【作者】 许文文；

【导师】 王青峰；

【作者基本信息】 燕山大学 ， 材料学， 2009， 硕士

【摘要】 本文为了研究低碳Mo-V-Ti-N微合金结构钢的焊接脆化倾向及其抑制方法,制备了典型增N(120ppm)、单独加B(12ppm)及N + B(210ppm+17ppm)复合添加等三种试验钢,并采用Gleeble3500热模拟机制备了各试验钢在不同t8/5(6～100s)下的模拟粗晶热影响区(CGHAZ)样品。针对各样品,观察了光学显微组织,统计了各组织形态参量;观察了马氏体-奥氏体(M-A)组元脆性相的透射(TEM)结构;测试了M-A及附近基体的纳米压痕硬度;测试了-20℃的V型缺口(CVN)试样的冲击功,观察了扫描(SEM)断口和二次裂纹形貌。结果表明,在Mo-V-Ti-N钢的各CGHAZ中均形成了含M-A的多相组织。其微观结构、硬度和断裂特征随t8/5增加的变化趋势是:贝氏体铁素体(BF)和针状铁素体(AF)的数量减少,先共析铁素体(PF)的数量增多;M-A的形态从条状向块状演化、尺寸增大、弥散度降低,其内部结构从板条马氏体+残余奥氏体向孪晶马氏体演化,与附近基体的硬度差增加;CVN冲击功降低,断口形貌从韧窝塑性断裂向准解理脆性断裂演化。另外,在Mo-V-Ti-N钢中添加微量的硼,在各t8/5下的CGHAZ中均可通过抑制PF的转变,促进针状铁素体和粒状贝氏体的形成,使M-A的含量降低、尺寸减小、弥散度提高,冲击韧性改善。因此,Mo-V-Ti-N钢的焊接脆化倾向取决于CGHAZ中含M-A组元的多相组织的微观结构和微观硬度特征,抑制方法是采用小线能量焊接或在钢中添加微量的硼。更多还原

【Abstract】 In this work, to investigate the brittleness susceptibility and metallurgical methods to depress it in the welding-induced coarse grain heat-affected zone (CGHAZ) of a low carbon Mo-V-Ti-N microalloyed structural steel, three experimental steels representing typical addition of N (120ppm), single addition of B (12ppm) and dual addition of N and B (210ppm+17ppm), respectively were prepared, by which the relevant CGHAZ samples simulated under different t8/5(6-100s, referring to the time interval for cooling from 800oC to 500oC ) were made using a Gleeble simulator. Their microstructures were examined quantitatively and the substructure of martensite-austenite constituents (M-A) as a second brittle phase was emphasized by transmission electron microscope (TEM) observation. The role of micromechanical aspects of M-A and its neighboring matrix phase in controlling the brittleness was also evaluated by the nanoindentation hardness measurements. The Charpy-V-Notch (CVN) impacts were additionally performed on each simulated CGHAZ at -20℃, and the fracture mode was revealed by scanning electron microscope (SEM) observations of the morphology of fractured surfaces and secondary cracking in sections perpendicular to them.It is shown that the M-A contained multi-phases microstructure formed in all the CGHAZs but in different morphologies. With the increasing t8/5, the amount of proeutectoid ferrite (PF) increased significantly at an expense of bainitic ferrite (BF) and acicular ferrite (AF). As a result, the densely distributed flaky M-A was gradually replaced by the more isolated massive M-A, which is accompanied with an increase in size and a change in substructure of M-A from the lath to the twin. The difference of nanohardness between the M-A constituents and the neighboring matrix was also evidenced to increase markedly with the increasing t8/5. Moreover, an increase in t8/5 causes the CVN impact energy of simuated CGHAZ to decrease and the fracture mode to change from dimple plastic fracture to quasi-cleavage brittle rupture, On the other hand, it is indicated that a trace addition of boron into the Mo-V-Ti-N steel resulted in a decreased fraction and size as well as a denser distribution of M-A by suppressing the transformation of PF in the CGHAZ obtained at each t8/5. The CVN toughness of CGHAZ was therefore significantly improved. With all the above results considered, the brittleness susceptibility in the CGHAZs of the Mo-V-Ti-N steel can be attributed to their microstructural and micromechanical features and as a metallurgical method to depress it, a relatively low heat input for welding process or a trace addition of boron in the steel is recommended.更多还原