JPH06172935A - High cr-p-added ferritic stainless steel excellent in weather resistance and rusting resistance - Google Patents

Abstract

PURPOSE:To produce high Cr-P-added ferritic stainless steel excellent in weather resistance and rusting resistance. CONSTITUTION:This high Cr-P-added ferritic stainless steel contains, by weight, <=0.02% C, <=1.0% Si, <=1.0% Mn, <=0.03% S, >20 to 40% Cr, <=0.015% N, 0.5% Al and >0.06 to 0.20% P, and the balance Fe with inevitable impurities. Furthermore, the steel may contain one or more kinds among the following groups (1) to (3): (1) <=4.0%Mo, (2) one or >= two kinds among <=1.0% Ti, <=1.05 Nb, <=1.0% Ta, <=1.0% V, <=1.0%W, <=1.0% Zr and <=0.01% B, and (3) one or >= two among <=1.0% Cu, <=5.0% Ni and <=1.0% Co.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high Cr, P-added ferritic stainless steel excellent in weather resistance, rust resistance and weld property. INDUSTRIAL APPLICABILITY The steel of the present invention can be widely used as a ferritic stainless steel to be used under the conditions where weather resistance and rust resistance are required, particularly for building exterior materials and some structures.

[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 good design, aesthetics, corrosion resistance, and weather resistance of stainless steel have been recognized, and in addition to the development of its construction technology, there is also a demand for it as a large building exterior material represented by a roof material. It has grown. in this case,
For example, as a roofing material, a coated stainless steel sheet obtained by painting and coloring a stainless steel sheet as a base material 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. Austenitic stainless steel, SUS304 (18Cr-8Ni), has been mainly used as the coating-use stainless steel plate because of its good workability. However, in the above-mentioned coated stainless steel plate, since the coating film is present on the surface, the designability due to the silver-white metallic luster that stainless steel originally has cannot be utilized, and the price increases because austenitic stainless steel contains a large amount of expensive Ni. It is also possible.

Further, since the coefficient of thermal expansion is about 1.5 to 2 times that of ferritic stainless steel, it is unsuitable for long products. Therefore, ferritic stainless steel is attracting attention as a building exterior material.When using ferritic stainless steel as an exterior material for buildings, especially as an unpainted roof material, red rust or sea Sufficient outdoor weathering and rust resistance that does not cause corrosion such as pitting due to the adhesion of salt particles is inevitably required. For this reason, Japanese Patent Laid-Open No.
As represented by No. 5-138058, it is possible to improve the corrosion resistance by reducing C and N in a high weatherability / high rust resistance ferritic stainless steel and by increasing the amount of Cr and the amount of Mo added. It has been tried. However, simply increasing the amount of Cr and increasing the amount of Mo results in a high alloy, which makes the price expensive and restricts from the economical aspect, and also reduces the formability due to hardening, and further the manufacturability due to the decrease in toughness. Deterioration becomes a problem. Therefore, Cr, Mo
Improvement of weather resistance and rust resistance can be expected by addition of elements other than the above, and further development of cheaper materials has been strongly desired.

On the other hand, when used in a region where welding is unavoidable in construction, the conventional high Cr steel and Mo-containing steel were not sufficient in toughness and rust resistance of the welded portion.

[0006]

SUMMARY OF THE INVENTION The present invention is aimed at satisfying such requirements, and a Cr content of more than 20% by weight is required.
By adding P in the range of more than 0.06% by weight and 0.2% by weight or less to the added steel, it is possible to provide a ferritic stainless steel that is cheaper than conventional steel and has excellent weather resistance and rust resistance. It is a thing. That is, the present invention provides a ferritic stainless steel capable of improving weather resistance and rust resistance by positively adding P, which has been conventionally regarded as a harmful element for steel, within a manufacturable range. It is intended to be provided.

[0007]

In order to achieve the above object, the present invention, in% by weight, is C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less,
S: 0.03% or less, Cr: over 20% to 40%,
N: 0.015% or less, Al: 0.5% or less,
Provided is a high Cr, P-added ferritic stainless steel containing P: more than 0.06% to 0.20% or less, with the balance being Fe and inevitable impurities, which is excellent in weather resistance and rust resistance.

