1.What is the root cause?
Steel has a body-centered cubic crystal structure. In this structure, the ability of dislocations to move between atoms decreases significantly as the temperature decreases.
When the temperature falls below a certain critical value (called the ductile-brittle transition temperature), the material transitions from ductile fracture (accompanied by significant plastic deformation) to brittle fracture (sudden fracture with almost no plastic deformation).
2.What are the exacerbating effects of the cold rolling process?
Work hardening: Cold rolling is a plastic deformation process at room temperature, which leads to grain distortion and increased dislocation density, thereby improving the strength and hardness of the material, but reducing plasticity and toughness. This inherent low plasticity state worsens further at low temperatures.
Internal stress: The cold rolling process leaves significant internal stress within the material, which may promote crack initiation and propagation at low temperatures.
3.What are the key influencing factors?
Chemical Composition:
Carbon Content: Higher carbon content generally leads to a higher ductile-brittle transition temperature and more pronounced low-temperature brittleness.
Impurity Elements: Impurity elements such as phosphorus, sulfur, and nitrogen significantly increase the cold brittleness tendency of steel.
Alloying Elements: Adding elements such as nickel and manganese can refine the grain size, lower the ductile-brittle transition temperature, and improve low-temperature toughness.
Microstructure:
Grain Size: Fine grains can improve toughness and lower the ductile-brittle transition temperature. Recrystallization annealing after cold rolling can further improve toughness.
Microstructure Type: Ferritic steels exhibit more pronounced low-temperature brittleness, while austenitic stainless steels (such as 304), due to their face-centered cubic structure, typically maintain good toughness at low temperatures.
Service Temperature: The lower the temperature, the more severe the decrease in toughness. Different grades of steel have their specific minimum service temperatures.
4.What are some countermeasures in engineering applications?
Material Selection: In low-temperature environments (such as outdoor structures in cold regions, cryogenic pressure vessels, and transportation equipment), cryogenic steel must be selected.
For example, Q345D/E (corresponding to the old grade 16MnDR) is a low-alloy high-strength steel. The "D" and "E" in the grade designation represent the impact toughness requirements at -20℃ and -40℃, respectively.
For cryogenic environments (such as liquefied natural gas equipment), austenitic stainless steel or nickel alloys are required.
Heat Treatment: Annealing cold-rolled steel can eliminate work hardening and restore plasticity and toughness.
Design and Manufacturing: Avoid sharp notches and stress concentrations, strictly control welding processes (to prevent heat-affected zone embrittlement), and conduct low-temperature impact tests to ensure safety.
5.How tough is cold-rolled coil in a low-temperature environment?
Due to its processing characteristics, cold-rolled coils inherently have lower toughness than hot-rolled or annealed coils. This decrease in toughness (increased brittleness) is even more pronounced at low temperatures. Therefore, when using cold-rolled steel in low-temperature conditions, material selection should not be based solely on its room-temperature performance. Its ductile-brittle transition temperature must be fully considered, and materials meeting the corresponding low-temperature toughness standards must be selected. Heat treatment may be necessary to improve performance. In practical applications, the requirements for low-temperature impact energy of materials in relevant industry standards and specifications (such as GB/T 150 "Pressure Vessels", EN 10028, etc.) should be strictly followed.