WO2024080657A1 - Tôles d'acier et leurs procédés de fabrication - Google Patents

Tôles d'acier et leurs procédés de fabrication Download PDF

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WO2024080657A1
WO2024080657A1 PCT/KR2023/015229 KR2023015229W WO2024080657A1 WO 2024080657 A1 WO2024080657 A1 WO 2024080657A1 KR 2023015229 W KR2023015229 W KR 2023015229W WO 2024080657 A1 WO2024080657 A1 WO 2024080657A1
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steel sheet
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steel
heat treatment
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PCT/KR2023/015229
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한상호
공종판
김은영
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주식회사 포스코
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
    • B21C47/02—Winding-up or coiling
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • 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
    • 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
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • 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

Definitions

  • the present invention relates to materials applied to automobile members, and more specifically, to a cold rolled steel sheet (and plated steel sheet) with excellent bending properties and a method of manufacturing the same.
  • composite structure steel sheets in which a ferrite phase and a low-temperature transformation phase such as martensite or bainite coexist are used as high-strength steel sheets with excellent workability.
  • Composite structure steel sheets simultaneously improve strength and workability by dispersing a hard, low-temperature transformation phase in soft ferrite.
  • HPF method hot press forming
  • the HPF method is widely used in parts manufacturing because it can secure high strength compared to the same thickness, but there are problems in application due to excessive facility investment and increased process costs, so the development of materials for cold stamping is necessary. . Therefore, the development of a cold rolled steel sheet that is suitable for use as a cold stamping material, has high strength and high yield ratio to ensure collision performance, and has excellent bending properties is required.
  • Patent Document 1 shows that the steel composition is C: 0.25 to 0.4%, Si: 1.0% or less, Mn: 1.5 to 2.5%, P: 0.02% or less, S: 0.003% or less, Al: 0.01 to 0.1%, N: 0.005%.
  • B 0.0005 to 0.005%
  • Ti 0.005 to 0.1%
  • Nb 0.005 to 0.1%
  • the metal structure is a martensite single phase structure.
  • steel sheets are manufactured by heating and holding in a temperature range from the Ae3 transformation point to 900°C, then rapidly cooling to 200°C or less at an average cooling rate of 300°C/s or more, and then tempering at 250°C or less.
  • shape (flatness) due to water cooling is poor, resulting in defects during molding.
  • Patent Document 2 is: C: 0.05% or more and 0.35% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.8% or more and 3.0% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less.
  • Patent Document 1 Japanese Application No. 2009-098534
  • Patent Document 2 Japanese Application No. 2018-143806
  • One aspect of the present invention is to overcome the limitations of the prior art described above, and its purpose is to optimize the steel composition and manufacturing process to provide a steel sheet with excellent shape and bending properties and ultra-high strength of 1500 MPa or more.
  • One embodiment of the present invention is,
  • a steel plate with a ratio (a/b ⁇ 100) of 75% or more (excluding 100%) is provided.
  • the steel sheet may contain 10% or less (excluding 0%) of one or more types of phases selected from the group consisting of ferrite and bainite as a microstructure and area percent in a region within 20 ⁇ m from the surface.
  • the steel sheet has a microstructure in the area within 20 ⁇ m from the surface, and the remainder may be martensite.
  • the steel sheet may contain 90 to 99% martensite as a microstructure and area percent in a region within 20 ⁇ m from the surface.
  • the t may be 0.6 to 2.5 mm.
  • a zinc-based plating layer may be included on the surface of the steel sheet.
  • the steel plate may have a tensile strength (TS) of 1500 MPa or more and a bendability (R/t) of 3.7 or less.
  • TS tensile strength
  • R/t bendability
  • Another embodiment of the present invention is,
  • Reheating the steel slab comprising: at a temperature of 1100-1300°C;
  • the annealing step may be performed by heat treatment at Ac3+10°C to Ac3+80°C for 30 seconds or more.
  • the step of over-aging heat treatment by reheating the secondary cooled steel sheet to 150-240°C may be further included.
  • the over-aging heat treatment step can be performed for 400 to 1000 seconds.
  • Figure 1 shows a photograph taken with a scanning electron microscope (SEM) of a cross-section in the thickness direction of a steel plate according to Inventive Example 1 of the present invention.
