Effect of Mn addition on eutectoid transformation of FeO
Structure control of iron oxide scale is an important index to evaluate the adhesion of scale,which directly affects the amount of powder loss and surface quality during product stamping.Currently,Mn element is commonly added to high-strength series steel to improve product strength level.In order to study the influence mechanism of Mn addition on the structure and eutectoid transformation of iron oxide scale,Girder steel with Mn mass percent of 1.8%-1.9%and low carbon steel with Mn mass percent of 0.15%-0.25%were used as the analysis objects.Firstly,the FeO transformation characteristics of Mn-containing steel at different temperatures were observed by thermograys and electron microscopy.It was found that with the increase of holding time at high temperature,the oxygen content in the FeO layer near the interface of Fe3O4 easily reached susaturated state and decomposed,forming the characteristics of pre-eutectoid Fe3O4 layer transformation.At low temperature,Fe ion migration and diffusion rate decrease in short distance and FeO decomposition change to petal-like Fe3O4 in the middle.At the same time,it is confirmed that the nose tip temperature of FeO eutectoid transformation of Mn-containing steel is around 450℃.It is found that the eutectoid reaction always starts at the interface of FeO and Fe3O4,because there are phase fluctua-tions and energy fluctuations at the interface of the two phase regions,which creates favorable conditions for the nucleation and growth of the new phase.The eutectoid decomposition of FeO is different between Mn containing steel(1.8%)and low Mn steel(0.18%)at 450℃for 60 min.The eutectoid decomposition ratio of Mn containing steel is only 18%,while that of Mn containing steel can reach 60%.The average interval between the eutectoid sheets of Mn-containing steel was 350 nm by SEM.The layer spacing of low Mn steel is 250 nm.It is found that FeO proportion of Mn steel in hot coil sheet increases under the same process conditions.At last,SEM and electron probe analysis showed that the eutectoid structure(Fe3O4 and α-Fe)of Mn-containing steel had obvious Mn emis-sion characteristics during the eutectoid transformation process,which led to the increase of Mn elements in the adja-cent undecomposed FeO regions.According to the binary phase diagram analysis,it is found that the dissolution of MnO in FeO can stabilize the FeO phase,reduce the driving force of phase transformation,and inhibit the eutectoid transformation of FeO containing Mn in general.Therefore,it can be seen that the effect of MnO-FeO on the struc-tural transformation of scale needs to be considered for Mn-containing high-strength steel(hot-cold system).From the perspective of easy descaling and pickling of cold-rolled high-strength steel,from the perspective of eutectoid scale ratio control of hot-rolled black-scale steel,combined with the characteristics of hot-rolled continuous cooling,the difference matching coiling temperature and cooling mode is the direction of further research.
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