Flow stress behaviour and static recrystallization characteristics of hot deformed austenite in microalloyed medium-carbon bainitic steels

Abstract In the past decade, efforts have been focused on developing very fine, medium-carbon bainitic steels via the low-temperature (typically 300–400 °C) ausforming process, which not only enables shorter isothermal holding times for bainitic transformation at low temperatures, but also offers significantly improved strength. This paper describes static recrystallization (SRX) characteristics of austenite in four medium-carbon 2%Mn-1.3%Si-0.7%Cr steels with and without microalloying intended for the development of these steels. The stress-relaxation method on a Gleeble simulator resulted in recrystallization times over a wide range of temperatures, strains and strain rates. Also, the occurrence of precipitation was revealed. Powers of strain (−1.7 to −2.7) and strain rate (−0.21 to −0.28) as well as the apparent activation energies (225–269 kJ/mol) were in the ranges reported in the literature for C-Mn and microalloyed steels with lower Mn and Si contents. The new regression equations established for estimating times for 50% SRX revealed the retardation effects of microalloying and Mo addition showing reasonable fits with the experimental data, whereas the previous model suggested for ordinary microalloyed steels tended to predict clearly shorter times on average than the experimental values for the present coarse-grained steels. The Boratto equation to estimate the non-recrystallization temperature was successfully modified to include the effect of Mo alloying and high silicon concentrations

Nanolath martensite-austenite structures engineered through DQ&P processing for developing tough, ultrahigh strength steels

Abstract A novel processing route comprising thermomechanical rolling followed by direct quenching and partitioning (DQ&P) was designed for developing tough, ductile, ultrahigh strength steels using 0.4 wt% carbon steels. A preliminary characterization of a laboratory-rolled, high-silicon DQ&P steel revealed an excellent combination of mechanical properties comprising high yield and tensile strengths (~1025 and ~2137 MPa, respectively), besides reasonable elongation (≈ 12%) and good impact toughness (T28J temperature ≈ −10 °C). The high mechanical properties are attributed to the formation of desired nanolath martensite-retained austenite (≈ 15%) structures. Features such as nano-twinned martensite, interlath austenite etc. were comprehensively characterized through high-resolution transmission electron microscopy. Attempts have been made to detect possible existence of metastable hexagonal omega phase within the boundaries of nano-twinned martensite and understand its nature of formation

Competitive mechanisms occurring during quenching and partitioning of three silicon variants of 0.4 wt.% carbon steels

Abstract Quenching and partitioning (Q&P) treated steels are traditionally alloyed with silicon (Si), but its precise role on microstructural mechanisms occurring during partitioning is not thoroughly understood. In this study, dilatometric analysis has been combined with detailed microstructural characterization to unravel the competing mechanisms occurring during partitioning either in parallel or in succession. Three 0.4 wt.% carbon steels with varying Si contents were quenched to 150 °C for ∼20 % untransformed austenite, and partitioned for 10–1000 s in the temperature range 200–300 °C. The steel with low Si content (0.25 wt.%) exhibited substantial bainitic transformation during partitioning at 300 °C and only 4% retained austenite (RA) at room temperature (RT) even after 1000 s hold. In contrast, a high Si fraction (1.5 wt.%) enabled retention of ∼18 % austenite under similar conditions. While η-carbides precipitated within the martensite laths in the high-Si steel, cementite precipitated in the low-Si variant. Furthermore, carbide precipitation and growth were strongly suppressed by high Si content. Secondary martensite formation occurred from carbon-enriched austenite during final cooling, irrespective of Si-content. Results illustrate that Si retards austenite decomposition at higher partitioning temperatures but does not improve carbon partitioning at lower temperatures

Constitutive flow behaviour of austenite at low temperatures and its influence on bainite transformation characteristics of ausformed medium-carbon steel

