WO2021125878A1 - 열간성형용 강재, 열간성형 부재 및 이들의 제조방법 - Google Patents

열간성형용 강재, 열간성형 부재 및 이들의 제조방법 Download PDF

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Publication number
WO2021125878A1
WO2021125878A1 PCT/KR2020/018658 KR2020018658W WO2021125878A1 WO 2021125878 A1 WO2021125878 A1 WO 2021125878A1 KR 2020018658 W KR2020018658 W KR 2020018658W WO 2021125878 A1 WO2021125878 A1 WO 2021125878A1
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Prior art keywords
less
hot
steel
steel sheet
hot forming
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
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PCT/KR2020/018658
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English (en)
French (fr)
Korean (ko)
Inventor
김성우
오진근
김상헌
전효식
배성범
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Posco Holdings Inc
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Posco Co Ltd
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Publication date
Priority claimed from KR1020200086574A external-priority patent/KR102379443B1/ko
Application filed by Posco Co Ltd filed Critical Posco Co Ltd
Priority to CN202080088989.XA priority Critical patent/CN114867883B/zh
Priority to US17/778,614 priority patent/US12385115B2/en
Priority to EP20901253.3A priority patent/EP4079913A4/en
Priority to JP2022530941A priority patent/JP7648620B2/ja
Priority to MX2022006587A priority patent/MX2022006587A/es
Publication of WO2021125878A1 publication Critical patent/WO2021125878A1/ko
Anticipated expiration legal-status Critical
Priority to JP2024029954A priority patent/JP2024063127A/ja
Priority to US19/268,585 priority patent/US20260009112A1/en
Ceased legal-status Critical Current

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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
    • 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
    • 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
    • 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
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/012—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of aluminium or an aluminium alloy
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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Definitions

  • the present invention relates to a steel material for hot forming, a hot forming member, and a manufacturing method thereof.
  • Hot-formed ultra-high-strength members have recently been widely applied to structural members of automobiles for the purpose of improving fuel efficiency and protecting passengers by reducing the weight of automobiles.
  • Patent Document 1 is proposed as a representative technique related to such hot forming.
  • Patent Document 1 proposes a technique for securing ultra-high strength with a tensile strength of over 1600 MPa by heating an Al-Si plated steel sheet to 850° C. or higher and then forming the structure of the member into martensite by hot forming and rapid cooling by pressing. are doing
  • the technique proposed in Patent Document 1 since it is molded at a high temperature, complex shapes can be easily molded, and a weight reduction effect can be expected due to high strength through an increase in strength due to rapid cooling in the mold.
  • low-strength steel for hot forming which is excellent in terms of energy absorption, has been developed and applied.
  • general steel for hot forming for TWB it is located within the actual part due to its low hardenability. It is not easy to secure uniform physical properties according to differences in cooling conditions, etc., and the tensile strength is only at the level of 500 to 600 MPa, so that additional weight reduction effect by improving strength is required.
  • Patent Document 1 US Patent No. 6296805
  • One aspect of the present invention is to provide a steel material for hot forming, a hot forming member, and a method for manufacturing the same, which can impart excellent collision energy absorption ability while having high strength to the member.
  • One embodiment of the present invention is by weight%, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1% , Cr: 0.01 to 0.8%, Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the remainder including Fe and unavoidable impurities, and the alloy factor expressed by the following relation 1 is 7 or more and 10 5 carbides/mm 2 or less of carbide having an equivalent circle diameter of 0.5 ⁇ m or more.
  • Another embodiment of the present invention is by weight%, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1% , Cr: 0.01 to 0.8%, Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the remainder including Fe and unavoidable impurities, and the alloy factor expressed by the following relation 1 is 7 or more It provides a hot-formed member having an equivalent circle diameter of 0.5 ⁇ m or more and 10 4 carbides/mm 2 or less.
