Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram. The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.     

Mechanical Properties of (20–30)Mn12Cr(0.56–0.7)CN Corrosion Resistant Austenitic TWIP Steels

New developed (20–30)Mn12Cr(0.56–0.7)CN TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800 MPa UTS), deformability (80–100% elongation), toughness (300 J ISO‐V), and impact wear resistance equivalent to that of Hadfield steel. In addition, they exhibit corrosion resistance by passivation in aqueous acidic media. Microstructure examination by SEM and EBSD at different degrees of deformation reveals that twinning takes place and is responsible for the high cold‐work hardening of the steels. Stacking fault energy measurement of three different developed steels locates them in the range of 30–40 mJ m^?2, being highly dependent on the N and Mn contents. Measurements carried out with digital image correlation indicate that at room temperature dynamic strain aging or Portevin–LeChatelier effect takes place. Measurements of impact toughness indicate that the steels have ductile to brittle transition at cryogenic temperatures as a consequence of the effect of nitrogen on the deformation mechanisms, resulting in a quasi‐cleavage fracture along the {111} planes at ?196°C.     

Influence of Manganese and Nickel on the α´ Martensite Transformation Temperatures of High Alloyed Cr‐Mn‐Ni Steels

The martensite start temperature (M_s), the martensite austenite re‐transformation start temperature (A_s) and the re‐transformation finish temperature (A_f) of six high alloyed Cr‐Mn‐Ni steels with varying Ni and Mn contents in the wrought and as‐cast state were studied. The aim of this investigation is the development of the relationships between the M_s, A_s, A_f, T₀ temperatures and the chemical composition of a new type of Cr‐Mn‐Ni steels. The investigations show that the M_s, A_s and A_f temperatures decrease with increasing nickel and manganese contents. The A_f temperature depends on the amount of martensite. Regression equations for the transformation temperatures are given. The experimental results are based on dilatometer tests and microstructure investigations.     

Numerical Modeling of δ‐Ferrite Dissolution Kinetics in Austenitic Stainless Steels

In the present paper a numerical model that describes the diffusion processes of Ni and Cr between ferrite and austenite in Fe‐Cr‐Ni ternary systems has been developed by using the finite difference method (FDM). The model employed the tie‐line relationships given by Kajihara and Kikuchi. The moving ferrite/austenite interface was determined by solving the mass balance equations using the Newton‐Raphson method. The model has been applied to predicting the dissolution kinetics of ferrite in austenitic stainless steels, which were processed by welding, continuous casting and strip casting, respectively. The accuracy of the model has been confirmed to be quite satisfactory as compared with measured values, with advantages over another numerical model developed by Vitek et al. This model was reproduced in the present work.     

Effect of Nitrogen on the Deformation Behaviour of High‐Nitrogen Austenitic Stainless Steels

The deformation behaviour of high‐nitrogen austenitic steels with the base composition of Fe‐18Cr‐10Mn containing various contents of nitrogen was investigated. Two deformation modes including deformation‐induced martensitic transformation (DIMT) and deformation twinning (DT) were observed depending on the nitrogen content. In the alloys with lower nitrogen contents, γ→?→α' martensitic transformation sequentially occurred, whereas DT acted as a main deformation mode and DIMT was suppressed in the alloys with increasing nitrogen content. Both DIMT and DT showed strong crystallographic orientation dependence. The competing mechanism between them was discussed in terms of the variation of stacking fault energy with nitrogen content.     

The continuous δ-γ transformation during cooling of ferritic-austenitic iron-chromium-nickel alloys

Following previously published isothermal TTT diagrams, continuous TTT diagrams for the δ-γ transformation of a series of different iron-chromium-nickel-alloys containing 23.0–35.0 Cr and 6.45–15.6 Ni are presented. The microstructures and transformation behaviors indicate the same nucleation and growth process as in the isothermal reactions. The critical upper cooling rate for complete undercooling of the δ-γ transformation decreases with increasing chromium- and decreasing nickel contents.     

