EP1994192A1

EP1994192A1 - Process for manufacturing steel sheet having very high strength, ductility and toughness characteristics, and sheet thus produced

Info

Publication number: EP1994192A1
Application number: EP07730968A
Authority: EP
Inventors: Sébastien Allain; Audrey Couturier; Thierry Iung; Christine Colin
Original assignee: ArcelorMittal France SA
Current assignee: ArcelorMittal France SA
Priority date: 2006-03-07
Filing date: 2007-02-14
Publication date: 2008-11-26
Anticipated expiration: 2027-02-14
Also published as: US10370746B2; BRPI0708649B1; KR101073425B1; CN101437975B; MA30261B1; JP5055300B2; US20090107588A1; BRPI0708649A2; US9856548B2; MX2008011274A; JP2009529098A; CA2645059A1; EP1832667A1; RU2008139605A; RU2397268C2; ZA200807519B; EP1994192B1; US20180010220A1; WO2007101921A1; KR20080106337A

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

PROCESS SHEETS OF MANUFACTURING STEEL ¹ VERY HIGH

CHARACTERISTICS OF RESISTANCE, DUCTILITY

AND TENACITY, AND SHEETS SO PRODUCED

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 / resistance ratio of less than 0.75. The automobile sector and the general industry are notably fields of application of these hot-rolled steel sheets. In the automotive industry, there is a continuing need for vehicle lightening and increased safety. This is how we proposed several families of steels offering different levels of resistance:

Steels having micro-alloy elements whose hardening is obtained simultaneously by precipitation and by grain size refinement were proposed first. 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 deform): 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. US Pat. No. 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 exemplary embodiments in the patent show that the resistance obtained barely exceeds 1200 MPa, together with a Re / R _m ratio 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.

US Pat. No. 4,472,208 also describes the production 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, as well as a precipitation of carbides. titanium 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.

Patent JP2004332100 describes the manufacture of hot-rolled sheet with a resistance greater than 800 MPa ₁ 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. JP2004190063 discloses the manufacture of high strength hot rolled 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 sulfur content.

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 object of the invention is also to provide a steel that is not very sensitive to damage when it is cut 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 more than 28 Joules at 20 ^° C. is sought. It also aims at having a steel having good weldability by means of the usual assembly processes in a thickness range from 1 to more than 30 millimeters, especially during spot or arc resistance welding, particularly in MAG ("Metal Active Gas") welding. 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 ₁ with 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%, with the proviso that 1% <Si + AI ≤2%, Cr + (3 x Mo)

≥0.3%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the development, the microstructure of the steel consisting of at least 75% bainite, residual austenite in quantity greater than or equal to 5%, and of marten $ ite in a quantity 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 mode, 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%.

Preferentially, 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% According to a preferred embodiment, the composition of the steel comprises; 1, 2% <If ≤

1, 8%.

Preferably, the composition of the steel comprises: 1.2% ≤ AI ≤ 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 being less than or equal to

50000 / mm ² . The subject of the invention is also a steel sheet comprising residual martensite-austenite islands, whose NMA number per unit area, of residual martensite-austenite islands whose maximum size L _max is greater than 2 micrometers and whose elongation factor is less than

4, is less than 14000 / mm ² .

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

- the casting of a half-product from this steel

the half-product is brought to a temperature greater than 115 ^° C., the semi-finished product is hot-rolled in a temperature range where the structure of the steel is entirely austenitic,

and then cooling the sheet thus obtained from a temperature T _D R above Ar 3 to a transformation temperature T _FR so that the primary cooling rate VR between TDR and TFR is between 50 and 90 ° C / s and that the temperature T _F R is between B's and Ms + 50 ° C, B's designating a temperature defined with respect to the bainitic transformation start temperature Bs, and M _s designating the start of transformation temperature martensitic and then

the sheet is cooled from the temperature T _F R with a secondary cooling rate V ' _R between 0.08 ^D C / min and 600 ^δ C / min to the ambient temperature,

- the temperature B's being equal to Bs when the speed VR is greater than or equal to 0.08 ^û C / min and less than or equal to 2 ° C / miπ

- the temperature B's being equal to Bs + 60 ° C when the speed VR is greater than 2 ° C / min and less than or equal to 60 ° 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: - one supplies a steel of composition above,

