KR20130110635A - Cold-rolled steel sheet and method of manufacturing the same - Google Patents
Cold-rolled steel sheet and method of manufacturing the same Download PDFInfo
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- KR20130110635A KR20130110635A KR1020120032748A KR20120032748A KR20130110635A KR 20130110635 A KR20130110635 A KR 20130110635A KR 1020120032748 A KR1020120032748 A KR 1020120032748A KR 20120032748 A KR20120032748 A KR 20120032748A KR 20130110635 A KR20130110635 A KR 20130110635A
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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
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
- C21D8/0426—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- 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
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
- C21D8/0436—Cold rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- 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
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
- C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- 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
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
- C21D8/0473—Final recrystallisation annealing
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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
- C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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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
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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Abstract
Description
The present invention relates to a cold rolled steel sheet and a method for manufacturing the same, and more particularly, to an ultra low carbon high strength cold rolled steel sheet having excellent moldability and high yield ratio characteristics through the control of alloy components and process conditions control.
The structural member or the reinforcement of the automobile forms the structure of the automobile body and requires excellent crash stability for the safety of the driver and passengers in the vehicle interior during an impact. In other words, the shape of the part is complicated and the collision stability can be improved by securing a high yield strength.
However, the conventional ultra low carbon high strength steel has a low wear ratio and a high elongation, which is advantageous for molding, but has a disadvantage in that it is disadvantageous for the collision stability as a structural member and a reinforcement material that requires high rigidity when used as a product after molding due to the low yield ratio. On the other hand, in the case of C-Mn-based low carbon steel has a high yield ratio, but low elongation or plastic anisotropy (r-value) has a disadvantage in molding.
Related prior art documents are Korean Patent Publication No. 10-0430982 (August 12, 2004), which discloses a method for producing a cold rolled steel sheet having excellent deep drawing properties.
An object of the present invention is to provide a method for producing a high yield ratio cold rolled steel sheet having excellent moldability through the control of alloy components and control of process conditions.
Another object of the present invention is prepared by the above method, tensile strength (TS): 400 ~ 550MPa, yield strength (YS): 300 ~ 450MPa, yield ratio (YR): 0.78 or more, elongation (El): 38% or more and Plastic anisotropy coefficient (r-value): To provide a cold rolled steel sheet having a 1.5 or more.
Cold rolled steel sheet manufacturing method according to an embodiment of the present invention for achieving the above object is (a) wt%, C: 0.001 ~ 0.005%, Si: 0.2 ~ 0.3%, Mn: 0.5 ~ 1.5%, P: 0.04 ~ Slab composed of 0.07%, Al: 0.01 to 0.10%, Ti: 0.01 to 0.03%, Nb: 0.02 to 0.06%, Cu: 0.1 to 0.3%, B: 0.0003 to 0.0015% and the remaining iron (Fe) and other unavoidable impurities Reheating the plate to SRT (Slab Reheating Temperature): 1150 ~ 1250 ℃; (b) finishing hot rolling of the reheated sheet at Finishing Delivery Temperature (FDT): Ar 3 to Ar 3 + 100 ° C .; (c) winding the hot rolled sheet to a cooling temperature (CT) of 500 to 600 ° C .; (d) pickling the wound sheet and then cold rolling at a reduction ratio of 50 to 80%; And (e) annealing the cold rolled plate at 830 to 880 ° C. for 20 to 30 hours.
Cold rolled steel sheet according to an embodiment of the present invention for achieving the other object by weight, C: 0.001 ~ 0.005%, Si: 0.2 ~ 0.3%, Mn: 0.5 ~ 1.5%, P: 0.04 ~ 0.07%, Al : 0.01 ~ 0.10%, Ti: 0.01 ~ 0.03%, Nb: 0.02 ~ 0.06%, Cu: 0.1 ~ 0.3%, B: 0.0003 ~ 0.0015% and the remaining iron (Fe) and other unavoidable impurities, tensile strength ( TS): 400 to 550 MPa, yield strength (YS): 300 to 450 MPa and yield ratio (YR): 0.78 or more.
In the present invention, it is possible to secure high elongation (EL) and plastic anisotropy coefficient (r-value) by controlling alloy components and controlling process conditions, and thus have excellent deep drawing properties and high yield ratios. This excellent high strength cold rolled steel sheet can be manufactured.
Therefore, the cold rolled steel sheet according to the present invention tensile strength (TS): 400 ~ 550MPa, yield strength (YS): 300 ~ 450MPa, yield ratio (YR): 0.78 or more, elongation (El): 38% or more and plastic anisotropy coefficient (r-value): It can satisfy 1.5 or more.
