Selection of PC steel quenching and tempering production line temperature

The selection of quenching temperature is based on the following points, which are determined after comprehensive consideration.

(1) Refer to the critical point Ac3 of the steel to select the quenching temperature. The quenching heating temperature of hypoeutectoid low-alloy steel is 30~50°C above Ac3. As the heating rate increases, the Ac3 point moves up, and its quenching heating temperature will increase. Move to 20~50°C. In the quenching process of the PC steel quenching and tempering production line, not only is it required to achieve austenitization by quenching, but the more important purpose is to obtain fine grains and quenched martensite structure in order to obtain high yield strength after tempering. , The tempered troostite structure with high yield ratio and certain plasticity is beneficial to improve the stress relaxation resistance of steel. In order to achieve the above objectives, the quenching temperature of PC steel should be reduced as much as possible. For this reason, the quenching temperature of the PC steel quenching and tempering production line should be 60~100°C above Ae, and the A3 of PC steel is shown in Table 7-12.

Table 7-12 Critical point temperature of PC steel (reference value) °C

Steel grade A”

30Mn 734 812

30SiMn 740 840

30MnB 732 847

35SiMn 750 830

(2) Choose the quenching temperature according to the mechanical properties of the product. PC steel products have their own characteristics in terms of mechanical properties. It requires steel to have high yield strength (2 1275MPa), high yield ratio (20.05), certain plasticity (25%), and good stress relaxation resistance. These mechanical properties ensure the long-term stability of PC steel. . Table 7-13 and Table 7-14 show the room temperature mechanical properties of typical PC steels after different traditional heating quenching and tempering treatments. The data listed in the table provides a scientific basis for analyzing the relationship between quenching temperature and mechanical properties. These data can provide the following basis.

① As the quenching temperature increases, the strength of the steel decreases and the plasticity increases. Therefore, increasing the quenching temperature will reduce the yield strength and yield ratio, which will have an unfavorable effect on the improvement of stress relaxation resistance.

②Appropriately reduce the quenching temperature (860~900°C). It can maintain a high yield strength and a high yield strength ratio. A low quenching temperature will help improve the surface quality of the steel, save energy, and maintain good resistance to stress relaxation.

To sum up, in order to obtain fine austenite grains and all quenched martensite structure of PC steel, the appropriate quenching temperature range of the quenching and tempering production line should be about 80°C above A*. For silicon-ytterbium PC steel, the quenching temperature is 900 to 950°C. It is equivalent to the quenching temperature of 880~920°C during traditional heating.