EP2855725B1 - Low-density hot- or cold-rolled steel, method for implementing same and use thereof - Google Patents

Description

The present invention relates to a rolled steel sheet having a mechanical strength greater than or equal to 600 MPa and an elongation at break greater than or equal to 20% and its manufacturing method.

• La première consiste à diminuer les épaisseurs des aciers tout en augmentant leurs niveaux de résistance mécanique. Hélas, cette solution trouve ses limites à cause d'une diminution de rigidité rédhibitoire à certaines pièces automobiles, et de l'apparition de problèmes acoustiques nuisibles au confort sonore du passager, sans compter l'incontournable perte de ductilité associée à la hausse de résistance mécanique.
• La seconde voie consiste à diminuer la densité des aciers en les alliant à d'autres métaux plus légers. Parmi ces alliages, ceux à basse densité dits Fer-Aluminium présentent des propriétés mécaniques et physiques intéressantes tout en permettant d'abaisser considérablement le poids. On entendra par faible ou basse densité, une densité inférieure ou égale à 7,3.

Ainsi, l'addition d'aluminium au fer, du fait de sa faible densité par rapport à ce dernier, a permis d'espérer de substantielles réductions de poids pour les pièces de structure automobile. C'est dans cette optique que la demande de brevet EP2128293 décrit une tôle laminée à chaud ou à froid de composition 0,2-0,8%C, 2-10%Mn, 3-15%Al, et une structure contenant moins de 99% de ferrite et plus d'1% d'austénite résiduelle. La tôle présente une résistance mécanique comprise dans l'intervalle 600-1000MPa et une densité inférieure à 7,2 et est revêtable. Le procédé de fabrication de la tôle à chaud consiste à réchauffer entre 1000 et 1200°C, laminer avec une température de fin de laminage comprise entre 700 et 850°C et à bobiner à une température inférieure à 600°C. Pour la tôle à froid, on lamine à froid la tôle à chaud avec une réduction comprise entre 40 et 90%, on réchauffe à une vitesse comprise entre 1 et 20°C/s à une température comprise entre la température de recristallisation et 900°C pendant 10 à 180 secondes. Cette demande de brevet vise à éviter le chiffonnage et l'apparition de criques au laminage en limitant le rapport Mn/AI à une valeur comprise entre 0,4 et 1,0. Il y apparait qu'au-delà d'un rapport de 1,0, la laminabilité à froid mène à l'apparition de fissures.Environmental constraints continuously drive car manufacturers to lower CO ₂ emissions from their vehicles. To achieve this, they have several options among which the main consist either to reduce the weight of vehicles or to improve the performance of their engine. Advances are often made in a combined way. The present invention relates to the first option, namely the reduction of the weight of motorized vehicles. In this very specific area, there is a two-way alternative:

• The first is to reduce the thicknesses of the steels while increasing their levels of mechanical resistance. Unfortunately, this solution finds its limits because of a reduction in rigidity unacceptable to certain auto parts, and the appearance of acoustic problems harmful to the passenger's comfort, not to mention the unavoidable loss of ductility associated with the increase in resistance mechanical.
• The second way is to reduce the density of steels by combining them with other lighter metals. Among these alloys, the low-density iron-aluminum alloys have interesting mechanical and physical properties while at the same time weight. Low or low density means a density of less than or equal to 7.3.

Thus, the addition of aluminum to iron, because of its low density relative to the latter, has allowed to expect substantial weight reductions for automotive structural parts. It is in this context that the patent application EP2128293 discloses a hot rolled or cold rolled sheet of composition 0.2-0.8% C, 2-10% Mn, 3-15% Al, and a structure containing less than 99% ferrite and more than 1% d residual austenite. The sheet has a mechanical strength in the range 600-1000 MPa and a density less than 7.2 and is coated. The method of manufacturing the hot-rolled sheet consists of heating at 1000 ° to 1200 ° C., rolling at a rolling end temperature of between 700 and 850 ° C and winding at a temperature below 600 ° C. For the cold-rolled sheet, the hot-rolled sheet is cold-rolled with a reduction of between 40 and 90%, and is heated at a rate of between 1 and 20 ° C./s at a temperature between the recrystallization temperature and 900.degree. C for 10 to 180 seconds. This patent application is intended to prevent creasing and the appearance of cracks rolling by limiting the ratio Mn / AI to a value between 0.4 and 1.0. It appears that beyond a ratio of 1.0, the cold laminability leads to the appearance of cracks.

The patent application JP2006118000 is a lightweight steel with high strength and good ductility. To do this, the composition of the proposed steel contains in weight percentage: 0.1 to 1.0% C, less than 3.0% Si, 10.0 to 50.0% Mn, less than 0.01 % P, less than 0.01% S, 5.0 to 15.0% Al and 0.001 to 0.05% N, the remainder being iron and unavoidable impurities, equation (1) below in front of be satisfied, the steel will have a density less than or equal to 7.0. $$\mathrm{VS}\le -0,020\mathrm{XMN}+\mathrm{HAVE}/15+0,53$$

It will have a microstructure containing ferrite and austenite. The product of the mechanical strength by the total elongation satisfying the following inequality: TSxEI ≥20000 (MPa x%). The laminability of steels with such high alloy levels in Mn and Al is known to be subject to high risk of occurrence of cracks.

The patent application WO2007 / 024092 is intended to provide easily rolled hot-rolled sheet. This application relates to a sheet containing 0.2-1% C, 8-15% Mn, with a product of mechanical strength by elongation of 24000 MPa. It appears that this application is a totally austenitic structure, but this type of microstructure is particularly difficult to roll.

• Une densité inférieure ou égale à 7,3
• Une résistance mécanique supérieure ou égale à 600 MPa
• Un allongement à rupture supérieur ou égal à 20%
• Une bonne aptitude au formage, particulièrement au laminage
• Une bonne soudabilité et une bonne revêtabilité

The invention aims to solve these difficulties by proposing hot-rolled or cold-rolled steel sheets simultaneously presenting:

• A density less than or equal to 7.3
• Mechanical strength greater than or equal to 600 MPa
• An elongation at break greater than or equal to 20%
• Good formability, especially rolling
• Good weldability and good coating

One of the aims of the invention is also to provide a method of manufacturing these sheets that is compatible with usual industrial applications while being insensitive to manufacturing conditions.

