Steel is one of the important materials in the world. From buses to building, from canned food to computers, almost everything we see around us is made of steel. The physical properties of steel make it a versatile structure for use. Steel is essential for development of modern society and it is almost the backbone of everything we use in our life. Steel can be truly recycled through efficient methods. Steel which we make today can be reused as steels by future generation by recycling. This is one of the sustainable materials in the world.
This paper addresses about the steels in structural applications. In this paper, we will discuss about the properties of steels used in structural applications and also about types of steels being used in different structural applications. In this paper, we will also discuss about optimization of base structural steels for improving strength and toughness.
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Steel is an alloy which is made from carbon and other elements such as manganese, chromium, vanadium and tungsten. Carbon content in steel ranges from .2 %to 2.1% by weight depending upon the grade of steels. Varying the amount of alloying elements will lead to different properties in the steel. There are many researches going on steels to make steels with high strength and low cost. Steels are being available for different applications such as marine, aerospace, construction, automotive etc.
Technique of Steel Making:
In the 15th century, steel was being made through cementation process. In this process, bars of wrought iron would be packed with powdered of charcoal and heated for several days. The wrought iron absorbs carbon and distribute carbon evenly within itself to form blister steel. This will be heated and forged to make required texture.
In modern era, the steelmaking is made by Bessemer process. The raw material used for making steel was pig iron. This method enabled a cheaper and larger production. This method was later improved and called as Gilchrist-Thomas process in which the converter was lined to remove the phosphorous. The method was again improved by the Linz-Donawitz process of basic oxygen steelmaking. In this process, the oxygen was pumped in to the furnace which helps in limiting the impurities which was entering through air. Today Electric arc furnace is being used for production of steel. In this process, electricity is being used to convert the pig iron into steel. It is cheaper process since lot of electricity is available at cheaper price.
Types of Steels:
Steels which are being used in structural applications can be classified as
High strength low alloy steels
Heat treated carbon steels
Heat treated constructional alloy steels
FIGURE 1: Typical stress- strain curve for structural steels
From the above figure of stress-strain curve, it is clear that carbon steels will have more strain for less stress, whereas heat-treated constructional alloy steels will have less strain for more stress.
Steel can be considered as carbon steel only if following condition are satisfied
The alloying element should not exceed the following maximum content: manganese-1.65%,silicon-0.60%,copper-0.60%
Copper content should not be less than 0.40%
According American society of technical and materials (ASTM), there are 4 different designations of carbon steels. They are
A36 is also called as principal carbon steel for bridges, buildings and many other constructional purposes. This steel has minimum yield point of 36 ksi in all structural shapes.
A529 steel has composition of carbon and manganese. It is for general structural purposes with limited size range available in shapes and plates. It has a minimum yield point of either 50-55ksi.
A573 is another type of carbon steel which has improved notch toughness and it is of three different grades for plate application.
A283 is another type of carbon steel which is available in large thickness and is mainly used for base plates. It has a minimum yield stresses from 24 to 33 ksi.
High Strength Low-Alloy Steel:
The steels which have minimum yield points more than 40ksi and which can achieve high strength by hot rolled conditions instead of heat treatment are called as high strength low alloy steels. Compared carbon steels, these steels have more strength. The HSLA steels can be splitted into 5 different types based on the applications.
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A242 steel is used for weathering purpose in structural application. It has minimum yield point of 50ksi, tensile strength of 70ksi for a plate thickness ¾ in shape. It has good resistance against corrosion.
A588 steel is also used for weathering steel in structural application. It has minimum yield stress of 50ksi, tensile elongation of 21ksi. A242 and A588 is called as weathering steels because when they are exposed to alternative wetting and drying condition, a tight oxide layer is being created above it to prevent further corrosion.
A572 is another type of HSLA steels which is being used for welded and bolted construction purposes other than bridges. It has minimum yield stress of 42ksi and tensile strength of 60ksi.
A992 is another type of HSLA steel which is being mostly for hot rolled shapes in structural applications. It has minimum yield stress of 50ksi and tensile strength of 65ksi.
A1043 is another type of HSLA steel, which has three grades used for rolled structural steel plates and shapes used for building framing and other structural purposes.
Heat-Treated Carbon and HSLA Steels:
For getting a yield point in the range of 50 to 75ksi in carbon and HSLA steels, they are heat treated. This gives a midway strength level in between as- rolled HSLA steels and the heat treated constructional alloy steels. This has 4 different designation according to ASTM, i.e.
A633 is being used in structural applications where enhanced notch toughness is required. It is of four different grades which has different chemical composition, minimum yield point from 42 to 60ksi subjecting to the shape, grade and thickness.
