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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 89 (2003), No. 1

  • To Save Environments and Resources

    pp. 1-2

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  • Recent Advance in Coated Steel Sheets

    pp. 3-17

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    The recent progress, mostly in the last ten years, in coated steel sheets is briefly reviewed for the major application such as automobiles, appliances, construction, and packaging. The progress in the novel process and that in testing and structural analysis are also introduced.
  • Effect of Surface Texture of Galvannealed Steel Sheet on Image Clarity after Painting

    pp. 18-22

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    Surface profiles at each producing step of galvannealed steel sheet and at each painting step were measured by stylus profilometer. Galvannealed steel sheet had fine unevenness with short wavelength components (λ<100μm) originated in the Fe-Zn alloying reactions, unevenness with relatively long wavelength components (λ>100μm) responded to the substrate cold rolled steel sheet. By skin-pass rolling, the amplitudes of almost all wavelength components were reduced. Unevenness of overall wavelength was reduced by painting. Fine unevenness with short wavelength (λ<100μm) disappeared completely. Unevenness with relatively long wavelength (λ>100μm) slightly remained. Good correlation between the NSIC-value, the index of image clarity, measured after middle coating and the amplitude of wavelength component from 100 to 2000 μm of galvannealed surfaces was indicated.
  • Influence of Steel Surface Property on Galvannealing Rate of Galvanized Steel Sheet

    pp. 23-30

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    Galvannealing rate becomes fast by heat treatment in N2 of the hot rolled sheet with scale. During the heat treatment, Fe inner oxides which contain the easily oxidizable metals are formed just under the scale by internal oxygen diffusion, and this inner oxides remain at the surface layer after pickling and cold rolling. The cause that of galvannealing rate becomes fast is shown in the following.
    (1) Amount of surface segregated oxide which is formed by reduction annealing extremely decrease, because the oxidation-reduction occurs inside the sheet, in other words, Fe-containing oxides produced during heat treatment changes to oxides of easily oxidizable metals during the annealing.
    (2) The concentration of easily oxidizable metals at surface layer decreases by the formation of this inner oxide.
  • Effects of Partial Pressure of Water Vapor in Annealing Atmosphere on Wettability of Mn, Si Containing Steel with Molten Zinc

    pp. 31-37

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    Effects of partial pressure of water vapor (dew point) in annealing atmosphere on wettability of 1.4 mass% Mn and 0.14-0.25 mass% Si containing steel with molten zinc was studied with considering the characteristics of surface oxide layer by cross sectional TEM microscopy.
    1.4%Mn-0.14%Si exhibited a good wettalbility with molten zinc after annealing in 20%H2-N2 mixed gas at 1133K for 120 s when the dew point of H2-N2gas is 237K. The wettability deteriorated while dew point was lower (209-227K) or higher (248-260K).
    It is considered that wettability was relevance to the form of oxides on the surface. After annealing in the dew point of 237K while wettability would be good, the oxide particles were dispersed on the surface and metallic surface was partially exposed. The internal oxide layer with small oxide particles (diameter of less than 100nm) was observed. And, Mn depletion was detected in that layer. On the other hand, the continuous oxide layer was observed on the surface having a poor wettability with the molten zinc.
    Dew point dependency of the forms of oxides was illustrated as a superposition of surface concentration of Mn, Si and total amount of oxides. In the dew point range of this experiment, MnO and MnSiO3 are predicted to generate. At the lower dew point, oxides would exhibit more concentration to the surface because oxidation depth was short. However, total amount of oxide would be increased at the higher dew point. Consequently, the wettability on 1.4%Mn-0.14%Si steel reached the peak at a specific dew point. But 1.4%Mn-0.25%Si steel did not exhibit good wettability in the dew point range of this experiment. This behavior was considered that the volume of MnSiO3 on the surface increased as the Si content.
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    3. Calculations of Internal Oxidation Rate Equations and Boundary Conditions between Internal and External Oxidation in Silicon Containing Steels MATERIALS TRANSACTIONS Vol.51(2010), No.3
  • Effect of the Substrate Compositions on the Growth of Fe-Al Interfacial Layer Formed during Hot Dip Galvanizing

