The Effects of Cooling Rate on Retained Austenite Characteristics of a 0.2C-1.5Si-1.5Mn-1.0Cr-0.05Nb TRIP-Aided Martensitic Steel

With the aim of increasing the volume fraction and stability of the retained austenite characteristics in a transformation-induced plasticity (TRIP)-aided steel with wider lath-martensite structure matrix, the effects of varying the post-hot-working cooling rate of a 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.05%Nb (mass%) steel on the retained austenite characteristics were investigated. When, after hot-working at 950°C, the steel was cooled to room temperature from 430°C above the martensite-start temperature using cooling rates lower than 3°C/s, the steel attained a higher volume fraction of metastable retained austenite and lower volume fractions of a finely dispersed martensite-austenite complex phase, carbide, and pro-eutectoid ferrite, although the volume fraction of bainitic ferrite increased. This was associated with a marked carbon-enrichment in the untransformed austenite, which was mainly due to the promoted bainitic ferrite, the initial lath martensite, and the refined prior austenitic grain.

Optimization of the Product of Strength and Plasticity of Hot-Stamped WHT1300HF High Strength Steel

The quenching and partitioning (Q&P) process was performed on high strength steel WHT1300HF at 250-350 °C for 30 to 90 s, respectively, for the improvement of its product of strength and plasticity (PSP). ε-carbide precipitation was observed in all the specimens partitioned at each temperature for different periods of time due to inadequate amount of Si in the composition of WHT1300HF steel. The volume fraction of retained austenite at room temperature in the partitioned specimens is extremely low due to the lack of carbon enrichment in untransformed austenite at the partitioning temperature as a result of the carbide precipitation. The decrease of tensile strength and increase of elongation are caused by the partitioning treatment, a maximum value of the PSP (17.6 GPa%) is achieved by partitioning at 300 °C for 60 s.

Microstructure Control of a Low Carbon Martensitic Stainless Steel by Quenching and Partitioning Heat Treatment

Quenching and partitioning (Q&P) treatment was applied to a commercial low carbon martensitic stainless steel, AISI Type 410 (Fe-12Cr-0.1C). The condition of partial quenching and partitioning was optimized with consideration of the untransformed austenite fraction and stability of austenite (carbon concentration in solid solution). As a result, the amount of retained austenite could be increased up to approximately 15 vol%. Tensile testing revealed that the specimens after Q&P heat treatment exhibited lower yield stress and larger work hardening rate compared with quench-and-tempered (Q&T) specimens under the same tensile strength level, resulting in a significantly better strength-ductility balance. It was confirmed that the TRIP effect had contributed to the mechanical property.

Austenite Stabilization by Quenching and Partitioning in a Low-Alloy Medium Carbon Steel

Innovative treatments like quenching and partitioning (Q&P) have been recently proposed to improve the combination of strength and ductility of high strength steels by stabilization of significant fractions of retained austenite in a microstructure of tempered martensite. The decomposition of austenite into bainite and carbides precipitation are the two main competitive processes that reduce the content of retained austenite achievable at room temperature. A medium carbon low-silicon steel (0.46% C and 0.25% Si) has been studied to identify in which limits the austenite can be enriched in C and stabilized by Q&P, although a silicon content well below 1.5%, commonly used to retard cementite precipitation, is adopted; indeed, high Si contents are detrimental to the surface quality of the product due to the formation of adherent scale in high temperature manufacturing cycles. The heat treatments have been carried out with a quenching dilatometer, investigating the carbon partitioning process mainly below Ms, where cementite precipitation is not activated. The dilatometric curves show the progressive enrichment of carbon in the untransformed austenite and the occurrence of austenite phase transformation during the isothermal holding below Ms. A range of temperatures and times has been found where a content of about 10% of retained austenite can be stabilized at room temperature, a percentage much lower than the theoretical maximum achievable with the carbon content of this steel.

