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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.
The effect of Nb on the phase transformation and microstructure refinement of 0.47% C carbon steel was investigated for two kinds of medium carbon steels without Nb and 0.06% Nb addition.The normalized microstructures of the two kinds of experimental steels were ferrite + pearlite , Nb microalloying can effectively refine the austenite grain size of medium carbon steel, resulting in a significant increase in the volume fraction of ferrite in the normalized structure.The yield strength of Nb-containing medium carbon steels is increased by 18% 70 MPa), the tensile strength basically remained unchanged, the impact toughness at -20 ℃ increased from 7 J to 19 J, showing a significant increase.In addition, the continuous cooling transformation (CCT) curve found that Nb microalloying carbon steel can be When the cooling rate is less than or equal to 10 ℃ / s, the ferrite with high volume fraction can be obtained, so that the workpiece does not appear bulky pearlite or bainite / martensite in a large cooling rate range. It is found that the Nb element is uniformly distributed in the steel in the state of minute precipitates Nb (C, N), and the Nb (C, N) precipitates effectively refine the austenite grains and thus increase the ferrite nucleation rate, which It is the main mechanism that Nb affects phase transformation and improves toughness in medium carbon steel.