Solidification mode and residual ferrite characteristics of high nickel 316L stainless steel continuous casting billet
The presence of residual ferrite in austenitic stainless steel significantly influences its performance.The characteristics of residual ferrite is related to composition,cooling rate,and solidification mode.It focuses on a 316L austenitic stainless steel continuous casting billet,which possesses a high nickel content and is in the eutectic point of composition.Moreover,its solidification mode exhibits susceptibility to alteration.The characteristics and distribu-tion of residual ferrite in the thickness direction of 316L austenitic stainless steel billet were investigated using optical microscopy(OM)and image-pro-plus software.The results reveal that the residual ferrite exhibits a morphology characterized by short rods,granules,skeletal structures,and network formations along the thickness direction.Fur-thermore,the distribution pattern of ferrite resembles an"M"type distribution which is similar to that observed in slabs.The ferrite content(volume percent)exhibits a fluctuation around 2%from the billet surface to a depth of 55 mm,reaching its highest value(4.77%)at a distance of 65mm from the surface before abruptly decreasing at 75 mm.Subsequently,there is an increase in ferrite content towards the center.Electron probe microanalysis(EPMA)was employed to investigate Cr,Ni,Mo,Si,Mn and other elements,revealing that the secondary austen-ite formed through solid phase transformation retains certain compositional characteristics inherited from ferrite.The ternary phase diagram of Fe-Cr-Ni and the equilibrium solidification process of the edge and center of the billet were calculated using a thermodynamic calculation software(Thermo-Calc).The results indicate that FA mode is observed at the edge while AF mode is observed at the center of the billet.Notably,the solidification mode deter-mined by residual ferrite morphology differs from the thermodynamic calculation results.The casting billet solidifies in the AF mode at both the edge and center,while a skeleton-like ferrite forms which indicate the FA mode within the columnar crystal region.Subsequently,an analysis of the formation mechanism for residual ferrite distribution is conducted.The lower content of ferrite observed in the surface fine crystal region can be attributed to the AF mode of solidification on the surface.The ferrite content reaches its maximum value at a distance of 65mm from the billet surface due to a transition in solidification mode from AF to AF+FA.The decrease in ferrite content at 75mm is attributed to the shift back to AF mode during solidification.From 75 mm to the center of the billet,the equiaxed crystal structure and the reduced cooling rate increase the diffusion distance required for solid phase transformation,resulting in an increase in ferrite content.
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