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Influence of Manufacturing Methods on Seamless Steel Tubes

Posted: 05/21/2023 19:33:24 Hits: 4

Manufacturing methods of seamless steel tubes include manufacturing methods of steel smelting, tube billet and steel pipe. The technical requirements directly affect the performance and quality of the seamless steel pipe.

1.2.1 The smelting method of steel
The smelting of steel is generally carried out with equipment such as electric furnaces or oxygen converters. The difference lies in whether and which external refining is used. The excellent properties and quality of steel are mainly manifested in high purity, good uniformity, small anisotropy, narrow range of alloy composition, which requires low impurity content in steel, especially non-metallic inclusions with low content and low grade. There is no or little difference in the longitudinal and transverse mechanical properties of the steel after rolling. The surface and internal quality of steel for steel pipes depend on its chemical composition (macroscopic purity), method of deoxidation (microscopic purity) and pouring conditions. There are four main methods of out-of-furnace refining.
1) Slag washing: This is a measure to improve the purity of the converter and electric furnace steel. Slag washing can effectively desulfurize, deoxidize and remove impurities, and reduce the harmful effect of secondary oxidation in the process.
2) Blow argon: Argon gas was blown through the vent brick specially installed at the bottom of the ladle, and relies on the argon bubbles floating in the molten steel to adhere to the emulsified slag droplets to remove harmful gases in the steel.
3) Vacuum degassing: The purpose is to reduce harmful gases in steel, especially hydrogen. There are many kinds of process methods, among which vacuum degassing VC can reduce the hydrogen content in the steel to be less than 2ppm (0.0002%), and the oxygen content about 20 ppm (0.002 0%); the DH method can remove 50% to 70% of the hydrogen content in the steel; the average content is 1.7 ppm (0.00017%), and the oxygen content is reduced to 20 to 40ppm (0.0020% to 0.004 0%); the RH method is also called the vacuum cycle degassing method. RH devices are widely used in China. This dehydrogenation method can reduce the hydrogen content in steel to below 2 ppm (0.0002%). It can also deoxidize and denitrogenate the molten steel. After vacuum degassing by the RH device, the purity can be improved; the anisotropy can be reduced, and the plasticity and toughness can be improved. The Chinese seamless steel pipe standard generally requires vacuum degassing.
4) Ladle powder spraying: It belongs to the method of adding solid materials, which is to spray the powdery reactants into the liquid molten pool of steel with a gas carrier. The powders injected are generally calcium, magnesium, or their alloys and compounds. Ladle powder spraying can not only achieve desulfurization, but also can reduce the content of non-metallic inclusions and change the shape of inclusions to improve the quality of steel.

1.2.2 Manufacturing methods of tube blanks
The tube blanks of seamless steel pipes can be produced by continuous casting machines; steel ingots can also be rolled into tube blanks by section steel rolling mills, and steel ingots can also be used directly. The continuous casting billet is not only economical and reasonable, but also has better quality than steel ingots. Usually, the defects of the tube blank will be transferred to the steel pipe, and the defects on the tube blank may be caused by steelmaking, or may be produced during rolling or drawing. Studies have shown that 40% of the defects of seamless steel pipes are caused by quality defects of the tube blank, and 60% are caused by incorrect operation of the rolling process. The main measures to reduce the defects of continuous casting billets are to improve the purity of steel and reduce non-metallic inclusions.

1.2.3 Manufacturing methods of seamless steel pipes
Larger-diameter seamless steel pipes are produced by hot rolling or thermal expansion. There are many methods to produce hot-rolled seamless steel pipes, but one of the main processes is to pierce the solid tube blank on the piercing machine. The quality of the finished tube depends greatly on the quality of the blank tube. One of the main defects during perforation is the folding of the inner and outer surfaces of the blank tube. The main reason for the folding is the quality of the original tube blank. This is related to the quantity and distribution of non-metallic inclusions in the tube blank. The characterization of folding is that there are thin metal layers on the inner and outer surfaces of the steel pipe. The shape of the layers is squamous, and the shape of the independent layers is like close curves, which are generally distributed over the entire length of the steel pipe. The research shows that there are gas holes around the non-metallic inclusions. In the perforation process, the gas holes are split to form inner folds. After being sized by the sizing machine, they are scale-like, and the non-metallic inclusions in the tube blank are brought from the steel ingot. The quality of steel ingots depends on the severity of non-metallic inclusions and banded segregation. Banded segregation is the segregation of phosphorus, sulfur, carbon and manganese. It can be seen from this that the importance of the content of phosphorus and sulfur in the chemical composition of seamless steel pipes affects the quality of steel pipes. Of course, the occurrence of folding defects in seamless steel pipes is also related to the casting process of steel ingots, that is, the casting strength and casting speed. This is a problem in the steel pipe manufacturing method and will not be discussed here.

1.2.4 Conclusion
From the above analysis of the seamless steel pipe manufacturing method, we can make four inferences that affect the quality.
1) The manufacturing methods of steel, tube billet and steel pipe in the seamless steel pipe standard directly affect the quality of the seamless steel pipe. Therefore, when selecting the seamless steel pipe standard, we must pay great attention to this technical requirement and analyze it carefully.
2) The smelting method of raw material steel of seamless steel pipe is directly related to the purity and uniformity of the material of seamless steel pipe, so the smelting method of steel must put forward the technical requirements of refining outside the furnace.
3) The defects of the tube blank will generally be transferred to the seamless steel pipe, so there should be standards for the quality control of the tube blank, and the strictness of the standard adopted directly affects the quality of the finished seamless steel tube.
4) The chemical composition segregation and non-metallic inclusions in the steel pipe raw material steel and the tube billet are important defects, which will be transferred to the finished seamless steel pipe in the future. Composition segregation and non-metallic inclusions in the form of a band structure also exist in the finished seamless steel pipe actually, so while minimizing the phosphorus and sulfur content in the chemical composition, attention should be paid to the steel pipe’s manufacturing method.

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