Method of dynamical adjustment for manufacturing a thermally treated steel sheet

Abstract

The present invention describes a method of dynamical adjustment for manufacturing a thermally treated steel sheet. The method includes: A. a control step, wherein at least one sensor detects a deviation happening during the thermal treatment, B. a calculation step performed when the deviation is detected during the thermal treatment such that a new thermal path TP.sub.target is determined to reach m.sub.target taking the deviation into account, such calculation step including: 1) a calculation substep, wherein at least two thermal path, TP.sub.x corresponding to one microstructure m.sub.x obtained at the end of TP.sub.x, are calculated based on TT and the microstructure m.sub.i of the steel sheet to reach m.sub.target, 2) a selection substep wherein one new thermal path TP.sub.target to reach m.sub.target is selected, TP.sub.target being chosen from said TP.sub.x and being selected such that m.sub.x is the closest to m.sub.target, C. a new thermal treatment step, wherein TP.sub.target is performed online on the steel sheet.

Claims

1. A method of dynamical adjustment for manufacturing a thermally treated steel sheet having a chemical steel composition and a microstructure m.sub.target comprising from 0 to 100% of at least one phase chosen among: ferrite, martensite, bainite, pearlite, cementite and austenite such that m.sub.target=Xferrite+Xmartensite+Xbainite+Xpearlite+Xcementite+Xaustenite, X being a phase fraction, in a heat treatment line, wherein a predefined thermal treatment TT including thermal treatment steps is performed on the steel sheet sequentially in the heat treatment line, such method comprising: performing at least one of the thermal treatment steps of the predefined thermal treatment TT on the steel sheet in the heat treatment line, A. a control step wherein at least one sensor detects a deviation happening in the heat treatment line during the performed at least one thermal treatment step, the deviation being such that the predefined thermal treatment TT is determined to produce a microstructure different from m.sub.target, B. a calculation step performed when the deviation is detected during the thermal treatment such that a new thermal path TP.sub.target, performed as at least one further heat treatment step in the heat treatment line sequentially downstream from the performed at least one thermal treatment step, is determined to reach m.sub.target taking the deviation into account, such calculation step comprising: 1) a calculation substep, wherein at least two thermal paths TP.sub.x, each performed as at least one further heat treatment step in the heat treatment line sequentially downstream from the performed at least one thermal treatment step and corresponding to one microstructure m.sub.x obtained at the end of TP.sub.x, are calculated based on TT, including the performed at least one thermal treatment step, and the microstructure m.sub.i of the steel sheet to reach m.sub.target, the calculation substep taking into consideration a thermal enthalpy H.sub.x released or consumed between m.sub.i and m.sub.target, the thermal enthalpy H.sub.x being calculated such that:
H.sub.x=(X.sub.ferrite*H.sub.ferrite)+(X.sub.martensite*H.sub.martensite)+(X.sub.bainite*H.sub.bainite)+(X.sub.pearlite*H.sub.pearlite)+(H.sub.cementite*X.sub.cementite)+(H.sub.austenite*X.sub.austenite) 2) a selection substep wherein one new thermal path TP.sub.target to reach m.sub.target is selected, TP.sub.target being chosen from one of the at least two thermal paths TP.sub.x calculated in substep B.1) and being selected such that m.sub.x is the closest to m.sub.target, C. performing a new thermal treatment step in the heat treatment line sequentially downstream from the performed at least one thermal treatment step by modifying at least one of a time, a temperature or rate of one of the thermal treatment steps of the predefined thermal treatment sequentially downstream from the performed at least one thermal treatment step, the performing of the new thermal treatment step including performing the selected new thermal treatment path TP.sub.target online on the steel sheet to produce a thermally treated steel sheet having a microstructure=Xferrite+Xmartensite+Xbainite+Xpearlite+Xcementite+Xaustenite with each phase X being within a predetermined threshold of the microstructure m.sub.target.

2. A method according to claim 1, wherein in step A, the deviation is due to a variation of one process parameter chosen from among: a furnace temperature, a steel sheet temperature, an amount of gas, a gas composition, a gas temperature, a line speed, a failure in the heat treatment line, a variation of a hot-dip bath, a steel sheet emissivity and a variation of the steel thickness.

