Chang Gil Lee

Effects of volume fraction and stability of retained austenite on formability in a 0.1C–1.5Si–1.5Mn–0.5Cu TRIP-aided cold-rolled steel sheet

The effects of volume fraction and stability of retained austenite on formability of a 0.1C–1.5Si–1.5Mn–0.5Cu (hereafter all in wt.%) TRIP-aided cold-rolled steel sheet was investigated after various heat treatments (intercritical annealing and isothermal treatment). Tensile tests and limiting dome height (LDH) tests were conducted on the heat-treated sheet specimens, and the changes of retained austenite volume fraction as a function of tensile strain were measured using an X-ray diffractometer. The results showed a plausible relationship between formability and retained austenite parameters such as stability and initial volume fraction. The formability was improved with increasing volume fraction of retained austenite. However, when the volume fraction of retained austenite was same, the better formability was obtained in the specimens with the higher stability of retained austenite. This indicated that the strain-induced transformation of retained austenite to martensite could be stably progressed, thereby leading to the improvement of formability. Thus, the heat-treatment conditions should be established in consideration of the maximum volume fraction and high stability of retained austenite, and the optimal conditions were found to be intercritical annealing in the temperature range at which the austenite volume fraction was about 50%, followed by isothermal treatment at Ms temperature.

Effect of Cu, Cr and Ni on mechanical properties of 0.15 wt.% C TRIP-aided cold rolled steels

Abstract Cold-rolled steels based on 0.15C–1.5Mn–1.5Si–0.5Cu containing Cr or/and Ni were prepared, and intercritical annealing and isothermal treatment, were carried out. The addition of Cu or Cu+Ni resulted in a large increase of the retained austenite volume fraction as well as an improvement of elongation and the strength-ductility balance. However, the addition of Cr or Cu+Ni showed a dual-phase deformation behavior having higher tensile strength and lower elongation.

On the crystal structure of Cr2N precipitates in high-nitrogen austenitic stainless steel

The crystal structure of Cr(2)N precipitates in high-nitrogen austenitic stainless steel was investigated by transmission electron microscopy (TEM). Based on the analyses of selected area diffraction (SAD) patterns, the crystal structure of Cr(2)N was confirmed to be trigonal (P31m) and was characterized by three sets of superlattice reflections: (001), ((11/33)0)and ((11/33)1). These could be explained in terms of the epsilon-type occupational ordering of nitrogen. The static concentration waves (SCWs) method was applied to describe the ordered superstructure of Cr(2)N. The occupation probability function (OPF) for describing the distribution of N atoms in the Cr(2)N superstructure was derived based on the superlattice reflections obtained in the SAD patterns and could be expressed as: n(r)=c-1/6eta1cos2piz+4/3eta3cos(2pi/3)(x+y+3z). The crystallographic models for epsilon-type ordering, mainly suggested in the Fe-N system, were discussed in comparison to the present model.

Observation of adiabatic shear bands formed by ballistic impact in aluminum-lithium alloys

Abstract The 2090 alloy and the Weldalite alloy with high strengths contained very few adiabatic shear bands even near the perforated region, whereas adiabatic shear bands were more easily formed in the 7039 alloy. In the front side of the impacted area, many thin delaminated sheets and a large amount of fragmentation were observed in the 2090 alloy and the Weldalite alloy, respectively. Microscopic observations of the adiabatic shear band were consistent with the fact that ballistic performance tends to increase with increasing yield strength. Particularly in the Weldalite alloy whose strength and fracture toughness were the highest of the three alloys, very few adiabatic shear bands were formed and their width was very narrow, indicating its excellent ballistic performance. For the 2090 alloy which contained heavily elongated grains and coarse grain boundary particles, many cracks formed near the perforated region and propagated along grain boundaries. These intergranular cracks possibly gave the beneficial effect on ballistic performance since they might prevent the formation of adiabatic shear bands.

Dilatometric Analysis on Phase Transformations of Intercritical Annealing of Fe-Mn-Si and Fe-Mn-Si-Cu Low Carbon TRIP Steels

Evaluations of austenite fraction and transformation kinetics upon intercritical annealing of low carbon TRIP steels were attempted using quantitative dilatometric analysis. The measured dilation curves were analyzed by taking the carbon distribution between austenite and its decomposed phases into account. The amount of austenite formed during intercritical annealing and its carbon content obtained by dilatometric measurement was compared with the values predicted by thermodynamic calculations under the ortho-equilibrium and para-equilibrium conditions. The kinetics of the reaustenization process including pearlite dissolution and non-isothermal and isothermal formation of austenite could be quantitatively characterized by means of a modified JMAK (Johnson-Mehl-Avrami-Kolmogrov) equation.

