High-strength cold-rolled steel sheet and method for manufacturing the same

Abstract

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

Description

Technical Field

[0001] The technical concept of this invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically to a high-strength cold-rolled steel sheet with high strength and excellent formability. Background Technology

[0002] High-strength steel for automotive applications is being developed to meet two key needs: vehicle weight reduction due to stricter energy and environmental regulations, and ensuring crash stability due to enhanced safety regulations. Steel sheets used as body structural components require tensile strengths of 1 GPa. Furthermore, a high yield ratio (YS / TS) is necessary to improve the crash performance of the vehicle body. To improve crash stability and formability in component manufacturing, materials with high yield strength and excellent ductility need to be developed. In particular, high-strength steel sheets for components with complex shapes require materials with excellent properties such as elongation and expansion capability. However, due to the trade-off between strength and elongation, component forming becomes difficult as material strength gradually increases, and various studies are underway to develop high-strength steels with excellent formability.

[0003] For example, there exist dual-phase (DP) steels, in which a large amount of soft ferrite and hard martensite are uniformly distributed in the microstructure, thus ensuring strength and elongation. However, DP steels are characterized by a low yield ratio (YS / TS) because mobile dislocations are introduced into the ferrite during the martensitic transformation. Transformation-induced plasticity (TRIP) steels are steels that utilize the phenomenon of transformation to martensite during the plastic deformation of retained austenite, and their advantage lies in the ability to utilize the properties of austenite (which has excellent formability) and the properties of hard martensite after forming. TRIP steels have high elongation but are characterized by low porosity values.

[0004] Therefore, in ultra-high strength steel plates with a strength of 1 GPa or higher, steel grades with high yield ratio, excellent elongation, and high hole expansion capability are required. Summary of the Invention

[0005] Technical issues The technical objective of this invention is to provide an ultra-high strength cold-rolled hot-dip galvanized steel sheet with excellent elongation and hole expansion properties, and a method for manufacturing the same.

[0006] However, such an objective is exemplary, and the technical concept of the present invention is not limited thereto.

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

[0008] According to one embodiment of the invention, the cold-rolled steel sheet contains, by weight %: carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): greater than 0% to 1.0%, phosphorus (P): greater than 0% and not greater than 0.02%, sulfur (S): greater than 0% and not greater than 0.01%, nitrogen (N): greater than 0% and not greater than 0.01%, boron (B): 0.001% to 0.005%, and the sum of at least one selected from titanium (Ti) and niobium (Nb): not greater than 0.1% (excluding 0%), and the balance iron (Fe) and other unavoidable impurities, and the final microstructure may consist of tempered martensite, bainite, fresh martensite, ferrite and retained austenite.

[0009] According to one implementation scheme, high-strength cold-rolled steel sheets can satisfy the following equations 1 and 2.

[0010] Formula 1): 10×C+0.4×(Si+6×Al)-0.4×Mn+15×(Ti+Nb)-52.5×B≤1.2 Formula 2): Yield ratio (YR) × elongation (EL) × porosity (HER) ≥ 400 (Where, the element symbols in Equation 1) represent the content of each element in terms of weight percentage.) According to one implementation scheme, high-strength cold-rolled steel sheets may have a total fraction of 60% to 80% tempered martensite and bainite, less than 10% fresh martensite, 10% to 30% ferrite, and 1% to 5% retained austenite by area fraction.

[0011] According to one implementation scheme, the ratio of fresh martensite (FM) to the sum of tempered martensite and bainite (TM+B) can be FM / (TM+B) < 0.25.

[0012] According to one implementation scheme, the high-strength cold-rolled steel sheet may also contain, by weight %, at least 0.5% (excluding 0%) of at least one selected from molybdenum (Mo), chromium (Cr), copper (Cu) and nickel (Ni).

[0013] According to one implementation, the grain size of tempered martensite and bainite can be 5 μm or smaller.

[0014] According to one implementation, the cold-rolled steel sheet may have a tensile strength (TS) of 980 MPa or greater and a tensile strength (TS) of 50,000 MPa% or greater multiplied by the expansion ratio (HER) (TS×HER).

[0015] According to one embodiment, titanium (Ti) may be included in an amount of 0.015% or more and 0.04% or less, and niobium (Nb) may be included in an amount of 0.06% or more and 0.085% or less.

[0016] According to another aspect of the present invention, a method for manufacturing high-strength cold-rolled steel sheets is provided.

[0017] According to one embodiment of the present invention, a method for manufacturing high-strength cold-rolled steel sheet comprises: (a) hot-rolling a steel material comprising, by weight %: C: 0.08% to 0.15%, Si: 0.8% to 1.5%, Mn: 2.0% to 3.0%, Al: greater than 0% to 1.0%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, N: greater than 0% and not greater than 0.01%, B: 0.001% to 0.005%, and the sum of at least one selected from Ti and Nb: not greater than 0.1% (excluding 0%), and the balance being Fe and other unavoidable impurities; (b) cold-rolling the hot-rolled steel material; (c) cold-rolling the steel sheet... The material is subjected to annealing heat treatment; (d) the annealed steel material is cooled to a temperature equal to or below Ms temperature; and (e) the cooled steel material is plated, wherein step (a) includes rolling at least one pass at a reduction rate of 40% or greater per pass, and the rolling is such that the total reduction rate of the first to third passes is higher than the total reduction rate of the remaining passes, and step (d) includes: a first cooling step of first cooling at a first cooling rate to a first cooling end temperature in the range of 620°C to 720°C, and a second cooling step of second cooling at a second cooling rate greater than the first cooling rate to a second cooling end temperature equal to or below Ms temperature after the first cooling.

