EP3862466A1

EP3862466A1 - Method for manufacturing metallic pipe, and method for washing metallic pipe

Info

Publication number: EP3862466A1
Application number: EP19868663.6A
Authority: EP
Inventors: Takamitsu Takagi; Hiromitsu Date; Eri HOSHIBA
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2018-10-04
Filing date: 2019-09-26
Publication date: 2021-08-11
Also published as: JPWO2020071224A1; CN112805409A; KR20210053949A; EP3862466A4; JP7157168B2; WO2020071224A1

Abstract

An object of the present disclosure is to achieve both energy conservation (low cost) and good cleaning performance in cleaning a metal tube with a chemical conversion coating film and a lubricating film on its surface. A manufacturing method of a metal tube (P) includes a preparing step (S1) of preparing a metallic material tube; a lubricating step (S2) of forming a chemical conversion coating film on a surface of the material tube and forming a lubricating film on the chemical conversion coating film; a cold drawing step (S3) of applying cold drawing to the material tube with the chemical conversion coating film and the lubricating film thereon to form the material tube into a metal tube (P) with specified dimensions; and a cleaning step (S4) of cleaning the metal tube (P) to remove the chemical conversion coating film and the lubricating film. The cleaning step (S4) includes: a step (S41) of soaking the metal tube (P) in a non-highly-heated alkaline degreasing fluid in an alkaline cleaning tank (11); and a step (S42) of soaking the metal tube (P) after soaked in the alkaline degreasing fluid in a washing liquid in an ultrasonic washing tank (21) while radiating ultrasonic waves into the washing liquid and generating fine bubbles in the washing liquid.

Description

TECHNICAL FIELD

The present disclosure relates to a manufacturing method and a cleaning method of a metal tube. The present disclosure particularly relates to a cleaning method of a metal tube formed by applying cold drawing to a material tube with a chemical conversion coating film or a lubricating film on its surface.

BACKGROUND ART

When cold drawing is carried out during a manufacturing process of a metal tube, a large frictional force occurs between the metallic material tube and the drawing tool. In order to decrease the frictional force, a lubricating treatment is applied to the material tube prior to the cold drawing.
In the lubricating treatment prior to the cold drawing, for example, a chemical conversion coating film such as a phosphate film, an oxalate film, a chromate film, or the like is formed on the surface of the material tube. On the chemical conversion coating film, a lubricating film is formed of a high fatty acid salt (soap) or the like. The chemical conversion coating film reduces frictional resistance by preventing a direct contact between the material tube and the drawing tool, and improves lubricity by enhancing adhesion between the material tube and the lubricating film.
After the material tube is formed by cold drawing into a metal tube with predetermined dimensions, the metal tube is cleaned for removal of the chemical conversion coating film and the lubricating film. These films are strongly coupled with the material of the metal tube. Therefore, it is difficult to remove these films from the metal tube.
As a way of cleaning metallic materials, etc., a cleaning method using ultrasonic waves is known. For example, Patent Literature 1 discloses an acid pickling method wherein a plurality of metal tubes are rubbed with one another in an acid liquid while the tube end portions are irradiated with ultrasonic waves. According to Patent Literature 1, in the portions of the metal tubes that are rubbed with one another, scale is removed by an abrasion effect brought by the rubbing and by a dissolution effect brought by the acid liquid. In the tube end portions that are not rubbed with one another, the dissolution effect brought by the acid liquid is strengthened by the ultrasonic waves, and thereby, scale is removed therefrom.
Patent Literature 2 discloses a method for removing scale from a hot-rolled steel plate by using ultrasonic waves and microbubbles. According to Patent Literature 2, microbubbles are supplied to an ultrasonic wave exposed portion of the hot-rolled steel plate, and cavitation is certainly caused. This certainly provides a scale removal effect.
Patent Literature 3 discloses a washing apparatus that uses ultrasonic waves and microbubbles. In the washing apparatus, ultrasonic waves are radiated into a washing liquid in a washing tank from an ultrasonic generator. A circulation route is connected to the washing tank. The washing liquid in the washing tank is introduced into a deaerator through the circulation route. The deaerator separates dissolved air from the washing liquid and generates microbubbles. The microbubbles are supplied to the washing tank through the circulation route, and thus, the concentration of air dissolved in the washing liquid is decreased. According to Patent Literature 3, maintaining the concentration of air dissolved in the washing liquid at a level equal to or lower than a predetermined value makes it possible to carry out efficient and good washing without causing a decrease in the sound pressure of the ultrasonic waves.

CITATION LIST

PATENT LITERATURES

Patent Literature 1: Japanese Patent Application Publication No. H03-177590
Patent Literature 2: Japanese Patent Application Publication No. 2000-256886
Patent Literature 3: Japanese Patent Application Publication No. 2007-29944

SUMMARY OF INVENTION

TECHNICAL PROBLEM

As described above, after a cold drawing process to form a metal tube, it is necessary to clean the metal tube for removal of a chemical conversion coating film and a lubricating film. The adhesion between the metal tube and the chemical conversion coating film and the lubricating film is extremely high. Therefore, by simply adopting the method and apparatus disclosed by Patent Literatures to clean the metal tube formed by cold drawing, it is difficult to completely remove these films from the metal tube.
In a conventional manner, a degreasing treatment and an acid pickling treatment are applied to a metal tube formed by cold drawing. Accordingly, first, the lubricating film is removed by the degreasing treatment, and thereafter, the chemical conversion coating film is removed by the acid pickling treatment. More specifically, the degreasing treatment is applied to the metal tube, for example, by using an alkaline degreasing fluid at a high temperature of 70 °C or higher, and thereafter, the acid pickling treatment is applied. In the degreasing treatment, a complex is produced from metallic soap components in the lubricating film, and other soap components are dissolved. In order to attain sufficient complex production and sufficient dissolution, it is necessary to use a high-temperature alkaline fluid. However, a large amount of energy is needed to heat the alkaline degreasing fluid to a high temperature, which causes a problem of an increase in the usage (consumption) of energy and accordingly a problem of an increase in cost. Meanwhile, unless the alkaline degreasing fluid is heated to a high temperature (70 °C or higher), the production of a complex and the dissolution of soap components are not facilitated, and the lubricating film may remain on the metal tube.
An object of the present disclosure is to achieve both energy conservation (low cost) and good cleaning performance in cleaning a metal tube with a chemical conversion coating film and a lubricating film on its surface.

SOLUTION TO PROBLEM

A metal tube manufacturing method according to the present disclosure includes: a preparing step of preparing a metallic material tube; a lubricating step of forming a chemical conversion coating film on the surface of the material tube and forming a lubricating film on the chemical conversion coating film; a cold drawing step of applying cold drawing to the material tube with the chemical conversion coating film and the lubricating film thereon to form the material tube into a metal tube with specified dimensions; and a cleaning step of cleaning the metal tube to remove the chemical conversion coating film and the lubricating film. The cleaning step includes: a step of soaking the metal tube in a non-highly-heated alkaline degreasing fluid in an alkaline cleaning tank; and a step of soaking the metal tube after soaked in the alkaline degreasing fluid in a washing liquid in an ultrasonic washing tank while radiating ultrasonic waves into the washing liquid and generating fine bubbles in the washing liquid.

ADVANTAGEOUS EFFECTS OF INVENTION

The present disclosure makes it possible to achieve both energy conservation (low cost) and good cleaning performance in cleaning a metal tube with a chemical conversion coating film and a lubricating film on its surface.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a flowchart showing a metal tube manufacturing method according to an embodiment.
[FIG. 2] FIG. 2 is an alkaline cleaning apparatus to be used at an alkaline cleaning step in the manufacturing method according to the embodiment.
[FIG. 3] FIG. 3 is a sectional view of the alkaline cleaning apparatus shown in FIG. 2 along line III-III.
[FIG. 4] FIG. 4 is a plan view of an ultrasonic washing apparatus to be used at an ultrasonic washing step in the manufacturing method according to the embodiment.
[FIG. 5] FIG. 5 is a sectional view of the ultrasonic washing apparatus shown in FIG. 4 along line V-V
[FIG. 6] FIG. 6 is an illustration diagram of an exemplary discharge mechanism that is employable in the ultrasonic washing apparatus shown in FIG. 4.
[FIG. 7] FIG. 7 is an illustration diagram of another exemplary discharge mechanism that is employable in the ultrasonic washing apparatus shown in FIG. 4.
[FIG. 8A] FIG. 8A is a schematic view of the surface of a metal tube after subjected to cold drawing.
[FIG. 8B] FIG. 8B is a schematic view of the surface of the metal tube after the alkaline cleaning step.
[FIG. 8C] FIG. 8C is a schematic view of the surface of the metal tube during the ultrasonic washing step.
[FIG. 9] FIG. 9 is a graph showing changes in the amount of carbon adhering to the inside surfaces of metal tubes in Example, Comparative Example 1, and Comparative Example 2.
[FIG. 10] FIG. 10 includes scanning electron microscopical images of the inside surfaces of the metal tubes subjected to cleaning of Example and cleaning of Comparative Example 1, respectively.

