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.     

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.     

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。     

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.     

汽车用先进高强度钢的开发及应用进展

 先进的高强度钢在汽车减重、节能、提高安全性、降低排放等方面展现出了广阔的前景,在新一代汽车伙伴计划、超轻钢车身——先进概念车等项目上得到了应用和推广。简述了先进高强度汽车用钢板的最新开发和应用进展情况。     

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)....     

Fracture Toughness to Understand Stretch-Flangeability and Edge Cracking Resistance in AHSS

The edge fracture is considered as a high risk for automotive parts, especially for parts made of advanced high strength steels (AHSS). The limited ductility of AHSS makes them more sensitive to the edge damage. The traditional approaches, such as those based on ductility measurements or forming limit diagrams, are unable to predict this type of fractures. Thus, stretch-flangeability has become an important formability parameter in addition to tensile and formability properties. The damage induced in sheared edges in AHSS parts affects stretch-flangeability, because the generated microcracks propagate from the edge. Accordingly, a fracture mechanics approach may be followed to characterize the crack propagation resistance. With this aim, this work addresses the applicability of fracture toughness as a tool to understand crack-related problems, as stretch-flangeability and edge cracking, in different AHSS grades. Fracture toughness was determined by following the essential work of fracture methodology and stretch-flangeability was characterized by means of hole expansions tests. Results show a good correlation between stretch-flangeability and fracture toughness. It allows postulating fracture toughness, measured by the essential work of fracture methodology, as a key material property to rationalize crack propagation phenomena in AHSS.     

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...     

Microstructural effects on the springback of advanced high-strength steel

The application of advanced high-strength steels (AHSS) has been growing rapidly in the automotive industry. Because of their high-strength, thinner sheet metals can be used for body components to achieve both weight savings and increased safety. However, this will lead to greater springback deviation from design after the forming operation. Fundamental understanding and prediction of springback are required for springback compensation and tooling design. While various types of continuum mechanics based models have been proposed to simulate the mechanical behavior of advanced high-strength steels, few of them consider microstructural effects such as material heterogeneity. In this study, through sheet thickness strength variation has been observed in DP 780 and TRIP 780 steels. Finite-element simulation indicates that the through thickness effect (TTE) can have a significant impact on the springback behavior of these sheet metals. This is verified through our experimental work using draw-bend testing. The results suggest that microstructural effects should be considered to accurately simulate springback of AHSS. Based on these results, implications of different microstructural designs will be discussed.     

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.     

国外新型汽车用钢的技术要求及研究开发现状

为了满足未来新一代汽车工业发展的需求,开发了大量的新钢铁材料,如超高强钢(AHSS)、高强钢(HSS)、双相钢(DP钢)、TRIP钢,TWIP钢等。介绍了这些钢铁材料,给出并分析了它们的成分、结构、工艺、性能和特点。     

Comparative study of resistance spot welding performance between cold-rolled DP980 and Q&P980 steels

Advanced high strength steel (AHSS) has been widely used in the automobile industry.The resistance spot welding performance of DP980 and Q&P980 steels was studied through comparing the two steels’welding current range,tensile shear strength (TSS),cross tension strength (CTS),weld spots’microhardness,etc.The following conclusions were achieved:It is easy for both DP980 and Q&P980 steels to get a nugget size bigger than 4 mm,they all have welding current ranges exceeding 2 kA and high weld strength.     

钒微合金化钢的技术进展与应用

介绍了钒微合金化技术的最新进展以及钒钢的开发与应用情况。氮是含钒钢中有效的合金元素,含钒钢中增氮,优化了钒在钢中的析出,显著提高沉淀强化效果。采用钒氮微合金化设计,配合适当的轧制工艺,促进V(C,N)在奥氏体中析出,起到了晶内铁素体形核核心作用,实现了含钒钢的晶粒细化。最新的研究成果表明钒微合金化可以提高双相钢、贝氏体钢、相变诱导塑性钢、孪晶诱导塑性钢、热成型马氏体钢等汽车用先进高强度钢的强度并改善使用性能,显示出良好的应用前景。钒氮微合金化技术在中国高强度钢筋、高强度型钢、非调质钢、薄板坯连铸连轧高强度带钢等产品中获得广泛应用,大大促进了中国钒微合金化钢的发展。     

