EP2291547A1 - Method for manufacturing very high strength, cold-rolled, dual phase steel sheets, and sheets thus produced - Google Patents

Abstract

The invention relates to an annealed, cold-rolled, dual phase steel sheet having a strength of 980 to 1100 MPa and an elongation at break greater than 9%, comprising the following composition (as expressed in wt.-%): 0.055% = C = 0.095%, 2% = Mn = 2.6%, 0.005% = Si = 0.35%, S = 0.005%, P = 0.05%, 0.1 = Al = 0.3%, 0.05% = Mo = 0.25%, 0.2% = Cr = 0.5%, assuming that Cr+2Mo = 0.6%, Ni = 0.1%, 0.01 = Nb = 0.04%, 0.01 = Ti = 0.050%, 0.0005 = B = 0.0025%, 0.002% = N = 0.007%, the remainder of the composition consisting of iron and inevitable impurities resulting from production.

Description

 METHOD FOR MANUFACTURING COLD LAMINATED DUAL PHASE STEEL SHEETS WITH VERY HIGH RESISTANCE AND SHEETS THUS

GOODS

The invention relates to the manufacture of cold-rolled and annealed sheets of so-called "dual-phase" steels having a very high strength and a deformability for the manufacture of parts by shaping, in particular in the automotive industry. . The dual-phase steels, whose structure includes martensite, possibly bainite, in a ferritic matrix, have developed a great deal because they combine high resistance with significant possibilities of deformation. In the delivery state, their yield strength is relatively low compared to their breaking strength, which gives them a very favorable ratio (yield strength / strength) during forming operations. Their consolidation capacity is very large, which allows a good distribution of deformations in the case of a collision and obtaining a significantly higher yield strength on the part after forming. It is thus possible to produce parts as complex as with conventional steels, but with higher mechanical properties, which allows a reduction in thickness to maintain identical functional specifications. In this way, these steels are an effective response to the requirements of lightening and safety of vehicles. In the field of hot-rolled sheets (of thickness ranging for example from 1 to 10 mm) or cold-rolled (thickness ranging for example from 0.5 to 3 mm), this type of steel finds particular applications for parts. structures and safety for motor vehicles, such as sleepers, longitudinal members, reinforcement pieces, or wheel sails.

The recent requirements for lightening and reducing energy consumption have led to an increased demand for very high strength dual-phase steels, that is to say whose mechanical strength R _m is between 980 and 1100 MPa. In addition to this level of resistance, these steels must have good weldability and good continuous galvanizing ability at the same time. tempered. These steels must also have good folding ability.

The manufacture of high strength dual phase steels is for example described in EP1201780 A1 relating to steels of composition: 0.01-0.3% C, 0.01 -2% Si, 0.05-3% Mn, <0.1% P, <0.01% S, 0.005-1% AI, with mechanical strength greater than 540 MPa, which exhibit good fatigue strength and hole expansion capability. However, most of the examples presented in this document disclose a resistance of less than 875 MPa. The rare examples in this document going beyond this value are relative to steels with a high carbon content (0.25 or 0.31%) for which the weldability and the expansion of the hole are not sufficient.

The document EP0796928A1 also describes cold-rolled Dual Phase steels with a resistance greater than 550 MPa, with a composition of 0.05-0.3% C, 0.8-3% Mn, 0.4-2.5% AI. , 0.01-0.2% Si. The ferritic matrix contains martensite, bainite and / or residual austenite. The examples presented show that the resistance does not exceed 660 MPa, even for a high carbon content (0.20-0.21%) JP11350038 describes dual phase steels with a resistance greater than 980 MPa, composition 0.10. -0.15% C, 0.8-1.5% Si, 1.5-2.0% Mn, 0.01-0.05% P, less than 0.005% S, 0.01-0.07 % AI in solution, less than 0.01% N, additionally containing one or more elements: 0.001-0.02% Nb, 0.001-0.02% V, 0.001-0.02% Ti. This high strength is, however, obtained at the cost of a significant addition of silicon which certainly allows the formation of martensite, but may nevertheless lead to the formation of surface oxides which deteriorate the coating on quenching. The object of the present invention is to provide a method of manufacturing dual-phase steel plates very high strength, cold rolled, bare or coated, not having the disadvantages mentioned above. It aims to provide Dual Phase steel sheets with a mechanical strength of between 980 and 1100 MPa together with an elongation greater than 9% rupture and good formability, including folding The invention also 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. The invention also aims to provide a sheet of steel easily fabricated by cold rolling, that is to say whose hardness after the hot rolling step is limited so that the rolling forces remain moderate during of the cold rolling step.

