Please cite this article as: R. Feng S. Li X. Zhu Q. Ao Microstructural characterization and formation mechanism of abnormal segregation band of hot rolled ferrite/pearlite steel, Journal of Alloys and Compounds (2015), Abstract: In order to further reveal the microstructural characterization and formation mechanism of abnormal segregation band of hot rolled ferrite/pearlite steel, the microstructure of this type steel was intensively studied with Scanning Auger Microprobe (SAM), etc. The results show that severe C-Mn segregation exists in the abnormal segregation band region at the center of hot rolled ferrite/pearlite steel, which results from the Mn segregation during solidification process of the continuous casting slab. The C-Mn segregation causes relative displacement of pearlite transformation curve and bainite transformation curve of C curve in the corresponding region, leading to bay-like shaped C curve. The bay-like shaped C curve creates conditions for the transformation from supercooling austenite to bainite at relatively lower cooling rate in this region. The Fe-Mn-C Atomic Segregation Zone (FASZ) caused by C-Mn segregation can powerfully retard the atomic motion, and increase the lattice reconstruction resistance of austenite transformation. These two factors provide thermodynamic and kinetic conditions for the bainite transformation, and result in the emergence of granular bainitic abnormal segregation band at the center of steel plate, which leads to lower plasticity and toughness of this region, and induces the layered fracture.
The influence of predeformation and stress on the isothermal bainite transformation has been investigated in G55SiMoV steel via microstructure observation and kinetic analysis. It was found that the bainite transformation became faster and at the end of isothermal holding the bainite fraction increased under the applied stress condition. When the stress increased to 150 MPa, the bainite distribution became to be nonrandom in G55SiMoV steel. Different deformation conditions, in which both promotion and inhibition occurred in the same steel, were created. The promoting and inhibiting factors affected bainite transformation comprehensively. 20% deformation could promote the bainite transformation when it deformed at 900°C, but prevent bainite transformation when it deformed at 750°C. Increase of ferrite nucleation rate caused by distortion and dislocation, would suppress the growth of bainite carbides and make most carbides without full growth be finer and shorter.
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