US20170275724A1 - Cold rolled high strength low alloy steel - Google Patents

Cold rolled high strength low alloy steel Download PDF

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Publication number
US20170275724A1
US20170275724A1 US15/504,941 US201515504941A US2017275724A1 US 20170275724 A1 US20170275724 A1 US 20170275724A1 US 201515504941 A US201515504941 A US 201515504941A US 2017275724 A1 US2017275724 A1 US 2017275724A1
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Prior art keywords
steel strip
sheet
strip
blank
mpa
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Calum McEwan
Paul BELLINA
Jean Joseph Campaniello
Johan Boezewinkel
Bernard Leo Ennis
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Tata Steel Ijmuiden BV
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Tata Steel Ijmuiden BV
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Assigned to TATA STEEL IJMUIDEN B.V. reassignment TATA STEEL IJMUIDEN B.V. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: BELLINA, Paul, ENNIS, BERNARD LEO, BOEZEWINKEL, JOHAN, CAMPANIELLO, Jean Joseph, MCEWAN, CALUM
Publication of US20170275724A1 publication Critical patent/US20170275724A1/en
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    • 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
    • C22C38/18—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
    • 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
    • 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
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    • 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
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    • 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
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • 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
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    • 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
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • 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
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    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
    • C23F17/00—Multi-step processes for surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by subclass C21D or C22F or class C25

