JPS6324022A - Method for preventing pickup to in-furnace roll of continuous annealing furnace - Google Patents

Abstract

PURPOSE:To effectively prevent the formation of pickups on the surface of in-furnace rolls by averting the sticking of Si foreign matter except iron and iron oxide on a steel sheet surface prior to passage of the thin steel sheet through a heat treatment furnace. CONSTITUTION:The sticking of Si to the steel sheet surface is eliminated by using NaOH as a cleaning agent in a degreasing stage before passage of the cold rolled thin steel sheet through the continuous annealing furnace. The in- furnace tension is set lower on the inlet side and higher toward the outlet to generate slight slips, by which the sticking of the pickups to the roll surface is prevented and the pickups sticks thereto are stripped. The pickups on the roll surface are dislodged by utilizing the difference in thermal expansion between the iron powder sticking to the roll surface and the rolls in a holding zone. The growth of the pickups occurring in the Si is prevented by the above- mentioned method, by which the ferrous pickups are made hardly stickable and the generation of press flaws to the product is averted.

Description

Detailed Description of the Invention (Industrial Field of Application) The invention is based on the problem of foreign matter sticking to the surface of the rolls in the furnace (called pickup or build-up) when thin steel sheets are continuously heat-treated in a non-oxidizing atmosphere. However, the present invention relates to a method for preventing pickup (hereinafter simply referred to as pickup).

(Prior art) When manufacturing thin steel sheets with a product thickness of 2.5 mm or less in a continuous annealing furnace, metal foreign matter adheres to the roll surface in the heating zone or soaking zone, and the foreign matter (pickup) ), the problem of internal furnace flaws that occur on the product surface can reduce product yield and cause major production failures.

As a method to prevent this, there is a known method of coating the surface of the roll in the furnace in the heating zone or soaking zone of the heat treatment furnace with a nitride, ceramic, etc. that has a low affinity for iron to prevent iron powder, etc. from adhering. It is.

On the other hand, the sources of roll pickup are iron powder, adhering scum, etc. that separate from the surface of the steel sheet being passed, and these deposits must be mechanically removed from the steel sheet using a polishing roll, grinding roll, brushing roll, etc. is disclosed, for example, in Japanese Patent Application Laid-Open No. 57-29522.

(Problems to be Solved by the Invention) Now, it is acknowledged that the above-mentioned technique of coating the furnace roll itself with a substance that is difficult to pick up is somewhat effective, but there is a risk of pick-up. It has to be applied to all rolls, which poses an economical problem.

On the other hand, the above-mentioned Japanese Patent Application Laid-Open No. 57, which mechanically polishes the surface of a steel plate,
Although the technology of No.-29522 is acknowledged to be effective to some extent,
This technology will increase costs due to additional polishing equipment, and although there is no problem with plated original plates that are hot-melted after surface polishing, this technology is applied to cold-rolled steel sheets where minute surface defects are a problem, such as automobile exterior panels. In order to do so, polishing control that does not leave any residue as a product would be a problem, and it would be unsuitable for quality control.

The object of the invention is to provide a method for preventing pick-up on a roll in a continuous annealing furnace, which solves the problems of the prior art described above.

(Means and effects to be solved by the invention) The gist of the present invention is that when a thin steel plate is continuously heat treated in a non-oxidizing atmosphere, iron and This is a method for preventing build-up on a roll in a continuous annealing furnace, which is characterized by avoiding the attachment of Sl-based foreign substances other than iron oxides.

The present invention will be explained in detail below.

The components of roll pickup are mainly iron and iron oxides, but Si, Mn, C, etc. may also be included in addition to iron. Iron and iron oxides generated from the steel plate itself passing through the continuous annealing furnace cannot be completely prevented, but iron or iron oxides that are carried into the continuous annealing furnace while remaining or attached to the surface of the steel plate cannot be completely prevented. Foreign substances other than iron oxides, especially S1 foreign substances, should be avoided.

In general, the cleaning agents used to clean steel plates include caustic soda (NcLOH) and sodium orthosilicate (which has strong cleaning power).
Nα20・5i02) is the mainstream. Cleaning with sodium orthosilicate has strong degreasing power, but it leaves S on the surface of the steel plate.
This Sl and Mn, Ca, etc. in the steel are taken up and grown as lumps on the roll surface in the annealing furnace.

