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.     

深冲用电镀钢板的摩擦特性及表面损伤的研究

采用扳成形动卷摩擦因数测试装置对两种铡扳(热浸镀 电镀,即GA-E钢板;IF钢电镀锌钢扳,即EG钢板）以及未电镀的裸板进行摩擦特性研究,着重研究了成形模具温度对摩擦的影响,并对两种镀层钢板经拉深后的表面损伤进行了分析。研究结果表明：(1)在相同的变形条件下,与EG钢板相比,GA-E钢板的摩擦因数随温度增加的变化幅度较小;(2)两种镀层钢板的摩擦因数均随冲头速度的增加而降低;(3)GA-E钢板接触表面损伤的主要形式为微裂纹,而EG钢板的接触表面损伤为犁划痕。     

Ra 及 Rpc 对无铬耐指纹热镀锌板表面性能的影响

目的研究热镀锌板表面平均粗糙度Ra和峰值数Rpc对无铬耐指纹热镀锌板表面耐指纹性、耐腐蚀性和涂装性的影响。方法将Ra=0.6~1.1μm和Rpc=55~100的热镀锌板制成耐指纹板,采用X射线光电子能谱仪分析其组成,采用分光光度仪分析其耐指纹性,采用硫酸铜混合溶液滴定和盐雾试验方法检测其耐腐蚀性,采用涂漆和冲击实验检测其涂装性。结果耐指纹膜和锌层表面形成Si—O—Zn金属键,耐指纹膜除了物理结合,还通过化学键合的方式与锌层表面结合。在充分交联固化和化学键合的情况下,无铬耐指纹热镀锌板耐指纹性不受热镀锌板Ra和Rpc的影响;热镀锌板Ra值越低,耐腐蚀性越好;耐指纹膜单位质量为1.1 g/m2,Ra≤1.1μm时,耐指纹板72 h盐雾试验白锈面积不大于5%;热镀锌板Rpc值越高,无铬耐指纹热镀锌板涂装性越好,Rpc≥80时,耐指纹热镀锌板涂漆冲压后,漆膜完好。结论 Ra和Rpc对热镀锌板耐指纹性影响不大,对耐磨蚀性和涂装性影响显著。     

Lubricity of volatile lubricants in sheet metal rolling

A series of experiments is carried out by using a rolling type tribometer to investigate the lubricity of the volatile lubricants at high speed forming. The roll material is the die steel alloy SKD11, and the workpiece material is the mild steel SPCE with a rough surface and the aluminum alloy A3004 with a smooth surface. Experimental results show that the friction coefficient decreases with increasing working velocity for both SPCE and A3004, in any lubricant. With an increase of reduction in thickness, the friction coefficient decreases for SPCE, but increases for A3004. Some volatile lubricants have the same lubricity as the generally used mineral oil with low-viscosity by judging from the value of friction coefficient, the surface appearance of rolled workpiece and the roll surface damage.     

Tribological testing of commercial CrN, (Ti,Al)N and CrC/C PVD coatings — Evaluation of galling and wear characteristics against different high strength steels

The increasing use of high strength steels in a variety of mechanical engineering applications has illuminated problems associated with galling in sheet metal forming operations. Galling is a tribological phenomenon associated with transfer of material from the steel sheet to the tool surface during forming resulting in seizure of the tool/steel sheet contact and extensive scratching of the steel sheet surface. As a result, a number of concepts have been developed in order to reduce the tendency of galling in sheet metal forming, including the development of new dry lubricants, new forming tool steel grades and improved surface engineering treatments such as the deposition of low friction CVD- and PVD-coatings. In the present study the potential performance of three commercial PVD coatings, including CrN, (Ti,Al)N and a CrC/C DLC-based coating, in the forming of hot and cold rolled high strength steel as well as electro and hot-dip galvanized high strength steel has been evaluated using pin-on-disc testing under lubricated contact conditions. Post-test examination of the tribosurfaces using FEG-SEM and EDS analyses was performed in order to evaluate the mechanisms controlling the tendency to material transfer and wear. The results show that in contact with the hot and cold rolled steel the material pick-up tendency of the PVD coatings tend to increase in the order CrC/C-CrN-(Ti,Al)N while in contact with the two galvanized steel sheets, the CrC/C and the (Ti,Al)N coating show a significantly lower material pick-up tendency as compared with the CrN coating. Further, the substrate hardness has a strong influence on the wear of the PVD coatings and consequently on the friction characteristics and galling tendency of the coating/substrate composite. Low substrate hardness, resulting in a low load bearing capacity, increases the tendency to cracking and subsequently chipping of the brittle coating.     

