Full-hard cold rolled steel and method of fabricating the same

Abstract

The present invention comprises the step of manufacturing a hot-rolled steel material comprising, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities; The present invention provides a method for manufacturing a full-hard cold-rolled steel, comprising the step of cold-rolling the hot-rolled steel at a cold reduction rate of 35% to 45% to produce a full-hard cold-rolled steel, wherein an annealing heat treatment is not performed after the cold-rolling, and wherein the full-hard cold-rolled steel finally produced by performing the cold-rolling has a yield strength of 450 MPa to 500 MPa, a tensile strength of 460 MPa to 510 MPa, and an elongation of 10% or more.

Description

Technology Field

[0001] The present invention relates to steel and a method for manufacturing the same, and more specifically, to a method for manufacturing full-hard cold-rolled steel. Background Technology

[0002] Generally, steel materials include hot-rolled steel produced by hot-rolling continuously cast products and cold-rolled steel produced by cold-rolling the said hot-rolled steel. Typically, cold-rolled steel immediately after cold rolling undergoes annealing heat treatment to improve workability.

[0003] In contrast, full-hard cold-rolled steel, which is cold-rolled steel that has not undergone separate annealing heat treatment after cold rolling, is a non-annealed steel. Full-hard cold-rolled steel has physical properties such as a tensile strength of 670 to 720 MPa and an elongation of 3 to 4%, and its workability is poor due to its relatively low elongation. Because of its high tensile strength, this full-hard cold-rolled steel is not easy to process in existing facilities that use cold-rolled steel of the 400 to 600 MPa class as a material. Furthermore, due to problems such as cracking occurring when bending 180 degrees because of its low elongation, a separate additional heat treatment process was required. Full-hard cold-rolled steel that has not undergone the additional heat treatment process was utilized as a base material by only a very small number of hot-dip galvanizing or forming companies. Therefore, there is a need for an alternative to increase the utilization of full-hard cold-rolled steel that has not undergone the aforementioned additional heat treatment process. Prior art literature

[0004] Korean Patent Application No. 2018-0115926 The problem to be solved

[0005] The problem that the present invention aims to solve is to solve the aforementioned problems and to provide a full-hard cold-rolled steel having tensile strength and elongation that are easy to process, and a method for manufacturing the same.

[0006] However, these tasks are exemplary, and the technical concept of the present invention is not limited thereto. means of solving the problem

[0007] A method for manufacturing a full-hard cold-rolled steel according to one embodiment of the present invention comprises the step of manufacturing a hot-rolled steel comprising, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities; The method comprises the step of manufacturing a full-hard cold-rolled steel by cold-rolling the hot-rolled steel at a cold reduction rate of 35% to 45%, wherein an annealing heat treatment is not performed after the cold-rolling, and the full-hard cold-rolled steel finally produced by performing the cold-rolling is characterized by having a yield strength of 450 MPa to 500 MPa, a tensile strength of 460 MPa to 510 MPa, and an elongation of 10% or more.

[0008] A method for manufacturing a full-hard cold-rolled steel according to another embodiment of the present invention comprises the step of manufacturing a hot-rolled steel comprising, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities; The method comprises the step of manufacturing a full-hard cold-rolled steel by cold-rolling the hot-rolled steel at a cold reduction rate of 30% to 70%, wherein an annealing heat treatment is not performed after the cold-rolling, and the full-hard cold-rolled steel finally produced by performing the cold-rolling is characterized by having a yield strength of 420 MPa to 600 MPa, a tensile strength of 430 MPa to 610 MPa, and an elongation of 5% or more.

[0009] In the above method for manufacturing full-hard cold-rolled steel, the full-hard cold-rolled steel finally produced by performing the cold rolling is characterized by having a microstructure consisting solely of ferrite.

[0010] In the above method for manufacturing full-hard cold-rolled steel, the step of manufacturing the hot-rolled steel can be performed under process conditions in which the finishing rolling temperature (FDT) is 900℃ to 940℃ and the coiling temperature (CT) is 670℃ to 710℃.

