Stabilization of retained austenite by the two-step intercritical heat treatment and its effect on the toughness of a low alloyed steel

Fine film-like stable retained austenite was obtained in a Fe–0.08C–0.5Si–2.4Mn–0.5Ni in weight percent (wt.%) steel by the two-step intercritical heat treatment. The first step of intercritical annealing creates a mixed microstructure of preliminary alloy-enriched martensite and lean alloyed intercritical ferrite, which is called as “reverted structure” and “un-reverted structure”, respectively. The second step of intercritical tempering is beneficial for producing film-like stable reverted austenite along the reverted structure. The stabilization of retained austenite was studied by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dilatometry and X-ray diffraction (XRD) analysis. The two-step austenite reverted transformation associated with intercritical partition of C, Mn and Ni is believed to be the underlying basis for stabilization of retained austenite during the two-step intercritical heat treatment. Stable retained austenite is not only beneficial for high ductility, but also for low temperature toughness by restricting brittle fracture. With 10% (volume fraction) of retained austenite in the steel, high low temperature toughness with average Charpy impact energy of 65 J at −80 °C was obtained.     

A medium-Mn steel processed by novel twin-roll strip casting route

A medium-Mn steel (Fe–0.3C–4Mn–1.82Al–0.6Si wt-%) was produced by a novel processing route involving twin-roll strip casting, hot rolling and intercritical annealing (IA). Macrosegregation was absent in the as-cast strip. The microstructure of the as-cast strip consisted of martensite and austenite (～10 vol.-%), and the solidification structure was characterised by dendritic structure. With an increase in IA temperature from 680 to 725 and to 755°C, austenite fraction in intercritically annealed steels was increased from 22 to 45% and then decreased to 27%. The 710°C intercritically annealed steel yielded excellent mechanical properties with a tensile strength of ～1007?MPa and total elongation of ～48%, achieved by a high volume fraction of austenite (～42%) with appropriate mechanical stability.     

Deformation-induced martensitic transformation kinetics and correlative micromechanical behavior of medium-Mn transformation-induced plasticity steel

An in situ high-energy X-ray diffraction (HE-XRD) technique was mainly used to investigate the micromechanical behavior of medium-Mn Fe-0.12C-10.16Mn-1.87Al (in wt%) transformation-induced plasticity (TRIP) steel subjected to intercritical annealing at 625 °C, 650 °C, 675 °C and 700 °C for 1 h. As the intercritical annealing temperature increased, the volume fraction of retained austenite (RA) and ultimate tensile stress (UTS) increased, while the Lüders strain and yield stress (YS) decreased. The incremental work-hardening exponent of experimental steel increased with increasing intercritical annealing temperature. The overall trend of the transformation kinetics of the RA with respect to the true strain followed the sigmoidal shape predicted by the Olson and Cohen (OC) model. Load partitioning occurred among the ferrite, austenite and martensite immediately after entering the yielding stage. Because the stability of the RA decreased with increasing intercritical annealing temperature, the load undertaken by the martensite increased. The moderate transformation kinetics of the RA and effective load partitioning among constituent phases were found to contribute to a favorable combination of strength and ductility for this medium-Mn TRIP steel.     

Design of an effective heat treatment involving intercritical hardening for high-strength–high elongation of 0.2C–1.5Al–(6–8.5)Mn-Fe TRIP steels: microstructural evolution and deformation behaviour

ABSTRACT

We propose an effective heat treatment involving a combination of intercritical hardening and tempering to obtain high strength–high ductility in hot-rolled 0.2C–1.5Al–(6–8.5)Mn–Fe transformation-induced plasticity (TRIP) steels. An excellent combination of high ultimate tensile strength of 1045–1380?MPa and total elongation of 34–39% was obtained when the steels were subjected to intercritical hardening at 630–650?°C and tempered at 200?°C. Intercritical hardening impacted the co-existence of austenite, ferrite and martensite, such that the deformation behaviour varied with the Mn content. The excellent properties of the steels were attributed to cumulative contribution of enhanced TRIP effect of austenite and ferrite and martensite constituents. The discontinuous TRIP e?ect during tensile deformation involves stress relaxation and led to consequent enhancement of ductility.     

