Jai-Hyun Kwak

Microstructural analysis of cracking phenomenon occurring during cold rolling of (0.1~0.7)C-3Mn-5Al lightweight steels

An investigation was conducted into the cracking phenomenon occurring during the cold rolling of lightweight steel plates. Four steels of varying C contents were fabricated and steel plates containing C contents of 0.5wt% or higher were cracked during the initial stage of the cold rolling. The steels were basically composed of ferrite grains and -carbides in a band shape, but the volume fraction and thickness of κ-carbide band increased as the C content increased. Microstructural observation of the deformed region of fractured tensile specimens revealed that deformation bands were homogeneously formed in wide areas of ferrite matrix in the steels containing C contents of 0.3 wt% or lower, while κ-carbide bands were hardly deformed or cracked. In the steels containing high C contents of 0.5 wt% or higher, on the other hand, microcracks were initiated mostly at fine proeutectoid ferrite located within κ-carbide bands, and were grown further to coalesce with other microcracks to form long cracks. To prevent the cracking, thus, the proeutectoid ferrite should be minimized by the hot rolling in the (α+γ) two phase region. As practical methods, the content of C below 0.5% or Al above 5% was suggested to expand the (α+γ) phase region.

Effect of C content on the microstructure and tensile properties of lightweight ferritic Fe-8Al-5Mn-0.1Nb alloy

Microstructure and tensile properties of ferritic Fe-8Al-5Mn-0.1Nb lightweight steels with different C contents (0.005, 0.02, and 0.05 wt.%) have been investigated in the present study. It shows that the microstructure becomes more elongated along the rolling direction, i.e., increasing propensity towards unrecrystallization with an increase in C content. This is mainly due to the effect of NbC on retarding the dynamic recrystallization of ferrite during hot rolling, which is active for higher C (0.02C and 0.05C) containing alloys. In the case of the 0.05C alloy, there is an additional precipitation of κ-carbide particles, which also retard the dynamic recrystallization of ferrite during hot rolling, resulting in a much more elongated structure in the 0.05C containing alloy than in the 0.02C alloy in as-hot rolled condition. Although κ-carbide particles retard the dynamic recrystallization of ferrite during hot rolling, they play an opposite role during final annealing, i.e., promoting static recrystallization by the operation of particle-stimulated nucleation mechanism, resulting in the development of homogeneously distributed fine grains in the 0.05C alloy. As a result, the 0.05C alloy shows higher strength and larger elongation than the lower C containing alloys.

Effect of initial microstructure on strain-stress partitioning and void formation in DP980 steel during uniaxial tension

Finite element analysis (FEA) was conducted to capture the effect of the initial microstructure on strain-stress partitioning and void formation in DP980 steel during uniaxial tension. By using the technique of direct mapping of a filtered microstructure into finite element meshes, FEA can capture the heterogeneity of strain-stress partitioning in DP980 steel through the thickness direction during uniaxial tension. The damage characteristics were studied through scanning electron microscope observations of polished sections of failed tensile specimens. FEA was also used to reveal the effect of microstructure heterogeneity on the hot spots for void formation during uniaxial tension.

Effects of Ti on the formability of Ti−IF steels

In the present study, the influence of precipitations and texture evolution on the sheet forming properties of Ti added IF (Interstitial Free) steels has been investigated. Precipitations in Ti−IF steels were examined using TEM. Equilibrium precipitation behavior was simulated to predict the proper amount of Ti addition to IF steels to fix interstitial atoms. Texture evolution was studied by employing X-ray pole figure measurement and ODF analysis. The formability of Ti−IF steels was improved by fixing interstitial atoms C, N, S with Ti. Additional improvement of formability was also found due to the existence of solute Ti of about 0.02 wt.%. A systematic analytical methodology for the precipitation behavior and texture evolution of Ti−IF steels was established for the development of IF steels.

Dramatic improvement of strain hardening and ductility to 95% in highly-deformable high-strength duplex lightweight steels

Ferrite + austenite duplex lightweight steels have been actively developed by adding low-density Al for overcoming a limitation of stiffness deterioration by a traditional approach to obtain a weight reduction. Multiple-stage deformation mechanism in lightweight steels, i.e., simultaneous formation of deformation-induced martensite and deformation twin and additional plasticity by twinning, has been nominated as an attractive strategy, but shows a steady flow behavior with early plastic instability. Here, we present a newly designed Fe-0.3C-9Mn-5Al steel in order to obtain an optimal level of stability of austenite and a resultant outstanding combination of tensile strength and ductility, e.g., 874 MPa and 72%, together with sufficiently high strain hardening. These enhanced properties are attributed to the decreased austenite stability by controlling the austenite size and alloying partitioning due to variation in austenite fraction inside duplex microstructures. The present work gives a promise for structural applications requiring both reduced specific weight and remarkable deformability.

Interpretation of dynamic tensile behavior by austenite stability in ferrite-austenite duplex lightweight steels

Phenomena occurring in duplex lightweight steels under dynamic loading are hardly investigated, although its understanding is essentially needed in applications of automotive steels. In this study, quasi-static and dynamic tensile properties of duplex lightweight steels were investigated by focusing on how TRIP and TWIP mechanisms were varied under the quasi-static and dynamic loading conditions. As the annealing temperature increased, the grain size and volume fraction of austenite increased, thereby gradually decreasing austenite stability. The strain-hardening rate curves displayed a multiple-stage strain-hardening behavior, which was closely related with deformation mechanisms. Under the dynamic loading, the temperature rise due to adiabatic heating raised the austenite stability, which resulted in the reduction in the TRIP amount. Though the 950 °C-annealed specimen having the lowest austenite stability showed the very low ductility and strength under the quasi-static loading, it exhibited the tensile elongation up to 54% as well as high strain-hardening rate and tensile strength (1038 MPa) due to appropriate austenite stability under dynamic loading. Since dynamic properties of the present duplex lightweight steels show the excellent strength-ductility combination as well as continuously high strain hardening, they can be sufficiently applied to automotive steel sheets demanded for stronger vehicle bodies and safety enhancement.