Systematically analyze and study the problem of low temperature impact performance failure of SA516Gr70 steel plate with a recent production thickness of 34-58mm, in order to determine the main cause of poor toughness of the steel plate, and improve the low temperature impact performance qualification rate of the steel plate through a series of process optimizations. At the same time, provide data support and theoretical guidance for the production of other similar steel grades.
The usage environment and mechanical properties of steel plates
Explosive composite panels, as steel for PTA (purified terephthalic acid) projects, will be used in corrosive environments. In the production of composite panels, SA516Gr70 steel plates are used as substrates and metal T composites, which can fully utilize the anti-corrosion properties of T plates and also provide support for SA516Gr70 steel plates.
Due to the location of the project being in a cold region, it is required that the steel plate still has good impact toughness in a low temperature environment of -35 ℃, and the impact energy after long-term simulated post weld heat treatment at (565+10) ℃ x17 h can still reach the minimum value of 60J, in order to meet the requirement that the average impact energy of the explosive composite plate after stress relief should be greater than 41J.
C is the main element second only to Fe, which can directly affect the strength, plasticity, toughness, and welding performance of steel. Considering the influence of C element on the hardenability of steel and the precipitation strengthening effect of carbides formed with trace elements Nb, V, etc., the content of C element is controlled within the range of 0.16% to 0.18%. Nb and V elements play a role in refining grain size and precipitation strengthening in controlled rolling. To fully utilize the role of Nb alloy, solid solution Nb precipitates and aggregates on dislocation lines during high-temperature rolling. As its concentration increases and its pinning effect is exerted, it exerts strong drag on dislocation climb, making it difficult to form nucleation points for recrystallization. Therefore, adding Nb can achieve high-temperature rolling in the non recrystallization zone (rolling temperature<950 ℃). The rolling process can form a large number of distortion bands to provide more nucleation points for ferrite phase transformation, promote ferrite phase transformation to form more refined grains, and improve the impact toughness of ferrite matrix. V elements mainly participate in the y → α transformation. During the process, the precipitation between phases and within the ferrite is strengthened, resulting in a higher volume fraction and density of the precipitated phase
Observe and analyze the metallographic structure of the unqualified batch of steel plates, as shown in Figure 1. From Figure 1, it can be seen that there is local non-uniformity in the metallographic structure, with a small amount of mixed crystals appearing. Mixed crystals have the greatest impact on the mechanical properties of the steel plate, especially the low-temperature impact toughness. At the same time, the strip structure of the unqualified batch of steel plates is more obvious, with a grain size of 8-9 grades.
