Machinery's Handbook, 31st Edition
Standard Steels
393
STANDARD STEELS Properties, Compositions, and Applications
Steel is the generic term for a large family of iron-carbon alloys that are malleable, within some temperature range, immediately after solidification from the molten state. The principal raw materials used in steelmaking are iron ore, coal, and limestone. These materials are converted in a blast furnace into a product known as “pig iron,” which con tains considerable amounts of carbon, manganese, sulfur, phosphorus, and silicon. Pig iron is hard, brittle, and unsuitable for direct processing into wrought forms. Steelmaking is the process of refining pig iron as well as iron and steel scrap by removing undesirable elements from the melt and then adding desirable elements in predetermined amounts. A primary reaction in most steelmaking is the combination of carbon with oxygen to form a gas. If dissolved oxygen is not removed from the melt prior to or during pouring, the gas - eous products continue to evolve during solidification. If the steel is strongly deoxidized by the addition of deoxidizing elements, no gas is evolved, and the steel is called “killed” because it lies quietly in the molds. Increasing degrees of gas evolution (decreased deox - idation) characterize steels called “semikilled,” “capped,” or “rimmed.” The degree of deoxidation affects some of the properties of the steel. In addition to oxygen, liquid steel contains measurable amounts of dissolved hydrogen and nitrogen. For some critical steel applications, special deoxidation practices as well as vacuum treatments may be used to reduce and control dissolved gases. By definition, steel is an iron-carbon alloy that contains less than 2 percent carbon and 1 percent manganese, which acts as a deoxidizer and facilitates hot-working. Silicon, phosphorus, and sulfur are also always present, if only in trace amounts. Other elements may be present, either as residuals that are not intentionally added but result from the raw materials or steelmaking practice, or as alloying elements added to effect changes in the properties of the steel. Steels can be cast to shape, or the cast ingot or strand can be reheated and hot-worked by rolling, forging, extrusion, or other processes into a wrought mill shape. Wrought steels are the most widely used of engineering materials, offering a multitude of forms, finishes, strengths, and usable temperature ranges. Standard Steel Classification.— Wrought steels may be classified into groups based on characteristics such as elemental composition and metallurgical structure; manufactur- ing, deoxidation, and finishing processes; product form; and so on. Chemical composi - tion is commonly used as a basis for identifying and assigning standard designations to wrought steels. Although carbon is the principal hardening and strengthening element in steel, no single element controls the steel’s characteristics. The combined effect of several elements influences microstructure, hardness, strength, response to heat treatment, form - ability, and corrosion resistance. Standard steels can be divided by composition into three main groups: plain carbon steels (mild steels), which have no appreciable alloying element content; low-alloy steels, containing up to 8 percent alloying elements; and high-alloy steels with more than 8 per- cent alloys. The general term alloy steel also may be used for any grade to which are added any amount of alloying elements other than what is found in plain carbon steel, within specific ranges, to enhance certain attributes of steels. Thermal treatments and other pro - cesses also are used to develop specific properties. Carbon Steels (Mild Steels): A steel qualifies as a carbon steel when its manganese content is limited to 1.65 percent (max), silicon to 0.60 percent (max), and copper to 0.60 percent (max). With the exception of deoxidizers and boron when specified, no other alloying elements are added intentionally, but they may be present as residuals. If any of these incidental elements are considered detrimental for special applications, maximum acceptable limits may be specified. Carbon steels are further divided into low-carbon (less than 0.2% carbon), medium-carbon (0.2% to 0.5%), and high-carbon (great than 0.5%) categories. In contrast to most alloy steels, carbon steels are most often used without
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