JPH05222449A - Annealing method of stainless steel - Google Patents

Abstract

PURPOSE:To control the thickness and the structure of the oxide film which grows during annealing and to improve the pickling property and the surface quality in the annealing and pickling operations of Fe-Cr or Fe-Cr-Ni cold rolled stainless steel plate. CONSTITUTION:When Fe-Cr cold rolled stainless strip is continuously annealed in the combustion gas atmosphere, the oxygen content in the annealing atmosphere is set <1% when the strip temperature is <=600 deg.C while the oxygen content is set >=1% and <=10% in the range over 600 deg.C and up to the target annealing temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous annealing method for cold rolled stainless steel strip. More specifically, when an Fe-Cr-based or Fe-Cr-Ni-based stainless steel strip is annealed, it improves the descaling property in pickling that is performed subsequent to the annealing and stabilizes the surface quality after pickling stably. The present invention relates to a method of annealing.

[0002]

2. Description of the Related Art As is well known, cold-rolled stainless steel strip is cold-rolled and annealed in the finish manufacturing process to obtain a product of a predetermined material. As the annealing method in this manufacturing process, except for bright annealing, which is annealing in a special atmosphere, a coiled steel strip is generally used with a combustible gas such as LPG or LNG and an air / fuel ratio of 1.0.
A method of performing continuous annealing in a combustion atmosphere in which the fuel is theoretically burned at a ratio of the amount of air required to completely burn the fuel to 1.0 or more is widely adopted. Oxide scale produced on the surface of the steel strip by such annealing not only impairs the original corrosion resistance and cosmetics of stainless steel but also adversely affects the processing such as pressing, so it is necessary to remove it beautifully and completely. is there. Therefore, after annealing, it is usual to carry out descaling by pickling or the like. However, it has been clarified that the annealing conditions have a great influence on the descaling property in pickling and also have various influences on the surface quality after pickling. That is, when the annealing conditions are not appropriate, the production cost rises, such as lengthening of the pickling equipment, lengthening of the pickling time, or increase of the acid basic unit, which causes a decrease in surface gloss and a decrease in abrasivity. There was a problem such as a cause.

Various proposals have been made to solve these problems. For example, JP-A-59-232233 discloses a method of annealing a stainless cold-rolled steel strip in an atmosphere in which a coke oven gas is burned. This is intended to form a scale that can be easily descaled by using a coke oven gas to shorten the descaling time. However, according to the research conducted by the present inventors, even when the coke oven gas is used, the conventional LPG and L
There is no great difference in the composition and structure of the oxide scale from the case of using NG, and it was revealed that a large effect of improving the descaling property cannot be obtained only by using the coke oven gas.

Therefore, as a method for solving the above problems, the present inventors set the annealing atmosphere as a coke oven gas combustion atmosphere and set the air / fuel ratio to 1.0 up to a plate temperature of 600.degree.
It was proposed that the annealing method should be 1.0 or more at a sheet temperature of less than 600 ° C. (Japanese Patent Application No. 2-69545). In this annealing method, the air / fuel ratio is set to less than 1.0 to suppress the growth of scale in the temperature range where oxidation does not adversely affect the surface quality after pickling, and the scale thickness is reduced to reduce the descaling property. In addition to improving the surface quality after pickling, in a high temperature region where oxidation progresses which directly affects the surface quality, the scale is grown but the surface quality is improved by setting the air / fuel ratio to 1.0 or more. The present invention has achieved improvement in descaling property and surface quality after pickling, as compared with the prior art in which the atmosphere is uniform in all temperature regions.

[0005]

However, this annealing method still has a point that it is not sufficient for stable operation in an actual production line. That is, it has been clarified that even if the air / fuel ratio is set to a certain value, it is difficult to control the atmosphere in the annealing furnace and the descaling property and the surface quality may not always be good. In particular, the boundary where the air / fuel ratio is 1.0 is a region where the atmosphere in the furnace becomes extremely unstable, and there remains a problem that descaling failure and surface quality deterioration are likely to occur. That is, air /
Controlling at a fuel ratio of 1.0 was very problematic on the production line for stable operation.

