Characteristics of The Production Process of Each Hot-rolling Mill Unit for Hot-rolled Seamless Steel Pipes

We usually call the wall thickness processing of capillary tube rolling. Pipe rolling is the most important process in the steel pipe forming process. The main task of this link is to thin the thick-walled capillary tube to a level suitable for the finished steel pipe according to the requirements of the finished steel pipe. That is to say, it must take into account the changes in wall thickness during the subsequent setting and diameter reduction processes. This link also needs to improve the capillary tube. The internal and external surface quality and wall thickness uniformity. The pipes that have passed through the wall-reducing and elongation process are generally called raw pipes. The basic feature of the pipe rolling method is to press a rigid mandrel inside the capillary tube and use an external tool (roller or die hole) to compress and reduce the wall thickness of the capillary tube. There are many production methods based on different deformation principles and equipment characteristics. It is generally customary to name the hot rolling mill unit according to the form of the pipe rolling machine. Pipe rolling machines are divided into single-stand and multi-stand. Single-stands include automatic pipe rolling mills, Asel rolling mills, ACCU-ROLL, etc. Cross-rolling pipe mills are all single-stand; continuous pipe rolling machines are all multi-stand. Usually 4~8 racks, such as MPM, PQF, etc. Currently, two pipe rolling processes are mainly used: continuous rolling (longitudinal rolling) and cross-rolling.

Several forms of continuous pipe rolling machines: After the long mandrel is inserted into the capillary tube, the continuous pipe rolling machine is arranged sequentially through multiple frames, and the roll gaps of adjacent frames are staggered (the roll gaps of the two rollers are staggered by 90°), a three-roller continuous rolling mill with roll gaps offset by 60° is rolled into steel pipes. It is the most widely used method of longitudinal rolling of steel pipes today. During the rolling process of the continuous pipe rolling mill, the deformation of the rolled piece is a process from circle to ellipse... ellipse, and then to circle again due to the repeated action of multiple sets (4 to 8 sets) of rollers and mandrels.

The development of continuous pipe rolling mills has a long history. As early as the end of the 19th century, attempts were made to roll pipes on long mandrels. However, due to various reasons, by 1950, there was only 6 continuous pipe rolling mills in the world. In the late 1990s, the three-roller continuous pipe mill (PQF) technology was launched, which brought the continuous pipe rolling process equipment to a higher level. Before the emergence of PQF, continuous pipe rolling mills were all two-roller type, that is, two rollers were used as a set to form a pass pattern. The two-roller frame was either arranged staggered at 45° with the ground or perpendicular to the ground. , horizontally staggered layout; PQF is a three-roller type, that is, three rollers form a set of pass patterns; MPM and PQF pass patterns are similar; when continuous rolling pipes, the metal at the top of the pass pattern is affected by the external pressure of the rollers and The pressure inside the mandrel causes axial extension and widens laterally to the circumference. The metal on the side wall of the hole does not contact the mandrel, but it is caused by the tensile stress added to it by the axially extending metal at the top. axial extension and axial contraction at the same time.

Floating mandrel continuous pipe rolling mill (or fully floating mandrel continuous pipe rolling mill): referred to as MM (Mandrel Mill), generally equipped with 8 stands. During the rolling process, the speed of the mandrel is not controlled. The mandrel is driven by the friction of the rolled metal to freely follow the pipe through the rolling mill. The running speed of the mandrel is uncontrolled; during the rolling process, the running speed of the mandrel changes with the pipe. There are fluctuations in the biting and steel throwing of each frame, which causes fluctuations in the wall thickness of the pipe; after the rolling is completed, the mandrel is rolled out with the raw pipe to the output roller table behind the continuous rolling mill. During rolling, the thin-walled pipe At this time, almost the entire length of the mandrel is in the waste tube, as shown in Figure 3; the waste tube with the mandrel moves laterally to the rod removal line, and the mandrel rod is pulled out of the waste tube by the rod removal machine for cooling, lubrication, and recycling. It is characterized by a fast-rolling rhythm and can roll 4 or more steel pipes per minute; however, the wall thickness accuracy of raw pipes is slightly lower, the process flow is longer due to the decoding machine, and the length of the mandrel is close to the length of the pipe. ; Suitable for producing seamless steel pipes of smaller specifications (outer diameter less than 177.8mm).

