Study on 800MPa high strength wear-resistant plate with excellent weldability in Japan
One of the research topics of Japan's "Super Steel" project is "Research on 800MPa class high-strength wear-resistant plates with excellent weldability". At present, the tensile strength of ferritic pearlite steel is below 500MPa, and the high-strength wear-resistant plate with tensile strength exceeding 500MPa is mainly formed by adding alloy components to form bainite, tempered martensite and other high-strength wear-resistant plates. However, with the increase of alloying elements, not only increases the cost of steel, but also increases the difficulty of steelmaking and refining, and more importantly, the welding performance deteriorates, and the fatigue strength of high-strength wear-resistant plate welded joints is only 60 ~ 100MPa(10% of the base metal matrix strength). Ferritic steel with the basic composition of C-Si-Mn has good weldability, but the strength of high-strength wear-resistant plates produced with current technology is limited, so the research objectives of this topic are: Based on the composition of C-Si-Mn, the grain size was refined from 10μm to 1μm by grain refining, and the strength and hardness balance of the ultra-fine ferrite 2 pearlite composite structure was obtained. The ultra-fine grain steel with tensile strength of 800MPa was developed. At the same time, the welding process of super wear-resistant plate with excellent welding performance was developed. To this end, this topic is studied from the following two aspects.
(1) The development of ultrafine crystal materials. Ultrafine ferrite grains can be prepared by using large plastic strain. However, when large plastic strain is applied to unidirectional deformation, the deformation of the material in the thickness direction is not uniform, resulting in the strain mainly concentrated in the central part of the sample. Nagai et al. use the "multidirectional deformation" technique to prepare ultrafine ferrite grains, which is characterized by the use of bidirectional or multidirectional deformation can significantly improve the uneven strain distribution, which is conducive to obtaining uniform ultrafine microstructure.
Nagai et al. studied the grain change of carbon steel with chemical composition (%) of 0.16C-0.4Si-1.4Mn by using a multidirectional deformation thermomechanical processing simulator developed in the laboratory. The results showed that the ultrafine grain steel prepared by the "multidirectional deformation" method had more uniform ultrafine crystal structure. The characteristic of the multidirectional thermal mechanical processing simulator is that the sample can be rotated 90° for each roll. Nagai et al. successfully prepared a bar with a size of Φ18mm×20000mm with low-carbon Si-Mn steel by using the multi-direction rolling technology of the laboratory mill. When the grain size of the steel was refined from 10μm to 0.5μm, the yield strength of the high-strength wear-resistant plate could be increased from 320MPa to 740MPa. When the grain size of 12mm×700mm×Cmm hot-rolled steel plate is refined to 1μm, the tensile strength of the high-strength wear-resistant plate reaches 800MPa, and the prepared hot-rolled high-strength wear-resistant plate has uniform ultra-fine grains in the thickness direction.
The anisotropy of mechanical properties of thick steel plates produced by large deformation, especially the reduction of toughness in some directions, is the focus of attention. To this end, Nagai et al. used a "large Angle cross rolling" method to change the crystalline orientation of the material. Through cross rolling, the texture or (100) pole index of the material can be effectively changed, so that the difference of the toughness and brittle-brittle-transition temperature in the transverse and rolling direction of the material is very small.
(2) Research on high-efficiency welding technology of ultra-fine grain steel. One of the most important problems in the industrial application of ultra-fine grain steel is the softening of HAZ. The traditional welding method will reduce the strength of the joint due to HAZ softening caused by grain coarsening. ReisukeITO et al. developed a new ultra-narrow gap gas shielded welding method. The second welding of 19mm thick steel plate, the chemical composition (%) of the steel plate is 0.15C-1.50Mn-0.20Si-0.02P-0.002S, the HAZ width is only 3mm, and the hardness of the joint is lower than HV250, so the welding cracks and stress corrosion cracks can be effectively prevented.
S. sukamoto et al. used 20kWCO2 high-power laser welding equipment to study the welding methods and joint characteristics of ultra-fine grain steel with chemical composition (%) of 0.049C-1.50Mn-0.981Si-0.021P-0.0009S, aiming to minimize the damage of ultra-fine crystal structure. At the same time, the properties of welded joints are obviously improved. AkihikoOHTA et al. developed a low transition temperature type welding wire with improved fatigue strength. The welding wire contains 10%Cr and 10%Ni, and the beginning temperature of austenite conversion to martensite is about 180℃, and the end temperature of transformation is room temperature. When martensitic transformation occurs, the expansion of weld metal results in compressive residual stress around the weld, which increases the fatigue strength of welded joint. The fatigue strength of the joint of the ultra-fine grain bar with a grain size of 1μm is up to 300MPa, which is 100MPa higher than that of the traditional welding wire.
