EP1819461B1 - Method of producing austentic iron/carbon/manganese steel sheets having very high strength and elongation characteristics and excellent homogeneity - Google Patents

Description

The present invention relates to the manufacture of hot-rolled and cold-rolled sheets of austenitic iron-carbon-manganese steels having very high mechanical characteristics, and in particular a very advantageous combination of mechanical strength and elongation at break combined with excellent homogeneity. mechanical properties.

In the automotive field, changes in the level of equipment in vehicles makes it even more necessary to lighten the metal structure itself. For this, each function must be redesigned to improve its performance and reduce its weight. Different families of steels have thus been developed in order to satisfy these ever-increasing requirements: in chronological order, mention will be made, for example, of high yield strength steels hardened by fine precipitation of niobium, vanadium or titanium, structural steels Dual-Phase (ferrite containing up to 25% martensite), “TRIP” steels composed of ferrite, martensite and austenite liable to transform under deformation (“Transformation Induced Plasticity”) For each type of structure, the tensile strength and deformability are antagonistic properties, so that it is generally not possible to obtain very high values for one of the properties without drastically reducing the other. Thus, for TRIP steels, it is difficult to simultaneously obtain a strength greater than 900 MPa and an elongation greater than 25%. Mention will also be made of steels with a bainitic or martensitic-bainitic structure, the resistance of which can reach 1200 MPa on hot-rolled sheets, but where the elongation is only of the order of 10%. If these characteristics can be satisfactory for certain applications, they nevertheless remain insufficient in the case where additional lightening is desired by the simultaneous combination of a high resistance and a great aptitude for the subsequent deformation operations and for the energy absorption.

In the case of hot-rolled sheets, that is to say of thickness ranging from from 1 to 10 mm, such characteristics are used to lighten ground connection parts, wheels, reinforcement parts such as door intrusion bars, or those intended for heavy vehicles (trucks , bus). For cold-rolled sheets (ranging from approximately 0.2 mm to 6 mm), the applications are aimed at manufacturing parts contributing to the safety and durability of motor vehicles or even external parts. To meet these simultaneous strength and ductility requirements, steels with an austenitic structure are known, such as Fe-C steels (up to 1.5%) - Mn (15 to 35%) (contents expressed by weight) and possibly containing other elements such as silicon, aluminum or chromium: At a given temperature, the mode of deformation of austenitic steels depends only on the energy of stacking defect or "EDE", physical quantity which itself only depends on the composition and the temperature: When the EDE decreases, one passes successively from a mode of deformation by sliding of the dislocations, then by twinning, and finally by martensitic transformation. Among these modes, the mechanical twinning makes it possible to obtain a great capacity of work hardening: by obstructing the propagation of the dislocations, the twins participate in the increase of the flow limit. The EDE increases in particular with the carbon and manganese content.

Austenitic steels Fe-0.6% C-22% Mn are thus known which are capable of deformation by twinning: Depending on the grain size, these steel compositions lead to tensile strength values ranging from 900 to 1150 MPa approximately. , in combination with a breaking strain ranging from 50 to 80%. There is, however, an unresolved need for hot or cold rolled steel sheets, with a strength significantly greater than 1150 MPa, also having good deformation capacity, and this without the addition of expensive alloys. It is sought to have steel sheets having a very homogeneous behavior during subsequent mechanical stresses.

FR-A-2 829 775 describes a process for manufacturing a welded tube, of the type comprising a final stretching or hydroforming step, characterized in that: an alloy is produced; a semi-finished product is then cast from this alloy, a) either in the form of an ingot which is then roughed by hot rolling to transform it into a slab, or directly in the form of a slab said slab then being hot rolled in the form of a strip and then wound, b) either in the form of a thin strip; the strip is then stripped if the strip is oxidized at the surface; we then proceed to manufacture the welded tube by progressive forming of a sheet metal cut from the previous strip to bring its edges until docking, then by welding of said edges, then by elimination of the weld bead, then by cold drawing or hydroforming.

The object of the invention is therefore to dispose of a sheet or a hot or cold rolled steel product, of economical manufacture, having a resistance greater than or equal to 1200 or even 1400 MPa in combination with a elongation such as product P: resistance (MPa) x elongation at rupture (%) is greater than 60,000 or 50,000 MPa% respectively at the resistance level mentioned above, a great homogeneity of mechanical properties during deformations or subsequent mechanical stresses and a structure free of martensite at any point during or after the deformation cold from this sheet or this product.

