1.Impact of carbon
It can be seen from Fig. 1 and Fig. 2 that the toughness of steel decreases with the increase of carbon content, and the hardness increases. This is mainly because carbon not only strengthens the matrix in steel, but also increases the hardenability of steel. Although Mr. Cheng’s lower bainite can divide and refine martensite to increase toughness, due to incomplete bainite transformation, the number of acicular structure product martensite increases with the increase of carbon content (see Fig. 3b and Fig. 3C), resulting in the decrease of toughness and the increase of hardness.
2.Effect of silicon
Silicon mainly plays the role of stabilizing austenite in steel. In the process of cooling transformation, it inhibits Fe3C precipitation and the inter strip of lower bainite ferrite substructure ε- The carbide is transformed into Fe3C carbide. From the test results, the amount of silicon in the steel increases, the retained austenite increases (see Fig. 3a and Fig. 3b), the toughness of the steel increases, and the hardness shows a downward trend (see Fig. 1 and Fig. 2).
3.Effect of isothermal quenching temperature
The influence of isothermal temperature on properties is complex, which changes with the amount of C and Si. The general trend is that with the increase of temperature, the toughness increases and the hardness decreases.
When the carbon is 0.38%, the toughness of 2.0% Si bainitic steel increases obviously with the increase of temperature, and the hardness changes little. This is mainly because the carbon content is low and silicon plays a leading role, increasing the retained austenite. When the silicon content decreases to 1.5%, the toughness changes slightly and the hardness increases to a certain extent. At 310 ℃, close to the “nose” point of the C-curve, the austenite inoculation time is short, a large amount of time is transformed into lower bainite, and a small amount of martensite is formed in the subsequent cooling, so the toughness is significantly increased and the hardness is slightly improved. At this time, the comprehensive mechanical properties are the best. At 340 ℃, bainite transformation is dominant, with a small amount of martensite and a part of retained austenite. Therefore, the toughness increases and the hardness decreases.
When the carbon is 0.48% and 0.43%, with the increase of temperature, the toughness decreases first and then increases, and the hardness increases first and then decreases. The mechanical properties at 310 ℃ are obviously better than those at other temperatures. This is mainly because the hardenability of steel increases with the increase of carbon content. There are both martensite with high hardness and bainite with good toughness in the structure, so the comprehensive mechanical properties are very good. At 340 ℃, due to the existence of a certain amount of upper bainite and retained austenite, its hardness is relatively reduced and its toughness is improved.