[0011] In the above method for manufacturing full-hard cold-rolled steel, the hot-rolled steel may further include titanium (Ti): 0.03% to 0.04% or niobium (Nb): 0.007% to 0.013% in weight%.

[0012] A full-hard cold-rolled steel according to one embodiment of the present invention is characterized in that, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder is iron (Fe) and other unavoidable impurities, and has a yield strength of 450 MPa to 500 MPa, a tensile strength of 460 MPa to 510 MPa, and an elongation of 10% or more.

[0013] A full-hard cold-rolled steel according to another embodiment of the present invention is characterized in that, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder is iron (Fe) and other unavoidable impurities, and has a yield strength of 420 MPa to 600 MPa, a tensile strength of 430 MPa to 610 MPa, and an elongation of 5% or more.

[0014] The above-mentioned full-hard cold-rolled steel is characterized by a microstructure that is finally realized consisting solely of ferrite.

[0015] The above full-hard cold-rolled steel may further contain titanium (Ti): 0.03% to 0.04% or niobium (Nb): 0.007% to 0.013% in weight%. Effects of the invention

[0016] According to the present invention, a full-hard cold-rolled steel having easy-to-process tensile strength and elongation and a method for manufacturing the same can be realized.

[0017] The effects of the present invention described above are illustrative and the scope of the present invention is not limited by these effects. Brief explanation of the drawing

[0018] FIGS. 1 to 3 are drawings showing the final physical properties according to the cold rolling rate in the method for manufacturing full-hard cold-rolled steel according to the first experimental example of the present invention. FIG. 4 is a diagram showing the microstructure of hot-rolled steel and the microstructure of full-hard cold-rolled steel according to the first experimental example of the present invention. Figure 5 is a photograph showing whether a crack occurred in Experimental Example 1 of Table 5, and Figure 6 is a photograph showing whether a crack occurred in Experimental Example 3 of Table 5. FIG. 7 is a photograph showing one example of a product made of full-hard cold-rolled steel according to an embodiment of the present invention. Specific details for implementing the invention

[0019] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical concept of the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the technical concept of the present invention to those skilled in the art. In this specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and areas in the drawings are depicted schematically. Accordingly, the technical concept of the present invention is not limited by the relative sizes or spacing depicted in the attached drawings.

[0020] The present invention discloses a full-hard cold-rolled steel, which is a cold-rolled steel that has not undergone a separate annealing heat treatment after cold rolling, and a method for manufacturing the same. Full-hard cold-rolled steel, which is a cold-rolled steel that has not undergone a separate annealing heat treatment after cold rolling, is a non-annealed steel. Full-hard cold-rolled steel has physical properties such as a tensile strength of 670 to 720 MPa and an elongation of 3 to 4%, and its workability is poor due to the relatively low elongation. Because such full-hard cold-rolled steel has a high tensile strength, it is not easy to process in existing facilities that use cold-rolled steel of the 400 to 600 MPa class as a material. Furthermore, due to problems such as cracking occurring when bending 180 degrees because of the low elongation, a separate additional heat treatment process was required. Full-hard cold-rolled steel that has not undergone an additional heat treatment process was utilized as a base material by only a very small number of hot-dip galvanizing or forming companies. Therefore, there is a need for an alternative that can increase the utilization of full-hard cold-rolled steel that has not undergone the aforementioned additional heat treatment process.

[0021] According to the full-hard cold-rolled steel and the method for manufacturing the same according to an embodiment of the present invention, the above-mentioned problems are solved, and a full-hard cold-rolled steel having tensile strength and elongation that are easy to process and a method for manufacturing the same are provided.

[0022] Hereinafter, a method for manufacturing full-hard cold-rolled steel according to the present invention will be described with reference to the attached drawings.

[0023] Method for manufacturing full-hard cold-rolled steel

[0024] A method for manufacturing a full-hard cold-rolled steel according to an embodiment of the present invention comprises the steps of: manufacturing a hot-rolled steel (S100) comprising, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities; and manufacturing a full-hard cold-rolled steel by cold-rolling the hot-rolled steel at a predetermined range of cold reduction rates (S200).

