Flow characteristics and hot workability of a typical low-alloy high-strength steel during multi-pass deformation

Heavy components of low-alloy high-strength(LAHS) steels are generally formed by multi-pass forging. It is necessary to explore the flow characteristics and hot workability of LAHS steels during the multi-pass forging process, which is beneficial to the formulation of actual processing parameters. In the study, the multi-pass hot compression experiments of a typical LAHS steel are carried out at a wide range of deformation temperatures and strain rates. It is found that the work hardening rate of the experimental material depends on deformation parameters and deformation passes, which is ascribed to the impacts of static and dynamic softening behaviors. A new model is established to describe the flow characteristics at various deformation passes. Compared to the classical Arrhenius model and modified Zerilli and Armstrong model, the newly proposed model shows higher prediction accuracy with a confidence level of 0.98565. Furthermore, the connection between power dissipation efficiency(PDE) and deformation parameters is revealed by analyzing the microstructures. The PDE cannot be utilized to reflect the efficiency of energy dissipation for microstructure evolution during the entire deformation process, but only to assess the efficiency of energy dissipation for microstructure evolution in a specific deformation parameter state.As a result, an integrated processing map is proposed to better study the hot workability of the LAHS steel, which considers the effects of instability factor(IF), PDE, and distribution and size of grains. The optimized processing parameters for the multi-pass deformation process are the deformation parameters of 1223–1318 K and 0.01–0.08 s^(-1). Complete dynamic recrystallization occurs within the optimized processing parameters with an average grain size of 18.36–42.3 μm. This study will guide the optimization of the forging process of heavy components.

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.

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.

Effect of Hot Dip Galvanized Steel Surface Chemistry and Morphology on Titanium Hexafluoride Pretreatment

Titanium hexafluoride pretreatments are known to improve paint adhesion and function as a barrier between the coating and the hot dip galvanized (HDG) steel surface. Interactions at the zinc/pretreatment interface are of utmost importance for the formation of pretreatment layers and the corrosion resistance of color coated hot dip galvanized steels. Removal rate of inert aluminum oxide from HDG steel samples by chemical dissolution was studied. XPS measurements showed that the surface Al2O3 layer thickness decreased rapidly already at mild alkaline cleaning, while complete removal of Al required severe etching. Low reactivity of an Al2O3-rich surface was confirmed by impaired formation of a titanium hexafluoride pretreatment layer. Grain boundaries and deformation twinnings were shown to be of importance for the reactivity of the HDG surface and for the precipitation of the pretreatment chemical. Helium ion microscopy images and electron probe microanalysis (EPMA) of a pretreated sample showed accumulation of the pretreatment chemical at the grain boundaries. Al removal rate was fast at the deformation twinnings at the grain plateaus. Slow Al removal was observed at dendritic valleys and grain boundaries. The results increase understanding of the reactivity of hot dip galvanized steel surface.

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.

Development of a hot-rolled 980 MPa advanced high-strength steel with excellent flangeability and bendability

A new 980 MPa advanced high-strength steel(AHSS) with excellent bendability and flangeability has been studied and industrially produced, typical of tensile strength, fractured elongation, and hole expansion ratio(HER) exceeding 980 MPa, 10%,and 30%,respectively.The 90° V-type bending perpendicular to the rolling direction can satisfy the R/t=1.0 requirement, indicating excellent bendability.Systematic evaluations of industrial trial-produced 980 MPa hot-rolled AHSS have been conducted, including microstructure, tensile properties in three directions, HER,bendability, fatigue limit strength, and forming limit.The microstructure of the newly developed 980 MPa AHSS primarily consists of fine bainite and a small amount of martensite-austenite constituent.The practical yield and tensile strength are higher than 800 and 980 MPa, respectively, with typical elongation of 13% and HER of around 40%.The good combination of the newly developed 980 MPa AHSS is primarily attributed to the fine bainitic microstructure, resulting in excellent flangeability and bendability.In addition, the newly developed 980 MPa AHSS has good fatigue and forming properties, making it suitable for the production of chassis and suspension components.

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.

The development of surface treatment products and technologies of steel sheets in Baosteel

The surface treatment technology can improve the corrosion resistance of steel sheets and increase the value of steel application.The surface treated steel sheets in Baosteel include zinc galvanized steel sheets,sin galvanized steel sheets,chromium galvanized steel sheets,and colour coating steel sheets with the application in the area from construction to automobiles,appliance,package,etc.The development,application and manufacturing technique of these products have become the core competition power of Baosteel. The article reviews the developments of surface treated steel sheets and surface treatment technologies in Baosteel in 10 years,especially hot dip galvanized high strength steel products used in vehicles.The article also looks forward into further developments of surface treatment steel products and technologies in Baosteel,which will focus on:①the new products of hot dip galvanized advanced high strength steels;②more environmental friendly products or functional coating products;③the development of surface treatment technologies to meet the requirement of higher surface quality in the marcket;④new type of surface treatment and coating technologies.

