JPS6321731B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD This invention relates to a continuous annealing method and continuous annealing equipment for cold-rolled steel strip, and in particular, to effectively prevent the occurrence of meandering and hit buckles in the continuous annealing equipment for the steel strip. Technical Background Generally, a vertical furnace is used as a continuous annealing furnace for cold-rolled steel strips due to the installation area and equipment cost. In such a rigid continuous annealing furnace, as shown in FIG. While the material is sequentially wound around these upper and lower hearth roll groups and passed through a roundabout path, a predetermined heat treatment necessary to obtain desired material properties is applied. However, when a steel strip is continuously heat-treated in a furnace as described above, depending on the shape of the steel strip, tension balance in the furnace, temperature conditions, etc., meandering may occur in the steel strip, making smooth operation impossible. Sometimes. Therefore, in order to prevent such meandering, crowned rolls 1' and 1'' as shown in FIGS. 2a and 2b are generally used as hearth rolls.This is due to the centering force of the crown roll, This method attempts to correct the meandering by applying a force that moves the steel strip toward the center in the width direction of the roll.However, if this centering force is too strong, the steel strip will buckle in the width direction. A defect called a heat buckle occurs. Therefore, in order to prevent both meandering and heat buckles from occurring, it is necessary to apply an appropriate amount of crown in advance to prevent both from occurring. For example, the higher the heat treatment temperature of the steel strip, the wider the strip width, the smaller the strip thickness, and the higher the strip threading speed, the more likely heat buckles will occur, so it is extremely difficult to select the appropriate amount of crown. Prior art and its problems As a solution to the above-mentioned crown amount problem, for example, Japanese Utility Model Application No. 55-172359 and Japanese Patent Application Laid-open No. 57
Hearth rolls with variable crowns have been proposed in Japanese Patent No. 177930 and Japanese Utility Model Application No. 105464/1983. However, controlling the crown amount requires a device to measure the crown amount for each hearth roll and a device to control the crown amount based on the measurement results, which incurs a large amount of cost. There were also problems with slow response times. By the way, cold-rolled steel strip for deep drawing is generally made of C.
Low carbon steel with a C content of 0.1% or less is used, and in recent years, due to advances in melting technology, ultra-low carbon steel with a carbon content of 0.005% or less is also used as a material for deep drawing steel sheets. However, these cold-rolled steel sheets for deep drawing are high-temperature annealed materials that are heated to temperatures of 800°C or higher, and are prone to heat buckling. Very noticeable. Recently, it has been used as a base plate for tinplate with a thickness of 0.2 mm.
Demand for the following ultra-thin materials is increasing, but even in such ultra-thin materials, there remains a problem in that heat buckles are more likely to occur as the sheet passing speed is increased in response to the above requirements. Furthermore, the problem of heat buckling also occurred in soft tin plates made of ultra-low carbon steel. Purpose of the Invention The present invention advantageously solves the above-mentioned problems, and effectively prevents the occurrence of heat buckles, which have a particularly large adverse effect on the yield and quality of products, as well as the occurrence of meandering in the annealing furnace. The purpose of the present invention is to propose a continuous annealing method for cold-rolled steel strips that can prevent such problems, together with suitable continuous annealing equipment for its implementation. Background of the elucidation of the solution Figure 3 schematically shows a conventional continuous annealing furnace that is suitable for continuous heat treatment of tin plate blanks. In the figure, number 3 is a heating zone, 4 is a soaking zone, 5 is a slow cooling zone, and 6 is a rapid cooling zone. A predetermined heat treatment is performed while the sheets are passed one after another. Now, Figures 4a and b show the results of an investigation into the meandering of the steel strip and the frequency of occurrence of heat buckles when using the above-mentioned continuous annealing furnace, respectively. and the relationship with the crown amount of the hearth roll for each rapid cooling zone. From the figure, the heat buckle is located in the latter half of the heating zone,
It can be seen that meandering tends to occur in high-temperature zones such as the soaking zone and slow cooling zone, while meandering is less likely to occur in these high-temperature zones. Next, FIG. 5 schematically shows a continuous annealing line including a conventional continuous annealing furnace mainly intended for deep drawing steel strips. In the place shown in the figure, the steel strip S is rewound by bay-off reels 7 and 7', and then subjected to pretreatment in input side equipment 8 such as a welder and cleaning device, and then passed through an input side looper 9. Continuous annealing furnace 10
sent inside. In this continuous annealing furnace 10, the steel strip S is divided into a preheating zone 11, a heating zone 12, a soaking zone 1
3. A predetermined heat treatment is performed while sequentially passing through the first cooling zone 14, the second cooling zone 15, the overaging treatment zone 16, and the third cooling zone 17, and then after passing through the outlet looper 18. Output processing device 1 such as shear
After being subjected to post-processing at step 9, it is wound up at tension reels 20, 20'. FIG. 6 shows the results of an investigation of the heat buckle generation rate when a steel strip for deep drawing was heat treated in such a continuous annealing furnace. In the figure, the horizontal axis represents the heating temperature of the steel strip, and the vertical axis represents the percentage of the number of coils generating heat buckles relative to the total number of coils processed. As is clear from the figure, the temperature of the steel strip is 780℃.
