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.
DETERMINING THE AMOUNT OF LÜDERS BAND
S. Rešković, I. Jandrlić, University of Zagreb, Faculty of Metallurgy, Sisak, Croatia, F. Vodopivec Institute of Metals and Technology, Ljubljana, Slovenia The article presents the results of measuring the amount of deformation during the formation and propagation of Lüders bands through the deformation zone. The amount of deformation was determined by applying the visioplasticity method with digital image correlation, and analyzing the captured images using the MatchID software package. Results show that during the formation of Lüders bands, the deformation of 0,004 mm/mm was achieved. Furthermore, it was determined that during the propagation of Lüders bands through deformation zone, the sample was deformed in the amount of 0,05 mm/mm.
Effect of solutes on lüders strain in low-carbon sheet steels
The effects of phosphorus, tin, and silicon on the magnitude of Lüders strain in low-carbon sheet steels have been studied. At equivalent grain sizes the Lüders strain was effectively reduced by the P or Sn additions. However, when various amounts of Sn were added to a steel containing 0.07 pct P, incremental effects of increasing Sn content could not be recognized; the combined effect of Sn and P on Lüders strain was nearly the same as the effect produced by a slightly higher P alone. Additions of Si to a 0.05 pct P-treated steel, even up to fairly high concentrations, produced very little effect on the Lüders strain. The magnitude of Lüders strain, as influenced by the grain size, is a linear function of d-1/2, this relation being similar in form to the Hall-Petch relation for yield or flow stress and grain size. The effect of P or Sn on this relation is to decrease the slope of the straight-line plot. These observations have been analyzed on the basis of Lüders strain in relation to the band-front velocity and the contribution of the mobile dislocation density and velocity product (ρv) to the strain rate. Solutes that effectively increase the density of mobile dislocations generated from the grain-boundary sources should decrease the magnitude of Lüders strain.