3. A method according to claim 1, wherein the at least one phase is defined by at least one element chosen from: a size, a shape and a chemical composition.

4. A method according to claim 1, wherein the microstructure m.sub.target is selected from a group consisting of: 100% of austenite, from 5 to 95% of martensite, from 4 to 65% of bainite, the balance being ferrite, from 8 to 30% of residual austenite, from 0.6 to 1.5% of carbon in solid solution, the balance being ferrite, martensite, bainite, pearlite and/or cementite, from 1% to 30% of ferrite and from 1% to 30% of bainite, from 5 to 25% of austenite, the balance being martensite, from 5 to 20% of residual austenite, the balance being martensite, ferrite and residual austenite, residual austenite and intermetallic phases, from 80 to 100% of martensite and from 0 to 20% of residual austenite, 100% martensite, from 5 to 100% of pearlite and from 0 to 95% of ferrite, or at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.

5. A method according to claim 1, wherein the steel sheet is selected from a group consisting of a Dual Phase steel, a Transformation Induced Plasticity steel, a Quenched & Partitioned steel, a Twins Induced Plasticity steel, a Carbide Free Bainite steel, a Press Hardening Steel, a TRIPLEX steel, or a DUPLEX steel.

6. A method according to claim 1, wherein in the calculation substep, the at least two thermal paths TP.sub.x are calculated such that: $T\left(t+\Delta t\right)=T\left(t\right)+\frac{\left({\phi }_{\mathrm{Convection}}+{\phi }_{\mathrm{radiance}}\right)}{\rho .Math.\mathrm{Ep}.Math.C_{\mathrm{pe}}}\Delta t\pm \frac{\mathrm{Hx}}{C_{\mathrm{pe}}},$ wherein Cpe: the specific heat of the phase (J.Math.kg.sup.−1.Math.K.sup.−1), ρ: the density of the steel (g.Math.m.sup.−3), Ep: thickness of the steel (m), φ: the heat flux (convective+radiative in W), H.sub.x (J.Math.kg.sup.−1), T: temperature (° C.) and t: time (s).

7. A method according to claim 1, wherein in the calculation substep, at least one intermediate steel microstructure m.sub.xint corresponding to an intermediate thermal path TP.sub.xint and the thermal enthalpy H.sub.xint are calculated.

8. A method according to claim 7, wherein in step in the calculation substep, TP.sub.x is the sum of all TP.sub.xint and H.sub.X is the sum of all H.sub.xint.

9. A method according to claim 1, wherein before the calculation substep, at least one targeted mechanical property P.sub.target chosen among yield strength YS, Ultimate Tensile Strength UTS, elongation hole expansion, and formability is selected.

10. A method according to claim 9, wherein m.sub.target is calculated based on P.sub.target.

11. A method according to claim 1, wherein in the calculation substep, process parameters undergone by the steel sheet before entering the heat treatment line are taken into account to calculate TP.sub.x.

12. A method according to claim 11, wherein the process parameters comprise at least one element chosen from among: a cold rolling reduction rate, a coiling temperature, a run out table cooling path, a cooling temperature and a coil cooling rate.

13. A method according to claim 1, wherein in the calculation substep, process parameters of the treatment line that the steel sheet will undergo in the heat treatment line are taken into account to calculate TP.sub.x.

14. A method according to claim 13, wherein the process parameters comprise at least one element chosen from among: a specific thermal steel sheet temperature to reach, a line speed, a cooling power of cooling sections, a heating power of heating sections, an overaging temperature, a cooling temperature, a heating temperature and a soaking temperature.

15. A method according to claim 1, wherein the thermal path, TP.sub.x, TT or TP.sub.target comprise at least one treatment chosen from: a heating, an isotherm or a cooling treatment.

16. A method according to claim 1, wherein every time a new steel sheet enters into the heat treatment line, a new iteration of the calculation substep is automatically performed.

17. A method according to claim 16, wherein an adaptation of the predefined thermal treatment TT is performed as the steel sheet enters into the heat treatment line on the first meters of the sheet.

18. A method according to claim 1, wherein an automatic calculation is performed during the thermal treatment to check if any deviation had appeared.

