EP1844173A1 - Method for producing austenitic iron-carbon-manganese metal sheets, and sheets produced thereby - Google Patents

Abstract

The invention relates to an austenitic iron-carbon-manganese metal sheet whose chemical composition comprises the following contents expressed in weight: 0.45 % = C = 0.75 %, 15 % = Mn = 26 %, Si = 3 %, Al = 0.050 %, S = 0.030 %, P= 0.080 %, N = 0.1 %, at least one metallic element selected from the group consisting of vanadium, titanium, niobium, chromium, molybdenum 0.050 % = V = 0.50 %, 0.040 % = Ti = 0.50 %, 0.070 % = Nb = 0.50 %, 0.070 % = Cr = 2 %, 0.14 % = Mo = 2 % and, optionally, one or more elements selected among 0.0005 % = B = 0.003 %, Ni = 1 %, Cu = 5 %, the remainder of the composition consisting of iron and of unavoidable impurities resulting from the processing, the quantity of said at least one metallic element in the form of precipitated carbides, nitrides or carbonitrides being: 0.030 % = Vp = 0.150 %, 0.030 %= Tip = 0.130 %, 0.040 % = Nbp = 0.220 %, 0.070 % = Crp = 0.6 %, 0.14 % = Mop = 0.44 %.

Description

SHEETS MANUFACTURING METHOD ^1-CARBON STEEL AUSTENTIC IRON-MANGANESE AND SHEETS PRODUCED

The invention relates to the manufacture of hot and cold rolled sheets of austenitic iron-carbon-manganese steels having very high mechanical characteristics, and in particular mechanical strength.

10 high combined with excellent resistance to delayed cracking.

It is known that certain applications, especially in the automotive field, require increased lightening and strength of the metal structures in the event of impact as well as good stamping ability: This requires the use of structural materials combining a high resistance to the

Rupture and high deformability. To meet these needs, patent FR 2 829 775 for example discloses austenitic alloys having as main elements: iron-carbon (up to 2%) manganese (between 10 and 40%) capable of being hot-rolled or cold, with a resistance likely to exceed 1200 MPa. The mode of

The deformation of these steels depends only on the stacking fault energy: for a sufficiently high stacking failure energy, a mode of mechanical deformation by twinning is observed, which makes it possible to obtain a high capacitance. hardening. By obstructing the spread of dislocations, twins participate in the increase of

25 flow limit. However, when the stacking fault energy exceeds a certain threshold, the sliding of the perfect dislocations becomes the dominant deformation mechanism and the work hardening capacity is less. The aforementioned patent therefore discloses Fe-C-Mn steel grades whose stacking failure energy is such that an important work hardening is

Observed, combined with a very high mechanical strength.

However, it is known that sensitivity to delayed cracking increases with mechanical strength, in particular after some cold forming operations since significant residual stresses are likely to remain after deformation. In combination with atomic hydrogen possibly present in the metal, these stresses are likely to lead to delayed cracking, that is to say occurring a certain time after the deformation itself. Hydrogen can gradually accumulate by diffusion in lattice defects such as matrix / inclusion interfaces, twin joints and grain boundaries. It is in these that hydrogen can become harmful when it reaches a critical concentration after a certain time. This delay results from the distribution field of the residual stresses and from the diffusion kinetics of hydrogen, the diffusion coefficient of hydrogen at room temperature being low, more particularly in alloys with austenitic structure where the average path per second of this element is of the order of 0.03 micrometers. In addition, hydrogen located at grain boundaries weakens their cohesion and promotes the appearance of delayed intergranular cracks. There is therefore a need for hot or cold rolled steel simultaneously having a high strength and high ductility, combined with a very high resistance to delayed fracture. There is also a need for such steels in economic conditions, that is to say with manufacturing conditions compatible with the productivity requirements of existing industrial lines, as well as with acceptable costs for this type of products. . It is known in particular that it is possible to significantly reduce the hydrogen content by specific thermal degassing treatments. In addition to their additional cost, the thermal conditions of these treatments eventually lead to grain enlargement or precipitation of cementite in these steels, sometimes incompatible with the requirements in terms of mechanical properties. The object of the invention is therefore to provide a hot-rolled or cold-rolled steel sheet or product of economical manufacture, having a resistance greater than 900 MPa, an elongation at break greater than 50%, particularly suitable for cold forming and having a very high resistance to delayed cracking, without the particular need for a specific heat treatment for degassing.

