High-strength cold rolled steel sheet and manufacturing method therefor

Abstract

According to one embodiment of the present invention, provided is a high-strength cold rolled steel sheet comprising, by wt%, 0.08-0.15% of carbon (C), 0.8-1.5% of silicon (Si), 2.0-3.0% of manganese (Mn), more than 0% and less than or equal to 1.0% of aluminum (Al), more than 0% and less than or equal to 0.02% of phosphorus (P), more than 0% and less than or equal to 0.01% of sulfur (S), more than 0% and less than or equal to 0.01% of nitrogen (N), 0.001-0.005% of boron (B), 0.1% or less (excluding 0%) in total of titanium (Ti) and / or niobium (Nb), and the balance of iron (Fe) and other inevitable impurities, wherein the final microstructure is composed of tempered martensite, bainite, fresh martensite, ferrite and retained austenite.

Description

High-strength cold-rolled steel sheet and its manufacturing method

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

[0002] High-strength steels for automotive applications are being developed to meet the dual needs of vehicle weight reduction due to strengthened energy resource and environmental regulations and enhanced crashworthiness due to strengthened safety regulations. Steel used in body structural components requires a tensile strength of 1 GPa. Furthermore, a high yield ratio (YS / TS) is essential for improving crashworthiness. To enhance crashworthiness and formability in parts, materials with high yield strength and excellent ductility are essential. High-strength steels used in complex-shaped components require steels with both excellent elongation and hole expandability. However, strength and elongation are trade-offs. As material strength increases, forming becomes more challenging. Therefore, extensive research is being conducted to develop high-strength steels with superior formability.

[0003] For example, there is DP (Dual Phase) steel that secures both strength and elongation by evenly distributing soft ferrite and hard martensite in the microstructure. However, DP steel has the characteristic of having a low yield ratio (YS / TS) due to the introduction of mobile dislocations in ferrite during martensitic transformation. TRIP (Transformation Induced Plasticity) steel is a steel that utilizes the phenomenon of retained austenite transforming into martensite during plastic deformation, and has the advantage of being able to utilize both the excellent formability properties of austenite and the properties of hard martensite after forming. TRIP steel has the characteristic of high elongation but low hole expandability value.

[0004] Therefore, a steel grade that has a high yield ratio, excellent elongation, and high hole expandability is required for ultra-high strength steel plates with a strength of 1 GPa or higher.

[0005] The technical problem to be achieved by the technical idea of ​​the present invention is to provide an ultra-high-strength cold-rolled galvanized steel sheet having excellent elongation and hole expandability and a method for manufacturing the same.

[0006] However, these tasks are exemplary and the technical idea of ​​the present invention is not limited thereto.

[0007] According to one aspect of the present invention, a high-strength cold-rolled steel sheet is provided.

[0008] According to one embodiment of the present invention, the cold rolled steel sheet contains, in wt%, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.01% or less, nitrogen (N): more than 0% and 0.01% or less, boron (B): 0.001% to 0.005%, a total of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities, and the final microstructure may be composed of tempered martensite, bainite, fresh martensite, ferrite, and retained austenite.

[0009] According to one embodiment, the high-strength cold-rolled steel sheet can satisfy the following equations 1) and 2).

[0010] Equation 1): 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B ≤ 1.2

[0011] Equation 2): Yield ratio (YR) × elongation (EL) × hole expandability (HER) ≥ 400

[0012] (Here, the element symbol in formula 1) represents the content of that element in weight%.)

[0013] In one embodiment, the high-strength cold-rolled steel sheet may have an area fraction of 60 to 80% of the total of the tempered martensite and bainite, less than 10% of the fresh martensite, an area fraction of the ferrite of 10 to 30%, and an area fraction of the retained austenite of 1 to 5%.

[0014] In one embodiment, the ratio of the fresh martensite (FM) and the sum of tempered martensite and bainite (TM+B) may be FM / (TM+B) < 0.25.

[0015] In one embodiment, the high-strength cold-rolled steel sheet may further contain, in weight percent, one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni), in a total amount of 0.5% or less (excluding 0%).

[0016] In one embodiment, the crystal grain size of the tempered martensite and bainite may be 5 μm or less.

[0017] In one embodiment, the cold rolled steel sheet may have a tensile strength (TS) of 980 MPa or more, and a product of the tensile strength (TS) and the hole expandability (HER) (TSХHER) of 50,000 MPa% or more.

[0018] In one embodiment, the titanium (Ti) may be contained in an amount of 0.015% or more and 0.04% or less, and the niobium (Nb) may be contained in an amount of 0.06% or more and 0.085% or less.

[0019] According to another aspect of the present invention, a method for manufacturing a high-strength cold-rolled steel sheet is provided.

[0020] According to one embodiment of the present invention, the method for manufacturing the high-strength cold-rolled steel sheet comprises: (a) a step of hot-rolling a steel material including, in wt%, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.01% or less, nitrogen (N): more than 0% to 0.01% or less, boron (B): 0.001% to 0.005%, a total of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; (b) a step of cold-rolling the hot-rolled steel material; (c) a step of annealing the cold-rolled steel; (d) a step of cooling the annealed steel to a temperature lower than or equal to Ms; and (e) a step of plating the cooled steel; wherein step (a) includes a step of performing at least one pass of rolling with a reduction ratio of 40% or more per pass, and rolling such that the total reduction ratio of the rolling of the first to third passes is higher than the total reduction ratio of the remaining passes, and step (d) may include a step of first cooling to a first cooling end temperature in a range of 620°C to 720°C at a first cooling rate; and a step of second cooling to a second cooling end temperature lower than or equal to Ms at a second cooling rate that is faster than the first cooling rate after the first cooling.

