Steel is the most common and widely used metallic material in today’s society. It can be cast or wrought into numerous forms and can be produced with tensile strengths exceeding 5 GPa. A prime example of the versatility of steel is in the automobile where it is the material of choice and accounts for over 60% of the weight of the vehicle. Steel is highly formable as seen in the contours of the automobile's outer body. Steel is strong and is used in the body frame, motor brackets, driveshaft, and door impact beams of the vehicle.
Steel is corrosion-resistant when coated with the various zinc-based coatings available today. Steel is dent resistant when compared with other materials and provides exceptional energy absorption in a vehicle collision. Steel is recycled and easily separated from other materials by a magnet. Steel is inexpensive compared with other competing materials such as aluminum and various polymeric materials.
In the past, steel has been described as an alloy of iron and carbon. Today, this description is no longer applicable since in some very important steels, e.g., interstitial-free (IF) steels and type 409 ferritic stainless sheets of steel, carbon is considered an impurity and is present in quantities of only a few parts per million. By definition, steel must be at least 50% iron and must contain one or more alloying elements. These elements generally include carbon, manganese, silicon, nickel, chromium, molybdenum, vanadium, titanium, niobium, and aluminum. Each chemical element has a specific role to play in the steelmaking process or in achieving particular properties or characteristics, e.g., strength, hardness, corrosion resistance, magnetic permeability, and machinability.
STEEL MANUFACTURE
In most of the world, steel is manufactured by integrated steel facilities that produce steel from basic raw materials, i.e., iron ore, coke, and limestone. However, the fastest growing segment of the steel industry is the “minimill” which melts steel scrap as the raw material. Both types of facilities produce a wide variety of steel forms, including sheet, plate, a structural, railroad rail, and bar products.
Ironmaking. When making steel from iron ore, a blast furnace chemically reduces the ore (iron oxide) with carbon in the form of coke. Coke is a spongelike carbon mass that is produced from coal by heating the coal to expel the organic matter and gases. Limestone (calcium carbonate) is added as a flux for easier melting and slag formation. The slag, which floats atop the molten iron, absorbs many of the unwanted impurities. The blast furnace is essentially a tall hollow cylindrical structure with a steel outer shell lined on the inside with special refractory and graphite brick.
The crushed or pelletized ore, coke, and limestone are added as layers through an opening at the top of the furnace, and chemical reduction takes place with the aid of a blast of preheated air entering near the bottom of the furnace (an area called the bosh). The air is blown into the furnace through a number of water-cooled copper nozzles called tuyeres. The reduced liquid iron fills the bottom of the furnace and is tapped from the furnace at specified intervals of time. The product of the furnace is called pig iron because in the early days the molten iron was drawn from the furnace and cast directly into branched mold configurations on the cast house floor. The central branch of iron leading from the furnace was called the “sow” and the side branches were called “pigs.” Today the vast majority of pig iron is poured directly from the furnace into a refractory-lined vessel (submarine car) and transported in liquid form to a basic oxygen furnace (BOF) for refinement into steel.
Steelmaking. In the BOF, liquid pig iron comprises the main charge. Steel scrap is added to dilute the carbon and other impurities in the pig iron. Oxygen gas is blown into the vessel by means of a top lance submerged below the liquid surface. The oxygen interacts with the molten pig iron to oxidize undesirable elements. These elements include excess carbon (because of the coke used in the blast furnace, pig iron contains over 2% carbon), manganese, and silicon from the ore and limestone and other impurities like sulfur and phosphorus. While in the BOF, the liquid metal is chemically analyzed to determine the level of carbon and impurity removal.
