KR101091282B1 - Method for manufacturing 780MPa grade hot rolled steel sheet having excellent weldability - Google Patents

Method for manufacturing 780MPa grade hot rolled steel sheet having excellent weldability Download PDF

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KR101091282B1
KR101091282B1 KR1020040077881A KR20040077881A KR101091282B1 KR 101091282 B1 KR101091282 B1 KR 101091282B1 KR 1020040077881 A KR1020040077881 A KR 1020040077881A KR 20040077881 A KR20040077881 A KR 20040077881A KR 101091282 B1 KR101091282 B1 KR 101091282B1
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steel
less
strength
weldability
cooling
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KR20060028954A (en
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김성규
김영봉
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주식회사 포스코
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

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  • Engineering & Computer Science (AREA)
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  • Mechanical Engineering (AREA)
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  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

본 발명은 용접성이 우수한 인장강도 780MPa급 고강도 열연강판의 제조방법에 관한 것이다.The present invention relates to a method for producing a high strength hot-rolled steel sheet of tensile strength 780MPa grade excellent in weldability.

본 발명은 중량%로, C: 0.05~0.13%, Si: 0.3~1.2%, Mn: 1.0~1.7%, S: 0.005% 이하, P: 0.03% 이하, Al: 0.05% 이하, Ti: 0.01~0.06%, Nb: 0.01~0.06%을 포함하여 이루어지는 강슬라브를 1150~1250℃의 범위에서 60~180분동안 재가열하고, Ar3+60℃ 이상에서 마무리열간압연한 다음 수냉하여 650~700℃범위에서 4~8초간 공냉하고, 이어 다시 수냉하여 250~350℃에서 수냉을 종료하는 것을 포함하여 이루어진다.In the present invention, by weight%, C: 0.05-0.13%, Si: 0.3-1.2%, Mn: 1.0-1.7%, S: 0.005% or less, P: 0.03% or less, Al: 0.05% or less, Ti: 0.01- Reheat the steel slab containing 0.06%, Nb: 0.01 ~ 0.06% for 60 ~ 180 minutes in the range of 1150 ~ 1250 ℃, finish hot rolling over Ar 3 + 60 ℃, and then cool it in water to 650 ~ 700 ℃ Air cooling at 4 to 8 seconds, followed by water cooling to complete the water cooling at 250 ~ 350 ℃.

본 발명은 용접성이 우수하고, 높은 강도와 상대적으로 높은 연신을 갖는 강재를 제공한다.
The present invention provides a steel having excellent weldability and having high strength and relatively high elongation.

용접성, 고강도, 열연강판, 가공성, 복합조직Weldability, high strength, hot rolled steel, processability, composite structure

Description

용접성이 우수한 인장강도 780㎫급 고강도 열연강판의 제조방법{Method for manufacturing 780MPa grade hot rolled steel sheet having excellent weldability} Method for manufacturing high strength hot rolled steel sheet of tensile strength 780MPa class with excellent weldability {Method for manufacturing 780MPa grade hot rolled steel sheet having excellent weldability}             

도 1은 본 발명에서 제시된 제조방법에 따라 제조된 소재의 강종, 권취온도에 따른 인장강도를 나타내는 그래프이다.1 is a graph showing the tensile strength according to the steel grade, winding temperature of the material produced according to the production method presented in the present invention.

도 2는 본 발명에서 제시된 제조방법에 따라 제조된 소재의 강종, 권취온도에 따른 연신율을 나타내는 그래프이다.
Figure 2 is a graph showing the elongation according to the steel grade, winding temperature of the material produced according to the production method presented in the present invention.

본 발명은 충돌시의 승객의 안전과 직접적인 관계를 가지는 자동차의 범퍼 보강재 혹은 도어내의 충격 흡수재에 사용되는 780MPa 이상의 초고강도를 가지는 자동차용 강판의 제조방법에 관한 것으로, 강도와 더불어 연신율이 우수하여 부품 성형이 용이하며, 성분 첨가량을 제한하여 용접성이 우수한 고강도 열연강판의 제조방법에 관한 것이다. The present invention relates to a method for manufacturing a steel sheet for automobiles having a very high strength of 780MPa or more used for bumper reinforcement of automobiles or shock absorbers in doors having a direct relationship with the safety of passengers in a collision. The present invention relates to a method for producing a high strength hot rolled steel sheet which is easy to mold and has an excellent weldability by limiting the amount of component addition.                         

