JPS5818411B2 - Annealing method for cold-rolled sandpaper strips - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of annealing cold rolled polished strip.

Cold-rolled polished strips such as cold-rolled steel sheets are subjected to softening annealing to restore workability, but conventionally such annealing has always been carried out in a non-oxidizing atmosphere.

In other words, the above-mentioned softening annealing involves batch annealing, in which a coil of cold-rolled sandpaper strip is wound into a tight or loose shape, is covered with a cover-type furnace, and atmospheric gas is circulated, and annealed, and cold-rolled sandpaper strip is wound into a coil shape. There is continuous annealing, in which the strip is unwound and annealed by passing it continuously through a furnace where atmospheric gas circulates.The annealing cycle consists of three processes: heating, soaking, and cooling. Gas that has been incompletely combusted to remove moisture), NX gas (
The entire process is made non-conductive using gas such as fuel gas (combusted fuel gas that has been incompletely combusted to remove CO2 and water), HNX gas (gas that has more CO2 and increased H2 than NX gas), and AX gas (gas that has decomposed ammonia). It has been kept in an oxidizing atmosphere.

In other words, in batch-type coil annealing, the coils are stacked on a base in several stages, and the inner cover and then the Araku cover are placed on top of that to seal the grounding parts of each. The coils are isolated from the atmosphere, but air is trapped inside the inner cover. Since it is sealed, atmospheric gas is passed through several hours before the start of heating to replace the air inside the inner cover with atmospheric gas, and then heating is started and the atmospheric gas is circulated until the annealing is completed.

In addition, water vapor is a particularly effective decarburizing substance when decarburizing annealing to remove strain caused by cold rolling and carbon from steel sheets. It is known to be particularly effective when steam is added to the atmosphere maintained in a non-oxidizing state as described above, but even in such cases, the steam never enters the atmosphere inside the furnace. We have been supplying this material under strict control to ensure that no problems occur.

For example, in Japanese Patent Publication No. 42-22202, steam is added to a high hydrogen-containing atmosphere only during the decarburization period, that is, at high temperatures; however, if the exhaust gas is decarburizing and non-oxidizing, steel Both the dew point and hydrogen content of the exhaust gas are measured and introduced, assuming that due to the high circulation rate and the high rate of water gas reaction that occurs in the furnace, a suitable reaction will occur regardless of the composition of the introduced gas. It maintains the equilibrium state of exhaust gas by changing the relative amount of hydrogen and water vapor in the gas.

Furthermore, Japanese Patent Publication No. 46-23817 proposes adding water vapor to the atmosphere throughout the annealing period based on the knowledge that nitrogen absorption in aluminum killed steel during annealing can be reduced by adding water vapor to the hydrogen-nitrogen annealing atmosphere. However, in order to obtain a dew point that is effective in reducing nitrogen absorption in steel in a reducing atmosphere, the dew point and hydrogen concentration of the atmosphere are maintained in an appropriate relationship.

However, the conventional annealing method described above has the disadvantage that a cold-rolled polished strip with excellent surface properties cannot be obtained.

That is, cold-rolled cold-rolled polishing strips contain oils such as rolling oil used in rolling, slag such as iron powder generated during rolling, or CJ'-, which was adsorbed during, for example, pickling treatment before rolling. Various organic and inorganic ions such as 804-1 remain, and in the conventional annealing method, this causes sooting and stains the cold-rolled l-IJ tube.

Depending on the requirements of the product, it is often the case that a cleaning treatment such as electrolytic cleaning is performed after cold rolling before annealing, but the above-mentioned residues cannot be completely removed by the commonly used industrial cleaning treatment.

Therefore, in order to sufficiently remove it, it is necessary to repeat the cleaning many times or use a special cleaning method, but due to the cost, this type of cleaning cannot be adopted by companies, and in the end, it is not possible to remove it sufficiently. The reality is that they are subjected to annealing without being annealed.

In addition, a new surface quality problem has recently been raised in which black stains due to crystalline graphite occur on the surface of cold-rolled polishing strips after annealing, and since it is not clear what the main cause is, we are struggling to find a countermeasure, but it is improving productivity. Considering the fact that there is no clue other than the phenomenon that cold-rolled Mikaki strips manufactured using expensive equipment have an increased tendency to occur, this is a latent problem that originally existed, and at least one factor is caused by the conventional annealing method itself. I have to say that it was.

The present invention provides an annealing method that prevents the occurrence of such stains and obtains a cold-rolled polished strip with excellent surface properties. It is characterized by being oxidizing to.

In other words, the present invention proposes that the oxidation of cold-rolled polished strips during annealing is a problem that should be discussed when the annealing is finally completed, and that oxidation during the process should be treated as long as no oxidation remains or even if it remains, it is not completely oxidized. Based on the idea that it should be allowed, the annealing atmosphere is made to be oxidizing to the cold-rolled polished l-IJ to the extent that a complete oxide film does not remain on the cold-rolled polished strip after annealing, and such As the oxygen source, oxidizing substances such as pure oxygen, air, water vapor, manganese oxide, and ichmacnesium oxide are used.

