Cold-rolled steel material and method for manufacturing the same

Abstract

The present invention provides a cold rolled steel material including, by weight, 0.04% to 0.07% of carbon (C), 0.1% to 0.3% of silicon (Si), 1.1% to 1.5% of manganese (Mn), 0.015% to 0.06% of aluminum (Al), more than 0% to 0.02% of phosphorus (P), more than 0% to 0.005% of sulfur (S), 0.02% to 0.04% of niobium (Nb), 0.17% to 0.23% of titanium (Ti), more than 0% to 0.1% of chromium (Cr), more than 0% to 0.005% of nitrogen (N), and a balance of iron (Fe) and other inevitable impurities, and having 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] This invention relates to steel materials, and more specifically to cold-rolled steel materials having a high yield ratio and high yield strength, and methods for manufacturing the same. Background Technology

[0002] In recent years, the automotive industry has been pursuing lightweighting to meet environmental regulations and improve fuel efficiency. High-strength steel is being used as a common method for lightweighting components, as the application of high-strength materials and reduction of thickness are frequently employed. Specifically, structural components and reinforcements in automobiles form the body structure and require excellent crash stability to ensure the safety of the driver and passengers in the event of a collision. That is, crash stability can be improved by achieving complex component shapes and ensuring high yield strength. Traditional ultra-low carbon high-strength steels have low yield ratios and high elongation (which are advantageous for forming); however, due to their low yield ratio, they are disadvantageous for structural components and reinforcements that require high stiffness when used in finished products after forming, as they are detrimental to crash stability.

[0003] Precipitation hardening 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 produce fine precipitates, thereby improving the steel's impact toughness and strength. Traditionally, research on precipitation hardening steel has primarily focused on hot-rolled steel materials, concentrating on models and experimental results regarding deformation-induced precipitation, interphase precipitation, and precipitation via dislocations after winding during the hot-rolling process. For cold-rolled steel materials, fundamental research results, such as studies on the precipitate formation mechanism, are very limited. In the case of hot-rolled precipitation hardening steel, a review of global standards indicates that the VDA standard lists yield strengths up to 700 MPa.

[0004] On the other hand, for cold-rolled precipitation-hardening steel, fundamental research results regarding strength improvement and precipitate formation mechanisms are limited compared to hot-rolled materials. A review of global standards indicates that the VDA standard only lists grades with yield strengths up to 460 MPa, and for strengths exceeding this, dual-phase and composite-phase steel standards are used. Some automakers require cold-rolled precipitation-hardening steel with a yield strength of 550 MPa; however, for components requiring strengths exceeding this level, the dual-phase steel standard is used, as mentioned above. Korean Patent Application No. 2012-0070333 is cited as prior art. Summary of the Invention

[0005] Technical issues The technical problem addressed by the technical concept of this invention is to provide a cold-rolled steel material with high yield ratio and high yield strength, and a method for manufacturing the same. For example, this invention provides a recovery-annealed cold-rolled precipitation-hardening steel characterized by a high yield ratio, enabling it to exhibit high dimensional stability suitable for seat rails, airbag assemblies, and components. However, such a problem is exemplary, and the invention is not limited thereto.

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

[0007] Cold-rolled steel materials contain, by weight percent, 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): greater than 0% and at most 0.02%, sulfur (S): greater than 0% and at most 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): greater than 0% and at most 0.1%, nitrogen (N): greater than 0% and at most 0.005%, and the balance iron (Fe) and unavoidable impurities, and have a yield strength (YS) of 600 MPa to 900 MPa, an elongation (EL) of 8% or greater, and a yield ratio (YR) of 95% or greater.

[0008] Cold-rolled steel materials may further contain boron (B): greater than 0% and at most 0.001% or molybdenum (Mo): greater than 0% and at most 0.06% by weight.

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

[0010] Cold-rolled steel materials can be cold-rolled precipitation-hardening steels with a ferrite single-phase structure and containing precipitates within the ferrite single-phase structure.

[0011] A method for manufacturing cold-rolled steel includes: reheating the steel material at a temperature of 1200°C to 1250°C, wherein the steel material contains, by 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): greater than 0% and at most 0.02%, sulfur (S): greater than 0% and at most 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, and chromium (Cr). ): greater than 0% and at most 0.1%, nitrogen (N): greater than 0% and at most 0.005%, and the balance iron (Fe) and unavoidable impurities; hot rolling of heated steel to finish rolling at Ar3 or higher; primary cooling of hot-rolled steel; winding of primary-cooled steel at a winding temperature of 580°C to 620°C; cold rolling of wound steel to form cold-rolled steel; annealing of cold-rolled steel at an annealing temperature of 740°C to 830°C; and secondary cooling of annealed cold-rolled steel.

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

[0013] In the manufacturing method of cold-rolled steel, the step of secondary cooling of the steel can include cooling at a cooling rate of 5°C / s to 100°C / s.

[0014] Advantages of the present invention According to the present invention, cold-rolled steel materials with high yield ratios and high yield strengths, and methods for manufacturing the same, can be realized. For example, recovery-annealed steels with a ferrite single-phase structure suitable for bending workability, yield strengths in the range of 600 MPa to 900 MPa, and yield ratios of 95% or greater can be realized.

[0015] The above-described effects of the present invention are given by way of example, and the scope of the present invention is not limited to these effects. Attached Figure Description

[0016] Figure 1 A process flow diagram illustrating a method for manufacturing cold-rolled steel material according to an embodiment of the present invention is provided.