For this purpose, the subject of the invention is an austenitic iron-carbon-manganese steel sheet, the chemical composition of which comprises the contents being expressed by weight: 0.45% <C <0.75%, 15% ≤ Mn <26%, Si <3%, Al <0.050%, S <0.030%, P <0.080%, N <0.1% at least one metal element selected from vanadium, titanium, niobium, chromium, molybdenum: 0.050% <V <0.50%, 0.040% <Ti ≤ 0.50%, 0.070% ≤ Nb ≤ 0, 50%, 0.070% ≤Cr <2%, 0.14% <Mo <2% and optionally one or more elements selected from 0.0005% <B <0.003%, Ni <1%, Cu <5%, the rest of the composition consisting of iron and unavoidable impurities resulting from the production, the amount of metal elements in the form of carbides, nitrides or carbonitrides precipitated being: 0.030% <V _P <0.150%, 0.030% < Ti _p ≤ 0.130%, 0.040% <Nb _p <0.220%, 0.070% <Cr _p <0.6%, 0.14% <Mo _p ≤ 0.44%.

Preferably, the composition of the steel comprises: 0.50% <C ≤ 0.70%

According to a preferred embodiment, the composition of the steel comprises: 17% <Mn <

24%

According to a preferred embodiment, the composition of the steel comprises 0.070% ≤V <0.40%, the amount of vanadium in the form of carbides, nitrides or carbonitrides precipitated being 0.070% <V _p <0.140%

As a preference, the composition of the steel comprises 0.060% ≤Ti <0.40%, the amount of titanium in the form of carbides, nitrides or carbonitrides precipitated being: 0.060% <Ti _p ≤ 0.110% The composition of the steel steel advantageously comprises 0.090% ≤Nb ≤ 0.40%, the amount of niobium in the form of carbides, nitrides or carbonitrides precipitated being: 0.090% <Nb _p <0.200%

Preferably, the composition of the steel comprises 0.20% ≤Cr≤ 1.8%, the amount of chromium in the form of precipitated carbides being 0.20% <Cr _p <0.5%

Preferably, the composition of the steel comprises 0.20% <Mo <1.8%, the amount of molybdenum in the form of precipitated carbides being 0.20% <Mo _p <0.35%

According to a preferred embodiment, the average size of the precipitates is between 5 and 25 nanometers, and more preferably between 7 and 20 nanometers

Advantageously, at least 75% of the population of said precipitates is located in intragranular position

The invention also relates to a method of manufacturing a sheet metal cold-rolled austenitic iron-carbon-manganese steel according to which is supplied a steel whose chemical composition comprises, the contents being expressed by weight: 0.45% <C <0.75%, 15% <Mn <26%, If <3%, Al <0.050%, S <0.030%, P <0.080%, N <0.1%, at least one metal element selected from vanadium, titanium, niobium, chromium, molybdenum: 0.050 % ≤V <0.50%, 0.040% ≤Ti <0.50%, 0.070% <Nb <0.50%, 0.070% ≤Cr <2%, 0.14% ≤Mo <2%, and optional one or more elements selected from 0.0005% ≤B <0.003%, Ni <1%, Cu <5%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the elaboration, it is proceeded to the casting of a semi-finished product from this steel, this semi-finished product is brought to a temperature of between 1100 and 1300 ^° C., this semi-finished product is hot-rolled to a higher end-of-rolling temperature or equal to 890 ^° C., the sheet obtained is reeled at a temperature of at least 580 ^° C., the sheet is cold-rolled and an annealing heat treatment is carried out comprising a heating phase with a heating rate Vc, a holding phase at a temperature Tm during a holding time tm, followed by a cooling phase at a cooling rate Vr, optionally followed by a holding phase at a temperature Tu during a holding time t _u , the parameters Vc, Tm, tm, Vr, Tu, t _u being adjusted to obtain the amount of precipitated metal elements mentioned above. According to a preferred embodiment, the parameters Vc, Tm, tm, Vr, Ty, t _u are adjusted so that the average size of the carbide, nitride or carbonitride precipitates after the annealing is between 5 and 25 nanometers, and preferentially between 7 and 20 nanometers.

The parameters Vc, Tm, tm, Vr, Tu, t _u are advantageously adjusted such that at least 75% of the population of the precipitates after the annealing is located in the intragranular position. In a preferred embodiment, a steel is procured whose chemical composition includes 0.050% <V <0.50%, is laminated to the hot semi-finished product to a temperature of the upper end lamination or equal to 950 ⁰ C, the sheet is reeled at a temperature below 500 ° C., the sheet is cold-rolled with a reduction ratio greater than 30%, an annealing heat treatment is carried out with a heating rate Vc of between 2 and 10 ° C / s, at a temperature Tm between 700 and 870 ⁰ C for a time between 30 and 180 s, and the sheet is cooled at a speed between 10 and 50 ° C / s. The heating rate Vc is preferably between 3 and 7 ° C./s. According to a preferred embodiment, the holding temperature Tm is between 720 and 850 ^° C.

