JP2005206863A - Cermet powder, and furnace roll having excellent build-up resistance and oxidation resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide cermet powder having excellent build-up resistance and oxidation resistance even in the case a large quantity of high-tensile strength steel is subjected to a heat treatment. <P>SOLUTION: Regarding the cermet powder, ceramic powder containing, to the total amount of the cermet powder, 10 to 20 mass% Cr<SB>23</SB>C<SB>6</SB>and 10 to 20 mass% Y<SB>2</SB>O<SB>3</SB>and ≤30 mass% in total is used, and heat-resistant alloy powder containing, to the total amount of the cermet powder, 4 to 6 mass% Al, 12 to 16 mass% Cr and ≤1 mass% Y, and the balance Co and/or Ni is used. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is a heat treatment furnace such as a continuous annealing furnace for steel sheets, and a cermet powder suitable for application to an in-furnace roll used for in-furnace conveyance of a material to be treated and build-up resistance obtained by spraying the cermet powder on the surface. And an in-furnace roll excellent in oxidation resistance.

In a continuous annealing line for steel strip, many heat-resistant rolls are used to support the traveling steel strip in a furnace. However, due to continuous use for a long period of time, oxides such as Mn and Si concentrated on the surface of the steel strip adhere to the surface of these in-furnace rolls, and so-called build-up is formed.
FIG. 1 shows a buildup 2 formed on the surface of the in-furnace roll 1, and the buildup 2 is formed in parallel in the circumferential direction along the passing portion of the steel strip 3.

  When such build-up occurs, quality defects such as steel strip surface defects occur. In severe cases, the operation is stopped and the roll surface is cleaned with a sheet material, and in more severe cases the furnace is opened. Therefore, it becomes necessary to perform grinding care or roll replacement on the roll surface.

Therefore, conventionally, various techniques for forming a protective film by thermal spraying on the roll surface have been proposed in order to prevent the occurrence of build-up as described above.
For example, Patent Document 1 proposes a technique in which a cermet sprayed material composed of ₅ to 20 mass% Cr ₂ O ₃ —Al ₂ O ₃ and 95 to 80 mass% CoNiCrAlY is sprayed onto the roll surface.
Patent Document 2 proposes a technique in which a cermet sprayed material composed of 51 to 95 vol% Al ₂ O ₃ and MCrAlY (M is Fe, Ni or Co) is sprayed on the roll surface.
Patent Document 3 proposes a technique in which chromium oxide is coated on the surface of a cermet sprayed coating made of 30 to 80 mass% ZrSiO ₄ and MCrAlY (M is Fe, Ni or Co).
Patent Document 4 proposes a technique for forming a multilayer coating of Al ₂ O ₃ —MgO and a binding metal, with Al ₂ O ₃ —MgO as the uppermost layer.
Patent Document 5 proposes a technique in which a mechanical alloy composite powder of 5 to 50 vol% boride and MCrAlY (M is Fe, Ni or Co) is sprayed on the roll surface.
Patent Document 6 contains 1 to 60% by volume (substantially 25% by volume or more) of borides such as CrB ₂ , ZrB ₂ , WB, and TiB ₂ , and Cr ₃ C ₂ , TaC, WC, There has been proposed a cermet coating containing 5 to 50% by volume (substantially 15% by volume or more) of carbides such as ZrC, TiC and NbC, with the balance being substantially made of metal (MCrAlY).
Patent Document 7 contains 3 to 8 mass% of Al in MCrAlY, 16 to 25 mass% of Cr, and 0.1 to 1 mass% of Y with respect to the total amount of cermet powder, and the balance is made of Co and / or Ni, and As a ceramic powder, a cermet powder for thermal spray coating in which 1 to 5 mass% of boride and / or 5 to 10 mass% of carbide is contained with respect to the total amount of cermet powder has been proposed. In the above, MCrAlY usually refers to a heat resistant alloy to which an appropriate amount of Cr, Al, Y is added based on at least one of Fe, Ni, Co.

