EP4079908A1 - Blattfederstahlblech mit ausgezeichneter ermüdungslebensdauer und verfahren zu seiner herstellung - Google Patents

Blattfederstahlblech mit ausgezeichneter ermüdungslebensdauer und verfahren zu seiner herstellung Download PDF

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Publication number
EP4079908A1
EP4079908A1 EP20902722.6A EP20902722A EP4079908A1 EP 4079908 A1 EP4079908 A1 EP 4079908A1 EP 20902722 A EP20902722 A EP 20902722A EP 4079908 A1 EP4079908 A1 EP 4079908A1
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Prior art keywords
steel sheet
hot
rolled material
vanadium
less
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EP20902722.6A
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English (en)
French (fr)
Inventor
Sun-Mi Kim
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Posco Holdings Inc
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Posco Co Ltd
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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
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B21B1/24—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • 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
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C21D1/32—Soft annealing, e.g. spheroidising
    • 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
    • C21D1/34—Methods of heating
    • C21D1/44—Methods of heating in heat-treatment baths
    • C21D1/46—Salt baths
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/02—Hardening by precipitation
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1216—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the working step(s) being of interest
    • C21D8/1222—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
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1216—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the working step(s) being of interest
    • C21D8/1233—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
    • 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
    • C22C38/02—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
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/607—Molten salts
    • 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
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present disclosure relates to a steel sheet for a part material used in an environment in which a constant load is repeated at regular intervals, and a method for manufacturing the same.
  • the seat belt of a vehicle has a structure in which a belt and a leaf spring are connected to each other to assist a user in easy attachment and detachment, and the fatigue lifespan of the leaf spring is an important factor influencing the durability of the seat belt.
  • the leaf spring provided for the seat belt should have excellent durability so as not to reach a fatigue limit during a life cycle of the vehicle, but development of a material thereof has not been undertaken.
  • Patent Document 1 proposes a steel sheet for a spring with improved strength using upper bainite.
  • the physical properties of the steel sheet are varied by the introduction of upper bainite, and thus the durability of the spring is rather reduced.
  • Patent Document 1 Korean Patent Application Publication No. 10-2008-0060619 (July 2, 2008 )
  • An aspect of the present disclosure is to provide a turbine. According to an aspect of the present disclosure, a steel material having excellent workability and a method for manufacturing the same may be provided.
  • a steel sheet for a leaf spring may include, by wt%, 0.7-1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities.
  • the steel sheet may include, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC), and an average prior austenite grain size may be 28 ⁇ m or less.
  • a content (Vs) of vanadium (V) of the steel sheet and a content (Vp) of vanadium (V) of the pearlite may satisfy Formular 1 below.
  • Vs denotes a content (wt%) of vanadium (V) of a steel sheet
  • Vp denotes a content (wt%) of vanadium (V) of a pearlite
  • a fraction of the vanadium carbide (VC) may be 0.002 area% or more.
  • An average interlamellar spacing of the pearlite may be 0.09 - 0.12 ⁇ m.
  • the average prior austenite grain size may be 2 ⁇ m or more.
  • a surface hardness of the steel sheet may be 410 (HB) or more based on Brinell hardness.
  • a thickness of the steel sheet is 3 mm or less (excluding 0 mm).
  • a method for manufacturing a steel sheet for a leaf spring may include providing a hot-rolled material including, by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities, isothermally holding the hot-rolled material by charging the hot-rolled material in a salt bath of 400 - 600°C after reheating the hot-rolled material, and providing a cold-rolled material by cold rolling the isothermally held hot-rolled material at a cumulative reduction ratio of 50% or more.
  • a hot-rolled material including, by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn),
  • the hot-rolled material may be reheated in a temperature range of 900°C or higher and less than 1000°C when a content of vanadium (V) of the hot-rolled material is less than 0.15%, and the hot-rolled material may be reheated in a temperature range of 900 - 1050°C when the content of vanadium (V) of the hot-rolled material is 0.15% or more.
