2 the influence of raw materials on the welding quality of steel pipes The main factors affecting the quality of raw materials are the unstable mechanical properties of steel belts, the surface defects of steel belts and the large deviation of geometric dimensions. So we should focus on these three aspects.
1) Influence of mechanical properties of steel strip on steel pipe quality. Carbon structural steel is often used to weld steel pipes, and the main brands are Q 195, Q2 15, Q235 SPCC SS400 SPHC, etc. The high yield point and tensile strength of steel strip will make it difficult to form the steel strip, especially the thick pipe wall, the large resilience of the material, the large deformation stress in steel pipe welding, and the cracks in the weld are easy to occur. When the tensile strength of steel strip exceeds 635 MPa and the elongation is lower than 10%, the weld of steel strip is prone to crack during welding. When the tensile strength is lower than 30 0MPa, the surface of steel strip is easy to wrinkle because of the soft material in the forming process. It can be seen that the mechanical properties of materials have a great influence on the quality of steel pipes, and the quality of steel pipes should be effectively controlled from the aspect of material strength.
) the influence of surface defects of steel strip on the quality of steel tube The common surface defects of steel strip include sickle bending, wave shape, longitudinal cutting and edge gnawing. Cold-rolled strip generally appears sickle bend and wave shape during rolling, which is caused by improper control of reduction. In the process of steel tube forming, sickle bending and wave shape will cause the strip to deviate or turn over, which will easily lead to lap welding of steel tube welds and affect the quality of steel tube. The edge gnawing of steel strip (that is, the edge of steel strip is jagged and uneven) generally appears on the belt of slitting machine, which is caused by the dull or sharp edge of the disc of slitting machine. Due to the edge gnawing of the steel strip, local meat shortage occurs from time to time, which makes the steel strip prone to cracks and cracking during welding, thus affecting the stability of weld quality.
3) The influence of the geometric size of steel strip on the quality of steel tube When the width of steel strip is less than the allowable deviation, the extrusion force decreases when welding steel tube, which makes the weld seam of steel tube unstable and leads to cracks or pipe opening; When the width of the steel strip is greater than the allowable deviation, the extrusion force increases when welding the steel pipe, and welding defects such as sharp mouth, lap welding or burr appear at the weld of the steel pipe. Therefore, the fluctuation of steel strip width not only affects the accuracy of steel tube outer diameter, but also seriously affects the surface quality of steel tube. For steel pipes that require the wall thickness difference of the same section not to exceed the specified value, that is, steel pipes with uniform wall thickness height, the fluctuation of steel strip thickness will transfer the allowable value that the wall thickness difference of the same coil of steel strip exceeds to the wall thickness difference of finished steel pipes, so that a large number of steel pipes exceed the allowable deviation and are rejected. The fluctuation of thickness not only affects the thickness accuracy of finished steel pipe, but also makes the extrusion force and welding temperature unstable when welding steel pipe because of the different thickness of steel strip, which leads to the unstable weld quality when welding steel pipe. In addition, there are material defects such as interlayer, impurities and trachoma in steel, which are also important factors affecting the quality of steel pipes. Therefore, before welding the steel strip, it is necessary to check the surface quality and geometric size of each coil of steel strip. If the quality of steel strip does not meet the standard requirements, don't produce it to avoid unnecessary losses.
3 Influence of high-frequency welding on the quality of steel pipe In the process of high-frequency welding of steel pipe, the control of welding technology and process parameters, and the placement of induction coil and resistor have great influence on the welding quality of steel pipe weld.
1) control of weld gap of steel pipe The steel strip enters the welded pipe mill, and after being shaped by the forming roller and guided by the guide roller, a round steel pipe blank with an open gap is formed. Adjust the extrusion amount of the extrusion roller, control the weld gap at 1 ~ 3 mm, and keep both ends of the weld flush. If the weld gap is too large, the weld will be poorly welded, resulting in incomplete fusion or cracking; If the weld gap is controlled too small, the weld will be burned and the molten metal will splash due to overheating, which will affect the welding quality of the weld.
