Cold-rolled steel material and method for producing same

Abstract

The present invention provides a cold-rolled steel material, wherein the cold-rolled steel material comprises, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities, and has a yield strength (YS) of 600 MPa to 900 MPa, an elongation (EL) of 8% or more, and a yield ratio (YR) of 95% or more.

Description

TECHNICAL FIELD

[0001] The present invention relates to steel materials, and more particularly, to a cold-rolled steel material having a high yield ratio and high yield strength, and a manufacturing method thereof.BACKGROUND ART

[0002] In recent years, the automotive industry has been pursuing weight reduction for the purpose of responding to environmental regulations and improving fuel efficiency. Since applying high-strength materials and reducing thickness is a common approach for light weighting components, high-strength steel materials are being adopted. In particular, structural members and reinforcements of automobiles form the structure of the vehicle body and are required to have excellent crash stability for the safety of the driver and passengers inside the vehicle in the event of a collision. That is, crash stability can be improved by achieving complex component shapes and securing high yield strength. Conventional ultra-low carbon high-strength steels have a low yield ratio and high elongation, which is advantageous for forming, however, due to the low yield ratio, they have the disadvantage of being unfavorable in terms of crash stability as structural members and reinforcements requiring high rigidity when used as products after forming.

[0003] Precipitation-hardened steel is a high-strength alloy in which trace amounts of carbonitride-forming elements such as titanium and niobium are added to ordinary carbon-manganese steel to generate fine precipitates, thereby improving the impact toughness and strength of the steel. Conventionally, research on precipitation-hardened steel has been conducted primarily on hot-rolled steel materials, with studies focused on models and experimental results concerning deformation-induced precipitation, interphase precipitation, and precipitation through dislocations after coiling during the hot-rolling stage, whereas for cold-rolled steel materials, fundamental research results such as studies on precipitate formation mechanisms have been conducted to a very limited extent. In the case of hot-rolled precipitation-hardened steel, a review of global standards reveals that grades up to a yield strength of 700 MPa are listed within the VDA standards.

[0004] On the other hand, for cold-rolled precipitation-hardened steel, fundamental research results regarding strength improvement and precipitate formation mechanisms have been conducted to a limited extent compared to hot-rolled materials. A review of global standards reveals that only grades up to a yield strength of 460 MPa are listed within the VDA standards, and for strengths beyond that, dual-phase steel and complex-phase steel standards are applied. Some automobile manufacturers require cold-rolled precipitation-hardened steel with a yield strength of 550 MPa; however, for components requiring strength beyond that level, dual-phase steel standards are applied as described above. Korean Patent Application No. 2012-0070333 is cited as a prior art document.SUMMARY OF THE INVENTIONTECHNICAL PROBLEM

[0005] The technical problem to be achieved by the technical concept of the present invention is to provide a cold-rolled steel material having a high yield ratio and high yield strength, and a manufacturing method thereof. For example, the present invention provides a recovery-annealed cold-rolled precipitation-hardened steel characterized by a high yield ratio capable of having high dimensional stability for application to seat rails, airbag components, and members. However, such problems are exemplary, and the present invention is not limited thereto.SOLUTION TO PROBLEM

[0006] According to one aspect of the present invention, a cold-rolled steel material and a manufacturing method thereof are provided.

[0007] The cold-rolled steel material comprises, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities, and has a yield strength (YS) of 600 MPa to 900 MPa, an elongation (EL) of 8% or more, and a yield ratio (YR) of 95% or more.

[0008] The cold-rolled steel material may further comprise, in weight%, boron (B): more than 0% and up to 0.001%, or molybdenum (Mo): more than 0% and up to 0.06%.

[0009] In the cold-rolled steel material, the total content of niobium (Nb) and titanium (Ti) may be 0.26% or less.

[0010] The cold-rolled steel material may be a cold-rolled precipitation-hardened steel having a ferrite single-phase structure, and comprising precipitates within the ferrite single-phase structure.

[0011] The manufacturing method of the cold-rolled steel material comprises: reheating a steel material comprising, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities, at a temperature of 1,200°C to 1,250°C; hot-rolling the heated steel material so as to finish at a temperature of Ar3 or higher; primary cooling the hot-rolled steel material; coiling the primary-cooled steel material at a coiling temperature of 580°C to 620°C; cold-rolling the coiled steel material to form a cold-rolled steel material; annealing the cold-rolled steel material at an annealing temperature of 740°C to 830°C; and secondary cooling the annealed cold-rolled steel material.

[0012] In the manufacturing method of the cold-rolled steel material, the step of primary cooling the steel material may comprise cooling to 580°C to 620°C at a cooling rate of 10°C / sec to 100°C / sec.

[0013] In the manufacturing method of the cold-rolled steel material, the step of secondary cooling the steel material may comprise cooling at a cooling rate of 5°C / sec to 100°C / sec.Advantageous Effects of Invention

[0014] According to the present invention, a cold-rolled steel material having a high yield ratio and high yield strength, and a manufacturing method thereof can be realized. For example, a recovery-annealed steel having a yield strength of 600 to 900 MPa grade and a yield ratio of 95% or more with a ferrite single-phase structure suitable for bending workability can be realized.

[0015] The effects of the present invention described above are set forth by way of example, and the scope of the present invention is not limited by such effects.BRIEF DESCRIPTION OF DRAWINGS

[0016] Fig. 1 is a process flowchart schematically illustrating the manufacturing method of a cold-rolled steel material according to an embodiment of the present invention. Fig. 2 is a graph showing the relationship between temperature and austenite fraction for the steel grades of the experimental examples shown in Table 1, as calculated by thermodynamic equilibrium calculation. Fig. 3 is a graph showing the relationship between stress and strain of cold-rolled steel materials of Examples and Comparative Examples as experimental examples of the present invention. Fig. 4 is a graph showing the relationship between yield strength and elongation for the steel grades of the experimental examples shown in Tables 1 to 3. Figs. 5 to 20 are photographs of the microstructures of the steel materials disclosed in Tables 2 and 3. MODES FOR CARRYING OUT THE INVENTION

[0017] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical concept of the present invention to those of ordinary skill 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 the present specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and regions in the drawings are schematically illustrated. Accordingly, the present invention is not limited by the relative sizes or spacings depicted in the accompanying drawings.

[0018] In recent years, the automotive industry has been pursuing weight reduction for the purpose of responding to environmental regulations and improving fuel efficiency. Since it is common practice to apply high-strength materials and reduce thickness in order to achieve component weight reduction, high-strength steel materials are being applied. In particular, structural members and reinforcements of automobiles form the structure of the automobile body, and are required to have excellent crash safety for the safety of the driver and passengers inside the vehicle in the event of a collision. That is, crash stability can be improved by securing a high yield ratio and yield strength.

[0019] The present invention is to provide a cold-rolled steel material and a manufacturing method thereof in which the yield ratio (= yield strength / tensile strength) of the steel material is improved by generating fine precipitates during the heat treatment process after cold rolling. Conventional precipitation-hardened steel materials caused precipitation of carbonitride elements in the hot rolling process to the maximum extent by setting the coiling temperature after hot rolling to exceed 620°C. However, when precipitates are formed during hot rolling, the precipitates coarsen during cold rolling, resulting in a decrease in the yield ratio.

[0020] In the present invention, a recovery-annealed cold-rolled precipitation-hardened steel having a ferrite single-phase structure with a yield strength of 600 to 900 MPa and a yield ratio of 95% or more, suitable for bending workability, has been proposed, and as described above, it can be applied to components such as seat rails and members that require high yield strength. Homogeneous material properties can be achieved with a ferrite single-phase microstructure, and in particular, a positive effect on hole expansion performance can be exhibited. In addition, a high yield ratio of 95% or more can bring optimal conditions during cold forming such as roll forming and bending.

[0021] Hereinafter, the cold-rolled steel material according to the present invention will be described in detail.

[0022] The cold-rolled steel material according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities.

[0023] Hereinafter, the role of each component included in the cold-rolled steel material according to the present invention and its content will be described as follows. In this case, the content of each component element refers to weight% with respect to the entire steel material.Carbon (C): 0.04% to 0.07%

[0024] Carbon (C) is added to secure the strength of the steel material and to control the microstructure. When the content is less than 0.04%, a sufficient precipitation effect cannot be obtained, making it difficult to secure the desired yield strength, and MC (M = Nb, Ti)-based carbides coarsen, resulting in a problem of reduced grain refinement effect. When the content exceeds 0.07%, material property degradation such as a decrease in strength may occur as the phase fraction of phases such as pearlite increases. In addition, when the carbon content increases, the pearlite structure increases, and upon annealing at high temperature after cold rolling, austenite transformation occurs in some pearlite structures, which may induce a two-phase structure. Therefore, it is preferable to add carbon in a content of 0.04% to 0.07%.Silicon (Si): 0.1% to 0.3%

[0025] Silicon not only contributes to strength improvement as a solid-solution strengthening element, but also, as a ferrite-stabilizing element, increases the degree of undercooling during ferrite transformation to suppress the formation of harmful carbides, refines grains, and suppresses the formation of pearlite, thereby increasing the reactivity between solid-solution C and M (M = Nb, Ti). When the silicon content is less than 0.1%, the effect of silicon addition is insufficient. When the silicon content exceeds 0.3%, oxides such as Mn 2 SiO 4 are formed, which impairs plating properties, deteriorates the surface properties of the steel material, and may increase the carbon equivalent, thereby reducing weldability. Therefore, it is preferable to add silicon in a content of 0.1% to 0.3% by weight of the entire steel material.Manganese (Mn): 1.1% to 1.5%

[0026] Manganese not only contributes to strength improvement as a solid-solution strengthening element, but can also control strength, toughness, and yield ratio depending on its content; however, when added in large amounts, it induces the formation of MnS inclusions and center segregation during casting, thereby reducing the toughness of the steel. When the Mn content is less than 1.1%, it is difficult to secure the desired yield strength of 800 MPa grade, and when it exceeds 1.5%, although it is advantageous for securing the desired strength, it may form inclusions or cause deterioration of workability and delayed fracture resistance due to segregation, and may increase the carbon equivalent, thereby reducing weldability, which is a major advantage of precipitation-hardened steel. In addition, transformation structures may be induced during annealing and cooling at high temperature after cold rolling. Therefore, it is preferable to add Mn in a content of 1.1% to 1.5%.Aluminum (Al): 0.015% to 0.06%

[0027] Aluminum is used as a deoxidizer and can help purify ferrite. When the Al content is less than 0.015%, the effect of addition is insufficient, and when it exceeds 0.06%, AlN is formed during slab manufacturing, which may induce cracks during casting or hot rolling. Therefore, it is preferable to add aluminum in a content of 0.015% to 0.06% by weight of the entire steel material.Phosphorus (P): more than 0% and up to 0.02%

