M.J. Santofimia

Effect of Prior Athermal Martensite on the Isothermal Transformation Kinetics Below M s in a Low-C High-Si Steel

Thermomechanical processing of Advanced Multiphase High Strength Steels often includes isothermal treatments around the martensite start temperature (Ms). It has been reported that the presence of martensite formed prior to these isothermal treatments accelerates the kinetics of the subsequent transformation. This kinetic effect is commonly attributed to the creation of potential nucleation sites at martensite-austenite interfaces. The aim of this study is to determine qualitatively and quantitatively the effect of a small volume fraction of martensite on the nucleation kinetics of the subsequent transformation. For this purpose, dilatometry experiments were performed at different temperatures above and below the Ms temperature for athermal martensite in a low-carbon high-silicon steel. Microstructural analysis led to the identification of the isothermal decomposition product formed above and below Ms as bainitic ferrite. The analysis of the transformation processes demonstrated that the initial stage of formation of bainitic ferrite at heat treatments below Ms is at least two orders of magnitude faster than above Ms due to the presence of martensite.

Volume Change Associated to Carbon Partitioning from Martensite to Austenite

Annealing of martensite/austenite microstructures leads to the partitioning of carbon from martensite to austenite until the chemical potential of carbon equilibrates in both phases. This work calculates the volume change associated with this phenomenon using theoretical models for the carbon partitioning from martensite to austenite. Calculations are compared with experimentally determined volume changes. This comparison reveals that in the case of steels with higher contents of austenite-stabilizing elements, reported volume changes are satisfactory predicted assuming a low mobilily martensite/austenite interface. In the case of a steel with lower additions of austenite-stabilizing elements, experimentally measured expansions are considerably larger than predicted ones. The large measured volume expansions probably reflect the decomposition of the austenite.

The Complexity of the Microstructural Changes during the Partitioning Step of the Quenching and Partitioning Process in Low Carbon Steels

The quenching and partitioning (Q&P) process is a novel heat treatment for the development of advanced high strength steels that is raising an elevated interest by steel makers and steel researchers around the world. The reason is that reported results on mechanical properties, showing promising levels of forming and strength, are proving this new type of steel as a serious competitor of TRIP, DP and martensitic steels. The Q&P heat treatment consists of an initial partial or full austenitisation, followed by a quench to form a controlled amount of martensite and an isothermal treatment to partition the carbon from the martensite to the austenite. Although the path of the heat treatment is simple, the investigations have shown that the evolution of the microstructure during the application of the Q&P process is rather complicated. Processes occurring during the partitioning step, such as the migration of the interfaces, the carbon accumulation near the austenite interfaces and the carbon diffusion through ferrite, have strong effects on the resulting microstructure. In this work, the most important microstructural changes found during the application and simulation of the partitioning step of the Q&P process are analysed and discussed. Procedures to control the microstructure development in the application of the Q&P process are proposed.

Thermodynamic aspects of carbon redistribution during ageing and tempering of Fe–Ni–C alloys

Carbon redistribution is known to occur during martensite ageing. The two associated processes most discussed in the literature are spinodal decomposition and carbon segregation to defects. In order to elucidate the topic, the ageing and tempering of two Fe–Ni–C alloys have been characterised by means of atom probe tomography and synchrotron radiation diffraction. Upon ageing at room temperature, carbon redistribution is clearly observed, where the process of carbon segregation to defects appears to be most likely to occur. Nevertheless, the possibility of spinodal decomposition is not entirely discarded, and the current work presents a series of discussion points that challenge our current understanding of the thermodynamic of ferrite in steels.

Influence of the Partitioning Treatment on the Mechanical Properties of a 0.3C-1.5Si-3.5Mn Q&P Steel

The Quenching and Partitioning (Q&P) process is a promising method for developing steels with superior mechanical properties. This process includes quenching an austenitic microstructure to form a controlled fraction of martensite, an isothermal treatment (partitioning step) aiming for the partitioning of carbon from martensite to austenite and a final quench to room temperature. This paper analyses the concurrent processes of carbon partitioning and martensite tempering during the partitioning step of a 0.3C-1.5Si-3.5Mn (wt.%) Q&P steel. The influence of the martensite tempering and the carbon partitioning on the tensile strength as well as on the uniform and post-uniform elongation of the developed Q&P microstructures is investigated.