Microstructure and tensile properties of a 0.20C–4.86Mn steel after short intercritical-annealing times

This work studies the microstructure and tensile properties of a cold-rolled Fe–0.20C–4.86Mn (mass %) steel after short intercritical annealing (IA) times using scanning and transmission electron microscopy, and uniaxial tensile tests. The short IA time is applied to represent the process characteristics of the industrial continuous annealing line. The experimental results show that IA temperature has a strong influence on the final microstructure and tensile properties while IA time has less. The fractions of retained austenite are much higher after IA at 650 and 675°C than the other IA temperatures, and thus improving elongation. Simulations using the DICTRA software and constitutive modelling are further performed to assist the understanding of the microstructure evolution and stress–strain curves.     

A high strength and ductility Mg–Zn–Al–Cu–Mn magnesium alloy

A high strength Mg–8.0Zn–1.0Al–0.5Cu–0.5Mn (wt.%) magnesium alloy with outstanding ductility was developed using a common casting technique and heat treatment. The microstructure of the as-cast alloy is composed of α-Mg, MgZn, MgZnCu and Al–Mn phases. After the solution treatment and subsequent two-step aging treatment, the yield strength (YS), ultimate tensile strength (UTS) and elongation of the alloy at peak hardness reach 228 MPa, 328 MPa and 16.0% at room temperature, respectively. The comprehensive mechanical properties of the alloy are superior to almost all other high performance casting Mg alloys.     

Effect of intercritical annealing on the Lüders strains of medium Mn transformation-induced plasticity steels

The present work systematically investigates the effects of starting microstructure and intercritical annealing temperature on the tensile properties and Lüders strain of transformation-induced plasticity steels containing 5 wt.% Mn. It is found that higher intercritical annealing temperature leads to smaller Lüders strain, lower yield strength and higher ultimate tensile strength. The starting cold-rolled microstructure produces much coarser microstructural constituents and larger Lüders strain than the martensitic one. It is concluded that the fraction and size of austenite grains and the amount of carbide formed during intercritical annealing are the most important microstructural factors to determine the Lüders strain rather than the fraction of retained austenite and the grain size of the ferritic phase.     

前驱体对含Cu低碳钢I&Q&P处理后组织性能的影响

采用IQP工艺和EPMA、SEM和XRD等手段,研究了3种前驱体对含Cu低碳钢残余奥氏体含量及力学性能的影响。结果表明,双相区保温初期试验钢奥氏体长大由C配分控制,后期由合金元素Mn、Cu配分控制;双相区保温奥氏体化后,双相区配分后形成弥散分布的局部高浓度Mn、Cu区域仍保留富集效果,在随后的淬火-碳配分阶段易于形成残余奥氏体。经IQP处理后,前驱体为P+F的钢室温组织中马氏体板条较粗,原始奥氏体晶界并不明显;前驱体为F+M钢得到的马氏体板条有序细密;前驱体为M的钢室温组织中马氏体板条更加细密。其中,前驱体组织为M的钢中残余奥氏体量最高,延伸率为24.1%,强塑积可达25 338 MPa·%,综合性能最好。     

Current opinion in medium manganese steel

Abstract

Medium Mn steels have been actively investigated due to their excellent balance between material cost and mechanical properties. The steels possess a single α′ martensite phase in hot and cold rolled states and multiphases after intercritical annealing. Many studies have focused on investigating the influences of chemical composition and annealing conditions on the microstructure, particularly the grain size and retained γ (γ_R), and on the tensile properties. The steels exhibit high strength and good ductility due to transformation induced plasticity occurring in γ_R, whose volume fraction is approximately 0·2–0·4. The present review summarises the important results of previous studies about the effects of both intercritical annealing conditions and alloying elements on the microstructure and tensile properties of medium Mn steels.     

