Internal folding defect of hot-rolled seamless steel tube
Internal folding defect of hot-rolled seamless steel tube
ABSTRACT: In the current production of hot-rolled seamless steel, the internal bending defect has always been an important factor affecting the quality of steel pipes. Its defects run through the whole piercing process and are closely related to the quality of continuous casting billet and the piercing process.
This paper focuses on the problems related to the internal folding defects of hot-rolled seamless steel tubes. First, the morphology of the internal folding and its causes are discussed, and then several preventive measures are put forward, hoping to be helpful to the workers of seamless steel tubes
Key words: hot-rolled seamless steel tube; internal folding defect; piercing method
Internal folding of hot-rolled seamless steel pipe refers to the appearance of spiral, semi-spiral or irregular zigzag folding on the inner surface of the pipe. Internal folding mainly exists at the head of the pipe, either regularly or irregularly. Some hot-rolled seamless steel pipes can be used by simple grinding after internal bending defect occurs. Serious cases need to be scrapped directly. In the current social production, the internal bending defect has always been an important factor affecting the service performance of hot-rolled seamless steel tube. How to effectively avoid the internal bending defect of hot-rolled seamless steel tube has always been the focus of attention of relevant scholars.
1. Hot-rolled seamless steel inner fold morphology and its causes
1.1 Hot-rolled Seamless Tube Folding Morphology
Hot-rolled seamless steel pipes have irregular inner fold morphology. The common morphology is small flake, but there are also large flake, straight line, fish scale morphology.
1.2 Reasons for Internal Bending of Hot Rolled Seamless Steel Pipe
When the steel pipe is obliquely pierced, the core of the billet of the continuous casting pipe will crack under the influence of tension stress, resulting in a cavity. Because of the uncertainty of cavity formation itself, the rough metal surface inside the cavity will be oxidized by cooling water and air of the plug, and then folded in the subsequent rolling process and attached to the inside of the steel pipe. Therefore, in order to effectively avoid the occurrence of internal folding, it is necessary to avoid the formation of cavity in the process of internal folding.
2. Quality and internal bending defects of continuous casting billet
At present, in the production of continuous casting billet, if we want to get the capillary without inner fold and inner and outer surface cracks, we can produce the finished pipe which meets the relevant standards. Under the influence of this idea, the level of smelting process becomes the key of the whole production. According to the current research results of continuous casting billet smelting, in order to ensure that the smelting process level can meet expectations, relevant personnel should carry out quality control according to the relevant contents of Table 1.
3. Perforation technology and internal folding defects
3.1 Piercing Temperature and Internal Defect
In the whole production of seamless steel tube, the perforation process is an important factor affecting the internal folding. If the temperature is too high or the heating time is too long, the perforation temperature will rise rapidly, and then the problem of internal folding will occur. At this time, fish-scale internal folding will occur in the continuous casting billet. If the temperature is too low, the heating time will be affected, which is not conducive to ensuring the metal plasticity of the steel pipe. The results show that the relationship between the perforation temperature and the deflection defect must be correctly recognized to avoid the occurrence of the deflection.
3.2 Rotary Speed of Piercing Roller and Defects of Internal Bending
When designing the rotational speed of piercing roll, relevant personnel must fully consider the influence of many factors, such as material properties, roll diameter and so on. There are obvious differences in rotational speed among different specifications of continuous casting billet. From the current actual situation, there are two kinds of common rotational speed, including higher and lower than the standard rotational speed. Under different production conditions, there are also obvious differences in the ratio of the internal inflection produced by these two rotational speeds. Relevant studies show that compared with low rotational speed, high rotational speed is more likely to have internal interruption. Consider the reasons for this phenomenon: at high drilling speed, increasing the roll speed will lead to an increase in the slip of steel pipe, resulting in a decrease in the axial efficiency, thereby increasing the risk of internal bending. Meanwhile supported by the results, it can be concluded that in order to effectively avoid the occurrence of internal folding, it is necessary to timely adjust the direction of the piercing machine and adopt the maximum bite angle for production. With the increase of biting angle, the slip of continuous casting billet can be improved, which will help to reduce the number of rotations during biting.
3.3 Plug and Internal Defect
It is well known that in the case of severe wear and tear on the top, its surface is uneven, which will scratch the inner wall in the process of perforation, leading to spiral internal folding in the pipe. At the same time, the shape of steel head has a certain influence on the quality of steel pipe.
(1) If the piercing machine's top is conical, it will lead to an increase in the reduction of the piercing machine's top from the tip to the pass part, which is not conducive to ensuring the quality of the barren pipe.