[0025] The method for manufacturing a full-hard cold-rolled steel according to an embodiment of the present invention is characterized by intentionally not performing an annealing heat treatment after the step (S200) of manufacturing a full-hard cold-rolled steel by cold rolling.

[0026] In the manufacturing method according to the present invention, the semi-finished product subject to the hot rolling and cold rolling processes may be, for example, a slab. The slab in the semi-finished product state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0027] Hot-rolled steel manufacturing step (S100)

[0028] In the hot-rolled steel manufacturing step (S100), a slab is prepared first. The slab may contain various compositions and contents, for example, the slab may contain an extremely low content of carbon. The slab may contain, for example, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder may be iron (Fe) and other unavoidable impurities.

[0029] The role and content of each component included in the full-hard cold-rolled steel according to the present invention are described below. In this case, the content of the component elements refers to weight percent.

[0030] Carbon (C): 0.0003% ~ 0.0034%

[0031] Carbon is the most important alloying element in steelmaking and serves primarily as a basic strengthening element. Within the alloy composition range described above, reducing the carbon content from approximately 400 ppm to 30 ppm can reduce the tensile strength of full-hard cold-rolled steel by approximately 50 to 100 MPa. If the carbon content is less than 0.0003%, there is a problem in that the cost of vacuum degassing treatment in the steelmaking process increases significantly. If the carbon content exceeds 0.0034%, fine carbides precipitate, increasing the peening force; consequently, grain growth is inhibited, workability is reduced, and it is difficult to achieve the reduction in tensile strength required by the present invention. Therefore, it is preferable that the carbon content be 0.0003% to 0.0034% of the total weight of the steel.

[0032] Silicon (Si): Greater than 0%, less than or equal to 0.02%

[0033] Silicon is an element that inhibits the formation of carbides (e.g., Fe3C) within ferrite and is well known as a ferrite stabilizing element that increases ductility by raising the ferrite fraction during cooling. If the silicon content exceeds 0.02%, workability such as elongation is reduced, and oxides (SiO2) may form on the surface of the steel during processing, leading to a decrease in plating performance due to poor wettability in that area. Therefore, it is desirable that the silicon content be greater than 0% and less than or equal to 0.02% of the total weight of the steel.

[0034] Manganese (Mn): Greater than 0% ~ 0.15% or less

[0035] Manganese is an excellent deoxidizer and can expand the austenite region, lower the transition temperature from austenite to polygonal ferrite, and improve the strength of steel by utilizing the solid solution strengthening action of manganese. However, if the manganese content exceeds 0.15%, it increases the carbon equivalent, significantly reducing weldability, and the formation of oxides (MnO) on the surface of the steel during processing can lead to a decrease in plating performance due to poor wettability in that area. Therefore, it is desirable that the manganese content be greater than 0% and less than or equal to 0.15% of the total weight of the steel.

[0036] Phosphorus (P): Greater than 0% ~ 0.02% or less

[0037] Phosphorus can play a role similar to silicon in steel. However, if phosphorus is added in an amount exceeding 0.02% of the total weight of the steel, it can reduce the weldability of the steel and increase brittleness, thereby causing material degradation. Therefore, it is desirable to limit the phosphorus content to more than 0% and less than or equal to 0.02% of the total weight of the steel.

[0038] Sulfur (S): Greater than 0% ~ 0.01% or less

[0039] Sulfur is an element that is inevitably contained during the manufacture of steel, and it impairs the toughness and weldability of steel and reduces its corrosion resistance and impact properties by combining with manganese (Mn) to form MnS. Therefore, it is desirable to limit the sulfur content to more than 0% and less than or equal to 0.01% of the total weight of the steel.

[0040] The remaining component of the above-mentioned full-hard cold-rolled steel is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the conventional steelmaking process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0041] Meanwhile, in addition to the alloy composition described above, the full-hard cold-rolled steel according to another embodiment of the present invention may further include titanium (Ti): 0.03% to 0.04% or niobium (Nb): 0.007% to 0.013% by weight.

