Effect of chromium on microstructure and mechanical properties of advanced high strength steels producted by quenching and carbon partitioning heat threatment

Regardless of whether the motor technology is fueled by fossil fuels or electricity, increasing the range of automobiles is the main goal of the automotive industry. To achieve this goal, low-weight vehicles are being developed without compromising safety standards by using high-strength materials. The quenching and partitioning (Q&P) steel technology has emerged as a prominent method in this field due to its many advantages, such as its applicability to low-alloy steels, suitability for automation in production, low energy consumption during heat treatment. The aim of this doctoral thesis is to investigate the effect of the chromium element on the mechanical and microstructural properties of Q&P steels. For the study, steel plates with a chemical composition that is either free of chromium or contains 0.6%, 0.9%, or 1.2% chromium by weight and with a low carbon content of 30 mm thickness were obtained using sand casting. The thickness of the plates was reduced to 16 mm using hot pressing and then to 4 mm using forging. Standard samples were taken from these plates for microstructure and mechanical property analysis. Q&P treatment was applied to each sample group at three different carbon partitioning temperatures and three different partitioning times according to numerical parameters such as temperature and holding time. Tensile and hardness tests were performed on these samples. These data were evaluated together with X-ray spectroscopy (EDS) and X-ray diffraction (XRD), optical microscope, and SEM observations to calculate the material's internal structure, fracture type, retained austenite content, and retained austenite carbon content. The results obtained were compared using statistical programs. As a result of microstructure analysis and mechanical tests, it was observed that increasing the chromium content increases the retained austenite content and tensile strength while having no significant effect on elongation values. Intensive ductile fracture and small amounts of brittle fracture surfaces were observed in all samples. As a result, it was found that the highest retained austenite content was obtained in the highest chromium-containing sample at 420°C for 200 s of holding time. Austenite carbon concentrations were observed in the samples processed at the highest temperature regardless of the process time. As a result of the study, samples with a wide range of strength-toughness multiplication values ranging from 15519 to 37100 Mpa.% were produced.