JP2000119839A - Boriding treating method for steel member surface - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the corrosion resistance and wear resistance of a steel member without causing its deformation and the cracking and peeling of a film by coating the steel member surface with a boriding treating agent, executing heating to form a compd. of iron and boron and introducing gas from the outside at one time of the heating or cooling. SOLUTION: The surface of a steel member such as large steel parts or the like is coated with a boriding treating agent contg. a boron supplying source and a boriding accelerator, which is heated at about 900 deg.C to form a compd. of iron and boron. As the boron feeding source, amorphous boron, FeB, B4C, borax or the like are used. Moreover, as the boriding accelerator, NH4, Cl, KBF4 or the like is used, and, preferably, it is used jointly with a boriding conditioner. In this boriding treatment, at the time of the heating and/or cooling, gas is introduced from the outside. This gas introduction is executed preferably at the time between the start of the boriding or the finishing temp. and the maximum treating temp. Moreover, as the introducing gas, inert gas or reducing gas is preferable. In this way, the prolongation of the service life of a steel member such as parts in a bath of a continuous hot dip metal plating device or the like can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a boring treatment method for a steel member surface, and more particularly to a boring treatment method for a large steel member surface used as a bath part such as a sink roll and a support roll in a continuous hot-dip metal plating apparatus. About the law.

[0002]

2. Description of the Related Art A continuous hot-dip metal plating apparatus is used for continuously passing a steel sheet to be plated through a plating bath made of a molten metal via a roll or the like and plating the surface of the steel sheet.

[0003] As materials for components such as support rolls and sink rolls used in a plating bath, steel materials such as cast iron, stainless steel and high chromium steel, which are generally excellent in corrosion resistance, have been used. Wear is severe due to the corrosive action of steel and friction with the steel plate. In order to improve this, cermets, ceramics, etc. are sprayed by chemical spraying or chemical vapor deposition (CV).
Attempts have been made to coat metal surfaces using D) or physical vapor deposition (PVD). For example, JP-A-61-3
There is a technique for producing a metal bath roll excellent in corrosion resistance, heat resistance, abrasion resistance, etc. by plasma spraying ceramics as seen in JP-A-7955. Attempts have been made to improve the high-temperature corrosion resistance of the metal base material by irradiating a laser beam to partially melt the ceramic or cermet coating formed on the surface of the metal base material.

[0004] As a method of forming a ceramic film on the surface of a steel material, many methods such as CVD, PVD and thermal spraying have been developed. However, in CVD or PVD, it is difficult to form a thick film having a thickness of several tens μm or more required for parts in a bath. In the plasma spraying method, a thick film can be formed. However, since a small pinhole exists in the coating, there is a possibility that molten metal may penetrate through the pinhole and corrode the base material. Further, the coating is easily peeled off and has poor reliability.

In order to protect the surface of parts in a bath such as a roll used in a plating bath, a film having a thickness of several tens μ
m or more coatings or layers must be formed. For this purpose, it is desirable that the base material and the surface coating or layer are bonded by a chemical reaction.

There are the following methods for forming a dense ceramic layer (nitride, boride or carbide). As a method of forming a nitride layer, there are a tuftride treatment and an ion nitridation treatment. Further, as a method for forming a boride layer, there is a powder method immersion boron treatment (for example,
1971-10) and a molten salt method immersion boron treatment (for example,
Abstract of the Japan Institute of Metals 1972-4). Further, as a method of forming a carbide layer, a method of coating titanium carbide by heating in a gas containing a halide [e.g., Ainihe Eigenshaften del Bergsstoff combinational stadium mitt titanium carbidberg, Metallobell flache 14 (1960) 2
Page 29: Einige Eigenschaften der Werkstoffkombinat
ion Stahl mit Titankarbiduberzug, Metalloberflache
14 (1960) 229] or a method in which a metal that forms carbide is melted in a molten bath, a steel material is immersed therein, and carbon is diffused from the steel material to combine with the metal to form carbide [for example, Toru Arai: Metallic materials 13, 3 (197
3) 98].

Although it is conceivable to make the whole in-bath parts such as rolls with ceramics, the in-bath parts are generally large and it is technically difficult and not economical to make the whole in ceramics.

