Carbon Content Of Retained Austenite Research Articles

Optimizing strength & ductility combination of medium-carbon TRIP steel via multi-stage work hardening

The effect of the retained austenite (RA) stability on mechanical properties of a medium-carbon transformation-induced-plasticity (TRIP) steel at different deformation temperatures was studied. Two steels were obtained by adjusting the annealing temperature from 780 °C to 820 °C and controlling the isothermal bainitic process, and the carbon content of RA in the steels was 1.2% and 1.4%, respectively. At 25 °C, ultimate tensile strength, yield strength and total elongation of the steel after intercritical annealing at 780 °C were 1390 MPa, 870 MPa and 40%, respectively. When the deformation temperature increased to 200 °C, yield strength nearly kept constant (860 MPa) while total elongation significantly increased to 73%. The stability of RA had a significant influence on the deformation mechanisms of the steel, hence leading to the distinct variation of the mechanical properties. The RA stability increased with increasing deformation temperature to 200 °C, reducing the rate of transformation from RA to martensite and making the TRIP effect more persistently. Meanwhile, due to low solubility of Cu atom and the large number of dislocations acting as fast diffusion channels for Cu and C atoms in transformed martensite, the joint action of the increasing temperature and the continuous TRIP effect promoted the formation of numerous nanosized precipitates including η carbides and Cu particles in the newly formed martensite. As a result, the work hardening rate was significantly increased, improving the ductility without sacrificing the strength. This study provides a strategy to control the deformation mechanisms of medium-carbon TRIP steel, which can tailor the stability of RA according to the requirement of application, so as to obtain a good combination of strength and ductility.

The abnormal carbon redistribution in lath martensite during tempering in Mn-patterned steels

Martensite tempering is a critical step to achieve high strength with decent ductility. It is well-known that, during tempering, the carbon is sequentially arranged as segregation, transition carbide and cementite. Abnormally, this study shows the absence of carbide precipitation in Mn-depleted martensite during tempering when Mn distribution is heterogeneous between austenite and martensite. After fast heating and short austenitization from Mn-depleted ferrite and Mn-enriched cementite, the high-temperature austenite with Mn heterogeneous distribution has formed, resulting in the formation of alternating Mn-depleted lath martensite and Mn-enriched film retained austenite (RA) after quenching. In comparison with the carbide precipitation in conventional lath martensite with Mn homogeneous distribution during tempering at 150–500°C, carbide formation is completely inhibited in Mn-depleted lath martensite. Instead, carbon atoms diffuse from Mn-depleted martensite to its neighboring Mn-enriched film RA, leading to an increased carbon content in RA. This is mainly attributed to the strong interaction between Mn and carbon, further assisted by high dislocation density and small width of lath martensite. Additionally, this RA can promote carbon diffusion in the interior due to its gradient Mn distribution.

Effect of austempering temperature on microstructure evolution, mechanical properties and wear resistance of carbon-free nano-bainite steel

Carbide-free nano-bainite steel holds significant potential for applications in mining as an ultra-high strength wear-resistant steel, but the wear mechanisms are not yet fully understood. This study aims to explore the effect of austempering temperature on microstructure, mechanical properties and wear resistance of carbide-free nano-bainitic steel. The relationship between the content and mechanical stability of retained austenite (RA) and mechanical properties was investigated to elucidate the mechanisms of pin-on-disc wear and impact wear. The results show that in the range of Ms∼300 °C, the phase transformation incubation period and completion time significantly increase with decreasing austempering temperature, which is mainly related to higher phase transformation driving force resulting from a larger subcooling degree and slow growth of carbon content in RA. Optimal strength and hardness of 2099 MPa and 615 HBW, respectively, are achieved after austempering at 210 °C for 83 h. Austempering at 270 °C for 11 h yields the best balance of strength and plasticity, with a strength-plasticity product of 28.8 GPa%. The micro-cutting mechanism of pin-on-disc wear leads to serious weight loss. The RA with poor mechanical stability accelerates the stress-induced martensite transformation, while internal stress support due to volume expansion reduces the penetration depth of abrasive particles. The weight loss in impact wear is primarily caused by shedding due to fatigue failure. With the increase of RA content and mechanical stability, the persistent stress-induced martensite transformation effectively hinders micro-crack propagation, thereby enhancing impact wear resistance.

