WO2017111428A1 - 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연강판, 용융아연도금강판 및 이들의 제조방법 - Google Patents
연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연강판, 용융아연도금강판 및 이들의 제조방법 Download PDFInfo
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- WO2017111428A1 WO2017111428A1 PCT/KR2016/014934 KR2016014934W WO2017111428A1 WO 2017111428 A1 WO2017111428 A1 WO 2017111428A1 KR 2016014934 W KR2016014934 W KR 2016014934W WO 2017111428 A1 WO2017111428 A1 WO 2017111428A1
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- C—CHEMISTRY; METALLURGY
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0273—Final recrystallisation annealing
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- 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
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- 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
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- 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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
- C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
- C23C2/0224—Two or more thermal pretreatments
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
- C23C2/026—Deposition of sublayers, e.g. adhesion layers or pre-applied alloying elements or corrosion protection
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
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- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- 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
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- 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
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- 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
- C21D2211/008—Martensite
Definitions
- the present invention relates to a high-strength steel sheet used for structural members of automobiles, and more particularly, high-strength cold-rolled steel sheet, hot dip galvanized with excellent hole expandability and elongation, excellent press formability and excellent phosphate treatment and spot weldability. It relates to a steel sheet and a manufacturing method thereof.
- the present invention has been made to solve the above-mentioned limitations of the prior art, and by using a reverse transformation phenomenon to form a unique structure by using a conventional alloy component, while having excellent ductility and hole expansion compared to the conventional method
- hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet which can improve the corrosion resistance and surface quality of the assembled parts as well as press forming by improving phosphate treatment and plating layer adhesion and plating quality. do.
- carbon (C) 0.05 to 0.3%
- silicon (Si) 0.6 to 2.5%
- aluminum (Al) 0.01 to 0.5%
- manganese (Mn) 1.5 to 3.0%
- the steel microstructure contains, in an area fraction, 60% or less of ferrite, 25% or more of needle bainite, 5% or more of martensite, and 5% or more of needle residual austenite,
- the ferrite has an average diameter of 2 ⁇ m or less
- the ferrite is characterized in that the ductility, porosity and surface treatment characteristics characterized in that the Fn2 defined by the following [Relationship 1] satisfy the 89% or more, and Fa5 defined by the following [Relationship 2] 70% or less.
- Fn2 defined by the following [Relationship 1] satisfy the 89% or more
- Fa5 defined by the following [Relationship 2] 70% or less.
- Fn2 [number of ferrite grains less than or equal to 2 ⁇ m / total number of ferrite grains] ⁇ 100
- Fa5 [ferrite grain area / total ferrite grain area of 5 ⁇ m or more] ⁇ 100
- Cr, Ni, Mo may further include one or two or more: 2% or less (where 0% is not included).
- Ti may further comprise 0.05% or less (here 0% is not included) and B is 0.003% or less (here 0% is not included).
- the Ni or Fe plating layer is formed in the surface with the adhesion amount of 5-40 mg / m ⁇ 2> .
- the present invention in the hot-dip galvanized steel sheet in which a hot-dip galvanized layer is formed on the surface of the cold-rolled steel sheet, Ni or Fe plating layer is formed with an adhesion amount of 100mg / m 2 or more between the cold-rolled steel sheet and the hot-dip galvanized layer.
- the present invention relates to a high strength hot dip galvanized steel sheet having excellent ductility, porosity, and surface treatment characteristics.
- an alloyed hot dip galvanized steel sheet obtained by alloying heat treatment on the hot dip galvanized steel sheet may be provided.
- the cold rolled steel sheet is preferably made of a microstructure before the second annealing step consisting of less than 20% ferrite and residual low-temperature transformation structure.
- the step of forming a Ni or Fe plating layer on the surface of the secondary annealing treatment 5 to 40mg / m 2 may further include.
- a Ni or Fe plating layer may be formed on the surface of the steel sheet with an adhesion amount of 5 to 40 mg / m 2 .
- the present invention after the first annealing, the surface of the steel sheet is coated with Ni or Fe at an adhesion amount of 100 mg / m 2 or more, followed by hot dip galvanized hot dip galvanized steel and the hot dip galvanized steel alloyed hot dip galvanized A plated steel sheet can be provided.
- ductile transformation tissue steel such as DP steel or TRIP steel and Q & P steel subjected to Q & P (Quenching & Partitioning) heat treatment
- tensile strength superior in ductility and hole expansion property is excellent in press formability of 980 MPa or more.
- Cold rolled steel sheet, hot dip galvanized steel sheet and alloyed hot dip galvanized steel sheet can be provided.
- Ni and Fe after the 1st and 2nd annealing heat treatment, it is excellent in phosphate treatment and plating the cold rolled steel sheet which is excellent in the adhesion of an electrodeposition coating layer, and Ni, Fe, etc. before the 2nd annealing, and there is no plating adhesiveness and unplated defect. Since the hot-dip galvanized steel sheet excellent in formability and corrosion resistance and excellent spot weldability can be manufactured, there is an advantage of long life and safety of components such as automobiles.
- the cold rolled steel sheet of the present invention has the advantage of high availability for industrial fields, such as building members, automotive steel sheet.
- FIG. 1 is a photograph illustrating the influence of the structure and geometry of the steel microstructure on the hole expandability and elongation with examples of the invention and comparative examples.
