JPS5811493B2 - Continuous annealing equipment for cold rolled steel strip - Google Patents

Description

[Detailed description of the invention] The present invention relates to continuous annealing equipment for cold rolled steel strip.

Conventionally, in the continuous annealing method of cold rolled steel strip, during heating,
Indirect heating is performed using a radiant tube burner.

When implementing this method, it is necessary to remove in advance cold rolling oil and iron powder generated during cold rolling, which are attached to the surface of the cold rolled steel strip and would deteriorate the surface quality of the product if annealed as is. Therefore, a cleaning section consisting of an alkaline soaking tank, a brush scrubber, an electrolytic degreasing tank, etc. is placed on the inlet side of the furnace section of the line. approx. 10%) and increase in operating costs (
(steam, water, alkali, etc.) are becoming a major problem.

In order to solve this problem, direct heating using a direct-fired burner (hereinafter referred to as "direct-fired heating") used in molten galvanizing lines or continuous annealing lines for electrical steel sheets has been developed.
If it can be introduced, (1) the above-mentioned cleaning section can be omitted due to the gas cleaning effect of the combustion flame and waste gas flow, reducing equipment costs and operating costs, and (11) it will be freed from restrictions on the heat-resistant temperature of the radiant pipe. This enables rapid heating at the blast furnace temperature, which in turn shortens the length of the furnace, reducing equipment costs including building costs, and (~) eliminates the cost of radiant pipe maintenance, which accounts for most of the furnace maintenance costs. It brings about many advantages such as:

However, when employing direct flame heating for continuous annealing of cold-rolled steel sheets, consideration must be given to the appearance and surface properties of the resulting product.

As mentioned in Saen, direct flame heating is used in hot-dip galvanizing lines and continuous annealing lines for electrical steel sheets, but since these products are coated with thick plating or insulating film, the appearance and surface properties of the steel sheets themselves may be affected. There are not very strict requirements regarding this.

However, in the case of cold-rolled thin steel, so-called "cold-rolled steel sheets"
Since it is often shipped as is, there are strict requirements regarding the surface quality.

Moreover, cold-rolled thin steel strips are not only shipped as cold-rolled steel sheets and painted and used by customers, but also processed into plated products such as electrogalvanized steel sheets and turn (Sn-Pb) plated steel sheets within the steelworks. Therefore, in addition to paintability (chemical conversion treatment properties, paint adhesion, and paint corrosion resistance), plating properties (plating adhesion and plating corrosion resistance) are also important aspects of surface properties.

Of course, it is also important that cold rolling oil and iron powder are not attached.

This is because incomplete removal of cold rolling oil and iron powder causes surface quality deterioration.

In addition to normal surface flaws, there are also problems with appearance, such as temper color (oxidized film generated by direct flame heating that remains due to insufficient reduction) and soot (cold rolling oil burn off residue or unsuitable direct flame atmosphere). These include carbon precipitation) and scale pick-up flaws (depression flaws caused by iron powder during cold rolling and oxide film generated by direct heating being picked up by the hearth roll).

The purpose of the present invention is to provide equipment by providing a vertical direct-fired heating zone that performs inclined combustion, a reduction zone that indirectly heats, a primary cooling zone that allows controlled cooling, and an overaging zone that is equipped with a heater element. The object of the present invention is to provide a continuous annealing facility that can significantly reduce operating costs and produce cold-rolled thin steel strips with excellent appearance and surface properties.

The present invention sequentially includes, from the entrance side of the furnace section, a preheating zone that utilizes combustion waste gas, a direct heating type heating zone equipped with a direct-fired burner controlled by inclined combustion equipment, a radiant tube type burner, and/or
or a primary cooling zone, an overaging zone, and a secondary cooling zone equipped with one or more of indirect heating type reduction zone equipped with a heater element, gas jet cooler, gas-liquid cooling, and boiling water cooling, and at least The gist of the present invention is a continuous annealing facility for cold-rolled steel strip, characterized in that the heating zone and subsequent parts are arranged in the form of a vertical furnace.

The present inventors have discovered that when direct flame heating is adopted for continuous annealing of cold rolled thin steel strips, copper at the exit of the direct flame heating furnace (hereinafter referred to as direct flame furnace) is an influential factor on the appearance and surface properties of the copper strip. There are the zone temperature, air ratio, and reduction conditions in the reduction zone, and (
It was confirmed that the direct-fired furnace outlet temperature and air ratio have a large influence on the steel sheet surface properties.

