Dual phase steel strips can be produced directly from the rolling mill heat through the application of a thermomechanical treatment compatible with the alloy composition.This kind of product was first developed in 1976(1). This approach had as its main objective the design of a steel which can develop a dual-phase microstructure direct from the rolling heat without great changes in the hot strip mill process.
The rolling finish temperature of this steel must be chosen immediately above the Ar3 temperature, thus maximizing the acceleration effect that the hot deformation promotes on the ferrite reaction and avoiding simultaneously the intercritical rolling that can result in the formation of work hardened ferrite that is deleterious to the cold formability of the product (2,3). Another important parameter is the coiling temperature, that must be kept below 600oC, in order to avoid the formation of pearlite which supresses the continuous yielding of the material during the tensile testing and lowers its work hardening rate(3).
The aim of this work was to study the effect of the finish rolling temperature and cooling rate on the development of the microstructure and mechanical properties of a Mn-Si-Cr-Mo dual-phase steel suitable for hot rolling.
- EXPERIMENTAL PROCEDURE
An 100 kg ingot of steel with 0.063% C, 0.87% Mn, 1.46% Si, 0.41% Cr and 0.38% Mo was produced in a vacuum furnace and forged in order to homogeinize its as-cast structure. Specimens were machined from the bars thus obtained, heated to 1200oC during 45 minutes and hot rolled according to a five-pass schedule from 25 to 5 mm using a laboratory hot rolling mill. Three finishing temperatures were used: 950, 900 and 850oC. The samples were then cooled in media with different quench severities: water, oil, aqueous solution of 0.55% polyacrilamide, air or diatomite. Another cooling pattern simulated the cooling cycle applied at the Hot Strip Mill: the samples were cooled in the aqueous solution of 0.55% polyacrilamide from the finishing temperature to a "coiling" temperature of 650 or 550oC, when the samples were introducted in a furnace previously heated to the chosen "coiling" temperature. They were soaked at this temperature for one hour. After that the furnace was powered off and the samples cooled into the furnace down to the room temperature. All the samples were analysed through optical quantitative metallography, scanning electronic microscopy and submitted to tensile testing.
- RESULTS AND DISCUSSION
Only the sample quenched in water from the finish temperature of 950oC showed a fully bainitic microstructure. All the other samples had a microstructure composed of polygonal ferrite matrix with a constituent of granular bainite/martensite dispersed as islands. Only samples quenched in water showed a very different value of this parameter: while these speciments had 72% and 42% of islands of granular bainite/martensite at finishing temperatures of 900oC and 850oC, respectively, the other samples showed values varying between 8 to 20%, with a very weak influence of the finishing temperature.
For slower cooling rates and higher finishing temperatures the ferritic grain size increased. However, the grain size of the constituent composed of bainite/martensite had a peculiar evolution: it showed a minimum value for intermediate cooling rates (cooling in the polyacrilamide solution); the finish temperature had no effect on this feature.
The yield and tensile strengths of the samples fell with the application of slower cooling rates; higher finish temperatures lowered only the yield strength.
In the case of samples submitted to interrupted quenching the "coiling" temperature effectively determined the type of second constituent present in the microstructure: for Tc=650oC pearlite was formed, and for Tc=550oC a constituent composed of bainite/martensite appeared. In the first case the yield during tensile testing was discontinuous and the samples showed and yield ratio of 0.8. In the other case the yielding was continuous and the yield ratio was lower: 0.6. This shows the fundamental effect of the coiling temperature on the development of the characteristic mechanical properties of the dual-phase steels.
A regression analysis showed that the yield strength can be mathematically described by an Hall-Petch-type equation using the mean ferritic free path - i.e., the mean spacing between the bainitic-martensitic islands - instead of the ferritic grain size:
Y.S. [MPa] = 203 + 855 L-0.5 [micrometers-0.5] r2 = 0.88
This demonstrates that the phase boundaries are more effective obstacles to the migration of dislocations than the ferrite grain boundaries in the specific case of dual-phase steels.
|Last Update: 14 August 1996|
|© Antonio Augusto Gorni|