[0028] Phosphorus is an impurity included in the steel manufacturing process, and while it can help improve strength through solid-solution strengthening, when contained in large amounts, it may cause low-temperature embrittlement and brittle fracture. Therefore, it is preferable to limit the phosphorus content to greater than 0% to 0.02% or less by weight of the entire steel material.Sulfur (S): more than 0% and up to 0.005%

[0029] Sulfur is an impurity included in the steel manufacturing process, and may form non-metallic inclusions such as FeS and MnS, thereby reducing toughness and weldability. Therefore, it is preferable to limit the sulfur content to greater than 0% to 0.005% or less by weight of the entire steel material.Niobium (Nb): 0.02% to 0.04%

[0030] Niobium is a powerful carbonitride-forming element that combines with carbon and nitrogen contained in the steel during hot rolling and annealing processes to form carbides or nitrides, and these Nb-based carbides or nitrides suppress recrystallization and grain growth during the annealing process after cold rolling, thereby refining grains and improving both the strength and toughness of the steel material. When the Nb content is less than 0.02%, it may be difficult to obtain a precipitation strengthening effect, and when it exceeds 0.04%, the annealing time must be extended or the annealing temperature must be increased to secure the elongation of the steel due to the recrystallization-retarding effect of Nb; therefore, it is preferable to add Nb in a content of 0.02% to 0.04%.Titanium (Ti): 0.17% to 0.23%

[0031] Titanium is a powerful carbonitride-forming element that combines with carbon and nitrogen contained in the steel during hot rolling and annealing processes to form carbides or nitrides, and these Ti-based carbides or nitrides suppress recrystallization and grain growth during the annealing process after cold rolling, thereby refining grains and improving both the strength and toughness of the steel material. When the Ti content is less than 0.17%, it may be difficult to obtain a sufficient recrystallization-retarding effect and precipitation strengthening effect, and when it exceeds 0.23%, the annealing time must be extended or the annealing temperature must be increased to secure the elongation of the steel due to the recrystallization-retarding effect of Ti; therefore, it is preferable to add Ti in a content of 0.17% to 0.23%.Chromium (Cr): more than 0% and up to 0.1%

[0032] It is preferable to add chromium (Cr) in a content ratio of 0.1% or less by weight of the entire precipitation-hardened steel material according to the present invention. When the chromium content exceeds 0.1% by weight of the entire steel material, there is a problem of reducing weldability or heat-affected zone (HAZ) toughness.Nitrogen (N): more than 0% and up to 0.005%

[0033] Nitrogen (N) is an element that inevitably remains in the steel during the steelmaking process. It has a faster diffusion rate and lower activation energy compared to carbon, so even a minute change in its amount causes a large variation in material properties, and aging is accelerated; therefore, it must be precipitated in the form of nitrides. In the present invention, when the nitrogen (N) content exceeds 0.005% and is added in large amounts, a large amount of TiN is formed, which adversely affects the formability of the steel. It is best to minimize it as much as possible; however, since it is an element that inevitably remains in the steelmaking process, it should not exceed 50 ppm.

[0034] The cold-rolled steel material according to the present invention may have a total content of niobium (Nb) and titanium (Ti) of 0.26 weight% or less, and for example, may be 0.20 to 0.26 weight%. When the total content of niobium (Nb) and titanium (Ti) is less than 0.20 weight%, it may be difficult to obtain a sufficient recrystallization-retarding effect and precipitation strengthening effect, and when the total content of niobium (Nb) and titanium (Ti) exceeds 0.26 weight%, the annealing time may need to be extended or the annealing temperature may need to be increased to secure the elongation of the steel due to the recrystallization-retarding effect.

[0035] Furthermore, the ratio of the value of weight% of carbon (C) to the value of the total content of weight% of niobium (Nb) and titanium (Ti) (= [C] / ([Nb] + [Ti])) may have a value of less than 0.23. If the value of [C] / ([Nb] + [Ti]) is 0.23 or more, a problem may arise in that the cementite fraction exists at 1% or more.

[0036] The remaining component of the cold-rolled steel material is iron (Fe). However, since unintended impurities may inevitably be incorporated from raw materials or the surrounding environment during a normal steelmaking process, they cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, all of their details are not specifically mentioned in the present specification.

[0037] Meanwhile, the cold-rolled steel material according to the present invention may further comprise, in weight%, boron (B): more than 0% and up to 0.001% or molybdenum (Mo): more than 0% and up to 0.06%.Boron (B): more than 0% and up to 0.001%

[0038] Boron (B) is added to prevent secondary work embrittlement that may occur due to the addition of phosphorus (P). In general, when boron is added in an amount exceeding 0.001%, material property variation due to segregation may occur; therefore, it is preferable to add boron in a range of 0.001% or less.Molybdenum (Mo): more than 0% and up to 0.06%

[0039] Molybdenum acts when Nb and Ti components combine with carbon to form precipitates, slowing the rate of formation and growth of fine precipitates, thereby obtaining a yield strength strengthening effect. When the molybdenum content exceeds 0.06%, the effect converges and price competitiveness is reduced; therefore, it is preferable to add molybdenum in a content of 0.06% or less.

[0040] The cold-rolled steel material manufactured by controlling the specific components of the alloy composition described above and their content ranges, and through the manufacturing method described below, can satisfy, for example, yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or more, and yield ratio (YR): 95% or more.

[0041] The cold-rolled steel material may include precipitates formed at grain boundaries, within grains, or both. The precipitates mainly include precipitates of the Ti,Nb(C) type. In addition, TiN, AlN, and the like may be included. The precipitates may have an average particle diameter of mainly 2 to 6 nm. The precipitates may be formed during the annealing heat treatment performed after cold rolling. By virtue of these precipitates, the cold-rolled steel material can simultaneously have high strength and a high yield ratio. For example, a portion of the Ti,Nb(C) is formed during hot rolling and coiling, and then the remaining 30 to 70% of Ti and Nb elements use the large number of dislocations generated during cold rolling as nucleation sites and precipitate as Ti,Nb(C) during the annealing process.

[0042] Hereinafter, a manufacturing method of the cold-rolled steel material according to the present invention will be described with reference to the accompanying drawings.Manufacturing Method of Cold-Rolled Steel Material

[0043] Fig. 1 is a drawing illustrating the temperature over time in the manufacturing method of a cold-rolled steel material according to an embodiment of the present invention and diagramming the microstructural changes of the steel material at each step.

[0044] The steel material comprises, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities.

[0045] The manufacturing method of a cold-rolled steel material according to an embodiment of the present invention comprises a reheating step (S110), a hot rolling step (S120), a primary cooling step (S130), a coiling step (S140), a cold rolling step (S150), an annealing heat treatment step (S160), and a secondary cooling step (S170).Reheating Step (S110)

[0046] The semi-finished product subject to the hot rolling process in the manufacturing method according to the present invention may be, for example, a slab. The slab in a semi-finished state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0047] In the reheating step (S110), the steel material having the above composition, for example a slab plate, is reheated at a slab reheating temperature (SRT) of 1,200°C to 1,250°C for 40 minutes to 1 hour. Through such reheating, re-dissolution of segregated components during casting and re-dissolution of precipitates may occur, and in particular, a sufficiently high temperature is required to re-dissolve precipitation hardening elements such as niobium so as to form fine precipitates during the annealing heat treatment after subsequent cold rolling. The fine precipitates may impede grain growth to achieve grain refinement, thereby increasing strength. When the reheating temperature is below 1,200°C or the reheating furnace residence time is less than 40 minutes, carbonitrides do not dissolve and become coarsened, making it impossible to obtain material properties exceeding the desired yield ratio; for example, maximum re-dissolution of precipitation hardening elements such as niobium contained in the steel material may not occur, and components segregated during casting may not be sufficiently uniformly distributed. When the reheating temperature exceeds 1,250°C or the reheating furnace residence time exceeds 1 hour, austenite grains may become coarsened, which may cause a decrease in yield strength. In addition, as the reheating temperature increases, there is a problem of increased manufacturing costs and decreased productivity due to heating costs and additional time required to achieve the hot rolling temperature.Hot Rolling Step (S120)

[0048] The heated steel material is first subjected to hot rolling after heating in order to adjust its shape. The hot rolling may be performed continuously by width rolling, roughing mill, and finishing mill. By the hot rolling step, the steel material may form a hot-rolled steel material. The hot-rolled steel material may be a hot-rolled steel sheet. The reduction ratio of hot rolling may be 50% or more. Work hardening (deformation) may occur during the hot rolling process, and furthermore, the work hardening may be eliminated through recrystallization and recovery.

[0049] The finishing mill may be completed at a finish rolling temperature (FRT) of Ar3 or higher. When the finish rolling temperature is below Ar3 (for example, below 870°C), rolling is performed in the two-phase region of austenite and ferrite, causing grain mixing, which results in non-uniform deformability and may lead to a decrease in rollability. When the finish rolling temperature exceeds Ar3 + 40°C (for example, exceeds 910°C), grains may become coarsened, resulting in a decrease in the strength of the final steel material.Primary Cooling Step (S130)

[0050] The primary cooling step (S130) may be performed on a run-out table (ROT). The run-out table (ROT) is a device that cools the steel sheet whose thickness and width have been determined by roughing rolling and finishing rolling. The run-out table is provided with a plurality of banks, each having a plurality of headers equipped with a row of nozzles, arranged above and below the steel sheet. A coiler for coiling the steel sheet into a coil form is provided at the rear end of the run-out table, and the cooling of the steel sheet must be controlled so that the temperature of the steel sheet immediately before entering the coiler, i.e., the coiling temperature, matches the target coiling temperature. The hot-rolled steel material is primarily cooled to 580°C to 620°C at a cooling rate of 10°C / sec to 100°C / sec. The cooling may be performed by air cooling or water cooling. When the cooling rate is less than 10°C / sec, the average particle size of precipitates may increase, making it difficult to secure strength. Conversely, when the cooling rate exceeds 100°C / sec, the microstructure of the steel material may become hardened, resulting in a decrease in impact toughness.Coiling Step (S140)

[0051] Upon completion of the primary cooling, the steel material is coiled at a coiling temperature (CT) of 580°C to 620°C. In the present invention, a low-temperature coiling process of 580°C to 620°C is applied to make the size of precipitates fine and to minimize the precipitation fraction as much as possible. When the coiling temperature is below 580°C, difficulties may arise in controlling the shape of the hot-rolled material, and the surface quality of the steel material may deteriorate due to the abrupt difference between the finishing rolling temperature and the coiling temperature. When the coiling temperature exceeds 620°C, it may contribute to increasing the strength of the hot-rolled material by improving the precipitation fraction; however, this is undesirable from the perspective of the final cold-rolled material properties because the precipitates become coarsened. That is, when the coiling temperature exceeds 620°C, carbonitride elements cannot be maintained in a solid solution state and may be formed as undesired precipitates.