In other words, if the temperature of the steel strip at the exit of the direct-fired furnace (the copper strip temperature at the final stage in the direct-fired heating zone) is too low, the removal of iron powder produced during cold rolling will be insufficient, while if it is too high, the copper strip produced in the direct-fired furnace will be removed. The oxide film on the surface becomes thick (and cannot be completely reduced in the next reduction zone and remains on the surface of the copper strip, resulting in a poor appearance called "temper color", and even if it is completely reduced, the oxide film that has formed once It has been revealed that the layer remains as a porous layer and causes pinholes during painting or plating, which basically deteriorates paint corrosion resistance and plating corrosion resistance.

Figure 4 shows the influence of the steel strip temperature at the outlet of the direct-fired furnace and the air ratio (described later) on the surface texture.

From FIG. 4, it can be seen that the appropriate range of the temperature of the steel strip at the outlet of the direct-fired furnace should be maintained at 400 to 900°C, judging from the oxide film thickness and the amount of residual iron powder.

The air ratio also has a large effect on the formation of the oxide film mentioned earlier.

As is clear from Fig. 4, when the air ratio is less than i, o, the atmosphere in the direct-fired furnace is a slightly oxidizing atmosphere, and oxidation proceeds slowly due to the reaction Fe+C02-+FeO+C0 Fe+H2O→FeO+H2.
．． If it exceeds 0, the surface of the steel sheet will be violently oxidized due to the reaction caused by the presence of free 0°.

Therefore, the air ratio at the outlet of the direct-fired furnace, where the plate temperature is high and oxidation tends to proceed, should not exceed 1.0.

On the other hand, if the air ratio is too low, thermal efficiency will decrease and fuel consumption will increase, so even if the inclined combustion method described below is implemented in the furnace, the air ratio at the outlet of the direct-fired furnace should be maintained at 0.6 or more.

Furthermore, in order to increase the thermal efficiency of the direct-fired furnace while ensuring product quality, the air ratio is gradually decreased from the inlet to the outlet of the direct-fired furnace as the temperature of the copper strip increases, as shown in Figure 5. It is necessary to adopt the inclined combustion method.

In the conventional method, the air ratio setting was constant in the direct-fired furnace, which caused problems between product quality and combustion efficiency. On the other hand, at the inlet of the direct-fired furnace where the copper zone temperature is low and the oxidation rate is slow, the air ratio is increased so that the air containing unburned matter flows from the outlet side with a low air ratio ( By adopting a tilted fuel method that fully burns the combustion waste gas, it is possible to maintain a high level of thermal efficiency as a whole of the direct-fired furnace, and to thin (control) the oxide film on the surface of the steel strip at the outlet of the direct-fired furnace. becomes possible.

In addition, in order to improve thermal efficiency, it is necessary to effectively utilize the sensible heat of the direct-fired furnace combustion waste gas, and in order to do so, according to the present invention, a preheating zone is provided on the inlet side of the direct-fired furnace, and the waste gas and the steel strip are opposed to each other. Heat exchange by flow, ie preheating of the copper strip.

At this time, in the preheating zone, the remaining unburned fuel is completely combusted by operating the direct-fired furnace at a low air ratio.

Note that it is advantageous to use the waste gas leaving the preheating zone to guide the recuperator and preheat the air for combustion in the direct-fired furnace.

As mentioned above, the copper strip whose oxide film formation has been suppressed during heating in a direct-fired furnace is then reduced in a reduction furnace.During this reduction, depending on the thickness of the oxide film, for example, hydrogen is
~15%, preferably 3 to 6% from the viewpoint of explosion prevention, and the balance is nitrogen (dew point is 10°C or lower, preferably 0°C or lower) for 10 seconds or more at a temperature of 600°C or higher, preferably 30°C. It is sufficient to hold it for more than a second.

Note that the upper limit of the holding time is preferably 90 seconds from the standpoint of equipment economy.

As described above (according to the present invention), the equipment consisting of a preheating zone, a direct-fired heating zone, a reduction zone, and a cooling zone is of the type of vertical furnace for the following reasons.

(1) In recent high-capacity, high-speed lines, if a horizontal furnace is used, the furnace length will be long and equipment costs, including building costs, will be relatively high.

On the other hand, using a vertical furnace allows for compact equipment and space savings.

(11) In the vertical furnace, the oxidation reaction in the direct-fired furnace and the reduction reaction in the reduction zone are performed uniformly on both sides of the copper strip, resulting in favorable product characteristics.

(rrr) In the vertical furnace, there is less slip between the hearth roll and the copper band in the direct-fired furnace/reduction zone (it is also advantageous for insulation to prevent scale pickup.

