Ultra-high strength cold rolled steel sheet and its manufacturing method

Abstract

The present invention provides an ultra-high tensile cold-rolled steel sheet having an improved balance of strength and ductility, and a method for manufacturing the same. According to one embodiment of the present invention, the ultra-high tensile strength cold rolled steel sheet contains, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): greater than 0% to 0.05%, a total of at least one selected from titanium (Ti), niobium (Nb), and vanadium (V): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.005%, nitrogen (N): greater than 0% to 0.006%, and the balance being iron (Fe) and other unavoidable impurities, and satisfies yield strength (YS): 850 MPa or more, tensile strength (TS): 1180 MPa or more, elongation (EL): 14% or more, hole expandability (HER): 25% or more, and TS×EL×HER / 1000: 500 or more.

Description

[Technical Field]

[0001] The technical concept of the present invention relates to a steel material, and more particularly to an ultra-high tensile cold rolled steel sheet having an improved balance between strength and ductility, and a method for manufacturing the same. [Background technology]

[0002] Recently, the automotive industry has been applying various ultra-high strength steel sheets to automobiles in order to meet conflicting goals such as reducing the weight of automobiles and ensuring collision safety. Although conventional ultra-high strength steel sheets tend to have lower ductility as their strength increases, the situation is such that both strength and ductility must be improved to meet these goals.

[0003] Conventional methods for producing ultra-high-tensile strength galvanized steel sheets of 1.0 GPa or higher include heat treatment in a continuous annealing line (CAL) or hot-dip galvanizing in a continuous galvanizing line (CGL), followed by final cooling of the cold-rolled steel sheet to below the martensitic transformation start temperature (Ms) to generate martensite, thereby ensuring strength. However, the martensite generated in this process is primarily fresh martensite, which is quenched martensite that does not contain iron-based carbides. While this contributes to improved strength, it significantly degrades hydrogen embrittlement resistance and toughness. Tempered martensite, formed by tempering the martensite, contains iron-based carbides and can contribute to improved hydrogen embrittlement resistance and toughness.

[0004] To produce tempered martensite, a steel sheet undergoes annealing, then is cooled to a temperature below the martensitic transformation start temperature (Ms), reheated, and tempered, followed by cooling. Galvanized cold-rolled steel sheets can be produced by immersing the tempered cold-rolled steel sheet in a hot-dip galvanizing bath for galvanization. The galvanized cold-rolled steel sheet then undergoes an alloying process and is then cooled to room temperature for final cooling to produce ultra-high-tensile galvanized cold-rolled steel sheets. However, this manufacturing method requires the modification of existing equipment or the construction of a new line to incorporate rapid cooling and reheating equipment. Furthermore, during the galvanizing and alloying processes, martensite formed in the previous cooling process may be over-tempered, resulting in material degradation. Furthermore, the stability of retained austenite may be reduced, potentially resulting in the failure to achieve the TRIP effect. Prior art documents include Korean Patent Application No. 10-2014-7010908. Summary of the Invention [Problem to be solved by the invention]

[0005] The technical problem to be achieved by the technical idea of ​​the present invention is to provide an ultra-high tensile cold rolled steel sheet having an improved balance between strength and ductility, and a method for manufacturing the same.

[0006] However, these problems are merely examples, and the technical idea of ​​the present invention is not limited to these. [Means for solving the problem]

[0007] According to one aspect of the present invention, there is provided an ultra-high strength cold rolled steel sheet and a method for manufacturing the same.

[0008] According to one embodiment of the present invention, the ultra-high tensile strength cold rolled steel sheet may contain, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): greater than 0% to 0.05%, a total of at least one selected from titanium (Ti), niobium (Nb), and vanadium (V): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.005%, nitrogen (N): greater than 0% to 0.006%, and the balance being iron (Fe) and other unavoidable impurities, and may satisfy a yield strength (YS): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, an elongation (EL): 14% or more, a hole expandability (HER): 25% or more, and a TS×EL×HER / 1000: 500 or more.

[0009] According to one embodiment of the present invention, the ultra-high strength cold rolled steel sheet may have a mixed structure of ferrite, retained austenite, bainite, fresh martensite, and tempered martensite, wherein the area fraction of the ferrite is in a range of 10% to 20%, the area fraction of the retained austenite is in a range of 5% to 20%, the area fraction of the bainite is in a range of 5% to 20%, and the sum of the area fractions of the fresh martensite and the tempered martensite is the remaining area fraction.

[0010] According to an embodiment of the present invention, a ratio (FM / TM) of the fresh martensite (FM) to the tempered martensite (TM) may be 0.1 to 0.6.

[0011] According to one embodiment of the present invention, the number density of iron-based carbides in the tempered martensite is 1.0×10 6 (pcs / mm 2 ) or more.

[0012] According to an embodiment of the present invention, the tempered martensite in the ultra-high strength cold rolled steel sheet may have a grain size of 5 μm or less.

[0013] According to an embodiment of the present invention, the ultra-high strength cold rolled steel sheet may further contain a total of chromium (Cr) and molybdenum (Mo): more than 0% to 1.0%.

[0014] According to one embodiment of the present invention, a method for producing the ultra-high strength cold-rolled steel sheet includes the steps of: producing a hot-rolled steel sheet having an alloy composition containing, by weight, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): greater than 0% to 0.05%, a total of at least one selected from titanium (Ti), niobium (Nb), and vanadium (V): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.005%, nitrogen (N): greater than 0% to 0.006%, and the balance being iron (Fe) and other inevitable impurities; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and rolling the cold-rolled steel sheet at a temperature of Ac3-30°C to 900°C. The method may include the steps of: performing a primary soaking treatment at a primary soaking temperature for 30 to 200 seconds; primarily cooling the primarily soaked cold-rolled steel sheet at a cooling rate of 5 to 15°C / second to a primary cooling temperature of 620 to 720°C; secondarily cooling the primarily cooled cold-rolled steel sheet at a cooling rate of 15 to 100°C / second to a secondary cooling temperature of 250 to 480°C; secondary soaking the secondarily cooled cold-rolled steel sheet at a secondary soaking temperature of 250 to 480°C for 50 to 300 seconds; tertiary cooling the secondarily soaked cold-rolled steel sheet to a tertiary cooling temperature of 150°C or less; and tertiary soaking the tertiary cooled cold-rolled steel sheet at a tertiary soaking temperature of 150 to 300°C for 100 to 30,000 seconds.

