JPS6140729B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention aims to provide a method for directly heating a steel strip and an apparatus suitable for carrying out the method.

A conventional method of brightly heating a steel strip using direct flame heating is a method using a combination of a direct flame heating furnace and a reduction zone in CGL. The furnace used in this method, usually referred to as a non-oxidizing furnace, is industrially capable of continuing reduction even when heated to high temperatures by operating at an air-fuel ratio of less than 1.0. It is certain that the degree of reduction of the surface of the steel strip thus obtained is sufficient since the steel strip can be galvanized. However, unless galvanized, the steel strip obtained in this way is different from ordinary cold-rolled steel sheets and often exhibits deterioration in surface quality, with the appearance of white powder adhering to it. Therefore, it has been virtually impossible to apply such steel strips to sheet materials. This tendency is particularly noticeable in high-temperature annealing (800°C or higher), and therefore, the above method is applied to cold-rolled steel sheets for deep drawing and high-strength cold-rolled steel sheets that require heating temperatures of 800°C to 850°C. That was impossible.

For this reason, the method disclosed in Japanese Patent Application Laid-Open No. 128711/1983 has been proposed as a method that can be applied to cold-rolled steel sheets for deep drawing as described above without deteriorating the surface properties. However, all of these methods involve combustion with the air ratio (or excess air ratio) set within a fixed range, and therefore involve the operation of controlling the air ratio (or excess air ratio), which is not necessarily easy.

The present invention was devised in view of the above-mentioned current situation, and provides a direct heating method capable of obtaining a steel strip with a relatively simple configuration and in which deterioration of surface properties is appropriately suppressed. Another object of the present invention is to provide an apparatus suitable for such a method.

Therefore, the basic features of the present invention are as follows:
A steel strip whose surface temperature has reached a range of approximately 550°C to approximately 800°C is heated to 1/20 to 1/3 of the time required to reach the surface temperature from approximately 300°C, depending on the surface temperature of the steel strip. The intermediate reduction is carried out for a time of at least An intermediate reduction zone is provided in a preheating zone or a heating zone in front of the reduction zone of a direct-fired heating furnace consisting of a heating zone and a reduction zone.

The basic structure of the present invention is that intermediate reduction is performed under certain conditions during heating before the surface of the steel strip reaches the final heating temperature, and then the steel strip is heated again to the final heating temperature, and then final reduction is performed. It was discovered that this makes it possible to ensure good surface properties of the steel strip after final reduction.
FIG. 1 shows an example of heating and reduction according to the present invention, where A is a preheating section, B and D are heating sections, C is an intermediate reduction section, and E is a final reduction section. When the temperature reaches T ₁ , intermediate reduction is performed for a time t ₂ compared to the previous heating time t ₁ , and then heating is performed again, and the surface temperature of the steel strip is further increased.
The final reduction is performed when T ₂ is reached.

Therefore, the present invention is characterized in that the above-mentioned intermediate reduction is performed, and this intermediate reduction needs to be performed under certain conditions. That is, the factors that determine the conditions for this intermediate reduction include the intermediate reduction temperature T ₁ and the intermediate reduction time t ₂ .

First, regarding the intermediate reduction temperature T ₁ , the reduction efficiency is low at very low temperatures, so at least
The temperature should be 550°C or higher, preferably 600°C or higher. However, if an intermediate reduction zone is provided on the high temperature side, as in the case of conventional high temperature annealing, the surface quality will deteriorate;
The temperature shall be:

Next, regarding the intermediate reduction time _t2 , it is reasonable to express it as a ratio to the heating time, but there is almost no formation of a surface oxide film when the steel strip temperature is below 300℃, and Since the length of the heating time does not affect the oxide film thickness, the time required to reach the temperature at the start of intermediate reduction from 300°C was used as the heating time to determine the intermediate reduction time, and experiments were conducted based on this. The results are shown in Figure 2. The figure shows the case where the material was heated to 850° C. after intermediate reduction, and then complete reduction was carried out, where the ◯ mark indicates good surface quality and the x mark indicates poor surface quality. According to this,
Ratio of intermediate reduction time t ₂ to the above heating time t ₁
If t ₂ /t ₁ is 1/10 or more when the intermediate reduction temperature is 700°C, the surface quality after final reduction is good, and as the intermediate reduction temperature increases by 100°C from 700°C, the above
The ratio is 1/2 times the ratio for 700℃, and 700
Good surface properties were obtained by setting the minimum intermediate reduction time t ₂ to a ratio of 2 times for every 100° C. drop from the temperature. That is, to state this specifically, the ratio of the intermediate reduction time t ₂ to the heating time t ₁ is 1/20 or more at a reduction temperature of 800°C, and at 600°C
It needs to be 1/5 or more at 550℃ and 1/3 or more at 550℃.

On the other hand, if the ratio of the intermediate reduction time t ₂ is made smaller than the above ratio, for example, the intermediate reduction temperature
1/20 at 700℃, 3/100 at 800℃, 600℃
When the temperature is 1/10 in the case of 550°C and 1/5 in the case of 550°C, good surface quality cannot be obtained.

Taking all the above into account, a steel strip whose surface temperature has reached a range of approximately 550℃ to approximately 800℃ due to heating with a heating zone can be heated from approximately 300℃ to approximately 300℃ depending on the surface temperature of the steel strip. At least 1/20 of the time required for
A good surface quality of the steel plate can be obtained by performing intermediate reduction for 1/3 to 1/3 of the time.

