Papers in JOURNAL OF THE JAPAN SOCIETY FOR TECHNOLOGY OF PLASTICITY

(Vol.41 No.468 January 2000)

Elastic Recovery Buckling Behavior in Diagonal Tensile Test of Square Specimen (Elastic Recovery Buckling Behavior of Sheet Metal Forming‐Ⅰ)

Tetsuro OHWUE, Matsuo USUDA, Kouji HASHIMOTO and Hisashi HAYASHI

(Received on September 22, 1998)

The width-bound diagonal tensile test of a square specimen and a simulation utilizing the finite-element method (FEM) program MARC, were carried out to investigate the elastic recovery buckling behavior. The results of FEM simulation coincides with the experimental results. In experiments, the greater yield stress (YS) becomes, the larger the size of the wrinkle. The wrinkle profile is assumed to follow a sine curve, by calculating the first and quadratic differential coefficients from a measured 3-dimensional profile. In the simulation, a material with higher YS has a large difference in stress (loaded stress minus unloaded stress). In a material with larger n-value, the calculated wrinkle height decreases with increasing 〓 (pulling strain).
Key words : diagonal tensile test, high strength steel sheets, buckling, elastic recovery, yield stress, n-value, FEM analysis, formability test, material testing

Effect of Deformation on Change of Yield Surface Shape for Al-Mg Alloy under Proportional Loading

Yasushi MAEDA, Shigeo HATTORI, Yasuhiro HAYASHIDA and Takashi ISHIKAWA

(Received on September 28, 1998)

This study aims to establish an empirical constitutive equation for aluminum alloy sheets. The flow law is usually assumed for the constitutive equation of plastic deformation. The yield surface shape and the hardening law are important mechanical properties for the flow law. In this study, changes in the yield surface shape and the strain direction, which increase with plastic work, are experimentally measured. The results show that the yield surface shape changes as plastic work increases and that the degree of change depends on the initial texture of the material. From these measurements, a new empirical yield function is proposed. To understand the mechanism of this phenomena, polycrystal models are introduced. From the model, it is demonstrated that the work hardening mechanism is more important for changing the yield surface shape than the crystal rotation mechanism. By using the model, it is confirmed that the degree of change of the yield surface shape decreases as recrystalized texture components increases.
Key words : sheet forming, aluminum alloy, crystallographic texture, constitutive equation, yield surface, numerical simulation

Superplastic Complex Working of Micro-Double Gear

Yasunori SAOTOME, Seiichi HATA and Kohji SAKAGUCHI

(Received on November 17, 1998)

Superplastic complex working was applied to the microforging and fabrication of a micro-double gear with a shaft. The material utilized was a fine-grained Al-78Zn superplastic alloy, and microformability tests were carried out using a micro-V-grooved die made of (100) silicon wafer. The throat width of the V-groove was 20〓m, and the groove was made by photolithography and the anisotropic-etching technique. Based on the results of the microformability tests, the microforming of a double gear was performed. The diameter of the pitch circle was 1 and 2mm〓 and the number of teeth was 10 and 20. The tooth width was 1 and 0.6mm. Two metal dies were machined with a wire electrical-discharge machine. The die for the larger gear part of the double gear was located on another die in a microdie set. Closed die forging was carried out at a temperature of 523K for 300sec. After microforming, the double gears were solid-state bonded with 0.4mm〓 steel shafts. As a result, the superplastic complex working process was confirmed to be useful for the forming and fabrication of milli- and micro-machine components.
Key words : forging, joining, complex working, microgear, formability test, forming property, suerplasticity, microforming, solid-phase joining

Effect of Skinpass Rolling Conditions on Surface Brightness of SUS430 Strip

Kazuhito KENMOCHI, Michio YAMASHITA and Ikuo YARITA

(Received on October 19, 1998)

Stainless cold-rolled strip is finished mainly with a mirror-polished roll under nonlubricating conditions in the skinpass rolling process. There have been few reports on the skinpass rolling, though the surface brightness of the strip is affected by the skinpass rolling conditions. In this study, the effect of skinpass rolling conditons on surface brightness of a SUS430 strip is examined quantitatively with a laboratory-scale mill, and stress in the roll bite is calculated by an elastic-plastic rolling theory using the Prandtl-Reuss equation. The results obtained are as follows: (1) Surface brightness improves with increasing rolling load and roll diameter and deteriorates with increasing tension. (2) Surface brightness is related to the surface roughness of the strip. Microdefects, such as scratches and oil-pits, are caused of surface roughness and are decreased by skinpass rolling, which flattens the strip surface and improves surface brightness. (3) Rolling pressure in the plastic deformation zone increases with increasing rolling load and roll diameter.
Key words : skinpass rolling, surface roughness, stainless steel, surface brightness, rolling pressure

Occurrence of Heat Streak in Cold Rolling of SUS430 Steel Strip

Kazuhito KENMOCHI, Osamu SONOBE and Ikuo YARITA

(Received on December 21, 1998)