  • the present inventors conducted extensive research to solve the problems of the prior art described above, and as a result confirmed through experiments that the target physical properties can be secured when the ingredients and operating conditions satisfy a specific relationship, and proposed the present invention. It has been completed.
  • the present invention in order to overcome the limitations of the prior art, it is possible to optimize the steel composition and manufacturing process to provide a steel sheet with excellent shape and bending characteristics and an ultra-high tensile strength of 1500 MPa or more.
  • the alloy composition of the steel sheet according to the present invention in weight percent, is carbon (C): 0.1 to 0.3%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 1.3 to 2.5%, chromium ( Cr): 0.2% or less (excluding 0%), molybdenum (Mo): 0.01 to 0.1%, boron (B): 0.0005 to 0.003%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S) : 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (Al): 0.01 to 0.1%, Niobium (Nb): 0.01 to 0.05%, Titanium (Ti): 0.01 ⁇ 0.05% and the balance includes Fe and unavoidable impurities. Unless otherwise specified in the present invention, the content of each element is based on weight percent.
  • Carbon is an interstitial solid solution element, the most effective and important element in improving the strength of steel, and is an element that must be added to secure the strength of martensitic steel.
  • the content exceeds 0.3%, the strength may rapidly increase due to excessive formation of martensite during cooling due to an increase in hardenability, and the elongation may worsen.
  • Silicon is known as a key element in TRIP (Transformation Induced Plasticity) steel, which acts to increase the retained austenite fraction and elongation. Additionally, in the present invention, the addition of Si is a factor that suppresses the occurrence of cracks during bending by suppressing precipitation of cementite. Therefore, in order to obtain the above-mentioned effect, 0% by weight is excluded from the Si content. However, if the Si content exceeds 0.5%, not only the weldability is inferior, but also the surface properties and plating properties of the steel sheet deteriorate, so the Si content is included at 0.5% by weight. Meanwhile, in terms of further improving the above-mentioned effect, the lower limit of the Si content may be 0.01%, or the upper limit of the Si content may be 0.3%.
  • Manganese is an element added to ensure strength. If the Mn content is less than 1.3%, the hardenability is low, and if the cooling rate is not fast enough during cooling after annealing, martensite is not formed, making it difficult to secure the level of strength required in the present invention. On the other hand, if the content exceeds 2.5%, the Ms temperature decreases during cooling after annealing, and as the final cooling temperature decreases, the shape of the steel sheet becomes poor and it becomes difficult to secure the initial martensite structure. In addition, during steelmaking/continuous casting operations, a segregation zone occurs in the length direction of the Mn-based slab, which acts as a factor that reduces bendability and sets the upper limit.
  • manganese is segregated in the thickness direction, forming a manganese band (Mn band) within the slab.
  • Mn band manganese band
  • the content of Mn is preferably set to 1.3 to 2.5%.
  • the lower limit of the Mn content may be 1.5%, or the upper limit of the Mn content may be 2.1%.
  • Chromium is an alloy element that facilitates securing a low-temperature transformation structure by suppressing ferrite transformation, and when using a continuous annealing process with slow cooling as in the present invention, it has the advantage of suppressing ferrite formation.
  • 0% by weight is excluded from the Cr content.
  • the Cr content exceeds 0.2%, delayed fracture resistance may deteriorate.
  • carbides such as CrC are formed, which impairs hole expandability and bending workability, and costs may increase due to excessive alloy input. Therefore, it is preferable that the Cr content is in the range of 0.2% or less.
  • the upper limit of the Cr content may be 0.15% or 0.1%.
  • there is no need to set a special lower limit range for the Cr content if it can be manufactured by optimizing the ingredients and operating conditions, but as an example, it may be 0.01%.
  • Molybdenum has the effect of improving the quenchability of steel, the effect of generating fine carbides containing Mo that become hydrogen trap sites, and the effect of improving delayed fracture resistance by refining martensite.
  • the Mo content exceeds 0.1%, the effect is not large compared to the increase in cost due to the addition of high-cost alloy elements, so it is preferable to set the upper limit to 0.1% or less.
  • the Mo content was less than 0.01%, the basic properties of Mo did not appear at all, and it was experimentally confirmed that there was no improvement effect on delayed failure, and the lower limit was set to 0.01% or more.
  • the lower limit of the Mo content may be 0.012%, or it is more preferable that the upper limit of the Mo content is 0.08%.