    pp. 38-45

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    It is important to clarify fundamentally the formation mechanism of a Fe-Al interfacial layer formed on a steel sheet during hot dip galvanizing to produce galvanized steel sheets with high strength. Effect of Si, Mn, and P in a pure iron sheet on the growth of the Fe-Al layer has been investigated using baths containing 0.15 mass% Al and 0.20 mass% Al. The amount of the Fe-Al layer formed at the interface between a zinc overlayer and a base metal containing Si decreased with increasing in Si content of the base metal both in the low Al added bath and in the high Al added bath. This was explained by the decrease in the reactivity of the base metal due to surface segregation of Si occurred during recrystallization annealing. Mn in a base metal restricted the formation of the Fe-Al layer in the high Al added bath, although Mn content in a base metal did not influence the amount of the Fe-Al layer in the low Al added bath. It was considered that particles of Mn oxide dispersed on the annealed base metal surface did not affect the dissolution of iron in the low Al added bath while they impeded the diffusion of Al through the thick Fe-Al layer in the high Al added bath. The amount of the Al-rich layer formed during galvanizing did not depend on P content in base metal in both baths.
  • The Effect of Si in Steel on Adhesion Strength of Galvannealed Steel

    pp. 46-53

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    The effect of Al content in galvanizing bath and chemical composition of steel sheet on the coating adhesion of galvannealed ultra-low carbon steel sheet was investigated using the lap-shear method and the gravel method.The exposed steel surfaces after both the lap-shear test and gravel test were examined so as to discuss the relationship between the geometrical shape of the coating/steel interface and the coating adhesion strength.In the case of ultra-low carbon Ti added steel, high Al addition to the galvanizing bath increased the adhesion strength of galvanealed coating, being previously announced.
    In the case of phosphorus added steel, high Al addition to the galvanizing bath didn't alter the morphology of coating/steel interface which is even and smooth, therefore the adhesion strength didn't increase.Si additions in the range of 0.05-0.10% to steel increasaed the adhesion strength of Galvanealed coating of phosphorus added steel, regardless of Al content in galvanizing bath.Zn invasion into the steel grain boundary was promoted by Si additions in the range of 0.05-0.10% to steel, and as a result, αFe grain boundary became brittle.Because the propagation course of crack in coating detoured round or branched, it was assumed that the peeling of coating/steel is relieved.Moreover, the addition of Si to steel has an excellent powdering resistance of galvannealed steel.
  • In-situ Observation of Zinc Electrodeposition on Iron Single Crystal Using Synchrotron Radiation X-ray Diffraction

    pp. 54-59

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    Continuous in-situ observations of changes in crystal orientation during zinc electrodeposition were performed using novel electrolysis cell that secure uniform current distribution through thin electrolyte layer.
    It has been clarified that electrodeposition can be separated into the two regions. First one is epitaxial deposition region in which orientations of deposited zinc and substrate keep following relations; Fe(100)//Zn(10·1) Fe(110)//Zn(00·2), Fe(111)//Zn(00·2). This region continued until the thickness of the deposit became about 0.5μm.
    Second one is bulk deposition region, in which the overpotential settled by electrolysis conditions determines the crystal orientation. Low overpotential leads to promoting Zn(00·2) deposition, and higher overpotential Zn(10·1) and Zn(11·0). Influences of the current density changes on the crystal orientation were also clarified. The use of an adsorbing organic additive influences both on the epitaxial growth region and on the bulk growth region and inhibits the deposition of the Zn basal plane.
  • Electrodeposition of Zn-SiO2 Composite from the Bath Containing Chained SiO2 Particles