19. A method according to claim 1, wherein the predetermined threshold is +/−3%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To illustrate the invention, various embodiments of non-limiting examples will be described, particularly with reference to the following Figures.

(2) FIG. 1 illustrates an example of an embodiment of the present invention.

(3) FIG. 2 illustrates a continuous annealing of a steel sheet comprising a heating step, a soaking step, a cooling step and an overaging step.

(4) FIG. 3 illustrates an example of an embodiment of the present invention.

(5) FIG. 4 illustrates an example of an embodiment according to the present invention, wherein a continuous annealing is performed on a steel sheet before the deposition of a coating by hot-dip.

DETAILED DESCRIPTION

(6) The following terms will be defined: CC: chemical composition in percentage in weight percent, m.sub.target: targeted value of the microstructure, m.sub.standard: the microstructure of the selected product, P.sub.target: targeted value of a mechanical property, m.sub.i: initial microstructure of the steel sheet, X: phase fraction in weight percent, T: temperature in degree Celsius (° C.), t: time (s), s: seconds, UTS: ultimate tensile strength (MPa), YS: yield stress (MPa), metallic coating based on zinc means a metallic coating comprising above 50% of zinc, metallic coating based on aluminum means a metallic coating comprising above 50% of aluminum, TT: thermal treatment, and thermal path, TT, TP.sub.target, TP.sub.x and TP.sub.xint comprises a time, a temperature of the thermal treatment and at least one rate chosen from: a cooling, an isotherm or a heating rate. The isotherm rate means a rate having a constant temperature and nanofluids: fluid comprising nanoparticles.

(7) The designation “steel” or “steel sheet” means a steel sheet, a coil, a plate having a composition allowing the part to achieve a tensile strength up to 2500 MPa and more preferably up to 2000 MPa. For example, the tensile strength is above or equal to 500 MPa, preferably above or equal to 1000 MPa, advantageously above or equal to 1500 MPa. A wide range of chemical composition is included since the method according to the invention can be applied to any kind of steel.

(8) The invention provides a method of dynamical adjustment for manufacturing a thermally treated steel sheet having a chemical steel composition and a microstructure m.sub.target comprising from 0 to 100% of at least one phase chosen among: ferrite, martensite, bainite, pearlite, cementite and austenite, in a heat treatment line wherein a predefined thermal treatment TT is performed on the steel sheet, such method comprising: A. a control step wherein at least one sensor detects any deviation happening during the thermal treatment TT, B. a calculation step performed when a deviation is detected during the thermal treatment such that a new thermal path TP.sub.target is determined to reach m.sub.target taking the deviation into account, such calculation step comprising: 1) a calculation substep, wherein at least two thermal path, TP.sub.x corresponding to one microstructure m.sub.x at the end of TP.sub.x, are calculated based on TT and the microstructure m.sub.i of the steel sheet to reach m.sub.target, 2) a selection substep, wherein one new thermal path TP.sub.target to reach m.sub.target is selected, TP.sub.target being chosen from TP.sub.X and being selected such that m.sub.X is the closest to m.sub.target, C. a new thermal treatment step wherein TP.sub.target is performed online on the steel sheet.

(9) Without willing to be bound by any theory, it seems that when the method according to the present invention is applied, it is possible to correct any deviation happening during a thermal treatment by providing a personalized heat treatment depending on each steel sheet. To do so, a precise and specific new thermal path TP.sub.target is calculated in a short calculation time taking into account m.sub.target, in particular the proportion of all the phases along the treatment, m.sub.i (including the microstructure dispersion along the steel sheet) and the deviation. Indeed, the method according to the present invention takes into account for the calculation the thermodynamically stable phases, i.e. ferrite, austenite, cementite and pearlite, and the thermodynamic metastable phases, i.e. bainite and martensite. Thus, a steel sheet having the expected properties with the minimum of properties dispersion possible is obtained.

(10) In some embodiments, the microstructures m.sub.x, m.sub.target and m.sub.i phases are defined by at least one element chosen from: the size, the shape and the chemical composition.

