Intercritical Annealing Temperature Research Articles

A review of recent progress in mechanical and corrosion properties of dual phase steels

New concepts proposed for processing of dual phase (DP) steels as one of the main classes of advanced high-strength steels (AHSSs) to enhance their mechanical properties (strength–ductility combination) and corrosion resistance were introduced. The current review covers (I) the processing routes to obtain the ferritic–martensitic microstructures, (II) parameters of intercritical annealing (IA) treatment, (III) primary thermomechanical treatments, and (IV) post processing. First, the principal heat treatment methods, i.e., step quenching, intermediate quenching, and intercritical annealing of ferritic–pearlitic steel, as well as the partitioning of manganese were critically discussed. Then, the effects of holding time at the intercritical annealing temperature on the austenitization, grain coarsening kinetics, abnormal grain growth, and volume fraction of martensite were summarized. Next, the importance of cold deformation (notably rolling) and heating rate for the development of fine-grained DP microstructures (with chain-networked martensitic islands) through recrystallization and modification of the preferred nucleation sites for the austenite phase was discussed. Moreover, the applications of severe plastic deformation techniques (such as constrained groove pressing), thermal cycling (multi-step or repetitive intercritical annealing), and spheroidization heat treatment were discussed. Finally, the impacts of tempering, quench aging, and bake hardening on the properties of DP steel were reviewed. This short overview shows the opportunities that the conventional and innovative processing routes can offer for the potential industrial applications of DP steels, especially in the lightweight car body for the automotive industry to address the safety, fuel consumption, and air pollution issues.

High elongation of medium-manganese steel containing 1.0 wt.% Al after a short intercritical annealing time

ABSTRACT This paper presents research on the phase transformation, microstructure, and tensile properties of medium-manganese steel containing 1.0 wt.% Al (1.0Al) after a short intercritical annealing (IA) time at different IA temperatures using thermodynamic simulation, scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and uniaxial tensile tests. The results show that the addition of 1.0Al can expand the size of the IA temperature range from 208 to 272 °C and shifts the range to higher temperatures, and this increases the fractions of the retained austenite in the steel when the IA time is short. Most of the retained austenite transforms to α’-martensite during the tensile test. Part of the retained austenite transforms to α’-martensite in the early stage and contributes little plasticity. The other part of the retained austenite transforms in the middle or late tensile deformation stage and improves the mechanical properties. The mechanical properties of the steel are remarkable, with a tensile strength above 1000 MPa, elongation above 35%, and strength × ductility above 35,000 MPa% when the IA temperature is 700 °C and the IA time is 3 min.

Microstructural Characteristics and Tensile Behavior of a Hot-Rolled Medium-Mn Steel (0.25C-8.5Mn-0.5Si-2.5Al) Processed by Intercritical Annealing Treatment

Microstructural characteristics and tensile behavior of a novel hot-rolled medium-Mn steel, 0.25C-8.5Mn-0.5Si-2.5Al (wt.%), under different intercritical annealing temperatures (TIA) were evaluated. The results show that an ultrafine duplex lamellar microstructure with a maximum volume fraction of retained austenite (RA) of 65 vol.% was obtained at TIA of 750 °C. It was found that both the stability of the RA and its stability range play critical role in controlling the sustainable TRIP effect. The product of ultimate tensile strength and total elongation as high as ~ 66 GPa% was obtained at TIA of 725 °C, which is much higher than those of previously investigated medium-Mn steels with similar Mn contents, indicating a great potential for the tested steel. The analysis of the fracture surface revealed that the steel exhibited a kind of micro-delamination fracture, which was found to be primarily nucleated at the interfaces of the transformation-induced martensite and ferrite, and the micro-delamination fracture became more significant with increasing TIA.