    pp. 60-63

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    Incorporation mechanism of the chained SiO2 particles into zinc electrodeposits have been discussed. The SiO2 particles have diameter of 30 nm and about five particles are chained. These chained SiO2 particles exist both on the (00·1)η and (10·1)η of hexagonal columnar zinc electrodeposits. The chained SiO2 particles precipitate in two ways on the (00·1)η of zinc electrodeposits: by lined up particles along the laterally growing macrosteps on the (00·1)η and by randomly dispersed particles on the (00·1)η. The sidewalls of the particles are incorporated into the macrosteps of the (00·1)η. The chained SiO2 particles randomly dispersed on the (10·1)η. These randomly dispersed chained SiO2 particles stick on the (10·1)η after macrosteps reached to the edge of the hexagonal columnar electrodeposits.
  • Verification of Deposition Mechanism of Zinc Alloys Using Non-aqueous Plating Solutions

    pp. 64-70

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    Electrodeposition of Zn alloys with iron-group metals, Cr and Mn was conducted in non-aqueous solutions such as methanol and N, Ndimethlyfromamide and the alloy deposition behavior was compared with that in aqueous sulfate solutions. The results obtained are described as follows:
    1) The electrodeposition behavior of Zn-iron-group metals and Zn-Cr alloys, in which the formation of Zn hydroxide due to pH rise in the cathode layer plays an important role during the codeposition process in aqueous solutions, was quite different between the non-aqueous and aqueous solutions.
    2) The deposition of Zn-iron-group-metal alloys in the non-aqueous solutions showed the typical feature of normal type codeposition, in which electrochemically more noble iron-group metal deposits preferentially over most plating conditions.
    3) In the codeposition of Zn with Cr, the deposits of about 90 mass% Cr was obtained in the non-aqueous solutions containing 80-90mol% of Cr3+, while the maximum Cr content was about 40 mass% in aqueous solutions.
    4) Since Zn hydroxide was not involved in the codeposition of Zn with Mn in aqueous solution, the deposition behavior of Zn-Mn alloy in the non-aqueous solutions was almost the same as that in aqueous solution.
  • Prevention of Sludge Generation in Sulfuric Acid Bath and Electrolytic Tin Plating by Sulfuric Acid Bath

    pp. 71-76

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    As for the electrolytic tinning bath used in tinplate manufacturing lines, the phenolsulfonic acid bath has been the mainstream in the world for many years. In recent years, some tinplate makers have applied the methanesulfonic acid bath. The author studied the sulfuric acid bath in order to develop a better plating bath. Although the problem of the sulfuric acid bath was sludge quantity caused by easy oxidation of stannous ion into stannic ion in the solution with the dissolved oxygen, it was found that only a small amount of sludge generated in the sulfuric acid bath in which specific phenolic antioxidants such as hydroquinone was added. The behavior of hydroquinone was discussed to be a radical scavenger. The tinplate obtained with the sulfuric acid bath stood comparison with that with the phenolsulfonic acid bath over a wide current density range.
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    1. Development of Methane Sulfonic Acid Tinplating Bath Tetsu-to-Hagané Vol.89(2003), No.1
  • Development of Methane Sulfonic Acid Tinplating Bath

    pp. 77-79

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    This paper describes the development and the performances of a new tin electroplating bath, which greatly reduces environmental loads and also improves the productivity. The conventional tin electroplating bath had a problem of heavy environmental loads. On the contrary, the methane sulphonic acid (MSA) based bath, which is used for soldered can material productions, has a merit of light environmental loads, but it is difficult to apply to the production of general tin electroplated steels because of the differences in required properties. Based on the MSA bath of light environmental loads, a new electroplating bath has been developed by finding new polyethylene glycol (PEG) contained additives, which make possible the balance of light environmental loads and productivity. The main characteristic of the new additive is that it contains two kinds of PEGs of high and low molecular weight, which dramatically improve the performance in a high current density range, although the bath containing either of the two PEGs is inferior to the conventional halogen based bath. This new tin electroplating bath containing two kinds of PEGs has been used in National Steel Corporation in US and has shown high performances.
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  • Effects of Organic Resins and Inorganic Inhibitors on Properties of Cr(VI)-free Thin Organic Composite Coated Steel Sheets

