Patent Publication Number: US-2023151452-A1

Title: Steel sheet having excellent toughness, ductility and strength, and manufacturing method thereof

Description:
The present disclosure concerns a method for manufacturing a hot-rolled and annealed steel sheet having high cold-rollability and toughness, and suitable for producing a cold-rolled and heat-treated steel sheet having a high combination of ductility and strength, and to a hot-rolled and annealed steel sheet produced by this method. 
     The present disclosure also relates to a method for manufacturing a cold-rolled and heat-treated steel sheet having a high combination of ductility and strength, and to a cold-rolled and heat-treated steel sheet obtained by this method. 
     BACKGROUND 
     In the automotive industry in particular, there is a continuous need to lighten vehicles, in order to improve their fuel efficiency in view of the global environmental conservation, and to increase safety, by using steels having a high tensile strength. Such steels may indeed be used to produce parts having a lower thickness whilst guaranteeing the same or an improved safety level. 
     To that end, steels have been proposed that have micro-alloying elements whose hardening is obtained simultaneously by precipitation and by refinement of the grain size. The development of such steels has been followed by those of higher strength called Advanced High Strength Steels which keep good levels of strength together with good cold formability. 
     For the purpose of obtaining even higher tensile strength levels, steels exhibiting TRIP (Transformation Induced Plasticity) behavior with highly advantageous combinations of properties (tensile strength/deformability) have been developed. These properties are associated with the structure of such steels, which consists of a ferritic matrix containing bainite and residual austenite. The residual austenite is stabilized by an addition of silicon or aluminium, these elements retarding the precipitation of carbides in the austenite and in the bainite. The presence of residual austenite gives an undeformed sheet high ductility. Under the effect of a subsequent deformation, for example when stressed uniaxially, the residual austenite of a part made of TRIP steel is progressively transformed to martensite, resulting in substantial hardening and delaying the appearance of necking. 
     To achieve an improved combination of strength and ductility, it was further proposed to produce sheets by the so-called “quenching and partitioning” process, wherein the sheets are annealed in the austenitic or in the intercritical domain, cooled down to a quenching temperature below the Ms transformation point, and thereafter heated to a partitioning temperature and maintained at this temperature for a given time. The resulting steel sheets have a structure comprising martensite and retained austenite, and optionally bainite and/or ferrite. The retained austenite has a high C content, resulting from the partitioning of carbon from the martensite during the partitioning, and the martensite comprises a low fraction of carbides. 
     All these steel sheets present good balances of resistance and ductility. 
     SUMMARY 
     However, new challenges appear when it comes to manufacture such sheets. Especially, the manufacturing process of such steel sheets generally comprises, before the heat-treatment imparting its final properties to the steel, casting a steel semi-product, hot-rolling the semi-product to produce a hot-rolled steel sheet, then coiling the hot-rolled steel sheet. The hot-rolled steel sheet is then cold-rolled to the desired thickness, and subjected to a heat-treatment chosen as a function of the desired final structure and properties, to obtain a cold-rolled and heat-treated steel sheet. 
     Owing to the composition of these steels, a high level of resistance is reached throughout the manufacturing process. Especially, the hot-rolled steel sheet exhibits, before cold-rolling, a high hardness impairing its cold-rollability. As a consequence, the range of available sizes for the cold-rolled sheets is reduced. 
     In order to solve this problem, it was proposed to subject the hot-rolled steel sheet, prior to cold-rolling, to a batch annealing, at a temperature generally comprised between 500° C. and 700° C., for a time of several hours. 
     The batch annealing indeed results in a decrease of the hardness of the hot-rolled steel sheet, and therefore improves its cold-rollability. 
     However, this solution is not entirely satisfactory. 
     Indeed, the batch annealing treatment generally leads to a decrease of the final properties of the steel, in particular its ductility and strength. 
     In addition, the hot-rolled steel sheet exhibits an insufficient toughness after batch annealing, which may be the cause of band breakage during further processing. 
     An object of the present disclosure therefore is providing a hot-rolled steel sheet, and a manufacturing method therefore, having an improved cold-rollability and toughness, whilst being suitable for producing a cold-rolled and heat-treated steel sheet having high mechanical properties, especially a high combination of ductility and strength. 
     Another object of the present disclosure is providing a cold-rolled and heat treated steel sheet and a manufacturing method thereof, having a high combination of mechanical properties, as compared to similar steel sheets produced by a method including a batch-annealing treatment before cold-rolling. 
     A method for manufacturing a steel sheet, comprises the steps of:
     casting a steel having a composition comprising, by weight percent:
   0.1% ≤ C ≤0.4%   3.5% ≤ Mn ≤ 8.0%   0.1% ≤ Si ≤ 1.5%   Al ≤ 3%   Mo ≤ 0.5%   Cr ≤ 1%   Nb ≤ 0.1%   Ti ≤ 0.1%   V ≤ 0.2%   B ≤ 0.004%   0.002% ≤ N ≤ 0.013%   S ≤ 0.003%   P ≤ 0.015%,   
the remainder being iron and unavoidable impurities resulting from the smelting, to obtain a steel semi-product,   reheating the steel semi-product to a temperature T reheat  comprised between 1150° C. and 1300° C.,   hot rolling the reheated semi-product at a temperature comprised between 800° C. and 1250° C., with a final rolling temperature T FRT  higher than or equal to 800° C., thereby obtaining a hot rolled steel sheet,   cooling the hot rolled steel sheet down to a coiling temperature T coil  lower than or equal to 650° C. at a cooling rate V c1  comprised between 1° C./s and 150° C./s, and coiling the hot-rolled steel sheet at the coiling temperature T coil , then   continuously annealing the hot-rolled steel sheet at a continuous annealing temperature T ICA  comprised between T lCAmin  and T ICAmax , with T ICAmin =650° C., and T lCAmax  being the temperature at which 30% of austenite is formed upon heating, the hot-rolled steel sheet being held at said continuous annealing temperature T ICA  for a continuous annealing time t ICA  comprised between 3 s and 3600 s, then,   cooling the hot-rolled steel sheet to room temperature, the hot-rolled steel sheet being cooled with an average cooling rate V ICA  between 600° C. and 350° C. of at least 1° C./s, thereby obtaining a hot-rolled and annealed steel sheet,   cold-rolling the hot-rolled and annealed steel sheet with a cold rolling reduction ratio comprised between 30% and 70%, thereby obtaining a cold-rolled steel sheet.   

     Preferably, the hot-rolled and annealed steel sheet has a structure consisting, in surface fraction, of:
     ferrite, the ferrite grains have an average size of at most 3 µm ,   at most 30% of austenite,   at most 8% of fresh martensite and   cementite, having an average Mn content lower than 25%.   

     Generally, the hot-rolled and annealed steel sheet has a Vickers hardness lower than 400 HV. 
     Preferably, the hot-rolled and annealed steel sheet has a Charpy energy at 20° C. of at least 50 J/cm 2 . 
     Preferably, the method further comprises, between the coiling and the continuous annealing and/or after the continuous annealing, a step of pickling the hot-rolled steel sheet. 
     Preferably, the continuous annealing time t ICA  is comprised between 200 s and 3600 s. 
     Preferably, the method further comprises, after cold-rolling:
     heating the cold-rolled steel sheet to an annealing temperature T anneal  comprised between 650° C. and 1000° C., and   holding the cold-rolled steel sheet at the annealing temperature T anneal  for an annealing time t anneal  comprised between 30 s and 10 min.   

     In a first embodiment, the annealing temperature T anneal  is comprised between T ICAmin  and Ae3. 
     In a second embodiment, wherein the annealing temperature T anneal  is comprised between Ae3 and 1000° C. 
     In an embodiment, the method further comprises a step of cooling the cold-rolled steel sheet from the annealing temperature T anneal  down to room temperature at a cooling rate Vc 2  comprised between 1° C./s and 70° C./s, to obtain a cold-rolled and heat treated steel sheet. 
     In another embodiment, the method further comprises, after holding the cold-rolled steel sheet at the annealing temperature T anneal , the successive steps of:
     cooling the cold-rolled steel sheet from the annealing temperature T anneal  down to a holding temperature T H  comprised between 350° C. and 550° C. at a cooling rate Vc 2  comprised between 1° C./s and 70° C./s,   maintaining the cold-rolled steel sheet at the holding temperature T H  for a holding time t H  comprised 10 s and 500 s, then,   cooling the cold-rolled steel sheet from the holding temperature T H  down to room temperature at a cooling rate V c3  comprised between 1° C./s and 70° C./s, to obtain a cold-rolled and heat treated steel sheet.   

     Preferably, the method further comprises a step of tempering the cold-rolled and heat treated steel sheet at a tempering temperature T T  comprised between 170° C. and 450° C. for a tempering time t T  comprised between 10 s and 1200 s. 
     Preferably, the method further comprises a step of coating the cold-rolled and heat treated steel sheet with Zn or a Zn alloy, or with Al or an Al alloy. 
     In another embodiment, the method further comprises the steps of:
     quenching the heated cold-rolled steel sheet from the annealing temperature T anneal  to a quenching temperature QT comprised between Mf+20° C. and Ms-20° C., at a cooling rate V c4  high enough to avoid the formation of ferrite and pearlite upon cooling,   reheating the cold-rolled steel sheet from the quenching temperature QT to a partitioning temperature T P  comprised between 350° C. and 500° C., and maintaining the cold-rolled steel sheet at the partitioning temperature T P  for a partitioning time t P  comprised between 3 s and 1000 s,   cooling the cold-rolled steel sheet to room temperature, to obtain a cold-rolled and heat treated steel sheet.   

