CAJ | 학술논문

Nb-V微合金钢在1200℃固溶0.5 h后淬火,在450℃回火不同时间,用三维原子探针(3DAP)研究了回火过程中合金碳化物的形核规律.结果显示,淬火态Nb-V微合金钢在450℃回火时合金碳化物处于形核阶段,合金元素可通过动态再分配,实现渗碳体到合金碳化物的原位转变,或者在位错等缺陷处直接与C结合,完成合金碳化物的单独成核长大,或者偏聚在残余奥氏体/基体和未溶的AlN粒子/基体界面处,实现合金碳化物的异质形核长大.
Nb-V미합금강재1200℃고용0.5 h후쉬화,재450℃회화불동시간,용삼유원자탐침(3DAP)연구료회화과정중합금탄화물적형핵규률.결과현시,쉬화태Nb-V미합금강재450℃회화시합금탄화물처우형핵계단,합금원소가통과동태재분배,실현삼탄체도합금탄화물적원위전변,혹자재위착등결함처직접여C결합,완성합금탄화물적단독성핵장대,혹자편취재잔여오씨체/기체화미용적AlN입자/기체계면처,실현합금탄화물적이질형핵장대.
During tempering of martensite a complex carbide precipitation sequence appeared in the steel particularly containing microalloyed elements such as V, Nb and Ti. The alloy car-bide, which usually precipitates following cementite precipitation in certain temperature range, has been designed to maximize the number density and to retard the coarsening for increasing soften resistance. During the nucleation stage of the alloy carbide, the dislocations and interfaces of distinct phases are the actively precipitated position. However, because of extremely small sizes, their characterization is restricted by the analytic resolution of conventional methods. The 3D atom probe (3DAP) is a particularly helpful instrument with atomic spatial resolution and high componential sensitivity in the characterization of the early stages of prccipitation reactions. In this paper, the 3DAP companied with TEM and micro-hardness test was applied to characterize the early nucleation stage of the alloy carbides precipitated during tempering of Nb V microalloyed steel after quenched from solution treatment at 1200 ℃ for 0.5 h. With the tempering time prolonged from 0.5 to 100 h at 450 ℃, the micro-hardness of the experimental steel changes with the microstructure recovery and carbide evolution (from cementite to alloy carbide). The two peak hardness values appeared at 4 and 100 h tempering are related to precipitate cementite and alloy carbide, respectively. The nucleation of the alloy carbides happens during 30 h tempering at 450 ℃. The alloyed elements dynamically redistributed in the existed remnant austenite, that is, non-carbide-forming elements such as Si and A1 diffuse to matrix from the cementite, whereas the carbide-forming elements such as Mo, Nb and V enriched in the cementite, resulting in in situ trans-formation of alloy carbides. The intragranular defects such as high density dislocation in martensite also act as nucleation sites of alloy carbide, at which V and Nb directly combine with C and lead to the formation of G.P. Zone before formation of alloy carbides. Besides, the interfaces of the remnant austenite/matrix and the undissolved A1N/matrix are also energetically favorable nucleation sites, re-sulting in heterogeneous nucleation of alloy carbides. With the decrease of dislocation density and the dissolutions of cementite and remnant austenite, the consumption of the potential nucleation sites ends the nucleation stage of alloy carbide when tempering for 100 h.