Advanced Cold Rolled Steels for Automotive Applications

Advanced high‐strength steels offer a great potential for the further development of automobile bodies‐in‐white due to their combined mechanical properties of high formability and strength. They represent the first choice in material selection for strength and crash‐relevant parts with challenging geometries. The intensive development of multiphase steels by ThyssenKrupp Steel has led to hot dip galvanizing concepts with an outstanding forming potential. Hot rolled, hot dip galvanized complex‐phase steels are currently produced in addition to cold rolled dual phase (DP) and retained austenite (RA) or transformation induced plasticity (TRIP) steels. New continuously annealed grades of steel are being developed with tensile strength levels of up to 1000 MPa in combination with sufficient ductility for the high demands of structural automobile components. These steels make use of the classic advantages of microalloying as well as the principles of DP steels and RA / TRIP steels. Further improvement of properties will be reached by the new class of high manganese alloyed steels.     

Effect of Bake‐Hardening on the Structure‐Property Relationship of Multiphase Steels for the Automotive Industry

The effect of a bake‐hardening (BH) treatment on the microstructure and mechanical properties has been studied in C‐Mn‐Si TRansformation Induced Plasticity (TRIP) and Dual Phase (DP) steels after: (i) thermomechanical processing (TMP) and (ii) intercritical annealing (IA). The steels were characterized using X‐ray diffraction, transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). All steels showed high BH response. However, the DP and TRIP steels after IA/BH showed the appearance of upper and lower yield points, while the stress‐strain behavior of the TRIP steel after TMP/BH was still continuous. This was due to the higher volume fraction of bainite and more stable retained austenite in the TMP/BH steel, the formation of plastic deformation zones with high dislocation density around the “as‐quenched” martensite and “TRIP” martensite in the IA/BH DP steel and IA/BH TRIP steel, respectively.     

Physical Metallurgy of Multi‐Phase Steel for Improved Passenger Car Crash‐Worthiness

The dynamic testing of high strength automotive steel grades is of great practical importance if their crash‐worthiness is to be evaluated. During forming operations, steels are processed in a controlled dynamic manner. In collisions, the deformation is different in the sense that the deformation is not controlled, i.e. both strain and strain rate are not pre‐determined. No clear standard testing procedures are currently available to test high strength steels dynamically, in order to evaluate their performance during car crashes. High tensile strength TRIP‐aided steels have been developed by the steel industry because of their promising high strain rate performance. The present contribution focuses on the effect of the strain rate and temperature on the mechanical behaviour of the low alloy high strength TRIP steel. The tests were carried out on the separated phases in order to determine their specific high strain rate deformation response. The temperature‐dependence of the transformation rate of the retained austenite is presented. It is argued that the adiabatic conditions present during high strain rate deformations have a beneficial effect on the behaviour of TRIP steel.     

TRIP steel with reduced silicon content

At present high strength thin sheet steel grades are gaining considerable market shares due to the intensified application in the automotive industry. Although several new high strength steels for cold forming have been developed and continuously improved for some decades there is still a necessity to increase the formability at a given high strength level. TRIP steels are a consistent further step to meet these requirements. Most of the published work concentrates on compositions with high C, Mn and Si contents which causes problems during production. Therefore in this work TRIP steels with reduced C and Si contents were produced and the influence of the annealing parameters on the microstructure and mechanical properties was investigated in detail for production via continuous annealing lines. Based on these investigations optimized heat cycles were proposed.     

Non‐Metallic Inclusions in High‐Manganese‐Alloy Steels

The possibility of applying new high‐strength steels with excellent forming behaviour (TRIP, TWIP and LIP steels) in automotive manufacturing is a significant potential for improvement in the area of reducing weight while simultaneously increasing crash safety. The present work investigates endogenous inclusions in some high‐alloy TRIP and TWIP steels because the most stringent product requirements are tightly related to cleanness. The expected formation of inclusions is discussed based on thermodynamic observations made with ThermoCalc. The solidification conditions were varied in experiments with the so‐called SSCT (submerged split chill tensile) apparatus. Furthermore, different treatment times were set in order to investigate this influence on the inclusions. A catalogue of endogenous inclusions in these new steel grades is currently being created with the help of the automated SEM/EDX inclusion analysis system at voestalpine Stahl GmbH in Linz. Further studies will follow to systematically determine the interactions between steel, slag and refractory materials.     

