Microstructure Evolution and Mechanical Behavior of a CMnSiAl TRIP Steel Subjected to Partial Austenitization Along with Quenching and Partitioning Treatment

The present study investigated the microstructure evolution and mechanical behavior in a low carbon CMnSiAl transformation-induced plasticity (TRIP) steel, which was subjected to a partial austenitization at 1183 K (910 °C) followed by one-step quenching and partitioning (Q&P) treatment at different isothermal holding temperatures of [533 K to 593 K (260 °C to 320 °C)]. This thermal treatment led to the formation of a multi-phase microstructure consisting of ferrite, tempered martensite, bainitic ferrite, fresh martensite, and retained austenite, offering a superior work-hardening behavior compared with the dual-phase microstructure (i.e., ferrite and martensite) formed after partial austenitization followed by water quenching. The carbon enrichment in retained austenite was related to not only the carbon partitioning during the isothermal holding process, but also the carbon enrichment during the partial austenitization and rapid cooling processes, which has broadened our knowledge of carbon partitioning mechanism in conventional Q&P process.     

Development of Multiphase Microstructure with Bainite, Martensite, and Retained Austenite in a Co-Containing Steel Through Quenching and Partitioning (Q&P) Treatment

The quenching and partitioning (Q&P) treatment of steel aims to produce a higher fraction of retained austenite by carbon partitioning from supersaturated martensite. Q&P studies done so far, relies on the basic concept of suppression of carbide formation by the addition of Si and/or Al. In the present study Q&P treatment is performed on a steel containing 0.32 C, 1.78 Mn, 0.64 Si, 1.75 Al, and 1.20 Co (all wt pct). A combination of 0.64 Si and 1.75 Al is chosen to suppress the carbide precipitation and therefore, to achieve carbon partitioning after quenching. Addition of Co along with Al is expected to accelerate the bainite transformation during Q&P treatment by increasing the driving force for transformation. The final aim is to develop a multiphase microstructure containing bainite, martensite, and the retained austenite and to study the effect of processing parameters (especially, quenching temperature and homogenization time) on the fraction and stability of retained austenite. A higher fraction of retained austenite (~13 pct) has indeed been achieved by Q&P treatment, compared to that obtained after direct-quenching (2.7 pct) or isothermal bainitic transformation (9.7 pct). Carbon partitioning during martensitic and bainitic transformations increased the stability of retained austenite.     

Overview of Mechanisms Involved During the Quenching and Partitioning Process in Steels

The application of the quenching and partitioning (Q&P) process in steels involves a microstructural evolution that is more complex than just the formation of martensite followed by carbon partitioning from martensite to austenite. Examples of this complexity are the formation of epitaxial ferrite during the first quenching step and the formation of bainite, carbides, and carbon gradients as well as migration of martensite/austenite interfaces during the partitioning step. In this work, recent investigations on the mechanisms controlling microstructural changes during the application of the Q&P process are evaluated, leading to phase-formation based concepts for the design of Q&P steels.     

Effect of Interfacial Mn Partitioning on Carbon Partitioning and Interface Migration During the Quenching and Partitioning Process

A so-called QP-LE model, in which interface condition is assumed to be Local Equilibrium (LE), has been proposed to evaluate the effect of interfacial Mn partitioning on interface migration and carbon partitioning during the Quenching and partitioning process (Q&P) of an Fe-0.3C-3.0Mn-1.5Si (wt pct) alloy. The predictions by the QP-LE model are compared with those by the conventional QP-PE model in which interface condition is assumed to be Paraequilibrium (PE). It is found that interfacial partitioning of Mn plays a significant role in carbon partitioning and the martensite/austenite interface migration during the Q&P process.     

