车用高强钢DP780电阻点焊接头微观组织和力学性能的研究

发布时间：2018-06-04 05:09

本文选题：DP780 + 不等厚板　；参考：《上海工程技术大学》2016年硕士论文

【摘要】：本文针对不等厚双相钢点焊接工艺窗口窄、极易在焊接过程中产生飞溅、结合线深入和缩孔等缺陷,电极磨损极为严重和热影响区易发生软化等问题。采用厚度分别为1.6 mm和2.0 mm的DP780钢,通过电阻点焊工艺试验、金相观测、拉剪试验(TS)和单向拉伸试验(CT);采用万能拉伸试验机、维氏显微硬度仪等测试手段对焊接接头的微观组织和力学性能进行了相关研究。本文研究发现,在DP780双相钢电阻点焊接头中,熔核区的组织为板条状马氏体和铁素体。越接近熔核区的区域组织晶粒越粗大,且主要为板条状马氏体;反之,靠近母材的临界区形成的是细小晶体,主要为针状马氏体和铁素体。随着焊接电流增大,熔核尺寸增大,熔核区的马氏体组织更加粗大,热影响区范围减小,热影响区的晶粒组织变得粗大。电极压力增大,熔核尺寸变大,熔核区马氏体组织大小没有明显变化,热影响区范围减小,热影响区的晶粒组织变得更细小。焊接时间的增加,熔核尺寸及热影响区宽度无明显变化,热影响区晶粒组织变化不明显,但是熔核区马氏体组织变得粗大。不等厚双相钢板DP780电阻点焊接头显微硬度分布情况为:熔核区和热影响区的显微硬度非常接近,平均显微硬度在400 HV左右,母材的显微硬度最低,平均显微硬度在320 HV左右。值得注意的是热影响区显微硬度存在突然大幅度降低,发生了明显的软化现象,软化区的最低显微硬度低于250 HV。当焊接电流和焊接压力增大,熔核区的显微硬度有所增大,焊接时间对点焊接头硬度变化不明显。电阻点焊失效模式主有界面撕裂和熔核剥离断裂,界面撕裂发生在熔核上,属于脆性断裂;熔核剥离断裂发生在母材上,属于韧性断裂。不等厚双相钢板DP780电阻点焊接头熔核向2.0 mm侧偏移,并且1.6 mm侧和2.0 mm侧的压痕率、焊透率、熔核偏移量和两侧电极磨损情况均不相等,点焊接头单向拉伸失效吸收的能量大于拉剪失效吸收的能量。最佳电阻点焊工艺参数为:焊接电流为12 k A,焊接时间为10cycle,电极压力为3.5 k N。在此电阻点焊工艺参数下,接头的最大拉剪力为32.70 k N,最大单向拉伸力为19.51 k N。另外论文通过对不等厚双相钢DP780进行上述研究,对今后DP780点焊研究积累经验,为未来双相钢点焊研究提供了参考。
[Abstract]:In this paper, due to narrow window of unequal thickness duplex steel spot welding process, spatter is easy to occur in welding process, wire depth and shrinkage hole are combined, electrode wear is very serious and heat affected zone is prone to soften. Using DP780 steel with thickness of 1.6 mm and 2.0 mm respectively, through resistance spot welding process test, metallographic observation, tensile shear test and uniaxial tensile test, using universal tensile testing machine, The microstructure and mechanical properties of welded joints were studied by Vickers microhardness tester. In this paper, it is found that the microstructure of the nucleation zone in the DP780 duplex steel resistance spot welding joint is strip martensite and ferrite. The grain size of the region closer to the nucleation zone is thicker, and it is mainly lath martensite. On the contrary, fine crystals are formed in the critical region near the base metal, mainly acicular martensite and ferrite. With the increase of welding current, the size of the nugget increases, the martensite structure of the nugget zone becomes more coarse, the range of the heat-affected zone decreases, and the grain structure of the heat-affected zone becomes coarse. With the increase of electrode pressure, the size of the nugget becomes larger, the microstructure of martensite in the nugget zone does not change obviously, the range of the heat-affected zone decreases, and the grain structure of the heat-affected zone becomes smaller. With the increase of welding time, the size of the nugget and the width of the HAZ do not change obviously, but the microstructure of martensite in the heat affected zone does not change obviously, but the microstructure of martensite in the zone becomes coarse. The distribution of microhardness of DP780 resistance spot welding joints with different thickness is as follows: the microhardness of nugget zone and heat affected zone are very close, the average microhardness is about 400HV, the microhardness of base metal is the lowest, and the average microhardness is about 320HV. It is worth noting that the microhardness of the heat-affected zone decreases suddenly and a significant softening occurs, and the minimum microhardness in the softening zone is less than 250 HV. The microhardness of nugget zone increases with the increase of welding current and welding pressure, and the welding time has no obvious change on the hardness of spot welding joint. The main failure modes of resistance spot welding are interface tearing and nugget stripping fracture, the interface tearing occurs on the nugget and belongs to brittle fracture, and the delamination fracture of nugget occurs on the base metal, which belongs to the ductile fracture. The nugget of DP780 resistance spot welding joint with different thickness is shifted to 2.0 mm side, and the indentation rate, weld penetration rate, nugget deviation and wear of both sides are not equal between 1.6 mm and 2.0 mm sides. The energy absorbed by uniaxial tensile failure of spot welding joint is larger than that by tensile shear failure. The optimum process parameters of resistance spot welding are as follows: welding current is 12 Ka, welding time is 10 cycle. electrode pressure is 3.5 kN. The maximum tensile shear force and uniaxial tensile force are 32.70 KN and 19.51 KN respectively. In addition, through the above research on DP780 of unequal thickness duplex steel, this paper has accumulated experience in the research of DP780 spot welding in the future, which provides a reference for the research of spot welding of dual phase steel in the future.
【学位授予单位】：上海工程技术大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG453.9

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