Patent Application
20200040426
The present invention relates to a method for manufacturing a thermally treated steel sheet having a chemical steel composition and a microstructure mtarget comprising from 0 to 100% of at least one phase chosen among: ferrite, martensite, bainite, pearlite, cementite and austenite, in a heat treatment line. The invention is particularly well suited for the manufacture of automotive vehicles.
It is known to use coated or bare steel sheets for the manufacture of automotive vehicles. A multitude of steel grades are used to manufacture a vehicle. The choice of steel grade depends on the final application of the steel part. For example, IF (Interstitial-Free) steels can be produced for an exposed part, TRIP (Transformation-Induced Plasticity) steels can be produced for seat and floor cross members or A-pillars, and DP (Dual Phase) steels can be produced for rear rails or roof cross members.
During the production of theses steels, crucial treatments are performed on the steel in order to obtain the desired part having expected 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. Usually, the treatment 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.
However, this method is only based on the selection and the application of well-known cooling cycles. It means that for one steel grade, for example TRIP steels, there is a huge risk that the same cooling cycle is applied even if each TRIP steel has its own characteristics comprising chemical composition, microstructure, properties, surface texture, etc. Thus, the method does not take into account the real characteristics of the steel. It allows for the non-personalized heat treatment of a multitude of steel grades.
Consequently, the heat 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 treated 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 mtarget to reach in a heat treatment line. In particular, an object of various embodiments of the present invention is to perform a treatment adapted to each steel sheet, such treatment being calculated very precisely in the lowest calculation time possible in order to provide a steel sheet having the expected properties, such properties having the minimum of properties dispersion possible.
The present invention provides a method for manufacturing a thermally treated steel sheet having a chemical steel composition and a microstructure mtarget 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. a preparation step comprising:
B. a thermal treatment step, wherein TPtarget is performed on the steel sheet.
In some embodiments, the predefined phases in step A.1) are defined by at least one element chosen from: a size, a shape, a chemical and a composition.
In some embodiments, the microstructure mtarget 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, or
at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.
In some embodiments, said predefined product types comprise Dual Phase, Transformation Induced Plasticity, Quenched & Partitioned, Twins Induced Plasticity, Carbide Free Bainite, Press Hardening Steel, TRIPLEX, DUPLEX and Dual Phase High Ductility steels.
In some embodiments, the differences between proportions of phase present in mtarget and mx is ±3%.
In some embodiments, in step A.2), the thermal enthalpy H released or consumed between mi and mtarget is calculated such that:
H
x=(Xferrite*Hferrite)+(Xmartensite*Hmartensite)+(Xbainite*Hbainite)+(Xpearlite*Hpearlite)+(Hcementite+Xcementite)+(Haustenite+Xaustenite),
X being a phase fraction.
In some embodiments, in step A.2), the all thermal cycle TPx is calculated such that:
with Cpe: the specific heat of the phase (J·kg−1·K−1), ρ: the density of the steel (g·m−3), Ep: thickness of the steel (m), φ: the heat flux (convective+radiative in W), Hx (J·Kg−1), T: temperature (° C.) and t: time (s).
In some embodiments, in step A.2), at least one intermediate steel microstructure mxint corresponding to an intermediate thermal path TPxint and the thermal enthalpy Hxint are calculated.
In some embodiments, in step in step A.2), TPx is the sum of all TPxint and Hx is the sum of all Hxint.
In some embodiments, before step A.1), at least one targeted mechanical property Ptarget chosen among yield strength YS, Ultimate Tensile Strength UTS, elongation hole expansion, and formability.
In some embodiments, mtarget is calculated based on Ptarget.
In some embodiments, in step A.2), process parameters undergone by the steel sheet before entering the heat treatment line are taken into account to calculate TPx.
In some embodiments, said 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.
In some embodiments, process parameters of the treatment line that the steel sheet will undergo in the heat treatment line are taken into account to calculate TPx.
In some embodiments, said 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 the cooling sections, a heating power of the heating sections, an overaging temperature, a cooling temperature, a heating temperature, and a soaking temperature.
In some embodiments, thermal path, TPx, TPxint, TPstandard or TPtarget, comprise at least one treatment chosen from: a heating, an isotherm or a cooling treatment.