[0052] Referring to the microstructural changes at each step of the hot rolling step (S120), primary cooling step (S130), and coiling step (S140) illustrated in Fig. 1, the SIP item refers to strain-induced precipitation, specifically precipitation caused by deformation occurring during the rolling process. PIP refers to phase-induced precipitation, specifically precipitates formed during the transformation process from austenite to ferrite. In addition, the Random item refers to precipitates that are widely precipitated within the ferrite microstructure during the cooling process after coiling.

[0053] Meanwhile, the γ→α phase transformation refers to the phase transformation from austenite to ferrite.Cold Rolling Step (S150)

[0054] A pickling treatment is performed to clean the coiled hot-rolled steel material with acid. Subsequently, cold rolling is performed on the pickled hot-rolled steel material at a cold reduction ratio of 50% to 80% to form a cold-rolled steel material. When the cold reduction ratio is less than 50%, the amount of nucleation for recrystallization during annealing is small, so grains may grow excessively during the annealing heat treatment described below, resulting in a sharp decrease in strength. As the cold reduction ratio increases beyond 50%, the number of sites at which fine precipitates can be formed increases, which is favorable for improving the yield ratio; however, when it exceeds 80%, the amount of nucleation becomes excessively large, causing the grains formed by annealing to be too fine, thereby reducing ductility and decreasing formability.Annealing Heat Treatment Step (S160)

[0055] The cold-rolled steel material is heat-treated in a continuous annealing furnace having a conventional slow cooling section. When continuously annealing the cold-rolled sheet, recovery recrystallization annealing is performed at a strip passing speed of 50 to 200 mpm and an annealing temperature of 740 to 830°C. The heat treatment may be referred to as annealing heat treatment. The annealing heat treatment is performed by heating at a temperature increase rate in the range of 1°C / sec to 10°C / sec and maintaining at a temperature in the range of 740°C to 830°C for 30 seconds to 1,000 seconds. At this time, the annealing heat treatment is one of the important process variables that determines the material properties of the final product. This recovery recrystallization annealing heat treatment results in a very fine final microstructure and generates fine precipitates of solid-solution carbonitride elements, thereby achieving a high yield ratio of the cold-rolled steel material. The carbonitrides may be, for example, NbC precipitates and AlN precipitates. When the annealing heat treatment temperature is below 740°C, recrystallization may not be sufficiently completed, and the target elongation may not be secured. When the annealing heat treatment temperature exceeds 830°C, grains may become coarsened, resulting in a decrease in yield strength and yield ratio. Therefore, in order to secure a yield strength of 600 to 900 MPa and a yield ratio of 95% or more, it is preferable to limit the annealing temperature to the above range.

[0056] Meanwhile, the strip passing speed during continuous annealing of cold-rolled sheets may be 90 to 110 mpm, which is the speed for producing steel for recrystallization at a general annealing temperature of 760 to 800°C when producing steel material with a thickness of 1.0 to 1.5t on a continuous galvanizing line (CGL). In the present invention, since the recrystallization fraction of the steel is distributed in the range of 31% to 81%, the range of the strip passing speed can be further widened to 50 to 200 mpm correspondingly.Secondary Cooling Step (S170)

[0057] The annealing heat-treated cold-rolled steel material is cooled at a cooling rate of 5°C / sec to 100°C / sec to a temperature in the range of, for example, 100°C to 300°C.

[0058] The cold-rolled steel material can be manufactured through the above process (S110 to S170), and the cold-rolled steel material may be a cold-rolled steel sheet.

[0059] If necessary, the cold-rolled steel material may be formed into a hot-dip galvanized steel material and an alloyed hot-dip galvanized steel material. The hot-dip galvanized steel material may be realized by immersing the cold-rolled steel material in a hot-dip galvanizing bath to form a hot-dip galvanized layer. The temperature of the plating bath may be in the range of 400°C to 520°C depending on the type and ratio of alloying elements constituting the plating layer and the composition system of the cold-rolled steel material. Under the plating bath conditions, a hot-dip galvanized layer is easily formed on the surface of the cold-rolled sheet, and the adhesion of the plating layer may be excellent. Subsequently, the hot-dip galvanized steel material may be formed by cooling to room temperature at a cooling rate of 1°C / sec to 100°C / sec.

[0060] The alloyed hot-dip galvanized steel material may be produced by subjecting the cold-rolled steel material on which the hot-dip galvanized layer has been formed to an alloying heat treatment. The alloying heat treatment may be performed at a temperature in the range of 500°C to 620°C for 10 seconds to 60 seconds. Under the above conditions, the hot-dip galvanized layer grows stably during the alloying heat treatment, and the adhesion of the plating layer may be excellent. When the alloying heat treatment temperature is below 500°C, alloying may not proceed sufficiently, resulting in a decrease in the integrity of the hot-dip galvanized layer. When the alloying heat treatment temperature exceeds 620°C, the temperature range may enter the two-phase region, causing changes in material properties. Thereafter, the alloyed hot-dip galvanized steel material may be formed by cooling.

[0061] In the zone where the reheating is performed, precipitation hardening elements such as niobium are re-dissolved. In the zone where the hot rolling is performed, precipitate phases such as niobium carbide may be formed within austenite grains. In the zone where the primary cooling is performed and the zone where the coiling is performed, precipitate phases such as niobium carbide are formed within ferrite grains; however, the amount of the precipitate phases may be reduced by the low coiling temperature. In the zone where the cold rolling is performed, niobium may be re-dissolved into the matrix. In the zone where the annealing heat treatment is performed, precipitate phases such as niobium carbide may be formed within ferrite grains. That is, by minimizing the amount of precipitate phases formed in the hot rolling process and inducing the formation of precipitate phases in the cold rolling process, the size of the precipitate phases can be refined. Accordingly, a high yield ratio and high yield strength can be achieved.

[0062] The cold-rolled steel material realized by applying the above-described alloy composition and process conditions can satisfy, for example, yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or more, and yield ratio (YR): 95% or more.

[0063] In the cold-rolled steel material realized by applying the above-described alloy composition and process conditions, the yield strength (unit: MPa) and yield ratio (unit: %) have a relationship approximately according to the following equations depending on the annealing heat treatment temperature (unit: °C) in the annealing heat treatment step (S160), the annealing heat treatment holding time (unit: seconds), and the titanium content (unit: weight%) constituting the steel material. 4286.5605 Ti wt % − 3.3482 SS ° C − 0.2952 t s + 2474.7647 = YP MPa 367.6854 Ti wt % − 0.1819 SS ° C − 0.0141 t s + 159.9337 = Yield Ratio %

[0064] In Equation 1 and Equation 2, Ti denotes the titanium content (unit: weight%), SS denotes the annealing heat treatment temperature (unit: °C) in the annealing heat treatment step (S160), t denotes the annealing heat treatment holding time (unit: seconds) in the annealing heat treatment step (S160), and YP denotes the yield strength (unit: MPa).

[0065] The cold-rolled steel material may include precipitates formed at grain boundaries, within grains, or both. The precipitates mainly include precipitates of the Ti,Nb(C) type. In addition, TiN, AlN, and the like may be included. The precipitates may have an average particle diameter of mainly 2 to 6 nm. The precipitates may be formed during the annealing heat treatment performed after cold rolling. Due to these precipitates, the cold-rolled steel material can simultaneously have high strength and a high yield ratio. For example, a portion of the Ti,Nb(C) is formed during the hot rolling and coiling processes, and the remaining 30 to 70% of Ti and Nb elements utilize the large number of dislocations generated during cold rolling as nucleation sites and precipitate as Ti,Nb(C) during the annealing process.Experimental Examples

[0066] Hereinafter, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are only intended to aid in understanding the present invention, and the present invention is not limited by the following experimental examples. Content not described herein can be technically inferred by those skilled in the art, and thus a description thereof will be omitted.

[0067] Steel materials having the compositions shown in Table 1 below were prepared, and cold-rolled steel sheets according to Examples and Comparative Examples were prepared through the hot rolling and cold rolling processes as described above.

[0068] Table 1 shows the compositions of the cold-rolled steel materials of the Examples and Comparative Examples. The content unit of each component is weight%, and the balance is iron (Fe). [Table 1]Steel GradeCSiMnPSCrMoTiNbVAlNA0.0510.221.300.0010.00070.004-0.2230.032-0.0410.0007B0.0510.211.300.0010.00060.004-0.1640.030-0.0420.0008C0.0540.201.310.0010.00050.004-0.1060.030-0.0420.0008

[0069] Referring to Table 1, Steel Grade A satisfies, in weight%, the composition ranges of carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe). In addition, Steel Grade A satisfies a total content of niobium (Nb) and titanium (Ti) of 0.26 wt% or less, and specifically, satisfies a range of 0.20 to 0.26 wt%. Furthermore, the ratio of the value of carbon (C) in wt% to the value of the total content of niobium (Nb) and titanium (Ti) in wt% (= [C] / ([Nb] + [Ti])) has a value of less than 0.23. In contrast, Steel Grade B and Steel Grade C do not satisfy the range of titanium (Ti): 0.17% to 0.23% and fall below it. In addition, Steel Grade B and Steel Grade C do not satisfy the range of 0.20 to 0.26 wt% for the total content of niobium (Nb) and titanium (Ti) and fall below it, and the ratio of the value of carbon (C) in wt% to the value of the total content of niobium (Nb) and titanium (Ti) in wt% (= [C] / ([Nb] + [Ti])) does not satisfy the range of less than 0.23 and exceeds it.

[0070] Fig. 2 is a graph showing the relationship between temperature and austenite fraction for the steel grades of the experimental examples shown in Table 1, as calculated by thermodynamic equilibrium calculation. In Fig. 2, A denotes Steel Grade A of Table 1, B denotes Steel Grade B of Table 1, and C denotes Steel Grade C of Table 1.

[0071] Referring to Fig. 2, in the case of Steel Grade A, it can be confirmed that the austenite fraction is nearly 0% up to approximately 820°C during thermodynamic equilibrium calculation. Since the probability of martensitic transformation is high upon cooling after austenite formation, it may be necessary to use Steel Grade A in order to maintain a single-phase ferrite microstructure. However, it may be taken into consideration that, during heating under actual annealing conditions, results that are delayed compared to the thermodynamic equilibrium calculation results appear, so a lower austenite transformation fraction may be exhibited.