DESCRIPTION OF EMBODIMENTS

A metal tube manufacturing method according to an embodiment includes: a preparing step of preparing a metallic material tube; a lubricating step of forming a chemical conversion coating film on the surface of the material tube and forming a lubricating film on the chemical conversion coating film; a cold drawing step of applying cold drawing to the material tube with the chemical conversion coating film and the lubricating film thereon to form the material tube into a metal tube with specified dimensions; and a cleaning step of cleaning the metal tube to remove the chemical conversion coating film and the lubricating film. The cleaning step includes: a step of soaking the metal tube in a non-highly-heated alkaline degreasing fluid in an alkaline cleaning tank; and a step of soaking the metal tube after soaked in the alkaline degreasing fluid in a washing liquid in an ultrasonic washing tank while radiating ultrasonic waves into the washing liquid and generating fine bubbles in the washing liquid.
In the metal tube manufacturing method according to the embodiment, after a lubricating film and a chemical conversion coating film, which is an underlayer of the lubricating film, are formed on the material tube, the material tube is cold-drawn and thereby formed into a metal tube with specified dimensions. The metal tube obtained by cold drawing is cleaned (subjected to alkaline degreasing) in the following way. The metal tube is first soaked in a non-highly-heated alkaline degreasing fluid. In the present disclosure, a non-highly-heated alkaline degreasing fluid typically means an alkaline degreasing fluid that is not positively heated by a heater or the like (for example, a steam heater or an electric heater)and is at natural temperature according to the atmospheric temperature around the alkaline degreasing facilities (air temperature or temperature in the factory building). In this case, there is no need to use (consume) energy to heat the alkaline degreasing fluid. However, when the temperature of the alkaline degreasing fluid is below 20 °C, the alkaline degreasing fluid is heated to about 20 to 40 °C. For example, in winter or a cold season, the temperature of the alkaline degreasing fluid may drop below 20 °C. In such a case, the alkaline degreasing fluid is positively heated so that the temperature of the fluid becomes about 20 to 40 °C. In this case, energy is used to heat the alkaline degreasing fluid; however, it is not necessary to heat the alkaline degreasing fluid to so high a temperature (70 °C or higher) as conventionally needed, and the usage of energy can be reduced.
By soaking the metal tube in a non-highly-heated alkaline degreasing fluid, production of a complex and dissolution are caused on some part of the lubricating film, and the lubricating film is partly removed from the surface of the metal tube. Next, the metal tube is soaked in a washing liquid provided with ultrasonic waves and fine bubbles. The lubricating film on the surface of the metal tube was partly removed by the alkaline degreasing treatment, and therefore, it is possible to peel off the remaining lubricating film from the metal tube by a physical action such as ultrasonic cavitation or the like.
The temperature of the alkaline degreasing fluid may be affected by reaction heat or circulation of the alkaline degreasing fluid in the cleaning apparatus during the cleaning process, but the warming of the alkaline degreasing fluid due to these effects is not considered as a positive heating treatment.
The ductility of the chemical conversion coating film is extremely low. Therefore, when the metal tube is cold-drawn, the chemical conversion coating film on the metal tube possibly cracks. Accordingly, by soaking the metal tube in the washing liquid supplied with ultrasonic waves and fine bubbles and by applying a physical action such as ultrasonic cavitation or the like to the metal tube, it is possible to peel off the chemical conversion coating film from the metal tube.
Thus, in the manufacturing method according to the embodiment, unlike in a conventional method, it is not necessary to heat an alkaline degreasing fluid to a high temperature at 70 °C or higher. Accordingly, both energy conservation (low cost) and good cleaning performance can be achieved.
In the manufacturing method, while the metal tube is soaked in the washing liquid in the ultrasonic washing tank, the concentration of oxygen dissolved in the washing liquid is preferably 5.2 mg/L or lower.
By maintaining the concentration of oxygen dissolved in the washing liquid, wherein ultrasonic waves are radiated, not more than 5.2 mg/L, it is possible to achieve good ultrasonic washing performance.
The metal tube cleaning method according to the embodiment is a method for cleaning a metal tube obtained by applying cold drawing to a material tube with a chemical conversion coating film and a lubricating film formed thereon. The cleaning method includes: a step of soaking the metal tube in a non-highly-heated alkaline degreasing fluid in an alkaline cleaning tank; and a step of soaking the metal tube after soaked in the alkaline degreasing fluid in a washing liquid in an ultrasonic washing tank while radiating ultrasonic waves into the washing liquid and generating fine bubbles in the washing liquid.
In the cleaning method, while the metal tube is soaked in the washing liquid in the ultrasonic washing tank, the concentration of oxygen dissolved in the washing liquid is preferably 5.2 mg/L or lower.
Embodiments of the present disclosure will hereinafter be described with reference to the drawings. In the drawings, the same or equivalent members are provided with the same reference signs, and repetitions of the same description will be avoided.
FIG. 1 is a flowchart showing a metal tube manufacturing method according to an embodiment. As shown in FIG. 1, the metal tube manufacturing method includes a preparing step S1, a lubricating step S2, a cold drawing step S3, and a cleaning step S4. The lubricating step S2 includes a chemical conversion coating film forming step S21 and a lubricating film forming step S22. The cleaning step S4 is a step of applying alkaline degreasing to the metal tube with a chemical conversion coating film and a lubricating film formed thereon, and the cleaning step S4 includes an alkaline cleaning step S41 and an ultrasonic washing step S42. First, an alkaline cleaning apparatus to be used at the alkaline cleaning step S41 and an ultrasonic washing apparatus to be used at the ultrasonic washing step S42 are described.

[Alkaline Cleaning Apparatus]

FIG. 2 is a schematic side view of an alkaline cleaning apparatus 10 to be used at the alkaline cleaning step S41. FIG. 3 is a sectional view of the alkaline cleaning apparatus 10 shown in FIG. 2 along line III-III.
As shown in FIGS. 2 and 3, the alkaline cleaning apparatus 10 includes an alkaline cleaning tank 11, a storage tank 12, and circulation piping 13.

(Alkaline Cleaning Tank)

The alkaline cleaning tank 11 includes a tank body 111 and a lid 112. The tank body 111 has an open upper side. The lid 112 is configured to cover the open side of the tank body 111.
The tank body 111 is configured to accommodate metal tubes P. In the tank body 111, usually, a plurality of metal tubes P are accommodated at the same time to be subjected to alkaline cleaning. An alkaline degreasing fluid is supplied to the tank body 111. The alkaline degreasing fluid is a known alkaline solution for use in common alkaline degreasing, such as sodium hydroxide (NaOH) aqueous solution, sodium silicate (Na₂SiO₃) aqueous solution, sodium carbonate (Na₂CO₃) aqueous solution, or the like. An interfacial active agent, a chelate agent and some other additives may be added to the alkaline degreasing fluid, as appropriate.
The tank body 111 is, for example, rectangular in a planar view. The tank body 111 includes a bottom 111a, and a peripheral wall 111b extending upward from the periphery of the bottom 111a. The bottom 111a has an inclined surface downwardly inclined from one end 111c to the other end 111d along the length direction of the tank body 111. The depth of the tank body 111 becomes greater with increasing distance from the end 111c and with decreasing distance from the end 111d.
A plurality of supports 113 may be provided in the tank body 111. The plurality of supports 113 are arranged in the length direction of the tank body 111 at intervals. The supports 113 are to support the metal tubes P such that the metal tubes P will not come into direct contact with the bottom 111a. For example, each of the supports 113 substantially has a U-shape.

(Storage Tank)

The storage tank 12 is located under the alkaline cleaning tank 11. The storage tank 12 is, for example, shaped like a hollow rectangular parallelopiped. In the storage tank 12, the alkaline degreasing fluid is stored. Near the end 111c of the tank body 111, the storage tank 12 communicates with the tank body 111 through the circulation piping 13. Near the end 111d of the tank body 111, the storage tank 12 communicates with the tank body 111 through a communication hole 14. The communication hole 14 is openable and closable.

