JPH07258740A - Method and apparatus for continuous heating steel slab - Google Patents

Abstract

PURPOSE:To remove the surface defect by restraining the development of oxide scale at the time of heating, oxidizing a steel slab after heating and forming the scale having good peeling property, in a furnace continuously heating the steel slab or steel billet, etc., with a combustion burner. CONSTITUTION:In the method for heating to a prescribed temp. with the combustion burner 2 by sequentially passing the steel slab S through the continuous heating furnace 1, the steel slab S is heated in the condition of restraining the development of the oxide scale and also, the oxidizing gas is blown near the outlet of the furnace to the steel slab S after heating. Further, this apparatus is provided with a heating means for restraining the development of the oxide scale and nozzles 6 for blowing the oxidizing gas to the steel slab near the outlet of the furnace and attached to a heat exchanger 4 in the combustion exhaust gas passing course in the outside of the furnace and arranged with piping 5 for introducing the oxidizing gas heated with this heat exchanger 4 to the nozzles 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for continuously heating steel slabs such as slabs and billets.

[0002]

2. Description of the Related Art When heating a steel slab such as a slab or billet during hot rolling, the steel slab is continuously passed through
A continuous heating furnace that heats to a predetermined temperature by a combustion burner is generally used. Conventionally, in a continuous heating furnace for billet, the in-furnace time is lengthened in order to uniformly heat the billet to a predetermined temperature, and a thick oxide scale is generated on the surface of the billet. Thick oxide scale had some problems such as being deposited in the furnace and its surroundings, deteriorating the environment and reducing the production yield of steel materials, but surface defects that existed before heating were removed by oxidation. It also has an advantage, and little attention was paid to measures against scale generation.

As a known document concerning the atmosphere of such a continuous heating furnace, there is, for example, Japanese Patent Laid-Open No. 63-109118, in which nitrogen oxide in combustion exhaust gas is removed by enriching combustion air of a burner with oxygen. Although it is described to suppress, the formation of billet oxide scale is not considered. On the other hand, by directly connecting the continuous heating furnace and the continuous casting equipment with a roller table or the like, it becomes possible to load the continuously cast high temperature steel slab into the heating furnace. Also in the heating furnace, the heating efficiency has been improved by improving the combustion burner. Furthermore, the combustion atmosphere can be adjusted to be oxygen-free or low-oxygen atmosphere. Therefore, by shortening the heating time of the billet and adjusting the heating atmosphere,
It is possible to suppress the generation of oxide scale.

As a high-efficiency combustion burner for a heating furnace, a heat storage type switching combustion burner is known from Japanese Utility Model Publication No. 3-46742. As shown in the example of FIG. 4, this burner is configured by connecting a pair of burners 11 and 21 and heat accumulators 12 and 22 with an air switching valve 15, and while one burner 21 is igniting. The other burner 11 sucks the exhaust gas in the furnace and stores heat in the regenerator 12. During this time, the exhaust gas passes through the connecting pipe 13 and is exhausted from the exhaust gas pipe 19 through the air switching valve 15, and the air enters the regenerator 22 through the air switching valve 15 and the connecting pipe 23 and is preheated and sent to the burner 21. To be Then, at a predetermined timing, the air switching valve 15 is rotated as shown by the arrow, and the burner 1
1 and the burner 21 are alternately switched to burn. Fuel gas is introduced alternately from the gas introduction pipes 14 and 24.

[0005]

In the continuous heating furnace for steel slabs, it is possible to heat in a short time by charging the hot slabs after continuous casting as described above or by using a high efficiency burner. Moreover, it is possible to suppress the production of oxide scale by adjusting the atmosphere in the furnace. But,
Although continuous casting technology has progressed, it is difficult to eliminate surface defects on steel slabs.Therefore, in the past, operation was carried out in a continuous heating furnace under heating conditions that generated oxide scale, and surface defects were It was removed by oxidation. For this reason, there is a problem in that scale is deposited in the furnace, and the scale causes flaws in the steel slab. The present invention, in the step of continuously heating steel slabs such as slabs and billets with a combustion burner, suppresses the formation of oxidized scale during heating, and oxidizes the steel slabs after heating to produce scale with good peelability and surface defects. It is an object of the present invention to provide a method and an apparatus which eliminates the above and solves the problem of scale deposition in a heating furnace.

