鳥塚 史郎

© 2020 ISIJ. Ultrafine ferrite + austenite steels with the chemical composition of 0.1%C-2%Si-5wt%Mn show excellent strength (TS=1 200 MPa) and high ductility (TEl=25%) balance, compared to conventional TRIP steels. This steel is expected as the third generation advanced high-tensile strength steels (AHSS). This steel can be produced by a simple intercritical annealing, however, longer annealing time is necessary to obtain appropriate ferrite + austenite structure. It is difficult to produce this steel by continuous annealing process. If the annealing time can be drastically reduced, this new TRIP steels can be commercialized. We focused on the effect of the prior microstructures before annealing on the formation of ferrite + austenite structure. The effect of the prior structure is not clear. Therefore, in this study, two kind of prior structures, ultrafine grained ferrite + cementite and martensite were used in 0.1%C-2%Si-5wt%Mn steels. It was found that the prior structure of ferrite + cementite can form large amount (20%) of austenite in a very short time (600 s). This is because cementite finely dispersed in the structure effectively acts as a preferential nucleation site of reverse transformed austenite and C and Mn are concentrated in cementite to enable a short time formation of austenite. Excellent strength-ductility balance (32 000 MPa%) which is superior to conventional TRIP steels is also obtained.

過去の研究では，Mn添加量の増加による引張強さと一様延性が増大する原因が明らかでなかった．そこで，本研究では組織観察や転位密度の測定を行い，加工硬化能との関係について調査し，以下の結論を得た．<br /> (1) Mnの添加量を増加すると，加工硬化能が向上した．よって，引張強さと一様延性が同時に向上したのは，加工硬化能が向上したからであると見出した．<br /> (2) Mnの添加量が増加するにつれてブロック幅が減少した．<br /> (3) 転位密度が1.5Mnにおいて最高荷重点に達する前に，ほとんど増大しなくなった．一方，5Mnにおいては増大し続けた．それは，転位配列の違いによるものであると考えられる．また，TEM観察においても，1.5Mnと5Mnの最高荷重点の転位配列の違いは，算出した結果と一致した．よって，加工硬化能の向上は，Mnによって転位セル構造の形成を抑制し転位密度を増加させることができたからであると考えられる．

超微細粒鋼は、合金元素添加なしで、高強度と高靭性を同時に発現できる資源循環型材料として極めて有望である。一方、結晶粒径１μｍ以下の超微細粒鋼は、実験室的サイズではECAP,ARP,HPTなどの方法によって達成されているが実用化に不可欠な材料の大型化は未達成であった。

The ductile fracture toughness of ferritic steel was assessed in terms of crack tip opening displacement (CTOD). The CTOD is composed of two parts: elastic and plastic. In the ductile fracture region, as compared to the elastic part, the fraction of the plastic part is dominant. The CTOD linearly increases with an increase in ferrite grain size, but grain coarsening simultaneously increases the possibility of cleavage fracture. As yield strength increases, the CTOD decreases due to the decreased plastic deformation ability.

A study was conducted to demonstrate physical interpretation of grain refinement-induced variation in fracture mode in ferritic steel. Tensile tests were performed at room temperature and at a crosshead rate of 0.4 mm/min with specimens of 4 mm in diameter and 10 mm in gage length to investigate the dependence of critical fracture strain on ferrite grain size. Round smooth tensile specimens were applied to conduct the investigations where they were in uniaxial stress state before the onset of necking and turned into triaxial state after necking. The stress triaxiality in the region ahead of the crack was higher than that in the round smooth tensile specimen for a sample with a crack as the crack tip was sharp. The fracture strain obtained from tensile tests was larger than the critical fracture strain in the local region ahead of the crack.

The effect of austenite grain size on martensitic transformation, particularly with regard to martensite structure, Ms/Mf temperatures, and mechanical properties was investigated in 0.1C-5Mn martensitic steel. Utilizing a newly developed experimental technique that makes it possible to examine phase transformation behavior and conduct tensile testing with the same specimen, we examined these relationships and obtained the following results. Ms temperature decreases as much as 40 K with a decrease in austenite grain size from 254 to 30 pm. Regarding martensite structure, the packet size and the block length decrease, while the lath width does not change, with the refinement of austenite grain size by about one tenth. Grain boundary density, especially high-angle grain boundary density, increases with decreasing austenite grain size. Tensile strength slightly increases though austenite grain size decreases about one tenth. However, reduction in area significantly improves particularly at refined grain sizes of 30 gm. True stress - true strain curves obtained up to fracture elucidates that the austenite refinement substantially improves true fracture strength and greatly increases true fracture strain of martensite, potentially invalidating the conventional concept of a trade-off between strength and ductility. Low C-5Mn martensitic steel produced from fine austenite shows a great possibility having an excellent total balance of strength, ductility and toughness.