(Circulation Piping)

The circulation piping 13 connects the tank body 111 and the storage tank 12 near the end 111c of the tank body 111 of the alkaline cleaning tank 11. The circulation piping 13 is configured to supply the alkaline degreasing fluid in the storage tank 12 to the tank body 111. For example, the circulation piping 13 includes a pump 131 (see FIG. 3) to send the alkaline degreasing fluid from the storage tank 12 into the tank body 111.

[Ultrasonic Washing Apparatus]

FIG. 4 is a plan view of an ultrasonic washing apparatus 20 to be used at the ultrasonic washing step S42. FIG. 5 is a sectional view of the ultrasonic washing apparatus 20 shown in FIG. 4 along line V-V.
As shown in FIG. 4, the ultrasonic washing apparatus 20 includes an ultrasonic washing tank 21, a supply mechanism 22, a plurality of discharge mechanisms 23, a plurality of ultrasonic radiation mechanisms 24, and a plurality of fine bubble generation mechanisms 25. The ultrasonic washing apparatus 20 further includes a plurality of cushions 26.

(Ultrasonic Washing Tank)

The ultrasonic washing tank 21 is configured to accommodate the metal tubes P. In the ultrasonic washing tank 21, usually, a plurality of metal tubes P are accommodated at the same time to be subjected to ultrasonic washing. A washing liquid for washing of the metal tubes P is stored in the ultrasonic washing tank 21. The kind of the washing liquid is not particularly limited, and it is possible to arbitrarily select and adopt one of well-known washing liquids. The washing liquid is, for example, water (tap water or industrial water).
The ultrasonic washing tank 21 is rectangular in a plan view. The ultrasonic washing tank 21 has an open upper side. The bottom surface of the ultrasonic washing tank 21 is an inclined surface downwardly inclined from one end to the other end along the length direction. The depth of the ultrasonic washing tank 21 becomes greater with increasing distance from the one end and with decreasing distance from the other end in the length direction.
The material of the ultrasonic washing tank 21 is not particularly limited. For example, metallic materials such as stainless steel, etc., plastic resin such as fiber-reinforced plastic (FRP), polypropylene (PP), etc., acid-proof brick, and the like are usable as the material of the ultrasonic washing tank 21. The above-described alkaline cleaning tank 11 and the storage tank 12 (see FIG. 2) can be formed of the same material as the material of the ultrasonic washing tank 21.

(Supply Mechanism)

The supply mechanism 22 supplies the washing liquid to the ultrasonic washing tank 21. The supply mechanism 22 includes at least one supply pipe 221. In the present embodiment, the supply mechanism 22 includes a plurality of supply pipes 221. The washing liquid is supplied to the ultrasonic washing tank 21 through the supply pipes 221. The plurality of supply pipes 221 are arranged at intervals. Therefore, the washing liquid is decentrally supplied to the ultrasonic washing tank 21. When there are three or more supply pipes 221, it is preferred that the intervals between the supply pipes 221 are substantially equal for uniform supply of the fresh washing liquid.
In the present embodiment, the plurality of supply pipes 221 are arranged along one of two lengthwise extending side walls of the ultrasonic washing tank 21. The number and arrangement of the supply pipes 221 are not particularly limited. Along each of the lengthwise extending side walls of the ultrasonic washing tank 21, one or more supply pipes 221 may be arranged. In addition to or instead of the supply pipes 221 arranged along one or both of the lengthwise extending side walls of the ultrasonic washing tank 21, one or more supply pipes 221 may be arranged along at least one of two widthwise extending side walls of the ultrasonic washing tank 21.

(Discharge Mechanism)

Each of the discharge mechanisms 23 discharges the washing liquid from the ultrasonic washing tank 21 when the amount of washing liquid in the ultrasonic washing tank 21 exceeds a predetermined amount. The plurality of discharge mechanisms 23 are arranged at intervals. Accordingly, the washing liquid is decentrally discharged from the ultrasonic washing tank 21. When there are three or more discharge mechanisms 23, it is preferred that the intervals between the discharge mechanisms 23 are substantially equal. However, only one discharge mechanism 30 may be provided.
In the present embodiment, the plurality of discharge mechanisms 23 are arranged along the lengthwise extending side wall of the ultrasonic washing tank 21 on the opposite side of the side wall where the supply pipes 221 are arranged. However, the number and arrangement of the discharge mechanisms 23 are not particularly limited. The discharge mechanisms 23 may be arranged along the lengthwise extending side wall of the ultrasonic washing tank 21 where the supply pipes 221 are arranged. In addition to or instead of the discharge mechanisms 23 arranged along one or both the lengthwise extending side wall of the ultrasonic washing tank 21, one or more discharge mechanisms may be arranged along at least one of two widthwise extending side walls of the ultrasonic washing tank 21.
FIG. 6 shows an exemplary discharge mechanism 23A that is employable in the ultrasonic washing apparatus 20. The discharge mechanism 23A includes a discharge outlet 231 and a discharge pipe 232.
The discharge outlet 231 is a hole made in a side wall of the ultrasonic washing tank 21. The discharge pipe 232 is located outside the ultrasonic washing tank 21 and connected to the discharge outlet 231. The ultrasonic washing liquid is discharged from the ultrasonic washing tank 21 through the discharge outlet 231 and the discharge pipe 232.
In the ultrasonic washing apparatus 20, a reference liquid surface level S is predetermined for the washing liquid in the ultrasonic washing tank 21. For washing of the metal tubes P, the washing liquid is supplied to the ultrasonic washing tank 21 until the surface of the washing liquid reaches the reference liquid surface level S. With respect to the depth direction of the ultrasonic washing tank 21, the position of the lower edge of the discharge outlet 231 is substantially same as the position of the reference liquid surface level S.
As indicated by alternate long and two short dashes line in FIG. 6, when the liquid surface of the washing liquid in the ultrasonic washing tank 21 becomes higher than the reference liquid surface level S, the washing liquid overflows through the discharge outlet 231 by an amount corresponding to the excess over the reference liquid surface level S. For example, in a state where the surface of the washing liquid in the ultrasonic washing tank 21 is on the same level with the reference liquid surface level S, when the supply mechanism 22 newly supplies the washing liquid to the ultrasonic washing tank 21, the washing liquid overflows through the discharge outlet 231 by an amount substantially same as the amount of supply.
In this way, the discharge mechanism 23A discharges the washing liquid from the ultrasonic washing tank 21 when the amount of washing liquid in the ultrasonic washing tank 21 exceeds the amount defined by the reference liquid surface level S
(the predetermined amount).
FIG. 7 shows another exemplary discharge mechanism 23B that is employable in the ultrasonic washing apparatus 20. The discharge mechanism 23B includes a discharge outlet 233, a discharge pipe 234, a discharge pump 235, and a liquid surface detector (not shown). As the liquid surface detector, a commercially available liquid level sensor or the like may be used.
The discharge outlet 233 is a hole made in a side wall of the ultrasonic washing tank 21. The discharge outlet 233 is located on the side wall of the ultrasonic washing tank 21, at a lower level than the reference liquid surface level S. The discharge pipe 234 is located outside the ultrasonic washing tank 21 and connected to the discharge outlet 233. The washing liquid is discharged from the ultrasonic washing tank 21 through the discharge outlet 233 and the discharge pipe 234.
The discharge pump 235 is located in the middle of the discharge pipe 234. When the surface of the washing liquid in the ultrasonic washing tank 21 exceeds the reference liquid surface level S, the discharge pump 235 is controlled to suck the washing liquid from the ultrasonic washing tank 21 by an amount corresponding to the excess over the reference liquid surface level S. For example, the discharge pump 235 is controlled as follows based on signals sent from the liquid surface detector located in the ultrasonic washing tank 21. When the surface of the washing liquid exceeds the reference liquid surface level S, the discharge pump 235 is driven, and when the surface of the washing liquid becomes lower than the reference liquid surface level S, the drive of the discharge pump 235 is stopped.
Thus, as with the discharge mechanism 23A (FIG. 6), the discharge mechanism 23B discharges the washing liquid from the ultrasonic washing tank 21 when the amount of washing liquid in the ultrasonic washing tank 21 exceeds a predetermined amount.

