JPH05140639A - Method for butch-annealing stainless steel strip - Google Patents

Abstract

PURPOSE:To establish a butch-annealing technique for steel strip reducing the load of de-scaling process and improving the surface characteristic of the steel strip. CONSTITUTION:At the time of executing the butch-annealing to the stainless steel strip after hot-rolling, by holding the atmosphere in an annealing furnace to reducing atmosphere having >=8% hydrogen concn. and <=-20 deg.C dew point, the oxide scale developed on the steel strip surface during the time when hot- rolling is effectively reduced, and the load by de-scaling treatment executed thereafter is reduced and the surface characteristic of the steel stripe is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a batch annealing method for stainless steel strips, and particularly proposes a batch annealing method which is advantageously applicable to ferritic stainless steel strips and martensitic stainless steel strips.

[0002]

2. Description of the Related Art A general process for manufacturing a stainless steel plate is
First, hot-roll a stainless steel slab into a hot-rolled steel strip,
Next, this steel strip is subjected to batch annealing in a heat treatment furnace called a bell-type annealing furnace, and thereafter, a step of removing oxide scale on the surface of the steel strip.

FIG. 2 is a schematic view showing a general manufacturing process of such a stainless steel plate (strip). As shown in this figure, a stainless steel slab heated in a heating furnace is first rolled into a steel strip in a hot rolling line indicated by reference numeral 1 and wound on a coiler 2 at the end of this line. Then, the wound steel strip is rewound again by the coil build-up facility 3 into a coil having a size suitable for annealing,
Three to four pieces are stacked in the bell-type annealing furnace 4 and annealed for about 80 hours. Then, the steel strip that has been subjected to the desired cooling is then subjected to a descaling treatment in an annealing pickling line (HAP) composed of an annealing furnace 5, a descaling device 6 and a pickling tank 7. Since the annealing process has already been completed, this HAP does not perform the annealing process.

FIG. 3 shows the structure of a known bell-type annealing furnace. Reference numeral 8 shown in the figure is a base (base) for stacking steel strips, 9 is a burner port for burning fuel gas, 10 is an outer cover for protecting furnace wall refractory, and 11 is combustion by a burner. Inner cover to prevent direct contact between gas and stainless steel coil and to keep the annealing atmosphere non-oxidizing, and 12 circulates the atmosphere gas in the inner cover to promote heat transfer to the steel strip It is a furnace air circulation fan for.

The annealing treatment of the stainless steel strip in this bell-type annealing furnace is carried out by stacking the steel strips on the base of the hearth and then covering them with the inner cover and the outer cover. However, in the annealing process of the stainless steel strip in this bell-type annealing furnace, since the treatment is performed for a long time of about 80 hours, depending on the characteristics of the atmosphere gas in the furnace, the surface of the steel strip produced during hot rolling may be different. However, there was a problem that the oxide scale of No. 1 grew more remarkably.

On the other hand, conventionally, in order to suppress the growth of the oxide scale, the atmosphere in the furnace has been made non-oxidizing. In particular, recently, since there are few oxidizing components and carbonizing components, and the manufacturing method is easy and inexpensive, HNX is
Gas (coke oven gas by-product), HN (AX + N ₂ ) gas, etc. are often used. In addition, it is also considered to increase the hydrogen concentration of this atmosphere gas as a movement in consideration of measures for improving production efficiency in batch annealing (“CAMP”).
-ISIJ "Vol.3).

[0007]

However, in the conventional batch annealing method, HN is used as the atmosphere gas in the furnace.
Even if a non-oxidizing gas such as X gas was used, if the annealing treatment was performed for a long time, oxide scale was always generated on the surface of the steel strip. This is because the moisture adhering to the steel strip when charging the stainless steel strip and the moisture in the atmosphere adsorbed on the refractory material in the base part are released as water vapor during heating, which causes annealing in the inner cover. This is because the dew point of the atmosphere has risen to about + 5 ° C.

As described above, under the conventional technique, oxide scale is unavoidably generated, and thus mechanical descaling treatment such as shot blasting has been required in a subsequent process. Moreover, a strong acid has to be used as the acid used for the pickling treatment, so that the purchase of the acid and the treatment of the waste acid require a great deal of cost. Further, there is a problem that the pickling rate is limited and the production efficiency cannot be increased. Moreover, the use of shot blast and the above strong acids, especially nitric hydrofluoric acid,
On the contrary, the problem of deteriorating the surface quality of the steel strip also occurred.

On the other hand, in the prior art which uses hydrogen gas at a high concentration, the object of annealing is a cold rolled steel sheet, and attention is paid to the high thermal conductivity of hydrogen gas, and the overheating time and cooling time of the steel strip are shortened. By doing so, the aim is to increase production efficiency.