Ultrasonic and conventional electromagnetic resonance fatigue tests were conducted on notched and smooth specimens of ultrafine-grained steels. The notched specimens developed no internal fractures but showed a fatigue limit. In this case, ultrasonic fatigue testing showed good agreement with the electromagnetic resonance. The smooth specimens showed internal fractures after ultrasonic fatigue testing, but not during electromagnetic resonance testing. In the smooth specimens, ultrasonic fatigue testing showed a longer fatigue life. These results show that the frequency effect is negligible in surface fractures of notched specimens but not in smooth specimens. The frequency effect causes a change in type of fracture from surface fracture to internal fracture. Concerning the gigacycle fatigue properties, the ultrafine-grained steel shows a fatigue limit with notched specimens, whereas the fatigue limit can disappear with smooth specimens because of the internal fracture. © 2012 Elsevier Ltd.

The work-hardening exponent is used to quantify the work-hardening of ultrafine-grained steels. The exponent for the types of steel with the carbon content below 0.22 wt% is proposed to be directly predicted by either yield strength or ferrite grain size. The exponent's value linearly decreases with increasing yield strength or D-1/2 (where D is the ferrite grain size), and becomes zero when yield strength is higher than 620 MPa (or D is less than 1 mu m). (C) 2011 Elsevier B.V. All rights reserved.

The crystal grain size of the work material is relatively large compared to the removal depth in micro-scale cutting. Therefore, the micromilling requires the small crystal grains in the material to machine accurate products in a stable manner. The study investigates the effect of crystal grain size on the cutting process in micromilling. The crystal grains of stainless steel in this study are downsized to an average size of 1.5m by repetition of material forming and phase transformation. The milling processes of ultra fine-grained steels were compared with those of normal grain steels. The milling tests were performed to measure the cutting force and the surface quality. The force component ratio of the ultra fine-grained steel is higher than that of the normal grain steel. The shearing force decreases in cutting of the ultra fine-grained steel meanwhile, the friction and/or the indentation forces increase. Burr formation can be reduced with the crystal grain size. In cutting of the normal grain steel, thrust component in the cutting force suddenly drops near the end of the grooves and a large burr is left on the edge of the groove. © 2012 The Authors.

Ultra fine grained steels have been developed by many researchers. However, studies on the effects of different grain size on processes and product functions have been limited because the size of the bulk material has been small for these products. The authors have developed a production process for thin ultra fine grained stainless steel coil, and the effects have been clarified. This paper first reports the effects on micro hole piercing by comparing materials with different grain sizes. Secondly, orifices are produced from these materials, and the liquid flow volume is measured as the functional effect of different grain sizes.

While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. Ultrafine-grained steels with different carbon contents from ultralow carbon to high carbon were produced through warm caliber rolling. It was found that the reduction in area- tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to martensite steels for all materials. Formability of ultrafine-grained steel is examined by applying to form a M 1.7 micro screw using these ultrafine-grained steels. Screws are formed through the process of cold heading and rolling. Relationship between cold heading, rolling, uniform elongation and reduction in area are investigated to clarify the formability of ultrafine-grained steels. Low-carbon ultrafine-grained steel has excellent cold headability and favorable rolling properties, i.e., excellent formability. Reduction in area is a measure to determine formability on cold heading. Ultrafine grained steel wire with length of several hundred meter were developed with the technology of warm continuous multi-directional rolling. This wire also have a good formability which can form microscrews. © (2012) Trans Tech Publications.

Fatigue strength is one of the key properties in the practical use of ultrafine grained steels. Fatigue tests were conducted on notched specimens by conventional electromagnetic resonance fatigue testing machines. The electromagnetic resonance fatigue testing was carried out at 150 Hz up to 10(7) cycles. The investigated steels had different levels of carbon, 0.15 wt%, 0.30 wt% and 0.60 wt% with tensile strengths of 850 MPa, 950 MPa and 1 105 MPa, respectively. With increasing in carbon content, the tensile strength increased and the total elongation decreased. The notched specimens never showed internal fracture and showed a clear fatigue limit. The notch fatigue limit increased with an increase in tensile strength from 850 MPa to 970 MPa, when the carbon content was 0.15 and 0.30 wt% with microstructures consisting of ultrafine ferrite grains and cementite particles. On the other hand, when the carbon content was 0.60 wt%, the notch fatigue limit decreased, though tensile strength increased to 1 105 MPa. Retained pearlite grains were observed in 0.60 wt%C steel in addition to ultrafine ferrite grains and cementite particles. These retained pearlite grains which were coarse and high angle grain boundary poor regions were attributed to the lower notch fatigue limit.

