Zhenli Mi

The oxidation behaviors of varying Cr-alloyed automotive beam steels—0.015 wt.% Cr, 0.15 wt.% Cr, and 1 wt.% Cr—were investigated using isothermal oxidation experiments. The morphologies of the oxide scale were characterized, and the formation mechanisms were analyzed to understand the change in the oxidation kinetics of the investigated steels. The results show that a small amount of Cr, up to 0.15 wt.%, can reduce oxidation kinetics; the addition of Cr at 1 wt.% causes the oxidation rate to decline at a low isothermal temperature, but the hindrance effect expires when the oxidation temperature is above 1050 °C. The oxidation scale, including the inner FeO layer, the intermediate Fe₃O₄ layer, and the outer Fe₂O₃ layer, exhibits a morphological evolution from marble-like to pore-like, then whisker-like, flocculation-like, fine oxide grains, and finally coarse oxide grains. With increasing Cr addition, the thickness of the FeO layer decreases significantly, leading to a reduction in the total thickness of the oxidation scale. During the oxidation process of the investigated steel with 0.15 wt.% Cr, a Cr-rich layer and FeO-(Cr, Fe, Mn)₃O₄ eutectic form; meanwhile, FeO-(Cr, Fe)₂O₃ eutectic and Si-rich oxides, as well as a (Cr, Si)-rich layer, occur in the oxidation scale when 1 wt.% Cr is added to the steel. The occurrence of voids in the (Cr, Si)-rich layer is responsible for the increasing oxidation kinetics of the 1 wt.% Cr steel when the isothermal temperature is above 1050 °C, and the optimal Cr concentration in automotive beam steel is 0.15 wt.%, considering both oxidation resistance and cost.

Friction stir processing (FSP) manufacturing technology was used to fabricate medium Mn advanced high-strength steel in this study. The mechanical properties and microstructure of the steel fabricated using FSP were investigated. The steel obtained a total elongation of 35.1% and a tensile strength of 1034.6 MPa, which is about 59% higher than that of the steel without FSP. After FSP, a gradient structure occurs along the thickness direction. Specifically, across the thickness direction from the base material zone to the transition zone and finally to the stirring zone, both the grain size and austenite fraction decrease while the dislocation density increases, which results from the simultaneous effect of severe plastic deformation and recrystallization during FSP. Due to the gradient structure, an obvious difference in the strain across the thickness direction of the steel occurs during the deformation process, resulting in significant hetero-deformation-induced (HDI) strengthening. The deformation mechanism analysis reveals that HDI strengthening and dislocation strengthening are the main factors in the improvement in the strength–ductility balance. The obtained knowledge sheds light on the process of fabricating medium Mn steels with excellent properties using FSP manufacturing technology.

The bending deformation recorded and fracture characterisation seen during the process of quenching and partitioning (Q&P) steel are investigated as a function of bending angle. According to experimental results, the mechanically induced martensitic transformation of the retained austenite occurred heterogeneously in the direction of thickness. Results from electron backscattered diffraction and finite element analysis confirmed that the heterogeneous phase transformation was caused by the stress difference in the direction of thickness. The results prove fracture behaviour is dominated by martensite cracking. The results provide a new insight into deformation characterisation of the Q&P steel under bending, which allows the design and optimisation of mechanical properties.

The dynamic tensile behaviour and deformation mechanism of the Fe–17Mn–0.4C–0.06V twinning-induced plasticity (TWIP) steel were investigated over a wide range of strain rates from 10⁻⁴ to 10³ s⁻¹. With increasing strain rate, the stacking fault energy increased due to the increase of adiabatic heating temperature, Δ T. At 10⁻⁴ to 10¹ s⁻¹, the transformation-induced plasticity (TRIP) effect coexisted with the TWIP effect and weakened with increasing strain rate. With the increase of strain rate in the range of 10⁻¹ to 10¹ s⁻¹, the TWIP effect strengthened gradually and intersected deformation twins were formed. When the strain rate was higher than 10¹ s⁻¹, the TRIP effect disappeared and the twinning was inhibited since the adiabatic heating effect elevated.

The hot deformation behaviour and processing maps of a typical Fe-Mn-Si-Cr steel for automotive body structures were investigated on a Gleeble3500 simulation tester by isothermal compression test in the temperature ranging of 950°C to 1150°C and a strain rate of 0.05–10 s⁻¹. The results show that dynamic recrystallisation softening mechanism is more evident at low strain rate, while dynamic recovery is more evident at high strain rate. The instability parameter ξ( ) values are negative and considered as unsafe deformation zones at low deformation temperature and high strain rates. The processing maps exhibit two safe domains for hot working: (a) 0.05–1 s⁻¹ and 1080–1150°C, the power dissipation values of η increase with the increase of strain rate. (b) 0.05–1 s⁻¹ and 950–1020°C, while strain is lower than 0.1, the values of η decrease with the increase of strain rate.