Through this, the cold rolled steel sheet according to the present invention is able to secure deep drawing properties and high strength, it is suitable for use as a structural member or reinforcing material for automobiles that require a lot of molding processing.
FIG. 1 is a process flow chart showing a cold-rolled steel sheet manufacturing method according to an embodiment of the present invention.
Fig. 2 is a photograph showing the microstructure of the specimen prepared according to Example 1. Fig.
Figure 3 is a photograph showing the microstructure for the specimen prepared according to Comparative Example 1.
Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.
Hereinafter, a cold-rolled steel sheet according to a preferred embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the accompanying drawings.
Cold rolled steel plate
Cold rolled steel sheet according to the present invention through the alloy component control and process conditions control, tensile strength (TS): 400 ~ 550MPa, yield strength (YS): 300 ~ 450MPa, yield ratio (YR): 0.78 or more, elongation (El) : 38% or more and plastic anisotropy (r-value): 1.5 or more.
To this end, the cold rolled steel sheet according to the present invention in weight%, C: 0.001 ~ 0.005%, Si: 0.2 ~ 0.3%, Mn: 0.5 ~ 1.5%, P: 0.04 ~ 0.07%, Al: 0.01 ~ 0.10%, Ti : 0.01 to 0.03%, Nb: 0.02 to 0.06%, Cu: 0.1 to 0.3%, B: 0.0003 to 0.0015% and the remaining iron (Fe) and other unavoidable impurities.
In this case, the steel sheet may include at least one of S: 0.002 to 0.010% by weight and N: 0.004% by weight or less.
Hereinafter, the role and content of each component contained in the cold-rolled steel sheet according to the present invention will be described.
Carbon (C)
Carbon (C) is an intrinsic solid solution strengthening element in steel. It is hardened not only in solid solution strengthening but also in austenite, and contributes to formation of martensite and increase of strength in cold rolling heat treatment.
The carbon (C) is preferably added in an amount of 0.001 to 0.005% by weight of the total weight of the steel sheet according to the present invention. When the content of carbon (C) is less than 0.001% by weight of the total weight of the steel sheet, the amount of NbC precipitates is reduced, resulting in coarse grain size, which causes a problem of surface defects such as orange peel during molding. On the contrary, when the content of carbon (C) exceeds 0.005% by weight of the total weight of the steel sheet, since the dissolved carbon significantly degrades the aging resistance, a large amount of expensive niobium (Nb) must be added to remove the dissolved carbon. There is a problem of an increase in manufacturing cost.
Silicon (Si)
Silicon (Si) plays a role in suppressing the formation of carbide and enhancing the hardening hardenability by increasing the solid carbon.
The silicon (Si) is preferably added in an amount of 0.2 to 0.3% by weight of the total weight of the steel sheet according to the present invention. When the content of silicon (Si) is less than 0.2% by weight, the deoxidation effect may not be properly exhibited. On the contrary, when the content of silicon (Si) exceeds 0.3% by weight, yield point stretching occurs, and the strength increases, but there is a problem that the ductility decreases.
Manganese (Mn)
Manganese (Mn) is a solid solution strengthening element that prevents MnS precipitates from reacting with sulfur (S) dissolved in a steel sheet to prevent hot shortness due to solid solution sulfur. It stabilizes austenite to obtain a two- And it is easy to generate martensite even at a low critical cooling rate.
The manganese (Mn) is preferably added in a content ratio of 0.5 to 1.5% by weight of the total weight of the steel sheet according to the present invention. When the content of manganese (Mn) is less than 0.5% by weight of the total weight of the steel sheet, it may be difficult to secure sufficient strength. On the contrary, when the content of manganese (Mn) exceeds 1.5% by weight of the total weight of the steel sheet, there is a problem that it is difficult to secure formability due to a decrease in elongation caused by excessively high strength of the steel sheet.
Phosphorus (P)
Phosphorus (P) is an element effective in strengthening the steel sheet by solid solution strengthening and suppressing the formation of carbide.
The phosphorus (P) is preferably limited to the content ratio of 0.04 to 0.07% by weight of the total weight of the steel sheet according to the present invention. When the content of phosphorus (P) is less than 0.04% by weight of the total weight of the steel sheet, the above effects cannot be properly exhibited. On the contrary, when the content of phosphorus (P) exceeds 0.07% by weight of the total weight of the steel sheet, it may cause a problem in which work brittleness occurs.
