MX2009002411A - Steel plate for producing light structures and method for producing said plate. - Google Patents

Abstract

The invention relates to a steel plate having a composition comprising 0.01 wt.-% ⿤ C ⿤ 0.2 wt.-%, 0.06 wt.-% ⿤ Mn ⿤ 3 wt.-%, Si ⿤ 1.5 wt.-%, 0.005 wt.-% ⿤ Al ⿤ 1.5 wt.-%, S ⿤ 0.03 wt.-%, P ⿤ 0.04 wt.-%, 2.5 wt.-% ⿤ Ti ⿤ 7.2 wt.-%, (0.45 xTi) - 0.35 wt.-% ⿤ B ⿤ (0.45 xTi) + 0.7 wt.-%, and, optionally, one or more of the following elements: Ni ⿤ 1 wt.-%, Mo ⿤ 1 wt.-%, Cr ⿤ 3 wt.-%, Nb ⿤ 0.1 wt.-%, V ⿤ 0.1 wt.-%, the rest of the composition comprising iron and unavoidable impurities resulting from production.

Description

STEEL PLATE FOR PRODUCING LIGHT STRUCTURES AND METHOD FOR PRODUCING THE INVENTION Description of the invention The invention relates to the manufacture of steel plates or structural parts that combine both a high elastic modulus E, a low density d and a high tensile strength. It is known that the mechanical performance of the structural elements varies as Ex / d, the coefficient x depends on the shape of the external load (for example in the traction or bending) or the geometry of the elements (plates, bars) . This illustrates the benefit of having materials that exhibit both a high elastic modulus and a low density. This requirement applies more particularly in the automotive industry where the concerns of lightness and safety in the vehicle are constant. In this way the objective is to increase the modulus of elasticity and reduce the weight of the steel parts by incorporating ceramic particles of different types, such as carbides, nitrides, oxides or borides. The reason for this is that this type of materials have a markedly higher elastic modulus, with intervals from approximately 250 to 550 GPa, than with the base steels, which is around 210 GPa, where they are incorporated. In this way the tempering is achieved by the change of REF. : 200566 the charge between the matrix and the ceramic particles under the influence of a charge. This tempering is further increased due to the refinement of the grain size of the matrix by the ceramic particles. In order to manufacture these materials comprising ceramic particles evenly distributed in a steel matrix, processes based on powder metallurgy are known: first, ceramic powders with controlled geometry are produced, these are mixed with steel powders, by means of this correspondence , for the steel, with an extrinsic addition of the ceramic particles. The powder mixture is compacted in a mold and then heated to a temperature such that this mixture undergoes sintering. In a variation of the process, the metallic powders are mixed in order to form the ceramic particles during the sintering phase. Regardless of the improved mechanical properties on steels do not contain a dispersion of ceramic particles, this type of process suffers from different limitations: - it requires the care of the melting and processing conditions in order not to cause a reaction with the atmosphere, taking into account the large specific surface area of the metal powders; - Even after the compaction and sintering operations, residual pores that probably act as sites of initiation during the application of a load cyclically; - the chemical composition of the matrix / particle interfaces, and therefore their cohesion, is difficult to control given the surface contamination of the powders before sintering (presence of oxides and carbon); - when the particles are added in large quantities, or when certain large particles are present, the elongation properties decrease; - this type of process is suitable for low volume production but can not meet the requirements of mass production in the automotive industry; and - the production costs associated with this type of production process are high. In the case of light alloys, the production processes are also known based on the extrinsic addition of the ceramic powders in the liquid metal. Again, these processes suffer from most of the disadvantages mentioned above. More particularly, the difficulty of the homogeneous dispersion of the particles can be mentioned, these particles have a tendency to agglomerate or settle or float in the liquid metal. Among the known ceramics that could used to increase the properties of steel is in particular titanium diboride TiB2, which has the following intrinsic characteristics: Elastic modulus: 565 GPa; Relative density: 4.52. However, because the production processes are based on the extrinsic additions of the TiB2 particles, they suffer from the disadvantages mentioned above. The object of the invention also solves the above problems, in particular the availability of mass produced steels economically with an elastic modulus increased by the presence of TiB2 particles. The object of the invention is in particular to provide a production process by continuous casting that does not present particular difficulties when the steels are melted. Another object of the invention is to provide steels having the