- FIG. 2 is a tissue photograph showing that cracks occur when holes are expanded in the tissue photograph of FIG. 1.
- Figure 3 is an illustration showing an example of the annealing heat treatment process according to the present invention (in Figure 1 (b) is a dotted line shows the thermal history during the molten alloy plating).
- Figure 4 is a photograph observing the microstructure in order to compare the difference between the tissue of the invention example and the comparative example in the Example.
- Figure 5 is a graph showing the difference between the observation frequency and ferrite grain size for the invention example and comparative example.
- FIG. 6 is a diagram showing the effect of Ni plating amount on the phosphate treatment.
- the hole expandability was not good in steels utilizing residual austenite to improve conventional elongation.
- the cooling rate is 20 ° C / s or more in order to obtain martensite structure in the first heat treatment process, but this is also localized as the cooling rate increases.
- press molding Because the plate is warped due to uneven cooling and the plate shape is not good.
- the inventors have confirmed through studies and experiments that the fine lath ferrite obtained by reverse transformation heat treatment, bainite, and residual austenite structure are important means to secure hole expansion and elongation at the same time. It was also confirmed that the particle size distribution of ferrite also plays an important role.
- the present invention has been completed by finding a means for solving the problems of defects, partial unplating, and weld cracking.
- the high strength cold rolled steel sheet having excellent ductility, porosity, and surface treatment characteristics of the present invention is, in weight%, carbon (C): 0.05 to 0.3%, silicon (Si): 0.6 to 2.5%, aluminum (Al): 0.01 to 0.5 %, Manganese (Mn): 1.5-3.0%, balance Fe and inevitable impurities.
- the alloy composition of the cold rolled steel sheet of the present invention and the reason for limitation thereof will be described in detail.
- the content of each component means weight% unless otherwise specified.
- Carbon (C) is an effective element for reinforcing steel, and is an important element added in the present invention for stabilizing residual austenite and securing strength. In order to obtain the above-mentioned effect, it is preferable to add it in 0.05% or more, but when the content exceeds 0.3%, the risk of casting defects increases. In addition, the weldability can also be greatly reduced, and there is also a problem because it is cooled to a lower temperature in order to obtain the martensite structure during the primary annealing. Therefore, the content of C in the present invention is preferably limited to 0.05 ⁇ 0.3%.
- Silicon (Si) is an element that suppresses the precipitation of carbides in ferrite, promotes diffusion of carbon in the ferrite into austenite, and consequently contributes to stabilization of residual austenite. In order to obtain the above-mentioned effect, it is preferable to add at 0.6% or more. However, when the content exceeds 2.5%, hot and cold rolling properties are inferior and there is a problem of inhibiting plating property by forming an oxide on the steel surface. have. Therefore, the content of Si in the present invention is preferably limited to 0.6 ⁇ 2.5%.
- Aluminum (Al) is an element that deoxidizes by combining with oxygen in the steel, for this purpose it is preferable to maintain the content of 0.01% or more.
- Al contributes to stabilization of retained austenite through suppression of carbide formation in ferrite as in Si.
- the content of Al in the present invention is preferably limited to 0.01 ⁇ 0.5%.
- Manganese (Mn) is an element effective in forming and stabilizing residual austenite while controlling the transformation of ferrite. If the Mn content is less than 1.5%, a large amount of ferrite transformation occurs, thereby making it difficult to secure the target strength. On the other hand, when the Mn content exceeds 3.0%, the phase transformation in the second annealing heat treatment step of the present invention is too delayed. As a large amount of martensite structure is formed, there is a problem that it is difficult to secure the intended ductility. Therefore, the content of Mn in the present invention is preferably limited to 1.5 ⁇ 3.0%.
- P is preferably 0.03% or less, and when it exceeds 0.03%, there is a problem that the weldability is lowered and the risk of brittleness of steel is increased.
- S is preferably 0.015% or less.
- Sulfur (S) is an impurity element inevitably contained in steel, and it is preferable to suppress the content as much as possible. Theoretically, the content of S is advantageously limited to 0%, but since it is inevitably contained in the manufacturing process, it is important to manage the upper limit, and if the content exceeds 0.015%, there is a possibility that the ductility and weldability of the steel sheet may be impaired. high.
- N is preferably 0.02% or less.
- Nitrogen (N) is an effective element for stabilizing austenite, but if the content exceeds 0.02%, the risk of brittleness of steel increases, and the quality of performance is increased as AlN is excessively precipitated by reacting with Al. There is a problem of deterioration.
- the cold rolled steel sheet of the present invention may further include at least one of Cr, Ni, Mo, Ti, and B for improving strength.
- one or two or more sums of Cr, Ni, and Mo: 2% or less (where 0% is not included) may be further included.
- the molybdenum (Mo), nickel (Ni) and chromium (Cr) are elements that contribute to the stabilization of the retained austenite, these elements are combined with C, Si, Mn, Al and the like to contribute to the stabilization of austenite. If the content of these elements is more than 2.0% in the case of Mo, Ni and Cr, there is a problem that the manufacturing cost is excessively increased, it is preferable to control not to exceed the content.
- Ti may be 0.05% or less (here 0% is not included), B may be 0.003% or less (here 0% is not included).
- Ti is preferably added at 0.05% or less when Al exceeds 0.05% or when B is added.
