| To develop the offshore oil and gas industry is an important industrial transformation of the national strategic level. At present, making localization of the offshore oil industry equipments and key materials is greatly significant to promote the progress of China’s marine engineering technology and to achieve the goal of strong country in marine economy. Starting from the goal of developing high-performance steels for marine engineering, the hardenability, strength&toughness of the ultra-heavy plate steel with high strength low alloy for marine engineering were studied with different research methods such as Thermo-Calc thermodynamic analysis combined with experimental observations, ANSYS numerical simulation combined with process experimental analysis and theoretical derivation combined with the microscopic statistical analysis with the help of the testing means such as OM, SEM, TEM, positron annihilation technique(PAT), AES and Jominy testing. The relationship between boron microalloyed technology and the hardenability of the low alloyed ultra-heavy plate steels, the effects of the sub-structures and heat treatment processes on the strength&toughness of the ultra-heavy plate steels were studied throughly in this paper.The study found that micro Ti and slightly higher content of Al could effectively inhibit BN to precipitate, ensure the solid solution of boron effectively, and strengthen the boron microalloying effect.But the inhibition of V on BN was very weak. The right segregation amount of boron on grain boundary was a precondition to obtain the desired hardenability of the steel, and the grain boundary segregation behavior of boron was affected by the quenching process. There was an optimal fit interval of quenching process, deviating from this optimal range, the amount of grain boundary segregation of boron was insufficient or excess, which would make a poor performance of hardenability adversely. During the single quenching, when the temperature was low (<920℃), the higher quenching temperature or appropriately extend the soaking time were conducive to the segregations of boron on the grain boundary, but when the temperature was too high, the segregation of boron declined. During the double quenching, only at the lower quenching temperature (≤920℃), reasonable double quenching could promote the segregation of boron.The quenching temperature, austenitizing soaking time, the double quenching and tempering temperature all had significantly influence on the strength&toughness of the steel. For the ultra-heavy plate steel with current component system, the best quenching temperature interval was890-920℃and the tempering interval was600-640℃, extending the soaking time appropriately and reasonable double quenching could effectively improve the hardenability and get a better matching of toughness and strength of the steel at the same time. ANSYS simulation results showed that the cooling rate of the center of the steel enhanced by increasing the surface cooling rate was limited during the quenching, the thickness was larger the limitation of the role was more obvious.In addition, after quenching and tempering, even if the surface of low alloy ultra-heavy steel got the high-temperature tempered martensite, the impact toughness level was still low. It was significantly different from the traditional law that high-temperature tempered martensite had a nice toughness, especially the toughness was extremely deteriorate in15mm of the surface zone in the steel. Combining with classical nucleation theory, a further study found that rapid cooling rate could make the nucleation rate of martensite phase transformation significantly reduce, which induced the sub-structures such as packets and blocks relatively crude and produced high density of dislocations. So it was one of most important causes for the surface effects of the ultra-heavy plate steel. |
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