[0015] According to one embodiment of the present invention, the step of manufacturing the hot-rolled steel sheet may include the steps of: reheating a steel material having the alloy composition at a reheating temperature of 1,150°C to 1,250°C; hot-rolling the reheated steel material; cooling the hot-rolled steel material at a cooling rate of 10°C / sec to 50°C / sec; and coiling the cooled steel material at a coiling temperature of 500°C to 700°C.

[0016] According to one embodiment of the present invention, the hot rolling step may include: a rough rolling step performed at 1,000°C to 1,150°C with a rolling reduction of 40% to 50% in the final pass; and a finish rolling step performed at a finish rolling end temperature of 880°C to 980°C with rolling in the final three stages of reduction rolls at 1,020°C or less so that the total rolling reduction is 40% or more, with the rolling reduction in one pass being in the range of 40% to 60%.

[0017] According to one embodiment of the present invention, in the hot rolling step, the steel sheet passing time through the final three stages of rolls in the finish rolling step may be in a range of 2.0 seconds or less (more than 0 seconds).

[0018] According to one embodiment of the present invention, the elapsed time from finish rolling to the start of cooling of the hot rolled steel may be 1.5 seconds or less.

[0019] According to an embodiment of the present invention, the method may further include a step of softening the hot-rolled steel sheet at a temperature in the range of 500°C to 650°C after the step of manufacturing the hot-rolled steel sheet.

[0020] According to an embodiment of the present invention, the method may further include hot-dip galvanizing the cold-rolled steel sheet after the second soaking step.

[0021] According to an embodiment of the present invention, the step of performing the secondary soaking treatment may be a step of hot-dip galvanizing the cold-rolled steel sheet.

[0022] According to an embodiment of the present invention, the method may further include a step of alloying treatment after the step of hot dip galvanizing. [Effects of the Invention]

[0023] According to the technical concept of the present invention, in the continuous heat treatment process or continuous hot-dip galvanizing heat treatment process, after the annealing heat treatment, plating treatment or plating alloying treatment, martensite is generated by cooling to Ms or less, and then reheating and isothermal holding are performed, whereby the martensite is appropriately tempered and the retained austenite is stabilized, preventing the martensite from being excessively tempered, making it possible to provide an ultra-high tensile cold-rolled steel sheet with an improved balance between strength and ductility.

[0024] The effects of the present invention described above are merely examples, and the scope of the present invention is not limited to these effects. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a process flowchart illustrating a method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention.

[0026] [Figure 2] 1 is a process flowchart illustrating a method for manufacturing an ultra-high tensile strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention.

[0027] [Figure 3] 1 is a graph showing a heat treatment history over time of an ultra-high strength cold rolled steel sheet according to an embodiment of the present invention.

[0028] [Figure 4] 1 is a graph showing a heat treatment history over time of an ultra-high tensile hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

[0029] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The present embodiments are provided to more completely explain the technical concept of the present invention to those skilled in the art. The following embodiments may be modified into various other forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more complete and complete and to fully convey the technical concept of the present invention to those skilled in the art. Throughout this specification, the same reference numerals refer to the same elements. Furthermore, various elements and regions in the drawings are shown schematically. Therefore, the technical concept of the present invention is not limited by the relative sizes and spacings shown in the accompanying drawings.

[0030] According to the technical concept of the present invention, in the continuous heat treatment process or continuous hot-dip galvanizing heat treatment process, after the annealing heat treatment, plating treatment or plating alloying treatment, the material is cooled to Ms or less to generate fresh martensite, and then tempered by reheating and isothermal holding, thereby converting the fresh martensite into tempered martensite.

[0031] Tempered martensite has an excellent balance of strength and toughness, but if the size of the tempered martensite is large or if the iron-based carbides present in the tempered martensite become coarse, the toughness may deteriorate. To prevent this, in the present invention, a constant amount of bainite is formed by isothermal holding in an appropriate temperature range during the annealing heat treatment process. The formed bainite disrupts austenite, and therefore, when the austenite transforms to martensite during subsequent cooling, the size of the martensite decreases, which in turn reduces the size of the tempered martensite formed after the tempering treatment.

[0032] In addition, the conditions of the tempering process after the annealing heat treatment are controlled to control the number or density of iron-based carbides so that excessive amounts of iron-based carbides are not formed when tempered martensite is formed, thereby ensuring desired performance.

[0033] Furthermore, to improve the formability of the final steel sheet, it is necessary to minimize the segregation of manganese (Mn) and other elements during the hot rolling process to obtain a uniform structure. To achieve this, it is necessary to minimize the segregation of manganese by controlling the temperature and reduction ratio during rough rolling and finish rolling.

[0034] The ultra-high strength cold rolled steel sheet according to the technical idea of ​​the present invention will be described in detail below.

[0035] An ultra-high tensile strength cold rolled steel sheet according to an embodiment of the present invention contains, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): greater than 0% to 0.05%, the total of at least one selected from titanium (Ti), niobium (Nb), and vanadium (V): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.005%, nitrogen (N): greater than 0% to 0.006%, and the balance being iron (Fe) and other unavoidable impurities.

[0036] The ultra-high tensile strength cold-rolled steel sheet may further include chromium (Cr): more than 0% to 1.0%. The ultra-high tensile strength cold-rolled steel sheet may further include the sum of chromium (Cr) and molybdenum (Mo): more than 0% to 1.0%.

[0037] The role and content of each element contained in the ultra-high strength cold rolled steel sheet according to the present invention will be described below. Here, the contents of the elemental elements are all expressed in wt% relative to the total weight of the steel sheet.