(Ultrasonic Radiation Mechanism)

With reference to FIG. 4 again, the ultrasonic radiation mechanisms 24 radiate ultrasonic waves into the washing liquid in the ultrasonic washing tank 21. As the ultrasonic radiation mechanisms 24, well-known ultrasonic vibrators commonly used in ultrasonic washing may be used.
The frequency of ultrasonic waves radiated from the ultrasonic radiation mechanisms 24 is preferably 20 kHz to 200 kHz. When the frequency of ultrasonic waves is 20 kHz or more, there is no fear that ultrasonic wave propagation in the washing liquid is blocked by large bubbles emitted from the surfaces of the metal tubes P, and washing performance degradation can be prevented. When the frequency of ultrasonic waves is 200 kHz or less, a decrease in the uniformity of washing due to the strong straightness of the travel of ultrasonic waves can be prevented. The frequency of ultrasonic waves is more desirably 20 kHz to 150 kHz, and still more desirably 25 kHz to 100 kHz.
The ultrasonic radiation mechanisms 24 preferably have a frequency sweeping function. With the frequency sweeping function, the ultrasonic radiation mechanisms 24 are capable of radiating ultrasonic waves into the washing liquid while sweeping the frequency within the range of ±0.1 kHz to ±10 kHz from a predetermined frequency. As the frequency of ultrasonic waves is being changed by the frequency sweeping function, the frequency-resonance diameter, which will be described later, changes, and it becomes possible to increase the number of microbubbles that contribute to cavitation washing.
When the wavelength of ultrasonic waves is a quarter of the thickness of the object to be exposed to the ultrasonic waves, the ultrasonic waves pass through the object. By changing the frequency of ultrasonic waves by using the frequency sweeping function, it becomes possible, in various portions of the metal tubes P, to satisfy the condition that the wavelength of ultrasonic waves is a quarter of the wall thicknesses of the metal tubes P. Accordingly, in various portions of the metal tubes P, ultrasonic waves can penetrate the metal tubes P from outside to inside.
In the present embodiment, at least one ultrasonic radiation mechanism 24 is set on the inside surface of each side wall of the ultrasonic washing tank 21. However, the number and arrangement of the ultrasonic radiation mechanisms 24 are not particularly limited. One or more ultrasonic radiation mechanisms 24 may be set on the bottom surface of the ultrasonic washing tank 21. When a plurality of ultrasonic radiation mechanisms 24 are set in the ultrasonic washing tank 21, it is preferred that the ultrasonic radiation mechanisms 24 are arranged in such a manner as to achieve uniform ultrasonic wave propagation in the entire ultrasonic washing tank 21. In this case, the ultrasonic radiation mechanisms 24 have the same load in oscillation, and interference between the ultrasonic waves radiated from the ultrasonic radiation mechanisms 24 can be prevented.

(Fine Bubble Generation Mechanism)

The fine bubble generation mechanisms 25 generate fine bubbles from gasses dissolved in the washing liquid in the ultrasonic washing tank 21. The fine bubble generation mechanisms 25 are located outside the ultrasonic washing tank 21. The plurality of fine bubble generation mechanisms 25 are arranged along a lengthwise extending side wall of the ultrasonic washing tank 21. However, the number and arrangement of the fine bubble generation mechanisms 25 are not particularly limited.
Each of the fine bubble generation mechanisms 25 includes pipes 251 and 252, and a fine bubble generator 253. The pipes 251 and 252 connect the ultrasonic washing tank 21 and the fine bubble generator 253. The washing liquid is introduced from the ultrasonic washing tank 21 to the fine bubble generator 253 through the pipe 251. The fine bubble generator 253 generates fine bubbles by using gasses dissolved in the washing liquid. The fine bubbles are returned to the ultrasonic washing tank 21 together with the washing liquid through the pipe 252.
The fine bubble generator 253 can be arbitrarily selected from known fine bubble generators. The known fine bubble generators are, for example, fine bubble generators that generate fine bubbles by shear of bubbles, passage of bubbles through fine holes, reduction in liquid pressure (change in pressure), dissolution of gasses under pressure, ultrasonic waves, electrolyzation, chemical reaction, etc. The fine bubble generator 253 is preferably such a thing as the bubble diameter and the density of fine bubbles can be easily controlled. For example, the fine bubble generator 253 may be a known fine bubble generator that generates fine bubbles by causing a pressure change of a liquid in the middle of circulation of the liquid.
Fine bubbles mean minuscule bubbles with an average bubble diameter of 100 µm or less. In some cases, fine bubbles with an average bubble diameter on the order of micrometers are referred to as microbubbles, and fine bubbles with an average bubble diameter on the order of nanometers are referred to as nanobubbles. The average bubble diameter is a diameter at which the number of samples is the largest in a number distribution of fine bubble diameter.
The average bubble diameter of the fine bubbles in the washing liquid is preferably equal to or greater than 0.01 µm in order to eliminate the necessity of enlarging the fine bubble generation mechanisms 25 and to facilitate control of the bubble diameter. Also, the average bubble diameter of fine bubbles is preferably equal to or less than 100 µm in order to prevent the fine bubbles from increasing in drift-up rate and from blocking the propagation of ultrasonic waves to the metal tubes P. More desirably, the fine bubbles are microbubbles with an average bubble diameter of 1 µm to 50 µm.
It is preferred that at least some of the fine bubbles in the washing liquid have bubble diameters equal to or less than a frequency-resonance diameter. The frequency-resonance diameter means a bubble diameter that resonates to the frequency of ultrasonic waves in the washing liquid. The fine bubble generation mechanisms 25 preferably generate fine bubbles in the washing liquid such that the ratio of the number of fine bubbles with bubble diameters equal to or less than the frequency-resonance diameter to the total number of fine bubbles is 70% or more. Considering that some fine bubbles expand immediately after the outbreak thereof, it is more desirable that the above-described ratio is within the range of 80% to 98%. This arrangement makes it possible to improve the propagation efficiency of ultrasonic waves in the washing liquid.
In order to improve the propagation efficiency of ultrasonic waves and ensure a sufficient number of nuclei for ultrasonic cavitation, the concentration (density) of fine bubbles in the washing liquid is preferably equal to or more than 10³ bubbles per mL. Also, in order to avoid the necessity of increasing the size of each of the fine bubble generation mechanisms 25 and the necessity of increasing the number of fine bubble generation mechanisms 25, the concentration (density) of fine bubbles in the washing liquid is preferably equal to or less than 10⁶ bubbles per mL.
The average bubble diameter and the concentration of fine bubbles can be measured by known devices, such as a particle counter for counting particles in liquid, a bubble diameter distribution measurement device, etc.

(Cushion)

The cushions 26 are located in the ultrasonic washing tank 21. The plurality of cushions 26 are arranged in the length direction of the ultrasonic washing tank 21.
As shown in FIG. 5, each of the cushions 26 has substantially a U-shape. The metal tubes P are placed on the cushions 26 in the ultrasonic washing tank 21. In the ultrasonic washing tank 21, the respective inside surfaces of the cushions 26 are positioned closer to the center of the ultrasonic washing tank 21 than the ultrasonic radiation mechanisms 24. Therefore, there is no fear that the metal tubes P may come into contact with the ultrasonic radiation mechanisms 24, and the ultrasonic radiation mechanisms 24 can be protected from the metal tubes P.

[Metal Tube Manufacturing Method]

In the method for manufacturing a metal tube P, the alkaline cleaning apparatus 10 (FIG. 2) and the ultrasonic washing apparatus 20 (FIG. 4) are used to clean the metal tube P. The method for manufacturing a metal tube P will hereinafter be described. With reference to FIG. 1 again, the method for manufacturing a metal tube P includes a preparing step S1, a lubricating step S2, a cold drawing step S3, and a cleaning step S4.

(Preparing Step)

At the preparing step S1, a material tube formed by hot working is prepared. Necking is applied to one end portion of the material tube for the cold drawing step S3, which will be described later. Specifically, hammering, drawing or the like is applied to one end portion of the material tube so that the end portion will be narrower than any other portion.
The material tube set for the preparing step S1 is, for example, a material tube made of stainless steel or a material tube made of a Ni-based alloy. The material tube made of stainless steel is a steel tube containing 10.5% or more Cr. When the material tube is a stainless steel tube, the chemical composition thereof preferably contains, in mass%, C: 0.01 to 0.13%, Si: 0.75% or less, Mn: 2% or less, P: 0.045% or less, S: 0.030% or less, Ni: 7 to 14%, and Cr: 16 to 20%, the balance being mainly Fe (typically the balance being Fe and impurities). The chemical composition may contain at least one of, in mass %, Nb: 0.2 to 1.1%, Ti: 0.1 to 0.6%, Mo: 0.1 to 3%, and Cu: 2.5 to 3.5% as substitute for part of Fe in the balance. Also, the chemical composition may contain, in mass %, B: 0.001 to 0.1% and N: 0.02 to 0.12% as substitute for part of Fe in the balance.
A Ni-based alloy tube is a tube in which the content of Ni is the highest of all the components of the alloy. When the material tube is a Ni-based alloy tube, the chemical composition thereof contains, for example, in mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 1% or less, P: 0.030% or less, S: 0.030% or less, Cr: 19.5 to 24.0%, Mo: 2.5 to 4.0%, Ti: 1.2% or less, and Fe: 22% or more, the balance being mainly Ni (typically, the balance being Ni and impurities). The chemical composition may contain, in mass%, at least one of Cu: 0.5% or less, Nb: 4.5% or less, Al: 2.0% or less as substitute for part of Ni in the balance.

