US20200032368A1 - A method for manufacturing a thermally treated steel sheet - Google Patents

A method for manufacturing a thermally treated steel sheet Download PDF

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US20200032368A1
US20200032368A1 US16/469,231 US201716469231A US2020032368A1 US 20200032368 A1 US20200032368 A1 US 20200032368A1 US 201716469231 A US201716469231 A US 201716469231A US 2020032368 A1 US2020032368 A1 US 2020032368A1
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cooling
target
soaking
standard
steel sheet
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Frédéric Bonnet
Yannick DOH
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ArcelorMittal SA
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/54—Furnaces for treating strips or wire
    • C21D9/56—Continuous furnaces for strip or wire
    • C21D9/573—Continuous furnaces for strip or wire with cooling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/55—Hardenability tests, e.g. end-quench tests
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/012—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of aluminium or an aluminium alloy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/667—Quenching devices for spray quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D11/00—Process control or regulation for heat treatments
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D11/00—Process control or regulation for heat treatments
    • C21D11/005—Process control or regulation for heat treatments for cooling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/12—Aluminium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C23C2/29—Cooling or quenching
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a method for manufacturing a thermally treated steel sheet having a microstructure m target in a heat treatment line.
  • the invention is particularly well suited for the manufacture of automotive vehicles.
  • theses steels During production of theses steels, crucial treatments are performed on the steel in order to obtain the desired part having excepted mechanical properties for one specific application. Such treatments can be, for example, a continuous annealing before deposition of a metallic coating or a quenching and partitioning treatment.
  • the cooling step is important because the microstructure and the mechanical properties of steels mostly depend on the performed cooling treatment.
  • the treatment including the cooling step to perform is selected in a list of known treatments, this treatment being chosen depending on the steel grade.
  • Patent application WO2010/049600 relates to a method of using an installation for heat treating a continuously moving steel strip, comprising the steps of: selecting a cooling rate of the steel strip depending on, among others, metallurgical characteristics at the entry and metallurgical characteristics required at the exit of the installation; entering the geometric characteristics of the band; calculating power transfer profile along the steel route in the light with the line speed; determining desired values for the adjustment parameters of the cooling section, and adjusting the power transfer of the cooling devices of the cooling section according to said monitoring values.
  • the cooling treatment is not adapted to one specific steel and therefore at the end of the treatment, the desired properties are not obtained. Moreover, after the treatment, the steel can have a big dispersion of the mechanical properties. Finally, even if a wide range of steel grades can be manufactured, the quality of the cooled steel is poor.
  • an object of various embodiments of the present invention is to solve the above drawbacks by providing a method for manufacturing a thermally treated steel sheet having a specific chemical steel composition and a specific microstructure m target to reach in a heat treatment line.
  • an object of various embodiments of the present invention is to perform a cooling treatment adapted to each steel sheet, such treatment being calculated very precisely in the lowest calculation time possible in order to provide a thermally treated steel sheet having the excepted properties, such properties having the minimum of properties dispersion possible.
  • the invention provides a method for manufacturing a thermally treated steel sheet having a microstructure m 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 thermal path TP target is performed, such method comprising:
  • a preparation step comprising:
  • the predefined phases in step A.1) are defined by at least one element chosen from: the size, the shape, a chemical and the composition.
  • TP standard further comprises a pre-heating step.
  • TP standard further comprises a hot-dip coating step, an overaging step a tempering step or a partitioning step.
  • the microstructure m target comprises:
  • said predefined product types include Dual Phase, Transformation Induced Plasticity, Quenched & Partitioned steel, Twins Induced Plasticity, Carbide Free Bainite, Press Hardening Steel, TRIPLEX, DUPLEX and Dual Phase High Ductility DP.
  • step A.2 the cooling power of the cooling system varies from a minimum to a maximum value.
  • step A.2 the cooling power of the cooling system varies from a maximum to a minimum value.
  • T soaking is a fixed number selected from the range between 600 to 1000° C.
  • T soaking varies from 600 to 1000° C.