Alternative Approach for the Intercritical Annealing of (Cr, Mo, V)-Alloyed TRIP-Assisted Steel before Austempering

TRIP-assisted C-Si-Mn steels are usually subjected to austempering with a preliminary intercritical annealing that is targeted at the multi-phase structure with 40–60 vol.% of proeutectoid ferrite. The kinetics and the mechanism of phase-structural transformations can be impacted due to the additional alloying of TRIP-assisted steel by the strong carbide forming elements, thus necessitating an alternative approach for the selection of intercritical annealing parameters. This issue is analyzed in the present work, which investigates the effect of the temperature of intercritical annealing on the “Structure/Properties” correlations in 0.2 wt.% C-Si-Mn-Nb steel additionally alloyed by 0.55 wt.% Cr, 0.20% Mo, and 0.11 wt.% V. The annealing temperature ranged from 770 °C to 950 °C, and austempering was performed at 350 °C for 20 min. It was observed that the addition of the (Cr, Mo, and V) complex significantly improved the steel hardenability. However, the annealing of steel at 770 °C (to gain 50 vol.% of proeutectoid ferrite) resulted in the precipitation of coarse cementite lamellas during bainite transformation, thus lowering the amount of retained austenite (RA) and decreasing the strength and ductility of the steel. At higher annealing temperatures, carbide-free bainite was formed, which presented a 2.5–3.5 times increase in the RA volume fraction and a 1.5 times increase in the RA carbon content. The optimal combination of the mechanical properties (UTS of 1040 MPa, TEL of 23%, V-notch impact toughness of 95 J/cm2, PSE of 23.9 GPa·%) referred to annealing at a temperature close to the Ac3 point, resulting in a structure with 5 vol.% ferrite and 9 vol.% RA (the residue was carbide-free bainite). This structure presented an extended manifestation of the TRIP effect with an enhanced strain hardening rate due to strain-induced martensite transformation. The impact of the alloying elements on the carbon activity in austenite served as the basis for the analysis of structure formation.

Universality of quenching-partitioning-tempering local equilibrium model

• Microstructural amount and C γ in low-C and medium-C Q-P-T steels were quantitatively characterized. • The QPT-LE model is a novel tool for the design of Q&P/Q-P-T steels. • The universality of QPT-LE model was firstly verified. The design of metastable retained austenite (RA) and its stability have always played a vital role in developing advanced high strength steels (AHSSs). The quenching-partitioning-tempering local equilibrium (QPT-LE) thermo-kinetic model was established by us, with consideration of carbide precipitation and interface migration. As a tool for designing AHSSs, the above model can well predict the microstructural amount and the carbon content in RA ( C γ ) in a high-C quenching-partitioning-tempering (Q-P-T) steel. In this work, QPT-LE model predicates the microstructural amount and C γ in low-C and medium-C Q-P-T steels more accurately than constrained carbon equilibrium (CCE) model without consideration of carbide precipitation and interface migration, and more accurately than quenching and partitioning-local equilibrium (QP-LE) model without consideration of carbide precipitation, which verifies the universality of QPT-LE model. The advantages of QPT-LE model are also discussed.

