David K. Matlock

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.

Dislocation substructure as a function of strain in a dual-phase steel

Dislocation structures in the ferrite of a C-Mn-Si dual-phase steel intercritically annealed at 810°C were characterized at various tensile strains by transmission electron microscopy At strains which corresponded to the second stage on a Jaoul-Crussard plot of strain hardening behavior, the dislocation density in the ferrite is inhomogeneous, with a higher density near the martensite. The third stage on a Jaoul-Crussard plot corresponds to the presence of a well-developed dislocation cell structure in the ferrite. The average cell size during this stage is smaller than the minimum size reported for deformed iron, and the cell size was inhomogeneous, with a smaller cell size near the martensite.

Void formation during tensile testing of dual phase steels

The effects of martensite volume fraction (MVF) and strain state on necking behavior, post-uniform elongation, and the nucleation and growth of voids in thin sheet dual phase steel, strained in tension, were investigated. Steel containing, in weight percent, 0.08C, 1.45Mn, and 0.21Si, was cold rolled 50 pct and intercritically annealed to produce dual phase microstructures. The effects of MVF were evaluated with a series of constant geometry tensile samples with martensite volume fractions between 5 and 40 pct. The effects of strain state within the neck were evaluated with a series of constant thickness samples with 20 pct MVF and with width variations between 3 and 25 mm. A transition from diffuse to localized necking, as well as a decrease in post-uniform elongation, occurred with both an increase in MVF and sample width. Metallographic analysis of deformed samples revealed that the void nucleation occurs primarily at martensite particles by three distinct mechanisms. The void size and density in the necked region increased toward the fracture surface in all samples and the void density was significantly higher for the samples which exhibited localized necking. However, independent of neck geometry, voids were nucleated uniformly throughout the samples, and were associated with the martensite. The difference in void size and density between the samples with different necking behavior indicates that void growth is a consequence of the strain gradient while the shape of the voids depends on both the strain state and strain gradient. The implications of the void structure analysis are interpreted based on the dual phase microstructure.

Intercritically Annealed and Isothermally Transformed 0.15 Pct C Steels Containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni" Part I. Transformation, Microstructure, and Room-Temperature Mechanical Properties

Steels containing 0.15 pct C and 1.2 pct Si-1.5 pct Mn or 4 pct Ni were intercritically annealed and isothermally transformed between 300 °C and 500 °C for 1 to 60 minutes. The specimens were subjected to tensile testing at room temperature, and the microstructures were evaluated by light microscopy, scanning and transmission electron microscopy (SEM and TEM, respectively), and X-ray diffraction (XRD). The microstructures consist of dispersed regions of bainite, martensite, and austenite in a matrix of ferrite, and a maximum of 11.6 pct austenite is retained after isothermal holding at 450 °C in the Si-Mn steel. In specimens where austenite transforms to martensite during quenching after isothermal holding, the stress-strain curves show continuous yielding, high ultimate tensile strength (UTS), and relatively low ductility. In specimens where higher volume fractions of austenite transform to bainite during isothermal holding, the stress-strain curves show discontinuous yielding, low UTS, and high ductility.

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.

Flow stress modeling and warm rolling simulation behavior of two Ti-Nb interstitial-free steels in the ferrite region

Abstract Axisymmetric compression tests and compact strip production (CSP) warm rolling simulations were carried out on two commercial Ti–Nb stabilized interstitial-free (IF) steels to investigate their flow behavior in the ferrite region. The comparison between the measured and predicted stress curves indicated that dynamic recovery was the dominant softening mechanism during axisymmetric compression deformation. The flow stress curves during CSP-warm rolling simulations indicated that static and dynamic recovery dominated the softening process for early passes and an apparent dynamic recrystallization contributed to the flow stress reductions for the last two passes. The microstructural evolution during the simulation confirmed that the very fine quasi-equiaxed ferrite grains obtained could be explained by apparent dynamic recrystallization, which was assisted by dynamic recovery at high temperature and a reduced Nb solute drag.

An etching technique for microalloyed dual-phase steels☆

Abstract A double etching technique consisting of picral etching followed by alkaline chromate staining has been applied to a Nb-microalloyed dual-phase steel. The etching technique is described in detail in this article. Evidence is presented which shows that the alkaline chromate stain delineates the size and shape of the austenite pool formed during intercritical annealing. Ferrite not transformed during annealing is stained gray and new ferrite, formed by the transformation of austenite on cooling, appears white. There is no grain boundary between the two types of ferrite. Martensite is stained black and other structures such as acicular ferrite and fine lamellar ferrite-carbide clusters are also clearly revealed.

Application of the bending-under-tension friction test to coated sheet steels

Application of the bending-under-tension-test to study the friction behavior of six zinc-based coated sheet steels is considered. The details of a laboratory test system are presented along with an analysis of the methods used to reduce the data. Several theoretical treatments exist in the literature; however, those that incorporate sheet bending have been shown to yield more accurate results. For coated sheet steels, the assumption that the friction coefficient is constant with contact pressure is considered in an analysis of the dependence of friction coefficient on contact pressure.