Introduction; manufacturing of steel, alloying, and definitions

CE 671 Behavior of Metal Structures Spring 2006

Introduction; manufacturing of steel, alloying, and definitionsCE 671 - Lecture #1Steel Steel is an alloy made primarily from iron and carbon (the alloy). Typically, the percent of carbon in steel is relatively low, less than 2% carbon. Many other elements within common structural steels such as manganese (1%) and small amounts of silicon, phosphorus, sulfur and oxygen. Composition controlled by ASTM Standards.

Henry Bessemer, a British inventor is typically credited with the invention of steel in 1856 (Bessemer Steel Company in Sheffield, England). Although steel was produced before that time, it was his patented Bessemer process which is still used today. In short, the process involves blowing air through molten pig iron to oxidize the material and separate impurities.

Steel is different than Wrought Iron and Case Iron.

Wrought Iron - Iron that is almost pure (less than 0.15% carbon). Can be shaped and forge welded with ease, but is soft and does not harden in the same way as Steel. The properties of Wrought Iron are partially attributed to the Slag inclusions that result from Puddling and Forge Welding. Widely used in Bridges, Axles and Ships plates before the development of Bessemer and Siemens Steel. The last commercial production of Wrought Iron in the U.K. ceased in 1976.

Cast Iron - Iron with a high Carbon content (above 2% to 2.5% but usually less than 6%). Identical, in most cases, to Pig Iron, it is easily cast to almost any shape and melts at a lower temperature to other type of iron and steel.

Cast Iron is extremely hard and brittle. Machining is difficult and it is easily shattered, revealing its crystalline structure. Chilled Cast Iron is even harder and is produced by cooling the castings to increase the speed at which the iron solidifies. Cast Iron is still is wide use for numerous casting, from drains covers through to engine blocks and water pipes. Can be Grey, White or Malleable.

Pig Iron - The name used for the iron directly produced from a blast furnace. Originally cast into 'pigs' around the base of the furnace, lasted casting machines were developed to produce pigs but iron is now generally transported while still molten and converted into steel on the same site. The name is derived from the impression given of piglets feeding from the sow by the iron being run off the furnace into the original style of sand moulds. Pigs were traditionally sized to be man handled, but size increase later. Pig Iron changes its name to Cast Iron when re-melted, although no actual processing takes place. Iron casings can be created directly from the blast furnace.

Back to Steel

Steel is made via two basic routes - from raw materials - iron ore, limestone and coke by the blast furnace and basic oxygen furnace (BOF) route. If a mill can produce virgin (brand new) steel as well as roll the steel, it is referred to as an integrated mill or plant.

However, today much steel (about 34% in 2003) is obtained from recycled scrap using the electric arc furnace (EAF) method. The second technique is much easier and faster since it only requires scrap steel. Recycled steel is introduced into a furnace and re-melted along with some other additions to produce the end product. Iron OreSimply rock that happens to contain a high concentration of iron. A few common ores are listed below.Common Iron Ores

Hematite - Fe2O3 - 70 percent iron

Magnetite - Fe3O4 - 72 percent iron

Limonite - Fe2O3 + H2O - 50 percent to 66 percent iron

Siderite - FeCO3 - 48 percent iron

Note these all have oxygen attached. To get the iron, we have to get rid of the Oxygen

The more advanced way to smelt iron is in a blast furnace. A blast furnace is charged with iron ore, coke (coke is charcoal made from coal) and limestone (CaCO3). Huge quantities of air are blasted into the bottom of the furnace. The calcium in the limestone combines with the silicates to form slag. At the bottom of the blast furnace, liquid iron collects along with a layer of slag on top. Periodically, you let the liquid iron flow out and cool.To create a ton of pig iron, you roughly start with 2 tons of ore, 1 ton of coke and half-ton of limestone. The fire consumes 5 tons of air. The temperature reaches almost 3000 degrees F (about 1600 degrees C) at the core of the blast furnace! The pig iron produced is tapped from the furnace. Recall Pig iron is high in carbon and is very brittle, so the carbon must be removed.Bessemer Furnace (Process) The process method of producing steel from a charge consisting mostly of pig iron, however limestone and iron ore are also added. The process is carried on in a large container called the egg-shaped Bessemer converter, which is made of steel and has a lining of silica and clay or of dolomite. The capacity is from 8 to 30 tons of molten iron; the usual charge is 15 or 18 tons. The wide end, or bottom, has a number of perforations through which the air is forced upward into the converter during operation and is set on pivots (trunnions) so that it can be tilted at an angle to receive the charge, turned upright during the "blow," and inclined for pouring the molten steel after the operation is complete. As the air passes upward through the molten pig iron, impurities such as silicon, manganese, and carbon unite with the oxygen in the air to form oxides; the carbon monoxide burns off with a blue flame and the other impurities form slag.

Open Hearth Furnace The pig iron, limestone and iron ore go into an open hearth furnace. Heated to about 1600 F (871 C). The limestone and ore forms a slag that floats on the surface. Impurities, including carbon, are oxidized and float out of the iron into the slag. When the carbon content is right, you have carbon steel.

