Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part I. Deformation modes

Zinc-based coatings are widely used for protection against corrosion of steel-sheet products in the automotive industry. The objective of the present article is to investigate the deformation modes at work in three different microstructures of a thin (8 μm) zinc coating on an interstitial-free steel substrate under tension, plane-strain tension, and expansion loading. Damage mechanisms are addressed in a companion article. The plastic slip and twinning activity in the zinc grains of an untempered cold-rolled coating (labeled NSK), a tempered cold-rolled coating (labeled SK), and a recrystallized coating are compared with the response of the corresponding bulk low-alloyed zinc material. The in-plane grain size in the NSK and SK coatings ranges from 300 to 600 μm, vs about 30 μm in the recrystallized coating and bulk material. The coatings exhibit a strong crystallographic texture, with the c-axis generally normal to the sheet plane. Basal slip is shown to be the main deformation mechanism in bulk zinc and the recrystallized coating, whereas pyramidal π₂ slip and mechanical twinning are found to be major modes in the NSK and SK coatings. These results, obtained from an extensive, quantitative slip-line analysis combined with electron backscattered diffraction (EBSD) measurements, are explained by the constraining effect of the substrate. This effect is successfully modeled using a simple Taylor-like polycrystalline approach. The recrystallized coating behaves much like the bulk material. The interpretation of this grain-size effect between the NSK and SK coating, on the one hand, and the recrystallized coating, on the other hand, requires a full three-dimensional finite-element analysis of the multicrystalline coating provided in this work. The simulations show that strong strain gradients can develop in the recrystallized coating from the interface to the surface, which is not the case in the NSK and SK coatings.     

热浸镀锌钢板镀层织构与耐蚀性

用X射线衍射和动电位扫描方法研究了热浸镀锌钢板镀层织构、相分布和耐蚀性的关系.实验结果表明,镀锌层中纯锌相的主要织构组分为{0002}基面织构,当镀锌层厚度增加时,纯锌相中{0002}基面织构密度水平增加,纯锌相自腐蚀电位升高、腐蚀电流密度减小以及耐蚀性增加.X射线衍射分析电化学剥离后的铁锌相,结果表明,镀锌层由纯锌相和合金相组成;电化学实验表明,镀锌层各相耐蚀性不同,合金相的耐蚀性高于纯锌相的耐蚀性,且Fe-Zn合金相的耐蚀性随铁含量的增加而增加.     

热镀锌板镀层表面缺陷及其消除措施

介绍热镀锌钢板镀层表面存在的诸如划伤、气刀条痕、辊印、波纹和条纹等缺陷的产生原因及其消除措施,同时还侧重介绍热镀锌钢板今后需要研究的锌渣缺陷、镀层表面质量与漆膜质量之间关系的课题。     

Resistance spot welding of galvanized steel: Part II. Mechanisms of spot weld nugget formation

Dynamic inspection monitoring of the weld current, voltage, resistance, electrode displacement, and force was performed in conjunction with a detailed study of the effects of material variations and weld process parameter modifications on resistance spot welding of coated and uncoated steels. In order to determine the mechanisms of weld nugget formation and growth, scanning electron microscopy photos were taken of the developing nugget. These physical changes were then related to the dynamic inspection curves and the welding current lobe. The effects of material variations and weld process modifications, the results of which were presented in Part I, can be explained through an understanding of these mechanisms.     

宝钢热镀锌钢板的开发与展望

随着经济的持续稳定发展,特别是汽车、家电和建筑行业的快速发展,对热镀锌钢板的需求持续旺盛,中国已成为世界上最大的热镀锌生产国,而随着近年来钢铁工业去产能化,热镀锌在钢铁中所占的比例仍将持续提高。简要回顾了热镀锌产品与技术的发展历史,结合中国热镀锌钢板的发展情况,介绍了宝钢在汽车、建筑和家电热镀锌产品方面的技术进步,重点介绍了先进高强度钢和镀层品种方面的进展。经过20多年的发展,宝钢已经形成了GI、GA、55Al-Zn、铝硅、锌铝镁和以此为基的表面处理系列,最高可供货强度达1 200 MPa,未来热镀锌产品将向更高强度、更高耐腐蚀性和表面功能性等方向发展。     

