Construction Steel This chapter focuses on the mechanical properties of construction steel, the cold working and strengthening of steel, and the standards and selection of steel. It introduces the corrosion reasons of steel and the measures to prevent corrosion. It simply introduces the fire protection of steel. Steel consists mostly of iron, with a carbon content under 2% and various other elements.
Construction steel refers to various steel materials used in construction projects, including various materials used for steel structures (such as round steel, angle steel, joint steel, and steel pipe), plates, and steel bars, steel wires, and strands used in concrete structure. Steel is the material produced under strict technical conditions, and it has the following advantages: even materials, stable properties, high strength, certain plasticity and toughness, and the properties to bear impacts and vibration loads, and can be welded, riveted, or screwed; the disadvantages are: easy to be corroded and high cost of repairs. These characteristics determine that steel is one of the important materials needed by economic construction departments. In construction, the steel structures consisted by steel in various shapes have high security and light deadweight, used for large-span and high-rise structures.
However, because every department needs a large amount of steel, the wide use of steel structure is limited to some extent. But though concrete structures have heavy deadweight, the usage of steel is decreased greatly, and it can overcome the corrosion and high cost of repairs of steel. Thus, steel is widely used in concrete structures.1 Classifications of Steel 8.1 By Smelting Processes Smelting is to oxidize the molten pig iron to reduce its carbon content to the scheduled range and to remove the other impurities to allowable range. During smelting, the removal degrees of impurities by different smelting methods are not the same, so the steel qualities are different.
Recently, there are three kinds of steel, including Bessemer steel (converter steel), Siemens-Martin steel, and electric steel. Bessemer Steel The smelting process of this steel is to use the molten pig iron as the raw material without any fuel and to make steel with air being blown through the molten iron (the raw material) from the bottom or the sides of the converter, called pneumatic converter steel; if pure oxygen is used to replace the air, it is called the oxygen converter steel. The disadvantage of pneumatic converter steel is that the nitrogen, hydrogen and other impurities in the air will interfuse easily, the smelting time is short, and the impurity content is difficult to control, so the quality is poor; the quality of oxygen converter steel is high, but the cost is a little higher. Siemens-Martin Steel The process of Siemens-Martin steel is to use solid or fluid pig iron, ore or waste steel as the raw materials and coal gas or heavy oil as the fuel and to remove the impurities from the iron by oxidation with the oxygen in ore or waste steel or the oxygen being blown through the iron.
Because the smelting time is long (4-12h), the impurities are removed clearly and the quality of steel is good. But the cost is higher than that of Bessemer steel. Electric Steel The process of electric steel is to make steel by electric heating. The heat source is high-tension arc, and the smelting temperature is high and can be adjusted freely, so the impurities can be removed clearly and the steel quality is good.
208 Building materials in civil engineering 8.2 By Deoxidation Methods Unavoidably, there will be part of ferric oxide left in molten steel during the smelting process, which reduce the steel quality. Thus, deoxidation is needed during the ingot casting. The steel made by different deoxidation methods has various properties. Therefore, there is rimmed steel, fully-killed steel, and semi-killed (or semi-deoxidized) steel.
Rimmed Steel It is the unkilled steel which is deoxidized only by ferromanganese, a weak deoxidizer. Because the remained FeO in the molten steel can generate CO with C, there are a lot of foams in the process of casting ingot, like boil, known as rimmed steel. Its organization is not dense enough and contains foams, so the quality is poor; but the rate of finished products is high and the cost is low. Fully-killed Steel This kind of steel is deoxidized thoroughly with a certain amount of silicon, manganese, and aluminum deoxidizers.
Because deoxidation is thorough, the molten steel can solidify calmly in ingot casting, known as fully-killed steel. Its organization is dense, chemical elements are even, and properties are stable, so its quality is good. However, the productivity is low, so the cost is high. It can be employed in the steel structures used to bear impacts, vibration or important welding.
Semi-killed Steel Its deoxidation degree and quality are between the above two.3 By Press-working Modes In the process of smelting and ingot-casting, there will be uneven structures, foams or other defects happening to the steel, so the steel used in industry should be processed by press to eliminate the defects. Meanwhile, there is requirement for shapes. The press-working modes include hot working and cold working. Hot-working Steel Hot working is to heat the steel ingot to a certain temperature and to conduct press-working to the steel ingot in the plastic state, such as hot rolling and hot forging.
Cold-working Steel The steel is processed under the normal temperature.4 By Chemical Elements Steel Classifications (GB/Tl3304-9 1), the Chinese standard, recommends two classification methods: one is to classify by chemical elements, and the other is to classify by quality degrees. By chemical elements, there is non-alloy steel, lean-alloy steel and alloy steel. 1) Non-alloy Steel: that is carbon steel with few alloy elements. 2) Lean-alloy Steel: that is the stcel with low alloy elements.