범퍼 보강재 혹은 도어내의 충격 흡수재는 차량의 충돌시 승객안전과 직접 관계되는 부품으로 인장강도 780MPa 이상의 초고강도 열연강판이 주로 사용되고 있으며, 높은 인장강도와 더불어 높은 연신율을 가져야 한다. 또한 점차 심각해지고 있는 환경 오염 규제에 대응하기 위해 연비를 증가시키고자 보다 높은 강도 부품의 고강도 강 사용 비율이 증가하고 있는데, 최근에는 780MPa 이상의 고강도 강의 상업화에 대한 연구가 증가하고 있다. 자동차용 고강도 강은 대표적으로 복합조직(Multi-Phase Steel)강, 변태유기소성(TRIP; Transformation Induced Plasticity)강과 이상조직(DP; Dual Phase)강이 있다.The bumper reinforcement or shock absorber in the door is a part directly related to passenger safety in the event of a vehicle collision. The super high strength hot rolled steel sheet with a tensile strength of 780 MPa or more is mainly used. In addition, the use of high strength steels of higher strength parts is increasing in order to increase fuel efficiency in order to cope with increasingly severe environmental pollution regulations. Recently, research on commercialization of high strength steels of 780 MPa or more is increasing. Automotive high-strength steels include multi-phase steels, transformation induced plasticity (TRIP) steels, and dual phase steels (DP).

제조공정은 크게 제조된 잉곳의 성분을 재고용하는 재가열, 최종 두께로 압연하는 열간 압연, 열간 압연된 판재를 상온으로 냉각/권취하는 냉각공정으로 구분되는데, 가열에서 나온 슬라브를 오스테나이트 구간에서 압연하고, 냉각과정에서 냉간종료온도를 Ms온도 보다 낮게 하여 오스테나이트를 마르텐사이트로 변태시키는 경우 이 강을 이상조직강이라고 하며, 전체 조직 중에서 마르텐사이트의 비율이 증가할수록 강도가 증가하고 페라이트 비율이 증가할수록 연성이 증가하는데, 강도 상승을 위하여 마르텐사이트 비율을 너무 커지면 상대적으로 페라이트 비율이 감소하여 오히려 연성이 저하된다. 그리고 저온에서 마르텐사이트를 형성하기 위해서 냉각속도를 크게 하여야 하는 단점이 있다.The manufacturing process is divided into reheating to re-use the components of the largely manufactured ingot, hot rolling to the final thickness, and cooling to wind / cold the hot rolled sheet at room temperature. The slab from the heating is rolled in the austenite section. In case of transforming austenite to martensite by lowering the cold end temperature to lower than Ms temperature in the cooling process, this steel is called an ideal tissue steel, and as the ratio of martensite increases, the strength increases and the ferrite ratio increases. Ductility increases, but if the martensite ratio is too large to increase the strength, the ferrite ratio decreases relatively, and the ductility decreases. In addition, there is a disadvantage in that the cooling rate must be increased to form martensite at low temperature.

한편, 상기 방법과 같이 압연 과정에서 오스테나이트를 형성한 후 냉각과정에서 냉각 속도와 냉각종료온도 등을 제어하여 상온에서 페라이트, 마르텐사이트와 일부 베이나이트 및 마르텐사이트/오스테나이트 혼합상을 형성함으로써, 상기 변태 조직강의 강도와 연성을 동시에 높게한 강이 복합조직(Multi-phase)강 이다. 복합조직 강의 경우 마르텐사이트 변태에 의한 저항복비 특성을 가지는 것이 아니므로 상대적으로 합금원소 첨가양이 적어 용접성이 우수하며, 항복강도가 높아 성형성에 다소 불리하나 높은 항복강도를 가져 많은 분야에 응용되고 있다.On the other hand, after forming austenite in the rolling process as described above by controlling the cooling rate and the cooling end temperature in the cooling process to form a ferrite, martensite and some bainite and martensite / austenitic mixed phase at room temperature, A steel having a high strength and ductility of the metamorphic tissue steel at the same time is a multi-phase steel. In the case of composite steel, it does not have resistance yield ratio due to martensitic transformation, so it has relatively low amount of alloying elements, which is excellent in weldability and high yield strength, which is somewhat disadvantageous in formability but has high yield strength and is applied in many fields. .

