Ultra-high-strength cold-rolled steel sheet and method for producing the same

Abstract

An ultra-high-strength cold-rolled steel sheet with a balanced improvement in strength and ductility and a manufacturing method thereof are provided. According to one embodiment of the present invention, the ultra-high-strength cold-rolled steel sheet comprises carbon (C): 0.1 to 0.3 wt%, silicon (Si): 1.0 to 2.0 wt%, manganese (Mn): 1.5 to 3.0 wt%, aluminum (Al): more than 0 wt% and up to 0.05 wt%, a combination of one or more selected from titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt% and up to 0.05 wt%, phosphorus (P): more than 0 wt% and up to 0.02 wt%, sulfur (S): more than 0 wt% and up to 0.005 wt%, nitrogen (N): more than 0 wt% and up to 0.006 wt%.-% and a balance of iron (Fe) and other unavoidable impurities, wherein the ultra-high-strength cold-rolled steel sheet satisfies: a yield strength (YS): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, an elongation (EL): 14% or more, a hole expansion ratio (HER): 25% or more, and TS×EL×HER / 1000: 500 or more.

Description

TECHNICAL FIELD

[0001] The present invention relates to a steel material, particularly an ultra-high-strength cold-rolled steel sheet having a balanced improvement in strength and ductility, and a manufacturing method thereof. TECHNICAL BACKGROUND

[0002] Recently, the automotive industry has been using various ultra-high-strength steel sheets in vehicles to achieve conflicting goals such as vehicle lightweighting and crashworthiness. In general, ultra-high-strength steel sheets exhibit a decrease in ductility with increasing strength. To achieve these goals, both strength and ductility must be improved.

[0003] A conventional method for producing ultra-high-strength galvanized steel sheet with a strength of 1.0 GPa or more involves annealing or hot-dip galvanizing a cold-rolled steel sheet in a continuous annealing line (CAL) or a continuous galvanizing line (CGL), and cooling the cold-rolled steel sheet to a martensite initiation temperature (Ms) or below during final cooling to form martensite, thereby achieving strength. However, the martensite formed at this time is mainly fresh martensite in a quenched state and does not contain iron carbides. This contributes to strength improvement but significantly deteriorates hydrogen embrittlement resistance or toughness. Tempered martensite, formed by tempering martensite, contains iron carbides and can thus contribute to improved hydrogen embrittlement resistance and toughness.

[0004] To form tempered martensite, after annealing, the steel sheet is cooled to a temperature of or below Ms, reheated for tempering, and then cooled again. For galvanized cold-rolled steel sheet, the tempered cold-rolled steel sheet can be immersed in a galvanizing bath for galvanizing. Furthermore, the galvanized cold-rolled steel sheet can be alloyed and then cooled to room temperature during final cooling to produce ultra-high-strength galvanized cold-rolled steel sheet. However, these processes require modifications to existing equipment or the construction of new lines to implement rapid cooling and reheating systems. Furthermore, the martensite formed in the previous cooling process can be excessively tempered during galvanizing and alloying, leading to material degradation and a decrease in the stability of the retained austenite.Therefore, the TRIP effect (transformation-induced plasticity effect) is not expected. The prior art document includes Korean Patent Application No. 2014-7010908. DETAILED DESCRIPTION OF THE INVENTION TECHNICAL PROBLEM

[0005] The present invention provides an ultra-high strength cold-rolled steel sheet having a balanced improvement in strength and ductility and a method for producing the same.

[0006] However, the above description is an example, and the scope of the present invention is not limited thereto. TECHNICAL SOLUTION

[0007] The present invention provides an ultra-high strength cold-rolled steel sheet and a method for producing the same.

[0008] According to one aspect of the present invention, there is provided an ultra-high-strength cold-rolled steel sheet comprising carbon (C): 0.1 wt% to 0.3 wt%, silicon (Si): 1.0 wt% to 2.0 wt%, manganese (Mn): 1.5 wt% to 3.0 wt%, aluminum (Al): more than 0 wt% and up to 0.05 wt%, a combination of one or more selected from titanium (Ti), niobium (Nb) and vanadium (V): more than 0 wt% and up to 0.05 wt%, phosphorus (P): more than 0 wt% and up to 0.02 wt%, sulfur (S): more than 0 wt% and up to 0.005 wt%, nitrogen (N): more than 0 wt% and up to 0.006 wt% and a balance of iron (Fe) and other unavoidable impurities, wherein the ultra-high-strength cold-rolled steel sheet has 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 expansion ratio (HER) of 25% or more, and TS×EL×HER / 1000 of 500 or more.

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

[0010] A ratio (FM / TM) of fresh martensite (FM) to tempered martensite (TM) can be 0.1 to 0.6.

[0011] A density of iron carbide particles in tempered martensite can be 1.0 × 10 6 particles / mm 2 or more.

[0012] A grain size of tempered martensite can be 5 µm or less.

[0013] The ultra-high-strength cold-rolled steel sheet may also comprise a combination of chromium (Cr) and molybdenum (Mo): more than 0 wt% and up to 1.0 wt%.