[0017] Figure 2 A graph showing the relationship between temperature and austenite fraction for the steel grades in the experimental examples shown in Table 1, calculated through thermodynamic equilibrium.

[0018] Figure 3A graph showing the relationship between stress and strain in cold-rolled steel materials that are experimental and comparative examples of the present invention.

[0019] Figure 4 A graph showing the relationship between yield strength and elongation for the steel grades in the experimental examples shown in Tables 1 to 3.

[0020] Figures 5 to 20 Photographs of the microstructures of the steel materials disclosed in Tables 2 and 3. Detailed Implementation

[0021] Preferred embodiments of the invention will be described in detail below with reference to the accompanying drawings. These embodiments are provided to more fully explain the technical concept of the invention to those skilled in the art. The following embodiments can be modified in various different ways, and the scope of the technical concept of the invention is not limited to these embodiments. Rather, these embodiments are provided to make the invention more credible and complete, and to fully convey the technical concept of the invention to those skilled in the art. In this specification, the same reference numerals denote the same elements. Furthermore, various elements and areas in the drawings are illustrated schematically. Therefore, the invention is not limited to the relative dimensions or spacing shown in the accompanying drawings.

[0022] In recent years, the automotive industry has been pursuing lightweighting to meet environmental regulations and improve fuel efficiency. Common practices for achieving component lightweighting include using high-strength materials and reducing thickness, thus high-strength steel is being applied. Specifically, structural components and reinforcements that form the body structure of a vehicle need to possess excellent crash safety to ensure the safety of the driver and passengers inside the vehicle in the event of a collision. That is, crash stability can be improved by ensuring a high yield ratio and yield strength.

[0023] This invention provides a cold-rolled steel material and its manufacturing method, wherein the yield ratio (yield strength / tensile strength) of the steel material is increased by generating fine precipitates during heat treatment after cold rolling. Conventional precipitation-hardening steel materials maximize the precipitation of carbonitride elements during hot rolling by setting the winding temperature after hot rolling to above 620°C. However, when precipitates form during hot rolling, they coarsen during cold rolling, leading to a decrease in the yield ratio.

[0024] In this invention, a recovery-annealed cold-rolled precipitation-hardening steel with a ferritic single-phase structure, possessing a yield strength of 600 MPa to 900 MPa and a yield ratio of 95% or greater, suitable for bending workability, is proposed. As described above, it can be applied to components such as seat rails and parts requiring high yield strength. The use of the ferritic single-phase microstructure enables uniform material properties, particularly exhibiting a positive effect on pore expansion properties. Furthermore, the high yield ratio of 95% or greater provides optimal conditions during cold forming processes such as roll forming and bending.

[0025] The cold-rolled steel material according to the present invention will be described in detail below.

[0026] According to one embodiment of the invention, the cold-rolled steel material comprises, by weight percent, 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): greater than 0% and at most 0.02%, sulfur (S): greater than 0% and at most 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): greater than 0% and at most 0.1%, nitrogen (N): greater than 0% and at most 0.005%, and the balance being iron (Fe) and unavoidable impurities.

[0027] In the following text, the role and content of each component in the cold-rolled steel material according to the present invention will be described. In this case, the content of each component element refers to a percentage of the total weight of the steel material.

[0028] Carbon (C): 0.04% to 0.07% The addition of carbon (C) is to ensure the strength of the steel and control its microstructure. When the content is below 0.04%, sufficient precipitation may not be achieved, making it difficult to ensure the desired yield strength, and MC (M=Nb,Ti)-based carbides may coarsen, resulting in a reduced grain refinement effect. When the content exceeds 0.07%, the increased phase fraction of phases such as pearlite may lead to deterioration of material properties (e.g., decreased strength). Furthermore, with increasing carbon content, the pearlite structure increases, and during high-temperature annealing after cold rolling, some of the pearlite structure undergoes austenite transformation, which may result in a two-phase structure. Therefore, it is preferable to add carbon at a content of 0.04% to 0.07%.

[0029] Silicon (Si): 0.1% to 0.3% Silicon not only contributes to strength enhancement as a solid solution strengthening element, but also acts as a ferrite stabilizing element, increasing undercooling during the ferrite transformation process to suppress the formation of harmful carbides, refine grains, and inhibit pearlite formation, thereby improving the reactivity between C and M (M=Nb, Ti) in the solid solution. When the silicon content is below 0.1%, the effect of silicon addition is insufficient. When the silicon content exceeds 0.3%, oxides such as Mn₂SiO₄ are formed, which impairs plating properties, deteriorates the surface properties of the steel, and may increase the carbon equivalent, thus reducing weldability. Therefore, it is preferable to add silicon at a content of 0.1% to 0.3% by weight of the total steel material.

[0030] Manganese (Mn): 1.1% to 1.5% Manganese not only contributes to strength enhancement as a solid solution strengthening element, but its content can also control strength, toughness, and yield ratio. However, when added in large quantities, it can induce the formation of MnS inclusions and central segregation during casting, thereby reducing the toughness of the steel. When the Mn content is below 1.1%, it is difficult to ensure the desired yield strength of 800 MPa, and when it exceeds 1.5%, although it is beneficial to ensure the desired strength, it may lead to the formation of inclusions or segregation, resulting in poor workability and resistance to delayed fracture, and may increase the carbon equivalent, thereby reducing weldability (a major advantage of precipitation hardening steel). In addition, structural transformation may be induced during high-temperature annealing and cooling after cold rolling. Therefore, it is preferable to add Mn at a content of 1.1% to 1.5%.

