Fe-Zn相图不同相区温度合金化锌镀层生长过程模拟

Simulation of the Growth Process of Galvannealed Coatings at Different Zone Temperature in Fe-Zn Phase Diagram

目的 优化GA镀层的成形性能,建立GA镀层物相生长模型,调控镀层物相组成,得出最佳合金化镀层物相组成对应的工艺参数,以指导生产。方法 依据最新的Fe-Zn相图,构建镀层合金化模型,模拟镀层物相η、ζ、δ和Γ生长过程及物相沿镀层截面分布、镀层合金化过程Fe含量变化。结果 成功模拟了Fe-Zn相图不同相区物相的生长过程,模拟530℃以下温度物相转变为η→ζ→δ→Γ,530℃以上温度物相转变为η→δ→Γ。模拟得到最佳镀层物相组成对应的合金化工艺为,510℃保温9.7s,540℃保温6.8s。研究得到了合金化过程中镀层Fe含量的变化规律,在合金化前期,Fe含量增加的速率较快,随着合金化程度的提高,镀层中Fe含量的增加速率减慢。结论 建立的GA镀层物相生长模型可以模拟得到不同合金化温度下最佳的工艺参数,为合金化热处理生产GA镀层提供了工艺参考。

Galvannealed steel(GA) is widely used in the automotive industry,household appliances and construction for its good weldability,paintability,corrosion resistance and heat resistance.However,compared to GI coatings,GA coatings often fail in the press forming because the brittle and high hardness Fe-Zn phases can easily cause coating to undergo powdering and flaking,which results in severe decrease of its corrosion resistance and quality.Therefore,in order to improve the formability of GA coatings,the work aims to establish a GA coating phases growth model to control the coating phase composition,and obtain the best galvannealing process parameters to guide production.Based on the latest Fe-Zn phase diagram,the phase zone boundary concentration equation was formulated and combined with the phase zone boundary concentration equation,the phase boundary movement equation and the phase growth equation,GA coating phase growth model was constructed.The galvannealing model was constructed to simulate the growth process of phases η,ζ,δ and Γ in different phase zones of GA coating,the distribution of the phases along GA coating cross-section,and the change of GA coating Fe content during the galvannealing process.The established model could successfully simulate the phase growth process in different phase zones of the Fe-Zn phase diagram,and compare the growth differences of different phases in different phase zones.The phase transformation below the galvannealing temperature of 530 ℃ was η→ζ→δ→Γ,while the phase transformation above the galvannealing temperature of 530 ℃ was η→δ→Γ.The galvannealing temperature was set at 510 ℃ and 540 ℃.According to the simulation results at 510 ℃,η phase was gradually consumed by ζ phase and δ phase,and disappeared at 8.2s.At the same time,ζ phase also grew to the surface of coating and stopped growing.Then the δ phase consumed the remaining ζ phase and grew rapidly to the coating surface at 9.7 s,Γ phase grew slowly by consuming δ phase and the thickness of Γ phase reached 1μm at 38 s.Simulation results at 540 ℃ indicated that η phase directly transformed to δ phase and disappeared at 6.8 s.δ phase stopped growing when reaching the surface of the coating at 6.8 s.With the extension of galvannealing time,Γphase grew slowly by consuming δ phase from the steel substrate and the thickness of Γ phase reached 1 μm at 17 s.The galvannealing process to obtain the best coating phases was simulated:510 ℃ for 9.7 s and 540 ℃ for 6.8 s.The study shows that the change rule of Fe content in GA coating during the galvannealing process is that the increase rate of Fe content is faster at the initial stage of galvannealing,and the increase rate of Fe content slows down with the increase of galvannealing degree,because the phase diffusion coefficient of Fe-Zn phase diagram is ζδΓ.Therefore,as galvannealing time goes on,phases with high Fe content in GA coating increase,GA coating phase diffusion coefficient decreases with the increase of Fe content and the diffusion rate of Fe-Zn atoms decreases.GA coating phases growth model can simulate the best process parameters at different galvannealing temperature,and provide a process reference for the production of GA coatings.