| Thick plate used for ship is one of the important strategic products, and there has been an increasing demand for thick ship plate with excellent weldability in high-heat input welding in recent years. During the thermal cycle of high-heat input welding, the fine microstructure of steel plates of heat affected zone(HAZ) is destroyed, resulting in the markedly reduced toughness of the HAZ. So improving the toughness of HAZ has been the key subject in the fields of steel research. Oxide metallurgy is considered a effective way to deal with such problem for oxide metallurgy can be used for the retardation of grain growth by the use of fine particles, which are stable at the high temperature. Mg has been proved to be one effective element in oxide metallurgy for the formation of dispersed and fine MgO particles that are stable at high temperature. However, the detailed and systemic research on steel with Mg treatment is seldom reported. So this thesis focused on the comprehensive study on the influence of Mg treatment on the base metal and HAZ of ship plate steel. In this study, EH36 ship plate steel was selected for its wide appliance and high quality requirement.Based on the alloy system of EH36 ship plate steel, the composition of each element in smelting steel was designed and activity coefficient of each element in the molten steel was worked out by the first order activity interaction coefficient. Then the equilibrium relation between [%O] and [%Mg] in the molten steel with Mg treatment was also calculated, which could provide theoretical basis for this experiment. According to the performance index of EH36 ship plate steel, the thermo-mechanical control process(TMCP) was designed. The experiment steels were made by vacuum melting furnace with traditional metallurgy technology and metallurgy technology with Mg treatment. Both the molten steel were then cast into ingots and then subsequently hot rolled to get test steel plates with same TMCP.This study found from comparative researches that Mg treatment has little effect on the microstructure and performance of base metal even though a lot of fine inclusions occurred in the steel after Mg addition. Whether on the direction prependicular or parallel to the rolling direction, the microstructure and morphology did not change. And both in the transversal and longitudinal direction of the steel with Mg treatment, the low temperature impact toughness, tensile strength and yeild strength were comparative to steel without Mg treatment.The toughness degradation mechanism of HAZ in plate steel after high-heat input welding was analysed. It was found that the microstructure of HAZ became coarse during experiencing long-time high temperature near the welding line, and meanwhile its toughness lowered sharply due to the formation of brittle zone during the air-cooled process. Furthermore, the impact toughness improvement mechanism of HAZ in Mg treatment steel was discussed. Fine inclusions deposited on the austenite grain boundary and retarded the growth of austenite grain as pinning particles during the themal cylcle of high-heat put welding. And simultaneously, the fine inclusions solid soluble in the austenite grains could induce the promotion of intragranular ferrite(IGF) during the solid phase transformation from austenite to ferrite, which would finally optimize the microstructure and improve the impact toughness of HAZ in plate steel.Plate steel with Mg treatment exhibited excellent low temperature impact toughness of HAZ compared with steel without Mg treatment. It was observed in the plate steel with Mg treatment that the impact fracture of HAZ was composed of a lot of tiny cleavage sections, while in the plate steel without Mg treatment, the impact fracture of HAZ was composed of larger area cleavage sections. It was measured that the average fracture energy of HAZ in plate steel with Mg treatment was 5 times more than that in plate steel without Mg treatment.The influence of Mg treatment on the composition, grain size and distribution density of fine inclusions in plate steel were studied by the combination of scanning electron microscope (SEM), transmission electron microscopy(TEM) and Energy Dispersive Spetrum (EDS) technology. It was shown that the amount of Al2O3 existing in inclusions(>0.2?m) with micro dimension decreased evidently and the size distribution concentrated in 0.2-1.5?m after the Mg was added to the steel. It was also found that Mg addition had introduced a great quantity of inclusions smaller than 200nm to the steel, which was 10 times higher than the quantity of inclusions(<0.2?m) existing in steel without Mg treatment.To comprehensively understand the improvement of fine inclusions on the impact toughness of HAZ in plate steel, confocal scanning laser microscope(CSLM) and electron back scattered diffraction (EBSD) technology as well as SEM, TEM and EDS technology were used. It was through CSLM technology that the grain growth and transformation of HAZ during welding thermal input were in situ observed. It was found that a great quantity of fine inclusions with the size smaller than 200 nm introduced by Mg addition had a effective pinning effect on the austenite and could retard the austenite grain growth at high temperature zone during high-heat input of welding. Under the completely same thermal input of welding, the average grain size of prior austenite of HAZ in plate steel with Mg treatment was 6 times smaller than that in plate steel without Mg treatment. The ferrite grain boundary misorientation difference of HAZ was analysed by EBSD technology, which found that the proportion of IGF with large angle boundary in plate steel with Mg treatment increased remarkably. From the observation of the microstructure in HAZ, it was found that a great number of fine inclusions with the size ranging of 0.2?m-1.5?m introduced by Mg treatment were very effective to induce the IGF nucleation and growth, which would finally lead to the microstructure with cross interlocking shape, large angle boundary and full of dislocations. Such a microstructure could effectively inhibit the crack spreading and improve the impact toughness of HAZ.The smelting process of ship plate steel with Mg treatment could improve the impact toughness of HAZ in plate steel with high-heat input welding effectively without destroying the microstructure and performance of base metal in this kind of shipbuilding steel. |
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