When ready, the BOF is tilted and the liquid steel is poured into a refractory-lined ladle. While in the ladle, certain alloying elements can be added to the steel to produce the desired chemical composition. This process takes place in a ladle treatment station or ladle furnace where the steel is maintained at a particular temperature by external heat from electrodes in the lid placed on the ladle. After the desired chemical composition is achieved, the ladle can be placed in a vacuum chamber to remove undesirable gases such as hydrogen and oxygen. This process is called degassing and is used for higher quality steel products such as railroad rail, sheet, plate, bar, and forged products. Stainless steel grades are usually produced in an induction or electric arc furnace, sometimes under vacuum. To refine stainless steel, the argon–oxygen decarburization (AOD) process is used. In the AOD, an argon–oxygen gas mixture is injected through the molten steel to remove carbon without a substantial loss of chromium (the main element in stainless steel).
Continuous Casting. Today, most steel is cast into solid form in a continuous-casting (also called strand casting) machine. Here, the liquid begins solidification in a water-cooled copper mold while the steel billet, slab, or bloom is withdrawn from the bottom of the mold. The partially solidified shape is continuously withdrawn from the machine and cut to length for further processing. The continuous-casting process can proceed for days or weeks as ladle after the ladle of molten steel feeds the casting machine. Some steels are not continuously cast but are poured into individual cast iron molds to form an ingot that is later reduced in size by forging or a rolling process to some other shape. Since the continuous-casting process offers substantial economic and quality advantages over ingot casting, most steel in the world is produced by continuous casting.
DEVELOPMENT OF STEEL PROPERTIES
In order to produce a steel product with the desired properties, basic metallurgical principles are used to control three things. This means that the steel composition and processing route must be closely controlled in order to produce the proper microstructure. The final microstructure is of utmost importance in determining the properties of the steel product. This section will explore how various microstructures are developed and the unique characteristics of each microstructural component in steel. The next section will discuss how alloy composition also plays a major role.
Iron–Carbon Equilibrium Diagram. Since most steels contain carbon, the basic principles of microstructural development can be explained by the iron–carbon equilibrium diagram. This diagram, shown in Fig. 1
Iron is an interesting chemical element in that it undergoes three phase changes when heated from room temperature to liquid. For example, from room temperature to 912∘C pure iron exists as ferrite (also called alpha iron), from 912 to 1394∘C it exists as austenite (gamma iron), from 1394 to 1538∘C it exists as ferrite again (delta iron), and above 1538∘C it is liquid. In other words, upon heating, iron undergoes allotropic-phase transformations from ferrite to austenite at 912∘C, austenite to ferrite at 1394∘C, and ferrite to liquid at 1538∘C. Each transformation undergoes a change in crystal structure or arrangement of the iron atoms in the crystal lattice.
It must be remembered that all chemical elements in their solid form have specific arrangements of atoms that are essentially the basic building blocks in producing the element in the form that we physically observe. These atomic arrangements form a latticework containing billions of atoms all aligned in a systematic way. Some of these lattices have a cubic arrangement, with an atom at each corner of the cube and another atom at the cube center. This arrangement is called body-centered-cubic (bcc). Others have an atom at each corner of the cube and atoms at the center of each face of the cube. This is called face-centered-cubic (fcc). Other arrangements are hexagonal, some are tetragonal, etc. As an example, pure iron as ferrite has a bcc arrangement. Austenite has an fcc arrangement. Upon heating, bcc ferrite will transform to fcc austenite at 912∘C. These arrangements or crystal structures impart different properties to steel. For example, a bcc ferritic stainless steel will have properties much different from an fcc austenitic stainless steel, as described.
Since pure iron is very soft and of low strength, it is of little interest commercially. Therefore, carbon and other alloying elements are added to enhance properties. Adding carbon to pure iron has a profound effect on ferrite and austenite, as discussed above. One way to understand the effect of carbon is to examine the iron-carbon diagram (Fig. 1). This is a binary (two-element) diagram of temperature and composition (carbon content) constructed under near-equilibrium conditions. In this diagram, as carbon is added to iron, the ferrite- and austenite-phase fields expand and contract depending upon the carbon level and temperature.