[0014] According to another aspect of the present invention, there is provided a manufacturing method of an ultra-high-strength cold-rolled steel sheet, the method comprising: manufacturing a hot-rolled steel sheet having an alloy composition of carbon (C): 0.1 wt% to 0.3 wt%, silicon (Si): 1.0 wt% to 2.0 wt%, manganese (Mn): 1.5 wt% to 3.0 wt%, aluminum (Al): more than 0 wt% and up to 0.05 wt%, a combination of one or more selected from titanium (Ti), niobium (Nb) and vanadium (V): more than 0 wt% and up to 0.05 wt%, phosphorus (P): more than 0 wt% and up to 0.02 wt%, sulfur (S): more than 0 wt% and up to 0.005 wt%, nitrogen (N): more than 0 wt% and up to 0.006 wt%, and a balance of iron (Fe) and other unavoidable impurities, producing a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet,primary soaking of the cold-rolled steel sheet at a primary soaking temperature of Ac3-30 °C to 900 °C for 30 seconds to 200 seconds, primary cooling of the primary soaked cold-rolled steel sheet at a cooling rate of 5 °C / s to 15 °C / s to a primary cooling temperature of 620 °C to 720 °C, secondary cooling of the primary cooled cold-rolled steel sheet at a cooling rate of 15 °C / s to 100 °C / s to a secondary cooling temperature of 250 °C to 480 °C, secondary soaking of the secondary cooled cold-rolled steel sheet at a secondary soaking temperature of 250 °C to 480 °C for 50 Seconds to 300 seconds, tertiary cooling of the secondary soaked cold-rolled steel sheet to a tertiary cooling temperature of 150 °C or less, and tertiary soaking of the tertiary cooled cold-rolled steel sheet at a tertiary soak temperature of 150 °C to 300 °C for 100 seconds to 30,000 seconds.

[0015] The manufacturing of the hot-rolled steel sheet may include: reheating a steel material having the alloy composition at a slab reheating temperature of 1150 °C to 1250 °C, hot rolling the reheated steel material, cooling the hot-rolled steel material at a cooling rate of 10 °C / s to 50 °C / s, and coiling the cooled steel material at a coiling temperature of 500 °C to 700 °C.

[0016] The hot rolling may include: a rough rolling process carried out at 1000 °C to 1150 °C with a reduction ratio of 40% to 50% in a final pass, and a finish rolling process carried out at a final conveying temperature of 880 °C to 980 °C, wherein rolling is carried out by a final three-roll stand at a temperature of 1020 °C or less and a total reduction ratio of 40% or more and a reduction ratio of 40% to 60% in a first pass.

[0017] For example, a time required for the steel sheet to pass through the final three-roll stand during the final rolling process is not longer than 2.0 seconds (and longer than 0 seconds).

[0018] For example, a time required from an end of the finish rolling process to a start of cooling of the hot-rolled steel material is not longer than 1.5 seconds.

[0019] The method may further comprise softening the hot-rolled steel sheet at a temperature in a range of 500 °C to 650 °C after the hot-rolled steel sheet has been produced.

[0020] The method may further comprise hot-dip galvanizing the cold-rolled steel sheet after the cold-rolled steel sheet has been secondarily soaked.

[0021] Secondary soaking can be hot-dip galvanizing of the cold-rolled steel sheet.

[0022] The method may further comprise alloying the cold-rolled steel sheet after the cold-rolled steel sheet has been hot-dip galvanized. BENEFICIAL EFFECTS

[0023] According to the present invention, a steel sheet annealed, galvanized, or galvanized and alloyed in a continuous annealing or galvanizing line can be cooled to the martensite initiation temperature (Ms) or below to form martensite, and then reheated and maintained at a constant temperature to appropriately temper martensite and stabilize the retained austenite to prevent excessive tempering of martensite. In this way, an ultra-high-strength cold-rolled steel sheet with a balanced improvement in strength and ductility can be provided.

[0024] The above-described effects of the present invention are examples, and the scope of the present invention is not limited thereto. DESCRIPTION OF THE DRAWINGS Fig.1 is a flowchart of a manufacturing method of an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention. Fig. 2 is a flowchart of a manufacturing method of an ultra-high-strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention. Fig. 3 is a graph showing the heat treatment history with time of an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention. Fig. 4 is a graph showing the time history of heat treatment of an ultra-high-strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention. MODE OF INVENTION

[0025] Hereinafter, the present invention will be described in detail by illustrating embodiments of the invention with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those of ordinary skill in the art. Like reference numerals refer to like elements throughout. Moreover, various elements and portions are shown schematically in the drawings. Therefore, the scope of the present invention is not limited by the relative sizes or distances shown in the accompanying drawings.

[0026] According to the present invention, a steel sheet annealed, galvanized, or galvanized and alloyed in a continuous annealing or galvanizing line is cooled to the martensite starting temperature (Ms) or below to form fresh martensite, and then reheated and kept at a constant temperature for tempering to convert fresh martensite into tempered martensite.

[0027] Tempered martensite is a structure with well-balanced strength and toughness, but toughness can deteriorate if the size is large or if iron carbides coarsen the tempered martensite. Therefore, in the present invention, to avoid the above-mentioned problem, a certain amount of bainite is formed by constantly maintaining a suitable temperature range during annealing. The formed bainite decomposes austenite, and consequently, the size of the martensite can be reduced as the austenite transforms into martensite during subsequent cooling. As such, martensite formed by tempering has a smaller size.

[0028] At the same time, by controlling the tempering conditions after annealing, the number or density of iron carbide particles in the tempered martensite can be controlled to prevent excessive formation and thus the desired performance can be achieved.

[0029] In addition, to improve the formability of the final steel sheet, a uniform structure must be obtained by minimizing the segregation of manganese (Mn) and the like during hot rolling. To this end, Mn segregation can be minimized by controlling the temperatures and reduction ratios of rough rolling and finish rolling.

[0030] An ultra-high strength cold-rolled steel sheet according to the present invention will now be described in detail.

