EP3791000B1 - Bande d'acier, feuille ou découpe présentant une formabilité améliorée et procédé pour produire une telle bande - Google Patents

Bande d'acier, feuille ou découpe présentant une formabilité améliorée et procédé pour produire une telle bande Download PDF

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EP3791000B1
EP3791000B1 EP19721632.8A EP19721632A EP3791000B1 EP 3791000 B1 EP3791000 B1 EP 3791000B1 EP 19721632 A EP19721632 A EP 19721632A EP 3791000 B1 EP3791000 B1 EP 3791000B1
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hot
steel strip
temperature
sheet
rolled
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EP3791000A1 (fr
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Rolf Arjan RIJKENBERG
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Tata Steel Ijmuiden BV
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Tata Steel Ijmuiden BV
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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
    • 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
    • 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
    • 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
    • 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
    • C22C38/12—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
    • 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
    • C21D2211/004—Dispersions; Precipitations
    • 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

Definitions

  • the invention relates to a hot rolled and cold rolled steel strip, sheet or blank having improved formability, and to a method for producing such a steel strip, sheet or blank.
  • Document WO 2017/050790 A1 is directed to hot rolled high strength formable steel sheets for automotive applications.
  • Document WO 2014/065623 A1 is directed to a batch type annealing facility and the use thereof for hot rolled sheets.
  • Document EP 3255164 A1 is directed to batch annealing of cold rolled sheets.
  • An object of the invention is to provide a formable steel strip, sheet or blank suitable for automotive chassis components.
  • a further object of the invention is to provide a formable steel strip, sheet or blank suitable for a tailor-rolled blank for automotive chassis components, which after flexible cold-rolling with reductions of 30% and higher, followed by batch annealing, has a variable thickness with a high yield strength and suitable tensile elongation (A50 in %).
  • a further object of the invention is to provide a batch-annealed formable steel strip or sheet according to the objects described hereinabove wherein the steel has a microstructure that consists of at least 50% recrystallized ferrite.
  • a further object of the invention is to provide a method of manufacturing a formable steel strip or sheet according to the objects described hereinabove.
  • one or more of these objects can be reached by a hot rolled and cold rolled steel strip, sheet or blank consisting of the following elements (in wt%):
  • the invention provides a formable steel strip or sheet with an excellent balance between recrystallization behaviour, strength, and ductility after batch-annealing and cold rolling.
  • the excellent balance between recrystallization and strength is obtained with an essential and substantial addition of V of at least 0.1 wt.% to a steel composition that further contains as precipitating elements Ti, Mo, and optionally Nb.
  • Aim of the present invention is that after hot rolling, the steel is cooled down and coiled at a relatively low temperature to suppress the formation of precipitates in general, and that of V-based precipitates in particular.
  • the intention is to have as much V in solid solution in the ferrite matrix of the hot-rolled intermediate feedstock prior to cold rolling (either to produce a uniform thickness via conventional cold rolling or a variable thickness via flexible rolling) and subsequent batch annealing within the ferrite phase region (below Ac1 transformation point).
  • the V in solid solution during the early stages of batch annealing will form - predominantly - V-based carbide precipitates in addition to V-based nitride and/or carbo-nitride precipitates.
  • the V precipitation during batch annealing is accelerated by the presence of dislocations induced by the cold-rolling operation as the increased dislocation density will increase the diffusional rate of elements.
  • the dislocations will act as preferential nucleation sites for precipitation during the early stages of batch annealing. This in turn will suppress annihilation of dislocations and hence reduce the degree of recovery as the newly formed V-based precipitates will pin dislocations and hinder their movement.
  • the reduced degree of recovery will increase the driving force for the nucleation of recrystallized ferrite grains and increase the density of ferrite nuclei, leading to more impingement during recrystallization and ultimately stimulate grain refinement of the final microstructure.
  • Subsequent growth of the newly formed ferrite nuclei / grains is promoted by ensuring a sufficiently high batch-annealing top temperature.
  • This top temperature should be equal to or above the dissolution temperature of V-based precipitates in order to dissolve the V-based precipitates and to lift their pinning force, allowing migration of grain boundaries.
  • a top temperature of 700 °C or higher is sufficient to promote substantial recrystallization.
  • Such a high top temperature during batch annealing will impair precipitation strengthening as precipitates in general will be prone to substantial coarsening at those temperatures.
  • the loss in precipitation strengthening at these temperatures can be suppressed by using Ti and Mo, optionally in combination with Nb.
  • the element Mo is known to combine with Ti and Nb to form composite carbide and/or carbo-nitride precipitates that have increased thermal stability and hence an increased resistance to coarsening.
  • the V brought in solid solution in the ferrite matrix during batch annealing after dissolution may again partially precipitate in the ferrite matrix upon cooling down, contributing to some extent to precipitate strengthening.
  • V addition provides grain refinement by stimulating recrystallization. This ensures that loss in precipitation strengthening due to the use of an elevated batch annealing temperature is mitigated by an increase in strength from grain refinement.
  • An alternative method to assess if substantial recrystallization of the microstructure has been achieved after batch annealing is to record the evolution of the yield strength after batch annealing for a range of cold rolling reductions.
  • the Rp0.2 after batch annealing will increase due to work hardening that is not (significantly) compromised as the amount of dislocations does not provide sufficient driving energy for recrystallization.
