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Medium-Mn (MMn) steels have received much research attention recently because their low austenitisation temperature enables low-temperature hot stamping (LTHS). However, the effect of the initial state of the material on the hot stamping performance is still unknown. In this study, the effect of different initial states on the deformation behaviour of a typical MMn steel during uniaxial tensile testing under LTHS conditions (deformation at 500–600 °C under strain rates of 0.01–1 s−1) are investigated using a Gleeble 3800 materials simulator; the final mechanical properties after austenitising and quenching are also examined. The microstructure of each material state before and after the LTHS heating cycle is characterised using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Of the three states investigated the hot rolled and annealed (HRA) state shows the best hot deformation performance represented by larger strain hardening exponent and higher total elongation, followed by the cold-rolled (CR) state, with the cold-rolled and annealed (CRA) state exhibiting the worst performance. The final mechanical properties, however, are very similar among the three states. In addition, the yield point phenomenon is found during hot deformation in both the CR and CRA states, and absent in the HRA state. The hot deformation behaviour has been discussed in terms of differences in microstructural properties, namely the grain size and its degree of heterogeneity.

Medium manganese (MMn) steels are a promising class of advanced high-strength steels (AHSS) with potential for application as vehicle panels in the automotive industry. In this study, the deformation behaviour of a representative cold-rolled MMn steel and its dependence on processing parameters are studied using uniaxial tensile testing under low-temperature hot stamping (LTHS) conditions, covering austenitisation soaking times of 60–600 s, deformation temperatures of 500–700 °C and strain rates of 0.01–5 s−1. A yield point phenomenon is observed for the first time in the early stage of deformation at such temperatures in this MMn steel, which has a submicron grain size. The extent of the yield point phenomenon is reduced, and strain hardening capability and total elongation are enhanced, with longer austenitisation soaking times, which give coarser-grained microstructures. The yield point phenomenon also tends to be weaker at higher deformation temperatures and lower strain rates; under these conditions, both the flow stress and the degree of strain hardening decrease, while the total elongation is insensitive to differences in deformation conditions. The mechanisms for the deformation behaviour and its dependence on test conditions are discussed. In addition, a set of unified constitutive equations is established and calibrated using the experimental data to predict the deformation behaviour of the MMn steel under uniaxial LTHS conditions, and close agreement with experiment, including the yield point phenomenon, is obtained.

The cooling system is a critical element in tooling for hot stamping high-strength aluminium alloys, for which very high quenching rates are required to ensure a supersaturated solid solution state in formed parts. To enhance cooling, ducts within the tools should be close and conformal to the surface die profile. Currently, ducts with curved profiles are made by drilling short straight lengths in die segments which are clamped together to form a complete die, which is expensive and hard to achieve the shape of duct with best cooling performance. To address these disadvantages, a novel method which enables efficient manufacture of conformal cooling systems by embedding a network of tubular cooling ducts within a cast matrix is presented in this paper. The feasibility of the proposed method of making ducts in the hot stamping die is demonstrated. Both experimental and computer-based die quenching tests using heated aluminium test pieces were undertaken to determine the cooling performance of a laboratory-scale tool set with cooling ducts. Simulations using the validated FE model were performed to investigate the effects of cross-sectional geometry, material and duct layout, on the quenching performance of the tools. It was found that ducts made of mild steel perform sufficiently well to make the use of high conductivity copper unnecessary. For a flat die surface, square section ducts provided highest cooling rates in comparison with circular and diagonal ones. The uniformity of die temperature increases with the decreased distance between neighbouring ducts, which indicates that a minimal gap is recommended without deteriorating tool strength. The developed tooling technology has the potential to provide a low-cost, highly efficient method of making conformal cooling ducts because the hot stamping die of high-strength aluminium alloy panels requires larger dimension, greater complexity and higher quenching rates.

In this study, the austenite transformation behaviour in a medium-Mn (MMn) steel is investigated during heat treatments that replicate those occurring in low-temperature hot stamping (LTHS), with the aim of better understanding this behaviour to optimise heat-treatment design. The austenitisation behaviour and critical phase transformation temperatures during the LTHS heating process and their dependence on heating conditions are investigated using dilatometry with a Gleeble 3800 thermal-mechanical simulator, covering heating rates of 1–25 °C s−1 and soaking temperatures of 630–900 °C. Both a higher heating rate and higher soaking temperature are found to be beneficial to shorten the time required for obtaining a given austenite fraction. The martensite start temperature (Ms) shows a rapid increase with increasing soaking temperatures when austenitisation is partial, and a slower increase after full austenitisation. Excellent ultimate tensile strength values of around 1750 MPa and total elongation values of around 9.3 % are obtained for the material after the LTHS heating process. A physically based model describing the austenitic transformation under these conditions has been adopted and calibrated. The model shows good agreement with austenitic transformation diagrams constructed from experimental data, and thus can function as a guide for selecting optimum heat-treatment parameters for LTHS.

Creep age forming (CAF) of an Al-Cu-Li alloy (AA2050-T34) considering its anisotropic creep-ageing behaviour has been experimentally and numerically investigated in this study. A series of uniaxial creep-ageing tests of AA2050-T34 has been carried out in both tension and compression conditions. The creep-ageing results in longitudinal and transverse samples show some anisotropic behaviour in both the room temperature yield strength and the creep strain, in which more strengthening occurs at the intermediate stage and less creep strain is produced in the transverse samples than the longitudinal samples under the same loading condition. It is also found that compressive loading results in much higher anisotropic behaviour than tensile loading. A material model has been adopted for the creep-ageing of AA2050 and shows a good fit to experimental data. Based on the model, a four-point bending virtual test has been developed in PAM-STAMP for simulating the CAF process of AA2050. The results indicate that the anisotropic behaviour contributes an apparent effect on both creep-aged properties and springback of the CAF process and need be considered for CAF manufacture of AA2050 components.