基于分裂增韧的层状超细晶钢板组织设计

Although low temperature nickel steels and medium manganese steels have excellent toughness, but these steels add a lot of alloys, such as Ni or Mn. So, there is a problem that huge resource is consumed and the cost is high. Delamination toughening is a method remarkably improving toughness. The applicant found laminated and ultrafine grained microstructures can be obtained in low carbon microalloyed steel plates via rolling at the low temperature in dual-phase region. And delamination toughening was introduced that excellent low-temperature toughness was obtained. At the same time, the upper shelf energy was high. Thus, resource-saving excellent toughness steel plates is expected to be gained. This project focuses on the microstructure design of laminated and ultrafine grained steel plates. The relationship between process parameters and microstructure characters, such as the degree of laminated morphology and grain size is established. The influence of each microstructure character on delamination and upper shelf energy, and the effect of delamination on toughness are analyzed in detail. The relationship between nucleation-propagation process of delaminated crack and microstructure characters, such as laminated microstructure morphology and grain orientation, are determined to clarify delamination mechanism. Thus, the microstructure design criteria of laminated and ultrafine grained steel plates can be obtained aimed at toughness optimization and the key generation technique is developed. This project can provide a new method for developing resource-saving excellent toughness steel plates.

虽然镍系低温钢和中锰钢等钢种具有优异的低温韧性，但均添加大量镍或锰等元素，存在资源消耗大、成本高的问题。分裂增韧是一种显著提高钢材低温韧性的方法。申请人前期研究发现，两相区低温段轧制工艺可在低碳微合金钢板中产生层状超细晶组织，从而引入分裂增韧，大幅度提高低温韧性；同时此种钢板具有高的上平台吸收功，从而有望开发出资源节约型优异韧性钢板。本项目围绕层状超细晶钢板的组织设计开展研究。建立工艺参数与组织层状结构程度和晶粒尺寸等特征的关系；详细分析各组织因素对分裂和上平台吸收功及分裂对韧性的影响，阐明分裂增韧的组织控制因素和上平台吸收功的影响因素；确定分裂裂纹形核和扩展过程中与组织的层状结构和晶粒取向等特征的关系，阐明分裂机理。以韧性优化为目标，提出层状超细晶钢板的组织设计准则，开发出优异韧性层状超细晶钢板的关键制备技术。本项目可为资源节约型优异韧性钢板的开发提供一条新的思路。

分裂增韧是一种不依靠添加N或Mn等合金元素，仅通过形成层状结构组织以改变裂纹扩展路径，大幅度降低裂纹尖端应力，从而提高韧性的方式。本研究拟通过临界区低温段轧制，在简单成分的低碳微合金钢中引入层状结构组织，研制出高韧性钢材。本项目系统研究了临界区轧制温度和变形量对钢材组织演变、冲击断口分裂、韧性和强度的影响规律，阐明了不同温度下的分裂机理、层状超细晶钢材的强化机制和退火过程中的组织演变规律。实验结果表明，温度低于810 ℃时，临界区形成的奥氏体含量低于20%；临界区轧制过程中，组织主要发生动态回复，随着轧制温度的升高，发生一定程度的动态再结晶，但均可获得层状结构组织；780 ℃和810 ℃轧制时奥氏体分别发生1.39%和6.89%的转变。随着变形量的增加，组织的层状结构程度逐渐增强；层状结构决定了冲击断口的分裂行为及韧性：层状结构较弱时，断口不分裂；层状结构增强，断口发生分裂，但在分裂裂纹尖端形成平行于冲击载荷方向的裂纹；层状结构更显著时，分裂裂纹后不再形成平行于冲击载荷方向的裂纹，并在分裂裂纹下方的组织中发生大范围、剧烈的塑性变形，显著提高韧性。温度较高时，通过在层状界面上形成微孔、并聚集长大的方式形成分裂裂纹；温度较低时，分裂裂纹直接在层状界面形核、扩展。层状超细晶钢板的主要强化机制为晶粒细化，占比可达45%。400 ℃以下退火时，位错密度降低，第二相基本保持不变；退火温度升至600 ℃时，珠光体和渗碳完成球化；进一步升至临界区时，由于奥氏体的C含量较高，空冷后形成马氏体。本项目系统研究了各工艺参数对分裂增韧型钢材组织演变、断口分裂及韧性和强度的影响机理，为此类钢材的制备提供了理论基础。