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.

Improvement of the microstructural features and mechanical properties of advanced high-strength steel DP590 welds

The effects of the welding current mode in resistance spot welding on the microstructure and mechanical properties of advanced high-strength steel dual-phase 590(DP590)sheets were investigated.Results showed that a rough martensitic structure was formed in the weld zone of the sample welded via the single-pulsed mode,whereas the microstructure in the heat-affected zone consisted of a very rough martensitic microstructure and rough ferrite.However,using the secondary pulse mode led to the formation of tempered martensite in the weld zone.The maximum load and the energy absorption to failure of the samples with the secondary pulsed cycle were higher than those of the samples with the single-pulsed mode.Tensile shear results indicated that the secondary pulsed mode could significantly change the mode of failure upon shear tension testing.Therefore,the obtained results suggest that the use of secondary pulsed mode can improve the microstructural feature and mechanical properties of advanced high-strength steel DP590 welds.

Delayed fracture of tensile strength 980 MPa grade steels for automotive applications

Three different types of tensile strength （TS） 980 MPa grade advanced high-strength steels used in automotive applications, namely, 980MS （ martensite steel ）, 980DP （ dual phase ） and 980QP （ quenching and partitioning） steels were examined. The delayed fracture resistance of the steels was evaluated using a U-bend test, slow strain rate test（SSRT） and a constant load tensile test. The results indicated that all the steels could pass the 300 h HC1 solution immersion test and none of the U-bend specimens was fractured in the test. However,the steels exhibited different susceptibilities to delayed fracture under SSRT and the constant load tensile tests. 980DP exhibited the highest resistance to delayed fracture among all the samples, while 980MS was found to be the most susceptible to delayed fracture.

Current Challenges and Opportunities Toward Understanding Hydrogen Embrittlement Mechanisms in Advanced High-Strength Steels: A Review

Hydrogen embrittlement(HE)is one of the most dangerous yet most elusive embrittlement problems in metallic materials.Advanced high-strength steels(AHSS)are particularly prone to HE,as evidenced by the serious degradation of their load-bearing capacity with the presence of typically only a few parts-per-million H.This strongly impedes their further development and application and could set an abrupt halt for the weight reduction strategies pursued globally in the automotive industry.It is thus important to understand the HE mechanisms in this material class,in order to develop effective H-resistant strategies.Here,we review the related research in this field,with the purpose to highlight the recent progress,and more importantly,the current challenges toward understanding the fundamental HE mechanisms in modern AHSS.The review starts with a brief introduction of current HE models,followed by an overview of the state-of-the-art micromechanical testing techniques dedicated for HE study.Finally,the reported HE phenomena in different types of AHSS are critically reviewed.Focuses are particularly placed on two representative multiphase steels,i.e.,ferrite–martensite dual-phase steels and ferrite–austenite medium-Mn steels,with the aim to highlight the multiple dimensions of complexity of HE mechanisms in complex AHSS.Based on this,open scientific questions and the critical challenges in this field are discussed to guide future research efforts.

A comparison study on interfacial properties of fluorine-bearing and fluorine-free mold flux for casting advanced high-strength steels

A comparison study on interfacial properties of a traditional fluorine-bearing(F-bearing)mold flux and a newly designed fluorine-free(F-free)mold flux to produce advanced high-strength steels(AHSSs)by compact strip production technology was conducted.The results showed that these two kinds of mold fluxes gradually spread out on the typical AHSS substrate when slags began to melt with the increase in heating temperature,and they had a good interfacial ability between the two mold fluxes and the AHSS substrate,and there was no other interfacial reaction except the oxidization of steel substrate by the mold fluxes.In comparison,the wettability of the designed F-free mold flux with the AHSS substrate was better than that of the F-bearing mold flux.The reason could be explained as the addition of B_(2)O_(3) would increase the complexity and polymerization degree of the melt structure and weaken the attractive force between the ions and ionic groups,then leading to a better wettability.Besides,B_(2)O_(3) is an effective flux,which can reduce the melting temperature obviously,and the surface tension of the liquid F-free mold flux would get reduced with the addition of B_(2)O_(3).