It also aims to have a steel sheet capable of depositing a metal coating, in particular by dip galvanizing according to the usual methods.

It also aims to have a steel having good weldability by means of conventional assembly methods such as spot resistance welding. The invention also aims to provide an economical manufacturing process by avoiding the addition of expensive alloying elements.

For this purpose, the subject of the invention is a dual-phase cold-rolled and annealed steel sheet having a strength of between 980 and 1100 MPa, an elongation at break of greater than 9%, the composition of which comprises the contents being expressed in terms of Weight: 0.055% <C <0.095%, 2% ≤Mn <2.6%, 0.005% <Si≤0.35%, S <0.005%, P <0.050%, 0.1 ≤AI <0.3% , 0.05% ≤Mo <0.25%, 0.2% <Cr <0.5%, with the proviso that Cr + 2Mo <0.6%, Ni≤0.1%, 0.01O≤Nb <0.040%, 0.01 O≤Ti ≤0.050%, 0.0005 ≤B <0.0025%, 0.002% <N <0.007%, the rest of the composition consisting of iron and unavoidable impurities resulting from the development. Preferably, the composition of the steel contains, the content being expressed by weight: 0.12% <Al <0.25%.

According to a preferred embodiment, the composition of the steel contains, the content being expressed by weight: 0.10% ≤ Si ≤ 0.30%. The composition of the steel preferably contains: 0.15% ≤ Si ≤ 0.28%. According to a preferred embodiment, the composition contains: P ≤ 0.015%.

The microstructure of the sheet preferably contains 35 to 50% of martensite in surface proportion. According to one particular embodiment, the complement of the microstructure consists of 50 to 65% of ferrite in surface proportion.

According to another particular mode, the complement of the microstructure consists of 1 to 10% of bainite and 40 to 64% of ferrite in surface proportion.

The surface fraction of non-recrystallized ferrite relative to the entire ferritic phase is preferably less than or equal to 15%.

The steel sheet preferably has a ratio between its elastic limit R _e and its resistance R _m such that: 0.6 <Re / R _m <0.8. In a particular embodiment, the sheet is galvanized continuously.

In another particular embodiment, the sheet has a galvannealed coating.

The subject of the invention is also a process for manufacturing a cold rolled and annealed dual phase steel sheet, characterized in that a composition steel is supplied according to any one of the above characteristics, and then

- the steel is cast as a semi-finished product, then

- one carries the semi-finished product at a temperature 1150 ° C <T _R <1250 ^° C, then

the semi-finished product is hot-rolled with an end-of-rolling temperature TFL ≥AΓ3 in order to obtain a hot-rolled product, and then

- one coil the hot rolled product at a temperature of 500 ⁰ C ≤ T _bO b≤ 570 ⁰ C., then pickled hot-rolled, followed by cold rolling with a reduction rate between 30 and 80% to obtain a cold-rolled product, and then - the cold-rolled is heated at a rate 1 ° C / s <V _c ≤5 ^° C / s down to a temperature T _M annealing as: Ad + 40 ° C <T _M ≤Ac3-30 ^o C where a maintenance is carried out for a duration: 30s≤t _M ≤300s so as to obtain a heated and annealed product with a structure comprising austenite, then

the product is cooled to a temperature below the temperature M ₅ with a speed V sufficient for the austenite to become totally martensite.

The subject of the invention is also a process for producing a cold rolled, annealed and galvanized dual phase steel sheet characterized in that supplying the heated and annealed product with a structure comprising austenite according to the above characteristic then,

the product heated and annealed is cooled with a speed V _R sufficient to prevent the transformation of the austenite into ferrite, until it reaches a temperature close to the dip galvanizing temperature T _Zn , then

the product is continuously galvanized by immersion in a bath of zinc or of Zn alloy at a temperature of 450 ° C.≤Zn≤480 ° C. to obtain a galvanized product, and then

- The galvanized product is cooled to room temperature with a speed VR greater than 4 ° C / s to obtain a cold rolled steel sheet, annealed and galvanized.

