Temperature and structural changes during hot rolling.

The 'published literature on strength, structural and thermal aspects of hot working has been reviewed with particular reference to their application to the hot rolling of stainless steel. Mild and stainless steel slabs. have been hot rolled on a mill instrumentated to measure load, torque and speed. Internal temperatures have been measured during rolling using embedded thermocouples whose outputs have been recorded on a U.V. recorder coupled with an automatic back-off system. As-rolled stainless steel slabs recrystallised very ~ slowly relative to the rate found by previous workers for simpler austenitic stainless steels. Additionally, slabs exhibited retarded recrystallisation at the surface compared with the centre whereas theoretical analysis predicts the reverse. These phenomena have been explained by rapid precipitation at temperatures ~950oC and limited electron microscopical evidence has been obtained to support this. Precipitation also increased the flow stresses at lower rolling temperatures and masked any effects that different substructures were expected to have on the hot strength. A finite difference computer programme has been developed to predict the temperature changes that occur through a two dimensional transverse section of a slab during a hot rolling SChedule. The air cooling and roll contact heat transfer coefficients have been derived by comparison of measured and computed temperature-time curves. This programme enabled a mean pass temperature to be derived from measured temperatures which then resulted in good agreement between hot strengths derived from rolling and torsion tests on stainless steel. The programme has been simplified to a one dimensional model for simulating temperature changes over central regions of wide sections during industrial hot rolling. Data have been presented to enable mean temperatures to be derived from measured surface temperatures and to illustrate the effects of various parameters on the heat loss during a pass.