Conclusion
The specific heat-treatment cycle for obtaining a two-phase ferritic-martensitic steel with specified properties depends upon the composition of the steel and the parameters of the heat-treatment equipment (heating rate and time, means of cooling the heated strip, capability of reheating for tempering, etc.). At the same time, based on the concepts presented above and the experimental data obtained, it is possible to recommend certain optimum conditions of treatment with heating to intercritical range temperatures which must be determined more accurately experimentally on the heat-treatment equipment available in the plant.
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1.
The heating temperature must correspond to Ac1+(60–90)°C in order to obtain 50–60% austenite in the structure with the given heating time. Subsequent cooling must provide the obtaining of 30–45% new ferrite with refinement of the martensite islands. As the result the structure of the steel must consist of about 20% martensite and 80% ferrite.
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2.
It is preferable to cool from intercritical range temperatures at moderate rates (5–30 deg/sec) if only in the area from the heating temperature to 400–500°C, which promotes an increase in plasticity of the steel and provides stability in obtaining the necessary quanlity of martensite (15–25%) within limits of ±3–5%.
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3.
The original structure of the steel must be as highly dispersed as possible. This promotes acceleration of austenitization and an increase in the number of centers of origin of austenite.
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4.
Tempering for increasing the plasticity of two-phase ferritic-martensitic steels intended for subsequent cold working must be done at 200–250°C with a time of not more than 10 min.
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5.
The criterion of correct choice of the basic heat-treatment parameters in obtaining a two-phase ferritic-martensitic steel must be assumed to be obtaining of a structure of 15–25% martensite and the remainder ferrite, the absence of a yield plateau on the tensile curve, and a σ0.2/σt ratio of 0.5–0.6.
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Additional information
I. P. Bardin Central Scientific-Research Institute for Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 46–50, August, 1985.
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Fonshtein, N.M. A heat treatment for obtaining a controlled ferritic-martensitic structure in steel. Met Sci Heat Treat 27, 610–616 (1985). https://doi.org/10.1007/BF00699362
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DOI: https://doi.org/10.1007/BF00699362