EP2155916B2 - Low density steel with good stamping capability - Google Patents

Description

The invention relates to a ferritic sheet of hot-rolled or cold-rolled steel, having a strength greater than 400 MPa and a density of less than about 7.3, and its manufacturing process.

• grâce à une augmentation des caractéristiques mécaniques des aciers constituant les pièces structurales ou les pièces de peau, ou
• à caractéristiques mécaniques données, grâce à une réduction de la densité des aciers.
• La première voie fait l'objet de nombreuses recherches, des aciers dont la résistance mécanique va de 800MPa à plus de 1000MPa ont été proposés par l'industrie sidérurgique. La densité de ces aciers reste cependant voisine de 7,8, qui est la densité d'aciers conventionnels.
• Une seconde voie passe par l'addition d'éléments susceptibles de réduire la densité des aciers (comme les aciers divulgués dans GB-A-1 044 801 , JP-A-2001-271 148 ou JP-A-04-056 748 ): Le brevet EP1485511 divulgue ainsi des aciers comportant des additions de silicium (2-10%) et d'aluminium (1-10%) de microstructure ferritique, et contenant également des phases carburées.

The reduction in the amount of CO ₂ emitted by motor vehicles includes the lightening of motor vehicles. This relief can be achieved:

• thanks to an increase in the mechanical characteristics of the steels constituting the structural parts or the pieces of skin, or
• with given mechanical characteristics, thanks to a reduction of the density of the steels.
• The first channel is the subject of much research, steels whose mechanical strength ranges from 800 MPa to more than 1000 MPa have been proposed by the steel industry. The density of these steels however remains close to 7.8, which is the density of conventional steels.
• A second way involves the addition of elements capable of reducing the density of the steels (such as the steels disclosed in GB-A-1,044,801 , JP-A-2001-271 148 or JP-A-04-056 748 ): The patent EP1485511 thus discloses steels comprising additions of silicon (2-10%) and aluminum (1-10%) ferritic microstructure, and also containing carburized phases.

However, the relatively high silicon content of these steels may in some cases pose problems of coating and ductility.

Steels containing an addition of approximately 8% of aluminum are also known: however, difficulties can be encountered during the manufacture of these steels, in particular during cold rolling. There may also be problems of scouring during the stamping of these steels. When these contain more than 0.010% C, precipitation of carburized phases may increase brittleness. The use of such steels for the manufacture of structural parts is then impossible.

• une densité inférieure à 7,3 environ
• une résistance R_m supérieure à 400MPa
• une bonne aptitude à la déformation, en particulier au laminage et une excellente résistance au chiffonnage,
• une bonne soudabilité et une bonne revêtabilité

The object of the invention is to propose hot-rolled or cold-rolled steel sheets having simultaneously:

• a density of less than about 7.3
• a resistance R _m greater than 400 MPa
• good deformability, in particular rolling and excellent resistance to creasing,
• good weldability and good coating

The object of the invention is also to provide a manufacturing method compatible with the usual industrial installations.

For this purpose, the subject of the invention is a ferritic hot-rolled steel sheet whose composition comprises, the contents being expressed by weight: 0.001 ≤ C ≤ 0.15%, Mn ≤ 1%, Si ≤ 1.5% , 6% ≤ Al ≤ 10%, 0.020% ≤ Ti ≤ 0.5%, S ≤ 0.050%, P ≤ 0, 1% and, optionally, one or more elements selected from: Cr ≤ 1%, Mo ≤ 1%, Ni ≤ 1%, Nb ≤ 0.1%, V ≤ 0.2%, B ≤ 0.01%, the rest of the composition consisting of iron and unavoidable impurities resulting from the preparation, the average size of ferrite grain d _IV measured on a surface perpendicular to the direction transverse to the rolling being less than 100 micrometers.

désignant la longueur totale des joints de grains comportant des précipités κ relativement à une surface (S) considérée, ${\displaystyle \sum _{\left(S\right)}}\mathit{Li}$

désignant la longueur totale des joints de grains relativement à la surface (S) considérée.The subject of the invention is also a cold rolled and annealed ferritic sheet obtained from a hot-rolled sheet above, characterized in that its structure consists of equiaxed ferrite whose average grain size _α is smaller. at 50 micrometers, and in that the linear fraction f of intergranular κ precipitates is less than 30%, the linear fraction f being defined by: $\mathrm{f}=\frac{{\displaystyle \underset{\left(S\right)}{\Sigma }}\mathit{di}}{{\displaystyle \underset{\left(S\right)}{\Sigma }}\mathit{Li}},{\displaystyle \underset{\left(S\right)}{\Sigma }}\mathit{di}$

designating the total length of the grain boundaries comprising precipitates κ relative to a surface (S) considered, ${\displaystyle \underset{\left(S\right)}{\Sigma }}\mathit{Li}$

designating the total length of grain boundaries relative to the surface (S) considered.

In a particular embodiment, the composition comprises: 0.001% ≤ C ≤ 0.010%, Mn ≤ 0.2%.

According to a preferred embodiment, the composition comprises: 0.010% <C ≤ 0.15%, 0.2% <Mn ≤ 1%.

Preferably, the composition comprises: 7.5% ≤ Al ≤ 10%.

Very preferably, the composition comprises: 7.5% ≤ Al ≤ 8.5%.

The carbon content in solid solution is preferably less than 0.005% by weight.

In a preferred embodiment, the strength of the sheet is greater than or equal to 400 MPa.