     In a first variant of this embodiment, the annealing temperature T anneal  is such that the cold-rolled steel sheet has a structure, upon annealing, consisting of, in surface fraction:
     between 10% and 45% of ferrite,   austenite, and   at most 0.3% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     In a second variant of this embodiment, the annealing temperature T anneal  is higher than Ae3, the cold-rolled steel sheet having a structure, upon annealing, consisting of:
     austenite, and   at most 0.3% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     After the maintaining of the cold-rolled steel sheet at the partitioning temperature T P , the cold-rolled steel sheet may be immediately cooled to the room temperature. 
     In a variant, between the maintaining of the cold-rolled steel sheet at the partitioning temperature T P  and the cooling of the cold-rolled steel sheet to the room temperature, the cold-rolled steel sheet is hot-dip coated in a bath. 
     Preferably, the Si content in the composition is of at most 1.4%. 
     A cold-rolled and heat treated steel sheet is also provided, made of a steel having a composition comprising, by weight percent:
     0.1% ≤ C ≤ 0.4%   3.5% ≤ Mn ≤ 8.0%   0.1% ≤ Si ≤ 1.5%   Al ≤ 3%   Mo ≤ 0.5%   Cr ≤ 1%   Nb ≤ 0.1%   Ti ≤ 0.1%   V ≤ 0.2%   B ≤ 0.004%   0.002% ≤ N ≤ 0.013%   S ≤ 0.003%   P ≤ 0.015%,   

     the remainder being iron and unavoidable impurities resulting from the smelting, wherein the cold-rolled steel sheet has a structure consisting of, in surface fraction:
     between 8 and 50% of retained austenite,   at most 80% of intercritical ferrite, the ferrite grains, if any, having an average size of at most 1.5 µm ,and   at most 1% of cementite, the cementite particles, if any, having an average size lower than 50 nm,   martensite and/or bainite.   

     In an embodiment, the structure comprises, in surface fraction, at least 10% of intercritical ferrite. 
     In another embodiment, the structure consists of, in surface fraction:
     between 8 and 50% of retained austenite,   at most 1% of cementite, the cementite particles, if any, having an average size lower than 50 nm,   martensite and/or bainite.   

     In an embodiment, the martensite consists of tempered martensite and/or fresh martensite. 
     In a first variant of this embodiment, the structure consists of, in surface fraction:
     between 8% and 50% of retained austenite, having an average C content of at least 0.4% and an average Mn content of at least 1.3 \*Mn%, Mn% designating the average Mn content in the steel composition,   between 40% and 80% of intercritical ferrite,   at most of 15% of martensite and/or bainite, and   at most 0.3% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     In a second variant of this embodiment, the structure consists of, in surface fraction:
     between 8% and 30% of retained austenite, having an average C content of at least 0.4%,   between 70% and 92% of martensite and/or bainite, and   at most 1% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     In another embodiment, the structure consists of, in surface fraction:
     at most 45% of intercritical ferrite,   between 8% and 30% of retained austenite,   partitioned martensite,   at most 8% of fresh martensite, and   at most 1% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     In a first variant of this embodiment, the structure consists of, in surface fraction:
     between 10% and 45% of intercritical ferrite,   between 8% and 30% of retained austenite,   partitioned martensite,   at most 8% of fresh martensite, and   at most 0.3% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     In a second variant of the embodiment, the structure consists of, in surface fraction:
     between 8% and 30% of retained austenite,   partitioned martensite,   at most 8% of fresh martensite, and   at most 1% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     Preferably, the Si content in the composition is of at most 1.4%. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will now be described in details and illustrated by examples without introducing limitations, with reference to the appended figures among which: 
         FIG.  1    is a micrograph illustrating the structure of a comparative hot-rolled and batch annealed steel sheet, 
         FIG.  2    is a micrograph illustrating the structure of a hot-rolled steel continuously annealed according to an embodiment of the present disclosure, 
         FIG.  3    is a graph comparing the mechanical properties of a cold-rolled and heat treated steel sheet produced either from a hot-rolled and batch annealed steel sheet, or from a hot-rolled and continuously steel sheet. 
     
    
    
     DETAILED DESCRIPTION 
     According to the present disclosure, the carbon content is between 0.1% and 0.4%. Carbon is an austenite-stabilizing element. Below 0.1%, high levels of tensile strength are difficult to achieve. If the carbon content is greater than 0.4%, the cold-rollability is reduced and the weldability becomes poor. Preferably, the carbon content is comprised between 0.1% and 0.2%. 
     The manganese content is comprised between 3.5% and 8.0%. Manganese provides a solid solution hardening and a refining effect on the microstructure. Manganese therefore contributes to increasing the tensile strength. In a content above 3.5%, Mn is used to provide an important stabilization of the austenite in the microstructure throughout the whole manufacturing process and in the final structure. Especially, with a Mn content above 3.5%, a final structure of the cold-rolled and heat treated steel sheet comprising at least 8% of retained austenite can be achieved. In addition, owing to the stabilization of the retained austenite with Mn, a high ductility can be obtained. Above 8.0%, weldability becomes poor, while segregations and inclusions deteriorate the damage properties. 
     Silicon is very efficient to increase the strength through solid solution and stabilize the austenite. Besides, silicon delays the formation of cementite upon cooling by substantially retarding the precipitation of carbides. That results from the fact that the solubility of silicon in cementite is very low and that Si increases the activity of carbon in austenite. Any formation of cementite will therefore be preceded by a step where Si is expelled at the interface. The enrichment of the austenite with carbon therefore leads to its stabilization at room temperature. 
     For this reason, the Si content is of at least 0.1 %. However the Si content is limited to 1.5%, because beyond this value, the rolling loads increase too much and hot rolling process becomes difficult. The cold-rollability is also reduced. In addition, at a too high content, silicon oxides form at the surface, which impairs the coatability of the steel. 
     Preferably, the Si content is of at most 1.4%. Indeed, a Si content of at most 1.4% reduces or even suppresses the occurrence of red scale (also called tiger stripes), caused by the existence of Fayalite (Fe 2 SiO 4 ), upon hot rolling. 
     Aluminum is a very effective element for deoxidizing the steel in the liquid phase during elaboration. Preferably, the Al content is not less than 0.003% in order to obtain a sufficient deoxidization of the steel in the liquid state. 
     Furthermore, like Si, Al stabilizes the residual austenite and delays the formation of cementite upon cooling. The Al content is however not higher than 3% in order to avoid the occurrence of inclusions, to avoid oxidation problems and to ensure the hardenability of the material. 
     The steel according to the present disclosure may contain at least one element chosen among molybdenum and chromium. 
     Molybdenum increases the hardenability, stabilizes the retained austenite, and reduces the central segregation which can result from the manganese content and which is detrimental to the formability. Above 0.5%, Mo may form too many carbides, which may be detrimental for the ductility. 
     When Mo is not added, the steel may however comprise at least 0.001% of Mo as an impurity. When Mo is added, the Mo content is generally higher than or equal to 0.05%. 
     Chromium increases the quenchability of the steel, and contributes to achieving a high tensile strength. A maximum of 1% of chromium is allowed. Indeed, above 1%, a saturation effect is noted, and adding Cr is both useless and expensive. When Cr is added, its content is generally of at least 0.01%. If no voluntary addition of Cr is performed, the Cr content may be present as an impurity, in a content as low as 0.001%. 
     Micro-alloying elements such as titanium, niobium and vanadium may be added in a content of at most 0.1% of Ti, at most 0.1% of Nb and at most 0.2% of V, in order to obtain an additional precipitation hardening. In particular titanium and niobium are used to control the grain size during the solidification. 
     When Nb is added, its content is preferably of at least 0.01%. Above 0.1%, a saturation effect is obtained, and adding more than 0.1% of Nb is both useless and expensive. 
     When Ti is added, its content is preferably of at least 0.015%. When the Ti content is comprised between 0.015% and 0.1%, precipitation at very high temperature occurs in the form of TiN and then, at lower temperature, in the form of fine TiC, resulting in hardening. Furthermore, when titanium is added in addition to boron, titanium prevents combination of boron with nitrogen, the nitrogen being combined with titanium. Hence, when boron is added, the titanium content is preferably higher than 3.42 N. However, the Ti content should remain lower than or equal to 0.1% to avoid precipitation of coarse TiN precipitates increasing the hardness of the hot-rolled steel sheet and the cold-rolled steel sheet during the manufacturing process. 
     Optionally, the steel composition comprises boron, to increase the quenchability of the steel. When B is added, its content is higher than 0.0002%, and preferably higher than or equal to 0.0005%, up to 0.004%. Indeed, above such limit, a saturation level is expected as regard to hardenability. 
     Sulfur, phosphorus and nitrogen are generally present in the steel composition as impurities. 
     The nitrogen content is generally of at least 0.002%. The nitrogen content must be of at most 0.013%, so as to prevent precipitation of coarse TiN and/or AIN precipitates degrading the ductility. 
     As for sulphur, above a content of 0.003%, the ductility is reduced due to the presence of excess sulphides such as MnS, in particular hole-expansion tests show lower values in presence of such sulphides. 
     Phosphorus is an element which hardens in solid solution but which reduces the spot weldability and the hot ductility, particularly due to its tendency to segregation at the grain boundaries or co-segregation with manganese. For these reasons, its content must be limited to 0.015%, in order to obtain good spot weldability. 
     The balance is made of iron and inevitable impurities. Such impurity may include at most 0.03% of Cu and at most 0.03% of Ni. 
     The method according to the present disclosure aims at providing a hot-rolled and annealed steel sheet having a high cold-rollability together with a high toughness, and which is suitable for producing a cold-rolled and heat-treated steel sheet having a high combination of ductility and strength. 
     The method according to the present disclosure also aims at manufacturing such a cold-rolled and heat-treated steel sheet. 
     The inventors have investigated the problems of low toughness of the hot-rolled and batch annealed steel sheets, and of degraded mechanical properties of the cold-rolled and heat-treated steel sheets manufactured from such hot-rolled and batch annealed steel sheets, as compared to sheets that would not have been subjected to annealing, and discovered that these problems result from four main factors. 
     Especially, the inventors have discovered that the batch annealing results in the formation of coarse cementite, highly enriched in manganese, which is therefore strongly stabilized in the hot-rolled and batch-annealed steel sheet. The inventors have further found that the cementite, thus stabilized, does not completely dissolve during the subsequent standard heat-treatment of the cold-rolled steel sheet. Consequently, part of the Mn of the steel remains trapped in cementite, its effect on the strength and ductility of the steel being thus inhibited. 
     The inventors have further discovered that the batch annealing also results in a coarsening of the structure of the hot-rolled and batch-annealed steel sheet, which results in a coarsening of the final structure of the cold-rolled and heat-treated steel sheet and degrades the mechanical properties. 
     In addition, the inventors have discovered that the micro-alloying elements that may be included in the steel composition, especially Nb, precipitate at an early stage during the batch-annealing as coarse precipitates, which do not harden the steel, and are consequently no longer available during the subsequent heat-treatment of the cold-rolled steel sheet to provide precipitation hardening. 
     Finally, the inventors have found that the batch annealing is performed at a temperature and for a time which induce temper embrittlement, resulting in a low toughness of the hot-rolled and batch-annealed steel sheet. 
     In order to solve these problems, the inventors have performed experiments by increasing the batch annealing temperature above the Ae1 transformation point of the steels. 
     However, the inventors have found that using higher batch annealing temperatures, though limiting the formation of cementite enriched in Mn, results in a coarsening of the microstructure thereby impairing the final properties of the cold-rolled and heat-treated steel sheet. 
     From these findings, the inventors discovered that the cold-rollability and the toughness can be highly improved, whilst guaranteeing the final properties of the cold-rolled and heat-treated steel sheets, if the hot-rolled steel sheet is annealed so as to have a microstructure comprising:
     ferrite, with an average ferritic grain size of at most 3 µm ,   at most 30% of austenite,   at most 8% of fresh martensite, and   cementite, having an average Mn content lower than 25%.   

