WO2023121187A1 - Tôle d'acier à haute résistance et à formabilité élevée ayant une excellente soudabilité par points, et son procédé de fabrication - Google Patents

Tôle d'acier à haute résistance et à formabilité élevée ayant une excellente soudabilité par points, et son procédé de fabrication Download PDF

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WO2023121187A1
WO2023121187A1 PCT/KR2022/020731 KR2022020731W WO2023121187A1 WO 2023121187 A1 WO2023121187 A1 WO 2023121187A1 KR 2022020731 W KR2022020731 W KR 2022020731W WO 2023121187 A1 WO2023121187 A1 WO 2023121187A1
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steel sheet
less
strength
spot weldability
rolling
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PCT/KR2022/020731
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English (en)
Korean (ko)
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김성규
한태교
박준호
조경래
한상호
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주식회사 포스코
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    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/667—Quenching devices for spray quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—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
    • 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
    • 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
    • C21D2211/002—Bainite
    • 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 present invention relates to a steel sheet used in automobiles, etc., and relates to a steel sheet having high strength and high formability as well as excellent spot weldability and a manufacturing method thereof.
  • DP steel dual phase steel
  • TRIP steel transformation induced plasticity steel
  • CP steel complex phase steel
  • FB steel ferrite-bainite steel Steel
  • LME liquid metal embrittlement
  • welding such as spot welding is widely used as a method of defective automobile parts.
  • the temperature of the heat-affected zone of the material rises during welding, melting of the coating layer occurs, and tensile stress due to electrode pressure is generated, cracks may occur due to embrittlement of liquefied metal.
  • the melting point is low, causing a problem that molten zinc causes liquefied metal embrittlement cracking of the welded part.
  • One aspect of the present invention is to provide a steel sheet having excellent strength and formability while ensuring excellent spot weldability and a method for manufacturing the same.
  • C 0.05 to 0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0 to 2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1% or less (excluding 0), Cr: 1.5% or less (excluding 0), P: 0.1% or less, S: 0.01% or less, the remainder including Fe and unavoidable impurities,
  • the microstructure at the 1/4th position of the steel plate thickness (t) includes 65 to 85% of the soft phase and the rest of the hard phase as an area fraction
  • It relates to a high-strength, high-formability steel sheet having excellent spot weldability, including a sound layer in the surface layer portion of the steel sheet, and the thickness of the sound layer is 5 to 50 ⁇ m.
  • C 0.05 to 0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0 to 2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1 % or less (excluding 0), Cr: 1.5% or less (excluding 0), P: 0.1% or less, S: 0.01% or less, the rest of the steel slab containing Fe and unavoidable impurities in the temperature range of 1100 to 1300 ° C heating with;
  • It relates to a method for producing a high-strength, high-formability steel sheet having excellent spot weldability.
  • a steel sheet having high strength and formability particularly excellent balance between strength and ductility (TS*El), thereby preventing processing defects such as cracks or wrinkles during press forming, and thus having a complex shape. It can be suitably applied to parts such as structures that require machining.
  • product quality can be improved by improving spot weldability and reducing the occurrence of liquefied metal embrittlement (LME).
  • 1 is a diagram showing cracks generated during spot welding of an alloyed hot-dip galvanized steel sheet.
  • Example 2 is a photograph of a surface layer portion of a steel sheet observed in Inventive Example 1 among Examples of the present invention.
  • Figure 3 is a photograph of the surface layer of the steel sheet observed in Comparative Example 1 among Examples of the present invention.
  • FIG. 4 is a schematic diagram illustrating an example of a method for measuring the aspect ratio of a hard phase.
  • 5 is a graph showing material temperature change when water cooling is applied and when water cooling is not applied during finish hot rolling in the present invention.
  • Figure 6 is a graph showing the heat treatment step of the continuous annealing process.
  • high-strength steels used as automobile materials are representative, such as dual phase steel (DP steel), transformation induced plasticity steel (TRIP steel), composite structure steel ( Complex Phase Steel (CP Steel) and Ferrite-Bainite Steel (FB Steel).