As a preference, the strength of the sheet is greater than or equal to 600 MPa. The subject of the invention is also a process for manufacturing a hot-rolled steel sheet according to which a steel of composition is supplied according to one of the above compositions, the steel is cast in the form of a semi-finished product. that is heated to a temperature greater than or equal to 1150 ° C. The semi-finished product is hot-rolled to obtain a sheet, by means of at least two rolling steps carried out at temperatures above 1050 ° C., the reduction rate of each of the steps being greater than or equal to 30%, the time flowing between each of the rolling steps, and the next rolling step being greater than or equal to 10 s. The rolling is completed at a temperature T _FL greater than or equal to 900 ° C, the sheet is cooled so that the time interval tp flowing between 850 and 700 ° C is greater than 3 s, to obtain a precipitation of precipitates κ, then the sheet is reeled at a temperature T _bob of between 500 and 700 ° C.

In a particular embodiment, the casting is carried out directly in the form of thin slabs or thin strips between contra-rotating rolls.

The invention also relates to a method of manufacturing a cold-rolled and annealed steel sheet according to which a hot-rolled steel sheet manufactured according to one of the above modes is supplied, then the sheet is cold-rolled. with a reduction ratio of between 30 and 90%, so as to obtain a cold-rolled sheet. The cold-rolled sheet is then heated to a temperature T 'with a speed V _c greater than 3 ° C./s, then the sheet is cooled at a speed V _R less than 100 ° C./s, the temperature T' and the speed V _R being selected so as to obtain complete recrystallization, a linear fraction f of intergranular precipitates κ of less than 30% and a carbon content in solid solution of less than 0.005% by weight.

The cold-rolled sheet is preferably heated to a temperature T 'of between 750 and 950 ° C.

According to a particular method of manufacturing a cold-rolled and annealed sheet, a sheet of composition is supplied: 0.010% <C ≤ 0.15%, 0.2% <Mn ≤ 1%, Si ≤ 1.5%, 6% ≤Al ≤ 10%, 0.020% ≤ Ti ≤ 0.5%, S ≤ 0.050%, P ≤ 0, 1% and, optionally, one or more elements selected from: Cr ≤ 1%, Mo ≤ 1 %, Ni ≤ 1%, Nb ≤ 0.1%, V ≤ 0.2%, B ≤ 0.01%, the rest of the composition consisting of iron and unavoidable impurities resulting from the preparation, and heating the cold-rolled sheet at a temperature T 'chosen so as to avoid the dissolution of precipitates κ.

According to a particular embodiment, supplying a sheet of composition above and heating the cold-rolled sheet to a temperature T 'of between 750 and 800 ° C.

The invention also relates to the use of steel sheets according to one of the above modes or manufactured in one of the above modes for the manufacture of skin parts or structural parts in the automotive field .

• La figure 1 définit schématiquement la fraction linéaire f de joints de grains ferritiques comportant une précipitation intergranulaire
• La figure 2 présente la microstructure d'une tôle d'acier laminée à chaud selon l'invention.
• La figure 3 présente la microstructure d'une tôle d'acier laminée à chaud fabriquée selon des conditions ne satisfaisant pas à l'invention
• Les figures 4 et 5 illustrent la microstructure de deux tôles laminées à froid et recuites selon l'invention.
• La figure 6 présente la microstructure d'une tôle d'acier laminée à froid et recuite fabriquée selon des conditions ne satisfaisant pas à l'invention La présente invention est relative à des aciers présentant une densité réduite, inférieure à 7,3 environ, tout en conservant des caractéristiques d'usage satisfaisantes.

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:

• The figure 1 schematically defines the linear fraction f of ferritic grain boundaries with intergranular precipitation
• The figure 2 presents the microstructure of a hot-rolled steel sheet according to the invention.
• The figure 3 presents the microstructure of a hot-rolled steel sheet manufactured under conditions not satisfying the invention
• The figures 4 and 5 illustrate the microstructure of two cold-rolled and annealed sheets according to the invention.
• The figure 6 DESCRIPTION OF THE INVENTION The present invention relates to steels having a reduced density, of less than about 7.3, while retaining the properties of the microstructure of a cold rolled and annealed steel sheet manufactured under conditions not satisfying the invention. satisfactory use characteristics.

The invention relates in particular to a manufacturing method for controlling the precipitation of intermetallic carbides, the microstructure, and the texture in steels including particular combinations of carbon, aluminum and titanium.

• Selon l'invention, la teneur en carbone est comprise entre 0,001% et 0,15% : au dessous de 0,001%, on ne peut obtenir un durcissement significatif. Lorsque la teneur en carbone est supérieure à 0,15%, l'aptitude au laminage à froid des aciers est faible.
• Lorsque la teneur en manganèse excède 1%, il existe un risque de stabilisation de l'austénite résiduelle à température ambiante en raison du caractère gammagène de cet élément. Les aciers selon l'invention ont une microstructure ferritique à température ambiante. Différents modes particuliers de l'invention peuvent être mis en oeuvre, en fonction de la teneur en carbone et en manganèse de l'acier :
• Lorsque la teneur en carbone est comprise entre 0,001 et 0,010% et lorsque la teneur en manganèse est inférieure ou égale à 0,2%, la résistance R_m minimale obtenue est de 400MPa.
• Lorsque la teneur en carbone est supérieure à 0,010% et inférieure ou égale à 0,15%, et lorsque la teneur en manganèse est supérieure à 0,2% et inférieure ou égale à 1%, la résistance minimale obtenue est de 600 MPa. Dans les gammes des teneurs en carbone présentées ci-dessus, les inventeurs ont mis en évidence que cet élément contribuait à un durcissement important par une précipitation de carbures (TiC ou précipités kappa) et par un affinement du grain ferritique. L'addition de carbone ne conduit qu'à une faible perte de ductilité si la précipitation de carbures n'est pas intergranulaire ou si le carbone n'est pas en solution solide.