Relationship between Work Hardening Behaviour and Deformation Structure in Ni‐free High Nitrogen Austenitic Stainless Steels

The work hardening behaviour of high nitrogen austenitic steel (HNS) depends not only on the nitrogen content but also on the addition of substitutional alloying elements such as Mn and Ni, although the effect of nitrogen content has been considered to be a main factor controlling the work hardening rate in HNS. In this study, two kinds of high nitrogen austenitic steels containing nearly 1 mass‐% of nitrogen with and without Mn (Fe‐25%Cr‐1.1%N and Fe‐21%Cr‐0.9%N‐23%Mn alloys) were tensile‐tested and their work hardening behaviour was investigated for the purpose of clarifying the effect of Mn on the work hardening behaviour. Then the results were related to the change in deformation substructure. In the Fe‐25Cr‐1.1N alloy, the work hardening rate kept high until fracture occurred, while in the Fe‐21Cr‐0.9N‐23Mn alloy it tended to decrease gradually with tensile deformation in the high strain region. It was concluded that the difference in work hardening behaviour between both alloys is attributed to the change in dislocation substructure from planar dislocation array to dislocation cell by the addition of Mn.     

Cold Work Hardening of High‐Strength Austenitic Steels

Mechanisms of cold work hardening in three austenitic steels containing (mass%) 12Mn and 1.2C (Hadfield steel denoted as C1.2); 21Cr, 23Mn, 2Ni and 0.9N (Böhler steel P‐560 denoted as N0.9); 18Cr, 18Mn, 0.345C, 0.615N (CARNIT steel denoted as CN0.96) were studied using mechanical tension tests and TEM studies of substructure formed in the course of plastic deformation. Hadfield steel C1.2 reveals the smallest yield and ultimate stresses and elongation but the highest cold work hardening. Similar yield and ultimate stresses were obtained for steels N0.9 and CN0.96 with a higher elongation and cold work hardening for the latter. The analysis of TEM results leads to the following conclusions: Cold work hardening of the carbon steel C1.2 is mainly due to intensive twinning with rather thick twins. Localized planar slip is a feature of the substructure in the nitrogen steel N0.9 and carbon+nitrogen steel CN0.96 at strains up to 10 %, whereas twinning is involved in deformation at strains in the range of 10 to 50%. The strain‐induced ∊ martensite is rarely observed in both of these steels at strains above 30 %. The substructure and cold work hardening are discussed in terms of stacking fault energy, short‐range atomic order and binding between interstitial atoms and dislocations.     

Effect of Surface Tension on Gas Atomization of a CrMnNi Steel Alloy

The aim of the current research is the experimental investigation of the mass median particle size d₅₀ as a function of surface tension for liquid Cr–Mn–Ni steel alloy with 16% Cr, 7% Mn, and 9% Ni. To modify the liquid steel design sulfur was add to the Cr–Mn–Ni steel in five steps up to a 1000 mass ppm. The surface tension of the liquid steel alloy was measured using maximum bubble pressure method and yttria stabilized capillary in a temperature range from 1701 to 1881 K. In addition, the same steel charges were sprayed to steel powder using a vacuum inert gas atomization using pure argon gas. The increase of sulfur in Cr–Mn–Ni steel will decrease the surface tension to 0.91 N m^?1. The temperature coefficient of surface tension is positive for all investigated Cr–Mn–Ni alloys due to a sulfur content ≥100 mass ppm. The final mass median particle size d₅₀ decreases from 54.3 µm for AISI 304 reference steel alloy to 17.1 µm for Cr–Mn–Ni steel alloy (16‐7‐9 S10) with the highest sulfur content and the lowest surface tension of all investigated liquid steels. It is concluded from the present work that surface tension is the decisive factor in adjusting d₅₀ at a constant spraying parameters.     

Simulation of the γ–α‐transformation using the phase‐field method

Phase transformation is a powerful tool to change the properties of steels. Of the known transformations especially the γ–α‐transformation is utilised. It occurs in a temperature range relevant for heat treating and hot deformation processes. In this paper an approach is presented in which the γ–α‐transformation is simulated with Micress. This software applies the multicomponent multiphase‐field model, which is based on the reduction of total free enthalpy. Two different steels have been selected for the simulations, an ULC and an IF steel. Dilatometric tests serve as a basis for the simulations. These tests have shown that the transformation behaviours of the two steel grades are governed by two different kinetics. The transformation kinetics of the IF grade is influenced by the microalloying concept applied, resulting in a very slow start of the transformation. This has also been incorporated in the simulations by choosing two different grain boundary mobilities, one main parameter of the simulation. The simulation results of the ULC grade show the huge influence of nucleation undercooling as another one of the main parameters. Both simulation results are satisfying. They show that the phase‐field method offers a strong simulation tool in the area of phase transformation.     

