JP2723759B2 - Batch annealing method and apparatus for steel - Google Patents

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 batch annealing steel materials, for example, a batch annealing method suitable for annealing a ferritic or martensitic stainless steel sheet after hot rolling in an H ₂ gas atmosphere. Method and apparatus.

[0002]

2. Description of the Related Art An outline of an annealing and pickling process for a hot-rolled ferritic or martensitic stainless steel sheet will be described with reference to FIG. A stainless steel slab heated in a heating furnace (not shown) is rolled by a hot rolling mill 10, and the hot-rolled steel sheet is wound into a coil by a coiler 12. Next, the coil is re-wound into a coil having a size convenient for annealing in the coil build-up facility 14, and three to four of these coils are stacked in a bell-type
Time annealing is applied. Since a large amount of scale adheres to the surface of the steel sheet after annealing, the scale is removed by an annealing pickling line (HAPL) including an annealing furnace 18, a mechanical scale removing device 20, and a pickling tank 22. You. However, since the annealing has already been performed in the bell-type annealing furnace 16, in this case, only the annealing furnace 18 passes, and in HAPL, the scale is substantially removed in the mechanical scale removing device 20 and the pickling tank 22. Is only applied.

Thus, for example, ferritic or martensitic stainless steel, ordinary steel, high carbon steel, etc.
After being subjected to hot rolling, most of it is annealed in a batch furnace and subjected to a descaling treatment. Next, an example of an annealing furnace used in this annealing will be described with reference to FIG.

FIG. 6 is a sectional view showing a schematic structure of the bell type annealing furnace 16 shown in FIG. The bell type annealing furnace 30 has an outer cover 34 which is a protective cover for the furnace wall refractory 32.
And a plurality of burners 36 attached to the furnace wall refractory 32 and the outer cover 34, and a base 38 for stacking the coiled steel sheets S. The burner 3 is provided inside the furnace wall refractory 32.
6, an inner cover 40 for preventing direct contact between the combustion gas and the steel sheet S and keeping the annealing atmosphere non-oxidizing, and circulating the atmosphere gas in the inner cover 40 to the steel sheet S. A furnace air circulation fan 42 for promoting heat conduction is provided.

In this bell type annealing furnace 30, steel sheets S are stacked on a base 38, and the steel sheets S are
After being covered with the outer cover 34 and the outer cover 34, annealing is started. Since this annealing process is performed for a long time of about 80 hours, the oxide scale on the steel sheet surface may grow significantly during the annealing depending on the characteristics of the atmosphere gas in the furnace.

Conventionally, in order to suppress the growth of the oxide scale, various non-oxidizing gases have been used as an annealing atmosphere gas. Oxidizing component recently, because of it, and a manufacturing method carbide component is small things and easy inexpensive, fever endothermic furnace gas (HNX gas, H ₂ content of 4% to 20%), and the like are widely used. However, there are two problems with the conventional batch annealing method even when a non-oxidizing gas such as the exothermic endothermic furnace gas is used as the atmospheric gas. The first is a problem that the annealing time is extremely long, and the second is a problem of oxide scale adhering to the steel sheet surface.

In order to solve the problem of the long time annealing and oxide scale, focusing on the high thermal conductivity and reducing the H ₂ gas, a method of using high-purity H ₂ gas was developed as the annealing atmosphere gas . Since the high thermal conductivity by using a high purity H ₂ gas is obtained, the heating time and reduce the cooling time allows the annealing time of the steel sheet as compared with the case of using HNX gas 50 as an atmospheric gas The problem was reduced to 60%, and one of the problems, that is, the problem of long-time annealing was solved.

[0008]

However, reduction of the scale made of oxides such as Cr and Fe formed on the surface of the steel sheet during rolling and prevention of generation and growth of the scale during annealing require the use of high-purity H ₂ gas. Just using it is not enough. For this reason, the scale removal treatment after annealing is performed by a mechanical scale removal device and a pickling tank. However, since the acid used for the pickling treatment is a strong acid, the purchase of the acid and the waste acid treatment are performed. However, the production efficiency cannot be increased because of the high cost and the limited pickling treatment speed. Furthermore, the use of shot blasting and the use of the above-mentioned strong acids, particularly nitric hydrofluoric acid, hinder improvement in surface properties.

