Intercritical Annealing Temperature Research Articles

Process Maps for Predicting Austenite Fraction (vol.%) in Medium-Mn Third-Generation Advanced High-Strength Steels.

Process maps were developed using a combination of microstructural analysis and DICTRA-based modeling to predict the austenite vol.% as a function of the intercritical annealing parameters and starting microstructure. The maps revealed a strong dependence of the calculated austenite fraction (vol.%) on the Mn content (4-12 wt.%) and intercritical annealing temperatures (600 °C to 740 °C). The calculations were carried out for constant carbon, Al, and Si contents of 0.2 wt.%, 1.5 wt.%, and 1.0 wt.%, respectively. A modified empirical equation proposed by Koistinen and Marburger was employed to calculate the room-temperature retained austenite vol.% as a function of the intercritical annealing temperature, including the effect of the austenite composition. The process maps offer valuable insights for designing intercritical treatments of medium-Mn steels, aiding in the optimization of steel properties for automotive applications.

Effect of intercritical annealing temperature on microstructure and mechanical properties of V-Ti-Mo microalloyed medium Mn steel

The effect of intercritical annealing temperature on the microstructure and mechanical properties of V-Ti-Mo microalloyed medium Mn steel were investigated using a scanning electron microscope, electron backscatter diffraction, X-ray diffraction and transmission electron microscope. The results showed that the microstructure of the experimental steel in the hot rolled state and after annealing at different temperatures consisted of martensite and reverted austenite. As the annealing temperature increased, the content of reverted austenite increased linearly and reached a maximum value of 29.08 % after annealing at 650 °C. It was difficult to occur austenitic reverted transformation at an annealing temperature of 580 °C. In addition, block-like reverted austenite appeared after annealing at 600 °C, and the austenite size had the largest after annealing at 650 °C. With the increase of annealing temperature, the mass fraction of M3C particles decreased gradually, while that of MC particles increased progressively. Specifically, the mass fraction of 1–5 nm sized MC particles increased first and then decreased, indicating the precipitation strengthening effect was enhanced initially and then weakened. As the annealing temperature rose, the yield strength and tensile strength of the steels decreased gradually, whereas the elongation and impact energy gradually increased. The steel annealed at 650 °C exhibited optimal comprehensive mechanical properties, with a strength-ductility product reaching 26.8 GPa· %.

Effect of Austenite Reversion and Its Stability on the Tensile and Impact Transition Behavior of High‐Strength Naval Steel

The present investigation correlates the austenite reversion and its stability during intercritical annealing (IA) with the tensile and Charpy impact properties of a low‐C naval grade steel. The as‐received steel is subjected to water quenching after austenitization at 950 °C, followed by IA at 620, 650, and 680 °C for 2 h. DICTRA simulation is used to predict the austenite reversion kinetics during the IA. Incorporation of cementite phase in the simulation correlates well with the experimental dilatometry results on the austenite growth kinetics. DICTRA simulation predicts a continuous rise in the reverted austenite fraction with increasing IA temperature. However, the percentage of reverted austenite, that gets retained after the subsequent quenching, decreases with increasing annealing temperature due to the reduction of austenite stability at elevated IA temperature. In other words, the amount of fresh untempered martensite increases rapidly with increasing IA temperature. Consequently, the sample annealed at an intermediate temperature of 650 °C (IA650) shows the best combination of strength, elongation, and low‐temperature impact properties as compared to the IA620 and IA680 steels due to an optimum balance of mechanically stable austenite and relatively lower untempered martensite fractions.