Low‐alloyed TRIP‐Steels with Optimized Strength,Forming and Welding Properties

To obtain the superior strength‐ductility‐balance of TRIP‐grades, a special chemical composition in combination with well adapted processing parameters are a prerequisite. Despite of their excellent formability performance in terms of drawability as characterized by high n‐ and elongation values, compared to mild steels TRIP‐grades are challenging in the press and the body shops. The high strength level in combination with the high work hardening of TRIP‐grades result in higher levels of spring back compared to mild steels and higher press forces are required. Furthermore, a higher sensitivity to failure for sharp bending radii and a deterioration of the formability of punched edges is reported for TRIP‐grades. While spring back can only be minimized by advanced forming processes supported by new simulation techniques with improved ability to predict spring back, the sensitivity to failure under special forming conditions can be influenced by optimizing microstructural features. Contrary to the forming behaviour, which is influenced significantly by the microstructure, the weldability is mainly governed by the chemical composition and the surface condition of the material. The high carbon content of TRIP‐grades compared to mild steels results in a higher hardening potential after welding. Additionally, a fracture behaviour untypical for mild steels after destructive testing of spot welds is sometimes observed for TRIP‐grades, which is assessed critically by some OEMs. In this work, after a discussion of the processing conditions, possibilities are demonstrated to improve the forming behaviour by an optimization of the microstructure and the spot weldability by adapting the chemical composition of low‐alloyed TRIP grades. First very promising results for TRIP‐grades with a minimum tensile strength level of 700 MPa are discussed.     

Martensite Transformation and Its Control in DP,TRIP and TWIP Steels

Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.     

Formability of High Strength Dual‐phase Steels

The application of ferritic‐martensitic dual‐phase (DP) steels has become an increasing trend in the automotive industry due to the possibility to achieve significant weight reduction and fuel efficiency with improved crash performance while keeping the manufacturing costs at affordable levels. In order to meet the different design requirements of individual auto‐body components, a wide variety of DP grades exhibiting different strength and ductility levels is currently industrially produced. Despite the numerous studies on the relationship between the mechanical properties and the microstructural characteristics of DP steels over the last decades, it is still a challenge to increase their formability at a constant strength level (or equivalently increasing the strength while maintaining a high ductility). One of the possibilities to increase strength is grain refinement. Ultrafine‐grained ferritic‐martensitic microstructures were produced by intercritical annealing of a cold‐rolled, pre‐processed dual‐phase steel. Ferrite mean grain sizes in the order of ~ 1.5 μm were obtained. The mechanical properties of these steels are studied, revealing the beneficial effect of grain refinement. Ultimate tensile strength above 800 MPa is achievable, while reaching remarkable high uniform and total elongations, which are only slightly affected by the martensite volume fraction. Moreover, the yield to tensile strength ratio can be adjusted between 0.4 and 0.5. Light and electron microscopy investigations, fracture profile and fracture surface analyses, hole expansion tests and additional ultramicrohardness measurements are used for the interpretation of the results and for the correlation of the mechanical properties and the formability characteristics with the microstructure of the steel.     

