Intercritical Annealing Process Research Articles

Experimental and Modelling Research on the Effect of Prior Ferrite on Bainitic Transformation in Medium-Carbon Bainitic Steel

In this study, we investigate the impact of prior ferrite on the bainite transformation kinetics and microstructure of medium-carbon steel interrupted by an intercritical annealing (IAA) process. It was found that the incubation time and completion time decreased from 687 s and 6018 s to 20 s and 4680 s, with the volume fraction of ferrite increasing from 9.5% to 28.6%, while the maximum transformation rate increased from 00271 μm/s to 0.0436 μm/s. The ferrite/austenite interface is introduced, and the nucleation sites are increased to accelerate the subsequent bainite transformation due to the formation of prior ferrite. However, there is a competitive relationship between the number and activation energy of bainite nucleation. According to the experimental results and theoretical calculations, the activation energy of the bainite transformation in the medium-carbon bainite steel decreases gradually with an increase in the volume fraction of prior ferrite.

Improving laser welded joint performance of medium-Mn steel through C and Mn partition by intercritical annealing process

In this paper, the fracture failure mechanism of laser welded (LW) joints of medium-Mn steel is studied. And the mechanical properties of the joints were greatly improved by designated intercritical annealing (IA) process. Experimental results showed that the elongation of the as-welded joint of medium-Mn steel is very low, and the fracture shows typical brittle fracture characteristics. Electron probe micro-analyzer (EPMA) test showed that there were many Mn segregation bands in the joint, and electron backscatter diffraction (EBSD) surface scanning test further demonstrated that many Mn23C6 carbides were produced at the martensite grain boundary (GB) of the fusion zone (FZ), which was the main reason for the occurrence of brittle fracture in the FZ. The IA process improves the segregation of Mn element at the GB of the joint and reduces the intergranular brittleness. At the same time, more austenite was distributed by C and Mn elements, and more Fe3C phases were found near austenite in the EBSD diagram, which provided nucleation centers for the formation of austenite. The product of strength and elongation (PSE) of the IA treated joint was dramatically increased by 2693.65%.

Influence of boron addition on microstructure and mechanical properties of medium-Mn advanced high-strength steel

This study investigates the effects of 0.12 wt% boron addition on the microstructure and mechanical properties of a Fe-0.08C-4.5Mn-2.5Cr–1Si medium-Mn steel in as-rolled conditions and after the intercritical annealing process. In the as-rolled conditions, boron addition increased the strength and reduced the alloy ductility. In as-hot rolled conditions, an ultimate tensile strength (UTS)/elongation of 1195 MPa/5.5% and 1008 MPa/7% was achieved in the boron-added and boron-free alloys, respectively. However, the effect of boron on the intercritically annealed specimens was diverse. The intercritical annealing had no tangible effect on the mechanical properties of the hot-rolled specimen. However, the intercritical annealing at 700 °C for 600 s led to a UTS/elongation of 1029 MPa/19.5% in the boron-added alloy, the highest UTS/elongation combination among all specimens. The formation of carbo-boride precipitates on lath boundaries of the boron-added alloy possibly locked the boundaries thereby a temper-resistant martensitic structure was attained. The lath structure of the annealed alloy caused an enhanced strength in the boron-added specimen. Nonetheless, in the boron-free alloy, martensite was normally tempered and a considerable reduction in the strength was observed.

Mitigating hydrogen embrittlement of advanced high-strength steel by controlling carbides (cementite and alloy carbides) and microstructural modification

This work aims to demonstrate that the resistance to hydrogen embrittlement of conventional advanced high-strength steel (AHSS) with a bainitic ferrite structure can be significantly enhanced through two metallurgical strategies. Firstly, a smaller fraction of cementite in a mixture of bainitic ferrite and lath-martensite structures is realized by an intercritical annealing process with rapid cooling. Secondly, a higher fraction of alloy carbides (MC) with a mean size of less than 30 nm is formed through the combined addition of Ti, Mo, and V as micro-alloying elements. The reduction of cementite precipitation in the matrix leads to an increased resistance to hydrogen evolution/adsorption on the surface under cathodic polarization. Moreover, a large number of V-bearing nanoprecipitates ((Ti,Mo,V)-carbides and V-carbides) provides intermediate trapping for hydrogen atoms, effectively delaying the diffusion kinetics of hydrogen towards potential crack-forming areas in the matrix. Consequently, the lower strain reduction during slow strain rate test and the higher resistance to cracking during the four-point bending test in acidic aqueous solutions are manifested in the newly designed AHSS.

