| High strength low alloy steels used in large oil storage tanks mainly depend on imports in our country. This unfavorable situation must be changed by localization of this steel. In the building process of oil storage tanks vertical position of stave sheets are welded using electro-gas welding (EGW) with high heat inputs, but after this operation the toughness of heat-affected zone (HAZ), especially coarse grain heat-affect zone (CGHAZ) can decrease sharply which has become a key problem for steels localization. Nb is an important alloy element that can improve mechanical properties, especially tensile strength, but the effect of Nb on the impact toughness of HAZ is still controversial. Therefore whether it is necessary to add Nb and how much is added is appropriate in the steel suffering high heat inputs deserve further study.For this reason, three experimental steels with different Nb content (0% Nb, 0.015% Nb, 0.026% Nb) are smelted and rolled. After the heat treatment of reheat-quenching and tempering (RQ-T), temper bainite and a small amount of ferrite are observed in base metal of the three steels. Microstructures are refined as a result of Nb addition. Tensile strengths of the three steels are higher than 610MPa, and high low temperature impact toughness at -40℃are also obtained. Based on the three steels CGHAZ continuous cooling transformation CCT diagrams are determined by thermal simulation method. The microstructures and mechanical properties of HAZ at different heat inputs and different peak temperatures are investigated by mechanical tests, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The influence mechanism of Nb on microstructure transformation of CGHAZ is investigated. The relationship of Nb content, heat input, the peak temperature and impact toughness is discussed. The effect of M-A constituent on impact toughness of CGHAZ is studied. The morphology, distribution and size of precipitation particles of CGHAZ are also investigated. The main results can be summarized as follows:The investigations on the continuous cooling transformation of CGHAZ combined with phase transformation kinetic for the steel without Nb and with 0.026% Nb indicate that transformation start temperature Ts decrease with the increase of cooling rate from 1°C/s to 125°C/s. Ts of the steel with 0.026% Nb are all lower than that without Nb and Nb addition push the transformation start to lower temperature.“S-shape”phase transformation kinetic curves of CGHAZ of the two steels are observed, and experiment data is well fitted to J-M-A equation.The investigations on microstructure and properties of simulated CGHAZ at different heat inputs and peak temperatures indicate that the Charpy impact energies decrease with the increase of Nb content (0%-0.015%-0.026%) at heat inputs range from 30kJ/cm to 100kJ/cm. At lower heat inputs (30~40kJ/cm), slight toughness decrease for the steels with Nb are observed and Nb promotes the formation of low carbon martensite with good toughness. At higher heat inputs (60~100kJ/cm), Nb addition can deteriorate impact toughness severely, and the mechanism of toughness decrease is considered as that the solute-drag effect of Nb in solid solution on ferrite grain boundaries is greater than bainite grain boundaries, and impedes ferrite growth kinetics, promotes the formation of granular bainite with poor toughness in CGHAZ. After welded by EGW with the heat input of 100kJ/cm, HAZ widths of the steel without Nb and with 0.026% Nb are all 7.5~8mm, but the CGHAZ width of the steel with 0.026% Nb increases approximately by 1.5mm compared to without Nb. The zone of toughness decreased drastically locates at 0~1mm away from the fusion line and impact toughness decrease with the increase of Nb content. The softening zones is observed at 5~6mm away from the fusion line and the softening degree decrease with the increase of Nb content.The investigations on the relationship of M-A constituent and impact toughness of simulated CGHAZ at high heat input of 100kJ/cm indicate that toughness values decrease with the increase of area percentage content of M-A constituent. The damage of massive M-A constituent on impact toughness is more severe than that of long strip. Impact toughness decreases with the increase of probability of massive M-A constituent. Considering the influence of alloy elements on the formation of M-A constituent, the relationship of area percentage contents of M-A constituent, probability of massive M-A constituent, impact toughness are analyzed by the method of multiple linear regressions and effective equations are obtained.The investigations on the particle coarsening behavior in simulated CGHAZ at different heat inputs indicate that cuboid TiN precipitation and spherical VC, M3C, M23C6 are observed in the base metal of the steel without Nb. After welding non-rectangular particles disappear and TiN particles coarsen with the increase of heat inputs that are considered related to dissolution and reprecipitation of particles and unrelated to isothermal growth of TiN. Spherical (Ti, Nb)(C, N) particles or cap-shaped attachment on near cuboid core particles are observed in the base metal of the steel with 0.026% Nb, and mean particles radius increase from 7.9nm of the steel without Nb to 9.32nm of the steel with 0.026% Nb and the morphologies of particles change to cuboid TiN with the increase of heat inputs. The phenomena of particles coarsening can be explained as the solid solution of TiC, NbC and NbN and dissolution of small particles and reprecipitation that can be explained by the Ostwald Ripening mechanism. Aiming at the thermal cycle used in welding thermal simulation, a new prediction equation for particle size of CGHAZ at different heat inputs is presented. The calculation of particle size is divided into three stages: isothermal holding, continuous heating and cooling, and the calculated results are in agreement with experimental data. |
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