Seamless steel pipe produces eccentricity (uneven thickness) for a reason

DataTime:[2019-3-27]

The finished products of seamless precision steel pipe will have the problem of uneven eccentric thickness. But how is it produced? Many people have not figured out. Today we will talk about cold drawn steel pipe, cold-rolled steel pipe, hot-rolled steel pipe, etc. The eccentricity of seamless seam steel pipe is generated. Why is the concentricity of seamless steel pipe not so ideal?

The picture above shows that we are measuring the thickness of the steel pipe.

The picture above shows the finished steel pipe produced by our company. You can see a little bit of eccentricity.

The above picture shows the seamless steel pipe produced by other steel pipe factories. It has obvious eccentricity and roundness.
Eccentric steel pipes are the easiest to produce in the production of hot rolled steel pipes, and most of the resulting links are produced during hot perforation:
According to the anatomical analysis of the steel pipe after the automatic pipe rolling mill, we believe that the uneven longitudinal wall thickness of the steel pipe after the perforated capillary is rolled by the automatic pipe rolling machine basically retains the uneven distribution of the wall thickness of the perforated capillary. That is, the steel pipe still has a spiral wall thickness unevenness after rolling, and the lateral wall thickness unevenness is remarkably increased.
The reasons for the uneven wall thickness of the automatic pipe mill are:

The existence form and severity of uneven wall thickness of perforated capillary tube directly affect the existence and severity of uneven wall thickness of steel pipe after rolling.
When rolling the pipe on the automatic pipe rolling machine, the top rod is bent away, the head position is deviated from the center of the hole type, resulting in uneven wall thickness, and the maximum wall thickness and minimum wall thickness position in the cross section of the pipe and the pipe head are almost fixed. The same is true; the unevenness of the wall thickness from the tube end to the tube head is gradually increased. Therefore, reducing the residual curvature of the ram and reducing the axial force of the ram when rolling the tube is significant for reducing the unevenness of the wall thickness. effect.
The larger the wall reduction, the more serious the wall thickness of the waste pipe is. When the wall reduction is small, the automatic pipe rolling machine has the effect of reducing the wall thickness unevenness of the perforated pipe. 4 hole type adjustment is not correct, when the roll gap is not parallel, the wall thickness of the waste pipe will be intensified.

Our seamless steel tube plant performed a Fourier transform on the measured wall thickness data of Φ400mm automatic tube rolling mill, perforation, secondary perforation (extension), automatic rolling and uniform rolling process, and obtained uneven wall thickness. The quantitative analysis and the reasons for its formation, and based on this, proposed ways to improve the uneven wall thickness of steel pipes:

After the secondary perforation (extension), the distribution of the uneven wall thickness on the waste pipe is retained to the finished pipe, so improving the secondary perforation (extension) is the key link to improve the wall thickness accuracy of the finished pipe. The main measure is to improve The tool is designed to increase the concentricity of the ram and the head with the rolling line during the rotation.
Improving the uneven thickness of the capillary tube after perforation is an important part. The main measures are to improve the heating uniformity of the tube blank, improve the precision of the centering hole, lengthen the length of the entire belt and the length of the reverse cone, and improve the ejector and the head. Concentricity with the rolling line during the rotation.
Although there is a serious symmetry wall thickness unevenness during rolling, it does not have a certain effect on reducing the spiral wall thickness. Therefore, two tubes should be rolled during the rolling process, and the waste pipe should be turned over 90° between the passes.
The leveling process can basically eliminate the symmetry wall thickness unevenness, but the effect of eliminating the spiral wall thickness unevenness is very small. Therefore, the capacity of the leveling machine should be improved.
Fourier transform is an effective means to study the uneven wall thickness during the cross-rolling process. This method can also be used to study the uneven wall thickness of other steel tube production units.

The picture above shows the seamless steel pipe perforation site. The eccentricity of many steel pipes is generated at this time, so it is very important to strictly control this perforation.
Finished steel pipe Cold drawn steel pipe can also cause eccentricity problems due to perforation:
Any factor that destroys the geometrical correctness of the deformation zone formed by the roll, the head and the guide plate will increase the uneven thickness of the capillary wall.
(1) Head.

Head shape design, the ideal head rolling cone should be parallel with the roll exit cone. If the head is designed according to the traditional Matviev formula, the rolling cone of the head is not parallel with the exit cone of the roll, the metal is Such a gradually enlarged deformation in the gap will inevitably result in insufficient tube wall rolling and uneven wall thickness of the capillary tube, and the wall thickness unevenness of the capillary tube is more serious with the increase of the feeding angle;
Due to the insufficient rigidity of the ram, bending occurs during the perforation process, so that the head cannot maintain the centering position, so that the wall thickness of the piercing capillary is uneven;
Uneven wear or damage to the head.

(2) Guide plate.

The distance between the guide plates is too large. During the perforation process, the centering of the perforations is maintained by the restriction of the guide plates. The distance between the guide plates is large, and the heads are greatly changed in the upper and lower positions, which makes the head unstable, resulting in uneven wall thickness of the capillary tubes.
The uneven wear of the upper and lower guide plates also aggravates the uneven thickness.

(3) Rolls.

Roll centerline deflection: During the production process, the screws on both sides of the puncher are not installed correctly, or the axial deflection occurs between the two rollers due to the thread and bearing wear. The feed angles of the two rolls are inconsistent. Distortion of the deformation zone results in uneven wall thickness.
Large feed angles cause the head to be more parallel with the rolling cone of the roll.
Improper roll speed will also affect the wall thickness accuracy.

(4) Centering and heating of the tube blank.
Centering hole eccentricity and uneven heating (yin and yang) will cause uneven wall thickness.
(5) Stiffness, structure and adjustment of the punch.
The rigidity of the punching machine is not enough, and the locking mechanism on it is not reliable; the centering device of the ejector is inaccurately adjusted, the operation is unreliable and the distance from the fuselage is far; the adjustment of the rolling center line is generally lower than the center line of the rolling mill. The purpose is to improve the stability of the rolled piece. If the adjustment is too large, the relative relationship between the tools in the deformation zone will change asymmetrically after the rolling line moves down, which will also affect the uneven wall thickness of the capillary.
Summarize:
In summary, the uneven thickness of the eccentricity of the seamless steel pipe is not completely avoided, and can only be strictly controlled step by step. If you have any thoughts on our analysis, please leave a message for us to discuss.
Source: China Seamless Steel Pipe Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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