The stainless steel of the present invention may further contain one or more of the following groups (1) to (3) in addition to the above steel components. (1) Mo: 4.0% or less (2) Ti: 1.0% or less, Nb: 1.0% or less Ta:
1.0% or less, V: 1.0% or less, W: 1.0% or less,
Zr: 1.0% or less and B: 0.01% or less, one or more types (3) Cu: 1.0% or less, Ni: 5.0% or less and Co: 1.0% or less 1 Seed or two or more

[0009]

The present invention will be described in more detail below. In the ferritic stainless steel of the present invention, P has been conventionally made efforts to reduce its content as a harmful element as much as possible, and P is added in an addition amount range that is producible without degrading the formability. It is characterized by doing. Regarding the effect of P on the corrosion resistance of stainless steel, it has almost no effect when the content is 0.1% or less in the complete solid solution state, and as the content of P increases, the pitting corrosion sensitivity increases, and as a result, the corrosion resistance increases. The reduction is described in the "Stainless Steel Handbook". As described above, it is considered that the reason for attempting to reduce the amount of P added to the conventional stainless steel as much as possible is the deterioration of the manufacturability accompanying 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 the welded portion. From this, it is considered that P is extremely harmful for ferritic stainless steel having a body-centered cubic structure and lower toughness than austenitic stainless steel, and studies have been made by a method of reducing the addition amount thereof as much as possible. As a matter of fact, the amount of P added is 0.03 for high Cr ferritic stainless steel such as SUS447J1 as specified in JIS G4304.
Limited to% or less. In addition, regarding other steel types, there is a regulation of 0.04% or less.

Therefore, the present inventors systematically investigated the effect of the positive addition of P to high Cr stainless steel on the weather resistance and rust resistance, and found that it exceeds the range specified in the conventional JIS. The present invention has been achieved by obtaining new knowledge that the weather resistance and the rust resistance can be improved by positively adding P to the steel.

Further, as a result of systematically investigating the influence of P on the toughness and rust resistance of the welded portion, surprisingly, P is T
It has been clarified that addition of appropriate amounts of i, Nb, V, Zr, Ta, W and B improves not only the base metal but also the weld rust resistance without lowering the weld toughness.

On the other hand, from the viewpoint of manufacturability, C, Mn, Mo,
The contents of Ni, Ti, Nb, Cu, N and Al were optimized.

As a merit of adding P, C is known to be extremely effective for improving weather resistance.
Since the added amounts of r and Mo can be replaced by inexpensive P,
It also has the merit that the cost can be low, and that the conventional cost reduction process can be eliminated or the process can be saved, so that the material cost and the cost for removing P can be greatly reduced. From the above, the steel of the present invention is expected to exert a very large industrial effect. In order to exert such effects, P is added in an amount of more than 0.06% by weight and 0.2% by weight or less.

The reasons for limiting the steel composition will be described below. (C, N) C and N are constituent elements that greatly affect the workability, toughness, rust resistance, and weld characteristics of ferritic stainless steel. In the steel of the present invention, the upper limit of C is 0.02% by weight and the upper limit of 0.01 is 0.01% so as not to cause deterioration of manufacturability due to deterioration of toughness and deterioration of weld toughness.
It was set to 5% by weight. Further, the effect of reducing these elements is not saturated, and the lower the content of C and N, the more preferable.

(Si) Si is usually added as a deoxidizing agent and is particularly effective for improving the oxidation resistance. Further, although it is also effective in improving weather resistance and rust resistance, addition of a large amount not only causes deterioration of the toughness and workability of the base metal due to its solid solution strengthening but also the toughness of the welded portion. In particular, when it exceeds 1.0% by weight, Ti, Nb, Zr, V, Ta, W, B described later
Addition, the toughness of the welded part is significantly reduced, so the upper limit was made 1.0% by weight.

(Mn) Mn is usually added as a deoxidizing agent, but it is an austenite stabilizing element and if added in excess,
Austenite is generated at high temperature, and not only a ferrite single-phase structure cannot be obtained, but also corrosion resistance deteriorates. Therefore, the upper limit is set to 1.0% by weight.

(S) S is an element that lowers the corrosion resistance, and its content is preferably small. Especially 0.03
When it exceeds the weight%, it is the main object of the present invention described above. Even if P is added, the corrosion resistance deteriorates significantly, so the upper limit is set to 0.
It was set to 03% by weight.

(Cr) Cr is an important element that determines the basic corrosion resistance of the steel of the present invention. Generally, the corrosion resistance is improved as the content thereof is increased, but if it is added in an amount of more than 40% by weight, the toughness is remarkably lowered and the manufacturability is remarkably deteriorated. On the other hand, if the content is 20% by weight or less, the generation of red rust cannot be sufficiently prevented in a salt-corrosive environment such as the coast, and the steel sheet aimed at by the present invention cannot be obtained. Therefore, the lower limit is more than 20% by weight. And

(Al) Al is added as a deoxidizing agent, but if added in excess of 0.50% by weight, the workability is deteriorated due to scattering of inclusions. Further, since a slag spot is likely to occur during welding, which is not preferable in terms of characteristics, the addition range is set to 0.50% by weight or less.