UPAYA PERBAIKAN SIFAT MEKANIK BAJA PLAT JOIN G 3131 SPHC MELALUI SIMULASI PROSES INTERCRITICAL ANNEALING DI INDUSTRI KONSTRUKSI BAJA

Improvement of the mechanical properties of G3131 SPHC equivalent to JIS G 3101 SS330 plate joint steel through intercritical annealing simulations has been carried out. The JIS G 3131 grade SPHC steel was gradually heated by intercritical annealing at temperatures of 750–850 oC for holding times of 1–15 minutes. After intercritical annealing treatment, the tensile strength and yield strength of steel increased compared to non-treatment samples. The average tensile strength obtained was 573 MPa with an increase of 76.85% compared to the tensile strength before the intercritical annealing process. Meanwhile, the yield strength of steel increased by 60% to 421 MPa. Increasing the annealing temperature and holding time will promote the formation of the martensite phase, as well as enlarge the austenite and ferrite grains. Optimal heating variable obtained at a temperature of 750 ℃ with a holding time of 15 minutes. Using these variables produced a tensile strength of 625 MPa and a yield strength of 434 MPa with an elongation of 23% which is standardly included in JIS G 3101 SS400.At the optimal value, the grain growth kinetic of K and n values were 0.280466778 and 0.6905, respectively, with an activation energy (Q) of 12.4268517 J/mol.

Influence of Heat Treatment on Microstructure and Mechanical Properties of Direct-Quenched Fe-0.06C-0.2Si-2.0Mn Steel

In this study, the effect of subsequent heat treatment applied to high-strength low-alloy steel (HSLA) on the structure–property relationships was investigated. Tempering and intercritical annealing processes are introduced to elucidate the influence of subsequent heat treatment on mechanical properties of direct-quenched Fe-0.06C-0.2Si-2.0Mn steel from a microstructural perspective. The tempering process results in a typical tempered martensite with uniformly dispersed cementite, whereas the intercritical annealing process forms a dual-phase microstructure composed of soft ferrite and hard martensite for the direct-quenched steel. In the intercritical annealed steel, a number of mobile dislocations at the interphase (martensite/ferrite) boundary significantly decrease the yield strength, and the large difference in strength between ferrite and martensite enhances work hardening. Charpy V-notch impact test results indicate that the tempering and intercritical annealing processes improved the absorbed energy by more than 100 J compared to the direct-quenched steel at room temperature, and at −50 °C, the intercritically annealed steel exhibited the highest absorbed energy of approximately 140 J. Additionally, the high fraction of high-angle grain boundaries and fine grains of the intercritically annealed steel increase the resistance to cleavage crack propagation, thereby reducing the ductile-to-brittle transition temperature.

Achieving 1.5 GPa grade medium Mn steel with high ductility via interrupted intercritical annealing process

In this study, we proposed an interrupted intercritical annealing (IIA) strategy to produce high-strength medium Mn steel with a nominal chemical composition of Fe–0.20C–8.0Mn–1.6Si–1.3Al (wt.%), which can promote the dissolution of carbides and make the C and Mn elements fully diffuse into austenite to enhance the stability of austenite. Furthermore, the content of retained austenite (RA) was significantly improved while avoiding excessive growth of RA by controlling the nucleation and growth of austenite. The concentration gradient of Mn was established to form the core-shell structured RA which had a higher Mn content in the shell than in the core. We used the IIA process to prepare medium Mn steel with an ultimate tensile strength (UTS) of 1536.93 MPa, a total elongation (TE) of 43.26 %, and a product of strength and elongation (PSE) of 66.49 GPa·%, which exhibits excellent mechanical properties. This study provides a theoretical reference for the preparation of high-strength medium Mn steel.

Heterogeneous microstructure design by pre-manipulating ferrite recrystallization in a cold rolled medium Mn steel

The medium Mn steel (MMS), which is regarded as the most representative candidate of the third generation advanced high strength steels, has received widely attention during the last several decades with respect to the exceptional advantage of low cost and excellent strength-ductility properties. In this study, a microstructural strategy of developing heterogeneous microstructures in a cold rolled MMS is presented. By pre-manipulating occurrence of the ferrite recrystallization, both the lamellar-shaped and granular-shaped ultra-fine retained austenite can be obtained after the two-step intercritical annealing process. Various amounts of recrystallized ferrite and difform martensite can be obtained by adjusting the pre-annealing temperature, which can effectively contribute to producing the two types of heterogeneous retained austenite, i.e., lamellar and granular in the following annealing process. The heterogeneous-structured retained austenite enables an excellent strength–ductility combination and reduced Lüders strain in the cold-rolled MMS.

