Thick-walled seamless steel pipe mainly used for high-strength structural steel. From the hierarchical defect mechanism, the general that the non-metallic inclusions in the tube will destroy the continuity and compactness of steel, severe mixed internally or stratification of the pipe. Another thought is hydrogen induced cracking, ie from accumulation of hydrogen in the steel caused by high gas pressure inside the metal, the inner tube is formed in the white circle point in the expansion of cracking during rolling, forming layered defects. In addition, two roll rotary piercing inhomogeneous deformation plasticity resulting stress exceeds the strength can also cause delamination.
In the smelting strictly controlled circumstances, many third situation occurs, the control measures:
1, to improve toughness plastic tube
Improve steel cleanliness and reduce harmful inclusions; increase the proportion of equiaxed crystal slab to reduce center segregation and center porosity; using reasonable cooling system to avoid internal cracks occur within the slab; right off the assembly line rolling slab or billet take slow cooling process, reducing the internal stress, in order to ensure billets and finished pipe and mechanical properties to meet the technical standards.
2, the reasonable control of the heating temperature
By measuring the thermoplastic curve, choose the best heating temperature. Also note that there is sufficient heating tube holding time, to reduce the plastic deformation resistance and improved toughness.
3, lower roll speed
Speed roll piercing process is a key parameter, roll speed the process of change from low to high, there is a critical stratification began to roll speed. Roll speed is low, the tube is easy to form cavity; roll speed is high, and capillary tube is easy to form a layered defects. To and capillary tube to eliminate layered defects should roll speed is reduced to the critical stratification began to appear less roll speed.
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