It can be seen from the results of the weld metallographic inspection that there is hardened martensite on the upper and lower surfaces of the high-frequency welded pipe weld. This is because the metal in the fusion zone at the edge of the high-frequency welded strip is in a molten state. After cooling by water penetration, the weld The upper and lower surfaces are quenched to obtain a martensite structure.
During the solidification process of the continuous casting slab, the alloying elements in the continuous casting slab are positive segregation, and the central segregation elements of 22 MnB5 steel are mainly C and Mn.

The high content of C and Mn moves the CCT curve to the lower right corner, improves the stability of high-temperature austenite during the welding process, and delays and prolongs the transformation time. The content of C and Mn in the center segregation is higher, which leads to the right shift of the C curve and the decrease of the critical cooling rate of quenching. The center segregation of hot-rolled steel strip is due to the longitudinal shear band located at the edge of the welded steel strip. After welding, the weld is cooled by water, the segregation zone forms a larger internal stress of the structure transformation, and the ductility of the quenched martensite is low. Under the action of welding thermal stress, the martensite structure on the upper and lower surfaces of the weld becomes the source of transverse cracks.

Measures to prevent transverse cracks in welds
According to the above analysis, the transverse cracks in the weld are caused by the central segregation during the solidification of the continuous casting slab. After rolling, the central segregation zone is located at the edge of the high-frequency welding raw material belt. During the high-frequency welding process, the weld The upper and lower surfaces form a rougher martensite structure.

One of the measures to prevent the transverse cracks of the hot rolled strip is to reduce the central segregation of the hot rolled strip during the continuous casting process, that is, to reduce the central segregation of the continuous casting billet during the continuous casting of the hot rolled strip, including a reasonable continuous casting temperature , Drawing speed, cooling water, etc., to reduce shrinkage cavity level, reduce center segregation, improve the internal quality of billet, and ensure uniform and stable composition.

Another measure to prevent transverse cracks in hot-rolled strip is to change the width of hot-rolled strip, that is, the longitudinal shear of hot-rolled strip is divided into odd-numbered strips that are not cut along the center line of hot-rolled strip, so that hot-rolled strip The central segregation zone of steel is not located at the edge of the high-frequency welding raw material steel strip. In this process, the hot-rolled strip with a width of 1176 mm after 1176 mm is divided into 1298 mm wide steel strips along the center line of the hot rolled strip, which can be adjusted to 1274 mm wide and 1398 mm wide steel strips. This will not cut longitudinally along the centerline of the hot-rolled steel strip, and the central segregation zone will not be located at the edge of the high-frequency welding raw material strip.

Conclusion
The reason for the transverse cracks is that there is component segregation in the hot-rolled steel strip of the raw material of the slitting strip, and the central component segregation of the steel strip is more serious. After the slitting, the central segregation zone is located at the edge of the high-frequency welding raw material strip.
When the weld is cooled by squeezing fusion water, the weld forms a rougher hardened martensite structure because the central segregation zone is located at the weld. Under the action of high-frequency welding thermal stress, the weld becomes the source of transverse cracks.
One of the measures to prevent the transverse cracks of the hot-rolled strip is to reduce the central segregation of the hot-rolled strip during the continuous casting process, that is, to reduce the central segregation of the continuous casting billet in the hot-rolled strip continuous casting process; second, to adjust the heat The width of the rolled strip is to be longitudinally cut into single strips that are not cut along the center line of the hot rolled strip, so that the center segregation zone of the hot rolled strip is not at the edge of the high-frequency welding raw material strip.

Tips: ASTM A500-Standard Specification for Cold-Formed Welded and Seamless Carbon Steel Structural Tubing in Rounds and Shapes.According to ASTM A500, the weld seam location for square, rectangle and other specially shaped tubing shall not be located within the corner radius, unless specified by the customer or manufacturer.Under ASTM A500, the corner radius for square and rectangular tubing shall not exceed three times the corner radius.