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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.

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

The microstructural characteristics, tensile properties and low-cycle fatigue properties of a dual-phase steel (DP780) were investigated following its joining by three methods: laser welding, tungsten inert gas (TIG) welding, and metal active gas (MAG) welding. Through this, it was found that the size of the welded zone increases with greater heat input (MAG > TIG > laser), whereas the hardness of the weld metal (WM) and heat-affected zone (HAZ) increases with cooling rate (laser > TIG > MAG). Consequently, laser- and TIG-welded steels exhibit higher yield strength than the base metal due to a substantially harder WM. In contrast, the strength of MAG-welded steel is reduced by a broad and soft WM and HAZ. The fatigue life of laser-and TIG-welded steel was similar, with both being greater than that of MAG-welded steel; however, the fatigue resistance of all welds was inferior to that of the non-welded base metal. Finally, crack initiation sites were found to differ depending on the microstructural characteristics of the welded zone, as well as the tensile and cyclic loading.

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.

The influence of pre-strain on the fatigue life behavior of Fe–18Mn twinning-induced plasticity (TWIP) steel was investigated by performing strain-controlled low-cycle fatigue (LCF) and stress-controlled high-cycle fatigue (HCF) tests with variation of the amount of pre-strain. The pre-strain to the as-received plate was achieved through a cold rolling process; thickness reductions of 10 and 30% were applied. The results revealed that both the LCF and HCF resistances were strongly dependent on the amount of pre-strain but showed vastly different effects; with increasing pre-strain, LCF resistance was degraded but HCF resistance was improved. An energy-based concept was adopted and modified to capture the pre-strain effect on the fatigue life behavior by considering the energy consumed by pre-straining. The developed model was further extended to establish a correlation between the LCF and HCF life data. The results showed that the HCF life behavior could be successfully predicted from the LCF life data.

The mechanical properties of austenitic high Mn Twinning Inducted Plasticity (TWIP) steel provide an excellent combination of strength and ductility when tested in uni-axial tension. The performance of TWIP steel during some critical formability tests such as deep drawing, bulge test and cutting edge stretching has not yet been studied extensively. In this contribution, the stretch-flangeability of Fe18Mn0.6C1.5Al TWIP steel and Ti Interstitial-Free (IF) steel were studied by means of hole expansion test. In-situ strain analysis and Infra-red (IR) thermography were carried out during the test. It was found that TWIP steel, despite having a higher uniform elongation in uniaxial tension, had poorer hole expansion properties than Ti IF steel. Strain distribution analysis revealed that the hole edge deformed in a deep drawing mode which was similar to a tensile deformation. Away from the hole edge, the deformation mode changed gradually from deep drawing to stretch forming mode. The IR-thermography of TWIP steel revealed a high degree of adiabatic heating which was absent in the case of IF steel. The crack associated with the edge fracture revealed a local temperature increase at the crack tip of up to 92°C. Two types of hole edge preparations were studied. A high quality hole edge finish resulted in a better hole expansion performance. The fractography of the crack plane surface of TWIP steel and Ti IF were also studied by SEM, and revealed a plastic failure mode in both cases.

The present study was undertaken to provide a more comprehensive understanding of the deformation modes of advanced fully austenitic steels exhibiting an enhanced combination of strength and ductility. For this purpose, a new plasticity, called microband induced plasticity (MBIP), was introduced. In addition, the origin of its superb combination of strength and ductility over the well-known transformation induced plasticity (TRIP) and twin induced plasticity (TWIP) was elucidated. With the aids of previously developed models, we focused on predicting the transitions among TRIP, TWIP, and MBIP, primarily in terms of the stacking fault energy. The analysis revealed that the TRIP-TWIP transition can be reasonably predicted by the energy balance for FCC austenite — HCP ɛ martensite transformation. The TWIP-MBIP transition can be addressed by the critical stress for mechanical twinning, which causes the infinite divergence of the Shockley partials. Lastly, the TWIP-MBIP transition model was validated by comparing it with the experimental data.

The body design with light weight and enhanced safety is a key issue in the car industry. Corresponding to this trend, POSCO is developing various automotive steel products with advanced performance. Conventional advanced high strength steels such as DP and TRIP steels are now expanding their application since the steels exhibit higher strength and ductility than those of conventional solution and precipitation strengthened high strength steels. Efforts have been made to enhance the mechanical performance of these steels such as ductility, hole expansion ratio, deep drawability, etc. Current research is focused on development of extra- and ultra-AHSS. Extra-AHSS are designed to utilize nano-scale retained austenite embedded in fine bainite and martensite. Ultra-AHSS are designed to have austenite as the major phase, and the ductility is enhanced primarily by continuous strain hardening generated during forming. These steels including extra- and ultra-AHSS are believed to be the next generation automotive steels which will replace the existing high strength steels due to their extremely high strength and ductility combinations.

In recent years, for automotive applications, the need for new advanced high-strength sheet steels (AHSSs) with high ductility has rapidly increased. This is mainly related to the need for more fuel-efficient (and therefore more environmentally friendly) cars, and the increasing consumer demand for safer vehicles. In this research, the transformation-induced plasticity (TRIP) that accompanies the mechanically induced martensitic transformation (MIMT) in TRIP-aided multiphase steel was analyzed. The analysis was performed using a computational model that takes the ductile fracture during tensile deformation into account. The TRIP and MIMT phenomena were calculated using the concept of variant selection, which is based on the Kurdjumov–Sachs (K–S) orientation relationship. To consider the localization of the plastic flow in the deforming material, the increase in void nucleation due to the martensitic transformation and the void growth based on the yield criterion for porous material were studied. The feasibility of the extra advanced high-strength sheet steel (X-AHSS) was assessed by analyzing the results obtained using various initial volume fractions and various stabilities of the retained austenite in the TRIP-aided multiphase steel. Subsequently, the optimum volume fraction and stability of the retained austenite in the TRIP-aided multiphase steel could be determined.

