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Analysis on cold-rolling cracking reason of 45# steel pipe

Longitudinal partial cracking of 45# steel occurs during cold rolling. The chemical composition, microstructure, fracture morphology and microhardness of 45# round bars and cracked tubes were analyzed using optical microscopy, microhardness tester, SEM and direct reading spectroscopy. The cold-rolled 45 steel work-hardening function curves were used to calculate the deformation parameters of the rolled tubes. The results show that 45# steel rounds are significantly work-hardened after piercing and rolling reducing toughness and plasticity, and the excessive deformation of cold-rolled steel tubes leads to large internal stresses resulting in cracking during rolling. The use of recrystallization annealing can eliminate the hardening of the steel tube, the reasonable design of the steel tube rolling deformation to improve the quality of the tube, avoiding the cracking of the body.

45# steel is a high-quality carbon structural steel, hardness is not high, easy cutting and processing, and after tempering has a better overall mechanical properties, widely used in various types of important structural parts. 45# steel seamless steel pipe is used to manufacture mechanical parts, such as cars, tractors, force parts, need to withstand tensile, compression, bending and vibration impact of a variety of forces, must ensure adequate strength and stiffness. A factory production of 45# seamless steel pipe, specification ф24.5mm × 4.6mm, the production process is: round steel billet → heating perforation → pickling → lubrication → cold rolling → oil removal → straightening → cutting → non-destructive testing. In the cold rolling phenomenon of longitudinal local cracking, the scrap rate reached 70%, the authors of this paper to determine the cause of cracking in a typical cracked tube and tube billet samples, analysis of chemical composition, microstructure, fracture morphology and microhardness of tissue, and analysis of the causes of cracking [1].

1. Physical and chemical tests and analysis

1.1 Macroscopic inspection and microscopic morphological observation

A section of the cracked tube with typical cracks observed macroscopic characteristics as shown in Figure 1, the crack along the longitudinal distribution of the tube body, the length of 60-70mm, and the axis of the tube into an angle of about 15 °, the surface of the tube body. There is no obvious scratch on the surface. The crack penetrated the tube wall, forming a penetrating crack, see Figure.1 (a), (b). Under the EVO18 German ZEISS scanning electron microscope to observe the crack at the morphology, the tube fracture morphology of multiple flush fracture surface has deconstruction characteristics, with obvious brittle fracture characteristics, see Figure.1 (c).
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Fig.1 Macro (a, b) and micro (c) morphologies in the crack of 45#steel tube

1.2 Material composition tests

The chemical composition of the two specimens was examined by Bruker Q4170 direct reading spectrometer, and the results are shown in Table.1. The other alloy compositions are within the range of the national standard.
Table.1 Chemical composition of 45#steel samples (wt%)
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1.3 Microstructure inspection

Samples of round steel and cracked pipe were taken along the cross and longitudinal sections, and after grinding, polishing and corrosion, the organization was observed under a Leica DM4000M metallurgical microscope. According to the ratio of ferrite and pearlite in the round steel and cracked pipe, the carbon content is in the upper limit of carbon content of common 45# steel, which is consistent with the results of chemical composition test. The round steel organization is pearlite and a small amount of needle-like distribution and ferrite, surface. There is a slight decarburization layer, see Figure.2 (a), (b). The cracked tube has lamellar pearlite and ferrite with white reticular, needle-like and massive distribution. There is a slight decarburization layer and cracks inside the tissue, see Fig. 2 (c) and (d) [2].

1.4 Hardness test

In order to detect the change of hardness of round steel after heating piercing and cold rolling in the pipe, MH-6 micro hardness tester was used to detect the micro hardness of ferrite and pearlite in round steel and cracked pipe respectively, and the results are shown in Table.2. In order to compare the overall hardness change of round steel and cracked pipe, the average hardness was detected by digital display Brinell hardness tester, the results are shown in Table.3.
Table.2 Microhardness of ferrite and pearlite in steel bar and cracking tube (HV0.1)
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Table.3 Average hardness of steel bar and cracking tube (HBW 5/750)

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Fig.2 Microstructure of the steel bar and cracking tube (a) core section of steel bar; (b) steel bar surface; (c) cracking tube surface; (d) tube cracking

2. Analysis and Discussion

2.1 Crack pattern and cause analysis

The chemical composition of the round steel and cracking tube can be tested, the carbon content in the national standard 45# steel carbon content of the upper limit. Carbon content increases the organization of pearlite too much, reducing the brittle fracture strength of steel, increasing the propensity of steel cracking [3]. The macroscopic observation of the crack is penetration type crack, which is a shear fracture of the metal unit after being subjected to complex multidirectional stress and plastic deformation beyond the strength limit of the pipe, and the warpage deformation at the edge and end of the crack is caused by residual tensile stress. The microscopic morphological observation and microstructure examination reveal that the fracture is characterized by deconstruction and there are cracks through the pearlite tissue inside the tissue, which is an obvious crystal penetration fracture. After piercing and cold rolling, a large amount of plastic deformation is produced, the lattice distortion is serious, the dislocation inside the grain increases sharply, the roughness and the resident slip zone are formed in large quantities, the strength of the grain itself decreases, the crack is easy to sprout from the inside of the grain, and then it becomes a penetration fracture. From the hardness test, the data in Tables 2 and 3 show 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, while the ferrite hardness does not change significantly. Plastic deformation caused by work hardening to improve the hardness of the steel pipe while plasticity, toughness decreased [4].