An experimental study on forming characteristics of pre-coated sheet metals

To analyze the forming and friction characteristics of pre-coated metals (PCMs) that are widely used in household appliances, rectangular deep drawing and friction tests were performed. There were four types of die materials (STD11(TiCN), STD11(TD), STD11, AMPCO) used in the deep drawing tests, and the results show that STD11 and AMPCO are better than others in respect to forming load and surface roughness. The friction mechanism, the thickness of materials and the effect of the roller’s diameter on the friction coefficient and surface roughness were examined in the friction test. The PCMs used in the tests had low friction coefficients (μ=0.15–0.20) in non-lubricated condition due to the coating film acting as lubrication. The friction coefficient decreased as the radius of the roller increased and the surface of product became better as the thickness of the material decreased.     

Evaluation of coefficients of friction in hot stamping by hot flat drawing test

The coefficient of friction in hot stamping was measured using a tribosimulator. Simulative experiments were carried out using SPHC steel and 22MnB5 steel under dry conditions. The coefficient of friction of 22MnB5 steel was higher than that of SPHC steel. The obtained coefficients of friction were effective for use in numerical simulations by finite element analysis. The obtained coefficients of friction under lubricated conditions for both steels were lower than those under dry conditions. It could be understood that the use of lubricants is effective for decreasing of the stamping load and die wear.     

Influence of the roughness geometry of tool surface and the flow stress of coated solid lubricants on tribo-conditions in cold forging

In cold die forging, a sever friction sliding is often locally caused on the tool surface. It is usually preferable to smoothly finish such a portion of the tool surface. Many studies have been performed to maintain good lubrication conditions by catching lubricants to micropockets of the tool surface. This research is aimed at the detailed FEM examination of how the tool surface roughness influences the friction coefficient, the local surface enlargement at the friction surface of material and solid lubricant thinning at the edge of tool–material interface.     

基于热混合润滑的钢/橡胶接触表面水润滑增强调控

目的 增强钢/橡胶摩擦副的润滑性能,为提高混合润滑状态下水润滑轴承的性能提供参考。方法 建立水润滑条件下钢/橡胶摩擦副的热混合润滑模型,讨论热效应对润滑性能的影响,并在此基础上进一步研究表面粗糙度、水基润滑剂黏度和供水压力对水润滑增强调控的作用。结果 与等温解相比,热效应使Stribeck曲线发生了右移,摩擦因数和载荷比增大,膜厚比降低。最高水膜温度随着转速的增加而升高,热效应对混合润滑性能的影响显著。减小摩擦副表面粗糙度,Stribeck曲线向左移动。在相同转速下,载荷比随着表面粗糙度的减小而降低,膜厚比反之。表面粗糙度越大,水膜温度越高,最高温度位于出口区,且钢的表面温度低于水膜和橡胶的表面温度。当水基润滑剂的黏度增大时,膜厚比增大,载荷比和最高水膜温度降低,Stribeck曲线发生左移。增加供水压力可以改善水膜压力分布,使水膜承载区增大、压力减小,粗糙峰接触压力和承载区减小,导致载荷比减小、膜厚比增加,Stribeck曲线向左偏移,水膜最高温度降低。当接触区由边界润滑向混合润滑过渡时,水膜最高温度出现拐点,且水膜最高温度拐点随着供水压力的增加而左移。结论 热效应会降低摩擦副的混合润滑性能,因此在混合润滑中不能忽略。考虑热效应时,通过减小表面粗糙度,或增加水基润滑剂黏度和供水压力,均有利于增强钢/橡胶接触表面水润滑的混合润滑性能。     

FEM analysis of contact mechanism in press-forming of lubricant pre-coated steel sheet

This paper describes the contact situation between the die and the lubricant pre-coated steel sheet in the press-forming by using FEM simulation. The FEM simulation is carried out by supposing that the lubricant pre-coated steel sheet consists of the lubrication layer and thick galvanized layer. Both the lubrication layer and galvanized layer are assumed as rigid–plastic material. The variations in the contact situation between the die and the lubricant pre-coated steel sheet are investigated by changing the friction coefficient between the die and lubrication layer, the thickness and hardness of lubrication layer, the velocity ratio of the relative sliding velocity to pressing velocity, etc. The simulated results show that the contact area ratio is influenced largely by the average contact pressure, the velocity ratio, the thickness of lubrication layer, and friction coefficient.     

