The technological properties of reinforcing bars include many items. Different requirements can be put forward according to the characteristics of different products, such as bending and reverse bending (reverse bending) tests for ordinary reinforcing bars, and repeated bending, torsion and winding tests for some prestressing steel bars.
All of these tests simulate to varying degrees the processing methods that may be involved in the actual use of materials, such as bending or bending of ordinary steel bars, winding of prestressing steel wires, etc. The purpose of these tests is to assess the material's ultimate bearing capacity for these specific plastic deformation, so the process performance is also a plastic requirement of materials, and is in line with the ductility (extension) mentioned above. Length) Requirements are common. Generally speaking, steel with high elongation has good technological properties.
However, compared with the unidirectional stress state under tension, the stress state of the process performance test is much more complex. The deformation type of the specimen is different from that of the specimen in all directions (axial and radial). The structure, grain size, harmful residual element content of the steel, especially any defect, such as cracks and inclusions, which affect the continuous deformation of the internal and surface, may affect and lead to the failure of the test. Therefore, in a sense, for the assessment of steel quality, it can be said that the process performance test is more stringent.
In addition, the reverse bending test of steel bar is essentially a strain aging sensitivity test, which is due to a certain amount of free nitrogen (N), also known as residual nitrogen, in molten steel. When the content is too high, the steel can be brittle at room temperature after plastic deformation.
Because steel bars often need to be used after bending forming, plastic deformation has been produced. If the material is brittle, the structure can not bear the additional load (such as earthquake) which makes the steel bars plastic deformation again. At present, the bending test is included in the steel bar standard as an important technical requirement at home and abroad, and the nitrogen content of the steel is limited (no more than 0.012%).
The results show that some elements used in steel microalloying, such as vanadium, titanium, niobium and so on, especially vanadium and nitrogen have excellent affinity. The addition of vanadium into steel can effectively combine free nitrogen, and the combination of vanadium and nitrogen can further enhance the strengthening effect of vanadium on steel. Therefore, some standards also indicate that "if there are enough elements that combine with nitrogen, the nitrogen content can be higher than the standard requirements".