Papers by Author: Jun Yanagimoto

Abstract: For the manufacture of good quality strips by rolling processes, the bending residual stress of the strip is one of the most important qualities to be controlled. Most reports on strip bending behavior have focused on thick plates produced under batch rolling conditions [1, 2], and few reports have examined continuous rolling of thin plates. In this report, we conducted a single-drive rolling test with a strip having the thickness of 0.23mm and a 2Hi test mill with work rolls 410mm in diameter. We also calculated the pressure distribution and warping curvature. The model used here was based on the Orowan model constructed by Morimoto and Yanagimoto [3]. In this calculation, the roll bite was separated into 3 sections, i.e., forward slip region, backward slip region and cross shear region. In the calculation results, the pressure distribution showed the existence of a pressure well in the cross shear region, as also reported by Zorowski et al. [4] The curvature in the calculation result increased as the reduction ratio increased up to 10%, but decreased with increasing reduction ratios larger than 10%. This tendency was the same as the measured curvature in the experiment, and was considered to be caused by the moment by friction force at the cross shear region and also the aspect ratio of the cross shear region.

Abstract: In the seamless pipe rolling process, the pipe wall thickness is largely determined at the mandrel mill or plug mill. It is possible to obtain the target thickness at these mills by defining the gap of a grooved roll and an inside tool such as a plug. However, the thickness of the free deformation part, which is not in contact with the roll and tool, had generally been estimated by experimental techniques. Although a number of analytical studies of mandrel mill rolling had been reported, few reports had examined plug mill rolling. Therefore, in this research, a finite element analysis model for plug mill rolling was developed by extending the rigid plasticity finite element model "Computational Rolling Mill (CORMILL)." Good agreement between the calculated results and experimental results was obtained for the wall thickness, and it was found that the thickness of the flange part decreases with reduction of the wall thickness at the grooved bottom. These results suggested that the wall thickness distribution of rolled pipes can be controlled by using a suitable inside tool and roll shape in each rolling pass, and the necessary shapes can be obtained by using the newly-developed model.

Abstract: Owing to the phase segregation occurred during semisolid process, the ferroalloys products manufactured by semisolid process always exhibit inhomogeneous microstructure and poor mechanical properties. In this study, the post heat treatments including quenching treatment and tempering treatment with various processing parameters were carried out to improve the quality of Cr-V-Mo steel (JIS SKD61, AISI H13, DIN 1.2344) which processed by RAP (recrystallization and partial melting) processed. The microstructural characteristics (phase transformation, morphology of carbides, and distribution of alloying elements) and mechanical properties (hardness, tensile strength, elongation, impact toughness, and resistance to high-temperature wear) of specimens processed by RAP and heat treatments under various experimental conditions were investigated experimentally. Being quenched from 1050 °C after isothermal holding for 480 s and then tempered twice at 560 °C for 2 h, the microstructural evolution took placed in both former solid-phase and liquid-phase regions of the RAP-processed Cr-V-Mo steel specimen. During this post heat treatment strategy, the weakening of phase segregation, the redistribution of carbides, and the release of residual stress occurred and resulted in the improvement of microstructure and a good combination of mechanical properties.

Abstract: T.M.C.P.(Thermo Mechanical Control Processing) has been widely used to improveplastic formability in steel strips. We have produced interstitial free steel(IF steel) strips and ferriticstainless-steel strips through T.M.C.P. rolling method. Optimizing conditions of hot rolling, hotrolled annealing, cold rolling and cold rolled annealing, we developed texture prediction model. Wecan predict rolling texture accurately using the conventional Taylor model. Moreover, we preciselypredict recrystallization texture classifying the total number of microscopic􀀁 slips which arecalculated using the Taylor model. We consider that these calculated results provednucleation-oriented model and two types of recrystallization and grain growth mechanisms exit inour studies. One mechanism is that grains which had the small total number of microscopic slips arepreferred orientation for the hot rolled and annealed ferritic stainless-steel strip. The othermechanism is that grains which had the high total number of microscopic slips are preferredorientation for the cold rolled and annealed IF steel strip.

Abstract: In this study, the authors propose a new refining process different from the rheorefining (a screen method) or the melt zone method. This new refining process is based on the gradient of solidification speed of the molten metal poured into a container. The molten metal in the cylindrical container firstly solidifies to columnar crystals from the wall of the container and a liquid phase gradually changes to semisolid state and progresses to the central portion of the container. After an intended semisolid state is achieved, a backward extrusion is carried out to extract the liquid phase component. The liquid component is known to contain a lot of impurity substances. For the refining trials, three aluminium alloys were selected: wrought aluminium alloy A2011, cast aluminium alloy AC4C and aluminium beverage can scraps. Backward extrusion (extraction) was applied twice, and the amount of pure Al increased from 90.59 % in the initial ingot to 96.34 % in the refined material. This new refining process can easily achieve the purification level of the rheorefining or the melt zone method.