JP-A-2-270955 Japanese Unexamined Patent Publication No. 63-199857 JP-A 63-47379 JP-A-60-56058 JP-A-3-226552 JP 7-11420 A Japanese Patent Publication No. 1-34866 (WO01-034866)

In the techniques disclosed in Patent Documents 1 to 6 described above, a significant reduction in build-up is recognized in heat treatment of general ordinary steel, and high strength steel materials (so-called high-tensile steel: usually 340 MPa for cold-rolled steel sheets). As described above, hot-rolled steel sheets usually have a tensile strength of 440 MPa or more), and there was no problem if the amount of treatment was small.
However, with the recent increase in high-tensile steel, these technologies are no longer effective for build-up.

  In this regard, the technology disclosed in Patent Document 7 was developed to cope with a case where a large amount of high-tensile steel is heat-treated based on the technologies of Patent Documents 1 to 6. Although the build-up property has been improved, it has been found that skin roughness and micro-peeling (chipping) occur due to oxidation of the roll surface with use. It has also been found that the roll life is shorter compared to the case of processing only conventional ordinary plain steel. In particular, it was recognized that the contained boride-based ceramics tend to oxidize with long-term use.

  The present invention advantageously solves the above problems, and even when high-tensile steel is heat-treated in large quantities, a cermet powder exhibiting excellent build-up resistance and oxidation resistance is formed on the surface of the cermet powder. It is intended to be proposed with sprayed in-furnace rolls.

The inventors have made extensive studies to obtain sufficient oxidation resistance even in heat treatment of high-tensile steel, and to obtain sufficient oxidation resistance that is unlikely to cause rough skin and micro-peeling over a long period of time. As a result, the intended purpose is advantageously achieved by reducing Al in MCrAlY, which has been generally used in the past, and by using Cr ₂₃ C ₆ instead of Cr ₃ C ₂ as a carbide in the ceramic. And gained knowledge.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. A cermet powder comprising a mixed powder of a ceramic powder and a heat-resistant alloy powder, wherein the ceramic powder is 10 to 20 mass% Cr ₂₃ C ₆ and 10 to 20 mass% Y ₂ O ₃ with respect to the total amount of the cermet powder, and Total of both: containing 30 mass% or less, while the heat-resistant alloy powder contains 4 to 6 mass% Al, 12 to 16 mass% Cr and 1 mass% or less Y with respect to the total amount of the cermet powder, the balance being Co And / or cermet powder comprising Ni.

2. Excellent build-up resistance and oxidation resistance, characterized by using the cermet powder described in 1 above and forming a sprayed layer with a coating thickness of 70 to 120 μm on the surface of the in-furnace roll of the heat treatment furnace. In-furnace roll.

  According to the present invention, particularly in heat treatment of high-tensile steel, it is possible to provide an in-furnace roll having excellent build-up resistance and excellent oxidation resistance, and in a production line having a heat treatment furnace such as a continuous annealing furnace. As a result of the time loss associated with roll maintenance and roll change, the line stop time can be shortened (expansion of operation time) and the cost required for roll maintenance can be reduced.

Hereinafter, the present invention will be described in detail.
Now, the cermet powder of the present invention is composed of a mixed powder of ceramic powder and heat-resistant alloy powder (MCrAlY). Here, MCrAlY refers to a heat-resistant alloy to which an appropriate amount of Cr, Al, Y is added based on at least one of Fe, Ni, and Co as described above.

First, the ceramic powder according to the present invention will be described.
The present invention has the greatest advantage in that Cr ₂₃ C ₆ is used as a carbide in the ceramic powder instead of the conventionally used Cr ₃ C ₂ .
Although this is a problem in terms of build-up resistance, it is possible to reduce Al and Cr, which had been forced to contain a relatively large amount in the protective film from the viewpoint of improving oxidation resistance. If Cr ₂₃ C ₆ instead of Cr ₃ C ₂ is used as the carbide, the oxidation resistance will not be reduced, so that it is possible to improve the build-up resistance without causing a reduction in oxidation resistance. This is based on what was found by the inventors' research.

