Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/46—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting
  • B21B1/463—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting in a continuous process, i.e. the cast not being cut before rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
  • B21B45/02—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for lubricating, cooling, or cleaning
  • B21B45/0203—Cooling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
  • B21C47/02—Winding-up or coiling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
  • C21D8/0436—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• 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.
• 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 second way is to reduce the density of steels by combining them with other lighter metals.
• 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.
• 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.
• the patent application EP2128293 describes a hot or cold rolled sheet of composition 0.2-0.8% C, 2-10% Mn, 3-15% AI, and a structure containing less 99% ferrite and more than 1% residual austenite.
• the sheet has a mechanical strength in the range 600-1 OOOMPa 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.
• 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 aims to prevent creasing and the appearance of cracks rolling by limiting the Mn / AI ratio 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 for a light steel and having a high strength and good ductility.
• 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.
• the patent application WO2007 / 024092 aims to provide easily rolled hot-rolled sheets.
• This application relates to a sheet containing 0.2-1% C, 8-5% 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.
• the invention aims to solve these difficulties by proposing hot-rolled or cold-rolled steel sheets simultaneously presenting:
• 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.
• 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:
• the composition comprises, the content being expressed by weight:
• the composition comprises, the content being expressed by weight:
• the composition comprises, the content being expressed by weight:
• the composition comprises, the content being expressed by weight:
• the composition comprises, the content being expressed by weight:
• 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%.
• 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:
• the last rolling pass will be at an end temperature of TFL rolling greater than or equal to 850 ° C.
• 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.
• the end of rolling temperature T F L is between 900 and 980 ° C.
• the cooling rate V re n is less than or equal to 55 ° C / s.
• the winding temperature is between 450 and 550 ° 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:
• the temperature T m is between 800 and 900 ° C.
• the cooling rate V re f2 is greater than or equal to 30 ° C / s.
• the cooling rate ⁇ re12 is maintained up to a temperature of between 500 ° C and 460 ° C.
• 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.
• FIG. 1 illustrates the microstructure of a hot rolled steel sheet according to the invention.
• FIG. 2 illustrates the microstructure of a hot-rolled steel sheet that does not satisfy the conditions according to the invention.
• FIG. 3 shows the mechanical behavior in hot traction representing the hot rollability as a function of the traction temperature in ° C.
• FIG. 4 illustrates the microstructure of a hot-rolled steel sheet not satisfying the conditions according to the invention.
• FIG. 5 illustrates the microstructure of a cold-rolled steel sheet according to the invention.
• FIG. 6 shows 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.
• FIG. 7 illustrates a microstructure of cold sheet which does not satisfy the conditions of the invention.
• FIG. 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.
• 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 AIC 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 must have a content of 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.
• 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.
• the addition of Mn to 10.0% will be limited.
• the Mn content will be 7.0% in order to reach the elongation of 20% more easily.
• 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, 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 it disappear in favor of ferrite and precipitates Kappa as shown in 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 an Mn
• 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.
• silicon is an element that reduces the density of steel and reduces the 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.
• 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.
• titanium and niobium make it possible to control grain size during solidification. A limitation is however necessary because beyond this, a saturation effect is obtained.
• cerium, boron, magnesium, or zirconium may 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.
• 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% 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.
• 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.
• less than 2% Kappa precipitates are contemplated. It is specified that the microstructure being uniform, the surface fraction is equal to the volume fraction.
• 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 half-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.
• a temperature of between 1000 ° C. and 1280 ° C. 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 easier it recrystallizes, and reheating temperatures above 1280 ° C. are avoided because they are industrially expensive and not very favorable for the recrystallization of ferrite. This can, on the other hand, amplify the phenomenon of ragging (also called "roping").
• 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.
• the reheating temperature is between 1150 and 1280 ° C.
• the steel sheet according to the invention has a noticeable drop in laminability as shown in Figure 3 which has the narrowing of test pieces subjected to hot traction 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 up to the winding temperature Tb 0.
• a cooling rate V re fi of less than or equal to 55 ° C./s is preferred for better control. the precipitation of kappa.
• the sheet is reeled at a temperature of between 450 and 550 ° C.
• 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 that 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. to ensure a recrystallization rate greater than 90% of the initial structure hardened.
• the sheet is then cooled at a speed V re f 2 up 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 the 5% in surface content.
• a cooling rate of greater than 30 ° C./s is preferred to better control the formation of the Kappa precipitates and not to exceed the 5% in surface content.
• 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 re f2 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.
• 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
• composition of the steels shown in Table 1 consists of iron and unavoidable impurities resulting from processing.
• Table 1 Composition of steels (% weight).
• T r ech is the reheating temperature
• Vre i is the cooling temperature after the last rolling pass.
• the sheets 11 and 12 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.
• 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.
• Table 3 Properties of hot-rolled sheets.
• the two steel sheets 11 and 12 correspond to the sheets according to the invention.
• the microstructure of the sheet 11 is illustrated in FIG. 1. None of these sheets has a 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 from observation with a transmission electron microscope (see 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 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 wound at a temperature above 600 ° C which led to a decomposition of the austenite Kappa and ferrite as shown in Figure 4. The elongation does not reach the required 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
• the remainder of the composition of the steels in Table 4 consists of iron and unavoidable impurities resulting from processing. Density measured by
• Table 4 Steel Composition (% Wt.) The invention The density of 16 was estimated at 7.1 by the curve of Figure 8.
• the products were first hot-rolled under the following conditions:
• Trech is the reheating temperature
• Vrefi is the cooling temperature after the last rolling pass.
• Rate is the reduction rate 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 kept at the temperature
• the sheets I3a, I3b, 14, 15 and 16 are sheets whose chemical composition and the method of implementation are according to the invention.
• 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.
• Measured Density refers to the density measured by pycnometry and shown in Figure 7.
• Fissure Refers to a crack clearly visible to the eye after rolling on the sheet.
• the cold-rolled steel sheets of Table 7 correspond to sheets according to the invention.
• the microstructure of the sheet I3a is illustrated in FIG. 5. None of these sheets has a 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 has been 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).

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