highest possible amount of TiB2 particles dispersed uniformly in the matrix. Another object of the invention is to provide steels with high tensile strength, the uniform stretching of these is equal to or greater than 8%, which can easily be attached to different welding processes, especially resistant welding. For this purpose, one object of the invention is a steel sheet, the chemical composition of this comprises, the content is expressed by weight: 0.010% < C < 0.20%; 0.06% < n < 3%; Yes < 1.5%; 0.005% < To < 1.5%; S < 0.030%; P < 0.040%; titanium and boron in quantities such that: 2.5% = Ti = 7.2%; (0.45xTi) - 0.35% < B < (0.45xTi) + 0.70%; optionally one or more elements chosen from: Ni < 1%; Mo = 1%; Cr = 3%; Nb = 0.1%; V = 1%, the rest of the composition consists of iron and unavoidable purities resulting from the fusion. Preferably, the content of titanium and boron, expressed in% weight, is such that - 0.22 = B - (0.45xTi) = 0.35%. Preferably, the content of titanium and boron, expressed in% weight, is such that - 0.35 = B - (0.45xTi) = 0.22%. Preferably, the titanium content is such that 4.6% < You < 6.9%. According to a particular embodiment, the titanium content is such that: 4.6% < You < 6% Preferably, the carbon content is such that C = 0.080%. According to a preferred embodiment, the carbon content is such that C = 0.050%. Preferably, the chromium content is such that Cr = 0.08%. The object of the invention is also a sheet metal steel with the above composition, comprising eutectic precipitates of TiB2 and optionally those of FeB2, the average size thereof being equal to or less than 15 micrometers and preferably equal to or less than 10 micrometers. Preferably, more than 80% by number of TiB2 precipitates are single crystal character. Another object of the invention is a steel sheet according to the above characteristics, the average grain size of the steel is equal to or less than 15 micrometers, preferably equal to or less than 5 micrometers and more preferably less than 3.5 micrometers. Another object of the invention is a steel sheet which is claimed in one of the above characteristics, the elastic modulus thereof, is measured in the rolling direction, is equal to or greater than 230 GPa, preferably equal to or greater than 240 GPa or preferably equal to or greater than 250 GPa. According to a particular embodiment, the tensile strength of the steel sheet is equal to or greater than 500 MPa and its uniform elongation is equal to or greater than 8%. Another object of the invention is an object made with a plurality of steel parts, with an identical or different composition or different thickness, at least one of the steel parts which is a steel plate according to any of the above characteristics, that is welded to at least one of the parts of this object, the composition or compositions of other steel parts comprise, by weight: 0.001% -0.25% C; 0.05% -2% Mn; Yes < 0.4%; To < 0.1; You < 0.1%; Nb < 0.1%; V < 0.1%, Cr < 3%; Mo < 1%; Ni < 1%; B < 0.003%, the complement of the composition consists of iron and relative impurities of the fusion. Another object of the invention is a process wherein a steel is provided with any of the above compositions and the steel is melted in the form of a semi-finished product, the melting temperature does not exceed more than 40 ° C above the temperature of steel liquid. According to a particular embodiment, the semi-finished product is melted in the form of a thin plate or thin strip between the counter-rotating rollers. The cooling rate during solidification of the melt is preferably equal to or greater than 0.1 ° C / sec. According to a particular embodiment, the semi-finished product is reheated before it is hot-rolled, the temperature and the duration of the superheat are both chosen such that the density of TiB2 and optionally the Fe2B eutectic precipitates, with a maximum size Lmax greater of 15 microns and an aspect ratio f > 5, are less than 400 / mm2. According to a particular embodiment, a hot rolling operation is performed on the semi-finished product, optionally a cold rolling operation and an annealing operation that is adjusted in such a way that a steel plate with a grain size is obtained is equal to or less than 15 micrometers, preferably equal to or less than 5 micrometers and more preferably less than 3.5 micrometers. Preferably, rolling is performed with a final rolling temperature below 820 ° C. According to a particular embodiment, at least one pressed blank is formed is cut from the steel sheet according to one of the above modalities, or is produced according to one of the above modalities, the pressed blank is deformed inside of the temperature range from 20 ° C to 900 ° C. Another object of the invention is a production process in which at least one steel sheet is welded according to one of the above modalities, or a steel sheet produced according to one of the above modalities. Another object of the invention is the use of a steel sheet or of an object according to