Phase transformation and mechanical behaviour of thermo-mechanically controlled processed high-strength multiphase steel

A novel low-alloy high-strength steel [Fe–0.20C–1.65Mn–1.40Si–1.50Al–1.30Cu–1.05Ni–1.07Co (wt%)] has been thermo-mechanically processed with a finish rolling temperature of 850 °C, followed by air cooling and water quenching in order to obtain a good combination of strength and ductility. Phase transformations of the above steel at different cooling rates have been studied and continuous cooling transformation (CCT) diagram has been constructed using data, obtained from dilatometric study. The phase field of CCT diagram indicates microstructure changes from a mixture of ferrite and bainite to fully martensite accompanied with the enhancement of hardness with increasing cooling rate. The microstructural investigation at lower cooling rate (≤5 °C/s) suggests the possibility of achieving pearlite-free microstructure by direct air cooling from the austenite region. Directly air-cooled steel has demonstrated primarily ferrite–bainite microstructure, which shows attractive tensile strength (>1050 MPa) and ductility (>15 %). On the other hand, directly water-quenched steels reveal predominantly lath martensitic microstructure with high dislocation density which exhibits higher tensile strength (>1600 MPa) and lower ductility (~12 %). The multiple stages of strain hardening behaviour of the investigated steel under different cooling conditions have been examined with respect to microstructural evolution.     

Quenching and partitioning steel produced through hot rolling,direct quenching and annealing

A novel type of quenching and partitioning steel was developed using direct quenching after hot finishing rolling, followed by intercritical annealing, quenching and partitioning (DQ–Q&P) process. The desirable combination of strength and ductility was obtained. The effect of various intercritical annealing temperatures on the microstructures and mechanical proprieties was studied. With the decreasing intercritical annealing temperature, the amount of acicular retained austenite increased, which exhibited a good work-hardening behaviour resulting in enhanced tensile strength and total elongation. After annealing at 740°C, superior mechanical properties, which were the ultimate tensile strength of 1015?MPa and total elongation of 32.22%, were achieved.     

Relative effect of C and Mn on strength-toughness of medium Mn steels

In order to find an alternative choice for structural load-bearing components, an effort was made to improve the impact toughness of medium Mn steel through inter-critical annealing treatment without significant reduction in strength. Besides, the relative effect of C and Mn contents on microstructure and mechanical properties of medium Mn steels was also studied. Fibrous microstructures with fine, alternate films of ferrite and martensite with retained austenite were obtained. The low-C, high-Mn steel showed a superior combination of yield strength, ductility and impact toughness due to finer microstructure and higher retained austenite fraction, as compared to high-C, low-Mn steel. Thus, the beneficial effect of Mn enrichment in stabilising retained austenite and improving mechanical properties by transformation induced plasticity effect becomes evident.     

Effect of inter-critical annealing parameters on ferrite recrystallization and austenite formation in DP 590 steel

This research investigates the effect of inter-critical annealing parameters on ferrite recrystallization and austenite formation during processing of a dual phase microstructure from a cold rolled low carbon steel. The main effort was to determine optimum annealing parameters for producing a desired ferrite-martensite dual phase microstructure in the steel for improved strength–ductility combination. A 57% cold rolled steel sheet was subjected to inter-critical annealing under different temperature–time conditions. Annealing temperatures were determined using Thermo-Calc. After annealing experiments, the resulting microstructures and corresponding hardness values were evaluated to determine ferrite recrystallization and austenite fraction under different conditions. The activation energy for ferrite recrystallization was 235.6?kJ/mol using standard Johnson–Mehl–Avrami–Kolmogorov analysis. Experiments showed that inter-critical annealing parameters affect the phenomenon of ferrite recrystallization and austenite formation. It was observed that both the rate of ferrite recrystallization and austenite formation increase with an increase in annealing temperature. Finally, steel was annealed under conditions similar to industrial processing in an annealing simulator with the selected annealing parameters to obtain improved strength–percentage elongation combinations. The steel under these conditions showed significant improvements in strength–ductility combination (610?MPa–26%; 680?MPa–15%) with an ideal yield strength to an ultimate tensile strength ratio of 0.5.     

