高V、N微合金钢动态CCT曲线及纳米碳氮化物析出冷速敏感性

Dynamic CCT curves and precipitation sensitivity of nano-sized carbonitride against cooling rate in the high-V, N micro-alloyed steel

摘要:

为实现对高V、N微合金钢轧制-冷却工艺过程中组织、析出相的精准控制,利用Gleeble-1500D热模拟实验机研究了轧后不同冷速下实验钢的热膨胀曲线、相变规律、过冷奥氏体动态连续冷却转变曲线(动态CCT曲线),并着重研究了微观组织演变行为、显微硬度和纳米碳氮化物析出行为对冷却速率的敏感性。结果表明：冷却速率低于3 ℃/s时,实验钢显微组织由铁素体和珠光体组成；当冷却速率位于3 ℃/s时,发生贝氏体相变,基体组织由铁素体、珠光体和贝氏体组成；冷却速率为8 ℃/s时,珠光体组织消失,马氏体组织开始出现,基体组织由沿晶铁素体、贝氏体和马氏体组成；当冷却速率达到20 ℃/s时,基体组织中马氏体占主,并由少量先共析铁素体和贝氏体组成。此外,冷却速率对纳米碳氮化物的析出行为也具有显著影响,冷速处于1 ℃/s以内时,多边形铁素体中纳米析出相直径和数密度具有较强的冷却速率敏感性,纳米析出相直径随冷速提升显著降低,数密度随冷速提升而提高；冷速由1 ℃/s增加至3 ℃/s时,纳米析出相直径进一步降低,而数密度趋于稳定；当冷速继续增至5 ℃/s时,纳米析出相直径保持稳定,数密度呈现下降趋势。研究还发现,贝氏体组织中纳米析出相较少,贝氏体不利于纳米相析出。基于上述组织演变与析出规律的研究,工业化试制出了屈服强度700MPa以上、满足抗震要求的高V、N微合金钢。

Abstract:

In order to achieve precise control of the structure and precipitates during the rolling-cooling process of the high V, N micro-alloyed steel, the Gleeble-1500D thermal simulated test machine was used to study the thermal expansion curve and phase transformation rules of the experimental steel at different cooling rates after rolling. Especially, the dynamic continuous cooling transformation curve (dynamic CCT curve) of supercooled austenite, and the microstructure evolution behavior, microhardness and sensitivity of the nano-sized carbonitride precipitation behavior to the cooling rate were studied. The results show that when the cooling rate is lower than 3 °C/s, the microstructure of the experimental steel is composed of ferrite and pearlite. As for the cooling rate is 3 °C/s, bainite transformation occurs, and the matrix structure is composed by ferrite, pearlite and bainite. On the other hand, the pearlite structure is disappeared, and the martensite structure begins to be formed at the cooling rate of 8 ℃/s, and the matrix structure is composed of intergranular ferrite, bainite and martensite. When the cooling rate reaches 20 °C /s, the matrix structure is dominated by martensite and it is mixed with a small amount of proeutectoid ferrite and bainite. Furthermore, the cooling rate also has a significant influence on the precipitation behavior of nano-sized carbonitrides. When the cooling rate is within 1 °C/s, the diameter and number density of the nanoprecipitates in the polygonal ferrite show strong sensitivity to the cooling rate. The diameter of the nanoprecipitates is apparently decreased with the increase of cooling rate, and the it is inverse for the change in number density. When the cooling rate increases from 1 ℃/s to 3 ℃/s, the diameter of the nanoprecipitates further decreases, and the number density tends to be stable. As the cooling rate is further increased from 3 ℃/s to 5 ℃/s, the diameter of the nanoprecipitates keeps constant, and the number density is decreased. It was also found that less nanoprecipitates is contained in the bainite structure and the bainite is not helpful for the precipitation. Based on the above research on the structure evolution and precipitation law, a high V, N micro-alloy steel with a yield strength of more than 700 MPa and meeting seismic requirements has been industrialized and trial-produced.