[0072] Table 2 shows the process conditions for forming the cold-rolled steel materials of the Examples and Comparative Examples. Process conditions not described in Table 2 are the same for both the Comparative Examples and the Examples, and have the same values within the range of the process conditions of the present invention described above. [Table 2]CategorySteel GradeHot RollingCold Rolling and AnnealingReheating Temperature (°C)FDT (°C)CT (°C)Cold Rolling Reduction (%)Annealing Temperature (°C)Annealing Time (sec)Inventive Example 1A122288560168740300Inventive Example 2A122288560168740500Inventive Example 3A1222885601687401000Inventive Example 4A122288560168770100Inventive Example 5A12228856016880030Inventive Example 6A12228856016880050Inventive Example 7A122288560168800100Inventive Example 8A12228856016883030Comparative Example 3B12219236056874050Comparative Example 4B12219236056877050Comparative Example 5B12219236056880050Comparative Example 6B12219236056883050Comparative Example 7C12238926026874050Comparative Example 8C12238926026877050Comparative Example 9C12238926026880050Comparative Example 10C12238926026883050

[0073] Referring to Table 2, Inventive Examples 1 to 8 have the composition of Steel Grade A of Table 1, and all satisfy the following process conditions: reheating temperature: 1,200°C to 1,250°C, finish rolling temperature (FDT): 870°C to 910°C, coiling temperature (CT): 580°C to 620°C, cold rolling reduction ratio: 50% to 80%, annealing temperature: 740°C to 830°C, and annealing time: 30 seconds to 1,000 seconds. In contrast, Comparative Examples 3 to 6 have the composition of Steel Grade B of Table 1, and do not satisfy the range of finish rolling temperature (FDT): 870°C to 910°C, exceeding it. Comparative Examples 7 to 10 have the composition of Steel Grade C of Table 1.

[0074] Table 3 shows the material properties of the cold-rolled steel materials of the Examples and Comparative Examples as experimental examples of the present invention. Fig. 3 is a graph showing the relationship between stress and strain of the cold-rolled steel materials of the Examples and Comparative Examples as experimental examples of the present invention. In Fig. 3 and Fig. 4, Inventive Materials 1 to 8 correspond to Inventive Examples 1 to 8 in Tables 2 and 3, respectively, and Comparative Materials 1 to 10 correspond to Comparative Examples 1 to 10 in Tables 2 and 3, respectively. [Table 3]Yield Strength (MPa)Yield Ratio (%)Elongation (%)Inventive Example 186699.88.6Inventive Example 277398.712.2Inventive Example 367995.214.1Inventive Example 477495.09.4Inventive Example 580899.511.0Inventive Example 670596.010.2Inventive Example 768695.913.3Inventive Example 866695.512.2Comparative Example 385391.94.6Comparative Example 459679.99.4Comparative Example 543069.015.1Comparative Example 633963.219.2Comparative Example 736164.116.6Comparative Example 827656.327.2Comparative Example 924352.629.0Comparative Example 1022450.133.1

[0075] Referring to Table 3 and Fig. 3, Inventive Examples 1 to 8 all satisfy yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or more, and yield ratio (YR): 95% or more. In contrast, Comparative Example 3 does not satisfy the ranges of elongation (EL): 8% or more and yield ratio (YR): 95% or more, falling below them. Comparative Examples 4 to 10 do not satisfy the ranges of yield strength (YS): 600 MPa to 900 MPa and elongation (EL): 8% or more, all falling below them. Furthermore, referring to Tables 1 to 3, it can be confirmed that Inventive Examples 1 to 8 have the correlations of Equation 1 and Equation 2 below. In Equation 1 and Equation 2, Ti denotes the titanium content (unit: wt%), SS denotes the annealing heat treatment temperature (unit: °C) in the annealing heat treatment step (S160), t denotes the annealing heat treatment holding time (unit: seconds) in the annealing heat treatment step (S160), and YP denotes the yield strength (unit: MPa). The calculated values for each equation and the actual values (Table 3) are very similar, and the R 2< values in the correlation equations are 0.922 for Equation 1 and 0.962 for Equation 2, indicating a very close relationship. In the correlation equations, R 2< represents the degree of variance in the results according to the regression analysis method, and is used as a criterion for judging the goodness of fit of the regression analysis model. 4286.5605 Ti wt % − 3.3482 SS ° C − 0.2952 t s + 2474.7647 = YP MPa 367.6854 Ti wt % − 0.1819 SS ° C − 0.0141 t s + 159.9337 = Yield Ratio %

[0076] Fig. 4 is a graph showing the relationship between yield strength and elongation for the steel grades of the experimental examples shown in Tables 1 to 3. In Fig. 4, A denotes Steel Grade A of Table 1, B denotes Steel Grade B of Table 1, and C denotes Steel Grade C of Table 1.

[0077] Referring to Fig. 4, in the graph of the relationship between yield strength and elongation, Steel Grade A exhibits different characteristics from Steel Grades B and C. That is, it can be confirmed that Steel Grade A is superior to Steel Grades B and C in both strength and elongation.

[0078] Meanwhile, the present inventors were able to confirm that as the Ti content increases, the hardness value increases under all conditions of the temperature (740°C to 830°C) and process time (30 to 1,000 seconds) of the annealing process.

[0079] Figs. 5 to 20 are photographs of the microstructures of the steel materials disclosed in Tables 2 and 3.

[0080] In this experimental example, the ferrite recrystallization fraction X(t) was measured by observing the microstructure of the steel material.

[0081] The ferrite recrystallization fraction X(t) can be expressed by the following equation. X t = HV 0 − HV t HV 0 − HV rex

[0082] Here, X(t) is the ferrite recrystallization fraction, Hv rex is the hardness measurement value of the fully recrystallized ferrite microstructure, HV 0 is the hardness measurement value of the ferrite microstructure before annealing, and HV t is the hardness measurement value of the ferrite microstructure under the specific condition in question.

[0083] Fig. 5 shows the microstructure of Inventive Example 1 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 1.92 µm and a ferrite recrystallization fraction (X) measured at 33%.

[0084] Fig. 6 shows the microstructure of Inventive Example 2 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.1 µm and a ferrite recrystallization fraction (X) measured at 51%.

[0085] Fig. 7 shows the microstructure of Inventive Example 3 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 3.49 µm and a ferrite recrystallization fraction (X) measured at 83%.

[0086] Fig. 8 shows the microstructure of Inventive Example 4 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.16 µm and a ferrite recrystallization fraction (X) measured at 52%.

[0087] Fig. 9 shows the microstructure of Inventive Example 5 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 1.76 µm and a ferrite recrystallization fraction (X) measured at 31%.

[0088] Fig. 10 shows the microstructure of Inventive Example 6 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 1.92 µm and a ferrite recrystallization fraction (X) measured at 58%.

[0089] Fig. 11 shows the microstructure of Inventive Example 7 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.22 µm and a ferrite recrystallization fraction (X) measured at 68%.

[0090] Fig. 12 shows the microstructure of Inventive Example 8 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.13 µm and a ferrite recrystallization fraction (X) measured at 81%.

[0091] Fig. 13 shows the microstructure of Comparative Example 3 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the recovery stage, with an average grain size of 1.92 µm and a ferrite recrystallization fraction (X) measured at 27%.

[0092] Fig. 14 shows the microstructure of Comparative Example 4 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.32 µm and a ferrite recrystallization fraction (X) measured at 57%.

[0093] Fig. 15 shows the microstructure of Comparative Example 5 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 4.33 µm and a ferrite recrystallization fraction (X) measured at 81%.

[0094] Fig. 16 shows the microstructure of Comparative Example 6 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the recrystallization stage, with an average grain size of 5.84 µm and a ferrite recrystallization fraction (X) measured at 89%.

[0095] Fig. 17 shows the microstructure of Comparative Example 7 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.11 µm and a ferrite recrystallization fraction (X) measured at 43%.

[0096] Fig. 18 shows the microstructure of Comparative Example 8 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the recrystallization stage, with an average grain size of 6.55 µm and a ferrite recrystallization fraction (X) measured at 90%.

[0097] Fig. 19 shows the microstructure of Comparative Example 9 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 9.42 µm and a ferrite recrystallization fraction (X) measured at 92%.

[0098] Fig. 20 shows the microstructure of Comparative Example 10 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the recrystallization stage, with an average grain size of 8.03 µm and a ferrite recrystallization fraction (X) measured at 95%.

[0099] The cold-rolled steel material and the manufacturing method thereof according to the present invention have been described above. As described above, it can be confirmed that, according to an embodiment of the present invention, by optimizing the Ti component, it is possible to provide a recovery-annealed steel having a single-phase ferrite microstructure suitable for bending workability, with a yield strength of 600 to 900 MPa and a yield ratio of 95% or more.

[0100] It will be apparent to those of ordinary skill in the art to which the technical concept of the present invention pertains that the technical concept of the present invention described above is not limited to the foregoing embodiments and the accompanying drawings, and that various substitutions, modifications, and changes are possible without departing from the technical concept of the present invention.

Examples

experimental examples

Experimental Examples

[0066]Hereinafter, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are only intended to aid in understanding the present invention, and the present invention is not limited by the following experimental examples. Content not described herein can be technically inferred by those skilled in the art, and thus a description thereof will be omitted.

[0067]Steel materials having the compositions shown in Table 1 below were prepared, and cold-rolled steel sheets according to Examples and Comparative Examples were prepared through the hot rolling and cold rolling processes as described above.

[0068]Table 1 shows the compositions of the cold-rolled steel materials of the Examples and Comparative Examples. The content unit of each component is weight%, and the balance is iron (Fe).

[Table 1]

Steel GradeCSiMnPSCrMoTiNbVAlN

A0.0510.221.300.0010.00070.004-0.2230.032-0.0410.0007

B0.0510.211.3...

TECHNICAL FIELD

[0001] The present invention relates to steel materials, and more particularly, to a cold-rolled steel material having a high yield ratio and high yield strength, and a manufacturing method thereof.BACKGROUND ART

[0002] In recent years, the automotive industry has been pursuing weight reduction for the purpose of responding to environmental regulations and improving fuel efficiency. Since applying high-strength materials and reducing thickness is a common approach for light weighting components, high-strength steel materials are being adopted. In particular, structural members and reinforcements of automobiles form the structure of the vehicle body and are required to have excellent crash stability for the safety of the driver and passengers inside the vehicle in the event of a collision. That is, crash stability can be improved by achieving complex component shapes and securing high yield strength. Conventional ultra-low carbon high-strength steels have a low yield ratio and high elongation, which is advantageous for forming, however, due to the low yield ratio, they have the disadvantage of being unfavorable in terms of crash stability as structural members and reinforcements requiring high rigidity when used as products after forming.