[0031] Aluminum (Al): 0.015% to 0.06% Aluminum is used as a deoxidizer and can help purify ferrite. When the Al content is below 0.015%, the effect is insufficient, and when it exceeds 0.06%, AlN will form during slab manufacturing, which may cause cracks during casting or hot rolling. Therefore, aluminum is preferably added at a content of 0.015% to 0.06% by weight of the entire steel material.

[0032] Phosphorus (P): greater than 0% and at most 0.02% Phosphorus is an impurity introduced during steelmaking, and although it can contribute to increased strength through solid solution strengthening, its presence in large quantities can lead to low-temperature embrittlement and brittle fracture. Therefore, the phosphorus content is preferably limited to greater than 0% by weight to 0.02% by weight or less in the entire steel material.

[0033] Sulfur (S): greater than 0% and at most 0.005%.

[0034] Sulfur is an impurity introduced during steelmaking and can form non-metallic inclusions (such as FeS and MnS), thereby reducing toughness and weldability. Therefore, the sulfur content is preferably limited to greater than 0% by weight to 0.005% by weight or less in the entire steel material.

[0035] Niobium (Nb): 0.02% to 0.04% Niobium is a potent carbonitride forming element that combines with carbon and nitrogen in steel during hot rolling and annealing to form carbides or nitrides. These Nb-based carbides or nitrides inhibit recrystallization and grain growth during annealing after cold rolling, thereby refining the grains and improving the strength and toughness of the steel. When the Nb content is below 0.02%, precipitation strengthening may be difficult to achieve, and when it exceeds 0.04%, the annealing time must be extended or the annealing temperature must be increased to ensure the elongation of the steel due to the delayed recrystallization effect of Nb. Therefore, it is preferable to add Nb at a content of 0.02% to 0.04%.

[0036] Titanium (Ti): 0.17% to 0.23% Titanium is a potent carbonitride-forming element. During hot rolling and annealing, it combines with carbon and nitrogen in steel to form carbides or nitrides. These Ti-based carbides or nitrides inhibit recrystallization and grain growth during annealing after cold rolling, thereby refining the grains and improving the strength and toughness of the steel. When the Ti content is below 0.17%, it may be difficult to obtain sufficient recrystallization delay and precipitation strengthening effects. When it exceeds 0.23%, the annealing time must be extended or the annealing temperature must be increased to ensure the elongation of the steel due to the recrystallization delay effect of Ti. Therefore, it is preferable to add Ti at a content of 0.17% to 0.23%.

[0037] Chromium (Cr): greater than 0% and at most 0.1% Chromium (Cr) is preferably added at a content ratio of 0.1% by weight or less of the entire precipitation-hardening steel material according to the invention. When the chromium content exceeds 0.1% by weight of the entire steel material, there is a problem of reduced weldability or heat-affected zone (HAZ) toughness.

[0038] Nitrogen (N): greater than 0% and at most 0.005% Nitrogen (N) is an element that inevitably remains in steel during the steelmaking process. Compared to carbon, nitrogen has a faster diffusion rate and a lower activation energy, so even small changes in its amount can lead to significant changes in material properties and accelerate aging. Therefore, nitrogen must precipitate in the form of nitrides. In this invention, when the nitrogen (N) content exceeds 0.005% and is added in large quantities, a large amount of TiN is formed, which adversely affects the formability of the steel. It is best to minimize nitrogen as much as possible; however, since nitrogen is an element that inevitably remains in the steelmaking process, its concentration should not exceed 50 ppm.

[0039] The total content of niobium (Nb) and titanium (Ti) in the cold-rolled steel material according to the present invention can be 0.26 wt% or less, for example, from 0.20 wt% to 0.26 wt%. When the total content of niobium (Nb) and titanium (Ti) is less than 0.20 wt%, it may be difficult to obtain sufficient recrystallization delay effect and precipitation strengthening effect, and when the total content of niobium (Nb) and titanium (Ti) exceeds 0.26 wt%, due to the recrystallization delay effect, it may be necessary to extend the annealing time or increase the annealing temperature to ensure the elongation of the steel.

[0040] Furthermore, the ratio of the weight percentage of carbon (C) to the weight percentage of the total content of niobium (Nb) and titanium (Ti) (= [C] / ([Nb]+[Ti])) can be less than 0.23. If the value of [C] / ([Nb]+[Ti]) is 0.23 or greater, there may be a problem of the cementite fraction being present at 1% or greater.

[0041] The remaining component of cold-rolled steel is iron (Fe). However, due to the unavoidable introduction of unintended impurities from raw materials or the surrounding environment during conventional steelmaking processes, these impurities may not be eliminated. Since these impurities are known to those skilled in the art in conventional manufacturing processes, not all details thereof are specifically described in this specification.

[0042] Furthermore, the cold-rolled steel material according to the present invention may also contain boron (B): greater than 0% and at most 0.001% by weight, or molybdenum (Mo): greater than 0% and at most 0.06%.

[0043] Boron (B): greater than 0% and at most 0.001% The addition of boron (B) is to prevent secondary processing embrittlement that may occur due to the addition of phosphorus (P). Generally, when boron is added in amounts exceeding 0.001%, changes in material properties due to segregation may occur; therefore, it is preferable to add boron in the range of 0.001% or less.