Effect of multi-directional forging and ageing on fracture toughness of Al–Zn–Mg–Cu alloys

Strength, ductility and fracture toughness are the most important mechanical properties of engineering materials. In this work, an Al–Zn–Mg–Cu alloy was subjected to multi-directional forging (MF) and ageing treatment. Microstructural evolution was studied by optical and electron microscopy and strength, ductility and fracture toughness were researched. After MF, the dislocation density was increased and the microstructure was refined. The strength and fracture toughness were increased, while the ductility was decreased sharply. Without compromising the strength, the ductility was improved significantly after ageing. The fracture toughness was increased further. The coarse and discontinuously distributed grain boundary precipitates were found to be responsible for higher fracture toughness of the fine-grained structure Al–Zn–Mg–Cu alloy.     

Microstructures and Mechanical Properties of Si-Al-Mn TRIP Steel with Niobium

Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP (transformation induced plasticity) steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10-2 to 10-4 s-1, the volume fraction of transformed martensite from retained austenite,as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.     

Microstructures and Mechanical Properties of Si-AI-Mn TRIP Steel with Niobium

Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP （transformation induced plasticity） steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10^-2 to 10^-4 s^-1, the volume fraction of transformed martensite from retained austenite, as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.     

Retained austenite stabilisation in low carbon high silicon steel during isothermal holding

We elucidate here the role of isothermal hold temperature of 300–500°C after intercritical annealing at 760°C on bainitic transformation and in governing the stabilisation of retained austenite in a 0.23C-1.35Si-1.82Mn steel. A critical analysis was attempted to explain the observations using displacive mechanism of bainite formation in the attempt to endeavour to understand the kinetics of bainitic transformation during isothermal holding. The model predicted that carbon enrichment in austenite was of particular significance in governing the stability of retained austenite. Thus, through the contribution of transformation induced plasticity effect of retained austenite, high tensile strength (964?MPa) and excellent ductility (uniform elongation of 24.5% and total elongation of 32%) was obtained on isothermal holding at 400°C.     

Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min^?1), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382     

Copper precipitation and its impact on mechanical properties in a low carbon microalloyed steel processed by a three-step heat treatment

The structure–mechanical property relationship, with particular focus on effect of tempering process on the microstructural evolution and mechanical properties was investigated in a low carbon Cu-bearing steel that was processed in three-steps, namely, intercritical annealing, intercritical tempering, and tempering heat treatment. The objective of adopting three steps was to elucidate the nature and evolution of microstructural constituents that contributed to high strength–ductility combination in the studied steel. The three-step processing led to a microstructure primarily comprising of ferrite, retained austenite, and small amount of bainite/martensite. The mechanical properties obtained were: yield strength > 720 MPa, tensile strength > 920 MPa, uniform elongation > 20%, total elongation > 30%, and low yield ratio of 0.78. The tempering step led to a significant increase in both yield and tensile strength and decrease in yield ratio, without reducing ductility, a behavior attributed to the precipitation of copper in retained austenite and ferrite. The precipitation of copper enhanced the stability of retained austenite and work hardening rate, leading to a high volume fraction of retained austenite (∼29%), with consequent increase in elongation and significant increase in yield and tensile strength during tempering.     

In Situ Observations of the Microstructural Evolution during Heat Treatment of a PH 13-8 Mo Maraging Steel

The standard heat treatment of PH 13-8 Mo maraging steels consists of solution annealing and subsequent aging. Herein, it is investigated how an additional intercritical annealing step prior to aging affects the microstructure, and, consequently, the mechanical properties of a PH 13-8 Mo maraging steel. In situ techniques by means of high-temperature electron backscatter diffraction and high-temperature X-ray diffraction are applied to study the microstructural changes during intercritical annealing and subsequent aging. In addition, high-resolution investigation methods, such as transmission electron microscopy and atom probe tomography supplemented by transmission Kikuchi diffraction, are used for an in-depth characterization of the microstructure. The results reveal that a diffusion-controlled martensite to austenite transformation accompanied by partitioning of the substitutional atoms Cr, Ni, and Mo takes place during intercritical annealing. As a result of partitioning during intercritical annealing, an inhomogeneous distribution of Ni remains in the microstructure after the martensitic transformation. Consequently, the formation of reverted austenite is facilitated during subsequent aging due to existing Ni-enriched zones in martensite. Since the fracture toughness is significantly enhanced compared to the standard heat treatment, it is suggested that this improvement is related to the increased phase fraction of reverted austenite.