EP1994192B1 - Process for manufacturing steel sheet having very high strength, ductility and toughness characteristics, and sheet thus produced - Google Patents

Abstract

Hot rolled steel sheet comprises (in %): carbon (0.1-0.25); manganese (1-3); aluminum (>= 0.015); silicon (1.985); molybdenum (= 0.3); chromium (= 1.5); sulfur (= 0.015); phosphorus (= 0.1); cobalt (= 1.5); boron (= 0.005); and iron and impurities (rest), where the sum of silicon and aluminum is 1-2 and the sum of chromium and molybdenum is greater than 0.3. The sheet has a strength of greater than 1200 MPa and a Re/Rm ratio of 0.75 (where Re is an elastic limit of the steel, and Rm is mechanical resistance of the steel) with elongation at rupture of greater than 25%. An independent claim is included for a process of manufacturing the steel comprising supplying the steel composition, casting the steel composition, heating the steel composition at greater than 1150[deg]C, hot rolling the semi-finished product until the microstructure of steel is entirely austenite in nature, cooling the obtained stainless steel at a temperature greater than the austenite transformation temperature i.e. at a cooling speed of 50-90[deg]C/second, a bainite transformation temperature or at Ms+50[deg]C, (where Ms is a transformation temperature of martensite) and cooling the stainless steel at a cooling speed of 0.08-600[deg]C/minute until the ambient temperature is reached, where the bainite transformation temperature is 0.08-2[deg]C/minute or is Bs+60[deg]C, when the speed is greater than 2-600[deg]C/minute.

Description

The invention relates to the manufacture of hot rolled sheets of so-called "multiphase" steels, simultaneously having a very high strength and a deformation capacity for carrying out cold forming operations. The invention more specifically relates to predominantly bainitic microstructure steels having a strength greater than 1200 MPa and a yield / strength ratio lower than 0.75. The automotive sector and the general industry are particularly fields of application for these hot-rolled steel sheets.

• On a tout d'abord proposé des aciers comportant des éléments de micro-alliage dont le durcissement est obtenu simultanément par précipitation et par affinement de la taille de grains. Le développement de ces aciers a été suivi par celui d'aciers « Dual-Phase » où la présence de martensite au sein d'une matrice ferritique permet d'obtenir une résistance supérieure à 450MPa associée à une bonne aptitude au formage à froid.

In particular, there is a continuing need in the automotive industry for lighter vehicles and increased safety. This is how we proposed several families of steels offering different levels of resistance:

• Steels were first proposed comprising micro-alloy elements whose hardening is obtained simultaneously by precipitation and by grain size refinement. The development of these steels was followed by that of "Dual-Phase" steels where the presence of martensite within a ferritic matrix makes it possible to obtain a strength greater than 450 MPa combined with a good cold forming ability.

In order to obtain even higher resistance levels, steels having a "TRIP" (Transformation Induced Plasticity) behavior have been developed with very advantageous combinations of properties (resistance-ability to deformation): these properties are related to the structure of these steels consisting of a ferritic matrix comprising bainite and residual austenite. The residual austenite is stabilized by the addition of silicon or aluminum, these elements delaying the precipitation of carbides in the austenite and in the bainite. The presence of residual austenite gives high ductility to an undeformed sheet. Under the effect of a subsequent deformation, for example during uniaxial loading, the residual austenite of a TRIP steel part is transformed progressively in martensite, which results in a significant consolidation and delays the appearance of a necking.

To reach an even higher resistance, ie a level greater than 800-1000 MPa, multiphase steels with predominantly bainitic structure have been developed: in the automobile industry or in the general industry, these steels are used with profit for structural parts such as bumper sleepers, uprights, various reinforcements, wear parts resistant to abrasion. The ability to shape these parts, however, requires simultaneously sufficient elongation, greater than 10% and a ratio (yield strength / resistance) not too high so as to have a sufficient plasticity reserve.

The patent US 6,364,968 describes the manufacture of niobium or titanium microalloyed hot-rolled sheets with a resistance greater than 780 MPa of bainitic or bainitomensitic structure comprising at least 90% of bainite with a grain size of less than 3 micrometers: the examples in the patent show that the resistance obtained hardly exceeds 1200 MPa, together with a ratio Re / R _m greater than 0.75. It is also noted that the carbides present in this type of very predominantly bainitic structure lead to mechanical damage in case of stress, for example in hole expansion tests.

The patent US 4,472,208 also describes the manufacture of titanium microalloyed hot-rolled steel sheet with a predominantly bainitic structure, comprising at least 10% of ferrite, and preferably 20 to 50% of ferrite, and a precipitation of titanium carbides TiC. Due to the large amount of ferrite, however, the strength of the grades made according to this invention is less than 1000 MPa, which may be insufficient for some applications.

The patent JP2004332100 describes the manufacture of hot-rolled sheet with a resistance greater than 800 MPa, with a predominantly bainitic structure, containing less than 3% of residual austenite. In order to obtain high values of resistance, however, expensive additions of niobium must be made.

The patent JP2004190063 discloses the manufacture of hot-rolled high strength steel sheet having a strength-elongation product of greater than 20000 MPa%, and containing austenite. These steels, however, contain expensive additions of copper, in relation to the in sulfur. A high ductility steel having 1% ≤Si + Al≤3% and a structure consisting of martensite and / or lower bainite as well as 5-30% residual austenity with optional participation in Ti, V, Zr or Nb is disclosed in EP725156 .

The present invention aims to solve the problems mentioned above. It aims at providing a hot-rolled steel having a mechanical strength greater than 1200 MPa together with good cold formability, a Re / R _m ratio of less than 0.75, an elongation at break greater than 10%. The invention also aims at providing a steel that is not very sensitive to damage during cutting by a mechanical method.

It also aims to have a steel with good toughness so as to withstand the sudden propagation of a defect, especially in case of dynamic solicitation. A Charpy V energy of rupture higher than 28 Joules is sought at 20 ° C. It also aims to have a steel having good weldability by means of the usual assembly processes in a thickness range of from 1 to more than 30 millimeters, in particular during resistance welding or spot welding. arc, in particular in MAG welding ("Metal Active Gas"). The invention also aims to provide a steel whose composition does not include expensive micro-alloy elements such as titanium, niobium or vanadium. In this way, the manufacturing cost is lowered and the thermomechanical manufacturing diagrams are simplified. It is also intended to provide a steel having a very high fatigue endurance limit. The invention further aims to provide a manufacturing method in which small variations in the parameters do not lead to significant changes in the microstructure or mechanical properties.

