Hydrogen Embrittlement of Automotive Advanced High-Strength Steels

Advanced high-strength steels (AHSS) have a better combination between strength and ductility than conventional HSS, and higher crash resistances are obtained in concomitance with weight reduction of car structural components. These steels have been developed in the last few decades, and their use is rapidly increasing. Notwithstanding, some of their important features have to be still understood and studied in order to completely characterize their service behavior. In particular, the high mechanical resistance of AHSS makes hydrogen-related problems a great concern for this steel grade. This article investigates the hydrogen embrittlement (HE) of four AHSS steels. The behavior of one transformation induced plasticity (TRIP), two martensitic with different strength levels, and one hot-stamping steels has been studied using slow strain rate tensile (SSRT) tests on electrochemically hydrogenated notched samples. The embrittlement susceptibility of these AHSS steels has been correlated mainly to their strength level and to their microstructural features. Finally, the hydrogen critical concentrations for HE, established by SSRT tests, have been compared to hydrogen contents absorbed during the painting process of a body in white (BIW) structure, experimentally determined during a real cycle in an industrial plant.     

Influence of Pre-Deformation on Springback Effect of Advanced High Strength Steel

In sheet metal forming process of automotive components,the springback effect is significant,in particular for Advanced High Strength Steels (AHSS),for example the Dual Phase (DP) steel.Most of construction parts of modern vehicles have very complex shapes and therefore multi-step procedures are necessary to form such a part.Steel sheets,which firstly undergo pre-deformation,can show considerable change in mechanical behavior during the forming process.However,at present there are limited sufficient data concerning pre-deformation effect on the springback available.In this work,a study of influences of different pre-strain levels on the springback of steel sheet made of AHSS materials has been carried out.The sheet specimens were firstly pre-stretched on a tensile testing machine and the pre-strain values were calculated based on the engineering strain.Furthermore,the steel sheets prepared parallel,transverse,and 45° to the rolling direction have been investigated.A modified U-shape forming was used to evaluate the degree of springback of the steel sheets under various conditions.In parallel,FE simulation of the U-shape forming was performed.Both isotropic model using stress-strain responses from tensile test of specimens with different directions and anisotropic Hill’s 48 model have been applied.The experimental results are compared with the sheet metal forming FE simulations.The primarily aim is to basically understand the springback mechanism by means of the simple models.And finally,conclusions with regard to the springback modeling will be presented.     

Analysis of Hot- and Cold-Rolled Loads in Medium-Mn TRIP Steels

Metallurgical and Materials Transactions B - The purpose of this work is to investigate the hot- and cold-rolling requirements to produce third-generation advanced high-strength steels (AHSS)....     

Ultra-high strength sheet steel property developments

Conventional advanced high strength steels(AHSS) such as dual-phase,TRIP and martensitic sheet products have undergone extensive development and substantially increased implementation in automobile structures over the past few years.Efforts are now underway to extend these improvements and develop steels with even greater strengths and formability.A variety of alloy/process/microstructure/property approaches aimed at next generation AHSS properties are highlighted here.One approach involves the quenching...     

Correlation of Local Constitutive Properties to Global Mechanical Performance of Advanced High-Strength Steel Spot Welds

Metallurgical and Materials Transactions A - Recent results have shown that the strength of resistance spot-welded joints made from advanced high-strength steels (AHSS) do not increase linearly...     

Experimental Analysis and Prediction of Springback in V-bending Process of High-Tensile Strength Steels

Experimental setup has been designed, to study the effects of thickness, width, bend angle and machine tool parameters on the springback, in two high-tensile strength steel grades, namely JSC440 and JSC590, during the V-bending process. Relationship between the springback and the parameters are analyzed using plots. Optimal combination of parameters for the minimum springback is evaluated. Analysis of variance has been carried out to analyze the magnitude of influence of these parameters on the springback. Using the experimental results, analytical models for the prediction of springback for the combinations of blank thickness, width, bend angle and machine tool parameters have been developed. Results reveal that in V-bending of JSC440, thickness and width are the dominant factors influencing the springback, whereas in JSC590 steel, insignificant change in springback is observed with the change in width of blank and using the hydraulic press with holding. However, thickness of steel sheet and bend angle influence significantly the springback in JSC590 steels.