高强钢材料性能对车身零件冲压回弹的影响

根据材料性能波动对某冲压件回弹的影响,结合某汽车车身高强钢零件的冲压结果,借助有限元仿真工具,比较了该零件的实验试冲结果与仿真分析结果。在两结果基本一致的基础上,研究了高强度钢板的屈服强度、应变硬化指数、摩擦因数及板料厚度的波动对该零件回弹影响的规律。应用带一个中心点的部分因子实验设计方法,考察了主要因素的影响规律,结果表明,对回弹波动的影响程度由强至弱依次为屈服强度、板料摩擦性能、硬化指数、板料厚度。

新能源电动汽车异种材料连接技术的挑战、趋势和进展

多材料混合结构在车身上的应用可以实现汽车安全性、轻量化水平的共同提升,也是如今新能源汽车工业发展的一个主要方向。先进高强钢(Advanced high strength steel,AHSS)、铝合金、工程塑料以及碳纤维增强复合材料(Carbon fiber reinforced polymer,CFRP)等作为轻质高强材料的代表已广泛应用于白车身、覆盖件以及复杂结构件的制造中。

AZ31镁合金与DP980高强钢的纯电塑性效应实验研究

通过单向拉伸试验研究了AZ31镁合金和DP980高强钢两种轻型材料的电塑性效应,研究表明:AZ31镁合金存在纯电塑性效应,而DP980高强钢不存在明显的纯电塑性效应。通过分析峰值电流密度和频率对AZ31镁合金流动应力的影响规律,在Fields-Backofen模型基础上建立了考虑电塑性效应的流动应力方程,此方程能很好的拟合小变形阶段的流变行为。

先进短流程——深加工新技术与高强塑性汽车构件的开发

在近年来国内外薄板坯连铸连轧TSCR和先进热成形处理AHFT技术发展的基础上,提出了先进的短流程与深加工技术相结合的工艺途径,为高强塑性汽车构件的生产制造开发了一种高效率、低能耗、低排放、低成本的新工艺。首先采用先进短流程工艺,包括以CSP、FTSR、ISP半无头轧制为主要特征的第2代TSCR技术和以ESP无头轧制技术为主要特征的第3代TSCR技术,生产高强度薄规格汽车板作为热冲压成形的原料;然后采用先进热成形处理AHFT技术,对短流程薄规格热轧酸洗板进行热冲压与热处理相结合的深加工,制作高强塑性汽车构件。本文简要介绍了短流程薄板坯连铸连轧TSCR技术的发展,先进热成形处理AHFT技术的强塑化工艺与主要技术特征。讨论了采用短流程TSCR新工艺生产先进高强度钢AHSS薄规格汽车板,为先进热成形处理AHFT提供热冲压成形板料的主要技术关键。介绍了在超短流程的双辊薄带连铸工艺线上开发高强塑性TWIP钢的试验进展。短流程TSCR新工艺与先进热成形处理AHFT深加工新技术相结合,开发与生产超高强塑性汽车构件,需要钢铁与汽车行业的密切合作。这项短流程—深加工新技术不仅可以满足新一代汽车对轻量化节能减排和抗冲撞安全的要求,而且可以显著降低汽车板构件生产制造过程中的能耗与温室气体排放。

我国汽车板生产现状及展望

介绍了当前汽车业主流车型对汽车板的要求和使用情况,新一代超轻钢和未来钢汽车在节能、环保、安全、舒适方面的结构特征,及其对各种新型汽车材料的技术要求和近期市场需求。概述了我国汽车板生产现状和宝钢、武钢、鞍钢、首钢等主要厂家的技术和产品发展。介绍了高品质和高等级汽车板的发展趋势与关键技术,包括冷轧汽车外板的典型表面缺陷形成机理与工艺控制,先进高强度钢AHSS汽车板的强塑化机理与连续退火工艺控制,以及热镀锌高强钢汽车板的发展。指出了利用短流程CSP—冷轧—罩式退火工艺中AlN固溶析出和{111}//RP织构发展,生产高r值冷轧深冲板的工艺优势。简述了汽车板深加工技术和产品开发的新进展——热冲压成形。指出我国汽车板生产面临着3个方面技术挑战:产品系列化和均质化、深加工与先期介入、新技术新产品的原创发明。

1200MPa级超高强度钢板冲压成形性能及其断裂机理的实验研究

通过圆筒拉深试验研究了1 200 MPa级超高强度双相钢板(DP1200)和纯马氏体钢板(M1200)的极限拉深成形性能,采用扫描电子显微镜观察分析了钢板的微观组织形貌与结构,探讨了其微观组织结构与断裂机理的关系.结果表明:DP1200和M1200的极限拉延比分别为2.03和1.99,其单向拉伸性能不能准确地反映钢板材料在复杂应力状态下的成形性能;2种钢板的断裂模式均为断口分布着大量韧窝的韧性断裂,但其裂纹扩展机理不同.DP1200中的裂纹是以沿铁素体/马氏体相界为主,兼有穿过马氏体相并扩展的失效模式;而M1200钢中的裂纹是沿马氏体晶界扩展的失效模式.

