Steel is the most versatile and most important of all metal alloys. The world's steel output in 2003 exceeded 945 million tones. Around 100 countries produce steel, Brazil ranking 9th in the world.
Steel is produced in a great variety of types and forms, each one efficiently suiting one or more applications. Such variety results from the continuous necessity of adjusting the product to the specific demands of newcoming applications, whether in the control of the chemical composition, or in granting specific features, or its final shape (plates, shapes, pipes, bars, etc.).
There are over 3,500 different types of steel and around 75% of those were developed over the past 20 years, showing the great evolution the sector has been undergoing.
Carbon steels have limited amounts of carbon, silicon, manganese, sulphur and phosphorus. Other chemical elements are present only in residual amounts.
The amount of carbon in the steel defines its classification. Low-carbon steels have a maximum 0.3% of this element and present high ductility. They are good for mechanical works and welding, non-hardenable, used in the construction of buildings, bridges, ships, cars, among others. Medium-carbon steels have between 0.3% - 0.6% of carbon and are used for gears and other mechanical components. These steels, hardened and drawtempered, reach good toughness and resistance. High-carbon steels have over 0.6% carbon and present high levels of hardness and resistance post-hardening. They are normally used in rails, coils, gears, agricultural components subject to weariness, small tools, etc.
In civil construction, the major interest is in the so-called medium- and high-mechanical resistance structural steels, a term adopted for all steels that, due to their hardness, ductility and other properties, are suitable to be used for construction elements subject to loads. The main requirements for structural steels are: high flow stress, high toughness, good welding properties, microstructural homogeneity, prone to cutting by flame without hardening, good workability in operations such as cutting, drilling and folding, without generating fractures or other flaws.
Strucutural steels may be classified in three main groups, depending on their minimum specified flow stress:
|Type||Minimum Flow Stress, MPa|
|Medium-resistance carbon steel||195 to 259|
|High-resistance low alloy steel||290 to 345|
|Steel alloys with thermal treatment||630 to 700|
The most used structural steel among those currently existing is ASTM A36, which is classified as medium mechanic resistance carbon steel. However, the current trend towards using larger structures has been leading engineers, designers and builders into using higher resistance steels, the so-called high-resistance low alloy steels, so as to avoid increasingly heavier structures.
High-resistance low alloy steels are used to:
Among the steels belonging to this category, high-resistance low alloy steels resistant to atmospheric corrosion must be mentioned. These steels were introduced in the US market in 1932, specifically for the construction of cargo cars. Other steels with similar features have been developed ever since, constituting the family of the so-called patinable steels.
Encompassed by several norms, such as Brazilian norms NBR 5008, 5920, 5921 and 7007 and US norms ASTM A242, A588 and A709, which specify limits for chemical compositions and mechanical properties, these steels have been used worldwide in the construction of bridges, viaducts, silos, energy transmission towers, etc. Their key advantage, besides not demanding painting in certain environments, is that they have higher mechanical resistance than carbon steels. In extremely hazardous environments, such as regions with high level of pollution by sulphur dioxide or those near the sea, painting grants them superior performance to carbon steels.
The differential of the new product from carbon steels, regarding resistance to corrosion, was the fact that, under certain environmental conditions, it could develop a protecting layer of adhering oxides, called patina, which reduced the pace of the attack of corrosive agents present in the environment. Figure 1 shows the typical corrosion resistance assessment curves of a patinable steel and of a common carbon steel exposed to industrial, urban, rural and marine environments.
Figure 1. Resistance to corrosion of a patinable steel (ASTM A242) and of a common carbon steel (ASTM A36) exposed to industrial (Cubatão, S.P.), marine (Bertioga, S.P.), urban (Santo André, S.P.) and rural (Itararé, S.P.) environments. Measurement is made in terms of loss of metallic mass per exposed time (in months). Source: Fabio Domingos Pannoni, M.Sc., Ph.D.