- Ti is an element that forms TiN, so it must be precipitated at a higher temperature than B or Al, so it is effective to add a lot, but there is a problem of clogging nozzles or raising costs during performance.
- Even in the upper limit of the Al and B addition amount of the present invention if Ti is added in the range of 0.05%, AlN or BN may not be formed and thus may act as a solid solution element, so the upper limit is made 0.05%.
- B boron
- the remaining component of the present invention is iron (Fe).
- iron Fe
- impurities which are not intended from raw materials or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned herein.
- the high-strength cold-rolled steel sheet having excellent ductility, porosity, and surface treatment characteristics of the present invention the steel microstructure, the area fraction, ferrite 60% or less, needle type bainite 25% or more, martensite 5% and needle type It comprises 5% or more of retained austenite. That is, the cold rolled steel sheet of this invention has the steel microstructure, and ferrite and needle-like bainite contain martensite and needle-shaped residual austenite.
- These structures are the steel sheet main structures of the present invention, which are advantageous for securing hole expansion, ductility, and strength, among which martensite structures are partially included in the steel structures due to heat treatment in the manufacturing process described later.
- the ferrite in the microstructure includes coarse polygonal ferrite and acicular ferrite, which is 60% or less in area% of the entire structure. If the ferrite structure exceeds 60%, the strength is lowered and the coarse polygonal ferrite fraction is increased, and the difference between the remaining metamorphic structure and the content of redistribution (partitioning) elements such as carbon and Mn is increased, thus during the hole expansion process. Since cracks are easily generated, there is a problem that the hole expandability is lowered.
- the bainite tissue is mostly needle-like and borders on surrounding ferrite, martensite and residual austenite. Since it has a medium strength between ferrite and two-phase structure (martensite and residual austenite), the interfacial separation between phases is reduced to improve pore expansion. It was made.
- the martensite structure is formed when the chemically unstable austenite is cooled to room temperature during final cooling, thereby lowering the elongation of the steel.
- the martensite structure is used as a means for improving the strength even by lowering the alloying element, and if the martensite structure is small, more alloying elements should be added, thereby causing a problem of cost increase.
- the lower limit of the martensite area ratio was 5%.
- the residual austenite is a very important structure for securing ductility and securing hole expandability. Therefore, the more, the better, but a large amount of austenite stabilized alloy elements such as carbon is added, there is a problem of lowering the cost rise and weldability.
- the needle-like residual austenite is made as in the present invention, since the stability of austenite is significantly increased in the same chemical component, it is not necessary to include a large amount as in the conventional method.
- at least 5% is required and the lower limit is 5%.
- the present invention it is important to control the fraction and size of the tissue of the ferrite. 1 and 2, coarse polygonal ferrite easily propagates cracks along the boundary of the neighboring second phase when the hole is expanded, but dispersing the needle-like ferrite inhibits crack propagation. The improvement in scalability can be understood. Therefore, the present invention is characterized by controlling the fraction and size of the ferrite using a heat treatment method described later.
- the ferrite has an average diameter of 2 ⁇ m or less, Fn2 defined by the following [Relationship 1] is 89% or more, and Fa5 defined by the following [Relationship 2] satisfies 70% or less. .
- Fn2 [number of ferrite grains less than or equal to 2 ⁇ m / total number of ferrite grains] ⁇ 100
- Fa5 [ferrite grain area / total ferrite grain area of 5 ⁇ m or more] ⁇ 100
- the needle-like ferrite means that the length ratio of the long side to the short side is 4 or more, and the size thereof was evaluated by an image analyzer having an analysis program (a method for measuring grains of ASTM E112) assuming that several hexagons are connected. As a result, the size and number of grains as shown in FIG. 5 were measured, and based on this, ferrite grain size and distribution of steel having excellent elongation and hole expansion property were determined.
- the hole expansion is excellent at 28% or more and at the same time the elongation is excellent at 20% or more. It is to confirm and present this technical configuration.
- the cold rolled steel sheet of the present invention that satisfies the size and distribution of the microstructure and ferrite described above has a tensile strength of 980 MPa or more, and has excellent hole expandability compared to the conventional TRIP steel manufacturing method, Q & P heat treatment method, and heat treatment method for reverse transformation. Softness can be secured at the same time.
- the cold rolled steel sheet having excellent ductility, hole processing property and surface treatment characteristics of the present invention includes a Ni or F plating layer formed on the surface thereof, and the plating deposition amount is preferably 5 to 40 mg / m 2 . If the plating deposition amount is less than 5mg / m 2 , as shown in FIG. 6, Mn or Si oxide is easily formed on the surface by fine oxidation during or after annealing, and as a result, a phosphate coating is not formed. It is because adhesiveness worsens. On the other hand, if the plating amount of Ni or Fe is more than 40mg / m 2 coarse phosphate crystals are reduced because fine phosphate irregularities are reduced, the adhesion is reduced.
- the present invention is not limited to the cold rolled steel sheet having the composition and structure described above, it can provide a hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on the surface of the cold-rolled steel sheet. In this case, however, it is preferable that a Ni or Fe plating layer is formed between 100 mg / m 2 and an adhesion amount between the cold rolled steel sheet and the hot dip galvanized layer.
- an alloyed hot dip galvanized steel sheet comprising an alloyed hot dip galvanized layer may be provided.
