Judging that the quality of the square tube produced by the directly forming to square pipe mill meets the standard is based on a large number of inspection data of the microhardness of the weld and the microhardness of the base material. The former is much higher than the latter. It is for this reason that the weld is usually not placed at a positive angle, but placed within t~(t+5mm) from the corner edge to reduce the risk of weld cracks. This requires effective control of the weld position. Control methods include forming roll method, guide roll method and sizing roll method.

The following is an introduction to the forming roll method and the guide roll method for the control methods of the welding seam position of the square rectangular tube produced by the directly forming to square pipe mill.

Forming roll method

The control principle is to roll a certain side of the tube billet more heavily by forming a solid roll, so that the longitudinal fibers of the parent material on this side can be extended, so that in the subsequent welding, cooling and forced equal length processes, more than the other is formed. The longitudinal strain and longitudinal tensile stress on one side, in this way, the weld will be pushed to the side where the longitudinal strain extension and stress are small, and then the purpose of controlling the position of the weld will be achieved. However, using the forming roll method to control the position of the weld is slow in response and it is not easy to accurately control the offset of the weld.

Guide roller method

By changing the pass of the guide roll and the position of the rolling center line, the position of the weld seam in the sizing section can be controlled. There are two types: deflection method and offset method.

(1) Deflection method. Based on the rolling center line, the guide roller is deflected to the left or right by a certain angle, so as to force the tube blank in the pass to deflect to the left or right.

The control principle is that because the pipe is welded with a rigid body, when a torsion force is applied to the pipe at the guide roller, the torsion is transmitted along the pipe body to the sizing direction, forcing the pipe body to twist in the same direction, thereby driving the weld deflection. The amount of deflection is closely related to the deflection angle, wall thickness, etc. A large deflection angle results in a large deflection amount; a thick wall and the same deflection angle can achieve a large amount of deflection. The advantage of the deflection method is high control sensitivity, the larger the tube, the thicker the wall, the more positive the response, and the deflection is easy to control; the disadvantage is that it sometimes affects the butt joint state of the weld and the removal of external burrs, and needs to be adjusted accordingly.

(2) Offset method. The principle of the offset method to control the position of the sizing weld is that if the guide roll is shifted to the right of the rolling centerline, the welded pipe will receive a leftward thrust in the squeeze roll, which is combined with the traction on the tube blank After that, a force to the left and front is formed. This force forces the pipe body, including the weld seam, to deflect to the left. The advantages and disadvantages of the offset method are similar to those of the deflection method, but the action response is more sensitive than that of the deflection method. From the adjustment practice, the offset method is the best way to control the position of the thin-walled pipe weld.

Adjust the welding seam position of the square tube produced by the directly forming to square pipe mill through the two control methods of the forming roller method and the guide roller method, but it must be judged and adjusted according to the actual situation.

What needs special explanation is:

The roll offset is generally only a few tenths of a millimeter to 1~2mm (the smaller the tube is, the smaller the value is), and usually will not crush the tube surface; if the tube surface is indented due to it, this method should be abandoned.

The above-mentioned methods for controlling the position of the weld seam can be used alone or in combination, not only for special-shaped pipes, but also for round pipes.