The present invention relates to a high strength steel sheet having high ductility and formability and to a method to obtain such steel sheet.

To manufacture various items such as parts of body structural members and body panels for automotive vehicles, it is known to use sheets made of DP (Dual Phase) steels or TRIP (Transformation Induced Plasticity) steels.

To reduce the weight of the automotive in order to improve their fuel efficiency in view of the global environmental conservation, it is desirable to have sheets having improved yield and tensile strengths. But such sheets must also have a good ductility and a good formability and more specifically a good stretch flangeability.

The purpose of the invention therefore is to provide a steel sheet reaching a yield strength of at least 700 MPa, a tensile strength of at least 900 MPa, a uniform elongation of at least 12% and a hole expansion ratio of at least 20%.

The object of the present invention is achieved by providing a steel sheet according to claim 1. The steel sheet can also comprise characteristics of anyone of claims 2 to 11. Another object is achieved by providing the method according to claim 12. The method can also comprise characteristics of anyone of claims 13 to 14. Another object is achieved by providing the spot welded joint according to claim 15 or 16.

The invention will now be described in details and illustrated by examples without introducing limitations.

Hereinafter, Ae1 designates the equilibrium transformation temperature below which austenite is completely unstable, Ae3 designates the equilibrium transformation temperature above which austenite is completely stable, Ar3 designates the temperature until which the microstructure remains fully austenitic upon cooling, TQ designates the temperature above which the cementite gets dissolved upon heating and Ms designates the martensite start temperature, i.e. the temperature at which the austenite begins to transform into martensite upon cooling.

All compositional percentages are given in weight percent (wt.%), unless indicated otherwise.

The composition of the steel according to the invention comprises, by weight percent:

- 0.03% £ C £ 0.25% for ensuring a satisfactory strength and improving the stability of the retained austenite which is necessary to obtain a sufficient elongation. Preferably, the carbon content is higher than or equal to 0.1 %. If the carbon content is too high, the hot rolled sheet is too hard to cold roll and the weldability is insufficient. If the carbon content is below 0.03%, the tensile strength will not reach the targeted values.

- 3.5% < Mn < 8% for ensuring a satisfactory strength and achieving stabilization of at least part of the austenite, to obtain a sufficient elongation. Below 3.5%, the final structure comprises an insufficient retained austenite fraction, and an insufficient Mn content in the retained austenite, so that the desired combination of ductility and strength is not achieved. The maximum is defined to avoid having segregation issues which are detrimental for the ductility. Preferably, the manganese content is higher than or equal to 3.7%.

- 0.1 % < Si £ 2.0% and 0.03% < Al < 1.2%. According to the invention Si and Al together play an important role: silicon delays the precipitation of cementite upon cooling below the equilibrium transformation temperature Ae3. Therefore, a Si addition of at least 0.1 % helps to stabilize a sufficient amount of retained austenite. Si further provides solid solution strengthening and retards the formation of carbides during carbon redistribution from martensite to austenite resulting from an immediate reheating and holding step performed after a partial martensitic transformation. At a too high content, silicon oxides form at the surface, which impairs the coatability of the steel. Therefore, the Si content is less than or equal to 2.0%.

Aluminum is a very effective element for deoxidizing the steel in the liquid phase during elaboration. In addition, Al is an alpha-former element that increases the Ae1 and Ae3 temperatures of the steel. Thus, owing to the addition of at least 0.03% of Al, the intercritical domain (i.e. between Ae1 and Ae3) is in a temperature range favoring the partitioning of Mn in the austenite, as described in further details below. The Al content is not higher than 1 .2% in order to avoid the occurrence of inclusions, to avoid oxidation problems and to ensure the hardenability of the material.

The steel according to the invention must contain at least one element chosen among niobium, titanium and vanadium, in a minimum combined content of at least 0.01 % Such addition will allow strengthening the recovered martensite by limiting the growth of martensitic laths through precipitation.

Nb £ 0.080% can be added in order to refine the austenite grains during hot-rolling and to provide precipitation strengthening. In a preferred embodiment, the minimum amount of niobium added is 0.010%. Above 0.080% of addition, yield strength, elongation and hole expansion ratio are not secured at the desired level.

