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

Abstract

High temperature confocal microscopy (using cooling rates of 1 and 20 C s À1 ) has been used in conjunction with segregation modeling (for cooling rates of 0.1-100 C s À1 ) to understand the implications that high cooling rates, representative of that seen for belt casting, has on compositional inhomogeneity and precipitation in high strength low alloy steels. Due to the increased level of segregation (and reduced back diffusion) precipitates are predicted, and shown, to form in the interdendritic solute rich regions at temperatures higher than would be expected under equilibrium conditions at the higher cooling rate. This results in a larger volume fraction of precipitates in samples held at 1000 C after being cooled at 20 C s À1 compared to those cooled at 1 C s À1 .

Effects of residual elements during the casting process of steel production: a critical review

“…Previous researches [74][75][76][91][92][93][94] have studied the significance of low cooling rates during the solidification stage of casting to minimise the inter-dendritic segregation of solute elements, which also decreases the susceptibility to hot-tearing problems. Cooling rate during solidification of conventional continuous casting of ∼250 mm thick slab casting (∼1°C s −1 ) is lower than that of ∼50 mm thin-slab casting (∼3.3°C s −1 ) over the temperature range of 900°C-1350°C [118].…”

“…Gui et al [72] evaluated average value of k for Si (0.585-0.646), S (0.040-0.037), Mn (0.723-0.691), C (0.147-0.149) and P (0.327-0.330) for different levels of S (0 wt-% to 0.35 wt-%). Slater et al [74] predicted increased inter-dendritic segregation of Nb solute (increase in ratio of Nb content in solute maximum to solute minimum region from 2.9 to 4) with an increase in cooling rate from 1°C s −1 to 100°C s −1 (in the study by Slater et al [74], cooling rate is defined for the stage of cooling from liquidus temperature to 1000°C). The higher cooling rate promotes higher segregation and minimises the time for back-diffusion to occur [74,75].…”

“…Slater et al [74] predicted increased inter-dendritic segregation of Nb solute (increase in ratio of Nb content in solute maximum to solute minimum region from 2.9 to 4) with an increase in cooling rate from 1°C s −1 to 100°C s −1 (in the study by Slater et al [74], cooling rate is defined for the stage of cooling from liquidus temperature to 1000°C). The higher cooling rate promotes higher segregation and minimises the time for back-diffusion to occur [74,75]. Dobrovska et al [76] estimated a back-diffusion parameter for different solute elements (C, Si, Mn, P and S) in steel based on different microsegregation models.…”

“…With an increase in cooling rate (0.1°C s −1 to 100°C s −1 ), T NZ and T NF decrease due to severe inter-dendritic segregation of solute elements (S, P and Si). As mentioned previously, the higher cooling rate promotes higher segregation and minimises the time for back-diffusion to occur [74,75]. Therefore, lower cooling rate (for the solidification stage of continuous casting process of steel) is effective to reduce inter-dendritic segregation of residual elements and minimise the chances of hot-tearing [91–94].…”

Interdendritic microsegregation development of high-strength low-alloy steels during continuous casting process

“…Recently, this has been used to account for segregation in HSLA steels. Notably, Slater et al [8] have studied Nb segregation during solidification and back diffusion, and Li et al [9] investigated elemental re-distribution at lower temperatures during solid state phase transformation γ → α in HSLA steel. Neither of these studies, however, has simulated the role of the peritectic reaction in alloying element microsegregation during solidification and cooling.…”