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Segregation and Homogenization of Alloying Elements in X70 Steels

  • Author / Creator
    Raj, Rishav

  • Solidification during continuous casting of X70 microalloyed steel results in two types of elemental segregation; microsegregation (interdendritic) and macrosegregation (at the centerline). Both micro and macrosegregation lead to deterioration of steel properties and, hence, a lower degree of segregation is highly sought after. The extent of segregation that occurs is dependent upon the continuous casting parameters and the composition of the steel. To reduce microsegregation, and to a lesser extent macrosegregation, homogenization of the cast material is undertaken. Homogenization is the process by which the cast steel is heated to a specific temperature and held for a period of time. During homogenization, diffusion of elements from regions of high concentration to low concentration occur. The ability to reduce segregation depends on the extent of segregation that occurs during solidification and on the homogenizing conditions.
    A quantitative study of element segregation was undertaken in order to understand the solidification behavior of alloying elements during continuous casting of microalloyed steels. Two X70 steels, S1 and S2, were studied to quantify the as-cast segregation and the effect of homogenization on segregation. The steels were chosen based on their different nominal compositions of Mn, Cr and Nb. A third steel, S3, was used to study the unidirectional solidification of microalloyed steel at two different cooling rates. The study presented in this work is comprised of two components; measurement and prediction of segregation during solidification, followed by the modeling and laboratory testing of the homogenization process.
    Electron microprobe analysis (EMPA) was used to map the concentration distribution of Mn, Cr, Nb and P, and to quantify their extent of segregation in industrial as-cast slabs. Segregation ratio (S.R.), defined as the ratio of measured concentration to the nominal composition, was used as a measure to quantify the extent of segregation. Bridgman directional solidification was used to study the effects of cooling rate on the partition coefficients for Mn, Cr, Nb and P. Two samples were solidified at a cooling rate of 0.4oC/s and 0.2oC/s. Laboratory homogenization heat treatments were performed at 1000, 1100 and 1200oC for 3, 6 and 12 h to study the effects of homogenization on segregation. ThermoCalc software was used to simulate solidification and diffusion of alloying elements. The Scheil module was used to model segregation during solidification and the DICTRA (Diffusion Controlled Transformation) module was used to model the diffusion of alloying elements during homogenization.
    Among the studied elements, Mn displayed the highest degree of microsegregation ratio (1.09-1.10), followed by Cr (1.04-1.08) and P (3.00) followed by Mn (1.20-1.50) and Cr (1.10-1.13). The precipitation of Nb in the form of carbonitrides during casting complicated the EMPA segregation measurements for Nb.
    The measured microsegregation concentration profiles of Mn, Cr and Nb in as-cast slabs compared equally well with both Gulliver-Scheil (GS) and modified GS calculations, while microsegregation profiles for P were better predicted using the modified GS calculations. The inclusion of back diffusion for P during modified GS calculations resulted in a closer match with the experimentally obtained values.
    A decrease in partition coefficient was observed for Mn, Cr and P with a decrease in cooling rate from 0.4oC/s to 0.2oC/s. The magnitude of the decrease in the partition coefficients was low for Mn and Cr (0.05). Formation of precipitates of Nb again inhibited the partition coefficient calculation for Nb. Being a more robust method as compared to S.R., the partition coefficient calculation method was also applied to as-cast and homogenized samples data for Mn to further explore its usage.
    The decrease in S.R. after homogenization was negligible for Mn and Cr
    in the interdendritic samples. The P microsegregation ratio decreased from 0.66 to 0.48 after 6 h at 1200oC. In centerline samples, the macrosegregation ratios for Mn and P decreased to 1.21
    from 1.45 and 2.06 from 3.90, respectively. The change in macrosegregation ratio for Cr was relatively low (<0.10). The extent of diffusion for P was considerably higher as compared with Mn, Cr and Nb in both EMPA and ThermoCalc results. The degree of diffusion of elements was lower in the DICTRA predictions as compared with the EMPA results from the homogenized samples.

  • Subjects / Keywords
  • Graduation date
    Fall 2020
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/r3-z874-sg41
  • License
    Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.