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Common Heat Treatment Methods and Defect Analysis of Stainless Steel Strip

The heat treatment of stainless steel strip is to eliminate the work hardening after cold rolling, so that the finished stainless steel strip  can achieve the specified mechanical properties.
In the production of stainless steel strip, 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 eliminate traces of the hot rolling process, austenitic, austenitic-ferritic and austenitic-martensitic hot-rolled strip steel must be quenched. The quenching operation is to heat the strip steel in a straight-through furnace first, and the heating temperature is generally 1050~1150°C, so that the carbides in the steel can be fully dissolved and a uniform austenite structure can be obtained. Then it is cooled rapidly, mainly with water. If it is cooled slowly after heating, it is possible to precipitate carbides from solid solution in the temperature range of 900 ~ 450 ° C, making stainless steel sensitive to intergranular corrosion.
Quenching of cold-rolled stainless steel strip can be used as intermediate heat treatment or final heat treatment. As the final heat treatment, the heating temperature should be in the range of 1100~1150°C.
(2) Annealing, martensite, ferrite and martensite-ferrite cold-rolled stainless steel coils need annealing. Annealing is performed in an electrically heated furnace or a gas hood furnace in air or protective gas. The annealing temperature of ferritic steel and martensitic steel is 750 ~ 900 ℃. Furnace cooling or air cooling is then performed.
(3) Cold treatment. In order to strengthen 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°C, and let it stand at this temperature for a period of time. Strong cooling (below the martensitic point Ms) transforms austenite into martensite. After cold treatment to reduce internal stress, temper (or age) 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.
Defects of heat treatment of stainless steel strip include:
(1) Gas corrosion is black dotted pits on the surface of the strip. If the residual emulsion, oil, salt, dirt, etc. on the surface of the strip are not cleaned up, part or the entire surface of the strip (staying in the furnace for a long time) will be corroded by the gas. At high temperatures, the corrosion of the gas on the strip surface is more serious.
(2) Overheating, the surface of the strip will turn dark brown when overheating. Although the iron oxide scale on the surface has fallen off, it is not easy to be cleaned by pickling. The cause of this defect is that the heating temperature of the metal is too high or the residence time in the furnace is too long. Excessive heat may cause intergranular corrosion.
(3) Underheating. When underheating, the surface of the strip steel has a light gray metallic luster. Iron oxide scale is difficult to wash off during the pickling process, and the strip steel is gray after pickling. The reason for insufficient heating is that the heating temperature is low or the speed of the strip passing through the furnace is too fast.

(4) Gutter damage, which refers to the black dot-shaped pits that are easy to see on the lower surface of the strip steel after pickling. This defect is that there are small bumps on the working surface of the roller table, which will damage the surface of the strip. Therefore, the rollers in the furnace must be ground and replaced regularly.