| High strength low carbon 3.5Ni-1.5Cr-0.38Mo steel containing 0.006% S was melted in the vacuum induction furnace. REM and/or CaSi were added into the melt to control the shape of sulfide inclusions. The specimens were taken in the longitudinal and transverse to the hot rolling direction and then heat treated. The effect of the sulfide inclusion shape control on the property anisotropy such as the impact absorption energy and the ductile-to-brittle fracture transition temperature were studied. Deoxydizing by a aluminum prior to the REM and/or Ca treatment refined the grain, and besides the sulfide or the oxide, complex oxysulfide inclusons are detected among the non-metallic inclusions after the shape control that diminished the inclusion shape factor, I_c (L) below 0.1 from 0.6-0.8. If the shape of sulfide inclusion is controlled by the REM and/or Ca treatment, the tensile property anisotropy is diminished since the ratio in the longitudinal and transverse to the rolling direction, T/L value are improved and T/L values of the impact absorption energy increased from 62.2% to 78.5-89.0%, and could decrease the ductile-to-brittle fracture transition temperature by about 10˚-30℃. The shape control of sulfide inclusion did not show any difference, regardless of the REM treatment or Ca treatment. |
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