one of the above embodiments, or a steel sheet produced according to one of the above modalities, for the production of the structural parts or elements of reinforcement in the automotive field. Other features and advantages of the invention will become apparent with the course of the next description, given by way of the non-limiting example and with reference to the appended figures in which: - Figures 1 and 2 respectively illustrate the microstructure of two steels according to the invention comprising a eutectic precipitation Fe-TiB2, in the foundry state; - Figure 3 illustrates the microstructure of a steel according to the invention in the cold rolled and annealed state; - Figures 4 and 5 illustrate the microstructure of two steels according to the invention, comprising eutectic precipitations of Fe-TiB2 and Fe-Fe2B, in the state of casting and the state hot-rolled respectively; and - Figures 6 and 7 illustrate the microstructure of the steel according to the invention, cooled with two cooling rates during solidification, in the melting state. Regarding the chemical composition of the steel, the carbon content is adapted for the purpose of economic achievements to a given level of elasticity limit or tensile strength. The carbon content also makes it possible to control the nature of the microstructure of the matrix of the steels according to the invention, this microstructure can be partial or completely ferritic, bainitic, austenitic or martensitic, or may comprise a mixture of these constituents in suitable portions to meet the required mechanical properties. A carbon content equal to or greater than 0.010% allows different constituents to be obtained. The carbon content is limited due to the weldability: resistance to cold cracking and tenacity in the HAZ (Zone Affected by Heat) decreases when the carbon content is greater than 0.20%. When the carbon content is equal to or less than 0.050% by weight, the resistance to solderability is particularly improved. Due to the titanium content of steel, the carbon content is preferably limited in order to avoid the primary precipitation of Tic and / or Ti (C, N) in the liquid metal. These precipitates, which are formed in the liquid, deteriorate the castability in the continuous casting process of the liquid steel. However, when this precipitation occurs in the solidification interval or in the solid phase, it has a favorable effect on structural tempering. The maximum carbon content is therefore preferably limited to 0.080% to produce the Ti and / or Ti (C, N) precipitates predominantly during eutectic solidification or solid phase. In an amount equal to or greater than 0.06% manganese it increases the hardenability and contributes to the hardening in solid solution and therefore increases the tensile strength. This is combined with any sulfur present, thus reducing the risk of hot cracking. However, above a manganese content of 3% by weight, there is a greater risk of forming a banded structural deterioration that arises with any segregation of the manganese during solidification. The silicon contributes effectively to increase the resistance to the tension thanks to the temper in solid solution. However, the excessive addition of silicon causes the formation of oxide adhesion which are difficult to remove during a pickling operation, and the possible appearance of the surface defects due in particular to the lack of wettability in galvanizing operation. by hot dip. To maintain good coating properties, the silicon content should not exceed 1.5% by weight. In an amount equal to or greater than 0.005% aluminum is a very effective element for the deoxidation of steel. However, above a content of 1.5% by weight, excessive primary precipitation of alumina occurs, causing problems of collability. In an amount greater than 0.030%, the sulfur has to precipitate in excessive amounts in the form of sulfur manganese which greatly reduces the ability to undergo hot or cold forming. Phosphorus is a known element to segregate at grain boundaries. Its content should not exceed 0.040% to maintain sufficient hot ductilityby this cracking is prevented, and hot cracking is prevented during welding. Optionally, nickel or molybdenum can be added, these elements increase the tensile strength of the steel. For economic reasons, these additions are limited to 1% by weight. Optionally, chromium can be added to increase the tensile strength. It also allows them to precipitate larger amounts of borides. However, its content is limited to 3% by weight in order to produce a not so expensive steel. A chromium content equal to or less than 0.080% preferably will be chosen. This is because an excessive addition of chromium causes more borides to precipitate, but these are then borides (Fe, Cr). Also optionally, niobium and vanadium can be added in an amount equal to or less than 0.1% to obtain complementary tempering in the form of precipitated carbonitrides. Titanium and boron play an important role in the invention.