The casting of the semi-finished product is advantageously carried out in the form of casting slabs or thin strips between counter-rotating steel rolls. The invention also relates to the use of an austenitic steel sheet described above or manufactured by a method described above, for the manufacture of structural parts, reinforcing elements or external parts. , in the automotive field.

Other features and advantages of the invention will become apparent from the description below, given by way of example. After numerous tests, the inventors have shown that the different requirements reported above can be satisfied by observing the following conditions: With regard to the chemical composition of steel, carbon plays a very important role in the formation of the microstructure and mechanical properties: it increases the stacking fault energy and promotes the stability of the austenitic phase. In combination with a manganese content ranging from 15 to 26% by weight, this stability is obtained for a carbon content greater than or equal to 0.45%. However, for a carbon content greater than 0.75%, it becomes difficult to avoid excessive precipitation of carbides during certain thermal cycles during industrial manufacture, a precipitation which degrades the ductility.

Preferably, the carbon content is between 0.50 and 0.70% by weight so as to obtain sufficient strength combined with optimum precipitation of carbides or carbonitrides. Manganese is also an essential element for increasing strength, increasing stacking fault energy and stabilizing the austenitic phase. If its content is less than 15%, there is a risk of formation of martensitic phases which significantly reduce the ability to deform. On the other hand, when the manganese content is greater than 26%, the ductility at room temperature is degraded. In addition, for questions of cost, it is not desirable that the manganese content be high.

Preferably, the manganese content is between 17 and 24% so as to optimize the stacking fault energy and to avoid the formation of martensite under the effect of a deformation. Moreover, when the manganese content is greater than 24%, the mode of deformation by twinning is less favored compared to the sliding mode of perfect dislocations. Aluminum is a very 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 disadvantage: in fact, manganese increases the solubility of nitrogen in the liquid iron. If too much aluminum is present in the steel, the nitrogen combining with the aluminum precipitates in the form of aluminum nitrides hindering the migration of the grain boundaries during the hot transformation and increases very significantly the risk of occurrence of cracks in continuous casting. In addition, as will be explained later, a sufficient amount of nitrogen must be available to form fine carbo-nitride precipitates for the most part. An Al content less than or equal to 0.050% avoids precipitation of AlN and maintains a sufficient nitrogen content for the precipitation of the elements mentioned below.

Correlatively, the nitrogen content must be less than or equal to 0.1% in order to prevent this precipitation and the formation of volume defects (blowholes) during solidification. In addition, in the presence of elements capable of precipitating in the form of nitrides, such as vanadium, niobium or titanium, the nitrogen content must not exceed 0.1% otherwise the risk of obtaining an ineffective coarse precipitation will be observed. with respect to the trapping of hydrogen. Silicon is also an effective element for deoxidizing steel as well as for hardening in the solid phase. However, beyond a content of 3%, it decreases the elongation, tends to form undesirable oxides during certain assembly processes and must therefore be kept below this limit. Sulfur and phosphorus are impurities that weaken the grain boundaries. Their respective content must be less than or equal to 0.030 and 0.080% in order to maintain sufficient hot ductility. Optionally, boron may be added in an amount of from 0.0005 to 0.003%. This element segregates at the austenitic grain boundaries and reinforces their cohesion. Below 0.0005%, this effect is not obtained. Above 0.003%, boron precipitates as borocarbons, and the effect is saturated. Nickel can be used as an option to increase the strength of the steel by hardening in solid solution. Nickel contributes to a high elongation break and increases in particular the toughness. However, it is also desirable for cost issues to limit the nickel content to a maximum content of less than or equal to 1%. Similarly, optionally, addition of copper to a content of 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 surface defects hot sheet. The metal elements capable of forming precipitates, such as vanadium, titanium, niobium, chromium, molybdenum, play an important role in the context of the invention.

