Classifications

• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
• • 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
  • C22C38/02—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

Definitions

• the invention relates to a hot-rolled steel sheet, with very high strength and low density, from a strip rolling train, as well as its manufacturing process.
• Reducing motor vehicles becomes a necessity due to the imposition of reducing C02 emissions to 140g / Km in 2008. This reduction can only be achieved by increasing the level of mechanical strength of steels to compensate for the decrease in sheet thickness. It is therefore necessary to increase the mechanical characteristics while reducing the thicknesses of the sheets with which the parts used are made. This approach finds its limits with the reduction in the rigidity of the parts and the appearance of unacceptable vibrations and noises in targeted applications in the automotive field where noise is an element of discomfort.
• the object of the invention is to provide users of hot-rolled steel sheets with a sheet of low density and having resistance levels comparable to steel sheets with high mechanical resistance currently used or even of a higher level, and this, in order to combine the two advantages of low density and high mechanical strength.
• the first object of the invention is a hot-rolled steel sheet of very high strength and low density, characterized in that its composition in% by weight comprises: 0.04% ⁇ carbon ⁇ 0.5%
• the steel comprises in its composition, in% by weight:
• the sheet according to the invention is such that the silicon content is between 3 and 6%, and that the aluminum content is between 1 and 2%.
• the sheet according to the invention is such that the silicon content is between 2 and 3%, and that the aluminum content is between 7 and 10%.
• the silicon and aluminum contents of the sheet according to the invention are such that:
• the sheet according to the invention can also have the following characteristics, alone or in combination:
• the sheet has a microstructure consisting of a primary ferrite phase and a secondary ferrite phase, the average grain size of said primary ferrite being greater than the average grain size of said secondary ferrite, said microstructure also containing carburetted phases,
• the sheet has a primary ferrite phase obtained during the heating of the steel carried out prior to hot rolling, and a secondary ferrite phase obtained after hot rolling, as well as carburetted phases, - the sheet comprises a primary ferrite phase whose average grain size greater than 5 ⁇ m, and a secondary ferrite phase are the average grain size less than 2 ⁇ m.
• the second object of the invention is a method of manufacturing a hot-rolled sheet which comprises the steps consisting in:
• the temperature of end of hot rolling being higher than the temperature AR3 of the austenitic phase formed during reheating, in order to carry out a rolling under austenitic conditions, thus transforming the austenitic phase into a phase secondary ferrite and fuel phases.
• FIG. 1 a curve showing the evolution of the density of a steel as a function of the silicon, aluminum and / or silicon plus aluminum content
• FIG. 2 the microstructure of a steel according to the invention comprising 0.04% of carbon (casting I)
• FIG. 3 the microstructure of a steel according to the invention comprising 0.160% of carbon (casting J)
• FIG. 4 the microstructure of a steel according to the invention comprising 0.268% of carbon (casting K)
• FIG. 5 the microstructure of a steel comprising 0.505% of carbon, shown for comparison (casting L ).
• the hot rolled steel on a strip train according to the invention has a high mechanical strength and a low density.
• molybdenum ⁇ 2% taken alone or in combination, the rest being iron and elements inherent in the preparation, comprises:
• the carbon content of the sheet according to the invention is between 0.04 and 0.5% by weight, preferably between 0.04 and 0.3% by weight.
• the evolution of the structure of the steel as a function of the carbon content is presented in FIGS. 2 to 5 and shows that the structure of the steel according to the invention (FIGS. 2 to 4) consists of primary ferrite with coarse grains, and a mixture of carburetted phase and fine secondary ferrite with smaller grains. If the carbon content falls below 0.04%, the microstructure does not include carburetted phases and loses in mechanical characteristics. On the other hand, if the carbon content exceeds 0.5% in weight, the structure becomes very fragile, and we observe that the microstructure no longer includes primary ferrite (see Figure 5).
• the steel according to the invention can reach levels of mechanical resistance ranging from 620 MPa to more than 1000 MPa and densities of the order of 7.55 and down to 7 g / cm3 depending on the contents.
• silicon and aluminum and additive elements as shown in Figure 1.
• the mechanical characteristics can be reinforced by the addition of micro alloy elements such as niobium, titanium, vanadium, the latter two, which are less dense than iron.
• the sheet according to the invention can be manufactured by any suitable process.
• This process first of all comprises reheating the slab at high temperature (preferably above 900 ° C.), prior to hot rolling.
• high temperature preferably above 900 ° C.
• the present inventors have discovered that during this reheating step, the slab has a microstructure composed of a so-called primary ferritic phase which forms at high temperature, and which co-exists with an austenitic phase.
• the austenitic phase is then completely transformed into a mixture of carburetted phase and secondary ferrite, the average grain size of which is smaller than that of the primary ferritic phase which remains.
• a carbon-manganese couple will advantageously be chosen in order to have an AR3 transformation temperature such that it is possible to guarantee rolling under austenitic conditions.
• Table 1 below presenting different analyzes according to the invention, shows the influence of the different elements on the characteristics of the steels.
• the flows A, C, F, H and L are given for comparison, while the flows B, D, E, G, I, J and K are according to the invention.
• the rolling temperature is 895 ° C, and the winding temperature of 600 ° C, with a cooling rate of 49 ° C / s, which gives the steel a mechanical resistance of 750 MPa.
• Lowering the winding temperature increases the level of mechanical resistance.
• the winding temperature of which is 20 ° C. with a cooling rate of 5 ° C./s, which makes it possible to reach a level of mechanical resistance of 902 MPa.
• the level of mechanical strength can also be adjusted by carbon and manganese contents and / or other addition elements as presented above.
• Certain operations such as, for example, re-rolling or a heat treatment such as annealing, can be used to modify or adjust the level of mechanical properties.
• the steel proposed meets two contradictory needs in the field of hot-rolled steels on the one hand, high mechanical properties and on the other hand, low density.
• the existing solutions for making steels with very high levels of mechanical strength are based on the use of addition elements which do not allow a large variation in density, and the existing solutions for making steels with low density are based on the use of addition elements which do not allow a high level of mechanical strength to be achieved.
• the steel of the invention combines these two properties, namely, a high level of mechanical resistance and a very low density for lightening of a part usable in the automobile.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Heat Treatment Of Steel (AREA)
• Laminated Bodies (AREA)
• Pressure Welding/Diffusion-Bonding (AREA)
• Heat Treatment Of Strip Materials And Filament Materials (AREA)

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• 2002
  • 2002-03-11 FR FR0202998A patent/FR2836930B1/fr not_active Expired - Lifetime
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