The aim of the present thesis is establishing a shorter and more energy-efficient process chain for manufacturing of high-quality tool steel products to replace conventional multi-pass rolling method, as long time and large energy is spend in the conventional route. Semi-solid processing technology, as an innovative forming technology of metal and alloys, was selected as the core technology to establish our innovative routes for manufacturing of high-quality tool steel products. Various semi-solid forming routes of ferrous alloys and the characteristics of semi-solid processed ferrous alloys were discussed. Ferrous alloys exhibit distinctive microstructure and characteristics in semi-solid state. In consideration of the spherical microstructure and flexible forming property of metals in a semi-solid state, semi-solid processing technology can improve the conventional route and shorten the process chain of tool steel products manufacturing.

Partial melting behaviors of commercial SKD61 tool steel and cast Cr-V-Mo steel were investigated experimentally. Based on the experimental results, a clear and complete understanding of the morphologies (including size of solid particles and liquid fractions) and deformation characteristics (including forming stress and liquid segregation) of ferrous alloys at various temperatures was obtained. Aiming at realizing the spherodization of semi-solid state tool steel, various methods were introduced and discussed. In consideration of the tool life and simplification of equipment, recrystallization and partial melting (RAP) method was selected to achieve the refinement of tool steel. Two innovative routes for manufacturing of high-quality tool steel products were proposed based on RAP method. The refinement of the microstructure and the homogeneous distribution of alloying elements are crucial for realizing such a short process, which can be considered as "semi-solid thermomechanical processing".

With the aim of investigating and verifying the feasibility of manufacturing tool steel products with excellent mechanical properties using improved route, RAP processing was studied systematically. As the definition of RAP, this processing includes two main stages, predeformation stage and partial melting stage. The main parameters of predeformation include predeformation degree and predeformation temperature. The main parameters of partial melting includes heating rate and isothermally holding time. The effects of parameters such as predefomration temperature, predeformation degree, heating rate, and holding time on the microstructure and mechanical properties of cast Cr-V-Mo steel were studied experimentally. Recrystallization, austenization, grain growth and partial melting occur during heating of predeformed cast billet. These behaviors refine the microstructure and improve the mechanincal properties. The refinement of microstructure and improvement of mechanical properties become more significant, when RAP is conducted with larger predeformation (50%), higher heating rate (50 °C/s) and shorter isothermal holding time (20 s).

According to the innovative route proposed in our study, subsequent heating treatments are quite important to adjust the mechanical properties of the ferrous alloy products processed by semi-solid processing. With the aim of improving the quality of semi-solid processed products, the optimal subsequent heat treatment strategy for semi-solid processed Cr-V-Mo steel was investigated. The effects of final heat treatments including quenching and tempering on the microstructure and mechanical properties of RAP processed Cr-V-Mo steel were clarified experimentally. The phase segregation of RAP-processed specimen results in an inhomogeneous microstructure and unstable hardness that cannot be improved by only quenching. Subsequent tempering treatment leads to the release of microstress, the diffusion of alloying elements, and changes in the morphology of carbides. This microstructural evolution results in more stable hardness and better ductility of the tempered specimens. When the tempering temperature is about 560 °C, secondary hardening occurs and a good combination of hardness and strength is obtained.

工具鋼の内部組織の適正な制御は,工具鋼の機械的特性を向上させるために必須の要件となっている.工具鋼の示す優れた耐熱性(結晶の熱的安定性)や耐摩耗性は,工具鋼の微細なミクロ結晶構造と,その粒界や粒内に形成される安定なナノサイズ炭化物(VC等)によってもたらされている.ミクロ結晶構造の制御とナノサイズ炭化物の分散および形態の制御は,現状では,ナノサイズ炭化物が安定しているが故に多数の工程とエネルギーを必要としている.具体的に言えば,現在の熱間ダイス鋼製造プロセスは,24時間の高温加熱でVを固溶させ,さらに再結晶による結晶粒の微細化を行うために20パス程度の熱間圧延を行う工程を利用することを強いられ,長時間と高エネルギーを浪費しているため,抜本的な対策が求められている.

本研究はMicrostructure Control of Cr-V-Mo Steel by Semi-Solid Processing (Cr-Mo-V鋼の半溶融処理による組織制御)と題し,熱間ダイス鋼SKD61(5Cr-1.2Mo-0.8V)鋼について,半溶融処理による組織制御についての研究結果をまとめている.第1章は序論,第2章はレビューであり,第3章(Partial melting behaviors of SKD61 tool steel)にはSKD61の半溶融処理中の組織変化を系統的に整理してまとめている.ここまでの研究をもとに,鋳造材に予加工を加えた後3時間程度の半溶融処理を行うことで,現在工業的に行われている24時間の高温加熱・20パス以上の熱間圧延(合計で50時間程度)を代替でき,時間とエネルギーを一桁程度短縮できることを見出し,第4章にて半溶融処理を利用した2つの新しい製造プロセス(Innovative Process Route)として提案した.第5章(Effect of process parameters of RAP on microstructure and mechanical properties of RAP processed cast Cr-V-Mo steel)では半溶融処理条件の内部組織,VC等の析出物分散状況,機械的特性に及ぼす影響,第6章(Effects of subsequent heat treatments on microstructure and mechanical properties of RAP processed)では,半溶融処理された材料の後続熱処理条件が内部組織と機械的特性に及ぼす影響を示した.第7章は総括である.

以上を要するに,本論文では,高合金鋼である熱間ダイス鋼SKD61を対象とした半溶融処理ならびに半溶融処理材の後続熱処理による,内部組織変化,VC等の析出物分散状況,機械的特性変化を,綿密な実験によって初めて明らかにしており,工学的に高く評価できる.またこれらの成果を熱間ダイス鋼SKD61の新たな製造プロセスの提案に繋げている点で,また,小型試験片での実験ではあるが新たに提案した製造プロセスによって得られる製品の内部組織や機械的特性について定量的に検討していることは,工業的にも評価できる.

よって本論文は博士(工学)の学位請求論文として合格と認められる.