Indeed, it is known that delayed cracking is caused by an excessive local concentration of hydrogen, in particular at the austenitic grain boundaries. The inventors have demonstrated that certain types of precipitates, the nature, quantity, size and distribution of which are precisely defined according to the invention, significantly reduce the sensitivity to delayed cracking, and this without reducing the properties ductility and tenacity. The inventors firstly demonstrated that carbides, nitrides or carbonitrides precipitated from vanadium, titanium or niobium, were very effective as hydrogen traps. Chromium carbides or molybdenum carbides can also play this role. At room temperature, the hydrogen is then irreversibly trapped at the interface between these precipitates and the matrix. It is however necessary, in order to ensure the trapping of the residual hydrogen which could be encountered under certain industrial conditions, that the quantity of metal elements in the form of precipitates is greater than or equal to a critical content, depending on the nature of the precipitates. The quantity of metal elements in the form of precipitates of carbides, nitrides, or carbonitrides is designated by V _p , Ti _p , Nb _p , respectively for vanadium, titanium and niobium, and Cr _p , Mo _p for chromium and molybdenum carbides.

As such, the steel comprises one or more metal elements chosen from: vanadium, in an amount of between 0.050 and 0.50% by weight, and whose quantity V _p in the form of precipitates is between 0.030% and 0.150 % in weight. Preferably, the vanadium content is between 0.070% and 0.40%, the amount V _p being between 0.070% and 0.140% by weight. titanium, in an amount Ti of between 0.040 and 0.50% by weight, the amount Ti _p in the form of precipitates being between 0.030% and 0.130%. Preferably, the titanium content is between 0.060% and 0.40%, the amount Ti _p being between 0.060% and 0.10% by weight. niobium, in an amount of between 0.070 and 0.50% by weight, the quantity Nb _p in the form of precipitates being between 0.040 and 0.220%. Preferably, the niobium content is between 0.090% and 0.40%, the amount Nb _p being between 0.090% and 0.200% by weight - chromium, in an amount of between 0.070% and 2% by weight, the amount Cr _p in the form of precipitates being between 0.070% and 0.6%. Preferably, the chromium content is between 0.20% and 1.8%, the amount Cr _p being between 0.20 and 0.5% - Molybdenum, in an amount between 0.14 and 2% weight, the amount Mo _p in the form of precipitates is between 0.14 and

0.44%. Preferably, the molybdenum content is between 0.20 and 1.8%, the amount Mo _p being between 0.20 and 0.35%. The minimum value expressed for these various elements (for example 0.050% for vanadium) corresponds to a quantity of addition necessary to form precipitates taking into account the thermal cycles of manufacture. A preferred minimum content (for example 0.070% for vanadium) is recommended, so as to obtain a larger quantity of precipitates. The maximum value expressed for these various elements (for example 0.50% for vanadium) corresponds to excessive precipitation, or in an inappropriate form, deteriorating the mechanical properties, or to an uneconomic implementation of the invention. A preferred maximum content (for example of 0.40% for vanadium) is recommended, so as to optimize the addition of the element.

The minimum value of metallic elements in the form of precipitates (for example 0.030% in the case of vanadium) corresponds to a quantity of precipitates for very effectively reducing the sensitivity to delayed cracking. A preferred minimum amount (for example 0.070% in the case of vanadium) is recommended, so as to obtain a particularly high resistance to delayed cracking.

The maximum value of metallic elements in the form of precipitates (for example 0.150% for vanadium) marks a deterioration of the ductility or the tenacity, the rupture starting on the precipitates. Furthermore, beyond this maximum value, intense precipitation occurs, which can prevent total recrystallization during continuous annealing thermal treatments after cold rolling. A preferred maximum content in the form of precipitates (for example 0.140% for vanadium) is recommended, so that the ductility is preserved as much as possible and that the precipitation obtained is compatible with the recrystallization under the usual annealing conditions. recrystallization. In addition, the inventors have demonstrated that a too large average size of precipitates reduces the efficiency of trapping. Here, mean size of precipitates is the size that can be measured, for example, from replicates with extraction, followed by observations by transmission electron microscopy: the diameter is measured (in the case of spherical or quasi-spherical precipitates) or the largest length (in the case of irregularly shaped precipitates) of each precipitate, then establishes a histogram of size distribution of these precipitates, the average of which is calculated from the count of a statistically representative number of particles. Beyond an average size of 25 nanometers, the efficiency of hydrogen scavenging decreases due to the decrease in the interface between precipitates and matrix. At a given precipitate amount, an average size of precipitates exceeding 25 nanometers also decreases the density of precipitates present, thereby excessively increasing the inter-site trapping distance. The trapping interfacial surface for hydrogen is also reduced. Preferably, the average size of precipitates is less than 20 nanometers in order to trap the largest amount of hydrogen possible.

