The production process of hot rolled seamless steel tube includes several basic processes, such as blank preparation before rolling, tube blank heating, piercing, rolling, sizing and reducing, tube cooling and finishing.

At present, there are three main deformation processes in the production of hot rolled seamless steel pipe: piercing, pipe rolling and sizing; Their respective technical purposes and requirements are as follows:

2. 1. 1 perforation: change the solid tube blank into hollow capillary; We can understand it as setting, that is, setting the cross section of the rolled piece as a ring; Its equipment is called a puncher. The requirements of piercing process are as follows: firstly, the wall thickness of piercing capillary should be uniform, the ellipticity should be small and the geometric dimension accuracy should be high; Secondly, the inner and outer surfaces of the capillary should be smooth, and there should be no defects such as scars, folds and cracks. Third, there should be corresponding piercing speed and rolling cycle to adapt to the production rhythm of the whole unit and make the final rolling temperature of the capillary meet the requirements of the pipe mill.

2. 1.2 tube rolling: thick-walled capillary tube is changed into thin-walled hollow tube (close to the wall thickness of finished product); We can regard it as a fixed wall, that is, the wall thickness of waste pipe in this process is determined according to the subsequent process reduction and empirical formula; This kind of equipment is called a pipe mill. The requirements for tube rolling process are as follows: firstly, when thick-walled capillary tube is changed into thin-walled hollow tube (wall reduction and wall expansion), it is necessary to ensure that the hollow tube has high wall thickness uniformity; Secondly, the waste water pipe has good internal and external surface quality.

2. 1.3 Sizing and reducing (including tension reducing): large circle becomes small circle, which is called sizing for short; The corresponding equipment is sizing (reducing) mill, whose main function is to eliminate the difference of outer diameter of waste pipes (same pipe or same batch) caused by the rolling process in the previous procedure, so as to improve the outer diameter accuracy and roundness of hot rolled finished pipes. The requirements of sizing reduction process are as follows: firstly, under the conditions of a certain total reduction rate and a small single stand reduction rate, the sizing purpose is achieved; Secondly, it can realize the task of producing finished pipes of various specifications with one specification tube blank; Thirdly, the quality of the outer surface of the steel pipe can be further improved.

At the end of 1980s, an attempt was made to cancel the pipe rolling process, and the seamless steel pipe (abbreviated as CPS, abbreviated as cross-rolling piercing tension reducing) was produced only by the method of piercing and constant reducing. The industrial test was carried out in Tosa plant in South Africa, and the steel pipe with an outer diameter of 33.4 ~ 179.8 mm and a wall thickness of 3.4 ~ 25 mm was produced, of which the minimum outer diameter of sizing was 65,438. The maximum outside diameter is 10 1.6 mm. Practice shows that the process quality is acceptable when producing steel pipes with wall thickness greater than 10mm, but when producing steel pipes with wall thickness less than 8mm, the spiral of perforated capillary cannot be completely eliminated by sizing and stretching, which affects the appearance quality of steel pipes. In the subsequent transformation, a small MPM(4 sets) must be added between the piercer and the reducing mill to ensure the product quality.

2.2 Characteristics of production process of each hot rolling unit

We usually call the processing of capillary wall thickness rolling. Tube rolling is the most important process in the process of steel tube forming. The main task of this link is to reduce the thickness of thick-walled capillary to the extent suitable for finished steel pipe according to the requirements of finished steel pipe, that is, the change of wall thickness in the subsequent sizing and reducing process must be taken into account, and this link should also improve the internal and external surface quality and wall thickness uniformity of capillary. The pipe after rolling wall reduction and extension process is generally called waste pipe. The basic feature of tube rolling wall reducing method is that a rigid mandrel is pressed into the capillary, and the capillary wall thickness is compressed and reduced by external tools (rollers or die holes). According to different deformation principles and equipment characteristics, it has a variety of production methods, as shown in table 1. It is customary to name a hot rolling mill according to its form. Pipe mill is divided into single stand and multi-stand. Single stand includes automatic pipe mill, pipe mill, ACCU roller, etc. The inclined pipe mill is a single stand. Even the pipe mill is multi-stand, usually 4~8 stands, such as MPM, PQF and so on. At present, continuous rolling (longitudinal rolling) and transverse rolling are mainly used for pipe rolling.

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