Therefore, this prior art is obviously not aimed at reducing the burden of the descaling step as aimed at by the present invention. Moreover, the dew point of the annealing atmosphere was not particularly taken into consideration, and there was no effect in suppressing the growth of oxide scale.

An object of the present invention is to solve the problems of the above-mentioned respective prior arts, and particularly to effectively reduce the scale formed of oxides of Cr and Fe formed on the surface of a steel strip during hot rolling. By doing so, it is an object of the present invention to provide a batch annealing method for a stainless steel strip that reduces the load of the descaling step and thereby improves the properties of the surface of the steel strip.

[0012]

Means for Solving the Problems As a result of intensive research for solving the above-mentioned problems, the inventors have come up with the invention of the following contents.

That is, according to the present invention, in batch annealing of a stainless steel strip after hot rolling, the atmosphere in the annealing furnace is maintained in a reducing atmosphere having a hydrogen concentration of 8% or more and a dew point of -20 ° C. It was decided to effectively reduce the oxides of Cr and Fe formed on the surface of the steel strip during hot rolling, and to reduce the burden of the subsequent descaling treatment. Further, in addition to the above, the method of the present invention further promotes the replacement of the oxidizing gas existing in the steel strip gap by repeating pressurization and depressurization in the furnace once or more, thereby further enhancing the reducing action. It is a technology.

[0014]

According to the method of the present invention, in the batch annealing of the stainless steel strip after hot rolling, the annealing atmosphere is kept at the hydrogen concentration of 8% or more and the dew point of -20 ° C or less. The oxide scale generated on the steel strip surface can be reduced during annealing for a long time. This is
It is effective in significantly reducing the subsequent descaling process. Furthermore, since the atmosphere gas in the furnace is pressurized and depressurized once or more, high-concentration hydrogen gas is supplied to the steel strip gap during pressurization, and H ₂ O generated during depressurization is quickly supplied from the steel strip gap. Can be removed. Therefore, the H ₂ O partial pressure in the gap between the steel strips can be maintained below the equilibrium partial pressure of the redox reaction of Fe or Cr, and the oxide scale can be reduced more effectively.

By the way, the surface of a steel strip obtained by hot rolling a ferritic and martensitic stainless steel slab is generally covered with an oxide scale layer containing hematite and chromite having a spinel structure as its main constituent elements. In this respect, in conventional batch annealing, since the dew point of the atmospheric gas is not taken into consideration, the hematite existing on the surface of the steel strip is reduced to wustite, but it is pure due to the small amount of water vapor contained in the atmospheric gas. It is not reduced to iron. It is also the main constituent of stainless steel
Cr is oxidized by water vapor to form chromite having a spinel structure, which promotes the formation of chromite scale. Since this scale is composed of fine particles and is very hard, the burden of the descaling process is further increased.

On the other hand, when batch annealing is performed under the annealing conditions according to the method of the present invention, most of hematite is reduced to pure iron, and the oxide scale remaining on the surface of the steel strip becomes very thin. Moreover, most of Cr oxides such as chromite are also reduced. FIG. 4 is a schematic sectional view of the surface of a steel strip annealed under the annealing conditions of the present invention.

As is clear from FIG. 4, the residual oxide scale has a rough surface and is fine inside because the molecular volume of pure iron produced by the reducing action of hydrogen is smaller than that of the oxide. There are many vacant holes. For this reason, this oxidized scale is brittle, and since its surface area is large, the contact between the acid and the scale is promoted in the pickling step, and a sufficient descaling effect can be obtained even if the immersion time is shortened.

That is, according to the method of the present invention, the oxide scale formed on the surface of the steel strip in the hot rolling step can be reduced during annealing, and as a result, the descaling step can be simplified.

[0019]

EXAMPLE A bell-type annealing furnace shown in FIG. 3 was used to carry out batch annealing of a ferritic stainless steel strip under the conditions shown in Table 1 while controlling the dew point of atmospheric gas and the internal pressure of the furnace. Further, FIG. 1 shows pressurization and depressurization of the furnace pressure under the condition 2.

[0020]

[Table 1]

Further, the thickness of the oxide scale existing on the surface of the steel strip after annealing was measured. The results are shown in Table 2. As is clear from the results shown in Table 2, the dew point is -10 ° C when the dew point is kept at -20 ° C under any conditions.
More reduction of oxide scale was observed, especially hydrogen concentration of 8
It can be seen that the degree of decrease is large when the percentage is above%.