Towards ultra-high strength dual-phase steel with excellent damage tolerance: The effect of martensite volume fraction

Ultra-high strength (> 1 GPa) dual-phase (DP) steels have been extensively investigated in literature. Yet, the damage tolerance of these DP steels remains mostly unexplored, whereas this parameter is crucial for the sheet formability and anti-crushing performance. In this work, a medium-Mn DP steel was proposed in order to develop strong but damage-resistant DP steel by refining the microstructure. Ultra-fine grained (< 1 µm) DP steels were obtained by a simple cold rolling and intercritical annealing process. The developed DP steel can achieve an outstanding ultimate tensile strength (UTS) of over 1470 MPa with appreciable elongation to failure (EtF) of over 9 % when the martensite volume fraction (Vm) is 55 %. Higher Vm results in higher strength. However, the fracture strain, which is connected with damage tolerance, decreases significantly with increasing Vm, and it is even much lower than that of a full-martensite steel. Furthermore, the necking behaviour transfers from diffuse plus localized necking to diffuse necking with increasing Vm. The sufficient ferrite content in the proposed DP1470 steel enables void growth after nucleation, consequently leading to the occurrence of localized necking and higher fracture strain. Thus, a moderate Vm ranging from 45 % to 65 % is suggested for the development of a 1470 MPa grade DP steel with excellent damage tolerance.

EFFECT OF COOLING-MEDIUM INDUCED INITIAL STRUCTURE BEFORE INTERCRITICAL ANNEALING ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LOW ALLOY DUAL-PHASE STEEL

The present research focused on determining the effect of cooling-medium-induced initial structure before the intercritical annealing induced dual-phase structure in the low alloy steel. Low carbon steel, which consists of containing 0.09 wt.% C was heated at 920 °C for 30 minutes to austenitization and then cooled in various media to provide the different initial structures before the IA (intercritical annealing) process. After austenization, the cooling process in the furnace and open-air provided a ferrite-pearlite phase, while the cooling process in water generated full martensite as the initial structure. Afterwards, the sample was intercritical-annealed at 750 °C (temperature between Ac1 and Ac3 lines or intercritical zone) for 10 minutes and then quenched in water. The water quenching after the austenitizing process improved the mechanical strength of steel (919 MPa), compared to the as-received state (519 MPa) due to martensite formation. As the cooling rate increased after the austenitizing process, the tensile strength increased and the elongation decreased. The different structures before intercritical annealing affected the martensite volume fraction and further correlated with improving mechanical properties. The ferrite and pearlite, as the initial structure before the IA process, provide a smaller fraction of martensite (18.36 vol.% for furnace cooling and 27.85 vol.% for open-air cooling). In contrast, the full martensite as the initial structure before IA generates a higher fraction of martensite (39.25 vol.%). The tensile strengths obtained were 512, 516, and 541 MPa with elongations of 29.8%, 30.1% and 32.6% for cooling furnace, open air and water, respectively. The strain-hardening behavior during the intercritical annealing is not affected by the initial process of the structure.

Improving the strength-ductility balance of medium-Mn Q&P steel by controlling cold-worked ferrite microstructure

The phase transformations, microstructure and properties of two Medium-Mn processed via quenching and partitioning steels were compared in this contribution and a new strategy for controlling mechanical properties by introducing and controlling cold-worked ferrite prior to heat treatment is proposed. It was found that during heating, the recovery and recrystallization of cold-worked ferrite compete with austenitization, thereby inhibiting the coarsening of austenite. The cold-worked ferrite interface will significantly delay the austenitization kinetics during the partitioning local equilibrium stage compared to martensite. These results lead to a diverse parent austenite, as well as a refined martensite substructure. As a result, the randomly distributed variants increase the number of effective grain boundaries, thus enhancing yield strength. The intercritical annealing process at a temperature of 860 °C resulted in the formation of fresh martensite-retained austenite (M/RA) constituents exhibiting a remarkably fine (<2 μm) and uniform grain morphology. Such microstructure yielded substantial improvement in both the strength and ductility of the steel. The proposed treatment led to excellent elongation (24%) at fracture, combined with very high ultimate tensile strength and yield strength of 1345 MPa and 1163 MPa, respectively, of the steel, resulting in a product of strength and elongation that exceed 32 GPa%.