  • Another embodiment of the present invention is by weight%, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1 %, Cr: 0.01 to 0.8%, Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the remainder including Fe and unavoidable impurities, the alloy factor represented by the following relation 1 is 7 Heating the above steel slab at 1050 ⁇ 1300 °C; obtaining a hot-rolled steel sheet by hot-rolling the heated steel slab at 800 to 950°C; winding the hot-rolled steel sheet at 500 to 700°C; cooling the wound hot-rolled steel sheet from the coiling temperature to 400°C at a cooling rate of 10°C/Hr or more; obtaining a cold rolled steel sheet by cold rolling the cooled hot rolled steel sheet; heating the cold-rolled steel sheet in a temperature range from 400° C.
  • annealing temperature at a rate of 20° C./s or less; annealing the heated cold-rolled steel sheet at 740 to 860°C; and cooling the annealed cold-rolled steel sheet from an annealing temperature to 660°C at a cooling rate of 1°C/s or more.
  • Another embodiment of the present invention comprises the steps of obtaining a blank using the above-described steel for hot forming; After heating the blank at Ac3 ⁇ 980 °C, maintaining 1 ⁇ 1000 seconds; and cooling the heated and maintained blank to room temperature after hot forming.
  • a steel material for hot forming capable of manufacturing a member having a high strength of 1000 MPa or more based on tensile strength, high impact energy absorption, and excellent material uniformity, a hot forming member using the same, and manufacturing thereof method can be provided.
  • CIE Chip initiation Energy
  • FIG. 2 is a schematic view showing a hot-formed member manufactured according to an embodiment of the present invention.
  • FIG 3 is a graph showing the collision energy absorption capacity according to the carbon content and alloy index of Inventive Examples 1 to 7 and Comparative Examples 3 to 9 according to an embodiment of the present invention.
  • the present inventors have studied in depth how to improve the impact energy absorption capacity of the hot-formed member. Therefore, the present inventors used the energy (area of the load-displacement curve) value up to the maximum load in the three-point bending test (VDA238-100) as an index that can well express the impact energy absorption capacity of the hot-formed member, and various components and manufacturing conditions , organization, etc. were evaluated.
  • the hot forming member It was concluded that the ability to absorb the collision energy of the polarizer can be maximized, and the present invention has been completed based on this conclusion.
  • the steel for hot forming according to the present invention is, by weight, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 0.8%, Mo: 0.01 to 0.5%, N: 0.02% or less, the balance may include Fe and unavoidable impurities.
  • Carbon (C) is an essential element added to increase the strength of the heat treatment member.
  • C Carbon
  • the content of C is preferably in the range of 0.06 to 0.1%.
  • the lower limit of the C content is more preferably 0.065%, and even more preferably 0.07%.
  • the upper limit of the C content is more preferably 0.095%, and even more preferably 0.09%.
  • Silicon (Si) not only has to be added as a deoxidizer in steelmaking, but also contributes to increase the strength of the hot-formed member as a solid solution strengthening element and a carbide formation inhibitory element, and is added as an effective element for material uniformity.
  • the Si content is preferably in the range of 0.05 to 0.6%.
  • the lower limit of the Si content is more preferably 0.1%, even more preferably 0.15%.
  • the upper limit of the Si content is more preferably 0.55%, even more preferably 0.5%.
  • Manganese (Mn) needs to be added to suppress ferrite formation during hot forming by improving hardenability as well as securing a solid solution strengthening effect. If the Mn content is less than 0.6%, there is a limit to obtaining the above effect, and when the Mn content is too low, other expensive alloying elements are excessively required for insufficient hardenability, which may cause a problem in that the manufacturing cost is greatly increased. On the other hand, when the Mn content exceeds 2%, not only the cold rolling property is deteriorated due to the increase in strength of the steel sheet before the hot forming process, but also the band-like structure arranged in the rolling direction in the microstructure deepens, so that the collision energy absorption ability is inferior.
  • the Mn content is preferably in the range of 0.6 to 2%.
  • the lower limit of the Mn content is more preferably 0.7%, even more preferably 0.8%, and most preferably 0.9%.
  • the upper limit of the Mn content is more preferably 1.8%, even more preferably 1.6%, and most preferably 1.4%.
