Deformation behavior of duplex austenite and ε-martensite high-Mn steel

Abstract

Deformation and work hardening behavior of Fe–17Mn–0.02C steel containing ε-martensite within the austenite matrix have been investigated by means of in situ microstructural observations and x-ray diffraction analysis. During deformation, the steel shows the deformation-induced transformation of austenite → ε-martensite → α′-martensite as well as the direct transformation of austenite → α′-martensite. Based on the calculation of changes in the fraction of each constituent phase, we found that the phase transformation of austenite → ε-martensite is more effective in work hardening than that of ε-martensite → α′-martensite. Moreover, reverse transformation of ε-martensite → austenite has also been observed during deformation. It originates from the formation of stacking faults within the deformed ε-martensite, resulting in the formation of 6H-long periodic ordered structure.     

Microstructure evolution during thermomechanical processing in 3Mn-0.1C medium-Mn steel

ABSTRACT

Medium-Mn steels are energetically investigated as a candidate of the third generation advanced high strength steels (AHSSs). However, their phase transformation and microstructaure evolution during various heat treatments and thermomechanical processing are still unclear. The present study first confirmed the kinetics of static phase transformation behaviour in a 3Mn-0.1C medium-Mn steel. Hot compression tests were also carried out to investigate the influence of high-temperature deformation of austenite on subsequent microstructure evolution. It was found that static ferrite transformation was quite slow in this steel, but ferrite transformation was greatly accelerated by the hot deformation in austenite and ferrite two-phase regions. Characteristic dual-phase microstructures composed of martensite and fine-grained ferrite were obtained, which exhibited superior mechanical properties.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Differential scanning calorimetry study of constrained groove pressed low carbon steel: recovery,recrystallisation and ferrite to austenite phase transformation

Abstract

Low carbon steel sheets are subjected to severe plastic deformation (SPD) via constrained groove pressing (CGP) up to five passes. As a result of this process, strain magnitude up to 5·8 is imposed to the sheets, which leads to grain size of 225 nm. These nanostructured steel sheets, due to their high dislocation density and ultrafine microstructure, are very sensitive to heating. In the present study, recovery, recrystallisation and ferrite to austenite phase transformation phenomena for the SPD steel are investigated using differential scanning calorimetry method. The results show that with increasing the strain in steel sheets, the deformed stored energy (released through recovery and recrystallisation) and enthalpy of ferrite to austenite phase transformation are significantly increased and varied in 38·5–85·8 and 109–156·1 MJ m^?3 ranges respectively. In addition, transformation temperature is decreased from 761 to 750°C after five CGP passes. However, recovery stored energy, recovery and recrystallisation peak temperatures are not changed, considerably. Experimental data show that with increasing the hardness, the stored energy is increased. One empirical equation is developed for relationship between hardness and stored energy of severely deformed low carbon steel. In addition, using the dislocation model, this mentioned relationship is justified.     

Microstructure in adiabatic shear bands in a pearlitic ultrahigh carbon steel

Abstract

Adiabatic shear bands, obtained in compression deformation at a strain rate of 4000 s^?1, in a pearlitic 1·3%C steel, were investigated. Shear bands initiated at 55% compression deformation with the width of the band equal to 14 μm. Nano-indentor hardness of the shear band was 11·5 GPa in contrast to the initial matrix hardness of 3·5 GPa. The high strength of the shear band is attributed to its creation from two sequential events. First, large strain deformation, at a high strain rate, accompanied by adiabatic heating, led to phase transformation to austenite. Second, retransformation upon rapid cooling occurred by a divorced eutectoid transformation (DET). The result is a predicted microstructure consisting of nano size carbide particles within a matrix of fine ferrite grains. It is proposed that the DET occurs in iron–carbon steels during high rate deformation in ball milling, ball drop tests and in commercial wire drawing.     

Effect of strain rate on quasistatic tensile flow behaviour of solution annealed 304 austenitic stainless steel at room temperature

Room temperature tensile test results of solution annealed 304 stainless steel at strain rates ranging between 5 × 10⁻⁴ and 1 × 10⁻¹ s⁻¹ reveal that with increase in strain rate yield strength increases and tensile strength decreases, both maintaining power–law relationships with strain rate. The decrease in tensile strength with increasing strain rate is attributed to the lesser amount of deformation-induced martensite formation and greater role of thermal softening over work hardening at higher strain rates. Tensile deformation of the steel is found to occur in three stages. The deformation transition strains are found to depend on strain rate in such a manner that Stage-I deformation (planar slip) is favoured at lower strain rate. A continuously decreasing linear function of strain rate sensitivity with true strain has been observed. Reasonably good estimation for the stress exponent relating dislocation velocity and stress has been made. The linear plot of reciprocal of strain rate sensitivity with true strain suggests that after some critical amount of deformation the increased dislocation density in austenite due to the formation of some critical amount of deformation-induced martensite plays important role in carrying out the imposed strain rate.     