  • the batch annealing parameters at some point the Rp0.2 after batch annealing will again start to decrease with increasing cold-rolling reduction as locally recrystallization will start to occur, leading to a loss in dislocation hardening.
  • the Rp0.2 may start to remain stable or increase again after a region in which it decreased with increasing cold-rolling reduction. This is the region of interest for the present invention and is the region in which recrystallization becomes dominant.
  • the increase in Rp0.2 with increasing cold-rolling reduction in this region is the result of increasing grain refinement, which results from recrystallization and the increasing amount of dislocations and hence increasing amount of potential nuclei that are present to form new, recrystallized ferrite grains. Hence, increased impingement and grain refinement will lead to an increase in Rp0.2 due to grain refinement.
  • the yield strength (Rp0.2) is constant or - preferably - increases with an increase in cold-rolling reduction (CR%) in the range of 30 to 60% or higher sufficient recrystallization is achieved to have sufficient formability for forming operations during manufacturing and to avoid or substantially suppress the issue of delamination or splitting as a result of shearing operations, including cutting or punching, when the steel is used to manufacture (automotive) components.
  • the role of the alloying elements for the present invention is as follows.
  • Carbon (C) is added to form carbide and/or carbo-nitride precipitates together with V, Ti, Mo and - in the present invention - optionally with Nb.
  • the amount of C depends on the amount of V, Ti, Nb and/or Mo used and should be at least 0.05 wt.%. However, the maximum content is 0.20 wt.% to prevent excessive segregation and to prevent a too high fraction of cementite and/or pearlite.
  • the fraction of pearlite and/or cementite in the microstructure of the batch-annealed steel is preferably at most 10%, or more preferably at most 5%, or most preferably at most 3%.
  • a more preferable C content range for the present invention is between 0.06 and 0.17 wt.%, or most preferably between 0.07 and 0.14 wt.%.
  • the microstructure thus contains at least 90% ferrite, which is the sum of recrystallised ferrite and non-recrystallised ferrite.
  • Silicon (Si) provides significant solid-solution strengthening, which is desirable as its contribution to strength is not compromised by the thermal cycle of the batch annealing process. Furthermore, it retards the formation of cementite and pearlite, thus suppressing the formation of coarse carbides, which can impair hole-expansion capacity. However, too high Si will lead to an undesired increase in rolling loads and may lead to surface issues and reduced fatigue properties. For these reasons, the Si content is between 0.30 and 0.60 wt.%.
  • Mn Manganese
  • Mn content should be at least 0.8 wt.%.
  • a too high Mn content may lead to excessive segregation, which can impair hole-expansion capacity and promote delamination or splitting during shearing operations.
  • a too high Mn content will suppress the ferritic transformation temperature and promote hardenability, leading to hard carbon-rich phase constituents in the intermediate hot-rolled feedstock (e.g., martensite and retained-austenite) which in turn can lead to unacceptable high strength and too high rolling loads for the cold mill.
  • a suitable maximum Mn content for the present invention is 2.5 wt.%.
  • a more preferable Mn content range for the present invention is between 0.9 and 2.30 wt.%, or most preferably between 1.20 and 2.00 wt.%.
  • Phosphorus (P) provides solid-solution strengthening, However, at high levels, P segregation will promote delamination or splitting during shearing operations and impair hole-expansion capacity. Therefore, the P content should be at most 0.06 wt.%, or preferably at most 0.04 wt.%, and more preferably at most 0.02 wt.%.
  • S Sulphur
  • the S content should be at most 0.01 wt.%, or preferably at most 0.005 wt.%, or more preferably at most 0.003 wt.%.
  • Al is added as a deoxidizer.
  • a suitable minimum Al content is 0.01 wt.%.
  • too high Al can be deleterious as it forms AIN particles during solidification of the molten steel, which can provoke surface issues during casting.
  • a too high Al content can impair hole-expansion capacity as it may lead to a too high fraction of Al x O y inclusions in the steel matrix, which can promote internal fractures upon shearing the steel.
  • the Al content should be at most 0.10 wt.%.
  • a suitable Al content range for the present invention is between 0.01 and 0.10 wt.%, or more preferably between 0.02 and 0.09 wt.%, and most preferably between 0.04 and 0.08 wt.%.
  • the Nitrogen (N) content should be low, i.e., at most 0.01 wt.%. Too high N content, in particular when too much N is free and in solid solution in the ferrite matrix, is deleterious for formability in general. Furthermore, too high N content in the presence of Ti can lead to an excessive amount of large cuboid TiN particles, which impair formability in general and hole-expansion capacity in particular.
  • N can be beneficial to promote nitride and/or carbo-nitride precipitates, which in general are more thermally stable than carbide precipitates. In this context, N can be beneficial to suppress coarsening during the thermal cycle of the batch annealing process.
  • a more preferable range for N content for the present invention is at most 0.008 wt.%, or most preferably between 0.002 and 0.007 wt.%.
  • Titanium (Ti) is used in the present invention to realise precipitation strengthening and to some degree grain refinement.
  • Ti is an essential element in the alloy composition of the present invention to achieve a desired strength level for the steel strip or sheet after batch annealing.
  • a suitable minimum Ti content is 0.07 wt.% or more preferably 0.08 wt.% or even 0.10 wt.%.
  • a too high Ti content can lead to undesired segregation-related phenomena, to too high rolling loads during hot rolling and subsequent cold rolling, and to too low formability due to insufficient recrystallization achieved after batch annealing. This insufficient recrystallization of the steel after batch annealing may lead to issues with splitting or delamination resulting from shearing the steel during manufacturing operations.
