Rintaro Ueji

Ultragrain refinement of plain low carbon steel by cold- rolling and annealing of martensite

Simple cold-rolling and annealing of martensite starting structure can produce ultrafine grained structure in carbon steel. The microstructural evolution during the process was studied in a 0.13%C steel. The ultrafine lamellar dislocation cells (LDCs) with mean thickness of 60 nm were mainly observed in a 50% cold-rolled specimen as well as the irregularly bent lamellas (IBLs) and the kinked laths (KLs). The LDCs and the IBLs had large local misorientations. The specimens annealed at temperatures from 723 to 773 K showed the multiphased ultrafine structure composed of equiaxed ultrafine ferrite grains with the mean grain size of 180 nm, nano-carbides distributed uniformly and small blocks of tempered martensite. The formation of the ultrafine grained structure was discussed from the viewpoint of characteristics of the martensite starting structure. It was concluded that the fine grained structure of martensite play an important role for ultrafine grain subdivision during plastic deformation.

A new and simple process to obtain nano-structured bulk low-carbon steel with superior mechanical property

Abstract A new process to obtain ultrafine grained bulk steel was developed. Plain low-carbon steel sheet with martensite starting microstructure was simply cold-rolled by 50% and annealed. The specimens annealed at intermediate temperatures such as 773 K revealed the multiphased nano-structure and showed superior mechanical properties.

Nanoscale crystallographic analysis of ultrafine grained IF steel fabricated by ARB process

Abstract Nanoscale crystallographic features of ultrafine grained interstitial free steel fabricated by accumulative roll-bonding (ARB) process have been studied by electron back-scattered diffraction in field-emission type scanning electron microscope. This work has clearly indicated that most of the elongated ultrafine grains in the ARB processed sheet are surrounded by high-angle grain boundaries. The characteristic textures in the ARB processed sheet were also clarified.

Effect of rolling reduction on ultrafine grained structure and mechanical properties of low-carbon steel thermomechanically processed from martensite starting structure

Abstract The present authors have invented a novel and simple thermomechanical processing to realize the ultrafine grained microstructure in carbon steels. The key of the process is to start from martensite structure. In the previous study, it has been clarified that conventional cold-rolling to a reduction in thickness of only 50% (equivalent strain of 0.8) and subsequent annealing at warm temperature around 500 °C fabricates the multi-phased ultrafine grained structure composed of the ultrafine ferrite grains with mean grain size of 180 nm, uniformly precipitated nano cementite and tempered martensite. In this study, the effect of the rolling reduction ranging from 25 to 70% (equivalent strains of 0.3–1.5) on the ultrafine grained structure and the mechanical properties of the plain low-carbon steel (Fe–0.13 wt% C) processed from martensite starting structure was studied. In the as-deformed specimen, the area fraction of the region showing the lamellar structure, which is typical for severely rolled metals, increased with increasing the rolling reduction and the strength also increased. After annealing at warm temperature around 500 °C, the multi-phased ultrafine grained microstructures were obtained in all the examined rolling reductions. The area fraction of the region showing the ultrafine ferrite grains increased with increasing the rolling reduction. At higher temperature, conventional recrystallization took place, and the recrystallization temperature became lower with increasing the reduction. Tensile test exhibited that the specimen rolled to the intermediate reduction (50%) performed the best strength-ductility balance (870 MPa of tensile strength and 9% of uniform elongation). The reason for the good strength-ductility balance of the specimen rolled to the intermediate reduction was discussed on the basis of the observed microstructures. q 2003 Elsevier Ltd. All rights reserved.

Grain Size Effect on the Martensite Formation in a High-Manganese TWIP Steel by the Rietveld Method

The aim of the present work was to study the effect of austenite grain size (AGS) on the martensite formation in a high-manganese twinning-induced plasticity (TWIP) steel. The results of a quantitative microstructural characterization of the steel by the whole X-ray pattern fitting Rietveld software, materials analysis using diffraction (MAUD), indicated that the volume fraction of a bcc -martensite increases with increasing AGS. However, the value of the stacking fault probability ( P sf ) does not show a large variation for samples with different values of AGS under water-quenching conditions.

