Investigation and improvement of tiny spot defects on hot-dip galvanized automotive steel sheets

The causes of tiny spot defects on the surface of hot-dip galvanized automotive steel sheets were studied using scanning electron microscopy(SEM)and energy dispersive spectrometer(EDS),and effective control measures were introduced.The results show that rubbing against the top roller after galvanizing is easy due to the local thickness of tiny spot defect location coating;therefore,the surface morphology is different from the normal part.Three kinds of defects,namely zinc slag,small slivers,and pitting,are likely to cause local thickening of the coating after galvanizing,leading to the formation of tiny spots.Therefore,resolving the three types of defects can effectively control the generation of tiny spot defects.Among them,due to the hereditary nature of the small sliver defect,focusing on its control and supervision is necessary.

Recent development of UHSS and its hot-dip galvanizing processes in Baosteel

Baosteel has excelled in automotive steel sheets in the past three decades.It has made a significant contribution to the development of China’s automotive industry by producing a wide range of high-quality steel products.Some milestones achieved by Baosteel automotive steel sheet were briefly reviewed.The current challenges in producing ultra-high strength steel(UHSS),especially hot-dip galvanized UHSS,were summarized.The most current advancements in UHSS and the corresponding hot-dip galvanizing processes were discussed.The galvanizability of Si-Mn-added QP steel and DP steel, Mn-added TWIP steel, and Al-added low-density steel has been improved by different techniques in Baosteel.

Effect of hot-dip galvanizing processes on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel

A C–Mn dual-phase steel was soaked at 800°C for 90 s and then either rapidly cooled to 450°C and held for 30 s(process A) or rapidly cooled to 350°C and then reheated to 450°C(process B) to simulate the hot-dip galvanizing process. The influence of the hot-dip galvanizing process on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel(DP600) was investigated using optical microscopy, scanning electron microscopy(SEM), transmission electron microscopy(TEM), and tensile tests. The results showed that, in the case of process A, the microstructure of DP600 was composed of ferrite, martensite, and a small amount of bainite. The granular bainite was formed in the hot-dip galvanizing stage, and martensite islands were formed in the final cooling stage after hot-dip galvanizing. By contrast, in the case of process B, the microstructure of the DP600 was composed of ferrite, martensite, bainite, and cementite. In addition, compared with the yield strength(YS) of the DP600 annealed by process A, that for the DP600 annealed by process B increased by approximately 50 MPa because of the tempering of the martensite formed during rapid cooling. The work-hardening coefficient(n value) of the DP600 steel annealed by process B clearly decreased because the increase of the YS affected the computation result for the n value. However, the ultimate tensile strength(UTS) and elongation(A80) of the DP600 annealed by process B exhibited less variation compared with those of the DP600 annealed by process A. Therefore, DP600 with excellent comprehensive mechanical properties(YS = 362 MPa, UTS = 638 MPa, A_(80) = 24.3%, n = 0.17) was obtained via process A.

The Influence of Al on the Surface Properties of the Hot-dip Galvanized Melt

Wettability of Zn-Al alloy melt on the pure iron substrate at 450℃was studied.The effect of Al content(Zn,Zn-1Al,Zn-2Al,Zn-3Al,Zn-4Al,and Zn-5Al)on the wetting behavior and interfacial reaction was investigated by high-temperature contact angle measuring device and scanning electron microscope(SEM).The results show that,with the increase of Al content,the initial contact angle of the molten alloy on the substrate decreases gradually and the wettability increases gradually.Compared with the initial contact angle,the final contact angle is slightly reduced,because the Fe-Al inhibition layer is preferentially formed at the interface when adding Al to the alloy.The presence of Al will promote the occurrence of the reactive wetting,leading to an insignificant wetting spreading process,and the final contact angle negligibly differs from the initial contact angle.The adhesion work and charge density distributions of interface systems were calculated based on the first-principles.The results show that the adhesion work of the Fe/Zn and Fe/(Zn-Al)interface model is 2.0171 J/m^(2)and 13.7944 J/m^(2),respectively.The addition of Al greatly increases the adhesion work between alloy melt and iron substrate.Compared with the Zn-Fe and Al-Fe interface models,it can be seen that a significant charge migration phenomenon occurs between the interfaces.The amount of charge migration in the Al-Fe interface model is much larger than that in the Zn-Fe interface model,indicating that the bonding between Al-Fe atoms can occur more easily and the interaction between Al-Fe interfaces is stronger.This is also the reason why the addition of Al increases the adhesion work between interfaces.