In high-speed and high-reduction cold rolling of a stainless steel strip, heat streak occurs on both roll and strip surfaces. Occurrence of heat streak in the cold rolling of a low carbon steel strip has been investigated to some degree, but there are few reports presented for that of a stainless steel strip. In this study, the effect of the cold rolling conditions of a ferritic SUS430 strip on the occurrence of heat streak is investigated. Inlet oil film thickness in a roll bite is calculated taking surface roughness of both the roll and the strip into consideration, and the temperature in the roll bite is calculated by a mixed lubrication model using the above inlet film thickness. The results obtained are as follows: (1) Heat streak occurs following a reduction in thickness of more than 21% and is little affected by rolling speed under this experimental condition. (2) Inlet oil film thickness increases with increasing rolling speed, but little affected the occurrence of heat streak. (3) Heat streak occurs under conditions of temperatures higher than 120℃, the maximum temperature to which a roll bite can be exposed. (4) Three types of heat streaks scratches due to roll roughness, pushed defects, and lumps of scum can be identified at the point where the heat streak begins.
Key words : cold rolling, lubrication, stainless steel, heat streak, rolling oil, surface roughness

Influence of Chemical Composition on Deformation-Induced Martensitic Transformation in Austenitic Stainless Steel

Manabu SANGA, Nobuki YUKAWA and Takashi ISHIKAWA

(Received on December 22, 1998)

To explain the effect of the chemical composition on the deformation-induced martensitic transformation in austenitic stainless steels, the Ni equivalent composed of many chemical elements was introduced thermodynamically. The Ni equivalent, (Ni), is the parameter expressing the stablity of austenite. In this work, the following equation was obtained. (Ni)＝Ni＋12.93C＋1.11Mn＋0.72Cr＋0.88Mo－0.27Si－0.24Ti－0.07Co＋0.19Nb＋0.53Cu＋0.90V＋0.70W－0.69Al＋7.55N－0.04(7－GSN), where the element symbols represent mass % of these elements in the steel, and GSN is the grain size number. To examine the validity of this Ni equivalent, a uniaxial tension test and a rolling test were carried out, and the amounts of martensite were measured. As a result, it was shown that the steels that had low Ni equivalents showed small amounts of 〓〓 martensite before the deformation tests. The amount of deformation-induced 〓〓 martensite increased in both the uniaxial tension test and the rolling test, when the Ni equivalent decreased. Therefore, the Ni equivalent that is introduced in this work can express the stablity of austenite in steels that composed of many chemical elements. Furthermore, this Ni equivalent was proved to be independent of stress states.
Key words : material testing, material, deformation-induced martensitic transformation, austenitic stainless steel

Effect of Rod Rolling Conditions on Bright Steel Bar Straightness －A Study of Higher Straightening and Lower Residual Stress Technology of Bar Products Ⅰ－

Motoh ASAKAWA

(Received on January 7, 1999)

Straightening bright steel bar products has become essential in the production of many mechanical parts． Recently，customers such as those associated with the automobile, precision machines and office automation industries, require a high degree of straightness and lower residual stress. However, straightening technology has not advanced in the past few years. The goal of this study is to systematically remedy this situation by developing methodologies to attain a higher degree straightness and lower residual stress in bar products. In this paper, the effects of the microstructure and the mechanical properties of a rolling rod on bar straightening were investigated. It was found that straightness was improved by using rods with lower and constant yield stress. These rods were produced by minimizing austenitic grain size by means of controlled rolling, and by controlling the ferrite-pearlite grain size via slow cooling to attain a regular microstructure.
Key words : straightening, leveling, bending, rod rolling, controlled rolling, drawing, ferrous material, microstructure, residual stress

Predicting the Wrinkle Limit Bending Radius of Aluminum Tubes in the Bending Process

Masatoshi YOSHIDA and Akibumi FUJIWARA

(Received on January 21, 1999)

Aluminum tubes are used for the structural body of automobiles. It is often necessary for the tubes to undergo the bending process before they can be used as automobile parts. Wrinkling is a problem in the bending process. A theoretical analysis model for estimating the wrinkle limit bending radius, which can be applied to the tubes with general cross-sections, was developed and the results by the model was compared to that by FEM analysis. The main results are as follows: (1)The wrinkle limit bending radius of tubes with arbitrary cross-sections can be predicted by the buckling limit of the constituent plates. (2)The nondimensional wrinkle limit bending radius R../H decreases with an increase in value of the parameters k(t/b)2, where k is the buckling constant on the elastic buckling that depends on the stress state and boundary conditions, and b/t is the aspect ratio of a rectangular plate. (3)The wrinkle limit bending radius obtained using Bleich's equivalent stiffness agrees well with that obtained by FEM.
Key words : bending, aluminum, plastic buckling, FEM, forming limit

Deep Drawability of Plastic Laminates and Paper in Ironing-Assisted Thermal Drawing Method

Kentaro YOSHIMATSU and Terufumi MACHIDA

(Received on July 6, 1999)

In order to form plastic laminates and paper into a cylindrical cup, deep drawing with local heating and ironing was successfully applied. Paper and its composites required a preliminary steam treatment to make the flange material moist and soft, thereby increasing its thickness. The forming limits, that is, the limiting drawing ratio (LDR) was greater than 2.5. The diameter and thickness distribution of the drawn cup wall was uniform, while at higher temperatures the shape of the drawn cup became unstable due to strain recovery.
Key words : ironing-assisted thermal drawing, limiting drawing ratio, accuracy, shape stability, polypropylene laminate, Al/paper composite, paper