Study On Intermediate Transformation For The Ultra-low Carbon Steel Non-containing Nb And Ti Elements

The effect of the amount of deformation at the high temperature,cooling rates and intermediate isothermal time on the phase composition,the state of microstructure and mechanical properties in the Fe-0.06C-1.6Mn-0.35Ni-0.3Si-0.28Mo(wt.%)steel was investigated by X-ray diffraction,optical microscopy,transmission electron microscopy(TEM)and mechanical test.The effect of the amount of deformation at 850℃ on the CCT curves in intermediate transformation area for experimental steel was researched,and the results showed that the intermediate transformation start temperature(Bs)decreased with increasing the amount of deformation.When the amount of deformation was less than 20%,the Bs was only slightly decreased,and the Bs was decreased rapidly under the more than 20% deformation.Due to the hindering effect of dislocation tangles on the phase transformation,the amount of formed bainite was reduced with increasing the amount of deformation.Experiments confirmed that20% was critical deformation amount significantly hinder intermediate transformation.The effect of cooling rates after deformation at 20% on microstructure and mechanical properties was researched,and the results showed that the proportion of sheaf-like bainite on entire organization was increased with increasing cooling rate when the cooling rate was less than 15℃/s,the tensile strength and impact toughness of the materials also increased.The content of sheaf-like bainite decreased when the cooling rate exceed 15℃/s,and the tensile strength and impact toughness of the materials decreased.Experiments confirmed that the suitable cooling rate of forming larger number of sheaf-like bainite was 15℃/s.The effect of isothermal time at 450℃after deformation at 20% on microstructure and mechanical properties was researched,and the results showed that the sheaf-like bainite microstructure becomes irregular with the extension of isothermal time under the isothermal time was less than 30 min,the main features of sheaf-like bainite was boundaries blurred.When the isothermal time was less than 20 min,corresponding to the tensile strength and impact toughness of the materials gradually increased with prolonged isothermal time.The tensile strength and impact toughness of the materials were reduced when isothermal time was longer than 20 min.Experiments confirmed that the isothermal time for experimental steel with higher tensile strength and impact toughness was 20 min.The fine microstructure of sheaf-like bainite was observed,and the results showed that the bainitic ferrite strip contains a high density of dislocation substructure under the isothermal time was less than 20 min,the width of bainitic ferrite strip with isothermal longer time was larger than that of isothermal shorter time,the features with fusion appeared between bainitic ferrite strips.