$$6,0\le \mathrm{Mn}\le 15,0\%$$

$$6,0\le \mathrm{Al}\le 15,0\%$$

et à titre optionnel, un ou plusieurs éléments choisis parmi : $$\mathrm{Si}\le 2,0\%$$

$$\mathrm{Ti}\le 0,2\%$$

$$\mathrm{V}\le \mathrm{0},\mathrm{6}\%$$

$$\mathrm{Nb}\le 0,3\%$$

le reste de la composition étant composé de fer et d'impuretés inévitables résultant de l'élaboration, le rapport du poids du manganèse sur celui d'aluminium étant tel que $\frac{\mathit{Mn}}{\mathit{Al}}>1,0,$

la microstructure de la tôle étant constituée de ferrite, d'austénite et jusqu'à 5% de précipités Kappa en fraction surfacique.The invention firstly relates to a rolled steel sheet whose density is less than or equal to 7.3 and whose composition comprises, the contents being expressed by weight: $$\mathrm{0},\mathrm{10}\le \mathrm{VS}\le \mathrm{0},\mathrm{30}\%$$

$$6,0\le \mathrm{mn}\le 15,0\%$$

$$6,0\le \mathrm{al}\le 15,0\%$$

and optionally, one or more elements selected from: $$\mathrm{Yes}\le 2,0\%$$

$$\mathrm{Ti}\le 0,2\%$$

$$\mathrm{V}\le \mathrm{0},\mathrm{6}\%$$

$$\mathrm{Nb}\le 0,3\%$$

the remainder of the composition being composed of iron and unavoidable impurities resulting from the elaboration, the ratio of the weight of manganese to that of aluminum being such that $\frac{\mathit{mn}}{\mathit{al}}>1,0,$

the microstructure of the sheet consisting of ferrite, austenite and up to 5% Kappa precipitates in surface fraction.

In a preferred embodiment of the invention, the composition comprises, the content being expressed by weight: $$0,18\le \mathrm{VS}\le 0,21\%$$

In another preferred embodiment of the invention, the composition comprises, the content being expressed by weight: $$7,0\le \mathrm{mn}\le 10,0\%$$

In another preferred embodiment of the invention, the composition comprises, the content being expressed by weight: $$6,0\le \mathrm{al}\le 12,0\%$$

In another preferred embodiment of the invention, the composition comprises, the content being expressed by weight: $$6,0\le \mathrm{al}\le 9,0\%$$

In another preferred embodiment of the invention, the composition comprises, the content being expressed by weight: $$\mathrm{Yes}\le 1\%$$

de manière encore préférée, le rapport est tel que $\frac{\mathit{Mn}}{\mathit{Al}}\ge 1,5,$

voire de manière encore plus préférée, le rapport est tel que $\frac{\mathit{Mn}}{\mathit{Al}}\ge 2,0.$

Preferably, the ratio of the weight of manganese to that of aluminum is such that: $\frac{\mathit{mn}}{\mathit{al}}\ge 1,1,$

even more preferably, the ratio is such that $\frac{\mathit{mn}}{\mathit{al}}\ge 1,5,$

even more preferably, the ratio is such that $\frac{\mathit{mn}}{\mathit{al}}\ge 2,0.$

In a still preferred manner, the sheet according to the invention is such that the tensile strength is greater than or equal to 600 MPa and the elongation at break is greater than or equal to 20%.

• Approvisionner un acier dont la composition est conforme à l'invention,
• Couler ledit acier pour former un demi produit,
• Réchauffer ledit demi-produit à une température Trech comprise entre 1000°C et 1280°C,
• laminer à chaud ledit demi-produit réchauffé avec au moins une passe en présence de ferrite pour obtenir une tôle,
• La dernière passe de laminage se fera à une température de fin de laminage TFL supérieure ou égale à 850°C.
• Refroidir ladite tôle à une vitesse de refroidissement Vref1 jusqu'à la température de bobinage Tbob inférieure ou égale à 600°C,
• Puis, bobiner ladite tôle refroidie jusque Tbob,

The subject of the invention is a method for manufacturing a rolled steel sheet having a density of less than or equal to 7.3, which comprises the steps of:

• Supply a steel whose composition is in accordance with the invention,
• Pouring said steel to form a half product,
• Heating said half-product to a temperature of between 1000 ° C. and 1280 ° C.,
• hot rolling said half-heated product with at least one pass in the presence of ferrite to obtain a sheet,
• The last rolling pass will be at an end temperature of TFL rolling greater than or equal to 850 ° C.
• Cooling said sheet at a cooling rate Vref1 to the winding temperature Tbob less than or equal to 600 ° C,
• Then, winding said cooled sheet until Tbob,

The invention also relates to a method of manufacturing a rolled sheet such that said semi-finished product is cast directly in the form of thin slabs or thin strips.

Preferably, the end of rolling temperature T _FL is between 900 and 980 ° C.

Preferably, the cooling rate V _ref1 is less than or equal to 55 ° C / s.

Preferably, the winding temperature is between 450 and 550 ° C.

• Approvisionner une tôle d'acier laminée, puis
• Laminer à froid ladite tôle laminée avec un taux de réduction compris entre 35 et 90% de façon à obtenir une tôle à froid, puis
• Chauffer ladite tôle avec une vitesse V_c jusqu'à une température de maintien T_m comprise entre 800 et 950°C pendant un temps t_m inférieur à 600 secondes, puis
• Refroidir ladite tôle à vitesse V_ref2 jusqu'à une température inférieure ou égale à 500°C.

The invention also relates to a method for manufacturing a cold-rolled and annealed steel sheet with a density of less than or equal to 7.3, which comprises the steps of:

• Supply a rolled steel sheet, then
• Cold rolling said laminated sheet with a reduction rate of between 35 and 90% so as to obtain a cold sheet, then
• Heat said sheet with a speed V _c to a holding temperature T _m of between 800 and 950 ° C for a time t _{m of} less than 600 seconds, then
• Cool said sheet at speed V _ref2 to a temperature of less than or equal to 500 ° C.

Preferably, the temperature T _m is between 800 and 900 ° C.

Preferably, the cooling rate V _ref2 is greater than or equal to 30 ° C / s.

Preferably, the cooling rate V _ref2 is maintained up to a temperature of between 500 ° C and 460 ° C.

Preferably, the cooled sheet is coated with zinc, a zinc alloy or a zinc-based alloy.

The steel sheets according to the invention may be used for the manufacture of structural parts or skin parts for motorized land vehicles.

• La figure 1 illustre la microstructure d'une tôle d'acier laminée à chaud selon l'invention.
• La figure 2 illustre la microstructure d'une tôle d'acier laminée à chaud ne satisfaisant pas aux conditions selon l'invention.
• La figure 3 présente le comportement mécanique en traction à chaud représentant la laminabilité à chaud en fonction de la température de traction en °C.
• La figure 4 illustre la microstructure d'une tôle d'acier laminée à chaud ne satisfaisant pas aux conditions selon l'invention.
• La figure 5 illustre la microstructure d'une tôle d'acier laminée à froid selon l'invention.
• La figure 6 présente un cliché de diffraction en axe de zone [110] ayant permis d'identifier le précipité Kappa sur une tôle d'acier laminée à chaud selon l'invention.
• La figure 7 illustre une microstructure de tôle à froid ne satisfaisant pas aux conditions de l'invention.
• La figure 8 illustre l'évolution de la densité en fonction de la teneur en aluminium.