A Novel Design to Eliminate Lüders Band in Medium-Mn Steel and Its Microstructure-Property Relationship

In the current work, we design a novel medium Mn steel with a superior mechanical property and no Lüders band. For industrial applications, a “low Mn addition” chemical composition and two kinds of different annealing processes with various initial microstructures were introduced. Consequently, the sample subjected to full austenitized quenching plus intercritical annealing process exhibited an outstanding mechanical property without the Lüders band. The microstructural evolution and austenite reverted transformation behavior were discussed in detail. In addition, austenite stability was estimated by chemistry stability and mechanical stabilization. It seemed that the austenite stability was significantly influenced by the morphological component. Thus, the sample with single lath-like ferrite and austenite exhibited the most excellent mechanical property. Furthermore, the “Lüders band” phenomenon was considered to rely on the restriction of martensitic recovery and recrystallization by lath-like morphology. The occurrence of the Lüders band was attributed to the low work-hardening ability caused by dynamic recovery. The formation of lath-like morphology could prevent the occurrence and propagation of the Lüders band by increasing the dislocation density and active TRIP effect.

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.

Partitioning of C and Mn During Intercritical Annealing and Transformation of Retained Austenite During Deformation in Medium Mn Steel

Herein, high‐strength medium Mn steel is obtained by intercritical annealing and quenching processes. Experimental steel with a chemical composition of Fe–0.2C–8.0Mn–1.6Si–1.3Al (wt%) is treated. The microstructures of the obtained samples comprise ferrite/martensite and retained austenite (RA) phases. The partitioning behavior of carbon and manganese during the intercritical annealing process is simulated using DICTRA. The sample annealed at 700 °C for 10 min has the best mechanical properties, with a product of strength and elongation as high as 47.25 GPa%. The deformation response of the RA is studied. The γ→α′ transformation mainly occurs in the early stage of deformation, whereas the γ→ε→α′ transformation mainly occurs in the later stage of deformation. The stress is mainly distributed at the deformed martensite and RA, and most of the stress is concentrated at the low‐angle grain boundary. During the deformation process, both the Kurdjumov–Sachs interface and the Nishiyama–Wasserman interface between the RA and deformed martensite decrease, but the Kurdjumov–Sachs interface decreases more obviously.

Research on Hot Stamping-Carbon Partition-Intercritical Annealing Process of Medium Manganese Steel.

In order to improve the plasticity of hot stamping parts, this paper combines the heat treatment process with the plastic forming of sheet metal, and creatively proposes a new process of hot stamping-carbon partitioning-intercritical annealing. The mechanical properties and microstructure are characterized under the newly proposed process, the quenching-partition (QP) process, and the intercritical annealing (IA) process, respectively. The new process firstly undergoes incomplete austenitizing treatment at 610 °C, then carries out distribution treatment while stamping at 300 °C, and finally conducts annealing treatment in critical zone at 680 °C in two-phase zone. The results show that a multi-phase refined microstructure composed of lath martensite, retained austenite, fresh martensite, and carbides are obtained by the new process. Most of the retained austenite is shaped in the thin film due to martensitic shear, in which carbon and manganese elements diffuse from martensite to austenite by heat treatment, thus stabilizing the retained austenite. Retained austenite with a volume fraction of 33.7% is obtained in the new process. The retained austenite with higher content and better stability is completely consumed during the stretching process, which gives full play to discontinuous TRIP effects, thus delivering the elongation of 36.8% and the product of strength and elongation (PSE) reached as high as 43.6 GPa%.

On the Formability of Medium Mn Steel Treated with Varied Thermal Processing Routes

In this contribution, we investigate the influence of thermal processing routes on the formability of medium Mn steel by assessing the strain hardening coefficient and anisotropy factor using the uniaxial tensile test. Medium Mn steel processed by intercritical annealing (IA) at 680 °C for 4 h demonstrates better formability than steel treated with a combination of IA at 800 °C for 10 min and quenching and partitioning (Q&P), based on the much higher strain hardening coefficient (n) and comparable anisotropy factor (r, rm, ∆r). The higher strain hardening coefficient of medium Mn steel with single IA treatment is ascribed to the enhanced transformation-induced plasticity (TRIP) effect resulting from the large amount of austenite that is transformed into martensite during deformation. In addition, the IA process allows for the production of medium Mn steel with high ductility, which is beneficial for its high formability and good 'part ductility' in lightweight automotive applications.

