基板表层组织对Fe-16Mn-0.7C-1.5AlTWIP钢可镀性的影响

Influence of Substrate Surface Structure on the Galvanizability of Fe-16Mn-0.7C-1.5Al TWIP Steel Sheet

以16%Mn-0.7%C-1.5%Al (质量分数) TWIP钢为研究对象,采用热镀锌模拟实验研究了2种基板表层组织对TWIP钢可镀性的影响。使用180°折弯检测了镀层附着性,使用GD-OES分析了退火及镀锌试样表面的元素深度分布,使用SEM观察了试样表面和截面微观形貌。结果表明,通过预处理得到的TWIP钢表面一层铁素体晶粒可以有效改善TWIP钢的可镀性。当轧硬态的TWIP钢直接连续退火并热镀锌时,Mn元素形成了明显的外氧化,严重阻碍了镀液中的Al和基板反应形成Fe-Al抑制层,不仅漏镀明显,而且镀层附着性差。当使用经过预处理、表面有一层铁素体的TWIP钢进行热镀锌时,TWIP钢表面的细晶粒铁素体层有效抑制了退火过程中Mn元素外氧化,从而显著改善了锌液对带钢的润湿性,在镀层/基板界面位置形成了充分的Fe-Al抑制层。通过预处理得到的表面铁素体层可以有效解决16%Mn-0.7%C-1.5%Al TWIP钢的可镀性差和镀层附着性差问题。

Twinning-induced plasticity(TWIP) steels with a high Mn content show an advanced combination of strength and formability among the commercially available advanced high-strength steels for automotive applications. However, applying zinc coatings on TWIP steels using the continuous hot-dip galvanizing process remains a great challenge owing to the selective oxidation of Mn that occurs during continuous annealing before hot dipping. In this study, a potential procedure for improving the galvanizability of TWIP steels is developed and its mechanism is discussed. Both as-received cold-rolled and pretreated 16%Mn-0.7%C-1.5%Al(mass fraction) TWIP steel sheets were galvanized using the hot-dip process in a laboratory, and the influence of the substrate surface structure on the galvanizability of the TWIP steel sheets was studied. The wettability of molten zinc on the TWIP steel sheets was examined,and the coating adhesion was tested by bending at 180°. The elemental depth profiles of both the annealed and galvanized panels were analyzed via glow discharge optical emission spectroscopy, and the surface and cross-sectional morphologies were observed via SEM. Results indicated that a thin layer of fine ferrite grains produced using the pretreatment process could effectively improve the galvanizability of the TWIP steel. When the as-received cold-rolled TWIP steel was galvanized using the hot-dip process,the dominant external oxidation of Mn was observed on the steel surface before hot dipping, which prevented the formation of an Fe-Al inhibition layer and further resulted in poor galvanizability and deteriorated coating adhesion. When a thin layer of fine ferrite grains covered the TWIP steel surface, the galvanizability was considerably improved even though the ferrite layer thickness was less than 1 μm. The presence of surface ferrite grains almost completely suppressed the external oxidation of Mn during the annealing process, resulting in a clean surface similar to that of an interstitial-free or bake-hardened steel.Therefore, the wettability of molten zinc on the TWIP steel sheet improved considerably and a sufficient Fe-Al inhibition layer was formed. The formation of a thin layer of surface ferrite grains on the 16%Mn-0.7%C-1.5%Al TWIP steel facilitates a novel technique for addressing problems associated with galvanizability and coating adhesion.