JPH0734205A - Ferritic stanless steel excellent in atmospheric corrosion resistance and crevice corrosion resistance - Google Patents

Abstract

PURPOSE:To provide a ferritic stainless steel excellent in atmospheric corrosion resistance and crevice corrosion resistance by actively adding P and properly regulating respective contents of Ca and Al. CONSTITUTION:This steel is a ferritic stainless steel consisting of, by weight, <=0.05% C, <=1.0% Si, 11-<20% Cr, <=1.0% Mn, <=0.10% N, <=0.03% S, 5-50ppm Ca, <=0.5% Al, >0.04-0.20% P, and the balance iron with inevitable impurities and excellent in atmospheric corrosion resistance and crevice corrosion resistance. Further, the above composition can contain <=6% Mo or at least one kind selected from <=1.0% Cu, <=3% Ni, and <=3% Co or at least one kind selected from <=1.0% Ti, <=1.0% Nb, <=1.0% V, <=1.0% W, <=1.0% Zr, <=1.0% Ta, and <=0.05% B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a P-containing ferritic stainless steel which is excellent in weather resistance and crevice corrosion resistance of a worked part. INDUSTRIAL APPLICABILITY The steel of the present invention is most suitable for applications when it is used as a material for building exteriors, members of electric appliances, panels, hot water bodies, etc., which are not particularly maintained for a long period of time. It can be widely used for ferritic stainless steel that requires crevice corrosion.

[0002]

2. Description of the Related Art Conventional stainless steel sheets are used for building exterior materials that require weather resistance, such as panels, sashes, and curtains.
It was often used in a relatively small area such as a wall.

However, in recent years, the design of stainless steel has
The good appearance, corrosion resistance, and weather resistance have been recognized, and in addition to the development of its construction technology, the demand for it as a large building exterior material represented by roofing materials and panel materials has increased. In this case, for example, as the roofing material, a painted stainless steel plate which is a stainless steel plate as a base material and which is painted and colored, or a stainless steel plate which is coated with a fluororesin is mainly used. This is to overcome the drawback that the conventional tin roofing material becomes unusable due to defects in the coating due to deterioration of the coating. This coating stainless steel sheet is mainly SU, which is austenitic stainless steel.
S304 (18Cr-8Ni) has been adopted because of its good workability.

However, in the case of the above-mentioned coated stainless steel plate or fluorine-coated material, in the case of an opaque coating material, the coating property is present on the surface, so the designability due to the silver-white metallic luster inherent in stainless steel cannot be utilized, and austenite Since stainless steel contains a large amount of expensive Ni, it is disadvantageous because the price is high. On the other hand, in the case of transparent fluorine coating, there is a problem that the appearance of the surface is impaired as the coating film deteriorates. Further, austenitic stainless steel has a coefficient of thermal expansion about twice that of ferritic stainless steel, and therefore is not suitable for long products.

Therefore, ferritic stainless steel is attracting attention as a building exterior material. However, when ferritic stainless steel is used as a building exterior material, especially as a roofing material without coating, it is red for a long time. Sufficient outdoor weatherability and rust resistance that does not cause corrosion such as pitting corrosion due to rust or sea salt particles are inevitably required. In addition, when used as an exterior material for building materials, such as a panel or a curtain wall, roll forming and press processing are performed,
Even in the processed part, the above characteristics, that is, weather resistance, corrosion resistance,
It goes without saying that crevice corrosion resistance is required.

For this reason, in the past, Japanese Patent Laid-Open No. 55-1
As represented by No. 38058, as a ferritic stainless steel having high weather resistance and high rust resistance, C and N can be reduced as much as possible, and the corrosion resistance can be increased by increasing the amount of Cr and the amount of Mo added. It has been tried. However, simply increasing the amount of Cr and the amount of Mo added results in a high alloy, which makes the price expensive and restricts from the economical aspect. In addition, there is a problem in that the moldability is deteriorated due to hardening and further the manufacturability is accompanied by addition of toughness. Therefore, the addition of elements other than Cr and Mo can be expected to improve weather resistance, rust resistance, and crevice corrosion resistance. Furthermore, there is no loss of formability of the material or corrosion resistance of the processed part, and the development of cheaper materials is strong. Was wanted.

