Joo Hyun Ryu

Medium-Alloy Manganese-Rich Transformation-Induced Plasticity Steels

The manganese concentration of steels which rely on transformation-induced plasticity is generally less than 2 wt pct. Recent work has highlighted the potential for strong and ductile alloys containing some 6 wt pct of manganese, but with aluminum additions in order to permit heat treatments which are amenable to rapid production. However, large concentrations of aluminum also cause difficulties during continuous casting. Alloy design calculations have been carried out in an effort to balance these conflicting requirements, while maintaining the amount of retained austenite and transformation kinetics. The results indicate that it is possible by adjusting the carbon and manganese concentrations to reduce the aluminum concentration, without compromising the mechanical properties or transformation kinetics. The deformation-induced transformation of retained austenite is explained quantitatively, for a range of alloys, in terms of a driving force which takes into account the very fine state of the retained austenite.

Effect of aluminium on hydrogen-induced fracture behaviour in austenitic Fe–Mn–C steel

It is known empirically that the addition of aluminium as a solute in high-Mn austenitic steels dramatically improves their resistance to hydrogen-induced embrittlement. A variety of experimental techniques, including the characterization of trapping sites and high-resolution observation of fracture facets, have been used to reveal the mechanism by which aluminium induces this effect. It is found that transgranular fracture is promoted by the segregation of hydrogen to mechanical twin interfaces and to any ε-martensite that is induced during deformation. Because aluminium increases the stacking fault energy of austenite, the tendency for mechanical twinning is reduced, and the formation of deformation-induced martensite eliminated. These two effects contribute to the resistance of the aluminium-alloyed steel to hydrogen embrittlement.

Performance of neural networks in materials science

Abstract Neural networks are now a prominent feature of materials science with rapid progress in all sectors of the subject. It is premature, however, to claim that the method is established. There are genuine difficulties caused by the often incomplete exploration and publication of models. The assessment presented here is an attempt to compile a loose set of guidelines for maximising the impact of any models that are created, in order to encourage thoroughness in publication to a point where the work can be independently verified.

Contribution of Microalloying to the Strength of Hot-Rolled Steels

Although the principles of microalloying are well established, the complexity of thermomechanical processing is such that it is difficult to deconvolute the contribution to strength of the microalloying additions as a function of the many variables involved. We report in this article the analysis of a large database on hot-rolled steels to create a neural network model which estimates the strength as a function of chemical composition and process variables. This model is then used to make comparisons against equivalent data in order to realize the role of minute additions of carbide formers in changing the properties of steels.

Influence of the Initial Microstructure on the Reverse Transformation Kinetics and Microstructural Evolution in Transformation-Induced Plasticity–Assisted Steel

Abstract The reverse transformation behavior upon heating to intercritical temperature was studied in Fe-0.21C-2.2Mn-1.5Si (wt pct) alloy with three initial microstructures. One is the cold-rolled (CR) structure and two others are martensite having different fractions of retained austenite. The CR structure exhibits slower reverse transformation kinetics than martensite due to the lesser population of potent nucleation sites and coarse cementite particles. The film type of retained austenite at the martensite lath boundary contributes to the earlier start of the reverse transformation, because it can proceed as the growth of pre-existing retained austenite, which makes the nucleation process less critical. Besides, the growth of interlath austenite plays an essential role in the evolution of fine lath-type reverse-transformed microstructure, which was difficult to obtain from similar initial microstructures of martensite having negligible fraction of interlath austenite.

Influence of Heating Rate on Annealing and Reverse Transformation Behavior of TRIP Steels Having Martensite as Starting Microstructure

The influence of heating rate on the annealing and transformation behavior is investigated in TRIP steel having martensite as the starting microstructure. A higher heating rate preserves the hierarchical structure of the initial microstructure before starting the reverse transformation. As the heating rate increases, the reversely transformed austenite has a propensity to develop a fine lath morphology, a consequence of the retention of pre-existing austenite and its growth along the lath boundary.

Influence of the Initial Microstructure on the Heat Treatment Response and Tensile Properties of TRIP-Assisted Steel

Microstructure evolution and mechanical properties were investigated in transformation-induced plasticity (TRIP) steel having a different initial microstructure. Compared with the cold-rolled structure that evolves into a typical microstructure of TRIP steel, the martensitic initial structure produces a more lath-type microstructure as the fraction of retained austenite increases in the initial microstructure. The interlath austenite after heat treatment contributes to improving the tensile properties by the enhanced stability and the refinement of the matrix phase.

Influence of initial microstructures on intercritical annealing behaviour in a medium Mn steel

ABSTRACT The present work reports the effect of different initial microstructures on reverse transformation kinetics and morphologies of austenite formed during intercritical annealing in Fe-0.14C-7Mn-1Si (wt-%) medium Mn steel. Three different initial microstructures were produced by cold-rolling and cold-rolling followed by austenitisation at 820°C and 900°C. The specimen austenitised at higher temperature shows lath-type austenite after intercritical annealing. The difference in austenitisation temperature leads to different Mn distribution in martensitic initial microstructures, thereby leading to a difference in morphology of austenite. The inhomogeneous Mn profiles in initial microstructures also affect reverse transformation kinetics of austenite upon intercritical annealing. The presence of Mn-enriched regions accelerates austenite growth at an early stage of intercritical annealing but retards the transformation kinetics afterwards. This paper is part of a Thematic Issue on Medium Manganese Steels.

Enhancement of tensile properties by room-temperature quenching and partitioning of 0.2C–10Mn–2Al steel

ABSTRACT Processing conditions better than those of conventional quenching and partitioning process are suggested for 0.2C–10Mn–2Al steel. The steel can retain 24% of austenite on quenching to room temperature and effectively partition carbon from martensite to austenite at 200°C. The resulting tensile properties were comparable to those produced by conventional quenching and partitioning. Moreover, the suggested processing condition resolves an issue of intercritically annealed medium Mn steels by improving the yield strength and eliminating yield point phenomenon as well as serrated flow. This paper is part of a Thematic Issue on Medium Manganese Steels.