US20110017363A1 - High strength thin steel sheet excelling in weldability and process for producing the same - Google Patents

High strength thin steel sheet excelling in weldability and process for producing the same Download PDF

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Publication number
US20110017363A1
US20110017363A1 US12/810,852 US81085208A US2011017363A1 US 20110017363 A1 US20110017363 A1 US 20110017363A1 US 81085208 A US81085208 A US 81085208A US 2011017363 A1 US2011017363 A1 US 2011017363A1
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steel sheet
less
thin steel
steel
high strength
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US12/810,852
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Hee Jae Kang
Jin Keun Oh
Kwang Geun Chin
Jong Sang Kim
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Posco Holdings Inc
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Posco Co Ltd
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Publication of US20110017363A1 publication Critical patent/US20110017363A1/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a high strength thin steel sheet having a tensile strength of about 800 MPa or more, mainly used for construction materials, home appliances, and automobiles, and a manufacturing method thereof, and more particularly, to a high strength thin steel sheet having excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio (HER) as well as high tensile strength, and a manufacturing method thereof.
  • a high strength thin steel sheet having a tensile strength of about 800 MPa or more, mainly used for construction materials, home appliances, and automobiles, and a manufacturing method thereof, and more particularly, to a high strength thin steel sheet having excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio (HER) as well as high tensile strength, and a manufacturing method thereof.
  • HER hole expansion ratio
  • a steel sheet for automobiles has required even higher strength to improve fuel economy or durability.
  • a high strength steel sheet having a high strength of about 800 MPa is increasingly used for a car's body structures or a reinforcing material in aspects of collision safety and passenger protection.
  • the high strength of a steel sheet causes a reduction in moldability and a welding characteristic, the development of a material for complementing this problem is highly required.
  • steel sheets of various composite structures such as ferrite-martensite dual phase steel or transformation-induced plasticity (TRIP) steel sheet using transformation-induced plasticity of retained austenite have been developed up to now.
  • Japanese Laid-Open Patent Publication No. 6-145892 proposes a method for manufacturing a steel sheet having excellent moldability by controlling chemical components and an amount of retained austenite.
  • Japanese Patent No. 2660644 and Japanese Patent No. 2704350 propose a method for manufacturing a high strength steel sheet having press moldability by controlling chemical components and fine structures of the steel sheet.
  • Japanese Patent No. 3317303 proposes a steel sheet including retained austenite of 5% or more and having an excellent moldability, particularly, excellent local elongation.
  • most of the above-described related arts have been developed to improve ductility. Sufficient considerations of bending workability, a hole expansion ratio, welding characteristic, etc., which are important standards during actual part processing, have not been made.
  • a most crucial characteristic of a steel sheet used for a car's body structure or a reinforcing material mainly requiring a steel sheet of high strength of 800 MPa or more is a spot welding characteristic.
  • the steel used for a car's body structure or a reinforcing material protects passengers by absorbing collision energy during a collision. If the strength of a spot welded portion is not sufficient, the portion will be destroyed and cut, so that a sufficient level of collision energy absorption cannot be obtained.
  • Japanese Laid-Open Patent Publication No. 2003-193194 there exists Japanese Laid-Open Patent Publication No. 2003-193194, but it does not meet a welding characteristic actually required by the market.
  • Japanese Laid-Open Patent Publication No. 2005-105367 proposes technology of securing a welding characteristic and ductility for steel of 780 MPa or more.
  • a cold rolling characteristic is remarkably reduced due to the high strength of a hot strip, which is an intermediate material.
  • a rapid cooling heat treatment condition should be applied during an annealing process, workability is also remarkably reduced.
  • Japanese Laid-Open Patent Publication No. 2005-105367 has no sufficient consideration of these problems.
  • the present invention has been made to solve the foregoing problems with the prior art, and therefore an object of the present invention is to provide a steel sheet having excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio in manufacturing a thin steel sheet having high tensile strength of 800 MPa or more. Also, another object of the present invention is to provide a method of securing workability of a steel sheet.
  • a steel sheet including, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as a remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of 5 ⁇ (Si/Mn+150B)/Sb ⁇ 20 and C+Mn/20+Si/30+2P+4S ⁇ 0.27.
  • a method for manufacturing a steel sheet including: reheating a slab of the steel sheet including, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as a remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of 5 ⁇ (Si/Mn+150B)/Sb ⁇ 20 and C+Mn/20+Si/30+2P+4S ⁇ 0.27, and rolling and winding the slab at a temperature
  • the steel sheet may have a structure including at least one selected from the group consisting of bainite and bainitic ferrite occupying 40% or more, and ferrite and martensite phases occupying the remainder.
  • the present invention can provide a steel sheet having excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio while having high a tensile strength of about 800 MPa or more, and a manufacturing method thereof that can secure the manufacturability of the steel sheet.
  • a steel sheet includes, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as the remainder, and Si, Mn, B, Sb, P, and S meet conditions of 5 ⁇ (Si/Mn+150B)/Sb ⁇ 20 and C+Mn/20+Si/30+2P+4S ⁇ 0.27.
