Study On Microstructure Evolution Of Ti-Bearing Microalloyed Steel Continuous Casting Slab During High Temperature And Heavy Reduction Rolling

High temperature and heavy reduction rolling is an innovative technology that provides the single-pass hot rolling to the slab after the final solidification point,which can effectively eliminate the internal defects such as shrinkage cavity,porosity in the continuous casting slab.In this paper,the dynamic recrystallization and static recrystallization behaviors of Ti-bearing microalloyed steel are systematically studied,corresponding to the viscoplastic temperature range of the slab.Combined with the recrystallization models and finite element simulation,the microstructure evolution of slab along the thickness direction is investigated during high temperature and heavy reduction rolling.The main research contents and achievements of this paper are as follows:(1)The high-temperature rheological behavior of the experimental steel was investigated by single-pass thermal simulation compression test.The strain-compensated flow stress constitutive model was established according to the flow stress curves.It is verified that this model can accurately predict the trend of flow stress.Then,the dynamic recrystallization behavior of experimental steel was studied,corresponding to the viscoplastic temperature range.And the critical strain model,kinetics model,and grain size model of the dynamic recrystallization were established.At last,the effect of deformation temperature and strain rate on the microstructure of dynamic recrystallization was analyzed.The result shows that the larger dynamic recrystallization grain was obtained when the higher deformation temperature and lower strain rate were performed in the tests.(2)The static recrystallization behavior of the experimental steel was studied based on the two-pass thermal simulation compression test,corresponding to the viscoplastic temperature range.With the increase of deformation temperature,strain and strain rate,or the reduction of initial grain size,the static recrystallization volume fraction turns larger.The single-pass thermal simulation compression test was carried out to analyze the effect of deformation parameters on static recrystallization microstructure.The smaller the initial grain size,the lower the deformation temperature,or the larger the strain rate and strain are help to refine the grain size of complete static recrystallization.The static recrystallization kinetics model and the static recrystallization grain size model of the experimental steel were established.(3)The three-dimensional thermomechanical coupled finite element model of high temperature and heavy reduction rolling was developed by Deform-3D software to analyze the microstructure evolution of slab in the thickness direction.In general,the volume fraction and grain size of dynamic recrystallization were gradually increased from the surface to the core of slab.With the increase of reduction amount,the volume fraction of dynamic recrystallization raises as well as the decreases of grain size.During the high temperature and heavy reduction rolling process,the volume fraction of dynamic recrystallization is small,which means the effect of grain refinement on the slab is limited.(4)The static recrystallization of the reduction of 50 mm was analyzed by finite element simulation.The volume fraction of static recrystallization gradually increases from the surface of the continuous casting slab to the core.The increasing speed of volume fraction of static recrystallization is higher when the observation point is closer to the core of slab.The core undergoes complete static recrystallization within 10s.The volume fraction of static recrystallization is much higher than that of dynamic recrystallization.The static recrystallization grain size gradually increases from the surface to the core,and the dynamic recrystallization grain size is smaller than that of static recrystallization.After the high temperature and heavy reduction rolling process,the average grain size reduces first and then increases from the surface to core.And the minimum value was obtained at the position about 40 mm away from the surface.Compared with dynamic recrystallization,static recrystallization is more effective in grain refining of the continuous casting slab.