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.     

Microstructure and properties of a C–Mn–Si–dual-phase steel

A study has been carried out on an Fe–0.11% C–1.58% Si–0.4% Mn-dual phase steel. The dual-phase microstructures and properties are significantly affected by both the intercritical temperature and cooling rate from (α + γ) field. Upon rapid cooling (water or oil quench) from the temperature range 735–820°C, the structure comprises ferrite + martensite. On the other hand, slow cooling (air cooling) from the temperature range 735–820°C produces microstructures containing ferrite + martensite + pearlite/bainite and more favourable mechanical properties as: σ_0,2 = 281–296 MPa, σ_UTS = 632–690 MPa, TE = 26–30% and continuous yielding behaviour.     

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.     

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.     

Bainite morphologies in sintered steels: influence of materials and process conditions

ABSTRACT

With the introduction of diffusion-bonded (d.b.) powders, metallographers found localised microstructure changes in sintered steels, and frequent presence of bainite. With the sinter-hardening process, the bainite fraction became a tool for rating the method. In this study, depending on different parameters, differences between bainitic structures of homogenous or not-homogeneous sintered steels have been analysed. When using pre-alloys, upper bainite is present with the same morphology. When using d.b. powders, variations of local composition lead to upper bainite of different morphology, each corresponding to a composition range. Quite high Ni local content gives rise to lower bainite, always mixed with martensite. Higher Ni contents lead to incoercible austenite. Upper bainite presents different morphologies, depending on local Ni and Cu amounts. The increase of sintering temperature leads to fewer morphologies of upper bainite, due to greater amount of diffusion processes. The lower bainite has always a typical morphology, whatever process parameters.     

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.     

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.     

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.     

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.     

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.