Einfluss von Temperatur und Vorverformung auf das plastische Werkstoffverhalten von modernen Karosseriestählen

Influence of temperature and prestraining on the plastic material behaviour of modern sheet steels for autobody applications Within the scope of a common research project of the automotive and steel industry, characteristic values describing the plastical behaviour of 20 sheet steels have been determined. In detail, quasistatic tensile tests at the testing temperatures ‐40 °C, 23 °C and 100 °C were carried out to obtain flow curves for the as delivered materials as well as for steels after a defined prestraining or heat treatment. Additionally, sheet metal testing led to forming limit diagrams and limiting drawing ratios including the working ranges for deep drawing. The results of the tensile tests showed significant differences between steel groups with regard to their strain hardening behaviour, which can be described by the ratio of yield and tensile strength R_p0,2/R_m or the Θ_IV‐value, and their temperature sensitivity. Within one steel group, consisting of steels with similar strain hardening behaviour, it might be possible to determine flow curves of one steel in a defined condition in order to calculate the flow curves of other steels with different strength. An advantage would be a lesser number of experimental tests which have to be performed in order to supply reliable input data for numerical material and component modelling.     

The influence of the temperature on the stress‐strain curves of a bearing steel and a case hardening steel

In the present paper the influence of the temperature and strain rate on the stress strain behaviour of two different steels were investigated. Two microstructures were considered: pearlitic and austenitic. Tensile tests with the bearing steel 100Cr6 and the case hardening steel 20MnCr5 were accomplished at various temperatures. For this purpose the Ludwik equation was used to describe the stress‐strain curve. The parameter of the constitutive equation was determined for each steel and microstructure. Especially for the austenitic state the parameters of the used material law were described as a function of the temperature.     

Promising effect of copper on the mechanical properties of transformation-induced plasticity steels

To put in a nut shell ‘Alloying with copper was found to results in promising benefits on mechanical properties of transformation-induced plasticity steels’. In this research, the CMnSiAlCu transformation-induced plasticity steels were annealed at two different temperatures. The unique combination of mechanical properties was obtained in new developed copper containing transformation-induced plasticity steels. It was also showed that by adding 2.4%Cu in transformation-induced plasticity steels and in both 770°C and 800°C annealing treated condition, the yield and tensile strengths were increased by more than 35% and 26%. Moreover, a detailed examination of deformation and strain hardening behaviour of the steels studied revealed that the lower annealing temperature leads to the more uniform strain distribution within the copper-added transformation-induced plasticity steel.     

Ermittlung von Berechnungskennwerten an Karosseriewerkstoffen – Bericht über ein Gemeinschaftsprojekt der Stahl‐ und Automobilindustrie

Determination of input data for numerical design of sheet steels – Report on a common research project of the steel and automotive industry Within the scope of a common research project of the steel and automotive industry, 20 sheet steels have been investigated to obtain input data for FE‐analysis. In detail, elastic, plastic and fatique characteristical values were determined by several testing institutes for a period of 3 years. The investigated sheet steels differ with regard to the microstructure and the steel concept. Beside several ferritic steels, multiphase steels like dual phase‐, complex phase‐ and TRIP steels as well as 2 austenitic stainless steels were characterised. The starting materials and selected steels with a defined predeformation and heat treatment were investigated. Within this project, the partners developed a testing and documentation precept in which the ways and means were fixed to reach the defined steel condition and to enable a standardised testing and data output for the material database, realised by the automotive industry. Before the actual elastic, plastic and fatique testing, a reception test for all steels was carried out to characterise the materials with regard to the microstructure, surface condition, chemical composition and mechanical properties, obtained in the quasistatic tensile test. The results of the different testing institutes (elastic, plastic and fatique) will be presented in separate publications in detail. As a result of this project it became obvious that the investigated steels can be divided into steel groups which show a similar strain hardening behaviour. Thus, a prediction of mechanical values and flow curves for cognate steels within one steel group seems to be possible. This subject will be the focus of further investigations within the scope of a new project started on January 1^st, 2003.     

