THICK-WALLED SMALL DIAMETER PIPE PRODUCING METHOD

Abstract

 A method of efficiently producing a thick-walled small diameter pipe having a smooth flawless inner peripheral surface, which can be suitably used for a fuel injection pipe for diesel engines, is provided. In producing a thick-walled small diameter pipe by applying a plurality of times of stretching to a seamless raw pipe, the method comprises the steps of cutting the work pipe into lengths for which the product size is taken into consideration, prior to the final stretching, applying cutting or grinding to the inner peripheral surface of such length, and effecting the final stretching by forging utilizing a pressing force provided by a press.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for manufacturing a thick wall small diameter tube, and more particularly to a method for manufacturing a thick wall small diameter tube capable of being used suitably as a fuel injection tube for a diesel engine, in an efficient manner.

BACKGROUND ART

[0002] For example, a thick wall small diameter tube used as a fuel injection tube for a diesel engine is required to have a high degree of smoothness on the inner circumferential surface of the tube, in order to reduce the flow resistance in the tube and to prevent blocking of the injection nozzle. Furthermore, it is also required that defects, and in particular, fine cracks, in the inner circumferential surface of the tube, are reduced, in such a manner that the tube is able to withstand the repeated fatigue of high pressure.

[0003] Here, the thick wall small diameter tube used in this application is normally based on a seamless stock tube formed by hot rolling, such as Mannesmann mill piercing, rolling in a mandrel mill, and the like. This seamless stock tube is then set to a prescribed diameter by a cold drawing process involving repetition of a tube drawing process using a draw bench, or the like, and a heat treatment process. The tube member adapted to the desired diameter is then cut to a product dimension, thereby completing manufacture of a thick wall small diameter tube for use as a fuel injection tube in a diesel engine.

[0004] However, in the case of the method of manufacture described above, a black skin layer containing cracks, and indent-shaped wrinkles caused by the Mannesmann mill piercing process, the mandrel mill rolling, or the like, involved in the hot rolling process, form on the inner and outer surfaces of the seamless stock tube. Furthermore, fine cracks caused by the repeated tube drawing processes using a draw bench, or the like, followed by heat treatment processing, also form on the inner and outer surfaces of the tube member that has been adjusted to the desired diameter.

[0005] Therefore, in order to obtain a tube member having an inner circumferential surface which is smooth and free of defects, cutting and grinding of the inner surface is carried out at various stages.

[0006] For example, Japanese Patent Publication No. S51-21391 discloses a method wherein the black skin layer on the inner circumferential surface of a seamless stock tube is removed by mechanical cutting, before the seamless stock tube formed by hot rolling is drawn into a thick wall small diameter tube forming a product.

[0007] Moreover, Japanese Patent Laid-open No. H5-154536 discloses a method wherein defects, such as fine cracks, and the like, which are present in the inner circumferential surface of the tube to be processed, are removed by mechanical cutting or grinding before the final tube drawing process, or a tube drawing process proximate thereto.

[0008] Furthermore, Japanese Patent Laid-open No. H9-57329 discloses a method wherein the inner circumferential surface of a seamless stock tube obtained by hot rolling is ground by a shot blasting process, either before the tube drawing process, or at an intermediate stage of a plurality of tube drawing processes.

[0009] Here, the process of cutting, grinding, or the like, of the inner circumferential surface of the tube described above is effective in manufacturing a thick wall small diameter tube having an inner surface that is smooth and free of defects, and in particular, cutting or grinding carried out at a stage as proximate as possible to the final stage of processing is very effective, since there is only slight generation of cracks, and the like, in the subsequent processing.

[0010] However, as the tube drawing process approaches the final stage, so the tube diameter becomes smaller and the tube length becomes longer. Cutting or grinding the inner circumference of the a long and thin tube member, no matter what the method employed for same, is a very difficult task indeed. Moreover, since the task is very difficult to perform, cutting or grinding is not carried out satisfactorily, and in the final product inspection, defects have been discovered in the inner circumferential surface, in a considerable number of cases.

[0011] The present invention was devised with the foregoing problems of the prior art in view, an object thereof being to provide a method whereby a thick wall small diameter tube, having an inner circumferential surface that is smooth and free of defects, which can be used suitably as a fuel injection tube for a diesel engine, can be manufactured in an efficient manner.