The cracking of zinc spangles on hot-dipped galvanized steel

The cracking of zinc spangles on hot dipped galvanized sheet steel was studied under controlled conditions. It was confirmed that there are two major modes of failure—boundary cracking and intragranular basal cleavage. There are four types of surface topographiesviz “mirror-like”, “feathery”, “dimpled” and “ridged”. Of these only the last type, “ridged”, was found to have its basal planes lying at an angle to the plane of the sheet. These spangles also tended to fracture most readily when strained. Hence a relationship between surface topography, crystallographic orientation and fracture behavior has been established. The implications of these findings for paint adherence is discussed. Formerly Senior Lecturer, University of Newcastle.     

连续热镀锌板镀层细微缺陷及防范技术

主要介绍热镀锌钢板镀层表面存在的诸如与钢基板、化学清洗、退火炉、锌液、气刀、平整等有关的细微缺陷的产生原因,同时还侧重介绍诸如采用工艺手段、沉没辊辊面喷涂技术、沉没辊辊面涂层封孔技术、流锌槽改善技术、锌液控制技术等,以减少热镀锌钢板镀层表面细微缺陷和露钢缺陷。     

Mechanical behavior and damage kinetics in nodular cast iron: Part I. Damage mechanisms

Numerous authors consider that in nodular cast iron, the interface between the graphitic nodule and the α-matrix is so weak that nucleation appears suddenly at the beginning of plastic strain. Nevertheless, the present experimental study indicates that the nucleation of voids is a continuous process as a function of the imposed mechanical parameters (Σ _m , ɛ _peq ). This result is explained in terms of interaction effects between closely spaced particles. A critical parameter for interaction is determined. Growth kinetics are also investigated and comparison with the Gurson-Tvergaard law allows identification of the q1 and q2 parameters of the potential. A critical void volume fraction f ^c is finally defined and is shown to depend on the stress triaxiality level. At last, the role of the various microstructural heterogeneity levels on the damage kinetics is emphasized. Then, it is clearly demonstrated that all these heterogeneity levels must be considered to estimate a critical interfacial stress.     

Processing and characterization of plasma-sprayed ceramic coatings on steel substrate: Part II. On coating performance

This investigation envisages the performance evaluation of a series of plasma-sprayed coatings from a few inexpensive powders, namely, alumina, plasma dissociated zircon (PDZ), zircon sand, and zircon-20 wt pct calcia. The hardness and adhesion (bond) strength of the coatings have been evaluated. All coatings offer an attractive thermal fatigue property. An added layer of yttria between the top and bond coats improves the thermal fatigue resistance appreciably. The wear performance of these coatings has also been evaluated using a “pin-on-disc” type wear testing setup. The Indian alumina coating offers a superior wear resistance and thermal fatigue property compared to an imported alumina coating. The coatings based on zirconia-silica offer an attractive thermal fatigue resistance along with an acceptable wear resistance. The latter improves considerably upon annealing of the coatings because of the removal of the residual stresses accumulated during deposition. Among these zirconia-silica-based coatings, the zircon-20 wt pct calcia is the most promising. The finishability of these coatings has been studied using a surface grinding setup equipped with a diamond wheel. The cutting forces and the surface roughness of the ground surface have been measured. The coatings have been found to possess a reasonably good grindability.     

Scanning auger microprobe study of hot-dipped regular-spangle galvanized steel: Part II. surface composition of chromated sheet

Zinc coatings produced on a hot-dipped-coating pilot line from a zinc bath containing small amounts of aluminum, antimony, and lead were treated with a commercial chromating solution and then examined with a scanning Auger microprobe. The results show that the chromating solution attacked the thin aluminum oxide precipitates in the zinc surface. The invisible conversion-coating film that was formed on the zinc matrix consisted mainly of oxygen, zinc, and chromium in order of decreasing atomic concentration. Surprisingly, the thickness and composition of the film was the same for treatment in 8 and 15 grams-per-liter chromate solutions. The film that was formed on precipitates of lead in the surface of the zinc coating was much thicker and richer in chromium than the film formed on the surrounding zinc. Examination of chromated surfaces exposed to water-saturated air at 100 °F and to normal atmospheres showed that the surfaces corroded rapidly on a microscale. After only a few hours exposure the surface had numerous mounds composed of equal atom concentrations of zinc and oxygen. In each instance corrosion sites were associated with a lead precipitate.     