[0042] Titanium (Ti): 0.03% ~ 0.04%

[0043] Titanium is added to improve machinability by precipitating solid solution carbon; by precipitating solid solution carbon using TiC or similar elements, non-aging properties and machinability are secured. As a carbonitride-forming element stronger than niobium, titanium precipitates solid solution carbon before niobium. When the amount of titanium added is less than 0.03%, the content of dissolved niobium is low, excluding the niobium required for precipitation, resulting in almost no effect on strength improvement. Therefore, it is desirable for the titanium content to be 0.03% or higher. Furthermore, when the amount of titanium added exceeds 0.04%, the amount of solid solution carbon decreases, and as the dissolved niobium increases, the yield strength increases, thereby weakening machinability. Therefore, it is desirable for the titanium content to be 0.04% or lower.

[0044] Niobium (Nb): 0.007% ~ 0.013%

[0045] Niobium (Nb) combines with carbon (C) to form NbC precipitates and improves the strength of the steel through solid solution strengthening in Fe. Additionally, it improves strength and toughness by suppressing grain growth during rolling and refining the grains. It is preferable that the niobium (Nb) be added in a content ratio of 0.007% to 0.013% of the total weight of the steel according to the present invention. If the niobium (Nb) content is less than 0.007%, the above-mentioned niobium addition effect cannot be properly exhibited. On the other hand, if the niobium (Nb) content exceeds 0.013%, excessive precipitation may reduce continuous casting, rollability, elongation, etc.

[0046] A slab having the composition described above is reheated at a slab reheating temperature (SRT) in the range of, for example, 1,000°C to 1,300°C. Through this reheating, the resolution of segregated components and precipitates may occur. If the reheating temperature is below 1,000°C, a problem may arise where the hot rolling load increases rapidly. If the reheating temperature exceeds 1,300°C, charging and discharging from the furnace may be difficult due to slab bending, and it may be difficult to secure the strength of the final product steel due to the coarsening of the initial austenite grains. The reheating temperature may vary depending on the slab.

[0047] Subsequently, the reheated slab is hot-rolled, and hot-rolling can be performed, for example, at a Roughing Delivery Temperature (RDT) of 940°C to 1000°C and a Finish Delivery Temperature (FDT) of 900°C to 940°C. If the Finish Delivery Temperature exceeds 940°C, there is a risk that the quality of the steel will deteriorate due to the formation of surface scale on the steel. In addition, if the Finish Delivery Temperature is less than 900°C, it may cause an increase in rolling load and a decrease in productivity. The hot-rolling can be performed in a single-phase region rather than a two-phase region.

[0048] Next, after cooling the hot-rolled steel, it is coiled at a coiling temperature (CT) in the range of, for example, 670°C to 710°C. The coiling temperature may vary depending on the steel. If the coiling temperature exceeds 710°C, an undesirable internal oxide layer may form on the hot-rolled steel or the coiled hot-rolled coil. Since the internal oxidation of the coiled hot-rolled coil varies, it may be difficult to uniformly control the thickness of the internal oxide layer. If the coiling temperature is below 670°C, an undesirable low-temperature structure may be formed.

[0049] Cold-rolled steel manufacturing step (S200)

[0050] In the cold-rolled steel manufacturing step (S200), the hot-rolled steel can be cold-rolled at a cold reduction rate within a predetermined range, and thereby, a full-hard cold-rolled steel can be manufactured. The method for manufacturing a full-hard cold-rolled steel according to the technical concept of the present invention is characterized by not performing annealing heat treatment after the cold-rolling.

[0051] The strength and elongation achieved in full-hard cold-rolled steel can be controlled according to the range of the above cold reduction rate.

[0052] For example, a full-hard cold-rolled steel finally produced by performing cold rolling with a cold reduction rate of 35% to 45% on hot-rolled steel can achieve material properties of yield strength: 450 MPa to 500 MPa, tensile strength: 460 MPa to 510 MPa, and elongation: 10% or more and less than 19%.

[0053] As another example, a full-hard cold-rolled steel finally produced by performing cold rolling with a cold reduction rate of 30% to 70% on hot-rolled steel can achieve material properties of yield strength: 420 MPa to 600 MPa, tensile strength: 430 MPa to 610 MPa, and elongation: 5% to 19%.

