Heat treatment line with a method of dynamical adjustment for manufacturing a thermally treated steel sheet

Abstract

The present invention provides a heat treatment line with a method of dynamical adjustment for manufacturing a thermally treated steel sheet.

Claims

1. A heat treatment line comprising: a heating section; a soaking section; and a cooling section including a cooling system, the heat treatment line implementing 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, wherein a predefined thermal treatment TT, comprising at least a heating, a soaking and a cooling steps, is performed, such method comprising: A. a control step wherein at least one detector detects any deviation happening during TT, B. a calculation step performed when a deviation is detected during TT 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 sub-step wherein through variation of the cooling power, new cooling paths CP.sub.x are calculated based on TT, the initial microstructure m.sub.i of the steel sheet to reach m.sub.target, the heating path, the soaking path comprising T.sub.soaking and T.sub.cooling, the cooling step of TT being recalculated using said CP.sub.x in order to obtain new thermal paths TP.sub.x, having the cooling step replaced by one CP.sub.x in order to obtain a thermal path TP.sub.x, each TP.sub.x corresponding to a microstructure m.sub.x, 2) a selection step wherein one TP.sub.target to reach m.sub.target is selected, TP.sub.target being chosen among the calculated thermal paths TP.sub.x and being selected such that m.sub.x is the closest to m.sub.target and C. a new thermal treatment step wherein TP.sub.target is performed online on the steel sheet.

2. The heat treatment line 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 the hot-dip bath, a steel sheet emissivity and a variation of the steel thickness.

3. The heat treatment line according to claim 1, wherein the phases are defined by at least one element chosen from: the size, the shape and the chemical composition.

4. The heat treatment line according to claim 1, wherein the microstructure m.sub.target 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%.

5. The heat treatment line according to claim 1, wherein the steel sheet comprises Dual Phase steel, Transformation Induced Plasticity steel, Quenched & Partitioned steel, Twins Induced Plasticity steel, Carbide Free Bainite steel, Press Hardening steel, TRIPLEX, DUPLEX or Dual Phase High Ductility steel.

6. The heat treatment line according to claim 1, wherein TT further comprises a pre-heating step.

7. The heat treatment line according to claim 1, wherein TT further comprises a hot-dip coating step, an overaging step or a partitioning step.

8. The heat treatment line according to claim 1, wherein in step B.1), the cooling power of the cooling system varies from a minimum to a maximum value.

9. The heat treatment line according to claim 1, wherein in step B.1), the cooling power of the cooling system varies from a maximum to a minimum value.

10. The method according to claim 1, wherein in step B.1), T.sub.soaking is a fixed number selected from a range between 600 to 1000? C.

11. The heat treatment line according to claim 1, wherein in step B.1), T.sub.soaking varies from 600 to 1000? C.

12. The heat treatment line according to claim 1, wherein the cooling system comprises at least one jet cooling, at least one cooling spray or at least both.

13. The heat treatment line according to claim 12, wherein when the cooling system comprises at least one jet cooling, the jet cooling spraying a gas, an aqueous liquid or a mixture thereof.

14. The heat treatment line according to claim 13, wherein the gas is chosen from air, HN.sub.x, H.sub.2, N.sub.2, Ar, He, steam water or a mixture thereof.

15. The heat treatment line according to claim 14, wherein the aqueous liquid is chosen from water or nanofluid.

16. The heat treatment line according to claim 14, wherein the jet cooling sprays air with a debit flow between 0 and 350000 Nm.sup.3/h.

17. The heat treatment line according to claim 1, wherein T.sub.cooling is the bath temperature when the cooling section is followed by a hot-dip coating section comprising a hot-dip bath.

18. The heat treatment line according to claim 17, wherein the bath is based on aluminum or a bath based on zinc.

19. The heat treatment line according to claim 1, wherein an adaptation of the cooling path is performed as the steel sheet entries into the cooling section of the heat treatment line on the first meters of the sheet.

20. The heat treatment line according to claim 1, wherein an automatic calculation is performed during the thermal treatment to check if any deviation had appeared.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figures.

[0075] FIG. 1 illustrates an embodiment of the present invention.

