A method for manufacturing a thermally treated steel sheet

Abstract

A method for manufacturing a thermally treated steel sheet is described. The method includes: A. preparation step containing: 1 ) a selection substep, wherein: a. m target and a chemical composition are compared to a list of predefined products, whose microstructure contains predefined phases and predefined proportion of phases, and a product having a microstructure m standard closest to m target and TP standard is selected, including at least a heating, a soaking and a cooling steps, to obtain m standard , b. a heating path, a soaking path including a soaking temperature T soaking , a power cooling of the cooling system and a cooling temperature T cooling are selected based on TP standard and 2 ) a calculation substep, wherein through variation of the cooling power, new cooling paths CP x are calculated based on the product selected in step A. 1 ) a and TP standard , the initial microstructure m i of the steel sheet to reach m target , the heating path, the soaking path comprising T soaking and T cooling , the cooling step of TP standard is recalculated using said CP x in order to obtain new thermal paths TP x , each TP x corresponding to a microstructure m x , 3 ) a selection substep wherein one TP target to reach m target is selected, TP target being chosen among the calculated thermal paths TP x and being selected such that m x is the closest to m target , and B. a thermal treatment step wherein TP target is performed on the steel sheet.

1 - 41 . (canceled) 
     
     
         42 : 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:
 1) a selection substep, wherein:
 a. m target  and a chemical composition are compared to a list of predefined products, whose microstructure comprises predefined phases and predefined proportion of phases, and a product having a microstructure m standard  closest to m target  and TP standard  is selected, comprising at least a heating, a soaking and a cooling steps, to obtain m standard , 
 b. a heating path, a soaking path including a soaking temperature T soaking , a power cooling of the cooling system and a cooling temperature T cooling  are selected based on TP standard  and 
 
 2) a calculation substep, wherein through variation of the cooling power, new cooling paths CP x  are calculated based on the product selected in step A. 1) a and TP standard , the initial microstructure m i  of the steel sheet to reach m target , the heating path, the soaking path comprising T soaking  and T cooling , the cooling step of TP standard  is recalculated using said CP x  in order to obtain new thermal paths TP x , each TP x  corresponding to a microstructure m x , 
 3) a selection substep wherein one TP target  to reach m target  is selected, TP target  being chosen among the calculated thermal paths TP x  and being selected such that m x  is the closest to m target , and 
   B. a thermal treatment step wherein TP target  is performed on the steel sheet.   
     
     
         43 : A method according to  claim 42 , wherein 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. 
     
     
         44 : A method according to  claim 42 , wherein TP standard  further comprises a pre-heating step. 
     
     
         45 : A method according to  claim 42 , wherein TP standard  further comprise a hot-dip coating step, an overaging step, a tempering step, or a partitioning step. 
     
     
         46 : A method according to  claim 42 , wherein the microstructure m target  comprises:
 100% of austenite,   from 5 to 95% of martensite, from 4 to 65% of bainite, the balance being ferrite,   from 8 to 30% of residual austenite, from 0.6 to 1.5% of carbon in solid solution, the balance being ferrite, martensite, bainite, pearlite and/or cementite,   from 1% to 30% of ferrite and from 1% to 30% of bainite, from 5 and 25% of austenite, the balance being martensite,   from 5 to 20% of residual austenite, the balance being martensite,   ferrite and residual austenite,   residual austenite and intermetallic phases,   from 80 to 100% of martensite and from 0 to 20% of residual austenite   100% martensite,   from 5 to 100% of pearlite and from 0 to 95% of ferrite, and   at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.   
     
     
         47 : A method according to  claim 42 , wherein said predefined product types comprise 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, a TRIPLEX, DUPLEX and Dual Phase High Ductility DP steels. 
     
     
         48 : A method according to  claim 42 , wherein in step A.2), the cooling power of the cooling system varies from a minimum to a maximum value. 
     
     
         49 : A method according to  claim 42 , wherein in step A.2), the cooling power of the cooling system varies from a maximum to a minimum value. 
     
     
         50 : A method according to  claim 42 , wherein in step A.1.b), T soaking  is a fixed number selected from the range between 600 to 1000° C. 
     
     
         51 : A method according to  claim 42 , wherein in step A.1.b), T soaking  varies from 600 to 1000° C. 
     