[0044] Molybdenum (Mo): greater than 0% and at most 0.06% When Nb and Ti components combine with carbon to form precipitates, molybdenum acts to slow down the formation and growth rate of fine precipitates, thereby achieving a yield strength enhancement effect. This effect converges and price competitiveness decreases when the molybdenum content exceeds 0.06%; therefore, it is preferable to add molybdenum at a content of 0.06% or less.

[0045] Cold-rolled steel materials manufactured by the manufacturing method described below, by controlling the specific components and content ranges of the alloy composition described above, can meet, for example, yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or greater, and yield ratio (YR): 95% or greater.

[0046] Cold-rolled steel materials can contain precipitates formed at grain boundaries, within grains, or both. These precipitates primarily include Ti,Nb(C) type precipitates. Additionally, they may include TiN, AlN, etc. The precipitates can have an average particle diameter primarily of 2 nm to 6 nm. These precipitates can form during the annealing heat treatment following cold rolling. Due to these precipitates, cold-rolled steel materials can simultaneously possess high strength and a high yield ratio. For example, a portion of Ti,Nb(C) is formed during hot rolling and winding, and then the remaining 30% to 70% of the Ti and Nb elements use the numerous dislocations generated during cold rolling as nucleation sites and precipitate as Ti,Nb(C) during annealing.

[0047] In the following description, a method for manufacturing cold-rolled steel material according to the present invention will be described with reference to the accompanying drawings.

[0048] Manufacturing method of cold-rolled steel materials Figure 1 To illustrate the temperature change over time in a method for manufacturing cold-rolled steel according to an embodiment of the present invention, and to graphically represent the changes in the microstructure of the steel material at each step.

[0049] The steel material contains, by weight percent, 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): greater than 0% and at most 0.02%, sulfur (S): greater than 0% and at most 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): greater than 0% and at most 0.1%, nitrogen (N): greater than 0% and at most 0.005%, and the balance iron (Fe) and unavoidable impurities.

[0050] The method for manufacturing cold-rolled steel material according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), a primary cooling step (S130), a winding step (S140), a cold rolling step (S150), an annealing heat treatment step (S160), and a secondary cooling step (S170).

[0051] Reheating step (S110) The semi-finished product subjected to the hot rolling process in the manufacturing method according to the present invention can be, for example, a slab. A slab in a semi-finished state can be obtained by a continuous casting process after obtaining molten steel with a predetermined composition through a steelmaking process.

[0052] In the reheating step (S110), the steel material (e.g., a billet) with the above composition is reheated at a slab reheating temperature (SRT) of 1200°C to 1250°C for 40 minutes to 1 hour. This reheating can lead to the redissolution of components segregated during casting and the redissolution of precipitates. In particular, sufficiently high temperatures are required to redissolve hardening elements (e.g., niobium), thereby forming fine precipitates during subsequent annealing heat treatment after cold rolling. These fine precipitates may hinder grain growth to achieve grain refinement, thus increasing strength. When the reheating temperature is below 1200°C or the reheat furnace dwell time is less than 40 minutes, the carbonitrides do not dissolve and become coarse, making it impossible to obtain material properties exceeding the desired yield ratio; for example, maximum redissolution of precipitation hardening elements (e.g., 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 1250°C or the reheating furnace dwell time exceeds 1 hour, the austenite grains may coarsen, which may lead to a decrease in yield strength. Furthermore, as the reheating temperature increases, there are issues of increased manufacturing costs and decreased productivity due to heating costs and the additional time required to reach the hot rolling temperature.

[0053] Hot rolling step (S120) After heating, the heated steel material is first hot-rolled to adjust its shape. Hot rolling can be carried out continuously through width rolling, roughing mills, and finishing mills. Through the hot rolling process, the steel material can be formed into hot-rolled steel material. Hot-rolled steel material can be hot-rolled steel sheet. The reduction rate of hot rolling can be 50% or greater. Work hardening (deformation) may occur during the hot rolling process; however, work hardening can be eliminated through recrystallization and recovery.

[0054] Finishing mills can be used at Ar3 or higher finishing rolling temperatures (FRT). When the finishing rolling temperature is below Ar3 (e.g., below 870°C), rolling occurs in the two-phase region of austenite and ferrite, resulting in grain mixing, which leads to uneven deformability and may cause a decrease in rollability. When the finishing rolling temperature exceeds Ar3 + 40°C (e.g., above 910°C), the grains may coarsen, resulting in a decrease in the strength of the final steel material.

[0055] One cooling step (S130) A primary cooling step (S130) can be performed on the output roller conveyor (ROT). The ROT is a device for cooling steel sheets whose thickness and width have been determined through roughing and finishing rolling. The ROT has multiple rows, each with multiple manifolds equipped with a row of nozzles positioned above and below the steel sheet. At the rear end of the ROT is a winding machine for winding the steel sheet into coils, and the cooling of the steel sheet must be controlled so that its temperature before entering the winding machine (i.e., the winding temperature) matches the target winding temperature. The hot-rolled steel is cooled to 580°C to 620°C in a single pass at a cooling rate of 10°C / s to 100°C / s. Cooling can be performed by air cooling or water cooling. When the cooling rate is less than 10°C / s, the average particle size of the precipitates may increase, thus ensuring strength. Conversely, when the cooling rate exceeds 100°C / s, the microstructure of the steel may harden, leading to a decrease in impact toughness.