Other features and advantages of the invention will become apparent through the present description. The attached figures attached are given by way of example and in a nonlimiting manner, they are such that:

• The figure 1 illustrates the microstructure of a hot-rolled steel sheet according to the invention.
• The figure 2 illustrates the microstructure of a hot-rolled steel sheet not satisfying the conditions according to the invention.
• The figure 3 presents the mechanical behavior in hot tension representing the hot rollability as a function of the tensile temperature in ° C.
• The figure 4 illustrates the microstructure of a hot-rolled steel sheet only not satisfying the conditions according to the invention.
• The figure 5 illustrates the microstructure of a cold-rolled steel sheet according to the invention.
• The figure 6 presents a zone-axis diffraction pattern [110] having made it possible to identify the Kappa precipitate on a hot-rolled steel sheet according to the invention.
• The figure 7 illustrates a microstructure of cold sheet that does not meet the conditions of the invention.
• The figure 8 illustrates the evolution of the density as a function of the aluminum content.

The present invention relates to hot-rolled or cold-rolled steel sheets having a reduced density relative to conventional steels and less than or equal to 7.3, while retaining mechanical properties of shaping, of mechanical strength. , weldability and satisfactory coating. The invention also relates to a manufacturing method for hot or cold rolling the steel of the invention to obtain a hot or cold sheet having a microstructure comprising ferrite, austenite and up to to 5% of Kappa precipitates in surface fraction.

• Le manganèse doit voir sa teneur comprise entre 6,0% et 15,0%. Cet élément est, lui aussi, gammagène. L'ajout du manganèse servira donc essentiellement à l'obtention d'une structure contenant de l'austénite en plus de la ferrite. Il a aussi un effet durcissant en solution solide et stabilisant sur l'austénite. Le ratio de la teneur en manganèse sur celle de l'aluminium aura une forte influence sur les structures obtenues en fin de laminage. Pour une teneur en Mn inférieure à 6,0%, l'allongement à rupture de 20% n'est pas atteint, en outre l'austénite sera insuffisamment stabilisée avec le risque de se transformer prématurément en martensite lors d'un refroidissement rapide, aussi bien en sortie de laminage à chaud que sur une ligne de recuit. Au dessus de 15,0%, du fait de son effet gammagène, le Mn augmente de manière excessive la fraction volumique d'austénite, réduisant de fait la concentration en carbone de la phase austénitique, ce qui empêcherait d'atteindre les 600 MPa de résistance. De manière préférée, on limitera l'addition de Mn à 10,0%. Pour la limite inférieure, de manière préférée, la teneur en Mn sera de 7,0% afin d'atteindre l'allongement de 20% plus facilement.
• En ce qui concerne l'aluminium, sa teneur doit aussi être comprise entre 6,0% et 15,0%. L'aluminium est un élément alphagène, il diminue donc le domaine austénitique et cet élément tend à promouvoir la formation de précipités Kappa en se combinant avec le carbone. L'aluminium présente une densité de 2,7 et influe fortement sur les propriétés mécaniques. Quand la teneur en aluminium augmente, la résistance mécanique et la limite élastique augmentent, alors que l'allongement à rupture diminue, ce qui s'explique par une diminution de la mobilité des dislocations. En dessous de 6,0%, l'effet de réduction de densité dû à la présence d'aluminium perd de son intérêt. Au dessus de 15,0%, une précipitation incontrôlée de Kappa avec une densité surfacique supérieure à 5% apparaît et nuit à la ductilité du matériau. On souhaite limiter, de manière préférentielle, la teneur en aluminium à strictement moins de 9,0% afin d'éviter une précipitation d'intermétalliques fragiles. La figure 7 illustre une microstructure dans laquelle les précipités Kappa se sont formés de manière incontrôlée.
• Le rapport de la teneur pondérale du manganèse sur celle de l'aluminium est primordial car il gouverne la stabilité de l'austénite et la nature des structures formées lors du cycle de fabrication. En dessous d'un rapport égal à 1,0 inclus, la nature des phases formées dépend trop fortement de la vitesse de refroidissement, aussi bien après le laminage à chaud qu'après le recuit de recristallisation pour la tôle à froid. On risque ainsi de former de la martensite à partir de l'austénite voire de voir disparaître cette dernière au profit de la ferrite et de précipités Kappa tel qu'illustré dans la figure 7. La microstructure de la tôle de l'invention écarte la présence de la martensite et assure la présence d'austénite stable. Ainsi, on ne souhaite pas avoir un rapport $\frac{\mathit{Mn}}{\mathit{Al}}\le 1,0$
  
  pour s'assurer d'avoir une bonne laminabilité et une tôle peu sensible aux conditions de fabrication.

To do this, the chemical composition of the steel is very important both for the mechanical behavior of the sheet as for its development. The contents of chemical composition elements which follow are given as a percentage of the weight.
According to the invention, the carbon content is between 0.10 and 0.30%. Carbon is a gamma element. It favors, with Mn, the appearance of austenite and, with aluminum, the formation of Kappa precipitates based on stoichiometry (Fe, Mn) ₃ AlC _x , where x is strictly less than 1. Below of 0.10%, the mechanical strength of 600 MPa is not reached. If the carbon content is greater than 0.30%, the formation of Kappa precipitates will be excessive because above 5% and the rolling of the steel sheet will lead to cracks. Preferably, it will limit the carbon content to 0.21% included to minimize the risk of occurrence of cracks rolling. Preferably, the minimum carbon content will also be greater than or equal to 0.18% to more easily reach the mechanical strength of 600 MPa.