     A fresh martensite fraction of at most 8% makes it possible to achieve a high toughness of the hot-rolled and annealed steel sheet. 
     Especially, the inventors have performed experiments by subjecting hot-rolled steel sheets made of several steels compositions to various annealing conditions leading to varying austenite and fresh martensite fractions after cooling down to room temperature, and measured the Charpy energy at 20° C. of the steel sheets thus obtained. 
     On the basis of these experiments, the inventors have found that the Charpy energy is an increasing function of the annealing temperature, and a decreasing function of the fresh martensite fraction. Furthermore, the inventors have discovered that a high Charpy energy, of at least 50 J/cm 2  at 20° C., is achieved if the hot-rolled and annealed steel sheet has a fresh martensite fraction of at most 8%. 
     Besides, a cementite having an average Mn content lower than 25% implies that the cementite dissolution is facilitated during the final heat treatment of the cold-rolled steel sheet, which improves ductility and strength during the further processing steps. By contrast, a cementite with an average Mn content above 25% would lead to a decrease in the mechanical properties of the cold-rolled and heat-treated steel sheet produced from the hot-rolled and annealed steel sheet. 
     In addition, having an average ferritic grain size of at most 3 µm allows producing a cold-rolled and heat-treated having a very fine microstructure, and increasing its mechanical properties. 
     The inventors have further found that the above microstructure allows achieving a hardness of the hot-rolled and annealed steel sheet lower than 400 HV, guaranteeing a satisfactory cold-rollability of the hot-rolled and annealed steel sheet. 
     The inventors have found that this microstructure and these properties of the hot-rolled and annealed steel sheet are achieved by performing on the hot-rolled steel sheet a continuous annealing at a continuous annealing temperature T ICA  comprised between a minimal continuous annealing temperature T ICAmin = 650° C. and a maximal continuous annealing temperature T ICAmax  which is the temperature at which 30% of austenite is formed upon heating, and for a time comprised between 3 s and 3600 s, and by subsequently cooling the hot-rolled steel sheet under particular cooling conditions. 
     Especially, the inventors have found that owing to the high continuous annealing temperature T ICA , an annealing time of at most 3600 s is sufficient to achieve sufficient tempering of the structure, thereby improving the cold-rollability of the hot-rolled and annealed steel sheet, whilst avoiding coarsening of the structure. 
     Moreover, annealing the sheet at a temperature higher than 650° C. allows the softening of the hot-rolled steel sheet, limiting the Mn enrichment of cementite particles below 25% and limiting the precipitation of the micro-alloying elements, if any, and preventing the coarsening of such precipitates, thereby retaining the effects of C, Mn and of the micro-alloying elements on the final mechanical properties. It also limits the segregation of embrittling impurities like P at the grain boundaries. 
     The manufacturing method will be now described in further details. 
     The method to produce the steel according to the present disclosure comprises casting a steel with the chemical composition of the present disclosure. 
     The cast steel is reheated to a temperature T reheat  comprised between 1150° C. and 1300° C. 
     When slab reheating temperature T reheat  is below 1150° C., the rolling loads increase too much and hot rolling process becomes difficult. 
     Above 1300° C., oxidation is very intense, which leads to scale loss and surface degradation. 
     The reheated slab is hot-rolled at a temperature between 1250° C. and 800° C., the last hot rolling pass taking place at a final rolling temperature T FRT  higher than or equal to 800° C. 
     If the final rolling temperature T FRT  is below 800° C., the hot workability is reduced. 
     After hot rolling, the steel is cooled at a cooling rate V c1  comprised between 1° C./s and 150° C./s, to a coiling temperature T coil  lower than or equal to 650° C. Below 1° C./s, a too coarse microstructure is created and the final mechanical properties deteriorate. Above 150° C./s, the cooling process is difficult to control. 
     The coiling temperature T coil  must be lower than or equal to 650° C. If the coiling temperature is above 650° C., deep intergranular oxidation is formed below scale leading to a deterioration of surface properties. 
     After coiling, the hot-rolled steel sheet is preferably pickled. 
     The hot-rolled steel sheet is then continuously annealed, i.e. the uncoiled hot-rolled steel sheet undergoes a heat treatment by continuously travelling within a furnace. 
     The hot-rolled steel sheet is continuously annealed at a continuous annealing temperature T ICA  comprised between the minimal continuous annealing temperature T ICAmin =650° C. and a maximal continuous annealing temperature T ICAmax  which is the temperature at which 30% of austenite is formed upon heating, and for a time comprised between 3 s and 3600 s. 
     Under these conditions, the microstructure of the steel created during the continuous annealing, before cooling down to room temperature, consists of;
     ferrite,   less than 30% of austenite   cementite having an average Mn content lower than 25%.   

     If the continuous annealing temperature is lower than 650° C., softening through microstructure recovery is insufficient during the continuous annealing treatment, so that the hardness of the hot-rolled and annealed steel sheet is above 400 HV. A continuous annealing temperature below 650° C. also enhances segregation of embrittling elements, like P, at the grain boundaries and leads to poor toughness values, which are critical for further processing the steel sheets. 
     If the continuous annealing temperature is higher than T ICAmax , a too high austenite fraction will be created during continuous annealing, which may result in an insufficient stabilization of the austenite and the creation of more than 8% of fresh martensite upon cooling. 
     If the continuous annealing time is lower than 3 s, the hardness of the hot-rolled and annealed steel sheet will be too high, especially higher than 400 HV, so that its cold-rollability will be unsatisfactory. The continuous annealing time is preferably of at least 200 s. 
     If the continuous annealing time is higher than 3600 s, the microstructure is coarsened; especially, the ferrite grains have an average size higher than 3 µm. Preferably, the continuous annealing time is of at most 500 s. 
     The austenite which can be created during the annealing is enriched in carbon and manganese, especially has an average Mn content of at least 1.3 \*Mn%, Mn% designating the Mn content of the steel, and an average C content of at least 0.4%. 
     The austenite is therefore strongly stabilized. 
     The hot-rolled steel sheet is then cooled down from the annealing temperature T ICA  to room temperature, with an average cooling rate V ICA  between 600° C. and 350° C. of at least 1° C./s. Under this condition, the temper embrittlement is limited. 
     If the cooling rate between 600° C. and 350° C. is lower than 1° C./s, segregation occurs in the hot-rolled and annealed steel sheet enhancing temper embrittlement, so that its cold-rollability is not satisfactory. 
     The hot-rolled and annealed steel sheet thus obtained has a structure consisting of:
     ferrite,   at most 30% of austenite,   at most 8% of fresh martensite,   cementite, having an average Mn content lower than 25%.   