[0003] Precipitation-hardened steel is a high-strength alloy in which trace amounts of carbonitride-forming elements such as titanium and niobium are added to ordinary carbon-manganese steel to generate fine precipitates, thereby improving the impact toughness and strength of the steel. Conventionally, research on precipitation-hardened steel has been conducted primarily on hot-rolled steel materials, with studies focused on models and experimental results concerning deformation-induced precipitation, interphase precipitation, and precipitation through dislocations after coiling during the hot-rolling stage, whereas for cold-rolled steel materials, fundamental research results such as studies on precipitate formation mechanisms have been conducted to a very limited extent. In the case of hot-rolled precipitation-hardened steel, a review of global standards reveals that grades up to a yield strength of 700 MPa are listed within the VDA standards.

[0004] On the other hand, for cold-rolled precipitation-hardened steel, fundamental research results regarding strength improvement and precipitate formation mechanisms have been conducted to a limited extent compared to hot-rolled materials. A review of global standards reveals that only grades up to a yield strength of 460 MPa are listed within the VDA standards, and for strengths beyond that, dual-phase steel and complex-phase steel standards are applied. Some automobile manufacturers require cold-rolled precipitation-hardened steel with a yield strength of 550 MPa; however, for components requiring strength beyond that level, dual-phase steel standards are applied as described above. Korean Patent Application No. 2012-0070333 is cited as a prior art document.SUMMARY OF THE INVENTIONTECHNICAL PROBLEM

[0005] The technical problem to be achieved by the technical concept of the present invention is to provide a cold-rolled steel material having a high yield ratio and high yield strength, and a manufacturing method thereof. For example, the present invention provides a recovery-annealed cold-rolled precipitation-hardened steel characterized by a high yield ratio capable of having high dimensional stability for application to seat rails, airbag components, and members. However, such problems are exemplary, and the present invention is not limited thereto.SOLUTION TO PROBLEM

[0006] According to one aspect of the present invention, a cold-rolled steel material and a manufacturing method thereof are provided.

[0007] The cold-rolled steel material comprises, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities, and has a yield strength (YS) of 600 MPa to 900 MPa, an elongation (EL) of 8% or more, and a yield ratio (YR) of 95% or more.

[0008] The cold-rolled steel material may further comprise, in weight%, boron (B): more than 0% and up to 0.001%, or molybdenum (Mo): more than 0% and up to 0.06%.

[0009] In the cold-rolled steel material, the total content of niobium (Nb) and titanium (Ti) may be 0.26% or less.

[0010] The cold-rolled steel material may be a cold-rolled precipitation-hardened steel having a ferrite single-phase structure, and comprising precipitates within the ferrite single-phase structure.

[0011] The manufacturing method of the cold-rolled steel material comprises: reheating a steel material comprising, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities, at a temperature of 1,200°C to 1,250°C; hot-rolling the heated steel material so as to finish at a temperature of Ar3 or higher; primary cooling the hot-rolled steel material; coiling the primary-cooled steel material at a coiling temperature of 580°C to 620°C; cold-rolling the coiled steel material to form a cold-rolled steel material; annealing the cold-rolled steel material at an annealing temperature of 740°C to 830°C; and secondary cooling the annealed cold-rolled steel material.

[0012] In the manufacturing method of the cold-rolled steel material, the step of primary cooling the steel material may comprise cooling to 580°C to 620°C at a cooling rate of 10°C / sec to 100°C / sec.

[0013] In the manufacturing method of the cold-rolled steel material, the step of secondary cooling the steel material may comprise cooling at a cooling rate of 5°C / sec to 100°C / sec.Advantageous Effects of Invention

[0014] According to the present invention, a cold-rolled steel material having a high yield ratio and high yield strength, and a manufacturing method thereof can be realized. For example, a recovery-annealed steel having a yield strength of 600 to 900 MPa grade and a yield ratio of 95% or more with a ferrite single-phase structure suitable for bending workability can be realized.

[0015] The effects of the present invention described above are set forth by way of example, and the scope of the present invention is not limited by such effects.BRIEF DESCRIPTION OF DRAWINGS

[0016] Fig. 1 is a process flowchart schematically illustrating the manufacturing method of a cold-rolled steel material according to an embodiment of the present invention. Fig. 2 is a graph showing the relationship between temperature and austenite fraction for the steel grades of the experimental examples shown in Table 1, as calculated by thermodynamic equilibrium calculation. Fig. 3 is a graph showing the relationship between stress and strain of cold-rolled steel materials of Examples and Comparative Examples as experimental examples of the present invention. Fig. 4 is a graph showing the relationship between yield strength and elongation for the steel grades of the experimental examples shown in Tables 1 to 3. Figs. 5 to 20 are photographs of the microstructures of the steel materials disclosed in Tables 2 and 3. MODES FOR CARRYING OUT THE INVENTION

[0017] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical concept of the present invention to those of ordinary skill 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 the present specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and regions in the drawings are schematically illustrated. Accordingly, the present invention is not limited by the relative sizes or spacings depicted in the accompanying drawings.

[0018] In recent years, the automotive industry has been pursuing weight reduction for the purpose of responding to environmental regulations and improving fuel efficiency. Since it is common practice to apply high-strength materials and reduce thickness in order to achieve component weight reduction, high-strength steel materials are being applied. In particular, structural members and reinforcements of automobiles form the structure of the automobile body, and are required to have excellent crash safety for the safety of the driver and passengers inside the vehicle in the event of a collision. That is, crash stability can be improved by securing a high yield ratio and yield strength.

[0019] The present invention is to provide a cold-rolled steel material and a manufacturing method thereof in which the yield ratio (= yield strength / tensile strength) of the steel material is improved by generating fine precipitates during the heat treatment process after cold rolling. Conventional precipitation-hardened steel materials caused precipitation of carbonitride elements in the hot rolling process to the maximum extent by setting the coiling temperature after hot rolling to exceed 620°C. However, when precipitates are formed during hot rolling, the precipitates coarsen during cold rolling, resulting in a decrease in the yield ratio.

[0020] In the present invention, a recovery-annealed cold-rolled precipitation-hardened steel having a ferrite single-phase structure with a yield strength of 600 to 900 MPa and a yield ratio of 95% or more, suitable for bending workability, has been proposed, and as described above, it can be applied to components such as seat rails and members that require high yield strength. Homogeneous material properties can be achieved with a ferrite single-phase microstructure, and in particular, a positive effect on hole expansion performance can be exhibited. In addition, a high yield ratio of 95% or more can bring optimal conditions during cold forming such as roll forming and bending.

[0021] Hereinafter, the cold-rolled steel material according to the present invention will be described in detail.

[0022] The cold-rolled steel material according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities.

[0023] Hereinafter, the role of each component included in the cold-rolled steel material according to the present invention and its content will be described as follows. In this case, the content of each component element refers to weight% with respect to the entire steel material.Carbon (C): 0.04% to 0.07%

[0024] Carbon (C) is added to secure the strength of the steel material and to control the microstructure. When the content is less than 0.04%, a sufficient precipitation effect cannot be obtained, making it difficult to secure the desired yield strength, and MC (M = Nb, Ti)-based carbides coarsen, resulting in a problem of reduced grain refinement effect. When the content exceeds 0.07%, material property degradation such as a decrease in strength may occur as the phase fraction of phases such as pearlite increases. In addition, when the carbon content increases, the pearlite structure increases, and upon annealing at high temperature after cold rolling, austenite transformation occurs in some pearlite structures, which may induce a two-phase structure. Therefore, it is preferable to add carbon in a content of 0.04% to 0.07%.Silicon (Si): 0.1% to 0.3%

[0025] Silicon not only contributes to strength improvement as a solid-solution strengthening element, but also, as a ferrite-stabilizing element, increases the degree of undercooling during ferrite transformation to suppress the formation of harmful carbides, refines grains, and suppresses the formation of pearlite, thereby increasing the reactivity between solid-solution C and M (M = Nb, Ti). When the silicon content is less than 0.1%, the effect of silicon addition is insufficient. When the silicon content exceeds 0.3%, oxides such as Mn 2 SiO 4 are formed, which impairs plating properties, deteriorates the surface properties of the steel material, and may increase the carbon equivalent, thereby reducing weldability. Therefore, it is preferable to add silicon in a content of 0.1% to 0.3% by weight of the entire steel material.Manganese (Mn): 1.1% to 1.5%

[0026] Manganese not only contributes to strength improvement as a solid-solution strengthening element, but can also control strength, toughness, and yield ratio depending on its content; however, when added in large amounts, it induces the formation of MnS inclusions and center segregation during casting, thereby reducing the toughness of the steel. When the Mn content is less than 1.1%, it is difficult to secure the desired yield strength of 800 MPa grade, and when it exceeds 1.5%, although it is advantageous for securing the desired strength, it may form inclusions or cause deterioration of workability and delayed fracture resistance due to segregation, and may increase the carbon equivalent, thereby reducing weldability, which is a major advantage of precipitation-hardened steel. In addition, transformation structures may be induced during annealing and cooling at high temperature after cold rolling. Therefore, it is preferable to add Mn in a content of 1.1% to 1.5%.Aluminum (Al): 0.015% to 0.06%

[0027] Aluminum is used as a deoxidizer and can help purify ferrite. When the Al content is less than 0.015%, the effect of addition is insufficient, and when it exceeds 0.06%, AlN is formed during slab manufacturing, which may induce cracks during casting or hot rolling. Therefore, it is preferable to add aluminum in a content of 0.015% to 0.06% by weight of the entire steel material.Phosphorus (P): more than 0% and up to 0.02%

[0028] Phosphorus is an impurity included in the steel manufacturing process, and while it can help improve strength through solid-solution strengthening, when contained in large amounts, it may cause low-temperature embrittlement and brittle fracture. Therefore, it is preferable to limit the phosphorus content to greater than 0% to 0.02% or less by weight of the entire steel material.Sulfur (S): more than 0% and up to 0.005%

[0029] Sulfur is an impurity included in the steel manufacturing process, and may form non-metallic inclusions such as FeS and MnS, thereby reducing toughness and weldability. Therefore, it is preferable to limit the sulfur content to greater than 0% to 0.005% or less by weight of the entire steel material.Niobium (Nb): 0.02% to 0.04%

[0030] Niobium is a powerful carbonitride-forming element that combines with carbon and nitrogen contained in the steel during hot rolling and annealing processes to form carbides or nitrides, and these Nb-based carbides or nitrides suppress recrystallization and grain growth during the annealing process after cold rolling, thereby refining grains and improving both the strength and toughness of the steel material. When the Nb content is less than 0.02%, it may be difficult to obtain a precipitation strengthening effect, and when it exceeds 0.04%, the annealing time must be extended or the annealing temperature must be increased to secure the elongation of the steel due to the recrystallization-retarding effect of Nb; therefore, it is preferable to add Nb in a content of 0.02% to 0.04%.Titanium (Ti): 0.17% to 0.23%

