内氧化对热镀锌高强钢镀层/基板界面的影响

 由于高强钢中添加的Si、Mn等合金元素会在退火时会发生选择性氧化,从而影响镀液与带钢之间的润湿性,因此热镀锌高强钢生产存在可镀性的难题。为了提高热镀锌高强钢的镀层附着性,以不同镀层附着性的连续热镀锌0.2%C-1.8%Si-1.8%Mn高强钢为研究对象,采用GD-OES、SEM、FIB、TEM等研究了镀层/基板界面抑制层与基板次表层内氧化层的特征,同时采用模拟退火试验研究了退火气氛露点对内氧化层形成的影响,揭示了退火气氛露点、内氧化层厚度、抑制层覆盖率与镀层附着性之间的相关性。结果表明,该热镀锌高强钢镀层附着性优劣由镀层/基板界面位置抑制层决定,当抑制层覆盖率达到80%以上时,可获得良好的镀层附着性。内氧化层厚度对抑制层覆盖率的影响存在临界厚度,约为0.58 μm。当内氧化层厚度由0逐渐增加至0.58 μm时,抑制层覆盖率由约10%增加至约80%;当内氧化层厚度由0.58进一步增加至3.85 μm时,抑制层覆盖率略有增加,介于80%~90%之间。提高退火气氛露点可以促进基板次表层形成内氧化,在退火温度分别为800和870 ℃、保温时间为120 s、退火气氛为N₂-5% H₂(体积分数)的试验条件下,当退火气氛露点由-40 ℃提高到+10 ℃时,内氧化层厚度由基本为0提高至3~5 μm。为了获得合适的内氧化层厚度,建议将0.2%C-1.8%Si-1.8%Mn高强钢的退火气氛露点控制在-20~-10 ℃范围内。

Influence of internal oxidation on coating/substrate interface of hot dip galvanized high strength steel sheet

Due to the selective oxidation of Si and Mn during the annealing process of Si-Mn added high strength steel which significantly deteriorates the wettability of strip with liquid zinc,the galvanizability of hot dip galvanized high strength steel is still a challenge. To improve the coating adhesion of hot dip galvanized high strength steel,characterizations of the inhibition layer on coating/substrate interface and the internal oxidation in the subsurface of hot dip galvanized 0.2%C-1.8%Si-1.8%Mn high strength steel sheets with different coating adhesion were conducted by GD-OES,SEM,FIB and TEM. Annealing simulation experiments were used to study the effect of annealing atmosphere dew point on the formation of internal oxidation. The correlation between dew point,internal oxidation thickness,inhibition layer coverage and coating adhesion was revealed. The coating adhesion is determined by the inhibition layer on the coating/substrate interface,good coating adhesion can be obtained when the coverage of inhibition layer is greater than 80%. There exists a critical internal oxidation thickness which is approximately 0.58 μm. When the internal oxidation thickness gradually increases from 0 to 0.58 μm,the inhibition layer coverage increases from 10% to 80% approximately. When it further increases from 0.58 μm to 3.85 μm,the inhibition layer coverage increases slightly in the range of 80%-90%. The internal oxidation is promoted when the dew point of the annealing atmosphere is increased. When it is annealed at 800 ℃ and 870 ℃ and soaked 120 s in an atmosphere of N₂-5% (volume percent)H₂,the internal oxidation thickness increases from 0 to 3-5 μm as the dew point increases from -40 ℃ to +10 ℃. Therefore,it is suggested to control the dew point of annealing atmosphere in the range of -20 ℃ to -10 ℃ to get a favorable internal oxidation thickness.