Stainless steel can be divided into five categories: coil, plate, profile, steel pipe and parts, the most important of which is coil and plate.
Profiles are all kinds of materials made of different shapes, such as Angle steel, flat steel, I-steel, channel steel and bar. Steel pipe mainly refers to seamless steel pipe and welded pipe, the main difference is whether it is formed at a time. Parts mainly refer to pipe fittings, elbows and flanges and other small things.
The coil and the plate are actually the same, the factory form is not the same, the coil is a roll, the plate is a sheet, if the factory is the plate, also known as the original plate, is generally thick plate, because it is very thick, it can not be rolled, generally more than 16 mm can not be rolled.
There are rolls, there are plates, many times it is a board, the price of the roll and the board is not the same. If it is the purchase volume, it is calculated according to the actual weighing, so the price is high, the board is calculated according to the theory, the price is low, the reason is that there is a lower difference, such as the actual 10mm thick may be 9.6mm, there will be a price difference in the middle.
Stainless steel plate is divided into hot rolling and cold rolling according to the rolling mill (rolling mill is the machine name of rolling plate) rolling process. Hot rolling is usually marked as NO.1, cold rolling is marked as 2B or BA(BA is better than the surface of 2B, to be bright, close to the mirror, the mirror does not need to be processed), such as 304 many times is the purchase board, which means that the volume must be opened into the form of a plate through the machine, this machine is the opening machine. Through the opening machine can be opened into different lengths, if not the conventional length is called fixed open, meaning that the size is set open, because the customer according to the actual needs, often put forward different length requirements, this time the volume is important, the conventional board can not meet the requirements.
Domestic cold rolling is generally below 3 mm, hot rolling is generally above 3 mm, and the thickness of 3 mm is both hot rolling and cold rolling, but the thickness of the imported cold rolled sheet can reach 4 mm or even 6 mm. Hot rolled 3 mm to 12 mm plate is called the plate, 12 mm or more is called the thick plate, can be thick to 120 mm or more, and called the hot rolled plate, and cold rolled is cold rolled sheet.
The corrosion of stainless steel at high temperature was mentioned earlier. Next, let's talk about the corrosion of stainless steel at room temperature. It should be known that the temperature has a great relationship with the oxidation resistance of stainless steel, because the surface oxidation temperature of different properties of stainless steel is not the same, so the correct selection of stainless steel should be considered at what temperature to use, and then consider other aspects of performance and requirements. Speaking of which, there should be a little understanding of stainless steel, stainless steel stainless steel is conditional. So how does electrochemical corrosion happen? First of all, it should be known that the elements and compounds of the material world have different electrode potentials (electrode potentials), that is, there is an electrode potential between different substances, once a solution or electrolyte is filled between the two substances, the battery circuit is created, forming a micro current. This means that electric corrosion is a universal phenomenon.
(1) Some factors of point corrosion
① Stainless steel surface inclusion: there are oxides, sulfides, silicates and other inclusions on the surface, if the surface encounters an aqueous solution attached to the above, water electrolysis between the two substances, there is a small current generation, hydroelectrolysis for hydrogen ions and hydroxide ions, and hydroxide ions can chemically react with chromium hydroxide and corrosion.
Stainless steel surface rough surface: there is a rough surface such as pitting or pockmarked surface. The rough surface is easy to store suspended particles in the air (dust), and the dust absorbs water in the air, so the existence of water, water in the electrode potential between the two substances under the action of electrolysis, decomposition of hydrogen ions and hydroxide ions, hydroxide ions and chromium reaction and corrosion.
③ Chloride ion environment: When the surface of stainless steel is in an atmosphere with chloride ions, chloride ions selectively destroy the surface of the dull stainless steel film, selectivity because there are inclusions, rough points, component segregation and mechanical properties differences on the surface of stainless steel, chloride ions are electrolytes, chloride ions and oxygen can generate hypochlorous acid, hydroxide ions react with chromium to generate chromium hydroxide. Once the passivation film is destroyed, there will be corrosion concentration, also known as pitting.
(2) Tissue corrosion:
Tissue corrosion is also called intergranular corrosion. In austenitic stainless steel or austenitic and ferrite duplex steel, the carbon element in the steel is dissolved in austenite with heating, such as in the hot rolling process, the steel is heated to 1000-1200 ° C, the carbon element begins to dissolve in austenite at 900 ° C, and the carbon element is basically completely dissolved in austenite at 1050 ° C. However, during the cooling process, the carbon is precipitated from the austenite and combined with chromium in the intergranular area to become the carbide of Cr23C6. The chromium content in the intercrystal is reduced, resulting in chromium deficiency phenomenon, and the function of rust prevention is removed.
⑶ Stress corrosion:
In the process of hot rolling deformation, cold drawing deformation, cold rolling deformation and heat treatment of stainless steel, the residual stress inside the steel base will cause the uneven performance of the strip. In particular, the cold-deformed surface is damaged when the superposition of additional stress is greater than the intergranular atomic force due to lattice anomaly and easy to be affected by external environment (such as friction, scratches, collision and etching).
