Application of Stainless Steel in Architectural Decoration
With the economic growth and the improvement of people’s living standards, the good comprehensive performance and the advantages of durability, beauty, long service life, and high cost-effectiveness of stainless steel have been recognized by people. , Municipal, bridges, tunnels, and other public facilities to residential houses; from decoration, roofs, and urban water supply projects to sinks and baths in residential houses, the application range of stainless steel is becoming more and more extensive.
(1) Application in decoration engineering. In addition to walls, cylinders, automatic retractable doors, roller shutter doors, step fence handrails, balcony handrails, rain downspouts, flag poles, street lamp poles, arcade frames, kitchen and bathroom countertops, sinks, brackets, etc., in residential houses, The application is increasing day by day.
(2) Application of roof. Early buildings that used stainless steel as the roof include the Savoy Hotel in London, the Eurostar Railway Station, the Chrysler Building in New York, the Empire State Building, etc., the Kuala Lumpur Twin Towers Pechners Tower, Shanghai Jin Mao Tower, and Shanghai Jin Mao Tower, built in Asia in the 1990s.
The Hong Kong Convention and Exhibition Center, the Kansai Airport Terminal in Japan, and the roof of the Sapporo Dome Stadium, newly built for the World Football Game in 2002, have successively adopted stainless steel as decorative materials.
The roof of a building not only withstands wind and rain, but it is also an important factor in determining the appearance of the building. In addition, with the development of coastal areas in recent years, exhibition halls and dome courts have begun to be used in large buildings under sea breeze erosion.
The roof not only requires decoration but also requires long-term maintenance in harsh environments. There are many examples of such roofs using stainless steel products. Due to the close integration of architectural styles and technological development, various products have been developed.
The development history and trend of stainless steel for roofs. Take Japan as an example. The use of stainless steel in exterior decorative building materials in Japan began in 1960. In the beginning, there were decorative items such as the building’s curtain walls, column covers, and shutters. Stainless steel was used as a roofing material after coated stainless steel went on the market in 1975. At that time, coated steel sheets were in the position of high-end products, generally for residential use.
The Development Trend of Stainless Steel for Architectural Decoration
1. From Austenitic Stainless Steel to Ferritic Stainless Steel
Almost all steel grades used as roofing materials were austenitic SUS304 or SUS316 10 years ago. However, when steel plates of this steel grade are used on buildings near the sea, they will rust in a relatively short period. , So began to develop stainless steel for roofs with good atmospheric corrosion resistance (corrosion resistance). Chromium and molybdenum among the elements added to stainless steel can greatly improve the atmospheric corrosion resistance.
Generally, Cr+3Mo is used as the corrosion resistance index of stainless steel and is called the pitting index. When compared under the same pitting index, the atmospheric corrosion resistance of the ferritic system is better than that of the austenitic system. Therefore, when the atmospheric corrosion resistance is improved by increasing the amount of Cr and Mo, the ferritic system has better cost characteristics than the austenitic system.
In recent years, there have been more and more large-sized roofs in terms of shape, rainproofing, and workability. Therefore, it is desirable to be able to suppress the expansion and contraction of the roof due to the temperature difference and to use a roofing material with a small thermal expansion. Compared with austenitic stainless steel, ferritic stainless steel has lower thermal expansion and is suitable for use as a large-size roofing material.
In this context, 22Cr ferritic stainless steel was developed. This steel grade was first adopted for large objects in the “Exposition Hall” in 1988, using 22Cr-0.8Mo steel. Later, in 1992, 22Cr-1.2Mo steel with higher corrosion resistance was developed, and its excellent cost characteristics were well received, and its usage increased sharply. The product is not only lower in cost than SUS316, but also better in corrosion resistance than SUS316, so now it is not only limited to the roof, as a representative steel grade for exterior decoration building materials, it is used by more objects.
2. Higher Corrosion Resistance is Required
The roof of a building is not only the roof surface but also generally includes curtain panels and eaves. In a seaside environment, the eaves and the parts of the walls under the eaves that are not exposed to rain are more likely to be corroded than the roof surface.
As we all know, this is due to the influence of the adhered sea salt particles, but these parts that are not exposed to rain, Even the 22Cr series steel cannot avoid rust. The product that is required to be used in such a location without rusting is 30Cr ferritic stainless steel. This type of steel hardly rusts even in the eaves horizontal plate and the wall part under the eaves.
