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 automobile 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 and Yield Stress 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.

Thus, the object of the invention is to solve the above drawbacks by providing a 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 20. Another object is achieved by providing a TWIP steel sheet according to claim 21.

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,

- Mtarget: target value of the mechanical property,

- Mrecrystaiiisation: mechanical property after a recrystallization annealing,

- Mcoid-roif mechanical property after a cold-rolling,

- UTS: ultimate tensile strength,

- TE: total elongation,

- P: pareq value,

- target value of pareq,

- FWHM: full width at half maximum of X-ray diffraction spectrum and

- FWH Mtarget: target value of the full width at half maximum of X-ray diffraction spectrum.

The invention relates to 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 _ta rget 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 width at mid height 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 FWMH 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 FWHMt _ar get corresponding to the M _ta rget is being determined,

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

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

C. a feeding step of a recrystallized steel sheet having a M _r ecrystaiiization _>

D. a cold-rolling step in order to obtain a steel sheet having a M _CO id-roii and

E. an annealing step performed at a temperature Target during a time t _tar get- 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 process parameters of the annealing step E) in order to acquire a recovered steel sheet, in particular a TWIP steel sheet, having the expected 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%.

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 calibration step A.I) wherein at least 2 samples of the steel sheet having undergone heat treatments between 400 and 900°C during 40 seconds to 60 minutes, corresponding to Pareq values P are prepared. In this step, 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:

Pareq = -0.67 ^* log(i-AH/RT)*dt)

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

Then, during the step A.ll), the samples are submitted to X-ray diffraction so as to obtain spectrums including a main peak whose the full width at half maximum FWHM is being measured. The X-ray diffraction is a non-destructive analytical technique which provides detailed information about the internal lattice of crystalline substances, including unit cell 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 crystallography 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 the full width at half maximum FWHM is measured corresponds to the Miller index [311]. Indeed, it is believed that this peak, being characteristic of the austenitic system, is the best representative of the dislocation density impact.

Then, during the step A.lll), M of such samples is being measured. Preferably, M is the Ultimate tensile Strength (UTS), the total elongation (TE) or both (UTS*TE).

After the recovery or recrystallization state of each sample is being measured during the step A. 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 A.V), a curve of M as a function of FWMH is being drawn in the domain where the samples are recovered from 0 to 100%, but not recrystallized. According to the present invention, a calculation step B) is realized. The calculation comprises a step B.I) wherein the value of FWHM _ta rget corresponding to the Mtarget is being determined. Preferably, FWHM _ta rget is above 1.0° and advantageously between 1.0 and 1.5°.

In one preferred embodiment wherein M is UTS, the determination of

FWHM is achieved with the following equation:

UTStarget = UTS _co ld-roll - (UTS _co ld-roll— UTSrecrystallization) * ( ^β Χρ((-

FWHM+2.3)/2.3)-1) ⁴ )

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

In another preferred embodiment wherein M is TE, the determination of FWHM during the calculation step B.I) is achieved with the following equation:

TE _tar get = TE _co ld-roll - (TE _re crystallizatior,- UTS cold-roll) ^* (exp((-FWHM+2.3)/2.3)- In this case, preferably, TE _tar get is above or equal to 15% and more preferably between 15 and 30%.

In another preferred embodiment, wherein M is UTS ^* TE, the determination of FWHM during the calculation step B.I) is achieved with the following equation:

UTS _t arget*TEtarget = 100000 ^* (1 -0.5FWHM)

In this case, preferably, UTS _t arget ^* TE _t arget is above 21000 and more preferably between 21000 and 60000, TE _tar get being maximum of 30%.

Then, the step B.ll), wherein the pareq value Ptarget of the heat treatment to perform to reach such M _ta rget is determined, is performed. Preferably, Ptarget is above 14.2, more preferably between 14.2 and 25 and more preferably, between 14.2 and 18.

After, the step B.lll), consisting in selecting a time t _ta rget and a temperature T°target corresponding to the Ptarget value, is realized. Preferably, Target is between 400 and 900°C and the target is between 40 seconds to 60 minutes.

Then, the method according to the present invention comprises a feeding step of a recrystallized a steel sheet having a M _re crystaiiization- Indeed, preferably, the steel sheet is recrystallized after a recrystallization annealing performed at a temperature between 700 and 900°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 _re crystaiiization 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 70%.

In another preferred embodiment, when M is TE ^* UTS, TE recrystaiiization ^* UTS recrystaiiization is above 16000, more preferably above 24000 and advantageously between 24000 and 98000.

Then, a cold-rolling step D) is realized in order to obtain a steel sheet having a M _CO id-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 _CO id-roii is above 1000, preferably above 1200MPa and advantageously above 1400MPa.

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 cow-roii ^* UTS cow-roii is above 2000, preferably 2400 and more preferably between 2400 and 70000.

Then, an annealing step E) is performed at a temperature T ⁰ _ta rg _et during a time ttarget-

After the second cold-rolling, 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

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

C (%) Mn (%) Si (%) P (%) Al (%) Cu (%) Mo (%) V (%) N (%) Nb (%) Cr (%) Ni (%)

0.583 21.9 0.226 0.03 0 0.031 0.01 0.206 0.0148 0 0.183 0.06

In this example, the recovered steel sheet had a target value of the mechanical property M _tar get is UTS _ta rget being of 1512MPa. Thanks to the calibration step A, the value of FMHM _ta rget corresponding to the UTS _ta rg _et was determined/the FMH Mtarget was of 1.096. The Pt _arge t of the heat treatment to perform to reach UTStarget was determined, it was of 14.39. Then, the selected time t _tar get was of 40 seconds and the selected temperature T° _ta rget was of 650°C.

Thus, firstly, Trials 1 and 2 were heated and hot-rolled at a temperature of 1200°C. The finishing temperature of hot-rolling was set to 890°C and the coiling was performed at 400°C after the hot-rolling. Then, a 1 ^st cold-rolling was realized with a cold-rolling reduction ratio of 50%. Thereafter, a recrystallization annealing was performed at 825°C during 180seconds. The value of UTSrecrystaiiization obtained was of 980 MPa. Afterwards, a 2 ^nd cold-rolling was realized with a cold-rolling reduction ratio of 30%. The value of UTS _C oid-roii obtained was of 1540 MPa.

Then, Trial 1 was annealed at 650°C during 40 seconds according to the present invention. After this annealing, Trial 1 was recovered. The UTS of Trial 1 was of 1512.5 MPa.

Trial 2 was annealed at 650°C during 90 seconds, i.e. ttarget and T° _ta rget determined by the method of the present invention were not respected. After this annealing, Trial 2 was recrystallized. UTS of Trial 2 was of 1415.15MPa. The FMHM of Trial 2 was of 0.989 and the P was of 14.12, i.e. outside the range of the present invention.

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