In a first embodiment, the content of weight expressed in percent of titanium or boron of steel is such that: 2.5% < You < 7.2%; and (0.45xti) - 0.35% < B < (0.45xTi) + 0.70%. The second relation can be expressed equivalently as: - 0.35 < B - (0.45xTi) < 0.70. The reasons for these limitations are the following: - when the content by weight of titanium is less than 2.5% precipitation of TiB2 does not occur in a sufficient amount. This is because the volume fraction of precipitated TiB2 is less than 5%, by means of this a significant change in the elastic modulus, which remains less than 220 GPa, is impossible; - when the titanium weight content is greater than 7. 2%, the precipitation of primary TiB2 coating occurs in the liquid metal and causes problems of castability in semi-finished products; if the weight content of titanium and boron is such that: B - (0.45xTi) > 0.70, there is excessive precipitation of Fe2B, which degrades ductility; and - if the content by weight of titanium and boron is such that: B - (0.45xTi) < - 0.35, the amount of titanium dissolved at room temperature in the matrix is greater than 0.8%. So you do not get a significant beneficial technical effect, Despite the higher cost of adding titanium. According to a second embodiment of the invention, the content of titanium and boron is such that - 0.22 = B - (0.45xTi) < 0.35: - when B- (0.45xTi) = 0.35, the precipitation of Fe2B is greatly reduced, thereby increasing the ductility; and - when B- (0.45xTi) = -0.22, the amount of titanium dissolved in the matrix is very low, which means that the titanium additions are particularly effective from an economic point of view. According to a particular embodiment of the invention, the content of titanium and boron is such that: -0.35 < B - (0.45XTÍ) < -0.22: - when the quantity B - (0.45xTi) is equal to or greater than -0.35 and lower -0.22, the amount of titanium dissolved at room temperature in the matrix is between 0.5% and 0.8% respectively. This amount proves that it is particularly suitable for obtaining precipitation composed solely of TiB2. According to a particular embodiment of the invention, the titanium content is such that: 4.6% = Ti = 6.9%. The reasons for these limitations are the following: - when the content by weight of titanium is equal to or greater than 4.6%, precipitation of TiB2 from such so that the fraction in precipitated volume is equal to or greater than 10%. The elastic modulus is then equal to or greater than about 240 GPa; and when the titanium weight content is equal to or less than 6.9%, the amount of precipitated TiB2 is mainly less than 3% by volume. The total precipitation of TiB2, consisting of possibly primary precipitates and eutectic precipitates, is less than 15% by volume. According to another preferred embodiment of the invention, the titanium content is such that: 4.6% = Ti = 6%. When the weight content of titanium is equal to or less than 6%, the castability after is particularly satisfactory due to the slight primary precipitation of TiB2 in the liquid metal. According to the invention, the eutectic precipitation of Fe-TiB2 occurs with solidification. The eutectic nature of the precipitation gives the formed microstructure a particular fineness and an advantageous homogeneity for the mechanical properties. When the amount of eutectic precipitate of TiB2 is greater than 5% by volume, the elastic modulus of the steel measured in the direction of rolling can exceed about 220 GPa. Above 10% by volume of TiB2 precipitates, the modulus can exceed about 240 GPa, thereby enabling appreciably light structures to be designed. This amount can be increased by 15% in volume in order to exceed approximately 250 GPa, in particular in the case of steels comprising alloying elements such as chromium or molybdenum. This is because when these elements are present, the maximum amount of TiB2 that can be obtained in the case of eutectic precipitation is increased. The content of boron and titanium according to the present invention prevents coarse primary precipitation of TiB2 in the liquid metal. The formation of these primary precipitates of occasionally large size (measured hundreds of times of micrometers) should be avoided due to their deleterious role with respect to damage or fracture mechanisms during subsequent mechanical loading. However, these precipitates present in the liquid metal, when they do not settle, are distributed locally and reduce the uniformity of the mechanical properties. This premature precipitation should be avoided as it can cause blockage of the nozzle when steel is continuously melted as a result of the agglomeration of the precipitate. As explained above, the titanium must be present in an amount sufficient to cause the endogenous formation of TiB2 in the form of Fe-TiB2 eutectic precipitation. According to the invention, titanium may also be present when it is dissolved at room temperature in the matrix in a superstoichiometric ratio in relation to boron, calculated based on TiB2. When the titanium content in the solid solution is less than 0.5%, the precipitation takes place in the form of two successive eutectic: first Fe-TiB2 and then Fe-Fe2B, this second endogenous precipitation of Fe2B takes place in greater or lesser amounts depending on the boron content of the alloy. The amount precipitated as Fe2B can have a range of 8% by volume. This second precipitation is also carried out in accordance with a eutectic scheme, making it possible to obtain a fine uniform distribution, thereby ensuring a good uniformity of the mechanical properties. The precipitation of Fe2B complements that of TiB2, the maximum amount of this is related to the eutectic. Fe2B plays a role similar to that of TiB2. This increases the elastic modulus and reduces the density. In this way it is possible to finely adjust the mechanical properties by changing the complement of the precipitation of Fe2B in relation to TiB2. This is a means that can be used in particular to obtain an elastic modulus greater than 250 GPa in the steel and an increase in the tensile strength of the product. When the steel contains an amount of Fe2B equal to or greater than 4% by volume, the elastic modulus is increased by more than 5 GPa. The elongation at break is then between 14% and 16% and the tensile strength reaches 590 MPa. When the amount of Fe2B is greater than 7.5% by volume, the elastic modulus is increased by more than 10 GPa, but the elongation at break is less than 9%. According to the invention, the average size of the eutectic precipitates of TiB2 or Fe2B is equal to or less than 15 micrometers in order to obtain greater elongation with breaking values and good fatigue properties. When the average size of these eutectic precipitates is equal to or less than 10 microns, the elongation at break can be greater than 20%. The inventors have shown that, more than 80% in number of the eutectic precipitates of TiB2 are of simple crystal character, the damage in the matrix / precipitate when the load is reduced mechanically and the risk of forming defects is lower due to the greater plasticity of the precipitate and its high level of cohesion with the matrix. In particular, it has been shown that the larger TiB2 precipitates form hexagonal crystals. Without wishing to be related to a particular theory, it is believed that this crystallographic character increases the possibility that these precipitates will deform when being blown under the effect of a mechanical load. This particular character of simple crystal, due to the precipitation of TiB2 in a eutectic form, is not found to a degree in the processes of the prior art, which are based on the extrinsic additions of the particles.

