HEARTH ROLL FOR CONTINUOUS ANNEALING FURNACES, AND METHOD FOR MANUFACTURING SAME

Abstract

The present invention provides a hearth roll for heat treatment furnaces, which has excellent build-up resistance, has a hexavalent-chromium-free thermal spray coating film formed on the roll surface thereof and is safe for the environment. A method for manufacturing a hearth roll for continuous annealing furnaces includes a first step of applying an aqueous solution containing chromium phosphate onto a thermal spray coating film formed on the roll surface of a hearth roll or impregnating the thermal spray coating film with the aqueous solution; and a second step of burning the hearth roll.

Claims

1. A method for manufacturing a hearth roll for continuous annealing furnaces, comprising: a first step of applying an aqueous solution containing chromium phosphate onto a thermal spray coating film formed on a roll surface of the hearth roll or impregnating the thermal spray coating film with the aqueous solution; and a second step of burning the hearth roll, wherein when an amount of the aqueous solution is 100% by mass, a concentration of chromium phosphate is 5% by mass to 30% by mass, and a concentration of chromium is 1.5% by mass to 15% by mass.

2. (canceled)

3. The method for manufacturing a hearth roll for continuous annealing furnaces according to claim 1, wherein the burning is performed once in the second step.

4. A hearth roll for continuous annealing furnaces, the hearth roll including a thermal spray coating film on a roll surface, wherein pores of the thermal spray coating film are sealed by a burned product obtained by burning an aqueous solution containing chromium phosphate, and a coating film surface of the thermal spray coating film is covered with the burned product, and when an amount of the burned product is 100% by mass, a chromium concentration is 15% by mass to 45% by mass, and a remainder is an oxide containing phosphorus.

5. (canceled)

6. The hearth roll for continuous annealing furnaces according to claim 4, wherein the burned product which covers the coating film surface of the thermal spray coating film has a thickness of 2 to 20 μm.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0022] FIG. 1 is a schematic view of a measurement device for measuring MN values.

DESCRIPTION OF EMBODIMENTS

[0023] The present invention will be more specifically described with reference to examples.

[0024] (Hexavalent Chromium Measurement Test)

[0025] An aqueous solution of chromic acid (comparative examples) placed in a ceramic container was burned to generate burned powder. The contents of chromium and hexavalent chromium in 2.5 g of this burned powder were measured, and the concentration of hexavalent chromium was calculated. Similarly, an aqueous solution of chromium phosphate (examples) placed in a ceramic container was burned to generate burned powder. The contents of chromium and hexavalent chromium in 2.5 g of this burned powder were measured, and the concentration of hexavalent chromium was calculated. The burning time was 3 hours. The burning was performed once. The burning temperature was 410° C. in Comparative Example 1, 460° C. in Comparative Example 2, 500° C. in Comparative Example 3, 600° C. in Comparative Example 4, 700° C. in Comparative Example 5, 410° C. in Example 1, 500° C. in Example 2, 600° C. in Example 3, and 700° C. in Example 4. The contents of chromium and hexavalent chromium were measured by diphenyl carbazide absorptiometry. The used measurement device was U-2000 double-beam spectrophotometer manufactured by Hitachi. The burned powder of Comparative Example 1 was black. The burned powders of Comparative Examples 2 and 3 were dark green. The burned powders of Comparative Examples 4 and 5 were bright green. All of the burned powders of Examples 1 to 4 were bright green. When the concentration of hexavalent chromium was 1000 ppm (0.1% by mass) or less, the evaluation “good” was given indicating that the content of hexavalent chromium is low. When the concentration of hexavalent chromium was more than 1000 ppm (0.1% by mass), the evaluation “poor” was given indicating that the content of hexavalent chromium is high.

TABLE-US-00001 TABLE 1 COMPARATIVE COMPARATIVE COMPARATIVE COMPARATIVE COMPARATIVE EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 BURNING 410° C. 460° C. 500° C. 600° C. 700° C. TEMPERATURE Cr CONTENT 6400 8000 8000 7600 7500 mg/kg(=ppm) HEXAVALENT 6400 7700 7800 7200 7300 CHROMIUM CONTENT mg/kg(=ppm) EVALUATION poor poor poor poor poor RESULTS

TABLE-US-00002 TABLE 2 EXAM- EXAM- EXAM- EXAM- PLE 1 PLE 2 PLE 3 PLE 4 BURNING 410° C. 500° C. 600° C. 700° C. TEMPERATURE Cr CONTENT LESS LESS LESS LESS mg/kg(=ppm) THAN 5 THAN 5 THAN 5 THAN 5 HEXAVALENT LESS LESS LESS LESS CHROMIUM THAN 5 THAN 5 THAN 5 THAN 5 CONTENT mg/kg(=ppm) EVALUATION good good good good RESULTS

[0026] As illustrated in Table 1, all of Comparative Examples 1 to 5 were evaluated as “poor” indicating that the content of hexavalent chromium was high. As illustrated in Table 2, all of Examples 1 to 4 were evaluated as “good” indicating that the content of hexavalent chromium was low. It is noted that the detection limit of the measurement device used for measuring the concentrations was 5 ppm. All of the concentrations of Examples were less than the detection limit.