    pp. 80-85

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    Despite the large amount of research of Cr(VI)-free coated steel sheets being carried out for many years, there have never been products that could provide corrosion resistance and electric conductivity. In this study, as a basic research on Cr(VI)-free thin organic coatings, effects of composition and thickness of organic coatings on corrosion resistance and electric resistance were investigated.
    (1) Regarding on barrier effect of organic resins, relationship between O2 permeability coefficient and corrosion resistance was investigated. Amine-modified epoxy resin, providing the lowest coefficient of O2 permeability, showed the best corrosion resistance. (2) In order to provide self-healing effect, effect of silica and phosphate compounds contained in organic coatings on corrosion resistance was investigated. Colloidal silica improved corrosion resistance at 20 to 30 wt%, while fumed silica deteriorated it. A condensed phosphate compound also provided good corrosion resistance. (3) Effect of coating thickness and compositions on electric resistance was investigated. As the coating thickness increased, the electric resistance was deteriorated. However it did not depend on coating compositions.
    From the facts described above, a basic concept of Cr(VI)-free thin organic coatings was obtained. In order to provide good corrosion resistance and electric resistance, thin organic composite coatings with barrier effect and self-healing effect are necessary.
  • Development of Cr(VI)-free Treated Steel Sheet for Electrical Appliances and Business Machines

    pp. 86-91

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    In the field of electrical appliances and business machines, a material used for the chassis needs a good fingerprint resistance and an electric conductivity in addition to good white rust resistance. Highly corrosion resistant chromate treated steel sheets, which contains Cr(VI), have been successfully used for a long period of time. Recently, the use of Cr(VI) has been recognized as one of harmful chemicals for human body. In reply to the social requirement of eliminating Cr(VI), Cr(VI)-free treated electrogalvanized steel sheets have been introduced by several Japanese steel makers. For the substitution of Cr(VI) treated steel sheets, a chemical resistance of Cr(VI)-free steel sheets is required as a new function in addition to the corrosion resistance and electric conductivity. New highly chemical resistant Cr(VI)-free coated steel sheet has been developed by hybridization of organic resins and inorganic reagents. The performance of the new products has been found to be excellent in comparison with the conventional Cr(VI) treated steel sheets.
  • Development of Cr(VI) Free Type Black Colored Steel Sheet

    pp. 92-96

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    Black colored steel sheet, produced by treating with Cr(VI) treatment and resin on Zn-Ni electroplated steel sheet with a blackening process, has been successfully used for office automation equipment, audio equipment and household electrical appliances. Recently, in reply to the social requirement of eliminating Cr(VI), which is one of the environmentally un-friendly materials, Cr(VI) free treated blackening Zn-Ni electroplated steel sheet has been developed. Newly developed Cr(VI) free type black colored steel sheet consists of over 1μm. thick organicinorganic composite film on black treated Zn-Ni electroplated steel sheet. This Cr(VI) free type black colored steel sheet shows the same appearance, corrosion resistance, change of color after press formation, friction coefficient, surface electrical resistance and resistance to alkaline degrease reagent to comparison with those of Cr(VI) type black colored steel sheet.
  • Properties of Environmentally Harmonized Precoated Steel Sheet for Automotive Fuel Tank Use

    pp. 97-102

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    A precoated steel sheet without dissolvable Cr(VI) for automotive fuel tank use has been developed. The newly developed Cr(VI)-free precoated steel sheet consists of thin organic resin coatings on an electrogalvanized steel sheet. One side of the sheet is coated by a 1μm thick lubricant resin layer used for the outside of the tank. The other side is coated by a 3μm thick gasoline resistance resin layer which contains metallic powders to provide good weldability. Cr(VI)-free precoated steel sheet showed good internal and external corrosion resistance, weldability, press formability in comparison with Pb-8%Sn alloy coated steel sheet.
  • Properties of Newly Developed Electrogalvanized Steel Sheet with Mg Containing Phosphate Film