The reason is considered as follows.
That is, Cr ₃ C ₂ transforms to Cr ₂₃ C ₆ at a high temperature of 800 ° C. or more, but at that time, tensile stress is generated in the film due to volume change, and the film peels off from the roll surface. As a result of partial occurrence of the portion, the oxidation resistance is lowered. In this regard, if Cr ₂₃ C ₆ is used as a carbide from the beginning, the volume change based on the high-temperature transformation as described above does not occur, so that the oxidation resistance does not deteriorate.

Here, if the amount of Cr ₂₃ C ₆ carbide added is less than 10 mass% with respect to the total amount of cermet powder, the above-mentioned effect of improving oxidation resistance is poor. On the other hand, if it exceeds 20 mass%, the film tends to become porous, so peeling It becomes easy to do. Therefore, the amount of Cr ₂₃ C ₆ added is limited to a range of 10 to 20 mass% with respect to the total amount of cermet powder.
Y ₂ O ₃ is an element useful for suppressing the outward diffusion of Al. However, if the content is less than 10 mass% with respect to the total amount of the cermet powder, the above effect is poor. If it exceeds 20 mass%, the film tends to be porous, so the amount of Y ₂ O ₃ added is limited to the range of 10 to 20 mass% with respect to the total amount of cermet powder.
If the amount of these high melting point ceramics is too large, unmelted ceramic powder remains and it becomes difficult to form a film, so Cr ₂₃ C ₆ and Y ₂ O ₃ must be limited to 30 mass% or less in total. is there.
In addition to the above-mentioned Cr ₂₃ C ₆ and Y ₂ O ₃ , the ceramic can tolerate mixing of Al ₂ O ₃ , Cr ₂ O _3, etc., in a small amount (1.0 mass% or less).

Next, the heat resistant alloy powder (MCrAlY) according to the present invention will be described.
Conventionally, it has been considered that Al in MCrAlY is required to improve build-up resistance and oxidation resistance by containing about 5 to 12 mass% in a protective film on the surface.
However, in the case of heat treatment of high-tensile steel, Al oxide in the protective film reacts with Mn concentrated on the surface of the steel strip to form a strong build-up (spinel type Mn ₂ AlO ₃ ) and grow. There was found.
Therefore, the inventors have conducted intensive research to solve the above problems.

  As a result, it was found that when high tensile steel is used, Al is preferably reduced as much as possible, and 4 to 6 mass% is an appropriate amount. That is, Al is an element that reacts most easily with Mn among the film components. When the Al content in the protective film exceeds 6 mass%, the reaction with Mn becomes apparent when the Al is not uniformly dispersed. On the other hand, if it is less than 4 mass%, the oxidation resistance in long-term use becomes insufficient, which causes the skin to become rough and chipped. Therefore, the amount of Al in the protective film, that is, the amount of Al relative to the total amount of cermet powder must be limited to a range of 4 to 6 mass%.

Next, since Cr in MCrAlY is an element that is likely to react with Mn next to Al, if contained in a large amount, it adversely affects build-up resistance. Therefore, it is desirable to reduce as much as possible from this viewpoint. However, Cr, on the other hand, is an extremely useful element in terms of oxidation resistance, and there is a concern about reduction in oxidation resistance when the amount of Cr is reduced.
Here, as described above, according to the inventors' research, regarding oxidation resistance, by using Cr ₂₃ C ₆ instead of Cr ₃ C ₂ conventionally used as a carbide in ceramics, It turns out that it can be fully supplemented.
Therefore, as a result of examining the Cr content based on this point, it was found that 12 to 16 mass% is appropriate. That is, when the Cr content in the protective film exceeds 16 mass%, the build-up resistance is lowered, whereas when it is less than 12 mass%, the oxidation resistance is lowered even when Cr ₂₃ C ₆ is added as a carbide. Therefore, the Cr amount in the protective film, that is, the Cr amount relative to the total amount of the cermet powder is limited to a range of 12 to 16 mass%.