The present invention can solve the above problems by Fe--C
A continuous annealing method for r-type stainless cold-rolled steel strips is provided, which is an annealing for stable production of steel strips having better descaling properties by pickling and good gloss after pickling as compared with the prior art. It is what gives way. Further, in the Fe-Cr-Ni-based stainless cold-rolled steel strip, the growth behavior of the oxide scale is Fe-.
Provided is an annealing method capable of reducing the descaling property in pickling and the surface quality after pickling, in particular, the grain boundary erosion depth, which is slightly different from that of a Cr-based stainless steel strip.

In order to solve the above problems, the present inventors have
A detailed laboratory study on the relationship between the annealing atmosphere and the descaling property and the surface quality after pickling, especially gloss, in the annealing using the combustion gas of the Fe-Cr-based or Fe-Cr-Ni-based stainless cold-rolled steel strip was conducted. went. As a result, it is possible to significantly improve the technique previously proposed by the present inventors by a method of directly controlling the O ₂ concentration in the combustion atmosphere within a specific range in each appropriate temperature range, not in the air / fuel ratio. Can be applied to the actual operation without problems,
The present invention has been accomplished by finding that it gives excellent gloss.

[0008]

The first invention of the present invention is as follows:
In an annealing method in which a cold-rolled Fe—Cr-based stainless cold-rolled steel strip is continuously annealed using a combustion gas and subsequently descaled, the O ₂ (oxygen) concentration in the annealing atmosphere is set to A method of annealing a Fe—Cr-based stainless steel strip is characterized in that the temperature (plate temperature) is less than 1% when the temperature is 600 ° C. or less, and is 1% or more and 10% or less in the region of more than 600 ° C. to the target annealing temperature. Is provided.

A second invention of the present invention is Fe-Cr.
-When continuously annealing a Ni-based stainless cold-rolled steel strip in a combustion gas atmosphere, the oxygen concentration in the annealing atmosphere depends on the strip temperature of the steel strip.
An annealing method for Fe-Cr-Ni-based stainless steel is characterized in that it is less than 1% at a temperature of 00 ° C or less, and is 1% or more and 10% or less in a region exceeding 800 ° C and reaching a target annealing temperature.

[0010]

The function of the present invention will be described below. In the present invention, a combustible gas is used as the annealing gas. As the combustible gas, generally used LPG or LNG may be used, and coke oven gas, which is a by-product of an iron mill, may be used. Since the present invention is a method of directly controlling the oxygen concentration of the atmosphere during combustion, the difference in the composition of the combustible gas itself does not affect the present invention, and the type of combustible gas is not limited.

In the first aspect of the present invention, the oxygen concentration in the annealing atmosphere of the Fe—Cr type stainless steel cold-rolled steel strip is less than 1% until the plate temperature of the steel strip reaches 600 ° C. and exceeds 600 ° C. In the temperature range, it is 1% or more and 10% or less. Each of FIGS. 1 to 3 uses SUS430 and the ultimate plate temperature is 600 ° C. and 8
The results of investigating the relationship between the O ₂ concentration in the atmosphere and the thickness of the oxide film formed when the temperature is 00 ° C. and 1000 ° C. are shown. From this result, the O ₂ concentration in the atmosphere was 1 up to the temperature of 600 ° C.
%, The growth of the oxide film is sufficiently suppressed,
On the contrary, if the O ₂ concentration is less than 1% at a temperature exceeding 600 ° C., the oxide film thickness remarkably increases, and conversely, if the O ₂ concentration is 1% or more, the oxide film thickness becomes thinner. I understand. Next, both of the above samples were pickled by a method of anodic electrolysis (10 A / dm ² ) in a 20% aqueous Na ₂ SO ₄ solution and further anodic electrolysis (10 A / dm ² ) in 10% nitric acid. The results of measuring the time required for descaling in the total treatment and the glossiness after pickling are shown in FIGS. 4 to 6. From these results, it was revealed that the thinner the oxide film is, the more efficient the descaling is, and that the gloss is significantly reduced when the temperature range exceeding 600 ° C. is annealed at an O ₂ concentration of less than 1%. Further, these results also show that the conventional air / fuel ratio of 1.0 (the residual O ₂ concentration in the atmosphere becomes zero) is not always an appropriate condition. In addition, O
_The condition where the concentration of ₂ exceeds 10% does not have the effect of improving the gloss, only increases the thickness of the oxide film, and increases the energy cost.