  • DP steel dual phase steel
  • TRIP steel transformation induced plasticity steel
  • CP Steel Complex Phase Steel
  • FB Steel Ferrite-Bainite Steel
  • LME liquid metal embrittlement
  • Causes of the LME include loads above critical loads, molten metals, and the appearance of austenite.
  • spot welding as current is applied, the temperature of the steel rises due to resistance heat, and zinc with a low melting point begins to melt first. Thereafter, the steel is transformed into austenite.
  • the lower the austenite formation temperature the longer the contact time between the molten zinc and the austenite structure of the steel at the surface layer.
  • thermal stress or external stress is applied, the austenite grain boundary in the part where the stress is concentrated glides and deforms.
  • the interface energy between the steel and the molten zinc is lower than the austenite grain boundary energy, intergranular cracks occur as the molten zinc penetrates the austenite grain boundary. This eventually leads to poor spot weldability.
  • Figure 1 shows the cracks that appear when welding 1200 MPa class alloyed galvanized high-strength steel on the market.
  • the alloy composition of the steel sheet according to the present invention is, by weight%, C: 0.05 to 0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0 to 2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1 % or less (excluding 0), Cr: 1.5% or less (excluding 0), P: 0.1% or less, S: 0.01% or less, the remainder including Fe and unavoidable impurities.
  • C 0.05 to 0.10%
  • Si 0.3% or less
  • Mn 2.0 to 2.5%
  • Ti 0.05% or less
  • Nb 0.1 % or less
  • Cr 1.5% or less
  • P 0.1% or less
  • S 0.01% or less
  • the C is an important element added for solid solution strengthening, and this C contributes to improving the strength of steel by forming fine precipitates in combination with precipitated elements.
  • the content of C exceeds 0.10%, hardenability increases, and as martensite is formed during cooling during steel production, strength excessively increases, while elongation may decrease. In addition, weldability is inferior, and there is a concern that welding defects may occur during processing into parts.
  • the C content is less than 0.05%, it may be difficult to secure a target level of strength. More advantageously, it is preferably 0.06 to 0.08%.
  • Si is a ferrite stabilizing element, and it is advantageous to secure a target level of ferrite fraction by accelerating ferrite transformation. In addition, it is effective in increasing the strength of ferrite due to its excellent solid solution strengthening ability, and is a useful element in securing strength without reducing the ductility of steel.
  • the Si content exceeds 0.3%, the solid solution strengthening effect is excessive, rather, the ductility is lowered, and surface scale defects are caused to adversely affect the plating surface quality and deteriorate the conversion processability. More advantageously, it is preferably 0.1% or less.
  • the Mn is an element that is advantageous for preventing hot brittleness due to the formation of FeS by precipitating sulfur (S) in steel as MnS, and for solid solution strengthening of steel. If the Mn content is less than 2.0%, the above effect cannot be obtained, and it is difficult to secure a target level of strength. On the other hand, when the content exceeds 2.5%, problems such as weldability and hot rolling are likely to occur, and at the same time, as martensite is more easily formed due to an increase in hardenability, there is a risk of deterioration in ductility. In addition, there is a problem in that the risk of occurrence of defects such as processing cracks increases due to excessive formation of a Mn-band of Mn oxide in the structure. In addition, during annealing, Mn oxide is eluted on the surface, which significantly impairs plating properties. More advantageously, it is preferably 2.2 to 2.4%.
  • Ti is an element that forms fine carbides and contributes to securing yield strength and tensile strength.
  • Ti has an effect of suppressing the formation of AlN in Al inevitably present in steel by precipitating N in steel as TiN, thereby reducing the possibility of cracking during continuous casting.
  • the Ti content exceeds 0.05%, coarse carbides are precipitated, and there is a risk of reduction in strength and elongation due to a decrease in carbon content in steel.
  • the Ti content is preferably 0.05% or less, and preferably more than 0%.
  • Nb is an element that is segregated at austenite grain boundaries to suppress coarsening of austenite crystal grains during annealing heat treatment and to form fine carbides to contribute to strength improvement.
  • the Nb content is preferably 0.1% or less, and preferably greater than 0%.
  • Cr is an element that facilitates the formation of bainite, suppresses the formation of martensite during annealing heat treatment, and forms fine carbides to contribute to strength improvement.
