Metallic properties
According to the different deoxidation methods used in the steelmaking process (melting, ingot casting and plate solidification), low carbon steel can be roughly divided into rimmed steel and aluminum-killed steel, as shown in the figure "Chemical composition of rimmed steel and aluminum-killed steel". Rimmed steel is deoxidized with carbon, while killed steel is deoxidized with aluminum. The difference between the two steels lies in the content of carbon, oxygen and aluminum. Rimmed steel has a high oxygen and carbon content, and most of them exist as non-metallic inclusions. Furthermore, the nitrogen in rimmed steel is not fixed, while the nitrogen in aluminum-killed steel is fixed by aluminum. Therefore, compared with the two, the aging deterioration phenomenon of rimmed steel (the phenomenon of low tensile strain and total elongation during forming) is difficult to avoid, while aluminum-killed steel is easy to form a crystalline aggregate structure and has the desired plastic anisotropy in deep drawing. Therefore, steel plates with high processing performance requirements (SPCD and SPCE, etc.) should use aluminum-killed steel plates, but rimmed steel plates are still used more because of their low production cost and easy surface beauty.
In addition to the above-mentioned identification of the properties of the two steel plates from the composition, they can also be identified from the metallographic perspective by comparing the grain formation state. In addition, the grain size can be controlled by the annealing conditions of the steel plate, and the boiling steel plate is more likely to change greatly. In particular, in order to improve the formability of boiling steel and the processability of enamel products (such as less prone to blistering), it is decarburized and annealed to produce cold-rolled steel plates with coarse grains and very soft texture. However, if the grains are too coarse, the forming limit of spinning may be reduced.
The following is a summary of the differences in the metal properties of the above-mentioned hot-rolled steel plates and cold-rolled steel plates. The non-metallic inclusions of hot-rolled steel plates are in the form of elongated ribbons, which are not well combined with other metals. Therefore, compared with the anisotropy that can be observed from the shape and size of the grains, the steel plate has a larger and potential anisotropy, resulting in poor ductility perpendicular to the rolling direction. Cold-rolled steel plates can alleviate their internal anisotropy through annealing.
Another major difference between the two steel plates is the difference in surface state. The surface of cold-rolled steel sheets is finely processed in the form of matte finishing (pear skin processing) and bright finishing due to the combination of cold rolling and tempering rolling. The surface of hot-rolled steel sheets used in ordinary processing is in a state where the scale generated during hot rolling is pickled, so it is very rough and does not have the uniform state of cold-rolled steel sheets. In addition, internal defects of the material and surface defects in manufacturing sometimes appear in the form of surface hairline. The influence of the surface characteristics of steel sheets on spinning has not been clarified, but there are cases reported that the workpiece breaks during deep drawing and spinning due to surface cracks caused during pre-transportation.
Mechanical properties
The table summarizes the tensile test values of typical hot-rolled steel sheets and cold-rolled steel sheets. Compared with the various steel sheets attached to the table for reference, the total elongation of steel sheets exceeds that of aluminum sheets, although the strength of steel sheets is two to four times that of aluminum. It can be said that this is the characteristic of steel sheets. In addition, it can be seen from the table that there are differences in r value (Lankford value) and n value (work hardening index). The n value of steel sheets is lower than that of aluminum sheets (except for hard aluminum sheets, i.e. H materials). It can be considered that this characteristic is suitable for spinning.
Comparing between steel plates, hot-rolled steel plates are generally slightly harder than cold-rolled steel plates, and their elongation is also lower. Among cold-rolled steel plates, the higher the grade, the softer the steel, with a lower yield point, larger n and r values, and a larger total elongation. In addition to the above-mentioned plastic deformation performance, the elastic modulus of steel plates is twice that of aluminum plates in terms of elastic properties. This is conducive to reducing the net elasticity of spinning and correspondingly improving the rigidity of the parts. This advantage is eye-catching.
