Summary of basic knowledge of stainless steel materials (part)

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This post was last posted by * nht1 edited on 2017-4-12 13:29 Basics of Stainless Steel 1. Overview With the development of world industry, people urgently need a steel type with special physical properties to meet the further development of industry. This kind of steel needs to be both producible and easy to process. ； It is not easy to rust and has special properties such as acid resistance, heat resistance, no peeling, non-magnetic, etc. In order to create and invent this type of steel, metallurgical producers and metallurgists from all over the world began careful research in the late 19th century. It was not until 1910 that the first batch of high-chromium alloys was produced. In 1913, at the beginning of the twentieth century, the United Kingdom reported that steel with a carbon content of 0.4% had good corrosion resistance when it contained 9 to 16% chromium. Then, in 1914, Germany reported that steel containing chromium and nickel could resist oxidation and acid. ； Canada obtained patents for the production of this type of steel in 1915 and the United States in 1916 ； In 1917, France also reported alloy steel containing 10 to 15% chromium and 20 to 40% nickel. It can be seen that stainless steel is almost at the same time in four or five * * Appear. Therefore, stainless steel is the result of careful research by metallurgical producers and metallurgists from all over the world. For more than half a century, in the major steel-producing * * Among them, stainless steel has exceeded 1% of the total steel output and 8% of the total value. Occupy a certain status in the economy. In our country, the output of stainless steel is less than 0.5% of the total output of steel. Stainless steel in our country and many * * , its production and application have only just begun. Because stainless steel has good physical and chemical properties, its production and application will continue to develop and expand in depth and breadth. In terms of chemical composition, stainless steel is a high-alloy steel based on iron and carbon and contains many alloying elements. Stainless steel is a special steel in terms of physical and chemical properties. After surface treatment (through pickling process) of stainless steel including acid-resistant steel, non-scaling steel, heat-resistant steel and resistance alloy, their common feature is that they can resist corrosion in air and weakly corrosive media - that is, they are not easy to rust. Because these steels are not prone to rust, they are collectively called stainless steel. Stainless steel is not absolutely free from rust and corrosion, but it has less rust and less corrosion. ； The process of rust and corrosion is slow and may not even be noticeable. Therefore, stainless steel only has a relative meaning, not an absolute one. 2. The performance and organization of stainless steel The performance and organization of stainless steel are mainly determined by various elements. Currently, there are more than 100 known chemical elements. The elements that have the greatest impact on the performance and structure of stainless steel are:: There are more than ten kinds of carbon, chromium, nickel, manganese, nitrogen, titanium, niobium, molybdenum, copper, aluminum, silicon, vanadium, tungsten, boron, etc. The addition of these elements causes changes in the internal structure of the steel, giving the steel special properties. In order to deepen our understanding of stainless steel, we must first understand the impact of various elements on the performance and structure of stainless steel. 1． Chromium - is the basic element that makes up stainless steel. Chromium is the most basic element that determines the corrosion resistance of stainless steel. In oxidizing media, chromium can quickly form a layer of chromium-rich oxide film on the surface of steel that is actually impermeable and insoluble to the corrosive medium. This oxide film is very dense and is basically combined with the metal very firmly, protecting the steel from further oxidative erosion by the external medium. ； Chromium can also effectively increase the electrode potential of steel. When the chromium content is not less than 12.5% ​​atoms, the electrode potential of the steel can suddenly change, rising from negative potential to positive electrode potential. Therefore, the corrosion resistance of steel can be significantly improved. The higher the chromium content, the better the corrosion resistance of the steel. When the chromium content reaches 25% and 37.5% atoms, the second and third mutations will occur, giving the steel higher corrosion resistance. 2． Nickel - alone cannot constitute stainless steel. The impact of nickel on the corrosion resistance of stainless steel can only be fully demonstrated when it is combined with chromium. Because, in order to obtain pure austenite structure for low carbon nickel steel, the nickel content needs to reach 24%. ； To significantly change the corrosion resistance of steel in certain media, the nickel content needs to be above 27%. Therefore, nickel alone cannot constitute stainless steel. Adding 9% nickel to steel containing 18% chromium can make the steel obtain a single austenite structure at room temperature and improve the resistance of the steel to non-oxidizing media (such as: dilute sulfuric acid, hydrochloric acid, phosphoric acid, etc.), and can improve the process performance of steel welding and cold bending. 