[0031] An ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention comprises: carbon (C): 0.1 wt% to 0.3 wt%, silicon (Si): 1.0 wt% to 2.0 wt%, manganese (Mn): 1.5 wt% to 3.0 wt%, aluminum (Al): more than 0 wt% and up to 0.05 wt%, a combination of one or more selected from titanium (Ti), niobium (Nb) and vanadium (V): more than 0 wt% and up to 0.05 wt%, phosphorus (P): more than 0 wt% and up to 0.02 wt%, sulfur (S): more than 0 wt% and up to 0.005 wt%, nitrogen (N): more than 0 wt% and up to 0.006 wt%, and the balance iron (Fe) and other unavoidable impurities.

[0032] The ultra-high-strength cold-rolled steel sheet may further comprise chromium (Cr): more than 0 wt% and up to 1.0 wt%. The ultra-high-strength cold-rolled steel sheet may further comprise a combination of Cr and molybdenum (Mo): more than 0 wt% and up to 1.0 wt%.

[0033] The functions and contents of the components of the ultra-high-strength cold-rolled steel sheet according to the present invention will now be described. The unit for the content of each component element in this case is wt% relative to the total weight of the steel sheet. Carbon (C): 0.1 wt% to 0.3 wt%

[0034] C is added to achieve strength and control the microstructure of the steel sheet. If the C content is less than 0.1 wt%, the desired strength cannot be easily achieved. If the C content is greater than 0.3 wt%, formability, such as elongation and hole expansion ratio, may decrease, and spot weldability may deteriorate. Therefore, the C content can be 0.1 to 0.3 wt% of the total weight of the steel sheet. Silicon (Si): 1.0 wt% to 2.0 wt%

[0035] S is a ferrite-stabilizing element that retards the formation of carbides in ferrite and tempered martensite and has a solid-solution strengthening effect. When the Si content is less than 1.0 wt%, the Si addition effect is insufficient. When the Si content is greater than 2.0 wt%, the formation of oxides such as Mn2SiO4 can impair coatability, and an increase in the C equivalent can reduce weldability. Therefore, the Si content can range from 1.0 wt% to 2.0 wt% of the total weight of the steel sheet. Manganese (Mn): 1.5 wt% to 3.0 wt%

[0036] Mn has a solid-solution strengthening effect and contributes to strength enhancement by increasing hardenability. Although strength, toughness, and yield strength can be controlled depending on the Mn content, an excessive amount of Mn can lead to the formation of MnS inclusions and cause center segregation during casting, thereby reducing the toughness of the steel. When the Mn content is less than 1.5 wt%, strength cannot be easily achieved due to insufficient hardenability, and the Mn addition effect is insufficient. When the Mn content is more than 3.0 wt%, the formation of inclusions such as MnS or the segregation of Mn can deteriorate formability, and the increase in C equivalent can reduce weldability. Therefore, the Mn content can be 1.5 wt% to 3.0 wt% of the total weight of the steel sheet. Aluminium (Al): more than 0 wt% and up to 0.05 wt%

[0037] Al is used as a deoxidizer and can contribute to ferrite purification. If the Al content exceeds 0.05 wt%, the formation of AlN during slab production can cause cracks during casting or hot rolling. Therefore, the Al content can be greater than 0 wt% and up to 0.05 wt% of the total weight of the steel sheet.

[0038] Combination of titanium (Ti), niobium (Nb) and vanadium (V): more than 0 wt% and up to 0.05 wt%

[0039] Ti, V, and Nb are important elements that precipitate in steel in the form of carbides. Ti, V, and Nb are added to achieve the stability of retained austenite and increase strength by refining initial austenite grains through the formation of precipitates, and to enable precipitation hardening through the refinement of ferrite grains and the presence of precipitates in ferrite. If the total content of Ti, V, and Nb exceeds 0.05 wt%, it may lead to deterioration of material properties and an increase in production costs. Therefore, the total content of Ti, Nb, and V can be greater than 0 wt% and as low as 0.05 wt% of the total weight of the steel sheet.

[0040] The steel sheet may contain at least one of Ti, Nb, and V. As such, the Ti content may be 0 wt% to 0.05 wt% of the total weight of the steel sheet, the Nb content may be 0 wt% to 0.05 wt% of the total weight of the steel sheet, and the V content may be 0 wt% to 0.05 wt% of the total weight of the steel sheet.

[0041] Combination of chromium (Cr) and molybdenum (Mo): more than 0 wt% and up to 1.0 wt%

[0042] Cr and Mo act as hardening elements and contribute to the formation of a two-phase structure. If the total content of Cr and Mo exceeds 1.0 wt%, the effect may converge, and production costs may increase. Therefore, the total content of Cr and Mo can range from 0 wt% to 1.0 wt% of the total weight of the steel sheet.

[0043] The steel sheet may also contain at least one of the elements Cr and Mo. As such, the Cr content may be more than 0 wt% and up to 1.0 wt% of the total weight of the steel sheet, and the Mo content may be more than 0 wt% and up to 1.0 wt% of the total weight of the steel sheet. Phosphorus (P): more than 0 wt% and up to 0.02 wt%

[0044] P is an impurity introduced during steelmaking and can contribute to strength enhancement through solid-solution strengthening. However, excessive P content can lead to low-temperature brittleness. Therefore, the P content must be limited to more than 0 wt% and up to 0.02 wt% of the total weight of the steel sheet. Sulphur (S): more than 0 wt% and up to 0.005 wt%

[0045] S is an impurity introduced during steelmaking that can reduce toughness and weldability through the formation of non-metallic inclusions such as FeS and MnS. Therefore, the S content must be limited to more than 0 wt% and up to 0.005 wt% of the total weight of the steel sheet. Nitrogen (N): more than 0 wt% and up to 0.006 wt%

[0046] N is an element inevitably introduced during steel production, and excessive N can lead to the precipitation of large amounts of nitrides and a decrease in ductility. Therefore, the N content must be limited to more than 0 wt% and up to 0.006 wt% of the total weight of the steel sheet.

