High-energy X-ray Diffraction Studies On The Microstructure And Mechanical Behavior Of Medium-Mn Steels

Owing to the increasing demand for fuel efficiency,environmental protection and safe design in the automobile industry,it is very necessary to develop the third generation advanced high strength steel,which offers a good trade-off between the mechanical properties and the cost of materials fabrication and final production.Medium-Mn steels provide the product of strength and elongation(PSE)over 30 GPa%are the most promising candidate for the third generation automotive steels.So far,researchers have conducted various studies to elucidate the effects of alloying elements and intercritical annealing conditions on the microstructures and mechanical properties of medium-Mn steels.However,the correlation between austenite stability and load partitioning among constituent phases of medium-Mn steels has not been systematically investigated.An in-depth understanding of microstructure evolution during Lders band propagation in medium-Mn steel is still missing.Besides,the PSE and work-hardening capability of most medium-Mn steels are relatively low in comparison with high-Mn TWIP steels.The development of medium-Mn steels with outstanding mechanical properties promises to enhance the design and optimization of medium-Mn steels.Microstructure and mechanical behavior of the Fe-0.1C-10Mn-0/2A1 were studied after different heat treatments.As the intercritical annealing temperature increased,the grain size of ferrite and austenite,the volume fraction of austenite,and ultimate tensile strength increased.The carbon concentration in austenite and yield strength decreased with increasing annealing temperature.Total elongation increased and then decreased as the intercritical annealing temperature increased.The OAl steel demonstrated the best PSE about 48.7 GPa%after intercritical annealing at 600 ? for 1h,while the 2A1 steel demonstrated the best PSE about 51.4 GPa%after intercritical annealing at 650 ? for 1h.When the micro structure and volume fraction of austenite in OA1 steel were similar to those of 2 A1 steel,the yield strength and ultimate tensile strength of OA1 steel were higher than those of 2A1 steel while the total elongation of OAl steel was lower than that of 2A1 steel.The micromechanical behavior of Fe-0.1C-1OMn-0/2Al steel was investigated using in situ high-energy X-ray diffraction(HE-XRD)with uniaxial tensile tests at temperatures of 100,25 and-50 ?.Load partitioning occurred among ferrite,austenite and martensite immediately after entering the yielding stage.The Luders bands were associated with sudden changes of lattice strain in austenite.Work hardening in the martensite phase played an important role in the plastic stability by accommodating stress compatibility between grains and phases.At 100 °C,austenite was too stable to transform to martensite,resulting in limited work-hardening capability and a relatively low strain to failure.At 25 ?,the austenite was found to transform in bursts during applied loading.These transformations correlated with stepwise peak broadening in the austenite phase and were attributed to Portevin-Le Chatelier(PLC)band propagation.At-50 ?,we observed a more intense TRIP effect which suppressed PLC band propagation.For the 0A1 steel,the???' and ??? phase transformation occurred during Luders band propagation and the ? ? ?' and ???' phase transformation occurred during PLC band propagation.While only the ???' phase transformation occurred during Luders band and PLC band propagation in the 2A1 steel.The stress of austenite in 0Al steel was more relaxed due to the complex phase transformation scenario.The in situ HE-XRD technique was used to investigate the relationship between austenite stability and micromechanical behavior of Fe-0.1C-10Mn-2Al steel fabricated by intercritical annealing at 625 ?,650 ?,675 ? and 700 ? for 1 h.The incremental work-hardening exponent of experimental steel increased with increasing intercritical temperature.The overall trend of the transformation kinetics of RA with respect to true strain followed the sigmoidal shape predicted by the Olson and Cohen(OC)model.An inverted U-shaped relationship between the martensite transformation rate and the true strain was observed.Load partitioning occurred among constituent phases immediately after entering the yielding stage.Because the stability of RA decreased with increasing intercritical annealing temperature,the martensite bore higher stress.Two-dimensional distributions of microstructural characteristics of austenite around the Luders band in medium-Mn TRIP steel fabricated by intercritical annealing 675 ? for 1 h were revealed.Luders band propagation led to significant changes in the volume fraction and lattice strains of austenite.The evolution of the lattice strain of austenite accorded well with the true yield behavior model and was used to estimate the angle of the Luders band front with respect to the tensile axis.The modified Williamson-Hall analysis showed that the dislocation density in austenite increased from 71×1014 m-2 to 1.5 ×1O15 m-2 because of a high volume fraction of the phase transformation from austenite to martensite inducing a large volume expansion during Luders band propagation.Large local gradients of strain/stress and microstructures around Luders band could provide effective localized work-hardening capability and contribute to the stability of Luders band propagation through the entire specimen without failure.Novel Fe-0.3C-9Mn-2Al-3Cu steel with outstanding mechanical properties was developed.The yield strength(824 MPa),ultimate tensile strength(1222 MPa),total elongation(55%)and the best PSE(67.2 GPa%)were achieved after intercritical annealing at 660 ? for 1 h,which were superior to the values reported for the medium-Mn steels.In comparison with high-Mn TWIP steels,our steels with lower alloying content demonstrated higher yield strength.The Cu-rich precipitation in ferrite led to the yielding lattice strain of ferrite higher than that of austenite.The addition of Cu increased the stacking fault energy of austenite.The TRIP effect and TWIP effect in austenite during deformation contributed to the large elongation of experimental steel.In summary,this dissertation established the relationship between austenite stability and micromechanical behavior of medium-Mn steels via in situ HE-XRD technique.Two-dimensional distributions of microstructural characteristics of austenite around the Luders band in medium-Mn TRIP steel were unraveled.Novel Cu-containing medium-Mn steel with outstanding mechanical properties was developed.This dissertation provided the experimental basis and useful insights on the design and optimization of medium-Mn steels.