JP2017002371A - Surface-treated metallic pipe for heat treatment furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent occurrence of scratches generating by contact of a wire rod with the inner wall of a pipe, or those generating by contacting with a hard carbide generated in the pipe to graze to be chipped when making the wire rod run in a metallic pipe in a continuous furnace carrying out a continuous heat treatment of the wire rod.SOLUTION: A metallic pipe is surface-treated by forming a film including hexagonal boron nitride as a main component on the inner wall or the whole surface of the metallic pipe. Thereby, the direct contact of the metallic pipe with a wire rod is prevented to inhibit occurrence of scratches, and further the scratch prevention work and pipe exchange frequency are reduced by reducing the tight adhesion of the pipe with an agglomerate of carbide occurred and accumulated on the inner wall of the pipe to enable easy removal of the agglomerate leading to scratches by air blow.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a member of a heat treatment furnace used for heat treating a metal wire and its surface treatment.

  Metal wire is a method of heat treatment by heating to a high temperature, and coiled wire or wire wound on a reel is unwound from there to be a single metal wire that is heated and cooled inline ( Called continuous furnace heat treatment). At this time, the wire is made of a stainless steel metal pipe with good thermal conductivity, or a pipe called muffle, into which a non-oxidizing or reducing atmosphere gas is introduced in order to suppress reactions such as surface oxidation at high temperatures. The wire is heated from the outside of the pipe through a metal pipe heated by electricity or gas. On the other hand, there is a method (called a batch type furnace) in which the wire is put into the furnace together with the coil (referred to as a batch furnace), but in comparison with this, the continuous furnace heat treatment can heat the wire uniformly and without heating. Since atmospheric gas can be efficiently introduced, it is economically superior.

In the heat treatment, the heating temperature and the temperature holding time are important parameters. When performing continuous furnace heat treatment, it is the temperature of the metal pipe, the moving speed of the wire, and the length of the metal pipe that correspond to this, in order to increase the moving speed of the wire considering the productivity, A sufficiently long metal pipe is required.

  In general, in the continuous furnace heat treatment, the length of the metal pipe is very large compared to the cross section of the wire. For example, in order to heat-treat a wire having a wire diameter of around 5 millimeters, a pipe of about 8 meters is required. In addition, in order to efficiently heat and maintain the temperature and to flow the above-mentioned gas necessary and sufficient from the viewpoint of quality and economy, for example, when the outer diameter is 34 mm, the thickness of the pipe is about 3.0 mm, When the outer diameter is 42.7 mm, the wall thickness is about 4.9 mm, the inner diameter is limited, and the bending rigidity of the metal pipe cannot be increased.

When a wire is passed through such a metal pipe, the inner diameter is small, so the wire is bent in the metal pipe and comes into contact with the inner wall. Further, even when the metal pipe has low rigidity and is straight at room temperature, it is deformed and meanders when heated and expanded, and is likely to come into contact with the wire.

  Generally, since metal softens when the temperature rises, the wire also softens in the pipe when heated. Moreover, since the inner walls of the wire and the metal pipe are in a non-oxidizing or reducing atmosphere, the oxide scale on the surface is reduced to expose a very active surface.

When a softened wire with an active surface at high temperature comes in contact with the inner wall of a metal pipe that is also activated, adhesion or welding occurs, and the wire moves, causing scuffing, which is serious for the wire. Bring about defects. In particular, when heat-treating a stainless steel wire that is the same material as the stainless steel pipe, adhesion and welding are accelerated, and such scuffs are likely to occur.

The higher the temperature of the heat treatment, the greater the deformation (meandering) of the metal pipe, for example, under the condition of being heated to 1000 ° C. or higher, and the softening of the wire and the activation of the surface progress, and scuffing is likely to occur. Become.

The wire may be cleaned before heat treatment to clean the surface, but calcium, sodium, oxalate, phosphate contained in the wire drawing lubricant used in the previous process Silicates, oils, and the like remain and adhere to some extent, and these are heated and reacted in the pipe to form solids (lumps) such as hard carbide and accumulate on the inner wall of the metal pipe.

When the solid such as hard carbide has a certain size or more and has a sharp corner, the softened wire generates an abraded (scratch-like) wrinkle on the surface when it comes into contact with the moving wire.

In order to prevent the occurrence of soot due to hard solids such as carbides accumulated on the inner wall of metal pipes, pipes were normally replaced at least once a year, but this lowered productivity and increased costs. It was a big burden.