初始表面粗糙度对沟槽织构摩擦性能的影响

目的探究初始表面粗糙度大小对激光沟槽织构化表面摩擦性能的影响规律。方法采用脉冲光纤激光器在不同粗糙度的45#钢试样表面制备具有不同深度、规则排列的沟槽织构,利用摩擦磨损试验机进行销-盘式往复摩擦试验,研究初始表面粗糙度对沟槽织构化表面摩擦系数的影响规律,以及不同初始表面粗糙度条件下,激光沟槽织构化表面的摩擦学行为。结果沟槽织构能够有效降低表面的摩擦系数,初始表面粗糙度、载荷和速度的大小对沟槽织构的润滑减摩性能有较大影响。在较低的载荷下,沟槽织构能有效提高表面的流体动压润滑效应;在较高的载荷下,沟槽织构能够有效改善表面的边界润滑性能。存在最优初始表面粗糙度,使得摩擦系数达到最小,初始表面粗糙度最优值的大小应根据载荷和速度大小来确定。结论根据摩擦副表面的载荷和速度工况条件,合理优化初始表面粗糙度能使沟槽织构获得较为理想的润滑减摩效果。     

温模条件下镀锌钢板摩擦行为研究

开发了温模条件下钢板摩擦行为的评估装置,在此基础上研究了模具温度变化对镀锌钢板成形表面摩擦行为的影响。结果表明,不同镀层特性的镀锌钢板的摩擦系数随模具温度变化呈现出不同的变化规律。当模具温度升高时,热镀纯锌、电镀纯锌和热镀锌铁合金的摩擦系数均有所升高。由于预磷化膜的存在,预磷化电镀锌钢板的摩擦系数随模具温度变化有所波动,且总体维持在较低的数值。电镀纯锌钢板表面摩擦系数在模具温度较低时变化不大,但在模具温度较高时大幅度上升。     

热镀锌板双硅烷无铬钝化膜性能的研究

目的为进一步提高热镀锌板无铬钝化膜的耐腐蚀性能。方法以偏钒酸铵和氟钛酸作为无机组分,添加主要成膜物质苯丙乳液,通过硅烷偶联剂KH-560和KH-570将两者紧密连接,在热镀锌板表面获得一层耐腐蚀性较强的双硅烷无铬钝化膜。采用中性盐雾试验、电化学试验、扫描电镜(SEM)、傅里叶红外光谱(FTIR)等表征钝化膜的耐腐蚀性能。结果热镀锌板-双硅烷无铬钝化膜经96 h中性盐雾试验,腐蚀面积在2%~4%,腐蚀电流密度比热镀锌板基体降低一个数量等级,这表明双硅烷无铬钝化膜能提供良好的耐腐蚀能力。SEM分析表明在热镀锌板表面生成了一层均匀、连续、致密的钝化薄膜,钝化膜的厚度大约在3μm。FTIR分析表明,双硅烷无铬钝化膜中存在—(CH_2)_n—、C=O、Si—O、Si—O—Si、Si—O—Zn键。结论在热镀锌板表面生成了一层均匀、致密的钝化膜,使热镀锌板呈现良好的耐腐蚀性能,且满足工业应用标准。     

一种船用低温钢板在干态室温下的往复摩擦特性研究

目的研究新型船用低温钢板的摩擦磨损性能。方法采用UMT-2型多功能摩擦磨损实验机,测试了船用低温钢板在室温干态环境、不同载荷(10、20、30 N)、不同频率(2、5 Hz)下的往复摩擦试验行为。采用光学显微镜(OM)、扫描电子显微镜(SEM)分析了船用低温钢板的磨痕表面形貌,用光学轮廓仪分析了磨损表面轮廓,用EDS对试样磨损表面进行了成分分析。结果随着试验法向载荷从10 N增加到30 N时,船用低温钢板的摩擦系数从0.51逐渐增加到0.63,磨损率先增加后降低,再逐渐增加。在相同载荷下,摩擦系数随着往复频率的提高而降低。载荷为30 N时,往复频率为5 Hz,摩擦2 h后,磨损断面轮廓宽度和深度分别为750μm和3871 nm。接触面从磨粒磨损转向疲劳磨损,接触面出现氧化层、表面硬化层和转移层。结论载荷较低时,船用低温钢磨损主要为氧化磨损和疲劳磨损;载荷增大时,接触面磨损出现疲劳磨损。同等载荷下,摩擦系数会随着移动速度的提高有所下降,接触面在摩擦热作用下形成的金属膜有助于降低表面粗糙度,减小摩擦系数。     