Abstract: The microstructural evolution and basic refinement mechanism of the semi-solid-state SKD61 tool steel fabricated by recrystallization and partial melting (RAP) method were investigated experimentally. The effects of different parameters in RAP processing were systematically clarified to achieve the grain refinement of cast SKD61 steel in the semi-solid state. Results showed that, the microstructure of RAP processed specimen were finer and more globular than the microstructure of starting material. The distribution of finer solid particles in the RAP processed specimen was affected by the distribution of strain during predeformation. Uniform and spherical solid particles inclined to locate in the regions suffered from larger strain. The specimens subjected to a plastic predeformation at a lower temperature have a more uniform and spherical semi-solid microstructure than those specimens undergoing predeformation with the same reduction at higher temperatures. In addition, the grain refinement mechanism of SKD61 tool steel processed by RAP method was discussed.

Abstract: The ferrite transformation kinetics of severely hot-deformed austenite has been studiedby considering ferrite nucleation from dislocation cell blocks inside austenite grains. The size ofdislocation cell blocks and ferrite grain size just after phase transformation are acknowledged to beinversely proportional to the square root of dislocation density. It is found that the ferrite nucleationrate in this area can reach the saturated state at a high temperature just under Ae3, and the ferritetransformation finishes within a very short time. The kinetics of ferrite volume fraction and theferrite grain growth after phase transformation for plain carbon (0.1%C, 0.2%Si, 1.0%Mn) steelhave been studied using a THERMECMASTER hot-compression testing machine. These modelscan be applied to the hot and warm forming processes of plain carbon steel to predict the ferritetransformation from severely deformed austenite.

Abstract: The feasibility of recycling machining grindings of aluminum alloys by semisolid process has been investigated. Machining grindings of A2011 aluminum alloy produced experimentally by lathe machining were used. The material is put into a metal mold and compressed up to 90 % of the true density at room temperature. The metal mold with the compressed machining grindings is heated up to a specified temperature. Afterwards, the metal mold is set into the extrusion container, and extrusion in hot and semisolid range was carried out. In this experimental study, extrusion load, internal structure of the product and mechanical properties (tensile strength, elongation, hardness) of the product are assessed. It was proven that semisolid extrusion is about 40% less extrusion load compared with that of hot extrusion, the shape of the machining grindings remained in the hot extrusion and the semisolid extrusion products extrusion ratios higher than 10 have excellent elongation property, which is comparable to the commercialized product.

Abstract: Thixoforming or Semi-Solid Metal Forming offers many advantages in comparison with casting and conventional forging. The purpose of the present study is to provide the basic microstructure and deformation data for austenitic and ferritic stainless steel under mushy state. As well known, the stainless steels solidify in different modes according to the different chemical compositions. In this paper, microstructural evolution of austenitic stainless steel type 304 which solidifies in FA mode ( L → L +δ → L +δ +γ →δ +γ →γ ),austenitic stainless steel type 310S which solidifies in A mode ( L → L +γ →γ ), and ferritic stainless steel type 430 which solidifies in F mode ( L → L +δ →δ )
are investigated during partial remelting by way of SIMA (Strain Induced Melted Activation). The results show that A and F mode of stainless steels melt directly at the grain boundary without phase transformation during reheating. A banded structure, originating from the primary dendritic segregation of the original ingots, is observed in type 310S steel during further heating. On the other hand, a perfect globular and insegregative two-phase semi-solid structure L +δ can be obtained while heated beyond the banded three-phase L +δ +γ semi-solid state in FA mode austenitic stainless steel type 304. This spheroidization can be attributed to the peritectic reaction occurred in the L +δ +γ semi-solid state. In addition, simple compression tests of these alloys in semi-solid state for varied combination of deformation rate and deformation temperature are conducted to examine the deformation behavior of stainless steel. Flow stress curves exhibit abrupt change in various alloys, even though in the same alloy such as type 304, various flow stresses are observed according to the difference in inner microstructure or morphology. Stress of type 310S steel shows the most reduction as the deformation temperature increasing at the same strain rate condition. The Liquid is centralized to periphery by the compression force in all deformed test pieces. Fracture, observed in all alloys except type 304 steel in globular L +δ semi-solid state, should be resulted from the lack of liquid in L +δ +γ state of type 304 steel and solidification crack in type 310S and type 430 steel. Deformation of solid particles occurs only in L +δ +γ state of type 304 steel. Last in this paper, various deformation mechanisms are proposed for various microstructures.

Abstract: The structural changes of the wrought magnesium alloy AZ31B in semisolid state were clarified using optical microscopy and hot-stage microscopy. The influence of heat treatment variables was assessed. Compression tests covering a range from room temperature to 673 K were carried out for mechanical property assessment; flow stress and breaking strain were determined. The following are the results: (1) The grain growth of the hot-extruded AZ31B without preprocessing sensitively reacted at temperature and retention time. (2) The hot-extruded AZ31B with 30 % preprocessing showed an almost perfectly spheroidized structure in a semisolid state under certain conditions. (3) Heating velocity markedly affected the spheroidizing rate of grains. (4) From the direct observation of the hot-extruded alloy AZ31B by hot-stage microscopy, spheroidization was observed in some crystal populations. (5) Spheroided materials in the semisolid temperature range had a lower flow stress and a larger breaking strain than nonspheroidized materials. These results indicate the possibility of manufacturing wrought magnesium alloy by cold working.