Manganese is intentionally present in most grades of steel and is a residual constituent of virtually all others. Aside from its historic importance as a desulfurizer and deoxidizer, manganese is undoubtedly the most prevalent alloying agent in steels, after carbon. Understandably, therefore, ferromanganese is the most widely used ferroalloy: average U.S. annual consumption should exceed 14 lbs. of manganese per ton of steel.
Standard ferromanganese is produced in blast furnaces and, more often, in submerged-arc electric furnaces. In addition, the high-manganese slags resulting from the EF process can be used to make silicomanganese.
The U.S. has considerable manganese ore reserves, but since these are far leaner than those available around the world, they have not been commercially exploited. As a result, all steelmaking manganese products are imported - normally as ferro- and silicomanganese. The leading producers, sharing over 85% of world capacity, are the CIS, the Republic of South Africa, Brazil and China.
Manganese is sold in an extensive variety of product forms. These can be classified into three major groups: ferromanganese, silicomanganese and manganese metal, with several standard or proprietary grades within each group.
Of the 15 steelmaking manganese products recognized in ASTM standards, the most important is high carbon (standard) ferromanganese. It is generally sold in two manganese concentrations: 74-76% and 78-82%, with the latter predominating. Carbon content will not exceed 7.5%, silicon will be less than 1.2% and sulfur and phosphorus, 0.050 and 0.35%, respectively. A low phosphorus variety is available as well. Medium carbon ferromanganese (80-85% Mn) is sold in several grades, varying broadly in silicon content. As usual, the product commands a higher price than the standard high carbon variety. More expensive still is low carbon ferromanganese containing 80-85% Mn or 85-90% Mn, depending on grade. Maximum silicon content here is usually less than 2%, but a product containing 5-7% Si has also been standardized. Low carbon ferromanganese is available with guaranteed maximum carbon contents of 0.070, 0.10, 0.15, 0.30, 0.50 or 0.75%.
Silicomanganese contains 65-68% Mn, <20% Si and 1.5, 2.0 or 3.0% C. Silicon and carbon contents are always inversely proportional. Silicomanganese has a higher deoxidizing power than ferromanganese.
Ferromanganese-silicon (63-66% Mn, 0.08% C, 28-32% Si) can be used as an alternative to low carbon ferromanganese, electrolytic manganese or ferrochrome-silicon (in stainless steel production) as part of the reducing mix. With the importance of ladle steelmaking nowadays, ferromanganese-silicon is used as a low carbon ladle addition, replacing more expensive low carbon ferromanganese and manganese metal. This is particularly the case in HSLA steel production provided, of course, that its relatively high silicon content can be tolerated.
Manganese metal addition agents can be subdivided into purity ranges: Electrolytic manganese (99.90% Mn, min.) is the purest commercial form and is usually reserved for stainless steels and nonferrous alloys. It is available in regular, intermediate and low H2 grades, as well as 4.5 and 6% N2 products.
A product often referred to as high density manganese contains 96 or 97% Mn, depending on grade. Iron, the principal impurity, varies accordingly. High density manganese is also sold in a nitrided form (5-8% N). Its composition and solubility make it a desirable addition agent for superalloys, stainless steel, non-ferrous alloys and low carbon, nitrogen strengthened HSLA steels.
Most manganese addition agents are sold in lump or crushed form in a series of size ranges from 8 in. x 4 in. to 20 mesh x D. Fines are sometimes briquetted or pelletized. The full series of available addition agents are tabulated in the appendix. Standard ferromanganese bears an ASTM friability rating of 4 (scale 1-6): there will be an "appreciable reduction in size of large pieces upon repeated handling", to quote the ASTM definition. Manganese addition agents (except high-silicon products) also have a tendency to oxidize in air, forming a dark surface film. This MnO film is actually beneficial, however, since it provides a wetting action in liquid steel that enhances solubility. It also improves slag fluidity in the furnace and can therefore reduce fluorspar consumption. For this reason, manganese ores can be used as substitutes for fluorspar.