3． Manganese and nitrogen - can replace nickel in chromium-nickel stainless steel. Manganese and nitrogen have similar effects to nickel in stainless steel. The role of manganese in stabilizing austenite is half that of nickel, while the role of nitrogen is much greater than that of nickel, about 40 times that of nickel. Therefore, manganese and nitrogen can replace nickel to obtain a single austenite structure. However, the addition of manganese will reduce the corrosion resistance of stainless steel with low chromium content. At the same time, high manganese austenitic steel is difficult to process. Therefore, manganese is not used alone in stainless steel, but only partially replaces nickel. 4． Carbon - has dual properties in stainless steel. The content of carbon in stainless steel and its distribution form largely affect the performance and structure of stainless steel.: On the one hand, carbon is a stabilizing austenite element, and its effect is very large, about 30 times that of nickel. (Martensitic) stainless steel with high carbon content can fully accept quenching strengthening, so that its mechanical properties can be improved. * * increase its strength ； On the other hand, due to the great affinity between carbon and chromium, chromium that takes up seventeen times the amount of carbon in stainless steel is combined with it to form chromium carbide. As the carbon content in steel increases, more chromium forms carbides with carbon, thereby significantly reducing the corrosion resistance of steel. Therefore, from the perspective of strength and corrosion resistance, the role of carbon in stainless steel is contradictory. In practical applications, in order to achieve corrosion resistance, the carbon content of stainless steel is generally low, mostly around 0.1%. In order to further improve the corrosion resistance of steel, especially the ability to resist intergranular corrosion, ultra-low carbon stainless steel is often used, with a carbon content of 0.03% or even lower. ； However, stainless steel used in the manufacture of rolling bearings, springs, tools, etc. requires high hardness and wear resistance, so the carbon content is relatively high, generally between 0.85 and 1.00%. Such as 9Cr18 steel, etc. 5. Titanium and niobium - can prevent intergranular corrosion of stainless steel. When stainless steel is heated to 450~800°C, the chromium content near the grain boundaries often decreases due to the precipitation of chromium carbides at the grain boundaries to form a chromium-depleted area, resulting in a decrease in the electrode potential near the grain boundaries, thereby causing electrochemical corrosion. This kind of corrosion is called intergranular corrosion. Common ones include intergranular corrosion that occurs in the heat-affected zone near the weld. Titanium and niobium are strong carbide-forming elements, and their affinity with carbon is much greater than that of chromium. Adding titanium or niobium to steel can cause the carbon in the steel to first form carbides with titanium or niobium instead of chromium, thereby ensuring that intergranular corrosion will not occur near the grain boundaries due to chromium deficiency. Therefore, titanium and niobium are often used to fix carbon in steel, improve stainless steel's ability to resist intergranular corrosion, and improve the welding performance of steel. The amount of titanium or niobium added depends on the carbon content, generally: The amount of titanium added is 5 times the carbon content, and the amount of niobium is 8 times the carbon content. 6． Molybdenum and copper - can improve the corrosion resistance of certain stainless steels to certain media. Molybdenum and copper can improve the corrosion resistance of stainless steels to corrosive media such as sulfuric acid and acetic acid. Molybdenum can also significantly improve the corrosion resistance in media containing chloride ions (such as hydrochloric acid) and organic acids. However, stainless steel containing molybdenum should not be used in nitric acid. The corrosion rate of stainless steel containing molybdenum in boiling 65% nitric acid is doubled compared with that of stainless steel without molybdenum. ； Adding copper to chromium-manganese-nitrogen stainless steel will accelerate the intergranular corrosion of stainless steel. Molybdenum has an adverse effect on the steel obtaining a single austenite structure. Therefore, in molybdenum-containing steel, in order to make the steel have a single austenite structure after heat treatment. The content of nickel in manganese and other elements should be increased accordingly. 