(11) In some embodiments, the microstructure m.sub.target to reach comprises: 100% of austenite, from 5 to 95% of martensite, from 4 to 65% of bainite, the balance being ferrite, from 8 to 30% of residual austenite, from 0.6 to 1.5% of carbon in solid solution, the balance being ferrite, martensite, bainite, pearlite and/or cementite, from 1% to 30% of ferrite and from 1% to 30% of bainite, from 5 and 25% of austenite, the balance being martensite, from 5 to 20% of residual austenite, the balance being martensite, ferrite and residual austenite, residual austenite and intermetallic phases, from 80 to 100% of martensite and from 0 to 20% of residual austenite, 100% martensite, from 5 to 100% of pearlite and from 0 to 95% of ferrite, and at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.

(12) In some embodiments, the steel sheets can be any kind of steel grade, including, e.g., Dual Phase DP, Transformation Induced Plasticity (TRIP), Quenched & Partitioned steel (Q&P), Twins Induced Plasticity (TWIP), Carbide Free Bainite (CFB), Press Hardening Steel (PHS), TRIPLEX, DUPLEX and Dual Phase High Ductility (DP HD) steels.

(13) The chemical composition depends on each steel sheet. For example, the chemical composition of a DP steel can comprise: 0.05<C<0.3%, 0.5≤Mn<3.0%, S≤0.008%, P≤0.080%, N≤0.1%, Si≤1.0%,
the remainder of the composition making up of iron and inevitable impurities resulting from the development.

(14) FIG. 1 illustrates an example of an embodiment according to the present invention, wherein a TT is performed on a steel sheet in a heat treatment line, such steel sheet having a chemical composition CC and m.sub.target to reach.

(15) According to an embodiment of the present invention, in step A), any deviation happening during the thermal treatment is detected. In some embodiments, the deviation is due to a variation of a process parameter chosen from among: a furnace temperature, a steel sheet temperature, an amount of gas, a gas composition, a gas temperature, a line speed, a failure in the heat treatment line, a variation of the hot-dip bath, a steel sheet emissivity and a variation of the steel thickness.

(16) A furnace temperature can be a heating temperature, a soaking temperature, a cooling temperature, an overaging temperature, in particular in a continuous annealing.

(17) A steel sheet temperature can be measured at any time of the heat treatment in different positions of the heat treatment line, for example: in a heating section preferably being a direct flame furnace (DFF), a radian tube furnace (RTF), an electrical resistance furnace or an induction furnace, in cooling section, in particular, in jets cooling, in a quenching system or in a snout and in isothermal section preferably being an electrical resistance furnace.

(18) To detect a temperature variation, the sensor can be a pyrometer or a scanner.

(19) Usually, heat treatments can be performed in an oxidizing atmosphere, i.e. an atmosphere comprising an oxidizing gas being for example: O.sub.2, CH.sub.4, CO.sub.2 or CO. They also can be performed in a neutral atmosphere, i.e. an atmosphere comprising a neutral gas being for example: N.sub.2, Ar or He. Finally, they also can be performed in a reducing atmosphere, i.e. an atmosphere comprising a reducing gas being for example: H.sub.2 or HN.sub.x.

(20) The variation of gas amount can be detected by barometer.

(21) The line speed can be detected by a laser sensor.

(22) For example, a failure in the heat treatment line can be: in a direct flame furnace: a burner not working anymore, in a radiant tube furnace: a radiant tube not working anymore, in an electrical furnace: a resistance not working anymore or in a cooling section: one or several jets cooling not working anymore.

(23) In such cases, sensor can be a pyrometer, a barometer, an electrical consumption or a camera.

(24) The variation of the steel thickness can be detected by a laser or an ultrasound sensor.

(25) When a deviation is detected, at least two thermal path TP.sub.x, corresponding to m.sub.x, are calculated based on TT and m.sub.i to reach m.sub.target, such TP.sub.x taking into account the deviation. The calculation of TP.sub.x is based on the thermal behavior and metallurgical behavior of the steel sheet compared to the conventional methods wherein only the thermal behavior is considered.

(26) FIG. 2 illustrates a continuous annealing of a steel sheet comprising a heating step, a soaking step, a cooling step and an overaging step. A deviation D due to a variation of T.sub.soaking is detected. Thus, a multitude of TP.sub.x is calculated to reach m.sub.target as shown only for the first cooling step in FIG. 2. In this example, the calculated TP.sub.x also includes the second cooling step and the overaging step.

