Carbon Steels & Special Low-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 with-
out manganese present. Manganese sulfides are usually present in plain and low-alloy 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 machin-
ability 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 Hand-
book, 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 as-rolled 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 are greater for steel in the normalized condition. This is because normalizing develops
a finer ferrite grain size. Yield and tensile strength are 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.

Special Low-Carbon Steels

These are the steels that are not classified in the aforementioned SAE table or listed in the afore-
mentioned ASM table. As mentioned earlier, carbon is not always beneficial in steels.

These are special steels with carbon contents below the lower level of the SAE/AISI 10xx steels. There
are a number of steels that are produced with very low carbon levels (less than 0.002% C), and
all the remaining free carbon in the steel is tied up as carbides. These steels are known as IF
steels, which means that the interstitial elements of carbon and nitrogen are no longer present
in elemental form in the iron lattice but are combined with elements such as titanium or niobium as carbides and nitrides (carbonitrides). Interstitial-free steels are required for exceptional
formability, especially in applications requiring deep drawability. Drawability is a property that
allows the steel to be uniformly stretched (or drawn) in thickness in a closed die without local-
ized thinning and necking (cracking or breaking). An example of a deep-drawn part would be a
compressor housing for a refrigerator. With proper heat treatment, IF steels develop a preferred
crystallographic orientation that favors a high plastic anisotropy ratio, or r value. High-r-value
steels have excellent deep-drawing ability and these steels can form difficult parts. Another
type of low-carbon steel is a special class called DQSK steel. This type of aluminum-treated
steel also has a preferred orientation and high r value. The preferred orientation is produced by
hot rolling the steel on a hot strip mill followed by rapid cooling. The rapid cooling keeps the
aluminum and interstitial elements from forming aluminum nitride particles (i.e., the Al and N
atoms are in solid solution in the iron lattice). After rolling, the steel is annealed to allow aluminumnitride to precipitate. The aluminum nitride plays an important role in the development
of the optimum crystallographic texture. The DQSK steel is used in deep-drawing applications
that are not as demanding as those requiring IF steel.
A new family of steels called bake-hardening steels also have a low, but controlled car-
bon content. These steels gain strength during the paint–bake cycle of automotive production.
Controlled amounts of both carbon and nitrogen combine with carbonitride-forming elements
such as titanium and niobium during the baking cycle (generally 175∘C for 30 min). The precipitation of these carbonitrides during the paint–bake cycle strengthens the steel by a process
called aging.
Enameling steel is produced with as little carbon as possible because during the enameling
process carbon in the form of carbides can react with the frit (the particles of glasslike material
that melts to produce the enamel coating) to cause defects in the coating. Thus, steels to be
used for enameling are generally decarburized in a special reducing atmosphere during batch
annealing. In this process, the carbon dissipates from the steel. After decarburization, the sheet
steel is essentially pure iron. Enamel coatings are used for many household appliances such
as washers and dryers, stovetops, ovens, and refrigerators. Also, steel tanks in most hot-water
heaters have a glass (or enameled) inside coating.
Electrical steels and motor lamination steels are also produced with as low a carbon con-
tent as possible. Dissolved carbon and carbides in these steels are avoided because the magnetic
properties are degraded. The carbides, if present in the steel, inhibit the movement of the
magnetic domains and lower the electrical efficiency. These steels are used in applications
employing alternating current (AC) in transformers and electric motors. Most electric motors
for appliances and other applications have sheet steel stacked in layers (called laminations)
that are wound in copper wire. Electrical steels used for transformers contain silicon, which is
added to enhance the development of a specific crystallographic orientation that favors electrical efficiency.