大線能量焊接用鋼CGHAZ韌性提升方法研究

大線能量焊接用鋼CGHAZ韌性提升方法研究Study on the Method of Improving the Toughness of CGHAZ for High Heat Input Welding Steels

通過研究焊接熱影響區(qū)（HAZ）沖擊值分布圖，構想出大線能量焊接用鋼焊接粗晶熱影響區(qū)（CGHAZ）韌性提升的新方法，即在峰值溫度不變的條件下，將焊接CGHAZ中晶界鐵素體（GBF）和大量針狀鐵素體（AF）組織改變成完全重結晶區(qū)（FGHAZ）多邊形鐵素體（PF）組織，并消除CGHAZ中破壞韌性的側板條鐵素體（FSP）組織。研究中以對比Ti-V-N與Al-Ti-V-N微合金焊接用鋼焊接CGHAZ組織和韌性為基礎，探討Al-Ti-V-N鋼焊接CGHAZ中PF轉變條件、形核機制，認為微米級氧化夾雜物是誘導焊接CGHAZ中大量PF形核的關鍵，納米級碳氮化物是拖曳、釘扎奧氏體與鐵素體晶界的關鍵，兩者的有效配合保證了焊接CGHAZ中大量PF組織生成，從而大幅提升焊接CGHAZ低溫沖擊韌性。

Compared with the low heat input welding steel structures, the high strength low alloy(HSLA) steel structures after high heat input welding keep high temperature with longer time, and the cooling speed is slower, then the austenite crystal grains of coarse-grained heat affected zones(CGHAZ) grow up sharply, and coarse upper bainite(UB) and ferrite side plate(FSP) are generated easily in original austenite crystal, thus toughness of CGHAZ deteriorates seriously. At present, the approach of improving toughness of CGHAZ is to produce massive interleaved acicular ferrite(AF) in the original austenite crystal. However, with the improvement of welding capability for thick plate, welding heat input will be greater, the hold time of high temperature will be more prolonged. In this case, AF coarsens much seriously, thus the improvement of CGHAZ toughness is limited severely. The paper proposes a new method of improving toughness of CGHAZ in high heat input welding steel by studying the distribution map of HAZ impact value. This new method changes the grain boundary ferrite(GBF) and AF of the CGHAZ to polygonal ferrite(PF) of the fine-grained heat affected zones(FGHAZ) at same peak temperature, which improves toughness of CGHAZ significantly. Comparing the microstructures and toughness of CGHAZ in Ti-V-N and Al-Ti-V-N micro alloy welding steels, the transformation condition and nucleation mechanism of PF in the CGHAZ of Al-Ti-V-N steel is studied. It is found that micron oxide inclusions is key factor to induce the nucleation of massive PF in CGHAZ, and nanoscale carbonitride is key factor to drag and pin the grain boundaries of austenite and ferrite. Therefore, the effective combination of above two factors guarantee the generation of a large number of PF, which improves the impact toughness greatly at low temperature.

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