In view of the above circumstances, it is an object of the present invention to reduce the scale made of oxides such as Cr and Fe formed on the surface of a steel sheet during hot rolling during annealing and to prevent the generation and growth of the scale. It is a first object of the present invention to provide a batch annealing method for steel materials in which the load of scale removal treatment after annealing is reduced. By the way, in order to achieve the first purpose of the batch annealing method, a decompression device is required as described later. Generally, a decompression device (for example, a booster pump, an ejector, etc.) is a large-scale facility. If a single decompression device is installed in one batch annealing furnace, the equipment cost increases, and the annealing device becomes complicated in terms of layout and measurement control.

Accordingly, a second object of the present invention is to provide a batch annealing apparatus having relatively low equipment costs and easy measurement control.

[0011]

Means for Solving the Problems The present inventor has found that, for example, moisture adhering to a stainless steel plate or moisture adsorbed by a refractory on a base portion is heated in an annealing furnace, and steam during annealing. As a result of conducting various experiments and studies, focusing on the fact that the dew point of the annealing atmosphere gas in the inner cover rises to a maximum of about + 5 ° C. and that the growth of oxide scale is further promoted, It has been found that reducing the pressure in the furnace after charging the steel material into the annealing furnace reduces the scale or prevents the growth of the scale, and has accomplished the present invention.

[0012] Specifically, the batch annealing method to achieve the first object of the present invention, (1) a batch furnace steel instrumentation after turning 0.3 kg / cm ² or more
A step of decompression down, (2) the batch furnace by the vacuum after the inert gas atmospheric
A step of pressure recovery until pressure above atmospheric pressure, (3)該復After depressurization of H ₂ gas and inert gas in the batch furnace
A step of annealing said steel is replaced with the scan with H ₂ gas atmosphere, (4) after said annealing is completed, the temperature of the steel product 100
Step of stopping the supply of H ₂ gas when the temperature falls below ℃
And characterized in that:

A batch annealing apparatus for achieving the second object of the present invention is a batch annealing apparatus including a plurality of bell-type annealing furnaces having an inner cover, wherein: (1) 0.3 kg / and a single decompression apparatus for decompressing the cm ² or less, (2) a gas passage for binding and said inner cover disposed in parallel and a plurality of single decompression apparatus the opening and closing the (3) each of said gas passage a shut-off valve which, (4) N ₂ gas and the condensate pressure inside each inner cover
A gas passage for supplying the H ₂ gas be further substituted with the N ₂ gas, it is characterized in that a shut-off valve for opening and closing the (5) each gas passage.

The shut-off valve may be electrically operated or manual. When using an electric shutoff valve, the control is further facilitated by providing a centralized control device for controlling the shutoff valve. Next, an experiment on which the present invention is based will be described. In this experiment, a bell-type annealing furnace having an inner cover was used, and a decompressor for reducing the pressure inside the inner cover was also used. This bell type annealing furnace has the same basic configuration as the bell type annealing furnace shown in FIG.

An experimental method will be described with reference to FIG. FIG. 1 shows the elapsed time on the horizontal axis and the furnace pressure on the vertical axis.
It is a graph which shows this experiment method. After the coil was stacked on the base (see FIG. 6) and the inner cover was set, the inside of the inner cover was depressurized by a decompressor. After decompression,
By supplying an inert gas such as N ₂ into the inner cover, the pressure in the inner cover is restored, and after this pressure becomes slightly higher than the atmospheric pressure, H ₂ gas is supplied into the inner cover. When the pressure inside the inner cover is slightly higher than the atmospheric pressure, the dew point and O ₂
The concentration was determined. The results of this investigation will be described with reference to FIGS.

Referring to FIG. 2, there are shown changes in pressure, N ₂ concentration, and H ₂ concentration with time in the inner cover.
Furthermore, changes over time of the O ₂ concentration and the dew point are shown in comparison with the conventional method. As shown in FIG. 2, subjected to vacuum in the inner cover about 10 minutes, and the inside inner cover supply pressure was regained, H ₂ gas by subsequent N ₂ gas. After decompression, N
_The reason why the pressure was restored by the _two gases is that if H ₂ gas is supplied immediately after the completion of the pressure reduction, there is a risk of explosion. The O ₂ concentration and dew point in the inner cover were measured from the time of pressure recovery with N ₂ gas, and compared with the O ₂ concentration and dew point in the conventional method in which the inner cover was not decompressed, as shown in FIG. It has been found that the above method is remarkably excellent, for example, which is advantageous for a reduction reaction of a ferritic or martensitic stainless steel sheet.