1000MPa级DP钢的成形特性试验

对宝钢产1000MPa级冷轧双相钢和热镀锌双相钢的材料特性和成形特性进行试验,深入分析钢板的力学性能、扩孔性能和成形极限。研究表明,宝钢1000MPa级超高强钢板具有较好的力学性能,其延伸率和扩孔率与国外同类产品相近,成形极限试验测量得到的FLD0为18%,低于采用软钢经验公式计算值。摩擦试验表明,冷轧GI板表面摩擦系数明显低于热镀锌CR板,此外,涂油对CR板表面影响较小,但对GI板表面有较大的影响。     

TRIP钢热镀锌可镀性研究进展

对国内外TRIP钢热镀锌可镀性研究的最新进展进行了介绍,分析了基板成分、退火工艺、热镀锌工艺对TRIP钢热镀锌可镀性的影响,同时阐述了改善TRIP钢可镀性的对策措施。最后指出,采用预氧化还原热镀锌工艺生产含硅TRIP钢,是未来TRIP钢热镀锌工业生产的主流方向。     

Prediction of effect of rolling speed on coefficient of friction in hot sheet rolling of steel using sliding rolling tribo-simulator

In order to reduce the rolling force and the roll wear, the lubricants have been used in hot rolling of steel. In order to evaluate the tribological behavior at the interface between roll and workpiece in hot steel rolling, it is important to measure the coefficient of friction and examine the effect of the tribological factors on the coefficient of friction. In this paper, the effect of the rolling speed on the coefficient of friction is investigated using the tribo-simulator testing machine for hot rolling developed by the authors. The workpiece used is SPHC. The roll material is SKD11 and the surface roughnesses are 0.05 μm, 0.2 μm and 0.8 μm. The rolling tests are carried out at a temperature of 800 °C during a rolling distance of 400 mm, changing the rolling speed from 15 to 70 m/min. The colza oil is used as a base oil and the emulsion concentrations are 0.1% and 3.0%. The coefficient of friction at an emulsion concentration of 3.0% is independent on the rolling speed. On the other hand, the coefficient of friction at an emulsion concentration of 0.1% decreases with increasing rolling speed in the lower range of rolling speed, but it increases in increasing rolling speed in the higher range of rolling speed.     

Determination of frictional behavior in sheet metals using orthogonal arrays

Frictional behavior of two sheet materials, aluminum-killed drawing-quality steel (AKDQ) and electro-galvanized (zinc) drawing-quality steel (AKDQ-EG), is examined under conditions of varying die material, die radius, crosshead speed, and lubricant. Tests are conducted using a special apparatus designed to measure front and back tension on uniform tensile strip specimens pulling over a circular die, simulating both frictionless and frictional conditions under certain sheet-metal-forming conditions. Use of a specially designed test apparatus with four contact angles for the same test condition minimizes the error associated with the use of single measurements for the determination of friction coefficient. Lubricant and die material play important roles among different factors examined in determining the coefficient of friction. Die radius has the most pronounced effect on the coefficient of friction. Implication of these results on actual sheet forming processes are discussed.     

Frictional behavior of the sliding interface between an A2 steel die and zinc-coated steel sheet

The forming of coated sheet steel products is performed for a wide range of processing conditions. This article examines the effects of die history on the frictional nature of the sheet-die interface. It has been found that as successive tests are performed on a die from the as-lapped condition to the “run-in” condition, the die surface morphology changes significantly and the coefficient of friction, μ, decreases. Changes in surface roughness as well as scratch formation on the die are believed to affect the nature of the die-workpiece, lubricant interaction. The findings in this study suggest that the extensive polishing procedures used in the manufacture of new stamping dies should be carefully reevaluated.     

奥氏体不锈钢变薄拉深过程的数值模拟

利用有限元软件Deform-2D,对奥氏体不锈钢变薄拉深过程进行热力耦合的数值模拟,研究了凸、凹模与坯料间摩擦和变形速度对变薄拉深过程的成形载荷和坯料损伤的影响。模拟结果显示：当凹模与坯料之间的摩擦增大,成形载荷和坯料损伤明显增大;当变形速度增大,坯料损伤增大而成形载荷减小。上述结论为奥氏体不锈钢变薄拉深工艺优化设计提供了可靠的理论依据。