JPS6260825A - Preheating method in continuous heat treatment of steel strip - Google Patents

Abstract

PURPOSE:To preheat at high temp., to prevent meandering of a steel strip and to make heating zone compact, by blowing gas heated by sensible heat of combustion exhausted gas at heating zone to the steel strip, then winding the strip around a heated roll, passing and preheating it. CONSTITUTION:Combustion exhausted gas of a radiant tube 5 of a heating furnace 4 for continuously heat treating the steel strip 1 is passed through a heat exchanger 7 by a suction fan 8 through a collecting duct 6. At this point, sensible heat of the exhausted gas is recovered by gas such as air and it is blown to the strip 1 from a hot wind chamber 11 at the first preheating part 2 by a circulation fan 10 to preheat the strip 1 to about 100-200 deg.C. Next, the strip 1 is passed through the second preheating part 3 made to nonoxidizing atmosphere and passed while being wound around heating rolls 15. A heating medium 12 such as fused salt heated at a heating apparatus 13 is passed to the rolls 15 to preheat the strip 1 to about 250-500 deg.C. Thereafter, the strip 1 is passed in the furnace 4 under reducing atmosphere to heat treatment.

Description

[Detailed Description of the Invention] (Industrial Application Field) Regarding preheating on the heating zone entry side in continuous heat treatment of steel strips, this specification describes the development of a method for gradually approaching the heating zone processing temperature through multi-stage preheating. Describe the results of the research.

The configuration of continuous heat treatment equipment for steel strips, such as continuous annealing equipment, is as follows:
It usually consists of a heating zone, a soaking zone, and a cooling zone. In the heating zone, the toner zone is heated to 650 to 860°C, but since it is necessary to create a cyclic atmosphere inside the heating zone, the heating zone is heated to 650-860°C.
Tube heating is used, and the heating zone equipment is large-scale.

Therefore, preheating at the entrance of the heating zone becomes important, aiming to make equipment more compact and improve production efficiency.

(Prior art) Regarding preheating on the entry side of the heating zone, JP-A-57-41880 and JP-A-58-73727 disclose a method in which the sensible heat of the combustion exhaust gas in the heating zone is transferred through heat exchange ((;; A method has been disclosed in which the steel strip is preheated by collecting the heat medium into a heat medium, guiding the heat medium to a roll having a flow path for the heat medium therein, and winding the steel strip around the roll.

In this method, the sensible heat of combustion exhaust gas of 800 to 850°C is recovered to the heat medium at a temperature of about 150°C where the acid dew point is not a problem.
If you go to C and preheat the steel strip, the bamboo strip will heat up to about 180~14
It is possible to raise the temperature to 0°Cg degree, and the exhaust heat recovery rate can be increased.

However, the steel strip preheated to about 180 to 140°C is
The main means of rapid heating to 850°C is radiation heating using radiant tubes, and since there is a limit to the heating rate, large and enormous equipment is required to increase production. Furthermore, since the preheating temperature is low, there is still room for improvement in thermal efficiency.

On the other hand, Japanese Unexamined Patent Publication No. 57-76183 describes a heating method using a roll heated by a heating induction coil without using combustion exhaust gas in a heating zone. This publication states that the heating roll is heated to 1000°C and the steel plate is heated from room temperature to 8°C.
An example of heating the steel plate to 00"C is shown, but because there is a large difference between the humidity of the steel plate when it is placed on the heating roll and the humidity of the heating roll, the temperature of the part where the steel plate is wrapped around the outer periphery of the heating roll decreases, causing a concave shape. A thermal crown occurs on the heated roll, causing the steel plate to meander.

In addition, even if the roll profile is manufactured from the beginning with a convex crown in order to stabilize the crown of the heating roll, the humidity of the steel plate will change depending on changes in plate thickness when the steel plate is passed, so the temperature of the steel plate will change. It is very difficult to maintain a stable crown when there is a large difference between the temperature of the crown and the heating roll.

Alternatively, using the sensible heat of the combustion exhaust gas in the heating zone, a preheating furnace is installed on the inlet side of the heating zone and the combustion exhaust gas is directly introduced into the preheating furnace, or a non-oxidizing furnace is installed on the inlet side of the heating zone and stripping is performed. A method of preheating the strip (direct fire method) is also known, but in the former method, the combustion exhaust gas comes into direct contact with the strip, resulting in surface defects such as oxidation of the surface and adhesion of impurities (sulfides, carbides, etc.) in the exhaust gas. In the latter case, equipment costs may increase.

Further, JP-A-80-135580 describes a method in which the sensible heat of the combustion exhaust gas in the heating zone is recovered by air, and the obtained hot air is blown onto the steel strip to preheat it. In this method, the preheating humidity of the steel strip is about 100 to 200°C, and preheating higher than this cannot be expected.

