| Micro-alloyed steel S355 was selected as the research steel and Xinyu steel company ultra-thickness slab caster was used as the test equipment.The effect of cooling rate on the microstructure transformation law of this steel was studied by using the thermal expansion analyzer.The influence of heat recycles on the high temperature plasticity was investigated by using of the hot simulation equipment Gleeble1500.The high temperature brittle mechanism at the third brittle zone was analyzed in detail.The influences of the temperature and cooling rate on the microstructure evolution were clarified.The reheating process was investigated to improve the high temperature plasticity through the hot tensile experiment.The optimum cooling temperature and cooling rate were proposed for the ultra thickness slab continuous casting.Finally,based on transformation mechanism during the continuous cooling and high temperature plasticity character research of the tested steel,according to slab solidification heat transferring theory,a new intensive cooling process at the vertical section and bending section was presented for the ultra thickness slab continuous caster.The nozzle type at the intensive cooling zone was optimized on the basis of the structure analysis of the second cooling spray system.The intensive cooling experiment plan was formulated for ultra thickness slab continuous casting second cooling process and the production experiment was executed.The experiment results show that the fine microstructure at the slab surface was obtained and the surface cracks was reduced.The thermal expansion analyzer,metallographic microscope,scanning electron microscope,transmission electron microscope and high temperature laser confocal equipment were used to investigate the microstructure evolution mechanism for the micro-alloyed steel S355.The results show that for the tested steel the critical cooling rate of bainite transformation is 2?/s,the critical cooling rate of martensite phase transformation is 15?/s,pearlite transformation will not occur if the cooling rate is higher than 7?/s,the uneven prior austenite grain size easily cause the formation of low temperature mischcrystal structure.The high temperature plastic behavior of micro-alloyed steel S3 55 was systematically studied under different temperature recycle by Gleeble thermal simulation equipment,the fracture microstructure and the precipitates distribution were studied by using scanning electron microscopy and transmission electron microscopy.Experimental results show that the proeutectoid ferrite film along prior austenite grain boundaries is the first reason for the occurrence of brittleness valley at the third brittle zone for the S355 steel.After cooling and self tempering,the hot plasticity improved largely.Meanwhile,the net-shape ferrite film was obviously reduced and intragranular ferrite formed.Most of the precipitates were uniformly distributed in grains,and the size of precipitates at dislocation cross position was larger.Based on the mathematical model simulation,the effect of intensive cooling process on the surface temperature of continuous casting slab was simulated and analyzed.The existing sector section spray system was calculated and checked;the nozzle selection of the intensive cooing zone was optimized.The best intensive cooling parameters were confirmed,such as the optimum intensive cooling location in the continuous caster and the cooling rate.The second intensive cooling experiment procedure was proposed and the production experiment was executed.The production test results show that the surface temperature of the slab was below 720? after intensive cooling and the slab surface microstructure was refined.The grain size uniformity was improved and the coarse dendrite structure basically eliminated.The grain size of the intensive cooling slab surface reached 11th level,and the depth of surface fine grain zone reached 5mm.Besides,the surface cracks rate decreased from original 7.53%to 3.29%.The purpose of the slab surface micro structure controlling and slab surface cracks controlling was achieved.Overall,the hot delivery rolling test results of the intensive cooling slab indicates the grain of the ultra-thickness slab surface and plate surface were refined.The core quality of ultra-thickness slab has been improved significantly.All the results show that the development of the intensive cooling process and the improvement of the spray system in the intensive cooling zone have been successfully performed.Formation of the innovation points are as follows:1)A new cooling process is proposed and validated,which is a process by using the intensive cooling to refine the slab surface microstructure and reduce the slab surface cracking rate in industrial production.2)The laboratory results show that proeutectoid ferrite film and its thickness is the key factors influencing the plasticity of S355 at the third brittle zone,the ferrite film and precipitates on the slab surface can be controlled by slab surface thermal history controlling.3)An industrial production experiment was carried out by using the intensive cooling process on ultra-thickness slab of S355,the results proved that the ferrite microstructure controlling process is correct and feasible. |
PDF Full Text Request