Vol 48, No 2 (2018)

Attention focuses on the processes in the mold of a continuous-casting machine when using a patented new cooling system. In particular, the temperature differences in the steel billet and in the wall over the mold height are of interest in modeling the casting processes, because those differences affect the quality of the billet produced. A literature review covers research on the slag-forming mixture, which affects the heat flux from the billet to the mold. Non-Russian authors highlight mild cooling of the mold in selecting the slagforming mixture. Improvement of billet cooling in the mold permits improvement in the surface quality of the slab, extension of mold life, and increase in productivity. According to numerous authors, that may be accomplished by mathematical modeling of the process. The mold cooling depends directly on the convective motion of liquid steel in the mold, a topic addressed by many non-Russian authors. Researchers have considered systems in which the heat pipes in the cooling system of the mold are based on porous material, with water and air as the working fluid, and those in which liquid droplets on nanostructured superhydrophilic surfaces are evaporated. Mold cooling at steel casting rates higher than 7 m/min, accompanied by increase in the heat-flux density, is of great importance, as reflected by the number of studies published. The relations between the basic process parameters are determined by means of Rayleigh dimensionality theory. The basic parameter selected is the temperature difference in the metal mold wall, which depends on the casting rate (the time that the billet is in the mold), the properties of the steel (specific heat, thermal diffusivity), the thermal conductivity of the mold wall, and the temperature difference in the cast steel. In determining the exponents in the dimensionless relations, the available experimental data regarding the dependence of the heatflux density on the casting rate and the parameter of the steel are taken into account. On the basis of the ratio Δt_me/t_me obtained (where Δt_me is the mean temperature difference over the wall thickness and t_me is the mean wall temperature) for molds with the existing and new (patented) cooling systems, the temperature difference in the steel billet may be determined. For the two cooling systems compared, Δt_me1 = 450°C and Δt_me2 = 231°C. Consequently, Δt_me1/Δt_me2 = 1.95. The smaller temperature difference Δtme2 indicates milder cooling of the mold with the new cooling system.

The elastoplastic transition in welded low-carbon steel samples is considered. Two methods of manual arc welding of floating-electrode type are employed: a traditional steady arc; and pulsed welding with controlled heat consumption. In terms of the structural characteristics and mechanical properties of the welded metal, the methods are identical. In both cases, extended elastoplastic transition by the nucleation and propagation of Luders bands is observed. However, the underlying process is different. In the traditional steady arc, the Luders bands are formed in the applied metal initially as diffuse regions where the deformation is localized. These regions fill the seam and convert it to the plastically deformed state. The moving fronts of the bands are finally shaped in the thermal-influence zones and pass to the basic metal. The velocity and morphology of the fronts match those of fronts in uniform objects of the same steel. When using a pulsed arc, Luders bands appear some distance from the weld seam, at the clamps of the loading device. Up to the thermal- influence zones, the velocity and morphology of the fronts correspond to those for the basic metal. At the fusion boundary, the front stops and forms the nucleus of a new band, which expands in the seam metal. This new band first transforms the applied metal to the deformed state and then creates a moving front in the opposite thermal-influence zone. The velocity of the front differs by an order of magnitude in the applied metal and the basic metal. The weld seam determines the nucleation of Luders bands. An explanation is offered for the different origins of the elastoplastic transition in the two welding methods. In traditional welding with a steady arc, the local long-range stress is considerably higher in the thermal-influence zones than in the basic metal. Therefore, the nucleation of Luders bands is a relaxational process in this case. In pulsed arc welding, the local long-range stress is higher in the basic metal, where the Luders bands appear. The results may be used in selecting the test conditions for power-system equipment.

The evolution of the carbide phase in the surface layers of bulk-quenched rails (after the passage of 500 and 1000 million t of traffic) and differentially quenched rails (after the passage of 691.8 million t) to a depth of 10 mm at the central axis of the rail cross section and at the nearby rounded section is studied by transmission electron-diffraction microscopy. The grains of plate pearlite, ferrite–carbide mixture, and structure-free ferrite are analyzed. The carbide phase in the surface layers of the steel changes in two mutually complementary processes during rail operation: (1) cleavage of cementite particles with subsequent entrainment in ferrite grains or plates (in the pearlite structure); (2) cleavage and dissolution of cementite particles, with transfer of carbon atoms to dislocations (in Cottrell atmospheres and in dislocational cores), which transport them to the ferrite grains (or plates), where cementite nanoparticles are formed again. In the previous location of the plates, fragmented dislocational substructure appears. The boundaries of the fragments are found at the positions previously occupied by cementite α-phase boundaries. The solution of cementite is mainly due to the energy of carbon atoms at dislocation cores and subboundaries in comparison with the cementite lattice. The binding energy of the carbon atom and the dislocations is 0.6 eV and the binding energy of the carbon atom and the subboundary is 0.8 eV, as against 0.4 eV for the carbon atom in cementite. Elastoplastic stress fields are formed; their stress concentrators are intra- and interphase boundaries of ferrite and pearlite grains, cementite plates and ferrite of the pearlite colonies, and globular cementite and ferrite particles. Those are also the basic sources of curvature and torsion in the crystal lattice of the rail steel. On approaching the contact surface, the number of stress concentrators increases, and the internal long-range stress fields are of greater amplitude.

A model is proposed for predicting the Lankford coefficient of steel sheet produced for the auto industry at PAO Severstal. The set of empirical model parameters is determined using an experimental database on the mean Lankford coefficient (for 138 strips of 13 steels differing greatly in composition) and additional databases on calculated steel microstructure parameters after hot rolling, and after heat treatment of cold-rolled strips. The mean error in calculations of the Lankford coefficient using the model is 7.1%. It is found that the Lankford coefficient is determined not only by the carbon content in the solid solution before cold rolling but also by the amount of carbon deposited at the ferrite grain boundaries in the form of tertiary cementite. The additional free carbon appearing in the solution when the tertiary cementite particles dissolve on heating of the cold-rolled sheet significantly affects the recrystallization and the texture formation.

The influence of alloying and microalloying elements on the mechanical properties of pipe-steel sheet (strength category K60) depends on the temperature in low-temperature controlled rolling, on account of the polymorphous γ → α and eutectoid transformation of austenite in the mill’s finishing stand.

The influence of deformation and heat treatment on the mechanical properties of rebar steel is investigated on a pilot system. Specifically, with intermittent quenching and subsequent self-tempering, the intense-cooling time determines the tempering temperature of the quenched steel and hence the final mechanical properties of the rebar. The influence of the water pressure in the intense-cooling chamber on the uniformity of the mechanical properties is studied for steel with 0.31, 0.32, and 0.35% C; the water pressure is varied from 0.2 to 0.6 MPa. On that basis, it is established that the water pressure in the cooling chamber must be no less than 0.3 MPa, and the carbon content in the steel must be more than 0.32% in order to ensure that the mechanical properties of the rebar steel conform to the AT500 strength class according to State Standard GOST 10884–2004.