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Development of High Strength Steel Plate for Automobile

Time:2016-10-19 17:24:50


With the high-strength steel plate in the automotive parts of the expansion of the scope of application, along with the requirements of the steel processing is also getting higher and higher. In particular, recent years have witnessed a rapid development of high-strength technology for steel sheets, and it is required to have high stretch-flangeability in addition to the strength-extension characteristics required in the past. In this regard, Kobe Steel has developed a strength, extension characteristics and extension flange of 980MPa grade cold-rolled steel. In order to further improve the tissue control technology than the conventional steels, the number of ferrite phases required to obtain a high elongation is ensured, and the stretch flangeability is improved by adding ferrite solid solution strengthening elements and a new understanding of martensite back Fire temperature, reduce the difference between the hardness of two-phase, has developed a high processing performance of the steel, the elongation and extension of the flange of a better than ordinary steel.
In order to expand the application of the high-strength steel sheet, in addition to requiring a good balance between strength and processability, various characteristics must be satisfied. One of the most important properties is weldability. In order to improve the strength of steel materials, generally need to use a large number of alloying elements, so with the strength, the chemical composition of steel will increase, the weldability will be worse. In this regard, Kobe Steel to find out the optimization of chemical composition in steel, especially to reduce the amount of C, but also to achieve the best high-strength composition design. Using this method, developed with good welding performance, low C 780,980 MPa grade cold rolled steel and GA steel plate, and has begun commercial production.
1 980MPa grade galvannealed hot dip galvanized steel sheet
As the low yield ratio of galvannealed steel plate. Kobe Steel has previously developed grades 590 MPa and 780 MPa GA. As a design concept, in order to ensure the required formability and weldability of automotive steel sheets, the idea of ​​obtaining a ferrite + martensite composite structure with a low-alloy composition has become a common design concept, and on the basis of this Technology established. When the strength of the steel sheet is increased to 980 MPa, the optimum composition system is designed based on this idea. Not only to ensure ductility, but also to improve the strength of the design is as follows:
1) Although Mn is effective as a solid solution strengthening element, it should be added as little as possible in order to ensure ductility of the ferrite phase.
2) Cr and Mo are elements that improve the quenching property and contribute to the increase of the volume fraction of martensite phase. In addition, C helps to improve the hardness of the martensitic phase. However, if any of the elements is excessively added, the weldability is deteriorated, and therefore, the amount required to increase the strength is compounded.
Since the developed steel is designed in accordance with the above-described components, stable characteristics can be obtained even in mass production.
Developed steels have a high elongation characteristic, comparable to cold rolled steel sheets. The forming limit curve of the developed steel is basically the same as that of the cold rolled steel sheet. The results show that the compression forming load of the developed steel is low, which is almost the same as that of the GA780MPa grade steel plate. , Deep drawing coefficient and cold-rolled steel sheet is basically the same.
The weldability of the developed steels and the weldability of the cold-rolled steel sheets were compared. The welding test was carried out on various test plates having a plate thickness of 1.4 mm by changing the welding current value. The relationship between the shear tensile strength of the welded joint and the nugget diameter determined after fracture was investigated. The results show that the critical diameter of the nugget fracture of the developed steel is larger than that of the cold rolled steel sheet. In addition, when the nugget fracture occurs, the joint strength greatly exceeds the JIS standard. Even if the strength of the base material is increased, the strength of the cross-shaped tensile strength is not changed even in the case of fracture and peeling-off in the nugget. Development of steel from the low current region to ensure a very strong strength, almost no fracture occurred within the nugget, until the high current value occurred nugget fracture. In addition, the propagation of the fracture is advanced, and the intensity increases with the current value, and there is no large intensity deviation between the two. Compared with the cold-rolled steel, the development of steel from nugget fracture to fracture propagation before the current value range. It is considered that this is because the effect of the component design which minimizes the increase in the amount of the added elements required for the high-strength enhancement is exhibited. In the high current region after propagation of the break, since the intensity fluctuation is large, the optimum current value should be set.
The developed steel was subjected to bending processing to produce a cap-shaped compression test material, and the compressive properties were investigated. The spot interval of spot welding is 50mm. The axial length of the parts for axial compression test was 300 mm, and the axial length of the parts for 3-point bending and compression test was 1000 mm. The hammer will fall from a certain height to test. The impact load is measured by a load cell under the test material, and the displacement output power is measured by a laser displacement meter. And the absorption energy before the displacement of 50 mm was obtained from the load-displacement curve. As a result, as the tensile strength was increased, the absorption energy was increased, and the strength of the parts was improved.
Ultra - fine grain ultra - high strength cold rolled steel sheet
In cold-rolled steel sheets for automobiles, ultra-high-strength steel sheets having a strength of more than 980 MPa have been used, such as shock absorbers and door bumper beams. More recently, regulations relating to crash safety have been strengthened, and ultra-high strength materials have been used in automotive body parts. In the application of ultra-high-strength steel expanded at the same time, hot stamping and high-frequency heating heat treatment technology has also begun to apply to ultra-high strength components. In the application of ultra-high strength steel, resistance to delayed fracture characteristics become a topic. When the tensile strength is increased to 1180MPa, the sensitivity of delayed fracture will increase rapidly and the delayed fracture will occur easily.
A 1470 MPa grade steel plate having a duplex structure (DP) was used as a comparative steel plate. Development of the extension of steel than the same strength of DP steel 1.5 times higher. For the local deformation characteristics, the bending test was carried out with the bending direction at right angles to the rolling direction. The bending angles were 60 ° and 90 °, and the bending radius varied from 1 to 5 mm. The V-bend test showed that the developed steels were bendable to a smaller bend radius than the DP steels. The reason why the developed steel has better bending workability than the DP steel is that the mother phase is a carbide-free and uniform carbon-free bainite phase occupies the majority, and the residual austenite is finely dispersed uniformly in the striped structure so that the deformation is not Will concentrate, is not easy to produce will lead to the origin of large cracks shrinkage.
The delayed fracture properties of the steel sheet are not only related to the strength, but also to the chemical composition and the microstructure. Various methods have been proposed for improving the resistance to delayed fracture of a steel sheet. In addition to the chemical composition and strength of the steel sheet, the degree of processing, the residual stress, and especially the corrosive environment in the actual use state, are important factors affecting the resistance to delayed fracture characteristics in actual parts due to a certain processing, factor. The resistance to delayed fracture was evaluated by a method in which a U-shaped bent material was immersed in hydrochloric acid. That is, an elongated specimen was bent at a right angle to the rolling direction at a bending radius of 10 mm and 15 mm, and the stress value of the bent portion was confirmed by a strain gauge after releasing the mold, Tight, and applied to the stress of 1000 ~ 2000MPa. The concentration of the impregnated hydrochloric acid was 5%, and the occurrence of cracks after 48 h immersion was investigated. Even with very high residual stresses, the developed steels are less prone to cracking when immersed for long periods of time, compared to DP steels of the same strength, indicating a very good resistance to delayed fracture. As described above, it is considered that this is because the mother phase of the developed steel is a carbide-free carbon-free bainite, and the retained austenite phase is finely dispersed to exhibit a hydrogen absorbing effect.
3 Conclusion
In the process of strengthening the regulations on anti-collision safety, it is becoming increasingly important to further develop a technique for reducing the weight of the vehicle body and to enlarge the application of the high-strength steel sheet. In the future should actively promote to meet the car manufacturers and parts manufacturers
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