There are no heat buckles in the following cases,
It can be seen that heat buckles rapidly occur as the temperature rises above 780°C. Further, FIG. 7 shows the results of an investigation regarding the occurrence of heat buckles when heat treatment was similarly performed on a tinplate original plate (thickness: 0.2 to 0.3 mm) made of ultra-low carbon steel. As is clear from Figure 7, the occurrence of heat buckles in ultra-low carbon tinplate plates, similar to deep-drawn steel plates, decreases dramatically as the processing temperature decreases, especially at 700
There were no cases below ℃. Based on the above experimental results, the inventors have conducted extensive studies on the prevention of heat buckles and meandering, and have found that hearth rolls, which are highly effective in preventing meandering, are also a major factor in generating heat buckles in the high-temperature zone of the furnace (other thermal (This is thought to be subtly influenced by the crown, decrease in material strength at high temperatures, threading speed, and thermal expansion of the steel strip.) Therefore, in the high-temperature zone of the furnace, such hearth rolls are While using a horizontal furnace that eliminates this risk, a vertical furnace with built-in hearth rolls is used for the low temperature zone where there is no risk of such a problem, and in particular, by appropriately controlling the steel strip temperature on the entrance and exit sides of the horizontal furnace. It was discovered that the intended purpose could be achieved very effectively. Structure of the Invention The present invention is derived from the above knowledge. That is, this invention provides continuous annealing treatment to a cold-rolled steel strip using continuous annealing equipment connected to a vertical furnace, a horizontal furnace, and then a vertical furnace while sequentially forming a heating zone, a soaking zone, and a cooling zone. This is a continuous annealing method for a cold-rolled steel strip, characterized in that the steel strip is introduced into a horizontal furnace and taken out at a temperature that does not cause heat buckles in the steel strip. The present invention also provides a continuous annealing facility equipped with a heating zone, a soaking zone, and a cooling zone, each of which is divided into a low temperature zone and a high temperature zone,
A cold-rolled steel characterized in that the low-temperature heating zone and the cooling zone, which are low-temperature zones, are each a vertical furnace, while the high-temperature zones, which are the high-temperature heating zone, soaking zone, and high-temperature cooling zone, are each a horizontal furnace. This is a continuous annealing facility for strips. In addition, as the heat treatment zone in the continuous annealing equipment according to the present invention, in addition to the above-mentioned heating zone, soaking zone, and cooling zone, a preheating zone can also be arranged in front of the heating zone. This invention will be specifically explained below. First, the continuous annealing equipment according to the present invention will be explained. Fig. 8 shows a preferred embodiment of the continuous annealing equipment for cooling steel strips for deep drawing.
1 is a pre-heating zone, 22 is a low-temperature heating zone, 23 is a high-temperature heating/soaking zone consisting of a horizontal furnace, followed by a high-temperature heating zone, followed by a soaking zone, and 24 continues to be horizontal and has a high-temperature cooling zone. These horizontal furnaces 23 and 24 are the primary cooling zone of the preheating zone 21.