For this purpose, the subject of the invention is a hot-rolled steel sheet with a resistance greater than 1200 MPa, a Re / R _m ratio of less than 0.75 and an elongation at break greater than 10%, the composition of which contains, the contents being expressed by weight: 0.10% ≤ C ≤ 0.25%, 1% ≤ Mn ≤ 3%, Al ≥ 0.015%, Si≤1.985%, Mo ≤ 0.30%, Cr ≤ 1.5%, S ≤ 0.015%, P ≤ 0.1%, Co≤1.5%, B ≤ 0.005%, it being understood that 1 % ≤Si + Al ≤2%, Cr + (3 x Mo) ≥0.3%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the elaboration, the microstructure of the steel consisting of at least 75% of bainite, of residual austenite in an amount greater than or equal to 5%, and of martensite in an amount of greater than or equal to 2%.

Preferably, the carbon content of the steel sheet is such that: 0.10% ≤ C ≤ 0.15%.

Preferentially, the carbon content is such that: 0.15% <C ≤ 0.17%.

According to a preferred embodiment, the carbon content is such that: 0.17% <C ≤ 0.22%.

Preferably, the carbon content is such that: 0.22% <C ≤ 0.25% According to a preferred embodiment, the composition of the steel comprises: 1% ≤Mn ≤ 1.5%.

Preferably, the composition of the steel is such that: 1.5% <Mn ≤ 2.3%.

As a preference, the composition of the steel comprises: 2.3% <Mn ≤ 3%

In a preferred embodiment, the composition of the steel comprises: 1.2% ≤Si ≤ 1.8%.

Preferably, the composition of the steel comprises: 1.2% ≤Al ≤ 1.8%.

According to a preferred embodiment, the composition of the steel is such that: Mo ≤0.010%.

The invention also relates to a steel sheet whose carbon content of the residual austenite is greater than 1% by weight.

The subject of the invention is also a steel sheet, comprising carbides between the bainite slats, the number N of interlayer carbides greater than 0.1 micrometers per unit area is less than or equal to 50000 / mm ² .

est inférieur àThe subject of the invention is also a steel sheet comprising residual martensite-austenite islands, the number of which is N _MA per unit area, of residual martensite-austenite islands whose maximum size L _max is greater than 2 micrometers and whose elongation factor $\frac{{\mathrm{The}}_{\max }}{{\mathrm{The}}_{\min }}$

is inferior to

4, is less than 14000 / mm ² .

• on approvisionne un acier de composition ci-dessus
• on procède à la coulée d'un demi-produit à partir de cet acier
• on porte le demi-produit à une température supérieure à 1150°C
• on lamine à chaud le demi-produit dans un domaine de température où la structure de l'acier est entièrement austénitique,
• puis on refroidit la tôle ainsi obtenue à partir d'une température T_DR située au dessus de Ar3 jusqu'à une température de transformation T_FR de telle sorte que la vitesse de refroidissement primaire V_R entre T_DR et T_FR soit comprise entre 50 et 90°C/s et que la température T_FR soit comprise entre B's et M_S+50°C, B'_S désignant une température définie par rapport à la température Bs de début de transformation bainitique, et M_S désignant la température de début de transformation martensitique, puis
• on refroidit la tôle à partir de la température T_FR avec une vitesse de refroidissement secondaire V'_R comprise entre 0,08°C/min et 600°C/min jusqu'à la température ambiante,
• la température B's étant égale à Bs lorsque la vitesse V'_R est supérieure ou égale à 0,08°C/min et inférieure ou égale à 2°C/min
• la température B's étant égale à Bs+60°C lorsque la vitesse V'_R est supérieure à 2°C/min et inférieure ou égale à 600°C/min

The subject of the invention is also a process for producing a hot-rolled steel sheet with a resistance greater than 1200 MPa, a Re / Rm ratio of less than 0.75 and an elongation at break greater than 10%, depending on which :

• supply a composition steel above
• a semi-finished product is cast from this steel
• the semi-finished product is heated to a temperature above 1150 ° C.
• the semi-finished product is hot-rolled in a temperature range where the structure of the steel is entirely austenitic,
• then the sheet thus obtained is cooled from a temperature T _DR located above Ar3 to a transformation temperature T _FR so that the primary cooling rate V _R between T _DR and T _FR is between 50 and 90 ° C / s and the temperature T _FR is between B's and M _S + 50 ° C, B ' _S designating a temperature defined with respect to the bainitic transformation start temperature Bs, and M _S denoting the temperature of beginning of martensitic transformation, then
• the sheet is cooled from the temperature T _FR with a secondary cooling rate V ' _R of between 0.08 ° C./min and 600 ° C./min up to the ambient temperature,
• the temperature B is equal to Bs when the speed V ' _R is greater than or equal to 0.08 ° C / min and less than or equal to 2 ° C / min
• the temperature B is equal to Bs + 60 ° C when the speed V ' _R is greater than 2 ° C / min and less than or equal to 600 ° C / min

• on approvisionne un acier de composition ci-dessus,
• on procède à la coulée d'un demi-produit à partir de cet acier
• on porte le demi-produit à une température supérieure à 1150°C et on le lamine à chaud dans un domaine de température où la microstructure de l'acier est entièrement austénitique, puis
• on refroidit la tôle ainsi obtenue à partir d'une température T_DR située au dessus de Ar3 jusqu'à une température intermédiaire T_I avec une vitesse de refroidissement V_R1 supérieure ou égale à 70°C/s, la température T_I étant inférieure ou égale à 650°C, puis
• on refroidit la tôle à partir de la température T_I jusqu'à une température T_FR, la température T_FR étant comprise entre B'_S et M_S+50°C, B'_S désignant une température définie par rapport à la température Bs de début de transformation bainitique, et M_S désignant la température de début de transformation martensitique,
  de telle sorte que la vitesse de refroidissement entre la température T_DR et la température T_FR soit comprise entre 20 et 90°C/s, puis
• on refroidit la tôle à partir de la température T_FR avec une vitesse de refroidissement secondaire V'_R comprise entre 0,08°C/min et 600°C/min jusqu'à la température ambiante,
• la température B's étant égale à Bs lorsque la vitesse V'_R est comprise entre 0,08 et 2°C/min
• la température B's étant égale à Bs+60°C lorsque la vitesse V'_R est supérieure à 2°C/min et inférieure ou égale à 600°C/min

The subject of the invention is also a process for producing a hot-rolled steel sheet with a resistance greater than 1200 MPa, a Re / Rm ratio of less than 0.75 and an elongation at break greater than 10%, depending on which :