Nichtrostende Stähle mit TRIP/TWIP/SBIP‐Effekt

Stainless steels with TRIP/TWIP/SBIP effect Economic austenitic steels with high energy absorption capability are in the focus of worldwide research activities, whereby the steels which show TRIP, TWIP and/or SBIP effects play a crucial role. New austenitic or austenitic‐martensitic stainless steels with a high cold workability and energy absorption capability are currently developed and tested in laboratory scale at the Institute of Iron and Steel Technology at the Technical University Bergakademie Freiberg. The mechanical properties of these steels are essentially influenced by the TRIP, TWIP and SBIP effect, becoming evident in hot formed and solution annealed steels as well as in as‐cast steels. The TRIP/TWIP/SBIP effects have a significant impact on the toughness and the strength of stainless steels consisting of metastable austenite. The TRIP effect owns a paramount position since it serves for a simultaneous increase of toughness and strength. The influences of alloying elements like manganese or nickel on the TRIP effect are in the centre of the investigations at the Institute of Iron and Steel Technology. These austenitic or austenitc‐martensitic stainless steels provide the ability for new applications fields due to their excellent mechanical properties. Exemplary, in the Collaborative Research Centre SFB 799 “TRIP‐Matrix‐Composites”, financed through the Deutsche Forschungsgemeinschaft DFG, the suitability of this new class of steels for cast components in ductile and transformation strengthened high performance (metal) ceramic composite materials will be investigated.     

Advanced cold rolled steels for automotive applications

Advanced multiphase steels offer a great potential for bodies‐in‐white through their combination of formability and achievable component strength levels. They are first choice for strength and crash‐relevant parts of challenging geometry. The intensive development of high‐strength multiphase steels by ThyssenKrupp 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 DP and RA steels. New continuously annealed grades with tensile strength levels of up to 1000 MPa in combination with sufficient ductility for applications mainly in the field of structural automobile elements make use of the classic advantages of microalloying as well as the principles of DP and TRIP steels. Further improvement of properties will be reached by the new class of high manganese alloyed steels.     

Numerical calculation of the main factors on cold cracking

Low alloy high‐strength steels are nowadays very common in industrial application offering a number of favourable characteristics. However, cold cracking is an increasing problem concerning the weldability of these materials. For the prediction of cold cracking susceptibility many different tests exist. The most important ones are the Tekken‐(Y‐groove restraint test), the controlled‐thermal‐severity test (CTS) and the implant test. But in spite of the same or similar welding conditions, one gets different minimum preheat temperatures dependent on the used test procedure. Therefore, a better evaluation of these tests seems to be necessary. Based on the commercial FEA‐tool SYSWELD three main factors influencing cold cracking were investigated for MAG‐welded Tekken specimens: the distribution of microstructure, the stress‐strain state and the hydrogen concentration. The analysis was performed for S690 low alloy high‐strength steel. Also, welding experiments were carried out to determine temperature field and microstructure.     

Enhancement of work‐hardening behavior of dual phase steel by heat treatment

The work‐hardening response and mechanical properties of dual phase steels originated from different initial microstructures under low and high martensite volume fractions were investigated using a typical carbon‐manganese steel. The modified Crussard‐Jaoul analysis was used for studying the work‐hardening stages and the deformation behavior of ferrite and martensite. It was revealed that the initial martensitic microstructure before intercritical annealing is much better than the full annealed banded ferritic‐pearlitic and spheroidized microstructures in terms of work‐hardening capacity and strength‐ductility trade off. By increasing the amount of martensite, via intercritical annealing at higher temperatures, the ductility decreased but the tensile toughness of dual phase steels increased toward reaching the domain of extra‐advanced high‐strength steels due to the enhancement of work‐hardening rate.     

Verschleißbeständige Fe‐Basislegierungen mit Niobkarbid

Wear Resistant Fe‐Base Alloys with Niobium Carbide Martensitic Fe‐base alloys from the system Fe‐Cr‐C are widely used as chilled cast irons and tool steels. Because of the low hardness of their FeCr‐carbides this paper reports about new alloys with primarily solidified harder niobium carbides. It focuses on a secondary hardenable welding alloy, a coating material for composite castings, a chilled casting and a corrosion resistant cold work tool steel, which are investigated with respect to their process related microstructure and abrasive wear behaviour.     

Effect of Strain Rate,Adiabatic Heating and Phase Transformation Phenomena on the Mechanical Behaviour of Stainless Steel

Abstract: The influence of strain rate on the stress–strain behaviour of an AISI 304 austenitic stainless steel sample was investigated. For this purpose, uniaxial tensile tests were performed at room temperature for different strain rates. Microstructural measurements of transformed martensitic phase as a function of plastic strain, and thermal analyses of the specimens were carried out as well. It was found that increasing the strain rate from 10^?4 to 10^?1 s^?1 leads to a 25% improvement in uniform elongation. Moreover, a ‘curve‐crossing’ phenomenon was observed for the hardening behaviour measured at different strain rates. These results were rationalized in terms of martensitic phase transformation suppressed by a temperature increase in the specimens deformed with high strain rates.     