- the casting of a half-product from this steel

the half-product is carried at a temperature above 1150 ^° C. and is 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 _D R located above Ar3 to an intermediate temperature Ti with a cooling rate Vm greater than or equal to 7 (TC / s, the temperature T 1 being lower or equal to 650 ^p C, then

- the plate is cooled from the temperature T to a temperature TFR, the temperature T _FR being between B's and M _s + 50 ° C, _B's denoting a temperature defined relative to the start temperature Bs bainitic transformation, and M ₅ denoting the martensitic transformation start temperature, such that the cooling rate between the temperature TDR and the temperature T _FR Is between 20 and 90 ^β C / s, then

the sheet is cooled from the temperature TFR with a secondary cooling rate V _R of between 0.08 ° C./min and 60 ° C./min until the ambient temperature,

the temperature B ¹ S being equal to Bs when the speed V ' _R is between 0, Q8 and 2 ° C / min

"The temperature B ¹ S being equal to Bs + 60 ° C when the speed VR is greater than 2 ° C / min and less than or equal to 600 ° C / min

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

- supply a composition steel above

- It proceeds to the casting of a half-product from this steel - the semi-finished product is brought to a temperature above 1150 ⁰ C

the semi-finished product is hot-rolled in a temperature range where the structure of the steel is entirely austenitic,

the primary cooling start temperature TDR above Ar3, the primary cooling end temperature TFR, the primary cooling speed VR between T _D R and T _F R, and the secondary cooling speed V _R are adjusted. such 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 _F R, the primary cooling rate V _R between T _D R and TFR, and the secondary cooling rate VR, so 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 TDR above Ar3, the primary cooling end temperature T _F R, the primary cooling rate Vp between TDR and T _F R are adjusted. , and the secondary cooling rate V ' _R so that the number of interlayer carbides greater than 0.1 micrometers per unit area is less than or equal to 50000 / mrn ² . The subject of the invention is also a method according to which the primary cooling start temperature T _D R above Ar3, the primary cooling end temperature T _FR , the primary cooling rate V _R between T _D R are adjusted. and T _FR , and the secondary cooling rate VR, such that the NMA number per unit area, residual martensite-austenite islands whose maximum size L _max is greater than 2 micrometers and whose elongation factor - is less than 4, less than 14000 / mrn 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. Other features and advantages of the invention will become apparent from the description below, given by way of example and with reference to the appended figures attached hereto according to which:

- Figure 1 shows a schematic description of an embodiment of the manufacturing method according to the invention, in connection with a transformation diagram from the austenite.

FIG. 2 shows an exemplary 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 bundles of slats, 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 Taustenite, 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. Preferentially, the manganese is between 1 and 1.5%: in this way a satisfactory curing is combined without any risk of damaging band structure formation. 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%. Beyond 3%, the risk of carbide precipitation or formation of structures in harmful 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 austéπite in the form of thin 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 + AI≤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 proutectoid 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 × Mo) ≥0.3%. ^'

The coefficients of chromium and molybdenum in this relationship reflect the greater or lesser ability of these two elements to retard 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 MB) ≥0.3%.

In an amount greater than 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 residual Paustenite. 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. FIG. 2 shows an example of a microstructure of a steel sheet according to the invention: The residual austenite A, in surface content here equal to 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 islets MA 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 1500 ° approximately on a surface that has a statistically representative population. It determines, for example by means of a known image analysis software itself, such as for example the Noesis ^® software the company Noesis, the maximum size L _my χ and minimum L min of each of the islands. The ratio between the maximum and minimum size characterizes the elongation factor of an island given. According to the invention, a particularly high ductility is obtained by reducing the NMA number of MA islands whose maximum length L _max is greater than 2 micrometers and whose elongation factor is less than 4. These large and large islands are prime areas of priming during a subsequent mechanical solicitation. According to the invention, the number of NMA islands 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 graduation greater 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.

The method of manufacturing a hot-rolled sheet according to the invention is as follows: - A steel of composition according to the invention is supplied - A semi-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. It is also possible to cast in the form of slabs of a few tens of millimeters thick, or thin strips, between contra-rotating steel cylinders. The cast half-products are first brought to a temperature above 115O ⁰ C to reach at any point a temperature favorable to the high deformations that will undergo the steel during rolling.