New Approach for Multiplying the Strength and the Formability of Cold Rolled BCC Based Structure Steel through Ultra Fine Structure Technique

Reduction in grain size of bcc based structure steel is still highly concerned in the cold rolled sheet to attain superior mechanical properties. As long as, the reduction of weight is much considered in the structure purposes, the strength/weight ratio of steel is highly demanded. In this study, an innovative technique was applied to attain ferrite grain size with hundreds of nanometer, in tandem with preserving the mechanical properties. In this approach, the micro-alloyed low carbon steel resulted from the thermomechanical process was followed by subcritical annealing regime prior to the first critical transformation temperature. To identify the effect of a micro-alloying element as vanadium, and the effect of subcritical annealing regime on the low carbon steel, two low carbon steel was subjected to studying in this research. The results refer that applying a subcritical annealing regime for the micro-alloyed low carbon steel after hot compression at intercritical annealing temperature can lead for attaining hundreds of nanometer ferrite grain size, which has a powerful effect on promoting the strength of the steel to exceed 1200 Mpa, in one hand with preserving the formability up to 20% as uniform elongation. Unexpectedly, the fine grain size obtained after the innovative technique promotes the impact toughness at room temperature, which is attributed to the fineness and the spheroid morphology of the secondary phase in conjugation with bcc ferrite structure.

Effect of continuous annealing process on various structure parameters of martensite of dual-phase steels

Industrial continuous annealing process routes for dual-phase steels are mostly found to be non-isothermal in nature. The present study is an effort to understand the importance of non-isothermal annealing process parameters and their impact on the various metallurgical phenomena; such as recrystallisation and phase transformation behaviours of duel-phase steel. These, in turn, are expected to influence various structure parameters of martensite phase which are critical in determining the strength of duel-phase steel. A dual-phase steel sheet in 67% cold rolled full hard condition was subjected to non-isothermal annealing treatment with varying heating rate and inter-critical annealing temperatures. After processing the samples were investigated for structural parameters of martensite phase using a scanning electron microscope, X-ray diffraction, and nano-indentation technique. It was observed that due to non-isothermal nature of continuous annealing process, the hardness of the martensite phase did not follow the hardness trends as determined from conventional carbon concentration. Further, lattice tetragonality of martensite was also affected by increasing its volume fractions. The annealing process apparently influenced the evolution of texture because of the increasing fraction of martensite in dual-phase steel.

Weldability of Dual Phase Steel Containing Boron

Carbon steel containing boron-dual phase was produced by casting and were heated to 1200 °C for hot forging to produce plates of 10 and 6 mm thick. The plates produced were heated to the inter-critical annealing temperature for 15 min. and then water quenched followed by tempering process. The microstructure of the produced steel is ferrite with islands of martensite. Welding was done to the heat treated steel plates (6mm) using SMAW process and applying AWS-E11016 electrode and as a result an over-tempered region in the heat affected zone was formed with a significant reduction of the ultimate tensile strength of the welded joint. Reduction of heat input resulted in an increase in the joint strength. Welding of the hot rolled plates with a subsequent heat treatment resulted in formation of homogenous joint with good mechanical and metallurgical properties. Application of Electron Beam Welding to this dual phase steel resulted in a welded joint with good mechanical properties comparable to that of the base metal. The results were discussed based on microstructure analysis and hardness distribution of the welded joints.

Mechanical property development of a 0.15C–6Mn–2Al–1Si third-generation advanced high strength steel using continuous galvanizing heat treatments

The effects of starting microstructure and intercritical annealing (IA) temperature on the mechanical properties of a prototype 0.15C–6Mn-1.5Al–1Si (wt%) third-generation medium manganese advanced high strength steel (3G AHSS) were determined. The general 3G AHSS mechanical property targets UTS × TE of 24,000–40,000 MPa% were met after a continuous galvanizing line (CGL)-compatible IA treatments at 710 °C and 740 °C for 120 s from an 80% martensite – 20% ferrite starting microstructure due to the as-annealed microstructure having large volume fractions of stable retained austenite. The deformation-induced retained austenite to martensite transformation – or TRansformation-induced plasticity (TRIP) – kinetics were a strong function of IA temperature and resulting retained austenite stability. In particular, the 665 °C intercritical anneal produced retained austenite which only partially transformed during deformation and the 740 °C intercritical anneal produced retained austenite which exhausted the TRIP effect at significantly lower strains than those observed for the 710 °C treatment. It was also shown that significant deformation-induced mechanical twinning occurred in the retained austenite, aiding in sustaining the work hardening rate. Overall, it was determined that medium-Mn 3G AHSS can be produced using CGL-compatible thermal cycles, which holds promise for cost-effective production of these alloys.

