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.     

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.     

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).

     

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.

     

The Formation of Martensitic Austenite During Nitridation of Martensitic and Duplex Stainless Steels

Isothermal martensite/ferrite-to-austenite phase transformations have been observed after low-temperature nitridation in the martensite and \(\updelta \)-ferrite phases in 15-5 PH (precipitation hardening), 17-7 PH, and 2205 (duplex) stainless steels. These transformations, in the region with nitrogen concentrations of 8 to 16 at. pct, are consistent with the notion that nitrogen is a strong austenite stabilizer and substitutional diffusion is effectively frozen at the paraequilibrium temperatures of our experiments. Our microstructural and diffraction analyses provide conclusive evidence for the martensitic nature of these phase transformations.     

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.     

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.     

Quenching and Partitioning of CMnSi Steels Containing Elevated Manganese Levels

Quenching and partitioning (Q&P) is receiving increased attention as a potential processing route to develop “Third Generation” advanced high strength steel (AHSS) properties. The Q&P process consists of quenching a steel from a reheat temperature to a pre‐determined temperature in the M_s ? M_f temperature region followed by a so‐called partitioning treatment at the quench temperature or higher. The present contribution investigates the Q&P response of laboratory‐processed CMnSi alloys with 3 or 5 wt% manganese. Samples were processed at quench temperatures between 70 and 250°C with partitioning temperatures of 400 and 450°C for times up to 100 s. Scanning electron microscopy and X‐ray diffraction were utilized to evaluate the Q&P microstructures. Attractive properties were generated with tensile strength/total elongation combinations such as 1500 MPa and 17%.     

Nitrogen Effects in Additively Manufactured Martensitic Stainless Steels: Conventional Thermal Processing and Comparison with Wrought

Metallurgical and Materials Transactions A - The microstructures of additively manufactured (AM) precipitation-hardenable stainless steels 17-4 and 15-5 were investigated and compared to those of...     

高N马氏体不锈轴承钢的热变形行为

利用Gleeble-3800热模拟试验机在温度为1 040~1 120℃,应变速率为1~20s-1的条件下进行了高N马氏体不锈轴承钢的热压缩变形试验。结合真应力-真应变曲线和热变形组织研究了变形参数对高N马氏体不锈轴承钢的热变形行为和碳氮化物演变规律的影响。结果表明:在该变形条件下,试验钢的真应力-真应变曲线为动态再结晶型。随着应变量的增大,碳化物的平均尺寸呈减小趋势,但数量有所增多。基于热变形方程计算得到的应变量为0.6时的热变形激活能Q为410.7kJ/mol。构建了包含应变量在内的流变应力方程,同时建立了高N马氏体不锈轴承钢的Zener-Hollomon参数本构方程。     

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...     

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

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

Dilatometry Analysis of Dissolution of Cr-Rich Carbides in Martensitic Stainless Steels

The dissolution of Cr-rich carbides formed in the martensitic constituent of a 13 pct Cr stainless steel was studied by dilatometry and correlative electron channeling contrast examinations. The dissolution of carbides subsequent to the martensite reversion to austenite was associated with a net volume expansion which in turn increased the dilatometry-based apparent coefficient of thermal expansion (CTE_a) during continuous heating. The effects of carbides fraction and size on the CTE_a variations during carbides dissolution are discussed.     

Processing Techniques for ODS Stainless Steels

Oxide dispersion-strengthened (ODS) alloys have been studied extensively, in part as a response to the need to develop the high-temperature materials that are required for energy efficient systems. ODS stainless steels, in particular, are of technological interest because of their high ductility and high strength at elevated temperatures, which when combined with their resistance to corrosion, render them ideal for many applications that require extreme performance quality. Survey of the published literature reveals that efforts have been devoted to incorporate specific strengthening mechanisms in order to meet demanding performance criteria for ODS alloys. As a result, a number of novel processing approaches have been developed in an effort to design distinct microstructures that can enhance the performance of ODS alloys. In the current paper, various techniques used to manufacture ODS stainless steel alloys are compared and summarized on the basis of a literature search, and results are contrasted with selected experimental data obtained by the authors. In particular, cryomilling, water-atomization, and melt-spinning processes are described in detail as three novel approaches that can be effectively used to distribute nanometric oxides in 316L stainless steel matrix. The microstructural features, mechanical properties, and sintering behavior of the fabricated materials are described and discussed. The critical processing parameters for each processing technique are discussed with reference to the properties of the material. The versatility and potential challenges of the individual techniques are also described and contrasted.     

Stress Induce Martensitic Transformations in Hydrogen Embrittlement of Austenitic Stainless Steels

In austenitic type stainless steels, hydrogen concentration gradients formed during electrochemical charging and followed by hydrogen loss during aging, at room temperature, surface stresses, and martensitic phases α′-BCC and ε-HCP developed. The basic relationship between the X-ray diffraction peak broadening and the hydrogen gradients, formed during charging and aging at room temperature in such austenitic stainless steels, were analyzed. The results demonstrate that the impact of stresses must be considered in the discussion of phase transformations due to hydrogenation. Austenitic stainless steels based on iron-nickel-chromium, have relatively low stacking fault energy γ_SFE and undergo: quenching to low temperatures, plastic deformation, sensitization heat treatments, high pressure (≥3–5 × 10⁹ Pa) by hydrogen or other gases, electrochemical charging (when the sample is cathode) and when is irradiation by various ions the samples in vacuum. All the above mentioned induce formation of ε and α′ in the face-centered cubic (FCC) austenite γ matrix. The highest stresses cause formation of mainly α′ phase and ε-martensite, and both are involved in plastic deformation processes and promoting crack propagation at the surface. In 310 steel, the crack propagation is based on deformation processes following ε-martensitic formation only. Formations of ε- and α′-martensites were noted along the fracture surfaces and ahead of the crack tip. The cracks propagated through the ε-martensitic plates, which formed along the active slip planes, while α′ phase was always found in the high-stress region on the ends of the ligaments from both sides of the crack surfaces undergoing propagation.