KR20000043762A - Method of manufacturing super high-strength cold-rolled steel sheet improved in ductility - Google Patents
Method of manufacturing super high-strength cold-rolled steel sheet improved in ductility Download PDFInfo
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- KR20000043762A KR20000043762A KR1019980060183A KR19980060183A KR20000043762A KR 20000043762 A KR20000043762 A KR 20000043762A KR 1019980060183 A KR1019980060183 A KR 1019980060183A KR 19980060183 A KR19980060183 A KR 19980060183A KR 20000043762 A KR20000043762 A KR 20000043762A
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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
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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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
- C21D8/0221—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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- 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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
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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
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0273—Final recrystallisation annealing
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- 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
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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Abstract
Description
본 발명은 자동차의 안전성을 향상시키기 위하여 사용되고 있는 충격보강재나 범퍼 소재용도의 인장강도 80-120kg/mm2급 초고강도 냉연강판에 있어서 열처리 방법을 개선함으로써 기존의 개발 강종에 비하여 연성을 향상시키는 초고강도 냉연강판의 제조방법에 관한 것이다.The present invention is to improve the ductility compared to the existing developed steel grade by improving the heat treatment method in the 80 ~ 120kg / mm 2 grade super high strength cold rolled steel tensile strength of impact reinforcement or bumper material used to improve the safety of automobiles It relates to a method for producing a high strength cold rolled steel sheet.
자동차용 초고강도 냉연강판의 종래의 열처리 방법으로는 회복소둔법, 급냉소입법, 연속소둔에 의한 오스템퍼링법(Austempering) 그리고 역변태 상소둔법 등이 있다. 먼저 회복소둔법은 냉간압연에 의한 가공경화를 이용하기 위하여 열처리 온도를 재결정 온도 이하로 하여 압연조직을 회복시키는 방법으로 강도확보를 위한 합금첨가량이 적기 때문에 용접성이 우수하다는 장점이 있으나 연성이 10% 이하로 비교적 낮기 때문에 프레스 가공이 불가능한 단점이 있다. 한편 급냉소입법은 Al 변태점 이상으로 가열할 때 얻어진 오스테나이트를 급속냉각하여 마르텐사이트나 베이나이트 등 강도가 높은 저온조직으로 변태시킴으로써 인장강도를 증가시키는 방법으로 가열온도와 냉각방법에 따라 저온조직의 종류 및 양이 변화하고 그에 따라 강도와 연성이 영향을 받는데 일반적으로 강도가 증가하면 연성은 반대로 감소하는 경향을 보인다. 이러한 급냉소입강은 회복소둔강에 비하여 연성이 우수한 반면 저온조직의 체적분율에 의하여 재질이 좌우되기 때문에 체적분율에 영향을 주는 성분 및 소둔온도의 엄격한 제어가 필요하다. 통상 이 방법에 의하면 소둔온도가 높을수록 소둔 가열직후 오스테나이트 양이 증가하기 때문에 냉각후 저온조직의 양이 증가하고 그 결과 강도가 상승한다. 또한 고온에서 형성된 오스테나이트를 냉각 과정에서 저온조직으로 변태시키기 위해서는 일정 속도 이상의 냉각이 필요하고 이 때 냉각속도를 임계 냉각속도라고 하는데 Mn, Cr, Mo등 합금원소를 첨가하거나 오스테나이트 결정립도를 조대하게 하면 임계 냉각속도가 감소하여 열처리시 비교적 느린 냉각속도에서도 이 강종을 제조할 수 있다. 급냉소입강의 경우 서냉하여 얻은 강이 급냉한 강에 비하여 연성이 우수한 반면 장출연신성과 용접성이 낮아지는 경향이 있다. 최근에는 초고강도 냉연강판의 연성을 크게 개선시키기 위하여 강중에 다량의 오스테나이트를 잔류시키는 열처리 방법이 개발되고 있는데 그 방법으로는 연속소둔에 의한 오스템퍼링법(Austempering)과 역변태 상소둔법 등이 있다. 잔류 오스테나이트는 상온에서 가공시 마르텐사이트로 변태(소성유기변태라고 함)하고 이 때 강도와 연성이 동시에 상승하는 현상(변태유기소성이라고 함)을 보이는데 이 때문에 이런 강을 소성유기변태강(TRIP강, Transformation Induced Plasticity가의 약어)이라고 한다. 소성유기변태강은 종래의 급냉소입강에 비하여 동일강도 대비 연신율이 높고 굽힘성이 우수한 특징을 나타내고 있다. 연속소둔에 의한 오스템퍼링법은 저탄소강에 Si 1.0-2.0%, Mn 1% 이상 다량 첨가하여 소둔 후 냉각과정에서 베이나이트 변태온도로 일정시간 유지시킴으로써 강중 탄소를 오스테나이트로 농화시키는 방법이다. 이 때 강중에 존재하는 Si가 철탄화물 형성을 억제함으로써 오스테나이트로 탄소가 농화되는 현상을 촉진시키는 역할을 한다(CAMP-ISIJ vol.1.p.877). 그러나 이 방법으로 제조된 강종의 인장강도는 주로 탄소함량에 의존하고 90kg/mm2이상의 강도를 확보하기 위해서는 탄소함량을 0.2% 이상으로 높여야 한다. 따라서 C, Si를 다량 첨가하기 때문에 프래쉬버트(Flash butt) 용접성이 저하하고 아연도금이 불가능한 단점이 있다. 한편 역변태 상소둔법은 오스테나이트 안정화 원소인 Mn을 다량 첨가한 강을 이용하여 열연후 얻어진 마르텐사이트와 베이나이트 혼합조직을 냉연후 상소둔하여 전조직의 래스(lath) 경계에 오스테나이트를 형성시킨 다음 냉각후 상온에 잔류시키는 방법이다. 