[0027] (Build-Up Resistance Test)

[0028] A prescribed sample was prepared, and measured for the MN value and the Fe attachment amount to evaluate build-up resistance. FIG. 1 is a schematic view of a measurement device for measuring MN values. As illustrated in FIG. 1, a thermal spray test piece 1 and a thermal spray test piece 1′ were stacked, and a build-up material 2 was placed therebetween (that is, between a thermal spray surface B and a thermal spray surface C). The build-up material 2 was also dispersed on a thermal spray surface A that is the upper surface of the thermal spray test piece 1. Then, reciprocating motions in an arrow X2 direction were performed while applying a load by pressing a half-moon-shaped roll 3 against the thermal spray surface A in an arrow X1 direction, thereby to evaluate the build-up states of the respective thermal spray surfaces A to C.

[0029] The test was performed with the temperatures and environmental conditions indicated in Table 3. The thermal spray surfaces A to C were formed with CoCrAlY-based (47% Co-17% Cr-10% Al-1% Y-25% Cr.sub.2O.sub.3 in terms of % by mass) cermet thermal spray coating films.

[0030] Each of the thermal spray surfaces A to C was evaluated on the basis of the total of points given according to the attachment state of build-up. When turning the thermal spray test pieces 1 and 1′ to the vertical direction caused the build-up material 2 to drop, three points were given indicating very favorable build-up resistance. When rubbing with gauze caused the build-up material 2 to drop, two points were given indicating mostly favorable build-up resistance. When rubbing with forceps caused the build-up material 2 to drop, one point was given indicating poor build-up resistance. When any of the above methods did not cause the build-up material 2 to drop, zero points were given indicating extraordinarily poor build-up resistance.

[0031] After the above-described reciprocating motions of the half-moon-shaped roll 3 were performed, the amounts of Fe attached to the thermal spray surfaces A to C were measured using a fluorescent X-ray measurement device, and the average value thereof was calculated. The test results are illustrated in Table 4. When the MN value was more than 7, and the Fe attachment amount was 2% by mass or less, the evaluation “very good” was assigned, indicating extraordinarily favorable build-up resistance. When the MN value was more than 4 and not more than 7, the evaluation “good” was assigned regardless of the Fe attachment amount, indicating favorable build-up resistance. When the MN value was 4 or less, the evaluation “poor” was assigned, indicating poor build-up resistance.

TABLE-US-00003 TABLE 3 ITEMS CONDITIONS TEMPERATURE(° C.) 1000° C. ATMOSPHERE 96% N.sub.2—4% H.sub.2 LOAD(kg) 10 BUILD-UP MATERIAL Fe.sub.3O.sub.4

TABLE-US-00004 TABLE 4 Fe THERMAL COMPOSITION Cr CONTENT THICKNESS ATTACH- SPRAY OF AQUEOUS OF BURNED OF BURNED MENT COATING SOLUTION FREQUENCY PRODUCT PRODUCT MN AMOUNT EVALU- FILM (% by mass) OF BURNING (% by mass) (μm) VALUE (% by mass) ATION EXAMPLE 5 CoCrAlY- 8% AQUEOUS ONCE 45% 8 8.5 0.90% very BASED SOLUTION OF good CERMETS CHROMIUM PHOSPHATE 15% AS Cr EXAMPLE 6 CoCrAlY- 10% AQUEOUS ONCE 42% 13 9.0 0.50% very BASED SOLUTION OF good CERMETS CHROMIUM PHOSPHATE + 0.05% SURFACTANT 13% AS Cr EXAMPLE 7 CoCrAlY- 5% AQUEOUS ONCE 38% 3 8.9 0.60% very BASED SOLUTION OF good CERMETS CHROMIUM PHOSPHATE + 0.05% SURFACTANT 11% AS Cr EXAMPLE 8 CoCrAlY- 8% AQUEOUS ONCE 43% 10 7.9 0.60% very BASED SOLUTION OF good CERMETS CHROMIUM PHOSPHATE 15% AS Cr EXAMPLE 9 CoCrAlY- 30% AQUEOUS ONCE 16% 18 8.1 0.50% very BASED SOLUTION OF good CERMETS CHROMIUM PHOSPHATE 5% AS Cr EXAMPLE 10 CoCrAlY- 30% AQUEOUS ONCE 23% 19 8.8 0.50% very BASED SOLUTION OF good CERMETS CHROMIUM PHOSPHATE + 0.05% SURFACTANT 9% AS Cr REFERENCE CoCrAlY- 3% AQUEOUS ONCE 36% 1 4.6 5.30% good EXAMPLE 1 BASED SOLUTION OF CERMETS CHROMIUM PHOSPHATE 10% AS Cr REFERENCE CoCrAlY- 10% AQUEOUS ONCE 12% 9 5.0 4.50% good EXAMPLE 2 BASED SOLUTION OF CERMETS CHROMIUM PHOSPHATE + 0.0001% SURFACTANT 0.5% AS Cr REFERENCE CoCrAlY- 10% AQUEOUS ONCE 14% 13 5.0 3.70% good EXAMPLE 3 BASED SOLUTION OF CERMETS CHROMIUM PHOSPHATE + 1.1% SURFACTANT 0.9% AS Cr REFERENCE CoCrAlY- 40% AQUEOUS ONCE 51% 23 4.3 5.30% good EXAMPLE 4 BASED SOLUTION OF CERMETS CHROMIUM PHOSPHATE + 0.05% SURFACTANT 16% AS Cr COMPARATIVE CoCrAlY- 15% AQUEOUS TWICE 26% 13 3.3 5.10% poor EXAMPLE 5 BASED SOLUTION OF CERMETS CHROMIUM SULFATE 20% AS Cr COMPARATIVE CoCrAlY- 23% AQUEOUS TWICE 31% 16 3.1 5.90% poor EXAMPLE 6 BASED SOLUTION OF CERMETS CHROMIUM CHLORIDE 26% AS Cr COMPARATIVE CoCrAlY- 17% AQUEOUS TWICE 23% 15 3.4 6.30% poor EXAMPLE 7 BASED SOLUTION OF CERMETS CHROMIUM NITRATE 8% AS Cr COMPARATIVE CoCrAlY- 15% AQUEOUS TWICE 56% 10 3.0 4.60% poor EXAMPLE 8 BASED SOLUTION OF CERMETS CHROMIC ACID 9% AS Cr COMPARATIVE CoCrAlY- 24% AQUEOUS TWICE 53% 12 3.9 6.80% poor EXAMPLE 9 BASED SOLUTION OF CERMETS CHROMIC ACID 15% AS Cr COMPARATIVE CoCrAlY- 15% AQUEOUS FOUR 55% 8 4.2 5.50% good EXAMPLE 10 BASED SOLUTION OF TIMES CERMETS CHROMIC ACID 9% AS Cr COMPARATIVE CoCrAlY- 24% AQUEOUS FOUR 54% 17 4.3 4.90% good EXAMPLE 11 BASED SOLUTION OF TIMES CERMETS CHROMIC ACID 15% AS Cr

