• 열처리공학회지
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• 제10권2호
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• pp.128-137
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• 1997

The formation processes of the retained austenite(${\gamma}_R$) in SHCP100 steel sheets were investigated in order to improve the transformation induced plasticity(TRIP) effect of ${\gamma}_R$. An excellent combination of elongation about 23% and high strength over 830 MPa was achieved by processing of intercritical annealing and isothermal holding. The mechanical properties of TRIP-aided dual phase steel was found to depend on the volume ratio of each phase and the volume fraction of ${\gamma}_R$. It was also noted that the proper mechanical stability of ${\gamma}_R$ improved the mechanical properties. In this work, the best balance of strength-ductility was obtained by holding the steel at $420^{\circ}C$ for 500sec. after annealing at $730^{\circ}C$ for 300 sec.

• Corrosion Science and Technology
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• 제11권1호
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• pp.29-36
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• 2012

The resistance spot welding of high strength steel degrades the weldability because of its high strength with rich chemical composition and coating layer to protect from corrosion. During the each resistance welding process the electrodes tip reacts with coating layer, then subsequently deteriorates and shorten electrode life. In this study, the Al-coated HPF (Hot Press Forming) steels and Zn-coated TRIP steels were used to investigate the electrode life for resistance spot welding. Experimental results show that the reactivity of Al-coating on HPF steels to electrode tip surface behaviors different from the conventional Zn-coated high strength steels. The electrode tip diameter and nugget size in electrode life test of Al-coated HPF steels are observed to be constant with respect to weld numbers. For Al-coated HPF steels, the hard aluminum oxide layer being formed during high temperature heat treatment process reduces reactivity with copper electrode during the resistance welding process. Eventually, the electrode life in resistance spot welding of Al-coated HPF steels has the advantage over the galvanized steel sheets.

• 동력기계공학회지
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• 제17권4호
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• pp.115-119
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• 2013

The resistance spot welding of high strength steel degrades the weldability because of its high strength with rich chemical composition and coating layer to protect from corrosion. And the weld Expulsion is prone to occur and severely affect the nugget guality when the initial gap between automatic borrowing galvanied steel sheets(SGARC35) and Zn-coateel trip steels(GA580TRIP and GA980 TRIP) exist in resistance spot welding(RSW). RSW is one of the most popular welding processes used to join sheet metals. but weld guality sometimes do creases due to welding condition. in this paper to verity tue weldability using spot welding with the hemispherically concaved electrode, tensile shear strength and cross-tensile strength were measured by a universal test machine. in addition, the nugget size on cross-sectional area of the weld was observed by optical and electron microscopy. As a result, the nugget size of this specimen is increased with increasing welding current and Max load of tensile-shear strength is increased with welding current is increasing.

• 한국산학기술학회논문지
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• 제19권3호
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• pp.52-60
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• 2018

최근 자동차 산업에서는 연비향상 및 안전규제 강화에 따라 차량 경량화가 필수적으로 요구됨에 따라 DP강(Dual Phase steel), CP강(Complex Phase steel), MS강(Martensitic Steel), TRIP강(Transformation Induced Plasticity steel), TWIP강(Twinning Induced Plasticity steel) 등과 같은 인장강도 700MPa 이상인 초고장력강(Ultra High Strength Steel)의 적용이 증가하고 있다. 초고장력강을 차체에 적용하기 위해서는 용접공정이 필수적이며, 원가 측면에서 유리한 전기저항점용접(Resistance Spot Welding, RSW)이 차체 용접에서 80%이상으로 가장 많이 적용되고 있다. 초고장력강은 강도향상을 위해 합금원소 함량을 늘이기 때문에 일반적으로 용접성이 열악한 것으로 알려져 있다. 이러한 초고장력강의 저항점용접의 경우 적정 용접조건 영역이 축소되고 용접부에서 계면파단 및 부분계면파단이 발생하는 것으로 보고되어 있어 결함 및 품질을 실시간으로 예측할 수 있는 용접품질 판정 연구가 활발히 진행되고 있다. 이에 따라 본 연구에서는 저항 점 용접을 수행할 때 검출되는 2차 회로 공정 변수를 이용하여 용접부의 동저항을 모니터링하고, 이 동저항 패턴에서 용접 품질 판단에 필요한 인자들을 추출하였다. 추출한 인자들을 상관분석하여 용접 품질과의 상관성을 파악하였으며, 상관성이 높은 인자들을 이용하여 회귀분석을 실시하였다. 이를 근거로 현장 적용이 가능한 회귀 모델을 제시하였다.