Enhanced Strength and Ductility by Introducing Nanobainitic Ferrite in Bainitic Steel Used in Sports Equipment

The mechanical properties of carbide-free bainitic steels used in sports equipment were investigated. The nanobainitic ferrite was introduced in bainitic steel to enhance the stability of blocky retained austenite (RA). The blocky RA formed in bainitic austempering process was coarse and led to poor mechanical properties. By introducing the nanobainitic ferrite into blocky RA, the yield strength was improved remarkably, which was increased from 706 to 1180 MPa. Furthermore, the total elongation was almost twice the value compared to the traditional bainitic treatment. The improved mechanical properties were attributed to the enhanced stability of blocky RA. Furthermore, the increased carbon content in RA derived from the carbon dissolved in bainitic ferrite and the carbon trapped in dislocation or Cottrell atmosphere.

The influence of the heating rate on the microstructure and mechanical properties of a peak annealed quenched and partitioned steel

In this study an Fe-0.28C-1.91Mn-1.44Si cold-rolled steel was subjected to conventional (10 °C/s) and ultrafast (100 °C/s - 700 °C/s) heating peak annealing treatments, followed by quenching and partitioning (Q&P). The microstructural characterization results showed that grain refinement of the parent austenite and its transformation products occurred with the increment of the heating rate from 10 °C/s to 100 °C/s, without further refining at 700 °C/s. The formation of complex microstructures after the end of the thermal treatment, accompanied by the reduction in the retained austenite carbon content, suggested that local chemical heterogeneities in austenite appear upon ultrafast heating. Regardless of the prior heating rate, similar mechanical properties and strain hardening were measured, revealing that both, the microstructure development and the extent of austenite stabilization during quenching and partitioning stage play a fundamental role on the mechanical behavior of the peak annealed Q&P steels.

Micromechanical Behavior of Transformation‐Induced Plasticity‐Assisted Annealed Martensitic Steel Using In Situ Neutron Diffraction

Herein, the transformation kinetics of retained austenite (RA) and the deformation mechanisms of constituent phases are quantified for a transformation‐induced plasticity (TRIP‐assisted) annealed martensitic (TAM) steel. This steel shows a good combination of strength and ductility (product of strength and elongation is 30.3 GPa%). The RA transformation coefficient k (=7.46) is at a medium level, indicating that RA has a suitable mechanical stability compared with other steels. An in situ neutron diffraction technique is used to experimentally study the stress partitioning among the constituent phases of the TAM steel. The results show that it is three times for stress partitioning occurring in TAM steel under uniaxial tensile test. When the engineering stress is just over the elastic limit, the stress from the annealed martensite is transferred to the hard phases and RA. After the specimen reaches the yield strength, the increasing rate of the lattice plane strain of RA declines because the stress from the RA transfers to the harder phase (twin martensite), resulting from the TRIP effect. When the engineering stress reaches 934 MPa, the increasing rate of the lattice plane strain of RA increases again possibly due to the increase in the carbon content of RA.

The Effect of Stress on Bainite Transformation, Microstructure, and Properties of a Low‐Carbon Bainitic Steel

The effects of applying external stress on bainite transformation, microstructure, and mechanical properties are investigated by metallography, dilatometry, etc. Results show that compressive stress during bainite transformation accelerates the bainite transformation and increases the final bainite amount due to the additional mechanical driving force. The lath‐like bainite becomes shorter, and smaller blocky martensite/austenite islands form under stress during the bainite reaction. Different from the results with stress during bainite transformation, applying stress after bainite transformation during isothermal holding cannot increase the volume fraction of bainite, and no obvious change in bainite morphology is observed. In addition, the subsequent martensite transformation is promoted during the cooling process. Moreover, the less amount of retained austenite (RA) but more carbon content in RA is obtained with applying stress after the bainite reaction. Finally, the tensile strength increases, while the elongation decreases when the external stress is imposed after bainite transformation.