[0021] In one embodiment, the steel material can satisfy the following equation 1).

[0022] Equation 1): 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B ≤ 1.2

[0023] (Here, the element symbol in formula 1) represents the content of that element in weight%.)

[0024] According to one embodiment, the method for manufacturing the cold rolled steel sheet may further contain, in weight %, one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni), in a total amount of 0.5% or less (excluding 0%).

[0025] In one embodiment, the secondary cooling end temperature may be a temperature of Ms - 140°C or higher and Ms - 30°C or lower.

[0026] In one embodiment, the titanium (Ti) may be contained in an amount of 0.015% or more and 0.04% or less, and the niobium (Nb) may be contained in an amount of 0.06% or more and 0.085% or less.

[0027] In one embodiment, the first cooling rate may range from 3°C / sec to 20°C / sec.

[0028] In one embodiment, the second cooling rate may range from 15°C / sec to 100°C / sec.

[0029] In one embodiment, the step (a) may include: (a-1) a step of reheating the steel at 1150 to 1300°C; (a-2) a step of hot-rolling the steel under conditions where the finishing rolling temperature is 850 to 950°C; and (a-3) a step of coiling the steel at 400 to 600°C.

[0030] In one embodiment, after step (a-2) and before step (a-3), a step of cooling to 680°C or lower at a cooling rate of 80°C / s or higher may be performed.

[0031] According to the technical concept of the present invention, a steel sheet having both high strength and excellent ductility can be manufactured, making it suitable for use in automotive structural components. Furthermore, compared to conventional DP steel, the yield ratio is higher, resulting in superior hole expandability, and the production of cold-rolled steel sheets with superior crashworthiness and formability. The aforementioned effects of the present invention are illustrative and should not be construed as limiting the scope of the present invention.

[0032] FIG. 1 is a flowchart schematically showing a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention, and FIG. 2 is a drawing schematically showing a heat treatment including annealing, cooling, and plating processes in a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention.

[0033] Figure 3 is a graph showing the properties of a cold-rolled steel sheet according to an experimental example of the present invention.

[0034] 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.

[0035] In this specification and claims, phase fraction refers to the area ratio (area%) derived from a microstructure photograph using an image analyzer. Additionally, unless otherwise specified, the content or concentration of a specific component refers to weight%.

[0036] The technical idea of ​​the present invention is to provide a cold-rolled high-strength steel sheet having excellent formability, in which the tensile strength (TS) is 980 MPa or more and the yield ratio (YR) × elongation (EL) × hole expandability (HER) is ≥ 400, and a method for manufacturing such a cold-rolled steel sheet.

[0037] In addition, according to an embodiment of the present invention, a cold-rolled steel sheet having a product of tensile strength (TS) and hole expandability (HER) (TS × HER) of 50,000 MPa% or more can be provided.

[0038] Below, the alloy amount and suitable heat treatment conditions for securing the tensile strength, elongation, yield ratio, and hole expandability targeted in the present invention are described.

[0039] First, a high-strength cold-rolled steel sheet according to the technical idea of ​​the present invention will be described in detail.

[0040] A high-strength cold-rolled steel sheet according to one embodiment of the present invention contains, in wt%, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.01% or less, nitrogen (N): more than 0% to 0.01% or less, boron (B): 0.001% to 0.005%, the sum of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities. Optionally, one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni) may be further included in an amount of 0.5% or less (excluding 0%).

[0041] Hereinafter, the role and content of each component included in the high-strength cold-rolled steel sheet according to the present invention will be described. Here, the content of each component element all refers to the weight % of the entire steel sheet.

[0042] Carbon (C): 0.08% to 0.15%

[0043] Carbon is added to ensure the appropriate fraction and stability of retained austenite. The carbon content ranges from 0.08% to 0.15% by weight. If the carbon content is less than 0.08%, the residual austenite fraction in the final microstructure is insufficient, making it difficult to achieve the desired ductility and forming fresh martensite. On the other hand, if the carbon content exceeds 0.15%, the strength increases excessively and carbides are easily formed, which can have a negative impact on weldability.

[0044] Silicon (Si): 0.8% to 1.5%

[0045] Silicon, a ferrite-stabilizing element, delays the formation of carbides within ferrite and has a solid-solution strengthening effect. The silicon content ranges from 0.8% to 1.5% by weight. If the silicon content is less than 0.8%, it is difficult to achieve the aforementioned effects. Conversely, if the silicon content exceeds 1.5%, oxides such as Mn2SiO4 are formed during the manufacturing process, hindering plating properties and increasing the carbon equivalent, which can reduce weldability.

[0046] Manganese (Mn): 2.0% to 3.0%

[0047] Manganese has a solid-solution strengthening effect and enhances hardenability, contributing to increased strength. The manganese content ranges from 2.0% to 3.0% by weight. Below 2.0%, the effect is insufficient, making it difficult to secure strength. Above 3.0%, the formation or segregation of inclusions such as MnS can lead to reduced workability and delayed fracture resistance. Furthermore, the increased carbon equivalent can impair weldability.

[0048] Aluminum (Al): greater than 0% to 1.0%

[0049] Aluminum is used as a deoxidizer and can help purify ferrite. Aluminum content ranges from 0% to 1.0% by weight. Without aluminum, the deoxidation effect is insufficient. Conversely, if aluminum content exceeds 1.0%, AlN can form during slab production in the continuous casting process, potentially causing cracks during continuous casting or hot rolling.