To this end, the subject of the invention is a hot-rolled sheet of austenitic iron-carbon-manganese steel whose resistance is greater than 1200 MPa, the product P of which (resistance (MPa) x elongation at break (%)) is greater than 65,000 MPa%, the nominal chemical composition of which includes, the contents being expressed by weight: 0.85% ≤ C ≤ 1.05%, 16% ≤ Mn ≤ 19%, Si ≤ 2%, Al ≤ 0.050%, S ≤ 0.030%, P≤ 0.050%, N ≤ 0.1%, and optionally, one or more elements chosen from: Cr ≤ 1%, Mo ≤ 1.50%, Ni ≤ 1%, Cu ≤ 5% , Ti ≤ 0.50%, Nb ≤ 0.50%, V ≤ 0.50%, the rest of the composition consisting of iron and unavoidable impurities resulting from the production, the surface fraction recrystallized from steel being equal to 100%, the surface fraction of precipitated carbides of the steel being equal to 0%, the average grain size of the steel being less than or equal to 10 microns.

The invention also relates to a cold-rolled and annealed sheet of austenitic iron-carbon-manganese steel whose strength is greater than 1250 MPa, whose product P (strength (MPa) x elongation at break (%)) is greater at 65,000 MPa%, the nominal chemical composition of which includes, the contents being expressed by weight: 0.85% ≤ C ≤ 1.05%, 16% ≤ Mn ≤ 19%, Si ≤ 2%, Al ≤ 0.050%, S ≤ 0.030%, P≤ 0.050%, N ≤ 0.1%, and optionally, one or more elements chosen from: Cr ≤ 1%, Mo ≤ 1.50%, Ni ≤ 1%, Cu ≤ 5%, Ti ≤ 0.50%, Nb ≤ 0.50%, V ≤ 0.50%, the rest of the composition consisting of iron and unavoidable impurities resulting from the production, the recrystallized surface fraction of the steel being equal to 100%, the average grain size of the steel being less than 3 microns.

The local carbon content C _L of the steel, and the local manganese content Mn _L , expressed by weight, at all points of the austenitic steel sheet, are such that:% Mn _L + 9.7% C _L ≥21.66 Preferably, the nominal silicon content of the steel is less than or equal to 0.6%

According to a preferred mode, the nominal nitrogen content of the steel is less than or equal to 0.050%.

Also preferably, the nominal aluminum content of the steel is less than or equal to 0.030%.

• on procède à la coulée d'un demi-produit à partir de cet acier
• on porte le demi-produit de la composition d'acier à une température comprise entre 1100 et 1300°C,
• on lamine le demi-produit jusqu'à une température de fin de laminage supérieure ou égale à 900°C
• on observe si nécessaire un temps d'attente de telle sorte que la fraction surfacique recristallisée de l'acier soit égale à 100%,
• on refroidit la tôle à une vitesse supérieure ou égale à 20°C/s,
• on bobine la tôle à une température inférieure ou égale à 400°C,

caractérisé en ce qu'on applique, sur la tôle laminée à chaud, refroidie après bobinage et déroulée, une déformation à froid avec un taux de déformation équivalente supérieur ou égal à 13% et inférieur ou égal à 17%According to a preferred mode, the nominal phosphorus content of the steel is less than or equal to 0.040%
The invention also relates to a process for manufacturing a hot-rolled sheet of austenitic iron-carbon-manganese steel whose resistance is greater than 1400 MPa, of which the product P ((resistance (MPa) x elongation at break ( %)) is greater than 50,000 MPa%, according to which a steel is produced whose nominal composition comprises, the contents being expressed by weight: 0.85% ≤ C ≤ 1.05%, 16% ≤ Mn ≤ 19%, Si ≤ 2%, Al ≤ 0.050%, S ≤ 0.030%, P≤ 0.050%, N ≤ 0.1%, and optionally, one or more elements chosen from: Cr ≤ 1%, Mo ≤ 1.50%, Ni ≤ 1%, Cu ≤ 5%, Ti ≤ 0.50%, Nb ≤ 0.50%, V ≤ 0.50%, the rest of the composition consisting of iron and unavoidable impurities resulting from the production ,

• we proceed to the casting of a semi-finished product from this steel
• the semi-finished product of the steel composition is brought to a temperature between 1100 and 1300 ° C.,
• the semi-finished product is rolled up to a temperature at the end of rolling greater than or equal to 900 ° C.
• a waiting time is observed if necessary so that the recrystallized surface fraction of the steel is equal to 100%,
• the sheet is cooled at a speed greater than or equal to 20 ° C / s,
• the sheet is coiled at a temperature less than or equal to 400 ° C.,

characterized in that a cold deformation is applied to the hot rolled sheet, cooled after winding and unwound, with an equivalent deformation rate greater than or equal to 13% and less than or equal to 17%

The invention also relates to a process for manufacturing a cold-rolled and annealed sheet of austenitic iron-carbon-manganese steel, the strength of which is greater than 1250 MPa, of which the product P (resistance (MPa) x elongation at rupture (%)) is greater than 60,000 MPa%, characterized in that a hot-rolled sheet obtained by the above process is supplied, at least one cycle is carried out, each cycle consisting of cold rolling the sheet in one or several successive passes and then perform recrystallization annealing, the average austenitic grain size before the last cold rolling cycle followed by recrystallization annealing, being less than 15 microns.

