The present invention relates to a stainless steel pipe of the bright annealing finish type (BA type), having a highly-smoothed inner surface, suitable as a clean pipe for use in an apparatus for the production of semiconductors, and as a method for producing the same.
It is well known that the inner surface roughness of a clean pipe is closely related to the production of impurities or fine particles and the discharge of water vapor from the inner surface of the pipe. In an apparatus which is required to have a high degree of cleanliness, clean pipes of the electrolytic polishing finish type whose inner surface roughness becomes lower and are expensive, are used.
In the production of a pipe having a smooth inner surface, there has been a conventional method employed in which a tubing material is subjected to cold plug drawing. Cold plug drawing is a method of processing in which a tubing material 85 is cold drawn with the outer and inner surfaces thereof constrained, as shown in FIG. 10, by a fixing die 86 having a round hole and a plug 81, and the outlet-side end of the tubing material 85 chucked (not illustrated). A chemical conversion treatment lubrication and oil lubrication are the general methods of lubrication between the tools (the die 86 and the plug 81) and the tubing material 85, and oil lubrication capable of forming a thin lubricating film is employed in order to obtain highly-smoothed inner and outer surfaces.
A material for a pipe such as a clean pipe which is required to have a more highly-smoothed inner surface is subjected to a highly-smoothing treatment such as electrochemical polishing after it is cold drawn by the above-described method.
Clean pipes are classified, according to the method of the production thereof, into the bright annealing finish type in which a stainless steel pipe after being cold drawn is subjected to bright annealing treatment; the electric polishing finish type (EP type) in which the inner surface of a stainless steel pipe of a bright annealing finish is further smoothed by means of electrochemical polishing; and like method.
As shown in FIG. 13, plug 101 connected with a supporting rod 107 is so tapered that the diameter of the plug is slightly increased toward the outlet side of drawing. The inside diameter of a tubing material 105 is increased by this plug 101 and a die 106, and, at the same time, the roundness of the inside of the tubing material 105 is improved. However, since the increase of the inside diameter is slightly in this method, the effect of squeezing the inner surface of the tubing material, which will be described later, is small, that is, only a thin layer of shear plastic deformation is formed on the inner surface of the tubing material. It is therefore impossible to obtain a highly-smoothed inner surface which is required for a clean pipe.
As illustrated in the figure, the diameter of the plug 101 is slightly increased toward the outlet side. However, with respect to the wall-thickness-processed part of the tubing material 105, there is no great difference between it and that part of the tubing material obtained by using the conventional plug as shown in FIG. 10(a) and FIG. 10(b). Therefore, although the roundness of the inside of a pipe can be improved by the plug of this shape, the inner surface roughness of a pipe cannot be improved.
For instance, Japanese Patent Publication No. 7244/1987 discloses a method in which a tubing material is processed by using a plug of a specific shape to form a work-hardened layer on the inner surface of a pipe so as to prevent the stainless steel pipe from being oxidized by water vapor.