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
• • 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
  • C21D8/0221—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 characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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
  • C22C38/18—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
  • 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
  • C21D2211/002—Bainite
• • 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
• • 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
  • C21D8/0247—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 characterised by the heat treatment
  • C21D8/0263—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 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
  • 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
  • C21D8/0278—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 involving a particular surface treatment 

Definitions

• the present invention relates to a hot-rolled steel with a very high strength, and a method of manufacturing strips of this steel, whose structure is bainito-martensitic and can contain up to 5% of ferrite.
• Very high strength steels have been developed in recent years, in particular to meet the specific needs of the automotive industry, which are in particular the reduction of weight and thus the thickness of parts, and the improvement of safety. which goes through the increase of the resistance to fatigue and the shock resistance of the parts. These improvements should not further deteriorate the fitness of the sheets used in the manufacture of parts.
• Improved impact resistance of shaped parts can be achieved in different ways and, in particular, by using steels having on the one hand a significant elongation A and on the other hand an E / Rm ratio having a low value, which allows after shaping and thanks to the consolidation capacity of the steel to increase its elastic limit.
• the fatigue life of the parts defines their service life according to the stresses undergone, and can be improved by increasing the tensile strength Rm of the steel. But, the increase in strength deteriorates the ability to shape the steel, thus limiting the workable parts, particularly with regard to their thickness.
• high-strength steel denotes a steel whose tensile strength Rm is greater than 800 MPa.
• a second family of very high strength steels consists of so-called dual phase steels with a structure comprising approximately 10% ferrite and 90% martensite. These steels have a very good formability, but resistance levels not exceeding 800 MPa.
• US6364968 discloses a method for producing a very high strength hot rolled steel sheet which comprises preparing and heating a slab of composition 0.05% ⁇ C ⁇ 0.3%, 1.5% ⁇ Mn ⁇ 3, 5%, 0.03% ⁇ S ⁇ 1%, Al ⁇ 0.07%, S ⁇ 0.05%, P ⁇ 0.02%, N ⁇ 0.0200%, 0.003% ⁇ Nb ⁇ 0.20% or / and 0.005% ⁇ Si ⁇ 0.20% remains iron and impurities resulting from processing at a temperature ⁇ 1200 ° C, hot rolling at a temperature> 800 ° C, cooling this sheet with a cooling rate of 20-150 ° C and winding said sheet at a winding temperature of 300-350 ° C and a steel having this composition.
• the object of the present invention is to overcome the drawbacks of the steels of the prior art by providing a hot rolled steel with very high strength, suitable for shaping, and having improved fatigue strength and impact resistance.
• the invention firstly relates to a hot rolled steel with very high strength, characterized in that its chemical composition comprises, by weight: 0 , 05 % ⁇ VS ⁇ 0 , 1 % 0 , 7 % ⁇ mn ⁇ 1 , 1 % 0 , 5 % ⁇ Cr ⁇ 1 , 0 % 0 , 05 % ⁇ Yes ⁇ 0 , 3 % 0 , 05 % ⁇ Ti ⁇ 0 , 1 % al ⁇ 0 , 07 % S ⁇ 0 , 03 % P ⁇ 0 , 05 % the rest being iron and impurities resulting from the elaboration, said steel having a bainito-martensitic structure which can contain up to 5% of ferrite.
• the chemical composition further comprises, by weight: 0 , 08 % ⁇ VS ⁇ 0 , 09 % 0 , 8 % ⁇ mn ⁇ 1 , 0 % 0 , 6 % ⁇ Cr ⁇ 0 , 9 % 0 , 2 % ⁇ Yes ⁇ 0 , 3 % 0 , 05 % ⁇ Ti ⁇ 0 , 09 % al ⁇ 0 , 07 % S ⁇ 0 , 03 % P ⁇ 0 , 05 % . the rest being iron and impurities resulting from the elaboration.
• the structure of the steel according to the invention consists of 70 to 90% of bainite, 10 to 30% of martensite and 0 to 5% of ferrite, and more particularly preferably from 70 to 85% of bainite, from 15 to 30% of martensite and from 0 to 5% of ferrite.
• the subject of the invention is also a method for producing a very high-strength hot-rolled steel strip according to the invention, in which a slab is hot-rolled, the composition of which comprises: 0 , 05 % ⁇ VS ⁇ 0 , 1 % 0 , 7 % ⁇ mn ⁇ 1 , 1 % 0 , 5 % ⁇ Cr ⁇ 1 , 0 % 0 , 05 % ⁇ Yes ⁇ 0 , 3 % 0 , 05 % ⁇ Ti ⁇ 0 , 1 % al ⁇ 0 , 07 % S ⁇ 0 , 03 % P ⁇ 0 , 05 % the rest being iron and impurities resulting from the elaboration, the rolling temperature being below 950 ° C., and the strip thus obtained is cooled to a temperature of less than or equal to 400 ° C., while maintaining a cooling rate greater than 50 ° C./s between 800 and 700 ° C.
• the composition of the slab is as follows: 0 , 08 % ⁇ VS ⁇ 0 , 09 % 0 , 8 % ⁇ mn ⁇ 1 , 0 % 0 , 6 % ⁇ Cr ⁇ 0 , 9 % 0 , 2 % ⁇ Yes ⁇ 0 , 3 % 0 , 05 % ⁇ Ti ⁇ 0 , 09 % al ⁇ 0 , 07 % S ⁇ 0 , 03 % P ⁇ 0 , 05 % the rest being iron and impurities resulting from the elaboration.
• the hot-rolled steel strip is coated with zinc or a zinc alloy, by immersion in a bath of zinc or molten zinc alloy, after winding and after being unreeled, then annealed.
• the method according to the invention consists first of all in hot rolling a slab of specific composition, in order to obtain a homogeneous structure.
• the rolling temperature is less than 950 ° C, and preferably less than 900 ° C.
• the strip thus obtained is cooled to a temperature of less than or equal to 400 ° C., while maintaining a cooling rate greater than 50 ° C./s between 800 and 700 ° C.
• This rapid cooling is carried out in such a way that less than 5% of ferrite, which is not desired, is formed because the titanium would precipitate preferentially in this phase.
• This cooling rate is preferably between 50 ° C / s and 200 ° C / s.
• the method then consists in winding the strip at a winding temperature of less than or equal to 250 ° C.
• We limit the temperature of this stage in order to avoid generating an income from martensite, which would reduce the mechanical strength and raise the elastic limit, resulting in a bad E / Rm ratio.
• composition according to the invention comprises carbon with a content of between 0.05% and 0.100%.
• This element is essential for obtaining good mechanical characteristics, but must not be present in too large quantities, because it could generate segregations.
• a carbon content of less than 0.100 makes it possible in particular to have good weldability, and to improve the shaping properties and the endurance limit.
• Manganese at a level of between 0.7% and 1.1%. Manganese improves the yield strength of steel while greatly reducing its ductility, for which it limits its content. A content of less than 1.1% also prevents segregation during continuous casting.
• the composition also comprises chromium at a level of between 0.50% and 1.0%.
• a minimum content of 0.50% makes it possible to promote the appearance of bainite in the microstructure. However, its content is limited to 1.0% because a high chromium content would favor the increase of the amount of ferrite formed above 5%, because of its alphagenic character.
• the composition also comprises silicon at a content of between 0.05% and 0.3%. It greatly improves the yield strength of steel while reducing its ductility slightly and deteriorating its coating, which is why its content is limited.
• the composition also comprises titanium at a content of between 0.05 and 0.1%.
• This element makes it possible to significantly increase the mechanical characteristics by a phenomenon of precipitation during rolling and cooling. It does not increase the hot hardness because of its moderate content. Its content is limited to 0.1% in order to avoid degrading the impact resistance properties, the hot hardness and the folding ability.
• composition may also comprise phosphorus at a content of less than 0.05%, since, beyond that, it could pose problems of segregation during continuous casting.
• the composition also comprises aluminum with a content of less than 0.07%, which occurs during the calming of the steel during its manufacture at the steel mill.
• a steel grade has been developed. Its composition is given in the following table: VS mn Cr Yes Ti S P al AT 0.78 0.95 0.79 0.233 0.094 0,001 0,038 0.048 The rest of the composition consists of iron and unavoidable impurities resulting from the elaboration.
• microstructure of test 1 is bainito-martensitic, whereas the microstructure of tests 2 and 3 is ferritic-bainitic.
• a winding temperature greater than 250 ° C. associated with a cooling rate between 800 and 700 ° C. of less than 50 ° C./s, increases the yield strength without increasing the mechanical strength.
• the ratio E / Rm is therefore too high.
• the steel according to the invention has a good ability to dip coating in a bath of molten metal, such as zinc or a zinc alloy, or aluminum or an alloy thereof.