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the present invention, the addition of P is aimed at improving the corrosion resistance, but the addition of P deteriorates the workability of the material, so cracks in the processed portion and measures to improve the corrosion resistance have been required.

The present invention aims to satisfy such requirements, and P is added to ferritic stainless steel in the range of more than 0.04% and 0.2% or less, which is one of the features of the present invention. By adjusting the amount of Ca and Al added, the amount, shape and distribution of non-metallic inclusions are regulated, which is cheaper than conventional steel and is particularly excellent in weatherability and crevice corrosion resistance in the machined part. The purpose is to provide steel.

That is, in the present invention, P, which has been known as a harmful element in the past, is positively added within the range of manufacturability due to the decrease in toughness, and the cleanliness is improved and the inclusion morphology and distribution are controlled. It is an object of the present invention to provide a ferritic stainless steel excellent in weather resistance and crevice corrosion resistance particularly in a processed part such as bending and squeezing by optimizing the amount of Ca and Al to be added. .

[0010]

[Means for Solving the Problems] When the effect of the combined addition of Ca and Al was investigated, it was found that there were large differences in the amount, shape and distribution of non-metallic inclusions as shown in the table below. The present invention has been developed.

That is, in the present invention, C: 0.05% or less, Si: 1.0% or less, Cr: 11% or more and less than 20%, Mn: 1.0% or less, N: 0.10% by weight. Hereinafter, S: 0.03% or less, Ca: 5 to 50 ppm, Al: 0.5% or less, P: more than 0.04% to 0.20%, and the balance contains iron and inevitable impurities, weather resistance, A ferritic stainless steel having excellent crevice corrosion resistance. Regarding the amount of Al added,
1: 0.1% or less is preferable, Al: 0.01
% Or more and 0.1% or less is more preferable.

Further, the stainless steel of the present invention may contain at least one of the following groups (1) to (3). (1) Mo is 6% or less (2) Cu: 1.0% or less, Ni: 3% or less and C
o: at least one selected from 3% or less (3) Ti: 1.0% or less, Nb: 1.0% or less, V:
1.0% or less, W: 1.0% or less, Zr: 1.0% or less, Ta: 1.0% or less, and B: 0.05% or less, at least one selected from

[0013]

The present invention will be described in more detail below. The ferritic stainless steel of the present invention is a range of manufacturable grades, which has been conventionally made efforts to reduce the content as a harmful element as much as possible, positively, without impairing the formability. By adding the appropriate amount of Ca and Al, the shape and distribution of the non-metallic inclusions are controlled, and the surface properties and cleanliness are improved, thereby improving the weather resistance and crevice corrosion resistance of the processed part. It is characterized by that. Regarding the effect of P on the corrosion resistance of stainless steel, in the "Stainless Steel Handbook", there is almost no effect when the content is 0.1% or less in the complete solid solution state, and as the content of P increases, pitting corrosion sensitivity increases. , As a result, it is described that corrosion resistance is reduced. The reason for attempting to reduce the P addition amount in conventional stainless steel as much as possible is as follows.
As described above, it is considered that the cause is the deterioration of the manufacturability associated with the decrease in the toughness of P.

That is, regarding manufacturability, it is generally known that P easily causes segregation, enhances hot crackability, and promotes crack susceptibility of welded portions. From this fact, it has been considered that P is extremely harmful to ferritic stainless steels, which have a conventional body-centered cubic structure and lower toughness than austenitic stainless steels, and efforts have been made to reduce the addition amount as much as possible. In fact, for high Cr ferritic stainless steel such as SUS447J1 as specified in JIS 430, the addition amount of P is 0.
It is specified to be 03% or less. Further, regarding other steel types, P is specified to be 0.04% or less because it lowers toughness and workability.

