Ferritic Stainless Steel Sheets Research Articles

ECAP・冷間圧延複合工程によるフェライト系ステンレス鋼板の加工組織と形成機構

ECAP・冷間圧延複合工程によるフェライト系ステンレス鋼板の加工組織と形成機構

Joint properties of ultra thin 430M2 ferritic stainless steel sheets by friction stir welding using pinless tool

In this study, friction stir butt welding of ultra thin 430M2 ferritic stainless steel sheets with satisfactory joint properties has been achieved using a pinless WC-Co tool. Friction stir welding was carried out at a tool rotational speed of 900rpm and a travel speed of 96mm/min. FSW joints were welded completely without any unwelded zone resulting from smooth material flow, despite of being adopted pinless tool. It was revealed that the maximum tensile strength of FSW welds was 468MPa, equivalent to the tensile strength of base metal. Notably, it was found that sigma phase formation in FSW welds is suppressed compared with that of arc welds, resulting in improving joint strength. These results suggest that the low input process of FSW is an effective substitute for the conventional fusion welding GTAW in ultra thin ferritic stainless steel plate.

Probing Formability Improvement of Ultra-thin Ferritic Stainless Steel Bipolar Plate of PEMFC in Non-conventional Forming Process

Formability increase in non-conventional forming profiles programmed in the servo-press was investigated using finite element analysis. As an application, forming experiment on a 0.15-mm-thick ferritic stainless steel sheet for a bipolar plate, a primary component of a proton exchange membrane fuel cell, was conducted. Four different forming profiles were considered to investigate the effects of forming profiles on formability and shape accuracy. The four motions included conventional V motion, holding motion, W motion, and oscillating motion. Among the four motions, the holding motion, in which the slide was held for a certain period at the bottom dead point, led to the best formability. Finite element simulations were conducted to validate the experimental results and to probe the formability improvement in the non-conventional forming profiles. A creep model to address stress relaxation effect along with tool elastic recovery was implemented using a user-material subroutine, CREEP in ABAQUS finite element software. The stress relaxation and variable contact conditions during the holding and oscillating profiles were found to be the main mechanism of formability improvement.

Role of hard martensite phase prior to cold-rolling on microstructure evolution after annealing in ferritic stainless steel

To improve the surface quality of 16mass%Cr ferritic stainless steel sheets processed by cold-rolling and subsequent annealing, the role of the hard martensite phase, in particular, the hardness ratio k between martensite and ferrite prior to cold-rolling on microstructure evolution was investigated by varying the k value through tempering specimens at different temperatures after intercritical annealing. In the specimen with the high ratio k (k>2.0), the heterogeneous deformation in ferrite around the hard martensite becomes prominent and the areas with the high KAM value are locally limited with less development of cold-rolling texture. The specimen with the small ratio k (k<2.0), however, shows rather uniform deformation structure, because tempered martensite deforms easily. Therefore, the recrystallized grain with the specimen k>2.0 becomes fine, because the presence of hard martensite provides the nucleation site of recrystallization leading to fragmenting the elongated {100}<011>colony structure, and the orientation is randomized. Consequently, the presence of the hard martensite phase prior to cold-rolling is effective for preventing the ridging of ferritic stainless steel sheets.

Application of central composite design for optimization of two-stage forming process using ultra-thin ferritic stainless steel

Two-stage forming process for manufacturing micro-channels of bipolar plate as a component of a proton exchange membrane fuel cell was optimized. The sheet materials were ultra-thin ferritic stainless steel (FSS) sheets with thicknesses of 0.1 and 0.075 mm. For the successful micro-channel forming in the two-stage forming approach, three process variables during the first stage were selected: punch radius, die radius, and forming depth. In this study, the effect of the three process variables on the formability of ultra-thin FSSs was investigated by finite element (FE) simulations, experiments, and central composite design (CCD) method. The optimum forming process designed by the CCD showed good agreement with those by experiments and FE simulations. The newly adopted optimization tool, CCD, was found to be very useful for optimization of process parameters in the multi-step sheet metal forming processes.