Naturally, in the case of 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 115 ^° 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 TFL, with reference to the appended FIG. This figure shows a thermomechanical manufacturing diagram 1 according to the invention, as well as a transformation diagram indicating the areas of ferritic transformation 2 baiπitic 3 and martensitic 4.

Controlled cooling is then performed, starting at a TDR temperature, located above Ar3 (ferritic transformation start temperature from austenite) and ending at a temperature T _F R (end-of-cooling temperature). of cooling between T _D R and T _FR is equal to V _R. This cooling and the associated VR speed are referred to as primary. According to the invention, the speed VR is between 50 and 90 ° C./s: When the cooling rate is lower than 50 ° C./s, pro-eutectoid ferrite is formed which is harmful for obtaining high characteristics of resistance. According to the invention, this avoids ferritic transformation from austenite. When the speed VR is greater than 90 ° C./s, there is a risk of forming martensite and of revealing a heterogeneous structure. The cooling range of the invention is advantageous from an industrial point of view because it is not necessary to cool the sheet quickly after the hot rolling, for example at a speed of about 200 ^ù 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 method can also be implemented according to the following variant: From the TDR temperature, a rapid cooling is carried out up to a temperature Ti less than or equal to 650 ^p C. The speed V _R i of this rapid cooling is greater at 7O ⁰ CVs. Cooling is then carried out to a TFR temperature so that the average cooling rate between T _D R and TF _R is between 20 and 90 ° C / s. This variant has the advantage of requiring slower cooling. on average between TDR and T _FR than in the previous variant, subject to performing faster cooling at the speed V _R- 1 from T _D R to ensure the absence of proeutectoid ferrite. 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. Ms denotes the martensitic transformation start temperature. The temperature B is defined with respect to the temperature Bs, bainitic transformation start temperature as follows: - When performing a very slow secondary cooling at a speed V ' _R between 0.08 ° C / min and 2 C / min, B ' _s = B _s , bainitic transformation start temperature. This temperature Bs can be determined experimentally or evaluated from the composition by means of formulas known per se. Figure 1 illustrates this first method of manufacture.

- When, from TFR, the hot rolled sheet is cooled at a speed comprised VR greater than 2 ° C / min and less than or equal to eOCTC / min, B s = Bs + 6OX.

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 higher 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 cooling termination temperature is less than Ms + 50 ^û C, 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 _F R between B's and Mg + 50 ° C makes it possible to control the bainitic transformation from austenite, to locally enrich this austenite in order 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 TDR and TFR, the end-of-cooling temperature TFR, the secondary cooling rate V ' _R , so that the microstructure of the steel is consisting 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%.

The parameters T _D R, T _F R, V _RI 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:

- TDR will be chosen higher than A _R3 to avoid the formation of proutectoid 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 VR 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 TFR 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 VR is slow, a variation of the temperature Tp _R in the range between B ' _s and Ms + 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 ^s . 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 , TVR, VR can also be adjusted so that the number N of bainitic carbides 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 1-1 to I-9 used in the manufacture 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. .

Table 1: Compositions of steels (% by weight) I = According to the invention. R≈référence

(*): Not in accordance with the invention.

Semi-finished products corresponding to the above compositions were warmed to 120 ° C and hot rolled to a thickness of 3 mm or 12 mm in a temperature range where the structure is fully austenitic. The cooling start temperatures TDR, between 820 and 945 ^° C., are also in the austenitic range. cooling rates VR between T _D R and T _FR , the cooling end temperatures T _F R, the secondary cooling rates V ' _R are given in Table 2. From the same composition, certain steels (1- 1, 1- 2, I-5, R-7) have been subject to different manufacturing conditions. References 1-1a, 1-1b and 1-1c, for example, designate three steel sheets manufactured under different conditions from the steel composition M. The steel sheets 1-1 a to c , 1-4, 1-5a and b, R-6, have a thickness of 12mm, the other sheets of 3mm. 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:

B _s ⁽⁰ C) = 830-270 (C) - 90 (Mn) - 37 (Ni) - 70 (Cr) - 83 (Mo) M ₅ ⁽⁰ C) - 561-474 (C) - 33 (Mn 17 (Ni) - 17 (Cr) - 21 (Mo) The various 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.