(2) When using the head of spherical perforator, it is helpful to ensure that the barren pipe is fed in the given direction, and reduce the reduction amount accordingly, so as to ensure the quality of the barren pipe. Of course, the best shape of piercing machine plug recognized in current production is to ensure the constant shape of downward pressure along the length direction of deformation zone. Under the action of piercing machine plug screw, the biting performance of continuous casting billet is guaranteed, and the effect of internal folding control is expected.
preventive measure
In order to effectively avoid the occurrence of internal folding defects, we must focus on the following work:
(1) Strictly control the properties of continuous casting billet, according to the relevant production requirements, expand the medium axle Crystal ratio of continuous casting billet, or shorten the length of columnar crystal accordingly, reduce the probability of internal folding.
(2) Strict heating system for continuous casting billet shall be formulated in accordance with production requirements. According to the billet diameter, length and production conditions, the adding temperature of the billet is determined, and the system of stopping rolling and cooling is implemented. In the temperature control, the heating temperature of the billet is equal to the piercing temperature-piercing temperature+outlet temperature.
(3) Use high-quality piercing and rolling tools. High quality piercing and rolling tools play an important role in preventing internal folding defects, so high quality rolling and piercing equipment must be used. For example, it is necessary to actively replace the hot centering punch and correct it in time after routine checking of centering holes to avoid affecting the quality of production. For the new piercing roll, the bigger parameters of head forward elongation and bite angle are adopted, and the quality control is carried out according to the low roll speed.
CONCLUSION:
This paper mainly studies the related problems of the internal bending defect of hot rolled seamless steel tube, and analyses the control methods of the internal bending defect. In order to effectively avoid the occurrence of internal folding defects in hot-rolled seamless steel tubes, we must start with material quality control, heating temperature, and the use of high-quality appliances, and do a good job of control to ensure that the occurrence of internal folding can be effectively avoided and the service performance of hot-rolled seamless steel tubes can be guaranteed.
ABSTRACT: In the current production of hot-rolled seamless steel, the internal bending defect has always been an important factor affecting the quality of steel pipes. Its defects run through the whole piercing process and are closely related to the quality of continuous casting billet and the piercing process.
This paper focuses on the problems related to the internal folding defects of hot-rolled seamless steel tubes. First, the morphology of the internal folding and its causes are discussed, and then several preventive measures are put forward, hoping to be helpful to the workers of seamless steel tubes
Key words: hot-rolled seamless steel tube; internal folding defect; piercing method
Internal folding of hot-rolled seamless steel pipe refers to the appearance of spiral, semi-spiral or irregular zigzag folding on the inner surface of the pipe. Internal folding mainly exists at the head of the pipe, either regularly or irregularly. Some hot-rolled seamless steel pipes can be used by simple grinding after internal bending defect occurs. Serious cases need to be scrapped directly. In the current social production, the internal bending defect has always been an important factor affecting the service performance of hot-rolled seamless steel tube. How to effectively avoid the internal bending defect of hot-rolled seamless steel tube has always been the focus of attention of relevant scholars.
1. Hot-rolled seamless steel inner fold morphology and its causes
1.1 Hot-rolled Seamless Tube Folding Morphology
Hot-rolled seamless steel pipes have irregular inner fold morphology. The common morphology is small flake, but there are also large flake, straight line, fish scale morphology.
1.2 Reasons for Internal Bending of Hot Rolled Seamless Steel Pipe
When the steel pipe is obliquely pierced, the core of the billet of the continuous casting pipe will crack under the influence of tension stress, resulting in a cavity. Because of the uncertainty of cavity formation itself, the rough metal surface inside the cavity will be oxidized by cooling water and air of the plug, and then folded in the subsequent rolling process and attached to the inside of the steel pipe. Therefore, in order to effectively avoid the occurrence of internal folding, it is necessary to avoid the formation of cavity in the process of internal folding.
2. Quality and internal bending defects of continuous casting billet
At present, in the production of continuous casting billet, if we want to get the capillary without inner fold and inner and outer surface cracks, we can produce the finished pipe which meets the relevant standards. Under the influence of this idea, the level of smelting process becomes the key of the whole production. According to the current research results of continuous casting billet smelting, in order to ensure that the smelting process level can meet expectations, relevant personnel should carry out quality control according to the relevant contents of Table 1.