However, when the average particle size is less than 5 nanometers, the precipitates will tend to form coherently with the matrix, thus reducing the trapping ability. The difficulty of controlling these very fine precipitates is also increased. These difficulties are optimally avoided when the average size of precipitates is greater than 7 nanometers. This average value can integrate the presence of many very fine precipitates, whose size is of the order of one nanometer. The inventors have also demonstrated that the precipitates are advantageously located in the intragranular position in order to reduce the sensitivity to delayed cracking: in fact, when at least 75% of the population of precipitates is located in the intragranular position, the distribution of hydrogen possibly present is more homogeneous, without accumulation at the austenitic grain boundaries which are potential sites of embrittlement. The addition of one of the aforementioned elements, in particular chromium, makes it possible to obtain a precipitation of various carbides such as MC, M ₇ C ₃ , M ₂₃ C ₆ , M ₃ C where M denotes not only the element metal but also Fe or Mn, elements present in the matrix. The presence of iron and manganese within the precipitates makes it possible to increase the quantity of precipitates at a lower cost, thus reinforcing the efficiency of the precipitation. The inventors have also demonstrated that additions of vanadium, which is precipitated in the form of VC vanadium carbides, vanadium nitrides VN, more or less complex carbonitrides V (CN), were particularly advantageous in the context of the invention.

Indeed, the object of the invention is to simultaneously dispose of steels with very high mechanical characteristics and insensitive to delayed fracture. As mentioned above in the context of the manufacture of a cold-rolled and annealed sheet, the steel should be completely recrystallized after the annealing cycle. Too early precipitation, for example at the stage of casting, hot rolling or winding, will be a potential brake on recrystallization and may harden the metal and increase the hot or cold rolling forces. It will also be less effective because it will intervene significantly on the austenitic grain boundaries. The size of these precipitates formed at high temperature will be larger, often greater than 25 nanometers. The inventors have shown that vanadium additions are particularly desirable insofar as the precipitation of this element hardly occurs during hot rolling or winding. In this way, the pre-existing adjustments of hot and cold rolling forces are not to be modified and all the vanadium is available for a very fine and homogeneous precipitation during the subsequent annealing cycle after cold rolling. The precipitation takes place in the form of VC and in the form of nanometric VN or V (CN) homogeneously distributed, the vast majority of the precipitates being located in the intragranular position, ie in the most desirable form for the entrapment of the nanoparticles. 'hydrogen. In addition, this fine precipitation limits the growth of the grain, a finer austenitic grain size can thus be obtained after annealing. The implementation of the manufacturing method according to the invention is as follows: A steel is produced whose composition comprises: 0.45% <C <0.75% 15% <Mn <26%, Si <3%, Al <0.050%, S <0.030, P <0.080%, N <0.1%, one or more elements selected from 0.050% <V <0.50%, 0.040% ≤Ti <0.50%, 0.070% <Nb <0.50%, 0.070% <Cr <2%, 0.14% <Mo <2%, and optionally one or more elements selected from 0.0005% <B <0.003%, Ni <1%, Cu <5%, the rest being iron and unavoidable impurities from the elaboration.

This development can be followed by casting in ingots, or continuously in the form of slabs of thickness of the order of 200 mm. It is also possible to advantageously perform the casting in the form of thin slabs, a few tens of millimeters thick, or thin strips of a few millimeters. When certain addition elements according to the invention such as titanium or niobium are present, the casting in the form of thin products will lead more particularly to a precipitation of nitrides or very thin and thermally stable carbonitrides, the presence of which reduces sensitivity to delayed cracking.

These cast semi-finished products are first brought to a temperature of between 1100 and 1300 ^° C. This is intended to achieve at all points the temperature ranges favorable to the high deformations which the steel will undergo during rolling. However, the reheating temperature must not be greater than 1300 ⁰ C, otherwise it will be too close to the solidus temperature that could be reached in possible zones enriched locally with manganese and / or carbon and cause a passage local by a liquid state that would be harmful for hot shaping. Naturally, in the case of direct casting of thin slabs, the hot rolling step of these semi-products starting between 1300 and 1000 ⁰ C can be done directly after casting without going through the intermediate heating step. The semi-finished product is hot-rolled, for example to obtain a thickness of hot rolled strip 2 to 5 millimeters thick, or even 1 to 5 mm in the case of semi-finished product from a thin slab casting. , or 0.5 to 3 mm in the case of a casting of thin strips. The low aluminum content of the steel according to the invention makes it possible to avoid excessive precipitation of AlN which would adversely affect the hot deformability during rolling. In order to avoid any problem of cracking due to lack of ductility, the end-of-lamination temperature must be greater than or equal to 890 ° C. After rolling, the strip must be wound at a temperature such that a precipitation of carbides, essentially intergranular cementitious (Fe, Mn) ₃ C), does not occur significantly, which would lead to a reduction of certain mechanical properties. This is obtained when the winding temperature is less than 580 ^° C. The elaboration conditions will also be chosen so that the product obtained is completely recrystallized. We can then proceed to a subsequent cold rolling followed by annealing. This additional step makes it possible to obtain a grain size smaller than that obtained on hot strip and thus to higher strength properties. It must naturally be implemented if one seeks to obtain products of thinner thickness, ranging for example from 0.2 mm to a few mm thick.

