Structure-Property Correlations of a Medium C Steel Following Quenching and Isothermal Holding above and below the M_s Temperature

Abstract

The processing of advanced multiphase high strength steels often includes isothermal treatments around the martensite start temperature (M s) for achieving a refined microstructure comprising bainiteaustenite and/or bainite-martensite-austenite phase constituents. The objective of this research work was to investigate the structure-property relationship for a medium carbon, high-silicon DIN 1.5025 steel (Fe-0.529C-1.67 Si-0.72Mn-0.12Cr (in wt.%)) following isothermal holding close to the M s temperature (~275°C) to enable low temperature austenite decomposition. For realizing multiphase microstructures, DIN 1.5025 steel samples were austenitized at 900°C for 5 min and then quenched to the isothermal holding temperatures 350 and 250°C for various times ranging from 5 to 3 600 s. Microstructural investigation corroborated the formation of multiphase microstructure comprising tempered martensite, bainite, retained austenite, and fresh martensite in both the samples isothermally held above (350°C) and below the M s (250°C) temperature. The sample isothermally held at 250°C showed a much more refined microstructure in comparison to that held at 350°C due to the presence of a fraction of initial martensite laths which acted as potential sites for bainite nucleation. Also, the evaluation of mechanical behaviour showed that the best tensile properties in terms of high tensile strength and good ductility were achieved in samples with high volume fractions of both interlath and blocky retained austenite, particularly those isothermally treated at 350°C for 200 s and at 250°C for 600 s, respectively.

Characteristics and Kinetics of Bainite Transformation Behaviour in a High-Silicon Medium-Carbon Steel above and below the Ms Temperature

“…Quenching to 250 and 200 • C facilitated initial martensite fractions of about 20 and 57%, respectively. The martensite fractions on quenching to different temperatures (250 and 200 • C) were estimated from the dilatation curves using the lever rule and more details are given in our previous work [5]. Dilatation measurements were performed for all steps during the quenching and isothermal holding processes, as well as final cooling to room temperature.…”

“…A large amount of initial athermal martensite renders a significantly lower amount of untransformed austenite prior to the start of bainite transformation and hence has a low driving force as well at a relatively low holding temperature of 200 °C. A detailed account is presented in our previous work [5].…”

“…In Figure 3b, the transformation rate increased as holding temperatures increased between 300 and 450 • C. Figure 3d reveals that the volume fraction of the bainite phase decreased as the holding temperature below the M s decreased from 250 to 200 • C, and this is due to the formation of significant initial martensite before the start of isothermal holding. A large amount of initial athermal martensite renders a significantly lower amount of untransformed austenite prior to the start of bainite transformation and hence has a low driving force as well at a relatively low holding temperature of 200 • C. A detailed account is presented in our previous work [5].…”

“…In the last few decades, advanced high-strength steels (AHSSs) with multiphase microstructures, essentially comprising fine phase mixtures of bainite (B), martensite (M) and retained austenite (RA), have attracted renewed interest due to their superb combinations of high strength and good ductility as well as high strain hardening capacity, and are being considered as potential candidates for use in automobile and industrial applications [1][2][3]. Steels processed at a temperature close to M s temperature via the quenching and partitioning (Q&P) route with essentially finely divided martensite-austenite-nanostructured bainite structures, as well as quenching and bainititizing (Q&B) treatment with mainly ultrafine/nanostructured bainite-austenite structures, are two such groups of multiphase third-generation AHSSs that show greatly improved mechanical property combinations, including good ductility imparted by transformation-induced plasticity (TRIP), an effect of the finely divided retained austenite (RA) in the steels [2,4,5]. In order to achieve a mixture of ultrafine bainite and finely distributed, carbon-enriched retained austenite, isothermal heat treatment procedures close to (both above and below) the M s temperature have been suggested [6,7].…”

Effect of Carbon Partitioning and Residual Compressive Stresses on the Lattice Strains of Retained Austenite During Quenching and Isothermal Bainitic Holding in a High‐Silicon Medium‐Carbon Steel

“…[4][5][6][7] The latest development of ultrahigh strength steels has adapted to the new concept of microstructural design involving multiphase, and multiscale microconstituents. [8][9][10] In the past decade, a few novel heat treatment processes, such as quenching and partitioning (Q&P), [11][12][13] quenching-partitioning-tempering (Q-P-T), [14] and lowtemperature bainitic transformation (close to or even below M s temperature), [15][16][17] have been designed to achieve an excellent combination of high/ultrahigh strength combined with good low temperature toughness and reasonable ductility. The volume fractions of different microstructural constituents, such as ferrite, bainite, martensite, and RA, determine the final mechanical properties in these groups of advanced high strength steels (AHSS).…”