[0054] In the method for manufacturing full-hard cold-rolled steel according to the technical concept of the present invention, it was confirmed that when cold rolling is performed by adjusting the cold reduction rate from 70% to 40%, the tensile strength of the finally achieved full-hard cold-rolled steel is reduced by about 10% to 15%.

[0055] The full-hard cold-rolled steel finally produced by performing the above cold rolling has a microstructure consisting solely of ferrite.

[0056] A full-hard cold-rolled steel according to one embodiment of the present invention, implemented by the method for manufacturing full-hard cold-rolled steel described above, is characterized in that, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder is iron (Fe) and other unavoidable impurities, and has a yield strength of 450 MPa to 500 MPa, a tensile strength of 460 MPa to 510 MPa, and an elongation of 10% or more.

[0057] In addition, the full-hard cold-rolled steel according to another embodiment of the present invention, implemented by the method for manufacturing full-hard cold-rolled steel described above, is characterized in that, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder is iron (Fe) and other unavoidable impurities, and has a yield strength of 420 MPa to 600 MPa, a tensile strength of 430 MPa to 610 MPa, and an elongation of 5% or more.

[0058] The full-hard cold-rolled steel according to the embodiments of the present invention described above has a carbon content reduced from about 400 ppm to 30 ppm and a cold rolling rate reduced from 70% to 40%, so that excessively high tensile strength is not exhibited, making it easy to process even in existing facilities that use cold-rolled steel of 400 to 600 MPa grade as a material, and the elongation is increased so that cracks can be prevented when bending 180 degrees.

[0059] In addition, the method for manufacturing full-hard cold-rolled steel according to the embodiments of the present invention can omit the annealing heat treatment process after cold rolling, so an additional effect of reducing manufacturing costs can be expected.

[0060] In addition, for example, when manufacturing a steel box for packaging, storage, or transporting machine parts as an ultra-thin material with a thickness of 1.6 mm or less using a 300 MPa grade hot-rolled material, there is a problem that production load is placed on the hot rolling process to realize the ultra-thin material. However, when the material of the steel box is replaced with a full-hard cold-rolled steel according to the embodiments of the present invention (see FIG. 7), it is not necessary to realize the ultra-thin material in hot rolling, so the production load of the hot rolling process can be relieved, and the effect of preventing wave phenomena or scale formation problems occurring in ultra-thin hot-rolled products can be expected.

[0061] That is, referring to FIG. 7, the steel box is a container for packaging, storing, or transporting machine parts and includes a lower frame and an upper frame spaced apart from each other and a support member that supports and connects the lower frame and the upper frame to form an internal space capable of accommodating the machine parts. The machine parts can be loaded onto the lower frame. At least one of the materials of the lower frame, the support frame, and the support member may be a full-hard cold-rolled steel having, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities, with a yield strength of 450 MPa to 500 MPa, a tensile strength of 460 MPa to 510 MPa, and an elongation of 10% or more.

[0062] Preferred experimental examples are presented below to aid in understanding the present invention. However, the following experimental examples are intended only to aid in understanding the present invention, and the present invention is not limited by the following experimental examples.

[0063] Experimental Example 1

[0064] Table 1 is a table showing the composition of full-hard cold-rolled steel according to the first experimental example of the present invention.

[0065] Prepare steel having the composition (unit: weight%) of Table 1 below, and prepare full-hard cold-rolled steel by undergoing the specified hot-rolling and cold-rolling processes. The remainder is iron (Fe) and other unavoidable impurities.

[0066] ingredient C Si Mn P S Ti Nb Content 0.0017 0.013 0.074 0.01 0.001 0.032 0.01

[0067] Table 2 is a table showing the process conditions of the method for manufacturing full-hard cold-rolled steel according to the first experimental example of the present invention. Other process conditions were also applied under the same conditions within the range of process conditions described in the method for manufacturing full-hard cold-rolled steel of the present invention described above.