Cu添加フェライト系ステンレス鋼板の再結晶挙動

Cu添加フェライト系ステンレス鋼板の再結晶挙動

Cold Formability of Automotive Sheet Metals: Anisotropy, Localization, Damage and Ductile Fracture

Experimental and numerical investigations on the characterization and prediction of cold formability of two different types of automotive steel sheets, a ferritic stainless steel sheet (AISI 439) and a dual-phase steel sheet (DP600) are performed in this study. Due to the different microstructure configurations, these two steels show significant differences in plasticity behavior as well as failure mechanisms. The ferritic stainless steel shows strong anisotropic plastic deformation in terms of both yielding and hardening, whereas the dual-phase steel behaves quite isotropic resulting from the mixture of two phases. However, unlike the localization dominant failure mechanism of the ferritic stainless steel, the incompatible deformation due to the distinctions of the mechanical properties of two phases naturally results in early damage and extensive damage development prior to localization, or ductile fracture without localization. In this study, all these features are taken into account for an accurate prediction of formability. A general modelling framework with specifications for these separate features is formulated and applied to the two steels.

Enhancement of Microstructural and Mechanical Properties by Pulse Mode of Metal Transfer in Welded Modified Ferritic Stainless Steel

The present study describes the enhancement of microstructural and mechanical properties by pulse mode of metal transfer in welded modified ferritic stainless steel (409 M) sheets (as received) of 4 mm thickness. The welded joints were prepared by varying modes of metal transfer at different heat input conditions (i.e. pulse mode at 0.5 kJ/mm and 0.9 kJ/mm and spray mode at 0.5 kJ/mm), using austenitic filler wire (i.e. 308 L) under Ar + 10% CO2 atmosphere. It has been observed that the pulse mode of metal transfer significantly alters the weld metal composition compare to spray mode which promotes comparatively stable austenite in the welds and also depicts significant enhancement in grain structure even with the higher heat input condition. Present study clearly shows that pulse mode enhances micro-hardness of welded joints and toughness values of weld metals compare to spray mode of metal transfer for a particular heat input.

フェライト系ステンレス鋼板の変形組織と再結晶に及ぼす冷間圧延前の1パスECAPの影響

Equal-channel angular pressing (ECAP) was applied to ferritic stainless steel sheets for one pass prior to cold rolling in order to improve formability and alleviate ridging of the sheets. Effect of the channel angle of ECAP (90° and 120°) on the deformation microstructure and the subsequent recrystallization was focused. One-pass ECAP indeed modified the cold-rolled microstructures, texture and subsequent recrystallization as compared with that in cold rolling alone. In particular, grain-scale shear bands are introduced during ECAP in otherwise hard-to-recrystallize <100>//ND grains by ECAP, and they facilitated the recrystallization. However, the effect of reducing the channel angle from 120°to 90°on the recrystallization and the formability was limited in spite of higher shear strain imposed on the sheets. The reduction of the channel angle affects an increase of misorientation between the shear band and the matrix and does not increase the density of shear band.

Optimization of strength of friction stir welded joints for AISI 430 ferritic stainless steels by genetic algorithm

In this paper, friction stir welding technique was used to join ferritic stainless steels, AISI 430. Ferritic stainless steel sheets were successfully joined considering both, the appearance of the welding bead and the strength of the welded joint by selecting proper weld parameters and tool material. The weld parameters such as revolutions, traverse speeds, compressive tool forces and tool tilt angles influenced the tensile strength of welded joints. Genetic algorithm model was developed to perceive the stronger joints by selecting proper welding parameter combinations. The strong welded joint higher than base material was obtained with appropriate welding parameter combinations, having a revolution of 1,120 rpm, compressive tool force of 4.5 kN, traverse speed of 125 mm min−1, and tool tilt angle of 0°. Best welding parameter combinations resulted in ultra fine grain structures in the welding zone and high-quality welded joint.

Effect of Welding Time in the Resistance Spot Welded Dissimilar Stainless Steels

In this study, the effect of welding time on joining capability of austenitic stainless steel (AISI 304) sheets and ferritic stainless steel (AISI 430) sheets by using resistance spot welding (RSW) process was investigated. Therefore, macrostructure, microstructure, microhardness, tensile shear strength, and failure mode of welded materials were determined for various welding times. The welding times values used in the resistance spot welding process were 0.4, 0.8, 1.2, and 1.6 s. Tensile shear tests carried out on the welded joints determined their strength and failure mode. The increase in the welding time resulted in an increase in the nugget size and the weld strength. Two distinct failure modes were observed during the tensile shear test: interfacial, pullout failure modes. Finally, an optimum welding time was obtained.