  • the boron is an element that suppresses the formation of ferrite, and accordingly, the present invention has the advantage of suppressing the formation of ferrite during cooling after annealing.
  • the B content exceeds 0.003%, ductility may be greatly reduced.
  • the B content is less than 0.0005%, there is no hardenability effect at all, and the target strength cannot be secured. Rather, ferrite is formed in the surface layer, which tends to deteriorate bendability, limiting its lower limit. Meanwhile, in terms of further improving the above-described effect, the lower limit of the B content may be 0.0008%, or the upper limit of the B content may be 0.0022%.
  • Phosphorus (P) 0.1% by weight or less (excluding 0%)
  • Phosphorus is an impurity element contained in steel.
  • the content of 0% is excluded.
  • the upper limit can be limited to 0.1% or less.
  • the lower limit of the P content may be 0.0001%, or the upper limit of the P content may be 0.03%.
  • S Sulfur
  • Sulfur like P, is an impurity that is inevitably included in steel, and is an element that impairs the ductility and weldability of steel sheets, so it is desirable to keep the content as low as possible. Therefore, in the present invention, it is preferable to limit the sulfur content to 0.01% or less. However, considering cases where it is inevitably included during the manufacturing process, 0% is excluded. Meanwhile, in terms of further improving the above-mentioned effect, the lower limit of the S content may be . Additionally, in order to further contribute to improving bendability by minimizing MnS precipitates in steel, the upper limit of the S content may be 0.008%, or 0.005%.
  • Nitrogen is an impurity element, and if the N content exceeds 0.01%, the risk of cracks occurring during playing due to AlN formation, etc. greatly increases, so it is desirable to limit the upper limit of the N content to 0.01%. However, considering cases where it is inevitably included during the manufacturing process, 0% is excluded. Meanwhile, in terms of further improving the above-described effect, the lower limit of the N content may be 0.0001%, or the upper limit of the N content may be more preferably 0.008%, and even more preferably 0.006%.
  • Aluminum can be added to remove oxygen in molten steel, and like Si, it is an element that is effective in stabilizing retained austenite by suppressing precipitation of cementite during the reheating and overaging stages. If the Al content is less than 0.01%, deoxidation of the steel material is not sufficiently achieved and the cleanliness of the steel material is impaired. On the other hand, if the Al content exceeds 0.1%, not only the castability of the slab deteriorates, but also the temperature required for single-phase heating during annealing increases, which may lead to production and equipment problems. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Al content may be 0.02%, or the upper limit of the Al content may be 0.05%.
  • Niobium (Nb) 0.01 to 0.05% by weight
  • Niobium is an element that segregates at austenite grain boundaries and suppresses the growth of austenite grains during annealing heat treatment, contributing to an increase in strength through a precipitation strengthening effect.
  • the Nb content exceeds 0.05%, precipitation of carbon, nitride, etc. increases, the machinability of the base material decreases, and the cost increases as the alloy input amount becomes excessive.
  • the Nb content is less than 0.01%, the lower limit is limited to 0.01% because it does not contribute at all to increasing strength. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Nb content may be 0.02%, or the upper limit of the Nb content may be 0.04%.
  • Titanium (Ti) 0.01 ⁇ 0.05% by weight
  • Titanium is a nitride forming element and is an element that scavenges by precipitating N in steel into TiN. If Ti is not added, there is a possibility that cracks may occur during continuous casting due to AlN formation. However, if the Ti content exceeds 0.05%, the strength of martensite may be reduced due to additional carbide precipitation in addition to the removal of dissolved N, and hole expansion and bending workability may be reduced due to the formation of carbon and nitrides such as TiC and TiN. may hinder. On the other hand, if the content of Ti is less than 0.01%, similar to the Nb element, it does not contribute at all to increasing strength, so the lower limit is set. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Ti content may be 0.02%, or the upper limit of the Ti content may be 0.04%.
  • the remainder includes iron (Fe), and unintended impurities may inevitably be introduced from raw materials or the surrounding environment during normal manufacturing processes, so this cannot be excluded. Since these impurities can be known to anyone skilled in the art during the manufacturing process, all of them are not specifically mentioned in this specification.
  • the ratio (a/b) of b) is more than 75% (excluding 100%).