- the cold rolled steel sheet according to the present invention may be manufactured by reheating-hot rolling-winding-cold rolling-annealing a steel slab that satisfies the composition of the composition proposed in the present invention, hereinafter for the conditions of the respective processes It explains in detail.
- the present invention prior to performing the hot rolling, it is preferable to undergo a step of reheating and homogenizing the steel slab having the composition as described above, more preferably at a temperature range of 1000 to 1300 ° C. .
- the rolling load may increase rapidly.
- the temperature exceeds 1300 ° C, not only energy costs increase, but the amount of surface scale may be excessive. Therefore, it is preferable to perform a reheating process at 1000-1300 degreeC in this invention.
- the reheated steel slab is hot rolled to produce a hot rolled steel sheet.
- hot finishing rolling is preferably performed at 800 to 1000 ° C. under normal conditions.
- the hot finish rolling temperature during hot rolling in the present invention is preferably limited to 800 ⁇ 1000 °C.
- the hot rolled steel sheet manufactured according to the above is wound, and the winding temperature is preferably in the range of 750 ⁇ 550 °C.
- the lower limit of the coiling temperature is not particularly limited, but considering the difficulty of subsequent cold rolling due to excessively high hot rolled sheet strength due to the formation of martensite, the lower limit was set to 550 ° C.
- the wound hot rolled steel sheet is pickled by a conventional method to remove an oxide layer, and then cold rolled to produce a cold rolled steel sheet in order to match the shape and thickness of the steel sheet.
- cold rolling is carried out to secure the thickness required by the customer, and there is no limitation on the reduction ratio, but cold rolling reduction is performed at 30% or more to suppress the formation of coarse ferrite grains during recrystallization in a subsequent annealing process. It is desirable to.
- the present invention is to produce a cold-rolled steel sheet comprising a needle-like ferrite and needle-shaped residual austenite phase as the main phase of the final microstructure of the major axis and minor axis ratio of 4 or more. It is important. Particularly, in the present invention, in order to secure a desired microstructure from redistribution of elements such as carbon and manganese during annealing, it is not a common cold rolling followed by a continuous annealing process, but a low temperature structure as described below. And then partitioning heat treatment is performed to ensure acicular ferrite and retained austenite during secondary annealing.
- the cold rolled steel sheet prepared above is annealed at a temperature of Ac3 or more, and then subjected to a primary annealing heat treatment to cool at a cooling rate of less than 20 ° C / s to a temperature of 350 ° C or less (see FIG. 3A).
- the ferrite with an area fraction of 20% or less and the remaining low temperature transformation structure (bainite and martensite) of the main phase of the microstructure of the cold rolled steel sheet subjected to the first annealing heat treatment. This is to ensure excellent strength and ductility of the cold rolled steel sheet manufactured through the final secondary annealing step. If ferrite is formed due to slow cooling after the first annealing, the ferrite fraction exceeds 20%, as described above. It may not be possible to obtain the cold rolled steel sheet of the present invention consisting of ferrite, residual austenite and low temperature structure.
- the cooling rate is not only the annealing temperature but also the cooling rate that is important for obtaining the structure through primary annealing. If the cooling rate is more than 20 °C / s, the plate shape is not good, such as the expansion of the steel due to the non-uniformly formed low-temperature transformation structure, the plate is warped, the wave is generated, the plate breaking may occur due to the plate turning.
- the cooling rate is preferably less than 20 ° C, and the lower limit is only required to obtain ferrite having an area fraction of 20% or less and the remaining low temperature transformation structure.
- the cooling end temperature or the constant temperature holding start temperature after cooling is preferably 350 ° C. or lower, because if higher than this, the precipitation of bainite increases and needle-like microstructure due to reverse transformation cannot be obtained.
- Ni or Fe plating may be applied to the surface of the steel sheet, and the plating amount thereof is preferably in the range of 5 to 40 mg / m 2 .
- Ni or Fe plated on the surface of the steel sheet may be dissipated by diffusion into the steel sheet during the subsequent secondary annealing, but Ni and Fe diffused on the surface of the steel sheet are preferable because they inhibit the oxidation of the steel sheet.
- the secondary cooling to maintain for 30 seconds or more Annealing heat treatment is performed (see FIG. 3B).
- the heating in the range of Ac1 ⁇ Ac3 is to form a fine ferrite and austenite in which the needle structure is maintained by the reverse transformation phenomenon by heating the low-temperature transformation structure obtained in the primary annealing in an ideal region.
- the heating in the range of Ac1 ⁇ Ac3 is to form a fine ferrite and austenite in which the needle structure is maintained by the reverse transformation phenomenon by heating the low-temperature transformation structure obtained in the primary annealing in an ideal region.
- the heating and maintaining at the temperature is intended to induce redistribution of alloying elements such as carbon and manganese together with reverse transformation of the formed low temperature structure (bainite and martensite) after the first annealing heat treatment.
- the redistribution at this time is called primary redistribution.
- the maintenance for the primary redistribution of the alloying elements may be carried out so that the alloying elements are sufficiently diffused toward the austenite, and the time is not particularly limited.
- the holding time is excessively excessive, there is a possibility that the productivity may be lowered, and the redistribution effect is also saturated, and in consideration of this, it is preferable to carry out in 2 minutes or less.
