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Why does the packaging steel strip tend to break neatly at the edges of the weld seam?
by：Hongmei
2021-03-15
Why does the packaging steel strip tend to break neatly at the edges of the weld seam?
In the actual production, sometimes there will be a phenomenon of neatly broken strips on the substrate at the edge of the weld, especially when welding thin-gauge high-strength steel strips. This is a hardened line that appears after the steel strip near the weld is rapidly cooled after being subjected to heat during welding, and breaks under the action of bending and stretching during the operation of the production line.
During welding, the temperature of the weld is very high, and the heat will also diffuse into the metal matrix near the weld through the weld, so that the temperature of this part will reach the quenching heating temperature (about 750°C) above, and the structure will be transformed into Austenitic high-temperature structure. During the cooling process after the welding wheel leaves, the thickness of the weld is thickened, and only the upper and lower sides can transfer heat in the air. The cooling rate is relatively slow, which can transform austenite into high temperature. The tempered structure has good overall performance. In the metal matrix near the weld, the temperature gradient in the temperature field distribution is very large, that is, the temperature near the weld is very high, and the temperature a little further away is very low , There is such a line near the weld that can transfer heat at a fast speed to a little far away, so that the cooling rate reaches the cooling rate of quenching, so the austenite structure is transformed into a hard and brittle martensite structure. There are two hardened lines with a width of only about 1.0mm on both sides of the seam. When welding steel strips with different thicknesses, this hardened line is generally on the side of the thinner steel strip. The structure and performance of this hardened wire are very different from the weld and the normal structure area on the other side. It is extremely brittle. As long as it is subjected to a certain tension and bending stress, it will break and cause a broken belt. For high-strength steel strip materials, there are strengthening elements with a certain composition, which will increase the quenching tendency of the material, so this defect is more obvious. After the welding seam and the metal matrix around it are annealed in the furnace, the structure and performance can be improved, so this type of band-breaking accident will occur in front of the heating furnace.
In the actual production, sometimes there will be a phenomenon of neatly broken strips on the substrate at the edge of the weld, especially when welding thin-gauge high-strength steel strips. This is a hardened line that appears after the steel strip near the weld is rapidly cooled after being subjected to heat during welding, and breaks under the action of bending and stretching during the operation of the production line.
During welding, the temperature of the weld is very high, and the heat will also diffuse into the metal matrix near the weld through the weld, so that the temperature of this part will reach the quenching heating temperature (about 750°C) above, and the structure will be transformed into Austenitic high-temperature structure. During the cooling process after the welding wheel leaves, the thickness of the weld is thickened, and only the upper and lower sides can transfer heat in the air. The cooling rate is relatively slow, which can transform austenite into high temperature. The tempered structure has good overall performance. In the metal matrix near the weld, the temperature gradient in the temperature field distribution is very large, that is, the temperature near the weld is very high, and the temperature a little further away is very low , There is such a line near the weld that can transfer heat at a fast speed to a little far away, so that the cooling rate reaches the cooling rate of quenching, so the austenite structure is transformed into a hard and brittle martensite structure. There are two hardened lines with a width of only about 1.0mm on both sides of the seam. When welding steel strips with different thicknesses, this hardened line is generally on the side of the thinner steel strip. The structure and performance of this hardened wire are very different from the weld and the normal structure area on the other side. It is extremely brittle. As long as it is subjected to a certain tension and bending stress, it will break and cause a broken belt. For high-strength steel strip materials, there are strengthening elements with a certain composition, which will increase the quenching tendency of the material, so this defect is more obvious. After the welding seam and the metal matrix around it are annealed in the furnace, the structure and performance can be improved, so this type of band-breaking accident will occur in front of the heating furnace.