Application of Quenching and Partitioning (Q&P) Processing to Press Hardening Steel

Press hardening steel (PHS) has been increasingly used for the manufacture of structural automotive parts in recent years. One of the most critical characteristics of PHS is a low residual ductility related to a martensitic microstructure. The present work proposes the application of quenching and partitioning (Q&P) processing to improve the ductility of PHS. Q&P processing was applied to a Si- and Cr-added Q&P-compatible PHS, leading to a press hardened microstructure consisting of a tempered martensite matrix containing carbide-free bainite and retained austenite. The simultaneous addition of Si and Cr was used to increase the retained austenite fraction in the Q&P-compatible PHS. The Q&P processing of the PHS resulted in a high volume fraction of C-enriched retained austenite, and excellent mechanical properties. After a quench at 543 K (270 °C) and a partition treatment at 673 K (400 °C), the PHS microstructure contained a high volume fraction of retained austenite and a total elongation (TE) of 17 pct was achieved. The yield strength (YS) and the tensile strength were 1098 and 1320 MPa, respectively. The considerable improvement of the ductility of the Q&P-compatible PHS should lead to an improved in-service ductility beneficial to the passive safety of vehicle passengers.     

Quenching and Partitioning (Q&P) Processing of Martensitic Stainless Steels

Austenite was stabilized in the martensitic stainless steel grade AISI 420 by means of quenching and partitioning (Q&P) processing. The effects of quenching temperature on the microstructure and mechanical properties were investigated. The specimens processed at low quench temperatures (regime I) had a microstructure consisting of tempered martensite and retained austenite. At high quench temperatures (regime II), fresh martensite was present too. The highest austenite fraction of about 0.35 was obtained at the quench temperature delineating regimes I and II. The amount of carbon in retained austenite increased as the quench temperature decreased. The carbon level of austenite was, however, much lower than the carbon concentrations expected from full partitioning assumption. This was mainly due to the extensive cementite formation in the partitioning step. Stabilization of austenite by Q&P processing was found not to be purely chemical. Austenite stabilization was also assisted by locking, because of local carbon enrichment, of potential martensite nucleation sites in the austenite/martensite boundaries and in austenite defects. The importance of the latter stabilization mechanism increased at higher martensite fractions. According to the tensile test results, the Q&P processed specimen with the highest austenite fraction was not associated with the best combination of strength and ductility. The mechanical stability of austenite was found to increase with its carbon concentration being the highest at the lowest quench temperature. The thermal stability, on the other hand, was almost inversely proportional to the retained austenite fraction, being low at intermediate quench temperatures where the retained austenite fraction was high.     

Effect of Retained Austenite on the Fracture Toughness of Quenching and Partitioning (Q&P)-Treated Sheet Steels

Fracture toughness K _IC was measured by double edge-notched tension (DENT) specimens with fatigue precracks on quenching and partitioning (Q&P)-treated high-strength (ultimate tensile strength [UTS] superior to 1200 MPa) sheet steels consisting of 4 to 10 vol pct of retained austenite. Crack extension force, G _IC, evaluated from the measured K _IC, is used to analyze the role of retained austenite in different fracture behavior. Meanwhile, G _IC is deduced by a constructed model based on energy absorption by martensite transformation (MT) behavior of retained austenite in Q&P-treated steels. The tendency of the change of two results is in good agreement. The Q&P-treated steel, quenched at 573 K (300 °C), then partitioned at 573 K (300 °C), holding for 60 seconds, has a fracture toughness of 74.1 MPa·m^1/2, which is 32 pct higher than quenching and tempering steel (55.9 MPa·m^1/2), and 16 pct higher than quenching and austempering (QAT) steel (63.8 MPa·m^1/2). MT is found to occur preferentially at the tips of extension cracks on less stable retained austenite, which further improves the toughness of Q&P steels; on the contrary, the MT that occurs at more stable retained austenite has a detrimental effect on toughness.     

Influence of Ni and Process Parameters in Medium Mn Steels Heat Treated by High Partitioning Temperature Q&P Cycles

In this work, two medium Mn steels (5.8 and 5.7 wt pct Mn) were subjected to a quenching and partitioning (Q&P) treatment employing a partitioning temperature which corresponded to the start of austenite reverse transformation (ART). The influence of a 1.6 wt pct Ni addition in one of the steels and cycle parameters on austenite stability and mechanical properties was also studied. High contents of retained austenite were obtained in the lower quenching temperature (QT) condition, which at the same time resulted in a finer microstructure. The addition of Ni was effective in stabilizing higher contents of austenite. The partitioning of Mn and Ni from martensite into austenite was observed by TEM–EDS. The partitioning behaviour of Mn depended on the QT condition. The lower QT condition facilitated Mn enrichment of austenite laths during partitioning and stabilization of a higher content of austenite. The medium Mn steel containing Ni showed outstanding values of the product of tensile strength (TS) and total elongation (TEL) in the lower QT condition and a higher mechanical stability of the austenite.