  • the providing of the hot-rolled material may include an operation of reheating a slab to a temperature range of 1000 - 1300°C, a rough rolling operation of providing an intermediate material by rough rolling the reheated slab at a cumulative reduction ratio of 60% or more, a finishing rolling operation of rolling the intermediate material at a cumulative reduction ratio of 60% or more, and providing a hot-rolled material by rolling the intermediate material at an exit temperature of 700 - 1000°C, a first cooling operation of performing cooling to a first cooling end temperature of 500 - 700°C by applying a slow cooling condition, a first holding operation of isothermally holding the hot-rolled material at the first cooling end temperature, and a second cooling operation of cooling the isothermally held hot-rolled material to room temperature by furnace cooling.
  • the present disclosure relates to a steel plate for a leaf spring having an excellent fatigue lifespan, and a method for manufacturing the same.
  • a steel plate for a leaf spring having an excellent fatigue lifespan and a method for manufacturing the same.
  • preferred embodiments of the present disclosure will be described.
  • the embodiments of the present disclosure can be modified to have various other forms, and the scope of the present disclosure should not be limited to the embodiments described below.
  • the embodiments of the present disclosure are provided to more fully explain the present disclosure to those skilled in the art.
  • the present inventors conducted an in-depth study on a method for improving a fatigue lifespan of a steel part used in an environment in which a constant load is repeatedly applied at regular intervals, and recognized that a fatigue lifespan of a steel material can be improved by suppressing generation and propagation of fatigue cracks when physical properties of the steel material itself are strengthened, and thus the present disclosure was derived.
  • the present inventors conducted an in-depth study of a method for effectively securing strength and hardness of a steel material through precipitation hardening and grain refinement while securing an effect of solid solution strengthening by including a predetermined level of carbon (C) or more, and confirmed that, when an appropriate level of carbon (C) is added to the steel material while vanadium carbide (VC) is used, the hardness and strength of the steel material may be effectively improved by maximizing effects of precipitation hardening and grain refinement, and thus the present disclosure was derived.
  • C carbon
  • VC vanadium carbide
  • a steel sheet for a leaf spring may include, by wt%, 0.7-1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities.
  • the steel sheet may include, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC), and an average prior austenite grain size may be 28 ⁇ m or less.
  • the steel material for a leaf spring may include, by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0%, of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities.
  • Carbon (C) is not only a representative hardenability improving element, but also an element effectively contributing to improvement of strength and hardness of steel by solid solution strengthening. Accordingly, the present disclosure may include 0.7% or more of carbon (C) for such an effect.
  • a preferable content of carbon (C) may be 0.75% or more, and a more preferable content of carbon (C) may be 0.8% or more.
  • carbon (C) may have a low solid solution limit in ferrite, and thus may react with a carbideforming element to form a precipitate or may combine with Fe to form cementite (Fe 3 C).
  • the present disclosure may limit an upper limit of a content of carbon (C) to 1.0%.
  • a preferable content of carbon (C) may be 0.95% or less, and a more preferable content of carbon (C) may be 0.9% or less.
  • Silicon (Si) is not only a ferrite stabilizing element, but also an element effectively contributing to improvement of strength and hardness of steel by refining an interlamellar spacing of pearlite by delaying a transformation rate of the pearlite. Accordingly, the present disclosure may include 0.1% or more of silicon (Si) for such an effect.
  • a preferable content of silicon (Si) may be 0.15% or more, and a more preferable content of silicon (Si) may be 0.2% or more.
  • silicon (Si) is excessively added, hot workability and toughness may be lowered as well as surface quality may be lowered, so the present disclosure may limit an upper limit of the content of silicon (Si) to 0.4%.
  • the preferable content of silicon (Si) may be 0.35% or less, and the more preferable content of silicon (Si) may be 0.3% or less.
  • Manganese (Mn) is an element not only contributing to improvement of hardenability of steel, but also an element effectively contributing to securing cleanliness of steel by deoxidation and desulfurization. Accordingly, the present disclosure may include 0.2% or more of manganese (Mn). A preferable content of manganese (Mn) may be 0.3% or more, and a more preferable content of manganese (Mn) may be 0.4% or more. However, when manganese (Mn) is excessively added, a segregation layer may be formed in a central portion of a steel sheet to lower workability, so that the present disclosure may limit an upper limit of the content of manganese (Mn) to 1.0%. The preferable content of manganese (Mn) may be 0.8% or less, and the more preferable content of manganese (Mn) may be 0.6% or less.