  • a suitable maximum Ti content is 0.25 wt.%.
  • a more preferable Ti maximum content for the present invention is 0.22 wt.%, or most preferably 0.20 wt.%.
  • Niobium (Nb) is used in the present invention to realise a certain degree of precipitation strengthening as well as to achieve grain refinement and hence strength via the Hall-Petch effect.
  • the degree of precipitation hardening is relatively limited compared to that of Ti.
  • a suitable minimum Nb content is 0.02 wt.%, or more preferably 0.03 wt.%, and a suitable maximum Nb content is 0.10 wt.%, more preferably 0.09 wt.%, and most preferably 0.08 wt.%.
  • Molybdenum is known to be a carbide-forming element and can form together with Ti, V and/or Nb composite carbide and/or carbo-nitride precipitates. These composite precipitates comprising Mo, are reported to be more thermally stable than their counterparts without Mo and hence more resistant to coarsening during exposure to a thermal cycle at temperatures above 600 °C. Hence, Mo is beneficial to suppress precipitate coarsening during batch annealing at top temperatures above 600 °C and to reduce the loss in precipitation strengthening due to batch annealing above 600 °C. The desired strength level of the final batch annealed steel in the end will determine to what extent Mo, which is an expensive alloy element, is required.
  • a suitable Mo content is at least 0.05 wt.% and at most 0.40 wt.%.
  • a more preferable Mo content range for the present invention is between 0.08 and 0.35 wt.%, or most preferably between 0.10 and 0.30 wt.%.
  • Vanadium (V) is an essential element for the present invention as it acts as an agent to stimulate recrystallization during batch annealing, providing grain refinement, and provides precipitation strengthening.
  • the former i.e., the aspect of recrystallization - is achieved by the formation of V-based carbide precipitates during the initial stages of batch annealing which nucleate on dislocations and hence pin dislocations, reducing their mobility and suppressing recovery.
  • the driving force for the onset of recrystallization is increased as the pool of surviving dislocations at the start of recrystallization increases.
  • V in solid solution in the ferrite matrix at elevated temperature during the batch annealing cycle may again precipitate later on during the final stages of the batch annealing cycle, contributing again to precipitation strengthening of the ferrite microstructure of the final steel strip or sheet after batch annealing.
  • the V in the present invention is not only believed to be beneficial to achieve strength via grain refinement and direct precipitation strengthening via the formation of freshly V-based precipitates during batch annealing as mentioned above, but also indirectly by suppressing the coarsening kinetics of Ti-based precipitates in the steel matrix during batch annealing.
  • the latter is believed to be the result of a relatively high V content in solid solution in the ferrite matrix, which will reduce Ti solubility and hence suppress coarsening kinetics of Ti-based precipitates.
  • part of the V that will precipitate during batch annealing will correspond with co-precipitation, i.e., V precipitating on existing Ti-based precipitates. This can promote a V-rich shell surrounding the Ti-based precipitate, which will acts as a barrier, suppressing coarsening of Ti-based precipitates covered with a V-rich shell.
  • the amount of V should be sufficiently high enough to promote a sufficient degree of recrystallization. Inventors found that a suitable minimum V content is 0.10 wt.%, or preferably 0.12 wt.%, and more preferably 0.13 wt.%. At the same time, the amount of V and the corresponding amount of VC precipitates should correspond with a dissolution temperature for VC precipitates in ferrite that is within the industrial capacity of the batch annealing furnace used.
  • T dis in °C the dissolution temperature
  • the value for T dis should be in line with the heating capacity of the batch annealing furnace in order to ensure that VC can be sufficiently dissolved during the batch annealing cycle to promote substantial recrystallization.
  • a suitable maximum V content is 0.35 wt.%, or more preferably 0.30 wt.%, or most preferably 0.25 wt.%.
  • Chromium (Cr) is an optional element for the present invention and can be used to promote the formation of ferrite, in particular when elevated levels of Mo and/or Mn are used that can suppress the formation of ferrite. If used, a suitable Cr content is 0.01 - 0.80 wt.%, or preferably 0.01 - 0.60 wt.%, or more preferably 0.01 - 0.40 wt.%.
  • Calcium is an optional element for the present invention and may be used to modify MnS-type of inclusions to improve formability and/or to modify Al x O y -type of inclusions to reduce the risk of clogging and to improve cast ability of the steel during steel making.
  • a too high Ca content can lead to excessive wear of the refractory lining in the installations of the steel-making plant
  • a suitable maximum Ca content is 50 ppm, or more preferable maximum 35 ppm.
  • a suitable minimum Ca content in the steel is 20 ppm.
  • the Ca content in the steel is at most 20 ppm, or preferably at most 10 ppm, or most preferably at most 5 ppm.
  • the steel strip, sheet or blank has a yield strength of 350 MPa or higher, preferably of 400 MPa or higher, more preferably of 450 MPa or higher, still more preferably of 500 MPa or higher, most preferably 550 MPa or higher after batch annealing.
  • a yield strength is suitable for automotive applications.
  • the steel strip, sheet or blank has a precipitation strengthened ferrite microstructure containing recrystallized ferrite, cementite and/or pearlite, and wherein the precipitates in said microstructure consist of Ti, V, Mo and optionally Nb after batch annealing.
