Investigation of Surface Damage in Forming of High Strength and Galvanized Steel Sheets

Powdering/exfoliating of coatings and scratching galvanized steels and high strength steels （HSS）, are the main forms of surface damage in the forming of which result in increased die maintenance cost and scrap rate. In this study, a special rectangular box was developed to investigate the behavior and characteristics of surface damage in sheet metal forming （SMF） processes. U-channel forming tests were conducted to study the effect of tool hardness on surface damage in the forming of high strength steels and galvanized steels （hot-dip galvanized and galvannealed steels）. Experimental results indicate that sheet deformation mode influences the severity of surface damage in SMF and surface damage occurs easily at the regions where sheet specimen deforms under the action of compressive stress. Die corner is the position where surface damage initiates. For HSS sheet, surface damage is of major interest due to high forming pressure. The HSS and hot-dip galvanized steels show improved ability of damage-resistance with increased hardness of the forming tool. However, for galvannealed steel it is not the forming tool with the highest hardness value that performs best.

Laser brazing with filler wire for galvanized steel sheets

The process properties and interface behavior of CO_2 laser brazing with automatic wire feed for galvanized steel sheets were investigated, in which the brazing filler metal was CuSi3 and no flux was used. As to the appearance quality of the brazing seams, the roles of the processing parameters, such as brazing speed, wire feeding rate, inclination and feeding direction of the wire, laser power, spot diameter and heating position, were assessed. The further investigation indicates that the behavior of the active elements Si, Mn and Zn are significantly influenced by energy input. At the interface, the microstructure of the base metal was composed of columnar crystals and the acicular α solid solution was found on the filler metal side.

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.

Surface topography evolvement of galvanized steels in sheet metal forming

U-channel forming tests were performed to investigate the surface topography evolvement of hot-dip galvanized(GI) and galvannealed(GA) steels and the effects of die hardness on sheet metal forming(SMF). Experimental results indicate that the surface roughness values of the two galvanized steels increase with the number of forming,i.e.,the surface topographies of galvanized steels are roughened in SMF. Moreover,GI steel has a better ability of damage-resistance than GA steel. The mechanisms of topography evolvement are different in the forming of GI and GA steels. Scratch is the main form of surface damage in the forming of GI steels. The severity of scratch can be decreased by increasing die hardness. GA steel results in exfoliating of the coating firstly and then severe scratching. The surface topography of galvannealed steels can be improved by increasing die hardness. However,the hardness should not be too high.

Effect of Variable Blank Holder Force on Rectangular Box Drawing Process of Hot-galvanized Sheet Steel

At first, a series of finite element method （FEM） simulation tests were used to find the critical forming conditions of hot-galvanized sheet steel during the rectangular box drawing processing when constant blank holder forces were applied. According the test results, the reasonable alteration scope of initial variable blank holder force （VBHF） was as 1.9-2.3 T. Then, based on the test productions of blank holder force, 12 typical VBHF curves were applied to perform the simulation tests by the simulation software of DYNAFORM. The simulation test results showed that VBHF had great effects on drawing formability of hot-galvanized sheet steel during the rectangular box drawing. However, the different VBHF curves were applied to control the whole drawing and it would get great different effects. At the same tine, the VBHF had great effects on the maximum thick thinning ratio, but had little effect on the maximum thick incrassation ratio. So, reasonable application of the VBHF would greatly decrease the fractures. When the VBHF profile is taken as curve L, the best effect of drawing formability could be obtained. When curve I is used, contrary effect could be gotten. The other types of curves would cause effects between above two types of VBHF curves. Finally, the actual tests were applied to check the validity of the FEM simulation tests. The results show that the FEM simulation tests are good ways for predicting and optimizing the VBHF.

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.

