WO2008029011A2

WO2008029011A2 - Steel plate for producing light structures and method for producing said plate

Info

Publication number: WO2008029011A2
Application number: PCT/FR2007/001401
Authority: WO
Inventors: Frédéric Bonnet; Olivier Bouaziz; Jean-Claude Chevallot
Original assignee: Arcelormittal France; Cala, Laure
Priority date: 2006-09-06
Filing date: 2007-08-27
Publication date: 2008-03-13
Also published as: CN101563476B; MX2009002411A; EP2064360A2; ES2659987T3; MA30698B1; EP1897963A1; JP2010502838A; RU2009108338A; CN101563476A; WO2008029011A3; TR201802707T4; CA2662741C; CA2662741A1; BRPI0716877B1; KR20090043555A; UA95490C2; PL2064360T3; BRPI0716877A2; JP5298017B2; EP2064360B1

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 SHEET FOR MANUFACTURING ALLEGE STRUCTURES AND METHOD OF MANUFACTURING SAME

The invention relates to the manufacture of steel sheets or structural parts simultaneously combining a high modulus of elasticity E, a reduced density and a high strength.

It is known that the mechanical performances of structural elements vary like E ^x / d, the coefficient x depending on the mode of external stress (traction or bending for example) or the geometry of the elements (sheets, bars). This illustrates the interest of have materials simultaneously having a high modulus of elasticity and a reduced density. This need exists especially in the automotive industry where vehicle lightening and safety are a constant concern. It has thus been sought to increase the modulus of elasticity and to reduce the weight of steel parts by incorporating particles of ceramics of different kinds, such as carbides, nitrides, oxides or borides. Indeed, these materials have a much higher modulus of elasticity, ranging from about 250 to 550 GPa, than that of base steels, of the order of 210 GPa, where they are incorporated. In this way, a hardening is achieved by a charge transfer between the matrix and the ceramic particles under the influence of a stress. The refinement of the grain size of the matrix by the ceramic particles further increases this hardening. In order to produce these materials comprising ceramic particles uniformly distributed in a steel matrix, processes are known which are based on powder metallurgy: firstly, the geometrically controlled ceramic powders are prepared. these with steel powders, which corresponds for steel to an exogenous contribution of ceramic particles. The whole is compacted in mold and then brought to a temperature such that the sintering of this mixture is observed. In a variant of the process, metal powders are mixed so as to obtain the formation of the ceramic particles during the sintering phase. Despite improved mechanical characteristics compared to steels not With no dispersion of ceramic particles, this type of process suffers from several limitations:

- It requires careful development and implementation conditions not to cause reaction with the atmosphere, given the high specific surface of the metal powders.

Even after the compaction and sintering operations, residual porosities may remain possibly capable of acting as priming sites during cyclic stresses.

- 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, carbon)

When the particles are added in a large quantity, or in the presence of certain large particles, the elongation properties decrease.

- This type of process is suitable for production in small quantities but can not meet the needs of the automotive industry on a very large scale.

- Manufacturing costs associated with this type of manufacturing process are high.

Also known in the case of light alloys, manufacturing processes based on the exogenous addition of powders of ceramics in the liquid metal. Again, these processes suffer from most of the defects mentioned above. More particularly, the difficulty of homogeneous dispersion of the particles, these having a tendency to agglomeration or decantation / flotation in the liquid metal. Among the ceramics that could be used to increase the properties of steels, TiB ₂ titanium diboride, which has the following intrinsic characteristics, is particularly known: Modulus of elasticity: 565 GPa Density: 4.52

However, the manufacturing processes based on exogenous additions of TiB ₂ particles, suffer from the drawbacks mentioned above. The invention aims to solve the above problems, in particular the large scale and economical provision of steels with increased modulus of elasticity by the presence of TiB2 particles. The invention aims in particular at providing a continuous casting manufacturing method which does not present any particular difficulties when casting steels.

It also aims to provide steels with a quantity of TiB ₂ particles as large as possible homogeneously dispersed in the matrix. It also aims to provide high strength steels, whose uniform elongation is greater than or equal to 8% and having a high ability to different welding processes, including resistance welding. For this purpose, the invention relates to a steel sheet whose chemical composition comprises, the contents being expressed by weight: 0.010% <C <0.20%, 0.06% <Mn <3%, Si < 1.5%, 0.005% <Al <1.5%, S <0.030%, P <0.040%, of titanium and boron in amounts such that: 2.5% ≤ Ti ≤ 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 composition consisting of iron and unavoidable impurities resulting from the preparation.

Preferably, the contents of titanium and boron, expressed in% by weight, are such that: -0.22 <B - (0.45x Ti) <0.35. As a preference, the contents of titanium and boron, expressed in% by weight, are such that: -0.35 <B - (0.45x Ti) <- 0.22.

The titanium content is preferably such that: 4.6% <Ti <6.9%. According to one particular embodiment, the titanium content is such that: 4.6% <Ti <6%. The carbon content is preferably such that: C <0.080%. According to a preferred mode, the carbon content satisfies: C <0.050%. The chromium content is preferably such that: Cr <0.08%.