  • a method for manufacturing the steel sheet includes: reheating a slab of the steel sheet including, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as the remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of 5 ⁇ (Si/Mn+150B)/Sb ⁇ 20 and C+Mn/20+Si/30+2P+4S ⁇ 0.27, and rolling and winding the slab at a temperature of a finish rolling exit side between Ar 3 transformation point and 950
  • the steel sheet includes at least one selected from the group consisting of bainite and bainitic ferrite occupying 40% or more, and ferrite and martensite occupying the remainder.
  • Carbon (C) is preferably in a weight % of 0.02-0.20 weight % (hereinafter simply referred to as %).
  • Carbon in steel is an element added in order to strengthen a transformation structure.
  • an amount of C exceeds 0.20%, a hole extension characteristic and a welding characteristic is reduced.
  • an amount of C is less than 0.02%, it is difficult to secure strength.
  • Silicon (Si) is preferably in 1.5% or less.
  • Silicon in steel is an element that can be effectively used in order to improve strength.
  • silicon not only causes surface scale defects but also reduces the surface characteristic of a plated steel sheet in connection with a surface characteristic. Also, silicon reduces a chemical treatment characteristic. Therefore, generally, silicon content is limited to 1.0% or less. Since recent progress in plating technology allows silicon content in steel up to 1.5% without a great problem during a manufacturing process, the content is limited to 1.5% or less.
  • Mn is preferably in 1.5-3.0%.
  • Mn in steel is an element having a very high solid-solution strengthening effect and simultaneously, facilitates the formation of a composite structure including ferrite and martensite.
  • Mn content in steel is less than 1.5%, it is difficult to secure the strength required by the present invention.
  • Mn content exceeds 3.0% there is a high possibility that problems in a welding characteristic and a hot rolling characteristic will occur.
  • P is preferably in 0.001-0.10%.
  • P in steel is an element having an effect of strengthening the steel.
  • P content is less than 0.001%, not only can the strengthening effect can be secured but a problem in manufacturing costs may also be generated.
  • press moldability may reduce and brittleness of steel may occur.
  • S is preferably in 0.010% or less.
  • S in steel is an impurity element, hindering the ductility and welding characteristic of a steel sheet.
  • S content in steel exceeds 0.01%, there is a high possibility of hindering the ductility and welding characteristic of a steel sheet.
  • Sol.Al is preferably in 0.01-0.4%.
  • Sol.Al in steel is an effective element to combine with oxygen in steel to perform a deoxidation operation, distribute carbon inside ferrite to austenite to improve martensite hardening ability.
  • Sol.Al content is less than 0.01%, such an effect cannot be secured.
  • Sol.Al content exceeds 0.4%, such an effect is saturated and manufacturing costs may increase.
  • N is preferably 0.020% or less.
  • N in steel is an element that effectively stabilizes austenite.
  • N content in steel exceeds 0.020%, the stability of austenite greatly increases to prevent the formation of bainite, which is a fine structure intended by the present invention.
  • Cr is preferably in 0.3-1.5%.
  • Cr in steel is an element added to improve the hardening ability of steel and to secure high strength.
  • Cr plays an important role of facilitating the formation of bainite.
  • Cr content in steel is less than 0.3%, such an effect is difficult to secure.
  • Cr content in steel exceeds 1.50%, such an effect is saturated and is disadvantageous economically.
  • Boron (B) is preferably in 0.0010-0.0060%.
  • Boron in steel is an element used to delay the transformation of austenite into pearlite during a cooling process of an annealing process.
  • B is added as an element of suppressing forming of ferrite and facilitating forming of bainite.
  • B content in steel is less than 0.0010%, such an effect is difficult to obtain.
  • B content in steel exceeds 0.0060%, excessive B is inspissated on a surface to cause deterioration in plating adhesion.
  • Sb is preferably 0.001-0.1%.
  • Sb in steel is an indispensable element added in order to secure an excellent plating characteristic in the present invention.
  • Sb has an outstanding effect in suppressing surface inspissation of oxides such as MnO, SiO 2 , Al 2 O 3 , etc. to reduce surface defects, and suppressing coarsening of surface inspissation materials by temperature rise and a change in a hot rolling process.
  • Sb content is less than 0.001%, such an effect is difficult to secure, and even when an added amount continuously increases, such an effect does not increase greatly and problems of manufacturing costs and moldability reduction may be generated. Therefore, Sb content is limited to 0.001-0.1%.
  • one or two or more materials selected from Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08% are added to the steel formed of the above elements to achieve a strength increase and miniaturization of grain diameters.
  • the steel of the present invention is formed with Fe and other inevitable impurities as the remainder besides the above-described elements.
  • an alloy constituent ratio of Si, Mn, B, Sb, P, and S may satisfy the following Math Figures 1 and 2 in designing an alloy of a steel sheet having the above-described component ranges.
  • Math Figure 1 is a component relation that can secure surface quality, obtained as an empirical numerical value. That is, Mn, Si, and B in steel are elements having a characteristic of forming inspissation materials on a surface during an annealing process. As inspissation materials of these elements increase, a plating characteristic is reduced. On the other hand, since Sb hinders a grain boundary diffusion of the above surface inspissation elements, Sb is very advantageous in an aspect of surface quality. For example, when a value calculated by Math Figure 1 is between 5 and 20, it means that a good surface quality can be secured.