    pp. 103-108

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    The press formability and corrosion resistance of electrogalvanized steel with Mg containing phosphate film were investigated. (1) The phosphate film could reduce the friction coefficient of electrogalvanized steel sheet and good press formability was obtained. It was seemed that the morphology of the phosphate film could be effective on friction property. (2) The phosphate film could improve the perforation corrosion resistance of electrogalvanized steel sheet. It was suggested that the film could delay the start of Zn corrosion and alsocould stabilize the corrosion products of Zn.
  • Chemical State Analysis of Conversion Coatings by SR-XPS and TEY-XANES

    pp. 109-115

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    Chromate coatings on galvanized steel have been studied by Synchrotron Radiation (SR) based techniques that include X-ray Photoelectron Spectroscopy (XPS) and Total-Electron-Yield X-ray Absorption Near Edge Structure (TEY-XANES). Non-destructive depth profiling of the coatings by SR-XPS reveals the enhancement of Cr6+ in the outer surface. TEY-XANES spectroscopy based on simple specimen current measurement is demonstrated as an effective technique for analyzing chemical states of conversion coatings on general bulk substrates. The sampling depth of this technique, which exceeds several tens of nanometer, is determined by the penetration length of Auger electrons excited by X-ray and the inelastic mean free path of secondary electrons excited by inelastically scattered Auger electrons. The chemical states of phosphoric acid added chromate coatings are studied using this technique. The phosphoric acid is taken into the chromate coatings as partially changed into zinc and chromium phosphates, and the remainder is as simply dried. A specimen negative bias technique is useful to increase the TEY-XANES signals.
  • Effect of Silica in Organic Coating on Corrosion Behavior of Galvanized Steel

    pp. 116-122

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    The behaviors of colloidal silica in organic coating on galvanized steel were studied in viewpoints of corrosion resistance. The samples coated with polyolefin resin including colloidal silica showed better corrosion resistance than that coated with resin only. Anodic polarization resistance showed a good relationship with corrosion resistance, and this was improved as colloidal silica increased in quantity or decreased in size. On the other hand, cathodic current density showed not so good correlations. The corrosion potentials of samples coated with resin including colloidal silica dropped once and then rose. On the other hand, those of the samples with only resin coat continue to decline. Colloidal silica in organic coating dissolves in water, and solubility also increased in quantity, as their particles decreased in diameter or pH rose above 8. At the pH below 4, colloidal silica did not dissolve at all. Some evidences to suggest the dissolution of silica in cathodic region and deposit on anodic site on zinc surface could be obtained, and the improvement of corrosion resistance seems to cause the film fbrmation on anodic site.
  • Influence of Organic Composite Coating Components on Various Properties of 55%Al-Zn Alloy Coated Steel Sheet

    pp. 123-127

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    The influence of the organic composite coating components on various properties of 55%Al-Zn alloy coated steel sheet have been investigated by salt spray test, draw bead test and the measurement of Cr fixation ratio. In addition, the relationship between these properties and the film structure was studied by SAM analysis.
    The coating, which was composed of newly developed Acrylic resin, phosphate and Cr compound was found to provide good corrosion resistance, formability, good appearance and film insolubility. Further, the results from the film structure analysis show that this coating was characterized by its gradient composition. The layer material in contact with the 55%Al-Zn alloy coated steel sheet is an insoluble passivation layer, rich in chromic compounds and phosphate, which provides superior corrosion resistance and film insolubility. The layer material on the outer surface consists primarily of organic resin and is sufficiently flexible to ensure good formability.
    This new organic composite coating on 55%Al-Zn alloy coated steel sheet is commercially produced for building materials applications.
  • Weatherability of HALS Copolymerized Acrylic Resin Precoated Steel Sheet

    pp. 128-134

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    The deterioration mechanism of polymer outdoors is known as autooxidation caused by radical chain reaction. Fluorocarbon resin is used to improve the weatherability of precoated metal (hereafter PCM). The binding energy and the hydohobicity of fluorocarobon resin are higher than those of other organic resins, therefore the generation of alkyl radical and the hydrolysis of bonding are reduced.
    We have investigated the improvement for the weatherability of acrylic base PCM without fluorocarbon. We have introduced hindered amine light stabilizers (hereafter HALS) to capture the radical, and introduced cyclohexyl metacrylate (hereafter CHMA) instead of styrene or methyl metacrylate to improve the light stability and hydrophobicity. HALS was copolymerized with CHMA to prevent the flow out form paint. The newly developed HALS-CHMA PCM showed not only good weatherbility but also good corrosion resistance.
  • Effects of Coatings and Substrates on Reflectiveness of the Solar Radiation for Pre-painted Steel Sheets

