780MPa级热镀锌双相钢微观裂纹成因及改进措施

Micro-crack origin and improvement measures in 780 MPa grade hot-dip galvanized dual-phase steel

780MPa级热镀锌双相钢表面块状色差缺陷的原因为基板表面大量开口微观裂纹。微裂纹开口导致Fe2Al5抑制层形成异常,宏观表现为块状色差。微裂纹主要分布在带钢中部板面中间等热轧卷散热较慢位置,裂纹处珠光体晶粒较粗大。通过分析此带钢全流程氧化特性,带钢表层在加热炉中形成锚状氧化铁皮,附着在带钢基体上不易去除;在热轧工序中Si、Cr元素向表面富集形成硬质氧化圆点,附着在带钢近基体上。Si、Cr氧化圆点与粗大珠光体在冷轧过程中形成微裂纹。带钢在镀锌工序退火过程中,表面微裂纹在大张力作用下成为开口状态。微裂纹开口增加了基板与锌液接触面积,增加了锌液与铁的反应速度;同时裂纹开口导致带钢表面破碎,Fe2Al5抑制层无法良好结晶,这两点因素导致Fe2Al5抑制层无法良好形成。降低板坯加热炉温度,减少'锚状'氧化铁皮的生成。提高粗轧出口温度,形成疏松铁皮,在冷轧酸洗过程中带走Si、Cr氧化圆点。降低热轧卷取温度,使热卷带板中位置形成细小珠光体,提高开裂抗力。降低镀锌工序退火炉张力,减小微裂纹开口几率。通过以上措施,可有效改善780MPa级热镀锌双相钢表面微裂纹,消除色差。

The reason of massive chromatic defects on the surface of 780 MPa grade hot-dip galvanized dual-phase steel is a large number of micro-cracks on the surface of the substrate.The opening micro-cracks lead to the abnormal formation of the Fe2Al5 inhibition layer,and the macroscopic performance is the block color difference.The micro-cracks are mainly distributed in the middle of the middle plate strip coil where cools slowly,and the pearlite grains at the cracks are coarse.Through the analysis of the whole process of the oxidation characteristics of this kind of strip,anchor-shaped iron scale forms in heating furnace and is not easy to remove in the strip steel substrate.During the hot rolling process,Si and Cr elements concentrate on the surface and form hard oxidized dots attaching to the strip near the matrix,then such Si,Cr oxidized dots and coarse pearlite form micro-cracks during cold rolling.During the annealing process of the galvanizing strip,the micro-cracks on the surface of the strip become open under the action of large tension.The opened micro-cracks increase the contact area between the substrate and the molten zinc as well as the reaction speed of zinc liquid and iron.Meanwhile,the opened cracks cause the surface of the steel strip to be broken,and the Fe2Al5 inhibition layer can not crystallize well.These two factors cause the Fe2Al5 inhibition layer can not form well.Reducing the temperature of the slab heating furnace can reduce the formation of the anchor-shaped scale.Through increasing the hot rolling inlet temperature,some Si and Cr oxide particles can be taken away during the pickling process before cold rolling.Reducing the hot rolling coiling temperature to form fine pearlite in the middle of hot coil plate can improve the cracking resistance.To reduce the opening probability of the micro-cracks,the tension of the annealing furnace of galvanizing process should also be reduced.The above measures can effectively improve the surface micro-crack of 780 MPa grade hot dip galvanized dual-phase steel and eliminate color difference.