(P) P is a particularly important element in the present invention, and is an element effective for improving weather resistance, rust resistance, and further rust resistance of the welded portion. The effect cannot be clearly expressed unless it exceeds 0.06%, so the lower limit was made 0.06% or more. On the other hand, if added in excess of 0.2%, weather resistance and rust resistance are rather lowered and toughness is lowered, and manufacturing becomes difficult, so the upper limit was made 0.20%.

(Mo) Mo is an additive element that improves weather resistance and rust resistance, and is effective depending on the amount added, but 4.0
If the content exceeds 10% by weight, not only the effect is saturated and it becomes uneconomical, but also the toughness of the base metal and the toughness of the welded portion are significantly reduced, so the upper limit was made 4.0% by weight. It should be noted that preferably 0.
From the viewpoint of corrosion resistance, addition of 1% by weight or more is preferable.

(Ti, Nb, V, Zr, W, Ta, B)
Ti, Nb, V, Zr, W, and Ta are carbonitride forming elements, and B is a nitride forming element. Therefore, it is effective in suppressing the grain boundary precipitation of Cr carbonitride due to the heat effect during welding, preventing nitriding by nitrogen picked up from the atmospheric gas during welding, and improving the toughness of the welded portion. In addition, due to the synergistic effect with the positive addition of P as in the present invention,
The rust resistance of the weld is significantly improved. The addition amount is Ti,
The upper limit of Nb, V, W, Zr, and Ta is 1.0% by weight, and the upper limit of B is 0.01% by weight.
Further, the preferable addition amount for obtaining the effect is T within the range of satisfying the following formula (I) in which each element is expressed by weight%.
It is preferable to add i, Nb, V, Zr, W, Ta, and B. 8 × (C + N) ≦ Ti + Nb + V + Zr + W + Ta + B ≦ 1 (I)

(Cu, Ni, Co) Cu, Ni, Co
Is an element effective in improving acid resistance, weather resistance and rust resistance. When Cu is contained in an amount of more than 1.0% by weight, hot ductility deteriorates, stress corrosion cracking susceptibility increases, and weld toughness deteriorates, so the upper limit was made 1.0% by weight. When Ni is contained in an amount of more than 5.0% by weight, workability is deteriorated, and a γ phase is generated during welding,
Since the rust resistance deteriorates, the upper limit was made 5.0% by weight. C
Regarding o, if it exceeds 1.0% by weight, the workability deteriorates, so the upper limit was made 1.0% by weight. In order to obtain the effects of these elements, it is preferable that the content thereof be 0.05% by weight or more.

Other unavoidable impurities and O, C
a, Mg, etc. are considered, but for O, 0.01
If the content exceeds 10% by weight, workability and corrosion resistance will decrease, so 0.0
It is preferably 1% by weight or less. Further, Ca and Mg are more likely to be mixed into the refractory material of the furnace in the steelmaking stage, and are used for mold flux during continuous casting and are effective for preventing nozzle clogging, but each exceeds 0.002% by weight. Since it causes deterioration of weather resistance and rust resistance, 0.0
02% by weight or less is preferable.

In the present invention, a high Cr P-added ferritic stainless steel excellent in weather resistance and rust resistance is obtained by containing a predetermined amount of P in the above chemical composition.

INDUSTRIAL APPLICABILITY The steel of the present invention can be widely used as a material for building exteriors, a water heater can body, and the like, in particular, a ferritic stainless steel used under conditions requiring weather resistance and rust resistance.
Further, the steel of the present invention is manufactured by a normal manufacturing process, that is, melting-hot rolling-
It can be manufactured by annealing-pickling-cold rolling-annealing- (pickling)-(temper rolling). Further, the steel of the present invention, whether used in a hot rolled annealed plate or cold rolled annealed plate (2D, 2B, BA, H
L, polishing finish), the effect is sufficiently obtained.

[0027]

EXAMPLES The present invention will be specifically described below based on examples.

