Bainitic Austempering Incorporating Quenching and Partitioning for Favorable Tuning of Microstructure and Mechanical Properties

Herein, martensite–ferrite dual‐phase high‐strength low‐alloy (HSLA) steels with a low yield ratio (YR), which is achieved through different heat‐treatment procedures, are evaluated in terms of multiple microstructures, e.g., grain size, soft‐phase proportion, and morphology. The extended austenitizing time exhibits a low YR by sacrificing both the yield and tensile strengths synchronously. By subtly regulating the tempering temperatures, the YR can be optimized from 0.93 to 0.89 after tempering at an applicable temperature. Moreover, by introducing the equivalent shape factor (ESF), the morphology effects of ferrite on the YR are investigated based on the microstructural observation and Abaqus finite element analysis. The YR is inversely proportional to the ESF of the microstructure, which can be used to guide the design of microstructural morphology. The correlation between the yield strength (YS), YR, and work hardening exponent is achieved using the Swift equation. The lower YR generates greater work hardening and reduced YS. By tuning multiple microstructures, high YS and relatively low YR are achieved via the secondary‐quenching intercritical heat treatment, which is beneficial for designing low‐yield‐ratio HSLA steels in industrial applications.

Section: Resultsmentioning

confidence: 97%

“…This may be ascribed to the phase nucleation and atom migration in the process of phase transition. [19,41] In addition, there are also distributed polygonal ferrites in both Q 0 -740 and Q 0 -700, as indicated by the red arrows.…”

Section: Effects Of the Intercritical Heat Treatment On Yrmentioning

confidence: 99%

Achieving Low Yield Ratio in High‐Strength Steel by Tuning Multiple Microstructures

Shi

Wang

Gao

et al. 2021

“…The integrated intensities of (200) γ , (220) γ , and (311) γ of austenite peaks and those of (200) α and (211) α of ferrite peaks were used to calculate the volume fraction of retained austenite, details can be found in previous work. [ 2 ]…”

Section: Methodsmentioning

confidence: 99%

“…Acceleration of bainite transformation has attracted wide interests in steel research. Strategies like alloy design, [ 1 ] austenite grain size control, [ 2–5 ] and deformation [ 6–9 ] have been proven to be effective ways promoting bainite transformation. Recently, effect of pre‐existing martensite, ferrite, and second phases on bainite transformation has increasingly drawn much attention.…”

Section: Introductionmentioning

confidence: 99%

Accelerated Isothermal Phase Transformation and Enhanced Mechanical Properties of Railway Wheel Steel: The Significant Role of Pre‐Existing Bainite

Gao

Tan

Tian

et al. 2021

Self Cite

Exploring possible ways to accelerate bainite transformation kinetics and enhance mechanical properties, especially for large‐scale parts, attracted much attention both in industrial manufacturing and fundamental research. Herein, prior continuous cooling transformed bainite is introduced to promote isothermal transformation and improve strength toughness matching of bainite railway wheel steel. Phase transformation study by dilatometer indicates that the time–temperature–transformation curves of tested steel swing back at lower isothermal temperature. This acceleration effect is due to the formation of leaf‐shaped bainite at initial isothermal transformation stage. By tailoring the cooling rate, leaf‐shaped bainite can also be obtained during continuous cooling. It is confirmed that the formation of leaf‐shaped bainite during continuous cooling stage accelerates the subsequently isothermal bainite transformation. In situ microstructure evolution observation demonstrates that prior bainite/ austenite interface provides favorable nucleation site for initial isothermal bainite transformation. Moreover, pre‐existing bainite formed during continuous cooling stage can effectively divide the original austenite grain size. Concurrent enhancement of tensile strength and impact toughness in bainite wheel steel can be achieved by controlling the cooling rate.

“…[ 1–3 ] The superior properties of these bainitic steels are determined by their unique chemical composition and processes. [ 4,5 ] They are the key to obtain carbide‐free bainite and film‐like retained austenite. [ 6 ] Strong bainite and numerous interfaces lead to significant improvements on strength of steels.…”

Section: Introductionmentioning

confidence: 99%

On the Significance of Thermal Deformation and Dynamic Recrystallization on Flow Behavior of 25Mn2Si2Cr Bainitic Alloy

Tan

Zhang

et al. 2020

Self Cite

Thermal deformation characteristics, dynamic recrystallization behavior, and processing map of 25Mn2Si2Cr high strength bainitic steels are elucidated during wide temperature range (850–1100 °C) and strain rate (0.1–10 s−1) from flow behavior. Thermomechanical analysis by Thermo‐Calc software indicates that microstructure during thermal deformation is austenite, which is conducive to dynamic recrystallization. Phenomenological constitutive equations of thermal deformation, dynamic recrystallization equation, and processing map are established to predict flow behavior. The experimental results indicate that deformation temperature and strain rate have significant effect on thermal deformation process. Flow stress increases with decrease in deformation temperature and increase in strain rate. Increasing deformation temperature and decreasing strain rate promote dynamic recrystallization occurrence. However, the grain size increases with increase in deformation temperature. Thermal simulations predict that optimum thermal deformation parameters are deformation temperature (950–1000 °C), strain rate (0.25–5 s−1) and deformation temperature (850–950 °C), strain rate (1–5 s−1) for processing of Mn–Si–Cr bainitic steels.