SIMULATION OF MICROSTRUCTURE EVOLUTION DURING HOT ROLLING OF ROUND BARS FROM MICROALLOYED STEEL

The paper presents a modified microstructure evolution model that better considers the influence of strain induced precipitation ( SIP) on static recrystallisation (SRX) kinetics. This is achieved by calculating the time for the 50 % softening t(0.5) (s) by static recrystallisation separately for the situation when it does not occur and separately when SIP occurs. This approach makes the resulting model more sensitive to grain coarsening in the high temperature rolling region, which is a consequence of the rapid progression of SRX and the larger interpass times during rolling on the country cross and continuous rolling mills. A method for deriving a modified model based on previously published two-stage softening curves is described. The modified model was used to design a rolling mode for a 90 mm diameter round bar. The modelling can be used to determine the necessary reduction in rolling temperature to induce SIP while the rolling is still in progress. This is because, during conventional rolling, SIP occurs after the last pass and thus has no effect on austenite grain size. Lowering the rolling temperature by 45 degrees C, it is possible to reduce the grain size from 80 to 45 mu m while increasing the mean flow stress (MFS) by only 15 % in the 8th pass. The resulting grain size for both rolling regimes is consistent with the operating results. A second controlled rolling mode is also presented (reduction of the rolling temperature by 100 degrees C), which leads to a grain refinement to 25 mu m, but at the cost of an increase in MFS of about 40 % in the 8th pass. The modified model can also be used to optimise the chemical composition of the steel.