Production of High Toughness Martensitic Stainless Steel using Partial Solution Treatment

为了开发马氏体不锈钢的部分固溶增韧工艺，提出了碳化物分散处理和分散碳化物颗粒控制铁素体成核的技术。部分固溶处理（奥氏体和M23C6调质后双相区再固溶处理）能使直径0.3 μm的碳化物均匀分散在奥氏体基体内，并能显著提高冷却时γ-α相变的成核频率。在此作用下，获得了晶粒取向随机分布的等轴细铁素体组织，显著提高了“高强低延性”的常规钢的延展性和成形性。通过九州大学的拉伸试验，研究了其强度-塑性平衡，明确了均匀伸长率、加工硬化和颈缩应变均显著增大。在日新制铁株式会社用深拉伸性评价了所开发钢的成形性。研究结果表明，所开发的钢板在拉深过程中不存在开裂问题，易于成形。此外，仅通过热处理而不进行变形工艺就可以进行显微组织控制，从而在实际生产线上确认大尺寸材料的生产没有任何问题。另一方面，通过透射电子显微镜和X射线分析揭示了微观组织发育的机理，并利用扩散理论推导的动力学对热处理条件进行了优化。

For the purpose of developing a process for toughening of martensitic stainless steels using Partial Solution treatment, techniques for the carbide dispersion treatment and the nucleation-site control of ferrite by dispersed carbide particles were proposed. Concretely, the Partial Solution treatment (re-solution treatment in dual-phase region of austenite and M23C6 after quench-and-tempering) enabled to disperse the carbide of O.3μm in diameter homogeneously within austenite matrix and to increase the frequency of nucleation markedly in γ-α transformation on cooling. By this effect, equiaxed fine ferritic structure with random crystallographic orientation distribution was obtained and the ductility and formability of conventional steel that "high strength but low ductility" was remarkably improved. The strength-ductility balance was investigated by means of tensile testing at Kyushu University, and then it was clarified that each of uniform elongation, work hardening and necking strain was enlarged significantly. The formability of the developed steel was evaluated in terms of deep drawability at Nisshin Steel co., ltd. And it was found that the developed steel could be easily formed without the problem of cracking during the deep drawing. In addition, it is possible to perform the microstructural control only by the heat treatment without deformation process so that large-sized materials were confirmed to be produced on the practical manufacturing line without any problem. On the other hand, the mechanism of the microstructure development was revealed by using transmission electron microscopy or X ray analysis, and then the heat treatment condition was also optimized with kinetics derived from diffusion theory.