Ni含量对高强度低合金钢淬透性影响的晶体学认识

Crystallographic Understanding of the Effect of Ni Content on the Hardenability of High-Strength Low-Alloy Steel

Ni作为可以同时提高强度和韧性的合金元素被广泛应用于高强度钢的生产中,但是当Ni的质量分数低于5%时,Ni元素的添加对于基体性能的改善不明显。然而对于低碳低合金钢而言,添加Ni会引起淬透性的变化,以及由此带来的协变相变产物的演变是不可忽略的。本工作设计了2种不同Ni含量的高强度低合金钢(0.92Ni钢和2.94Ni钢),通过末端淬火实验和热模拟实验研究了Ni含量对0.92Ni钢和2.94Ni钢淬透性及相变温度的影响,利用SEM和EBSD表征了0.92Ni钢和2.94Ni钢协变相变产物的显微组织和晶体学特征。结果表明,Ni含量的增加可以显著提高2.94Ni钢的淬透性,降低其相变温度。在0.5℃/s的低冷速下,2.94Ni钢的组织为板条贝氏体和少量呈薄膜状弥散分布的马氏体/奥氏体岛(M/A岛),形成以密排面分组(CP分组)为主导的相变模式,大角晶界密度、板条束(block)边界密度和V1/V2变体对含量较高,硬度也较高;而0.92Ni钢的组织为粒状贝氏体和呈粗大块状分布的M/A岛,形成以Bain分组为主导的相变模式,大角晶界密度、block边界密度和变体对含量较低,硬度较低。热力学和动力学分析表明,在0.5℃/s的低冷速下,Ni含量增加显著提高了2.94Ni钢的相变驱动力,转变速率更快;提高了未转变奥氏体中最大C含量的上限,促进了贝氏体的完全转变,减少M/A岛的含量。

A matrix structure with high strength,such as lath martensite/bainite is created via quenching to achieve conventional high-strength low-alloy ultra-heavy plates.Subsequently,this structure is tempered to improve its toughness.However,it is usually impossible to avoid the low cooling rate in the center of the ultra-heavy plates during cooling,causing inhomogeneous microstructure and mechanical properties along the normal direction.Therefore,it is necessary to enhance the hardenability of the alloy.At lower cooling rates,granular bainite/ferrites are formed in the center of the plates with low hardenability.While this leads to the incompletely transformed martensite/austenite islands(M/A islands),which often cause cracks,fewer high angle grain boundaries(HAGBs)are also formed,which can effectively impede crack propagation.Therefore,improving the strength,toughness,and hardenability is crucial for the development of high-strength low-alloy steel.The addition of nickel can improve the hardenability as well as the toughness of the heavy plates.In this study,two high-strength low-alloy steels with different nickel contents are designed.In addition,the effect of nickel content on hardenability and phase transition temperature is tested using end quenching and thermal mechanical simulation testing.The effects of nickel content on the microstructure and crystallographic characteristics of coherent phase-transformed products are characterized using SEM and EBSD.The results reveal that the increased nickel content greatly improves the hardenability and significantly reduces the phase transition temperature.At a low cooling rate of 0.5℃/s,the microstructure of 2.94Ni steel is lath bainite,and the M/A islands are dispersed on a thin film,forming a phase transformation mode with higher HAGB density,block boundary density and V1/V2 variant pair content,and high hardness.This mode is dominated by the close-packed plane group.While the microstructure of 0.92Ni steel is granular bainite and the M/A islands are distributed in coarse blocks,forming a phase transformation mode with lower HAGB density,block boundary density and V1/V2 variant pair content,and significantly low hardness.Moreover,this mode is dominated by the Bain group.Additionally,the results demonstrate that at the cooling rate of 0.5℃/s,as nickel content increases,the driving force of phase transformation is greatly improved to obtain a higher transformation rate than the steel with low nickel content.The maximum carbon content of untransformed austenite is higher,which promotes the complete transformation of bainite and produces fewer M/A islands.Therefore,this research possesses great potential for the composition design and process control of high-strength low-alloy steel.