Table 1 Data sheet for quality control of continuous casting billet
| Sequence Number | Key Points of Quality Control |
| 1 | In production, it is necessary to strictly control the casting speed so as to avoid low drawing speed or mismatch of molten steel temperature, and pay attention to avoid high heat and large central shrinkage. All these factors are the main causes of introversion. |
| 2 | The superheat temperature of molten steel should be controlled strictly according to the production specifications. Generally, it is better to control the internal folding rate by matching the drawing speed. |
| 3 | Due to the biting stage after perforation, the core of the loosely centered billet will crack and affect the subsequent perforation and rolling effect, resulting in defects. Therefore, in production, attention should be paid to control the quality of the core and avoid the application of loose tube blanks in production. |
3. Perforation technology and internal folding defects
3.1 Piercing Temperature and Internal Defect
In the whole production of seamless steel tube, the perforation process is an important factor affecting the internal folding. If the temperature is too high or the heating time is too long, the perforation temperature will rise rapidly, and then the problem of internal folding will occur. At this time, fish-scale internal folding will occur in the continuous casting billet. If the temperature is too low, the heating time will be affected, which is not conducive to ensuring the metal plasticity of the steel pipe. The results show that the relationship between the perforation temperature and the deflection defect must be correctly recognized to avoid the occurrence of the deflection.
3.2 Rotary Speed of Piercing Roller and Defects of Internal Bending
When designing the rotational speed of piercing roll, relevant personnel must fully consider the influence of many factors, such as material properties, roll diameter and so on. There are obvious differences in rotational speed among different specifications of continuous casting billet. From the current actual situation, there are two kinds of common rotational speed, including higher and lower than the standard rotational speed. Under different production conditions, there are also obvious differences in the ratio of the internal inflection produced by these two rotational speeds. Relevant studies show that compared with low rotational speed, high rotational speed is more likely to have internal interruption. Consider the reasons for this phenomenon: at high drilling speed, increasing the roll speed will lead to an increase in the slip of steel pipe, resulting in a decrease in the axial efficiency, thereby increasing the risk of internal bending. Meanwhile supported by the results, it can be concluded that in order to effectively avoid the occurrence of internal folding, it is necessary to timely adjust the direction of the piercing machine and adopt the maximum bite angle for production. With the increase of biting angle, the slip of continuous casting billet can be improved, which will help to reduce the number of rotations during biting.
3.3 Plug and Internal Defect
It is well known that in the case of severe wear and tear on the top, its surface is uneven, which will scratch the inner wall in the process of perforation, leading to spiral internal folding in the pipe. At the same time, the shape of steel head has a certain influence on the quality of steel pipe.
(1) If the piercing machine's top is conical, it will lead to an increase in the reduction of the piercing machine's top from the tip to the pass part, which is not conducive to ensuring the quality of the barren pipe.
(2) When using the head of spherical perforator, it is helpful to ensure that the barren pipe is fed in the given direction, and reduce the reduction amount accordingly, so as to ensure the quality of the barren pipe. Of course, the best shape of piercing machine plug recognized in current production is to ensure the constant shape of downward pressure along the length direction of deformation zone. Under the action of piercing machine plug screw, the biting performance of continuous casting billet is guaranteed, and the effect of internal folding control is expected.
preventive measure
In order to effectively avoid the occurrence of internal folding defects, we must focus on the following work:
(1) Strictly control the properties of continuous casting billet, according to the relevant production requirements, expand the medium axle Crystal ratio of continuous casting billet, or shorten the length of columnar crystal accordingly, reduce the probability of internal folding.
(2) Strict heating system for continuous casting billet shall be formulated in accordance with production requirements. According to the billet diameter, length and production conditions, the adding temperature of the billet is determined, and the system of stopping rolling and cooling is implemented. In the temperature control, the heating temperature of the billet is equal to the piercing temperature-piercing temperature+outlet temperature.
(3) Use high-quality piercing and rolling tools. High quality piercing and rolling tools play an important role in preventing internal folding defects, so high quality rolling and piercing equipment must be used. For example, it is necessary to actively replace the hot centering punch and correct it in time after routine checking of centering holes to avoid affecting the quality of production. For the new piercing roll, the bigger parameters of head forward elongation and bite angle are adopted, and the quality control is carried out according to the low roll speed.
CONCLUSION:
This paper mainly studies the related problems of the internal bending defect of hot rolled seamless steel tube, and analyses the control methods of the internal bending defect. In order to effectively avoid the occurrence of internal folding defects in hot-rolled seamless steel tubes, we must start with material quality control, heating temperature, and the use of high-quality appliances, and do a good job of control to ensure that the occurrence of internal folding can be effectively avoided and the service performance of hot-rolled seamless steel tubes can be guaranteed.
