Abstract
Problem Statement (Relevance): It is the metal microstructure that primarily determines what combination of mechanical and performance properties the final steel products will have, as well as their processability and applicability. The technical and patent literature looks at the relationship between the metallurgical quality of an as-cast billet and the process and heat treatment parameters and the structure and properties of the wire rod for welding. However, in a number of cases the choice of process conditions that would efficiently lead to the formation of a homogeneous ferritic structure with completely or partially excluded negative bainitic-martensitic areas in a low-carbon alloy wire rod is not a straightforward choice. This article looks at increasing the plasticity of wire rod at the steel-making stage and during the wire rod heat treatment process rather than during the actual wire rod production, which is the conventional approach. Objectives: The article aims to analyze the effect of metallurgical factors and heat treatment on the structure of the wire rod designed for the manufacture of welding electrodes and copper-coated wire for semi- and automatic welding, as well as to develop an efficient end-to-end process from steel production and refining to continuous casting, rolling and the Stelmor line heat treatment process, which would provide a high-performance annealing-free wire drawing operation with the above-mentioned welding products obtaining the desired performance characteristics. Methods Applied: The chemical composition of the steel was determined with the help of standard spectrometers. The metallographic tests were based on optical and scanning and transmission electron microscopy. The microsegregation of chemical elements and the chemical composition of non-metallic inclusions were studied with the help of energy and wavelength dispersive X-ray spectrometers within a scanning electron microscope by electron microanalysis. Micro hardness testers were applied for structural component microhardness measurements. Tensile tests were carried out for the wire rod. A CCT diagram was developed using a dilatometric technique. Originality: The authors have developed scientifically proven solutions for optimising the chemical composition of low-carbon steel alloyed with manganese, chromium, nickel, molybdenum and vanadium at different combinations and concentrations. The authors have defined the conditions for efficient micro-alloying of steel with boron correlated with the nitrogen concentration. The authors have also found an efficient temperature range for the low carbon alloy wire rod austenitization process ensuring an efficient quasi-isothermal cooling rate during the air phase. The rate values were determined when analysing a CCT diagram. Findings: The study helped establish the following scientifically sound relationships: The boron to nitrogen ratio of 0.8 +/- 0.15 should be used for boron microalloying to prevent the impacts of free boron and nitrogen, with free boron entailing an increased hardenability and free nitrogen leading to ferrite strengthening and the ageing effect while at the same time minimizing the hardening element concentration resulting in their minimum grade-determined concentration defined based on the equivalent carbon and/or manganese content (their values should be no greater than 0.55 and 2.10% respectively). Heat treatment of the Sv-08G2S and Sv-08KhG2SMF steels involves austenization at 950-980°C and cooling which takes place under insulation caps at the rate of 0.15 to 0.30°C/s and results in the ferrite structure formation with the minimum number (no more than 5% and 15% respectively) of bainitic-martensitic areas at the cross section. This provided a high processability with no need for softening treatments. Practical Relevance: This gives the best combination of the wire rod structure and properties meeting regulatory and customers’ requirements and ensuring a high-performance wire drawing process.Thus, in the case of the Sv-08G2S steel the tensile strength dropped from 750 to 500 MPa, the contraction ratio increased from 35 to 75%, the number of bainitic-martensitic areas saw an 8-fold reduction (from 40 to 5%). This also saved two intermediate recrystallization annealing operations when drawing a 0.8 mm wire out of a 5.5 mm wire rod.
Keywords
Metallurgical factors, micro-alloying, boron, wire rod, Stelmor line, heat treatment, cooling rate, structure, properties, bainitic-martensitic areas.
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