Strength and strain-hardening enhancement by generating hard delta-ferrite in twinning-induced plasticity steel

ABSTRACT

A dual-phase (12?vol.% delta-ferrite?+?78?vol.% austenite) high manganese twinning-induced plasticity (TWIP) steel was produced by hot rolled and annealing treatment. In comparison with the fully austenitic TWIP steel, both the yield and ultimate tensile strength of the dual-phase TWIP steel reinforced by hard delta-ferrite are significantly increased. It was found that the delta-ferrite in dual-phase steel exhibits a high hardness owing to the formed DO₃ structured intermetallic phase within ferrite. The presence of delta-ferrite dramatically improves the strain-hardening ability of TWIP steel. This is principally attributed to the effects of strain partitioning between hard delta-ferrite and softer austenite on the kinetics of deformation twinning and/or additional geometrical necessary dislocation (GND) during the deformation process.     

Strain rate effects on tensile deformation behavior of Fe-3.5Mn-0.3C-5Al ferritic based lightweight steel

Tensile deformation behavior of Fe-3.5Mn-0.3C-5Al ferritic based lightweight steel was studied in a large range of strain rate (0.001 s⁻¹–1200 s⁻¹). Microstructures of the steel before and after tension were observed. The results show that Fe-3.5Mn-0.3C-5Al lightweight steel has a good strength (820 MPa) and plasticity (40 %) and exhibits excellent combinations of specific strength and ductility (>32000 MPa %) at the strain-rate of 0.001 s⁻¹ after annealing at 850 °C for 5 minutes then directly quenching into water. The austenite in the steel tested was transformed into α′-martensite during the tensile deformation process. With an increase in strain rate from 0.001 s⁻¹ to 1200 s⁻¹, tensile strength of the steel investigated increased from 820 MPa to 932 MPa, while its elongation first decreased from 40 % to 15 %, and then increased from 15 % to 29 %. At the strain rate of 1200 s⁻¹, adiabatic heating resulted in temperature rising in matrix, suppressed the transformation of austenite to α′-martensite. Comparing with transformation induced plasticity steel, the austenite in 3.5Mn lightweight steel is obviously unstable and cannot provide progressive phase transition.     

Microstructure and texture of high manganese steel subjected to dynamic impact loading

ABSTRACT

Dynamic impact response of high Mn-steel at a strain rate of 3000?s^?1 was investigated using the Split Hopkinson Pressure bar. The investigated steel depicted continuous yielding at high strain rates. Additionally, the yield stress displayed a positive strain-rate sensitivity with an increasing strain rate. Microstructural evaluations displayed that strain-induced martensitic transformation and dislocation multiplication during slip were dominant plastic deformation mechanisms in the absence of deformation twinning which contributes to the strain hardening. Adiabatic shear band and martensite to austenite reversion or dynamic recrystallisation were also attributed to strain softening during impact deformation. The {001}<110> R-cube, {011}<110> R-Goss, and ({111}<110>) E texture components were strengthened after impact loading compared with as-received condition, while the intensities of Cube, Cupper, Brass, and S texture components were decreased.     

Optimisation of hot rolling schedule for direct charging of thin slabs of Nb–V microalloyed steel

Abstract

A recently developed continuous casting simulator and the ‘Wumsi’ hot deformation simulator have been used to carry out laboratory simulation tests to determine the as cast microstructure and the recrystallisation behaviour of a Nb–V microalloyed steel during the process of direct charging. By variation of the initial specimen thickness (between 25 and 60 mm) different values of total strain Φ_Σ could be imposed to improve the coarse as cast microstructure. For a series of deformation schedules the total strain was divided systematically into two components: an austenite grain refining strain Φ_γGF (above the recrystallisation stop temperature T_RS) and an austenite strengthening strain Φ_γS (below T_RS). After hot deformation slow and accelerated cooling with simulated coiling were employed. It was found that a total strain Φ_Σ>1·4 is required to ensure mechanical properties that were comparable or even superior to those found using the conventional cold charging process. The coarse as cast austenite microstructure can be refined significantly when Φ_γGF=0·3–0·6. The austenite strengthening strain Φ_γS represents the dominant component of the total strain if a satisfactory toughness is to be achieved. Strength properties are less sensitive to the applied strain.