[0038] Carbon (C): 0.1%~0.3%

[0039] Carbon is added to ensure the strength of the steel sheet and to control the microstructure. If the carbon content is less than 0.1%, it is difficult to achieve the target strength. If the carbon content exceeds 0.3%, formability such as elongation and hole expandability may be reduced, and spot weldability may also be reduced. Therefore, it is preferable to add carbon in an amount of 0.1% to 0.3% of the total weight of the steel sheet.

[0040] Silicon (Si): 1.0% to 2.0%

[0041] Silicon is a ferrite stabilizing element that delays the formation of carbides in ferrite and tempered martensite, thereby enhancing solid solution strengthening. If the silicon content is less than 1.0%, the effect of silicon addition is insufficient. If the silicon content exceeds 2.0%, oxides such as Mn2SiO4 are formed, which can impair galvanic properties and increase the carbon equivalent, which can reduce weldability. Therefore, it is preferable to add silicon in an amount of 1.0% to 2.0% of the total weight of the steel sheet.

[0042] Manganese (Mn): 1.5% to 3.0%

[0043] Manganese has a solid solution strengthening effect and can contribute to improving strength by increasing hardenability. Strength, toughness, and yield ratio can be controlled by the manganese content. However, adding large amounts of manganese reduces the toughness of steel by inducing the formation of MnS inclusions and center segregation during casting. If the manganese content is less than 1.5%, the hardenability is insufficient, making it difficult to ensure strength, and the effect of adding manganese is insufficient. If the manganese content exceeds 3.0%, the formability may be reduced due to the formation and segregation of inclusions such as MnS, and the carbon equivalent may be increased, reducing weldability. Therefore, it is preferable to add manganese in an amount of 1.5% to 3.0% of the total weight of the steel sheet.

[0044] Aluminum (Al): Over 0% to 0.05%

[0045] Aluminum is used as a deoxidizer and can help purify ferrite. If the aluminum content exceeds 0.05%, it may form AlN during slab production, which may induce cracks during casting or hot rolling. Therefore, it is preferable to add aluminum in an amount greater than 0% to 0.05% of the total weight of the steel plate.

[0046] Sum of titanium (Ti), niobium (Nb) and vanadium (V): over 0% to 0.05%

[0047] Titanium, vanadium, and niobium are the main elements that precipitate in the form of carbides in steel. The purpose of adding titanium, vanadium, and niobium is to ensure the stability of retained austenite and improve strength by refining primary austenite grains through the formation of precipitates, and to refine ferrite grains and promote precipitation hardening through the presence of precipitates in ferrite. If the total content of titanium, vanadium, and niobium exceeds 0.05%, this may result in a deterioration in material quality and an increase in manufacturing costs. Therefore, the total content of titanium, niobium, and vanadium is preferably greater than 0% and less than 0.05% of the total weight of the steel sheet.

[0048] The steel plate may contain at least one of titanium, niobium, and vanadium, whereby the titanium may be 0% to 0.05% of the total weight of the steel plate, the niobium may be 0% to 0.05% of the total weight of the steel plate, and the vanadium may be 0% to 0.05% of the total weight of the steel plate.

[0049] Total of chromium (Cr) and molybdenum (Mo): Over 0% to 1.0%

[0050] Chromium and molybdenum act as hardening elements and contribute to the formation of a two-phase structure. If the total content of chromium and molybdenum exceeds 1.0%, the effects converge, which may increase the manufacturing cost. Therefore, the total content of chromium and molybdenum is preferably 0% to 1.0% of the total weight of the steel plate.

[0051] The steel plate may further contain at least one of chromium and molybdenum, whereby the chromium may be more than 0% to 1.0% of the total weight of the steel plate, and the molybdenum may be more than 0% to 1.0% of the total weight of the steel plate.

[0052] Phosphorus (P): Over 0% to 0.02%

[0053] Phosphorus is an impurity contained in the steel manufacturing process and can help improve strength through solid solution strengthening, but if contained in large amounts, it can cause low-temperature embrittlement. Therefore, it is preferable to limit the phosphorus content to more than 0% to 0.02% of the total weight of the steel sheet.

[0054] Sulfur (S): More than 0%~0.005%

[0055] Sulfur is an impurity contained in the steel manufacturing process and can form non-metallic inclusions such as FeS, MnS, etc., which can reduce toughness and weldability. Therefore, it is preferable to limit the sulfur content to more than 0% to 0.005% of the total weight of the steel plate.

[0056] Nitrogen (N): Over 0% to 0.006%

[0057] Nitrogen is an element that is inevitably contained during steel production, and if present in excess, a large amount of nitrides will precipitate, which can deteriorate ductility. Therefore, the nitrogen content is preferably limited to more than 0% to 0.006% of the total weight of the steel sheet.

[0058] The remaining component of the ultra-high strength cold-rolled steel sheet is iron (Fe). However, in a typical steelmaking process, unintended impurities are inevitably mixed in from raw materials or the surrounding environment, and it is not possible to eliminate these impurities. These impurities are known to any engineer of a typical manufacturing process, and therefore, the full details of these impurities will not be specifically mentioned in this specification.

[0059] An ultra-high tensile strength cold-rolled steel sheet produced by controlling the specific components and content ranges of the alloy composition described above and using the production method described below can, for example, satisfy a yield strength (YS) of 850 MPa or more, a tensile strength (TS) of 1180 MPa or more, an elongation (EL) of 14% or more, a hole expandability (HER) of 25% or more, and a TS×EL×HER / 1000 ratio of 500 or more.The ultra-high tensile strength cold-rolled steel sheet can, for example, satisfy a yield strength (YS) of 850 MPa to 1000 MPa, a tensile strength (TS) of 1180 MPa to 1300 MPa, an elongation (EL) of 14% to 20%, a hole expandability (HER) of 25% to 40%, and a TS×EL×HER / 1000 ratio of 500 to 900.