  • Chromium (Cr) is an element contributing to improvement of hardenability of steel.
  • chromium (Cr) is an element not only forming fine carbide, but also an element effectively contributing to improvement of strength and hardness of steel by refining the interlamellar spacing of the pearlite. Accordingly, the present disclosure may include 0.05% or more of chromium (Cr).
  • a preferred content of chromium (Cr) may be 0.07% or more.
  • the present disclosure may limit an upper limit of the content of chromium (Cr) to 2.0%.
  • the preferred content of chromium (Cr) may be 1.5% or less, and a more preferred content of chromium (Cr) may be 1.0% or less.
  • V Vanadium (V): 0.07 - 0.2%
  • vanadium (V) is an element essentially added to improve strength and hardness of steel. Vanadium (V) is not only an element having easy heat treatment properties and having low reactivity with oxygen, but also an element reacting with carbon (C) in steel to precipitate fine vanadium carbide (VC) and effectively contribute to grain refinement of austenite. Accordingly, the present disclosure may include 0.07% or more of vanadium (V). A more preferable content of vanadium (V) may be 0.1% or more. However, when a content of vanadium (V) exceeds a predetermined level, an effect of adding vanadium (V) may be saturated, whereas vanadium (V), a relatively expensive element, may not be preferred in terms of economic feasibility. The present disclosure may limit an upper limit of the content of vanadium (V) to 0.2%. The more preferable content of vanadium (V) may be 0.18% or less.
  • P Phosphorus
  • S Sulfur
  • Phosphorus (P) and sulfur (S) are representative impurity elements, and the present disclosure seeks to minimize contents of the elements in terms of securing cleanliness of steel. However, in consideration of economic feasibility in a conventional steelmaking process, the present disclosure may limit upper limits of contents of phosphorus (P) and sulfur (S) to 0.03%, respectively.
  • the steel sheet for a leaf spring according to an aspect of the present disclosure may include a balance of Fe and other inevitable impurities in addition to the above-described elements.
  • unintended impurities from a raw material or a surrounding environment may be inevitably mixed therewith in a general manufacturing process, so that the unintended impurities may not be entirely excluded.
  • the impurities are known to those skilled in the art, and thus all contents thereof are not specifically mentioned in the present specification.
  • the addition of effective elements other than the above-described composition is not excluded.
  • the steel sheet for a leaf spring according to an aspect of the present disclosure may include, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC).
  • a lower limit of a preferable fraction of vanadium carbide (VC) may be 0.002 area% or more. That is, the steel sheet for a leaf spring according to an aspect of the present disclosure may have a microstructure including a predetermined level of vanadium carbide (VC) in a pearlite single-phase structure.
  • a content (Vs, wt%) of vanadium (V) included in the steel sheet and a content (Vp, wt%) of vanadium (V) included in the pearlite may satisfy Formula 1 below.
  • Vs denotes a content (wt%) of vanadium (V) included in the steel sheet
  • Vp denotes a content (wt%) of vanadium (V) included in the pearlite
  • the steel sheet for a leaf spring according to an aspect of the present disclosure does not aim for complete solid solution of vanadium (V) in the pearlite structure, and thus may mean that some vanadium (V) is precipitated as vanadium carbide (VC). Accordingly, in the steel sheet for a leaf spring according to an aspect of the present disclosure, an effect of improving strength and hardness of the steel sheet by precipitation of vanadium carbide (VC) may be expected.
  • an average prior austenite grain size (average grain size at an austenitization temperature) may be 28 ⁇ m or less, and a preferred average prior austenite grain size may be 2 ⁇ m or more.