The subject of the invention is also a process for producing a cold-rolled and galvannealed dual phase steel sheet, characterized in that the heated and annealed product is supplied with a structure comprising austenite according to the characteristic above, then,

the product heated and annealed is cooled with a speed V _R sufficient to prevent the transformation of said austenite into ferrite, until it reaches a temperature close to the dip galvanizing temperature T _Zn , then

- is continuously galvanized product by immersion in a bath of zinc or Zn alloy at a temperature of 450 ^o C≤T _Zn ≤480 ° C to obtain a galvanized product, then

the galvanized product is heated at a temperature of between 490 and 550 ° C. for a period of between 10 and 40 seconds to obtain a galvannealed product, and then the galvannealed product is cooled to ambient temperature at a speed V "R greater than 4 ° C / s, to obtain a cold-rolled and galvannealed steel sheet.

The invention also relates to a manufacturing method according to one of the above characteristics, characterized in that the temperature T _M is between 760 and 83O ⁰ C.

According to a particular mode, the cooling rate V _R is greater than or equal to 15 ° C / s. The invention also relates to the use of a steel sheet according to any one of the above characteristics, or manufactured by a process according to any one of the above characteristics, for the manufacture of structures or safety for motor vehicles. 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 accompanying figures in which:

FIG. 1 shows an example of microstructure of a steel sheet according to the invention. FIGS. 2 and 3 show examples of microstructure of steel sheets not in accordance with the invention.

The invention will now be described in a more precise but nonlimiting manner, considering its various characteristic elements: With regard to the chemical composition of steel, carbon plays an important role in the formation of the microstructure and in the mechanical properties: below 0.055% by weight, the resistance becomes insufficient. Beyond 0.095%, a lengthening of 9% can no longer be guaranteed. The weldability is also reduced. In addition to a solid solution hardening effect, manganese is an element that increases quenchability and reduces carbide precipitation. A minimum content of 2% by weight is necessary to obtain the desired mechanical properties. However, beyond 2.6%, its gammagenic character leads to the formation of a band structure too marked. Silicon is a component involved in the deoxidation of liquid steel and hardening in solid solution. This element also plays an important role in the formation of the microstructure by preventing the precipitation of carbides and by promoting the formation of martensite which enters the structure of the Dual Phase steels. It plays an effective role beyond 0.005%. An addition of silicon in an amount greater than 0.10%, preferably greater than 0.15%, makes it possible to achieve the highest levels of resistance to which the invention relates. However, an increase in the silicon content degrades the dip coating ability by promoting the formation of adherent oxides on the surface of the products: its content must be limited to 0.35% by weight, and preferably 0.30% to obtain a good coating. In addition, the silicon decreases the weldability: a content of less than 0.28% makes it possible simultaneously to ensure very good weldability as well as good coating. Beyond a sulfur content of 0.005%, the ductility is reduced due to the excessive presence of sulfides such as MnS which decrease the ability to deform, especially during hole expansion tests. Phosphorus is an element that hardens in solid solution but decreases spot weldability and hot ductility, particularly because of its ability to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content must be limited to 0.050%, and preferably to 0.015% in order to obtain a good spot welding ability. Aluminum plays an important role in the invention by preventing the precipitation of carbides and promoting the formation of martensitic constituents upon cooling. These effects are obtained when the aluminum content is greater than 0.1%, and preferably when the aluminum content is greater than 0.12%. In the form of AlN, aluminum limits grain growth during annealing after cold rolling. This element is also used for the deoxidation of the liquid steel in an amount usually less than about 0.050%. It is usually considered that higher levels increase the erosion of refractories and the risk of plugging the nozzles. In excessive amounts, aluminum reduces hot ductility and increases the risk of defects in continuous casting. It is also sought to limit inclusions of alumina, in particular in the form of clusters, in order to ensure sufficient elongation properties. However, the inventors have demonstrated, in connection with the other elements of the composition, that an amount of aluminum up to 0.3% by weight could be added without adverse effect vis-à-vis other properties required particularly with respect to the deformability, and also provided the desired microstructural and mechanical properties. Beyond 0.3%, there is a risk of interaction between the liquid metal and the slag during casting continuous, which leads to the possible appearance of defects. An aluminum content of up to 0.25% by weight makes it possible to ensure the formation of a fine microstructure without large martensitic islands which would play a detrimental role on the ductility. The inventors have shown that, surprisingly, it was possible to obtain a high level of resistance, between 980 and 1100 MPa, even in spite of the limitation of additions of aluminum and silicon. This is achieved by the particular combination of the alloying or microalloying elements according to the invention in particular by virtue of the additions of Mo, Cr, Nb ₁ Ti, B.

