2.3 Ferro Alloys for stainless steelMetalist
Among high-alloyed steels, stainless steels are a special case because of their large production amount and central position as users of ferrochromium.
Furthermore, other ferroalloys are also used extensively for stainless steel production. The history of stainless steel is quite short, as the effect of chromium on the improved corrosion resistance of iron was only recognized in the early 1800s. However, at that time it was not yet possible to produce chromium or chromium ferroalloy had to be produced industrially. Reducing chromium from chromite ore (FeO-Cr2O3) is difficult and needs high temperatures ~1700C for efficient reduction and liquid slag formation during the process.
High temperature also sets strict requirements for a furnace and its lining material. The first trials to produce “stainless steels” were performed soon after 1900. The “stainless era” is usually said to have begun in 1912 when austenitic Cr-Ni stainless steel was patented by the company Krupp, followed by ferritic and martensitic stainless steels created by other inventors (Tylecote, 1984). Acid resistant Mo-containing stainless steels were introduced later in the 1920s. In the following decade, fundamentals of main types of stainless steels and their properties were duly examined, but production grew slowly because of the obstacle of a too-high carbon content.
Today it is evident that in most stainless steels, carbon content should be very low (typically below 0.05%) to avoid the formation of carbides, which impair corrosion resistance and cause the “sensitization” phenomenon (a type of grain boundary corrosion). In the steelmaking process, the carbon problem is linked to chromium: when carbon is oxidized for removal, Cr also starts to oxidize. This meant that high-carbon FeCr could not be used for stainless steel without drastic chromium losses during steelmaking. A lot of chromium should be added after decarburization by using low-carbon FeCr, which is more expensive than a high-carbon grade.
Low-carbon FeCr should also have a higher Cr:Fe ratio to minimize the amount of alloying required, but so-called charge FeCr typically had rather low Cr:Fe (containing only 50% to 55% Cr). Therefore, to produce low-carbon FeCr, ores with a high chromium content are preferred. Special processes were developed to decarburize FeCr for example. single or double treatment with slag to make “affine” (<4% C) and “sur-affine” (<0.5% C) FeCr grades. The stainless steel production exceeded 1 Mt/year in 1950. Stainless steel production received a boost with the invention of the argon-oxygen decarburization (AOD) process, taken into operation in the 1960s. It made possible the use of high-carbon FeCr and to decarburize the melt by diluting oxygen by Ar gas and thus lowering the partial pressure of CO, which is formed (Krivsky, 1973).
In 1976, the world production of stainless steels exceeded 5 MT, reaching 10 MT in 1988, 20 MT in 2002, and 30 MT in 2010 (based on data from www.worldstainless.org, 2012). Stainless steels can be divided in several groups depending on their alloying and structure. Austenitic stainless steels (ASTM classification 300 series) form the biggest group, accounting for 55% to 60% of all stainless. A classic example is 18/8 steel with 18% Cr and 8% Ni. If better corrosion resistance, especially against pit corrosion, is required, the nickel content is raised to 11% to 14% and molybdenum is added (2% to 4%). The carbon maximum is normally 0.03% C (“L” grades), but in high temperature grades may be typically up to 0.08% C (“H” grades).
The second major group is represented by ferritic stainless steels, some 30% of all the stainless (400 series). Here, chromium content is from 10.5% to 30%, and it can contain some Mo and even Ni. Carbon content is limited, like in austenitic steels. A relatively new group of stainless steels is “duplex stainless steels,” which have approximately same fraction of austenite and ferrite in their structure. An example analysis is 21% to 23% Cr, 4.5% to 6.5% Ni, 2.5% to 3.5% Mo, 2% Mn, and Cmax 0.03%. Duplex steels have better mechanical properties because of their structure.
Martensitic stainless steels belong to ferritic steels but have a higher carbon content (0.1 to 1% C), which makes them harden-able when quenching after heat treatment. Corrosion resistance is not as good as it is in corresponding low-carbon grades but good mechanical properties make these steels applicable in different tools, dental and surgical instruments, cutlery, and so on. A typical martensitic stainless steel has 12% to 14% Cr, <2% Ni, 0.2% to 1% Mo, 1% Mn, 1% Si, and 0.3% C. Because of the high contents of alloying elements in stainless steels, their price is sensitive to changes in alloying costs. When nickel prices climbed up, increased attention was paid to finding substitutes for nickel in austenitic 300 series stainless steels. A common austenite stabilizer is manganese.
In the 200 series stainless steels, Cr content is kept to 16% to 19%, but Ni content is decreased to around 5% (in some grades only 1% to 2% Ni) and Mn content is raised to 5% or 10% Mn. Also copper (Cu) and nitrogen (N) can partly substitute for nickel. Significant production of these Cr-Mn-Ni stainless steels, known since the 1930s, began in the United States in the 1950s (ASM Handbook, 1990). After the year 2000, production of the 200 series steels increased, especially in China and India, and exceeded 10% of the total stainless steel production in 2005; it is currently on the level of 12% to 14%.