Ti < 0.080% can be added to provide precipitation strengthening. In a preferred embodiment, the minimum amount of titanium added is 0.010%. However, when its amount is above or equal to 0.080%, yield strength, elongation and hole expansion ratio are not secured at the desired level

V < 0.2% can be added to provide precipitation strengthening. In a preferred embodiment, the minimum amount of vanadium added is 0.010%. However, when its amount is above or equal to 0.2%, yield strength, elongation and hole expansion ratio are not secured at the desired level.

The remainder of the composition of the steel is iron and impurities resulting from the smelting. In this respect, Ni, Cu, S, P and N at least are considered as residual elements which are unavoidable impurities. Therefore, their contents are less than 0.05% for Ni, 0.03% for Cu, 0.010% for S, 0.020% for P and 0.008% for N.

Some elements can optionally be added to the composition of the steel according to the invention:

0.1 % < Mo < 0.5%. Molybdenum increases the hardenability, stabilizes the retained austenite thus reducing austenite decomposition during partitioning, and reduces the central segregation which can result from the high manganese content and which is detrimental to the hole expansion ratio. Furthermore, Mo helps refining the structure. Above 0.5%, the addition of Mo is costly and ineffective in view of the properties which are sought after.

- 0.01 % < Cr < 1 % to delay the dissolution of carbides and stabilize the retained austenite. A maximum of 1 % of chromium is allowed, above a saturation effect is noted, and adding chromium is both useless and expensive.

- 0.0005% < B < 0.004%. in order to increase the quenchability of the steel.

Preferably, the composition of the steel is such that the steel has a carbon equivalent Ceq lower or equal to 0.4%, the carbon equivalent being defined as Ceq = C% + Si%/55 + Cr%/20 + Mn%/19 - Al%/18+ 2.2*P% - 3.24*B% - 0.133*Mn% * Mo%.

The microstructure of the cold-rolled and heat-treated steel sheet according to the invention will be now described.

The cold-rolled and heat-treated steel sheet has a structure consisting of, in surface fraction:

- between 10% and 30% of retained austenite, said retained austenite being present as films having an aspect ratio of at least 3 and as MA islands, less than 8% of such MA islands having a size above 0.5 pm,

- at most 10% of fresh martensite and

- recovered martensite containing precipitates of at least one element chosen among niobium, titanium and vanadium.

The surface fractions and aspect ratio are determined through the following method: a specimen is cut from the cold-rolled and heat-treated, polished and etched with a reagent known per se, so as to reveal the microstructure. The section is afterwards examined through optical or scanning electron microscope, for example with a Scanning Electron Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification greater than 5000x, coupled to an Electron Backscatter Diffraction (“EBSD”) device and to a Transmission Electron Microscopy (TEM). The determination of the surface fraction of each constituent are performed with image analysis through a method known per se. The retained austenite fraction is for example determined by X-ray diffraction (XRD).

The microstructure of the cold-rolled and heat-treated steel sheet includes at least 10% of austenite which is, at room temperature, retained austenite. When present in surface fraction of at least 10%, retained austenite contributes to increasing ductility. Above 30%, the required level of hole expansion ratio HER according to ISO 16630:2009 is lower than 20%.

The retained austenite is present as films having an aspect ratio of at least 3 and as MA (Martensite Austenite), islands less than 8% of such MA islands having a size above 0.5 pm.

The specific minimum value of aspect ratio of the residual austenite films and the maximum percentage of MA islands having a size above 0.5 pm have to be respected to obtain the required level of hole expansion ratio HER according to ISO 16630:2009.

In a preferred embodiment, the cold-rolled and heat-treated steel sheet according to the invention is such that the fraction ratio between MA islands having a size above 0.5 pm and the austenite film is below 1 .0 or, even better, below 0.5.

In another preferred embodiment, the cold-rolled and heat-treated steel sheet according to the invention is such that less than 5% of such MA islands have a size above 0.5 pm.

