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 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
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
  • C21D6/00—Heat treatment 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
  • 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
  • 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
  • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • 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
  • C21D2211/008—Martensite

Definitions

• the invention relates to a method of manufacturing steel sheets with a thickness of less than 3 millimeters with a totally martensitic structure with a mechanical strength greater than that which could be obtained by a simple quenching treatment with martensitic quenching, and resistance properties. mechanical and elongation allowing their application to the manufacture of energy absorbing parts in motor vehicles.
• (C) denotes the carbon content of the steel, expressed as a percentage by weight.
• a method of manufacture is thus sought which makes it possible to obtain an ultimate tensile strength of 50 MPa at expression (1), ie a strength greater than 3220 ( C) + 958 MPa for this steel. It seeks to have a method for the manufacture of sheet with a very high yield strength, that is greater than 1300 MPa. It is also sought to have a method for the manufacture of directly usable sheets, that is to say without the imperative need of a tempering treatment after quenching.
• the present invention aims to solve the problems mentioned above. It aims in particular at providing sheets with a thickness of less than 3 millimeters with a yield strength greater than 1300 MPa, tensile strength, expressed in megapascals, greater than (3220 (C) +958) MPa, and preferably a total elongation of greater than 3%.
• the average size of austenitic grains is less than 5 micrometers.
• the sheet is subjected to a subsequent thermal treatment of tempering at a temperature T 4 of between 150 and 600 ° C. for a period of between 5 and 30 minutes.
• the subject of the invention is also a sheet of steel with a thickness of less than 3 millimeters and no yield strength greater than 1300 MPa, obtained by a method according to one of the above methods of manufacture, with a totally reduced structure. martensitic, having an average slat size of less than 1.2 micrometers, the average elongation factor of slats being between 2 and 5.
• the subject of the invention is also a steel sheet having a thickness of less than 3 millimeters obtained by the process with the above-mentioned tempering treatment, the steel having a totally martensitic structure with an average slat size of less than 1.2. micrometer, the average elongation factor of the slats being between 2 and 5.
• Manganese lowers the initial formation temperature of martensite and slows the decomposition of austenite. In order to obtain sufficient effects, the manganese content must not be less than 1.5%. Moreover, when the manganese content exceeds 3%, segregated zones are present in excessive amount which impairs the implementation of the invention. A preferred range for the implementation of the invention is 1.8 to 2.5% Mn.
• the silicon content must be greater than 0.005% so as to participate in the deoxidation of the steel in the liquid phase.
• the silicon must not exceed 2% by weight because of the formation of surface oxides which significantly reduce the coating ability, in the case where it would be desirable to coat the sheet by passing through a metal coating bath, in particular by continuous galvanizing.
• the aluminum content of the steel according to the invention is not less than 0.005% so as to obtain sufficient deoxidation of the steel in the liquid state.
• the aluminum content is greater than 0.1% by weight, casting problems can occur. It is also possible to form inclusions of alumina in too large quantities or sizes which play a detrimental role on toughness.
• the sulfur and phosphorus contents of the steel are respectively limited to 0.05 and 0.1% in order to avoid a reduction in the ductility or toughness of the parts or sheets produced according to the invention.
• the steel also contains niobium in an amount between 0.025 and 0.1%, and optionally titanium in an amount between 0.01 and 0.1%.
• Chromium and molybdenum are very effective elements for delaying the transformation of austenite and can be used optionally for the implementation of the invention. These elements have the effect of separating the ferrito-pearlitic and bainitic transformation domains, the transformation Ferritic-pearlitic occurring at temperatures above the bainitic transformation. These transformation domains are then in the form of two distinct "noses" in an isothermal transformation diagram (Transformation-Temperature-Time)
• the chromium content must be less than or equal to 4%. Beyond this content, its effect on the quenchability is practically saturated; an additional addition is then expensive without corresponding beneficial effect.
• the molybdenum content must not exceed 2% because of its excessive cost.
• the steel can also contain boron: indeed, the significant deformation of the austenite can accelerate the conversion to ferrite on cooling, a phenomenon that should be avoided. Addition of boron in an amount of between 0.0005 and 0.005% by weight makes it possible to guard against early ferritic transformation.