On the basis of the above results, in the first invention of the present invention, a porous oxide film is grown thinly at a plate temperature of up to 600 ° C. with an O ₂ concentration of less than 1% in the atmosphere.
O ₂ concentration of 1% or more and 1 up to a predetermined temperature (usually 1000 ° C. or less for Fe-Cr stainless steel).
Annealing is performed to obtain a good gloss with 0% or less. It should be noted that the present invention is not limited to the maximum reached plate temperature, time, etc. during annealing, and conditions that are appropriately determined depending on the steel type, steel strip (plate) thickness, etc. may be used. Regarding the pickling conditions after annealing, it is important to take care not to reduce the gloss in the pickling step in order to maximize the effect of the present invention. Therefore, the alkali molten salt treatment or neutral salt treatment After the electrolytic treatment, a method of dipping in nitric acid or electrolytic treatment is preferable. Thus, the alkali molten salt has a composition of NaOH: NaNO ₃ = 1: 1 and a temperature of 400.
~ 500 ° C, neutral salt electrolysis is 15-25 wt% Na
₂ SO ₄ aqueous solution at 70 to 90 ° C., current density 6 to 12 A
/ Dm ² and then in 5-20% by weight of HNO ₃ ,
Dipping at 45-65 ° C or 6-15A / dm ²
It is preferable to electrolyze at a current density of.

In the second aspect of the present invention, Fe-Cr-N is used.
O in the annealing atmosphere of i-type stainless cold-rolled steel strip₂ Concentration of steel
It is less than 1% until the strip temperature reaches 800 ° C,
In the temperature range exceeding ° C, it is set to 1% or more and 10% or less. Figure
7 to 9 use SUS304 and set the reached plate temperature to 6 respectively.
Atmosphere O at 00 ° C, 800 ° C, 1100 ° C ₂ 
The results of examining the relationship between the concentration and the thickness of the oxide film formed
Show. From this result, it was found that O
₂ If the concentration is less than 1%, the growth of the oxide film is sufficiently suppressed.
If the temperature exceeds 800 ° C, on the contrary, O₂ Dark
If the degree is less than 1%, the oxide film thickness will increase remarkably.
To O₂ If the concentration is 1% or more, the oxide film thickness will be thinner.
I understand. Next, for the above sample, 20% Na₂ SO
_Four 10 A / dm in aqueous solution² After anodic electrolysis for 15 seconds,
NO₃ 50g / l + HF10g / l nitric hydrofluoric acid water solution
Glass when pickled by immersion in liquid at 55 ° C
Descaling time in hydrofluoric acid immersion and grain boundary penetration of pickling materials
The results of measuring the food depth are shown in FIGS. 10, 11 and 12.
You From these results, descaling shows that the oxide film is thin.
High efficiency and temperature range over 800 ℃
O₂ Grain boundary erosion depth after pickling when annealed at a concentration less than 1%
Became very large. Also this
The result was 1.0 (with the conventional air / fuel ratio) (remaining in the atmosphere.
O₂ (Concentration becomes zero) is not always the proper condition
It also shows that. In addition, O₂ Articles whose concentration exceeds 10%
The problem is that it does not have the effect of improving grain boundary erosion,
Not only thicker but also increase energy cost
Since it can be added, it should be 10% or less.

Based on the above results, in the present invention, 800 ° C.
Up to 800 ° C. at a predetermined temperature (usually 105% for Fe-Cr-Ni-based stainless steel) while a thin porous oxide film was grown at an O ₂ concentration of less than 1% in the atmosphere.
(0 to 1150 ° C.) is set to an O ₂ concentration of 1% or more and 10% or less to perform annealing capable of reducing the grain boundary erosion depth.
It should be noted that the present invention does not limit the maximum reached plate temperature, time, etc. during annealing, and conditions that are appropriately determined depending on the steel type, the steel strip (plate) thickness, etc. may be used.

