WO2020128591A1

WO2020128591A1 - Method for the manufacture of a recovered steel sheet having an austenitic matrix

Info

Publication number: WO2020128591A1
Application number: PCT/IB2018/060434
Authority: WO
Inventors: Nicolas CHARBONNIER; Jean-Christophe HELL; Thierry Iung; Blandine REMY
Original assignee: Arcelormittal
Priority date: 2018-12-20
Filing date: 2018-12-20
Publication date: 2020-06-25

Abstract

The present invention relates to a method for manufacturing a recovered steel sheet having an austenitic matrix presenting at least one mechanical property (M), being Ultimate tensile Strength (UTS), the total elongation (TE) or both (UTS*TE), equal or above a target value M_target whose composition comprises, in weight: 0.1 < C < 1.2%, 13.0 < Mn < 25.0%, S < 0.030%, P < 0.080%, N < 0.1 %, Si < 3.0%, and on a purely optional basis, one or more elements such as Nb < 0.5 %, B < 0.005%, Cr < 1.0%, Mo < 0.40%, Ni < 1.0%, Cu < 5.0%, Ti < 0.5%, V < 2.5%, Al < 4.0%, Sn < 0.2, the remainder of the composition making up of iron and inevitable impurities resulting from the development.

Description

Method for the manufacture of a recovered steel sheet having an austenitic matrix

The present invention relates to a method for producing a recovered steel sheet having an austenitic matrix. The invention is particularly well suited for the manufacture of automotive vehicles.

With a view of saving the weight of vehicles, it is known to use high strength steels for the manufacture of automotive vehicle. For example for the manufacture of structural parts, mechanical properties of such steels have to be improved. However, even if the strength of the steel is improved, the elongation and therefore the formability of high steels decreased. In order to overcome these problems, recovered steel sheets, in particular, twinning induced plasticity steels (TWIP steels) having good formability have appeared. Even if the product shows a very good formability, mechanical properties such as Ultimate Tensile Stress (UTS) and Yield Stress (YS) may not be high enough to fulfill automotive application.

To improve the strength of these steels while keeping good workability, it is known to induce a high density of twins by cold-rolling followed by a recovery treatment removing dislocations but keeping the twins.

However, by applying such methods, there is a risk that the expected mechanical properties are not obtained. Indeed, the man skilled in the art can only follow the known methods and then measure the mechanical properties of the obtained steel sheet to see if the desired mechanical properties are achieved. It is not possible to adapt the conditions of the method in order to obtain expected mechanical properties.

The patent application WO2017/203350 discloses a method for manufacturing a recovered steel sheet having an austenitic matrix presenting at least one mechanical property (M) equal or above a target value M_target whose composition comprises, in weight: 0.1 < C < 1.2%, 13.0 < Mn < 25.0%, S < 0.030%, P < 0.080%, N < 0.1 %, Si < 3.0%, and on a purely optional basis, one or more elements such as Nb < 0.5 %, B < 0.005%, Cr < 1.0%, Mo < 0.40%, Ni < 1.0%, Cu < 5.0%, Ti < 0.5%, V < 2.5%, Al < 4.0%, the remainder of the composition making up of iron and inevitable impurities resulting from the development, such method comprising the steps consisting in:

A. a calibration step wherein:

I. at least 2 samples of said steel having undergone heat treatments between 400 and 900°C during 40 seconds to 60 minutes, corresponding to Pareq values P are prepared,

II. said samples are submitted to X-ray diffraction so as to obtain spectrums including a main peak whose Full Width at Half Maximum (FWHM) is being measured,

III. M of such samples is being measured,

IV. the recovery or recrystallization state of each sample is being measured,

V. the curve of M as a function of FWHM is being drawn in the domain where the samples are recovered from 0 to 100%, but not recrystallized,

B. a calculation step wherein:

I. the value of FWH Mtarget corresponding to the Mtarget is being determined,

II. the pareq value P_target of the heat treatment to perform to reach such M_target is being determined and

III. a time t_target and a temperature Target corresponding to the Ptarget value are being selected,

C. a feeding step of a recrystallized steel sheet having a M_recrystaiiization,

D. a cold-rolling step in order to obtain a steel sheet having a Mcold-roll and

E. an annealing step performed at a temperature Target during a time t_target.

However, in this method, the calculation step can be further optimized to reach M_target· Indeed, it seems that the value of FWH M_target is not enough precise to reach the best M_target·

Thus, the object of the invention is to solve the above drawbacks by providing a more precise and optimized method for manufacturing a recovered steel sheet presenting at least one expected mechanical property, such mechanical property being improved. Another object is to provide a recovered steel sheet having such improved mechanical properties. This object is achieved by providing a method for the manufacture of a TWIP steel sheet according to claim 1 . The method can also comprise characteristics of claims 2 to 14.

