Dual-phase steel (DP steel) is a high-strength

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...

that has a

ferritic A ferrite is a ceramic material made by mixing and firing large proportions of iron(III) oxide (Fe2O3, rust) blended with small proportions of one or more additional metallic elements, such as strontium, barium, manganese, nickel, and zinc. Th ...

–

martensitic Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation. Properties Mart ...

microstructure. DP steels are produced from low or medium

carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, cobalt ...

s that are quenched from a temperature above A₁ but below A₃ determined from

continuous cooling transformation A continuous cooling transformation (CCT) phase diagram is often used when heat treating steel.. These diagrams are used to represent which types of phase changes will occur in a material as it is cooled at different rates. These diagrams are ofte ...

diagram. This results in a

microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymers ...

consisting of a soft ferrite matrix containing islands of martensite as the secondary phase (martensite increases the

tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...

). Therefore, the overall behaviour of DP steels is governed by the volume fraction, morphology (size, aspect ratio, interconnectivity, etc.), the grain size and the carbon content.. For achieving these microstructures, DP steels typically contain 0.06–0.15 wt.% C and 1.5-3% Mn (the former strengthens the martensite, and the latter causes solid solution strengthening in ferrite, while both stabilize the austenite), Cr & Mo (to retard pearlite or bainite formation), Si (to promote ferrite transformation), V and Nb (for precipitation strengthening and microstructure refinement).. The desire to produce high strength steels with

formability Formability is the ability of a given metal workpiece to undergo plastic deformation without being damaged. The plastic deformation capacity of metallic materials, however, is limited to a certain extent, at which point, the material could experienc ...

greater than

microalloyed steel Microalloyed steel is a type of alloy steel that contains small amounts of alloying elements (0.05 to 0.15%), including niobium, vanadium, titanium, molybdenum, zirconium, boron, and rare-earth metals. They are used to refine the grain microstruct ...

led the development of DP steels in the 1970s... DP steels have high ultimate tensile strength (UTS, enabled by the martensite) combined with low initial yielding stress (provided by the ferrite phase), high early-stage strain hardening and macroscopically homogeneous plastic flow (enabled through the absence of Lüders effects). These features render DP steels ideal materials for automotive-related sheet forming operations. The steel melt is produced in an

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

top blowing process in the converter, and undergoes an alloy treatment in the secondary metallurgy phase. The product is

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

- killed steel, with high tensile strength achieved by the composition with

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...

and

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

. Their advantages are as follows:. *Low

yield strength In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and wi ...

*Low yield to tensile strength ratio (yield strength /

= 0.5) *High initial strain hardening rates *Good uniform elongation *A high strain rate sensitivity (the faster it is crushed the more energy it absorbs) *Good fatigue resistance Due to these properties DP steels are often used for automotive

body panel A quarter panel (British English: rear wing) is the body panel (exterior surface) of an automobile between a rear door (or only door on each side for two-door models) and the trunk (boot) and typically wraps around the wheel well. The similar f ...

s, wheels, and bumpers.

References

Notes

Bibliography

*. * *. *. * . *{{Citation , last = Abid , first = Najmul H. , last2 = Abu Al-Rub , first2 = Rashid K. , last3 = Palazotto , first3 = Anthony N. , title = Computational Modeling of the Effect of Equiaxed Heterogeneous Microstructures on Strength and Ductility of Dual Phase Steels , publisher = Elsevier , journal = Computational Materials Science , year = 2015 , volume = 103 , pages= 20–37 , doi=10.1016/j.commatsci.2015.02.051. Steels Vehicle technology