Al-free Steel Research Articles

Effects of increasing the strain rate on mechanical twinning and dynamic strain aging in Fe-12.5Mn-1.1C and Fe–24Mn-0.45C–2Al austenitic steels

Understanding the relationship between mechanical twinning and dynamic strain aging (DSA) in austenitic steels is necessary for steel development. DSA can be manipulated by changing the strain rate during tension tests, or by alloying austenitic steels with aluminum. In the current study, 12.5%Mn-1.1%C austenitic steel and 24%Mn-0.45%C–2%Al austenitic steel were investigated under different strain rates. Increases in the strain rate resulted in an enhanced resistance of the steels to the plastic deformation only during early stages of strain (at 3% strain) because of mechanical twinning promotion. On one hand, at 25% strain under low strain rate deformation, the Al-alloyed steel showed a tendency to form more twin plates than the Al-free steel as indicated by the transmission electron microscopy. On the other hand, at 25% strain under a high strain rate deformation, the Al-free steel showed an enhanced twin plates propagation due to a stronger self-segregation/depletion of the interstitial atoms (the carbon concentration ranged between 0 and 11 at.%) as revealed by the 3D atom probe tomography. Furthermore, the Al-alloyed steel showed a reduced twin volume fraction with a strain rate increase due to shifting of the stacking fault energy to higher values.

Effect of Finish Rolling Temperature on Microstructure and Mechanical Properties of Low-Si Steel After Quenching and Nonisothermal Partitioning

The quenching and nonisothermal partitioning process has been found to achieve excellent combinations of strength and ductility through the retention of austenite in the microstructure. Therefore, in the present study, the effect of grain size on phase transformations and ensuing mechanical properties has been investigated. A low-Si and Al-free steel was finish rolled at different temperatures which was subsequently quenched to a constant temperature below Ms for the formation of martensite with remaining untransformed austenite. This was followed by slow cooling to room temperature to simulate the condition of nonisothermal partitioning of carbon from martensite to the austenite. The results show that there is not much change in the average grain size with variation in the finish rolling temperature, as well as, no measurable differences are observed in the microstructural features and retained austenite content. The microstructure shows the presence of martensite, carbides, and a small amount of retained austenite. However, in terms of mechanical properties, the finish rolling at a lower temperature of 900 °C lead to higher hardness and strength with comparable ductility in comparison to the sample which has been finish rolled at a relatively higher temperature (940 °C). The above results will thus be helpful in designing the industrial processing schedule for low-Si steel to achieve enhanced performance.

Impact of Al addition on deformation behavior of Fe–Cr–Ni–Mn–C austenitic stainless steel

The influence of Al on the temperature dependence of deformation mechanisms in metastable austenitic stainless steels was investigated by tensile tests in the temperature range of -196 – 300 °C. The deformed microstructures of the Al-free and Al-added steels at −140 °C consisted of planar glide features such as deformation-induced α′-martensite and deformation twins, with a larger fraction of α′-martensite in the Al-free steel. The formation of α′-martensite was mediated by deformation twins. The delayed kinetics of α′-martensite formation in the Al-added steel implies an increase in the stacking fault energy upon Al addition. This was also supported by the higher work hardening rate of the Al-free steel at deformation temperatures up to 150 °C. The work hardening rates at 300 °C were influenced by the dynamic strain aging. The more pronounced dynamic strain aging in the Al-added steel caused a higher work hardening rate compared to the Al-free steel. According to the tensile test results, the addition of Al caused a shift in the curves representing the temperature dependence of tensile elongation and tensile strength to lower temperatures. For the Al-free steel, the temperature associated with the maximum tensile elongation almost exactly coincided with the Mdγ→α′ temperature, namely an immediate deterioration of tensile elongation occurred upon the α′-martensite formation. For the Al-added steel, on the other hand, the maximum tensile elongation was achieved at a temperature well below its respective Mdγ→α′ temperature. This indicates an enhancement of tensile elongation in spite of the deformation-induced α′-martensite formation. Since deformation-induced micro-mechanisms in both steels were quite similar, the difference in the sensitivity of steels to the deformation-induced α′-martensite formation was interpreted in terms of an inhomogeneous distribution of alloying elements and local variations in the stacking fault energy and deformation-induced micro-mechanisms.