In addition, when the A-band wetness after preheating is in the range of 100 to 200°C, for example, if the steepness of the steel strip is 1%, the defective shape of the steel strip is carried over and remains in the first half of the heating zone. Meandering occurs when the steel strip is threaded, making it impossible to increase the threading speed and reducing production efficiency.

(Problems to be solved by the invention) Multi-stage preheating of the processed original plate at the entrance side of the heating zone achieves high temperature preheating and prevention of meandering of the steel strip, making the heating zone more compact and improving production efficiency. It is an object of this invention to do so.

(Means for Solving the Problems) This invention provides a method for heat-treating a control zone in a radiant tube type heating zone, by using a heat exchanger to recover the sensible heat of the combustion exhaust gas in the heating zone. This method consists of a sequential combination of first-stage preheating by spraying onto the steel strip, then winding the steel strip around a heated roll, and then performing second-stage preheating at a higher temperature than the first-stage preheating. This is a preheating method for continuous heat treatment of a strip.

Now, the steel strip heating device shown in FIG. 1 will be explained.

The steel strip 1 is heated to a temperature of 100 to
250°C in the second preheating section 8 which performs second stage preheating to 200°C.
Preheated to N5'00°C.

First, the combustion exhaust gas from the radiant tube 5 of the heating zone 4 is collected into the exhaust gas collection duct 6 and introduced into the heat exchanger 7, where the sensible heat of the combustion exhaust gas is transferred to and received heat from a gas such as air to lower the temperature, and the exhaust gas is sucked. Heat-resistant temperature of 7 Anne 8 and chimney 9 (
Discharge at temperatures below 400°C (usually around 400°C).

Gas heated by heat exchange (hereinafter referred to as hot air) is circulated by a hot air circulation fan 10 and blown onto the steel strip 1 from the hot air chamber 11 installed in the first preheating section 2.
Heat to 00-200°C.

In addition to air, the injection gas used in the first preheating section 2 is preferably nitrogen or nitrogen mixed with a slight amount of hydrogen.

On the other hand, in the second preheating section 3, the heat medium 12 is heated by the heat medium heating device 18.
It is heated to a predetermined temperature and circulated and supplied into the roll 15 from the circulation pump 14. The roll 15 is heated by the flow of the heat medium 12, and the steel strip 1 from the first preheating section 2 is wound around the roll 15 and passed through to heat the steel strip 1 to 250-500°C. The heat medium 12 that has passed through the roll 15 returns to the tank 17 via the return pipe 1B.

In addition to thermo-oil and metallic sodium, heat carriers include nitrate-based nitrates such as sodium nitrate and potassium nitrate, and molten chloride-based molten salts such as calcium chloride and IJum. A molten salt of is preferred.

In addition, a plate thermometer 18 is installed on the exit side of the first preheating section 2, and
The humidity of the steel strip should be monitored to ensure that it is normally in the range of 1oo to 2υ0°C.If the humidity of the steel strip exceeds 250°C, a thick oxide film may form on the surface of the strip and the surface quality may deteriorate. , hot air circulation fan 1 blowing hot air onto the steel strip
The temperature should not exceed 250°C by controlling the rotation speed at zero or using a damper installed in the piping.

Furthermore, a plate thermometer 19 is installed on the outlet side of the second preheating section 8, and the flow rate and temperature of the heat medium 12 flowing into the roll 15 are adjusted so as to maintain the plate temperature of 250 to 500''C on the outlet side of the second preheating section. , and the wrapping angle of the steel strip.

(Function) Next, the results of the investigation regarding thermal efficiency, meandering of the steel strip, surface treatment properties, and equipment investment are shown in Figures 2 to 9.

The experiment was conducted under the following conditions.

Recording band: General cold rolled steel plate Steel strip thickness: 0.5 to 1.6 Temperature band width near 00 to 1600 m5 Steel strip passing speed: 100 to 800 m/min Steepness of the recording band on the entry side: 0.5-1.6% steel strip tension = 0.5-1
．． 5PC9/w+” Steel strip temperature on the exit side of the first preheating section: 11
10-2 Ll fJ "C second preheating section (""-L heating: 250-5UIJ".

Outlet side steel strip temperature Heat medium: nitrate Heat medium temperature: 200 to 600°C In addition to examples of compliance with the present invention, in Figs. Hot air blowing is shown using a type preheating furnace + radiant tube furnace.

First, as shown in FIG. 2, the thermal efficiency is greatly improved by performing the second stage preheating using roll heating mainly based on heat conduction through metal contact.

Figure 8 is a graph showing the decrease in production efficiency due to the meandering that occurs in front of the heating zone when a steel strip with a steepness of 0.5 to 1.0% is passed before heat treatment. The superiority of this can be confirmed.

Further, FIG. 4 shows a reduction in production efficiency due to meandering similar to that in FIG. 3 in comparison with Comparative Example B. Meandering can be prevented by rapidly raising the temperature to about 500°C by heating the rolls.

