JP3411084B2 - Ferritic stainless steel for building materials - Google Patents

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 for building materials which is soft (hardness characteristic), excellent in rust resistance and excellent in ductility (elongation characteristic).

[0002]

2. Description of the Related Art Ni-free ferritic stainless steel is inexpensive and is immune to stress corrosion cracking in a chloride environment. Therefore, it is widely used in chemical plants and interior / exterior products. Further, as a feature of ferritic stainless steel, there is an advantage that the coefficient of thermal expansion is smaller than that of austenitic stainless steel.

Conventionally, austenitic stainless steels such as SUS304 and SUS316 have been used as roofing stainless steels by the welding method and the folding method, but the rust resistance is not always sufficient with the active development of the waterfront. On the other hand, for long roofs, ferritic stainless steel having excellent rust resistance is desired because of the problem of fatigue due to thermal expansion and contraction. Further, it is demanded that it be easy to cut with scissors, that is, soft, and that it has good cold workability in terms of bending and the like in terms of workability.

[0004] Increasing the amounts of Cr and Mo is effective for improving the corrosion resistance of ferritic stainless steels, but it is known that increasing the concentrations of these components makes them harder and reduces the elongation. . Further, in order to improve the corrosion resistance of the welded portion, reduction of C and N and C and N such as Ti and Nb
For the ferritic stainless steel containing Ti and Nb, which have excellent corrosion resistance at the welded part together with the base metal part, which is known to be effective to add the stabilizing element of
-21102, Japanese Patent Publication No. 62-19972,
JP-B-62-22210, JP-B-59-5222
No. 6 and Japanese Patent Publication No. 1-159319. The stainless steel was developed for applications such as chemical industrial equipment and heat exchangers using seawater, and in the methods disclosed in the above-mentioned publications except Japanese Patent Publication No. 55-21102, ferritic stainless steels are used. Cu or Ni that inhibits the stress corrosion cracking resistance, which is a characteristic of the above, is added, and according to the research conducted by the present inventors, these elements show little effect of improving rust resistance, and JP-B-55-55.
The method disclosed in the publication No. 21102 is insufficient in rust resistance and T
A large amount of i and Nb are added, which is expensive and does not take into consideration the softness and cold workability, which are required properties for building materials, especially as roofing materials, and satisfies cold workability and rust resistance. There was a problem that was insufficient.

[0005]

SUMMARY OF THE INVENTION The object of the present invention is to provide a cooling material having the properties required for building materials without impairing the low thermal expansion and stress corrosion cracking resistance, which are the characteristics of ferritic stainless steel as a building material stainless steel. It ensures inter-workability and improves the rust resistance of the base metal and welded parts in highly corrosive environments in coastal areas, especially the roof eaves.

[0006]

Means for Solving the Problems The present invention has been made possible as a result of earnest research on the components of ferritic stainless steels for building materials.
At C: 0.005 to 0.020%, Si: 0.
1 to 1.0%, Mn: 0.1 to 1.0%, S
: 0.001 to 0.010%, Cr: 21.5 to 3
1.0%, Mo: 0.3 to 4.0%, N: 0.0.
005 to 0.020%, Ti: 0.10 to 0.20
%, Nb: 0.15 to 0.25%, the balance being Fe and inevitable impurities, and the pitting corrosion resistance index P of the formula (1).
I value is 28 or more and the softness index HI value of the formula (2) is 3
When it is 1 or less, it is a ferritic stainless steel for building materials which is soft and has excellent rust resistance of the base material portion and the welded portion. The present invention further comprises Ce in an amount of 0.005 to 0.050%.
By containing it, the ductility can be improved. PI = Cr + 1.7Mo ……………………………… (1) HI = Cr + 2.4Mo ………………………… (2)

[0007]

The present inventors have earnestly studied the ferritic stainless steel for building materials which is soft and has excellent rust resistance of the base material portion and the welded portion and also has ductility, and as a result, obtained the following findings. As a result of research on rust resistance, it was quantitatively understood that chlorine ions are more likely to be concentrated in the eaves backside in the critical area due to the arrival of sea salt particles compared to the rainwater hitting area such as the roof top surface. Figure 1 is 1 at a distance of 50m from the coast of Okinawa.
It shows the rusting condition of the material which was subjected to the annual outdoor exposure test. In the exposure test, # 500 emery-polished material was placed on an exposure test stand simulating a roof structure. The rusting condition can be arranged by the above-mentioned PI value (Cr + 1.7Mo), and the rust resistance was improved as the PI value increased, and the PI value was 28 or more to obtain a score of 1 without rusting.