2.2 Rolling process analysis

From the knowledge of metallurgy, tensile strength is equal to 3.5 times the Brinell hardness. Literature [5] table. It is clear that the cold rolled 45# steel work hardening function curve is: S = 660.39×0.7528, where: S – tensile strength, x – elongation coefficient, x = 1/(1-Z), Z – cold rolled section shrinkage. According to the above relationship, the test of the perforated tube specifications for ф51mm × 5.5mm, cold-rolled tube specifications for ф24.5mm × 4.6mm, it is known that the cold-rolled section shrinkage Z = 63.4%, the elongation coefficient of x = 2.732, tensile strength S = 1406.63MPa, theoretically rolled steel pipe hardness of 401.7HBW, while the test cracking tube hardness The company specified that the deformation is too large, resulting in a large internal stress in the pipe, resulting in cracking when rolling.

3. Improvement measures and effects

3.1 Improvement measures

In order to eliminate the impact of work hardening after perforation to take recrystallization annealing process. As the carbon content of this batch of steel is close to the upper limit of 45# steel national standard, the pearlite is relatively more steel hardness, and because the hardness of the pearlite is related to its layer spacing, the larger the layer spacing the lower the hardness, the slower the cooling speed when annealing, the larger the layer spacing of the pearlite. So before rolling using recrystallization annealing process to improve the plasticity and toughness of steel, to eliminate the effects of process hardening, recrystallization annealing temperature of 730 ℃, to 80-100 ℃ / h speed cooling to 160 ℃ out of the furnace air cooling [6-9].
In order to meet the premise of the strength and hardness of the steel pipe, the use of cold rolled 45# steel hardening function curve: S = 660.39×0.7528 design a reasonable amount of deformation, the larger the amount of deformation will make the steel pipe strength, hardness is too high, the steel pipe produces a large internal stress in the rolling or straightening cracking, but also unfavorable to the processing and use.

3.2 Implementation effect

Through the above analysis of 45# steel seamless pipe rolling cracking causes, the ф40mm × 5.5mm perforated raw tube rolled into a finished tube specifications ф24.5mm × 4.6mm, the steel pipe section shrinkage Z = 51.7%, than the use of ф51mm × 5.5mm perforated tube rolled finished tube deformation is reduced by 12 percentage points, and in the cold rolling before the raw tube The recrystallization annealing process described above. After the follow-up of the production of 45# seamless steel pipe, the material of the pipe body was improved, and the hardness of the finished pipe was tested to be about 256 HB, and no more cracking of the rolled pipe body occurred, thus proving the effectiveness of the improvement measures.

4. Conclusion

  • 1) The carbon content of the steel pipe is close to the upper limit of the national standard and the pearlite is too much, the lattice distortion is serious, cracks sprout from inside the grain to form a grain penetration fracture, the fracture is brittle. The overall hardness of the steel pipe reached 326.3HBW, and the pearlite hardness in the organization reached 325.0 HV0.1.
  • 2) The work hardening phenomenon of 45# steel rounds after piercing and rolling reduces the toughness and plasticity of the steel, while the rolling deformation of the steel pipe is too large to reach 64.3%, which causes large internal stresses inside the steel pipe resulting in cracking during rolling.
  • 3) In order to eliminate the process hardening phenomenon of steel tubes using recrystallization annealing process: the temperature of 730 ℃, 80-100 ℃ / h speed cooling to 160 ℃ out of the furnace air cooling, and the use of ф40mm × 5.5mm perforated burr tube for rolling, reducing the amount of rolling deformation. The quality of the tube body was improved by tracking the production of steel tubes, and the phenomenon of rolling cracking did not occur.

Authors: Ding Ye, Sun Weilian, Zhang Yunqian, Sun Pu

Source: China 45# Steel Pipe Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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  • [6]He Jianzhong, Feng Xiaodong. Analysis of the causes of rolling cracking of seamless steel tubes for hydraulic supports [J]. Journal of University of Science and Technology Beijing, 2012, 34(Suppl.):90-92.
  • [7] Liu Chunlin. Analysis of cold-drawn seamless steel pipe cracking [J]. Journal of Ningbo Engineering College, 2007, 19(2):29-30.
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