RELATION BETWEEN DISCONTINUOUS PLASTIC FLOW AND STRAIN HARDENING OF THE LOW-CARBON STEEL

Abstract

Purpose. Determination of the conditions for the disappearance of a section of intermittent flow and its effect on the strain hardening of low-carbon steel.
 Actuality. The use of low carbon steels with intermittent flow for stamping has a significant limitation. Based on this, the issues of the effect of ferrite grain size on the occurrence of intermittent flow are relevant for determining the optimal structural state of steels intended for deep drawing.
 Research methods. The structure of the samples was studied under a light microscope, the grain size of the ferrite was determined by the methods of quantitative metallography. The mechanical properties of steel were determined in tension at room temperature and a strain rate of 10-3 s-1. The characteristics of the initiation of plastic flow and the parameters of strain hardening were determined from the analysis of tension curves in logarithmic coordinates. In the region of uniform strain hardening, the deformation of the violation of the directly proportional relation lgσ-lgε was determined as the moment of formation of dislocation structures with a certain periodicity.
 Results. An increase in the grain size of ferrite is accompanied by a decrease in the region of intermittent flow and a shift in the moment of formation of a dislocation cellular structure towards small plastic deformations. The strain hardening rate in the region of uniform work hardening and the Lüders strain are related by an inversely proportional relationship. With excessively large grain sizes of ferrite, the difficulty of maintaining conditions for a uniform distribution of slip lines is one of the reasons for the disappearance of the intermittent flow region on the deformation curves of low-carbon steel.
 Scientific novelty. With an increase in the grain size of ferrite, the deformation of the beginning of the decay of a uniform distribution of dislocations into periodic structures is shifted to a decrease. At the same time, an increase in the ability of the metal to work hardening during the formation of a deformation band contributes to a decrease in the length of the intermittent flow section.
 Practical value. Determining the nature of the influence of the grain size of low-carbon steel ferrite on the characteristics of work hardening and the formation of dislocation periodic structures can be useful in determining the optimal structural state of steel for deep drawing.