Microstructure Transformation And Control Of Stainless Steel Under Near-rapid Solidification

Twin-roll strip cast process is a advanced technology in the fields of current metallurgy, that the metal melt is directly cast into a pool made of two rotating rolls, solidified under chill cooling of the rolls and formed a strip with the rolls.An interfacial heat transfer simulation apparatus is used to study the austenitic stainless steel microstructure under near-rapid solidification. The samples have larger secondary dendnte arm space and take place morphology transition from dendrite growth to cell growth with the increasing of immerging time. Similar to the twin-roll strip cast process, they have same kind of interfacial heat transfer ,solidification type and also the same cooling rate. The microstructure transition of stainless steel cast strip is observed dynamically at real time with the Lasertech Confocal Scanning Laser Microscope. During 600-900 ℃,the carbides are precipitated first in grains inside, then grains interfaces. However, the precipitated carbides is dissolved slowly into substrate when the strip is heated higher than 1000℃.While stainless steel is melt, the strip produced by strip cast process is melt first mainly in grains inside, but stainless steel produced by conventional processes first mainly in grains interface.Through stainless steel strip microstructure analysis, it can be found that the inclusions have bigger diameter size and most distributing equably as spheres, no inclusions gathering has been found. The carbides precipitate in grains interface and phases boundary between ferrite and austenite, uncontinuously distributing as spot or line. The ferrite in stainless steel mainly distributes as acicula or cosh, the counts are effected by alloy components, solidification cooling rate and so on, it's a linearity relation with alloy's Cr/Ni ratio. Under rapid cooling rate, lathy ferrite will be formed among the austenite dendrite branches, badly effect on the stainless steel microstructure and properties. A Ni, Mn elements segregation is found in strip thickness direction, related to the strip cast process due to the effect of solidification diffusion and meltflow.