Hot Deformation Behaviors of Fe–30Mn–3Si–3Al TWIP Steel during Compression at Elevated Temperature and Strain Rate

The hot deformation behavior of twinning‐induced plasticity (TWIP) steel was investigated at 973–1373 K and strain rates of 0.01–20 s^?1 by hot‐compression experiments performed on a Gleeble‐3800 thermo‐simulation test system. Microstructural evolution during recrystallization in the hot deformed TWIP steels was investigated by metallurgical analysis. The hot‐flow behavior can be represented by a Zener–Hollomon parameter in the hyperbolic‐sine equation. The hot‐deformation activation energy is 436.813 kJ mol^?1. Deformation bands are initially generated in the deformed austenitic grains during the dynamic recrystallization (DRX) of TWIP steel. With increasing temperature, the recrystallized grains emerge at the boundary junctions after the disappearance of the deformation bands. Subsequently, they gradually spread along the austenitic boundaries and exhibit a necklace shape. The dynamic recrystallized grains continuously grow until they finally reach equilibrium. The DRX mechanism of TWIP steel is a boundary bulge mechanism. The optimum hot‐working technology parameters (especially for rolling) for the TWIP steel is the deformation temperature range of 1223–1323 K, and strain rate range of 1–10 s^?1.     

Multiscale Characterization of the Work Hardening Mechanisms in Fe–Mn Based TWIP Steels

When strained in tension, high‐manganese austenitic twinning induced plasticity (TWIP) steels achieve very high strength and elongation before necking. The main hypotheses available in the literature about the origin of their excellent work hardening include deformation twinning and dynamic strain ageing. In order to provide some answers, various experiments at different scales were conducted on Fe–Mn–C steels and the Fe–28 wt%Mn–3.5 wt%Al–2.8 wt%Si alloy. At a macroscopic scale, tensile tests were performed on all the studied grades. It was shown that, though the Fe–Mn–Al–Si based alloy retains very high elongation, the Fe–Mn–C steels properties are even more extraordinary. Tensile tests at different strain rates with the help of digital image correlation were also performed on the Fe–20 wt%Mn–1.2 wt%C steel to study the PLC effect occurring in this type of steel. It is suggested that supplementary hardening could come from reorientation of Mn–C pairs in the cores of the dislocations. At a microscopic scale, the Fe–20 wt%Mn–1.2 wt%C TWIP steel and the Fe–Mn–Al–Si grade were thoroughly investigated by means of in situ TEM analysis. In the Fe–Mn–C steel, the formed twins could also lead to a composite effect, since they contain plenty of sessile dislocations. In the Fe–Mn–Al–Si alloy, mechanical twins are thicker and contain fewer defects, leading to a lower work hardening than the other grade.     

Oxidation of Fe–18%Mn–0.6%C Steels in Air and a N2–CO2–O2 Mixed Gas Atmosphere at 1273–1473 K

Austenitic Fe–18 wt% Mn–0.6 wt% C steels were oxidized at 1273, 1373, and 1473 K for up to 2 h in either atmospheric air or an 85%N₂–10%CO₂–5%O₂ gas mixture. The alloys oxidized faster in air than in the mixed gas, but the morphology and composition of the oxide scale formed were similar in both atmospheres. The scales that consisted primarily of FeO, Fe₂O₃, and MnFe₂O₄ were highly susceptible to cracking and spallation due to the severe oxidation condition. Since Mn was consumed to form MnFe₂O₄, the original γ‐matrix changed to an α‐matrix in the subscale area, in which Mn‐rich internal oxide precipitates formed locally.     

Mechanical Behavior of Deformation‐Induced α′‐Martensite and Flow Curve Modeling of a Cast CrMnNi TRIP‐Steel

For the modeling of the mechanical behavior of a two phase alloy with the rule of mixture (RM), the flow stress of both phases is needed. In order to obtain these information for the α′‐martensite in high alloyed TRIP‐steels, compression tests at cryogenic temperatures were performed to create a fully deformation‐induced martensitic microstructure. This martensitic material condition was subsequently tested under compressive loading at ?60, 20, and 100°C and at strain rates of 10^?3, 10⁰, and 10³ s^?1 to determine the mechanical properties. The α′‐martensite possesses high strength and surprisingly good ductility up to 60% of compressive strain. Using the flow stress behavior of the α′‐martensite and that of the stable austenitic steel AISI 316L, the flow stress behavior of the high alloyed CrMnNi TRIP‐steel is modeled successfully using a special RM proposed by Narutani et al.     

