Transformation Induced Plasticity "TRIP" Steel

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[edit] Introduction

Fig 1: Effect of cold work on the strength and ductility of a typical metal

Transformation Induced Plasticity (TRIP) steel, is a type of steel alloy which exhibits excellent strength and ductility. Transformation induced plasticity refers to the transformation of retained austeniteW to martensiteW during plastic deformation. This property allows TRIP steels to have a high formability, while retaining excellent strength. In the processing of metals, there is generally a compromise that must be made between strength and ductility.[1] This is demonstrated by the graph in Figure 1, which shows the compromise between strength and ductility during cold work. The advantage of TRIP steels is that they have much higher ductility than other steels with similar strength.[2] The ductility and strength of TRIP steels make them an excellent candidate for automotive applications. Indeed, structural components can be made thinner because TRIP steels have the ductility necessary to withstand high deformation processes such as stamping, as well as the strength and energy absorption characteristics to meet safety regulations.


[edit] Underlying materials science principles

[edit] TRIP Steel Composition

Fig 2: Diagram of the grain structure for a typical TRIP steel
Fig 3: SEM micrograph showing the typical grain structure of a TRIP steel[3]

Trip steels are hypoeutectoid iron carbon alloys which typically contain 0.1 – 0.4 % carbon by weight. TRIP steels also contain alloying elements which prevent the precipitation of the high carbon cementiteW phase which is present in typical steels at room temperature. This raises the carbon concentration of the austenite phase, which becomes stable at room temperature. Silicon and Aluminium are the two most common elements used to stabilize the austenite phase at room temperature.[4] Other alloying elements such as titanium, niobium, vanadium…ect. can also be added to improve the strength of the alloy.

[edit] Processing method

Fig 4: Iron carbon phase diagram showing the intercritical anealing process
Fig 5: Diagram showing the carbon concentration in the bainitic ferrite phase and the austenite phase

In order to produce a strong and ductile TRIP steel, an intercritical annealing process is used to obtain the correct phase distribution.[5] During intercritical annealing, the steel is brought to a temperature above the eutectoidW, where the material is composed of a solid austenite phase and a solid ferrite phase. The austenite phase is a high temperature solid phase which only exists in equilibrium at temperatures above 727 degrees Celsius.[6] The material is then isothermally cooled at a temperature of approximately 400 degrees Celsius,[4] in order to allow the austenite to form a banitic ferrite phase. During the eutectoid transformation, excess carbon is produced by the formation of the low carbon ferrite phase. In a typical steel alloy, the excess carbon would form a high carbon cementite phase. However, the silicon and aluminium prevent the formation of cementite. In consequence, the excess carbon diffuses to the remaining austenite phase. In order to obtain the correct microstructure, it is important that the isothermal transformation be completed at a temperature where the formation of bainitic ferrite is slow enough to allow the carbon to diffuse to the austenite. The carbon enriched austenite phase eventually reaches a high enough carbon content that it is stable at room temperature.[7] The result of the intercritical annealing process is a material composed primarily of ferrite, and bainiteW formed from the austenite phase during intercritical annealing, as well as dispersed retained austenite, and martensite phases. The grain microstructure can be seen in Figure 2 which shows a schematic of the phases, and Figure 3 which shows a micrograph taken with a scanning electron microscopeW. Figure 4 depicts the intercritical annealing process on the iron phase diagram. Figure 5 demonstrates the carbon concentration of the ferrite and austenite phases during the intercritical annealing process.

[edit] The “TRIP” effect

The transformation induced plasticity phenomenon occurs when the retained austenite transforms to martensite during plastic deformation.[1] The transformation of retained austenite produces a high carbon martensite phase which is very brittle. However, the retained austenite is very finely dispersed in the ferrite phase. This fine dispersion allows TRIP steels to retain their strength. The transformation of austenite into martesite is almost instantaneous and completely diffusionless.[6] In TRIP steels, plastic deformation forms matensite nucleation sites in microscopic areas seeing large deformations.[8] These nucleation sites trigger the formation of the martensite phase. The nucleation areas are known as shear bands, where crystallographic defectsW such as twins or stacking fault bundles are located.