  Next, Y is a useful element having an action of improving the bondability between the ceramic and the heat-resistant alloy and strengthening the protective film. However, if it is added in excess of 1 mass%, the peel strength of the film will be reduced. Therefore, the Y amount in the protective film, that is, the Y amount relative to the total amount of the cermet powder is set to 1 mass% or less. The lower limit of the Y amount is not particularly defined, but is preferably 0.1 mass% or more, and more preferably 0.5 to 1.0 mass%.

Further, the remainder of the heat-resistant alloy is made of Co or Ni or an alloy thereof in order to ensure heat resistance and oxidation resistance. In addition, the case where inevitable impurities are mixed is also within the scope of the present invention.
From the viewpoint of the adhesion of the thermal spray coating, it is somewhat advantageous that the balance is Co or a Co—Ni alloy.

Next, a thermal spraying method using the above cermet powder will be described.
The above ceramic powder and heat-resistant alloy powder are mixed to obtain a mixed powder having a particle size of about 10 to 100 μm.
Next, spraying is performed on the surface of the in-furnace roll made of heat-resistant cast steel or the like. However, the spraying method and the spraying conditions are not particularly limited, and may be performed by conventionally known methods and conditions.
For example, an explosive spraying method (D-GUN method), a high-speed gas combustion spraying method (D-JET method), a gas plasma spraying method, etc. are advantageously adapted as the spraying method.

  Further, if the thickness of the sprayed coating is less than 70 μm, a sufficient durability life cannot be obtained. On the other hand, if the thickness is more than 120 μm, peeling due to thermal fatigue tends to occur. Therefore, the thickness of the sprayed coating is preferably 70 to 120 μm.

Mixing ceramic powder and heat-resistant alloy powder with various addition amounts of Cr ₂₃ C ₆ and Y ₂ O _{3 with} the component ratios shown in Table 1 by mixing method, the particle size is 50-100 μm Cermet powder was prepared. The content of the heat-resistant alloy component and the content of Cr ₂₃ C ₆ and Y ₂ O ₃ are both ratios relative to the total amount of cermet powder.
Each of the above cermet powders was sprayed on the surface of a SUS substrate having dimensions of 25 mm × 25 mm × 10 mm by an explosive spraying method (D-GUN method) to form a sprayed coating having a thickness of 100 μm. Next, the surface of the sprayed coating was finished by grinding.
Two steel plates with sprayed coating thus obtained were prepared, and as shown in FIG. 2, with the sprayed surface facing inward, high-tensile steel (Mn: 1.2 mass%, Si: 0.05 mass%, Al: 0.04 mass%) Was included) to obtain one test piece.
In the figure, reference numeral 4 is a SUS substrate, 5 is a sprayed coating, and 6 is high-tensile steel.

A sintering test was performed in which the test piece thus prepared was held at 900 ° C. in an experimental furnace in an atmosphere of 3% by volume H ₂ -97% by volume N ₂ for 180 hours. After carrying out the sintering test in the experimental furnace, the test piece is taken out, the high-tensile steel is removed, the surface of the sprayed surface is quantified with EDX (energy dispersive X-ray analyzer), and the high-tensile steel with SEM (scanning electron microscope). A photograph of the cross section of the sprayed coating of the part from which was removed was taken.
At the same time, after preparing a 100 μm thick spray coating on the surface of a SUS substrate with dimensions of 50 mm × 50 mm × 10 mm by the explosive spraying method (D-GUN method), a test piece whose surface is ground is prepared. Then, after being heated in an experimental furnace at 1000 ° C. for 30 seconds, it was taken out and subjected to a peel resistance test in which it was cooled with water.