Microstructures and Impact Wear Behavior of Al-Alloyed High-Mn Austenitic Cast Steel After Aging Treatment

The microstructural evolution and impact wear behavior of Al-alloyed ultra-high-Mn austenitic cast steel were investigated after different heat treatments, including quenching (Q) and quenching + aging (Q + A). Another ultra-high-Mn austenitic cast steel without Al was compared under the same experimental condition. After Q + A, Al addition contributed to the precipitation of κ-carbides which improves the yield strength and wear resistance of the austenite matrix. However, the mechanical properties and wear resistance of the Al-free steel were strongly deteriorated due to the precipitation of plate-like M3C-type carbides, as well as the worn surface of the Al-free steel had the feature of micro-scratches. After Q + A (2 h), the Al-alloyed steel obtained optimal wear resistance and the main wear feature was strain fatigue. But, excessive aging time (4 h) would lead to a large fragile peeling from the worn surface and the wear resistance was decreased. Also, the worn subsurface microstructures with different heat treatments were observed by transmission electron microscopy (TEM). These results show that the deformation microstructures of the Al-free steel contained stacking faults, dislocation tangle and deformation twins. The deformation character of Al-alloyed steel is dominated by planar glide, and with increase in the aging time, planar dislocation substructures gradually evolved, relating to dislocation pileups, Taylor lattices and micro-bands.

Influence of Aluminium on Castability of V-Microalloyed Steels

The influence of aluminium content on the hot ductility behaviour in V-N steels was investigated. Cylindrical specimens were subjected to thermal cycles and strain rates approximating those experienced by a conventional slab surface during continuous casting. The resulting microstructures were examined using light optical and electron microscopy and correlated with measured reduction-in-area (RA) values, calculated precipitate chemistries and volume fractions, as well as the flow stress behaviour. It was found that removal of aluminium significantly improved the hot ductility. However, increasing the total [V][N] product in Al-free steels reduces RA. Poor hot ductility is caused by low austenite grain boundary mobility characterized by high work hardening rates. The fracture mode in brittle specimens is intergranular along thin ferrite films. AlN appears to inhibit austenite grain boundary mobility in V-high N steels when the cooling rate and strain rate are both very slow as experienced during unbending. SEM analysis of fracture surfaces revealed the presence of MnS-AlN particles in microvoids. TEM-EDAX spectra showed that the larger particles observed in the Al-containing steels are mostly a constitution of duplex/triplex grain boundary precipitates, i.e., MnS-AlN and V(C,N). Conversely, good ductility in austenite is associated with high grain boundary mobility that produces fine, recrystallised grains and subsequent dimple fracture after plastic tensile stress.

Oxidation of an austenitic stainless steel with or without alloyed aluminum in O2 + 10% H2O environment at 800 °C

Oxidation behavior in O2+10% H2O at 800°C of an austenitic stainless steel with or without aluminum is investigated. The Al-free steel enters into breakaway oxidation stage after only 15h exposure, while the one with 2.7wt% aluminum shows high oxidation resistance. This minor amount of aluminum is lower than the critical content to form an intact alumina scale, but it promotes the formation of a thin Al-rich inner layer which is favor in prohibiting holes formation due to volatilization of CrO2(OH)2 and Kirkendall effect. A compact scale develops and protects the Al-containing steel very well.