Werkstoffverhalten und Mikrostrukturentwicklung hochfester Mn‐Al‐Si‐Leichtbaustähle unter Zugbelastung

Material behaviour and microstructure evolution of high‐strength Mn‐Al‐Si‐light‐weight steels under tensile loading Because of their extraordinary combination of high strength and superior ductility high‐strength high Mn‐steels with reduced density and additions of aluminium and silicon are interesting candidates for structural applications. The initial microstructure consisting of stable austenite or austenite and ε‐ and α'‐martensite was achieved by alloying. During plastic deformation intense strain induced martensitic transformation and / or mechanical twinning was observed. These deformation mechanisms are used to extend the limited forming capability and contribute to a high energy absorption (in impact tests) up to very high strain rates. Tensile tests reveal that the properties are maintained up to strain rates of about 1000 1/s. The flow stress behaviour is strongly influenced by the initial microstructure and their evolution during deformation processes is determined by the rates of martensite and twin formation, respectively.     

Thermal Effects During Tensile Deformation of Ti‐6Al‐4V at Different Strain Rates

An in‐depth analysis of the effect of heat generated by plastic work on the observed tensile behaviour of Ti­6Al­4V at different strain rates is presented. Special emphasis is put on the transition from isothermal to adiabatic conditions and how this transition is affected by several process parameters such as material properties, environmental conditions and sample geometry. Experiments are performed in isothermal conditions at moderate temperatures, from ?10 to 70 °C, as well as at strain rates from quasi‐static speeds to more than 1000 s^?1 using a split Hopkinson tensile bar setup. This experimental data is used in conjunction with numerical simulations to determine the evolution of temperature during the experiments and the temperature and strain rate sensitivity of the material, as well as the Taylor–Quinney coefficient. Finally, a full model of the material behaviour is presented and used to define clear limits for adiabatic and isothermal conditions.     

Zum Einfluss des Schweißens auf das Kriechverhalten moderner Stähle für die thermische Energieerzeugung

The influence of welding on creep behaviour of modern steels for thermal power generation Un‐ and low alloyed ferritic/bainitic Chromium steels as well as high alloyed ferritic/martensitic 9–12 % Chromium steels are widely used for high temperature components in thermal power generation. Welding in all its variety is the major repair and joining technology for such components. The weld thermal cycle has significant influence on the base material microstructure and its properties. The Heat Affected Zone is often regarded as the weakest link during high temperature service. While weldments of un‐ and low alloyed ferritic Chromium steels can show significant susceptibility to Reheat Cracking in the coarse grained heat affected zone, weldments of high alloyed ferritic Chromium steels generally fail by Type IV Cracking in the fine grained heat affected zone during long term service. In this paper the influence of the weld thermal cycle on the base material microstructure is described. Long‐term creep behaviour of weldments is directly related to the main failure mechanisms in creep exposed ferritic weldments and implications for industries using heat resistant ferritic steels are shown.     

Effect of carbon content on plastic flow behaviour of plain carbon steels at elevated temperature

Abstract

In the present study the effect of carbon composition on the hot flow behaviour of two different plain carbon steels is analysed. For this purpose the constitutive equations describing the stress–strain (σ–?) relationships at a given strain rate ? and temperature T were determined for each steel. Uniaxial hot compression tests were performed to characterise the mechanical behaviour of the alloys. It was observed that irrespective of the test conditions, the low carbon steel displayed similar flow stresses to the high carbon steel. Comparison of the characteristic parameters of the constitutive equations describing the high temperature flow behaviour of these steels, together with values reported in the literature enabled determination of the effect of carbon content on flow behaviour. It has been found that flow stresses can be rationalised as a balance between work hardening and softening processes (basically dynamic recovery). At high temperatures and small strain rates, the high carbon steel showed lower hardening rates and slower dynamic recovery kinetics than the low carbon steel. In contrast, at low temperatures and large strain rates, the high carbon steel displayed higher hardening rates and recovery rates than the low carbon steel.     

Einfluss des Anlassens auf die Warmhärte von P92 Blechmaterial

Influence of tempering on the hot hardness of P92 sheet metal The measurement of hardness at elevated temperatures allows a first estimation of the material concerning its high temperature strength. The advantage of this testing method is to determine the hardness as function of the temperature in a wide temperature range in a short time on a small specimen. Especially the hot hardness testing is suited to compare the influence of different heat treatments. In this work the influence of tempering time and temperature on the hot hardness of P92 were investigated. The results show a mathematical relation between the hardness at different temperatures and the Hollomon‐Jaffe‐Parameter, which allows in a certain range a replace of tempering time and tempering temperature.     