• supplying a steel of composition above,
• a semi-finished product is cast from this steel
• the semi-finished product is heated to a temperature above 1150.degree. hot rolled in a temperature range where the microstructure of the steel is entirely austenitic, then
• the sheet thus obtained is cooled from a temperature T _DR located above Ar3 to an intermediate temperature T _I with a cooling rate V _R1 greater than or equal to 70 ° C / s, the temperature T _I being lower or equal to 650 ° C, then
• the sheet is cooled from the temperature T _I to a temperature T _FR , the temperature T _FR being between B ' _S and M _S + 50 ° C, B' _S denoting a temperature defined with respect to the temperature Bs beginning of bainitic transformation, and M _S denoting the martensitic transformation start temperature,
  such that the cooling rate between the temperature T _DR and the temperature T _FR is between 20 and 90 ° C / s, then
• the sheet is cooled from the temperature T _FR with a secondary cooling rate V ' _R of between 0.08 ° C./min and 600 ° C./min up to the ambient temperature,
• the temperature B is equal to Bs when the speed V ' _R is between 0.08 and 2 ° C / min
• the temperature B is equal to Bs + 60 ° C when the speed V ' _R is greater than 2 ° C / min and less than or equal to 600 ° C / min

• on approvisionne un acier de composition ci-dessus
• on procède à la coulée d'un demi-produit à partir de cet acier
• on porte le demi-produit à une température supérieure à 1150°C
• on lamine à chaud le demi-produit dans un domaine de température où la structure de l'acier est entièrement austénitique,
• on ajuste la température de début de refroidissement primaire T_DR située au dessus de Ar3, la température de fin de refroidissement primaire T_FR, la vitesse de refroidissement primaire V_R entre T_DR et T_FR, et la vitesse de refroidissement secondaire V'_R de telle sorte que la microstructure de l'acier soit constituée d'au moins 75% de bainite, d'austénite résiduelle en quantité supérieure ou égale à 5%, et de martensite en quantité supérieure ou égale à 2%.

The invention also relates to a method for manufacturing a hot-rolled steel sheet according to which

• supply a composition steel above
• a semi-finished product is cast from this steel
• the semi-finished product is heated to a temperature above 1150 ° C.
• the semi-finished product is hot-rolled in a temperature range where the structure of the steel is entirely austenitic,
• adjusting the primary cooling start temperature T _DR above Ar3, the primary cooling end temperature T _FR , the primary cooling rate V _R between T _DR and T _FR , and the secondary cooling rate V _R so that the microstructure of the steel consists of at least 75% bainite, residual austenite in an amount greater than or equal to 5%, and martensite in a quantity greater than or equal to at 2%.

The invention also relates to a manufacturing method according to which the primary cooling start temperature T _{DR is} set above Ar3, the primary cooling end temperature T _FR , the primary cooling rate V _R between T _DR and T _FR , and the secondary cooling rate V ' _R , such that the carbon content of the residual austenite is greater than 1% by weight.

The invention also relates to a method according to which the primary cooling start temperature T _{DR is} set above Ar3, the primary cooling end temperature T _{FD is} the primary cooling rate V _R between T _DR and T _FR , and the secondary cooling rate V ' _R such that the number of interlayer carbides larger than 0.1 micrometers per unit area is less than or equal to 50000 / mm ² .

est inférieur à 4, soit inférieur à 14000/mm².The invention also relates to a method according to which the primary cooling start temperature T _{DR is} set above Ar3, the primary cooling end temperature T _FR , the primary cooling rate V _R between T _DR and T _FR , and the secondary cooling rate V ' _R , such that the number N _MA per unit area; of residual martensite-austenite islands whose maximum size L _max is greater than 2 micrometers and whose elongation factor $\frac{{\mathrm{The}}_{\max }}{{\mathrm{The}}_{\min }}$

is less than 4, ie less than 14000 / mm ² .

The invention also relates to the use of a hot-rolled steel sheet according to the characteristics described above, or manufactured by a method according to one of the above modes, for the manufacture of structural parts. or reinforcing elements, in the automotive field.

The invention also relates to the use of a hot-rolled steel sheet according to the characteristics described above, or manufactured by a method according to one of the above modes, for the manufacture of reinforcements and parts. structure for general industry, and abrasion resistance parts.

• La figure 1 présente une description schématique d'un mode de réalisation du procédé de fabrication selon l'invention, en relation avec un diagramme de transformation à partir de l'austénite.
• La figure 2 présente un exemple de microstructure d'une tôle d'acier selon l'invention.

Other features and advantages of the invention will become apparent from the description below, given by way of example and with reference to the attached figures, in which:

• The figure 1 presents a schematic description of an embodiment of the manufacturing method according to the invention, in relation to a transformation diagram from austenite.
• The figure 2 shows an example of microstructure of a steel sheet according to the invention.

Under usual cooling conditions after hot rolling, a steel containing about 0.2% C and 1.5% Mn is converted, during a cooling from the austenite, bainite composed of ferrite slats and carbides. In addition, the microstructure may contain a greater or lesser amount of pro-eutectoid ferrite formed at a relatively high temperature. However, the flow limit of this component is low, so that it is not possible to obtain a very high level of resistance when this constituent is present. The steels according to the invention do not include pro-eutectoid ferrite. In this way, the mechanical strength is significantly increased beyond 1200 MPa. Thanks to the compositions according to the invention, the precipitation of interlayer carbides is also delayed, the microstructure then consists of bainite, residual austenite, and martensite resulting from the transformation of the austenite. The structure also has an appearance of thin bainitic packs (a package designating a set of parallel slats within the same austenitic former grain) whose strength and ductility are superior to those of polygonal ferrite. The size of the bainite slats is of the order of a few hundred nanometers, the size of the batten packets, of the order of a few micrometers.

With regard to the chemical composition of steel, carbon plays a very important role in the formation of the microstructure and in the mechanical properties: From an austenitic structure formed at high temperature after rolling of a hot sheet a bainitic transformation occurs, and bainitic ferrite slats are initially formed within a matrix still predominantly austenitic. Due to the solubility Very lower carbon in ferrite compared to that in austenite, the carbon is rejected between slats. Thanks to certain alloying elements present in the compositions according to the invention, in particular thanks to the combined additions of silicon and aluminum, the precipitation of carbides, in particular cementite, occurs in a very limited manner. Thus, the untransformed austenite interlayer is progressively enriched in carbon substantially without significant precipitation of carbides intervening at the austenite-bainite interface. This enrichment is such that the austenite is stabilized, that is to say that the martensitic transformation of most of this austenite practically does not occur during cooling to room temperature. A limited amount of martensite appears as islets, contributing to increased resistance.

Carbon also delays the formation of pro-eutectoid ferrite, the presence of which must be avoided to obtain high levels of mechanical strength.

According to the invention, the carbon content is between 0.10 and 0.25% by weight: Below 0.10%, sufficient strength can not be obtained and the stability of the residual austenite is not not satisfactory. Beyond 0.25%, the weldability is reduced by the formation of low-tenacity microstructures in the heat-affected zone or in the melted zone under autogenous welding conditions.

According to a first preferred embodiment, the carbon content is between 0.10 and 0.15%: within this range, the weldability is very satisfactory and the toughness obtained is particularly high. Continuous casting is particularly easy because of a favorable solidification mode.

According to a second preferred embodiment, the carbon content is greater than 0.15% and less than or equal to 0.17%: within this range, the weldability is satisfactory and the toughness obtained is high.

