冷却速率对Ti-Zr处理钢针状铁素体转变的影响

 大线能量焊接技术在低合金钢中的广泛使用降低了低合金钢的低温韧性,向低合金钢中添加适量的钛、锆元素可以促进针状铁素体形核,细化晶粒组织从而提高低合金钢的低温韧性。为了研究Ti-Zr处理钢中针状铁素体转变机理,使用25 kg真空感应炉中熔炼试验所需钢种,向低合金钢中添加了0.04%钛元素和0.014%锆元素;利用高温激光共聚焦显微镜原位观察了冷却速率对针状铁素体转变行为的变化;使用扫描电镜观察了Ti-Zr处理钢中的夹杂物成分和针状铁素体在夹杂物表面形核,使用光学显微镜观察不同冷速下的微观组织变化差异。试验发现,随着冷却速率从1增加至10 ℃/s,侧板铁素体的开始转变温度从770.2降低至632.4 ℃,针状铁素体的开始转变温度从731.5降低至612.6 ℃,针状铁素体的面积分数从47.91%增加至68.04%,针状铁素体与侧板铁素体的面积分数比值从1.34增加至3.54,针状铁素体与侧板铁素体的开始转变温度与结束转变温度差的比值从0.52增加至0.83,针状铁素体与侧板铁素体的面积分数之比与其转变温度区间之比存在正比例关系;不同冷却速率下,Ti-Zr处理钢中的主要夹杂物为ZrO₂-TiN-MnS,并且ZrO₂-TiN-MnS夹杂物可以有效促进针状铁素体形核;一方面是因为生成的贫锰区增加了铁素体转变驱动功;另一方面是因为ZrO₂-TiN-MnS夹杂物与铁素体有良好的晶格匹配关系降低了针状铁素体在夹杂物表面形核的界面能。

Effect of cooling rates on transformation of acicular ferrite in a Ti-Zr treated steel

The toughness of the low alloy steel is reduced by the wide use of high heat input welding technology and the addition of Ti and Zr elements can promote the nucleation of acicular ferrite which refine the microstructure and improve the properties of the low alloy steel. To study the transformation mechanism of acicular ferrite in Ti-Zr treated steel,the steel required for the experiment was smelted in a 25 kg vacuum induction furnace,0.04% Ti and 0.014% Zr were added to low alloy steel;the effect of different cooling rates on the transformation of acicular ferrite was observed by high temperature confocal microscope;the inclusion of Ti-Zr treated steel and the nucleation of acicular ferrite on the surface of the inclusions was observed by scanning electron microscope;the microstructure under different cooling rates was observed by optical microscope. Through experiments,it is found that with cooling rate increased from 1 ℃/s to 10 ℃/s,the start transformation temperature of side plate ferrite decreased from 770.2 ℃ to 632.4 ℃,the onset transformation temperature of acicular ferrite decreased from 731.5 ℃ to 612.6 ℃,and the area percent of acicular ferrite increased from 47.91% to 68.04%,the ratio of the area fraction for acicular ferrite to side plate ferrite increased from 1.34 to 3.54,and the ratio of the difference between the start and end transformation temperatures for acicular ferrite to side plate ferrite increased from 0.52 to 0.83. There is a proportional relationship between the ratio of area percent and the ratio of the transition temperature range on acicular ferrite and side plate ferrite. The main inclusions in Zr-treated steel were ZrO₂-TiN-MnS and the ZrO₂-TiN-MnS inclusion could effectively promote the nucleation of acicular ferrite. The nucleation mechanism is that transformation driving work increased by Mn-depleted zone the and inclusion has a good lattice matching with ferrite that interface energy was reduced.