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
  • C21D8/0221—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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS 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
  • 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 present invention relates to a process for producing a sheet from a steel slab, having very high mechanical strength, a formability and excellent weldability.
• the present invention also relates to a steel sheet obtained by this process.
• the wheel sails have thinned parts which constitute zones of weakness for these wheels.
• manufacturers are moving towards steel having an increasingly high mechanical resistance as well as excellent stamping and welding properties.
• the steel sheets used to make this type of part are generally part of the range of hot-rolled flat products whose mechanical characteristics are obtained by controlled rolling on a rolling mill with wide strips.
• HLE high yield strength steel
• HLE steel is a microalloyed steel which has an elastic limit between 315 and 690 MPa, but which has a poor formability due in particular to a ratio between the elastic limit Re and the high mechanical resistance Rm , i.e. greater than 0.85.
• DUAL PHASE steels are also known, the structure of which is of the ferrite-martensite type and which have remarkable formability.
• HR steels are also part of steels with high mechanical characteristics and are carbon-manganese steels which undergo a sufficiently rapid cooling after rolling associated with a winding at around 400-450 ° C.
• the steel sheets thus produced have a ferrito-bainitic structure of intermediate formability between the HLE steel sheets and the DUAL PHASE.
• TRIP steels are also known, that is to say with induced plastic transformation, which combine a very high mechanical resistance, greater than 800 MPa with high elongations which give them excellent formability.
• the object of the present invention is to remedy the drawbacks mentioned above by proposing a method for producing a steel sheet having at the same time a very high mechanical strength, a good formability and an excellent weldability.
• the steel sheets which are the subject of the present invention are obtained by hot rolling of slabs from, for example, a continuous casting installation.
• the nitrogen content of 80 ppm verifies the 3.5N ⁇ Ti relationship, the rest being made up of iron and impurities.
• the steel slabs from the continuous casting are heated in an oven at a temperature between 1200 and 1250 ° C and, more precisely at 1250 ° C, then introduced into a rolling mill with wide strips comprising a roughing train and a finishing train, each consisting of several rolling mill stands.
• the essential function of this rolling is to reduce the thickness of the slab.
• the blank obtained is then engaged in the finishing train in which a work hardening is carried out with a reduction rate varying from 70 to 95% thus producing a refining of the austenite grains.
• the rolled blank in the finishing train is permanently engaged in one of the cages of said train where the recrystallization phenomenon is delayed.
• the sheet that is obtained has a thickness of between 2 and 10 mm and its temperature, also called the end of rolling temperature, is between 800 and 900 ° C. and for example equal to 845 °. vs.
• the casting of thin slabs makes it possible in particular to suppress the passage of slabs in a roughing train.
• the thin slab is directly engaged in the finishing train in which the hardening is carried out with a reduction rate varying from 70 to 95%.
• the combined effect of boron and niobium makes it possible to inhibit the germination of the ferrite and thus the austenite begins its transformation into bainite at temperatures lower than the usual temperatures.
• Niobium is completely effective from 0.04% but, beyond 0.06%, the shaping properties of steel deteriorate.
• the optimum boron is between 10 and 40ppm.
• microalloy elements of steel are not used as above, but to inhibit ferritic and pearlitic transformation.
• the choice of the end of rolling temperature makes it possible to generate more or less defects, called dislocations, in the crystal lattice and in which the bainite germinates.
• the austenite is cooled at a speed of between 50 and 150 ° C / s, preferably equal to 80 ° C / s.
• the cooled sheet is then wound at a temperature between 500 and 600 ° C, preferably between 560 and 590 ° C, and for example equal to 580 ° C.
• the Applicant has had the merit of realizing that molybdenum allows the formation of highly fragmented complex carbides at a high winding temperature above 500 ° C and which maintain a high mechanical resistance Rm between 800 and 900 MPa.
• a silicon content of less than 0.5% makes it possible to improve the elongation and therefore the shaping properties and makes it possible to avoid the formation of surface defects which are called fayalite during the heating of the slabs in the ovens .
• the fayalite Fe2SiO4 forms with FeO an eutectic whose melting point is at 1177 ° C, thus this eutectic penetrates along the gamma grain boundaries and the incrustations during rolling in the roughing train are revealed during the pickling.
• the elongation of such a proportional test piece taken in the long direction from a steel sheet is greater than 15% and for example equal to 18.2%.
• the elongation is greater than 13% and for example equal to 15%.
• the low Re / Rm ratio of the steel thus obtained and its elongation favorably allow its use for the manufacture of hollow profiles and in particular of tubes with high mechanical resistance.
• the sulfur content must be less than 0.01% and this element must be globularized.
• the presence of a low carbon content gives the steel excellent weldability, but it is advisable not to exceed the value of 0.05% beyond which there is an increase in the dispersion of the mechanical characteristics.
• the low carbon content also prevents the formation of martensite which is rather harmful to the ductility of steel.
• titanium is present in steel to protect boron from nitrogen and allow synergy between boron and niobium.
• the sheet After the winding operation, the sheet has all its mechanical characteristics of use.
• pickling is carried out, for example by immersion in one or more hydrochloric acid baths in order to remove all traces of oxide on the face of said sheet.
• the sheet can be cut into strips by shears before delivery.
• the strips thus obtained also have a tight flatness for which the corrugation deflection is less than 1% of the length of the corrugation and in all cases less than 10mm.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials 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 (2)

Publications (2)

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Family Applications (1)

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Cited By (1)

Citations (3)

• 1992
  • 1992-02-28 FR FR9202407A patent/FR2688009B1/fr not_active Expired - Fee Related
• 1993
  • 1993-02-04 EP EP19930400287 patent/EP0564309B1/de not_active Revoked
  • 1993-02-04 DE DE1993602950 patent/DE69302950T2/de not_active Revoked

Patent Citations (3)

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