[0006]

Means for Solving the Problems The method of the present invention for achieving the above-mentioned object is a method of passing steel pieces one after another in a continuous heating furnace and heating them to a predetermined temperature by a combustion burner, in which an oxide scale of the steel pieces is obtained. It is a continuous heating method for a steel slab, characterized in that the steel slab is heated under the condition that the generation is suppressed, and an oxidizing gas is blown to the heated steel slab near the outlet in the furnace.

Further, the apparatus of the present invention is provided with a heating means for suppressing the formation of oxide scale in a continuous heating furnace in which steel pieces are passed through one after another and heated to a predetermined temperature by a combustion burner, and an oxidizing gas is sprayed on the steel pieces. A nozzle is provided in the vicinity of the outlet in the furnace, a heat exchanger is attached to the combustion exhaust gas path outside the furnace, and a pipe for guiding the oxidizing gas heated by the heat exchanger to the nozzle is provided. It is a continuous heating device for billets. Further, it is preferable that the heating means is a heat storage type switching combustion burner.

[0008]

The present invention will be described below with reference to the drawings. FIG. 1 shows an example of an apparatus of the present invention, in which a steel slab S such as a slab is
In the continuous heating furnace 1, in the direction of the arrow, that is, from right to left in the figure,
It is conveyed one after another by a walking beam (not shown) or the like, and heated to a predetermined temperature by the combustion burner 2. The combustion exhaust gas in the furnace is discharged from a duct 3 provided at the outlet of the continuous heating furnace 1. According to the method of the present invention, in such a method for heating a steel slab in a continuous heating furnace, the steel slab is heated under the condition that generation of oxide scale is suppressed. As conditions for suppressing the generation of oxide scale, heating is performed for a short time by using a known regenerative combustion burner as shown in FIG. 3, and in various combustion burners, the air ratio is reduced to anoxic or low oxygen atmosphere. Adjustment or direct application of the reducing flame of the combustion burner to the billet can be adopted.

Further, according to the method of the present invention, an oxidizing gas is blown onto the heated steel slab S in the vicinity of the outlet in the furnace. A plurality of nozzles 6 for spraying are arranged toward the upper surface and the lower surface of the steel slab S in the width direction of the furnace. A high temperature oxidizing gas is supplied to each nozzle 6 from a heat exchanger 4 attached to the exhaust gas duct 3 through a pipe 5. As oxidizing gas,
Steam, high temperature air, oxygen or the like can be used.
According to such a method of the present invention, generation of oxide scale during heating is suppressed, and oxide scale is generated by blowing a high-temperature oxidizing gas to the steel slab after being heated to a predetermined temperature. Since the generation of the oxide scale is controlled in a high temperature oxidizing gas atmosphere, it is uniform and has good peeling property.
The thickness of the oxide scale can be controlled by adjusting the spraying amount according to the type and depth of the surface flaws such as the beard and sliver existing in the steel piece before heating. The steel slab that has generated the oxide scale is extracted outside the furnace, and then the scale is peeled off by a scale breaker before hot working to remove surface flaws.

With respect to the steel slab that has produced oxide scale near the outlet in the furnace, even if a part of the scale may fall off during the transportation process after extraction to the outside of the furnace, the scale will almost always fall off in the furnace. Since it does not occur, scale does not accumulate in the furnace as in the conventional case. Further, since the combustion exhaust gas flows toward the duct 2 in the traveling direction of the steel slab S near the outlet in the furnace, the sprayed oxidizing gas does not affect the heating atmosphere in the furnace, and the steel after heating is heated. It acts only on the surface of the piece S.