As a result of investigating the properties of the pickups attached to the rolls in the furnace in relation to the cleaning agent, the present inventors found that the composition of the pickups generated on the roll surface is different between sodium orthosilicate and caustic soda, as shown in Table 1. It was found that there are differences in melting point.

Table 1 Differences in melting point due to differences in the composition of the pick-up produced on the surface of the roll in the furnace have a great effect on the formation of the pick-up #
It gives

Generally, it is said that sintering temperature ('K) = 0.7X melting point (0K), and the melting point is 1050' (:, 51-Mn-
In case of Ca type pickup 1. The sintering temperature is 653°C.

On the other hand, when a cold-rolled steel sheet is manufactured by a continuous annealing method, the steel sheet is annealed at 700°C or higher.

That is, when an Sx-Mn-Ca-based pickup source is present, it is easy to bond F and a fairly strong pickup is generated. However, in the case of iron-based pickups, the sintering temperature is about 968°C, which is an impossible temperature for annealing cold-rolled steel sheets. (The maximum annealing temperature of a cold-rolled steel plate is about 850°C.) That is, even if an iron-based pick-up source is present, it is difficult to stick.

On the other hand, even if the adhesion of S1-based foreign matter is avoided, the steel plate itself, which is constantly passing through the continuous annealing furnace, is made of iron, and during the process of contact between the steel plate and the roll, some iron powder may be absorbed from the steel plate. It is inevitable that it will flake off and stick to the roll.

However, even if iron powder adheres to the roll, it is difficult to stick to the roll surface as a pick-up as mentioned above, and even if it sticks as a big amplifier, it will cause micro-slip caused by fluctuations in the tension in the furnace and fluctuations in the furnace temperature. Due to the difference in thermal expansion, the disadvantage of spontaneous peeling and damage to the finished product is avoided.

In addition, in order to reliably remove the roll pickup, use the method of removing it using mechanical force as recommended below.
Alternatively, it would be possible to remove the particles more effectively by adding a method of removing them based on the difference in thermal expansion.

As a method for removing by mechanical force, for example, when considering polishing the roll surface with a jig such as a grindstone,
There are concerns about issues such as sealing performance and thermal deformation of the jig.

On the other hand, in order to stably pass a steel plate at high speed, an appropriate tension is applied to the steel plate. Therefore, this tension should be used as a direct method.

When considering the level of tension applied to a steel plate, the continuous annealing method is certified by considering two points compared to the conventional method.

Heat buckle for thin and wide materials, and walk (meandering) for narrow materials. To put it qualitatively, it is desirable that the tension be higher in order to prevent heat buckling. That is, in order to satisfy contradictory conditions, the appropriate tension range is limited.

The relationship between roll pickup and tension remains unclear until now, and no concrete knowledge has been obtained.

Therefore, the inventors of the present invention have conducted extensive studies and experiments regarding the relationship between roll pickup and tension, and have reached the following conclusion.

That is, the tension should be low at the entrance of the furnace and set higher toward the exit. However, in this case, settings should be made with sufficient consideration to prevent the aforementioned heat buckling and walking from occurring.

Generally, roll pickup occurs frequently in the heating zone and soaking zone, and special attention must be paid to the tension setting in these zones. Since the plate temperature in the soaking zone can reach a maximum of 800'C, the tension in the soaking zone generally tends to be set low for fear of heat buckling. However, it is not good to set it too low. In particular, the thickness of the plate requires consideration when setting the tension.

One of the effects of tension is crushing of roll pickup due to high tension. For example, in the case of a steel plate with a thickness of 16 sheets, the tension in the heating zone and soaking zone should be 0.5 K7/- or more. . Another tension effect of lowering the tension on the furnace entry side and increasing the tension on the furnace exit side is the scraping effect of roll pickup due to minute slip between the steel plate and the roll. For example, in the case of a steel plate with a thickness of 16 mm, between the heating zone and the soaking zone,
The difference should be at least O,l K4 /lij.

Basically, the speed is controlled so that there is no slip between the roll and the steel plate. However, the allowable minute slip is within the range of o, o1% or less, and is not something that can be handled by electrical control. By making the tension of the steel plate on the exit side higher than the tension of the steel plate on the input side of the roll, the elongation of the steel plate on the exit side is longer than that on the input side1, and becomes slightly faster than the roll circumferential speed, causing a slight slip. . (O, 1% or less) As a result, it is possible to prevent the pickup from sticking and to scrape it off the roll surface.