Starting from a hot rolled product obtained by the process described above, a cold rolling is carried out after possibly carrying out a preliminary etching in the usual manner. After this rolling step, the grain is very hardened, and it is necessary to perform a recrystallization annealing: this treatment has the effect of restoring the ductility and to obtain a precipitation according to the invention. This annealing, preferably carried out continuously, comprises the following sucessive steps:

A heating phase characterized by a heating rate Vc; a holding phase at a temperature Tm during a holding time tm;

A cooling phase at a cooling rate Vr,

Optionally a holding phase at a temperature Tu during a holding time t _u Before the optional phase of maintaining the temperature Tu, the product can optionally be cooled to room temperature. This phase of maintaining the temperature You can possibly be carried out within a separate device, for example a furnace for the static annealing of steel coils. The precise choice of the parameters Vc, Tm, tm, Vr, Tu, t _u is usually carried out in such a way that the desired mechanical properties are obtained, in particular thanks to a complete recrystallization. In addition, in the context of the invention the person skilled in the art will adjust in particular according to the cold rolling ratio, these so that the amount of metal elements (V, Ti, Nb, Cr, Mo) present in the form of carbides, nitrides or carbonitrides precipitated after annealing is included within the contents mentioned above ((V _p , Ti _p , Nb _p , Cr _p , Mo _p )

Those skilled in the art will also adjust these annealing parameters so that the average size of these precipitates is between 5 and 25 nanometers, and preferably between 7 and 20 nanometers.

These parameters can also be adjusted so that a large majority of the precipitation occurs homogeneously in the matrix, that is to say that the precipitates are at least 75% in the intragranular position. In particular, the invention will advantageously be used thanks to additions of vanadium. For this purpose, a steel of composition: 0.45% <C <0.75%, 15% ≤ Mn <26%, Si <3%, Al <0.050%, S <0.030%, P <0.080%, N <0.1%, 0.050% <V ≤ 0.50%, and optionally one or more elements selected from 0.0005% <B <0.003%, Ni <1%, Cu <5%, optimally a steel sheet according to the invention by casting a half-product, bringing it to a temperature between 1100 and 1300 ⁰ C, by hot rolling this half-product to a temperature of end of rolling greater than or equal to 950 ⁰ C, then conducting a winding at a temperature below 500 ⁰ C.

The sheet is cold rolled with a reduction rate greater than 30% (the reduction ratio being defined by: (thickness of the sheet before cold rolling - thickness of the sheet after cold rolling) / (thickness of the front plate The rate of 30% corresponds to a minimum deformation so as to obtain a recrystallization.An annealing heat treatment is then carried out with a heating rate Vc of between 2 and 10 ° C./s (preferably between 3 and 7 ° C.). ° C / s), at a temperature Tm between 700 and 870 ° C (preferably between 720 and 850 ° C) for a time between 30 and 180s and the sheet will be cooled at a speed between 10 and 50 ° C / s

In this way, a steel is obtained whose resistance is greater than 1000 MPa, whose elongation at break is greater than 50%, offering excellent resistance to delayed cracking due to the very fine and homogeneous precipitation of vanadium carbonitrides. In the case of additions of Cr or Mo according to the invention, it will be advantageous to carry out a temperature maintenance treatment subsequent to the recrystallization annealing so that the precipitation of nanometric carbides of chromium or molybdenum does not interact with recrystallization. This can be done on continuous annealing plants within a survivor zone immediately following the cooling phase mentioned above. Those skilled in the art will therefore adjust the parameters of this holding phase (Tu temperature, holding time t _u ) so as to obtain the precipitation of chromium and molybdenum carbides according to the invention. It is also possible to achieve this precipitation through annealing later in coils.

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

Example: Steels were produced whose composition is shown in the table below (compositions expressed in percentages by weight In addition to the steels 11 and 12, according to the invention, the composition of reference steels was indicated for comparison: R1 steel has a very low vanadium content.A cold rolled steel sheet of R2 steel, under the conditions detailed below, contains too much precipitates (see Table 2). R3 has an excessive content of vanadium.