[0031] Titanium is a powerful carbonitride-forming element that combines with carbon and nitrogen contained in the steel during hot rolling and annealing processes to form carbides or nitrides, and these Ti-based carbides or nitrides suppress recrystallization and grain growth during the annealing process after cold rolling, thereby refining grains and improving both the strength and toughness of the steel material. When the Ti content is less than 0.17%, it may be difficult to obtain a sufficient recrystallization-retarding effect and precipitation strengthening effect, and when it exceeds 0.23%, the annealing time must be extended or the annealing temperature must be increased to secure the elongation of the steel due to the recrystallization-retarding effect of Ti; therefore, it is preferable to add Ti in a content of 0.17% to 0.23%.Chromium (Cr): more than 0% and up to 0.1%

[0032] It is preferable to add chromium (Cr) in a content ratio of 0.1% or less by weight of the entire precipitation-hardened steel material according to the present invention. When the chromium content exceeds 0.1% by weight of the entire steel material, there is a problem of reducing weldability or heat-affected zone (HAZ) toughness.Nitrogen (N): more than 0% and up to 0.005%

[0033] Nitrogen (N) is an element that inevitably remains in the steel during the steelmaking process. It has a faster diffusion rate and lower activation energy compared to carbon, so even a minute change in its amount causes a large variation in material properties, and aging is accelerated; therefore, it must be precipitated in the form of nitrides. In the present invention, when the nitrogen (N) content exceeds 0.005% and is added in large amounts, a large amount of TiN is formed, which adversely affects the formability of the steel. It is best to minimize it as much as possible; however, since it is an element that inevitably remains in the steelmaking process, it should not exceed 50 ppm.

[0034] The cold-rolled steel material according to the present invention may have a total content of niobium (Nb) and titanium (Ti) of 0.26 weight% or less, and for example, may be 0.20 to 0.26 weight%. When the total content of niobium (Nb) and titanium (Ti) is less than 0.20 weight%, it may be difficult to obtain a sufficient recrystallization-retarding effect and precipitation strengthening effect, and when the total content of niobium (Nb) and titanium (Ti) exceeds 0.26 weight%, the annealing time may need to be extended or the annealing temperature may need to be increased to secure the elongation of the steel due to the recrystallization-retarding effect.

[0035] Furthermore, the ratio of the value of weight% of carbon (C) to the value of the total content of weight% of niobium (Nb) and titanium (Ti) (= [C] / ([Nb] + [Ti])) may have a value of less than 0.23. If the value of [C] / ([Nb] + [Ti]) is 0.23 or more, a problem may arise in that the cementite fraction exists at 1% or more.

[0036] The remaining component of the cold-rolled steel material is iron (Fe). However, since unintended impurities may inevitably be incorporated from raw materials or the surrounding environment during a normal steelmaking process, they cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, all of their details are not specifically mentioned in the present specification.

[0037] Meanwhile, the cold-rolled steel material according to the present invention may further comprise, in weight%, boron (B): more than 0% and up to 0.001% or molybdenum (Mo): more than 0% and up to 0.06%.Boron (B): more than 0% and up to 0.001%

[0038] Boron (B) is added to prevent secondary work embrittlement that may occur due to the addition of phosphorus (P). In general, when boron is added in an amount exceeding 0.001%, material property variation due to segregation may occur; therefore, it is preferable to add boron in a range of 0.001% or less.Molybdenum (Mo): more than 0% and up to 0.06%

[0039] Molybdenum acts when Nb and Ti components combine with carbon to form precipitates, slowing the rate of formation and growth of fine precipitates, thereby obtaining a yield strength strengthening effect. When the molybdenum content exceeds 0.06%, the effect converges and price competitiveness is reduced; therefore, it is preferable to add molybdenum in a content of 0.06% or less.

[0040] The cold-rolled steel material manufactured by controlling the specific components of the alloy composition described above and their content ranges, and through the manufacturing method described below, can satisfy, for example, yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or more, and yield ratio (YR): 95% or more.

[0041] The cold-rolled steel material may include precipitates formed at grain boundaries, within grains, or both. The precipitates mainly include precipitates of the Ti,Nb(C) type. In addition, TiN, AlN, and the like may be included. The precipitates may have an average particle diameter of mainly 2 to 6 nm. The precipitates may be formed during the annealing heat treatment performed after cold rolling. By virtue of these precipitates, the cold-rolled steel material can simultaneously have high strength and a high yield ratio. For example, a portion of the Ti,Nb(C) is formed during hot rolling and coiling, and then the remaining 30 to 70% of Ti and Nb elements use the large number of dislocations generated during cold rolling as nucleation sites and precipitate as Ti,Nb(C) during the annealing process.

[0042] Hereinafter, a manufacturing method of the cold-rolled steel material according to the present invention will be described with reference to the accompanying drawings.Manufacturing Method of Cold-Rolled Steel Material

[0043] Fig. 1 is a drawing illustrating the temperature over time in the manufacturing method of a cold-rolled steel material according to an embodiment of the present invention and diagramming the microstructural changes of the steel material at each step.

[0044] The steel material comprises, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities.

[0045] The manufacturing method of a cold-rolled steel material according to an embodiment of the present invention comprises a reheating step (S110), a hot rolling step (S120), a primary cooling step (S130), a coiling step (S140), a cold rolling step (S150), an annealing heat treatment step (S160), and a secondary cooling step (S170).Reheating Step (S110)

[0046] The semi-finished product subject to the hot rolling process in the manufacturing method according to the present invention may be, for example, a slab. The slab in a semi-finished state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0047] In the reheating step (S110), the steel material having the above composition, for example a slab plate, is reheated at a slab reheating temperature (SRT) of 1,200°C to 1,250°C for 40 minutes to 1 hour. Through such reheating, re-dissolution of segregated components during casting and re-dissolution of precipitates may occur, and in particular, a sufficiently high temperature is required to re-dissolve precipitation hardening elements such as niobium so as to form fine precipitates during the annealing heat treatment after subsequent cold rolling. The fine precipitates may impede grain growth to achieve grain refinement, thereby increasing strength. When the reheating temperature is below 1,200°C or the reheating furnace residence time is less than 40 minutes, carbonitrides do not dissolve and become coarsened, making it impossible to obtain material properties exceeding the desired yield ratio; for example, maximum re-dissolution of precipitation hardening elements such as niobium contained in the steel material may not occur, and components segregated during casting may not be sufficiently uniformly distributed. When the reheating temperature exceeds 1,250°C or the reheating furnace residence time exceeds 1 hour, austenite grains may become coarsened, which may cause a decrease in yield strength. In addition, as the reheating temperature increases, there is a problem of increased manufacturing costs and decreased productivity due to heating costs and additional time required to achieve the hot rolling temperature.Hot Rolling Step (S120)

[0048] The heated steel material is first subjected to hot rolling after heating in order to adjust its shape. The hot rolling may be performed continuously by width rolling, roughing mill, and finishing mill. By the hot rolling step, the steel material may form a hot-rolled steel material. The hot-rolled steel material may be a hot-rolled steel sheet. The reduction ratio of hot rolling may be 50% or more. Work hardening (deformation) may occur during the hot rolling process, and furthermore, the work hardening may be eliminated through recrystallization and recovery.

[0049] The finishing mill may be completed at a finish rolling temperature (FRT) of Ar3 or higher. When the finish rolling temperature is below Ar3 (for example, below 870°C), rolling is performed in the two-phase region of austenite and ferrite, causing grain mixing, which results in non-uniform deformability and may lead to a decrease in rollability. When the finish rolling temperature exceeds Ar3 + 40°C (for example, exceeds 910°C), grains may become coarsened, resulting in a decrease in the strength of the final steel material.Primary Cooling Step (S130)

[0050] The primary cooling step (S130) may be performed on a run-out table (ROT). The run-out table (ROT) is a device that cools the steel sheet whose thickness and width have been determined by roughing rolling and finishing rolling. The run-out table is provided with a plurality of banks, each having a plurality of headers equipped with a row of nozzles, arranged above and below the steel sheet. A coiler for coiling the steel sheet into a coil form is provided at the rear end of the run-out table, and the cooling of the steel sheet must be controlled so that the temperature of the steel sheet immediately before entering the coiler, i.e., the coiling temperature, matches the target coiling temperature. The hot-rolled steel material is primarily cooled to 580°C to 620°C at a cooling rate of 10°C / sec to 100°C / sec. The cooling may be performed by air cooling or water cooling. When the cooling rate is less than 10°C / sec, the average particle size of precipitates may increase, making it difficult to secure strength. Conversely, when the cooling rate exceeds 100°C / sec, the microstructure of the steel material may become hardened, resulting in a decrease in impact toughness.Coiling Step (S140)

[0051] Upon completion of the primary cooling, the steel material is coiled at a coiling temperature (CT) of 580°C to 620°C. In the present invention, a low-temperature coiling process of 580°C to 620°C is applied to make the size of precipitates fine and to minimize the precipitation fraction as much as possible. When the coiling temperature is below 580°C, difficulties may arise in controlling the shape of the hot-rolled material, and the surface quality of the steel material may deteriorate due to the abrupt difference between the finishing rolling temperature and the coiling temperature. When the coiling temperature exceeds 620°C, it may contribute to increasing the strength of the hot-rolled material by improving the precipitation fraction; however, this is undesirable from the perspective of the final cold-rolled material properties because the precipitates become coarsened. That is, when the coiling temperature exceeds 620°C, carbonitride elements cannot be maintained in a solid solution state and may be formed as undesired precipitates.

[0052] Referring to the microstructural changes at each step of the hot rolling step (S120), primary cooling step (S130), and coiling step (S140) illustrated in Fig. 1, the SIP item refers to strain-induced precipitation, specifically precipitation caused by deformation occurring during the rolling process. PIP refers to phase-induced precipitation, specifically precipitates formed during the transformation process from austenite to ferrite. In addition, the Random item refers to precipitates that are widely precipitated within the ferrite microstructure during the cooling process after coiling.