  • Phosphorus (P) exists as an impurity in steel, and when its content exceeds 0.05%, it may greatly embrittle the weldability of the hot-formed member. Meanwhile, the lower limit of P as an impurity may not be particularly limited, but it may be limited to 0.001% or more because a large manufacturing cost may be required to control the P content to less than 0.001%.
  • S Sulfur
  • S exists as an impurity in steel, and since it is an element that inhibits ductility, impact properties and weldability of hot-formed members, the maximum content may be limited to 0.02%.
  • S is an impurity, and the lower limit thereof may not be particularly limited, but may be limited to 0.0001% or more because a large manufacturing cost may be required to control the S content to less than 0.0001%.
  • Aluminum (Al), along with Si, is an element that increases the cleanliness of steel by deoxidizing in steelmaking.
  • the content of Al is preferably in the range of 0.01 to 0.1%.
  • the lower limit of the Al content is more preferably 0.015%.
  • the upper limit of the Al content is more preferably 0.08%, even more preferably 0.07%, and most preferably 0.06%.
  • Chromium (Cr) is added to secure hardenability of steel like Mn. If the Cr content is less than 0.01%, it may be difficult to secure sufficient hardenability. On the other hand, when the content exceeds 0.8%, the effect of improving the hardenability compared to the added amount is insignificant, and it promotes the formation of coarse iron carbide, thereby making the collision energy absorbing ability inferior, so the upper limit can be limited to 0.8%. Therefore, the content of Cr is preferably in the range of 0.01 to 0.8%.
  • the lower limit of the Cr content is more preferably 0.015%, and even more preferably 0.02%.
  • the upper limit of the Cr content is more preferably 0.75%, and even more preferably 0.7%.
  • Molybdenum (Mo) not only has the effect of improving the hardenability of steel like Cr and Mn, but also can obtain effects such as increase in bendability by refining grains through the formation of fine precipitates.
  • the Mo content exceeds 0.5%, the upper limit may be limited to 0.5% because it causes an excessive increase in ferroalloy cost compared to the effect.
  • the Mo content is preferably in the range of 0.5% or less (excluding 0%).
  • the Mo content is more preferably 0.45% or less, more preferably 0.4% or less, and most preferably 0.35% or less.
  • the N is included as an impurity in the steel.
  • the N content exceeds 0.02%, there is a problem in that slab cracks are likely to occur due to AlN formation as in the case of Al.
  • the N as an impurity may not be particularly limited with respect to the lower limit thereof, but may be limited to 0.001% or more because a large manufacturing cost may be required to control the N content to less than 0.001%.
  • the steel material for hot forming according to an aspect of the present invention in addition to the alloy components described above, selectively Ni: 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less of one or more may include more.
  • Nickel (Ni) is an austenite stabilizing element and may improve hardenability of steel through addition of Ni.
  • the Ni content is preferably in the range of 0.5% or less.
  • the lower limit of the Ni content is more preferably 0.01%, more preferably 0.03%, and most preferably 0.05%.
  • the upper limit of the Ni content is more preferably 0.45%, even more preferably 0.4%, and most preferably 0.35%.
  • Niobium (Nb) is an element that can obtain a precipitation strengthening effect through the formation of fine precipitates, and through this, an effect of improving the bendability by increasing the strength and refining the grains can be obtained. In addition, by suppressing excessive grain growth during heating for hot forming, it is possible to achieve robustness against variations in heat treatment conditions.
  • the content of Nb is preferably in the range of 0.1% or less.
  • the lower limit of the Nb content is more preferably 0.005%, even more preferably 0.01%, and most preferably 0.015%.
  • the upper limit of the Nb content is more preferably 0.09%, even more preferably 0.08%, and most preferably 0.07%.
  • Titanium (Ti) is an element that is sometimes added together when B is added to secure hardenability by combining with nitrogen remaining as an impurity in steel to form TiN.
  • precipitation strengthening and grain refinement effects can be expected through the formation of TiC precipitates.
  • the Ti content is preferably in the range of 0.1% or less.