  • the invention provides a steel strip or sheet with a predominantly ferrite microstructure that is strengthened with precipitates consisting of Ti, V, Mo, and optionally Nb after batch annealing.
  • the indication "predominantly” in this case means that the ferrite microstructure comprises at least 90%, or preferably at least 95%, or more preferably at least 97% ferrite, or most preferably 100% ferrite.
  • a predominantly ferrite microstructure may consist of at most 10%, or preferably at most 5%, or more preferably of at most 3% of cementite and/or pearlite.
  • the steel strip, sheet or blank contains Nb, Ti, and Mo represented by weight percentage (wt.%) satisfying the equation of 0.8 ⁇ Nb 93 + Ti 48 Mo 96 ⁇ 2.2 or preferably 0.8 ⁇ Nb 93 + Ti 48 Mo 96 ⁇ 1.3
  • the steel after hot rolling and annealing has a yield ratio of at least 0.9, and/or a tensile strength of 900 MPa or higher, preferably 950 MPa or higher, and/or an elongation A50/t 0.2 that is 9% or higher.
  • a yield ratio of at least 0.9, and/or a tensile strength of 900 MPa or higher, preferably 950 MPa or higher, and/or an elongation A50/t 0.2 that is 9% or higher.
  • Such mechanical properties are often required by the automotive industry for high strength steel.
  • the steel after cold rolling with a cold rolling reduction of at least 30% and annealing has a yield strength of 350 MPa or higher, preferably of 400 MPa or higher, more preferably of 450 MPa or higher, still more preferably of 500 MPa or higher, most preferably 550 MPa or higher, and an elongation A50 / t 0.2 that is 14% or higher.
  • yield strength 350 MPa or higher, preferably of 400 MPa or higher, more preferably of 450 MPa or higher, still more preferably of 500 MPa or higher, most preferably 550 MPa or higher, and an elongation A50 / t 0.2 that is 14% or higher.
  • fraction recrystallized ferrite at 1 ⁇ 4 depth is at least 50%, preferably at least 60%, more preferably at least 70%, most preferably at least 80%.
  • Such a microstructure can provide the mechanical properties that are looked for.
  • a method for producing the steel according to the first aspect of the invention comprising the steps of:
  • the reheating temperature of the slabs in the furnaces of the hot-strip mill prior to rolling should be high enough h to ensure that practically all carbide and carbo-nitride precipitates containing Ti and V, and optionally Nb, have dissolved in the steel matrix. This is required to maximise the amount of Ti and V, and optionally Nb, in solid solution prior to hot rolling and further down-stream processing.
  • the optimum reheating temperature depends on the amount of Ti and V, and optionally Nb. However, inventors found that a suitable range of the reheating temperature is between 1150 and 1300 °C.
  • Finish rolling in the hot-strip mill should be done at Ar3 transformation point or higher in order to finish the hot rolling sequence in the austenite phase region prior to actively cooling down the steel strip or sheet to enforce austenite-to-ferrite phase transformation.
  • the average cooling rate on the run-out-table of the hot-strip mill to cool the steel strip or sheet just after finish rolling should be in the range of 10 to 150 °C/s.
  • the temperature to coil the steel strip or sheet in the hot-strip mill should be low enough to suppress precipitation in general, but in particular that of V. At the same time, the coiling temperature should not be too low as this leads to too much transformation hardening.
  • the microstructure of the intermediate hot-rolled feedstock in the present invention is preferably ferrite and/or bainitic in nature, preferably without the presence of a substantial amount of martensite. Inventors found that a suitable coiling temperature of the steel strip or sheet in the hot-strip mill is between 450 and 580 °C.
  • the heating of the hot rolled strip for at least 1 hour at a top temperature between 550 and 700 °C promotes the formation of new precipitates and thus increases the yield strength of the hot rolled strip to provide an improved balance between strength and formability.
  • the batch annealing process for at least 8, 10 or 14 hours results in a at least 50% recrystallized cold rolled strip, resulting in a precipitation strengthened ferrite microstructure with an adequate balance between strength and formability.
  • the hot-rolled steel strip is hot-rolled with a finish rolling temperature of 870 °C or higher, preferably with a finish rolling temperature of 900 °C or higher, more preferably with a finish rolling temperature of 940 °C or higher, and most preferably with a finish rolling temperature of 980 °C or higher.
  • the temperature set for finish rolling may be chosen higher. Another benefit of a higher finish rolling temperature is its beneficial influence on texture development and hence mechanical properties and isotropy.
  • the finish rolling temperature should preferably be 900 °C or higher, or more preferably 940 °C or higher, or most preferably 980 °C or higher.
  • the hot-rolled steel strip after finish rolling is cooled to the coiling temperature with an average cooling rate of 40 to 100 °C/s.
  • the hot-rolled steel strip is coiled in the temperature range between 480 and 560° C, or more preferably between 500 and 540 °C to provide the preferred microstructure of the intermediate feedstock.
  • the hot-rolled steel strip is batch annealed after hot rolling for at least 1 hour at a top temperature between 600 and 700 °C, more preferably at a top temperature between 650 and 700 °C.
  • the heating of the hot rolled strip promotes the formation of new precipitates and thus increases the yield strength of the hot rolled strip to provide an improved balance between strength and formability.
  • the cold-rolled steel strip or sheet is thus batch annealed, preferably in an inert and protective atmosphere, consisting of hydrogen and/or nitrogen, to prevent excessive decarburisation and/or Fe-based oxide scale formation, and/or to promote more efficient heat transfer during batch annealing.