Further, in the method of the present invention, as shown in FIG. 2, the exhaust duct 3 is installed on the inlet side of the furnace, and the combustion exhaust gas is caused to flow in the direction opposite to the moving direction (the direction of the arrow) of the steel slab S, thereby increasing the thermal efficiency. It can also be applied to a heating furnace with improved temperature. However, in this case, in order to prevent the sprayed oxidizing gas from diffusing in the inlet side direction of the furnace and oxidizing the atmosphere inside the furnace, a partition wall 26 as shown in FIG. The outlet door 27 is provided in addition to the one provided with the combustion burner 2, the oxidizing gas blowing direction is inclined to the outlet direction, the outlet door 27 is opened at the same time as the blowing, and the steel slab S is transported to the outside of the furnace at the end of the blowing. Adopt a system that does. For the partition wall 26 and the opening / closing door 27, refractory brick or plastic refractory can be adopted. Further, for the partition wall 26, a skirt-shaped structure made of zirconia-based or alumina-based ceramic fiber can also be used.

In the method of the present invention, the nozzles 6 for blowing an oxidizing gas are provided in a distributed manner in a direction intersecting with the conveying direction of the steel slab to remove surface flaws on the steel slab before charging in the heating furnace or in the heating furnace. By detecting the position and type and controlling the amount of oxidizing gas sprayed from each nozzle according to the detection result, it is possible to minimize the amount of oxide scale produced and remove surface defects.

Next, the apparatus of the present invention is provided with a heating means for suppressing the production of oxide scale in the continuous heating furnace 1 as shown in the examples of FIGS. 1 and 2. As a heating means that suppresses the generation of oxide scale, a known heat storage type switching combustion burner as shown in FIG. 4 is adopted as the combustion burner 2.
Among various combustion burners, those having a reduced air ratio and adjusted to an oxygen-free or low-oxygen atmosphere, or those arranged so that the reducing flame of the combustion burner directly hits the steel billet may be employed.

Further, in the apparatus of the present invention, a heat exchanger 4 is attached to a combustion exhaust gas path outside the furnace such as the duct 3 of FIGS. 1 and 2, and a nozzle 6 for blowing an oxidizing gas to the steel slab S is installed in the furnace. A pipe 5 is provided in the vicinity of the outlet of the inside to guide the oxidizing gas heated by the heat exchanger 4 to the nozzle 6. The nozzles 6 are provided above and below the steel slab S so as to be distributed in a direction intersecting the transportation direction of the steel slab so that the oxidizing gas is blown onto the entire surface of the steel slab S. It is also possible to provide a flaw detection device for the steel slab S on the front surface of the continuous heating furnace 1 or in the furnace, and control the amount of oxidizing gas sprayed from each nozzle 6 according to the detection result.

FIG. 3 shows an example in which a heat storage type switching combustion burner is used as the heating means in the apparatus of the present invention. A plurality of heat storage type switching combustion burners 7 and 8 (four in each figure are shown) are arranged above and below a steel slab S conveyed in the continuous heating furnace 1 in the direction of the arrow. The heat storage type switching combustion burners 7 and 8 are respectively burners 11 and 21.
And the heat accumulators 12 and 22, and are connected by the connecting pipes 13 and 23 to the air switching valve 15 and the gas introducing pipes 14 and 24 to the gas switching valve 16. The air switching valve 15 is connected to the air pipe 17 via the air adjusting valve 20, and the gas switching valve 16 is connected to the gas pipe 18 via the gas adjusting valve 25. Both switching valves 15 and 16 operate at the same time to alternately combust the regenerative switching combustion burners 7 and 8.

When burning the upper combustion burner 1,
Fuel gas is supplied from the gas pipe 18 to the combustion burner 7 via the gas regulating valve 25, the gas switching valve 16 and the gas introducing pipe 14, and the air is fed from the air pipe 17 to the air regulating valve 20, the air switching valve 15 and the connection. It is supplied to the combustion burner 7 via the pipe 13 and preheated in the heat storage device 12. During this time, the lower combustion burner 8 sucks the exhaust gas in the furnace, stores heat in the heat storage device 22, and guides it to the exhaust gas pipe 19 via the air switching valve 15.
A heat exchanger 4 is attached to the path of the exhaust gas pipe 19, and a nozzle 6 for blowing a high temperature oxidizing gas to the steel slab S through a pipe 5 is provided near the outlet in the furnace.

The temperature inside the furnace is controlled by operating the gas adjusting valve 25 and the air adjusting valve 20 on the basis of the information obtained in advance to adjust the supply amounts of fuel gas and air to control combustion. Further, the switching valves 15 and 16 are operated at an appropriate timing according to the supply amounts of the fuel gas and the air to switch the combustion of both combustion burners 7 and 8. It is effective that the switching timing is shorter than the time when the temperature of the heat storage device 12 or 22 is saturated.

