High-Temperature Oxidation Behavior Of Hot-Rolled Stainless Steel Sheets

In the hot-rolled annealing process,due to the high-temperature oxidation,the surface of the stainless steel is coated with a complex oxide layer and chromium-depleted layer.Their compositions and distribution law directly affect the subsequent pickling process and the surface quality of the stainless steel.Therefore,the study on the high-temperature oxidation behavior,especially the formation of oxide scales and the distribution of Cr-depleted layer holds great significance.In this paper,the 304 austenitic stainless steel and 430 ferritic stainless steel were taken as the research object.The high-temperature oxidation behavior at various annealing temperature and oxygen pressure were investigated by oxidation kinetic experiment,XRD and SEM-EDS analysis.The oxidation mechanism and transformation rule of oxides at different oxidation stages were proposed as well as the distribution of Cr-depleted layer.Under the experimental conditions of this paper,the following main conclusions were obtained:(1)After oxidation at annealing temperature for 7h,the oxide scales formed on the austenitic stainless steel and ferritic stainless steel were stratified.,The composition of oxides were as following.The outer layer was mainly composed of Fe2O3,the middle layer was Fe-Cr spinel-type oxides such asFeCr2O4and FeFe2-xCrxO4,and the inner layer was mainly Cr2O3.(2)The high temperature oxidation process of stainless steel can be divided into three stages:an initial antioxidant stage with the weight gain curve obeyed the parabola law;a period of linear oxidation-the instability oxidation stage;the dual oxidations stage which also obeyed the parabola law.(3)During the dual oxidation stage,as oxygen continues to diffuse inside the matrix,Cr2O3 was formed again at the oxide/matrix interface,and when the content of Cr is insufficient,FeCr2O4 and FeFe2xCrxO4 was formed again.The process was carried out cyclically,leading to the staggered distribution of the chromium-depleted and chromium-rich regions in the oxide layer.This phenomenon was common for the 430 ferritic stainless steel which finished at least one oxidation circle.For the 304 austenitic stainless steel,this phenomenon was only detected at the oxidation nodule.(4)During stainless steel high temperature oxidation,a chromium-depleted layer with a thickness of about 1.5-2?m was formed at the oxide/matrix interfaces.Chromium depleted layer was always located in the front of the inner oxide layer.The composition and morphology of the chromium depleted layer were highly affected by the inner oxide layer.In front of the oxide nodule,the "chromium-depleting" phenomenon in the chromium-depleted layer could be alleviated if SiO2 was formed.