Another object is achieved by providing a TWIP steel sheet according to claim 15.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

The following terms will be defined:

- M: mechanical property,

- Mtar_get: target value of the mechanical property,

- Mrecrystaiiisation: mechanical property after a recrystallization annealing,

- Mcoi_d-roii: mechanical property after a cold-rolling,

- UTS: Ultimate Tensile Strength,

- TE: Total Elongation,

- P: Pareq value,

- Ptar_get: target value of Pareq,

- FWHM: full width at half maximum of a diffraction peak in the X-ray diffraction spectrum,

- FWH Mtar_get- target value of the full width at half maximum of a diffraction peak in the X-ray diffraction spectrum,

- FWFIMmeasure_d: value of the full width at half maximum of a diffraction peak in the X-ray diffraction spectrum measured,

- FWFIMcoi_d-roii: value of the full width at half maximum of a diffraction peak in the X- ray diffraction spectrum after the cold-rolled steel sheet and

- FWFIMrecrystaiiization: value of the full width at half maximum of a diffraction peak in the X-ray diffraction spectrum of the recrystallized steel sheet.

The invention relates to a method for manufacturing a recovered steel sheet having an austenitic matrix presenting at least one mechanical property (M), being Ultimate Tensile Strength (UTS), the total elongation (TE) or both (UTS^*TE), equal or above a target value M_target whose composition comprises, in weight:

0.1 < C < 1 .2%,

13.0 < Mn < 25.0%,

S < 0.030%, P < 0.080%,

N < 0.1 %,

Si < 3.0%,

and on a purely optional basis, one or more elements such as

Nb < 0.5 %,

B < 0.005%,

Cr < 1.0%,

Mo < 0.40%,

Ni < 1.0%,

Cu < 5.0%,

Ti < 0.5%,

V < 2.5%,

Al < 4.0%,

Sn < 0.2,

the remainder of the composition making up of iron and inevitable impurities resulting from the development,

such method comprising the following steps:

A. a calculation step wherein:

I. a target value being FWHM_target, being a target value of Full Width at Half Maximum (FWHM) of the main peak of an X-ray diffraction spectrum of the steel sheet, corresponding to M_target is being determined by one of the following equation:

^■ when M is UTS, the determination of FWHM_target is determined with the following equation:

^■ when M is TE, the determination of FWHM_target is achieved with the following equation:

when M is TE^*UTS, the determination of FWHM_target is achieved with the following equation: wherein when M is TE^*UTS, FWHM_target has to satisfy the following equation:

II. a Pareq value P_target corresponding to an annealing performed between 400 and 900°C during 1 second to 60minutes, to perform to reach such M_target is being determined based on FWHM_target and

III. a time t_target and a temperature T_target corresponding to the P_target value are being selected,

B. a feeding step of a steel sheet that has undergone a first cold-rolling,

C. a recrystallization annealing of the steel sheet obtained in step B) to obtain a recrystallized steel sheet,

D. a second cold-rolling step and

E. an annealing step performed at a temperature T_target during a time _rget- Without willing to be bound by any theory it seems that when the method according to the present invention is applied, it makes it possible to obtain more precise process parameters of the annealing step E) in order to acquire a recovered steel sheet, in particular a TWIP steel sheet, having the desired improved mechanical properties.

Regarding the chemical composition of the steel, C plays an important role in the formation of the microstructure and the mechanical properties. It increases the stacking fault energy and promotes stability of the austenitic phase. When combined with a Mn content ranging from 13.0 to 25.0% by weight, this stability is achieved for a carbon content of 0.5% or higher. In case there are vanadium carbides, a high Mn content may increase the solubility of vanadium carbide (VC) in austenite. However, for a C content above 1.2%, there is a risk that the ductility decreases due to for example an excessive precipitation of vanadium carbides or carbonitrides. Preferably, the carbon content is between 0.4 and 1.2%, more preferably between 0.5 and 1.0% by weight so as to obtain sufficient strength.