[0053] Meanwhile, the γ→α phase transformation refers to the phase transformation from austenite to ferrite.Cold Rolling Step (S150)

[0054] A pickling treatment is performed to clean the coiled hot-rolled steel material with acid. Subsequently, cold rolling is performed on the pickled hot-rolled steel material at a cold reduction ratio of 50% to 80% to form a cold-rolled steel material. When the cold reduction ratio is less than 50%, the amount of nucleation for recrystallization during annealing is small, so grains may grow excessively during the annealing heat treatment described below, resulting in a sharp decrease in strength. As the cold reduction ratio increases beyond 50%, the number of sites at which fine precipitates can be formed increases, which is favorable for improving the yield ratio; however, when it exceeds 80%, the amount of nucleation becomes excessively large, causing the grains formed by annealing to be too fine, thereby reducing ductility and decreasing formability.Annealing Heat Treatment Step (S160)

[0055] The cold-rolled steel material is heat-treated in a continuous annealing furnace having a conventional slow cooling section. When continuously annealing the cold-rolled sheet, recovery recrystallization annealing is performed at a strip passing speed of 50 to 200 mpm and an annealing temperature of 740 to 830°C. The heat treatment may be referred to as annealing heat treatment. The annealing heat treatment is performed by heating at a temperature increase rate in the range of 1°C / sec to 10°C / sec and maintaining at a temperature in the range of 740°C to 830°C for 30 seconds to 1,000 seconds. At this time, the annealing heat treatment is one of the important process variables that determines the material properties of the final product. This recovery recrystallization annealing heat treatment results in a very fine final microstructure and generates fine precipitates of solid-solution carbonitride elements, thereby achieving a high yield ratio of the cold-rolled steel material. The carbonitrides may be, for example, NbC precipitates and AlN precipitates. When the annealing heat treatment temperature is below 740°C, recrystallization may not be sufficiently completed, and the target elongation may not be secured. When the annealing heat treatment temperature exceeds 830°C, grains may become coarsened, resulting in a decrease in yield strength and yield ratio. Therefore, in order to secure a yield strength of 600 to 900 MPa and a yield ratio of 95% or more, it is preferable to limit the annealing temperature to the above range.

[0056] Meanwhile, the strip passing speed during continuous annealing of cold-rolled sheets may be 90 to 110 mpm, which is the speed for producing steel for recrystallization at a general annealing temperature of 760 to 800°C when producing steel material with a thickness of 1.0 to 1.5t on a continuous galvanizing line (CGL). In the present invention, since the recrystallization fraction of the steel is distributed in the range of 31% to 81%, the range of the strip passing speed can be further widened to 50 to 200 mpm correspondingly.Secondary Cooling Step (S170)

[0057] The annealing heat-treated cold-rolled steel material is cooled at a cooling rate of 5°C / sec to 100°C / sec to a temperature in the range of, for example, 100°C to 300°C.

[0058] The cold-rolled steel material can be manufactured through the above process (S110 to S170), and the cold-rolled steel material may be a cold-rolled steel sheet.

[0059] If necessary, the cold-rolled steel material may be formed into a hot-dip galvanized steel material and an alloyed hot-dip galvanized steel material. The hot-dip galvanized steel material may be realized by immersing the cold-rolled steel material in a hot-dip galvanizing bath to form a hot-dip galvanized layer. The temperature of the plating bath may be in the range of 400°C to 520°C depending on the type and ratio of alloying elements constituting the plating layer and the composition system of the cold-rolled steel material. Under the plating bath conditions, a hot-dip galvanized layer is easily formed on the surface of the cold-rolled sheet, and the adhesion of the plating layer may be excellent. Subsequently, the hot-dip galvanized steel material may be formed by cooling to room temperature at a cooling rate of 1°C / sec to 100°C / sec.

[0060] The alloyed hot-dip galvanized steel material may be produced by subjecting the cold-rolled steel material on which the hot-dip galvanized layer has been formed to an alloying heat treatment. The alloying heat treatment may be performed at a temperature in the range of 500°C to 620°C for 10 seconds to 60 seconds. Under the above conditions, the hot-dip galvanized layer grows stably during the alloying heat treatment, and the adhesion of the plating layer may be excellent. When the alloying heat treatment temperature is below 500°C, alloying may not proceed sufficiently, resulting in a decrease in the integrity of the hot-dip galvanized layer. When the alloying heat treatment temperature exceeds 620°C, the temperature range may enter the two-phase region, causing changes in material properties. Thereafter, the alloyed hot-dip galvanized steel material may be formed by cooling.

[0061] In the zone where the reheating is performed, precipitation hardening elements such as niobium are re-dissolved. In the zone where the hot rolling is performed, precipitate phases such as niobium carbide may be formed within austenite grains. In the zone where the primary cooling is performed and the zone where the coiling is performed, precipitate phases such as niobium carbide are formed within ferrite grains; however, the amount of the precipitate phases may be reduced by the low coiling temperature. In the zone where the cold rolling is performed, niobium may be re-dissolved into the matrix. In the zone where the annealing heat treatment is performed, precipitate phases such as niobium carbide may be formed within ferrite grains. That is, by minimizing the amount of precipitate phases formed in the hot rolling process and inducing the formation of precipitate phases in the cold rolling process, the size of the precipitate phases can be refined. Accordingly, a high yield ratio and high yield strength can be achieved.

[0062] The cold-rolled steel material realized by applying the above-described alloy composition and process conditions can satisfy, for example, yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or more, and yield ratio (YR): 95% or more.

[0063] In the cold-rolled steel material realized by applying the above-described alloy composition and process conditions, the yield strength (unit: MPa) and yield ratio (unit: %) have a relationship approximately according to the following equations depending on the annealing heat treatment temperature (unit: °C) in the annealing heat treatment step (S160), the annealing heat treatment holding time (unit: seconds), and the titanium content (unit: weight%) constituting the steel material. 4286.5605 Ti wt % − 3.3482 SS ° C − 0.2952 t s + 2474.7647 = YP MPa 367.6854 Ti wt % − 0.1819 SS ° C − 0.0141 t s + 159.9337 = Yield Ratio %

[0064] In Equation 1 and Equation 2, Ti denotes the titanium content (unit: weight%), SS denotes the annealing heat treatment temperature (unit: °C) in the annealing heat treatment step (S160), t denotes the annealing heat treatment holding time (unit: seconds) in the annealing heat treatment step (S160), and YP denotes the yield strength (unit: MPa).

[0065] The cold-rolled steel material may include precipitates formed at grain boundaries, within grains, or both. The precipitates mainly include precipitates of the Ti,Nb(C) type. In addition, TiN, AlN, and the like may be included. The precipitates may have an average particle diameter of mainly 2 to 6 nm. The precipitates may be formed during the annealing heat treatment performed after cold rolling. Due to these precipitates, the cold-rolled steel material can simultaneously have high strength and a high yield ratio. For example, a portion of the Ti,Nb(C) is formed during the hot rolling and coiling processes, and the remaining 30 to 70% of Ti and Nb elements utilize the large number of dislocations generated during cold rolling as nucleation sites and precipitate as Ti,Nb(C) during the annealing process.Experimental Examples

[0066] Hereinafter, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are only intended to aid in understanding the present invention, and the present invention is not limited by the following experimental examples. Content not described herein can be technically inferred by those skilled in the art, and thus a description thereof will be omitted.

[0067] Steel materials having the compositions shown in Table 1 below were prepared, and cold-rolled steel sheets according to Examples and Comparative Examples were prepared through the hot rolling and cold rolling processes as described above.

[0068] Table 1 shows the compositions of the cold-rolled steel materials of the Examples and Comparative Examples. The content unit of each component is weight%, and the balance is iron (Fe). [Table 1]Steel GradeCSiMnPSCrMoTiNbVAlNA0.0510.221.300.0010.00070.004-0.2230.032-0.0410.0007B0.0510.211.300.0010.00060.004-0.1640.030-0.0420.0008C0.0540.201.310.0010.00050.004-0.1060.030-0.0420.0008

[0069] Referring to Table 1, Steel Grade A satisfies, in weight%, the composition ranges of carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe). In addition, Steel Grade A satisfies a total content of niobium (Nb) and titanium (Ti) of 0.26 wt% or less, and specifically, satisfies a range of 0.20 to 0.26 wt%. Furthermore, the ratio of the value of carbon (C) in wt% to the value of the total content of niobium (Nb) and titanium (Ti) in wt% (= [C] / ([Nb] + [Ti])) has a value of less than 0.23. In contrast, Steel Grade B and Steel Grade C do not satisfy the range of titanium (Ti): 0.17% to 0.23% and fall below it. In addition, Steel Grade B and Steel Grade C do not satisfy the range of 0.20 to 0.26 wt% for the total content of niobium (Nb) and titanium (Ti) and fall below it, and the ratio of the value of carbon (C) in wt% to the value of the total content of niobium (Nb) and titanium (Ti) in wt% (= [C] / ([Nb] + [Ti])) does not satisfy the range of less than 0.23 and exceeds it.

[0070] Fig. 2 is a graph showing the relationship between temperature and austenite fraction for the steel grades of the experimental examples shown in Table 1, as calculated by thermodynamic equilibrium calculation. In Fig. 2, A denotes Steel Grade A of Table 1, B denotes Steel Grade B of Table 1, and C denotes Steel Grade C of Table 1.

[0071] Referring to Fig. 2, in the case of Steel Grade A, it can be confirmed that the austenite fraction is nearly 0% up to approximately 820°C during thermodynamic equilibrium calculation. Since the probability of martensitic transformation is high upon cooling after austenite formation, it may be necessary to use Steel Grade A in order to maintain a single-phase ferrite microstructure. However, it may be taken into consideration that, during heating under actual annealing conditions, results that are delayed compared to the thermodynamic equilibrium calculation results appear, so a lower austenite transformation fraction may be exhibited.

[0072] Table 2 shows the process conditions for forming the cold-rolled steel materials of the Examples and Comparative Examples. Process conditions not described in Table 2 are the same for both the Comparative Examples and the Examples, and have the same values within the range of the process conditions of the present invention described above. [Table 2]CategorySteel GradeHot RollingCold Rolling and AnnealingReheating Temperature (°C)FDT (°C)CT (°C)Cold Rolling Reduction (%)Annealing Temperature (°C)Annealing Time (sec)Inventive Example 1A122288560168740300Inventive Example 2A122288560168740500Inventive Example 3A1222885601687401000Inventive Example 4A122288560168770100Inventive Example 5A12228856016880030Inventive Example 6A12228856016880050Inventive Example 7A122288560168800100Inventive Example 8A12228856016883030Comparative Example 3B12219236056874050Comparative Example 4B12219236056877050Comparative Example 5B12219236056880050Comparative Example 6B12219236056883050Comparative Example 7C12238926026874050Comparative Example 8C12238926026877050Comparative Example 9C12238926026880050Comparative Example 10C12238926026883050

[0073] Referring to Table 2, Inventive Examples 1 to 8 have the composition of Steel Grade A of Table 1, and all satisfy the following process conditions: reheating temperature: 1,200°C to 1,250°C, finish rolling temperature (FDT): 870°C to 910°C, coiling temperature (CT): 580°C to 620°C, cold rolling reduction ratio: 50% to 80%, annealing temperature: 740°C to 830°C, and annealing time: 30 seconds to 1,000 seconds. In contrast, Comparative Examples 3 to 6 have the composition of Steel Grade B of Table 1, and do not satisfy the range of finish rolling temperature (FDT): 870°C to 910°C, exceeding it. Comparative Examples 7 to 10 have the composition of Steel Grade C of Table 1.

