Analysis of the causes of eccentricity in seamless steel pipes

The finished products of seamless steel pipes and precision steel pipes will have the problem of uneven eccentric thickness, but how does it happen? Many people don't understand. Today we will talk about how the eccentricity of seamless steel pipes such as cold-drawn steel pipes, cold-rolled steel pipes, and hot-rolled steel pipes occurs. Why is the concentricity of seamless steel pipes not so ideal?

Eccentric steel pipes are most likely to occur during the production process of hot-rolled steel pipes. Most of them occur during hot piercing:

Based on the anatomical analysis of the steel pipes rolled by the automatic pipe rolling machine, we believe that after the perforated capillary pipe is rolled by the automatic pipe rolling machine, the uneven longitudinal and transverse wall thickness of the steel pipe retains the distribution characteristics of the uneven wall thickness of the perforated capillary pipe. That is, the rolled steel pipe still has spiral wall thickness unevenness, and the transverse wall thickness unevenness increases significantly.

The reasons for uneven wall thickness in automatic pipe rolling machines are:

①The existence form and severity of uneven wall thickness of perforated capillary tubes directly affect the existence form and severity of uneven wall thickness of rolled steel pipes.

② When rolling pipes on an automatic pipe rolling machine, due to the bending of the ejector rod, the position of the ejector's head deviates from the center of the pass shape, resulting in uneven wall thickness. The maximum and minimum wall thickness positions in the pipe and on each cross-section of the pipe head are almost fixed. However, the unevenness of wall thickness from the end of the pipe to the head gradually increases. Therefore, reducing the residual curvature of the ejector rod and the axial force of the ejector rod during pipe rolling will significantly reduce the unevenness of the wall thickness. Effect.

③The greater the amount of wall reduction, the more serious the uneven wall thickness of the raw pipe. When the amount of wall reduction is small, the automatic pipe rolling machine has the effect of reducing the uneven wall thickness of the perforated capillary tube. ④The adjustment of the hole shape is incorrect. When the roll gaps are not parallel, the uneven wall thickness of the raw pipe will be aggravated.

Our seamless steel pipe factory conducted a Fourier transform on the measured wall thickness data of raw pipes in the four rolling processes of Φ400mm automatic pipe rolling unit, piercing, secondary piercing (extension), automatic pipe rolling, and leveling, and obtained the uneven wall thickness. Quantitative analysis of and its causes, and based on this, a way to improve the uneven wall thickness of steel pipes is proposed:

① After secondary perforation (extension), the uneven distribution of spiral wall thickness on the raw pipe remains until the finished pipe. Therefore, improving the secondary perforation (extension) is a key link in improving the wall thickness accuracy of the finished pipe. The main measure is to improve The tool design to improve the concentricity of the ejector pin and the ejector's head with the rolling line during rotation.

②Improving the uneven wall thickness of the capillary tube after perforation is an important step. The main measures are to improve the heating uniformity of the tube blank, improve the accuracy of the centering hole, lengthen the length of the uniform belt of the plug and the length of the reverse taper, and improve the connection between the ejector rod and the plug. Concentricity with the rolling line during rotation.

③ Although serious symmetrical wall thickness unevenness will occur during pipe rolling, it can play a certain role in reducing spiral wall thickness unevenness. Therefore, the pipe should be rolled in two passes, and the raw pipe should be turned over 90° between passes.

④ The leveling process can eliminate symmetrical wall thickness unevenness, but it has little effect on eliminating spiral wall thickness unevenness. Therefore, the ability of the leveling machine should be improved.

⑤ Fourier transform is an effective means to study uneven wall thickness in the cross-rolling process. This method can also be used to study uneven wall thickness of other steel pipe production units.

The eccentricity of many steel pipes occurs at this time, so it is very important to strictly control this perforation link.

Precision-rolled steel pipes and cold-drawn steel pipes can also have eccentricity problems due to perforation:

Any factor that destroys the geometric accuracy of the deformation zone formed by the roller, the head, and the guide plate will aggravate the uneven capillary wall thickness.

(1) Head.

① Shape design of the plug. The ideal rolling cone of the plug should be parallel to the exit cone of the roll. If the plug is designed according to the traditional Matveyev formula, the rolling cone of the plug will not be parallel to the exit cone of the roll. The metal will Deformation in such a gradually expanding gap will inevitably cause insufficient rolling of the tube wall, resulting in uneven capillary wall thickness. Moreover, the uneven capillary wall thickness becomes more serious as the feed angle increases;

② Due to the insufficient stiffness of the ejector rod, it bends during the piercing process, making the ejector head unable to maintain its centered position, resulting in uneven wall thickness of the capillary tube that passes through;

③Uneven wear or damage of the plug.

(2) Guide plate.

① The distance between the guide plates is too large. During the perforation process, the restriction of the guide plates is relied on to maintain the center line of the perforation. The distance between the guide plates is large and the top changes greatly in the up and down position, making the top unstable and causing uneven capillary wall thickness.

② Uneven wear of the upper and lower guide plates will also aggravate the uneven wall thickness.

(3) Roller.

①Roll center line deflection: During the production process, due to incorrect installation of the pressing screws on both sides of the punching machine, or due to thread and bearing wear, the axial direction between the two rollers is horizontally deflected, and the feed angles of the two rollers are inconsistent. Distortion occurs in the deformation zone, resulting in uneven wall thickness.

② A large feed angle causes the rolling cone of the head and the roller to become less parallel.

③Improper roll speed will also affect wall thickness accuracy.

(4) Centering and heating of tube blanks.

The eccentricity of the centering hole and uneven heating (yin and yang sides) will cause uneven wall thickness.

(5) Stiffness, structure, and adjustment of the punch.

The body of the punching machine is not rigid enough and the locking mechanism on it is unreliable; the centering device of the ejector rod is inaccurately adjusted, unreliable, and far away from the machine body; the rolling center line is generally adjusted to be lower than the rolling mill center line. , its 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 downward, which will also affect the uneven wall thickness of the capillary tube.

Summarize: To sum up, there is no way to completely avoid the uneven eccentric thickness of seamless steel pipes. It can only be strictly controlled step by step to minimize it. If you have any other ideas about our analysis, please leave us a message for discussion.