Layered defects of structural pipes and control measures
2013-09-06
From a mechanism perspective, it is generally believed that non-metallic inclusions in the pipe blank can damage the continuity and compactness of 45# structural steel pipes, and severe inclusions can even cause delamination within the 45# structural steel pipe. Another view is hydrogen-induced cracking, which occurs due to hydrogen accumulation in the steel causing excessive gas partial pressure inside the metal, forming white spots in the round pipe blank. During the rolling process, cracks may expand, ultimately leading to delamination defects. Additionally, uneven deformation caused by two-roll skew rolling perforation can generate stresses that exceed plastic strength, also resulting in delamination.
Under strict smelting control conditions, a third situation often arises, with control measures as follows:
1. Improve the plastic toughness of 45# structural steel pipes.
Increase the cleanliness of molten steel to reduce harmful inclusions; increase the equiaxed crystal ratio of continuous casting billets to reduce central segregation and central porosity; adopt reasonable cooling systems to avoid internal cracks in cast billets; apply slow cooling processes for offline cast billets or continuous rolling billets to reduce internal stress, thereby ensuring that the structure and mechanical properties of both pipe blanks and finished 45# structural steel pipes meet technical standards.
2. Reasonably control heating temperature.
By measuring the thermoplastic curve, select the optimal heating temperature. It is also important to ensure sufficient holding time during pipe blank heating to reduce deformation resistance and improve the plastic toughness of 45# structural steel pipes.
3. Reduce roll speed.
Roll speed is a key parameter in perforation technology. As roll speed changes from low to high, there exists a critical roll speed at which delamination begins to occur. At lower roll speeds, it is easier for cavities to form in the pipe blank; at higher roll speeds, delamination defects are more likely to occur in both pipe blanks and 45# structural steel pipes. To eliminate delamination defects in pipe blanks and 45# structural steel pipes, roll speed should be reduced below the critical roll speed where delamination starts.
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Shenyang Tengfei Iron Corporation
Shenyang Tengfei Steel Pipe Co., Ltd.
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