Fatigue crack growth in two TWIP steels with different stacking fault energies

Fatigue crack growth tests with R=0 and 0.4 were carried out on Fe-22Mn-0.6C and Fe-22Mn-0.6C-3Al (wt.%) Twinning-Induced Plasticity (TWIP) steels with stacking fault energies around 21.5 and 37mJ/m2, respectively. The former exhibits more crack closure effects, partly due to stronger asperity-induced closure. Strain-controlled push-pull tests followed by scanning electron microscope observations show that both steels are prone to mechanical twinning under low-cycle fatigue, associated with an increasing kinematic hardening. Twinning is however more profuse in Fe-22Mn-0.6C steel. Elastic-plastic finite elements simulations of crack growth, using specific constitutive equations able to capture the increasing kinematic hardening suggest that plasticity-induced crack closure is lower in Fe-22Mn-0.6C steel. Even after closure corrections, the Al-free steel, exhibits a lower resistance to fatigue crack growth, which is attributed to a pronounced strain localisation at the crack tip, and maybe also to environment effects.

The critical factors governing mechanical properties in hot-rolled Fe-0.2C-11Mn-(0-6)Al TRIP steels and fracture behavior

In the present study, we fundamentally explore the reasons underlying mechanical property variation in hot-rolled 11Mn transformation-induced plasticity (TRIP) steels with varying Al-content. With increase in Al-content, the ultimate tensile strength (UTS) was decreased, while total elongation (TE) was increased. The variation in UTS was mainly associated with the fraction of constituent phases and stability of austenite, while the increase in total elongation (TE) with increase in Al-content was related to the increase in the fraction of ferrite. Moreover, the Al-free steel, quenched from intercritical region experienced brittle fracture. When the Al-content was increased to 2wt%, the total elongation increased to ~22–32% and is also attributed to TRIP effect. In 4Al steel, the tensile elongation was in the range of ~19–40%, and was associated with TRIP effect and cooperative dislocation slip. In 6Al steel, the cumulative contribution of transformation-induced plasticity (TRIP), twinning-induced plasticity (TWIP), and cooperative deformation of δ-ferrite led to high elongation of ~37–65%. The increase in the contribution of TRIP and TWIP with increase in Al-content led to dramatic change in the mode and characteristics of fracture.

Austenite stabilization and high strength-elongation product of a low silicon aluminum-free hot-rolled directly quenched and dynamically partitioned steel

Microstructures composed of lath martensite and retained austenite with volume fraction between 8.0vol.% and 12.0vol.% were obtained in a low-C low-Si Al-free steel through hot-rolling direct quenching and dynamical partitioning (HDQ&DP) processes. The austenite stabilization mechanism in the low-C low-Si Al-free steel under the special dynamical partitioning processes is investigated by analyzing the carbon partition behavior from martensite to austenite and the carbide precipitation-coarsening behavior in martensite laths combining with the possible hot rolling deformation inheritance. Results show that the satisfying retained austenite amount in currently studied low-Si Al-free HDQ&DP steel is caused by the high-efficiency carbon enrichment in the 30–80nm thick regions of austenite near the interfaces in the hot-rolled ultra-fast cooled structure and the avoidance of serious carbides coarsening during the continuous cooling procedures. The excellent strength-elongation product reaching up to 26,000MPa% shows that the involved HDQ&DP process is a promising method to develop a new generation of advanced high strength steel.

Microstructure and mechanical properties of Al-alloyed Fe–Cr–Ni–Mn–C stainless steels

