In cold-rolled steel sheets for automobiles, such as shock absorbers and door anti-collision stringers, ultra-high strength steel sheets with strength exceeding 980MPa have been used. Recently, laws and regulations related to crash safety have been strengthened, and ultra-high strength materials have been applied to automobile body parts. While the application of ultra-high strength steel plates has expanded, heat treatment technologies such as hot stamping and high-frequency heating have also begun to be applied to ultra-high strength components. In the application of ultra-high strength steel plate, the resistance to delayed fracture has become a subject. With the increase of strength, the delayed fracture sensitivity also increases. When the tensile strength increases to more than 1180MPa, the delayed fracture sensitivity will increase rapidly, and delayed fracture is easy to occur.
1470MPa grade steel plate with dual phase (DP) structure is used as comparison steel plate. The elongation property of the developed steel is about 1.5 times higher than that of the DP steel with the same strength. With regard to local deformation characteristics, V-bend test method is adopted. The bending direction is at right angles to the rolling direction, and the bending angle is 60° And 90°, The bending radius varies from 1 to 5mm. V bending test shows that compared with DP steel, the developed steel can be bent to a smaller bending radius. The reason why the bending workability of the developed steel is better than that of the DP steel is that the parent phase is carbide-free and homogeneous carbon-free bainite phase, which accounts for most of the total, and the residual austenite is uniformly dispersed in the strip structure, so that the deformation will not be concentrated, and it is not easy to produce large shrinkage holes that will lead to the origin of cracking.
The delayed fracture resistance of steel plate is not only related to strength, but also to chemical composition and microstructure. In order to improve the delayed fracture resistance of steel plates, various methods have been proposed. In addition, in actual parts, due to some processing, in addition to the chemical composition and strength of the steel plate, the degree of processing, residual stress, and especially the corrosion environment in actual use have become important factors affecting the delayed fracture resistance. The delayed fracture resistance of U-bend material is evaluated by immersing it in hydrochloric acid. That is, bend the long strip sample with a bending radius of 10mm and 15mm in the direction at right angles to the rolling direction. After demoulding, confirm the stress value of the bending part with a strain gauge on one side, clamp it with bolts on the other side, and apply a stress of 1000~2000MPa. The concentration of hydrochloric acid impregnated was 5%. The occurrence of cracks after 48 hours of immersion was investigated. Compared with DP steel with the same strength, the developed steel is not easy to crack after immersion for a long time, even if it is bent to a small extent, even if the residual stress is high, which indicates that it has very good resistance to delayed fracture. As mentioned above, it can be considered that this is because the parent phase of the development steel is carbide-free carbon-free bainite, and the residual austenite phase plays a role in hydrogen absorption after fine dispersion.
