John G. Speer

Carbon partitioning into austenite after martensite transformation

Abstract A model is developed to describe the endpoint of carbon partitioning between quenched martensite and retained austenite, in the absence of carbide formation. The model assumes a stationary α/γ interface, and requires a uniform chemical potential for carbon, but not iron, in the two phases, leading to a metastable equilibrium condition identified here as “constrained paraequilibrium” or CPE. The model is explained with example calculations showing the characteristics of the constrained paraequilibrium condition, and applications are discussed with respect to new microstructures and processes, including a new “quenching and partitioning,” or Q&P process, to create mixtures of carbon-depleted martensite, and carbon-enriched retained austenite. Important new implications with respect to fundamental elements of the bainite transformation are also discussed.

Third Generation of AHSS: Microstructure Design Concepts

In recent years there has been an increased emphasis on the development of new advanced high strength sheet steels (AHSS), particularly for automotive applications. Descriptive terminology has evolved to describe the “First Generation” of AHSS, i.e. steels that possess primarily ferrite-based microstructures, and the “Second Generation” of AHSS, i.e. austenitic steels with high manganese contents which include steels that are closely related to austenitic stainless steels. First generation AHSS have been referred to by a variety of names including dual phase (DP), transformation induced plasticity (TRIP), complex-phase (CP), and martensitic (MART). Second generation austenitic AHSS include twinninginduced plasticity (TWIP) steels, Al-added lightweight steels with induced plasticity (L-IP®), and shear band strengthened steels (SIP steels). Recently there has been increased interest in the development of the “Third Generation” of AHSS, i.e. steels with strength-ductility combinations significantly better than exhibited by the first generation AHSS but at a cost significantly less than required for second generation AHSS. Approaches to the development of third generation AHSS will require unique alloy/microstructure combinations to achieve the desired properties. Results from a recent composite modeling analysis have shown that the third generation of AHSS will include materials with complex microstructures consisting of a high strength phase (e.g. ultra-fine grained ferrite, martensite, or bainite) and significant amounts of a constituent with substantial ductility and work hardening (e.g. austenite). In this paper, design methodologies based on considerations of fundamental strengthening mechanisms are presented and evaluated to assess the potential for developing new materials. Several processing routes will be assessed, including the recently identified Quenching & Partitioning (Q&P) process developed in the authors’ own laboratory.

Microstructures and properties of direct-cooled microalloy forging steels

Abstract In comparison to conventionally processed quenched-and-tempered steels, direct-cooled microalloy steels offer the potential for significant cost savings. However, direct material substitutions have often been limited based on toughness considerations at the required hardness levels. In an effort to improve toughness, direct-cooled microalloyed forging steels have evolved from precipitation strengthened ferrite–pearlite steels to steels with non-traditional bainitic microstructures that may contain a significant amount of retained austenite. As a consequence of recent developments, the use of direct-cooled microalloyed steels has increased and there is current interest in the use of these steels processed to obtain higher hardness levels. In this paper, processing approaches for the production of direct-cooled forging steels are considered with an emphasis on features that control strength and toughness.

Critical Assessment 7: Quenching and partitioning

Abstract Quenching and partitioning is a relatively new heat treatment concept to generate microstructures containing retained austenite stabilised by carbon partitioning from martensite. Research on quench and partitioning has been conducted by numerous groups, and this critical assessment provides some of the authors’ perspectives on progress and understanding in the field, with particular focus on the physical metallurgy and transformation mechanisms, process variations, mechanical behaviour, and industrial implementation. While much progress has been made, the field provides rich opportunity for further understanding and development.

