Straight seam steel pipe

Straight seam steel pipe is a steel pipe with welds parallel to the longitudinal direction of the steel pipe. It is usually divided into metric electric welded steel pipe, electric welded thin-walled pipe, transformer cooling oil pipe, and so on.
Production process: Straight seam high-frequency welded steel pipe has the characteristics of a relatively simple process and rapid continuous production, and is widely used in civil construction, petrochemical, light industry, and other departments. It is mostly used to transport low-pressure fluid or make various engineering components and light industrial products.
1. Production process of straight seam high frequency welded steel pipe
The straight seam welded steel pipe is made by rolling a certain specification of long steel strip into a round pipe through a high-frequency welding unit and welding the straight seam into a steel pipe. The shape of the steel pipe can be round, square, or irregular, depending on the sizing and rolling after welding. The main materials of welded steel pipes are low carbon steel and low alloy steel or other steel with σs≤300N/mm2 and σs≤500N/mm2. The production process of high-frequency welding of straight seam steel pipe is as follows:
2. High-frequency welding
High-frequency welding is based on the principle of electromagnetic induction and the skin effect, proximity effect, and eddy current thermal effect of AC charges in the conductor, so that the steel at the edge of the weld is locally heated to a molten state, and the butt weld is extruded by a roller to achieve crystal indirect Together, to achieve the purpose of welding seam welding. High-frequency welding is a kind of induction welding (or pressure contact welding). It does not need weld filler, no welding spatter, narrow welding heat-affected zone, a beautiful welding shape, and good welding mechanical properties. Therefore, it is favored in the production of steel pipes. Wide range of applications. The high-frequency welding of steel pipes uses the skin effect and proximity effect of alternating current. After the steel (strip steel) is rolled and formed, a circular tube blank with a broken section is formed, which rotates a circle near the center of the induction coil in the tube blank. Or a group of resistors (magnetic rods), the resistor, and the opening of the tube blank form an electromagnetic induction loop. Under the action of the skin effect and the proximity effect, the edge of the tube blank opening produces a strong and concentrated thermal effect, making the edge of the weld seam Rapidly heated to the temperature required for welding and squeezed by the pressure roller, the metal in the molten state realizes intergranular bonding, and forms a firm butt weld after cooling.
3. High-frequency welded pipe unit
The high-frequency welding process of straight seam steel pipe is completed in the high-frequency welded pipe unit. The high-frequency welded pipe unit is usually composed of rolling forming, high-frequency welding, extrusion, cooling, sizing, flying saw cutting, and other components. The front end of the unit is equipped with a material storage looper, and the rear end of the unit is equipped with a steel pipe turning frame; The electrical part is mainly composed of a high-frequency generator, DC excitation generator, and instrument automatic control device. Taking the φ165mm high-frequency welded pipe unit as an example, its main technical parameters are as follows:
4. High-frequency excitation circuit
The high-frequency excitation circuit (also known as the high-frequency oscillation circuit) is composed of a large electron tube and an oscillation tank installed in the high-frequency generator. It uses the amplification effect of the electron tube to connect the anode to the anode The output signal is positively fed back to the gate to form a self-excited oscillation circuit. The magnitude of the excitation frequency depends on the electrical parameters (voltage, current, capacitance, and inductance) of the oscillation tank.
5. Straight seam steel pipe high-frequency welding process
5.1 Control of weld gap The strip steel is sent into the welded pipe unit, and rolled by multiple rollers, the strip steel is gradually rolled up to form a circular tube billet with an opening gap, and the reduced amount of the extrusion roller is adjusted to make the weld gap Control it at 1~3mm, and make both ends of the welding joint flush. If the gap is too large, the proximity effect will be reduced, the heat of the eddy current will be insufficient, and the intergranular bonding of the weld will be poor, resulting in a lack of fusion or cracking. If the gap is too small, the proximity effect will increase, and the welding heat will be too large, causing the weld to burn; or the weld will form deep pits after extrusion and rolling, which will affect the surface quality of the weld.
5.2 Welding temperature control The welding temperature is mainly affected by the high-frequency eddy current thermal power. According to the formula (2), the high-frequency eddy current thermal power is mainly affected by the current frequency, and the eddy current thermal power is proportional to the square of the current excitation frequency; while the current excitation frequency is affected by the excitation voltage, current and capacitance and inductance. The formula of excitation frequency is f=1/[2π(CL)1/2]...(1) where: f-excitation frequency (Hz); C-capacitance (F) in the excitation circuit, capacitance=power/ Voltage; L-inductance in the excitation circuit, inductance = magnetic flux/current, the above formula shows that the excitation frequency is inversely proportional to the square root of the capacitance and inductance in the excitation circuit, or proportional to the square root of the voltage and current, as long as the capacitance in the circuit is changed, Inductance, voltage, and current can change the size of the excitation frequency, to achieve the purpose of controlling the welding temperature. For low-carbon steel, the welding temperature is controlled at 1250~1460°C, which can meet the penetration requirements of the pipe wall thickness of 3~5mm. In addition, the welding temperature can also be achieved by adjusting the welding speed. When the input heat is insufficient, the edge of the heated weld cannot reach the welding temperature, and the metal structure remains solid, forming incomplete fusion or incomplete penetration; when the input heat is insufficient, the heated edge of the weld exceeds the welding temperature, resulting in Overburning or dripping causes the weld to form a molten hole.