On the other hand, as an example in which P is positively added to ferritic stainless steel, JP-A-60-248868 is known.
No. 6, JP-A-61-2825, JP-B-57-3090
There is No. 3, but positive addition of P is secondary workability,
It is said that it only improves the descaling property and high temperature characteristics, and does not affect the corrosion resistance.

Further, for example, when ferritic stainless steel is subjected to roll forming or panel processing as a building material application, the processed portion causes surface roughness or minute cracks due to the cleanliness of the material, and these become rust initiation points,
There is concern that rusting such as red rust may occur and weather resistance may deteriorate. On the other hand, in the case of joining with a bolt or the like, a gap is generated, so that crevice corrosion resistance is required.

Therefore, in addition to systematically investigating the effect of the positive addition of P to ferritic stainless steel, which has not been noticed in the present invention, on weather resistance, rust resistance and crevice corrosion resistance, and Ca By adding Al and Al, the shape and distribution of non-metallic inclusions are controlled, the cleanliness and surface properties are improved, and the decrease in manufacturability and workability due to the addition of P is supplemented, which is standardized by the conventional JIS. It is said that P can be added positively over the range to improve weather resistance and crevice corrosion resistance, and in particular, as an exterior material for buildings with bending parts, etc., there is little red rust and it is possible to provide an aesthetically superior material. The present invention was obtained based on new findings.

That is, in the present invention, C: 0.05% or less, Si: 1.0% or less, Cr: 11% or more and less than 20%, Mn: 1.0% or less, N: 0.10% by weight. Hereinafter, S: 0.03% or less, Ca: 5 to 50 ppm, Al: 0.5% or less, P: more than 0.04 to 0.20%, and the balance is composed of iron and inevitable impurities. A ferritic stainless steel having excellent crevice corrosion resistance is provided. In addition, Al in the above range is 0.1
% Or less. The Al content is preferably 0.1% or less, and more preferably 0.01% to 0.1%.

Further, the stainless steel of the present invention may contain at least one of the following groups (1) to (3). (1) Mo is 6% or less (2) Cu: 1.0% or less, Ni: 3% or less and C
o: at least one selected from 3% or less (3) Ti: 1.0% or less, Nb: 1.0% or less, V:
1.0% or less, W: 1.0% or less, Zr: 1.0% or less, Ta: 1.0% or less, and B: 0.05% or less, at least one selected from

As a merit of adding P, in addition to the above, when a material having the same weather resistance and crevice corrosion resistance is manufactured, the amount of addition of elements such as Cr and Mo, which are effective for improving these characteristics, can be changed by inexpensive P. The cost is low because it is used as an alternative, and it is possible to expect the reduction of raw material cost and the cost of P removal because the conventional low P process can be eliminated or the process can be saved.

From the above, the steel of the present invention is expected to exert a very large industrial effect.

The reason why the steel composition is limited as described above will be explained below. The unit is% by weight. However, only Ca is ppm.

C, N C, N are constituent elements that greatly affect hot workability, toughness, and rust resistance. Since the manufacturability of the steel of the present invention is lowered by adding P, the upper limit is 0.05% for C and 0.
It was set to 10%. Further, the lowering effect of these elements is not saturated, and the lower the content of C and N is, the more preferable. However, from the viewpoint of actual production, it can be said that C ≧ 10 ppm and N ≧ 20 ppm are preferable.

Cr Cr is a heavy element that determines the basic corrosion resistance of the steel of the present invention. Corrosion resistance improves with the increase of this content,
If it is added in excess of 20%, the workability of the material, particularly ductility, in the P-added steel such as the steel of the present invention decreases, roll forming and panel processing become difficult, and cracks easily occur in the processed part, so the upper limit is set. It was less than 20%. On the other hand, if it is less than 11%, sufficient corrosion resistance and weather resistance cannot be obtained, so the lower limit was made 11%. The preferable range of the amount of Cr is 15% to 18%.