Abstract Austenitic, ferritic, and duplex Fe–17Cr–6Mn–0.45C– x Ni– y Al (3Ni alloy: x =3, y =4 and 9Ni series: x =9, y =0,4,7) cast stainless steels were obtained by adjusting the Al and Ni concentrations. The Al-free steel containing 9 wt% Ni exhibited an austenitic microstructure. The addition of 4 wt% Al to the 9Ni series of alloys retained the microstructure austenitic. Increasing the Al content to 7 wt%, however, led to the development of a fully ferritic matrix with a high hardness and poor ductility. The alloy containing 4 wt% Al but a reduced Ni content of 3 wt% exhibited a duplex microstructure consisting of nearly 20% ferrite in an austenitic matrix. Dilatometry indicated unusually high thermal expansion coefficients for the 7 wt% Al ferritic steel at temperatures above the Curie temperature of the alloy. This anomaly was interpreted in terms of the formation of high concentrations of thermal vacancies possibly due to the presence of intermetallic compounds. The tensile tests conducted at room temperature indicated a concurrent enhancement of tensile strength and total elongation of the Al-free austenitic alloy by the addition of 4 wt% Al. This advantage is magnified by the lower density of the Al-alloyed variant. The extended elongation and the high strength were achieved in spite of the decreasing work hardening rate of the Al-alloyed steel variant at high strains, interpreted as a gradual transition from planar to wavy glide mode of dislocations. The high glide planarity at low strains of the Al-alloyed austenitic alloy was confirmed by the TEM observation of dislocation pile-up arrangements. Substantial strengthening compared to austenitic steels was achieved in the duplex steel. This strengthening was associated with a marginal loss of ductility as 20% ferrite was introduced in the microstructure.

Inconsistent effects of mechanical stability of retained austenite on ductility and toughness of transformation-induced plasticity steels

The tensile and impact properties of two low-carbon bainitic transformation-induced plasticity (TRIP) steels, one being Al-free and the other being Al-containing, have been investigated. The two steels are featured by the existence of different sizes and shapes of retained austenite showing different mechanical stabilities. Compared with the Al-containing steel, the Al-free steel shows significantly improved tensile strength, uniform strain and total elongation, but exhibits much decreased impact toughness. These results demonstrate conflicting effects of strain-induced martensitic transformation of retained austenite on tensile ductility and impact toughness of TRIP steels. It is indicated that a wide distribution of size and shape of retained austenite is favorable for sustained high strain hardening rate and increased uniform tensile strain; however, thin films of retained austenite, which is mechanically more stable, tends to enhance impact toughness.

Effect of Aluminum and Titanium Content on Grain Growth, Texture and Magnetic Properties in 3%Si Non-Oriented Electrical Steel

The effect of annealing temperature on grain growth, texture development and magnetic properties of Al-free and Al-1% added non-oriented electrical steel were investigated. Normal grain growth occurred in Al-free steel. On the other hand, abnormal grain growth occurred in Al-added steel which was annealed at 800°C for 24h. Precipitates in these two steels were different. TiN precipitated in Alfree steel, but in the case of Al-added steel, AlN and TiC precipitated. The TiC in Al-added steel was so fine that it inhibited the normal grain growth and finally caused the abnormal grain growth. Main textures of both steels were near {111}<112>, but the intensity of near {111}<112> in the abnormal grain growth was higher than that in the normal grain growth. Magnetic flux density (B50/Bs) was decreased by the grain growth. Especially B50/Bs in the abnormal grain growth was lower than that in normal grain growth. B50/Bs in these steels can be estimated by their three-dimensional textures in vector method.

The effects of Al and Si additions on the microstructures of precipitates in low carbon steels containing Sn

The microstructures of low carbon steels with Sn additions were investigated using scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and energy dispersive X-ray spectroscopy. Four steels based on Fe-0.9Nb-0.3Sn-0.05C (wt%) with different levels of Al and Si additions were prepared by arc melting under an argon atmosphere. The effects of heat treatment and the level of alloying elements Al and Si on the precipitation of Sn-rich phases were studied. After ageing at 1150°C and 850°C, NbC precipitates were found in all samples, as well as AlN in the higher Al content steels. The concentration of Al in steel was also found to affect the formation of Sn-rich compounds after heat treatment at 850°C for 96 hours. In the lower Al or Al-free steels, a η-Fe2Nb3 phase, which dissolves a significant amount of Si, was observed. In the higher Al steels, a Fe2Nb-based Laves phase, which dissolves both Si and Sn was detected. A mechanism based on both size factors and thermodynamic considerations is described, which accounts for the experimental observations.