Aluminum (Al)
Aluminum (Al) reacts with nitrogen (N) to form fine AlN precipitates, thereby improving the strength by precipitation strengthening as well as grain refinement.
The aluminum (Al) is preferably added in a content ratio of 0.01 to 0.10% by weight of the total weight of the steel sheet according to the present invention. When the content of aluminum (Al) is less than 0.01% by weight of the total weight of the steel sheet, the amount of AlN precipitates may be relatively reduced, so that it may be difficult to secure sufficient strength. On the contrary, when the content of aluminum (Al) is excessively added in excess of 0.10% by weight of the total weight of the steel sheet, there is a problem in that it is difficult to play, thereby lowering the productivity and excessively increasing the yield strength (YS).
Titanium (Ti)
In the present invention, titanium (Ti) is a TiC, TiN precipitate forming element, and precipitates solid carbon and solid solution nitrogen such as TiC, TiN upon reheating. In addition, titanium serves to precipitate solid carbon and solid nitrogen to improve inferiority and processability.
The titanium (Ti) is preferably added in a content ratio of 0.01 to 0.03% by weight of the total weight of the steel sheet according to the present invention. When the content of titanium (Ti) is less than 0.01% by weight of the total weight of the steel sheet, the titanium addition effect may not be properly exhibited. On the contrary, when the content of titanium (Ti) exceeds 0.03% by weight of the total weight of the steel sheet, TiC, TiN precipitates, etc. are coarsened, so that the effect of suppressing grain growth is reduced, which may cause surface defects of the manufactured steel sheet. .
Niobium (Nb)
Niobium (Nb) is a strong carbonitride-forming element. It reacts with carbon (C) and nitrogen (N) present in steel during hot rolling to form fine NbC and NbN precipitates and suppress grain growth. Further, niobium (Nb) has an effect of improving the strength and suppressing the secondary machining brittleness through grain refining effect.
The niobium (Nb) is preferably added in an amount ratio of 0.02 to 0.06% by weight of the total weight of the steel sheet according to the present invention. When the content of niobium (Nb) is less than 0.02% by weight of the total weight of the steel sheet, a certain amount of the solid carbon is excessive and the yield point drawing and aging phenomenon is accelerated. On the contrary, when the content of niobium (Nb) exceeds 0.06% by weight of the total weight of the steel sheet, the amount of solid solution carbon decreases, making it hard to secure baking hardening, and the yield strength increase due to the refinement of grains is accelerated, and the solid solution in the ferrite is accelerated. There is a problem of being in a state of being lowered rather than toughness.
Copper (Cu)
Copper (Cu) is an element which improves the hardenability and corrosion resistance of steel.
The copper (Cu) is preferably added in a content ratio of 0.1 to 0.3% by weight of the total weight of the steel sheet according to the present invention. When the content of copper (Cu) is less than 0.1% by weight of the total weight of the steel sheet, the above effects cannot be exerted properly. On the contrary, when the content of copper (Cu) exceeds 0.3% by weight of the total weight of the steel sheet, there is a problem of lowering the surface quality of the steel.
Boron (B)
Boron (B) is a strong incipient element, which plays a role in blocking segregation of phosphorus (P) and improving strength. If segregation of phosphorus (P) occurs, secondary processing brittleness may occur, so boron (B) is added to block segregation of phosphorus (P) to increase resistance to process embrittlement.
The boron (B) is preferably added in an amount of 0.0003 to 0.0015% by weight of the total weight of the steel sheet according to the present invention. When the content of boron (B) is less than 0.0003% by weight, the above-mentioned effect cannot be properly exhibited because the amount of addition is insignificant. On the contrary, when the content of boron (B) is added in excess of 0.0015% by weight, the formation of boron oxide may cause a problem of inhibiting the surface quality of the steel sheet.
Sulfur (S)
Sulfur (S) reacts with manganese (Mn) to form precipitates of fine MnS to improve processability.
The sulfur (S) is preferably limited to the content ratio of 0.002 to 0.010% by weight of the total weight of the steel sheet according to the present invention. When the content of sulfur (S) is less than 0.002% by weight of the total weight of the steel sheet, not only the amount of precipitated MnS is small but also the number of precipitates precipitated may be very small. On the contrary, when the content of sulfur (S) exceeds 0.010% by weight of the total weight of the steel sheet, the content of the solid solution of sulfur (S) is too high, the ductility and moldability may be significantly lowered, there is a fear of red brittleness.