7. Silicon and aluminum - can improve the anti-oxidation ability of stainless steel. Silicon has a significant effect on improving the anti-oxidation ability of chromium steel. The anti-oxidation ability of steel containing 5% chromium and 1% silicon can be equal to that of 12% chromium steel. To make steel resistant to oxidation at 1000°C, 22% chromium is needed when containing 0.5% silicon. If 2.5 to 3% silicon is added, only 12% chromium is needed. Some information also states that by adding 2.5% silicon to Cr15Ni20 chromium-nickel steel, the oxidation resistance can be equivalent to that of Cr15Ni60 chromium-nickel alloy. Adding aluminum to high-chromium steel can also significantly improve the oxidation resistance, and its function is similar to that of adding silicon. The purpose of adding silicon and aluminum to high chromium steel: One is to further improve the oxidation resistance of steel, and the other is to save chromium. Although silicon and aluminum play a great role in improving the oxidation resistance of chromium steel, they also have many shortcomings. The most important thing is that it increases the grain coarsening and brittleness tendency of steel. 8. Tungsten and vanadium are added to steel, and their main function is to improve the thermal strength of steel. 9. Adding 0.005% boron to boron high-chromium ferritic stainless steel (Cr17MO2Ti) can improve the corrosion resistance of the steel in boiling 65% acetic acid. ； Adding a trace amount (0.0006~0.0007%) of boron to austenitic stainless steel can improve the hot plasticity of the steel. ； Boron has a good effect on improving the thermal strength of steel, and can significantly improve the thermal strength of stainless steel. ； Boron-containing chromium-nickel austenitic stainless steels have special uses in the atomic energy industry. However, the boron content in stainless steel will reduce the plasticity and impact toughness of the steel. 10. In addition to the above elements, some stainless steels also add rare metal elements and rare earth elements to improve the properties of the steel. The above mentioned the basic element that constitutes stainless steel - chromium, and the important element that affects the structure and performance of stainless steel - carbon, as well as the added elements that improve the performance and structure of stainless steel - nickel, manganese, nitrogen, titanium, niobium, molybdenum, copper, silicon, aluminum, tungsten, vanadium, boron and more than ten elements. In stainless steel that is actually used in industry, there are several to more than a dozen alloying elements in many steels at the same time. When several elements coexist in the unity of stainless steel, the structure of stainless steel is determined by the sum of the effects of various elements. The influence of various elements on the structure of stainless steel, based on their commonalities, can be summarized into two categories.: One type is the elements that form or stabilize austenite. They are carbon, nickel, manganese, nitrogen, and copper. Carbon and nitrogen play the largest role. ； The other type is the elements that form ferrite, which are chromium, tungsten, molybdenum, niobium, silicon, titanium, vanadium, aluminum, etc. The role of this type of elements in forming ferrite, if compared with chromium as 1, the role of other elements is greater than that of chromium. When these two types of elements coexist in stainless steel, the structure of the stainless steel depends on the result of their mutual interaction. If the role of austenite-stabilizing elements is dominant, the structure of stainless steel will be dominated by austenite, with little or no ferrite. ； If their degree of action cannot keep the austenite of the steel at room temperature, this unstable austenite will undergo martensite transformation during cooling, and the structure of the steel will be martensite. ； If the role of the elements that form ferrite becomes the main aspect, the structure of steel will be dominated by ferrite. In addition to process factors, the performance of stainless steel mainly depends on the composition of its internal structure, and the structure of stainless steel is the sum of various alloying elements in the steel. Therefore, the performance of stainless steel is, in the final analysis, mainly determined by alloy elements. Stainless steel containing only chromium is usually called "stainless iron" and "chromium stainless steel" in industry. This type of steel is magnetic, and their metallographic structure is ferrite, martensite or a complex structure composed mainly of ferrite and martensite. This type of steel has the ability to resist the atmosphere and weak corrosive media, or has higher corrosion resistance and oxidation resistance, or can be quenched for use. However, their mechanical properties or process properties are poor, and they have almost no shortcomings in welding performance and so on. This type of steel is far from meeting the special requirements of industrial steel. In the matrix of chromium grade stainless steel, an appropriate amount of nickel is added. For example, after adding 8% nickel to low carbon (0.12% or less carbon) ferritic steel containing 18% chromium, the most ideal pure austenite structure can be obtained at room temperature. This kind of steel is what people usually call non-magnetic stainless steel. Compared with ferrite or martensitic acid-resistant stainless steel with the same chromium content, this type of chromium-nickel stainless steel not only has higher corrosion resistance, but more importantly, they have good cold deformation hardening performance and welding performance. They have high plasticity and impact toughness at normal or low