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
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • 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
  • C21D8/0273—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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
• • 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
• • 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 present invention relates to a method of manufacturing a dual-phase steel strip with a ferrito-martensitic structure, cold-rolled, and the strip obtainable by this method, which is more particularly intended for the manufacture of automobile parts. by deep drawing.
• 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.
• the object of the present invention is to overcome the disadvantages of the steels of the prior art by providing a steel strip capable of deep drawing, and having both excellent mechanical characteristics and excellent characteristics of anisotropy .
• the invention firstly relates to a method of manufacturing a dual-phase steel strip with a ferrite-martensitic structure, cold-rolled, characterized in that a slab is hot-rolled, the chemical composition of which comprises , in weight :
• the cooling rate between 600 ° C and the ambient temperature being between 100 ° C / s and 1500 ° C / s,
• the chemical composition further comprises, by weight:
• the method according to the invention may also comprise the following characteristics, alone or in combination:
• the strip is hot-rolled at a temperature above 850.degree. C., the hot strip is reeled at a temperature of between 550.degree.
• the strip is cold-rolled with a reduction ratio of between 70 and 80%
• the continuous annealing of the cold rolled strip comprises a temperature rise phase and then a holding phase at a predetermined temperature
• the holding temperature is between Ad and 900 ° C.
• the holding temperature is between 750 and 850 ° C
• cooling to room temperature comprises a first slow cooling between the holding temperature and 600 ° C, during which the cooling rate is less than 50 ° C / s, then a second cooling at a higher speed and between 100 ° C / s and 1500 ° C / s, up to room temperature.
• the second subject of the invention is a dual-phase steel strip with a cold-rolled ferrito-martensitic structure, the chemical composition of which comprises, by weight:
• composition of the strip is as follows: 0.020% ⁇ C ⁇ 0.060%
• the strip according to the invention may also comprise the following characteristics, alone or in combination: it has a tensile strength Rm greater than 450 MPa,
• the invention has the third object the use of a steel strip according to the invention, for the manufacture of parts for automotive deep drawing.
• the method according to the invention consists of hot rolling a slab of specific composition, then winding the hot strip obtained at a temperature between 550 and 850 ° C.
• This winding at high temperature is indeed favorable to the development of what is called a texture, ie an anisotropic structure.
• a texture ie an anisotropic structure.
• Such a winding indeed makes it possible to coalesce the precipitation of Fe3C cementite, and to reduce the amount of carbon redissolved during annealing, which is harmful to the development of the recrystallization texture.
• the process then involves cold rolling the web with a reduction ratio of 60 to 90%, and then continuously annealing the web in the intercritical range.
• Intercritical annealing makes it possible to re-dissolve the majority of the carburized phases formed during the winding after the recrystallization.
• the fact that the austenitization and dissolution of the carburized phases occurs after recrystallization makes it possible to retain the carbon trapped during the recrystallization and to release it once the texture of the recrystallized ferrite is developed. The texture will therefore not be affected by the carbon in solid solution, as in the case of a low temperature winding, but only altered by the isotropic nature of the martensite formed.
• the method then consists in cooling the strip to room temperature, in one or more stages, the cooling rate between 600 ° C. and the ambient temperature being between 100 ° C./s and 1500 ° C./s, and optionally to incur an income at a temperature below 300 ° C.
• the martensitic phase fraction formed can be reduced by lowering the holding temperature to lower values in the intercritical range, or by performing slow cooling before quenching.
• the income optionally practiced according to the invention consists in precipitating a part of the solid solution carbon trapped in martensite, without decreasing the proportion of this martensite.
• the maximum temperature of this income is 300 ° C, preferably 250 ° C, and more preferably 200 ° C.
• composition according to the invention comprises carbon with a content of between 0.010% and 0.100%. This element is essential to obtain good mechanical properties, but should not be present in too large quantities, because it would generate the formation of a large proportion of martensitic phase.
• the composition also comprises chromium at a content of between 0.010% and 1.0% which aids in the desired formation of martensite.
• the composition also comprises silicon at a content of between 0.010% and 0.50%. It greatly improves the yield strength of steel while reducing its ductility slightly and deteriorating its coating.
• the composition also comprises phosphorus at a content of between 0.001% and 0.20%, which hardens the microstructure without affecting its texture.
• the composition also comprises aluminum at a content of between 0.010% and 0.10% which avoids aging by trapping nitrogen.
• compositions consist of iron and unavoidable impurities resulting from the elaboration.
• the hot-rolled strips were then cooled by quenching with water at a cooling rate of the order of 25 ° C./s until the winding temperature was reached.
• Grade A was coiled at 720 ° C while one sample of Grade B was coiled at 550 ° C and the other at 720 ° C.
• the different samples were then cold-rolled to a 75% reduction rate, then subjected to an annealing treatment at a holding temperature of 750 ° C for some samples, and 800 ° C for 'other. Cooling to room temperature is then carried out at a rate of about 25 ° C./s, by quenching with water.
• the overall anisotropy of a steel is determined by the average normal anisotropy coefficient r: rT + rL + 2 (r45 °))
• FIG. 1 shows the relationship between the average coefficient r and the martensite ratio formed% m for the grades A and B. It can be seen that the higher the martensite content, the more steel is isotropic.
• FIG. 2 shows the microstructure obtained with the grade A, wound at 720 ° C., then annealed at 750 ° C. to finally obtain 12% of martensite.
• ferrite and formed martensite There is a good distinction between ferrite and formed martensite.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Metal Rolling (AREA)

Priority Applications (7)

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Publications (1)

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Cited By (6)

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Citations (6)

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• 2003
  • 2003-02-05 FR FR0301358A patent/FR2850671B1/fr not_active Expired - Lifetime
• 2004
  • 2004-01-30 EP EP04706710.3A patent/EP1592816B1/fr not_active Expired - Lifetime
  • 2004-01-30 WO PCT/FR2004/000209 patent/WO2004079022A1/fr not_active Ceased
  • 2004-01-30 JP JP2006505664A patent/JP4528769B2/ja not_active Expired - Lifetime
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  • 2004-01-30 KR KR1020057014238A patent/KR101091021B1/ko not_active Expired - Lifetime
  • 2004-01-30 RU RU2005127577/02A patent/RU2341566C2/ru active
  • 2004-01-30 PL PL377834A patent/PL206109B1/pl unknown
  • 2004-01-30 MX MXPA05008189A patent/MXPA05008189A/es active IP Right Grant
  • 2004-01-30 HU HUE04706710A patent/HUE052206T2/hu unknown
  • 2004-01-30 ES ES04706710T patent/ES2831249T3/es not_active Expired - Lifetime
  • 2004-01-30 CA CA2514736A patent/CA2514736C/fr not_active Expired - Lifetime
  • 2004-01-30 US US10/544,206 patent/US20060144482A1/en not_active Abandoned
  • 2004-01-30 BR BR0407236-7A patent/BRPI0407236A/pt active IP Right Grant
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• 2005
  • 2005-07-25 ZA ZA200505968A patent/ZA200505968B/en unknown

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