Causes of Hot Rolling Cracks in N80 Oil Casing

    Hot-rolled seamless steel pipe production is one of the important processes, the rational development of its process plan or not, a direct impact on the size of the steel pipe after rolling the surface of the precision and quality,

    Unreasonable process plan will cause the inner surface of the steel pipe to produce the defects such as folding, crack and concave.

    N80 steel casing is a standard oil casing commonly used in oil (for the seamless steel pipe), the standard provisions of the finished pipe heat treatment process for quenching and tempering.

    N80 casing production process is as follows: furnace refining → continuous casting → annular furnace → perforation → continuous rolling → stepping furnace → sizing → sawing → tempering treatment → straightening → physical and chemical testing → testing → thread processing → Coupling tight → water pressure test → measuring length weighing, marking → packaging storage. The billet was heated in an annular furnace for about 2.5 hours, perforated at about 1250 °C and then subjected to continuous rolling. The sintering temperature was 950 °C and the finish rolling temperature was 900 °C. The chemical composition of the tube from the test results can know the chemical composition of the tube is not the cause of the formation of cracks. From the metallographic, The results of EDS analysis show that both the crack tip and the middle part contain iron oxide inclusions. The crack is a non-quenching crack and should be produced before the heat treatment. This is probably caused by the perforation and hot rolling of the steel tube.

    In the process of perforation of steel pipe, because temperature non-uniformity and uneven deformation have a large additional stress, the blank in the perforation easy to crack; And the heating temperature is too high, will also be due to reduce the intergranular binding force cracks; if the heating temperature is too low, due to lower plasticity, deformation resistance and deterioration of the perforation conditions, will also crack. In addition, the perforation process, due to the presence of additional deformation, when the billet surface will produce longitudinal shear stress and transverse shear shear stress. Therefore, there is a large lateral and longitudinal additional stress in the outer layer of the steel tube. In the case of non-uniform heating of the tube, when the superposition of the transverse basic stress and the additional tensile stress exceeds the intergranular bonding force, Small cracks, these cracks in the subsequent rolling and sizing process, along the rolling direction is elongated and extended to form the final crack. Therefore, in the future production should pay special attention to the perforation temperature, sizing temperature and finishing temperature of the monitoring and control, as far as possible the temperature fluctuations in the control within ± 20 ℃, and should increase the light tube end of the test strength, If necessary, increase the cut length of the pipe end.

    N80 casing pipe end crack is in the steel pipe perforation and rolling process; when the billet is hole, due to uneven heating temperature, at the same time in the transverse and longitudinal additional stress, resulting in crack formation and expansion.