• Manganese content should be between 6.0% and 15.0%. This element is also gamma. The addition of manganese will therefore essentially serve to obtain a structure containing austenite in addition to ferrite. It also has a hardening effect in solid solution and stabilizing on the austenite. The ratio of the manganese content to that of aluminum will have a strong influence on the structures obtained at the end of rolling. For an Mn content of less than 6.0%, the elongation at break of 20% is not reached, in addition the austenite will be insufficiently stabilized with the risk of prematurely turning into martensite during rapid cooling, both hot roll output and a annealing line. Above 15.0%, due to its gammagenic effect, Mn excessively increases the volume fraction of austenite, effectively reducing the carbon concentration of the austenitic phase, which would prevent reaching the 600 MPa of resistance. Preferably, the addition of Mn to 10.0% will be limited. For the lower limit, preferably, the Mn content will be 7.0% in order to reach the elongation of 20% more easily.
• As regards aluminum, its content must also be between 6.0% and 15.0%. Aluminum is an alphagene element, so it decreases austenitic domain and this element tends to promote the formation of Kappa precipitates by combining with carbon. Aluminum has a density of 2.7 and strongly influences the mechanical properties. As the aluminum content increases, the mechanical strength and the yield point increase, while the elongation at break decreases, which is explained by a decrease in the mobility of the dislocations. Below 6.0%, the density reduction effect due to the presence of aluminum loses its interest. Above 15.0%, an uncontrolled Kappa precipitation with a surface density greater than 5% appears and adversely affects the ductility of the material. It is desired to limit, preferably, the aluminum content to strictly less than 9.0% in order to avoid a precipitation of fragile intermetallics. The figure 7 illustrates a microstructure in which Kappa precipitates formed uncontrollably.
• The ratio of the weight content of manganese to that of aluminum is essential because it governs the stability of the austenite and the nature of the structures formed during the manufacturing cycle. Below a ratio equal to 1.0 inclusive, the nature of the phases formed depends too much on the cooling rate, both after the hot rolling and after the recrystallization annealing for the cold-rolled sheet. It is thus possible to form martensite from austenite or even to see the latter disappear in favor of ferrite and Kappa precipitates as illustrated in FIG. figure 7 . The microstructure of the sheet of the invention eliminates the presence of martensite and ensures the presence of stable austenite. So, we do not want to have a report $\frac{\mathit{mn}}{\mathit{al}}\le 1,0$
  
  to ensure good laminability and a sheet that is not sensitive to manufacturing conditions.

• Au même titre que l'aluminium, le silicium est un élément permettant de réduire la densité de l'acier et réduit l'énergie de défaut d'empilement. Cette réduction permet d'obtenir un effet TRIP connu de l'homme de métier. Néanmoins sa teneur est limitée à 2,0%, car au-delà, cet élément a tendance à former des oxydes fortement adhérents générant des défauts de surface. En effet, la présence d'oxydes de surface mène à des défauts de mouillabilité lors d'une éventuelle opération de dépôt de zinc au trempé par exemple. Préférentiellement, on limitera le Si à 1%.
• des éléments de micro alliages tels que le titane, le vanadium et le niobium peuvent être ajoutés en quantité respectivement inférieures à 0,2%, 0,6% et 0,3% afin d'obtenir un durcissement supplémentaire par précipitation. En particulier le titane et le niobium permettent de contrôler la taille de grain au cours de la solidification. Une limitation est cependant nécessaire car au-delà, on obtient un effet de saturation.

Above a ratio of the weight content of manganese to that of the aluminum equal to 1.0, the sheet produced is insensitive to the manufacturing conditions while being easily laminated both hot and cold. This decrease in sensitivity is improved by increasing the ratio, so it is preferred a ratio greater than or equal to 1.1, preferably, a ratio greater than or equal to 1.5 or even more preferably, a higher ratio or equal to 2.0.

• Similar to aluminum, silicon is an element that reduces the density of steel and reduces stacking fault energy. This reduction makes it possible to obtain a TRIP effect known to those skilled in the art. Nevertheless its content is limited to 2.0%, because beyond this element tends to form strongly adherent oxides generating surface defects. Indeed, the presence of surface oxides leads to wettability defects during a possible zinc deposition operation by dipping, for example. Preferably, the Si will be limited to 1%.
• micro alloy elements such as titanium, vanadium and niobium may be added in amounts of less than 0.2%, 0.6% and 0.3%, respectively, in order to obtain additional hardening by precipitation. In particular, titanium and niobium make it possible to control grain size during solidification. A limitation is however necessary because beyond this, one obtains a saturation effect.

Other elements such as cerium, boron, magnesium, or zirconium can be added alone or in combination in the following proportions: Ce ≤ 0.1%, B ≤ 0.01, Mg ≤ 0.010, and Zr ≤ 0.010. Up to the maximum levels indicated, these elements make it possible to refine the ferritic grain during solidification.

• La microstructure de la tôle selon l'invention est constituée de ferrite, d'austénite et jusqu'à 5% de précipités Kappa en fraction surfacique. La ferrite présente une solubilité du carbone croissante avec la température. Or, le carbone en solution solide est très fragilisant pour les aciers à basse densité, car il réduit davantage la mobilité des dislocations déjà basse du fait de la présence de l'aluminium. Une saturation de carbone dans la ferrite peut donc conduire à l'activation d'un mécanisme de maclage au sein de cette dernière. Ainsi, sans être lié par cette théorie, les inventeurs avancent que l'austénite et les précipités servent de pièges à carbone efficaces et facilitent le laminage dans le domaine intercritique. Cette approche est surprenante car on pourrait croire qu'il faudrait éviter de former ces phases dures pour faciliter le laminage mais la solubilité du carbone dans l'austénite et dans les précipités est plus élevée que dans la ferrite. Cette combinaison de structure contenant de la ferrite, de l'austénite jusqu'à 5% de précipités Kappa en fraction surfacique confère donc à la tôle la ductilité nécessaire autant à sa laminabilité lors du laminage que lors de fabrication de pièces de structure. Il est précisé que le taux de recristallisation de la ferrite après le recuit ou après le bobinage sera supérieur à 90% et idéalement égal à 100%. Si la fraction recristallisée de ferrite est inférieure à 90%, la tôle obtenue ne présentera pas les 20% d'allongement requis par l'invention.

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

• The microstructure of the sheet according to the invention consists of ferrite, austenite and up to 5% of Kappa precipitates in surface fraction. Ferrite exhibits increasing carbon solubility with temperature. However, carbon in solid solution is very weak for low-density steels because it further reduces dislocation mobility already low due to the presence of aluminum. A saturation of carbon in the ferrite can therefore lead to the activation of a twinning mechanism within the latter. Thus, without being bound by this theory, the inventors argue that austenite and precipitates serve as effective carbon traps and facilitate rolling in the intercritical domain. This approach is surprising because one might think that it would be necessary to avoid forming these hard phases to facilitate the rolling but the solubility of the carbon in the austenite and in the precipitates is higher than in the ferrite. This combination of structure containing ferrite, austenite up to 5% of Kappa precipitates in surface fraction thus confers on the sheet the ductility necessary as much for its laminability during rolling as during the manufacture of structural parts. It is specified that the recrystallization rate of the ferrite after annealing or after winding will be greater than 90% and ideally equal to 100%. If the recrystallized fraction of ferrite is less than 90%, the resulting sheet will not have the 20% elongation required by the invention.

Numerous experiments and metallographic studies have enabled the inventors to demonstrate that the localized presence of Kappa type precipices in the form of a border around the ferritic grain boundaries reduces the laminability of the sheet.