    pp. 135-141

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    Exteriors made of steel sheets for architectural applications have many superior features compared with ceramics, for example, lightness, good formability and so on, however, have large heat conductivity. As a result, in the summer daytime, the solar radiant heat transfers into the interior of structures easily, then, it occurs high interior temperatures, and the energy required for cooling increases.
    Therefore, pre-painted steel sheets which can restrain the solar radiant heat, reduce or omit heat insulators and conserve the energy for cooling, moreover, which can be also colored without restraint, were required.
    In this report, guidelines for pre-painted steel sheets with reflectiveness of the solar radiation were given for conserving energy, especially, by evaluating the effects of coatings and substrates.
  • The Mechanism of Wet Adhesion of the PET-laminated Steel Sheet

    pp. 142-148

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    In recent years the production of PET-film laminated steel sheets for can materials have been rapidly growing to replace lacquered cans because of an environmental concern. To use the film laminated steel cans for beverage and food usage, wet adhesion of the film is one of the most important properties because the film delamination should be suppressed during retort treatments. In this paper, the delamination mechanisms have been studied by investigating the steel surfaces after the films are peeled off in the retort atmosphere, and two factors have been found to suppress the delamination. Firstly, the cohesion failure of the PET film should be suppressed by reducing the thickness of an amorphous PET layer, which is formed on the steel substrate during heat lamination processes. Secondly, the interfacial failure between the film and steel substrate should be suppressed by using adhesion-enhanced steel with appropriate surface finishing such as sufficient amount of electrolytic chromium coating.
  • Effect of Film Crystallinity on Environmental Stress Cracking in Polyester Film Laminated on Steel Sheets for Interior Building Materials

    pp. 149-154

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    Polyvinyl chloride coated steel sheets were generally used as interior building materials because of their embossing decorativeness, bending formability and durability. However the polyvinyl chloride generates harmful gases when it's disposed of. Hence substitution of the resin was demanded from the environmental point of view.
    We have developed polyester film laminated steel sheets as the substitution and already reported that especially polybutylene terephthalate (PBT) film laminated ones were superior to the polyvinyl chloride coated ones as conventional materials.
    In some tests for durability, however, we found that crack occurred at the bent portion of the film when it was dipped in alkaline solution. A degree of the crack changed by laminating conditions of the film and aging time after bending. This phenomena implies that the transition of crystallinity in the film due to the laminating condition relates to the generation of the cracking. Here, the crack is considered to be a kind of environmental stress cracking.
    In this paper, we investigated the cracking at the bent portion of the PBT film laminated steel sheets, which was caused by dipping into alkaline solution. The crack size of the bent portion was larger than that occurred at the plane portion. The more the crystallinity of the film had, the larger the crack size was. In low crystallinity, the crack was inclined to decrease with aging after bending. At 40°C aging, the crack rapidly decreased when comparing to that at 25°C aging.
  • Impact Resistance of Biaxially Oriented Polyester Film Laminated on Steel

    pp. 155-160

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    Electrolytically chromium/chromium oxide coated steel and lightly tin coated steel sheets laminated by biaxially-oriented-polyester-film are applied for various kinds of can use and highly evaluated not only from the cost performance but also from the environmental point of view.
    These laminated steel sheets are demanded various characteristics such as formability, adhesive property, corrosion resistance and impact resistance. The characteristics depend on laminating process, which is characterized by partial melting of the supplied films that are pressed toward both sides of heated strip through a couple of cooled rubber rolls. We have already reported the adhesive property and the corrosion resistance could be controlled by a degree of biaxial orientation of the film and the thickness of amorphous layer. However, there are very few investigations for impact resistance of the laminated steel sheets that are important characteristics other than adhesive property and corrosion resistance for can use.
    This report deals with the effect of a degree of biaxial orientation and thickness of the amorphous layer after laminating on impact resistance for the laminated steel sheets produced in various laminating conditions. Finally, we found that impact resistance of the laminated steel sheets for can use are also controlled by the degree of biaxial orientation and the thickness of amorphous layer of the laminated film.
  • The Corrosion Resistance of Zn-11%Al-3%Mg-0.2%Si Hot-dip Galvanized Steel Sheet

