Get the automotive offer application

for your smartphone or tablet now.

Europe product catalogue
Flat Carbon Europe website EN | FR | DE | ES

TRIP (TRansformation Induced Plasticity) steels

Description

TRIP steels offer an outstanding combination of strength and ductility as a result of their microstructure. They are thus suitable for structural and reinforcement parts of complex shape. The microstructure of these steels is composed of islands of hard residual austenite and carbide-free bainite dispersed in a soft ferritic matrix. Austenite is transformed into martensite during plastic deformation (TRIP: TRansformation Induced Plasticity effect), making it possible to achieve greater elongations and lending these steels their excellent combination of strength and ductility.

These steels have high strain hardening capacity. They exhibit good strain redistribution and thus good drawability. As a result of strain hardening, the mechanical properties, and especially the yield strength, of the finished part are far superior to those of the initial blank.

High strain hardening capacity and high mechanical strength lend these steels excellent energy absorption capacity. TRIP steels also exhibit a strong bake hardening (BH) effect following deformation, which further improves their crash performance.

The TRIP range of steels comprises 2 cold rolled grades in both uncoated and coated formats (TRIP 690 and TRIP 780) and one hot rolled grade (TRIP 780), identified by their minimum tensile strength expressed in MPa.

Applications

As a result of their high energy absorption capacity and fatigue strength, TRIP steels are particularly well suited for automotive structural and safety parts such as cross members, longitudinal beams, B-pillar reinforcements, sills and bumper reinforcements.

ArcelorMittal has extensive data on the forming and service characteristics of the TRIP family of steels. To integrate these steels at the design stage, a team of experts is available to carry out specific studies based on modeling and experimental tests.

B-pillar reinforcement in electrogalvanized TRIP 780 (thickness: 1.2 mm)

Bumper cross member in electrogalvanized TRIP 780 (thickness: 1.6 mm)

Designation and standard

 EuronormsVDA 239-100
TRIP 690HCT690T (+ZE,+Z)CR400Y690T-TR (-UNC,-EG,-GI)
TRIP 780HCT780T (+ZE, +ZF)CR450Y780T-TR (-UNC,-EG,-GA)
 

Euronorms
Uncoated (EN 10338 :2015): Steel grade name
Electrogalvanized (EN 10338 :2015 + EN 10152 :2009): Steel grade name+ZE
Galvannealed (EN 10346 :2015): Steel grade name+ZF
Extragal® (EN 10346 :2015): Steel grade name+Z

VDA 239-100
Uncoated: Steel grade name-UNC
Electrogalvanized: Steel grade name-EG
Galvannealed: Steel grade name-GA
Extragal®: Steel grade name-GI

 
 Hot rolled     Cold rolled

While the ArcelorMittal grades conform perfectly well to the indicated EN standards, ArcelorMittal grades generally offer tighter mechanical properties (see table below).

The above indicative table summarizes the European and VDA standards corresponding to the ArcelorMittal product range.

Technical characteristics

Mechanical properties
Guaranteed for 20x80 mm ISO tensile specimens (uncoated sheets) with the tensile axix parallel to the rolling direction

 YS (MPa)UTS (MPa)ef (%)
L0 = 80 mm
th < 3 mm
nBH2 (MPa)
 TRIP 690410 - 510690 - 800≥ 25≥ 0,1940
 TRIP 780*450 - 550780 - 900≥ 23≥ 0,1840
 
 Hot rolled     Cold rolled

* The product is also available with a minimum yield stress of 500 MPa

Typical cold rolled electrogalvanized TRIP 780 microstructure (residual austenite fraction about 18%)

Typical hot dip galvanized TRIP 690 microstructure (residual austenite fraction about 10%)

Chemical composition (%)

 CMnAl+Si
 MaxMaxMax
TRIP 6900,2002,02,0
TRIP 7800,2502,02,0
 
 Hot rolled     Cold rolled

Available coatings

 UncoatedElectrogalvanizedExtragal®GalvannealedJetgal®
 EURNAMSAMRSACHIEURNAMSAMRSACHIEURNAMSAMRSACHIEURNAMSAMRSACHIEURNAMSAMRSACHI
TRIP 690                    
TRIP 780                 

 Hot rolled     Cold rolled

Available in non-visible part quality Undergoing customer testing Under development Available in visible and non-visible part quality (Z)

EUR : Europe Region - NAM : North America Region - SAM : South America Region - RSA : South Africa Region - CHI : China

X Available - O Under development

Recommendations for use and secondary processing

Forming

TRIP steels offer high ductility for their tensile strength. For example, cold rolled TRIP 780 has uniform elongation comparable to that of an ArcelorMittal 04.