5.3 Control of extrusion force After the two edges of the tube blank are heated to the welding temperature, under the extrusion of the extrusion roller, the common metal grains are formed to penetrate and crystallize each other, and finally form a firm weld. If the extrusion force is too small, the number of common crystals formed will be small, the strength of the weld metal will decrease, and cracks will occur after being stressed; if the extrusion force is too large, the molten metal will be squeezed out of the weld, which will not only reduce The strength of the weld is reduced, and a large number of internal and external burrs will be generated, and even defects such as welding laps will be caused.
5.4 Adjustment of the position of the high-frequency induction coil should be as close as possible to the position of the extrusion roller. If the induction coil is far away from the extrusion roller, the effective heating time is longer, the heat-affected zone is wider, and the strength of the weld seam decreases; otherwise, the edge of the weld seam is insufficiently heated, and the shape after extrusion is poor.
5.5 The impedance is one or a group of special magnetic rods for welded pipes. The cross-sectional area of the impedance should generally not be less than 70% of the internal diameter of the steel pipe. Its function is to form an electromagnetic induction loop between the induction coil, the weld edge of the pipe blank, and the magnetic rods. , Proximity effect is generated, and the eddy current heat is concentrated near the edge of the weld of the tube blank, heating the edge of the tube blank to the welding temperature. The impedance is dragged in the tube blank by a steel wire, and its central position should be relatively fixed close to the center of the extrusion roller. When starting up, due to the rapid movement of the tube blank, the resistor is worn out by the friction of the inner wall of the tube blank and needs to be replaced frequently.
5.6 Weld scars will be produced after welding and extrusion, which need to be removed. The removal method is to fix the tool on the frame and scrape the weld scars flat by the rapid movement of the welded pipe. The burrs inside the welded pipe are generally not removed.
6. Technical requirements and quality inspection of high-frequency welded pipes According to the provisions of the GB3092 "Welded Steel Pipes for Low-Pressure Fluid Transmission", the nominal diameter of the welded pipe is 6~150mm, the nominal wall thickness is 2.0~6.0mm, and the length of the welded pipe is usually 4~150mm. 10 meters, can be delivered according to fixed length or double length. The surface quality of the steel pipe should be smooth, and defects such as folding, cracks, delamination, and lap welding are not allowed. The surface of the steel pipe is allowed to have minor defects such as scratches, scratches, weld misalignment, burns, and scars that do not exceed the negative deviation of the wall thickness. The thickening of the wall thickness at the weld and the existence of inner seam welding reinforcement are allowed.
Welded steel pipes should be subjected to mechanical performance tests, flattening tests, and flaring tests, and must meet the requirements of the standard. The steel pipe should be able to withstand a certain internal pressure, and if necessary, carry out a 2.5Mpa pressure test to keep it leak-free for one minute. The method of eddy current flaw detection is allowed to replace the hydrostatic test. The eddy current flaw detection is carried out according to the standard of GB7735 "Steel tube eddy current flaw detection inspection method". The eddy current flaw detection method is to fix the probe on the frame, keep a distance of 3~5mm between the flaw detection and the weld seam, and conduct a comprehensive scan of the weld seam by the rapid movement of the steel pipe. The flaw detection signal is automatically processed and sorted by the eddy current flaw detector. To achieve the purpose of flaw detection. It is a steel pipe made of steel plate or steel strip after crimping and welding. Welded steel pipe has a simple production process, high production efficiency, many varieties and specifications, and less equipment investment, but its general strength is lower than that of seamless steel pipe. Since the 1930s, with the rapid development of continuous rolling production of high-quality strip steel and the advancement of welding and inspection technology, the quality of welds has been continuously improved, and the varieties and specifications of welded steel pipes have increased day by day and replaced seamless steel pipes in more and more fields. Seam steel pipe. Welded steel pipes are divided into straight seam welded pipes and spiral welded pipes according to the form of the weld seam. The production process of straight seam welded pipe is simple, the production efficiency is high, the cost is low, and the development is rapid. The strength of the spiral welded pipe is generally higher than that of the straight seam welded pipe, and the welded pipe with a larger diameter can be produced with a narrower billet, and welded pipes with different diameters can be produced with the same width billet. But compared with the straight seam pipe of the same length, the weld length is increased by 30~100%, and the production speed is lower. The welded pipe after the flaw detection is cut off according to the specified length with a flying saw, and it is rolled off the assembly line through the turning frame. Both ends of the steel pipe should be chamfered with flat ends, printed with marks, and the finished pipes are packed in hexagonal bundles before leaving the factory.