- the average cooling rate is preferably less than 20 °C / s, this is also to uniform the shape of the plate. Even if the austenite is sufficiently stabilized and slow cooled by the primary redistribution, polygonal ferrite is not formed during cooling. However, a cooling rate of 5 ° C / s or more is preferable because the productivity decreases when cooling is too slow.
- the cooling end temperature is preferably in the temperature range of Ms ⁇ Bs, because the supersaturation is less than Bs does not cause secondary partitioning, the diffusion is very slow at the temperature below Ms, the time required for partitioning is significantly increased.
- the partitioning time is preferably 30 seconds or more in the Ms to Bs section.
- the cooling rate means the average temperature from the temperature of the heat treatment cracking to the end of the cooling temperature.
- Ni or Fe plating may be performed on the surface of the steel sheet after the second annealing, and the plating deposition amount may be in the range of 5 to 40 mg / m 2 .
- the Ni or Fe plating layer thus formed is improved in the subsequent phosphate treatment property to be excellent in electrodeposition coating property, and also excellent in welding properties.
- the present invention heats and maintains the formed low temperature structure in the range of Ac1 to Ac3 after the primary annealing process, and induces primary redistribution of alloying elements such as carbon and manganese with rapid reverse transformation.
- alloying elements such as carbon and manganese with rapid reverse transformation.
- reheating and inducing secondary redistribution a fine acicular microstructure as in Fig. 4 is obtained compared to the tissue obtained by the conventional method, and excellent pore expansion and elongation can be secured simultaneously.
- the primary annealing heat-treated cold-rolled steel sheet may be plated using a hot dip plating process or an alloyed hot dip plating process as a secondary annealing process, and the plating layer formed therefrom is preferably zinc-based.
- the hot dip galvanizing bath may be manufactured as a hot dip galvanized steel sheet, and in the case of the hot dip galvanizing method, an alloy may be manufactured by performing a conventional alloy hot dip plating process.
- the present invention it is preferable to perform hot dip galvanizing after Ni or Fe plating is performed on the surface of the steel sheet after the first annealing with an adhesion amount of 100 mg / m 2 or more.
- This is to prevent the occurrence of Mn or Si oxides formed on the surface and the surface thickening of these elements by plating more powerful Ni or Fe on the surface of the cold rolled steel sheet.
- the wettability of the base steel sheet and the hot-dip galvanized steel having almost no surface oxide layer is increased, and thus a hot-dip galvanized steel sheet can be produced.
- the amount of Ni or Fe plating is less than 100 mg / m 2 , as shown in FIG. 7, unplating occurs and intensive corrosion later occurs in the unplated surface.
- the molten metal having the composition shown in Table 1 below was prepared in a 90 mm thick, 175 mm wide ingot through vacuum melting. Subsequently, it was reheated for 1 hour at 1200 ° C. for homogenization treatment, and hot-rolled and rolled at 900 ° C. or higher, which is a temperature of Ar 3 or higher, to prepare a hot rolled steel sheet. Thereafter, the hot rolled steel sheet was cooled, charged into a furnace preheated to 600 ° C., maintained for 1 hour, and then cold rolled to simulate hot rolled winding. Then, the hot rolled sheet was cold rolled at a cold reduction rate of 50 to 60%, and then subjected to annealing heat treatment under the conditions shown in Table 2 to produce a final cold rolled steel sheet.
- the cold rolled steel sheet having the composition was annealed under the heat treatment conditions as shown in Table 2 below, and Ms and Bs at this time were calculated and shown in Table 2 below.
- the chemical element means the weight percent of the added element
- Bs is the bainite transformation start temperature
- Ms is the martensite transformation start temperature.
- Ms and Bs were calculated by the following equation.
- CR refers to the cooling rate
- F refers to the ferrite area fraction in the tissue after the first annealing.
- the cooling rates were all 12 ° C./s, and the holding time at the cooling end temperature was 120 seconds except for Comparative Example 7.
- Comparative Example 7 since Mn content was high, constant temperature was maintained for 300 seconds to sufficiently cause bainite transformation. Yield strength, tensile strength, elongation and hole expandability (HER) were measured on the cold rolled steel sheet after secondary annealing, and the results are also shown in Table 2 above. At this time, the tensile test piece was used in JIS 5, HER was evaluated as 120x150mm.
- HER is a hole expandability, when a hole is punched at a clearance of 12% with a punch of 10 mm, then a burr generation surface is brought to the top so that a crack can be seen on the processing surface with a cone of 60 degrees from the bottom. It is the value obtained by the following relational expression 3 after machining.
- HER (%) (hole diameter after processing-hole diameter before processing, 10mm) / hole diameter before processing
- ferrite, bainite, residual austenite, and martensite were analyzed by backscattering electron diffraction (EBSD) on the specimen after the second heat treatment, where ferrite, residual austenite, and bainite were analyzed for IQ distribution of EBSD.
- EBSD backscattering electron diffraction
- phase separation was performed by taking the kernel mean misorientation at the inflection point.
- the grain size of the ferrite was evaluated by an image analyzer with a built-in analysis program that assumes that several hexagons are connected (the grain measuring method of ASTM E112).
- the tissue analysis differences between the inventive examples and the comparative examples are shown in Table 3 below.
- F means ferrite
- B bainite
- M martensite
- G residual austenite
- GS refers to the average grain size of ferrite
- Fn2 refers to the above-mentioned relational expression 1
- Fa5 refers to the relational expression 2.
