Hardness specification of structural steel used in the Republic of Kazakhstan

. Monitoring was conducted for the structural steel types used in the Republic of Kazakhstan for construction of buildings using the metal frames. For each type of steel it was the first time determined for hardness specification by Brinell methods. Hardness specification is applied to determine the yield strength, breaking strength and the relative deformation at rupture. These parameters of structural steel are considered to determine compliance with requirements of the National Annex of the Republic of Kazakhstan Eurocode 1993, which is used to design metal structures in the Republic of Kazakhstan. Previously, such issues have not been solved yet. Practical significance of the work consists in possibility of using local types of steel for design according to Eurocode 1993.


Introduction
The possibility of determining mechanical properties from hardness measurements was first shown by I. Brinell. Subsequent numerous experimental researches [1] have confirmed these conclusions. It was shown that there is a correlation of hardness with strength, yield strength, ductility and wear resistance characteristics. Theoretical foundations of static and dynamic hardness testing have been developed, which allows connection to be made between hardness parameters and other properties of the studied materials [2][3][4][5].
The Brinell hardness values of material are usually measured using the special instruments, followed by processing of the results in statistical analysis and theoretical studies. In [2], on the basis of experimental data and theoretical analysis, not only the relationship between Brinell hardness and mechanical properties of the measured material is discussed, but also all sources of measurement uncertainty are analyzed. The force loading characteristics of Brinell hardness measurements are described and a finite element analysis is performed for the entire Brinell hardness measurement process. Next, the Monte Carlo method for simulating Brinell hardness experiments is applied to obtain theoretical measurement uncertainties and reliable Brinell hardness values. Finally, several series of experiments are performed on HB-3000C and the Brinell hardness values are assumed to be reliable.
Work [3] is devoted to determining the hardness of stainless steel by the Brinell method. Study [4] provides for the rail steel hardness outcome. The Brinell hardness measurements and ultrasonic measurements show the change of material properties as a result of the rail manufacturing process. This is a successful example of combination of different ways to determine mechanical properties of the material.
Work [5] includes different methods -uniaxial tension test, Brinell hardness test and instrumental indentation method. There was also comparative analysis of strength characteristics of the sheet steels performed. It is established that the difference of strength characteristics Rp0,2 and Rm values of researched steels by Brinell method in comparison with corresponding strength characteristics under uniaxial tension do not exceed 10,4 %.
Hardness measurement methods are used to determine the strength properties of rocks [6], and in [7] for aluminum alloys. The article [7] presents data on Brinell hardness of forged aluminum alloy EN AW-2618A. Thus, the Brinell method is a universal method for determining the hardness of many materials.
The abovesaid indicates that the hardness method can be widely used to obtain not only the standard but also special mechanical properties, and it should be assumed that the hardness method has as yet unexplored possibilities for determining the mechanical properties of various materials, including structural steels.
The use of the Brinell hardness measurement method is governed by European standards. For example, German -DIN EN ISO 6506 -1:2015-02 Metallic Materials -Brinell Hardness Test -Part1: test Method Beuth Verlag GMBH, Berlin.
It should be noted that there are various ways of estimating the values of strength properties of structural steel on the basis of the brinelless methods according to the hardness characteristics [1]. They are all based on correlation between Brinell, Vickers, Rockford or Mayer hardness parameters and the strength properties of the steel. Experimental and theoretical approaches of deformable body mechanics are used.
The objectives are to: -Using the Brinell method to determine the hardness values of structural steel used in construction practice in the Republic of Kazakhstan.
-On the basis of the hardness values of structural steel to determine parameters of the yield strength and value of the relative breaking strength for 7 types of structural steel.
-Verify the conformity of structural steel used in the Republic of Kazakhstan to requirements of Eurocode 93 [8].
Such issues in the Republic of Kazakhstan are solved for the first time. This is connected to the fact that in the Republic of Kazakhstan, since 2015, a new regulatory framework in construction based on the Eurocodes is applied.

Methods and objects
The experimental study is assumed for 7 samples of structural steel ( Table 1). Specification of the structural steel is taken from the factory certificates of conformity. The hardness of steel is investigated by the Brinell method. The hardness values are then used to determine the yield strength, breaking strength and relative deformation at rupture.
Limit values of the ratio of temporary resistance to yield strength, relative elongation at rupture and the relative deformation of steel must meet the following requirements: Relative elongation at rupture not less than 15% It is not difficult to see that requirements of the National Annexes are about 18% stricter than requirements of the main text of Eurocode 1993.
Further research will consist of experimental verification of compliance with requirements of the Eurocode and National Annexes for the types of structural steel used in the Republic of Kazakhstan.

Outcomes
Seven structural steel samples from Table 1 were tested for hardness by the Brinell method using certified TKM-359 metal hardness tester. Therefore, the HB hardness values were determined. Table 2 shows the HB Brinell hardness values for 7 types of steel from Table 1. In Table 3, the Brinell hardness values are given for the breaking strength, yield strength and relative elongation at rupture. In calculations, the method of determining the properties of structural steel from [9][10][11][12] was used. Analysis of Table 3 shows that conditions (1) and (2) are fulfilled. Condition (3) in terms of the Brinell hardness values is quite difficult to assess.
Thus, obtained the first positive results of assessing the quality of structural steel samples used in the Republic of Kazakhstan of local and Russian production.

Consideration
Using MATHCAD and MATLAB mathematical models we approximated the experimental data. Graphical representation of the obtained results is shown in Fig. 1 -3. Linear dependence (straight line regression) obtained by minimizing the error sum of squares is acceptable for approximation: = −87.249 + 2.812 (4) = 129.944 + 2.592 (5) = 39.864 − 0.118 (6) Equations (4) can be solved with respect to BH hardness values:  Table 4 shows the BH hardness values corresponding to the normative steel grades from Eurocode 1993 and obtained by successive calculation using expressions (7), (5), (6). Let's solve the inverse task. Using the values of yield strength from Table 1, determine the hardness BH, tensile strength, hardness values and elongation at rupture. In this way, we check the accuracy of the mathematical model. If we assume that the ultimate strength in the factory tests is determined correctly (Table  1), we can recalculate the error in calculation method according to formulas (4)- (7). The average value of the error is 9.02%.
In conclusion, we add that other methods of the hardness determination can be used for quality control of hardness values. It is possible to use, for example, the Rockwell method. There are quite a few ways of its application have been developed [12].

Conclusion
1. Hardness tests of structural steel samples with thickness of 8-20 mm using the certified hardness tester TKM-359M were carried out, and the Brinell HB hardness values were determined. It was found that in all 7 series of tests, conditions (1) and (2) are satisfied for the obtained HB values. Further it is supposed to carry out a comparative evaluation of different methods of determination of yield or breaking strength values by Vickers (HV), Rockwell (HRA) and Shore (HSD).
2. To assess the accuracy of determining the properties of structural steel, it is advisable to conduct experimental tensile testing of structural steel samples in accordance with ISO standard.
4. Within the limits of this model, the requirements of interstate standard GOST 27772-2021 and National Annexes of the Republic of Kazakhstan Eurocode 1993 in relation (1) and value of deformation at rupture are completely satisfied.
5. Values of relative elongation after rupture, calculated by hardness tests, less than the values indicated on the factory certificates, but fully consistent with the requirements of Eurocode 1993. The outcome show the high ductility of domestic steel.