The invention also relates to a process for manufacturing a cold-rolled and annealed sheet of austenitic iron-carbon-manganese steel whose resistance is greater than 1400 MPa, of which the product P (resistance (MPa) x elongation at break (%)) is greater than 50,000 MPa%, characterized in that, after the final recrystallization annealing, cold deformation is carried out with an equivalent deformation rate greater than or equal to 6%, and less than or equal to 17%.

The subject of the invention is also a method of manufacturing a cold-rolled sheet of austenitic iron-carbon-manganese steel whose resistance i is greater than 1400 MPa, of which the product P (resistance (MPa) x elongation at break ( %)) is greater than 50,000 MPa%, characterized in that a cold rolled and annealed sheet is supplied according to the invention, and that a cold deformation of this sheet is carried out with a higher equivalent deformation rate or equal to 6%, and less than or equal to 17%.

The invention also relates to a process for manufacturing an austenitic steel sheet, characterized in that the conditions for casting or heating said semi-finished product, such as the temperature for casting said semi-finished product. semi-finished product, the mixing of the liquid metal by electromagnetic forces, the heating conditions leading to a homogenization of carbon and manganese by diffusion, are chosen so that, at any point on the sheet, the local carbon content C _L and the local manganese content Mn _L , expressed by weight, are such that:% Mn _L + 9.7% C _L ≥21.66

According to a preferred embodiment, the casting of the semi-finished product is carried out in the form of casting slabs or thin strips between counter-rotating steel cylinders.

The invention also relates to the use of an austenitic sheet steel for the manufacture of reinforcing or structural elements or of external parts, in the automotive field.

The invention also relates to the use of an austenitic steel sheet manufactured by means of a process described above, for the manufacture of reinforcing or structural elements or external parts, in the automotive field.

Other characteristics and advantages of the invention will appear during the description below, given by way of example and made with reference to the figure 1 annexed which presents the theoretical variation of the stacking defect energy at room temperature (300 ° K) as a function of the carbon and manganese content.

After numerous tests, the inventors have shown that the various requirements reported above are satisfied by observing the following conditions:
Regarding the chemical composition of steel, carbon plays a very important role on the formation of the microstructure and the mechanical properties obtained: In combination with a manganese content ranging from 16 to 19% by weight, a nominal content in carbon greater than 0.85% provides a stable austenitic structure. However, for a nominal carbon content greater than 1.05%, it becomes difficult to avoid a precipitation of carbides which occurs during certain thermal cycles of industrial manufacture, in particular during cooling on the winding, and which degrades the ductility and tenacity. In addition, increasing the carbon content decreases the weldability.

Manganese is also an essential element for - increasing resistance, increasing the energy of stacking defect and stabilizing the austenitic phase. If its nominal content is less than 16%, there is, as will be seen below, a risk of formation of martensitic phase which decreases most notably the ability to deform. Furthermore, when the nominal manganese content is greater than 19%, the twinning deformation mode is less favored compared to the sliding mode of perfect dislocations. In addition, for cost reasons, it is undesirable that the manganese content is high.

Aluminum is a particularly effective element for the deoxidation of steel. Like carbon, it increases the stacking fault energy. However, its excessive presence in steels with a high manganese content has a drawback. Manganese increases the solubility of nitrogen in liquid iron, and if too much aluminum is present in the steel, the nitrogen combining with the aluminum precipitates in the form of aluminum nitrides. interfering with the migration of grain boundaries during hot processing and very significantly increases the risk of cracks appearing. A nominal content of Al less than or equal to 0.050% makes it possible to avoid precipitation of AIN. Correlatively, the nominal nitrogen content must be less than or equal to 0.1% in order to avoid this precipitation and the formation of volume defects during solidification. This risk is particularly reduced when the nominal aluminum content is less than 0.030% as well as when the nominal nitrogen content is less than 0.050%.

Silicon is also an effective element for deoxidizing steel as well as for hardening in the solid phase. However, beyond a nominal content of 2%, it reduces the elongation and tends to form undesirable oxides during certain assembly processes and must therefore be kept below this limit. This phenomenon is greatly reduced when the nominal silicon content is less than 0.6%.