Properties of Baosteel inorganic-lubricant-coated galvanized steel sheet for automotive body panels

An inorganic lubrication treatment for a galvanized steel sheet was developed to improve the press formability of automotive body panels. The properties and application of Baosteel＇s inorganic-lubricant-coated galvanized steel sheet are demonstrated. Compared with galvanized steel sheets without the inorganic lubricant coating,the Baosteel inorganic-lubricant-coated galvanized steel sheet has a lower friction coefficient and excellent lubricating ability,more homogeneous tribological and stamping behaviors,lower sensitivity to stamping parameters,excellent spot weldability,good adhesivity,and perfect phosphate compatibility and paintability. These results verify that the Baosteel inorganic-lubricant-coated galvanized steel sheet has excellent press formability and a huge market application potential.

The development of advanced Cr-free organic composite coated electrogalvanized steel sheet with excellent grounding property

In recent years,the concern about environmental protection is increasing on a world scale.Major manufacturers of home appliances and OA equipment have introduced so-called 'green procurement schemes' for reducing environmentally harmful substances in manufacturing process and end products. Under such background,a new type of chromate-free organic composite coated electro-galvanized steel sheet with high conductivity was developed by Baosteel,which meets the EU RoHS Directive(Restriction of the Hazardous Substances) and other related laws and regulations on environmental safety.It also provides excellent surface electrical conductivity,corrosion resistance,fingerprint resistance,solvent resistance,coating adhesion, heat resistance,formability and other special properties to meet the demand of manufacture's process of OA machine. Compared with previously developed anti-fingerprinting coated electro-galvanized steel sheet,this newly developed product has a good balance between high corrosion resistance(time to 5%white rust in salt spray test is 120 h for flat panel and 72 h for worked potion) and conductivity(surface electro-resistivity in accordance with LOREAST is less than 0.1 milliohm ) due to the special design of coating's structure.Besides,It also provides the properties of grounding and shielding against electromagnetic waves.The evaluation of surface performances of new product showed that it is comparable or even better than the similar products. Currently,the newly developed product has been commercialized. In this paper,the major properties are discussed,such as corrosion resistance,surface electrical conductivity, fingerprint resistance,solvent resistance,coating adhesion(ink/melamine alkyd paint),heat resistance and formability.Furthermore,the application is also briefly described.

Effects of surface treatments of galvanized steels on projection welding procedure

A group of projection welding experiments and joints tension-shear tests are carried out for cold-rolled steel sheets, galvanized steel sheets (GSS) without treatment, GSS with phosphating and GSS with surface greasing, respectively. The experimental results are regressively analyzed on the computers, then the projection welded joint tension-shear strength curve and the perfect welding currents range of each material are obtained. The results show that surface treatments of galvanized steels have effects on their spot weldabilities. Among the four kinds of materials, GSS with surface greasing have the worst spot weldability, for they need higher welding current and have a narrow welding current range.

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.

热浸镀Zn-Al合金钝化工艺及耐蚀性能研究

在钢板热浸镀Zn-5%Al合金表面制备了钼酸盐钝化膜,探讨了溶液pH、θ及t对钝化膜性能的影响。采用电化学方法对钝化膜耐腐蚀性能进行测试。结果表明,Zn-5%Al合金钝化膜阳极极化曲线呈现钝化特征。用扫描电镜对钝化膜的表面形貌进行观察,发现钝化膜呈片层状结构,用环境扫描电镜配套的能谱仪对钝化膜成分进行分析,结果表明钝化膜由Mo、P、O、Zn和Al等元素组成。

Small-invasive determination of iron content in coating of galvanized steel sheets by laser-induced breakdown spectroscopy