Another way to remove iron powder adhering to the roll surface is to use differential thermal expansion. It is to make use of it. If the iron powder sticks to the roll surface for some reason, the difference in expansion coefficient between the iron powder and the roll can be used. The linear expansion coefficient of iron powder is (700~800)'Cf1sx
1o-'/℃, l 4 X I 0-67 at 500℃
It is about C. On the other hand, the furnace roll material is usually made of stainless steel, whose linear expansion coefficient is (700~
(20-21) XIO-6/°C,
At 500°C, it is about (18-19)X 1 o-6/°C.

Usually, the furnace temperature in the soaking zone is (700~Boo)'C, so
During cooling from this operating temperature to 500'C4, the roll side shrinks better than the iron powder, and during heating from 500°C to 700°C, the roll side expands better than the iron powder.

As a result, the pickups on the roll surface fall off. In this case, it is even more effective if a ceramic spray coating is applied to the surface of the roll in the furnace.

That is, the linear expansion coefficient of the ceramic material is 400 to s o
O'C is (4-5)

Since a sufficient effect cannot be obtained unless the cooling temperature is 500'C or less, it was decided to cool to a temperature below this temperature.

The cooling rate and heating rate are not particularly limited, but in consideration of productivity, a rate of 100''C/Hr or more is desirable.

As mentioned above, using the roll pickup as a crushing or scraping means,
Although two methods using tension and thermal expansion have been discussed, other known removal methods can also be used.

(Example 1) When manufacturing a 1.6M thick cold rolled steel plate in a continuous annealing furnace, two types of cleaning agents, sodium orthosilicate and caustic soda, were used in the degreasing process before passing through the continuous annealing furnace, and the operation was started. After 1 hour, 24 hours, and 720 hours have passed, the line is stopped and moved to a soaking area (700 hours) where the pick-up rate is high.
The presence or absence of adhesion of the pickup on the roll in the furnace was observed.

When all the caustic soda was used, no abnormality was observed on the roll surface. On the other hand, when sodium orthosilicate was used, adhesion was observed on the roll surface after one hour, and concave flaws due to transfer were also detected on the surface of the product.

By the way, what is the composition of the pick-up part that occurs on the roll surface when using sodium orthosilicate as the cleaning agent? As a result of analysis at the time of roll change, it was found to be Mn-3i-C.

(Example 2) When manufacturing a cold rolled steel sheet with a thickness of 1.6 cm in a continuous annealing furnace, caustic soda and sodium orthosilicate were used as cleaning agents in the degreasing process before passing through the continuous annealing furnace, and the tension in the furnace was set as follows. I set it like this.

In the case of caustic soda, no pick-up was observed even after one month under high load in the latter stage, but in the normal pattern, some foreign matter was observed on the roll surface, but no damage to the finished product occurred. - On the other hand, in the case of sodium orthosilicate, there was no effect of Zhang Kabataan.

(Example 3) When producing a cold rolled steel sheet with a thickness of 1.6 mm in a continuous annealing furnace, caustic soda and sodium orthosilicate were used as cleaning agents in the degreasing process before passing through the continuous annealing furnace, and the soaking zone was cooled. The surface of the roll was observed after one month.

As a result, in the case of caustic soda, when the soaking zone is cooled,
Although no pick-up was observed, some foreign matter was observed on the roll surface, but no scratches were observed on the finished product. On the other hand, in the case of sodium orthosilicate, there was no effect of lowering the furnace temperature.

(Effects of the Invention) The present invention prevents the adhesion of Sl-based foreign substances other than iron and iron oxides to the surface of the thin steel sheet before passing it through the heat treatment furnace when continuously heat-treating the thin steel sheet in a non-oxidizing atmosphere. When the roll is passed through a heat treatment furnace, no S1 type pickups, which tend to grow as pickups, are generated on the surface of the roll inside the furnace.

In addition, even if it grows into a pick amplifier, it will be an iron-based pickup that easily peels off, so in addition to well-known mechanical means, it can be easily peeled off by tension control, furnace temperature control, etc., and the finished product This is an effective method that does not cause any damage to the surface.

Claims (1)

[Claims] Continuous annealing characterized by avoiding the adhesion of Si-based foreign substances other than iron and iron oxides to the surface of the thin steel sheet before passing it through the heat treatment furnace, when the thin steel sheet is continuously heat treated in a non-oxidizing atmosphere. Method for preventing pick-up on rolls inside the furnace.