  • the present inventors have found that, compared to the point (1/4) ⁇ t (where t is the total thickness of the steel sheet) from the surface (in the thickness direction of the steel sheet), the The present invention was completed by discovering a tendency for bending properties to improve when the total content of C and Mn satisfies a specific ratio. Therefore, according to the present invention, the ratio of the total content of C and Mn in the area within 20 ⁇ m from the surface of the steel sheet compared to the point (1/4) ⁇ t (where t is the total thickness of the steel sheet) from the surface is 75. If it is less than %, ferrite or bainite may be excessively formed in the form of clusters in the surface layer, causing cracks to occur at the boundary between the ferrite and martensite phases, which may cause a problem of reduced bendability.
  • the lower limit of the ratio (a/b) may be 78%, or the upper limit of the ratio (a/b) may be 87%.
  • 10% or less of one or more phases selected from the group consisting of ferrite and bainite (excluding 0%) as microstructure and area% ) can be included.
  • the ratio (A) of one or more phases selected from the group consisting of ferrite and bainite exceeds 10%, soft ferrite or bainite is excessively formed around the hard martensite. Cracks may occur when bending.
  • the lower limit of the ratio (A) there is no particular limitation on the lower limit of the ratio (A), but it is advantageous if it can be managed as low as possible within a practically manufacturable range.
  • the remainder other than the above-described ferrite and bainite may be martensite as the microstructure.
  • the area within 20 ⁇ m from the surface may contain 90 to 99% of martensite as a microstructure and area percent.
  • the lower limit of the fraction of martensite in area% in the area within 20 ⁇ m from the surface may be 95%, or in the area within 20 ⁇ m from the surface, the lower limit of the fraction of martensite in area% may be 95%.
  • the upper limit of the fraction may be 98%.
  • t may be 0.6 to 2.5 mm.
  • the steel sheet of the present invention may further include a plating layer.
  • a plating layer there is no particular limitation on the plating layer, so not only the type such as zinc-based plating or aluminum-based plating, but also the method such as hot-dip plating or electroplating is not limited. In other words, it is sufficient as long as it can be used in the technical field to which the present invention pertains.
  • the plating layer may be a zinc-based plating layer.
  • an ultra-high strength steel sheet with excellent bending properties having a tensile strength (TS) of 1500 MPa or more and a bendability (R/t) of 3.7 or less can be provided.
  • TS tensile strength
  • R/t bendability
  • the tensile strength (TS) is 1500 MPa or more
  • the elongation (El) is 3% or more (or a more preferable range of elongation is 5% or more, and in particular, the upper limit is higher. It is advantageous and is not calculated), and a steel sheet with a bendability (R/t) of 3.7 or less can be provided.
  • a slab of steel having the above-described composition system is reheated at a temperature of 1100 to 1300°C. This process is performed to smoothly perform the subsequent hot rolling process and sufficiently obtain the target physical properties of the steel sheet. At this time, if the reheating temperature is less than 1100°C, a problem occurs in which the hot rolling load rapidly increases. On the other hand, if the reheating temperature exceeds 1300°C, the amount of surface scale increases and the yield of the material decreases, so it is limited.
  • the reheated slab is hot rolled.
  • hot rolling can be performed at Ar3 ⁇ 1000°C.
  • the finishing hot rolling temperature of the reheated slab is limited to Ar3 (the temperature at which ferrite begins to appear when austenite is cooled) or higher. This is because below Ar3, rolling is performed in the two-phase region of ferrite + austenite or in the ferrite region, resulting in a mixed texture. This is limited because there is a risk of malfunction due to fluctuations in the hot rolling load.
  • the hot rolled steel sheet is wound at a temperature range of 400 to 600°C. If the coiling temperature exceeds 600°C, the oxide film on the surface of the steel sheet may be excessively generated, causing defects, and the surface properties of the plating material may deteriorate, so the upper limit is limited. In addition, it is desirable to maintain a low coiling temperature in order to secure material uniformity in the overall length and overall width by forming the structure of the hot-rolled sheet as much as possible into a single-phase structure rather than a composite structure.
  • the lower limit of the coiling temperature may be 420°C, or the upper limit of the coiling temperature may be 520°C, and water cooling treatment may be performed after winding.