     

Impact of Si on Microstructure and Mechanical Properties of 22MnB5 Hot Stamping Steel Treated by Quenching & Partitioning (Q&P)

Based on 22MnB5 hot stamping steel, three model alloys containing 0.5, 0.8, and 1.5 wt pct Si were produced, heat treated by quenching and partitioning (Q&P), and characterized. Aided by DICTRA calculations, the thermal Q&P cycles were designed to fit into industrial hot stamping by keeping partitioning times ≤ 30 seconds. As expected, Si increased the amount of retained austenite (RA) stabilized after final cooling. However, for the intermediate Si alloy the heat treatment exerted a particularly pronounced influence with an RA content three times as high for the one-step process compared to the two-step process. It appeared that 0.8 wt pct Si sufficed to suppress direct cementite formation from within martensite laths but did not sufficiently stabilize carbon-soaked RA at higher temperatures. Tensile and bending tests showed strongly diverging effects of austenite on ductility. Total elongation improved consistently with increasing RA content independently from its carbon content. In contrast, the bending angle was not impacted by high-carbon RA but deteriorated almost linearly with the amount of low-carbon RA.     

Benefits of Intercritical Annealing in Quenching and Partitioning Steel

Compared to the quenching and partitioning (Q&P) steel produced by full austenization annealing, the Q&P steel produced by the intercritical annealing shows a similar ultimate tensile stress but a larger tensile ductility. This property is attributable to the higher volume fraction and the better mechanical stability of the retained austenite after the intercritical annealing. Moreover, intercritical annealing produces more ferrite and fewer martensite phases in the microstructure, making an additional contribution to a higher work hardening rate and therefore a better tensile ductility.     

Nanoscale Analyses of High-Nickel Concentration Martensitic High-Strength Steels

Austenite reversion in martensitic steels is known to improve fracture toughness. This research focuses on characterizing mechanical properties and the microstructure of low-carbon, high-nickel steels containing 4.5 and 10 wt pct Ni after a QLT-type austenite reversion heat treatment: first, martensite is formed by quenching (Q) from a temperature in the single-phase austenite field, then austenite is precipitated by annealing in the upper part of the intercritical region in a lamellarization step (L), followed by a tempering (T) step at lower temperatures. For the 10 wt pct Ni steel, the tensile strength after the QLT heat treatment is 910 MPa (132 ksi) at 293 K (20 °C), and the Charpy V-notch impact toughness is 144 J (106 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). For the 4.5 wt pct Ni steel, the tensile strength is 731 MPa (106 ksi) at 293 K (20 °C) and the impact toughness is 209 J (154 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). Light optical microscopy, scanning electron and transmission electron microscopies, synchrotron X-ray diffraction, and local-electrode atom-probe tomography (APT) are utilized to determine the morphologies, volume fractions, and local chemical compositions of the precipitated phases with sub-nanometer spatial resolution. The austenite lamellae are up to 200 nm in thickness, and up to several micrometers in length. In addition to the expected partitioning of Ni to austenite, APT reveals a substantial segregation of Ni at the austenite/martensite interface with concentration maxima of 10 and 23 wt pct Ni for the austenite lamellae in the 4.5 and 10 wt pct Ni steels, respectively. Copper-rich and M₂C-type metal carbide precipitates were detected both at the austenite/martensite interface and within the bulk of the austenite lamellae. Thermodynamic phase stability, equilibrium compositions, and volume fractions are discussed in the context of Thermo-Calc calculations.     