  • step A.2 a further calculation substep is performed wherein:
  • the selected TP target further includes the value of T soaking .
  • step A.3) when at least two CP x have their m x equal, the selected TP target selected is the one having the minimum cooling power needed.
  • step A.2 in step A.2), the differences between proportions of phase present in m target and m x is ⁇ 3%.
  • step A.2 the thermal enthalpy H released between m i and m target is calculated such that:
  • H released ( X ferrite *H ferrite )+( X martensite *H martensite )+( X bainite *H bainite )+( X pearlite *H pearlite )+( H cementite +X cementite )+( H austenite +X austenite ), X being a phase fraction.
  • step A.2) the all cooling path CP x is calculated such that:
  • T ⁇ ( t + ⁇ ⁇ ⁇ t ) T ⁇ ( t ) + ( ⁇ Convection + ⁇ radiance ) ⁇ ⁇ Ep ⁇ C pe ⁇ ⁇ ⁇ ⁇ t ⁇ H released C pe
  • step A.2 at least one intermediate steel microstructure m xint corresponding to an intermediate cooling path CP xint and the thermal enthalpy H xint are calculated.
  • CP x is the sum of all CP xint and H released is the sum of all H xint .
  • At least one targeted mechanical property P target is chosen among yield strength YS, Ultimate Tensile Strength UTS, elongation hole expansion, formability is selected.
  • m target is calculated based on P target .
  • step A.2) the process parameters undergone by the steel sheet before entering the heat treatment line are taken into account to calculate CP x .
  • 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.
  • step A.2) 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 x .
  • 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.
  • the cooling system comprises at least one jet cooling, at least one cooling spray or at least both.
  • the cooling system comprises at least one jet cooling
  • the jet cooling comprises spraying a gas, an aqueous liquid or a mixture thereof.
  • the gas is chosen from air, HN x , H 2 , N 2 , Ar, He, steam water or a mixture thereof.
  • the aqueous liquid is chosen from water or a nanofluid.
  • T cooling is the bath temperature when the cooling section is followed by a hot-dip coating section comprising a hot-dip bath.
  • the bath is based on aluminum or based on zinc.
  • T cooling is the quenching temperature T q .
  • T cooling is between 150 and 800° C.
  • a new calculation step A.2) is automatically performed based on the selection step A. 1) performed beforehand.
  • 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.
  • the present invention also provides a coil including said predefined product types including DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX and DP HD, obtainable from the method methods described above, the coil having a standard variation of mechanical properties below or equal to 25 MPa between any two points along the coil. In some embodiments, a standard variation of the coil is below or equal to 15 MPa between any two points along the coil. In some embodiments, a standard variation of the coil is below or equal to 9 MPa between any two points along the coil.
  • the present invention further provides a thermal treatment line for the implementation of the methods described above.
  • the present invention provides a computer program product comprising at least a metallurgical module, an optimization module and a thermal module cooperating together to calculate TP target such modules comprising software instructions that when implemented by a computer implement the method according to claims.
  • FIG. 1 illustrates an example of an embodiment of a method according to the present invention.
  • FIG. 2 illustrates an example of an embodiment of a method according to the present invention, wherein a continuous annealing of a steel sheet comprising a heating step, a soaking step, a cooling step and an overaging step is performed.
  • FIG. 3 illustrates a preferred embodiment according to the invention.
  • FIG. 4 illustrates an example of an embodiment according to the invention, wherein a continuous annealing is performed on a steel sheet before the deposition of a coating by hot-dip.
  • 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.
  • 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.
  • the invention provides a method for manufacturing a thermally treated steel sheet having a microstructure m 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 thermal path TP target is performed, such method comprising:
  • A. preparation step comprising:
  • a method according to various embodiments of the present invention allows for a precise and specific cooling path which takes into account m target , in particular the proportion of all the phases during the cooling path and m i (including the microstructure dispersion along the steel sheet).
  • the method according to various embodiments 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.
  • TP standard further comprises a pre-heating step.
