34CrMo4 steel is an alloy structural steel with high lasting strength and creep resistance at high temperature, low temperature impact toughness, good permeability, no overheating tilt, low deformation, cold deformation plasticity and better machinability. This alloy structural steel is widely used in the production of seamless steel pipes. In the production and processing of seamless steel pipes, defects such as inner folds, outer folds and holes will appear on the seamless steel pipe or surface. The inner bending defect of the steel pipe is usually connected to the metal surface. The inner bending defect is a straight tooth or spiral tooth defect on the inner surface of the steel pipe. For the alloy of the steel pipe, this defect is an irregular block distribution at the inlet. Some hot giant seamless steel pipes can be repaired by simple repairs after internal folding defects, and serious ones need to be handled. In the current actual production, the inner bending error has always been an important factor affecting the performance of hot-pressed seamless steel pipes. This article analyzes the causes of inner surface defects of seamless steel pipes for reference.

Introduction
Due to its excellent oxidation resistance, corrosion resistance, heat resistance and worm resistance, P92 has become the preferred pipe for four key pipelines such as main steam pipelines, high-temperature and high-temperature pipelines, bypass pipelines and connecting pipelines. At present, the hot processing methods for domestic production of P92 seamless pipes are mainly extrusion, fast forging and oblique rolling. Among them, the oblique rolling production method is one of the main domestic production methods due to its high yield rate, low production cost and high production efficiency. P92 pipes have high alloy content and have the characteristics of high deformation resistance, low plasticity and narrow deformation temperature range during hot processing production.

1 Causes of internal folding defects in 34CrMo4 seamless steel pipes
Internal cracks are a common defect type of seamless steel pipes. How to effectively prevent the occurrence of defects in water-cooled wall tubes has always been an important issue of concern to relevant scholars. Underwater bubbles are the main cause of surface defects in steel pipes. Underwater bubbles are formed during the smelting process and are transferred to the boiler and peroxide parts during the decarburization of the pipeline, resulting in a lot of residues in the center of the plate; thus leading to internal defects in the steel pipe when it is perforated at high temperature, oxidation occurs between the steel base and the metal oxide layer, and the internal folding defects are mainly related to the truncation of the center and size between the main factors that cause bending defects in the tube, including the relaxation center of the material, the center hole, the internal structure and the presence of voids, and the microstructure of chromium and molybdenum is optimized by adjusting the cooling rate; therefore, the bainite structure is more resilient, which provides a theoretical basis for future production and optimized modulation treatment. When the material in the steel is Ca spheroidized, the cleanliness of the steel can be improved, thereby reducing the steel pipe cracks generated at the interface of the hot rolling layer and the oxide layer after copper, tin heat treatment. The oxidation and polarization of copper, arsenic and tin on the gamma boundary give priority to this wealth, while the secondary heating increases the rich state of copper, arsenic and tin back to the steel base, thereby deteriorating the plasticity of the steel. By comparing four different heat treatment processes, it is speculated that the sub-heating + thermal radiation (IT) treatment can optimize the strength and plasticity of 34CrMo4 steel materials, making it have the best comprehensive mechanical properties.

2 Metallographic analysis of inner surface cracks
The metallographic analysis of the inner surface defects of the steel pipe was carried out. From the metallographic structure of the defect, it can be seen that the P92 matrix is ​​a tempered martensite structure, and there are more white phases near the cracks. The scanning electron microscope energy spectrum (EDS) analysis was carried out around the crack, and the results showed that this white phase was a high-temperature 6-ferrite phase containing high Cr and high W. The presence of high-temperature 6-ferrite phase is easy to affect the surface quality of stainless steel and heat-resistant steel seamless steel pipes. The high temperature 8-ferrite phase is a brittle phase. Under the shear stress of two-roller oblique rolling and piercing, the austenite and ferrite phases have different high temperature plasticity and deformation resistance, and the two phases deform in an uncoordinated manner, which leads to tearing at the junction of austenite and ferrite and the formation of fine cracks.

3 Process Optimization
3.1 Billet Heating System Optimization
Since the high temperature deformation resistance of P92 steel is higher than that of carbon steel and low alloy steel, the heating temperature and holding time of the original P92 billet are higher and longer than those of carbon steel and low alloy billets of the same specifications. In order to avoid and reduce the occurrence of cracks on the inner wall of P92 pipes and ensure that the maximum working load of the piercing machine is not exceeded during the piercing of P92 pipe billets, the heating system of P92 pipe billets is optimized as follows: (1) The heating temperature of the billet is increased from 1220~1250℃ of the original process to 1190~1220℃ according to the billet specifications; (2) The total heating and insulation time of the billet in the heating furnace is reduced to 80% of the original process, thereby reducing the oxidation and decarburization of the surface of the central through hole of the pipe billet.
3.2 Optimization of piercing process
The oblique rolling piercing process parameters have an important influence on the rolling quality of P92 steel. Literature research shows that reducing the roll speed of the piercing machine and increasing the roll entrance angle are beneficial to improving the rolling quality of difficult-to-deform alloy seamless steel pipes. To this end, the roll speed of the P92 steel piercing machine is adjusted to 75%~80% of carbon steel and low alloy steel according to the billet specifications, and the roll spacing is appropriately enlarged and the head extension is reduced to reduce the degree of deformation when the material is bitten.
4 Analysis of the cause of defect formation
The surface sample of the defect was observed using a stereo microscope. There were two black lines on the surface along the rolling direction, which were in the shape of a gully with a low center and high sides. The surface was observed using a scanning electron microscope to be rough and uneven, with a large number of uneven pits distributed. The defect surface and transverse and longitudinal section analysis showed that this defect was a defect existing in the ingot. Combined with the rolling process of the seamless steel pipe, a comprehensive analysis was conducted and it was inferred that this defect was a central shrinkage cavity on the ingot. During the subsequent high-temperature rolling process, oxidation occurred, and the shrinkage cavity continuously deformed to form uneven pits. The cause of the shrinkage cavity is that high-temperature pouring causes columnar crystals to develop and form a "bridge", and the molten steel in the bridge cannot be replenished. When the molten metal solidifies from the surface to the inside, liquid shrinkage[5], solidification shrinkage and solid shrinkage occur; when the liquid shrinkage and solidification shrinkage exceed the solidification shrinkage of the solidified metal, the liquid will separate from the solidified metal surface to form a shrinkage cavity.
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