[0060] Here, "TS×EL×HER / 1000" means the value obtained by multiplying the tensile strength, elongation, and hole expandability and dividing the result by 1000.

[0061] The ultra-high strength cold rolled steel sheet may have a mixed structure of ferrite, retained austenite, bainite, fresh martensite, and tempered martensite.

[0062] The area fraction of the ferrite may be, for example, in the range of 10% to 20%. The area fraction of the retained austenite may be, for example, in the range of 5% to 20%. The area fraction of the bainite may be, for example, in the range of 5% to 20%. The remaining area fraction is made of martensite, and the martensite includes both fresh martensite and tempered martensite. The area fractions refer to area ratios derived from a microstructure photograph using an image analyzer.

[0063] The value (FM / TM) obtained by dividing the area ratio of the tempered martensite (TM) by the area ratio of the fresh martensite (FM) may be in the range of 0.1 to 0.6.

[0064] The number density of the iron-based carbides in the tempered martensite is, for example, 1.0 × 10 6 (pcs / mm 2) or more, for example, 1.0 × 10 6 ~20×10 6 (pcs / mm 2 ) may also be used.

[0065] The tempered martensite grain size may be, for example, 5 μm or less, for example, 1 μm to 5 μm. The grain size is measured using EBSD as a circle-equivalent diameter of a region surrounded by a boundary having a misorientation of 10° or more, and can be referred to as the grain size of the grain.

[0066] Hereinafter, a method for manufacturing an ultra-high strength cold rolled steel sheet according to the present invention will be described with reference to the accompanying drawings.

[0067] Manufacturing method for ultra-high strength cold rolled steel sheet

[0068] FIG. 1 is a process flow chart that schematically shows a method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention, and relates to a method for manufacturing an ungalvanized cold-rolled steel sheet.

[0069] Referring to FIG. 1, a method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention includes a hot-rolled steel sheet manufacturing step (S110), a cold-rolled steel sheet manufacturing step (S120), a first soaking step (S130), a first cooling step (S140), a second cooling step (S150), a second soaking step (S160), a third cooling step (S170), and a third soaking step (S180).

[0070] Hot-rolled steel sheet manufacturing step (S110)

[0071] In the step S110 of producing the hot-rolled steel sheet, a steel material containing, by weight, 0.1% to 0.3% carbon (C), 1.0% to 2.0% silicon (Si), 1.5% to 3.0% manganese (Mn), 0% to 0.05% aluminum (Al), 0% to 0.05% total of at least one selected from niobium (Nb), titanium (Ti), and vanadium (V), 0% to 0.02% phosphorus (P), 0% to 0.005% sulfur (S), 0% to 0.006% nitrogen (N), and the balance being iron (Fe) and other unavoidable impurities is prepared. The steel material may further contain 0% to 1.0% total of chromium (Cr) and molybdenum (Mo).

[0072] In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling process may be, for example, a slab. The semi-finished slab can be obtained through a continuous casting process after molten steel having a predetermined composition is obtained through a steelmaking process.

[0073] The steel material, for example, a slab plate, is reheated at a slab reheating temperature (SRT) of, for example, 1,150°C to 1,250°C for, for example, 1 hour to 5 hours. This reheating process can redissolve elements that segregated during casting and precipitates, homogenizing the steel and making it suitable for hot rolling. If the reheating temperature is less than 1,150°C, elements that segregated during casting may not be fully redissolved and may not be uniformly distributed. If the reheating temperature exceeds 1,250°C, austenite grains may coarsen, resulting in a decrease in yield strength. Furthermore, higher reheating temperatures can increase manufacturing costs and reduce productivity due to heating costs and the additional time required to adjust the hot rolling temperature. If the reheating time is less than 1 hour, the segregation zone may not be reduced sufficiently, while if it exceeds 5 hours, the grain size may increase, increasing process costs.

[0074] Next, the reheated steel is first hot-rolled after heating to adjust its shape. The hot-rolling may be performed successively by rough rolling and finish rolling. Through the hot-rolling step, the steel can be formed into a hot-rolled steel. The hot-rolled steel may be a hot-rolled steel plate.

[0075] As described above, in order to improve the formability of the final steel sheet, it is necessary to obtain a uniform structure by suppressing the segregation of manganese (Mn) and the like as much as possible during the hot rolling process. To this end, by appropriately controlling the control conditions such as the temperature and reduction rate during rough rolling and finish rolling, the segregation of manganese is controlled, the formation of a band-like structure is suppressed, and manganese is uniformly diffused, thereby making it possible to produce an ultra-high tensile steel containing a hard phase with uniform hardness after final tempering heat treatment.

[0076] To this end, the rough rolling step may be carried out at, for example, 1,000°C to 1,150°C. Rough rolling may be carried out over multiple passes while reciprocating back and forth across rough rolling rolls, and the reduction rate in the final pass may be 40% or more, for example, in the range of 40% to 50%. In the rough rolling step, by increasing the reduction rate in the final pass to 40% or more, austenite can be refined and manganese (Mn) segregation can be reduced.

[0077] Furthermore, in the finish rolling step, rolling with the final three roll stages is performed at 1020°C or less, for example, in the range of 880°C to 1020°C, but the reduction in the first pass must be 40% or more, and the total reduction in the final three roll stages must be controlled to 40% to 60%. In this case, the time for passing the steel sheet through the final three roll stages must be kept as short as possible, for example, controlled to 2.0 seconds or less (more than 0). By controlling the reduction in this finish rolling step, segregation of manganese (Mn) and phosphorus (P) can be reduced.

[0078] The finish rolling temperature is in the range of 880°C to 980°C. If the finish rolling temperature is less than 880°C, the rolling load increases sharply, which may result in a decrease in productivity. If the finish rolling temperature is more than 980°C, the crystal grains become coarse, which may result in a decrease in the strength of the final steel material.