  • TP standard further comprises a hot-dip coating step, an overaging step a tempering step or a partitioning step.
  • the microstructure m target to reach comprises: 100% of austenite
  • the chemical composition and m target are compared to a list of predefined products.
  • the predefined products can be any kind of steel grade.
  • they may include 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).
  • the chemical composition depends on each steel sheet.
  • the chemical composition of a DP steel can comprise:
  • Each predefined product comprises a microstructure including predefined phases and predefined proportion of phases.
  • the predefined phases in step A.1) are defined by at least one element chosen from: the size, the shape and the chemical composition.
  • m standard includes predefined phases in addition to predefined proportions of phase.
  • m i , m x , m target include phases defined by at least one element chosen from: the size, the shape and the chemical composition.
  • the predefined product having a microstructure m standard closest to m target is selected as well as TP standard to reach m standard
  • m standard comprises the same phases as m target .
  • m standard also comprises the same phases proportions as m target .
  • FIG. 1 illustrates an example according to an embodiment of the present invention, wherein the steel sheet to treat has the following CC in weight: 0.2% of C, 1.7% of Mn, 1.2% of Si and of 0.04% Al.
  • m target comprises 15% of residual austenite, 40% of bainite and 45% of ferrite, from 1.2% of carbon in solid solution in the austenite phase.
  • CC and m target are compared to a list of predefined products chosen from among products 1 to 4 .
  • CC and m target correspond to product 3 or 4 , such product being a TRIP steel.
  • Product 3 has the following CC 3 in weight: 0.25% of C, 2.2% of Mn, 1.5% of Si and 0.04% of Al.
  • m 3 corresponding to TP 3 , comprises 12% of residual austenite, 68% of ferrite and 20% of bainite, from 1.3% of carbon in solid solution in the austenite phase.
  • Product 4 has the following CC 4 in weight: 0.19% of C, 1.8% of Mn, 1.2% of Si and 0.04% of Al.
  • m 4 corresponding to TP 4 , comprises 12% of residual austenite and 45% of bainite and 43 of ferrite, from 1.1% of carbon in solid solution in the austenite phase.
  • Product 4 has a microstructure m 4 closest to m target since it has the same phases as m target in the same proportions.
  • two predefined products can have the same chemical composition CC and different microstructures.
  • Product 1 and Product 1′ are both DP600 steels (Dual Phase having a UTS of 600 MPa).
  • One difference is that Product 1 has a microstructure m i and Product, has a different microstructure min.
  • the other difference is that Product 1 has a YS of 360 MPa and Product, has a YS of 420 MPa.
  • the power cooling of the cooling system, the heating path, the soaking path including the soaking temperature T soaking and the cooling temperature T cooling to reach are selected based on TP standard .
  • new cooling paths CP x are calculated based on the selected product in step A.1.a) and TP standard , m i to reach m target , the heating path, the soaking path comprising T soaking and T cooling , the cooling step of TP standard being recalculated using said CP x in order to obtain new thermal paths TP x , each TP x corresponding to a microstructure m x .
  • the calculation of CP x takes into account the thermal behavior and metallurgical behavior of the steel sheet when compared to the conventional methods wherein only the thermal behavior is considered.
  • product 4 is selected because m 4 is the closest to m target , m 4 and TP 4 being respectively m standard and TP standard .
  • 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 multitude of CP x is calculated so to obtain news thermal paths TP x and therefore one TP target .
  • the cooling power of the cooling system varies from a minimum to a maximum value.
  • the cooling power can be determined by a flow rate of a cooling fluid, a temperature of a cooling fluid, the nature of cooling fluid and the thermal exchange coefficient, the fluid being a gas or a liquid.
  • the cooling power of the cooling system varies from a maximum to a minimum value.
  • the cooling system comprises at least one jet cooling, at least one cooling spray or at least both.
  • 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.
  • the gas is chosen from air, HN x , H 2 , N 2 , Ar, He, steam water or a mixture thereof.
  • the aqueous liquid is chosen from: water or nanofluids.