(Lubricating Step)

When the material tube is an austenitic stainless steel tube or a Ni-based alloy tube, the material tube has excellent strength, but the load (frictional force) at the time of cold drawing is great. Therefore, after the preparing step S1 and before the cold drawing step S3, the lubricating step S2 is carried out. The lubricating step S2 includes a chemical conversion coating film forming step S21 and a lubricating film forming step S22.

(Chemical Conversion Coating Film Forming Step)

At the chemical conversion coating film forming step S21, a chemical conversion coating film is formed on the surface of the material tube. At the chemical conversion coating film forming step S21, scale that was produced during hot working or the like is removed from the material tube in a known acid pickling process, and thereafter, the material is soaked in a chemical conversion treatment liquid for a predetermined time. Thereby, a chemical conversion coating film is formed on the surface of the material tube.
The chemical conversion treatment liquid used at the chemical conversion coating film forming step S21 is not particularly limited, but the chemical conversion treatment liquid is, for example, an oxalate solution. In this case, on the surface of the material tube, an oxalate film consisting mainly of iron oxalate (II) is formed.

(Lubricating Film Forming Step)

After the chemical conversion coating film forming step S21, the material tube is washed with water and neutralized, and the lubricating film forming step S22 is carried out. At the lubricating film forming step S22, a lubricating film is formed on the chemical conversion coating film formed on the surface of the material tube at the chemical conversion coating film forming step S21. At the lubricating film forming step S22, the material tube with the chemical conversion coating film formed thereon is soaked in a lubricating treatment liquid for a predetermined time. Thereafter, the material tube is taken out from the lubricating treatment liquid, and is completely dried. In this way, a lubricating film is formed on the chemical conversion coating film.
As the lubricating treatment liquid, a high fatty acid salt (soap) is usable. In this case, the lubricating treatment liquid reacts to the chemical conversion coating film, and as a result, a metallic soap is produced. Therefore, the lubricating film includes a metallic soap layer on the chemical conversion coating film, and a soap layer on the metallic soap layer. For example, when the chemical conversion coating film is an oxalate film and when the lubricating treatment liquid is sodium stearate, the metallic soap layer consists mainly of iron stearate, and the soap layer consists mainly of sodium stearate.

(Cold Drawing Step)

After a chemical conversion coating film and a lubricating film are formed on the material tube at the lubricating step S2, the cold drawing step S3 is carried out. At the cold drawing step S3, known cold drawing is applied to the material tube to form the material tube into a metal tube with specified dimensions. For example, the necked end portion of the material tube is inserted in dies (not shown) and held by a gripper (not shown), and the gripper is moved to draw the material tube out from the dies. Thereby, the outside diameter of the material tube is finished to a predetermined outside diameter. If the wall thickness of the material tube needs to be adjusted, the drawing is performed with a mandrel inserted in the material tube.

(Cleaning Step)

The metal tube with specified dimensions obtained by the cold drawing step S3 is cleaned at the cleaning step S4 so that the chemical conversion coating film and the lubricating film are removed. The cleaning step S4 includes an alkaline cleaning step S41 and an ultrasonic washing step S42.

(Alkaline Cleaning Step)

With reference to FIGS. 2 and 3 again, the alkaline cleaning step S41 is a step of soaking a metal tube P in an alkaline degreasing fluid in the alkaline cleaning tank 11 of the alkaline cleaning apparatus 10. At the alkaline cleaning step S41, first, the metal tube P is placed in the tank body 111. Usually, a plurality of metal tubes P are placed in the tank body 111, but only one metal tube P may be placed in the tank body 111. The metal tubes P are placed in the alkaline cleaning tank 11 by a crane or the like. Specifically, the metal tubes P are put on the supports 113. Accordingly, there is a space between the metal tubes P and the bottom 111a of the tank body 111.
By the time the metal tubes P are placed in the tank body 111, the alkaline degreasing fluid has not been supplied to the tank body 111. After the metal tubes P are placed in the empty tank body 111, the lid 112 is put on the tank body 111 to cover the upper side thereof.
Next, the alkaline degreasing fluid in the storage tank 12 is supplied to the alkaline cleaning tank 11 through the circulation piping 13. When the pump 131 is driven, the alkaline degreasing fluid in the storage tank 12 is sent upward to the alkaline cleaning tank 11. The alkaline degreasing fluid flows into the tank body 111 from the end 111c side and flows toward the end 111d side. The alkaline degreasing fluid flows through the respective inside of the metal tubes P, through the space between the metal tubes P and the bottom 111a, and the space between the metal tubes P and the peripheral wall 111b.
While the alkaline degreasing fluid is being supplied to the tank body 111, the communication hole 14 near the downstream end 111d is kept closed. Accordingly, the alkaline degreasing fluid is stored in the tank body 111.
In the storage tank 12, a non-highly-heated alkaline degreasing fluid is stored. Therefore, the non-highly-heated alkaline degreasing fluid is supplied to the tank body 111. As mentioned above, the non-highly-heated alkaline degreasing fluid typically means an alkaline degreasing fluid that has not been positively heated by a heater or the like (for example, a steam heater or an electric heater) and is at a temperature corresponding to the atmospheric temperature around the alkaline cleaning apparatus 10. However, for example, when the atmospheric temperature around the alkaline cleaning apparatus 10 drops below 20 °C in winter or a cold season, the alkaline degreasing fluid may be heated to a temperature of about 20 °C to 40 °C. However, even in winter or a cold season, the temperature of the alkaline degreasing fluid is usually maintained at 20 °C or higher because the pump 131 for sending the alkaline degreasing fluid from the storage tank 12 to the alkaline cleaning tank 11 generates heat. On the other hand, in summer or a hot season, the temperature of the alkaline degreasing fluid sometimes rises above 40 °C. In this case, it is not necessary to cool down the alkaline degreasing fluid to 40 °C or lower. Even when the temperature of the alkaline degreasing fluid naturally rises to a high temperature (above 40 °C) without being positively heated (with no energy consumed for heating of the alkaline degreasing fluid), it is possible to achieve both energy conservation (low cost) and good cleaning performance. In the case in which any positive heating treatment is not applied to the alkaline degreasing fluid, no upper limit is set to the temperature of the alkaline degreasing fluid. Generally, however, the temperature of the alkaline degreasing fluid rises to about 60 °C at the highest without any positive heating treatment applied. Thus, typically, an alkaline degreasing fluid at a temperature corresponding to the atmospheric temperature around the alkaline cleaning apparatus 10 (i.e., a non-heat-treated alkaline degreasing fluid) is supplied to the tank body 111 (the alkaline cleaning tank 11) and stored therein. However, when the temperature of the alkaline degreasing fluid drops below 20 °C, the alkaline degreasing fluid is heated to a temperature not more than 40 °C is supplied to the tank body 111 (the alkaline cleaning tank 11) and stored therein.
When the alkaline cleaning tank 11 is filled with the non-highly-heated alkaline degreasing fluid, the supply of the alkaline degreasing fluid to the tank body 111 is stopped. The metal tubes P are held in the non-highly-heated alkaline degreasing fluid in the alkaline cleaning tank 11 for a predetermined time. The holding time of the metal tubes P may be arbitrarily set as appropriate, and the holding time is, for example, 1 to 5 minutes.
Thereafter, the communication hole 14 is opened so that the alkaline degreasing fluid is discharged from the tank body 111 to the storage tank 12. In this regard, in the tank body 111, the alkaline degreasing fluid flows from the end 111c side toward the end 111d side. The alkaline degreasing fluid flows through the respective inside and circumference of the metal tubes P. All the alkaline degreasing fluid in the tank body 111 is retrieved in the storage tank 12.
The supply of the alkaline degreasing fluid from the storage tank 12 to the alkaline cleaning tank 11, the holding of the metal tubes P in the alkaline degreasing fluid, and the discharge of the alkaline decreasing fluid from the alkaline cleaning tank 11 to the storage tank 12 are defined as one cycle, and this cycle is repeated a predetermined number of times. It is possible to achieve higher cleaning performance by repeating the cycle more. However, it is preferred to determine the number of cycles to be carried out in consideration of the balance between cleaning performance and operation efficiency. The number of cycles to be carried out may be, for example, 2 to 5. The time for one cycle depends on the capacity of the alkaline cleaning tank 11, etc., and may be, for example, about 5 to 15 minutes.
After the above-described cycle is repeated the predetermined number of times, the lid 112 is put off from the tank body 111, and the metal tubes P are pulled up from the tank body 111 by a crane or the like. Then, the alkaline cleaning step S41 is completed.
At the alkaline cleaning step S41, it is preferred to cause a flow of the alkaline degreasing fluid in the alkaline cleaning tank 11. Then, alkaline cleaning of the metal tubes P is carried out by the physical action of the flow of the alkaline degreasing fluid in addition to the chemical action of the alkaline degreasing fluid itself. In the present embodiment, in the process of supplying and discharging the alkaline degreasing fluid, a flow of the alkaline degreasing fluid in the alkaline cleaning tank 11 is caused. In the present embodiment, when the alkaline cleaning tank 11 is filled with the alkaline degreasing fluid, the flow of the alkaline degreasing fluid is once stopped, but the alkaline degreasing fluid may be kept flowing in the alkaline cleaning tank 11. However, the alkaline cleaning step S41 may be carried out without causing a flow of the alkaline degreasing fluid, that is, with the alkaline degreasing fluid kept stationary.