With regard to the chemical composition of steel, carbon plays an important role in the formation of the microstructure and in the mechanical properties:

• According to the invention, the carbon content is between 0.001% and 0.15%: below 0.001%, significant curing can not be obtained. When the carbon content is greater than 0.15%, the cold rollability of the steels is low.
• When the manganese content exceeds 1%, there is a risk of stabilization of the residual austenite at room temperature because of the gamma-genic nature of this element. The steels according to the invention have a ferritic microstructure at ambient temperature. Different particular embodiments of the invention may be implemented, depending on the carbon and manganese content of the steel:
• When the carbon content is between 0.001 and 0.010% and when the manganese content is less than or equal to 0.2%, the minimum resistance R _m obtained is 400 MPa.
• When the carbon content is greater than 0.010% and less than or equal to 0.15%, and when the manganese content is greater than 0.2% and less than or equal to 1%, the minimum resistance obtained is 600 MPa. In the ranges of carbon contents presented above, the inventors have demonstrated that this element contributes to a significant hardening by a precipitation of carbides (TiC or kappa precipitates) and a refinement of the ferritic grain. The addition of carbon only leads to a small loss of ductility if the precipitation of carbides is not intergranular or if the carbon is not in solid solution.

• Au même titre que l'aluminium, le silicium est un élément permettant de réduire la densité de l'acier. Cependant, une addition excessive de silicium, au delà de 1,5%, provoque la formation d'oxydes fortement adhérents et l'apparition éventuelle de défauts de surface, conduisant notamment à un manque de mouillabilité dans les opérations de galvanisation au trempé. De plus, cette addition excessive diminue la ductilité.
• L'aluminium est un élément important de l'invention : lorsque sa teneur est inférieure à 6% en poids, une réduction suffisante de la densité ne peut être obtenue. Lorsque sa teneur est supérieure à 10%, il existe un risque de formation de phases intermétalliques fragilisantes Fe₃Al et FeAl. Préférentiellement, la teneur en aluminium est comprise entre 7,5 et 10% : au sein de cette gamme, la densité de la tôle est inférieure à 7,1 environ. Préférentiellement, la teneur en aluminium est comprise entre 7,5 et 8,5% : dans cette gamme, on obtient un allégement satisfaisant sans diminution de la ductilité.
• L'acier contient également une teneur minimale en titane de 0,020% qui contribue à limiter la teneur en carbone en solution solide en quantité inférieure à 0,005% en poids, grâce à une précipitation de TiC. Le carbone en solution solide a un effet néfaste sur la ductilité du fait qu'il réduit la mobilité des dislocations. Au delà de 0,5% de titane, la précipitation de carbures de titane intervient en quantité trop importante, et la ductilité est réduite.
• Une addition éventuelle de bore limitée à 0,010% contribue également à une réduction du carbone en solution solide.
• La teneur en soufre est inférieure à 0,050% de façon à limiter une précipitation éventuelle de TiS qui diminuerait la ductilité.
• Pour des raisons de ductilité à chaud, la teneur en phosphore est également limitée à 0,1%.

In these composition ranges, the steel has a ferritic matrix at any temperature during the manufacturing cycle, that is to say from the solidification from the casting.

• Like silicon, silicon is an element that reduces the density of steel. However, excessive addition of silicon, above 1.5%, 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. In addition, this excessive addition decreases ductility.
• Aluminum is an important element of the invention: when its content is less than 6% by weight, a sufficient reduction of the density can not be obtained. When its content is greater than 10%, there is a risk of formation of Fe ₃ Al and FeAl embrittling intermetallic phases. Preferably, the aluminum content is between 7.5 and 10%: within this range, the density of the sheet is less than about 7.1. Preferably, the aluminum content is between 7.5 and 8.5%: in this range, satisfactory lightening is obtained without reducing the ductility.
• The steel also contains a minimum titanium content of 0.020% which helps to limit the carbon content in solid solution in an amount of less than 0.005% by weight, thanks to a precipitation of TiC. Carbon in solid solution has a deleterious effect on ductility because it reduces the mobility of dislocations. Beyond 0.5% titanium, the precipitation of titanium carbides occurs in too large a quantity, and the ductility is reduced.
• A possible addition of boron limited to 0.010% also contributes to a reduction of carbon in solid solution.
• The sulfur content is less than 0.050% so as to limit any precipitation of TiS which would decrease the ductility.
• For reasons of hot ductility, the phosphorus content is also limited to 0.1%.

• du chrome, du molybdène, ou du nickel en quantité inférieure ou égale à 1%. Ces éléments apportent un durcissement complémentaire par solution solide.
• Des éléments de micro-alliage, comme le niobium et le vanadium en quantité respectivement inférieure à 0,1 et 0,2% en poids, peuvent être ajoutés pour obtenir un durcissement complémentaire par précipitation.

As an option, steel may also contain, alone or in combination:

• chromium, molybdenum, or nickel in an amount of 1% or less. These elements provide additional hardening by solid solution.
• Microalloy elements, such as niobium and vanadium in amounts of less than 0.1 and 0.2% by weight, respectively, can be added to obtain additional hardening by precipitation.

The rest of the composition consists of iron and unavoidable impurities that result from the elaboration.

The structure of the steels according to the invention comprises a homogeneous distribution of highly disoriented ferritic grains: the strong disorientation between neighboring grains makes it possible to avoid the crimping defect: this defect is characterized, during the cold forming of sheets, by the localized and premature appearance of strips in the direction of rolling, forming a relief. This phenomenon is due to the presence of recrystallized and weakly disoriented grain groups, since they originate from the same original grain before recrystallization. A scuff sensitive structure is characterized by spatial texture distribution.

When the scuffing phenomenon is present, the cross-machine mechanical properties (especially the uniform elongation) and the formability are greatly reduced. The steels according to the invention do not show any sensitivity to creasing during the shaping, because of their favorable texture.