냉연강판을 역변태 상소둔법에 의하여 제조하는 방법은 열연판을 역변태 상소둔하는 것과 달리 냉간압연시 도입된 다량의 전위에 의해 고온소둔시에는 시 C, Mn 원소의 확산이 촉진되어 냉각시 오스테나이트의 불안정성이 증가하고 그 결과 강도는 상승하지만 연성이 상대적으로 저하하고 저온소둔시에는 강도는 감소하고 연성은 30% 이하로 낮은 문제점이 있다.Conventional heat treatment methods for the ultra-high strength cold rolled steel sheet for automobiles include recovery annealing, rapid quenching, continuous annealing, austempering and reverse transformation annealing. First, recovery annealing is a method of recovering a rolled structure by using a heat treatment temperature below the recrystallization temperature in order to use the work hardening by cold rolling, and has an advantage of excellent weldability because the alloying amount for securing strength is small. Since it is relatively low below, there is a disadvantage that press working is impossible. Rapid quenching is a method of increasing the tensile strength by rapidly cooling austenite obtained by heating above the Al transformation point and transforming it into a high-strength low temperature structure such as martensite or bainite. Kinds and amounts change and strength and ductility are affected accordingly. In general, as strength increases, ductility tends to decrease. Such quenched steel has superior ductility compared to recovery annealed steel, but since the material depends on the volume fraction of the low temperature structure, strict control of the components and the annealing temperature is required. In general, according to this method, as the annealing temperature increases, the amount of austenite immediately after the annealing heating increases, so that the amount of the low-temperature structure after cooling increases, and as a result, the strength increases. In addition, in order to transform the austenite formed at a high temperature into a low temperature structure in the cooling process, it is required to cool more than a certain speed. At this time, the cooling rate is called a critical cooling rate. An alloy element such as Mn, Cr, Mo, or the like is added to the austenite grain size The lower critical cooling rate can be used to produce these steels at relatively slow cooling rates during heat treatment. In the case of quenched quenched steel, steel obtained by slow cooling tends to have lower ductility and weldability than quenched steel. Recently, in order to greatly improve the ductility of ultra-high strength cold rolled steel sheet, a heat treatment method for retaining a large amount of austenite in steel has been developed. The methods include austempering by continuous annealing and reverse transformation annealing. . Residual austenite transforms into martensite (called plastic organic transformation) when processed at room temperature, and at the same time shows strength and ductility to rise (called metamorphic organic plasticity). Transformation Induced Plasticity). Plastic organic transformation steel is characterized by higher elongation compared to the same strength and excellent bendability compared to conventional quenched steel. Osteotempering by continuous annealing is a method of thickening carbon in austenite by adding a large amount of Si 1.0-2.0%, Mn 1% or more to low carbon steel and maintaining it at a bainite transformation temperature for a certain time during cooling after annealing. At this time, Si present in the steel suppresses the formation of iron carbide, thereby promoting the phenomenon of carbon enrichment with austenite (CAMP-ISIJ vol. 1.p.877). However, the tensile strength of the steel produced by this method depends mainly on the carbon content, and the carbon content should be increased to 0.2% or more in order to secure the strength of 90kg / mm 2 or more. Therefore, since a large amount of C and Si are added, the flash butt weldability is lowered and zinc plating is impossible. On the other hand, inverse transformation annealing method uses a steel containing a large amount of Mn, an austenite stabilizing element, to form austenite on the lath boundary of the entire structure by cold annealing the martensite and bainite mixed structure obtained after hot rolling. It is the method of remaining at room temperature after cooling. The method of manufacturing the cold rolled steel sheet by reverse transformation annealing method is different from the reverse transformation annealing of the hot rolled steel sheet, which promotes diffusion of C and Mn elements during high temperature annealing due to the large amount of potential introduced during cold rolling. Instability of the knight increases and as a result the strength is increased, but the ductility is relatively low, the strength is reduced at low temperature annealing, the ductility is less than 30% has a problem.