Elemental partitioning in medium Mn steel during short-time annealing: An in-situ study using synchrotron x-rays

In this study, high energy synchrotron radiation was used to perform an in-situ diffraction experiment in medium Mn Fe-6Mn-0.5C-1Al alloy to study the elemental partitioning and consequent austenite phase evolution at different stages, namely, during heating, holding and cooling. It has been observed that the austenite phase fraction significantly increases on annealing at the inter-critical annealing temperature and remains stable at room temperature. The austenite phase stability at room temperature is due to the rapid partitioning of manganese (Mn) and carbon (C) during annealing. The relative change in d-spacing (Δd/d) during heating-cooling confirms the partitioning of alloying elements during inter-critical annealing.

Transformations During Intercritical Annealing and Their Implications for Microstructure and Mechanical Properties of Medium Mn Transformation-Induced Plasticity Steel in Continuous Annealing Line

Phase transformation during intercritical annealing (IA) and the relationship between microstructure and mechanical properties of the intercritical annealed 0.2C-5MnTRIP steels were investigated using a combined method of dilatometry, scanning electron microscope, transmission electron microscope, x-ray diffraction, and tensile testing. With the increase in IA temperature and holding time, the fraction of reversed austenite increases while the chemical stability of reversed austenite decreases, leading to the transformation of reversed austenite to martensite in the cooling stage. IA temperature is found to have a greater influence on the stability of austenite than IA time. The microstructures mainly comprised of ferrite and retained austenite, with granular carbide and martensite also observed in some specimens. The carbide precipitates formed during hot rolling remain after cold rolling and annealing process and gradually dissolve at higher IA temperature or with longer IA time. The optimal IA parameter, i.e., 650 °C for 15 min, contributes to a good product of tensile strength and elongation (997 MPa × 29.25%) due to an excellent TRIP effect. The present work gives a clear insight in controlling the mechanical properties of medium Mn TRIP steel in the continuous annealing line by a reasonable design of the IA process.

Regulating tensile properties through bainitic transformation temperature in a hot-rolled δ-TRIP steel

This study aims to clarify effects of isothermal bainitic transformation (IBT) temperature on microstructure and mechanical properties of a high Al–low Si δ-TRIP steel. For this purpose, the alloy was austenitised at the intercritical annealing temperature of 820°C for 600 s and then was isothermally cooled down into a salt bath and held at the temperatures between 350 and 450°C for 360 s. X-ray diffraction indicates that by increasing the IBT temperature, the volume fraction of retained austenite increases. In addition, it was found that the optimum combination of strength and ductility, i.e. tensile strength of 1046 MPa and total elongation of 54%, was achieved in the sample which was isothermally held at 450°C for 360 s.

New Insights into the Microstructural Changes During the Processing of Dual-Phase Steels from Multiresolution Spherical Indentation Stress–Strain Protocols

In this study, recently established multiresolution spherical indentation stress–strain protocols have been employed to derive new insights into the microstructural changes that occur during the processing of dual-phase (DP) steels. This is accomplished by utilizing indenter tips of different radii such that the mechanical responses can be evaluated both at the macroscale (reflecting the bulk properties of the sample) and at the microscale (reflecting the properties of the constituent phases). More specifically, nine different thermo-mechanical processing conditions involving different combinations of intercritical annealing temperatures and bake hardening after different amounts of cold work were studied. In addition to demonstrating the tremendous benefits of the indentation protocols for evaluating the variations within each sample and between the samples at different material length scales in a high throughput manner, the measurements provided several new insights into the microstructural changes occurring in the alloys during their processing. In particular, the indentation measurements indicated that the strength of the martensite phase reduces by about 37% when quenched from 810 °C compared to being quenched from 750 °C, while the strength of the ferrite phase remains about the same. In addition, during the 10% thickness reduction and bake hardening steps, the strength of the martensite phase shows a small decrease due to tempering, while the strength of the ferrite increases by about 50% by static aging.

Hydrogen embrittlement behavior of high strength low carbon medium manganese steel under different heat treatments

Hydrogen embrittlement (HE) behavior was investigated in a low carbon medium Mn steel with three different volume fraction of retained austenite (RA), which was obtained after different heat treatments. The hydrogen permeation test showed a higher permeability for directly water quenched specimen compared to quench-tempered specimens. Melt extraction test showed hydrogen concentration increased with hydrogen charging current density in the order of directly quenched specimen, QLA, quenched with low-temperature annealed specimens and QHA quenched with high-temperature annealed specimens. Slow strain-rate tensile test was employed to examine the HE behavior, the HE indices decreased with the increase of RA irrespective of increased hydrogen concentration. HE susceptibility can be suppressed by raising intercritical annealing temperature because Mn enrichment increases the stability of RA.