According to a third preferred embodiment, the carbon content is greater than 0.17% and less than or equal to 0.22%: this range of compositions optimally combines strength properties on the one hand, ductility, toughness and weldability on the other hand.

According to a fourth preferred mode, the carbon content is greater than 0.22% and less than or equal to 0.25%: in this way the highest levels of mechanical strength are obtained at the cost of a slight decrease in toughness. .

In an amount of between 1 and 3% by weight, an addition of manganese, a gammagenic element, stabilizes the austenite by lowering the transformation temperature Ar 3. Manganese also helps to deoxidize steel during liquid phase processing. The addition of manganese also contributes to effective solid solution hardening and increased strength. Preferably, the manganese is between 1 and 1.5%: in this way a satisfactory curing is combined without risk of formation of harmful band structure. Preferentially, the manganese content is greater than 1.5% and less than or equal to 2.3%. In this way, the effects sought above are obtained without, however, excessively increasing the quenchability in the welded joints. Also preferably, the manganese is greater than 2.3% and less than or equal to 3%. Above 3%, the risk of carbide precipitation
or formation of structures in nefarious bands, becomes too important. Under the conditions defined according to the invention, in combination with the additions of molybdenum and / or chromium, a resistance greater than 1300 MPa can be obtained.

Silicon and aluminum, together, play an important role according to the invention.

Silicon inhibits the precipitation of cementite during cooling from austenite by considerably delaying the growth of carbides: this is due to the fact that the solubility of silicon in cementite is very low and that this element increases the activity of carbon in austenite: in this way, if a possible cementite seed is formed at the ferrite-austenite interface, the silicon is rejected at the interface. The carbon activity is then increased in this austenitic zone enriched in silicon. The growth of the cementite is then slowed down since the carbon gradient between the cementite and the surrounding austenitic zone is reduced. An addition of silicon therefore helps to stabilize a sufficient quantity residual austenite in the form of fine films which locally increase the resistance to damage and which prevent the formation of embrittling carbides.

Aluminum is a very effective element for the deoxidation of steel. As such, its content is greater than or equal to 0.015%. Like silicon, it is very slightly soluble in cementite and stabilizes the residual austenite.

It has been shown that the effects of aluminum and silicon on the stabilization of austenite are very similar: When the silicon and aluminum contents are such that: 1% ≤Si + Al≤2%, a stabilization satisfactory austenite is obtained, which allows to form the desired microstructures while retaining satisfactory use properties. Given that the minimum aluminum content is 0.015%, the silicon content is less than or equal to 1.985%.

Preferably, the silicon content is between 1.2 and 1.8%: in this way, the precipitation of carbides is avoided and excellent weldability is obtained; there is no cracking in MAG welding, with sufficient latitude in terms of welding parameters. Spot resistance welds are also free from defects. Moreover, since silicon stabilizes the ferritic phase, an amount of less than or equal to 1.8% makes it possible to avoid the formation of undesirable pro-eutectoid ferrite. An excessive addition of silicon also causes the formation of strongly adherent oxides and the possible appearance of surface defects, leading in particular to a lack of wettability in dip galvanizing operations.

Preferentially, these effects are obtained when the aluminum content is between 1.2 and 1.8%. At equivalent content, the effects of aluminum are indeed very similar to those noted above for silicon. The risk of occurrence of superficial defects is however reduced.

Molybdenum retards bainitic transformation, contributes to hardening by solid solution and also refines the size of the bainitic slats formed.

According to the invention, the molybdenum content is less than or equal to 0.3% to prevent the excessive formation of quenching structures.

In less than 1.5%, chromium has a substantially similar effect to molybdenum since it also helps to prevent the formation of pro-eutectoid ferrite and the hardening and refinement of the bainitic microstructure.

According to the invention, the contents of chromium and molybdenum are such that:

Cr + (3 x Mo) ≥0.3%.

The coefficients of chromium and molybdenum in this relation reflect the respective greater or lesser ability of these two elements to retard the ferritic transformation: when the above inequality is satisfied, the formation of pro-eutectoid ferrite is avoided in the specific cooling conditions according to the invention.

However, molybdenum is an expensive element: the inventors have demonstrated that it is possible to manufacture a steel particularly economically by limiting the molybdenum content to 0.010% and compensating for this reduction by adding chromium to respect the relationship. : Cr + (3 x Mo) ≥0.3%.

In excess of 0.015%, sulfur tends to precipitate excessively in the form of manganese sulphides which greatly reduce the shaping ability.

Phosphorus is a known element to segregate at grain boundaries. Its content must be limited to 0.1% in order to maintain sufficient hot ductility. The sulfur and phosphorus limitations also make it possible to obtain good weldability in spot welding.

The steel may also comprise cobalt: in an amount of less than or equal to 1.5%, this hardening element makes it possible to increase the carbon content in the residual austenite. The quantity must also be limited for reasons of cost.

The steel may also include boron in an amount less than or equal to 0.005%. Such addition increases quenchability and contributes to the removal of pro-eutectoid ferrite. It also allows to increase the levels of resistance.

The rest of the composition consists of unavoidable impurities resulting from the preparation, such as, for example, nitrogen.

According to the invention, the microstructure of the steel consists of at least 75% of bainite, of residual austenite in an amount greater than or equal to 5%, and of martensite in an amount of greater than or equal to 2%, these contents being referring to surface percentages. This bainitic majority structure, without proeutectoid ferrite, gives a very good resistance to further mechanical damage.

The microstructure of the hot-rolled sheet according to the invention contains a quantity greater than or equal to 5% of residual austenite, which is preferred rich in carbon, stabilized at ambient temperature, in particular by the additions of silicon and aluminum. The residual austenite is in the form of islands and interlayer films in the bainite, ranging from a few hundredths of a micrometer to a few micrometers.

A residual austenite amount of less than 5% does not allow interlayer films to significantly increase the resistance to damage.

Preferably, the carbon content of the residual austenite is greater than 1% in order to reduce the formation of carbides and to obtain residual austenite sufficiently stable at ambient temperature.

The figure 2 shows an example of a microstructure of a steel sheet according to the invention: The residual austenite A in surface area content of 7%, appears in white, in the form of islands or films. Martensite M, in area content here equal to 15%, is here in the form of very dark constituent on a bainitic matrix B appearing in gray.