Next, with reference to FIG. 3, there is shown a relationship between the minimum pressure in the inner cover when the pressure is reduced and the O ₂ concentration in the inner cover when about 5 hours have elapsed after the start of annealing. FIG. 3 shows the minimum pressure in the inner cover at the time of decompression on the horizontal axis, and the dew point and O ₂ concentration in the inner cover at about 5 hours after the start of annealing on the vertical axis. From FIG.
About 0.3 kg / cm ² or less, more preferably about 0.1 k / cm ²
It was found that a good furnace atmosphere was obtained by reducing the pressure to g / cm ² or less.

Next, the results of measurement of the dew point and O ₂ concentration in the inner cover at the time corresponding to the lapse of 5 to 20 hours after the start of annealing are shown in Table 1 in comparison with the dew point and O ₂ concentration of the conventional method. .

[0019]

[Table 1] 炉 Furnace dew point O ₂ concentration (℃) ( ppm) Conventional method After 5 hours -30 60 After 20 hours -40 50 Main experiment After 5 hours -60 3 After 20 hours -60 ────────────────────わ か る As can be seen from Table 1, the dew point and O ₂ concentration in the furnace were both superior in this experiment compared to the conventional method, It turned out to be a very favorable condition for the problem.

[0020]

According to the batch annealing method of the present invention, since the pressure in the batch furnace is reduced before the start of annealing, oxidizing substances such as water and air adhering to the steel material and the batch furnace are removed, The dew point and oxygen concentration during annealing can be reduced as much as possible, and formation of oxide scale and the like can be prevented. The annealing atmosphere is H
_{Since two} gases are used, the oxide scale is reduced during annealing, and the subsequent pickling treatment can be greatly reduced.

In the batch annealing apparatus according to the present invention, it is possible to select a bell-type annealing furnace in which the inside of a plurality of bell-type annealing furnaces is depressurized by one decompressor, and the pressure is reduced by opening and closing a shutoff valve. For this reason, a batch annealing apparatus that is low in cost and easy to control can be obtained.

[0022]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 4 is a schematic configuration diagram showing an outline of the batch annealing apparatus of the present invention. The batch annealing apparatus 50 includes 8
There are provided two bell-type annealing furnaces 52a to 52h. Since the basic configuration of each of the bell-type annealing furnaces is the same as the configuration of the bell-type annealing furnace shown in FIG. 6 described above, the detailed description of the structure is omitted. I do. Further, the batch annealing apparatus 50 is provided with a pressure reducing device 54 for reducing the pressure in the inner covers (see FIG. 6) of the eight bell-type annealing furnaces 52a to 52h. The decompressor 54 and the respective inner covers of the eight bell-type annealing furnaces 52 a to 52 h are communicated via a gas pipe 56. The gas pipe 56 is provided with shut-off valves 58a to 58h. By selectively opening and closing the shut-off valves 58a to 58h, the pressure inside the inner cover of any bell-type annealing furnace can be reduced. ing. Further, a shutoff valve 60 is provided in a gas pipe near the pressure reducer 54.
A gas pipe 6 for supplying H ₂ gas and N ₂ gas from the gas supply device (not shown) into the inner cover of each bell type annealing furnace.
The gas pipe 62 has a shut-off valve 64a.
~ 64h.

In the batch annealing apparatus 50, as described above, the shutoff valves 58a-58 are connected to the gas pipes 56 and 62, respectively.
h, since the shutoff valves 64a to 64h are provided, only an arbitrary bell-type annealing furnace can be used. For example,
When only the bell type annealing furnaces 52a and 52b are used, only the shutoff valves 58a, 58b and 64a, 64b and 60 are opened, and the other shutoff valves are closed. As described above, in the batch annealing apparatus of the present embodiment, one decompression device that can be shared by a plurality of bell-type annealing furnaces is arranged, and only the inside of the inner cover of the bell-type annealing furnace used for annealing can be depressurized. Therefore, it is possible to use an annealing furnace according to the production amount.

Next, an embodiment in which the hot-rolled stainless steel is annealed using this batch annealing apparatus 50 will be described together with a comparative example with reference to Table 2.

[0025]

[Table 2]

In the present embodiment, the pressure in the inner cover before the annealing was reduced to about 0.3 kg / cm ² by the pressure reducer 54, and then the pressure was restored with N ₂ gas and annealing was performed in an H ₂ gas atmosphere. In the conventional method, H _{2 is used} without depressurizing the inside of the inner cover.
About 9% of the gas was annealed, and the rest was annealed in a N ₂ gas atmosphere. Table 2
In the middle, it was visually determined whether or not the scale remained.

