针对无Nb和添加0.06%Nb的2种中碳钢, 研究了Nb对0.47%C中碳钢相变及组织细化的影响规律. 2种实验钢正火组织均为铁素体+珠光体, Nb微合金化能够有效细化中碳钢的奥氏体晶粒, 从而导致正火后组织中铁素体体积分数明显增加. 含Nb中碳钢的屈服强度相对无Nb钢提高了18% (70 MPa), 抗拉强度基本保持不变, -20 ℃冲击韧性则由7 J提高到19 J, 呈现显著提高. 此外, 由连续冷却转变(CCT)曲线发现, Nb微合金化中碳钢可在冷速≤10 ℃/s时获得较高体积分数的铁素体, 因此, 可保证工件在较大冷速范围内不出现大块珠光体或贝氏体/马氏体组织. 结合TEM观察发现, Nb元素以微小析出物Nb(C, N)的状态均匀分布在钢中. Nb(C, N)析出物能有效细化奥氏体晶粒, 并因此提高铁素体形核率, 这是Nb在中碳钢中影响相变并提高韧性的主要机制.

Medium carbon steel is widely used in structural steels because of its favorable strength, but lack of toughness is a limitation in industrial applications. Among the different strengthening mechanism, grain refinement is the only method to improve both strength and toughness simultaneously. The toughness of steels can be affected by micro-alloying elements and microstructures, for medium carbon wheel steel, the fracture toughness is proportional to the square root of ferrite fraction and inversely proportional to the cube root of prior austenitic grain size. In this work, Nb micro-alloying is used to improve mechanical properties of medium carbon steel. Microstructures and mechanical properties of Nb-bearing medium carbon steel were studied in contrast with traditional Nb-free steel. The continuous cooling transformation (CCT) curves of investigated steels were drawn by adopting dilatometry and metallographic method. The typical microstructures were observed by OM and SEM with EDS. The morphologies of precipitates were obtained by TEM. The effects of cooling rates on microstructure and hardness of the steel were studied with the above experimental methods. The results showed that the typical microstructure of medium carbon steel was ferrite and pearlite and the volume fraction of ferrite was increased from 4% to 24% by adding 0.06%Nb with refined microstructure. The yield strength of Nb-bearing steel was improved from 385 to 455 MPa and the Charpy V-notch energy at -20 ℃ was increased from 7 to 19 J in the condition of almost no reduction in tensile strength. It is because of Nb addition, which makes the transformation products of medium carbon steel be composed of ferrite and pearlite in a wider region of cooling rates (≤10 ℃/s) and a broader temperature range (530~690 ℃), with the hardness lower than 300 HV. With the calculation of Thermal-Calc software and solid solubility formula, Nb exists in medium carbon steel in the form of precipitate. The result of observation by TEM indicates the size of Nb precipitates was distributed in 20~50 nm. To sum up, the grain refining and precipitation strengthening are the main mechanism of Nb to promote the ferrite-pearlite transformation and improve toughness in medium carbon steel.