${\displaystyle \sum _{\left(S\right)}}\mathit{di}$

désigne la longueur totale des joints de grains comportant des précipités κ, relativement à la surface (S) considérée. ${\displaystyle \sum _{\left(S\right)}}\mathit{Li}$

représente la longueur totale des joints de grains relativement à la surface (S) considérée.According to one form of the invention, the microstructure at ambient temperature of the steels consists of an equiaxed ferrite matrix whose average grain size is less than 50 micrometers. Aluminum is mainly in solid solution in this matrix based on iron. These steels contain kappa precipitates ("κ") which are a ternary Fe ₃ AlC _x intermetallic phase. The presence of these precipitates in the ferritic matrix leads to a significant hardening. These κ precipitates, however, must not be present in the form of a marked intergranular precipitation under penalty of a significant reduction in ductility: the inventors have shown that the ductility was reduced when the linear fraction of ferritic grain boundaries which present κ precipitation was greater than or equal to 30%. The definition of this linear fraction f is given to the figure 1 : If we consider a particular grain whose contour is limited by successive grain boundaries of length L ₁ , L ₂ , .. L _i , observations by microscopy show that this grain may have precipitates κ along the joints over a length d ₁ , ..d _i ... Considering a surface (S) statistically representative of the microstructure, for example composed of more than 50 grains, the linear fraction comprising precipitates κ is defined by the expression f: $$\mathrm{f}=\frac{{\displaystyle \underset{\left(S\right)}{\Sigma }}\mathit{di}}{{\displaystyle \underset{\left(S\right)}{\Sigma }}\mathit{Li}}$$

${\displaystyle \underset{\left(S\right)}{\Sigma }}\mathit{di}$

denotes the total length of the grain boundaries comprising precipitates κ relative to the surface (S) considered. ${\displaystyle \underset{\left(S\right)}{\Sigma }}\mathit{Li}$

represents the total length of the grain boundaries relative to the surface (S) considered.

The expression f thus translates the degree of recovery of ferritic grain boundaries by κ precipitation.

• On approvisionne un acier de composition selon l'invention.
• On procède à la coulée d'un demi-produit à partir de cet acier. Cette coulée peut être réalisée en lingots, ou en continu sous forme de brames d'épaisseur de l'ordre de 200mm. On peut également effectuer la coulée sous forme de brames minces de quelques dizaines de millimètres d'épaisseur, ou de bandes minces, entre cylindres d'acier contra-rotatifs. Ce mode de fabrication sous forme de produits minces est particulièrement avantageux, car il permet d'obtenir plus facilement une structure fine qui favorise la réalisation de l'invention comme on le verra plus loin. Au moyen de ses connaissances générales, l'homme du métier saura déterminer les conditions de coulée satisfaisant à la fois la nécessité d'obtenir une structure fine et équiaxe après la coulée, et celle de satisfaire les exigences usuelles d'une coulée industrielle.

According to another form of the invention, the ferritic grain is not equiaxed but its average size d _IV is less than 100 micrometers. d _IV denotes the grain size measured by the method of linear intercepts on a representative surface (S) perpendicular to the direction transverse to the rolling. The measurement of d _IV is made in the direction perpendicular to the thickness of the sheet. This non-equiaxial grain morphology, having an elongation in the direction of rolling, may for example be present on hot-rolled steel sheets according to the invention. The method of manufacturing a hot-rolled sheet according to the invention is implemented as follows:

• A steel composition is provided according to the invention.
• A semi-finished product is cast from this steel. This casting may be carried out in ingots, or continuously in the form of slabs of thickness of the order of 200 mm. The casting can also be carried out in the form of thin slabs of a few tens of millimeters thick, or thin strips, between contra-rotating steel rolls. This method of manufacture in the form of thin products is particularly advantageous, because it makes it easier to obtain a fine structure which favors the realization of the invention as will be seen later. By means of his general knowledge, the skilled person will determine the casting conditions satisfying both the need to obtain a fine and equiaxed structure after casting, and that of meeting the usual requirements of an industrial casting.

The cast semifinished products are first brought to a temperature above 1150 ° C. in order to reach at all points a temperature favorable to the high deformations that the steel will undergo during the various rolling steps.

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

• On lamine à chaud le demi-produit pour obtenir une tôle, par une succession d'étapes de laminage. Chacune des étapes correspond à une réduction d'épaisseur du produit par le passage au sein de cylindres de laminoir. Dans des conditions industrielles, ces étapes sont réalisées lors du dégrossissage du demi-produit sur un train à bandes. Le taux de réduction associé à chacune de ces étapes est défini par : (épaisseur du demi-produit après étape de laminage- épaisseur avant laminage)/(épaisseur avant laminage) Selon l'invention, au moins deux de ces étapes sont réalisées à des températures supérieures à 1050°C, le taux de réduction de chacune d'elles est supérieur ou égal à 30%. L'intervalle de temps t_i entre chacune des déformations de taux supérieur à 30% et la déformation ultérieure est supérieur ou égal à 10 s de façon à obtenir une recristallisation totale à l'issue de cet intervalle de temps t_i. Les inventeurs ont mis en évidence que cette combinaison particulière de conditions conduisait à un affinement très important de la structure à chaud. On promeut ainsi une recristallisation grâce à des températures de laminage supérieures à la température de non-recristallisation Tnr. Les inventeurs ont également mis en évidence qu'une structure initiale fine, telle que celle obtenue après une coulée directe, était favorable pour accélérer la recristallisation.
• On achève le laminage à une température T_FL supérieure ou égale à 900°C, de façon à obtenir une recristallisation complète.
• On refroidit ensuite la tôle obtenue : les inventeurs ont mis en évidence qu'une précipitation particulièrement efficace de précipités κ et de carbures TiC était obtenue lorsque l'intervalle de temps t_p s'écoulant au refroidissement entre 850 et 700°C était supérieur à 3 s. On obtient de la sorte une précipitation intense favorable au durcissement.
• On bobine ensuite la tôle à une température T_bob comprise entre 500 et 700°C. Cette étape achève la précipitation de TiC.

