Influence of Niobium on the Beginning of the Plastic Flow of Material during Cold Deformation
Investigations were conducted on low-carbon steel and the steel with same chemical composition with addition of microalloying element niobium. While tensile testing was carried out, the thermographic measurement was tacking place simultaneously. A specific behavior of niobium microalloyed steel was noticed. Test results have shown that, in the elastic deformation region, thermoelastic effect occurs, which is more pronounced in niobium microalloyed steel. Start of plastic flow in steel which is not microalloyed with niobium begins later in comparison to the microalloyed steel, and it is conducted so that, at the point of maximum stress, deformation zone is formed within which stresses grow. In steel microalloyed with niobium after proportionality limit, comes the occurrence of the localized increase in temperature and the occurrence of Lüders band, which propagate along the sample forming a deformation zone.
Effect of strain rate on Lüders band propagating velocity and Lüders strain for annealed mild steel under uniaxial tension
For annealed mild steel that exhibits yield-point phenomena under uniaxial tension, physical equations expressing the strain-rate dependencies of the strain and the Luders-band velocity have been proposed in this work. In these expressions, both the Luders strain and the Luders-band velocity increase with gauge-length strain rate in the form of exponent functions. The proposed equations have been verified by our experimental data of uniaxial tension on mild steels, as well as by those reported by some other researchers. The proposed equations may provide a simple method to determine the stress-rate sensitivity exponent, in the equation of stress-dependent dislocation velocity, which is usually difficult to be measured by experiments.
Effect of Precipitate Size and Dispersion on Lankford Values of Titanium Stabilized Interstitial-free Steels
A quantitative study was undertaken to characterise the size, dispersion and microchemistry of precipitates and the texture of two hot-dip galvanised Ti stabilised interstitial free steels of comparable chemistry and grain size but with a distinct difference in Lankford values. Small angle neutron scattering technique was used complementary to electron optical techniques to resolve the ultra-fine precipitates (<5 mm). The specimen with a dense dispersion of fine precipitates exhibited a weak intensity of {111} texture and a low Lankford value. In order to clarify whether the development of {111} texture was suppressed by increased solute carbon in the matrix or by the Zener pinning force exerted by the ultra-fine precipitates during recrystallisation growth, internal friction measurements were carried out. The effect of ultrafine precipitates in retarding the grain boundary mobility is confirmed. The possible roles of solute carbon effect, and particle pinning effect on the evolution of {111} texture are discussed.