JPH06346196A - Highly corrosion resistant ferritic stainless steel excellent in antidazzle characteristic - Google Patents

Abstract

PURPOSE:To obtain a highly corrosion resistant ferritic stainless steel usable even in the condition of polished surface or dull finished surface by forming a passivating film increased in the percentages of Cr and Mo. CONSTITUTION:This ferritic stainless steel has a composition containing, by weight, <=0.05% C, <=1.0% Si, <=1.0% Mn, <=0.04% P, <=0.03% S, 1.0% Ni, 20-35% Cr, 1-8% Mo, 0.1-1.0% Nb, and <=0.05% N and satisfying (Cr+3Mo)>=37%. Further, one or >=2 kinds among 0.05-0.3% Ti, 0.01-0.5% Al, 0.1-1.0% Cu, and 0.05-0.3% Zr can be incorporated. By this method, a passivating film increased in the percentages of Cr and Mo can be formed, even in the case of polished surface or dull finished surface, on the surface of the steel material. This steel material can be used as an exterior material excellent in antidazzle characteristic.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel having excellent antiglare property, pitting corrosion resistance and weather resistance.

[0002]

2. Description of the Related Art Stainless steel is used as a typical corrosion resistant material in various building materials, structural materials and the like. Among them, ferritic stainless steel is widely used as an inexpensive corrosion resistant material because it does not contain expensive nickel and has no problem in welding. However, taking SUS430, which is a representative steel type of ferritic stainless steel, as an example, the corrosion resistance and weather resistance are not sufficient so that red rust is generated in a short period even in a rural area where the corrosive environment is mild.
Further, there is a drawback that intergranular corrosion is likely to occur due to heating and cooling during welding.

In order to improve weather resistance, increase of Cr content and M
It is effective to add o. The decrease in toughness with the increase of Cr and Mo is improved by reducing C and N. The reduction of C and N is also effective for improving the intergranular corrosion resistance. However, there is a limit to the reduction of C and N, and currently,
At the industrially attainable C and N content levels, the intergranular corrosion susceptibility cannot be completely eliminated. Therefore, the adverse effect of C and N on the intergranular corrosion is eliminated by adding a stabilizing element such as Ti or Nb capable of fixing C and N, either alone or in combination.

[0004]

Based on these technical backgrounds, for example, low carbon / low nitrogen 30Cr-2.0.
Mo-containing high Cr steels having excellent weather resistance have been developed, such as Mo-Nb steels. However, when exposed to a corrosive environment containing chlorine ions, such as in coastal areas and the sea, red rust may be generated in a short period of time depending on weather conditions such as temperature and humidity. In red rust generation, a part of the passive film is destroyed by chlorine ions,
This is due to the dissolution of the stainless steel components and the formation of Fe corrosion products. That is, the surface of stainless steel is usually covered with a passivation film that suppresses rusting such as red rust. However, in the coastal area and the sea, the passive film is destroyed, and the corrosion progresses due to the arrival of sea salt particles containing chlorine ions, which have the effect of hindering the regeneration.

Further, as stainless steel plates used for applications such as exterior materials, dull-processed and polished finishes are often used because antiglare properties with suppressed surface gloss are desired. However, the dull and polished skins are inferior in weather resistance to those finished by bright annealing or temper rolling. This is because the passivation film formed on the material of dull skin or polished skin has many defects, and moreover, since there are irregularities, the attached chlorine ions do not easily flow down. Even for those that have been subjected to bright annealing or the like, if defects such as flaws generated during processing are present in the surface layer portion, the weather resistance may deteriorate and red rust may occur early. The present invention has been devised to solve such a problem, and by the alloy design that improves the characteristics of the passive film, it exhibits excellent corrosion resistance and weather resistance even in a corrosive environment containing chlorine ions, It is an object of the present invention to provide a ferritic stainless steel plate which has excellent glare and can be used as a polished skin, a dull skin, etc.

[0006]

The high corrosion resistance ferritic stainless steel of the present invention has a C:
0.05 wt% or less, Si: 1.0 wt% or less, Mn:
1.0% by weight or less, P: 0.04% by weight or less, S: 0.
03% by weight or less, Ni: 1.0% by weight, Cr: 20 to 3
5% by weight, Mo: 1 to 8% by weight, Nb: 0.1 to 1.0
% By weight and N: 0.05% by weight or less, Cr + 3M
It satisfies o ≧ 37% by weight. This ferritic stainless steel further contains Ti: 0.05 to 0.3% by weight and Al:
0.01-0.5% by weight, Cu: 0.1-1.0% by weight
And Zr: 0.05 to 0.3% by weight of one kind or two or more kinds may be contained.

