NUMERICAL ESTIMATION OF THE PROBABILITY  OF METAL FAILURE UNDER HOT PLASTIC DEFORMATION BY MEANS  OF THE COCKCROFT — LATHAM CRITERION

The paper presents a new method for evaluating the potential destruction of metal during hot rolling, providing for joint use of physical and numerical simulation. We have applied this technique in rolling pipe line steel of strength category X42 on a continuous mill 2000 and K60 on a reversing mill 5000. The Gleeble 3800 simulator allowing to model the majority of thermomechanical processes was used to determine hot ductility. The Deform 3DTM software was used for determining the limit values of the Cockcroft-Latham failure criterion and for failure probability analysis during hot rolling. AusEvol+ software was used for studying the relaxation processes taking place during the rough and the final stages of rolling for edge and average temperatures. It was established that the degree of stress relaxation had to be taken into account in mathematical simulations of rolling modes in the Deform 3DTM software and in determining the limit values of the Cockcroft-Latham criterion by the average temperature during the rough stage of rolling. In view of this, the values of the accumulated effective strain were zeroed after each pass in the rough stage of rolling when calculating the Cockcroft-Latham failure criterion. In the numerical simulation in calculating the Cockcroft-Latham failure criterion for the finish rolling passes as well as for the edge area of the sheet during rough rolling, strain accumulation was taken into account if stress relaxation did not occur. It was established that the steels under consideration have a high ductility and failure should not occur either in the bulk of the metal or at the edge throughout the rolling cycle. Failure possibility can be initiated by the existence defects in initial slab and not optimized temperature and deformation schedules. However, failure of metal is possible during hot rolling due to the presence of defects in the initial slab and due to the violation of the temperature and deformation regimes. The proposed technique can be used for other types of pressure processing of metal.