JPH0827515A - Annealing method excellent in atmosphere controlling property - Google Patents

Abstract

PURPOSE:To inhibit the change with time in the decomposition of NH, and the fluctuations in the amount of nitriding of a steel sheet by regulating the atmosphere in an annealing furnace, in which metals such as steel sheet are nitrided, to a reducing atmosphere reducing the metallic surface of an electric heater, preventing the acceleration of the decomposition of NH3. CONSTITUTION:In an annealing furnace in which metals such as steel sheet are nitrided and an electric heater made of a Fe-Cr-Ni alloy is used, the degree of oxidation of an atmospheric gas in the furnace in the course of nitriding is regulated to a value in the range satisfying 2X10<-2>>=(PH2O/PH2)>=3X10<-3>. By this method, the steel sheet, causing neither changes in NH, concentration in the furnace at the time of nitriding treatment nor the change with time in the amount of nitriding of the steel sheet and having always stable quality characteristic, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an annealing method for nitriding grain-oriented electrical steel sheets or cold-rolled steel sheets.

[0002]

2. Description of the Related Art Generally, in an annealing furnace, a heat-removed product is heated in a heating zone and is kept and soaked at a predetermined temperature in a soaking zone. For this heating action, when the steel sheet to be annealed is required to have strict surface properties or to obtain high-grade quality characteristics, the heating in the annealing is easy to control the temperature as disclosed in JP-A-1-142031. In addition, the electric heater made of an alloy such as Ni, Cr, Fe, or the like, which does not easily pollute the atmosphere in the furnace, is used.

The above-mentioned annealing is used, for example, for nitriding a cold-rolled steel sheet for highly strengthening a steel sheet and nitriding for forming an inhibitor of a grain-oriented electrical steel sheet. In such annealing, heating is performed by an electric heater, and nitrogen is infiltrated into the steel from an atmospheric gas such as NH _{3 for} nitriding.

[0004]

When strengthening such a cold-rolled steel sheet or forming an inhibitor of a grain-oriented electrical steel sheet by a conventional annealing method, the degree of nitriding varies, and There may be variations in the amount of improvement and the amount of inhibitors formed, and the product characteristics may vary. Also,
Nitriding to a predetermined nitriding amount is difficult, and it may be difficult to obtain a desired product.

[0005]

[Means for Solving the Problems] An experiment was conducted to investigate the cause of fluctuations in nitriding of a steel sheet in a decarburization annealing furnace for grain-oriented electrical steel sheets. The nitriding capacity gas, for example, NH ₃ reacts with the steel sheet. However, if the nitriding amount changes, the concentration of the gas having nitriding capacity such as NH ₃ in the atmosphere as shown in FIG. Was found to have changed and decreased over time. It was also found that the change occurs even if the supply amount of the nitriding gas is constant. Further, when the transition of the dew point in the furnace from the start of nitriding was examined, it was found that the dew point gradually decreased as shown in FIG. However, the composition of the atmosphere gas at that time was H ₂ 75% and N ₂ 25%.

As a result, the dew point in the furnace is lowered and a reducing atmosphere is created.
Since the oxide on the surface of the electric heater made of an alloy of r and Fe is reduced and the metal surface is exposed and activated, and acts as a catalyst, the nitriding gas such as NH ₃ supplied to the furnace is used. It was found that the decomposition was accelerated and the concentration of the nitriding gas in the furnace was lowered.

The present invention has been made on the basis of the above findings, and its gist is to perform annealing / nitriding in a annealing furnace using an electric heater made of an alloy of Ni, Cr and Fe for nitriding a steel sheet. The degree of oxidation of the atmosphere gas in the furnace is 0.0
The annealing method is characterized in that the range is 2 ≧ (PH ₂ O / PH ₂ ) ≧ 0.003.

