The Influence of Segregation of Mn on the Recrystallization Behavior of C-Mn Steels
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RECRYSTALLIZATION in steel still remains one of the key approaches to refining grain structure during steel production and has been studied for many decades.[1–3] Empirical equations, such as the Dutta–Sellars model,[4,5] developed to predict recrystallization behavior, are widely used and provide an understanding of the governing mechanisms and sensitivities that define the recrystallization rate and final recrystallized grain size. The demand for improving properties of materials continues and, thus, there is a need to increase the accuracy in prediction of the full grain structure as this affects strength and toughness. Recently, full grain size distributions, rather than mode size,[6] have been used in recrystallization modeling, and it is known that the fine grains recrystallize first followed by the larger grains in the distribution, with the largest grains in the distribution having the biggest influence on toughness. Ensuring that the extremes of the grain size distribution are correctly represented in terms of recrystallization will help process optimization and product development. An
aspect that has not yet been considered is the effect that segregation during casting might have on the recrystallization kinetics and grain size during hot deformation. Many steels display the effects of casting in the form of segregation, for example, in microstructural banding of the ferrite and martensite in dual-phase steels[7] and in inhomogeneous Nb precipitation in HSLA steel,[8] to name just two. In common, C-Mn automotive forging grades segregation can be seen by local variations in Mn and Cr levels, and a banded ferrite + pearlite structure in the soft condition.[9] Mn is known to have in impact on solid solution strengthening with the empirical equations for strengthening indicating a typical increase in yield strength of 32 to 43 MPa/wt pct Mn in ferrite.[10] Cr, on the other hand, has little to no impact on the solution strengthening of steel.[11] Differences in local strength levels will have an effect during deformation, for example, via strain inhomogeneity during hot rolling or forging. This in turn may have a significant impact on recrystallization behavior due to the strong sensitivity of recrystallization time (e4) and recrystallized grain size (e1) to strain,[3] and is the subject of this paper.
C. SLATER, A. MANDAL, and C. DAVIS are with the University of Warwick, Coventry, CV4 7AL, U.K. Contact e-mail: c.d.slater@ warwick.ac.uk Manuscript submitted November 20, 2019. Article published online May 15, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B
VOLUME 50B, AUGUST 2019—1627
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the segregated and homogenized samples, were heated to 1100 C at the rate of 20 C/s and held for 5 minutes. The sample was then cooled to 900 C at the rate of 10 C/s, held for 30 seconds before being subjected to a compressive strain of 0.3 at a strain rate of 1 s1; tests with varying post deformation holding times ranging from 30 to 300 seconds were carried out prior to an air quenching at the rate of approximately 70 C
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