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.     

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).

     

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.     

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.     

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.     

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.     

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.     

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.

     

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.     

Q&P工艺研究现状

 介绍一种马氏体钢淬火＋分配（Quenching and Partitioning）的热处理工艺即钢经奥氏体化后淬火到Ms-Mf间的某一温度形成一定量的马氏体和未转变奥氏体,然后在这个温度或高于此温度保温,使马氏体中的碳扩散至未转变奥氏体使之稳定化,最后淬火至室温得到由马氏体和残余奥氏体组成的混合组织,可以改善钢的性能。本文就该工艺的提出、热力学、动力学、试验参数对残余奥氏体的量和形态的影响以及力学性能等方面进行概述。     

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。     

On Various Aspects of Decomposition of Austenite in a High-Silicon Steel During Quenching and Partitioning

Using a Gleeble thermomechanical simulator, a high-silicon steel (Fe-0.2C-1.5Si-2.0Mn-0.6Cr) was laboratory hot-rolled, re-austenitized, quenched into the M _s–M _f range, retaining 15 to 40 pct austenite at the quench stop temperature (T _Q), and annealed for 10 to 1000 seconds at or above T _Q in order to better understand the mechanisms operating during partitioning. Dilatometer measurements, transmission electron microscopy, and calculations showed that besides carbon partitioning, isothermal martensite and bainite form at the partitioning temperature. While isothermal martensite formation starts almost immediately after quenching with the rate of volume expansion dropping all the time, the beginning of bainite formation is marked by a sudden increase in the rate of expansion. The extent of its formation depends on the partitioning temperature following TTT diagram predictions. At the highest partitioning temperatures martensite tempering competes with partitioning. Small fractions of bainite and high-carbon martensite formed on cooling from the partitioning temperature. The average carbon content of the austenite retained at room temperature as determined from XRD measurements was close to the carbon content estimated from the M _s temperature of the martensite formed during the final cooling.     

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.     

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.

     

Resolving the Martensitic Transformation in Q&P Steels In-Situ at Dynamic Strain Rates Using Synchrotron X-ray Diffraction

The dynamic deformation response of two quenching and partitioning (Q&P) steels was investigated using a high strain rate tension pressure bar and in-situ synchrotron radiography and diffraction. This allowed for concurrent measurements of the martensitic transformation, the elastic strains/stresses on the martensite and ferrite, and the bulk mechanical behavior. The steel with the greater fraction of ferrite exhibited greater ductility and lower strength, suggesting that dislocation slip in ferrite enhanced the deformability. Meanwhile, the kinetics of the martensitic transformation appeared similar for both steels, although the steel with a greater ferrite fraction retained more austenite in the neck after fracture.

     

In situ X-Ray Diffraction Analysis of Carbon Partitioning During Quenching of Low Carbon Steel

In situ X-ray diffraction investigations of phase transformations during quenching of low carbon steel were performed at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) at beamline ID11. A dynamic stabilization of the retained austenite during cooling below martensite start was identified, resulting in an amount of retained austenite of approximately 4?vol pct. The reason for this dynamic stabilization is a carbon partitioning occurring directly during quenching from martensite (and a small amount of bainite) into retained austenite. A carbon content above 0.5?mass pct was determined in the retained austenite, while the nominal carbon content of the steel was 0.2?mass pct. The martensitic transformation kinetic was compared with the models of Koistinen-Marburger and a modification proposed by Wildau. The analysis revealed that the Koistinen-Marburger equation does not provide reliable kinetic modeling for the described experiments, while the modification of Wildau well describes the transformation kinetic.     

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.     

双相区形变对含Cu低碳钢Cu配分行为及组织性能影响

采用双相区形变+IQP及IQP(双相区等温-奥氏体化-淬火-碳配分)热处理工艺,研究了双相区形变对一种含Cu低碳钢Cu配分行为及其组织性能的影响。采用电子探针(EPMA)、扫描电镜(SEM)及透射电镜(TEM)等手段对元素配分行为及组织演变进行了表征。结果表明:实验钢经2种工艺处理后均出现Cu元素向逆转奥氏体的配分行为,采用双相区形变+IQ(双相区保温淬火)处理的组织中富Cu最高的区域面积为12.9%,比IQ工艺下富Cu区域提高108%;双相区形变+IQP工艺处理后实验钢的晶粒明显细化,且组织中块状残余奥氏体较多;与单一IQP工艺相比,双相区形变+IQP工艺处理的实验钢抗拉强度由1 253MPa提高到1 293MPa,伸长率由16.9%提高到18.3%,残余奥氏体体积分数由11.6%提高到13.8%,表明双相区30%的形变处理实现了促进Cu配分行为诱导残余奥氏体含量增加和细晶强化的双重效果。     

Microstructual Evolution of a Nb‐Microalloyed Advanced High Strength Steel Treated by Quenching‐Partitioning‐Tempering Process

The recently developed “quenching and partitioning” heat treatment and “quenching‐partitioning‐tempering” heat treatment are novel processing technologies, which are designed for achieving advanced high strength steels (AHSS) with combination of high strength and adequate ductility. Containing adequate amount of austenite phase is an important characteristic of the above steel, and the partitioning treatment is a key step in Q&P or Q‐P‐T process during which the austenite phase is enriched with carbon and achieves thermal stability. However, the microstructural evolution of the steel during the partitioning process is rather complicated. In present study, evolution of complex microstructure in a low carbon steel containing Nb during the Q‐P‐T process has been studied in detail. The microstructural evolution of the steel was investigated in terms of X‐ray diffraction, scanning electron microscope and transmission electron microscope. The experimental results show that the Nb‐microalloyed steel demonstrates a complex multiphase microstructure which consists of lath martensite with high dislocation density, retained austenite, alloy carbide, transition carbide, and a few twin martensite after the Q‐P‐T process. The experimental results can be helpful for the design of Q‐P‐T heat treatment and for the control of mechanical properties of Q‐P‐T steel.