Mn is also an essential element for increasing the strength, for increasing the stacking fault energy and for stabilizing the austenitic phase. If its content is less than 13.0%, there is a risk of martensitic phases forming, which very appreciably reduce the deformability. Moreover, when the manganese content is greater than 25.0%, formation of twins is suppressed, and accordingly, although the strength increases, the ductility at room temperature is degraded. Preferably, the manganese content is between 15.0 and 24.0% and more preferably between 17.0 and 24.0% so as to optimize the stacking fault energy and to prevent the formation of martensite under the effect of a deformation. Moreover, when the Mn content is greater than 24.0%, the mode of deformation by twinning is less favored than the mode of deformation by perfect dislocation glide.

Al is a particularly effective element for the deoxidation of steel. Like C, it increases the stacking fault energy which reduces the risk of forming deformation martensite, thereby improving ductility and delayed fracture resistance. However, Al is a drawback if it is present in excess in steels having a high Mn content, because Mn increases the solubility of nitrogen in liquid iron. If an excessively large amount of Al is present in the steel, the N, which combines with Al, precipitates in the form of aluminum nitrides (AIN) that impede the migration of grain boundaries during hot conversion and very appreciably increases the risk of cracks appearing in continuous casting. In addition, as will be explained later, a sufficient amount of N must be available in order to form fine precipitates, essentially of carbonitrides. Preferably, the Al content is below or equal to 2%. When the Al content is greater than 4.0%, there is a risk that the formation of twins is suppressed decreasing the ductility. Preferably, the amount of Al is above 0.06% and more preferably above 0.7%.

Correspondingly, the nitrogen content must be 0.1 % or less so as to prevent the precipitation of AIN and the formation of volume defects (blisters) during solidification. In addition, when elements capable of precipitating in the form of nitrides, such as vanadium, niobium, titanium, chromium, molybdenum and boron, the nitrogen content must not exceed 0.1 %

According to the present invention, the amount of V is below or equal to 2.5%, preferably between 0.1 and 1.0%. Preferably, V forms precipitates. Preferably, the volumic fraction of such elements in steel is between 0.0001 and 0.025%. Preferably, vanadium elements are mostly localized in intragranular position. Advantageously, vanadium elements have a mean size below 7 nm, preferably between 1 and 5nm and more preferably between 0.2 and 4.0 nm

Silicon is also an effective element for deoxidizing steel and for solid-phase hardening. However, above a content of 3%, it reduces the elongation and tends to form undesirable oxides during certain assembly processes, and it must therefore be kept below this limit. Preferably, the content of silicon is below or equal to 0.6%.

Sulfur and phosphorus are impurities that embrittle the grain boundaries. Their respective contents must not exceed 0.030 and 0.080% so as to maintain sufficient hot ductility.

Some Boron may be added up to 0.005%, preferably up to 0.001 %. This element segregates at the grain boundaries and increases their cohesion. Without intending to be bound to a theory, it is believed that this leads to a reduction in the residual stresses after shaping by pressing, and to better resistance to corrosion under stress of the thereby shaped parts. This element segregates at the austenitic grain boundaries and increases their cohesion. Boron precipitates for example in the form of borocarbides and boronitrides.

Nickel may be used optionally for increasing the strength of the steel by solution hardening. However, it is desirable, among others for cost reasons, to limit the nickel content to a maximum content of 1.0% or less and preferably between below 0.3%.

Likewise, optionally, an addition of copper with a content not exceeding 5% is one means of hardening the steel by precipitation of copper metal. However, above this content, copper is responsible for the appearance of surface defects in hot-rolled sheet. Preferably, the amount of copper is below 2.0%. Preferably, the amount of Cu is above 0.1 %. Titanium and Niobium are also elements that may optionally be used to achieve hardening and strengthening by forming precipitates. However, when the Nb or Ti content is greater than 0.50%, there is a risk that an excessive precipitation may cause a reduction in toughness, which has to be avoided. Preferably, the amount of Ti is between 0.040 and 0.50% by weight or between 0.030% and 0.130% by weight. Preferably, the titanium content is between 0.060% and 0.40 and for example between 0.060% and 0.110% by weight. Preferably, the amount of Nb is above 0.01 % and more preferably between 0.070 and 0.50% by weight or 0.040 and 0.220%. Preferably, the niobium content is between 0.090% and 0.40% and advantageously between 0.090% and 0.200% by weight.