The invention also relates to a steel sheet of the above composition, comprising eutectic precipitates of TiB ₂ and possibly Fe ₂ B, whose average size is less than or equal to 15 micrometers, and preferably less than or equal to 10 micrometers. Preferably, more than 80% by number of TiB ₂ precipitates have a monocrystalline character. The invention also relates to a steel sheet according to the above characteristics, whose average grain size is less than or equal to 15 microns, preferably less than or equal to 5 microns, very preferably less than 3.5 microns. The invention also relates to a steel sheet according to one of the above characteristics, whose modulus of elasticity measured in the rolling direction is greater than or equal to 230GPa, preferably greater than or equal to 240GPa, or preferentially greater than or equal to 250GPa According to a particular mode, the strength of the steel sheet is greater than or equal to 500 MPa and its uniform elongation is greater than or equal to 8%. The subject of the invention is also an article manufactured from a plurality of steel parts, of identical or different composition, of identical or different thickness, at least one of the steel parts being a steel sheet. according to any of the above characteristics, welded to at least one of the other parts of this object, the composition or compositions of the other steel parts comprising by weight: 0.001-0.25% C, 0.05-2 % Mn, Si <0.4%, Al <0.1%, Ti <0.1%, Nb <0.1%, V <0.1%, Cr <3%, Mo <1%, Ni < 1%, B <0.003%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation. The invention also relates to a method according to which a steel is supplied according to any one of the above compositions, and the steel is cast in the form of a semi-finished product, the casting temperature not exceeding more than 40 ° C the liquidus temperature of the steel. According to a particular embodiment, the semi-finished product is cast as slabs or thin products between counter-rotating rolls. The cooling rate during the solidification of the casting is preferably greater than or equal to 0.1 ° C./s.

According to a particular mode, the semi-finished product is heated before hot rolling, the temperature and the duration of the heating being chosen so that that the density of eutectic precipitates of TiB ₂ and possibly Fe ₂ B, maximum size L _ma χ greater than 15 micrometers and form factor f> 5, is less than 400 / mm ² .

According to one particular embodiment, the semi-finished product is thermally rolled, optionally cold rolling and annealing, the rolling and annealing conditions being adjusted so that a steel sheet whose size is obtained is obtained. grain average is less than or equal to 15 microns, preferably less than or equal to 5 microns, very preferably less than 3.5 microns. The hot rolling is preferably carried out with an end-of-rolling temperature of less than 820 ^° C.

According to a particular embodiment, at least one blank is cut from a steel sheet according to one of the above modes, or manufactured according to one of the above modes, and the blank is deformed in a temperature range from 20 ° to 900 ^° C.

The invention also relates to a manufacturing method according to which one welds at least one steel sheet according to one of the above modes, or a steel sheet manufactured according to one of the modes above. The invention also relates to the use of a steel sheet or an object according to one of the above modes, or manufactured according to one of the modes above, for the manufacture of parts of structure or reinforcement elements in the automotive field.

Other features and advantages of the invention will emerge during the description below, given by way of non-limiting example and with reference to the appended figures in which:

FIGS. 1 and 2 respectively illustrate the microstructure of two steels according to the invention comprising eutectic Fe-TB2 precipitation, in the raw casting state.

- Figure 3 illustrates the microstructure of a steel according to the invention in the cold rolled and annealed state.

FIGS. 4 and 5 illustrate the microstructure of two steels according to the invention comprising eutectic precipitations Fe-TiB ₂ and Fe-Fe ₂ B, respectively in the raw state of casting and hot rolled. FIGS. 6 and 7 illustrate the microstructure of a steel according to the invention, cooled according to two cooling speeds during solidification, in the raw state of casting

With regard to the chemical composition of the steel, the carbon content is adapted for the purpose of economically attaining a given level of yield strength or resistance. The carbon content also makes it possible to control the nature of the microstructure of the matrix of steels according to the invention, which can be partially or totally ferritic, bainitic, austenitic or martensitic or comprise a mixture of these constituents in proportion adapted to satisfy the mechanical properties required. A carbon content greater than or equal to 0.010% makes it possible to obtain these various constituents.

The carbon content is limited because of the weldability: cold cracking resistance and heat-affected zone toughness decrease when the C content is greater than 0.20%. When the carbon content is less than or equal to 0.050% by weight, the resistance weldability is particularly improved.

Given the titanium content of the steel, the carbon content is preferably limited so as to avoid a primary precipitation of TiC and / or Ti (C ₁ N) in the liquid metal. These precipitates which form in the liquid are detrimental to the flowability in the continuous casting process of the liquid steel. On the other hand, when this precipitation occurs in the solidification or solid phase interval, it has a favorable effect on the structural hardening. The maximum carbon content must therefore preferably be limited to 0.080% so as to reveal the TiC and / or Ti (C, N) precipitates, mainly during the eutectic or solid phase solidification.