[0007]

The present inventors investigated and studied the relationship between the passivation film formed on the surface of ferritic stainless steel and the corrosion resistance and weather resistance from various points of view. As a result, when the proportion of Cr and Mo contained in the passivation film becomes high, corrosion resistance,
It was clarified that very excellent properties such as weather resistance can be obtained. The reason why Cr and Mo in the passivation film improve the corrosion resistance, weather resistance, etc. is presumed as follows. When a large amount of Cr is present in the passivation film, even if the passivation film is destroyed, the self-repairing action is strong and repassivation is likely to occur.
However, in the passivation film containing Cr and Fe as main components,
There are active defects that are the origin of corrosion. Mo complements the defective portion existing in this passive film and suppresses the occurrence of corrosion. In addition, the action of suppressing the cathode reaction is Mo.
Therefore, even if corrosion occurs, the progress of corrosion is suppressed. As a result, corrosion resistance and weather resistance are improved.

Cr and Mo in the passivation film are greatly influenced by the composition of the ferritic stainless steel as the base material. Therefore, the effect of the alloy design of ferritic stainless steel on the composition of the passive film was investigated. As a result, C
20 so that the condition of r + 3Mo ≧ 37% by weight is satisfied
To 35 wt% Cr and 1 to 8 wt% M
It has been found that when o is included, a passivation film with a stable high Cr and Mo ratio is formed. When the passivation film contains a large amount of Cr and Mo, the corrosion resistance and pitting corrosion resistance are significantly improved. Further, even if there is a defect such as a processing flaw, the progress of pitting corrosion is suppressed. Moreover, such a passivation film is formed even on the as-polished steel material surface without requiring pickling finish. Therefore, it can be used as a material having dull skin or polished skin which is effective for antiglare property.

The alloy components and contents of ferritic stainless steel used as the base material will be described below. C, N: Elements inevitably contained in stainless steel. When the C and N contents are reduced, the content becomes soft, the workability is improved, and the generation of carbides is reduced. Also, C, N
With the decrease in the content, the weldability and the corrosion resistance of the welded part are also improved. Therefore, the upper limits of the C and N contents are both 0.0
It was defined as 5% by weight. Si: An element harmful to hot cracking of the weld and toughness of the weld. In addition, since it makes stainless steel hard, Si
The lower the content, the better. Therefore, the upper limit of the Si content is defined as 1.0% by weight.

Mn: A harmful element that reduces the weather resistance by forming a soluble sulfide MnS by combining with a small amount of S existing in stainless steel. So M
The upper limit of the n content is defined as 1.0% by weight. P: The lower the P content is, the less the base material and the toughness of the welded portion are. However, it is difficult to remove P from Cr-containing steel such as stainless steel, and extremely reducing the P content causes an increase in manufacturing cost. Therefore, P
The upper limit of the content is specified as 0.04% by weight. S: A lower S content is preferable because it is a harmful element that adversely affects weather resistance and hot cracking of the welded portion. Therefore, the upper limit of the S content is specified to be 0.03% by weight.

Ni: An alloying element effective in improving the toughness of ferritic stainless steel. However, the inclusion of a large amount of Ni causes a high cost. In the present invention, the upper limit of the Ni content is set to 1.0% by weight, which is mixed as an unavoidable impurity in ordinary ferritic stainless steel. Cr: A main element that enhances the corrosion resistance of stainless steel, and significantly improves weather resistance, pitting corrosion resistance, crevice corrosion resistance and general corrosion resistance. The action of Cr on improving the corrosion resistance is
If it is less than 20% by weight, it is insufficient. However, if the Cr content exceeds 35% by weight, significant embrittlement occurs, which causes difficulties during thin plate manufacturing, product processing, and the like. Therefore, the Cr content is set in the range of 20 to 35% by weight.