Preparation and hot-dip galvanizing application of CeO_2/Zn nanocomposite

CeO2/Zn nanocomposite was fabricated by high-energy ball milling and subsequent hot-press sintering under vacuum. The produced nanocomposite powders and bulks were characterized using X-ray diffraction （XRD）, transmission electron microscopy （TEM） and scanning electron microscopy （SEM） with an energy dispersive X-ray spectrum （EDS）. The as-fabricated nanocomposite bulks were added successfully into the hot-dip galvanizing bath to improve the corrosion resistance of coatings. The results show that after milling for 120min, nano-sized CeO2 particles are distributed homogeneously on the surface of fine Zn particles to form spherical-like nanocomposite powder with narrow grain size distribution. The subsequent sintering does not lead to a dramatic grain growth, and the distribution of CeO2 nanoparticles in the composite bulk is also homogeneous. With the addition of as-prepared bulks, the corrosion resistance of the galvanized coatings is improved obviously. It provides a feasible route for adding CeO2 nanoparticles into the hot-dip galvanized coating.

Hot-Dip Galvanized Sheet Production and Application

Hot-dip galvanized sheet is wildly used in construction,household appliances,ship,vehicle and vessel building and machinery,etc.In last ten years,with the development of automobile industry,the anti-eorrosion requirements for car body are increasingly strict,by which the rapid development in technology has been promoted.The application of hot-dip galvanized sheet,technological progress in production and some Chinese large units were introduced.

Analysis of causes and influence factors of small black spot defects in hot-dip galvanized pure Zn products

In this study,the factors affecting the formation of small black spot defects on the surface of a hot-dip galvanized pure zinc strip are analyzed. These defects are primarily formed w hen the iron-aluminum inhibition layer is incompletely formed or fails to form or when foreign matter adheres to the surface or embeds in the zinc layer.The defects can be reduced by controlling the cleanliness and activity of the strip base before coating,regulating the zinc pot process parameters,and ensuring the cleanliness of the passline and workshop.

Development of Cr-free anti-fingerprint hot-dip galvanized sheet for household appliances

China has become the maximum household appliances manufacturing base worldwide.It is a new challenge facing the steel industry to develop the low-cost and environment-friendly sheet for household appliance industry,so Cr-free anti-fingerprint hot-dip galvanized sheet for household appliances is the upgraded product which satisfies the requirement of China household appliance industry' s nuisanceless treatment.The paper discusses the technical key points of development on Cr-free anti-fingerprint galvanized sheet for household appliances,including the development of the Cr-free anti-fingerprint chemical and the production technology of Cr-free anti-fingerprint galvanized sheet for household appliances.The paper elaborates the main compositions,physical features and properties of the Cr-free anti-fingerprint chemical, surface quality control technology,anti-fingerprint coating quality control technology,influence of anti-fingerprint film weight on corrosion resistance,conductibility,high-temperature discolouration resistance,acid resistance,alkaline resistance and solvent resistance of Cr-free anti-fingerprint hot-dip galvanized sheet, influence of anti-fingerprint coating uniformity on paintability of the sheet,influence of strip surface roughness on corrosion resistance and conductivity of the sheet and influence of strip surface quality on corrosion resistance of the sheet.Adopting the suitable Cr-free anti-fingerprint chemical and the reasonable production process parameters,the properties of the Cr-free anti-fingerprint hot-dip galvanized sheet conform to the delivery technical norms and the Cr-free anti-fingerprint hot-dip galvanized sheet becomes the new material for household appliances such as computer case.

Evolution of appearance profile of hot-dip galvanized car-body steel sheet during deformation process