  • the lower limit of the Ti content is more preferably 0.005%, even more preferably 0.01%, and most preferably 0.015%.
  • the upper limit of the Ti content is more preferably 0.08%, even more preferably 0.06%, and most preferably 0.05%.
  • Boron (B) is an element that can improve hardenability even with a small amount of addition, and can effectively suppress the brittleness of the hot-formed member due to segregation at the grain boundary of P or / and S by segregation at the grain boundary of prior austenite. .
  • the content of B is preferably in the range of 0.01% or less.
  • the lower limit of the B content is more preferably 0.0001%, even more preferably 0.0003%, and most preferably 0.0005%.
  • the upper limit of the B content is more preferably 0.009%, even more preferably 0.007%, and most preferably 0.005%.
  • the remainder may include Fe and unavoidable impurities. Inevitable impurities may be unintentionally mixed in a typical steel manufacturing process, and this cannot be entirely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning.
  • the present invention does not entirely exclude the addition of a composition other than the above-mentioned steel composition.
  • the steel material for hot forming according to an aspect of the present invention satisfies the above-described component system, and at the same time, maximizes the collision energy absorption capacity in the hot forming member and minimizes the hardness deviation.
  • the alloy index is less than 7, it is difficult to secure sufficient hardenability, so that a large hardness deviation may occur in the hot-formed member.
  • grain boundary ferrite is formed in the surface layer portion of the hot-formed member, so that the collision energy absorption ability may be greatly inferior.
  • the alloy index is more preferably 7.5 or more, and more preferably 8 or more. In the present invention, as long as the alloy index is 7 or more, the effect desired by the present invention can be secured, so the upper limit is not particularly limited.
  • the alloy index may be 40 or less, more preferably 30 or less in terms of manufacturing cost reduction.
  • Relational Equation 1 shows that the present inventors used a number of ferroalloys whose contents of main alloying elements were changed based on the same carbon content, and after heating to an austenite region, hardening for each element through a final hardness change test for each cooling rate It is an expression derived through linear regression analysis on the effect on performance.
  • energy absorption capacity is one of the important characteristics, and this energy absorption capacity is affected by strength and bending characteristics. That is, the higher the strength and the better the bending properties, the better the energy absorption ability.
  • the factor that has the greatest influence on strength after hot forming of steel for hot forming is the martensite fraction, and in particular, when martensite is the main structure, it is greatly affected by the carbon content.
  • the bending properties depending on the composition of the tissue it usually exhibits excellent properties when it is composed of a single phase, and when it is composed of two or more phases, the smaller the difference in strength between the phases, the better the characteristic.
  • the number of carbides having an equivalent circle diameter of 0.5 ⁇ m or more is 10 5 pieces/mm 2 or less.
  • the present inventors have concluded that it is important to properly secure the strength and bendability of the material in order to maximize the excellent impact energy absorption capacity of the hot-formed member, and through various experiments, by appropriately controlling the number density of coarse carbides from the steel sheet, bending It was confirmed that it was possible to secure sex.
  • the coarse carbide having an equivalent circle diameter of 0.5 ⁇ m or more exceeds 10 5 pieces/mm 2 , some iron carbides are re-dissolved during heating for hot forming, and other portions remain in the member after hot forming.
  • Coarse carbide remaining without being completely dissolved in this way acts as a cracking initiation point during bending deformation, so it becomes a factor to decrease bendability and ultimately deteriorates the ability to absorb impact energy.
  • the steel material for hot forming may include at least one of ferrite: 50 to 90 area%, pearlite: 30 area% or less, bainite: 20 area% or less, and martensite: 20 area% or less have.
  • the ferrite is an effective structure for reducing the blanking process load of a steel sheet when manufacturing a blank as a soft phase, and in order to obtain the above effect, it is preferable to secure 50 area% or more.
  • the ferrite preferably has a range of 50 to 90 area%.
  • cementite may be incompletely dissolved after hot forming to reduce strength or cause material non-uniformity.
  • bainite and martensite each exceed 20 area %, the strength of the steel sheet is excessively increased, which may cause problems such as mold wear during blank manufacturing.