It is generally said that the oxide film on a cold-rolled polished strip during annealing becomes visible when its thickness exceeds several angstroms (approximately 6 angstroms); however, in the present invention, a complete oxide film is defined by visual inspection. This refers to an oxidized film that is observed, and the present invention allows this to occur during the soaking process, preferably during the heating period.

In other words, once a complete oxide film is generated, it is later reduced to form an invisible oxide film, and for this reason, a reduction region is provided at least at the end of the soaking period.

The shorter this reduction zone is provided, the longer the complete oxide film will exist, but on the other hand, the risk of incomplete reduction increases. It is more realistic to achieve complete reduction, and it is therefore desirable to keep the reduction range as wide as possible, covering most or all of the high-temperature soaking period, or even longer. It is desirable that the atmosphere during this period be non-oxidizing to the cold rolled polishing strip.

This is because both the efforts to maintain a relatively small amount of a complete oxide film for as long as possible and the efforts to generate as much complete oxide film as possible in a relatively short time are the main objectives of the present invention, such as sooting and blackening. This is because the same is true for removing dirt.

In other words, residues such as rolling oil on the surface of the cold-rolled polishing strip are removed by reacting with oxygen in the atmosphere and gasifying, and the resulting complete oxide film prevents the formation of crystalline graphite. Although it is responsible for the action, in any case, the complete oxide film will be reduced later.

The gasification of the residue is an oxidation reaction during the complete oxidation of the cold-rolled polishing strip and does not require any special explanation, and the gaseous material leaves the furnace together with the exhaust atmosphere over time. This is also easily understood.

On the other hand, when it comes to inhibiting the formation of crystalline graphite, the mechanism is not necessarily clear, but it is thought that at least a complete oxide film has some effect on so-called amorphous carbon before it becomes crystalline graphite.

In other words, the residue on the surface of the cold-rolled polishing strip, which is considered to be an important carbon source not only for sooting but also for black stains, is still carbon-free, even if it is not a problem because it has already been removed when a complete oxide film is formed. Carbon in the cold-rolled polishing strip is thought to be the source, and a complete oxide film is thought to gasify it with its own oxygen.

In this way, sooting and black staining can be prevented by making the atmosphere oxidizing to the cold rolled sanding strip, preferably during the heating period, ideally during the soaking process, but the ultimate purpose of the present invention is to The object of the present invention is to provide an annealing method for obtaining a cold-rolled polished strip with excellent surface properties, and to obtain a cold-rolled polished strip surface with excellent rust prevention properties.

This is possible by making the atmosphere oxidizing to the cold-rolled polishing strip even during the cooling period, and produces an extremely thin invisible oxide film that does not form a complete oxide film.

Such a film can be formed by creating a weakly oxidizing atmosphere, and the resulting film is hard to break and exhibits a remarkable rust-preventing effect.

Therefore, the atmosphere transition in the present invention is as follows, but it is sufficient that the atmosphere during the soaking period is reducing, and can be reduced and decarburized, reduced and denitrified, or reduced and decarburized and denitrified, etc. It is suitable for annealing for any purpose without any problems.

In addition, the oxygen source that makes the atmosphere oxidizing may be used alone or in combination, but when air is used, air is made to exist in the furnace, so the air source before heating in batch annealing starts. There is no need to expel the annealing process, which shortens the annealing process by several hours, and when using air and steam, not only does the work before heating with air be omitted, but the atmosphere becomes reducing and weakly decarburizing when the steam is soaked. If maintained, it is possible to obtain the effect that the cold-rolled Migakist+J tube can be prevented from being nitrided without being decarburized.

That is, the use of the oxygen source in the present invention is not necessarily limited to the period when the annealing atmosphere is made oxidizing to the cold-rolled sandpaper re-top, but it is necessary to use the oxygen source in order to maintain the annealing atmosphere at a sufficient reducing level during the soaking period. If there is no problem, it can be used throughout the entire annealing period, and as a control method, a mode in which a constant amount of oxygen source is always supplied to the furnace or a mode in which the supplied oxygen source is changed according to each period can be adopted as appropriate. However, in any case, whether the annealing atmosphere becomes oxidizing or non-oxidizing for cold-rolled milled IJ tubes is related to the amount of hydrogen in the atmosphere, the amount of oxygen brought in by the oxygen source, and the annealing temperature. is easily achieved by known methods, for example by measuring the CO extremity in the exhaust atmosphere gas and thereby controlling the amount of oxygen source included in the inlet atmosphere.

On the other hand, if an oxygen source is used only during the heating or cooling period, the annealing atmosphere can be changed to the cold-rolled sandpaper strip using simple QN-(DFF) control, in which the supply path is fully opened and closed depending on the annealing temperature. It can be made oxidizing.

The expression "control" here does not only mean automatic control, but also includes manual operation, but regardless of the form of control, the embodiments of the present invention are as shown in Figure 1. .