[0076] FIG. 2 illustrates a continuous annealing of a steel sheet comprising a heating step, a soaking step, a cooling step and an overaging step.

[0077] FIG. 3 illustrates a preferred embodiment according to the present invention.

[0078] FIG. 4 illustrates one 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

[0079] The following terms will be defined: [0080] CC: chemical composition in percentage in weight percent, [0081] m.sub.target: targeted value of the microstructure, [0082] m.sub.standard: the microstructure of the selected product, [0083] P.sub.target: targeted value of a mechanical property, [0084] m.sub.i: initial microstructure of the steel sheet, [0085] X: phase fraction in weight percent, [0086] T: temperature in degree Celsius (? C.), [0087] t: time (s), [0088] s: seconds, [0089] UTS: ultimate tensile strength (MPa) [0090] YS: yield stress (MPa) [0091] metallic coating based on zinc means a metallic coating comprising above 50% of zinc, [0092] metallic coating based on aluminum means a metallic coating comprising above 50% of aluminum and [0093] TT: thermal treatment and [0094] thermal path, TT, TP.sub.target and TP.sub.x comprises a time, a temperature of the thermal treatment and at least one rate chosen from: a cooling, an isotherm or a heating rate, [0095] a heating path comprises a time, a temperature and a heating rate, [0096] a soaking path comprises a time, a temperature and a soaking rate, [0097] CP.sub.x and CP.sub.xint comprise a time, a temperature and a cooling rate and [0098] nanofluids: fluid comprising nanoparticles.

[0099] 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.

[0100] 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 comprising a heating section, a soaking section and a cooling section including a cooling system, wherein a predefined thermal treatment TT, comprising at least a heating, a soaking and a cooling steps, is performed, such method comprising: [0101] A. a control step wherein at least one detector detects any deviation happening during TT, [0102] B. a calculation step performed when a deviation is detected during TT 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: [0103] 1) a calculation sub-step wherein through variation of the cooling power, new cooling paths CP.sub.x are calculated based on TT, the initial microstructure m.sub.i of the steel sheet to reach m.sub.target, the heating path, the soaking path comprising T.sub.soaking and T.sub.cooling, the cooling step of TT being recalculated using said CP.sub.x in order to obtain new thermal paths TP.sub.x, each TP.sub.x corresponding to a microstructure m.sub.x, [0104] 2) a selection step wherein one TP.sub.target to reach m.sub.target is selected, TP.sub.target being chosen among the calculated thermal paths TP.sub.x and being selected such that m.sub.x is the closest to m.sub.target and [0105] C. a new thermal treatment step wherein TP.sub.target is performed online on the steel sheet.

[0106] Without willing to be bound by any theory, it appears that when a 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 comprising a personalized cooling path which depends on each steel sheet. Thus, a precise 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 during the cooling path, m.sub.i (including the microstructure dispersion along the steel sheet) and the deviation. Indeed, the method according to an embodiment of 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.

[0107] In some embodiments of the present invention, 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.

[0108] In some embodiments of the present invention, TT further comprises a pre-heating step. More preferably, TT further comprises a hot-dip coating step, an overaging step or a partitioning step.

[0109] In some embodiments of the present invention, the microstructure m.sub.target to reach comprises: [0110] 100% of austenite, [0111] from 5 to 95% of martensite, from 4 to 65% of bainite, the balance being ferrite, [0112] 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, [0113] from 1% to 30% of ferrite and from 1% to 30% of bainite, from 5 and 25% of austenite, the balance being martensite, [0114] from 5 to 20% of residual austenite, the balance being martensite, [0115] ferrite and residual austenite, [0116] residual austenite and intermetallic phases, [0117] from 80 to 100% of martensite and from 0 to 20% of residual austenite [0118] 100% martensite, [0119] from 5 to 100% of pearlite and from 0 to 95% of ferrite and [0120] at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.

[0121] Advantageously, the steel sheets can be any kind of steel grade including 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).

[0122] 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.

[0123] FIG. 1 illustrates an example according to the 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.

[0124] According to an embodiment of the present invention in step A), any deviation happening during the thermal treatment is detected. Preferably, 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.

[0125] A furnace temperature can be a heating temperature, a soaking temperature, a cooling temperature, an overaging temperature.