  • An aluminum-based plating layer may be formed on at least one surface of the steel material for hot forming according to an aspect of the present invention.
  • the aluminum-based plating layer is not particularly limited, but as a non-limiting embodiment, Si: 6 to 12%, Fe: 1 to 4%, the remainder Al and unavoidable impurities may be included in wt%.
  • the hot forming member according to an aspect of the present invention may be manufactured by hot press forming the above-described steel for hot forming.
  • the hot-formed member according to an aspect of the present invention preferably has the above-described alloy composition and alloy index. Further, it is preferable that the number of carbides having an equivalent circle diameter of 0.5 ⁇ m or more is 10 4 pieces/mm 2 or less. Since carbides existing in the steel sheet before hot forming are dissolved into the steel in the heating step for hot forming, the number density and size of the carbides are reduced compared to the state of the steel sheet before hot forming.
  • the hot-formed member according to an aspect of the present invention may have a martensitic single-phase microstructure or a mixed structure including martensite and bainite in an area of 40% or less. Since the martensite is an effective structure for securing the strength targeted by the present invention, the microstructure of the present invention may be a martensitic single-phase structure. On the other hand, although bainite has a slightly lower strength than martensite, it does not significantly reduce bendability when formed in a martensite matrix, and is an advantageous structure for securing strength. In the present invention, together with the martensite, 40 area% or less It may have a mixed structure containing bainite of However, when the fraction of bainite is less than 40 area %, it may be difficult to secure the target strength in the present invention.
  • the microstructure may further include at least one of 10% by area or less of ferrite and 5% or less of retained austenite.
  • the ferrite and retained austenite are structures that may be unavoidably contained in the manufacturing process. When the ferrite structure exceeds 10 area %, the strength may be reduced as well as the bending properties may be greatly inferior, and if the retained austenite structure exceeds 5 area %, the strength may be reduced or atmospheric gas during hot forming Hydrogen inflow is increased, which may increase the possibility of hydrogen embrittlement.
  • CIE Crack initiation Energy
  • the hot-formed member measures hardness at arbitrary points of the hot-formed member, and the difference between the maximum value and the minimum value is called the hardness deviation, and the value obtained by dividing the hardness deviation by the average hardness of the hot-formed member
  • the deviation level may be 0.3 or less.
  • the number of the arbitrary points is not particularly limited, and, for example, hardness may be measured for any 9 points.
  • the hot-formed member according to an aspect of the present invention may have a yield strength (YS): 800 MPa or more, a tensile strength (TS): 1000 MPa or more, and an elongation (El): 5% or more.
  • Yield strength 800 MPa or more
  • TS tensile strength
  • El elongation
  • the steel slab heating temperature is preferably 1050 ⁇ 1300 °C.
  • the lower limit of the heating temperature of the steel slab is more preferably 1070 °C, even more preferably 1100 °C.
  • the upper limit of the heating temperature of the steel slab is more preferably 1280 °C, even more preferably 1250 °C.
  • finish hot rolling the heated steel slab at 800 ⁇ 950 °C to obtain a hot rolled steel sheet.
  • the finish hot rolling temperature is less than 800° C., it may be difficult to control the plate shape because a mixed structure is generated in the surface layer portion of the steel sheet according to the abnormal rolling.
  • the finish hot rolling temperature exceeds 950° C., there is a problem in that grain coarsening due to hot rolling easily occurs. Therefore, the finish hot rolling temperature is preferably 800 ⁇ 950 °C.
  • the lower limit of the finish hot rolling temperature is more preferably 810°C, and even more preferably 820°C.
  • the upper limit of the finish hot rolling temperature is more preferably 940°C, even more preferably 930°C.
  • the hot-rolled steel sheet is wound at 500 to 700°C.
  • the coiling temperature is less than 500° C.
  • martensite is formed in whole or in part of the steel sheet, making it difficult to control the shape of the plate, and also, due to the increase in strength of the hot-rolled steel sheet, there may be a problem in that the rollability in the subsequent cold rolling process is deteriorated.