and the upper part of the vertical furnace forming the low temperature heating zone 22. and 25 is the secondary cooling zone, 26
2 is an overaging treatment zone, and 27 is a tertiary cooling zone, both of which are comprised of a vertical furnace. Now, in such continuous annealing equipment, the steel strip S is
For example, heat treatment is performed according to a heat pattern as shown by symbol A in FIG. 9 to impart predetermined material properties. That is, the steel strip S first passes through a tropical zone 21 and then a low-temperature heating zone 22 to be heated to a certain temperature, and then passes through a high-temperature zone heating/soaking zone 23, which is a horizontal furnace placed above these furnace zones. It is introduced and subjected to a predetermined heat treatment, and then guided to the first cooling zone 24 where the temperature is lowered to a predetermined temperature. By passing through the tertiary cooling zone 27, desired material properties are imparted. As can be seen from the results shown in Figure 6 above, it is necessary to keep the steel strip temperature at the entrance and exit sides of the horizontal furnaces 23 and 24 below 780°C from the viewpoint of preventing heat buckling. be. As a heating means, heating/soaking zone 22, 2
3 is preferably a radiant tube burner, and the preheating zone 21 is preferably heated directly by the exhaust gas from the heating/soaking zones 22 and 23 or heated by air heat exchanged with the exhaust gas. and tertiary cooling zones 24, 25,
27, it is preferable to use gas jet cooling using atmospheric gas, and for the overaging treatment zone 26, it is preferable to use radiation heating using an electric heater or a radiant tube. Another embodiment of continuous annealing equipment suitable for use in annealing such cold-rolled steel strips for deep drawing is shown in FIG. Whether to use the type shown in FIG. 10 or the type shown in FIG. 8 above is determined depending on the heating rate, temperature at the entrance of the horizontal furnace, etc. Next, FIG. 11 shows a preferred embodiment of continuous annealing equipment for deep drawing cold rolled steel strips made of ultra-low carbon steel. In this preferred example, except that the latter half vertical furnace consists only of the secondary cooling zone 25',
The gist of the configuration is the same as the equipment shown in Figure 10 above. In the place shown in FIG. 11, the steel strip S is
According to the heat pattern shown by B in FIG. 9 above, the preheating zone 21, the low temperature heating zone 22,
Desired material properties are imparted by heat treatment through the high temperature heating/soaking zone 23, the high temperature cooling zone 24, and the low temperature cooling zone 25'. Here, the temperature of the steel strip S at the entrance and exit sides of the horizontal furnaces 23 and 24 must also be 780° C. or lower. Furthermore, Fig. 12 shows another example of continuous annealing equipment suitable for annealing ultra-thin materials such as tinplate blanks made of ultra-low carbon steel and tinplate blanks with a thickness of 0.2 mm or less. shows. The gist of the configuration in this example is almost the same as that shown in Figure 11 above, but the primary cooling zone 2 of the horizontal furnace
4 is a slow cooling zone, and the following secondary cooling zone 25 of the vertical furnace is a rapid cooling zone. Now, in such equipment, the steel strip S is heat treated according to the heat pattern shown by C in FIG. As shown in FIG. 7 above, it is necessary to control the temperature of the steel strip at the side and outlet side to 700° C. or lower, at which no heat buckles occur. In this way, in the method of this invention, it is important to introduce and remove the steel strip into the horizontal furnace at a temperature that does not generate heat buckles, but this temperature varies greatly depending on the material and thickness of the steel strip. Therefore, it cannot be determined unambiguously. By the way, in this invention, the high-temperature zone in continuous annealing is a horizontal furnace, while the low-temperature zones before and after the horizontal furnace are vertical furnaces to prevent the occurrence of heat buckles and meandering in the high-temperature zone. By using the rolls at the end and the exit end as steering rolls, it is possible to further improve the effectiveness of preventing meandering. Further, in a horizontal furnace, if the amount of drooping of the steel strip is a problem, support rolls, floaters, etc. may be installed as appropriate. In the above-mentioned embodiments, the case where a preheating zone is provided has been mainly explained, but it goes without saying that there is no problem even if the preheating zone is not provided. Next, an embodiment of the annealing method according to the present invention will be described. Using the continuous annealing equipment shown in Figure 8, deep-drawing cold-rolled steel strips made of ultra-low carbon steel and having the sizes shown in Table 1 below are heated and soaked in the high-temperature area as shown in Table 1. Then, 40 coils were annealed under various strip-threading conditions in which the introduction and exit temperatures of the primary cooling zone and the line speed were varied. Table 1 also shows the results of an investigation into the occurrence of heat buckles and meandering during the annealing process.

