2 Ironmaking • Steelmaking Chapter 11Iron and SteelIronmaking • Steelmaking
3 The blast furnace is used to convert iron ore into pig iron. Blast furnace reduction is the reduction of iron ore to pig iron in a blast furnace. Pig iron is an impure form of iron. Reduction is the removal of the chemically combined oxygen in the iron ore. Reduction is performed in a blast furnace, which is a complex refractory-lined vertical structure more than 45 m (150) high. See Figure 11-1.
4 Elements present in pig iron must be refined to allowable amounts to meet steelmaking specifications.Pig iron is very impure and unsuitable for any practical purpose. It contains excessive amounts of carbon, phosphorus, and sulfur. These elements, and also manganese and silicon, must be refined to allowable amounts so the iron can be used in steelmaking. See Figure 11-2.
5 The microstructure of wrought iron consists of ferrite grains incorporating a large quantity of silicate slag elongated in the working direction.Wrought iron contains little carbon (<0.1% C) and manganese (<0.07% Mn), and a relatively high amount of silicate slag (approximately 2% Si). The microstructure consists of elongated silicate slag stringers in a matrix of almost pure iron. A stringer is an elongated concentration of foreign material aligned in the direction of working. The principal qualities of wrought iron, such as ease of welding and corrosion resistance, are dependent on the silicate slag. See Figure 11-3.
6 In the basic oxygen furnace, oxygen is lanced into the molten pig iron and scrap steel. The basic oxygen furnace is a large pear-shaped steel barrel lined with refractory. See Figure The furnace is tilted so that the charge of molten pig iron and scrap steel can be poured into its mouth from the scrap-charging car. Depending on the furnace size, the charge weight can be from 80 to 350 tons, with 200 tons being the most common. An oxygen lance (pipe) is lowered into the furnace about six feet above the charge, and oxygen is blown in at very high velocities. This makes the oxygen ignite. The submerged injection (Q-BOP) process uses the basic oxygen furnace where the oxygen is blown through the bottom of the furnace.
7 The open hearth furnace can make large amounts of steel in one heat. The open hearth furnace is a large, refractory-lined shallow basin with an arched roof. See Figure The furnace is charged with a mixture of liquid pig iron, solid pig iron, scrap steel, and fluxes. As much as 600 tons of metal may be charged in a single heat.
8 The electric-arc furnace is the most common furnace used in making many alloys of steel. The electric furnace differs from the basic oxygen and open hearth furnaces in that it uses electricity to supply the heat rather than gaseous or liquid fuel. There are several types of electric furnaces. The most common for steelmaking is the electric-arc furnace. See Figure It resembles a giant, refractory-lined steel kettle with a spout at one end. The roof pivots and swings to one side so that the charge and flux can be loaded. Three large retractable electrodes extend up through the roof of the furnace.
9 Each level of deoxidation practice exhibits a significant influence on the macrostructure of the ingot.During deoxidation, a controlled amount of oxygen is removed from the molten steel. The deoxidation practice determines the amount of deoxidation performed and the basic steel type that is produced in the steelmaking operation. There are four levels of deoxidation practice. They influence the macrostructure of the ingot and result in the production of rimmed steel, capped steel, semikilled steel, and killed steel. See Figure 11-7.
10 A shroud is used to protect the molten steel from picking up atmospheric gases as it is poured from the ladle into the mold.Protection of the pouring stream is a refining technique performed once ladle refining operations are complete. The stream of molten steel poured from the ladle is protected from air. This minimizes the oxygen that is picked up by the molten steel. One way of achieving this is to employ a shroud between the ladle and the tundish and between the tundish and the mold. The shrouds exclude air entry while the molten steel is being poured. See Figure 11-8.