(27) In some embodiments, at least 10 TP.sub.x are calculated, more preferably at least 50, advantageously at least 100 and more preferably at least 1000. For example, the number of calculated TP.sub.x is between 2 and 10000, preferably between 100 and 10000 and preferably between 1000 and 10000.

(28) In step B.2), one new thermal path TP.sub.target to reach m.sub.target is selected. TP.sub.target is chosen from TP.sub.x and being selected such that m.sub.x is the closest to m.sub.target. Thus, in FIG. 1, TP.sub.target is chosen from a multitude of TP.sub.x. Preferably, the differences between phases proportions of each phase present in m.sub.target and m.sub.x is +3%.

(29) In some embodiments, in step B.1), the thermal enthalpy H released or consumed between m.sub.i and m.sub.target is calculated such that:
H.sub.x=(X.sub.ferrite*H.sub.ferrite)+(X.sub.martensite*H.sub.martensite)+(X.sub.bainite*H.sub.bainite)+(X.sub.pearlite*H.sub.pearlite)+(H.sub.cementite*X.sub.cementite)+(H.sub.austenite*X.sub.austenite)
X being a phase fraction.

(30) Without willing to be bound by any theory, H represents the energy released or consumed along the all thermal path when a phase transformation is performed. It is believed that some phase transformations are exothermic and some of them are endothermic. For example, the transformation of ferrite into austenite during a heating path is endothermic whereas the transformation of austenite into pearlite during a cooling path is exothermic. Preferably, H.sub.x is taken in account in the calculation of TP.sub.x.

(31) In one embodiment, in step B.1), the all thermal cycle TP.sub.x is calculated such that:

(32) $T\left(t+\Delta t\right)=T\left(t\right)+\frac{\left({\phi }_{\mathrm{Convection}}+{\phi }_{\mathrm{radiance}}\right)}{\rho .Math.\mathrm{Ep}.Math.C_{\mathrm{pe}}}\Delta t\pm \frac{\mathrm{Hx}}{C_{\mathrm{pe}}}$
with Cpe: the specific heat of the phase (J.Math.kg.sup.−1.Math.K.sup.−1), ρ: the density of the steel (g.Math.m.sup.−3), Ep: the thickness of the steel (m), φ: the heat flux (convective and radiative in W), H.sub.x(J.Math.kg.sup.−1), T: temperature (° C.) and t: time (s).

(33) In some embodiments, in step B.1), at least one intermediate steel microstructure m.sub.xint corresponding to an intermediate thermal path TP.sub.xint and the thermal enthalpy H.sub.xint are calculated. In this case, the calculation of TP.sub.x is obtained by the calculation of a multitude of TP.sub.xint. Thus preferably, TP.sub.x is the sum of all TP.sub.xint and H.sub.x is the sum of all H.sub.xint. In this preferred embodiment, TP.sub.xint is calculated periodically. For example, it is calculated every 0.5 seconds, preferably 0.1 seconds or less.

(34) FIG. 3 illustrates an embodiment of the present invention, wherein in step B. 1), m.sub.int1 and m.sub.int2 corresponding respectively to TP.sub.xint1 and TP.sub.xint2 as well as H.sub.xint1 and H.sub.xint2 are calculated. H.sub.x during the all thermal path is determined to calculate TP.sub.x. according to the present invention, a multitude, i.e more than 2, of TP.sub.xint, m.sub.xint and H.sub.xint are calculated to obtain TP.sub.x.

(35) In some embodiments, before step B.2), at least one targeted mechanical property P.sub.target chosen among yield strength YS, Ultimate Tensile Strength UTS, elongation hole expansion, formability is selected. In these embodiments, preferably, m.sub.target is calculated based on P.sub.target.

(36) Without willing to be bound by any theory, it is believed that the characteristics of the steel sheet are defined by the process parameters applied during the steel production. Thus, In some embodiments, in step B.1), the process parameters undergone by the steel sheet before entering the heat treatment line are taken into account to calculate TP.sub.x. For example, the process parameters comprise at least one element chosen from among: a cold rolling reduction rate, a coiling temperature, a run out table cooling path, a cooling temperature and a coil cooling rate.