     A fresh martensite fraction of at most 8% is achieved owing to the stabilization of the austenite with Mn, which therefore does not transform or only to a small extent into fresh martensite upon cooling. 
     The retained austenite of the hot-rolled and annealed steel sheet has an average Mn content of at least 1.3 \*Mn%, wherein Mn% designates the Mn content of the steel, and has an average C content of at least 0.4%. 
     A tempering treatment is optionally performed so as to further limit the fresh martensite fraction. 
     In addition, the ferrite grains have an average size of at most 3 µm. Indeed, the continuous annealing, performed during a relatively short time as compared to batch annealing, did not result in a coarsening of the structure and therefore allows achieving a hot-rolled and annealed sheet having a very fine structure. 
     At this stage, the hot-rolled and annealed sheet has improved cold-rollability and toughness, as compared to the hot-rolled steel sheet before annealing. In addition, the hot-rolled and annealed steel sheet is suitable for producing a cold-rolled and heat treated steel sheet having high mechanical properties, especially high ductility and strength. 
     In particular, the hot-rolled and annealed sheet has a Vickers hardness lower than 400 HV, and has therefore a very good cold-rollability. 
     In addition, the hot-rolled and annealed steel sheet has a Charpy energy at 20° C. of at least 50 J/cm 2 . Therefore, the hot-rolled and annealed steel sheet has a very good processability and the risks of band breakage during further processing is strongly decreased as compared to hot rolled steel sheets that would have been batch annealed. Moreover, the inventors have discovered that not only is the Charpy energy of the hot-rolled and annealed steel sheet higher than hot rolled and batch annealed steel sheets, but it is also generally higher than the Charpy energy of the hot-rolled steel sheet from which the hot-rolled and annealed steel sheet was produced. 
     After cooling down to room temperature, the hot-rolled and annealed steel sheet is optionally pickled. However, this step may be omitted. Indeed, owing to the short duration of the continuous annealing, no or little internal oxidation occurs during the continuous annealing. Preferably, the hot-rolled and annealed steel sheet is pickled at this stage if no pickling was performed between the hot-rolling and the continuous annealing. 
     The hot-rolled steel sheet is then cold-rolled, with a cold-rolling reduction ratio comprised between 30% and 70%, to obtain a cold-rolled steel sheet. Below 30%, the recrystallization during subsequent heat-treatment is not favored, which may impair the ductility of the cold-rolled steel sheet after heat-treatment. Above 70%, there is a risk of edge cracking during cold-rolling. 
     The cold-rolled steel sheet is then heat-treated on a continuous annealing line to produce a cold-rolled and heat-treated steel sheet. 
     The heat-treatment performed on the cold-rolled steel sheet is chosen depending on the final mechanical properties targeted. 
     In any case, the heat-treatment comprises the steps of heating the cold-rolled steel sheet to an annealing temperature T anneal  comprised between 650° C. and 1000° C., and holding the cold-rolled steel sheet at the annealing temperature T anneal  for an annealing time t anneal  comprised between 30 s and 10 min. 
     In addition, the annealing temperature T anneal  is such that the structure created upon annealing comprises at least 8% of austenite. 
     If the annealing temperature is lower than 650° C., cementite will be created in the structure during the annealing, resulting in a degradation of the mechanical properties of the cold-rolled and heat-treated steel sheet. 
     The annealing temperature T anneal  is of at most 1000° C. in order to limit the coarsening of the austenitic grains. 
     The reheating rate Vr to the annealing temperature T anneal  is preferably comprised between 1° C./s and 200° C./s. 
     According to a first embodiment, the annealing is an intercritical annealing, the annealing temperature T anneal  being lower than Ae3 and such that the structure created upon annealing comprises at least 8% of austenite. 
     According to a second embodiment, the annealing temperature T anneal  is higher than or equal to Ae3, so as to obtain, upon annealing, a structure consisting of austenite and at most 1% of cementite. 
     In the first embodiment, at the end of the holding at the annealing temperature, the austenite has a C content of at least 0.4% and an average Mn content of at least 1.3 \*Mn%. 
     The cold-rolled and annealed steel sheet is then cooled down to room temperature, either directly, i.e. without any holding, tempering or reheating step between the annealing temperature T anneal  and room temperature, or indirectly, i.e. with holding, tempering and/or reheating steps, to obtain a cold-rolled and heat-treated steel sheet. 
     In any case, the cold-rolled and heat-treated steel sheet has a structure (hereinafter final structure) comprising:
     between 8% and 50% of retained austenite,   martensite, which may include fresh martensite and/or partitioned or tempered martensite, and optionally bainite,   at most 80% of intercritical ferrite, and   at most 1% of cementite.   

     The retained austenite generally has an average C content of at least 0.4% and generally an average Mn content of at least 1.3 \*Mn%. 
     Owing to the Mn content in cementite of at most 25% in the microstructure of the hot-rolled and annealed steel sheet, cementite is easily dissolved upon annealing. Depending on the heat-treatment performed, a small fraction of cementite may remain in the final structure. However, the cementite fraction in the final structure will in any case remain lower than 1%. In addition, the cementite particles, if any, have an average size lower than 50 nm. 
     The martensite may comprise fresh martensite and partitioned martensite or tempered martensite. 
     As explained in further details below, partitioned martensite has an average C content strictly lower than the nominal C content of the steel. This low C content results from the partitioning of carbon from the martensite, created upon quenching below the Ms temperature of the steel, to the austenite, during the holding at a partitioning temperature T P  comprised between 350° C. and 500° C. 
     By contrast, tempered martensite has an average C content which equals the nominal C content of the steel. Tempered martensite results from a tempering of the martensite created upon quenching below the Ms temperature of the steel. 
     Partitioned martensite can be distinguished from tempered martensite and fresh martensite on a section polished and etched with a reagent known per se, for example Nital reagent, observed by Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD). 
     The structure may comprise bainite, especially carbides free bainite, containing less than 100 carbides per surface unit of 100 mm 2 . 
     The ferrite fraction depends on the annealing temperature during the heat-treatment. 
     The ferrite, when present in the final structure, is intercritical ferrite. 
     Therefore, the ferrite, when present, is inherited from the structure of the hot-rolled and annealed steel sheet, which is then cold-rolled and recrystallized. As a result, the ferrite has an average grain size of at most 1.5 µm . 
     The preferred heat-treatments performed on the cold-rolled steel sheets will now be described in further details. 
     In a first preferred heat-treatment, after holding at the annealing temperature T anneal  lower than or higher than Ae3, the cold-rolled steel sheet is cooled down to room temperature at a cooling rate Vc 2  comprised between 1° C./s and 70° C./s. 
     The cold-rolled steel sheet is cooled at the cooling rate Vc 2  to the room temperature, or cooled, at the cooling rate Vc 2 , to a holding temperature T H  comprised between 350° C. and 550° C. and held at the holding temperature T H  for a time between 10 s and 500 s. It was shown that such a thermal treatment, which facilitates the Zn coating by hot dip process for instance, does not affect the final mechanical properties. After the optional holding at the holding temperature T H , the cold-rolled steel sheet is cooled down to room temperature at a cooling rate Vc 3  comprised between 1° C./s and 70° C./s 
     Optionally, after cooling down to the room temperature, the cold rolled and heat-treated steel sheet is tempered at a temperature T t  comprised between 170 and 450° C. for a tempering time t t  comprised between 10 and 1200 s. 
     This treatment enables the tempering of martensite, which may be created during cooling to room temperature after the annealing. The martensite hardness is thus decreased and the ductility is improved. Below 170° C., the tempering treatment is not efficient enough. Above 450° C., the strength loss becomes high and the balance between strength and ductility is not improved anymore. 
     The structure of the cold-rolled and heat-treated steel sheet obtained with the first preferred heat-treatment consists of, in surface fraction:
     between 8% and 50% of retained austenite, having an average C content of at least 0.4%,   at most 80% of intercritical ferrite,   at most of 92% of martensite and/or bainite,   at most 1% of cementite.   

