THE EFFECT OF PLASTIC DEFORMATION ON THE STRUCTURE FORMATION OF LOW-ALLOY STEEL

Sergiy Shejko1, Vadim Shalomeev1, Valerii Mishchenko2

1Zaporozhye National Technical University (Zaporizhzhia, Ukraine) 2Zaporozhye National University (Zaporizhzhia, Ukraine)

Abstract This study determines the effect of plastic deformation on the structure formation and

the process of phase transformations in low-alloy steel 10ХФТБч (Standard of Ukraine). The deformation temperature was varied from 770 оС to 950 оС, the degree and rate of deformation remained constant. It was found that the optimal hot deformation mode for low-alloy steels is in the temperature range from 850…950 °С, at the deformation rate u = 100 s-1

with the degree of deformation ln ε = 1,2. The amount of phases is 80…58% ferrite and 20…42% perlite, which significantly increases the strength of the alloy steel, while maintaining sufficient ductility.

Introduction Hot plastic deformation is still considered as one of the most promising ways of

obtaining a fine-grained structure of metals, which is capable of providing a high level of mechanical properties. In this study, the influence of selected deformation parameters at variable temperature on the structure and properties of low-alloy steel 10ХФТБч (Standard of Ukraine) is investigated [1-3]. The chemical composition of the studied low-alloy steel, wt. %: С – 0,08-0,12; Si – 0,10-0,50; Mn – 0,15-0,50; Cr – 0,14-0,16; V – 0,10-0,15; Ti – 0,12-0,15; Nb – 0,07-0,15; barium – 0,0005-0,0015; rare earth elements – 0,001-0,010; while the content of sulfur and phosphorus in the steel does not exceed 0,035%.

Tests of the hot steel were carried out with a modern plastometer "Gleeble 3800" and a dilatometer. Working parameters of the plastometer were:

− test temperature - 20…950 °C; − speed of the punch – up to 2000 mm/sec; − degree of logarithmic deformation - ;2,1...01,0сomp 15,0...01,0dist .

When tested on the plastometer, samples with a ratio of d x h = 10 x 12 mm were placed in a chamber in which air was evacuated and a vacuum was created to prevent oxidation of the metal. Special computer programs carried out control over the plastometer by temperature, speed and degree of deformation. At certain intervals during the loading process, the yield stress and the logarithmic deformation were recorded.

Table I shows the thermomechanical parameters of the deformed samples.

Table I. Deformation parameters

Samples 1 2 3 4 5 Temperature, °C 770 800 850 900 950

Deformation rate, 1sec 100 100 100 100 100

Degree of deformation, ln ε 1,2 1,2 1,2 1,2 1,2

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Contributed Papers from Materials Science and Technology 2017 (MS&T17)October 8 – 12, 2017, David L. Lawrence Convention Center, Pittsburgh, Pennsylvania USA

Keywords: plastic deformation, structure, phase composition, low-alloy steel

Copyright © 2017 MS&T17®DOI 10.7449/2017/MST_2017_238_244

The size of the actual grain and the ratio of the structural components of the ferrite-perlite samples were determined using a software-hardware complex, which includes the "AXIOVERT 200 MAT" light inverted microscope. To quantify the microstructure, the “Sigmaplot” computer program that process digital images of structural components by constructing histograms of the distribution of the brightness level, corresponding to different details of the structures was used. The confidence interval was 0.95. As a result of processing, the ratios of the areas, occupied by the corresponding structural components were determined.

The Main Researches Fig. 1 presents diagrams of sample compression at different temperatures, showing

the dependence of the yield stress on the compression deformation logarithm at different stages of shaping. The Hensel-Spittel mathematical model was used to describe the change in the yield stress, depending on the logarithmic deformation, temperature and deformation rate. To calculate the correlation coefficient, the corresponding function in the “Excel” program was used. It is established that the computed Hensel-Spittel model is characterized by the following correlation coefficients: 0.9713; 0.9661; 0.9613, that adequately reflects the existing interrelations and can be used to model the rheological properties of 10ХФТБч steel.

Figure 1. Dependence of the deformation resistance on the degree of deformation at various temperatures at a speed of 100 s-1

To develop plastic deformation modes that provide the maximum possible grinding of the structure of low-carbon low-alloy steels, detailed information on the effect of austenite deformation regimes on formation of optimal structure of the steel in the state of delivery is needed. To determine the critical degrees of deformation, which provide formation of recrystallized austenite grains, studies of the effect of the degree and temperature of deformation on the structure of low-alloyed low-carbon steel were made. The deformation temperature was varied from 770 °C to 950 °C, the degree and rate of deformation were kept constant (ln ε = 1.2, deformation rate - 100 s-1). Microstructures and distribution diagrams by the grain-size number are shown in Fig. 2 and Fig. 3.