Regarding the pickling conditions after annealing, in order to maximize the effect of the present invention, the descaling property is not deteriorated in the pickling step, and rough skin is caused by grain boundary erosion and the like. It is important to be careful not to
Therefore, it is preferable that after the molten alkali salt treatment or the neutral salt electrolytic treatment is performed, it is immersed in a mixed acid of nitric acid and hydrofluoric acid, and then immersed in nitric acid or electrolyzed.

At this time, the molten alkali salt is NaOH: Na.
The composition is NO ₃ = 1: 1, the temperature is 450 to 530 ° C., and the neutral salt electrolysis is 15 to 25 wt% in Na ₂ SO ₄ aqueous solution.
The current density is preferably 6 to 12 A / dm ² at 70 to 90 ° C. In addition, 4-10% by weight of HNO ₃ ,
40 in a mixed acid prepared by mixing 0.5 to 5% by weight of HF,
Immerse at ~ 60 ° C, then in 5-20% by weight nitric acid, 4
It is preferable to immerse at 5 to 65 ° C. or to electrolyze at a current density of 6 to 15 A / dm ² .

[0017]

EXAMPLES Examples of the present invention will be described below. Example 1 Using an SUS430 cold-rolled steel strip having a plate thickness of 1.0 mm, experimental firing
The present invention shown in Table 1 in an LPG combustion gas atmosphere in a blunt furnace.
And O consisting of conventional technology₂ It depends on the conditions of concentration and annealing temperature.
Annealing was performed at a high reaching plate temperature of 820 ° C. O in the atmosphere₂ Dark
Degree is 0₂ Use the method of analyzing with a densitometer
I was there. These annealed materials are treated with 20 wt% Na ₂ SO_Four Aqueous solution
Medium, liquid temperature 80 ℃, 10A / dm² Current density of 15 seconds
10% by weight HNO after anodic electrolysis₃ Aqueous solution
Medium, 50 ° C, 10A / dm² Anodic electrolytic treatment for 5 seconds
Descaling was performed by pickling. The results are shown in Table 1.
You From the results in Table 1, according to the annealing of the present invention, descaling
The surface quality after pickling is stable and extremely good.
On the other hand, the comparative technology has a descaling property,
Defects such as poor surface quality or poor manufacturing stability
It is clear that there is.

[0018]

[Table 1]

Example 2 Using an SUS444 cold-rolled steel strip having a plate thickness of 0.8 mm and using an experimental annealing furnace in a coke oven gas combustion atmosphere, the O ₂ concentration and annealing according to the present invention and the prior art shown in Table 2 were performed. Annealing was performed under the temperature conditions with the maximum reached plate temperature of 1000 ° C. (Composition of coke oven gas: CO ₂ = 3-5%, CH ₄ = 20-3
0%, C ₂ H ₂ + C ₂ H ₄ + C ₂ H ₆ = 3 to 5%, H ₂
= 40~50%, CO = 5~10% , N 2 = 5~10
%) The O ₂ concentration in the atmosphere was measured in the same manner as in Example 1.

These annealed materials were treated with NaOH: NaNO ₃ =
After performing a molten alkali salt immersion treatment at 1: 1, 450 ° C. for 50 seconds, subsequently, in a 15 wt% HNO ₃ aqueous solution at 50 ° C.,
Pickling and descaling were performed by a method of anodic electrolysis at 15 A / dm ² for 5 seconds. The results are shown in Table 2. From the results in Table 2, it is clear that the annealing method of the present invention has stable and very good descaling property and gloss after pickling, as compared with the conventional technique.

[0021]

[Table 2]

Example 3 Using an SUS304 cold-rolled steel strip having a plate thickness of 1.0 mm, in an experimental annealing furnace, in an LPG combustion gas atmosphere, the O ₂ concentration and the annealing temperature according to the present invention and the prior art shown in Table 3 were obtained. Under the conditions described above, annealing was performed at the maximum reached plate temperature of 1150 ° C. O ₂ in the atmosphere
The concentration was determined by extracting the gas in the furnace and analyzing it with an O ₂ concentration meter. These annealed materials were treated with NaOH: NaNO ₃ = 1:
In Example 1, the molten alkali salt immersion treatment was performed at 480 ° C. for 45 seconds, and then pickling and descaling was performed by a method of immersing in a nitric hydrofluoric acid aqueous solution containing 5% HNO ₃ + 1.5% HF at 55 ° C. for 35 seconds. The results are shown in Table 3. From the results shown in Table 3, it is clear that the annealing of the present invention provides stable and good descaling properties and good surface quality with shallow intergranular erosion depth as compared with the prior art.