[0050] Phosphorus (P): 0% or more but 0.02% or less

[0051] Phosphorus is an impurity contained in steel manufacturing processes. While it can help improve strength through solid solution strengthening, high levels can cause low-temperature embrittlement. Therefore, it is recommended that the phosphorus content be limited to 0.02% or less (weight percent).

[0052] Sulfur (S): 0% or more and 0.01% or less

[0053] Sulfur is an impurity contained in steel manufacturing processes. It 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 0.01% or less (weight percent).

[0054] Nitrogen (N): More than 0% and less than or equal to 0.01%

[0055] Nitrogen is an unavoidable element in steel manufacturing. If its content exceeds 0.01%, it can delay recrystallization and reduce elongation. Therefore, it is desirable to minimize its content as much as possible. Therefore, it is recommended to limit the nitrogen content to between 0% and 0.01% of the total weight of the steel sheet.

[0056] Boron (B): 0.001% to 0.005% or less

[0057] Boron is a quenching element that inhibits the formation of polygonal ferrite and facilitates the formation of fresh martensite. Boron content ranges from 0.001% to 0.005% by weight. Below 0.001%, the effect is minimal, while above 0.005%, workability deteriorates.

[0058] Total of titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%)

[0059] Titanium and niobium are strong carbonitride-forming elements. During hot rolling, they form fine precipitates with carbon and nitrogen present in steel, inhibiting grain growth and enhancing strength. The combined content of titanium and niobium should be within the range of 0.1% by weight. If neither titanium nor niobium is present in the steel, the precipitation strengthening effect is difficult to achieve. Conversely, if the content exceeds 0.1%, the strength becomes excessively high and ductility deteriorates.

[0060] At this time, when adding titanium, it is preferable to add the titanium content at 0.015% to 0.04%. In order to fully obtain the effect of boron, it is preferable to induce precipitation of TiN by adding titanium in a certain amount or more, because the effect of boron cannot be obtained if boron combines with nitrogen in the steel and is lost as BN. However, if the titanium content exceeds 0.04% at this time, defects such as nozzle clogging may occur due to the formation of coarse TiN, which may reduce continuous castability.

[0061] The remaining component of the above high-strength cold-rolled steel sheet 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.

[0062] Meanwhile, the high-strength cold-rolled steel sheet may optionally further include at least one selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni) in a total amount of 0.5% or less (excluding 0%) in weight percent.

[0063] Sum of one or more of molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni): 0.5% or less (excluding 0%)

[0064] Molybdenum enhances hardenability and inhibits pearlite formation. It also refines martensite. However, when molybdenum segregates at grain boundaries, ferrite grain growth is halted, reducing the ferrite fraction. To suppress this, molybdenum content should be limited to 0.50% or less.

[0065] Chromium, like manganese, has a solid-solution strengthening effect and contributes to increased strength by enhancing hardenability. When the chromium content exceeds 0.5%, hardenability increases, reducing the fraction of retained austenite and increasing the fraction of martensite, thus reducing ductility.

[0066] Copper is an effective element for ensuring corrosion resistance, blocking corrosive environments through surface enrichment. Adding copper in amounts exceeding 0.5% can increase the corrosion rate of corroded steel plates.

[0067] Nickel, when added together with copper in steel plates, enhances corrosion resistance. When nickel is added at levels below 0.5%, the density of the copper-concentrated layer within the steel plate increases, thereby enhancing corrosion resistance.

[0068] The steel sheet according to the present invention having the above alloy composition may have a carbon equivalent defined as 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B (wherein C, Si, Al, Mn, Ti, Nb, and B are weight ratios of carbon, silicon, aluminum, manganese, titanium, niobium, and boron, respectively) of 1.2 or less.

[0069] Microstructure of steel plate

[0070] The microstructure of a high-strength cold-rolled steel sheet according to the technical idea of ​​the present invention may include tempered martensite, bainite, fresh martensite, ferrite, and retained austenite. In this case, the combined area fraction of tempered martensite and bainite may be 60 to 80%, the area fraction of fresh martensite may be less than 10%, the area fraction of ferrite may be 10 to 30%, and the area fraction of the retained austenite may be 1 to 5%.

[0071] The above area fraction refers to the area fraction derived from an image of the microstructure of the steel obtained by electron backscatter diffraction (EBSD) using an image analyzer. EBSD specimens can be prepared by polishing a cold-rolled steel sheet to remove surface defects. The above area fraction can be derived by calculating the image area fraction of the image obtained by EBSD using field-effect scanning electron microscopy (FE-SEM).

[0072] Since the hardness of tempered martensite and bainite is higher than that of ferrite and lower than that of fresh martensite, when the total fraction of tempered martensite and bainite is secured at 60 to 80%, voids between the hard and soft phases are less likely to occur.

[0073] Fresh martensite is a hard phase and, because it is difficult to deform during shearing, can suppress the occurrence of defects in press-formed products. However, if the area fraction of fresh martensite exceeds 10%, voids are likely to form during processing, reducing workability. For this reason, the area fraction of fresh martensite is kept below 10%.

[0074] Ferrite is a soft phase, and it is effective to structure the metal structure with ferrite crystal grains that have excellent ductility and low dislocation density. To achieve this effect, the area ratio should be 10% or more. On the other hand, if the area ratio exceeds 30%, ferrite is easily deformed, which can cause defects during press processing. Therefore, the area ratio of ferrite is set at 10 to 30%.

[0075] The retained austenite phase transforms into martensite during processing such as punching, which is likely to result in poor hole expandability. Therefore, it is recommended to control the retained austenite phase to 5% or less. Furthermore, it is preferably 1% or more.