본 발명은 이와같은 종래의 역변태 상소둔법에 의한 냉연강판 제조시 문제점을 해결하기 위하여 다량의 안정된 잔류 오스테나이트를 확보할 수 있는 열처리 방법에 관한 것으로 성분의 추가하지 않고 강도 및 연성이 우수한 초고강도 냉연강판의 제조방법을 제공하는 것을 목적으로 한다.The present invention relates to a heat treatment method that can secure a large amount of stable residual austenite in order to solve the problem in manufacturing a cold rolled steel sheet by the conventional reverse transformation phase annealing method, ultra-high strength excellent in strength and ductility without adding components It is an object to provide a method for producing a cold rolled steel sheet.
본 발명을 상기 목적을 달성하기 위하여, 중량%로 C: 0.05-0.2%, Si: 0.2-0.5% 이하, Mn: 4-7%, P: 0.01% 이하, S: 0.005% 이하, 산가용성 Al: 0.02-0.10% 의 기본조성에 Mo: 0.1-0.3% 이하, V: 0.1%이하, Nb: 0.1% 이하, Ti: 0.1% 이하의 조건으로 1 성분을 첨가하고 잔부 Fe 및 기타 불가피한 불순물로 이루어진 강을 통상적으로 열간압연하는 단계와; 500-600℃ 이하의 온도에서 권취하여 상온까지 냉각하는 단계와; 550-600℃에서 5-24시간 가열 후 노냉하는 단계와; 30-60%의 압하율로 냉간압연 하는 단계와; 600-650℃의 온도에서 30분 이상 24시간 이내로 가열하는 단계와; 분당 10-300℃의 속도로 냉각하는 단계와로 이루어진 연성이 향상된 초고강도 냉연강판의 제조방법을 제공하는 것을 특징으로 한다.In order to achieve the object of the present invention, by weight% C: 0.05-0.2%, Si: 0.2-0.5% or less, Mn: 4-7%, P: 0.01% or less, S: 0.005% or less, acid-soluble Al : 0.02-0.10% of basic composition, Mo: 0.1-0.3% or less, V: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less Typically hot rolling a steel; Winding at a temperature of 500-600 ° C. or lower to cool to room temperature; Furnace-cooling after 5-24 h heating at 550-600 ° C .; Cold rolling at a reduction ratio of 30-60%; Heating at a temperature of 600-650 ° C. for at least 30 minutes and within 24 hours; It characterized by providing a method for producing an ultra-high strength cold rolled steel sheet with improved ductility consisting of cooling at a rate of 10-300 ℃ per minute.
도 1은 본 발명강의 2차 열처리후 냉각속도와 기계적 성질과의 관계를 도시한 그래프도.1 is a graph showing the relationship between the cooling rate and the mechanical properties after the secondary heat treatment of the present invention steel.
이하, 본 발명을 더욱 상세히 설명하기로 한다.Hereinafter, the present invention will be described in more detail.
본 발명은 중량%로 C: 0.05-0.2%, Si: 0.2-0.5% 이하, Mn: 4-7%, P: 0.01% 이하, S: 0.005% 이하, 산가용성 Al: 0.02-0.10% 의 기본조성에 Mo: 0.1-0.3% 이하, V: 0.1%이하, Nb: 0.1% 이하, Ti: 0.1% 이하의 조건으로 1 성분을 첨가하고 잔부 Fe로 제조된 강을 통상적으로 열간압연후 500-600℃ 이하의 온도에서 권취하여 상온까지 냉각하고 550-600℃에서 5-24시간 가열 후 노냉한 다음 30-60%의 냉간압연 후 600-650℃의 온도에서 30분 이상 24시간 이내로 가열한 다음 분당 10-300℃의 속도로 냉각하는 것을 특징으로 하는 초고강도 냉연강판의 연성을 향상시키는 열처리 방법에 관한 것이다.The present invention is based on the weight% C: 0.05-0.2%, Si: 0.2-0.5% or less, Mn: 4-7%, P: 0.01% or less, S: 0.005% or less, acid-soluble Al: 0.02-0.10% Steels made of the balance Fe after adding one component under conditions of Mo: 0.1-0.3% or less, V: 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less are usually hot-rolled to 500-600 Wind up at room temperature or below, cool to room temperature, heat for 5 to 24 hours at 550-600 ° C, and then cool down. After 30-60% cold rolling, heat at a temperature of 600-650 ° C for more than 30 minutes or less within 24 hours. It relates to a heat treatment method for improving the ductility of ultra-high strength cold rolled steel sheet, characterized in that cooling at a rate of 10-300 ℃.