강도와 함께 우수한 연신율을 얻기위해서 열간압연 후의 냉각조건을 제어하는 것이 중요하며, 냉각중에 공냉을 통해 페라이트를 얻고자 냉각패턴을 적정화하고 성분을 최적화하여야 한다. 복합조직강은 일정 분율 이상(약 50 vol% 이상)의 페라이트를 유지하는 것이 중요한데, 이를 위해서 실리콘, 망간 등을 첨가하며, 미량 첨가원소인 나이오븀(Nb)과 타이타늄(Ti)을 첨가하여 결정립을 미세하게 하는 것이 중요하다. 또한 우수한 용접성을 얻기 위해 탄소당량을 0.36이하로 유지하는 것이 중요하다.
It is important to control the cooling conditions after hot rolling in order to obtain excellent elongation along with the strength. In order to obtain ferrite through air cooling during cooling, it is necessary to optimize cooling patterns and optimize components. It is important to maintain ferrite with a certain fraction or more (about 50 vol% or more) in composite steel. For this purpose, silicon, manganese, etc. are added. It is important to make it fine. It is also important to keep the carbon equivalent below 0.36 in order to obtain good weldability.

본 발명에서는 용접성이 우수하고 780MPa 이상의 인장강도와 20% 이상의 연신율을 가지는 고강도 열연복합조직강을 개발하고자 하였다. 우수한 용접성을 얻기 위해 탄소당량을 0.36이하를 만족하는 성분계를 바탕으로 강의 주요성분인 탄소, 망간, 실리콘 등의 성분을 제어하고, 결정립미세화를 위해 나이오븀과 타이타늄을 첨가하였으며, 적정 제조조건에 따른 재질 특성을 평가하였다.
In the present invention, to develop a high strength hot-rolled composite steel having excellent weldability and having a tensile strength of 780MPa or more and an elongation of 20% or more. Based on the component system satisfying the carbon equivalent of 0.36 or less in order to obtain excellent weldability, the carbon, manganese, silicon, etc., the main components of steel, were controlled, and niobium and titanium were added for grain refinement. Material properties were evaluated.

상기한 목적을 달성하기 위한 본 발명은 중량%로, C: 0.05~0.13%, Si: 0.3~1.2%, Mn: 1.0~1.7%, S: 0.005% 이하, P: 0.03% 이하, Al: 0.05% 이하, Ti: 0.01~0.06%, Nb: 0.01~0.06%을 포함하여 이루어지는 강슬라브를 1150~1250℃의 범위에서 60~180분동안 재가열하고, Ar3+60℃ 이상에서 마무리열간압연한 다음 수냉하여 650~700℃범위에서 4~8초간 공냉하고, 이어 다시 수냉하여 250~350℃에서 수냉을 종료하는 것을 포함하여 이루어는 용접성이 우수한 780MPa급 고강도 열연강판의 제조방법에 관한 것이다.
The present invention for achieving the above object by weight, C: 0.05 ~ 0.13%, Si: 0.3 ~ 1.2%, Mn: 1.0 ~ 1.7%, S: 0.005% or less, P: 0.03% or less, Al: 0.05 Reheat the steel slab comprising% or less, Ti: 0.01 ~ 0.06%, Nb: 0.01 ~ 0.06% for 60 ~ 180 minutes in the range of 1150 ~ 1250 ℃, and finish hot rolling over Ar 3 + 60 ℃ The present invention relates to a method for producing a high strength hot rolled steel sheet having excellent weldability, including cooling water for 4 to 8 seconds in a range of 650 to 700 ° C., followed by water cooling to terminate water cooling at 250 to 350 ° C.

이하 본 발명의 화학성분 및 제조조건에 대하여 상세히 설명한다.Hereinafter, the chemical components and the preparation conditions of the present invention will be described in detail.