     
         52 : A method according to  claim 51 , wherein after step A.2), a further calculation sub-step is performed wherein:
 a. T soaking  varies from in a predefined range value chosen from 600 to 1000° C. and   b. For each T soaking  variation, 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  is recalculated using said CP x  in order to obtain new thermal paths TP x , each TP x  corresponding to a microstructure m x .   
     
     
         53 : A method according to  claim 52 , wherein in the selection step A.3), the selected TP target  further includes the value of T soaking . 
     
     
         54 : A method according to 53, wherein in step A.3), when at least two CP x  have their m x  equal, the selected TP target  is the one having the minimum cooling power needed. 
     
     
         55 : A method according to  claim 42 , wherein in step A.2), the differences between proportions of phase present in m target  and m x  is ±3%. 
     
     
         56 : A method according to  claim 42 , wherein in 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.   
     
     
         57 : A method according to  claim 42 , wherein in step A.2), the all cooling path CP x  is calculated such that: 
       
         
           
             
               
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         with C pe : the specific heat of the phase (J·kg −1 ·K −1 ), ρ: the density of the steel (g·m −3 ), E p : thickness of the steel (m), φ: the heat flux (convective and radiative in W), H realeased  (J·kg −1 ), T: temperature (° C.) and t: time (s). 
       
     
     
         58 : A method according to  claim 56 , wherein in 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. 
     
     
         59 : A method according to  claim 58 , wherein in step A.2), CP x  is the sum of all CP xint , and H released  is the sum of all H xint . 
     
     
         60 : A method according to  claim 42 , wherein before step A.1.a), at least one targeted mechanical property P target  chosen among yield strength YS, Ultimate Tensile Strength UTS, elongation hole expansion, and formability is selected. 
     
     
         61 : A method according to  claim 60 , wherein m target  is calculated based on P target . 
     
     
         62 : A method according to  claim 42 , wherein in 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 . 
     
     
         63 : A method according to  claim 62 , wherein 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. 
     
     
         64 : A method according to  claim 42 , wherein in 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 . 
     
     
         65 : A method according to  claim 64 , wherein 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. 
     
     
         66 : A method according to  claim 42 , wherein the cooling system comprises at least one jet cooling, at least one cooling spray or at least both. 
     
     
         67 : A method according to  claim 66 , wherein when the cooling system comprises at least one jet cooling, the jet cooling sprays a gas, an aqueous liquid or a mixture thereof. 
     
     
         68 : A method according to  claim 67 , wherein the gas is chosen from air, HN x , H 2 , N 2 , Ar, He, steam water or a mixture thereof. 
     
     
         69 : A method according to  claim 68 , wherein the aqueous liquid is chosen from water or a nanofluid. 
     
     
         70 : A method according to  claim 68 , wherein the jet cooling sprays air with a debit flow between 0 and 350000 Nm 3 /h. 
     
     
         71 : A method according to  claim 42 , wherein T cooling  is the bath temperature when the cooling section is followed by a hot-dip coating section comprising a hot-dip bath. 
     
     
         72 : A method according to  claim 71 , wherein the bath is based on aluminum or based on zinc. 
     
     
         73 : A method according to  claim 42 , wherein T cooling  is the quenching temperature T q . 
     
     
         74 : A method according to  claim 42 , wherein T cooling  is between 150 and 800° C. 
     
     
         75 : A method according to  claim 42 , wherein 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. 
     
     
         76 : A method according to  claim 75 , wherein an adaptation of the cooling path is performed as the steel sheet enters into the cooling section of the heat treatment line on the first meters of the sheet. 
     
     
         77 : A coil made of a steel sheet comprising a predefined product types comprising DP, TRIP, Q&P, TWIP, CFB, PHS, TRIPLEX, DUPLEX and DP HD steels, said steels obtained by a method according to  claim 42  and having a standard variation of mechanical properties below or equal to 25 MPa between any two points along the coil. 
     
     
         78 : A coil according to  claim 77  having a standard variation is below or equal to 15 MPa between any two points along the coil. 
     
     
         79 : A coil according to  claim 78  having a standard variation is below or equal to 9 MPa between any two points along the coil. 
     
     
         80 : A coil according to  claim 77  covered by a metallic coating based on zinc or based on aluminum. 
     
     
         81 : A thermal treatment line for the implementation of the method according to  claim 42 , the thermal treatment line comprising a heating section, a soaking section and a cooling section comprising a cooling system. 
     
     
         82 : 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 a method according to  claim 42 .