이하에는 본 발명의 화학성분 및 제조조건의 한정이유에 대하여 더욱 상세히 설명하기로 한다.Hereinafter will be described in more detail with respect to the reasons for the limitation of the chemical components and manufacturing conditions of the present invention.
강중 탄소는 열연후 래스조직을 가지는 마르텐사이트나 베이나이트의 형성과 역변태 상소둔시 형성되는 오스테나이트 양 및 안정화에 영향을 미치는데 탄소양이 0.05% 이하로 너무 낮으면 래스조직의 형성이 불안정하고 소둔 후 오스테나이트 안정성도 감소하게 되어 연성과 강도가 저하하고 반대로 탄소양이 0.2% 이상이 되면 냉간 압연하중이 증가하고 용접성이 저하되기 때문에 그 범위를 0.05-0.2%로 제한한다.Carbon in steel affects the formation of martensite or bainite having a lattice structure after hot rolling and the amount and stabilization of austenite formed during reverse transformation annealing. After the annealing, the austenite stability is also reduced, and the ductility and strength are lowered. On the contrary, when the carbon content is 0.2% or more, the cold rolling load increases and the weldability is reduced, so the range is limited to 0.05-0.2%.
강중 Si은 제강시 개재물의 부상분리를 원활하게 하고 또한 용접시 용접금속의 유동성 증가를 위하여 첨가하였으며 Si양이 0.2% 이하에서는 제강시 개재물 및 MnS 형성에 영향을 미칠 수 없고 0.5% 이상 첨가하면 열간압연성 및 내식성을 약화시킬 뿐 아니라 Mn/Si비가 낮아져 프래쉬버트 용접성이 열화되는 문제가 있으므로 적정범위를 0.2-0.5%로 한다.In steel, Si was added to facilitate floating separation of inclusions during steelmaking and to increase the fluidity of weld metal during welding.If Si content is less than 0.2%, it does not affect inclusions and MnS formation during steelmaking. In addition to weakening the rollability and corrosion resistance, the Mn / Si ratio is low, there is a problem of deterioration of the weld butter weldability, so the appropriate range is 0.2-0.5%.
Mn은 열연 권취 이후 냉각조건에서도 래스조직을 얻기 위하여 소입성을 증가시키는 효과와 역변태 상소둔시 래스조직에서 오스테나이트가 형성되는 온도범위를 확장하기 이하여 첨가하였다. 마르텐사이트를 얻기 위한 최소 냉각속도는 Mn당량(=Mn% + 0.45Si% + 2.67Mo%)에 의하여 log(임계 냉각속도, 단위 ℃/초) = 3.95 - 1.73 Mn 당량의 관계식으로 주어진다. 본 발명강의 경우 권취후 냉각속도가 0.005℃/초 (18℃/시간)이상이므로 최소한 Mn 당량으로 3.6%가 최소한 필요하고 한편 역변태 상소둔시 역변태 온도범위를 50℃ 이상으로 확보하기 위해서는 4% 이상의 Mn이 필요하다. 또한 Mn은 오스테나이트 안정화 원소이기 때문에 소둔시 형성된 오스테나이트를 잔류시키는데 매우 효과적인 원소이다. 그러나 Mn이 7% 이상이면 용접성이 악화되고 제강시 슬래그의 조성이 변화하여 내화물 침식이 증가하고 열간압연 전에 가열단계에서 강괴의 표면층 부근에서 입계에 Mn 산화물을 형성하여 열간압연 후 표면결함을 유발한다. 따라서 Mn은 4-6%로 한정하였다.Mn was added after the hot rolling to increase the hardenability to obtain the lattice structure even under cooling conditions and to extend the temperature range in which austenite is formed in the lattice structure during reverse transformation annealing. The minimum cooling rate for obtaining martensite is given by the relation of log (critical cooling rate, unit ° C / sec) = 3.95-1.73 Mn equivalent by Mn equivalent (= Mn% + 0.45Si% + 2.67Mo%). In the present invention, since the cooling rate after winding is more than 0.005 ° C / sec (18 ° C / hour), at least Mn equivalent is required at least 3.6%, and in order to secure the reverse transformation temperature range at 50 ° C or higher during reverse transformation annealing, 4 At least Mn is required. In addition, since Mn is an austenite stabilizing element, it is a very effective element for retaining austenite formed during annealing. However, if Mn is more than 7%, weldability is deteriorated and the composition of slag is changed during steelmaking to increase refractory erosion, and Mn oxide is formed at the grain boundary near the surface layer of the steel in the heating step before hot rolling, causing surface defects after hot rolling. . Therefore, Mn was limited to 4-6%.
강중 P와 S는 슬라브내 편석이 심하고 열간취성을 일으키는 원인이 되며 강재의 연성을 악화시키기 때문에 0.01%와 0.005% 이하로 제한한다.P and S in steel are limited to 0.01% and 0.005% or less because segregation in slab is severe and causes hot brittleness and worsens ductility of steel.