Investigation on the resistance spot-welded austenitic/ferritic stainless steel

In this work, the effect of weld current on joining capability of austenitic stainless steel (AISI 304) and ferritic stainless steel (AISI 430) sheets with application of resistance spot welding process was investigated. Macrostructure, microstructure, microhardness, tensile shear strength, and failure mode of welded materials were evaluated for different weld currents. The values of weld current were 2.5, 3.75, and 5 kA. It was found that when the weld current increased, the nugget size and the weld strength were increased. Two distinct failure modes including interfacial and pullout were observed during tensile shear test. Finally, an adequate weld current was obtained.

Homogenization of Microstructure of Castings from the Alloy Fe-40at.%Al-Zr-B

The samples intended for the study of static recrystallization on plastometer Gleeble were prepared from the laboratory castings of iron aluminide containing 24.6 Al 0.17 Mn 0.16 Zr 0.026 B 0.004 C (in wt. %, remainder Fe). Nevertheless, the structure analysis discovered that the results were excessively influenced by the huge heterogeneity of the as-cast microstructure, mostly of the grain size. Combination of the hot forming and recrystallization process during the long-term high-temperature annealing was selected for the necessary structure homogenization. As the tested intermetallic alloy is extremely brittle and susceptible to surface cracking, the original method was applied for its processing. The method consists in hot rolling in the protective capsules welded from the ferritic stainless steel sheet. The castings were rolled to 2/3 of their thickness by 4 reductions with the inter-stage heating, and then annealed at the temperature of 1200 °C in the vacuum furnace for several periods. Metallographic analysis revealed that annealing lasting 7 hours was essential for the uniform coarsening of the recrystallized grains. Material processed in this way proved successful for the subsequent metallographical study of static recrystallization.

Modeling Mechanical Properties of 21-Cr Ferritic Stainless Steel with Variation of Stress Ratio

A phenomenological material model describing anisotropic plastic behavior of 21-Cr ferritic stainless steel sheet was investigated using a non-quadratic anisotropic yield function. Uniaxial tension tests in rolling, diagonal and transverse directions, equi-biaxial bulge test and disk compression test were conducted for determining parameters of the yield criterion. Especially, Rb(ɛTD /ɛRD) values obtained from disk compression test and equi-biaxial bulge test were compared. For modeling of yield anisotropy of a metal sheet, the yield function, Yld2000-2d has been widely used by assuming that the shape of its initial yield locus does not change during deformation. In this study, in order to consider variation of stress ratios, coefficients (α1∼α8) of Yld2000-2d model were assumed to be functions of the equivalent plastic strain and the corresponding yield function was implemented into the user material subroutine in the FE software ABAQUS. Effects of variation of stress ratios during deformation were studied using single element FE simulations. Results showed that Yld2000-2d model with varying coefficients could describe the stress-strain curves under both uniaxial tension and equi-biaxial bulge better than that with constant coefficients.

Effect of Heat Input on Macro, Micro and Tensile Properties of Flux Cored Arc Welded Ferritic Stainless Steel Joints

This paper discusses the influence of Flux Cored Arc Welding Process parameters such as welding current, travel speed, voltage on bead profile, metallurgical and mechanical properties of welds of 2 mm thick 409M ferritic stainless steel sheets. The study reveals that, grain coarsening, volume fraction of martensite, hardness of heat affected zone and % of delta ferrite in ER 309 weld metal increases with increase in heat input. However, the results show that variation of heat input does not make any significant effect on tensile strength of the joint. Hence, welding parameters that provide uniform bead profile for the weld are recommended for fabrication.

