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EP-4735651-A1 - METHOD FOR DETERMINING AN AMOUNT OF BAINITIC FERRITE FORMED DURING A STEEL PROCESSING OPERATION, ASSOCIATED MONITORING OR CONTROL METHODS

EP4735651A1EP 4735651 A1EP4735651 A1EP 4735651A1EP-4735651-A1

Abstract

The method comprises: - s1) determining a bainitic ferrite growth rate proportional to: exp(- Q*/RT ), Q* being an activation energy for an austenite to bainitic ferrite transformation, and proportional to a moderation coefficient which approaches zero when a free enthalpy of a bainitic ferrite phase approaches a free enthalpy of an austenite phase that depends on a Carbon content ( C γ ) in the austenite phase, - s2) determining a cementite growth rate, depending at least on the Carbon content in the austenite phase, - s3) determining values at a next time step for bainitic ferrite, austenite and cementite phases fractions (f α , f γ , f c ), based on the bainitic ferrite growth rate and on the cementite growth rate, and updating the Carbon content in the austenite phase based on the values of said phases fractions.

Inventors

  • CHARBONNIER, Nicolas
  • JACOLOT, Ronan

Assignees

  • ArcelorMittal

Dates

Publication Date
20260506
Application Date
20240605

Claims (19)

  1. 1 . A method for determining an amount of bainitic ferrite (f a ) formed in a steel semi- product (1 ) during a steel processing operation, the steel semi-product having a chemical composition CC, the method comprising the following steps, executed by an electronic device: - Acquiring the chemical composition CC of the steel of said steel semi-product, - s1 ) determining a bainitic ferrite growth rate, as being proportional to: o a density of bainitic ferrite nucleation centers (n nucl a ), o exp(—Q*/RT) where T is the temperature of the steel, R is the universal gas constant and Q* is an activation energy for an austenite to bainitic ferrite transformation, and to o a moderation coefficient, which approaches zero when a difference AG y ^ a between a free enthalpy of a bainitic ferrite phase and a free enthalpy of an austenite phase approaches zero, the enthalpy of the austenite phase depending at least on a Carbon content (C y ) in the austenite phase, the value of the Carbon content in the bainitic ferrite phase employed for computing AG y ^ a being the same as the value of the Carbon content in the austenite phase (C y ), - s2) determining a cementite growth rate, depending at least on the chemical composition CC of the steel and on the Carbon content (C y ) in the austenite phase, - s3) determining values at a next time step for bainitic ferrite, austenite and cementite phases fractions (f a , f Y , f c ), based on the bainitic ferrite growth rate and on the cementite growth rate, and determining a value at the next time step for the Carbon content in the austenite phase (C y ) based on the values of said phases fractions (f a , f Y , f c ), the set of steps s1 , s2 and s3 being executed several times successively.
  2. 2. A method according to claim 1 wherein the moderation coefficient: - is proportional to | &G Y ^ a \/RT when | AG Y ^ a \/RT approaches zero, and - is bound to a constant value when | &G Y ^ a | /RT increases.
  3. 3. A method according to claim 2 wherein the moderation coefficient is equal or substantially equal to tanh .
  4. 4. A method according any of the preceding claims wherein Q* is equal or substantially equal to Q o + K 1 . |AG y ^ a |, and where Q o and Ki depend on the chemical composition CC of the steel but do not vary when the Carbon content (C y ) in the austenite phase evolves, during said processing.
  5. 5. A method according to claim 4 wherein Q o is equal or substantially equal to A+B.(T- T M ), A and B being two constants and T M being the Martensite start temperature for said steel.
  6. 6. A method according to any of the preceding claims wherein the density of bainitic ferrite nucleation centers (n nuc( a ) is equal or substantially equal to: - an overcooling T B -T, where T B is the Bainite start temperature, multiplied by - a coefficient a b , that depends on the chemical composition CC of the steel and that is proportional to 1/d y , d Y being a dimension of austenite grains in said steel.
  7. 7. A method according to any of the preceding claims wherein the bainitic ferrite growth rate is computed according to equation eqn 1 below where - f a is the fraction of bainitic ferrite in said steel, - Cmod is the moderation coefficient, - n nuci,a is the density of bainitic ferrite nucleation centers, v = kT/h « 10 13 s -1 , - f' a is equal to f a , or possibly to f a + f F , f F being the fraction of ferrite in said steel, - fy being the fraction austenite in said steel, - and A is an auto-catalysis coefficient proportional to a dimension d Y of austenite grains.