  • the Cr content exceeds 1.5%, bainite is excessively formed and elongation decreases.
  • carbides are formed at grain boundaries, strength and elongation may be inferior, and manufacturing cost increases. Therefore, the Cr content is preferably 1.5% or less, and preferably greater than 0%.
  • Phosphorus (P) 0.1% or less
  • P is a substitutional element having the greatest solid-solution strengthening effect, and is an element that is advantageous for improving in-plane anisotropy and securing strength without significantly deteriorating formability.
  • the amount of P is 0.1% or less, and 0% may be excluded in consideration of an unavoidable level.
  • S is an element that is unavoidably added as an impurity element in steel, and since it inhibits ductility, it is preferable to manage its content as low as possible. In particular, since S has a problem of increasing the possibility of generating red heat brittleness, it is preferable to manage its content to 0.01% or less. However, considering the level that is unavoidably included, 0% can be excluded.
  • the rest includes iron (Fe), and since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the art during the manufacturing process, not all of them are specifically mentioned in the present specification.
  • the microstructure of a steel sheet refers to what is observed at a point in the thickness direction 1/4t (1/4 position of the steel sheet thickness (t)), and is used to describe physical properties such as strength and formability based on this.
  • the LME characteristics generated by the surface microstructure of the steel sheet and the internal microstructure that determines physical properties such as strength are described, and the microstructure of the surface layer and the internal microstructure are separately described.
  • the internal microstructure refers to the microstructure in 1/4t, and unless otherwise specified, the microstructure refers to the internal microstructure.
  • the microstructure of steel is determined immediately after rolling depending on whether the temperature at which the rolling proceeds is the austenite phase or the ferrite phase, and then transformation occurs according to cooling conditions to form the final microstructure.
  • the hot rolling process is a step in which dynamic recrystallization occurs during rolling.
  • the microstructure immediately after rolling is a recrystallized austenite single-phase structure when the rolling temperature is high, and a recrystallized austenite/ferrite mixture when the rolling temperature is low. phase, and if the temperature is very low, a ferrite single phase can be obtained.
  • one embodiment of the present invention relates to a technique for securing strength and formability by controlling the internal microstructure and improving the LME by controlling the microstructure of the surface layer.
  • the steel sheet includes a sound layer in a surface layer portion.
  • the dry layer is a ferrite main structure composed of 95% or more of ferrite in area fraction, and it is effective that the crystal grains of the ferrite have a size of 6 to 20 ⁇ m.
  • the thickness of the dry layer is 5 to 50 ⁇ m.
  • the sound layer of the surface layer is less than 5 ⁇ m, it is difficult to improve the LME, and when it exceeds 50 ⁇ m, it is difficult to sufficiently achieve physical properties such as strength of the steel sheet.
  • 2 and 3 are observations of the surface layer of Example 1 and Comparative Example 1 in Examples to be described later, respectively. In FIG. 2, a sound layer having coarse crystal grains was confirmed in the surface layer, but this cannot be observed in FIG. 3 .
  • the microstructure of the steel sheet (position 1/4 of the thickness of the steel sheet (t)) is composed of a hard phase and a soft phase, and in particular, ferrite recrystallization is maximized by an optimized annealing process, and finally, the recrystallized ferrite base has a hard phase of bainite and It is preferable to include a structure in which the martensite phase is uniformly distributed.
  • the hard phase is mainly martensite, and a small amount of bainite is included to mean a mixed phase, and the soft phase means a ferrite phase.
  • the deformation characteristics determine the formability of the soft phase and the strength of the hard phase.
  • the hard phase preferably contains 15 to 35% in area fraction. If the fraction of the hard phase is too high, the strength is high but the elongation is low, and if the fraction of the soft phase is high, the elongation is high but the strength is low. In order to secure the strength of 780 MPa or more provided by the present invention, it is preferable that the hard phase contains 15% or more in area fraction, and it is preferable not to exceed 35% to secure formability.
  • the area fraction of the soft phase is 65 to 85%.