  • the oxidation layer formed on the surface of the hot-rolled steel sheet wound after the hot rolling is removed through a pickling process, and then cold rolling is performed at a reduction ratio of 30 to 80%. If the reduction ratio of the cold rolling is less than 30%, not only is it difficult to secure the target thickness, but there is a risk that the remaining hot rolled grains may affect austenite generation and final physical properties during annealing heat treatment. In addition, if the reduction ratio of the cold rolling exceeds 80%, there is a problem that material deviation of the final steel sheet may occur due to uneven rolling reduction in the length and width directions due to work hardening that occurs during cold rolling. It may be difficult to secure the target thickness due to the load.
  • annealing temperature range of Ac3+10°C to Ac3+80°C. Since the Ac3 temperature varies depending on the component, it is determined by Equation 1 below. If the annealing temperature is less than Ac3+10°C, a mixed structure may be formed over the entire length of the coil by annealing in a two-phase region rather than a single-phase region, which has a negative effect on the material. Therefore, the lower limit is set to Ac3+10°C. define. On the other hand, if the annealing temperature exceeds Ac3+80°C, equipment problems may occur due to overload of the annealing furnace, so the upper limit is set to Ac3+80°C. Meanwhile, more preferably, the lower limit of the annealing temperature may be 823°C, or the upper limit of the annealing temperature may be 916°C.
  • the heat treatment during annealing may be performed (maintained) for more than 30 seconds at a temperature range of Ac3+10°C to Ac3+80°C. If the heat treatment time at Ac3+10°C ⁇ Ac3+80°C is less than 30 seconds, there may be a problem that the structure is not sufficiently single-phase heat treated, making it difficult to ultimately secure the martensite structure.
  • the annealed steel sheet is subjected to primary cooling to the primary cooling end temperature range of 680 to 749°C at an average cooling rate of 10°C/s or less (exceeding 0°C/s).
  • the ratio of phase structures other than martensite one or more phases selected from the group consisting of ferrite and bainite
  • the total content (a) of C and Mn in the area exceeding 10% and/or within 20 ⁇ m from the surface is (1/4) ⁇ t point from the surface (where t is the total amount of the steel sheet)
  • the ratio (a/b) becomes less than 75%, and eventually the bendability (R/t) evaluation value may exceed 3.7, which results in poor formability This could get worse.
  • the primary cooling end temperature should be between 680 and 749°C. limited to Meanwhile, more preferably, the lower limit of the primary cooling end temperature may be 700°C, or the upper limit of the primary cooling end temperature may be 730°C.
  • the upper limit of the average cooling rate of primary cooling is set to 10°C/s.
  • the lower limit of the average cooling rate of primary cooling may be as low as possible due to the equipment configuration, so the lower limit may not be set separately, so it is set to exceed 0°C/s (or more than 1°C/s).
  • the lower limit of the average cooling rate of the primary cooling may be 4.3°C/s, or the upper limit of the average cooling rate of the primary cooling may be 7.7°C/s.
  • the primary cooled steel sheet is subjected to secondary cooling (rapid cooling) from 100°C to Mf temperature at an average cooling rate of 60 to 160°C/s.
  • the Mf refers to the finish temperature (Mf) of martensite transformation, and is measured using a dilatometer.
  • the average cooling rate of the secondary cooling is less than 60°C/s, some bainite structure may be formed during cooling, making it difficult to secure the target strength.
  • the average cooling rate of the secondary cooling exceeds 160°C/s, problems with the shape of the steel sheet and material deviation in the width direction may occur due to the rapid martensite transformation rate at the time of secondary cooling.
  • the lower limit of the average cooling rate of the secondary cooling may be 80°C/s, or the upper limit of the average cooling rate of the secondary cooling may be 153°C/s.
  • the cooling end temperature of the secondary cooling is 100°C to Mf temperature, and if the cooling end temperature of the secondary cooling exceeds the Mf temperature, martensite transformation is not sufficiently achieved, and the microstructure desired by the present invention is may be difficult to secure.
  • the cooling end temperature of the secondary cooling is less than 100°C, the cooling is too low, which is disadvantageous in terms of shape and exceeds the manufacturing process range in terms of equipment, so the lower limit is limited to 100°C.
  • the lower limit of the cooling end temperature of the secondary cooling may be 106°C, or the upper limit of the cooling end temperature of the secondary cooling may be 152°C.
  • the method for manufacturing a steel plate according to an embodiment of the present invention can satisfy the following relational expression 1.