The working characteristics of the floating mandrel continuous pipe rolling machine are: since the mandrel speed is not controlled during rolling, the mandrel speed changes multiple times during the entire rolling process. For example, on an 8-stand continuous pipe rolling mill, when the metal enters the first stand, the mandrel runs at a rolling speed close to the first stand under the action of friction; when the metal enters the second stand When entering the third stand, the mandrel speed will change and run at a speed between the rolling speeds of the first and second stands; when entering the third stand, the mandrel speed has changed to the first, second and second stands. A certain speed between the rolling speed of the third stand; and so on, until entering the eighth stand, the mandrel speed has changed 8 times, running at a speed between 1 and 8 stands, and entering a Relatively stable rolling stage. At this stage, the rolling speed of the front stand is slower than the mandrel speed (called a slow stand), and the rolling speed of the rear stand is faster than the mandrel speed (called a fast stand). If a machine is in the middle the rolling speed of the frame is the same as the running speed of the mandrel, it is called a synchronous frame. Then, when the metal is gradually rolled out from the relevant rack, the mandrel speed changes to a speed between 2 and 8 racks; when the metal is rolled out from the second stand, the mandrel speed changes to the third to the third rack. A certain speed between the eighth stand, and so on until the metal is rolled out of the eighth stand.

It can be seen from the above that during the rolling process of steel pipes, the speed of the mandrel will change at least 15 times. Changes in the speed of the mandrel will lead to changes in the metal flow conditions. The floating mandrel continuous pipe rolling machine changes the metal flow due to the change in the mandrel speed during the rolling process. The irregularity of the metal flow causes changes in the longitudinal wall thickness and diameter of the steel pipe. Although many measures have been taken and achieved A certain effect is obtained when the changes in rolling conditions still exist and the dimensional accuracy of the product tube is always inferior to that of the limited mandrel rolling mill. In addition, the long mandrel increases manufacturing costs and makes manufacturing difficult, and the weight of the long mandrel is also very large. Running the steel pipe on the roller table with an overweight mandrel will cause surface damage to the steel pipe. Therefore, floating mandrel continuous pipe rolling machines are currently used in small units.

During continuous pipe rolling, the raw pipe can be regarded as being formed by continuous rolling between rolls of different diameters. The mandrel inserted in the steel pipe can be regarded as an inner roller with an infinite radius of curvature. During floating mandrel rolling, the mandrel has no other external force except the force transmitted from the roller through the rolled piece. After the head of the rolled piece bites through the first stand, as the rolled piece moves toward the extended stands one by one, the number of stands acting on the mandrel increases one after another, and the bar speed continues to increase. This stage is called the "biting in" "stage. When the head of the rolled piece enters the last stand, the entire rolled piece is in the rolling process of all stands of the continuous pipe rolling mill, and the speed of the mandrel remains unchanged, which is called the "stable rolling" stage. When the tail of the rolled piece leaves the first stand, the speed of the mandrel increases step by step until the rolling out is extended, which is called the "rolling out" stage. The working peripheral speed of the roll is set under the "stable rolling" state. During the rolling process, the rolled piece follows the law of constant volume. However, the increase in the speed of the rolling stock caused by the mandrel will inevitably increase the amount of metal flowing into the subsequent frames. That is to say, the mandrel feeds more metal into the subsequent frames than the set roll circumferential speed allows. If there is more metal, there will be metal accumulation that increases the cross-sectional area. The larger cross-section caused by this gradual inflow of additional metal, although processed on the final frame, still leads to larger diameters and thicker wall thicknesses in some parts of the barren pipe. This phenomenon is called "bamboo knots" ". In principle, "bamboo knots" may appear on the entire steel pipe. Obviously, the "bamboo knot" phenomenon is an uneven longitudinal wall thickness, which is detrimental to subsequent tensioner rolling and should be prevented as much as possible. To prevent or reduce the formation of "bamboos", when allocating the reduction amount in the hole design, the reduction amount of the first few frames should be appropriately increased while ensuring that the total extension remains unchanged. In this way, the jump in mandrel speed can be weakened in the following stands, thereby reducing the impact of mandrel speed changes. Good mandrel lubrication is beneficial to extension reduces energy consumption, and can also reduce the formation of bamboo knots. Electronic control technology can also be used to prevent the occurrence of bamboo knots. It is preset by the electronic computer and the roller speed changes as required. When the rolling stock passes, the rollers are calibrated so that the exit speed of each stand can adapt to the changes in the speed of the mandrel. Floating mandrel continuous pipe rolling mill units became popular in the 1970s. Due to the limitation of the weight of the mandrel, this type of unit can only produce steel pipes with a diameter of less than 177.8mm so far.