Influence of Thermo‐Mechanical Processing Parameters and Chemical Composition on Bake Hardening Ability of Hot Rolled Martensitic Steels

In recent years, the increasing demand for advanced high‐strength steels (AHSS) has mainly been driven by the automotive industry and the need to reduce weight and to improve safety. Besides good ductility and high strength, AHSS have a high bake hardening and ageing effect, giving additional contribution to the strength of structural parts, subjected to the paint baking process. This paper investigates their bake hardening behaviour in dependence of hot rolling parameters and chemical composition, however, focussing on martensitic steels. Tosimulate the finishing steps of the hot rolling process with slight changes in reduction and temperature and their influence on the final mechanical properties of hot rolled martensitic steels, different thermo‐mechanical paths were applied. The increase in strength due to bake hardening was determined for different thermo‐mechanical schedules. Additionally, samples of different chemical compositions within the characteristic industrial tolerance range were studied under variation of pre‐load conditions, simulating the thermo‐mechanical hot rolling process. The samples were then subjected to bake hardening to study the varying chemical composition on this effect. Furthermore, the local use of bake hardening and ageing in hot rolled multiphase steels was investigated. It could be shown that characteristic values integrally describing the ageing effect, depend on the deformation path and the degree of pre‐strain, as well as on temperature and duration of the subsequent heat treatment. This partial ageing is stable and has a potential to be used for local strengthening of the steels.     

Effect of Thermo-Mechanical Processing on Precipitate Formation in Next Generation HSLA Steel

Development of advanced high strength steels (AHSS) using a conventional rolling setup is one of the biggest challenges to steel industry.It has been found that fine precipitation in a soft matrix,formed after hot rolling,can markedly improve the mechanical properties.In this work,three dimensional atom probe tomography (3D-APT) has been used to study the formation of precipitates in thermomechanically simulated steel.3D-APT data reveals co-existence of numerous nano clusters with precipitates.Also,quantitative analysis of the nano clusters and precipitates shows clusters are as small as 1nm in size.Precipitates are found to be disc shaped with the composition of equilibrium precipitates (TiMo)C.Thus,3D-APT is seen as an ideal technique to complement TEM to understand the nanoscale features in thermomechanically processed steel for further improvements in the mechanical properties of AHSS.     

国外汽车用先进高强度钢板及其标准综述

简要介绍了9种先进高强钢的特点和用途以及国外先进高强钢标准的概况,重点分析了欧洲、美国和日本先进高强钢标准的特点和标准间的异同。针对我国的现状,提出了先进高强钢的发展设想以及先进高强钢国家标准的制定原则和实施方法。     

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。     

汽车用第3代先进高强度钢的研发进展

近几十年来,汽车用先进高强度钢(AHSS-Advanced High Strength Steel)是材料的研发重点。第1代以铁素体为基的AHSS钢的强塑积为15 GPa·%,第2代以奥氏体为基的AHSS钢的强塑积为50 GPa·%,其合金含量高和生产工艺控制困难导致成本高,因此正研发第3代多相AHSS钢,通过多相、亚稳和多尺度的组织精细调控,其强塑积为30 GPa·%。第3代AHSS钢以提高第1代AHSS钢强度、塑性和降低第2代AHSS合金含量、生产成本两方面进行研发。本文介绍了超细DP(双相)钢,改进型TRIP(相变诱发塑性)钢,淬火-碳分配(Q&P)钢,超细晶贝氏体钢,超快加热和冷却的贝氏体-铁素体-马氏体钢,高锰铁索体-奥氏体钢和中锰亚稳奥氏体-超细晶基体钢等第3代AHSS钢的研究进展。     

汽车用先进高强度钢开发和研究的进展

先进高强度钢已经在汽车上得到了广泛应用,并在汽车的减重、安全、节能、环保等方面表现出广阔的前景。强度更高、成型性更好的新型先进高强度钢的开发也得到越来越多的重视。对正在开发的新型先进高强度钢开发和研究现状进行了简述,并对下一代高强度钢的研究进行了预测。     

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.