The effect of chromium addition on microstructure and mechanical properties of medium manganese advanced high strength steels

Since the end of the twentieth century, the priorities of the automotive industry have changed, and factors such as fuel consumption and passenger safety have played a decisive role in the design of automobiles. These factors compelled the development of new types of steels, and advanced high-strength steels were developed that showed high strength and improved elongation. Increasing demands of the industry and strict safety rules have increased the need for new types of AHSS over time and have pushed the steel industry to find ways to produce the next generation of AHSS types. Some types of steel developed have responded to the demands of the industry. One of these steel types, which can be the new generation AHSS, is medium manganese steels. This study was carried out to determine the effect of chromium (Cr) addition on microstructure and mechanical properties in medium manganese steels. For this purpose, four different medium manganese steel plates containing different amounts of Cr (0, 0.4, 0.8, 1.2 wt.%) were cast, and these plates were formed into sheet metal by the hot forging method. Different heat treatment processes have been applied to the samples taken from these sheets, and these processes have been extensively discussed with heat treatment parameters such as intercritical annealing temperature (650, 675, 700 and 800 °C), intercritical annealing time (30, 60 min.), quenching medium (water and air), tempering temperature (200, 250, 300 °C) and tempering time (20, 40, 60 min.). Microstructural differences in the samples were examined with optical microscope (OM) and scanning electron microscope (SEM) micrographs, and the composition and volume fractions of the retained austenite phase were calculated using the data obtained from energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analyzes. The mechanical properties determined by tensile and hardness tests were interpreted in relation to the microstructures of the samples. Finally, the fracture behavior of the tensile specimens was investigated with SEM micrographs and macro images taken from the fracture surfaces. The results revealed that low Cr additions increased the thermal stability of the retained austenite phase and improved the strength-elongation combination in medium manganese steels. In addition, it was seen that the intercritical annealing process was quite decisive for the advanced property combination, and the best mechanical property combinations were obtained in the 1.2% Cr added samples annealed at 650 °C (⁓ 32 GPa.%). In the images taken from the fracture surfaces, it was seen that the quenching medium was effective on the fracture behavior. In this respect, it was observed that the fracture occurred as the mixed-type in air-cooled samples, and it was determined that the brittle fracture mechanism was more effective at the water quenched samples.