Cold-rolled steel material and method for producing same

Abstract

The present invention provides a cold-rolled steel material comprising, 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 the balance being 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

Cold rolled steel and its manufacturing method

[0001] The technical idea of ​​the present invention relates to steel, and more specifically, to cold-rolled steel having a high yield ratio and high yield strength and a method for manufacturing the same.

[0002] The automotive industry is currently pursuing lightweight construction to address environmental regulations and improve fuel efficiency. The typical approach for weight reduction involves using high-strength materials and reducing thickness, leading to the adoption of high-strength steels. In particular, structural members and reinforcing materials in automobiles form the structure of the car body and require superior crashworthiness to ensure the safety of drivers and passengers in the vehicle during impacts. Therefore, crashworthiness can be enhanced by ensuring complex component shapes and high yield strength. Conventional ultra-low-carbon, high-strength steels possess high yield ratios and elongation, which are advantageous for forming. However, their low yield ratios hinder their impact safety as structural members and reinforcing materials that require high rigidity after forming.

[0003] Precipitation hardening steels are high-strength alloys that are formed by adding trace amounts of carbonitride-forming elements such as titanium and niobium to general carbon-manganese steels to form micro-precipitates, thereby enhancing the steel's impact toughness and strength. Previously, research on precipitation hardening steels focused on hot-rolled steels, focusing on models and experimental results of precipitation phenomena such as strain-induced precipitation, interphase precipitation, and dislocation after coiling during the hot rolling process. However, for cold-rolled steels, fundamental research, such as studies on the precipitate formation mechanism, has been very limited. Global standards for hot-rolled precipitation hardening steels include a yield strength of up to 700 MPa within the VDA standard.

[0004] On the other hand, in the case of cold-rolled precipitation hardening steel, fundamental research results such as strength improvement and precipitate formation mechanism have been limited compared to hot-rolled steel. Looking at global standards, only the yield strength of 460 MPa is listed in the VDA standard, and for strengths higher than that, two-phase steel and composite phase steel standards are used. Some automobile manufacturers require cold-rolled precipitation hardening steel with a yield strength of 550 MPa, but for parts that require higher strength, the two-phase steel standard is applied as mentioned above. Prior art literature includes Korean Patent Application No. 2012-0070333.

[0005] The technical goal of the present invention is to provide cold-rolled steel with a high yield ratio and high yield strength, and a method for manufacturing the same. For example, the present invention provides a cold-rolled precipitation-hardened steel of the recovery annealing type, characterized by a high yield ratio and high dimensional stability, for applications such as seat rails, airbag components, and members. However, these goals are exemplary, and the technical goal of the present invention is not limited thereto.

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

[0007] The above cold rolled steel contains, in wt%, 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% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% to 0.1% or less, nitrogen (N): more than 0% to 0.005% or less, and the remainder is iron (Fe) and other unavoidable impurities, and has a yield strength (YS): 600 MPa to 900 MPa, an elongation (EL): 8% or more, and Yield Ratio (YR): Satisfies 95% or more.

[0008] The above cold rolled steel may further include, in weight %, boron (B): more than 0% and less than 0.001% or molybdenum (Mo): more than 0% and less than 0.06%.

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

[0010] The above cold rolled steel may be a cold rolled precipitation hardening steel having a ferrite single phase structure and including precipitates within the ferrite single phase structure.

[0011] The method for manufacturing the above cold rolled steel comprises the steps of reheating the steel, which contains, in wt%, 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% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% to 0.1% or less, nitrogen (N): more than 0% to 0.005% or less, and the remainder being iron (Fe) and other unavoidable impurities, at a temperature of 1,200°C to 1,250°C; It includes a step of hot rolling the heated steel to a temperature of Ar3 or higher; a step of first cooling the hot rolled steel; a step of coiling the first cooled steel at a coiling temperature of 580°C to 620°C; a step of cold rolling the coiled steel to form cold rolled steel; a step of annealing the cold rolled steel at an annealing temperature of 740°C to 830°C; and a step of secondarily cooling the annealed cold rolled steel.

[0012] In the above method for manufacturing cold rolled steel, the step of first cooling the steel may include a step of cooling to 580°C to 620°C at a cooling rate of 10°C / sec to 100°C / sec.

[0013] In the above method for manufacturing cold rolled steel, the step of secondary cooling the steel may include a step of cooling at a cooling rate of 5°C / sec to 100°C / sec.

[0014] According to the technical concept of the present invention, cold-rolled steel 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 and a yield ratio of 95% or higher and a ferrite single-phase structure suitable for bending processing characteristics can be realized.

[0015] The effects of the present invention described above are illustrative, and the scope of the present invention is not limited by these effects.

[0016] Figure 1 is a process flow diagram schematically showing a method for manufacturing cold rolled steel according to an embodiment of the present invention.

[0017] Figure 2 is a graph showing the relationship between temperature and austenite fraction for the steel grades of the experimental examples shown in Table 1, using thermodynamic equilibrium calculations.

[0018] Figure 3 is a graph showing the relationship between stress and strain of cold-rolled steel of examples and comparative examples as an experimental example of the present invention.

[0019] Figure 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.

[0020] Figures 5 to 20 are photographs of the microstructure of the steel materials disclosed in Tables 2 and 3.

