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Causes of Hot Rolling Cracks in N80Q Oil Casing

Date:2018-03-15    View:809      Tag:Causes of Hot Rolling Cracks in N80Q Oil Casing
Hot-rolled seamless steel tubes are one of the important processes in production. Reasonable development of their process schemes directly affects the dimensional accuracy and quality of the surface after steel tube rolling.
Unreasonable process schemes can cause defects such as folds, cracks, and concaves on the inner surface of the steel pipe.

The N80Q steel casing is a standard oil casing used for oil (seamless steel pipe), and the heat treatment process for the finished steel pipe is a standard requirement for quenching and tempering.

The production process of N80Q steel tube is as follows: Outer furnace refining → Continuous casting → Ring furnace → Perforation → Continuous rolling → Walking furnace → Sizing → Saw → Tempering treatment → Straightening → Physical and chemical testing → Testing → Threading → Coupling close → Press water test → measure and weigh, mark → pack and store. The billets were heated in a ring furnace for about 2.5 hours, perforated at about 1250°C, and then continuously rolled. The sintering temperature is 950°C and the finishing temperature is 900°C. The test results show that the chemical composition of the steel pipe can know that the chemical composition of the pipe is not the cause of crack formation. From the metallographic analysis, it can be seen that both the tip and the middle of the crack contain iron oxide inclusions. Cracks are non-quenched cracks and should be generated before heat treatment. This may be due to perforated steel tubes and hot rolling.

In the process of perforating the steel pipe, due to the non-uniform temperature and uneven deformation have a greater additional stress, in the perforated easily cracked blank; and the heating temperature is too high, but also due to reduce the intercrystalline bond strength cracks; if the heating temperature is too Low, cracks will occur due to lower plastic deformation resistance and worse perforating conditions. In addition, during the perforation process, due to the presence of additional deformation, longitudinal shear stress and transverse shear stress are generated on the surface of the blank. Therefore, the outer layer of the steel pipe has greater lateral and longitudinal additional stress. In the case of non-uniform heating of the pipe, when the superposition of the transverse basic stress and the additional tensile stress exceeds the intergranular bond strength, the cracks in the rolling direction gradually extend and expand to form the final crack during the subsequent rolling and sizing process. . Therefore, in the future production should pay special attention to the perforation temperature, sizing temperature and finishing monitoring and control temperature, as much as possible temperature fluctuations controlled at 20 °C ±, should increase the necessary test intensity, light pipe end, increase the length of the pipe end cutting .

The crack at the end of the N80Q steel casing is during the piercing and rolling process of the steel tube. When the blank hole, due to the uneven heating temperature, the transverse force and the longitudinal force add to the force, which leads to the formation and expansion of the crack.

It is suggested that the perforation and hot rolling process parameters be adjusted in time to strengthen the control of the tube heating temperature so that the heating temperature within the tube fluctuates within a reasonable range.

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