Process for manufacturing a cold rolled trip steel product

Abstract

The present invention is related to a process comprising a cold rolling step, for the production of uncoated, electro-galvanised or hot dip galvanised TRIP steel products,hot rolling a slab of a specific composition, wherein the finishing rolling temperature is higher than the Ar3 temperature, to form a hot-rolled substrate,cooling said substrate to a coiling temperature (CT) between 500° C. and 680° C.,coiling said substrate at said coiling temperature,pickling said substrate to remove the oxides,cold rolling said substrate to obtain a reduction of thickness, with a minimum reduction of 40%.

Description

FIELD OF THE INVENTION[0001]The present invention is related to a steel composition comprising phosphor, to be used for the production of TRIP steel products. The invention is equally related to the process of production of said products, and to the end products themselves.STATE OF THE ART[0002]In the automotive industry there is a need for weight reduction, which implies the use of higher strength materials in order to be able to decrease the thickness of the parts without giving up safety and functional requirements. Ultra high strength steel (UHSS) sheet products and in particular TRIP steel products, showing a remarkable combination of high strength and good formability, can provide the solution for this problem. Additionally, an increased corrosion resistance of these steel sheet products by means of electro or hot dip galvanising, is frequently asked for.[0003]Several documents are describing such UHSS products. EP-A-1096029 is related to the production of a tempered martensit...

AI Technical Summary

Benefits of technology

The present invention is related to a cold rolled TRIP steel composition that is suitable for various applications such as uncoated, electro-galvanised or hot dip galvanised materials. The invention is characterized by a specific combination of elements such as P, Si, Al, and C, which allows for better strength levels and weldability. The invention also includes a process for manufacturing the cold rolled TRIP steel product, which involves preparing a steel slab, hot rolling it, cooling it, and pickling it. The invention is further related to a steel product produced according to the process, which has a microstructure comprising ferrite, bainite, retained austenite, and possibly martensite. The invention is also related to a steel product with a carbon content between 1300 ppm and 2300 ppm, which has a yield strength between 320 MPa and 480 MPa, a tensile strength above 590 MPa, an elongation A80 higher than 26% and a n-value higher than 0.2.

Problems solved by technology

This extra process step will not only increase the total processing cost, but it will also complicate logistics as well as weldability at the entrance of the continuous annealing or hot dip galvanising line.

Cracks that are initiated in the weld or heat affected zone, will easily propagate into the hard and brittle martensitic structure, leading to a high risk on complete rupture of the weld between two coils.

The second major drawback is related to the rather high Si content in these steels.

From about 0.5% Si on, these high Si contents are well known to provoke problems as to surface quality because of the presence of Si-oxides which after pickling create a surface with irregular and very high roughness.

Moreover, in view of corrosion protection, hot dip galvanising of such a high Si-containing substrate in general leads to insufficient surface appearance for automotive applications, with moreover a very high risk on the presence of bare spots on the surface.

The high Si content in this invention will however again impair hot dip galvanisability resulting in an insufficient surface appearance and a very high risk on bare spots.

The occurrence of red scale which is difficult to remove, on the hot strip, due to the higher Si content, is also expected to cause processing difficulties.

However, Al, as opposed to Si, does not produce a strong solid solution strengthening effect.

These high Al levels are however known to cause stickers during continuous casting and impair weldability due to the presence of Al-oxides in the welded area.

This is especially detrimental for the crash behaviour of welded structures.

In order to avoid the casting problems an adapted very fine casting powder is required that could cause health problems.

Steelmaking plants are therefore in general not willing to produce this kind of compositions because the workers have to carry masks and a lot of special precautions have to be taken.

All of them require the steel sheet to be held for 2 sec to 10 min. in an atmosphere containing not less than 2% ammonia in the temperature range 550-800° C. It is clear that this nitriding step makes processing a lot more difficult and requires complicated technical modifications to existing installations.

At the moment this process is internationally not considered to be industrially feasible.

Furthermore the very low alloying content of this steel grade, does not allow to reach tensile strength levels above 650 MPa.

However, as Al and Si are both ferrite stabilizers, their sum is limited in order to avoid over-stabilizing the retained austenite.

The latter is considered to give rise to hot dip galvanising difficulties (bad wettability by molten Zn) and a deteriorated surface appearance (bare spots).

None of these example steels contained micro-alloying additions as in high Si-TRIP steels, these are known to markedly increase the hot strip hardness, leading to strongly increased cold rolling forces.

A large disadvantage of these compositions is the necessity of adding Cu and Ni, elements which are considered as impurities in bulk flat carbon steel production.

If a steelmaking plant has to cast this, extra logistic problems with scrap recycling occur.

Moreover, the use of Ni, Cu and Cr makes the alloying cost much more expensive.

Elongation values are in all cases rather low, which can be explained by the limited addition of Al and C, rendering the retained austenite less stable.

This can lead to a non-compatibility between different galvanising lines (with e.g. different lengths of the levelling zone around 490-460° C.) or even to strongly thickness-dependent mechanical properties.

This can be explained by the insufficient stabilisation of the retained austenite and the shift of the optimum overageing time to noticeably longer times due to the too low Al-additions.

Furthermore the high Al-content requires the use of an adapted very fine casting powder that can give rise to health problems.

Furthermore the weldability can be impaired due to the presence of Al-oxides in the welded area, a consequence of the high Al-contents.

As already explained before, the low-Al steel will suffer from mechanical properties that are very sensitive to process parameter variations such as line speed and overageing temperature.

This can lead to a non-compatibility between different galvanising lines or even to strongly thickness-dependent mechanical properties.

The high-Al steel on the other hand again requires the use of an adapted casting powder that can give rise to health problems.

Furthermore the weldability will be impaired due to the presence of Al-oxides in the welded area.

As no specifications were given related to the Al / Si ratio in this analysis, it is not possible to conclude about the galvanisability.

The mentioned P-content is insufficient for obtaining the desired mechanical properties (high tensile strength in combination with high elongation), whilst maintaining a good weldability (sufficient carbon content reduction).