[0021] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Embodiments of the present invention are provided to more completely explain the technical idea of ​​the present invention to those skilled in the art. The following embodiments may be modified in various different forms, and the scope of the technical idea of ​​the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely convey the technical idea of ​​the present invention to those skilled in the art. Like reference numerals throughout this specification denote like elements. Furthermore, various elements and areas in the drawings are schematically drawn. Therefore, the technical idea of ​​the present invention is not limited by the relative sizes or intervals drawn in the attached drawings.

[0022] The automotive industry is currently pursuing lightweight construction to address environmental regulations and improve fuel efficiency. To achieve this, the standard approach is to use high-strength materials and reduce thickness, leading to the adoption of high-strength steels. In particular, structural members and reinforcing materials form the structure of the car body and require superior crashworthiness to ensure the safety of drivers and passengers in the event of an impact. Therefore, ensuring high yield ratios and yield strengths can enhance crashworthiness.

[0023] The technical idea of ​​the present invention is to provide a cold rolled steel and a manufacturing method thereof, which improves the yield ratio (=yield strength / tensile strength) of the steel by forming fine precipitates during a heat treatment process after cold rolling. Conventional precipitation-strengthened steels have coiling temperatures exceeding 620°C after hot rolling, thereby maximizing the precipitation of carbonitride elements during the hot rolling process. However, when precipitates are formed during hot rolling, they coarsen during cold rolling, resulting in a decrease in the yield ratio.

[0024] In the present invention, a cold-rolled precipitation hardening steel with a recovery annealing process is proposed, which has a yield strength of 600 to 900 MPa and a yield ratio of 95% or higher, and is suitable for bending processing characteristics. As described above, this can be applied to parts such as seat rails and members that require high yield strength. Homogeneous material properties can be achieved with a ferrite single-phase microstructure, and it can show a positive effect on hole expansion performance in particular. In addition, a high yield ratio of 95% or higher can bring about optimal conditions for cold forming such as roll forming and bending.

[0025] Hereinafter, cold rolled steel according to the technical idea of ​​the present invention will be described in detail.

[0026] A cold rolled steel according to an embodiment of the present invention contains, in wt%, 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% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% to 0.1% or less, nitrogen (N): more than 0% to 0.005% or less, and the remainder is iron (Fe) and other unavoidable impurities.

[0027] Hereinafter, the role and content of each component included in the cold rolled steel according to the present invention will be described. In this case, the content of each component element all refers to the weight % of the entire steel.

[0028] Carbon (C): 0.04% ~ 0.07%

[0029] Carbon (C) is added to secure the strength of steel and control the microstructure. If the content is less than 0.04%, it is difficult to secure the target yield strength because the precipitation effect is not sufficient, and the MC (M=Nb, Ti) carbide becomes coarse, which reduces the grain refinement effect. If it exceeds 0.07%, the material properties, such as strength decrease due to the increase in the fraction of phases such as pearlite, may deteriorate. In addition, as the carbon content increases, the pearlite structure increases, and when annealed at high temperatures after cold rolling, some pearlite structures may transform to austenite, which can induce a two-phase structure. Therefore, it is recommended to add carbon in a content of 0.04% to 0.07%.

[0030] Silicon (Si): 0.1% to 0.3%

[0031] Silicon not only contributes to increasing strength as a solid solution strengthening element, but also acts as a ferrite stabilizing element, increasing the degree of supercooling during ferrite transformation, thereby suppressing the formation of harmful carbides, refining grains, and suppressing the formation of pearlite, thereby enhancing the reactivity with solid solution C and M (M = Nb, Ti). When the silicon content is less than 0.1%, the effect of adding silicon 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 sheet, and increases the carbon equivalent, which may lower weldability. Therefore, it is preferable to add silicon in an amount of 0.1% to 0.3% of the total weight of the steel. Therefore, it is preferable to add silicon in an amount of 0.1% to 0.3%.

[0032] Manganese (Mn): 1.1% ~ 1.5%

[0033] Manganese is a solid-solution strengthening element that not only contributes to increasing strength, but also controls strength, toughness, and yield ratio depending on the content. However, when added in large quantities, it causes the formation of MnS inclusions and center segregation during casting, which reduces the toughness of the steel. If the Mn content is less than 1.1%, it is difficult to secure the target yield strength of 800 MPa, and if it exceeds 1.5%, it is advantageous for securing the target strength, but it causes the formation of inclusions or segregation, which reduces workability and delayed fracture resistance, and increases the carbon equivalent, which can reduce the weldability, which is the main advantage of precipitation hardening steel. In addition, it can induce transformation structure during annealing and cooling at high temperatures after cold rolling. Therefore, it is desirable to add Mn in a content of 1.1 to 1.5%.

[0034] Aluminum (Al): 0.015% to 0.06%

[0035] Aluminum is used as a deoxidizer and can help purify ferrite. If the aluminum content is less than 0.015%, the effect of addition is insufficient. If it exceeds 0.06%, AlN may form during slab manufacturing, which can cause cracks during casting or hot rolling. Therefore, it is recommended to add aluminum at a level of 0.015% to 0.06% of the total weight of the steel.

