Shiro Torizuka

Refinement of Ferrite-Pearlite Structures through Transformation from Heavily Deformed Austenite in a Low Carbon Si-Mn Steel

In a low-carbon Si-Mn steel without any other alloying elements, an ultra-fine ferrite-pearlite microstructure with a ferrite grain size of 2.3 μm was created through thermomechanical treatment. The feature of the process was one-pass heavy reduction at a low temperature in unrecrystallized austenite region followed by controlled cooling at 10 K/s. Crystallographic orientation of each ferrite grain was randomly distributed and most of the grain boundaries were high-angle ones.

High Z - Large Strain Deformation Processing and Its Applications

Ultrafine-grained structures formed dynamically through simple compression at warm deformation temperatures were investigated in a 0.15%C- 0.4%Si-1.5%Mn steel. The effects of strain, strain rate and deformation temperature on the microstructural evolution were examined using an isothermal plane strain compression technique with a pair of anvils. The maximum strain was 4, the deformation temperature was below the AC1 temperature, and the Zener-Hollomon parameter (Z) ranged between 1012 s-1 and 1016 s-1. Ultrafine ferrite grains surrounded by high angle boundaries are generated by simple compression when the strain exceeded a critical value. The number of newly generated ultrafine grains increased with the strain; however, the average sizes were found to be independent of strain. The grain size, `d`, was found to depend on Z parameter. An equation, d (μm) =102.07Z-0.16, was found to satisfy the experimentally obtained data. This study demonstrates the possibility of obtaining ultrafine ferrite through multi-pass caliber rolling as a high Z- large strain deformation technique for producing bulk engineering components. It was also noted that the empirical relation established based on single pass compression tests is valid for multi-pass caliber rolling.

Effect of initial grain size on evolved ferrite grain size during high Z large strain deformation

Abstract The effect of initial grain size on the evolved ferrite grain size during high Z large strain deformation was studied in ultralow carbon steel with two widely varying initial microstructures: a coarse grained (200 μm) and an ultrafine grained (0·7 μm) microstructures. Plane strain compression was used for imposing large strain in the test specimen and a thermomechanical simulator was used for varying the strain rate and temperature precisely. Electron backscattered diffraction was used for the analysis of deformed microstructure and for grain size measurements. Deformation conditions were varied to cover wide range of Zener–Hollomon parameters. It was noted that the evolved ferrite grain size solely depends on the Zener–Hollomon parameter and does not depend on the initial grain size. The relation d=10Z–0·12 (Q gb=155 kJ mol–1) was obtained for two initial microstructures with grain sizes varying by three orders of magnitude.

Formability of Ultrafine Grained Steel

Ultrafine grain refinement to 1 m deteriorates the uniform elongation in the tensile tests of steels. Such loss of ductility has been argued to be an inherent feature of the ultrafine-grained steels. 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 and evaluated for their stress-strain behavior along with the reduction in area. It was found that the reduction in area- tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to bainitic steels for all materials tested in the present study. Formability of ultrafine grained steel is examined by applying to make a micro screw. Good formability was verified by this process.

Micro Hole Piercing for Ultra Fine Grained Steel

Ultra fine grained steels have been developed by many researchers. However, the study of influence on processes and product functions from different grain size are limited because the size of bulk material was small for these products. Authors have developed the production process of thin ultra fine grained stainless steel coil, and the effects are able to be clarified. This paper will firstly report the influence on micro hole piercing by comparing different grain size materials. Secondly, orifices are produced from these materials, and the liquid flow volume is measured as the functional effect of different grain size. The effects of grain size differences were discussed with observing the hole conditions and measuring flow volume. The effects of reduction of the grain size were summarized as follows: (1) Accurate small hole is produced when ultra fine grained stainless steel is employed. (2) Product functional improvement is possible, and the phenomena are useful for liquid control devices.

Mechanical Properties of Ultrafine Grained Steels Processed by Large Strain-High Z Deformation - A Review

Ultrafine-grained steels with a grain size of about one micron offer the prospect of high strength coupled with high toughness among conventional steel compositions and are attracting the attention of researchers worldwide. Application of these ultrafine-grained steels to potential engineering structures demands extensive study of their mechanical properties. While there are many studies on the development of ultrafine-grained microstructures through various deformation processing techniques on a spectrum of compositions, fewer studies were reported on the more important aspect of evaluating their mechanical properties. This is to verify the basic assumption that the microstructural refinement at bulk level indeed improves the mechanical properties offering the prospect of a realistic replacement of the existing conventional steels in the near future. As we move towards the ultimate goal of applying these advanced high strength materials, this review article attempts to present a comprehensive picture on the mechanical properties of ultrafine-grained steels with varying carbon contents fabricated by large strain warm deformation. Finally, it is believed that time is ripe for exploring the possible applications of these materials for structural applications.

Effects of strain on grain boundary characteristics and their correlation to mechanical properties in low carbon steel processed by warm multipass calibre rolling

Abstract The effect of strain on the grain boundary characteristics and their correlation to mechanical properties is studied in detail in low carbon steel processed by warm multipass calibre rolling. It was noted that strength properties are controlled by the total grain boundary length but not grain boundary character or their fraction. On the other hand, impact properties are controlled by the length fraction of high angle grain boundaries. Finally, it was noted that low angle boundaries are found to be helpful in improving the strength whereas they are harmful for impact properties.

High Strength Microscrew with Ultrafine Grained Structure

While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. It was found that the reduction in area - tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to martensitic and bainitic steels. Formability of ultrafine-grained steel is examined by applying to form a M1.7 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. High strength microscrew with ultrafine grained structure was obtained.

Effect of Austenite Grain Size on the Mechanical Properties in Air-Cooled 0.1C-5Mn Martensitic Steel

In 0.1C-5Mn steels, 5%Mn addition increases hardening ability and makes 100% martensitic transformation even in air cooling without water quenching. Their Ms and Mf temperatures are in the range of 350-250°C, and subzero treatment is not needed. This makes it possible to measure Ms and Mf temperatures accurately by dilatometry. 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 with identical specimens and obtained the following results. Ms temperature decreases as much as 40 K with a decrease in austenite grain size from 254 to 30 m. 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. 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 balance between strength and ductility.

Ultrafine Grained Steel Wire and its Formability

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.