Apart from the favorable effect of a dispersion of endogenous particles on the tensile properties, the inventors have shown that limiting grain size is a very effective means of increasing tensile properties: when the average grain size is equal to or less than 15. micrometers, the tensile strength can exceed approximately 560 MPa. In addition, when the grain size is equal to or less than 3.5 micrometers, the crack resistance is particularly high: the Charpy toughness tests with a thickness of 3 mm at -60 ° C show that the ductile area in the fractured test specimens is greater than 90% The process for the production of a sheet according to the invention is implemented as follows: a steel is provided with the composition according to the invention; and the steel then melts into a semi-finished product. This casting can be made to form ingots or is continuously made to form sheets with a thickness of around 200 mm. It is also possible to melt the steel in the form of thin plates a few tens of millimeters thick or thin strips a few millimeters thick between the counter-rotating rollers. The last method of implementation is particularly advantageous to obtain a fine eutectic precipitation and prevent the formation of primary precipitates. By increasing the cooling rate during solidification, the fineness of the obtained microstructure is increased. Of course, casting can be done in a format that allows the production of products having different geometries, in particular in the form of billets to produce long products. The fineness of TiB2 and Fe2B precipitation increases the tensile strength, ductility, toughness, formability and mechanical behavior in the HAZ. The fineness of the precipitation is increased thanks to a low melting temperature and a higher cooling speed. In particular, it has been found that a melting temperature limited to 40 ° Carriba of the liquid temperature causes fine microstructures to be obtained. The melting conditions will also be chosen in such a way that the melting rate during solidification is equal to or greater than 0.1 ° C / sec so that the size of the precipitates of TiB2 and FeB2 are particularly fine. The inventors have also shown that the morphology of the eutectic precipitates of TiB2 and Fe2B plays a role in damage during subsequent mechanical solidification. After observing the precipitates under the optical microscope and the amplification intervals of 500x 1500x approximately on a surface having a statistically representative population, the maximum size Lmax and the minimum size Lmin of each precipitate is determined using image analysis computation programs known per se, such as for example Scion® image analysis computer programs . The ratio of the maximum size to the minimum size Lmax / Lmin characterizes the aspect ratio f of a given precipitate. The inventors have shown that large precipitates (Lmax> 15 micrometers) and elongate shape (f> 5) reduce uniform elongation and the coefficient of hardening by cold deformation n. According to the invention, after the semi-finished product has melted, the temperature and reheat time for the semi-finished product before the subsequent hot rolling is chosen to cause most of the harmful precipitates to spheridize. In particular, the temperature and reheat time are chosen in such a way that the density of elongated eutectic precipitates (f > 5) with a size Lmax > 15 micrometers is less than 400 / mm2. The semi-finished product then undergoes hot rolling, possibly followed by rolling. Optionally, the cold rolled and annealing is done in order to obtain thinner plates. The conditions of hot rolling, rolling, cold rolling and annealing in such a way as to obtain the steel sheet with an average grain size equal to or less than 15 microns, preferably less than 5 microns and more preferably less than 3.5 microns. A finer grain size is obtained by: - hardening by substantial deformation before the end of hot rolling and before the allotropic transformation (? -a) with quenching; a low temperature at the end of rolling, preferably below 820 ° C; - accelerated cooling after the transformation (? -a) in order to limit the growth of ferritic grain; a cooling operation with a relatively low temperature; and - after the possible cold rolling, the annealing temperature and the annealing time is limited for the purpose of completing the recrystallization without exceeding the temperature and time values necessary for this recrystallization. A final temperature of hot rolling below 820 ° C in particular provides an effective means of obtaining a fine grain size. A particular effect of the TiB2 and Fe2B precipitates on the nucleation and crystallization of the microstructures in the steels according to the invention has been demonstrated. Specifically, when steel is deformed in accordance with In the invention, the significant difference in mechanical behavior between the precipitates and the matrix causes a greater deformation around the precipitates. This intense local deformation reduces the temperature without recrystallization. A low final rolling temperature promotes ferritic nucleation around the precipitates and limits granular growth. In the same way, the greater field of deformation around the precipitates promotes the nucleation of the grain during the restoration / recrystallization that follows the cold rolling, originating the refinement of the grain. The steel sheet obtained in this way thus exhibits very good formability. Without wishing to relate to a particular theory, it is believed that the eutectic precipitates present within a highly deformable matrix play a similar role to that played by the martensitic or bainitic phases within the ferrite in the "dual phase" steels. The steels according to the invention have a ratio of (yield strength Re / tensile strength Rm) favorable to a variety of forming operations. Depending on the content of carbon and quenching elements, and depending on the cooling rate below the Arl temperature (this temperature denotes the beginning of the transformation with the austenite cooling), it is possible to obtain hot-rolled sheets or cold-rolled and annealed sheets comprising matrices with different microstructures - these can be totally ferritic, bainitic, martensitic or austenitic. For example, a steel containing 0.04% C, 5.9% Ti and 2.3% B will have, after cooling from 1200 ° C, a Vickers hardness with range from 187 to 327 for a cooling interval from 5 to 150 ° C / s. The highest hardness levels correspond in this case to a completely bainitic matrix composed of slightly disoriented carbide free strips. If it is desired to produce a part by a shaping operation, a blank formed with the sheet is cut and deformed by means such as drawing or bending in a temperature range between 20 and 900 ° C. The tempering phases of TiB2 and Fe2B exhibit very good thermal stability up to 1100 ° C. Due to the thermal stability of the dispersed particles in the matrix and the adaptation to the different cold, warm and hot forming processes, parts of the complex geometry with a higher elastic modulus can be produced according to the invention. Furthermore, the increase in the elastic modulus of the steels according to the invention reduces the elastic recovery after the shaping operations and by means of this increases the dimensional accuracy of the finished parts.