  • the soft phase ferrite may be classified into recrystallized ferrite and non-recrystallized ferrite. As shown in FIG. 4, the difference between recrystallized ferrite and non-recrystallized ferrite can be distinguished by the aspect ratio of grain size with respect to the rolling direction. Unrecrystallized ferrite has a large aspect ratio, as shown in FIG. 4(b), and when analyzed in detail, a linear deformed structure within the ferrite grains is observed.
  • recrystallized ferrite is advantageous in securing formability, it is preferable that recrystallized ferrite in the soft phase is 60% or more, and non-recrystallized ferrite is soft, but when the fraction is high, formability is reduced, so it is preferably 5% or less.
  • the aspect ratio of the hard phase is 1.2 or less.
  • the aspect ratio means the ratio (b/a) of the long axis (b) and the short axis (a) of the crystal grain size in the rolling direction
  • the aspect ratio of the hard phase is the hard phase.
  • This is the aspect ratio of the tissue formed by stretching the phase in the rolling direction.
  • An increase in the aspect ratio of the hard phase adversely affects bending, which is important for deformation resistance in the thickness direction.
  • the aspect ratio of the hard phase is increased, the hole expandability is lowered. Therefore, since it is important to manage the aspect ratio of the hard phase as low as possible, it is preferable not to exceed 1.2.
  • the steel sheet of the present invention has a high tensile strength (TS) of 780 MPa or more and an elongation of 18% or more, so that excellent strength and formability can be secured.
  • TS tensile strength
  • the steel sheet of the present invention may further include a plating layer for improving corrosion resistance, for example, a zinc-based plating layer.
  • a plating layer for improving corrosion resistance for example, a zinc-based plating layer.
  • Most steel sheets for automobiles can form hot-dip plating and electroplating layers on base steel sheets, and the present invention relates to a technology that can include both a plating layer formed by hot-dip plating and a plating layer formed by electroplating.
  • the thickness of the plating layer may vary as needed, but may be, for example, 10 ⁇ m or less.
  • the steel sheet of the present invention may be manufactured by first preparing a steel slab, heating it, performing hot rolling, then winding and cooling, cold rolling, and annealing. Meanwhile, a process of forming a plating layer may be further included as needed.
  • hot rolling is controlled, and after deformation in cold rolling, an appropriate structure is formed in the annealing process.
  • each step will be described in detail.
  • the above-mentioned alloy composition that is, by weight, C: 0.05 ⁇ 0.10%, Si: 0.3% or less (excluding 0), Mn: 2.0 ⁇ 2.5%, Ti: 0.05% or less (excluding 0), Nb: 0.1%
  • the heating process conditions are not particularly limited, and any method or condition commonly used in the art to which the present invention belongs may be used. As an example, it is preferable to heat to a temperature range of 1100 ⁇ 1300 °C.
  • the heated steel slab is hot-rolled to produce a hot-rolled steel sheet.
  • a method for obtaining an appropriate surface layer portion a method of differentiating the temperature of the surface of the steel slab and the thickness 1/4 position during hot rolling is proposed.
  • the temperature of the center (thickness 1/4 position), that is, the temperature of the material itself, finish hot rolling is performed in the temperature range of Ar3 to 1000 ° C, and the surface temperature of the material is lower than Ar3 for a predetermined time during finish hot rolling. It is preferable to perform it so that it may become a temperature.
  • the temperature at the exit side of the material itself during the finish hot rolling is less than Ar3, the strength of the material increases and the hot deformation resistance during rolling rapidly increases, and when it exceeds 1000 ° C, the rolling load is relatively reduced, which is advantageous for productivity, A thick oxide scale may be generated to form defects in the surface layer portion. More preferably, it can be performed in the temperature range of 760-940 degreeC.
  • the surface temperature of the material is an important process to form a sound layer in the surface layer portion, and the surface layer temperature is lower than Ar3 so that ferrite is easily formed by recrystallization during hot rolling, and the thickness 1/4 position is excessive rolling load prevent this from happening. That is, rolling is simultaneously performed so that the surface temperature is lower than Ar3 for a predetermined period of time, so that ferrite recrystallizes in the surface layer portion to form a coarse ferrite sound layer.
  • the method for making the surface temperature lower than Ar3 is not particularly limited, but for example, a method of spraying water during or between rolling passes so that the surface temperature is lower than Ar3 for a predetermined time can be applied. there is.
