Abstract
As-hot-rolled, ferrite-martensite dual-phese steels of rather simple chemistry can be produced by "DPR Process" which uses a low finish rolling temperature and a very low coiling temperature. Laboratory DPR experiments have been carried out using C-Mn steels and these with Cr or Si additions, to examine the effects of alloying and processing factors on the structure and mechanical properties of the processed steels. Major results obtained are as follows:
(1) To attain a sufficiently low yield-to-tensile strength ratio, the final finish pass temperature should be at about Ar3 point which varies depending on the chemistry, so as to bring about early phase separation of alpha from gamma before cooling starts. The coiling after a rapid cooling should be done at a temperature lower than about 200°C, almost regardless of the steel composition, to suppress auto-tempering of the transformed martensite and aging of the ferrite.
(2) Both Cr and Si additions enhance the hardenability of partitioned austenite, allowing a slower cooling rate to obtain martensite phases. However, Cr addition is prone to hinder the early phase separation, making the gamma-to-alpha transformation sluggish. Si addition accelerates the phase separation so that a wide range of finishing temperature is available.
