Study Of Hot Charge On Microstructure And Precipitation Of A Welded Cylinders Steel

In recent years,the technology of hot charge of continuous casting slabs has been widely used in the steel industry.However,low alloyed steel slabs tend to form more surface cracks with hot charge process than cold charge process.In this paper,through the laboratory simulation of slab hot charging of HP295 welded gas cylinder steel,the microstructure evolution of HP295 welded gas cylinder steel and the precipitation of precipitated phase at different hot charging conditions were analyzed.The formation mechanism of hot cracks was discussed.The following conclusions are obtained:(1)The CCT static curve and CCT dynamic curve of HP295 steel was plotted and results show that as the cooling rate increased,the size of ferrite and pearlite decreased gradually,and the amount of pro-eutectoid ferrite decreased,while the number of pearlites gradually increased.When the cooling rate exceeds 30°C/s,the widmanstatten structure appears in the tissue.When the cooling rate is increased to 40°C/s,acicular ferrite appears in the tissue,even if the cooling rate reaches 50°C/s,martensite still does not appear in the microstructure;(2)The hot ductility law of HP295 steel was measured.The results indicate that the area reduction of the specimens exceeded 77% in the experimental temperature range,and the plasticity of sample was overall good in the test temperature range.The curve of high temperature hot ductility showed a plastic groove between 750°C and 850°C,and the area of reduction of the sample reached a minimum value of 77.98% at 800°C.It is vital to avoid this temperature range for slab straightening in the actual production and the slab should be straightened at the temperature above 900°C.In addition,the high temperature strength of HP295 steel decreased with increasing temperature.(3)When the hot charging temperature was below 650°C,the charging transformation had almost no effect on transformation.The microstructure consisted of ferrite,pearlite and widmanstatten structure.When the hot charging temperature was 700°C,750°C and 800°C,the transformation temperature increased during the secondary cooling.The microstructure consisted of ferrite and pearlite.When the hot charging temperature rose to 850°C,the phase transition temperature and microstructure were the same as those below 650 °C.In addition,the precipitation between 700~800°C deteriorated the toughness and plasticity of the steel.Especially,the larger precipitated particles would seriously damage the workability of the steel,and might even cause cracks during rolling.It was one of reasons of cracks in HP295 steel.