The Effect of Ultra-Fast Heating on the Microstructure, Grain Size and Texture Evolution of a Commercial Low-C, Medium-Mn DP Steel

Abstract

The effect of ultra-fast heating on the microstructures of steel has been thoroughly studied over the last year as it imposes a suitable alternative for the production of ultra high strength steel grades. Rapid reheating followed by quenching leads to fine-grained mixed microstructures. This way the desirable strength/ductility ratio can be achieved while the use of costly alloying elements is significantly reduced. The current work focuses on the effect of ultra-fast heating on commercial dual phase grades for use in the automotive industry. Here, a cold-rolled, low-carbon, medium-manganese steel was treated with a rapid heating rate of 780 °C/s to an intercritical peak temperature (760 °C), followed by subsequent quenching. For comparison, a conventionally heated sample was studied with a heating rate of 10 °C/s. The initial microstructure of both sets of samples consisted of ferrite, pearlite and martensite. It is found that the very short heating time impedes the dissolution of cementite and leads to an interface-controlled α → γ transformation. The undissolved cementite affects the grain size of the parent austenite grains and of the microstructural constituents after quenching. The final microstructure consists of ferrite and martensite in a 4/1 ratio, undissolved cementite and traces of austenite while the presence of bainite is possible. Finally, it is shown that the texture is not strongly affected during ultra-fast heating, and the recovery and recrystallization of ferrite are taking place simultaneously with the α → γ transformation.

Simulation and characterisation of the microstructure of ultra-fast heated dual-phase steel

“…This retained austenite was observed using the EBSD. Small fractions of retained austenite (around 1%) were observed in both samples as reported previously in [21] and in Figure 9(b). With a closer look (Figures 9c,d), it appears that the austenite remains preferentially inside the prior pearlitic areas in close proximity to the lamellar or spheroidised cementite (Figure 9a).…”

“…As cementite is only partially dissolved, the areas near or inside the pearlitic colonies are expected to have higher carbon content in comparison to the areas that are in greater proximity to any carbon source. This heterogeneity can lead to different transformations during the cooling stage, as it is shown by Papaefthymiou et al [3] and Banis et al [21]. Therefore, mixed bainitic/martensitic microstructures are expected during quenching and, depending on the treatment, various fractions of ferrite and retained austenite.…”

“…Kaluba et al [9-11] have shown that high heating rates lead to a bainite-like, massive ferrite to austenite transformation that is not controlled by the diffusion of carbon (C) anymore, but by the movement of the interfaces. As previous work of the group has shown [3-7,21], during UFH, the dissolvement of pearlite and dissolution of cementite ( θ ) is averted. This also impedes the diffusion of carbon in the microstructure and creates a gradient in the microstructure and, thus, heterogeneity in the composition of the parent austenite.…”

“…The samples for the microstructure characterisation were cut from the homogeneously heat-treated zone of the Gleeble specimens. Then they were prepared using the standard procedure [21]. For TEM, disc samples were cut from the Gleeble specimens.…”

Fine microstructure formation in steel under ultrafast heating and cooling

“…This is because a high cooling rate results in a decrease in the Ms. The factors that lower the Ms point are an increase in the amount of carbon in the γ phase 38 , an increase in the dislocation density in the γ phase 39 , and the refinement of the γ grains 40 – 43 . Our previous report has shown that rapid heating increased the dislocation density in the high-temperature γ phase and formed the finer γ grains 16 .…”