Therefore, this grade of stainless steel is called “super stainless steel”. Super stainless steels include austenitic 20-25Cr-18-25Ni-5-6Mo steels, etc., but as mentioned above, they are not as good as ferritic steels in terms of thermal expansion performance. In addition, due to the significant increase in Ni resource consumption, even for workability A good Austenitic system cannot avoid violent price fluctuations.
In 1994, SUS447J (30Cr-2Mo) was used in the “Roof of Kansai Airport”. This is the first case of this kind of stainless steel being used in a large building. As mentioned above, this kind of stainless steel can be effective in parts that cannot be exposed to rain. Therefore, in recent years, the use of frontal walls and shutters in Okinawa and areas with severe salt damage has increased.
3. Higher Decorative Requirements
The processing of stainless steel used as building materials generally includes hairline processing and mirror processing, but these processings have a too high gloss and are not suitable for processing roof materials. Initially, when SUS304 and SUS316 were used on the roof, they were generally processed with 2D and 2B surface anti-glare processing.
Ferritic stainless steel, even with 2D surface processing, does not form a white anti-glare surface like SUS304 but has a rather shiny processed surface. Therefore, a special passivation surface treatment has been developed to achieve 2DR processing with anti-glare properties.
Through this processing, an anti-glare product equivalent to the original 2D processing of SUS304 can be obtained. However, it is difficult for stainless steel to eliminate the metallic feeling. For buildings such as exhibition halls and gymnasiums, the temperament of stainless steel gives people a modern image and is very coordinated, but it is difficult to say that it is suitable for places with solemn requirements such as cultural halls and art galleries.
In the past, coated stainless steel was used to solve this requirement, but the difference between coated products and generally coated steel plates is difficult to distinguish, so the demand has not made much progress.
Galvanized stainless steel is a product developed to meet the needs of such a solemn image of the building. The ferritic stainless steel is plated with a thick layer of zinc, and after phosphate treatment, it has a black hue that is smoked with pine leaves, forming an appearance that cannot be achieved with a coating.
Since it is coated with clear varnish to eliminate gloss, no protective film is required during molding. This product has been adopted by many buildings. The Sagawa Art Museum, which was built in 1998, is a representative “roof-view” building. The color of galvanized stainless steel expresses its solemn appearance.
Material Selection Requirements
Stainless steel for architectural exterior decoration is selected from the following aspects: the price of stainless steel, the corrosion resistance of stainless steel, the light reflectivity of stainless steel, the expansion coefficient of stainless steel, and the processing performance of stainless steel, and the surface quality of stainless steel.
1. Chemical Composition
Compared with austenitic stainless steel 304 and 316L, 445R does not contain Ni itself, and the cost of its alloy composition is much lower. In order to improve the corrosion resistance of the material, the Cr content is above 22%, and 1.2-2.0% Mo is added at the same time. To improve the intergranular corrosion of the material, Nb and Ti bistable elements are added
2. Physical Properties
As ferritic stainless steel, 445R has a smaller linear expansion coefficient than austenitic stainless steel 304 and 316L. The use of exterior decoration in buildings reduces the deformation of materials caused by temperature differences. And the density is smaller than that of austenitic stainless steel 304 and 316L. Under the same weight and thickness, the actual area used is larger than 304 and 316L, which can save materials.
3. Mechanical Properties
As ferritic stainless steel, the actual yield strength of 445R is significantly greater than that of austenitic material, which is equivalent to stainless steel 304 and 316L. Compared with other similar alloy materials, it has high strength and good wind resistance;
4. Corrosion Resistance
Figure 1 Corrosion resistance comparison
5. Surface Selection
| Surface | Surface Definition | Use |
|---|---|---|
| 2B | Cold rolled surface, after continuous annealing and pickling, smooth | Structure |
| BA | Bright cold-rolled surface, after bright annealing, bright and smooth | Curtain wall |
| HL | Grinding and polishing the surface, annealing, and pickling, abrasive belt grinding and polishing | Curtain wall |
| NO.4 | Grinding and polishing the surface, annealing, and pickling, abrasive belt grinding | Curtain wall |
| Matte | Rolled surface with matt roller, pickled or bright annealed | Roof, curtain wall |
| Embossing | Cold rolled surface, annealed and pickled, cold rolled and embossed | Curtain wall |
Table 4 Commonly used surfaces for surface decoration
The above is the surface selection for mass production in steel mills. In practical applications, the variety of stainless steel surfaces is the mainstream choice of designers due to the requirements of building style design. The surface treatment plant also has many surfaces, 8K, sandblasting, etching, titanium plating, embossing, and another follow-up deep processing of stainless steel.