In an amount greater than 0.05% by weight, molybdenum plays an effective role on quenchability and delays the enlargement of ferrite and the appearance of bainite. However, a content greater than 0.25% excessively increases the cost of the additions. In an amount greater than 0.2%, chromium, by its role on quenchability, also contributes to delay the formation of proeutectoid ferrite. Beyond 0.5%, the cost of the addition is too excessive.

The joint effects of chromium and molybdenum on quenchability are taken into account in the invention according to their specific characteristics; according to the invention, the chromium and molybdenum contents are such that: Cr + (2 × Mo) <0.6%. The coefficients in this relation reflect the respective influence of these two elements on the quenchability in order to favor the obtaining of a fine ferritic structure. Titanium and niobium are microalloy elements used together according to the invention:

In an amount of between 0.010 and 0.050%, titanium combines essentially with nitrogen and carbon to precipitate in the form of nitrides and / or carbonitrides. These precipitates are stable during reheating of the slabs at 1150-1250 ^° C. before hot rolling, which makes it possible to control the size of the austenitic grain. Beyond a 0.050% titanium content, there is a risk of forming coarse titanium nitrides precipitated in the liquid state, which tend to reduce ductility.

- In an amount greater than 0.010%, niobium is very effective in forming Fine precipitates of Nb (CN) in the austenite or ferrite during hot rolling, or during annealing in a temperature range close to the intercritical transformation interval. It retards recrystallization during hot rolling and annealing and refines the microstructure. However, an excessive amount of niobium decreasing weldability should be limited to 0.040%.

The titanium and niobium contents above make it possible to ensure that the nitrogen is completely trapped in the form of nitrides or carbonitrides, so that the boron is in free form and can play an effective role on the quenchability. The effect of boron on quenchability is fundamental. By limiting the activity of carbon, boron indeed makes it possible to control and limit the diffusive phase transformations (ferritic or pearlitic transformation during cooling) and to form hardening phases (bainite or martensite) necessary for obtaining high mechanical strength characteristics. The addition of boron is therefore an important component of the present invention, it also makes it possible to limit the addition of quenching elements such as Mn, Mo, Cr and to reduce the analytical cost of the steel grade. The minimum boron content to ensure effective quenchability is 0.0005%. Beyond 0.0025%, the effect on the quenchability is saturated and there is a detrimental effect on the coating and hot ductility. In order to form a sufficient amount of nitrides and carbonitrides, a minimum content of 0.002% nitrogen is required. The nitrogen content is limited to 0.007% to avoid the formation of BN which would decrease the amount of free boron required for the hardening of the ferrite.

Optional addition of nickel may be performed to provide additional hardening of the ferrite. This addition is, however, limited to 0.1% for cost reasons. The implementation of the method of manufacturing a rolled sheet according to the invention comprises the following successive steps:

A steel of composition according to the invention is supplied

- It proceeds to the casting of a half-product from this steel. This casting can be carried out in ingots or continuously in the form of thick slabs of the order of 200mm. It is also possible to perform the casting in the form of slabs of a few tens of millimeters thick or thin strips between contra-rotating steel rolls.

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

However, if the temperature T _R is too high, the austenitic grains increase undesirably. In this temperature range, the only precipitates likely to effectively control the size of the austenitic grain are titanium nitrides, and the reheating temperature should be limited to 1250 ^° C. in order to maintain a fine austenitic grain at this stage.

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-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: if TFL is lower than the starting temperature of transformation from austenite to cooling A _F3 , the ferrite grains are hardened by rolling and ductility is reduced.

Preferably, a rolling end temperature of greater than 850 ° C. will be chosen.