[0068] Finish Rolling Temperature (FDT) Winding temperature (CT) 920℃ 690℃

[0069] Table 3 is a table showing the final physical properties according to the cold reduction rate in the method for manufacturing full-hard cold-rolled steel according to the first experimental example of the present invention. The initial thickness of the steel before performing cold rolling is 4.0 mm. Figures 1 to 3 are graphs showing the data in Table 3.

[0070] Final thickness (mm) Cold rolling rate (%) Yield strength (MPa) Tensile strength (MPa) Elongation rate (%) 4.0 0 180 284 58 3.4 15 345 357 33 2.8 30 424 432 19 2.4 40 461 469 13 2.0 50 509 517 7 1.6 60 554 564 7 1.2 70 594 601 5 1.0 75 619 628 4 0.8 80 653 666 4 0.6 85 669 695 3

[0071] Referring to Figures 1 to 3 and Table 3, it can be seen that in the method for manufacturing full-hard cold-rolled steel according to the first experimental example of the present invention, as the cold reduction rate increases, the yield strength and tensile strength increase, and the elongation decreases.

[0072] Furthermore, it can be confirmed that the strength and elongation achieved in full-hard cold-rolled steel can be controlled according to the range of the above cold reduction rate.

[0073] For example, it can be confirmed that the full-hard cold-rolled steel finally produced by cold rolling with a cold reduction rate of 35% to 45% on hot-rolled steel can achieve material properties of yield strength: 450 MPa to 500 MPa, tensile strength: 460 MPa to 510 MPa, and elongation: 10% or more and less than 19%.

[0074] As another example, it can be confirmed that the final full-hard cold-rolled steel produced by cold rolling with a cold reduction rate of 30% to 70% on hot-rolled steel can achieve material properties of yield strength: 420 MPa to 600 MPa, tensile strength: 430 MPa to 610 MPa, and elongation: 5% to 19%.

[0075] According to the experimental results described above, it can be understood that by controlling the carbon content and the cold rolling rate in the method for manufacturing full-hard cold-rolled steel according to the embodiment of the present invention, various products made of existing hot-rolled materials can be replaced with the full-hard cold-rolled steel of the present invention. For example, racks, mold bases, container shoring, and container house walls made of existing 300 to 400 MPa grade hot-rolled products can be replaced with 450 to 600 MPa grade full-hard cold-rolled steel. In addition, the material of a steel box made of 300 MPa grade hot-rolled products can be replaced with 450 MPa grade full-hard cold-rolled steel.

[0076] Figure 4 is a diagram showing the microstructure of a full-hard cold-rolled steel and a hot-rolled steel according to the first experimental example of the present invention.

[0077] Referring to Figure 4, it can be seen that as the cold rolling reduction rate increases to 40%, 60%, and 80%, the grain size in the microstructure of the cold-rolled steel decreases, and the final microstructure of the full-hard cold-rolled steel is composed of ferrite.

[0078] 2nd Experimental Example

[0079] Table 4 is a table showing the composition of full-hard cold-rolled steel according to the second experimental example of the present invention.

[0080] Prepare steel having the composition (unit: weight%) of Table 4 below, and prepare full-hard cold-rolled steel by undergoing the specified hot-rolling and cold-rolling processes. The remainder is iron (Fe) and other unavoidable impurities.

[0081] C Si Mn P S Cho Seong-A 0.04 0.023 0.132 0.01 0.001 Composition B 0.0017 0.013 0.074 0.01 0.001

[0082] Referring to Table 4, composition B satisfies the compositional range of carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder being iron (Fe).

[0083] In contrast, composition A does not satisfy the range of carbon (C): 0.0003% to 0.0034% and silicon (Si): greater than 0% and less than or equal to 0.02%.

[0084] Table 5 shows the process conditions, physical properties, and whether cracks occur in the method for manufacturing full-hard cold-rolled steel according to the second experimental example of the present invention. Other process conditions were also applied under the same conditions within the range of process conditions described in the method for manufacturing full-hard cold-rolled steel of the present invention described above. For example, the hot rolling process applied the conditions disclosed in Table 2.