Chromium and Molybdenum may be used as optional element for increasing the strength of the steel by solution hardening. However, since chromium reduces the stacking fault energy, its content must not exceed 1.0% and preferably between 0.070% and 0.6%. Preferably, the chromium content is between 0.20 and 0.5%. Molybdenum may be added in an amount of 0.40% or less, preferably in an amount between 0.14 and 0.40%.

Optionally, tin (Sn) is added in an amount below 0.2% by weight without willing to be bound by any theory, it is believed that since tin is a noble element and does not form a thin oxide film at high temperatures by itself, Sn is precipitated on a surface of a matrix in an annealing prior to a hot dip galvanizing to suppress a pro-oxidant element such as Al, Si, Mn, or the like from being diffused into the surface and forming an oxide, thereby improving galvanizability. However, when the added amount of Sn exceeds 0.2%, the added Sn causes hot shortness to deteriorate the hot workability. Therefore, the upper limit of Sn is limited to 0.2% or less.

The steel can also comprise inevitable impurities resulting from the development. For example, inevitable impurities can include without any limitation: O, H, Pb, Co, As, Ge, Ga, Zn and W. For example, the content by weight of each impurity is inferior to 0.1 % by weight.

Furthermore, without willing to be bound by any theory, it seems that precipitates of vanadium, titanium, niobium, chromium and molybdenum can reduce the sensitivity to delayed cracking, and do so without degrading the ductility and toughness properties. Thus, preferably, at least one element chosen from titanium, niobium, chromium and molybdenum under the form of carbides, nitrides and carbonitrides are present in the steel.

According to the present invention, the method comprises a calculation step A.l) wherein the value of FWHM_target of the steel sheet according to the present invention corresponding to M_target is being determined. X-ray diffraction is a non destructive analytical technique which provides detailed information about the internal lattice of crystalline substances, including lattice dimensions, bond- lengths, bond-angles, and details of site-ordering. Directly related is single-crystal refinement, where the data generated from the X-ray analysis is interpreted and refined to obtain the crystal structure. Usually, an X-ray diffractometer is the tool used for identifying such crystal structure. According to the present invention, the steel sheet has an austenitic matrix, the austenitic matrix having a face-centered cubic system. Thus, preferably, the main peak whose full width at half maximum FWHM is considered corresponds to the Miller index [31 1 ], i.e. the diffraction peak corresponding to the {31 1 } planes of the austenitic phase. Indeed, it is believed that this peak, being characteristic of the austenitic system, is the best representative of the dislocation density impact.

When M is UTS, the determination of FWFIM target is determined with the following equation:

In this case, preferably, the UTS_target is above or equal to 1430MPa and more preferably between 1430 and 2000MPa.

When M is TE, the determination of FWFIM_target is achieved with the following equation:

In this case, preferably, TE_target is above or equal to 15% and more preferably between 15 and 30%. When M is TE^*UTS, the determination of FWHM_target is achieved with the following equation:

wherein FWHM_target has to satisfy the following equation:

In a preferred embodiment when M is TE^*UTS, FWFIM_target has to satisfy the following equation:

More preferably when M is TE^*UTS, FWFI Mt_arget has to satisfy the following equation:

In this case, preferably, UTSt_arget*TEt_arget is above 21000 and more preferably between 21000 and 60000, TE_target being maximum of 30%.

Then, in step A. I I), the Pareq value Pt_arget of the annealed to perform to reach M_target is determined based on FWFI Mtarget- For example, a curve of FWFI Mtarget as a function of Ptarget is being drawn. The parameter called Pareq is determined to be able to compare different heat treatments carried out at different temperatures for different times, it is defined by:

With DH: energy of diffusion of iron in iron (equal to 300 kJ/mol), T = temperature of the cycle in Kelvin, the integration being over the heat treatment time in hour. The hotter or longer the heat treatment, the lower the Pareq value. Two different heat treatments having an identical Pareq value will give the same result on the same grade of steel. Preferably, the Pareq value is above 14.2, more preferably between 14.2 and 25 and more preferably between 14.2 and 18.