[0056] Winding step (S140) After a cooling cycle, the steel is wound at a winding temperature (CT) of 580°C to 620°C. In this invention, a low-temperature winding process of 580°C to 620°C is used to minimize the size of the precipitates and reduce the precipitate fraction as much as possible. When the winding temperature is below 580°C, it may be difficult to control the shape of the hot-rolled material, and the surface quality of the steel may deteriorate due to the significant difference between the finishing temperature and the winding temperature. When the winding temperature exceeds 620°C, it may help to increase the strength of the hot-rolled material by increasing the precipitate fraction; however, this is undesirable from the perspective of the properties of the final cold-rolled material, as the precipitates become coarser. That is, when the winding temperature exceeds 620°C, carbonitride elements cannot remain in a solid solution state and may form undesirable precipitates.

[0057] Reference Figure 1The microstructure changes in each step of the hot rolling step (S120), the primary cooling step (S130), and the winding step (S140) are illustrated. The SIP term represents strain-induced precipitation, specifically precipitation caused by deformation occurring during rolling. PIP represents phase-induced precipitation, specifically precipitates formed during the transformation from austenite to ferrite. Furthermore, the random term represents precipitates that widely precipitate within the ferrite microstructure during cooling after winding.

[0058] Meanwhile, the γ→α phase transformation refers to the phase transformation from austenite to ferrite.

[0059] Cold rolling step (S150) Pickling is performed to clean the coiled hot-rolled steel material. Subsequently, the pickled hot-rolled steel material is cold-rolled at a reduction rate of 50% to 80% to form cold-rolled steel material. When the reduction rate is less than 50%, the amount of nucleation sites for recrystallization during annealing is small, thus grain overgrowth may occur during the annealing heat treatment described below, leading to a sharp decrease in strength. When the reduction rate increases to more than 50%, the number of sites for the formation of fine precipitates increases, which is beneficial for improving the yield ratio; however, when it exceeds 80%, the nucleation amount becomes excessive, resulting in overly fine grains formed by annealing, thereby reducing ductility and formability.

[0060] Annealing heat treatment step (S160) Cold-rolled steel materials are heat-treated in a continuous annealing furnace with a conventional slow cooling zone. When continuously annealing cold-rolled sheets, recrystallization annealing is performed at a strip throughput rate of 50 mpm to 200 mpm and an annealing temperature of 740°C to 830°C. This heat treatment can be referred to as annealing heat treatment. Annealing heat treatment is performed by heating at a heating rate ranging from 1°C / s to 10°C / s and holding at a temperature ranging from 740°C to 830°C for 30 to 1000 seconds. Here, annealing heat treatment is one of the important process variables determining the material properties of the final product. This recrystallization annealing heat treatment results in a very fine final microstructure and produces fine precipitates of solid solution carbonitride elements, thereby achieving a high yield ratio in cold-rolled steel materials. These carbonitrides can be, for example, NbC precipitates and AlN precipitates. When the annealing heat treatment temperature is below 740°C, recrystallization may not be fully completed, and the target elongation may not be guaranteed. When the annealing heat treatment temperature exceeds 830°C, the grains may coarsen, resulting in a decrease in yield strength and yield ratio. Therefore, to ensure a yield strength of 600 MPa to 900 MPa and a yield ratio of 95% or greater, the annealing temperature is preferably limited to the above range.

[0061] Meanwhile, the strip throughput speed during the continuous annealing process of cold-rolled sheet can be from 90 mpm to 110 mpm, which is the speed at which recrystallized steel is produced at a typical annealing temperature of 760°C to 800°C when producing steel with a thickness of 1.0 t to 1.5 t on a continuous galvanizing line (CGL). In this invention, since the recrystallization fraction of the steel is distributed in the range of 31% to 81%, the range of strip throughput speed can be further broadened to 50 mpm to 200 mpm.

[0062] Secondary cooling step (S170) The annealed heat-treated cold-rolled steel material is cooled to a temperature in the range of, for example, 100°C to 300°C at a cooling rate of 5°C / s to 100°C / s.

[0063] Cold-rolled steel materials can be manufactured by the above processes (S110 to S170), and cold-rolled steel materials can be cold-rolled steel sheets.

[0064] If necessary, cold-rolled steel materials can be formed into hot-dip galvanized steel materials and alloyed hot-dip galvanized steel materials. Hot-dip galvanized steel materials are formed by immersing cold-rolled steel materials in a hot-dip galvanizing bath to form a hot-dip galvanized layer. Depending on the type and proportion of alloying elements constituting the coating and the composition of the cold-rolled steel material, the bath temperature can be in the range of 400°C to 520°C. Under bath conditions, a hot-dip galvanized layer is easily formed on the surface of the cold-rolled steel sheet, and the adhesion of the coating can be excellent. Subsequently, the hot-dip galvanized steel material can be formed by cooling to room temperature at a cooling rate of 1°C / s to 100°C / s.

[0065] Alloyed hot-dip galvanized steel is produced by alloying heat treatment of cold-rolled steel with an already formed hot-dip galvanized coating. The alloying heat treatment can be performed at a temperature ranging from 500°C to 620°C for 10 to 60 seconds. Under these conditions, the hot-dip galvanized coating grows stably during the alloying heat treatment, and the coating adhesion can be excellent. When the alloying heat treatment temperature is below 500°C, alloying may not be sufficient, leading to a decrease in the integrity of the hot-dip galvanized coating. When the alloying heat treatment temperature exceeds 620°C, the temperature range may enter the two-phase region, causing changes in material properties. Afterward, the alloyed hot-dip galvanized steel can be formed by cooling.