     The martensite consists of tempered martensite and/ or fresh martensite. 
     The structure may comprise bainite, especially carbides free bainite, containing less than 100 carbides per surface unit of 100 mm 2 . 
     The average size of the cementite particles is lower than 50 nm. 
     The ferrite and austenite fractions depend on the annealing temperature during the heat-treatment. 
     In a first variant of the first preferred heat-treatment, the annealing temperature T anneal  is lower than Ae3, and preferably such that the structure created upon annealing comprises between 40% and 80% of ferrite. 
     In this first variant, the final structure preferably comprises, in surface fraction:
     8% to 50% of retained austenite, having an average C content of at least 0.4% and an average Mn content of at least 1.3 \*Mn%,   40% to 80 % of intercritical ferrite, the ferrite grains having an average size of at most 1.5 µm,   at most 15% of martensite (consisting of tempered martensite and/ or fresh martensite) and/or bainite,   at most 0.3% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     In a second variant of the first preferred heat-treatment, the annealing temperature is higher than or equal to Ae3. 
     In this second variant, the final structure consists of:
     8% to 30% of retained austenite, having an average C content of at least 0.4%,   70% to 92% of martensite (consisting of tempered martensite and/ or fresh martensite) and/or bainite,   at most 1% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     In a second preferred heat-treatment, the cold-rolled steel sheet is subjected to a quenching and partitioning process. 
     To that end, after holding at the annealing temperature T anneal , the cold-rolled steel sheet is quenched from the annealing temperature T anneal  to a quenching temperature QT lower than the Ms transformation point of the austenite, at a cooling rate Vc 4  high enough to avoid the formation of ferrite and pearlite upon cooling. 
     The cooling rate Vc 4  to the quenching temperature QT is preferably at least 2° C./s. 
     During this quenching step, the austenite partly transforms into martensite. 
     The quenching temperature is selected between Mf+20° C. and Ms-20° C., depending on the desired final structure, especially on the fractions of partitioned martensite and retained austenite desired in the final structure. For each particular composition of the steel and each structure, one skilled in the art knows how to determine the Ms and Mf start and finish transformation points of the austenite by dilatometry. 
     If the quenching temperature QT is lower than Mf+20° C., the fraction of partitioned martensite in the final structure is too high. Moreover, if the quenching temperature QT is higher than Ms-20° C., the fraction of partitioned martensite in the final structure is too low, so that a high ductility will not be reached. 
     One skilled in the art knows how to determine the quenching temperature adapted to obtain a desired structure. 
     The cold-rolled steel sheet is optionally held at the quenching temperature QT for a holding time tQ comprised between 2 s and 200 s, preferably between 3 s and 7 s, so as to avoid the creation of epsilon carbides in martensite, that would result in a decrease in the ductility of the steel. 
     The cold-rolled steel sheet is then reheated to a partitioning temperature T P  comprised between 350° C. and 500° C., and maintained at the partitioning temperature T P  for a partitioning time t P  comprised between 3 s and 1000 s. During this partitioning step, the carbon diffuses from the martensite to the austenite thereby achieving an enrichment in C of the austenite. 
     If the partitioning temperature T P  is higher than 500° C. or lower than 350° C., the elongation of the final product is not satisfactory. 
     Optionally, the cold-rolled steel sheet is hot-dip coated in a bath at a temperature for example lower than or equal to 480° C. Any kind of coatings can be used and in particular, zinc or zinc alloys, like zinc-nickel, zinc-magnesium or zinc-magnesium-aluminum alloys, aluminum or aluminum alloys, for example aluminum-silicium. 
     Immediately after the partitioning step, or after the hot-dip coating step, if performed, the cold-rolled steel sheet is cooled to the room temperature, to obtain a cold-rolled and heat treated steel sheet. The cooling rate to the room temperature is preferably higher than 1° C./s, for example comprised between 2° C./s and 20° C./s. 
     The final structure of the cold-rolled and heat-treated steel sheet obtained through the second preferred heat-treatment mainly depends on the annealing temperature T anneal  and on the quenching temperature QT. 
     However, the structure of the cold-rolled and heat-treated steel sheet thus obtained generally consists of, in surface fraction:
     between 8% and 30% of retained austenite,   at most 45% of intercritical ferrite,   partitioned martensite,   at most 8% of fresh martensite,   at most 1% of cementite.   

     The retained austenite is enriched in carbon, especially has an average C content of at least 0.4%. 
     The ferrite, if any, is intercritical ferrite, and has an average grain size of at most 1.5 µm. 
     The fraction of fresh martensite in the structure is lower than or equal to 8%. Indeed, a fraction of fresh martensite higher than 8% would impair the hole expansion ratio HER. 
     In this second preferred heat-treatment, a small fraction of cementite may be created upon cooling from the annealing temperature and during partitioning. However, the cementite fraction in the final structure will in any case remain lower than 1% and the average size of the cementite particles in the final structure remains lower than 50 nm. 
     In a first variant of the second preferred embodiment, the annealing temperature T anneal  is such that the cold-rolled steel sheet has a structure, upon annealing, consisting of, in surface fraction:
     between 10% and 45 % of ferrite,   austenite, and   at most 0.3% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     In this first variant, the final structure preferably comprises, in surface fraction:
     between 10% and 45% of intercritical ferrite, having an average grain size of at most 1.5 µm   between 8% and 30% of retained austenite,   partitioned martensite,   at most 8% of fresh martensite, and   at most 0.3% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     The retained austenite is enriched in Mn and C. Especially, the average C content in the retained austenite is of at least 0.4%, and the average Mn content in the retained austenite is of at least 1.3 \*Mn%. 
     In a second variant of the second preferred embodiment, the annealing temperature T anneal  is higher than or equal to Ae3, so that that the cold-rolled steel sheet has a structure, upon annealing, consisting of austenite and at most 0.3% of cementite. 
     In this second variant, the quenching temperature QT is preferably selected so as to obtain, just after quenching, a structure consisting of at most between 8% and 30% of austenite, at most 92% of martensite and at most 1% of cementite. 
     In this second variant, the final structure consists of, in surface fraction:
     between 8% and 30% of retained austenite,   partitioned martensite,   at most 8% of fresh martensite, and   at most 1% of cementite, the cementite particles, if any, having an average size lower than 50 nm.   

     The retained austenite is enriched in C, the average C content in the retained austenite being of at least 0.4%. 
     The microstructural features described above are for example determined by observing the microstructure with a Scanning Electron Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification greater than 5000x, coupled to an Electron Backscatter Diffraction (“EBSD”) device and to a Transmission Electron Microscopy (TEM). 
     EXAMPLES 
     As examples and comparison, sheets made of steels compositions according to table I, have been manufactured, the contents being expressed by weight percent. 
     
       
         
          TABLE 1
           
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 Steel 
                 C (%) 
                 Mn (%) 
                 S (%) 
                 P (%) 
                 Si (%) 
                 Al (%) 
                 Mo (%) 
                 Cr (%) 
                 Nb (%) 
                 Ti (%) 
                 B (%) 
                 N (%) 
               
             
            
               
                 I1 
                 0.174 
                 3.8 
                 0.001 5 
                 0.0130 
                 1.52 
                 0.757 
                 0.2 
                 0 
                 0.03 
                 &lt;0.00 5 
                 &lt;0.000 5 
                 0.012 7 
               
               
                 I2 
                 0.114 
                 4.78 
                 &lt;0.00 1 
                 0.014 
                 0.46 5 
                 1.58 
                 &lt;0.00 5 
                 &lt;0.00 5 
                 0.03 
                 &lt;0.00 5 
                 &lt;0.000 5 
                 0.003 
               
               
                 I3 
                 0.188 
                 4.04 
                 0.001 2 
                 0.013 
                 1.19 
                 0.781 
                 0.2 
                 0.505 
                 0.022 
                 0.04 
                 0.0022 
                 0.004 7 
               
               
                 I4 
                 0.109 
                 5.17 
                 0.003 
                 0.015 
                 0.507 
                 1.81 
                 &lt;0.00 5 
                 &lt;0.00 5 
                 &lt;0.00 2 
                 &lt;0.01 
                 &lt;0.000 5 
                 0.005 
               
               
                 I5 
                 0.127 
                 4.96 
                 0.001 9 
                 &lt;0.01 
                 0.51 
                 1.76 
                 &lt;0.00 5 
                 &lt;0.00 5 
                 0.027 
                 &lt;0.01 
                 &lt;0.000 5 
                 0.002 
               
               
                 I6 
                 0.18 
                 4.01 
                 0.002 3 
                 &lt;0.01 
                 1.51 
                 0.03 3 
                 0.207 
                 &lt;0.00 5 
                 &lt;0.00 2 
                 0.017 
                 0.0026 
                 0.002 8 
               
               
                 I7 
                 0.146 
                 3.78 
                 0.001 
                 0.009 
                 1.46 
                 0.79 
                 0.187 
                 &lt;0.00 5 
                 0.058 
                 &lt;0.01 
                 &lt;0.000 5 
                 0.005 
               
            
           
         
       
     
     In a first experiment, steels I1, I2, I3, I6 and I7 were cast so as to obtain ingots. The ingots were reheated at a temperature T reheat  of 1250° C., de-scaled and hot-rolled at a temperature higher than Ar3 to obtain hot rolled steels. 
     The hot-rolled steels were then cooled at a cooling rate V c1  comprised between 1° C./s and 150° C. to a coiling temperature T coil  and coiled at this temperature T coil . 
     Some of the hot-rolled steels were then either continuously annealed or batch annealed at an annealing temperature T A  for an annealing time t A  then cooled down to room temperature with an average cooling rate V ICA  between 600° C. and 350° C. 
     The manufacturing conditions of the hot-rolled and annealed steel sheets are reported in Table 2 below, as well as the austenite fraction created upon annealing. 
     