(37) In some embodiments, the process parameters of the treatment line that the steel sheet will undergo in the heat treatment line are taken into account to calculate TP.sub.x. For example, the process parameters comprise at least one element chosen from among: a specific thermal steel sheet temperature to reach, the line speed, cooling power of the cooling sections, heating power of the heating sections, an overaging temperature, a cooling temperature, a heating temperature and a soaking temperature.

(38) In some embodiments, the thermal path, TP.sub.x, TP.sub.xint, TT or TP.sub.target comprise at least one treatment chosen from: a heating, an isotherm or a cooling treatment. For example, the thermal path can be a recrystallization annealing, a press hardening path, a recovery path, an intercritical or full austenitic annealing, a tempering path, a partitioning path, isothermal path or a quenching path.

(39) In some embodiments, a recrystallization annealing is performed. The recrystallization annealing comprises optionally a pre-heating step, a heating step, a soaking step, a cooling step and optionally an equalizing step. In one embodiment, it is performed in a continuous annealing furnace comprising optionally a pre-heating section, a heating section, a soaking section, a cooling section and optionally an equalizing section. Without willing to be bound by any theory, it is believed that the recrystallization annealing is the thermal path the more difficult to handle since it comprises many steps to take into account comprising cooling and heating steps.

(40) In some embodiments, every time a new steel sheet enters into the heat treatment line, a new calculation step B.1) is automatically performed. Indeed, the method according to the present invention adapts the thermal path TP.sub.target to each steel sheet even if the same steel grade enters in the heat treatment line since the real characteristics of each steel often differs. The new steel sheet can be detected and the new characteristics of the steel sheet are measured and are pre-selected beforehand. For example, a sensor detects the welding between two coils.

(41) In some embodiments, the adaptation of the thermal path is performed as the steel sheet entries into the heat treatment line on the first meters of the sheet in order to avoid strong process variation.

(42) In some embodiments, an automatic calculation is performed during the thermal treatment to check if any deviation had appeared. In these embodiments, periodically, a calculation is realized to verify if a slight deviation had occurred. Indeed, the detection threshold of sensor is sometimes too high which means that a slight deviation is not always detected. The automatic calculation, performed for example every few seconds, is not based on a detection threshold. Thus, if the calculation leads to the same thermal treatment, i.e. the thermal treatment performs online, TT will not change. If the calculation leads to a different treatment due to a slight deviation, the treatment will change.

(43) FIG. 4 illustrates one example according to the present invention, wherein a continuous annealing is performed on a steel sheet before the deposition of a coating by hot-dip. With the method according to one embodiment of the present invention, when a deviation D appears, TP.sub.x is calculated based on m.sub.i, the selected product, TT and m.sub.target. In this example, intermediate thermal paths TP.sub.xint1 to TP.sub.xint4, corresponding respectively m.sub.xint1 to m.sub.xint4, and H.sub.xint1 to H.sub.xint4 are calculated. H.sub.x is determined in order to obtain TP.sub.x. In this Figure, the represented TP.sub.target has been chosen from TP.sub.x.

(44) With the method according to an embodiment of the present invention, when a deviation appears, a new thermal treatment step comprising TP.sub.target is performed on the steel sheet in order to reach m.sub.target.

(45) The present invention also provides a coil made of a steel sheet including said predefined product types, including, e.g., DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX, DP or HD steels, such coil having a standard variation of mechanical properties below or equal to 25 MPa, preferably below or equal to 15 MPa, more preferably below or equal to 9 MPa, between any two points along the coil. Indeed, without willing to be bound by any theory, it is believed that the method including the calculation step B.1) takes into account the microstructure dispersion of the steel sheet along the coil. Thus, TP.sub.target applied on the steel sheet allows for a homogenization of the microstructure and also of the mechanical properties.

(46) The low value of standard variation is due to the precision of TP.sub.target. In some embodiments, the mechanical properties are chosen from YS, UTS or elongation.

(47) In some embodiments, the coil is covered by a metallic coating based on zinc or based on aluminum.

(48) In some embodiments, in an industrial production, the standard variation of mechanical properties between 2 coils made of a steel sheet including said predefined product types, including, e.g., DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX, DP HD steels, measured and successively produced on the same line is below or equal to 25 MPa, preferably below or equal to 15 MPa, more preferably below or equal to 9 MPa.