(iv) When a vertical furnace is used, the direct-fired furnace and the reduction zone are connected only through the lower throat portion, and the atmosphere between the direct-fired furnace and the reduction zone can be easily separated by controlling the furnace pressure.

The furnace height of a vertical furnace is determined from the viewpoint of equipment costs and operational aspects (plate breakage,
Considering the possibility of fluttering, etc., the distance between the upper and lower mousse roll cores is preferably about 15 to 25 m.

An embodiment of the installation according to the invention is shown in FIG.

1 is a winding machine, 2 is an entrance shearing machine, 3 is a welding machine, 4 is an entrance looper, 5 is a preheating zone, 6 is a direct-fired heating zone (direct-fired furnace), 1
8 is a reduction zone, 8 is a primary cooling zone, 9 is an overaging zone, 10 is a secondary cooling zone, 11 is an exit side looper, 12 is a temper rolling mill, 13
14 is an inspection and finishing section, 14 is an output shearing machine, and 15 is a winding machine.

The furnace parts 5 to 10 are constructed so as to realize the annealing cycle shown in FIG. 6, which will be described later.

The preheating zone 5 uses the combustion waste gas from the direct-fired furnace 6 as a heat source.
is equipped with an open flame burner, and the reduction zone 7 consists of one or both of a heating zone equipped with a radiation-opening burner and a soaking furnace equipped with a heating element.

For the primary cooling zone 8, one or several types of cooling methods such as gas jet cooler cooling, gas-liquid cooling, or boiling water cooling are used.

The overaging zone 9 is equipped with a heater element, and the secondary cooling zone 10 uses one or more types of cooling such as a gas jet cooler, cold water cooling, or airplast.

Although the direct-fired furnace 6 is shown as having one pass in FIG. 1, multiple passes are also possible.

Plastic bricks or the like are used as the fireproof material.

As a countermeasure against copper strip breakage during an emergency stop, measures such as relaxing the tension of the copper strip and cooling with N2 injection are taken.

FIG. 2 is a diagram showing details of the preheating zone and the direct heating zone.

21 is a steel strip, 22 is a preheating zone, 23 is a direct heating zone, 24
25 is a soaking zone, 26 is a reduction zone, 27 is a hearth roll compartment, 28 is a seal roll, 29 is a waste gas outlet, 30 is a direct burner, 31 is a heating zone 23 and a burner 30. A zone partition wall partitioning each group, 32 is a radiant tube burner disposed within the radiant tube heating zone 24, 33 is a heater element disposed within the soaking zone 25, 34 is a recuperator, and 135 is a waste gas blower. 136 is a combustion air blower, 37 is a fuel gas, 38 is a preheated air, and 39 is a waste gas.

FIG. 3 is a diagram showing an example of a control method for implementing the inclined combustion method according to the present invention in the direct-fired heating zone 33.

A plurality of burners 30 facing each other and having axes parallel to the surface of the copper strip to be threaded are arranged in the direct-fired furnace 23, and these burners are divided into a plurality of zones for each group of burners by a zone partition wall 31. It is divided into

Further, each zone is connected to an air and fuel supply source through a supply conduit connecting an air valve 40 and a fuel valve 41.

Further, an air ratio controller 42 for adjusting an air valve 40 and a fuel valve 41 is provided for each zone.

Thus, the air ratio controllers 42 of each zone are all connected to a common zonal air ratio distribution setter 43.

The setting device 43 instructs each air ratio regulator 42 to a predetermined air ratio, and each regulator 42 adjusts the air valve 40 and fuel valve 41 so that the predetermined air ratio is achieved.

The air distribution ratio setting for each zone is variable depending on the heat cycle in the direct-fired furnace.

Next, an example of operation using the equipment of the present invention will be explained.

FIG. 6 shows an example of an annealing cycle when operating using the continuous annealing equipment of the present invention.

C0.05%, Si0.01%, Mn0.23%, PO
A cold rolled steel strip obtained by cold rolling a capped steel having a composition of .014% and 80.012% was passed through the continuous annealing equipment of the present invention.

As mentioned above, the temperature at the exit of the steel strip of the heating zone of the direct heating method should be 400° C. or higher and 900° C. or higher from the viewpoint of the generated oxide film and residual iron powder.

In FIG. 6, the temperature was set at 600°C.

In this case, the combustion air ratio is 1.0 at the initial stage of the heating zone.
~1.6, and less than 0.6 to 1.0 in the final stage, so in the case of the annealing cycle shown in Figure 6, the combustion air ratio is 1.4 in the initial stage and 0.6 in the final stage.
It was set as 9.