面向汽车轻量化的先进高强度钢成型技术

汽车轻量化是现代汽车节能减排的重要方向,采用先进高强度钢是其主要途径,当前的技术难点在于先进高强度钢成型困难。本文详细分析五种先进高强度钢的机理特性、成型技术及其应用。

基于数字图像相关法的第三代先进高强钢QP980成形性能评价

对第三代先进高强钢QP980和第一代先进高强钢DP980的成形性能进行分析研究。采用数字化图像相关方法(DIC)进行标准试样的单向拉伸试验,对变形区域全场轴向应变和横向应变进行分析,并比较了标距区域和变形区域最大点的应力—应变曲线;基于Marciniak方法采用DIC方法进行两种材料成形极限试验,并对材料在不同应变下的残余奥氏体含量进行了测试。结果表明:QP980的综合成形性明显高于DP980,表现为QP980材料的均匀延伸率高出DP980约9%,达到17.26%;加工硬化指数接近0.18,比DP980高80%;成形极限FLD0为0.18,比DP980高出50%;在均匀变形阶段,QP980中大部分残余奥氏体发生向马氏体的转化,提高其加工硬化能力,表现为比DP980材料更高的均匀延伸率;但进入局部变形阶段后,残余奥氏体含量很低,继续变形相变诱导强化能力明显减弱,表现为两种材料相当的局部变形能力。

超高强度钢薄板扭曲回弹特性的数值模拟与试验分析

为了研究超高强度钢复杂件冲压过程中出现的扭曲回弹现象,设计了可用同一副模具成形的两端开放和一端封闭的2类S梁零件,对其成形后的扭曲回弹特性进行了数值模拟和试验验证.提出了零件整体最大扭曲量的概念,并用其作为评价扭曲回弹程度的指标,分析了压边力、摩擦以及板料轧制方向对于零件扭曲程度的影响.结果表明,在所研究的压边力范围内,扭曲程度随着压边力的减小而减小,摩擦对于2类S梁零件的扭曲回弹的影响呈现不同的规律,在较大压边力和摩擦系数下,采用垂直轧制方向板料成形导致更大的扭曲.研究结果可为预测与减少复杂零件拉深成形扭曲回弹控制提供有益的指导.

高强度钢辊弯成型回弹分析

随着计算机性能的提高,试制成本的降低也带动了板料成形费用的降低。现在,在真正的金属成型生产之前,应用计算机对成型过程完全仿真,可优化成型的条件。成型预测技术发展过程中,在成型包括先进高强度钢内的许多金属出现了回弹问题。回弹是一种内部应力释放的过程,它对金属成型的精度有很大影响。尤其对一些加工硬化明显的材料,如高强度钢,回弹成为金属成型仿真的难点问题。本文对辊弯成型中使用高强度钢时回弹做了初步的有限元仿真分析,得出模型参数对回弹影响的基本规律。

Effect of Heat Treatment on Microstructures and Mechanical Properties of Hot-Dip Galvanized DP Steels

In order to simulate the hot-dipped galvanizing of dual-phase （DP） steel （wt%） 0.15C-0.1Si-1.7Mn, the DP steels were obtained by different annealing schedules. The effects of soaking temperature, time, and cooling rate on ferrite grain, volume fraction of martensite, and the fine structure of martensite were studied. Results showed that the yield strength （YS） of DP steel is sensitive to annealing schedule, while total elongation has no noticeable dependence on annealing schedule. Increasing soaking temperature from 790 to 850 ℃, the YS is the lowest at soaking temperature of 850 ℃. Changing CR1 from 6 to 24 ℃/s, the YS is the highest when CR1 is 12 ℃/s. Increasing soaking time from 30 to 100 s, the YS is the lowest at soaking time of 100 s. Besides, it was found that sufficient movable dislocations within ferrite grains and high martensite volume fraction can eliminate yield point elongation, decrease the YS, and increase ultimate tensile strength. Through TEM observations, it was also found that increasing annealing temperature promotes austenite transformation into twin martensite, and increases volume fraction of martensite at sufficient cooling rate. With increasing the martensite volume fraction, the deformation substructure in the ferrite is well developed.