[0023]

[Table 3]

[0024]

As described above, according to the present invention, Fe-
During annealing and pickling of a Cr-based or Fe-Cr-Ni-based stainless cold-rolled steel sheet, the O ₂ concentration during annealing was changed with the sheet temperature to control the thickness and structure of the oxide film grown during annealing. As a result, it has become possible to stably improve pickling ability and surface quality under actual production. At the same time, it has become possible to reduce the energy cost during annealing.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an O ₂ concentration in an atmosphere of an Fe—Cr-based stainless steel at an annealing temperature of 600 ° C. and an oxide film thickness.

FIG. 2 is a diagram showing the relationship between the O ₂ concentration in the atmosphere of Fe—Cr stainless steel at an annealing temperature of 800 ° C. and the oxide film thickness.

FIG. 3 is a diagram showing the relationship between the O ₂ concentration in the atmosphere of Fe—Cr stainless steel at an annealing temperature of 1000 ° C. and the oxide film thickness.

FIG. 4 is a diagram showing the O ₂ concentration in the atmosphere during annealing of Fe—Cr-based stainless steel at 600 ° C., the time required for descaling during pickling, and the result of gloss measurement after pickling.

FIG. 5 is a diagram showing the O ₂ concentration in the atmosphere during annealing of Fe—Cr-based stainless steel at 800 ° C., the time required for descaling during pickling, and the result of gloss measurement after pickling.

FIG. 6 is a diagram showing the O ₂ concentration in the atmosphere during annealing of Fe—Cr-based stainless steel at 1000 ° C., the time required for descaling during pickling, and the result of gloss measurement after pickling.

FIG. 7 is a diagram showing the relationship between the O ₂ concentration in the atmosphere of Fe—Cr—Ni stainless steel at an annealing temperature of 600 ° C. and the oxide film thickness.

FIG. 8 is a diagram showing a relationship between an O ₂ concentration in an atmosphere of Fe—Cr—Ni stainless steel at an annealing temperature of 800 ° C. and an oxide film thickness.

FIG. 9 Fe—Cr—Ni at an annealing temperature of 1100 ° C.
In the atmosphere of stainless steel ₂ Concentration and oxide film thickness
It is a figure which shows a relationship.

FIG. 10: Fe—Cr—Ni stainless steel at 600 ° C.
FIG. 5 is a diagram showing the O ₂ concentration in the atmosphere during annealing, the time required for descaling during pickling, and the results of grain boundary erosion depth measurement after pickling.

FIG. 11 800 ° C. of Fe—Cr—Ni system stainless steel
FIG. 5 is a diagram showing the O ₂ concentration in the atmosphere during annealing, the time required for descaling during pickling, and the results of grain boundary erosion depth measurement after pickling.

FIG. 12: Fe—Cr—Ni system stainless steel 1100
℃ is a diagram showing the intergranular corrosion depth measurement result after descaling duration and pickling at atmospheric O ₂ concentration and pickling during annealing at.

Claims (2)

[Claims] 1. When continuously annealing a cold rolled Fe—Cr stainless steel strip in a combustion gas atmosphere, the oxygen concentration in the annealing atmosphere is less than 1% when the strip temperature is 600 ° C. or less, 60
1% or more and 10% in the range from 0 ° C to the target annealing temperature
An Fe-Cr-based stainless steel annealing method characterized by the following. 2. When continuously annealing a cold rolled Fe—Cr—Ni stainless steel strip in a combustion gas atmosphere, the oxygen concentration in the annealing atmosphere is less than 1% when the strip temperature of the steel strip is 800 ° C. or less, 800 1 in the region above ℃ and reaching the target annealing temperature
% To 10% Fe-Cr-
Annealing method for Ni-based stainless steel.