JPS59226113A - Hearth roll for heat treating furnace having excellent resistance to build-up and exfoliation - Google Patents

Abstract

PURPOSE:To form a coating layer which has excellent resistance to exfoliation and prevents formation of build-up on the surface of a hearth roll for heat treatment of a steel plate by spraying thermally a binding metal layer, a mixed layer of mullite and a binding metal and a mullite layer on the surface of said hearth roll. CONSTITUTION:A hearth roll in a heat treating furnace for a steel plate is formed by spraying thermally a binding metal layer consisting of an Ni or Co alloy having >=0.1mm. thickness on the suface of said roll. A mixed layer of said binding metal and mullite is thermally sprayed thereon in >=2 layers at >=0.1mm. thickness on the surface thereof. The mixing ratio of the mullite with respect to the binding metal is made larger toward the upper layer. A layer of mullite is finally thermally sprayed again at >=0.1mm. thickness. The hearth roll having the coating layer which is hardly exfoliated by thermal expansion owing to the presence of the binding metallic layer and the plural mixed layers and is free from formation of build-up by oxide of Fe, etc. is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nozzle roll for a heat treatment furnace for steel plates, and particularly to a nozzle roll for a heat treatment furnace that is less likely to cause build-up and less likely to have a thermally sprayed coating layer peeled off.

Generally, a material such as 80H-22, which is a high Cr--Ni type material and has excellent creep strength at high temperatures, is used as a nozzle roll for a heat treatment furnace. However, in the case of hearth rolls made of these materials, civil azo occurs on the roll surface due to various oxides etc. during the process of conveying the steel plate.The build-up that occurs on the roll surface damages the surface of the steel plate and reduces the commercial value of the steel plate. significantly decreases. Therefore, in order to prevent build-up from occurring on rolls, research has been carried out on roll materials and various surface treatments. Currently, a countermeasure against this problem is to provide a surface coating layer by thermal spraying. However, thermal sprayed materials that are difficult to pill and crack have poor peeling resistance, while materials that are excellent in peeling resistance have poor pill and crack resistance.Currently, no material has been found that satisfies both requirements.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and provide a nozzle roll for a heat treatment furnace that is less likely to cause build-up and has an excellent peeling resistance of the coating layer.

The coating layer of a hearth roll for a heat treatment furnace must not allow oxides such as scale or their reduction products to adhere to the surface, and must be dense and not easily peeled off. This invention was selected from among many materials' pill and horn resistance tests.
A203.5tO2) was selected for thermal spraying, and a mixed layer of mullite and a bonding metal was formed as an intermediate layer to improve the peeling resistance of the coating layer.

That is, the gist of the present invention is to have a thermally sprayed intermediate layer made of a mixture of mullite and a bonding metal and a thermally sprayed uppermost layer of mullite on the surface of the roll base material via a bonding metal layer, is for heat treatment furnaces with excellent build-up resistance and peeling resistance, characterized by consisting of at least two layers in which the concentration of mullite is gradually increased from the surface side of the roll base material to the top layer of thermal spray coating of mullite. It's in Hearthroll.

The present invention will be explained in detail below.

Nlite has excellent adhesion resistance with metal oxides, etc., and also has excellent heat resistance. It also has excellent thermal spraying properties and forms an extremely dense thermal spray coating layer. According to the present invention, the mullite thermal spray coating layer applied to the roll surface via the bonding metal layer and the intermediate layer preferably has a thickness of 0.1 tug or more from the viewpoint of durability.

The roll of the present invention is constructed as follows in order to improve the peeling resistance of the mullite vaporized coating layer.

As for the thermal spraying method, the plasma method is recommended from the viewpoint of the density of the mullite thermal spray coating layer. This mullite spray coating layer is a bonding metal layer sprayed onto the roll surface, such as Ni.
However, since the base material and bonding metal have different coefficients of thermal expansion from mullite, peeling is likely to occur due to thermal shock. In the roll of the present invention, at least two intermediate layers consisting of a mixture of a bonding metal and mullite are thermally sprayed in order to eliminate such difficulties and to further improve the peeling resistance. At this time, the influence of the difference in coefficient of thermal expansion is reduced by changing the concentration of mullite in this intermediate layer so as to increase stepwise from the roll surface side to the mullite spray coating layer side. This intermediate layer has at least two layers and a thickness of 0.1+
It is preferable that it be equal to or higher than as. In addition, when spraying an intermediate layer consisting of a mixture of mullite and a bonding metal, in order to avoid uneven mixing due to the difference in specific gravity between the two, it is not supplied in a mixed state in advance; It is desirable to vary the supply amount by supplying from separate containers so that the mixing ratio is as follows.

Using the thermal spraying method described above, a roll of the present invention having the following configuration was manufactured and subjected to a real root test for build-up resistance. The results are shown together with the actual machine test results for the conventional roll and comparative roll. The working atmosphere temperature is 750℃.
Tested for several months.

Table-1 Actual machine test results (Note: The bonding metal is a 6frht-containing Ni-Cr alloy) As is clear from Table-1, the conventional roll and comparative roll 1,
In contrast to No. 2, pilled horns were formed to varying degrees, whereas pilled horns were not formed at all in the roll of the present invention having the mullite sprayed coating layer.

Next, as a test material, a bonding metal sprayed layer with a thickness of 0.1 tan was applied as the first layer on the surface of a steel pipe with an outer diameter of 120 m, as in the case of the roll of the present invention used in the actual machine test described above.
The 2nd to 4th layers each consist of mullite whose mullite concentration is gradually changed from 20 to 70q6 and a bonding metal.
．． A thermally sprayed mixed layer with a thickness of 1 mm is applied, and a mullite thermally sprayed coating layer (thickness of 0.1 tan) is applied on the outermost surface by plasma spraying.
Same as the roll equivalent material of the present invention obtained by applying <120 mn
The first layer is 0.1 mm thick on the surface of the steel pipe by regular thermal spraying.
Using a bonding metal sprayed layer of
A thermal shock test of heating and cooling was conducted by heating the upper part of the test material with a gas flame and simultaneously cooling the lower part with water.

As a result, in the case of the roll phased material of the present invention, no peeling of the mullite spray coating layer occurred even after 8 hours;
For the comparison material, after 1 hour, parts of the zirconia sprayed coating layer peeled off in 3 places with a maximum size of 5φ, and after 3 hours, the test was discontinued because peeling occurred on the entire surface.

As detailed above, according to the present invention, it is possible to provide a hearth roll for heat treatment furnaces which is extremely excellent in pill lag resistance and peeling resistance, and therefore has great industrial benefits.

Claims (1)

[Claims] A thermally sprayed intermediate layer made of a mixture of mullite and a bonding metal and a thermally sprayed top layer of mullite are formed on the surface of the roll base material via a bonding metal layer, and the thermally sprayed intermediate layer is coated with mullite from the surface side of the roll base material. A hearth roll for a heat treatment furnace having excellent build-up resistance and peeling resistance, characterized by comprising at least two layers in which the concentration of mullite is gradually increased toward the top layer of the thermal spray coating.