SPEED DEPENDENCE OF ACOUSTIC VIBRATION PROPAGATION FROM THE FERRITIC GRAIN SIZE IN LOW-CARBON STEEL

Dep. «Technology of Materials», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (056) 373 15 56, e-mail dnuzt_texmat@ukr.net, ORCID 0000-0002-7353-1916 Dep. «Technology of Materials», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (056) 373 15 56, e-mail 7435892@gmail.com, ORCID 0000-0003-1805-4616 DS Ltd, Scientific-industrial company, B. Morskay St., 63, Mykolaiv, Ukraine, 54001, tel. +38 (0512) 35 44 83, e-mail ds@mksat.net, ORCID 0000-0002-0338-5976 China machinery investment group Ltd, Anli Road, 60, Chaoyang District, Beijing, China, 100101, tel. 86 106 482 7530, e-mail xxhai2004@163.com, ORCID 0000-0001-7051-6254


Introduction
In modern conditions of the industry development, the intensification of production is impossible without the development of automated control systems of technological processes [1,10].
Based on this, the requirements are quite reasonably increasing on the accuracy and speed evaluation of the metallic material properties [2], or the degree defectiveness during the using of the product [9], in which is necessary for the timely adjustment of parameters of technological processes.These automated systems in most cases based on the use of nondestructive methods for determining the properties of metals and alloys [6].The numerical technique for modeling the propagation of elastic waves in materials received the application [12], methods for measuring physical properties of metallic materials, which include methods of acoustic measurement [13,14], acoustic emission, magnetic [11], and other properties.

The state of the question
To some of the known acoustic methods for assessing the properties of metals and alloys include the method of measuring the speed propagation of acoustic vibration (V ) [8].These specifications are structure-sensitive value to the internal structure of the metallic material.By the results of research show the sensitivity of the speed propagation of acoustic vibration to changes in the strength characteristics [13], the structural state of metal [14].On the other hand, for example, carbon steel the same level of strength can be achieved by varying the ratio of morphology and dispersion of particles of the minor phase, the grain size of the metal matrix, the presence of substructure [4] and others.Based on this, rather complex overall impact of structural components in multiphase alloys magnitude to speed propagation of acoustic vibration [8] indicates the necessity for continued research.Making a detailed analysis of the nature of the expected dependency rate speed propagation of acoustic vibration from the characteristics of the internal structure of the metal may be useful for the development of non-destructive testing methods, particularly in the difficult conditions of loading system "wheel -rail" of railway transport [7].

Purpose
It is determining the nature of the ferrite grain size influence of low-carbon alloy steel on the speed propagation of acoustic vibrations.

Methodology
The material for the research served a steel sheet of thickness 1.4 mm.Steel type H18T1 had a content of chemical elements within grade composition: 0, 12 % C, 17, 5 % Cr, 1 % Mn, 1, 1 % Ni, 0, 85 % Si, 0, 9 % Ti.The specified steel belongs to the semiferritic class on the accepted classification.
The specified steel belongs to the semiferritic class on the accepted classification.The structural state of the metal for the study was obtained by cold plastic deformation by rolling at a reduction in the size range of 20-30 % and subsequent recrystallization annealing at 740 -750 ° C. Different degrees of cold plastic deformation was obtained by pre-selection of the initial strip thickness so that after a desired amount of rolling reduction receives the same final thickness.
The microstructure was observed under a light microscope, the ferrite grain size was determined using a quantitative metallographic technique.[3].In order to understand the mechanism of ferrite grain size influence propagation of acoustic vibration in the metal determined the parameters of its fine crystal structure.The using of X-ray structural analysis techniques [5] allowed determining the level of second-order distortion of the crystal latitude of the ferrite.
The speed propagation of acoustic vibrations was measured using a special device such as an ISP-12 with a working frequency of pulses 1.024 kHz [8].As the characteristic of strength used the hardness was evaluated by the Brinell's method.[3].