The formation of the patina is due to three types of factors. The first are related to the steel's own chemical composition. The main alloy elements contributing to the increase in resistance to atmospheric corrosion, favoring the formation of patina are copper and phosphorus. Chromium, nickel and silicon also provide secondary effects. It must be observed though that phosphorus must be kept at low grades (< 0.1%), due to the risk of impairing certain mechanical properties of the steel as well as its welding properties.
Secondly, there are environmental factors, among which the presence of sulphur dioxide and sodium chloride in the atmosphere, the temperature, force (direction, speed, frequency) of the winds, wet and dry cycles, etc. Thus, while the presence of sulphur dioxide to some extent favors the development of the patina, sodium chloride in suspension in marine atmospheres impairs its protecting properties. The use of unprotected patinable steels in areas with sulphur dioxide higher than 168mgSO2/m2 day (US and UK) and in marine atmospheres with chloride disposal rate higher than 50mg/m2.day (US) or 10 mg/m2.day (UK) is not recommended.
There are finally factors related to the geometry of the part, explaining why different structures made of the same steel placed side by side may be attacked differently. This phenomenon occurs due to the influence of open/closed sections, correct rainwater drainage and other factor acting directly on the wet/dry cycles. Thus, for example, under continuous wetting conditions, determined by unsatisfactory drying, the formation of patina becomes severely damaged. In many of these situations, the pace of corrosion of patinable steel is similar to that found in carbon steels. Examples include water-immersed patinable steels, or those buried underground or covered by vegetation.
Table 1 lists the chemical compositions and mechanical properties of a medium mechanical resistance carbon steel (ASTM A36), a high-resistance low alloy steel (ASTM A572 Grade 50) and two high-resistance low alloy steels resistant to atmospheric corrosion (ASTM A588 Grade B and ASTM A242).
High-resistance low alloy steels resistant to corrosion are produced in Brazil by several steel companies. Table 2 lists the producers and their patinable steels. For further information, please visit the company's website.
|% C max.||0.26||0.23||0.20||0.15|
|% Mn||... (1)||1.35 max.||0.75-1.35||1.00 max.|
|% P max.||0.04||0.04||0.04||0.15|
|% S max.||0.05||0.05||0.05||0.05|
|% Si||0.40||0.40 max.3||0.15-0.50||...|
|% Ni||...||...||0.50 max.||...|
|% Cu||0.202||...||0.20-0.40||0.20 min.|
|(% Nb + %V)||...||0.02-0,15||...||...|
|Flow Limit (MPa)||250 min.||345 min.||345 min.||345 min.|
|Resistance limit (MPa)||400-550||450 min.||485 min.||480 min.|
|Elongation after rupture, % (lo = 200mm)||20 mín.||18 mín.||18 min.||18 min.|
(1): For shapes heavier than 634 kg/m, the manganese grade must be between 0.85% and 1.35% and the silicon grade between 0.15% and 0.40%
(2): Minimum when copper is specified.
(3): For shapes no heavier than 634 kg/m.
(4): Thicknesses of 20 mm and less.
|Grupo Usiminas||COS AR COR 300, COS AR COR 350, ASTM A242, ASTM A588||www.usiminas.com.br|
|CSN||CSN CSN-COR 420, CSN-COR 500||www.csn.com.br|
|Gerdau Açominas||ASTM A588||www.gerdau.com.br|
|USIMINAS||USI-SAC-300, USI-SAC-350, USI-FIRE-350, ASTM A242, ASTM A588||www.usiminas.com.br|
|V&M do Brasil SA||VMB 250 COR, VMB 300 COR, VMB 350 COR||www.vmtubes.com.br|
Read the article "Fire protection for metallic structures".
You will find below a list of books published by Zigurate Editora (www.zigurate.com.br) in architecture, engineering and construction. These are publications with differentiated technological contents, approaching specific subjects, such as steel construction technology, as well as didactic subjects related to the study of architecture and engineering:
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