(Ultrasonic Washing Step)

After the alkaline cleaning step S41, the ultrasonic washing step S42 is carried out. The ultrasonic washing step S42 is a step of soaking a metal tube P after soaked in the alkaline degreasing fluid in a washing liquid supplied with ultrasonic waves and fine bubbles. At the ultrasonic washing step S42, the ultrasonic washing apparatus 20 (FIG. 4), which is different from the alkaline cleaning apparatus 10 (FIG. 2) used at the alkaline cleaning step S41, is used. Therefore, after the alkaline cleaning step S41 is completed, the metal tube P is conveyed from the alkaline cleaning apparatus 10 to the ultrasonic washing apparatus 20. After the alkaline cleaning step S41 and before the ultrasonic washing step S42, the metal tube P may be washed with water. This washes off the alkaline degreasing fluid from the metal tube P.
With reference to FIG. 4 again, prior to the ultrasonic washing of the metal tubes P, the washing liquid is stored in the ultrasonic washing tank 21. The washing liquid is supplied to the ultrasonic washing tank 21 by the supply mechanism 22. However, any other means than the supply mechanism 22 may be used to supply the washing liquid to the empty ultrasonic washing tank 21. The washing liquid to be supplied to the ultrasonic washing tank 21 preferably has an oxygen concentration of about 7 mg/L to 11 mg/L. The washing liquid is typically water (tap water or industrial water). When the washing liquid is water (tap water or industrial water) at a temperature of 10 to 35 °C, the concentration of oxygen dissolved in the washing liquid is within the range of 7 mg/L to 11 mg/L. The washing liquid to be supplied to the ultrasonic washing tank 21 more desirably has an oxygen concentration of about 8 mg/L to 10 mg/L. When the washing liquid is water (tap water or industrial water) at a temperature of 15 to 25 °C, the concentration of oxygen dissolved in the washing liquid is within the range of 8 mg/L to 10 mg/L. The oxygen concentration serves as an index of the amount of gasses dissolved in the washing liquid.
When the liquid surface of the washing liquid in the ultrasonic washing tank 21 exceeds the reference liquid surface level S (see FIG. 6 or FIG. 7), the discharge mechanisms 23 start discharging the washing liquid. The supply mechanism 22 continues supplying the washing liquid to the ultrasonic washing tank 21 even after the liquid surface of the washing liquid reaches the reference liquid surface level S. Therefore, discharge of the washing liquid from the ultrasonic washing tank 21 is carried out in parallel with supply of the washing liquid to the ultrasonic washing tank 21. In this regard, the amount of discharge of the washing liquid is substantially equal to the amount of supply of the washing liquid.
The metal tubes P that was conveyed from the alkaline cleaning apparatus 10 (FIGS. 2 and 3) to the ultrasonic washing apparatus 20 is soaked in the washing liquid stored in the ultrasonic washing tank 21 for a predetermined time. The metal tubes P can be placed in the ultrasonic washing tank 21 by a crane or the like to be soaked in the washing liquid. Typically, a plurality of metal tubes P are soaked in the washing liquid at the same time; however, only one metal tube P may be soaked in the washing liquid at a time.
While the metal tube P is soaked in the washing liquid, the washing liquid is newly supplied to the ultrasonic washing tank 21 continuously by the supply mechanism 22. The washing liquid is typically water (tap water or industrial water). In the meantime, the washing liquid is continuously discharged from the ultrasonic washing tank 21 by the discharge mechanisms 23 by an amount corresponding to the excess over the reference liquid surface level S. This makes it possible to prevent the washing performance from degrading due to excessive contamination of the washing liquid in the ultrasonic washing tank 21. The amount (per unit time) of washing liquid supplied to the ultrasonic washing tank 21 by the supply mechanism 22 may be determined in consideration of the amount of washing liquid stored in the ultrasonic washing tank 21 and the degree of contamination of the washing liquid. The washing liquid discharged by the discharge mechanisms 23 from the ultrasonic washing tank 21 is subjected to a specified wastewater treatment and discarded.
While the metal tube P is soaked in the washing liquid, ultrasonic waves are radiated from the ultrasonic radiation mechanisms 24 into the washing liquid, and fine bubbles are supplied by the fine bubble generation mechanisms 25.
At the ultrasonic washing step S42, the fine bubble generation mechanisms 25 uses gasses dissolved in the washing liquid to generate bubbles, and accordingly, the concentration of oxygen dissolved in the washing liquid becomes lower. The fine bubble generation mechanisms 25 decrease the concentration of oxygen dissolved in the washing liquid in the ultrasonic washing tank 21 to 5.2 mg/L or lower. The fine bubble generation mechanisms 25 decrease the concentration of oxygen dissolved in the washing liquid in the ultrasonic washing tank 21 desirably to 4.5 mg/L or lower, and more desirably to 4.2 mg/L or lower.
Specifically, the supply mechanism 22 supplies the washing liquid with an oxygen concentration of about 7 mg/L to 11 mg/L, and preferably about 8 mg/L to 10 mg/L, to the ultrasonic washing tank 21. When the washing liquid passes through the fine bubble generators 253 of the fine bubble generation mechanisms 25, gasses dissolved in the washing liquid are caused to come out as fine bubbles, and the concentration of oxygen dissolved in the washing liquid becomes lower. As the washing liquid is circulated between the ultrasonic washing tank 21 and the fine bubble generation mechanisms 25, the concentration of oxygen dissolved in the washing liquid in the ultrasonic washing tank 21 becomes 5.2 mg/L or lower, desirably 4.5 mg/L or lower, and more desirably 4.2 mg/L or lower. This makes it possible to achieve good ultrasonic washing performance in a wide sound pressure region. In order to certainly achieve good washing performance under such a dissolved oxygen concentration, it is preferred that the ultrasonic waves have a sound pressure of 120 mV or higher.
The concentration of oxygen dissolved in the washing liquid in the ultrasonic washing tank 21 is usually equal to or higher than 2.0 mg/L. However, it is not necessary to adjust or control the lower limit of the concentration of oxygen dissolved in the washing liquid in the ultrasonic washing tank 21.
The dissolved oxygen concentration [mg/L] is a value measured by a commercially available dissolved oxygen meter (LAQUA OM-71 made by Horiba, Ltd.). The sound pressure [mV] is a value measured by a commercially available ultrasonic sound pressure meter (sound pressure level monitor 19001D made by KAIJO Corporation) in a measurement mode to calculate an average value for 5 seconds with a probe (vibration transmission bar with a piezoelectric element) inserted 100 mm deep in the liquid from the liquid surface. In the present disclosure, measured values obtained in this way are considered as the dissolved oxygen concentration and the sound pressure, respectively.
After the metal tubes P are held in the washing liquid in the ultrasonic washing tank 21 for a specified time, the metal tubes P are pulled up from the ultrasonic washing tank 21. At the ultrasonic washing step S42, placing the metal tubes P in the ultrasonic washing tank 21, holding the metal tubes P in the washing liquid, and pulling up the metal tubes P from the ultrasonic washing tank 21 are defined as one cycle, and the cycle is repeated a predetermined number of times. The holding time of the metal tubes P in the cycle and the number of cycles to be carried out can be determined such that the total time of soaking the metal tube P in the washing liquid will be a specified time or longer. The total soaking time of the metal tubes P may be arbitrarily determined according to the amount of films on the metal tubes P, and the like. The total soaking time of the metal tubes P is, for example, 30 seconds or longer, and preferably, 1 minute or longer.
When the metal tubes P are pulled up from the ultrasonic washing tank 21, the metal tubes P are preferably inclined from the horizontal surface. Thereby, the washing liquid is drained from the inside of the metal tubes P. When the above-described cycle is repeated a plurality of times, it is preferred that the direction of inclination of the metal tubes P is changed for each cycle.
After the metal tubes P are soaked in the washing liquid for the predetermined total soaking time or longer, the metal tubes P are retrieved from the ultrasonic washing tank 21 by a crane or the like. At the moment, it is preferred to pull up the metal tubes P while inclining the metal tubes P. This prevents the washing liquid from remaining in the metal tubes P.
With the retrieval of the metal tubes P, the ultrasonic washing step S42 is completed. Meanwhile, in the ultrasonic washing tank 21, ultrasonic waves and fine bubbles are provided to the washing liquid, and supply and discharge of the washing liquid are continued. Therefore, it is possible to successively carry out ultrasonic washing of other metal tubes P.
When ultrasonic washing of other metal tubes P is successively carried out, there is fear that an alkaline degreasing fluid, water, or the like containing no fine bubbles may be brought into the ultrasonic washing tank 21 by the metal tubes P, which will heighten the concentration of oxygen dissolved in the washing liquid in the ultrasonic washing tank 21. When the concentration of oxygen dissolved in the washing liquid becomes higher, it is preferred to stop ultrasonic washing of metal tubes P until the concentration of oxygen dissolved in the washing liquid is sufficiently decreased by the operation of the fine bubble generation mechanisms 25. The ultrasonic washing of metal tubes P after the alkaline cleaning step S41 should be restarted when the concentration of oxygen dissolved in the washing liquid becomes 5.2 mg/L or lower, 4.5 mg/L or lower, or 4.2 mg/L or lower.
In the present embodiment, after the washing liquid is stored in the ultrasonic washing tank 21, the metal tubes P are placed therein. However, the metal tubes P may be placed in the ultrasonic washing tank 21 in an empty sate, and thereafter, the washing liquid may be stored in the ultrasonic washing tank 21.