[0047] The remainder of ultra-high-strength cold-rolled steel sheet is iron (Fe). However, due to the inevitable introduction of unintentional impurities from the raw materials or the environment during the typical steelmaking process, the addition of impurities cannot be completely ruled out. These impurities are known to the average person skilled in the art and are therefore not specifically mentioned in this description.

[0048] An ultra-high-strength cold-rolled steel sheet produced by controlling specific components of the alloy composition described above and their content ranges and performing the following manufacturing process can achieve, for example, a yield strength (YS): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, an elongation (EL): 14% or more, a hole expansion ratio (HER): 25% or more, and a TS×EL×HER / 1000: 500 or more. The ultra-high-strength cold-rolled steel sheet can achieve, for example, a YS: 850 MPa to 1000 MPa, a TS: 1180 MPa to 1300 MPa, an EL: 14% to 20%, a HER: 25% to 40%, and a TS×EL×HER / 1000: 500 to 900.

[0049] Where “TS×EL×HER / 1000” refers to the product of tensile strength, elongation and hole expansion ratio divided by 1000.

[0050] The ultra-high-strength cold-rolled steel sheet can have a mixed structure of ferrite, retained austenite, bainite, fresh martensite and tempered martensite.

[0051] The area fraction of ferrite, for example, can range from 10% to 20%. The area fraction of retained austenite, for example, can range from 5% to 20%. The area fraction of bainite, for example, can range from 5% to 20%. The remaining area fraction can consist of martensite, and the martensite can include both fresh martensite and tempered martensite. The area fraction refers to a percentage of the area derived from a microstructure image using an image analysis device.

[0052] The value (FM / TM) obtained by dividing the area fraction of tempered martensite (TM) by the area fraction of fresh martensite (FM) can range from 0.1 to 0.6.

[0053] The density of the iron carbide particles in tempered martensite can be, for example, 1.0 × 10 6 particles / mm 2 or more, in particular 1.0 × 10 6 particles / mm 2 up to 20 × 10 6 particles / mm 2 .

[0054] For example, the grain size of tempered martensite can be 5 µm or less, more precisely 1 µm to 5 µm. The grain size in this case is measured as the equivalent circle diameter of a region enclosed by boundaries with an orientation difference of 10 degrees or more, using electron backscatter diffraction (EBSD).

[0055] A manufacturing method of an ultra-high strength cold-rolled steel sheet according to the present invention will now be described with reference to the accompanying drawings. Manufacturing process of ultra-high-strength cold-rolled steel sheet

[0056] Fig.1 is a flowchart of a manufacturing method of an ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention and relates to a manufacturing method of an uncoated cold-rolled steel sheet.

[0057] Referring to Fig. 1, the method according to an embodiment of the present invention comprises a hot-rolled steel sheet manufacturing step S110, a cold-rolled steel sheet manufacturing step S120, a primary soaking step S130, a primary cooling step S140, a secondary cooling step S150, a secondary soaking step S160, a tertiary cooling step S170, and a tertiary soaking step S180. Hot-rolled steel sheet production (S110)

[0058] In the hot-rolled steel sheet manufacturing step S110, a steel material comprising C: 0.1 wt% to 0.3 wt%, Si: 1.0 wt% to 2.0 wt%, Mn: 1.5 wt% to 3.0 wt%, Al: more than 0 wt% and up to 0.05 wt%, a combination of one or more selected from Nb, Ti, and V: more than 0 wt% and up to 0.05 wt%, P: more than 0 wt% and up to 0.02 wt%, S: more than 0 wt% and up to 0.005 wt%, N: more than 0 wt% and up to 0.006 wt%, and the balance Fe and other unavoidable impurities is prepared. The steel material may further comprise a combination of Cr and Mo: more than 0 wt% and up to 1.0 wt%.

[0059] In the method according to the present invention, the semi-finished product to be hot-rolled may, for example, be a slab. The slab provided as a semi-finished product may be produced by continuously casting molten steel with a specific composition obtained through a steelmaking process.

[0060] The steel material, e.g., a slab, is reheated at a slab reheating temperature (SRT) of, e.g., 1150°C to 1250°C for, e.g., 1 hour to 5 hours. The reheating process allows the components and precipitates segregated during casting to redissolve. As such, the steel material is homogenized and ready for hot rolling. If the SRT is below 1150°C, the components segregated during casting may not be sufficiently redissolved, resulting in uneven distribution. If the SRT is above 1250°C, coarsening of austenite grains may cause a decrease in yield strength. In addition, the heating costs and the additional time required to reach the hot rolling temperature may lead to higher production costs and lower productivity with increasing SRT. If the reheating time is shorter than 1 hour, the segregation zones may not be sufficiently reduced.If the reheating time is longer than 5 hours, the grain size may increase and the process cost may increase.

[0061] The reheated steel material is then heated and hot-rolled to adjust its shape. The hot rolling process can be carried out continuously through rough rolling and finish rolling. Due to the hot rolling process, the steel material can be formed into a hot-rolled steel material. The hot-rolled steel material can be a hot-rolled steel sheet.

[0062] As described above, to improve the formability of the final steel sheet, a uniform structure must be achieved by minimizing the segregation of Mn and the like during the hot rolling process. To this end, the segregation of Mn can be controlled by appropriately adjusting control conditions, such as the temperatures and reduction ratios of rough rolling and finish rolling, and Mn can be evenly distributed by inhibiting the formation of a band structure. Consequently, an ultra-high-strength steel can be produced that has a hard phase with uniform hardness after final tempering.