     

Role of Niobium in Advanced High Strength Steels for Automotive Applications

Two major drivers for the use of newer steels in the automotive industry is fuel efficiency and increased safety performance.Fuel efficiency is mainly a function of weight of steel parts,which in turn,is controlled by gauge and design.Safety is determined by the energy absorbing capacity of the steel used to make the part.All of these factors are incentives for the U.S.automakers to use Advanced High Strength Steels (AHSS) to replace the conventional steels used to manufacture automotive parts in the past.AHSS is a general term used to describe various families of steels.The most common AHSS is the dual-phase steel that consists of a ferrite-martensite microstructure.These steels are characterized by high strength,good ductility,low tensile to yield strength ratio and high bake-hardenability.Another class of AHSS is the multi-phase steel which have a complex microstructure consisting of various phase constituents and a high yield to tensile strength ratio.Transformation Induced Plasticity (TRIP) steels is the latest class of AHSS steels finding interest among the U.S.automakers.These steels consist of a ferrite-bainite microstructure with significant amount of retained austenite phase and show the highest combination of strength and elongation,so far,among the AHSS in use.High level of energy absorbing capacity combined with a sustained level of high n value up to the limit of uniform elongation as well as high bake hardenability make these steels particularly attractive for safety critical parts and parts needing complex forming.Finally,martensitic steels with very high strengths are also in use for certain parts.The role of Niobium in all of the above families of advanced steels for the automotive industry will be discussed in this paper.     

微合金高强钢纳米相间析出行为研究进展

 提高强度的同时不降低韧性,是人们在高强钢研发过程中追求的目标。相较于其他强化方式,细化晶粒可以使材料的强度和韧性同时提高,但目前为止钢铁材料晶粒尺寸最小可控制到3～5 μm,所能带来的强化效果有限。新型铁素体基高强钢通过相间析出使铁素体基体上分布着有规则排列的纳米尺寸碳化物,大大提高强韧性能、可成型性和焊接性,广泛应用于工程机械、石油管线、汽车零部件以及高层建筑领域。近几年随着钢铁行业的发展,对于相间析出的了解也越来也深入,各种微合金钢的纳米相间析出特征已有大量报道,主要集中在析出物微观结构特征与强度贡献的研究上。然而,相间析出机制以及模型还存在许多争议,通过工艺控制实现稳定的相间析出还比较困难,纳米相间析出高强钢的性能稳定性还较差。随着科技的发展,相关先进分析仪器也在更新换代,为新型铁素体基高强钢纳米尺度碳化物相间析出行为的深入研究提供了条件。综述了铁素体基高强钢纳米尺度碳化物相间析出的国内外研究动态,阐明了台阶理论与溶质消耗理论的局限性;对铁素体基高强钢的成分设计进行了分析,揭示了合金化方式对相间析出行为的影响规律;介绍了目前国际上研究纳米尺度碳化物相间析出行为的先进仪器及技术,最后对新型铁素体基高强钢的发展进行了展望,指出复合微合金化表现出的优势将会是以后微合金钢发展的必然趋势。     

首钢热轧酸洗先进高强钢的开发与进展

介绍了首钢热轧酸洗先进高强钢的开发与进展。根据客户的使用要求,结合生产设备,首钢研发了双相钢、高扩孔钢、复相钢、相变诱导塑性钢等系列高强钢。分析了双相钢、高扩孔钢、复相钢以及相变诱导塑性钢等产品的化学成分和力学性能,并利用金相显微镜和扫描电镜等手段对微观组织特征进行了描述。首钢热轧酸洗先进高强钢因出色的力学性能已经在汽车控制臂和纵梁等零件上得到广泛应用,未来将在乘用车减重和安全等方面发挥更大作用。     

Influence of strain rate on tensile properties and fracture behaviour of DP600 and DP780 dual-phase steels

The influences of the strain rate on the tensile properties and fracture behaviour of DP600 and DP780 advanced high-strength sheet steels have been studied. The variation of their mechanical properties depending on the strain rate have been researched by applying uniaxial tensile tests at three different strain rates (0.001, 0.01, 0.06?s^?1). The influences of strain rate on fracture behaviour have been investigated by displaying the fracture surfaces of the material. Strain rate increase has been determined to increase the yield strength, tensile strength, total elongation and hardening rate. The strain hardening coefficient has been found not to be significantly affected by the strain rate. It has been determined that, the fracture has occurred faster during necking while load-carrying capacity has increased with strain rate increase.     