[Effects of Embodiment]

According to the present embodiment, a metal tube P after subjected to cold drawing is soaked in a non-highly-heated alkaline degreasing fluid, and thereafter, the metal tube P is soaked in a washing liquid provided with ultrasonic waves and fine bubbles. Thereby, the chemical conversion coating film and the lubricating film formed on the metal tube P can be removed with no use of a highly-heated alkaline degreasing fluid.
With reference to FIGS. 8A to 8C, the effects of the cleaning step S4 of the manufacturing method of the metal tube P according to the present embodiment will hereinafter be described. FIG. 8A is a schematic view of the surface of the metal tube P after subjected to cold drawing. FIG. 8B is a schematic view of the surface of the metal tube P after the alkaline cleaning step S41. FIG. 8C is a schematic view of the surface of the metal tube P during the ultrasonic washing step S42.
As shown in FIG. 8A, the metal tube P after cold drawing has a chemical conversion coating film 31 on its surface. Further, the metal tube P has a lubricating film 32 on the chemical conversion coating film 31. The lubricating film 32 includes a metallic soap layer 321 and a soap layer 322. For example, as the chemical conversion coating film 31, an iron oxalate (II) film with a thickness of 1 to 100 µm, and preferably 5 to 40 µm, is formed. As the lubricating film 32, a metallic soap (iron stearate) layer 321 and a soap (sodium stearate) layer 322 are formed to have a total thickness of 10 to 1000 µm, and preferably 50 to 200 µm.
The metal tube P after cold drawing has cracks in the chemical conversion coating film 31. This is considered to occur for the following reason. The ductility of the chemical conversion coating film 31 is extremely low, and during the cold drawing, therefore, the chemical conversion coating film 31 does not expand and cracks. Meanwhile, the lubricating film 32 expands by heat generated by the cold drawing and covers the cracked chemical conversion coating film 31.
As shown in FIG. 8B, when the metal tube P after cold drawing is soaked in a non-highly-heated alkaline degreasing fluid, the lubricating film 32, which covers the chemical conversion coating film 31, is partly removed. Specifically, the metallic soap layer 321 partly reacts to the alkaline degreasing fluid and forms a complex, and the soap layer 322 is partly dissolved in the alkaline degreasing fluid. Accordingly, after the alkaline cleaning step S41, the lubricating film 32 on the metal tube P has been partly removed.
When ultrasonic washing is applied to the metal tube P from which the lubricating film 32 has been partly removed, as shown in FIG. 8C, the lubricating film 32 is peeled off from the metal tube P with the part where the lubricating film 32 has been partly removed serving as points of origin. When the lubricating film 32 covering the chemical conversion coating film 31 is peeled off, the chemical conversion coating film 31 is peeled off from the metal tube P with the cracks caused during the cold drawing serving as points of origin. The chemical conversion coating film 31 and the lubricating film 32 are peeled off from the metal tube P by the physical action such as ultrasonic cavitation, etc.
The cleaning step S4 of the manufacturing method of the metal tube P according to the present embodiment makes it possible to remove the chemical conversion coating film 31 and the lubricating film 32 from the metal tube P after cold drawing without using a highly-heated alkaline degreasing fluid. Accordingly, energy conservation (low cost) and good cleaning performance can be achieved.
Since the chemical conversion coating film 31 is chemically bound with materials of the metal tube P, it is conventionally necessary to carry out acid pickling to remove the chemical conversion coating film 31 from the metal tube P. Specifically, it is conventionally necessary to remove the lubricating film 32 from the metal tube P by using a highly-heated alkaline degreasing fluid and thereafter to carry out acid pickling of the metal tube P to remove the chemical conversion coating film 31. In the cleaning method according to the embodiment, however, not only the lubricating film 32 but also the chemical conversion coating film 31 can be removed by only alkaline cleaning and ultrasonic washing. Thus, it is no longer necessary to carry out acid pickling to remove the chemical conversion coating film 31, and the cleaning process of the metal tube P can be simplified.
According to the present embodiment, at the ultrasonic washing step S42, fine bubbles are generated in the washing liquid. Thereby, ultrasonic waves in the washing liquid are dispersed and propagated three-dimensionally. Then, the performance in washing the metal tube P is improved. At the ultrasonic washing step S42, the fine bubbles are generated from gasses dissolved in the washing liquid, and the concentration of oxygen dissolved in the washing liquid is decreased to 5.2 mg/L or lower, desirably to 4.5 mg/L or lower, and more desirably to 4.2 mg/L or lower. Therefore, excellent ultrasonic washing performance can be achieved in a wide sound pressure region.
Although an embodiment of the present disclosure has been described, the present disclosure is not limited to the embodiment above, and it is possible to modify the embodiment in various ways without departing the gist of the present disclosure.
For example, in the above-described embodiment, acid pickling to remove the chemical conversion coating film 31 can be omitted. However, the embodiment is not to exclude acid pickling, and acid pickling may be carried out after the ultrasonic washing. After the washing, when a metal tube P is pulled up from the ultrasonic washing tank 21, dirt (some part of the films removed from the metal tube during the washing, etc.) floating in the ultrasonic washing tank 21 may adhere to the metal tube P. In order to remove the dirt that has adhered to the metal tube surface again, acid pickling may be carried out. Even in this case, as compared with a conventional method, it takes only a short time to remove the dirt from the metal tube surface by acid pickling.

EXAMPLES

The present disclosure will be described in more detail by providing some examples. However, the present disclosure is not limited to the examples below.

[Example]

In order to confirm the effects of the metal tube cleaning according to the present disclosure, a cleaning test of metal tubes P after subjected to cold drawing was conducted by using the alkaline cleaning apparatus 10 shown in FIG. 2 and the ultrasonic washing apparatus 20 shown in FIG. 4. Specifically, in the alkaline cleaning apparatus 10, the alkaline cleaning step by use of a non-highly-heated alkaline degreasing fluid was carried out, and thereafter, in the ultrasonic washing apparatus 20, the ultrasonic cleaning step by use of a washing liquid was carried out with ultrasonic waves and fine bubbles provided to the washing liquid. In this Example, the temperature of the alkaline degreasing fluid used at the alkaline cleaning step was a temperature corresponding to the atmospheric temperature around the alkaline cleaning apparatus 10, and specifically about 25 °C. In Example, after the alkaline cleaning step and before the ultrasonic washing step, the metal tubes P were washed with water.