  • an inert and protective atmosphere consisting of hydrogen and/or nitrogen
  • the batch annealing cycle preferably uses an overall slow heating rate to top temperature to provide sufficient time and thermal energy to promote precipitation in general and V precipitation in particular, in order to pin dislocations to suppress recovery.
  • the preferred top temperature of the batch annealing cycle depends partially on the amount of V and C in solid solution in the ferrite matrix available for the formation of VC precipitates and consequently is linked to the minimum temperature needed to dissolve the VC precipitates formed during the initial stages of the batch annealing.
  • a top temperature of 700 °C or higher will provide an adequate degree of recrystallization in order to ensure that the object of the present invention is realised;
  • the yield strength (Rp0.2) of the steel strip or sheet after batch annealing is constant or preferably increases with an increase in cold-rolling reduction (CR%) of 30% and higher, or preferably 30% to 60%.
  • a dwell time of at least 14 hours at a top temperature of 700 °C or higher during the batch annealing cycle is preferred to reach aforementioned objects of the present invention.
  • the batch annealing cycle requires an overall slow cooling rate from top temperature to circa 200 °C to provide sufficient time and thermal energy to promote again precipitation in general and V precipitation in particular.
  • the steel strip or sheet can optionally be provided with a zinc-based coating to the surface of the steel strip or sheet to provide corrosion protection.
  • a zinc-based coating to the surface of the steel strip or sheet to provide corrosion protection.
  • This can be done via a coating process, such as heat-to-coat or electro-galvanizing, wherein a zinc or zinc alloy coating is applied to the surface of the steel strip or sheet.
  • the alloy preferably contains Aluminium and/or Magnesium as its main alloying elements.
  • Figure 1 shows the time-temperature curve of the batch annealing cycle use in the examples.
  • Examples are performed using laboratory cast ingots.
  • Steels 1A to 1H having chemical compositions shown in Table 1.1 were hot rolled after reheating the ingots to 1250 °C for 45 minutes to ensure optimum dissolution of carbide and carbo-nitride precipitates, which, depending on alloy composition, comprise Mo, Ti, Nb and V.
  • the hot-rolled steel sheets were rolled in 5 passes from a thickness of 35 to 3.5 ⁇ 0.5 mm with an exit temperature for the final rolling pass in the range of circa 900 to 1000 °C.
  • the steel sheet was transferred to a run-out-table (ROT) and cooled down from a start ROT temperature in between 850 to 900 °C with an average cooling rate of circa 40 to 50 °C/s to an exit ROT temperature around 600 or 540 °C.
  • ROT run-out-table
  • the hot-rolled steel sheet was transferred to a furnace to replicate slow coil cooling from a start temperature of 600 or 540 °C to ambient temperature.
  • the hot-rolled steel sheets were pickled prior to tensile testing of the hot-rolled steel sheet or further processing in terms of cold rolling and subsequent batch annealing followed by tensile testing of cold-rolled and batch-annealed steel sheets.
  • steel 1A, 1B, 1D, 1E, 1F and 1H are comparative examples, since they contain less than 0.10 wt% V.
  • Batch annealing was done on hot-rolled steel sheets with no cold-rolling reduction (CR% equals 0%) and on cold-rolled steel sheets after a 10, 20, 30, 40, 50, or 60% cold-rolling reduction post hot rolling.
  • CR% cold-rolling reduction
  • plates were wrapped in stainless steel foil and a protective H 2 atmosphere was used in the batch anneal furnace.
  • the tensile properties were in all cases, i.e., for hot-rolled as well as batch-annealed steel sheets, measured parallel to rolling direction by means of taking out A50 test pieces and applying a tensile load to the test pieces according to EN 1002-1/ISO 6892-1 (Rp0.2 is the 0.2% offset proof or yield strength; Rm is the ultimate tensile strength; Ag is the uniform elongation; A50 is the total tensile elongation).
  • Hot-rolled steel sheets Table 1.3 gives the A50 tensile properties of the hot-rolled steel sheets coiled at 600 °C of steels 1A to 1D (corresponding hot-rolled steel sheets labelled as 1A-HR600, 1B-HR600, 1C-HR600, 1D-HR600).
  • Table 1.4 gives the A50 tensile properties of the hot-rolled steel sheets coiled at 540 °C of steels 1A to 1H with labelling of the corresponding hot-rolled steel sheets in a similar fashion as done in Table 1.3.
  • Batch-annealed steel sheets Tables 1.5 and 1.6 give the tensile properties of batch-annealed steel sheets without any intermediate cold rolling (CR% equals 0%) and corresponding with hot-rolled steel sheets coiled at 600 and 540 °C, respectively.
  • Table 1.4 shows data for steels 1A to 1D corresponding with the difference in Rp0.2 and Rm between coiling at 600 and 540 °C.
  • the data demonstrates that lowering the coiling temperature from 600 to 540 °C leads to a reduction in strength, in particular for Rp0.2.
  • This decrease in Rp0.2 with a decrease in coiling temperature is most pronounced for steels 1B and 1D with - compared with steel 1A - increased content in Ti and Mo as well as for steel 1C with increased Ti, Mo, and V.
  • This reduction in strength is largely attributed to a loss in precipitation strengthening as the reduction in coiling temperature reduces the kinetics needed to nucleate and form precipitates.