[0126] A steel sheet temperature can be measured at any time of the heat treatment in different positions of the heat treatment line, for example: [0127] in a heating section preferably being a direct flame furnace (DFF), a radian tube furnace (RTF), an electrical resistance furnace or an induction furnace, [0128] in cooling section, in particular, in jets cooling, in a quenching system or in a snout and [0129] in isothermal section preferably being an electrical resistance furnace.

[0130] To detect a temperature variation, the detector can be a pyrometer or a scanner.

[0131] Usually, heat treatments can be performed in an oxidizing atmosphere, i.e. an atmosphere comprising an oxidizing gas being for example: O.sub.2, 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, He or Xe. 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.

[0132] The variation of gas amount can be detected by barometer.

[0133] The line speed can be detected by a laser detector.

[0134] For example, a failure in the heat treatment line can be: [0135] in a direct flame furnace: a burner not working anymore, [0136] in a radiant tube furnace: a radiant tube not working anymore, [0137] in an electrical furnace: a resistance not working anymore or [0138] in a cooling section: one or several jets cooling not working anymore.

[0139] In such cases, detector can be a pyrometer, a barometer, an electrical consumption or a camera.

[0140] The variation of the steel thickness can be detected by a laser or an ultrasound detector.

[0141] When a deviation is detected, through variation of the cooling power, new cooling paths CP.sub.x are calculated based on TT, m.sub.i to reach m.sub.target, the heating path, the soaking path comprising T.sub.soaking and T.sub.cooling, the cooling step of TT being recalculated using said CP.sub.x in order to obtain new thermal paths TP.sub.x, each TP.sub.x corresponding to a microstructure m.sub.x. The calculation of CP.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.

[0142] 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 CP.sub.x and therefore TP.sub.x are calculated such that TP.sub.standard, being selected among TP.sub.x, reach m.sub.target as shown only for the first cooling step in FIG. 2. In this example, the calculated CP.sub.x also includes the second cooling step (not shown).

[0143] In some embodiments, in step B.1), the cooling power of the cooling system varies from a minimum to a maximum value or from a maximum to a minimum value. For example, the cooling system comprises at least one jet cooling, at least one cooling spray or at least both. Preferably, the cooling system comprises at least one jet cooling, the jet cooling spraying a fluid being a gas, an aqueous liquid or a mixture thereof. For example, the gas is chosen from air, HN.sub.x, H.sub.2, N.sub.2, Ar, He, steam water or a mixture thereof. For example, the aqueous liquid is chosen from: water or nanofluids.

[0144] In some embodiments, jets cooling spray gas with a flow rate between 0 and 350000 Nm.sup.3/h. The number of jets cooling present in the cooling section depends on the heat treatment line, it can vary from 1 to 25, preferably from 1 to 20, advantageously from 1 to 15 and more preferably between from 1 and 5. The flow rate depends on the number of jets cooling. For example, the flow rate of one jet cooling is between 0 and 50000 Nm.sup.3/h, preferably between 0 and 40000 Nm.sup.3/h, more preferably between 0 and 20000 Nm.sup.3/h.

[0145] When the cooling section comprises jets cooling, the variation of cooling power is based on the flow rate. For example, for one jet cooling, 0 Nm.sup.3/h corresponds to a cooling power of 0% and 40000 Nm.sup.3/h corresponds to a cooling power of 100%.

[0146] Thus, for example, the cooling power of one jet cooling varies from a 0 Nm.sup.3/h, i.e. 0%, to 40000 Nm.sup.3/h, i.e. 100%. The minimum and maximum value of the cooling power can be any value chosen in the range of 0 to 100%. For example, the minimum value is of 0%, 10%, 15% or 25%. For example, the maximum value is of 80%, 85%, 90% or 100%.

[0147] When the cooling section comprises at least 2 jets cooling, the cooling power can be the same or different on each jet cooling. It means that each jet cooling can be configured independently of one other. For example, when the cooling section comprising 11 jets cooling, the cooling power of the three first jets cooling can be of 100%, the cooling power of the following four can be of 45% and the cooling power of the last four can be of 0%.