The surface density of the Kappa precipitates can be up to 5% because above 5%, the ductility drops and the 20% breaking elongation of the invention is not reached. In addition, there is also a risk of precipitation uncontrolled Kappa around the ferritic grain seals, which would increase the rolling forces of the sheet of the invention with the usual tools of steel rolling on an industrial scale. Thus, preferably, less than 2% Kappa precipitates are contemplated. It is specified that the microstructure being uniform, the surface fraction is equal to the volume fraction.

• On approvisionne un acier de composition selon l'invention
• On procède à la coulée d'un demi-produit à partir de cet acier. La coulée peut s'effectuer soit en lingot, soit en continu soit sous forme de brames minces ou bandes minces. C'est-à-dire avec une épaisseur allant d'environ 220 mm pour les brames et pouvant aller jusque quelques dizaines de mm pour les bandes minces.
• Les demi-produits coulés sont ensuite réchauffés à une température comprise entre 1000°C et 1280°C afin d'avoir en tout point une température favorable aux fortes déformations de laminage. Au-delà de 1280°C, on risque de former des grains ferritiques particulièrement grossiers, les nombreux essais des inventeurs ont indiqué une corrélation entre la taille de grain ferritique initiale et la capacité de ces derniers à recristalliser lors du laminage à chaud. Plus la taille de grain ferritique initiale est grande, moins il recristallise facilement, ainsi on évite des températures de réchauffage au-delà de 1280°C car celles-ci sont industriellement couteuses et peu favorables à la recristallisation de la ferrite. Cela peut, d'autre part, amplifier le phénomène de chiffonnage (encore appelé « roping »). Il est précisé que le chiffonnage est dû à un ensemble de grains de petite taille, faiblement désorientés, au sein de grains de plus grande taille. Ce phénomène est visible par une localisation préférentielle des déformations au sein de bandes dans la direction de laminage. Il est dû à la présence de grains non recristallisés restaurés. On le mesure par un faible allongement réparti dans la direction transverse.

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

• A composition steel is supplied according to the invention
• A semi-finished product is cast from this steel. The casting can be carried out either in ingot, or continuously or in the form of slabs or thin strips. That is to say with a thickness ranging from about 220 mm for slabs and up to a few tens of mm for thin strips.
• The cast semi-finished products are then heated to a temperature of between 1000 ° C. and 1280 ° C. in order to have at all points a temperature favorable to the large rolling deformations. Above 1280 ° C., it is possible to form particularly coarse ferritic grains, the numerous tests of the inventors have indicated a correlation between the initial ferritic grain size and the capacity of these latter to recrystallize during hot rolling. The larger the initial ferritic grain size, the less it recrystallizes easily, thus reheating temperatures above 1280 ° C are avoided because they are industrially expensive and unfavorable for the recrystallization of ferrite. This can, on the other hand, amplify the phenomenon of ragging (also called "roping"). It is specified that the crimping is due to a set of small grains, weakly disoriented, within grains of larger size. This phenomenon is visible by a preferential localization of the deformations within strips in the rolling direction. It is due to the presence of restored non-recrystallized grains. It is measured by a small elongation distributed in the transverse direction.

Below 1000 ° C., it becomes increasingly difficult to have an end-of-rolling temperature above 850 ° C. Preferably, the reheating temperature is between 1150 and 1280 ° C.

• Il est nécessaire d'effectuer le laminage avec une moins une passe de laminage en présence de ferrite, c'est-à-dire dans le domaine partiellement ou totalement ferritique. Ceci afin d'éviter une saturation de carbone dans la ferrite pouvant mener au maclage. L'austénite sert ainsi de pièges à carbone efficace car la solubilité du carbone dans l'austénite est plus élevée que dans la ferrite.
• La dernière passe de laminage est effectuée à une température supérieure à 850°C car en dessous de cette température, la tôle d'acier selon l'invention présente une chute notable de laminabilité comme le montre la figure 3 qui présente la striction d'éprouvettes soumises à une traction à chaud à différentes températures. Une température de fin de laminage comprise entre 900 et 980°C est préférée afin d'avoir une structure propice à la recristallisation et laminable.
• On refroidit ensuite la tôle obtenue à une vitesse de refroidissement jusqu'à la température de bobinage T_bob. De manière préférentielle, on préférera une vitesse de refroidissement V_ref1 inférieure ou égale à 55°C/s afin de mieux contrôler la précipitation des kappa.
• On bobine ensuite la tôle à une température de bobinage inférieure à 600°C car au dessus, on risque de ne pas pouvoir contrôler la précipitation de kappa, et d'avoir plus de 5% de ce dernier suite à une décomposition importante l'austénite tel qu'illustré dans les figures 2 et 4. De manière préférentielle, on bobine la tôle à une température comprise entre 450 et 550°C.

The following steps prevent the phenomenon of crumpling and have good ductility and good drawability:

• It is necessary to perform the rolling with at least one rolling pass in the presence of ferrite, that is to say in the partially or totally ferritic domain. This is to avoid carbon saturation in the ferrite that can lead to twinning. Austenite thus serves as effective carbon traps because the solubility of carbon in austenite is higher than in ferrite.
• The last rolling pass is carried out at a temperature above 850 ° C because below this temperature, the steel sheet according to the invention has a notable drop in laminability as shown in FIG. figure 3 which exhibits the necking of specimens subjected to hot tension at different temperatures. An end-of-rolling temperature of between 900 and 980 ° C is preferred in order to have a structure that is suitable for recrystallization and laminatable.
• The sheet obtained is then cooled at a cooling rate to the winding temperature T _bob . Preferably, a cooling rate V _{ref1 of} less than or equal to 55 ° C / s will be preferred in order to better control the kappa precipitation.
• The sheet is then reeled at a winding temperature of less than 600 ° C because above, there is a risk of not being able to control the kappa precipitation, and to have more than 5% of the latter following a decomposition important austenite as illustrated in the figures 2 and 4 . Preferably, the sheet is reeled at a temperature between 450 and 550 ° C.

• On effectue un laminage à froid avec une réduction d'épaisseur comprise entre 35 et 90%.
• On chauffe ensuite la tôle laminée à froid à une vitesse de chauffe V_c que l'on préfère supérieure à 3°C jusqu'à une température de maintien T_m comprise entre 800 et 950°c pendant un temps inférieur à 600 secondes afin de s'assurer d'un taux de recristallisation supérieur à 90% de la structure initiale fortement écrouie.
• On refroidit ensuite la tôle à une vitesse V_ref2 jusqu'à une température inférieure ou égale à 500°C, on préfère une vitesse de refroidissement supérieure à 30°C/s pour mieux contrôler la formation des précipités Kappa et ne pas dépasser les 5% en teneur surfacique. En dessous de 500°C, un traitement thermique supplémentaire afin de faciliter un dépôt de revêtement au trempé avec par exemple du zinc ne changera pas les propriétés mécaniques de la tôle de l'invention. Les inventeurs ont pu montrer qu'en arrêtant le refroidissement à la vitesse V_ref2 entre 500 et 460°C, pour effectuer un maintien avant trempe dans un bain de zinc, les propriétés visées par la tôle de l'invention restent inchangées. A titre illustratif et non limitatif, les essais suivants vont montrer les caractéristiques avantageuses pouvant émaner de la mise en oeuvre de tôles d'acier selon l'invention.