temperatures, and are non-magnetic. The disadvantage of this type of stainless steel is that its mechanical properties are relatively low and it is sensitive to intergranular corrosion and stress corrosion, but it can be improved or eliminated through appropriate alloy additives or process measures. In the basic of chromium grade stainless steel, stainless steel with manganese as the main alloying element is added. It is what people usually call "chromium manganese nitrogen" stainless steel. In addition to having higher strength than chromium-nickel steel, this type of stainless steel is not as good as chromium-nickel austenitic stainless steel in terms of corrosion resistance and process performance. Except for a few cases, the production of this type of stainless steel is mainly to save the expensive nickel element, so this type of steel is also called nickel-saving stainless steel. 3. Classification and uses of stainless steel Stainless steel is a wide range of special steel series. There are more than 100 stainless steel grades produced in our country. However, in terms of its main alloy composition, metallographic structure and main industrial uses, it can generally be classified as follows: 1． Stainless steel can usually be divided into the following three categories according to its main alloy composition: (1) Chromium stainless steel: In addition to the iron base, the main alloying element of this type of stainless steel is chromium. Some also contain one or more elements such as silicon, aluminum, tungsten, molybdenum, nickel, titanium, vanadium, etc. The content of these elements in steel is between 1 and 3% respectively. (2) Chromium-nickel stainless steel: In addition to the iron base, the main alloying elements of this type of stainless steel are chromium and nickel. Some also contain one or more elements such as titanium, silicon, molybdenum, tungsten, vanadium, boron, etc. The content of these elements in steel is less than 4% to trace amounts. (3) Chromium manganese nitrogen stainless steel: In addition to the iron base, the main alloying elements of this type of stainless steel are chromium and manganese. Most steels also contain less than 0.5% nitrogen. Some also contain one or more elements such as nickel, silicon, and copper. The content of these elements in steel is only less than 5%. 2． Stainless steel is usually divided into the following three categories according to its structure (metallographic structure): (1) Ferritic - stainless steel containing chromium but not nickel. Cold working of this type of steel can harden it to a certain extent, but heat treatment cannot. This type of steel is always magnetic. (2) Martensitic - this type of stainless steel contains only a small amount of nickel except for a few steel grades. Most steel grades only contain chromium. The advantage is that heat treatment can harden it. This type of steel is always magnetic. (3) Austenitic - stainless steel containing elements such as chromium nickel or chromium nickel manganese or chromium manganese nitrogen. This type of steel can only be hardened by cold working ； Heat treatment can only soften it. It is non-magnetic in the annealed state. After cold processing, some will become magnetic. The above three classifications are only based on the matrix structure of steel, because the effects of elements that stabilize austenite and form ferrite in steel cannot balance each other. Therefore, the structure of stainless steel actually used in industry is also: Transitional complex phase stainless steels such as martensite-ferrite, austenite-ferrite, austenite-martensite, and stainless steel with martensite-carbide structure. 3． Stainless steel can be divided into: There are four types: stainless steel, acid-resistant steel, peel-proof steel, and heat-resistant steel. Some use electric heating alloys as a component of stainless steel. Their main functions and representative steel grades are listed as follows:: (1) Stainless steel - resistant to corrosion in air and weakly corrosive media. Steel with this function is called stainless steel. Its representative steel grades include: OCr13, 1Cr13, 2Cr13, 3Cr13, 4Cr13, Cr14, Cr15, 9Cr18, 9Cr18MOV, Cr14MO, etc. (2) Acid-resistant steel - can resist corrosion in various highly corrosive media (such as acid, alkali, salt solution, etc.). Its representative steel grades include: Cr17, Cr28, Cr17Ti, Cr18Ti, Cr25Ti, Cr17Mn9, Cr17Ni2, OCr18Ni9, 1Cr18Ni9, 2Cr18Ni9, 2Cr13Ni4Mn9, Cr14Mn14Ni, Cr18Mn8Ni5, Cr18Mn10Ni5M O3, 1Cr18Ni9Ti, Cr17MO2Ti, Cr25MO3Ti, Cr18Ni11Nb, Cr18Ni12MO2Ti, Cr18Ni12MOTi, Cr18Ni9Cu3Ti, Cr18Ni18MO2Cu2Ti, Cr18Ni20MO2Cu2Nb, etc. (3) Non-scaling steel (also called anti-oxidation steel) - has sufficient thermal stability at high temperatures and can resist gas corrosion without peeling off the iron oxide scale. Simply put, it is steel that resists gas erosion and does not peel at high temperatures. To sum up, stainless steel is not only a widely used corrosion-resistant material, but also has good heat resistance, including oxidation resistance and high-temperature strength. Therefore, it is also an important type of heat-resistant material. Stable austenitic stainless steel can still maintain high impact toughness at low temperatures, so it is also a good low-temperature structural material. ； This