[0076] Furthermore, the grain size of tempered martensite and bainite can be less than 5 μm. In this case, the grain size is measured using EBSD as the equivalent diameter of a circle in a region surrounded by boundaries with an orientation difference of 10 degrees or more.

[0077] The ratio of the fresh martensite (FM) and the sum of tempered martensite and bainite (TM+B), FM / (TM+B), is set to less than 0.25. If the FM / (TM+B) value is 0.25 or more, voids between the hard phase and the soft phase can easily occur during processing of cold-rolled steel sheets, which may reduce processability.

[0078] The high-strength cold-rolled steel sheet of the present invention can be implemented as a cold-rolled high-strength steel sheet with excellent formability, having a tensile strength (TS) of 980 MPa or more and a yield ratio (YR) × elongation (EL) × hole expandability (HER) ≥ 400.

[0079] In addition, the cold-rolled steel sheet may preferably have a product of tensile strength (TS) and hole expandability (HER) (TS × HER) of 50,000 MPa% or more.

[0080] Hereinafter, with reference to the attached drawings, a method for manufacturing a high-strength cold-rolled steel sheet having the above-described composition range, which is a cold-rolled steel sheet according to the technical idea of ​​the present invention, will be described.

[0081] Manufacturing method of cold rolled steel sheet

[0082] 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.

[0083] Figure 1 is a flowchart showing step-by-step a method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention.

[0084] A method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention comprises the steps of hot-rolling a steel material having the above composition (S10); cold-rolling the hot-rolled steel material (S20); annealing the cold-rolled steel material (S30); first cooling the annealed steel material (S40); second cooling the steel material (S50); and plating the cooled steel material (S60).

[0085] Hot rolling stage

[0086] The steel slab having the above alloy composition is reheated to a temperature of 1150-1300℃. The slab is manufactured in the form of a semi-finished product by continuous casting of molten steel obtained through the steelmaking process, and the reheating process homogenizes the component segregation that occurred during the casting process and makes it ready for hot rolling. If the slab reheating temperature (SRT) is below 1150℃, there is a problem that the segregation of the slab is not sufficiently reused, and if it exceeds 1300℃, the size of the austenite grains increases, which may increase the process cost. The slab reheating can be carried out for 1 to 2 hours. If the reheating time is less than 1 hour, the segregation zone is not sufficiently reduced, and if it exceeds 2 hours, the grain size increases, which may increase the process cost.

[0087] After the above reheating, hot rolling including rough rolling and finish rolling is performed, and hot finish rolling is performed at a finish delivery temperature (FDT) in the range of, for example, 850°C to 950°C to manufacture a hot rolled steel sheet. If the finish rolling temperature is lower than 850°C, the rolling load increases rapidly, which reduces productivity, and if it exceeds 950°C, the grain size may increase, which may reduce strength.

[0088] The above rough rolling temperature is preferably set to be higher than the temperature (Tnr) at which austenite recrystallization stops, and can be performed at, for example, 1000°C to 1150°C. In addition, when rough rolling is performed in two or more passes, the reduction ratio of the last pass is preferably 40% or higher. Here, the reduction ratio (%) means {[thickness of material before rolling - thickness of material after rolling] / thickness of material before rolling} × 100.

[0089] During rough rolling, the grain size of the recrystallized structure due to the initial rolling is subject to high temperatures. However, when the final pass is performed, grain growth is delayed, so the reduction rate of the final pass significantly affects the grain size of the final microstructure. In addition, if the reduction rate of the final pass in rough rolling is low, sufficient strain is not transmitted to the core, which may lead to a decrease in toughness due to coarsening of the core. Therefore, it is desirable to set the reduction rate of the final pass to 40% or more for the purpose of refining austenite and reducing manganese segregation.

[0090] The above rough-rolled steel is subjected to finishing rolling in 5 to 7 passes. When performing finishing rolling, the reduction ratio of the first pass is set to 40% or more, and the total reduction ratio of the rolling of the 1st to 3rd passes is set higher than the total reduction ratio of the remaining passes. This is for the purpose of refining austenite grains and reducing manganese segregation. If the total reduction ratio of the initial 1st to 3rd passes is lower than the total reduction ratio of the remaining passes, coarse austenite grains are formed, making it impossible to obtain the desired microstructure, making it difficult to secure strength, and making it impossible to obtain a uniform microstructure.

[0091] After hot rolling, it is cooled to a temperature of 400-600℃ and then coiled. If the coiling temperature is below 400℃, the strength increases, which increases the rolling load during cold rolling. If it exceeds 600℃, ferrite and pearlite structures are excessively formed, which may result in poor hole expandability and may cause defects in subsequent processes due to surface oxidation, etc.

[0092] In the cooling process after the above hot rolling, cooling (rapid cooling) can be performed at a rapid cooling rate of 80°C / s or more from the finishing rolling end temperature to, for example, a temperature of 680°C, and then cooling (slow cooling) can be performed at a low cooling rate of 5°C / s or more to a coiling temperature of 600°C or lower.

[0093] The above rapid cooling step is intended to prevent excessive ferrite transformation. Furthermore, pearlite may be formed at temperatures above 680°C, and structural inhomogeneity may occur in subsequent processes, resulting in poor hole expandability. Therefore, it is preferable to apply the rapid cooling step up to a temperature of 680°C.