FIG. 2 is a diagram showing the relationship between this PI value and the pitting potential in the pitting potential test according to JIS G0577. In order to secure a PI value of 28 or more that does not cause rusting in the above exposure test, the pitting electron Vc ' ₁₀₀ is 1000 mV (vs
It is necessary that the potential exceeds SCE) and pitting corrosion does not occur.

The hardness was evaluated by the difficulty of cutting when a material with a thickness of 1 mm with different alloy components was cut with a gold scissors. As shown in FIG. 3, which shows the relationship between Vickers hardness and cutting property, the Vickers hardness evaluated to be relatively easy to cut was 200 or less. FIG. 4 shows the relationship between the Vickers hardness and the softness index HI value (Cr + 2.4Mo). As shown in the figure, there is a good correlation between the HI value and the Vickers hardness, and the Vickers hardness of 200 or less can be obtained when the HI value is 31 or less.

Next, the reason for defining the component range of each alloy element will be described. C and N are elements that deteriorate the intergranular corrosion resistance of the welded portion and are preferably low, but Ti
It can be rendered harmless by fixing C and N by addition of Nb and Nb. However, it is necessary to increase the amounts of Ti and Nb in accordance with the increase of the amounts of C and N, and since C and N deteriorate the toughness, the C amount of 0.005 to 0, which can be easily reached by the ordinary steelmaking method, is used. 0.02%, N content 0.005
It was set to 0.02%. Preferably both elements are 0.015%
The following is desirable.

Si is necessary for refining as a deoxidizing agent,
Since it increases the Vickers hardness and becomes harder,
It was 1.0%. A desirable range is 0.2 to 0.5%. Mn acts as a deoxidizer similarly to Si, but addition of more than 1% deteriorates rust resistance, so the content was made 0.1 to 1.0%. It is preferably 0.1 to 0.6%.

[0012] Cr and Mo are effective elements for enhancing pitting corrosion resistance to chlorine ions contained in sea salt particles and improving rust resistance, and the rust resistance improves as the amount of Cr and Mo increases. The pitting corrosion resistance index PI of the formula (1) needs to be 28 or more to suppress the rusting of the eaves back part, but these elements become hard as shown by the softness index HI of the formula (2), and Hv200 To obtain the following, the HI value must be 31 or less. From these things, Cr,
Mo amount is Cr: 21.5 to 31.0%, Mo: 0.3 to
The range was set to satisfy the expressions (1) and (2) at 4.0%.

Ti and Nb are added in order to prevent intergranular corrosion due to the formation of chromium carbide in the heat-affected zone of welding, but Ti enhances rust resistance due to the concentration effect in the passive film, and When the metal inclusions are only Nb additions, they tend to be the starting points of MnS corrosion, but the addition of 0.10% or more of Ti results in a chemically stable T
Although it has the effect of forming i-based inclusions, excessive addition tends to cause flaws on the product surface.
The amount was 0.10 to 0.20%. Nb is added in an amount of 0.15% or more in order to improve intergranular corrosion resistance and to improve toughness by adding an appropriate amount, but even if the Nb content exceeds 0.25%, the effect is saturated. And since it becomes expensive, it was set to 0.15 to 0.25%.

S is preferably as low as possible because it forms non-metallic inclusions and deteriorates the cleanliness. It is not necessary to set the lower limit, but 0.001 to 0.010% is the range that can be achieved at the industrial level. And Preferably 0.00
It is 6% or less.