[0148] For example, the variation of the cooling power has an increment between 5 to 50%, preferably between 5 to 40%, more preferably between 5 to 30% and advantageously between 5 to 20%. The cooling power increment is, for example, of 10%, 15% or 25%.

[0149] When the cooling section comprises at least 2 jets cooling, the cooling power increment can be the same or different on each jet cooling. For example, in step B.1), the cooling power increment can be of 5% on all the jets cooling. In another embodiment, the cooling power increment can be of 5% for the three first jets, 20% for the following four and 15% for the last four. Preferably, the cooling power increment is different for each jet cooling, for example 5% for the first jet, 20% for the second jet, 0% for the third jet, 10% for the fourth jet, 0% for the fifth jet, 35% of the sixth jet, etc.

[0150] In some embodiments, the cooling systems are configured depending on the phase transformation independently of one other. For example, when the cooling system comprises 11 jets cooling, the cooling power of the three first jets cooling can be configured for the transformation, the cooling power of the following four can be configured for the transformation of austenite into perlite and the cooling power of the last four can be configured for the transformation of austenite into bainite. In another embodiment, the cooling power increment can be different for each jet cooling.

[0151] In some embodiments, in step B.1), T.sub.soaking is a fixed number selected from the range between 600 to 1000? C. For example, T.sub.soaking can be of 700? C., 800? C. or 900? C. depending on the steel sheet.

[0152] In some embodiments, T.sub.soaking varies from 600 to 1000? C. For example, T.sub.soaking can vary from 650 to 750? C. or from 800 to 900? C. depending on the steel sheet.

[0153] In some embodiments, when T.sub.soaking varies, after step B.1), a further calculation sub step is performed wherein: [0154] a. T.sub.soaking varies from in a predefined range value being between 600 and 1000? C. and [0155] b. For each T.sub.soaking variation, new cooling paths CP.sub.x are calculated, based on TT, m.sub.i to reach m.sub.standard and T.sub.cooling, the cooling step of TT being recalculated using said CP.sub.x in order to obtain new thermal paths TP.sub.x, each TP.sub.x corresponding to a microstructure m.sub.x.
Indeed, with the method according to the present invention, the variation of T.sub.soaking is taken into consideration for the calculation of CP.sub.x. Thus, for each temperature of soaking, a multitude of new cooling paths CP.sub.x and therefore new TP.sub.x are calculated.

[0156] In some embodiments, at least 10 CP.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 CP.sub.x is between 2 and 10000, preferably between 100 and 10000, more preferably between 1000 and 10000.

[0157] In some embodiments, in step B.2), one 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. Preferably, the differences between phases proportions present in m.sub.target and m.sub.x is ?3%.

[0158] In some embodiments, in step B.2), when at least two CP.sub.x have their m.sub.x equal, the selected TP.sub.target selected is the one having the minimum cooling power needed.

[0159] In some embodiments, when T.sub.soaking varies, the selected TP.sub.target further includes the value of T.sub.soaking to reach m.sub.target, TP.sub.target being chosen from TP.sub.x.

[0160] In some embodiments, in step B.2), the thermal enthalpy H .sub.released between m.sub.i and m.sub.target is calculated such that:

H.sub.released=(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.

[0161] Without willing to be bound by any theory, H represents the energy released 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.

[0162] In some embodiments, in step B.2), the all thermal cycle CP.sub.x is calculated such that:

[00002]$T\left(t+?t\right)=T\left(t\right)+\frac{\left({?}_{\mathrm{Convection}}+{?}_{\mathrm{radiance}}\right)}{?.Math.\mathrm{Ep}.Math.C_{\mathrm{pe}}}?t?\frac{\mathrm{Hreleased}}{C_{\mathrm{pe}}}$

with C.sub.pe: the specific heat of the phase (J.Math.kg.sup.?1.Math.K.sup.?1), ?: the density of the steel (g.Math.m.sup.?3), Ep: the thickness of the steel (m), ?: the heat flux (convective and radiative in W), H.sub.realeased (J.Math.kg.sup.?1), T: temperature)(? C.) and t: time (s).