Also, there are fields consisting of two phases, e.g., ferrite plus austenite. Since carbon has a small atomic diameter when compared with iron, it is called an interstitial element because it can fill the interstices between the iron atoms in the cubic lattice. Nitrogen is another interstitial element. On the other hand, elements such as manganese, silicon, nickel, chromium, and molybdenum have atomic diameters similar to iron and are called substitutional alloying elements. These substitutional elements can thus replace iron atoms at the cube corners, faces, or center positions. There are many binary-phase diagrams (Fe–Mn, Fe–Cr, Fe–Mo, etc.) and tertiary-phase diagrams (Fe–C–Mn, Fe–C–Cr, etc.) showing the effect of interstitial and substitutional elements on the phase fields of ferrite and austenite. These diagrams are found in the handbooks listed at the end of the chapter.
Being an interstitial or a substitutional element is important in the development of steel properties. Interstitial elements such as carbon can move easily about the crystal lattice whereas a substitutional element such as manganese is much more difficult to move. The movement of elements in a crystal lattice is called diffusion.
Diffusion is a controlling factor in the development of microstructure. Another factor is solubility, which is a measure of how much of a particular element can be accommodated by the crystal lattice before it is rejected. In metals, when two or more elements are soluble in the crystal lattice, a solid solution is created (somewhat analogous to a liquid solution of sugar in hot coffee). For example, when added to iron, carbon has very limited solubility in ferrite but is about 100 times more soluble in austenite, as seen in the iron-carbon diagram in Fig. 2 (a limited version of the diagram in Fig. 1). The maximum solubility of carbon in ferrite is about 0.022% C at 727∘C while the maximum solubility of carbon in austenite is 100 times more, 2.11% C at 1148∘C. At room temperature, the solubility of carbon in iron is only about 0.005%.
Any amount of carbon over the solubility limit is rejected from a solid solution and is usually combined with iron to form an iron carbide compound called cementite. This hard and brittle compound has the chemical formula Fe3C and a carbon content of 6.7%. This is illustrated in the following two examples. The first example is a microstructure of very low-carbon steel (0.002% C), shown in Fig. 3a. The microstructure consists of only ferrite grains (crystals) and grain boundaries. The second example is a microstructure of low-carbon steel containing 0.02% C, in Fig. 3b. In this microstructure, cementite can be seen as a particle at the ferrite grain boundaries. The excess carbon rejected from the solid solution of ferrite formed this cementite.
As the carbon content in steel is increased, another form of cementite appears as a constituent called pearlite, which can be found in most carbon steels. Examples of pearlite in low-carbon (0.08%C) and medium-carbon (0.20% C) steels are seen in Figs. 4a and 4b. Pearlite has a lamellar (parallel-plate) microstructure, as shown at higher magnification in Fig. 5, and consists of layers of ferrite and cementite. Thus, in these examples, in increasing the carbon level from 0.002–0.02 to 0.08–0.20%, the excess carbon is manifested as a carbide phase in two different forms, cementite particles and cementite in pearlite. Both forms increase the hardness and strength of iron. However, there is a trade-off; cementite also decreases ductility and toughness.
Pearlite forms on cooling austenite through a eutectoid reaction as seen below:
Austenite ↔ Fe3C + ferrite
A eutectoid reaction occurs when a solid phase or constituent reacts to form two different solid constituents on cooling (a eutectic reaction occurs when a liquid phase reacts to form two solid phases). The eutectoid reaction is reversible on heating. In steel, the eutectoid reaction (under equilibrium conditions) takes place at 727∘C and can be seen on the iron-carbon diagram (Fig. 1) as the “V” at the bottom left side of the diagram. A fully pearlitic microstructure forms at 0.77% C at the eutectoid temperature of 727∘C (the horizontal line on the left side of the iron-carbon diagram). Steels with less than 0.77% C are called hypoeutectoid steels and consist of mixtures of ferrite and pearlite with the amount of pearlite increasing as the carbon content increases.
