The Effects of Ti and S in Direct Rolled Thin Slab Cast Low Carbon Steel

Abstract

The effects of sulphur on the metallurgy of mild steel produced via the Thin Slab Direct Rolling (TSDR) and Cold Charge Rolling (CCR) routes were compared. Particular attention was given to the precipitation behaviour of MnS and its influence on the microstructure, mechanical properties and susceptibility to cracking of the hot band. The sulphur content was varied from 0.003wt% to 0.04wt% and three levels of Mn (0.10, 0.24 and 0.64 wt%) were used. A number of casts with Ti additions of 0.024 and 0.036wt% were also made. CCR produced an homogenous, polygonal grain structure while TSDR product displayed banding of the microstructure into coarse and fine ferrite grains. The banding was less severe at low S contents. The average grain size of the TSDR product became finer than that of the CCR product as the S content was increased. Yield strength was greater in the TSDR material and increased to a maximum at a sulphur content which was inversely related to the level of Mn. Mechanisms responsible for the observed change in mechanical properties with S content include ferrite grain refinement, due to botn the refinement of the austenite grain size through precipitation of MnS on the austenite grain boundaries, and strain induced precipitation of MnS during rolling, which retarded recrystallisation and increased retained strain prior to transformation. Surface defects in the form of edge cracking were observed in all TSDR strip with the general exception of low S and high Mn steels. The cracking was most severe in the low Mn TSDR steels where it occurred at very low S levels. It was found that an isothermal hold at 1150°C for 20 minutes prior to rolling eliminated cracking. The severity of the cracking appeared to be dependent on the amount of S in solution prior to rolling. Small additions of Ti significantly reduced the degree of edge cracking, due to the formation of a Ti-based sulphide and heterogeneous nucleation of MnS on Ti-based oxide particles.