In another preferred embodiment, the cold-rolled and heat-treated steel sheet according to the invention is such that the surface fraction of austenite films having an aspect ratio above 3 is at least 8%.

The microstructure of the cold-rolled and heat-treated steel sheet includes at most 10% of fresh martensite. Indeed, a fraction of fresh martensite higher than 10% would lead to a hole expansion ratio HER according to ISO 16630:2009 lower than 20%.

In another preferred embodiment, the cold-rolled and heat-treated steel sheet according to the invention is such that the surface fraction of fresh martensite is below 5%. The microstructure of the cold-rolled and heat-treated steel sheet includes recovered martensite containing precipitates of at least one element chosen among niobium, titanium and vanadium. If such precipitates are not present, the steel grade can not reach the minimum value of tensile strength targeted by the invention.

Recovered martensite can be distinguished from fresh martensite on a section polished and etched with a reagent known per se, for example Nital reagent, observed by Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD).

The steel sheet according to the invention can be produced by any appropriate manufacturing method and the man skilled in the art can define one. It is however preferred to use the method according to the invention comprising the following steps:

Hot rolled sheet having a thickness between, for example, 1 .8 to 6 mm can be produced by casting a steel having a composition as mentioned above so as to obtain a slab, reheating the slab at a temperature T _re heat comprised between 1 150°C and 1300°C, and hot rolling the reheated slab, the final rolling temperature being higher than Ar3, to obtain a hot rolled steel.

The final rolling temperature is preferably of at most 1000°C, in order to avoid coarsening of the austenitic grains.

The hot-rolled steel is then cooled, at a cooling rate for example comprised between 1 °C/s and 120°C/s, and coiled at a temperature Tcoii comprised between 20°C and 600°C.

After the coiling, the sheet can be pickled.

The hot-rolled steel sheet is then annealed, in order to improve the cold- rollability and the toughness of the hot-rolled steel sheet, and in order to provide a hot-rolled and annealed steel sheet which is suitable for producing a cold-rolled and heat-treated steel sheet having high mechanical properties, in particular a high strength and a high ductility.

In a preferred embodiment, the annealing performed on the hot-rolled steel sheet is a batch annealing, performed at a temperature comprised between 500°C and 680°C, during 1000 s to 50000 s.

The hot-rolled and annealed steel sheet is then optionally pickled. The hot-rolled and annealed steel sheet is then cold-rolled to obtain a cold rolled steel sheet having a thickness that can be, for example, between 0.7 mm and 3 mm, or even better in the range of 0.8 mm to 2 mm.

The cold-rolling reduction ratio is preferably comprised between 20% and 80%. Below 20%, the recrystallization during subsequent heat-treatment is not favored, which may impair the ductility of the cold-rolled and heat-treated steel sheet. Above 80%, there is a risk of edge cracking during cold-rolling.

The cold-rolled steel sheet is then heat treated on a continuous annealing line.

The heat treatment comprises the steps of:

- reheating the cold-rolled steel sheet to a first annealing temperature above 860°C and maintaining the cold-rolled steel sheet at said annealing temperature for a holding time comprised between 30 s and 600 s, so as to obtain, upon annealing, a fully austenitic structure, The reheating rate to the first annealing temperature is preferably comprised between 1 °C/s and 200°C/s.

- quenching the cold-rolled steel sheet at a cooling rate comprised between 0.5°C/s and 200°C/s, to a quenching temperature comprised between 20°C and Ms-50°C and maintaining it at said quenching temperature for a holding time comprised between 1 and 200 s,

The cooling rate is chosen so as to avoid the formation of pearlite upon cooling. For each particular composition of the steel and each structure, one skilled in the art knows how to determine the Ms start transformation point of the austenite by dilatometry.

During this quenching step, the austenite partly transforms into martensite.

If the quenching temperature is lower than 20°C, the fraction of recovered martensite in the final structure is too high to stabilize a sufficient amount of retained austenite above 10%. Besides, if the quenching temperature is higher than Ms-50°C, the fraction of recovered martensite in the final structure is too low to obtain the desired whole expansion ratio.

