Analysis of the causes of cracking during cold rolling of 45# steel pipe

1. Physical and chemical inspection and analysis

1.1 Macro inspection and microscopic morphology observation

A section of cracked pipe with typical cracks was selected to observe the macroscopic characteristics. The cracks were locally distributed along the longitudinal direction of the pipe body, with a length of 60-70 mm and an angle of about 15° with the axis of the pipe. There were no obvious scratches on the surface of the pipe body. The cracks penetrated the pipe wall to form a penetrating crack. The morphology of the crack was observed under a scanning electron microscope. The multiple flush fracture surfaces of the steel pipe fracture morphology had cleavage characteristics and obvious brittle fracture characteristics.

1.2 Material Composition Test

The samples were cut from the round steel billet and the cracked pipe body, and the chemical composition of the two specimens was detected by a direct reading spectrometer. By comparing GB/T1591-2008 "Low Alloy High Strength Structural Steel", it can be seen that the carbon content of the two specimens is close to the upper limit of the national standard carbon content, and other alloy components are within the range required by the national standard.

1.3 Microstructure inspection

Sampling was performed along the transverse and longitudinal sections of the round steel and cracked pipes. After grinding, polishing, and corrosion, the structures were observed under a metallographic microscope. The structures of the round steel and cracked pipes are both ferrite and pearlite. According to the ratio of the number of ferrite and pearlite in the round steel and cracked pipes, their carbon content is at the upper limit of the carbon content of ordinary 45 steel, which is consistent with the test results of the chemical composition of the materials. The structure of the round steel is pearlite and a small amount of ferrite in a network distribution and a small amount of needle-shaped ferrite, with a slight decarburization layer on the surface. The structure of the cracked pipe is lamellar pearlite and ferrite distributed in white network, needle, and block shapes, with a slight decarburization layer on the surface and cracks inside the structure.

1.4 Hardness test

To detect the hardness change of the structure in the pipe after the round steel is heated, perforated, and cold rolled, a microhardness tester is used to detect the microhardness of ferrite and pearlite in the round steel and cracked pipe fittings. To compare the overall hardness changes of round steel and cracked pipe, a digital Brinell hardness tester was used to detect their average hardness.

2. Analysis and discussion

2.1 Crack morphology and cause analysis

The chemical composition of round steel and cracked pipe was tested and it was found that their carbon content was at the upper limit of the carbon content of national standard 45 steel. The increase in carbon content caused excessive pearlite in the organization, which reduced the brittle fracture strength of the steel and increased the tendency of steel cracking. From the macroscopic observation, the crack was a penetrating crack, which was a shear fracture of the metal unit after the plastic deformation exceeded the strength limit of the pipe after being subjected to complex multi-directional stress. The warping deformation at the edge and end of the crack was caused by residual tensile stress. From the microscopic morphology observation and microstructure inspection, it was found that the fracture had cleavage characteristics and cracks were passing through the pearlite organization inside the organization, which was an obvious transgranular fracture. After perforation and cold rolling, the steel pipe produces a lot of plastic deformation, severe lattice distortion, and a sharp increase in dislocations inside the grains. After a large number of roughness and resident slip bands are formed, the strength of the grains themselves decreases, and cracks easily initiate from the inside of the grains, which then become transgranular fractures. From the hardness test, it can be seen that the hardness of the cracked pipe fittings is 132.3HBW higher than that of the round steel, and the pearlite hardness in the cracked pipe fittings is 95.6HV0.1 higher than that of the round steel, and the ferrite hardness has not changed significantly. The work hardening caused by plastic deformation increases the hardness of the steel pipe while reducing plasticity and toughness.

2.2 Rolling Process Analysis

From metallurgical knowledge, it is known that the tensile strength is equal to 3.5 times the Brinell hardness. The literature shows that the function curve of the work hardening of cold-rolled 45 steel is: S=660.39x0.7528, where: S—tensile strength, x—elongation coefficient, x=1/(1-Z), Z—cold rolling section shrinkage. According to the above relationship, the specifications of the perforated rough pipe in this test are 51mmX5.5mm, and the specifications of the cold-rolled pipe are 24.5mmX4.6mm. It can be seen that the cross-sectional shrinkage rate of cold rolling is Z=63.4%, the elongation coefficient is x=2.732, and the tensile strength is S=1406.63MPa. Theoretically, the hardness of the steel pipe after rolling is 401.7HBW, while the hardness of the cracked pipe detected is 326.3HBW. The deformation is too large, and a large internal stress is generated in the steel pipe, resulting in cracking during rolling.

3. Improvement measures and effects

3.1 Improvement measures

To eliminate the influence of work hardening after perforation, a recrystallization annealing process is adopted. Since the carbon content of this batch of steel is close to the upper limit of the national standard of 45 steel, the pearlite is relatively more and the hardness of the steel is higher. Because the hardness of pearlite is related to its interlamellar spacing, the larger the interlamellar spacing, the lower the hardness, the slower the cooling rate during annealing, and the larger the interlamellar spacing of pearlite. Therefore, the recrystallization annealing process is used before rolling to improve the plasticity and toughness of the steel and eliminate the influence of work hardening. The recrystallization annealing temperature is 730℃, and it is cooled to 160℃ at a rate of 80-100℃/h and then air-cooled out of the furnace. Under the premise of meeting the strength and hardness of the steel pipe, the cold-rolled 45 steel work hardening function curve: S=660.39x0.7528 is used to design a reasonable deformation amount. A large deformation amount will make the strength and hardness of the steel pipe too high, and a large internal stress will be generated in the steel pipe, causing cracking during rolling or straightening, which is also not conducive to processing and use.

3.2 Implementation Effect

Through the above analysis of the causes of cracking during the rolling of 45 steel seamless steel pipe, the 40mmX5.5mm perforated rough pipe was rolled into a finished pipe with a specification of 24.5mmX4.6mm. At this time, the cross-sectional shrinkage rate of the steel pipe Z=51.7%, which was 12 percentage points less than the deformation of the finished pipe rolled with a 51mmX5.5mm perforated pipe, and the rough pipe was subjected to the above-mentioned recrystallization annealing process before cold rolling. After the subsequent production tracking of 45 steel seamless steel pipes, the material of the pipe body was improved, the hardness of the finished pipe was about 256HB, and there was no cracking during the rolling of the pipe body, which proved that the improvement measures were effective.

4. Conclusion

1) The carbon content of the steel pipe is close to the upper limit of the national standard and there is too much pearlite, the lattice distortion is serious, the cracks initiate from the inside of the grain to form transgranular fracture, and the fracture is brittle. The overall hardness of the steel pipe reached 326.3HBW, and the hardness of the pearlite in the organization reached 325.0HV0.1.

2) The work hardening phenomenon of 45 steel round steel after perforation and rolling reduces the toughness and plasticity of the steel. At the same time, the rolling deformation of the steel pipe is too large, reaching 64.3%, which causes large internal stress inside the steel pipe and causes cracking during rolling.

3) To eliminate the work hardening phenomenon of the steel pipe, the recrystallization annealing process is adopted: the temperature is 730℃, cooled to 160℃ at a speed of 80-100℃/h, and air-cooled out of the furnace, and a perforated rough pipe of 40mmX5.5mm is used for rolling, which reduces the rolling deformation. The quality of the steel pipe body has been improved in the follow-up production, and no rolling cracking has occurred.