Steel scrap is raw materials of the recycled steels refined in electric arc furnaces, and this recycle process is good for keeping our living environment. Authors focus in the toughness of the recycled steels that contain much tramp elements and nitrogen. Chemical compositions of specimens are 0.17C-0.20Si-1.15Mn,Cu : 0.08 0.50mass%,Ti : 0.006 0.031mass%. This report contains properties of HAZs (heat affected zones) in thermal cycled mother steels, and these HAZs were estimated a heat input of 45-600 kJ/cm. The following experimental results are obtained. TiN particles fasten on grain boundaries, and improve the toughness of HAZs. A grain boundary ferrite is restrained by Cu, and ferrite grains become fine, too.
The base materials used were TMCP385B and SA440B with a plate thickness of 40 mm. For the backing plate, a SN490B blast-furnace steel sheet with a thickness of 9 mm and two types of electric furnace steel plates with different nitrogen contents were used; a blastfurnace steel plate stock was also used for comparison. JIS Z 3312 G59JA1UC3M1T was used as the welding wire. MAG welding was used as the welding method. It was determined that the material of the electric furnace steel sheet, i.e., SN490B, which is the current backing-plate material, did not cause any deterioration in the quality of the weld metal properties. This behavior is attributed to the formation of intragranular ferrites by Ti oxide.
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