11 The four types of semifinished forms (blooms, billets, slabs, and rounds) are defined by shape and dimensions.A bloom is a square-shaped, semifinished form that is greater than 20 cm × 20 cm (8" × 8"). Where a bloom is too large for rolling into the finished product form, billets are used. A billet is a square-shaped, semifinished form that is less than 20 cm × 20 cm (8" × 8"). A slab is a rectangular-shaped, semifinished form that has a width-to-thickness ratio of 2:1 or greater. Slabs are usually 5 cm (2") thick or greater. A round is a semifinished form that has a circular section of any diameter. See Figure 11-9.
12 The ingot mold is tapered to facilitate its removal from the steel ingot after solidification. An ingot mold is shaped like a box. It is a tall container made of cast iron and weighs as much as one and a half times the weight of the ingots cast in it. The ingot mold is usually tapered to facilitate stripping (removal) of the ingot. The two basic types of molds are big-end-down and big-end-up. There are two variations of the big-end-down mold and three of the big-end-up mold. See Figure The mold is set on a stool, and the molten steel is poured into the top of the mold.
13 The macrostructure of a cast ingot consists of a chill zone of fine equiaxed grains adjacent to the mold wall, columnar grains that grow perpendicular to the wall, and equiaxed grains toward the center.During ingot solidification the molten metal to first contact the mold wall freezes rapidly and consists of small equiaxed (equal dimensions in all directions) grains in a band about 1.25 cm (1/2") wide. Columnar structures then appear. Columnar structures are large dendrites that grow inward principally along the longitudinal axis perpendicular to the mold wall. See Figure The ingot is chemically inhomogeneous and is extremely segregated, with inclusions located between the dendrites and near the top of the ingot.
14 Primary rolling reduces the ingot to semifinished forms of desired cross section. After solidification, the mold is stripped from the ingot, which is placed in a furnace known as a soaking pit. The burners soak the ingots, or supply heat so that the temperature equalizes through their cross sections. The ingots are removed from the soaking pit and hot rolled in a primary rolling mill. A rolling mill consists of a series of parallel rolls through which products are passed in order to reduce their cross section. The first stage of ingot reduction is performed in a primary mill. A primary mill produces semifinished forms of convenient cross section, breaks down and homogenizes the cast structure of the ingot, and seals internal voids in the ingots. It may take a number of passes to achieve the wanted cross section. See Figure
15 Continuous casting is a direct and continuous method of producing blooms, billets, slabs, and rounds.The solidified steel emerges from the bending rolls as a bloom, billet, slab, or round. This depends on the shape and size of the water-cooled copper mold. The solidified product then passes through a straightener. The straightened product is then cut into convenient lengths for further processing. Three variations of continuous casting are vertical, curve mold, and bent strand. See Figure
16 Pipe is a shrinkage cavity located in the upper portion of the ingot. A pipe is a central shrinkage cavity located in the upper portion of an ingot. It is formed during solidification of the ingot. See Figure Pipe is most likely to be found in killed steels, where there is no vigorous gas evolution to offset the shrinkage. Hot tops are used to prevent pipe. A hot top is a refractory-lined container that is placed on top of an ingot mold. It absorbs heat less rapidly than the ingot mold and therefore maintains a reservoir of molten steel. The reservoir feeds the solidifying metal below it. The hot top material is cropped off and discarded after primary rolling.
17 Heavy press forging is performed in a large hydraulic press in order to reduce the size of the ingot to a semifinished form larger than a standard bloom.Heavy press forging is hot working between dies in presses at temperatures high enough to avoid strain hardening. See Figure It is used to directly reduce the cross section of ingots. Heavy press forging is performed in large hydraulic presses and is used when the very large starting size of the ingot must be reduced to produce the desired shape.