[0079] Next, the hot-rolled steel material is cooled. The time elapsed from the final pass of finish rolling until the start of cooling is preferably as short as possible; for example, the elapsed time must be controlled to 1.5 seconds or less (more than 0). The cooling can be either air-cooling or water-cooling, and can be performed at a cooling rate of, for example, 10°C / s to 50°C / s. The cooling is preferably performed to a coiling temperature of, for example, 500°C to 700°C. If the cooling rate is less than 10°C / s, the average particle size of precipitates increases, making it difficult to ensure strength. Conversely, if the cooling rate exceeds 50°C / s, the structure of the steel material becomes hard, and impact toughness may decrease.

[0080] Next, the hot-rolled steel sheet is coiled at a coiling temperature (CT) in the range of, for example, 500°C to 700°C. If the coiling temperature is less than 500°C, the surface quality of the steel may deteriorate due to a sharp difference between the finish rolling temperature and the coiling temperature, and the strength may increase, which may increase the rolling load during cold rolling. If the coiling temperature exceeds 700°C, the carbonitride elements may not be maintained in a solid solution state and may form as undesired precipitates, which may cause defects in subsequent processes due to surface oxidation, etc. The coiled steel sheet may be cooled to room temperature.

[0081] Softening heat treatment step

[0082] Optionally, after the step of producing the hot-rolled steel sheet is performed, the hot-rolled steel sheet may be softened at a temperature in the range of, for example, 500°C to 650°C for, for example, 1 hour to 10 hours. The softening step can effectively control the influence of the microstructure of the hot-rolled steel sheet after hot rolling and before cold rolling.

[0083] Typically, changes in the coiling temperature during the hot rolling process affect the microstructure and physical properties of the hot-rolled steel sheet. The same effect can also be caused by the cooling rate of the coil after coiling. It is difficult to uniformly control the coiling temperature across the entire width / longitudinal direction of the steel sheet. Furthermore, during cooling in the yard after coiling, the cooling rate can vary depending on seasonal factors or the presence or absence of stacked coils, which can result in significant deviations in the material properties of the hot-rolled steel sheet. Such changes in the physical properties of the hot-rolled steel sheet continue to have an impact on the subsequent cold rolling process, significantly affecting the quality of the final product. To eliminate such deviations in the material properties or the influence of the microstructure of the hot-rolled steel sheet, the softening heat treatment step can be performed.

[0084] The softening heat treatment can soften the hot-rolled steel sheet, reducing the rolling load in the subsequent cold rolling, thereby reducing thickness deviation that often occurs when cold rolling high-tensile steel, and facilitating shape control.

[0085] If the softening heat treatment temperature is less than 500°C, the hot-rolled steel sheet is not softened sufficiently, and the influence of the structure after hot rolling on the finally obtained cold-rolled steel sheet cannot be eliminated. Moreover, the structure after the softening heat treatment may be formed to be non-uniform.

[0086] If the softening heat treatment temperature exceeds 650°C, a non-uniform austenite phase is formed, and unwanted phases are generated during the cooling process, which may affect the annealing conditions of the final steel sheet.

[0087] Furthermore, when softening heat treatment is performed for a long time at 600°C or higher, various alloy carbides are precipitated during the heat treatment, and it becomes difficult to redissolve these alloy carbides during the subsequent continuous annealing, which may result in the failure to obtain the desired mechanical properties. Therefore, it is preferable to perform the softening heat treatment for 10 hours or less.

[0088] Cold-rolled steel sheet manufacturing step (S120)

[0089] In the cold-rolled steel sheet manufacturing step (S120), the hot-rolled steel sheet is cold-rolled to adjust the thickness to the final steel sheet. The coiled hot-rolled steel sheet is then pickled with acid. The pickled hot-rolled steel sheet is then cold-rolled at a cold reduction of 40% to 60% to form a cold-rolled steel sheet. If the cold reduction is less than 40%, the amount of nuclei generated for recrystallization during the subsequent soaking treatment is small, which can lead to excessive grain growth during the soaking treatment and a rapid decrease in strength. If the cold reduction is more than 60%, the amount of nuclei generated is excessive and the grains formed during the soaking treatment are excessively fine, which can reduce ductility and formability.

[0090] After cold rolling is completed, a predetermined heat treatment process is performed to obtain the desired final microstructure. Figure 3 is a graph showing the heat treatment history of an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention. Hereinafter, the heat treatment steps of the cold-rolled steel sheet will be described in detail with reference to Figures 1 and 3.

[0091] First soaking step (S130)

[0092] In the first soaking step (S130), the cold-rolled steel sheet may be soaked in a continuous annealing furnace having a conventional slow cooling section. In the first soaking, the cold-rolled steel sheet is heated to a first soaking temperature of, for example, Ac3-30°C to 900°C at a temperature increase rate of, for example, 1°C / sec or more, for example, 1°C / sec to 10°C / sec. The steel sheet is held at the first soaking temperature for, for example, 30 seconds to 200 seconds. The first soaking can ensure a target austenite fraction. If the first soaking temperature is lower than Ac3-30°C or the holding time is shorter than 30 seconds, it may be difficult to form sufficient austenite, resulting in a high ferrite fraction and a decrease in strength. If the first soaking temperature exceeds 900°C or the holding time exceeds 200 seconds, the austenite grain size may become coarse or productivity may be excessively reduced.

[0093] First cooling step (S140)

[0094] In the primary cooling step (S140), the cold-rolled steel sheet that has undergone the primary soaking treatment is primarily cooled to a primary cooling temperature of, for example, 620°C to 720°C at a cooling rate of, for example, 5°C / sec to 15°C / sec. The cooling may be performed by air cooling or water cooling. The primary cooling may be referred to as a slow cooling step. The primary cooling is performed to ensure a certain amount of ferrite in the final microstructure to ensure plasticity. If the primary cooling temperature is less than 620°C, excessive ferrite transformation may occur, resulting in a decrease in strength. If the primary cooling temperature exceeds 720°C, the temperature difference with the subsequent secondary cooling section may increase significantly, resulting in quality issues or reduced productivity.