Many methods have been studied so far in order to prevent these two types of scuffs. First, in order to prevent direct contact between the wire and the inner wall of the metal pipe, it was considered to apply a tensile force (tension) to the wire in order to reduce the deflection, but the softened wire was stretched and the cross section As a result, the dimension was reduced, resulting in a defective dimension, and a sufficiently large tensile force could not be applied.

In order to avoid contact between the wire and the inner wall of the metal pipe, an attempt has been made to insert a short ceramic pipe having an outer diameter equal to or smaller than the inner diameter of the metal pipe into the metal pipe. However, a hard lump of the same carbide or the like was fixedly formed on the inner surface of the ceramic pipe by the residual lubricant brought in by the wire, and it was not possible to prevent scuffing. (Patent Document 1)

For the same purpose, it was considered that many ceramic balls were inserted into the pipe and rolling was used to prevent rubbing. However, carbides formed on the ceramic balls caused scratches on the softened wire. I couldn't get rid of the habit. (See Patent Document 2)

Furthermore, in order to prevent direct contact between the wire and the inner wall of the pipe, it has been considered to insert a heat-resistant cloth or a ceramic sheet into the metal pipe to prevent the wire from rubbing. However, this method requires an increase in the amount of reducing gas to prevent the oxidation of the ceramic surface, resulting in a problem of cost increase. In particular, the temperature of the heat treatment is remarkable in the treatment exceeding 1000 ° C., and the ceramic sheet may be damaged. (See Patent Document 3)

Japanese Utility Model Publication No. 6-76355 JP 2004-36981 A JP 2008-50645 A

In a continuous heat treatment furnace for wire, when the wire is run through a metal pipe in order to efficiently heat, maintain and cool the wire while flowing a non-oxidizing or reducing atmosphere gas, the wire is applied to the inner wall of the pipe. It is an object of the present invention to suppress the generation of scraping generated by contact and welding, or generated by contact with a solid such as a hard carbide generated in a metal pipe and scraping by scraping.

The methods of inserting and removing members such as heat-resistant cloth, ceramic sheets, and ceramic balls, which have been used as countermeasures against rubbing so far, into a pipe are time-consuming and troublesome operations. As a result, it was necessary to increase the amount of atmospheric gas. It is an object to suppress the generation of wrinkles without inserting such a member into the pipe.

Residue of the lubricant brought in by the wire is evaporated at a high temperature and accumulates in the metal pipe to form a hard carbide lump, which generates scuffs on the moving wire. Since the resulting lump was in close contact with the inner wall of the pipe and could not be easily removed from the metal pipe, the metal pipe itself was replaced. Without removing the metal pipe from the heat-treating furnace and replacing it as much as possible, it accumulates in the metal pipe and removes lumps of hard carbide etc. accumulated in the metal pipe, reducing the frequency of replacement of the metal pipe and reducing the occurrence of soot It is a problem to suppress.

In the past, during the heat treatment operation, the state of the inner wall of the metal pipe could not be evaluated from the viewpoint of preventing wrinkles generated in the wire, so the metal pipe was replaced early. By making it possible to monitor and evaluate the condition of the inner wall of the metal pipe, it is possible to predict the appropriate replacement time of the metal pipe and to suppress the occurrence of soot while performing efficient metal pipe replacement. Let it be an issue.

In a heat treatment furnace for continuously heat-treating a metal wire, in the heat-resistant metal pipe used for heat treatment in a non-oxidizing or reducing atmosphere by inserting and passing the wire, on the inner surface or the entire surface thereof, A metal pipe formed with a film mainly composed of hexagonal boron nitride is used.

A metal pipe having a hexagonal boron nitride film formed using the hexagonal boron nitride having a purity of 98% or more is used.

In the above hexagonal boron nitride, a metal pipe having a film thickness of 0.2 μm to 5.0 μm is used.

By measuring the electrical resistance between the metal pipe on which the hexagonal boron nitride film is formed and the wire, the film thickness of the hexagonal boron nitride film is obtained, and the ability and remaining life of the hexagonal boron nitride film are predicted. Metal pipe that can be used.

The structure of the surface-treated metal pipe and continuous heat treatment furnace in the present invention is shown. The conventional subject in the metal pipe of this invention is shown. Means for solving the conventional problems of the metal pipe of the present invention will be described. The conventional subject in the metal pipe of this invention is shown. Means for solving the conventional problems of the metal pipe of the present invention will be described. 1 shows a system for measuring the state of wear deterioration of hexagonal boron nitride and predicting the remaining life in the present invention.