[0018] According to one implementation scheme, the steel material can satisfy the following equation 1).

[0019] Formula 1): 10×C+0.4×(Si+6×Al)-0.4×Mn+15×(Ti+Nb)-52.5×B≤1.2 (Where, the element symbols in Equation 1) represent the content of each element in terms of weight percentage.) According to one embodiment, in the method for manufacturing cold-rolled steel sheet, the steel material may also contain, by weight %, at least one selected from Mo, Cr, Cu and Ni in a total amount not exceeding 0.5% (excluding 0%).

[0020] According to one implementation, the second cooling end temperature can be Ms-140°C or higher and Ms-30°C or lower.

[0021] According to one embodiment, titanium (Ti) may be included in an amount of 0.015% or more and 0.04% or less, and niobium (Nb) may be included in an amount of 0.06% or more and 0.085% or less.

[0022] According to one implementation, the first cooling rate can range from 3°C / s to 20°C / s.

[0023] According to one implementation, the second cooling rate can range from 15°C / s to 100°C / s.

[0024] According to one embodiment, step (a) may include: (a-1) reheating the steel material at 1150°C to 1300°C; (a-2) hot rolling the steel material at a finishing rolling temperature of 850°C to 950°C; and (a-3) winding the steel material at 400°C to 600°C.

[0025] According to one implementation, 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 greater can be performed.

[0026] Beneficial effects According to the technical concept of the present invention, steel sheets with high strength and excellent ductility can be manufactured, and therefore can be appropriately used as materials for automotive structural components, etc. Furthermore, cold-rolled steel sheets with a higher yield ratio compared to conventional DP steel, thus exhibiting excellent hole-expanding properties, excellent impact resistance, and excellent formability, can be manufactured. The above-described advantageous effects of the present invention have been described by way of example, and the scope of the invention is not limited to these effects. Attached Figure Description

[0027] Figure 1 A flowchart illustrating a method for manufacturing cold-rolled steel sheet according to an embodiment of the present invention is provided. Figure 2 A diagram illustrating the outline of a heat treatment process including annealing, cooling, and plating in a method for manufacturing cold-rolled steel sheet according to an embodiment of the present invention.

[0028] Figure 3 A graph illustrating the properties of cold-rolled steel sheet according to an experimental embodiment of the present invention. Detailed Implementation

[0029] Preferred embodiments of the invention will be described in detail below with reference to the accompanying drawings. These embodiments are provided to provide a more complete explanation of the technical concept of the invention to those skilled in the art. Various other forms may be modified from these embodiments, and the scope of the technical concept of the invention is not limited to these embodiments. Rather, these embodiments are provided so that the invention is comprehensive and complete, and fully conveys the technical concept of the invention to those skilled in the art. In this specification, the same reference numerals denote the same elements. Furthermore, various elements and regions in the drawings are depicted schematically. Therefore, the technical concept of the invention is not limited to the relative dimensions or spacing shown in the accompanying drawings.

[0030] In this specification and claims, phase fraction refers to the area ratio (area %) obtained by analyzing microstructure images using an image analyzer. Furthermore, unless otherwise stated, the content or concentration of a particular component refers to weight %.

[0031] The technical concept of the present invention provides a cold-rolled high-strength steel sheet and a method for manufacturing such a cold-rolled steel sheet, the cold-rolled high-strength steel sheet having a tensile strength (TS) of 980 MPa or greater and a yield ratio (YR) × elongation (EL) × expansion ratio (HER) ≥ 400 (which has very good formability).

[0032] Furthermore, according to an embodiment of the present invention, a cold-rolled steel sheet can be provided in which the product of tensile strength (TS) and hole expansion ratio (HER) (TS×HER) is 50000 MPa% or greater.

[0033] The following describes the amount of alloy and heat treatment conditions suitable for ensuring target tensile strength, elongation, yield ratio and porosity in this invention.

[0034] First, the high-strength cold-rolled steel sheet according to the technical concept of the present invention will be described in detail.

[0035] According to one embodiment of the invention, the high-strength cold-rolled steel sheet comprises, by weight percent, carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): greater than 0% to 1.0%, phosphorus (P): greater than 0% and not greater than 0.02%, sulfur (S): greater than 0% and not greater than 0.01%, nitrogen (N): greater than 0% and not greater than 0.01%, boron (B): 0.001% to 0.005%, and the total of at least one selected from titanium (Ti) and niobium (Nb): not greater than 0.1% (excluding 0%), and the balance iron (Fe) and other unavoidable impurities. Optionally, it may also contain at least one selected from molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni) in a total of not more than 0.5% (excluding 0%).

[0036] The role and content of each component in the high-strength cold-rolled steel sheet according to the present invention will be described below. In this context, the content of each component element refers to a percentage of the total weight of the steel sheet.

