JPS62253731A - Method for preventing deterioration of hot-treating roll - Google Patents

Abstract

PURPOSE:To prevent the deterioration in the roll pickup resistance of a graphite roll by controlling the gaseous CO concn. in the furnace atmosphere to such a concn. as to prevent the reduction reaction of Cr2O3 when the graphite roll contg. Cr2O3 in the pore is used in a non-decarbonizing atmosphere. CONSTITUTION:The reduction reaction of Cr2O3 by C in the graphite roll is shown by equation I. When the gaseous CO partial pressure (atm) in the furnace atmosphere is designated as PCO and the furnace temp. ( deg.K) as T, equation II is obtained as the equilibrium equation. The gaseous CO concn. in the furnace atmosphere is controlled in this invention to a concn. higher than the value obtained by equation II, and the proceeding of the reduction reaction of Cr2O3 shown by equation I can be prevented. For example, when the furnace temp. is controlled to 700-1,000 deg.C, the gaseous CO concn. in the furnace atmosphere is adjusted to a value higher than that shown by table I to prevent the reduction reaction of Cr2O3. As a result, the deterioration in the roll pickup resistance of the graphite roll can be prevented.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is directed to a heat treatment furnace, particularly for heat treating various metal plates, such as tin plates, electrical steel sheets, etc., in a reducing atmosphere, that is, a non-decarburizing atmosphere. This invention relates to a method for preventing deterioration of graphite rolls used in furnaces.

(Prior art) Conventionally, in a heat treatment furnace having an in-furnace roll, 5ooc
When electromagnetic steel sheets and stainless steel sheets that require high-temperature annealing as described above are continuously processed, deposits from the steel sheets that come into contact with the rolls accumulate on the roll surfaces, which causes indentations in the steel sheets. In order to prevent this so-called roll pickup, as shown in Japanese Patent Publication No. 47-13165, a chromic acid impregnated graphite roll, that is, a graphite roll, is treated with chromic acid anhydride, heavy Rolls are used in which chromium oxide is present in the pores of graphite rolls by immersing them in aqueous solutions such as ammonium chromate, or by injecting these aqueous solutions into graphite rolls under reduced pressure or pressure. .

As described in the above-mentioned publication, this chromic acid-impregnated graphite roll has excellent roll pick-up resistance in a weakly oxidizing atmosphere such as H, -N, -H, O, etc., that is, in a decarburizing atmosphere. show.

By the way, conventionally, in the production of high-grade non-oriented electrical steel sheets, molten steel whose composition has been adjusted is made into a slab by a continuous casting method or an ingot blooming method, followed by hot rolling and cold rolling to determine the final thickness and k. Then, the final annealing is performed in a weakly oxidizing atmosphere (decarburizing atmosphere) with a high dew point to reduce the C content of the product to 0.
．． A series of processes were used to keep the amount below 0.03%.

However, in recent years, as shown in Japanese Patent Publication No. 58-30368, the amount of C is reduced to the value required by the final product (C≦0.003%) during the steelmaking stage, and the final annealing is performed using H. , -N, a method for producing non-oriented electrical steel sheets in a reducing atmosphere has been put into practical use.

Furthermore, in the production of low-grade non-oriented electrical steel sheets, the decarburization level at the steel manufacturing stage, for example, the C content is 0.0200 to 0.00.
When the steel is cast or ingot-formed at 20% and processed according to a known combination of hot rolling, cold rolling, and annealing, decarburization is often not performed in the annealing process.

On the other hand, in the production of grain-oriented electrical steel sheets, the composition is adjusted to a specific range of C content, e.g. Reduce the amount of C in the atmosphere to 0.003% or less.

As described above, it has become necessary to switch the annealing atmosphere to decarburizing or non-decarburizing depending on the material to be treated and the intended use.

(Problems to be Solved by the Invention) Therefore, when the chromic acid impregnated graphite roll, which had conventionally been suitably used in a decarburizing atmosphere, was used in a non-decarburizing atmosphere, many pickups occurred. A high percentage of the pickups were caused by deep scratches on the steel sheet, which would not have been seen in conventional decarburizing atmospheres.

The present invention shows that even when a graphite roll sealed with chromium oxide present in its pores is used in a furnace in a non-decarburizing atmosphere, it is more effective than when it is used in a decarburizing atmosphere. An object of the present invention is to provide a method for preventing deterioration of rolls for heat treatment furnaces, which provides comparable roll pick-up resistance.

(Means for Solving the Problems) The present invention provides a method for using a graphite roll in which chromium oxide is present in the pores in a non-decarburizing atmosphere.
By setting the O gas concentration to a concentration sufficient to prevent the progress of the reduction reaction of chromium oxide, the roll pick-up resistance of the graphite roll is prevented from deteriorating.

In order to clarify the cause of the deterioration of roll pick-up resistance when a chromic acid-impregnated graphite roll is used in a non-decarburizing atmosphere, the present inventors conducted various research studies and found that pick-up substances on the roll surface When analyzed, metallic chromium was detected in it.

Based on this, further investigation revealed that in a non-decarburizing atmosphere, the chromium oxide that seals the pores of the graphite roll is reduced, and the effect of the chromic acid impregnation treatment is lost. 1 as the pickup performance decreases.
It was also found that the amount of 0-L consumption also increased. At this time, chromium oxide Cr.