Investigation into the Incidence of Severe Rusting and Pitting Corrosion in Imported Hot-Rolled AISI 430 Ferritic Stainless Steel Coils

Metallurgical investigations were directed to probe into the incidence of inordinate rusting and pitting in imported AISI 430 grade hot-rolled ferritic stainless steel sheet coils. Visual examination, electron microprobe analyses (EPMA), scanning electron microscopy (SEM) and electrochemical potentiokinetic reactivation (EPR) were concomitantly employed to investigate the problem. Studies revealed that the unprecedented degree of corrosion in ferritic stainless steel coils, during the short span of shipment time, was attributable to the ingress of sea water and its retention within the tight folds/ wraps of the steel coils during their shipment. The abundance of moisture and chloride (from the entrapped saline electrolyte) on the steel surface together with depleted O2 supply within the tight folds are presumed to have created conditions akin to an actively-corroding crevice, by way of passive film instability and its eventual breakdown on the stainless steel surface. As a consequence, the coils are believed to have suffered an accelerated and intensified chloride-induced corrosion attack and damage within the short span of shipment time. The investigations also revealed that the corrosive conditions were further exacerbated by the vulnerability and susceptibility of ferritic stainless steel to intergranular corrosion (IGC) due to its inherent sensitized condition. The paper thus throws light on an unusual precedent of chloride-induced corrosion in ferritic stainless steel and highlights the investigative metallographic work and corrosion failure analysis that led to above revelations.

Mechanical modeling of macroscopic behavior for anisotropic and heterogeneous metal alloys

The present investigative work was focused on modeling the effect of through-thickness texture gradients on the global mechanical behavior of a rolled ferritic stainless steel sheet. The material was experimentally characterized according to the needs of the analysis. First, a homogeneous rolled sheet was analytically described, based on Hill’s formalism of generalized materials. Then, a heterogeneous sheet was analyzed through two analytic approaches. The predictive capability of the resulting approaches was also proven in relation to the choice of the pseudo-anisotropic coefficients selected from fictitious materials. Finally, the application of a simple method, called continuum mechanics of textured polycrystals, taking into account crystallographic considerations, was adopted. As a result, this method was found to be an effective way to model the mechanical behavior of an anisotropic and heterogeneous sheet, replicating the evolution of experimental yield stress and plastic strain ratio either in terms of evolution or in those of level values. An attempt to estimate the impact of low-texture gradients on the heterogeneity of any industrial metal sheet is also made.

Quantification of Ridging in Ferritic Stainless Steel Sheets by Electron Backscattered Diffraction R-Value Maps

In ferritic stainless steel (FSS), undesirable surface defects of ridging appear during deep drawing. The formation of these defects is attributed to the inhomogeneous distribution of orientations of individual grains. In the present work, a new electron backscattered diffraction R(α)-value map was introduced, and the dependence of the tensile directions on the formation of ridging in an FSS sheet was discussed using this map. The results showed that large grain colonies in the R(α)-value maps lead to the formation of severe ridging in an FSS sheet.

Formability of austenitic and ferritic stainless steels at warm forming temperature

The warm temperature drawability of austenitic (STS 304) and ferritic stainless (STS 430) steel sheet samples was evaluated using the cup drawing test. The measured limiting drawing ratios (LDRs) increased about 44% and 25% for STS 304 and STS 430, respectively, compared to the isothermal case by introducing optimal temperature gradients between punch and die/blank holder. The drawability improvement was more drastic with STS 304 than with STS 430 using this temperature gradient approach. Fully coupled thermo-mechanical finite element (FE) analyses of the warm cup drawing tests were also carried out and validated using experimental data. For an accurate FE modeling, the mechanical properties measured in uniaxial and balanced biaxial tension tests were accounted for. For STS 304, an isotropic, temperature-dependent, yield function led to a reasonable prediction of the cup drawing performance at all temperature conditions. However, for STS 430, an anisotropic yield function was necessary to achieve good accuracy, in particular, for the prediction of the fracture location and cup height. Besides drawability, the earing profiles of the STS 430 cups were also simulated using three different yield functions, namely, von Mises, Hill48 and the non-quadratic anisotropic Yld2000-2d. Comparisons with experimental measurements indicated that the latter resulted in the best earing profile predictions for various temperature conditions.

開発裏話～微量 Sn 添加によりレアメタル(Cr)を削減した高純度フェライト系ステンレス鋼板の開発

開発裏話～微量 Sn 添加によりレアメタル(Cr)を削減した高純度フェライト系ステンレス鋼板の開発