The cold brittleness (or low-temperature embrittlement tendency) of the cold-rolled precision bright steel pipe is expressed by the toughness-brittle transition temperature Tc. High purity iron (0.01% C) has a Tc at 100 C. Below this temperature, it is completely in an embrittled state. Most alloying elements in cold-rolled precision bright steel tubes increase the toughness-brittle transition temperature of cold-rolled precision bright steel tubes, increasing the tendency of cold and brittleness. When the ductile fracture occurs above room temperature, the fracture of the cold-rolled precision bright steel pipe is a dimple-type fracture, and the fracture is a cleavage fracture at a low temperature. The reason for the low temperature embrittlement of cold-rolled precision bright steel pipe is: (1) The theory that the local stress exceeds the cold-rolled precision bright steel pipe when the dislocation generated by the dislocation source is blocked by the obstacle (such as grain boundary and the second equivalent). The strength produces microcracks. (2) Several dislocations of the plug build a microcrack at the grain boundary. (3) The two slip belts react at the intersection, causing the movable dislocations to be wedge-shaped microcracks.
Factors that increase the cold and brittleness of cold-rolled precision bright steel pipes are:
1. Solid solution strengthening elements. Phosphorus increases toughness and brittleness. The transformation temperature is the strongest; there are also molybdenum, titanium and vanadium; the elements with low content and low content and high toughness and brittle transition temperature are silicon, chromium and copper; reducing toughness and brittleness. There is nickel in the conversion temperature, and manganese is firstly lowered to increase the toughness-brittle transition temperature.
2. Form the elements of the second phase. In the second phase, the most important element for cold rolling of cold-rolled precision bright steel pipe is carbon. With the increase of carbon content in cold-rolled precision bright steel pipe, the pearlite content in cold-rolled precision bright steel pipe increases, and the average pearlite volume increases by 1%. The toughness-brittle transition temperature increased by an average of 2.2 °C. Figure 2 shows the effect of carbon content on brittleness in ferritic-pearlite steel. The microalloying elements such as titanium, niobium and vanadium are added to form a dispersed nitride or carbonitride, which causes the toughness-brittle transition temperature of the cold-rolled precision bright steel tube to rise.
3. Grain size affects the toughness-brittle transition temperature. As the grain coarsens, the toughness-brittle transition temperature increases. Refining the grains reduces the cold-brittle tendency of cold-rolled precision bright steel tubes, which is a widely used method.