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　　According to the characteristics of seamless steel pipe production process, seamless steel pipe products are organized according to the outer diameter and wall thickness series. At present, the seamless steel pipe size series in all countries in the world is not yet uniform, and all countries have their own seamless steel pipe size series standards. Seamless steel pipe standards that are widely used in the world include ASME B 36.0 in the United States, DIN 2448 in Germany, BS 3600 in due course, and ISO 4200 in the International Organization for Standardization.

　　Although there is a JIS standard in Japan, in order to enter the international market, seamless steel pipes are also produced according to the above-mentioned standards of the United States, the United Kingdom, and Germany.

　　In the series of ASTM A106 Seamless Steel Pipe for Oil and Gas Line outer diameter sizes in various countries in the world, China, Japan, Germany, and the International Organization for Standardization use mm to represent the outer diameter size, while the United States has two methods of representation, using mm and in respectively.

　　Although the external diameter dimensions of seamless steel pipes in foreign countries are not exactly the same, when DN4in, except for a few large differences in external diameters, the difference in the external diameter sizes of seamless steel pipes with other nominal diameters is small and does not affect interchangeability. Starting from DN4in, the diameter of the seamless steel pipe is equal to the nominal diameter. For example, DN14in has an external diameter of 14 in or 355.6, mm(14 X25.4 mm).

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