[0037] Carbon (C): 0.08% to 0.15% Carbon is added to ensure an appropriate fraction and stability of the retained austenite. The carbon content ranges from 0.08 wt% to 0.15 wt%. If the carbon content is below 0.08%, the fraction of retained austenite in the final microstructure is insufficient, making it difficult to achieve the target ductility and to form fresh martensite. On the other hand, if the carbon content exceeds 0.15%, the strength increases excessively, and carbides are easily formed, which may be detrimental to weldability.

[0038] Silicon (Si): 0.8% to 1.5% Silicon is a ferrite stabilizing element, delaying the formation of carbides in ferrite and providing solid solution strengthening. The silicon content ranges from 0.8% to 1.5% by weight. If the silicon content is below 0.8%, the aforementioned effects are difficult to achieve, while if the silicon content exceeds 1.5%, oxides such as Mn₂SiO₄ may form during manufacturing, impairing plating applicability, and the carbon equivalent may increase, thus reducing solderability.

[0039] Manganese (Mn): 2.0% to 3.0% Manganese has a solid solution strengthening effect and helps improve strength by increasing hardenability. The manganese content ranges from 2.0 wt% to 3.0 wt%. If the manganese content is below 2.0%, the effect is insufficient, making it difficult to ensure strength. If the manganese content exceeds 3.0%, machinability may decrease, resistance to delayed fracture may decrease, and the carbon equivalent may increase due to the formation or segregation of inclusions such as MnS, thereby reducing weldability.

[0040] Aluminum (Al): greater than 0% to 1.0% Aluminum is used as a deoxidizer and can help purify ferrite. The range of aluminum is greater than 0% by weight to 1.0% by weight. If aluminum is not included, the deoxidation effect is insufficient, and if aluminum exceeds 1.0%, AlN may form during slab production in the continuous casting stage, which may lead to cracking during continuous casting or hot rolling.

[0041] Phosphorus (P): greater than 0% and not greater than 0.02% Phosphorus is an impurity introduced during the steelmaking process, and although it can help improve strength through solid solution strengthening, high phosphorus content can lead to low-temperature brittleness. Therefore, the phosphorus content is preferably limited to 0.02% by weight or less.

[0042] Sulfur (S): greater than 0% and not greater than 0.01% Sulfur is an impurity introduced during the steelmaking process and can form non-metallic inclusions such as FeS and MnS, thereby reducing toughness and weldability. Therefore, the sulfur content is preferably limited to 0.01% by weight or less.

[0043] Nitrogen (N): greater than 0% and not greater than 0.01% Nitrogen is an unavoidable element in the steelmaking process, and if nitrogen exceeds 0.01%, it may lead to a decrease in elongation due to delayed recrystallization. Therefore, it is preferable to minimize nitrogen content. Thus, the nitrogen content is preferably limited to greater than 0% to 0.01% of the total weight of the steel plate.

[0044] Boron (B): 0.001% to 0.005% Boron is a hardening element that inhibits the formation of polygonal ferrite and promotes the formation of fresh martensite. The boron content ranges from 0.001% to 0.005% by weight. If the boron content is below 0.001%, the effect is not significant, while if the boron content exceeds 0.005%, machinability decreases.

[0045] The total amount of at least one selected from titanium (Ti) and niobium (Nb) is not greater than 0.1% (excluding 0%). Titanium and niobium are strong carbonitride forming elements. During hot rolling, they react with carbon and nitrogen present in the steel to form fine precipitates, thereby inhibiting grain growth and increasing strength. The total content of titanium and niobium should not exceed 0.1% by weight. If the steel does not contain titanium and niobium, it is difficult to exhibit precipitation strengthening effect, while if the total content of titanium and niobium exceeds 0.1%, the strength becomes too high and the ductility decreases.

[0046] In this case, when titanium is added, the titanium content is preferably added at 0.015% to 0.04%. To fully obtain the aforementioned boron effect, if boron combines with nitrogen in the steel and is lost as BN, the boron effect cannot be achieved. Therefore, it is preferable to induce TiN precipitation by adding a certain amount or more of titanium. However, if the titanium content exceeds 0.04%, defects such as nozzle clogging may occur due to the formation of coarse TiN, which may reduce continuous casting performance.

[0047] The remaining component of high-strength cold-rolled steel sheet is iron (Fe). However, in conventional steelmaking processes, unintended impurities from raw materials or the surrounding environment may inevitably be introduced, and therefore may not be completely eliminated. Since these impurities are known to those skilled in the art of conventional manufacturing processes, they are not specifically mentioned in this specification.

[0048] In addition, the high-strength cold-rolled steel sheet may further selectively contain, by weight, at least 0.5% (excluding 0%) of at least one of molybdenum (Mo), chromium (Cr), copper (Cu) and nickel (Ni).

[0049] The total amount of at least one of molybdenum (Mo), chromium (Cr), copper (Cu), and nickel (Ni) is not greater than 0.5% (excluding 0%). Molybdenum improves hardenability and inhibits pearlite formation. It also refines martensite. Simultaneously, when molybdenum segregates at grain boundaries, ferrite grain growth ceases, leading to a decrease in the ferrite fraction. To suppress this phenomenon, when molybdenum is included, its content is limited to 0.50% or less.