The present invention generates heat on the surface of the wire during the treatment in a continuous heat treatment furnace in which heat treatment is performed while flowing atmospheric gas through the pipe through which the wire passes to suppress oxidation of the surface of the wire during heat treatment of the wire. The present invention relates to a metal pipe that has been surface-treated to prevent wrinkles.

In FIG. 1, in a heating furnace 7 of a continuous heat treatment furnace, the wire 1 to be heat-treated, the metal pipe 2 in the continuous heat treatment furnace for heating the wire 1, and the metal pipe 2 are heated from the outside to heat the wire 1. 1 schematically shows a heater 3 and an atmosphere gas inlet 4. The atmospheric gas refers to a gas used for preventing oxidation and preventing excessive oxidation scale, such as argon gas or a modified reducing gas. A plurality of (approximately 10) metal pipes 2 are installed in the furnace in parallel, but only one of them is shown here for easy understanding.

The metal pipe 2 is generally heated to 800 ° C. or higher by the heater 3 to heat the wire 1 to a predetermined temperature. For this reason, the material used for the metal pipe 2 is mainly stainless steel such as SUS310, which has high corrosion resistance at high temperatures, but other materials can be used as long as the corrosion resistance at high temperatures is sufficient. It is.

The length S of the metal pipe 2 is larger than the diameter T of the wire 1 (the ratio of S to T is about 2000), and the inner diameter U of the metal pipe 2 is not so large as compared to the diameter T of the wire 1 (The ratio of U and T is 10 or less). The wire 1 is actually bent by gravity as shown in FIG. 2 (a) in the furnace and is in contact with the inner wall 21 of the metal pipe shown in FIG. 2 (b). Yes. Further, the metal pipe 2 itself expands due to thermal expansion due to the heating by the heater 3, but at this time, the metal pipe 2 is bent or meandered because it is restricted in the length direction by the heating furnace 7. As a result, the wire 1 and the inner wall 21 of the metal pipe come into contact with each other due to factors other than gravity, and the wire 1 moves in the traveling direction W. Therefore, relative rubbing between them is inevitable.

In order to prevent oxidation of the surface 11 of the wire, heating is performed in the atmospheric gas V introduced from the atmospheric gas inlet 4. Therefore, the surface 11 of the wire and the inner wall 21 of the metal pipe are very chemically active. When contacted, adhesion O is generated as shown in FIG. 2B, and the wire 1 moves in the traveling direction W. As a result, as shown in FIG. 2C, a rubbing crease P due to adhesion is generated on the surface 11 of the wire.

FIG. 3 shows a metal pipe 2 in which a hexagonal boron nitride film 5 according to the present invention for preventing this is formed on the inner wall 21 of the metal pipe.

Boron nitride includes cubic boron nitride (c-BN) made at high pressure and hexagonal boron nitride (h-BN) made at normal pressure. The former is a compound with a hard structure next to synthetic diamond. It is used for cutting tools and the like. Cubic boron nitride does not have a structure that reduces the friction coefficient, and therefore, when rubbed, the surface 11 of the wire rod is rubbed.

  On the other hand, hexagonal boron nitride, like graphite, molybdenum disulfide, and mica, has a layered structure in which two-dimensionally linked flat structures are stacked with weak bonding force (van der Waals force). It has a structure (laminated structure), and a flat structure easily shears and slides to show a small coefficient of friction. As a result, even if the heated softened wire 1 is rubbed thereon, the hexagonal boron nitride film 5 is sheared and deformed, so that the surface 1 of the wire as shown in FIG. There is nothing. That is, the hexagonal boron nitride film 5 enters between the surface 11 of the wire rod and the surface 21 of the metal pipe, prevents direct contact between metals, prevents adhesion, and has a small friction coefficient and good sliding property. Even if the wire 1 is rubbed on the hexagonal boron nitride film 5, the surface 11 of the wire is not wrinkled.

Graphite, molybdenum disulfide, and mica also show the same structure and show a small coefficient of friction as an excellent solid lubricant. However, oxidation proceeds in the air at 500 ° C. or lower, and in a non-oxidizing atmosphere at about 800 ° C. A structural change occurs, the laminated structure is broken, and the function is significantly reduced. On the other hand, hexagonal boron nitride is stable up to 900 ° C. even in the air, and is said to be stable up to 1800 ° C. in a non-oxidizing atmosphere and 3000 ° C. in a nitrogen atmosphere. This time, the solution heat treatment temperature of the stainless steel wire which is the main target is 1000 ° C. to 1200 ° C., and has sufficient structural stability and good slipperiness in this temperature range.