In an amount greater than or equal to 0.06%, manganese increases the quenchability, contributes to hardening in solid solution and thus to obtaining increased strength. It combines with any sulfur present, reducing the risk of hot cracking. However, beyond a content of 3% by weight of manganese, the risk of formation of a harmful band structure that would come from a possible segregation of manganese during solidification. Silicon effectively contributes to increasing strength through solid solution hardening. However, an excessive addition of silicon causes the formation of adherent oxides that are difficult to remove during a stripping operation, and the possible appearance of surface defects due in particular to a lack of wettability in the dip galvanizing operations. In order to maintain good coating properties, the silicon content should not exceed 1.5% by weight. In an amount greater than or equal to 0.005%, aluminum is a very effective element for the deoxidation of steel. Beyond a content of 1, 5% by weight, excessive primary precipitation of alumina occurs, however, causing flowability problems. In excess of 0.030%, sulfur tends to precipitate excessively in the form of manganese sulfides which greatly reduce hot or cold forming ability.

Phosphorus is a known element to segregate at grain boundaries. Its content must not exceed 0.040% so as to maintain a sufficient hot ductility by avoiding the creasability and to avoid hot cracking during welding.

As an option, nickel or molybdenum can be added to increase the strength of the steel. For economic reasons, these additions are limited to 1% by weight. As an option, chromium can be added to increase the strength. It also makes it possible to precipitate borides in a larger quantity. However, its content is limited to 3% by weight to make a less expensive steel.

Preferably, a chromium content of less than or equal to 0.080% will be chosen. In fact, an excessive addition of Cr leads to more borides being precipitated, but these are borides of (Fe, Cr)

Also optionally, niobium and vanadium may be added in an amount less than or equal to 0.1%, so as to obtain a complementary hardening in the form of precipitation of fine carbonitrides. Titanium and boron play an important role in the invention:

In a first embodiment, the weight contents expressed in percent, titanium and boron of the steel are such that:

2.5% <Ti <7.2% (0.45 xTi) - 0.35% ≤ B <(0.45 xTi) + 0.70%

The second relationship is equivalent - 0.35 <B- (0.45 xTi) <0.70 The reasons for these limitations are:

When the weight content of titanium is less than 2.5%, a precipitation of TiB ₂ does not occur in sufficient quantity; in fact, the volume fraction of TiB 2 precipitated is less than 5%, which does not allow to obtain a significant modification of the modulus of elasticity which remains below 220GPa.

When the weight content of titanium is greater than 7.2%, a coarse primary precipitation of TiB 2 occurs in the liquid metal and causes problems of flowability of the semi-finished products.

- If the weight contents of titanium and boron are such that:

B- (0.45 xTi)> 0.70, there is an excessive precipitation of Fe ₂ B which degrades the ductility. - If the weight contents of titanium and boron are such that:

B- (0.45 xTi) <-0.35, the content of titanium dissolved at room temperature in the matrix is greater than 0.8%. No significant beneficial technical effect is then obtained despite the higher cost of titanium addition.

According to a second embodiment of the invention, the titanium and boron contents are such that: -0.22 <B - (0.45 × Ti) <0.35

When: B- (0.45 xTi) ≤ 0.35, the precipitation of Fe ₂ B is very low, which increases the ductility.

- When: B- (0.45 xTi) ≥ -0.22, the content 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 titanium and boron contents are such that: -0.35 ≤ B - (0.45 × Ti) - 0.22 When the amount: B- (0.45xTi) is greater than or equal to -0.35 and less than -0.22, the content of titanium dissolved at room temperature in the matrix is between 0.5% and 0 respectively, 8%. This amount is particularly suitable for obtaining a precipitation composed only of TiB ₂ .

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:

- When the weight content of titanium is greater than or equal to 4.6%, a precipitation of TiB ₂ occurs such that the volume fraction precipitated is greater than or equal to 10%. The modulus of elasticity is then greater than or equal to about 240 GPa.

When the weight content of titanium is less than or equal to 6.9%, the amount of primary precipitates of TiB ₂ is less than 3% by volume. The total precipitation of TiB ₂ , consisting of possible primary precipitates and eutectic precipitates, is then 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 less than or equal to 6%, flowability is then particularly satisfactory due to the low precipitation of primary TiB ₂ in the liquid metal.

According to the invention, a eutectic Fe-TiB ₂ precipitation occurs during solidification. The eutectic character of the precipitation confers on the microstructure formed a particular character of fineness and homogeneity that is advantageous for the mechanical properties. When the amount of TiB ₂ eutectic precipitates is greater than 5% by volume, the modulus of elasticity of the steel measured in the rolling direction may exceed about 220 GPa. Beyond 10% by volume of TiB2 precipitates, the module can exceed about 240 GPa, which makes it possible to design structures with notable lightening. This amount can be increased to 15% by volume to exceed about 250 GPa, especially in the case of steels comprising alloying elements such as chromium or molybdenum. The presence of these elements indeed increases the maximum amount of TiB2 that can be obtained in the case of eutectic precipitation.

