Method for producing a TWIP steel sheet having an austenitic microstructure

Abstract

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.sub.target is provided. The method includes a calibrating process involving heat treatments on at least 2 samples of the steel, corresponding to Pareq values P, submitting said samples to X-ray diffraction so as to obtain spectrums including a main peak whose width at mid height FWHM is being measured, measuring a mechanical property (M) of said samples, measuring a recovery or recrystallization state of each sample, drawing a curve of M as a function of FWMH in a domain where the samples are recovered from 0 to 100%, but not recrystallized. The method further includes calculating a FWHM.sub.target corresponding to a target mechanical property M.sub.target, determining a pareq value P.sub.target of the heat treatment to perform to reach M.sub.target and selecting a time t.sub.target and a temperature T.sub.target corresponding to the P.sub.target value. The method further includes feeding a recrystallized steel sheet having a M.sub.recrystallization, cold-rolling the recrystallized steel sheet in order to obtain a steel sheet having a M.sub.cold-roll and annealing the cold rolled steel sheet at a temperature T.sub.target during a time t.sub.target.

Claims

1. 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.sub.target whose composition comprises, in weight: 0.1<C<1.2%, 13.0Mn<25.0%, S0.030%, P0.080%, N0.1%, Si3.0%, the remainder of the composition making up of iron and inevitable impurities resulting from development, the method comprising: A. calibrating, wherein the calibrating includes: I. performing heat treatments between 400 and 900 C. during 40 seconds to 60 minutes on at least 2 samples of said steel, the heat treatments having Pareq values P, Pareq being defined by an equation:
Pareq=0.67*log(H/RT)*dt), with H: energy of diffusion of iron in iron, T=temperature of the heat treatment, the integration being over the heat treatment time, the heat treatments being different heat treatments carried out at different temperatures for different times, II. submitting said samples to X-ray diffraction so as to obtain spectrums including a main peak whose width at mid height FWHM is being measured, III. measuring a mechanical property (M) of said samples, IV. identifying a recovery or recrystallization state of each sample, V. drawing a curve of M as a function of FWHM in a domain where the samples are recovered from 0 to 100%, but not recrystallized, B. calculating, wherein the calculating includes: I. determining, from the curve, a value FWHM.sub.target corresponding to a target mechanical property M.sub.target, II. determining a pareq value P.sub.target of the heat treatment to perform to reach M.sub.target based on the M and Pareq values P determined during the calibrating, and III selecting a time t.sub.target and a temperature T.sub.target for the P.sub.target value using the equation, C. feeding a recrystallized steel sheet having a M.sub.recrystallization, D. cold-rolling the recrystallized steel sheet in order to obtain a steel sheet having a M.sub.cold-roll and E. annealing the cold rolled steel sheet at the selected temperature T.sub.target for the selected time t.sub.target to achieve at least one mechanical property M equal or above the target value M.sub.target.

2. The method according to claim 1, wherein the composition further comprises one or more of Nb0.5%, B0.005%, Cr1.0%, Mo0.40%, Ni1.0%, Cu5.0%, Ti0.5%, V2.5%, and/or Al4.0%.

3. The method according to claim 1, wherein the steel sheet is recrystallized after a recrystallization annealing realized between 70 and 900 C.

4. The method according to claim 1, wherein the cold-rolling is realized with a reduction rate between 1 and 50%.

5. The method according to claim 1, wherein during the calibration step A.II), the main peak whose width at mid height FWHM is measured corresponds to a Miller index [311].

6. The method according to claim 1, wherein M is an Ultimate tensile Strength (UTS), a total elongation (TE), or UTS*TE.

7. The method according to claim 6, wherein when M is UTS, the determination of FWHM during the calculation step B.I) is achieved with the following equation:
UTS.sub.target=UTS.sub.cold-roll(UTS.sub.cold-rollUTS.sub.recrystallization)*(exp((FWHM+2.3)/2.3)1).sup.4).