The effects of heat-treatment parameters on the mechanical properties and microstructures of a low-carbon dual-phase steel

The austenite decomposition phase transformation in a low-carbon dual-phase (DP) steel is studied as a function of inter-critical annealing parameters: annealing time, annealing temperature, and cooling rate. The austenization kinetics are obtained by dilatometric analyses, water-quenched metallography, and thermodynamic calculations, and the austenite formed at different inter-critical annealing temperatures influences the subsequent formation of secondary phases during continuous cooling. Results indicate that the austenite volume fraction increases with both annealing temperature and time; although their effect on the final microstructure and mechanical properties lessens when the cooling rate decreases to the slow cooling regime below air cooling. Interestingly, the yield strength (YS) values of samples cooled at ∼1 °C/s with sand cooling, are 75∼100 MPa greater than the corresponding values at a rate of ∼6 °C/s with air-cooling. The results indicate that the yield strength is positively correlated with yield-point elongation but negatively correlated with the ultimate tensile strength. The effects of the cooling rate on the mechanical properties can be explained by the formation of Cottrell atmospheres of carbon together with the increase of the pearlite volume fraction. Based on the secondary phase components and strength evolutions, there are four possible outcomes for the different cooling rates: (I) martensite (M); (II) ferrite-martensite (FM); (III) coexistence of martensite and pearlite (MP); and (IV) ferrite-pearlite (FP). Our results reveal that the microstructure-property correlation can be illustrated schematically as a function of cooling rates and can thereby direct the heat-treatment parameters required to obtain desirable mechanical properties.

Alloy composition and process design based on thermodynamic and kinetic simulation: The case of medium Mn steel

This paper presents a design method for alloy materials that can be applied to medium Mn steel containing Al and/or Si. By using thermodynamic calculations based on CALPHAD, a wide range of alloy composition spaces were systematically studied, and the relationship between retained austenite fraction and its stability with intercritical annealing temperature was investigated. Alloys that meet the Process Window (PW) were screened and their various characteristics of alloy elements within the process window were determined. Additionally, the study performs three different kinetic simulations of austenite growth and solute partitioning during isothermal annealing on alloys that meet PW. The prediction of the model is verified by comparing with the experimental data in the literature. The results show that the remote model is more suitable for the actual production of low temperature isothermal intercritical annealing. Therefore, the method provided in this work makes a guiding contribution to alloy development, which can determine the alloy composition and heat treatment process satisfying PW and optimize the overall properties.

On the role of Manganese Partitioning in Metastable Austenite by Quenching and Partitioning Treatment

With the aim to study the role of “frozen” concentration gradient of manganese (Mn) element in stability of retained austenite (RA) with multiple‐stage martensite transformation, a series of intercritical annealing (IA) temperatures is conducted before quenching and partitioning (Q&P) treatment. Morphology and distribution of RA are observed by field emission gun scanning electron microscope and electron back‐scatter diffraction. The volume fraction (7%–16%) and stability of metastable RA is found to be affected profoundly by IA temperature. Thermodynamic and kinetic analysis are conducted to elucidate the evolution of RA in process of IAQP treatment. The predicted levels of RA are in good accordance with measurements. It is found that the inhomogeneous partitioning of Mn in period of IA, combining with the incomplete partitioning of carbon during Q&P, radically regulated the Q&P microstructure. The incomplete partitioning of carbon in RA, with excess carbon segregation at dislocations and boundaries, lead to partition‐less bainite transformation owing to the average carbon content in RA lower than the “To” threshold.

A low-cost strategy to improve strength-ductility-toughness balance in a low-carbon steel

ABSTRACT The current research introduces a low-cost strategy to improve strength-ductility-toughness balance in low-carbon steel containing 0.08 wt-% carbon by simple heat treatment. As the intercritical annealing temperature increased, the fraction of martensite enhanced. By increasing the annealing temperature from 770 to 830°C, the hardness of dual-phase steel was increased from 183 HV to 212 HV, which was due to the increment of martensite fraction from 0.28 to 0.49. The strength of all dual-phase steel samples produced by intercritical annealing treatment was higher than that of the initial sample owing to the presence of hard martensite in the soft ferrite. The interesting point was that the ductility of the dual-phase steels was almost the same as the initial sample. This led to a remarkable increase in the toughness of dual-phase steels compared to the initial sample. All dual-phase steels revealed a perfect ductile fracture.