  • Tables 1.5 and 1.6 show the tensile data of hot-rolled steel sheets 1A to 1H coiled at 600 and 540 °C, respectively, when subjected to a batch annealing with different values for T top and t hold and without intermediate cold rolling.
  • the Rp0.2 decreases after batch annealing with t hold of 3 or 10 hours. This decrease in Rp0.2 is seen for all steels, i.e., steels 1A to 1H, and is believed to be related to a loss in precipitation strengthening due to substantial precipitate coarsening above 700 °C.
  • the observations above imply that it is possible to control precipitation during batch annealing by stimulating nucleation and growth of freshly formed precipitates during batch annealing on the one hand and by promoting coarsening of precipitates on the other hand.
  • the Rp0.2 of the steel sheet after batch annealing can in this way be increased or decreased compared with that of the Rp0.2 of the corresponding hot-rolled steel sheet.
  • This control over the degree of precipitation strengthening during batch annealing may be used to control the strength of the final batch-annealed steel sheet after hot-rolling without any intermediate cold-rolling step, but can also be used to control and improve recrystallization behaviour of cold-rolled steel sheets during batch annealing, i.e., promote substantial/partial (i.e., ⁇ 50%) or - preferably - full recrystallization already at a relatively low cold-rolling reduction (e.g., CR% ⁇ 30%).
  • This onset of substantial recrystallization is indicated by an increase in yield strength and tensile elongation as a function of cold-rolling reduction, e.g., CR% ⁇ 30%.
  • Table 1.1 Composition of steels (in wt.%) Steel C Mn Si P S Al Nb V Ti Mo N Ratio A* Ratio B* 1A 0.079 1.820 0.430 0.001 0.0015 0.065 0.037 ⁇ 0.001 0.096 0.150 0.0040 1.53 0.00 1B 0.105 1.820 0.430 ⁇ 0.001 0.0013 0.060 0.036 ⁇ 0.001 0.180 0.240 0.0060 1.65 0.00 1C 0.097 1.820 0.430 ⁇ 0.001 0.0013 0.060 0.036 0.150 0.120 0.240 0.0050 1.15 0.96 1D 0.097 1.810 0.430 ⁇ 0.001 0.0013 0.062 ⁇ 0.001 0.002 0.180 0.240 0.0060 1.50 0.02 1E 0.079 1.855 0.456 0.002 0.0020 0.074 0.041 0.005 0.118 0.154 0.0057 1.81 0.04 1F 0.097 1.861 0.462 0.002 0.0017 0.081 0.042 0.008 0.204
  • Example 2 Steels 2A to 2G having chemical compositions shown in Table 2.1 were hot rolled and further processed in a similar fashion as reported in Example 1. The tensile properties were measured in an identical way as reported in Example 1
  • steel 2A and 2B are comparative examples, since they contain less than 0.10 wt% V.
  • fraction recrystallized ferrite and the average grain size of the recrystallized ferrite are as follows.
  • the microstructures were characterised with Electron Back Scatter Diffraction (EBSD). To this purpose the following procedures were followed with respect to sample preparation, EBSD data collection and EBSD data evaluation.
  • EBSD Electron Back Scatter Diffraction
  • the EBSD measurements were conducted on cross sections parallel to the rolling direction (RD-ND plane) mounted in a conductive resin and mechanically polished to 1 ⁇ m. To obtain a fully deformation free surface, the final polishing step was conducted with colloidal silica (OPS).
  • OPS colloidal silica
  • the Scanning Electron Microscope (SEM) used for the EBSD measurements is a Zeiss Ultra 55 machine equipped with a Field Emission Gun (FEG-SEM) and an EDAX PEGASUS XM 4 HIKARI EBSD system.
  • EBSD scans were collected on the RD-ND plane of the sheets. The samples were placed under a 70° angle in the SEM. The acceleration voltage was 15kV with the high current option switched on. A 120 ⁇ m aperture was used and the working distance was 15 mm during scanning. To compensate for the high tilt angle of the sample, the dynamic focus correction was used during scanning.
  • the EBSD scans were captured using the TexSEM Laboratories (TSL) software OIM (Orientation Imaging Microscopy) Data Collection version 7.2 Typically, the following data collection settings were used: Hikari camera at 6 x 6 binning combined with standard background subtraction. The scan area was in all cases located at a position of 1 ⁇ 4 the sample thickness and care was taken to avoid as much as possible to include non-metallic inclusions in the scan area.
  • TSL TexSEM Laboratories
  • OIM Orientation Imaging Microscopy
  • the EBSD scan size was in all cases 100 x 100 ⁇ m, with a step size of 0.1 ⁇ m, and a scan rate of approximately 100 frames per second. Fe( ⁇ ) was used to index the Kikuchi patterns.
  • the Hough settings used during data collections were: Binned pattern size of circa 96; theta set size of 1; rho fraction of circa 90; maximum peak count of 10; minimum peak count of 5; Hough type set to classic; Hough resolution set to low; butterfly convolution mask of 9 x 9; peak symmetry of 0.5; minimum peak magnitude of 10; maximum peak distance of 20.
  • the EBSD scans were evaluated with TSL OIM Analysis software version "8.0 x64 [12-14-16]". Typically, the data sets were 90° rotated over the RD axis to get the scans in the proper orientation with respect to the measurement orientation.