Findings and discussion
During the cold plastic rolling deformation draft the increasing of shrinkage accompanied by a steady rising in the number of accumulated defects in the crystal structure.With further annealing at the expense of recrystallization processes is the formation of homogeneous microstructure of ferrite grain size structure ( d ), which are inversely proportional to the degree of plastic deformation [4].With increasing ferrite grain size the steel hard- ness, like most of the strength characteristics [4,15], decreases (Fig. 2).The shown dependence is well obeys the equation of Hall-Petch type relation where 0 HB -the steel hardness at the infinitely large ferrite grain size, k -the angular dependency ratio.From the analysis of the ratio (Fig. 2) were determined constant of equation ( 1 HB in absolute values ap- proaching the shear stress of the ferrite crystal lattice, whereas similar magnitude exceeds by more than an order of magnitude [4].Elevated values k are likely due to different stress state of the metal.Thus, measuring the hardness under the indenter is formed volumetric stress state, whereas in most studies devoted to the analysis of diagrams of the Hall-Petch relation, the tests were carried out in the uniaxial stress state at tensile.Experimental studies determined that under certain conditions during heat treatment in the steel structure of the specified can remain the volume fraction of residual austenite.To assess the possible influence of austenite on the steel hardness, determined the size of the lattice phase distortions for ferrit ( µ ).The comparative analysis of the magnitude µ of the steel hardness showed the existence of the directly-proportional ratio between them (Fig. 3).Given the persistent structural state of metal ferrite grain size reduction is accompanied by quite natural growth of phase distortions.On this basis, it can be assume that for investigated the structural state of metal structure in the presence of austenitic phase does not lead to violations of the nature of ferrite grain size influence to the hardness and phase distortions.
The measurement of the velocity of propagation of sound vibrations in the metal from the grain size (Fig. 4) showed the existence of influence the texture of cold plastic deformation by rolling.From the comparative analysis of the given correlations can determine that the dependence of the velocity of sound waves along (Vvp ) and across (Vpp ) in the direction from d remained un- changed and is accountable directly proportional to the ratio: where 0 V -the constant value and α -angular dependency ratio.The influence of texture is reflected not only on the absolute values of the speed propagation of acous-  ).The result is additional evidence of the impact of rolling texture on the value V of the tested metal.Although based on microstructural studies the influence after rolling recrystallization texture is difficult to define (fig.1).This is due by formed almost homogeneous microstructure of grain structure with no signs of forced orientation of the grains.Thus, only a negligible impact from parts of the texture that is left after recrystallization, can be determined by measuring the speed propagation of acoustic vibrations.The analysis of experimental results confirmed the given situation that the influence of the remnants of the texture of cold rolling and subsequent annealing of increasing ferrite grain size is reflected in the change V to a greater extent.
On the basis of numerous experimental data it is known that with increasing degree of cold plastic deformation occurs progressive increase in the number of centers of nucleation recrystallising grains.On this basis, it is quite clear that expectation of grain refinement of ferrite during annealing.
On the other hand, by reducing the degree of cold plastic deformation significantly increases the likelihood of development of processes of polygonization subsequent heating [4].For this reason, after an inadequate level of plastic deformation may develop processes of polygonization could greatly complicate the formation of germ recrystallising grains.Thus, for the investigated steel the influence of texture from preliminary cold plastic deformation by rolling with the growth of the grain size must be manifested to a greater degree.As shown in the diagram it should be considered that the smaller degree of plastic deformation is subjected to the metal, the larger the grain size will be obtained after recrystalization annealing and will be more retained by the influence of cold rolling texture.For the purpose of determining the absence of the influence of the rolling texture you need to use the dependencies Vvp and Vpp from the ferrite grain size (fig.4).Extrapolation of these dependencies in the area of small grain size to the point of intersection (whenVvp ≈ Vpp ) allows you to define the grain size, below which it should expect from the lack of influence of the rolling texture.The ferrite grain size has roughly equal 30-33mkm.Thus, to achieve almost complete absence of anisotropy properties after recrystallization annealing before cold-rolled is necessary to increase the degree of cogging.In this case, the processes of recrystallization will occur more fully and, as a consequence, there will be a more short-grain structure.To justify the submitted proposals were used constructed value correlation Vvp and Vpp from the steel hardness (fig.5).Implementation of extrapolation in areas of high hardness values just as it was done for according to the ferrite grain size (Fig. 4   , that actually corresponds 30-33mkm.A similar grain size values obtained from the analysis of dependencies which are shown on figure 4.
Thus, in terms of the engineering industry applications of the measuring method of speed propagation of acoustic vibrations will allow to evaluate the degree of anisotropy of the properties of metallic materials, without costly testing the mechanical properties.

Originality and practical value
1. On the basis of research defined directly proportional to the impact of ferrite grain size on the propagation of acoustic vibrations in the lowcarbon alloy steel.
2. It is shown that at increasing in size of recrystallising ferrite grain, the degree of texture influence from preliminary cold plastic deformation by rolling is grown up.
The results obtained by the determination of the ferrite grain size influence on the speed propagation of acoustic vibrations may be useful in the development of methods of nondestructive testing of metallic materials.The particular value is the specified method of measurement of gains in the ongoing manufacture of metal structures.

Conclusions
1. Analysis of the hardness dependence of steel from the ferrite grain size showed that the hardness component, which determines the state of a solid solution substantially equal to the shear stress of the crystal lattice of the ferrite.
2. The angular dependency ratio of steel hardness from the ferrite grain size is determined by the influence of the remnants of the rolling texture after annealing recristalization.
3. Shredding ferrite grain lowers the effect textures of rolling low carbon alloy steel after recrystallization annealing.

Fig. 1 .
Fig.1.The microstructure of steel H18T1 after cold plastic flow by the rolling and annealing at a temperature of 750 C. Magnification is 100.The magnitude of the grain size varied in the range 43-65 µm.The typical microstructure of investigated steel is shown on Figure 1.Normal distribution of grains in size is a confirmation of the completion process of building recrystallization.With increasing ferrite grain size the steel hard- of the obtained characteristics with known parameters for most steels shows that 0

Fig. 2 .
Fig.2.The dependence of the steel hardness from the ferrite grain size

Fig. 3 .
Mutual change of size µ and hardness.

Fig. 4 .
Fig.4.The dependence of the speed propagation acoustic vibrations from the grain size and the rolling direction:■ -Vvp ♦and ♦-Vpp Indeed, assessing the sensitivity of the speed propagation of acoustic vibrations in the steel prior to the change of grain size of ferrite in angular ratio ( ) V f d = (fig.4), it can be determine that the propagation of acoustic vibrations in the direction of rolling the magnitude of the sensitivity to changes d ( 1 Vvp d ∆ α = ∆ ) about in 2.5 times exceeds the similar characteristic to the rolling direction transversely ( 2