- optionally holding the quenched sheet at the quenching temperature for a holding time comprised between 1 s and 200 s, preferably between 3 s and 7 s, so as to avoid the formation of epsilon carbides in martensite, that would result in a decrease in the elongation of the steel.

- reheating the cold-rolled steel sheet to a second annealing temperature comprised between TQ and 720°C, and maintaining the cold-rolled steel sheet at said annealing temperature for a time comprised between 100 s and 2000 s,

During this second annealing step, the cementite gets dissolved and the carbon and Mn diffuse from the martensite to the austenite, thereby achieving an enrichment in carbon and Mn of the austenite and recovering the martensite.

- optionally hot-dip coating the sheet in a bath at a temperature lower than or equal to 480°C. Any kind of coatings can be used and in particular, zinc or zinc alloys, like zinc-nickel, zinc-magnesium or zinc-magnesium- aluminum alloys, aluminum or aluminum alloys, for example aluminum-silicon.

- immediately after the second annealing step, or immediately after the hot-dip coating step, if performed, cooling the cold-rolled steel sheet to the room temperature, to obtain a cold-rolled and heat treated steel sheet. The cooling rate is preferably higher than 1 °C/s, for example comprised between 2°C/s and 20°C/s.

During this cooling step, part of the austenite may transform into fresh martensite. However, the surface fraction of the fresh martensite remains lower than or equal to 10%, owing to the stabilization of austenite with carbon and manganese.

- optionally, after cooling down to the room temperature, if the hot-dip coating step has not been performed, the sheet can be coated by electrochemical methods, for example electro-galvanizing, or through any vacuum coating process, like PVD or Jet Vapor Deposition. Any kind of coatings can be used and in particular, zinc or zinc alloys, like zinc-nickel, zinc- magnesium or zinc-magnesium-aluminum alloys. Optionally, after coating by electro-galvanizing, the sheet may be subjected to degassing. Examples

Three grades, which compositions are gathered in table 1 , were cast in semi-products and processed into steel sheets following the process parameters gathered in table 2, going through heating, controlled hot rolling and subsequent water cooling, achieved by quenching and self-tempering.

Table 1 - Compositions

The tested compositions are gathered in the following table wherein the element contents are expressed in weight percent:

Steel A and B are according to the invention whereas steel C is a comparative example.

For a given steel, one skilled in the art knows how to determine Ae1 , Ae3 and TQ temperatures through dilatometry tests and metallography analysis.

o

Table 2 - Process parameters

Steel semi-products, as cast, were reheated at 1250°C, hot rolled and then coiled at 550°C, pickled, annealed at 600°C

o during 5 h, pickled and cold rolled with a 50% reduction rate. They were then processed under the following conditions:

n

H

w o o o m

The resulting samples were then analyzed and the corresponding microstructure elements and mechanical properties were respectively gathered in table 3 and 4. Table 3 - Microstructure and precipitates

The phase percentages of the microstructures of the obtained steel sheet were determined:

* : trials according to the invention.

Y : stands for residual austenite surface fraction

Y aspect ratio : stands for the aspect ratio of austenite films

MA : stands for MA islands surface fraction with a size above 0.5 pm

FM :stands for fresh martensite surface fraction

RM :stands for recovered martensite surface fraction

RF stands for recrystallized ferrite surface fraction. Precipitates in RM : stands for presence of precipitates of Nb in recovered martensite

Table 4 - Mechanical properties

Mechanical properties of the tested samples were determined and gathered in the following table:

* : trials according to the invention.

The yield strength YS, the tensile strength TS and the uniform elongation UE are measured according to ISO standard ISO 6892-1 , published in October 2009. The hole expansion ratio HER is measured according to ISO standard 16630:2009. Due to differences in the methods of measure, the values of the hole expansion ratio HER according to the ISO standard 16630:2009 are very different and not comparable to the values of the hole expansion ratio l according to the JFS T 1001 (Japan Iron and Steel Federation standard).

The examples show that the steel sheets according to the invention, namely examples 1 to 4 and 8 to 11 are the only one to show all the targeted properties thanks to their specific composition and microstructures.