MST/1872     

Effect of plastic strain on grain size of ferrite transformed from deformed austenite in Si-Mn steel

Abstract

The effect of plastic strain on the grain size of ferrite transformed from deformed and unrecrystallised austenite has been investigated for a low carbon Si-Mn steel. An explicit finite element technique was used to evaluate the plastic strain distribution introduced by deformation in a specimen. The specimen was compressed by a pair of anvils controlled to keep the strain rate constant. The contact condition between the anvil and the specimen was determined by the experimental result from an identically deformed screw set in the specimen. The interrelation between the equivalent plastic strain epsiloneq, numerically obtained in the range 0.1<epsiloneq<4.2, and the experimental grain size of 2-9 m was analysed. The change in ferrite grain size caused by epsiloneq in the unrecrystallised austenite region has been discussed.     

Transformation of austenite remaining after intercritical rolling to ferrite

Abstract

The impact of austenite deformation in the intercritical range on the rate of transformation in continuous cooling to ferrite, pearlite, bainite or martensite has been studied. The austenite associated with the rolled ferrite is much higher in carbon content, which does not influence the pearlite transformation but retards bainite and martensite. Furthermore, in comparison with rolling of stable austenite the increased strain hardening of the intercritically cooled austenite accelerates the formation of ferrite and pearlite (+ 10–30°C) and refines them but retards the bainite and martensite transformations (?20–40°C). At the intermediate cooling rate near 16 K s^?1, these several influences combined with near doubling of the ferrite production give rise to the suppression of bainite formation and to maximum increased delay of martensite start.     

Strain hardening behavior of a TRIP/TWIP steel with 18.8% Mn

Tensile tests were carried out to study the strain hardening behavior of a TRIP/TWIP steel with 18.8% manganese. The results indicated that the true stress-strain curve can be divided into 4 stages in tension testing. Material is in an elastic region when the true strain is below 0.06. In the initial stage of the plastic deformation (? = 0.06-0.14), ?-martensite was preliminarily formed, and that austenite transformed to α-martensite through the ?-martensite formation. When the true strain was between 0.14 and 0.35, the stacking fault energies were elevated by the increase of strain energy, deformation twinning occurred instead of the ?-martensite formation. The second derivative of the stress-strain curve satisfied the condition d²σ/d?² > 0. Twinning induced plasticity dominated this stage. In the last plastic deformation stage (? = 0.35-0.45), γ → α transformation occurred at the crossing of twins, and α-martensite grew along the thickness of the twinned regions.     

Dynamic recrystallisation of as cast austenite in 18–8 stainless steel

Abstract

Dynamic recrystallisation behaviour of an as cast 0Cr18Ni9Ti stainless steel during hot deformation was investigated by hot compression test at a temperature range of 950–1200°C and strain rate of 5 × 10^-3–1 × 10^-1 s^-1. Change of austenite grain size owing to dynamic recrystallisation was also studied by microstructural observation. The experimental results showed that the hot deformation conditions, such as temperature, strain, and strain rate determine the dynamic recrystallisation behaviour for the as cast stainless steel, and the dynamically recrystallised grain size is determined by the deformation conditions and is independent of the strain.     

Analysis of deformation induced martensitic transformation in stainless steels

Abstract

Many studies monitoring the formation of martensite during the tensile deformation of austenite report data which are, in principle, affected by both the applied stress and the resulting plastic strain. It is not clear in these circumstances whether the transformation is stress induced (i.e. the stress provides a mechanical driving force) or whether the generation of defects during deformation helps nucleate martensite in a scenario better described as strain induced transformation. The authors demonstrate in the present work that a large amount of published data relating the fraction of martensite to plastic strain can in fact be described in terms of the pure thermodynamic effect of applied stress.     

Transformation characteristics of medium carbon V – Ti – N microalloyed steel for non-quenched/tempered oil well tubes

Abstract

The phase transformation points of a medium carbon V - Ti - N microalloyed steel were determined, as were the continuous cooling transformation curves of austenite heated at 1100 ° C without deformation and heated at 1200 ° C with two pass deformation. The data have been further used to analyse the real production scheme of N80 grade hot rolled non-quenched/tempered seamless tubes. The results have showed clearly that the commonly used 'in line normalisation' is not always necessary in the non-quenched/tempered production process of N80 grade hot rolled seamless oil well tubes. The above viewpoint has been further demonstrated by the microstructural examination of specimens sampled from industrial production.     