Nitrogen (N)
In the present invention, nitrogen (N) is an unavoidable impurity, and there is a problem in that internal quality of the steel sheet is reduced by forming inclusions such as AlN and TiN.
In the present invention, it is preferable to control the nitrogen (N) in a very small amount, in which case the manufacturing cost increases and there is difficulty in management. Therefore, in the present invention, the content of nitrogen (N) is limited to 0.004% by weight or less based on the total weight of the steel sheet.
Cold rolled steel sheet manufacturing method
FIG. 1 is a process flow chart showing a cold-rolled steel sheet manufacturing method according to an embodiment of the present invention.
Referring to Figure 1, the cold rolled steel sheet manufacturing method shown is a slab reheating step (S110), hot rolling step (S120), cooling / winding step (S130), pickling step (S140), cold rolling step (S150), annealing heat treatment Step S160 and cooling step S170 are included. At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to carry out the step to derive effects such as reuse of precipitates.
In the method for manufacturing a cold rolled steel sheet according to the present invention, the slab sheet material of the semi-finished state, which is the target of the hot rolling process, is in weight%, C: 0.001 to 0.005%, Si: 0.2 to 0.3%, Mn: 0.5 to 1.5%, and P: 0.04 to 0.07%, Al: 0.01 to 0.10%, Ti: 0.01 to 0.03%, Nb: 0.02 to 0.06%, Cu: 0.1 to 0.3%, B: 0.0003 to 0.0015% and the rest of iron (Fe) and other unavoidable impurities have.
At this time, the slab plate may include one or more of S: 0.002 ~ 0.010 wt% and N: 0.004 wt% or less.
Reheat slab
In the slab reheating step S110, the slab plate having the above composition is reheated to a slab reheating temperature (SRT) of 1150 to 1250 ° C. Here, the slab plate can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. At this time, in the slab reheating step (S110), the slab plate obtained through the continuous casting process is reheated to reuse the segregated components during casting.
In this step, when the slab reheating temperature (SRT) is less than 1150 ° C, coarse precipitates generated during continuous casting may remain completely insoluble and may cause a problem of generating coarse precipitates even after hot rolling. On the other hand, if the SRT reheating temperature (SRT) exceeds 1250 ° C, the austenite crystal grain size may increase and the strength of the steel sheet may be difficult to secure, and the manufacturing cost of the steel sheet may be increased due to the excessive heating process.
Hot rolling
In the hot rolling step (S120) the slab plate is hot rolled to FDT (Finishing Delivery Temperature): Ar 3 ~ Ar 3 + 100 ℃. In this step, the specific temperature of Ar 3 may be 900 to 950 ° C.
If the finish rolling temperature (FDT) is too low below Ar 3 , there is a problem that workability is lowered due to a problem that a mixed structure due to abnormal reverse rolling occurs. On the other hand, when the finishing rolling temperature (FDT) exceeds the range of Ar 3 + 100 ° C, the austenite grains become coarse, which may make it difficult to secure strength.
Cooling / Winding
In the cooling / winding step (S130), the hot rolled sheet is cooled by winding to CT (Coiling Temperature): 500 to 600 ° C.
In this step, when the coiling temperature is less than 500 ° C, there is a problem in that the surface quality of the steel sheet is lowered due to a sharp difference between the finish rolling temperature and the coiling temperature. On the contrary, when the coiling temperature exceeds 600 ℃, precipitates grow too coarse to reduce the grain refining effect may be difficult to secure sufficient strength.
On the other hand, in the cooling / winding step (S130), the cooling rate is preferably carried out at 20 ~ 30 ℃ / sec. In this step, when the cooling rate is less than 20 ℃ / s may be difficult to secure the strength due to the problem that the average particle size of the precipitate exceeds approximately 0.2㎛. On the contrary, when the cooling rate exceeds 30 ° C / s, there is a problem that the structure becomes hard and the impact toughness decreases.
Pickle
In the pickling step (S140) is pickled (acid pickling) to remove the scale of the wound sheet, that is, the hot rolled coil manufactured by the hot rolling process. Although not shown in the drawings, after the pickling step (S140), a step of applying oil to prevent oxidation of the surface of the steel sheet may be further performed.
Cold rolling
In the cold rolling step (S150), the pickled plate is cold rolled.
