Dual-phase (DP) steels

8.2.5 Dual-phase (DP) steels

Much research into the deformation behavior of speciality steels has been aimed at producing improved strength while maintaining good ductility. The conventional means of strengthening by grain refinement, solid-solution additions (Si, P, Mn) and precipitation hardening by V, Nb or Ti carbides (or carbonitrides) have been extensively explored and a conventionally treated HSLA steel would have a lower yield stress of 550 MN m −2 , a TS of 620 MN m −2 and a total elongation of about 18%. In recent years an improved strength–ductility relationship has been found for low-carbon, low-alloy steels rapidly cooled from an annealing temperature at which the steel consisted of a mix- ture of ferrite and austenite. Such steels have a microstructure containing principally low-carbon, fine-grained ferrite intermixed with islands of fine martensite and are known as dual-phase steels.

Typical properties of this group of steels would be a TS of 620 MN m −2 , a 0.2% offset flow stress of 380 MN m −2 and a 3% offset flow stress of 480 MN m −2 , with a total elongation ≈28%.

The implications of the improvement in mechanical properties are evident from an examination of the nominal stress–strain curves, shown in Figure 8.3. The dual-phase steel exhibits no yield discontinuity but work-hardens rapidly so as to be just as strong as the conventional HSLA steel when both have been deformed by about 5%. In contrast to ferrite–pearlite steels, the work-hardening rate of dual-phase steel increases as the strength increases. The absence of discontinuous yielding in dual-phase steels is an advantage during cold-pressing operations and this feature, combined with the way in which they sustain work hardening to high strains makes them attractive materials for sheet-forming operations. The flow stress and tensile strength of dual-phase steels increase as the volume fraction of hard phase increases with a corresponding decrease in ductility; about 20% volume fraction of martensite produces the optimum properties.

The dual phase is produced by annealing in the (α + γ) region, followed by cooling at a rate which ensures that the γ-phase transforms to martensite, although some retained austenite is also usually present, leading to a mixed martensite–austenite (M–A) constituent. To allow air cooling after annealing, microalloying elements are added to low-carbon–manganese–silicon steel, particularly vanadium or molybdenum and chromium. Vanadium in solid solution in the austenite increases the hardenability but the enhanced hardenability is due mainly to the presence of fine carbonitride

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700 W H rate (MN m

600 0.1 0.2 0.3 冪f/d (mm ⫺ )

Figure 8.4 Dependence of work-hardening rate on (volume fraction f/particle size) 1/2 for a dual-phase steel at strain values of 0.2 and 0.25 (after Balliger and Gladman, 1981).

precipitates which are unlikely to dissolve in either the austenite or the ferrite at the temperatures employed and thus inhibit the movement of the austenite/ferrite interface during the post-anneal cooling.

The martensite structure found in dual-phase steels is characteristic of plate martensite having internal microtwins. The retained austenite can transform to martensite during straining, thereby contributing to the increased strength and work hardening. Interruption of the cooling, following intercritical annealing, can lead to stabilization of the austenite with an increased strength on sub- sequent deformation. The ferrite grains ( ≈5 µm) adjacent to the martensite islands are generally observed to have a high dislocation density resulting from the volume and shape change associated with the austenite to martensite transformation. Dislocations are also usually evident around retained austenitic islands due to differential contraction of the ferrite and austenite during cooling.

Some deformation models of DP steels assume both phases are ductile and obey the Ludwig relationship, with equal strain in both phases. Measurements by several workers have, however, clearly shown a partitioning of strain between the martensite and ferrite, with the mixed (M–A) constituent exhibiting no strain until deformations well in excess of the maximum uniform strain. Models based on the partitioning of strain predict a linear relationship between yield stress, TS and volume fraction of martensite, but a linear relationship is not sensitive to the model. An alternative approach is to consider the microstructure as approximating to that of a dispersion-strengthened alloy.