Sulfur and phosphorus are impurities which weaken grain boundaries. Their respective nominal content must be less than or equal to 0.030 and 0.050% in order to maintain sufficient hot ductility. When the nominal phosphorus content is less than 0.040%, the risk of brittleness is particularly reduced.

Chromium can be used as an option to increase the strength of the steel by hardening in solid solution. However, since chromium decreases the stacking defect energy, its nominal content must be lower or equal to 1%. Nickel increases the stacking defect energy and contributes to obtaining a significant elongation at break. However, it is also desirable, for cost reasons, to limit the nominal nickel content to a maximum content less than or equal to 1%. Molybdenum can also be used for similar reasons, this element further delaying the precipitation of carbides. For efficiency and cost reasons, it is desirable to limit its nominal content to 1.5%, and preferably to 0.4%.

Likewise, optionally, adding copper to a nominal content less than or equal to 5% is a means of hardening the steel by precipitation of metallic copper. However, beyond this content, copper is responsible for the appearance of hot sheet surface defects. Titanium, niobium and vanadium are also elements which can be used optionally to obtain hardening by precipitation of carbonitrides. However, when the nominal Nb or V or Ti content is greater than 0.50%, excessive precipitation of carbonitrides can cause a reduction in ductility and drawability, which should be avoided.

The implementation of the manufacturing process according to the invention is as follows: A steel is produced, the composition of which has been set out above. This production can be followed by casting in ingots, or continuously in the form of slabs with a thickness of the order of 200 mm. The casting can also be carried out in the form of thin slabs a few tens of millimeters thick, or of thin strips, between counter-rotating steel cylinders. Of course, if the present description illustrates the application of the invention to flat products, it can be applied in the same way to the manufacture of long products made of Fe-C-Mn steel.

These cast semi-finished products are first brought to a temperature of between 1100 and 1300 ° C. This is intended to reach at all points the temperature ranges favorable to the high deformations that the steel will undergo during rolling. However, the temperature should not be higher than 1300 ° C, on pain of being too close to the solidus temperature which could be reached in possible areas segregated into manganese and / or carbon, and of causing a start of local passage by a liquid state which would be harmful for hot forming. In the case of a direct casting of thin strips between counter-rotating cylinders, the stage of hot rolling of these semi-finished products starting between 1300 and 1100 ° C. can be done directly after casting so that a stage of reheating intermediary is not necessary in this case.

The conditions for the preparation of semi-finished products (casting, reheating) have a direct influence on the possible segregation of carbon and manganese, this point will be detailed later.

The semi-finished product is hot rolled, for example to obtain a thickness of hot rolled strip of a few millimeters. The low aluminum content of the steel according to the invention makes it possible to avoid excessive precipitation of AIN which would harm hot deformability during rolling. In order to avoid any cracking problem due to lack of ductility, the end of rolling temperature must be greater than or equal to 900 ° C.

The inventors have demonstrated that the ductility properties of the sheets obtained are reduced when the recrystallized surface fraction of the steel is less than 100%. Consequently, if the hot rolling conditions have not led to a total recrystallization of the austenite, the inventors have demonstrated that it is necessary to observe, after the hot rolling phase, a time of waiting so that the recrystallized surface fraction is equal to 100%. This isothermal high-temperature maintenance phase after rolling thus causes total recrystallization.

For hot-rolled sheets, it has also been shown that it is necessary to prevent precipitation of carbides (essentially cementite (Fe, Mn) ₃ C, and perlite) from occurring. which results in a deterioration of the mechanical properties in particular by a decrease in ductility and an increase in the elastic limit. For this purpose, the inventors have discovered that a cooling rate after the rolling phase (or after the possible waiting time necessary for recrystallization) greater than or equal to 20 ° C./s makes it possible to completely avoid this precipitation. . This cooling phase is followed by a winding. It has also been demonstrated that the winding temperature should be less than 400 ° C, also to avoid precipitation.

For steel compositions according to the invention, the inventors have used evidence that particularly high strength and elongation at break properties are obtained when the average austenitic grain size was less than or equal to 10 microns. Under these conditions, the breaking strength of the hot sheets thus obtained is greater than 1200 MPa and the product P (resistance x elongation at break) is greater than 65000 MPa%.

