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Practical knowledge of stainless steel

From AA09 Industrial Design Network

Stainless steel is steel that does not rust easily. In fact, some stainless steels have both stainless steel and acid resistance (corrosion resistance). Stainless steel's stainless and corrosion resistance is due to the formation of chromium-rich oxide film (passivation film) on its surface. This stainless property and corrosion resistance are relative. The results show that the corrosion resistance of steel increases with the increase of chromium content in weak medium such as atmosphere and water and oxidizing medium such as nitric acid. When the chromium content reaches a certain percentage, the corrosion resistance of steel changes abruptly, that is, from easy to difficult to rust, from never corrosion resistance to corrosion resistance. There are many ways to classify stainless steel. According to the microstructure classification at room temperature, there are martensitic, austenitic, ferritic and duplex stainless steels; According to the main chemical composition classification, it can be basically divided into two systems:chromium stainless steel and chromium nickel stainless steel; According to the application, there are nitric acid-resistant stainless steel, sulfuric acid-resistant stainless steel, seawater-resistant stainless steel, etc. According to the corrosion-resistant type, it can be divided into pitting corrosion-resistant stainless steel, stress corrosion-resistant stainless steel, intergranular corrosion-resistant stainless steel, etc. According to the functional characteristics, it can be divided into non-magnetic stainless steel, free-cutting stainless steel, low-temperature stainless steel, high-strength stainless steel and so on. Stainless steel has been widely used in heavy industry, light industry, daily necessities industry and architectural decoration industry because of its excellent corrosion resistance, formability, compatibility and strength and toughness in a wide temperature range.
I. Hot rolled stainless steel plate

Hot rolled stainless steel plate is a stainless steel plate produced by hot rolling process. Thin plates with thickness not more than 3mm are thin plates, and thick plates with thickness greater than 3mm are used for manufacturing corrosion-resistant parts, containers and equipment in chemical, petroleum, machinery, shipbuilding and other industries. Their classification and designation are as follows:
1. Austenitic section steel
(1) 1Cr17Mn6Ni15N;
(2) 1Cr18Mn8Ni5N;
(3) 1Cr18Ni9;
(4) 1Cr18Ni9Si3;
(5) 0Cr18Ni9;
(6) 00Cr19Ni10;
(7) 0Cr19Ni9N;
(8) 0Cr19Ni10NbN;
(9) 00Cr18Ni10N;
(10) 1Cr18Ni12;
(11) 0Cr23Ni13;
(12) 0Cr25Ni20;
(13) 0Cr17Ni12Mo2;
(14) 00Cr17Ni14Mo2;
(15) 0Cr17Ni12Mo2N;
(16) 00Cr17Ni13Mo2N;
(17) 1Cr18Ni12Mo2Ti;
(18) 0Cr18Ni12Mo2Ti;
(19) 1Cr18Ni12Mo3Ti;
(20) 0Cr18Ni12Mo3Ti;
(21) 0Cr18Ni12Mo2Cu2;
(22) 00Cr18Ni14Mo2Cu2;
(23) 0Cr19Ni13Mo3;
(24) 00Cr19Ni13Mo3;
(25) 0Cr18Ni16Mo5;
(26) 1Cr18Ni9Ti;
(27) 0Cr18Ni10Ti;
(28) 0Cr18Ni11Nb;
(29) 0Cr18Ni13Si4;
2. Austenitic-ferritic section steel
(30) 0Cr26Ni5Mo2;
(31) 00Cr18Ni5Mo3Si2;
3. Ferrite section steel
(32) 0Cr13Al;
(33) 00Cr12;
(34) 1Cr15;
(35) 1Cr17;
(36) 1Cr17Mo;
(37) 00Cr17Mo;
(38) 00Cr18Mo2;
(39) 00Cr30Mo2;
(40) 00Cr27Mo
4. Martensitic section steel
(41) 1Cr12;
(42) 0Cr13;
(43) 1Cr13;
(44) 2Cr13;
(45) 3Cr13;
(46) 4Cr13;
(47) 3Cr16;
(48) 7Cr17
5. Precipitation hardening section steel
(49) 0Cr17Ni7Al

II. Cold rolled stainless steel plate
Cold rolled stainless steel plate is a stainless steel plate produced by cold rolling process. Thickness is not more than 3mm for thin plate, and thickness is greater than 3mm for thick plate. Used for making corrosion-resistant parts, petroleum and chemical pipelines, containers, medical instruments, marine equipment, etc., their classification and brands are as follows:
1. Austenitic section steel
In addition to the same as the hot rolled part (29 kinds), there are:
(50) 2Cr13Mn9Ni4;
(51) 1Cr17Ni7;
(52) 1Cr17Ni8
2. Austenitic-ferritic section steel
In addition to the same as the hot rolled part (2 kinds), there are:
(53) 1Cr18Ni11Si4AlTi;
(54) 1Cr21Ni5Ti
3. Ferrite section steel
In addition to the same as the hot rolled part (9 kinds), there are:
(55) 00Cr17
4. Martensitic section steel
In addition to the same as the hot rolled part (8 kinds), there are:
(56) 1Cr17Ni2
5. Precipitation hardening section steel:the same as hot rolled section