[0050] Similar to manganese, chromium has a solid solution strengthening effect and helps to increase strength by improving hardenability. If the chromium content exceeds 0.5%, hardenability becomes too high, the fraction of retained austenite decreases, the fraction of martensite increases, and ductility decreases.

[0051] Copper is an effective element for ensuring corrosion resistance and can block corrosive environments through surface concentration. However, adding more than 0.5% copper may increase the corrosion rate of corrosion-resistant steel sheets.

[0052] When added to steel sheets along with copper, nickel enhances corrosion resistance. Adding nickel at 0.5% or less increases the density of the Cu-rich layer in the steel sheet, further improving corrosion resistance.

[0053] The steel plate with the above alloy composition according to the present invention can have a carbon equivalent of 1.2 or less, 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 the weight ratios of carbon, silicon, aluminum, manganese, titanium, niobium and boron, respectively).

[0054] Microstructure of steel plate The microstructure of the high-strength cold-rolled steel sheet according to the technical concept of the present invention may include tempered martensite, bainite, fresh martensite, ferrite, and retained austenite. In this case, the total fraction of tempered martensite and bainite, in terms of area fraction, may be 60% to 80%, the fraction of fresh martensite may be less than 10%, the fraction of ferrite may be 10% to 30%, and the fraction of retained austenite may be 1% to 5%.

[0055] Area fraction refers to the area ratio obtained by analyzing microstructure images acquired through electron backscatter diffraction (EBSD) using an image analyzer. EBSD specimens can be prepared by polishing cold-rolled steel sheets and removing surface defects. The area fraction can be obtained using FE-SEM by calculating the fraction of the image area from the images acquired through EBSD.

[0056] Since the hardness of tempered martensite and bainite is greater than that of ferrite but less than that of fresh martensite, it is unlikely that voids will appear between the hard and soft phases when the total fraction of tempered martensite and bainite is fixed at 60% to 80%.

[0057] Fresh martensite is a hard phase, and because it is difficult to deform during shearing, it can suppress the occurrence of defects in stamped products. However, if the area fraction of fresh martensite is 10% or greater, voids are easily generated during processing, and machinability decreases. Therefore, the area fraction of fresh martensite is set to be less than 10%.

[0058] Ferrite is a soft phase and is effective in forming the metal microstructure with low dislocation density using ferrite grains that have excellent ductility. To achieve this effect, the area fraction is set to 10% or more. On the other hand, if the area fraction exceeds 30%, defects may occur during stamping because the ferrite tends to deform. Therefore, the area fraction of ferrite is set to between 10% and 30%.

[0059] The retained austenite phase transforms into martensite during processes such as stamping, which significantly reduces porosity. Therefore, the retained austenite phase is preferably controlled to 5% or less. Furthermore, it is preferably 1% or more.

[0060] Furthermore, the grain size of tempered martensite and bainite can be 5 μm or smaller. In this case, the grain size is measured using EBSD as the equivalent circle diameter of the region surrounded by grain boundaries with an orientation difference of 10 degrees or greater.

[0061] The ratio FM / (TM+B), which is the sum of fresh martensite (FM) and tempered martensite and bainite (TM+B), is set to be less than 0.25. When the value of FM / (TM+B) is 0.25 or greater, voids between the hard and soft phases are easily generated during the processing of cold-rolled steel sheets, and machinability may decrease.

[0062] The high-strength cold-rolled steel sheet of the present invention can be realized as a cold-rolled high-strength steel sheet with excellent formability, having a tensile strength (TS) of 980 MPa or greater, and a yield ratio (YR) × elongation (EL) × expansion ratio (HER) ≥ 400.

[0063] Furthermore, the product of the tensile strength (TS) and the hole expansion ratio (HER) of the cold-rolled steel sheet (TS×HER) can preferably be 50,000 MPa% or greater.

[0064] In the following description, a method for manufacturing high-strength cold-rolled steel sheets having the above-described compositional range according to the technical spirit of the present invention will be described with reference to the accompanying drawings.

[0065] Method for manufacturing cold-rolled steel sheets In the manufacturing method according to the invention, the semi-finished product subjected to the hot rolling process can be, for example, a slab. After obtaining molten steel with a predetermined composition through a steelmaking process, a slab in a semi-finished state can be obtained through a continuous casting process.

[0066] Figure 1 A flowchart is provided to illustrate step by step a method for manufacturing high-strength cold-rolled steel sheets according to an embodiment of the present invention.

[0067] A method for manufacturing high-strength cold-rolled steel sheet according to an embodiment of the present invention includes: a step of hot rolling a steel material having the above-described composition (S10); a step of cold rolling the hot-rolled steel material (S20); a step of annealing the cold-rolled steel material (S30); a step of first cooling the annealed steel material (S40); a step of second cooling the steel material (S50); and a step of plating the cooled steel material (S60).