산가용성 Al은 용강중 탈산을 위하여 첨가되며 0.02% 이하에서는 탈산이 불완전하고 0.1% 이상에서는 Al에 의한 제강성 결함이 발생하거나 슬라브 균열의 원인이 되므로 제한범위를 0.02%-0.1%로 한다.Acid-soluble Al is added for deoxidation in molten steel, and deoxidation is incomplete at less than 0.02%, and steelmaking defects caused by Al may occur at 0.1% or more, or it may cause slab cracking. Therefore, the limit range is 0.02% -0.1%.
Mo은 소입성 및 강도 상승을 위하여 0.1% 이상 첨가하였으며 과도한 첨가시는 연성을 크게 저하시키기 때문에 0.3%로 제한한다.Mo is added at 0.1% or more to increase the hardenability and strength, and when added excessively, Mo is limited to 0.3% because it greatly reduces the ductility.
V, Nb, Ti은 열연조직의 미세화와 강도상승을 위하여 첨가하였으며 0.1% 이상 첨가시에는 강도의 과도한 상승으로 냉간압연의 부하가 증가하기 때문에 한정하였다.V, Nb, and Ti were added to refine the hot rolled structure and increase the strength. When 0.1% or more was added, the cold rolling load increased due to excessive increase in strength.
본 발명은 열간압연후 얻어진 래스 조직을 냉간압연 부하를 경감하기 위하여 1차 열처리를 통하여 템퍼드 마르테사이트와 잔류 오스테나이트의 래스 조직으로 만들고 다시 냉간압연을 실시한 후 2차 열처리시 가속냉각을 하여 다량의 안정된 잔류 오스테나이트를 형성시키는 것을 특징으로 하고 있다. 열간압연은 통상의 방법과 같이 슬라브 가열후 900℃ 이상에서 마무리 압연하고 살수방식(water spray)으로 냉각하여 권취하는데 권취온도가 500℃ 이하에서는 판형상이 나빠지고 강도가 상승하여 냉연시 작업성이 저하하고 600℃ 이상인 경우 밴드상의 베이나이트 조직이 조대하게 형성되어 소둔 조직을 불균일하게 함으로써 가공성을 저하시키기 때문에 권취온도는 500-600℃로 제한하였다. 열연판은 마르텐사이트와 베이나이트의 혼합조직으로 인장강도가 130kg/mm2이상으로 매우 높기 때문에 냉간압연의 부하가 급증하는 문제가 있다. 따라서 예비 열처리를 실시하여 강도를 90kg/mm2이하로 낮추면 냉간압연이 용이하므로 냉간압연전 1차 열처리를 실시하였다. 1차 열처리는 강도 하향이 목적이기 때문에 저온조직을 연화시키기 위해 소둔온도를 템퍼링(tempering) 온도 구간인 500-600℃로 하였으며 이 온도 범위에서는 조직이 템퍼드 마르텐사이트와 소량의 잔류 오스테나이트로 구성되어 강도가 저하하지만 550℃ 이하에서는 조온조직의 연화가 충분히 일어나지 않고 600℃ 이상에서는 잔류 오스테나이트의 양이 증가하여 압연시 균열이 발생하기 때문에 제한하였다. 또한 소둔시간은 길수록 저온조직의 연화에 유리하며 최소 5시간 이상 처리가 필요하고 24시간 이상 장시간 처리하는 경우에는 잔류 오스테나이트가 증가하므로 한정하였다. 열연판은 염산에서 산세후 냉간압하율은 30-60%로 하였으며 30% 이하에서는 냉간압연에 의한 두께 감소효과가 작고 60% 이상이면 압연부하가 증가하여 압연이 불가능하게 되기 때문에 제한하였다. 2차 소둔온도는 역변태에 의하여 래스 조직에서 오스테나이트가 형성되는 온도로 600℃이하에서는 오스테나이트 형성양이 적어 소둔 후 강도 및 연성 증대를 위한 다량의 오스테나이트를 얻기 어렵고 650℃ 이상에서는 냉간압연시 도입된 전위에 의하여 성분원소의 확산이 촉진되기 때문에 오스테나이트 안정성이 저하하여 다량의 오스테나이트를 얻기 어렵다. 따라서 소둔온도를 600-650℃로 한정하였으며 소둔시간은 소둔온도에서 평형상태를 얻기 위하여 필요한 시간으로 30분 이하에서는 오스테나이트의 핵생성 및 성장이 불완전하여 다량의 잔류 오스테나이트를 얻을 수 없고 24시간은 오스테나이트가 평형상태에 충분히 도달할 수 있기 때문에 그 이상 소둔하는 것은 경제적으로 비효율적이기 때문에 한정하였다. 한편 소둔후 냉각속도는 역변태에 의하여 형성된 오스테나이트의 안정성에 가장 중요한 변수로 냉연강판과 같이 압연에 의하여 도입된 전위가 존재하는 경우 냉연전의 래스조직이 파괴되고 그 결과 오스테나이트의 형태가 짧은 막대형상의 미세한 조직으로 변화하고 냉각시 C, Mn의 조직간 배분배가 촉진된다. 따라서 상소둔 후 냉각을 일정한 속도 이상으로 하여 C, Mn 의 재분배를 억제하지 않으면 다량의 안정된 잔류 오스테나이트는 연성에도 기여하지만 소성유기변태 후에는 경질의 조직인 마르텐사이트가 되기 때문에 강도 상승에도 매우 효과적이므로 소둔후 가속냉각처리는 다량의 안정된 잔류 오스테나이트를 확보할 수 있어서 노냉재에 비하여 항복강도는 낮아지고 인장강도와 연성은 증가하는 효과가 있다. 냉각속도가 분당 10℃ 이하에서는 C, Mn의 재분배가 일어나 오스테나이트의 안정성이 저하하고 분당 300℃ 이상에서는 판형상 및 불균일한 냉각속도에 의한 판형상 불량 및 다량의 냉각공기에 의한 표면산화가 일어나기 때문에 제한하였다.In the present invention, in order to reduce the cold rolling load, the lattice obtained after hot rolling is made into a lattice structure of tempered martensite and residual austenite through primary heat treatment, and then cold-rolled and accelerated cooling during secondary heat treatment. It is characterized by forming a stable residual austenite. Hot rolling is finished rolling at 900 ℃ or higher after slab heating and cooling by water spraying as usual, but the winding temperature is lower than 500 ℃ and the plate shape worsens and the strength increases, so the workability during cold rolling decreases. In the case of 600 ° C. or higher, the band-shaped bainite structure is coarsened, and thus the workability is reduced by making the annealing structure uneven. Thus, the winding temperature is limited to 500-600 ° C. Hot rolled