  • jets cooling spray gas with a flow rate between 0 and 350000 Nm 3 /h In some embodiments, jets cooling spray gas with a flow rate between 0 and 350000 Nm 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.
  • the flow rate of one jet cooling is between 0 and 50000 Nm 3 /h, preferably between 0 and 40000 Nm 3 /h, more preferably between 0 and 20000 Nm 3 /h.
  • the variation of cooling power is based on the flow rate. For example, for one jet cooling, 0 Nm 3 /h corresponds to a cooling power of 0% and 40000 Nm 3 /h corresponds to a cooling power of 100%.
  • the cooling power of one jet cooling varies from a 0 Nm 3 /h, i.e. 0%, to 40000 Nm 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%.
  • the minimum value is of 0%, 10%, 15% or 25%.
  • the maximum value is of 80%, 85%, 90% or 100%.
  • 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%.
  • 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%.
  • the cooling power increment can be the same or different on each jet cooling.
  • the cooling power increment can be of 5% on all the jets cooling.
  • the cooling power increment can be of 5% for the three first jets, 20% for the following four and 15% for the last four.
  • 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.
  • the cooling systems are configured depending on the phase transformation independently of one other.
  • 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
  • the cooling power of the last four can be configured for the transformation of austenite into bainite.
  • the cooling power increment can be different for each jet cooling.
  • T soaking is a fixed number selected from the range between 600 to 1000° C.
  • T soaking can be of 700° C., 800° C. or 900° C. depending on the steel sheet.
  • T soaking varies from 600 to 1000° C.
  • T soaking can vary from 650 to 750° C. or from 800 to 900° C. depending on the steel sheet.
  • a further calculation substep is performed such that:
  • T soaking varies from in a predefined range value chosen from 600 to 1000° C. and b.
  • new cooling paths CP x are calculated, based on the selected product in step A. 1 . a ) and TP standard , the initial microstructure m i of the steel sheet to reach m standard and T cooling , the cooling step of TP standard being recalculated using said CP x in order to obtain new thermal paths TP x , each TP x corresponding to a microstructure m x .
  • the variation of T soaking is taken into consideration for the calculation of CP x .
  • CP x is calculated for each temperature of soaking.
  • At least 10 CP X are calculated, more preferably at least 50, advantageously at least 100 and more preferably at least 1000.
  • the number of calculated CP x is between 2 and 10000, preferably between 100 and 10000, more preferably between 1000 and 10000.
  • step A.3) one TP target to reach m target is selected, TP target being chosen among the calculated TP x and being selected such that m x is the closest to m target .
  • the differences between proportions of phase present in m target and m x is ⁇ 3%.
  • the selected TP target is the one having the minimum cooling power needed.
  • the selected TP target when T soaking varies, further includes the value of T soaking to reach m target , TP target being chosen from TP x .
  • step A.2 the thermal enthalpy H released between m i and m target is calculated such that:
  • H released ( X ferrite *H ferrite )+( X martensite *H martensite )+( X bainite *H bainite )+( X pearlite *H pearlite )+( H cementite +X cementite )+( H austenite +X austenite )
  • 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.
  • step A.2) the all thermal cycle CP x is calculated such that:
  • T ⁇ ( t + ⁇ ⁇ ⁇ t ) T ⁇ ( t ) + ( ⁇ Convection + ⁇ radiance ) ⁇ ⁇ Ep ⁇ C pe ⁇ ⁇ ⁇ ⁇ t ⁇ Hreleased C pe
  • C pe the specific heat of the phase (J ⁇ kg ⁇ 1 ⁇ K ⁇ 1 )
  • p the density of the steel (g ⁇ m ⁇ 3 )
  • Ep the thickness of the steel (m)
  • ⁇ the heat flux (convective and radiative in W)
  • H realeased J ⁇ kg ⁇ 1
  • T the temperature (° C.)
  • t the time (s).
  • step A.2 at least one intermediate steel microstructure m xint corresponding to an intermediate thermal path CP xint and the thermal enthalpy H xint are calculated.