There are applications where it is desired to obtain even higher resistance characteristics on hot-rolled sheets, at a level greater than or equal to 1400 MPa. The inventors have demonstrated that such characteristics are obtained by giving the hot-rolled steel sheets described above, a cold deformation with an equivalent deformation rate greater than or equal to 13%, and less than or equal at 17%. This cold deformation is therefore imparted to a sheet which is cooled after winding, unwound, and usually pickled. This deformation of a relatively low rate leads to the manufacture of a product with reduced anisotropy without affecting the subsequent processing. Thus, although the process includes a cold deformation step, the sheet produced can be qualified as “hot rolled sheet” insofar as the rate of cold deformation is very minimal in comparison with the usual rates achieved during rolling. cold before annealing for the production of thin sheets, and insofar as the thickness of the sheet thus produced is within the usual range of thicknesses of hot-rolled sheets. However, when the equivalent cold deformation rate is greater than 17%, the reduction in elongation becomes such that the parameter P (resistance R x elongation at break A) cannot reach 50,000 MPa%. Under the conditions of the invention, despite its very high resistance, the sheet retains a good elongation capacity since the product P of the sheet thus obtained is greater than or equal to 50,000 MPa%.

For cold-rolled and annealed sheets, the inventors have also demonstrated that the structure must be completely recrystallized after annealing in order to achieve the desired properties. Simultaneously, when the average grain size is less than 5 microns, the resistance exceeds 1200 MPa, and the product P is greater than 65000 MPa%. When the average grain size obtained after annealing is less than 3 microns, the resistance exceeds 1250 MPa, the product P always being greater than 65000MPa%.

• Un laminage à froid en une ou plusieurs passes sucessives
• Un recuit de recristallisation,

la taille moyenne de grain austénitique avant le dernier cycle de laminage à froid subi d'un recuit de recristallisation étant inférieure à 15 microns.The inventors have also discovered a process for manufacturing cold-rolled and annealed steel sheets of resistance greater than 1250 MPa and of product P greater than 60,000 MPa%, this being carried out by supplying hot-rolled sheets according to the process described above. above, then by performing at least one cycle, each cycle consisting of the following steps:

• Cold rolling in one or more successive passes
• A recrystallization annealing,

the average austenitic grain size before the last cold rolling cycle undergoing recrystallization annealing being less than 15 microns.

We may wish to obtain a cold rolled sheet with an even higher resistance, greater than 1400 MPa. The inventors have demonstrated that such properties could be obtained by supplying a cold-rolled sheet having the characteristics according to the invention described above, or by supplying a cold-rolled sheet obtained by the process according to the invention described below. -above. The inventors have discovered that the application of cold deformation to such a sheet with an equivalent deformation rate greater than or equal to 6%, and less than or equal to 17%, makes it possible to achieve a resistance greater than 1400 MPa and a product P greater than 50,000 MPa%. When the equivalent cold deformation rate is greater than 17%, the reduction in elongation becomes such that the parameter P cannot reach 50,000 MPa%.

We will now detail the particularly important role played by carbon and manganese in the context of the present invention. We will refer for this to the figure 1 , which presents, in a carbon-manganese diagram (and iron complement) the calculated iso-energy curves of stacking defect whose values range from 5 to 30mJ / m ² . At a deformation temperature and for a given grain size, the deformation mode is theoretically identical for any Fe-C-Mn alloy having the same EDE. The area of occurrence of martensite has also been shown in this diagram.

The inventors have shown that, in order to assess the mechanical behavior, it is necessary to consider not only the nominal chemical composition of the alloy, for example its nominal or average content. carbon and manganese, but also its local content.

We know that, during the production of steel, solidification causes more or less marked segregation of certain elements. This is because the solubility of an element in the solid phase is different from that in the liquid phase. We will thus frequently witness the formation of solid seeds whose solute content is lower than the nominal composition, the last phase of solidification involving a residual liquid phase enriched in solute. This primary solidification structure can take on different morphologies (for example dendritic or equiaxed) and be more or less marked. Even if these characteristics are modified by rolling and subsequent heat treatments, an analysis of the local elementary content indicates a fluctuation around a value corresponding to the average or nominal content of this element.

By local content is meant here the content measured by means of a device such as an electronic probe. A linear or surface scan using such a device makes it possible to appreciate the variation in the local content.

We thus measured the variation of the local content of an Fe-C-Mn alloy whose nominal composition is: C = 0.23%, Mn = 24%, Si = 0.203%, N = 0.001%. The inventors have demonstrated a co-segregation of carbon and manganese, the areas locally enriched (or depleted) in carbon also correspond to the areas enriched (respectively depleted) in manganese. Each measured point having a local concentration of carbon (C _L ) and manganese (Mn _L ) has been reported within the figure 1 , the assembly forming a segment representing the local variation in carbon and manganese in the steel sheet, centered on the nominal content (C = 0.23%, Mn = 24%). In this case, it appears that the variation of the local carbon and manganese content results in a variation of the stacking defect energy, since this value goes from 7mJ / m ² for the zones richest in C and in Mn up to around 20 mJ / m ² for the richest areas. We also know that twinning occurs as a preferred deformation mode at room temperature when the EDE is around 15-30mJ / m ² . In the exposed case, this preferred mode of deformation may not be absolutely present throughout the steel sheet and certain particular zones may possibly exhibit behavior. mechanical different from that expected for a steel sheet of nominal composition, in particular a reduced ability to deformation by twinning within certain grains. More generally, it is understood that, under very specific conditions depending for example on the deformation or stress temperature, on the grain size, the local carbon and manganese content can be reduced to the point of locally causing an induced martensitic transformation. by deformation.

