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Industrial and Metallurgical Applications

Manganese and Steelmaking

Steel is basically an alloy of iron and carbon, consisting of an iron phase and iron carbides. Crude steel produced from iron contains an undesirable amount of oxygen and some sulphur. Manganese plays a key role because of two important properties : its ability to combine with sulphur and its powerful deoxidation capacity. When there is insufficient manganese the sulphur combines with iron to form a low melting point sulphide, which melts at hot rolling temperatures, causing a surface cracking phenomenon known as “hot shortness”. Desulphurisation processes reduce the need for manganese in this respect. Some 30% of the manganese used today is still used for its properties as a sulphide former and deoxidant.

The other 70% of the manganese is used purely as an alloying element. These alloying uses depend on the desired properties of the steel being made. Steel, as has been noted, contains iron and carbon. At room temperature, iron crystallises into a body-centred cubic structure named alpha iron (ferrite). At a high temperature (above 910 degrees C), the structure is transformed into a face-centred cubic form, which is called a gamma iron (austenite). When the steel is cooled down slowly, the carbon, soluble in austenite, precipitates as an iron carbide called cementite, the austenite transforms to ferrite and they precipitate together in a characteristic lamellar structure known as pearlite.

Manganese plays an important role as it lowers the temperature at which austenite transforms into ferrite, thus avoiding cementite precipitation at ferrite grain boundaries, and by refining the resulting pearlitic structures. The strength and toughness of steel depend, first of all, on the grain size and the volume fraction of pearlite contained. Alloying elements, including manganese, also contribute some solution-hardening of the ferrite, but this effect is limited compared to that of carbon, nitrogen, phosphorus and even silicon. When the cooling process is accelerated by quenching, austenite transforms into structures with high strength such as bainite and martensite.

Manganese improves the response of steel to quenching by its effect on the transformation temperature. Manganese is also a weak carbide former. Both properties are advantageous in heat-treated steels specified by mechanical engineers. Another important property of manganese is its ability to stabilize the austenite in steel, as does nickel. Since manganese is not as powerful as nickel in its ability to stabilize austenite, more manganese is required to achieve the same effect. However, manganese has the advantage of being much less expensive. The effect of manganese in forming austenite can be reinforced by combining it with nitrogen, which is also an austenite-forming element. Manganese also increases hardenability rate, used to significant advantage, depending on the steel type and the end product, to improve mechanical properties.

Manganese Content in Steel Today

The bulk of steel production results in multi-purpose low carbon steels containing from 0.15% to 0.8% manganese. A large proportion consists of low carbon steel sheets with less than 0.3% manganese, some with even less than 0.2% for extra deep drawing qualities. High strength steels with a yield strength over 500MPa, and representing 3 to 4% of the tonnage of steel produced, contain over 1% manganese. A large number of these are high strength low alloy steels (HSLA). They are low carbon, controlled-rolled steels containing higher manganese levels (1.0% to 1.8%), taking advantage of its beneficial effect on the austenitic transformation temperature to obtain a very fine ferrite structure. Micro-alloying additions help to refine the structure or to strengthen the steel through carbide or nitride precipitates which are evenly distributed in the ferrite matrix. These steels are widely used for oil/gas pipelines, shipbuilding and in transportation equipment in order to reduce weight.

Engineering steels comprise either HSLA or heat-treated grades ; either pure chromium-manganese grades, or with nickel, chromium, molybdenum, vanadium additives and often 0.6 to 0.8% manganese. A few grades containing 1.0% to 1.5% manganese (as well as chromium or boron) are popular with the automobile industry.

Stainless steels which represent less than 2% of total world steel production make use of chromium and nickel. They also contain about 1% manganese. There are also manganese-stainless steels, where nickel is replaced partly or entirely by manganese, giving a manganese content of 4 to 16%. These are not yet produced in large quantities, but they could develop in the future depending on the evolution of the nickel price compared with manganese and on the marketing effort devoted to them. Large amounts were produced in India in the 1980’s.

Any overview of high Mn steels must include Hadfield steel, named after its 19th century UK inventor and the first alloy steel ever invented. This steel contains 13% or more manganese. It has unique properties which make it indispensable for applications in which great toughness and wear resistance are required. Among these can be cited gyratory crushers, jaw-crusher plates, railway points and crossover components, teeth for earth-moving equipment, etc. High manganese (10-12%) non-magnetic steels are used for such products as retainer rings for turbo alternators and collars on oil rigs. Grades with a similar chemistry are used as cryogenic steels. A high manganese stainless "memory" steel has been developed.