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, ...

The evolution of texture and stored energy was studied for Ti bearing and Ti-Nb bearing IF high strength steel undergoing a double cold rolling and annealing treatment by using Orientation Imaging Microscopy. Ti bearing IF high strength steel showed a similar texture evolution of the α and γ fiber components to that of IF steel. On the contrary, Ti-Nb bearing IF high strength steel displayed a different texture evolution from the one observed in the former steels. The difference in texture and stored energy evolution between both high strength steels seemed to be affected by the segregation behavior of solute alloying elements such as P.

In general, plastic strain occurs over a certain stress, called yield stress. However, it has been reported that the permanent strain could happen during boundary migrating even under the extremely slight externally applied stress. In this study, we performed dilatometry experiments under the various compressive stresses and measured the amount of recrystallization and growth induced permanent strain. A new empirical constitutive equation was suggested to describe the recrystallization and growth induced plasticity. This equation was verified by comparing the calculated values with dilatometric experimental data under the various compressive stresses.

A present study was performed to investigate the effect of volume fraction of undissolved cementite on the high-cycle fatigue (HCF) properties of high carbon steels. High carbon steels were heat treated to have three distinctive volume fractions of undissolved cementite phase in the martensite matrix. With increasing holding time at the austenitizing temperature of 800 °C, the volume fraction of undissolved cementite gradually decreased from 9% to 1%. A series of tensile and micro-hardness tests showed that the specimen with 1% volume fraction of undissolved cementite exhibited the highest strength, which was mainly attributed to the increased carbon content in the martensite matrix. Though of its highest tensile strength, however, the fatigue strength was the lowest in the specimen with 1% volume fraction of undissolved cementite, which was the reverse trend as compared to the general relationship between fatigue strength and tensile strength. This was attributed to the higher propensity of crack initiation in the specimen containing fewer amounts of undissolved cementite, where cracks were observed to nucleate at the interfaces between nonmetallic inclusions (Al 2 O 3 and SiO 2 ) and martensite matrix during HCF tests. A probabilistic model was developed to predict fatigue strength of heat treated high carbon steel and compared with the experimental data.

The microstructural evolution and the softening behavior of hot rolled and 60% cold rolled 0.85wt% carbon pearlitic steels during spheroidization annealing have been investigated by using the textural and microstructural information contained in the Orientation Imaging Microscopy (OIM) scans. The local boundary energy map, recently suggested by the present authors, is used to monitor the changes of stored plastic strain energy distribution in ferrite during the annealing process, which shows that the spheroidization process of cementite is finished before the completion of recrystallization of the 60% cold-rolled high carbon pearlitic steel.

A finite element model was developed to simulate the deformation, temperature and phase transformation behavior in high carbon steels. The heat capacity of each phase and the heat evolution due to phase transformation were obtained from the thermodynamic analysis of S45C, 50CrV4 and SK85 steels. Phase transformation kinetics of the steels were derived from continuous cooling experiments. An additivity technique was applied to a modified Johnson-Mehl-Avrami equation to analyze continuous cooling curve. To predict the strain due to TRansformation Induced Plasticity (TRIP), a variant selection model for diffusionless transformation and an accelerative creep model for diffusional transformation were adopted. In order to calculate the deformation behavior, the elastic strain, the volumetric strain due to thermal contraction and phase transformation, the plastic strain and the TRIP strain were taken into account. Using the finite element model developed in this study, the temperature-phase-deformation behavior of the high carbon steels was calculated.

By applying a double cold rolling and annealing treatment, the evolution of the α and γ fiber components differed from the ones observed in conventional processing. This is attributed to the difference of the initial texture. An increased reduction of stored energy of the {111} component was found by monitoring the change of the stored energy during annealing, indicating that the onset of the nucleation stage of recrystallization by relaxation and annihilation of dislocations occurred mainly on the {111} component with its higher stored energy. The detailed texture evolution of the double cold rolled specimen during 2nd annealing is described by coupling the theory of oriented nucleation and orientation pinning, which is experimentally confirmed by OIM scan.

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 (∊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 γ 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 ∊vM= 2.4 and 4.0.

The effect of sheet thickness on shear deformation and texture for the ferrite rolling has been studied. The shear deformation was estimated from the distorted shape of inserted wire before rolling and FEM simulation. The deformed shape of inserted wires and FEM results showed that the shear strain decreased with increasing initial sheet thickness. The measured and simulated textures also showed that the Goss component changed to Dillamore component as the thickness increased. From rolling simulation of 20 % reduction, the minimum shear deformation was found to exist for the given roll bite geometry and friction coefficient.

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 AHSS forming. Application of conventional forming technologies has been restricted more and more by relatively low formability of AHSS with high tensile-strength. To overcome the limitation in the forming, POSCO has recently developed new forming technologies such as hydro-forming, hot press forming, roll forming and form forming. In this paper, tailored strength HPF, hydroformed torsion beam axle and multi-directional roll forming are introduced as examples of new forming technologies.