[0036] Phosphorus (P): 0% or more to 0.02% or less

[0037] Phosphorus is an impurity contained during the steel manufacturing process. While it can help improve strength through solid solution strengthening, high levels can cause low-temperature embrittlement and brittle fracture. Therefore, it is recommended to limit the phosphorus content to between 0% and 0.02% of the total steel weight.

[0038] Sulfur (S): 0% or more to 0.005% or less

[0039] Sulfur is an impurity contained during the steel manufacturing process, and can form non-metallic inclusions such as FeS and MnS, thereby reducing toughness and weldability. Therefore, it is desirable to limit the sulfur content to between 0% and 0.005% of the total steel weight.

[0040] Niobium (Nb): 0.02% ~ 0.04%

[0041] Niobium is a strong carbonitride-forming element. It combines with carbon and nitrogen contained in steel during hot rolling and annealing (annealing) processes to form carbides or nitrides. These Nb-based carbides or nitrides suppress recrystallization and grain growth during the annealing process after cold rolling, thereby refining the grains and improving both the strength and toughness of the steel. When the Nb content is less than 0.02%, it may be difficult to obtain the precipitation strengthening effect, and when it exceeds 0.04%, the annealing time must be extended or the annealing temperature must be raised to secure the elongation of the steel due to the recrystallization delay effect of Nb. Therefore, it is desirable to add the content in the range of 0.02 to 0.04%.

[0042] Titanium (Ti): 0.17% ~ 0.23%

[0043] Titanium is a strong carbonitride-forming element. It combines with carbon and nitrogen contained in steel during hot rolling and annealing (annealing) processes to form carbides or nitrides. These Ti-based carbides or nitrides suppress recrystallization and grain growth during the annealing process after cold rolling, thereby refining the grains and improving both the strength and toughness of the steel. When the Ti content is less than 0.17%, it may be difficult to obtain sufficient recrystallization delay effect and precipitation strengthening effect, and when it exceeds 0.23%, due to the recrystallization delay effect of Ti, the annealing time must be extended or the annealing temperature must be increased to secure the elongation of the steel. Therefore, it is desirable to add the content in the range of 0.17 to 0.23%.

[0044] Chromium (Cr): 0% or more but less than 0.1%

[0045] It is preferable to add chromium (Cr) in an amount of 0.1% or less of the total weight of the precipitation hardening steel sheet according to the present invention. If the chromium content exceeds 0.1% of the total weight of the steel sheet, there is a problem of deteriorating weldability or heat affected zone (HAZ) toughness.

[0046] Nitrogen (N): More than 0% and less than or equal to 0.005%

[0047] Nitrogen (N) is an element that inevitably remains in steel during the steelmaking process. Compared to carbon, it has a faster diffusion rate and low activation energy, so even small changes in amount cause significant material deviation and accelerate aging. Therefore, it must be precipitated in the form of nitrides. In the present invention, if the nitrogen (N) content exceeds 0.005%, a large amount of TiN is formed, which has a negative effect on the formability of the steel. It is best to minimize it as much as possible, but since it is an element that inevitably remains during the steelmaking process, it should not exceed 50 ppm.

[0048] The cold rolled steel according to the present invention may have a sum of niobium (Nb) and titanium (Ti) of 0.26 wt% or less, for example, 0.20 to 0.26 wt%. If the sum of niobium (Nb) and titanium (Ti) is less than 0.20 wt%, it may be difficult to obtain a sufficient recrystallization delay effect and precipitation strengthening effect, and if the sum of niobium (Nb) and titanium (Ti) exceeds 0.26 wt%, the annealing time may be extended or the annealing temperature may be increased to secure the elongation of the steel due to the recrystallization delay effect.

[0049] Furthermore, the ratio of the weight % of carbon (C) to the sum of the weight % of niobium (Nb) and titanium (Ti) (=[C] / ([Nb] + [Ti])) may have a value less than 0.23. If the value of [C] / ([Nb] + [Ti]) is 0.23 or more, a problem may arise in which the cementite fraction is present at 1% or more.

[0050] The remaining component of the above cold-rolled steel is iron (Fe). However, during the typical steelmaking process, unintended impurities from raw materials or the surrounding environment can inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the typical manufacturing process, their full content is not specifically addressed in this specification.

[0051] Meanwhile, the cold rolled steel according to the present invention may further include, in weight %, boron (B): more than 0% and 0.001% or less and / or molybdenum (Mo): more than 0% and 0.06% or less.

[0052] Boron (B): More than 0% and less than or equal to 0.001%

[0053] Boron (B) is added to prevent secondary processing embrittlement that may occur due to phosphorus (P) addition. In general, boron is preferably added in a range of 0.001% or less, as segregation may cause material deviation when added in amounts exceeding 0.001%.

[0054] Molybdenum (Mo): More than 0% 0.06%

[0055] Molybdenum acts when niobium and titanium combine with carbon to form precipitates, slowing the formation and growth of fine precipitates and enhancing yield strength. Molybdenum levels exceeding 0.06% are detrimental to the steel's performance, reducing price competitiveness. Therefore, it is recommended to keep the molybdenum content below 0.06%.

[0056] By controlling the specific components of the alloy composition described above and the content range thereof, and by using the manufacturing method described below, cold-rolled steel can satisfy, for example, a yield strength (YS): 600 MPa to 900 MPa, an elongation (EL): 8% or more, and a yield ratio (YR): 95% or more.

