The results of the causal analysis of welded steel pipe waste show that the factors affecting the quality of raw materials are mainly three aspects: unstable mechanical properties of steel strips, surface defects of steel strips, and large deviations in geometric dimensions. Therefore, key control should be carried out from these three aspects.
1. The influence of the mechanical properties of the steel strip on the quality of the steel pipe
The steel type commonly used for welded steel pipes is carbon structural steel, and the yield point and tensile strength of the steel strip are too high, which will cause difficulties in forming the steel strip. Especially when the pipe wall is thick, the resilience of the material is large, and the steel pipe has a large deformation stress during welding, and the weld is prone to cracks. When the tensile strength of the steel strip exceeds 635MPa and the elongation is lower than 10%, the weld seam of the steel strip is prone to cracking during the welding process.When the tensile strength is lower than 300MPa, the surface of the steel strip is prone to wrinkle due to the soft material during the forming process. Especially when welding steel pipes with a diameter of more than 60mm, obvious and regular ring-shaped wrinkles appear on the surface of the steel pipes. When the wrinkles are serious, the entire batch of steel pipes will be scrapped. 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.
2. The influence of steel strip surface defects on steel pipe quality
Common surface defects of steel strips include sickle bends, waves, and slitting edges. Camber bends and waves generally appear in the rolling process of cold-rolled steel strips, which are caused by improper control of reduction. During the forming process of the welded steel pipe, the camber and wave shape will cause the deviation or overturn of the strip steel, which will easily cause lap welding of the weld seam of the steel pipe and affect the quality of the steel pipe. The gnawing edge of the steel belt (that is, the jagged and uneven phenomenon on the edge of the steel belt) generally occurs on the slitting belt, and the reason is that the disc blade of the slitting machine is blunt or not sharp. Due to the edge gnawing of the steel strip, there is a local lack of meat from time to time, which makes the steel strip prone to cracks and cracks during welding, which affects the stability of the weld quality.
3. The influence of the geometric size of the steel strip on the quality of the steel pipe
When the width of the steel strip is less than the allowable deviation, the extrusion force when welding the steel pipe is reduced, so that the welding seam of the steel pipe is not strong, and cracks or open pipes appear. When the width of the steel strip is greater than the allowable deviation, the extrusion force when welding the steel pipe increases, and welding defects such as sharp mouth, lap welding or burrs appear at the weld of the steel pipe. Therefore, the fluctuation of the width of the steel strip not only affects the accuracy of the outer diameter of the steel pipe, but also seriously affects the surface quality of the steel pipe. For steel pipes with high wall thickness uniformity, the fluctuation of steel strip thickness will transfer the allowable value of thickness difference of the same steel strip to the wall thickness difference of finished steel pipes, so that a large number of steel pipes will be rejected if the thickness exceeds the allowable deviation. The fluctuation of thickness not only affects the thickness accuracy of the finished steel pipe. At the same time, due to the different thickness of the steel strip, the extrusion force and welding temperature are unstable when the steel pipe is welded, resulting in unstable weld quality when the steel pipe is welded.
In addition, due to the existence of interlayers, impurities, trachoma and other material defects inside the steel, it is also an important factor affecting the quality of steel pipes. Therefore, before the steel strip is welded, it is necessary to check the surface quality and geometric dimensions of each coil of steel strip. If the quality of the steel strip does not meet the standard requirements, do not carry out production to avoid unnecessary losses.
1. The influence of the mechanical properties of the steel strip on the quality of the steel pipe
The steel type commonly used for welded steel pipes is carbon structural steel, and the yield point and tensile strength of the steel strip are too high, which will cause difficulties in forming the steel strip. Especially when the pipe wall is thick, the resilience of the material is large, and the steel pipe has a large deformation stress during welding, and the weld is prone to cracks. When the tensile strength of the steel strip exceeds 635MPa and the elongation is lower than 10%, the weld seam of the steel strip is prone to cracking during the welding process.When the tensile strength is lower than 300MPa, the surface of the steel strip is prone to wrinkle due to the soft material during the forming process. Especially when welding steel pipes with a diameter of more than 60mm, obvious and regular ring-shaped wrinkles appear on the surface of the steel pipes. When the wrinkles are serious, the entire batch of steel pipes will be scrapped. 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.
2. The influence of steel strip surface defects on steel pipe quality
Common surface defects of steel strips include sickle bends, waves, and slitting edges. Camber bends and waves generally appear in the rolling process of cold-rolled steel strips, which are caused by improper control of reduction. During the forming process of the welded steel pipe, the camber and wave shape will cause the deviation or overturn of the strip steel, which will easily cause lap welding of the weld seam of the steel pipe and affect the quality of the steel pipe. The gnawing edge of the steel belt (that is, the jagged and uneven phenomenon on the edge of the steel belt) generally occurs on the slitting belt, and the reason is that the disc blade of the slitting machine is blunt or not sharp. Due to the edge gnawing of the steel strip, there is a local lack of meat from time to time, which makes the steel strip prone to cracks and cracks during welding, which affects the stability of the weld quality.
3. The influence of the geometric size of the steel strip on the quality of the steel pipe
When the width of the steel strip is less than the allowable deviation, the extrusion force when welding the steel pipe is reduced, so that the welding seam of the steel pipe is not strong, and cracks or open pipes appear. When the width of the steel strip is greater than the allowable deviation, the extrusion force when welding the steel pipe increases, and welding defects such as sharp mouth, lap welding or burrs appear at the weld of the steel pipe. Therefore, the fluctuation of the width of the steel strip not only affects the accuracy of the outer diameter of the steel pipe, but also seriously affects the surface quality of the steel pipe. For steel pipes with high wall thickness uniformity, the fluctuation of steel strip thickness will transfer the allowable value of thickness difference of the same steel strip to the wall thickness difference of finished steel pipes, so that a large number of steel pipes will be rejected if the thickness exceeds the allowable deviation. The fluctuation of thickness not only affects the thickness accuracy of the finished steel pipe. At the same time, due to the different thickness of the steel strip, the extrusion force and welding temperature are unstable when the steel pipe is welded, resulting in unstable weld quality when the steel pipe is welded.
In addition, due to the existence of interlayers, impurities, trachoma and other material defects inside the steel, it is also an important factor affecting the quality of steel pipes. Therefore, before the steel strip is welded, it is necessary to check the surface quality and geometric dimensions of each coil of steel strip. If the quality of the steel strip does not meet the standard requirements, do not carry out production to avoid unnecessary losses.