Manganese addition agents can often be used interchangeably, depending on conditions. Because the situation will vary from practice to practice and even heat to heat, it is best to outline a few fundamentals:
Manganese is a mild deoxidizer and desulfurizer. Its major uses in steelmaking over the past century relied heavily on these properties, and still do to a large extent. With the advent of hot-metal desulfurization (where called for in the product) and ladle deoxidation (to reduce expensive furnace time, among other reasons) these traditional uses for manganese have declined. They have, incidentally, been supplanted by increased manganese consumption as a true alloying constituent, mainly in HSLA steels. Nonetheless, the efficiency of any manganese addition - no matter when or why added - will depend on the steel's oxygen and sulfur contents. Bath temperature is important not only as it affects the carbon/oxygen balance, but also in relation to chill factors, especially when large manganese additions must be made.
Processing factors are also important: long tap times or extended holding in the furnace or ladle may provide opportunity for reoxidation, for example. Slag composition and the type and condition of refractories must also be considered. The addition of stronger deoxidizers than manganese (e.g. aluminum or silicon) can cause reversion of Mn from slag to metal, just as they do for phosphorus. Finally, cost and availability of addition agents must be weighed against all of the above factors as they apply to the heat process. A fast response chemical analysis system coupled to a computer is obviously helpful, especially given increasingly more stringent quality and productivity requirements, but it must be noted that the skill and experience of individual operators enabled them to solve these complex problems intuitively for many decades.
All other factors considered, however, manganese addition agents are generally chosen on the basis of carbon content. Inexpensive standard ferromanganese is used when the steel is well oxidized (low in carbon) or when higher residual carbon contents are allowable. As maximum steel carbons become more restricted it is necessary to switch to the more costly low-or medium carbon ferroalloys. The final aim chemistry will determine which addition agents can or cannot be used. Silicon content is often a second deciding factor.
In basic electric furnace practice, furnace addition of ferromanganese is usually not needed, assuming sulfur has been adequately controlled by proper charge selection or other means. Silicomanganese can be a cheap and useful reducing agent, but some steels require that aluminum or ferroaluminum be used instead.
Ladle additions will consist of one or more of the standard products depending on the factors listed above. Sizes on the order of 5 in. x 2 in. or 4 in. x 1/2 in. are preferred for their higher solution rates and relative absence of fines. Timing of the additions depends on their function. Deoxidation additions are made early, sometimes along with ferrosilicon. Trim or alloying additions must come later, and standard, medium- or low carbon products may be used. Metallic manganese addition agents are used for premium grades such as stainless steels, specialty alloy and interstitial-free (IF) steels. They are used exclusively as alloying additions since they are too expensive to use as deoxidizers. High density manganese is added to the ladle, the AOD or the degasser. Electrolytic manganese has a tendency to ball up and form floaters when added to the ladle and is therefore better applied as an induction furnace addition during the production of specialty alloys. It can also be added to the AOD or the degasser and, if done in such a way that dissolution proceeds uniformly, floaters will be avoided.
Manganese improves hot workability by preventing the formation of low-melting iron sulfide, FeS. Steels with a Mn/S ratio of at least 8/1 will not exhibit hot shortness. Mn/S, which forms preferentially to FeS, has a high melting point and appears in ingots as discrete and randomly distributed globules. (However, see Sulfur for a discussion of segregation.) Although solid at hot working temperatures, the MnS inclusions are soft enough to deform into elongated stringers during rolling or forging. Their presence may be harmful, beneficial or benign, depending on the product form and its application. Manganese sulfides are necessary in any steel that must be machined since they are effective chip breakers. On the other hand, long manganese sulfide stringers are detrimental to the transverse and through-thickness ductility and impact properties of flat-rolled products. Where these properties are not critical, MnS is essentially harmless.
