What’s TWIP Steel?

In last article, we introduced the TRIP steel for automotive application, today we continue to discuss one of the AHSS, TWIP steel. TWIP Steel is a high Mn Al-Si Austenitic high strength steel first discovered by Sir Robert Hadfield (1888). In 1997, Grassel et al. found in the experimental study of Fe-Mn-Si-Al TRIP steel that when the manganese content reached 25wt %, aluminum exceeded 3wt %, and the silicon ranged from 2wt % to 3wt %, the product of tensile strength (Rm) and elongation (A) of the steel was more than 50000MPa %, which was twice as high strength and toughness TRIP steel.


The matrix structure of TWIP steel after rolling, water quenching and annealing treatment is austenite with a large number of annealing twins. When crystal twin deformation under the action of shear stress occurs when part of the crystal along the twin surface and twin direction relative to the other part of the crystal to make uniform shear, crystal lattice types do not change, but it make uniform shear zone in the crystal orientation is changed, become and not shear zone crystal orientation of mirror symmetry. The crystal in the deformed part changes to a new favorable orientation, which can further stimulate the slip. This makes TWIP steel more plastic.


TWIP steel has a good combination of high strength, high ductility and damage resistance and is widely used in the automotive and mechanical engineering industries. High strength allows carmakers to reduce body weight, while high malleability and formability make car designs more complex. TWIP steel is considered to cause high flow stress (600-1100 MPa) and abnormal elongation (60-95%).In addition, the addition of Al can effectively reduce the specific gravity of these steels (6.8-7.3g/cm3, depending on Al content) and reduce carbon emissions and fuel consumption. At present, TWIP steel in industrial production, such as TWC450Y950T in fiat standard MS. 50002, has tensile strength of 950MPa and elongation of 47%.


There are several widely used TWIP steels: Fe-Mn-C TWIP, Fe-Mn-C-Al TWIP, Fe-Mn-Al-Si TWIP series. The rough range of alloying elements proposed in TWIP steel is reasonable, since the cementite precipitation occurs in austenite, the carbon content above 1.2 % by weight is useless. The chemical composition range of TWIP steel (wt.%) is:

C

0010010 nbsp;

Mn

0010010 nbsp;

Al

0010010 nbsp;

Si

0010010 nbsp;

N

0010010 nbsp;

Ti, 0010010 nbsp; V, Cu, Nb, Cr

0.5-1.2

0010010 nbsp;

15-30

0010010 nbsp;

2.0-3.0

0010010 nbsp;

0-3

0010010 nbsp;

0010010 lt;0.21

0010010 nbsp;

0010010 lt;0.1

0010010 nbsp;

0010010 nbsp;

Generally speaking, TWIP steel has a high Mn content (12~30%) and a small amount of C ( 0010010 lt; 1%), Si ( 0010010 lt;3%) or Al ( 0010010 lt;3%).The structure at room temperature is a single austenite and a small amount of annealing twins. A large number of Mn is essential to maintain the austenitic structure of the Fe-Mn-al ternary alloy system, and is beneficial to control the lamellar fault energy (SFE) of the Fe-Mn-Al ternary alloy system. The addition of aluminum significantly increases SFE, thus stabilizing austenite and preventing Fe-Mn alloy from undergoing phase transition during deformation. At the same time, aluminum can strengthen austenite grain by solid solution strengthening.

0010010 nbsp;

Advantages and disadvantages of TWIP steel



Although TWIP steel has excellent mechanical properties and processing properties, such as high strength, high elongation, complex molding capacity, high fatigue performance and other characteristics. However, TWIP steel still has some disadvantages such as:

1. High cost caused by high content of alloy

2. Weldability caused by high content of alloy

3. Hidden danger of delayed fracture (hydrogen induced fracture)

4. The mechanism is still not very clear

5. Very high requirements for process control