Showing papers by "S. Sankaran published in 2021"

Influence of rolling temperature and strain on the microstructural evolution and mechanical properties in quench and partition (Q&P) steels

Abstract: Three different compositions of C–Mn–Si–Al type Q&P steels, A, B and C, were subjected to thermo-mechanically controlled processing (TMCP) at two different rolling temperatures viz. 1075 °C and 1150 °C, using two different levels of strain 0.51 and 1.1, given in single pass, followed by direct quench and partitioning (DQP) to produce a martensite – austenite structure. It was observed that Si and Al significantly influenced the formation of prior austenite grains (PAGs) during TMCP and thereby the evolution of microstructure after the subsequent Q&P treatment. With the help of constitutive models, it was found that mainly dynamic recrystallization (DRX) along with/without meta-dynamic recrystallization (MRX) were likely to occur during TMCP and their effects would be retained by immediate quench from the high temperature of processing. Electron back-scattered diffraction (EBSD) results confirmed that the PAGs were essentially DRX grains. Rolling at the lower temperature produced finer PAGs, thereby favouring the formation of larger amount of retained austenite. Effect of strain, which enhanced the enrichment of carbon in the austenite, was more pronounced at the lower rolling temperature and at higher applied strain. More than the grain refinement, dislocation density was found to be the dominant strengthening mechanism in the martensite. A maximum dislocation density of 2.18 × 1015 m−2 was observed for the steel rolled with 1.1 strain at 1150 °C. Higher rolling temperature favoured higher dislocation density in the steels, owing to the attainment of higher martensite carbon content. Consequently, TMCP with a higher rolling temperature favoured higher strength properties. In fact, the maximum strength of 1593 MPa, with yield strength of 1042 MPa and total elongation of 13% were achieved in Steel-A rolled at 1150 °C with 1.1 strain (1.1-DQP-1150 °C Steel A). After evaluating the different strengthening mechanisms operating in these steels, it was concluded that the maximum strength achieved in Steel A was primarily due to its alloy composition.

Temperature dependent partitioning mechanisms and its associated microstructural evolution in a CMnSiAl quenching and partitioning (Q&P) steel

Abstract: The effect of temperature (350 °C i M S i 450 °C) on the partitioning mechanisms and the final microstructure evolution in a CMnSiAl quenching and partitioning (Q&P) steel was investigated. The microstructure of both the Q&P specimens, comprised of distorted BCC or pseudo tetragonal martensite structure with two different characteristics namely (i) tempered or carbon depleted martensite that formed during initial quenching (M f i 240 °C i M S ) and partitioning step and (ii) carbon enriched fresh martensite that formed after partitioning step and final quenching (RT) together with blocky and inter-lath films of retained austenite. In addition, packets of M/A constituents were observed in Q&P-350-1min specimen and some traces of carbide and plate martensite were observed in Q&P-450-1min specimen. The increase in partitioning temperature led to nearly 2% increase in the amount of retained austenite (both blocky and inter-lath) with increased carbon content of 0.27 wt.%. Along with carbon partitioning, slight interface mobility/isothermal martensite formation was also observed in the case of specimen partitioned at 350 °C, whereas tempering effect was predominantly seen in the case of specimen partitioned at 450 °C. Irrespective of the partitioning temperature, the amount of carbon required to stabilize the retained austenite at RT was found to be about 1.15 wt.% and was confirmed through APT analysis.