The hot-rolled product is then rolled at a temperature T _bO b of between 500 and 570 ^° C. This temperature range makes it possible to obtain a complete bainitic transformation during the quasi-isothermal maintenance associated with the winding. This range leads to a morphology of Ti and Nb precipitates which are sufficiently fine in order to allow the exploitation of their hardening and quenching power during the subsequent steps of the manufacturing process. A winding temperature greater than 57O ⁰ C leads to the formation of coarser precipitates, whose coalescence during continuous annealing significantly reduces the efficiency.

When the winding temperature is too low, the hardness of the product is increased, which increases the efforts required during cold rolling at a later time.

The hot-rolled product is then etched according to a process known per se, followed by cold rolling with a reduction ratio preferably comprised between 30 and 80%.

The cold-rolled product is then heated, preferably in a continuous annealing installation, with an average heating rate Vc of between 1 and 5 ° C./sec. In relation to the TM annealing temperature below, this heating rate range makes it possible to obtain a fraction of non-recrystallized ferrite less than or equal to 15%.

The heating is carried out up to an annealing temperature T _M between the temperature A _0I (allotropic transformation start temperature at heating) + 40 ° C, and A ^ (end of allotropic transformation temperature at heating) - 30 ^° C. C, that is to say in a particular temperature range of the intercritical range: when TM is less than (A _c i + 40 ° C), the structure may also comprise non-recrystallized ferrite zones whose surface fraction may reach 15 %. This proportion of non-recrystallized ferrite is evaluated as follows: after having identified the ferritic phase within the microstructure, the surface percentage of non-recrystallized ferrite relative to the entire ferritic phase is quantified. The inventors have demonstrated that these non-recrystallized zones play a detrimental role on the ductility and do not make it possible to obtain the characteristics targeted by the invention. An annealing temperature T _M according to the invention makes it possible to obtain an amount of austenite sufficient to subsequently form the cooling of the martensite in an amount such that the desired characteristics are attained. A temperature TM lower than (Ac ³ - 30 ^° C.) also makes it possible to ensure that the carbon content of the austenite islands formed at the temperature T _M indeed leads to a subsequent martensitic transformation: when the annealing temperature is too high, the carbon content of the austenite islands becomes too low, which leads to a subsequent transformation into bainite or unfavorable pearlite. In addition, too high a temperature leads to an increase in the size of niobium precipitates which lose some of their curing ability. The final mechanical strength is then decreased.

Preferably will be chosen for this purpose a TM temperature between 760 ⁰ C and 83O ⁰ C.

A minimum holding time t _M of 30s at the temperature T _M allows the dissolution of the carbides, a partial transformation into austenite is carried out. The effect is saturated beyond a duration of 300 s. A holding time greater than 300s is also difficult to comply with the productivity requirements of continuous annealing equipment, in particular the speed of scrolling. The holding time tM is between 30 and 300s. The following process steps differ depending on whether an uncoated, or continuously galvanized, or galvannealed steel sheet is manufactured:

In the first case, at the end of the maintenance of annealing, cooling is carried out to a temperature below the temperature M _s (martensite formation start temperature) with a cooling rate V sufficient for the austenite formed during annealing is totally transformed into martensite.

This cooling can be carried out from the temperature T _M in one or several steps and may involve in the latter case different cooling modes such as cold or boiling water baths, jets of water or gas . These possible accelerated cooling modes can be combined to obtain a complete martensitic transformation of the austenite. After this martensitic transformation, the sheet is cooled to room temperature. The microstructure of the cooled bare sheet then consists of a ferritic matrix with islands of martensite whose surface proportion is between 35 and 50%, and is free of bainite.