Within certain islands, the local carbon content and thus the local hardenability may vary. Residual austenite is then locally associated with martensite within these islets, which are referred to as "MA" islands, associating Martensite and residual Austenite. In the context of the invention, it has been demonstrated that a specific morphology of islets MA was to be particularly sought. The morphology of the islets MA can be revealed by means of appropriate chemical reagents known per se: after chemical attack, the MA islands appear for example in white on a bainitic matrix more or less dark. These islets are observed by light microscopy at magnitudes ranging from 500 to 1500x approximately on a surface that has a statistically representative population.

caractérise le facteur d'élongation d'un îlot donné. Selon l'invention, une ductilité particulièrement élevée est obtenue en réduisant le nombre N_MA d'îlots M-A dont la longueur maximale L_max est supérieure à 2 micromètres et dont le facteur d'élongation est inférieur à 4.Is determined, for example by means of an image analysis software known in itself, such as for example the Visilog ^® software of the company Noesis, the maximum size L _max and minimum L _min of each of the islands. The ratio between maximum and minimum size $\frac{{\mathrm{The}}_{\max }}{{\mathrm{The}}_{\min }}$

characterizes the elongation factor of a given island. According to the invention, a particularly high ductility is obtained by reducing the number N _MA of islets MA whose maximum length L _max is greater than 2 micrometers and whose elongation factor is less than 4.

These massive islands and large are revealed priming areas privileged during a subsequent mechanical solicitation. According to the invention, the number of islets N _MA per unit area must be less than 14000 / mm ² .

The structure of the steels according to the invention also contains, in addition to the bainite and the residual austenite, martensite in an amount greater than or equal to 2%: this characteristic allows additional hardening which makes it possible to obtain superior mechanical strength. at 1200 MPa.

Preferably, the number of carbides located in position interlatts, generally coarser, size greater than 0.1 micrometer, is limited.

These carbides can be observed for example in optical microscopy at a magnification higher than or equal to 1000x. It has been demonstrated that N, the number of interlayer carbides greater than 0.1 micrometers per unit area, should be less than 50000 / mm ² , otherwise the damage becomes excessive in case of subsequent solicitation, for example during hole expansion tests. In addition, the excessive presence of carbides can cause early initiation of fracture and reduced toughness.

• On approvisionne un acier de composition selon l'invention
• On procède à la coulée d'un demi-produit à partir de cet acier. Cette coulée peut être réalisée en lingots, ou en continu sous forme de brames d'épaisseur de l'ordre de 200mm. On peut également effectuer la Coulée sous forme de brames minces de quelques dizaines de millimètres d'épaisseur, ou de bandes minces, entre cylindres d'acier contre-rotatifs.

The method of manufacturing a hot-rolled sheet according to the invention is implemented as follows:

• A composition steel is supplied according to the invention
• A semi-finished product is cast from this steel. This casting may be carried out in ingots, or continuously in the form of slabs of thickness of the order of 200 mm. We can also perform the casting in the form of Thin slabs of a few tens of millimeters thick, or thin strips, between counter-rotating steel cylinders.

The cast semifinished products are first brought to a temperature higher than 1150 ° C. to reach at any point a temperature favorable to the high deformations which the steel will undergo during rolling.

Naturally, in the case of a direct casting of thin slabs or thin strips between contra-rotating rolls, the hot rolling step of these semi-finished products starting at more than 1150 ° C. can be done directly after casting so well. that an intermediate heating step is not necessary in this case.

The semi-finished product is hot-rolled in a temperature range where the structure of the steel is totally austenitic up to an end-of-rolling temperature T _FL , with reference to the figure 1 attached. This figure shows a thermomechanical manufacturing diagram 1 according to the invention, as well as a transformation diagram indicating the zones of ferritic transformation 2 bainitic 3 and martensitic 4.

Controlled cooling is then performed, starting at a temperature T _DR , located above Ar3 (ferritic transformation start temperature from austenite) and ending at a temperature T _FR (end-of-cooling temperature). between T _DR and T _FR is equal to V _R. This cooling and the associated speed V _R are called primary. According to the invention, the speed V _R is between 50 and 90 ° C / s: When the cooling rate is less than 50 ° C / s; pro-eutectoid ferrite is formed which is detrimental to high resistance characteristics. According to the invention, the ferritic transformation is thus avoided from the austenite. When the speed V _R is greater than 90 ° C / s, there is a risk of forming martensite and revealing a heterogeneous structure. The cooling range according to the invention is advantageous from an industrial point of view since it is not necessary to cool the sheet very rapidly after hot rolling, for example at a speed of the order of 200 ° C. / s, which avoids the need for expensive specific installations. The cooling rate range according to the invention can be obtained by spraying water or air-water mixture, depending on the thickness of the sheet.

The process may also be carried out according to the following variant: From the temperature T _DR , rapid cooling is carried out up to a temperature T _{I of} less than or equal to 650 ° C. The speed V _R1 of this rapid cooling is greater than 70 ° C / s. Cooling is then carried out to a temperature T _FR so that the average cooling rate between T _DR and T _FR is between 20 and 90 ° C / s. This variant has the advantage of requiring slower cooling on average between T _DR and T _FR than in the previous variant, subject to performing a faster cooling at the speed V _R1 from T _DR to guarantee the absence of proeutectoid ferrite.

• Lorsqu'on effectue un refroidissement secondaire très lent à une vitesse V'_R comprise entre 0,08°C/min et 2°C/min, B'_S= Bs, température de début de transformation bainitique. Cette température B_S peut être déterminée expérimentalement ou évaluée à partir de la composition au moyen de formules connues en elles-mêmes. La figure 1 illustre ce premier mode de fabrication.
• Lorsque, à partir de T_FR, on refroidit la tôle laminée à chaud à une vitesse comprise V'_R supérieure à 2°C/min et inférieure ou égale à 600°C/min, B'_S= Bs+ 60°C.

After this first rapid cooling phase carried out according to one of the two preceding variants, a slower, so-called secondary cooling phase is started, which starts at a temperature T _FR of between B ' _S and M _S + 50 ° C. and which ends at room temperature. The secondary cooling rate is designated V ' _R. M _S denotes the martensitic transformation start temperature. The temperature B ' _S is defined with respect to the temperature B _S , bainitic transformation start temperature as follows:

• When performing a very slow secondary cooling at a rate V ' _R between 0.08 ° C / min and 2 ° C / min, B' _S = Bs, bainitic transformation start temperature. This temperature B _S can be determined experimentally or evaluated from the composition using formulas known per se. The figure 1 illustrates this first method of manufacture.
• When, from T _FR , the hot-rolled sheet is cooled at a speed V ' _R greater than 2 ° C / min and less than or equal to 600 ° C / min, B' _S = Bs + 60 ° C.

The first case corresponds to the manufacture of thin sheets of thickness, up to about 15mm, hot-wound, and thus cooled slowly after the winding operation. The second case corresponds to the manufacture of sheets of greater thickness non-hot rolled: according to the thickness of the sheets, the cooling rates greater than 2 ° C / min and lower or equal to 600 ° C / min correspond to slightly accelerated cooling or air cooling.

When the end of cooling temperature is greater than B ' _S , the carbon enrichment of the austenite is not sufficient: after complete cooling, carbides or islands of martensite are formed.