Definitions

  • the invention relates to a high strength low alloy steel strip, sheet or blank.
  • the invention also relates to a method for producing such a high strength low alloy steel strip.
  • High strength low alloy steel is well known in the art.
  • HSLA steels are often used in the automotive industry.
  • HSLA steels are for instance defined in the specification of the Verband Der Automobilindustrie (VDA). Reference is made to the VDA 239-100 Material specification of August 2011.
  • VDA Verband Der Automobilindustrie
  • cold rolled HSLA steels are indicated with a steel grade number, for instance CR420LA, wherein CR stands for cold rolled, the number 420 stands for the lower limit of the yield strength Rp0,2 in longitudinal direction, and LA stands for low alloy.
  • the VDA specification gives a chemical composition for HSLA steels containing Ti and Nb, apart from the standard alloying elements C, Mn, Si and Al, to provide for the high strength.
  • Thin HSLA steel strip, sheet or blank is usually coated with an aluminium coating or a zinc coating. If a zinc coating is used, the coating is often applied as a hot dip galvanised or hot dip galvannealed coating.
  • Cold rolled HSLA steels at higher strength levels have the drawback that, due to their high strength, the hot rolled strip is difficult to cold roll to a relatively thin gauge at wide dimensions.
  • a high strength low alloy steel strip, sheet or blank, coated with zinc or a zinc alloy having the following composition in weight %:
  • the inventors have found that when Ti and V are used as a combination of alloying elements, instead of the combination of Ti and Nb as known from the VDA specification, a steel is produced that provides lower mill loads.
  • the Ti and V levels have to be used in combination with a specific level for C, Mn and Si, as specified according to the invention. Within the ranges of the invention, it is possible to achieve a yield strength Rp0,2 of at least 420 MPa.
  • the HSLA steel according to the invention contains no added Cr, Cu, Mo and Nb. These elements are not needed to provide a HSLA steel with the required yield strength.
  • Vanadium provides precipitation strengthening and some grain refinement. At a concentration lower than 0.04 wt % V the volume of vanadium-carbide precipitates is not sufficient to provide enough additional precipitation strengthening to reach a strength of 420 MPa for Rp0,2. At concentrations higher than 0.15 wt % V recrystallisation is suppressed during annealing. This limits elongation.
  • Titanium also provides precipitation strengthening and some grain refinement. At concentrations higher than 0.07 wt % Ti the work hardening during cold rolling will rise significantly, limiting a high cold reduction. On the other hand, the inventors have found that a concentration lower than 0.02 wt % Ti will decrease the total elongation of the steel strip, sheet or blank. The combination of the right amount of Ti and V appears to generate a special microstructure providing both a high strength and elongation.
  • Carbon is useful to increase the solution strengthening and thus gain more strength. Therefore, at least 0.03 wt % C should be added. However a too high concentration will limit the cold rolling and will decrease the elongation. For this reason, the amount of carbon is limited to 0.07 wt %.
  • Nitrogen has an effect similar to that of C.
  • This element will combine preferentially with Al and Ti to form AlN and TiN precipitates.
  • TiN precipitates are formed at high temperatures already in the re-heating oven, but also during hot rolling and during coiling. They are large precipitates (several microns) that do not increase the strength.
  • AIN can also form at high temperature. Nevertheless with a fast cooling and a coiling temperature lower than 650° C., their precipitation can be partly stopped, keeping in solid solution a source of Al and N that precipitates during the continuous annealing and that may contribute to the precipitation strengthening. If a large amount of N is added (>0.008 wt %) elongation is degraded and cracking of slabs occurs.
  • Silicon is used for solution strengthening, but at a high concentration (>0.2 wt % Si) it will deteriorate the surface quality.
  • the smelting cost to remove the Si becomes too high if the concentration is below 0.01 wt % Si.
  • Phosphorus is used for solution strengthening, but a high concentration will deteriorate the steel ductility. Therefore, the concentration should be below 0.03 wt % P.
  • Aluminium is used as deoxidizer in steel and its minimum amount should be 0.005 wt % Al to ensure the deoxidation. At a concentration higher than 0.1 wt % Al, the occurrence of surface defects resulting from alumina clusters increases.
  • Niobium is kept as low as possible and even avoided because it will increase significantly the work hardening and thus limit the cold reduction of wide strip. Moreover at a concentration higher than 0.03 wt % Nb, it has a great effect on the recrystallisation temperature which makes the use of a high annealing temperature necessary (higher than 800° C.) to obtain reasonably recrystallised HSLA.
  • one or more of the alloying elements can be present is a limited amount, as follows:
  • Narrowing the Carbon range gives the best elongation for a given strength level. Increasing the minimum C level increases the proof stress of the material. Reducing the upper C level minimises cold rolling loads and achieves the best combination of maximum width and elongation at this higher strength level.
  • Narrowing the Aluminium range improves the deoxidation and limits the risk on surface defects.
  • Titanium retards recrystallisation. Minimising the maximum Ti level can assist in optimising the elongation for a given strength level.
  • Niobium level further assist in being able to roll wider at a given strength level of the cold rolled and annealed product
  • Minimising the remaining elements further assist in improving elongation at a given strength level.
  • the steel strip, sheet or blank has a yield strength Rp0,2 in longitudinal direction of at least 460 MPa, more preferably a yield strength Rp0,2 of at most 580 MPa.
  • the automotive industry prefers to use HSLA steel having such a yield strength, in accordance with the VDA specification.