Cold-rolling technologies of advanced high strength steels in Ansteel

Low - carbon becomes a high - frequency and fashionable word which gets the greatest concern in the world.Low - carbon refers to a minimal output of greenhouse gas emissions into the biosphere, specifically refers to the greenhouse gas carbon dioxide.To reduce energy consumption of automobile,more and more high strength steels are used by vehicle companies.To meet the request of vehicle companies, various high strength steels are developed in steel companies all over the world.Ansteel can provide,ultra-low carbon bake hardening(BH) steels,dual phase(DP) steels and transformation - induced plasticity (TRIP) steels with the grades of under 780 MPa,up to now.AHSS steels have much different composition, microstructure and strenght than conventional vehicle steels,so there are some trouble in producing in cold rolling mills,for example,difficult to join,poor thickness and flatness,accurate temperature and velocity control and so on.To reduce the opportunity of strip breakage,we have done many welding experiments and special research.Now,DP and TRIP steels can be continuously produced in Ansteel.To assure thickness and flatness of strips,we optimized the hot rolling parameter to get low deformation resistance,optimized rolling oil to get fit frictional coefficient and fix on the targat rolling curve.To get more accurate and repetitive results over the production shifts,the Mathematical Model(MM) is used in Ansteel.The MM analyses information transmitted by various sensors and transmitters,compares the collected data with the chosen parameters and adjusts the settings of the various pieces of equipment to hone on the parameter setpoints.At present,the key production technologies of AHSS were grasped by cold rolling mill Ansteel,and Ansteel is the first company to apply the TRIP with the grade of 780 MPa.     

A Review of the Physical Metallurgy related to the Hot Press Forming of Advanced High Strength Steel

The automotive industry requirements for vehicle weight reduction, weight containment, improved part functionality and passenger safety have resulted in the increased use of steel grades with a fully martensitic microstructure. These steel grades are essential to improve the anti‐intrusion resistance of automotive body parts and the related passenger safety during car collisions. Standard advanced high strength steel (AHSS) grades are notoriously difficult to be press formed; they are characterized by elastic springback, poor stretch flangeability and low hole expansion ratios. Hot press forming has therefore received much attention recently as an alternative technology to produce AHSS automotive parts. In this contribution, the physical metallurgy principles of the hot press forming process are reviewed. The effect of composition on CCT curves of standard CMnB hot press forming steels is discussed taking the deformation during press forming into account. Furthermore,the effect of the static strain ageing processes occurring during the paint baking cycle on the in‐service mechanical properties of press hardened steel will be presented. The influence of temperate and strain rate on the flow stress during press forming and the final room temperature mechanical properties will be discuss ed. Moreover, the issues related to coatings on B‐alloyed CMn hot press forming steel will be critically reviewed. In particular the combined effects of thermal cycle and deformation on the degradation of the Al‐10%Si coating will be discussed in detail. Finally, the properties of both Al‐based and Zn‐based coating systems are compared, and the possibility of the formation of a diffusion barrier during press forming is discussed.     

Shear Fracture of Advanced High Strength Steels

Failure experiments were carried out through a stretch-bending test system for advanced high strength steels, i.e. dual-phase （DP） steels and martensitic steels （MS）. The die radius in this system was designed from 1 to 15 mm to investigate the failure mode under different geometries. Two failure modes were observed during the ex- periments. As a result, critical relative radii （the ratio of inner bending radius R to sheet thickness t） for DP590 and DP780 steels were obtained. The stretch-bending tests of DP980 display some trends unlike DP590 and DP780 steels, and curve of DP980 in different thicknesses does not coincide well. High blank holder force exhibits more possibility of shear fracture tendency than low blank holder force. The unique character of high strength martensitic steel （1500MS） is that no shear fracture is found especially over small bending radius （R =2 mm） under the same experi- mental conditions. Microstructure analysis indicates that there are obviously elongated grains on shear fracture sur- face. It shows smaller diameter and shallower depth of the dimples than the necking failure.     