[0057] The cold rolled steel may include precipitates formed at grain boundaries, within grains, or both. The precipitates mainly include Ti, Nb(C) type precipitates. In addition, TiN, AlN, etc. may be included. The precipitates may mainly have an average grain size of 2 to 6 nm. The precipitates may be formed during an annealing heat treatment process after cold rolling. Due to these precipitates, the cold rolled steel may have high strength and a high yield ratio at the same time. For example, after some of the Ti, Nb(C) is formed during the hot rolling and coiling processes, 30 to 70% of the remaining Ti, Nb elements serve as nucleation sites for a large amount of dislocations generated during cold rolling and are precipitated as Ti, Nb(C) during the annealing process.

[0058] Hereinafter, a method for manufacturing cold rolled steel according to the present invention will be described with reference to the attached drawings.

[0059] Manufacturing method of cold rolled steel

[0060] FIG. 1 is a drawing illustrating temperature over time and structural changes of the steel at each stage in a method for manufacturing cold-rolled steel according to an embodiment of the present invention.

[0061] The above steel contains, in wt%, 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% to 0.02%, sulfur (S): more than 0% to 0.005%, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% to 0.1% or less, nitrogen (N): more than 0% to 0.005% or less, and the remainder is iron (Fe) and other unavoidable impurities.

[0062] A method for manufacturing cold rolled steel according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), a first cooling step (S130), a coiling step (S140), a cold rolling step (S150), an annealing heat treatment step (S160), and a second cooling step (S170).

[0063] Reheating step (S110)

[0064] In the manufacturing method according to the present invention, the semi-finished product subject to the hot rolling process may be, for example, a slab. The slab in semi-finished form can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0065] In the reheating step (S110), the steel having the above composition, for example, a slab plate, is reheated at a reheating temperature (Slab Reheating Temperature, SRT) of 1,200°C to 1,250°C for 40 minutes to 1 hour. Through this reheating, the components segregated during casting and the precipitates can be re-dissolved, and in particular, a sufficiently high temperature is required to re-dissolve precipitation-hardening elements such as niobium to form fine precipitates during the annealing heat treatment after cold rolling. The fine precipitates can hinder the growth of crystal grains, thereby achieving grain refinement, thereby increasing the strength. If the reheating temperature is less than 1,200℃ or the reheating time is less than 40 minutes, carbonitrides do not dissolve and coarsen, making it difficult to obtain material properties higher than the desired yield ratio. For example, maximum re-dissolution of precipitation hardening elements such as niobium contained in the steel may occur, and components segregated during casting may not be sufficiently evenly distributed. If the reheating temperature exceeds 1,250℃ or the reheating time exceeds 1 hour, austenite grains may coarsen, which may cause a decrease in the yield strength. In addition, as the reheating temperature increases, there are problems such as increased manufacturing costs and decreased productivity due to heating costs and additional time required to match the hot rolling temperature.

[0066] Hot rolling stage (S120)

[0067] The above-mentioned heated steel is first heated to adjust its shape and then hot-rolled. The hot-rolling may be performed sequentially through width rolling, roughing milling, and finishing milling. Through the hot-rolling step, the steel may be formed into hot-rolled steel. The hot-rolled steel may be a hot-rolled steel sheet. The reduction ratio of the hot-rolling may be 50% or more. During the hot-rolling process, work hardening (strain) may occur, and further, the work hardening may be removed through recrystallization and recovery.

[0068] The above hot rolling, i.e., the above-mentioned finishing rolling, may be completed at a finish rolling temperature (FRT) of Ar3 or higher, for example, Ar3 or higher. If the finish rolling temperature is lower than Ar3 (e.g., lower than 870°C), rolling may occur in an ideal region of austenite and ferrite, causing grain mixing, resulting in uneven deformability and thus a reduction in rollability. If the finish rolling temperature exceeds Ar3 + 40°C (e.g., exceeds 910°C), grains may become coarser, which may result in a reduction in the strength of the final steel.

[0069] 1st cooling stage (S130)

[0070] The primary cooling step (S130) can be performed on a run-out table (ROT). The run-out table (ROT) is a device for cooling steel sheets whose thickness and width have been determined by rough rolling and finish rolling. The run-out table is provided with multiple banks having multiple headers equipped with a row of nozzles, one above the other, on the steel sheet. A coiler is provided at the rear end of the run-out table to coil the steel sheet into a coil, 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 is first cooled to 580°C to 620°C at a cooling rate of 10°C / sec to 100°C / sec. The cooling can be performed by air cooling or water cooling. If the cooling rate is less than 10°C / sec, the average particle size of the precipitate increases, making it difficult to secure strength. Conversely, if the cooling rate exceeds 100℃ / sec, the steel structure may become hardened and the impact toughness may decrease.

[0071] Winding stage (S140)

[0072] After the primary cooling is completed, the steel 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 the precipitates fine and to minimize the precipitate fraction. If the coiling temperature is lower than 580°C, it may be difficult to control the shape of the hot-rolled steel, and the surface quality of the steel may deteriorate due to the sharp difference between the finishing rolling temperature and the coiling temperature. If the coiling temperature exceeds 620°C, it may contribute to increasing the strength of the hot-rolled steel by improving the precipitate fraction, but it is not desirable from the perspective of the final cold-rolled steel material because the precipitates become coarse. That is, if the coiling temperature exceeds 620°C, the carbonitride element cannot be maintained in a solid solution state, and may be formed as unwanted precipitates.

