某公司生产的规格为PL20 mm×3 000 mm×12 000 mm的S355ML+Z35钢板在使用前复验检出厚度方向（z向）性能不合格。采用断口宏观分析、化学成分分析、力学性能测试、金相检验等方法，对S355ML+Z35钢板z向性能不合格原因进行了分析。结果表明：z向性能不合格是由该板材试样不当的焊接操作造成的。建议规范CO₂气体保护电弧焊的焊接操作，减少焊缝中的扩散氢。

Thickness direction (z-direction) performance of S355ML+Z35 plate with PL20 mm×3 000 mm×12 000 mm was not qualified before using. The reasons for the disqualification of z-direction performance of S355ML+Z35 steel plate were analyzed by macro fracture analysis, chemical composition analysis, mechanical properties test, metallographic test and so on. The results show that the disqualification of z-direction performance was due to incorrect welding operation for the specimens. It is suggest that the CO₂ gas shielded arc welding operation should be standardized to reduce the diffusion hydrogen in the weld.