탄소[C]: 0.5-0.15 wt%Carbon [C]: 0.5-0.15 wt%

강 중 탄소[C]는 철강재료에서 가장 중요한 성분으로 강도는 물론 인성, 내식성 등의 모든 물리적, 화학적 특성과 밀접한 관계를 가지며, 강의 특성에 가장 큰 영향을 미치는 성분이다. 특히 탄소의 양이 너무 적으면 제2상의 분율이 감소하여 강도가 감소하므로 최소 0.5 wt% 이상으로 하였다. 과도하게 첨가되는 경우, 용접성 저하, 제 2상 분율의 급격한 증가로 인한 가공성의 격감하는 등의 단점이 있으며, 최대 0.15 이하로 제한하였다.
Carbon [C] in steel is the most important component in steel materials. It is closely related to all physical and chemical properties such as strength, toughness and corrosion resistance, and is the most important component in steel. In particular, if the amount of carbon is too small, the fraction of the second phase decreases and the strength decreases, so that the amount is at least 0.5 wt%. When excessively added, there are disadvantages such as deterioration of weldability, deterioration of workability due to a sharp increase in the second phase fraction, and the like is limited to a maximum of 0.15 or less.

실리콘[Si]: 0.3-1.2 wt%Silicon [Si]: 0.3-1.2 wt%

실리콘은 페라이트에 고용되는 페라이트 안정화 원소로 강도에 기여하며, 탈 산제로 첨가되는 경우가 일반적이다. 실리콘은 복합조직강에서 냉각시 오스테나이트-페라이트 변태를 촉진시켜 페라이트 분율을 증가시키는 원소이다. 실리콘이 0.3 wt% 이하인 경우 페라이트 강도가 감소하고, 탄화물 억제 효과가 감소하므로 0.3 wt% 이상을 첨가하여야 한다. 또한 용강의 유동성을 증대시키는 원소로 망간이 다량 첨가된 강에서 MnS 개재물의 부상 분리에 효과적이고, 망간/실리콘 비가 5-30인 범위에서 프래시버트 용접성을 개선시키지만, 열연 스캐일을 유발시킬 뿐 아니라, 가장 중요하게는 용접성이 열화되는 문제점이 있으므로 최대 1.2 wt% 이하로 성분을 제한하는 것이 바람직하다.
Silicon is a ferrite stabilizing element which is dissolved in ferrite and contributes to strength, and is usually added as a deoxidizer. Silicon is an element that increases the ferrite fraction by promoting austenite-ferrite transformation upon cooling in composite steel. If the silicon is 0.3 wt% or less, the ferrite strength is reduced and the carbide inhibiting effect is decreased, so 0.3 wt% or more should be added. It is also effective in the flotation separation of MnS inclusions in steels with a large amount of manganese as an element that increases the flowability of molten steel, and improves the weld weld weldability in the range of 5-30 manganese / silicon ratio, but also causes hot rolled scales. Most importantly, the weldability is deteriorated, so it is desirable to limit the component to a maximum of 1.2 wt% or less.

망간[Mn]: 1.0~1.7wt%Manganese [Mn]: 1.0-1.7 wt%

망간은 에시큘라(acicular) 페라이트 및 베이나이트와 같은 저온 변태상 형성을 용이하게 하는, 즉 경화능을 크게 하여 강도를 증가시키는 원소로 오오스테나이트 안정화 원소이다. 탄소와 실리콘이 0.7-0.13 wt%와 0.5-1.0 wt% 범위로 첨가되는 경우 780MPa 이상의 강도를 가지기 위해서는 최소한 1.0 wt% 이상 첨가되는 것이 바람직하다. 과도하게 많이 첨가되는 경우 용접성이 저하되고, 개재물 형성에 의해 수소유기 취성을 야기하며, 열간압연시 판재 중앙에 편석대를 형성하므로 최대 1.7 wt% 이하로 제한한다.
Manganese is an austenite stabilizing element that facilitates the formation of low-temperature transformation phases such as acicular ferrite and bainite, that is, increases hardenability and increases strength. When carbon and silicon are added in the range of 0.7-0.13 wt% and 0.5-1.0 wt%, it is preferable to add at least 1.0 wt% to have a strength of 780 MPa or more. When excessively added, weldability is degraded, hydrogen organic embrittlement is caused by inclusion formation, and a segregation zone is formed at the center of the sheet during hot rolling, so it is limited to a maximum of 1.7 wt% or less.

