Patent Grant
11692237
The present invention relates to a method of dynamical adjustment 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.
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 member.
During the production of these 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. These treatments are performed in an adapted furnace line.
During these treatments, some unplanned deviations can appear online. For example, a temperature in the furnace, the thickness of the steel sheet, the line speed can vary.
U.S. Pat. No. 4,440,583 relates to a method of controlled cooling for steel strip implemented by use of a cooling apparatus comprising a plurality of nozzles disposed in the direction in which strip travels, the nozzles spraying coolant against the hot running strip, and a flow-rate control valve attached to the pipe that supplies the coolant to the nozzles. By using an equation containing the thickness of strip, the cooling starting and finishing temperatures, and the desired cooling rate, the heat transfer rate needed to obtain the desired cooling rate is calculated, and the obtained heat transfer rate is corrected according to the effect of natural cooling in idle-pass zones preceding and following the coolant spray zone. Then, the flow rate of the coolant is derived, and set, from its pre-established relationship with the heat transfer rate. The length of the coolant spraying zone along the strip travel path is calculated using the running speed of the strip, the cooling starting and finishing temperatures, and the desired cooling rate. The nozzles are set to turn on and off so that coolant is sprayed from only such a number of nozzles as correspond to the calculated value. When strip thickness varies while controlled cooling is being effected, the heat transfer rate is re-calculated, on the basis of the above settings, to correct the coolant flow rate accordingly. When strip speed varies, the length of the coolant spraying region is re-calculated to correct the on-off pattern of the nozzles.
In this method, when a deviation appears, the heat transfer rate or the length of the coolant spraying region is re-calculated to correct the deviation. This method does not take into account the steel sheet characteristics comprising chemical composition, microstructure, properties, surface texture, etc. Thus, there is a risk that the same correction is applied to any kind of steel sheet even if each steel sheet has its own characteristics. The method allows for a non-personalized cooling treatment of a multitude of steel grades.
Consequently, the correction 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 of dynamical adjustment 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.
Another object of the present invention is to adjust a thermal path online by providing a treatment adapted to each steel sheet, such treatment being calculated very precisely in the lowest calculation time possible.
Another object of the present invention is to provide a steel sheet having the expected properties, such properties having the minimum of properties dispersion possible.
The present invention provides a method of dynamical adjustment 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, wherein a predefined thermal treatment TT is performed on the steel sheet, such method comprising:
A. a control step wherein at least one sensor detects a deviation happening during the thermal treatment,
B. a calculation step performed when the deviation is detected during the thermal treatment such that a new thermal path TPtarget is determined to reach mtarget taking the deviation into account, such calculation step comprising:
1) a calculation substep, wherein at least two thermal path, TPx corresponding to one microstructure mx obtained at the end of TPx, are calculated based on TT and the microstructure mi of the steel sheet to reach mtarget,
2) a selection substep wherein one new thermal path TPtarget to reach mtarget is selected, TPtarget being chosen from said TPx and being selected such that mx is the closest to mtarget,
C. a new thermal treatment step, wherein TPtarget is performed online on the steel sheet.
In some embodiments, 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.
In some embodiments, the phases are defined by at least one element chosen from: a size, a shape and a chemical composition.
In some embodiments, the microstructure mtarget comprises:
In some embodiments, the steel sheet is a Dual Phase steel, a Transformation Induced Plasticity steel, a Quenched & Partitioned steel, a Twins Induced Plasticity steel, a Carbide Free Bainite steel, a Press Hardening Steel, or a TRIPLEX, DUPLEX and Dual Phase High Ductility steel.
In some embodiments, the differences between phases proportions of phase present in mtarget and mx is ±3%.
In some embodiments, in step B.1), the thermal enthalpy H released or consumed between mi and mtarget is calculated such that:
HX=(Xferrite*Hferrite)+(Xmartensite*Hmartensite)+(Xbainite*Hbainite)+(Xpearlite*Hpearlite)+(Hcementite*Xcementite)+(Haustenite*Xaustenite), X being a phase fraction.
In some embodiments, in step B.1), the all thermal cycle TPx is calculated such that:
wherein 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 B.1), 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 B.1), TPx is the sum of all TPxint and Hx is the sum of all Hxint.
In some embodiments, before step B.1), at least one targeted mechanical property Ptarget chosen among yield strength YS, Ultimate Tensile Strength UTS, elongation hole expansion, formability is selected.
In some embodiments, mtarget is calculated based on Ptarget.
In some embodiments, in step B.1), process parameters undergone by the steel sheet before entering the heat treatment line are taken into account to calculate TPx.
In some embodiments, 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.
In some embodiments, in step B.1), 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, the process parameters comprise at least one element chosen from among: a specific thermal steel sheet temperature to reach, a line speed, a cooling power of 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, the thermal path, TPx, TPxint, TT 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 B.1) is automatically performed.
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.
In some embodiments, an automatic calculation is performed during the thermal treatment to check if any deviation had appeared.