Manganese may be present in other inclusions, as well. It forms complex and sometimes mutually soluble oxides, sulfides, oxysulfides and silicates with a number of elements. The more important of these are the ones used for inclusion shape control, i.e., calcium, titanium, zirconium and the rare earth metals.
Manganese has a negligible solid solution strengthening effect in austenite and only a moderate effect in ferrite. Mn increases strength and toughness after rolling by lowering the austenite decomposition temperature during cooling to give ferritic grain refinement and a reduction in grain size. Manganese increases the work hardening rate in austenite and actually reduces work hardening (through enhancement of dislocation cross slip) in ferrite, at least at ordinary concentrations. Since manganese increases a steel's resistance to deformation, manganese steels will be stiffer during rolling or forging.
Manganese very strongly retards the transformation of austenite and therefore promotes deep hardening in heat treatable steels. Manganese also lowers the transformation temperature and the eutectoid carbon content. These properties - especially the first two - account for the wide use of manganese in steels in which transformation must be controlled.
Because it is the most cost-effective hardenability intensifier (hardenability factor divided by cost), manganese is present in all standard AISI/SAE heat treatable steels. Up to about 1% Mn is specified in these steels. However, manganese is also important in flat-rolled steels. The lower transformation temperature produced by manganese addition promotes finer grain sizes, either as-rolled or normalized. As grain size is reduced (either ferrite, bainite or pearlite) yield strength increases and impact properties improve. As an added benefit, pearlite content increases with increasing manganese concentration for a given carbon content. This raises strength, without sacrificing weldability. Depending on which carbon equivalent formula is applicable for the steel in question, manganese is only 1/6 to 1/20 as detrimental to weldability as carbon itself.
Very high manganese contents suppress the g - a transformation entirely, and such steels (see below) will be fully austenitic at room temperature. In all cases, manganese lowers the Ms temperature, and high manganese steels will tend to contain residual austenite.
Although it forms a carbide that is similar to cementite, manganese produces no secondary hardening during tempering. Manganese does enhance susceptibility to temper embrittlement when present in excess of 0.30% and care should be taken to avoid the critical temperature range (375-575 C, 700-1070 F) during tempering. Heavy sections should be quenched from the tempering temperature, if it is above this range.
Besides the heat treatable steels already mentioned, manganese is present in a wide variety of steels, for an equally wide variety of reasons. Flat rolled carbon and HSLA steels contain up to 2% Mn for microstructural refinement and resulting improved mechanical properties. Solid solution strengthening is also important here. These steels may contain Mn-V-C, Mn-V-N, Mn-Mo-Cb or other combinations of elements, depending on grade, product form and application.
Manganese may be substituted for part of the nickel content in austenitic stainless steels (200 series). Such steels gained importance during times of critical nickel shortage, such as World War II. They will contain between 5.5 and 10% Mn. One of the earliest, and most interesting, types of alloy steels contains 10 to 14% Mn and 1.0 to 1.4% C. These are the so-called Hadfield steels, which were originally developed before the 1900's. When quenched from above 1000 C (1832 F), they remain fully austenitic at room temperature. Their utility is based on their extremely high work hardening rate, which makes them useful as, for example, railroad frogs and earthmoving and mining equipment components. When the surface of a Hadfield's steel is deformed it becomes very hard, resisting further deformation. The underlying metal, however, remains soft and ductile.
Carburizing steels of the AISI/SAE family contain up to about 1.0% Mn, but considerably less may be present in certain grades. Nitriding steels contain typically 0.55% Mn, but the range within the several commercial grades extends from residual traces to almost 1.0% Mn. Manganese does form a nitride, but its use in these steels is mainly based on heat treatment (hardenability) effects.
Similarly, tool and die steels rely on manganese for deep hardening. This is important when high concentrations of strong carbide formers are present, as these can withdraw carbon from solid solution, thereby reducing hardenability.
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