- Comparative Example 5-7 which does not satisfy the composition range of the composition presented in the present invention, it can be seen that the tensile strength, elongation or HER is low even when reverse transformation heat treatment is performed. Comparative Example 5 having low Si or Mn has both low tensile strength and HER. In Comparative Examples 6 and 7, where C or Al and Mn are very high, only strength was obtained very high, but HER or elongation was low.
- the primary partitioning is performed in the coexistence temperature range of ferrite and austenite during cracking, and then the secondary partitioning is performed by constant temperature heat treatment in the bainite transformation temperature region, thereby performing the secondary annealing condition of the present invention.
- Comparative Examples 10, 12, and 14 satisfy all of the first and second annealing conditions, but after the cracking of the first annealing, the cooling rate is 5 ° C./s so that coarse ferrite is formed in the cooling process.
- the area fraction of ferrite grains having an area of ferrite exceeding 60% or a size of 5 ⁇ m or more was about 80% or more, and thus the tensile strength or HER was not high.
- the grains of ferrite are fine, and in particular, having a needle-like structure can increase both the mechanical properties which are incompatible with hole expansion and elongation while having high strength.
- FIG. 1 is a tissue photograph showing the influence of the structure and geometry of the tissue on the hole expansion and elongation.
- Figure 1 (a) corresponds to Comparative Example 11 is annealing by a conventional heat treatment method. After the annealing of the reverse station, the mixture was cooled and kept at 440 ° C. where bainite transformation occurred. Coarse ferrite is due to the formation of polygonal ferrite and austenite at the time of the annealing of the reverse region, and since the bainite transformation occurs in austenite after cooling, the remaining austenite is stabilized at the same time, thereby obtaining a structure as shown in FIG. 1 (a). It is.
- Example 1 (b) contained 8% and 11% of retained austenite, respectively, and the elongation reached 24.6 and 26.5%, respectively.
- Inventive Example 1 (b) having a fine structure was high in strength and excellent in elongation. It can be confirmed from the tissue photograph of FIG. 4 observed with a secondary electron microscope that needle-like ferrite and polygonal ferrite having a long side-to-short side ratio of 4 or more are significantly developed as compared with the conventional manufacturing method.
- Inventive Example 2 has a very high density of fine needle-like ferrites of about 1 ⁇ m, while in Comparative Example 12 there are many polygonal ferrite grains of 1 to 3 ⁇ m, and crystal grains of 3 to 5 ⁇ m are also relatively high. .
- Table 3 shows the analysis of the structural properties of the steel composition of Table 1 and each of the specimens subjected to the heat treatment conditions of Table 2.
- the ferrite has an average diameter of 2 ⁇ m or less, and among the ferrites, Fn2 defined by the above-mentioned relational formula 1 is 89% or more, and Fa5 defined by the above-mentioned formula 2 satisfies 70% or less.
- Fn2 defined by the above-mentioned relational formula 1
- Fa5 defined by the above-mentioned formula 2 satisfies 70% or less.
- Figure 6 shows the effect of Ni plating amount on the phosphate treatment.
- the Ni plating amount was changed to 50 mg / m 2 , respectively.
- Ni plating solution was used as a nickel lactate, and the plating amount was changed by controlling the current under a constant PH conditions.
- the film was formed in a phosphate solution at 45 ° C. for 150 seconds, and after washing and drying, the film crystals were observed by a secondary electron microscope, while surface components were obtained by GDS analysis on Ni plating samples of 3 mg / m 2 and 30 mg / m 2 .
- Figure 6 (b) shows the results of GDS analysis for the specimen of Ni plating amount of 3mg / m 2 and 30mg / m 2 .
- the surface of the base steel sheet had many surface oxides and internal oxides, and the concentration of Si and Mn was large and the concentration of oxygen on the surface was high.
- the Ni plating 30mg / m 2 specimens had a low oxygen concentration due to the oxygen blocking action of the surface Ni, and as a result, the amount of surface concentrated Si, Mn was not high.
- FIG. 7 shows hot dip galvanization after Ni plating at 10 and 150 mg / m 2 after primary annealing and before secondary hot dip galvanizing annealing.
- 10 mg / m 2 specimen some oxides were present on the surface during the second annealing, and the unplated layer was observed.
- the 150 mg / m 2 specimen had a beautiful plating surface and no unplated defects were observed. This is because the plating of more powerful Ni on the surface prevented the generation of Mn or Si oxides formed on the surface and the surface thickening of these elements.
- FIG. 8 shows spot cracks after welding by spot welding after 10-300 mg / m 2 Ni plating before the first annealing and the second hot dip galvanizing annealing.
- the pressing force was 4 kN and the welding current was 7 kN.
- weld cracks did not occur in the specimen coated with 100 mg / m 2 of Ni. This is because Ni diffuses into the surface of the steel and the plating layer melts and raises the melting temperature of the plating layer.
- the welding crack is a phenomenon in which molten zinc penetrates into the grain boundaries of the base steel sheet under stress. This is because it increases the penetration temperature of liquid zinc.
- the cold rolled steel sheet produced according to the present invention can not only secure tensile strength and excellent elongation of 980 MPa or more, but also excellent in phosphate treatment and plating adhesion.
