Patent Description:
To manufacture various equipments 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.

It is also known to use steels having a bainitic structure, free from carbides precipitates, with retained austenite, containing about <NUM>% of C, about <NUM>% of Mn, about <NUM>% of Si, with a yield strength of about <NUM> MPa, a tensile strength of about <NUM> MPa, a total elongation of about <NUM>%. These sheets are produced on continuous annealing lines by cooling from an annealing temperature higher than the Ac<NUM> transformation point, down to a holding temperature above the Ms transformation point and maintaining the sheet at the temperature for a given time.

For example, <CIT> discloses a method for manufacturing a TRIP steel sheet, comprising <NUM>% to <NUM>% of the sum of ferrite and bainite, <NUM>% to <NUM>% of retained austenite, <NUM>% to <NUM>% of martensite and up to <NUM>% of pearlite. This method comprises the steps of annealing a hot-rolled or cold-rolled steel sheet, cooling the steel sheet to a cooling stop temperature and holding the sheet at this temperature for <NUM> to <NUM>. During the holding at the cooling stop temperature, the austenite first partly transforms into bainite. Then, the carbon partitions from the bainite to the austenite. However, according to the examples of <CIT>, no martensite is formed after the cooling to the cooling stop temperature and before the holding at this temperature. As a result, the martensite present in the structure, resulting from the final cooling, is not partitioned, and retains a relatively high C content, leading to a high yield strength and an unsatisfactory formability.

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.

In this respect, it is desirable to have coated or non-coated sheets having a tensile strength TS of at least <NUM> MPa, a total elongation TE of at least <NUM>%, preferably of at least <NUM>%, still preferably at least <NUM>%, and a hole expansion ratio HER of more than <NUM>%. The tensile strength TS and the total elongation TE are measured according to ISO standard ISO <NUM>-<NUM>, published in October <NUM>. It must be emphasized that, due to differences in the methods of measurement, in particular due to differences in the geometries of the specimen used, the values of the total elongation TE according to the ISO standard are very different, and are in particular lower, than the values of the total elongation measured according to the JIS Z <NUM>-<NUM> standard. The hole expansion ratio HER is measured according to ISO standard <NUM>:<NUM>. Due to differences in the methods of measure, the values of the hole expansion ration HER according to the ISO standard <NUM>:<NUM> are very different and not comparable to the values of the hole expansion ratio λ according to the JFS T <NUM> (Japan Iron and Steel Federation standard).

It is also desirable to have coated or non coated steel sheets having mechanical properties or features as mentioned above, in a thickness range from <NUM> to <NUM>, and more preferably in the range of <NUM> to <NUM>.

Therefore, the present invention aims at providing a sheet with the mechanical features and properties mentioned above and a method to produce it.

For this purpose, the invention relates to a method for producing a steel sheet according to claim <NUM>.

Preferably, the tempered martensite has a C content of at most <NUM>%.

Preferably, the quenched sheet has, just before the heating to the partitioning temperature PT, a structure consisting of, in area fraction, between <NUM>% and <NUM>% of ferrite, at least <NUM>% of retained austenite, at least <NUM>% of martensite and at most <NUM>% of lower bainite.

According to an embodiment, the method comprises, between the annealing step and the quenching step, a step of slow cooling the sheet to a temperature higher than or equal to <NUM> at a cooling rate lower than <NUM>/s.

In this embodiment, the ferrite comprises, in area fraction with respect to the whole structure, between <NUM>% and <NUM>% of intercritical ferrite, and between <NUM>% and <NUM>% of transformation ferrite, said transformation ferrite being formed during the slow cooling step, it being understood that the ferrite fraction, which is the sum of the intercritical ferrite and the transformation ferrite fractions, is comprised between <NUM>% and <NUM>%.

According to a particular embodiment, the step of providing said cold-rolled steel sheet comprises:.

Preferably, after the sheet is quenched to the quenching temperature QT and before the sheet is heated to the partitioning temperature PT, the sheet is held at the quenching temperature QT for a holding time comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

Preferably, the chemical composition of the steel satisfies at least one of the following conditions: C ≥ <NUM>%, C ≤ <NUM>%, Mn ≥ <NUM>%, Mn ≤ <NUM>%, <NUM>% ≤ Nb, Mo ≤ <NUM>% or Mo ≥ <NUM>%, Cr ≤ <NUM>%, or Cr ≥ <NUM>%.