[0068] Hot rolling process The steel billet with the aforementioned alloy composition is reheated to a temperature of 1150°C to 1300°C. The slab is manufactured as a semi-finished product by continuous casting of molten steel obtained through the steelmaking process, and homogenized through a reheating process to eliminate component segregation generated during casting and bring it to a state suitable for hot rolling. If the slab reheating temperature (SRT) is below 1150°C, there is a problem of insufficient redissolution of segregation in the slab, and if the slab reheating temperature (SRT) exceeds 1300°C, the austenite grain size increases, and the process cost may increase. The slab reheating can be carried out for 1 to 2 hours. If the reheating time is less than 1 hour, the reduction of segregation is insufficient, and if the reheating time exceeds 2 hours, the grain size increases, and the process cost may increase.

[0069] After reheating, hot rolling is performed, which includes roughing and finishing rolling. For example, hot finishing rolling can be carried out at a finishing exit temperature (FDT) ranging from 850°C to 950°C to produce hot-rolled steel sheets. If the finishing temperature is below 850°C, the rolling load increases sharply, leading to a decrease in productivity, and if the finishing temperature exceeds 950°C, the grain size increases and the strength may decrease.

[0070] The roughing temperature is preferably set to be equal to or higher than the recrystallization stop temperature (Tnr) of austenite, and can be performed, for example, at 1000°C to 1150°C. When roughing is performed in two or more passes, the reduction in the last pass is preferably 40% or greater. Here, the reduction (%) is defined as {[thickness of material before rolling - thickness of material after rolling] / thickness of material before rolling} × 100.

[0071] During rough rolling, the microstructure recrystallized through the initial rolling undergoes grain growth due to high temperatures. However, in the final pass, grain growth slows down, thus the reduction rate of the final pass significantly affects the grain size of the final microstructure. Furthermore, if the reduction rate of the rough rolling passes is reduced, the deformation transmitted to the center may be insufficient, resulting in reduced toughness due to coarsening of the center. Therefore, to refine austenite grains and reduce manganese segregation, it is preferable to set the reduction rate of the final pass to 40% or greater.

[0072] The rough-rolled steel is then finished in 5 to 7 passes. During finish rolling, the reduction rate of the first pass is set to 40% or greater, and the total reduction rate of passes 1 through 3 is set higher than the total reduction rate of the remaining passes. This is to refine the austenite grains and reduce manganese segregation. If the total reduction rate of passes 1 through 3 is lower than the total reduction rate of the remaining passes, coarse austenite grains will form, making it impossible to obtain the desired microstructure and ensuring both strength and a uniform microstructure.

[0073] After hot rolling, the steel is cooled to 400°C to 600°C and then wound. If the winding temperature is below 400°C, the strength will increase and the rolling load during cold rolling will increase. If the winding temperature exceeds 600°C, excessive ferrite and pearlite microstructure will be generated, which may lead to poor porosity and defects in subsequent processes due to surface oxidation, etc.

[0074] During the cooling process after hot rolling, the temperature is cooled from the finishing rolling end temperature to, for example, 680°C at a high cooling rate of 80°C / s or greater (rapid cooling), and then cooled to a winding temperature of 600°C or lower at a low cooling rate of 5°C / s or greater (slow cooling).

[0075] The rapid cooling step is to prevent excessive ferrite transformation. Furthermore, since pearlite may form at temperatures of 680°C or higher, and microstructural inhomogeneities may occur in subsequent processes, thus worsening porosity, a rapid cooling step up to 680°C is preferably applied.

[0076] Cold rolling process Hot-rolled steel sheets are pickled to remove the surface oxide layer by cleaning with acid. They are then cold-rolled at an average reduction rate of, for example, 30% to 80%, to form cold-rolled steel sheets. A higher average reduction rate results in a more significant improvement in formability due to microstructure refinement. If the average reduction rate is less than 30%, it is difficult to obtain a uniform microstructure. If the average reduction rate exceeds 80%, the rolling force increases, thereby increasing the process load. The microstructure of cold-rolled steel sheets can be an elongated structure compared to the microstructure of hot-rolled steel sheets.

[0077] After cold rolling, the cold-rolled steel sheet undergoes annealing heat treatment and plating process.

[0078] Figure 2 A diagram illustrating the outline of a heat treatment process including annealing, cooling, and plating in a method for manufacturing cold-rolled steel sheet according to an embodiment of the present invention.

[0079] In the following text, reference will be made to Figure 2 The heat treatment after cold rolling is described step by step.

[0080] Annealing heat treatment steps Figure 2 S11 to S12 in the text represent the annealing heat treatment steps.

[0081] See Figure 2 The cold-rolled steel sheet is heated from room temperature to 800°C to 900°C (S11). The heating step (S11) is a step of nucleation from martensite, which is the initial microstructure, to austenite, thereby determining the shape of the microstructure. Considering productivity, the heating rate is preferably 2°C / s or greater.

[0082] Subsequently, the steel sheet is subjected to a first homogenization at a temperature of 800°C to 900°C (S12). During annealing, the low-temperature phase undergoes a reverse transformation to ferrite / austenite, and carbon (C) and manganese (Mn) redistribute into the austenite. A longer annealing time is preferred for sufficient reverse transformation and alloy element redistribution; however, if the annealing time becomes too long, productivity may decrease. Therefore, the annealing holding time is limited to between 30 and 180 seconds.

[0083] The annealed steel sheet then undergoes a first cooling (S13) and a second cooling (S14). The first cooling zone (S13) can be classified as the slow cooling zone (SCS), and the second cooling zone (S14) can be classified as the rapid cooling zone (RCS).