  • Math Figure 2 is a component relation that can secure a desirable welding characteristic, obtained as an empirical numerical value. That is, C, Mn, Si, P, and S in steel raise a carbon equivalent. As well known in the art, when a carbon equivalent is high, a welding characteristic is reduced. Setting a condition by which a welding defect is not generated during spot welding, which is a welding method primarily performed when steel of the present invention is used through repeated experiments provides Math Figure 2. When a value calculated using Math Figure 2 exceeds 0.27, it means that there is a high possibility that a welding defect may be generated.
  • a steel sheet of the present invention has a structure in which one or more selected from bainite and bainitic ferrite occupy 40% or more, and ferrite and martensite phases occupy the remainder. Ferrite and martensite may occupy 25% or less and 35% or less, respectively.
  • a slab whose components have been formed using the above-described alloy designing method is reheated and hot rolling is performed.
  • Finish rolling in the hot rolling may be performed at a temperature of an exit side between the Ar 3 transformation point and 950° C. That is, at a hot finish rolling temperature below the Ar 3 transformation point, there is a high possibility that hot transformation resistance rapidly increases, and a problem in manufacturing may be generated.
  • a temperature exceeding 950° C. not only may excessively thick oxidation scales occur, but there is also high possibility that a steel sheet may be coarsened.
  • a hot rolled steel sheet manufactured using the above process is pickled and cold-rolled.
  • a reduction ratio of the cold rolling may be 40-80%.
  • a reduction ratio is less than 40%, recrystallization driving force is weakened, so that there is possibility that a problem may be generated in obtaining good recrystalline grain.
  • a reduction ratio exceeds 80%, a rolling load increases rapidly.
  • the above obtained cold rolled steel sheet is continuously annealed at a temperature of preferably 740-860° C.
  • a danger that non-recrystallization grain is formed increases.
  • temperature exceeds 860° C. a large grain may be formed and simultaneously, a strip passing ability may be defective due to a high temperature annealing operation.
  • the cold rolled steel sheet is continuously cooled down to a temperature of 250-600° C. at a cooling rate allowing a value calculated by following Math Figure 3 to exceed 30 within the cooling rate (CR) of 3-150° C./s, and then is gradually cooled down at a cooling rate of 5° C./min. or more.
  • a high strength thin steel sheet having tensile strength of 800 MPa and having good plating characteristic, welding characteristic, and hole expansion ratio can be easily manufactured by continuously annealing the thin steel sheet under the above condition.
  • cooling may be performed in the cooling rate (CR) of 3-150° C./s as described above.
  • a cooling rate allowing a value calculated by Math Figure 3 to exceed 30 should be applied. That is, when a value calculated by Math Figure 3 is less than 30, bainite or bainitic ferrite phase, in which the steel of the present invention intends to obtain as its fine structure, is difficult to obtain by as much as 40% or more. When the bainite-based structure is obtained by as much as 40% or more, a product having excellent bending workability and hole expansion ratio while having high strength of about 800 MPa, which are the characteristics of the steel according to the present invention, can be manufactured.
  • a cooling final temperature for a cooling operation may be a temperature between 250 and 600° C.
  • a cooling final temperature is less than 250° C.
  • a danger that a large amount of martensite will be formed increases.
  • a cooling final temperature exceed 600° C. a large amount of soft phases of ferrite or pearlite, etc., are formed, so that an intended material is difficult to accomplish.
  • the above-described manufacturing method can be likewise applied to a plated product such as a hot-dip galvanized material (GI) and a galvannealed material (GA) as well as a cold rolled steel sheet.
  • a plated product such as a hot-dip galvanized material (GI) and a galvannealed material (GA) as well as a cold rolled steel sheet.
  • a slab having the component composition of the present invention is heated to a temperature of 1200° C. and extracted, and then rolling is performed at a cold reduction ratio of 55% using, as a material, a hot rolled steel sheet manufactured by hot-rolling the slab under a condition of a finish rolling temperature of 900° C.
  • Continuous annealing heat treatment is performed (CR) at the annealing temperature and cooling condition of Table 2.
  • a plated product is manufactured by performing hot-dip galvanizing (GI) and galvannealing (GA) processes. Conditions and galvannealing process time applied during continuous annealing are given below.
  • plating characteristics (appearance and adhesion characteristic) and the quality of a material (tensile strength, hole expansion ratio, and bending workability) are measured and results thereof are shown together with a comparison material.
  • a plated appearance is represented by non-plating or ⁇ for a case not including other plating defects.
  • a defect name is written for a case where a plating defect is generated.
  • a plating adhesion appraisal has been made in the following way, in which: a plated sheet is cut off by 20 mm ⁇ 50 mm, a bending test is performed on the plated sheet, the plated sheet is unfolded again, a tape is attached on the folded portion of the plated sheet, and the width of a plated layer detached from the plated sheet is appraised using the following criteria.
