The heat treatment of stainless steel strip is to eliminate the work hardening after cold rolling, so that the finished stainless steel strip can reach the specified mechanical properties.
In the production of stainless steel strips, three most heat treatment methods are commonly used.
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For austenitic, austenitic-ferritic and austenitic-martensitic stainless steel strips, quenching is a softening heat treatment operation.
Austenitic, austenitic-ferritic and austenitic-martensitic hot-rolled stainless steel strips must all be quenched in order to eliminate traces of the hot rolling process.
The quenching operation is to first heat the stainless steeel strip in a straight-through furnace, generally at a heating temperature of 1050-1150 °C, so that the carbides in the steel are fully dissolved and a uniform austenite structure is obtained.
Then cool quickly, mostly water. If it is cooled slowly after heating, carbides may be precipitated from the solid solution in the temperature range of 900 to 450 °C, making stainless steel sensitive to intergranular corrosion.
Quenching of cold-rolled stainless steel strip can be used as both an intermediate heat treatment and a final heat treatment. As the final heat treatment, the heating temperature should be in the temperature range of 1100 to 1150 °C.
Martensite, ferrite and martensite-ferrite cold-rolled stainless steel strips need to be annealed.
Annealing is carried out in an electric or gas bell furnace with air or protective gas.
The annealing temperature of ferritic steel and martensitic steel is 750 to 900°C. Then furnace cooling or air cooling.
In order to maximize the strengthening of martensitic steel, ferritic-martensitic steel, and austenitic-martensitic steel, cold treatment is required.
Cold treatment is to immerse the cold-rolled or heat-treated stainless steel strip into a low-temperature medium of -40 to -70 °C, and let it stand for a certain period of time at this temperature.
Intense cooling (below the martensite point Ms) transforms austenite into martensite.
After the cold treatment, in order to reduce the internal stress, tempering (or aging) is performed at a temperature of 350-500 °C.
Liquid or solid carbon dioxide, liquid oxygen, liquid nitrogen or liquefied air are commonly used as cooling media.