The boron and titanium contents according to the invention make it possible to avoid a coarse primary precipitation of TiB ₂ in the liquid metal. The formation of these primary precipitates of sometimes large size (several tens of micrometers) must be avoided because of their harmful role vis-à-vis mechanisms of damage or rupture during subsequent mechanical stresses. Moreover, these precipitates appeared in the liquid metal, when they do not decant, are distributed in a localized manner and reduce the homogeneity of the mechanical properties. This early precipitation must be avoided because it can lead to a plugging of nozzles from the continuous casting of steel following the agglomeration of precipitates. As stated, titanium must be present in sufficient quantity to lead to the endogenous formation of TiB ₂ in the form of eutectic Fe-TiB ₂ precipitation. According to the invention, the titanium may also be present dissolved at room temperature in the matrix in superstoichiometric proportion relative to boron, calculated from TiB ₂ . When the solid solution titanium content is less than 0.5%, the precipitation takes place in the form of two successive eutectics: Fe-TiB ₂ in the first place, then Fe-Fe ₂ B, this second endogenous Fe ₂ B precipitation intervenes in greater or lesser quantity depending on the boron content of the alloy. The amount precipitated as Fe ₂ B can be up to 8% by volume. This second precipitation also occurs according to a eutectic scheme to obtain a fine and homogeneous distribution, which ensures a good homogeneity of the mechanical characteristics.

The precipitation of Fe ₂ B completes that of TiB ₂ , the maximum amount of which is linked to the eutectic. Fe ₂ B has a role similar to that of TiB ₂ . It increases the modulus of elasticity and decreases the density. It is thus possible to adjust the mechanical properties in a fine way by adjusting the precipitation complement of Fe ₂ B with respect to the precipitation of TiB ₂ . This is a means that can be used in particular to obtain a modulus of elasticity greater than 250 GPa in steel as well as an increase in the mechanical strength of the product. When the steel contains a quantity of Fe ₂ B in volume greater than or equal to 4%, the modulus of elasticity increases by more than 5 GPa. The elongation at break is then between 14% and 16% and the mechanical strength reaches 590 MPa. When the amount of Fe ₂ B is greater than 7.5% by volume, the modulus of elasticity is increased by more than 10 GPa but the elongation at break is then less than 9%.

According to the invention, the average size of the eutectic precipitates of TiB ₂ or Fe ₂ B is less than or equal to 15 microns so as to obtain increased elongation characteristics and good fatigue properties. When the average size of these eutectic precipitates is less than or equal to 10 microns, the elongation at break may be greater than 20%. The inventors have demonstrated that, when more than 80% of the number of TiB ₂ eutectic precipitates have a monocrystalline character, the matrix-precipitated damage during a mechanical stress is reduced and the risk of defect formation is lower. because of the greater plasticity of the precipitate and its great cohesion with the matrix. In particular, it has been demonstrated that larger TiB ₂ precipitates have hexagonal crystallization. Without wishing to be bound by theory, it is believed that this crystallographic character confers an increased possibility of deformation by twinning of these precipitates under the effect of a mechanical stress.

This particular character of monocrystallinity, related to the precipitation of TiB ₂ in a eutectic form, is not encountered to such a degree for the processes of the prior art based on exogenous contributions of particles. In addition to the favorable effect of a dispersion of endogenous particles on the mechanical properties of traction, the inventors have shown that the limitation of the grain size was a very effective way to increase the mechanical characteristics of traction: When the average size grain is less than or equal to 15 micrometers, the resistance may exceed 560 MPa approximately. In addition, when the grain size is less than or equal to 3.5 micrometers, the resistance to cleavage is particularly high: Charpy resilience tests of thickness 3 mm at -60 ⁰ C, reveal that the proportion of ductile zone in the broken test pieces is greater than 90%. The implementation of the method of manufacturing a sheet according to the invention is as follows:

A steel of composition according to the invention is supplied

- It proceeds to the casting of a half-product from this steel. This casting can be carried out in ingots or continuously in the form of slabs of the order of 200mm thickness. It is also possible to perform the casting in the form of slabs of a few tens of millimeters thick or thin strips, a few millimeters thick, between contra-rotating rolls. The latter mode is particularly advantageous for obtaining fine eutectic precipitation and for preventing the formation of primary precipitates. An increase in the cooling rate upon solidification increases the fineness of the resulting microstructure.

Naturally, the casting may be carried out in a format allowing the manufacture of products of various geometries, in particular in the form of a billet for the manufacture of long products.

The fineness of the precipitation of TiB ₂ and Fe ₂ B increases the strength, ductility, resilience, formability and mechanical behavior in the Heat Affected Zone. The fineness of the precipitation is increased by a low casting temperature and a higher cooling rate. In particular, it has been found that a casting temperature limited to 40 ° C beyond the liquidus temperature, led to the obtaining of such fine microstructures.

The casting conditions will also be chosen so that the cooling rate at the time of solidification is greater than or equal to 0.1 ° C./s so that the size of the precipitates of TiB ₂ and Fe ₂ B is particularly fine.

The inventors have also demonstrated that the morphology of the eutectic precipitates of TiB ₂ and Fe ₂ B plays a role in the damage during a subsequent mechanical solidification. After observing the precipitates by optical microscopy at magnitudes ranging from 500 to 150Ox approximately on a surface which has a statistically representative population, it is determined by means of an image analysis software known per se such as, for example, the image analysis software Scion®., The maximum size L ^ _13x and minimum L _min of each precipitate. The ratio between the maximum and minimum size characterizes the

form of a given precipitate. The inventors have shown that precipitates of large size (L _max > 15 micrometers) and elongated (f> 5) reduce the distributed elongation and the coefficient of hardening n.