  • the present inventors have confirmed that excellent properties in strength and bendability can be secured by satisfying a specific relationship as shown in Equation 1 below between the cooling end temperatures during primary and secondary cooling. .
  • T1 represents the cooling end temperature (°C) of primary cooling
  • T2 represents the cooling end temperature (°C) of secondary cooling.
  • the secondary cooled steel sheet is reheated at 150 to 240° C. to perform overaging heat treatment.
  • the martensite obtained through the rapid secondary cooling process described above can be transformed into tempered martensite to increase the yield strength.
  • the overaging heat treatment temperature is less than 150°C, there is a disadvantage in that the yield strength is low and sufficient toughness cannot be secured due to insufficient tempering.
  • the overaging heat treatment temperature exceeds 240°C, there is a disadvantage in that bending workability is deteriorated due to a large amount of precipitation and coarsening of carbides.
  • the lower the lower limit of the overaging heat treatment temperature the more advantageous the bendability.
  • 150°C or higher is recommended, and more preferably 170°C or higher.
  • the upper limit of the overaging heat treatment temperature is preferably 200°C, and more preferably 198°C.
  • the method for manufacturing a steel plate according to an embodiment of the present invention can satisfy the following relational expression 2.
  • the present inventors have secured a steel sheet with superior properties in terms of strength and bendability by satisfying a specific relationship as shown in Equation 2 below between the cooling end temperature of secondary cooling and the overaging heat treatment temperature. Additionally, it was discovered that it could be done.
  • T2 represents the cooling end temperature of secondary cooling (°C)
  • T3 represents the temperature of overaging heat treatment (°C).
  • the overaging heat treatment may be performed for 400 seconds or more.
  • the over-aging heat treatment time refers to the time maintained in the over-aging heat treatment temperature range.
  • the overaging heat treatment time is less than 400 seconds, tempering is not sufficiently achieved, and the yield strength may be lowered.
  • the upper limit of the over-aging heat treatment time there is no particular limitation on the upper limit of the over-aging heat treatment time, but it is difficult to exceed 1000 seconds due to the nature of continuous annealing equipment. Therefore, the over-aging heat treatment time may be performed for 400 to 1000 seconds, and in terms of further improving the above-mentioned effect, the lower limit of the over-aging heat treatment time may be 428 seconds, or the upper limit of the over-aging heat treatment time may be 428 seconds. It could be 600 seconds.
  • the plate can be subjected to temper rolling or tension leveling to improve its shape.
  • the plating method includes a dip plating method in which a plating bath is installed and the steel sheet is dipped into a hot dip plating solution, and a method of electroplating in an electrolyte solution after annealing is completed.
  • the plating conditions are not particularly limited as long as they are generally known in the technical field to which the present invention pertains.
  • Molten steel having the alloy composition shown in Table 1 below was cast into an ingot and then sized and rolled to produce a steel slab.
  • This steel slab was heated to a temperature of 1200°C, held for 1 hour, finished hot rolled at 900°C, charged into a heated furnace set under various conditions, held for 1 hour, and then furnace cooled to simulate hot rolling.
  • cold rolling at a cold reduction rate of 50%, annealing heat treatment, primary cooling (slow cooling), secondary cooling (rapid cooling), reheating, and overaging heat treatment were performed under the conditions shown in Table 2 below.
  • a cold rolled steel sheet was manufactured and electrogalvanized treatment was performed using normal conditions.
  • the microstructure was evaluated through optical microscopy and SEM structure observation.
  • 3000x SEM tissue was used to observe the tissue of the superficial layer corresponding to the area within 20 ⁇ m from the surface, and the fraction for each tissue phase was analyzed by analyzing the area ratio through image analysis of each phase, and the average value of three analyzes was taken as a representative value. It was done as follows.
  • the local C and Mn concentrations were analyzed for each of 5 points within an approximate diameter of 20 ⁇ m, and the quantitative concentration ratio (%) of C and Mn was analyzed and the arithmetic average was used to obtain representative values.
  • the line profile technique is used for relative comparison to obtain the average total content ratio of C and Mn in the area within 20 ⁇ m from the surface compared to the (1/4) ⁇ t point from the surface. was accurately evaluated.
  • Tensile strength (TS) and yield strength (YS) were measured by collecting a tensile test specimen of JIS No. 5 size in the direction perpendicular to the rolling direction and performing a tensile test at a strain rate of 0.01/s.