[0163] In some embodiments, in step B.2), at least one intermediate steel microstructure m.sub.xint corresponding to an intermediate thermal path CP.sub.xint and the thermal enthalpy H.sub.xint are calculated. In this case, the calculation of CP.sub.X is obtained by the calculation of a multitude of CP.sub.xint. Thus, preferably, CP.sub.x is the sum of all CP.sub.xint and H.sub.released is the sum of all H.sub.xint. In this preferred embodiment, CP.sub.xint is calculated periodically. For example, it is calculated every 0.5 seconds, preferably 0.1 seconds or less.

[0164] FIG. 3 illustrates a preferred embodiment wherein in step B.1), m.sub.int1 and m.sub.int2 corresponding respectively to CP.sub.xint1 and CP.sub.xint2 as well as H.sub.xint1 and H.sub.xint2 are calculated. H.sub.released during the all thermal path is determined to calculate CP.sub.x. In this embodiment, a multitude, i.e more than 2, of CP.sub.xint, m.sub.xint and H.sub.xint are calculated to obtain CP.sub.x (not shown).

[0165] In some embodiments, before step A.1), 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 this embodiment, preferably, m.sub.target is calculated based on P.sub.target.

[0166] 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, advantageously, in step B.1), the process parameters undergone by the steel sheet before entering the heat treatment line are taken into account to calculate CP.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.

[0167] In another embodiment, the process parameters of the treatment line that the steel sheet will undergo in the heat treatment line are taken into account to calculate CP.sub.x. For example, the process parameters comprise at least one element chosen from among: the line speed, a specific thermal steel sheet temperature to reach, heating power of the heating sections, a heating temperature and a soaking temperature, cooling power of the cooling sections, a cooling temperature and an overaging temperature.

[0168] In some embodiments, T.sub.cooling is the bath temperature when the cooling section is followed by a hot-dip coating section comprising a hot-dip bath. Preferably, the bath is based on aluminum or based on zinc. In a preferred embodiment, the bath based on aluminum 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.

[0169] In some embodiments, the zinc-based bath comprises 0.01-8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn.

[0170] 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.

[0171] In some embodiments, T.sub.cooling is the quenching temperature Tq. Indeed, for the Q&P steel sheet, an important point of a quenching & partitioning treatment is T.sub.q.

[0172] In some embodiments, T.sub.cooling is between 150 and 800? C.

[0173] In some embodiments, every time a new steel sheet enters into the heat treatment line, a new calculation step B.2) is automatically performed. Indeed, the method according to the present invention adapts the cooling path 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 detector detects the welding between two coils.

[0174] 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.

[0175] In some embodiments, an automatic calculation is performed during the thermal treatment to check if any deviation had appeared. In this embodiment, periodically, a calculation is realized to verify if a slight deviation had occurred. Indeed, the detection threshold of detector 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.

[0176] FIG. 4 illustrates 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. With the method according to an 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 embodiment, intermediate thermal paths CP.sub.xint1 to CP.sub.xint4, corresponding respectively m.sub.xint1 to m.sub.xint4, and H.sub.xint1 to H.sub.xint4 are calculated. H.sub.realeased is determined in order to obtain CP.sub.x and therefore TP.sub.x. In this Figure, TP.sub.target is illustrated.

[0177] With the method according to an embodiment of the present invention, when a deviation appears, a new a thermal treatment step TP.sub.target is performed on the steel sheet.

[0178] Thus, a coil made of a steel sheet including said predefined product types include, e.g., DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX, DP HD is obtained, 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.

[0179] In some embodiments, the mechanical properties are chosen from YS, UTS or elongation. The low value of standard variation is due to the precision of TP.sub.target.

[0180] In some embodiments, the coil is covered by a metallic coating based on zinc or based on aluminum.

[0181] In some embodiments, in an industrial production, between two coils made of a steel sheet including said predefined product types include DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX, DP HD, the 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.

[0182] 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 include DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX, DP HD measured 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.

[0183] A thermally treatment line for the implementation of a method according to an embodiment of 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.

[0184] Finally, the present invention relates to a computer program product comprising at least a metallurgical module, an optimization module and a thermal module that cooperate together to determine TP.sub.target such modules comprising software instructions that when implemented by a computer implement a method according to the present invention.

[0185] The metallurgical module predicts the microstructure (m.sub.x, 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.