According to a first particular embodiment, between the maintaining of the sheet at the partitioning temperature PT and the cooling of the sheet to the room temperature, the steel sheet is hot-dip coated at a temperature not more than <NUM>, the partitioning temperature PT is comprised between <NUM> and <NUM> and the partitioning time Pt is comprised between <NUM> and <NUM>.

According to a second particular embodiment, after the maintaining of the sheet at the partitioning temperature PT, the sheet is immediately cooled to the room temperature, the partitioning temperature PT is comprised between <NUM> and <NUM> and the partitioning time Pt is comprised between <NUM> and <NUM>.

In this embodiment, after the step of cooling down the steel sheet to the room temperature, the steel sheet is for example coated by an electrochemical method or through a vacuum coating process.

For example, the steel sheet is coated with Zn or a Zn alloy.

According to a particular embodiment, the chemical composition of the steel is such that C+Si/<NUM> ≤ <NUM>% and Al ≥ <NUM> (C+Mn/<NUM>)-<NUM>%.

In this embodiment, the chemical composition of the steel is preferably such that <NUM>% ≤ Si ≤ <NUM>% and <NUM>% < Al ≤ <NUM>%, still preferably such that <NUM>% ≤ Si < <NUM>% and <NUM>% ≤ Al ≤ <NUM>%.

According to another particular embodiment, the chemical composition of the steel is such that <NUM>% ≤ Si ≤ <NUM>% and <NUM>% ≤ Al ≤ <NUM>%.

The invention also relates to a process for producing a resistance spot weld of at least two steel sheets, said process comprising:.

The invention also relates to a steel sheet according to claim <NUM>.

According to an embodiment, the ferrite comprises, with respect to the whole structure, between <NUM>% and <NUM>% of intercritical ferrite and between <NUM>% and <NUM>% of transformation ferrite.

Preferably, the C content in the retained austenite is comprised between <NUM>% and <NUM>%.

Especially, a yield strength below <NUM> MPa allows guaranteeing an excellent formability.

The chemical composition of the steel preferably satisfies at least one of the following conditions: C ≥ <NUM>%, C ≤ <NUM>%, Mn ≥ <NUM>%, Mn ≤ <NUM>%, Mo ≤ <NUM>% or Mo ≥ <NUM>%, <NUM>% ≤ Nb, Cr ≤ <NUM>% or Cr ≥ <NUM>%.

According to another particular embodiment the chemical composition of the steel is such that <NUM>% ≤ Si ≤ <NUM>% and <NUM>% ≤ Al ≤ <NUM>%.

The steel sheet is for example coated with Zn or a Zn alloy, the coating resulting from a coating at a temperature less than <NUM>.

Preferably, the thickness of said steel sheet is comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

The invention also relates to a welded structure comprising at least ten resistance spot welds of at least two steel sheets, wherein a first steel sheet is according to the invention, coated with Zn or a Zn alloy and such that C+Si/<NUM> ≤ <NUM>% and Al ≥ <NUM> (C+Mn/<NUM>)-<NUM>%, and a second steel sheet has a composition such that C+Si/<NUM> ≤ <NUM>% and Al ≥ <NUM> (C+Mn/<NUM>)-<NUM>%, the mean number of cracks per resistance spot weld being less than <NUM>.

Preferably, the second steel sheet is according to the invention, and is coated with Zn or a Zn alloy.

The invention further relates to the use of a steel sheet produced according to the method of the invention, or of a steel sheet according to the invention, for the manufacture of structural parts in motor vehicles.

The invention further relates to the use of a resistance spot weld manufactured according to the process of the invention, or of a welded structure according to the invention, for the manufacture of structural parts in motor vehicles.

The invention will now be described in details with reference to the appended Figure, but without introducing limitations.

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

A certain amount of aluminum is combined with oxygen as Al<NUM>O<NUM>, and with nitrogen as AlN; this amount depends on O and N contents and remains less than <NUM>%. The remainder if it exists is not combined and consists in "free aluminum".

