Study On Microstructure Evolution And Charging Process Of Microstructure Slab After Reduction Process

Delivery and charge means that the continuous casting steel is sent to the soaking pit for heating to the rolling temperature after it leaves the continuous casting machine and cools down to room temperature.The hot delivery and hot charge is another process during which the casting steel is delivered to the heating furnace directly or indirectly and then heated to the rolling temperature.Investigations indicate that,after HR,structure of the micro-alloyed casting steel and the ferrite is refined.This provides more nucleation sites for the formation of austenite during delivery and charge,which contributes to refine the austenite structure after delivery and charge.Based on the practical HR process,the present paper takes the micro-alloyed casting steel slab as the research object and systematically studies the effect of reduction amount on the structure evolution at different positions of the casting steel before,during and after the delivery and charge.A reasonable heating route for hot charging was then established.The main research contents are as follows:(1)A thermal-mechanical coupled model was developed based on the practical heating process.The temperature variation at different positions of the micro-alloyed casting steel was then determined,which provides theoretical basis for conducting corresponding offline simulation.The thermal stress of the micro-alloyed casting steel during different delivering and charging process was investigated.By comparing the thermal stress and high temperature strength for the casting steel after HR and SR under the same condition of heating rate,it was found that the temperature widow of delivering and charging for the casting steel after HR is wider than that for the traditional casting steel.Meanwhile,the risk of hot charging cracks for casting steel after HR is lower.The suitable heating and charging process for micro-alloyed continuous casting slab after HR was then determined.(2)The original austenite grain size of the micro-alloyed casting steel gradually decreases with the reduction amount increased.Statistical results indicate that the average original austenite grain size decreases by 60%from 1150 ?m to 700 ?m with the reduction amount increased from 9 mm to 25 mm.Compared with the average austenite grain size of 50 ?m after charging for the casting steel with a reduction amount of 9 mm,this value decreases by 56%and reaches 28 ?m.(3)The refinement of austenite grain for the micro-alloyed casting steel is mainly caused by three factors:? the smaller ferrite grains size after HR provides more nucleation sites for the formation of austenite during delivering and charging;?Compared with the traditional casting slab,more second phase particles precipitate during HR,which enhances the pinning effect of second phase particles on the growth of austenite grain;? The reduction process can promote the nucleation for ferrite grains at austenite grain boundaries prior,and the orientation interface between ferrite grains and prior austenite becomes smaller,this situation can divide the prior austenite grain boundary and make the austenite to nucleate easily.(4)After charging at 700?,the precipitates in micro-alloyed casting steel with a reduction amount of 25 mm distributed more uniformly compared with that in the casting steel with a reduction amount of 9 mm.Additionally,the grain size is smaller,and the grain equiaxedness is better,which decreases the risk of mirco cracks at the grain boundaries and meets the requirement of hot delivering and charging.Compared with the traditional micro-alloyed continuous casting slab,the required temperature for the micro-alloyed continuous casting slab after HR increases by?50 ? and reaches a value of>700?.