According to the invention, after casting of the semi-finished product, the temperature and the reheating time of the semi-finished product are chosen before the subsequent hot rolling so as to cause a globulization of the most harmful precipitates. In particular, the temperature and the reheating time will be chosen so that the density of eutectic precipitates with a size L _m χ ^> 5 microns and elongated (f> 5) is less than 400 / mm ² .

The semi-finished product is then hot-rolled, optionally followed by a winding. Optionally, cold rolling and annealing are carried out to obtain sheets of lesser thickness. The conditions of hot rolling, winding, cold rolling and annealing are chosen such that a steel sheet is obtained whose average grain size is less than or equal to 15 micrometers, preferably less than 5 microns. micrometers, very preferably less than 3.5 micrometers. A finer grain size is obtained by: - an important work hardening before the end of hot rolling and before the allotropic transformation (γ-α) occurring during cooling

a low end-of-rolling temperature, preferably less than 820 ^° C.

an accelerated cooling after the transformation (γ-α) so as to limit the growth of the ferritic grain

- a relatively low temperature winding

after a possible cold rolling, a limitation of the annealing temperature and of the annealing time in order to obtain a complete recrystallization, without exceeding the temperature and the time beyond the values which are necessary for this recrystallization.

In particular, an end-of-hot-rolling temperature below 82 ^° C. is an effective means for obtaining a fine grain size. We put in Obviously, in the steels according to the invention, a particular effect of the precipitates of TiE $ 2 and Fβ2B on the germination and the recrystallization of the microstructures: indeed, during a deformation of the steels according to the invention, the significant difference in mechanical behavior between the precipitates and the matrix leads to greater deformation around the precipitates. This intense local deformation decreases the non-recrystallization temperature: a low rolling end temperature promotes ferritic germination around the precipitates and limits grain growth. Likewise, the higher deformation field around the precipitates favors the germination of the grains during the restoration / recrystallization which follows the cold rolling, resulting in a refinement of the grain. The steel sheet thus obtained thus has a very good formability: without wishing to be bound by theory, it is believed that the eutectic precipitates present within a highly deformable matrix play a role similar to that played by the martensitic or bainitic phases within the ferrite in "Dual-Phase" type steels. The steels according to the invention have a ratio (elasticity limit Re / resistance Rm) favorable to various shaping operations. Depending on the carbon content and the quenching elements, and depending on the cooling rate below the temperature Ar1 (this temperature designates the start of transformation on cooling from the austenite), hot-rolled or hot-rolled sheets can be obtained. cold and annealed having matrices with various microstructures: these may be totally or partially ferritic, bainitic, martensitic or austenitic.

For example, a steel containing 0.04% C, 5.9% Ti, 2.3% B will have, after cooling from 1200 ⁰ C, a hardness ranging from 187 to 327 HV for a cooling rate ranging from at 15O ⁰ CVs. The highest levels of hardness correspond in this case to a totally bainitic matrix composed of slats with low disorientation, without carbides.

In the case where it is desired to produce a part comprising a shaping operation, a blank is cut from the sheet and deformation is carried out by means such as stamping, folding in a box. temperature range between 20 and 900 ° C. Very good thermal stability of the TiB ₂ and Fe ₂ B hardening phases is observed up to 1100 ^° C.

Given the thermal stability of the particles dispersed in the matrix and the good aptitude for the various cold, hot or hot forming processes, pieces of complex geometry with an increased modulus of elasticity can be produced according to the invention. . In addition, the increase of the modulus of elasticity of the steels according to the invention decreases the springback after the shaping operations and thus increases the dimensional accuracy on finished parts.

Structural elements are also advantageously manufactured by welding steels according to the invention, of identical or different composition or thickness so as to obtain in the final stage parts whose mechanical characteristics vary within them and are adapted locally. to subsequent solicitations.

In addition to iron and the inevitable impurities, the composition by weight of steels that can be welded to the steels according to the invention will comprise, for example: 0.001-0.25% C, 0.05-2% Mn, Si≤ O.4%, AI <0.1%, Ti <0.1%, Nb <0.1%, V <0.1%, Cr <3%, Mo <1%, Ni <1%, B < 0.003%, the balance of the composition consisting of iron and unavoidable impurities resulting from the elaboration.

In the melted zone, given the high temperature reached, there is a partial dissolution of the precipitates and their reprecipitation cooling. The amount of precipitates in the melted zone is very comparable to that of the base metal. Within the Heat Affected Zone (ZAC) of welded joints, eutectic precipitates are not dissolved and may even act as a brake on the growth of the austenitic grain and possible germination sites during the subsequent cooling phase. In a welding implementation of the steels according to the invention, a homogeneity of the amount of TiB ₂ and Fe ₂ B precipitates is thus obtained, which goes from the base metal to the molten metal through the ZAC, which ensures that the mechanical properties referred (module, density) will also be provided continuously in the case of welded connections. By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention.

Example 1

Steels were made whose composition is shown in Table 1 below, expressed in weight percent.

