Formation of textures in a C–Si–V dual-phase steel

A vanadium microalloyed steel (0.1 C, 1.50 Si, 0.1 V) was subjected to initial heat treatments and intercritical annealing at 750 and 810°C to produce dual-phase structures of different distribution. Intercritically annealed materials were cold-rolled to a reduction of 60% in thickness and small samples taken from them were recrystallisation annealed at two temperatures of 650 and 800°C for various lengths of time. The (110) pole-figures for the cold-rolled materials with different dual-phase distribution showed a strong {111}< 112 > and a rather weak {111}< 110 > texture components. The O.D.F. (orientation distribution function) plots also showed the major texture components, {111}< 112 > and {111}< 110 > along with the minor components, like, {337}< 110 >, {337}< 776 >, {112}< 111 > and {112}< 110 >. No complete {111} fibre has been observed in the present investigation. Further the orientations{11, 11,4}< uvw > and {337}< uvw > have been found to be present as weak and incomplete fibre. The (110) pole-figures of the recrystallised materials have shown similar features (with reduced pole densities) as compared to the cold-deformed materials. Similarly, no {111} fibre has been observed in the recrystallised materials. The behaviour of the other two components, namely {11, 11,4}< uvw >, and {337}< uvw > have been found to be similar to that in the cold deformed material.     

Forming response of retained austenite in a C‐Si‐Mn high strength TRIP sheet steel

TRIP sheet steels typically consist of ferrite, bainite, retained austenite, and martensite. The retained austenite is of particular importance because its deformation‐induced transformation to martensite contributes to excellent combinations of strength and ductility. While information is available regarding austenite response in uniaxial tension, less information is available for TRIP steels with respect to the forming response of retained austenite in complex strain states. Therefore, the purpose of this work was to study the austenite transformation behaviour in different strain paths by determining the amount of retained austenite before and after forming. Forming experiments were performed on a high strength 0.19C‐1.63Si‐1.59Mn TRIP sheet steel 1.2 mm in thickness in two different strain conditions, uniaxial tension (ε₁ = ‐2ε₂) and balanced biaxial stretching (ε₁ = ε₂). Specimens were formed to strains ranging from zero to approximately 0.2 effective (von Mises) strain. Specimens were tested both longitudinally and transverse to the rolling direction in uniaxial tension, and subtle mechanical property differences were found. The volume fraction of austenite, determined with X‐ray diffraction subsequent to forming, was found to decrease with increasing strain for both forming modes. Some modification in the crystallographic texture of the ferrite was observed with increasing strain, in specimens tested in the balanced biaxial stretch condition. This trend was not evident in the uniaxial tensile test results. Slight differences were found in the transformation behaviour of the austenite when formed in different strain conditions. More austenite transformed in specimens tested parallel to the rolling direction than transverse to the rolling direction in uniaxial tension. The amount of austenite transformed during biaxial stretching was determined to be greater than the amount transformed in uniaxial tension for specimens tested transverse to the rolling direction at an equivalent von Mises strain. The amount of austenite that transformed in biaxial tension, however, was comparable to the amount of austenite that transformed in specimens tested longitudinal to the rolling direction in uniaxial tension.     

Thermodynamics of Mn–Ca–j and Mn–C–j system

Solubility experiments of Ca and C in Mn melts with different contents of third elements j at different temperatures were carried out in Mo-wire-heated furnace. With these data the first and second order activity interaction coefficients of j upon Ca and C, based on the same activity and the same concentration method and also In γ_Ca⁰ and In γ_C⁰, were evaluated. The solubility of Ca and C in liquid Mn formulated in relation to temperature was determined and the standard free energy of solution of Ca and C in liquid Mn based on 1 wt.% solution standard was evaluated, respectively.     

Acicular ferrite morphologies in a medium-carbon microalloyed steel

The influence of time and isothermal transformation temperature on the morphology of acicular ferrite in a medium-carbon microalloyed steel has been studied using optical and transmission electron microscopy (TEM). This study has been carried out with the analysis of the microstructures obtained with one- and two-stage isothermal treatments at 400 °C and 450 °C, following austenitization at 1250 °C. The heat treatments were interrupted at different times to observe the evolution of the microstructure at each temperature. The results show that a decrease in the isothermal transformation temperature gives rise to the development of sheaves of parallel ferrite plates, similar to bainitic sheaves, but intragranularly nucleated. These replace the face-to-edge nucleation that dominates the transformation at higher temperatures. The TEM observations reveal that the plates correspond to upper acicular ferrite and the sheaves to lower acicular ferrite. In this last case, cementite precipitates are present at the ferrite unit interiors and between the different platelets.     

Phosphorus segregation in austenite in Fe–P–C,Fe–P–B and Fe–P–C–B alloys

The equilibrium grain boundary segregation of phosphorus was investigated in Fe–P–C, Fe–P–B and Fe–P–C–B alloys after austenitising at temperatures ranging from 825–1100 °C. The grain boundary concentrations were determined by Auger electron spectroscopy on intergranular fracture surfaces. Phosphorus, carbon and boron segregate to the austenite grain boundaries. The segregation of P in austenite occurs mainly in equilibrium, but some additional segregation takes place during quenching. Boron and, in a lesser degree, carbon were found to decrease the grain boundary concentration of phosphorus. The results can be explained by assuming equilibrium segregation and mutual displacement of these elements in austenite.     