A novel ultra-strong hot stamping steel treated by quenching and partitioning process

ABSTRACTIn the present study, we design a novel hot stamping steel containing high amounts of C and Si and micro-alloying element (i.e. Nb). The steel was subjected to quenching and partitioning (Q&P) process. The new Q&P treated hot stamping steel exhibits a significantly improved mechanical property in terms of strength, ductility and impact toughness compared with the traditional 22MnB5 hot stamping steel. The influence of partitioning time on the microstructure and mechanical properties was investigated. The retained austenite (RA) fraction and the carbon content of RA significantly increased with higher partitioning times. With increasing partitioning time, the uniform elongation, total elongation, strength-ductility balance and impact energy was also remarkably enhanced. The maximum strength-ductility balance achieves around 23 GPa %.

Effects of Temperature and Time of Isothermal Holding on Retained Austenite Stability in Medium-Mn Steels

Effects of isothermal holding time and temperature on the stability of retained austenite in medium manganese bainitic steels with and without Nb microaddition were investigated. The amount of retained austenite for various variants of thermomechanical processing was determined by X-ray diffraction. Relationships between processing conditions and microstructure were revealed using light microscopy and scanning electron microscopy techniques. The isothermal holding temperatures changed from 500 to 300 °C and the time was from 60 to 1800 s. The optimal time and temperature of isothermal holding for all the investigated steels were 400 °C and 300 s, respectively. The relationships between the Mn content, amount of retained austenite, and carbon enrichment of the retained austenite (RA) were observed. The noticeable effect of Nb microaddition on the amount of retained austenite was not observed. In general, the carbon content in RA was slightly lower for the steels containing Nb. The optimum gamma phase amount was up to 18% for the 3% Mn steels, whereas it was c.a. 13% for the steels with 5% Mn. It was found that the morphology of blocky/interlath retained austenite depends substantially on the isothermal holding temperature.

Austenite stability and mechanical properties of a low-alloyed ECAPed TRIP-aided steel

Equal channel angular pressing (ECAP) and post-ECAP heat treatment were conducted on a Fe-1.38Mn-0.67Si-0.56Al-0.014 Nb-0.18 C transformation-induced plasticity (TRIP) aided steel. The microstructure of unECAPed TRIP sample was mainly composed of polygonal ferrite, bainite, retained austenite (RA) and lath martensite, while lath martensite was not observed in ECAPed TRIP samples. The stability of retained austenite (RA) for unECAPed TRIP sample was mainly influenced by the carbon content in RA, while for ECAPed TRIP steel, it was dominated by the size of RA. Comparing the mechanical properties of the unECAPed samples with the ECAPed samples, the ultimate tensile strength (UTS) changed slightly, but the total elongation (TEL) increased from 25.6% to 39.8%. The 4 passes ECAPed TRIP sample exhibited excellent mechanical properties with the product of tensile strength and total elongation (PSE) of 29.4 GPa %, which was a consequence of TRIP effect and cooperative deformation of ferrite, bainite and austenite.

Effects of Austenitization Temperature and Compressive Stress During Bainitic Transformation on the Stability of Retained Austenite

The effects of stress and austenitization temperature on the carbon content and morphology of retained austenite (RA) have been investigated. The stability of RA has been analyzed. An interesting finding is that the increase in the amount of bainite is not accompanied by the increase in the carbon content in RA under the effect of stress. This does not match with what is expected from the bainitic transformation theory. The amount of bainitic transformation and stress both affect the carbon content in RA. In addition, the stress during bainitic transformation helps to increase the stability of RA by decreasing the amount of blocky RA, whereas it has little effect on the carbon content in RA. Moreover, the increase in austenitization temperature is beneficial to increasing the stability of RA, whereas it has no significant effect on the morphology of RA.