8. The method according to claim 6, wherein when the M is UTS, the UTS.sub.target is above or equal to 1430 MPa.

9. The method according to claim 8, wherein the UTS.sub.target is between 1430 and 2000 MPa.

10. The method according to claim 6, wherein when M is TE, the determination of FWHM during the calculation step B.I) is achieved with the following equation:
TE.sub.target=TE.sub.cold-roll(TE.sub.recrystallizationUTS.sub.cold-roll)*(exp((FWHM+2.3)/2.3)1).sup.2.5).

11. The method according to claim 6, wherein when M is TE, TE.sub.target is above or equal to 15%.

12. The method according to claim 11, wherein TE.sub.target is between 15 and 30%.

13. The method according to claim 6, wherein when M is TE*UTS, the determination of FWHM during the calculation step B.I) is achieved with the following equation:
UTS.sub.target*TE.sub.target=100000*(10.5FWHM).

14. The method according to claim 1, wherein when M is TE*UTS, UTS.sub.target*TE.sub.target is above 21000, TE.sub.target being maximum of 30%.

15. The method according to claim 14, wherein UTS.sub.target*TE.sub.target is between 21000 and 60000, TE.sub.target being maximum of 30%.

16. The method according to claim 1, wherein FWHM.sub.target is above or equal 1.0.

17. The method according to claim 15, wherein FWHM.sub.target is between 1.0 and 1.5.

18. The method according to claim 1, wherein P.sub.target is above 14.2.

19. The method according to claim 18, wherein P.sub.target is between 14.2 and 25.

20. The method according to claim 19, wherein P.sub.target is between 14.2 and 18.

21. The method according to claim 20, wherein T.sub.target is between 40 and 900 C. and the t.sub.target is between 40 seconds to 60 minutes.

Description

DETAILED DESCRIPTION

(1) The following terms will be defined: M: mechanical property, M.sub.target: target value of the mechanical property, M.sub.recrystallisation: mechanical property after a recrystallization annealing, M.sub.cold-roll: mechanical property after a cold-rolling, UTS: ultimate tensile strength, TE: total elongation, P: pareq value, P.sub.target: target value of pareq, FWHM: full width at half maximum of X-ray diffraction spectrum and FWHM.sub.target: target value of the full width at half maximum of X-ray diffraction spectrum.

(2) The present 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.sub.target whose composition comprises, in weight: 0.1<C<1.2%, 13.0Mn<25.0%, S0.030%, P0.080%, N0.1%, Si3.0%, and on a purely optional basis, one or more elements such as Nb0.5%, B0.005%, Cr1.0%, Mo0.40%, Ni1.0%, Cu5.0%, Ti0.5%, V2.5%, Al4.0%, 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 40 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 FWHM.sub.target corresponding to the M.sub.target is being determined, II. the pareq value P.sub.target of the heat treatment to perform to reach such M.sub.target is being determined and III. a time t.sub.target and a temperature T.sub.target corresponding to the P.sub.target value are being selected, C. a feeding step of a recrystallized steel sheet having a M.sub.recrystallization, D. a cold-rolling step in order to obtain a steel sheet having a M.sub.cold-roll and E. an annealing step performed at a temperature T.sub.target during a time t.sub.target.

(3) 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.

(4) 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.

(5) 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.

(6) 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 (AlN) 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%.

(7) Correspondingly, the nitrogen content must be 0.1% or less so as to prevent the precipitation of AlN 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%

(8) According to embodiments of 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 5 nm and more preferably between 0.2 and 4.0 nm

(9) 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%.

(10) 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.

(11) 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.

(12) 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%.

(13) 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%.

(14) 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.

(15) 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%.

(16) 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.

(17) According to embodiments of 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 40 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(H/RT)*dt)
With H: 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.

(18) Then, during the step A.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 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.

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

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

(21) 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.

(22) According to an embodiment of the present invention, a calculation step B) is realized. The calculation comprises a step B.I) wherein the value of FWHM.sub.target corresponding to the M.sub.target is being determined. Preferably, FWHM.sub.target is above 1.0 and advantageously between 1.0 and 1.5.

(23) In one preferred embodiment wherein M is UTS, the determination of FWHM is achieved with the following equation:
UTS.sub.target=UTS.sub.cold-roll(UTS.sub.cold-roll-UTS.sub.recrystallization)*(exp((FWHM+2.3)/2.3)1).sup.4)

(24) In this case, preferably, the UTS.sub.target is above or equal to 1430 MPa and more preferably between 1430 and 2000 MPa.