Temperature Dependent Deformation Mechanisms of a High Nitrogen‐Manganese Austenitic Stainless Steel

The influence of temperature on the deformation behaviour of a Fe‐16.5Cr‐8Mn‐3Ni‐2Si‐1Cu‐0.25N (wt%) austenitic stainless steel alloy was investigated using transmission electron microscopy and X‐ray diffraction measurements. Recrystallized samples were deformed under tension at ?75°C, 20°C, and 200°C and the microstructures were characterized after 5% strain and after testing to failure. Deformation to failure at ?75°C resulted in extensive transformation induced plasticity (TRIP) with over 90% α′‐martensite. The sample deformed to 5% strain at ?75°C shows that the austenite transformed first to ?‐martensite which served to nucleate the α′‐martensite. Transformation induced martensite prohibits localized necking providing total elongation to failure of over 70%. At room temperature, in addition to some TRIP behaviour, the majority of the deformation is accommodated by dislocation slip in the austenite. Some deformation induced twinning (TWIP) was also observed, although mechanical twinning provides only a small contribution to the total deformation at room temperature. Finally, dislocation slip is the dominant deformation mechanism at 200°C with a corresponding decrease in total elongation to failure. These changes in deformation behaviour are related to the temperature dependence on the relative stability of austenite and martensite as well as the changes in stacking fault energy (SFE) as a function of temperature.     

The isothermal δ-γ transformation of ferritic-austenitic iron-chromium-nickel alloys

Quenching from ferritizing temperatures of 1350 and 1250 °C the isothermal δ-γ transformation of two iron-chromium-nickel alloys with 26.0% Cr, 6.9% Ni resp. 30.0% Cr, 7.05% Ni at temperatures between 700 and 1200 °C is studied. Three different transformation stages are revealed indicating grain boundary diffusion controlled growth in the first and last stage while volume diffusion seems to act at least partially in the second stage. Isothermal TTT diagrams are established and activation energies calculated.     

Hot Mechanical Properties of 24Cr‐14Ni High Alloy Billets

24Cr‐14Ni alloys have gained importance in high temperature applications. Because of δ‐ferrite and α phase formation, 24Cr‐14Ni austenitic stainless steel billets are difficult to hot work. The mechanical properties at high temperature of such stainless steels are investigated on a hot tensile test machine according to hot‐rolling conditions, under different time and temperature regimes. These 24Cr‐14Ni stainless steels were also hot rolled under various reduction ratios. The influences of the reduction ratio on the hot mechanical properties and phase transformation from δ‐ferrite into σ phase in 24Cr‐14Ni stainless steels are discussed in detail. The results obtained can be a contribution to improve the hot rolling of this high alloy stainless steel.     

The Development of a Processing Window for the Continuous Galvanizing of a Mn–Cr–Si Martensitic Steel

Martensitic or complex phase steels are leading candidates for automotive impact management applications. However, achieving high strengths while obtaining high quality coatings via continuous galvanizing is a challenge due to cooling rate limitations of the processing equipment and selective oxidation of alloying elements such as Cr, Mn, and Si adversely affecting reactive wetting. The galvanizability of a Cr? Mn? Si steel with a target tensile strength above 1250 MPa was investigated within the context of the continuous galvanizing line. The continuous cooling transformation behavior of the candidate alloy was determined, from which intercritical and austenitic annealing thermal cycles were developed. The evolution of substrate surface chemistry and oxide morphology during these treatments and their subsequent effect on reactive wetting during galvanizing were characterized. The target strength of 1250 MPa was achieved and high quality coatings produced using both intercritical (75% γ) and austenitic (100% γ) annealing using a conventional 95%N₂–5%H₂, ?30°C dew point process atmosphere and 0.20 wt% dissolved (effective) Al bath, despite the presence of significant Mn and Cr oxides on the substrate surfaces. It is proposed that complete reactive wetting by the Zn(Al, Fe) bath was promoted by in situ aluminothermic reduction of the Mn and Cr‐oxides by the dissolved bath Al.     

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.