Si Si is an element which is added as a deoxidizer and is particularly effective for improving the oxidation resistance and cleanliness. Further, although effective effects for improving weather resistance and rust resistance are also recognized, if the addition amount is too large, elongation and toughness decrease due to solid solution strengthening, so the upper limit is 1.
It was set to 0%.

Mn Mn is an element that forms austenite at high temperature, and is an element that forms martensite by cooling after high-temperature heat treatment. It is used as a deoxidizer in the steelmaking process, but if added in excess of 1.0%, it is harmful to hot workability, so the upper limit was made 1.0%. The preferable range of the amount of Mn is 0.3% or less.

S S is not only an element harmful to mechanical properties and weldability, but inclusions such as MnS serve as a starting point of rusting, so that it deteriorates weather resistance and rust resistance. Therefore, the smaller the content, the better. In particular, if it exceeds 0.03%, weather resistance, rust resistance,
Since the crevice corrosion resistance is significantly reduced, the upper limit is 0.03.
%. The preferable range of the amount of S is 0.007% or less.

Al Al has a deoxidizing effect and is added as a deoxidizing material. Also A
l is MnO, FeO which promotes erosion of refractory materials,
Since the formation of silicate is suppressed, oxide inclusions due to the corrosion of the refractory material are reduced, and the manufacturability and workability of steel are improved. However, excessive addition of more than 0.5% promotes the generation of macroscopic inclusions and causes the workability to decrease due to scattering of inclusions, so the upper limit of the addition amount was made 0.5%. The preferable amount of Al is 0.1% or less.
Further, if the effect of Al is less than 0.01%, the effect cannot be expected so much, so the lower limit is preferably 0.01% or more.

Ca Ca is an element that improves the cleanliness and surface properties of the steel of the present invention, contributes to the improvement of the characteristics of the steel, and is an important element for adjusting the shape and distribution of nonmetallic inclusions. That is, Ca has the effect of adjusting the shape and distribution of non-metallic inclusions in deoxidized steel, forming a so-called monotectic inclusion that is rich in ductility without forming a continuous brittle layer of inclusions, It is an element effective in improving workability. It can be seen that this effect reduces cracks caused by inclusions in the processed portion by adding 5 ppm or more, and further improves weatherability and crevice corrosion resistance by adding P in combination. However, if the addition amount is too large, the surface quality is deteriorated and the corrosion resistance is mainly deteriorated mainly due to CaO, so the upper limit was made 50 ppm. The preferable addition range of Ca is 3 ppm to 15 ppm.

P P is an element effective for improving corrosion resistance, weather resistance and crevice corrosion resistance. The effect does not clearly appear unless it exceeds 0.04%, so the lower limit was made 0.04% or more. On the other hand, 0.2%
If it is added in an amount exceeding 0.2%, not only the workability and manufacturability are deteriorated but also the rust resistance is rather deteriorated.
And The preferable range of the amount of P is more than 0.04% to 0.1%.
Is.

Mo Mo is an additive element that has the effect of significantly increasing the corrosion resistance and weather resistance of the steel of the present invention, and is an extremely effective element for improving rust resistance, pitting corrosion resistance, and crevice corrosion resistance. Further, since the effect is further promoted by increasing the amount of Cr, it is an element that should be added by all means. However, if the addition amount exceeds 6.0%, the toughness is lowered and the manufacturability is remarkably lowered, and the economical efficiency is deteriorated. Therefore, the addition range is limited to 6.0% or less. The preferable range of the amount of Mo is 2.0% or less.

