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).

     

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.

     

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.     

Combined Intercritical Annealing and Q&P Processing of Medium Mn Steel

The microstructure and mechanical properties of intercritically annealed medium Mn steel are dependent on the selection of the intercritical annealing (IA) temperature. While the yield strength (YS) decreases with increasing IA temperature, the ultimate tensile strength increases with increasing IA temperature. Strain aging phenomena, both static and dynamic, are also often observed. The present contribution shows that, by combining IA with the quench and partitioning processing of the intercritical austenite, it is possible to obtain non-aging mechanical properties which combine a high YS with an ultra-high tensile strength. These properties are particularly suitable for automotive parts related to passenger safety.     

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.     

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.     

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.

     

超低碳贝氏体钢热变形时组织演变的有限元模拟

分析了(％)0．07C-0．056Nb-0．01Ti-0．42Cu-0．26Mo-0．0031B微合金化超低碳贝氏体钢热压缩过程的动、静态再结晶数学模型。利用有限元分析软件MSC．Autoforge，采用热-力耦合的弹塑性有限元方法(FEM)，模拟了压缩过程中的应变场和温度场。在此基础上，通过编写用户子程序，将热弹塑性有限元模型与组织演变模型结合起来，计算了在此过程中再结晶晶粒尺寸和再结晶分数，结果表明，对尺寸测量值的范围为34．3～65．3μm的再结晶晶粒，其晶粒尺寸模拟值的范围为32．4～61．1μm，对应值相对误差小于11．8％。     

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.     

Yield Strength Enhancement by Carbon Trapping in Ferrite of the Quenching and Partitioning Steel

The split quenching and partitioning (S-QP) process allows researchers to investigate microstructure and properties separately, i.e., before and after partitioning. After the partitioning process, the yield strength increases by approximately 300 MPa in the ferrite-bearing δ-quenching and partitioning (δ-QP) steel. We propose that carbon trapping in dislocations at the ferrite grain boundaries during partitioning process is responsible for the yield strength enhancement of ferrite. Combined transmission electron microscopy and 3D atom probe tomography observations demonstrate carbon atoms segregating in dislocations. The mechanisms for the high yield strength of ferrite presented QP steels are clarified for the first time in this research.

     

Q&P钢在冷却过程中铁素体的相变规律

 为了研究冷却过程中Q&P钢(quenching and partition steel)铁素体的相变规律,在热膨胀仪上以846 ℃均热200 s为冷却的初始条件,检测了成分(质量分数)为0.2%C、1.25%Si、2.0%Mn的Q&P钢在不同冷却速率下铁素体的相变热膨胀数据,应用杠杆定律将数据处理为相变规律与温度的关系,通过光学显微镜检测热处理后金相中的铁素体相体积分数和铁素体晶粒尺寸,得出了饱和位置形核条件下铁素体的形核率,基于混合控制模型得出了铁素体相变的相界迁移速率。结合相变开始温度,利用混合控制模型计算了相变结束温度和铁素体晶粒尺寸在相变过程中的演变规律,铁素体晶粒尺寸计算值与实测值吻合程度较高,相变结束温度的计算值与实测值的误差在±15 ℃以内,所获得的铁素体相变规律可以用于控制Q&P钢在冷却过程中的铁素体相变体积分数。     

Isothermal Phase Transformations in a Low Carbon Steel During Single and Two-Step Partitioning

Metallurgical and Materials Transactions A - Single and two-step heat treatments were performed on hot-rolled 0.2C-2Mn-1.4Si-0.3Mo steel. The microstructure was characterized by means of optical...     

Enhancement of Impact Toughness Via Tailoring Deformation Compatibility of Constituent Phases in Duplex Q&P Steel with Excellent Strength and Ductility

Metallurgical and Materials Transactions A - Q&P steels possessing a combination of ultra-high product of strength and tensile elongation (PSE) and superior impact toughness was obtained by...     

Finite-Element Analysis of Steel Bridge Distortion-Induced Fatigue

Welded plate girder bridges built before the mid-1980s are often susceptible to fatigue cracking driven by out-of-plane distortion. However, methods for prediction of secondary stresses are not specifically addressed by bridge design specifications. This paper presents a finite-element study of a two-girder bridge that developed web gap cracks at floortruss-girder connections. The modeling procedures performed in this research provide useful strategies that can be applied to determine the magnitude of distortion-induced stresses, to describe the behavior of crack development, and to assess the effectiveness of repair alternatives. The results indicate severe stress concentration at the crack initiation sites. The current repair method used at the positive moment region connections is found acceptable, but that used at the negative moment region connections is not satisfactory, and additional floortruss member removal is required. Stress ranges can be lowered below half of the constant amplitude fatigue threshold, and fatigue cracking is not expected to recur if the proposed retrofit approach is carried out.     

On the Spheroidized Carbide Dissolution and Elemental Partitioning in High Carbon Bearing Steel 100Cr6

We report on the characterization of high carbon bearing steel 100Cr6 using electron microscopy and atom probe tomography in combination with multi-component diffusion simulations. Scanning electron micrographs show that around 14 vol pct spheroidized carbides are formed during soft annealing and only 3 vol pct remain after dissolution into the austenitic matrix through austenitization at 1123 K (850 °C) for 300 seconds. The spheroidized particles are identified as (Fe, Cr)₃C by transmission electron microscopy. Atom probe analysis reveals the redistribution and partitioning of the elements involved, i.e., C, Si, Mn, Cr, Fe, in both, the spheroidized carbides and the bainitic matrix in the sample isothermally heat-treated at 773 K (500 °C) after austenitization. Homogeneous distribution of C and a Cr gradient were detected within the spheroidized carbides. Due to its limited diffusivity in (Fe, Cr)₃C, Cr exhibits a maximum concentration at the surface of spheroidized carbides (16 at. pct) and decreases gradually from the surface towards the core down to about 2 at. pct. The atom probe results also indicate that the partially dissolved spheroidized carbides during austenitization may serve as nucleation sites for intermediate temperature cementite within bainite, which results in a relatively softer surface and harder core in spheroidized particles. This microstructure may contribute to the good wear resistance and fatigue properties of the steel. Good agreement between DICTRA simulations and experimental composition profiles is obtained by an increase of mobility of the substitutional elements in cementite by a factor of five, compared to the mobility in the database MOBFE2.     

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.