Possible substances that reduce Os are H7 in the furnace atmosphere and C in the graphite roll, but the reduction reaction by H2 CrtO5(s)+3H2(g)=2Cr(II)+3
Town 0 (g) ------- (1) is the usage condition of the heat treatment furnace when roll pickup occurs frequently, that is, the furnace temperature is 700 to 1000C.

The atmosphere inside the furnace is H2-N with about 20 inches of H2, and a dry atmosphere (
Thermodynamic calculations revealed that this does not occur at a dew point of -100).

On the other hand, the reduction reaction of chromium oxide by C on the graphite roll Cr20s (s) + 3C (s) = 2Cr (s) + 3G
O(g) ... In (2), PcO
When T is the partial pressure (atmospheric pressure) of CO gas in the furnace atmosphere and T is the furnace temperature (0K), equation (3) holds true as an equilibrium reaction equation.

J3og PcO= 8.992-Eng 4 animals 1
゛“°(3) However, the calculation of equation (3) and the reduction reaction of equation (1) are
To verify that this would not occur under the above conditions, we referred to data from the Iron and Steel Handbook, Basic Edition (1981 Maruzen) edited by the Japan Iron and Steel Institute.

The present invention reduces the CO gas concentration in the furnace atmosphere to the above (3).
By setting the value to be smaller than the value determined by the formula, the chromium oxide reduction reaction of formula (2) is prevented from proceeding. For example, in the range of the above-mentioned furnace temperature of 700 to 1000 C, in order to prevent the reduction reaction of chromium oxide, it is sufficient that the CO gas concentration in the furnace atmosphere is equal to or higher than the value shown in Table 1.

Table 1 Methods for introducing ω gas into the heat treatment furnace include pure ω gas
The gas may be diluted into the furnace atmosphere gas.

A quick method is to flow the CO gas into the furnace from a cylinder through the furnace wall, but bottled CO gas is expensive, and if it does not contain harmful components to the materials to be heat treated, the gas The effect is the same even if a fuel gas containing is used instead.

In addition, Cr20. As the reduction reaction, in addition to the warm type, for example, the formula (4) can be considered.

2 Cr, O, (s) + 7 C(s) = Cr
, C(s) + 600(g) ・−・・(4
) However, this reduction reaction of formula (4) is shown in Table 1.
It can be prevented even if the amount of GO is smaller than the amount.

By the way, when using the same heat treatment furnace by switching between a non-decarburizing atmosphere and a decarburizing atmosphere depending on the type of heat treatment, according to the method of the present invention, the furnace can be used in a non-decarburizing atmosphere. By flowing the polishing gas into the furnace, the chromic acid-impregnated graphite roll can be suitably used in both decarburizing and non-decarburizing atmospheres.

Furthermore, it is extremely effective when the first half of the continuous annealing furnace is decarburized at a relatively low temperature and the second half is subjected to grain growth annealing at a relatively high temperature in a non-decarburizing atmosphere. If this relatively high temperature, for example non-decarburizing atmosphere annealing at a furnace temperature of 1050 C is required, the ω gas in the atmosphere may be maintained at about 4.8% or more.

Of course, if the amount of CO gas in the annealing atmosphere becomes extremely large, there is a risk that the material to be heat treated will be carburized, but this is unlikely to occur in a short period of time and under normal conditions.

(Example) Material to be heat treated: C0,003, Si0.10-3.3
-Non-oriented electrical steel sheet, plate thickness 0.35~0.5011E
Furnace temperature: 700 to 900 tZ' Furnace atmosphere: About 20% H2, -N, ω gas is introduced into the non-decarburizing atmosphere, and the gloss gas concentration in the furnace atmosphere is adjusted to
0.1% at furnace temperature 700-800C, furnace temperature 800-9
In 00C, 0.1 to 0.3% was used.

In addition, the CO gas concentration includes about ω8, and in addition, H2
The fuel gas was prepared by flowing into the furnace a fuel gas of about 30% N2, about 10% 5Co2, and the balance CH.

Table 2 shows the frequency and contents of replacement of the chromic acid-impregnated graphite roll when the heat treatment furnace is used under the above conditions, compared with the case where no gas is introduced.

Table 2 In this example, a fuel gas containing CH4 was used as the gas containing gas, but the results of analysis and investigation of the amount of C in the heat-treated material showed that there was no carburization of the steel plate.

(Effects of the Invention) According to the present invention, the following effects can be obtained. When using a graphite roll with chromium oxide present in its pores in a non-decarburizing atmosphere, the ω gas concentration in the furnace atmosphere must be set to a concentration sufficient to prevent the reduction reaction of chromium oxide from proceeding. This prevents the chromic acid impregnation effect from being lost and the roll pick-up resistance of the graphite roll from deteriorating.

Claims (1)

[Claims] When using a graphite roll in which chromium oxide is present in the pores in a non-decarburizing atmosphere, the CO gas concentration in the atmosphere is set to a concentration sufficient to prevent the reduction reaction of chromium oxide from proceeding. A method for preventing deterioration of a roll for a heat treatment furnace, characterized by preventing a decrease in the roll pick-up resistance of the graphite roll.