이 우수한 극후물 강판의 제조방법.
According to claim 5,
The finish hot rolling is a method for producing an ultra-thick steel sheet having excellent low-temperature impact toughness, which is terminated in the temperature range of 820 ~ 900 ℃.
상기 조압연 후 잔압하량은 25~35%인 저온 충격인성이 우수한 극후물 강판의 제조방법.
According to claim 5,
Method for producing an ultra-thick steel sheet having excellent low-temperature impact toughness with a residual pressure reduction of 25 to 35% after the rough rolling.
상기 강판은 100~200mm의 두께를 가지는 저온 충격인성이 우수한 극후물 강판의 제조방법.According to claim 5,
The steel sheet is a method for producing an ultra-thick steel sheet having excellent low-temperature impact toughness having a thickness of 100 to 200 mm.
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