This would be appropriate when the martensite does not deform and still be a good approximation when the strain difference between the two phases is large. Such a model affords an explanation of the high work-hardening rate, as outlined in Chapter 6, arising from the interaction of the primary dislocations with the dense ‘tangle’ of dislocations generated in the matrix around the hard phase islands.

Several workers have examined DP steels to determine the effect of size and volume fraction of the hard phase. Figure 8.4 shows the results at two different strain values and confirms the linear relationship between work-hardening rate (dσ/dε) and (f /d) 1/2 predicted by the dispersion-hardening theory (see Chapter 6). Increasing the hard-phase volume fraction while keeping the island diameter constant increases the work-hardening rate, increases the TS but decreases the elongation. At constant volume fraction of hard phase, decreasing the mean island diameter produces no effect on the tensile strength but increases the work-hardening rate and the maximum uniform elongation (Figure 8.5).

Advanced alloys 453 1 ) ⫺ 800

m 700

600 WH rate (MN

Tensile strength (MN 500

6 7 [d ] (mm) (c)

15 Uniform elongation (%)

[d ] (mm)

(b)

Figure 8.5 Effect of second-phase particles size d at constant volume fraction f on: (a) work-hardening rate, (b) elongation and (c) tensile strength (after Balliger and Gladman, 1981).

Total stress (MN

0.02 0.04 c 0.06 0.08 0.10

Total strain

Figure 8.6 Bauschinger tests for a 0.06% C–1.5% Mn–0.85% Si dual-phase steel (courtesy of

D. V. Wilson).

Thus, the strength is improved by increasing the volume fraction of hard phase, while the work hardening and ductility are improved by reducing the hard-phase island size. Although dual-phase steels contain a complex microstructure it appears from their mechanical behavior that they can be considered as agglomerates of non-deformable hard particles, made up of martensite and/or bainite and/or retained austenite, in a ductile matrix of ferrite. Consistent with the dispersion-strengthened model, the Bauschinger effect, where the flow stress in compression is less than that in tension, is rather large in dual-phase steels, as shown in Figure 8.6, and increases with increase in martensite content up to about 25%. The Bauschinger effect arises from the long-range back-stress exerted by the martensite islands, which adds to the applied stress in reversed straining.

The ferrite grain size can give significant strengthening at small strains, but an increasing proportion of the strength arises from work hardening and this is independent of grain-size changes from about

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3 to 30 µm. Solid-solution strengthening of the ferrite (e.g. by silicon) enhances the work-hardening rate; P, Mn and V are also beneficial. The absence of a sharp yield point must imply that the dual-phase steel contains a high density of mobile dislocations. The microstructure exhibits such a dislocation density around the martensite islands, but why these remain unpinned at ambient temperature is still in doubt, particularly as strain ageing is significant on ageing between 423 and 573 K. Intercritical annealing allows a partitioning of the carbon to produce very low carbon ferrite, while aluminum- or silicon-killed steels have limited nitrogen remaining in solution. However, it is doubtful whether the concentration of interstitials is sufficiently low to prevent strain ageing at low temperature; hence it is considered more likely that continuous yielding is due to the residual stress fields surrounding second- phase islands. Two possibilities then arise: (1) yielding can start in several regions at the same time rather than in one local region, which initiates a general yield process catastrophically, and (2) any local region is prevented from yielding catastrophically because the glide band has to overcome a high back-stress from the M–A islands. Discontinuous yielding on ageing at higher temperatures is then interpreted in terms of the relaxation of these residual stresses, followed by classical strain-ageing.

In dual-phase steels the n value ≈0.2 gives the high and sustained work-hardening rate required when stretch formability is the limiting factor in fabrication. However, when fracture per se is limiting, dual-phase steels probably perform no better than other steels with controlled inclusion content. Tensile failure of dual-phase steels is initiated either by decohesion of the martensite–ferrite interface or by cracking of the martensite islands. Improved fracture behavior is obtained when the martensite islands are unconnected, when the martensite–ferrite interface is free from precipitates to act as stress raisers, and when the hard phase is relatively tough. The optimum martensite content is considered to be 20%, because above this level void formation at hard islands increases markedly.