After numerous tests, the inventors have demonstrated that it was possible to avoid the problem of scouring and to obtain very good drawability and good ductility, by means of the manufacturing process comprising the following steps:

• The semi-finished product is hot rolled to obtain a sheet, by a succession of rolling steps. Each of the steps corresponds to a reduction in the thickness of the product by passing through rolling mill rolls. Under industrial conditions, these steps are performed when roughing the semi-finished product on a band train. The reduction rate associated with each of these steps is defined by: (thickness of the semi-finished product after rolling step-thickness before rolling) / (thickness before rolling) According to the invention, at least two of these steps are carried out at temperatures above 1050 ° C, the reduction rate of each of them is greater than or equal to 30%. The time interval t _i between each of the rate deformations greater than 30% and the subsequent deformation is greater than or equal to 10 s so as to obtain a total recrystallization at the end of this time interval t _i . The inventors have demonstrated that this particular combination of conditions led to a very important refinement of the hot structure. This promotes recrystallization through rolling temperatures above the non-recrystallization temperature Tnr. The inventors have also demonstrated that a fine initial structure, such as that obtained after direct casting, was favorable to accelerate the recrystallization.
• The rolling is completed at a temperature T _FL greater than or equal to 900 ° C, so as to obtain a complete recrystallization.
• The sheet obtained is then cooled: the inventors have demonstrated that a particularly effective precipitation of κ precipitates and TiC carbides was obtained when the time interval t _p flowing on cooling between 850 and 700 ° C. was greater than 3 sec. In this way, an intense precipitation is obtained which is favorable to hardening.
• The sheet is then reeled at a temperature T _bob of between 500 and 700 ° C. This step completes the precipitation of TiC.

• On approvisionne une tôle laminée à chaud, fabriquée selon le procédé décrit ci-dessus. Naturellement, si l'état de surface de la tôle l'exige, on effectuera un décapage au moyen d'un procédé connu en soi.
• On effectue ensuite un laminage à froid, le taux de réduction étant compris entre 30 et 90%
• On chauffe ensuite la tôle laminée à froid avec une vitesse de réchauffage V_c supérieure à 3°C/s, ceci afin d'éviter une restauration qui diminuerait la capacité à la recristallisation ultérieure. Le réchauffage est effectué jusqu'à une température de recuit T' qui sera choisie de façon à obtenir une recristallisation complète de la structure initiale fortement écrouie.

At this stage, a hot-rolled sheet is obtained, the thickness of which is, for example, from 2 to 6 mm. If it is desired to manufacture a sheet of lower thickness, for example from 0.6 to 1.5 mm, the manufacturing process is as follows:

• A hot-rolled sheet is supplied, manufactured according to the method described above. Naturally, if the surface state of the sheet requires it, stripping will be carried out by means of a method known per se.
• Cold rolling is then carried out, the reduction ratio being between 30 and 90%
• The cold-rolled sheet is then heated with a heating rate V _c greater than 3 ° C./s, in order to avoid a restoration which would reduce the capacity for subsequent recrystallization. Reheating is performed up to an annealing temperature T 'which will be chosen so as to obtain a complete recrystallization of the initial structure hardened.

The sheet is then cooled at a speed V _{R of} less than 100 ° C./s so as not to cause any embrittlement by excess of carbon in solid solution. This result is particularly surprising in that it could be thought that a rapid cooling rate would be favorable to reduce embrittling precipitation. However, the inventors have demonstrated that slow cooling, at a cooling rate of less than 100 ° C./s, led to a significant precipitation of carbides which thus reduced the carbon content in solid solution: this precipitation has the effect of increase the resistance without any detrimental effect on ductility.

• Une recristallisation complète
• Une fraction linéaire f de précipités intergranulaires κ inférieure à 30%
• Une teneur en carbone en solution solide inférieure à 0,005%.

The annealing temperature T 'and the speed V _R will be chosen so as to obtain on the final product:

• A complete recrystallization
• A linear fraction f of intergranular precipitates κ less than 30%
• Carbon content in solid solution less than 0.005%.

A temperature T 'of between 750 and 950 ° C. is preferably chosen to obtain complete recrystallization.

More particularly, when the carbon content is greater than 0.010% and less than or equal to 0.15% and when the manganese content is greater than 0.2% and less than or equal to 1%, the temperature T 'of in addition to preventing the dissolution of κ precipitates present before the annealing. In fact, if these precipitates are dissolved, the subsequent precipitation at slow cooling will take place in embrittling intergranular form: a too high annealing temperature would lead to the redissolution of the precipitates κ formed during the manufacture of the hot-rolled sheet and reduce the mechanical strength. . For this purpose, a temperature T 'of between 750 and 800 ° C. will preferably be chosen.

By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention.

Example 1: Hot-rolled sheets

Casting steels were produced in the form of half-products with a thickness of about 50 mm. Their compositions, expressed in weight percent, are shown in Table 1 below. Table 1 Compositions of steel (% by weight). I = according to the invention. R = reference Underlined values: Not in accordance with the invention. landmark VS Yes mn al Ti Cr MB Or S P Nb I1 0.005 0,013 0.108 8.55 0.096 0,007 0,025 0.005 0.012 0.016 0,004 I2 0,009 0,013 0.108 8.5 0.097 0,008 0,027 0.005 0,013 0.016 0.005 I3 0,080 0,275 0.485 8.24 0.096 0,009 0,026 0.005 0.012 0.016 0.005 R1 0,010 0,170 0.09 6.8 0006 0,032 - 0.005 0,001 0,009 - R2 0.079 1.44 1.21 3.25 - - - - 0,010 0,009 - R3 0.005 0,010 0,010 14.5 0.104 - - - 0,010 0,009 - R4 0.19 0,018 1.45 12.6 0.084 0.006 0,026 0.006 0,009 0,009 - R5 0.197 0,010 1.7 10.2 - - - 0,010 0,009 - R6 0.19 0,022 0.98 12.2 0.098 2.2 0.27 - 0,010 0.006 -

The semi-finished products were heated to a temperature of 1220 ° C. and hot-rolled to obtain a sheet having a thickness of about 3.5 mm.

