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.     

Interaction of Hydrogen and Retained Austenite in Bainite／Martensite Dual-Phase High Strength Steel

SymbolList　　DA———Apparentdiffusioncoefficient;　　DL———Latticediffusioncoefficient;　　EaD———Diffusionactivationenergyofhydrogeninnormallattice ;　　EaT———Trapactivationenergy ;　　EB———Trapbindingenergy ;　　ES———Saddlepointenergy ;     

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.     

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。     

淬火-配分马氏体钢的组织特征

淬火-配分(Quenching and Partitioning,简称QP)工艺是针对马氏体钢提出的热处理新工艺。利用QP工艺处理40Si2Ni2钢,并通过扫描电镜观察其微观组织特征。结果表明,与传统淬火+回火工艺得到的组织不同,QP组织为低碳(回火态)和高碳马氏体(淬火态)共存,其中高碳马氏体呈现为有规则几何形状、边界清晰、无析出物析出的块状组织,淬火温度(QT)对高碳马氏体(淬火态)量有影响。     

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.     

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.     

Anisotropy of Retained Austenite Stability during Transformation to Martensite in a TRIP‐Assisted Steel

Retained austenite as a key constituent in final microstructure plays an important role in TRansformation Induced Plasticity (TRIP) steels. The volume fraction, carbon concentration, size, and morphology of this phase are the well‐known parameters which effects on the rate of transformation of retained austenite to martensite and the properties of steel, are studied by many researchers. Of the transformation of retained austenite to martensite under strain in a TRIP steel is studied in this paper. The experimental results show that the transformation rate of retained, austenite with similar characteristics, to martensite in differently processed TRIP steel samples, exhibits an anisotropic behavior. This phenomenon implies a kind of variant selection of martensitic reaction of retained austenite under strain and is explained by ferrite texture developed in steel.     

高铝TRIP钢的微观组织与残余奥氏体稳定性研究

主要研究了高Al TRIP钢的显微组织与残余奥氏体的稳定性。通过光学显微镜、SEM、TEM观察了其微观组织。通过TEM观察了钢中马氏体与贝氏体的形貌。通过电子衍射斑分析,得出了残余奥氏体与马氏体的位向关系为K-S位向关系,奥氏体母相与贝氏体的位向关系为N-W位向关系。为研究残余奥氏体机械稳定性,对试验用钢进行了不同应变量的单向拉伸,用X射线测量了残余奥氏体体积分数。结果表明,真应变小于0.11时残余奥氏体体积分数随应变量增加而减少。真应变量大于0.11后,残余奥氏体体积分数随应变量增加变化不大。为了研究残余奥氏体热稳定性,将试验用钢冷却至不同的温度。发现高Al TRIP钢残余奥氏体热稳定性很高,深冷至-196℃条件下不发生马氏体转变。     

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.

     

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.     

Characterization of the Factors Influencing Retained Austenite Stability in Q&P Steels via In Situ EBSD

Metallurgical and Materials Transactions A - The current work studies the correlations between microstructure and retained austenite (RA) transformation, in a single-quenched and partitioned...     

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).

     

A Finite-Element Approach for the Partitioning of Carbon in Q&P Steel

Metallurgical and Materials Transactions B - This paper reports a Galerkin finite-element analysis of carbon partitioning from martensite into austenite during the quenching and partitioning...     

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.     

1 500 MPa级经济型贝氏体/马氏体复相钢的组织与性能

在清华大学贝氏体研究及推广中心以往工作的基础上,设计了一种新的1500MPa级经济型贝氏体／马氏体复相钢,该钢奥氏体化后空冷,就可获得具有较复杂精细结构的无碳化物贝氏体／马氏体复相组织,经低温回火后,其σb≥1500MPa,σ0.2≥1200MPa,δ5≥14%,ψ≥57%,αku≥100J/cm^2,并且组织细化,可进一步改善钢的氢脆敏感性。     

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.