Stainless steel in air, water, acid, salt aqueous solution and other oxidizing atmosphere has high chemical stability, with its good corrosion resistance, high temperature resistance, low temperature resistance, wear resistance and beautiful appearance and other characteristics, is widely used in aerospace, atomic energy, petrochemical, Marine development and household and other aspects.
According to the alloy composition, it can be roughly divided into Cr steel and Cr-Ni steel. According to metallographic structure, it can be divided into five series: austenite type, ferrite type, austenite – ferrite iron type (biphase type), Martensite type, precipitate hardening type. In GB1220-92 stainless steel bar, 64 grades are listed, 33 austenitic type, such as 1Cr18Ni9 (ASTM, JIS302), 7 ferritic type, such as 0Cr13Al (ASTM, JIS405), 3 duplex type, Such as 0Cr26Ni5Mo2 (American ASTM, JIS329), Martensite 18, such as 2Cr13 (American ASTM, JIS420), precipitation hardening type 3, such as 0Cr17Ni4Cu4Nb (American ASTM, JIS630), etc.
In 2006 the world stainless steel output 28.4 million t, China’s stainless steel output jumped to the first in the world, has reached 5.3 million T, including 300 series 3.004 million T, 400 series 1.185 million T, 200 series 811,000 T. In 2007, the world stainless steel production was 27.6 million tons, down 2.9% year on year, and China’s stainless steel production was 7.2 million tons, up 36% year on year .
1) Austenitic stainless steel. In order to enlarge the γ region, the α-Fe stainless steel was completely eliminated by adding Ni, Mn and other elements of face-centered cubic lattice to fe-Cr alloy. This kind of steel is non-magnetic and has good corrosion resistance due to its high Ni and Cr content. Since there is no phase change during heat treatment, it cannot be hardened, but it can be hardened by cold working. Partial substitution of Ni with Mn or N is an effective way to reduce the cost of such steels. This kind of steel is often called 18-8 steel (generally 18% Cr-8% Ni, C≤ 0.1%), the United States, Japan steel grade grade is 300 series, typical steel grade is 304, high yield, wide use, has good processing performance, mechanical properties, corrosion resistance and welding performance. Austenitic stainless steel also includes nickel section type 200 series steel grades. Figure 5 shows the development process of Cr-Ni austenitic stainless steel .
2) ferritic stainless steel. It is composed of α-Fe with the structure of body-centered cubic lattice, and the content of Cr varies from 12% to 30%. Due to the absence of phase transition, this steel cannot be strengthened by heat treatment, but can be cold worked and annealed. Ferritic stainless steel is resistant to chloride high stress crack corrosion and has high plastic embrittlement transition temperature. Many steel grades have been developed by adding some other elements, making them more widely used. American and Japanese steel grade is 400 series. Figure 6 shows the development process of ferritic stainless steel .
3) Martensitic stainless steel, add C to fe-Cr alloy to expand the γ zone, like carbon steel to make the austenite into martensitic, its Cr content of 13% ~ 18%, C content of 0.1% ~ 1%. This kind of steel can be strengthened and improved hardness through heat treatment, but its corrosion resistance is not as good as ferrite and austenitic stainless steel, is widely used in cutting tools, brake pads, bearings, turbine blades, structural parts and wear-resistant appliances. American and Japanese steel grade is 400 series.
4) austenitic ferrite stainless steel, composed of austenite and ferrite two-phase organization, that is, in stainless steel, there are two opposite forces at the same time: ferrite forming elements continue to form ferrite, austenite forming elements continue to form austenite. The final crystal structure depends on the relative amounts of the two types of additive elements. Cr is a ferrite forming element, so there is a competitive relationship between Cr and austenite forming element in the formation of stainless steel crystal structure. Because Fe and Cr are ferritic forming elements, 400 series stainless steels are fully ferritic and magnetic. In the process of adding austenite forming element Ni to Fe-Cr stainless steel, with the increase of Ni composition, the formation of austenite will gradually increase, until all the ferrite structure is transformed into austenite structure, so the formation of 300 series stainless steel. If only half the amount of Ni is added, it forms 50% ferrite and 50% austenite, a structure known as duplex stainless steel. It has the advantages of both austenitic and ferritic steels, characterized by good corrosion resistance, acid resistance and high strength.
5) Precipitate hardened stainless steel, composition and austenite shape similar, but the content of Ni is low and added a small amount of Al, Ti, Cu and other elements. Although austenitic stainless steel has good corrosion resistance, it can not be strengthened by heat treatment, while martensitic stainless steel has a high strength, but the ability to resist environmental erosion is weak, so the iron base corrosion resistance alloy using aluminum base, magnesium base and nickel base alloy effective precipitation hardening or aging hardening method; Due to the need for strength, C was added to the steel, and the alloy with low carbon martensite as matrix was developed and its strength was enhanced by precipitation hardening. The role of a small amount of alloying elements is to have the ability of aging strengthening during heat treatment, and can distribute the dispersion strengthening phase in the austenite matrix to form a high strength series of steel grades, which can be used as springs, gaskets, shafts, steam turbine parts, high strength vessels and other parts requiring high strength and good elasticity. American and Japanese steel grade is 600 series. The production process
There are three methods for stainless steel smelting: electric furnace one-step smelting method, rarely used at present; Using electric furnace or converter (mainly electric furnace) and AOD (or VOD, etc.) two-step smelting method (secondary refining), this method is commonly used at present; A three-step process, i.e., eAF + bof combined blowing (or AOD) +VOD, is also used for stainless steel smelting. The comparison of AOD and VOD refined stainless steels is shown in Table 2. Stainless steel continuous casting, the foam ceramic filter technology (for tundish weir dam), tundish heating technology, no oxidation pouring technology, mould liquid level height automatic control technology, the electromagnetic stirring technology (mold electromagnetic stirring in the M – EMS, secondary zone electromagnetic stirring S – EMS, solidification end electromagnetic stirring F – EMS) as well as thin slab continuous casting technology. Hot rolling strip mainly adopts steckel mill, planetary mill, hot rolling mill and so on. Cold rolling strip mainly uses 4 high cold rolling machine, multi – high cold rolling machine (such as ZR20 Senjimil mill), cold rolling mill, etc. Rod and wire rolling generally use no twist rolling mill, continuous annealing, pickling and so on.
The combined bof and RHOB refining furnace, namely LD-RHOB process, is divided into five steps (see Figure 7) : KR desulfurization treatment, bof melting, steel extraction and slag discharge, molten steel back to bof melting, vacuum oxygen blowing decarburization and deoxidation.