  • a preferred average interlamellar spacing of the pearlite may be 0.09 - 0.12 ⁇ m. That is, the steel sheet for a leaf spring according to an aspect of the present disclosure may be manufactured by heat-treating a hot-rolled material with added vanadium (V), and thus it can be seen that a structure is refined by vanadium carbide (VC) present in a state of the hot-rolled material. Accordingly, in the steel sheet for a leaf spring according to an aspect of the present disclosure, an effect of improving strength and hardness of the steel sheet by refining the structure may be expected.
  • a thickness of the steel sheet for a leaf spring according to an aspect of the present disclosure is not particularly limited, but may preferably have a thickness of 3 mm or less (excluding 0 mm).
  • a surface hardness of the steel sheet for a leaf spring according to an aspect of the present disclosure may be greater than or equal to 410 (HB) based on Brinell hardness.
  • HB 410
  • HB denotes a Brinell hardness of a steel sheet surface
  • 2Nf denotes a fatigue lifespan
  • E in Formula 2 above denotes an elastic modulus, and a fixed value of 210 GPa is applied in the present disclosure.
  • a method for manufacturing a steel sheet for a spring may include providing a hot-rolled material including, by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities, isothermally holding the hot-rolled material by charging the hot-rolled material in a salt bath of 400 - 600°C after reheating the hot-rolled material, and providing a cold-rolled material by cold rolling the isothermally held hot-rolled material at a cumulative reduction ratio of 50% or more.
  • a hot-rolled material including, by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn),
  • the hot-rolled material may be reheated in a temperature range of 900°C or higher and less than 1000°C when a content of vanadium (V) of the hot-rolled material is less than 0.15%, and the hot-rolled material may be reheated in a temperature range of 900 - 1050°C when the content of vanadium (V) of the hot-rolled material is 0.15% or more.
  • the providing of the hot-rolled material may include an operation of reheating a slab to a temperature range of 1000 - 1300°C, a rough rolling operation of providing an intermediate material by rough rolling the reheated slab at a cumulative reduction ratio of 60% or more, a finishing rolling operation of rolling the intermediate material at a cumulative reduction ratio of 60% or more, and providing a hot-rolled material by rolling the intermediate material at an exit temperature of 700 - 1000°C, a first cooling operation of performing cooling to a first cooling end temperature of 500 - 700°C by applying a slow cooling condition, a first holding operation of isothermally holding the hot-rolled material at the first cooling end temperature, and a second cooling operation of cooling the isothermally held hot-rolled material to room temperature by furnace cooling.
  • the slab of the present disclosure may have an alloy composition corresponding to the alloy composition of the steel sheet described above, and thus descriptions of the alloy composition of the slab of the present disclosure are replaced with descriptions of the alloy composition of the steel sheet described above.
  • a slab reheating temperature of the present disclosure is not particularly limited, but reheating of the slab may be performed at 1000 - 1300°C in consideration of uniformity of a material and a rolling load of a subsequent hot-rolling process.
  • An intermediate material may be provided by rough rolling the reheated slab at a cumulative reduction ratio of 60% or more, and then finishing rolling may be performed to provide a hot-rolled material by rolling the intermediate material at a cumulative reduction ratio of 60% or more.
  • an upper limit of an exit temperature of finishing rolling may be limited to 1000°C, and a lower limit of the exit temperature may be limited to 700°C in consideration of the rolling load.
  • An upper limit of a preferable exit temperature may be 920°C, and a lower limit of the preferable exit temperature may be 830°C.
  • two-stagecondition cooling may be performed. That is, after first cooling is performed to a first cooling end temperature of 500 - 700°C by applying a slow cooling condition, isothermal holding may be performed at the first cooling end temperature, and then secondary cooling of the hot-rolled material held isothermally by furnace cooling may be performed to room temperature.
  • a cooling rate of the first cooling may be 20 - 100°C/s.
  • the isothermal holding may be performed in a state of a hot-rolled coil, and isothermal holding time may be 30 - 90 minutes.
  • a cooling rate of the secondary cooling may be 5°C/ s or less.
  • a process of isothermal holding may be performed by charging the hot-rolled material in a salt bath set to a pearlitization temperature.
  • a hot-rolled material reheating temperature may be selectively applied according to a content of vanadium (V) included in the hot-rolled material.
