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

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 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. By comparing GB /T 1591-2008 "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, and 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 ratio of ferrite and pearlite in round steel and cracked tubes, 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 structure of round steel is pearlite and ferrite distributed in a network and a small number of needles, with a slight decarburization layer on the surface. The structure of the cracked pipe is lamellar pearlite and ferrite distributed in a white network, needle, and block, 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 of the pipe 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 respectively. To compare the overall hardness change of round steel and cracked pipe, the digital display Brinell hardness tester is 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 is tested, and it is known that their carbon content is at the upper limit of the carbon content of national standard 45 steel. The increase in carbon content leads to excessive pearlite in the structure, which reduces the brittle fracture strength of steel and increases the tendency of steel to crack. From the macroscopic observation, the crack is penetrating. It 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. From the microscopic morphology observation and microstructure inspection, it is found that the fracture has cleavage characteristics and cracks are passing through the pearlite structure inside the organization, which is an obvious transgranular fracture. After perforation and cold rolling, the steel pipe has a large amount of plastic deformation, severe lattice distortion, and a sharp increase in the positional dislocation in the grain. After the roughness and the resident slip band are formed in large quantities, the strength of the grain itself decreases, and the crack is easy to initiate from the inside of the grain, and then becomes a transgranular fracture. From the hardness test, it can be seen that the hardness of the cracked pipe is 132.3 HBW higher than that of the round steel, and the pearlite hardness in the cracked pipe is 95.6 HV0.1 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, we know that tensile strength is equal to 3.5 times the Brinell hardness. Relevant literature shows that the function curve of cold-rolled 45 steel work hardening is: S = 660.39x 0.7528, where: S—tensile strength, x—elongation coefficient, x =1 /(1-Z), Z—cold-rolled cross-sectional shrinkage. According to the above relationship, the perforated rough pipe in this test is 51mm × 5.5 mm, and the cold-rolled pipe is 24.5 mm × 4.6 mm. It can be seen that the cold-rolled section shrinkage Z = 63.4%, the elongation coefficient x = 2.732, the tensile strength S = 1406.63 MPa, and the theoretical hardness of the steel pipe after rolling is 401.7 HBW, while the hardness of the cracked pipe tested is 326.3 HBW. 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 more and the hardness of the steel is higher. Because the hardness of pearlite is related to its lamellar spacing, the larger the lamellar spacing, the lower the hardness. The slower the cooling rate during annealing, the larger the lamellar 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 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 = 660.39x 0.7528 is used to design a reasonable deformation. A larger deformation will make the strength and hardness of the steel pipe too high, and a larger 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 pipe, the perforated rough pipe with a specification of 40 mm × 5.5 mm was rolled into a finished pipe with a specification of 24.5 mm × 4.6 mm. At this time, the cross-sectional shrinkage rate of the steel pipe was Z = 51.7%, which was 12 percentage points less than the deformation of the finished pipe rolled with a perforated pipe with a specification of 51mm × 5.5 mm. The rough pipe was subjected to the above recrystallization annealing process before cold rolling. After tracking the production of subsequent 45 steel seamless steel pipes, the material of the pipe body was improved, the hardness of the finished pipe was tested to be about 256 HB, and there was no cracking of the pipe body during rolling, 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 326.3 HBW, and the pearlite hardness in the organization reached 325.0 HV0.1.
2) The work hardening phenomenon of 45 steel round steel after perforation and rolling reduced the toughness and plasticity of the steel. At the same time, the rolling deformation of the steel pipe was too large, reaching 64.3%, which caused a large internal stress inside the steel pipe and caused cracking during rolling.
3) To eliminate the work hardening phenomenon of the steel pipe, a recrystallization annealing process was adopted: the temperature was 730 ℃, and it was cooled to 160 ℃ at a rate of 80~100 ℃/h and then air-cooled out of the furnace. A perforated rough pipe with a size of 40 mm × 5.5 mm was used for rolling, which reduced the rolling deformation. The quality of the steel pipe body was improved in the follow-up production, and no rolling cracking occurred.