Table 1: Composition of steels 11-2: according to the invention. R1-3: Reference

Semi-finished products of these steels were heated to 118O ⁰ C, hot rolled to a temperature of 950 ⁰ C to bring them to a thickness of 3mm and then wound at a temperature of 500 ° C.

The steel sheets thus obtained were then cold-rolled with a reduction rate of 50% up to a thickness of 1.5 mm, and then annealed under the conditions presented in Table 2. The quantity of metallic elements was determined. precipitated in the form of carbides, nitrides or carbonitrides, in these different sheets by chemical extraction and selective dosing. Given the compositions and the manufacturing conditions, these potential precipitates are here based on vanadium, mainly vanadium carbonitrides. The amount of vanadium V _p in the form of precipitates was reported in Table 2 as well as the average size of the precipitates measured from extracted replicas observed by transmission electron microscopy.

Table 2: Annealing conditions after cold rolling State of precipitation after annealing, (*): Excluding the invention

Table 3 shows the mechanical tensile properties: strength and elongation at break, obtained under these conditions. In addition, circular blanks with a diameter of 55 mm were cut in the cold-rolled and annealed sheets. These blanks were then embossed by swallowing in the form of flat-bottomed cups (swift shrinkage tests) using a 33mm diameter punch. In this way, the factor β characterizing the severity of the test (ratio between the initial blank diameter and the diameter of the punch) is 1.66. The possible presence of micro-cracks was then noted either immediately after shaping, or after a waiting period of 3 months, thus characterizing a possible sensitivity to delayed cracking. The results of these observations were also reported in Table 3.

Table 3: Mechanical tensile properties obtained on cold rolled and annealed sheets, and characteristics of drawability and sensitivity to delayed cracking n.d: not determined

In the case of the reference steel R3, the total vanadium content (0.865%) is excessive, and it is impossible to obtain a recrystallization even after annealing at 85O ⁰ C. The elongation properties are then very insufficient . In the case of R2 steel, even though the size of the precipitates is adequate, the precipitation of vanadium occurs in excessive amounts (0.219% of vanadium precipitate) which causes a deterioration of the elongation at break and insufficient characteristics.

In the case of steel R1, the desired precipitation is not present and there is a sensitivity to delayed failure.

Steels 11 and 12 according to the invention comprise precipitates of suitable size and nature. These are located at more than 75% in intragranular position. These steels combine excellent mechanical characteristics (resistance greater than 1000 MPa, elongation greater than 55% and a high resistance to delayed fracture. This last property is obtained even without specific heat treatment of degassing. The hot-rolled or cold-rolled sheets according to the invention are advantageously used in the automobile industry in the form of structural parts, reinforcing elements or external parts which, because of their very high strength and their high ductility, contribute to a very effective reduction of vehicle weight while increasing safety in case of impact.

