Volgograd State Technical University, Volgograd, Russia:

V. F. Petrova, Cand. Eng., Associate Prof., Dept. of Materials Technology, e-mail: tecmat@vstu.ru

A. V. Drozdov, Undergraduate Student

Volgograd State Technical University, Volgograd, Russia¹ ; EXPERTIZA Ltd., Volgograd, Russia²:
S. O. Gevlich, Cand. Eng., Associate Prof.¹, Technical Director²

JSC Volzhsky Pipe Plant, Volzhsky, Russia:
A. A. Guseva, Process Engineer of the 1^st category, Laboratory of Metal Science and Heat Treatment of Pipes

The paper considers the effect of rolling heating on the formation of the microstructure of bainite steel. The effect of hot deformation and cooling rate on the grain size and the forming structure is shown. The grain size decreases from 75 microns, in a workpiece with a low degree of deformation, to 45 microns with a higher degree of deformation. The paper simulates the effect of different cooling rates during a thermal cycle simulating rolling heating on the structural state of samples from the steel under study. It is established that the excess phase in the form of ferrite located along the grain boundaries of the main phase is formed during cooling at a rate within 20 °C/min. With an increase in the degree of deformation from 7 to 40 %, the amount of ferrite decreases from 13.7 % to 6.5 %. The work reveals the determining factor of destabilization of the impact strength of steel 40KHMFA pipe billet, which consists in the release of excess ferrite along the grain boundaries during hot deformation, thereby obtaining a twophase structure. An assumption is made about the influence of the two-phase structure on the instability of the impact strength indicators. The formation of microcracks in ferrite, which persist during subsequent heat treatment (improvement), is considered as a mechanism of destabilization. For the stability of the impact strength, it is necessary that the deformation and calibration of the workpieces take place in a single-phase austenitic region, i.e. the temperature of its end was not lower than the point Ac3 (825 °C). At the same time, it is necessary to control the cooling rate of the workpiece at a level of more than 20 °C/min and less than 93 °C/min.

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