18 Structural shapes are formed in several rolling sequences, known as roughing, intermediate rolling, and finishing.A structural shape is a hot-rolled, flanged shape having at least one cross-sectional dimension of 3 or greater. They are used for bridges, buildings, ships, and for numerous other construction purposes. They are produced by passing billets or blooms through a series of grooved rolls. A portion of the shape is represented by the grooves in one roll, and the remaining portion is represented by the grooves in the opposite roll. Hot rolling of structural shapes is performed in three broad stages, which are roughing (rough forming), intermediate rolling, and finishing. See Figure
19 Wire is drawn in several stages, known as drafts, on a wire drawing machine. After each draft the wire is coiled.Wire is produced in several stages. The wire rod used to make wire is first descaled. It is then drawn through the tapered hole of one or several wire drawing dies. The number of dies and the number of drafts depend on the final size required. A draft is a single drawing step. To begin the drawing process, one end of the wire rod is pointed, inserted in the die, and then pulled by a powerful block through the die. Between drafts, the wire product is coiled. See Figure After the final draft, the wire is often given a metallic coating for decoration and/or protection. Coatings include zinc (galvanizing), tin, and aluminum.
20 Tubing products may be produced as seamless or welded. Tubular products may be seamless or welded. See Figure Seamless tubular products are made from forged cylindrical billets, which are pierced through the center to make them hollow and then rolled or extruded and drawn to size. Welded tubular products are made from skelp. A skelp is a flat sheet used as the starting stock for producing welded tubing or pipe. The skelp is first uncoiled and then curved about its longitudinal axis into a circular configuration. The longitudinal edges of the skelp are welded together to form pipe or tubing. The skelp may then be reduced in size by cold drawing. Various welding process may be used to fuse the skelp. For example, electric resistance welding, with no welding filler metal, or tungsten inert gas welding, using welding filler metal, can be used.
21 Inclusions become elongated in the working direction of the steel and lead to directional mechanical properties, known as anisotropy.In as-cast steel, the inclusions are located at grain boundaries or between dendrites. Hot or cold working elongates the inclusions in the working direction if they are plastic at the working temperature. This leads to anisotropy. See Figure
22 The AISI-SAE designation system indicates the alloying elements and the partial chemical composition.The AISI-SAE designation system is a system of alloy identification consisting of a four-digit classification that is partially descriptive of the composition. See Figure The first digit indicates the family to which the steel belongs. Thus, 1 indicates a carbon steel, 2 a nickel steel, 3 a nickel-chromium steel, and so on. In the case of simple alloy steels, the second digit indicates the percentage of the principal alloying element. Usually, the second, third, and fourth digits indicate the average carbon content in points (hundredths of a percent). For example, 2340 is a nickel steel with approximately 3% Ni and 0.4% C.
23 The end use of carbon steel is strongly influenced by the carbon content. The 1xxx group is divided into the three categories of low-, medium-, and high-carbon steels. Low-carbon steels contain less than approximately 0.20% C and are the weakest, most ductile, and most easily welded group. Medium-carbon steels contain approximately 0.20% C to 0.45% C and are stronger and less ductile. The approximate upper carbon limit for weldability without the need for preheating and post-weld heat treatment is 0.35% C. High-carbon steels contain greater than approximately 0.45% C and are significantly stronger and less ductile. High-carbon steels are weldable with careful attention to preheating, interpass temperature, post-weld heat treatment, and cooling rate from the welding temperature. The applications of low-, medium-, and high-carbon steels are strongly dependent upon the carbon content. See Figure
24 Generic ASTM specifications indicate common characteristics of families of product forms and prevent excessive repetition in the individual product specifications.ASTM specifications are the most widely used for the procurement of steels. ASTM specifications have the prefix letter A followed by a number. For example, ASTM A106 is a specification for seamless carbon steel pipe that is often specified for boilers. Specifications for steel products are written at two levels. The first level consists of generic requirements for a family of products, and the second level consists of individual specifications for particular product forms. The generic specifications reduce the length of individual specifications by collecting requirements in one place that are common to a family of products. See Figure
25 Quality descriptors in ASTM standards are used to distinguish products well suited to specific applications.The title of the ASTM specification usually includes a quality descriptor. This indicates the characteristics that make the product particularly suited to a specific application or subsequent fabrication operation. An example of a quality descriptor is specification ASTM A646 Premium Quality Alloy Steel Blooms and Billets for Aircraft and Aerospace Forgings. The quality descriptor is the term “Premium Quality” that describes the quality level of the blooms and billets. See Figure