       
         
          TABLE 2
           
               
               
               
               
               
               
               
               
             
               
                 Example 
                   
                 Steel 
                 Tcoil (°C) 
                 T ICA  (°C) 
                 Austenite fraction upon annealing (%) 
                 t ICA  (s) 
                 V ICA  (°C/s) 
               
             
            
               
                 1 
                 I1A 
                 I1 
                 450 
                 no annealing 
               
               
                 2 
                 I1B 
                 I1 
                 450 
                 500 
                 0 
                 25200 
                 0.028 
               
               
                 3 
                 I1C 
                 I1 
                 450 
                 600 
                 0 
                 25200 
                 0.028 
               
               
                 4 
                 I1D 
                 I1 
                 450 
                 650 
                 5 
                 25200 
                 0.028 
               
               
                 5 
                 I1E 
                 I1 
                 450 
                 680 
                 11 
                 25200 
                 0.028 
               
               
                 6 
                 I1F 
                 I1 
                 450 
                 700 
                 25 
                 120 
                 30 
               
               
                 7 
                 I1G 
                 I1 
                 450 
                 720 
                 34 
                 120 
                 30 
               
               
                 8 
                 I2A 
                 I2 
                 450 
                 no annealing 
               
               
                 9 
                 I2B 
                 I2 
                 450 
                 500 
                 2.2 
                 25200 
                 0.028 
               
               
                 10 
                 I2C 
                 I2 
                 450 
                 600 
                 8.7 
                 25200 
                 0.028 
               
               
                 11 
                 I2D 
                 I2 
                 450 
                 650 
                 22.6 
                 25200 
                 0.028 
               
               
                 12 
                 I2H 
                 I2 
                 20 
                 650 
                 0 
                 720 
                 70 
               
               
                 13 
                 I2J 
                 I2 
                 20 
                 700 
                 28.5 
                 3600 
                 70 
               
               
                 14 
                 I2K 
                 I2 
                 450 
                 700 
                 26.9 
                 120 
                 70 
               
               
                 15 
                 I3A 
                 I3 
                 450 
                 no annealing 
               
               
                 16 
                 I3B 
                 I3 
                 450 
                 500 
                 0 
                 25200 
                 0.028 
               
               
                 17 
                 I3C 
                 I3 
                 450 
                 600 
                 0 
                 25200 
                 0.028 
               
               
                 18 
                 I3D 
                 I3 
                 450 
                 650 
                 9.8 
                 25200 
                 0.028 
               
               
                 19 
                 I3E 
                 I3 
                 450 
                 680 
                 23.8 
                 25200 
                 0.028 
               
               
                 20 
                 I3L 
                 I3 
                 20 
                 550 
                 0 
                 720 
                 70 
               
               
                 21 
                 I3H 
                 I3 
                 20 
                 650 
                 0 
                 720 
                 70 
               
               
                 22 
                 I3M 
                 I3 
                 20 
                 700 
                 n.d. 
                 120 
                 70 
               
               
                 23 
                 I3N 
                 I3 
                 20 
                 700 
                 n.d. 
                 360 
                 70 
               
               
                 24 
                 I3O 
                 I3 
                 20 
                 700 
                 n.d. 
                 720 
                 70 
               
               
                 25 
                 I3P 
                 I3 
                 20 
                 700 
                 n.d. 
                 1800 
                 70 
               
               
                 26 
                 I3J 
                 I3 
                 20 
                 700 
                 18.2 
                 3600 
                 70 
               
               
                 27 
                 I3Q 
                 I3 
                 20 
                 750 
                 45 
                 120 
                 70 
               
               
                 28 
                 I6A 
                 I7 
                 450 
                 no annealing 
               
               
                 29 
                 I6C 
                 I7 
                 450 
                 600 
                 0 
                 25200 
                 0.028 
               
               
                 30 
                 I6D 
                 I7 
                 450 
                 650 
                 15 
                 25200 
                 0.028 
               
               
                 31 
                 I6K 
                 I7 
                 450 
                 700 
                   
                 120 
                 70 
               
               
                 32 
                 I7A 
                 I8 
                 450 
                 no annealing 
               
               
                 33 
                 I7C 
                 I8 
                 450 
                 600 
                 0 
                 25200 
                 0.028 
               
               
                 34 
                 I7D 
                 I8 
                 450 
                 650 
                 6 
                 25200 
                 0.028 
               
               
                 35 
                 I7K 
                 I8 
                 450 
                 700 
                 n.d. 
                 120 
                 70 
               
               
                 36 
                 I2L 
                 I2 
                 20 
                 660 
                 4.3 
                 300 
                 0.03 
               
               
                 37 
                 I2M 
                 I2 
                 20 
                 660 
                 4.3 
                 300 
                 0.05 
               
               
                 38 
                 I2N 
                 I2 
                 20 
                 660 
                 4.3 
                 300 
                 0.1 
               
               
                 39 
                 I2O 
                 I2 
                 20 
                 660 
                 4.3 
                 300 
                 1 
               
               
                 40 
                 I2P 
                 I2 
                 20 
                 660 
                 4.3 
                 300 
                 2.5 
               
               
                 41 
                 I2Q 
                 I2 
                 20 
                 660 
                 4.3 
                 300 
                 5 
               
               
                 42 
                 I2R 
                 I2 
                 20 
                 660 
                 4.3 
                 300 
                 10 
               
               
                 43 
                 I6L 
                 I6 
                 20 
                 660 
                 12 
                 300 
                 0.03 
               
               
                 44 
                 I6M 
                 I6 
                 20 
                 660 
                 12 
                 300 
                 0.05 
               
               
                 45 
                 I6N 
                 I6 
                 20 
                 660 
                 12 
                 300 
                 1 
               
               
                 46 
                 I6O 
                 I6 
                 20 
                 660 
                 12 
                 300 
                 2.5 
               
               
                 47 
                 I6P 
                 I6 
                 20 
                 660 
                 12 
                 300 
                 5 
               
               
                 48 
                 I6Q 
                 I6 
                 20 
                 660 
                 12 
                 300 
                 10 
               
            
           
         
       
     
     In Table 2, the underlined values are not according to the present disclosure, and “n.d.” means “not determined”. 
     The inventors have investigated the microstructures of the hot-rolled and optionally annealed steel sheets thus obtained with a Scanning Electron Microscope with a Field Emission Gun (“FEG-SEM”) at a magnification of 5000x, coupled to an Electron Backscatter Diffraction (“EBSD”) device and to a Transmission Electron Microscopy (TEM). 
     Especially, the inventors measured the ferrite grain size, the surface fraction of fresh martensite (FM), the surface fraction of austenite (RA) and the average Mn content in the cementite (Mn% in cementite). 
     The inventors have further measured the Charpy energy at 20° C. and the Vickers hardness of the hot-rolled steel sheets. The features of the microstructures and the mechanical properties are reported in Table 3 below. 
     
       
         
          TABLE 3
           
               
               
               
               
               
               
               
               
             
               
                 Example 
                   
                 Ferrite grain size (µm) 
                 FM (%) 
                 Austenite fraction at the end of soaking (%) 
                 Mn% in cementite (%) 
                 Charpy toughness at 20° C. (J/cm 2 ) 
                 Hardness 
               
             
            
               
                 1 
                 I1A 
                 &lt;3 
                 &lt;8 
                 n.d. 
                 n.d. 
                 40 
                 424 
               
               
                 2 
                 I1B 
                 &lt;3 
                 &lt;8 
                 0 
                 58 
                 18 
                 364 
               
               
                 3 
                 I1C 
                 &lt;3 
                 &lt;8 
                 0 
                 44 
                 19 
                 328 
               
               
                 4 
                 I1D 
                 &gt; 3 
                 &lt;8 
                 5 
                 32 
                 20 
                 272 
               
               
                 5 
                 I1E 
                 6 
                 &lt;8 
                 11 
                 24 
                 45 
                 255 
               
               
                 6 
                 I1F 
                 &lt;3 
                 &lt;8 
                 25 
                 15 
                 65 
                 340 
               
               
                 7 
                 I1G 
                 &lt;3 
                 9 
                 34 
                 n.d. 
                 39 
                 430 
               
               
                 8 
                 I2A 
                 &lt;3 
                 &gt; 8 
                 n.d. 
                 n.d. 
                 98 
                 429 
               
               
                 9 
                 I2B 
                 &lt;3 
                 &lt;8 
                 2.2 
                 70 
                 43 
                 363 
               
               
                 10 
                 I2C 
                 &lt;3 
                 &lt;8 
                 8.7 
                 41 
                 45 
                 320 
               
               
                 11 
                 I2D 
                 &gt; 3 
                 &lt;8 
                 22.6 
                 n.d. 
                 84 
                 298 
               
               
                 12 
                 I2H 
                 &lt;3 
                 &lt;8 
                 0 
                 17.7 
                 108 
                 337 
               
               
                 13 
                 I2J 
                 &lt;3 
                 2 
                 28.5 
                 n.d. 
                 175 
                 311 
               
               
                 14 
                 I2K 
                 &lt;3 
                 &lt;8 
                 26.9 
                 n.d. 
                 140 
                 334 
               
               
                 15 
                 I3A 
                 &lt;3 
                 &gt; 8 
                 n.d. 
                 n.d. 
                 70 
                 458 
               
               
                 16 
                 I3B 
                 &lt;3 
                 &lt;8 
                 0 
                 49 
                 12 
                 n.d. 
               
