Evolution Mechanism And Inlfuence Factors Of Oxide Scale Of Austenitic Stainless Steel In Multipass Hot Rolling Process

Finish rolling of austenitic stainless steel uses generally Steckel mills in factory. Duringthis process, surface on austenitic stainless steel easily forms thick oxide scale. The existenceof this kind of oxide scale may effect on surface quality and pickling process, so multipass hotrolling test was performed in the laboratory in order to simulate Steckel hot rolling process.This method was used to investigate evolution mechanism and influence factors of oxidescale of austenitic stainless steel in multipass hot rolling process. The results can be used tocontrol surface quality, and then provide theoretical bases to optimize product quality.In this paper, hot rolling device composed of two roll which was developed underinduction heating coil in THERMORESTOR-W test machine chamber, simultaneously, theexternal of testing machine designed a set of the gas distribution system, gas may enter intothe chamber by cooling system to oxidize austenitic stainless steel. Tests were performed onabove testing system using304austenitic stainless steel of as-cast condition. Scanningelectron microscope and X-ray photoelectron spectroscopy were used to analyze themorphology and phase types of the oxide scale, then evolution mechanism of oxide scale ofaustenitic stainless steel in multipass hot rolling process were investigated by them. On thebasis of evolution mechanism of oxide scale, hot rolling tests were conducted underconditions of changing heating temperature and atmosphere, by analysis of morphologychanges of oxide scale and Cr concentration within the alloy at oxide-metal interface, rootcauses of heating temperature and atmosphere influence evolution of oxide scale of austeniticstainless steel in multipass hot rolling process were investigated. At the same time, in order toinvestigate hot rolling effect evolution of oxide scale, austenitic stainless steel were not berolled and oxidized, by comparing morphology of oxide scale and Cr concentration within thealloy at oxide-metal interface to hot-rolled austenitic stainless steel, root causes of hot rollinginfluence evolution of oxide scale of austenitic stainless steel was investigated. Ultimately,comprehensive consideration of temperature, atmosphere, and hot rolling to influenceevolution of oxide scale, the schemes to control evolution of oxide scale were got. The mainconclusions are drawn as follows:1. The surface of austenitic stainless steel in multipass hot rolling process mainly formsingle layered scale, block oxide scale, and multilayer oxide scale. For single layered scale,forming process of it starts with nucleation of iron-rich oxides, subsequently, kineticconditions are satisfied, and continuous Cr₂O₃layer grow in metallic matrix. For block oxidescale, it forms by cracking of multilayer oxide scale when it has been rolled, so phase types and phase distribution in block oxide scale just like in multilayer oxide scale. For multilayeroxide scale, forming process of it starts with nucleation of iron-rich oxides，subsequently，oxidation process is controlled by protective oxidation and Cr₂O₃particles grow in metallicmatrix. With oxidation progress，Cr₂O₃in nearby metallic matrix become unstable, and theyconvert to stable phase FeCr₂O₄by breakaway oxidation which form stable healing layer.Since then oxidation process is controlled by duplex oxidation, the diffusion of iron frommetallic matrix toward the surface form iron oxides, simultaneously, oxygen diffuse inwardand internal oxidation occur. When Cr₂O₃become unstable again, the process above repeatsitself, finally leading to a complex multilayer oxide scale. After multipass hot rolling process,evolution mechanism of oxide scale is circulation oxidation patterns, with progress ofoxidation, protective oxidation, breakaway oxidation, and duplex oxidation are repeatthemselves deeper into metal matrix forming complex multilayer oxide scale.2.Heating temperature ranging from950℃to1050℃, forming multilayer oxide scaleon hot-rolled austenitic stainless steel when it was oxidized by74.5%N₂，9%CO₂，5%O₂，and11.5%H2O, but at condition of950℃,1pass rolled austenitic stainless steel’s surface formssingle layered scale. Multilayer oxide scale has lots of caverns. With elevated heatingtemperature and increased rolling pass, thickness of multilayer oxide scale increase.During hot rolling process, when heating temperature raised, Cr concentration within thealloy at oxide-metal interface will reduce, and then chemical failure or breakaway oxidationoccur, finally thick multilayer oxide scale form on surface of austenitic stainless steel.3. Morphology of oxide scale on multipass hot-rolled austenitic stainless steel relates toexist of water vapour in oxidizing gas, and have nothing to do with hot rolling passes andheating temperature. Hot-rolled austenitic stainless steel was oxidized by gas contain watervapour, multilayer oxide scale forms, while it was oxidized by gas which is absent of watervapour, single layered scale forms. Increased hot rolling passes and heating temperature mayincrease thickness of oxide scale, but which can not lead to change of morphology.4. Hot rolling has remarkable effect on oxidation of austenitic stainless steel. Hot-rolledand without being rolled austenitic stainless steel were oxidized by mixture gas, they havedifferent morphology of oxide scale. For hot-rolled austenitic stainless steel, thick multilayeroxide scale forms on its surface during short time’s oxidation, and this kind of oxide scale’sthickness increases when heating temperature increased. For undeformed austenitic stainlesssteel, its surface tends to form single layered scale，which always is not easy to distinguish byscanning electron microscope, this kind of oxide scale’s thickness and morphology have notobvious variation when heating temperature increased.5. In order to optimizing surface quality of the industrial hot Steckel rolling products, breakaway oxidation must be avoided or weakened by reducing level of surface deformation,by reducing the furnace temperature to950℃around, by removing water vapour in furnace,and reducing rolling pass.