【作者】 贾坤宁；

【导师】 王国栋；

【作者基本信息】 东北大学 ， 材料加工工程， 2008， 博士

【摘要】 目前,高强度桥梁钢在国外已经达到较高的水平,且形成了大量的专利技术。而在我国高强度桥梁钢的开发和研究却处在起步阶段,因此,国内急需开发高强度桥梁钢。本文在首钢总公司—东北大学“钢种开发”项目的支持下开展研究,对开发的高强度桥梁钢Q460q的焊接性能进行了系统研究。论文主要工作如下：1)采用热输入量较大的焊接方法即埋弧焊对Q460q进行不同工艺参数条件下的焊接,确定合理的焊接工艺参数,并对Q460q的使用焊接性和工艺焊接性进行评价。结果表明：高强度桥梁钢Q460q具有良好的使用焊接性能,即焊接接头的强度不低于母材,-60℃-20℃条件下韧性虽稍低于母材但均表现出较好的韧性,且焊接接头中焊缝、熔合区和热影响区的DBTT均在较低的温度范围内；高强度桥梁钢Q460q在焊接过程中产生冷裂纹的倾向不大,故其工艺焊接性较好。2)采用热模拟方法全面研究了高强度桥梁钢Q460q和Q460qNH不同焊接热输入条件下热影响区的组织性能。结果表明：Q460q和Q460qNH在t8/5为30s-200s条件下(E相当于38～117k J/cm)模拟热影响区粗晶区的组织主要为贝氏体,具有板条和粒状两种形态,贝氏体中的M-A组元的形态、分布和数量随着t8/5的改变而变化并对粗晶区的韧性产生显著影响；随着t8/5的增加,粗晶区原始奥氏体晶粒尺寸不断增大,有效晶粒尺寸的增大使粗晶区韧性显著变差。因此为提高焊接热影响区粗晶区的韧性,应采取合适的方法来控制粗晶区晶粒尺寸。3)测定了高强度桥梁钢Q460q和Q460qNH的SH-CCT图,为研究高强度桥梁钢焊接物理冶金规律、制定适宜的焊接工艺奠定了基础。4)利用R>0的正弦波对高强度桥梁钢Q460q焊接接头的疲劳S-N曲线、疲劳裂纹中速扩展区Paris公式材料常数C、指数m以及裂纹门槛值△Kth进行了测试,并对Q460q焊接接头疲劳裂纹萌生机理进行了研究。结果表明：Q460q在R>0时的疲劳极限为470MPa,在较高应力幅下的疲劳裂纹起源于试样表面的某种缺陷,在较低应力幅下疲劳裂纹起源于大尺寸夹杂物。焊接接头中焊缝的裂纹门槛值△Kth高于热影响区和母材,而裂纹扩展速率低于热影响区和母材。焊缝金属随着疲劳载荷下循环次数的增加,依次形成位错纠结→位错墙→位错胞→亚晶,亚晶界开裂从而形成裂纹。5)在较大热输入量条件下(t8/5>80),高强度桥梁钢热影响区粗晶区出现脆化,为了改善粗晶区的韧性,首次研究了第二相粒子对微钙钢焊接热影响区奥氏体晶粒尺寸的影响。通过对母材进行Ca处理,在钢中产生弥散细小的第二相粒子CaO、CaS.一方面充分利用微钙钢中细小的高温热稳定的CaO、CaS粒子在1350-1450℃对奥氏体晶界迁移的钉扎作用,有效地减少焊接热影响区粗晶区的组织粗化,细化粗晶区原始奥氏体晶粒,提高韧性；另一方面利用焊接热影响区中较大尺寸的CaO粒子作为针状铁素体的形核点,促进晶内高强韧性的针状铁素体的形成。结果表明：在大热输入量条件下,即t8/5为80S、200s(相当于E为87.117kJ/cm),粗晶区原始奥氏体晶粒尺寸得到明显细化,且随着t8/5的增加,晶粒长大趋势小,韧性得到明显改善。6)采用真实焊接和热模拟方法研究了微钙钢不同条件下热影响区的组织性能,并确定了微钙钢较小的热影响区奥氏体晶粒长大倾向。结果表明：t8/5=8s时的热影响区粗晶区奥氏体平均晶粒尺寸约20μm;t8/5=20s时,奥氏体晶粒尺寸约40μm;t8/5=40s时,奥氏体晶粒尺寸约60μm;t8/5=80s时,奥氏体晶粒尺寸约80μm;t8/5=200s时,奥氏体晶粒尺寸约110μm。结果表明：微钙钢与一般低合金高强钢相比焊接热影响区粗晶区的原始奥氏体晶粒长大程度明显降低,微钙钢焊接热影响区晶粒细化效果显著。更多还原