plate is a mixed structure of martensite and bainite, so the tensile strength of 130kg / mm 2 or more is very high, there is a problem that the load of cold rolling is rapidly increased. Therefore, if the strength is lowered to 90kg / mm 2 or less by preliminary heat treatment, cold rolling is easy, and thus, primary heat treatment before cold rolling is performed. In order to soften the low temperature structure, the annealing temperature was set to 500-600 ° C, which is a tempering temperature range, in order to soften the low temperature structure. In this temperature range, the structure is composed of tempered martensite and a small amount of retained austenite. The strength was lowered, but the softening of the temperature structure was not sufficiently occurred at 550 ° C. or lower, and the amount of retained austenite was increased at 600 ° C. or higher, so that cracking occurred during rolling. In addition, the longer the annealing time, the more favorable the softening of the low-temperature tissue, the treatment is required at least 5 hours or more, and the long-term treatment over 24 hours was limited because the residual austenite increases. The hot rolling rate of the hot rolled plate was 30-60% after pickling in hydrochloric acid, and the thickness reduction effect was less than 30% due to the cold rolling. The secondary annealing temperature is the temperature at which austenite is formed in the lattice structure due to reverse transformation, and the amount of austenite formation is less than 600 ℃, so it is difficult to obtain a large amount of austenite for strength and ductility after annealing, and cold rolling above 650 ℃. Since the diffusion of the component elements is accelerated by the potential introduced during the test, the austenite stability is lowered, and a large amount of austenite is hardly obtained. Therefore, the annealing temperature is limited to 600-650 ℃, and the annealing time is the time required to obtain equilibrium at the annealing temperature. In 30 minutes or less, the nucleation and growth of austenite are incomplete, so that a large amount of retained austenite cannot be obtained. Annealing was limited because austenite could reach equilibrium sufficiently because further annealing was economically inefficient. On the other hand, the cooling rate after annealing is the most important variable for the stability of austenite formed by reverse transformation.If there is a potential introduced by rolling, such as cold rolled steel sheet, the lattice structure before cold rolling is destroyed and as a result, the rod of short austenite form It changes into a fine structure of the shape, and upon cooling, the distribution of C and Mn is promoted. Therefore, if the cooling is more than a certain speed after the annealing and the redistribution of C and Mn is not suppressed, a large amount of stable retained austenite contributes to the ductility, but after plastic organic transformation, it becomes very hard to increase the strength since it becomes a hard tissue martensite. Accelerated cooling after annealing can secure a large amount of stable retained austenite, resulting in lower yield strength and increased tensile strength and ductility compared to furnace coolant. When the cooling rate is 10 ℃ or less per minute, redistribution of C and Mn occurs, and the stability of austenite decreases. At 300 ℃ or more per minute, plate shape defects due to plate shape and non-uniform cooling rate and surface oxidation by a large amount of cooling air occur. It was limited.