- the corrosion resistance of the parts is improved and the fatigue life of the assembly parts is extremely excellent because no welding cracks are generated, and thus cold forming for applying to structural members is more easily performed than steel materials manufactured through the conventional Q & P heat treatment process. It can be seen that there is an advantage that the durability of the component is significantly improved.
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Abstract
Description
강번 | C | Si | Mn | P | S | Al | Cr | Ni | Mo | Ti | B | N | 구분 |
1 | 0.08 | 0.7 | 1.5 | 0.008 | 0.003 | 0.02 | 0.5 | 0.02 | 0.002 | 0.003 | 발명강 | ||
2 | 0.14 | 1.5 | 2 | 0.012 | 0.005 | 0.14 | 0.02 | 0.02 | 0.05 | 0.004 | 발명강 | ||
3 | 0.22 | 1.5 | 1.8 | 0.011 | 0.006 | 0.48 | 0.01 | 0.11 | 0.025 | 0.0017 | 0.004 | 발명강 | |
4 | 0.18 | 1.8 | 2.5 | 0.008 | 0.004 | 0.03 | 0.5 | 0.02 | 0.023 | 0.0015 | 0.006 | 발명강 | |
5 | 0.07 | 0.3 | 1.4 | 0.011 | 0.006 | 0.04 | 0.02 | 0.02 | 0.004 | 비교강 | |||
6 | 0.35 | 1 | 1.2 | 0.009 | 0.006 | 0.8 | 0.01 | 0.01 | 0.003 | 비교강 | |||
7 | 0.2 | 0.8 | 3.5 | 0.008 | 0.004 | 0.02 | 0.02 | 0.02 | 0.004 | 비교강 |
구분 | 강번 | 소둔 조건 (℃) | Ms(℃) | Bs(℃) | 물성 | ||||||||
1차 | 2차 | ||||||||||||
균열 | 냉각종료 | CR(℃/s) | F(%) | 균열 | 냉각종료 | YS(MPa) | TS(MPa) | El(%) | HER(%) | ||||
발명예1 | 1 | 850 | 330 | 18 | 12 | 830 | 400 | 442 | 638 | 567 | 983 | 26.5 | 37 |
발명예2 | 2 | 840 | 350 | 15 | 7 | 820 | 420 | 400 | 607 | 590 | 1003 | 24.9 | 39 |
발명예3 | 3 | 830 | 310 | 14 | 5 | 810 | 390 | 385 | 605 | 633 | 1089 | 27.8 | 31 |
발명예4 | 4 | 840 | 300 | 12 | 2 | 820 | 400 | 353 | 521 | 685 | 1214 | 20.3 | 28 |
비교예5 | 5 | 850 | 320 | 20 | 64 | 820 | 400 | 463 | 685 | 608 | 925 | 19.4 | 33 |
비교예6 | 6 | 825 | 280 | 14 | 3 | 810 | 400 | 373 | 628 | 703 | 1151 | 21.3 | 18 |
비교예7 | 7 | 830 | 300 | 5 | 0 | 800 | 390 | 336 | 461 | 722 | 1445 | 8.2 | 43 |
비교예8 | 1 | 810 | 450 | 15 | 83 | - | - | 442 | 638 | 350 | 683 | 31.7 | 56 |
비교예9 | 2 | 820 | 420 | 16 | 74 | - | - | 400 | 607 | 422 | 760 | 25.2 | 24 |
비교예10 | 2 | 840 | 350 | 5 | 42 | 820 | 420 | 400 | 607 | 453 | 840 | 26.1 | 22 |
비교예11 | 3 | 830 | 440 | 18 | 67 | - | - | 385 | 605 | 521 | 923 | 24.6 | 6 |
비교예12 | 3 | 830 | 310 | 5 | 31 | 810 | 390 | 385 | 605 | 580 | 1054 | 26.5 | 13 |
비교예13 | 4 | 810 | 400 | 17 | 66 | - | - | 353 | 521 | 511 | 962 | 20.8 | 8 |
비교예14 | 4 | 840 | 300 | 5 | 28 | 820 | 400 | 353 | 521 | 536 | 997 | 21.9 | 16 |
구분 | F | B면적분율(%) | M면적분율(%) | G면적분율(%) | |||
GS(μm) | 면적분율(%) | Fa5 (%) | Fn2 (%) | ||||
발명예1 | 1.3 | 52.1 | 68.4 | 91.5 | 28.1 | 8.7 | 11.1 |
발명예2 | 1 | 36.7 | 22.4 | 91 | 43.8 | 8.6 | 10.9 |
발명예3 | 1.2 | 48.1 | 65.9 | 93.8 | 30.6 | 9.5 | 11.8 |
발명예4 | 1.2 | 46.1 | 51.7 | 92.9 | 32.2 | 11.3 | 10.4 |
비교예5 | 1.4 | 20 | 52.1 | 81.7 | 54.3 | 20.3 | 5.4 |
비교예6 | 1.3 | 10.6 | 38.7 | 79.7 | 62.9 | 18.6 | 7.9 |
비교예7 | 1.2 | 26.5 | 71.3 | 72.8 | 55.7 | 14.7 | 3.1 |