[0063] For this purpose, the rough rolling process can be carried out at temperatures between 1000 °C and 1150 °C, for example. The rough rolling process can be performed in multiple passes, moving back and forth through the rough rolling mill. In this case, the reduction ratio in the final pass can be 40% or more, e.g., 40% to 50%. By setting a high reduction ratio of 40% or more in the final pass of the rough rolling process, austenite can be refined and Mn segregation can be reduced.

[0064] In the final rolling process, rolling through the final three-roll stand is carried out at a temperature of 1020°C or less, for example, 880°C to 1020°C. The reduction ratio in the first pass is 40% or more, and the total reduction ratio through the final three-roll stand must be controlled to be between 40% and 60%. In this case, the time required for the steel sheet to pass through the final three-roll stand must be controlled to be as short as possible, for example, no longer than 2.0 seconds (and no longer than 0 seconds). By controlling the reduction ratio in the final rolling process as described above, the segregation of Mn and P can be reduced.

[0065] The final feed temperature (FDT) ranges between 880 °C and 980 °C. If the FDT is below 880 °C, the roll load can increase rapidly, causing a reduction in productivity. If the FDT is higher than 980 °C, grain coarsening can occur, causing a decrease in strength.

[0066] The hot-rolled steel material is then cooled. The time required from the final pass through the finish rolling until the start of cooling must be controlled to be as short as possible, e.g., no longer than 1.5 seconds (and longer than 0 seconds). The cooling process can be carried out using air cooling or water cooling at a cooling rate of, for example, 10 °C / s to 50 °C / s. The cooling process can be carried out up 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 may increase, and consequently, strength cannot be easily obtained. On the other hand, if the cooling rate is more than 50 °C / s, the microstructure of the steel material may become harder, and consequently, the impact toughness may decrease.

[0067] The hot-rolled steel sheet is then coiled at a coiling temperature (CT) ranging from 500°C to 700°C, for example. If the CT is lower than 500°C, the significant difference between FDT and CT may degrade the surface quality of the steel material, and the increased strength may increase the rolling load during cold rolling. If the CT is higher than 700°C, carbonitride elements may not be retained in a solid solution form, undesirable precipitates may form, and defects due to surface oxidation or the like may occur in subsequent processes. The coiled steel material can be cooled to room temperature. softening

[0068] Optionally, after the hot-rolled steel sheet is produced, the hot-rolled steel sheet can be softened at a temperature within a range of, for example, 500 °C to 650 °C for, for example, 1 hour to 10 hours. This softening step can effectively control the influence on the microstructure of the hot-rolled steel sheet after hot rolling and before cold rolling.

[0069] In general, the coiling temperature during the hot rolling process affects the microstructure and properties of the hot-rolled steel sheet. Furthermore, the cooling rate of the coil after coiling can have a similar effect. The coiling temperature may not be easily controlled to be uniform along the entire width / length of the steel sheet. Furthermore, during the cooling in the warehouse after coiling, seasonal factors or the proximity of other coils can vary the cooling rate, resulting in significant material property variations of the hot-rolled steel sheet. These variations in material properties can continuously influence the subsequent cold rolling process, significantly affecting the quality of the final product. To eliminate the material property or microstructure influence of the hot-rolled steel sheet, the softening step can be performed.

[0070] Softening can soften the hot-rolled steel sheet, reducing the rolling load during the subsequent cold rolling process. Furthermore, the thickness variation commonly observed in cold rolling of high-strength steel can be reduced, and shape control is easily achieved.

[0071] If the softening temperature is below 500 °C, the hot-rolled steel sheet is not sufficiently softened, and the impact on the microstructure of the cold-rolled steel sheet obtained after hot rolling may not be eliminated. Furthermore, the microstructure may become uneven after softening.

[0072] If the softening temperature is above 650 °C, an uneven austenite phase may form, and unnecessary phases may form during cooling, affecting the annealing conditions of the finally produced steel sheet.

[0073] If softening is carried out for a long period at 600 °C or higher, various alloy carbides may precipitate during heat treatment and cannot be easily redissolved during subsequent continuous annealing, preventing the desired mechanical properties from being achieved. Therefore, the softening time can be within 10 hours. Cold-rolled steel sheet production (S120)

[0074] The cold-rolled steel sheet manufacturing step S120 is performed to obtain the thickness of the finally produced steel sheet using the hot-rolled steel sheet. The coiled hot-rolled steel sheet is acid pickled. Then, a cold-rolled steel sheet is formed by cold-rolling the pickled hot-rolled steel sheet at a cold-rolling reduction ratio of 40% to 60%. If the cold-rolling reduction ratio is less than 40%, grains may grow excessively during soaking because nucleation is insufficient for recrystallization during subsequent soaking, and consequently, strength may rapidly decrease. If the cold-rolling reduction ratio is more than 60%, grains formed during soaking may become excessively fine because nucleation occurs excessively, and ductility and formability may decrease.