  • R/t (bending characteristics) is obtained by processing a cold-rolled steel sheet into a specimen of 100 mm wide The reliability of the values has been improved.
  • the shape was scanned for each section using a 3D scanner technique for the full width after cutting 200 mm in the longitudinal direction, and then the flatness was evaluated. In general, in the present invention, if the value is 3 mm or less, it is judged to be a satisfactory level.
  • Table 1 shows the range of ingredients for manufacturing the invention steel and comparative steel
  • Table 2 summarizes the operating conditions for the invention steel and comparative steel.
  • inventive steels operating conditions outside the scope of the present invention were marked with *, and in the comparative steels, cases outside the scope of the present invention were also marked with *.
  • the inventive examples of the present invention have a tensile strength of 1500 MPa or more, a bendability (R/t) of 3.7 or less, and a flatness of 3 mm or less, and thus have ultra-high strength. , it was confirmed to have excellent shape and bendability.
  • Figure 1 shows a photograph taken with a scanning electron microscope (SEM) of a cross-section in the thickness direction of a steel plate according to Inventive Example 1 of the present invention.
  • SEM scanning electron microscope
  • Comparative Examples 1 and 2 when the first and second cooling end temperatures and cooling rates are outside the range required by the present invention, the flatness or bendability is outside the range required by the present invention, which is due to ferrite and bay in addition to martensite structure within 20 ⁇ m of the surface layer. It can be seen that the ratio of one or more types of mixed structures selected from the group consisting of nitrate exceeds 10%, or the total content ratio of C and Mn is outside the required range of the present invention, resulting in poor bendability.
  • Comparative Examples 3 and 4 were manufactured under conditions where the coiling temperature and annealing temperature were outside the range of the present invention, and it can be seen that the bendability was inferior, and in Comparative Examples 5 to 8, the rigidity was outside the target range, and the bendability was also inferior. You can see that it is.

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Abstract

La présente invention concerne un matériau appliqué à des éléments de véhicule et, plus précisément, une tôle d'acier laminée à froid et une tôle d'acier plaquée, ayant d'excellentes propriétés de flexion, et leurs procédés de fabrication.
PCT/KR2023/015229 2022-10-13 2023-10-04 Tôles d'acier et leurs procédés de fabrication WO2024080657A1 (fr)

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Citations (5)

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Publication number Priority date Publication date Assignee Title
KR20120074798A (ko) * 2010-12-28 2012-07-06 주식회사 포스코 인장강도 1.5ＧＰａ급의 초고강도 강판의 제조방법 및 이에 의해 제조된 강판
KR101767780B1 (ko) * 2015-12-23 2017-08-24 주식회사 포스코 고항복비형 고강도 냉연강판 및 그 제조방법
KR20210147255A (ko) * 2020-05-28 2021-12-07 현대제철 주식회사 냉연 도금 강판 및 그 제조방법
KR20210149841A (ko) * 2019-05-16 2021-12-09 제이에프이 스틸 가부시키가이샤 고강도 부재, 고강도 부재의 제조 방법 및 고강도 부재용 강판의 제조 방법
KR102403767B1 (ko) * 2020-11-25 2022-05-30 현대제철 주식회사 초고강도 냉연강판 및 그 제조방법

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Publication number Priority date Publication date Assignee Title
KR20090098534A (ko) 2008-03-14 2009-09-17 서경석 음식물류 폐기 발생폐수 순산소발효 공정

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20120074798A (ko) * 2010-12-28 2012-07-06 주식회사 포스코 인장강도 1.5ＧＰａ급의 초고강도 강판의 제조방법 및 이에 의해 제조된 강판
KR101767780B1 (ko) * 2015-12-23 2017-08-24 주식회사 포스코 고항복비형 고강도 냉연강판 및 그 제조방법
KR20210149841A (ko) * 2019-05-16 2021-12-09 제이에프이 스틸 가부시키가이샤 고강도 부재, 고강도 부재의 제조 방법 및 고강도 부재용 강판의 제조 방법
KR20210147255A (ko) * 2020-05-28 2021-12-07 현대제철 주식회사 냉연 도금 강판 및 그 제조방법
KR102403767B1 (ko) * 2020-11-25 2022-05-30 현대제철 주식회사 초고강도 냉연강판 및 그 제조방법

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