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Thermomechanical response of a TWIP steel during monotonic and non-monotonic uniaxial loading
Majidi, O.,De Cooman, B.C.,Barlat, F.,Lee, M.G.,Korkolis, Y.P. Elsevier Sequoia 2016 Materials science & engineering. properties, micro Vol.674 No.-
<P>The tensile properties of a Fe-18%Mn-0.6%C-1.5%Al Twinning-Induced Plasticity (TWIP) steel were investigated at different strain rates in three loading modes, i.e. uniaxial monotonic loading, stress relaxation and loading-unloading-reloading. Infrared thermography was used to investigate the effect of the dynamic strain aging, the strain rate and the temperature on the flow stress. In addition to the standard, i.e., non-isothermal tensile tests, isothermal uniaxial tensile tests were performed at 25 degrees C, 45 degrees C and 65 degrees C. While the non-monotonic loading modes resulted in an increase of the total elongation at a low strain rate of 10(-3) s(-1), no increase was observed for strain rates higher than 6 x 10(-3) s(-1). The temperature gradients observed during non-isothermal tests were reduced when non-monotonic loading conditions were used. Temperature changes were found to influence the hardening behavior, and consequently the ductility, of the TWIP steel. Deformation twinning also had a significant influence on the results as its kinetics in TWIP steel are determined by the temperature dependence of the stacking fault energy. (C) 2016 Elsevier B.V. All rights reserved.</P>
Development of Continuous Galvanization-compatible Martensitic Steel
( Y. F. Gong ),( T J. Song ),( Han S. Kim ),( J H Kwak ),( B. C De Cooman ) 한국부식방식학회 2012 Corrosion Science and Technology Vol.11 No.1
The development of martensitic grades which can be processed in continuous galvanizing lines requires the reduction of the oxides formed on the steel during the hot dip process. This reduction mechanism was investigated in detail by means of High Resolution Transmission Electron Microscopy (HR-TEM) of cross-sectional samples. Annealing of a martensitic steel in a 10% H2 + N2 atmosphere with the dew point of-35˚C resulted in the formation of a thin c-xMnO.Si02 (x>l) oxide film and amorphous a-xMnO.SiO2 oxide particles on the surface. During the hot dip galvanizing in Zn-0.13%Al, the thin c-xMnO.SiO2 (x>1) oxide film was reduced by the Al. The a-xMnO.SiO2 (x<0.9) and a-S102 oxides however remained embedded in the Zn coating close to the steel/coating interface. No Fe2AIZn inhibition layer formation was observed. During hot dip galvanizing in Zn-0.20%AI, the c-xMnO.Si02 (x>l) oxide film was also reduced and the amorphous a-xMnO.SiO2, and a-SiO2 particles were embedded in the Fe2M5Zn inhibition layer formed at the steel/coating interface during hot dipping. The results clearly show that Al in the liquid Zn bath can reduce the crystalline c-xMn, SiO2 (x>l) oxides but not the amorphous a-xMnO.SiO2 (x<0.9) and a-Si02 oxides. These oxides remain embedded in the Zn layer or in the inhibition layer, making it possible to apply a Zn or Zn-alloy coating on martensitic steel by hot dipping. The hot dipping process was also found to deteriorate the mechanical properties, independently of the Zn bath composition.