The present invention also provides a coil made of a steel sheet comprising a predefined product types comprising DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX, DP, or HD, the steel 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 an 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 invention provides a method of dynamical adjustment 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 wherein a predefined thermal treatment TT is performed on the steel sheet, such method comprising:
Without willing to be bound by any theory, it seems that when the method according to the present invention is applied, it is possible to correct any deviation happening during a thermal treatment by providing a personalized heat treatment depending on each steel sheet. To do so, a precise and specific new thermal path TPtarget is calculated in a short calculation time taking into account mtarget, in particular the proportion of all the phases along the treatment, mi (including the microstructure dispersion along the steel sheet) and the deviation. Indeed, the method according to the present invention takes into account for the calculation the thermodynamically stable phases, i.e. ferrite, austenite, cementite and pearlite, and the thermodynamic metastable phases, i.e. bainite and martensite. Thus, a steel sheet having the expected properties with the minimum of properties dispersion possible is obtained.
In some embodiments, the microstructures mx, mtarget and mi phases are defined by at least one element chosen from: the size, the shape and the chemical composition.
In some embodiments, the microstructure mtarget to reach comprises:
In some embodiments, the steel sheets can be any kind of steel grade, including, e.g., Dual Phase DP, Transformation Induced Plasticity (TRIP), Quenched & Partitioned steel (Q&P), Twins Induced Plasticity (TWIP), Carbide Free Bainite (CFB), Press Hardening Steel (PHS), TRIPLEX, DUPLEX and Dual Phase High Ductility (DP HD) steels.
The chemical composition depends on each steel sheet. For example, the chemical composition of a DP steel can comprise:
According to an embodiment of the present invention, in step A), any deviation happening during the thermal treatment is detected. In some embodiments, the deviation is due to a variation of a process parameter chosen from among: a furnace temperature, a steel sheet temperature, an amount of gas, a gas composition, a gas temperature, a line speed, a failure in the heat treatment line, a variation of the hot-dip bath, a steel sheet emissivity and a variation of the steel thickness.
A furnace temperature can be a heating temperature, a soaking temperature, a cooling temperature, an overaging temperature, in particular in a continuous annealing.
A steel sheet temperature can be measured at any time of the heat treatment in different positions of the heat treatment line, for example:
To detect a temperature variation, the sensor can be a pyrometer or a scanner.
Usually, heat treatments can be performed in an oxidizing atmosphere, i.e. an atmosphere comprising an oxidizing gas being for example: O2, CH4, CO2 or CO. They also can be performed in a neutral atmosphere, i.e. an atmosphere comprising a neutral gas being for example: N2, Ar or He. Finally, they also can be performed in a reducing atmosphere, i.e. an atmosphere comprising a reducing gas being for example: H2 or HNx.
The variation of gas amount can be detected by barometer.
The line speed can be detected by a laser sensor.
For example, a failure in the heat treatment line can be:
In such cases, sensor can be a pyrometer, a barometer, an electrical consumption or a camera.
The variation of the steel thickness can be detected by a laser or an ultrasound sensor.
When a deviation is detected, at least two thermal path TPx, corresponding to mx, are calculated based on TT and mi to reach mtarget, such TPx taking into account the deviation. The calculation of TPx 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.
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 and preferably between 1000 and 10000.
In step B.2), one new thermal path TPtarget to reach mtarget is selected. TPtarget is chosen from TPx and being selected such that mx is the closest to mtarget. Thus, in
In some embodiments, in step B.1), the thermal enthalpy H released or consumed between mi and mtarget is calculated such that:
Hx=(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. Preferably, Hx is taken in account in the calculation of TPx.
In one embodiment, in step B.1), 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: 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 B.1), 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 preferably, TPx is the sum of all TPxint and Hx is the sum of all Hxint. In this preferred embodiment, TPxint is calculated periodically. For example, it is calculated every 0.5 seconds, preferably 0.1 seconds or less.
In some embodiments, before step B.2), at least one targeted mechanical property Ptarget chosen among yield strength YS, Ultimate Tensile Strength UTS, elongation hole expansion, formability is selected. In these embodiments, 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 B.1), 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 cold rolling reduction rate, a coiling temperature, a run out table cooling path, a cooling temperature and a coil cooling rate.
In some embodiments, the process parameters of the treatment line that the steel sheet will undergo in the heat treatment line are taken into account to calculate TPx. For example, the process parameters comprise at least one element chosen from among: a specific thermal steel sheet temperature to reach, the line speed, cooling power of the cooling sections, heating power of the heating sections, an overaging temperature, a cooling temperature, a heating temperature and a soaking temperature.
In some embodiments, the thermal path, TPx, TPxint, TT 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 path, a partitioning path, 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 one embodiment, it is performed in a continuous annealing furnace comprising optionally a pre-heating section, a heating section, a soaking section, a cooling section and optionally an equalizing section. Without willing to be bound by any theory, it is believed that the recrystallization annealing is the thermal path the more difficult to handle since it comprises many steps to take into account comprising cooling and heating steps.
In some embodiments, every time a new steel sheet enters into the heat treatment line, a new calculation step B.1) is automatically performed. Indeed, the method according to the present invention adapts the thermal path 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 sensor detects the welding between two coils.