[0085] furtherance Cold rolling reduction rate Process types TS(MPa) EL(%) Cracks occur Experimental Example 1 Cho Seong-A 70% FH 703 2.7 generation Experimental Example 2 Composition B 70% FH 601 5 Non-occurrence Experimental Example 3 Composition B 40% FH 469 13 Non-occurrence

[0086] In Table 5, FH refers to a manufacturing process for full-hard cold-rolled steel in which hot rolling and cold rolling are performed sequentially without annealing heat treatment, TS refers to tensile strength, EL refers to elongation, and crack occurrence indicates whether cracks occur as a result of a 180-degree bending test. Figure 5 is a photograph showing whether cracks occurred in Experimental Example 1 of Table 5, and Figure 6 is a photograph showing whether cracks occurred in Experimental Example 3 of Table 5.

[0087] Referring to Table 5, it can be seen that when the carbon content in Experimental Examples 1 and 2 is reduced from 400 ppm to 17 ppm, the tensile strength decreases by approximately 100 MPa, and the elongation increases by up to 5%. Furthermore, when the cold rolling reduction ratio in Experimental Examples 2 and 3 is reduced from 70% to 40%, the tensile strength decreases by approximately 150 MPa, and the elongation increases by up to 13%. In addition, as a result of the 180-degree bending test, cracks occur in Experimental Example 1, whereas no cracks occur in Experimental Example 3, indicating good performance.

[0088] Although the present invention has been described above with reference to embodiments, various changes and modifications may be made by those skilled in the art. Such changes and modifications are considered to be within the scope of the present invention as long as they do not depart from the scope of the present invention. Accordingly, the scope of rights of the present invention should be determined by the claims set forth below.

Claims

Claim 1 A step of manufacturing a hot-rolled steel material comprising, in weight percent, carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, titanium (Ti): 0.03% to 0.04%, niobium (Nb): 0.007% to 0.013%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities; The method comprises the step of manufacturing a full-hard cold-rolled steel by cold-rolling the hot-rolled steel at a cold reduction rate of 30% to 70%; wherein the step of manufacturing the hot-rolled steel is performed under process conditions of a reheating temperature: 1000℃ to 1300℃, a finishing rolling temperature (FDT): 900℃ to 940℃, and a coiling temperature (CT): 670℃ to 710℃, and is characterized by not performing an annealing heat treatment after the cold rolling, wherein the full-hard cold-rolled steel finally produced by performing the cold rolling has a microstructure consisting only of ferrite, and the full-hard cold-rolled steel finally produced by performing the cold rolling has a yield strength of 420MPa to 600MPa, a tensile strength of 430MPa to 610MPa, and an elongation of 5% or more. Manufacturing method. Claim 2 A method for manufacturing a full-hard cold-rolled steel according to claim 1, wherein the step of manufacturing the full-hard cold-rolled steel comprises the step of manufacturing the full-hard cold-rolled steel by cold-rolling the hot-rolled steel at a cold reduction rate of 35% to 45%; and wherein the full-hard cold-rolled steel finally realized by performing the cold-rolling has a yield strength of 450 MPa to 500 MPa, a tensile strength of 460 MPa to 510 MPa, and an elongation of 10% or more. Claim 3 delete Claim 4 delete Claim 5 delete Claim 6 Full-hard cold-rolled steel, characterized in weight percent as follows: carbon (C): 0.0003% to 0.0034%, silicon (Si): greater than 0% and less than or equal to 0.02%, manganese (Mn): greater than 0% and less than or equal to 0.15%, titanium (Ti): 0.03% to 0.04%, niobium (Nb): 0.007% to 0.013%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, and the remainder being iron (Fe) and other unavoidable impurities; yield strength: 420 MPa to 600 MPa, tensile strength: 430 MPa to 610 MPa, elongation: 5% or more; and the finally realized microstructure consisting only of ferrite. Claim 7 Full-hard cold-rolled steel according to claim 6, characterized by yield strength: 450MPa ~ 500MPa, tensile strength: 460MPa ~ 510MPa, and elongation: 10% or more. Claim 8 delete Claim 9 delete

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