[0075] After completion of cold rolling, the desired final microstructure can be obtained by certain heat treatment processes. Fig. 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. The heat treatment processes for the cold-rolled steel sheet will now be described with reference to Fig. 1 and Fig. 3 described in detail. Primary warming (S130)

[0076] In the primary soaking step S130, the cold-rolled steel sheet can be soaked in a typical continuous annealing furnace with a slow cooling period. In the primary soaking process, the cold-rolled steel sheet is heated to a primary soaking temperature of, for example, Ac3-30 °C to 900 °C at a heating rate of, for example, 1 °C / s or more, in particular, 1 °C / s to 10 °C / s. The steel sheet is held at the primary soaking temperature for, for example, 30 to 200 seconds. Through the primary soaking process, the desired austenite content can be obtained. If the primary soaking temperature is below Ac3-30 °C or the holding time is shorter than 30 seconds, sufficient austenite cannot be easily formed, and the increased ferrite content can lead to a decrease in strength.If the primary soaking temperature is above 900 °C or the holding time is longer than 200 seconds, the austenite grain size may coarsen or the productivity may decrease excessively. Primary cooling (S140)

[0077] In the primary cooling step S140, the primary soaked cold-rolled steel sheet is primary cooled at a cooling rate of, for example, 5 °C / s to 15 °C / s to a primary cooling temperature of, for example, 620 °C to 720 °C. The cooling process can be performed using air cooling or water cooling. The primary cooling process can also be referred to as the slow cooling process. The primary cooling process is performed to achieve plasticity by acquiring a certain amount of ferrite in the final microstructure. If the primary cooling temperature is below 620 °C, excessive ferrite transformation can lead to a decrease in strength. If the primary cooling temperature is above 720 °C, the significant temperature difference from the subsequent secondary cooling process can lead to quality degradation or reduced productivity. Secondary cooling (S150)

[0078] 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 / s to 100 °C / s to a secondary cooling temperature of, for example, 250 °C to 480 °C. The secondary cooling process can also be referred to as a rapid cooling process. During the secondary cooling process, further ferrite transformation must be prevented. Secondary warming (S160)

[0079] In the secondary soaking step S160, the secondary-cooled cold-rolled steel sheet is subjected to secondary soaking at a secondary soaking temperature of 250°C to 480°C for 50 seconds to 300 seconds. In the secondary soaking step, a certain amount of bainite is formed by maintaining the temperature constant in the range of 250°C to 480°C. The bainite formed at this time decomposes austenite. Therefore, when austenite transforms into martensite during the subsequent cooling process, the size of the martensite may decrease. Thus, the size of the tempered martensite formed after tempering may also decrease, contributing to the refinement of the tempered martensite. Tertiary cooling (S170)

[0080] In the tertiary cooling step S170, the cold-rolled steel sheet is tertiarily cooled to a tertiary cooling temperature of, for example, room temperature (e.g., 0 °C to 40 °C) to 150 °C. In the tertiary cooling step S170, austenite transforms into fresh martensite while being cooled to a temperature at or below Ms. Tertiary warming (S180)

[0081] In the tertiary soaking step S180, the tertiary-cooled cold-rolled steel sheet is heated to a tertiary soak temperature of, for example, 150°C to 300°C at a heating rate of, for example, 1°C / s or more, particularly 1°C / s to 10°C / s, and the tertiary soak temperature is maintained for, for example, 100 seconds to 30,000 seconds. In the tertiary soak step, fresh martensite formed during the secondary cooling step transforms into tempered martensite, the number of carbide particles can be controlled, and C can be enriched in retained austenite for stabilization. In this way, high strength and high elongation can be achieved, and the configuration of the final microstructure can be maintained.If the tertiary soaking temperature is below 150 °C or the holding time is shorter than 100 seconds, martensite may be insufficiently tempered or retained austenite may be insufficiently stabilized. If the tertiary soaking temperature is higher than 300 °C or the holding time is longer than 30,000 seconds, excessive tempering of martensite may lead to a decrease in strength, and phase transformation of retained austenite may cause a deterioration in ductility.

[0082] After completion of the tertiary soaking step S180, the cold-rolled steel sheet is cooled to room temperature, e.g., 0 °C to 40 °C, at a cooling rate of 1 °C / s to 100 °C / s.

[0083] According to one embodiment of the present invention, after the primary soaking step S130, instead of directly lowering the temperature to Ms or below during rapid cooling after slow cooling, secondary soaking is performed to form a certain amount of bainite, and then cooling to Ms or below is performed.

[0084] Fig. 2 is a flowchart of a manufacturing method of 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 showing the time history of heat treatment of an ultra-high-strength hot-dip galvanized cold-rolled steel sheet according to an embodiment of the present invention. Fig. 4 shows both a hot-dip galvanizing step S161 and an alloying step S162.

[0085] In the present embodiment, steps S110 to S150 are the same as in the above-described manufacturing method of an uncoated ultra-high-strength cold-rolled steel sheet.

[0086] In the present embodiment, as shown in Fig. 4, the cold-rolled steel sheet is introduced into a galvanizing bath after completion of the secondary soaking step and hot-dip galvanized (see S161 in Fig. 2).

[0087] Alternatively, the cold-rolled steel sheet can be introduced into the galvanizing bath immediately after reaching the secondary soaking temperature and hot-dip galvanized. In this case, the secondary soaking step S160 can be considered the hot-dip galvanizing step.

[0088] If necessary, an additional alloying step can be carried out after the hot-dip galvanizing step (see S165 in Fig.4) to form a hot-dip galvanized steel material and an alloy hot-dip galvanized steel material. Hot-dip galvanizing (S161)

[0089] In the hot-dip galvanizing step, the cold-rolled steel sheet is immersed in the galvanizing bath to form a hot-dip galvanized layer. The temperature of the galvanizing bath can vary depending on the type and ratio of alloying elements used to configure the galvanized layer and the composition system of the cold-rolled steel sheet, and can 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 above-mentioned galvanizing bath conditions, the hot-dip galvanized layer can be easily formed on the surface of the cold-rolled steel sheet, and the adhesion of the galvanized layer can be improved.