[0084] The cooling process of annealed steel sheets may include a slow cooling zone, depending on the heat treatment equipment. When a slow cooling zone is included, the end temperature of the slow cooling should be between 620°C and 720°C to control the ferrite phase fraction, and the cooling rate can range from 3°C / s to 20°C / s. If the temperature and cooling rate are outside these ranges, the ferrite fraction will decrease, and the ductility will decline.

[0085] Following a first cooling step (S13) involving slow cooling, a second cooling step (S14) involves rapid cooling of the steel plate. The cooling end temperature of the second cooling step (S14) can be from Ms-140°C to Ms-30°C. The cooling rate of the second cooling step (S14) can be set to be greater than that of the first cooling step (S13), for example, at a rate of 15°C / s or greater. If the cooling rate during the cooling process is less than 15°C / s, polygonal ferrite or pearlite will be generated during cooling, resulting in poor tensile properties of the final steel. Therefore, the cooling rate can range from, for example, 15°C / s to 100°C / s. The second cooling end temperature can effectively improve austenite stability.

[0086] In this invention, Ms can be determined, for example, by the following formula 3), but may vary slightly depending on process conditions, etc.

[0087] Equation 3): Ms (°C) = 539 - 423C - 30.4Mn - 12.1Cr - 17.7Ni - 7.5Mo (Where, the element symbols in Equation 3) represent the content of each element in terms of weight %.) If the cooling end temperature of the second cooling step (S14) exceeds Ms-30°C, the redistribution of alloying elements will be insufficient, and austenite stability may not be adequately ensured. The higher the cooling end temperature, the more ferrite transformation occurs, which reduces strength and elongation.

[0088] After reaching the end temperature of the second cooling, a second homogenization process (S15) is performed, holding the temperature within ±20°C for 10 to 100 seconds. In the initial stage of holding after rapid cooling, the temperature of the steel is homogenized, and during the isothermal holding process, some of the retained austenite may transform into lower bainite, etc.

[0089] Subsequently, after reheating to a temperature of 350°C to 550°C, the steel plate is held in this temperature range for 30 to 500 seconds (S16).

[0090] The steel material is then immersed in a hot-dip galvanizing bath for hot-dip galvanizing (GI) (S17-1). The temperature of the hot-dip galvanizing bath can range from 400°C to 600°C. If necessary, the steel material immersed in the bath undergoes alloying heat treatment (GA) (S17-2) at a temperature, for example, ranging from 500°C to 570°C. Following this process, a final cooling to 100°C or lower is performed at a cooling rate of 10°C / s or greater.

[0091] In this case, the plating (S17-1) time and alloying heat treatment (S17-2) time can also be included in the reheat holding time.

[0092] If necessary, tempering rolling may also be performed. The elongation during tempering rolling is in the range of 0.1% to 1.0%.

[0093] The final microstructure of the cold-rolled steel sheet obtained by the above manufacturing method may include tempered martensite, bainite, fresh martensite, ferrite, and retained austenite. In this case, the total fraction of tempered martensite and bainite, in terms of area fraction, may be 60% to 80%, the fraction of fresh martensite may be less than 10%, the fraction of ferrite may be 10% to 30%, and the fraction of retained austenite may be 1% to 5%.

[0094] Furthermore, the ratio of fresh martensite (FM) to the sum of tempered martensite and bainite (TM+B), FM / (TM+B), can be less than 0.25. When the value of FM / (TM+B) is 0.25 or greater, voids between the hard and soft phases are easily generated during the processing of cold-rolled steel sheets, and the machinability may decrease.

[0095] Within the component system described in this invention, the steel grade with the microstructure described above, obtained by the above heat treatment process, can be realized as a cold-rolled high-strength steel sheet with excellent formability, having a tensile strength (TS) of 980 MPa or greater, and a yield ratio (YR) × elongation (EL) × expansion ratio (HER) ≥ 400.

[0096] In addition, the cold-rolled steel sheet may preferably have a tensile strength (TS) of 50,000 MPa% or greater and a perforation ratio (HER) product (TS×HER).

[0097] Experimental Examples Preferred experimental embodiments are shown below to aid in understanding the invention. However, the following experimental embodiments are only for the purpose of aiding in understanding the invention, and the invention is not limited to the following experimental embodiments.

[0098] For the analysis and measurement of this experimental embodiment, the microstructure was analyzed using scanning electron microscopy (SEM), and the retained austenite fraction and carbon content in the retained austenite were analyzed using XRD. Mechanical properties were evaluated by tensile testing using a Zwick / Roell CorpZ100 according to KS 5 standard.

[0099] Steels having the composition (unit: wt%) shown in Table 1 below were prepared, and cold-rolled steel sheets according to the Examples and Comparative Examples were produced by predetermined hot rolling, cold rolling, and heat treatment processes. The balance is iron (Fe). In the steel grade column, Invention 1 to Invention 6 refer to Invention Steel 1 to Invention Steel 6, and Comparative 1 to Comparative 9 refer to Comparative Steel 1 to Comparative Steel 9.

[0100] [Table 1] Table 2 shows the hot rolling process conditions for the steel grades in Table 1. In the category column, Examples 1 to 6 represent Examples 1 to 6, and Comparative Examples 1 to 11 represent Comparative Examples 1 to 11.