The inventors have sought the specific conditions for obtaining very high mechanical characteristics simultaneously with a great homogeneity of these characteristics within a steel sheet. As explained above, the combination of carbon (0.85% -1.05%) and manganese (16-19%) associated with the other characteristics of the invention leads to resistance values greater than 1200MPa and to a product (resistance x elongation at break) greater than 60,000, or even 65,000 MPa%. We will observe at the figure 1 that these steel compositions are in a field where the EDE is of the order of 19-24 mJ / m ² , that is to say favorable to deformation by twinning. However, the inventors have also demonstrated that a variation in the local carbon or manganese content has a much smaller influence than that mentioned in the previous example. Indeed, measurements of variations in local contents (C _L , Mn _L ) carried out on different compositions of austenitic steels Fe-C-Mn revealed, under identical manufacturing conditions, co-segregation of carbon and manganese very close from that illustrated in figure 1 . Under these conditions, a variation of the local contents (C _L , Mn _L ) has only little consequence with regard to the mechanical behavior, since the segment representing this co-segregation is located in a direction substantially parallel to the curves d 'iso-EDE.

In addition, the inventors have shown that it was absolutely necessary to avoid the formation of martensite during deformation operations or the use of sheets under penalty of heterogeneity of mechanical characteristics on the parts. The inventors have determined that this condition is satisfied when, at any point on the sheets, the local carbon and manganese contents of the sheet are such that:% Mn _L + 9.7% C _L ≥21.66. Thus, thanks to the characteristics of the nominal chemical composition defined by the invention and those defined by the local carbon and manganese contents, austenitic steel sheets are produced which not only have very high mechanical characteristics but also a very low dispersion of these characteristics.

By means of his knowledge, the person skilled in the art will adapt the manufacturing conditions so as to satisfy this relationship concerning the local contents, in particular by means of the casting conditions (casting temperature, stirring of the liquid metal by electromagnetic forces) or heating conditions leading to homogenization of carbon and manganese by diffusion.

In particular, it will be advantageous to use semi-finished product casting methods in the form of thin slabs (a few centimeters thick) or of thin strips, since these methods are generally associated with a reduction in the heterogeneities of local compositions.

By way of nonlimiting example, the following results will show the advantageous characteristics conferred by the invention.

Example:

Steels of the following nominal composition were developed (contents expressed as a percentage by weight): Table 1: Nominal composition of steels Steel VS Mn Yes S P Al Cu Cr Or Mo NOT I According to the invention 0.97 17.6 0.51 0.001 0.005 0.030 0.005 0.025 R1 Reference 0.61 21.5 0.49 0.001 0.016 0.003 0.02 0.053 0.044 0.009 0.01 R2 Reference 0.45 17.5 0.3 0.001 0.005 0.030 0.01

After casting, a semi-finished product of steel I according to the invention was reheated to a temperature of 1180 ° C and hot rolled to a temperature above 900 ° C to reach a thickness of 3 mm. A waiting time of 2 s was observed after rolling for complete recrystallization, then cooling was carried out at a speed greater than 20 ° C / s, followed by winding at room temperature.

The reference steels were reheated to a temperature above 1150 ° C, rolled to a rolling end temperature above 940 ° C and then coiled at a temperature below 450 ° C.

The recrystallized surface fraction is 100% for all steels, the fraction of precipitated carbides is equal to 0%, the average grain size between 9 and 10 microns.

The tensile characteristics of hot-rolled sheets are as follows: Table 2: Mechanical tensile properties of hot-rolled sheets Steel Resistance Elongation at break P = Strength x Elongation at break According to the invention I 1205 MPa 64% 77000 MPa% Reference R1 1010 MPa 65% 66180 MPa% Reference R2 1050 MPa 45% 47,250 MPa%

Compared to a reference steel R1, the mechanical properties of which are already high, the steel according to the invention makes it possible to obtain an increased resistance of approximately 200 MPa with very comparable elongation.