Effect of Holding Temperature on Microstructure and Mechanical Properties of High‐Strength Multiphase Steel

Isothermal holding following intercritical annealing is usually used in microstructure control, e.g., fractions and stabilities of retained‐austenite (RA). Fe–0.22C–2.5Mn–0.47Si–0.41Cr–0.02Nb (mass%) steel is subjected to intercritical annealing and isothermal treatment at 250, 300, 350, and 400 °C to elucidate the impact on microstructures and mechanical properties by means of electron microscopy and uniaxial tensile test, respectively. The results show that the isothermal holding temperature is vital for the formed phases, including the morphology, volume fraction, and carbon content of RA in the processed steels. The tensile test results indicate that the mechanical properties including Ultra‐tensile strength (UTS), Yield strength (YS), as well as Total Elongation (TEL) are attributed to the synthetic action of all constituents of phase morphology and corresponding fractions, e.g., hard‐to‐soft phase ratio, morphology and fraction of RA, dispersed precipitates. An excellent combination of strength–ductility of the present multiphase steel has been explained in terms of their specific microstructure.

Effects of retained austenite volume fraction, morphology, and carbon content on strength and ductility of nanostructured TRIP-assisted steels

With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C–0.30Si–1.76Mn–1.52Al (weight percent, wt%). After intercritical annealing and bainitic holding, a combination of ultimate tensile strength (UTS) of 1100MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including the magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governing microstructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels.

Microstructure and Mechanical Properties of a Low-Carbon Mn-Si Multiphase Steel Based on Dynamic Transformation of Undercooled Austenite

The microstructure evolution of 0.20C-2.00Mn-2.00Si steel treated by the thermomechanical process to manufacture hot-rolled, transformation-induced plasticity (TRIP) steel based on dynamic transformation of undercooled austenite was investigated using a Gleeble 1500 (Dynamic Systems, Inc., Poestenkill, NY) hot simulation test machine in combination with light microscope (LM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The mechanical properties of this steel with different multiphase microstructures were also analyzed using room-temperature tensile tests. The results indicated that the multi-phase microstructures consisting of fine-grained ferrite with a size of 1–3 μm, bainite packets, and retained austenite and martensite were formed for the used steel by a thermo-mechanical process involving dynamic transformation of undercooled austenite, controlled cooling, isothermal bainite treatment and water-quenching. With the increase in the strain of hot deformation of undercooled austenite, the fraction of ferrite increased, that of bainite decreased, and that of martensite increased. At the same time, the fraction of retained austenite (RA), as well as the carbon content of RA, first increased and then decreased. For the used steel treated by such process, the tensile strength is about 1200 MPa with a total elongation of about 20 pct, and the product of tensile strength and total elongation can be up to 25,000 MPa × pct.

Quantitative assessment of the effects of microstructure on the stability of retained austenite in TRIP steels

The stability of retained austenite (RA) in transformation-induced plasticity steels (TRIP) is affected by many factors, including chemical composition, RA grain size, neighboring microconstituents of RA and temperature. The composition and microstructure factors are interrelated, so it is difficult to separate out the influence of each individually. In this investigation, methods were developed to study the effects of RA grain size and neighboring microconstituents in a silicon-alloyed low-carbon (0.2C–1.6Si–1.6Mn) TRIP steel. Uniaxial tensile tests, performed at −20°C, room temperature (20°C) and 40°C, were interrupted at several strain levels. Scanning electron micrographs were obtained from each condition, and RA was quantified using a novel technique called the categorical chord-length distribution (CCLD), which enables microstructural quantification based on specific neighboring microconstituents. The results show that RA adjacent to bainitic ferrite (BF) and fine RA grain size are correlated with higher RA stability. A modified Burke–Matsumura–Tsuchida stability model was developed to kinetically analyze the effects of microstructure on RA transformation. The CCLD and kinetic modeling analysis indicates that the stability of RA inside BF is less sensitive to the testing temperature than RA inside polygonal ferrite (PF), and thermodynamic analysis of the driving force for transformation as a function of temperature and carbon content implies that there is a higher carbon content in RA inside BF. Additionally, nanohardness tests showed that the hardnesses of BF and PF are not significantly different after moderate amounts of deformation. Thus, the enhanced stability of RA inside BF compared to PF is more strongly related to the elevated carbon content of RA inside BF rather than stress partitioning differences for RA adjacent to BF or PF.