Modelling the effect of strain rate on the thermomechanical behaviour of AISI 304 stainless steel

The present work deals with the interdependences between the strain rate and the strain hardening on stainless austenitic steels. Uniaxial tension tests were conducted on a 304 stainless steel at different strain rates in order to analyse the influence of this parameter on the strain hardening and on the material formability. For the strain rates levels analysed (10^‐4 to 10^‐1 s^‐1) it was also observed that increasing the strain rate from 10^‐4 up to 10^‐1 s^‐1 leads to a 25 % difference in uniform tension elongation revealing the curve‐crossing phenomenon. Namely, strain rates equal or higher than 10^‐2 lead to a stagnation of strain hardening after a tensile strain of about 0.2. In order to investigate the results obtained, microstructural and thermal analyses were conducted and numerical simulations were performed. It was observed that the decreasing of formability of the material is essentially due to thermal aspects. In the discussion, the experimental and numerical results are analysed in terms of thermal softening, phase transformation and strain rate sensibility.     

Towards improved ODS steels: A comparative high‐temperature low‐cycle fatigue study

Within the frame of this work, the mechanical behaviour of a bimodal ferritic 12Cr‐ODS steel as well as of a ferritic‐martensitic 9Cr‐ODS steel under alternating load conditions was investigated. In general, strain‐controlled low‐cycle fatigue tests at 550°C and 650°C revealed similar cyclic stress response. At elevated temperatures, the two steels manifest transitional stages, ie, cyclic softening and/or hardening corresponding to the small fraction of the cyclic life, which is followed by a linear cyclic softening stage that occupies the major fraction of the cyclic life until failure. However, it is clearly seen that the presence of the nano‐sized oxide particles is certainly beneficial, as the degree of cyclic softening is significantly reduced compared with non‐ODS steels. Besides, it is found that both applied strain amplitude and testing temperature show a strong influence on the cyclic stress response. It is observed that the degree of linear cyclic softening in both steels increases with increasing strain amplitude and decreasing test temperature. The effect of temperature on inelastic strain and hence lifetime becomes more pronounced with decreasing applied strain amplitude. When analysing the lifetime behaviour of both ODS steels in terms of inelastic strain energy calculations, it is found that comparable inelastic strain energies lead to similar lifetimes at 550°C. At 650°C, however, the higher inelastic strain energies of 12Cr‐ODS steel result in significant lower lifetimes compared with those of the 9Cr‐ODS steel. The strong degradation of the cyclic properties of the 12Cr‐ODS steel is obviously linked to the fact that the initial hardening response appears significantly more pronounced at 650°C than at 550°C. Finally, the obtained results depict that the 9Cr‐ODS steel offers higher number of cycles to failure at 650°C, compared with other novel ODS steels described in literature.     

Struktur der Randschicht nichtrostender Stähle nach Tieftemperatur‐Nitrierung – eine Mößbauerstudie

Case structure of stainless steel after low temperature nitriding – a Mössbauer study Due to the nitriding of stainless steel at temperatures between 300 °C and 400 °C cases of high hardness and nitrogen contents ranges between 8wt.% and 12 wt.% could be prepared. In the present work the nitriding was performed by gas and plasmanitriding. The phase generation was investigated by use of the Conversion‐Mössbauer‐specrtoscopy (CEMS and XCMS) and the X‐ray diffraction. The chemical composition was determined by GDOS and the hardness test using the Martens‐hardness under load. In spite of different principles of action no significant differences between the structure and properties of the gas and plasmanitrited samples could be observed within the bounds of the used test methods. Clear differences were found in the nitriding behaviour of different steels.     

Ermittlung von Umformgrenzen beim Freiformschmieden

Determination of forming limits during open die forging The fundamentals of determining the forming limits during open die forging have been investigated in the research project “Vermeidung von Oberflächenfehlern beim Freiformschmieden” (“Prevention of surface defects during open die forging processes”) which was supported by the AiF (“Arbeitsgemeinschaft industrieller Forschungsvereinigungen”). The industrially produced material was analysed in the form of cast ingots of two high‐alloyed steels (1.2367 and 1.6957) by compression, tension and torsion tests. The tests showed that the sampling point has a small influence on the materials’ plasticity for the present dimensions of the ingots. Furthermore transformation and precipitation behaviour of the both steels have been determined and additional metallographic investigations have been made to find out reasons of variation of measurements for low forming temperatures. Forming limit diagrams are generated to predict the forming limits of open die forging processes. For this purpose, compression tests and numerical simulations of the compression tests and forging processes were made for crack‐critical temperatures. The points in time of crack initiation of the samples during the compression tests were determined by the acoustic emission analysis and show a high variation of measurements. Thus the forming limit diagrams also have high scatter, an unique forming limit cannot be determined. Instead of that a maximum allowed height‐reduction was determined and checked in open die forging tests in the laboratory under industrial conditions. The results show that the determined limits are very safe because no cracks were generated during the forging of both materials.