EP4394076A1 - Tôle d'acier laminée à froid ayant une aptitude au soudage, une résistance et une aptitude au formage excellentes, et son procédé de fabrication - Google Patents

Tôle d'acier laminée à froid ayant une aptitude au soudage, une résistance et une aptitude au formage excellentes, et son procédé de fabrication Download PDF

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EP4394076A1
EP4394076A1 EP22861736.1A EP22861736A EP4394076A1 EP 4394076 A1 EP4394076 A1 EP 4394076A1 EP 22861736 A EP22861736 A EP 22861736A EP 4394076 A1 EP4394076 A1 EP 4394076A1
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European Patent Office
Prior art keywords
steel sheet
cold
rolled steel
less
temperature
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EP22861736.1A
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German (de)
English (en)
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EP4394076A4 (fr
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Young-Roc Im
Sang-Ho Uhm
Young-Ha Kim
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Posco Holdings Inc
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Posco Co Ltd
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Publication of EP4394076A1 publication Critical patent/EP4394076A1/fr
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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
    • 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
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    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0278—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment 
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • 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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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
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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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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    • 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
    • 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
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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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • An aspect of the present disclosure is to provide a cold-rolled steel sheet having excellent weldability, strength, and formability, and a method of manufacturing the same.
  • Boron (B) may be an element added to secure hardenability.
  • Mn is added alone, a very large amount of Mn exceeding the Mn content range of the present disclosure should be added. This problem may be solved by adding 0.0001% or more of B.
  • B content exceeds 0.0001%, B may be excessively accumulated on a surface, impairing plating adhesion of a plating material. Therefore, the B content may range 0.0001 to 0.001%.
  • a lower limit of the B content is more preferably 0.00010%, and an upper limit of the B content is more preferably 0.0005%.
  • Titanium (Ti) may be an element added to secure strength of a steel sheet and refine a structure thereof. When less than 0.001% of Ti is added, it may be difficult to achieve strength improvement and structure refinement effects. When the Ti content exceeds 0.05%, castability may be impaired due to excessive formation of TiN, and recrystallization may be delayed due to local grain fixation, thereby impairing uniformity of the structure. Therefore, the Ti content may range 0.001 to 0.05%. A lower limit of the Ti content is more preferably 0.015%, or an upper limit of the Ti content is more preferably 0.03%.
  • Phosphorus (P) may exist as an impurity in steel, and it may be advantageous to control its content as low as possible. Therefore, a lower limit of the P content may exclude 0% (i.e., exceed 0%), taking into account cases where P is inevitably included. P may be sometimes added intentionally to increase strength of the steel. When P is added excessively, toughness of the steel may deteriorate. Therefore, to prevent this, the present disclosure may restrict an upper limit thereof to 0.04%.
  • the lower limit of the P content is more preferably 0.002%, or the upper limit of the P content is more preferably 0.0173%.
  • S Sulfur
  • S may exist as an impurity in steel, and it may be advantageous to control its content as low as possible. Therefore, a lower limit of the S content may exclude 0% (i.e. exceed 0%), taking into account cases where S is inevitably included. Since S deteriorates ductility and impact properties of the steel, an upper limit thereof may be restricted to have 0.01%.
  • the lower limit of the S content is more preferably 0.0009%, or the upper limit of the S content is more preferably 0.0021%.
  • nitrogen (N) may be included in a steel material as an impurity, and it may be advantageous to control its content as low as possible. Therefore, a lower limit of the N content excludes 0% (i.e. exceeds 0%), taking into account cases where N is inevitably included.
  • An upper limit of the N content may be restricted to have 0.01%. It is more preferable that the lower limit of the N content is 0.0005%. Additionally, the upper limit of the N content is more preferably 0.007%, even more preferably 0.006%, and most preferably 0.0052%.
  • the remainder may include Fe and inevitable impurities.
  • the inevitable impurities may be unintentionally mixed in a normal steel manufacturing process, and may not thus be completely excluded, and any engineer in the normal steel manufacturing field may easily understand their meaning.
  • the present disclosure may not completely exclude addition of compositions other than the steel compositions mentioned above.
  • the cold-rolled steel sheet may further include optionally one or more selected from the group consisting of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).
  • Copper (Cu) and nickel (Ni) may be elements that increase strength of a steel material.
  • the elements may be elements that increase strength and hardenability of the steel material. When the elements are added in excessive amounts, the elements may exceed a target strength grade. Since the elements may be expensive elements, from an economic standpoint, upper limits thereof may be limited to 0.1% or less, respectively. Since Cu and Ni act as solid solution strengthening elements, when adding one or more of Cu and Ni, a solid solution strengthening effect may be minimal when less than 0.03% is added. Therefore, it is preferable to add 0.03% or more of each.