  • FIG. 5 shows a time-temperature graph of those in which water cooling is applied so that the surface temperature is equal to or less than Ar3 as in the present invention during hot rolling and those in which it is not.
  • FIG. 5 when water cooling is not applied, both the center and the surface are rolled at a temperature of Ar3 or higher, but when water cooling is applied, it can be seen that the surface temperature is lowered to Ar3 or lower for a predetermined time.
  • the hot-rolled steel sheet manufactured by the hot rolling may be wound into a coil shape.
  • the winding may be performed in a temperature range of 400 to 700 °C.
  • the coiling temperature is less than 400° C.
  • excessive formation of martensite or bainite causes an excessive increase in strength of the hot-rolled steel sheet, which may cause problems such as shape defects due to load during subsequent cold rolling.
  • the coiling temperature exceeds 700 ° C., surface scale may increase and pickling performance may be deteriorated.
  • the rolled hot-rolled steel sheet it is preferable to cool the rolled hot-rolled steel sheet to room temperature at an average cooling rate of 0.1° C./s or less (excluding 0).
  • the rolled hot-rolled steel sheet may be cooled after passing through processes such as transfer and stacking, and the process prior to cooling is not limited thereto.
  • a hot-rolled steel sheet in which carbides serving as austenite nucleation sites are finely dispersed can be obtained.
  • a process of pickling the surface of the hot-rolled steel sheet to remove surface scale may be additionally performed prior to subsequent cold rolling.
  • the pickling method is not particularly limited, and it is sufficient to perform it in a method commonly performed in the technical field to which the present invention belongs.
  • the hot-rolled steel sheet wound as described above may be cold-rolled at a constant reduction ratio at room temperature to produce a cold-rolled steel sheet.
  • the method of performing the cold rolling is not particularly limited in the present invention, and any method can be applied as long as it is performed in the technical field to which the present invention belongs.
  • TCM Total Cold Rolling Mill
  • ZRM Sendzimir rolling mill
  • TCM is a reversible rolling, and since low manufacturing cost and mass production are possible, it has the advantage of excellent productivity, but has the disadvantage of being somewhat restricted in applying a rolling force.
  • ZRM is a reversible batch type, and has the disadvantage of low productivity, but has the advantage of being somewhat easy to apply the pressing force.
  • the reduction rate of the cold rolling is an important operation factor for improving various physical properties by improving the phase transformation of steel, controlling the reduction rate is particularly important for securing quality.
  • the manufactured cold-rolled steel sheet is subjected to continuous annealing.
  • the continuous annealing treatment may be performed, for example, in a continuous annealing furnace (CAL).
  • An example of the heat treatment step of the continuous annealing process is shown as a graph in FIG.
  • the temperature of each section measures the temperature attached to the point at which each section ends, the temperature means the temperature at the point where each section ends.
  • the temperature of the quench zone (RCS) is the temperature of the section where the quench zone ends, and is indicated by 4 in the case of FIG. 6 .
  • the steel sheet is heated at a constant temperature increase rate, and as the temperature of the steel sheet increases, dislocation recovery, cementite precipitation, ferrite recrystallization, and reverse transformation occur.
  • the sheet-threading speed varies depending on the thickness and width of the steel sheet, and the change in the microstructure for each temperature section may vary according to the initial hot-rolled structure and the cold rolling reduction rate.
  • the crack zone (SS) section When entering the cracking zone (SS) section, it is maintained at a constant temperature for a certain period of time, and at this time, the reverse transformation of austenite or single-phase austenite is observed according to the annealing temperature.
  • the crack zone (SS) section is known as one of the sections that consume the most energy in an annealing furnace.
  • SCS slow cooling zone
  • RCS rapid cooling zone
  • Bainite may be formed during cooling depending on the RCS set temperature and hardenability. there is.
  • the temperature of the crack zone (SS) is closely related to the phase transformation. Factors that affect phase transformation and change in the state of matter include temperature, pressure, composition, and the like, and when the composition is determined, it can be adjusted through temperature and pressure. In particular, the higher the temperature and pressure, the faster the phase transformation during heating in the annealing furnace, but the higher the temperature, the higher the energy cost and the higher the carbon emissions such as carbon dioxide after combustion, which is not environmentally friendly.