Table 3 shows the scale removal treatment after annealing.
3 levels 1 to 3 shown in the table. Table 3
MSB refers to a mechanical scale breaker, sulfuric acid refers to about 20% sulfuric acid, and nitric hydrofluoric acid refers to 15% nitric acid and 5% hydrofluoric acid.

[0028]

[Table 3]

In this embodiment, since the dew point and the O ₂ concentration can be reduced as described above, the generation and growth of scale during annealing are prevented, and the reduction reaction of oxide scale is promoted during annealing. Even if the line speed was increased by increasing the pickling speed, or even if shot blasting, sulfuric acid, and nitric hydrofluoric acid were partially omitted (or only a half tank was used), good results were obtained in the scale characteristics.

[0030]

As described in detail above, according to the batch annealing method of the present invention, the inside of the furnace is depressurized and vacuum purged before annealing, so that residual oxygen and moisture are reduced, and the reduction reaction in stainless steel is performed. In the case of high carbon steel and general steel, an annealing atmosphere which is advantageous for the surface properties can be obtained. As a result, in the stainless steel sheet annealed by the present method, the scale removal treatment after the annealing is facilitated, so that the speed of the pickling line can be increased and the unit consumption of the pickling can be reduced.

According to the batch annealing apparatus for steel materials of the present invention, even if there are a plurality of annealing furnaces, only one decompressor is required, and the equipment cost can be greatly reduced.

[Brief description of the drawings]

FIG. 1 is a graph showing an outline of an experimental method which led to the present invention.

FIG. 2 is a graph showing changes over time in pressure, N ₂ concentration, and H ₂ concentration in an inner cover, and changes over time in O ₂ concentration and dew point, as compared with a conventional method.

FIG. 3 is a graph showing the relationship between the minimum pressure in the inner cover when the pressure is reduced and the O ₂ concentration in the inner cover when the pressure is about 5 hours after the start of annealing.

FIG. 4 is a schematic configuration diagram showing an outline of a batch annealing apparatus according to one embodiment of the present invention.

FIG. 5 is a schematic view showing an outline of an annealing and pickling step of a hot-rolled ferritic or martensitic stainless steel sheet.

FIG. 6 is a sectional view showing a schematic structure of a conventional bell-type annealing furnace.

[Explanation of symbols]

 Reference Signs List 40 inner cover 50 annealing device 52a to 52h bell type annealing furnace 54 pressure reducer 56, 62 gas pipe 58a to 58h shutoff valve

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Katsumi Sakuma 1st Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Corp. Inside Chiba Works (72) Inventor Yoshikazu Kiino 1st Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Corp. Company Chiba Works (72) Inventor Tadao Nishiyama 1 Kawasaki-cho, Chuo-ku, Chiba City Kawasaki Works Chiba Works (56) References JP-A-4-107222 (JP, A) JP-A-52-42407 ( JP, A) Japanese Patent Publication No. 49-20449 (JP, B1) Japanese Patent Publication No. 55-21816 (JP, B2)

Claims (2)

(57) [Claims] 1. A batch annealing process of the steel, comprising the steps of applying reduced <br/> batch furnace steel instrumentation after turning to 0.3 kg / cm ² or less, an inert gas after the vacuum of the batch furnace large Pressure
Recovering the pressure to a pressure higher than the pressure, and placing the inert gas in the batch furnace into H ₂ gas after the pressure recovery.
A step of annealing said steel with H ₂ gas atmosphere by conversion, after the annealing is completed, the temperature of the steel product 100 ° C. or less
And c. Stopping the supply of H ₂ gas at the time when the temperature of the steel sheet has become equal to. In batch annealing apparatus having a plurality of bell furnace with a wherein the inner cover, and a single decompression apparatus for decompressing the inside of the inner cover to 0.3 kg / cm ² or less, the single pressure reduction a gas passage for binding and said inner cover disposed on the machine and a plurality of parallel, and shut-off valve for opening and closing the respective gas passage, N ₂ gas and said the recovery pressure the inside of the inner cover
A gas passage for supplying the H ₂ gas be further substituted with N ₂ gas, batch annealing apparatus which is characterized in that a shut-off valve for opening and closing the respective gas passage.