• the steel may also contain calcium in an amount between 0.0005 and 0.005%: by combining with oxygen and sulfur, calcium prevents the formation of large inclusions that are harmful to the ductility of the sheets or parts thus manufactured.
• the rest of the composition of the steel consists of iron and unavoidable impurities resulting from the elaboration.
• the steel sheets manufactured according to the invention are characterized by a totally martensitic slatted structure of great fineness: due to the specific thermomechanical cycle and composition, the average size of the martensitic slats is less than 1.2 micrometers and their average elongation factor is between 2 and 5.
• These microstructural characteristics are determined, for example, by observing the microstructure by Scanning Electron Microscopy by means of a field effect gun ("MEB-FEG" technique) at a magnification. greater than 1200x, coupled to an EBSD detector ("Electron Backscatter Diffraction"). It is defined that two contiguous slats are distinct when their disorientation is greater than 5 degrees.
• the average slat size is defined by the known intercepts method: the average size of slats intercepted by defined lines is evaluated. random with respect to the microstructure. The measurement is performed on at least 1000 martensitic slats in order to obtain a representative average value. The morphology of the individual slats is then determined by image analysis using software known in itself: the maximum I max and minimum I min dimension of each martensitic slat and its elongation factor are determined. l max l min . In order to be statistically representative, this observation concerns at least 1000 martensitic slats. The average elongation factor l max l min is then determined for all of these slats observed.
• the invention is not limited to this geometry and to this type of product, and can also be adapted the manufacture of long products, bars, profiles, by successive stages of hot deformation.
• the steel sheets may be used as such or subjected to a heat treatment of tempered temperature T 4 between 150 and 600 ° C for a period of between 5 and 30 minutes.
• This treatment of income generally has the effect of increasing the ductility at the price of a decrease of the limit of elasticity and the resistance.
• the inventors have however demonstrated that the method according to the invention, which gives a tensile strength of at least 50 MPa higher than that obtained after conventional quenching, retained this advantage even after a tempering treatment with temperatures ranging from from 150 to 600 ° C. The fineness characteristics of the microstructure are preserved by this income treatment.
• the sheets were rolled in this temperature range in 5 passes with a cumulative reduction rate ⁇ b of 76%, ie up to a thickness of 2.8 mm, then cooled. then to room temperature with a speed of 80 ° C / sec so as to obtain a completely martensitic microstructure.
• steel sheets of the above composition were heated at a temperature of 1250 ° C., held for 30 minutes at this temperature and then cooled with water so as to obtain a completely martensitic microstructure (reference condition).
• the yield strength Re By means of tensile tests, the yield strength Re, the breaking strength Rm and the total elongation A have been determined for sheets obtained by these different methods of manufacture.
• Steel B does not contain enough niobium: it does not reach a yield strength of 1300 MPa, both after simple martensitic quenching (test B2) and in the case of rolling with roughing and finishing at temperature.
• T 3 (test B1)
• microstructure of the plates obtained by Scanning Electron Microscopy was also observed by means of a field effect gun ("MEB-FEG" technique) and EBSD detector, and quantified the average size. slats of the martensitic structure and their average elongation factor l max l min .
• the method according to the invention makes it possible to obtain a martensitic structure with an average slat size of 0.9 micrometres and an elongation factor of 3. This structure is considerably thinner than that observed after simple martensitic quenching, whose average slat size is of the order of 2 micrometers.
• the plates produced according to the invention because of their lower carbon content, have good weldability by the usual processes, in particular spot resistance welding. They also have good ability to be coated, for example by galvanizing or continuous dipping aluminization.
• the invention allows the manufacture of sheets of thickness less than 3 millimeters or bare or coated with very high mechanical characteristics, under very satisfactory economic conditions.

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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 Steel (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Manufacturing Of Steel Electrode Plates (AREA)

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• 2011
  • 2011-05-12 WO PCT/FR2011/000295 patent/WO2012153009A1/fr active Application Filing
• 2012
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