In this way, it is possible to obtain a steel having a "dual-phase" structure but whose combination of properties (strength-ductility) is lower than that of the invention. These structures also have a greater sensitivity to damage than those of the invention.

When the end-of-cooling temperature is less than Ms + 50 ° C, the carbon enrichment of the austenite is excessive. Under certain industrial conditions, there is a risk of formation of a marked band structure and excessive martensitic transformation.

Thus, under the conditions according to the invention, the process has a low sensitivity to a variation of the manufacturing parameters.

The secondary cooling associated with a temperature T _FR between B ' _S and M _S + 50 ° C makes it possible to control the bainitic transformation from austenite, to locally enrich this austenite so as to stabilize it, and to obtain a ratio (bainite / residual austenite / martensite) appropriate.

In the context of the invention, it is also possible to adjust the primary speed V _R between T _DR and T _FR , the end of cooling temperature T _FR , the secondary cooling rate V ' _R , so that the microstructure of the steel consists of at least 75% bainite, residual austenite in an amount greater than or equal to 5%, and martensite in an amount greater than or equal to 2%.

• T_DR sera choisie supérieure à A_R3 pour éviter la formation de ferrite pro-eutectoïde, tout en évitant une croissance exagérée du grain austénitique et affiner la microstructure finale
• La vitesses de refroidissement V_R sera choisie de façon à être la plus rapide possible pour éviter une transformation perlitique (ce qui conduirait à une teneur en austénite résiduelle insuffisante) et ferritique tout en restant au sein des capacités de contrôle d'une ligne industrielle de façon à obtenir une homogénéité microstructurale dans le sens longitudinal et transversal de la tôle laminée à chaud. La vitesse de refroidissement V_R doit être cependant limitée pour éviter la formation d'une microstructure hétérogène dans l'épaisseur de la tôle.
• La vitesse de refroidissement V'_R est essentiellement dépendante des capacités de production des sites industriels et de l'épaisseur des tôles.
• Indépendamment de V'_R, T_FR sera choisie suffisamment basse de façon à éviter une transformation perlitique, ce qui se traduirait par une transformation bainitique incomplète et une teneur en austénite résiduelle inférieure à 5%,
• De plus, si la vitesse V'_R est rapide, la température T_FR sera choisie suffisamment élevée pour laisser le temps à la transformation bainitique de se dérouler au dessus du domaine martensitique. On évite alors la formation de plus de 20% de martensite par un passage trop rapide dans le domaine martensitique. Cette dernière transformation se produirait aux dépens de la transformation bainitique et de la stabilisation de l'austénite résiduelle.
• Dans le cas où la vitesse V'_R est lente, une variation de la température T_FR dans le domaine entre B'_S et M_S+50°C, aura peu d'influence sur la microstructure finale.

The parameters T _DR , T _FR , V _R , V ' _R , adjusted to obtain at least 75% of bainite, at least 5% of austenite and at least 2% of martensite, will be chosen as follows:

• T _DR will be chosen higher than A _R3 to avoid the formation of pro-eutectoid ferrite, while avoiding excessive growth of the austenitic grain and refining the final microstructure
• The cooling rate V _R will be chosen so as to be as fast as possible to avoid a pearlitic transformation (which would lead to an insufficient residual austenite content) and ferritic while remaining within the control capabilities of an industrial line so as to obtain a microstructural homogeneity in the longitudinal and transverse direction of the hot-rolled sheet. The cooling rate V _R must however be limited to avoid the formation of a heterogeneous microstructure in the thickness of the sheet.
• The cooling rate V ' _R is essentially dependent on the production capacities of the industrial sites and the thickness of the sheets.
• Independently of V ' _R , T _FR will be chosen sufficiently low so as to avoid a pearlitic transformation, which would result in an incomplete bainitic transformation and a residual austenite content of less than 5%,
• Moreover, if the speed V ' _R is fast, the temperature T _FR will be chosen high enough to allow time for the bainitic transformation to take place above the martensitic domain. The formation of more than 20% of martensite is then avoided by passing too fast in the martensitic domain. This last transformation would occur at the expense of bainitic transformation and the stabilization of residual austenite.
• In the case where the speed V ' _R is slow, a variation of the temperature T _FR in the range between B' _S and M _S + 50 ° C will have little influence on the final microstructure.

These parameters can also be adjusted to obtain a particular morphology and nature of the MA islands, in particular chosen so that the number N _MA of islands of martensite-residual austenite whose size is greater than 2 micrometers and whose elongation factor is less than 4, ie less than 14000 / mm ² . These parameters can also be adjusted so that the carbon content of the residual austenite is greater than 1% by weight. In particular, a cooling rate V _R will be chosen which is not too high so as to avoid excessive formation of coarse MA islands. The parameters V _R , T _FR , V ' _R can also be adjusted so that the number N of bainitic carbides of size greater than 0.1 micrometer by unit area is less than or equal to 50000 / mm ² .

Example:

Steels have been developed, the composition of which is given in the table below, expressed in percentage by weight. In addition to the steels I-1 to I-9 used for the production of sheets according to the invention, the composition of steels R-1 to R-9 used for the manufacture of reference sheets has been indicated for comparison purposes. .

Semi-products corresponding to the above compositions were heated to 1200 ° C and hot rolled to a thickness of 3 mm or 12 mm in a temperature range where the structure is fully austenitic. Cooling start temperatures T _DR , between 820 and 945 ° C, are also in the austenitic range. The cooling rates V _R between T _DR and T _FR , the cooling end temperatures T _FR , the secondary cooling rates V ' _R are given in Table 2. From the same composition, certain steels (I-1 , I-2, I-5, R-7) have been subject to different manufacturing conditions. The references I-1a, I-1b and I-1c designate for example three steel sheets manufactured under different conditions from the steel composition I-1. The steel sheets I-1a to c, 1-4, I-5a and b, R-6, have a thickness of 12mm, the other sheets of 3mm.

$${\mathrm{M}}_{\mathrm{s}}\left(\mathrm{°C}\right)=561-474\left(\mathrm{C}\right)-33\left(\mathrm{Mn}\right)-17\left(\mathrm{Ni}\right)-17\left(\mathrm{Cr}\right)-21\left(\mathrm{Mo}\right)$$

Table 2 also indicates the transformation temperatures B ' _S and M _S + 50 ° C calculated from the chemical compositions using the following expressions, the compositions being expressed in percentage by weight: $${\mathrm{B}}_{\mathrm{s}}\left(\mathrm{°\; C}\right)=830-270\left(\mathrm{VS}\right)-90\left(\mathrm{mn}\right)-37\left(\mathrm{Or}\right)-70\left(\mathrm{Cr}\right)-83\left(\mathrm{MB}\right)$$

$${\mathrm{M}}_{\mathrm{s}}\left(\mathrm{°\; C}\right)=561-474\left(\mathrm{VS}\right)-33\left(\mathrm{mn}\right)-17\left(\mathrm{Or}\right)-17\left(\mathrm{Cr}\right)-21\left(\mathrm{MB}\right)$$

The different microstructural constituents measured by quantitative microscopy were also reported: surface fraction of bainite, residual austenite by X-ray diffraction or sigmametry, and martensite.

