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.     

Evaluation of Carbon Partitioning in New Generation of Quench and Partitioning (Q&P) Steels

Quenching and partitioning (Q&P) and a novel combined process of hot straining (HS) and Q&P (HSQ&P) treatments have been applied to a TRIP-assisted steel in a Gleeble®3S50 thermomechanical simulator. The heat treatments involved intercritical annealing at 800 °C and a two-step Q&P heat treatment with a partitioning time of 100 seconds at 400 °C. The “optimum” quench temperature of 318 °C was selected according to the constrained carbon equilibrium (CCE) criterion. The effects of high-temperature deformation (isothermal and non-isothermal) on the carbon enrichment of austenite, carbide formation, and the strain-induced transformation to ferrite (SIT) mechanism were investigated. Carbon partitioning from supersaturated martensite into austenite and carbide precipitation were confirmed by means of atom probe tomography (APT) and scanning transmission electron microscopy (STEM). Austenite carbon enrichment was clearly observed in all specimens, and in the HSQ&P samples, it was significantly greater than in Q&P, suggesting an additional carbon partitioning to austenite from ferrite formed by the deformation-induced austenite-to-ferrite transformation (DIFT) phenomenon. By APT, the carbon accumulation at austenite/martensite interfaces was observed, with higher values for HSQ&P deformed isothermally (≈ 11 at. pct), when compared with non-isothermal HSQ&P (≈ 9.45 at. pct) and Q&P (≈ 7.6 at. pct). Moreover, a local Mn enrichment was observed in a ferrite/austenite interface, indicating ferrite growth under local equilibrium with negligible partitioning (LENP).

     

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 “T_o” threshold.     

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.     

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.     

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.     

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.

     

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.     

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.

     

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.     

Investigation into Competitive Mechanism of Carbon Partitioning and Pseudospinodal Decomposition of Supersaturated Martensite in Q&P Steel Containing High Silicon

In this study, the competing mechanisms of carbon partitioning and concurrent pseudospinodal decomposition of supersaturated martensite, forming superlattice-ordered α″-Fe₁₆C₂, are elucidated in a quenching and partitioning (Q&P) steel containing high silicon based on various microstructural characterizations. Our results demonstrate that the fluctuation of carbon content caused by high-density dislocations and transformation residual stress in martensite may stimulate the pseudospinodal decomposition. Furthermore, the sluggish diffusion kinetics of silicon and nickel inhibits further transformation from α″-Fe₁₆C₂ to carbide precipitation. The experimental results provide new insights into the pseudospinodal decomposition and carbon redistribution mechanism during the carbon partitioning process.

     

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.     

Austenite Stability and Strain Hardening in C-Mn-Si Quenching and Partitioning Steels

Metallurgical and Materials Transactions A - Quenching and partitioning (Q&amp;P) processing of third-generation advanced high strength steels generates multiphase microstructures containing...     

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

Quench and Partitioning Steel: A New AHSS Concept for Automotive Anti‐Intrusion Applications

A new type of high strength, high toughness, martensitic steel, based on a newly proposed Quench and Partitioning (Q&P) process, is presented. This high strength martensitic grade is produced by the controlled low temperature partitioning of carbon from as‐quenched martensite laths to retained inter‐lath austenite under conditions where both low temperature transition carbide formation and cementite precipitation are suppressed. The contribution focuses on both the current understanding of the fundamental processes involved and includes a discussion of the technical feasibility of large‐scale industrial production of these steels as sheet products. The Q&P process, which is carried out on steels with a lean composition, should be implemented easily on some current industrial continuous annealing and galvanizing lines. In addition, martensitic Q&P sheet steel is characterized by very favourable combinations of strength, ductility and toughness, which are particularly relevant for high strength anti‐intrusion automotive parts.     

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.     

Kinetic Transition During Ferrite Growth Induced by Interfacial Solute Segregation in an Fe–C–Mn–Si Alloy

The kinetic transition of partitionless proeutectoid ferrite transformation from austenite, experimentally reported earlier in an Fe–C–Mn–Si alloy, is simulated incorporating interfacial segregation of carbon and alloy elements. The time-dependent diffusion equations of solutes are solved within the α/γ interface to evaluate the transient effects of solute accumulation on the migration of interface. The carbon concentration at the interface in the matrix decreased faster and the interface migration ceased, or the so-called stasis occurred, when the carbon concentration gradient in the immediate front of the interface turned to null or reversed. This can happen earlier than the partitionless-to-partitioned growth transition predicted from conventional theory in the absence of interfacial segregation, depending upon austenite grain size, i.e., the extent of soft impingement of carbon diffusion fields in the matrix in which a large carbon supersaturation remained. The subsequent transformation may be resumed accompanying the bulk partitioning of Mn (and probably Si) and/or nucleation of new ferrite crystals.

     

淬火-贝氏体区配分工艺及钢的组织性能

 Q&P（Quenching and Partitioning, 淬火配分）工艺在CCE条件下,通过采用[Ms]和[Mf]点之间的最佳淬火温度和低于[Ms]点的配分温度,避免配分阶段的贝氏体形成最终可以得到最高含量的残余奥氏体组织。但试验中得到不足体积分数8%的残余奥氏体含量限制了钢塑性的提高。通过提出淬火-贝氏体区配分工艺,并应用在(0.21～0.29)C-(1.5~2.0)Si-(1.5～2.1)Mn成分钢,得到了体积分数12%左右的残余奥氏体含量和25%左右的伸长率,同时强度保持在1 000～1 100 MPa,强塑积最高达到36.6 GPa·%。不同的淬火温度和配分温度试验结果表明,工艺变化对强度影响较低,伸长率和强塑积随着配分温度的提高而提高,其中270 ℃的淬火温度试样的提高幅度高于245 ℃淬火试样,采用Q&PB工艺得到了无碳贝氏体＋马氏体＋残余奥氏体的三相组织。淬火和贝氏体区配分得到了优异的强度和塑性的结合,为新一代汽车用钢的发展提供新的思路。     

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.