After the intermediate reduction as described above, the steel strip is heated again, and then the final reduction is carried out, and the final reduction takes at least 1.5 times, preferably about twice the time of the intermediate reduction. It takes . That is, FIG. 3 shows the remaining state of the oxide film on the surface of the steel strip when the final reduction is performed without intermediate reduction, and the vertical axis represents the heating time before the intermediate reduction at the final reduction time t' ₂ . Ratio t′ ₂ /t ₁ to t ₁
This shows that. In the figure, ○ indicates no oxide film,
The x mark indicates the presence of an oxide film, and as is clear from the figure, if reduction is performed for approximately twice the intermediate reduction time shown in Figure 2 at each final reduction temperature, an oxide film will be formed. The persistence of these substances is largely preventable. On the other hand, the circle in parentheses in Figure 3 indicates the case where intermediate reduction was performed at 600°C and then final reduction was performed at 700°C. In this case, a certain degree of oxide film was formed during intermediate reduction. Because it has been removed,
Even if the final reduction time is about 1.5 times the intermediate reduction time, the remaining oxide film can be prevented. Taking all of the above into consideration, the final return should be 2 times the intermediate return time for safety reasons.
If it is doubled, the remaining oxide film can be almost completely prevented.

Next, FIG. 4 shows a direct-fired heating device used to carry out the above method, in which 1 indicates the roll, 2 indicates the steel strip, and the x mark indicates the position of the burner. First, it consists of a pass or pre-heating zone 4, and an afterburner 3 between the passes.
The unburned part of the waste gas that flows in from the latter stage is burned and used for preheating, and the steel strip 2 is heated to around 300°C in this preheating zone 4. The pass and the upper half of the pass also consist of a preheating zone 4', and this preheating zone 4' absorbs waste gas having a composition equivalent to an air ratio (or air excess ratio) of less than 1.0, which flows in from the heating zone 5 in the latter stage. Make use of it. The lower half of the pass and the lower half of the pass are composed of a heating zone 5, the upper half of the pass is composed of an intermediate reduction zone 6, and the upper 2/3 of the pass is composed of a heating zone 5'. 5'. In this example, the steel strip 2 reaches a temperature of 700°C before entering the intermediate reduction zone 6.
Heated to nearby temperatures. Also the bottom of the path
1/3 consists of the final reduction zone 7, where steel strip 2
The final reduction will take place.

FIG. 5 shows an example of heating and reduction using the above-mentioned apparatus. According to this, intermediate reduction is performed when the surface temperature of the steel strip 2 reaches 700°C, and here the steel strip is heated and reduced. The number of passes required for the surface temperature of No. 2 to rise from 300°C to the above 700°C is 2.5 passes and up to half of the passes, and the number of intermediate reduction passes is 0.5, so the intermediate reduction time of 300°C
The ratio to the heating time from .degree. C. to 700.degree. C. is 0.2, which is sufficient compared to the embodiment shown in FIG.

In addition, according to the above-described apparatus, the roll 1 between the passes is heated to the steel strip temperature by surface contact with the steel strip 2, but in the intermediate reduction zone in the upper half of the passes, the steel Since most of the oxide film formed on the band 2 is removed, the formation of oxide deposits on the roll 1 is properly prevented.

In addition, in all of the above embodiments, intermediate reduction is performed during heating in the heating zone, but depending on the case, it is also possible to perform intermediate reduction between the preheating zone and the heating zone, or in the preheating zone. .

According to the present invention as described above, by performing intermediate reduction under certain conditions during heating of the steel strip, it is possible to improve the surface quality of the steel strip obtained after the final reduction. This invention is industrially highly effective because it enables the production of high-quality cold-rolled steel sheets for deep drawing and high-strength cold-rolled steel sheets, and because it allows such features to be obtained with a relatively simple structure. It is.

[Brief explanation of the drawing]

FIG. 1 schematically shows an example of heating and reduction according to the present invention. FIG. 2 shows the influence of intermediate reduction on the surface properties of the steel strip after final reduction. Third
The figure mainly shows the state of the oxide film remaining on the steel strip obtained when intermediate reduction is not performed. FIG. 4 is an explanatory diagram schematically showing an embodiment of the apparatus according to the present invention. Figure 5 shows the heating and
This shows an example of reduction. In the figure, 2 is a steel strip, 4 and 4' are preheating zones,
5 and 5' are heating zones, 6 is an intermediate reduction zone, and 7 is a final reduction zone.

Claims (1)

[Claims] 1. In a direct heating method for steel strip, a steel strip whose surface temperature is in the range of about 550°C to about 800°C is heated from about 300°C to about 300°C depending on the surface temperature of the steel strip. A direct firing method for steel strip, which is characterized by performing intermediate reduction for 1/20 to 1/3 or more of the time required to reach the surface temperature, then heating again, and then performing final reduction. Heating method. 2. A direct-fired heating device for steel strip, characterized in that an intermediate reduction zone is provided in the preheating zone or heating zone in front of the reduction zone of the direct-fired heating furnace consisting of a preheating zone, a heating zone, and a reduction zone. Direct-fire heating device for steel strips.