Phase transformation mechanism and kinetics during step quenching of st37 low carbon steel

Step quenching of st37 low carbon steel was studied for understanding the phase transformation kinetics. During step quenching, the amount of martensite (formerly austenite) decreased by increasing the holding time at the intercritical annealing temperature. Based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis using the hardness data, Avrami exponents were determined in the range of 0.3 to 0.8, which indicate that the austenite to ferrite transformation in the intercritical temperature region is diffusion controlled. The activation energy of 84.2 kJ mol−1 was obtained, which is close to the activation energy for the diffusion of carbon in α-iron (87.4 kJ mol−1). Consequently, the diffusion of carbon in α-iron was found to be responsible for the microstructure formation via transformation of austenite to ferrite during step quenching. In fact, the grain-boundary ferrite phase grows into the surrounding austenite phase during step quenching while the austenite phase forms in place of pearlite and also nucleates on the ferrite grain boundaries during intercritical annealing of ferrite-pearlite microstructure. Based on these findings, the slower kinetics of the microstructure formation during step quenching (as compared to the conventional intercritical annealing) was related to the atomic diffusion mechanisms, where the diffusion of carbon in α-iron was calculated to be slower than that in γ-iron at 850 °C.

Effect of the heat treatment on the microstructure and mechanical properties of medium-Mn-steels

ABSTRACTThe heat-treatment (HT) schedule and selected annealing parameters have a substantial effect on the microstructure and mechanical properties of medium-Mn-steels. The structure morphology depends on the fact, whether the austenite-reverted transformation takes place from deformed (one-step HT) or non-deformed (two-step HT) microstructures. Depending on the intercritical annealing temperature, the stability of the retained austenite can be altered to a large extent. As a result, the mechanical properties can be adjusted from high strength with excellent ductility to very high strength with reasonable ductility. The present contribution, therefore, elucidates the dependence of the microstructural characteristics and material behaviour on the HT parameters for medium-Mn alloy compositions with different Mn-contents.This paper is part of a Thematic Issue on Medium Manganese Steels.

On the strain hardening abilities of α+β titanium alloys: The roles of strain partitioning and interface length density

In this study, the strain hardening ability of an α+β titanium alloy (Ti–6Al–4V) was systematically investigated by comparing the tensile properties of different microstructures at room temperature, including the lamellar microstructure, bi-lamellar microstructure, equiaxed microstructure and bimodal microstructure. The lamellar and equiaxed microstructures, mostly comprised of α phase (either α lamellae or equiaxed α grains) and a few retained β phase, were characterized by limited strain hardening abilities, regardless of the colony size and grain size. On the other hand, the bi-lamellar and bimodal microstructures obtained by intercritical annealing of the lamellar and equiaxed microstructures in α+β two-phase region followed by water quenching, generally possessed higher strain hardening abilities than the lamellar and equiaxed microstructured counterparts. In addition, the evolution tendencies of uniform elongation (as determined by the strain hardening rate and true stress) with intercritical annealing temperature were different between the bi-lamellar and bimodal microstructures. In the bi-lamellar microstructure, the uniform elongation continuously decreased with the increase of the intercritical annealing temperature while in the bimodal microstructure, a peak uniform elongation was obtained at an intermediate annealing temperature (910 °C). Both microstructures can be regarded as ‘dual-phased’ microstructures, as composed of primary α lamellae/grains and transformed β regions (an aggregate of secondary α lamellae and retained β). Due to the different nano-hardness between primary α lamellae/grains and transformed β regions, a plastic strain partitioning between the two components was found in both microstructures. The strain gradient at the interface between the two components introduced geometrically necessary dislocations, which contributed to the strain hardening ability. It is therefore believed that the interface length density was a critical microstructural parameter in determining the strain hardening abilities of bi-lamellar and bimodal microstructures, which explained the different intercritical annealing temperature dependences of uniform elongation in both microstructures.