[0066] In the reheating region, precipitated hardening elements (such as niobium) redissolve. In the hot-rolling region, precipitates such as niobium carbide may form within the austenite grains. In the primary cooling region and the winding region, precipitates such as niobium carbide form within the ferrite grains; however, the amount of precipitates can be reduced by lowering the winding temperature. In the cold-rolling region, niobium may redissolve into the matrix. In the annealing heat treatment region, precipitates such as niobium carbide may form within the ferrite grains. That is, by minimizing the amount of precipitates formed during hot rolling and inducing the formation of precipitates during cold rolling, the size of the precipitates can be refined. Therefore, high yield ratios and high yield strengths can be obtained.

[0067] Cold-rolled steel materials obtained by applying the above alloy composition and process conditions can meet, for example, yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or greater, and yield ratio (YR): 95% or greater.

[0068] In cold-rolled steel materials obtained by applying the above alloy composition and process conditions, the yield strength (unit: MPa) and yield ratio (unit: %) are approximately related according to the following equation, depending on the annealing heat treatment temperature (unit: °C), annealing heat treatment holding time (unit: seconds) in the annealing heat treatment step (S160), and the titanium content (unit: weight %) constituting the steel material.

[0069] Equation 1: 4286.5605 Ti (wt%) - 3.3482 SS (°C) - 0.2952 t (s) + 2474.7647 = YP (MPa) Equation 2: 367.6854 Ti (wt%) -0.1819 SS (°C) -0.0141 t (s) + 159.9337 = Yield Ratio (%) In Equations 1 and 2, Ti represents the titanium content (unit: weight %), SS represents the annealing heat treatment temperature in the annealing heat treatment step (S160) (unit: °C), t represents the annealing heat treatment holding time in the annealing heat treatment step (S160) (unit: seconds), and YP represents the yield strength (unit: MPa).

[0070] Cold-rolled steel materials can contain precipitates formed at grain boundaries, within grains, or both. These precipitates primarily include Ti,Nb(C) type precipitates. Additionally, TiN, AlN, etc., may also be included. The precipitates can have an average particle diameter primarily of 2 nm to 6 nm. These precipitates can form during annealing heat treatment following cold rolling. Due to these precipitates, cold-rolled steel materials can simultaneously possess high strength and a high yield ratio. For example, a portion of Ti,Nb(C) is formed during hot rolling and winding, and the remaining 30% to 70% of the Ti and Nb elements utilize the numerous dislocations generated during cold rolling as nucleation sites, precipitating as Ti,Nb(C) during the annealing process.

[0071] Experimental Examples Preferred experimental embodiments are shown below to aid in understanding the invention. However, the following experimental embodiments are intended only to aid in understanding the invention, and the invention is not limited to the following experimental embodiments. Those skilled in the art can technically infer what is not described herein, and therefore its description will be omitted.

[0072] Steel materials having the compositions shown in Table 1 below were prepared, and cold-rolled steel sheets according to the Examples and Comparative Examples were prepared by hot rolling and cold rolling processes as described above.

[0073] Table 1 shows the composition of the cold-rolled steel materials of the Examples and Comparative Examples. The content of each component is in % by weight, and the balance is iron (Fe).

[0074] [Table 1] Referring to Table 1, steel grade A meets the following composition range (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): greater than 0% and at most 0.02%, sulfur (S): greater than 0% and at most 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): greater than 0% and at most 0.1%, nitrogen (N): greater than 0% and at most 0.005%, and the balance iron (Fe). Furthermore, steel grade A meets the requirement that the total content of niobium (Nb) and titanium (Ti) is 0.26% by weight or less, specifically, within the range of 0.20% to 0.26% by weight. Furthermore, the ratio of carbon (C) by weight to the total content of niobium (Nb) and titanium (Ti) by weight (=[C] / ([Nb]+[Ti])) is less than 0.23. In contrast, steel grades B and C do not meet the range of titanium (Ti): 0.17% to 0.23%, and are below that range. In addition, the total content of niobium (Nb) and titanium (Ti) in steel grades B and C does not meet the range of 0.20% to 0.26% by weight, and is below that range; and the ratio of carbon (C) by weight to the total content of niobium (Nb) and titanium (Ti) by weight (=[C] / ([Nb]+[Ti])) does not meet the range of less than 0.23, and exceeds that range.

[0075] Figure 2 A graph illustrating the relationship between temperature and austenite fraction for the steel grades in the experimental examples shown in Table 1, calculated through thermodynamic equilibrium. Figure 2 In the table, A represents steel grade A in Table 1, B represents steel grade B in Table 1, and C represents steel grade C in Table 1.

[0076] Reference Figure 2 In the case of steel grade A, it can be confirmed that the austenite fraction is almost 0% at temperatures as high as approximately 820°C during thermodynamic equilibrium calculations. Since the possibility of martensitic transformation during cooling after austenite formation is high, steel grade A may be necessary to maintain the single-phase ferrite microstructure. However, it may be necessary to consider that the heating process under actual annealing conditions results in a delayed outcome compared to the thermodynamic equilibrium calculations, potentially exhibiting a lower austenite transformation fraction.

[0077] Table 2 shows the process conditions used 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 described above in this invention.