Influence of intercritical annealing temperature on microstructure, microtexture, and tensile behavior of TRIP-assisted medium manganese steel

The present work investigates, the effect of intercritical annealing temperature on the microstructure, microtexture, tensile behavior, and mechanical stability of austenite in thermomechanically processed Fe-0.18C-4Al-7Mn (wt.%) medium manganese steel. The steel is subjected to thermomechanical processing (hot forging and hot rolling) followed by intercritical annealing at various temperatures (700, 750 and 800 °C) for 1 hour. The development of a complex microstructure with blocky and lath-type reverted austenite (RA), fine polygonal and blocky type intercritical ferrite (IF), coarse elongated ferrite (EF), and lath martensite (M) is revealed by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses. The steel annealed at 750 °C exhibits an excellent ultimate tensile strength (UTS) of 802 MPa and total elongation (TE) of 61% owing to its high RA content, high transformation-induced plasticity (TRIP) effect and low mechanical stability of RA. Further, to understand the dislocation phenomena during tensile deformation, the tensile samples are analyzed using TEM, which reveals dislocation interactions, dislocation tangles, and dislocation accumulations at the IF grain boundaries and martensite lath boundaries. The manifestation of the TRIP effect is elucidated by XRD and EBSD studies. In addition, to study the texture evolution and the role of texture components on the TRIP effect, the orientation distribution function is employed. The major texture component, i.e., Ms Brass {011} 〈211〉 indicates the TRIP effect resulting in the deformation-induced martensitic transformation under tensile deformation. Additionally, Ms Brass component shifts towards G/B (Goss/Brass) twin {011} 〈111〉 component by grain rotation to accommodate the deformation.

Austenite Nucleation and Growth as a Function of Starting Microstructure for a Fe–0.15C–5.56Mn–1.1Si–1.89Al Medium‐Mn Steel

The effects of starting microstructure and intercritical annealing temperature on the phase‐transformation kinetics and microstructural evolution of a prototype Fe–0.15C–5.56Mn–1.1Si–1.89Al medium‐Mn third‐generation advanced high‐strength steel are examined. The starting microstructures comprise 1) an as‐received cold‐rolled (CR) microstructure containing a significant fraction of ferrite and tempered martensite and 2) an austenitized and quenched martensite–ferrite (MF) microstructure. Based on the microstructural observations, two different scenarios for austenite formation during intercritical annealing are proposed. For the CR starting microstructure, austenite can nucleate at ferrite/cementite interfaces, and at ferrite grain boundaries. In the case of the MF starting microstructure, which contains thin films of interlath retained austenite (RA), austenite forms on the martensite lath boundaries or grows directly from the existing interlath RA. The studies are interpreted using the DICTRA module of Thermo‐Calc. At 665 °C, the austenite reversion kinetics of the MF samples are faster than the transformation kinetics in the CR structure. At higher temperatures where most of the cementite has dissolved, the austenite fraction in both starting microstructures is very similar.

Effect of Intercritical Annealing on the Properties of Dual Phase Steel via Finite Element Method

Dual phase (DP) steels are rapidly becoming more and more popular for automotive applications. They offer a weight reduction with a combination of energy absorption for crash zones. Rails, reinforcements, back panels, cross members, and pillars can be given as application examples. DP steels microstructure consists of a soft ferrite matrix with hard martensite islands. The hard martensite islands provide strength while the ductile ferrite provides formability. The strength level of DP steel is related to the amount of martensite in the microstructure, and the martensite amount can be arranged via intercritical annealing. In this work, thermodynamic analysis of St52 steel was carried out with Thermo-Calc software. A1 and A3 temperatures were determined by calculating the temperature-dependent phase fractions. Intercritical annealing temperatures were determined according to the calculated critical temperatures (A1 and A3). The intercritical annealing process was modelled by using Simheat NxT software. In this modelling and simulation work, the effect of intercritical annealing temperature on the final microstructure and hardness of DP steel was investigated.