From the same composition, some steels have been subjected to different hot rolling conditions. The references I1-a, I1-b, I1-c, I1-d, I1-e designate for example five steel sheets manufactured under different conditions from the composition I1.

• Le nombre N d'étapes de laminage effectuées à une température de laminage à chaud supérieure à 1050°C
• Parmi celles-ci, le nombre N_i d'étapes de laminage dont le taux de réduction est supérieur à 30%
• Le temps t_i s'écoulant entre chacune des étapes N_i, et l'étape de laminage succédant immédiatement à chacune de celles-ci
• La température de fin de laminage T_FL
• L'intervalle de temps tp s'écoulant au refroidissement entre 850 et 700°C
• La température de bobinage T_bob

Tableau 2 : Conditions de fabrication lors du laminage à chaud Repère N N_I t_i(s) TFL
(°C) tp
(s) Tbob
(°C) I1a I 4 3 14,5
20,6
26,8 900 21 700 I1b R' 6 2 2
2 900 21 700 I1c R 4 1 8 900 0 1,3 700 I1d I 5 3 26,5
23,5
20 900 21 700 I1e R 7 5 7,7
5,2
3,5
3
2,5 1050 20 700 I3a I 4 2 10
11 950 20 700 I3b R 4 1 5 950 20 700 I= Selon l'invention. R= référence
Valeurs soulignées : Non conformes à l'invention. For the steel I1 to I3, Table 2 details the conditions of the successive hot rolling stages:

• The number N of rolling steps performed at a hot rolling temperature greater than 1050 ° C.
• Among these, the number N _i of rolling steps whose reduction rate is greater than 30%
• The time t _i flowing between each of the steps N _i , and the rolling step succeeding immediately to each of these
• The end temperature of rolling T _FL
• The time interval tp flowing on cooling between 850 and 700 ° C.
• The winding temperature T _bob

Table 2: Manufacturing conditions during hot rolling landmark NOT N _I t _i (s) TFL
(° C) tp
(s) Tbob
(° C) I1a I 4 3 14.5
20.6
26.8 900 21 700 I1b R ' 6 2 2
2 900 21 700 I1c R 4 1 8 900 0 1.3 700 i 1d I 5 3 26.5
23.5
20 900 21 700 I1E R 7 5 7.7
5.2
3.5
3
2.5 1050 20 700 i3a I 4 2 10
11 950 20 700 i3b R 4 1 5 950 20 700 I = according to the invention. R = reference
Underlined Values: Not in accordance with the invention.

Table 3 shows the density measured on the plates of Table 2 and certain mechanical and microstructural characteristics. The resistance Rm, the uniform elongation A _u , the elongation at break A _t, have thus been measured in the cross-machine direction with respect to rolling. The _IV grain size was also measured by the method of linear intercepts according to the NF EN ISO 643 standard on a surface perpendicular to the direction transverse to the rolling. The measurement of d _IV was carried out in the direction perpendicular to the thickness of the sheet. In order to obtain increased mechanical properties, it is more particularly desired to obtain a grain size of _IV less than 100 microns. Table 3: Properties of hot-rolled sheets obtained from I1 and I3 steels. landmark Rm (MPa) At (%) At (%) Density D _IV I1a I 505 10.7 25.4 7.05 75 I1b R 507 nd nd 7.05 200 I1c R 474 nd nd 7.05 450 i 1d I 524 nd nd 7.05 40 I1E R 504 nd nd 7.05 120 i3a I 645 nd nd 7.07 70 i3b R 628 nd nd 7.07 400 I = according to the invention. R = reference nd = not determined
Underlined Values: Not in accordance with the invention.

The steel sheets according to the invention, the microstructure of which is illustrated for example in FIG. figure 2 for sheet I1d, are characterized by a grain size _IV less than 100 micrometers and have a strength ranging from 505 to 645 MPa.

The sheets I1b and I1e were rolled with a time interpasse too short. Their structure is then coarse and not recrystallized or insufficiently recrystallized as shown by figure 3 relating to Sheet I1e. As a result, the ductility is decreased and the sheet is more sensitive to the lack of creasing. Similar conclusions can be drawn for sheet I3b.

The sheet I1c was laminated with an insufficient number of rolling steps with a rate greater than 30%, a time interpasse and a time interval t _p too short. The consequences are identical to those noted on sheets I1b and I1e. Since the time interval t _p is too low, a hardening precipitation of κ precipitates and TiC carbides occurs only partially, which makes it impossible to take full advantage of the curing possibilities.

The semi-finished products made from the reference steels R1 to R6 were rolled to manufacture hot-rolled sheets under manufacturing conditions identical to those of the steel 13a of Table 2. The properties obtained on these sheets are brought to table 4. Table 4: Mechanical properties of hot-rolled sheets obtained from steels R1 to R6. landmark Re
(Mpa) rm
(MPa) At
(%) Has
(%) Density R1 nd nd nd nd 7.2 R2 nd nd nd nd 7.44 R3 nd 450 0.1 0.1 6.48 R4 725 786 0.6 0.6 6.67 R5 596 687 2.7 2.7 6.9 R6 853 891 0.7 0.7 6.7 I = according to the invention. R = reference nd = not determined
Underlined Values: Not in accordance with the invention.

