特厚规格低合金高强海工用钢近表面应变时效冲击性能研究

Study on Near Surface Strain Aging Impact Properties of Low Alloy High Strength Ultra Heavy Plates for Marine Engineering

研究了TMCP+正火+NCC工艺生产的70〜85 mm特厚规格S355NL+Z35近表面7%应变时效-40℃冲击不合的原因。结果表明,NCC条件下造成近表面应变时效冲击不合的原因主要为正火后加速冷速过程中近表面冷速较大,组织以上贝氏体和粗大的粒状贝氏体为主,这种组织导致韧脆转变温度升高,低温韧性恶化。而厚度1/4处组织为正火后加速冷却形成的多边形铁素体、粒状贝氏体以及针状铁素体等构成的混合组织,这种复合组织有利于低温韧性的提高,使得板厚1/4位置处的-40℃应变时效冲击性能够稳定达到200 J以上。采用低碳当量微合金化+少量的Ni的成分设计,降低近表面组织的淬透性,获得了以针状组织+细化的粒状贝氏体为主的组织,有效提高了低温韧性,使得-40℃应变时效冲击性能达到100J以上,并且组织性能稳定,满足了下游客户的需求。

The reason of unqualified impact of 70〜85 mm heavy plate S355NL+Z35 near surface with 7% strain aging at-40℃ produced by TMCP+normalizing+NCC process was studied.The results showed that the main reason for the near surface strain aging impact incompatibility under NCC condition was that the near surface cooling rate was larger during the process of accelerated cooling after normalizing,and the microstructure was mainly composed of upper bainite+massive granular bainite,which led to the increase of ductile brittle transition temperature and the deterioration of low temperature toughness.Formed by accelerated cooling after normalizing,the microstructure of 1/4 of the plate was composed of polygonal ferrite,granular bainite and acicular structure,which could improve low temperature toughness,and the strain aging impact property of 1/4 plate at -40℃ could reach more than 200 J.With the optimized composition of low carbon equivalent with low carbon microalloying and a small amount of Ni used,the hardenability of near surface microstructure could be reduced,and the microstructure of acicular structure+refined granular bainite obtained,which effectively improved the low temperature toughness,and made the impact property of strain aging at -40℃ reach more than 100 J.And the microstructure and properties were stable enough to meet the needs of downstream customers.