In some embodiments, every time a new steel sheet enters into the heat treatment line, a new calculation step A.2) is automatically performed based on the selection step A.1) performed beforehand.
In some embodiments, an adaptation of the thermal path is performed as the steel sheet enters into the heat treatment line on the first meters of the sheet.
Another object is achieved by providing a coil made of a steel sheet comprising said predefined product types comprising DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX and DP HD steels, said steels obtained by a method 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 is below or equal to 15 MPa between any two points along the coil. In some embodiments, a standard variation is below or equal to 9 MPa between any two points along the coil.
The present invention further provides a thermal treatment line adapted for the implementation of the methods described above.
The present invention further provides a computer program product comprising at least a metallurgical module, an optimization module and a thermal module cooperating together to determine TPtarget, such modules comprising software instructions that when implemented by a computer implement a method according to the embodiments described above.
Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.
To illustrate the invention, various embodiments of non-limiting examples will be described, particularly with reference to the following Figures.
The following terms will be defined:
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.
The present invention provides a method for manufacturing a thermally treated steel sheet having a chemical steel composition and a microstructure mtarget 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. a preparation step comprising:
B. a thermal treatment step wherein TPtarget is performed on the steel sheet.
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 obtain a personalized heat treatment for each steel sheet to treat in a short calculation time. Indeed, the method according to various embodiments of the present invention allows for a precise and specific heat treatment which takes into account mtarget, more precisely the proportion of all the phases along the treatment and mi (including the microstructure dispersion along the steel sheet). Indeed, 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. Thus, a steel sheet having the expected properties with the minimum of properties dispersion possible is obtained.
In some embodiments, the microstructure mtarget 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%.
In some embodiments, during the selection sub step A.1), the chemical composition and mtarget are compared to a list of predefined products. The predefined products can be any kind of steel grade. For example, 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) steels.
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.
Each predefined product comprises a microstructure including predefined phases and predefined proportion of phases. In some embodiments, the predefined phases in step A.1) are defined by at least one element chosen from: the size, the shape and the chemical composition. Thus, mstandard includes a predefined phase in addition to predefined proportions of phase. In some embodiments, mi, mx, mtarget include phases defined by at least one element chosen from: the size, the shape and the chemical composition. In some embodiments, the predefined product having a microstructure mstandard closest to mtarget is selected as well as thermal path TPstandard to reach mstandard, mstandard comprises the same phases as mtarget. In some embodiments, mstandard also comprises the same phases proportions as mtarget.
Product 3 has the following CC3 in weight: 0.25% of C, 2.2% of Mn, 1.5% of Si and 0.04% of Al. m3 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 CC4 in weight: 0.19% of C, 1.8% of Mn, 1.2% of Si and 0.04% of Al. m4 comprises 12% of residual austenite, 45% of bainite and 43% of ferrite, from 1.1% of carbon in solid solution in the austenite phase.
Product 4 has a microstructure closest to mtarget since it has the same phases as mtarget in the same proportions.
As shown in
During the calculation sub step A.2), at least two thermal paths TPx are calculated based on the selected product of step A.1) and mi to reach mtarget. The calculation of TPx 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. In the example of
In some embodiments, at least 10 TPx are calculated, more preferably at least 50, advantageously at least 100 and more preferably at least 1000. For example, the number of calculated TPx is between 2 and 10000, preferably between 100 and 10000, more preferably between 1000 and 10000.
In step A.3), one thermal path TPtarget to reach mtarget is selected. TPtarget is chosen from TPx such that mX is the closest to mtarget. Thus, in
In some embodiments, in step A.2), the thermal enthalpy H released or consumed between mi and mtarget is calculated such that:
H
x=(Xferrite*Hferrite)+(Xmartensite*Hmartensite)+(Xbainite*Hbainite)+(Xpearlite*Hpearlite)+(Hcementite+Xcementite)+(Haustenite+Xaustenite),
X being a phase fraction.
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.
In some embodiments, in step A.2), the all thermal cycle TPx is calculated such that:
with Cpe: the specific heat of the phase (J·kg−1·K−1), ρ: the density of the steel (g·m3), Ep: the thickness of the steel (m), φ: the heat flux (convective and radiative in W), Hx (J·kg−1), T: temperature (° C.) and t: time (s).