  • the cold-rolled steel sheet may optionally further include V: 0.05% or less (excluding 0%).
  • Vanadium (V) may increase strength of a steel material even with addition of a small amount thereof, but its effect on improving elongation may not be significant. Therefore, it is desirable to control its content to 0.05% or less.
  • the V content is more preferably 0.04% or less, and even more preferably 0.03% or less.
  • the cold-rolled steel sheet may be provided to secure excellent formability even at a tensile strength (TS) of 980 MPa or higher, and in particular, to obtain high local formability, the steel sheet should reduce a difference in hardness between microstructure phases.
  • TS tensile strength
  • the composition is controlled such that a value defined by the following Relationship 1 satisfies 70 or more while satisfying the above-described alloy composition under typical annealing heating conditions, it is confirmed that an austenite single phase is obtained and a ferrite fraction may be reduced to 10 area% or less. When the ferrite fraction exceeds 10 area%, there may be risks that yield strength is lowered and a hole expansion ratio deteriorates.
  • a lower limit of the ferrite fraction may be 2 area%, and an upper limit of the ferrite fraction may be 7 area%.
  • the value defined by Relationship 1 may be satisfied to have 70 or more, to avoid a soft ferrite phase.
  • a bainite phase which may be soft, next to ferrite, is not sufficiently introduced, it may be difficult to secure ductility of a steel material.
  • a lower limit of the value defined by Relationship 1 may be 75.7, or an upper limit of the value defined by Relationship 1 may be 90.
  • the value defined by Relationship 2 may be controlled to satisfy 270 or more and 330 or less while satisfying the above-described alloy composition.
  • bainite which has an MA phase (martensite-austenite aggregate) as a second phase, may be formed in 35 area% or more to less than 70 area%, making it possible to further improve hole expansion ratio. It is believed that the reason why strength of the bainite phase is secured close to that of martensite is because it contains a relatively hard second phase MA phase internally through carbon distribution.
  • a lower limit of the value defined by Relationship 2 is 286, or an upper limit of the value defined by Relationship 2 is 311.
  • the retained austenite may be a structure that increases elongation of the steel material through a TRIP effect. As a fraction thereof increases, elongation thereof may increase, and a fraction of the retained austenite may exceed 1 area% to obtain the required level of elongation. To obtain austenite exceeding 5 area%, a large amount of C and Si should be added, and in this case, spot welding LME resistance may deteriorate. Therefore, in the present disclosure, the fraction of the retained austenite may be controlled to 5 area% or less. In this case, in terms of further improving the above-mentioned effect, more preferably, a lower limit of the fraction of the retained austenite is 2 area%, or an upper limit of the fraction of the retained austenite is 4 area%.
  • a fraction of the bainite may be 35 area% or more and less than 70 area%.
  • the fraction of martensite or ferrite may be relatively high, which may cause a problem of low hole expansion ratio, and when the fraction of the bainite is 70 area% or more, the fraction of the martensite may be lowered, which may cause a problem of low insufficient strength.
  • a lower limit of the fraction of the bainite is 45 area%, or an upper limit of the fraction of the bainite is 63%.
  • the cold-rolled steel sheet may further include other phases in addition to the microstructure described above.
  • the other phases may include martensite-austenite (MA) or the like, and for example, martensite-austenite (MA) or the like present in bainite may exist.
  • spot weldability may deteriorate, and in particular, when spot welding is performed on a galvanized steel sheet, liquid metal embrittlement (LME) may be caused.
  • LME liquid metal embrittlement
  • spot welding of a steel material may be performed on or below a minimum current at which expulsion occurs, and the minimum current at which expulsion occurs may be referred to as a condition that provides the highest amount of heat input when performing actual spot welding.
  • an AE value defined as a difference between a minimum current value for generating LME minus a minimum current value for generating scattering, may be a positive value.
  • welding may be performed on or below the minimum current value for generating scattering during actual spot welding, and at this time, when LME does not occur, it may be determined that the AE value is 0 or more.
  • the AE value may have a unit of kA.
  • an alloy component condition having excellent LME resistance i.e., the AE value is 0 or more
  • a content relationship between C, Si, and Al needed to be controlled such that a value defined by Relationship 3 below satisfies 1.8 or less: 5 ⁇ C + Si + 0.5 ⁇ Al (In Relationship 3 above, [C], [Si], and [Al] represent weight percentages of the elements in parentheses, respectively.)
  • the above-described cold-rolled steel sheet may have a tensile strength (TS) of 980 MPa or more (preferably 980 to 1150 MPa, more preferably 980 to 1075 MPa), a yield strength (YS) of 740 to 950 MPa (more preferably, 790 to 920 MPa), a hole expansion ratio (HER) of 45% or more (more preferably, 50 to 65%), and an elongation (El) of 12% or more (more preferably, 12 to 20%), to secure excellent strength, ductility, and a hole expansion ratio at the same time.