[0073] Looking at the structural changes of the steel in each of the hot rolling stage (S120), first cooling stage (S130), and coiling stage (S140) illustrated in Fig. 1, the SIP item refers to strain-induced precipitation, and specifically refers to precipitation caused by strain that occurs during the rolling process. PIP refers to phase-induced precipitation, and specifically refers to precipitation that occurs during the transformation process from austenite to ferrite. In addition, the Random item refers to precipitates that are widely precipitated within the ferrite structure during the cooling process after coiling.

[0074] Meanwhile, the γ→ α phase transformation refers to the phase transformation from austenite to ferrite.

[0075] Cold rolling stage (S150)

[0076] The above-mentioned coiled hot-rolled steel is subjected to a pickling treatment to clean it with acid. Subsequently, the pickled hot-rolled steel is cold-rolled at a cold reduction ratio of 50% to 80% to form cold-rolled steel. If the cold reduction ratio is less than 50%, the amount of nuclei generated for recrystallization during annealing is small, so that the crystal grains grow excessively during the annealing heat treatment described later, which may rapidly reduce the strength. As the cold reduction ratio increases beyond 50%, the sites where fine precipitates can be generated increase, which is desirable for improving the yield ratio. However, if it exceeds 80%, the amount of nuclei generated becomes excessively large, so that the crystal grains formed by annealing are rather too fine, which may reduce the ductility and deteriorate the formability.

[0077] Annealing heat treatment step (S160)

[0078] The above cold-rolled steel is heat-treated in a continuous annealing furnace with a conventional slow-cooling section. When continuously annealing the cold-rolled sheet, recovery recrystallization annealing is performed at a sheet speed of 50 to 200 mpm and an annealing temperature of 740 to 830°C. The above heat treatment may be referred to as annealing heat treatment. The annealing heat treatment is performed by heating at a heating rate in the range of 1°C / sec to 10°C / sec and maintaining the heat treatment at a temperature in the range of 740°C to 830°C for 30 to 1000 seconds. At this time, the annealing heat treatment is one of the important process variables that determine the material of the final product. This recovery recrystallization annealing heat treatment can realize a high yield ratio of the cold-rolled steel by creating a very fine structure in the end and fine precipitates of solid-solution carbonitride elements. The carbonitride may be, for example, NbC precipitates and AlN precipitates. If the annealing heat treatment temperature is lower than 740°C, recrystallization may not be sufficiently completed, and the target elongation may not be achieved. If the annealing heat treatment temperature exceeds 830°C, the grains may coarsen, resulting in a decrease in the yield strength and yield ratio. Therefore, it is desirable to limit the annealing temperature to the above range to secure a yield strength of 600 to 900 MPa and a yield ratio of 95% or higher.

[0079] Meanwhile, the sheet conveying speed during continuous annealing of cold-rolled sheets is typically 90 to 110 mpm, which is the speed used to produce steel with a thickness of 1.0 to 1.5 tons at a typical annealing temperature of 760 to 800°C for recrystallization of steel during continuous galvanizing line (CGL) production. In the present invention, since the recrystallization fraction of steel is distributed in the range of 31 to 81%, the range of sheet conveying speed can be further expanded to 50 to 200 mpm accordingly.

[0080] Secondary cooling stage (S170)

[0081] The cold rolled steel subjected to the above annealing heat treatment is cooled at a cooling rate of 5°C / sec to 100°C / sec, for example, to a temperature in the range of 100 to 300°C.

[0082] Cold rolled steel can be manufactured through the above process (S110 to S170), and the cold rolled steel can be a cold rolled steel sheet.

[0083] If necessary, the cold rolled steel may be formed into a hot-dip galvanized steel and an alloyed hot-dip galvanized steel. The hot-dip galvanized steel may be formed by immersing the cold rolled steel in a hot-dip galvanized bath to form a hot-dip galvanized layer. The temperature of the plating bath may range from 400°C to 520°C depending on the type and ratio of alloy elements for forming the plating layer and the composition of the cold rolled steel. Under the plating bath conditions, a hot-dip galvanized layer may be 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 may be formed by cooling to room temperature at a cooling rate of 1°C / sec to 100°C / sec.

[0084] The above-mentioned alloyed hot-dip galvanized steel can be obtained by alloying heat-treating cold-rolled steel having the above-mentioned hot-dip galvanized layer formed thereon. The alloying heat-treating can be performed at a temperature in the range of 500°C to 620°C for 10 to 60 seconds. When the alloying heat-treating is performed under the above-mentioned conditions, the hot-dip galvanized layer can grow stably, and the adhesion of the plating layer can be excellent. When the alloying heat-treating temperature is less than 500°C, alloying may not progress sufficiently, and the soundness of the hot-dip galvanized layer may deteriorate. When the alloying heat-treating temperature exceeds 620°C, it may pass into an abnormal temperature range, causing a change in material properties. Thereafter, the alloyed hot-dip galvanized steel can be formed by cooling.

