Research and development progress of new cold rolled sheet steels of car body

The automobile industry has entered a new stage of development, with new changes in steel materials for car bodies and increasingly stringent requirements for energy conservation and emission reduction. This paper summarizes the recent research and development progress of new cold-rolled sheet steels for the car body. The technical routes, product advantages, and application of dual-phase steel with improved formability (DH steel), zinc–aluminum–magnesium (Zn–Al–Mg) coated steel, and automobile outer panels with high distinctness of image have been emphasized. In DH steel, an appropriate amount of retained austenite with high stability greatly improves the total elongation and work hardening rate, which is effective in solving stamping cracking. In addition, DH steel has a higher tensile strength under the same elongation in comparison to traditional DP steel. Thus, it is possible to make the body lightweight by applying thinner sheets. The special phase structure of Zn–Al–Mg coated steel determines its excellent corrosion resistance and formability compared with pure zinc-coated steel, which is widely used in automobile inner and outer plates. The surface waviness of parts is an important index of an automobile’s outer plate with a high distinctness degree of image. Shougang has developed the evolution mechanism and control technology of surface waviness and solved the problem of coordinated control of steel sheet surface roughness and waviness. Simultaneously, it is noted that the research and development of DH steel should focus on some production and application problems, such as continuous casting difficulties and surface defects due to high Al and Si content, mechanical property fluctuations caused by high alloy content, hydrogen embrittlement of high-strength products, and liquid metal embrittlement (LME) of the coated sheet during welding. The basic data of welding, coating, bonding, and forming of Zn–Al–Mg coated steel sheets are not perfect, and the high corrosion resistance needs to be inspected and recognized by more users. The high distinctness of automobile outer plate needs to further reduce the number and size of surface defects and decrease the longwave surface contour in the future to achieve a high degree of cleanliness and homogenization of the automobile outer panel surface. Continuously improving the performance advantages of the above materials through a combination of theoretical research, process progress, and equipment upgrading, as well as expanding their application in the automotive field, requires the joint efforts of iron and steel production enterprises, material application enterprises, and scientific research institutions.