The ferrite phase is called a pro eutectoid phase because it forms prior to the eutectoid transformation that occurs at 727∘C. A typical example of eutectoid ferrite is shown in Fig. 6. In this photomicrograph, the ferrite (the white-appearing constituent) formed on the prior austenite grain boundaries of hypoeutectoid steel with 0.60% C. The remaining constituent (dark appearing) is pearlite. Steels between 0.77% C and about 2% C are called hypereutectoid steels and consist of pearlite with pro eutectoid cementite. Cementite forms a continuous carbide network at the boundaries of the prior austenite grains. Because there is a carbide network, hypereutectoid steels are characterized as steels with little or no ductility and very poor toughness. This means that in the commercial world the vast majority of carbon steels are hypoeutectoid steels.
Thus, according to the iron-carbon diagram, steels that are processed under equilibrium or near-equilibrium conditions can form-
(a) pure ferrite at very low carbon levels generally under 0.005% C,
(b) ferrite plus cementite particles at slightly higher carbon levels between
0.005% C and 0.022% C,
(c) ferrite plus pearlite mixtures between 0.022% C and 0.77%C,
(d) 100% pearlite at 0.77% C, and
(e) mixtures of pearlite plus cementite networks between 0.77% C and 2% C.
The higher the percentage of cementite, the higher the hardness and strength and lower the ductility and toughness of the steel.
Departure from Equilibrium (Real World). Industrial processes do not occur at equilibrium, and only those processes that take place at extremely slow heating and cooling rates can be considered near equilibrium, and these processes are quite rare. Therefore, under real conditions, the iron-carbon diagram can only be used as a rough guideline since the equilibrium transformation temperatures shift to lower temperatures on cooling and to higher temperatures on heating. If steels are cooled at very fast rates, e.g., quenching in water, the iron-carbon diagram can no longer be used since there is a major departure from equilibrium.
In fact, during the quenching of steel, new constituents form that are not associated with the iron-carbon diagram. Therefore, at fast cooling rates the concept of time–temperature transformation (TTT) diagrams must be considered. These diagrams are constructed under isothermal (constant) temperature (called IT diagrams) or continuous-cooling conditions (called CT diagrams). It is important to know how these diagrams are constructed so that we can understand the development of non-equilibrium microstructures, which are so important in carbon and alloy steels.
Isothermal Transformation Diagram. This diagram is formed by quenching very thin specimens of steel in salt baths set at various temperatures. For example, thin specimens of 0.79% C steel can be quenched into seven different liquid salt baths set at 650, 600, 550, 500, 450, 400, and 200∘C.
The specimens are held for various times at each temperature, then pulled from the bath and quickly quenched in cold water. The result will be a diagram called an isothermal transformation (IT) diagram, Isothermal transformation diagram of SAE/AISI 1080 steel showing the beginning and end of transformation curves with temperature and time. (Source: ASM Handbook, Vol. 1, Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, Materials Park, OH, 1990, p. 128.) Reprinted with permission of ASM International. essentially a map showing where various constituents form. For example, at 650∘C, austenite (A) begins to transform to pearlite if held in the bath for 10 s. The curve drawn through this point is the pearlite transformation start temperature and is labeled the beginning of a transformation. At about 100 s the pearlite transformation is finished. The second curve represents the pearlite transformation finish temperature and is labeled the end of the transformation.
In this steel, pearlite forms at all temperatures along the start of the transformation curve from 727∘C (the equilibrium temperature of the iron-carbon diagram) to 540∘C (the “nose” of the curve). At the higher transformation temperatures, the pearlite interlamellar spacing (the spacing between cementite plates) is very coarse and decreases in spacing as the temperature is decreased, i.e., the nose of the IT diagram is approached. This is an important concept since steel with a coarse pearlite interlamellar spacing is softer and of lower strength than steel with a fine pearlite interlamellar spacing. Commercially, rail steels are produced with a pearlitic microstructure, and it has been found that the finer the interlamellar spacing, the harder the rail and the better the wear resistance.