[0085] In the above reheating zone, precipitation hardening elements such as niobium are re-dissolved. In the above hot rolling zone, precipitation phases such as niobium carbide may be formed within the austenite grains. In the primary cooling zone and the coiling zone, precipitation phases such as niobium carbide are formed within the ferrite grains, but the amount of the precipitation phase may be reduced by the low coiling temperature. In the cold rolling zone, niobium may be dissolved again in the matrix. In the annealing heat treatment zone, precipitation phases such as niobium carbide may be formed within the ferrite grains. That is, by minimizing the amount of precipitation phases formed in the hot rolling process and inducing the formation of precipitation phases in the cold rolling process, the size of the precipitation phases may be refined. Accordingly, a high yield ratio and a high yield strength may be achieved.

[0086] Cold rolled steel produced 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.

[0087] The cold rolled steel produced by applying the above-described alloy composition and process conditions has a yield strength (unit: MPa) and yield ratio (unit: %) that are approximately related to the following formula depending on the annealing heat treatment temperature (unit: ℃), the annealing heat treatment holding time (unit: sec) in the annealing heat treatment step (S160), and the titanium content (unit: weight%) that constitutes the steel.

[0088] Equation 1:

[0089] 4286.5605 Ti(wt%) - 3.3482 SS(℃) - 0.2952 t(s)+2474.7647 = YP (MPa)

[0090] Equation 2:

[0091] 367.6854 Ti(wt%) - 0.1819 SS(℃) - 0.0141 t(s)+159.9337 = Yield ratio(%)

[0092] In Equations 1 and 2, Ti represents the titanium content (unit: wt%), SS represents the annealing heat treatment temperature (unit: ℃) in the annealing heat treatment step (S160), t represents the annealing heat treatment holding time (unit: seconds) in the annealing heat treatment step (S160), and YP represents the yield strength (unit: MPa).

[0093] The cold rolled steel may include precipitates formed at grain boundaries, within grains, or both. The precipitates mainly include Ti, Nb(C) type precipitates. In addition, TiN, AlN, etc. may be included. The precipitates may mainly have an average grain size of 2 to 6 nm. The precipitates may be formed during an annealing heat treatment process after cold rolling. Due to these precipitates, the cold rolled steel may have high strength and a high yield ratio at the same time. For example, after some of the Ti, Nb(C) is formed during the hot rolling and coiling processes, 30 to 70% of the remaining Ti, Nb elements serve as nucleation sites for a large amount of dislocations generated during cold rolling and are precipitated as Ti, Nb(C) during the annealing process.

[0094] Experimental example

[0095] Below, preferred experimental examples are presented to aid understanding of the present invention. However, these examples are provided solely to aid understanding of the present invention and are not intended to limit the present invention. Any details not described herein are technically feasible for those skilled in the art, and therefore, their description will be omitted.

[0096] Steel having the composition shown in Table 1 below was prepared, and cold-rolled steel sheets according to examples and comparative examples were prepared through hot-rolling and cold-rolling processes as described above.

[0097] Table 1 shows the composition of cold-rolled steels of examples and comparative examples. The content unit of each component is weight %, and the remainder is iron (Fe).

[0098] 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

[0099] Referring to Table 1, steel grade A satisfies the composition range 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% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% to 0.1% or less, nitrogen (N): more than 0% to 0.005% or less, and the remainder iron (Fe). In addition, steel grade A has a sum of niobium (Nb) and titanium (Ti) of 0.26 wt% or less, specifically, satisfies the range of 0.20 to 0.26 wt%. Furthermore, the ratio of the value of the weight% of carbon (C) to the sum of the weight% of niobium (Nb) and titanium (Ti) (= [C] / ([Nb] + [Ti])) has a value less than 0.23. In contrast, steel grades B and C do not satisfy the range of titanium (Ti): 0.17% to 0.23% and fall below it. In addition, steel grades B and C do not satisfy the range of 0.20 to 0.26 wt% for the sum of niobium (Nb) and titanium (Ti), and the ratio of the value of the wt% of carbon (C) to the sum of the wt% of niobium (Nb) and titanium (Ti) (= [C] / ([Nb] + [Ti])) does not satisfy the range of less than 0.23, and exceeds it.

[0100] Figure 2 is a graph showing the relationship between temperature and austenite fraction for the steel grades of the experimental examples shown in Table 1, using thermodynamic equilibrium calculations. In Figure 2, A refers to steel grade A in Table 1, B refers to steel grade B in Table 1, and C refers to steel grade C in Table 1.

[0101] Referring to Fig. 2, for steel type A, it can be confirmed that the austenite fraction is almost 0% up to around 820℃ when calculating the thermodynamic equilibrium. Since the probability of martensite transformation during cooling after austenite formation is high, it may be necessary to use steel type A to maintain a single-phase ferrite structure. However, it can be considered that when heating under actual annealing conditions, the results may be delayed compared to the thermodynamic equilibrium calculation results, so a lower austenite transformation fraction may appear.

[0102] Table 2 shows the process conditions for forming cold-rolled steel for examples and comparative examples. Process conditions not listed in Table 2 are the same for the comparative examples and examples, and have the same values ​​within the range of the process conditions of the present invention described above.