[0094] Cold rolling stage

[0095] The hot-rolled steel sheet is subjected to an acid pickling treatment to remove a surface scale layer. Subsequently, the hot-rolled steel sheet is cold-rolled at an average reduction ratio of, for example, 30% to 80% to form a cold-rolled steel sheet. The higher the average reduction ratio, the more effective the formability is due to the microstructure refinement effect. When the average reduction ratio is less than 30%, it is difficult to obtain a uniform microstructure. When the average reduction ratio exceeds 80%, the roll force increases, thereby increasing the process load. The structure of the cold-rolled steel sheet may have a structure in which the structure of the hot-rolled steel sheet is elongated.

[0096] After cold rolling is completed, the cold rolled steel sheet undergoes annealing heat treatment and plating processes.

[0097] FIG. 2 is a drawing showing an outline of a heat treatment process including annealing, cooling, and plating processes in a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention.

[0098] Referring to Figure 2 below, the heat treatment after cold rolling is described step by step.

[0099] Annealing heat treatment stage

[0100] s11 to s12 in Fig. 2 represent the annealing heat treatment steps.

[0101] Referring to Figure 2, a cold-rolled steel sheet is heated from room temperature to 800°C to 900°C (S11). The heating step (S11) determines the microstructure shape by nucleating austenite from the initial microstructure, martensite. Considering productivity, the heating rate is preferably 2°C / s or higher.

[0102] Continuing, primary cracking is performed at a temperature of 800℃ to 900℃ (S12). During annealing, the low-temperature phase undergoes reverse transformation to ferrite / austenite, and carbon (C) and manganese (Mn) are redistributed within the austenite. A longer annealing time is desirable for sufficient reverse transformation and alloying element redistribution. However, excessively long annealing times may lead to decreased productivity, so the annealing holding time is limited to 30 to 180 seconds.

[0103] Afterwards, the annealed steel plate is subjected to primary cooling (S13) and secondary cooling (S14). The primary cooling section (S13) can be divided into a slow cooling section (SCS), and the secondary cooling section (S14) can be divided into a rapid cooling section (RCS).

[0104] When cooling the annealed steel sheet, a slow cooling section may be included depending on the heat treatment equipment. If a slow cooling section is included, the slow cooling end temperature should be 620°C to 720°C to control the ferrite phase fraction, and the cooling rate may range from 3°C / sec to 20°C / sec. If the temperature and cooling rate exceed the above, the ferrite fraction decreases, resulting in a decrease in ductility.

[0105] A steel plate that has undergone a first cooling step (S13) of slow cooling is subjected to a second cooling step (S14) of rapid cooling. The cooling end temperature of the second cooling step (S14) may be Ms-140°C to Ms-30°C. The cooling rate of the second cooling step (S14) may be set to be greater than the cooling rate of the first cooling step (S13), and may be cooled at a rate of, for example, 15°C / s or more. If the cooling rate is less than 15°C / s, polygonal ferrite or pearlite is generated during cooling, which may result in deterioration of the tensile properties of the final steel. Therefore, the cooling rate may be in the range of, for example, 15°C / s to 100°C / s. The end point temperature of the second cooling can effectively increase austenite stability.

[0106] In the present invention, Ms can be determined by the following equation 3) as an example, but may vary somewhat depending on process conditions, etc.

[0107] Equation 3): Ms(℃) = 539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo

[0108] If the cooling end point temperature of the second cooling stage (S14) exceeds Ms-30℃, the redistribution of alloy elements is insufficient, making it difficult to sufficiently secure austenite stability. In addition, as the cooling end point temperature increases, ferrite transformation occurs, which causes a decrease in strength and elongation.

[0109] After the secondary cooling end point temperature is reached, secondary cracking is performed, maintaining the temperature within a range of ±20°C for 10 to 100 seconds (S15). During the initial period of holding after rapid cooling, temperature homogenization of the steel occurs, and during the isothermal holding, some of the retained austenite may transform into lower bainite, etc.

[0110] Next, reheat to a temperature of 350 to 550°C and maintain it in that temperature range for 30 to 500 seconds (S16).

[0111] Thereafter, the steel material is immersed in a molten zinc plating bath to perform plating (GI) (S17-1). The temperature of the molten zinc plating bath may range from 400 to 600°C. If necessary, the steel material immersed in the plating bath is subjected to alloying heat treatment (GA) (S17-2) at a temperature range of, for example, 500 to 570°C. After the above process, the steel material is finally cooled to 100°C or lower at a cooling rate of 10°C / s or higher.

[0112] At this time, the plating (S17-1) and alloying heat treatment (S17-2) times may also be included in the reheating holding time.

[0113] Temper rolling can also be performed if necessary. The elongation during temper rolling is in the range of 0.1% to 1.0%.

[0114] The final microstructure of the cold-rolled steel sheet produced by the above-described manufacturing method may include tempered martensite, bainite, fresh martensite, ferrite, and retained austenite. In this case, the total area fraction of tempered martensite and bainite may be 60 to 80%, the area fraction of fresh martensite may be less than 10%, the area fraction of ferrite may be 10 to 30%, and the area fraction of the retained austenite may be 1 to 5%.

[0115] Furthermore, the ratio of the fresh martensite (FM) to the sum of tempered martensite and bainite (TM+B), FM / (TM+B), may be less than 0.25. If the FM / (TM+B) value is 0.25 or more, voids between the hard phase and the soft phase may easily occur during processing of the cold-rolled steel sheet, which may reduce the processability.

[0116] Within the composition range described in the present invention, a steel grade composed of the heat treatment process described above and the microstructure obtained therefrom can be realized as a cold-rolled high-strength steel sheet having excellent formability, with a tensile strength (TS) of 980 MPa or more and a yield ratio (YR) × elongation (EL) × hole expandability (HER) ≥ 400.

