【作者】 王学林；

【导师】 尚成嘉；

【作者基本信息】 北京科技大学， 材料科学与工程， 2018， 博士

【摘要】 随着海洋结构不断向更深的海域发展,用来建造海洋结构的工程用钢其强度与厚度都随之增大。与此同时,钢结构的建造对母材焊接性能提出了越来越苛刻的要求。但在焊接过程中,影响焊接区组织转变的因素包括母材和焊材成分、焊接工艺(热输入量、层间温度、冷速及道次间热循环等)以及焊后热处理等,焊接过程可变参数多,可控参数少,无论是焊接热影响区还是焊缝熔覆金属其组织转变均很复杂。可是,对于钢铁材料任何复杂的过程均有其规律,在认清其演变过程、影响因素的前提下均可得到有效的控制。因此,研究海洋工程用钢焊接过程的物理冶金原理及焊接性能均是十分重要的。本文首先从适用于低温服役环境的高强高韧埋弧焊丝的研发入手,探究了焊丝中合金成分Mn、Ni和Mo配比对焊缝金属显微组织和力学性能的影响。结果表明,焊缝组织主要为精细的针状铁素体、少量的先共析晶界铁素体、侧板条铁素体和弥散分布的细小马氏体/奥氏体(M/A)岛状颗粒。焊缝金属中0.2%Mo可以有效抑制先共析晶界铁素体及侧板条铁素体生成。此外,Mn和Ni的适量增加会促进针状铁素体形成,显著提高焊缝金属低温韧性。但Mn、Ni配比不当而超过某个范围时将会导致马氏体或其他低温相变产物形成,削弱低温韧性。对于K65管线钢而言,当焊缝金属获得1.5%-2.0%Mn、0.9%-1.2%Ni和0.2%-0.25%Mo时,可以确保其具有高强度的同时低温冲击韧性优异,且在Mn与Ni配比含量不越过马氏体形成线(Ms线)的前提下,可以采用“加Mn减Ni”的方法配比其合金含量。针对36mm厚550MPa级海洋工程用钢,研究了焊接工艺(层间温度)对多层多道焊接接头组织和性能的影响,并利用热模拟手段模拟了焊接道次间组织演变情况,建立了各类显微组织与冲击韧性的定性关系。结果表明,层间温度过高会导致铁素体粗化,柱状晶晶粒尺寸和M/A的体积分数显著增加,焊缝金属重加热区及热影响区的宽度增加,强度和硬度降低。层间温度过低则诱发盖帽区形成较高含量的马氏体,硬度波动大,冷裂倾向性高。焊缝冲击韧性最佳的层间温度为130℃左右,但均值仍不足70J。原因是,链状M/A在焊缝金属二次热循环的临界区、临界粗晶区形成,且实际焊缝金属冲击样品不可避免的包含上述脆性区,进而拉低整个焊缝金属的低温韧性。但,多次热循环的自热处理可以促进链状M/A分解,对改善低温韧性有益。为了改善厚板多道焊接头低温韧性,研究了传统回火和临界热处理对接头组织和性能的影响。研究发现,传统回火热处理工艺(回火温度500-740℃,回火时间30min)对改善55mm厚板多层多道焊缝金属低温韧性的作用不明显。但,采用临界热处理(淬火+临界退火+回火或临界退火+回火)可以显著提高焊缝金属低温韧性和均匀延伸率。临界热处理过程显著地促进了链状M/A回转,并通过Mn和Ni合金元素的两步富集而形成了有利于提高韧性的稳定残余奥氏体。经过上述两种工艺热处理后,焊缝金属-40℃冲击功可由39.2J提高到97.7J或83.5J。此外,研究表明,针对母材与焊缝成分,设计合理的焊后热处理工艺,可以实现母材与焊缝金属综合性能的整体提高。针对海洋工程用钢焊接粗晶热影响区显微组织的晶体学研究及定量化表征显示,晶体学是材料的本征属性,晶体学结构决定着其冲击断裂韧性的高低。降低冷速或者减小特定条件下奥氏体晶粒尺寸,会促进贝氏体变体选择加强,变体选择模式由CP(close-packed plane)分组向Bain分组转变,并显著降低由V1/V2变体对构成的block界面密度。此外,冲击韧性与晶体学结构定量化关系的建立显示,冲击韧性的变化主要与晶体学的block尺寸密切相关。而在特定的冷却条件下,奥氏体晶粒的不均也会导致变体选择模式多样化,裂纹扩展机制不一,冲击韧性波动大。更多还原