In recent years, the waterborne free intermediate coating process has been widely used in the automotive industry. Because the baking times and coating thickness are decreased, the surface covering capability of the painting process is reduced, which directly affects the appearance quality（ long-and short-wave values） of the body paint. Thus, there are correspondingly higher requirements for the white body surface profile prior to painting. The surface profile of the white body is mainly affected by the plate material, the surface profile, and the deformation process. So,the change rule for the surface profile during deformation of the steel plate is a key factor in coating appearance optimization. In this paper, we first analyze the typical deformation of the outer cover of a car body. Then ,we examine the change tendency of the surface profile of steel plates with respect to different deformation rates, specifically for a steel plate comprising a hot-dip galvanized bake-hardened steel sheet. Based on our analysis of the influence of the deformation on the coating appearance,we selected 3% ,5% ,and 8% deformation rates in this research. We found the roughness （Ra） value in the typical deformation range （3% -8% ） of the car body to exhibit a decreasing trend at first and then an increasing trend. The Ra value of the 8% deformation is not more than the original plate test value. When the Pc value of the original plate is in the lower range （ about 60）, it exhibits a slight increasing trend in the deformation process （3 % -8 % ）. And when the Pc value of the original plate is in the higher range （ about 120 ）, it exhibits no increasing trend in the deformation process （ 3% -8% ）. In contrast,the waviness （WCA） value in the car body＇s typical deformation range （3%-8%） shows a significant growth trend.

Application of WC cermet sprayed composite coating in hot-dip galvanizing lines

With the rapid development of the automobile industry, the use of galvannealed and galvanized steel sheets in automobiles is on the rise. These sheets must meet very high surface quality requirements. The surface treatment of line rolls is known to have a great impact on strip quality. To prevent dusts such as zinc ash from pressing into the strip surface, we used a composite thermal spray surface treatment technique to treat rolls. The successfully developed tungsten carbide （WC） ＋ Ni-P composite plating technology improved the quality of the tungsten carbide thermally sprayed WC roll surface. This technique is also helpful to control defects such as adhered foreign materials in hot-dip galvanized automobile outer panel surfaces.

Lead and Cadmium Distribution in Tubes of Galvanized Steel by Hot-Dip Used for Drinking Water Supply

The main objective of this work was to evaluate the presence of lead and cadmium in galvanized steel tubes by hot-dip used exclusively for drinking water supply.In this research,samples of galvanized tubes were removed from walls of residential buildings with 30 years of use.In the process of hot galvanizing,the manufacturing standards allow or for low cost conveniences,and the use of primary zinc ingot with 1.6%metallic impurities,such as lead and cadmium,is allowed,enabling the incorporation of these metals into the zinc coating deposited on the carbon steel.The associated corrosion of these tubes can release these toxic metals,contaminating the drinking water used by residents.The samples taken from the tubes were evaluated by a square-wave anodic stripping voltammetry technique,and the results for lead and cadmium suggested randomly distributed levels,indicating that the contents of these contaminants depend mainly on the quality of the added primary zinc and the operational conditions of the immersion bath.The metallographic evaluations performed on the samples of the galvanized tubes showed irregularities in thickness and even a lack of the zinc layer deposited on the carbon steel surface.These facts suggest that galvanized tubes should not be used for drinking water.

连续热镀锌板漏镀原因分析及预防措施

对连续热镀锌板漏镀的原因进行分类分析,利用金相显微镜、扫描电镜及能谱仪精确确定漏镀的原因,并提出改进措施,提高了镀锌板的表面质量,减少了漏镀的发生。

三次渗碳体和游离渗碳体对低碳钢弯曲性能的影响

生产和使用中发生了两起低碳钢产品弯曲开裂:SPHC钢异形门框弯曲开裂和Q195钢热镀锌C形钢弯曲开裂。对开裂的异形门框和C形钢进行了宏观检验、金相检验和断口分析,并对热镀锌在线退火工艺进行了模拟试验。结果表明:低碳钢中的三次渗碳体和游离渗碳体与基体之间易形成孔隙和微裂纹,降低钢的弯曲性能,在弯曲力的作用下钢发生开裂;热镀锌SPHC和Q195钢带中的渗碳体来源于热轧钢带的组织遗传;为控制三次渗碳体的数量,热轧低碳钢带的终轧温度应控制在860~880℃,卷取温度控制在580~610℃,卷取后钢带应避免缓冷。

延长连续热镀锌生产线锌锅辊使用寿命的研究

在连续热镀锌生产中经常发现因锌锅辊系中的沉没辊表面黏渣而导致辊系磨损和镀锌板表面质量缺陷的现象。鉴于此,为了保证镀锌板产品质量和生产效率,设法延长锌锅辊系使用寿命。重点分析影响连续热镀锌生产线锌锅辊使用寿命的原因,并针对性地提出延长锌锅辊系寿命的技术措施。