A review of friction stir welding of steels： Tool, material flow, microstructure, and properties

Considerable progress has been achieved in friction stir welding （FSW） of steels in every aspect of tool fab- rication, microstructure control and properties evaluation in the past two decades. With the development of reliable welding tools and precise control systems, FSW of steels has reached a new level of technical maturity. High-quality, long welds can be produced in many engineering steels. Compared to traditional fusion welding, FSW exhibits unique advantages producing joints with better properties. As a result of active control of the welding temperature and/or cooling rate, FSW has the capability of fabricating steel joints with excellent toughness and strength. For example, unfavorable phase transformations that usu- ally occur during traditional welding can be avoided and favorable phase fractions in advanced steels can be maintained in the weld zone thus avoiding the typical property degradations associated with fusion welding. If phase transformations do occur during FSW of thick steels, optimization of microstructure and properties can be attained by controlling the heat input and post-weld cooling rate.

冷却制度对铸态先进高强度钢组织及断裂韧性的影响

通过设计一套可开模的水冷铜模装置,研究了不同连续冷却工艺对先进高强度钢显微组织和断裂韧性的影响。为研究试验钢的裂纹形成及扩展机理,对试样中不同类型夹杂物在基体中所产生的径向应力进行了计算以及应用了一种新方法来表征试验钢裂纹的扩展行为。结果表明:试验钢在固相线附近进行缓慢冷却有利于形成更多的MnS+Al2O3复合夹杂物,与单一的Al2O3夹杂相比,复合夹杂周围的径向应力明显降低。试验钢在临界铁素体转变温度以下缓慢冷却,有助于获得更多的晶内多边形铁素体,晶内多边形铁素体对裂纹扩展有抑制作用,因此试验钢的断裂韧性更高。

Influence of lanthanum on inclusions and as-cast microstructures in a low-alloy high-strength steel

Advanced high-strength steel ingots with total lanthanum(TLa)contents of 0,15×10^(–6),86×10^(–6)and 360×10^(–6)were prepared through laboratory experiments.The modification of inclusions and the variation of the as-cast microstructure with the content of lanthanum in the high-strength steel were analyzed.The result showed that with the increase in the TLa content in the steel from 0 to 360×10^(–6),the modification path of inclusions in the as-cast steel was Al2O_(3)and calcium aluminate→LaAlO_(3)→La_(2)O_(2)S→La_(2)O_(2)S–La2O_(3).The addition of La in the high-strength steel significantly refined the solidification structures.With the increase in the TLa content in the steel from 0 to 360×10^(–6),the ratio of the equiaxed crystal region in the macrostructure increased from 30.1%to 50.7%,the proportion of the high-angle grain boundary in the microstructure increased from 36.9%to 69.8%,and the area fraction of the acicular ferrite and the bainite increased from 0 to 93.3%.Inclusions of LaAlO_(3),La_(2)O_(2)S and La2O_(3)in the La-containing steel could act as heterogeneous nucleation cores ofα-Fe during the solidification.With the increase in the TLa content in the steel,the number density of inclusions that could act as effective heterogeneous nucleation cores in the steel gradually increased,which enlarged the ratio of the equiaxed crystal region and the proportion of intragranular acicular ferrite,and refined the as-cast microstructure of the high-strength steel.

几种典型第三代汽车用先进高强度钢技术浅析

针对第一代先进高强度钢塑性低与第二代先进高强度钢成本高、工艺性较差,国内外对第三代先进高强度钢开展了大量研究。通过对相关报道分析,第三代先进高强度钢是以高强度BCC结构为基体,在基体上有一定量的较高稳定性FCC结构的复合组织。对第三代先进高强度钢如δ-TRIP钢、DSHT工艺、纳米贝氏体钢、中锰TRIP钢、Q&P钢、HS-Q&P钢进行了介绍及展望。