Abstract In order to impart superior mechanical properties to medium carbon carbide-free bainitic steels, an innovative approach has been adopted to extensively refine the bainitic ferrite plate thickness. Unlike controlled deformation in the no-recrystallization regime above the Ar3 temperature, an attempt has been made in this study to carry out low temperature ausforming in the bay between ferrite and bainite C-curves at 500 °C in order to impart high dislocation densities in the austenite prior to phase transformation. Two experimental high-silicon, medium carbon steels were suitably designed and processed for this study, with one steel containing small additions of 0.3Mo and 0.03Nb. Flow stress measurements were made using single-hit compression tests in the temperature range 300–900 °C in steps of 100 °C at different strain rates in the range 0.1–10 s⁻¹ on a Gleeble simulator. Samples ausformed at 500 °C were isothermally held for 1 h at different transformation temperatures in the range of 300–400 °C to complete the bainitic transformation. Influence of strain induced bainite transformation on flow stress was obvious at 0.01 s⁻¹, particularly at 300 and 400 °C. Despite enhanced nucleation in fine-grained steel B containing Nb + Mo, growth of bainite sheaves was much slower. Dilatation behaviour was comparable for the two steels at <350 °C, but at higher temperatures, the effect of Nb + Mo on slower transformation kinetics was obvious. The microstructure of both steels showed extremely fine bainitic ferrite below 325 °C, but at higher temperatures, coarse bainite with M/A constituents and extensive martensite formed in steels without or with Nb + Mo constituents. A correlation between hardness data and retained austenite contents has been established in both the steels. The paper presents the first account of the flow stress and transformation behaviour including the influence of Nb + Mo alloying and the details concerning the carbon-enriched austenite retained at room temperature and hardness variation as a function of isothermal holding temperature

Static recrystallization characteristics and kinetics of high-silicon steels for direct quenching and partitioning

Abstract In the direct quenching and partitioning (DQ&P) process,tough ultra-high-strength steel is made by combining thermomechanical processing with quenching and partitioning to obtain martensite toughened by thin films of retained austenite. The hot rolling stage with deformation and recrystallization between the rolling passes affects the state of the austenite before quenching and partitioning. This paper describes the static recrystallization kinetics of two steels with compositions suited to DQ&P processing, viz. (in wt.%) 0.3C-1Si-2Mn-1Cr and 0.25C-1.5Si-3Mn. The stress relaxation technique on a Gleeble thermomechanical simulator provided recrystallization times over a wide range of temperature, strain, strain rate and initial grain size. The higher levels of Si and Mn made the recrystallization kinetics less sensitive to strain, strain rate and temperature. The equations derived to describe the recrystallization kinetics can be used in the design of the rough rolling part of thermomechanical processing

Design of tough, ductile direct quenched and partitioned advanced high-strength steel with tailored silicon content

Abstract The novel processing concept of direct quenching and partitioning (DQ&P) has been explored with a medium-carbon (0.4 wt.% C) steel to evaluate and optimize the processing route for excellent property combinations. New compositional design approach was based on physical simulation studies aiming to understand the influence of varying silicon contents (1.5, 0.75 and 0.25 wt.%) and Q&P processing parameters on microstructural development including carbide formation and retained austenite stabilization. Optimized Q&P parameters were selected to design a DQ&P processing route for laboratory hot-rolling trials based on the analyses of physical simulation data. The overall aim of the study was to produce ultrahigh-strength structural steels with yield strength ≥1100 MPa combined with high uniform and total elongation and impact toughness, achieved through designing a low-temperature quenching and partitioning route with effective carbon partitioning. The DQ&P steels gained an excellent combination of mechanical properties comprising of high yield strength of ∼1000–1200 MPa and tensile strength ∼2100–2300 MPa, good elongation (∼11–13%), and moderate impact toughness transition temperature T28J ∼ (−5 to +12 °C). Straining of austenite prior to DQ&P led to an extensive refinement of the final martensitic-austenitic nanostructure. Formation of nanoscale lath-martensite and fine film-like retained austenite structures enabled the observed improvement of mechanical properties. Besides the formation of nano-twinned martensite, inter-lath austenite and transitional carbides were comprehensively characterized. No adverse effect of prolonged partitioning during slow cooling, simulating coiling in actual industrial practice, has been noticed suggesting new possibilities for developing tough, ductile structural steels both for strip/plate products