EFFECTS OF METALLURGICAL FACTORS AND HEAT TREATMENT ON THE STRUCTURE OF WIRE ROD DESIGNED FOR STEEL CORD PRODUCTION
It is primarily the microstructure that determines the mechanical properties and performance of steel products, as well as their processability. The technical literature describes the effects of metallurgical quality of primary steel sections and of process and heat treatment parameters on the structure and properties of high carbon wire rod. However, sometimes it is not easy to identify the best processes to get a finely dispersed pearlitic structure, the required decarbonization and other properties in cast and rolled steel to be used for steel cord production. The purpose of this work is to study the effects of metallurgical factors and heat treatment on the structure of wire rod designed for steel cord production, and to develop an efficient end-to-end process technology comprising steel making, refining, continuous casting, rod production and heat treatment processes using a Stelmor line which ensures a required scope of steel cord properties and a highly efficient thin wire drawing and stranding process. Among the techniques applied, standard spectrometers were used to determine the chemical composition of steel. The metallographic study was performed through scanning optic and electron microscopy. The chemical segregation and the chemical composition of nonmetallic inclusions were analyzed through energy-dispersive and wavelength dispersive X-ray spectroscopy on an electron probe micro analyzer (EPMA). Microhardness was measured at different loads using a microhardness tester. The wire rod was tested on tensile-testing machines. Dilatometric techniques were applied to build a CCT diagram. The originality of the study includes development of proven solutions for austenization temperature for high carbon wire rod to ensure an efficient cooling rate at the air-cooling stage. The efficient cooling rates were identified when analyzing the CCT diagram. The following proven patterns have been established. The steel is microallyed with boron at the boron to nitrogen ratio of < 0.4 to prevent the negative impact of free boron and nitrogen when boron causes increased hardenability and nitrogen leads to ferrite strengthening and steel aging. Steel contamination with nonmetallic inclusions was studied and effective solutions were offered on how to modify them. The heat treatment process of 70-85 steel involves austenization at 950-980<sup>о</sup>С and cooling at the rate of 25-30<sup>о</sup>С/s with sorbitic pearlite formed covering at least 70% of rod section. This results in the best combination of wire rod structure and properties with high processability meeting both the regulatory and consumers' requirements.