Carbon Steels

Carbon steels (also called plain-carbon steels) constitute a family of iron–carbon–manganese alloys. In the SAE/AISI system, the carbon steels are classified as follows:

Nonresulfurized carbon steels 10xx series Resulfurized steels 11xx series Rephosphorized and resulfurized steels 12xx series High-manganese carbon steels 15xx series A four-digit SAE/AISI number is used to classify the carbon steels with the first two digits being the series code and the last two digits being the nominal carbon content in points of carbon (1 point  0.01% C). For example, SAE/AISI 1020 steel is a carbon steel containing 0.20% C (actually 0.18–0.22% C). The chemical composition limits for the above SAE/

AISI 10xx series of carbon steels for semifinished products, forgings, hot- and cold-finished bars, wire, rods, and tubing are listed in SAE Materials Standards Manual (SAE HS-30, 1996). There are slight compositional variations for structural shapes, plates, strip, sheet, and welded tubing (see SAE specification J403). The SAE Manual gives the SAE/AISI number along with the UNS number. The carbon level spans the range from under 0.06% C to 1.03% C.

Because of the wide range in carbon content, the SAE/AISI 10xx carbon steels are the most commonly used steels in today’s marketplace. All SAE/AISI 10xx series carbon steels contain manganese at levels between 0.25 and 1.00%. For a century, manganese has been an important alloying element in steel because it combines with the impurity sulfur to form manganese sulfide (MnS). MnS is much less detrimental than iron sulfide (FeS), which would form without manganese present. Manganese sulfides are usually present in plain and lowalloy steels as somewhat innocuous inclusions. The manganese that does not combine with sulfur strengthens the steel. However, with the development of steelmaking practices to produce very low sulfur steel, manganese is becoming less important in this role.

The SAE/AISI 11xx series of resulfurized steels contain between 0.08 and 0.33% sulfur. Although in most steel, sulfur is considered an undesirable impurity and is restricted to less than 0.05%, in the SAE/AISI 11xx and 12xx series of steels, sulfur is added to form excess manganese sulfide inclusions. These are the free-machining steels that have improved machinability over lower sulfur steels due to enhanced chip breaking and lubrication created by the MnS inclusions.

The SAE/AISI 12xx series are also free-machining steels and contain both sulfur (0.16– 0.35%) and phosphorus (0.04–0.12%). The SAE/AISI 15xx series contain higher manganese levels (up to 1.65%) than the SAE/AISI 10xx series of carbon steels. 

Typical mechanical properties of selected SAE/AISI 10xx and 11xx series of carbon steels are listed in the first part of the table on pp. 20–23, Section 4, of the ASM Metals Handbook, Desktop Edition, 1985, for four different processing conditions (as rolled, normalized, annealed, and quenched and tempered). These properties are average properties obtained from many sources, and thus this table should only be used as a guideline. The asrolled condition represents steel before any heat treatment was applied. Many applications

utilize steel in the as-rolled condition. As can be seen from the aforementioned ASM table, yield and tensile strength is greater for steel in the normalized condition. This is because normalizing develops a finer ferrite grain size. Yield and tensile strength is lowest for steels in the annealed condition. This is due to a coarser grain size developed by the slow cooling rate from the annealing temperature. In general, as yield and tensile strength increase, the percent elongation decreases. For example, in the ASM table, annealed SAE/AISI 1080 steel has a tensile strength of 615 MPa and a total elongation of 24.7% compared with the same

steel in the normalized condition, with a tensile strength of 1010 MPa and a total elongation of 10%. This relationship holds for most steel.

READ MORE.......

Bruce L. Bramfitt

International Steel Group, Inc.

Research Laboratories

Bethlehem, Pennsylvania

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

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