Papers in JOURNAL OF THE JAPAN SOCIETY
FOR TECHNOLOGY OF PLASTICITY

(Vol.41 No.474 July 2000)

Austenite Grain Refinement and Strengthening of Carburized
Low Alloy Steel with Warm Forming

Toshiaki TANAKA, Munehisa MATSUI and Atsushi DANNO

(Received on July 9, 1999)

A thermomechanical treatment process has been experimentally studied to obtain finer austeneite grains and higher fatigue strength of carburized and quench hardened steel parts. The combination of two operaions, the cooling operation to pearlite transformation temperature after warm forming for a reduction in height of about 50% and the subsequent short-time reheating operation to a temperature immediately above the austenite transformation temperature, was effective in refining austenite grains. The austenite grain size of 3 to 5μm was obtained with precarburized low alloy steels, such as SCr420, SCM420 and SNCM420m. The bending strength and fatigue strength of these steels with such fine austenite grain was 50% higher and 10% to 30% higher, respectively, than those of conventional steels. The deflection of these steels in the bending test was about twice as large as that conventional steels.
Key words : form rolling, forging, thermomechanical treatment, warm forming, ferrous metals, carburizing, austenite grain, strength, fatique strength

Improvement of Dimensional Accuracy of Rectangular Extruded Pipes by Die Mandrel Shapes of Porthole Dies

Toshiyuki INAGAKI, Satoshi MURAKAMI, Norio TAKATSUJI,
Kenji MATSUKI, Mitsuyuki ISOGAI and Katsuhiko TOGAMI

(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

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

Manabu SANGA, Nobuki YUKAWA and Takashi ISHIKAWA

(Received on August 2, 1999)

To explain the effect of the deformation path on the deformation-induced martensitic transformation in austenitic stainless steels, the uniaxial tension test and rolling were carried out, and the amount of deformation-induced martensite was measured. The amount of martensite existing before deformation or martensite formation before deformation was changed by the aging of 77K. The influence of martensite existing before deformation or martensite formation before deformation and the strain path of deformation-induced martensitic transformation were examined. The following conclusions were obtained. The amount of deformation-induced martensitic transformation differed according to the deformation path. The deformation-induced martensitic transformation follows the hydrostatic stress theory when the martensite exists before deformation. However, the deformation-induced martensitic transformation is contrary to the hydrostatic stress theory when the initial martensite does not exist. The growth of martensite obeys the hydrostatic stress theory, and the nucleation of martensite obeys the autocatalytic effect theory.
Key words : material testing, material, deformation-induced martensitic transformation, austenitic stainless steel

Study on Deburring Conditions by Abrasive Magnetic Machining and the Availability to Assembled Products

Isamu AOKI, Toshinori TAKAHASHI and Kiyoshi SUZUKI

(Received on August 17, 1999)

In this study, we suggest a finishing technology using grit and cut-wire shot as magnetic abrasives, and a new finishing procedure applicable for deburring and polishing complex-shape products. The grit and the cut- wire shot exclusively used in shot blasting were used as magnetic polishing media. These media can be supplied at low cost and fabricated with ease. Three types of sheet metals, aluminum, copper and stainless steel are polished using these media in addition to traditional magnetic abrasives to investigate the polishing ability. In the polishing of assembled products, newly suggested characteristic feature is that the multi-layer disk is used as the magnetic pole, whilst the right and left disks are offset in the direction of the central axis of the disks. This therefore offsets the magnetic brush formed between the right and left plates. The practical applicability of the method was confirmed by actual edge finishing tests using a product assembled from three plates with long slots. The corners of the slots of all the plates could be finished to a rounded edge.
Key words : blanking, deburring, assembled product, magnetic abrasive machining, edge quality

Edge Crack of Sheared Aluminum Alloy Sheets during
Cold Rolling and Its Formability Improvement

Toshihiro YANO, Naoki ASANO, Tatsuo YOKOTE and Ryuta ONODERA

(Received on September 8, 1999)