DISCLOSURE OF THE INVENTION

[0012] In order to achieve the aforementioned object, the present invention is such that, in manufacturing a thick wall small diameter tube by performing a plurality of tube drawing processes on a seamless stock tube, prior to a final tube drawing process, the tube being processed is cut to a length that accounts for the product dimension, and a cutting or grinding process is performed on the inner circumferential surface of the cut tube.

[0013] According to the method for manufacturing a thick wall small diameter tube relating to the present invention described above, since the cutting or grinding process of the inner circumferential surface is performed at a stage where only the final tube drawing process remains, it is possible to provide a thick wall small diameter tube having an inner circumferential surface that is smooth and free of defects. Moreover, since the cutting or grinding process is carried out on a cut piece which has been cut to a length that accounts for the product dimension, the length of the piece is short and the cutting or grinding process can be performed readily, in a highly accurate manner.

[0014] Furthermore, according to the method for manufacturing a thick wall small diameter tube relating to the present invention described above, since the tube being processed is cut to a length that accounts for the product dimension, prior to the final tube drawing process, then this final tube drawing process can be carried out readily by means of a forging process using a pressing force created by a press, or a forging process using a parts former employed in the formation of various parts, such as bolts, nuts, or the like, for example, this process being carried out in a short period of time in comparison with a tube drawing process using a draw bench, as in the prior art, and furthermore, there being no need for chucking in order to perform drawing, thus allowing the product yield rate to be improved.

[0015] Reference to the "final tube drawing process" in the present invention described above does not indicate a single final tube drawing process, but rather, signifies a final stage of tube drawing processes.

[0016] In other words, the operation of drawing a tube from a seamless stock tube until it assumes a product dimension is divided into a plurality of stages. This is because the amount by which the tube can be extended in one drawing operation, in other words, the rate of processing of the external diameter per operation, Re [=[[external diameter of tube before processing - external diameter of tube after processing] / external diameter of tube before processing] x 100], cannot be set in such a manner that the tube drawing is performed in a single action, as the pulling force exerted on the tube must be set at or below the breaking load of the tube at its passage through the dice. To give a representative example, in the case of a carbon steel tube, drawing is implemented at an external diameter processing rate Re = 20 - 30%.

[0017] Therefore, in the case of a carbon steel tube, reference to "the final tube drawing process" as stated in the present invention signifies a stage where the product dimension is reached, using a tube drawing process where the external processing rate for the subsequent drawing operation is Re = 20 - 30%.

[0018] Consequently, cases where this final tube drawing process is performed over a plurality of tube drawing process steps, by means of a forging process using a parts former, for example, are also included in the technical scope of the present invention described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] 

Fig. 1 is a diagram showing a conceptual view of a processes of a method for manufacturing thick wall small diameter tube relating to the present invention. Fig. 2 is a diagram showing a conceptual view of one example of a final tube drawing process.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] Below, an embodiment of a method for manufacturing thick wall small diameter tube according to the present invention is described in detail, but the present invention is not limited to the embodiment described below.

[0021] Firstly, seamless carbon steel stock tube (JIS G 3455) having an external diameter of 34 mm and a wall thickness of 4.5 mm, formed by hot rolling involving Mannesmann mill piercing, rolling in a mandrel mill, or the like, is taken and is subjected to cold drawing consisting of repetitions of tube drawing work using a draw bench and a heat treatment process, as implemented in the prior art, thereby obtaining a tube member having an external diameter of 11 mm and a wall thickness of 2.0 mm (Fig. 1(a)).

[0022] Desirably, the steps of cutting or grinding the inner circumferential surface of the tube as performed in the prior art are carried out during this process, since they are easy to carry out when the diameter of the tube is still relatively large, but these are not essential steps in the present invention.

[0023] Subsequently, the aforementioned tube member of 11 mm external diameter and 2.0 mm wall thickness is cut to a length of 150 mm by a cutting machine (see Fig. 1(b)).

[0024] Here, the length to which the tube member is cut is such that the tube will assume a product length after subsequent tube drawing processing.

[0025] Thereupon, cutting or grinding is performed on the inner surface of the cut tube (Fig. 1(c)).

[0026] For the means for cutting or grinding the inner surface of the cut tube, in addition to cutting or grinding means such as a lathe, broaching machine, honing machine, or the like, which are commonly known in the prior art, it is also possible to use means such as shot blasting, fluid grinding, or the like, or to incorporate a suitable combination of the aforementioned means.

[0027] Whatever the means adopted, the object of the cutting or grinding according to the present invention is a short piece having a length of approximately 150 mm, as described above, and therefore these steps can be performed readily and in a highly accurate manner.