  • ⁇ Width of detached plating is within about 1-3 mm
  • ⁇ Width of detached plating is within about 3-5 mm
  • Width of detached plating is about 5 mm or more
  • a hole expansion ratio (HER) is obtained by making a hole having a diameter of 10 mm in a test piece having a size of 120 ⁇ 120 mm, expanding the hole using a punch having a forming portion angle of 60 degrees until a crack is generated, and calculating a ratio of an expanded hole to the initial hole of 10 mm in diameter. Also, in Table 2, bending workability has been appraised by performing a bending test on a test piece using a 90 degree V-shaped punch, and measuring a smallest punch radius (mm) that does not cause breakage.
  • FIG. 2 Steel No. C Si Mn P S Al N Ti Nb Mo Cr B Sb value value 1 0.06 0.1 2.5 0.01 0.004 0.035 0.005 0.02 0.05 0.03 0.9 0.0018 0.02 15.5 0.22 2 0.07 0.15 2.2 0.015 0.003 0.05 0.004 0.025 0.055 0.05 0.7 0.0023 0.03 13.8 0.23 3 0.05 0.05 2.1 0.007 0.003 0.22 0.003 0.015 0.045 0.01 0.5 0.0013 0.02 10.9 0.18 4 0.03 0.1 2.5 0.008 0.004 0.043 0.005 — 0.06 — 0.7 0.0019 0.04 8.1 0.19 5 0.15 0.1 2.7 0.005 0.003 0.052 0.003 0.04 — — 1.0 0.0021 0.03 11.7 0.31 6 0.08 0.5 2.1 0.009 0.003 0.35 0.007 0.02 0.03 — 0.8 0.0032 0.04 18.0 0.23 7 0.07 0.20 2.1 0.0
  • FIG. 1 (Si/Mn + 150B)/SbMath
  • FIG. 2 C + Mn/20 + Si/30 + 2P + 4S
  • Math FIG. 1 (Si/Mn + 150B)/SbMath
  • FIG. 2 C + Mn/20 + Si/30 + 2P + 4S
  • a high strength thin steel sheet having tensile strength of about 800 MPa or more, having excellent surface characteristic and mechanical characteristic compared to an existing comparison material, and having excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio can be manufactured.
  • the steel sheet has a structure in which one selected from bainite and bainitic ferrite occupies 40% or more, and ferrite and martensite occupy 25% or less and 35% or less, respectively.

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Abstract

Provided are a high strength thin steel sheet having tensile strength of about 800 MPa or more, and a manufacturing method thereof. The thin steel sheet is mainly used for construction materials, home appliances, and automobiles. The thin steel sheet has excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio. The thin steel sheet includes, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, SoLAl: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and includes Fe and other inevitable impurities as a remainder. Here, Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27. Also, the manufacturing method can secure workability of the thin steel sheet.

Description

    TECHNICAL FIELD
  • The present invention relates to a high strength thin steel sheet having a tensile strength of about 800 MPa or more, mainly used for construction materials, home appliances, and automobiles, and a manufacturing method thereof, and more particularly, to a high strength thin steel sheet having excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio (HER) as well as high tensile strength, and a manufacturing method thereof.
    • BACKGROUND ART
  • Recently, a steel sheet for automobiles has required even higher strength to improve fuel economy or durability. A high strength steel sheet having a high strength of about 800 MPa is increasingly used for a car's body structures or a reinforcing material in aspects of collision safety and passenger protection. However, since the high strength of a steel sheet causes a reduction in moldability and a welding characteristic, the development of a material for complementing this problem is highly required. In response to this requirement, steel sheets of various composite structures such as ferrite-martensite dual phase steel or transformation-induced plasticity (TRIP) steel sheet using transformation-induced plasticity of retained austenite have been developed up to now.
  • For example, Japanese Laid-Open Patent Publication No. 6-145892 proposes a method for manufacturing a steel sheet having excellent moldability by controlling chemical components and an amount of retained austenite. Japanese Patent No. 2660644 and Japanese Patent No. 2704350 propose a method for manufacturing a high strength steel sheet having press moldability by controlling chemical components and fine structures of the steel sheet. Also, Japanese Patent No. 3317303 proposes a steel sheet including retained austenite of 5% or more and having an excellent moldability, particularly, excellent local elongation. However, most of the above-described related arts have been developed to improve ductility. Sufficient considerations of bending workability, a hole expansion ratio, welding characteristic, etc., which are important standards during actual part processing, have not been made.
  • Among the required characteristics of a steel sheet, a most crucial characteristic of a steel sheet used for a car's body structure or a reinforcing material mainly requiring a steel sheet of high strength of 800 MPa or more is a spot welding characteristic. The steel used for a car's body structure or a reinforcing material protects passengers by absorbing collision energy during a collision. If the strength of a spot welded portion is not sufficient, the portion will be destroyed and cut, so that a sufficient level of collision energy absorption cannot be obtained. For technology regarding high strength steel sheet with consideration of a welding characteristic, there exists Japanese Laid-Open Patent Publication No. 2003-193194, but it does not meet a welding characteristic actually required by the market.