Influence of silicon,aluminium, phosphorus and boron on hot ductility of TRansformation Induced Plasticity assisted steels

Abstract

The hot ductility of steels having high aluminium or phosphorus contents, which are currently being considered as possible replacements for the conventional high silicon TRansformation Induced Plasticity (TRIP) steel, has been examined. Tensile specimens were cast in situ and tested in the temperature range 750 - 1000 ° C at a strain rate of 3 × 10^-3 s^-1. The ductility trough for the conventional high silicon TRIP steel was controlled by the austenite - ferrite transformation, intergranular failure occurring when a thin band of the softer ferrite phase formed around the austenite grains. Void formation at the sulphides situated in the soft ferrite at the boundaries then occurred, and the strain concentrated locally there. The thin bands of ferrite were deformation induced and, as such, formed at temperatures above Ar ₃ and could form at as high a temperature as Ae ₃. Adding ferrite formers such as silicon, phosphorus and aluminium increased the Ae ₃ temperature and thus widened the trough. The high aluminium (2%) TRIP steel exhibited good ductility throughout the temperature range examined, since large amounts of ferrite were always present, preventing strain concentration, and the AlN particles were too coarse to influence the hot ductility. In contrast, the 1%Al containing steel gave poor ductility below 850 ° C, the band of strain induced ferrite being extremely thin. The ductility trough in the titanium containing high phosphorus steel was poor, owing to fine precipitation of TiN. Adding boron to the steel and reducing the manganese content from 1.4 to 1% resulted in better ductility. Generally, the TRIP type steels had superior ductility to the conventional niobium containing high strength low alloy steel.     

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.     

Tensile deformation of a duplex Fe-20Mn-9Al-0.6C steel having the reduced specific weight

The room temperature deformation characteristics of a duplex Fe-20Mn-9Al-0.6C steel with the reduced specific weight of 6.84 g/cm³ in the fully solutionized state were described in conjunction with the deformation mechanisms of its constituent phases. The phase fraction was insensitive to annealing temperature in the range of 800-1100 °C. The ferrite grain size was also nearly unaltered but the austenite grain size slightly increased with increasing annealing temperature. This revealed that there is little window to control the microstructure of the steel by annealing. The steel exhibited a good combination of strength over 800 MPa and ductility over 45% in the present annealing conditions. Ferrite was harder than austenite in this steel. Strain hardening of both phases was monotonic during tensile deformation, but the strain hardening exponent of austenite was higher than that of ferrite, indicating the better strain hardenability of austenite. In addition, the strain hardening exponent of austenite increased but that of ferrite remained unchanged with increasing annealing temperature. The overall strain hardening of the steel followed that of austenite. Considering element partitioning by annealing, the stacking fault energy of austenite of the steel was estimated as ∼70 mJ/m². Even with the relatively high stacking fault energy, planar glide dominantly occurred in austenite. Neither strain induced martensite nor mechanical twins formed in austenite during tensile deformation. Ferrite exhibited the deformed microstructures typically observed in the wavy glide materials, i.e. dislocation cells. The mechanical properties of the present duplex steel were compared to those of advance high strength automotive steels recently developed.     

Micro-defect effect on gigacycle fatigue S-N property and very slow crack growth of high strength low alloy steel

Abstract

Transformation induced plasticity (TRIP) assisted steels contain a small quantity of carbon enriched retained austenite, which transforms into martensite during the course of plastic deformation. Transformation of this kind can be induced by both stress and plastic strain. The detailed mechanism by which the martensite is induced is different for these two scenarios. An attempt is made here to discover the relative importance of these mechanisms and it is found that stress affected transformation can explain much of the variation in retained austenite content as a function of plastic strain.     

Modelling of isothermal formation of pearlite and subsequent reaustenitisation in eutectoid steel during continuous heating

Abstract

Three different morphologies of pearlite have been formed isothermally at three different temperatures in a eutectoid steel. Moreover, the interlamellar spacing of the pearlite was calculated using the Zener and Hillert theoretical method. Experimental results suggest that the growth of pearlite is mainly controlled by volume diffusion of carbon in austenite, in the temperature range studied in this steel. In addition, a model that describes pearlite to austenite transformation during continuous heating in a eutectoid steel has been developed. The influence of structural parameters, such as interlamellar spacing and edge length of pearlite colonies, on the transformation kinetics has been experimentally studied and considered in the modeling. It has been found that the kinetics of pearlite to austenite transformation are slower the coarser the initial pearlite microstructure. Experimental validation of this model has been carried out and a good agreement (an accuracy level of higher than 90% in square correlation factor) between the experimental and calculated values has been found.