In the cold rolling step (S150), the cold reduction rate is preferably carried out at 50 to 80%. When the cold rolling reduction rate is less than 50%, there is a problem that the amount of annealed recrystallized nuclei is small, so that the crystal grains are excessively grown during the annealing heat treatment to be described later, and the strength is rapidly lowered. On the other hand, when the cold rolling reduction is more than 80%, the amount of nucleation becomes too large, so that the annealing grains are rather too fine to reduce the ductility and deteriorate the formability.
Annealing heat treatment
In the annealing heat treatment step (S160), the cold-rolled sheet is subjected to annealing for 5 to 15 minutes at 830 ~ 880 ℃.
At this time, annealing heat treatment is one of the important process variables that determine the material of the final product. Such annealing heat treatment is preferably carried out at a temperature of 830 ~ 880 ℃, which is to induce the final structure is very fine and the average particle size of NbC precipitates and AlN precipitates of 0.05㎛ or less.
In this step, when the annealing heat treatment temperature is less than 830 ℃, or when the annealing heat treatment time is less than 5 minutes, recrystallization is not completed sufficiently may be difficult to secure the target ductility. On the contrary, when the annealing heat treatment temperature exceeds 880 ° C or when the annealing heat treatment time exceeds 15 minutes, there is a problem that the strength decreases due to coarsening of the recrystallized grains.
In particular, in this step, the heating rate of the annealing heat treatment is preferably carried out at 20 ~ 40 ℃ / sec. If the heating rate is less than 20 ℃ / sec, precipitates such as NbC, AlN may be re-grown, it may be difficult to secure a high yield ratio. On the contrary, when the heating rate exceeds 40 ° C./sec, growth of precipitates such as NbC and AlN does not occur, but it may be difficult to secure a target strength by coarsening of recrystallized grains.
Cooling
In the cooling step (S170) is cooled to 550 ~ 650 ℃ plate recrystallized by the annealing heat treatment process. At this time, the cooling is preferably cooled to 550 ~ 650 ℃ and indirect water cooling or indirect gas jet cooling (Gas Jet Cooling: GJC) to 550 ℃ to room temperature. In this step, the cooling rate may be about 1 ~ 100 ℃ / sec, but is not limited thereto.
Cold-rolled steel sheet produced by the above process (S110 ~ S170) is low Mn and P content, and by adding additional Cu as a hardenability improving element, it is possible to secure the moldability and high yield ratio characteristics.
Through this, the cold rolled steel sheet produced by the method according to the present invention is tensile strength (TS): 400 ~ 550MPa, yield strength (YS): 300 ~ 450MPa, yield ratio (YR): 0.78 or more, elongation (El): 38% Ideal and plastic anisotropy (r-value): can satisfy 1.5 or more.
Therefore, since the cold rolled steel sheet according to the present invention can secure deep drawing properties and high strength, the cold rolled steel sheet according to the present invention is suitable for use as structural members or reinforcing materials for automobiles, which require a lot of molding processing.
Example
Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.
Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.
1. Preparation of specimens
Specimens according to Examples 1 and 2 and Comparative Examples 1 to 4 were prepared under the compositions of Tables 1 and 2 and the process conditions of Table 3.
[Table 1] (unit:% by weight)
[Table 2] (unit:% by weight)
[Table 3]
2. Evaluation of mechanical properties
Table 4 shows the mechanical property evaluation results for the specimens according to Examples 1 and 2 and Comparative Examples 1 to 4.
[Table 4]
Referring to Tables 1 to 4, the specimens prepared according to Examples 1 to 2 are tensile strengths (TS): 400 to 550 MPa, yield strength (YS): 300 to 450 MPa, yield ratio (YR) corresponding to the target value. : 0.78 or more, elongation (El): 38% or more and plastic anisotropy (r-value): 1.5 or more.
On the other hand, in the case of specimens prepared according to Comparative Examples 1 to 2 showing a difference in terms of alloying components in comparison with Example 1, tensile strength (TS) and yield strength (YS) satisfied the target value, but in Example 1 In comparison, the yield strength (YS) is relatively low, indicating that the yield ratio does not reach the target value. In addition, in the case of specimens prepared according to Comparative Examples 1 and 2, it can be seen that the elongation (El) and the plastic anisotropy coefficient (r-value) do not reach the target values.
In addition, in the case of specimens prepared according to Comparative Examples 3 to 4 showing a difference in terms of alloying components in comparison with Example 1, the tensile strength (TS) satisfies the target value, but is due to the yield strength is less than the target value It can be seen that the yield ratio has a significantly low value. In addition, in the case of specimens prepared according to Comparative Examples 3 to 4, it can be seen that the elongation (El) and the plastic anisotropy coefficient (r-value) do not reach the target values.