[0090] Although Fig.4 shows both the hot-dip galvanizing step and the alloying step. If the alloying step is not performed after the hot-dip galvanizing step, the galvanized steel sheet removed from the galvanizing bath directly undergoes the tertiary cooling step S170. Therefore, austenite transforms into martensite in this step. Subsequently, the tertiary soaking step S180 is performed. The description of the tertiary soaking step S180 has already been given above and will therefore not be repeated here to avoid redundancy. Alloying (S165)

[0091] If necessary, the cold-rolled steel sheet can be alloyed with the hot-dip galvanized layer. For the alloying step, the galvanized steel sheet taken from the galvanizing bath can be loaded into a heat treatment facility and alloyed. The alloying step can be carried out by maintaining a temperature of, for example, 500°C to 600°C for, for example, 1 second to 20 seconds. If the alloying step is carried out under the above conditions, the hot-dip galvanized layer can grow stably, and the adhesion of the galvanized layer can be improved. If the alloying temperature is below 500°C, insufficient alloying may compromise the integrity of the hot-dip galvanized layer. If the alloying temperature is higher than 600°C, reaching the intercritical temperature range may change the material properties. The alloyed steel sheet undergoes the tertiary cooling step S170.Therefore, austenite transforms into martensite in this step. Subsequently, the tertiary soaking step S180 is performed. The description of the tertiary soaking step S180 has already been given above and will therefore not be repeated here to avoid redundancy. Test examples

[0092] To better understand the present invention, test examples will now be described. However, the following test examples are merely intended to better understand the present invention, and the present invention is not limited thereto. Details not described herein can be easily deduced by one of ordinary skill in the art, and therefore, further explanations are omitted.

[0093] Steel materials with the compositions (unit: wt%) shown in Tables 1 and 2 were prepared. In Tables 1 and 2, the remainder consists of Fe and impurities inevitably introduced during the steelmaking process or the like. The unit for the content of each component is wt%. [Table 1] Steel type C Si Mn P S Al N Cr A 0,163 1,02 2,66 0,011 0,0023 0,030 0,0037 0 B 0,230 1,88 2,02 0,009 0,0020 0,015 0,0024 0 C 0,116 0,63 3,32 0,016 0,0030 0,030 0,0025 0 D 0,212 1,49 2,17 0,012 0,0015 0,024 0,0031 0 E 0,186 1,81 2,76 0,014 0,0016 0,037 0,0033 0,04 [Table 2] Steel type Nb Ti V B Ac1(°C) Ac3(°C) Ms(°C) A 0 0 0,028 0,0005 724 843 361 B 0 0 0 0,0005 756 848 380 C 0 0 0 0,0005 706 793 389 D 0,012 0 0 0,0005 743 836 383 E 0,001 0,018 0,003 0,0007 747 852 376

[0094] Referring to Tables 1 and 2, steel types A, B, D, and E meet the composition range of the present invention. Steel type C has a Si content below the lower limit of the composition range of the present invention and a Mn content above the upper limit of the composition range of the present invention.

[0095] Cold-rolled steel sheets were produced by hot rolling and cold rolling of steel types A to E.

[0096] Table 3 shows the hot rolling condition values ​​of the ultra-high strength cold-rolled steel sheets of the test samples. [Table 3] Nr. Steel type Oven Rough rolling Finishing rollers Wrap softening SRT(°C) RDT(°C) Last-pass reduction ratio FDT(°C) Last 3-roll reduction ratio (%) 1.Pass-through reduction ratio (%) Time from end of roll to winding start (s) Cooling rate (°C / s) CT(°C) Temp.(°C) Test example 1 A 1220 1090 42 960 45 41 0,88 50 600 - Test example 2 A 1220 1060 41 920 42 42 0,88 50 600 - Test example 3 B 1250 1010 40 940 53 46 0,88 50 600 - Test example 4 B 1250 1005 43 900 55 48 0,88 50 600 - Test example 5 C 1230 1005 45 900 48 52 0,88 50 570 600 Test example 6 C 1230 1030 45 900 49 51 0,88 50 570 600 Test example 7 D 1210 1050 41 940 45 43 0,88 50 530 - Test example 8 D 1210 1050 42 920 43 45 0,88 50 530 - Test example 9 E 1207 1080 45 957 49 46 0,88 50 500 600 Test example 10 E 1205 1090 43 934 52 43 0,88 50 500 600

[0097] Referring to Table 3, Test Examples 1 to 10 were prepared under the hot rolling conditions proposed in the present invention. Test Examples 5, 6, 9, and 10 were prepared by further performing softening at 600 °C for 2 hours.

[0098] Table 4 shows the heat treatment condition values ​​of the test samples after cold rolling.

[0099] Referring to Table 4, of Test Examples 1 to 10, the final product of Test Example 9 was a cold-rolled steel sheet (CR) without a galvanized layer, and the final product of the others was an alloy hot-dip galvanized steel sheet (GA). Test Examples 1, 7, 9, and 10 were manufactured in compliance with all the process conditions proposed in the present invention. The galvanizing temperature in Table 4 indicates the temperature of the molten zinc in the galvanizing bath.