A new method was presented to determine the iron content in the coating of galvanized steel sheet based on laser-induced breakdown spectroscopy.The zinc-iron coating was characterized with a series of single laser pulses irradiated on the traversing sheet steel,each on a different steel sheet position.The influences of laser fluence and elemental depth distribution were studied and analyzed.To protect the corrosion performance of the coating and meet requirements for small-invasive measurement,the ablation size of the crater under different laser fluences was studied.Under the optimized experimental parameters,the diameter of ablation craters is about 50μm,and then,the Fe content in the coating was calibrated and analyzed by the linear standard calibration method.The calibration result,however,is not good.Considering that the Zn content in the coating was high and relatively constant,curve calibration was then carried out with the intensity ratio(IFe404.58/Izn468.01)instead of the net line intensity of Fe,and then,the determination coefficient of calibration curve increases from 0.7713 to 0.9511,and the root-mean-square error decreases from 0.4832%to 0.1509%.The results prove that the laser-induced breakdown spectroscopy is an effective way for the analysis of the Fe content in the coating of galvanized steel sheet.

Effect of Cu Addition on the Microstructure and Mechanical Properties of U-MIG Welds on Galvanized Steel Sheets

Ultrasonic-assisted metal inert gas(U-MIG)welding experiments were performed to analyze the effect of Cu addition on the microstructure and mechanical properties of galvanized steel sheet welds,and first-principles calculations were undertaken to predict the effect of Cu on the mechanical properties ofα-Fe based structures.As shown by scanning electron microscopy analysis,Cu is conducive to the refinement and uniformity of the microstructure in the weld zone.Energy-dispersive spectrometry data indicated the presence of Cu in the welds after welding with Cu powder.The elastic moduli of the welds without and with Cu powder,obtained from nanoindentation tests,were(217.66±5.11)GPa and(223.13±9.12)GPa,respectively,which were close to the calculated results.The mechanical properties of the crystal structures ofα-Fe andα-Fe_(1.9)Cu_(0.1)were calculated using Materials Studio software.Both the experimental and calculated results showed that Cu doping reduced the bulk modulus of the weld but increased its shear modulus.

热镀锌钢板烘烤硬化值检测的影响因素

为了研究热镀锌钢板烘烤硬化值检测的影响因素,主要介绍了HC180BD+Z钢板烘烤硬化值的检测方法与检测结果,分析并探讨了试验温度、保温时间、加热装置、拉伸预应变等烘烤条件对烘烤硬化值检测的影响因素。结果表明:采用相同加热装置时,将保温时间控制为20 min,试验温度由160℃上升至180℃,烘烤硬化值一直保持上升趋势,但当试验温度为160~170℃时,烘烤硬化值的上升趋势要比试验温度为170~180℃的变化明显;采用相同加热装置时,控制试验温度为170℃,当保温时间由16 min延长至20 min时,烘烤硬化值随保温时间的延长而上升,但当保温时间由20 min延长至30 min时,烘烤硬化值无明显变化;采用相同的试验温度和保温时间,不同的加热装置时,烘烤硬化值的检测结果无明显差异;当拉伸预应变由1%增大至2%时,烘烤硬化值的变化趋势与拉伸预应变同步,均有上升趋势,当拉伸预应变由2%增大至3%时,烘烤硬化值却随着拉伸预应变的增大呈下降趋势。

冷轧厂薄带钢高效生产的问题及解决方案分析

热镀锌板具有抗腐蚀强、易于深加工、使用寿命长、成本低以及表面美观等特点,主要用于家用电器、汽车等产品的内部构件和面板应用。随着家电板、汽车板工业的发展,家电和汽车所用的镀锌板占全国钢产量的比例逐步提升。随着国内外家电板、汽车板轻质化和高质量的要求,客户对产品表面质量要求越来越严格。针对生产线在提速增产过程中出现的入口跑偏、工艺段带钢表面质量缺陷和薄带钢薄锌层产速受限问题,提出了相应的改进措施。优化后,不仅提高了产量,而且提高了产品质量。