[0074] Table 3 shows the material properties of the cold-rolled steel materials of the Examples and Comparative Examples as experimental examples of the present invention. Fig. 3 is a graph showing the relationship between stress and strain of the cold-rolled steel materials of the Examples and Comparative Examples as experimental examples of the present invention. In Fig. 3 and Fig. 4, Inventive Materials 1 to 8 correspond to Inventive Examples 1 to 8 in Tables 2 and 3, respectively, and Comparative Materials 1 to 10 correspond to Comparative Examples 1 to 10 in Tables 2 and 3, respectively. [Table 3]Yield Strength (MPa)Yield Ratio (%)Elongation (%)Inventive Example 186699.88.6Inventive Example 277398.712.2Inventive Example 367995.214.1Inventive Example 477495.09.4Inventive Example 580899.511.0Inventive Example 670596.010.2Inventive Example 768695.913.3Inventive Example 866695.512.2Comparative Example 385391.94.6Comparative Example 459679.99.4Comparative Example 543069.015.1Comparative Example 633963.219.2Comparative Example 736164.116.6Comparative Example 827656.327.2Comparative Example 924352.629.0Comparative Example 1022450.133.1

[0075] Referring to Table 3 and Fig. 3, Inventive Examples 1 to 8 all satisfy yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or more, and yield ratio (YR): 95% or more. In contrast, Comparative Example 3 does not satisfy the ranges of elongation (EL): 8% or more and yield ratio (YR): 95% or more, falling below them. Comparative Examples 4 to 10 do not satisfy the ranges of yield strength (YS): 600 MPa to 900 MPa and elongation (EL): 8% or more, all falling below them. Furthermore, referring to Tables 1 to 3, it can be confirmed that Inventive Examples 1 to 8 have the correlations of Equation 1 and Equation 2 below. In Equation 1 and Equation 2, Ti denotes the titanium content (unit: wt%), SS denotes the annealing heat treatment temperature (unit: °C) in the annealing heat treatment step (S160), t denotes the annealing heat treatment holding time (unit: seconds) in the annealing heat treatment step (S160), and YP denotes the yield strength (unit: MPa). The calculated values for each equation and the actual values (Table 3) are very similar, and the R 2< values in the correlation equations are 0.922 for Equation 1 and 0.962 for Equation 2, indicating a very close relationship. In the correlation equations, R 2< represents the degree of variance in the results according to the regression analysis method, and is used as a criterion for judging the goodness of fit of the regression analysis model. 4286.5605 Ti wt % − 3.3482 SS ° C − 0.2952 t s + 2474.7647 = YP MPa 367.6854 Ti wt % − 0.1819 SS ° C − 0.0141 t s + 159.9337 = Yield Ratio %

[0076] Fig. 4 is a graph showing the relationship between yield strength and elongation for the steel grades of the experimental examples shown in Tables 1 to 3. In Fig. 4, A denotes Steel Grade A of Table 1, B denotes Steel Grade B of Table 1, and C denotes Steel Grade C of Table 1.

[0077] Referring to Fig. 4, in the graph of the relationship between yield strength and elongation, Steel Grade A exhibits different characteristics from Steel Grades B and C. That is, it can be confirmed that Steel Grade A is superior to Steel Grades B and C in both strength and elongation.

[0078] Meanwhile, the present inventors were able to confirm that as the Ti content increases, the hardness value increases under all conditions of the temperature (740°C to 830°C) and process time (30 to 1,000 seconds) of the annealing process.

[0079] Figs. 5 to 20 are photographs of the microstructures of the steel materials disclosed in Tables 2 and 3.

[0080] In this experimental example, the ferrite recrystallization fraction X(t) was measured by observing the microstructure of the steel material.

[0081] The ferrite recrystallization fraction X(t) can be expressed by the following equation. X t = HV 0 − HV t HV 0 − HV rex

[0082] Here, X(t) is the ferrite recrystallization fraction, Hv rex is the hardness measurement value of the fully recrystallized ferrite microstructure, HV 0 is the hardness measurement value of the ferrite microstructure before annealing, and HV t is the hardness measurement value of the ferrite microstructure under the specific condition in question.

[0083] Fig. 5 shows the microstructure of Inventive Example 1 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 1.92 µm and a ferrite recrystallization fraction (X) measured at 33%.

[0084] Fig. 6 shows the microstructure of Inventive Example 2 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.1 µm and a ferrite recrystallization fraction (X) measured at 51%.

[0085] Fig. 7 shows the microstructure of Inventive Example 3 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 3.49 µm and a ferrite recrystallization fraction (X) measured at 83%.

[0086] Fig. 8 shows the microstructure of Inventive Example 4 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.16 µm and a ferrite recrystallization fraction (X) measured at 52%.

[0087] Fig. 9 shows the microstructure of Inventive Example 5 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 1.76 µm and a ferrite recrystallization fraction (X) measured at 31%.

[0088] Fig. 10 shows the microstructure of Inventive Example 6 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 1.92 µm and a ferrite recrystallization fraction (X) measured at 58%.

[0089] Fig. 11 shows the microstructure of Inventive Example 7 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.22 µm and a ferrite recrystallization fraction (X) measured at 68%.

[0090] Fig. 12 shows the microstructure of Inventive Example 8 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.13 µm and a ferrite recrystallization fraction (X) measured at 81%.

[0091] Fig. 13 shows the microstructure of Comparative Example 3 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the recovery stage, with an average grain size of 1.92 µm and a ferrite recrystallization fraction (X) measured at 27%.

[0092] Fig. 14 shows the microstructure of Comparative Example 4 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.32 µm and a ferrite recrystallization fraction (X) measured at 57%.

[0093] Fig. 15 shows the microstructure of Comparative Example 5 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 4.33 µm and a ferrite recrystallization fraction (X) measured at 81%.

[0094] Fig. 16 shows the microstructure of Comparative Example 6 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the recrystallization stage, with an average grain size of 5.84 µm and a ferrite recrystallization fraction (X) measured at 89%.

[0095] Fig. 17 shows the microstructure of Comparative Example 7 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 2.11 µm and a ferrite recrystallization fraction (X) measured at 43%.

[0096] Fig. 18 shows the microstructure of Comparative Example 8 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the recrystallization stage, with an average grain size of 6.55 µm and a ferrite recrystallization fraction (X) measured at 90%.

[0097] Fig. 19 shows the microstructure of Comparative Example 9 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the partial recrystallization stage, with an average grain size of 9.42 µm and a ferrite recrystallization fraction (X) measured at 92%.

[0098] Fig. 20 shows the microstructure of Comparative Example 10 from Tables 2 and 3, in which a single-phase ferrite microstructure was observed at the recrystallization stage, with an average grain size of 8.03 µm and a ferrite recrystallization fraction (X) measured at 95%.

[0099] The cold-rolled steel material and the manufacturing method thereof according to the present invention have been described above. As described above, it can be confirmed that, according to an embodiment of the present invention, by optimizing the Ti component, it is possible to provide a recovery-annealed steel having a single-phase ferrite microstructure suitable for bending workability, with a yield strength of 600 to 900 MPa and a yield ratio of 95% or more.

[0100] It will be apparent to those of ordinary skill in the art to which the technical concept of the present invention pertains that the technical concept of the present invention described above is not limited to the foregoing embodiments and the accompanying drawings, and that various substitutions, modifications, and changes are possible without departing from the technical concept of the present invention.

Examples

experimental examples

Experimental Examples

[0066]Hereinafter, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are only intended to aid in understanding the present invention, and the present invention is not limited by the following experimental examples. Content not described herein can be technically inferred by those skilled in the art, and thus a description thereof will be omitted.

[0067]Steel materials having the compositions shown in Table 1 below were prepared, and cold-rolled steel sheets according to Examples and Comparative Examples were prepared through the hot rolling and cold rolling processes as described above.

[0068]Table 1 shows the compositions of the cold-rolled steel materials of the Examples and Comparative Examples. The content unit of each component is weight%, and the balance is iron (Fe).

[Table 1]

Steel GradeCSiMnPSCrMoTiNbVAlN

A0.0510.221.300.0010.00070.004-0.2230.032-0.0410.0007

B0.0510.211.3...

Claims

1. A cold-rolled steel material comprising, in weight%: carbon (C): 0.04% to 0.07%; silicon (Si): 0.1% to 0.3%; manganese (Mn): 1.1% to 1.5%; aluminum (Al): 0.015% to 0.06%; phosphorus (P): more than 0% and up to 0.02%; sulfur (S): more than 0% and up to 0.005%; niobium (Nb): 0.02% to 0.04%; titanium (Ti): 0.17% to 0.23%; chromium (Cr): more than 0% and up to 0.1%; nitrogen (N): more than 0% and up to 0.005%; and the balance being iron (Fe) and unavoidable impurities, wherein the cold-rolled steel material has a yield strength (YS) of 600 MPa to 900 MPa; an elongation (EL) of 8% or more; and a yield ratio (YR) of 95% or more.

2. The cold-rolled steel material of claim 1, further comprising, in weight%: boron (B): more than 0% and up to 0.001%; or molybdenum (Mo): more than 0% and up to 0.06%.

3. The cold-rolled steel material of claim 1, wherein the cold-rolled steel material has a ferrite single-phase structure, and the ferrite single-phase structure includes precipitates.

4. A method of manufacturing a cold-rolled steel material, the method comprising: reheating a steel material comprising, in weight%, carbon (C): 0.04% to 0.07%, silicon (Si): 0.1% to 0.3%, manganese (Mn): 1.1% to 1.5%, aluminum (Al): 0.015% to 0.06%, phosphorus (P): more than 0% and up to 0.02%, sulfur (S): more than 0% and up to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% and up to 0.1%, nitrogen (N): more than 0% and up to 0.005%, and the balance being iron (Fe) and unavoidable impurities, at a temperature of 1,200°C to 1,250°C; hot-rolling the heated steel material so as to finish at a temperature of Ar3 or higher; primary cooling the hot-rolled steel material; coiling the primary-cooled steel material at a coiling temperature of 580°C to 620°C; cold-rolling the coiled steel material to form a cold-rolled steel material; annealing the cold-rolled steel material at an annealing temperature of 740°C to 830°C; and secondary cooling the annealed cold-rolled steel material.

5. The method of manufacturing a cold-rolled steel material of claim 4, wherein the primary cooling of the steel material comprises cooling to 580°C to 620°C at a cooling rate of 10°C / sec to 100°C / sec.

6. The method of manufacturing a cold-rolled steel material of claim 4, wherein the secondary cooling of the steel material comprises cooling at a cooling rate of 5°C / sec to 100°C / sec.

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