  • a standard grain dilation clean-up was performed (Grain Tolerance Angle (GTA) of 5°, a minimum grain size of 5 pixels, criterion used that a grain must contain multiple rows for a single dilation iteration clean up).
  • GTA Gram Tolerance Angle
  • a pseudo-symmetry clean-up (GTA 5, axis ang 30@111) was applied.
  • Hot-rolled steel sheets Table 2.2 gives the A50 tensile properties of the hot-rolled steel sheets coiled at 600 or 540 °C of steels 2A to 2G. Labelling of the corresponding hot-rolled steel sheets is done in a similar fashion as previously in Example 1.
  • Tables 2.3 gives the tensile properties of batch-annealed steel sheets without any intermediate cold rolling (CR% equals 0%) and corresponding with hot-rolled steel sheets coiled at 600 and 540 °C, respectively.
  • Tables 2.4 and 2.5 provide the tensile properties of the batch-annealed steel sheets with intermediate cold-rolling reductions of 0 to 60% for associated with hot-rolled feedstock coiled at 600 and 540 °C, respectively.
  • Tables 2.4 and 2.5 provide the fraction recrystallized ferrite (in %) and the average grain size (in ⁇ m) of the recrystallized ferrite based on EBSD measurements.
  • Tables 2.4 and 2.5 show - apart from the tensile data - the fraction recrystallized ferrite and average ferrite grain size of the recrystallized ferrite of all batch-annealed steel sheets.
  • the former microstructural parameter is a clear and direct indication of the degree of recrystallization realized with batch-annealing.
  • Another indicator of recrystallization is the evolution of Rp0.2 after batch annealing as a function of cold-rolling reduction.
  • Table 2.1 Composition of steels (in wt.%) Steel C Mn Si P S Al Nb V Ti Mo N Ratio A Ratio B 2A 0.082 1.849 0.454 0.011 0.0013 0.071 0.044 0.006 0.125 0.156 0.0042 1.89 0.05 2B 0.081 1.873 0.456 0.011 0.0014 0.068 0.046 0.005 0.129 0.147 0.0054 2.08 0.04 2C 0.105 1.858 0.462 0.013 0.0010 0.072 0.046 0.149 0.133 0.245 0.0053 1.28 0.88 2D 0.102 1.850 0.452 0.012 0.0005 0.069 0.046 0.147 0.133 0.150 0.0049 2.09 0.72 2E 0.094 1.877 0.466 0.011 0.0010 0.071 0.046 0.101 0.134 0.248 0.0050 1.27 0.85 2F 0.080 1.422 0.450 0.012 0.0011 0.061 0.045 0.100 0.136 0.151 0.0046 2.11 0.93 2G

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Claims (14)

  1. Bande, tôle ou flan d'acier laminés à chaud et laminés à froid constitué des éléments suivants (en % en poids) :
    0,05 à 0,20 de C
    0,30 à 0,60 de Si
    0,80 à 2,50 de Mn
    0,01 à 0,10 d'Al
    0,07 à 0,25 de Ti
    0,10 à 0,35 de V
    0,05 à 0,40 de Mo
    0,02 à 0,10 de Nb
    éventuellement 0,01à 0,80 de Cr
    au plus 0,06 de P
    au plus 0,01 de S
    au plus 0,01 de N
    au plus 0,005 de Ca
    le reste étant constitué d'impuretés inévitables et de Fe,
    ledit acier comportant une microstructure d'au moins 90 % de ferrite, le reste étant de la cémentite et/ou de la perlite,
    et ledit acier comportant une microstructure de ferrite renforcée par précipitation contenant de la ferrite recristallisée, de la cémentite et/ou de la perlite et lesdits précipités dans ladite microstructure étant constitués de Ti, V, Mo et Nb.
  2. Bande, tôle ou flan d'acier selon la revendication 1, pour un ou plusieurs éléments, ladite plage suivante étant valable :
    0,06 à 0,17 de C
    0,90 à 2,30 de Mn
    0,02 à 0,09 d'Al
    0,08 à 0,22 de Ti
    0,12 à 0,30 de V
    0,08 à 0,35 de Mo
    0,03 à 0,09 de Nb
    éventuellement 0,01 à 0,60 de Cr
    au plus 0,04 de P
    au plus 0,005 de S
    au plus 0,008 de N
    au plus 0,003 de Ca.
  3. Bande, tôle ou flan d'acier selon la revendication 1 ou 2, pour un ou plusieurs éléments ladite plage suivante étant valable :
    0,07 à 0,14 de C
    1,20 à 2,00 de Mn
    0,04 à 0,08 d'Al
    0,10 à 0,20 de Ti
    0,13 à 0,25 de V
    0,10 à 0,30 de Mo
    0,03 à 0,08 de Nb
    éventuellement 0,01 à 0,40 de Cr
    au plus 0,02 de P
    au plus 0,003 de S
    0,002 à 0,007 de N
    au plus 0,001 de Ca.
  4. Bande, tôle ou flan d'acier selon l'une quelconque des revendications précédentes, Nb étant présent en une quantité comprise entre 0,03 et 0,09 %, de préférence entre 0,03 et 0,08 %.
