The bainite-martensite-ferrite steels (tri-phase steels) were made in the laboratory by intercritical annealing, bainite transformation and oil quenching in sequence. With bainite inclusions, ductility was improved substantially without significant reduction of tensile strength. The ductility increase was found to be due to large deformation after necking and increased workhardening.EinfluB des Bainits auf die mechanischen Eigenschaften von Dualphasen-Stiihlen. Bainitisch-martensitisch-ferritische Stahle (Dreiphasen Stahle) wurden durch interkritisches GIGhen im Zweiphasen-Gebiet (ct+y) mit nachfolgender isothermer Teilumwandlung in der Bainitstufe und abschlieBendem Abschrecken der Proben im Olbad erzeugt. Steigende Gehalte an Bainit in der zweiten Phase fGhren zu einer hoheren Zahigkeit, ohne daB die Zugfestigkeit wesentlich abgesenkt wird. Die verbesserte Zahiqkeit ist zuruckzufuhren auf eine erhohte Verfestigung und ein hoheres Einschnurvermoqen. Table 1. Chemical composition (wt.-%) of the steel investigated Tafel 1. Chemische Zusammensetzung (Gew.-%) des untersuchten StahlsDual-phase steels consist of martensite particles dispersed in ferrite matrix. They can be produced either by intercritical annealing or controlled rolling. The cooling rate in controlled rolling processes tends to become slower as the steel temperature approaches bainite transformation temperature. Very often, therefore, bainite is included in as-rolled dualphase steels. When bainite was purposely introduced by adjusting the cooling schedule, ductility and fatigue strength were reported to improve while specimens retained some characteristics of dual-phase steels I).In this paper the effect of bainite inclusion on strength, ductility and work-hardening behaviour of dual-phase steels will be reported from investigation results with laboratory prepared steels. Experimental procedure C 0.057 a) Mn 1.38 Si 1.51 Cr 1.12The steel composition is given in table 1. A laboratory melted ingot was forged before being hot drawn to 12 mm diameter bars. The temperature range of bainite-martensite transformation was identified by use of a time-tern peraturetransformation diagram as between 600 and 400°C. Cylindrical standard tensile specimens (B 8 x 40) were machined from the 12 mm bars. The specimens were austenitized for 30 minutes at 900°C in nitrogen atmosphere, and then maintained at 800°C for 30 minutes in a salt bath. Then, the intercritically annealed specimens were transferred to another salt bath of 550°C. After maintaining at 550°C for varying periods of time, the specimens were quenched in a stirred oil bath, the cooling rate of which was measured as 28°C/s on an average between 650 and 350°C. The microstructure of the bainite-martensite-ferrite mixture was revealed by etching in Lel'era-solutiorr'). In this paper the bainite-martensite-ferrite mixture is refered to as "tri-phase steel". b) Figure 1. Light micrographs of tri-phase steels. LePera etch a) TP6, 16% martensite and 84% ferrite, 3 minutes at 550°C b) TP5, 8% martensite, 8% bainite...

The final heat treatment of austenitic stainless steels of types X 5 CrNi 18 9 (1.4301) and X 2 CrNi 18 10 (1.4306) normally is annealing at 1050°C and subsequent water quenching. The resulting structure is of a metastable fee-type. Plastic deformation, especially at low temperature, causes martensitic transformation of these metastable structure. The transformation is accompanied by a substantial flow stress increase. This strengthening mechanism should be used in prac tice, e.g. for saving weight /1,2/. The deformed structure consists of tetragonal a'-martensite, austenite and hep c-martensite. Whereas a 1 -martensite increases continuously with deformation, the content of c-martensitc reaches a maximum value at about 5% plastic strain at 77 K. The hep phase is only detectable by means of x-ray analysis whilst a'-martensite can be determined quantitatively by saturation magnetisation measurement. The flow stress increase in low temperature deformation of meta stable austenitic stainless steels is based on normal work-har dening by dislocation accumulation, in addition to a distinct amount of work-hardening due to martensitic transformation. Analysis of the work-hardening behavior in the range of stable deformation (T > M ) can be used to predict the amount of normal work-hardening when deformation is performed in the instable temperature regime. On the basis of figure 1 the corresponding flow curves can then be predicted /3/. Separation of the flow stress contributions according to the procedure described above enables the possible savings in weight to be predicted when using cryogenically stretched instable austenitic steels in comparison with stable grades deformed under the same conditions.