The aluminum which is combined with oxygen results from the deoxidation in the liquid stage. It is detrimental for to the ductility properties and therefore, its content has to be limited as much as possible.

The aluminum which is combined with nitrogen slows down the austenitic grains growth during annealing. Nitrogen is a residual element resulting from the smelting and is less than <NUM>% in the steel sheet.

After heating in the austenitic range, the inventors have found that Si and free Al stabilize the austenite by delaying the formation of carbides. This occurs in particular if the steel sheet is cooled at a temperature so as to obtain a partial martensitic transformation, and immediately reheated and maintained at a temperature PT during which the carbon is redistributed from martensite to austenite. If Si and free Al content additions are in sufficient amount, the carbon redistribution occurs without significant carbides precipitation. For this purpose Si + Al has to be more than <NUM>% in weight (but less than <NUM>%). Moreover, Si provides a solid solution strengthening and improves the hole expansion ratio. But the Si content has to be limited to <NUM>% to avoid the formation of silicon oxides at the surface of the sheet which would be detrimental to the coatability.

Moreover, the inventors have found that when Si/<NUM> > <NUM>% - C (Si and C being expressed in weight percentage), due to the LME (liquid metal embrittlement phenomenon), silicon is detrimental to the spot welding of coated sheets and particularly to galvanized or galvannealed or electrogalvanized sheets. LME occurrence causes cracks at grain boundaries in the heat affected zones and in the weld metal of welded joints. Therefore (C + Si/<NUM>) has to be maintained less than or equal to <NUM>%, especially if the sheet is to be coated.

They have also found that to reduce the LME occurrence, for the domain of composition which is considered, the Al content has to be higher than or equal to <NUM>(C+Mn/<NUM>) - <NUM>%.

Thus, according to a first embodiment, particularly when LME is not likely to appear, Al is added only to deoxidize or optionally to control the austenitic grains growth during annealing, its content being less than or equal to <NUM>%, for example less than <NUM>%, but at least <NUM>%. According to this first embodiment, the Si content is between <NUM>% and <NUM>%. In this embodiment, C +Si/<NUM> can be for example higher than <NUM>%.

According to a second embodiment, particularly when the problem of LME has to be considered, in particular when the sheet is coated with Zn or Zn alloys, C +Si/<NUM> has to be less than or equal to <NUM>%. It means that Si must remain less than <NUM>% at least when C is higher than <NUM>%. Thus, Al is added in more important quantities in order to replace at least partly Si to stabilize austenite and to reduce the LME sensitivity when C and/or Mn contents are too high. In this second embodiment, the Al content is such that Al ≥ <NUM>(C+Mn/<NUM>) - <NUM>% and Si + Al ≥ <NUM> %; therefore, Al is preferably comprised between <NUM>% and <NUM>% and Si is comprised between <NUM>% and <NUM>% preferably <NUM>% and <NUM>%. Preferably, the Al content is higher than or equal to <NUM>%. However, the Al content is limited to <NUM>% in order to prevent the increase of the Ac3 transformation temperature, which would imply higher cost when heating at high temperature for obtaining austenitization of the steel sheet in the annealing step.

The balance is iron and residual elements resulting from the steelmaking. In this respect, Ni, Cu, Ti, V, B, S, P and N at least are considered as residual elements which are unavoidable impurities. Therefore, their contents are less than <NUM>% for Ni, <NUM>% for Cu, <NUM>% for V, <NUM>% for B, <NUM>% for S, <NUM>% for P and <NUM>% for N. The Ti content is limited to <NUM>% because above such values, large-sized carbonitrides would precipitate mainly in the liquid stage, and the formability of the steel sheet would decrease, making the <NUM>% target for the total elongation more difficult to reach.

When the sheets are coated with Zn or Zn alloys, the spot weldability can be affected by the LME phenomenon (Liquid Metal Embrittlement).

The sensitivity of a particular steel to this phenomenon can be evaluated by tensile test performed at high temperature. In particular, this hot tensile test can be performed using a Gleeble RPI thermal simulator, such device being known per se in the art.

This test which is named "Gleeble LME test" is described as follows:.