[0008]

Among ceramics such as nitrides, borides and carbides, borides are the most excellent in terms of corrosion resistance to molten metal. The present invention is intended to improve the corrosion resistance and wear resistance of a rolled steel member by boring only a necessary portion of a surface of a part in a bath such as a roll.

However, as a result of applying an iron boride film to the surface of a large steel material based on Japanese Patent Application Laid-Open No. Hei 7-18884, deformation, cracking and peeling of the film occurred. That is, as a result of boring a large steel material in the same process as a small steel material, cracks occurred on the surface.

[0010] Therefore, there is a need to develop a boring technology that can be applied to large components. An object of the present invention is to provide a boring treatment technique which is free from deformation, film cracking and peeling, and can be applied to large steel parts. Another object of the present invention is to achieve a longer life of a component in a bath of a hot-dip metal plating apparatus by using the component of the present invention.

[0011]

An object of the present invention is to provide a boride treatment for forming a compound of iron and boron on the surface of a steel member by covering the surface of the steel member with a boride treatment agent and heating the material.
This is achieved by introducing a gas from the outside at least at one time during heating and / or cooling during the boring treatment.

[0012] The above-mentioned problem is also caused by the fact that in a boride treatment of forming a compound of iron and boron on the surface of a steel member by covering the surface of the steel member with a boride treatment agent and heating, boring of the steel member starts upon heating. This is achieved by introducing a gas from the outside from a temperature in the vicinity to the maximum processing temperature.

[0013] Another object of the present invention is to provide a boride treatment for forming a compound of iron and boron on the surface of a steel member by covering the surface of the steel member with a boride treatment agent and heating the steel member. This is achieved by introducing a gas from the outside up to a temperature near the end of boration.

Another object of the present invention is to provide a boride treatment for forming a compound of iron and boron on the surface of a steel member by covering the surface of the steel member with a boride treatment agent and heating the steel member. This is achieved by introducing a gas from the outside to a temperature close to the maximum processing temperature, and then introducing a gas from the outside during the cooling to a temperature close to the end of the decomposition of boron from the processing agent from the maximum processing temperature. You.

In the boring treatment of the present invention, the temperature at which boring of the steel member starts or the temperature at which boring of the steel member ends is a temperature higher than 700 ° C. and lower than 800 ° C. The temperature near the start or end of boration is around 750 ° C. Further, the maximum treatment temperature of the boride treatment is 900
A temperature of 870 ° C or more and 1000 ° C or less centered on ° C is optimal.

In the present invention, the object to be borated is a member made of a steel material such as carbon steel, alloy steel, stainless steel, cast steel and cast iron.

In the present invention, the gas may be introduced at the time of heating or at the time of cooling, but it is desirable to introduce the gas at both the time of heating and the time of cooling. Regarding the type of gas to be introduced, when a gas containing oxygen is introduced, the surface of the base material is oxidized and a sound boride layer is not formed. Therefore, an inert gas such as argon, a reducing gas such as hydrogen, a nitrogen gas, or the like. It is desirable to use

In the present invention, as the boride treating agent, a mixed powder containing a boron source and a boride accelerator is used. As a boron source, amorphous boron, ferroboron (Fe
B), boron carbide (B ₄ C), borax (Na ₂ B ₂ O ₇ ), and the like. As the boration accelerator, ammonium chloride (NH ₄ C)
1), boron borofluoride (KBF ₄ ) or the like can be used. Of these, boron carbide (B ₄ C) is preferable as a boron source practically and widely used, and boron borofluoride (KBF ₄ ) is preferable as a boride accelerator. It is desirable to further blend a boride modifier such as silicon carbide (SiC) with the boride treating agent.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following experiment was conducted as a process leading to the present invention.

[Experimental example 1] Outer diameter 250 mm, inner diameter 210
A large test piece made of carbon steel (S45C) having a length of 300 mm and a length of 300 mm was used as a test piece.