On-line control of surface quality for an axially moving steel strip partially immersed in fluid under tension

An on-line control method of surface quality for continuous hot-dip galvanized steel strip after cooling is presented, which combines analytical dynamics theory of a thin plate with the finite element method. The inherent characteristics of the non-immersed and partially immersed strip in liquid zinc were calculated on the basis of theoretical analysis and numerical simulation, respectively. Multi-parameter fitting of the deviation between results using different methods was performed. To optimize the strip excitation frequency away from the resonance region, on-line vibration control of the strip near the air knife under full product conditions was achieved by changing the field production parameters based on the field test results. The results indicate that although the axial velocity has little effect on the inherent characteristics of the strip compared with other manufacturing parameters such as the steel specifications and tension, it induces external excitations,including moving the aerodynamic load and bearing vibration. To some degree, the vibration near the air knife can be reduced by strengthening the support stiffness of the contact rolls. A total on-line control program of surface quality for continuous hot-dip galvanized pure Zn and galvannealed steel sheet in the cooling section is proposed.

桥梁缆索用87MnSi钢线材微观组织演变对力学性能的影响

针对桥梁缆索用87MnSi钢线材的异常断裂现象,对其盘条和生产工艺过程中钢丝的微观组织和力学性能演变规律进行系统分析。结果表明,盘条组织由珠光体和少量的先共析铁素体组成,且渗碳体片层随机分布。冷拉拔后,渗碳体片层发生扭转弯曲或减薄纤维化,且部分渗碳体发生破碎溶解,溶解量为1.40%(占比9.69%)。热镀锌后,溶解的渗碳体发生球化。盘条(329.8HV)在冷拉拔-热镀锌-稳定化处理过程中的显微硬度呈现先增大(450.2HV)后减小(447.9HV)再增大(469.6HV)的变化趋势;冷拉拔过程中溶解的渗碳体使得钢丝硬度值上升至468.3HV,而热镀锌过程渗碳体的球化,导致硬度值下降至439.8HV,并恶化拉伸性能。当边部的渗碳体球化程度明显时,使得钢丝内外组织和力学性能存在较大差异,并在稳定化处理过程中因受力不均匀而导致断裂。

热轧工艺对家电用热镀锌超深冲IF钢性能影响的研究

文章结合生产实践,研究了热轧关键工艺参数对家电用热镀锌超深冲IF钢性能的影响,分别开展了880℃和915℃两种精轧终轧温度、680℃和710℃两种卷取温度下试验钢的织构、金相组织及力学性能分析。研究结果揭示了不同热轧工艺参数与家电用热镀锌超深冲IF钢成品织构、金相组织及力学性能的相关性,为通过制定合理的热轧工艺来提高热镀锌超深冲IF钢产品性能提供了参考。

基于聚类和GBDT的镀锌钢卷力学性能预测

热镀锌钢卷力学性能影响因素之间关系复杂,限制了模型精度的提升。采用k-means算法利用化学成分属性对镀锌钢卷数据集进行聚类,将数据聚成三种模式簇实现样本的优选。利用梯度提升树算法,开展各模式数据集与不划分模式的全数据集下的力学性能建模研究,最后结合网格搜索与交叉验证方法进行模型参数优化。研究结果表明,分模式下模型MAE误差相比于全数据集建模平均减小0.85 MPa。参数优化后,各模式下MAE误差平均减少5.19 MPa,RMSE误差平均减少3.63 MPa,提高了预测模型精度。

770 MPa级调质铸钢液态金属致脆失效原因及相应防护对策

采用金相显微镜、电子扫描显微镜等对某大型截面铸钢工件裂纹成因进行了分析。结果表明:开裂高强度调质铸钢,在热浸锌过程中,基体产生了垂直于表面的裂纹,裂纹长度约为3 mm,且裂纹沿晶开裂。能谱结果表明,裂纹中富含大量锌,为典型的液态金属致脆现象。针对此情况,在热浸锌工艺、焊接工艺、成型工艺、涂装体系等方面给出了避免产生液态金属致脆的防护对策。

Hot Deformation and Corrosion Resistance of High-Strength Low-Alloy Steel

The hot deformation characteristics and the corrosion behavior of a high-strength low-alloy(HSLA) steel were investigated at deformation temperatures ranging from 800 to 1100 ℃ and strain rates ranging from 0.1 to 10 s-1 using an MMS-200 thermal simulation testing machine. Based on the flow curves from the experiment, the effects of temperature and strain rate on the dynamic recrystallization behavior were analyzed. The flow stress decreased with increasing deformation temperature and decreasing strain rate. With the assistance of the process parameters, constitutive equations were used to obtain the activation energy and hot working equation. The hot deformation activation energy of HSLA steel in this work was 351.87 kJ/mol. The work hardening rate was used to determine the critical stress(strain) or the peak stress(strain). The dependence of these characteristic values on the Zener-Hollomon parameter was found. A dynamic recrystallization kinetics model of the tested HSLA steel was constructed, and the validity of the model was confirmed by the experimental results. Observation of the microstructures indicated that the grain size increased with increasing deformation temperature,which led to a lowered corrosion resistance of the specimens.