After, in step A.lll), a time rget and a temperature T_target corresponding to the Ptarget value, are selected. Preferably, T_target is between 400 and 900°C and the ta_rget is between 1 second to 60 minutes.

Then, the method according to the present invention comprises a step B) being the feeding of a steel sheet that has undergone a first cold-rolling.

In a step C), a recrystallization annealing to obtain a recrystallized steel sheet is performed. Preferably, the steel sheet is recrystallized after a recrystallization annealing performed at a temperature between 700 and 1000°C. For example, the recrystallization is realized during 10 to 500 seconds, preferably between 60 and 180 seconds.

In one preferred embodiment, when M is UTS, UTS_recrystaiiization is above 800 MPa, preferably between 800 and 1400MPa and more preferably between 1000 and 1400MPa.

In another preferred embodiment, when M is TE, TE recrystaiiization is above 20%, preferably above 30% and more preferably between 30 and 80%.

In another preferred embodiment, when M is TE*UTS, TE recrystaiiization *UTS recrystaiiization is above 1 6000, more preferably above 24000 and advantageously between 24000 and 98000.

Then, a second cold-rolling step D) is realized in order to obtain a steel sheet having a M_COid-roii· Preferably, the reduction rate is between 1 to 50%, preferably between 1 and 25% or between 26 and 50%. It allows the reduction of the steel thickness. Moreover, the steel sheet manufactured according to the aforesaid method, may have increased strength through strain hardening by undergoing this rolling step. Additionally, this step induces a high density of twins improving thus the mechanical properties of the steel sheet.

In one preferred embodiment, when M is UTS, UTS_Coid-roii is above 1000, preferably above 1450MPa and advantageously above 1600MPa.

In another preferred embodiment, when M is TE, TE coid-roii is above 2%, more preferably between 2 and 50%. In another preferred embodiment, when M is TE^*UTS, TE coid-roii ^*UTS coid-roii is above 2000, preferably 2400 and more preferably between 2400 and 70000.

Then, an annealing step E) is performed at a temperature T_target during a time ttarget-

Preferably, a calibration step F) is performed to obtain the formulas of steps A. I). In this case, before the calculation step A), a calibration step F) comprising the sub-following steps is performed:

I. at least 1 sample of the steel sheet according to the present invention undergoes the following treatment:

i. a first cold-rolling,

ii. a recrystallization annealing to obtain a recrystallized steel sheet having a Mrecrystaiiization

iii. a second cold-rolling in order to obtain a steel sheet having a

Mcoid-roii snd

iv. an annealing step, such step being performed between 400 and 900°C during 1 second to 60 minutes, corresponding to Pareq values P are prepared,

I I . the at least 1 sample is submitted to X-ray diffraction so as to obtain spectrums including a main peak whose width at mid height FWFIM wherein, FWFI M_recrystaiiized after step F.l.ii), FWFIM_C0|d.r0iied after step F.l.iii) and FWFI Manneaied after step F.l.iv) are determined,

III. M of such sample is being measured after step F.l.ii) to obtain Mrecrystaiiization_j after step F.l.iii) to obtain Mcoid-roii and after step F.l.iv) to obtain M_anneaied being determined,

IV. the recovery or recrystallization state of each sample is being measured,

V. the curve of M_anneaied as a function of FWFIM_anneaied is being drawn in the domain where the samples are recovered from 0 to 100%.

Then, during the step F. II), the samples are submitted to X-ray diffraction so as to obtain spectrums including a main peak whose the full width at half maximum FWFIM is being measured. Preferably, the main peak whose full width at half maximum FWFIM is measured corresponds to the Miller index [31 1 ] Then, during the step F.lll), M of such samples is being measured. M is the Ultimate Tensile Strength (UTS) or the Total Elongation (TE) or both (UTS^*TE).

After, the recovery or recrystallization state of each sample is being measured during the step F.IV). Preferably, such states are measured with Scanning Electron Microscope (SEM) and EBSD (Electron Back Scattered Diffraction) or Transmission Electron Microscope (TEM).

Then, during step F.V), a curve of M_anneaied as a function of FWFI Manneaied is being drawn in the domain where the samples are recovered from 0 to 100%. The curve is then used to find FWFIMtarget and M_target·

Advantageously, after step D) or step E) or during step E), a hot-dip coating step G) can be performed. Preferably, the step G) is realized with an aluminum- based bath or a zinc-based bath.