- In the case where it is desired to manufacture a galvanized sheet continuously quenched, at the end of the maintenance of annealing, the product is cooled to a temperature close to the temperature T _Zn dip galvanizing, the speed of VR cooling is fast enough to avoid the transformation of austenite to ferrite. For this purpose, the cooling rate V _R is preferably greater than 15 ⁰ CVs. Galvanizing is carried out by immersion in a bath of zinc or alloy of zinc whose temperature Tz _n is between 450 and 480 ^° C. A partial transformation of austenite into bainite occurs at this stage, which leads to the formation of 1 to 10% of bainite, this value being expressed in surface proportion. The maintenance in this temperature range must be less than 80s so as to limit the surface proportion of bainite to 10% and thus obtain a sufficient proportion of martensite. The galvanized product is then cooled at a rate V ' _R greater than 4 ° C./s up to room temperature in order to completely convert the remaining austenite fraction into martensite: in this way a sheet of aluminum is obtained. Cold-rolled, annealed and galvanized steel containing 40-64% ferrite, 35-50% martensite and 1-10% bainite in surface proportion. - In the case where it is desired to manufacture a cold-rolled and "galvannealed" dual-phase steel sheet, that is to say galvanized-alloyed, the product is cooled at the end of the maintenance of annealing until reaching a temperature close to the dip galvanizing temperature T _Zn , the cooling rate V _R being fast enough to avoid the transformation of austenite to ferrite. For this purpose, the cooling rate V _R is preferably greater than 15 ° C / s. Galvanizing is carried out by dipping by immersion in a bath of zinc or zinc alloy whose temperature T _Zn is between 450 and 480 ^° C. A partial transformation of the austenite into bainite occurs at this stage, which leads to the formation of 1 to 10% of bainite, this value being expressed in surface proportion. The maintenance in this temperature range must be less than 80s so as to limit the proportion of bainite to 10%. After leaving the zinc bath, the galvanized product is heated to a temperature T _G of between 490 and 550 ⁰ C for a period XQ of between 10 and 40s. This causes the interdiffusion of iron and the thin layer of zinc or zinc alloy deposited during the immersion, which allows to obtain a galvannealed product. This product is cooled to room temperature with a speed V " _R greater than 4 ° C./s: a galvannealed sheet of ferritic matrix steel is obtained in this way, containing 40-64% of ferrite in surface proportion, 35-50% of martensite and 1-10% of bainite Martensite is typically in the form of islands of average size less than 4 micrometers, or even two microns, these Islets presenting, for more than 50% of them, a massive morphology rather than an elongated morphology. The morphology of a given island is characterized by the ratio between its maximum size L _max and minimum L _min . A given island is considered to have a

5 massive morphology when its ratio is less than or equal to 2.

In addition, the inventors have found that small variations in the manufacturing parameters within the conditions defined according to the invention do not lead to significant modifications of the microstructure or the mechanical properties, which is an advantage for the stability of the characteristics of manufactured industrial products.

The present invention will now be illustrated on the basis of the following nonlimiting examples:

Example:

Steels have been developed whose composition is shown in the table below, expressed as a percentage by weight. In addition to the steels IX to IZ used for the manufacture of sheets according to the invention, it was indicated for comparison the composition of a steel R used for the manufacture of reference sheets.

Table 1 Compositions of steel (% by weight). R = Reference Underlined values: Not in accordance with the invention. 0

Cast half-products corresponding to the above compositions were heated to 123O ⁰ C and then hot rolled to a thickness of 2.8-4 mm in a field where the structure is entirely austenitic. The manufacturing conditions of these hot-rolled products (rolling end temperature T _F L, winding temperature T _bO b) are shown in Table 2.

Table 2 Manufacturing conditions for hot-rolled products

The hot-rolled products were then pickled and then cold-rolled to a thickness of 1.4 to 2 mm, a reduction rate of 50%. From the same composition, some steels have been subject to different manufacturing conditions. References 1X1, IX2 and IX3 denote for example three steel sheets manufactured under different conditions from the steel composition IX. The sheets were galvanized by dipping in a zinc bath at a temperature T _Zn of 460 ^° C., others were further subjected to a galvannealing treatment. Table 3 shows the manufacturing conditions for annealed sheet after cold rolling:

. Heating speed V _c

- Annealing temperature T _M.

- Annealing hold time t _M - Cooling speed after annealing V _R

- Cooling rate after galvanizing VR

- Galvannealing temperature T _G

- Duration of galvannealing {Q

- Cooling rate V "R after galvannealing treatment The transformation temperatures A ₀₁ and Ac ₃ were also reported in Table 3.