[edit] Material Properties of TRIP Steels

Fig 6: Typical engineering stress-strain curve for TRIP steels

A typical engineering stress-strain curve for TRIP steels is shown in Figure 6. As can be seen, TRIP steels have a large amount of work hardening. The high work hardening can be attributed to the TRIP effect, as well as the fact that TRIP steels are primarily composed of soft ferrite and hard bainite. This "dual phase" nature allows for local deformation of the ferrite phase while maintaining a high tensile strength. Indeed, their tensile strengthW is typically twice the value of their yield strengthW. [2] This means that TRIP steels also exhibit very stable work hardening, where the onset of necking occurs at relatively high elongation values (over 25%). This makes TRIP steels ideal for forming operations such as stamping or bending. Forming operations are often limited by the loss of strength of the component due to wall thinning, or rupture because the material reached its forming limit. TRIP steels are ideal for such operations because they have a high formability limit and have stable yield point elongation which increases the structural integrity of formed components.

[edit] Improving the performance of TRIP steels

[edit] Improving the galvanized surface finish of TRIP steels

Hot-dip galvanizingW is a widely used surface treatment for steels. During the process, molten zinc bonds with the iron to form a layer which protects against corrosion. Original TRIP steels only contained silicon as the alloying element used to suppress the formation of the cementite phase. The content of silicon in these alloys was approximately 1.5 % by weight. This relatively high silicon content formed silicon oxide at the surface of the steel prior to the galvanizing process. [9] This oxide severely degraded the properties of the galvanized surface coating.[4] Newer TRIP steels have partially or completely replaced silicon with aluminium as an alloying component. The aluminium plays the same role as the silicon, but does not have negative effects on the surface finish during galvanizing. Therefore, the silicon content can be reduced while maintaining the TRIP properties of the steel.

[edit] High strength micro-alloying of TRIP steels

Basic TRIP steels have a tensile strength of approximately 600 MPa. However, by varying the alloy content, TRIP steels can have tensile strengths above 800 MPa. This was first accomplished by raising the carbon content of the alloy to approximately 0.4 % by weight.[1] However, this high carbon content leads to poor weldability. As well, the retained austenite becomes more stable due to the increased carbon content, which diminishes the formability of the TRIP steel. Instead of increasing the carbon content, alloying elements such as titanium, niobium, and vanadium can be used to give TRIP steels added tensile strength. [1] These alloying elements increase the strength of the steel through precipitation hardeningW, while having a minimal effect on weldability and formability.

[edit] Improving fuel efficiency of automobiles

TRIP steels are an ideal choice for structural materials in automobiles. They have the ductility and stable work hardening necessary to withstand high deformation processes such as stamping. As well, their high tensile strength makes them ideal for highly stressed components. Finally, they have excellent energy absorption properties because of their ductility and strength, which can improve vehicle safety during a crash. Because of these beneficial properties, TRIP steels could be used in smaller quantities to replace current steel components. This is known as "down-gauging", where thinner sheets of steel are used to form components.

[edit] Weight reduction estimate

Approximately 55 percent of the mass of an average passenger car is made of steel.[10] It has been shown that the volume of a mild steel formed sheet can be reduced by 20 percent by using TRIP steel, while maintaining the same stiffness. [11] Therefore, it can be assumed that the use of TRIP steel could reduce the mass of steel on a vehicle by 20 percent, and the total vehicle mass by 11 percent.

[edit] Effect of weight reduction on fuel efficiency

The percentage of consumed fuel that goes directly into recovering inertial losses from braking is 5.8 percent. [12] The weight savings from using TRIP steel would reduce the amount of inertial losses by 11 percent, because kinetic energy is directly proportional to mass. This means that using TRIP steels could lead to a reduction in total fuel consumption of 0.64 percent.

Please note this is an extremely conservative assumption as rolling resistanceW is also dependent on mass and was not included in this example.

