Gyosung Kim

Diffusion-controlled recrystallization and grain growth-induced plasticity of steel under externally applied stress

Permanent deformation, which occurs during recrystallization and grain growth under an applied stress much smaller than the yield stress of the material, was observed by dilatometric measurement and modelled on the basis of a migrating grain boundary diffusion mechanism. The observation confirmed that the transformation-induced deformation accompanying the interface migration is described by migrating boundary-induced plasticity rather than internal stress caused by volume change and plastic yielding. In the model, atomic diffusion along the migrating grain boundary was assumed to cause recrystallization and grain growth-induced plasticity. A constitutive equation for recrystallization and grain growth-induced plasticity was derived as a function of the grain growth rate, temperature and externally applied stress. The predictions were compared with the dilatometric measurements made during recrystallization and grain growth of an extra low-carbon steel under various levels of uniaxial compressive stress, with good agreement being found between the calculated permanent strain and the experimentally derived strain.

Factors governing hole expansion ratio of steel sheets with smooth sheared edge

Stretch-flangeability measured using hole expansion test (HET) represents the ability of a material to form into a complex shaped component. Despite its importance in automotive applications of advanced high strength steels, stretch-flangeability is a less known sheet metal forming property. In this paper, we investigate the factors governing hole expansion ratio (HER) by means of tensile test and HET. We correlate a wide range of tensile properties with HERs of steel sheet specimens because the stress state in the hole edge region during the HET is almost the same as that of the uniaxial tensile test. In order to evaluate an intrinsic HER of steel sheet specimens, the initial hole of the HET specimen is produced using a milling process after punching, which can remove accumulated shearing damage and micro-void in the hole edge region that is present when using the standard HER evaluation method. It was found that the intrinsic HER of steel sheet specimens was proportional to the strain rate sensitivity exponent and post uniform elongation.

Evaluation of stored energy after accumulative roll bonding of an interstitial-free steel by means of orientation microscopy data

The stored energy of warm-rolled interstitial-free steels, produced in an accumulative roll bonding process, is evaluated by using the textural and microstructural information contained in orientation imaging microscopy (OIM) scans which were measured after accumulated von Mises strains (e vM ) of 0.8, 1.6, 2.4 and 4.0, respectively. It is assumed that the plastic strain energy is stored in a cellular network of local boundaries of low and high misorientations. The presence of intracellular dislocations which do not contribute to a local crystal orientation gradient is ignored in the present analysis. On the basis of the Read-Shockley equation, the local misorientation can be associated with a local boundary energy which can be expressed as a local stored energy by taking into account the radius of curvature of the cellular network. The validity of this procedure was verified by comparing the integrated average stored energy of the sample with the Vickers hardness data, which produced a reasonable correspondence. The present analysis also allowed the calculation of the stored energy distribution in the areas of the orientation representation space which were sufficiently populated by sample crystal orientations, i.e. the y fibre ((111)∥ND) of the present deformation texture. The distribution of stored energy along this fibre displayed a maximum on the {111}(211) texture component, particularly after a von Mises strain of 1.6, whereas the {111}(110) component displayed a local minimum of stored energy after e vM = 2.4 and 4.0.

Inverse characterization method for mechanical properties of strain/strain-rate/temperature/temperature-history dependent steel sheets and its application for hot press forming

In order to measure the flow curves of steel sheets at high temperatures, which are dependent on strain and strain rate as well as temperature and temperature history, a tensile test machine and specimens were newly developed in this work. Besides, an indirect method to characterize mechanical properties at high temperatures was developed by combining experiments and its numerical analysis, in which temperature history were also accounted for. Ultimately, a modified Johnson-Cook type hardening law, accounting for the dependence of hardening behavior with deterioration on strain rate as well as temperature, was successfully developed covering both pre- and post-ultimate tensile strength ranges for a hot press forming steel sheet. The calibrated hardening law obtained based on the inverse characterization method was then applied and validated for hot press forming of a 2-D mini-bumper as for distributions of temperature history, thickness and hardness considering the continuous cooling transformation diagram. The results showed reasonably good agreement with experiments

Lightweight Steel Solutions for Automotive Industry

Recently, improvement in fuel efficiency and safety has become the biggest issue in worldwide automotive industry. Although the regulation of environment and safety has been tightened up more and more, the majority of vehicle bodies are still manufactured from stamped steel components. This means that the optimized steel solutions enable to demonstrate its ability to reduce body weight with high crashworthiness performance instead of expensive light weight materials such as Al, Mg and composites. To provide the innovative steel solutions for automotive industry, POSCO has developed AHSS and its application technologies, which is directly connected to EVI activities. EVI is a technical cooperation program with customer covering all stages of new car project from design to mass production. Integrated light weight solutions through new forming technologies such as TWB, hydroforming and HPF are continuously developed and provided for EVI activities. This paper will discuss the detailed status of these technologies especially light weight steel solutions based on innovative technologies.

Mn-deprived Phase Transformation in High-Mn Steel during the Dew-point Control Process

Phase transformation by the Mn-deprivation was observed in the high-Mn twinning-induced plasticity-aided steel. Mn-depletion was induced by the formation of Mn-O oxide during the dew-point control process at temperature above , which changed austenitic parent phase to multi-grained ferrite. Mixture of Al-O, Al-Mn-Si-O oxides were observed at the grain boundaries of transformed ferrite.

New Forming Technologies for Autobody at POSCO

As development of car body with light weight and enhanced safety became one of the hottest issues in the auto industry, the advanced high strength steels have been broadly applied to various automotive parts over the last few years. Corresponding to this trend, POSCO has developed various types of cold and hot rolled AHSSs such as DP, TRIP, CP and FB, and continues to develop new types of steel in order to satisfy the requirement of OEMs more extensively. To provide optimal technical supports to customers, POSCO has developed more advanced EVI concept, which includes the concept of CE/SE, VA/VE, VI and PP as well as the conventional EVI strategy.To realize this concept, POSCO not only supports customers with material data, process guideline, and evaluation of formability, weld‐ability, paint‐ability and performance but also provides parts or sub‐assemblies which demand highly advanced technologies. Especially, to accelerate adoption of AHSS in autobody, POSCO has tried to come up with optimal solutions to...