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a b

c d

f g

Figure 2. Microstructures of low-alloy steel 10ХФТБч after deformation: а, b - 770 оС; c, d - 800 оС; f, g - 850 оС; а, c, f - x3000; b, d, g - x5000; Degree of deformation is ln ε = 1,2 and deformation rate is 100 s-1.

After deformation at the temperature of 770 °C, a ferrite-pearlite structure with a main

grain-size number of 9 ... 11, 2.42% - grain-size number of 14, and 28.34% with a grain size number of 10 (Fig. 3, a) is formed in the steel. There are 81.5% of ferrite and 18.5% of perlite formed in the steel. This is because the deformation takes place in a two-phase region with formation of a significant fraction of ferrite (Fig. 3, b).

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а

b Figure 3. Results of a study of the microstructure of steel, х250: а – distribution by the grain-size number; b – phase distribution; after deformation ln ε = 1,2 at the temperature of 770 оС and deformation rate of 100 s-1.

After the deformation at the temperature of 850 оС, a structure with the main grain-size number of 11 ... 12 is formed. There are 33.35% of grains with grain-size number of 11 and 31.09% with grain-size number of 12 in the structure (Fig. 4, a). At the same time, 62.04% of ferrite and 37.95% of perlite are formed in the structure (Fig. 4, b). This indicates grinding of the γ-phase as a result of recrystallization and formation of a subgrain structure inside the grain.

A further increase in temperature to 950 °C leads to an insignificant increase in the average size of the structural element after deformation ln ε = 1,2. The structure consists of 24.52% of grains with the grain-size number of 11 and 20.16% of grains with the grain-size number of 12. The highest grain-size number of 14 is 10.50%. At the same time, 58,46% of ferrite and 41,53% of perlite are formed in the structure.

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а

b Figure 4. Results of a study of the microstructure of steel, х250: а – distribution by the grain-size number; b– phase distribution; after deformation ln ε = 1,2 at the temperature of 950 оС and deformation rate of 100 s-1.

When looking at the grain size distribution histograms, it can be seen that the distribution pattern is similar at different deformation temperatures. Analysis of the phase distribution histograms established the differences between them. At the deformation temperature of 770 °C, the distribution peak is the share of ferrite, which is more than 80%. At higher temperatures the share of ferrite decreases and lies within the range of 58 ... 62%, which indicates a change in the ratio of the structural constituents of ferrite (with predominantly large-angle disorientations between grains) [4, 5].

Microhardness was measured on the studied samples. Dependences of the change in microhardness and average grain size on the deformation temperature was established. Microhardness of the sample deformed with the degree of deformation ln ε = 1,2 at the temperature of 770 оС, is the lowest and amounts to 260 HV. Increase in temperature to 850 оС leads to increase in hardness up to 320 HV. Further increase in temperature does not affect the microhardness.

The minimal microhardness was obtained on samples deformed at the temperature of 770 °C. The most fine-grained structure is observed in these samples. This is due to the fact that plastic deformation occurs in the two-phase region and the share of ferrite is the greatest - 80% of ferrite and 20% of perlite.

The obtained data is in line with modern concepts of the structure formation mechanisms depending on the deformation temperature of steel. According to these concepts

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[6-9], fragmentation is the dominant mechanism of the structure grinding at the deformation temperature of 770 °C. The effect of this mechanism is partial recrystallization of ferrite and change of the initial orientation to disoriented subgrains (fragments) with small-angle dislocation boundaries of deformation origin.

At higher temperatures, two competing mechanisms are implemented: fragmentation and initial processes of dynamic austenite recrystallization. Due to formation of ferrite at the temperature of 770 °C, minimal hardness is observed, while maximal hardness in combination with small grain size is observed after deformation at 850 оС with the deformation degree ln ε = 1,2.

Thus, the appropriate choice of modes for the 10ХФТБч type steel allows obtaining higher strength and plastic properties. This makes it possible to control obtaining of a given set of properties, and in the future - steel with the same chemical composition for producing after sheet rolling of different categories of strength with increased plastic properties.

The obtained test results for the studied steel with different grain size and calculated stress σi can be used later to construct the dependence of σт on d using the Petch-Hall model. It is known that this dependence follows a well-known relationship [10]:

2

1

dki , (1)

where σт - is the tensile yield strength or the flow stress; d – grain size; σi and k – parameters characterizing given material

Thus, an experimental-theoretical method has been developed for determining the interrelations between the stress-strain state of a metal, the grain size d, and the yield strength σт. To get a full picture of the effect of plastic deformation on the changes in structural-phase transformations in the new 10ХФТБч steel grade, more comprehensive studies with using of modern plastometric methods of testing the stress-strain flow of a metal and its mathematical description are required.

Conclusions The study of experimental 10ХФТБч type steel on a plastometer using special

computer programs made it possible to establish critical points of phase transitions and optimal energy parameters of hot deformation that allows choosing a hot deformation temperature in the range of 850-950 °С.

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