  • the coiling temperature exceeds 700° C., coarse carbides are formed, and the collision energy absorbing ability of the hot-formed member may be reduced. Therefore, the coiling temperature is preferably 500 ⁇ 700 °C.
  • the lower limit of the coiling temperature is more preferably 520°C, and even more preferably 550°C.
  • the upper limit of the coiling temperature is more preferably 680°C, and even more preferably 650°C.
  • the wound hot-rolled steel sheet is cooled from the coiling temperature to 400°C at a cooling rate of 10°C/Hr or more.
  • the cooling rate is 10° C./Hr or more.
  • the cooling rate is more preferably 12°C/Hr or more, and even more preferably 15°C/Hr or more.
  • the upper limit of the cooling rate is not particularly limited.
  • the cooling rate may be 500° C./Hr or less, more preferably 45° C./Hr or less, and even more preferably 400° C./Hr or less.
  • the process of pickling the cooled hot-rolled steel sheet before cold rolling may be further included.
  • the product surface quality can be improved by removing the scale formed on the surface of the steel sheet through the pickling process.
  • the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
  • a reduction ratio of 30 to 80% may be applied to obtain a target thickness of the steel material.
  • continuous annealing and aluminum-based plating may be performed on the cold-rolled steel sheet, or aluminum-based plating may be performed on the cooled hot-rolled steel sheet immediately after pickling.
  • the cold-rolled steel sheet it is preferable to heat the cold-rolled steel sheet in a temperature range from 400° C. to an annealing temperature at a rate of 20° C./s or less.
  • the heating rate exceeds 20°C/s from 400°C to the annealing temperature, there is not enough time for the carbide precipitated in the hot-rolling step to be re-dissolved, so coarse carbide may remain, and the final resultant hot-formed member collides Energy absorption capacity may be reduced. Therefore, the heating rate from 400° C. to the annealing temperature is preferably 20° C./s or less. The heating rate is more preferably 18°C/s or less, and even more preferably 15°C/s or less.
  • the lower limit of the heating rate is not particularly limited.
  • the heating rate may be 0.5°C/s or more, more preferably 1°C/s or more, and even more preferably 1.5°C/s or more.
  • the heating rate is not particularly limited in the temperature range from the cold rolling temperature to less than 400 °C, because even if the heating rate is controlled, the effect on the re-dissolution of carbides is insignificant.
  • the heated cold-rolled steel sheet is preferably annealed at 740 ⁇ 860 °C. If the annealing temperature is less than 740° C., recrystallization of the cold-rolled tissue is not sufficiently performed, resulting in poor plate shape or excessively high strength after plating, which may cause mold wear during the blanking process. On the other hand, when the annealing temperature exceeds 860° C., Si, Mn, etc. may form a surface oxide during the annealing process, resulting in a poor plating surface. Therefore, the annealing temperature is preferably 740 ⁇ 860 °C. The lower limit of the annealing temperature is more preferably 750°C, and even more preferably 760°C. The upper limit of the annealing temperature is more preferably 850°C, and even more preferably 840°C.
  • the atmosphere during the continuous annealing is preferably a non-oxidizing atmosphere, for example, a hydrogen-nitrogen mixed gas may be used, and in this case, the dew point temperature of the atmospheric gas may be -70 ⁇ -30 °C. .
  • the dew point temperature In order for the dew point temperature to be less than -70°C, additional equipment for control is required, which increases the manufacturing cost.
  • the dew point exceeds -30°C, an excessive amount of annealed oxide is formed on the surface of the steel sheet during annealing. It may cause defects such as plating. Therefore, it is preferable that the dew point temperature of the atmospheric gas during the continuous annealing is -70 to -30°C.
  • the lower limit of the dew point temperature of the atmospheric gas is more preferably -65°C, and even more preferably -60°C.
  • the upper limit of the dew point temperature of the atmospheric gas is more preferably -35°C, and even more preferably -40°C.
  • the annealed cold-rolled steel sheet is cooled from the annealing temperature to 660°C at a cooling rate of 1°C/s or more.
