Effect Of Reduction Annealing On Iron Scale On Steel Surface

The experimental steel grades in this paper are SPHC plain carbon steel.The GLEEBLE3800 test machine was used to study the transformation history of iron oxide scale during the heating process.The thermal transformation method was used to study the microstructure transformation of hot rolled strip during the heating process.The tube-type reduction resistance furnace was used to carry out the experiment of the influence of the reduction temperature on the scale of the iron oxide scale,and the reducing gas H₂/N₂mixed gas was introduced to reduce the surface scale of the hot-rolled strip.After the experiment,the surface and the cross section of the samples were examined and analyzed by optical microscopy,electron microscopy and electron probe.The following conclusions were obtained:Through the research on the transformation law of iron oxide scale during the heating process,the original structure of the hot-rolled steel oxide scale is composed of the outer layer of Fe₃O₄ and the inner layer of eutectoid structure?Fe₃O₄/Fe?,which is in the oxidation scale during the heating process.The analysis of the organization prioritizes the eukaryotic reverse transformation,and then the economization organization continues to react.The preferential reaction of the eutectoids during the heating process is completed in two steps.The first step is the dissolution of Fe,the second step is the reverse transformation of the eutectoid structure,and Fe₃O₄ and Fe form FeO.With the increase of heating temperature,the eutectoid organization continues to undergo reverse transformation,and Fe₃O₄ changes to FeO.The final scale structure forms a two-layer structure consisting of the outer layer of Fe₃O₄ and the inner layer of FeO.It is known that the surface reduction product is porous iron at a reduction temperature of 500?.It is loose and porous,and the pores are dendritic,similar to the lamellar eutectoid structure.At a reduction temperature of 600?,the surface partially becomes dense,and the dense iron is transformed,and the pores become large.A thin layer of dense iron is observed from the cross section,and porous iron is distributed inside.At a reduction temperature of 700?,the surface has covered a large area of dense iron,and elemental iron is precipitated in a granular form,and a large area of dense iron is observed from the cross section.At the reduction temperature of 800?,the surface-covered reduction product is already dense iron,there are many pores,and the existence of grain boundaries can be observed.It is suspected that elemental iron grains are sprouted and grown,and the iron sheet has been reduced to Dense iron,there are a large number of holes,and the combination with the substrate is poor.When the reduction temperature is 900?,the surface is reduced in quality,the roughness is small,bright white,only a few pores exist,there are large grains,and the iron oxide scale is reduced to dense iron from the cross section.The reduction product elemental iron and the matrix are well combined.The effect of coiling conditions on the microstructure and thickness of the iron oxide scale of the wheel steel shows that the higher the coiling temperature,the longer the cooling time,the more the eutectoid transformation,the less the residual FeO in the scale,and the thicker the thickness of the iron sheet.The faster the wearing speed,the more unfavorable the eutectoid transformation,the iron oxide structure is the first eutectoid Fe₃O₄ and the eutectoid Fe,the residual FeO is more,and the thinner the thickness of the scale.