HSLA钢冶金工艺技术的进展

20世纪90年代以来,薄板坯连铸连轧技术及中厚板坯炉卷轧制工艺,进入了低合金高强度钢(HSLA)的生产流程;并在该流程中对HSLA钢热机械控制轧制工艺(TMCP)取得了不少有益的经验。目前薄板坯连铸连轧CSP(Compact Strip Production)工艺所开发的钢种有:(1)ECC、UCC和IF软钢;(2)高强度多相DP和TRIP钢;(3)X80级管线钢。介绍了薄板坯连铸连轧和中厚板坯炉卷轧制工艺开发的HSLA钢品种的工艺特点和工艺优化。     

Strain Hardening and Strain Rate Sensitivity Behaviors of Advanced High Strength Steels

 The mechanical properties of commercial dual phase (DP), transformation induced plasticity (TRIP), and high strength low alloy (HSLA-340) steel sheets are investigated and compared at various strain rates ranging from 0. 0017 to 0. 17 s-1 at ambient temperature. TRIP steel outperforms the other two materials, having comparable ductility and twice as large strength relative to DP steel. TRIP has larger strength and much larger ductility than HSLA-340. The exceuent ductility of TRIP800 is due to its high strain hardening capability, which promotes stable plastic deformation. It is observed that the strain hardening rate in TRIP800 does not decrease to zero at failure, as common in most materials in which failure is preceded by necking.     

Energy Absorption Behaviour of Austenitic and Duplex Stainless Steels in a Crash Box Geometry

The improvement of the passive safety plays an important role in the development of new steels for automotive parts. At the same time aspects of weight reduction as well as the industrial feasibility have to be considered. Powered by these objectives, the development and application of new steel concepts for various purposes is promoted. For the present investigation especially weight reduction combined with an improvement of the passive safety are emphasised. As example one representative part of the body structure, the crash box, is considered. At the moment different steel grades (dual phase‐, TRIP‐and HSLA‐steels) as well as fibre reinforced materials are applied. New materials for this special purpose have to exhibit outstanding formability, a high capacity to absorb energy during a possible crash and should be cost effective compared to already existing material concepts. During this project different grades of austenitic stainless steels with varying stability were compared to duplex stainless steels and a TRIP grade with regard to their possible application as crash‐box material. The austenitic grades show excellent gradual formability according to their strength level. All of them exhibit an extraordinary strain hardening behaviour. The duplex grades show a lower formability but on a much higher yield level. Besides the determination of classical material data such as uni‐ and multi‐axial flow curves, dynamic tensile tests and forming tests for the determination of forming limit curves were performed. The material data were used in the simulation of a drop tower test which is commonly used to evaluate the performance of different materials in car components. The results were then evaluated with regard to the absorbed energy, the folding behaviour and the resulting forces.     

鞍钢先进高强汽车用钢的研制开发

介绍了鞍钢先进高强汽车用钢开发研制情况,包括以980 MPa级DP钢、TRIP钢、TWIP钢和QP钢为代表的热轧、冷轧和热镀锌先进高强钢系列产品,以及鞍钢开发的先进高强钢热镀锌生产技术,并对鞍钢先进高强汽车钢未来的发展方向进行了展望。     

Local Use of Ageing Effects in Multiphase Steels for Designing Local Properties of Constructional Elements

This paper describes processes leading to local bake hardening (BH) effects in multiphase steels. The investigations are part of a comprehensive project which investigates the influence of the deformation path and of the temperature and duration of thermal treatments on strengthening in modern multiphase steels, in regard to both local and bulk properties of steel structures. Dual phase (DP), retained austenite steels (TRIP) ‐ both hot and cold rolled ‐ and complex phase (CP) steel are investigated to examine the effect of thermomechanical processing parameters on local bake hardening ability. For this purpose two ways to achieve a local BH effect, i.e. local deformation and local heat treatment, are studied, as well as ageing stability of the adjusted strength. Hardness increased after local deformation through bending as result of work hardening and bake hardening effect. The local heat treatment leads to an improvement of mechanical properties (hardness and strength) and to local strengthening of material. The stability of the local bake hardening effect could be confirmed.     

