Low Temperature Mechanical Properties of S355J2 Steel and Q345D Steel

S355J2 steel and Q345D steel two kinds of low-alloy high-strength steel have good comprehensive mechanical properties, and are widely used in bridges, ships, power stations, boilers, pressure vessels, petroleum storage tanks, lifting and transportation machinery. Glory Steel Work could supply S355J2 steel square bar and flat bar as crane rail. S355J2 steel and Q345D steel two alloys are the main materials for the tower section of wind turbines, so the study of the low temperature mechanical properties of this steel is of great significance to the production.

For both S355 and Q345D steel materials, the strength gradually increases as the test temperature decreases. This phenomenon occurs because both materials belong to the body-centered cubic metal. In addition, it is difficult for the metal to slip at the low temperature. For the above two reasons, both metals have a sharp increase in yield strength and tensile strength with decreasing temperature. During the low temperature tensile test, it was found that the notched tensile strength of the two metal materials S355 and Q345D was higher than that of the smooth sample at the same temperature. Both S355J2 and Q345D low-strength low-carbon low-alloy steels in the test have higher fracture toughness at room temperature. The platform area of the S355 sample is wider, and the ductile-brittle transition temperature is lower, lower than -80°C. The drop between the upper and lower platforms of the Q345D sample is larger, and the transition temperature range of the fracture toughness is narrower. Compared with Q345D, the S355 low temperature fracture toughness is significantly higher than that of Q345D. By observing the metallographic picture, it is found that the grain of Q345D is significantly larger than that of S355J2. It is concluded that both S355J2 and Q345D belong to low-carbon low-alloy steel and have the same heat treatment state, the same microstructure and similar chemical composition, so the difference in the plateau region is not large. Due to the fine grain size of S355J2, the total grain boundary area increases, and there is also a phase interface between ferrite and austenite. The degree of segregation of harmful impurity elements at the grain boundary and phase interface is relatively reduced, so at low temperature the fracture toughness is superior to Q345D with coarse grain at low temperature.

Compared with room temperature, the yield strength and tensile strength of the two metal materials increased with the decrease of temperature, which increased by about 20% to 25%. As the temperature decreased, the elongation and the area shrinkage and the data at room temperature did not change much, and no significant temperature sensitivity occurred. The elongation of S355J2 and Q345D is about 32%, and the reduction in area is 72%. The tensile strength of the two metal notches gradually increases with decreasing temperature. The tensile strength of the notched material is higher than that of the smooth sample due to the good plasticity of the material. In the fracture toughness test, the fracture toughness of the two materials decreases as the temperature decreases.