Bauschinger Effect in Microalloyed Steels: Part I. Dependence on Dislocation-Particle Interaction
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RODUCTION
THERE are two principal Bauschinger effect theories (Figure 1): back stress and Orowan theory.[1,2] During forward plastic deformation, moving dislocations interact with different obstacles (e.g., other dislocations, grain boundaries, and precipitates), preventing their further propagation. This generates a back stress around the contact point resisting further progress of dislocations. During reverse deformation, this back stress repels the dislocations from the obstacles in the opposite direction, namely in the direction of reverse strain. Thus, the stress field helps to move the dislocations in the direction of reverse strain and the reverse yield stress drops by the level of the back stress. Material with a larger number of dislocations (rolled below the recrystallization stop temperature) and obstacles (precipitates) will show a larger yield drop after reverse cold deformation due to the presence of a larger number density of dislocation-obstacle interaction sites. Experimental investigations have shown that, in microalloyed steels, the Bauschinger effect does not depend on grain size,[3] but depends on steel chemistry[4–7] and phase balance.[8] Recent research has shown a Bauschinger effect dependence on microalloying element particles[9] and dislocation structure:[10] with an increase in particle volume fraction and dislocation density, the yield stress drop in the direction of reverse strain increases. To predict the strength change, in practice, one needs a quantitative model of dislocation-particle ANDRII G. KOSTRYZHEV, Research Fellow, School of Electronic, Electrical and Computer Engineering, MARTIN STRANGWOOD, Senior Lecturer, and CLAIRE L. DAVIS, Professor of Ferrous Metallurgy, are with the School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom. Contact e-mail: [email protected] Manuscript submitted August 16, 2009. Article published online March 18, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
interaction influence on the yield stress. This is discussed in the present article.
II.
MATERIALS
For the investigation, two microalloyed steels with a banded ferrite-pearlite microstructure in as-rolled and annealed conditions were used. Annealing at 400 °C and 550 °C for 30 minutes was applied to modify the dislocation structure and microalloying element particle distributions, without affecting the grain size and the second-phase content. Original steel plates of 10-mm thickness were provided by Corus plc (Table I). Optical microscopy of the as-rolled steel plates has shown the grain size to be 2.0 to 3.0 lm in both studied steels and the second-phase content to decrease from 9.5 pct in the C-Nb to 4.6 pct in the C-Nb-V steel, due to decreasing carbon content in steel composition.[9,10] III.
EXPERIMENTAL TECHNIQUES
Thermodynamic modeling of the steel microstructures was carried out using version L of Thermo-Calc (Royal Institute of Technology, Stockholm, Sweden) with the bulk alloy composition as an input. Equilibrium phase balances within
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