In the figure, A indicates the state before the start of heating; in the case of batch annealing, the coils are stacked in several stages on the base and covered with an inner cover, the grounding part of which is sealed, and the coil is isolated from the atmosphere, and in the case of continuous annealing, it is the state before heating starts. This is the state immediately before the strip is introduced into the heating furnace.In conventional methods, in order to make the strip non-oxidizing, the air inside the inner cover is replaced with reducing or inert atmosphere gas over several hours in batch annealing. This is the period during which

B indicates a heating period, C indicates a soaking period, and D indicates a cooling period.

T indicates the temperature, and the decimal digit is 0 or room temperature, l or about 50-55
0°G, 2 is about 550-750°C, 3 is about 150-5
It is 00℃.

HNX means not only HNX gas but also any other reducing atmosphere gas used for annealing.

N20 represents water vapor, and 02 represents pure oxygen as well as air and oxides such as manganese oxide and magnesium oxide.

Hot IT is HNX when 02, especially pure oxygen or air, is actively supplied as an oxygen source during heating period B.

This setting is especially designed to avoid explosions caused by hydrogen in gases with high hydrogen concentration.If the temperature is below about 550℃, the maximum oxidation that can be completely reduced during soaking period C at temperature T2 is set. The amount of 02 produced can be supplied without exploding.

The reason for setting a lower limit for T1 is to allow 02 to sufficiently react with the residue on the surface of the cold-rolled polishing strip, and cannot be expected to occur below about 50°C.

The temperature T is a condition for forming an invisible oxide film, and although an invisible oxide film can be obtained at a temperature of about 500° C. or higher, it is difficult to obtain a uniform one.

Further, if 02, particularly pure oxygen or air, is actively used as an oxygen source, there is a risk of explosion as described above.

On the other hand, if the temperature is 150° C. or lower, an invisible oxide film having a rust-preventing effect cannot be sufficiently obtained.

Example 1 Plate thickness 0.6 to 0.8 mvt, plate width 700 to 14001 m
, cold-rolled steel strip coils with a unit weight of 12 to 16 tons are stacked in 2 to 5 stages in a batch furnace that allows monitoring of furnace wall temperature and pace temperature as shown in Figure 2, and air is sealed in the furnace. Heating was started while the temperature was still in the condition, and air was passed through the inner cover at a rate of 15,771"/hr. When the pace temperature reached 380°C, the air flow was stopped and N2: 4.5%, N2: Annealing was carried out by passing 95.5% H, NX gas and maintaining the furnace wall temperature at 760-800°G1 pace temperature at 640-760°C for 20-30 hours.

After that, in the cooling process, 02 was
0 ppm was added and cooled.

The results are shown in Table 1 along with comparative examples.

In Figure 2, 1 is a coil, 2 is a convector plate, 3 is a differential user, 4 is a fan, 5 is an inner cover, 6 is an outer cover, 7 is a thermoelectric lamp, 8 is an atmospheric gas supply path, and 9 is an oxygen source 10 represents a supply path and an exhaust gas flow path.

Example 2 Cold-rolled plate with matte finish, 0.8-1.2mm thick.

A cold-rolled steel plate with a thickness of SOO~1400 mm was passed through an electrolytic cleaning line and wound into a coil having a unit weight of 15 to 18 tons, and this was annealed in the same batch furnace as in Example 1 as follows.

If the coils are stacked in 2 to 5 layers on the pace, covered with an inner cover, and the ground contact part is sealed, N2: 4.50
%, N2: 94.36%, N20: 1.14 atmosphere gas was passed through the chamber, and when all the enclosed air was replaced with the atmosphere gas, the outer cover was covered and heating was started. It was annealed using the same annealing cycle as 1.

The water vapor was kept constant at the above amount until the annealing was completed, but this was because the atmosphere was kept at a constant temperature of 530°C or less during the heating and cooling periods, except for the extremely low steam inert region with a dew point of 9°C. Make it oxidizing.

The results are shown in Table 2 along with comparative examples.

Comparative Example Annealing was performed under the same conditions as in Example 1, except that a HNX gas of N277%N2:93 was used from before the start of heating to the end of annealing, and a reducing atmosphere was maintained throughout the period.Tables 1 and 2 show the results. Shown below.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention, and FIG. 2 is an explanatory diagram of an embodiment of the first embodiment. ' 1... Coil, 2... Convector plate,
3... Differential user, 4... Fan, 5... Inner cover, 6... Outer cover, 7... Thermocouple, 8... Atmosphere gas supply path, 9... Oxygen source supply road,
10...Exhaust gas flow path.

Claims (1)

[Claims] 1 The atmosphere during the annealing process up to soaking by heating is made oxidizing with a newly prepared atmosphere of pure oxygen, air, steam, manganese oxide, magnesium oxide, carbon dioxide, etc. for the cold-rolled polished strip, and the surface of the strip is heated. A cold-rolled polishing strip characterized by forming an oxide film, then creating a reducing atmosphere, and further making the cold-rolled polishing strip weakly oxidizing in the cooling process to generate an invisible oxide film on the surface of the strip. Annealing square circle