    pp. 161-165

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    For the improvement on corrosion resistance of Zn-5mass%-0.1mass%Mg hot-dip galvanized steel sheet, it was investigated that the influence of Al and Mg contents and Si addition in coating on the corrosion resistance of the coating.
    It was found that the corrosion resistance of Zn-11mass%Al-3mass%Mg-0.2mass%Si galvanized steel sheet is more than 5 times superior to Zn-5mass%Al-0.1mass%Mg galvanized steel sheet in the corrosion loss after the SST, and also superior to that in the outdoor exposure test.
    According to the polarization measurements in the 5mass% NaCl solution, further additions of Al, Mg and Si in the Zn-5mass%Al-0.1mass%Mg coating decreased the both anode and cathode currents. This effect is regarded as one of reasons of enhancement of corrosion resistance in the SST.
    It is supposed that the increased polarization is related to the thickness and stability of basic zinc chloride as the corrosion product formed on the Zn-11%Al-3%Mg-0.2%Si coating.
  • Corrosion Products of Hot-dip Zn-6%Al-3%Mg Coated Steel Sheet Subjected to Atmospheric Exposure

    pp. 166-173

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    Atmospheric corrosion of hot-dip Zn-6%Al-3%Mg alloy coated steel sheet after 5 years of exposure in a severe marine and a rural sites have been investigated compared with Zn-0.2%Al and Zn-4.5%Al-0.1%Mg alloy coated steel sheets.
    The corrosion resistance of Zn-6%Al-3%Mg alloy coated steel sheet is more superior than that of Zn-4.5%Al-0.1%Mg alloy coated steel sheet in both exposure sites. An extensive study using XRD and TEM revealed that the corrosion products consisting of amorphous, finecrystal Zn5(OH)8Cl2·H2O and Zn6Al2(OH)16CO3·4H2O are formed on Zn-6%Al-3%Mg alloy coated steel sheet after 1 year of exposure in a severe marine site. Even after 5 years of exposure the formation of zinc oxide, which have little protective action for coatings, is suppressed on Zn-6%Al-3%Mg alloy coated steel sheet.
    It is concluded that the surface of Zn-6%Al-3%Mg alloy coated steel sheet is covered with the protective corrosion products in the early stage of exposure, resulting in the remarkably improvement of atmospheric corrosion resistance.
  • Corrosion Resistance of Hot Dip Zn-Mg Galvanized Steel Sheet

    pp. 174-179

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    Hot dip Zn-Mg galvanized steel sheet having excellent corrosion resistance was developed aiming to elongate the life time of hot-dip galvanized steel sheet. This pre-coated steel sheet also revealed the customers cost down due to the reduction of post-dip galvanized process. It has excellent corrosion performance in various corrosion tests and atmospheric exposure tests especially improves corrosion resistance in salt damaged area, which are problematic now. And, it has superior sliding abrasion in forming due to the relative high surface hardness. Therefore, this coated steel sheet has a promising future as a environmentally-harmonizing and resource-saving coated steel sheet. The microstructure and corrosion behavior of hot dip Zn-Mg galvanized steel sheet were also investigated by some surface analysis, corrosion tests and electrochemical methods. The corrosion products of Zn-Mg were more compact and stable than that of Zn. For instance, in the case of test environment containing relative high Cl, ZnCl2·4Zn(OH)2 formed on Zn-Mg coatings was very stable for a long time. Especially, cathodic reaction (oxygen reduction reaction) was restricted according to the polarization curve.
  • Atmospheric Corrosion Resistance of Hot-dip Aluminized SUH409L Steel Sheet