type of stainless steel is not ferromagnetic and is therefore a non-magnetic material. High-carbon martensitic stainless steel is not only corrosion-resistant, but also has good wear resistance. Therefore, stainless steel is another type of wear-resistant material. Therefore, stainless steel has a wide range of superior properties, and its commonality is stainlessness. Because stainless steel has the characteristics of stainless steel, corrosion resistance, high temperature resistance, non-magnetic, wear resistance and so on. Therefore, stainless steel is widely used in the food industry, textile printing and dyeing industry, medical industry, instrumentation industry, shipbuilding industry, chemical industry, aviation industry, defense industry, scientific research and daily industry and other sectors. Such as these industrial tools, equipment, equipment, parts and civilian commodities, etc. 4. Identification of Stainless Steel We talked about the organization, performance and main uses of stainless steel in the two sections "Performance and Organization of Stainless Steel" and "Classification and Use of Stainless Steel" respectively. We talked about the characteristics of stainless steel, such as stainless steel, acid resistance, high temperature resistance, wear resistance and austenite, which is generally non-magnetic. Give us a basic concept of stainless steel ； Plus stainless steel is our common high-grade alloy steel. Pens, all-steel watch cases, white metal watch bands, etc. in our daily necessities are generally made of stainless steel. There are also stainless steel knives, stainless steel tableware, etc. These daily contacts have laid the foundation for us to identify stainless steel. However, most of the stainless steel we see every day is 1-2 Cr18Ni9 and 1Cr18Ni9Ti, which are basically the same nickel-chromium stainless steel and 1-4 Cr13 chromium stainless steel. In my country, there are more than a hundred varieties (steel grades) of stainless steel, and the elements and contents in the steel grades are different, and their performance and value are also very different. Therefore, the identification of stainless steel requires careful investigation and research, clarifying the steel number mark, and comprehensive testing to correctly identify it. Currently, we mainly use the following methods to identify stainless steel:: (-) Identification of stainless steel: The sources of stainless steel whole materials are generally orders from steel mills or imported from abroad, as well as processed products that are overstocked and overstocked by the society. The sources of stainless steel materials are different, and the methods of identification are also different. 1． To identify stainless steel products imported or ordered by steel mills, generally you only need to check the marks on the steel products or packaging according to the quality certificate (referred to as the warranty) from the import or steel mill. The quality certificate is the supplier's confirmation and guarantee of the inspection results of the batch of products. Therefore, the quality certificate not only states the name, specifications, number of delivered pieces, weight and delivery status of the material, etc. In addition, all inspection results of the specified guarantee items must be stated. Likewise, in order to facilitate management, avoid confusion and prevent usage accidents caused by confusion. The manufacturer marks the grade, batch number, status, specifications, quantity and manufacturer code on the materials or packaging. The mark marked on it should be consistent with the mark on the quality certificate. Commonly used marking methods mainly include the following three types:: (1) Coloring - Paint the specified parts of the material with a color indicating its grade ； (2) Printing - stamp or spray print on the specified parts of the material to indicate the material grade, specifications, furnace number, etc. Stamping is often used on thick steel plates or large and medium-sized steel sections. (3) Tag-hanging a tag indicating the grade, batch number, specification, quantity, etc. on the bundles or boxes of materials. Stainless steel signs generally use two types: printing and listing. 2． The society has overstocked stainless steel materials. Depending on the length of the backlog and the quality of storage, there are generally two situations.: First, the overstocking time is not long, and the maintenance (maintenance) management is good. The processing unit not only keeps the original quality certificate or copy, but also has complete and clear signs on the steel or including it. For the identification of this kind of stainless steel, you only need to check the mark on the steel or packaging according to the warranty. The other kind is that the backlog has been long and the storage is incomplete. There is no quality certificate, and the marks on the steel or packaging are no longer obvious or have fallen off. For the identification of this type of stainless steel, it is mainly necessary to conduct in-depth investigation and research and consult the original documents from the processing unit. Generally speaking, the steel number, etc. are noted on the original documents such as invoices. If there is no steel number to wear, you must also check