双相钢热镀锌前热处理工艺研究

对DP600钢进行热镀锌前的热处理工艺研究,探讨不同再加热工艺条件下DP600钢的组织和力学性能。通过扫描电子显微镜和透射电子显微镜观察,分析快速冷却温度、再加热过程保温时间对组织演变的影响。拉伸试验表明,不同的微观结构组成表现出不同的力学性能和加工硬化行为,随着快速冷却温度从250℃降低到150℃,粒状残余奥氏体晶粒的体积分数增加,抗拉强度从658 MPa降低到643 MPa;快速冷却温度为200℃时,随着保温时间的增加,屈服强度(YS)、拉伸强度(TS)和屈服强度与拉伸强度的比值(YS/TS)均呈下降趋势。此外,在两阶段的加工硬化行为中,保温时间为10 s的样品与30 s和50 s样品相比,在阶段Ⅰ和阶段Ⅱ皆略高,这归因于未转化奥氏体在较短保温时间内稳定性较低,在随后的冷却过程中已转化为马氏体。

镀锌钢管环氧粉末涂层附着力的研究

简述了钢管热浸镀锌-环氧粉末复合防护涂层体系的特点,详细分析了钢管热镀锌后水冷及钝化、镀锌层表面处理对粉末涂层附着力的影响情况。分析认为,应根据镀锌层风化状态制定热镀锌水冷、钝化及表面处理流程及操作规程,建议采取清扫级喷砂清理清洁锌层表面,以获取合适的表面粗糙轮廓,改善粉末涂层附着力。

Q235钢和热镀锌钢在棕壤土环境中的腐蚀行为研究

为研究接地网设备常用金属材料Q235钢和热镀锌钢在棕壤土环境中的初期土壤腐蚀行为,在湖北襄阳地区选取东津、乔营站内外共4个土壤腐蚀实验点进行实验,采用宏观形貌分析、微观形貌分析、腐蚀产物分析和电化学测试等手段进行试样分析。形貌分析结果表明,襄阳土壤环境中碳钢试样腐蚀1 a后表面为非均匀腐蚀,可见明显的麻坑状腐蚀坑,镀锌钢材料未见明显红锈且表面镀锌层未被破坏。腐蚀产物分析表明,襄阳地区棕壤土为高含水率的碱性土壤,Q235钢土壤埋地试样1 a后表面锈层的主要成分铁氧化物和微量的SiO_(2),镀锌钢表面腐蚀产物主要为锌氧化物和微量SiO_(2)。腐蚀速率与土壤理化性质相关性分析结果表明,在襄阳土壤环境中,2种试样土壤腐蚀速率与土壤理化性质均呈正相关,其中土壤pH值对土壤腐蚀速率影响最为显著。电化学结果表明,镀锌钢表面镀锌层为基材提供良好的保护性,镀锌钢的腐蚀电流密度较Q235钢小1~2个数量级。综上,在襄阳地区棕壤土环境中镀锌钢材料较Q235钢材料表现出更为优异的耐蚀性,值得注意的是,该地区土壤pH值显著影响镀锌钢材料的土壤腐蚀速率。

420 MPa级热基镀锌钢板表面爆皮的原因分析

420 MPa级热基镀锌钢板表面产生爆皮缺陷。采用扫描电子显微镜、能谱仪和差热分析仪分析了镀锌板表面爆皮的特征和原因。结果表明:实质上爆皮是热轧板表面形成了疏松和分层的氧化铁皮,主要为FeO;该镀锌钢板的抗高温氧化性能不良,在990和1180℃左右的氧化质量增加速率急剧增大,导致钢板爆皮。通过优化粗轧机除鳞水、增加精轧机架间冷却水流量和调整卷取工艺等改进措施,表面发生爆皮的420 MPa级热基镀锌钢板大大减少。

热渗锌工艺对钢制螺栓力学性能的影响研究

为了研究热渗锌工艺对钢制螺栓的力学性能影响。通过硬度检测、拧脱耐磨试验、拉伸强度检测及微观分析等测试技术,分析评价了钢制螺栓表面热渗锌层的力学性能。结果显示,经热渗锌工艺处理后,钢制材料的组织结构及化学成分并没有改变,且经过处理后的螺栓抗拉强度均符合标准规定,表明热渗锌工艺对螺栓力学强度影响较小,但热渗锌工艺能够显著提升钢制螺栓的拧脱耐磨性能及显微硬度,这为钢制螺栓大规模使用热渗锌工艺提供了一种解决方案。

1960 MPa级Ф15.20 mm高强度低松弛预应力镀锌钢绞线研制

介绍1 960 MPa级Ф15.20 mm低松弛预应力镀锌钢绞线研发工艺。采用的Ф13.0 mm 82B专用盘条添加Cr和V元素,采用盐酸酸洗和锌系磷化表面处理工艺,中频加热温度380℃,张力120 kN,捻制速度40 m/min,生产的镀锌钢绞线力学性能和表面质量均符合GB/T 33363—2016要求,满足客户对产品的技术使用要求。