Table 3 Manufacturing conditions for cold-rolled and annealed sheets Underlined values: not in accordance with the invention

The mechanical tensile properties obtained (elastic limit Re, resistance Rm, elongation at break A were given in Table 4 below), the ratio Re / Rm was also indicated, and the microstructure of the steels was determined. the matrix is ferritic The surface fractions of bainite and martensite were quantified after Picral and LePera reagent etching respectively, followed by an image analysis using Aphelion ™ software, and the non-recrystallized ferrite surface fraction was determined by to observations in optical microscopy and scanning electron where the ferritic phase has been identified, then quantified the recrystallized fraction within this ferritic phase The non-recrystallized ferrite is generally in the form of islands elongated by rolling. The folding ability was quantified as follows: sheets were folded in a block on themselves in several turns. In this way, the bending radius decreases each turn. The foldability is then evaluated by noting the presence of cracks on the surface of the folded block, the rating being expressed from 1 (low foldability) to 5 (very good ability). satisfactory.

Table 4 Results obtained on cold-rolled and annealed sheets Underlined values: not in accordance with the invention

The steel sheets according to the invention have a set of microstructural and mechanical characteristics enabling the advantageous manufacture of parts, in particular for structural applications: resistance of between 980 and 1100 MPa, ratio R _e / R _m of between 0.6 and 0.8, elongation at break of greater than 9%, good folding ability. Figure 1 illustrates the morphology of the 1X1 steel sheet, where the ferrite is completely recrystallized.

The sheets according to the invention have good weldability, in particular resistance, the equivalent carbon being less than 0.25. In particular, the weldability range as defined by the standard I S018278-2, in spot welding is very wide, of the order of 3500A. It is increased relative to a reference grade of the same grade. In addition, cross-tension or tensile-shear tests carried out on welded points of sheets according to the invention reveal that the resistance of these welded points is very high with regard to the mechanical characteristics. By comparison, the reference plates do not offer these same characteristics:

The steel plates IX3 (galvanized) and IX6 (galvannealed) were annealed at a temperature T _M too low: consequently, the fraction of non-recrystallized ferrite is excessive as well as the martensite fraction. These microstructural features are associated with a decrease in elongation and foldability. FIG. 2 illustrates the microstructure of the steel sheet IX3: there is the presence of non-recrystallized ferrite in the form of elongate islands (marked (A)) coexisting with recrystallized ferrite and martensite, the latter constituting appearing darker on micrography. A scanning electron micrograph (FIG. 3) makes it possible to finely distinguish the zones of non-recrystallized ferrite (A) from those recrystallized (B).

Sheet IX5 is a galvannealed sheet annealed at a temperature T _M too high: the carbon content of austenite at high temperature then becomes too low and the appearance of bainite is favored at the expense of the formation of martensite. Coalescence of niobium precipitates is also observed, which causes a loss of hardening. The resistance is then insufficient, the ratio Re / R _m being too high. The Galvannealed 1X7 sheet was cooled at a rate V _R too slow after the annealing step: the transformation of the austenite formed into ferrite then occurs in this cooling step excessively, the steel sheet containing at the stage final a proportion of bainite too important and a proportion of martensite too low, which leads to insufficient resistance.

The composition of the steel sheet R does not correspond to the invention, its carbon content being too high, and its content of manganese, aluminum, niobium, titanium, boron being too low. As a result, the martensite fraction is too weak so that the mechanical strength is insufficient. The steel sheets according to the invention will be used profitably for the manufacture of structural parts or safety in the automotive industry.