Ni, Co, Cu Ni, Co and Cu All of these elements have weather resistance,
It is an element effective in improving corrosion resistance, acid resistance, and crevice corrosion resistance. Further, Ni and Co also have an effect of improving toughness. However, if Cu is added in excess of 1.0%, the hot workability deteriorates and the material becomes hard. Further, if Ni and Co are added in excess of 3.0%, the workability is deteriorated and the economical efficiency is deteriorated. Therefore, the contents of these elements are Ni and Co: 3% or less and Cu: 1.0%, respectively. The following was set. The preferable ranges of the amounts of Ni, Co and Cu are N
i; 1.0% or less, Co; 1.0% or less, Cu; 0.6
% Or less.

Nb, Ti, V, Zr, Ta, W, B Nb, Ti, V, Zr, Ta, W and B are all carbide and nitride forming elements, and have weather resistance, improved formability and welded parts. Exerts the effect of improving the corrosion resistance of. However, when Nb, Ti, V, Zr, Ta and W are added in excess of 1.0% and B is added in excess of 0.05%, the effect is saturated and the workability is impaired. Therefore, Nb, Ti, V, Zr, Ta, and W are 1.0% or less, while B is an element effective for further improving the secondary workability, but if 0.05% or more, the effect is Since it saturates and conversely the workability decreases, the range was made 0.05% or less. Nb, Ti, V, Z
The preferred ranges of the amounts of r, Ta, W and B are Nb;
0.5% or less, Ti; 0.3% or less, V; 0.2% or less, Zr; 0.3% or less, Ta; 0.3% or less, W;
0.2% or less, B: 0.02% or less.

P which is excellent in weather resistance and crevice corrosion resistance of the present invention
The added ferritic stainless steel can be widely used for building exterior materials to be processed (roof materials, exterior panels, etc.), water heater cans, and coating base plates. Further, the steel of the present invention is a normal manufacturing process of the steel produced by the above-mentioned component system, that is, melting-hot rolling-annealing-pickling-cold rolling-
It can be manufactured by annealing- (pickling) -finish rolling (temper rolling).

Further, the steel of the present invention may be either a hot rolled annealed plate or a cold rolled annealed plate (2D, 2B, BA, HL,
In particular, when the material is processed by roll forming, etc., it is possible to obtain the effect on the corrosion resistance, weather resistance and crevice corrosion resistance of the processed part.

[0036]

EXAMPLES The present invention will be specifically described below based on examples. (Example) A 30 kg small steel ingot having the composition shown in Table 1 was melted in a vacuum high-frequency furnace, heated at 1250 ° C. for 1 hour, then formed into a hot rolled sheet having a thickness of 4 mm, and then allowed to cool to obtain a hot rolled annealed sheet. The plate was shot blasted, further pickled to remove surface scale, and then cold rolled to a thickness of 0.6 mm.
After reheating for 30 seconds in the temperature range of 0 ° C to 1150 ° C, the mixture was immediately cooled.

This material was processed into a material as shown in FIG. 1 and then subjected to an atmospheric exposure test (J
IS Z 2381) and weather resistance (Rust area ratio)
I conducted a survey. The details of the test method were in accordance with JIS, but the test pieces were exposed to a pedestal with an angle of 36 degrees to the south for two years for each of the two steel types, and the amount of rusting on the flat plate was used as the rusting area ratio. The evaluation was performed and the presence or absence of rusting in the processed portion and the cross cut portion was investigated. Table 2 shows the test results after 3 years from the atmospheric exposure test, that is, the presence or absence of rusting at the 180 degree bent portion and the cross cut portion, the rusting area ratio of the flat plate portion, and 10% ferric chloride solution-3% saline solution. The results of the crevice corrosion occurrence test are also shown. Further, in FIG. 2, 13Cr containing Ca and Al
The influence of P on the change with time of the corrosion area ratio of -0.5Mo-0.2Nb-0.02 (C + N) steel is shown. From this figure, the weather resistance of the steel type in which P exceeds 0.04% is superior to the steel in which no P is added, as compared with the 0.04% or less P-added steel specified in JIS.

