Uncover the production process of stainless steel

Stainless Steel is the abbreviation of stainless steel and acid resistant steel. The steel that is resistant to weak corrosion media such as air, steam and water or has stainless property is called stainless steel; The steel resistant to chemical medium corrosion (acid, alkali, salt and other chemical etching) is called acid resistant steel. Due to the difference in chemical composition, their corrosion resistance is different. Ordinary stainless steel is generally not resistant to chemical medium corrosion, while acid resistant steel is generally stainless.

Historical origin

The invention and use of stainless steel can be traced back to the First World War. British scientist Buhengli Brerley was commissioned by the British Government Military Department Arsenal to study the improvement of weapons. At that time, the rifle chamber used by soldiers was very easy to wear, and Brawley wanted to invent an alloy steel that was not easy to wear.

Brawley and his assistant collected various types of steels and alloy steels with different properties produced at home and abroad, carried out performance tests on machines with different properties, and then selected more suitable steels to make guns.

One day, they tested a domestic alloy steel containing a large amount of chromium. After the wear test, it was found that this alloy was not wear-resistant, indicating that it could not be used to manufacture guns. So they recorded the test results and threw the matter in the corner. One day a few months later, an assistant ran excitedly with a piece of shiny steel and said to Brawley, “Sir, this is the alloy steel sent by Mr. Mullah when I was cleaning the warehouse. Do you want to experiment and see what its special role is!” “Good!” Braille looked at the bright steel and said happily.

The experimental results show that it is a stainless steel that is not afraid of acid, alkali and salt. This kind of stainless steel was invented by the German Mullah in 1912. However, the Mullah did not know what the purpose of this stainless steel was. Braille thought to himself, “This kind of steel, which is not wear-resistant but corrosion-resistant, cannot be used to make guns. Can it be used as tableware?” He said he would do what he said, and made stainless steel fruit knives, forks, spoons, fruit trays and folding knives.

Main types

The term “stainless steel” not only refers to one kind of stainless steel, but also refers to more than 100 kinds of industrial stainless steels. Each type of stainless steel developed has good performance in its specific application field. The key to success is to understand the purpose first, and then determine the correct steel grade. There are usually only six types of steel related to the application field of building construction. They all contain 17~22% chromium, and better steel grades also contain nickel. The addition of molybdenum can further improve the atmospheric corrosion, especially the corrosion resistance to the atmosphere containing chloride.

Stainless steels are usually divided into martensitic steels, ferritic steels, austenitic steels, austenitic ferritic (duplex) stainless steels and precipitation hardening stainless steels according to the structure state. In addition, it can be divided into chromium stainless steel, chromium nickel stainless steel and chromium manganese nitrogen stainless steel according to composition

Main features

Weldability

The requirements for welding performance vary with the use of products. Class I tableware generally does not require welding performance, even some pot enterprises. But the vast majority of products need raw materials with good welding performance, such as Class II tableware, insulated cups, steel pipes, water heaters, drinking fountains, etc.

Corrosion resistance

The vast majority of stainless steel products require good corrosion resistance, such as Class I and II tableware, kitchenware, water heaters, drinking fountains, etc. Some foreign businessmen have also conducted corrosion resistance tests on products: heated NACL aqueous solution to boiling, poured out the solution after a period of time, washed and dried, and weighed the weight loss, To determine the degree of corrosion (Note: when the product is polished, the abrasive cloth or abrasive paper contains Fe, which may cause rust spots on the surface during the test)

Polishing performance

In today’s society, stainless steel products are generally polished during production, and only a few products, such as water heaters, water dispenser liners, do not need polishing. Therefore, it requires good polishing performance of raw materials. The main factors affecting polishing performance are as follows:

① Surface defects of raw materials. Such as scratches, pitting, pickling, etc.

② Raw material quality problem. If the hardness is too low, it is not easy to polish when polishing (poor BQ property), and if the hardness is too low, orange peel is easy to appear on the surface during deep drawing, which affects the BQ property. BQ with high hardness is relatively good.

③ After deep drawing, small black spots and RIDGING will appear on the surface of the area with great deformation, which will affect the BQ property.

Heat resistance

Heat resistance means that stainless steel can still maintain its excellent physical and mechanical properties at high temperatures.

Influence of carbon: carbon is strongly formed and stabilized in austenitic stainless steel. Elements that define austenite and expand the austenite zone. The ability of carbon to form austenite is about 30 times that of nickel. As a interstitial element, carbon can significantly improve the strength of austenitic stainless steel through solution strengthening. Carbon can also improve the stress and corrosion resistance of austenitic stainless steel in highly concentrated chloride (such as 42% MgCl2 boiling solution).

However, in austenitic stainless steel, carbon is often regarded as a harmful element. This is mainly because under some conditions (such as welding or heating at 450~850 ℃) in the corrosion resistance of stainless steel, carbon can form high chromium Cr23C6 type carbon compound with chromium in the steel, which leads to local chromium depletion, reducing the corrosion resistance of steel, especially the resistance to intergranular corrosion. So. Most of the newly developed Cr Ni austenitic stainless steels since the 1960s are of ultra-low carbon type with carbon content less than 0.03% or 0.02%. It can be seen that with the reduction of carbon content, the susceptibility of steel to intergranular corrosion decreases. The most obvious effect is when the carbon content is less than 0.02%. Some experiments also point out that carbon will also increase the tendency of pitting corrosion of Cr Ni austenitic stainless steels. Due to the harmful effect of carbon, not only should the carbon content be controlled as low as possible in the smelting process of austenitic stainless steel, but also in the subsequent hot, cold working and heat treatment processes to prevent carburization of the stainless steel surface and chromium free carbide precipitation.

Corrosion resistance

When the number of chromium atoms in the steel is not less than 12.5%, the electrode potential of the steel will suddenly change from negative potential to positive electrode potential. Prevent electrochemical corrosion.