[0095] Secondary cooling step (S150)

[0096] In the secondary cooling step (S150), the primarily cooled cold-rolled steel sheet is secondarily cooled at a cooling rate of, for example, 15°C / sec to 100°C / sec to a secondary cooling temperature of, for example, 250°C to 480°C. The secondary cooling can be referred to as a rapid cooling step. Additional ferrite transformation must be suppressed during the secondary cooling.

[0097] Secondary soaking step (S160)

[0098] In the second soaking step (S160), the second-cooled cold-rolled steel sheet is subjected to a second soaking treatment at a second soaking temperature of 250°C to 480°C for 50 to 300 seconds. In the second soaking step, a constant amount of bainite is formed by isothermal holding in the temperature range of 250°C to 480°C. The bainite formed at this time disrupts austenite, and therefore, when the austenite transforms into martensite during subsequent cooling, the size of the martensite is reduced. This also reduces the size of the tempered martensite formed after the tempering treatment, contributing to refinement of the tempered martensite.

[0099] Third cooling step (S170)

[0100] In the tertiary cooling step (S170), the cold-rolled steel sheet is subjected to tertiary cooling, for example, to a tertiary cooling temperature of room temperature (0°C to 40°C) to 150°C. In the tertiary cooling step (S170), the austenite is transformed into fresh martensite by cooling to a temperature equal to or lower than Ms.

[0101] Third soaking step (S180)

[0102] In the third soaking step (S180), the tertiarily cooled cold-rolled steel sheet is heated to a third soaking temperature of, for example, 150°C to 300°C at a heating rate of, for example, 1°C / sec or more, for example, 1°C / sec to 10°C / sec, and held at the third soaking temperature for, for example, 100 seconds to 30,000 seconds. In the third soaking step, the fresh martensite generated in the second cooling transforms into tempered martensite, which can control the number of carbides and concentrate and stabilize the retained austenite with carbon. This ensures high strength and elongation and maintains the final microstructure. If the third soaking temperature is less than 150°C or the holding time is less than 100 seconds, the martensite may not be tempered sufficiently or the retained austenite may not be stabilized sufficiently. If the tertiary soaking temperature exceeds 300°C or the holding time exceeds 30,000 seconds, the martensite may be excessively tempered, resulting in a decrease in strength, and the phase transformation of retained austenite may proceed, resulting in a decrease in formability.

[0103] After the third soaking step (S180) is completed, the cold-rolled steel sheet is cooled to room temperature, for example, 0°C to 40°C, at a cooling rate of 1°C / sec to 100°C / sec.

[0104] According to an embodiment of the present invention, after the first soaking step (S130), a slow cooling step is performed, and instead of immediately lowering the temperature to below Ms through a rapid cooling step, a second soaking step is performed in between to form bainite with a predetermined content, and then a step of cooling to below Ms is performed.

[0105] Fig. 2 is a process flow chart that schematically illustrates a method for manufacturing an ultra-high strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention, and Fig. 4 is a graph that illustrates the heat treatment history of an ultra-high strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention over time. Fig. 4 illustrates both the hot-dip galvanizing step (S161) and the alloying step (S162).

[0106] In this embodiment, steps S110 to S150 are the same as the above-described method for manufacturing an unplated ultra-high tensile strength cold-rolled steel sheet.

[0107] In this embodiment, as shown in FIG. 4, the cold-rolled steel sheet that has undergone the secondary soaking treatment is put into a galvanizing bath and subjected to the hot-dip galvanizing step (S161 in FIG. 2).

[0108] Alternatively, the steel sheet may be placed in a galvanizing bath immediately after the temperature reaches the second soaking temperature, and the hot-dip galvanizing step may be performed. In this case, the second soaking step (S160) may be considered as the hot-dip galvanizing step.

[0109] If necessary, an alloying step (S165 in FIG. 4) may be additionally performed after the hot-dip galvanizing step to form a hot-dip galvanized steel material and a galvannealed hot-dip galvanized steel material.

[0110] Hot-dip galvanized step (S161)

[0111] The hot-dip galvanizing step involves immersing the cold-rolled steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer. The temperature of the galvanizing bath may vary depending on the types and ratios of alloy elements constituting the galvanized layer and the composition of the cold-rolled steel sheet, and may be, for example, 450°C to 480°C. The cold-rolled steel sheet is immersed in the galvanizing bath and held for, for example, 30 seconds to 100 seconds. Under the conditions of the galvanizing bath, a hot-dip galvanized layer is easily formed on the surface of the cold-rolled steel sheet, and the adhesion of the galvanized layer is excellent.

[0112] 4 shows both the hot-dip galvanizing step and the alloying step, but if the alloying step is not performed after the hot-dip galvanizing step, the steel sheet that has been coated and exits the coating tank is immediately subjected to a tertiary cooling step (S170), in which austenite transforms to martensite. The tertiary soaking step (S180) is then performed. Since the tertiary soaking step (S180) has already been described above, its description will be omitted here to avoid redundancy.

[0113] Alloying step (S165)

[0114] If necessary, a step of subjecting the cold-rolled steel sheet having the hot-dip galvanized layer to an alloying heat treatment can be further performed. To perform the alloying step, the steel sheet that has been completely coated and exited the coating bath can be introduced into a heat treatment device for the alloying heat treatment. The alloying heat treatment can be performed at a temperature of, for example, 500°C to 600°C, for example, 1 to 20 seconds. When the alloying heat treatment is performed under these conditions, the hot-dip galvanized layer grows stably and exhibits excellent adhesion. If the alloying heat treatment temperature is less than 500°C, alloying may not proceed sufficiently, resulting in a decrease in the integrity of the hot-dip galvanized layer. If the alloying heat treatment temperature is more than 600°C, changes in material properties may occur as the steel sheet transitions into the two-phase temperature range. After the alloying heat treatment is completed, the steel sheet undergoes a third cooling step (S170), in which austenite transforms to martensite. Thereafter, the third soaking step (S180) is carried out. Since the third soaking step (S180) has already been described above, a description thereof will be omitted here to avoid duplication.