In addition to the 1-1 and I-2 steels according to the invention, the composition of a reference steel R1 containing no endogenous eutectic precipitates of TiB ₂ or Fe ₂ B has been indicated for comparison.

These steels were produced by casting semi-finished products from the liquid state, the additions of titanium and boron being carried out for steels 1-1 and I-2 in the form of ferroalloys. The casting temperature is 133O ⁰ C, an excess of 40 ^° C. with respect to the liquidus temperature.

Table 1: Compositions of steels (% weight). I = according to the invention.

R = reference. (*): Not in accordance with the invention

The microstructure in the raw state of casting, illustrated in FIGS. 1 and 2, relating respectively to steels 1-1 and I-2, shows a fine and homogeneous dispersion of endogenous TiB ₂ precipitates within a ferritic matrix. Boron precipitates as a Fe-TiB ₂ binary eutectic.

The volume amounts of precipitates were measured by means of an image analyzer and are respectively 9% and 12.4% for I-1 and I-2 steels. The amount of TiB ₂ in the form of primary precipitates is less than 2% by volume and promotes good flowability. The average sizes of the eutectic precipitates of TiB ₂ are respectively 5 and 8 microns for the 1-1 and I-2 steels. Among the population of these precipitates, more than 80% in number have a monocrystalline character. After reheating to 1150 ^° C., the semi-finished products were then hot-rolled in the form of sheets to a thickness of 3.5 mm, the end-of-rolling temperature being 94O ⁰ C. Hot rolling was followed by a winding at 700 ^° C. It was also carried out heating treatments at 123O ⁰ C on I-2 steel before hot rolling, for varying periods of 30 to 120 minutes. The morphology of the precipitates was then observed. It has been demonstrated that a treatment at 123O ⁰ C for a duration greater than or equal to 120 minutes makes it possible to globulise the precipitates so that the density of the eutectic precipitates of large size (Lmax ^ δ micrometers) and elongated (f> 5) is less than 400 / mm ² . The distributed elongation Au and the coefficient of hardening n are then significantly increased since they pass respectively from 11% and 0.125 (reheating time: 30 minutes) to 16% and 0.165 (reheating time 120 minutes) thanks to the treatment. of globulization of the precipitates. Moreover, in the case of steel I-2, a sheet was hot-rolled with a rolling end temperature of 810 ^° C.

These hot-rolled sheets were then pickled according to a method known per se and then cold-rolled to a thickness of 1 mm. A recrystallization annealing was then carried out at 800 ^° C. for 1 minute of maintenance, followed by cooling in air.

The observations made by Scanning Electron Microscopy reveal no decohesion at the eutectic precipitates / matrix interface or any damage to the precipitates themselves as a result of hot rolling or cold rolling.

After hot rolling, the average grain size of the steel 1-1 is 12 micrometers whereas it is 28 micrometers for the reference steel. In the case of I-2 steel, a low end-of-rolling temperature (810 ° C) leads to a very fine average grain size (3.5 micrometers) after hot rolling.

After cold rolling and annealing, the structure of steels 1-1 and I-2 is recrystallized, as shown in Figure 3 for steel 1-1. The photo was made with a scanning electron microscope in crystalline contrast, which attests to the totally recrystallised nature of the structure. Most precipitates are eutectic precipitates. Compared to conventional steel R-1, precipitates of TiB ₂ cause a significant refinement of the microstructure: The average grain size is 3.5 micrometers for steel 1-1 according to the invention whereas is equal to 15 micrometers for the reference steel R-1.

Measurements by pycnometry indicate that the presence of precipitates of TiB ₂ and Fe ₂ B is associated with a significant reduction in the density d since this passes from 7.80 (conventional steel R-1) to 7.33 (steel I-2) The elastic moduli of steels 1-1 and I-2 measured in the rolling direction are 230 GPa and 240 GPa, respectively. The modulus of elasticity of the reference steel R-1 is 210 GPa. For bending-stressed sheets whose performance index varies as E ^1/3 / d, the use of the steels according to the invention allows a weight reduction of greater than 10% compared with conventional steels.

The tensile mechanical properties measured (conventional yield strength Re measured at 0.2% deformation, resistance Rm, uniform elongation Au, elongation at break At) were given in Table 2 (hot-rolled sheets) or 3 (sheets). cold rolled and annealed) below.

Table 2: Mechanical tensile characteristics of hot-rolled sheets, (direction parallel to rolling)

Table 3: Mechanical tensile characteristics of cold-rolled and annealed sheets (parallel to rolling) The ratio Re / Rm of the hot-rolled or cold-rolled sheets according to the invention is close to 0.5, reflecting a mechanical behavior approaching that of a Dual-Phase steel and a good aptitude for subsequent shaping. Spot resistance welding tests were carried out on cold-rolled steel 1-1 sheets: failure in tensile-shear tests occurs systematically by unsticking. It is known that this is a preferred mode of rupture because associated with high energy. The presence of eutectic precipitates according to the invention is also observed in welded melted zones, which contributes to a homogeneity of mechanical properties in welded joints. Satisfactory properties are also obtained in LASER welding and arc welding.