Next, the reduction time t1 in the reduction zone depends on the hydrogen concentration in the atmosphere, but the retention time is 10
It is necessary to set the time to at least 30 seconds, preferably at least 30 seconds and at most 90 seconds.

In FIG. 6, the hydrogen concentration was 4% and the time was 60 seconds.

Soaking temperature T2 and soaking time t2 should be determined from the metallurgical aspect of recrystallization and grain growth, and are usually 600°C or higher and 90°C or higher.
10 seconds or more, preferably 20 seconds or more at 0°C or below6
The heating time is 0 seconds or less, but in FIG. 6, the heating time is 700° C. and 40 seconds.

Following the above heat treatment, if the secondary cooling zone and overaging zone are passed through and cooled in the secondary cooling zone, it is possible to produce ultra-low carbon materials that have been vacuum degassed, materials that have C fixed with Ti, etc., or composite structure types. In the case of high-tensile steel plates, annealing is completed.

On the other hand, in the case of ordinary cold-rolled steel sheets such as the above-mentioned capped steel or aluminum guild steel, it is necessary to perform an over-aging treatment to prevent carbide precipitation. , the overaging start temperature T3 was set to 500
~400°C, the overaging end temperature T4 is 400~250°C, and the overaging time t3 is 30 seconds or more, preferably 1 minute or more and 4 minutes or less.

In Figure 6, T3: 450℃, T4: 300℃, t
3:3 minutes.

After this annealing is completed, except for composite structure type high-strength steel sheets, etc., which are used as products as is, ordinary cold-rolled steel sheets such as the above-mentioned capped steel undergo removal of yield point elongation, shape correction, or surface treatment. To adjust the roughness, a skin pass mill connected to the cooling zone through a looper is used to adjust the roughness to 0.3
Temper rolling is performed at an elongation rate of % to 7%, usually 0.5% to 1.5%.

At this time, if the steel strip temperature is too high, the yield point elongation will recover immediately and flatness cannot be ensured due to the thermal crown of the work roll, while if the steel strip temperature is too low, moisture in the atmosphere will reach the surface of the copper strip. Since dew condensation occurs, the temper rolling temperature T5 should be 30°C or more and 50°C or less.

In the case of FIG. 6, the temperature was 40°C. In this way, a cold-rolled steel sheet with excellent appearance and surface properties is obtained.

As is clear from the above explanation, the equipment of the present invention can produce high-quality products in large quantities at low cost when continuously annealing cold-rolled thin steel strips, so it is extremely beneficial to the industry. It is.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the arrangement of the continuous annealing equipment for cold rolled thin steel strips of the present invention, and Figure 2 is the same (detailed diagram of the pre-heating zone, direct-fire heating zone, radiant tube heating zone, and soaking zone). Figure 3 is a diagram showing how the combustion air ratio is controlled by zone in a direct-fired heating zone, Figure 4 is a diagram showing appropriate operating conditions in a direct-fired heating zone, and Figure 5 is a diagram showing combustion in a direct-fired heating zone. A diagram showing the air ratio distribution, and FIG. 6 is a diagram showing an annealing cycle suitable for operation of the equipment of the present invention. 1... Unwinding machine, 2... Entry side shearing machine, 3... Welding machine, 4...
... Entrance looper, 5... Preheating zone, 6... Direct-fired heating zone,
7... Reduction zone, 8... Primary cooling zone, 9... Overaging zone, 1
0... Secondary cooling zone, 11... Output side looper, 12... Temper rolling mill, 13... Inspection and refinement department, 14... Output side shearing machine,
15... Winding machine, 21... Steel strip, 22... Preheating zone, 23
...Direct fire type heating zone, 24...Radiation tube type heating zone, 25...
Soaking zone, 26...Reduction zone, 27...Hath roll branch,
28... Seal roll, 29... Waste gas outlet, 30...
... direct flame burner, 31 ... zone partition wall, 32 ... radiation tube burner, 33 ... heater element, 34 ...
...Recuperator-135...Waste gas blower, 36...
Combustion air blower, 31...fuel gas, 38...preheating air, 39...waste gas.

Claims (1)

[Claims] 1 From the entrance side of the furnace section, a preheating zone that utilizes combustion waste gas, a direct heating heating zone equipped with a direct burner controlled by inclined combustion equipment, a radiant tube burner, and/or a heating element are installed. A primary cooling zone, an overaging zone, and a secondary cooling zone equipped with one or more types of indirect heating type reduction zone, gas jet cooler, gas-liquid cooling, and boiling water cooling are provided, and at least after the heating zone, a vertical cooling zone is provided. Continuous annealing equipment for cold rolled steel strip characterized by being arranged in the form of a mold furnace.