A boride treatment agent comprising a powder obtained by mixing B ₄ C as a boron supply source, KBF ₄ as a boride accelerator, and SiC as a boride regulator has an outer diameter of 330 mm and an inner diameter of 310 mm.
A stainless steel (SUS316) case with a length of 450 mm is packed, and a large test piece is put in the case.
After heating at 0 ° C / h and holding at 900 ° C for 7 hours,
As a result of cooling a large test piece at a cooling rate of 100 ° C./h,
As shown in the photograph of FIG. 1, cracks occurred over almost the entire surface.

The large test piece is cut into K ₃ [Fe (C
_{N)] 6, K 4 [} Fe (CN)] 6, KOH, results of corroded section using a mixed reagent of H ₂ O was observed with an optical microscope (magnification: 200 times), the test as shown in FIG. 2 A two-layer iron boride film having a thickness of 120 to 170 μm is formed on the surface of the piece, and a black, corroded, 30 to 70 μm porous hard and brittle FeB layer is present on the outermost surface, and the inner surface is formed therefrom. Then, it was found that a dense Fe ₂ B layer corroded brightly was present. Further, as a result of examining the depth of the crack, it was found that the crack remained in the outermost FeB film.

From the results of this experiment, it was found that, in the case of large members, tensile thermal stress is generated on the surface during cooling, so that it is necessary to reduce the cooling rate and reduce the thermal stress in order to prevent cracking of the FeB film. Was.

[Experimental Example 2] Using a large test piece of the same material and dimensions as in Experimental Example 1, heating was the same as in Experimental Example 1,
The cooling rate from 900 ° C to 650 ° C was maintained at 650 ° C for 5 hours at a rate of 30 ° C / h. Further, it was cooled to around room temperature at a cooling rate of 30 ° C./h.

As a result, cracks on the surface could be prevented. This is a process for reducing the tensile thermal stress generated in the boride layer. By maintaining the steel member at a temperature near the start of the pearlite transformation of the steel member until the steel member has a predetermined temperature distribution, the boride is reduced. It has been found that harmful cracks generated in the layer can be prevented.

However, as shown in the photograph of FIG. 3, a large number of granular peelings occurred. As a result of measuring the peeling depth, 30 to
It was 80 μm. As a result of observing the cross-sectional structure in the same manner as in Experimental Example 1, as shown in FIG.
A two-layer iron boride film of 40 to 190 μm is formed,
On the outermost surface, there is a porous hard and brittle FeB layer of 40 to 100 μm which has been corroded black, and there is a bright and corroded dense Fe ₂ B layer inside thereof, and F
It was found that peeling occurred in the eB layer and near the interface between the FeB and Fe ₂ B layers. This is because the FeB layer and F
Since the physical, mechanical, and thermal characteristics of the e ₂ B layer are different, it is interpreted that the e ₂ B layer is caused by shear stress generated at the interface.

Therefore, it was found from the results of this experiment that it is necessary to make the FeB layer as thin as possible or to form a single Fe ₂ B layer.

[Experimental Example 3] Diameter 10 mm, length 20 mm
An experiment was conducted to examine the effects of the temperature on the start of boride and the temperature on FeB formation using a small test piece made of carbon steel (S45C). The same result should be obtained for FeB formation even with a large specimen.

A boride treating agent composed of a powder obtained by mixing B ₄ C as a boron supply source, KBF ₄ as a boride accelerator, and SiC as a boride regulator is packed in a case, and a small test piece is put in the case and heated. Heat at 300 ° C / h at 600 ° C, 70
5 hours at each of 0 ° C, 800 ° C, 900 ° C, and 1000 ° C
After the holding, the small test piece was cooled at a cooling rate of 100 ° C./h.

The small test piece was cut and cut into K ₃ [Fe (C
_{N)] 6, K 4 [} Fe (CN)] 6, KOH, a corroded section using a mixed reagent of H ₂ O and observed under an optical microscope (magnification: 200 times), the thickness of the FeB layer and FeB + Fe ₂ B FIG. 5 shows the result of measuring the thickness of the layer.

From FIG. 5, no boride layer is formed below 700 ° C., and at temperatures above 800 ° C., the higher the processing temperature, the thicker the boride layer. From this result, it is found that the temperature near the start of boride is a temperature higher than 700 ° C. and lower than 800 ° C. The boride treatment proceeds by a boron diffusion reaction, and the longer the holding time, the thicker the boride layer.