Table 2: Production conditions and microstructure of the hot-rolled sheets obtained I = according to the invention. R = reference (*): Not in accordance with the invention, n: Not 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 rupture energy has been determined at 20 ^° C. from the V test specimens.

Furthermore, it was evaluated damage related to a cut (shearing or punching for example) which could possibly reduce the ability of subsequent deformation of a cut piece, for this purpose, was cut by shearing specimens of size 20 x 80 mm ² . 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 εi 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. Cutting sensitivity is evaluated by the damage factor: Δ = εi (cut edges) - ^ (polished edges) / E ₁ (polished edges). The arc weldability (MAG) and spot resistance properties of these steel sheets were also evaluated.

Table 3; Mechanical properties of the hot-rolled sheets obtained.

I = according to the invention. R = reference {*): Not according to the invention, n: Not determined

The steel sheets 1-1 to I-9 according to the invention have a particularly advantageous combination of mechanical properties: on the one hand a mechanical strength greater than 1200 MPa ₁ 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 dilots NMA below 140Q0 / mm ^z. In particular, the steel sheets 1-2a and 1-5a have a low 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 19, the cooling between T _DR (880 ^ù C) and T _F R (485 ^P C) (see Table 2) was also performed according to the following variant: after a first cooling at a speed V _R1 = 80 ° C / s to a temperature T, 590 ⁰ C, the sheet was cooled so that the average speed between 88O ⁰ C and 485 ° C is 37 ° C / s. The observed mechanical properties are then very close to those shown in Table 3, Example 19. The steel R-1 has an insufficient content of chromium and / or molybdenum. The cooling conditions relating to steels R-1 to R-3 (V _R too high, T _F R 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 NMA 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 ^z ) 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 consequently 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; as a result, residual austenite is highly enriched in carbon and martensite formation could not occur. The resistance obtained is therefore insufficient, The steel sheet R-9 has been cooled at an excessive speed until a temperature of end of cooling 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

1. Hot-rolled steel sheet having a strength greater than 1200 MPa, yield strength / resistance ratio Re / R _m less than 0.75, elongation at break greater than 10%, the composition of which contains the contents being expressed in 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% with the understanding that

1% <Si + AI <2%,

Cr + (3 x Mo) ≥0.3%, the remainder of the composition consisting of iron and inevitable impurities resulting from the elaboration, the microstructure of said steel consisting of at least 75% of bainite, austenite residual in an amount greater than or equal to 5%, and martensite in an amount greater than or equal to 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% <C ≤ 0.15%

3. Sheet steel according to claim 1, characterized in that the composition of said steel contains, the content being expressed by weight:

0.15% <C <0.17%

4. Sheet steel according to claim 1, characterized in that the composition of said steel contains, the content being expressed by weight: 0.17% <C <0.22%

5. Sheet steel according to claim 1, characterized in that the composition of said steel contains, the content being expressed by weight;

0.22% <C ≤ 0.25%

6. Sheet steel 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% ≤Mn ≤ 1.6%

7. Sheet steel 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% <Mn <2.3%

8. Sheet steel 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% <Mn ≤ 3%

9. Sheet steel 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% ≤Si <1.8%

10. Sheet steel according to any one of claims 1 to 8, characterized in that the composition of said steel contains, the content being expressed in weight:

1.2% ≤A1 ≤ 1.8%

11. Sheet steel according to any one of claims 1 to 10, characterized in that the composition of said steel contains, the content being expressed by weight:

Mo≤O.010%

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. Sheet steel according to any one of claims 1 to 12, comprising carbides between bainite slats, characterized in that the number N said interlayer carbides larger than

0.1 micrometer per unit area is less than or equal to 50000 / mm ² .