(Example 1) Tables 1-1, 1-2 and 1-
A 50 kg small steel ingot having the composition shown in FIG. 3 was melted in a vacuum high-frequency furnace, heated at 1250 ° C. for 1 hour, and then a hot rolled sheet having a thickness of 4 mm was annealed to obtain a hot rolled annealed sheet. This plate is shot blasted and further pickled to remove surface scale and then 0.6
It was cold-rolled to a thickness of mm and reheated in the temperature range of 950 to 1150 ° C. for 30 seconds to obtain a cold rolled annealed plate. Atmospheric exposure test (JIS) at a distance of 5 m from the coastline using a sample obtained by polishing No. 500 with emery paper on this material.
Z2381), CASS test (JIS
-D0201) for 240 hours and change the degree of rusting from A to D
(A: No rusting at all, B: Small rusting degree, C: Medium rusting degree, D: Large rusting degree).

The dissolution behavior in the active state measured by the anodic polarization curve was evaluated by the active state peak current density measured by the anodic polarization curve. Here, the anodic polarization curve was measured by the potentiodynamic method of 20 mV / min.

The details of the test method are in conformity with JIS. However, in the atmospheric exposure test for investigating the weather resistance of the actual environment, a test piece with a size of 15 cm × 10 cm is attached to each steel number on a platform with an angle of 36 ° to the south. Each of them was exposed for 2 years and the amount of rusting was quantitatively evaluated.

FIGS. 1 and 2 show 24 wt% Cr steel (Steel Nos. 1 to 7 in Table 1-1) and 24 wt% Cr-2 wt% Mo steel (Table 1).
-2 shows the effect of P addition on the rusting area ratio of the medium steel Nos. 1 to 8) after the atmospheric exposure test (however, the rusting area ratio described here is not red rust but stain-like rusting Area ratio). When the amount of P exceeds 0.06% by weight, the red rust generation is suppressed in the 24% by weight steel, and 24% by weight Cr-2% by weight M
It can be seen that in the o steel, the area ratio of the spotted rust is reduced and the weather resistance is remarkably improved. However, it can also be seen that if the P content exceeds 0.2% by weight, the weather resistance will start to decrease.

On the other hand, 22% by weight Cr-0.2 is shown in FIGS.
The rusting area ratio and rust resistance evaluation of the wt% Nb steel are shown. It can be seen that the weather resistance is remarkably improved by adding P in an amount of more than 0.06 wt%. However, if it exceeds 0.2% by weight, the weather resistance will start to decrease.

FIG. 5 shows the effect of P on the active state peak current density obtained from the anodic polarization curve investigated for 24 wt% Cr-2 wt% Mo steel. From these results, it is similarly understood that there is a region where the active state peak current density is the lowest when P is added in an amount of more than 0.06% by weight and 0.2% by weight or less. The numbers in the figure show the steel numbers in Table 1-2.

The results of the atmospheric exposure test for 2 years and the 240-hour CASS test are shown in Table 2 (2-1-1,2-1-).
2, 2-2), but for the atmospheric exposure test, the presence or absence of red rust (○: No, X: Red rust)
The CASS test has the same evaluation as that in FIG. Further, the corrosion resistance of the welded portion was evaluated by the presence or absence of through holes after immersion for 1000 hours, and the toughness of the welded portion was evaluated by a Charpy impact test (-20 ° C, 0 ° C). The results are also shown in Table 2. Note) Atmospheric exposure test ○: No red rust is generated ×: Red rust is generated Corrosion resistance of weld zone Presence or absence of through-hole after 1000 hours immersion ○: None ×: Yes Toughness of weld zone ○: Ductile fracture surface △: Ductility and brittleness mixed
×: Brittle fracture surface

FIG. 6 similarly shows the effect of Mo on the rust area ratio of 24 wt% Cr-0.1 wt% P steel after the atmospheric exposure test. Although the rust area ratio decreases due to the increase of Mo,
It can be seen that the addition of 0.5% by weight results in about 1/2 the amount before addition.

Further, a 26 wt% Cr steel base (C /
0.003 to 0.008% by weight, N / 0.006 to 0.
007% by weight, the other component S is P, Nb,
Hot rolling, annealing, shot, pickling, cold rolling of a 50 kg small steel ingot (high-frequency vacuum melting) in which Ti, V, Zr, and B are changed,
It was annealed to obtain a plate having a plate thickness of 1.0 mm. Further, the surface of the steel sheet was polished with Emery No. 500, and butt welding was performed by TIG welding. This weld is 80 ℃, (1160pp
mCl ⁻ + 800ppmCu ⁺⁺ ) in an aqueous solution (p
The immersion test was conducted for 10 days under H3.5). The maximum pitting depth generated in the test piece at that time is shown in FIG. 7, and Nb, Ti,
It can be seen that the addition of appropriate amounts of V, Zr, and B and the addition of P in excess of 0.06% by weight and 0.2% by weight or less significantly improve the rust resistance of the welded portion. That is, P and (N
It can be seen that the combined addition of b, Ti, V, Zr, W, Ta and B) significantly improves the rust resistance of the welded part.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