[Table] As is clear from the results shown in Table 1, 780℃
Heat buckling could be prevented by heating, soaking, and primary cooling in the above-mentioned high-temperature range in a horizontal furnace. In addition, we were able to prevent meandering by increasing the hearth roll crown of the low-temperature vertical furnace, where heat buckles are less likely to occur, and by installing a steering device at the introduction and exit portions of the horizontal furnace. As described above, according to the present invention, by using a horizontal furnace for the high-temperature zone and a vertical furnace for the low-temperature zone, it is possible to effectively prevent the occurrence of heat buckles and meandering. It is as follows. (1) The installation area of continuous annealing equipment can be reduced. (2) Productivity is improved because the threading speed can be increased without worrying about the occurrence of heat buckles. (3) Tension fluctuations have traditionally been a problem, particularly in high-temperature zones, but by using a horizontal furnace for such high-temperature zones, tension fluctuations can be alleviated by changing the amount of droop of the steel strip. (4) Pick-up flaws, which are particularly easy to form in high-temperature zones, occur when foreign matter on the surface of the steel strip adheres to the hearth roll.
It is possible to reduce defects caused by deposits and transfer to the steel strip.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a rigid continuous annealing furnace, Figures 2a and b are front views of the crown roll, and Figure 3
The figure is a schematic diagram of a conventional continuous annealing equipment for tinplate blanks, and Figures 4a and 4b show the influence of the crown amount of the hearth roll on the meandering of the steel strip and the frequency of heat buckles, respectively, in the first half of the heating zone. , the second half of the same, a graph comparing the results of the investigation for each soaking zone, slow cooling zone, and rapid cooling zone, Figure 5 is a schematic diagram of a conventional continuous annealing equipment for cold-rolled steel strips for deep drawing, Figure 6 is a graph showing the relationship between the annealing temperature and the heat buckle generation rate of cold-rolled steel strip for deep drawing, and Figure 7 is the graph showing the relationship between the annealing temperature and heat buckle generation rate in a tin plate made of ultra-low carbon steel. Figures 8 and 10 are both schematic diagrams of continuous annealing equipment according to the present invention for cold-rolled steel strips for deep drawing, and Figure 9 is a graph showing the relationship between the Figure 11 is a schematic diagram of continuous annealing equipment for deep drawing cold-rolled steel strips made of ultra-low carbon steel, and Figure 12 is a diagram showing the heat pattern of the strip. FIG. 2 is a schematic diagram of continuous annealing equipment suitable for annealing thin tin plate blanks. 21...Pre-preparation zone, 22...Low temperature heating zone, 23...High temperature heating/soaking zone, 24...Primary cooling zone, 25...
Secondary cooling zone, 26... Overaging treatment zone, 27... Third
Next cooling zone.

Claims (1)

[Claims] 1. Continuously annealing a cold-rolled steel strip using continuous annealing equipment connected to a vertical furnace, a horizontal furnace, and then a vertical furnace while sequentially forming a heating zone, a soaking zone, and a cooling zone. 1. A continuous annealing method for a cold-rolled steel strip, characterized in that the steel strip is introduced into a horizontal furnace and taken out at a temperature at which heat buckles do not occur in the steel strip. 2 Continuous annealing equipment equipped with a heating zone, a soaking zone, and a cooling zone, in which the heating zone and the cooling zone are each divided into a low temperature zone and a high temperature zone, and the low temperature zone is a low temperature zone heating zone and a low temperature zone cooling zone. Continuous annealing equipment for cold-rolled steel strip, characterized in that each of the vertical furnaces is a vertical furnace, and the high-temperature zones, namely a high-temperature heating zone, a soaking zone, and a high-temperature cooling zone, are each horizontal furnaces. 3. The continuous annealing equipment according to claim 2, wherein the cooling zone comprises a primary cooling zone of a horizontal furnace, a secondary cooling zone, an overaging treatment zone, and a tertiary cooling zone of a vertical furnace. . 4. The continuous annealing equipment according to claim 2, wherein the cooling zone comprises an annealing zone of a horizontal furnace and a quenching zone of a vertical furnace. 5. The continuous annealing equipment according to claim 2, 3 or 4, wherein the introduction and lead-out rolls on the horizontal furnace entry side and exit side are steering rolls.