(49) A thermal treatment line for the implementation of a method according to the present invention is used to perform TP.sub.target. For example, the thermally treatment line is a continuous annealing furnace, a press hardening furnace, a batch annealing or a quenching line.

(50) Finally, the present invention provides a computer program product comprising at least a metallurgical module, a thermal module and an optimization module that cooperate together to determine TP.sub.target such modules comprising software instructions that when implemented by a computer implement the method according to the present invention.

(51) The metallurgical module predicts the microstructure (m.sub.y, m.sub.target including metastable phases: bainite and martensite and stables phases: ferrite, austenite, cementite and pearlite) and more precisely the proportion of phases all along the treatment and predicts the kinetic of phases transformation.

(52) The thermal module predicts the steel sheet temperature depending on the installation used for the thermal treatment, the installation being for example a continuous annealing furnace, the geometric characteristics of the band, the process parameters including the power of cooling, heating or isotherm power, the dynamic thermal enthalpy H released or consumed along the all thermal path when a phase transformation is performed.

(53) The optimization module determines the best thermal path to reach m.sub.target, i.e. TP.sub.target following the method according to the present invention using the metallurgical and thermal modules.

EXAMPLES

(54) In the following examples, DP780GI having the following chemical composition was chosen:

(55) TABLE-US-00001 C Mn Si Cr Mo P Cu Ti N (%) (%) (%) (%) (%) (%) (%) (%) (%) 0.145 1.8 0.2 0.2 0.0025 0.015 0.02 0.025 0.06

(56) The cold-rolling had a reduction rate of 55% to obtain a thickness of 1.2 mm.

(57) m.sub.target to reach comprised 12% of martensite, 58% of ferrite and 30% of bainite, corresponding to the following P.sub.target:YS of 460 MPa and UTS of 790 MPa. A cooling temperature T.sub.cooling of 460° C. has also to be reached in order to perform a hot-dip coating with a zinc bath. This temperature must be reached with an accuracy of +/−2° C. to guarantee good coatability in the Zn bath.

(58) The thermal treatment TT to perform on the steel sheet is as follows: a pre-heating step wherein the steel sheet is heated from ambient temperature to 680° C. during 37.5 seconds, a heating step wherein the steel sheet is heated from 680° C. to 780° C. during 40 seconds, soaking step wherein the steel sheet is heated at a soaking temperature T.sub.soaking of 780° C. during 24.4 seconds, a cooling step wherein the steel sheet is cooled with 11 jets cooling spraying HN.sub.x as follows:

(59) TABLE-US-00002 Jets Jet 1 Jet 2 Jet 3 Jet 4 Jet 5 Jet 6 Jet 7 Jet 8 Jet 9 Jet 10 Jet 11 Cooling 10 10 9 5 9 22 50 18 18 21 11 rate (° C./s) Time (s) 1.89 1.89 1.89 1.89 1.68 1.8 1.8 1.63 1.63 1.63 1.63 T (° C.) 754 734 718 708 693 653 563 533 504 481 463 Cooling 0 0 0 0 0 0 28 100 100 100 100 power (%) a hot-dip coating in a zinc bath a 460° C., the cooling of the steel sheet until the top roll during 27.8 s at 300° C. and the cooling of the steel sheet at ambient temperature.

Example 1: Deviation of T.SUB.soaking

(60) When the soaking temperature T.sub.soaking decreased from 780° C. to 765° C., a new thermal path TP.sub.target1 is determined to reach m.sub.target taking the deviation into account. To this end, a multitude of thermal path TP.sub.x was calculated based on TT, m.sub.i of DP780GI to reach m.sub.target and the deviation.