[0115] Experimental example

[0116] In the following, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are merely provided to aid in understanding the present invention, and the present invention is not limited to these experimental examples. Contents not described here can be fully inferred by those skilled in the art, and therefore, explanations thereof will be omitted.

[0117] Steel materials having the compositions (unit: weight %) shown in Tables 1 and 2 below were prepared. In Tables 1 and 2, the balance consists of iron (Fe) and impurities that are inevitably contained in the steelmaking process, etc. The content unit of each component is weight %.

[0118] [Table 1]

[0119] [Table 2]

[0120] Referring to Tables 1 and 2, steel types A, B, D, and E satisfy the composition range of the present invention. Steel type C differs in that the silicon content is lower than the lower limit of the composition range of the present invention and the manganese content is higher than the upper limit of the composition range of the present invention.

[0121] Steel types A to E were subjected to hot rolling and cold rolling processes to form cold-rolled steel sheets, respectively.

[0122] Table 3 shows the condition values ​​of the hot rolling process of the ultra-high strength cold rolled steel sheets of the experimental examples.

[0123] [Table 3]

[0124] Referring to Table 3, Experimental Examples 1 to 10 were manufactured by satisfying the hot rolling process conditions proposed in the present invention. However, Experimental Examples 5, 6, 9, and 10 were further subjected to softening heat treatment at 600°C for 2 hours.

[0125] Table 4 shows the condition values ​​of the heat treatment process after cold rolling in the experimental examples.

[0126] [Table 4]

[0127] Referring to Table 4, among Experimental Examples 1 to 10, in Experimental Example 9, the final product was a cold-rolled steel sheet (CR) without a coating layer, while in the remaining Experimental Examples, the final product was a galvannealed steel sheet (GA). Experimental Examples 1, 7, 9, and 10 were manufactured by satisfying all of the process conditions proposed in the present invention. The coating temperature in Table 4 refers to the temperature of molten zinc in the coating bath.

[0128] Experimental Example 2 differs in that the tertiary cooling temperature is higher than the upper limit of the process range of the present invention; Experimental Example 3 differs in that the tertiary soaking temperature is higher than the upper limit of the process range of the present invention; Experimental Example 4 differs in that the first soaking temperature is lower than the lower limit of the process range of the present invention and the tertiary soaking temperature is higher than the upper limit of the process range of the present invention; Experimental Example 5 differs in that the tertiary soaking temperature is higher than the upper limit of the process range of the present invention; Experimental Example 6 differs in that the tertiary soaking holding time is lower than the lower limit of the process range of the present invention; and Experimental Example 8 differs in that the tertiary soaking temperature is higher than the upper limit of the process range of the present invention and the tertiary soaking holding time is lower than the lower limit of the process range of the present invention.

[0129] Table 5 shows the results of measuring the mechanical properties of the experimental examples, including tensile strength (TS), elongation (EL), hole expansion capacity (HER), and the product of tensile strength, elongation, and hole expansion capacity (TS × EL × HER / 1000).

[0130] [Table 5]

[0131] Referring to Table 5, Experimental Examples 1, 7, 9, and 10, which met all of the process conditions proposed in the present invention, met the target ranges of the present invention for tensile strength (TS), elongation (EL), hole expandability (HER), and the product of tensile strength, elongation, and hole expandability (TS×EL×HER / 1000).

[0132] In contrast, in Experimental Examples 2, 3, 4, 5, 6 and 8, which did not meet the process conditions proposed in the present invention, the mechanical properties did not meet the target range of the present invention.

[0133] Specifically, Experimental Examples 2, 4, and 5 showed tensile strength and TS×EL×HER / 100 values ​​lower than the lower limit of the target range of the present invention, Experimental Example 3 showed tensile strength values ​​lower than the lower limit of the target range of the present invention, Experimental Example 6 showed elongation and TS×EL×HER / 1000 values ​​lower than the lower limit of the target range of the present invention, and Experimental Example 8 showed tensile strength values ​​higher than the upper limit of the target range of the present invention and TS×EL×HER / 1000 values ​​lower than the lower limit of the target range of the present invention.

[0134] Table 6 shows the microstructure of the experimental examples, including the area fractions of ferrite, retained austenite, bainite, fresh martensite (FM), and tempered martensite (TM), the value obtained by dividing the area ratio of fresh martensite (FM) by the area ratio of tempered martensite (TM) (FM / TM), and the number of tempered martensite particles with a size of 5 μm or more.

[0135] [Table 6]

[0136] Experimental Examples 1, 7, 9, and 10 are examples of the present invention, and satisfied the target ranges of the present invention for the area fractions of ferrite, retained austenite, bainite, fresh martensite (FM), and tempered martensite (TM), the number of tempered martensite particles of 5 μm or more, and the FM / TM ratio.

[0137] In contrast, the bainite content and FM / TM ratio in Experimental Example 2 were higher than the upper limit of the target range of the present invention. Experimental Example 2 had a higher tertiary cooling temperature than the upper limit of the process range of the present invention, and therefore the relatively high bainite content was analyzed to have resulted in low mechanical strength.

[0138] Experimental Example 3 differs in that the tertiary soaking temperature was higher than the upper limit of the process range of the present invention, and the ferrite area fraction was higher than the upper limit of the target range of the present invention, and the mechanical properties were lower than the target range.

[0139] Experimental Example 4 was processed under the conditions that the first soaking temperature was lower than the lower limit of the process range of the present invention and the third soaking temperature was higher than the upper limit of the process range of the present invention. As a result, the area fraction of ferrite was 77%, which showed a microstructure in which ferrite was essentially the main phase, and therefore showed very low tensile strength and TS×EL×HER value.

[0140] Experimental Example 5 had a lower silicon content and a higher manganese content than the composition of the present invention, and the third soaking temperature was higher than the upper limit of the process range of the present invention, resulting in a lower FM / TM ratio and lower tensile strength and TS×EL×HER / 1000 values ​​than the lower limit of the target range of the present invention.

