Study of the Phase Transition Kinetics in the Experimental Melting of Rolled Coils of K55 Grade Strength Steel for Pipes Manufacturing

Experimental results of melting and in-laboratory rolling of steel of K55 grade strength of new chemical composition with 0.32% C, 1.1% Mn and with addition of vanadium are presented. The phase transition kinetics of melting of new chemical composition by method of building the thermokinetic diagram of transformation of deformed austenite at continuous cooling is explored. Based on the analysis of the constructed thermokinetic diagram, the recommendations for obtaining the target ferrite-perlite microstructure at mill 2000 of PJSC “Severstal” are given: cooling rate up to 20°C/s at a coiling temperature above 580°C. The microstructure was studied by optical microscopy and scanning electron microscopy. All samples had a ferrite-pearlite microstructure. The mechanical properties of the experimental rolled strips after correcting the cooling modes meet the target requirements for K55 steel: σ Y = 379–552 N/mm 2 , σ UTS ≥ 655 N/mm 2 , δ 5 = 22%, KV +21 ≥ 27 J.