  • the steel strip, sheet or blank has an elongation A80mm in longitudinal direction of at least 15%. This is the elongation a CR460LA steel grade should possess according to the VDA specification.
  • the steel strip, sheet or blank has a tensile strength Rm in longitudinal direction of at least 480 MPa, more preferably a tensile strength Rm of at least 520 MPa, more preferably a tensile strength Rm of at most 680 MPa.
  • tensile strengths are preferred for by the automotive industry, in accordance with the VDA specification.
  • the zinc or zinc alloy coating is a hot dip galvanized or hot dip galvannealed coating. These are the generally used zinc coatings in the automotive industry.
  • the zinc alloy coating comprises 0.5 to 4 wt % Al and 0.5 to 3.2 wt % Mg, the remainder being zinc and traces of other elements.
  • the coating preferably has a thickness between 5 and 15 ⁇ m per side, more preferably a thickness between 6 and 13 ⁇ m per side.
  • This is a so-called AlMgZn coating providing an improved corrosion protection in comparison to the usual zinc coatings.
  • the other elements that can be present are Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi.
  • Pb, Sn, Bi and Sb are usually added to form spangles. These elements can be present in small amounts, less than 0.5 wt % each, usually less then 0.2 wt % each, often less then 0.2 wt % in total.
  • the coiling temperature is affecting the precipitation of V and mainly VC.
  • a small amount of VC is present that helps the cold rolling (less work hardening).
  • the volume of VC precipitates will increase, increasing the work hardening and thus making the cold rolling more difficult, which at the end will limit the width of the strip to be cold rolled at the defined cold reduction.
  • the VC precipitates will start to coarsen and then the benefit of the precipitation strengthening in the cold rolled annealed end material will be lowered.
  • Bainite will increase cold rolling loads. It is preferential to avoid bainite, hence temperatures below 500° C. are not recommended.
  • the upper limit of the annealing temperature is governed by the coarsening/dissolution of VC precipitates. This upper limit should be at least 20° C. lower than the solubility temperature of VC precipitates.
  • the solubility of VC precipitates is depending on the V (and C) concentration. In counter part, the volume of VC precipitates will affect the recrystallisation of the steel; the greater the VC volume, the higher the recrystallisation temperature is.
  • V concentration in the steel composition a balance should be found between the cold rolling reduction and the concentration of C, Mn, N and Ti, in order to define a annealing temperature.
  • the annealed strip is hot dip coated with a zinc or zinc alloy coating.
  • the continuous annealing is directly followed by the hot dip coating with zinc or a zinc alloy.
  • the coated strip is cold rolled in a temper mill with a reduction of 0.1-3.0%, preferably 0.2-2.0%.
  • the temper rolling provides the strip with an improved surface quality. At higher levels of temper rolling an increased yield strength is seen as well as the removal of yield point elongation (Luders lines).
  • the strip is cold rolled at a width of at least 1400 mm, preferably at a width of at least 1600 mm, more preferably at a width of at least 1800 mm, with a gauge of 0.7-2.0 mm.
  • the HSLA with Ti and V has an improved ductility compared to HSLA with Ti and Nb or with Nb and V.
  • the coiling temperature of the hot rolled strip is between 550° C. and 600° C. and/or the overall cold rolling reduction is 60-70% and/or the annealing temperature is between 760° C. and 800° C.
  • the steel used in the method has a composition as provided by the preferred embodiment of the composition according to the first aspect of the invention.
  • the produced steel strip has a yield strength Rp0,2 of at least 420 MPa, preferably a yield strength Rp0,2 of at least 460 MPa, more preferably a yield strength Rp0,2 of at most 580 MPa.
  • the produced steel strip has an elongation A80 mm of at least 15%.
  • a number of strips has been produced as full production material. Samples of these strips are indicated with the numbers 1, 2, 3 and 4. For each sample a variant A and B is tested, wherein the variants A and B each time have the same composition, see Table 1, but for which variants A and B different coiling temperatures and different temper rolling reductions are used. The information about the coiling temperature and temper rolling reduction, together with the cold reduction percentage and the annealing temperature, is given in Table 2.
  • Table 2 shows that for a composition in accordance with the invention it is possible to reach a yield strength Rp0,2 of at least 420 MPa for a cold reduction of 60%, and with the right choice of composition, coiling temperature and annealing temperature it is even possible to reach a yield strength Rp0,2 of at least 460 MPa, see samples 2, 3 and 4.
  • the temper rolling reduction for these samples has been at most 1%.
  • Table 2 also shows that the elongation A80 mm is usually at least 15% for the samples tested. Only for sample 4A, which has the highest yield strength Rp0,2, the elongation A80 mm is slightly lower than 15%.
  • Table 3 shows laboratory samples 5 and 6 from the same production material as used for sample 1A and 1B, which samples 5 and 6 have been processed with annealing temperatures near the limits or outside the range provided according to the invention
  • Sample 5 shows that with an annealing temperature that is too high, the Rp0,2 will be too low.
  • Sample 6 shows that when the annealing temperature is quite low, the elongation A80 mm is lower than desired. Samples 5 and 6 thus show that the annealing temperature is quite critical for reaching the desired properties.
US15/504,941 2014-08-25 2015-08-24 Cold rolled high strength low alloy steel Abandoned US20170275724A1 (en)

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WO2021180964A1 (en) * 2020-03-13 2021-09-16 Tata Steel Ijmuiden B.V. Hot rolled steel strip having improved properties
CN111575592B (zh) * 2020-06-28 2021-10-29 马鞍山钢铁股份有限公司 一种屈服强度460MPa级的低合金高强钢及生产方法
WO2023062210A1 (en) * 2021-10-15 2023-04-20 Tata Steel Ijmuiden B.V. Hybrid high strength low alloy cold-rolled and annealed steel strip and method for producing it

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