Effect Of Heating Conditions On The Structure And Descaling Of Primary Iron Oxide Scale Of Typical Stainless Steel Casting Billets

Stainless steel casting billet is subjected to high temperature and oxidizing atmosphere in the heating furnace,and complex primary oxide scales will be formed on the surface,which is easy to be pressed into the steel matrix during the hot rolling process,resulting in the deterioration of the surface quality of the steel.For this reason,two typical stainless steel casting billets of 304 austenitic and 430 ferrite are used as the research objects,and the influence of heating conditions on the primary oxide scale structure and descaling of the two stainless steel casting billets is analyzed in this paper.Firstly,the influence of heating temperature and heating atmosphere on the structure,phase composition and formation mechanism of the primary oxide scales of two stainless steel casting slabs were studied by the isothermal continuous weight gain method.Second,the bonding force of the primary iron oxide scale and the substrate under different heating conditions was studied by the water quenching method and the micro-scratch method,and the surface roughness of the steel/oxide interface was measured by the optical profiler,revealing the relationship between of the primary iron oxide scale structure and descaling.The research results provided data reference and theoretical basis for the optimization of casting billet heating and descaling process.The main conclusions were listed as follows:When the two stainless steels are oxidized in the air at 1200?1280?,the oxidation weight gain curve followed the parabolic law,and the oxidation rate of 430 stainless steel was significantly higher than that of 304 stainless steel.Both stainless steel oxide scales were of three-layer structure,which consisted primarily of the outer Fe₂O₃ and Fe₃O₄,the inner Fe Cr₂O₄,and the transition layer Fe Cr₂O₄ and metal matrix.However,the morphologies of inner oxide layer and transition layer of the two stainless steels were different,the inner oxide layer of 304 stainless steel was dense,and the transition layer was distributed in a block shape,which has poor adhesion to the matrix;the inner oxide layer of 430 stainless steel had many holes and cracks,and the transition layer was distributed in a continuous root shape,which enhanced the bonding force between the iron oxide scale and the matrix.When the two stainless steels were oxidized in an atmosphere of O₂ and O₂+H₂O at 1200?,the oxidation weight gain curves also followed the parabolic law,and the introduction of water vapor accelerated the oxidation rate of the stainless steel.In the two heating atmospheres,the 304 stainless steel oxide scale completely peeled off,while the 430 stainless steel did not peel off.The 430 stainless steel generated irregular oxidation nodules in O₂,and the iron oxide scales generated in O₂+H₂O was thicker and uniform,and internal oxidation was more serious.The water quenching experiment results showed that the thickness and morphology of the transition layer were important factors that affected the bond between the iron oxide scale and the matrix.With the increased of heating temperature,heating time and the surface roughness of the original sample,the thickness of the transition layer,the surface roughness of the steel/oxide interface and the bonding force of the iron oxide scale and the matrix all increased.The micro-scratch test results showed that the bonding force of 430 stainless steel iron oxide scale and substrate was greater than that of 304 stainless steel,and iron oxide scale was more difficult to remove.In order to reduce the oxidation burning loss of the stainless steel casting billet and improve the descaling performance,the heating and holding temperature of the two stainless steels should not exceed 1240?,the water vapor content in the heating furnace should be reduced,and the surface roughness of the casting billet should be reduced as much as possible.