The results of examining the build-up resistance, oxidation resistance, and peel resistance by the above-mentioned experiment are also shown in Table 1.
The occurrence of buildup was evaluated by the amount of Mn measured by EDX. Broadly speaking, “Large” in Table 1 indicates that the Mn amount is 30 mass% or more, “Middle” indicates that the Mn amount is 15 mass% or more and less than 30 mass%, “Small” indicates that the Mn amount is 5 mass% or more and less than 15 mass%, and “Minimum” "Is the case where the amount of Mn is less than 5 mass%.
In addition, the oxidation resistance is evaluated by the thickness of the oxidized scale of the surface measured by SEM. In Table 1, “Large” means the average scale thickness is 20 μm or more, “Medium” means the average scale thickness is 10 μm or more and less than 20 μm, “Small” means that the scale average thickness is 5 μm or more and less than 10 μm, and “Minimum” means that the scale average thickness is less than 5 μm.
Further, the peel resistance was evaluated by the number of times until the film was peeled off, with heating and cooling in the above peel resistance test as one cycle.

  As shown in the table, when using the cermet powder according to the present invention, the occurrence of buildup is slight, the amount of oxide scale is small, and the number of times until peeling is 40 times or more, It can be seen that there is a significant improvement. Thus, it was confirmed that by using the cermet powder of the present invention, a sprayed coating excellent in oxidation resistance as well as build-up resistance can be obtained.

Next, the cermet powder according to the present invention (Al: 5 mass%, Cr: 16 mass% and Y: 0.8 mass% with respect to the total amount of the cermet powder, with the balance being Cr heat-resistant alloy powder material, Cr ₂₃ C ₆ 15 mass% and cermet powder mixed with 15 mass% of Y ₂ O ₃ ) are sprayed onto the surface of the in-furnace roll (800mmφ × 2000mmL) of continuous annealing line using D-GUN method (film thickness: 100 μm) The in-furnace roll of the present invention was prototyped and evaluated with an actual machine.
For comparison, a conventional cermet powder (Al: 6 mass%, Cr: 25 mass%, and Y: 0.8 mass% with respect to the total amount of cermet powder, the balance being a heat-resistant alloy powder of Co and boride) The cermet powder containing ₃ mass% ZrB ₂ and 14 mass% Y ₂ O ₃ was sprayed onto the surface of the in-furnace roll (800 mmφ × 2000 mmL) using the D-GUN method, as in the present invention example (film thickness: 100 μm) to obtain a conventional in-furnace roll.

Line speed (steel strip conveying speed): Max 500 mpm, Furnace temperature: Max 950 ° C., Furnace atmosphere: 3 vol% H ₂ -97 vol% N ₂ A conventional furnace roll was applied. This line is a so-called sheet CAL that performs high-tensile steel processing for 100,000 km / month or more.
As a result, with conventional rolls, micro-peeling gradually began to appear after 30 months, and was forced to replace the roll after 42 months, whereas when using the roll of the present invention, 48 months No build-up occurred even after the lapse of time, and no micro-detachment that was attributed to film oxidation was observed.

It is a front view of the roll in a furnace where buildup occurred. 2 is a cross-sectional view of a test piece used in Example 1. FIG.

Explanation of symbols

1 In-furnace roll 2 Build-up 3 Steel strip 4 SUS substrate 5 Thermal spray coating 6 High-tensile steel

Claims (2)

A cermet powder comprising a mixed powder of a ceramic powder and a heat-resistant alloy powder, wherein the ceramic powder is 10 to 20 mass% Cr ₂₃ C ₆ and 10 to 20 mass% Y ₂ O ₃ with respect to the total amount of the cermet powder, and Total of both: containing 30 mass% or less, while the heat-resistant alloy powder contains 4 to 6 mass% Al, 12 to 16 mass% Cr and 1 mass% or less Y with respect to the total amount of the cermet powder, the balance being Co And / or cermet powder comprising Ni. Using the cermet powder according to claim 1, a sprayed layer having a coating thickness of 70 to 120 μm is formed on the surface of an in-furnace roll of a heat treatment furnace. Excellent furnace roll.

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