Usually, it is considered that when the minimum critical displacement is less than <NUM> at a temperature between <NUM> and <NUM>, the probability of occurrence of LME in resistance spot welding is high, and when the minimum critical displacement is at least <NUM>, the probability to observe many LME cracks in resistance spot welding is low.

In this respect, the inventors have discovered that for steels corresponding to the present invention or similar to these steels, if the composition is such that (C+Si/<NUM>) is less than or equal to <NUM>%, and Al is higher than or equal to <NUM>(C+Mn/<NUM>)-<NUM>%, the minimum critical displacement is at least <NUM>, and when (C+Si/<NUM>) is more than <NUM>% and/or Al is lower than <NUM>(C+Mn/<NUM>)-<NUM>%, the minimum critical displacement is less than <NUM>, and even less than <NUM>.

As examples, Gleeble LME tests have been made with steels having the following compositions:.

For S1, C+Si/<NUM> = <NUM>% and the minimum critical displacement is <NUM>.

For S2, C+Si/<NUM> = <NUM>% and the minimum critical displacement is <NUM>.

Another method for evaluating the spot weldability of the coated sheets is a "LME spot welding test" which allows determining the probability to have cracked welds among an important number of resistance spot welds, for example in an industrial production of products comprising parts which are assembled by resistance spot welding such as, for example, car bodies.

This "LME spot welding test" is derived from the electrode life test for resistance spot welding in which a plurality of resistance spot welds, for example <NUM>, are performed on three sheets superposed together: the sheet to be tested and two support sheets made of galvanized low carbon sheets, for example DX54D+Z according to EN <NUM>. The thicknesses of the sheets are <NUM> and the resistance spot welds are made according to the ISO Standard <NUM> - <NUM> for heterogeneous assemblies. The parameters are:.

For this test, in order to determine the eventual occurrence of cracks in the resistance spot welds, the samples are cut and polished. The resistance spot welds are then etched with picric acid, and observed by microscope, for example with a 200x magnification, in order to determine the number of cracks in each observed resistance spot weld and the sum of the length of the cracks in each resistance spot weld.

For the examples S1 and S2, the proportions of the numbers of cracks for each resistance spot weld are as follows:.

If the mean number of cracks in each resistance spot weld is considered, the results are as follows:.

Hot rolled sheet having a thickness between <NUM> and <NUM> can be produced in a known manner from the steel composition of the invention mentioned above. As an example, the reheating temperature before rolling can be comprised between <NUM> and <NUM>, preferably about <NUM>, the finish rolling temperature is preferably comprised between Ar3 and <NUM>, and preferably higher than <NUM>, and the coiling is performed at a temperature preferably comprised between <NUM> and <NUM>. Preferably, if Si > <NUM>%, the coiling temperature is lower than or equal to <NUM>.

After the coiling, the sheet has a ferrito-pearlitic or ferrito-pearlito-bainitic structure.

After the coiling, the sheet is optionally batch annealed in order to reduce the hardness of the steel sheet and therefore improve the cold-rollability of the hot-rolled and coiled steel sheet.

For example, the hot-rolled and coiled steel sheet is batch annealed at a temperature between <NUM> and <NUM>, for example between <NUM> and <NUM>, for a time between <NUM> and <NUM> days, preferably between <NUM> and <NUM> days. This time includes the heating to the batch annealing temperature and the cooling from the batch annealing temperature to ambient temperature.

This batch annealing is preferably performed in the first embodiment of the steel composition, in particular if the steel comprises more than <NUM>% Si. In the second embodiment of the composition, the batch annealing step may be omitted.

The sheet can be pickled and cold rolled to obtain a cold rolled sheet having a thickness between <NUM> and <NUM>, for example in the range of <NUM> to <NUM>.

Then, the sheet is heat treated on a continuous annealing line, or, if the sheet is hot-dip coated, it is preferably treated on a combined continuous annealing and hot-dip coating line.

The heat treatment and optional coating comprise the steps of:.

The sheet is maintained at the annealing temperature i.e. maintained between TA - <NUM> and TA + <NUM>, for an annealing time tA preferably of more than <NUM>, still preferably more than <NUM>, but which does not need to be of more than <NUM>.