The hexagonal boron nitride film 5 can be formed relatively easily. Boron nitride powder does not easily dissolve in water or solvent, but it dissolves in them by using an appropriate dispersant, and it has moderate wettability to stainless steel (SUS310), which is the material of the metal pipe 2. Since it has, it spreads uniformly at room temperature and can perform the stable surface treatment (film formation).

As the water dispersant, potassium oxybisphosphonate, carboxymethyl, sodium cellulose and the like are suitable. Nitrocellulose and isopropyl alcohol are effective as the solvent dispersant. All of these are suitably used at 2 to 6% with respect to 20% of hexagonal boron nitride. If the amount is too small, the dispersion is insufficient. If the amount is too large, unevenness such as segregation of the hexagonal boron nitride film 5 occurs, and stable surface treatment (film formation) cannot be performed.

When the heater 3 of the heating furnace 7 is turned on and the temperature of the metal pipe 2 exceeds 500 ° C., the dispersant is completely evaporated and only the hexagonal boron nitride film 5 remains on the surface. The purity of the boron nitride powder used is preferably 98% or more so as not to hinder the function at high temperature.

The hexagonal boron nitride film 5 thus formed is chemically stable at a high temperature of 1000 ° C. or more, and at the same time has a uniform film thickness X and stands between the surface 11 of the wire and the inner wall 21 of the pipe. The wire 1 that suppresses adhesion O as shown in 2 (c) and passes through the metal pipe 2 on which the hexagonal boron nitride film 5 is formed may generate a rubbing flaw P on the surface 11 of the wire. Absent.

If the film thickness X of the hexagonal boron nitride is too thin, the wear life may not be sufficient, and 0.2 μm or more is necessary. On the other hand, if it is too thick, the elastic modulus of hexagonal boron nitride 5 is about twice that of a metal pipe (390 GPa), so that stress may be generated on the surface and the interface to cause peeling. From the results of the bending test, a stable effect is exhibited by setting the maximum film thickness X to 5 μm or less.

In FIG. 4 (a), due to vaporization K of the lubricant (calcium, sodium, oxalate, phosphate, silicate, etc.) remaining in the wire 1, they adhere to the metal pipe 2 at a high temperature. A state where a lump L of hard carbide or the like is formed and deposited on the pipe inner wall 21 is shown.

When the hexagonal boron nitride film 5 is not present, the lump L such as carbide is strongly fixed by reacting with the surface 21 of the metal pipe, and the wire 1 is rubbed against the lump L such as carbide. As shown in FIG. 4 (c), rubbing wrinkles Q are generated in the wire 12 that has passed.

Even if an attempt is made to scrape off a lump of carbide or the like while the metal pipe 2 is attached to the furnace in order to prevent the rubbing habit Q, it is not easy to remove it because the adhesion is strong. For this reason, it is necessary to periodically replace the metal pipe 2, and this work is a heavy burden in terms of production and cost.

The hexagonal boron nitride film 5 is more stable at high temperatures than other solid lubricants (eg, graphite, molybdenum disulfide, etc.), calcium, sodium, oxalate, phosphate, silicate, etc. Almost no reaction with carbides of 1000 ° C or higher. Therefore, the adhesion force between the mass M of carbide or the like on the hexagonal boron nitride film of FIG. 5A and the hexagonal boron nitride film 5 is not chemical and becomes very small.

When the hexagonal boron nitride film 5 is formed on the metal pipe 2, the adhesion force of the mass M such as carbide on the hexagonal boron nitride film generated by vaporization K of the remaining lubricant is reduced. Therefore, as shown in FIG. 5B, the lump M such as carbide on the hexagonal boron nitride film 5 is removed from the pipe end face 22 by air blow N at a pressure of about 5 kg / cm ² , for example. can do. Thereby, rubbing wrinkles Q can be prevented without exchanging the metal pipe.

If the hexagonal boron nitride film 5 remains above a certain level, the use can be continued, so that the frequency of replacement of the metal pipe decreases. In addition, after the hexagonal boron nitride film 2 is worn, the same metal pipe 2 is used to perform surface treatment again, and the hexagonal boron nitride film 5 can be re-deposited. To do.

Since the friction coefficient between the wire 1 and the hexagonal boron nitride film 5 is small, the rate of decrease of the film thickness X due to wear is small, and if the film thickness is within the appropriate film thickness X range, it does not peel off suddenly. The wear rate is stable. If the rate of decrease in film thickness due to wear is known, the remaining film thickness of the hexagonal boron nitride film 2 can be obtained, and the function of preventing flaws in the metal pipe 2 on which the hexagonal boron nitride film 2 is formed remains. The service life can be predicted, and the replacement time of the metal pipe 2 becomes clear.