Claims

- Austenitic iron-carbon-manganese steel sheet, the chemical composition of which comprises the contents being expressed by weight: 0.45% <C <0.75% 15% <Mn <26% If <3% Al <0.050% S <0.030% P <0.080% N <0.1%, at least one metal element selected from vanadium, titanium, niobium, chromium, 0.050% molybdenum ≤V <0.50%, 0.040% ≤Ti <0.50% 0.070% <Nb <0.50% 0.070% <Cr ≤ 2% 0.14% <Mo <2% and optionally one or more elements selected from 0.0005% <B <0.003% Ni <1% Cu <5%, the rest of the composition consisting of iron and unavoidable impurities resulting from the production, the amount of said at least one metal element in the form of carbides precipitated nitrides or carbonitrides being: 0.030% ≤V _p ≤ 0.150%, 0.030% ≤Ti _p <0.130% 0.040% <Nb _p <0.220% 0.070% <Cr _p <0.6% 0.14% <Mo _p <0.44%, Steel sheet according to claim 1, characterized in that the composition of said steel comprises, the content being expressed by weight 0.50% <C <0.70% 3 - steel sheet according to one of claims 1 or 2, characterized in that the composition of said steel comprises, the content being expressed by weight 17% <Mn <24% 4 - Steel sheet according to any one of claims 1 to 3, characterized in that the composition of said steel comprises 0.070% <V <0.40%, the amount of vanadium in the form of carbides, nitrides or carbonitrides precipitated being 0.070% <V _p <0.140% 5 - steel sheet according to any one of claims 1 to 4, characterized in that the composition of said steel comprises 0.060% ≤Ti < 0.40%, the amount of titanium in the form of carbides, nitrides or carbonitrides precipitated being 0.060% <Tip <0.10% 6 - Steel sheet according to any one of claims 1 to 5, characterized in that the composition of said steel comprises, 0.090% ≤Nb <0.40%, the amount of niobium in the form of carbides, nitrides or carbonitrides precipitated being 0.090% <Nb _p <0.200% 7 - steel sheet according to any one of claims 1 to 6, characterized in that the composition of said steel comprises 0.20% ≤Cr≤ 1, 8%, the amount of chromium in the form of precipitated carbides being 0.20% <Cr _p <0.5% 8 - Steel sheet according to one of claims 1 to 7, characterized in that the composition of said steel comprises 0.20% ≤Mo ≤1.8%, the amount of molybdenum in the form of precipitated carbides being 0.20% <M ≤ 0.35% 9 - Steel sheet according to any one of claims 1 to 8, characterized in that the average size of said precipitates is between 5 and 25 nanometers 10 - Steel sheet according to any one of claims 1 to 9 characterized in that the average size of said precipitates is between 7 and 20 nanometers 11 - Steel sheet according to any one of claims 1 to 10 characterized in that at least 75% of the population of said precipitates is located in intragranular position 12 - Process for manufacturing a cold-rolled sheet of austenitic iron-carbon-manganese steel according to which a steel is supplied whose chemical composition comprises, the contents being expressed by weight: 0.45% <C <0.75% 15% <Mn <26% If <3% Al <0.050% S <0.030% P≤ 0.080% N ≤ O.1%, at least one metal element selected from vanadium, titanium, niobium, chromium, molybdenum 0.050% ≤V <0.50%, 0.040% ≤Ti <0.50% 0.070% <Nb <0.50% 0.070% ≤Cr <2% 0.14% ≤Mo <2%, and optionally one or more elements selected from 0.0005% ≤B <0.003% Ni ≤ 1% Cu <5%, the rest of the composition consisting of iron and unavoidable impurities resulting from the preparation, a semi-finished product is cast from this steel; said half-product is brought to a temperature of between 1100 and 1300 ⁰ C, said semi-finished product is hot rolled to a rolling end temperature of greater than or equal to 89 ^° C., said sheet is reeled at a temperature of less than 580 ^° C. said sheet is cold-rolled said sheet is subjected to an annealing heat treatment, said heat treatment comprising a heating phase with a heating rate Vc, a holding phase at a temperature Tm during a holding time tm, followed by a cooling phase at a cooling rate Vr, optionally followed by a holding phase at a temperature Tu during a holding time t _u , the parameters Vc, Tm, tm, Vr, Tu, t _u being adjusted to obtain the quantity of said at least a precipitated metal element according to any of claims 1 to 8 13 - Process according to claim 12, characterized in that the parameters Vc, Tm, tm, Vr, Tu, t _u are adjusted so that the average size of said precipitates of carbides, nitrides or carbonitrides after said annealing is between 5 and 25 nanometers 14 - Process according to any one of claims 12 or 13, characterized in that the parameters Vc, Tm, tm, Vr, Tu, t _u are adjusted so that the average size of said precipitates after said annealing is between 7 and 20 nanometers 15 - Process according to any one of claims 12 to 14, characterized in that the Vc parameters Tm, tm, Vr, You ₁ t _u are adjusted such that at least 75% of the population of said precipitates after said annealing is located in an intragranular position 16 - A method of manufacturing a cold-rolled sheet of iron-carbon-manganese steel according to claim 12, characterized in that a steel is supplied whose chemical composition comprises 0.050% <V <0.50%, that said semi-finished product is hot-rolled to a rolling end temperature greater than or equal to 95 ^° C., said sheet being reeled at a temperature of less than 500 ^° C., said sheet being cold-rolled with reduction rate greater than 30%, it performs an annealing heat treatment with a heating rate Vc of between 2 and 10 ° C / s, at a temperature Tm between 700 and 870 ° C for a time between 30 and 180 s, and that said sheet is cooled at a speed of between 10 and 50 ° C / s 17 - Process for manufacturing a cold rolled sheet according to claim 16, characterized in that the heating rate Vc is between 3 and 7 ° C / s 18 - Process for manufacturing a cold rolled sheet according to one of claims 16 or 17, characterized in that the holding temperature Tm is between 720 and 850 ⁰ C 19 - Manufacturing method according to any one of claims 12 to 18, characterized in that the casting of said semi-product is carried out in the form of casting slabs or thin strips between contra-rotating steel cylinders Use of an austenitic steel sheet according to any one of claims 1 to 11, or manufactured by a process according to any one of claims 12 to 19, for the manufacture of structural parts, reinforcement elements or external parts, in the automotive field.