Monotonic and cyclic stress strain curves and strain fatigue‐life curves of a normalized carbon steel Cf 53 N, two quenched and tempered steels Ck 45 QT, 34 CrMoS 4 QT and three microalloyed medium‐carbon precipitation‐hardening steels 27 MnSiVS 6 + Ti BY, 38 MnSiVS 5 BY and 44 MnSiVS 6 + Ti BY, have been evaluated. Similar strain hardening was observed in the monotonic tensile tests whereas different hardening or softening was found under cyclic loading conditions. QT steels reveal pronounced cyclic softening over the entire strain range investigated, the ferritic pearlitic steels show only a slight decrease in the cyclic proof stress and cyclic hardening at larger strains. Strain fatigue‐life curves result in a common scatterband of all steels investigated with the microalloyed steels 27 MnSiVS 6 + Ti BY and 44 MnSiVS 6 + Ti BY lying at the upper limit. Crack initiation probability of the microalloyed medium‐carbon precipitation‐hardening steels in the low‐cycle fatigue range is equivalent or lower than for the normalized carbon steel and the QT‐steels.

Zur Beschreibung der Fließkurve des untersuchten Baustahls wurde ein allgemeines phänomenologisches Modell zur Entwicklung der Gesamtversetzungsdichte mit der plastischen Vergleichsdehnung herangezogen. Spezielle Formen dieser Evolutionsgleichung liefern bekannte empirische Fließgesetze. Unter Berücksichtigung der Randbedingungen bei der Herleitung dieser Gesetze sind deren Koeffizienten physikalisch plausibel zu interpretieren. Der gebräuchlichen Hollomon‐Gleichung ähnliche Potenzfunktionen, die eine additive Konstante enthalten, sind auf der Basis dieser Modellvorstellung als Fließgesetze ohne Berücksichtigung von Erholungsprozessen zu verstehen. Die Güte der Approximation ist folglich stark von der Versuchstemperatur abhängig. Die Voce‐Gleichung stellt die exakteste Beschreibung der Fließkurve des untersuchten Stahls dar. Aufgrund der Berücksichtigung von Erholungsvorgängen enthält die Voce‐Gleichung, im Gegensatz zu den anderen Ansätzen, eine Grenz‐ oder Sättigungsspannung, deren Temperaturabhängigkeit unter Zugrundelegung eines thermisch aktivierten Versetzungsprozesses quantifiziert werden kann. Abweichungen am Beginn homogener plastischer Verformung sind auf die Wirkung eines zweiten Strukturparameters, der Dichte der beweglichen Versetzungen zurückzuführen. Mit Hilfe neuerer Modellvorstellungen kann dieser Bereich quantitativ beschrieben werden.

The final heat treatment of austenitic stainless steels of types X 5 CrNi 18 9 (1.4301) and X 2 CrNi 18 10 (1.4306) normally is annealing at 1050°C and subsequent water quenching. The resulting structure is of a metastable fee-type. Plastic deformation, especially at low temperature, causes martensitic transformation of these metastable structure. The transformation is accompanied by a substantial flow stress increase. This strengthening mechanism should be used in prac tice, e.g. for saving weight /1,2/. The deformed structure consists of tetragonal a'-martensite, austenite and hep c-martensite. Whereas a 1 -martensite increases continuously with deformation, the content of c-martensitc reaches a maximum value at about 5% plastic strain at 77 K. The hep phase is only detectable by means of x-ray analysis whilst a'-martensite can be determined quantitatively by saturation magnetisation measurement. The flow stress increase in low temperature deformation of meta stable austenitic stainless steels is based on normal work-har dening by dislocation accumulation, in addition to a distinct amount of work-hardening due to martensitic transformation. Analysis of the work-hardening behavior in the range of stable deformation (T > M ) can be used to predict the amount of normal work-hardening when deformation is performed in the instable temperature regime. On the basis of figure 1 the corresponding flow curves can then be predicted /3/. Separation of the flow stress contributions according to the procedure described above enables the possible savings in weight to be predicted when using cryogenically stretched instable austenitic steels in comparison with stable grades deformed under the same conditions.