Significance of intercritical annealing temperature on the impact behavior of Fe-7Mn-4Al-0.18C steel

The current study examines the impact behavior of thermomechanically processed Fe-6.78Mn-3.92Al-0.18C (wt. %) steel after intercritical annealing at 700, 750 and 800 °C. The formation of a multi-phase microstructure with blocky and lath-type reverted austenite (RA)/martensite (M), fine intercritical ferrite (IF), and coarse elongated ferrite (EF) is revealed by electron backscatter diffraction (EBSD). When subjected to the Charpy-impact test, the sample annealed at 750 °C shows the highest impact energy owing to high transformation-induced-plasticity (TRIP) effect. To further comprehend the micro-mechanism of fracture, the fracture surface is also examined by scanning electron microscopy (SEM).

In-situ investigation of strengthening and strain hardening mechanisms of Cu-added medium-Mn steels by synchrotron-based high-energy X-ray diffraction

A novel Cu-added medium-Mn steel with a chemical composition of Fe–0.27C–9.1Mn–1.86Al–3.3Cu (wt.%) was designed and subjected to intercritical annealing (IA) temperature range from 620 °C to 680 °C for 1 h. The ultimate tensile strength (UTS) increases and the yielding strength (YS) decreases with the IA temperature increasing. The YS of 824 MPa, UTS of 1222 MPa, total elongation (TE) of 55%, and product of strength and elongation (PSE) of 67.2 GPa·% are achieved after IA at 660 °C. Transmission electron microscopy confirmed that Cu-rich nanoparticles precipitate in the ferrite. The in-situ high-energy X-ray diffraction (HE-XRD) experiments show that at the beginning of plastic deformation, both austenite and ferrite bear the applied load. The load is mainly undertaken by martensite with effective transformation-induced plasticity (TRIP) effect triggered. The YS of ferrite is significantly higher than that of austenite. The individual contribution of solid solution strengthening, grain refinement strengthening, dislocation strengthening, and precipitation strengthening in ferrite and austenite is analyzed. The discrepancy between the YS of ferrite and austenite is mainly attributed to the precipitation strengthening due to the Cu-rich nanoparticles precipitation. The moderate mechanical stability and the collaboration of TRIP and twinning-induced plasticity (TWIP) effects of austenite contributed to the enhanced strain hardening capability and resulted in large ductility.

Microstructure and mechanical properties of medium manganese steel in intercritical annealing

A new type of cold-rolled medium manganese steel was investigated. The test steel was treated by Q&P process. With the increase in annealing temperature, the grain size gradually increases, the carbides dissolve progressively and the fine lath grains transform into massive or equiaxed grains. Both the volume fraction and the stability of the retained austenite are continuously decreasing, which together affect the change of mechanical properties. At the intercritical annealing temperature of 680°C, the specimens have higher retained austenite content and transformation rate. At the same time, due to the discontinuous TRIP effect caused by the retained austenite stability gradient inside the specimen, it exhibits excellent comprehensive mechanical properties, and the product of strength and elongation is 43.5GPa·%.

Influence of Initial Structures on the Mechanical Properties in Cold‐Rolled Transformation‐Induced Plasticity Steel

A novel transformation‐induced plasticity steel is analyzed to study the effects of initial structures and the intercritical annealing temperatures on austenization behavior, strain distribution, and the characteristics of retained austenite (RA). The resulting microstructure and its relationship with mechanical properties are measured using dilatometry, scanning electron microscopy, electron backscatter diffraction, X‐ray diffraction, and tensile testing. The results show that the initial structure is composed of martensite (M) or tempered martensite (TM) and can significantly refine the final microstructure and its strain distribution is more homogeneous compared to the ferrite + perlite (F + P) initial structure samples. However, it is found that the larger size of the RA mainly distributes at the ferrite grain boundaries while the smaller size mainly distributes in the bainite matrix and outside martensite. Meanwhile, most of the RA in the bainite matrix are films and the carbon concentration is high. In addition, the initial M samples have many homogeneous lath interfaces and the ferrite has a strong carbon discharge ability during high‐temperature annealing, so the fraction of RA is the highest at 23.1% when annealed at 820 °C, and the corresponding carbon concentration is 1.16 wt%, so the final combination of strength and ductility is up to 32.1 GPa%.