A678 is used in structural application which requires excellent notch toughness. It is quenched and tempered plate steels which has minimum yield point from 50 to 75ksi.
A852 is used in weathering purpose in structural application. It is a quenched and tempered plate. It has minimum yield point of 70ksi in thickness up to 4inch. It has a resistance to corrosion which is 4 times that of carbon steels.
A913 is produced by quenching and self-tempered process for construction of bridges, building and other structures. It has a minimum yield point 50 to 70ksi. It has maximum carbon for improving the weldability.
Heat-Treated Constructional Alloy Steels:
Steels which have excess of limits for the carbon and are heat treated to get a combination of high strength and toughness are called as constructional alloy steels. It has yield strength of 100ksi and these considered as strongest steels among the others. It has only one designation of ASTM i.e. A514 which has minimum yield point of 100ksi and tensile strength of 110 to 130ksi. It is used for weathering purpose in structural application. It has four times resistance to corrosion compared to carbon steels.
The above discussed details about the different types of steels are made into table and displayed below.
TABLE 1: Minimum properties of structural steels
Tensile Properties of Structural steels:
Typical stress-strain curve for steels is given below. When steel is subjected to a load, an initial elastic range is detected where there is no permanent deformation in the steel. So if this load is removed from the steel, it will return back to normal dimension. Modulus of elasticity is the ratio of stress and strain in elastic range. The strains beyond than elastic range in tension test are termed the inelastic range. For high strength low-alloy steel, this range is divided in to 2 parts. When strain increases with less increase in stress, is termed as plastic range. This later followed by strain hardening range, in which strain increase is accompanied by a substantial increase in stress.
FIGURE 2: Stress-Strain curve for a Structural Steel
The proportional limit is determined from stress-strain curve. It is the stress equal to the first seen withdrawal from linear-elastic behaviour. Since the withdrawal is gradual from elastic action, the proportional limit rest on individual judgement and on accuracy and sensitivity of the strain measuring device.
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Ductility is the important property of steel. It permits the redistribution of stresses in uninterrupted members and at points of high local stresses, such as those at holes or other dimensions or at other interrupted places.
In tension test, ductility is determined from percentage of elongation over the given length or percentage of compression of cross-sectional area. Ductility is repressed under strain conditions where restraint is present.
Tensile properties in Cold working Effect:
In the fabrication of structures, steels are made into desired shape at room temperature. These cold forming operations will cause in-elastic deformation, since the steels retains the designed shape. To understand the behaviour of strength and ductility of steel, when assume carbon steel is subjected to plastic deformation and the resulting tensile loading is discussed below.
If steel is subjected to load which results in plastic deformation or strain hardened range and then unloaded. The curve of unloading follows the trail to the elastic portion of the stress strain curve.
If the amount of plastic deformation is less than or sufficient to cause strain hardening, the tensile strength remains the same. But the ductility is decreased. The diagram of stress-strain for the effect of strain hardening effect is shown below.
FIGURE 3: Diagram of stress-strain for the effect of strain hardening steel
Similarly, if a steel specimen has been loaded to strained hardening range, unloaded and kept to ageing for several days at room temperature. Then the tensile strength will increase while the ductility will decrease.
FIGURE 4: Diagram of stress-strain for the effect of strain hardening steel after ageing
Tensile Properties Due To Strain Rate Effect:
In order to get appropriate information for designing a structural steel, tensile property is found from the slow strain rate. The rapid tension test is done on a carbon steel, 2 HSLA steel and a constructional alloy steel. The test at three different temperature i.e. -50F, room temperature and 600F. The test is also conducted at three different strain rates. The result is shown in diagram below. From the diagram we come to know, in Figure A and B, the curve is 0.2% offset of all the steels, this says that tensile strength increase as the strain rate increases at -50F and room temperature. Increase in tensile strength of A514 steel is 15%, whereas increase in yield strength for A515 carbon steel is 48%. For 600F temperature, the increase in strain rate has small influence in the yield strength. As discussed earlier, the ductility decreases with strain rate.
FIGURE 5: Effect of strain rate on yield and tensile strength of structural steels at different temperatures
Strengthening Mechanism in Structural Steels:
In order to improve the strengthening mechanism of the steels, the following are the ways to improving the strengthening mechanism in structural steels:
Refining the ferrite grain size
Solid solution strengthening
Ferrite grain refinement:
The Hall and Petch made the foundation for refining the ferrite grain. The Hall-Petch equation is given below which is mostly used in metallurgy of ferrous.
=friction stress which opposes dislocation movement
= a constant
D= ferrite grain size
Thus the refinement of ferrite grain size will result in increasing the strength of steel. And it is shown in the figure given below.
