Study Of ShaJin Defect About SUS430Stainless Steel And Development Of Process

This thesis takes the SUS430stainless steel supplied by a steel factory as the studying object, and systematical investigations have been carried out on the Shajin defect of SUS430stainless steel, including the main mechanism of Shajin and the influnces of hot-rolled sheets from different sources and the production processes on SUS430stainless steel. Annealing process and pickling process were also researched.,There are four main research results as follows.(1) Carbide precipitation was studied through experiments of SEM and EDS, The results indicated that SUS430stainless steel sheets with Shajin defect have a large number of carbide precipitates along the rolling direction and they aggregate around the grain boundaries. Too many carbides make the grain boundary lack of the element of Cr, in another word chromium depletion appears. As a result, the phenomena of intergranular corrosion appeared in the pickling. In fact, it is intergranular corrosion that leads to the binding force between the grain and grain boundary drops sharply. That is why Shajin defect appears.(2) Through detecting SUS430stainless steel hot-rolled sheets from different sources, it is known that hot-rolled sheets from Kawasaki company in Japan and Pohang Iron and Steel company in South Korea have V element, and accordingly the probability of Shajin defect in these sheets is relatively low; while V element could not be detected in the domestic hot-rolled sheets in which apparent Shajin defect appears. Therefore, V element in SUS430stainless steel hot-rolled sheets is closely related to the Shajin defect.(3) By tracking the annealing process of SUS430stainless steel in3AP process, it is found that the positions of SUS430stainless steel and the number of coils have a significant impact on Shajin defect. The annealing process was simulated and the improved annealing process is that the annealing temperature is840centigrade and the holding time is30hours.(4) The grain sizes of No.1after pickling are different between Shajin defect sheets and qualified sheets through microstructur analysis. In addition, the surface roughness of Shajin defect sheet is bigger than that of qualified sheets through site tracking. So the pickling line of SUS430stainless steel has a serious impact on Shajin defect. Through simulation of the pickling process, the law of concentration of HNO3and HF ratio, the sheet speed and the pickling temperature can be gained. The on-site pickling process properly can be adjusted combining the results of the simulation.The adjustment process is:increasing the sheet speed to50m/min, increasing the concentration of mixed acid, the concentratio of HNO3is140g/L, and HF is12g/L, while the amount of blasting is increased.(5) Through simulating the processes of steel sheet with Shajin defect produced in April of2011by BP neural network as well as comparing measured values with predicted ones, the success rate of the mode is83.3%. So the applying of BP Artificial Neural Network technology into the production process of SUS430stainless steel can predict the incidence of Shajin reasonably in3AP pickling and shot peening processes.