Analysis of cold-rolling cracking reason of 45 steel tube

1. Physical and chemical inspection and analysis

1. 1 Macro inspection and microscopic morphology observation: A section of cracked tube with typical cracks was selected to observe the macroscopic characteristics. The cracks were locally distributed along the longitudinal direction of the tube body, with a length of 60-70 mm and an angle of about 15° with the axis of the tube. There were no obvious scratches on the surface of the tube body. The cracks penetrated the tube wall to form a penetrating crack. The morphology of the crack was observed under the EVO18 German ZEISS 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: Samples were cut from round steel billets and cracked tubes, and the chemical composition of the two specimens was detected by Bruker Q4170 direct reading spectrometer. The results are shown in Table 1. Compared with 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: Round steel and cracked tubes were sampled along the transverse and longitudinal sections. After grinding, polishing, and corrosion, the organization was observed under a Leica DM4000M metallographic microscope. The organization of round steel and cracked tubes is ferrite and pearlite. According to the quantitative ratio of ferrite and pearlite in round steel and cracked tubes, it can be seen that their carbon content is at the upper limit of the carbon content of ordinary 45 steel, which is consistent with the material chemical composition test results. The organization of round steel is pearlite and a small amount of ferrite is distributed in a network and needle-shaped, with a slight decarburized layer on the surface. The cracked tube structure is lamellar pearlite and ferrite distributed in white mesh, needle, and block shapes. There is a slight decarburization layer on the surface and cracks inside the structure.

1.4 Hardness test: To detect the hardness change of the tube structure after the round steel is heated, perforated, and cold rolled, the MH-6 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 change of the round steel and the cracked tube, the digital Brinell hardness tester is used to detect the average hardness.

2. Analysis and discussion

2.1 Crack morphology and cause analysis: The chemical composition of the round steel and the cracked tube is tested, and it is known that the carbon content is at the upper limit of the carbon content of the national standard 45 steel. The increase in carbon content leads to excessive pearlite in the organization, which reduces the brittle fracture strength of the steel and increases the tendency of the steel to crack. From the macroscopic observation, the crack is a penetrating crack, which is a shear fracture caused by the plastic deformation of the metal unit after being subjected to complex multi-directional stress and exceeding the strength limit of the pipe. The warping deformation at the edge and end of the crack is caused by residual tensile stress. Microscopic morphology observation and microstructure inspection revealed that the fracture had cleavage characteristics and cracks passing through the pearlite structure inside the structure, which was an obvious transgranular fracture. After perforation and cold rolling, the steel pipe produced a large amount 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 were formed, the strength of the grains themselves decreased, and cracks easily initiated from inside the grains, thus becoming transgranular fractures. From the hardness test, it can be seen from the data in Tables 2 and 3 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 the plasticity and toughness.

2.2 Rolling process analysis: From metallurgical knowledge, it can be seen that the tensile strength is equal to 3.5 times the Brinell hardness. The literature shows that the function curve of cold-rolled 45 steel work hardening 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 perforated rough pipe specifications in this test are 51mm×5.5mm, and the specifications of the cold-rolled pipe are 24.5mm×4.6mm. It can be seen that the cold rolling section shrinkage is Z=63.4%, the elongation coefficient is x=2.732, the tensile strength S=1406.63MPa, and the theoretical hardness of the steel pipe after rolling is 401.7HBW, while the hardness of the cracked pipe detected is 326.3HBW. This shows that the deformation specified by the company is too large, which produces a large internal stress 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 for 45 steel, the pearlite is relatively abundant and the hardness of the steel is high. 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, 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 rolling cracking of 45 steel seamless steel pipes, the 40mm×5.5mm perforated rough pipe was rolled into a finished pipe with a specification of 24.5mm×4.6mm. 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 51mm×5.5mm perforated pipe, and the rough pipe was subjected to the above-mentioned recrystallization annealing process before cold rolling. After tracking the subsequent production of 45 steel seamless steel pipes, the pipe body material was improved, the hardness of the finished pipe was about 256HB, and there was no rolling cracking 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 pearlite hardness 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 after being taken out of the furnace. The perforated rough pipe with a size of 40mm×5.5mm 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.