FIGURE 2: Effect of ferrite grain size on yield strength and impact properties
The strengthening effect will lead to a decreasing the toughness. If thr cooling to ambient temperature ferrite grain are refined, then strengthening effect will not lead to decrease of toughness. Refinement of grains can be attained in many ways. The fine grained steels usually contain about 0.03 % Al which remains in solution at 1250Í¦c throughout the rolling and after cooling to ambient temperature. But, the reheating the ferrite range to the mormalizing , the Al combines with the nitrogen and forms AlN which is a fine dispersion. Later, these particles pin the austenite grain boundaries at a temperature ranging from850-920Í¦c. This will result in fine ferrite grain size because fine austenite grain size. Similar to austenite grain size, there are other elements which play major role in refining the ferrite grain size. Addition of carbon and manganese or by increasing the cooling rate of the austenite temperature will also help in the refinement.
Solid Solution strengthening:
The solid solution strengthening effect of the common alloying elements is shown in the below figure. From the figure and Pickering and gladman work tells us that carbon, nitrogen and phosphorous will reduce the toughness of the steel and on the other hand, addition of phosphorous will increase the resistance of atmospheric corrosion. Silicon and manganese will solid solution strengtheners but silicon is added to steels to act as deoxidizing agent.
FIGURE 2: Solid solution strengthening effect in ferrite -pearlite high strength low alloy
In ferrite-pearlite steels, Precipitation strengthening can be induced by niobium, vanadium and titanium. These elements have great attraction toward carbon and nitrogen. So they are added in small amounts in steels. Niobium is being added to the solution at slab of 1250Í¦c. On cooling, these Nb will precipitate at the austenite-ferrite interface during transformation. During reheating, small amount of Nb will get dissolved and the undissolved particles will help in restricting the austenite grain development and will lead to the formation of fine ferrite grain size.
Vanadium will dissolve more easily than Niobium and reacts with carbon to form which happens at 920Í¦c. If the temperature is increased, then Vanadium will react with nitrogen to form VN which will act as grain refining agent. Therefore vanadium plays a significant role in precipitation strengthening effects. The solubility of NbC and VN is shown in figure below.
FIGURE 3: Solubility of NbC and VN in austenite at various temperature.
Addition of elements and faster cooling rates decrease the temperature of transformation of austenite to ferrite and it will be sufficient for transformation to bainite or martensite. The result is shown in figure. Due to this transformation, there is increase in strength but there is decrease in toughness and ductility. The addition of molybdenum and boron will increase the hardenability and vanadium for improving the tempering resistance.
FIGURE 4: Relationship between 50percent transformation temperature and tensile strength
Low Carbon Structural Steels in Bridges and Multi-storey building:
In UK, only 20% of the bridge is being made from structural steel whereas in Japan around 80% of the bridges are made from steels. This is because corrosion, designing the bridges with steel is complex and not easy to use. The code of design mostly deals with avoidance against brittle fracture. Steels for structural application in bridges are mostly A709. Under specification it can divided in to 4 different grades i.e.36, 50, 70 and 100 which has yield strength of 36,50,70 and 100ksi respectively. It is shown in below.
For multi-storey building, the steel is used to make sure there is fire protection and corrosion resistance. Cost of the steel plays an important factor in this.
TABLE 2: Toughness property of A709
Recent Development in Steel:
The care towards innovative steels at low cost increased considerably in the last years. The research is mainly focussing upon the development of duplex stainless steels.
Duplex Stainless steels (DSS):
Duplex stainless steel is one of the types of stainless steel categorized by a biphasic microstructure with about equal amounts of austenite and ferrite. DSS has good resistance against corrosion, high strength, good weldability and low temperature. DSS is optimised with some properties of austenitic and ferritic stainless steel. DSS are more delicate to minor variations in chemical composition or processing than austenitic stainless steels. It is cheaper than austenitic stainless steel because of less nickel content. Today the most commonly used duplex stainless steel grade is 2205 whose composition is 22%Cr, 5%Ni, 3%Mo and 0.16%N. This is a nitrogen boosted duplex stainless steel. The 2205 DSS gives corrosion resistance much more than that of AISI type 304,316 and 317 stainless steels. This DSS is widely used in welded sheets and welded pipe. For environmental related applications, Mo, Cu and W alloying elements are added. The development of lean duplex stainless steel with N and Mn content is to replace the Ni. The chemical composition of lean is shown in table below.
TABLE 14: Chemical composition of lean duplex stainless steel
In this paper, we have discussed about all the types of steels which are available for structural applications, tensile properties of steel, strengthening mechanism of structural steels and also about the recent development in steels. Steels are being used all around us. Steels have become backbone of the development in any application due to its strength and toughness.