Analysis of Microstructure Evolution in Quenching and Partitioning Automotive Sheet Steel

Extensive research efforts are underway globally to develop new steel microstructure concepts for high-strength sheet products, driven largely by the need for lightweight automotive structures in support of designs to enhance occupant safety and energy efficiency. One promising approach, involving the quenching and partitioning (Q&P) process, was introduced in the predecessor to this paper series, Austenite Formation and Decomposition, 2003.[1] Development of the Q&P process has continued through to the present, and the current status is highlighted in this article, along with some alternative approaches that are also receiving attention. Special emphasis is placed on the synthesis and interpretation of the fundamental phase transformation responses, perspectives related to alloying and processing, and the resulting microstructure and properties. Key mechanistic issues are discussed, including carbide formation and suppression, migration of the martensite/austenite interface, carbon partitioning, and partitioning kinetics.     

Quenched and Partitioned Microstructures Produced <Emphasis Type="Italic">via</Emphasis> Gleeble Simulations of Hot-Strip Mill Cooling Practices

Previous researchers reported on quenched and partitioned (Q&P) microstructures produced via carbon partitioning from martensite into austenite during isothermal annealing after quenching to develop a partially martensitic initial structure. However, the thermal profile used in previous studies is not well suited to creating Q&P microstructures directly from a hot-strip mill. In this work, the commonly employed Q&P thermal profile (i.e., having an isothermal partitioning step) was modified to evaluate nonisothermal partitioning that might instead occur during cooling of a wound coil. Thus, it was possible to assess the potential for creating Q&P microstructures and properties directly off of the hot mill. Gleeble thermal simulations representative of a hot-strip mill cooling practice were used to create dual-phase, Q&P, transformation-induced plasticity (TRIP), and conventional microstructures by varying the quench/coiling temperatures (CTs) using a 0.19C-1.59Mn-1.63Si (wt pct) steel. Microstructural and mechanical property data indicate that hot rolling might be a viable processing route for high-strength Q&P steels.     

Microstructure-strength relations in a hardenable stainless steel with 16 pct Cr, 1.5 pct Mo,and 5 pct Ni

Metallographic studies have been conducted on a 0.024 pct C-16 pct Cr-1.5 pct Mo-5 pct Ni stainless steel to study the phase reactions associated with heat treatments and investigate the strengthening mechanisms of the steel. In the normalized condition, air cooled from 1010 °C, the microstructure consists of 20 pct ferrite and 80 pct martensite. Tempering in a temperature range between 500 and 600 °C results in a gradual transformation of martensite to a fine mixture of ferrite and austenite. At higher tempering temperatures, between 600 and 800 °C, progressively larger quantities of austenite form and are converted during cooling to proportionally increasing amounts of fresh martensite. The amount of retained austenite in the microstructure is reduced to zero at 800 °C, and the microstructure contains 65 pct re-formed martensite and 35 pct total ferrite. Chromium rich M₂₃C₆ carbides precipitate in the single tempered microstructures. The principal strengthening is produced by the presence of martensite in the microstructure. Additional strengthening is provided by a second tempering treatment at 400 °C due to the precipitation of ultrafine (Cr, Mo) (C,N) particles in the ferrite.     

An investigation of the high-temperature and solidification microstructures of PH 13-8 Mo stainless steel

Differential thermal analysis (DTA), high-temperature water-quench (WQ) experiments, and optical and electron microscopy were used to establish the near-solidus and solidification microstructures in PH 13-8 Mo. On heating at a rate of 0. 33 °C/s, this alloy begins to transform from austenite to δ-ferrite at ≈1350 °C. Transformation is complete by ≈1435 °C. The solidus is reached at ≈1447 °C, and the liquidus is ≈1493 °C. On cooling from the liquid state at a rate of 0. 33 °C/s, solidification is completed as δ-ferrite with subsequent transformation to austenite beginning in the solid state at ≈1364 °C. Insufficient time at temperature is available for complete transformation and the resulting room-temperature microstructure consists of matrix martensite (derived from the shear decomposition of the austenite) and residual δ-ferrite. The residual δ-ferrite in the DTA sample is enriched in Cr (≈16 wt pct), Mo (≈4 wt pct), and Al (≈1. 5 wt pct) and depleted in Ni (≈4 wt pct) relative to the martensite (≈12. 5 wt pct Cr, ≈2 wt pct Mo, ≈1 wt pct Al, ≈9 wt pct Ni). Solid-state transformation of δσ γ was found to be quench-rate sensitive with large grain, fully ferritic microstructures undergoing a massive transformation as a result of water quenching, while a diffusionally controlled Widmanstätten structure was produced in air-cooled samples.     

