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TS590N/mm2 hot rolled steel showing a good balance of stretch formability and stretch flangeability has been newly developed for manufacturing high strength automotive parts. A laboratory study revealed that an introduction of a small amount of martensite in ferrite · bainite microstructure remarkably improves uniform elongation of ferrite · bainite steel giving little influence on its good localized elongation. Characteristics in ductile property of this “Tri-Phase” type steel were confirmed in press forming tests.

The characteristics of DC butt welding of high strength hot rolled steel was studied in relation to the production of wheel rims. A thermal distribution analysis of DC butt welding indicated the importance of the optimization of welding condition as the maximum temperature attained in DC butt welding significantly differed from place to place along the welding line. Insufficient heat input or overheating might cause cold weld or solidification cracking. Particularly,the welding current and welding cycle should be strictly controlled in order to make a weld with good cold formability. The influence of steel chemistry on weld hardness was also investigated. High strength steel for DC butt rim welding should be produced with the least Ceq. because a relatively early stage bead trimming enhances hardenability of the weld,which increases sensitivity to cracking.

In this paper the tensile deformation behavior of dual-phase steel and the effect of mechanical properties of various types of high strength steel sheets on surface wrinkles during forming have been studied. A continuum model based on the “rule of mixture” is proposed to explain the tensile deformation behaviour of dual-phase steels which are composed of ferrite and martensite. In the model, the size of martensite and the strain ratio of martensite to ferrite are considered. Moreover, one of the assumptions of this theory is that the flow curves of the ferrite and martensite follow the equations σ = K∊n and σ = A ln∊ + B∊ + C respectively, where K, n, A, B and C are constants. Agreement between the calculated results and the experimental results is good for low volume fraction of martensite. The occurrence and growth of wrinkles under non-uniformly distributed stretching load depended mainly on the yield ratio and n-value in a small strain range.