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, the commonly used heat treatment methods are as follows:
(1) Quenching, for austenitic, austenitic-ferritic and austenitic-martensitic stainless steels, quenching is a softening heat treatment operation.
In order to remove traces of the hot rolling process, austenitic, austenitic-ferritic and austenitic-martensitic hot-rolled strips must all be quenched. The quenching operation is to heat the strip steel in the through furnace first. The heating temperature is generally 1050~1150℃, so that the carbides in the steel are fully dissolved and a uniform austenite structure is obtained. It is then cooled rapidly, mostly by water. If it is cooled slowly after heating, it is possible to precipitate carbides from 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 an intermediate heat treatment or as a final heat treatment. As the final heat treatment, the heating temperature should be in the range of 1100~1150℃.
(2) Annealing, martensitic, ferritic and martensitic-ferritic cold rolled stainless steel coils require annealing. Annealing is carried out in an electrically heated furnace or hood furnace with air or protective gas. The annealing temperature for ferritic and martensitic steels is 750 to 900°C. Furnace cooling or air cooling is then performed.
(3) Cold treatment: In order to strengthen the martensitic steel, ferritic martensitic steel, and austenitic martensitic steel to a greater extent, cold treatment is required. Cold treatment is to immerse the cold-rolled or heat-treated stainless steel strip in a low temperature medium of -40 ~ -70 ℃, and let it stand for a period of time at this temperature. Strong cooling (below the martensite point Ms) transforms austenite into martensite. After cold treatment, reduce internal stress and temper (or age) at a temperature of 350 ~ 500 ℃. Liquid or solid carbon dioxide, liquid oxygen, liquid nitrogen or liquefied air are commonly used as cooling media.