비교예8 | 4.2 | 73.1 | 94.6 | 45.2 | 14.2 | 2.1 | 10.6 |
비교예9 | 3.3 | 68.9 | 87.5 | 58.1 | 19.5 | 5.3 | 6.3 |
비교예10 | 2.7 | 62.2 | 83.8 | 77.1 | 24.4 | 3.8 | 9.6 |
비교예11 | 2.2 | 64.6 | 83.4 | 62.3 | 17.3 | 9.9 | 8.2 |
비교예12 | 1.9 | 57.3 | 80.1 | 84.9 | 23.2 | 8.3 | 11.2 |
비교예13 | 2.3 | 61.8 | 82.2 | 66.7 | 20.1 | 10.1 | 8 |
비교예14 | 1.8 | 55.3 | 79.9 | 85.8 | 26.5 | 8.7 | 9.5 |
Claims (14)
- 중량%로, 탄소(C):0.05~0.3%, 실리콘(Si):0.6~2.5%, 알루미늄(Al): 0.01~0.5%, 망간(Mn):1.5~3.0%, 잔부 Fe 및 불가피한 불순물을 포함하고,강 미세조직이, 면적분율로, 페라이트 60%이하, 침상 베이나이트 25%이상, 마르텐사이트 5%이상 및 침상 잔류 오스테나이트 5%이상을 함유하며,상기 페라이트는 평균 직경 2 ㎛ 이하이고,상기 페라이트는, 하기 [관계식 1]에 의해 정의되는 Fn2가 89%이상, 그리고 하기 [관계식 2]에 의해 정의되는 Fa5가 70%이하를 만족하는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연 강판.[관계식 1]Fn2 = [2 ㎛ 이하의 페라이트 결정립 개수/전체 페라이트 결정립 개수] × 100[관계식 2]Fa5 = [5 ㎛ 이상의 페라이트 결정립 면적/전체 페라이트 결정립 면적] ×100
- 제 1항에 있어서, Cr, Ni, Mo를 1종 또는 2종 이상의 합:2%이하(여기에서 0%는 미포함)를 추가로 포함하는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연 강판.
- 제 1항에 있어서, Ti를 0.05%이하(여기에서 0%는 미포함), B를 0.003%이하(여기에서 0%는 미포함)를 추가로 포함하는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연 강판.
- 제 1항에 있어서, 그 표면에 Ni 또는 Fe 도금층이 5~40mg/m2의 부착량으로 형성되어 있는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연 강판.
- 제 1항의 냉연강판 표면에 용융아연도금층이 형성되어 있는 용융아연도금강판에 있어서, 상기 냉연강판과 용융아연도금층 사이에는 Ni 또는 Fe 도금층이 100mg/m2 이상의 부착량으로 형성되어 있는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 용융아연도금강판.
- 제 5항의 용융아연도금강판을 합금화 열처리함으로써 얻어지는 합금화 용융아연도금강판.
- 중량%로, 탄소(C): 0.05~0.3%, 실리콘(Si): 0.6~2.5%, 알루미늄(Al): 0.01~0.5%, 망간(Mn): 1.5~3.0%, 잔부 Fe 및 불가피한 불순물을 포함하는 강슬라브를 마련한 후, 이를 재가열하는 단계;상기 재가열된 강 슬라브를 통상의 열간압연 조건으로 압연한 후, 750~550℃ 의 온도범위에서 권취하는 단계;상기 권취된 열연강판을 냉간 압연하여 냉연강판을 제조하는 단계;상기 냉연강판을 Ac3 이상의 온도로 가열한 후, 20℃/s 미만의 냉각속도로 350℃이하 까지 냉각하는 1차 소둔 단계; 및상기 1차 소둔 후 Ac1~Ac3 범위의 온도로 가열·유지한 후, 20℃/s 미만의 냉각속도로 Ms ~ Bs의 온도 범위까지 냉각하고, 이어, 30초 이상 유지한 후 최종 냉각하는 2차 소둔 단계;를 포함하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연강판의 제조방법.
- 제 7항에 있어서, Cr, Ni, Mo를 1종 또는 2종 이상의 합:2%이하(여기에서 0%는 미포함)를 추가로 포함하는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연강판의 제조방법.
- 제 7항에 있어서, Ti를 0.05%이하(여기에서 0%는 미포함), B를 0.003%이하(여기에서 0%는 미포함)를 추가로 포함하는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연강판의 제조방법.
- 제 7항에 있어서, 상기 1차 소둔 후, 2차 소둔 전에 강판의 표면에 5~40mg/m2의 부착량으로 Ni 또는 Fe 도금층을 형성하는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연강판의 제조방법.
- 제 7항에 있어서, 상기 냉연강판은 2차 소둔 단계 이전의 미세조직이 면적분율로 20% 이하의 페라이트와 잔여 저온 변태조직으로 이루어져 있음을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연강판의 제조방법.
- 제 7항에 있어서, 상기 2차 소둔 처리된 강판 표면에 5~40mg/m2의 부착량으로 Ni 또는 Fe 도금층을 형성하는 단계;를 추가로 포함하는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 고강도 냉연강판의 제조방법.
- 제 7항의 1차 소둔된 강판의 표면에 100mg/m2이상의 부착량으로 Ni 또는 Fe 도금을 실시한 후 용융아연도금처리하는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 용융아연도금강판의 제조방법.
- 제 13항의 용융아연도금강판에 합금화 열처리하는 것을 특징으로 하는 연성, 구멍가공성 및 표면처리 특성이 우수한 합금화 용융아연도금강판 제조방법.
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