It is well known that the formability of sheared sheets is extremely poor during cold working. In this study, in order to examine the effect of shearing on edge cracking in Al alloy sheets, cold rolling tests were carried out on 2017 Al alloy using sheared specimens which had been aged and overaged. In addition, the effectiveness of “double shearing”, which involves reshearing the sheared edge only slightly in order to improve the poor formability, was investigated. The obtained results were as follows. (1)Edge cracks of sheared Al alloy sheets, in both aged and overaged specimens, were attributed to the generation and activity of shear bands with the increase of rolling reduction. (2)Shearing influenced the worked hardness and shepe of the sheared edge, particularly in the case of overaged specimens, so that the overaged specimens exhibited extremely poor formability. (3)The formability of overaged specimens was remarkably improved by double shearing, although that of aged specimens was not.
Key words : edge crack, shear band, rolling, shearing, Al alloy, forming property

A New Punching Method for Preventing Centerline Cracking
in High-Tensile-Strength Steel

Toshio NAKAJIMA and Hiroaki WADA

(Received on September 9, 1999)

High-tensile-strength steel is liable to incur lamellar cracking (centerline cracking) caused by centerline segregations at the center of the plate thickness when the workpiece is punched after cold working. The following punching method was developed as a preventive countermeasure. This novel process consists of two stages:the first stage of partial shearing by negative clearance, and the second stage of punchthrough by conventional shearing. In this technique, centerline cracking can be suppressed during the first stage, because the tensile stress is not generated in the thickness direction, for the material is pushed out by compression. Also, in the second stage, centerline cracking does not occur, as compressive residual stress is applied in the vicinity of the partial shearing position. It is possible that centerline cracking is prevented by applying this method to cold-worked high-tensile-strength steel whose original tensile strength is 590MPa.
Key words : shearing, punching, accuracy, two-stage punching, partial shearing, negative clearance, high-tensile-strength steel, centerline cracking, centerline segregations

Effect of Thermal Cycles on Two Way Memory Characteristics
of Ti-Ni Shape Memory Alloy

Toshio YOKOYAMA, Yoshihide HOSHINO and Yuhsuke MASHIMO

(Received on September 10, 1999)

Two-way memory characteristics of Ti-50at%Ni wire of 0.5mm in diameter were experimentally tested. The straight-line shape was memorized as the shape at high temperature, by applying a tensile strain of 0 to 30% to the wire at room temperature, holding it for 30 minutes at 450℃, and then quenching the wire in water. A curved line shape was memorized as the shape at low temperature, by winding the wire around circular cylinders of 2.5, 5, and 10mm in diameter at room temperature. Thermal cycles were carried out by alternately immersing the alloy in about 20℃ water and 100℃ water. Tensile tests were performed for the alloys after they had undergone 100 thermal cycles. The following results were obtained from the present experiment. (1) The shape recovery ratio at high temperature is increased as the tensile strain during the shape memory treatment is increased and as the bending strain applied at low temperature is decreased. In contrast, the shape recovery ratio at low temperature is increased as the tensile strain is decreased and the bending strain is increased. (2) The shape recovery ratio at high temperature is decreased with an increased number of cycles when thermal cycles are applied for an alloy shape fixed to the low-temperature shape. (3) The degree of work hardening based on the number of thermal cycles is increased as the tensile strain applied at high temperature is decreased.
Key words : bending, shape memory effect, Ti-Ni alloy, thermal cycle, work hardening, two-way shape memory, martensitic transformation

Theoretical Analysis of Tandem Stand Rolling Using Three-Dimensional Finite Element Method

Takashi KUBOKI, Haruhiro FURUTA,
Shinji MURAKAMI and Kouichi KURODA

(Received on September 13, 1999)

An algorithm for tandem rolling, which can calculate inter-stand tension, has been developed. The algorithm expands the applicability of a three-dimensional rigid-plastic finite element method model for single-stand rolling: “CORMILL system” to tandem-stand rolling. The developed algorithm can predict the effects of rolling conditions on the rolled shape, the inter-stand tension, the rolling velocity, the rolling load and so on. In this paper, the effects of gap operation at each stand on the rolled shape were investigated with respect to bar rolling. Calculated results showed that the gap operation of the first and last stands is much more effective on the lateral spread of the rolled bar than that of the other stands. These results were verified by laboratory experiments and it became clear that the newly developed algorithm can be used as an effective tool for predicting the behavior of the tandem-stand rolling.
Key words : rolling, numerical analysis, tandem, FEM, inter-stand tension, precision, bar, wire

