Effect of Heat Treatment on Mechanical Properties of Steels of Strength Class K65 (X80)

High-strength superlow-carbon pipe steels microalloyed with V, Nb and Ti possess a unique set of mechanical properties due to ultrafine grains (about 2 – 3 m) and a proportioned content of ferrite and bainite martensite formed as a result of controlled rolling and accelerated cooling [1, 2]. A typical feature of the base metal of welded pipes from these steels is an elevated dislocation density in the ferrite ( d = 5 10 cm – 2 ) and in the bainite martensite ( d = 1 10 cm – ) [2, 3] acquired in the production of the sheet and of the pipe. The accelerated cooling fixes a specific concentration of the atoms of carbon and alloying elements in the solid solution of ferrite, which, in addition to the elevated dislocation density, is responsible for the susceptibility of these steels to deformation aging [4, 5]. In the steels with superfine grains this effect is much more pronounced [6 – 8]. In the opinion of some researchers [9] strain aging should cause degradation of the strength, ductility and toughness properties, which has been discovered for steels of strength class X70 and lower strength classes with ferrite-perlite structure [10]. Steels of strength class X80 have been developed relatively recently, and the experience of operation of pipes produced from them is limited, which attracts interest to the possibility of degradation of mechanical properties. At the same time, it is known that the possibility of raising strength in the production of sheets for gas pipes by the method of controlled rolling and accelerated cooling has almost been exhausted [11]. All these moments attract interest to application of various modes of heat treatment to steels of high strength classes. The aim of the present work was to study the evolution of mechanical properties ( y , r , KCV ) of steels of strength class X80 (K65) after heating in a wide temperature range from 150°C to the bottom part of the intercritical range.