At this stage, a hot-rolled sheet is obtained and if it is desired to obtain a cold-rolled sheet with a thickness of less than 5 mm, the following steps are carried out:

• Cold rolling is carried out with a thickness reduction of between 35 and 90%.
• The cold-rolled sheet is then heated to a heating rate V _c which is greater than 3 ° C. up to a holding temperature T _m of between 800 and 950 ° C. for a time of less than 600 seconds in order to ensure a recrystallization rate greater than 90% of the initially hardened initial structure.
• The sheet is then cooled at a speed V _ref2 to a temperature of less than or equal to 500 ° C., a cooling rate of greater than 30 ° _C./s is preferred to better control the formation of the Kappa precipitates and not to exceed 5 ° C. % in surface content. Below 500 ° C, additional heat treatment to facilitate a dip coating deposit with for example zinc will not change the mechanical properties of the sheet of the invention. The inventors have been able to show that by stopping the cooling at the speed V _ref2 between 500 and 460 ° C., to carry out a maintenance before quenching in a zinc bath, the properties targeted by the sheet of the invention remain unchanged. By way of illustration and not limitation, the following tests will show the advantageous characteristics that can emanate from the implementation of steel sheets according to the invention.

Example 1: Hot-rolled sheets

Semi-finished products have been developed from steel castings. The compositions of the semi-finished products, expressed in percentage by weight, are shown in Table 1 below:

The remainder of the composition of the steels shown in Table 1 consists of iron and unavoidable impurities resulting from processing. Table 1: Composition of steels (% weight). VS mn al Yes Ti V Nb Mn / Al I1 0,193 14.9 6.52 <0.030 0.096 <0.030 <0.030 2.29 I2 0.188 8.28 7.43 <0.030 <0.030 <0.030 <0.030 1.11 R1 0,186 3.4 9.7 <0.030 <0.030 <0.030 <0.030 0.35 R2 0.117 4.78 7.6 <0.030 <0.030 <0.030 <0.030 0.63 R3 0.2 7.01 8.07 0.25 <0.030 <0.030 <0.030 0.87 I = invention / R = Reference / the underlined values are not in accordance with the invention.

• T_rech : est la température de réchauffage
• T_FL : est la température de fin de laminage
• V_ref1 : est la température de refroidissement après la dernière passe de laminage.
• T_bob : est la température de bobinage

Tableau 2 : Conditions de fabrication des tôles laminées à chaud à partir des demi-produits. T_rech (°C) T_FL (°C) V_ref1 T_bob (°C) I1 1180 950 air 500 I2 1230 964 air 500 R1 1300 950 air 500 R2a 1230 975 air 700 R2b 1150 954 eau ambiante R3 1220 927 50°C/s 500 I=invention / R=Référence / les valeurs soulignées sont non-conformes à l'invention. The products have been hot-rolled to obtain hot-rolled sheets and the manufacturing conditions are shown in Table 2 below with the following abbreviations:

• T _rech : is the reheat temperature
• T _FL : is the end of rolling temperature
• V _ref1 : is the cooling temperature after the last rolling pass.
• T _bob : is the winding temperature

Table 2: Conditions of manufacture of hot-rolled sheet from semi-finished products T _rech (° C) T _FL (° C) V _ref1 T _bob (° C) I1 1180 950 air 500 I2 1230 964 air 500 R1 1300 950 air 500 2a 1230 975 air 700 2b 1150 954 water ambient R3 1220 927 50 ° C / s 500 I = invention / R = Reference / the underlined values are not in accordance with the invention.

The sheets I1 and I2 are sheets whose chemical composition and the method of implementation are according to the invention. The two chemical compositions are different and have different Mn / Al ratios. The sheets referenced R1, R2 and R3 have chemical compositions which do not satisfy the conditions according to the invention respectively for the content of Mn, for the contents of C and Mn or for the Mn / Al ratio. R2a and R2b are two tests from the same grade R2 in Table 1. The hot rolling was carried out with at least one rolling pass in the presence of ferrite. Air cooling has a cooling rate of less than 55 ° C / sec.

• Ferrite : désigne la présence ou non de ferrite recristallisée avec un taux de recristallisation supérieur à 90% dans la microstructure de la tôle après le bobinage.
• Austénite : désigne la présence ou non de d'austénite dans la microstructure de la tôle après le bobinage.
• K : désigne la présence de précipités Kappa dans la microstructure avec une fraction surfacique inférieure à 5 %. Cette mesure est effectuée grâce à un microscope électronique à balayage.
• Rm (MPa) : la résistance mécanique dans un essai de traction en sens longitudinal par rapport à la direction de laminage.
• Atot(%) : désigne l'allongement à rupture dans un essai de traction en sens longitudinal par rapport à la direction de laminage.
• Densité estimée : sur la base de la figure 8 selon la teneur en Al.
• Fissure : Désigne si une fissure clairement visible à l'oeil nu est apparue après le laminage à chaud sur la tôle.
• X : Indique que la mesure n'a pas été faite.

Tableau 3 : Propriétés des tôles laminées à chaud. Ferrite Austenite K Rm (MPa) Atot (%) Densité mesurée Fissure I1 OUI OUI OUI 647 41 <7,3 NON I2 OUI OUI OUI 683 34,1 <7,3 NON R1 OUI OUI OUI X X <7,3 OUI R2a OUI OUI OUI 560 2,9 <7,3 NON R2b OUI OUI OUI 664 13 <7,3 NON R3 OUI OUI OUI 810 14,1 <7,3 OUI I=invention / R=Référence / les valeurs soulignées sont non-conformes à l'invention Table 3 has the following characteristics:

• Ferrite: refers to the presence or not of recrystallized ferrite with a recrystallization rate greater than 90% in the microstructure of the sheet after winding.
• Austenite: refers to the presence or absence of austenite in the microstructure of the sheet after winding.
• K: denotes the presence of Kappa precipitates in the microstructure with a surface fraction less than 5%. This measurement is made using a scanning electron microscope.
• Rm (MPa): the mechanical strength in a longitudinal tensile test with respect to the rolling direction.
• Atot (%): denotes the elongation at break in a longitudinal tensile test with respect to the rolling direction.
• Estimated density: based on the figure 8 according to the Al content.
• Crack: Designates if a crack clearly visible to the naked eye appeared after hot rolling on the sheet.
• X: Indicates that the measurement has not been made.