(25) 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.sub.target=TE.sub.cold-roll(TE.sub.recrystallizationUTS.sub.cold-roll)*(exp((FWHM+2.3)/2.3)1).sup.2.5)

(26) In this case, preferably, TE.sub.target is above or equal to 15% and more preferably between 15 and 30%.

(27) 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.sub.target*TE.sub.target=100000*(10.5FWHM)

(28) In this case, preferably, UTS.sub.target*TE.sub.target is above 21000 and more preferably between 21000 and 60000, TE.sub.target being maximum of 30%.

(29) Then, the step B.II), wherein the pareq value P.sub.target of the heat treatment to perform to reach such M.sub.target is determined, is performed. Preferably, P.sub.target is above 14.2, more preferably between 14.2 and 25 and more preferably, between 14.2 and 18.

(30) After, the step B.III), consisting in selecting a time t.sub.target and a temperature T.sub.target corresponding to the P.sub.target value, is realized. Preferably, T.sub.target is between 40 and 900 C. and the t.sub.target is between 40 seconds to 60 minutes.

(31) Then, the method according to an embodiment of the present invention comprises a feeding step of a recrystallized a steel sheet having a M.sub.recrystallization. Indeed, preferably, the steel sheet is recrystallized after a recrystallization annealing performed at a temperature between 70 and 900 C. For example, the recrystallization is realized during 10 to 500 seconds, preferably between 60 and 180 seconds.

(32) In one preferred embodiment, when M is UTS, UTS.sub.recrystallization is above 800 MPa, preferably between 800 and 1400 MPa and more preferably between 1000 and 1400 MPa.

(33) In another preferred embodiment, when M is TE, TE.sub.recrystallization is above 20%, preferably above 30% and more preferably between 30 and 70%.

(34) In another preferred embodiment, when M is TE*UTS, TE.sub.recrystallization*UTS recrystallization is above 16000, more preferably above 24000 and advantageously between 24000 and 98000.

(35) Then, a cold-rolling step D) is realized in order to obtain a steel sheet having a M.sub.cold-roll. 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.

(36) In one preferred embodiment, when M is UTS, UTS.sub.cold-roll is above 1000, preferably above 1200 MPa and advantageously above 1400 MPa.

(37) In another preferred embodiment, when M is TE, TE.sub.cold-roll is above 2%, more preferably between 2 and 50%.

(38) In another preferred embodiment, when M is TE*UTS, TE.sub.cold-roll*UTS cold-roll is above 2000, preferably 2400 and more preferably between 2400 and 70000.

(39) Then, an annealing step E) is performed at a temperature T.sub.target during a time t.sub.target.

(40) 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.

(41) 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.

(42) 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.

(43) 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.

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

(45) 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.

(46) Example

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

(48) TABLE-US-00001 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

(49) In this example, the recovered steel sheet had a target value of the mechanical property M.sub.target is UTS.sub.target being of 1512 MPa. Thanks to the calibration step A, the value of FMHM.sub.target corresponding to the UTS.sub.target was determined, the FMHM.sub.target was of 1.096. The P.sub.target of the heat treatment to perform to reach UTS.sub.target was determined, it was of 14.39. Then, the selected time t.sub.target was of 40 seconds and the selected temperature T.sub.target was of 650 C.

(50) 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.sup.st cold-rolling was realized with a cold-rolling reduction ratio of 50%. Thereafter, a recrystallization annealing was performed at 825 C. during 180 seconds. The value of UTS.sub.recrystallization obtained was of 980 MPa. Afterwards, a 2.sup.nd cold-rolling was realized with a cold-rolling reduction ratio of 30%. The value of UTS.sub.cold-roll obtained was of 1540 MPa.

(51) 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.

(52) Trial 2 was annealed at 650 C. during 90 seconds, i.e. t.sub.target and T.sub.target determined by the method of the present invention were not respected. After this annealing, Trial 2 was recrystallized. UTS of Trial 2 was of 1415.15 MPa. 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.

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