  • Also, Japanese Laid-Open Patent Publication No. 2005-105367 proposes technology of securing a welding characteristic and ductility for steel of 780 MPa or more. In the case of manufacturing a steel sheet having a high strength of 800 MPa or more in a real process, a cold rolling characteristic is remarkably reduced due to the high strength of a hot strip, which is an intermediate material. Also, since a rapid cooling heat treatment condition should be applied during an annealing process, workability is also remarkably reduced. Japanese Laid-Open Patent Publication No. 2005-105367 has no sufficient consideration of these problems.
    • DISCLOSURE OF INVENTION Technical Problem
  • The present invention has been made to solve the foregoing problems with the prior art, and therefore an object of the present invention is to provide a steel sheet having excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio in manufacturing a thin steel sheet having high tensile strength of 800 MPa or more. Also, another object of the present invention is to provide a method of securing workability of a steel sheet.
    • Technical Solution
  • According to an aspect of the present invention, there is provided a steel sheet including, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as a remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27.
  • According to another aspect of the present invention, there is provided a method for manufacturing a steel sheet, the method including: reheating a slab of the steel sheet including, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as a remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27, and rolling and winding the slab at a temperature of a finish rolling exit side between the Ar3 transformation point and 950° C.; pickling a wound hot rolled steel sheet and performing cold rolling on the same at a reduction ratio of 40-80%; and performing continuous annealing on an obtained cold rolled steel sheet at a temperature range of 740-860° C., cooling the cold rolled steel sheet down to 250-600° C. at a cooling rate satisfying a condition of −5 Log CR+25C−17Si+40Cr+13,000B>30 in a cooling rate range of 3-150° C./s, and cooling the same at a cooling rate of 5° C./minute or more.
  • The steel sheet may have a structure including at least one selected from the group consisting of bainite and bainitic ferrite occupying 40% or more, and ferrite and martensite phases occupying the remainder.
    • ADVANTAGEOUS EFFECTS
  • The present invention can provide a steel sheet having excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio while having high a tensile strength of about 800 MPa or more, and a manufacturing method thereof that can secure the manufacturability of the steel sheet.
    • BEST MODE FOR CARRYING OUT THE INVENTION
  • A steel sheet includes, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as the remainder, and Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27.
  • Also, a method for manufacturing the steel sheet includes: reheating a slab of the steel sheet including, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as the remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27, and rolling and winding the slab at a temperature of a finish rolling exit side between Ar3 transformation point and 950° C.; pickling a wound hot rolled steel sheet and performing cold rolling on the same at a reduction ratio of 40-80%; and performing continuous annealing on an obtained cold rolled steel sheet at a temperature range of 740-860° C., cooling the cold rolled steel sheet down to 250-600° C. at a cooling rate satisfying a condition of −5 Log CR+25C−17Si+40Cr+13,000B>30 in a cooling rate range of 3-150° C./s, and cooling the same at a cooling rate of 5° C./minute or more.
  • The steel sheet includes at least one selected from the group consisting of bainite and bainitic ferrite occupying 40% or more, and ferrite and martensite occupying the remainder.
  • Hereinafter, the present invention will be described in detail.
  • Carbon (C) is preferably in a weight % of 0.02-0.20 weight % (hereinafter simply referred to as %).
  • Carbon in steel is an element added in order to strengthen a transformation structure. However, when an amount of C exceeds 0.20%, a hole extension characteristic and a welding characteristic is reduced. On the other hand, when an amount of C is less than 0.02%, it is difficult to secure strength.
  • Silicon (Si) is preferably in 1.5% or less.
  • Silicon in steel is an element that can be effectively used in order to improve strength. However, since silicon not only causes surface scale defects but also reduces the surface characteristic of a plated steel sheet in connection with a surface characteristic. Also, silicon reduces a chemical treatment characteristic. Therefore, generally, silicon content is limited to 1.0% or less. Since recent progress in plating technology allows silicon content in steel up to 1.5% without a great problem during a manufacturing process, the content is limited to 1.5% or less.
  • Mn is preferably in 1.5-3.0%.
  • Mn in steel is an element having a very high solid-solution strengthening effect and simultaneously, facilitates the formation of a composite structure including ferrite and martensite. When Mn content in steel is less than 1.5%, it is difficult to secure the strength required by the present invention. When Mn content exceeds 3.0%, there is a high possibility that problems in a welding characteristic and a hot rolling characteristic will occur.
  • P is preferably in 0.001-0.10%.
  • P in steel is an element having an effect of strengthening the steel. When P content is less than 0.001%, not only can the strengthening effect can be secured but a problem in manufacturing costs may also be generated. On the other hand, when P content is excessively added, press moldability may reduce and brittleness of steel may occur.
  • S is preferably in 0.010% or less.
  • S in steel is an impurity element, hindering the ductility and welding characteristic of a steel sheet. When S content in steel exceeds 0.01%, there is a high possibility of hindering the ductility and welding characteristic of a steel sheet.
  • Sol.Al is preferably in 0.01-0.4%.