[0117] In addition, the cold-rolled steel sheet may preferably have a product of tensile strength (TS) and hole expandability (HER) (TS × HER) of 50,000 MPa% or more.

[0118] Experimental example

[0119] Below, preferred experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are provided solely to aid understanding of the present invention, and the present invention is not limited to the following experimental examples.

[0120] In relation to the analysis and measurement of this experimental example, the microstructure was analyzed using a scanning electron microscope (SEM), and XRD analysis was used to analyze the retained austenite fraction and carbon content within the retained austenite. The mechanical properties were evaluated by performing a tensile test according to KS No. 5 using a Zwick / Roell Corp. Z100.

[0121] Steel having the composition (unit: weight%) shown in Table 1 below was prepared, and cold-rolled steel sheets according to examples and comparative examples were prepared through the specified hot rolling, cold rolling, and heat treatment processes. The remainder is iron (Fe).

[0122] Steel type CSiMnAlPSTiNbCrMoBMs(℃)Formula 1Inventive steel 10.100.92.50.030.0100.0020.020- 0.3- 0.00204170.6Inventive steel 20.121.32.70.030.0120.0020.020- - - 0.00204060.9Inventive steel 30.120.92.50.030.0100.0020.020- - - 0.00204120.8Inventive steel 40.120.82.40.0250.0120.0020.0150.020.20.40.00134101.1Inventive steel 50.110.82.60.0250.0120.0020.0150.020.50.20.00144060.9Inventive steel 60.120.82.40.0250.0120.0020.0150.020.50.20.00154081.1Comparative steel 10.130.92.50.030.0120.0020.0200.0250.3- 0.00054041.4Comparative Steel 20.160.92.50.030.0100.0020.0200.025- - 0.00083951.7Comparative Steel 30.140.62.30.0470.0120.0020.0400.020.20.20.00184061.6Comparative Steel 40.120.62.30.50.0120.0020.010 0.80.10.00084081.8Comparative steel50.130.62.30.0540.0120.0020.0400.020.20.060.00194111.5Comparative steel60.140.62.20.50.0120.0020.015 0.80.10.00074022.1Comparative steel70.100.52.40.50.0120.0020.015 0.80.10.00054131.6Comparative steel80.140.62.60.040.0120.0020.0300.020.2 0.00093981.4Comparative Strength90.140.62.40.040.0120.0020.0200.020.20.10.00064041.3

[0123] Table 2 shows the hot rolling process conditions for the steel grades in Table 1.

[0124] Steel type heat research, branch heating furnace, rough rolling, finish rolling, coiling, slab heating temperature (℃), rough rolling, exit temperature (℃), rough rolling, last pass reduction ratio (%), finish exit temperature (℃), 1st pass reduction ratio (%), 1~3 pass reduction ratio total (%), remaining pass reduction ratio total (%), 1st cooling rate (℃ / s), coiling temperature (℃), Invention steel 1 1 2 0 0 1 0 9 0 4 5 9 2 5 4 4 1 1 9 1 0 1 3 3 5 6 0, Experimental example 1, Invention steel 2 1 1 9 7 1 0 6 0 4 6 9 2 0 4 5 1 2 0 1 0 9 7 5 7 0, Experimental example 2, Invention steel 3 1 1 9 8 1 0 4 5 4 6 9 1 5 4 5 1 2 9 8 1 2 8 5 5 0, Experimental example 3, Invention steel 4 1 2 0 0 1 0 7 8 4 5 9 0 5 4 6 1 2 3 9 7 9 3 5 4 0, Experimental Example 4 Invention 5 1 2 0 2 1 0 8 2 4 5 9 0 8 4 5 1 1 8 1 0 2 1 2 6 5 7 0 Experimental Example 5 Invention 6 1 2 0 6 1 0 6 4 6 9 1 1 4 6 1 1 9 1 0 1 8 6 5 7 0 Experimental Example 6 Comparative Example 1 Comparative Example 2 1 1 9 1 0 5 9 4 6 9 0 6 4 5 1 2 0 1 0 1 1 5 5 5 0 Comparative Example 2 Comparative Example 3 1 1 9 9 1 0 80459044512199116550Comparative Example 3Comparative Strength 412011090469124612595123550Comparative Example 4Comparative Strength 5120110634590945119101121550Comparative Example 5Comparative Strength 612021078449084512298119550Comparative Example 6Comparative Strength 712061085459034412199 134560Comparative Example 7Comparative Steel 8119810714690546120100128550Comparative Example 8Comparative Steel 9119910804690445119101126550Comparative Example 9Inventive Steel 2119710604690735105115134570Comparative Example 10Inventive Steel 311981045469034512199133550Comparative Example 11

[0125] After hot rolling the slabs of the above-mentioned steel grades according to the conditions in Table 2, the surface oxide scale was removed through pickling and cold rolled at a reduction ratio of 30 to 80%. Thereafter, the cold-rolled steel sheets were heat treated according to the conditions in Table 3.