2) Control of the position of the high-frequency induction coil The induction coil should be placed on the same centerline as the steel pipe, and the distance between the front end of the induction coil and the centerline of the extrusion roll should be as close as possible according to the specifications of the steel pipe without burning the extrusion roll. If the induction coil is far away from the extrusion roll, the effective heating time will be long and the heat affected zone will be wide, which will reduce or incomplete the weld strength of steel pipe. On the contrary, the induction coil is easy to burn the extrusion roller.
3) The position of the resistor is controlled by one or a group of special magnetic bars for welded pipes, and the cross-sectional area of the resistor should generally not be less than 70% of the cross-sectional area of the inner diameter of the steel pipe. Its function is to make the induction coil, the weld edge of the tube blank and the magnetic bar form an electromagnetic induction loop to produce proximity effect, and the eddy current heat is concentrated near the weld edge of the tube blank, so that the tube blank edge is heated to the welding temperature. The resistor should be placed in the heating section of the V-shaped area, with the front end in the center of the extrusion roll, so that its center line is consistent with the center line of the pipe. If the position of the resistor is not well placed, it will affect the welding speed and quality of the welded pipe and cause cracks in the steel pipe.
4) High-frequency welding process parameters-control of input heat The heat input by high-frequency power supply to steel pipe weld is called input heat. When converting electric energy into heat energy, the formula for inputting heat energy is:
Q=KI2 Rt ( 1)
Where q refers to the heat input of the tube blank; K- energy conversion efficiency; I- welding current; R loop impedance; T- heating time.
Heating time: t=Lv (2)
Where l refers to the distance from the front end of the induction coil or electrode tip to the center of the extrusion roller; V- welding speed.
When the heat input by high frequency is insufficient and the welding speed is too fast, the edge of the heated pipe body can not reach the welding temperature, and the steel still maintains its solid structure and cannot be welded, forming an incomplete fusion or incomplete penetration crack; When the high-frequency heat input is too large and the welding speed is too slow, the edge of the heated pipe body exceeds the welding temperature, which is easy to overheat or even burn, resulting in weld fracture, metal splashing and shrinkage cavity formation. According to formulas (1) and (2), the high-frequency input heat can be controlled by adjusting the high-frequency welding current (voltage) or welding speed, so that the weld of the steel pipe should be penetrated or not, thus obtaining a steel pipe with excellent welding quality.
4 the influence of roll adjustment on the quality of steel pipe can be seen from the cause analysis diagram of steel pipe waste that roll adjustment belongs to the operation process of steel pipe. In the production process, when the rollers are seriously damaged or worn, some rollers need to be replaced on the unit, or a variety of rollers need to be replaced if enough rollers are continuously produced. At this time, the rollers should be adjusted to obtain good steel pipe quality. If the roller is not properly adjusted, it will easily lead to defects such as torsion, lap welding, edge wave, bulging, indentation or scratch, large ellipticity of steel pipe and so on. Therefore, the adjustment skills of rollers should be mastered when changing rollers.
1) When changing the specifications of steel pipes, a complete set of rollers is usually replaced. The method of roll adjustment is: use steel wire to pull a center line from the entrance to the exit of the unit, and adjust it so that the pass of each stand is on a center line and the molding bottom line meets the technical requirements. After changing the roll specifications, the forming roll, guide roll, extrusion roll and sizing roll are adjusted comprehensively, and then the closed pass, guide roll and extrusion roll of the forming roll are adjusted emphatically.
2) The function of the guide roller is to control the direction of the pipe seam and the height of the bottom line of the pipe blank, relieve the edge extension, control the rebound of the edge of the pipe blank, and ensure that the pipe seam is straight and not screwed into the extrusion roller. If the guide roller is not properly adjusted, it will easily cause welding defects such as torsion, lap welding and side wave during steel pipe welding.
3) The extrusion roller is the key equipment of the welded pipe mill, and its function is to complete the pressure welding of the pipe whose edge is heated to the welding temperature under the extrusion pressure of the extrusion roller. In the production process, the opening angle of the extrusion roller should be controlled. When the extrusion force is too small, the strength of weld metal decreases and cracks will occur after stress. When the extrusion force is too large, the welding strength decreases and the external burr increases, which is easy to cause welding defects such as lap welding.
4) During the slow start-up of the welded pipe mill, we should pay close attention to the rotation of the rollers in various parts and adjust the rollers at any time to ensure the welding quality and process size of the welded pipe meet the specified requirements.