Mo: It is an effective alloying element that enhances weather resistance together with Cr, and its effect increases as the amount of Cr increases. At the Cr content level specified in the present invention, 1
If the Mo content is less than wt%, the effect of improving weather resistance is small,
On the other hand, if Mo is added in excess of 8%, the ductility will be lowered and the processing will be difficult. Therefore, the Mo content is set in the range of 1 to 8% by weight. Mo content is C between Cr content
It is necessary that the relationship of r + 3Mo ≧ 37% by weight is established. This formula was obtained as a result of experiments by the present inventors and is an effective index for maintaining good weather resistance even for polished skin and dull skin. That is, C
In the alloy design satisfying the relationship of r + 3Mo ≧ 37% by weight, the proportion of Cr and Mo oxides in the passivation film formed on the surface of the steel material increases, and the corrosion resistance and weather resistance are excellent even in the condition of polished skin and dull skin. improves. Cr and Mo are basic components for improving the weather resistance, and Mo has a larger contribution to the improvement of the weather resistance than Cr.
Doubled

Nb: Prevents intergranular corrosion in the ferritic stainless steel having the C content level specified in the present invention.
In order to obtain this effect, a Nb content of 0.1% by weight or more is necessary. However, since excessive Nb addition impairs weld toughness, the upper limit of the Nb content is defined as 1.0% by weight. The ferritic stainless steel of the present invention has T
One or more of i, Al, Cu and Zr can be contained as an optional component. The effects of these alloying elements are
It is as follows. Ti: The passivation film is strengthened, and excellent corrosion resistance and weather resistance can be obtained even with a composition having a relatively low amount of Cr and Mo. In addition, addition of Ti also suppresses intergranular corrosion and exhibits the effect of fixing C and N. In order to obtain such an effect, it is necessary to contain 0.05% by weight or more of Ti. However, when Ti is contained in excess of 0.3% by weight, the surface quality of the material is deteriorated and local corrosion tends to be strengthened. Therefore, when Ti is contained, its content is set in the range of 0.05 to 0.3% by weight.

Al: A component added as a deoxidizing agent, but also has the function of densifying the passive film. Such an effect becomes remarkable when the Al content is 0.01% by weight or more. However, if Al is added in excess of 0.5% by weight,
The surface quality of the material is deteriorated and the weldability deteriorates.
Therefore, when Al is contained, its content is 0.0
It is set in the range of 1 to 0.5% by weight. Cu, Zr: Cu and Zr added as optional components
Are both alloying elements effective for improving weather resistance. To obtain such effects, a Cu content of less than 0.1% by weight or a Zr content of less than 0.05% by weight is insufficient. . However, if it is too large, the toughness of the welded part is impaired, so
Therefore, when Cu or Zr is contained, the upper limit of the Cu content is set to 1.0% by weight and the upper limit of the Zr content is set to 0.3% by weight.

In this alloy system, a passive film containing a large amount of Cr and Mo is formed on the as-polished steel surface. Looking at the ratio of each element and the ratio of Cr + 3Mo in the total amount of Fe, Cr and Mo contained in the passivation film in terms of atomic number, the ratio of Cr satisfies the requirement that the composition of the steel material defines in the present invention. The limit is 25.0% or more. Further, the proportion of Mo is 4% or more, and the proportion of Cr + 3Mo is 42% or more. On the other hand, the proportion of Fe is
With the increase of Cr and Mo, it decreased to 70% or less. Since such a large amount of Cr and Mo is contained in the passivation film, even if there are some defects such as work flaws in the steel material surface layer, the self-repairing action works and the progress of corrosion is suppressed. On the other hand, in the case of the abrasive containing less than 37% by weight of Cr + 3Mo, the proportion of Fe becomes high and a relatively porous passivation film is formed. Both weather resistance and pitting corrosion resistance are inferior.

[0016]

[Examples] Various stainless steels having the chemical compositions shown in Table 1 were melted and hot-rolled to produce hot-rolled sheets having a plate thickness of 3.5 mm. After that, cold rolling to a plate thickness of 1.0 mm,
After annealing at 1000 to 1050 ° C., a test material having a length of 100 mm and a width of 100 mm was cut out. Note that some of the test materials were dull after annealing and pickling. In Table 1, group A steels are stainless steels that satisfy the components and compositions regulated by the present invention. Among them, A1 and A
5 is based on 25Cr steel, A2 and A4 are 30Cr
Based on steel, A3 is 31Cr-3Mo steel. On the other hand, Group B steels are stainless steels in which Cr + 3Mo does not reach 37% by weight.