Friction stir welding of hypereutectoid steel (SK5) below eutectoid temperature

Abstract The welded joints of the hypereutectoid steel (SK5, AISI-SAE W1-8), which is a high carbon steel with 0·85 wt-%C, were studied by friction stir welding below and above the A1 point (eutectoid temperature 723°C). Defect free joints can be successfully fabricated, and the joint structure and the mechanical properties were investigated for both conditions. The microstructures of the above A1 joints mainly consisted of martensite. Therefore, the microhardness of the above A1 joints is significantly increased to more than 1000 HV. The tensile tests results are scattered in which some of the joints are fractured at the stir zone while the others are fractured at the base metal. On the other hand, the microstructures of the below A1 joints consist of a ferrite with globular cementite, and grain refinement occurred because no phase transformation to the martensite structures occurred. The microhardness of the below A1 joints is slightly greater than the hardness of the base metal. All of the below A1 joints are fractured at the base metal during the tensile test.

High Speed Deformation of Ultrafine Grained TWIP Steel

Tensile properties of twinning induced plasticity (TWIP) steels (31%Mn-3%Al-3%Si-Fe) with various mean grain sizes ranging from ultrafine grain size (1.1μm) to conventional one (35.5μm) at a wide range of strain rates from 10-3sec-1 to 103sec-1 were studied. The ultrafine grained TWIP steel exhibits a large work hardening and keeps an adequate elongation at any strain rate. The strength held to the Hall-Petch relationship at each strain rate and the Hall-Petch slopes do not change largely.

Enhanced mechanical properties of 70/30 brass joint by multi-pass friction stir welding with rapid cooling

Abstract 70/30 (Cu/Zn) brass plates with a 2 mm thickness were jointed by repeated rapid cooling friction stir welding. The joints from each FSW cycle showed the typical construction of a microstructure which includes the stir zone and the thermo-mechanically affected zone, but the morphology and boundary characteristics in these zones changed with the different cycles. In the stir zone, the grain size decreased and the number fraction of the high angle boundaries increased with the increasing number of FSW cycles. The texture analysis suggested that the post-annealing effect, which frequently occurred after the FSW process, was remarkably restricted by the liquid CO2 cooling, which accelerated the refinement of the microstructure. As a result, a joint with an ultrafine grained structure (0·8 μm) and an excellent strength ductility matching (548 MPa and 34% respectively) can be achieved by multi-pass rapid cooling FSW process.

Grain Size Dependence of the Flow Stress of TWIP Steel

The effect of grain size on the flow stress in TWinning Induced Plasticity (TWIP) steel was investigated via the X-ray diffraction (XRD) measurements of dislocation density. The results indicated that the hardening behavior of fine grained samples (mean grain sizes in the range of 2.1-3.8μm) can be described as typical dislocation interactions. However in coarse grained samples (mean grain sizes in the range of 4.7-38.5μm) where extensive mechanical twinning occurs, another strengthening mechanism is required. Consequently, the effect of grain size on the flow stress parameters of the proposed equation was considered and it was found that in the fine grained samples, the Holloman analysis can describe the hardening behavior. However, in coarse grained samples, a second hardening term due to the strengthening effect of mechanical twin boundaries needs to be added to the Holloman equation.

Structural Analyses of Fractional Monolayer (GaAs)m/(AlAs)n Superlattices by X-ray Resonant/Off-Resonant Scattering

The structure of (GaAs)m/(AlAs)n superlattices with a period length of a fractional number in monolayer (ML) units, which is fabricated by shutter control in molecular beam epitaxy (MBE) growth, is studied by X-ray diffraction (XRD) analysis using synchrotron radiation. Satellite peaks originating from the fractional superlattice (SL) periods, 1.69 monolayers (ML), are clearly observed in XRD profiles. XRD simulation, which is based on the model of modulated composition distribution, can reproduce the satellite peak positions. We also measure X-ray resonant scattering at the Ga K-edge and estimate the atomic composition in the SL region.