이하 실시예를 통하여 본 발명을 구체적으로 설명한다.Hereinafter, the present invention will be described in detail through examples.
실시예Example
표 1에 나타낸 바와 같은 화학성분을 가지는 발명강(1번강-7번강)의 잉고트(ingot)를 제조하여 1200℃의 온도에서 60분간 가열한 뒤 압연 마무리 온도 950℃로 2.6mm 두께까지 압연하고 560℃까지 냉각 후 1시간 등은 유지 후 노냉하였다. 냉각된 열연판은 산세 후 580℃에서 12시간 유지 후 노냉한 다음 1.4mm로 냉간압연하여 얻은 냉연판을 표 2에 나타낸 조건으로 열처리하였다. 비교재는 발명강의 같은 성분을 가진 강을 열간압연후 역변태 처리 또는 열연판을 1차 열처리를 실시하지 않고 바로 냉간압연한 다음 역변태 처리 후 노냉하였다. 본 발명재의 1차 열처리 후 기계적 성질과 역변태 처리를 위한 열처리 조건 및 기계적 성질을 비교강과 같이 표 2에 나타내었으며 2차 열처리 후 냉각속도와 연신율 관계를 그림 1에 나타내었다. 표 2와 그림 1에서 알 수 있듯이 본 발명재는 동일 성분계에서 열연후 1차 열처리에 의하여 강도저하가 현저하고 비교재에 비해서 최종적인 기계적 성질이 항복강도는 7-8kg/mm2낮고 인장강도는 6-8kg/mm2상승하며 연신율도 6-12% 증가하는 우수한 특징을 보였다. 이러한 효과는 냉간압연에 의한 조직 미세화와 가속냉각에 따라 잔류 오스테나이트가 안정화되기 때문이다. 그러므로 본 발명재는 인장강도 90kg/mm2 이상을 가지면서 연성이 30% 이상으로 기존의 초고강도 냉연강판에 비하여 매우 우수하고 동시에 작업성이 현저하게 개선되는 효과가 있다.An ingot of the inventive steel (steel No. 1-7) having a chemical composition as shown in Table 1 was prepared, heated at a temperature of 1200 ° C. for 60 minutes, and then rolled to a thickness of 2.6 mm at a rolling finishing temperature of 950 ° C. and 560. After cooling to 1 ° C., the furnace was cooled after holding for 1 hour. After cooling, the hot rolled sheet was annealed at 580 ° C. for 12 hours, and then cooled. The cold rolled sheet obtained by cold rolling to 1.4 mm was heat-treated under the conditions shown in Table 2. The comparative material was cold-rolled immediately after hot rolling the steel having the same component of the inventive steel without performing reverse transformation treatment or hot-rolled sheet, and then cold-treated after reverse transformation treatment. The mechanical properties after the first heat treatment and the heat treatment conditions and mechanical properties for reverse transformation treatment of the present invention are shown in Table 2 as in comparison steel, and the relationship between the cooling rate and the elongation after the second heat treatment is shown in FIG. As can be seen from Table 2 and Figure 1, the present invention has a significant decrease in strength due to the first heat treatment after hot rolling in the same component system, and the final mechanical properties have a lower yield strength of 7-8 kg / mm 2 and a tensile strength of 6 than the comparative material. -8 kg / mm 2 rises, the elongation was also shown to increase by 6-12%. This effect is due to stabilization of the retained austenite due to the microstructure of the cold rolling and accelerated cooling. Therefore, the present invention has a tensile strength of 90kg / mm 2 or more and the ductility is more than 30% compared to the existing ultra-high strength cold rolled steel sheet is very excellent and at the same time workability is significantly improved.
본 발명은 종래의 역변태 상소둔에 의하여 소성유기변태강을 제조하는 방법에 비해 냉연전 1차 열처리를 통하여 냉간압연시 부하를 경감하므로써 보다 얇고 폭이 넓은 강판을 생산할 수 있는 이점이 있을 뿐 아니라 소둔 후 가속냉각에 의하여 연성이 향상되면서 동시에 강도 상승이 일어나기 때문에 합금원소 첨가를 저감할 수 있으며 또한 항복강도도 낮기 때문에 프레스 가공성이 개선되는 효과가 있다.The present invention has the advantage of producing a thinner and wider steel sheet by reducing the load during cold rolling through the primary heat treatment before cold rolling, compared to the conventional method for producing plastic organic transformation steel by reverse transformation annealing. Since the ductility improves at the same time by the accelerated cooling and the strength rises, the addition of alloying elements can be reduced, and the yield strength is also low, so that press workability is improved.
