Study On Microstructure Evolution And Heredity In Nb-Ti Microalloyed Continuously Cast Slab During Hot-core Heavy Reduction Rolling

The internal defects of continuously cast slab mainly include center-microsegregation,shrinkage and porosity,etc.For large-section steel products,these defects cannot be effectively eliminated in subsequent rolling due to insufficient compression ratio,and the mechanical properties of the final steel products are significantly decreased,lead to the final steel products is unable to meet the usage requirement.In recent years,a novel technology called HHR2(Hot-core Heavy Reduction Rolling)has been developed by Northeastern University,which directly roll continuously cast slab by single pass compressing with large reduction amount and large temperature gradient through located the rolling mill at the exit of the caster,with the aim to improve the internal quality of continuously cast slab.Currently,there have been many researches on the reduction technology,which mainly focused on center-segregation,porosity and shrinkage of the continuously cast slab,however,the microstructure evolution and microstructure heredity of continuously cast slab during HHR2,subsequent reheating and hot rolling process is limited.In view of HHR2 technology,this paper focused on microstructure evolution,and systematically studied dynamic recrystallization behavior,dynamic recrystallized austenite grain evolution and precipitation behavior of Nb-Ti microalloyed steel at viscoplastic zone,besides,the microstructure tracking,monitoring and evaluating system of Nb-Ti microalloyed continuously cast slab was established to study effect of hot-core heavy reduction rolling and different charging process on the slab microstructure,reheated microstructure,hot-rolled microstructure and mechanical properties,which provided theoretical basis for optimizing HHR2 process parameters.The main content and results of this paper are as follows:(1)The material parameters and constitutive model of steel at ultra-high temperature(above 1200?)and low strain rate are limited,thus,single pass compression experiments were conducted to study flow behavior of Nb-Ti microalloyed steel at viscoplastic zone of 1000?1350?,and viscoplastic constitutive model,dynamic recrystallization model and dynamic recrystallization grain size model were established.Then the dynamic recrystallization volume fraction and austenite grain size during HHR2 process can be well reflected by the established models,besides,the deformation theory of steel at ultra-high temperature is supplemented.(2)Double pass compression experiments were conducted to study the effect of deformation parameters on precipitation behavior in Nb-Ti microalloyed steel at temperatures in the range of 850?1300?,besides,theoretical models were established to calculate recrystallization driving force Fr and pinning force Fp,and illustrated the interaction between recrystallization and precipitation.The results indicated that precipitation will occur before recrystallization when the deformation temperature was in the range of 850? to 1000?.Moreover,when the deformation temperature was in the range of 1100 to 1300?,recrystallization will be prior occurred compared with precipitation.(3)Viscoplastic finite element model was established by Deform-3D software to simulate HHR2 process of Nb-Ti microalloyed continuously cast slab,and the dynamic recrystallization volume fraction and recrystallized grain size along thickness direction of continuously cast slab during HHR2 process was quantitatively analyzed based on the established dynamic recrystallization model and austenite grain size model.The results indicated that rolling with a big temperature gradient and large reduction amount can make the deformation deeply transferred into the center position,resulted in austenite grains at the slab center can be significantly refined,and microstructure homogeneity of continuously cast slab was also significantly improved.(4)Hot-core heavy reduction rolling experiment of Nb-Ti microalloyed continuously cast slab was conducted.The results indicated that the ferrite grain size of the CC slab was 82,105 and 128 ?m,respectively,and the ferrite grain size of CC-HHR2 slab was 69,63 and 65 ?m,respectively.At the quarter thickness of CC-HHR2 slab,the number of precipitates with a size range of 6?20 nm was higher than that of CC slab.The hot-core heavy reduction rolling experiment confirmed that microstructure homogeneity of Nb-Ti microalloyed continuously cast slab can be significantly improved.(5)Plant trials of hot-core heavy reduction rolling,reheating and controlled rolling and controlled cooling were simultaneously conducted to study the reheated microstructure and microstructure heredity of Nb-Ti microalloyed continuously cast slab.The results indicated that the reheated and hot rolled austenite grains in CC-HHR2 slab were significantly refined compared with CC slab,and it was confirmed that there was a certain degree of heredity in the slab after reheating,besides,the refined microstructure caused by HHR2 can also be remained to the final hot rolled steel,lead to the mechanical properties homogeneity of the final hot rolled steel can be significantly improved.(6)Charging process experiment of Nb-Ti microalloyed CC slab and CC-HHR2 slab was conducted to study the microstructure evolution,precipitation behavior and mechanical properties.Compared with cold charging process of original CC slab,CC-HHR2 slab with directly rolling process and hot charging process cannot reduce micro structure homogeneity and mechanical properties homogeneity,as well as can better realize high effiency,energy saving and optimize production.