Example 2

Table 4 below shows the composition of three steels according to the invention.

Table 4 Steel compositions according to the invention (% by weight)

The steels were produced by casting semi-finished products, the additions of titanium and boron being carried out in the form of ferroalloys. The casting temperature is 40 ^° C. above the liquidus temperature. In comparison with the steels 1-1 and I-2, the steels I-3 to I-5 have an excess of boron with respect to the stoichiometry of TiB ₂ so that eutectic co-precipitation of TiB ₂ and Fe ₂ B occur. The volume amounts of eutectic precipitates are shown in Table 5.

Table 5: Precipitation content (% by volume) relative to I-3-4-5 steels

The eutectic precipitates have an average size of less than 10 micrometers. FIG. 4 illustrates, in the case of steel I-3, the coexistence of TiB ₂ and Fe ₂ B precipitates. The Fe ₂ B precipitates appearing in light gray and the darker TiB ₂ precipitates are dispersed. within the ferritic matrix.

The semi-finished products were hot-rolled under the same conditions as those described in Example 1. Again, there is no damage to the precipitated-matrix interface. Figure 5 illustrates the microstructure of steel I-5. Characteristics of these hot-rolled steels are shown in Table 6.

Table 6: Mechanical tensile characteristics of hot rolled sheets (direction parallel to rolling) and density.

With respect to the steels 1-1 and I-2, a complementary eutectic precipitation of Fe ₂ B in a volume amount ranging from 3 to 7.9% increases the modulus of elasticity from 5 to 15 GPa.

The additional precipitation of Fe ₂ B increases the mechanical strength. When this precipitation occurs in excessive proportions, the uniform elongation can however be significantly lower than 8%. Example 3

Semi-finished products of steel of composition I-2 were cast at a temperature of 1330 ^° C. By varying the intensity of the cooling flow of these semi-finished products, and the thickness of the cast semi-finished products, two cooling rates were achieved, ie 0.8 and 12 ° C / s. The microstructures shown in FIGS. 6 and 7 illustrate that an increased cooling rate makes it possible to very significantly refine the eutectic Fe-TiB ₂ precipitation.

Example 4

Composite steel sheets 1-2 of 2.5mm thick were welded by LASER CO ₂ under the following conditions: Power: 5.5kW, welding speed: 3m / min. Micrographic observations in the melted zone show that a eutectic Fe-TiB ₂ precipitation occurs in a very fine form during cooling from the liquid state. The amount of precipitates in the melted zone is close to that of the base metal. Depending on the local cooling conditions at the time of solidification (local gradient G of temperature, speed of displacement R of the isotherms), the solidification takes place in dendritic form or in cellular form. The dendritic morphology is more likely to be associated with the Heat Affected Zone, given the local solidification conditions (high G gradient, low R speed). The TiB ₂ precipitates are therefore present in the various zones of the bond (base metal, ZAC, molten zone), thus the increase of the modulus of elasticity and the reduction of the density are carried out in the whole of the welded joint.

A 1-2 steel sheet was also welded by LASER without any difficulty in operation with a soft, stampable steel sheet whose composition contains (% by weight): 0.003% C, 0.098% Mn, 0.005% Si, 0.059% AI , 0.051% Ti, 0.0003% B, as well as unavoidable impurities resulting from the elaboration. The melted zone still contains Fe-TiB ₂ eutectic precipitation, which is naturally less important than in the case of welding. autogenous. In this way, it is possible to manufacture metal structures whose rigidity properties vary locally and whose mechanical characteristics more specifically correspond to the local requirements for implementation or maintenance in service.

Example 5

1-2 cold-rolled and annealed steel sheets according to the invention, with a thickness of 1.5 mm, were assembled in spot resistance welding under the following conditions: - Assembly force: 650 daN

- Welding cycle: 3 x (7 periods of current flow at an intensity I + 2 periods without current flow)

The welding domain expressed in intensity I is between 7 and 8.5 kA. The two terminals of this domain correspond on the one hand to obtaining a core diameter greater than 5.2mm (lower limit in intensity) and on the other hand to the appearance of the spark when welding (terminal The steel according to the invention thus has good resistance to spot welding with a sufficiently wide weldability range of 1.5 kA. The invention thus enables the manufacture of structural parts or reinforcing elements with an increased level of performance, both in terms of intrinsic lightening and the increase of the modulus of elasticity. The easy implementation by welding of the steel sheets according to the invention makes their incorporation possible within more complex structures, in particular by means of connections with pieces of steels of different composition or thickness. We will take particular advantage of these different characteristics in the automotive field.

