Study On Evolution Of Inclusions In Stainless Steel During Heat Treatment And Rolling Processes

The type,morphology,size and quantity of non-metallic inclusions in stainless steel have significant influence on the properties of steel.After heat treatment and rolling,the characteristics of inclusions in solid steel are often different from the initial ones that formed in molten steel.The main reason is that heat treatment and rolling processes can not only change the shape and structure of slab,but also cause solid-state reactions and extrusion deformation between inclusions and solid steel matrix,resulting in the modification and deformation of inclusions.It is feasible to control the characteristics of non-metallic inclusions in solid stainless steel by appropriate heat-treatment and hot-deformation processes.In this study,the evolution behavior of non-metallic inclusions in 18Cr-8Ni stainless steel during heat treatment and rolling processes were studied first by thermo-mechanical treatment experiments,and the influence of process parameters such as steel composition,heat-treatment temperature,and reduction on the change of inclusions was clarified.Next,the evolution behavior,changing rules,influencing factors,and transformation mechanism of rare earth oxide inclusions during thermo-mechanical treatment of yttrium-treated stainless steel were explored and discussed.Finally,the mechanism and influence law of the solid-state reaction between MnO-SiO2 inclusions and steel matrix during heat treatment were investigated using diffusion couple method.And a dynamic theoretical model was constructed to predict the solid-state reaction between inclusions and steel matrix,and the modification of inclusions.The results show that:1)The changing behavior of MnO-SiO2 inclusions in 18Cr-8Ni stainless steel during heat treatment is related to the Si and Mn contents of stainless steel.When the Si content in stainless steel is lower than the critical Si content,MnO-SiO2 inclusions will change to MnO-Cr2O3 oxides.Otherwise,it does not change.Increasing the Mn content in stainless steel will reduce the value of the critical Si content of inclusion transition;2)In the temperature range of 1273 K-1573 K,increasing heat-treatment temperature will promote the transition of MnO-SiO2 type inclusions to MnO-Cr2O3 in stainless steel with low Si content.However,it has no effect on the inclusions in stainless steel with high Si content;3)The silicate inclusions in 18Cr-8Ni stainless steel show good deformability during hot rolling.A large number of elongated inclusions are observed in the hot-rolled sheet.During cold rolling process,the elongated inclusions are fractured and formed multiple fine oxide particles.With increasing cold rolling reduction,the size of the oxide particles gradually decreases,but the spacing between the particles increases.In the cold rolling process,the critical size of inclusion particles with no more fracture is about 0.5 ?m;4)Homogeneous spherical Al2O3-Y2O3-SiO2-MnO-Cr2O3 inclusions in yttrium-treated stainless steel will transform into irregular complex inclusions with Y-rich phase(Y2O3-SiO2)+Al-rich phase(A12O3-MnO-Cr2O3)during heat treatment.With the increase in heat treatment temperature,the transformation of the yttrium-based inclusions is promoted first and then reduced.Among them,the transformation of the inclusions at 1373 K is the most significant;5)The transformation mechanism of yttrium-based rare earth inclusions during heat treatment is considered to be the mutual effect of(i)internal transformation of inclusions owing to the crystallization of glassy oxide and(ii)reaction between inclusions and solid steel;6)During hot deformation,the yttrium-based oxides(Al2O3-Y2O3-SiO2-MnO-Cr2O3)will also change to heterogeneous inclusions with Y-rich and Al-rich phases.As the reduction increases,the transformation of rare earth inclusions is promoted.The effect of strain rate on the transformation of rare earth inclusions can be neglected.In the temperature range of 1223 K-1623 K,increasing the deformation temperature will accelerate the transformation of the rare earth inclusions;7)Yttrium-based rare earth oxide inclusions deform during hot deformation process.With increasing reduction,the true elongation of inclusions increases,but the deformability of inclusions decreases.In the range of 0.01s-1 to 1.0s-1,the true elongation and deformation index of the inclusions increase with the increase in deformation rate.In the range of 1223 K?1623 K,the true elongation and deformation index of the inclusions increase first and then decrease with increasing deformation temperature.The deformability of the inclusions is the best at 1323 K;8)During the heat treatment of MnO-SiO2 type oxide/steel matrix diffusion couple,the excess oxygen generated from the decomposition of FeO in oxide diffuses into the iron-based alloy and reacts with Mn and Si,causing the decreasing in Mn and Si contents in iron-based alloys and the precipitation of fine oxide particles.Thus,forming Mn depleted-zone(MDZ)and particle precipitation region(PPZ).Increasing heat-treatment time or heat-treatment temperature will promote the solid-state reaction between the oxide and steel matrix;9)A dynamic theoretical model for predicting the solid-state reaction between MnO-SiO2 oxide inclusions and Si-Mn deoxidized steel during heat treatment is conducted.