Investigate on ultrahigh-strength TRIP-aided autosteel plate under the framework of ICME

A novel low-carbon (C) low-alloy transformation-induced plasticity (TRIP) steel with a nominal composition of Fe-0.3C-1.9Mn-1.0Si-0.7Al wt.% was successfully designed by combining high-throughput thermodynamic and kinetic calculations and feature engineering based on the Integrated Computing Materials Engineering (ICME) framework. The designed TRIP steel was subjected to a series of two-stage heat treatments according to the target intercritical annealing temperature determined by estimation in an attempt to optimize phase constituents to obtain an ideal strength-plasticity balance. After annealing at 780 °C, the ultimate tensile strength (UTS), total elongation (TEL) and the product of strength and elongation (PSE) of the experimental steel can reach ∼980 MPa, ∼30% and ∼30 GPa•% respectively, because of the appropriate phase fraction and size of retained austenite (RA). Annealing at 820 °C, when the isothermal bainitic transformation (IBT) temperature is low, the short distance diffusion of C made it enriched in a small region, the stability of different RA grains varied greatly, which was not conducive to ensure stable and continuous TRIP effect. Moreover, by evaluation of the effect multiphase on yield strength (YS) and UTS, the presence of more bainite led to samples having higher tensile strength but deteriorated their plasticity.

Enhancing mechanical properties of hot-rolled Al-alloyed medium-Mn steel by novel double-step intercritical annealing

The novel heat treatment strategy for advanced medium-Mn steel is presented in the work. Conventional intercritical annealing (IA) was followed by short IA to improve the strength-ductility product of Al-alloyed medium-Mn steel without significant prolonging the duration of heat treatment process. The proper design of the new heat treatment schedules required both experimental and modelling approaches to optimize the microstructure and mechanical properties of investigated steel. The secondary IA step was performed in a temperature range from 750 °C to 850 °C. The obtained microstructures are composed of ferrite and fine lath-type austenite in different proportions. Specimens annealed at 800 °C and 850 °C display also some fraction of martensite. No martensite was observed for the specimen held at 750 °C. An increase of second IA temperature resulted in a gradual increase of tensile strength due to a higher martensite fraction and a reduced fraction of ferrite in the microstructure. The optimal martensite-austenite ratio resulted in combination of high strength and moderate plasticity for the specimen heat treated at 800 °C. The strength-ductility product was about 20.5 GPa%. The C and Mn partitioning from ferrite to austenite and dissolution of cementite are main factors affecting the stability of retained austenite during second IA step and work hardening behavior. The tensile behavior of specimens after two-step IA was affected by serrations resulting from the Portevin-Le Chatelier (PLC) effect.

Effects of different intercritical annealing processes on microstructure and cryogenic toughness of newly designed medium-Mn and low-Ni steel

Through the idea of replacing Ni with Mn, a shortened process of ultra-fast cooling and two-step intercritical annealing for medium-Mn and low-Ni steel after rolling was designed. Microstructures were characterized through optical microscopy, electron probe micro analyzer, scanning electron microscopy, electron backscattered diffraction, and X-ray diffractometer. Furthermore, the effects of different intercritical annealing temperatures on austenite including content, stability, and the impact toughness of experimental steel were investigated. The microstructures of the experimental steel comprised retained austenite (RA), fine-grained ferrite, and part of martensite. The results show that fine grain structure and austenite with high stability enriched by Mn and Ni provided excellent mechanical properties for experimental steel with yield strength of 645 MPa, tensile strength of 745 MPa, total elongation of 32%, and high impact energy of 133 J at −150 °C. As the intercritical annealing temperature increased, the microstructures of the matrix coarsened, and the volume fraction and size of RA increased, while the stability decreased significantly. RA with high stability provided long-lasting work hardening ability through continuous deformation-induced transformation (TRIP effect) during the deformation of steel, which played an important role in toughening the multiphase system.