[0126] Steel grade, heat treatment, notes, 1st crack, 1st cooling, 2nd cooling, 2nd crack, reheating, plating, temperature holding time, temperature speed, temperature speed, holding time, temperature holding time, invention steel 18408068015280353545055GA, experiment example 1, invention steel 28308068012270353545055GA, experiment example 2, invention steel 38408068015280353545055GA, experiment example 3, invention steel 4830806505300283544055GA, experiment example 4, foot Myunggang 5815806505350233546055GA Experimental Example 5 Inventive Steel 6830806505330233546055GA Experimental Example 6 Comparative Steel 18408068015250393545055GA Comparative Example 1 Comparative Steel 28108068010280353545055GA Comparative Example 2 Comparative Steel 38308066 05320253548055GAComparative Example 3Comparative Steel 4815806206300233546055GAComparative Example 4Comparative Steel 5810806505450153548055GAComparative Example 5Comparative Steel 6840806506350203546055GAComparative Example 6Comparative Steel 78508065064501535460 55GA Comparative Example 7 Comparative Steel 8 8 30 80 6 5 0 6 35 0 2 2 3 5 4 8 0 5 5 GA Comparative Example 8 Comparative Steel 9 8 5 0 8 0 6 7 0 6 35 0 2 3 5 4 6 0 5 5 GA Comparative Example 9 Inventive Steel 2 8 30 8 0 6 8 0 1 2 7 0 3 5 3 5 4 5 0 5 5 GA Comparative Example 10 Inventive Steel 3 8 4 0 8 0 6 8 0 1 5 4 0 2 5 3 5 4 5 0 5 5 GI Comparative Example 11

[0127] Table 4 shows the microstructure and mechanical properties of the cold-rolled steel sheet finally manufactured by completing the process according to Tables 2 and 3. In Table 4, the structure TM+B means that it is composed of tempered martensite and bainite, F means ferrite, FM means fresh martensite, RA means retained austenite, and FM / (TM+B) means the ratio of fresh martensite (FM) to the sum of tempered martensite and bainite (TM+B).

[0128] In addition, it represents yield strength (YS), tensile strength (TS), elongation (EL), and hole extensibility (HER), and YRХELХHER represents yield ratio (YR) × elongation (EL) × hole extensibility (HER).

[0129] No. Microstructure Mechanical properties Remarks TM+BFFMRAFM / (TM+B)YSTSELHERYRХELХHER Inventive steel 17614820.1189099012.062669 Experimental example 1 Inventive steel 27513930.1278899712.653528 Experimental example 2 Inventive steel 37218910.1387398010.557533 Experimental example 3 Inventive steel 47215850.119 58109012.143457Experimental Example 4Inventive Steel 57414930.12892110312.750514Experimental Example 5Inventive Steel 67514920.12872106212.847494Experimental Example 6Comparative Steel 18212330.04788103012.737359Comparative Example 1Comparative Steel 258211830.3161297414.732296Comparative Example 2 Comparative example 3818920.11863111812.138355Comparative example 3Comparative example 428422911.0451993015.125211Comparative example 4Comparative example 530333701.23519104012.522137Comparative example 5Comparative example 627383321.22517104612.921134Comparative example 6Comparative example 735362900.8351 698714.220148Comparative Example 7Comparative Steel 841243320.8057996014.527236Comparative Example 8Comparative Steel 938243710.9756497015.625227Comparative Example 9Inventive Steel 27711930.12832102513.137393Comparative Example 10Inventive Steel 354182440.4461290714.131295Comparative Example 11

[0130] Figure 3 is a graph showing the properties of a cold-rolled steel sheet according to an experimental example of the present invention.

[0131] Referring to Table 1 and Figure 3, invention steels 1 to 6 satisfy the alloy composition proposed in the present invention, thereby satisfying the range of formula 1) to be achieved in the present invention.

[0132] On the other hand, Comparative Steel 1 to Comparative Steel 9 do not satisfy the alloy composition proposed in the present invention, and thus are outside the range of Formula 1) that the present invention seeks to achieve.

[0133] In addition, referring to Table 4 and Figure 3, all of comparative steels 1 to 9 showed yield ratio (YR) × elongation (EL) × hole expandability (HER) of less than 400, and all of invention steels 1 to 6 showed yield ratio (YR) × elongation (EL) × hole expandability (HER) of 400 or more.

[0134] Referring to Table 2, Comparative Example 10 deviates from the process conditions proposed by the present invention because the sum of the reduction rates of the 1st to 3rd passes during the final rolling is lower than the sum of the reduction rates of the remaining passes. Referring to Tables 1 and 3, Comparative Example 11 does not satisfy the temperature range of Ms-140°C to Ms-30°C (264°C to 374°C) proposed by the present invention for the end temperature of the secondary cooling.

[0135] Referring to Table 4, when comparing Experimental Example 2 and Comparative Example 10 using the same steel type, Invention Steel 2, Experimental Example 2 satisfied all the mechanical properties to be achieved in the present invention, whereas Comparative Example 10 showed a yield ratio (YR) × elongation (EL) × hole expandability (HER) of less than 400. This is believed to be because Comparative Example 10 exceeds the reduction ratio suggested in the present invention, and thus the austenite grain refinement effect is minimal.

[0136] In addition, when comparing Experimental Example 3 and Comparative Example 11 using the same steel type, Invention Steel 3, Experimental Example 3 satisfied all the mechanical properties to be achieved in the present invention, whereas Comparative Example 11 did not satisfy the tensile strength (TS): 980 MPa or more to be achieved in the present invention, and the yield ratio (YR) × elongation (EL) × hole expandability (HER) was less than 400.

[0137] In addition, while Experimental Example 3 satisfies the target fraction of the area fraction of each microstructure component in the present invention, Comparative Example 11 does not satisfy the composition requirements of the microstructure proposed in the present invention: a total of tempered martensite and bainite: 60 to 80% and fresh martensite less than 10%, FM / (TM+B) < 0.25.

[0138] This is because in Comparative Example 11, the secondary cooling end point temperature exceeded Ms-30℃, causing ferrite transformation, which reduced strength and elongation, and austenite stability was not sufficiently secured, so the desired microstructure was not secured.