This means that rails will last longer in track if produced with the finest spacing allowable. Most rail producers employ an accelerated cooling process called head hardening to obtain the necessary conditions to achieve the finest pearlite spacing in the rail head (the point of wheel contact). If the specimens are quenched to 450∘C and held for various times, pearlite does not form. In fact, pearlite does not isothermally transform at transformation temperatures (in this case, salt pot temperatures) below the nose of the diagram.
The new constituent is called bainite, which consists of ferrite laths with small cementite particles (also called precipitates). An example of the microstructure of bainite is shown in Fig. 8. This form of bainite is called upper bainite because it is formed in the upper portion below the nose of the IT diagram (between about 540 and 400∘C). Lower bainite, a finer ferrite–carbide microstructure, forms at lower temperatures (between 400 and about 250∘C). Bainite is an important constituent in tough, high-strength, low-alloy steel. If specimens are quenched into a salt bath at 200∘C, a new constituent called martensite will form.
The start of the martensitic transformation is shown in Fig. 7 as Ms (at 220∘C). Martensite is a form of ferrite that is supersaturated with carbon. In other words, because of the very fast cooling rate, the carbon atoms do not have time to diffuse from their interstitial positions in the bcc lattice to form cementite particles. An example of martensite. Steel products produced with an as-quenched martensitic microstructure are very hard and brittle, e.g., a razor blade. Most martensitic products are tempered by heating to temperatures between about 350 and 650∘C. The tempering process allows some of the carbon to diffuse and form as a carbide phase from the supersaturated iron lattice. This softens the steel and provides some ductility. The degree of softening is determined by the tempering temperature and the time at the tempering temperature. The higher the temperature and the longer the time, the softer the steel. Most steels with martensite are used in the quenched and tempered condition.
Continuous-Cooling Transformation Diagram. The other more useful form of a time temperature transformation diagram is the continuous-cooling transformation (CT) diagram.
This differs from the IT diagram in that it is constructed by cooling small specimens at various cooling rates and measuring the temperatures at which transformations start and finish using a device called a dilatometer (a machine that measures dilation). Each phase transformation undergoes a distinct volume change (positive on cooling and negative on heating) that can be measured by a sensitive length measuring device in the dilatometer. A CT diagram has similar features to the IT diagram shown in Fig. 7 but is produced by continuous cooling rather than isothermal conditions. A continuous-cooling diagram is applicable for most industrial processes and should be used in lieu of an IT diagram. A CT diagram can also be constructed by quenching one end of a Jominy bar described below.
Hardenability Concept. In thick products, e.g., large-diameter bars, thick plates, and heavy forgings, the through-thickness properties are achieved through hardenability. Hardenability is the ability to induce depth of hardness in a steel product. The hardness level is obtained by controlling the amount of martensite in the microstructure. To increase the depth of hardness, certain alloying elements are added to the steel for increased hardenability. Elements, such as nickel, chromium, and molybdenum, shift the pearlite nose of the IT and CT diagrams to the right (longer times). With the nose out of the way on cooling, martensite can be formed over a wider range of cooling rates when compared with a steel without alloying elements.
There is a fairly simple test to measure the hardenability of steel called the Jominy test. A 25.4-mm-diameter and 102-mm-long bar is austenitized to 845∘C for 1 h and then water is quenched at one end of the bar. The quenching takes place in a specially designed fixture where the bar is suspended in the vertical position and water is directed against the machined bottom end face of the bar. After quenching, parallel flats 0.38 mm deep are machined on opposite sides of the bar. Hardness is measured at 1.6-mm (1 16 -in.) intervals from the quenched end.
The hardness is plotted against depth from the quenched end to produce a hardenability curve or band. A hardenability band for medium-carbon Society of Automotive Engineers/American Iron and Steel Institute (SAE/AISI) 1045 steel. The two curves that form the band represent the maximum and minimum hardness values from many Jominy tests. To illustrate the concept of hardenability, compare the hardenability band for SAE/AISI 1045 steel to low-alloy SAE/AISI 4145 steel in Fig. 10b. These steels are similar except that the low-alloy steel has chromium and molybdenum additions.