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.
In some embodiments, an automatic calculation is performed during the thermal treatment to check if any deviation had appeared. In these embodiments, periodically, a calculation is realized to verify if a slight deviation had occurred. Indeed, the detection threshold of sensor is sometimes too high which means that a slight deviation is not always detected. The automatic calculation, performed for example every few seconds, is not based on a detection threshold. Thus, if the calculation leads to the same thermal treatment, i.e. the thermal treatment performs online, TT will not change. If the calculation leads to a different treatment due to a slight deviation, the treatment will change.
With the method according to an embodiment of the present invention, when a deviation appears, a new thermal treatment step comprising TPtarget is performed on the steel sheet in order to reach mtarget.
The present invention also provides a coil made of a steel sheet including said predefined product types, including, e.g., DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX, DP or HD steels, such coil having a standard variation of mechanical properties below or equal to 25 MPa, preferably below or equal to 15 MPa, more preferably below or equal to 9 MPa, between any two points along the coil. Indeed, without willing to be bound by any theory, it is believed that the method including the calculation step B.1) takes into account the microstructure dispersion of the steel sheet along the coil. Thus, TPtarget applied on the steel sheet allows for a homogenization of the microstructure and also of the mechanical properties.
The low value of standard variation is due to the precision of TPtarget. In some embodiments, the mechanical properties are chosen from YS, UTS or elongation.
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 steels, measured and successively produced on the same line is below or equal to 25 MPa, preferably below or equal to 15 MPa, more preferably below or equal to 9 MPa.
A thermal 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, a thermal module and an optimization 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 (my, 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 dynamic 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.
In the following examples, DP780GI having the following chemical composition was chosen:
The cold-rolling had a reduction rate of 55% to obtain a thickness of 1.2 mm.
mtarget to reach comprised 12% of martensite, 58% of ferrite and 30% of bainite, corresponding to the following Ptarget:YS of 460 MPa and UTS of 790 MPa. A cooling temperature Tcooling 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 thermal treatment TT to perform on the steel sheet is as follows:
When the soaking temperature Tsoaking decreased from 780° C. to 765° C., a new thermal path TPtarget1 is determined to reach mtarget taking the deviation into account. To this end, a multitude of thermal path TPx was calculated based on TT, mi of DP780GI to reach mtarget and the deviation.
After the calculation of TPx, one new thermal path TPtarget1 to reach mtarget was selected, TPtarget1 being chosen from TPx and being selected such that mx is the closest to mtarget. TPtarget1 is as follows:
A new steel sheet DP780GI entered into the heat treatment line so a calculation step was automatically performed based on the following new CC:
The new thermal path TPtarget2 was determined to reach mtarget taking the new CC into account. TPtarget2 is as follows:
Table 1 shows the steel properties obtained with TT, TPtarget1 and TPtarget2:
With the method according to the various embodiments of the present invention, it is possible to adjust a thermal TT when a deviation appears or when a new steel sheet having a different CC enters into the heat treatment line. By applying the new thermal paths TPtarget1 and TPtarget2, it is possible to obtain a steel sheet having the desired expected properties, each TPtarget being precisely adapted to each deviation.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/IB2016/001790 | Dec 2016 | WO | international |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2017/058189 | 12/20/2017 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2018/116194 | 6/28/2018 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4440583 | Ikegami et al. | Apr 1984 | A |
| 5891275 | Elfmark et al. | Apr 1999 | A |
| 6185970 | Latzel | Feb 2001 | B1 |
| 20030089431 | Gramckow et al. | May 2003 | A1 |
| 20040205951 | Kurz et al. | Oct 2004 | A1 |
| 20050016712 | Franz et al. | Jan 2005 | A1 |
| 20070198122 | Weinzierl | Aug 2007 | A1 |
| 20080135137 | Okamoto et al. | Jun 2008 | A1 |
| 20090265146 | Franz et al. | Oct 2009 | A1 |
| 20140175713 | Roessiger et al. | Jun 2014 | A1 |
| 20190352738 | Bonnet et al. | Nov 2019 | A1 |
| Number | Date | Country |
|---|---|---|
| 102851474 | Jan 2013 | CN |
| 105838869 | Aug 2016 | CN |
| 106011435 | Oct 2016 | CN |
| 1970457 | Sep 2008 | EP |
| 2287345 | Feb 2011 | EP |
| H01298114 | Dec 1989 | JP |
| 2006-193759 | Jul 2006 | JP |
| 2007231409 | Sep 2007 | JP |
| 2020509241 | Mar 2020 | JP |
| 2363740 | Aug 2009 | RU |
| 2017050311 | Mar 2017 | WO |
| WO2017050311 | Mar 2017 | WO |
| Entry |
|---|
| International Search Report issued in connection with International application No. PCT/IB2017/058189 dated Feb. 27, 2018. |
| International Preliminary Report on Patentability issued in connection with International application No. PCT/IB2017/058189 dated Apr. 16, 2019. |
| Number | Date | Country | |
|---|---|---|---|
| 20190352739 A1 | Nov 2019 | US |