  • the content of vanadium (V) included in the hot-rolled material may be interpreted as meaning corresponding to that of a content of vanadium (V) included in the slab.
  • reheating of the hot-rolled material may be performed in a temperature range of 900°C or higher and less than 1000°C.
  • reheating of the hot-rolled material may be performed in a temperature range of 900 - 1050°C. That is, in order to allow a predetermined level of vanadium carbide (VC) to remain in a final steel sheet, the present disclosure may selectively apply a reheating temperature range according to the content of vanadium (V).
  • the present disclosure does not particularly limit hot-rolled material reheating time, but the hot-rolled material reheating time may be 1 minute to 10 minutes in consideration of effects of preventing coarsening of a microstructure and preventing complete solid solution of a carbide.
  • a lower limit of preferred hot-rolled material reheating time may be 3 minutes, and an upper limit of the preferred hot-rolled material reheating time may be 5 minutes.
  • the reheated hot-rolled material may be isothermally held to provide a microstructure of the final steel sheet as a single-phase pearlite structure.
  • the present disclosure does not particularly limit isothermal holding time, but in order to prevent a fatigue lifespan from being lowered due to local formation of a lowtemperature structure, isothermal holding may be performed for 30 seconds or more.
  • preferred isothermal holding time may be 60 to 150 seconds in consideration of implementation and economic feasibility of a desired final structure.
  • Cold rolling may be performed on the isothermally held hot-rolled material under a condition of a cumulative reduction ratio of 50% or more, and a steel sheet having a final thickness of 3 mm or less may be provided.
  • the steel sheet for a leaf spring manufactured by the above-described manufacturing method may include, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC), and a content (Vs, wt%) of vanadium (V) of the steel sheet and a content (Vp, wt%) of vanadium (V) of the pearlite may satisfy Formula 1 below.
  • Vs denotes a content (wt%) of vanadium (V) included in a steel sheet
  • Vp denotes a content (wt%) of vanadium (V) included in pearlite
  • an average prior austenite grain size (average grain size at an austenitization temperature) may be 28 ⁇ m or less, and an average interlamellar spacing of the pearlite may be 0.09 - 0.12 ⁇ m or more.
  • the steel sheet for a leaf spring manufactured by the above-described manufacturing method may have a hardness of 410 (HB) or more based on Brinell hardness.
  • HB hardness of 410
  • HB denotes a Brinell hardness of a steel sheet surface
  • 2Nf denotes a fatigue lifespan
  • E in Formula 2 above denotes an elastic modulus, and a fixed value of 210 GPa is applied in the present disclosure.
  • 2% nital etching was performed on a polished specimen surface, and then a prior austenite grain size was observed by performing 200 times magnification with an optical microscope, and a structure composition and a pearlite lamellar spacing were observed by performing 500 - 50,000 times magnification with a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • specimens 5 to 9 satisfying an alloy composition and a process condition of the present disclosure satisfied a desired microstructure and physical properties
  • specimens 1 to 4 that do not satisfy one or more among the alloy composition and the process condition of the present disclosure did not satisfy the desired microstructure and physical properties.
  • specimen 5 satisfied a content of vanadium (V) limited by the present disclosure, but re-dissolution of vanadium carbide (VC) present in a hot-rolled material specimen occurred due to a hot-rolled material reheating temperature exceeding a range limited by the present disclosure.
  • V vanadium
  • VC vanadium carbide
  • Specimens 1 to 3 are photographic images illustrating prior austenite grains of specimens 4, 5, and 6 observed using an optical microscope, respectively, from which, it can be confirmed that specimen 4 had a largest average prior austenite grain size, and specimen 6 had a smallest average prior austenite grain size.
  • Specimens 4 to 6 are photographic images illustrating pearlite structures of specimens 4, 5 and 6 observed using an SEM, from which, it can be confirmed that average pearlite interlamellar spacings of specimens 5 and 6 were formed narrower than that of specimen 4.

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EP20902722.6A 2019-12-20 2020-12-10 Blattfederstahlblech mit ausgezeichneter ermüdungslebensdauer und verfahren zu seiner herstellung Pending EP4079908A1 (de)

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