[Comparative Example 1]

As Comparative Example 1, alkaline cleaning by use of a highly-heated alkaline degreasing fluid was applied to metal tubes P after subjected to cold drawing. After the high-temperature alkaline cleaning, the metal tubes P were washed with water. In Comparative Example 1, the alkaline cleaning apparatus 10, which was also used in Example, was used. In Comparative Example 1, ultrasonic washing was not carried out after the alkaline cleaning.

[Comparative Example 2]

As Comparative Example 2, washing (ultrasonic washing) by use of a washing liquid was carried out with ultrasonic waves and fine bubbles provided to the washing liquid. In Comparative Example 2, alkaline cleaning was not carried out before the ultrasonic washing.

[Test Conditions]

In each of Example, Comparative Example 1 and Comparative Example 2, 20 metal pipes P were used. Each of the metal tubes P had a chemical composition containing, in mass%, C: 0.08%, Si: 0.2%, Mn: 0.8%, Cu: 3.0%, Ni: 8.8%, Cr: 18.5%, and Nb: 0.5%, the balance being Fe and impurities. Each of the metal tubes P had the following dimensions: an outside diameter of 50.8 mm, a thickness of 7.2 mm, and a length of 8000 mm. On the surface of the metal tubes P, as the chemical conversion coating film 31, an iron oxalate (II) film with a thickness of 10 µm was formed, and as the lubricating film 32, a metallic soap (iron stearate) layer 321 and a soap (sodium stearate) layer 322 with a total thickness of 100 µm were formed on the iron oxalate (II) film. Other test conditions are shown in TABLE 1.

TABLE 1

	Conditions and Method
Step	Example	Comparative Example 1	Comparative Example 2
1. Alkaline Cleaning	Performed	Performed	Not Performed
	(1) Alkaline degreasing fluid	(1) Alkaline degreasing fluid
	- liquid temperature: ordinary temperature	- liquid temperature: 80°C or higher
	- NaOH: about 4% - interfacial active	- NaOH: 3.2% or more
	agent: 0.03% (2) 10-minute flow cycle × 3 times	- interfacial active agent: 0.2% or more (2) 10-minute flow cycle × 3 times
2. Water Washing (Neutralization)	Performed	Performed	Not Performed
	(1) liquid temperature: ordinary temperature	(1) liquid temperature: 50°C
	(2) hanging soaking for 2 to 3 minutes	(2) hanging soaking for 2 to 3 minutes
3. Ultrasonic Washing	Performed	Not Performed	Performed

The 10-minute flow cycle in TABLE 1 means that it took 10 minutes to carry out one cycle of supplying an alkaline degreasing fluid to the alkaline cleaning tank 11, holding the metal tubes P in the alkaline degreasing fluid, and discharging the alkaline degreasing fluid from the alkaline cleaning tank 11. In Example and in Comparative Example 1, during the alkaline cleaning, the 10-minute flow cycle was repeated three times.

[Evaluation]

Regarding Example, Comparative Example 1 and Comparative Example 2, the performance in cleaning the metal tubes P was evaluated. FIG. 9 is a graph showing changes in the amount of carbon (C) [mg/m²] adhering to the inside surfaces of the metal tubes P in Example, Comparative Example 1, and Comparative Example 2. The amount of carbon was measured by a commercially available measuring instrument (multiphase carbon-hydrogen/moisture analyzer RC612 made by LECO Japan Corporation).
As shown in FIG. 9, in Comparative Example 2 in which only ultrasonic washing was carried out, the amount of carbon was hardly decreased as compared with that before the ultrasonic washing. This shows that it is impossible to remove the chemical conversion coating film and the lubricating film from each metal tube P only by ultrasonic washing.
In Comparative Example 1, the amount of carbon was decreased by alkaline cleaning using a highly-heated alkaline degreasing fluid. This shows that the lubricating film was mostly removed by the highly-heated alkaline degreasing fluid. However, it is considered that the chemical conversion coating film was not removed and that the lubricating film partly remained as described below. Therefore, the metal tubes P must be subjected to acid pickling after the high-temperature alkaline cleaning so that the remaining soap constituent and the chemical conversion coating film can be removed.
In Example, the amount of carbon was slightly decreased by alkaline cleaning using an alkaline degreasing fluid at about 25 °C. As described with reference to FIG. 8B, this is because the lubricating film covering the chemical conversion coating film was partly removed. The amount of carbon at the completion of the alkaline cleaning using the alkaline degreasing fluid at about 25 °C was greater than the amount of carbon at the completion of the high-temperature alkaline cleaning in Comparative Example 1. The reason is possibly that the lubricating film could be removed only partly by the alkaline cleaning using the alkaline degreasing fluid at about 25 °C. However, the amount of carbon was greatly decreased (decreased to several tens of milligrams per square meter) by the ultrasonic washing carried out thereafter, and the amount of carbon decreased in Example was much greater than the amount of carbon decreased in Comparative Example 1. In Comparative Example 1, the lubricating film was mostly removed. On the other hand, in Example, by carrying out ultrasonic washing after the alkaline cleaning using a low-temperature (about 25 °C) alkaline degreasing fluid, it becomes possible to remove not only the lubricating film but also the chemical conversion coating film from each metal tube P. Unlike Comparative Example 1, in Example, it is not necessary to carry out acid pickling thereafter. However, in order to remove the chemical conversion coating film more certainly, acid pickling may be carried out.
FIG. 10 shows scanning electron microscopical (SEM) images of the inside surfaces of the metal tubes P subjected to the cleaning of Example and the cleaning of Comparative Example 1, respectively. As shown in FIG. 10, in the metal tube P subjected to the cleaning of Comparative Example 1, white soap constituent remained. This shows that the lubricating film was mostly removed from the metal tube P but partly remained in the metal tube P after the cleaning of Comparative Example 1. On the other hand, in the metal tube P subjected to the cleaning of Example, no white soap constituent was not seen. This shows that the lubricating film was removed from the metal tube P by the cleaning of Example. Also, since the amount of carbon was greatly decreased in Example as compared with Comparative Example 1 as shown in FIG. 9, it is considered that not only the lubricating film but also the chemical conversion coating film was removed from each metal tube P in Example.
These results show that good cleaning performance is obtained by carrying out washing using ultrasonic waves and fine bubbles after carrying out alkaline cleaning using a non-highly-heated alkaline degreasing fluid.

REFERENCE SIGNS LIST

10: alkaline cleaning apparatus
11: alkaline cleaning tank
20: ultrasonic washing apparatus
21: ultrasonic washing tank
P: metal tube

Claims

A manufacturing method for manufacturing a metal tube, the manufacturing method comprising:
a preparing step of preparing a metallic material tube;

a lubricating step of forming a chemical conversion coating film on a surface of the material tube and forming a lubricating film on the chemical conversion coating film;

a cold drawing step of applying cold drawing to the material tube with the chemical conversion coating film and the lubricating film thereon to form the material tube into a metal tube with specified dimensions; and

a cleaning step of cleaning the metal tube to remove the chemical conversion coating film and the lubricating film,

wherein:
the cleaning step includes:
a step of soaking the metal tube in a non-highly-heated alkaline degreasing fluid in an alkaline cleaning tank; and

a step of soaking the metal tube after soaked in the alkaline degreasing fluid in a washing liquid in an ultrasonic washing tank while radiating ultrasonic waves into the washing liquid and generating fine bubbles in the washing liquid.
The manufacturing method according to claim 1, wherein while the metal tube is soaked in the washing liquid, a concentration of oxygen dissolved in the washing liquid is 5.2 mg/L or lower.
A cleaning method for cleaning a metal tube formed by applying cold drawing to a material tube with a chemical conversion coating film and a lubricating film on its surface, the cleaning method comprising:
a step of soaking the metal tube in a non-highly-heated alkaline degreasing fluid in an alkaline cleaning tank; and

a step of soaking the metal tube after soaked in the alkaline degreasing fluid in a washing liquid in an ultrasonic washing tank while radiating ultrasonic waves into the washing liquid and generating fine bubbles in the washing liquid.
The cleaning method according to claim 3, wherein while the metal tube is soaked in the washing liquid, a concentration of oxygen dissolved in the washing liquid is 5.2 mg/L or lower.