Steel sheet according to any one of Claims 1 to 13, comprising residual martensite-austenite islands, characterized in that the NMA number per unit area, of said residual martensite-austenite islands whose maximum size L _max is greater than to 2 micrometers and whose elongation factor (maximum size L _ma χ / minimum size L _min ) is less than 4, is less than 140007mm ²

15. A process for producing a hot-rolled steel sheet with a resistance greater than 1200 MPa ₁ with a Re / Rm ratio of less than 0.75 and an elongation at break greater than 10%, according to which: composition steel according to any one of claims 1 to 11, - the casting of a semi-finished product from this steel said half-product is brought to a temperature greater than 115O ⁰ C

said half-product is hot-rolled in a temperature range in which the microstructure of the steel is entirely austenitic, and then the sheet thus obtained is cooled from a temperature T _D R located above Ar 3 up to a transformation temperature T _FR in such a way that the primary cooling rate Vr between T _0R and TFR is between 50 and 90 ° C / s and the temperature T _F R 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 martensitic transformation start temperature, and then

said 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 to ambient temperature,

said temperature B is equal to Bs when said speed V ' _R is between 0.08 and 2 ° C / min

- said temperature B's being equal to Bs + 60 ° C when said speed VR is greater than 2 ° C / min and less than or equal to 600 ° C / min

16. A process for manufacturing a hot-rolled steel sheet with a resistance greater than 1200 MPa, a Re / Rm ratio of less than 0.75 and a tensile elongation greater than 10%, wherein:

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

- the casting of a half-product from this steel

said half-product is brought to a temperature above 1150 ° C.

said half-product is hot-rolled in a temperature range where the microstructure of the steel is entirely austenitic, and then

the sheet thus obtained is cooled from a temperature T _D R located above Ar3 to an intermediate temperature T | with a cooling rate Vm greater than or equal to 70 ° C / s, said temperature Ti being less than or equal to 65O ⁰ C, then

said sheet is cooled from said temperature Ti to a temperature T _FR, said temperature T _FR being between B ' _s and Ms + 50 ° C, B' being a temperature defined with respect to the temperature Bs of beginning of bainitic transformation, and Ms designating the martensitic transformation start temperature, such that the cooling rate between said temperature T _D Rβt said temperature T _FR SOit between 20 and 90 ° C / s, then cooled said sheet from the temperature T _FR with a secondary cooling rate V _'R between 0.08 ° C / min and 6Q0 ^q C / min to room temperature,

said temperature B is equal to Bs when said speed VR is between 0.08 and 2 ° C / min - said temperature B's being equal to Bs + 60 ^β C when said speed

VR is greater than 2 ° C / min and less than or equal to 600 ^û C / min

17. A method of manufacturing a hot-rolled steel sheet according to which: a composition steel is supplied according to any one of claims 1 to 11,

- the casting of a half-product from this steel

said half-product is brought to a temperature greater than 115O ⁰ C

said semi-finished product is hot-rolled in a temperature range where the structure of the steel is entirely austenitic,

the temperature of the primary cooling start T _{DR is} set above Ar3, the temperature of. primary cooling end T _FR , the primary cooling rate V _R between TD _R and T _FR , and the secondary cooling rate VR, such that the microstructure of said steel consists of at least 75% of bainite, of residual austenite in an amount greater than or equal to 5%, and martensite in an amount greater than or equal to 2%

18. Method according to any one of claims 15 or 17, characterized in that adjusts the primary cooling start temperature TDR above Ar3, the primary cooling end temperature TFR, the primary cooling rate V _R between TDR and T _F R, and the secondary cooling rate V ' _R1 so that the carbon content of the residual austenite is greater than 1% by weight

19. A method according to any one of claims 15, 17 or 18, characterized in that one adjusts the primary cooling start temperature T _DR located above Ar3, la. primary cooling end temperature T _FR) the primary cooling rate V _R between T _DR and T _F R, and the secondary cooling rate V ' _R , such that the number of interlayer carbides larger than 0.1 micrometer per unit area is less than or equal to 50000 / mm ²

20. Method according to any one of claims 15 or 17 to 19, characterized in that adjusts the primary cooling start temperature T _DR located above Ar3, the primary cooling end temperature TFR, the speed primary coolant V _R between T _DR and T _FR , and the secondary cooling rate V _R , so that the number N _MA per unit area, residual martensite-austenite islands whose maximum size L _max is greater than 2 micrometers and whose elongation factor is less than 4, less than 14000 / mm ²

21. Use of a hot-rolled steel sheet according to any one of claims 1 to 14, or manufactured by a method according to any one of claims 15 to 20, for the manufacture of structural parts or reinforcement 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 method according to any one of claims 15 to 20, for the manufacture of reinforcements and structural parts for General industry, and parts of abrasion resistance