The present invention will be described in detail below.
When nitriding the decarburized and annealed steel sheet with a gas having a nitriding ability such as NH ₃ gas at a temperature of 750 ° C., the degree of oxidation PH ₂ O / PH ₂ of the atmosphere gas during the nitriding treatment is changed to change the NH in the furnace. FIG. 3 shows the amount of change in the NH ₃ concentration in the furnace after 100 hr treatment from the start of nitriding as the change in ₃ concentration over time. PH ₂ O
It has been found that as / PH ₂ increases, the amount of change over time in the NH ₃ concentration in the furnace decreases, and there is almost no change when PH ₂ O / PH ₂ is 3 × 10 ^-3 or more. Again 100hr
The nitriding amount of the steel sheet that has been subjected to the nitriding treatment in the annealing furnace for 30 seconds is shown in FIG. 4, and it has been found that it becomes a substantially constant value when PH ₂ O / PH ₂ is 3 × 10 ⁻³ or more.

This is because when the degree of oxidation PH ₂ O / PH ₂ in the atmosphere in the furnace during the nitriding treatment is large, the surface of the metal heater is coated with an oxide film and the catalytic action of NH ₃ decomposition is small.
If PH ₂ O / PH ₂ is small, is reduced oxide film on the surface of the metal heater is activated, act strongly as a catalysis of the NH ₃ decomposing, NH ₃ results decomposition is promoted, NH in the furnace
₃ It is considered that the concentration decreases. On the other hand, PH ₂ O /
When PH ₂ exceeds 0.02, the steel sheet is oxidized and affects the appearance of the surface.
It is desirable that 02 ≧ PH ₂ O / PH ₂ ≧ 0.003. As a method of increasing the degree of oxidation of the atmospheric gas, it is effective to add water or steam to the atmospheric gas.

[0010] By means of the present invention described above, when the nitriding treatment with a gas having a nitriding capacity, such as NH _3, without NH ₃ concentration in the furnace changes with time, a constant reduction also nitrided amount of the steel sheet We were able to.

[0011]

EXAMPLES Next, examples will be described. Cold rolled 0.3 mm grain-oriented electrical steel sheet at 750 ° C for 30 seconds under H ₂
Under a gas composition of 75% and N ₂ 25%, NH ₃ was mixed in an amount of 15 Nm ³ / Hr under various atmospheric conditions and nitrided. NH ₃ in the furnace at the initial stage of nitriding and after 100 Hr continuous nitriding treatment
The concentration state and the nitriding amount of the steel sheet after 30 seconds of nitriding were measured, and the results are shown in Table 1. No. In Nos. 4, 5 and 6, a stable nitriding amount could be obtained for a long time.

[0012]

[Table 1]

[0013]

According to the present invention, as seen in the examples, there is no change in the NH ₃ concentration in the furnace during the nitriding treatment,
In addition, since the amount of steel sheet nitriding and the change over time do not occur, it is possible to always produce a steel sheet having stable quality characteristics.

[Brief description of drawings]

FIG. 1 is a diagram showing a change with time of a NH ₃ concentration in an atmosphere gas in a furnace during a nitriding treatment of a steel sheet.

FIG. 2 is a diagram showing a transition of a dew point in a furnace during nitriding treatment of a steel sheet.

FIG. 3 is a diagram showing the amount of change in the NH ₃ concentration in the furnace from the initial value after 100 hours have passed from the start of nitriding treatment in various atmospheres.

FIG. 4 is a diagram showing a nitriding amount of a steel sheet that has been subjected to a nitriding treatment for 30 seconds in an annealing furnace that has been subjected to a nitriding treatment under various atmospheres.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C21D 9/56 101 C C23C 8/26

Claims (2)

[Claims] 1. When annealing in an annealing furnace for nitriding a metal such as a steel ingot or a thin plate using an electric heater made of an alloy of Ni, Cr, and Fe, the degree of oxidation of atmospheric gas in the furnace during nitriding is set to 0. An annealing method with excellent atmosphere controllability, characterized in that the range is 02 ≧ (PH ₂ O / PH ₂ ) ≧ 0.003. 2. Ni and C in a decarburization annealing furnace for grain-oriented electrical steel.
When performing annealing and nitriding using an electric heater made of an alloy of r and Fe, the degree of oxidation of the atmosphere gas in the furnace during nitriding is set in the range of 0.02 ≧ (PH ₂ O / PH ₂ ) ≧ 0.003. An annealing method with excellent atmosphere controllability.