Dual-phase high strength steels are used by most car manufacturers for better crash protection, higher comfort and reduced fuel consumption. Thanks to their strain-hardening properties DP steels get even  stronger when the strengthis needed, in the case of an accident for example. TRIP steels are said to have the same advantage.

Which is the choice for your application? Karl-Inge Nilsson, product manager for Docol, the cold-reduced steels that are produced by SSAB Tunnplåt gives his answer to this question.

“Within SSAB we focus on the production of high  strength cold-reduced DP steels”, says Karl-IngeNilsson. “DP-steel can be produced very ef.ciently and reach the high demands regarding quality.”

This is, in his opinion, a problem for TRIP steels.

“It is very difficult to manufacture TRIP steels as these require very precise control of the annealing temperatures to achieve the desired mechanical properties”, Karl- Inge Nilsson explains.

“The spreading in yield strength of TRIP steels are double compared to dualphase steels. This causes more problems with springback by forming with TRIP compared to DP.”

The biggest advantage of TRIP-steel is that when the material reaches its speci- fications it has very good formability.

“Unfortunately tests in laboratories do not give the same result as practical experiences”, says Karl-Inge Nilsson. ”We have seen trials in real  production where our DP-steel Docol 600 DL gives similar results as TRIP steels in corresponding grades.”

In the automotive industry DP-steels are often used because of their energy absorption.

“Our DP-steels have a higher strain hardening compared to TRIP steels”, states Karl-Inge Nilsson. “Because of that I think that the energy absorption properties for dual- phase steels are equivalent to TRIP steels.”

When it comes to welding there are some important differences between DP and TRIP:

In general Dual-phase steels have a better weldability when spot-welding is used.

“We have seen trials with Docol 800 DP compared with TRIP 800 where the spot welded TRIP steel suffered from partial plug failures”, says Karl-Inge Nilsson. “Also with laser welding  DP-steelshave an advantage as the  hardness of the weld can cause trouble in TRIP-steel. There is no big difference between those steel grades when MAG-welding is used.”

Text: News Magazine No 2, 2005