니오늄[Nb]: 0.01~0.06 wt%Nionium [Nb]: 0.01 to 0.06 wt%

일반적으로 니오늄은 열간압연시 오스테나이트에 고용되거나 석출상을 형성하 여 오스테나이트의 결정립을 줄여 페라이트 결정립을 미세하는 원소이다. 고용 및 석출 효과를 위해 최소 0.01 wt% 이상 첨가되는 것이 바람직하며, 과도하게 많이 첨가되는 경우 연속주조시 크랙을 유발하고, 제조 원가를 상승시키므로 최대 0.06 wt% 이하로 제한된다.
In general, nionium is an element that refines ferrite grains by reducing the grains of austenite by solid solution of austenite or forming a precipitated phase during hot rolling. It is preferable to add at least 0.01 wt% or more for the solid solution and precipitation effect, and when excessively added, it causes a crack during continuous casting and increases the manufacturing cost, so it is limited to a maximum of 0.06 wt% or less.

타이타늄[Ti]: 0.01~0.06 wt%Titanium [Ti]: 0.01 ~ 0.06 wt%

일반적으로 타이타늄은 나이오븀과 같이 오스테나이트에 고용되거나 석출상을 형성하여 오스테나이트의 결정립을 줄여 페라이트 결정립을 미세하는 원소이다. 고용 및 석출 효과를 위해 최소 0.01 wt% 이상 첨가되는 것이 바람직하며, 과도하게 많이 첨가되는 경우 원가가 상승하고, 재가열온도를 과도하게 증가시켜야 하므로 최대 0.06 wt% 이하로 제한된다.
In general, titanium is an element that solidifies ferrite grains by reducing the grains of austenite by dissolving in austenite or forming a precipitated phase, such as niobium. It is preferable to add at least 0.01 wt% or more for the solid solution and precipitation effect, and if it is added too much, the cost is increased and the reheating temperature must be excessively increased.

황[S]:0.005 wt% 이하Sulfur [S]: 0.005 wt% or less

황[S]이 0.005 wt% 초과이면 열연판에 조대한 TiS와 MnS가 생성되어 가공성과 인성을 저하시키므로 가능한 줄이는 것이 효과적이다.If sulfur [S] is more than 0.005 wt%, coarse TiS and MnS are formed in the hot-rolled sheet, which degrades the workability and toughness, so it is effective to reduce it as much as possible.

이상의 성분에 철강에 일반적으로 포함되는 불순물로 0.03wt% 이하의 인, 탈산제로 첨가되는0.05wt% 이하의 알루미늄, 0.006 wt%이하의 질소를 함유한다. The above components generally contain not more than 0.03 wt% of phosphorus and not more than 0.05 wt% of aluminum added as deoxidizer, and not more than 0.006 wt% of nitrogen.

합금설계시 탄소당량 계산을 위해 사용한 식은 아래와 같다. 대개 우수한 용접성을 갖기 위해 탄소당량을 0.36이하로 제한하는 것이 바람직하다.The formula used to calculate the carbon equivalent in the alloy design is as follows. It is usually desirable to limit the carbon equivalent to 0.36 or less in order to have good weldability.

Ceq = C + Mn/6 + (Cr+Mo+V)/3 + (Cu+Ni)/152 Ceq = C + Mn / 6 + (Cr + Mo + V) / 3 + (Cu + Ni) / 152                     

상기와 같이 조성되는 슬라브는 제강공정을 통해 용강을 얻은 다음에 주괴 또는 연속주조공정을 통해 만든다. 이 슬라브를 가열로에서 주조시 편석된 성분을 재고용하는 공정과, 판재의 두께를 제어하는 열간 압연공정, 재질을 확보하고, 상온으로 판재를 냉각하는 냉각공정 및 권취공정으로 제조하는데, 아래 각 공정별 제조조건을 구체적으로 설명한다.
The slabs formed as described above are obtained through ingot or continuous casting process after obtaining molten steel through steelmaking process. The slabs are manufactured by a process of reusing segregated components during casting in a heating furnace, a hot rolling process to control the thickness of the plate, a cooling process and a winding process of securing the material to room temperature, and the following processes. The manufacturing conditions for each star will be described in detail.

가열로 공정Furnace process

슬라브를 가열하는 공정은 주조시 편석된 성분을 재고용하기 위한 것으로, 재가열온도가 낮은 경우 편석된 성분이 재고용되지 못하며, 과도하게 높게 하면, 오스테나이트 결정입도가 증가하여 페라이트 입도가 조대화되면서 강도가 감소하게 된다. 따라서 본 강의 경우 1180~1220℃가 바람직하다.
The slab heating process is for reusing the segregated components during casting. If the reheating temperature is low, the segregated components cannot be reclaimed. When the slab is excessively high, the austenite grain size increases and the ferrite grain size is coarsened to increase the strength. Will decrease. Therefore, 1180-1220 degreeC is preferable in this steel.

