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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.

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.

Fatigue tests were performed on five kinds of fillet welded joints on 2.6 mm thick HSLA steel sheets, to evaluate the fatigue life of automotive HSLA chassy frame with different joint geometries under various loading conditions. Specimens were made by CO2 gas arc welding with 1.2 mm diameter wire. During the fatigue tests at stress ratio (R)=−1, dynamic strain was measured using a 3 mm strain gage cemented on the sheet surface nearest to the toe. Data were discussed by nominal stress as well as by local strain.