30CrMo gas cylinder steel pipe has high strength, high toughness and good processing properties after heat treatment. It is one of the commonly used materials for manufacturing high-pressure gas cylinder tubes.
The cooling rate of samples measured in the laboratory in air is 0.8~1.0°C/s, and the critical cooling rate for steel to completely undergo ferrite + pearlite phase transformation is 0.50°C/s[2]. Observe the metallographic structure of the sample. The structure of the steel after normalization is an integrated structure of bainite + ferrite pearlite. From the structural photos, we can see the metallographic structure of the sample after normalizing and holding for 50 minutes and 90 minutes. The difference is not big; Judging from the mechanical property test results, the mechanical properties of the specimen after normalizing after two heat treatment processes have not changed much compared with the rolling state, and the elongation has slightly increased, but neither of them can meet the user's requirements; Yes It can be seen that the normalizing cooling rate of the small laboratory sample is greater than the critical cooling rate for the complete phase transformation of ferrite + pearlite. The emergence of bainite structure is the fundamental reason why the material strength and elongation cannot meet the requirements of the cold drawing process.
Since the mechanical properties and metallographic structure of the material cannot meet the relevant requirements after the normalizing heat treatment process of the sample, the laboratory conducted an annealing heat treatment process test on 30CrMo steel. The critical temperature of pearlite transformation in 30CrMo steel is 693°C. The sample was heated to 870°C for 50 minutes, then cooled to 680°C in the furnace and removed for air cooling. The microstructure and mechanical properties of the annealed samples were tested.
After testing, the yield strength of the material was reduced to 365 MPa, which is about 200 MPa lower than the rolling yield strength, and the elongation reached 26%. The structure of the steel after annealing is ferrite + pearlite. After annealing using this process, the mechanical properties and structure of 30CrMo steel can meet the user's cold drawing process requirements.