[0186] 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 thermal enthalpy H released or consumed along the all thermal path when a phase transformation is performed.

[0187] 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. The invention will now be explained in trials carried out for information only. They are not limiting.

EXAMPLES

[0188] In the following examples, DP780GI having the following chemical composition was chosen:

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

[0189] The cold-rolling had a reduction rate of 55% to obtain a thickness of 1.2 mm.

[0190] m.sub.target to reach comprises 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.

[0191] The thermal treatment TT to perform on the steel sheet, is as follows: [0192] a pre-heating step wherein the steel sheet is heated from ambient temperature to 680? C. during 37.5 seconds, [0193] a heating step wherein the steel sheet is heated from 680? C. to 780? C. during 40 seconds, [0194] soaking step wherein the steel sheet is heated at a soaking temperature T.sub.soaking of 780? C. during 24.4 seconds, [0195] a cooling step wherein the steel sheet is cooled with 11 jets cooling spraying HN.sub.x as follows:

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 rate (? C./s) 10 10 9 5 9 22 50 18 18 21 11 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 power(%) 0 0 0 0 0 0 28 100 100 100 100 [0196] a hot-dip coating in a zinc bath ? 460? C., [0197] the cooling of the steel sheet until the top roll during 27.8 s at 300? C. and [0198] the cooling of the steel sheet at ambient temperature.

Example 1: Deviation of T.SUB.soaking

[0199] 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 CP.sub.x is calculated based on TT, m.sub.i of DP780GI to reach m.sub.target, the heating path, the soaking path comprising T.sub.soaking and T.sub.cooling.

[0200] The cooling step of TT was recalculated using said CP.sub.x in order to obtain new thermal paths TP.sub.x. After the calculation of TP.sub.x, one TP.sub.target to reach m.sub.target was selected, TP.sub.target being chosen from the recalculated TP.sub.x and being selected such that m.sub.x is the closest to m.sub.target. TP.sub.target1 is as follows: [0201] 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, [0202] a cooling step CP.sub.1 including: [0203] a cooling step wherein the steel sheet is cooled with 11 jets cooling spraying HN.sub.x as follows:

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 rate (? C./s) 9 9 10 15 32 28 31 11 10 7 8 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 power(%) 0 0 0 25 50 50 45 45 45 45 45 [0204] a hot-dip coating in a zinc bath ? 460? C., [0205] the cooling of the steel sheet until the top roll during 27.8 s at 300? C. and [0206] the cooling of the steel sheet at ambient temperature.

Example 2: Steel Sheet Having a Different Composition

[0207] A new steel sheet DP780 entered into the heat treatment line so a calculation step was automatically performed based on the following new CC:

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

[0208] 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: [0209] a pre-heating step wherein the steel sheet is heated from ambient temperature to 680? C. during 37.5 seconds, [0210] a heating step wherein the steel sheet is heated from 680? C. to 780? C. during 40 seconds, [0211] a soaking step wherein the steel sheet is heated at a soaking temperature T.sub.soaking of 780? C. during 24.4 seconds, [0212] a cooling step CP.sub.3 including:

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 rate (? C./s) 17 17 9 6 6 6 38 30 18 17 10 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 power(%) 100 100 30 0 0 0 100 100 100 100 100 [0213] a hot-dip coating in a zinc bath ? 460? C., [0214] the cooling of the steel sheet until the top roll during 26.8 s at 300? C. and [0215] the cooling of the steel sheet at ambient temperature.

[0216] Table 1 shows the steel properties obtained with TT, TP.sub.target1 and TP.sub.target2.

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: X.sub.ferrite: X.sub.ferrite: X.sub.ferrite: thermal path 55% 61% 55% 58% X.sub.bainite: X.sub.bainite: X.sub.bainite: X.sub.bainite: 33% 27% 32% 30% Deviation X.sub.martensite: X.sub.martensite: X.sub.martensite: (?cart) with 0% 0% 2% respect to X.sub.ferrite: X.sub.ferrite: X.sub.ferrite: m.sub.target 3% 3% 3% X.sub.bainite: X.sub.bainite: X.sub.bainite: 3% 3% 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)

[0217] With the method according to 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 depending on each deviation.