Quenching and Partitioning Steel Heat Treatment

Quenching and partitioning (QP property advancements continue to be made through research on this emerging technology. Early investigations [1] also proposed a corresponding thermodynamic model for Q&P steel and its heat treatment, which is now referred to as constrained carbon equilibrium [3]. Since first proposed in 2003, Q&P steel has gained interest for its potential to enhance properties of strength and ductility with compositions similar to transformationinduced plasticity (TRIP) steel and has been proposed as a third-generation automotive steel (Fig. 1) [4]. Many researchers [5–17] have investigated the relationship between properties and microstructures of Q&P steels subjected to various heat treatments and showed that the ultrahigh strength of Q&P steel results from martensite laths, while its good ductility is attributed to TRIP-assisted behavior of retained austenite during deformation. De Moor et al. [14] examined the stability of retained austenite and showed that the TRIP effect occurs in Q&P steels, thereby effectively contributing to the significant strain hardening. Santofimia et al. [15, 16] and Takahama et al. [17] analyzed microstructural evolution during annealing Editor’s Note The following is a preview chapter from the upcoming volume Steel Heat Treating Fundamentals and Processes, Volume 4A, ASM Handbook, Jon Dossett and George Totten, editors. The volume is scheduled for publication later this year.

Processing Opportunities for New Advanced High-Strength Sheet Steels

Currently there is considerable interest in developments leading to new advanced high-strength sheet steels (AHSS) for automotive and other transportation applications that demand high strength, light weight materials. Design requirements will involve material properties with strengths greater than currently available dual phase and transformation-induced plasticity (TRIP) steels (steels in this group are referred to as the “First Generation” of AHSS), with good ductility and formability, but produced at a cost less than the high ductility stainless steels or high manganese TWIP steels (materials referred to as the “Second Generation” of AHSS). Recent studies have shown that materials that satisfy the required property/cost combinations will include complex microstructures containing high amounts of retained austenite in combination with a high strength constituent that may be ultrafine grained ferrite, martensite, bainite, and/or combinations of ferrite-based constituents. In this article, selected methodologies leading to the production of new AHSS materials will be reviewed and assessed to provide a framework for consideration of new concepts and processing routes that will be required in production operations.

Influence of interface migration during annealing of martensite/austenite mixtures

Tempering of martensite in the absence of carbide precipitation leads to carbon partitioning into retained austenite. If the martensite/austenite interface is assumed to remain stationary during this process, the phase compositions reach a condition that has recently been called constrained carbon equilibrium. If iron atoms are sufficiently mobile at the interface, longer partitioning times may lead to migration of the ferrite/austenite interface. The interface may be expected to move in either direction, depending on the specific details of the phase fractions and compositions controlling the chemical potential of iron at the interface. If interface migration occurs during carbon partitioning, the situation is more complicated and conditions could exist where the interface moves first in one direction and then the other.

New Microalloyed Steel Applications for the Automotive Sector

Developments related to the use of microalloy additions, primarily of Ti, Nb, and V, and controlled processing are reviewed to illustrate how steels with tailored microstructures and properties are produced from either bar or sheet steels for new automotive components. Microalloying additions are shown to control the necessary strengthening mechanisms to produce high strength materials with the desired toughness or formability for a specific application. Selected examples of direct cooled forging steels, microalloyed carburizing steels, and advanced high strength sheet (AHSS) steels are discussed.

Recent developments in low-carbon sheet steels

This overview provides examples of recent areas of research related to physical metallurgy of low-carbon sheet steels. Development of new alloys and microstructures, understanding mechanical behavior in new loading regimes, and understanding microstructure evolution in response to new processing methods are shown to provide continued challenges and opportunity. Ferrous physical metallurgy remains an active and fruitful field of research.

Mechanical properties of high-Si plate steel produced by the quenching and partitioning process

The microstructures and mechanical properties of a high-Si (1.5 wt.%) steel produced by a novel process of quenching and partitioning (Q & P) were compared with those obtained using traditional heat treatments (i.e. austempering, intercritical annealing for dual phase, quench and tempering). Plate steel was included for exploration of the Q & P process in applications requiring strength and toughness (such as an API line pipe), where retained austenite may contribute to the overall toughness via the TRIP phenomenon at a crack top. The Q & P process is based on the partial transformation of austenite to martensite, followed by partitioning of carbon from martensite into austenite, which leads to an untypical microstructure. Retained austenite amounts up to 6 vol.% with a carbon content of up to 0.88 wt.% were achieved in 0.1% carbon steel using Q & P. Superior impact toughness at higher yield strength levels was found after Q & P compared to other traditional heat treatments with equivalent partitioning, austempering or tempering conditions.