In microalloyed steel, industrial candidate for application to parts of automobiles, we investigated strengthening mechanism by vanadium carbide (VC) precipitation in the pearlite. The specimens, containing 0.1-0.65% carbon (C) and 0-0.4% vanadium (V), were quenched from 1200 to 650-550 degrees C and kept at this temperature. The more V and the lower transformation temperature, the higher strength can be obtained. The maximum 0.2% proof strength is determined to be 1450 MPa for 0.65%C-0.4%V steel and 1200 MPa for 0.65%C-0.2%V steel. VC is found to precipitate at interphase boundaries between the austenite and the ferrite in the pearlite. When the transformation temperature is lowered, the VC size becomes finer and the precipitation density increases. These results suggest that the strengthening mechanism by VC is the Orowan-type. The amount of precipitation strengthening by VC was calculated by Ashby-Orowan equation. Both the calculated and the experimental results were determined to have good correlation. Strengthening mechanism by VC and that by pearlite lamellar would be competitive, however the amount of precipitation strengthening by VC in pearlite was close to that in ferrite.

Steel bars having a cross section of 18mm square with uniform distribution of ultrafine ferrite grains were produced through a multi-pass warm caliber rolling process in a 0.15%C-0.3%Si-1.5%Mn steel. The average ferrite grain sizes of 0.43μm, 0.70μm and 1.2 μm were obtained in the isothermal warm caliber rolling processes at 773K, 823K and 873K respectively. Even though caliber rolling results in inhomogeneous strain distribution, multi-pass caliber rolling to large cumulated strains of 2 or 3 can be uniformly introduced in to the bar samples. Strain accumulation due to the multi-pass warm deformations was confirmed by comparing microstructural evolution through the multi-pass deformations with that of single pass deformation. The size of ultrafine grains formed through warm deformation was found to depend on the Zener-Hollomon parameter. The similarity of the microstructural evolution with single pass deformation reveals that the multi-pass warm deformation is an effective method to obtain ultrafine grained ferrite structure in bulk materials. It is proposed that compressive strain-Z parameter plots along with grain size-Z parameter plots help in establishing the processing conditions for obtaining products with a desired microstructure and grain size. Finally, such "processing maps" developed for a variety of materials serve useful purpose in bridging the science and technology of developing bulk ultrafine grained materials in semi-finished / finished products. © (2011) Trans Tech Publications, Switzerland.

The paper discusses the effect of the crystal grain size of materials on the micro cutting processes. The cutting process of ultra fine grain carbon steel, which consists of 0.5 μm sized grains, is compared with those of the larger grain steels. The micro cutting tests were performed to measure the cutting force with changing the cutting thickness. The cutting force of the ultra fine grain steel is larger than that of the large grain steel corresponding to the hardness change of materials. The specific cutting forces remarkably increase when the cutting thickness becomes less than 10 μm. The shear angle of the ultra fine grain steel is larger than that of larger grain steels in small cutting thickness. The uniform chip formation is also observed with fine surfaces in cutting of the ultra fine grain steel. © 2011 American Institute of Physics.

Some commercially applicable processes for nanostructured steel wires, bars and strips are introduced in this chapter. They are warm multi-pass caliber rolling, warm continuous oval to square rolling, cold strip rolling including cladding, and repetition of cold strip rolling with reverse phase transformation. The basic idea of how to apply the amount of strain necessary for introducing a nanostructure differs depending on the product shape and material. Microstructures with a grain size of several hundred nanometers and the mechanical properties of these materials are presented.Several application examples of these nanostructured steels such as small screws, orifice plates for liquid spray and shafts are explained. The advantage of nanostructured material is not only in strength but also in formability and machinability. The applications take advantages of these unique features of nanostructured steel properly and they are at the stage of mass production. © 2011 Woodhead Publishing Limited All rights reserved.

The cutting processes of ultra fine grain stainless steels are compared to that of normal grain steel in microscale cutting with a single point tool made of single crystal diamond. Cutting force is measured with oscillation in the dynamic component. Vibration in cutting force is reduced with the grain size. Shear angles are measured to discuss the cutting process with changes in depth of cutting. The large shear angle is observed when the grain size becomes small. The surface finish is also improved when the ultra fine grain steel is machined.