The figure below shows examples of forming limit curves for cold rolled TRIP 690 and TRIP 780 steels in 1.5 mm thickness. Their formability is superior to that of a lower strength Dual Phase 600 steel.

Forming limit curves for TRIP 690 and 780 (thickness: 1.5 mm)

Please consult us for more information about the forming of TRIP steels.

In order to fully exploit the potential of TRIP steels, the metal characteristics after forming rather than those of the initial blank should be used in the design stage.

Because of their very good drawability, this family of steels can be used to make safety and structural parts with both simple and complex geometries, provided springback is taken into account at the design stage.

Welding

Resistance spot welding

TRIP steels can be readily welded using conventional welding processes, provided the parameters are adjusted. Because of the high carbon equivalent, electrode forces must be increased and welding cycles adjusted to obtain high-quality weld spots. The risk of interface fracture, which can occur in TRIP-TRIP welds, can be reduced by optimizing the welding parameters.

The table below gives examples (indicative only) of spot welding parameters for TRIP 690 Extragal® and TRIP 780 electrogalvanized matching joints, determined according to the ISO 18278-2 standard:

 CoatingThickness (mm)Max. intensity (kA)Nugget diameter (mm)Pure tensile (kN)Tensile-shear (kN)
TRIP 690Extragal®1.08.36.56.713
TRIP 780Electrogalvanized1.07.76.75.513.7
 
 Hot rolled     Cold rolled

MAG arc welding

MAG (Metal Active Gas) arc welding employs a filler wire in a protective gas shield. It can be used for thicknesses greater than 0.8 mm. MAG weldability of TRIP 780 has been assessed using CMOS technology according to the EN 288 and EN 25817 standards for 1.5 mm thick butt joints. Heat input is of the order of 2 kJ/cm.

As a result of its chemical composition, TRIP 780 typically has a relatively high carbon equivalent of about 0.50. However, no particular precautions are needed to prevent cold cracking. The small thicknesses employed (< 2 mm) minimize restraint stresses during welding.

The most appropriate combination for MAG welding of TRIP 780 in a thickness range of about 1.5 mm is the following:

  • Filler: G3Si1 type in accordance with EN 440;
  • Protective gas: Ar + 8% CO2.
(M21 according to EN 439).

The CMOS evaluation shows satisfactory overall weld behavior meeting the mechanical strength criteria set out in the standards, given that:

  • bends are good up to 120° and crack on the reverse side at 180°;
  • all tensile test failures occur in the base metal, even with G3Si1 filler metal, as a result of the associated dilution.

Laser welding

Laser welding tests have revealed no particular difficulties.

Laser lap welding is particularly suitable for TRIP/TRIP joining.Based on long shop-floor experience in characterizing its products, ArcelorMittal can provide technical assistance in adjusting the welding parameters for all steels in the TRIP range.

Fatigue strength

Due to their high mechanical strength, TRIP grades have significantly better fatigue properties than conventional steels.

Examples of Wöhler curves for a variety TRIP grades are shown in the two graphs below. The curves plot maximum stress versus number of cycles to failure. They are calculated for two loading ratios: tension-compression R=-1 and tension-tension R=0.1.

Wöhler curves or S-N curves for TRIP steels

The graph below shows the low-cycle fatigue or E-N curves for the same steels. The curves plot strain amplitude versus number of reversals to failure (one cycle corresponds to two reversals). Other high and low cycle fatigue data can be provided on request.

Low-cycle fatigue or E-N curves for TRIP steels

ArcelorMittal can make a TRIP steel fatigue database available to its customers.

Impact strength

As a result of their very high tensile strength, TRIP steels are particularly suitable for parts designed to absorb energy in an impact.

TRIP steels have been characterized in dynamic axial compression tests using an omega structure with a spot-welded closure plate at an impact velocity of 56 kph. These tests have demonstrated the very good impact behavior of these steels.

Mass reduction potential compared to that of an HSLA 380 steel (reference)

These results are obtained for specimens produced by bending. Strain hardening during drawing enhances the energy absorption capacity of this grade. In order to fully exploit the potential of TRIP steels, the metal characteristics after forming (hardening) rather than those of the initial blank should be used in the design stage. Crushing tests have shown a 9% gain in energy absorption of drawn parts compared to parts obtained by bending.

Sign up to receive Automotive news alerts by email