               
                 17 
                 I3C 
                 &lt;3 
                 &lt;8 
                 0 
                 39 
                 4 
                 n.d. 
               
               
                 18 
                 I3D 
                 &gt; 3 
                 &lt;8 
                 9.8 
                 31 
                 21 
                 n.d. 
               
               
                 19 
                 I3E 
                 &gt; 3 
                 &gt; 8 
                 23.8 
                 23 
                 24 
                 n.d. 
               
               
                 20 
                 I3L 
                 &lt;3 
                 &lt;8 
                 0 
                 &lt;25 
                 24 
                 435 
               
               
                 21 
                 I3H 
                 &lt;3 
                 &lt;8 
                 0 
                 20 
                 50 
                 380 
               
               
                 22 
                 I3M 
                 &lt;3 
                 &lt;8 
                 n.d. 
                 &lt;15 
                 65 
                 386 
               
               
                 23 
                 I3N 
                 &lt;3 
                 &lt;8 
                 n.d. 
                 &lt;15 
                 82 
                 n.d. 
               
               
                 24 
                 I3O 
                 &lt;3 
                 &lt;8 
                 n.d. 
                 &lt;15 
                 89 
                 n.d. 
               
               
                 25 
                 I3P 
                 &lt;3 
                 &lt;8 
                 n.d. 
                 &lt;15 
                 95 
                 n.d. 
               
               
                 26 
                 I3J 
                 &lt;3 
                 2 
                 18.2 
                 &lt;15 
                 86 
                 n.d. 
               
               
                 27 
                 I3Q 
                 &lt;3 
                 29 
                 45 
                 nd 
                 26 
                 461 
               
               
                 28 
                 I6A 
                 &lt;3 
                 &lt;8 
                 nd 
                 nd 
                 65 
                 484 
               
               
                 29 
                 I6C 
                 &lt;3 
                 &lt;8 
                 0 
                 33 
                 14 
                 293 
               
               
                 30 
                 I6D 
                 n.d. 
                 n.d. 
                 15 
                 23 
                 31 
                 240 
               
               
                 31 
                 I6K 
                 &lt;3 
                 &lt;8 
                 n.d. 
                 n.d. 
                 n.d. 
                 n.d. 
               
               
                 32 
                 I7A 
                 &lt;3 
                 &lt;8 
                 nd 
                 nd 
                 71 
                 444 
               
               
                 33 
                 I7C 
                 &lt;3 
                 &lt;8 
                 0 
                 45 
                 6.8 
                 344 
               
               
                 34 
                 I7D 
                 n.d. 
                 n.d. 
                 6 
                 35 
                 28 
                 271 
               
               
                 35 
                 I7K 
                 &lt;3 
                 &lt;8 
                 n.d. 
                 n.d. 
                 n.d. 
                 n.d. 
               
               
                 36 
                 I2L 
                 &lt;3 
                 &lt;8 
                 4.3 
                 &lt;25 
                 37 
                 302 
               
               
                 37 
                 I2M 
                 &lt;3 
                 &lt;8 
                 4.3 
                 &lt;25 
                 38 
                 305 
               
               
                 38 
                 I2N 
                 &lt;3 
                 &lt;8 
                 4.3 
                 &lt;25 
                 41 
                 307 
               
               
                 39 
                 I2O 
                 &lt;3 
                 &lt;8 
                 4.3 
                 &lt;25 
                 50 
                 311 
               
               
                 40 
                 I2P 
                 &lt;3 
                 &lt;8 
                 4.3 
                 &lt;25 
                 51 
                 311 
               
               
                 41 
                 I2Q 
                 &lt;3 
                 &lt;8 
                 4.3 
                 &lt;25 
                 52 
                 311 
               
               
                 42 
                 I2R 
                 &lt;3 
                 &lt;8 
                 4.3 
                 &lt;25 
                 53 
                 311 
               
               
                 43 
                 I6L 
                 &lt;3 
                 &lt;8 
                 12 
                 &lt;25 
                 46 
                 286 
               
               
                 44 
                 I6M 
                 &lt;3 
                 &lt;8 
                 12 
                 &lt;25 
                 49 
                 290 
               
               
                 45 
                 I6N 
                 &lt;3 
                 &lt;8 
                 12 
                 &lt;25 
                 75 
                 301 
               
               
                 46 
                 I6O 
                 &lt;3 
                 &lt;8 
                 12 
                 &lt;25 
                 85 
                 301 
               
               
                 47 
                 I6P 
                 &lt;3 
                 &lt;8 
                 12 
                 &lt;25 
                 88 
                 301 
               
               
                 48 
                 I6Q 
                 &lt;3 
                 &lt;8 
                 12 
                 &lt;25 
                 90 
                 301 
               
            
           
         
       
     
     In this Table, n.d. means “not determined”. The underlines values are not according to the present disclosure. 
     These experiments shows that only when the hot-rolled steel sheets annealed under the conditions of embodiments of the present disclosure are the targeted microstructure and the targeted mechanical properties of the hot-rolled and annealed steel sheets achieved. 
     By contrast, examples I1A, I2A, I3A, I6A and I7A were not subjected to any annealing. 
     As a result, their hardness is higher than 400 HV, so that the cold-rollability of these hot-rolled steel sheets is insufficient. 
     Examples I1B, I2Band I3Bwere batch annealed at a temperature of 500° C. for a time of 25200 s. The batch annealing resulted in a decrease in hardness as compared to examples I1A, I2Aand I3A respectively, not subjected to any annealing. However, the batch annealing resulted in a decrease in the Charpy energy, so that the processability of examples I1B, I2Band I3Bis insufficient. In addition, the batch annealing resulted in the creation of cementite highly enriched in Mn. 
     Example I1C, I2C, I3C, I6Cand 7C were also subjected to a batch annealing, at a temperature of 600° C. for 25200 s. As a result of the batch annealing, the hardness of these examples decreased, as compared to examples I1A, I2A, I3A, I6Aand I7Arespectively, and further decreased as compared to examples I1B, I2Band I3B. However, the Charpy energy remained lower than 50 J/cm 2 , and the batch annealing resulted in the creation of cementite highly enriched in Mn. 
     The inventors then performed experiments by increasing the batch annealing temperature to 650° C., above the Ae1 transformation point (examples I1D, I2D, I3D, I6D and I7D). This higher batch annealing temperature resulted in an increase in the Charpy energy of the sheets, and to a decrease in the average Mn content in cementite, as compared to examples I1C, I2C, I3C, I6Cand I7C respectively. 
     Nevertheless, the batch annealing at a temperature above Ae1 resulted in a coarsening of the microstructure, the ferrite grain size being higher than 3 µm. 
     The inventors further increased the batch annealing temperature to 680° C. (examples I1E and I3E). This increase in the batch annealing temperature resulted in a further increase of the Charpy energy and to a further decrease of the average Mn content in cementite. However, this increase in the batch annealing temperature also resulted in a further undesired increase in the ferrite grain size. 
     These examples thus show that, even if the batch annealing reduces the hardness of the hot-rolled steel sheet, the Chary energy of the hot-rolled and batch annealed steel sheets is generally insufficient to ensure a high processability of the steel sheets. In addition, the batch annealing results in an undesired creation of cementite highly enriched in Mn. These examples further show that, though the increase of the batch annealing temperature may result in an increase in the Charpy energy and to a decrease in the average Mn content in the cementite, the Charpy energy remains in most of the cases lower than the targeted value of 50 J/cm 2 , and the increase in the batch annealing temperature leads to an undesired coarsening of the microstructure. 
     Example I3Lwas subjected to a continuous annealing, with however a continuous annealing temperature lower than 650° C. Consequently, softening through microstructure recovery was insufficient, so that the hardness of example I3Lis higher than 400 HV and the Charpy energy insufficient. 
     Examples I1G and I3Q were continuously annealed with an annealing temperature such that more than 30% of austenite was created upon annealing. As a result, the fresh martensite fraction in the hot-rolled and annealed steel sheets is higher than 8%, so that the hardness of these examples is higher than 400 HV and their Charpy energy lower than 50 J/cm 2 . 
     Examples I1F, I2H, I2J, I2K, I3H, I3M, I3, I3O, I3P, I3J, I6Kand I7Kwere subjected to a continuous annealing under the conditions of embodiments of the present disclosure. Consequently, the hot-rolled and annealed steel sheets have a Charpy energy at 20° C. of at least 50 J/cm2 and a hardness lower than or equal to 400 HV. These hot-rolled and annealed steel sheets have therefore satisfactory cold-rollability and processability. In addition, the microstructure of these examples is such that the average ferrite grain size is lower than 3 µm, and the average Mn content in the cementite is lower than 25%. Consequently, these hot-rolled steel sheets are suitable for producing cold-rolled and heat-treated steel sheets having high mechanical properties. 
     The microstructures of the hot-rolled and annealed steel sheet thus obtained were observed. 
     The microstructure of examples I1E and I1F are shown on  FIGS.  1  and  2    respectively. 
     As visible on these figures, the microstructure of steel I1F, produced with a continuous annealing according to an embodiment of the present disclosure, is much finer than the microstructure of steel I1E, produced with a batch annealing above Ae1. 
     These experiments demonstrate that unlike the batch annealing, the continuous annealing according to an embodiment of the present disclosure results in a very fine microstructure. 
     The inventors have further performed experiments to evaluate the final properties of cold-rolled and heat-treated steels produced from batch annealing at a temperature lower than Ae1 or higher than Ae1, or subjected to a continuous annealing according to an embodiment of the present disclosure before cold-rolling. 
     Especially, steels I1, I2, I4, I5, I6 and I7 were cast so as to obtain ingots. The ingots were reheated at a temperature T reheat  of 1250° C., descaled and hot-rolled at a temperature higher than Ar3 to obtain a hot rolled steel. 
     The hot-rolled steel sheets were then coiled at a temperature T coil . 
     The hot-rolled steels sheets were then either batch annealed or continuously annealed. 
     The hot-rolled and annealed steel sheets were then cold-rolled with a cold-rolling reduction ratio of 50%, and subjected to various heat-treatments, comprising annealing then cooling down to room temperature at a cooling rate Vc 1 . 
     The yield strength, the tensile strength, the uniform elongation and the hole expansion ratio of the cold-rolled and heat-treated steel sheets thus obtained where then measured. 
     The manufacturing conditions and the measured properties are reported in Tables 4 and 5. 
     In these tables, T coil  designates the coiling temperature, T A  and t A  are the batch or continuous annealing temperature and time, HBA refers to batch annealing, ICA refers to the continuous annealing according to an embodiment of the present disclosure, T anneal  is the annealing temperature, t anneal  is the annealing time and VC 1  the cooling rate (or the cooling conditions). 
     The measured properties reported in Tables 4 and 5 are the yield strength YS, the tensile strength TS, the uniform elongation UE and the hole expansion ratio HER. 
     In these tables, “n.d.” means “not determined”. The underlined values are not according to the present disclosure. 
     