On the other hand, Figure 2 is a photograph showing the microstructure for the specimen prepared according to Example 1, Figure 3 is a photograph showing the microstructure for the specimen prepared according to Comparative Example 1.
Referring to FIG. 2, in the case of the specimen prepared according to Example 1, grain growth occurred after recrystallization of a rolled structure, and thus, tissue growth after annealing heat treatment proceeded well.
On the other hand, referring to Figure 3, in the case of the specimen prepared according to Comparative Example 1 it can be seen that the carbon precipitates are mixed due to the relatively high carbon content compared to Example 1.
At this time, even in the case of ultra low carbon steel having a carbon content of 0.001 to 0.005% by weight, the grains of the hot rolled sheet material have different shapes depending on the content of Mn and P, which causes Mn and P to delay recrystallization. It can be seen that they appear in different shapes as well. In particular, in the case of the specimen prepared according to Comparative Example 1, recrystallization is delayed and recovery does not occur sufficiently, thereby adversely affecting the cold rolling by excessively improving the strength of the hot rolled sheet. On the other hand, as in the specimen prepared according to Example 1, when the content of Mn and P is lowered, sufficient recrystallization and recovery occurs, the strength of the hot rolled sheet is found to have an advantage in cold rolling.
Based on the above test results, the specimens according to Examples 1 to 3 have high yield strength, high elongation and high plastic anisotropy coefficient after the annealing heat treatment step, and thus have excellent deep drawing properties and high yield ratios. It confirmed that it was excellent.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.
S110: Slab reheating step
S120: Hot rolling step
S130: cooling / winding step
S140: Pickling Step
S150: cold rolling stage
S160: annealing heat treatment step
S170: cooling stage
Claims (8)
(b) finishing hot rolling of the reheated sheet at Finishing Delivery Temperature (FDT): Ar 3 to Ar 3 + 100 ° C .;
(c) winding the hot rolled sheet to a cooling temperature (CT) of 500 to 600 ° C .;
(d) pickling the wound sheet and then cold rolling at a reduction ratio of 50 to 80%; And
(e) annealing the cold rolled plate at 830 to 880 ° C. for 20 to 30 hours.
The slab plate
S: 0.002 to 0.010% by weight and N: 0.004% by weight or less of the cold rolled steel sheet manufacturing method characterized in that it is included.
In the step (c)
Cooling rate is 20 ~ 30 ℃ / sec cold rolled steel sheet manufacturing method characterized in that.
In the step (e)
Method for producing a cold rolled steel sheet, characterized in that the heating rate of the annealing heat treatment is 20 ~ 40 ℃ / sec.
After the step (e)
(f) cooling the annealing heat-treated plate to 550 ~ 650 ℃; cold rolled steel sheet manufacturing method further comprising.
Tensile strength (TS): 400 ~ 550MPa, Yield strength (YS): 300 ~ 450MPa and Yield ratio (YR): Cold rolled steel sheet characterized in that it has more than 0.78.
The steel sheet
S: 0.002 to 0.010% by weight and N: 0.004% by weight or less of the cold rolled steel sheet comprising a.
The steel sheet
Elongation (El): 38% or more and plastic anisotropy coefficient (r-value): cold rolled steel sheet characterized in that it has 1.5 or more.
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| KR1020120032748A KR20130110635A (en) | 2012-03-29 | 2012-03-29 | Cold-rolled steel sheet and method of manufacturing the same |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| KR1020120032748A KR20130110635A (en) | 2012-03-29 | 2012-03-29 | Cold-rolled steel sheet and method of manufacturing the same |
Publications (1)
| Publication Number | Publication Date |
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| KR20130110635A true KR20130110635A (en) | 2013-10-10 |
Family
ID=49632420
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| KR1020120032748A KR20130110635A (en) | 2012-03-29 | 2012-03-29 | Cold-rolled steel sheet and method of manufacturing the same |
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| Country | Link |
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| KR (1) | KR20130110635A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR20230030973A (en) * | 2021-08-26 | 2023-03-07 | 현대제철 주식회사 | Cold-rolled steel sheet with excellent plastic anisotropy and strength and method of manufacturing the same |
-
2012
- 2012-03-29 KR KR1020120032748A patent/KR20130110635A/en not_active Application Discontinuation
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR20230030973A (en) * | 2021-08-26 | 2023-03-07 | 현대제철 주식회사 | Cold-rolled steel sheet with excellent plastic anisotropy and strength and method of manufacturing the same |
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