Advanced High Strength Steels IV: Session II
Sponsored by: TMS Structural Materials Division, TMS: Steels Committee
Program Organizers: Ana Araujo, Vesuvius USA; Mary O'Brien, Los Alamos National Laboratory; Tilmann Hickel, Bam Federal Institute For Materials Research And Testing; Amy Clarke, Los Alamos National Laboratory; Kester Clarke, Los Alamos National Laboratory; C. Tasan, Massachusetts Institute of Technology; MingXin Huang, University of Hong Kong
Tuesday 2:00 PM
February 25, 2020
Room: Mission Hills
Location: Marriott Marquis Hotel
Session Chair: Ana Araujo, AK Steel; Kester Clarke, Colorado School of Mines
2:00 PM
In-situ Quantitative Assessment of the Role of Silicon During the Quenching and Partitioning of a 0.2C Steel: Pierre Huyghe1; Sylvain Dépinoy2; Cedric Georges3; Matteo Caruso4; Stephane Godet2; 1AGC Research Center; 2Université Libre de Bruxelles; 3CRM Group; 4Engie Laborelec
Silicon is an essential alloying element added in quenching & partitioning (Q&P) steels to delay carbide precipitation. However, there is a strong industrial interest to reduce the silicon content. This work investigates by in-situ High-Energy XRD (HEXRD) the effect of silicon on the microstructure evolution during quenching & partitioning of a commercial 0.2C-2.3Mn grade. This study highlights the role of the bainite transformation during the re-heating and partitioning steps. Silicon influences the kinetics of austenite decomposition into bainite and promotes the stabilization of austenite. This is explained by its ability to suppress carbide precipitation (i) at the interface between bainite and austenite and (ii) in the martensite matrix. Carbide precipitation at the bainite/austenite interface decreases the amount of carbon that diffuses from bainite to austenite, subsequently accelerating the bainite transformation kinetics and preventing austenite stabilization. Carbide precipitation in martensite reduces the amount of carbon available for partitioning in austenite.
2:20 PM
Austenite Stability of an 1180 MPa Quenched-and-partitioned Steel: Ana Araujo1; Jun Hu1; Erik Pavlina1; 1AK Steel Research and Innovation Center
One approach to produce steels with a combination of high strength and high formability is via a multiphase microstructure containing metastable austenite. Metastable austenite transforms to martensite differently under different loading conditions, testing temperature, and strain rates. A quenched-and-partitioned (Q&P) steel with a minimum ultimate tensile strength of 1180 MPa was chosen in this study to demonstrate this behavior. Five loading conditions, including uniaxial tension, uniaxial compression, simple shear, equi-biaxial tension, and plain strain tension, testing temperatures ranging from -25 to 225 °C, and nominal strain rates ranging from 0.001 to 1000/s were used. Retained austenite content was evaluated via x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) for each condition and at various strain levels. Lastly, a phenomenological model was proposed to fit the transformation behavior coupled with plastic strain under these different conditions.
2:40 PM
Influence of Prior Processing on the Response to Quenching & Partitioning: Casey Gilliams1; John Speer1; Kip Findley1; Richard Thiessen2; 1Colorado School of Mines; 2ThyssenKrupp Steel
Quenched and partitioned (Q&P) steels are martensite/austenite high strength, formable stee1s employed for manufacture of lighter vehicle structures. Optimization of heat treat process variables has involved efforts to maximize the amount of retained austenite and the carbon concentration of the austenite. Few studies have been reported that examine the influence of “upstream” processing steps on the heat treating response, microstructure and performance of Q&P steels. Thus, the influence of retained austenite characteristics on the properties of Q&P steels were investigated using a 0.17C-2.8Mn-1.5Si steel with systematic variations in key processing conditions (hot band coiling temperature, and hot band thickness and associated cold reduction). The influence of prior processing and microstructure on the heat treating response are investigated using Q&P processing conditions representative of continuous galvanizing. Resulting microstructures and tensile properties are reported.
3:00 PM Cancelled
Toughening Mechanisms of Quenching and Partitioning Steels by Carbon Management: Zhou Wang1; MingXin Huang1; 1University of Hong Kong
To study the toughening mechanisms of quenching and partitioning (Q&P) steels, partitioning conditions are varied to obtain different microstructures yet similar tensile strengths around 1500 MPa. It is found that carbide precipitation during partitioning can reduce solute carbon content in both martensite matrix and retained austenite. Despite the similar tensile strength and ductility, the J-R curves show that the fracture toughness increases significantly with the decreasing solute carbon content. The results show that in the Q&P steels, microcracks initiate at brittle fresh martensite, then propagate into the martensite matrix. The reduction of carbon in the retained austenite decrease the brittleness of the transformed fresh martensite, which delays the crack initiation. And the carbon reduction softens the martensite matrix, which hinders crack propagation. Therefore, the concept of “carbon management” is developed to provide a practical way to enhance the fracture toughness of high-strength-level Q&P steels.
