Characterization Of Initial Solidification Shrinkage And Crack Susceptibility Of Peritectic Steels Based On Roughness

Longitudinal surface crack is a quality defect of continuous casting slabs of peritectic steel,and the formation of crack is directly related to the peritectic transformation shrinkage of steel at the initial solidification stage.At present,the effect of composition on the initial solidification shrinkage of steel has been mainly studied.However,the molten steel solidifies under the non-equilibrium conditions in the practical casting process,and here the peritectic transformation shrinkage and crack occurring of steel are also affected by the cooling rate.Therefore,it is significant to study the peritectic transformation shrinkage and the crack susceptibility of steel under the non-equilibrium condition,so as to provide theoretical and technical support for controlling the longitudinal surface crack on the peritectic steel slabs.In this paper,a new method characterizing the peritectic transformation shrinkage of steel was proposed by analyzing the shrinkage wrinkles,surface roughness and shrinkage strain index.Based on the analysis of initial solidification,peritectic transformation and surface roughness,the peritectic transformation shrinkage variation was studied under the non-equilibrium solidification.The crack susceptibility of continuously cast slab was studied according to the surface roughness due to the fact that the surface roughness was changed by the composition and cooling rate.In addition,the carbon equivalent formula was evaluated based on the surface roughness experiment,which provides a reference for people to calculate the carbon equivalent under different cooling condition.The peritectic transformation(???)caused the surface roughness variation during the peritectic steel solidification process.Through the two-stage solidification experiment,the change of surface roughness on the convex grains and peritectic transformation shrinkage index were compared.It was found that the surface roughness variation can reflect the peritectic transformation shrinkage.The surface roughness value was affected by the composition and cooling rate.Here,the surface roughness results are reproducible,the error is small and the experimental can be carried out at high cooling rate.Based on these results,surface roughness was firstly proposed to characterize the peritectic transformation shrinkage degree of steel.Combining with the solidification process and surface roughness variation,the effects of cooling rates on the initial solidification and peritectic transformation shrinkage of 0.10%C steel were discussed.The increase of cooling rate first increased the carbon concentration at the front of solidification and reduced the solidification rate when the cooling rate increased from 5?/s to 20?/s,which leaded to the lag of liquid solidification.At the same time,most of the supercooled ? phase occur the phase transition in the crack brittle temperature range with the massive transformation.The sudden quickly phase transition increased the initial solidification shrinkage.There were high carbon concentration gradient,high temperature gradient and high undercooling existed in ? phase when the massive transformation occurred.The peritectic transformation shrinkage(surface roughness)increased first and then decreased with the cooling rate increasing from 2.5?/s to 80?/s.The change of shrinkage degree provides two ideas to reduce the longitudinal surface crack of continuous casting slab of peritectic steel:one is the slow cooling strategy,which has been used in the traditional continuous casting of peritectic steel;the other is strong cold,that is greatly increasing the initial cooling rate to decrease the peritectic transformation shrinkage and the crack susceptibility.Surface roughness(peritectic transformation shrinkage)was predicted through the mathematical model of carbon equivalent,cooling rate and surface roughness,and then the crack susceptibility of steel was analyzed according to the relationship between surface roughness and peritectic transformation shrinkage.The crack susceptibility of carbon steel was judged according to the surface roughness(Ra)predicted.Here,the crack susceptibility of steel is weak if Ra was less than 22?m;the crack sensitivity of steel is general if Ra was between 22?m and 28?m;the crack susceptibility of steel is strong if Ra was larger than 28?m.The analysis results of surface roughness prediction are consistent with the statistical data of slab cracks,and the influence of alloy composition and cooling rate were considered in the analysis process.Compared with other methods,the method is adapted to different continuous casting process conditions(cooling rates),which provides a reference for the design of steel grades,the adjustment of caster parameters and the selection of mold powder.The carbon equivalent formula of continuous casting steel were evaluated through the surface roughness experiment,and the corresponding carbon equivalent formula was recommended according to the cooling condition.Howe 1993,BSSTC and Yasumoto formulas were recommended for the carbon equivalent calculation under the equilibrium conditions.For the traditional continuous casting,it was suggested to choose the Presoly formula to calculate carbon equivalent,which can analyzes the steel with high Si,Mn and Al contents.A new carbon equivalent formula was fitted and suggested for the thin slab continuous casting,which is CE=[%C]-0.044[%Si]+0.039[%Mn]+0.023[%Ni]-0.004[%Mo]-0.01[%S]+0.04[%P],the carbon equivalent of crack susceptibility region is 0.10%?0.17%.