In a preferred embodiment, the hot-dip galvanizing step is performed with an aluminum-based bath comprises less than 15% Si, less than 5.0% Fe, optionally 0.1 to 8.0% Mg and optionally 0.1 to 30.0% Zn, the remainder being Al.

In another preferred embodiment, the hot-dip galvanizing step is performed with a zinc-based bath comprises 0.01 -8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn.

The molten bath can also comprise unavoidable impurities and residuals elements from feeding ingots or from the passage of the steel sheet in the molten bath. For example, the optionally impurities are chosen from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the content by weight of each additional element being inferior to 0.3% by weight. The residual elements from feeding ingots or from the passage of the steel sheet in the molten bath can be iron with a content up to 5.0%, preferably 3.0%, by weight.

For example, an annealing step can be performed after the coating deposition in order to obtain a galvannealed steel sheet.

Thus, a recovered steel sheet having an austenitic matrix at least one expected and improved mechanical property is obtained by applying the method according to the present invention. Example 1

In this example, steel sheets having the following weight composition were used:

In this example, the aim is to obtain a recovered steel sheet having a mechanical property target M_target being UTStar_get^*TE_tar_get of 21500MPa%, i.e. UTStarget Of 1600 MPa and TE_target Of 13.5%.

Firstly, the calibration step F) was performed. In step F.l), several samples of said steel have undergone the following treatment:

i. a first cold-rolling,

ii. a recrystallization annealing to obtain a recrystallized steel sheet having a Mrecrystallization

iii. a second cold-rolling in order to obtain a steel sheet having a M_COid-roii and

iv. an annealing step at a temperature between 400 and 900°C during 1 seconds to 60 minutes. The pareq value corresponding to each annealing performed was determined using the following equation:

with DH: energy of diffusion of iron in iron (equal to 300 kJ/mol), T = temperature of the cycle in Kelvin, the integration being over the heat treatment time in hour.

In a step F.ll), steel sheets are submitted to X-ray diffraction so as to obtain spectrums including the peak [31 1 ] whose FWFIM is being measured. Thus, F W H M recrystaii ized , FWIHMcoid-roiied snd FWH Manneaied sre determined.

In a step F. Ill), the mechanical properties UTS and TE of all the steel sheets are measured to obtain Mrecrystallization Mcold-roll snd Mannealed-

In a step F.IV), the recovery or recrystallization state of each sample is being measured.

Then in a step F.V), the curve of M_anneaied being UTS^*TE as a function of FWHManneaied is being drawn in the domain where the samples are recovered from 0 to 100%. Thanks to the calibration step, UTS_recrystaiiization, TE_recrystalliZed, FWHM_{recrystalliZed,} UTS_¥id-r0ii, TE_C0|d-r0ii, FWHM_C0|d.r0ii corresponding to the target are determined as shown in the following Table 1 :

Then, a calculation step A) was performed. In a step A. I), the value of FWH Mtarget was calculated thanks to the following equation:

With the above equation, FWHM_target has 2 solutions, one being 1.57 and the other one being 0.65.

/ FWHMt_arg_et FWHM₂ndcold-roll

has to be below or equal to 1. Thus, the solution

\FWHM_recry_Staiii_zation— FWHM ₂ndcold-r oil )

0.65 is chosen.

Then, FWHM_target is calculated with the following equation:

Then, in step A. II), Ptarget of the annealing to perform to reach such M_target was determined based on a FWHM_target. Indeed, a curve of FWHM_target as a function of P_target was drawn. Ptarget was of 14.22. In step A. Ill), a time _rget and a temperature T_target corresponding to the Ptarget value were selected. The selected time target was of 200 seconds and the selected temperature Ttarget was of 625°C.

Then in a step B), 3 steels sheets, called Trial 1 , 2 and 3, that have undergone a 1^st cold-rolling realized with a cold-rolling reduction ratio of 50% were provided. Thereafter, in a step C), a recrystallization annealing was performed at 825°C during 180seconds.

In a step D), a 2^nd cold-rolling was realized with a cold-rolling reduction ratio of 30%.

In a step E), an annealing step was performed at a temperature T_target during a time target for Trial 2. Trial 1 and 3 were annealed during different time.

The results are in Table 2 below:

^*according to the present invention

Results show that when the method according to the present invention is applied, a recovered steel sheet having expected mechanical properties is obtained.

