Search Results

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 response to an increasing demand of high strength steel sheets for automobiles, study was carried out to develop TS 980MPa (100kgf/mm2) grade cold rolled dual phase steel sheet which can be used for the manufacturing of, for instance, bumper reinforcement and door impact beams. First, the process conditions of water-quenching-type continuous annealing were investigated to establish the concepts on the optimum dual phase microstructure which guarantees high strength and good ductility. Secondly, solution hardening by typical chemical elements: carbon, silicon, manganese and phosphorus, was discussed to find the most beneficial way to maintain good ductility in the strengthening of the steel sheet. Mill production of the TS 980MPa grade steel sheet proved to have good ductility which now can replace the conventional TS 784MPa grade steel sheet.

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.

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.