Optimizing Mechanical Properties of a 0.3C-1.5Si-3.5MnQuenched and Partitioned Steel

The Quenching and Partitioning (Q&P) process is known as a promising method for producing steels with superior mechanical properties. Developing Q&P steels with optimized mechanical properties requires well understanding of the relation between their microstructural and mechanical properties. The microstructural evolution during different Q&P processes in a 0.3C-1.5Si-3.5Mn (wt.%) steel was analysed. Mechanical properties of the developed microstructures were measured by using microtensile test. The influence of volume fractions and carbon contents of the phases on the ductility and strength of the microstructures was investigated. Furthermore, the effect of the specimen size on the tensile properties was discussed and a correction procedure was applied to convert the measured microtensile properties to the standard ones. A comparison with the measured mechanical properties of other type of Advanced High Strength Steels (AHSS) shows the improved properties of the Q&P steels.

Toughness of Advanced High Strength Bainitic Steels

Carbide free bainite has achieved the highest strength and toughness combinations to date for bainitic steels in as-rolled conditions. By alloying designing and with the help of phase transformation theory, it was possible to improve simultaneously the strength and toughness because of the ultra-fine grain size of the bainitic ferrite plates. Ultimate tensile strengths ranging from 1600 MPa to 1800 MPa were achieved while keeping a total elongation higher than 10 %. Their toughness at room temperature matches tempered martensitic steels, known to be the best-in-class regarding this property. However, it has been observed that the presence of coalesced bainite leads to a dramatic deterioration in toughness in these novel high strength bainitic steels.

Microstructure and Properties of Ultrafast Annealed High Strength Steel

The grain size, recrystallization, phase transformation and mechanical properties of a cold-rolled high-strength steel (HSS) are studied after annealing with high (~140°C/s) and ultra-high (~1500°C/s) reheating rate, followed by subsequent water quenching without isothermal soaking. By monitoring the hardness and microstructure, it was shown that the increase of the reheating rate from 140°C/s to 1500°C/s causes grain refinement from 5 µm to 1 µm in diameter and the final ferrite grain size depends significantly on the reheating temperature and reheating rate. It was observed that after an extreme reheating rate of ~1500°C/s the α-γ phase transformation starts before the completion of recrystallization in the recovered matrix. The crystallographic texture of the ultrafast reheated and water-quenched high-strength steel inherits the cold-rolled deformation texture with well pronounced RD and ND texture fibres, even after the α-γ-α′ phase transformations. It was found that the ultrafast reheating results in a very fine non-equilibrium ferrite-martensite structure with an excellent ultimate tensile strength of ~1400 MPa and an acceptable elongation at fracture. The observed data are very promising from industrial application point of view and open up possibilities for further structural refinement and alternative texture control.

Model for the Annealing of Partial Martensite-Austenite Microstructures in Steels

The microstructure formed in a steel after a partial martensitic transformation contains martensite-austenite assemblies with similar chemical composition in the two phases. In the absence of carbide precipitation, further annealing or tempering of this microstructure is believed to promote carbon partitioning from the martensite to the austenite. Experimental observations suggest that this carbon diffusion might take place in combination with migration of martensite-austenite interfaces. In this work, the effect of the martensite and austenite dimensions on the interaction between the carbon partitioning from martensite to austenite and the interface migration during annealing of martensite-austenite microstructures is analyzed. With that aim, simulations have been done by using a model in which the chemical potentials of carbon in martensite and austenite are assumed to be the same at the interface and motion of the phase interface is occurring when an appropriate driving force is present. Carbide precipitation is precluded in the model, and three different assumptions about interface mobility are considered, ranging from a completely immobile interface to the relatively high mobility of an incoherent ferrite–austenite interface.