The Influence of Segregation of Mn on the Recrystallization Behavior of C-Mn Steels

Abstract

The influence of Mn segregation that occurs during casting on recrystallization kinetics has been explored for a C-Mn automotive steel. A homogenization heat treatment was used to remove Mn segregation, while maintaining a similar initial austenite grain size to the segregated condition, to provide base-line comparison data. Deformation trials were carried out in a Gleeble thermomechanical simulator to 0.3 strains at 900°C, and significant differences in both recrystallization times and final grain sizes were seen between the two conditions. The segregated sample incurred a longer recrystallization time and a wider recrystallized grain size distribution than the homogenized sample. A finite element model was used to predict the local deformation strains based on variations in material properties caused by segregation; the model used data from XRF elemental mapping of Mn to define the local mechanical properties based on differences in solid solution strengthening. Local strain variations ranging from 0.19 to 0.49 were predicted in the segregated sample for an overall applied 0.3 strain (compared to 0.26 to 0.35 in the homogenized sample); using these strains to predict the recrystallized grain sizes gave a much better prediction for the grain size distribution.

Implications of Accelerated Solidification Rates Seen in Belt Casting on Precipitation in Nb Bearing Steels

“…In addition to Equation , Figure shows images taken from the CSLM during cooling of the sample at 1 and 20 °C s −1 . Due to the direction of cooling, the secondary dendrite arms growth is out of the plane imaged (primary dendrite grow parallel to the droplet surface) . Equation predicts the SDAS to be 148 and 47 µm at the 1 and 20 °C s −1 solidification rates, respectively, and Figure shows that these values used are in agreement, with the experimentally determined SDAS being 155 and 42 μm, respectively.…”

“…Casting and Thermal Profiles A high temperature confocal scanning laser microscope (CSLM) was used to simulate different cooling rates (1 and 20 °C s −1 ) to investigate precipitation in the steel. A full description of how the CSLM works can be found in the literature . For this work, samples of 2 × 2 × 2 mm cubes were prepared and placed in an alumina crucible.…”

Near Net Shape Casting: Is It Possible to Cast Too Thin?

“…A previous study has shown that the surface measurements of SDAS in the CSLM are the same as observed in the bulk material. [19] Figure 2 shows the secondary dendrite arm spacing (SDAS) as a function of the cooling rate and a typical image of the steel surface in the CSLM where the solidifying steel secondary dendrite arm tips can be seen; as the dendrite grow approximately parallel to the steel surface, the spacing between the observed tips gives the SDAS. [19] The SDAS with cooling rate data has been compared to the literature [20] for a similar steel (0.17C-1.5Mn-0.4Si) and good agreement between the two can be seen in Figure 2.…”

The Effect of Micro-Segregation on the Quantification of Microstructural Parameters in Grade 91 Steel

“…Reliable quantitative microstructural data are also required to feed into creep damage models for these materials. [27] It is clear that micro-segregation can have significant effects in other steel types such as those detailed for high-strength low-alloy (HSLA) steels [28][29][30] and in automotive grades, [31] where the effects of segregation on precipitation and grain structure affecting properties and property scatter have been identified. In the case of HSLA steels, a study by Wang et al regarding the segregation of Nb showed a subsequent formation of an increased amount of Nb-rich precipitates in the interdendritic regions resulting in a banded microstructure with a bimodal grain size distribution.…”

“…[32] Regarding C-Mn automotive steels, Slater et al found a Mn-segregated steel sample which only showed a partially recrystallized microstructure for the same holding times of an unsegregated sample. [31] In both examples, the objective was to investigate the variations in mechanical properties due to inhomogeneous compositions and microstructures. The objective of this paper is to study the implications of micro-segregation of Mo and its effect on the quantification of the intermetallic Laves phase in Cr.91 steel, a key detail that to the author's knowledge has not been previously appreciated or studied.…”