[0141] Experimental Example 6 had a lower silicon content, a higher manganese content, and a tertiary soaking time lower than the lower limit of the process range of the present invention, which resulted in insufficient tempering and a higher FM / TM ratio than the upper limit of the target range of the present invention, resulting in lower elongation and TS×EL×HER / 1000 than the lower limit of the target range of the present invention.

[0142] In Experimental Example 8, the tertiary soaking temperature was higher than the upper limit of the process range of the present invention, and therefore the area fraction of bainite was higher than the upper limit of the target range of the present invention due to excessive tempering, and as a result, the tensile strength and TS×EL×HER / 1000 value were lower than the target range of the present invention.

[0143] It will be apparent to those 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 accompanying drawings, and that various substitutions, modifications and changes are possible within the scope of the technical idea of ​​the present invention.

Claims

1. In weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% to 0.05%, the total of at least one selected from titanium (Ti), niobium (Nb) and vanadium (V): more than 0% to 0.05%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.005%, nitrogen (N): more than 0% to 0.006%, and the balance including iron (Fe) and other unavoidable impurities, An ultra-high tensile cold-rolled steel sheet that satisfies the following requirements: yield strength (YS): 850 MPa or more, tensile strength (TS): 1180 MPa or more, elongation (EL): 14% or more, hole expandability (HER): 25% or more, and TS x EL x HER / 1000: 500 or more.

2. The ultra-high tensile cold rolled steel sheet is A mixed structure containing ferrite, retained austenite, bainite, fresh martensite, and tempered martensite, the area fraction of the ferrite is in the range of 10% to 20%; the area fraction of the retained austenite is in the range of 5% to 20%, the area fraction of bainite is in the range of 5% to 20%, 2. The ultra-high strength cold rolled steel sheet according to claim 1, wherein the sum of the area fractions of the fresh martensite and the tempered martensite is the remaining area fraction.

3. The ultra-high tensile cold rolled steel sheet according to claim 2, wherein a ratio (FM / TM) of the fresh martensite (FM) to the tempered martensite (TM) is 0.1 to 0.

6.

4. The number density of the iron-based carbides in the tempered martensite is 1.0 × 10 ６ (pcs / mm ２ 3. The ultra-high tensile strength cold rolled steel sheet according to claim 2, wherein the tensile strength is 100% or more.

5. The ultra-high tensile cold rolled steel sheet is 3. The ultra-high strength cold rolled steel sheet according to claim 2, wherein the tempered martensite has a crystal grain size of 5 μm or less.

6. The ultra-high tensile cold rolled steel sheet is The ultra-high tensile strength cold rolled steel sheet according to claim 1, further comprising a total of chromium (Cr) and molybdenum (Mo): more than 0% to 1.0%.

7. manufacturing a hot-rolled steel sheet having an alloy composition containing, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% to 0.05%, the total of at least one element selected from titanium (Ti), niobium (Nb) and vanadium (V): more than 0% to 0.05%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.005%, nitrogen (N): more than 0% to 0.006%, and the balance being iron (Fe) and other unavoidable impurities; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; A step of subjecting the cold-rolled steel sheet to a primary soaking treatment at a primary soaking temperature of Ac3-30°C to 900°C for 30 seconds to 200 seconds; a step of primarily cooling the cold-rolled steel sheet that has been subjected to the primary soaking treatment to a primary cooling temperature of 620°C to 720°C at a cooling rate of 5°C / sec to 15°C / sec; Secondary cooling the primarily cooled cold-rolled steel sheet at a cooling rate of 15°C / sec to 100°C / sec to a secondary cooling temperature of 250°C to 480°C; subjecting the second-cooled cold-rolled steel sheet to a second soaking temperature of 250°C to 480°C for 50 seconds to 300 seconds; Tertiary cooling the cold-rolled steel sheet that has been subjected to the secondary soaking treatment to a tertiary cooling temperature of 150°C or less; and subjecting the tertiarily cooled cold-rolled steel sheet to a tertiary soaking temperature of 150°C to 300°C for 100 seconds to 30,000 seconds.

8. The step of manufacturing the hot-rolled steel sheet includes: Reheating the steel material having the alloy composition at a reheating temperature of 1,150°C to 1,250°C; hot rolling the reheated steel material; Cooling the hot-rolled steel material at a cooling rate of 10°C / sec to 50°C / sec; and coiling the cooled steel material at a coiling temperature of 500°C to 700°C. The hot rolling step includes: a rough rolling step carried out at 1,000°C to 1,150°C with a rolling reduction of 40% to 50% in the final pass; and a finish rolling step in which the finish rolling is performed at a finish rolling end temperature of 880°C to 980°C, and rolling with the final three stages of rolling rolls is performed at 1020°C or less so that the total reduction is 40% or more, and the reduction in one pass is in the range of 40% to 60%.

9. In the finish rolling step, The method for producing an ultra-high tensile cold rolled steel sheet according to claim 8, wherein the steel sheet passing time through the final three roll stages is in the range of 2.0 seconds or less (more than 0).

10. The method for producing an ultra-high strength cold rolled steel sheet according to claim 8, wherein the time elapsed from finish rolling to the start of cooling of the hot rolled steel material is 1.5 seconds or less.

11. After the step of manufacturing the hot-rolled steel sheet is performed, The method for producing an ultra-high tensile cold rolled steel sheet according to claim 7, further comprising the step of subjecting the hot rolled steel sheet to softening heat treatment at a temperature in the range of 500°C to 650°C.

12. After the second soaking step, The method for producing an ultra-high strength cold rolled steel sheet according to claim 7, further comprising the step of hot-dip galvanizing the cold rolled steel sheet.

13. The method for producing an ultra-high strength cold rolled steel sheet according to claim 12, wherein the step of performing the secondary soaking treatment is a step of hot-dip galvanizing the cold rolled steel sheet.

14. The method for producing an ultra-high tensile cold rolled steel sheet according to claim 12 or 13, further comprising a step of performing an alloying treatment after the step of hot dip galvanizing.

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