열간 압연 공정Hot rolling process

상기와 같이 제조되는 슬라브를 Ar3+60℃ 이상에서 마무리 압연하는데, Ar3 직상에서 마무리압연하면 압연롤의 부하가 크게 증가하여 작업의 어려움이 증가하며, Ar3 이하에서 압연하면 열간 압연 중에 형성된 페라이트 내에 많은 전위가 도입되고 이러한 페라이트가 냉각 혹은 권취중에 성장하여 표면 조대립이 형성된다. 그러나 과도하게 높은 온도에서 마무리 압연하는 경우 페라이트 결정입도가 증가하여 강도가 감소하므로 Ar3+60℃ 이상이 적당하다. To roll finish the slab to be produced as described above in more than Ar 3 + 60 ℃, if the finish rolling at the Ar 3 immediately above, and the difficulty of the task increases the rolling roll load increased significantly, when rolling at Ar 3 or less is formed during hot rolling Many dislocations are introduced into the ferrite and these ferrites grow during cooling or winding to form surface coarse grains. However, when the finish rolling at an excessively high temperature, the ferrite grain size is increased to decrease the strength, Ar 3 + 60 ℃ or more is suitable.

냉각 및 권취공정Cooling and Winding Process

상기 열연판은 강도와 연신율이 동시에 확보되어야 하므로, 강도는 베이나이트와 마르텐사이트의 분율로 제어하고, 연신율은 주로 페라이트 분율로 제어하는 것이 바람직하다. 약 50 부피분율의 페라이트를 얻기 위해 압연시 수냉하고 공냉구간을 확보하여 페라이트 분율을 얻고, 다시 수냉으로 급냉하였다. 일정분율 이상의 베이나이트를 얻기 위해 연속냉각곡선상에서 베이나이트 변태 구간을 통과하고, 냉각종료시의 온도를 250~350℃로 하여 마르텐사이트를 얻고자하였다. 냉각종료온도가 250℃ 미만이면 과도하게 많은 마르텐사이트가 형성되어 강도는 높으나 연신율이 감소하고, 350℃을 초과이면 탄화물이 너무 조대하게 되어 연신율은 높으나 마르텐사이트 분율이 감소하여 강도가 낮게 된다.
Since the hot rolled sheet should be secured simultaneously with the strength and elongation, the strength is preferably controlled by the fraction of bainite and martensite, and the elongation is mainly controlled by the ferrite fraction. In order to obtain about 50 volume fractions of ferrite, water was cooled during rolling and an air cooling section was secured to obtain a ferrite fraction, followed by quenching with water cooling. In order to obtain more than a certain percentage of bainite, it was passed through a bainite transformation section on a continuous cooling curve, and the martensite was obtained by setting the temperature at the end of cooling to 250 to 350 ° C. If the cooling end temperature is less than 250 ℃ excessive martensite is formed is high strength, but elongation is reduced, if it exceeds 350 ℃ carbide is too coarse and elongation is high, but the martensite fraction is reduced and the strength is low.

이하 실시예를 통하여 본 발명을 보다 구체적으로 설명한다.The present invention will be described in more detail with reference to the following Examples.

[실시예][Example]

표 1에 발명강의 화학성분을 나타내었다.
Table 1 shows the chemical composition of the inventive steel.

구분division CC MnMn SiSi PP SS Sol.AlSol.Al NN NbNb TiTi CeqCeq AA 0.10.1 1.51.5 0.500.50 0.010.01 0.0030.003 0.030.03 0.0050.005 -- -- 0.350.35 BB 0.10.1 1.51.5 0.800.80 0.010.01 0.0030.003 0.030.03 0.0050.005 -- -- 0.350.35 CC 0.10.1 1.51.5 1.001.00 0.010.01 0.0030.003 0.030.03 0.0050.005 -- -- 0.350.35 DD 0.10.1 1.51.5 1.001.00 0.010.01 0.0030.003 0.030.03 0.0050.005 0.0400.040 -- 0.350.35 EE 0.10.1 1.51.5 1.001.00 0.010.01 0.0030.003 0.030.03 0.0050.005 0.0400.040 0.040.04 0.350.35 FF 0.10.1 1.51.5 1.001.00 0.010.01 0.0030.003 0.030.03 0.0050.005 0.0400.040 0.080.08 0.350.35