Advantageously, the structural elements are also produced by welding steels according to the invention, which have identical or different compositions, or identical or different thicknesses, as well as in the final stage to obtain parts whose mechanical properties vary within these and It adapts locally to subsequent loads. In addition to the iron and the unavoidable impurities, the composition by weight of the steels that can be welded with the steels according to the invention comprises, for example: 0.001-0.25% C; 0.05-2% Mn; Yes < 0.4%; To < 0.1% Ti < 0.1%; Nb < 0.1%; V < 0.1%; Cr < 3%; Mo < 1%; Ni < 1%; B < 0.003%, the rest of the composition consists of iron and the inevitable purities that result from the casting. In the melting zone, due to the high temperature reached, the precipitates partially dissolve and then re-precipitate with cooling. The amount of precipitates in the melting zone is very comparable to that of the base metal. Within the HAZ of the welded joints, the eutectic precipitates do not dissolve and can still serve to decrease the growth rate of the austinitic grain and of the possible nucleation sites during the subsequent cooling phase.

During a welding operation performed on the steels according to the invention, the concentration of precipitates of TiB2 and Fe2B is therefore uniform, transiting from the base metal to the molten metal passing through HAZ, by means of this is guaranteed, in the case of welded joints, the projected mechanical properties (modulus, density) will be continuous through all joints. To give a non-limiting example, the following results demonstrate the advantageous features conferred by the invention.

Example 1: Steels with the composition given in Table 1 below were produced, expressed in percentages by weight. In addition to the steels 1-1 and 1-2 according to the invention, this table indicates, as a comparison, the composition of a reference steel R-1 which does not contain endogenous TiB2 or Fe2B eutectic precipitates. The steels were produced by melting the semi-finished products of the liquid state, the additions of titanium and boron are made in the case of steels 1-1 and 1-2 in the form of ferro-alloys. The melting temperature was 1330 ° C, that is 40 ° C above the liquid temperature.

Table 1: Steel compositions (% by weight) I = In accordance with the invention; R = reference; (*) = not according to the invention The microstructure in the melting state, illustrated in Figures 1 and 2, relates to the steels 1-1 and 1-2 respectively, show a fine uniform dispersion of the endogenous TiB2 precipitates within a ferritic matrix. The boron precipitated in the form of a binary eutectic Fe-TiB2. The volume quantities of the precipitates were measured by means of an image analyzer and are 9% and 12.4% for steels 1-1 and 1-2 respectively. The amount of TiB2 in the form of primary precipitates is less than 2% by volume and promotes good castability. The average size of eutectic precipitates TiB2 is 5 and 8 micrometers for steels 1-1 and 1-2 respectively. Among the population of these precipitates, more than 80% in number have a simple crystal character. After reheating to 1150 ° C, the semi-finished products Afterwards they were hot-rolled in the form of sheets with a thickness of 3.5 mm, the final rolling temperature is 940 ° C. Hot rolling was followed by cooling to 700 ° C. The treatments were also carried out by reheating the steel 1-2 to 1230 ° C before hot rolling, several times varying from 30 to 120 minutes. The morphology of the precipitates was then observed. They showed that a treatment at 1230 ° C for a time of 120 minutes or more allows the precipitates to form spheres in such a way that the density of the large eutectic precipitates (Lmax> 15 micrometers) elongated (f > 5) is less than 400 / mm2. The uniform elongation Au and the hardening coefficient by deformation n then grew significantly, since they were from 11% to 0.125 (reheating time: 30 minutes) to 16% and 0.165 (reheating time: 120 minutes) thanks to the treatment of formation of spheres of the precipitate. However, in the case of steel 1-2, a sheet was rolled hot with a final rolling temperature of 810 ° C. The hot-rolled plates were then decapitated using a process known per se and then cold rolled to a thickness of 1 mm. Then they underwent recrystallization by annealing at 800 ° C, with 1 minute of soaking, before cooling with air.

The SEM (Scanning Electron Microscopy) observation showed no loss of cohesion at the interface of the matrix / eutectic precipitate or damage to the precipitates alone after hot or cold rolling. After hot rolling, the average grain size of steel 1-1 was 12 microns, since it was 28 microns in the case of the reference steel. In the case of steel 1-2, a low final rolling temperature (810 ° C) resulted in a finer average grain size (3.5 micrometers) after hot rolling. After cold rolling and annealing, the structure of steels 1-1 and 1-2 was recrystallized, as indicated in figure 3 which relates to steel 1-1. The micrograph was taken using the electron scanning microscope in the crystalline contrast mode, by means of which the completely recrystallized character of the structure is attested. The precipitates are very predominant eutectic precipitates. Compared with conventional steel Rl, and TiB2 precipitates cause substantial refinement of the microstructure - the average grain size is 3.5 micrometers for steel 1-1 according to the invention considered to be 15 micrometers in steel case of reference Rl. Pycnometry measurements indicate that the presence of the TiB2 and Fe2B precipitates is associated with a significant reduction in the relative density d since it goes from 7.80 (conventional steel R-l) to 7.33 (steel 1-2). The elastic modules of the steels 1-1 and 1-2 measured in the direction of rolling were 230 GPa and 240 GPa respectively. The elastic modulus of the reference steel R-1 was 210 GPa. For the sheets with loads applied in the bend, the efficiency index of these was obtained, they varied with respect to E1 / 3 / d, the use of the steels according to the invention allowed a weight reduction greater than 10% over the conventional steels The tensile properties measured (yield point Re measured at 0.2% strain, tensile strength Rm, uniform elongation Au and elongation at break At) are given in Table 2 (hot-rolled sheets) or Table 3 ( cold rolled and annealed sheets) next.