The effects of double intercritical annealing on microstructure and mechanical properties of 0.107C-2.39Mn-0.453Si dual phase steel

In this study, double intercritical annealing was applied to modify the mechanical properties of 0.107%C-2.39%Mn-0.453%Si dual phase steel. The effects of the double intercritical annealing (DIA) method were investigated via microstructure observation and tensile test, and then compared with the single intercritical annealing (SIA) method. By increasing the intercritical annealing temperature, yield and tensile strengths increase while ductility decreases primarily due to the increase of martensite fraction. DIA leads to a slight reduction of the ferrite size and the martensite fraction regardless of the intercritical annealing temperature. Tensile results showed that DIA increases ductility without losing significant amount of strength. The outcome implies that the DIA method can be used to modify the mechanical properties of DP steels without adding excessive complexity to the process.

Effect of Intercritical Temperature on the Microstructure and Mechanical Properties of a Ferritic-Martensitic Dual-Phase Low-Alloy Steel with Varying Nickel Content.

Dual-phase low-alloy steels combine a soft ferrite phase with a hard martensite phase to create desirable properties in terms of strength and ductility. Nickel additions to dual-phase low-alloy steels can increase the yield strength further and lower the transformation temperatures, allowing for microstructure refining. Determining the correct intercritical annealing temperature as a function of nickel content is paramount, as it defines the microstructure ratio between ferrite and martensite. Likewise, quantifying the influence of nickel on the intercritical temperature and its synergistic effect with the microstructure ratio on mechanical properties is vital to designing dual-phase steels suitable for corrosive oil and gas services as well as hydrogen transport and storage applications. In this work, we used a microstructural design to develop intercritical annealing heat treatments to obtain dual-phase ferritic-martensitic low-alloy steels. The intercritical annealing and tempering temperatures and times were targeted to achieve three different martensite volume fractions as a function of nickel content, with a nominal content varying between 0, 1, and 3-wt% Ni. Mechanical properties were characterized using tensile testing and microhardness measurements. Additionally, the microstructure was studied using scanning electron microscopy coupled with electron backscatter diffraction analysis. Tensile strength increased with increasing martensite ratio and nickel content, with a further grain refinement effect found in the 3-wt% Ni steel. The optimal heat treatment parameters for oil and gas and hydrogen transport applications are discussed.

Influence of Partitioning Temperature on Microstructure and Mechanical Performance of Medium Manganese Fe-C-Mn-Cu-Cr-Mo-Nb Steel

A hot-rolled medium manganese Q&P steel was annealed at an intercritical annealing temperature (670 °C) and quenched to four partitioning temperatures (300 °C, 280 °C, 260 °C, and 240 °C). The effect of the partitioning temperature on the microstructure and mechanical properties of the steel was studied. The microstructure evolution at different partitioning temperatures was characterized using scanning electron microscopy (SEM), a transmission electron microscope (TEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The results showed that, as the partition temperature increased, the fractions and the size of the fresh massive martensite (M2) increased, the initial lamellar martensite (M1) was coarsened, and the amount of M1 decreased. Moreover, the amounts of lath-like retained austenite (RA) decreased and the amounts of bulky austenite increased. The tested steel partitioned at 260 °C had relatively greater fractions of RA, in which there were sufficient contents of carbon diffused from M1. Therefore, the test steel partitioned at 260 °C showed fine comprehensive mechanical properties. The combination of tensile strength and elongation reached 30.3 GPa.%, and its work-hardening plateau was the longest, showing excellent ductility as it had sufficient fractions of the retained austenite with good stability.