However, the thickness of the FeB layer hardly changes even when the processing temperature increases. Therefore, the ratio of FeB [(FeB / (FeB + Fe ₂ B)] decreases as the treatment temperature increases, because the diffusion rate of boron into the steel material is low at low temperatures, and the amount of boron in the surface layer is low. More FeB
It is considered that the number of

[Experimental Example 4] Using test pieces of the same material and dimensions as in Experimental Example 3, the cooling rate from 900 ° C to 650 ° C was kept at 650 ° C for 5 hours at a rate of 30 ° C / h. Further, it was cooled to around room temperature at a cooling rate of 30 ° C./h.

The test piece was cut to obtain K ₃ [Fe (CN)] ₆ ,
A cross section corroded with a mixed reagent of K ₄ [Fe (CN)] ₆ , KOH, and H ₂ O was observed with an optical microscope (magnification: 200).
Times), the thickness of the FeB layer and the thickness of the FeB + Fe ₂ B layer were measured. The results are shown in FIG. 6, and for comparison, the results of the test pieces cooled at a cooling rate of 900 ° C./h from 900 ° C. shown in FIG. 5 are also shown.

From FIG. 6, it was found that the rate of FeB was extremely large when the cooling rate was slower than when it was fast. This is considered to be a result of a layer containing a large amount of boron being formed on the surface during the cooling process and being cooled without being directly diffused into the steel material. Therefore, it is necessary to cool the temperature range in which the decomposition of boron is performed as quickly as possible. However, as shown in Experimental Example 1, when the cooling rate is high, cracks occur in the boride layer in large parts, so that measures for preventing cracks are necessary.

According to the experimental examples described above, the reaction between iron and boron is suppressed in the process of heating to the proper processing temperature and in the process of cooling from the processing temperature, so that boron can be added to the surface layer even if the heating or cooling rate is reduced. It is considered that if the generation of a large amount of FeB can be reduced, deformation, cracking, peeling, and the like of a large component can be prevented.

A method of heating a steel material by packing it in a case together with a boride treatment agent comprising a powder obtained by mixing B ₄ C as a boron supply source, KBF ₄ as a boride accelerator, and SiC as a boride regulator, and heating the solid material It is known that the reaction of producing FeB by iron and boron is locally accompanied by a gasification reaction. The present invention has been achieved by considering the introduction of a gas that does not contribute to the boride treatment.

[0038]

EXAMPLES Examples of the present invention will be described below, but the present invention is not limited to the scope of these examples.

Example 1 The outer diameter was 250 as in Example 1.
mm, inner diameter 210mm, length 300mm carbon steel (S4
5C) large test piece was used as the test piece.

A boride treatment agent comprising a powder obtained by mixing B ₄ C as a boron supply source, KBF ₄ as a boride accelerator, and SiC as a boride regulator, has an outer diameter of 330 mm and an inner diameter of 310 mm.
A stainless steel (SUS316) case with a length of 450 mm is packed, and a large test piece is put in the case.
It was heated at 0 ° C / h to a maximum treatment temperature of 900 ° C. During the heating from 750 ° C., which is the temperature near the start of boring of the steel member, to 900 ° C., a 2 l / min argon gas was introduced from the lower part of the case.

The temperature is maintained at 900 ° C. for 10 hours,
The cooling rate from 30 ° C to 650 ° C is 30 ° C / h and 65
It was kept at 0 ° C. for 5 hours. Further, it was cooled to around room temperature at a cooling rate of 30 ° C./h. However, 750 ° C., which is a temperature near the end of boring of steel members from the maximum processing temperature of 900 ° C.
During cooling down to the same temperature as when heating,
1 / min of argon gas was introduced.

As a result, as shown in the photograph of FIG. 7, a borated part having no surface cracking or peeling and little deformation was obtained. Further, the test piece was cut into K ₃ [Fe (CN)] ₆ ,
The result of observing a cross section corroded using a mixed reagent of K ₄ [Fe (CN)] ₆ , KOH and H ₂ O with an optical microscope (magnification:
200 times), as shown in FIG.
In spite of the formation of the two thick iron boride films of 0 to 210 μm, the FeB layer corroded black on the outermost surface is 10 to 30 μm, which is extremely reduced as compared with FIG.