Synergistic effect of chromium content and intercritical annealing process on corrosion-resistant improvement of Ni-Cr-Mo low alloy

Synergistic effect of chromium content and intercritical annealing process on corrosion-resistant improvement of Ni-Cr-Mo low alloy

Effect of a New Prestrain, Intercritical Annealing, Quenching, and Partitioning Process on the Microstructure and Mechanical Properties of Medium‐Manganese Steels

A new process combining 4% prestrain, intercritical annealing, quenching, and partitioning (prestrain–IA–Q&amp;P) is used to investigate the effect of tensile number on the microstructure and mechanical properties of medium‐manganese steels. A microstructural model is built based on the specimens’ microstructure and mechanical properties. The amount of tiny brilliant white carbide particles precipitated in the microstructure seems to grow with the number of stretches. The test steel's tensile strength first decreases and then increases. However, the product of tensile strength and elongation exhibits the opposite trend. The new process improves test steel elongation compared with IA–Q&amp;P. This might be because multiple stretches have destroyed the original microstructure of the test steel. Consequently, the test steel generated more deformation bands and dislocation energy. Simultaneously, grain refinement and carbide precipitation in the test steel increase the nucleation point of austenite, resulting in a diffuse distribution of the retained austenite. The tested steel is stretched numerous times, enhancing the capacity of the microstructure's ferrite/martensite/retained austenite to accept dislocations, and there is increased dislocation slip and back stress production between the microstructures. As a result, test steel's ductility is increased by the transformation induced plasticity effect of retained austenite and the synergistic deformation of ferrite.

Batch annealing enhances the yield strength of a high quenching temperature Q&P steel through accelerated bainitic transformation

Hot-dip galvanized quenching and partitioning (Q&P) steels exhibit excellent corrosion resistance and strength-ductility synergies. However, production using conventional continuous galvanizing lines (CGLs) is challenging because these lines cannot meet the required quenching and reheating capacity. The presence of ferrite and absence of sufficient primary martensite led to a substantial decrease in the yield strength of the material. To solve this problem, we propose a simple batch annealing (BA) treatment prior to continuous annealing and galvanization. The BA treatment resulted in spheroidization of cementite in the coiled samples and enrichment of Mn. During the intercritical annealing process, the higher Mn content in the cementite of the BA sample retards cementite dissolution. The retained cementite particles served as nucleation sites and significantly promoted bainite formation during quenching and partitioning. Consequently, the yield strength increased by approximately 90%, from 438 MPa to 821 MPa. The total elongation of the BA samples was not affected due to the simultaneous stabilization of the retained austenite. The effect of batch annealing time was also investigated, and the balance between Mn enrichment in cementite and the number density of spheroidized cementite before Q&P treatment was revealed.

Microstructure and Mechanical properties of Medium Manganese Steels

Medium manganese steel is a 3rd generation advanced high strength steel, which contains 3–12 wt% manganese. This steel is highly suitable material for the automobile industry due to good balance between the cost of material and the tensile properties. It has got the optimum amount of strength and elongation. Initially the microstructure of this steel after cold rolling is deformed martensite (ά). Intercritical annealing is done to get moderate amount of retained austenite (γr) having good stability, which leads to higher strength and better ductility due to transformation-induced plasticity (TRIP). The formation of stable retained austenite (γr) by intercritical annealing process is our prime interest. Medium Manganese steel usually exhibits austenitic-ferrite microstructure after the intercritical annealing process, which makes the steel suitable for 3rd generation advanced high strength steels as this microstructure gives good amount of elongation. From this study we get best combination of intercritical annealing and quenching to get the required microstructures and properties. Thermo-calc software is used to find the optimum temperature for intercritical annealing at which we get optimum amount of retained austenite (γr) containing maximum manganese. Tensile tests were carried out at different pre-strain level from the samples taken in both rolling and transverse directions. Samples were prepared for SEM and optical microscope characterization to see the changes from initial microstructure of the steel. X-ray diffraction was also done to find out the amount of retained austenite that we are getting from tensile sample after pre-strain it to different level. It was concluded that after intercritical annealing at 650 °C for 2 h the steel sample have approximately 22% retained austenite and ultimate tensile strength (UTS) of 843 MPa and total elongation of around 36%.