冷轧厂钝化烘干系统优化探析

冷轧镀锌板材生产完成后,在潮湿环境中镀层表面与水发生反应生成氢氧化锌,在这种条件下,很容易形成白锈等腐蚀产物。为了解决这一难题,生产线增加了后处理塔钝化系统工艺,在板带两面顺涂或逆涂一层铬溶液,然后经过热风烘干,最终在带钢表面形成一层钝化膜,防止钢卷在中间储存过程中产生白锈现象,达到防腐的目的。但是在实际生产中遇到一系列问题,通过增加远程控制系统、增加点火烧嘴的放散管道等措施对设备进行优化改进,效果显著。

国标连续镀锌钢带(S450)双立柱光伏支架设计优化策略研究

本研究旨在利用S450高强度钢材作为光伏支架的主要材料,并结合钢结构有限元分析软件3D3S进行结构优化,探讨如何实现光伏支架设计材料使用优化和成本效益最大化。通过对双立柱支架的设计分析,成功降低了该项目每兆瓦用钢量约15%,实现了严格的成本控制目标,为光伏支架的经济性设计提供了新的思路和方法。

Features and control methods of the galvanneal furnace

The recent rapid developments in the automobile industry have demanded the extensive use of gaivannealed （GA） steel sheets.In particular,the development of lightweight automobiles is putting increasingly higher requirements on the strength of GA steel sheets.The galvanneal furnace,which is used for processing galvannealed steel sheets,is typically composed of the induction heating section,holding section and fog cooling section.This paper described the structural characteristics of each component of the galvanneal furnace,and analyzed temperature control methods of the galvanneal furnace that are important for the successful production of high-strength GA steel sheets for automotive applications.

Bottom dross defect and its transformation behavior during the galvannealing process

Bottom dross obtained from zinc bath and the typical bottom dross defects on galvanized and galvannealed coating were metallographically investigated.Galvannealing simulations with different holding time were conducted,and the bottom dross transformation behavior during the galvannealing process was revealed.The fast diffusion between bottom dross particles and the surrounding zinc occurs during the galvannealing process,resulting in the formation of an Al-containing Zn-Fe intermetallic outburst structure at the defect position.The thicker and harder coating will cause a shiny spot on the coating surface and a deep dent on the substrate after being temper rolled.

Influence of Ni-electrodeposited Pretreatment on Galvanized Coatings of Reactive Steels

Four types of steel sheets containing 0.04%, 0.09%, 0. 14% and 0.36% Si, respectively, were electrodeposited with a nickel layer of 3 tam in thickness and then galvanized in molten Zn at 450℃ for various periods of time. The formation and growth of intermetallic compound layers on the surface of the samples were investigated by SEM and EDS. The experimental results show that the method of Ni-electrodeposited pretreatment can distinctively restrain the over-growth of the galvanized coatings of reactive steels and get eligible coatings with a proper thickness, bright appearance and strong adherence. EDS results indicate that a series of Ni-Zn intermetallic compounds γ′, γ and δ, are first formed on the surface of the samples. With a prolonged immersion time, the F2-Fe-Zn-Ni and δ-Fe-Zn are formed accompanied by the gradual disappearance of γ′, γ and δ2 layer. After a longer immersion time, the lumpy ζ- Fe-Zn occurs between δ and liquid Zn and the F-Fe-Zn does between steel substrate and δ. Subsequently, ζ is in the form of a continuous and compact layer. The method of Ni-electrodeposited pretreatment changes the formation of Fe-Zn intermetallic compounds, which delay the growth of lumpy （and promote the growth of compact δ. Consequently, the abnormal growth of reactive steels is eliminated.

Improvement the Corrosion Resistance for the Galvanized Steel by Adding Sn

The chemical composition of the zinc bath can strongly influence on the hot-dip galvanized coatings. In this work, the effects of tin addition on the surface morphology, and the corrosion resistance of hot-dip galvanized steel were investigated. The corrosion behavior of steel samples galvanized with zinc and Zn-Sn alloys containing different wt% Sn was analyzed by various corrosion tests such as potentiodynamic polarization Tafel lines and electrochemical impedance spectroscopy (EIS) techniques. Salt spray test was employed in order to study the corrosion products of the specimens. Surface morphology, the composition of coating layers and nature of the corrosion products were also investigated using field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX) and X-Ray diffraction (XRD) techniques, respectively. The results indicated that the addition of small amounts of Sn (0.1 wt%) to the molten zinc galvanizing bath can improve the corrosion resistance of hot-dip galvanized steel.