  • the cooling rate from the annealing temperature to 660° C. is preferably 1° C./s or more, and the cooling rate is If it is less than 1° C./s, a large amount of coarse carbides may be formed, so that the finally obtained hot-formed member may have a reduced ability to absorb impact energy. Therefore, it is preferable that the cooling rate is 1°C/s or more.
  • the cooling rate is more preferably 1.5°C/s or more, and even more preferably 2°C/s or more.
  • the upper limit of the cooling rate is not particularly limited.
  • the cooling rate may be 50° C./s or less, more preferably 45° C./s or less, and still more preferably 40° C./s or less.
  • the method may further include forming an aluminum-based plating layer by immersing the cooled cold-rolled steel sheet in an Al-based plating bath.
  • the composition and plating conditions of the Al-based plating bath are not particularly limited.
  • the composition of the plating bath may include Si: 6 to 12%, Fe: 1 to 4%, the remainder Al and other unavoidable impurities, and the plating amount is commonly applied in the art. It may be 30 ⁇ 130g/m 2 based on one side being used.
  • the Si content in the plating bath composition is less than 6%, the plating bath temperature is excessively increased to deteriorate the equipment, and when it exceeds 12%, the heating time for hot forming must be prolonged by excessively delaying alloying.
  • the Fe content is less than 1%, plating adhesion or spot weldability may be inferior, and if it exceeds 4%, dross in the plating bath may be excessively generated and cause poor surface quality.
  • Coating weight in this case single-sided standard 30g / m 2 is less than may be difficult to secure the corrosion resistance of the desired hot forming member, 130g / m 2 If exceeded, the due to the excessive coating weight not only manufacturing costs to rise coil the coating weight on steel It may not be easy to uniformly plated the entire width and length.
  • a steel material for hot forming manufactured by the above-described manufacturing method is prepared, and a blank for hot forming is prepared using the steel material for hot forming.
  • the blank is heated to a temperature range of austenite single-phase region or higher, more specifically, Ac3 temperature or higher and 980°C or lower. If the blank heating temperature is less than the Ac3 temperature, it may be difficult to secure a predetermined strength due to the presence of untransformed ferrite. On the other hand, when the heating temperature exceeds 980°C, it may be difficult to secure spot weldability due to excessive oxide formation on the member surface. Therefore, the blank heating temperature is preferably Ac3 ⁇ 980 °C.
  • the lower limit of the blank heating temperature is more preferably Ac3+5°C, and even more preferably Ac3+10°C.
  • the upper limit of the said blank heating temperature it is more preferable that it is 970 degreeC, and it is still more preferable that it is 960 degreeC.
  • the heated blank is preferably maintained in the temperature range for 1 to 1000 seconds. If the holding time is less than 1 second, the temperature may not be uniform throughout the blank, which may cause material differences for each part, and if the holding time exceeds 1000 seconds, spot weldability is secured by excessive oxide generation on the member surface like excessive heating temperature. It can be difficult to do. Therefore, the holding time is preferably 1 to 1000 seconds.
  • the lower limit of the holding time is more preferably 30 seconds, and still more preferably 60 seconds.
  • the upper limit of the said holding time it is more preferable that it is 900 second, and it is still more preferable that it is 800 second.
  • the heated and maintained blank is hot-formed and cooled to room temperature to finally prepare a hot-formed member.
  • the specific conditions for the hot forming are not particularly limited, and a hot forming method commonly known in the art to which the present invention belongs may be applied as it is.
  • a steel slab having a thickness of 40 mm having the alloy composition shown in Table 1 was prepared through vacuum melting.
  • the steel slab was heated to 1250°C, and then hot-rolled to a finish hot rolling temperature of 900°C to obtain a hot-rolled steel sheet.
  • the conditions described in Table 2 below were applied to the coiling temperature and the cooling rate from the coiling temperature to 400° C. for each steel type using a simulated heat treatment furnace, and hot rolling was performed so that the final hot-rolled thickness was all 3 mm.