连续退火工艺对冷轧热镀锌双相钢780DP性能的影响

研究了冷轧连续退火过程中退火温度和缓冷段温度对冷轧热镀锌双相钢780DP组织与性能的影响。结果表明,通过调整退火工艺,可以得到强韧性能配合较好的铁素体—马氏体双相钢组织;在一定的温度范围内,随着退火温度和缓冷温度的升高,双相钢780DP抗拉强度有不同程度的下降,而延伸率有所上升,缓冷温度在600℃左右可以得到较理想的实物性能。     

Resistance spot welding of galvanized steel: Part I. Material variations and process modifications

Material variations and process modifications have been studied to determine their effects on the acceptable range of resistance spot welding conditions for galvanized steel sheet. The material variations studied include zinc coating integrity, structure, composition, thickness, roughness, oil, and the amount and type of Fe-Zn intermetallics. Process modifications studied include upsloping and downsloping of the weld current, preheat current, postheat current, electrode tip geometry, and applied force. It was found that hot-dipped galvanized materials with coatings which have a very thin Fe-Zn alloy layer have a wider range of acceptable welding conditions than the commercial galvannealed products, which have a fully alloyed Fe-Zn coating. The decreased lobe width of the galvannealed material is due to the discontinuous Fe-Zn coating structure and morphology. Small variations in the thickness of the coatings studied have no significant effect on the welding current range. Surface roughness of the coating has no effect on lobe width. Upsloping and downsloping of the weld current increase the welding range of hot-dipped products when using truncated cone electrodes, whereas sloped current has no advantage for galvannealed or uncoated materials. Radiused electrodes can increase the lobe width of hot-dipped products but are not beneficial when using sloped current or when welding galvannealed or uncoated materials.     

汽车用镀锌板预磷化和磷化技术

主要介绍诸如浸渍式、喷淋式和辊涂式等3种典型的汽车车身用镀锌板预磷化与磷化技术。进入21世纪,为了不断改进汽车车身的防腐技术,要求进一步缩短磷化工艺时间、降低工艺成本,并使锌盐磷化技术更具环保性。同时还侧重介绍一种全新的能够满足汽车制造商最新要求的磷化技术及其发展趋势与几点看法。     

Oxidation of low carbon steel in multicomponent gases: Part II. Reaction mechanisms during reheating

Oxidation behavior of low carbon steel during reheating in an industrial walking-beam steel reheat furnace was investigated. It was observed that scaling (oxidation) rates were reduced by reducing the input air/fuel ratio to the furnace, thereby lowering concentrations of free oxygen in the combustion products from about 3 to 1.5 pct. Laboratory experiments involving isothermal and nonisothermal oxidation were carried out in atmospheres consisting of oxygen, carbon dioxide, water vapor, and nitrogen. A general equation for the prediction of weight gains due to oxidation during reheating, using isothermal oxidation rate constants, was developed. The prediction of weight gains from nonisothermal oxidation conducted in the laboratory was poor, owing to a separation of the scale from the metal substrate which took place at about 900 °C. The predicted weight gains during reheating in the industrial reheat furnace indicated that oxidation rates during reheating were intermediate between linear and parabolic, especially during reheating with high air/fuel ratio. However, the linear mechanism predominated. Laboratory isothermal experiments for oxidation in atmospheres containing free oxygen showed that the magnitude of the linear oxidation rates was determined by the oxygen concentration in the atmosphere. It was concluded that the observed reduction in scaling rates during reheating of low carbon steel in the industrial reheat furnace was a result of the lower free oxygen level in the furnace atmosphere.     