Microstructural evolution during continuous annealing of a 980 MPa cold rolled steel grade

The development of advanced high strength steels (AHSS) is motivated by automotive industry demands for higher strength steels with better formability and specific stretch-flange formability to avoid fractures during parts forming. In response to this, the new steel grades must have special microstructures in terms of constitution, morphology, distribution and hardness. Thus, a better understanding of formation mechanisms and the characterization of these microstructures has shown to be of fundamental importance for the development of new steels. In this study the influence of heating rate and intercritical temperature on microstructural evolution and hole expansion ratio of a cold rolled 980 MPa AHSS grade was evaluated, using a Dilatometer and a Gleeble machine. The stretch-flange formability was evaluated by hole expansion tests and the microstructures were characterized using scanning electron microscopy, electron backscatter diffraction (EBSD) and nanohardness measurements. The increase of heating rate resulted in an overlap between ferrite recrystallization and austenite formation, changing the morphology of austenite from random to fibrous distribution. On the other hand, higher intercritical annealing temperature resulted in more homogeneous microstructures and higher hole expansion ratios. This microestructural homogeneity, evaluated by nanohardness, suggested that the difference in hardness between ferrite and second-phase affects micro-void formation and crack propagation during hole expansion test.

Correlation of ferrite and martensite micromechanical behavior with mechanical properties of ultrafine grained dual phase steels

Thermomechanical treatment was employed to produce ultrafine grained dual-phase (UFG DP) steels consisting of different volume fractions of martensite and different ferrite grain sizes. The effects of intercritical annealing temperature on the microstructure and deformation behavior of DP steels were investigated. The modified Crussard-Jaoul analysis illustrated four-stage deformation of the investigated DP steels. The ferrite nanoindentation response in the microstructures showed that increasing the intercritical annealing temperature from 780 to 820 °C, resulted in an increase in the ferrite nanohardness by about 46% due to a decrease in the average ferrite grain size by about 34%. The average martensite nanohardness decreased by about 18% with decreasing its carbon content by about 18%. The mechanical properties of UFG DP steels, such as strength, elongation, energy absorption, and strain hardening characteristics were well correlated with the ferrite and martensite micromechanical behavior.

Effect of deep cryogenic pretreatment on microstructure and mechanical properties of warm-deformed 7 Mn steel after intercritical annealing

Microstructure and mechanical properties of 7Mn steel undertaken different intercritical annealing (IA) with or without deep-cryogenic treatment (DCT) were investigated in this work. It is found that the additional DCT process can lead to the changes in the size and composition of austenite of the steel after IA. With the decrease of the IA temperature, the improvement effect of the DCT on mechanical properties is more remarkable. The product of ultimate tensile strength (UTS) and total elongation (TE) of the pre-DCT specimen IA at 600°C (DCT + IA600) is nearly twice as much as that of the non-DCT specimen. Austenite and ferrite grains exhibit different morphologies, lamellar and equiaxed, and bimodal size distribution at different IA temperatures. With the decrease of IA temperature, the occurring of serration behavior on engineering stress-strain curves is postponed, and the density of serration as well as the mean value of work hardening (WH) rate decrease, both of which are caused by the increasing stability of austenite. The volume fraction, morphology and composition of austenite can be tailored by pre-DCT and IA process to achieve the optimum combination of the retained austenite (RA) volume fraction and stability. The DCT + IA645 specimen exhibits an outstanding combination of 1167 MPa UTS and 35.4% TE.

Intercritical annealing temperature dependence of hydrogen embrittlement behavior of cold-rolled Al-containing medium-Mn steel

The present investigation attempts to evaluate the influence of intercritical annealing temperature (TIA) on the hydrogen embrittlement (HE) of a cold-rolled Al-containing medium-Mn steel (Fe-0.2C-4.88Mn-3.11Al-0.62Si) by using electrochemical hydrogen-charging, slow strain rate tensile test and scanning electron microscope. The results show that an excellent combination of strength and ductility (the product of ultimate tensile strength and total elongation) up to ∼53 GPa·% was obtained for the specimen intercritically annealed at an intermediate temperature of 730 °C, whereas the HE index increases significantly with an increase in TIA up to 850 °C. Being different from the typical dimple ductile fracture for the uncharged specimen, the hydrogen-charged specimen exhibits a mixed brittle interface decohesion and ductile intragranular fracture mode in the crack initiation region and the brittle fracture fraction increases with increasing TIA. Both the stability and amount of austenite play a critical role in governing the HE behavior of TRIP-assisted medium-Mn steel. Thus, it is suggested that suitable TIA should be explored to guarantee the safety service of automotive parts made of this type of steel in addition to acquiring excellent mechanical properties.

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

ABSTRACTA 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.