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Investigation of the structural state of pipe steel during thermo mechanical processing

The article is devoted to the research of the structural state of pipe steel during thermo mechanical processing on the line of the 5000 thick-sheet mill. The study of layer-by-layer structure formation depending on the temperature of the rolling end and the trajectory of accelerated cooling of the workpiece in the process line was carried out using theoretical thermodynamic methods. The technological parameters allowing to increase the dispersion of the microstructure and, accordingly, the strength characteristics of thick-sheet pipe steel are determined. It is revealed that the austenite grain is crushed due to the use of controlled rolling and the decay of a significant part of austenite in the temperature range of bainite transformation


Introduction
Consumer properties (strength, ductility, impact strength, cold brittleness) of steels are determined by the phase and structural state. The key task in developing new and improving existing technologies for the production of tube steels is to select a controlled rolling and heat treatment mode that ensures the necessary phase and structural state. Understanding the mechanisms that control the kinetics of austenite decay, the course of phase transformations and the formation of structures in steel is of great practical importance [1,2,3].
In most cases, high properties are achieved not as a result of isolation of equilibrium products of phase transformations, but due to the formation of certain metastable structures [1][2]. In low-carbon steels of classes Х80-Х100, high mechanical strength is provided due to the purity of the steel by impurities, the absence of perlite, the small size of the micro structural components and the optimal combination of all the mechanisms for strengthening the steel. Grinding of austenitic and ferritic grains is achieved by applying controlled rolling and performing the decomposition of a significant part of austenite in the temperature range of bainite transformation [4][5][6][7][8].
The construction of physically based models describing the processes of austenite decay and formation of the structural state can significantly speed up and reduce the cost of developing new steels.
The purpose of this work is to analyze the processes occurring during thermo mechanical processing of pipe steel on the line of the thick-sheet mill 5000 and develop mathematical models describing these processes.

Methodology
To obtain the best consumer properties of pipe steels, small micro structural components, the predominance of the bainite structure and its like are required. Of great importance is the content of bainite in the surface layers of the workpiece.
For model calculations of the final microstructure, the corresponding IT solution was used, developed by LLC ITC "Ausferr" (Magnitogorsk) in conjunction with Nosov MSTU and MMK PJSC's 5000 sheet mill automated control system [9][10][11][12][13][14]. The model implements a phenomenological form of layer-by-layer prediction of structure formation depending on the temperature of the rolling end and the trajectory of accelerated cooling of the workpiece in the process line [15][16][17][18][19]. The initial data for the calculations were taken technological parameters of the production of pipe steel of the 06G2MB brand, corresponding to the standard strength class K60 [11]. The parameters of the accelerated cooling mode are shown in table 1. Temperature of end of rolling, o C 820 Speed at the accelerated cooling line, m/с 1,994  (Table.2, Fig. 1). These results, in general, correlate with our studies of the microstructure and crystallographic texture of the results of a number of experimental production modes [2,3]. The cooling mode of the workpiece lower surface is quite intense; the share of bainite on the lower surface of the workpiece was near 98%.

Result and discussion
On the lower surface (Table 3), an increment of the bainite fraction is also observed, but by a smaller amount of near 8%. This is due to the high content of bainite in the initial state. The ferrite content also is decreased by an equivalent amount. The proportion of martensite remains insignificant. This dynamics of the content of micro structural components on the lower surface of the workpiece increases the strength characteristics of rolled products.
The average distribution of micro structural components over the entire thickness of the workpiece is changed slightly, but for the lower (more dispersed) bainite it increased by near 5% displacing the proportion of the upper bainite and ferrite / perlite by the corresponding amount. At the same time, the share of ferrite/perlite fell by near 2%. This distribution and other results of calculations indicate a fairly significant degree of heterogeneity of the microstructure of the thick sheet in thickness. The average content of ferrite/perlite practically does not exceed 40% and the main share of it falls on the central layers.

Summary
Using the results of atomistic modeling and methods of theoretical thermodynamics, the analysis of physical processes occurring in the rolling workpiece during its passage through the technological line of thick sheet mill 5000 of MMK PJSC was carried out.
Controlling the workpiece speed of driving at the accelerated cooling line allows achieving a significant increase in the dispersion of the microstructure. Increasing the dispersion of the microstructure increases the strength characteristics of thick sheet workpiece. The use of methods for describing the influence of technological parameters on the formation of micro structural components allows you to adjust the technological process in order to increase its efficiency.