In order to evaluate the structural and mechanical homogeneity during a deformation, deep-drawn buckets were produced on which the microstructure was examined by X-ray diffraction. In the case of the reference steel R2, we note the appearance of martensite as soon as the deformation rate exceeds 17%, the total drawing operation leading to rupture. An analysis indicates that the characteristic:% Mn _L + 9.7% C _L ≥21.66 is not fulfilled at all points ( figure 1 ).

In the case of the steel of the invention, no trace of martensite is revealed, a similar analysis indicates that the characteristic:% Mn _L + 9.7% G _L ≥21.66 is satisfied at all points which avoids any appearance of martensite.

The steel sheet according to the invention was then subjected to a slight cold deformation by rolling with an equivalent deformation of 14%. The resistance of the product is then 1420 MPa, its elongation at break 42%, or a product P = 59640 MPa%. This product with exceptionally high mechanical characteristics offers great possibilities of subsequent deformation due to its reserve of plasticity and its low anisotropy.

Furthermore, after the winding, unwinding and pickling step, hot-rolled steel sheets according to the invention and steel R1 were then cold-rolled and then annealed so as to obtain a completely recrystallized structure. The average austenitic grain size, the resistance, the elongation at break were indicated in the table below. Table 3: Mechanical characteristics of cold-rolled and annealed sheets Steel Average grain size Resistance Elongation at break Product P (resistance x elongation at break) According to the invention I 4 microns 1289 MPa 58% 74760 MPa% Reference R1 3 microns 1130 MPa 55% 62150 MPa%

The steel sheet produced according to the invention, whose average grain size is 4 microns, therefore offers a particularly advantageous resistance-elongation combination and a significant increase in resistance compared to the reference steel. As with hot rolled sheets, these characteristics are obtained with very high homogeneity on the product, no trace of martensite is present after deformation.

Equibiaxial expansion tests on a hemispherical punch 75 mm in diameter carried out on a cold-rolled and annealed sheet 1.6 mm thick according to the invention, reveal a limit stamping height of 33 mm, which highlights a excellent deformability. Bending tests carried out on this same sheet also show that the critical deformation before the appearance of cracks is greater than 50%.

The steel sheet produced according to the invention was subjected to cold deformation by rolling with an equivalent deformation rate of 8%: The resistance of the product is then 1420 MPa, its elongation at break of 48%, ie a product P = 68 160 MPa%.

Thus, because of their particularly high mechanical characteristics, their very homogeneous mechanical behavior and their microstructural stability, hot-rolled or cold-rolled steels according to the invention will be used with advantage for applications where capacity is sought. significant deformation and very high strength. In the case of their use in the automotive industry, advantage will be taken of their advantages for the manufacture of structural parts, reinforcing elements or even external parts.

Claims (14)

1. Hot-rolled sheet made of iron-carbon-manganese austenitic steel, the strength of which is greater than 1,200 MPa, the product P of which (strength (MPa) x breaking elongation (%)) is greater than 65,000 MPa%, the nominal chemical composition of which comprises, the contents being expressed by weight:

   0.85% ≤ C ≤ 1.05%

   16% ≤ Mn ≤ 19%

   Si ≤ 2%

   Al ≤ 0.050%

   S ≤ 0.030%

   P ≤ 0.050%

   N ≤ 0.1%

   and optionally, one or more elements chosen from

   Cr ≤ 1%

   Mo ≤ 1.50%

   Ni ≤ 1%

   Cu ≤ 5%

   Ti ≤ 0.50%

   Nb ≤ 0.50%

   V ≤ 0.50%,

   the remainder of the composition being constituted by iron and unavoidable impurities resulting from the smelting, the recrystallised surface fraction of said steel being equal to 100%, the surface fraction of precipitated carbides of said steel being equal to 0%, the average grain size of said steel being less than or equal to 10 microns, and at any point of said sheet, the local content, of said steel, of carbon C_L and the local content of manganese Mn_L, expressed by weight, being such that %Mn_L + 9.7%C_L ≥ 21.66.
2. Cold-rolled and annealed sheet made of iron-carbon-manganese austenitic steel, the strength of which is greater than 1,250 MPa, the product P (strength (MPa) x breaking elongation (%)) is greater than 65,000 MPa%, of nominal chemical composition according to claim 1, the recrystallised surface fraction of the steel being equal to 100%, the average grain size of said steel being less than 3 microns and, at any point of said sheet, the local content of said steel of carbon C_L and the local content of manganese Mn_L, expressed by weight, being such that %Mn_L + 9.7%C_L ≥ 21.66.
3. Steel sheet according to any of the claims 1 to 2, characterised in that the nominal content of silicon of said steel is less than or equal to 0.6%.
4. Steel sheet according to any of the claims 1 to 3, characterised in that the nominal content of nitrogen of said steel is less than or equal to 0.050%.
5. Steel sheet according to any of the claims 1 to 4, characterised in that the nominal content of aluminium of said steel is less than or equal to 0.030%.
6. Steel sheet according to any of the claims 1 to 5, characterised in that the nominal content of phosphorus of said steel is less than or equal to 0.040%.
7. Production method of a hot-rolled sheet made of iron-carbon-manganese austenitic steel, the strength of which is greater than 1,400 MPa, the product P of which (strength (MPa) x breaking elongation (%)) is greater than 50,000 MPa%, according to which a steel is smelted, the nominal composition of which comprises, the contents being expressed by weight:

   0.85% ≤ C ≤ 1.05%

   16% ≤ Mn ≤ 19%

   Si ≤ 2%

   Al ≤ 0.050%

   S ≤ 0.030%

   P ≤ 0.050%

   N ≤ 0.1%

   and optionally, one or more elements chosen from

   Cr ≤ 1%

   Mo ≤ 1.50%

   Ni ≤ 1%

   Cu ≤ 5%

   Ti ≤ 0.50%

   Nb ≤ 0.50%

   V ≤ 0.50%,

   the remainder of the composition being constituted by iron and unavoidable impurities resulting from the smelting,

   - casting of a semi-finished product from this steel is implemented,

   - said semi-finished product of said steel composition is brought to a temperature between 1,100 and 1,300°C,

   - said semi-finished product is rolled until at a temperature at the end of rolling greater than or equal to 900°C,

   - if necessary a waiting time is observed such that the recrystallised surface fraction of the steel is equal to 100%,

   - said sheet is cooled at a speed greater than or equal to 20°C/s,

   - said sheet is coiled at a temperature less than or equal to 400°C,

   - there is applied, on said hot-rolled sheet, cooled after coiling and unrolled, a cold-deformation with a deformation rate equivalent to, greater than or equal to 13% and less than or equal to 17%.
8. Production method of a cold-rolled and annealed sheet made of iron-carbon-manganese austenitic steel, the strength of which is greater than 1,250 MPa, the product P of which (strength (MPa) x breaking elongation (%)) is greater than 60,000 MPa%, characterised in that:

   there is supplied a cooled and coiled hot-rolled sheet obtained by the method according to claim 7,

   at least one cycle is effected, each cycle consisting of:

   - cold-rolling said sheet in one or more successive passes,

   - effecting a recrystallisation annealing,

   the average austenitic grain size before the last cycle of cold-rolling followed by recrystallisation annealing being less than 15 microns.
9. Production method of a cold-rolled sheet made of iron-carbon-manganese austenitic steel, the strength of which is greater than 1,400 MPa, the product P of which (strength (MPa) x breaking elongation (%)) is greater than 50,000 MPa%, characterised in that:

   - there is provided a cooled and coiled hot-rolled sheet obtained by the method according to claim 7,

   - at least one cycle is effected, each cycle consisting of:

   - cold-rolling said sheet in one or more successive passes,

   - effecting a recrystallisation annealing, the average austenitic grain size before the last cold-rolling cycle followed by a recrystallisation annealing being less than 15 microns,

   - there is effected, after the final recrystallisation annealing, a cold-deformation with a deformation rate equivalent to, greater than or equal to 6%, and less than or equal to 17%.
10. Production method of a cold-rolled sheet made of iron-carbon-manganese austenitic steel, the strength of which is greater than 1,400 MPa, the product P of which (strength (MPa) x breaking elongation (%)) is greater than 50,000 MPa%, characterised in that a cold-rolled and annealed sheet according to any of the claims 2 to 6 is supplied and in that a cold-deformation of said sheet is effected with a deformation rate equivalent to, greater than or equal to 6%, and less than or equal to 17%.
11. Production method of an austenitic steel sheet according to any of the claims 7 to 10, characterised in that the casting or reheating conditions of said semi-finished product, such as the casting temperature of said semi-finished product, the agitation of the liquid metal by electromagnetic forces, the reheating conditions leading to homogenisation of the carbon and of the manganese by diffusion, are chosen so that, at any point of said sheet, the local content of carbon C_L and the local content of manganese Mn_L, expressed by weight, are such that: %Mn_L + 9.7%C_L ≥ 21.66.
12. Production method according to any of the claims 7 to 11, characterised in that the casting of said semi-finished product is effected in the form of casting of slabs or of thin strips between counter-rotating steel cylinders.
13. Use of an austenitic steel sheet according to any of the claims 1 to 6, for production of structural parts, reinforcing elements or even exterior parts, in the automobile field.
14. Use of an austenitic steel sheet produced by means of a method according to any of the claims 7 to 12 for production of structural pieces, reinforcing elements or even exterior parts, in the automobile field.

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