    pp. 180-187

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    A 10-year outdoor exposure test has been carried out to investigate atmospheric corrosion behavior of hot-dip aluminized SUH409L steel sheets in an industrial and a marine environments. Corrosion current density distribution at scribed coating area was measured by scanning vibrating electrode technique (SVET) to study corrosion mechanism. In both environments aluminized SUH409L steel sheets have kept good appearance without red-rust even at bare edges and scribes. Corrosion of aluminized SUH409L steel sheets was slight and the whole surfaces of them including bare edges and scribes were covered with corrosion products mainly consisting of Al, S and O. It was found that these corrosion products adhered to the surfaces suppress the cathodic reaction at bare edges and scribes. It is concluded that the excellent atmospheric corrosion resistance of aluminized SUH409L steel sheets is caused by the sacrificial protection of coatings and the barrier effect of their corrosion products.
  • Corrosion Resistance of Light Gauge Steels for Steel Framed House

    pp. 188-195

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    Atmospheric corrosion on the inhabited steel framed house was investigated with Fe-Ag galvanic type atmospheric corrosion monitoring (ACM) sensors. As the environmental conditions, temperature, humidity and currents of the ACM sensors were continuously measured in several parts of the house for five years. Corrosion rates of hot-dip galvanized steel sheet, Zn-5%Al coated steel and 55%Al-Zn coated steel for the light gauge steels were also determined on the basis of exposure tests. Service life of the galvanized steels and dew condensation in the house were discussed.
    Amount of deposited sea salt influences the corrosion resistance of the coated steels. Under the indoor conditions of the inhabited house, the amount of deposited sea salt was negligibly small. It is difficult to evaluate the corrosion resistance of coated steel sheets using convenient corrosion tests because of too much sea salt.
  • Outdoor Exposure Test on Prepainted 55%Al-Zn Coated Steel Sheet Subjected in 15-year

    pp. 196-203

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    We carried out a 15-year outdoor exposure test to investigate the corrosion behavior of prepainted 55%Al-Zn coated steel sheet in comparison with prepainted galvanized steel sheet. The edge creep of prepainted 55%Al-Zn coated steel sheet is larger than that of prepainted galvanized steel sheet in the early stages of the exposure test. However, the progress of edge creep of prepainted 55%Al-Zn coated steel sheet slows down year after year then stopps, while that of prepainted galvanized steel sheet remains almost constant. The edge creep widths of both sheets were almost the same at 6 to 8 years. After that point, the edge creep width of prepainted 55%Al-Zn coated steel sheet was smaller than that of prepainted galvanized steel sheet. Also, the scribed part and T-bend part of prepainted 55%Al-Zn coated steel sheet shows better corrosion resistance than those of prepainted galvanized steel sheet. The effect of pretreatment and primer on the corrosion of prepainted 55%Al-Zn coated steel sheet was not clearly noticed.
  • Effect of Change of Surface Morphology on Press-formability of Steel Sheets

    pp. 204-209

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    To clarify the influence of plastic deformation of asperities under contact with forming tool on press-forming behavior, the relationship between drawing force and bead holding force was evaluated for various types of steel strips. The result showed that the friction coefficient between the steel strip and the forming-tool decreased as the bead holding force was increased. However it was found that, particularly for lowstrength steel strips, the friction coefficient decreased considerably as contact pressure became near to the yield-strength. A number of microholes were observed after the drawing tests of low strength steels, which were formed by the plastic deformation during the drawing process. We conclude that these holes acted as micro oil reservoirs during the drawing and were responsible for drop of the friction coefficient. The above-mentioned dependency of friction coefficient on bead holding force was not observed for galvannealed steels (GA) and organic lublicant film coated GA. To evaluate the effect of contact pressure dependent friction coefficient on the drawing force, a finite element method analysis, which incorporated the effect of contact pressure dependent friction coefficient was developed. Using this finite element analysis, we confirmed that the increment of the drawing force could be suppressed by choosing a suitable range of the bead holding force.

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