Strain Hardening of Low-Carbon Steel in the Area of Jerky Flow

Purpose. The aim of this work is to assess the effect of ferrite grain size of low-carbon steel on the development of strain hardening processes in the area of nucleation and propagation of deformation bands. Methodology. Low-carbon steels with a carbon content of 0.06–0.1% C in various structural states were used as the material for study. The sample for the study was a wire with a diameter of 1mm. The structural studies of the metal were carried out using an Epiquant light microscope. Ferrite grain size was determined using quantitative metallographic techniques. Different ferrite grain size was obtained as a result of combination of thermal and termo mechanical treatment. Vary by heating temperature and the cooling rate, using cold plastic deformation and subsequent annealing, made it possible to change the ferrite grain size at the level of two orders of magnitude. Deformation curves were obtained during stretching the samples on the Instron testing machine. Findings. Based on the analysis of stretching curves of low-carbon steels with different ferrite grain sizes, it has been established that the initiation and propagation of plastic deformation in the jerky flow area is accompanied by the development of strain hardening processes. The study of the nature of increase at dislocation density depending on ferrite grain size of low-carbon steel, starting from the moment of initiation of plastic deformation, confirmed the existence of relationship between the development of strain hardening at the area of jerky flow and the area of parabolic hardening curve. Originality. One of the reasons for decrease in Luders deformation with an increase of ferrite grain size of low-carbon steel is an increase in strain hardening indicator, which accelerates decomposition of uniform dislocations distribution in the front of deformation band. The flow stress during initiation of plastic deformation is determined by the additive contribution from the frictional stress of the crystal lattices, the state of ferrite grain boundaries, and the density of mobile dislocations. It was found that the size of dislocation cell increases in proportion to the diameter of ferrite grain, which facilitates the development of dislocation annihilation during plastic deformation. Practical value. Explanation of qualitative dependence of the influence of ferrite grain size of a low-carbon steel on the strain hardening degree and the magnitude of Luders deformation will make it possible to determine the optimal structural state of steels subjected to cold plastic deformation.

RELATIONSHIP BETWEEN MAGNETIC PROPERTIES AND MICROSTRUCTURE OF FERRITES DURING SINTERING IN RADIATION AND RADIATION-THERMAL CONDITIONS

The studies of correlation between magnetic properties and microstructure were conducted on samples of lithium-substituted ferrite, sintered in radiation and radiation-thermal conditions. Radiation-thermal sintering was performed for compacts irradiated with a pulsed electron beam with energy of (1.5–2.0) MeV, beam current per pulse of (0.5-0.9) A, irradiation pulse duration of 500 μs, pulse repetition rate of (5–50) Hz, and compact heating rate of 1000 C/min. Sintering in thermal furnaces (T-sintering) was carried out in a preheated chamber electric furnace. The paper shows that magnetic induction does not depend on the ferrite grain size. In this case, the coercive force is inversely proportional to the grain size and depends on the intragranular porosity of ferrite samples. In contrast to thermal sintering, radiation-thermal sintering does not cause capturing of intergranular voids by growing grains and enhances coagulation of intragranular pores.

THE IMPACT OF SHEAR GAP SIZE ON THE QUALITY OF THE SHEARED SURFACE IN ELECTRICAL STEEL SHEET BLANKING

The quality of the sheared surface when blanking, also known as die-cutting, is the result of several factors. Based on current knowledge about blanking, the following technological parameters – shear gap size, blunting of the shearing tool, lubrication in the shearing process, and deformation rate – can be considered as decisive parameters on the quality of the sheared surface. The main material characteristics include yield strength, tensile strength, ductility, and ferrite grain size. The paper is focused on the influence of the shear gap on the quality of the shear surface of electrical sheets with different chemical composition and different mechanical properties. The quality of the cutting surface was characterized by the size of the plastic cutting area. The relationships between the size of the shear gap, which ranged from 1 to 7% of the thickness of the cut material and the size of the plastic shear area, were evaluated and measured macroscopically.

New Approach for Multiplying the Strength and the Formability of Cold Rolled BCC Based Structure Steel through Ultra Fine Structure Technique

Reduction in grain size of bcc based structure steel is still highly concerned in the cold rolled sheet to attain superior mechanical properties. As long as, the reduction of weight is much considered in the structure purposes, the strength/weight ratio of steel is highly demanded. In this study, an innovative technique was applied to attain ferrite grain size with hundreds of nanometer, in tandem with preserving the mechanical properties. In this approach, the micro-alloyed low carbon steel resulted from the thermomechanical process was followed by subcritical annealing regime prior to the first critical transformation temperature. To identify the effect of a micro-alloying element as vanadium, and the effect of subcritical annealing regime on the low carbon steel, two low carbon steel was subjected to studying in this research. The results refer that applying a subcritical annealing regime for the micro-alloyed low carbon steel after hot compression at intercritical annealing temperature can lead for attaining hundreds of nanometer ferrite grain size, which has a powerful effect on promoting the strength of the steel to exceed 1200 Mpa, in one hand with preserving the formability up to 20% as uniform elongation. Unexpectedly, the fine grain size obtained after the innovative technique promotes the impact toughness at room temperature, which is attributed to the fineness and the spheroid morphology of the secondary phase in conjugation with bcc ferrite structure.

Influence of continuous annealing soaking temperature on fish-scaling resistance of ultra-low carbon steel for porcelain enameling

The relationship between fish-scaling resistance of ultra-low carbon (ULC) enamel steel and continuous annealing soaking temperature was rarely studied before. In this paper, the influence of continuous annealing soaking temperature on microstructure, precipitation behavior and fish-scaling resistance of ULC steel for porcelain enameling was investigated. The results obtained were as follows: for ULC enamel steel, with the continuous annealing soaking temperature increasing from 750 °C to 840 °C, the mean diameter of second phase particle increased from 41 nm to 52 nm, the corresponding volume fraction decreased from 5.8 × 10−3 m3/m3 to 2.9 × 10−3 m3/m3, and the ferrite grain size grew from 13.4 µm up to 17.3 µm, the hydrogen permeation value (TH value) decreased from 32 min/mm2 to 12 min/mm2. The grain boundary and precipitates are main hydrogen traps, as the ferrite grain size increased and the volume fraction of second phase particles decreased, the fish-scaling resistance of ULC enamel steel decreased.

Effect of Cooling Path on Microstructure Features and Tensile Properties in a Low Carbon Mo-V-Ti-N Steel

The two-stage controlled rolling and cooling of a low carbon Mo-V-Ti-N steel at different cooling paths was simulated through a Gleeble 3500 system. The microstructure and tensile properties of each sample were examined by estimating their dependence on the cooling paths. It was indicated that a mixed microstructure of polygonal ferrite (PF), acicular ferrite (AF), granular bainitic ferrite (GBF), and a martensite-austenite (M-A) constituent was developed in each sample. Results showed that application of the reduced cooling rate and elevated finishing cooling temperature led to the increases in the effective ferrite grain size and the precipitate amount despite a decrease in dislocation density, which eventually resulted in the overall yield strength. It also led to an increasing amount of M-A constituent, which lowered the yield ratio and, thereby, enhanced the capacity for strain hardening. In addition, the underlying mechanism for the correlations among the cooling path, the microstructure, and the yield strength was considered.