[0078] [Table 2] Referring to Table 2, Examples 1 to 8 of the invention have the composition of steel grade A in Table 1 and all meet the following process conditions: reheating temperature: 1200°C to 1250°C, finishing rolling temperature (FDT): 870°C to 910°C, winding temperature (CT): 580°C to 620°C, cold rolling reduction: 50% to 80%, annealing temperature: 740°C to 830°C, and annealing time: 30 seconds to 1000 seconds. In contrast, Comparative Examples 3 to 6 have the composition of steel grade B in Table 1 and do not meet the finishing rolling temperature (FDT) range of 870°C to 910°C (exceeding this range). Comparative Examples 7 to 10 have the composition of steel grade C in Table 1.

[0079] Table 3 shows the material properties of the cold-rolled steel materials used in the experimental and comparative embodiments of the present invention. Figure 3 A graph showing the stress-strain relationship of cold-rolled steel materials used as experimental and comparative examples of the present invention. Figure 3 and Figure 4 In the table, inventive materials 1 to 8 correspond to inventive embodiments 1 to 8 in Tables 2 and 3, respectively, and comparative materials 1 to 10 correspond to comparative embodiments 1 to 10 in Tables 2 and 3, respectively.

[0080] [Table 3] Refer to Table 3 and Figure 3 Examples 1 to 8 of the invention all satisfy the following conditions: yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or greater, and yield ratio (YR): 95% or greater. In contrast, Comparative Example 3 does not satisfy the range of elongation (EL): 8% or greater and yield ratio (YR): 95% or greater (below it). Comparative Examples 4 to 10 do not satisfy the range of yield strength (YS): 600 MPa to 900 MPa and elongation (EL): 8% or greater (both below it). Furthermore, referring to Tables 1 to 3, it can be confirmed that Examples 1 to 8 of the invention have the following correlation between Equations 1 and 2. In Equations 1 and 2, Ti represents the titanium content (unit: weight %), SS represents the annealing heat treatment temperature in the annealing heat treatment step (S160) (unit: °C), t represents the annealing heat treatment holding time in the annealing heat treatment step (S160) (unit: seconds), and YP represents the yield strength (unit: MPa). The calculated values ​​for each equation are very close to the actual values ​​(Table 3), and the R values ​​in the relevant equations are also very close. 2 The values ​​of 0.922 (Equation 1) and 0.962 (Equation 2) indicate a very close relationship. In the relevant equations, R... 2It indicates the degree of variance of the results based on the regression analysis method and is used as a standard to judge the goodness of fit of the regression analysis model.

[0081] Equation 1: 4286.5605 Ti (wt%) - 3.3482 SS (°C) - 0.2952 t (s) + 2474.7647 = YP (MPa) Equation 2: 367.6854 Ti (wt%) -0.1819 SS (°C) -0.0141 t (s) + 159.9337 = Yield Ratio (%) Figure 4 A graph showing the relationship between yield strength and elongation for the steel grades in the experimental examples shown in Tables 1 to 3. Figure 4 In the table, A represents steel grade A in Table 1, B represents steel grade B in Table 1, and C represents steel grade C in Table 1.

[0082] Reference Figure 4 In the curve showing the relationship between yield strength and elongation, steel grade A exhibits different characteristics compared to steel grades B and C. In other words, it can be confirmed that steel grade A is superior to steel grades B and C in both strength and elongation.

[0083] Furthermore, the inventors were able to demonstrate that, under all conditions of annealing temperature (740°C to 830°C) and process time (30 to 1000 seconds), the hardness value increases with increasing Ti content.

[0084] Figures 5 to 20 Photographs of the microstructures of the steel materials disclosed in Tables 2 and 3.

[0085] In this experimental embodiment, the ferrite recrystallization fraction X(t) is measured by observing the microstructure of the steel material.

[0086] The ferrite recrystallization fraction X(t) can be expressed by the following formula. Where X(t) is the ferrite recrystallization fraction, and Hv rex HV0 represents the hardness measurement of the fully recrystallized ferrite microstructure, while HV represents the hardness measurement of the ferrite microstructure before annealing. t This represents the hardness measurement of the ferrite microstructure under the specific conditions considered.

[0087] Figure 5 The microstructures of Embodiment 1 of the invention shown in Tables 2 and 3 are shown, wherein a single-phase ferrite microstructure was observed in the partial recrystallization stage, with an average grain size of 1.92 μm and a measured ferrite recrystallization fraction (X) of 33%.

[0088] Figure 6 The microstructures of Embodiment 2 of the invention are shown in Tables 2 and 3, wherein a single-phase ferrite microstructure was observed in the partial recrystallization stage, with an average grain size of 2.1 μm and a measured ferrite recrystallization fraction (X) of 51%.

[0089] Figure 7 The microstructure of Example 3 of the invention is shown in Tables 2 and 3, wherein a single-phase ferrite microstructure was observed in the partial recrystallization stage, with an average grain size of 3.49 μm and a measured ferrite recrystallization fraction (X) of 83%.

[0090] Figure 8 The microstructure of Embodiment 4 of the invention is shown in Tables 2 and 3, wherein a single-phase ferrite microstructure was observed in the partial recrystallization stage, with an average grain size of 2.16 μm and a measured ferrite recrystallization fraction (X) of 52%.

[0091] Figure 9 The microstructures of Embodiment 5 of the invention are shown in Tables 2 and 3, wherein a single-phase ferrite microstructure was observed in the partial recrystallization stage, with an average grain size of 1.76 μm and a measured ferrite recrystallization fraction (X) of 31%.