[0101] [Table 2] After hot rolling the slabs of the above steel grades under the conditions in Table 2, the surface oxide scale is removed by pickling, and then cold rolling is performed with a reduction rate of 30% to 80%. Then, the cold-rolled steel sheets are heat-treated under the conditions in Table 3.

[0102] [Table 3] Table 4 shows the microstructure and mechanical properties of the final cold-rolled steel sheets manufactured after completing the processes according to Tables 2 and 3. In Table 4, the microstructure TM+B means that it consists of tempered martensite and bainite, F represents ferrite, FM represents fresh martensite, RA represents retained austenite, and FM / (TM+B) represents the ratio of fresh martensite (FM) to the sum of tempered martensite and bainite (TM+B).

[0103] In addition, yield strength (YS), tensile strength (TS), elongation (EL), and porosity (HER) are shown, and YR×EL×HER represents yield ratio (YR)×elongation (EL)×porosity (HER).

[0104] [Table 4] Figure 3 A graph illustrating the properties of cold-rolled steel sheet according to an experimental embodiment of the present invention.

[0105] See Table 1 and Figure 3 The steels 1 to 6 of the invention satisfy the range of Formula 1) which the invention aims to achieve by satisfying the alloy composition proposed in the present invention.

[0106] On the other hand, comparative steels 1 to 9 deviate from the scope of Formula 1) that the present invention aims to achieve because they fail to meet the alloy composition proposed in the present invention.

[0107] In addition, see Table 4 and Figure 3 Comparative steels 1 to 9 showed yield ratios (YR) × elongation (EL) × pore size (HER) of less than 400, while all inventive steels 1 to 6 showed yield ratios (YR) × elongation (EL) × pore size (HER) of 400 or greater.

[0108] Referring to Table 2, Comparative Example 10 deviates from the process conditions proposed in this invention because the total reduction rate of the first to third passes during finishing rolling is lower than the total reduction rate of the remaining passes. Referring to Tables 1 and 3, Comparative Example 11 does not meet the Ms-140°C to Ms-30°C second cooling end temperature range proposed in this invention.

[0109] Referring to Table 4, when comparing Example 2 and Comparative Example 10, which used the same inventive steel 2, Example 2 satisfied all the mechanical properties intended to be achieved by the present invention, while Comparative Example 10 showed a yield ratio (YR) × elongation (EL) × porosity (HER) of less than 400. This appears to be because Comparative Example 10 deviated from the reduction ratio proposed by the present invention, and therefore the effect of austenite grain refinement was not significant.

[0110] Furthermore, when comparing Example 3 and Comparative Example 11, which use the same inventive steel 3, Example 3 satisfies all the mechanical properties that the present invention aims to achieve, while Comparative Example 11 does not satisfy the tensile strength (TS) of 980 MPa or greater that the present invention aims to achieve, and shows a yield ratio (YR) × elongation (EL) × porosity (HER) of less than 400.

[0111] Furthermore, Example 3 satisfies the area fraction of each microstructure component under the fraction targeted by the present invention, while Comparative Example 11 does not meet the microstructure composition requirements proposed by the present invention: the sum of tempered martensite and bainite is 60% to 80%, fresh martensite is less than 10%, and FM / (TM+B) < 0.25.

[0112] This appears to be because in Comparative Example 11, the second cooling end temperature exceeded Ms-30°C, resulting in a ferrite transformation, which reduced strength and elongation and may not ensure sufficient austenitic stability, making it impossible to obtain the target microstructure.

[0113] The second cooling end temperature of Comparative Examples 5 and 7 exceeded Ms-30°C, thus failing to adequately ensure austenite stability and not meeting the area fraction of residual austenite required by the present invention: 1% to 5%.

[0114] Therefore, when all the alloy composition and process conditions proposed in this invention are met, steel plates with high strength and excellent ductility can be manufactured, and cold-rolled steel plates with high yield ratio, excellent hole expansion, excellent corrosion resistance and excellent formability can be manufactured.

[0115] The technical concept of the present invention is not limited to the above-described embodiments and accompanying drawings, and it will be apparent to those skilled in the art that various substitutions, changes and modifications can be made without departing from the technical concept of the present invention.

Claims

1. A high-strength cold-rolled steel sheet, said high-strength cold-rolled steel sheet comprising, by weight %: carbon (C): 0.08% to 0.15%, silicon (Si): 0.8% to 1.5%, manganese (Mn): 2.0% to 3.0%, aluminum (Al): greater than 0% to 1.0%, phosphorus (P): greater than 0% and not greater than 0.02%, sulfur (S): greater than 0% and not greater than 0.01%, nitrogen (N): greater than 0% and not greater than 0.01%, boron (B): 0.001% to 0.005%, and the sum of at least one selected from titanium (Ti) and niobium (Nb): not greater than 0.1% (excluding 0%), and the balance being iron (Fe) and other unavoidable impurities. in, The final microstructure consists of tempered martensite, bainite, fresh martensite, ferrite, and retained austenite.