III. Introduction of Ferrite, Austenite and Martensite
As we all know, solid metals and alloys are crystals, that is, atoms in them are arranged according to certain laws. There are generally three ways of arrangement:body-centered cubic lattice structure, face-centered cubic lattice structure and close-packed hexagonal lattice structure. Metals are composed of polycrystals, and their polycrystalline structure is formed during the crystallization of metals. Fe-C alloys are composed of two kinds of iron lattice structures:a-iron with body-centered cubic lattice structure below 910 ℃ and y-iron with face-centered cubic lattice structure above 910 ℃. If carbon atoms are squeezed into the lattice of iron without destroying iron

has a lattice structure, such a substance is called
Solid solution. The solid solution formed by dissolving carbon into α-iron is called ferrite, which has extremely low carbon dissolving ability and the maximum solubility does not exceed 0.02%. The solid solution formed by dissolving carbon into γ-iron is called austenite, and its carbon dissolving ability is relatively high, up to 2%. Austenite is the high temperature phase of iron-carbon alloy. Austenite formed in steel at high temperature becomes unstable undercooled austenite when it is undercooled below 727 ℃. If it is supercooled to below 230 ℃ at a great cooling rate, the carbon atoms in austenite have no possibility of diffusion, and austenite will directly transform into a supersaturated α solid solution containing carbon, which is called martensite. Due to the supersaturation of carbon content, the strength and hardness of martensite increase, the plasticity decreases and the brittleness increases. The corrosion resistance of stainless steel mainly comes from chromium. Experiments show that the corrosion resistance of steel will be greatly improved only when the chromium content exceeds 12%, so the chromium content in stainless steel is generally not less than 12%. Because of the increase of chromium content, it also has a great influence on the microstructure of steel. When chromium content is high and carbon content is small, chromium will make iron-carbon balance, and the γ phase area on the diagram will shrink or even disappear. This stainless steel has ferritic microstructure and does not undergo phase transformation when heated, so it is called ferritic stainless steel. When the chromium content is low (but higher than 12%) and the carbon content is high, the alloy is easy to form martensite when cooled from high temperature, so this kind of steel is called martensitic stainless steel. Nickel can expand the gamma phase region and make steel have austenite structure. A steel is called austenitic stainless steel if the nickel content is sufficient so that the steel also has an austenitic structure at room temperature.

IV. Application of stainless steel in various fields

1. During the 40 years from 1960 to 1999, the output of stainless steel in western countries increased sharply from 2.15 million tons to 17.28 million tons, an increase of about 8 times, with an average annual growth rate of about 5.5%. Stainless steel is mainly used in kitchen, household appliances, transportation, construction and civil engineering. In kitchen appliances, there are mainly water washing tanks and electrical and gas water heaters, and household appliances mainly include rollers of automatic washing machines. From the point of view of environmental protection such as energy saving and recycling, the demand for stainless steel is expected to further expand. In the field of transportation, there are mainly exhaust systems of railway vehicles and automobiles. The stainless steel used for exhaust systems is about 20-30kg in each vehicle, and the annual demand in the world is about 1 million tons, which is the largest application field of stainless steel. In the construction sector, there has been a recent surge in demand, such as the use of about 5000 tons of stainless steel exterior decorating materials for the protective devices of Singapore subway stations. Another example is Japan after 1980, the stainless steel used in the construction industry increased about 4 times, mainly used as roofs, interior and exterior decoration of buildings and structural materials. In 1980s, 304 type unpainted materials were used as roof materials in coastal areas of Japan, and gradually changed to painted stainless steel from rust prevention. In 1990s, ferritic stainless steel with high corrosion resistance of over 20% and high Cr was developed, which was used as roof material. At the same time, various surface finishing technologies were developed for aesthetics. In the field of civil engineering, stainless steel is used for dam absorption towers in Japan. In cold areas of Europe and America, salt is needed to prevent freezing of expressways and bridges, which accelerates the corrosion of steel bars, so stainless steel bars are used. In North American roads, about 40 stainless steel bars have been used in recent 3 years, and the usage amount of each place is 200-1000 tons. In the future, stainless steel will make a difference in this field market.
2. The key to expand the application of stainless steel in the future is environmental protection, long life and the popularization of IT.
With regard to environmental protection, first of all, from the viewpoint of atmospheric environmental protection, the demand for heat-resistant and high-temperature corrosion-resistant stainless steel for high-temperature waste incineration plants, LNG power generation plants and high-efficiency power generation plants using coal will expand. It is also estimated that the battery cases of fuel cell vehicles that will be put into practical use in the early 21st century will also use stainless steel. From the viewpoint of water quality and environmental protection, stainless steel with excellent corrosion resistance will also expand its demand in water supply and drainage treatment devices. With regard to long life, the use of stainless steel in existing bridges, highways, tunnels and other facilities in Europe is increasing, and this trend is expected to spread all over the world. In addition, the life span of general residential buildings in Japan is particularly short, which is 20-30 years, and waste treatment has become a major problem. Recently, buildings with the goal of 100-year life have begun to appear, so the demand for materials with excellent durability will increase. From the point of view of earth environmental protection, it is necessary to discuss how to reduce maintenance cost from the design stage of introducing new concepts while reducing civil and building waste materials for long life. With regard to the popularization of IT, in the process of IT development and popularization, functional materials play a great role in equipment hardware, and the requirements for high-precision and high-functional materials are very great. For example, in mobile phones and microcomputer components, the high strength, elasticity and non-magnetism of stainless steel are flexibly applied, which makes the application of stainless steel expand. In addition, stainless steel with good cleanliness and durability plays an important role in the manufacturing equipment of semiconductors and various substrates. Stainless steel has many excellent properties that other metals do not have. It is a material with excellent durability and recycling. In the future, corresponding to the changes of the times, stainless steel will be widely used in various fields.

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