Table 2: Traction properties of hot-rolled sheets (parallel to the rolling direction) Steel Re (MPa) Rm (MPa) Au (%) At (%) 1-2 300 558 15 22 1-2 244 527 14 20 Table 3: Traction properties of cold-rolled and annealed sheets (parallel to the rolling direction) The Re / Rm ratio of hot rolled or cold rolled sheets according to the invention approaches 0.5, causing the mechanical behavior to approach that of a dual phase steel and good subsequent forming capacity. The resistance spot welding tests were carried out on cold-rolled steel plates 1-1: in the tensile shear tests, the failure systematically occurs by peeling. It is known that this is a preferred form of fracture since it is associated with great energy. It has also been shown that, within the melted zones in the weld, the eutectic precipitates according to the invention are present, by means of this the mechanical properties in the welded assemblies are helped to be uniform. Satisfactory properties are also obtained with laser welding and arc welding.

Example 2: Table 4 below shows the compositions of three steels according to the invention.

Table 4: Compositions of steels according to the invention (% by weight) The steels were produced by casting the semi-finished products, additions of titanium and boron taking into account the shape of ferro-alloys. The melting temperature was 40 ° C above the liquid temperature. Compared with steels 1-1 and 1-2, steels 1-3 to 1-5 have an excess amount of boron compared to TiB2 stoichiometrically, so that the complementary eutectic Fe2B TiB2 precipitations take place and afterwards. The volume quantities of the eutectic precipitates are given in Table 5.

Table 5: Content of precipitates (% vol.) Of steels The eutectic precipitates had an average size of less than 10 micrometers. Figure 4 illustrates, in the case of steel 1-3, the coexistence of precipitates of TiB2 and Fe2B. The light gray Fe2B precipitates and the darker TiB2 precipitates are dispersed within the ferritic matrix. The wetted products were hot rolled under conditions identical to those presented in Example 1. Again, no damage was observed to the interface of the precipitate-matrix. Figure 5 illustrates the microstructure of steel 1-5. The properties of these hot rolled steels are given in the Table Table 6: Traction properties (parallel to the rolling direction) and relative density of hot-rolled sheets Compared with steels 1-1 and 1-2, a complementary eutectic precipitation of Fe2B in a volume amount with a range of 3 to 7.9% increases the elastic modulus with an amount with a range of 5 to 15 GPa. The complementary precipitation of Fe2B increases the tensile strength. When this precipitation occurs in excessive proportions, however, the uniform elongation is markedly less than 8%.

Example 3: The semi-finished products made of steel with composition 1-2 were melted at a temperature of 1330 ° C. Two cooling speeds were achieved by changing the intensity of the flow by cooling the semi-finished products and the thickness of the molten semi-finished products. say 0.8 ° C / s and 12 ° C / s. The microstructures given in Figures 6 and 7 illustrate that a higher cooling rate refines the Fe-TiB2 eutectic precipitation with greater significance.

Example 4: Steel sheets with the composition 1-2, 2.5 mm thick were welded by means of laser welding with C02 under the following conditions: Energy: 5.5 KW; Welding speed: 3 m / min. The micrographs of the melted zone show that the eutectic precipitation of Fe-TiB2 is carried out in a finer form with the cooling of the liquid state. The amount of precipitates in the fusion zone is close to that of the base metal. Depending on the local cooling conditions during solidification (local temperature gradient G, travel speed R of the isotherms), solidification takes place in dendritic form or in cellular form. The dendritic morphology is more easily found in the union with the HAZ, given the local conditions of solidification (high gradient G and low speed R) · The precipitates of TiB2 are therefore present in different areas of the union (base metal, the HAZ and the fusion zone). In this way, the increase in the elastic modulus and the reduction in density is present throughout the entire welding line A sheet steel 1-2 was also welded with laser welding without any difficulty of operation with a sheet of mild steel that can be stretched, the composition of this had (in% by weight): 0.003% C, 0.098% Mn , 0.005% Si, 0.059% Al, 0.051% Ti, 0.003% B and the inevitable impurities that result from the melting. The fusion zone also had eutectic Fe-TiB2 precipitates, of course with a lower proportion in the case of autogenous welding. It is therefore possible to produce metal structures whose stiffness properties vary locally and whose mechanical properties correspond more specifically to local processing or performance requirements in service.