In some embodiments, in step A.2), at least one intermediate steel microstructure mxint corresponding to an intermediate thermal path TPxint and the thermal enthalpy Hxint are calculated. In this case, the calculation of TPx is obtained by the calculation of a multitude of TPxint. Thus, in some embodiments, TPx is the sum of all TPxint and Hx is the sum of all Hxint. In these embodiments, TPxint is calculated periodically. For example, it is calculated every 0.5 seconds, preferably 0.1 seconds or less.
In one embodiment, before step A.1), at least one targeted mechanical property Ptarget chosen among yield strength YS, Ultimate Tensile Strength UTS, elongation, hole expansion, formability is selected. In this embodiment, preferably, mtarget is calculated based on Ptarget.
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 A.2), the process parameters undergone by the steel sheet before entering the heat treatment line are taken into account to calculate TPx. For example, the process parameters comprise at least one element chosen from among: a final rolling temperature, a run out table cooling path, a coiling temperature, a coil cooling rate and cold rolling reduction rate.
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 TPx. 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, an overaging temperature.
In some embodiments, the thermal path, TPx, TPxint, TPstandard or TPtarget 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 or partitioning path, an isothermal path or a quenching path.
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 this case, 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.
In some embodiments, every time a new steel sheet enters into the heat treatment line, a new calculation step A.2) is automatically performed based on the selection step A.1) performed beforehand. Indeed, the method according to certain embodiments of the present invention adapts the thermal path TPtarget 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.
In some embodiments, an 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.
In some embodiments, mtarget can be the expected microstructure at any time of a thermal treatment. In other words, mtarget can be the expected microstructure at the end of a thermal treatment as shown in
With the method according to the present invention, it is possible to obtain 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 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. 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, TPtarget applied on the steel sheet in step B) allows for a homogenization of the microstructure and also of the mechanical properties.
In some embodiments, the mechanical properties are chosen from YS, UTS and elongation. The low value of standard variation is due to the precision of TPtarget.
In some embodiments, the coil is covered by a metallic coating based on zinc or based on aluminum.
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, 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 TPtarget. For example, the thermally treatment line is a continuous annealing furnace, a press hardening furnace, a batch annealing or a quenching line.
Finally, the present invention provides a computer program product comprising at least a metallurgical module, an optimization module and a thermal module that cooperate together to determine TPtarget, such modules comprising software instructions that when implemented by a computer implement the method according to the present invention.
The metallurgical module predicts the microstructure (mx, mtarget 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 mtarget, i.e. TPtarget following the method according to the present invention using the metallurgical and thermal modules.
The invention will now be explained in the examples carried out. They are not limiting.
In this example, DP780GI having the following chemical composition was chosen:
The cold-rolling had a reduction rate of 50% to obtain a thickness of 1 mm.
mtarget to reach comprised 13% of martensite, 45% of ferrite and 42% of bainite, corresponding to the following Ptarget:YS of 500 MPa and a UTS of 780 MPa. A cooling temperature Tcooling of 460° C. had 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.
Firstly, the steel sheet was compared to a list of predefined products in order to obtain a selected product having a microstructure mstandard closest to mtarget. The selected product was a DP780GI having the following chemical composition:
The microstructure of DP780GI, i.e., mstandard, comprises 10% martensite, 50% ferrite and 40% bainite. The corresponding thermal path TPstandard comprises:
Then, a multitude of thermal paths TPx were calculated based on the selected product DP780 and TPstandard and mi of DP780 to reach mtarget.
After the calculation of TPx, one thermal path TPtarget to reach mtarget was selected, TPtarget being chosen from TPx and being selected such that mx is the closest to mtarget. TPtarget comprises:
Table 1 shows the properties obtained with TPstandard and TPtarget on the steel sheet:
Table 1 shows that with the method according to the present invention, it is possible to obtain a steel sheet having the desired expected properties since the thermal path TPtarget is adapted to each steel sheet. On the contrary, by applying a conventional thermal path, TPstandard, the expected properties are not obtained.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/IB2016/001786 | Dec 2016 | IB | international |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2017/058186 | 12/20/2017 | WO | 00 |