the local price regulations at that time to determine the steel number based on the price. This is a more effective method to determine the steel number, but it is often easy to be ignored. Therefore, it is necessary to master the identification method of physical objects. (2) Physical identification of stainless steel: Physical identification is a specific method to use the senses to determine whether it is stainless steel and which type of stainless steel it belongs to based on the inherent physical and chemical properties of the commodity (including whole materials, residual materials, waste materials, etc.) without knowing the steel type (number) with the help of simple instruments. It should be pointed out: Sensory identification cannot distinguish the specific steel (type) number, but can only basically distinguish the three major categories of chromium stainless steel, chromium-nickel stainless steel and chromium-manganese-nitrogen stainless steel. The identification method is as follows: 1． The difference in color: Pickled stainless steel, silvery white and smooth surface: Chrome nickel stainless steel color silvery white jade color ； Chrome stainless steel color white slightly gray luster weak ； The color of chromium-manganese-nitrogen stainless steel is similar to chromium-nickel stainless steel and slightly lighter. Surface color of unpickled stainless steel: Chrome-nickel steel is brown-white ； Chrome steel is brown-black ； Chromium manganese nitrogen is black (these three colors refer to the more heavily oxidized colors). Cold-rolled unannealed chromium-nickel stainless steel with silvery white surface and reflective surface. Except for stainless steel, stainless steel is generally pickled to appear white. 2． Identify with copper sulfate: The method is to remove the oxide layer on the steel, put a drop of water on it, and rub it with copper sulfate. If it does not change color after rubbing, it is usually stainless steel. ； Turning purple: Non-magnetic steel is high manganese steel, and magnetic steel is generally ordinary steel or low alloy steel. 3． Use magnets to identify: Magnets can basically distinguish between two types of stainless steel. Because chromium stainless steel can be attracted by magnets in any state ； Chromium-nickel stainless steel is generally non-magnetic in the annealed state, but some may become magnetic after cold working. However, high manganese steel with higher manganese content is non-magnetic. ； The magnetic situation of chromium-nickel-nitrogen stainless steel is more complicated: Some are non-magnetic, some are magnetic, and some are non-magnetic in the longitudinal direction but magnetic in the transverse direction. Therefore, although magnets can basically distinguish between chromium stainless steel and chromium-nickel stainless steel, they cannot correctly distinguish some steel types with special properties, let alone specific steel grades. For steel types with special properties, we still need to adopt the following three methods to identify them. 1) Identification of grinding flowers: To identify the grinding pattern, grind the stainless steel on a grinder and observe the sparks. If the sparks are streamlined and have many dense knots, it is high manganese steel or manganese nitrogen steel with higher manganese content. ； If there are no knots, it is chromium steel or chromium-nickel stainless steel. 2) Identification by annealing method: If the cold-processed chromium-nickel stainless steel is magnetic, you can burn a small piece in a fire and let it cool naturally or put it into water (annealing). Generally speaking, the magnetism will be significantly weakened or completely disappeared after annealing. However, some chromium-nickel stainless steels, such as Cr18Ni11Si4AlTi steel and Cr21Ni5Ti steel, because the steel contains more ferrite elements, a considerable part of its internal structure is ferrite. Therefore, it is magnetic even in the hot-processed state. 3) Identification by chemical qualitative method: Chemical qualitative method is an identification method to identify whether magnetic stainless steel contains nickel. The method is:: Dissolve a small piece of stainless steel in aqua regia, dilute the acid with purified water, add ammonia to neutralize it, and then gently inject the nickel reagent. If there is a red velvet-like substance floating on the liquid surface, it indicates that the stainless steel contains nickel. ； If there is no red velvet-like substance, it proves that there is no nickel in the stainless steel. (However, due to the low nickel content in stainless steel, generally only a few percent, the nickel content is not easy to reveal or determine. Generally, it can only be grasped after many experiments with standard samples. ) Description of the above identification methods: Using a sensor tube to identify stainless steel requires not only comprehensive testing using several methods, but the test results can only determine a certain type of stainless steel, but cannot determine which alloying elements and specific contents are contained in the steel. Therefore, the method of sensory identification is currently extremely imperfect, and some may be wrong. There are also many physical phenomena that are only known but not why, and need to be further explored.