[edit] Magnitude of effect on a global scale

If we assume that an average passenger vehicle consumes 10L/100km (23.5 mpg), and that an average car travels 20 000 km (12500 miles) in one year, the weight savings translate to an annual fuel consumption reduction of 12.8 liters (3.33 gallons). In 2007, there were 136 billion passenger cars in the United States. [13] This means that using TRIP steels in vehicle fabrication has the potential to reduce fuel consumption by 1.74 billion liters (460 million gallons). This represents a carbon dioxide emissions reduction of 4.18 billion kg (1.9 billion lbs), which is a reduction of 0.07% of the total United States carbon dioxide emissions.[14]

[edit] Economics of TRIP steels

Steel is commonly used because of its strength, formability and low cost relative to other metals. Metals such as titanium, magnesium, and aluminum have a higher strength to weight ratio and could offer significant weight savings in automobile components. However, they are much more expensive due to their lower abundance, higher production costs, and higher machining costs. As well, worldwide supply of these metals is fairly limited. These factors prevent these metals from being commonly used in low end production cars, which the majority of the population drives. TRIP steels do not face any of these difficulties, because they are a low alloy steel. Implementing the intercritical annealing step in steel processing should not prove to be difficult. This means that TRIP steels could be produced for the same price as other high strength steels. The most important barrier TRIP steels have faced in their market integration is the poor galvanizing surface finish. The galvanizing process is used on a large number of automotive components, because it is easy, inexpensive, and effective. With the recent discovery of aluminium as a replacement for silicon, TRIP steels no longer face past difficulties with galvanizing. TRIP steels can now be effectively protected from corrosion in an economically viable way, which means we may be seeing commercial production of TRIP steel products in the near future.

[edit] References

  1. 1.0 1.1 1.2 1.3 M. Zhang & Al., "Continuous cooling transformation diagrams and properties of micro-alloyed TRIP steels", Materials Science and Engineering A 438-440, 2006.
  2. 2.0 2.1 U.S. Steel TRIP Steels (2009) Available: http://xnet3.uss.com/auto/tech/grades/TRIP_main.htm
  3. A. Mark, "Microstructural Effects on the Stability of Retained Austenite in Transformation Induced Plasticity Steels", Ph.D. Thesis, Queen's University, 2007.
  4. 4.0 4.1 4.2 S. Chatterjee & Al., "Delta TRIP steel", Materials Science and Technology, Vol 23 No 7,819-827, 2007.
  5. E. Emadoddin & Al. "Effect of cold rolling reduction and intercriticalannealing temperature on the bulk texture oftwo TRIP-aided steel sheets", Journal of Materials Processing Technology 203, 293-300, 2008.
  6. 6.0 6.1 William D. Callister, "Materials Science and Engineering An Introduction", 7th edition, Wiley, 2007. p.292
  7. Qiang Liu & Al., "Research and development of 780 MPa cold rolling TRIP-aided steel", International Journal of Minerals, Metallurgy and Materials, Vol 16, Num 4, 399-406, 2009.
  8. G.B. Olson, Morris Cohen, "Kinetics of Strain-Induced Martensitic Nucleation", Metallurgical Transactions A, Vol 6A, 971, 1975.
  9. Wolfgang Bleck, “Using the TRIP effect – the dawn of a promising group of cold formable steels”, International Conference on TRIP-Aided High Strength Ferrous Alloys
  10. "How Much Steel is there in the Average Car?", Driving Today, September 15 (1999), Available:http://www.drivingtoday.com/wcco/news_this_week/1999-09-15-306-driving/index.html
  11. W. Li & Al., “Application of TRIP Steel to Replace Mild Steel in Automotive Parts”, International Conference on Advanced High Strength Sheet Steels for Automotive Applications Proceedings, 31-36, June 2004
  12. "Advanced Technologies and Fuel Efficiency", Available: http://www.fueleconomy.gov/FEG/atv.shtml
  13. ”Number of U.S. Aircraft, Vehicles, Vessels, and Other Conveyances”, Bureau of Transportation Statistics, Available: http://www.bts.gov/publications/national_transportation_statistics/html/table_01_11.html
  14. ”CO2 Emissions per Capita United States”, Google Public Data, Available: http://www.google.com/publicdata?ds=wb-wdi&met=en_atm_co2e_pc&idim=country:USA&q=us+carbon+dioxide+emissions
Mat.png This page was developed as part of a project for MECH370, a Queen's University class on materials processing. It is now open edit.