Effect of subzero thermal shock treatment on deep drawability and mechanical properties of galvanized dx54 steel

Low-alloy steels, widely used in the automotive and manufacturing industries, are known for their excellent deep drawability. Due to their diverse applications, these steels may be exposed to environmental conditions ranging from -40°C to +40°C. In this thesis, the effects of sub-zero thermal shock, ranging from -40°C to +40°C, on the microstructure, mechanical properties, and deep drawability of DX54 grade galvanized steel were investigated. The cold thermal shock process was defined as each transition between -40°C and +40°C being considered one cycle, and the galvanized DX54 steel samples were subjected to 250 cycles, 500 cycles, 750 cycles, and 1000 cycles, as well as a control group with no treatment. After the sub-zero thermal shock process, the samples were analyzed using microstructure examination, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), microhardness testing, electron backscatter diffraction (EBDS), tensile testing, and Erichsen cupping tests. The results obtained were compared with each other. The tensile test results showed that, compared to the untreated samples, the 1000-cycle samples had a yield strength increase of 25±8.5 MPa, a tensile strength increase of 19±8.1 MPa, and a decrease in elongation by 3.8±1.5%. In the microhardness test results, the untreated samples showed an average hardness of 100.7±1.9 HV0.1, while the 1000-cycle samples exhibited an average hardness of 107.1±3.9 HV0.1. In microstructure grain size measurements, conducted according to ASTM E112 standards, the untreated sample had a grain size of 8.55. With an increasing number of thermal cycles, the average grain size of the samples decreased, and in the 1000-cycle sample, the grain size was measured as 9.34. Microstructural examinations revealed that changes in the microstructure of the galvanized and base materials were not significant up to 750 cycles, but the effects became noticeable at 1000 cycles. The Erichsen cupping test results showed an average indentation depth of 10.24±0.2 mm and an average force of 13.7±0.5 kN in the untreated samples. The indentation depth decreased slightly up to 1000 cycles, with the 1000-cycle sample showing an indentation depth of 9.58±0.1 mm and an average force of 14.98±0.2 kN. The studies concluded that the threshold for the negative impact of the thermal shock process between -40°C and +40°C on the microstructure, mechanical properties, and deep drawability is 1000 cycles.