  • Sol.Al in steel is an effective element to combine with oxygen in steel to perform a deoxidation operation, distribute carbon inside ferrite to austenite to improve martensite hardening ability. When Sol.Al content is less than 0.01%, such an effect cannot be secured. On the other hand, when Sol.Al content exceeds 0.4%, such an effect is saturated and manufacturing costs may increase.
  • N is preferably 0.020% or less.
  • N in steel is an element that effectively stabilizes austenite. When N content in steel exceeds 0.020%, the stability of austenite greatly increases to prevent the formation of bainite, which is a fine structure intended by the present invention.
  • Cr is preferably in 0.3-1.5%.
  • Cr in steel is an element added to improve the hardening ability of steel and to secure high strength. In the present invention, Cr plays an important role of facilitating the formation of bainite. When Cr content in steel is less than 0.3%, such an effect is difficult to secure. When Cr content in steel exceeds 1.50%, such an effect is saturated and is disadvantageous economically.
  • Boron (B) is preferably in 0.0010-0.0060%.
  • Boron in steel is an element used to delay the transformation of austenite into pearlite during a cooling process of an annealing process. B is added as an element of suppressing forming of ferrite and facilitating forming of bainite. However, when B content in steel is less than 0.0010%, such an effect is difficult to obtain. When B content in steel exceeds 0.0060%, excessive B is inspissated on a surface to cause deterioration in plating adhesion.
  • Sb is preferably 0.001-0.1%.
  • Sb in steel is an indispensable element added in order to secure an excellent plating characteristic in the present invention. Sb has an outstanding effect in suppressing surface inspissation of oxides such as MnO, SiO2, Al2O3, etc. to reduce surface defects, and suppressing coarsening of surface inspissation materials by temperature rise and a change in a hot rolling process. When Sb content is less than 0.001%, such an effect is difficult to secure, and even when an added amount continuously increases, such an effect does not increase greatly and problems of manufacturing costs and moldability reduction may be generated. Therefore, Sb content is limited to 0.001-0.1%.
  • According to the present invention, one or two or more materials selected from Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08% are added to the steel formed of the above elements to achieve a strength increase and miniaturization of grain diameters.
  • When an added amount of Ti, Nb, and Mo is less than 0.003% in its lower limit, an effect of achieving a strength increase and miniaturization of grain diameters is difficult to secure. When an added amount exceeds 0.08% in its upper limit, manufacturing costs may be increased and ductility may be remarkably reduced due to excessive eduction materials.
  • The steel of the present invention is formed with Fe and other inevitable impurities as the remainder besides the above-described elements.
  • According to the present invention, an alloy constituent ratio of Si, Mn, B, Sb, P, and S may satisfy the following Math Figures 1 and 2 in designing an alloy of a steel sheet having the above-described component ranges.

  • MathFigure1

  • 5<(Si/Mn+150B)/Sb<20  [Math.1]

  • MathFigure2

  • C+Mn/20+Si/30+2P+4S<0.27  [Math.2]
  • Math Figure 1 is a component relation that can secure surface quality, obtained as an empirical numerical value. That is, Mn, Si, and B in steel are elements having a characteristic of forming inspissation materials on a surface during an annealing process. As inspissation materials of these elements increase, a plating characteristic is reduced. On the other hand, since Sb hinders a grain boundary diffusion of the above surface inspissation elements, Sb is very advantageous in an aspect of surface quality. For example, when a value calculated by Math Figure 1 is between 5 and 20, it means that a good surface quality can be secured.
  • Meanwhile, Math Figure 2 is a component relation that can secure a desirable welding characteristic, obtained as an empirical numerical value. That is, C, Mn, Si, P, and S in steel raise a carbon equivalent. As well known in the art, when a carbon equivalent is high, a welding characteristic is reduced. Setting a condition by which a welding defect is not generated during spot welding, which is a welding method primarily performed when steel of the present invention is used through repeated experiments provides Math Figure 2. When a value calculated using Math Figure 2 exceeds 0.27, it means that there is a high possibility that a welding defect may be generated.
  • A steel sheet of the present invention has a structure in which one or more selected from bainite and bainitic ferrite occupy 40% or more, and ferrite and martensite phases occupy the remainder. Ferrite and martensite may occupy 25% or less and 35% or less, respectively.
  • Hereinafter, a method for manufacturing steel sheet formed of the above components using a cold rolled steel sheet will be described in detail.
  • A slab whose components have been formed using the above-described alloy designing method is reheated and hot rolling is performed. Finish rolling in the hot rolling may be performed at a temperature of an exit side between the Ar3 transformation point and 950° C. That is, at a hot finish rolling temperature below the Ar3 transformation point, there is a high possibility that hot transformation resistance rapidly increases, and a problem in manufacturing may be generated. At a temperature exceeding 950° C., not only may excessively thick oxidation scales occur, but there is also high possibility that a steel sheet may be coarsened.
  • A hot rolled steel sheet manufactured using the above process is pickled and cold-rolled.
  • A reduction ratio of the cold rolling may be 40-80%. When a reduction ratio is less than 40%, recrystallization driving force is weakened, so that there is possibility that a problem may be generated in obtaining good recrystalline grain. When a reduction ratio exceeds 80%, a rolling load increases rapidly.