That is, by introducing a gas that suppresses the boration reaction during the heating and cooling processes, FeB can be reduced even when a large component is gradually heated and cooled. It became possible to make.

In this embodiment, the gas was introduced during both heating and cooling. However, it was confirmed that introducing the gas only during heating or only during cooling was effective. However, it is desirable to introduce a gas during both heating and cooling.
The type of gas to be introduced was also studied. As a result, when a gas containing oxygen is introduced, the surface of the base material is oxidized and a sound boride layer is not formed. Therefore, it is preferable to use an inert gas such as argon, a reducing gas such as hydrogen, or a nitrogen gas. understood.

Embodiment 2 FIG. 9 shows an example of parts in contact with a molten zinc bath. That is, in a facility for continuously applying hot-dip galvanizing to the strip 1, the strip 1 is guided to the plating bath 4 in the plating bath 3 via the snout 2, changed direction by the sink roll 5, and passed through the support roll 6. After being drawn out of the plating bath 4, the plating thickness is adjusted by the wiping nozzle 7. Among the components used in the hot-dip galvanizing apparatus, the sink roll 5 rotating in contact with the plating bath, its bearing portion 8, and the support roll 6 and its bearing portion 9 are often melted and worn. Erosion,
As the wear progresses, the vibration of the strip 1 increases during the running of the steel sheet, and uniform zinc deposition becomes impossible. According to the method of the present invention, it has become possible to form a uniform boride layer even on a large product that is difficult to manufacture with ceramics integrally.

In order to confirm the results of the present invention, results applied to a sink roll and a support roll for continuous galvanizing will be described. The sink roll has a body diameter of 700 mm,
The support roll had a body diameter of 200 mm, the body length was 1600 mm for both rolls, and the material used was S45C. Both rolls were held at 900 ° C. for 10 hours as a pretreatment and then cooled in a furnace. Thereafter, machining was performed. Boride process sets the roll in the center of a cylindrical case made of stainless steel (SUS316), B ₄ C as a boron source, KBF ₄ as boride accelerator, from a powder of a mixture of SiC as a boride modifier Fill the gap between the roll and the case
The heating rate was 300 ° C./h. During heating from 750 ° C., which is the temperature near the start of boring of the steel member to the maximum treatment temperature of 900 ° C., 15 l / min for the sink roll and 5 l / min for the support roll from the bottom of the case.
min argon gas was introduced. 1 at 900 ° C
The temperature was maintained for 0 hour, and the cooling rate from 900 ° C. to 650 ° C. was maintained at 650 ° C. for 5 hours at a rate of 30 ° C./h. Furthermore, 3
It cooled to near room temperature at a cooling rate of 0 ° C./h. However, during cooling from the maximum treatment temperature of 900 ° C. to 750 ° C., which is a temperature near the end of boring of the steel member, argon gas was introduced from the lower part of the case in the same manner as during heating.

However, the thickness of the mixed powder of the boride treating agent is 25
mm, and processed by a hanging method. As a result, the amount of deformation of the sink roll was 20 μm and the amount of deformation of the support roll was 50 μm, and a sound roll free of cracks and peeling was obtained. As a result of measuring the thickness of the boride layer at the body end of each roll, the thickness was 160 μm to 180 μm, which was a deformation amount that could be corrected by finish polishing.

These rolls were incorporated into a continuous hot-dip galvanizing apparatus at 470 ° C. and used continuously for 100 hours.
The shaft portions of both rolls were not worn at all, and the body portions were good with no adhesion of dross or the like.

[0049]

According to the present invention, a boride treatment which could not be applied to a large component due to deformation or cracking or peeling of the boride layer can be performed, and a sound boride layer can be formed on a large component. Can be done.

Thus, the in-bath components used in the plating bath have high corrosion resistance and abrasion resistance, and have a long life. In addition, as a result of using it as a roll in a continuous galvanizing bath, it is easier to handle than conventional ceramic parts,
A stable galvanized steel sheet can be obtained with less roll exchange and less contamination of the zinc bath. Furthermore, not only parts in the plating bath but also other large parts have a sound boride layer, so that they are used as parts requiring corrosion resistance and wear resistance.