Microtexture Analysis of Friction Stir Welded Al 6061-T651 Plates

Microstructural characteristics of friction-stir-welded Al 6061-T651 with varying rotating and advancing speed were examined by the electron backscattering diffraction (EBSD) installed in field emission-scanning electron microscopy (FE-SEM). It was found that FSW produced an equiaxed fine-grained microstructure in weld zone and the grain size in weld zone decreased up to about 4~6 µm with decreasing rotating speed. The primary textures developed in weld zone were {100}<001>, {110}<001> and {111}<110>. In thermo-mechanical affected zone, the change in grain size was not significant, however, large number of low angle grain boundaries were observed, which seems to be concerned with the formation of subgrains due to the development of dislocation cells.

Effects of Cooling Conditions on Microstructure, Tensile Properties, and Charpy Impact Toughness of Low-Carbon High-Strength Bainitic Steels

In this study, four low-carbon high-strength bainitic steel specimens were fabricated by varying finish cooling temperatures and cooling rates, and their tensile and Charpy impact properties were investigated. All the bainitic steel specimens consisted of acicular ferrite, granular bainite, bainitic ferrite, and martensite-austenite constituents. The specimens fabricated with higher finish cooling temperature had a lower volume fraction of martensite-austenite constituent than the specimens fabricated with lower finish cooling temperature. The fast-cooled specimens had twice the volume fraction of bainitic ferrite and consequently higher yield and tensile strengths than the slow-cooled specimens. The energy transition temperature tended to increase with increasing effective grain size or with increasing volume fraction of granular bainite. The fast-cooled specimen fabricated with high finish cooling temperature and fast cooling rate showed the lowest energy transition temperature among the four specimens because of the lowest content of coarse granular bainite. These findings indicated that Charpy impact properties as well as strength could be improved by suppressing the formation of granular bainite, despite the presence of some hard microstructural constituents such as bainitic ferrite and martensite-austenite.

Effect of heat treatment on formability in 0.15C−1.5Si−1.5Mn multiphase cold-rolled steel sheet

The effects of volume fraction and the stability of retained austenite on the formability of a 0.15C−1.5Si−1.5Mn (hereafter all in wt.%) TRIP-aided multiphase cold-rolled steel sheet were investigated after various heat treatments. The steel sheets were intercritically annealed at 800°C, and isothermally treated at 400°C and 430°C. Microstructural observation, tensile tests and limiting dome height (LDH) tests were conducted on the heat-treated sheet specimens, and the changes in retained austenite volume fraction as a function of tensile strain were measured using an X-ray diffractometer. The results showed a plausible relationship between formability and retained austenite stability. Although the same amount of retained austenite was obtained after isothermal holding at different temperatures, better formability was obtained in the specimens with the higher stability of retained austenite. If the stability of the retained austenite is high, the strain-induced transformation of retained austenite to martensite can be stably progressed, resulting in a delay of necking to the high strain region and improvement in formability.

Improvement of Formability and Spring-Back of AA5052-H32 Sheets Based on Surface Friction Stir Method

A process to improve formability and spring-back was developed for AA5xxx-H temper sheets based on the surface friction stir (SFS) method. In the SFS method, a rotating probe stirs the sheet surface so that material flow and heat, which result from plastic deformation and friction, change the microstructure and macroscopic mechanical properties of the stirred zone and therefore, ultimately, the formability and spring-back performances of the whole sheet. When applied to AA5052-H32 sheets, the process improved formability and spring-back, as experimentally and numerically confirmed in the limit dome height and unconstrained bending tests.

The ordering behavior of supersaturated metastable phases in β-Ti alloys

A phase resulting from quenching solution treated beta-Ti alloys has long been recognized as a metastable structure containing unusual phenomena, and is not well understood. Tweed structure consisting of the metastable phase was observed by transmission electron microscopy, and spot satellites inflicted by streaks were also observed in the lattice diffraction. The results of microstructural studies and diffraction pattern surveys have identified the metastable structure as the elastic strain zone caused by the supersaturation of solute atoms, which may be induced by the electron charge distribution and atomic bonding between solvent matrix and solute atoms, causing the tweed structure. This paper presents the main finding of a microstructural abnormality, and compares the results with ordering behavior such as premartensitic transformation, and reviews a new regularity generated in the metastable phase of the supersaturated state.