The MA islets have been highlighted by Klemm's reagent. Their morphology was examined using image analysis software to determine the parameter N _MA . In some cases, the presence of carbides greater than 0.1 micron in the bainitic phase was investigated by Nital etching and observation at high magnification. The number N (/ mm ² ) of interlayer carbides larger than 0.1 micrometer was determined.

The tensile mechanical properties obtained (yield strength Re, resistance Rm, uniform elongation Au, elongation at break At) were given in Table 3 below. The Re / Rm ratio was also indicated. In some cases, the KCV fracture energy at 20 ° C was determined from V resilient test pieces.

Furthermore, damage due to cutting (shearing or punching for example) has been evaluated which could possibly reduce the ability of subsequent deformation of a cut piece. For this purpose, 20 × 80 mm ² specimens were cut by shearing. Some of these The test pieces were then polished at the edges. The specimens were coated with photodeposited grids and then subjected to uniaxial traction until rupture. The values of the principal deformations ε ₁ parallel to the direction of the stress have been measured as close as possible to the initiation of the rupture from the deformed grids. This measurement was carried out on the specimens with mechanically cut edges and on the specimens with polished edges. The sensitivity to cutting is evaluated by the damage factor: Δ = ε ₁ (cut edges) -ε ₁ (polished edges) / ε ₁ (polished edges).

The arc weldability (MAG) and spot resistance properties of these steel sheets were also evaluated.

The steel sheets I-1 to I-9 in accordance with the invention have a particularly advantageous combination of mechanical properties: on the one hand a mechanical strength greater than 1200 MPa, on the other hand an elongation at break greater than 10%. and a ratio Re / Rm of less than 0.75 ensuring good formability. The steels according to the invention also have a Charpy V fracture energy at room temperature greater than 28 Joules. This high tenacity allows the manufacture of parts resistant to the sudden propagation of a defect especially in case of dynamic stresses. The microstructures of steels according to the invention have a number of islands N _MA less than 14,000 / mm ^2.

In particular, the steel sheets I-2a and I-5a have a small surface proportion of large and large islets of MA, respectively 10500 and 13600 compounds per mm ² .

The steels according to the invention also have good resistance to damage in the event of cutting, since the damage factor Δ is limited to -12 or -13%.

These steels also have a good homogenous weldability MAG: for weld parameters adapted to the thicknesses reported above, the welded seams are free from cold or hot cracks. A similar observation can be made for homogeneous spot resistance welding.

In the case of steel I9, the cooling between T _DR (880 ° C) and T _FR (485 ° C) (see Table 2) was also carried out according to the following variant: after a first cooling at a speed V _R1 = 80 ° C / s up to a temperature T _I of 590 ° C, the sheet was cooled so that the average speed between 880 ° C and 485 ° C is 37 ° C / s. The observed mechanical properties are then very close to those presented in Table 3, Example I9.

R-1 steel is deficient in chromium and / or molybdenum. The cooling conditions relating to steels R-1 to R-3 (V _R too high, T _FR too low) are not suitable for the formation of a fine bainitic structure. The absence of martensite does not allow sufficient hardening, the resistance is significantly lower than 1200 MPa and the ratio Re / R _m is excessive.

In the case of steel sheets R-4 and R-5, the cooling rate too fast after rolling does not provide a sufficiently large amount of bainite. The MA islands formed are relatively coarse. In the case of steel sheet R-4, the number of N _MA compounds is 14700 / mm ² . The bainitic fraction and the resistance of these steels are insufficient. R-4 steel sheet with a large number of carbides (N> 50000 / mm ² ) has an excessive sensitivity to damage as evidenced by the value of the damage factor: Δ = -48%.

R-6 steel has an excessive carbon content, leading to a high martensite content due to its high hardenability; its bainite and austenite content is insufficient. The steel sheet R-6 therefore has insufficient resistance to the sudden propagation of a defect since its Charpy V fracture energy at 20 ° C is much lower than 28 Joules.

Steel sheets R-7a and R7-b also have an excessive carbon content. The transition temperature at the 28 Joule level, estimated from specimens of reduced thickness, is higher than the ambient temperature, testifying to poor toughness. Welding ability is reduced. It will be noted that, despite their higher carbon content, these steel sheets do not have a greater mechanical strength than that of the steels of the invention.

R-8 steel sheet with excessive carbon content has been cooled too slowly, so residual austenite is highly carbon enriched and martensite formation could not occur. The resistance obtained is therefore insufficient.

The R-9 steel sheet was cooled at an excessive speed until the cooling end temperature was too low. As a result, the structure is almost completely martensitic and the elongation at break is insufficient.

Thus, the invention enables the manufacture of bainitic matrix steel sheets without the addition of expensive microalloy elements. These combine very high strength and high ductility. Thanks to their high strength, these steel sheets are suitable for the manufacture of elements undergoing cyclic mechanical stresses. The steel sheets according to the invention are used profitably for the manufacture of structural parts or reinforcement elements in the automotive field and general industry.

Claims (22)

1. Hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, a yield strength/tensile strength ratio R_e/R_m of less than 0.75 and an elongation at break of greater than 10%, the composition of which contains, the contents being expressed by weight: $$0.10\%\le \mathrm{C}\le 0.25\%$$

   

   $$1\%\le \mathrm{Mn}\le 3\%$$

   

   $$\mathrm{Al}\ge 0.015\%$$

   

   $$\mathrm{Si}\le 1.985\%$$

   

   $$\mathrm{Mo}\le 0.30\%$$

   

   $$\mathrm{Cr}\le 1.5\%$$

   

   $$\mathrm{S}\le 0.015\%$$

   

   $$\mathrm{P}\le 0.1\%$$

   

   $$\mathrm{Co}\le 1.5\%$$

   

   $$\mathrm{B}\le 0.005\%$$

   

   
   it being understood that $$1\%\le \mathrm{Si}\mathrm{+}\mathrm{Al}\le 2\%$$

   

   $$\mathrm{Cr}+\left(3\times \mathrm{Mo}\right)\ge 0.3\%,$$

   

   
   the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of said steel consisting of at least 75% bainite, residual austenite in an amount equal to or greater than 5% and martensite in an amount equal to or greater than 2%.
2. Steel sheet according to Claim 1, characterized in that the composition of said steel contains, the content being expressed by weight: $$0.10\%\le \mathrm{C}\le 0.15\%\mathrm{.}$$

   
3. Steel sheet according to Claim 1, characterized in that the composition of said steel contains, the content being expressed by weight: $$0.15\%<\mathrm{C}\le 0.17\%\mathrm{.}$$