  • the variable compared to the pressure in the steel manufacturing process is the cold reduction rate. If the cold reduction rate is increased at the same temperature, the phase transformation proceeds quickly. Using this principle, in the present invention, the cold rolling reduction is performed at 70 to 90% higher than the conventional method.
  • the soaking zone temperature in a typical annealing process is generally in the range of Ac1+30°C to Ac3-30°C.
  • the annealing process of the present invention heats to a temperature range of Ac1 ⁇ Ac1 + 30 °C, It is desirable to keep The present invention can reduce hardness and improve workability through recrystallization and phase transformation even in the above temperature range.
  • the stepwise cooling may be performed in a slow cooling zone (SCS) and a rapid cooling zone (RCS).
  • SCS slow cooling zone
  • RCS rapid cooling zone
  • the end temperature of the slow cooling is less than 650 ° C, the diffusion activity of carbon is low due to too low temperature, and the carbon concentration in ferrite increases, while the fraction of hard phase becomes excessive as the carbon concentration in austenite decreases, increasing the yield ratio, , thereby increasing the tendency to crack during machining.
  • a problem in that the shape of the plate becomes non-uniform may occur because the temperature difference with the cracking zone is too large.
  • the end temperature exceeds 700° C., there is a disadvantage in that an excessively high cooling rate is required during subsequent cooling (rapid cooling).
  • the average cooling rate during slow cooling exceeds 10 ° C / s, carbon diffusion cannot sufficiently occur, and it is preferable to cool at an average cooling rate of 1 ° C / s or more in consideration of productivity.
  • the quench cooling end temperature is less than 300 ° C, there is a concern that the cooling deviation occurs in the width and length directions of the steel sheet, resulting in poor plate shape, and if it exceeds 580 ° C, it is impossible to sufficiently secure a hard phase and the strength is lowered.
  • the average cooling rate during the rapid cooling is less than 5 ° C / s, there is a risk that the fraction of the hard phase will be excessive, and if it exceeds 50 ° C / s, there is a risk that the hard phase will be insufficient.
  • overaging treatment (OAS) may be performed if necessary.
  • the overaging treatment is a process of holding for a certain period of time after the quenching end temperature.
  • the overaging treatment does not perform a separate treatment, and can be regarded as the same as a kind of air cooling treatment.
  • the overaging treatment may be performed for 200 to 800 seconds.
  • a plating layer may be formed through a plating process.
  • the plating includes a hot-dip plating method in which a plating bath is installed during annealing to immerse the steel sheet in a molten plating solution, and a method of electroplating in an electrolyte solution after annealing is completed. LME generated during spot welding occurs when there is molten zinc. Therefore, it is irrelevant to the manufacturing method of coated steel sheet.
  • the plating conditions are generally not particularly limited as long as those known in the art to which the present invention pertains.
  • each steel slab was heated at 1200 ° C. for 1 hour, and then , The material center temperature was set to a temperature of 800 ⁇ 920 °C under the conditions of the finish rolling temperature in Table 2 below, and a process of spraying water during finish rolling was applied to the surface layer.
  • the prepared hot-rolled steel sheet was cooled at a cooling rate of 0.1° C./s and wound at 650° C. Thereafter, the rolled hot-rolled steel sheet was cold-rolled at a reduction ratio of 40% and 80% to manufacture a cold-rolled steel sheet.
  • the annealing temperature was heated to a temperature range of 730 to 860 ° C., and heat treatment was performed under the annealing temperature conditions in Table 2.
  • Table 2 shows the temperatures of each step in the heating zone (HS), soaking zone (SS), slow cooling zone (SCS), rapid cooling zone (RCS), and overaging zone (OAS) in FIG. 1.
  • slow cooling SCS section in Table 2
  • rapid cooling RCS section in Table 2
  • the tensile test for each test piece was performed at a strain rate of 0.01 / s after taking a JIS No. 5 size tensile test piece in the direction perpendicular to the rolling direction.
  • each fraction was measured using a SEM and an image analyzer after nital etching.