热镀锌无铬耐指纹钢板涂装前处理工艺探讨

在一定生产工艺条件下,通过试验揭示了热镀锌无铬耐指纹产品涂装前处理工艺的变化对耐指纹皮膜的影响规律,进一步研究膜重对产品应用性能（包括导电性、耐摩擦性、耐指纹性、耐热性、涂装性及耐蚀性等）的影响规律,并提供相关的技术参考。结果表明,经前处理后,耐指纹皮膜质量有一定的损失,而膜重是影响产品最终应用性能决定性的因素,将钢板出厂膜重控制在1.0～1.3g/m2范围内,可生产出综合性能优良的热镀锌耐指纹产品。     

Thermomechanical fatigue,oxidation, and creep: Part i. Damage mechanisms

Isothermal fatigue tests and both out-of-phase and in-phase thermomechanical fatigue tests were performed in air and in helium atmospheres. A wide range of temperatures from 20 ‡C to 700 ‡C was considered in these tests on 1070 steel specimens. A procedure for inert atmosphere testing using encapsulated specimens is described. Results indicate that the fatigue lives are 2 to 12 times greater in helium than in air. Interrupted tests were performed to characterize the pro-gression of damage in the material. Results indicate that oxidation-induced crack nucleation and crack growth are detrimental at high temperatures for isothermal and out-of-phase thermome-chanical fatigue tests. In these tests, transgranular cracking is observed. However, creep-induced intergranular cracking is the dominant damage mechanism during in-phase thermomechanical fatigue tests.     

Effect of surface roughness on the surface lubrication performance of galvanized steel sheets with a self-lubricated coating

Four kinds of galvanized steel sheets having different surface roughness values were used to prepare the steel sheets with a self-lubricated coating.The effects of surface roughness on the surface lubrication performance of the steel sheets were examined using a friction coefficient tester.Results revealed large dynamic friction coefficients for the galvanized steel sheets,which increased remarkably with surface roughness.Once the self-lubricated coating was applied,significant drops in the dynamic friction coefficients were measured.After the first stage of the friction test,the coefficients were almost unchanged,which reflected a weak dependence on the surface roughness of the self-lubricated steel sheets.However,the dynamic friction coefficients gradually increased as the test progressed,where these increase clearly correlated with the surface roughness of the self-lubricated steel sheets.     

Scanning auger microprobe study of hot-dipped regular-spangle galvanized steel: Part I. surface composition of As-produced sheet

Untreated bright and dull zinc coatings produced on a hot-dipped-coating pilot line from a zinc bath containing small concentrations of aluminum, antimony, and lead were examined with a scanning Auger microprobe. The results show that the entire surface of the zinc coating was covered by a very thin, continuous film comprised of aluminum and oxygen. The thickness of this film, of the order of 5 nanometers, was the same for both bright and dull coatings and independent of zinc spangle roughness. Beneath this film the zinc surface contained numerous small precipitates that occupied a considerable portion of the total coating surface area, perhaps as much as 25 pct for some spangles. In all, five distinct types of precipitates were observed in the coatings. Aluminum was common to each type except for one which was essentially pure lead. In the others the principal element combined with aluminum was oxygen in one case, antimony in the second, iron in the third, and zinc in the fourth case. Based on the results of Auger electron spectrometry, these appear to be A1₂O₃, AlSb, one of the aluminum-rich iron-aluminum intermetallic compounds, and the zinc-rich solid solution of zinc in aluminum, respectively. The size of these various precipitate particles tended to be larger and their frequency greater in rough than in smooth spangles.     

Processing and characterization of plasma-sprayed ceramic coatings on steel substrate: Part I. On coating characteristics

This investigation envisages the processing of a series of plasma-sprayed coatings from a few commercially available and inexpensive powders, namely, alumina (commercial grade, Indian), plasma-dissociated zircon (PDZ), zircon sand, and zircon-20 wt pct calcia. These powders do not belong to the so-called “plasma sprayable” grade, expensive powders. The microstructures and several properties of these coatings have been studied to evaluate their potential as thermal barrier and wear-resistant coatings. With an appropriate choice of processing condition, a sound and adherent ceramic coating is achievable using such powders. In some coatings, a layer of yttria has been applied between the top and bond coats with an aim to improve its thermal barrier properties. Such a layer does not disrupt the interfacial continuity of the coatings. The powders have been found to undergo phase transformations during spraying, subsequent annealing, and also during tribological testing of the coatings. An understanding of such phase transformations is important for the interpretation of coating behavior during performance tests as wear-resistant and thermal barrier coatings. These responses are dealt with in Part II of this series of articles.