[0028] It is necessary that after cutting or grinding, the inner circumferential surface is such that the black skin layer formed during hot rolling has been removed completely from same, and furthermore, that defects (having a size of 50 im or above) such as indent-shaped wrinkles arising during the tube drawing work, or fine cracks, or the like, formed on the basis of these indent-shaped wrinkles, have been removed completely.

[0029] Thereupon, the cut piece which has undergone cutting or grinding on the inner surface thereof as described above is then subjected to a final tube drawing process in such a manner that the product dimensions of 8 mm external diameter, 2.0 mm wall thickness and 230 mm length are obtained (see Fig. 1(d)).

[0030] Since this final tube drawing process is performed on a short tube of approximately 150 mm length, it is possible to carry out drawing and forming using a parts former, which is used in the formation of various types of parts, such as bolts, nuts, and the like.

[0031] For example, as shown in Fig. 2, the tube to be processed A is inserted into a dice 1 having a pierce hole of restricted diameter, by means of press equipment using water pressure, hydraulic oil pressure or a clamp mechanism, and the tube to be processed A is drawn by means of a forging process.

[0032] Element 3 in Fig. 2 is a plug which is inserted inside the tube to be processed A.

[0033] This final tube drawing process is carried out in a short period of time in comparison with conventional tube drawing processes using a draw bench, and moreover, it requires no chucking for the purpose of drawing, and hence the product yield rate can be improved.

[0034] There is a possibility that the thick wall small diameter tube manufactured in this manner may contain slight cracks (of maximum size 10 im) generated by the final tube drawing process, but it is a thick wall small diameter tube having an inner circumferential surface which is sufficiently smooth and free of defects, and it can be used appropriately as a fuel injection tube for a diesel engine.

[0035] The foregoing description is related to an embodiment of the present invention, but the present invention is not limited in any way to the embodiment described above.

[0036] For example, the method for manufacturing a thick wall small diameter tube according to the present invention is also applicable to thick wall small diameter tubes used in applications other than fuel injection tubes for a diesel engine, and may be employed appropriately for the manufacture of any thick wall small diameter tube having strict requirements with regard to the properties of the inner circumferential surface thereof.

[0037] Furthermore, it goes without saying that the method of the present invention may also be applied to the manufacture of thick wall small diameter tube of any type of steel, such as carbon steel, alloy steel, stainless steel, and the like.

[0038] Moreover, as stated previously, the final tube drawing process may be implemented by dividing it over a plurality of tube drawing steps.

[0039] According to the method for manufacturing a thick-wall small diameter tube relating to the present invention as described above, since a cutting or grinding process on the inner circumferential surface is carried out at a stage where only the final tube drawing process remains, it is possible to provide a thick wall small diameter tube having an inner circumference that is smooth and free of defects, in addition to which, since the cutting or grinding process is carried out on a cut piece that has been cut to a length which accounts for the product dimension, then a merit is obtained in that, since the length of the piece is short, the cutting or grinding process can readily be carried out with good accuracy.

[0040] Moreover, according to the method for manufacturing a thick wall small diameter tube relating to the present invention, since the tube to be processed is cut to a length which accounts for the product dimension, before the final tube drawing process, it is possible to carry out the final tube drawing process readily by means of a forging process using a pressing force created by a press, or by means of a forging process using a parts former employed to form various parts, such as bolts, nuts, or the like, for example, and this process can be carried out in a short period of time in comparison with a tube drawing process using a draw bench, as in the prior art, in addition to which, chucking for the purpose of drawing is not required and hence a merit is obtained in that the product yield rate can also be improved.

INDUSTRIAL APPLICABILITY

[0041] As described above, the method for manufacturing a thick wall small diameter tube relating to the present invention is applicable to cases where thick wall small diameter tubes having strict requirements with respect to the properties of their inner surface, such as fuel injection tubes for a diesel engine, are manufactured in an efficient manner.

Claims

1. A method for manufacturing a thick wall small diameter tube, wherein, in manufacturing a thick wall small diameter tube by performing a plurality of tube drawing processes on a seamless stock tube, prior to a final tube drawing process, the tube being processed is cut to a length that accounts for the product dimension, and a cutting or grinding process is performed on the inner circumferential surface of the cut tube.

2. The method for manufacturing a thick wall small diameter tube according to claim 1, wherein said final tube drawing process is performed on said cut tube that has undergone a cutting or grinding process on the inner circumferential surface thereof, by means of a forging process using a pressing force generated by a press.

Drawing