   
4. Steel sheet according to Claim 1, characterized in that the composition of said steel contains, the content being expressed by weight: $$0.17\%<\mathrm{C}\le 0.22\%\mathrm{.}$$

   
5. Steel sheet according to Claim 1, characterized in that the composition of said steel contains, the content being expressed by weight: $$0.22\%<\mathrm{C}\le 0.25\%\mathrm{.}$$

   
6. Steel sheet according to any one of Claims 1 to 5, characterized in that the composition of said steel contains, the content being expressed by weight: $$1\%\le \mathrm{Mn}\le 1.5\%\mathrm{.}$$

   
7. Steel sheet according to any one of Claims 1 to 5, characterized in that the composition of said steel contains, the content being expressed by weight: $$1.5\%<\mathrm{Mn}\le 2.3\%\mathrm{.}$$

   
8. Steel sheet according to any one of Claims 1 to 5, characterized in that the composition of said steel contains, the content being expressed by weight: $$2.3\%<\mathrm{Mn}\le 3\%\mathrm{.}$$

   
9. Steel sheet according to any one of Claims 1 to 8, characterized in that the composition of said steel contains, the content being expressed by weight: $$1.2\%\le \mathrm{Si}\le 1.8\%\mathrm{.}$$

   
10. Steel sheet according to any one of Claims 1 to 8, characterized in that the composition of said steel contains, the content being expressed by weight: $$1.2\%\le \mathrm{Al}\le 1.8\%\mathrm{.}$$

    
11. Steel sheet according to any one of Claims 1 to 10, characterized in that the composition of said steel contains, the content being expressed by weight: $$\mathrm{Mo}\le 0.010\%\mathrm{.}$$

    
12. Steel sheet according to any one of Claims 1 to 11, characterized in that the carbon content of the residual austenite is greater than 1% by weight.
13. Steel sheet according to any one of Claims 1 to 12, containing carbides between the bainite laths, characterized in that the number N of said inter-lath carbides of size greater than 0.1 micron per unit area is equal to 50 000/mm² or less.
14. Steel sheet according to any one of Claims 1 to 13, containing martensite/residual austenite islands, characterized in that the number N_MA per unit area of said martensite/residual austenite islands having a maximum size L_max greater than 2 microns and having an elongation factor (maximum size L_max/minimum size L_min) less than 4 is less than 14 000/mm².
15. Process for manufacturing a hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, an R_e/R_m ratio of less than 0.75 and an elongation at break of greater than 10%, in which:

    - a steel of composition according to any one of Claims 1 to 11 is supplied;

    - a semi-finished product is cast from this steel;

    - said semi-finished product is heated to a temperature above 1150°C;

    - said semi-finished product is hot-rolled in a temperature range in which the microstructure of the steel is entirely austenitic; then

    - the sheet thus obtained is cooled from a temperature TDR lying above Ar3 down to a transformation temperature TFR in such a way that the primary cooling rate VR between TDR and TFR is between 50 and 90°C/s and the temperature TFR is between B'S and MS + 50°C, B's denoting a temperature defined relative to the bainite transformation start temperature BS, and MS denoting the martensite transformation start temperature, then

    - said sheet is cooled from the temperature TFR at a secondary cooling rate V'R between 0.08°C/min and 600°C/min down to the ambient temperature;

    - said temperature B'S being equal to BS when said rate V'R is between 0.08 and 2°C/min; and

    - said temperature B'S being equal to BS + 60°C when said rate V'R is greater than 2°C/min but does not exceed 600°C/min.
16. Process for manufacturing a hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, an R_e/R_m ratio of less than 0.75 and an elongation at break of greater than 10%, in which:

    - a steel of composition according to any one of Claims 1 to 11 is supplied;

    - a semi-finished product is cast from this steel;

    - said semi-finished product is heated to a temperature above 1150°C;

    - said semi-finished product is hot-rolled in a temperature range in which the microstructure of the steel is entirely austenitic; then

    - the sheet thus obtained is cooled from a temperature TDR lying above Ar3 down to an intermediate temperature TI at a cooling rate VR1 of 70°C/s or higher, said temperature TI not exceeding 650°C; then

    - said sheet is cooled from said temperature TI down to a temperature TFR, said temperature TFR being between B'S and MS + 50°C, B'S denoting a temperature defined relative to the bainite transformation start temperature BS, and MS denoting the martensite transformation start temperature, in such a way that the cooling rate between said temperature TDR and said temperature TFR is between 20 and 90°C/s; then

    - said sheet is cooled from the temperature TFR at a secondary cooling rate V'R between 0.08°C/min and 600°C/min down to the ambient temperature;

    - said temperature B'S being equal to BS when said rate V'R is between 0.08 and 2°C/min; and

    - said temperature B'S being equal to BS + 60°C when said rate V'R is greater than 2°C/min but does not exceed 600°C/min.
17. Process for manufacturing a hot-rolled steel sheet according to Claim 15, characterized in that the primary cooling start temperature T_DR lying above Ar3, the primary cooling finish temperature T_FR, the primary cooling rate V_R between T_DR and T_FR and the secondary cooling rate V'_R are adjusted in such a way that the microstructure of said steel consists of at least 75% bainite, residual austenite in an amount equal to or greater than 5% and martensite in an amount equal to or greater than 2%.
18. Process according to either of Claims 15 and 17, characterized in that the primary cooling start temperature T_DR lying above Ar3, the primary cooling finish temperature T_FR, the primary cooling rate V_R between T_DR and T_FR and the secondary cooling rate V'_R are adjusted in such a way that the carbon content of the residual austenite is greater than 1% by weight.
19. Process according to any one of Claims 15, 17 and 18, characterized in that the primary cooling start temperature T_DR lying above Ar3, the primary cooling finish temperature T_FR, the primary cooling rate V_R between T_DR and T_FR and the secondary cooling rate V'_R are adjusted in such a way that the number of inter-lath carbides having a size greater than 0.1 microns per unit area does not exceed 50 000/mm².
20. Process according to any one of Claims 15 and 17 to 19, characterized in that the primary cooling start temperature T_DR lying above Ar3, the primary cooling finish temperature T_FR, the primary cooling rate V_R between T_DR and T_FR and the secondary cooling rate V'_R are adjusted in such a way that the number N_MA per unit area of martensite/residual austenite islands having a maximum size L_max greater than 2 microns and an elongation factor Lmax/Lmin less than 4 is less than 14 000/mm².
21. Use of a hot-rolled steel sheet according to any one of Claims 1 to 14, or manufactured by a process according to any one of Claims 15 to 20, for the manufacture of structural parts or reinforcing elements in the automotive field.
22. Use of a hot-rolled steel sheet according to any one of Claims 1 to 14, or manufactured by a process according to any one of Claims 15 to 20, for the manufacture of reinforcements and structural parts for general industry and of abrasion-resistant parts.

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