3:20 PM
The Influence of Transformation Induced Plasticity on Damage Development in Third Generation Advanced High Strength Steels: Javad Samei1; Concetta Pelligra1; David Wilkinson1; 1McMaster University
Recent advances in heat treatment optimization to retain austenite at ambient temperatures has led to an industrial break-through in attaining advanced high strength steels with exceptional strengths and ductility. Considerable research has been invested in creating these steels; however, little is known on the extent to which transformation-induced plasticity (TRIP) in 3G steels can suppress damage. X-ray computed microtomography (µXCT) was used to quantify 3D damage evolution as a function of strain. Damage-strain curves on a variety of medium to high strength DP and TRIP steels suggests that a refined microstructure and TRIP shows the most delayed fracture. Investigations isolating the impact of TRIP on damage was attained through the comparison of damage evolution in a 3G Medium Mn steel subjected to slight changes in intercritical annealing. Extensive use of µXCT on a single Medium Mn steel has enabled an accurate assessment of the influence of TRIP kinetics on damage.
3:40 PM Break
4:00 PM
Effect of Strain Path on the Deformation Characteristics of Austenite-containing Advanced High Strength Steels: Melissa Thrun1; Amy Clarke1; Magnus Ahlfors2; Christopher Finfrock1; Kester Clarke1; 1Colorado School of Mines; 2Quintus Technologies
Third generation Advanced High Strength Steels (AHSSs) such as Quenched and Partitioned (Q&P), and Transformation Induced Plasticity (TRIP) bainitic ferrite (TBF) steels achieve high strength and ductility through the transformation of metastable austenite to martensite during deformation. Q&P heat treatment parameters, such as quench temperature, partitioning temperature and time, and ambient pressure can result in varying amounts of retained austenite with varying thermal and mechanical stability. The rate of the transformation of the austenite retained is dependent on variables such as temperature, strain path, strain rate, and surrounding microstructure; therefore, the deformation behavior of Q&P and TBF steels are also dependent on these variables. This work investigates the rate of transformation from austenite to martensite by varying the deformation conditions of steels containing metastable austenite to understand the microstructural evolution and stability of austenite and how it relates to the mechanical properties of the sheet.
4:20 PM
Crystal Plasticity Finite Element Simulations of the Planar Anisotropy of Q&P Steels: Deepika Tirumalasetty1; Sankaran S1; Anand Kanjarla1; 1Indian Institute of Technology Madras
The increasing demand for weight reduction and crash safety in the automobile industry has promoted the application of AHSS. The quenched and partitioned(Q&P) steel is of considerable interest due to their exceptional properties with a minimal alloying element. The Q&P steel gains its enhanced strength from the hierarchical microstructure of martensite and improved ductility from transformation induced-plasticity of retained austenite. The hierarchical microstructure of martensite consists of laths, blocks and packets within the parent austenite grains. Despite exhibiting high strength-ductility combination, the formability of the steel is limited. We developed a multiphase and multiscale microstructure model using crystal-plasticity based constitutive relations considering the individual phase properties to study the mechanical behaviour of the steel. Using CP-FEM simulations, we report on the effect of different microstructural features such as volume fraction of the phases, morphology, the ratio of prior grain size to austenite islands and lath thickness on the planar anisotropy.
4:40 PM
Development of Quenching and Partitioning Plate Steel Intended For Wear and Toughness Applications: Travis Marsh1; John Speer1; Rainer Fechte-Heinen2; Fred Fletcher3; Cory Alexander4; 1Colorado School of Mines; 2thyssenkrupp Steel Europe AG; 3ArcelorMittal; 4Deere & Company
In recent years, the 3rd generation of advanced high-strength steels (AHSS) has been developed for automotive sheet applications using quenching and partitioning (Q&P) as one of the heat treatments, developing a microstructure of martensite and retained austenite (RA). There is interest in exploring the application of a Q&P process to low-alloy plate steel because a microstructure of martensite and RA may have enhanced wear resistance and toughness as compared to tempered martensite microstructures generated by traditional quench and temper (Q&T) processes. In this work, Q&P and Q&T heat treatments are applied to 18 mm steel plates with nominal composition 0.2C-1.5Si-1.5Mn-0.4Cr-0.25Mo to obtain a bulk hardness >400 HBW. The conditions are compared via microstructural characterization and mechanical properties.
5:00 PM Cancelled
Dual Effects of Retained Austenite for Third Generation Advanced High Strength Steels: Xuejun Jin1; Lianbo Luo1; Wei Li1; Yu Gong1; Qi Lu2; Charles Enloe3; Jason Coryell3; Jianfeng Wang2; 1Shanghai Jiao Tong University; 2General Motors China; 3General Motors
High ductility of the advanced high strength steels (AHSS) is usually achieved via the transformation induced plasticity (TRIP), i.e., the phase transformation of metastable retained austenite to martensite. However, the deformation-induced martensite is often twinned martensite enriched with carbon, which is susceptible to the embrittlement. Several cold-rolled third generation AHSS with nominal tensile strengths of the range from 1000 MPa to 1500 MPa were first pre-strained, and then assessed for impact toughness by means of the stacked Charpy V-Notch impact testing. Some pre-strained samples were also tempered prior to the impact test to simulate the paint baking process. Associated with sophisticated microstructure analysis such as 3DAP and TEM, it is believed that carbon re-distribution and transition carbide (or cluster) formation during the baking step is responsible for the toughness recovery in the pre-strained retained austenite containing steels.