Control of Microstructure in Heat Affected Zone of Welded Metal using Oxide, Sulfide and Nitride Fine Particles

众所周知，在大热容焊接过程中，焊接金属热影响区(HAZ)的韧性会因奥氏体晶粒长大而变差。为了防止这一恶化，研究了用氧化物、硫化物和氮化物微粒控制焊缝金属组织的方法。在钢的凝固过程中，均匀弥散在钢液中的氧化物和硫化物微粒成为奥氏体晶核的形核中心。因此，可以控制显微组织的形态。利用焊接热循环模拟器，在类似于大热容焊接过程的热条件下对该样品进行加热。阐明了一次夹杂物和二次夹杂物(凝固过程中析出的硫化物和碳氮化物)的成分、尺寸分布和弥散性对热影响区晶粒长大的影响。研究了夹杂物的钉扎效应、偏析元素的溶质拖曳效应以及析出的硫化物和碳氮化物的溶解和长大对热影响区晶粒长大控制的影响。讨论了夹杂物颗粒组成和尺寸分布对晶间铁素体组织和形核的影响。在Ar气氛中熔炼Fe-C合金，控制初始氧、硫和氮含量。加入Fe-X合金(X=Al、Mg、Zr、Ti、Si)可形成初生夹杂物颗粒。研究了初生夹杂物颗粒在凝固和变质过程中对组织形核的影响。研究了初生夹杂物颗粒组成和尺寸分布与组织形态(柱状、立方)的关系。由于发现二次夹杂物(硫化物、碳氮化物)析出在一次夹杂物颗粒上，因此讨论了多元夹杂物的形成及其对IGF形成的影响。

It is well known that the toughness at the heat affected zone of welding metal (HAZ) becomes wrong due to austenite grain growth after large-heat capacity welding process. In order to prevent this deterioration, control of microstructure in welded metal by using oxide, sulfide and nitride fine particles has been investigated.Fine oxide and sulfide particles, which were dispersed uniformly in liquid steel, became nucleation sites of austenite grains during solidification of steel. As a result, morphology of microstructure could be controlled. This sample was heated under the thermal condition similar to a large-heat capacity welding process by using welding thermal cycle simulator. The influence of composition, size distribution and dispersion of primary inclusion and secondary inclusions (sulfide and carbonitride precipitated during solidification) on grain growth in HAZ was clarified. The Influences of pinning effect of inclusion, solute drag effect of segregated elements, and dissolution and growth of precipitated sulfide and carbonitride on the control of grain growth at HAZ were also investigated. The influences of composition and size distribution of inclusion particles on the microstructure and nucleation of inter granular ferrite (IGF) were discussed.Fe-C alloy was melted in Ar atmosphere, and the initial oxygen, sulfur and nitrogen contents were controlled. The primary inclusion particles were formed by the addition of Fe-X alloy (X=Al,Mg,Zr,Ti,Si). The effect of these primary inclusion particles on the nucleation of microstructure during solidification and modification was examined. Relationships between the morphology of microstructure (columnar, cubic) and the composition and size distribution of primary inclusion particles was investigated. Since it was found that secondary inclusion (sulfide, carbonitride) was precipitated on the primary inclusion particle, the formation of multi-composed inclusion and its effect on IGF formation was discussed.