[0017]

[Table 1]

Each of the test materials was first # 120 and then # 60.
After mechanical polishing of 0, it was finished by manual polishing of # 600. The test materials with dull skin were used as they were. XP on the surface layer of the passive film formed on the abrasive
S analysis was performed to quantify Fe, Cr and Mo. When the total of Fe, Cr, and Mo is calculated as 100 in terms of the number of atoms, the proportion of each element and Cr atom + 3 (Mo atom)
Was calculated. As is clear from Table 2 showing the calculation results, all the test materials of the A group satisfying the condition of Cr + 3Mo ≧ 37 wt% have a high ratio of Cr atom + 3 (Mo atom). On the other hand, Cr + 3Mo is 3
In the sample materials of Group B, which did not reach 7% by weight, the ratio of Cr atom + 3 (Mo atom) showed a low value.

[0019]

[Table 2]

Each abrasive was attached to a stainless steel angle through a bakelite washer with an inclination of 5 degrees so that the exposed surface faces north on the roof of a three-story building in Ginowan City, Okinawa Prefecture. , Subjected to atmospheric exposure test. Then, the weather resistance of each test material was judged by the rate of occurrence of red rust on the surface of the test material after the exposure test. Red rust occurrence rate is JIS D 02
It was calculated based on 01. When the red rust occurrence rate was sorted by Cr + 3Mo, as shown in FIG. 1, the red rust occurrence rate sharply decreased at the boundary of Cr + 3Mo = 37% by weight. Table 2
Further, from FIG. 1, it is confirmed that Cr + 3Mo ≧ 37 wt% has a high ratio of Cr and Mo in the passivation film and is effective for improving the weather resistance.

Next, the pitting corrosion potentials of the abrasive and the dull material were measured. The measurement was performed by keeping the liquid temperature at 80 ° C. and degassing Ar 20.
% NaCl solution was used to perform a potentiodynamic method with a sweep rate of 0.1 mV / min, and the potential at which the anode current density reached 200 μA / cm ² was measured as the pitting potential. The results of organizing the measured pitting potential with Cr + 3Mo are shown in FIG. As is clear from FIG. 2, also with respect to pitting corrosion resistance, Cr + 3
A sharp upward trend is seen at the boundary of Mo = 37% by weight. On the other hand, in the region where Cr + 3Mo is less than 37% by weight,
The increase in pitting potential commensurate with the increase in Cr + 3Mo was small.

[0022]

As described above, in the present invention, the alloy design of ferritic stainless steel, especially Cr +
By setting 3Mo to 37% by weight or more, a large amount of Cr and Mo are contained in the passivation film formed on the steel surface. This passive film, in combination with the component system of the base material,
It exhibits excellent corrosion resistance and weather resistance even in the condition of polished skin and dull skin. Therefore, polishing finish and dull finish, which are effective in improving the antiglare property, can be adopted, and by utilizing the excellent weather resistance and pitting corrosion resistance, it can be widely used as an exterior structural material for roof materials, exterior materials, outdoor tanks such as hot water storage tanks, etc. Used in the field.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of Cr + 3Mo on the occurrence rate of red rust.

FIG. 2 is a graph showing the relationship between pitting potential and Cr + 3Mo.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ikuhiro Sugimoto 4976 Nomuraminami-cho, Shinnanyo-shi, Yamaguchi Prefecture Steel Research Laboratory, Nisshin Steel Co., Ltd. Steel Research Co., Ltd.

Claims (2)

[Claims] 1. C: 0.05% by weight or less, Si: 1.0
Wt% or less, Mn: 1.0 wt% or less, P: 0.04 wt% or less, S: 0.03 wt% or less, Ni: 1.0 wt%, Cr: 20-35 wt%, Mo: 1 ~ 8% by weight, N
b: 0.1 to 1.0% by weight and N: 0.05% by weight or less, high corrosion-resistant ferritic stainless steel excellent in antiglare property and satisfying Cr + 3Mo ≧ 37% by weight. 2. Ti: 0.05 to 0.3% by weight, Al:
0.01-0.5% by weight, Cu: 0.1-1.0% by weight
And Zr: 0.05 to 0.3% by weight of one or more of the high corrosion-resistant ferritic stainless steel according to claim 1.