Aging and tempering behavior of iron-nickel-carbon and iron-carbon martensite

The aging at room temperature (RT) and the tempering behavior in the temperature range 293 to 973 K of ternary iron-nickel-carbon martensite (containing 14.4 at. pct Ni and 2.35 at. pct C) was investigated principally by using X-ray diffractometry to analyze changes in the crystalline structure and differential scanning calorimetry to determine heats of transformation and activation energies. These techniques also were used in the parallel study performed in this work of the tempering behavior of FeC martensite (containing about 4.4 at. pct C) in the temperature range 298 to 773 K. Analysis of the structural changes revealed that in both FeNiC and FeC the following processes occurred: (1) formation of carbon enrichments and development of a periodic arrangement of planar carbon-rich regions up to 423 K; (2) precipitation of ε/η transition carbide and transformation of a part of the austenite into ferrite under simultaneous enrichment with carbon of the remaining austenite (between 423 and 523 K); (3) decomposition of the retained austenite into ferrite and cementite between 523 and 723 K (only partly for FeNiC); (4) precipitation of cementite between 523 and 723 K; and (5) for FeNiC, reformation of austenite from ferrite and cementite above 773 K. A short comparative discussion concerning the first stage of martensite decomposition for FeC, FeNiC, FeN, and FeNiN martensites is given.     

Toughness Improvement in a Novel Martensitic Stainless Steel Achieved by Quenching–Tempering and Partitioning

In the present work, a novel medium carbon martensitic stainless steel (MCMSS) with an excellent combination of strength, ductility, and impact toughness was designed on the basis of quenching-tempering and partitioning (Q–T&P) technology. Q–T&P is an identical heat treatment with a standard quenching and tempering (Q–T) process but has the same role with quenching and partitioning (Q&P) on microstructure control, i.e., promoting carbon-rich retained austenite via inhibiting carbide precipitation. Results show that, without compromise on strength, the total elongation and room temperature impact toughness, i.e., 9.6 pct and 90 J cm⁻², of the proposed alloy (23Cr13MnSi) increase by 14 and 110 pct, respectively, as compared to those of the commercial AISI 420. The significant improvement of ductility and impact toughness in the proposed alloy is mainly a result of the gradual transformation induced plasticity (TRIP) effects, which are caused by carbon-rich retained austenite with heterogeneous stability and carbide-free martensite formed in the Q–T&P process.

     

Enhancement of the Mechanical Properties of a V–Ti–N Microalloyed Steel Treated by a Novel Precipitation-Quenching & Partitioning Process

In this study, a novel precipitation-quenching & partitioning (P-Q&P) process was proposed by combining a proper intermediate holding treatment with the Q&P process, which successfully increased the strength of a V–Ti–N microalloyed steel without sacrificing the plasticity. However, the impact toughness of the P-Q&P samples is lower than that of the Q&P sample. Compared to the Q&P sample, the P-Q&P samples have more retained austenite. In addition, coarser substructures of martensite and bainite were formed in the P-Q&P samples. All the P-Q&P and Q&P samples contain two types of carbonitrides, which are the large-size particles (enriched in Ti) formed or undissolved in austenite and the small-size particles (enriched in V) formed in martensite and bainite. The P-Q&P samples have a smaller size and larger volume fraction of the large-size particles than the Q&P sample. The increase in the strength of the P-Q&P samples is attributed to the precipitation strengthening of the carbonitrides formed in austenite during the intermediate holding treatment. And the maintained elongation is mainly caused by the higher austenite content in the P-Q&P samples. The poor toughness of the P-Q&P samples is mainly resulted from the coarser substructures.