Table 3: Properties of hot-rolled sheets. Ferrite austenite K Rm (MPa) Atot (%) Density measured Rift I1 YES YES YES 647 41 <7.3 NO I2 YES YES YES 683 34.1 <7.3 NO R1 YES YES YES X X <7.3 YES 2a YES YES YES 560 2.9 <7.3 NO 2b YES YES YES 664 13 <7.3 NO R3 YES YES YES 810 14.1 <7.3 YES I = invention / R = Reference / the underlined values are not in accordance with the invention

The two steel sheets I1 and I2 correspond to the sheets according to the invention. The microstructure of the sheet I1 is illustrated by the figure 1 . None of these sheets has crack after rolling. The mechanical strengths are greater than 600 MPa, their elongation at break is well above 20% and they are weldable and can be coated. The presence of ferrite and austenite was confirmed by a scanning electron microscope and the presence of Kappa precipitates was confirmed by the indexing of the diffraction pattern. obtained after observation with a transmission electron microscope (cf. figure 6 ).

The sheet R1 has an Mn content of less than 6%, an Mn / Al ratio of less than 1 and a reheat temperature of greater than 1280 ° C. The sheet, after hot rolling, showed cracks. The laminability of this steel is insufficient. The letter "X" means that there has been no traction test.

The sheets R2a and R2b come from the sheet R2 and have an Mn / Al ratio of less than 1 and a manganese content of less than 6%. R2a was coiled at a temperature above 600 ° C which led to a decomposition of the austenite into Kappa and ferrite as illustrated by the figure 4 . The lengthening does not reach the necessary 20%.

The sheet R2b has undergone rolling conditions according to the invention but the chemical composition does not satisfy the conditions referred to, that is to say that the Mn / Al ratio is below 1, the elongation of 20% n is not reached.

Sheet R3 has an Mn / Al ratio of less than 1.0; despite rolling conditions according to the invention and alloying elements in the ranges covered by the invention, cracks appeared during hot rolling.

Example 2: Cold-rolled and annealed sheets

Semi-finished products were developed from a steel casting. The chemical composition of the semi-finished products, expressed in percentage by weight, is shown in Table 4 below:

The remainder of the composition of the steels in Table 4 consists of iron and unavoidable impurities resulting from processing. Table 4: Composition of steel (% weight) .I = invention VS mn al Yes Ti V Nb Mn / Al Density measured by pycnometry I3 0.21 8.2 7.4 0.26 <0.030 <0.030 <0.030 1.11 7.04 I4 0.21 8.6 6.1 0 <0.030 <0.030 <0.030 1.41 7.17 I5 0.2 8.6 6.1 0.89 <0.030 <0.030 0.1 1.41 7.12 I6 0.19 8.7 7.2 0 <0.030 <0.030 <0.030 1.21 not measured

The density of I6 was estimated at 7.1 thanks to the curve of the figure 8 .

The products were first hot-rolled under the following conditions: Table 5: Hot Rolling Conditions T _rech (° C) T _FL (° C) V _ref1 T _bob (° C) i3a 1180 905 50 ° C / s 500 i3b 1180 964 50 ° C / s 500 I4 1150 935 55 ° C / s 450 I5 1150 952 55 ° C / s 450 I6 1150 944 50 ° C / s 450

• T_rech : est la température de réchauffage
• T_FL : est la température de fin de laminage
• V_ref1 : est la température de refroidissement après la dernière passe de laminage.
• T_bob : est la température de bobinage
• Taux : est le taux de réduction lors du laminage à froid
• V_c : est la vitesse de chauffe jusqu'à la température de maintien T_m.
• T_m : est la température de maintien de recristallisation.
• t_m : est le temps pendant lequel la tôle est maintenue à la température T_m.
• V_ref2 : est la vitesse de refroidissement jusqu'à une température inférieure à 500°C.

Tableau 6 : Conditions de fabrication des tôles laminées à froid et recuites. I=invention Taux (%) V_c (°C/s) T_m (°C) t_m (sec) V_ref2 I3a 74 15 830 136 50 I3b 74 15 850 136 50 I4 75 15 905 136 55 I5 75 15 910 136 55 I6 75 15 909 136 55 The sheets were then cold rolled and annealed. The manufacturing conditions are shown in Tables 5 and 6 with the following abbreviations:

• T _rech : is the reheat temperature
• T _FL : is the end of rolling temperature
• V _ref1 : is the cooling temperature after the last rolling pass.
• T _bob : is the winding temperature
• Rate: is the rate of reduction during cold rolling
• V _c : is the heating rate up to the holding temperature T _m .
• T _m : is the recrystallization maintenance temperature.
• t _m : is the time during which the sheet is maintained at the temperature T _m .
• V _ref2 : is the cooling rate up to a temperature below 500 ° C.

Table 6: Production conditions for cold-rolled and annealed sheets. I = invention Rate (%) V _c (° C / s) T _m (° C) t _m (sec) V _ref2 i3a 74 15 830 136 50 i3b 74 15 850 136 50 I4 75 15 905 136 55 I5 75 15 910 136 55 I6 75 15 909 136 55

The sheets I3a, I3b, I4, I5 and I6 are sheets whose chemical composition and the method of implementation are according to the invention.

• Ferrite : désigne la présence ou non de ferrite recristallisée avec un taux de recristallisation supérieur à 90% dans la microstructure de la tôle recuite.
• Austénite : désigne la présence ou non de d'austénite dans la microstructure de la tôle après le bobinage.
• K : désigne la présence de précipités Kappa dans la microstructure avec une fraction surfacique inférieure à 5 %. Cette mesure est effectuée grâce à un microscope électronique à balayage. Quand il est écrit « NON », les précipités kappa sont absents.
• Rm (MPa) : la résistance mécanique dans un essai de traction en sens longitudinal par rapport à la direction de laminage.
• Atot(%) : désigne l'allongement à rupture dans un essai de traction en sens longitudinal par rapport à la direction de laminage.
• Densité mesurée: désigne la densité mesurée par pycnométrie et illustrée sur la figure 7.
• Fissure : Désigne si une fissure clairement visible à l'oeil nu est apparue après laminage sur la tôle.

