MATERIAL CHARACTERISTICS

Dual phase steel can be produced directly from the rolling mill by the use of a thermo-mechanical treatment (TMT) well suited to the alloy

Coiling window

composition^[23]. The finishing temperature of the rolling process must be close to the upper critical temperature to maximise the acceleration effect that the hot deformation has on the ferrite reaction, but intercritical rolling that can result in the formation of work hardened ferrite (that is

detrimental to the cold formability of the steel) must be avoided. The coiling temperature is another important factor, and must be kept below 600°C in order to avoid the formation of pearlite which suppresses the continuous yielding of the metal during tensile testing and lowers its work hardening rate. Amongst the various hardening mechanisms of steels, the distribution of a hard second phase in the ferritic matrix is one of the best methods to optimise the strength/ductility ratio i.e. its formability^[24]. To achieve weight reductions and fuel saving in automobiles, designers have concentrated on dual phase steels (DPS) containing ferrite and martensite phases. It has been documented that the volume fraction and shape of grains in dual phase steels strongly influences the material’s stress-strain behaviour^[25].

The ferrite-martensite structure is obtained by heating the steel to approximately 800°C, which is in the austenite-ferrite region, and quenching it back to room temperature. The structure that is quenched contains ferrite and austenite that is enriched in carbon. The exceptional properties of dual phase steel are low yield strength and a high work hardening rate. The importance of a high work hardening rate is that the strength of the steel increases rapidly as it is deformed. A high work hardening rate is considered to be typical of a mixed microstructure consisting of a hard phase in a soft matrix^[26]. It has been found that dual phase steels frequently contain, in addition to ferrite and martensite,

between 2% and 9% of retained austenite. These particles of retained austenite increase the work hardening rate during the first few percent strain during a tensile test. This is due to strain-induced transformation of the retained austenite into martensite^[27].

2.14.1 Chemical composition

The chemical composition of the dual phase steel used in this study is shown in Table 2.2.

Table 2.2: Chemical composition of dual phase steel (DPS)

% Carbon 0,09 % Copper 0,025

% Silicon 0,22 % Tin 0,004

% Manganese 0,90 % Vanadium 0,001

% Sulphur 0,04 % Aluminium 0,04

% Chromium 0,71 % Titanium 0,004

% Nickel 0,04 % Boron 0,0003

% Molybdenum 0,01 % Iron 96,696

% Phosphorus 0,04

The main difference in chemical composition as compared to C2 steel (conventional steel used for manufacture of wheel rims) is given in Table 2.3. Both C2 and dual phase steel are currently used for the manufacture of the same wheel rims and C2 material are sometimes used in place of dual phase steel as it is easier to obtain.

Table 2.3: Difference in main alloying elements between C2 and dual phase steel

Alloying element C2 Dual Phase Steel

% Silicon 0,03 0,22

%Phosphorus 0,015 0,04

% Chromium 0,05 0,71

The comparison to the C2 chemical composition is just to indicate the main difference in steel composition and hence the different

microstructures obtained. The matrix microstructure of C2 consists of ferrite and pearlite compared to the ferrite and martensite matrix of dual phase steel. Elements such as nickel, aluminium, silicon and copper are

all found largely dissolved in ferrite^[4]. The carbide forming tendencies of some of these elements are only apparent when there is a significant amount of carbon present. Any element dissolved in ferrite increases its hardness and strength due to solid solution strengthening. The

strengthening effect of the dissolved elements contributes relatively little to the overall strength of the steel^[26].

The change in critical temperature produced by the presence of alloying elements is important in the heat treatment of alloy steels, since it will either raise or lower the hardening temperature compared to plain-carbon steel. All alloying elements tend to reduce the plain-carbon content of the eutectoid, but only nickel and manganese reduce the eutectoid temperature.

2.14.2 Mechanical properties

The mechanical properties, as determined by the tensile test, are given in Table 2.4.

Table 2.4: Mechanical properties of dual phase steel 0,2% Proof

stress

Tensile strength

Modulus of Elasticity

% Elongation % Reduction of cross-sectional area

341,3 MPa 577,8 MPa 198,7 GPa 23,8 52,5

The average hardness of the material in the as-received condition was found to be approximately HV0,5 157. From Table 2.3 it is clear that the material has a high ductility, based on percentage elongation and percentage reduction in cross-sectional area.

It is well known that the dual phase steels developed over the past decades show low yield strengths, continuous yielding, high work hardening rate, good formability and good ductility^[28]. This work

hardening process in dual phase steels under axial loading is complex.

The work hardening process can be divided into three strain regions,

each region exhibiting a different work hardening rate. The rapid work hardening in the first phase leads to the removal of residual stresses and the rapid build-up of back stress in the ferrite caused by plastic instability of the two phases. During the second phase, the work hardening rate decreases due to the constrained deformation of the ferrite caused by the presence of the rigid martensite phase. The third stage shows the formation of dislocation cell structures and further deformation in the ferrite is governed by dynamic recovery and cross-slip and by ultimate yielding of the hard martensite phase.

It has been shown in previous work that the initiation of microviods occurs by decohesion of ferrite-martensite interfaces or by the shear cracking of martensite particles^[28]. It has also been observed that the majority of microvoids are formed at the ferrite-martensite interface rather than at the cracked martensite, and eventually these microvoids

coalesce to cause failure during subsequent tensile loading. It is therefore preferred that the martensite produced by heat treatment be kept as deformable as possible to remain coherent with the matrix during

cold forming operations. Amongst the various martensite morphologies produced by heat treatment, the fibrous lath martensite, with a high degree of structural coherency with the surrounding ferrite, leads to the lowest microvoid density during cold deformation^[28].