1. Introduction
The search for new technical solutions directly linked with the study of the physics and mechanics of the metal forming processes, rheology behavior of the material and the formation of the metal structure. The most important characteristic of the material is the strain resistance
2. Determination of the Influence of Thermomechanical Parameters of the Tensile Process on the Strain Resistance
The staff of the Metal Forming Department of Ural Federal University (Ekaterinburg, Russia) together with the scientists of Czestochowa Technological University (Czestochowa, Poland) conducted studies of the dependence of the thermomechanical parameters of the tensile processon the strain resistance of lowcarbon steel. For this purpose, the samples were tested on the torsion plastometer «STD 812»of Czestochowa Technological University. Chemical composition of the steel is presented in the Table 1.
The determinants varied in the following range: deformation temperature
Table 1


C  Mn  Si  P  S  Cr  Ni  Cu  Al  Mo 
0,21  0,97  0,10  0,014  0,009  0,26  0,07  0,17  0,024  0,014 
N  Pb  Al  As  Cb  V  Ti  B  Zn  Sn 
0,0119  0,001  0,020  0,007  0,002  0,004  0,047  0,0030  0,018  0,012 
Equations (1) and (2) were used to calculate the degree and speed of torsional deformation and to determine the strain resistance equation (3) was used [24]:
(1)
(2)
(3)
where: r – sample radius, L – sample length, N – number of sample rotations,
Figure 1 shows deformation hardening curves of steel depending on the strain degree and strain rate at temperature 1200
Similar dependences are observed for other thermomechanical parameters of the process. It is found that at the beginning of the test there is an intensive increase in the strain resistance, and when strain degree
3. Processing of the plastometer tests results for determination of strain resistance
For determination of strain resistance
(4)
where
(5)
The parameters of the model (5) are convenient to represent in normalized form:
where
The regression equation takes the following form:
Parameters that affect
Table 2


Factors  1  BL  +1 

X

0,25  0,45  0,65  0,2 
X

5  10  15  5 
X

900  1000  1100  100 
A secondorder central composition plan was used, which as a "core" has a matrix of linear orthogonal total factorial experiment 2
The orthogonality of the central compositional plan is provided by the appropriate selection of the star shoulder α (for three factors α = 1,215) and special transformation of quadratic variables x
Due to the orthogonality of the planning matrix, its coefficients are calculated using the following formulas:
By the calculations results were determined:
Table 3
The significance of the coefficients is checked by the Student criterion
for that the reproducibility dispersion is determining by five parallel experiments in the central point of the plan
The dispersion of the regression equation coefficients is determined by the following formulas:
By the calculations results were determined:
The Student ttest for each of the coefficients was:
The response values
The adequacy of the equation is checked by Fisher criterion:
The equation is adequate, since composed Fratio is less than the theoretical F
then the mathematical model
can be considered adequate and recommended for technological and strength calculations.
4. Summary
Studies of the dependence on the strain resistance of lowcarbon steel in dependence of the thermomechanical parameters of the tensile process were conducted. It is found that at the beginning of the test there is an intensive increase in the strain resistance, and when strain degree reaches a certain value, the metal is practically not strengthened, and with further deformation, the strain resistance decreases, i.e., the material is softening. The correction algorithm of the strain resistance value of mathematical model is presented. A mathematical model of the relation between the strain resistance and the thermomechanical parameters of the tensile process was obtained. The adequacy of the model and the possibility of its use to study the influence on the strain resistance of thermomechanical parameters independently of each other were proved based on the application of statistical analysis methods.