低碳中锰热轧TRIP钢退火工艺及组织演变

研究了第三代高强度高塑性TRIP钢的退火工艺对性能的影响和组织演变规律.热轧后形成的原始马氏体与临界退火时形成的残余奥氏体使TRIP钢具有良好的强度和塑性.结果表明:实验用钢可获得1000MPa以上的抗拉强度和30%以上的断后延伸率,且强塑积>30 Gpa·%;退火温度和保温时间对钢的力学性能具有显著影响,热轧TRIP钢临界退火温度为630℃,保温时间18 h时,实验用钢能获得最佳的综合力学性能.      

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.     

Development of Ultrahigh Strength TRIP Steel Containing High Volume Fraction of Martensite and Study of the Microstructure and Tensile Behavior

A new transformation induced plasticity (TRIP) steel containing high volume fraction of martensite was produced by austempering heat treatment cycle. Microstructure and tensile properties of this TRIP steel were investigated and compared to those of a dual phase (DP) steel with high martensite volume fraction. Microstructural analysis showed a mixture of ferrite, bainite, retained austenite and about 25–30 vol% of martensite in the TRIP steel. As a result of the strain induced transformation of retained austenite to martensite, the TRIP steel showed a strength elongation balance of 86% higher than that for the DP steel. In comparison to the commercial TRIP780 steel, the current TRIP steel showed a 15% higher ultimate tensile strength value while maintaining the same level of ductility. TRIP steel also had a larger work hardening exponent than DP steel at all strains.     

Dynamic Tensile Behaviour of TRIP and DP Steels at Different Temperatures

The stress‐strain response of TRIP 700 and DP 600 steels was studied at a wide range of strain rates and temperatures using a special high/low temperature tensile Hopkinson Split Bar (THSB) device. The mechanical properties of the studied steels, especially of the TRIP steel, were found to be strongly affected by both temperature and strain rate. The beneficial TRIP effect in the studied steel reached its maximum at temperatures between 75‐150 °C. The transformation behaviour of the retained austenite in the TRIP steel was studied by XRD, revealing that the phase transformation rate increases with decreasing temperature and decreases with increasing strain rate. A phenomenological numerical model was also presented to describe the behaviour of the TRIP and DP steels at different temperatures and strain rates.     

高强TRIP钢的动态力学性能

将Si-Mn系双相钢（DP钢）作为对比钢种,分析研究了高应变速率下600 MPa级Si-Mn系TRIP钢及含Al、Ni的1000 MPa级TRIP钢的显微组织及其动态力学性能.对DP钢而言,其抗拉强度随着应变速率的增大而升高,断裂延伸率则由于绝热温升的作用也呈上升趋势;对TRIP钢而言,随着应变速率的增大,其抗拉强度不断增大,断裂延伸率先减小后增大,但无法达到其静态拉伸时的塑性水平,这是由于在动态拉伸条件下奥氏体向马氏体的渐进式转变被抑制造成的.此外,在相同应变速率下测得的TRIP钢的绝热温升始终比DP钢高,而这部分高出的热量应当来自于在动态变形条件下TRIP钢中发生TRIP效应后释放的相变潜热.      

超轻型汽车用钢的研究与开发

随着人们对能源、环境问题和安全性要求的不断提高,汽车用材料正在发生很大的变化。汽车轻量化已成为当今汽车工业的一项关键技术。为了满足超轻型汽车发展的需求,开发了大量的先进高强度钢,如DP钢、TRIP钢、CP钢、TWIP钢等。简要介绍了这些钢铁材料,分析了它们的成分、组织结构、性能特点和生产工艺等。     

合金钢的现状与发展趋势

在目前和可预见的未来,合金钢仍然是社会和经济发展所需要的重要材料。新型合金钢必须满足社会发展对钢铁生产、加工、使用和回收等环节提出的节约能源、节约资源、保护环境的要求,为此,合金钢将向高性能和多品种方向发展。本文概述了高强度低合金钢、合金结构钢、超高强度钢、不锈耐蚀钢、耐热钢、工具钢、模具钢、轴承钢等合金钢的现状和发展趋势。