[0103] ClassificationSteel gradeHot rolled, cold rolled and annealed materialHeating temperature (℃)FDT (℃)CT (℃)Cold rolling reduction ratio (%)Annealing temperature (℃)Annealing time (sec)Inventive material 1A122288560168740300Inventive material 2A122288560168740500Inventive material 3A1222885601687401000Inventive material 4A122288560168770100Inventive material 5A12228856016880030Inventive material 6A12228856016880050Inventive material 7A122288560168800100Inventive material 8A12228856016883030Comparative material 3B12219236056874050Comparative material 4B12219236056877050Comparative material 5B12219236056880050Comparative material 6B12219236056883050Comparative material 7C12238926026874050Comparative material 8C12238926026877050Comparative material 9C12238926026880050Comparative material 10C12238926026883050

[0104] Referring to Table 2, inventive materials 1 to 8 have the composition of steel grade A in Table 1, and satisfy all of the process conditions of reheating temperature: 1,200℃ to 1,250℃, finish rolling temperature (FDT): 870 to 910℃, coiling temperature (CT): 580℃ to 620℃, cold rolling reduction ratio: 50 to 80%, annealing temperature: 740 to 830℃, and annealing time: 30 seconds to 1,000 seconds. In contrast, comparative materials 3 to 6 have the composition of steel grade B in Table 1, and do not satisfy but exceed the range of finish rolling temperature (FDT): 870 to 910℃. Comparative materials 7 to 10 have the composition of steel grade C in Table 1.

[0105] Table 3 shows the material properties of cold-rolled steels of examples and comparative examples as experimental examples of the present invention. Figure 3 is a graph showing the relationship between stress and strain of cold-rolled steels of examples and comparative examples as experimental examples of the present invention.

[0106] Yield strength (MPa) Yield ratio (%) Elongation (%) Inventive material 186699.88.6 Inventive material 277398.712.2 Inventive material 367995.214.1 Inventive material 477495.09.4 Inventive material 580899.511.0 Inventive material 670596.010.2 Inventive material 768695.913.3 Inventive material 866695.512.2 Comparative material 385391.94.6 Comparative material 459679.99.4 Comparative material 543069.015.1 Comparative material 633963.219.2 Comparative material 736164.116.6 Comparative material 827656.327.2 Comparative material 924352.629.0 Comparative material 1022450.133.1

[0107] Referring to Table 3 and Figure 3, inventive materials 1 to 8 all satisfy the following conditions: yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or more, and yield ratio (YR): 95% or more. In contrast, comparative material 3 does not satisfy the following conditions: elongation (EL): 8% or more and yield ratio (YR): 95% or more, and falls below these values. Comparative materials 4 to 10 do not satisfy the following conditions: yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or more, and falls below these values. Furthermore, referring to Tables 1 to 3, it can be confirmed that inventive materials 1 to 8 have the following correlations of Equations 1 and 2. In Equations 1 and 2, Ti represents the titanium content (unit: wt%), SS represents the annealing heat treatment temperature (unit: ℃) in the annealing heat treatment step (S160), t represents the annealing heat treatment holding time (unit: sec) in the annealing heat treatment step (S160), and YP represents the yield strength (unit: MPa). The calculated and actual values ​​(Table 3) for each equation are very similar, and R in the correlation equation 2 The values ​​are 0.922 for Equation 1 and 0.962 for Equation 2, which are very close relationships. In the correlation equation, R 2 It indicates the degree of dispersion in the results according to the regression analysis method and is used as a criterion for judging the model fit of the regression analysis.

[0108] Equation 1:

[0109] 4286.5605 Ti(wt%) - 3.3482 SS(℃) - 0.2952 t(s)+2474.7647 = YP (MPa)

[0110] Equation 2:

[0111] 367.6854 Ti(wt%) - 0.1819 SS(℃) - 0.0141 t(s)+159.9337 = Yield ratio(%)

[0112] Figure 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 Figure 4, A means steel grade A of Table 1, B means steel grade B of Table 1, and C means steel grade C of Table 1.

[0113] Referring to Figure 4, in the graph of the relationship between yield strength and elongation, steel grade A exhibits different characteristics from steel grades B and C (convergence). That is, it can be confirmed that steel grade A is superior to steel grades B and C in both strength and elongation.

[0114] Meanwhile, the inventors of the present invention were able to confirm that as the Ti content increases, the hardness value increases under all conditions of the temperature (740 to 830°C) and process time (30 to 1000 seconds) of the annealing process.

[0115] Figures 5 to 20 are photographs of the microstructure of the steel materials disclosed in Tables 2 and 3.

[0116] In this experimental example, the microstructure of the steel was observed and the ferrite recrystallization fraction X(t) was measured.

[0117] The ferrite recrystallization fraction X(t) can be expressed by the following equation.

[0118]

[0119] Here, X(t) is the ferrite recrystallization fraction, and Hv rex is a hardness measurement of the ferrite structure after recrystallization, HV0 is a hardness measurement of the ferrite structure before annealing, and HV t is a hardness measurement of the ferrite structure under the specific conditions.

[0120] Fig. 5 is a microstructure of the invention material 1 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 1.92 ㎛, and the ferrite recrystallization fraction (X) was measured to be 33%.

