抄録
On observing macrostructures and sulfur print figures around hot tears in many steel ingots it was found that tears were always surrounded by abnormally equiaxal grain structure. By X-ray analysis, it was proved that a hot tear started from inversely segregated sulfides in the outer skin of steel castings. Hot tearing is considered to occur at two stages : primary tearing occurs during formation of a solid skin, the gap being filled with liquid containing higher solute concentration from which sulfur rich liquid is segregated at the last stage of solidification; secondary tearing probably occurs at the sulfur rich liquid film. A weak point in the solid skin was mostly formed by discontinuous rising of molten metal in ingot mold. The tendency of hot tearing decreased with increase of carbon, silicon and manganese content, and with decrease of sulfur content. The influence of copper and phosphor on the hot tearing was not obvious. In order to prevent hot tearing, pouring needs to be made at lower temperature and at lower rate. Reducing atmospher in the mold during pouring prevents the primary tearing in solid skin. Mn/S ratio over 30 is also very effective.