Dynamics of the Quenching and Partitioning (Q&P) Process

The novel heat treatment concept of Quenching and Partitioning (Q&P) offers exciting prospects for the production of higher strength steel products with enhanced formability from a microstructure containing retained austenite and martensite. The Q&P process hinges on an interrupted quench and partitioning step at intermediate temperatures whereby the untransformed austenite can be thermodynamically stabilised by enrichment of carbon from the supersaturated martensite. Although the concept is similar to that producing carbide-free bainite in TRIP-assisted steel, Q&P offers the advantage of separating the ferrite formation and austenite enrichment stages of the process. While the concept is readily understood, the details of microstructural evolution during interrupted quenching and partitioning steps are difficult to study and are generally inferred from dilatometry or metallographic examination after a final quench back to room temperature. Consequently, in this study, alloying has been used to develop a model alloy in which the sequential steps of heat treatment can be separated for closer, more direct inspection by neutron diffraction techniques.

Microstructural Evolution during the Novel Quenching and Partitioning (Q&P) Heat Treatment of Steel

The novel non-equilibrium heat treatment procedure known as Quenching and Partitioning (Q&P) may offer the prospect of higher strength steel products with enhanced formability based upon martensitic microstructures containing controlled quantities of carbon-enriched retained austenite. The Q&P process requires an interrupted quench and isothermal annealing (partitioning) step at intermediate temperatures, whereby untransformed austenite can be thermodynamically stabilised by carbon migration from supersaturated martensite regions. The concept is comparable to that producing carbide-free bainite, for example, in TRIP-assisted steel, although Q&P allows separation of the ferrite formation and austenite enrichment stages of the process. However, although the Q&P concept is readily understood, evolution of the microstructure during interrupted quenching and partitioning has been inferred indirectly from dilatometer studies and metallographic examination after final quenching to room temperature. Consequently, a model alloy was developed in which the sequential steps of heat treatment could be separated for direct inspection by conventional metallography, X-ray diffraction and neutron diffraction techniques.

The Effects of Si and Deformation on the Phase Transformation in Dual Phase Steels

The effect of Si on phase transformation was well known in dual phase steels. Si promoted the ferrite transformation and the enriched C in untransformed austenite prohibited the transformation at intermediate temperature range resulting in the formation of lower bainite and martensite at low temperature range. In addition, during continuous cooling with fast cooling rate, it was very hard to differentiate one phase from the others. In order to clarify the effects of Si on the austenite-to-ferrite transformation quantitatively, the start temperatures of bainite(BS) and martensite(MS) as well as ferrite(Ae3) and pearlite(Ae1) were calculated by thermodynamic analysis. LVDT measured by dilatometer and 1st differentiation peaks of LVDT were examined with microstructures, which gives a possibility of the phase separation. In CCT diagrams, it was also found that large austenite grain size(AGS) widened the gap between the transformation start(Ts) and end(Tf) when Si was added.

Microstructural Features of 'Quenching and Partitioning': A New Martensitic Steel Heat Treatment

The microstructure following a new martensite heat treatment has been examined, principally by high-resolution microanalytical transmission electron microscopy and by atom probe tomography. The new process involves quenching to a temperature between the martensite-start (Ms) and martensite-finish (Mf) temperatures, followed by ageing either at or above, the initial quench temperature, whereupon carbon can partition from the supersaturated martensite phase to the untransformed austenite phase. Thus the treatment has been termed ‘Quenching and Partitioning’ (Q&P). The carbon must be protected from competing reactions, primarily carbide precipitation, during the first quench and partitioning steps, thus enabling the untransformed austenite to be enriched in carbon and largely stabilised against further decomposition to martensite upon final quenching to room temperature. This microstructural objective is almost directly opposed to conventional quenching and tempering of martensite, which seeks to eliminate retained austenite and where carbon supersaturation is relieved by carbide precipitation. This study focuses upon a steel composition representative of a TRIP-assisted sheet steel. The Q&P microstructure is characterised, paying particular attention to the prospect for controlling or suppressing carbide precipitation by alloying, through examination of the carbide precipitation that occurs.