汽车车身常用板材耐腐蚀性能研究

车身常用板材的耐腐蚀性能是车身板材选用的重要参考标准,因此,对已加工为零件的热浸镀铝硅镀层板、未加工为零件的热浸镀铝硅镀层板、电镀锌板、热镀锌板、热成型钢裸板和冷轧板分别进行裸板材板和电泳板的中性盐雾对比试验。结果表明:有镀层的板材耐腐蚀性能明显优于无镀层板材;热浸镀铝硅镀层板加工成零件后其镀层会被破坏,导致其裸板耐腐蚀性能下降,但其电泳板的耐腐蚀性能加工前后变化不大,划痕扩蚀情况反而提升;热成型钢裸板因加工过程形成的氧化膜和脱碳层,在中性盐雾条件下4 h即出现锈蚀,其电泳板和电泳划痕板中性盐雾试验均出现严重的起泡问题。

镀锌钢板的CO_2激光焊焊接性

以裁焊板工业生产为背景 ,对汽车用镀锌钢板的CO2 激光切割板坯的CO2 激光焊焊接性进行了试验研究。对于不等厚和等厚的镀锌板拼接进行CO2 激光焊接 ,在激光功率 4 0 0 0W时其焊接速度可达 0 .1m/s。对镀锌钢板激光切口拼接的激光焊接与铣口拼接的激光焊接的焊缝形貌和硬度进行了比较 ,它们产生的焊缝宽度在 0 .5～1.0mm范围 ,激光焊缝最大硬度比母材硬度高出 1～ 1.5倍 ,激光焊接对激光切口拼接与铣口拼接两者都可获得等同的宏观焊接质量。

热镀锌钢板成形后镀层脱落原因分析

对成形后镀层脱落的热镀锌钢板镀层组织进行了观察,从抑制层形成机制、热浸镀工艺等方面对镀层脱落的原因进行解析。结果表明,镀层发生脱落原因是由于基板表面抑制层不完整,发生铁锌合金反应生成脆性的铁锌合金相,在变形过程中,镀层从铁锌合金相处破坏并产生剥离脱落。建议通过优化以下关键工艺以提高产品质量:提高带钢表面清洁度、严格控制锌液中铝含量、合理控制热浸镀工艺等。经过以上工艺优化与严格控制,最终得到了抑制层连续致密的合格镀锌板产品。

无机-有机硅烷复合钝化膜的性能

为了扩大硅烷与无机盐复合钝化的应用范围,简化工艺,降低成本,以硅烷偶联剂为主成膜物,磷酸盐、氟钛酸盐作钝化剂,Na2MoO4和NH4VO3作缓蚀剂,一步钝化制备了无机-有机硅烷复合钝化膜。采用扫描电子显微镜、硫酸铜点滴腐蚀、盐水浸泡、中性盐雾试验、电化学测试等技术对钝化膜层的微观形貌、性能进行了分析。结果表明:无机-有机硅烷复合钝化膜在锌层表面起到了化学和物理的屏障作用,可以阻止腐蚀过程中的极化反应和锌的溶解,增强镀锌钢板的耐蚀性;无机-有机硅烷复合钝化膜的腐蚀面积小于5%,极化阻抗达到9 kΩ,耐蚀性能接近铬酸盐钝化膜。

镀锌板水性环氧树脂复合钝化膜的耐蚀性能

镀锌板经环氧树脂系有机/无机复合钝化处理后耐蚀性能大幅提高,目前对其研究不够深入。将水性环氧树脂、Ni(NO3)2.6H2O,H2O2及有机酸等配制成复合钝化液,在镀锌板表面制备了有机/无机复合钝化膜。采用Tafel极化曲线、交流阻抗(EIS)和中性盐雾试验(NSS)等方法对单一环氧树脂钝化膜和复合钝化膜的耐蚀性进行了对比研究,采用扫描电镜(SEM)和能谱分析(EDS)对复合钝化膜的表面形貌和组成进行了分析。结果表明:无机盐﹑有机酸与环氧树脂在镀锌板表面形成的有机/无机协同缓蚀复合膜具有优良的耐蚀性和较好的工业实用推广价值。