  5. Bande, tôle ou flan d'acier selon l'une quelconque des revendications précédentes, lesdites teneurs en Nb, Ti et Mo représentées par un pourcentage en poids (% en poids) satisfont l'équation 0,8 ≤ Nb 93 = Ti 48 Mo 96 ≤ 2,2
    Figure imgb0034
     ou de préférence 0,8 ≤ Nb 93 + Ti 48 Mo 96 ≤ 1,3
    Figure imgb0035
  6. Bande, tôle ou flan d'acier selon l'une quelconque des revendications précédentes, lesdites teneurs en C, N, Ti, Mo, V et éventuellement Nb représentées en pourcentage en poids (% en poids) satisfont à l'équation 0,8 ≤ V 51 C * 12 ≤ 1,5
    Figure imgb0036
     avec C * = C − 12 × Ti * 48 − 12 × Nb 93 − 12 × Mo 96
    Figure imgb0037
     et Ti * = Ti − 48 × N 14 .
    Figure imgb0038
  7. Bande, tôle ou flan d'acier selon l'une quelconque des revendications précédentes, ledit acier après laminage à chaud et recuit comportant un taux d'élasticité supérieur ou égal à 0,9, et/ou une résistance à la traction supérieure ou égale à 900 MPa, de préférence supérieure ou égale à 950 MPa, et/ou un allongement A50/t0,2 supérieur ou égal à 9 %.
  8. Bande, tôle ou flan d'acier selon l'une quelconque des revendications précédentes, ledit acier après laminage à froid avec une réduction par laminage à froid supérieure ou égale à 30 % et recuit comportant une limite d'élasticité supérieure ou égale à 350 MPa, de préférence supérieure ou égale à 400 MPa, mieux encore supérieure ou égale à 450 MPa, toujours mieux encore supérieure ou égale à 500 MPa, idéalement supérieure ou égale à 550 MPa et un allongement A50/t0,2 qui est supérieur ou égal à 14 %.
  9. Bande, tôle ou flan d'acier selon l'une quelconque des revendications précédentes, ladite fraction de ferrite recristallisée à 1/4 de profondeur étant supérieure ou égale à 50 %, de préférence supérieure ou égale à 60 %, mieux encore supérieure ou égale à 70 %, idéalement supérieure ou égale à 80 %.
  10. Procédé permettant la production d'une bande d'acier comprenant les étapes de :
    • coulage d'une brame comportant la composition selon l'une quelconque des revendications précédentes,
    • réchauffage de la brame solidifiée à une température comprise entre 1150 et 1300°C,
    • terminaison du laminage à chaud à une température de laminage à chaud final d'un point de transformation Ar3 ou supérieur,
    • refroidissement de la bande d'acier laminée à chaud à la température d'enroulement à une vitesse de refroidissement moyenne de 10 à 150°C/s,
    • enroulement de la bande d'acier laminée à chaud dans la plage de températures comprise entre 450 et 580°C
    • ladite bande d'acier laminée à chaud étant recuite par lots après laminage à chaud pendant au moins 1 heure à une température maximale comprise entre 550 et 700°C,
    ou
    • ladite bande d'acier laminée à chaud étant laminée à froid et recuite par lots
    • pendant au moins 8 heures à une température maximale supérieure ou égale à 740°C, ou
    • pendant au moins 10 heures à une température maximale supérieure ou égale à 720°C, ou
    pendant au moins 14 heures à une température maximale supérieure ou égale à 700°C.
  11. Procédé selon la revendication 10, ladite bande d'acier laminée à chaud étant laminée à chaud avec une température de laminage final supérieure ou égale à 870°C, de préférence avec une température de laminage final supérieure ou égale à 900°C, mieux encore avec une température de laminage final supérieure ou égale à 940°C, et idéalement avec une température de laminage final supérieure ou égale à 980°C.
  12. Procédé selon la revendication 10 ou 11, ladite bande d'acier laminée à chaud après laminage final étant refroidie à la température d'enroulement avec une vitesse de refroidissement moyenne de 40 à 100°C/s.
  13. Procédé selon la revendication 10, 11 ou 12, ladite bande d'acier laminée à chaud étant enroulée dans la plage de températures comprise entre 480 et 560°C.
  14. Procédé selon l'une quelconque des revendications 10 à 13, la bande d'acier laminée à chaud étant recuite par lots après laminage à chaud pendant au moins 1 heure à une température maximale de 600 et 700°C, de préférence pendant au moins 1 heure à une température maximale comprise entre 650 et 700°C.
EP19721632.8A 2018-05-08 2019-05-07 Bande d'acier, feuille ou découpe présentant une formabilité améliorée et procédé pour produire une telle bande Active EP3791000B1 (fr)

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US7959747B2 (en) * 2004-11-24 2011-06-14 Nucor Corporation Method of making cold rolled dual phase steel sheet
EP1918396B1 (fr) * 2005-08-05 2014-11-12 JFE Steel Corporation Feuille d'acier à forte résistance à la traction et son procédé de production
KR101253703B1 (ko) * 2012-10-26 2013-04-12 주식회사 에스에이씨 배치식 소둔 열처리 설비
JP6032300B2 (ja) * 2015-02-03 2016-11-24 Ｊｆｅスチール株式会社 高強度冷延鋼板、高強度めっき鋼板、高強度溶融亜鉛めっき鋼板および高強度合金化溶融亜鉛めっき鋼板、並びにそれらの製造方法
CA3000554A1 (fr) * 2015-09-22 2017-03-30 Tata Steel Ijmuiden B.V. Tole d'acier a haute resistance laminee a chaud et pouvant etre formee au rouleau possedant une excellente aptitude au formage de bord tombe, et procede de production dudit acier

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