In a first embodiment, the sheet is immediately cooled to the room temperature after the partitioning step, without being hot dip coated. In this first embodiment, the partitioning temperature PT is comprised between <NUM> and <NUM>, and preferably between <NUM> and <NUM>, and the partitioning time Pt is comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>. A partitioning temperature PT comprised between <NUM> and <NUM> and a partitioning time Pt comprised between <NUM> and <NUM> make it possible to obtain a total elongation according to ISO <NUM>-<NUM> of at least <NUM>% when the sheet is not hot-dip coated.

In a second embodiment, the sheet is hot-dip coated just after the step of maintaining the sheet at the partitioning temperature PT, and then cooled to the room temperature. The hot-dip coating step is taken into account when selecting the partitioning temperature PT and the partitioning time Pt. In this second embodiment, the partitioning temperature PT is comprised between <NUM> and <NUM>, and preferably between <NUM> and <NUM>, and the partitioning time Pt is comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>. When the sheet if hot-dip coated, if the partitioning temperature PT is higher than <NUM> or lower than <NUM>, the elongation of the final coated product is not satisfactory.

The hot dip coating can be, for example, galvanizing but all metallic hot dip coating is possible provided that the temperatures at which the sheet is brought to during coating remain less than <NUM>. When the sheet is galvanized, it is performed with the usual conditions, for example through a Zn-bath with temperature ranging from <NUM> to <NUM>. The steel according to the invention can be galvanized with Zn or with a Zn alloy, as for example zinc-magnesium or zinc-magnesium-aluminum.

This heat treatment makes it possible to achieve a final structure (i.e. after partitioning, optional hot-dip coating and cooling to the room temperature) consisting of, in area fraction:.

A fraction of retained austenite of at least <NUM>%, together with a fraction of ferrite of at least <NUM>%, allows obtaining a total elongation of at least <NUM>%, and always of at least <NUM>% when the sheet is not hot-dip coated, the elongation being measured according to ISO Standard ISO <NUM>-<NUM>.

Furthermore, this treatment allows obtaining an increased C content in the retained austenite, which is of at least <NUM>%, preferably even of at least <NUM>%, and up to <NUM>%.

The martensite comprises fresh martensite, and tempered martensite.

The tempered martensite, which is partitioned martensite, has a C content of at most <NUM>%, this content resulting from the partitioning of carbon from martensite towards austenite during the partitioning step. Especially, this content results from the partitioning of carbon from the martensite formed during the quenching towards the austenite.

A C content in the tempered (or partitioned) martensite of at most <NUM>% is necessary to guarantee a sufficient stabilization of the austenite, and therefore a total elongation of at least <NUM>%. In addition, a C content in the tempered martensite higher than <NUM>% would lead to the precipitation of carbides within the martensite, increasing the yield strength. Therefore, a C content in the martensite of at most <NUM>% allows achieving a yield strength of at most <NUM> MPa, and therefore a high formability of the steel sheet.

The C content in the tempered martensite is generally of at most <NUM>%. A C content in the tempered martensite of at most <NUM>% guarantees an optimum stabilization of the austenite, which does not transform into martensite during the hole expansion ratio test, and therefore guarantees a hole expansion ratio HER of at least <NUM>%.

The fresh martensite, which results from the transformation of enriched austenite into martensite after the partitioning step, has a C content which is of at least <NUM>% and generally less than <NUM>%. The fraction of fresh martensite in the structure is lower than or equal to <NUM>%. Indeed, a fraction of fresh martensite higher than <NUM>% would lead to a hole expansion ratio lower than <NUM>% according to ISO Standard ISO <NUM>:<NUM>.

With this heat-treatment, steel sheets having a yield strength YS of at least <NUM> MPa, a tensile strength TS of at least <NUM> MPa, a total elongation TE according to the ISO standard <NUM>-<NUM> of at least <NUM>%, and even higher than <NUM>%, and a hole expansion ratio HER according to the ISO standard <NUM>:<NUM> of at least <NUM>%, and even at least <NUM>%, can be obtained.