[0121] Fig. 6 shows the microstructure of the invention material 2 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 2.1 ㎛, and the ferrite recrystallization fraction (X) was measured to be 51%.

[0122] Fig. 7 shows the microstructure of invention material 3 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 3.49 ㎛, and the ferrite recrystallization fraction (X) was measured to be 83%.

[0123] Fig. 8 is a microstructure of the invention material 4 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 2.16 ㎛, and the ferrite recrystallization fraction (X) was measured to be 52%.

[0124] Fig. 9 shows the microstructure of the invention material 5 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 1.76 ㎛, and the ferrite recrystallization fraction (X) was measured to be 31%.

[0125] Fig. 10 shows the microstructure of the invention material 6 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 1.92 ㎛, and the ferrite recrystallization fraction (X) was measured to be 58%.

[0126] Fig. 11 shows the microstructure of invention material 7 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 2.22 ㎛, and the ferrite recrystallization fraction (X) was measured to be 68%.

[0127] Fig. 12 shows the microstructure of the invention material 8 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 2.13 ㎛, and the ferrite recrystallization fraction (X) was measured to be 81%.

[0128] Fig. 13 shows the microstructure of Comparative Material 3 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the recovery stage, the average grain size was 1.92 ㎛, and the ferrite recrystallization fraction (X) was measured to be 27%.

[0129] Fig. 14 shows the microstructure of Comparative Material 4 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 2.32 ㎛, and the ferrite recrystallization fraction (X) was measured to be 57%.

[0130] Fig. 15 shows the microstructure of Comparative Material 5 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 4.33 ㎛, and the ferrite recrystallization fraction (X) was measured to be 81%.

[0131] Fig. 16 shows the microstructure of Comparative Material 6 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the recrystallization stage, the average grain size was 5.84 ㎛, and the ferrite recrystallization fraction (X) was measured to be 89%.

[0132] Fig. 17 shows the microstructure of Comparative Material 7 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, the average grain size was 2.11 ㎛, and the ferrite recrystallization fraction (X) was measured to be 43%.

[0133] Fig. 18 is a microstructure of Comparative Material 8 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the recrystallization stage, the average grain size was 6.55 ㎛, and the ferrite recrystallization fraction (X) was measured to be 90%.

[0134] Fig. 19 is a microstructure of Comparative Material 9 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the partial recrystallization stage, and the average grain size was 9.42 ㎛, and the ferrite recrystallization fraction (X) was measured to be 92%.

[0135] Fig. 20 is a microstructure of Comparative Material 10 of Tables 2 and 3, in which a single-phase ferrite structure was observed in the recrystallization stage, the average grain size was 8.03 ㎛, and the ferrite recrystallization fraction (X) was measured to be 95%.

[0136] So far, cold-rolled steel and its manufacturing method according to the technical concept of the present invention have been described. As described above, according to one embodiment of the present invention, it can be confirmed that a recovery annealing steel having a yield strength of 600 to 900 MPa and a yield ratio of 95% or more, with a ferrite single-phase structure, can be provided so as to optimize the Ti component to be suitable for bending processing characteristics.

[0137] It will be apparent to a person skilled in the art to which the technical idea of ​​the present invention pertains that the technical idea of ​​the present invention described above is not limited to the above-described embodiments and the attached drawings, and that various substitutions, modifications, and changes are possible within a scope that does not depart from the technical idea of ​​the present invention.

Claims

1. 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% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% to 0.1% or less, nitrogen (N): more than 0% to 0.005% or less, and the remainder contains iron (Fe) and other unavoidable impurities. Yield strength (YS): 600 MPa to 900 MPa, elongation (EL): 8% or more, and yield ratio (YR): 95% or more, satisfying Cold rolled steel.

2. In paragraph 1, In weight %, further comprising boron (B): more than 0% but not more than 0.001% or molybdenum (Mo): more than 0% but not more than 0.06%, Cold rolled steel.

3. In paragraph 1, The above cold rolled steel has a ferrite single phase structure and includes precipitates within the ferrite single phase structure. Cold rolled steel.

4. A step of reheating a steel material containing, 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): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.005% or less, niobium (Nb): 0.02% to 0.04%, titanium (Ti): 0.17% to 0.23%, chromium (Cr): more than 0% to 0.1% or less, nitrogen (N): more than 0% to 0.005% or less, and the remainder being iron (Fe) and other unavoidable impurities, at a temperature of 1,200°C to 1,250°C; A step of hot rolling the above-mentioned heated steel to a temperature of Ar3 or higher; A step of first cooling the hot-rolled steel material; A step of coiling the above-mentioned first-cooled steel at a coiling temperature of 580℃ to 620℃; A step of cold rolling the above-mentioned coiled steel to form cold rolled steel; A step of annealing the above cold rolled steel at an annealing temperature of 740°C to 830°C; and A step of secondary cooling of the above annealed cold-rolled steel; Method for manufacturing cold rolled steel.

5. In paragraph 4, The first cooling step of the above steel is: Cooling from 580℃ to 620℃ at a cooling rate of 10℃ / sec to 100℃ / sec. Method for manufacturing cold rolled steel.

6. In paragraph 4, The second cooling step of the above steel is: Cooling at a cooling rate of 5℃ / sec to 100℃ / sec, Method for manufacturing cold rolled steel.

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