The steel R1 has an insufficient titanium content which leads to a solid solution carbon content that is too high: the folding ability is then reduced.

Steel R2 has an insufficient aluminum content which does not allow to obtain a density lower than 7.3.

R3, R4, R5 and R6 steels contain too much aluminum and possibly carbon: their ductility is reduced due to the excessive precipitation of intermetallic phases or carbides

Example 2: Cold-rolled and annealed sheets

From the sheets of hot-rolled steel I1-a and I3-a (according to the invention) and 11-c and I-3b (not satisfying the conditions of the invention), a cold rolling was carried out with a reduction of 75% to obtain sheets approximately 0.9mm thick. The cold rolling ability was noted during this step. An annealing characterized by a heating rate V _c = 10 ° C./s. The annealing temperatures T 'and the cooling rates V _R are given in Table 5. Under these conditions, annealing results in complete recrystallization.

From the same hot-rolled sheet, some steels have undergone different conditions of cold rolling and annealing. The references I3a1, 13a2, I3a3, I3a4, for example, designate four steel sheets manufactured under different conditions of cold rolling and annealing from the hot-rolled sheet I3a. Table 5: Production conditions for cold-rolled and annealed sheets landmark Ability to cold rolling T ' V _R I1a1 I satisfactory 900 ° C 13 ° C / s I1a2 R satisfactory 900 ° C 150 ° C / s I1c1 R satisfactory 900 ° C 13 ° C / s I3a1 I satisfactory 800 ° C 13 ° C / s 13a2 R satisfactory 800 ° C 150 ° C / s I3a3 R satisfactory 900 ° C 13 ° C / s 13a4 R satisfactory 900 ° C 150 ° C / s i3b R Unsatisfactory (cracks in the transverse direction) I = according to the invention. R = reference
Underlined Values: Not in accordance with the invention.

Table 6 presents certain mechanical, chemical, microstructural and density characteristics of the sheets of Table 5. The yield strength Re, the resistance Rm, uniform elongation Au, elongation at break A _t . By means of scanning electron microscopy observations, the presence of cleavage facets on the rupture surfaces of the test specimens was noted.

The soil _sol carbon content in solid solution was also measured. The ability to bend and press was evaluated. It was also noted the possible presence of scuffing consecutive deformations. The microstructure of these recrystallized sheets consists of equiaxed ferrite whose average _α- grain size was measured in the transverse rolling direction. The rate of recovery of ferritic grain boundaries by κ precipitation was also measured using the Aphelion ^™ image analysis ^software . Table 6: Mechanical properties of cold-rolled and annealed sheets obtained from I1 and I3 steels. landmark Re
(MPa) rm
(MPa) At
(%) At (%) Breaking mode d _α C _sol
(%) f
(%) crease Ability to bend and tip-ssage Density I1a1 I 390 four hundred ninety seven 18 31 Ductile 27 0,002 0 No Yes 7.05 I1a2 R 405 510 17 29 Ductile / brittle 27 0.005 0 nd Yes 7.05 I1c1 R 437 552 13.8 25 Ductile 53 nd nd Yes No 7.05 I3a1 I 531 633 16.5 28.8 Ductile 11 0,003 2 No Yes 7.07 I3a2 R 532 627 13.8 19 Ductile / brittle 11 0,010 0 No nd 7.07 I3a3 R 513 612 13 14 Ductile / brittle 12 nd 60 nd No 7.07 I3a4 R 613 687 12.8 16 Brittle 12 0,060 17 nd No 7.07 I = according to the invention. R = reference. Nd: not determined
Underlined Values: Not in accordance with the invention.

The steel sheets I1a1 and I3a1 have a solid solution carbon content, a ferritic equiaxed grain size, and a grain boundary recovery ratio that satisfies the conditions of the invention. As a result, the ability to bend, stamping, scratch resistance of these sheets, is high.

The figure 4 illustrates the microstructure of the steel sheet I1a1 according to the invention.

The figure 5 illustrates the microstructure of another steel sheet according to the invention, I3a1: note the presence of κ precipitates of which only a small amount is present in intergranular form, which allows to maintain a high ductility.

In comparison, the steel sheet I1a2 was cooled at a too high speed after annealing: the carbon is then completely in solid solution, which causes a reduction in ductility of the matrix resulting in the local presence of fragile areas on the plates. facies of rupture. Similarly, the sheet I3a2 has been cooled too fast and also leads to an excessive content of solid solution.

The figure 6 illustrates the microstructure of the sheet I3a3: it was annealed at too high a temperature T ': the κ precipitates present before the annealing were dissolved, their subsequent precipitation after cooling intervened in an excessive amount of intergranular form. This results in the local presence of fragile beaches on fracture facies.

The sheet 13a4 was also annealed at a temperature which causes partial dissolution of the κ precipitates. The carbon content in solid solution is excessive.

The steel sheet I1c1 was made from a hot-rolled sheet not satisfying the requirements of the invention: the equiaxial grain size is too large, the crimping resistance and the stamping ability are insufficient.

The hot-rolled sheet I3b, which does not satisfy the criteria of the invention, is not suitable for deformation since transverse cracks appear during cold rolling.

Spot resistance weldability tests were carried out on the I1a1 steel sheet, either in homogeneous welding (welding of two sheets of the same composition) or in heterogeneous welding (welding with a sheet of steel without interstitial composition, expressed in weight percent: 0.002% C, 0.01% Si, 0.15% Mn, 0.04% Al, 0.015% Nb, 0.026% Ti) Examinations show that the welded joints are free from defects.

In the case of subsequent heat treatment of the welded joints, the addition of 0.096% Ti guarantees the absence of solid solution carbon in the heat-affected zone.

The steels according to the invention have good continuous galvanizing properties, in particular during an annealing cycle at 800 ° C. with a dew point temperature higher than -20 ° C.