On the other hand, when the furnace pressure is constant by tight annealing,
Although the degree of reduction of oxide scale is inferior to that of open annealing, it can be seen that when the atmosphere in the furnace is repeatedly pressurized and depressurized as in condition 2, the reduction of oxide scale is similar to that of open annealing in condition 3. This is due to the fluctuation of pressure
This is because gas replacement in the gap between the steel strips is promoted, that is, H ₂ O produced by the reduction of the oxide scale is rapidly removed and the reduction reaction is promoted.

[0023]

[Table 2]

Next, the annealed steel strip was pickled to evaluate the pickling property of the oxide scale. The results are shown in Table 3. As is clear from Table 3, at the dew point of -10 ° C, the pickling property of the oxide scale after annealing is poor.

On the other hand, in the annealing method in which the dew point of the gas in the furnace is -20.degree. C., it is possible to reduce the oxide by setting the hydrogen concentration in the furnace to 50% or more, and use hydrofluoric acid which is expensive. Descaling can be done by pickling with sulfuric acid without doing.

[0026]

[Table 3]

As described above, in order to reduce the load of the descaling step, the atmosphere gas in the furnace is kept in a reducing atmosphere having a hydrogen concentration of 8% or more and a dew point of -20 ° C. or less, and preferably the furnace atmosphere is further reduced. Repeat pressurization and depressurization at least 1
By performing the rolling, C generated on the surface of the steel strip during hot rolling
It is understood that the oxides of r and Fe can also be effectively reduced.

The pressure in the pressurized state and the pressure in the depressurized state are 1 kg / cm ² (gauge pressure) and 0 shown in the examples, respectively.
mm H ₂ O (gauge pressure), and the holding time of the pressurizing state and the depressurizing state is not limited to 1 hour,
Appropriate pressure, holding time, number of pressurization and depressurization may be selected depending on the annealing time determined by the steel type and the winding state of the steel strip.

[0029]

As described above, according to the batch annealing method for a stainless steel strip according to the present invention, the hydrogen concentration of the atmosphere gas when annealing the steel strip is 8% or more and the dew point is -20 ° C.
By keeping it below, the oxide scale generated on the surface of the steel strip during hot rolling can be reduced in the annealing treatment step.

As a result, it is possible to significantly reduce the load of the descaling treatment which is a post-process, and therefore the costs for the purchase of acid and the treatment of waste acid can be greatly reduced. Moreover, the production efficiency, which was limited by the pickling treatment speed, can be significantly improved. Furthermore, mechanical descaling such as shot blasting and pickling with a strong acid such as nitric hydrofluoric acid can be omitted, and thus the surface properties of the steel strip can be improved.

By infiltrating hydrogen gas having a high heat transfer coefficient into the gap between the steel strips, the heat transfer is improved, the time required for overheating and cooling the steel strip is shortened, and the production efficiency is improved. There is also.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a pressurized and depressurized state in a furnace.

FIG. 2 is a schematic view showing a manufacturing process of a stainless steel plate.

FIG. 3 is a side view of a bell-type annealing furnace.

FIG. 4 is a schematic view of a steel strip surface after being annealed under batch annealing conditions according to the present invention.

[Explanation of symbols]

 1 Hot rolling process 2 Tension reel 3 Coil build-up equipment 4 Bell type annealing furnace 5 Annealing furnace 6 Descaling device 7 Pickling tank 8 Base 9 Burner port 10 Outer cover 11 Inner cover 12 Furnace air circulation fan 13 Oxide scale 14 iron

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kuniaki Sato 1 Kawasaki-cho, Chiba-shi, Chiba Inside the Kawasaki Steel Co., Ltd. Chiba Works (72) Inventor Kojijima 1-Kawasaki-cho, Chiba-shi Kawasaki Steel Co., Ltd. Chiba Inside the Steel Works (72) Inventor Yoshiaki Wada 1 Kawasaki-cho, Chiba-shi, Chiba Inside the Kawasaki Steel Co., Ltd. (72) Inventor Hiroyuki Kakiuchi 1 Kawasaki-cho, Chiba-shi Inside the Chiba Steel Co., Ltd.

Claims (2)

[Claims] 1. When performing batch annealing of a stainless steel strip after hot rolling, the atmosphere in the annealing furnace has a hydrogen concentration of 8% or more and a dew point of −.
A batch annealing method for a stainless steel strip characterized by holding in a reducing atmosphere at 20 ° C or lower. 2. When performing batch annealing of a stainless steel strip after hot rolling, the atmosphere in the annealing furnace has a hydrogen concentration of 8% or more and a dew point of −.
A batch annealing method for a stainless steel strip, which is characterized by maintaining a reducing atmosphere at 20 ° C or lower and repeating pressurization and depressurization in the furnace at least once.