  • The above obtained cold rolled steel sheet is continuously annealed at a temperature of preferably 740-860° C. When temperature is less than 740° C. during continuous annealing, a danger that non-recrystallization grain is formed increases. When temperature exceeds 860° C., a large grain may be formed and simultaneously, a strip passing ability may be defective due to a high temperature annealing operation.
  • After the continuous annealing, the cold rolled steel sheet is continuously cooled down to a temperature of 250-600° C. at a cooling rate allowing a value calculated by following Math Figure 3 to exceed 30 within the cooling rate (CR) of 3-150° C./s, and then is gradually cooled down at a cooling rate of 5° C./min. or more. A high strength thin steel sheet having tensile strength of 800 MPa and having good plating characteristic, welding characteristic, and hole expansion ratio can be easily manufactured by continuously annealing the thin steel sheet under the above condition.

  • MathFigure3

  • 5 Log CR+25C−17S+40Cr+13,000B>30  [Math.3]
  • where CR is a cooling rate.
  • When a cooling rate is lowered to less than 3° C./s after the continuous annealing, ferrite or pearlite is formed, so that strength intended by the present invention is difficult to secure. Also, if the cooling rate is too higher than 150° C./s, hard phase of martensite, etc. is excessively formed, so that bending workability and a hole expansion ratio is greatly reduced, and also reduction in a strip passing ability due to a shape defect during a process is greatly worried. Therefore, cooling may be performed in the cooling rate (CR) of 3-150° C./s as described above.
  • Also, to accomplish excellent bending workability and hole expansion ratio, which are the characteristics of steel according to the present invention, a cooling rate allowing a value calculated by Math Figure 3 to exceed 30 should be applied. That is, when a value calculated by Math Figure 3 is less than 30, bainite or bainitic ferrite phase, in which the steel of the present invention intends to obtain as its fine structure, is difficult to obtain by as much as 40% or more. When the bainite-based structure is obtained by as much as 40% or more, a product having excellent bending workability and hole expansion ratio while having high strength of about 800 MPa, which are the characteristics of the steel according to the present invention, can be manufactured.
  • Meanwhile, a cooling final temperature for a cooling operation may be a temperature between 250 and 600° C. When a cooling final temperature is less than 250° C., a danger that a large amount of martensite will be formed increases. When a cooling final temperature exceed 600° C., a large amount of soft phases of ferrite or pearlite, etc., are formed, so that an intended material is difficult to accomplish.
  • The above-described manufacturing method can be likewise applied to a plated product such as a hot-dip galvanized material (GI) and a galvannealed material (GA) as well as a cold rolled steel sheet.
    • MODE FOR THE INVENTION
  • Hereinafter, the present invention is described in more detail using an embodiment thereof.
  • As illustrated in Table 1, a slab having the component composition of the present invention is heated to a temperature of 1200° C. and extracted, and then rolling is performed at a cold reduction ratio of 55% using, as a material, a hot rolled steel sheet manufactured by hot-rolling the slab under a condition of a finish rolling temperature of 900° C. Continuous annealing heat treatment is performed (CR) at the annealing temperature and cooling condition of Table 2. A plated product is manufactured by performing hot-dip galvanizing (GI) and galvannealing (GA) processes. Conditions and galvannealing process time applied during continuous annealing are given below.
      • Annealing furnace atmosphere: N2-10% H2O (dew point −32° C.)
      • Annealing furnace heating rate: 3° C./sec
      • Annealing time: 90 sec
      • Plating temperature: 460° C.
      • Galvannealing time: 24 sec (in case of GA product)
  • As illustrated in Table 2, plating characteristics (appearance and adhesion characteristic) and the quality of a material (tensile strength, hole expansion ratio, and bending workability) are measured and results thereof are shown together with a comparison material.
  • In Table 2, a plated appearance is represented by non-plating or ◯ for a case not including other plating defects. A defect name is written for a case where a plating defect is generated.
  • In Table 2, a plating adhesion appraisal has been made in the following way, in which: a plated sheet is cut off by 20 mm×50 mm, a bending test is performed on the plated sheet, the plated sheet is unfolded again, a tape is attached on the folded portion of the plated sheet, and the width of a plated layer detached from the plated sheet is appraised using the following criteria.
  • ⊚: No detached plating or width of detached plating is within about 1 mm
  • ◯: Width of detached plating is within about 1-3 mm
  • Δ: Width of detached plating is within about 3-5 mm
  • X: Width of detached plating is about 5 mm or more
  • In Table 2, a hole expansion ratio (HER) is obtained by making a hole having a diameter of 10 mm in a test piece having a size of 120×120 mm, expanding the hole using a punch having a forming portion angle of 60 degrees until a crack is generated, and calculating a ratio of an expanded hole to the initial hole of 10 mm in diameter. Also, in Table 2, bending workability has been appraised by performing a bending test on a test piece using a 90 degree V-shaped punch, and measuring a smallest punch radius (mm) that does not cause breakage.
