Study regarding the mechanical characteristics of high-strength steels

Due to the continuous development of the automotive and machine building industry, there has been an urgent need to develop steels with increasingly high strength, without compromising toughness and ductility characteristics. On the other hand, it is necessary for these steels to show high strength even at high temperatures. In this context, there is great interest in high-strength steels, with numerous studies and research in this field of research. The numerical values of the mechanical characteristics of a steel are determined through a series of tests differentiated by the type of macroscopic deformation generated, the method of applying the load, the testing temperature, etc. The most common tests used are static tensile tests and dynamic impact bending tests. This paper presents an optimization of mechanical characteristics based on the chemical composition of the steel, various heat treatment options, and testing conditions.


Introduction
The Alloyed steels contain multiple alloying elements, but one of them is the one that determines its main utilization property, while the others helping either in purity or in behavior during plastic or thermal processing, or to mechanical requests.
The numerical values of the mechanical characteristics of a steel are determined through a series of tests differentiated [1] by: -the type of macroscopic deformation generated, traction, compression, torsion, bending, hardness, etc. are some of the tests used to determine the mechanical characteristics of steel.These tests involve subjecting the steel specimen to specific types of macroscopic deformations in order to measure its response and evaluate its mechanical properties.
-mode of application of the load static, dynamic, variable in magnitude and direction: -static load where is applied steadily over time without significant changes.
-dynamic load, then load varies in magnitude or direction over time, introducing cyclic or impact loading.
-variable load when load changes both in magnitude and direction during the testing process.-test temperature can have a significant impact on the mechanical properties of steel.Mechanical tests can be performed at ambient temperature or at elevated or cryogenic temperatures to assess the steel's behaviour under different thermal conditions.
The most common tests used for determining the mechanical characteristics of steel are presented in following paragraph [2][3][4].
Static Tensile Testing.This test involves applying a static load to a steel specimen until it fractures.It measures the tensile strength, yield strength, elongation, and other properties related to the material's response to stretching or pulling forces.
Dynamic Charpy Impact Testing.This test evaluates the steel's resistance to brittle fracture under dynamic loading conditions.A notched specimen is subjected to a sudden impact load, and the energy absorbed during fracture is measured.This test provides information about the steel's toughness and ability to withstand sudden shocks or impacts [5].
Static Hardness Testing.Various hardness testing methods, such as Brinell, Rockwell, or Vickers, are used to determine the hardness of a steel material.Hardness is a measure of the material's resistance to indentation or scratching and is an indicator of its strength and wear resistance.

Study of the problem
Have been developed high-strength and ultra-high-strength steels in order to combine high strength with appropriate toughness.These construction steels are capable of achieving strengths greater than 1000 MPa.By combining high strength with suitable toughness and ductility, these advanced steels offer improved performance and allow for the design of lighter, more efficient structures that can withstand demanding loading conditions [1], [2], [6].Toughness refers to a material's ability to absorb energy and deform plastically before fracturing.In the context of high-strength steels, toughness is crucial because it ensures that the material can withstand sudden impacts or dynamic loading without catastrophic failure.
The typical chemical composition of such grades of steel is presented in the following table.These steels belong to the Cr-Mo-V steel class, designed for the manufacturing of parts with high strength requirements [5], [7].

Steel class
Chemical composition [%] High-strength steels with adequate ductility can undergo significant elongation or deformation before reaching their ultimate strength, enabling them to withstand large deformations without failure.
These steels must have excellent mechanical characteristics, high strength and toughness, over a very wide temperature range, for example -75 o C …+500 o C, toughness, high fatigue resistance and high reliability.
Another requirement for these heavily stressed steels is a high degree of isotropy in their mechanical characteristics.To achieve a high level of isotropy, the manufacturing and processing of these steels should be carefully controlled.Factors such as grain structure, heat treatment, and material homogeneity play important roles in promoting isotropic behaviour.By ensuring a high degree of isotropy, these steels can meet the demanding requirements of their applications and provide reliable and predictable performance in various loading conditions [7], [8].

Experiments and results of mechanical tests on semifinished product
The processing of steel is done in electric arc furnaces, with basic lining, according to the general instructions for processing -casting.Considering the high level of purity imposed on the steel, it is recommended that its elaboration be done in the duplex system -electric furnace -vacuum treatment facility.
Utilization of a duplex system, facilitates the production of high-purity steel with precise composition control.It enables the steel to meet the stringent requirements for high-stress applications, providing excellent mechanical properties, purity, and reliability [9].
In first part of this work presents the results of mechanical testing conducted on the semifinished product.Test specimens were extracted longitudinally and transversely from the slab and few results are presented in figures 1, 2, 3 and 4. The testing on the longitudinal and transverse specimens from the slab provides valuable information about the mechanical properties of the steel in different orientations.This allows for a comprehensive understanding of how the steel performs under various loading conditions and helps assess its anisotropic behaviour.