Contact Problems with Variational Inequality Formulation and Application to Sheet Metal Forming Simulation

Kiyoshi KAMITA and Akitake MAKINOUCHI

(Received on September 26, 1999)

In this paper we propose a new method of solving the contact problems which arise during sheet metal forming simulations using a static-explicit finite-element method (FEM) code. We have identified the factors which determine the contact status of the nodes which have very small reactive force, and we propose an algorithm to solve the nonlinear contact problems using variational inequalities. Furthermore, we have enhanced the static-explicit FEM code ITAS3D using this formulation, to demonstrate that this approach is effective and stable.
Key words : sheet metal forming, numerical analysis, elastoplastic FEM, contact, lagrangian method, static-explicit FEM, variational inequality

Controlling Edge Drop by Tapered-Crown Work Roll Shifting
and Crossing Mill in Cold Strip Rolling

Jun-ichi TATENO, Kazuhito KENMOCHI, Ikuo YARITA,
Tomohiro KANEKO and Teruhiro SAITO

(Received on October 13, 1999)

In order to technologically advance the control the edge drop in cold strip rolling, the control capability of the edge drop and deformation of the strip were investigated through a combination of work roll shifting and crossing. The experiments were carried out on a laboratory-scale mill, in which work roll shifting and crossing devices were installed. In the work roll shifting and crossing mill, the metal flow at strip edges occurs before the roll bite, and the edge drop is improved according to the contour of the work roll taper in the roll bite. This allows a much higher capability of controlling the edge drop than the sum of the work roll shifting and the work roll crossing capabilities. The amount of improvement of the edge drop increases quadratically with increase in the effective roll gap, which is defined as the roll gap on the basis of the 100mm edge point. The flatness of the rolled strip achieved by the work roll shifting and crossing mill is much better than those achieved by the work roll shifting mill or the work roll crossing mill. Therefore, the work roll shifting and crossing mill is the most effective for simultaneously controlling the edge drop and strip flatness in cold strip rolling.
Key words : cold rolling, rolling mill, thickness, edge drop, work roll shifting, work roll crossing, strip flatness

Manufacture of Cluster-Diamond-Dispersed Aluminum Composite by Dynamic Compaction

Noboru NAKAYAMA, Masao MAYUZUMI, Kotaro HANADA,
Toshio SANO and Hiroyuku TAKEISHI

(Received on November 11, 1999)

The thin-film forming of cluster-diamond (graphite cluster diamond)-dispersed aluminum composite is carried out by a dynamic compaction method. The friction coefficient, density, mechanical properties and microstructures of compacted powders are investigated. The stress wave control is also attempted in order to obtain a high compacting stress by the superposition of incident and reflected stress waves in the punches. Dynamic powder compaction is carried out at 300-873K in vacuum by an electromagnetic induction method. A density measurement reveals that the compacted cluster-diamond (graphite cluster diamond)-dispersed aluminum composite has a relative density of 0.90‐0.97. The addition of cluster-diamond to matrix systems reduces the friction coefficient.
Key words : high-energy-rate forming, electromagnetic forming, powder forming, composite material, tribology

Ultrahigh Temperature Forging of Steel
- Development of Near-Net-Shape
Forging of Complicated Parts Ⅰ -

Takeshi MIKI, Osamu KADA and Masahiro TODA

(Received on June 30, 1999)

Forging at temperatures near the solidus limit has previously been avoided. One of the reasons for this is because melting occurs in the part of the material and the other is that the microstructure of the material after forging grows considerably and the mechanical properties become distorted. However, near-net-shape forging of complicated parts is urgently needed for cost reduction. In this paper, ultrahigh-temperature forging near the solidus limit is attempted in order to enable forging at the lowest flow stress of steel. Under simple compression of a cylindrical specimen at the temperature immediately above the solidus limit, the free surface of the specimen breaks due to melting at the grain boundary. Therefore, during forging, the free surface should be restricted by forging tools. High hydrostatic pressure caused by the tool restriction supresses breakage and refines the microstructure of the forged part because of a large reduction in the cross section. These results indicate the possibility of near-net-shape forging at ultrahigh-temperatures.
Key words : forging, forging steel, tensile strength, near-net-shape, ultrahigh temperature