  • the depth of the surface layer sound portion of the prepared steel sheet was measured with an optical microscope.
  • LME was spot welded under the same conditions, then the spot weld was cut and the cross section was observed with an optical microscope to confirm whether or not cracks were generated in the surface layer by the LME.
  • YS yield strength
  • TS tensile strength
  • LME tensile strength
  • FIG. 2 is an observation of the surface layer of Example 1, and it can be confirmed that a sound layer is formed.
  • Comparative Examples 1, 2, and 4 to 6 were manufactured by an existing process in which water cooling was not performed during hot rolling, and a sound layer was not formed on the surface layer, so they were sensitive to LME during spot welding and formed defects.
  • ferrite recrystallization is insufficient and austenite is formed, so that strength is secured but elongation is low.
  • FIG. 3 is a photograph of the microstructure observed on the surface layer of Comparative Example 1, and it can be seen that no sound layer is formed on the surface layer.
  • Comparative Example 8 water was sprayed during hot rolling to form a healthy ferrite layer (a healthy layer) on the surface, and no LME cracks were observed. It was confirmed that the cold rolling was performed at a high reduction ratio, and the continuous annealing temperature was annealed at a high temperature exceeding Ac1 + 30 ° C., so that the internal hard phase fraction was high and the elongation rate was inferior.
  • Comparative Examples 9 to 11 were manufactured by an existing process in which water cooling was not performed during hot rolling, so that a sound layer was not formed on the surface layer and was sensitive to LME during spot welding to form defects.
  • cold rolling was performed at a high reduction ratio, but the continuous annealing temperature exceeded Ac1 + 30 ° C., and the internal hard phase fraction was high, resulting in poor elongation.

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Abstract

La présente invention concerne une tôle d'acier destinée à être utilisée dans des automobiles, etc, et une tôle d'acier qui présente une résistance élevée et une aptitude au formage élevée et est supérieure en termes de soudabilité par points, ainsi que son procédé de fabrication.
PCT/KR2022/020731 2021-12-21 2022-12-19 Tôle d'acier à haute résistance et à formabilité élevée ayant une excellente soudabilité par points, et son procédé de fabrication WO2023121187A1 (fr)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2005105367A (ja) * 2003-09-30 2005-04-21 Nippon Steel Corp 溶接性と延性に優れた高降伏比高強度冷延鋼板および高降伏比高強度溶融亜鉛めっき鋼板、並びに、高降伏比高強度合金化溶融亜鉛めっき鋼板とその製造方法
KR100778680B1 (ko) * 2006-12-07 2007-11-22 주식회사 포스코 열간압연장치 및 그 방법
KR20130074503A (ko) * 2011-12-26 2013-07-04 주식회사 포스코 용접성 및 굽힘가공성이 우수한 초고강도 냉연강판 및 그 제조방법
KR20140041833A (ko) * 2011-07-29 2014-04-04 신닛테츠스미킨 카부시키카이샤 굽힘성이 우수한 고강도 아연 도금 강판 및 그 제조 방법
KR20160041850A (ko) * 2013-04-15 2016-04-18 제이에프이 스틸 가부시키가이샤 고강도 열연 강판 및 그의 제조 방법

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2005105367A (ja) * 2003-09-30 2005-04-21 Nippon Steel Corp 溶接性と延性に優れた高降伏比高強度冷延鋼板および高降伏比高強度溶融亜鉛めっき鋼板、並びに、高降伏比高強度合金化溶融亜鉛めっき鋼板とその製造方法
KR100778680B1 (ko) * 2006-12-07 2007-11-22 주식회사 포스코 열간압연장치 및 그 방법
KR20140041833A (ko) * 2011-07-29 2014-04-04 신닛테츠스미킨 카부시키카이샤 굽힘성이 우수한 고강도 아연 도금 강판 및 그 제조 방법
KR20130074503A (ko) * 2011-12-26 2013-07-04 주식회사 포스코 용접성 및 굽힘가공성이 우수한 초고강도 냉연강판 및 그 제조방법
KR20160041850A (ko) * 2013-04-15 2016-04-18 제이에프이 스틸 가부시키가이샤 고강도 열연 강판 및 그의 제조 방법

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