【Abstract】 With the continuous development of marine structures from shallow sea to deep sea regions,the strength and thickness of the steel used for the construction of the marine structure increase gradually.Meanwhile,the construction of steel structure has more stringent requirements for the welding performance of the base material.But in the welding process,factors affecting the microstructure transformation included chemical composition of the base metal and welding material,welding procedures(heat input,interpass temperature,cooling rate and welding thermal cycle),post-weld heat treatment and others,are variable and less controllable.Thus,the microstructure transformed either in welding heat affected zone or weld metal is very complex.Nevertheless,for any complex process of the steel formation,it has its laws and can be effectively controlled on the premise of identifying its evolution process and influencing factors.Therefore,it is very important to study the physical metallurgy principle and welding performance of the offshore engineering steel.In this paper,the submerged arc welding wire with high strength and high toughness was developed to use for low temperature environment,and the effect of Mn,Ni and Mo proportion on microstructure and properties of weld metal was studied.The results showed that the microstructure of weld metal primarily comprised of fine acicular ferrite(AF),proeutectoid grain boundary ferrite(GBF),ferrite side plates(FSP)and small martensite/austenite(M/A)constituents.The weld metal with 0.2%Mo can effectively restrain the formation of GBF and FSP.The increased Mn and Ni enhanced the low temperature toughness of weld metal by increasing the fraction of acicular ferrite.However,the concentration of Mn and Ni should be control under a critical value,as much more Mn and Ni additions would promote the formation of martensite or other low temperature microstructural features,which is detrimental to low temperature toughness.For K65 pipeline steel,the optimum combination of alloying element content was 1.5%-2.0%Mn,0.9%-1.2%Ni,0.2%-0.25%Mo.Excellent strength and toughness can be obtained through replacing Ni by Mn in the terms of the concentration of Mn and Ni being above the Ms(martensite start)line.The influence of interpass temperature on the microstructure and mechanical properties of multi-pass weld joints(up to 36-mm thickness)in a 550MPa grade offshore engineering steel was studied.Moreover,the microstructural evolution in weld metal during thermal cycle was also investigated through thermal simulation,and then the qualitative relationship between various microstructure and impact toughness was established.The results showed that increasing the interpass temperature leads to the coarsen of ferrite.Moreover,the grain size of columnar grain and the volume fraction of M/A increase significantly.In addition,the width of reheated zone in weld metal and heat affected zone increase,accompanying by a decrease in strength and toughness.The low interpass temperature promotes a larger volume fraction of martensite to form in the cap,which,in turn,scatters the hardness and enhances the tendency of cold cracking.Optimal mechanical properties were obtained at the interpass temperature of～130℃,while the average impact toughness is still less than 70J.Because the machined notch of actual impact sample contained one or more brittle reheated zones.In these brittle reheated zones,a large amount of M/A constituent necklacing prior austenite grains was formed in the reheated zone of all weld metals and was mainly responsible for lower toughness of the entire weld metal.Fortunately,this deterioration in toughness could be reduced as decomposition of necklace type M/A constituent occurred due to the later welding passes.To enhance the toughness of multi-pass weld joint,conventional tempering and new intercritical heat treatment were designed.The results suggested that there was insignificant effect on toughness through conventional tempering(tempering at 500-740℃ and holding for 30min),but obvious improvement through combination of quenching plus intercritical annealing and tempering.The intercritical heat treatments that promotes the reversion of necklace M/A constituent and formation of～5-6vol.%retained austenite by the enrichment of Mn and Ni in reversed austenite during intercritical tempering process,significantly enhance the low temperature(-40℃)impact energy from 39.2J in weld metal to 97.7J or 83.5J.Moreover,the comprehensive performances of base metal and weld metal could be improved significantly through a reasonable post-weld heat treatment designed based on alloy compositions.Crystallographic study on the microstructure of coarse grained heat affected zone of offshore engineering steel indicated that crystallography is the intrinsic property of materials,and the crystallographic structure determines its impact fracture toughness.Decreasing the cooling rate or prior austenite grain size under a specific condition,will enhance the variant selection of bainite.Meanwhile,the mode of variant selection changes from CP(close-packed plane)grouping to Bain grouping,which significantly reduces the density of block boundary formed by V1/V2 variants.In addition,the establishment of the quantitative correlation between impact toughness and crystallographic structure suggested that the variation of impact toughness was mainly correlated to the block size.Furthermore,under certain cooling conditions,the uneven size of austenite grain will also lead to the diversification of variant selection modes,which,in turn,diversifies the mechanism of crack propagation and increases the impact toughness scatter.更多还原