[0032] In Examples 5 to 10, the MN value was more than 7, and the Fe attachment amount was 2% by mass or less. Therefore, the build-up resistance was evaluated as “very good.” That is, since Examples 5 to 10 satisfied a chromium phosphate concentration of 5% by mass to 30% by mass and a chromium concentration of 1.5% by mass to 15% by mass, which are preferable conditions of the present invention, the evaluation for build-up resistance became “very good.” In Reference Example 1, the Fe attachment amount increased due to the low chromium phosphate concentration. However, the MN value was more than 4. Therefore, the build-up resistance was evaluated as “good.” In Reference Example 2, since the low surfactant concentration led to the reduced permeation of the aqueous solution into the thermal spray coating film, the Fe attachment amount increased. However, the MN value was more than 4. Therefore, the build-up resistance was evaluated as “good.” In Reference Example 3, since the concentration of a surfactant was excessively high, the surfactant was carbonized and dropped off after burned, and the Fe attachment amount increased. However, the MN value was more than 4. Therefore, the build-up resistance was evaluated as “good.” In Reference Example 4, although preferable conditions of the present invention were not satisfied, the MN value was more than 4. Therefore, the build-up resistance was evaluated as “good.” In Comparative Examples 5 to 7, an aqueous solution of trivalent chromium salt was used instead of an aqueous solution of chromium phosphate. Therefore, sealing effect to the thermal spray coating film was low, and the Fe attachment amount increased. The MN value was 4 or less. Therefore, the build-up resistance was evaluated as “poor.” In Comparative Examples 8 to 9, the MN value was 4 or less. Therefore, the build-up resistance was evaluated as “poor”.

[0033] As apparent from the above-described tests, the build-up resistance was improved by the pores-sealing treatment with the aqueous solution of chromium phosphate compared to the case by a known pores-sealing treatment with an aqueous solution of chromic acid. That is, it was found that the hearth roll according to the invention of the present application is safe for the environment in terms of the absence of hexavalent chromium, and has extraordinarily excellent build-up resistance. Furthermore, it was found that the build-up resistance is drastically improved by satisfying preferable conditions of the present invention (chromium phosphate concentration: 5% by mass to 30% by mass, chromium concentration: 1.5% by mass to 15% by mass). It is noted that the present inventor conducted tests similar to the above-described tests by using MnO instead of Fe.sub.3O.sub.4 as the build-up material 2, and confirmed that results mostly similar to the above-described results were obtained.

[0034] Furthermore, in the examples, extraordinarily high build-up resistance was obtained by performing the burning treatment (that is, the applying step and the burning step) once. On the other hand, in Comparative Examples 5 to 9, sufficient build-up resistance was not obtained even by performing the burning treatment twice. Also, as illustrated in Comparative Examples 10 toll, the burning treatment had to be performed four times for raising the evaluation for build-up resistance from “poor” to “good.”

REFERENCE SIGNS LIST

[0035] 1, 1′: thermal spray test piece [0036] 2: build-up material [0037] 3: half-moon-shaped roll