Basic Oxygen Furnace

The process method of producing steel from a charge consisting mostly of pig iron. The charge is placed in a furnace similar to the one used in the Bessemer process of steelmaking except that pure oxygen instead of air is blown into the charge to oxidize the impurities present. One desirable feature of this process is that it takes less than an hour, and is thus much faster than the open-hearth process, another important method of steelmaking. A second advantage is that a major byproduct is carbon monoxide, which can be used as a fuel or in producing various chemicals, such as acetic acid. The basic oxygen process also produces less air pollution than methods using air.

Electric Arc Furnace (EAF) Scrap is melted in an electric arc furnace (100%).

The raw material fed into the furnace may be selected but untreated scrap (old machine parts, for example), or may be delivered as sorted, crushed and calibrated scrap with a minimum iron content of 92 percent.

The produces the molten steel, which then undergoes the same refining and grading processes as pig iron.

The raw materials must be carefully selected for each different steel grade. Selection depends on the type of "impurities" that any metal or ore in the scrap might contain. About 40% of all steel in the US is from EAF

Coke

Combustible substance obtained by the dry distillation (gasification of undesirable components) of coal in a coke oven.

Coke is virtually pure carbon, with a porous structure and highly resistant to crushing.

Burned in the blast furnace, it provides the heat and gases needed to melt reduce iron ore.

Sintering Plant The sinter plant is where the iron ore is prepared

The iron ore is crushed and calibrated into grains which are "sintered", or bonded together.

The sintered iron ore is then crushed and fed, in alternating layers with coke, into the blast furnace.

Continuous Caster

Molten steel is poured continuously into a bottomless mold. As it is drawn, the steel comes into contact with the water-cooled interior surface of the mold, and begins to solidify. The cast metal is then drawn downwards, guided by a series of rollers, while it continues to cool. The cross-section can be controlled and several basic shapes exist.

By the time it reaches the end, the steel is completely solidified, and is immediately cut into the required lengths

Alloying of SteelSteels are easily the most common alloy in civil engineering structures and there are many alloying elements added to the basic steel alloy in order to achieve the desired properties.

An iron-based mixture is considered to be an alloy steel when manganese is greater than 1.65%, silicon over 0.5%, copper above 0.6%, or other minimum quantities of alloying elements such as chromium, nickel, molybdenum, or tungsten are present. An enormous variety of distinct properties can be created for the steel by substituting these elements in the recipe. Some common alloying elements are listed below, but there are many more.Carbon has a major effect on steel properties. Carbon is the primary hardening element in steel. Hardness and tensile strength increases as carbon content increases up to about 0.85% C as shown in the figure above. Ductility and weldability decrease with increasing carbon.

Manganese is generally beneficial to surface quality especially in resulfurized steels. Manganese contributes to strength and hardness, but less than carbon. The increase in strength is dependent upon the carbon content. Increasing the manganese content decreases ductility and weldability, but less than carbon. Manganese has a significant effect on the hardenability of steel.

Phosphorus increases strength and hardness and decreases ductility and notch impact toughness of steel. The adverse effects on ductility and toughness are greater in quenched and tempered higher-carbon steels. Phosphorous levels are normally controlled to low levels. Higher phosphorus is specified in low-carbon free-machining steels to improve machinability.

Sulfur decreases ductility and notch impact toughness especially in the transverse direction. Weldability decreases with increasing sulfur content. Sulfur is found primarily in the form of sulfide inclusions. Sulfur levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added to improve machinability.

Silicon is one of the principal deoxidizers used in steelmaking. Silicon is less effective than manganese in increasing as-rolled strength and hardness. In low-carbon steels, silicon is generally detrimental to surface quality.

Copper in significant amounts is detrimental to hot-working steels. Copper negatively affects forge welding, but does not seriously affect arc or oxyacetylene welding. Copper can be detrimental to surface quality. Copper is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20%. Weathering steels are sold having greater than 0.20% Copper.

Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance, to increase hardenability, or to improve high-temperature strength. As a hardening element, Chromium is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength. Chromium is a strong carbide former. Complex chromium-iron carbides go into solution in austenite slowly; therefore, sufficient heating time must be allowed for prior to quenching.

Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in ferrite, strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of steels.

Molybdenum increases the hardenability of steel. Molybdenum may produce secondary hardening during the tempering of quenched steels. It enhances the creep strength of low-alloy steels at elevated temperatures.

Titanium is used to retard grain growth and thus improve toughness. Titanium is also used to achieve improvements in inclusion characteristics. Titanium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.

Vanadium increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of Niobium can significantly increase the strength of steels. Vanadium is one of the primary contributors to precipitation strengthening in micro-alloyed steels. When thermo-mechanical processing is properly controlled the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition temperature also increases when vanadium is added.

Some Useful ASTMs

A36/A 36M Specification for Carbon Structural Steel

A514/A 514M Specification for High-Yield-Strength, Quenched and Tempered Alloy Steel Plate, Suitable for Welding

A588/A 588M

Specification for High-Strength Low-Alloy Structural Steel with 50 ksi [345 MPa] Minimum Yield Point to 4 in. [100 mm] Thick

A572/A 572M Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel A370 Test Methods and Definitions for Mechanical Testing of Steel Products

A307 Specification for Carbon Steel Bolts and Studs, 60000 psi Tensile StrengthA502Specification for Rivets, Steel, StructuralA668/A668M Specification for Steel Forgings, Carbon and Alloy, for General Industrial UsePAGE 1