Claims

1. Steel sheet whose chemical composition includes, the contents being expressed by weight: 0.010% ≤ C ≤ 0., 20%

0.06% <Mn <3%

If <1, 5% 0.005% <Al <1.5%

S <0.030% P <0.040%, titanium and boron in quantities such that

2.5% <Ti <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 composition consisting of iron and unavoidable impurities resulting from the elaboration

2. Sheet steel according to claim 1, characterized in that the titanium and boron contents are such that: -0.22 <B - (0.45x Ti) <0.35

3. Sheet steel according to claim 1, characterized in that the titanium and boron contents are such that:

-0.35 <B - (0.45x Ti) <- 0.22

4. Sheet steel according to any one of claims 1 to 3 characterized in that the titanium content is such that: 4.6% <Ti <6.9%

5. Sheet steel according to claim 4 characterized in that the titanium content is such that: 4.6% <Ti <6%

6. Sheet steel according to any one of claims 1 to 5, characterized in that its composition comprises, the content being expressed by weight: C <0.080%

7. Sheet steel according to any one of claims 1 to 6, characterized in that its composition comprises, the content being expressed by weight:

C <0.050%

8. Sheet steel according to any one of claims 1 to 7, characterized in that its composition comprises, the content being expressed by weight:

Cr <0.08%

9. Steel sheet according to any one of claims 1 to 8, characterized in that it comprises eutectic precipitates of TiB2βt possibly Fβ ₂ B, whose average size is less than or equal to 15 micrometers

10. Steel sheet according to any one of claims 1 to 9, characterized in that it comprises eutectic precipitates of T1B2 and optionally Fβ ₂ B, 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 said TiB ₂ precipitates have a monocrystalline character

12. Sheet steel according to any one of claims 1 to 11, characterized in that the average grain size of said steel is less than or equal to 15 micrometers

13. Sheet steel according to any one of claims 1 to 12, characterized in that the average grain size of said steel is less than or equal to 5 micrometers

Steel sheet according to any one of claims 1 to 13, characterized in that the average grain size of said steel is less than or equal to 3.5 micrometers

15. Steel sheet according to any one of claims 1 to 14, characterized in that its modulus of elasticity measured in the rolling direction is greater than or equal to 230GPa

16. Steel sheet according to any one of claims 1 to 15, characterized in that its modulus of elasticity measured in the direction of rolling is greater than or equal to 240GPa

Steel sheet according to any one of claims 1 to 16, characterized in that its modulus of elasticity measured in the rolling direction is greater than or equal to 250GPa.

Steel sheet according to any one of Claims 1 to 16, characterized in that its strength is greater than or equal to 500 MPa and its uniform elongation is greater than or equal to 8%.

19. Object manufactured from a plurality of steel parts, of identical or different composition, of identical or different thickness, characterized in that at least one of said steel pieces is a steel sheet according to the invention. any one of claims 1 to 18, welded to at least one of the other said pieces of steel, the composition or compositions of the other said pieces of steel comprising by weight: 0.001 -0.25% C, 0.05-2% Mn, Si <0.4%, Al <0 , 1%, TiO, 1%, Nb <0.1%, V <0.1%, Cr <3%, Mo <1%, Ni <1%, B <0.003%, the rest of the composition being constituted iron and unavoidable impurities resulting from the elaboration

20. Manufacturing process according to which a steel is supplied according to any one of compositions 1 to 8, said steel is cast in the form of a semi-finished product, the casting temperature not exceeding by more than 40 ^° C. the temperature of liquidus of said steel

21. Manufacturing process according to claim 20, characterized in that said semi-product is cast as a thin slab or thin strip between contra-rotating rolls.

22. The manufacturing method according to claim 20 or 21, characterized in that the cooling rate during solidification of said casting is greater than or equal to 0.1 ° C / s.

23. The manufacturing method according to any one of claims 20 to 22, characterized in that said semi-product is heated before hot rolling, the temperature and the duration of said heating being chosen so that the density of eutectic precipitates TiB ₂ and optionally Fe ₂ B, maximum size L _max greater than 15 micrometers and form factor f> 5, less than 400 / mm ² , and that said half-product is hot-rolled

24. A method according to any one of claims 20 to 23, characterized in that a hot rolling of said semi-product is carried out, optionally cold rolling and annealing, the rolling and annealing conditions being adjusted to such a degree. so that we obtain a steel sheet whose average grain size is less than or equal to 15 micrometers

25. A method according to any one of claims 20 to 24, characterized in that a hot rolling of said semi-product is carried out, optionally cold rolling and annealing, the rolling and annealing conditions being adjusted to such a degree. so that one obtains a steel sheet whose average grain size is less than or equal to 5 micrometers

26. Process according to any one of claims 20 to 25, characterized in that a hot rolling of said semi-product is carried out, optionally a cold rolling and an annealing, the rolling and annealing conditions being adjusted to such a degree. so that one obtains a steel sheet whose average grain size is less than or equal to 3.5 micrometers

27. Process according to any one of claims 23 to 26, characterized in that said hot rolling is carried out with an end-of-rolling temperature of less than 820 ^° C.

28. A method of manufacturing structural parts, characterized in that is cut at least one blank from a steel sheet according to any one of claims 1 to 18, or manufactured according to any one of claims 20 to 27, and said at least one blank is deformed in a temperature range of from 20 ° to 900 ^° C.

29. Process for the production of structural parts, characterized in that at least one steel sheet according to any one of Claims 1 to 18 or made according to any one of Claims 20 to 27 is welded.

30. Use of a steel sheet according to any one of claims 1 to 18, or an object according to claim 19, or manufactured by a process according to any one of claims 20 to 29, for the manufacture structural parts or reinforcement elements in the automotive field.