  8. 8. A method according to any of the preceding claims wherein the cementite growth rate is kept equal to zero as long as the Carbon content (C y ) in the austenite phase remains below a Carbon content threshold for cementite precipitation (C y ^ c ) which depends on the chemical composition CC of the steel.
  9. 9. A method according to any of the preceding claims wherein the Carbon content (C y ) in the austenite phase is computed while neglecting a Carbon content in the bainitic ferrite phase (C a ) with respect to the Carbon content (C y ) in the austenite phase, and with a carbon content in the cementite phase (C c ) that is constant.
  10. 10. A method according to any of the preceding claims wherein Carbon, Manganese, Silicon, Aluminum and Chromium contents in said steel, noted respectively as C o , x Mn , x S i, XAI and x Cr and expressed in weight %, are in the following ranges: C o <0.5 ; x Mn < 3 ; Xsi<2 ; XAI<1.5 ; Xcr^4.
  11. 1 1 . A method for monitoring a steel processing operation during which a steel semi-product (1 ) is processed in a steel processing installation (4), the method comprising the following steps: - Acquiring at least an initial temperature (Ti) of the steel semi-product at beginning of the steel processing operation, said temperature being measured by a sensor (24) of the steel processing installation, Determining an amount of bainitic ferrite (f a ) formed in the steel semi-product during the steel processing operation, according to the method of any of claims 1 to 10, said method for determining being executed while taking into account that the steel temperature at the beginning of the steel processing operation is the initial temperature (Ti) measured by said sensor.
  12. 12. A method according to claim 1 1 , further comprising - Acquiring thermal path data, representative of a thermal path (TP) followed by the steel semi-product during the steel processing operation, And wherein, when determining the amount of bainitic ferrite formed according to the method of any of claim 1 to 10, at each stime step, the evolution of the bainitic ferrite phase fraction is determined taking into account a temperature (T) of the steel according to said thermal path.
  13. 13. A method according to claim 1 1 , further comprising: - Acquiring data representative of heating or cooling conditions (HCC) applied to the steel semi-product (1 ) during the steel processing operation, And wherein, when determining the amount of bainitic ferrite formed according to the method of any of claim 1 to 10, at each stime step, an updated value of the temperature (T) of the steel is determined taking into account said heating or cooling conditions and taking into account the respective phase fractions of bainitic ferrite, austenite and cementite (f a , f Y , f c ) in said steel.
  14. 14. A method according to claim 13, wherein heating or cooling actuators (8, 9) of the steel processing installation (4) are controlled depending on: - a temperature (T) of the steel expected at an end of the steel processing operation, as determined according to claim 13, and depending on - a target temperature, to be reached by the steel semi-product (1 ) at the end of the steel processing operation.
  15. 15. A method according to any of claims 1 1 to 13, wherein - An estimated final property of the steel semi-product is determined, taking into account the amount of bainitic ferrite formed during the steel processing operation, and - process parameters are adjusted, depending on said estimated final property and depending on a target property for the steel semi-product (1 ), to be obtained at the end of the steel processing operation, the process parameters thus adjusted being transmitted to actuators (8, 9) of the steel processing installation for achieving the steel processing operation according to said process parameters.
  16. 16. A method according to any of claims 1 1 to 15, wherein at least one value of the bainitic ferrite fraction determined during said method for determining is output using a human- computer interface, or is recorded in a production database.
  17. 17. A method for controlling a steel processing installation, the method comprising the following steps: Determining a thermal path, to be followed by the steel semi-product during the steel processing operation, in order to obtain a given target property for the steel semi-product at an end of said processing, said thermal path being determined taking into account bainitic ferrite formation during said processing, an amount of bainitic ferrite formed during said processing being determined according to the method of any of claim 1 to 10, - Controlling actuators of the steel processing installation for achieving the steel processing operation according to the thermal path determined.
  18. 18. An electronic device (15 ; 15’ ; 5) comprising at least a processor and a memory, configured for executing the method according to any of claims 1 to 17.
  19. 19. A computer program, comprising instructions whose execution on a computer make the computer to execute the method according to any of claims 1 to 17.