Tensile and Springback Behavior of DP600 Advanced High Strength Steel at Warm Temperatures

 In recent years, the use of advanced high strength steels in automotive industry has been increased remarkably. From advanced high strength steels, dual-phase (DP) steels have gained a great attention due to a combination of high strength and good formability. However, high strength usually increases springback behavior of material which creates problem for the parts during the assembly. In this study, uniaxial tensile deformation and springback behaviors of DP600 advanced high strength steel were investigated at rolling (0o), diagonal (45o), and transverse (90o) directions in the temperature range from room (RT) to 300 oC. All tests were performed at 25 mm/min deformation speed. A V-shape die (60o) was used for springback measurements. Results indicate that the formability and springback of the material were decreased with increasing the temperatures. The material showed complex behaviors at different directions and temperatures.     

高强钢损伤容限的微观结构设计

先进高强钢以其优良的机械性能,在生产生活的各个领域得到了广泛的应用。近年来,工业上对先进高强钢损伤的关注日益增加。对于这种先进高强钢,由于它复杂的微观结构及相关的变形机制,损伤概念需重新审定。概要介绍了损伤的概念、尺度及损伤机制,对比了材料损伤的实验评估及数值模型,并基于容损设计方法给出了提高材料损伤容限的理论。     

Effect of Aluminum Content on the Dynamic Recrystallization of Fe18MnxAl0.74C Steels During Hot-Forging Treatments

Metallurgical and Materials Transactions A - In the present work, the dynamic recrystallization and microstructural evolution of the family of advanced high-strength steels Fe18MnxAl0.74C are...     

Design of microstructure for damage tolerance in high strength steels

With the extensively wide application of advanced high strength steels(AHSS) in various fields for the excellent mechanical properties,the industrial interest on the damage of AHSS is increasing in the recent years.For these modern steels,due to the complex microstructure and the relevant deformation mechanisms,the damage concept needs to be reexamined.In this paper,the definition,length scale and different mechanisms of damage are introduced.Both experimental evaluation of damage and the numerical damage models are briefly viewed and compared.The approaches to improve the damage tolerance are given in the framework of damage tolerance design principle.     

Robustness of the sheet metal springback cup test

The robustness of a proposed test for elastic springback characterization of sheet metal has been examined using a matrix of defined experimental errors. A series of flat bottom deep drawn cups made from AISI 1010 steel sheet were examined. It was found that misalignment of the blank over the forming tool and error in the vertical location where the springback ring was cut from the cup sidewall had the largest effect on the resulting springback opening. Other experimental errors involving cup height and ring width were found to be less important. The effect of in-plane anisotropy of mechanical properties on springback was negligible. The results are examined in terms of measured through thickness residual stresses and elastic bending of beams with circumferential thickness gradients.     

High-strain-rate behavior of low-alloy multiphase aluminum- and silicon-based transformation-induced plasticity steels

High-strength, low-alloy transformation-induced plasticity (TRIP) steels are advanced multiphase steel grades that combine high-strength levels with an excellent ductility, making them ideally suited for application in crash-relevant parts of automotive car bodies. The enhanced plastic hardening and deformability are due to a complex interaction between the microstructural phases and to the transformation of metastable austenite to martensite during plastic deformation. During high-strain-rate loading, not only the material but also the transformation will be influenced by adiabatic heating. The impact-dynamic properties of CMnAl- and CMnSi-TRIP steels were determined in the range of 500 to 2000 s⁻¹ using a split Hopkinson tensile bar (SHTB) setup. Bake-hardening treatments were applied to study the effect of strain aging. The experiments show that strain-rate hardening is superior to thermal softening: yield stresses, deformation, and energy dissipation increase with the strain rate. Phenomenological material models were investigated to describe the strain-rate and temperature-dependent behavior of TRIP steels. Both the Johnson-Cook model and an extended version of the Ludwig model were found to give good agreement with the experimental data.     

Strain Limit of Extra Galvannealed InterstitialFree and Bake Hardened Steel Sheets Under Different Stress Conditions

The formability of bake hardened steel （thickness 0.82 mm）, and the extra galvannealed IF steel （thickness 0.82 mm） have been studied. The suitability of the above steels for forming applications has been critically examined. The microstructure, tensile properties, and formability parameters of the above sheet metals were determined. The manufacturing process of the steels and the significance with reference to its formability were studied.