Kyoo-Young Lee

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.

Extraordinary ductility in Al-bearing δ-TRIP steel

An iron-based alloy system has been developed that exhibits impressive combinations of tensile strength and elongation that are not available with current steels used in the manufacture of automobiles. Furthermore, the heat treatments required to achieve these properties are consistent with practical production processes. The alloys rely on significant concentrations of ferrite-stabilizing solutes so that δ-ferrite which forms during solidification is retained in the microstructure.

Non-equilibrium solidification and ferrite in δ-TRIP steel

Abstract Microscopy and microanalysis experiments on two cast alloys, designed on the basis of equilibrium to contain substantial amounts of δ-ferrite, reveal zero or much reduced fractions of this phase in the solidified condition. It appears that the solid state transformation of δ-ferrite into austenite occurs without the required partitioning of solutes and that this is responsible for the development of non-equilibrium microstructures. This conclusion is supported by microanalytical data and through calculations of limiting phase diagrams based on paraequilibrium rather than equilibrium. Kinetic simulations confirm that this interpretation is consistent with the majority of austenite growing in the solid state without the partitioning of the substitutional solutes.

Stabilisation of ferrite in hot rolled δ-TRIP steel

Abstract A steel has recently been designed to benefit from the deformation induced transformation of retained austenite present in association with bainitic ferrite. It has as its major microstructural component, dendrites of δ-ferrite introduced during solidification. The δ-ferrite replaces the allotriomorphic ferrite present in conventional alloys of this kind. The authors examine here the stability of this δ-ferrite during heating into a temperature range typical of hot rolling conditions. It is found that contrary to expectations from calculated phase diagrams, the steel becomes fully austenitic under these conditions and that a better balance of ferrite promoting solutes is necessary in order to stabilise the dendritic structure. New alloys are designed for this purpose and are found suitable for hot rolling in the two-phase field over the temperature range 900–1200°C.

Spot weldability of TRIP assisted steels with high carbon and aluminium contents

Abstract We examine here the spot welding characteristics of transformation induced plasticity assisted steels, which contain δ-ferrite as a consequence of their aluminium concentrations of 3·5 or 5·6 wt-% and which also have high carbon contents of 0·3 or 0·4 wt-% when compared with conventional automotive steels. The resistance spot welds are tested in both shear and cross-tensile tests in order to determine the so called ductility ratio, which is a parameter associated with the fitness of such welds for automotive applications. With an increase in the δ-ferrite fraction from 0·19 to 0·5, the hardness variation across the weld and heat affected zone is decreased approximately from 400 to 150 HV. It seems that the presence of stable δ-ferrite is helpful in reducing hardness variations and in achieving a significant ductility ratio of 0·39.

Spot weldability of δ-TRIP steel containing 0·4 wt-%C

Abstract Strong steels are usually difficult to resistance spot weld because of the tendency to form hard phases. This applies particularly to the transformation induced plasticity (TRIP) assisted steels with relatively high carbon equivalents. A new development in this context is the δ-TRIP steel, designed to retain δ-ferrite as a stable phase at all temperatures below melting. Fully martensitic regions are therefore avoided, making it possible to weld in spite of the high carbon concentration. The authors present here the first spot welding tests on the novel alloy system.

Relative effects of Mo and B on ferrite and bainite kinetics in strong steels

Abstract Well–designed cementite–free bainitic steels are important in contributing to unique combinations of strength, toughness and cost. We examine here the relative effects of molybdenum and boron on the kinetics of transformation of austenite particularly into allotriomorphic ferrite and bainite. There are some surprising results on the role of boron, which is found in some circumstances to accelerate the transformation to allotriomorphic ferrite. This, and other features of transformation behaviour are assessed in the context of phase transformation mechanisms.

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.

Evaluation of stored energy after accumulative roll bonding of an interstitial-free steel by means of orientation microscopy data

The stored energy of warm-rolled interstitial-free steels, produced in an accumulative roll bonding process, is evaluated by using the textural and microstructural information contained in orientation imaging microscopy (OIM) scans which were measured after accumulated von Mises strains (e vM ) of 0.8, 1.6, 2.4 and 4.0, respectively. It is assumed that the plastic strain energy is stored in a cellular network of local boundaries of low and high misorientations. The presence of intracellular dislocations which do not contribute to a local crystal orientation gradient is ignored in the present analysis. On the basis of the Read-Shockley equation, the local misorientation can be associated with a local boundary energy which can be expressed as a local stored energy by taking into account the radius of curvature of the cellular network. The validity of this procedure was verified by comparing the integrated average stored energy of the sample with the Vickers hardness data, which produced a reasonable correspondence. The present analysis also allowed the calculation of the stored energy distribution in the areas of the orientation representation space which were sufficiently populated by sample crystal orientations, i.e. the y fibre ((111)∥ND) of the present deformation texture. The distribution of stored energy along this fibre displayed a maximum on the {111}(211) texture component, particularly after a von Mises strain of 1.6, whereas the {111}(110) component displayed a local minimum of stored energy after e vM = 2.4 and 4.0.

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.