In FIG. 6, the horizontal axis represents the difference between the heat medium temperature and the adjustment temperature at the exit side of the second preheating section, and the vertical axis represents the meandering amount per heating roll. It can be seen that when the temperature difference exceeds aOOoC, the meandering becomes severe and is not practical.

Therefore, stepwise heating with little temperature difference is suitable.

The reason why the meandering becomes severe is that when the temperature difference is large, the center of the heating roll is cooled from both sides, increasing the concave crown that occurs on the roll, and the copper sheet tends to move in the direction of increasing tension, so the copper sheet is This is because the object is not stable at the center and tends to move toward the edges.

Figure 6 shows the relationship between the steel strip temperature and the oxide film thickness when the steel strip is heated by blowing air onto the strip in the first preheating section.If the steel strip humidity exceeds 250'C, the oxide film becomes thicker. It can be seen from FIG. 7 that even if reduction is performed after the heating zone, the deterioration of the surface quality cannot be removed.

In addition, FIG. 7 is an observation of the surface state when a chemical conversion treatment was performed after continuous heat treatment, pickling and degreasing. The surface condition after this treatment is shown as surface treatment property.

Moreover, FIG. 8 shows the surface treatment properties when the inside of the second preheating section is made of air and when it is made of a non-oxidizing atmosphere.

When the steel strip is heated to 250 to 500°C in the second preheating section, the oxide film becomes very thick in the air atmosphere.
Even after reduction in the heating zone, the oxide film remains uneven and the surface treatment properties deteriorate.

Furthermore, from Fig. 9, the temperature of the steel strip on the exit side of the first preheating section is 200.
When the temperature exceeds °C, both the investment amount index and the depreciation period index increase, meaning that equipment such as hot air circulation fans, motors, heat exchangers, etc. become huge, and both the investment amount and the depreciation period increase.

Under the same conditions, the temperature of the steel strip on the exit side of the heating strip was 750°C.
The calorific value and flow rate of each gas were as follows.

Combustion gas in radiant tube of O heating zone: 15XI
U kcaj/h 850ONm/h Heating gas from O combustion furnace: 4 X 10 koal/
h2200 (l Nm/h) When the sensible heat of this heated gas was transferred to the air used in the first preheating section via a heat exchanger, the humidity of the heated gas after heat transfer was 600°C to 850°C. became.

On the other hand, the amount of air circulating in the first preheating section is 5ooo.

Nm'/h, and by heat exchange 2 X 106k
A heat amount of cal/h was obtained, and the air humidity at the inlet side of the heat exchanger was 250°C, but it became 880°C at the outlet side.

(Example) A steel strip was heated under the following conditions using the heating apparatus shown in FIG.

(1) 1st preheating section steel strip thickness 0*6~LOm"! Plate width 900~1200 SAN I Threading speed 200~auo +++/min 1st preheating section heating length 5Q+a Circulating hot air m5uouo Nm/Hr (2ooo C) Hot air blowing pressure 50-70° R20 Ivy blowing hot air temperature 250-850°C Heat transfer coefficient: Table 1 shows the adjustment temperature on the exit side of the first preheating section when the temperature is 60 kcal/m2h.

Table 1 (II) Second preheating section heating medium: Glass salt # Temperature: 500°C Heat transmission coefficient between the adjustment zone and the heating medium: 100 (1 to 150 υk
cat/mgh-”C When winding onto a roll, the angle is 120° or more, and the strip is passed through a strip of 0.8-thick sheet at a speed of 2 U.
When heated at 0m/min, the temperature at the entrance was 200°C.

(Effects of the Invention) According to the present invention, it is possible to prevent meandering of the steel strip and achieve high-temperature 7JO heat, thereby improving production efficiency and making equipment more compact.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the heating device, Figure 2 is a graph showing the thermal efficiency of various preheating methods, Figures 8 and 4 are graphs showing the relationship between preheating methods and efficiency reduction due to meandering, and Figure 5 is a heating medium. Figure 6 is a graph showing the relationship between steel strip temperature and oxide film. Figure 7 is a graph showing the relationship between surface treatment and steel strip temperature. Figure 8 is a graph showing the relationship between surface treatment and steel strip temperature. Figure 9 is a graph showing the relationship between atmosphere and surface treatment properties, and Figure 9 is a graph showing the relationship between steel strip temperature and investment and depreciation index.

Claims (1)

[Claims] 1. When heat-treating a steel strip in a radiant tube type heating zone, the sensible heat of the combustion exhaust gas in the heating zone is recovered by a heat exchanger, and a gas is blown onto the steel strip. A preheating method for continuous heat treatment of a steel strip consisting of the following steps: first stage preheating, then wrapping the steel strip around a heated roll and passing it through to perform second stage preheating at a higher temperature than the first stage preheating. . 2. The method according to 1, wherein the second stage preheating is performed under a non-oxidizing atmosphere.