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利用热处理炉对热轧抗拉强度700 MPa级Nb-Ti微合金钢的激光焊接接头进行了回火处理,研究了回火温度对焊接接头各个微区显微组织和硬度的影响规律。结果表明,未处理样品焊接接头各个微区的显微组织分别为焊缝区和粗晶区均为板条马氏体;细晶区和混晶区均为铁素体和M-A组元的混合组织,但细晶区的显微组织更为精细;母材的显微组织为铁素体和沿着铁素体晶界分布的碳化物。当回火温度在400~500℃之间时,焊缝区的硬度变化不明显,均在310 HV左右,随着回火温度的升高板条马氏体束之间析出的碳化物逐渐增多,粗晶区的硬度出现明显的降低,由未处理样品的350 HV降低至315HV左右;当回火温度达到550℃时,焊缝区和粗晶区的硬度均出现陡增,硬度上升幅度约为35 HV左右,其主要原因为该区域中固溶的Nb、Ti和C元素发生了二次析出提高了其硬度;当回火温度超过550℃至650℃时,焊缝区和粗晶区的硬度均出现了明显的降低,主要是由于板条马氏体发生了明显的再结晶。在本文所研究的回火温度范围内(400~650℃),回火温度对细晶区和混晶区的组织和硬度影响不明显。
The laser welded joint of hot rolled Nb-Ti microalloyed steel with tensile strength of 700 MPa was tempered by heat treatment furnace. The influence of tempering temperature on the microstructure and hardness of each micro-zone was studied. The results show that the microstructures of the welded sections of the untreated samples are lath martensite in the weld zone and the coarse grain zone, respectively. Both the fine grain zone and the mixed grain zone are the mixture of ferrite and MA However, the microstructure of the fine-grained zone is more refined. The microstructure of the base metal is composed of ferrite and carbides distributed along the ferrite grain boundaries. When the tempering temperature is between 400 ℃ and 500 ℃, the hardness of the weld zone does not change obviously, all around 310 HV. With the increase of tempering temperature, the precipitation of carbides between martensite beams increases , The hardness of the coarse grain region obviously decreases from 350 HV of the untreated sample to about 315 HV; when the tempering temperature reaches 550 ℃, the hardness of the weld zone and the coarse grain area all increase sharply, and the hardness increases by about Is about 35 HV, the main reason is that the secondary precipitation of solid solution Nb, Ti and C elements in the region increases the hardness; when the tempering temperature exceeds 550 ℃ to 650 ℃, the weld zone and the coarse grain zone The hardness of the slab was obviously decreased, mainly due to the significant recrystallization of lath martensite. In the tempering temperature range studied in this paper (400 ~ 650 ℃), the influence of tempering temperature on the microstructure and hardness of the fine grain and mixed grain region is not obvious.