[0041]

[Table 5]

[0042]

[Table 6]

[0043]

[Table 7]

[0044]

[Table 8]

[0045]

[Table 9]

[0046]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, the ferritic stainless steel of the component type melted in accordance with the present invention is excellent in weather resistance, rust resistance, and weld property. Moreover, there is a merit that it can be manufactured at low cost,
Industrially useful effects are provided.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of the addition of P on the rust area ratio in 24% Cr steel.

FIG. 2 is a diagram showing the effect of the addition of P on the rust area ratio in a 24% Cr-2% Mo steel.

FIG. 3 is a diagram showing the influence of the amount of P on weather resistance.

FIG. 4 is a diagram showing the effect of P content on rust resistance.

FIG. 5 is a diagram showing the effect of addition of P on the active state peak current density in a 24% Cr-2% Mo steel.

FIG. 6 is a diagram showing the effect of addition of Mo on the rust area ratio in 24% Cr-0.1% P steel.

FIG. 7 is a diagram showing the effect of the amount of P on the rust resistance of welded portions.

Front page continuation (72) Inventor Fusao Togashi, 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Division, Kawasaki Steel Corporation (72) Inventor Keiichi Yoshioka, 1 Kawasaki-cho, Chuo-ku, Chiba Address Kawasaki Iron & Steel Co., Ltd. Technical Research Division (72) Inventor Sadao Hasuno 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Iron & Steel Co., Ltd. Chiba Works

Claims (8)

[Claims] 1. In% by weight, C: 0.02% or less, Si: 1.0% or less, M
n: 1.0% or less, S: 0.03% or less, C
r: more than 20% to 40%, N: 0.015% or less, A
1: 0.5% or less, P: more than 0.06% to 0.2
A high Cr, P-added ferritic stainless steel containing 0% or less and the balance being Fe and inevitable impurities, which is excellent in weather resistance and rust resistance. 2. A high-Cr, P-added ferritic stainless steel excellent in weather resistance and rust resistance according to claim 1, which further contains Mo: 4.0% or less. 3. Ti: 1.0% or less, Nb: 1.0
% Or less, Ta: 1.0% or less, V: 1.0% or less, W:
1.0% or less, Zr: 1.0% or less and B: 0.01
% Or less, and the high Cr, P-added ferritic stainless steel according to claim 1, which is excellent in weather resistance and rust resistance. 4. Further, Cu: 1.0% or less, Ni: 5.0
% Or less and Co: 1.0% or less, and a high Cr, P-added ferritic stainless steel excellent in weather resistance and rust resistance according to claim 1. 5. Mo: 4.0% or less is contained, and further T
i: 1.0% or less, Nb: 1.0% or less, Ta: 1.0
% Or less, V: 1.0% or less, W: 1.0% or less, Zr:
1% or less of 1.0% or less and B: 0.01% or less
The high-Cr, P-added ferritic stainless steel having excellent weather resistance and rust resistance according to claim 1, containing at least one species. 6. Mo: 4.0% or less is contained, and further C
u: 1.0% or less, Ni: 5.0% or less, and Co:
1. 1% or more of 1.0% or less is contained.
High-Cr, P-added ferritic stainless steel excellent in weather resistance and rust resistance described in. 7. Ti: 1.0% or less, Nb: 1.
0% or less, Ta: 1.0% or less, V: 1.0% or less,
W: 1.0% or less, Zr: 1.0% or less and B: 0.
The composition according to claim 1, which contains one or two or more of 01% or less and one or more of Cu: 1.0% or less, Ni: 5.0% or less, and Co: 1.0% or less. High-Cr, P-doped ferritic stainless steel with excellent weather resistance and rust resistance. 8. Mo: 4.0% or less is contained, and further T
i: 1.0% or less, Nb: 1.0% or less, Ta: 1.0
% Or less, V: 1.0% or less, W: 1.0% or less, Zr:
1% or less of 1.0% or less and B: 0.01% or less
The weather resistance and the rust resistance according to claim 1, which contain one or more kinds of Cu: 1.0% or less, Cu: 1.0% or less, Ni: 5.0% or less, and Co: 1.0% or less. Excellent high Cr, P
Added ferritic stainless steel.