Study On The Microstructure Evolution Of S50C Steel During CSP Process

As the relatively high carbon content in S50C steel,segregation usually tends to occur.The CSP(Compact Strip Production)process is characterized with high casting speed,high cooling strength,coarsened austenite grains before hot rolling,large deformation during a single hot rolling pass,and high strain rate.Compared with conventional strip production process,the extraordinary thermo-mechanical history of CSP is advantageous for the homogenization of chemical composition and microstructure.This work studies the element segregation and evolution of micro structure for S50C steel during the CSP process of Jiuquan Iron and Steel Corporation(JISCO).Under the CSP process of JISCO,the continuous casting process is not equipped with electromagnetic stirring equipment,and a rough rolling mill is not equipped before hot rolling.After the slab is discharged from the soaking furnace,it directly enters the 6 finishing rolling.It is found that the solidified structure of S50C steel by CSP is fine and the distribution of element is uniform indicating a high inner quality of the S50C slabs.However,the hot rolled strip of S50C steel produced by CSP exhibits high mechanical properties and low elongation leading to the breakage of cold rolled sheet when supplied with the hot rolled strip.To optimize the properties of hot rolled strip and avoid the breakage during cold rolling,studies of element segregation and microstructure evolution during CSP process for S50C steel are conducted in present work.Base on the study,the mechanism of element segregation and evolution of microstructure is explored.The main results and innovation are summarized as follows:Firstly,industrial research of S50C steel produced by CSP process under different superheat and mechanical reduction was carried out.The results of pickling test and analysis by OPA-100 show that to obtain a high inner quality of S50C slab,it is reasonable to control superheat of the S50C steel at 43℃ and reduction with 20mm during CSP process.In order to control the carbon segregation during solidification,chemical composition should be realized by taking the upper limit of carbon and manganese,the lower limit of silicon and sulphur according to the designed chemical composition.The mechanical property test of S50C hot rolled strip shows that the yield strength of the strip with the thickness of 4.0mm,3.5mm and 3.0mm is 480～500 MPa,and the tensile strength is 720～800 MPa,which are higher than the steel grades with similar composition produced under the conventional process,but its elongation is only about 20%.Secondly,the solidified structure and element segregation of S50C slab in CSP process were studied.The solidified structure of slabs produced by CSP process is fine,and the primary dendrite spacing(λp)and the secondary dendrite spacing(λs)range in 80～240μm and 30～110 μm,respectively.As the solidified structure is refined under CSP process,the permeability(Kp)is reduced,which is only 0.1～2.3 μm2。The predicted model for λp and λs under CSP process are developed by combining the heat transfer model with the solidification growth model and verified by measured data,which realized the combination of microstructure evolution and CSP process parameters.The mushy zone coefficient A mush was characterized based on the predicted model of λs,and by analyzing the solidification and fluid flow behavior in funnel-shaped mold zone with Fluent software,the predicted model was verified and the value of Amush was suggested in 108～109 kg·m-3·s-1.Through optimized Amush,the thickness of solidified shell was predicted and compared with the value calculated by the empirical formula,the difference between which is less than 1 mm,therefore the model was furtherly validated.The analysis of element segregation shows that,the distribution of elements in CSP process is relatively uniform on the whole.The formation of central segregation of element C is mainly caused by negative pressure formed by solidification shrinkage and bulging,which drives the flow of liquid steel with solute enriched between dendrites.After controlling the composition and bulging,the segregation index of carbon ranges only between 1.0～1.1.The evolution process of solidified structure of S50C steel produced by CSP at high-temperature was observed in situ by using the ultra-high-temperature confocal scanning laser microscope combined with SEM and Matlab picture processing.It is found that the evolution of solidification structure of S50C steel at high-temperature can be characterized by the diffusion of carbon in the interdendritic space,and its diffusion process can be described by Fick’s second law.With the help of SEM,Matlab processing and inverse problem method,the integrative diffusion coefficient of carbon in S50C steel produced by CSP during solidified structure evolution is determined with D=15 μm2/s by numerical calculation.Finally,aiming at settling the breakage problem during cold rolling,the microstructure evolution during hot rolling under CSP process was investigated.According to the experimental results,the original austenite size of S50C steel slab by CSP before hot rolling is 50～475 μm.The results of EBSD show that the grain size of hot-rolled strip is 3.6～4.8 μm,and there are a large number of low angle grain boundaries in the grains,indicating there are lots of defects and substructures in the deformed austenite grains.The mechanical property test of S50C by CSP hot rolled strip(R1)shows that its tensile strength is 850 MPa,yield strength is 595 MPa,and elongation is only 19.5%leading to the cold rolled sheet breakage.The microstructure evolution under different deformation conditions was simulated by Gleeble-3500 thermal simulator.In order to simulate the coarsened austenite rolling process in CSP,the evolution of austenite at different temperatures was observed by means of ultrahigh temperature confocal microscope.The results show that when austenitized at 1300℃ for 3 min,the size of austenite basically could meet the CSP simulating requirements.The analysis of sample micro structure under thermal simulation test shows that the refinement of coarsened austenite under CSP process is caused by austenite recrystallization and enhanced phase transformation:on the one hand,at high temperature and low strain rate,dynamic recrystallization occurs.On the other hand,at low temperature and high strain rate,due to the high distortion energy,the driving force of dynamic recrystallization increases,and the grains undergo dynamic recrystallization insufficiently.However,due to the low temperature,it is difficult for grains to grow,and the deformation is obvious,and the size is uneven.Besides,under low temperature and high strain rate,the internal substructure and defects of grains increase,which provides more nucleation sites resulting in enhancement of phase transformation during the subsequent cooling,so that the nucleation ratio is greatly improved and the structure is refined.Based on the above results,the parameters for hot rolling of R1 were optimized by adjusting the coiling temperature to 730℃(R2),and the results show that the ferrite content is increased to about 25%,and the adjacent cementite is increased to 0.84 μm.Correspondingly,the mechanical property test of R2 indicates that the tensile strength and yield strength reduced to700 MPa and 400 MPa,respectively,and the elongation is increased to about 30%.As the mechanical properties of the hot rolled strip are optimized,the cold rolled sheet breakage were avoided effectively.