The effect of high reductions per pass on material properties in the hot rolling of low-alloyed steel

Abstract

The production of thick plates of ordinary steel usually starts from ingots, because continously-cast slabs do not give sufficient reduction in thickness during the rolling. A reduction ration of around 4:1 is necessary with low-alloyed steels in order to be able to achieve good mechanical properties and structure. For plate thickness above 50–70 mm problems with the necessary reduction ratios can be expected. There are, however, indications that a high reduction per pass, >40%, will give a better stress-state and hydrostatic pressure in the material than will a normal reduction per pass and that this may eventually result in better segregation equalization, structure breakdown and porosity closure. If better mechanical properties can be achieved there is then the possibility of using smaller total reduction ratios and perhaps an extended use of continuously-cast slabs for thick plates. Rolling with a high reduction per pass usually leads to gripping problems: these can be overcome either by increasing the roll diameter or by pushing the workpiece into the roll gap. Tests with a pusher have been carried out which show that it is possible to subject billets to a reduction of 90% in one pass. At the Metal Working Research Plant (BTF) in Luleå rolling experiments have been carried out with high reductions per pass using a pusher and a 5 MN pilot-plant mill in order to investigate the possibility of using high reductions in the hot rolling of plate. The experiments have shown that high reductions - compared to conventional rolling - give no significant improvements in porosity closure and structure breakdown, or improvements in mechanical properties of the as-rolled material. As the mechanical properties usually determine the total reduction ratio necessary, it is not likely that a smaller total reduction can be used when rolling with high reductions per pass. Furthermore, the increase in rolling force is rather large, which indicates that there would be restrictions in the industrial rolling of plates with high reductions per pass.