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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)

Applications Claiming Priority (3)

Publications (2)

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• 2003
  • 2003-01-15 FR FR0300371A patent/FR2849864B1/fr not_active Expired - Lifetime
• 2004
  • 2004-01-14 US US10/542,107 patent/US7699947B2/en active Active
  • 2004-01-14 WO PCT/FR2004/000058 patent/WO2004070064A2/fr not_active Ceased
  • 2004-01-14 UA UAA200508007A patent/UA79531C2/uk unknown
  • 2004-01-14 JP JP2006502099A patent/JP4505055B2/ja not_active Expired - Lifetime
  • 2004-01-14 MX MXPA05007580A patent/MXPA05007580A/es active IP Right Grant
  • 2004-01-14 CN CNB2004800021197A patent/CN100366759C/zh not_active Expired - Lifetime
  • 2004-01-14 BR BRPI0406731-2A patent/BRPI0406731B1/pt not_active IP Right Cessation
  • 2004-01-14 PL PL378236A patent/PL209154B1/pl unknown
  • 2004-01-14 CA CA2513096A patent/CA2513096C/fr not_active Expired - Lifetime
  • 2004-01-14 KR KR1020057013089A patent/KR101065781B1/ko not_active Expired - Lifetime
  • 2004-01-14 EP EP04701978A patent/EP1587963B1/fr not_active Expired - Lifetime
  • 2004-01-14 ES ES04701978T patent/ES2374188T3/es not_active Expired - Lifetime
  • 2004-01-14 AT AT04701978T patent/ATE528414T1/de active
  • 2004-01-14 RU RU2005125717/02A patent/RU2333284C2/ru active
• 2005
  • 2005-06-24 ZA ZA2005/05161A patent/ZA200505161B/en unknown

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