2. The high-strength cold-rolled steel sheet according to claim 1, wherein the high-strength cold-rolled steel sheet satisfies the following formulas 1) and 2). Formula 1): 10×C+0.4×(Si+6×Al)-0.4×Mn+15×(Ti+Nb)-52.5×B≤1.2 Formula 2): Yield ratio (YR) × elongation (EL) × porosity (HER) ≥ 400 (in, The element symbols in Formula 1) represent the content of each element in terms of weight %.

3. The high-strength cold-rolled steel sheet according to claim 1, wherein, In terms of area fraction, the total fraction of tempered martensite and bainite is 60% to 80%, fresh martensite is less than 10%, ferrite is 10% to 30%, and retained austenite is 1% to 5%.

4. The high-strength cold-rolled steel sheet according to claim 1, wherein, The ratio of fresh martensite (FM) to the sum of tempered martensite and bainite (TM+B) is FM / (TM+B) < 0.

25.

5. The high-strength cold-rolled steel sheet according to claim 1, wherein the high-strength cold-rolled steel sheet further comprises, by weight %, at least one selected from molybdenum (Mo), chromium (Cr), copper (Cu) and nickel (Ni) in a total amount not exceeding 0.5% (excluding 0%).

6. The high-strength cold-rolled steel sheet according to claim 1, wherein, The grain size of tempered martensite and bainite is 5 μm or smaller.

7. The high-strength cold-rolled steel sheet according to claim 1, wherein, The tensile strength (TS) is 980 MPa or greater, and the product of the tensile strength (TS) and the porosity (HER) (TS×HER) is 50000 MPa% or greater.

8. The high-strength cold-rolled steel sheet according to claim 1, wherein, With 0.015% or greater and It contains 0.04% or less of titanium (Ti) and 0.06% or more and 0.085% or less of niobium (Nb).

9. A method for manufacturing high-strength cold-rolled steel sheet, the method comprising: (a) A steel material is hot-rolled, the steel material comprising, by weight %: C: 0.08% to 0.15%, Si: 0.8% to 1.5%, Mn: 2.0% to 3.0%, Al: greater than 0% to 1.0%, P: greater than 0% and not greater than 0.02%, S: greater than 0% and not greater than 0.01%, N: greater than 0% and not greater than 0.01%, B: 0.001% to 0.005%, the sum of at least one selected from Ti and Nb: not greater than 0.1% (excluding 0%), and the balance Fe and other unavoidable impurities; (b) Cold rolling hot-rolled steel materials; (c) Annealing heat treatment of cold-rolled steel materials; (d) Cooling the annealed steel to a temperature equal to or below Ms temperature; and (e) The cooled steel material is plated. Step (a) includes rolling at least one pass at a reduction rate of 40% or greater per pass, and the rolling is performed such that the total reduction rate of the first to third passes is higher than the total reduction rate of the remaining passes. Step (d) includes: A first cooling step involving initial cooling at a first cooling rate to a first cooling end temperature ranging from 620°C to 720°C; and A second cooling step is performed after the first cooling, with a second cooling rate greater than the first cooling rate, to a second cooling end temperature equal to or lower than Ms temperature.

10. The method for manufacturing high-strength cold-rolled steel sheet according to claim 9, wherein, The steel material satisfies the following formula 1): Formula 1): 10×C+0.4×(Si+6×Al)-0.4×Mn+15×(Ti+Nb)-52.5×B≤1.2 (wherein, the element symbols in Formula 1 represent the content of each element in terms of weight %).

11. The method for manufacturing high-strength cold-rolled steel sheet according to claim 9, wherein the high-strength cold-rolled steel sheet further comprises, by weight %, at least 0.5% (excluding 0%) of at least one selected from Mo, Cr, Cu and Ni.

12. The method for manufacturing high-strength cold-rolled steel sheet according to claim 9, wherein, The second cooling cycle ends at a temperature of Ms-140°C or higher and Ms-30°C or lower.

13. The method for manufacturing high-strength cold-rolled steel sheet according to claim 9, wherein, With 0.015% or greater and It contains 0.04% or less of titanium (Ti) and 0.06% or more and 0.085% or less of niobium (Nb).

14. The method for manufacturing high-strength cold-rolled steel sheet according to claim 9, wherein, The first cooling rate ranges from 3°C / s to 20°C / s.

15. The method for manufacturing high-strength cold-rolled steel sheet according to claim 9, wherein, The second cooling rate ranges from 15°C / s to 100°C / s.

16. The method for manufacturing high-strength cold-rolled steel sheet according to claim 9, wherein, Step (a) includes: (a-1) Reheat the steel material at 1150°C to 1300°C; (a-2) Hot rolling of steel at a finishing temperature of 850°C to 950°C; and (a-3) The steel material is wound at 400°C to 600°C.

17. The method for manufacturing high-strength cold-rolled steel sheet according to claim 16, wherein, After step (a-2) and before step (a-3), a cooling step is performed to a temperature of 680°C or lower at a cooling rate of 80°C / s or greater.

18. The method for manufacturing high-strength cold-rolled steel sheet according to claim 9, wherein, Ms temperature is defined by the following equation (3): Equation 3): Ms = 539 - 423C - 30.4Mn - 12.1Cr - 17.7Ni - 7.5Mo (wherein, the element symbols in Equation 3) represent the content of each element in terms of weight %).

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