Liquid equilibria in the Fe–Ni–Mn system

New literature results on the liquid equilibria in the three edge binary systems make necessary a reconsideration and correction of liquidus surfaces of the γ and δ solid solutions hitherto outlined in the literature. Therefore, with respect to the critically reinterpreted edge binary systems, the shape of the stable liquidus surface of the γ and δ solid solutions has been newly outlined.     

Effects of Widmanstätten ferrite on the mechanical properties of a 0.2 pct C-0.7 pct Mn steel

Laboratory melted and rolled C-Mn steel plates were austenitized at either 925 °C or 1150 °C to produce nominal austenite grain sizes of 60 and 200 μm, resspectively. The plates were then cooled at rates in the range of about 2 °C/min to 400 °C/min to produce mixed polygonal ferrite/Widmanst?tten ferrite/pearlite microstructures. The percentage of Widmanst?tten structure (a Widmanst?tten ferrite/pearlite aggregate) increases with increasing prior austenite grain size and cooling rate. Both yield strength and impact toughness increase with decreasing austenite grain size and increasing cooling rate. This simultaneous improvement in strength and toughness is attributed to overall refinement of both the polygonal ferrite and Widmanst?tten structure. Both yield and tensile strength increase with an increase in the volume fraction of Widmanst?tten ferrite and a reduction in ferrite grain size. In contrast, the toughness level achieved in these polygonal ferrite/Widmanst?tten ferrite/pearlite microstructures depends largely on the ferrite grain size; the finer the grain size, the better the toughness.     

Validation of a Model of Plastic Deformation of C‐Mn Steels in the Two‐Phase Temperature Region

The paper describes the validation of a thermal‐mechanical‐microstructural model of deformation of carbon‐manganese steels in the two‐phase temperature region. The model has been developed on the basis of dilatometric and plastometric tests performed for a wide range of temperatures and strain rates. Dilatometric tests were used to identify the phase transformation model and plastometric tests were used to identify the flow stress model. Inverse analysis was applied to find the parameters of the models which were further implemented into a finite element code. Numerical simulations of the deformation of steels in the two‐phase temperature range were performed. Multi‐stage plane strain compression tests were performed to validate the model. The samples were quenched after subsequent stages of the tests and metallographic analysis was performed. Predicted loads, grain size and volume fractions of the microstructural components were compared with measurements and the coefficients in the models were updated.     

Formation of austenite in 1.5 pct Mn steels

The formation of austenite from different microstructural conditions has been studied in a series of 1.5 pct Mn steels that had been heated in and above the intercritical (α+ γ) region of the phase diagram. The influence of variables such as cementite morphology, initial structural state of the ferrite and the carbon content has been assessed in terms of their respective effects on the kinetics of austenite formation and final microstructure. Austenite was found to form preferentially on ferrite-ferrite grain boundaries for all initial structures. The results of this study have shown that the 1.5 pct Mn has lowered both the AC₃ and AC₁, lines causing large amounts of austenite to form in low carbon steel. The kinetics of austenite formation at 725 °C were not only very slow but also were approximately independent of the amount formed. Austenite appeared to form slightly more rapidly from cold rolled ferrite than from recrystallized ferrite or ferrite-pearlite structures.     

Berechnung der Gleichgewichtszusammensetzungen und Rußgrenzen im System C–CH4–H2–CO–H2O–CO2

Rechenprogramm zur Ermittlung der thermodynamischen Gleichgewichte im System C–CH₄–H₂–CO–H₂O–CO₂. Hinweise auf die Anwendungsmöglichkeiten der Rechnungen, und zwar auf die Kohlevergasung, auf die Spaltung von Kohlenwasserstoffen zur Reduktionsgaserzeugung, auf Oxidations- und Reduktionsvorgänge oder auf Ent- und Aufkohlungsreaktionen. Berechnung der Gleichgewichtszusammensetzungen in der Gasphase, der Rußgrenzen und der Aktivitäten von Kohlenstoff, Wasserstoff und Sauerstoff in Abhängigkeit von Temperatur und Gesamtdruck. Erörterung des Verlaufs der Rußgrenzen mit Hilfe der Gleichgewichte für den Methanzerfall und die Boudouard-Reaktion.     