Tableau 7 : Propriétés des tôles laminées à froid et recuites. I=invention Ferrite Austenite K Rm (MPa) Atot (%) Densité mesurée Fissure I3a OUI OUI NON 831 23 7,04 NON I3b OUI OUI NON 800 26 7,04 NON I4 OUI OUI NON 685 34 7,17 NON I5 OUI OUI NON 742 30 7,12 NON I6 OUI OUI NON 704 22 7,1* NON * la densité d'I6 a été estimée. Table 7 shows the following characteristics:

• Ferrite: refers to the presence or not of recrystallized ferrite with a recrystallization rate greater than 90% in the microstructure of the annealed sheet.
• Austenite: refers to the presence or absence of austenite in the microstructure of the sheet after winding.
• K: denotes the presence of Kappa precipitates in the microstructure with a surface fraction less than 5%. This measurement is made using a scanning electron microscope. When it is written "NO", the kappa precipitates are absent.
• Rm (MPa): the mechanical strength in a longitudinal tensile test with respect to the rolling direction.
• Atot (%): denotes the elongation at break in a tensile test in longitudinal direction relative to the rolling direction.
• Density measured: denotes the density measured by pycnometry and illustrated on the figure 7 .
• Crack: Designates if a crack clearly visible to the naked eye appeared after rolling on the sheet.

Table 7: Properties of cold-rolled and annealed sheets. I = invention Ferrite austenite K Rm (MPa) Atot (%) Density measured Rift i3a YES YES NO 831 23 7.04 NO i3b YES YES NO 800 26 7.04 NO I4 YES YES NO 685 34 7.17 NO I5 YES YES NO 742 30 7.12 NO I6 YES YES NO 704 22 7.1 * NO * The density of I6 has been estimated.

The cold-rolled steel sheets of Table 7 correspond to sheets according to the invention. The microstructure of the sheet I3a is illustrated by the figure 5 . None of these sheets has crack after rolling. The mechanical strengths are greater than 600 MPa, their elongation at break is greater than 20% and they are weldable and the I3a sheet was coated with Zn by a quenching process in a Zn bath at 460 ° C, called the galvanizing process by soaking. The sheet, both bare and coated, has good weldability. The steels according to the invention thus have good continuous galvanizing properties, in particular.

The steels according to the invention have a good combination of properties of interest for structural or skin parts in the automobile (low density, good deformability, good mechanical properties, good weldability and good resistance to corrosion with coating).

Claims (22)

1. Rolled steel sheet having a density of 7.3 or lower and composition, with contents expressed by weight, comprising: $$0.10\le C\le 0.30\%$$

   

   $$$$

   

   $$$$

   

   and optionally one or more elements selected from among: $$\mathrm{Si}\le 2.0\%$$

   

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

   

   $$V\le 0.6\%$$

   

   $$\mathrm{Nb}\le 0.3\%$$

   

   $$\mathrm{Nb}\le 0.3\%$$

   

   the remainder of the composition being composed of iron and inevitable processing impurities, provided that $\frac{\mathrm{Mn}}{\mathrm{Al}}>1.0,$

   

   the microstructure of the sheet being formed of ferrite, austenite and up to 5 % surface density of Kappa precipitates .
2. The steel sheet according to claim 1 having a composition, with contents expressed by weight, comprising: $$0.18\le C\le 0.21\%\mathrm{.}$$

   
3. The steel sheet according to claims 1 or 2 having a composition, with contents expressed by weight, comprising: $$7.0\le \mathrm{Mn}\le 10.0\%$$

   
4. The steel sheet according to any of claims 1 to 3 having a composition, with contents expressed by weight, comprising: $$6.0\le \mathrm{Al}\le 12.0\%\mathrm{.}$$

   
5. The steel sheet according to any of claims 1 to 4 having a composition, with contents expressed by weight, comprising: $$6.0\le \mathrm{Al}\le 9.0\%\mathrm{.}$$

   
6. The steel sheet according to any of claims 1 to 5 having a composition, with contents expressed by weight, comprising: $$\mathrm{Si}\le 1\%\mathrm{.}$$

   
7. The steel sheet according to any of claims 1 to 6 wherein the surface density of Kappa precipitates is 2 % or lower.
8. The steel sheet according to any of claims 1 to 7 wherein the mechanical tensile strength is 600 MPa or higher and ultimate elongation is 20 % or higher.
9. The steel sheet according to any of claims 1 to 8 wherein the ratio of Mn content to Al content is such that: $\frac{\mathrm{Mn}}{\mathrm{Al}}\ge \mathrm{1.1.}$

   
10. The steel sheet according to any of claims 1 to 9 wherein the ratio of Mn content to Al content is such that: $\frac{\mathrm{Mn}}{\mathrm{Al}}\ge \mathrm{1.5.}$

    
11. The steel sheet according to any of claims 1 to 10 wherein the ratio of Mn content to Al content is such that: $\frac{\mathrm{Mn}}{\mathrm{Al}}\ge \mathrm{2.0.}$

    
12. A process to manufacture rolled steel sheet having a density of 7.3 or lower, wherein:

    - steel having the composition according to any of claims 1 to 11 is provided;

    - said steel is cast to form a semi-finished product;

    - said semi-finished product is optionally reheated to a temperature T_heat of between 1000°C and 1280°C;

    - said heated semi-finished product is rolled with at least one rolling pass in the presence of ferrite to obtain sheet;

    - the finish rolling temperature T_FR is 850°C or higher;

    - said sheet is cooled at a cooling rate of V_ref1 down to a coiling temperature T_coil of 600°C or lower;

    - said cooled sheet is coiled.
13. The process to manufacture rolled sheet according to claim 12 wherein said semi-finished product is cast directly in the form thin slabs or thin strips.
14. The manufacturing process according to any of claims 11 or 13 wherein the finish rolling temperature T_FR is between 900 and 980°C.
15. The manufacturing process according to any of claims 11 to 14 wherein the cooling rate V_ref1 is 55°C/s or lower.
16. The manufacturing process according to any of claims 11 to 15 wherein the coiling temperature is between 450 and 550°C.
17. A process to manufacture cold rolled and annealed steel sheet having a density of 7.3 or lower wherein

    - rolled steel sheet according to any of claims 11 to 16 is provided; then

    - said rolled sheet is cold-rolled with a reduction rate of between 35 and 90 % to obtain cold-rolled sheet; then

    - said sheet is heated at a rate Vc up to a hold temperature T_hold of between 800 and 950°C for a time t_hold of less than 600 seconds, then

    - said sheet is cooled at a rate V_ref2 down to a temperature of 500°C or lower.
18. The manufacturing process according to claim 17 wherein the temperature T_hold is between 800 and 900°C.
19. The manufacturing process according to any of claims 16 or 18 wherein the cooling rate V_ref2 is 30°C/s or higher.
20. The manufacturing process according to any of claims 16 to 19 wherein the cooling V_ref2 is maintained down to a temperature of between 500°C and 460°C.
21. The manufacturing process according to any of claims 11 to 20 wherein the sheet is then coated with zinc, a zinc alloy or zinc-based alloy.
22. The use of steel sheets according to any of claims 1 to 11 or able to be obtained according to any of claims 12 to 21 for the production of structural parts or skin parts for land motor vehicles.

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