[Brief description of the drawings]

FIG. 1 is a photograph of the surface of a large cracked steel material that has been borated in Experimental Example 1.

FIG. 2 is a cross-sectional optical micrograph (magnification: 200 times) of a large steel material borated in Experimental Example 1.

FIG. 3 is a photograph of the surface of a large-sized steel material having a peel that has been borated in Experimental Example 2.

FIG. 4 is a cross-sectional optical micrograph of a large steel material borated in Experimental Example 2 (magnification: 200 times).

FIG. 5 shows the thickness of FeB layer and FeB + F in Experimental Example 3.
Measurement result of e ₂ B layer thickness

FIG. 6 shows the thickness of FeB layer and FeB + F in Experimental Example 4.
Measurement result of e ₂ B layer thickness

FIG. 7 is a photograph of the surface of a large-sized steel material that has been borated and sounded in Example 1.

FIG. 8 is an optical micrograph of a cross section of a large steel material borated in Example 1 (magnification: 200 times).

FIG. 9 is an explanatory view showing an example of parts in contact with a molten zinc bath.

[Description of Signs] 1 Strip 2 Snout 3 Plating Bathtub 4 Plating Bath 5 Sink Roll 6 Support Roll 7 Wiping Nozzle 8 Sink Roll Bearing 9 Support Roll Bearing

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Yukio Saito 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Within Hitachi Research Laboratory, Hitachi, Ltd. (72) Inventor Junji Sakai 7-chome, Omika-cho, Hitachi City, Ibaraki Prefecture No. 1-1 Inside Hitachi, Ltd.Hitachi Research Laboratories (72) Inventor Osamu Shimomura 3-1-1 Sachimachi, Hitachi City, Ibaraki Prefecture Inside Hitachi, Ltd.Hitachi Plant (72) Inventor Takehisa Kimura Ibaraki Prefecture Hitachi 1-1, Yokocho, Hitachi-shi, Hitachi, Ltd.Hitachi Plant (72) Inventor Hironori Shimogama 3-1-1, Yachimachi, Hitachi-City, Hitachi, Ibaraki Pref. Hitachi, Ltd.Hitachi Plant (72) Inventor Tsutomu Nishiba 3-13-10 Tamachi, Kawasaki-ku, Kawasaki-shi, Kanagawa Pref.

Claims (8)

[Claims] In a boration treatment for forming a compound of iron and boron on the surface of a steel member by covering the surface of the steel member with a boride treatment agent and heating the material, at least heating and / or cooling during the boration treatment is performed. A boring treatment method for a steel member surface, wherein a gas is externally introduced at one time. 2. The method for boring a steel member surface according to claim 1, wherein a gas is externally introduced from a temperature near the start of boring of the steel member during heating to a maximum processing temperature. 3. The method of boring a steel member surface according to claim 1, wherein a gas is externally introduced during cooling from a maximum processing temperature to a temperature near the end of boring of the steel member. 4. The method according to claim 1, wherein a gas is introduced from the outside from a temperature near the start of boring of the steel member during heating to a maximum processing temperature, and further, the boring of the steel member is reduced from the maximum processing temperature during cooling. A boring treatment method for a steel member surface, wherein a gas is introduced from the outside up to a temperature near the end. 5. The method according to claim 1, wherein an inert gas and / or a reducing gas is used as the gas to be introduced.
The boring treatment method for a steel member surface according to any one of the above. 6. The boride treating method for a steel member surface according to claim 1, wherein the boride treating agent contains a boron source and a boride accelerator. 7. A boron source comprising amorphous boron, ferroboron (FeB), boron carbide (B ₄ C) or borax (Na ₂ ).
B ₂ O ₇ ), ammonium chloride (NH4
7. The method for boring a steel member surface according to claim 6, wherein Cl) or boron borofluoride (KBF ₄ ) is used. 8. The boring treatment method for a steel member surface according to claim 1, wherein the steel member is a part in a bath used in a continuous hot-dip metal plating apparatus.