3) Alloy Steel: that is the steel added with more alloy elements to improve some properties of the steel.5 By Quality Degrees According to quality degrees, the steel can be classified into: common steel, quality stcel and advanced quality steel.6 By Purposes The steel can be classified by purposes, such as construction steel, railway steel, and pressure vessel steel. The construction steel can be classified by purposes into the steel for steel structures and that for concrete structures. At present, the steel commonly used in constructions includes carbon structural steel and lean-alloy and high-strength structural steel.2 Characteristics of Steel 8.1 Characteristics of Steel The characteristics of steel include strength, elasticity, plasticity, toughness and rigidity. Tensile Strength The tensile strength of construction steel includes: yield strength, ultimate ,tensile strength, and fatigue strength.
210 Building materials in civil engineering (1) Yield Strength or Yield Limit Subjected to the dead load, steel starts to lose the ability to resist deformation and generates a great deal of stress in plastic deformation. As shown in Figure 8.1, at the yield stage, the corresponding stress of the highest point on the hackle is called the upper yield point ( Bup); the corresponding stress of the lowest point is called the lower yield point ( BdOw ). Because the yield points are unstable, the Chinese Standard regulates that the stress of the lower yield point is the yield strength of the steel, expressed by a,.Medium carbon steel and high carbon steel have no obvious yield points, so 0.2% of the stress of the residual deformation is the yield strength, expressed by shown in Figure 8. Yield strength is very important to the use of steel.
When the actual stress of a structure reaches the yield point, there will be irretrievable deformation which is not allowed in constructions. Thus, yield strength is the main base to determine the allowable stress of the steel.1 Stretching of Low Carbon Steel Q - E I The elastic stage, expressed by 0,; I1 The yield stage. expressed by 0, 111 The reinforcement stage, expressed by 0,;Iv The necking stage.2 The Assigned Yield Point of Hard Steel 8 Construction Steel 211 (2) Ultimate Tensile Strength (Simply Called Tensile Strength) It is the ultimate tensile stress that the steel can bear under the role of tension, shown in Figure 8.1, the highest point of stage 111. Tensile strength cannot be the calculated base directly, but the ratio of yield strength to tensile strength is the yield ratio, namely, 5 which is very important in constructions.
The smaller the yield ratio is, the more reliable the structure is, that is, the higher the potential to prevent the damage of the structure is; but if the ratio is too small, the available utilization ratio of the steel will be too low, and the reasonable yield ratio should lie between 0. Therefore, the yield strength and the tensile strength are the major test indexes of the mechanical properties of steel. (3) Fatigue Strength Under the role of alternating loads, steel will be damaged suddenly when the stress is far below the yield strength, and this damage is called fatigue failure. The value of stress at which failure occurs is called fatigue strength, or fatigue limit.
The fatigue strength is the highest value of the stress at which the failure never occurs. Generally, the biggest stress that the steel bears alternating loads for 106-107 times and no failure occurs is called the fatigue strength.1 shows that the steel is subjected to the dead load and the ratio of the stress to the strain at stage OA is the elastic stage. This deformation property is ’ called elasticity. At this stage, the ratio of the stress to the strain is the modulus a of elasticity, that is, E = - with MPa as the unit.
& The modulus of elasticity is the index to measure the ability of the steel to resist deformation. The bigger E is, the higher the stress that causes its deformation is; and under the certain stress, the smaller the elastic deformation will be. In projects, the modulus of elasticity reflects the rigidity of the steel which is an important value to calculate the deformation of a structure under stress. The elastic modulus of 4235, the carbon structural steel commonly used in constructions, is calculated as follows: E=(2.
212 Building materials in civil engineering 3. Plasticity The construction steel should have good plasticity. In projects, the plasticity of the steel is usually expressed by the elongation (or -the reduction of cross-section area) and cold bending. 1) Elongation refers to the ratio of the increment of the gauge length to the original gauge length when the specimen is stretched off, expressed by S(%), shown in Figure 8.3 Elongation of Steel 2) Reduction of cross-section area is the percentage of the cross-section shrinkage quantity of the neck-shrinking part to the original cross-section area when the specimen is stretched off, expressed by qj (%).
For the sake of measurement, elongation is often used to express the plasticity of steel. Elongation is the important index to measure the plasticity of steel. The bigger the elongation is, the better the plasticity of steel is. The elongation is related to the gauge length, and usually 6,and S,, are used to express the elongation when lo=5a and lo=lOa respectively.
For the same steel, 6, > S,,. 3) Cold bending is the property that the steel bears the bending deformation under the normal conditions. The cold bending is tested by checking whether there are cracks, layers, squamous drops and ruptures on the bending part after the specimen goes through the regulated bending. Generally, it is expressed by the ratio of the bending angle a and the diameter of the bending heart d to the thickness of the steel or the diameter of the steel a.4 shows that the bigger the bending angle is, the smaller the ratio of d to a is, and the better the cold bending property is.
8 Construction Steel 213 Figure 8.4 Cold Bending Test of Steel d.'diarneter of the bending heart; a. the thickness or the diameter of the specimen; a.