In particular, when the sheets are not hot-dip coated, the sheets have a yield strength YS of at least <NUM> MPa, a tensile strength TS of at least <NUM> MPa, a total elongation TE according to ISO standard <NUM>-<NUM> of at least <NUM>%, and even higher than <NUM>%, and a hole expansion ratio HER according to the ISO standard <NUM>:<NUM> of at least <NUM>%, and even at least <NUM>%.

When the sheets are hot-dip coated, the sheets have a yield strength YS of at least <NUM> MPa, a tensile strength TS of at least <NUM> MPa, a total elongation TE according to ISO standard <NUM>-<NUM> of at least <NUM>%, and even higher than <NUM>%, and a hole expansion ratio HER according to the ISO standard <NUM>:<NUM> of at least <NUM>%, and even at least <NUM>%.

As examples and comparison, sheets made of steel compositions according to table I, have been manufactured, the elements being expressed in weight. The transformation temperatures such as Ac1 and Ac3 are reported in table I. Ac1 and Ac3 were measured by dilatometry.

In this Table, "res. " means that the element is only present as a residual, and that no voluntary addition of this element was made, and "nd" means that the value was not determined.

The sheets were hot-rolled, then coiled at <NUM> (steels I-III and V) or <NUM> (steel IV). Some of the sheets were batch annealed for <NUM> days at <NUM> or <NUM>. The sheets, after coiling or batch-annealing, were pickled and cold rolled to obtain sheets having a thickness of <NUM>, <NUM> or <NUM>, annealed, quenched, partitioned and cooled to the room temperature. Some of the sheets were hot-dip coated by galvanizing at <NUM> between the partitioning and the cooling to the room temperature.

The conditions of treatment are reported in Table II for the non coated sheets, and in Table III for the hot-dip coated sheets.

In these tables, Tcoil designates the coiling temperature, th designates the thickness of the sheet after cold-rolling, THBA designates the batch annealing temperature, TA is the annealing temperature, tA is the annealing time, QT the quenching temperature, PT the partitioning temperature, Pt the partitioning time. In Table III, Ms designates the martensite start temperature of the austenite resulting from the annealing.

Measured properties are the Hole expansion ratio HER measured according to the standard ISO <NUM>:<NUM>, the yield strength YS, the tensile stress TS, the uniform elongation UE and the total elongation TE. The yield strength YS, the tensile stress TS, the uniform elongation UE and the total elongation TE were measured according to the ISO standard ISO <NUM>-<NUM>, published in October <NUM>, except for example <NUM>*, which is identical to example <NUM>, and examples <NUM>, <NUM>, <NUM> and <NUM>, for which these properties were measured according to the JIS Z <NUM>-<NUM> standard.

The mechanical properties and the microstructures obtained for the sheets which were not hot-dip coated are reported in Table IV. F is the area fraction of ferrite, TM is the area fraction of tempered martensite, FM is the area fraction of fresh martensite, RA is the area fraction of retained austenite and B is the area fraction of bainite.

In all these examples <NUM> to <NUM>, the C content of the tempered martensite is of at most <NUM>%.

These examples show that with a method according to the invention, when no hot-dip coating is performed, steel sheets having a tensile strength of at least <NUM> MPa and a total elongation according to ISO <NUM>-<NUM> of at least <NUM>%, and even of more than <NUM>% can be obtained. These steel sheets have also a yield strength of at least 600MPa, and lower than <NUM> MPa, a uniform elongation of at least <NUM>%, and generally of more than <NUM>%, and a hole expansion ratio HER according to ISO <NUM>:<NUM> of at least <NUM>%, and even of more than <NUM>%.

The comparison of examples <NUM>-<NUM> shows that the method is very robust with variations of the quenching temperature QT and the partitioning temperature PT. In particular, example <NUM> shows that when the sheet is not hot-dip coated, choosing a partitioning temperature PT comprised between <NUM> and <NUM> and a partitioning time Pt comprised between <NUM> and <NUM>, in particular higher than <NUM> make it possible to achieve a total elongation of at least <NUM>%.

Besides, the comparison of the total elongations measured for examples <NUM> and <NUM>* demonstrate that the values of the total elongation TE according to the ISO standard are lower, in this case about <NUM>% lower, than the values of the total elongation according to the JIS Z <NUM>-<NUM> standard.