Experiments and results of mechanical tests on heat treated samples
Additionally, mechanical tests were also performed on heat-treated and tempered samples, characteristics of heat treatment are presented in table 3. Mechanical tests on heat-treated and tempered samples is crucial for evaluating the steel's response to specific heat treatment processes.Heat treatment can significantly alter the mechanical properties of the steel, including its strength, hardness, and toughness [1], [5], [9].
The tensile and impact bending tests were conducted on samples taken from a slab with a diameter of 80 mm, with the following dimensions: 35 x 300 mm, 30 x 300 mm, 20 x 250 mm.The total number of tests performed on the test specimen for mechanical characteristics was 52 (26 for air cooling and 26 for oil cooling).These bars were subjected to heat treatment according to the data presented in table 3.
After performing the heat treatments, specimens for the tensile test and specimens for the shock bending test were made from the respective samples.Both tests were performed in this case at room temperature.
Ultimate tensile strength or breaking strength represents the maximum stress or force a material can sustain before it fractures or breaks.
Yield strength refers to the amount of stress or force a material can withstand before it starts to deform plastically, meaning it undergoes permanent deformation without fracturing.
Both yield strength and ultimate tensile strength are critical in assessing the mechanical performance and structural integrity of materials and are presented in figure 5. Elongation at break refers to the ability of a material to deform plastically before breaking [4], [5].This property is important in applications where deformability and ductility are key requirements, such as in construction or the manufacture of components that require significant deformability.In figure 6 are presented variation of elongation and fracture toughness depending of temperature of tempering.Fracture toughness is a measure of a material's toughness.The higher the fracture toughness value, the greater the material's resistance to crack propagation and its ability to absorb energy during fracture.This aspect is important in applications where resistance to cracking and material resilience are crucial, such as in construction or the aerospace industry.
For evaluating the resilience of steel, impact bending tests on notched specimens have found widespread practical use.The Charpy impact test, also known as the pendulum impact test, involves fracturing a freely supported specimen with a U-or V-shaped notch using a pendulum hammer.The tests were conducted on U-notched specimens using the Charpy pendulum hammer [3], [4], [8].
The Charpy impact test provides valuable information about the toughness and resistance to brittle fracture of a material.It is particularly useful in assessing the behavior of steel under sudden impact or shock-loading conditions, such as in structural applications or when dealing with potential failure scenarios.
Is an optimal tempering temperature range for each specific type of high-strength steel.The resilience was determined in cases of two types of cooling, in air respectively in oil, on a total number of 96 samples (41 for air cooling and 45 for oil cooling).In both cases, the heating temperature was 550, 580, 610, 640, 670 and 700 degrees Celsius.Temperatures that are too low may lead to insufficient tempering and reduced toughness, while temperatures that are too high can result in excessive softening and decreased strength.Selecting the appropriate tempering temperature is crucial in achieving the desired balance between strength and resilience in high-strength steels [2], [3].
In figure 7 are represented the effective values of the measurements for the maximum 12 samples at different temperatures.Faster cooling rates, such as those achieved with oil quenching, promote the formation of a higher volume fraction of martensite, which can lead to increased strength and hardness.Air cooling provides a milder quenching effect compared to oil cooling.
In figure 8 is presented a comparison between the resilience determined on the samples in the two variants of cooling in oil and in air.The variation of resilience with temperature is represented by polynomial curves of order 3. Finally decision is typically based on a combination of material specifications, application requirements, and engineering considerations [1], [3], [6].

Results, discussion and conclusions
The structure-property relationship in steel is well-established, and heat treatments are crucial for achieving the desired combination of mechanical, physical, and chemical properties required for specific applications.
As a conclusion, in figures 9, 10 and 11, variations of the mechanical characteristics are presented for different variants of heat treatment, for samples taken from semi-finished products respectively with heat treatment.Modifying the structure is the main objective of heat treatments, as it allows for targeted changes in the properties of the material.In fact, we can say that the primary purpose of heat treatments is to alter the structure, which in turn influences the variation in properties of the steel.
During tempering, the hardness, internal stresses, and amount of residual austenite are reduced, while the elongation, fracture toughness, and resilience are increased at the expense of strength.
The characteristics of the tempering structures are clearly superior to those of equilibrium (annealing) due to the fact that the degree of dispersion of the structure is greater and the shape of the carbides and constituents is fine globular.
The temperature of heating is an important factor that influence the results obtained, while the cooling rate during tempering (air, oil) has a lesser influence.These factors are carefully controlled and optimized to achieve the desired combination of strength, ductility, and resilience required for a particular application.Correlating the values of resilience, fracture toughness, and yield strength with the microstructures throughout the tempering range, it can be concluded that the tempering range of 630-650°C can be considered optimal for the final heat treatment of hot-rolled semifinished products.

Figure 5 .
Figure 5. Steel breaking limit and steel yield strenght limit depending of temperature and medium of tempering

Figure 6 .
Figure 6.Elongation and fracture toughness depending of temperature and medium of tempering

Figure 7 .
Figure 7. Variation of resilience KCU for air and oil cooling depending of number of test specimens

Figure 8 .
Figure 8. Comparative variation of resilience KCU for air and oil cooling

Figure 9 .
Figure 9. Comparative variation of breaking limit and yield stress depending on studied cases

Figure 11 .
Figure 11.Comparative variation of resilience depending on studied cases

Table 3 .
Heat treatment applied