[0139] Comparative Examples 5 and 7 did not satisfy the target area fraction of retained austenite of 1 to 5% of the present invention because the secondary cooling end point temperature exceeded Ms-30℃ and austenite stability was not sufficiently secured.

[0140] Through this, it can be seen that when the alloy composition and process conditions proposed in the present invention are all satisfied, a steel plate having high strength and excellent ductility can be manufactured, a cold-rolled steel plate having excellent hole expandability due to a high yield ratio, and excellent crash resistance and formability can be manufactured.

[0141] 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): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.01% or less, nitrogen (N): more than 0% to 0.01% or less, boron (B): 0.001% to 0.005%, the sum of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder including iron (Fe) and other unavoidable impurities. The final microstructure consists of tempered martensite, bainite, fresh martensite, ferrite, and retained austenite. High strength cold rolled steel sheet.

2. In paragraph 1, Characterized in that it satisfies the following equations 1) and 2). High strength cold rolled steel sheet. Equation 1): 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B ≤ 1.2 Equation 2): Yield ratio (YR) × elongation (EL) × hole expandability (HER) ≥ 400 (Here, the element symbol in formula 1) represents the content of that element in weight%.) 3. In paragraph 1, In terms of area fraction, the total fraction of the tempered martensite and the bainite is 60 to 80%, the fresh martensite is less than 10%, the fraction of the ferrite is 10 to 30%, and the fraction of the retained austenite is 1 to 5%. High strength cold rolled steel sheet.

4. In paragraph 1, The ratio of the fresh martensite (FM) and the sum of the tempered martensite and the bainite (TM+B) is FM / (TM+B) < 0.

25. High strength cold rolled steel sheet.

5. In paragraph 1, Containing, in weight %, one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni), in a total amount of 0.5% or less (excluding 0%), High strength cold rolled steel sheet.

6. In paragraph 1, The crystal grain size of the above tempered martensite and the above bainite is 5㎛ or less. High strength cold rolled steel sheet.

7. In paragraph 1, The tensile strength (TS) is 980 MPa or more, and the product of tensile strength (TS) and hole extensibility (HER) (TSХHER) is in the range of 50,000 MPa% or more. High strength cold rolled steel sheet.

8. In paragraph 1, The above titanium (Ti) is contained in an amount of 0.015% or more and 0.04% or less, and niobium (Nb) is contained in an amount of 0.06% or more and 0.085% or less. High strength cold rolled steel sheet. 9.(a) A step of hot rolling a steel material including, in wt%, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): more than 0% to 1.0%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): more than 0% to 0.01% or less, nitrogen (N): more than 0% to 0.01% or less, boron (B): 0.001% to 0.005%, the sum of at least one selected from titanium (Ti) and niobium (Nb): 0.1% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; (b) a step of cold rolling the hot-rolled steel; (c) a step of annealing the cold rolled steel; (d) a step of cooling the annealed heat-treated steel to a temperature below the Ms temperature; and (e) a step of plating the cooled steel material; including, Step (a) above, It includes a step of performing at least one pass of rolling at a reduction ratio of 40% or more per pass, and performing rolling so that the total reduction ratio of the rolling of the first to third passes is higher than the total reduction ratio of the remaining passes. Step (d) above, A first cooling step having a first cooling end temperature in the range of 620℃ to 720℃ at the first cooling rate; and A step of performing a second cooling to a second cooling end temperature of Ms or lower at a second cooling rate faster than the first cooling rate after the first cooling; including; Method for manufacturing high-strength cold rolled steel sheet.

10. In paragraph 9, The above steel satisfies the following equation 1): Method for manufacturing high-strength cold rolled steel sheet. Equation 1): 10×C + 0.4×(Si + 6×Al) - 0.4×Mn + 15×(Ti+Nb) - 52.5×B ≤ 1.2 (Here, the element symbol in formula 1) represents the content of that element in weight%.) 11. In paragraph 9, Containing, in weight %, one or more selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni), in a total amount of 0.5% or less (excluding 0%), Method for manufacturing high-strength cold rolled steel sheet.

12. In paragraph 9, The above secondary cooling end temperature is a temperature of Ms - 140℃ or higher and Ms - 30℃ or lower. Method for manufacturing high-strength cold rolled steel sheet.

13. In paragraph 9, The above titanium (Ti) is contained in an amount of 0.015% or more and 0.04% or less, and niobium (Nb) is contained in an amount of 0.06% or more and 0.085% or less. Method for manufacturing high-strength cold rolled steel sheet.

14. In paragraph 9, The above first cooling rate has a range of 3°C / sec to 20°C / sec. Method for manufacturing high-strength cold rolled steel sheet.

15. In paragraph 9, The second cooling rate has a range of 15°C / sec to 100°C / sec. Method for manufacturing high-strength cold rolled steel sheet.

16. In paragraph 9, Step (a) above, (a-1) a step of reheating the above steel at 1150 to 1300℃; (a-2) a step of hot rolling the above steel material under the condition that the finishing rolling temperature is 850 to 950℃; and (a-3) a step of coiling the steel material at 400 to 600°C; including; Method for manufacturing high-strength cold rolled steel sheet.

17. In paragraph 16, After step (a-2) above and before step (a-3), A step of cooling to 680℃ or lower at a cooling rate of 80℃ / s or higher is performed. Method for manufacturing high-strength cold rolled steel sheet.

18. In paragraph 9, The above Ms temperature is defined by the following equation 3). Method for manufacturing high-strength cold rolled steel sheet. Equation 3): Ms = 539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo (Here, the element symbol in Equation 3) represents the content of that element in weight%.)

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