       
         
          TABLE 4
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 Ex. 
                 T coil  (°C) 
                 TA (°C) 
                 t A  (min) 
                 T anneal  (°C) 
                 tanneal (s) 
                 Vc 1  (°C/s) 
                 YS (MPa) 
                 TS (MPa) 
                 UE (%) 
                 HER (%) 
               
             
            
               
                 I1Fa 
                 450 
                 700 (ICA) 
                 2 
                 730 
                 240 
                 25 
                 748 
                 1229 
                 14.1 
                 n.d. 
               
               
                 I1Fb 
                 450 
                 700 (ICA) 
                 2 
                 710 
                 240 
                 25 
                 775 
                 1043 
                 22 
                 n.d. 
               
               
                 I2Vc 
                 450 
                 600 (HBA) 
                 900 
                 720 
                 120 
                 20 
                 814 
                 965 
                 17.6 
                 23 
               
               
                 I2Kc 
                 450 
                 700 (ICA) 
                 2 
                 902 
                 1024 
                 19.6 
                 22 
               
               
                 I2Vd 
                 450 
                 600 (HBA) 
                 900 
                 730 
                 120 
                 20 
                 758 
                 982 
                 16 
                 19 
               
               
                 I2Kd 
                 450 
                 700 (ICA) 
                 2 
                 870 
                 1071 
                 17.9 
                 18 
               
               
                 I2Ve 
                 450 
                 600 (HBA) 
                 900 
                 740 
                 120 
                 20 
                 734 
                 1045 
                 14.6 
                 15 
               
               
                 I2Ke 
                 450 
                 700 (ICA) 
                 2 
                 817 
                 1098 
                 16.8 
                 16 
               
               
                 I4Tf 
                 550 
                 600 (HBA) 
                 300 
                 710 
                 120 
                 Air 
                 739 
                 810 
                 17.3 
                 n.d. 
               
               
                 I4Tq 
                 550 
                 600 (HBA) 
                 300 
                 730 
                 120 
                 Air 
                 650 
                 953 
                 17.2 
                 n.d. 
               
               
                 I4Ug 
                 550 
                 700 (ICA) 
                 2 
                 733 
                 955 
                 21.5 
                 n.d. 
               
               
                 I4Th 
                 550 
                 600 (HBA) 
                 300 
                 740 
                 120 
                 Air 
                 624 
                 989 
                 16.9 
                 n.d. 
               
               
                 I4Uh 
                 550 
                 700 (ICA) 
                 2 
                 690 
                 1015 
                 18.2 
                 n.d. 
               
               
                 I4Ti 
                 550 
                 600 (HBA) 
                 300 
                 750 
                 120 
                 Air 
                 528 
                 1021 
                 10.5 
                 n.d. 
               
               
                 I4Ui 
                 550 
                 700 (ICA) 
                 2 
                 611 
                 1070 
                 15.4 
                 n.d. 
               
               
                 I4Ti 
                 550 
                 600 (HBA) 
                 300 
                 760 
                 120 
                 Air 
                 453 
                 1076 
                 10.6 
                 n.d. 
               
               
                 I4Tk 
                 550 
                 600 (HBA) 
                 300 
                 770 
                 120 
                 Air 
                 516 
                 1138 
                 8.7 
                 n.d. 
               
               
                 I5Wd 
                 600 
                 600 (HBA) 
                 300 
                 730 
                 120 
                 20 
                 877 
                 1066 
                 18.2 
                 19.2 
               
               
                 I5Xd 
                 20 
                 600 (HBA) 
                 300 
                 868 
                 1063 
                 17.8 
                 22 
               
               
                 l5Kd 
                 450 
                 700 (ICA) 
                 2 
                 914 
                 1034 
                 21.7 
                 18.6 
               
               
                 I5We 
                 600 
                 600 (HBA) 
                 300 
                 740 
                 120 
                 20 
                 843 
                 1091 
                 17.1 
                 16.4 
               
               
                 I5Xe 
                 20 
                 600 (HBA) 
                 300 
                 824 
                 1078 
                 16 
                 19 
               
               
                 I5Ke 
                 450 
                 700 (ICA) 
                 2 
                 807 
                 1102 
                 15.6 
                 15.3 
               
               
                 I5Wl 
                 600 
                 600 (HBA) 
                 300 
                 750 
                 120 
                 20 
                 776 
                 1111 
                 15.3 
                 17 
               
               
                 I5Xl 
                 20 
                 600 (HBA) 
                 300 
                 809 
                 1100 
                 18.1 
                 13.4 
               
               
                 I5Kl 
                 450 
                 700 (ICA) 
                 2 
                 849 
                 1056 
                 20.2 
                 14 
               
               
                 I6Kb 
                 450 
                 700 (ICA) 
                 2 
                 710 
                 240 
                 25 
                 778 
                 1352 
                 16 
                 nd 
               
               
                 I6Fm 
                 450 
                 700 (ICA) 
                 2 
                 690 
                 240 
                 25 
                 918 
                 1169 
                 22.3 
                 nd 
               
               
                 I7Ka 
                 450 
                 700 (ICA) 
                 2 
                 730 
                 240 
                 25 
                 844 
                 1235 
                 14.4 
                 nd 
               
               
                 I7Kb 
                 450 
                 700 (ICA) 
                 2 
                 710 
                 240 
                 25 
                 932 
                 1105 
                 19.4 
                 nd 
               
            
           
         
       
     
     
       
         
          TABLE 5
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 Ex. 
                 T coil  (°C) 
                 TA (°C) 
                 t A  (min ) 
                 T anneal  (°C) 
                 t anneal  (s) 
                 Vc 1  (°C/s) 
                 TQ (°C) 
                 PT (°C) 
                 tP (s) 
                 YS (MPa) 
                 TS (MPa) 
                 UE (%) 
                 HER (%) 
               
             
            
               
                 I3Yn 
                 450 
                 600 (HBA) 
                 300 
                 840 
                 120 
                 10 
                 150 
                 450 
                 220 
                 1216 
                 1332 
                 11 
                 24.5 
               
               
                 I3Zo 
                 450 
                 700 (ICA) 
                 10 
                 770 
                 120 
                 10 
                 40 
                 450 
                 220 
                 1098 
                 1291 
                 12.3 
                 nd 
               
               
                 I3Zp 
                 450 
                 700 (ICA) 
                 10 
                 830 
                 120 
                 10 
                 90 
                 450 
                 220 
                 1318 
                 1361 
                 10.8 
                 26.8 
               
               
                 I3Zq 
                 450 
                 700 (ICA) 
                 10 
                 130 
                 450 
                 220 
                 1247 
                 1356 
                 11.6 
                 26 
               
            
           
         
       
     
     The properties of the examples made of steel I4 are reported on  FIG.  3    (UTS designating the tensile strength and UEI designating the uniform elongation). 
     On this figure, each curve corresponds to an annealing condition after hot-rolling (black squares: batch annealing at 600° C. for 300 min; white squares: continuous annealing at 700° C. for 2 min), and each point of each curve reports the tensile strength and the uniform elongation obtained with a particular annealing temperature, it being understood that the higher the annealing temperature, the higher the tensile strength. 
     The results reported on  FIG.  3    and in Table 4 demonstrate that performing the continuous annealing of the present disclosure allows achieving an improved combination of tensile strength and elongation as compared to batch annealing. 
     Thus, the steel sheets manufactured according to the present disclosure can be used with profit for the fabrication of structural or safety parts of vehicles.