Claims (1)
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| Application Number | Priority Date | Filing Date | Title |
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| KR1019980060183A KR20000043762A (en) | 1998-12-29 | 1998-12-29 | Method of manufacturing super high-strength cold-rolled steel sheet improved in ductility |
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| KR1019980060183A KR20000043762A (en) | 1998-12-29 | 1998-12-29 | Method of manufacturing super high-strength cold-rolled steel sheet improved in ductility |
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Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
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| KR20020054531A (en) * | 2000-12-28 | 2002-07-08 | 이계안 | A process method of bumper back beam with high strength |
| KR100496565B1 (en) * | 2000-12-20 | 2005-06-23 | 주식회사 포스코 | The making method for the cold rolled high strength sheet steel with excellent ductility |
| KR100554754B1 (en) * | 2001-12-27 | 2006-02-24 | 주식회사 포스코 | Method for Manufacturing Cold-rolled Steel Sheets with Ultra High Strength |
| KR100748116B1 (en) * | 2001-06-29 | 2007-08-10 | 주식회사 포스코 | Annealing method for transformation Induced Plasticity of bainite |
| WO2009142362A1 (en) * | 2008-05-20 | 2009-11-26 | Posco | High strength steel sheet and hot dip galvanized steel sheet having high ductility and excellent delayed fracture resistance and method for manufacturing the same |
| US9194030B2 (en) | 2008-05-19 | 2015-11-24 | Posco | High strength thin steel sheet for the superior press formability and surface quality and galvanized steel sheet and method for manufacturing the same |
| US11203795B2 (en) | 2015-11-02 | 2021-12-21 | Posco | Ultra-high strength steel plate having excellent formability and hole-expandability, and method for manufacturing same |
| CN115605626A (en) * | 2020-07-24 | 2023-01-13 | 安赛乐米塔尔公司（Lu） | Cold rolled and annealed steel sheet and method for manufacturing same |
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| JPS55161028A (en) * | 1979-05-30 | 1980-12-15 | Kobe Steel Ltd | Manufacture of high strength cold rolled sheet having superior press moldability and weather resistance |
| KR890010262A (en) * | 1987-12-28 | 1989-08-07 | 정명식 | Ultra high strength cold rolled steel sheet and manufacturing method |
| KR19980045322A (en) * | 1996-12-10 | 1998-09-15 | 김종진 | Manufacturing method of super high strength cold rolled steel sheet with excellent workability |
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| JPS55161028A (en) * | 1979-05-30 | 1980-12-15 | Kobe Steel Ltd | Manufacture of high strength cold rolled sheet having superior press moldability and weather resistance |
| KR890010262A (en) * | 1987-12-28 | 1989-08-07 | 정명식 | Ultra high strength cold rolled steel sheet and manufacturing method |
| KR19980045322A (en) * | 1996-12-10 | 1998-09-15 | 김종진 | Manufacturing method of super high strength cold rolled steel sheet with excellent workability |
Cited By (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR100496565B1 (en) * | 2000-12-20 | 2005-06-23 | 주식회사 포스코 | The making method for the cold rolled high strength sheet steel with excellent ductility |
| KR20020054531A (en) * | 2000-12-28 | 2002-07-08 | 이계안 | A process method of bumper back beam with high strength |
| KR100748116B1 (en) * | 2001-06-29 | 2007-08-10 | 주식회사 포스코 | Annealing method for transformation Induced Plasticity of bainite |
| KR100554754B1 (en) * | 2001-12-27 | 2006-02-24 | 주식회사 포스코 | Method for Manufacturing Cold-rolled Steel Sheets with Ultra High Strength |
| US9194030B2 (en) | 2008-05-19 | 2015-11-24 | Posco | High strength thin steel sheet for the superior press formability and surface quality and galvanized steel sheet and method for manufacturing the same |
| US9598753B2 (en) | 2008-05-19 | 2017-03-21 | Posco | High strength thin steel sheet for the superior press formability and surface quality and galvanized steel sheet and method for manufacturing the same |
| WO2009142362A1 (en) * | 2008-05-20 | 2009-11-26 | Posco | High strength steel sheet and hot dip galvanized steel sheet having high ductility and excellent delayed fracture resistance and method for manufacturing the same |
| US9109273B2 (en) | 2008-05-20 | 2015-08-18 | Posco | High strength steel sheet and hot dip galvanized steel sheet having high ductility and excellent delayed fracture resistance and method for manufacturing the same |
| US11203795B2 (en) | 2015-11-02 | 2021-12-21 | Posco | Ultra-high strength steel plate having excellent formability and hole-expandability, and method for manufacturing same |
| CN115605626A (en) * | 2020-07-24 | 2023-01-13 | 安赛乐米塔尔公司（Lu） | Cold rolled and annealed steel sheet and method for manufacturing same |
| CN115605626B (en) * | 2020-07-24 | 2024-03-08 | 安赛乐米塔尔公司 | Cold-rolled and annealed steel sheet and method for manufacturing same |
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