표 1 조성의 잉곳을 1200℃에서 1시간 가열하여 860℃에서 마무리 열간 압연한 다음 670℃로 급냉하여 약 6초 정도 유지하여 공냉하고 다시 수냉하였다. 수냉 후 냉각종료온도를 제어하여 제조된 시편을 이용하여 강도와 연신율을 측정하였다. 이를 정리하면 표 2와 같다.
The ingot of the composition of Table 1 was heated at 1200 ° C. for 1 hour, hot rolled at 860 ° C. for finishing, then quenched to 670 ° C. for about 6 seconds, air-cooled, and water cooled again. After cooling, the strength and elongation were measured using a specimen prepared by controlling the cooling end temperature. This is summarized in Table 2.

강종Steel grade 냉각종료온도(℃)Cooling end temperature (℃) YS
(MPa)YS
(MPa) TS
(MPa)TS
(MPa) Uni.-El.
(%)Uni.-El.
(%) Tot.-El.
(%)Tot.-El.
(%) YR
(%)YR
(%) TS*El
(MPa*%)TS * El
(MPa *%) 비고Remarks AA 250250 445.6445.6 692.4692.4 14.014.0 23.223.2 64.464.4 16035.516035.5 비교예Comparative example 350350 468.0468.0 661.2661.2 13.213.2 23.423.4 70.870.8 15448.315448.3 비교예Comparative example BB 250250 454.7454.7 692.2692.2 15.715.7 25.425.4 65.765.7 17567.917567.9 비교예Comparative example 350350 527.7527.7 670.1670.1 14.214.2 25.225.2 78.778.7 16920.316920.3 비교예Comparative example CC 250250 537.4537.4 679.6679.6 17.717.7 26.826.8 79.179.1 18220.618220.6 비교예Comparative example 350350 512.0512.0 690.4690.4 15.315.3 23.323.3 74.274.2 16066.116066.1 비교예Comparative example DD 250250 624.5624.5 815.2815.2 12.712.7 19.219.2 76.676.6 15667.515667.5 비교예Comparative example 350350 687.4687.4 749.7749.7 13.413.4 21.421.4 91.791.7 16041.016041.0 발명예Inventive Example EE 250250 693.4693.4 860.5860.5 14.614.6 23.923.9 80.680.6 20579.520579.5 발명예Inventive Example 350350 742.0742.0 803.3803.3 14.214.2 22.422.4 92.492.4 17969.717969.7 발명예Inventive Example FF 250250 718.2718.2 869.2869.2 11.711.7 18.618.6 82.682.6 16155.316155.3 비교예Comparative example 350350 767.2767.2 838.4838.4 11.811.8 19.219.2 91.591.5 16083.616083.6 비교예Comparative example

표 2를 보면, A, B, C강의 경우 Si이 과소 첨가되어 강도가 낮으며, D, E, F강의 경우 본 발명에서 추구하는780MPa이상의 강도를 가진다. 재료의 기계적 특성, 즉 인장특성은 미세조직에 의존하며 미세조직은 성분과 제조조건에 따라 달라진다. 그러나 Ti가 과도하게 첨가된 F강의 경우 제2상의 분율이 과도하게 증가하여 강도는 높으나 연신율이 낮다.As shown in Table 2, in the case of A, B, and C steels, Si is under-added and the strength is low, and in the case of D, E, and F steels, the steel has a strength of 780 MPa or more. The mechanical properties of the material, ie the tensile properties, depend on the microstructure and the microstructure depends on the composition and the manufacturing conditions. However, in the case of the F steel in which Ti is excessively added, the fraction of the second phase is excessively increased, so that the strength is high but the elongation is low.

D강의 경우 권취온도가 낮아지면 마르텐사이트 분율이 과도하게 증가하여 연신율이 낮다. E강은 적정량의 타이타늄 첨가로 권취온도가 낮아도 페라이트 입도가 미세하여 연신율이 우수하다. 한편 F강은 제2상의 분율이 증가하여 강도는 높으나 연신율이 낮다.In the case of steel D, when the coiling temperature is lowered, the martensite fraction is excessively increased and the elongation is low. E steel has excellent elongation due to its fine ferrite grain size even when the winding temperature is low due to the addition of an appropriate amount of titanium. F steel, on the other hand, increases the fraction of the second phase, resulting in high strength but low elongation.

