Tempering has been used as a method to develop a range of dual phase steels with the same martensite morphology and volume fraction, but containing phases with different relative strengths. These steels were used to examine the strain partitioning between the two constituent phases experimentally through mechanical testing and numerically through finite element modelling. It was found that increasing the differential in strength between the two phases not only produces regions of high strain, but also regions of low strain. On average, a larger difference in strength between the phases increased the strain carried by the softer phase. There was no discernible preferential strain localisation to the ferrite/martensite interface, with the regions of strain localisation being determined by the morphology of the microstructure. A direct correlation between the average strain in the ferrite, and the measured ductility has been found.

Strain partitioning in dual-phase steels containing tempered martensite

Granular bainite, where M-A constituents dispersed in bainitic ferrite matrix usually presents at the half thickness region in thermo-mechanically processed heavy gauge offshore platform steel. In the present work, the decomposition of M-A constituents during tempering at 600 °C was firstly revealed by transmission electron microscopy (TEM) analysis, which primarily involves the precipitation of cementite, recovery and recrystallization of highly dislocated ferrite matrix. Then, the effect of tempering on mechanical properties was investigated by tempering at different temperature for 60 min. Results indicated that, at tempering temperature of 500–600 °C, large quantity of micro-alloying carbides precipitated and partially compensated the loss of strength mainly due to the decomposition of M-A constituents. Compared with the as-rolled state, the decomposition of M-A constituents and softening of bainitic ferrite matrix after tempering have resulted in higher density of microvoids and substantial plastic deformation before impact failure.

Investigation on tempering of granular bainite in an offshore platform steel

Among generations of advanced high-strength steel alloys, dual-phase steels exhibit a unique combination of strength and formability making them excellent candidates for use in the automotive industry. In this study, we seek to establish a relation between mechanical properties and microstructure of DP980. Electron backscatter diffraction (EBSD)and nanoindentation are used to identify and characterize martensite and ferrite phases. Spatial distributions of martensite and ferrite phases of subjected to various annealing treatments are found using a 2-point correlation function. Micro- and macro-mechanical properties are measured with nanoindentation, Vickers hardness and tensile tests and the results are used to determine the relation between martensite and ferrite phases and the strength of the metal. During the annealing/recovery process, the strength of the martensite phase decreases, the dislocation structure relaxes in the phase boundary region of the ferrite, and the martensite alignment along the rolling direction decreases resulting in the observed metal strength reduction. It is also shown that the higher the annealing temperature, the more homogeneous and equiaxed the distribution of martensite.

Morphology and distribution of martensite in dual phase (DP980) steel and its relation to the multiscale mechanical behavior

Tempering treatment is conducted on a commercial dual phase (DP) 980 steel at 250 °C and 400 °C for 60 min each. Ferrite and martensite grains are distinguished using electron backscatter diffraction (EBSD) and scanning probe microscopy (SPM), and the martensite volume fractions (MVF) are determined based on the image quality (IQ) map. Indentation tests combined with a newly developed inverse method are used to obtain the individual phase flow properties in each sample. The results show that, i) tempering significantly reduces martensite yield strength, while it slightly reduces the ferrite yield strength; ii) tempering temperature has a more significant influence on the work hardening exponent of ferrite than that of martensite. As a validation, a simple rule-of-mixtures is used to verify the above-predicted individual phase flow stresses with the experimentally obtained overall true stress vs. true strain curves.

/pdf/quantifying-the-effects-of-tempering-on-individual-phase-1t31w9rsbp.pdf

Quantifying the effects of tempering on individual phase properties of DP980 steel with nanoindentation

In this paper, the behavior of auto-tempering in hot rolled plain C-Mn dual phase steels was investigated. The steels with thickness of 11 mm were fabricated on the compact strip production line and coiled at 150 °C and 260 °C, respectively. The auto-tempering in the steel with coiling temperature of 260 °C caused the softening of ferrite and martensite and the reduction in hardness difference between these two phases, ultimately led to the decrease in tensile strength and strain hardening ability and the increase in post-necking elongation. The Charpy impact test results indicated that the toughness of auto-tempered steel was superior to the non-tempered steel. When the tensile test or impact test was conducted on the auto-tempered steel, both ferrite grains and tempered martensite islands were readily elongated as fibrous structure. The acquired fibrous structure could inhibit the formation of microvoids and propagation of cracks in tensile test, and delayed the tensile fracture. In addition, the fibrous structure could inhibit the propagation of main crack during impact test, whereas induced the branching cracks, leading to high impact energy.

Effects of auto-tempering on microstructure and mechanical properties in hot rolled plain C-Mn dual phase steels

In this study, affect of tempering temperature on the microstructure and mechanical properties of dual phase steels was studied. The C–Mn steel specimens were put under intercritical annealing treatment (ICT) at 760 °C and quenched in water to obtain 31% martensite and then tempered within the range of 100–600 °C. The tempering changes the mechanical properties of the dual phase steels through influencing both martensite and dislocation density of ferrite. Low tempering temperature results in formation of fine carbides in the martensite phase. Density of carbides increases and then carbon concentration of martensite decreases as tempering temperature increases. The continuous yield behavior of dual phase steel remains until tempering up to 300 °C. At tempering temperatures above 300 °C the discontinuous yielding and upper–lower yield points return which means degradation in mechanical properties. Yield strength increases by tempering up to 200 °C slightly, but it decreases at higher tempering temperatures. Ultimate tensile strength does not change at tempering temperature up to 200 °C considerably, but it decreases at higher tempering temperatures. Uniform elongation and total elongation increased slightly with increasing tempering time. Hardness of martensite decreases after tempering but this is not the case with ferrite resulting in a decreased hardness difference between martensite and ferrite. According to obtained results it seems that the tempering temperatures lower than 300 °C (1 h) is suitable for attaining continuous yielding and optimum strength and ductility.

A. Anazadeh Sayed

Papers

Affect of the tempering temperature on the microstructure and mechanical properties of dual phase steels