FIG. 3 shows 18Cr-0.2Mo-0.005.
The influence of Al and Ca on the pitting potential (the processing method is the same as that described with reference to FIG. 4 below) in the 180 ° bent (R = 1 mm) portion of C-0.01N-steel is shown. P from 0.06
The effect of Al is more remarkable in the steel containing 0.07 and containing 15 ppm of Ca, and the effect of the combined addition of Al and Ca appears.

On the other hand, in FIG.
Bending test was conducted at intervals of 20 degrees up to 0 degree, and then the pitting potential was measured. Here, the pitting potential is measured by JI
According to SG 0577, it was measured in a 3.5% NaCl solution at 30 ° C. Moreover, the current density is 10 μA / c.
It is represented by the potential reaching m ² . Steel 6 (Ca-less, Al:
0.05%: Comparative steel) and Steel 7 (Ca; 23 ppm, A)
(1; 0.06% composite added steel: steel of the present invention), the larger the bending angle, the more the pitting corrosion resistance of the steel differs, and the Ca addition steel has better corrosion resistance.

From the above results, P standardized by JIS
It was found that by adding P in excess of 0.04%, which is the range of addition, and further adding Ca and Al, the corrosion resistance of not only the flat plate but also the processed part was improved.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

[Table 4]

[0045]

[Table 5]

[0046]

[Table 6]

[0047]

EFFECTS OF THE INVENTION In this way, the component-based ferritic stainless steels containing Ca, Al, and P compounded in accordance with the present invention have a lower alloy than the conventional steel types, and have better weather resistance and rust resistance in the machined part. It is also excellent in crevice corrosion resistance and can be manufactured at low cost, and is expected to have an industrially useful effect.

[Brief description of drawings]

FIG. 1 is a diagram showing the shape of a test piece.

FIG. 2 is a graph showing the effect of P addition on the rust area ratio after 6 months and 5 years in the air exposure test.

FIG. 3 is a graph showing the effect of combined addition of Ca and Al on the pitting potential of a 180 ° bend (R = 1 mm).

FIG. 4 is a graph showing the effect of Ca and Al combined addition on the pitting potential and the bending angle of the test piece.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Fusao Togashi, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba, Kawasaki Steel Corporation Technical Research Division (72) Inventor Keiichi Yoshioka, Chuo-ku, Chiba, Chiba Kawasaki-cho 1 Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Satoshi Owada 2-3 2-3 Uchisaiwaicho, Chiyoda-ku, Tokyo Kawasaki Steel Co., Ltd. Tokyo Head Office

Claims (4)

[Claims] 1. By weight%, C: 0.05% or less,
Si: 1.0% or less Cr: 11% or more and less than 20%, Mn: 1.0% or less N: 0.10% or less, S: 0.03% or less Ca: 5 to 50 ppm, Al: 0.5% The following P: ferritic stainless steel containing P: more than 0.04% to 0.20% with the balance being iron and unavoidable impurities and having excellent weather resistance and crevice corrosion resistance. 2. In addition to the component according to claim 1, M
Ferritic stainless steel containing 6% or less of o, and the balance consisting of iron and unavoidable impurities and having excellent weather resistance and crevice corrosion resistance. 3. In addition to the component according to claim 1 or 2, Cu: 1.0% or less, Ni: 3% or less and C.
o: Ferritic stainless steel containing one or more selected from 3% or less, and the balance consisting of iron and unavoidable impurities, which is excellent in weather resistance and crevice corrosion resistance. 4. In addition to the component according to any one of claims 1, 2 and 3, Ti: 1.0% or less, Nb: 1.
0% or less, V: 1.0% or less, W: 1.0% or less, Zr
1.0% or less, Ta: 1.0% or less and B: 0.05
%, A ferritic stainless steel containing one or more selected from the following, and the balance consisting of iron and unavoidable impurities and having excellent weather resistance and crevice corrosion resistance.