In such an apparatus of the present invention, rapid heating can be carried out by the heat storage type switching combustion burner, so that during heating, generation of oxidized scale of the steel slab is suppressed,
After heating, an oxide scale having a thickness corresponding to the type and depth of the flaw can be produced. In the present invention, when the heat storage type switching combustion burner is adopted, the combustion burners alternately burned may be paired up and down as in the example of FIG. 3, or may be paired on both sides of the continuous heating furnace 1. Yes, continuous heating furnace 1
Various combinations can be adopted depending on the capacity and heating temperature of the furnace, for example, by providing combustion burners on the ceiling and pairing the burners on the ceiling, or pairing the burners on the ceiling and the side burners.

[0019]

EXAMPLE In a continuous heating furnace as shown in FIG. 1, a mild steel continuous cast slab was heated to 1000 ° C. in a low oxygen atmosphere by adopting a heat storage type switching combustion burner, and then at the furnace outlet, the slab conveying direction was changed. Water vapor at 200-300 ° C for 20-30m from a full cone nozzle installed orthogonally every 1m.
Sprayed at a flow rate of / sec. Oxide scale was formed on the entire surface of the slab extracted from the furnace, and it did not peel off in the furnace, and the entire surface was easily peeled off by the scale breaker after extraction. As a result of observing the surface defects of the slab before heating and adjusting the spraying time according to the type and depth of the surface defects, no surface defects were observed in the slab after scale peeling.

[0020]

According to the present invention, in the step of heating a steel slab such as a slab or billet with a combustion burner in a continuous heating furnace, the production of oxide scale is suppressed during heating, and the oxide scale is thickened at the furnace outlet after heating. It can be generated by adjusting the height. The produced oxide scale has good peelability,
Surface defects of slabs can be easily removed as scales, and the thickness of scales can be partially controlled according to the type and depth of surface defects, thus minimizing yield loss due to scale loss. . Since the generated scale is difficult to peel in the furnace, there is no problem of scale deposition in the heating furnace. Further, since oxide scale is generated at the outlet of the furnace, there is no heating loss due to the scale layer in the furnace.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing an example of a device of the present invention.

FIG. 2 is a vertical cross-sectional view showing another example of the device of the present invention.

FIG. 3 is a vertical cross-sectional view showing another example of the device of the present invention.

FIG. 4 is a cross-sectional view of a known heat storage type switching combustion burner.

[Explanation of symbols]

 1: Continuous heating furnace 2: Combustion burner 3: Duct 4: Heat exchanger 5: Piping 6: Nozzle 7,8: Heat storage type switching combustion burner 10: Furnace wall 11,21: Burner 12,22: Heat storage device 13, 23 : Connection pipes 14, 24: Gas introduction pipes 15: Air switching valve 16: Gas switching valve 17: Air pipe 18: Gas pipe 19: Exhaust gas pipe 20: Air adjusting valve 25: Gas adjusting valve 26: Partition wall 27: Open / close door S: Steel billet

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location F23D 14/66 B F27B 9/04 F27D 7/02 A 7727-4K

Claims (3)

[Claims] 1. A method in which steel billets are passed through a continuous heating furnace one after another and heated to a predetermined temperature by a combustion burner, wherein the billet is heated under the condition that the oxide scale formation of the billet is suppressed and A continuous heating method for a steel slab, which comprises spraying an oxidizing gas on the steel slab near an outlet in a furnace. 2. A continuous heating furnace in which steel pieces are passed through one after another and heated to a predetermined temperature by a combustion burner, a heating means for suppressing generation of oxide scale is provided, and a nozzle for blowing an oxidizing gas to the steel pieces is provided in the furnace. Of the steel slab provided with a heat exchanger provided in the vicinity of the outlet of the furnace and attached to the combustion exhaust gas path outside the furnace, and a pipe for guiding the oxidizing gas heated by the heat exchanger to the nozzle. Heating device. 3. The continuous heating device for billet according to claim 2, wherein the heating means is a regenerative combustion burner.