Introduction
Cold rolled steel sheets and strips are steel products produced from hot rolled steel sheets or strips through a number of cooling, annealing and/or cold rolling processes. Cold rolled steel sheets and strips are an important material used in many industries because of their high strength-to-weight ratio and superior formability. The Chinese national standard GB/T 708-1988 governs cold rolled steel sheets and strips, providing standards for material designation, product size, production, inspection and testing requirements.
Material Designation
As laid out in the standard, cold rolled steel sheets and strips are designated by the letter S, followed by a seven-digit number. The first two digits of the number indicate the nominal yield strength of the material, in kilograms per square millimetre (kg/mm²). For example, an S15 grade steel has a nominal strength of 150 kg/mm². The last four digits of the designation indicate thickness or width of the material. Thickness or width are measured in millimetres. For example, a 1000 x 2000 mm sheet of S15-1200 material would be 1.2 mm thick, while a strip of S15-2550 material 5.5 mm wide.
Material Size
The standard also outlines the size requirements for cold rolled steel sheets and strips. The width of cold rolled sheets is a minimum of 25 mm and a maximum of 1050 mm. Strips should have a width of no less than 5 mm and no more than 760 mm. The thickness of cold rolled sheets should be between 1.0 mm and 8 mm, while the thickness of cold rolled strips should range from 0.2 mm to 3 mm.
Production Process
The production process of cold-rolled steel sheets and strips begins with the hot rolling of steel. This process is conducted at temperatures above 1000°C, resulting in a hot-rolled sheet or strip. The sheet or strip is then subjected to a number of processes including annealing, cold rolling and tempering. The annealing process is conducted at temperatures below the recrystallization temperature to relieve stresses and improve the ductility of the material. The cold rolling process reduces the thickness of the material and induces strength. Finally, the tempering process is conducted to further increase the strength of the material.
Inspection and Testing
Inspection and testing of cold rolled steel sheets and strips is carried out according to the requirements of GB/T 708-1988. Visual inspection is conducted to assess the surface condition of the material, as well as to identify any defects or irregularities. Non-destructive testing (NDT) is also conducted to detect any internal defects. NDT methods used include magnetic particle inspection (MPI), Ultrasonic Testing (UT), Eddy current testing (ECT) and X-Ray testing (XRT). Destructive mechanical testing is also used to assess the strength and ductility of the material.
Conclusion
In conclusion, cold rolled steel sheets and strips are an important material used in many industries. The Chinese national standard, GB/T 708-1988, provides requirements for cold rolled steel sheets and strips, including material designation, product size, production process, inspection and testing. Adhering to the requirements of this standard is important for ensuring the quality and performance of cold rolled steel products.