Description

Method for determining an amount of bainitic ferrite formed during a steel processing operation, associated monitoring or control methods The technical field concerns phase transformation determination, for a steel semi- product undergoing a steel processing operation, in particular bainitic ferrite formation prediction. Such a prediction is useful in particular for monitoring, controlling, designing or tuning such a steel processing operation. T e c h n i c a I b a c kg r o u n d Recent developments in steel for automotive and other applications lead to consider bainite microstructures as good candidates to provide an alternative balance of strength and ductility. Being able to determine how much bainite is being formed in a steel undergoing cooling, or more generally undergoing a thermal treatment, is thus very useful, to better control the bainite content of the steel finally obtained. Being able to determine the bainite growth rate in a steel is also very useful for the control of a cooling operation itself, or for the control of a thermal treatment. Indeed, as the bainite formation is exothermal, taking into account how much bainite is being formed enables a more accurate temperature control. Regarding bainite formation, different models have been developed in the last years to predict Time-Temperature-Transformation (TTT) curves, or to predict bainite formation kinetics and final fractions. Some of these models perform well for so called ‘carbide free’ grades (for which there is no or little cementite formation due to high Silicon or Aluminium content), but less accurately for grades with a low silicon content. The article “Modelling Simultaneous Formation of Bainitic Ferrite and Carbide in TRIP Steels” by Fateh Fazeli and Matthias Militzer (ISIJ International, 2012, Volume 52, Issue 4, Pages 650-658, Print ISSN 0915-1559) describes a method for predicting fractions of bainite formed during cooling for a high silicon content. Other models predict well bainite formation for lean-silicon steels, where there may be substantial carbide precipitation, but less accurately for carbide free grades (in particular regarding the final bainite fraction). In this context, there is a need for a method enabling to determine how much bainite is being formed in a steel whose temperature is lowered below the bainite start temperature TB, both for ‘carbide-free’ grade and lean-silicon ones, and with a good comprise between computing time and accuracy. Summary of the invention The invention is achieved by providing a method for determining an amount of bainitic ferrite formed in a steel semi-product during a steel processing operation, the steel semi- product having a chemical composition CC, the method comprising the following steps, executed by an electronic device: - Acquiring the chemical composition CC of the steel of said steel semi-product, - s1 ) determining a bainitic ferrite growth rate, as being proportional to: o a density of bainitic ferrite nucleation centers, o exp(— <2*/RT) where T is the temperature of the steel, R is the universal gas constant and Q* is an activation energy for an austenite to bainitic ferrite transformation, and to o a moderation coefficient, which approaches zero when a difference AGy^a between a free enthalpy of a bainitic ferrite phase and a free enthalpy of an austenite phase approaches zero, the enthalpy of the austenite phase depending at least on a Carbon content in the austenite phase, - s2) determining a cementite growth rate, depending at least on the chemical composition CC of the steel and on the Carbon content in the austenite phase, - s3) determining values at a next time step for bainitic ferrite, austenite and cementite phases fractions, based on the bainitic ferrite growth rate and on the cementite growth rate, and determining a value at the next time step for the Carbon content in the austenite phase based on the values of said phases fractions, the set of steps s1 , s2 and s3 being executed several times successively. In the computation of the bainitic ferrite formation, the moderation coefficient employed in the claimed method enables to slow down this formation when the free enthalpy in the bainitic ferrite phase approaches the free enthalpy in the austenite phase. This enables to obtain final bainitic ferrite fractions that are in good agreement with experimental results, for ‘carbide-free’ grades. Besides, this moderation coefficient does not modify substantially the dynamic of formation, when the difference between said free enthalpies is substantial, at the beginning of bainite formation. This enables to obtain also good agreements regarding the formation kinetics. This moderation coefficient mainly reflects a carbon-content driven equilibrium between the bainitic ferrite phase and the austenite phase. For lean-silicon steels, cementite precipitation modifies the carbon content in the austenite phase, and thus modifies the equilibrium in question. Taking int