The Microstructure and Mechanical Properties of Ultrafine Grained Plain C‐Mn Steels

An ultrafine microstructure was produced in plain C‐Mn steels with different carbon contents (0.15 ‐ 0.3 mass% C) by heavy warm deformation. The rolling was simulated by the plane strain compression test with a simulated post rolling coiling. The final microstructure consists of an ultrafine grained ferrite matrix with the average grain size of 1.1 ‐ 1.4 μm and spheroidized cementite particles of two different size groups. The fraction of high‐angle grain boundaries maintained in the range of 60% to 65%. With the increase of C content from 0.15 mass% to 0.3 mass% the strength increases by about 100 MPa, while the total elongation of 23% hardly changes. The (specific) upper shelf energy decreases from 320 J/cm² to 236 J/cm² but a rather low ductile‐to‐brittle transition temperature (DBTT) of about 206 K does not rise with increasing C content. The ultrafine steel with higher C content (0.3 mass%) exhibits a superior strength‐toughness combination.     

Hydrogen induced interior crack-tip morphologies in high strength steel

In this study WOL specimens of high strength 4340 steel were used to obtain information on the interior, or near plane strain, crack-tip morphology resulting from hydrogen assisted crack growth, including the position of secondary cracks in relation to the region of maximum triaxial stress, crack-tip plastic zone size, and grain size. The interior, or near plane strain, regions were revealed by removing thin sections, by a grinding and polishing procedure, from the surfaces of specimens that underwent hydrogen assisted crack growth. The results appear to indicate that advance of the crack occurs by growth of protrusions, or steps, from the main crack front, followed by linkup of these protrusions by sideways growth along grain boundaries.     

Modelling the Stress Relaxation Kinetics of Intercritically Deformed Austenite and Ferrite in C‐Mn Steel

In this study the softening kinetics of intercritically deformed C‐Mn steel have been characterised using the stress relaxation technique. In addition, the progress of softening has been monitored via optical microscopy of quenched samples. Physically based models for the softening kinetics of the separate phases were combined using the simple rule of mixtures, to predict the stress relaxation kinetics following intercritical deformation. Moreover, the strain and stress distribution developed during deformation has been taken into account using an analytical approach from the literature. Comparison of the model with experiments showed significant deviations. These were thought to be due to two effects concerning the role of phase interactions. Firstly, there was a region in the austenite phase having a low strain, leading to a fraction of non‐recrystallizing austenite. Secondly, the number of recrystallization nucleation sites was reduced. These two effects were tested by modifying the original model. The best agreement with experimental data was obtained when assuming the presence of a significant fraction of austenite that did not recrystallize.     

A Physical Analysis of the Stress Relaxation Kinetics of Deformed Austenite in C‐Mn Steel

The softening kinetics following hot deformation of austenite have been characterised using the stress relaxation technique. Samples were deformed in compression for a variety of temperatures, strains and strain rates. At low strains where recovery was the only softening mechanism, the stress relaxation kinetics have been analysed using a recovery model previously proposed in the literature, the main parameters being activation energy and activation volume. The activation energy for recovery was found to be 314 kJ/mol, whilst the activation volume was inversely proportional to the internal stress. At higher strains where austenite recrystallization occurred as well, the stress relaxation kinetics were modelled using the recovery model combined with a single grain model for recrystallization. Reasonable agreement was obtained between model and experiment for a variety of deformation conditions. Analysis of the model parameters and experimental data indicated that the nucleation density for recrystallization depended only on the applied strain for the range of deformation conditions imposed. In addition the mobility of recrystallizing boundaries was best explained by solute drag due to manganese atoms.     

Strain Rate Sensitivity of C‐alloyed,High‐Mn,Twinning‐induced Plasticity Steel

The mechanical properties of twinning‐induced plasticity (TWIP) steels are often assumed to be solely due to the reduction of the mean free path of glide dislocations resulting from deformation twinning. Other mechanisms may also play an essential role: Mn‐C cluster formation, planar glide, pseudo‐twinning, short range ordering, and dynamic strain ageing. The present contribution offers a critical analysis of the mechanical properties of high‐Mn TWIP steels, especially in terms of Dynamic Strain Aging (DSA) and Static Strain Aging (SSA). The presentation offers new insights into the properties of TWIP steels which were obtained by using new experimental techniques such as in‐situ strain analysis and high sensitivity infrared thermo‐graphic imaging.     

An improved method for determining the continuous cooling transformation diagram of C‐Mn steels

Dilatometry is often used to study the decomposition of austenite in steels, but the analysis of dilatometric data is often limited to the determination of transformation temperatures. The well‐known lever rule is not applicable when more than one phase transformation occurs. A model accounting for the carbon partitioning effects was developed to extract the phase transformation kinetics of a C‐Mn steel cooled using a wide range of cooling rates. The model is shown to be suitable to analyze the phase transformations in C‐Mn steels and it can be used to obtain a detailed CCT diagram for those steels.     

Theory of modelling the isothermal austenite grain growth in a Si‐Mn TRIP steel

The development of a model to predict the isothermal austenite grain growth during soaking of a low carbon Si‐Mn TRIP steel is described. After reviewing the existing models for isothermal grain growth, a general model dⁿ=d₀ⁿ + K₁t exp(K₂/T) was selected and a procedure was delineated to calculate the values of the different constants of the equation starting with the real three‐dimensional austenite grain size. This paper also deals with an improved etching technique to reveal the austenite grain boundaries.