따라서 그림 1, 2를 보면, D강의 경우 350℃에서 권취한경우, E강의 경우 250℃, 350℃에서 권취한 경우 우수한 특성을 가짐을 알 수 있다.
Therefore, in Figures 1 and 2, it can be seen that in the case of coiling at 350 ° C. for D steel, and winding at 250 ° C. and 350 ° C. for E steel, it has excellent characteristics.

자동차용 재료의 경우 대부분 용접에 의해 제조되므로 우수한 용접성과 프레스 성형에 필요한 연신율을 가져야 한다. 페라이트 기지조직과 더불어 2상으로 베이나이트와 마르텐사이트를 가지는 복합조직강의 경우, 이상조직강, 변태유기소성강과 같이 많은 성분을 함유하지 않으므로 용접성이 우수하고, 높은 강도와 상대적으로 높은 연신을 가진다. 일반적으로 780MPa 이상의 고강도 강의 경우 가공성을 위해 20% 이상의 연신율이 필요하고, 용접성 확보를 위해 탄소당량을 0.36 이하로 유지하여야 한다.Since most automotive materials are manufactured by welding, they must have good weldability and elongation required for press molding. In the case of composite tissue steel having bainite and martensite in two phases together with ferrite matrix, it does not contain many components such as abnormal tissue steel and metamorphic organic plastic steel, so it has excellent weldability, high strength and relatively high elongation. In general, for high strength steel of 780MPa or more, elongation of 20% or more is required for workability, and carbon equivalent should be kept below 0.36 to secure weldability.

Claims (2)

중량%로, C: 0.05~0.13%, Si: 0.3~1.2%, Mn: 1.0~1.7%, S: 0.005% 이하(0은 제외), P: 0.03% 이하(0은 제외), Al: 0.05% 이하(0은 제외), Ti: 0.01~0.04%, Nb: 0.01~0.06%을 포함하여 이루어지는 강슬라브를 1150~1250℃의 범위에서 60~180분동안 재가열하고, Ar3+60℃ 이상에서 마무리열간압연한 다음 수냉하여 650~700℃범위에서 4~8초간 공냉하고, 이어 다시 수냉하여 250~350℃에서 수냉을 종료하는 것을 포함하여 이루어지는 용접성이 우수한 인장강도 780MPa급 고강도 열연강판의 제조방법.By weight%, C: 0.05-0.13%, Si: 0.3-1.2%, Mn: 1.0-1.7%, S: 0.005% or less (excluding 0), P: 0.03% or less (excluding 0), Al: 0.05 Reheat the steel slab comprising% or less (excluding 0), Ti: 0.01 to 0.04%, and Nb: 0.01 to 0.06% for 60 to 180 minutes in the range of 1150 to 1250 ° C, and at Ar 3 + 60 ° C or higher. The method of manufacturing a high-strength hot-rolled steel sheet having high tensile strength 780MPa grade, including finishing hot rolling, followed by water cooling, air cooling for 4-8 seconds in the range of 650 to 700 ° C., followed by water cooling to terminate water cooling at 250 to 350 ° C. . 제 1항에 있어서, 상기 열연강판은 탄소당량이 0.36 이하인 것을 특징으로 하는 용접성이 우수한 인장강도 780MPa급 고강도 열연강판의 제조방법.The method of claim 1, wherein the hot rolled steel sheet has a carbon equivalent of 0.36 or less.
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JPH10298645A (en) 1997-04-24 1998-11-10 Sumitomo Metal Ind Ltd Manufacture of hot rolled high tensile strength steel plate
KR100340580B1 (en) * 1999-12-27 2002-06-12 이구택 A TENSILE STRENGTH OF 780MPa GRDE HOT ROLLED STEEL SHEET FOR AUTOMOBILE AND A METHOD FOR MANUFACTURING IT

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH10298645A (en) 1997-04-24 1998-11-10 Sumitomo Metal Ind Ltd Manufacture of hot rolled high tensile strength steel plate
KR100340580B1 (en) * 1999-12-27 2002-06-12 이구택 A TENSILE STRENGTH OF 780MPa GRDE HOT ROLLED STEEL SHEET FOR AUTOMOBILE AND A METHOD FOR MANUFACTURING IT

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