Ce is added especially when bendability is required. FIG. 5 shows the effect of the amount of Ce addition on the ductility of 23.9Cr-3Mo steel, but the addition of 0.005% or more of Ce improves the ductility, and the effect saturates even if added over 0.05%. Therefore, 0.005-0.05
It was set to 0%. Although Ce is an element that forms S and non-metallic inclusions, it does not deteriorate the rust resistance because it does not become the starting point of rusting. Due to the combination and limitation of the above steel components, it is possible to obtain a ferritic stainless steel which is required for a building material and has excellent rust resistance and ductility.

[0016]

EXAMPLES The present invention will be described based on examples. Table 1 shows the components of the present invention steels (No. 1 to 7) and comparative steels (No. 8 to 17), calculated values of HI and PI values, measured values of Hv hardness, pitting corrosion potential and rust resistance by exposure test. The results of evaluation of the properties and the measured values of elongation by a tensile test are shown. After melting in a vacuum melting furnace, a 45 kg ingot was cast and hot-rolled into a hot-rolled plate having a thickness of 5 mm, which was held at 1050 ° C. for 3 minutes and then air-cooled. Subsequently, after pickling, a cold-rolled sheet having a thickness of 1 mm was heat-treated by heating for 1 minute at 1050 ° C. and then air-cooling. After pickling, the base material was polished with # 500 emery polishing paper, and the welding material was # 5 after TIG welding
It was subjected to 00 emery polishing and subjected to a pitting potential test.
In the exposure test, a # 500 emery-polished base material test piece was attached to an exposure table simulating a roof structure, and 50 from the coast of Okinawa.
The rust resistance was evaluated by conducting an exposure test for 1 year at a place distant from m. In addition, Hv hardness is J
It was measured by IS13B test piece.

Steel types Nos. 1 to 7 of the steels of the present invention have a pitting potential of 1000 mV or more in both the base material and the welded material, have no rust in the exposure test in Okinawa, and have hardness of Hv 200 or less, which satisfies both characteristics. In addition, steel type number 5 with Ce added
Nos. 6 and 7 have improved ductility.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

INDUSTRIAL APPLICABILITY As described above, the steel of the present invention is soft and excellent in rust resistance of the base material portion and welded portion and excellent in ductility without impairing the characteristics of ferritic stainless steel, and has the characteristics required for building materials. And, the industrial profit is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a rusting degree and a pitting corrosion resistance index PI value obtained from a one-year exposure test result with a test piece simulating a roof structure in Okinawa facing the sea.

FIG. 2 is a diagram showing a relationship between a pitting potential and a rust resistance index PI value by a JIS pitting potential test (JIS G0577).

FIG. 3 is a diagram showing the relationship between the cutting property by scissors and Vickers hardness.

FIG. 4 is a diagram showing a relationship between Vickers hardness and a softness index HI value.

FIG. 5 is a diagram showing the relationship between the amount of Ce added and elongation.

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-5-320764 (JP, A) JP-A-5-70899 (JP, A) JP-A-6-49539 (JP, A) JP-A-57- 98656 (JP, A) Special Table 4-504140 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00

Claims (2)

(57) [Claims] 1. By weight%, C: 0.005-0.02%, Si: 0.1-1.0%, Mn: 0.1-1.0%, S: 0.001-0. 010%, Cr: 21.5 to 31.0%, Mo: 0.3 to 4.0%, N: 0.005 to 0.020%, Ti: 0.10 to 0.20%, Nb: 0. 0.1 to 0.25%, the balance consisting of Fe and inevitable impurities,
The pitting corrosion resistance index PI of the formula (1) is 28 or more, and the softness index HI value of the formula (2) is 31 or less, which is soft and excellent in rust resistance of the base metal portion and the welded portion. Ferritic stainless steel for building materials. PI = Cr + 1.7Mo ……………………………… (1) HI = Cr + 2.4Mo ………………………… (2) 2. The steel of the preceding paragraph further contains Ce in an amount of 0.00% by weight.
5 to 0.050% is contained, The ferritic stainless steel for building materials of Claim 1 characterized by the above-mentioned.