【Abstract】 High strength bridge steel has been studied widely and substantive patents have been obtained in foreign countries at present. However, the study on high strength bridge steel in China has just been started recently. So, the development of high strength bridge is urgent to carry out in China. With the support of the project of "development of steels" taken on by Capital Steel Company and Northeastern University, the weldability of a high strength bridge steel Q460q was systematically studied in this thesis, and the main work is as follows:(1) In order to determine reasonable welding parameters, and to estimate the service weldability and fabrication weldability, Q460q was welded under different parameters using submerged-arc welding during which heat input is high. The results proved that, Q460q has high service weldability. The strength of the weld joint is not lower than the base metal, good toughness is obtained at -60℃～20℃, and the DATT of the welding line, bond and heat affected zone are low. In addition, cold crack is not formed during welding, proving the good fabrication weldability of Q460q.(2) The microstructure of heat affected zone(HAZ) in Q460q and Q460qNH with different heat input was studied through thermo mechanical simulation experiments. The results showe that bainite including lathing and granular formed in the coarse grain heat affected zone(CGHAZ) when t8/5 is in the range of 30s～200s. The morphology, distribution and quantity of the M-A constituents in bainite changed with t8/5 and has obvious effect on the toughness of coarse grain zone. The original austenite grain grows with the increase of t8/5-The increase of effective grain size deteriorate the toughness. Thus, in order to improve the toughness, the grain size of CGHAZ should be controlled with appropriate method.(3) The SH-CCT curve was determined, and it is greatly helpful to study the physical metallurgy during welding and to set down appropriate welding parameters. (4) The fatigue S-N curve of the welding joint, the material constant C, exponent m and the crack threshold value AKth in Paris Formula were determined with sine wave (R>0), and the initiation mechanism of fatigue crack was also investigated. The results showe that the fatigue value was 470MPa. The fatigue crack under higher stress amplitude originated from some defect at the surface of the sample, and originated from inclusion under lower stress amplitude. The AKth of welding line in the welding joint is higher than that of the base metal, while crack extension rate is lower than that of HAZ and base metal. The microstructure changes as follows with the increase of cycle index:dislocation tangle→dislocation wall formed→dislocation cell formed→subgrain formed. Then the crack developes when the subgrain boundary split.(5) The CGHAZ embrittlemented as t8/5>80. To improve the toughness of the CGHAZ, the effect of oxide on the physical metallurgy in low alloy high strength steel was firstly studied. Dispersed second-phase CaO and CaS formed by low Ca treatment. On one hand, The dispersed and steady CaO and CaS would pin the austenite boundary migrating at 1350～1450℃. As a result, the microstructure is refined in coarse grain zone and the toughness is improved. On the other hand, acicular ferrite can nucleate at larger CaO, leading to the formation of acicular ferrite with high toughness. It is observed that when t8/5 is 80s and 200s(E=87 and 117kJ/cm), the original austenite grains are refined evidently. The growing rate of grains slow down with the increase of t8/5, and the toughness is improved obviously.(6) The microstructure and properties as well as the growth tendency of austenite grains in HAZ of micro-added Ca steel were investigated by real welding and thermo mechanical simulation. Experiments were carried out under different conditions. The results showe that as t8/5= 8s,20s,40s,80s, and 200s, the average austenite grain diameter in CGHAZ is about 20μm,40μm,60μm,80μm and 110μm, respectively. Compared with the low alloy high strength steel, the austenite grain growth rate in coarse grain zone decrease dramatically in micro-added Ca steel. Grain refinement in HAZ is apparently effective in micro-added Ca steel.更多还原