Example 5: The cold rolled and annealed sheets of steel 1-2 according to the invention, with a thickness of 1.5 mm, were joined by spot welding with strength under the following conditions: bond strength: 650 daN; - welding cycle: 3 (7 periods with current flow 1 + 2 period without current flow). The welding interval, expressed in terms of current I, was between 7 and 8.5 kA. The two unions for This interval corresponds, on the one hand, to obtain a core diameter greater than 5.2 mm (connection with lower current) and, on the other hand, the presence of sparks during welding (upper connection). The steel according to the invention therefore shows good weldability with spot welding with resistance with a weldability range of 1.5 kA, sufficiently wide. In this way the invention allows the production of structural parts or reinforcement elements with an improved level of performance, both from the point of view of the intrinsic lightness and the increase in the elastic modulus. The easy processing of the steel sheets according to the invention by welding makes it possible to incorporate them into more complex structures, in particular by means of joints with parts made of steels with different compositions or different thicknesses. The automotive field will benefit most in particular with these different characteristics. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (27)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. Steel sheet, characterized in that its chemical composition comprises the amounts expressed in weight: 0.010% < C < 0.20% 0.06% < Mn < 3% Yes < 1.5% 0.005% < To < 1.5% S < 0.030% P < 0.040%, of titanium and of boron in amounts such as: 2.5 < You < 7.2% (0.45 xTi) - 0.35% < B < (0.45 xTi) + 0.70% optionally one or more elements chosen from: Ni < 1% Mo < 1% Cr < 3% Nb < 0.1% V < 0.1%, the rest of the iron composition and inevitable impurities resulting from the processing being constituted.
2. 2. Steel sheet in accordance with the claim 1, characterized in that the amounts in titanium and boron are such as: -0.22 < B - (0.45x Ti) < 0.35 3. Steel sheet according to claim 1, characterized in that the amounts in titanium and boron are such as: -0.35 < B - (0.45x Ti) < 0.22 4. Steel sheet according to any of claims 1 to 3, characterized in that the amount of titanium is such that: 4.6% < You < 6.9% 5. Steel sheet according to claim 4, characterized in that the amount of titanium is such that: 4.6% < You < 6% steel sheet according to any of claims 1 to 5, characterized in that the composition comprises, the quantity expressed by weight: C < 0.080% 7. Steel sheet according to any of claims 1 to 6, characterized in that its composition comprises, the amount expressed by weight: C < 0.050% 8. Steel sheet according to any of claims 1 to 7, characterized in that its composition comprises, the amount expressed in weight: Cr < 0.08% 9. Steel sheet according to any of claims 1 to 8, characterized in that it comprises eutectic precipitates of TiB2 and occasionally of Fe2B, whose average size is less than or equal to 15 micrometers. Steel sheet according to any of claims 1 to 9, characterized in that it comprises eutectic precipitates of TiB2 and occasionally of Fe2B, whose average size is less than or equal to 10 micrometers. 11. Steel sheet according to claim 10, characterized in that more than 80% by number of the TiB2 precipitates have a nanocrystalline character. Steel sheet according to any of claims 1 to 11, characterized in that the average grain size of the steel is less than or equal to 15 micrometers. 13. Steel sheet according to any of claims 1 to 12, characterized in that the average grain size of the steel is less than or equal to 5 micrometers. Steel sheet according to any of claims 1 to 13, characterized in that the average grain size of the steel is less than or equal to 3.5 micrometers. 15. Steel sheet in accordance with any of the claims 1 to 14, characterized in that its modulus of elasticity measured in the direction of the lamination is greater than or equal to 230 GPa. Steel sheet according to any of claims 1 to 15, characterized in that its modulus of elasticity measured in the direction of the lamination is greater than or equal to 240 GPa. 17. Sheet steel according to any of claims 1 to 16, characterized in that its modulus of elasticity measured in the direction of the lamination is greater than or equal to 250 GPa. 18. Sheet steel according to any of claims 1 to 16, characterized in that its strength is greater than or equal to 500 MPa and its uniform stretching is greater than or equal to 8%. 19. Manufacturing process characterized in that a steel is supplied in accordance with any of the compositions 1 to 8, the steel is cast in the form of semi-product, the temperature of the steel not exceeding the temperature of more than 40 ° C. liquid. 20. Manufacturing process according to claim 19, characterized in that the semi-product is cast in the form of a thin or thin-band ingot between counter-rotating cylinders. 21. Manufacturing procedure in accordance with claim 19 or 20, characterized in that the cooling rate during solidification of the casting is greater than or equal to 0.1 ° C / s. 22. Manufacturing process according to claim 19 to 21, characterized in that the semi-product is heated before hot rolling, the temperature and duration of the heating being chosen so that the density of eutectic precipitates of TiB2 and occasionally of Fe2B, of maximum size Lm¾x greater than 15 micrometers and of the form factor f > 5, is greater than 400 / mm2 and the semi-product is hot-rolled. 23. Manufacturing process according to any of claims 19 to 22, characterized in that a hot rolling of the semi-product is carried out, optionally a cold rolling and annealing, the rolling and annealing conditions being adjusted in such a way that A steel sheet whose average grain size is less than or equal to 15 micrometers is obtained. Method of manufacture according to any of claims 19 to 23, characterized in that a hot rolling of the semi-product is carried out, optionally a cold rolling and annealing, the rolling and annealing conditions being adjusted in such a way that A steel plate is obtained whose average grain size is less than or equal to 5 micrometers. 25. Manufacturing process according to any of claims 19 to 24, characterized in that a hot rolling of the semi-product is carried out, optionally a cold rolling and annealing, the rolling and annealing conditions being adjusted so that it is obtained a steel plate whose average grain size is less than or equal to 3.5 micrometers. Method according to any of claims 22 to 25, characterized in that hot rolling is carried out with a final laminate temperature of less than 820 ° C. 27. Use of a steel sheet according to any of claims 1 to 18, or manufactured by a method according to any of claims 19 to 26, for the manufacture of structural parts or reinforcement elements in the domain automotive