(61) After the calculation of TP.sub.x, one new thermal path TP.sub.target1 to reach m.sub.target was selected, TP.sub.target1 being chosen from TP.sub.x and being selected such that m.sub.x is the closest to m.sub.target. TP.sub.target1 is as follows: a soaking step wherein the steel sheet is heated at a soaking temperature T.sub.soaking of 765° C. during 24.4 seconds due to a deviation in the soaking section of the heat treatment line, a cooling step wherein the steel sheet is cooled with 11 jets cooling spraying HN.sub.x as follows:

(62) TABLE-US-00003 Jets Jet 1 Jet 2 Jet 3 Jet 4 Jet 5 Jet 6 Jet 7 Jet 8 Jet 9 Jet 10 Jet 11 Cooling 9 9 10 15 32 28 31 11 10 7 8 rate (° C./s) Time (s) 1.89 1.89 1.89 1.89 1.68 1.8 1.8 1.63 1.63 1.63 1.63 T (° C.) 742 725 706 679 625 574 518 500 483 472 459 Cooling 0 0 0 25 50 50 45 45 45 45 45 power (%) a hot-dip coating in a zinc bath a 460° C., the cooling of the steel sheet until the top roll during 27.8 s at 300° C. and the cooling of the steel sheet at ambient temperature.

Example 2: Steel Sheet Having a Different Composition

(63) A new steel sheet DP780GI entered into the heat treatment line so a calculation step was automatically performed based on the following new CC:

(64) TABLE-US-00004 C Mn Si Cr Mo P Cu Ti N (%) (%) (%) (%) (%) (%) (%) (%) (%) 0.153 1.830 0.225 0.190 0.0025 0.015 0.020 0.025 0.006

(65) The new thermal path TP.sub.target2 was determined to reach m.sub.target taking the new CC into account. TP.sub.target2 is as follows: a pre-heating step wherein the steel sheet is heated from ambient temperature to 680° C. during 37.5 seconds, a heating step wherein the steel sheet is heated from 680° C. to 780° C. during 40 seconds, a soaking step wherein the steel sheet is heated at a soaking temperature T.sub.soaking of 780° C. during 24.4 seconds, a cooling step wherein the steel sheet is cooled with 11 jets cooling spraying HN.sub.x

(66) TABLE-US-00005 Jets Jet 1 Jet 2 Jet 3 Jet 4 Jet 5 Jet 6 Jet 7 Jet 8 Jet 9 Jet 10 Jet 11 Cooling 17 17 9 6 6 6 38 30 18 17 10 rate (° C./s) Time (s) 2.2 2.2 2.2 2.2 1.96 2.1 2.1 1.9 1.9 1.9 1.9 T (° C.) 737 705 688 677 667 655 586 537 508 481 464 Cooling 100 100 30 0 0 0 100 100 100 100 100 power (%) a hot-dip coating in a zinc bath a 460° C., the cooling of the steel sheet until the top roll during 26.8 s at 300° C. and the cooling of the steel sheet at ambient temperature.

(67) Table 1 shows the steel properties obtained with TT, TP.sub.target1 and TP.sub.target2:

(68) TABLE-US-00006 Expected TT TP.sub.target1 TP.sub.target2 properties T.sub.cooling 461 458 462 460 obtained (° C.) Microstructure X.sub.martensite: X.sub.martensite: X.sub.martensite: X.sub.martensite: obtained at 12% 12% 14% 12% the end of the X.sub.ferrite: 55% X.sub.ferrite: 61% X.sub.ferrite: X.sub.ferrite: thermal path X.sub.bainite: 33% X.sub.bainite: 27% 55% 58% X.sub.bainite: X.sub.bainite: 32% 30% Deviation X.sub.martensite: X.sub.martensite: X.sub.martensite: — (écart) with 0% 0% 2% respect to X.sub.ferrite: 3% X.sub.ferrite: 3% X.sub.ferrite: m.sub.target X.sub.bainite: 3% X.sub.bainite: 3% 3% X.sub.bainite: 2% YS (MPa) 453.5 465 462 460 YS deviation 6.5 5 2 — with respect to P.sub.target (MPa) UTS (MPa) 786.8 790 804 790 UTS deviation 3.2 0 14 — with respect to P.sub.target (MPa)

(69) With the method according to the various embodiments of the present invention, it is possible to adjust a thermal TT when a deviation appears or when a new steel sheet having a different CC enters into the heat treatment line. By applying the new thermal paths TP.sub.target1 and TP.sub.target2, it is possible to obtain a steel sheet having the desired expected properties, each TP.sub.target being precisely adapted to each deviation.