• 한국재료학회지
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• 제15권2호
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• pp.115-120
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• 2005

The aim of this research is to develop the TRIP aided high strength low carbon steels using reverse transformation process. The $4\~8\%$ Mn steel sheets were reversely transformed by slow heating to intercritical temperature region and furnace cooling to room temperature. The stability of retained austenite depends on the enrichment of carbon and manganese by diffusion during the reverse transformation. The amount of retained austenite formed after reversely transformed at $625^{\circ}C$ for 6 hrs was about $50\;vol.\%$ in the $8\%Mn$ steel. The change in volume fraction of retained austenite with a holding temperature was consistent with the changes in elongation and the strength-ductility combination. The maximum strength-ductility combination of 40,000 $MPa{\cdot}\%$ was obtained when the $8\%Mn$ steel reversely transformed at $625^{\circ}C$ for 12 hrs. However, it's property was significantly decreased at higher holding temperature of $675^{\circ}C$ resulting from the decrease of ductility.

• Corrosion Science and Technology
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• 제9권2호
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• pp.81-86
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• 2010

The mechanisms of selective oxidation of intercritically annealed CMnAl TRIP steels in a Continuous Galvanizing Line (GCL) were studied by cross-sectional observation of the surface and sub-surface regions by means of High Resolution Transmission Electron Microscopy (HR-TEM). The selective oxidation and nitriding of an intercritically annealed CMnAl TRIP steel in a controlled dew point 10%$H_2+N_2$ atmosphere resulted in the formation of c-xMnO.$MnO_2$ (1${\leq}$x<3) and c-xMnO.$Al_2O_3$ ($x{\geq}1$) particles on the steel surface. Single crystal c-xMnO.$SiO_2$ ($2{\leq}x{\leq}4$) oxide particles were also observed on the surface. A thin film of crystalline c-xMnO.$SiO_2$ (2${\leq}$x<3) and c-xMnO.$Al_2O_3$ ($x{\geq}1$) was present between these particles. In the sub-surface region, internal oxidation, nitriding and intermetallic compound formation were observed. In the first region, large crystalline c-xMnO.$SiO_2$ ($1{\geq}x{\geq}2$) and c-xMnO.$Al_2O_3$ ($x{\geq}1$) oxides particles were present. In the second region, c-AlN particles were observed, and in a third region, small $MnAl_x$ (x>1) intermetallic compound particles were observed.

• 한국재료학회지
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• 제14권2호
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• pp.126-132
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• 2004

The high strength steel sheets has been widely used as the automobile parts to reduce the weight of a vehicle. The aim of this research is to develop the TRIP aided high strength low carbon steels using reverse transformation process. The 0.15C-4Mn and 0.15C-6.5Mn steel sheets were reversely transformed by slow heating to intercritical temperature region and air cooling to room temperature. The stability of retained austenite depends on the enrichment of carbon and manganese by diffusion during the reverse transformation. The amount of retained austenite formed after reversely transformed at $645^{\circ}C$ for 12 hrs. was about 46vol.% in hot rolled 0.lC-6.5Mn steel. The change in volume fraction of retained austenite with a holding temperature was consistent with the changes in elongation and the strength-ductility combination. The tendency of tensile strength to increase with increasing the holding temperature was due to the decrease of retained austenite after cooling from the higher temperature of $670 ^{\circ}C$. The maximum strength-ductility combination was about 4,250 kg/$\textrm{mm}^2$ㆍ% when the hot rolled 0.lC-6.5Mn steel was reversely transformed at $645^{\circ}C$ for 12 hrs.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2001년도 추계학술대회 논문집
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• pp.75-80
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• 2001

The purpose of this study is to investigate the characteristics of springback for various process conditions of the 2D draw-bending operation. The process variables are the forming temperature, the geometry of tools such as punch profile radius(Rp) and die profile radius(Rd). Especially, in order to control the springback, the use of the warm forming method is applied. For the warm draw-bending, five steps of temperature ranges, from room temperature to $200^{\circ}C$, were adopted. And two kinds of steel sheets, namely SCP1 and TRIP(transformation-induced plasticity), the newly developed high strength Steel, were adopted. As a result, the springback was affected by the elevated temperature and the geometry of tools in two kinds of steel sheets.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2003년도 추계학술발표대회 개요집
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• pp.30-32
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• 2003

• 한국레이저가공학회:학술대회논문집
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• 한국레이저가공학회 2005년도 춘계학술발표대회 논문집
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• pp.57-60
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• 2005