Claims

1. A method for manufacturing a recovered steel sheet having an austenitic matrix presenting at least one mechanical property (M), being Ultimate Tensile Strength (UTS), the Total Elongation (TE) or both (UTS^*TE), equal or above a target value M_target whose composition comprises, in weight:

0.1 < C < 1.2%,

13.0 < Mn < 25.0%,

S < 0.030%,

P < 0.080%,

N < 0.1 %,

Si < 3.0%,

and on a purely optional basis, one or more elements such as

Nb < 0.5 %,

B < 0.005%,

Cr < 1.0%,

Mo < 0.40%,

Ni < 1.0%,

Cu < 5.0%,

Ti < 0.5%,

V < 2.5%,

Al < 4.0%,

Sn < 0.2,

such method comprising the following steps:

A. a calculation step wherein:

I. a target value being FWH M_target, being a target value of Full Width at Half Maximum (FWFIM) of the main peak of an X-ray diffraction spectrum of said steel sheet, corresponding to M_target is being determined by one of the following equation: ^■ when M is UTS, the determination of FWHM_target is determined with the following equation:

^■ when M is TE^*UTS, the determination of FWFIM_target is achieved with the following equation:

wherein when M is TE^*UTS, FWFIM_target has to satisfy the following equation:

II. a Pareq value P_target, corresponding to annealing performed between 400 and 900°C during 1 second and 60mminutes, to perform to reach M_target is being determined based on FWFIM_target and

III. a time t_target and a temperature T_target corresponding to P_target value are being selected,

B. a feeding step of a steel sheet that has undergone a first cold-rolling,

D. a second cold-rolling step and

E. an annealing step performed at a temperature T_target during a time t_target-

2. Method according to claim 1 , wherein before the calculation step A), a calibration step F) is performed, said calibration step comprising the sub following steps:

I. at least 1 sample of said steel has undergone the following treatment:

i. a first cold-rolling,

iii. a second cold-rolling in order to obtain a steel sheet having a

Mcoid-roii snd

I I . the at least 1 sample is submitted to X-ray diffraction so as to obtain spectrums including a main peak whose width at mid height FWHM wherein, FWFI M_recrystaiiized after step F.l.ii), FWFIM_CO|d.roiied after step F.l.iii) and FWFI Manneaied after step F.l.iv) are determined,

I I I . M of such sample is being measured after step F.l.ii) to obtain Mrecrystaiiization_j after step F.l.iii) to obtain Mcoid-roii and after step F.l.iv) to obtain M_anneaied being determined,

IV. the recovery or recrystallization state of each sample is being measured,

V. the curve of M_anneaied as a function of FWFIManneaied is being drawn in the domain where the samples are recovered from 0 to 100%.

3. Method according to claim 1 or 2, wherein in step C) and in step F.l.ii), the steel sheet is recrystallized at a temperature between 700 and 1000°C.

4. Method according to anyone of claims 1 to 3, wherein in step D) and in step F.l.iii), the second cold-rolling is realized with a reduction rate between 1 and 50%.

5. Method according to anyone of claims 1 to 4, wherein during the calibration step B. II), FWFIM is measured corresponds to the Miller index [311 ].

6. Method according to anyone of claims 1 to 5, wherein when the M is UTS, the UTS_target is above or equal to 1430MPa.

7. Method according to claim 6, wherein the UTS_target is between 1430 and 2000MPa;

8. Method according to anyone of claims 1 to 7, wherein when M is TE, TE_target is above or equal to 15%.

9. Method according to claim 8, wherein TE_target is between 15 and 30%.

10. Method according to anyone of claims 1 to 9, wherein when M is TE^*UTS, UTS_target ^*TE_target is above 21000, TE_target being maximum of 30%.

1 1. Method according to claim 10, wherein UTSt_arget*TE_target is between 21000 and 60000, TE_target being maximum of 30%.

12. Method according to anyone of claims 1 to 1 1 , wherein FWH M_target has to satisfy the following equation:

13. Method according to anyone of claims 1 to 1 2, wherein P_target is equal or above 14.2.

14. Method according to claim 13, wherein T_target is between 400 and 900°C and the _rget is between 1 second to 60 minutes.

15. A recovered TWIP steel sheet having an austenitic matrix obtainable from the method according to anyone of claim 1 to 14.