Study On Scale Defect And Precipitation Behavior Of BN1TL Stainless Steel

As an indispensable steel,stainless steel has been widely used in daily appliances,buildings,automobiles and special industries.In daily production,after the austenitic stainless steel becomes the finished product of cold rolled plate through continuous casting,hot rolling and cold rolling,the existence of long stripe scale defects will be found in some areas of the surface,especially in the edges.Scale defects often appear on the surface of BN1TL austenitic stainless steel produced by a company,which have negative effects on the mechanical properties,corrosion resistance,secondary processing properties and other main properties of cold-rolled plates,and usually can only be treated as defective products by cutting out the scale defects.Therefore,the incidence of scale defects will directly affect the yield,and then affect the economic benefits of stainless steel companies.It is found that the main cause of scale defect is the grain boundary weakening of continuous casting billet,so the grain boundary precipitation behavior of continuous casting billet is studied.While studying the scale defect,it was found that there was serious segregation in the cold-rolled sheet of BNITL austenitic stainless steel.Then,the segregation of the coldrolled sheet and its formation reasons were studied.According to the current research,there are many reasons for the formation of scale defects on the surface of cold-rolled sheet,such as excessive residual ferrite content,slag entrapment in molten steel,precipitation phase and segregation.Therefore,it is necessary to study the material itself and the actual production situation in the factory,and find out the specific reasons for the formation of scale defects on the surface of BN1TL austenitic stainless steel cold-rolled plate.In this paper,taking BN1TL austenitic stainless steel produced by a company as the research object,the scale defect on the surface of cold-rolled plate was systematically studied.On the basis of clarifying the microstructure,composition and basic properties of scale defects on the surface of cold-rolled sheet,the continuous casting billet was tested and characterized,and the relationship between the quality of continuous casting billet and scale defects of coldrolled sheet was analyzed.The effects of hot charging and cold charging process on precipitation behavior of BN1TL austenitic stainless steel were studied by thermal simulation experiment.The research results can provide reference and theoretical support for the research of scale defect of austenitic stainless steel cold rolled sheet and the quality of continuous casting billet.The main contents and results of the research are as follows:(1)the morphology and composition of scale defect on the surface of BN1TL stainless steel cold-rolled sheet were studied.the results show that scale defect is a kind of interlayer defect,which consists of scale structure,oxide and cold-rolled sheet matrix in order from surface to interior.The depth of the defect is about 10μm～50μm,mainly concentrated in 20 μm～40 μ m.Oxide is contained in the cold-rolled sheet,and the main component is iron oxide.(2)The microstructure and element segregation of surface and core of BN1TL stainless steel continuous casting billet were studied.The results show that the corrosion resistance of intragranular and grain boundaries on the surface of slab is quite different,and there are pits where precipitated phases fall off There is a serious grain boundary weakening phenomenon in the surface layer of continuous casting billet;There is a serious composition segregation region in the core of continuous casting billet,which is mainly composed of Cr,Mn and N.(3)The formation mechanism of scale defects in BN1TL austenitic stainless steel is discussed.There is a strong relationship between the structure of continuous casting billet and scale defects.The causes of scale defects are that the grain boundaries on the surface of continuous casting are weakened,the grain boundaries on the surface of continuous casting billet are oxidized during hot rolling,and scale defects are formed after cold rolling.The layered segregation in the middle of cold-rolled plate comes from the core segregation of continuous casting billet.(4)Combined with the actual heating system of factory production,the influence of hot and cold loading process on precipitation behavior of BN1TL austenitic stainless steel was studied by using thermal simulation experimental machine.The results show that the intergranular precipitates begin to precipitate at about 1000℃ and gradually increase with the decrease of temperature.The precipitation behavior continues at 400℃ but the speed is slow.The grain size of continuous casting billet is the main factor affecting the number of precipitates.The more the number of grain boundaries,the more the number of precipitates.