The mechanical properties and the microstructure obtained for the hot-dip coated sheets are reported in Table V. As previously, TE is measured according to ISO <NUM>-<NUM> and HER according to ISO <NUM>:<NUM>. Moreover, F is the area fraction of ferrite, TM is the area fraction of tempered martensite, FM is the area fraction of fresh martensite, RA is the area fraction of retained austenite and B is the area fraction of bainite.

In examples <NUM>-<NUM>, the C content of the tempered martensite is of at most <NUM>%.

These examples show that with a method according to the invention, hot-dip coated steel sheets having a tensile strength of at least <NUM> MPa and a total elongation according to ISO <NUM>-<NUM> of at least <NUM>%, and even of more than <NUM>% can be obtained. These steel sheets have a yield strength of at least <NUM> MPa, and lower than <NUM> MPa, a uniform elongation of at least <NUM>% and a hole expansion ratio according to ISO <NUM>:<NUM> HER of at least <NUM>%, and even of more than <NUM>%.

Regarding the spot weldability, the sheets according to the invention have a low LME sensitivity when the composition is such that C+Si/<NUM> ≤ <NUM>%. It means that which such steels it is possible to produce structures comprising resistance spot welds, such as car bodies, for which the probability of the number of cracks in the resistance spot welds is such that the mean value is less than <NUM> cracks per resistance spot weld and the probability to have less than <NUM> cracks is <NUM>%.

In particular, a welded structure, including resistance spot weld, of at least two steel sheets, can be produced by producing a first steel sheet by a method according to the invention, the first sheet being such that +Si/<NUM> ≤ <NUM>% and Al ≥ <NUM>(C+Mn/<NUM>) - <NUM>% and being coated with Zn or a Zn alloy, providing a second steel sheet having a composition such that C+Si/<NUM> ≤ <NUM>% and Al ≥ <NUM>(C+Mn/<NUM>) - <NUM>%, and resistance spot welding the first steel sheet to the second steel sheet. The second steel sheet may for example be produced by a method according to the invention, and coated with Zn or a Zn alloy.

Thus, a welded structure having a low LME sensitivity is obtained. For example, for such a welded structure comprising at least ten resistance spot welds, the mean number of cracks per resistance spot weld is less than <NUM>.

Claim 1:
A method for producing a steel sheet having a yield strength of at least <NUM> MPa and lower than <NUM> MPa, a tensile strength of at least <NUM> MPa, a total elongation according to ISO standard <NUM>-<NUM> of at least <NUM>% and a hole expansion ratio according to ISO standard <NUM>:<NUM> HER of at least <NUM>%,
wherein the method comprises the following successive steps:
- providing a cold-rolled steel sheet, the chemical composition of the steel containing in weight %: <MAT> <MAT> <MAT> <MAT> with <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT>
the remainder being Fe and unavoidable impurities,
- annealing the steel sheet at an annealing temperature TA comprised between Ac1 and Ac3 so as to obtain a structure comprising at least <NUM>% of austenite and at least <NUM>% of intercritical ferrite,
- quenching the sheet from a temperature of at least <NUM> at a cooling rate of at least <NUM>/s down to a quenching temperature QT comprised between <NUM> and <NUM>,
- heating the sheet up to a partitioning temperature PT between <NUM> and <NUM> and maintaining the sheet at this partitioning temperature PT for a partitioning time Pt comprised between <NUM> and <NUM>, the partitioning time Pt being comprised between <NUM> and <NUM> if the partitioning temperature PT is comprised between <NUM> and <NUM>, and comprised between <NUM> and <NUM> if the partitioning temperature PT is comprised between <NUM> and <NUM>,
- cooling the sheet down to the room temperature,
the steel sheet having a final microstructure consisting, in area fraction, of:
- at least <NUM>% of tempered martensite, the tempered martensite having a C content of at most <NUM>%,
- between <NUM>% and <NUM>% of retained austenite,
- between <NUM>% and <NUM>% of ferrite,
- at most <NUM>% of fresh martensite, having a C content of at least <NUM>% and less than <NUM>%,
- at most <NUM>% of bainite.