  • TABLE 1
    Math Math
    FIG. 1 FIG. 2
    Steel No. C Si Mn P S Al N Ti Nb Mo Cr B Sb value value
    1 0.06 0.1 2.5 0.01 0.004 0.035 0.005 0.02 0.05 0.03 0.9 0.0018 0.02 15.5 0.22
    2 0.07 0.15 2.2 0.015 0.003 0.05 0.004 0.025 0.055 0.05 0.7 0.0023 0.03 13.8 0.23
    3 0.05 0.05 2.1 0.007 0.003 0.22 0.003 0.015 0.045 0.01 0.5 0.0013 0.02 10.9 0.18
    4 0.03 0.1 2.5 0.008 0.004 0.043 0.005 — 0.06 — 0.7 0.0019 0.04 8.1 0.19
    5 0.15 0.1 2.7 0.005 0.003 0.052 0.003 0.04 — — 1.0 0.0021 0.03 11.7 0.31
    6 0.08 0.5 2.1 0.009 0.003 0.35 0.007 0.02 0.03 — 0.8 0.0032 0.04 18.0 0.23
    7 0.07 0.20 2.1 0.012 0.003 0.04 0.003 — — 0.04 0.9 0.0017 0.02 17.5 0.22
    8 0.15 0.2 2.7 0.015 0.008 0.043 0.005 — — — — 0.0012 — — 0.35
    9 0.11 0.3 2.4 0.015 0.008 0.043 0.005 — — 0.04 — — — — 0.30
    10 0.18 0.2 1.8 0.011 0.005 0.038 0.004 — 0.05 — — — — — 0.32
    Math FIG. 1 = (Si/Mn + 150B)/SbMath FIG. 2 = C + Mn/20 + Si/30 + 2P + 4S When a value calculated by Math FIG. 1 is betwwen 5-20, and a value calculated by Math FIG. 2 is less than 0.27, an alloy design condition of the present invention is satisfied.
  • TABLE 2
    Continuous Hole
    annealing Cooling Math Tensile expansion Bending
    Steel temperature rate FIG. 3 Plated Plating strength ratio workability Bainite
    No. Product (° C.) (° C./sec) value appearance adhesion (MPa) (%) (mm) ratio Remark
    1 CR 840 8 54.7 — — 1045 39 0 R 55 Steel of the
    2 GA 830 20 50.6 ◯ ⊚ 995 45 0 R 60 present invention
    3 GA 855 20 30.8 ◯ ⊚ 830 68 0 R 45
    4 GI 860 30 44.4 ◯ ⊚ 874 54 0 R 65
    5 GA 810 20 62.8 ◯ ⊚ 1076 48 0 R 60
    6 CR 820 10 62.1 — — 1012 60 0 R 55
    7 GA 820 20 49.9 — — 982 54 0 R 45
    8 GA 810 20 — Non-plated X 1087 12 2 R 15 Steel of comparison
    9 GA 810 20 — ◯ Δ 990 8 2 R 10
    10 GA 810 20 — Non-plated Δ 1040 7 2 R 10
    Math FIG. 3 = −5LogCR + 25C − 17Si + 40Cr + 13,000B*A value calculated by Math FIG. 3 is 30 or more, the manufacturing condition of the present invention is satisfied.
  • As shown in Table 2, when a steel sheet is manufactured according to the method of the present invention, a high strength thin steel sheet having tensile strength of about 800 MPa or more, having excellent surface characteristic and mechanical characteristic compared to an existing comparison material, and having excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio can be manufactured.
  • According to the steel of the present invention, the steel sheet has a structure in which one selected from bainite and bainitic ferrite occupies 40% or more, and ferrite and martensite occupy 25% or less and 35% or less, respectively.

Claims (6)

1. A high strength thin steel sheet having an excellent welding characteristic, the thin steel sheet comprising: in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as a remainder,
wherein Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27.
2. The thin steel sheet of claim 1, wherein the steel sheet has a structure comprising at least one selected from the group consisting of bainite and bainitic ferrite occupying about 40% or more, and ferrite and martensite phases occupying the remainder.
3. The thin steel sheet of claim 1, wherein the steel sheet comprises a hot-dip galvanized (GI) layer or a galvannealed (GA) layer on a surface of the steel sheet.
4. A method for manufacturing a high strength thin steel sheet having an excellent welding characteristic, the method comprising:
reheating a slab of the steel sheet comprising, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as a remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27, and rolling and winding the slab at a temperature of a finish rolling exit side between the Ar3 transformation point and 950° C.;
pickling a wound hot rolled steel sheet and performing cold rolling on the same at a reduction ratio of 40-80%; and
performing continuous annealing on an obtained cold rolled steel sheet at a temperature range of 740-860° C., cooling the cold rolled steel sheet down to 250-600° C. at a cooling rate satisfying a condition of −5 Log CR+25C−17Si+40Cr+13,000B>30 in a cooling rate range of 3-150° C./s, and cooling the same at a cooling rate of 5° C./minute or more.
5. The method of claim 4, wherein the steel sheet has a structure comprising at least one selected from the group consisting of bainite and bainitic ferrite occupying 40% or more, and ferrite and martensite phases occupying the remainder.
6. The method of claim 4, further comprising performing hot-dip galvanizing (GI) or galvannealing (GA).
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