Study on Salt Spray Delayed Cracking Performance of Ultra-high Strength Martensitic Steel

The delayed cracking performance of three kinds of automotive ultra-high strength martensite steels, MS980, MS1180 and MS1300, was studied by the salt spray delayed cracking test of deep drawing and punching cup, and the relationship between delayed cracking performance and microstructure was analyzed. The results show that the best of anti-delayed fracture resistance is MS980 steel. The worst of anti-delayed fracture resistance is MS1300 steel. In addition, the results also show that there is a critical forming ratio of three MS steels (MS980 steel is 1.7, MS1180 steel is between 1.5 and 1.6, and MS1300 steel is between 1.3 and 1.4). Below the critical forming ratio, there is no delay cracking in deep drawing specimen. And the critical forming ratio can be used as a basis for evaluating the delay fracture of ultra high-strength steel under some condition.