  • cold rolling was performed at a cold rolling reduction ratio of 50% to obtain a cold-rolled steel sheet.
  • the cold-rolled steel sheet is heated by controlling the heating rate from 400° C.
  • the annealing temperature under the conditions shown in Table 2 below, annealed in an atmosphere of 5% hydrogen-95% nitrogen, and then cooled from the annealing temperature to 660° C.
  • a cold-rolled steel sheet was manufactured by controlling the speed. Then, after cooling the cold-rolled steel sheet, Al-based plating was performed. At this time, the composition of the Al-based plating bath was composed of Al-9%Si-2%Fe and the rest of the unavoidable impurities, and the plating adhesion amount was 80 g/m 2 based on one side.
  • the steel sheet thus manufactured was manufactured as a blank, and then hot-formed using a mold for hot forming to prepare a hot-formed member having the form shown in FIG. 2 .
  • the heating temperature of the blank was 900° C.
  • the holding time was 6 minutes
  • the transfer time from the heating furnace to molding was applied in the same way as all 10 seconds.
  • the number of carbides having an equivalent circle diameter of 0.5 ⁇ m or more was measured by observing 10 fields of view at a magnification of 10000 times using a transmission electron microscope (TEM) after preparing a thin foil specimen.
  • TEM transmission electron microscope
  • the microstructure was measured using a scanning transfer microscope after etching the surface of the steel sheet using nital.
  • Yield strength (YS), tensile strength (TS), and elongation (El) were measured by taking ASTM standard specimens in a direction parallel to the rolling direction of the steel sheet, and then performing a tensile test.
  • the impact energy absorption capacity was evaluated by measuring the area until reaching the maximum load (CIE: Crack Initiation Energy) from the load-displacement curve obtained from a three-point bending test according to the VDA standard (VDA238-100). , a case higher than 25000 Nm, which is the CIE value of a typical 1500 MPa class steel for hot forming, was evaluated as good, and a case lower than that was evaluated as bad.
  • CIE Crack Initiation Energy
  • Comparative Examples 6 to 8 are cases in which the C content exceeds the conditions of the present invention, and although there is an effect of increasing the strength according to the increase of the C content, it can be seen that the bendability is greatly decreased compared to the increase in strength, so that the collision energy absorption ability is rather inferior.
  • Comparative Example 9 is a case in which the C content does not meet the conditions of the present invention, and it can be seen that not only did not secure the target strength, but also the collision energy absorption ability was deteriorated due to this.
  • FIG. 3 is a graph showing the collision energy absorption capacity according to the carbon content and alloy index of Inventive Examples 1 to 7 and Comparative Examples 3 to 9; As shown in Figure 3, the C content and alloy index are directly related to the impact energy absorption capacity of the hot-formed member, and good impact energy absorption capacity can be secured only when the C content and alloy index proposed by the present invention are satisfied. can be known

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PCT/KR2020/018658 2019-12-20 2020-12-18 열간성형용 강재, 열간성형 부재 및 이들의 제조방법 Ceased WO2021125878A1 (ko)

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CN202080088989.XA CN114867883B (zh) 2019-12-20 2020-12-18 热成型用钢材、热成型部件及它们的制造方法
US17/778,614 US12385115B2 (en) 2019-12-20 2020-12-18 Steel for hot forming, hot-formed member, and manufacturing methods therefor
EP20901253.3A EP4079913A4 (en) 2019-12-20 2020-12-18 STEEL FOR HOT FORMING, HOT FORMED ELEMENT AND METHOD OF MANUFACTURE THEREOF
JP2022530941A JP7648620B2 (ja) 2019-12-20 2020-12-18 熱間成形用鋼材、熱間成形部材及びこれらの製造方法
MX2022006587A MX2022006587A (es) 2019-12-20 2020-12-18 Acero para formacion en caliente, miembro formado en caliente y metodos de manufactura para los mismos.
JP2024029954A JP2024063127A (ja) 2019-12-20 2024-02-29 熱間成形用鋼材、熱間成形部材及びこれらの製造方法
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