  • TS tensile strength
  • YiS yield strength
  • HER hole expansion ratio
  • El elongation
  • the cold-rolled steel sheet of the present disclosure may have a hot-dip galvanized layer formed on at least one surface.
  • a hot-dip galvanized layer formed on at least one surface.
  • the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer alloyed with some alloy components of the steel sheet.
  • a slab having the above-described alloy composition may be heated.
  • a heating temperature may be 1150 to 1250°C.
  • the slab heating temperature is less than 1150°C, it is not possible to perform hot-rolling, which may be the next step.
  • the slab heating temperature exceeds 1250°C, a lot of energy may be unnecessarily consumed to increase the slab temperature. Therefore, the slab heating temperature ranges from 1150 to 1250°C.
  • a lower limit of the slab heating temperature is more preferably 1170°C, and even more preferably 1180°C.
  • An upper limit of the slab heating temperature is more preferably 1230°C, and even more preferably 1220°C.
  • the hot-rolled steel sheet may be coiled at 450 to 700°C.
  • the coiling temperature (hereinafter also referred to as 'CT') exceeds 700°C, there may be a disadvantage in that coarse internal oxidation of hot-rolling occurs and surface properties deteriorate.
  • the coiling temperature is less than 450°C, it may correspond to a transition boiling range, which has a disadvantage of worsening controllability of the coiling temperature and deteriorating a shape of the steel sheet.
  • a lower limit of the coiling temperature is more preferably 480°C, and even more preferably 500°C.
  • An upper limit of the coiling temperature is more preferably 670°C, and even more preferably 640°C.
  • cooling may be performed to the coiling temperature at an average cooling rate of 10 to 100°C/s.
  • the average cooling rate is less than 10°C/s, hot-rolling productivity may be low and a cooling medium with low cooling ability should be deliberately selected during actual production, and when the average cooling rate exceeds 100°C/s, there may be disadvantageous that a temperature deviation in the steel sheet is not uniform to deteriorate a shape and excessively increase strength of the steel sheet. Therefore, the average cooling rate may range 10 to 100°C/s.
  • the coiled hot-rolled steel sheet may be cold-rolled.
  • a cold-rolling reduction rate may be 30 to 60%.
  • the cold-rolling reduction rate is less than 30%, it may be difficult to secure target thickness accuracy and it may be difficult to correct a shape of the steel sheet.
  • the cold-rolling reduction ratio exceeds 60%, possibility of cracks occurring at an edge of the steel sheet may increase, and the cold-rolling load may excessively increase. Therefore, the cold-rolling reduction ratio may be 30 to 60%.
  • the continuously annealed steel sheet may be cooled to a primary cooling end temperature of 450 to 600°C (hereinafter also referred to as 'SCS') at an average cooling rate of less than 10°C/s (more preferably, between from 1°C/s or more to less than 10°C/s).
  • the primary cooling end temperature may be defined as a time point at which secondary cooling (quick cooling) is initiated by additionally applying quenching equipment that was not applied in primary cooling.
  • a primary cooling rate When a primary cooling rate is less than 1°C/s, an amount of precipitation of the ferrite phase increases during cooling, making it difficult to obtain high-strength steel, and when a primary cooling rate exceeds 10°C/s, an amount of cooling in the secondary cooling may increase to increase a final temperature deviation and a material deviation. More preferably, in terms of improving the above-mentioned effect, a lower limit of the primary cooling rate may be 3°C/s, and an upper limit of the primary cooling rate may be 8°C/s.
  • the reheated steel sheet may be additionally subjected to a hot-dip galvanizing process, an alloyed hot-dip galvanizing process, and a temper-rolling process.
  • plating the reheated steel sheet in a zinc plating bath in a temperature range of 450 to 470°C may be further included.
  • the slab was reheated at 1180 to 1220°C, and subjected to hot-rolling, coiling, annealing, primary cooling, secondary cooling, reheating, and hot-dip galvanizing (GI) under conditions illustrated in Table 2 below, to manufacture a cold-rolled steel sheet.
  • GI hot-dip galvanizing
  • some steel sheets were alloyed and heat treated under alloying heat treatment temperature (GA) conditions listed in Table 2 below.
  • martensite-austenite (MA) present in bainite of a cold-rolled steel sheet obtained in Inventive Example 1 of the present application a photograph of a cross-section in a thickness direction was observed at 5,000X magnification with a scanning electron microscope (SEM) is illustrated in FIG. 1 .
  • SEM scanning electron microscope

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EP22861736.1A 2021-08-26 2022-08-25 Tôle d'acier laminée à froid ayant une aptitude au soudage, une résistance et une aptitude au formage excellentes, et son procédé de fabrication Pending EP4394076A4 (fr)

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