Quenching and Partitioning of Martensite and Other Advancements in Steels: Session 6
Program Organizers: Emmanuel De Moor, Colorado School of Mines

Friday 2:00 PM
July 14, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Anton Muehlemann, University of California, Berkeley

2:00 PM  
Multi-Mechanism Design of Strength and Structure by High-Temperature-Tempering of Lath Martensite: George Krauss1; 1Colorado School of Mines
    High-Temperature-Tempering (HTT) of as-quenched lath martensite in carbon steels can provide excellent combinations of strength and toughness and resistance to hydrogen embrittlement, but deeper understanding of the multiple atomic mechanisms operating simultaneously during HTT, around 600 C, should optimize microstructure and properties for many applications. The supersaturation of carbon and other elements, the very high dislocation density, and the extremely fine crystal size of as-quenched lath martensite provide driving forces for the many atomic mechanisms operating during HTT, including recovery, crystal boundary elimination and stabilization, diffusion, alloy element enrichment of cementite, secondary hardening, and in some cases recrystallization. All of these mechanisms are dependent on alloying and time and temperature of tempering. This talk will show examples of the effects of various operating mechanisms, emphasize many areas where characterization and understanding are incomplete, and hopefully encourage systematic research to resolve some of the outstanding questions.

2:20 PM  
Design of the 3rd Generation Advanced High Strength Steels by Tailored Martensitic Transformation: Chenchong Wang1; Wei Xu1; 1Northeastern University
    In the development of the 3rd generation advanced high strength steels for automotive applications, the retained austenite and its transformation to martensite play the key role in achieving a better combination of strength and toughness, upon either component processing or collision. The stability of retained austenite depends on various factors including its composition, size and morphology etc. In the present study, thermodynamic and kinetic modeling was performed to analyze the composition and morphology evolution of retained austenite employing by Thermo-Calc and DICTRA. The transformation of retained austenite to martensite upon deformation was evaluated by modified Olson-Cohen model. Dedicated experiments were carried out to validate the modeling results. Based on models mentioned above, new 3g AHSS composition and corresponding processing parameters were computational designed, so as to tailor the stability of retained austenite and realize the ‘martensite (transformation) by design’.

2:40 PM  
The Martensite Transformation Behavior of Delta+Martensite Dual Phase Steels with Low Density Steels: Xuemin Wang1; Xiangyu Xu1; Chengjia Shang1; 1Collaborative innovation center of steel technology, University of Science and Technology Beijing
    The influence of martensite on the mechanical properties for low density steels has been investigated with the aid of optical microscope and Scanning Electronic microscope. The transition electron microscope was also employed to study the effect of NbC and TiC on the grain refinement of delta ferrite and austenite. The experimental results show that during the tempering after the accelerated cooling the NbC and TiC precipitated, the NbC and Tic can impede the recrystallization of delta ferrite and refine the delta ferrite grains, and the toughness of steels was raised. With the cooling rate increases the martensite from gamma to alpha increases and the strength of steels was improved.

3:00 PM  
Effect of Hot-rolling Direct Quenching and Partitioning Treatment on the Microstructure and Mechanical Properties of a Low Carbon Steel: Jun Xu1; Feng Huang1; Zhenghong Guo1; Jianfeng Gu2; 1School of Materials Science and Engineering; 2Institute of Materials Modification and Modeling
    Effects of rolling temperature and following cooling manner on the microstructure and mechanical property of Fe-0.2C-1.5Mn-1.5Si quenching and partitioning steel were investigated. The results show that both the packets and blocks were refined significantly when martensite formed in the recrystallized prior austenite, but only blocks were refined when martensite formed in the unrecrystallized one. The rolling temperature has little effect on the amount of retained austenite. When the two-step Q&P treatment was replaced by natural cooling after rolled specimen was quenched to Ms temperature, the amount of retained austenite was decreased dramatically. Tensile and impact tests indicate cooling manner is more critical than rolling temperature in determining mechanical properties. Rolling at a temperature slightly higher than recrystallization temperature plus two-step Q&P treatment is an optimized process to obtain best comprehensive mechanical property by improving both the strength and impact toughness without scarifying plasticity. The related mechanism was discussed briefly.

3:15 PM  
Reverted Austenite Transformation Behavior from Martensite and Its Effect on Retained Austenite and Properties in a Low Carbon 3Mn Steel: Chengjia Shang1; Gang Han1; Zhenjia Xie1; Lin Xiong1; 1University of Science and Technology Beijing
    The influences of intercritical annealing temperature and time on retained austenite and properties were investigated in a 3Mn low carbon low alloy steel. Results showed that multi-phase microstructures consisted of intercritical ferrite, martensite/bainite, retained austenite and fine dispersed precipitations. The volume fraction of retained austenite reached a maximum value of 14% at 680oC for 30 min. Retained austenite was stabilized by controling reverted austenite tranformation and the intercritical partition of Mn, Ni into reverted austenite. The reverted austenite tranformation and partition behaviors of Mn, Ni during intercitical annealing were studied by TEM, STEM and discussed by DICTRA simulation. Due to the combination contribution of multi-phase microstructure, retained austenite and nanoscale precipitation, excellent properties of high yield strength (751 MPa), low yield to tensile ratio (0.77) and ductility (12% and 25% in uniform and total elongation) were achieved at 680oC for 30 min.

3:30 PM Break