[0092] Figure 10 The microstructures of Embodiment 6 of the invention are shown in Tables 2 and 3, wherein a single-phase ferrite microstructure was observed in the partial recrystallization stage, with an average grain size of 1.92 μm and a measured ferrite recrystallization fraction (X) of 58%.

[0093] Figure 11 The microstructures of Embodiment 7 of the invention are shown in Tables 2 and 3, wherein a single-phase ferrite microstructure was observed in the partial recrystallization stage, with an average grain size of 2.22 μm and a measured ferrite recrystallization fraction (X) of 68%.

[0094] Figure 12 The microstructures of Embodiment 8 of the invention are shown in Tables 2 and 3, wherein a single-phase ferrite microstructure was observed in the partial recrystallization stage, with an average grain size of 2.13 μm and a measured ferrite recrystallization fraction (X) of 81%.

[0095] Figure 13 The microstructures of Comparative Example 3 are shown in Tables 2 and 3, in which a single-phase ferrite microstructure was observed during the recovery phase, with an average grain size of 1.92 μm and a measured ferrite recrystallization fraction (X) of 27%.

[0096] Figure 14The microstructures of Comparative Example 4 are shown in Tables 2 and 3, in which a single-phase ferrite microstructure was observed during the partial recrystallization stage, with an average grain size of 2.32 μm and a measured ferrite recrystallization fraction (X) of 57%.

[0097] Figure 15 The microstructures of Comparative Example 5 are shown in Tables 2 and 3, in which a single-phase ferrite microstructure was observed during the partial recrystallization stage, with an average grain size of 4.33 μm and a measured ferrite recrystallization fraction (X) of 81%.

[0098] Figure 16 The microstructures of Comparative Example 6 are shown in Tables 2 and 3, in which a single-phase ferrite microstructure was observed during the recrystallization stage, with an average grain size of 5.84 μm and a measured ferrite recrystallization fraction (X) of 89%.

[0099] Figure 17 The microstructures of Comparative Example 7 are shown in Tables 2 and 3, in which a single-phase ferrite microstructure was observed during the partial recrystallization stage, with an average grain size of 2.11 μm and a measured ferrite recrystallization fraction (X) of 43%.

[0100] Figure 18 The microstructures of Comparative Example 8 are shown in Tables 2 and 3, in which a single-phase ferrite microstructure was observed during the recrystallization stage, with an average grain size of 6.55 μm and a measured ferrite recrystallization fraction (X) of 90%.

[0101] Figure 19 The microstructures of Comparative Example 9 are shown in Tables 2 and 3, in which a single-phase ferrite microstructure was observed during the partial recrystallization stage, with an average grain size of 9.42 μm and a measured ferrite recrystallization fraction (X) of 92%.

[0102] Figure 20 The microstructures of Comparative Example 10 are shown in Tables 2 and 3, in which a single-phase ferrite microstructure was observed during the recrystallization stage, with an average grain size of 8.03 μm and a measured ferrite recrystallization fraction (X) of 95%.

[0103] The cold-rolled steel material and its manufacturing method according to the present invention have been described above. As mentioned above, it can be confirmed that, according to embodiments of the present invention, by optimizing the Ti composition, a recovery-annealed steel with a single-phase ferrite microstructure suitable for bending workability can be provided, which has a yield strength of 600 MPa to 900 MPa and a yield ratio of 95% or greater.

[0104] It will be apparent to those skilled in the art to which the technical concept of this invention pertains that the above-described technical concept of this invention is not limited to the foregoing embodiments and accompanying drawings, and that various substitutions, modifications and variations are possible without departing from the technical concept of this invention.

Claims

1. A cold-rolled steel material, said cold-rolled steel material comprising, by weight percent: 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): greater than 0% and at most 0.02%; Sulfur (S): greater than 0% and at most 0.005%; Niobium (Nb): 0.02% to 0.04%; Titanium (Ti): 0.17% to 0.23%; Chromium (Cr): greater than 0% and at most 0.1%; Nitrogen (N): greater than 0% and at most 0.005%; and The remaining iron (Fe) and unavoidable impurities, in, The cold-rolled steel material has a yield strength (YS) of 600 MPa to 900 MPa, an elongation (EL) of 8% or greater, and a yield ratio (YR) of 95% or greater.

2. The cold-rolled steel material according to claim 1, wherein the cold-rolled steel material further comprises, by weight percent: Boron (B): greater than 0% and at most 0.001%; ​​or Molybdenum (Mo): greater than 0% and at most 0.06%.

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

4. A method for manufacturing cold-rolled steel material, the method comprising: The steel material is reheated at a temperature of 1200°C to 1250°C, the steel material comprising, by 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): greater than 0% and at most 0.02%, sulfur (S): greater than 0% and at most 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): greater than 0% and at most 0.1%, nitrogen (N): greater than 0% and at most 0.005%, and the balance being iron (Fe) and unavoidable impurities; The heated steel material is hot-rolled to allow it to be finished at an Ar3 temperature or higher; The hot-rolled steel material is cooled once. The steel material that has been cooled once is wound at a winding temperature of 580°C to 620°C; Cold rolling is performed on wound steel materials to form cold-rolled steel materials; The cold-rolled steel material is annealed at an annealing temperature of 740℃ to 830℃; as well as The annealed cold-rolled steel material undergoes secondary cooling.

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

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

Images