The effects of thermomechanical processing on the precipitation in an industrial dual-phase steel microalloyed with tita
- PDF / 2,683,027 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 58 Downloads / 153 Views
ODUCTION
II. EXPERIMENTAL PROCEDURE
CLASSICAL dual-phase steels (nonmicroalloyed) offer some very attractive strength-ductility properties that make them attractive for cold stamping operations.[1,2] Their microstructure consists of a dispersion of martensitic islands in a ductile ferrite matrix. The amount of martensite present will depend not only on the initial carbon content but also on the intercritical annealing temperature in the ferrite ⫹ austenite region just prior to rapid quenching. The strength of unalloyed dual-phase steels is given by the rule-of-mixtures:[1,2] D ⫽ ␣V␣ ⫹ MVM , where is yield strength and V is volume fraction, and the subscripts ␣, M, and D represent ferrite, martensite, and dual-phase steel, respectively. Typically, D is around 380 MPa, a value not as high as required for some specific applications such as wheels. On the basis of the ferrite-martensite microstructure alone, any further increase in strength can only be obtained by an increase of the volume fraction of martensite, VM. This, however, might be done at the expense of ductility and elongation. Another way to increase the strength of such steels is to add a dispersoid-forming element such as titanium to the steel melt, which would form small precipitates and hinder dislocation movement in ferrite. The objective of this work, therefore, was to study the precipitation at each step of the thermomechanical treatment of an industrial dual-phase steel microalloyed with titanium and to estimate the contribution of each type of precipitate to yield strength.
The chemical composition of the commercial microalloyed dual-phase steel (DP80) studied is shown in Table I. Also shown in Table I is the chemical composition of the classical nonmicroalloyed steel (DP60) used simply to compare austenitic grain size with the microalloyed steel. Several types of samples were analyzed in order to completely study the effect of processing on precipitation. Depending on the step of processing, the analysis was done either on an industrial sample or on a sample obtained by simulation. This is due to physical constraints in cases such as that of the effect of rolling on precipitation, where it is quite impossible to stop the industrial rolling mill after each rolling stand to obtain a sample for analysis. A list of the origin of samples for each analysis is given in Table II. Similar thermomechanical treatments were applied on both the industrial and simulation samples. A schematic is shown in Figure 1. After reheat, the slab is rolled to its final thickness in two major steps: roughing and finishing. After the final rolling pass, and while still in the austenite (␥) phase field, the steel is rapidly quenched to an intercritical temperature in the ␣ ⫹ ␥ zone. An air cooling step is applied to allow for the formation of ferrite. A rapid quench is then applied to a temperature below the martensite start (Ms) temperature to transform the remaining austenite into martensite. In these steels, the volume fraction of martensite was close to 15 pct. Th
Data Loading...
