1. Analysis of crack characteristics
It can be seen that there are many transverse cracks distributed along the longitudinal direction of the steel pipe. The cracks are arranged neatly. Each crack has a wavy feature, with a slight deflection in the longitudinal direction and no longitudinal scratches. As shown in Figure 2, the cracks have a certain deflection angle with the surface of the steel pipe and a certain width. There are oxides and decarburization at the edge of the crack. The bottom is blunt and there is no sign of expansion. The matrix structure is normal ferrite + pearlite, which is distributed in a band and has a grain size of 8. The cause of the cracks is related to the friction between the inner wall of the steel pipe and the inner mold during the production of the steel pipe. According to the analysis of the production process, the factors affecting the friction between the inner wall of the steel pipe and the inner mold are mainly the quality of lubrication and are also related to the plasticity of the steel pipe. If the plasticity of the steel pipe is poor, the possibility of drawing cracking is greatly increased, and the poor plasticity is related to the intermediate stress relief annealing heat treatment. It is inferred that the cracks may be generated in the cold drawing process. In addition, since the crack is not open to a large extent and there is no obvious sign of expansion, it means that after the crack is formed, it has not been affected by the secondary drawing deformation. Therefore, it is further inferred that the most likely time for the crack to be generated should be the second cold drawing process. The most likely influencing factors are poor lubrication and/or poor stress relief annealing. To determine the cause of the crack, a crack reproduction test was carried out in cooperation with the steel pipe manufacturer. Based on the above analysis, the following test was carried out: Under the condition that the perforation and hot rolling diameter reduction process remain unchanged, the lubrication and/or stress relief annealing heat treatment conditions are changed, and the drawn steel pipes are inspected to try to reproduce the same defects.
2. Test plan
Nine test plans were proposed by changing the lubrication process and annealing process parameters. Among them, the normal phosphating and lubrication time requirements are 40 minutes, the normal intermediate stress relief annealing temperature requirements are 830℃, and the normal insulation time requirements are 20 minutes. The test process uses a 30t cold drawing unit and a roller bottom heat treatment furnace.
3. Test results
Through the inspection of the steel pipes produced by the above 9 schemes, it was found that except for schemes 3, 4, 5, and 6, other schemes all had jitter or transverse cracks to varying degrees. Among them, scheme 1 had an annular step; schemes 2 and 8 had transverse cracks, and the crack morphology was very similar to that found in production; schemes 7 and 9 had jitter, but no transverse cracks were found.
4. Analysis and discussion
Through the above comparative analysis, it is shown that in the two cases of poor lubrication + no intermediate annealing and poor lubrication + low intermediate annealing temperature, transverse cracks will occur, while in the cases of poor lubrication + good intermediate annealing, good lubrication + no intermediate annealing, and good lubrication + low intermediate annealing temperature, although jitter defects will occur, transverse cracks will not be caused on the inner wall of the steel pipe. Poor lubrication is the main cause of transverse cracks, and poor intermediate stress relief annealing is the auxiliary cause. Since the drawing stress of the steel pipe is proportional to the friction force, poor lubrication will lead to an increase in the drawing force and a decrease in the drawing rate. When the steel pipe is first drawn, the speed is low. If the speed is lower than a certain value, the bifurcation point is reached, and the mandrel will produce self-excited vibration, resulting in chattering. In the case of insufficient lubrication, the axial friction between the surface (especially the inner surface) metal and the die is greatly increased during drawing, resulting in work hardening. If the subsequent stress relief annealing heat treatment temperature of the steel pipe is insufficient (such as about 630℃ set in the test) or no annealing, it is easy to cause surface cracks. According to theoretical calculations (minimum recrystallization temperature ≈ 0.4×1350℃), the recrystallization temperature of 20 steel is about 610℃. If the annealing temperature is close to the recrystallization temperature, the steel pipe fails to fully recrystallize, and the work hardening is not eliminated, resulting in poor material plasticity, metal flow is blocked during friction, and the inner and outer layers of metal are severely deformed unevenly, thereby generating a large amount of axial additional stress. As a result, the axial stress of the inner surface metal of the steel pipe exceeds its limit, thereby generating cracks.
5. Conclusion
The transverse cracks on the inner wall of 20 seamless steel pipes are caused by the combined effect of poor lubrication during the drawing process and insufficient intermediate stress relief annealing heat treatment (or no annealing). Among them, poor lubrication is the main cause, and poor intermediate stress relief annealing (or no annealing) is the auxiliary cause. To avoid the occurrence of similar defects, manufacturers should require workshop operators to strictly follow the relevant technical regulations for lubrication and heat treatment processes during production. In addition, since the roller bottom continuous annealing furnace is a continuous annealing furnace, although it is convenient and quick to load and unload, it is difficult to control the temperature and speed of materials of different specifications and sizes in the furnace. If it is not strictly implemented by the regulations, it is easy to cause uneven annealing temperature or too short a time, resulting in insufficient recrystallization, leading to defects in subsequent production. Therefore, manufacturers who use roller bottom continuous annealing furnaces for heat treatment should control the various requirements and actual operations of heat treatment.
Post time: Nov-06-2024