Hexagonal boron nitride has a very high electrical resistance and is said to be approximately 1 × 10 ¹⁶ Ωm. FIG. 6 shows a circuit 6 formed by connecting the wire 1 and the metal pipe 2 on which the hexagonal boron nitride film 5 is formed by using the high insulation property of the hexagonal boron nitride, and the electric resistance of the circuit 6 is changed. Is a conceptual diagram of a system that measures the above, obtains the film thickness X of the film Z in which the hexagonal boron nitride film 5 remains, and predicts the remaining lifetime h of the film. The circuit 6 includes a contact 61 with the wire 1, a contact 62 with the metal pipe 2, a DC voltage 63 and an ammeter 64. The contact 62 can be provided at a plurality of locations on the metal pipe as shown in FIG. 6 in consideration of measurement accuracy. When the current value is i and the voltage value is e, the resistance R between the wire 1 and the metal pipe 2 on which the hexagonal boron nitride film 5 is formed, and the resistance r of the other circuit 6 are assumed.
R = e / i−r
It can be asked. The larger the resistance R, the larger the film thickness X of the remaining film Z. On the contrary, the film thickness X decreases as the resistance R decreases. Since the resistance R is substantially proportional to the film thickness X, the proportionality coefficient a corresponding to the wire 1 to be heat-treated and the heat treatment conditions can be obtained to obtain the film thickness X of the remaining film Z. For example, R is a resistance between “wire 1”, “hexagonal boron nitride film 5”, and “metal pipe 2”, t is a value of film thickness X of hexagonal boron nitride film 5, and proportionality factor a is make use of,
t = a ・ R
From this, t can be obtained as the value of the film thickness X. In addition, a quadratic expression relating to R, a polynomial, or the like can be used instead of the linear expression.

The limit value c of the film thickness X that does not generate the scraping soot P due to adhesion and the scraping soot Q with a lump of carbide or the like is set from experiments and the like. In comparison, the remaining life h of the metallic pipe can be predicted by using the decrease rate v of the film. Ie
h = (t−c) / v
It is expressed as If the remaining life h is known, the metallic pipe 2 can be efficiently replaced at an appropriate time. Thereby, the replacement frequency of the metal pipe 2 which has conventionally been performed once per year can be significantly reduced.

The hexagonal boron nitride film 5 may be not only the inner wall 21 of the metal pipe but also the entire circumference including the outer periphery of the metal pipe 2. This is because the hexagonal boron nitride film 5 is stable at high temperatures even in an oxidizing atmosphere and has good thermal conductivity.

DESCRIPTION OF SYMBOLS 1 Wire 2 Metal pipe 3 Heater 4 Atmospheric gas inlet 5 Hexagonal boron nitride film 6 Circuit 7 Heating furnace of continuous heat treatment furnace

DESCRIPTION OF SYMBOLS 11 Wire surface 12 Passed wire 21 Metal pipe inner wall 22 Pipe end surface 61 Contact with wire 62 Contact with metal pipe 63 DC voltage 64 Ammeter

O adhesion
P Rub due to adhesion
Q rub
K Vaporization of remaining lubricant
L Mass of carbide
M Mass of carbide etc. on hexagonal boron nitride film
N Air blow R Resistance
S pipe length
T Wire diameter
U inner diameter
V Atmospheric gas
W direction of travel
X film thickness Z remaining film

a Proportional coefficient c Limit value i Current value e Voltage value h Remaining life r Other circuit resistance t Film thickness value v Wear rate

Claims (4)

In a heat treatment furnace for continuously heat-treating a metal wire, a metal pipe used for heat treatment in a non-oxidizing or reducing atmosphere by inserting and passing the wire, at least on its inner wall, hexagonal crystal A metal pipe that has been surface-treated by forming a film mainly composed of boron nitride. 2. The metal pipe according to claim 1, wherein the hexagonal boron nitride has a purity of 98% or more and is surface-treated by forming a film of hexagonal boron nitride. The surface-treated metal pipe according to claim 1 or 2, wherein the film thickness of the hexagonal boron nitride is in the range of 0.2 µm to 5.0 µm. By providing means for measuring the electrical resistance between the metal pipe on which the hexagonal boron nitride film is formed and the wire, the film thickness of the hexagonal boron nitride film is obtained, and the lifetime of the hexagonal boron nitride film is increased. The surface-treated metal pipe according to claim 1, which can be predicted.

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