Claims

1. Cold-rolled, annealed, double-strength steel sheet having a strength of between 980 and 1100 MPa, elongation at break greater than 9%, the composition of which includes the contents being expressed by weight: 0.055% ≤C <0.095% 2% ≤Mn <2.6% 0.005% <Si≤ 0.35% S <0.005% P <0.050% 0.1 <AI <0.3% 0.05% <Mo <0.25% 0.2% <Cr≤0.5% with the proviso that Cr + 2Mo <0.6% Ni ≤ 0.1% 0.010 <Nb <0.040% 0.010 <Ti <0.050% 0.0005 ≤B <0.0025% 0.002% ≤N <0.007%, the balance of the composition consisting of iron and unavoidable impurities resulting from the elaboration 2. Steel sheet according to claim 1, characterized in that the composition of said steel contains, the content being expressed by weight: 0.12% <AI 0 0.25% 3. Sheet steel according to claim 1 or 2, characterized in that the composition of said steel contains, the content being expressed by weight: 0.10% <If ≤ 0.30% 4 steel sheet according to claim 1 or 2, characterized in that the composition of said steel contains, the content being expressed by weight: 0.15% <If <0.28% Steel sheet according to any one of claims 1 to 4, characterized in that the composition of said steel contains, the content being expressed by weight: P <0.015% 6 steel sheet according to any one of claims 1 to 5, characterized in that its microstructure contains 35 to 50% of martensite in surface proportion 7 steel sheet according to claim 6, characterized in that the complement of said microstructure consists of 50 to 65% of ferrite in surface proportion 8 steel sheet according to claim 6, characterized in that the complement of said microstructure consists of 1 to 10% of bainite and 40 to 64% of ferrite in surface proportion 9 steel sheet according to any one of claims 1 to 8 characterized in that its surface fraction of non-recrystallized ferrite relative to the entire ferritic phase, is less than or equal to 15% 10 steel sheet according to any one of claims 1 to 9 characterized in that the ratio between its elastic limit R _e and its resistance R _m is such that: 0.6 <Re / R _m <0.8 11 Steel sheet according to any one of claims 1 to 6 or 8 to 10, characterized in that it is continuously galvanized Steel sheet according to one of Claims 1 to 6 or 8 to 10, characterized in that it comprises a galvannealed coating 13 Process for manufacturing a cold rolled and annealed dual phase steel sheet, characterized in that a composition steel is supplied according to any one of Claims 1 to 5, and then said steel is cast in the form of a semi-finished product, then said semi-finished product is brought to a temperature of 1150 ° C./TR <1250 ° C., and said semi-finished product is then hot-rolled with an end-of-rolling temperature T _F L ≥ Ar3 to obtain a hot rolled product, then said hot rolled product is rolled at a temperature Tbob such that: 500 ⁰ C <T _b ob 57 570 ⁰ C, then said hot-rolled product is scraped off, then cold rolling is carried out with a reduction ratio of between 30 and 80% to obtain a cold-rolled product, then said cold-rolled product is heated at a rate of 1 ° C./s <Vc≤5 ° C./s up to an annealing temperature T _M such that: Ad + 40 ° C≤TM≤AC3-30 ° C where a maintenance is carried out for a period: 30s≤tM ≤300s so as to obtain a heated and annealed product with a structure comprising austenite, then said product is cooled to a temperature below the temperature M ₅ with a speed V sufficient for said austenite to be totally transformed into martensite Process for producing a cold-rolled, annealed and galvanized Dual Phase steel sheet characterized in that said heated and annealed product is supplied with a structure comprising austenite according to claim 13 and then said heated and annealed product is cooled with a sufficient speed VR in order to avoid the transformation of said austenite into ferrite, until it reaches a temperature close to the dip galvanization temperature Tz _n , then - Continuously galvanizing said product by immersion in a bath of zinc or Zn alloy at a temperature 450 ° C <T _Z n≤480 ^o C to obtain a galvanized product, then said galvanized product is cooled to ambient temperature with a speed V ' _R greater than 4 ° C./s to obtain a cold-rolled, annealed and galvanized steel sheet A process for producing a cold-rolled and galvannealed dual phase steel sheet, characterized in that said heated and annealed product is supplied with a structure comprising austenite according to claim 13 and then said heated and annealed product is cooled with a speed V _R sufficient to prevent the transformation of said austenite into ferrite, until it reaches a temperature close to the dip galvanizing temperature T _Zn , then - Continuously galvanizing said product by immersion in a bath of zinc or Zn alloy at a temperature of 450 ° C <T _Zn ≤480 ^o C to obtain a galvanized product, then said galvanized product is heated at a temperature TQ of between 490 and 550 ^° C. for a period XQ of between 10 and 40 s to obtain a galvannealed product, then said galvannealed product is cooled to ambient temperature at a speed V "R greater than 4 ° C./s, to obtain a cold rolled steel sheet and galvannealed Manufacturing method according to any one of claims 13 to 15, characterized in that said temperature TM is between 760 and 830 ⁰ C Manufacturing method according to claim 14 or 15, characterized in that said cooling rate VR is greater than or equal to 15 ° C / sec. 18 Use of a steel sheet according to any one of claims 1 to 12, or manufactured by a method according to any one of claims 13 to 17, for the manufacture of structural parts or safety for motor vehicles