The steels according to the invention thus have a combination of properties (density, mechanical strength, deformability, weldability, coating) particularly interesting. These steel sheets are used with advantage for the manufacture of skin parts or structure in the automotive field.

Claims (16)

1. Hot-rolled ferritic steel sheet, the composition of which contains, the contents being expressed by weight, $$0.001\le \mathrm{C}\le 0.15\%$$

   

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

   

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

   

   $$6\%\le \mathrm{Al}\le 10\%$$

   

   $$0.020\%\le \mathrm{Ti}\le 0.5\%$$

   

   $$\mathrm{S}\le 0.050\%$$

   

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

   

   
   and, optionally, one or several elements selected from: $$\mathrm{Cr}\le 1\%$$

   

   $$\mathrm{Mo}\le 1\%$$

   

   $$\mathrm{Ni}\le 1\%$$

   

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

   

   $$\mathrm{V}\le 0.2\%$$

   

   $$\mathrm{B}\le 0.010\%$$

   

   
   the remainder of the composition consisting of iron and inevitable impurities resulting from the smelting,
   the average ferrite grain size d_IV measured on a surface perpendicular to the transverse direction with respect to the rolling direction being less than 100 micrometers, said sheet comprising a precipitation of kappa precipitates and TiC carbides.
2. Cold-rolled and annealed ferritic steel sheet obtained up from a hot-rolled steel sheet according to claim 1, characterized in that structure consists of equiaxed ferrite, the average grain size d_α of which is less than 50 micrometers, and in that the linear fraction f of intergranular κ precipitates is less than 30%, said linear fraction f being defined by: $f=\frac{{\displaystyle \sum _A}\mathit{di}}{{\displaystyle \sum _A}\mathit{Li}},$

   

   
   ${\displaystyle \sum _A}\mathit{di}$

   

   denoting the total length of the grain boundaries including κ precipitates relatively to an area (A) in question, ${\displaystyle \sum _A}\mathit{di}$

   

   referring to the total length of the grain boundaries relatively to said area (A) in question.
3. Steel sheet according to claim 1 or 2, characterized in that its composition comprises, the contents being expressed by weight, $$0.001\le \mathrm{C}\le 0.010\%$$

   

   $$\mathrm{Mn}\le 0.2\%\mathrm{.}$$

   
4. Steel sheet according to claim 1 or 2, characterized in that its composition comprises, the contents being expressed by weight, $$0.010\%\le \mathrm{C}\le 0.015\%$$

   

   $$0.2\%<\mathrm{Mn}\le 1\%\mathrm{.}$$

   
5. Steel sheet according to any of claims 1 to 4, characterized in that its composition comprises, the contents being expressed by weight, $$7.5\%\le \mathrm{Al}\le 10\%\mathrm{.}$$

   
6. Steel sheet according to any of claims 1 to 4, characterized in that its composition comprises, the contents being expressed by weight, $$7.5\%\le \mathrm{Al}\le 8.5\%\mathrm{.}$$

   
7. Steel sheet according to any of claims 1 to 6, characterized in that the carbon content in solid solution is less than 0.005% by weight.
8. Steel sheet according to any of claims 1 to 7, characterizing that its strength R_m is greater than or equal to 400 MPa.
9. Steel sheet according to claim 4, characterized in that its strength R_m is greater than or equal to 600 MPa.
10. Process for manufacturing a hot-rolled steel sheet, wherein

    - a steel with a composition according to any of claims 1 to 6 is supplied,

    - said steel is cast in the form of a semi-finished product; then

    - said semi-finished product is brought to a temperature greater than or equal to 1,150°C; then

    - said semi-finished product is hot-rolled so as to obtain a sheet, by means of at least two rolling steps carried out at temperatures above 1,050°C, the reduction rate of each of said at least two steps being greater than or equal to 30%, the time elapsed between each of said at least two rolling steps, and the following rolling step, being greater than or equal to 10 s, and then

    - rolling is completed at a temperature T_ER greater than or equal to 900°C, and then

    - said sheet is cooled so that the time interval t_p elapsed between 850 and 700°C is more than 3 s, so as to obtain a precipitation of κ precipitates, and then

    - said sheet is coiled at a temperature T_coil comprised between 500 and 700°C.
11. The method for manufacturing a hot-rolled sheet according to claim 10, characterized in that said casting is directly carried out as a casting of thin slabs or thin strips between counter-rotating cylinders.
12. A method for manufacturing a cold-rolled and annealed steel sheet, wherein

    - a hot-rolled steel sheet manufactured according to claim 10 or 11 is supplied;
    then

    - said sheet is cold-rolled with a reduction rate comprised between 30 and 90%, so as to obtain a cold-rolled sheet; then

    - said cold-rolled sheet is heated to a temperature T' with a rate V_h of more than 3°C/s, and then

    - said sheet is cooled at a rate V_C of less than 100°C/s

    - said temperature T' and said rate V_C being selected so as to obtain complete recrystallization, a linear fraction of intergranular κ precipitates of less than 30% and a carbon content in solid solution of less than 0.005% by weight.
13. The manufacturing method according to claim 12, characterized in that said cold-rolled sheet is heated to a temperature T' comprised between 750 and 950°C.
14. The manufacturing method according to claim 12, characterized in that a sheet with a composition according to claim 4 is supplied and in that said cold-rolled sheet is heated to a temperature T' selected so as to avoid dissolution of κ precipitates.
15. The manufacturing method according to claim 12, characterized in that a sheet with a composition according to claim 4 is supplied and in that said cold-rolled sheet is heated to a temperature T' comprised between 750 and 800°C.
16. The use of steel sheets according to any of claims 1 to 9, or manufactured according to any of claims 10 to 15 for manufacturing skin parts or structural parts in the automotive field.

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