  • the calculation of CP x is obtained by the calculation of a multitude of CP xint .
  • CP x is the sum of all CP xint and H released is the sum of all H xint .
  • CP xint is calculated periodically. For example, it is calculated every 0.5 seconds, preferably 0.1 seconds or less.
  • FIG. 3 illustrates an embodiment, wherein in step A.2), m int1 and m int2 corresponding respectively to CP xint1 and CP xint2 as well as H xint1 and H xint2 are calculated. H released during the all thermal path is determined to calculate CP x . In this embodiment, a multitude, i.e more than 2, of CP xint , m xint and H xint can be calculated to obtain CPx (not shown).
  • At least one targeted mechanical property P target chosen among yield strength YS, Ultimate Tensile Strength UTS, elongation, hole expansion, formability is selected.
  • m target is calculated based on P target .
  • the characteristics of the steel sheet are defined by the process parameters applied during the steel production.
  • the process parameters undergone by the steel sheet before entering the heat treatment line are taken into account to calculate CP x .
  • 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.
  • 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 x .
  • 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, an overaging temperature.
  • T cooling is the bath temperature when the cooling section is followed by a hot-dip coating section comprising a hot-dip bath.
  • the bath is based on aluminum or based on zinc.
  • 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.
  • the bath based on zinc comprises 0.01-8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn.
  • the molten bath can also comprise unavoidable impurities and residuals elements from feeding ingots or from the passage of the steel sheet in the molten bath.
  • 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.
  • T cooling is the quenching temperature Tq. Indeed, for the Q&P steel sheet, an important point of a quenching & partitioning treatment is T q .
  • T cooling is between 150 and 800° C.
  • a new calculation step A.2) is automatically performed based on the selection step A.1) performed beforehand.
  • 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.
  • 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.
  • a CP x is calculated based on m i , the selected product and m target .
  • intermediate thermal paths CP xint1 to CP xint3 corresponding respectively to m xint1 to m xint3 , and H xint1 to H xint3 are calculated.
  • H released is determined in order to obtain CP x and therefore TP x .
  • TP target is illustrated.
  • a thermal treatment step TP target is performed on the steel sheet.
  • the invention also provides a coil made of a steel sheet including said predefined product types, including DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX, DP HD, 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.
  • the method including the calculation step A.2) takes into account the microstructure dispersion of the steel sheet along the coil.
  • TP target applied on the steel sheet in step allows for a homogenization of the microstructure and also of the mechanical properties.
  • the mechanical properties are chosen from YS, UTS or elongation.
  • the low value of standard variation is due to the precision of TP target .
  • the coil is covered by a metallic coating based on zinc or based on aluminum.
  • 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.
  • a thermally treatment line for the implementation of a method according to the present invention is used to perform TP target .
  • the thermally treatment line is a continuous annealing furnace.
  • the invention also provides a computer program product comprising at least a metallurgical module, a thermal module and an optimization module cooperating together to determine TP target , such modules comprising software instructions that when implemented by a computer implement a method according to the present invention.
  • the metallurgical module predicts the microstructure (m x , m 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.
  • 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.
  • the optimization module determines the best thermal path to reach m target , i.e. TP target following the method according to the present invention using the metallurgical and thermal modules.
  • DP780GI having the following chemical composition was chosen:
  • the cold-rolling had a reduction rate of 50% to obtain a thickness of 1 mm.
  • m target to reach comprises 13% of martensite, 45% of ferrite and 42% of bainite, corresponding to the following P target : YS of 500 MPa and a UTS of 780 MPa.
  • a cooling temperature T 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.
  • the steel sheet was compared to a list of predefined products in order to obtain a selected product having a microstructure m standard closest to m target .
  • the selected product was also a DP780GI having the following chemical composition:
  • the microstructure of DP780GI i.e. m standard , comprises 10% martensite, 50% ferrite and 40% bainite.
  • the corresponding thermal path TP standard is as follows:
  • TP target is as follows:
  • Table 1 shows the properties obtained with TP standard and TP target on the steel sheet:

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