The Research On Microstructure And Solidification Mode Of Low-Nickel Austenitic Stainless Steel

Cr-Mn-N based austenitic stainless steel is extensive attentioned because of their low-nickel and low-cost characteristics.But because of the composition is different with the traditional Cr-Ni based austenitic stainless steel,these series of stainless steel appeared edge crack during the process of rolling.This kind of edge crack related with solidification mode.Austenitic stainless steel solidification mode decided by the chemical composition,as well as effecting by cooling rate.In this paper, Cr15Mn9Cu2Ni1N which is a kind of new Cr-Mn-N based austenitic stainless steel was selected as experimental materials.Control cooling rate and adjust Cr,Mn content solidification experiment was made by using THERMORESTOR-W type thermal simulation of welding machine.The effect of cooling rate and Cr,Mn content on the microstructure and solidification mode of steel was studied with the metallographic microscope and EPMA test method of material.At first,the method of solidification experiments taking in the weld thermal simulation testing machine was empoldered.It was found that the holes appeared during solodification experiment bacause of the larger current coming into a greater electromagnetic stirring force.This problem can solve though any key operations such as strictly control the location of thermocouples,reducing the current of heating coil, appropriatly to extend the melting holding time and so on.Control cooling rate experiments show that when cooling rate is smaller, microstructure was vermicular ferrite distribution on the austenite matrix,solidification mode was FA mode.When cooling rate is higher,solidification mode changed as most liquid crystallized as ferrite,the residual liquid phase directly crystallized as austenite. With cooling rate increasing,ferrite morphology changed from vermicular,reticulate, dendritic to lath.Adjusting Cr and Mn content experiments show that when Cr content greater than 17.8%,the solidification mode changed from most liquid crystallized as ferrite then the residual liquid phase directly crystallized as austenite mode to FA mode.When content of Cr is 18.8%,the microstructure of samples with lower N content was duplex stainless steel with ferrite and austenite about 50%each other.The microstructure of simples with higher Cr content still was reticulate ferrite distribution on the austenitic matrix. Content greater than 11%,Mn is austenization element rather than ferrite element,solidification mode changed from most liquid crystallized as ferrite then the residual liquid phase directly crystallized as austenite mode to FA mode.Higher Mn content increased the composition undercooling degree of the forefront of solid-liquid interface,which made crystal morphology changes from cellular to dendritic.