[0100] Test Example 2 has a tertiary cooling temperature higher than the upper limit of the operation range of the present invention, Test Example 3 has a tertiary soaking temperature higher than the upper limit of the operation range of the present invention, Test Example 4 has a primary soaking temperature lower than the lower limit of the operation range of the present invention and a tertiary soaking temperature higher than the upper limit of the operation range of the present invention, Test Example 5 has a tertiary soaking temperature higher than the upper limit of the operation range of the present invention, Test Example 6 has a tertiary soaking holding time lower than the lower limit of the operation range of the present invention, and Test Example 8 has a tertiary soaking temperature higher than the upper limit of the operation range of the present invention and a Tertiary warm-up holding time,which is lower than the lower limit of the operation range of the present invention.,

[0101] Table 5 shows the measurement results of the mechanical properties of the test samples, such as tensile strength (TS), elongation (EL), hole expansion ratio (HER), and a product of tensile strength, elongation, and hole expansion ratio (TS×EL×HER / 1000). [Table 5] Nr. Steel type Galvanizing TS(MPa) EL(%) HER(%) TS×EL×HER / 1000(MPa% 2 ) Remarks Test example 1 A GA 1185 16 32 607 Inventive example Test example 2 A GA 1151 16 18 331 Comparison example Test example 3 B GA 1078 22 35 830 Comparison example Test example 4 B GA 690 30 20 414 Comparison example Test example 5 C GA 1036 17 21 370 Comparison example Test example 6 C GA 1280 8 20 205 Comparison example Test example 7 D GA 1230 16 33 649 Inventive example Test example 8 D GA 1320 14 15 277 Comparison example Test example 9 E CR 1223 14 35 599 Inventive example Test example 10 E GA 1219 15 28 512 Inventive example

[0102] Referring to Table 5, Test Examples 1, 7, 9 and 10 satisfy all the process conditions proposed in the present invention and have TS, EL, HER and TS×EL×HER / 1000 values ​​that fall within the target ranges of the present invention.

[0103] On the contrary, Test Examples 2, 3, 4, 5, 6 and 8 do not satisfy the process conditions proposed in the present invention and show mechanical properties that are outside the target ranges of the present invention.

[0104] Specifically, Test Examples 2, 4, and 5 show TS and TS×EL×HER / 100 values ​​lower than the lower limits of the target ranges of the present invention, Test Example 3 shows a TS value lower than the lower limit of the target range of the present invention, Test Example 6 shows EL and TS×EL×HER / 1000 values ​​lower than the lower limits of the target ranges of the present invention, and Test Example 8 shows a TS value higher than the upper limit of the target range of the present invention and a TS×EL×HER / 1000 value lower than the lower limit of the target range of the present invention.

[0105] Table 6 shows the microstructures of the test samples, e.g., the area fractions of ferrite, retained austenite, bainite, fresh martensite (FM) and tempered martensite (TM), a fresh martensite / tempered martensite (FM / TM) ratio and the number of tempered martensite particles ≥ 5 µm. [Table 6] Nr. Steel type Ferrite (area%) Resta ustenite (area%) Getem perterMartensite(Area%) Fresh artensite (area%) Bainite (area%) Number of annealed Martensite particles ≥ 5 µm (10 6 p / mm 2 ) FM / T M ratio Remarks Test example 1 A 18 10 47 8 17 2 0,2 Inventive example Test example 2 A 18 10 25 17 30 0 0,7 Comparison example Test example 3 B 34 15 35 3 13 3 0,1 Comparison example Test example 4 B 77 2 5 5 11 0 1,0 Comparison example Test example 5 C 10 6 70 6 8 10 0,1 Comparison example Test example 6 C 10 3 10 70 7 10 7,0 Comparison example Test example7 D 5 14 53 10 18 2 0,2 Inventive example Test example8 D 5 12 15 45 23 3 3,0 Comparison example Test example 9 E 15 10 58 6 11 4 0,1 Inventive example Test example 10 E 14 10 52 6 18 2 0,1 Inventive example

[0106] Test Examples 1, 7, 9 and 10 correspond to embodiments of the present invention and show area fractions of ferrite, retained austenite, bainite, fresh martensite (FM) and tempered martensite (TM), the number of tempered martensite particles ≥ 5 µm and a fresh martensite / tempered martensite (FM / TM) ratio that are within the target ranges of the present invention.

[0107] In contrast, Test Example 2 exhibits a bainite content and FM / TM ratio that exceed the upper limits of the target ranges of the present invention. It is analyzed that Test Example 2 exhibits low mechanical strength due to the relatively high bainite content, because the tertiary cooling temperature is above the upper limit of the process range of the present invention.

[0108] Test Example 3 has a tertiary soak temperature higher than the upper limit of the process range of the present invention and exhibits mechanical properties lower than the target range due to the area ratio of ferrite higher than the upper limit of the target range of the present invention.

[0109] Test Example 4 has a primary soak temperature lower than the lower limit of the inventive process range and a tertiary soak temperature higher than the upper limit of the inventive process range, and therefore shows very low TS and TS×EL×HER values ​​due to the 77% area fraction of ferrite, indicating that ferrite is the dominant phase of the microstructure.

[0110] Test Example 5 has a Si content lower than and a Mn content higher than the composition of the present invention, and a tertiary soak temperature higher than the upper limit of the process range of the present invention. As such, Test Example 5 exhibits a low FMITM ratio and TS and TS×EL×HER / 1000 values ​​lower than the lower limits of the target ranges of the present invention.

[0111] Test Example 6 has a Si content lower than and a Mn content higher than the composition of the present invention, and a tertiary soak-hold time lower than the lower limit of the operation range of the present invention.

[0112] As such, Test Example 6 shows an FMITM ratio higher than the upper limit of the target range of the present invention due to insufficient annealing, and therefore shows EL and TS×EL×HER / 1000 values ​​lower than the lower limits of the target ranges of the present invention.

[0113] Test Example 8 has a tertiary soak temperature higher than the upper limit of the target range of the present invention. As such, Test Example 8 exhibits an area fraction of bainite higher than the upper limit of the target range of the present invention due to excessive annealing, and consequently exhibits TS and TS×EL×HER / 1000 values ​​lower than the target ranges of the present invention.

[0114] Although the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the following claims. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] KR 2014-7010908

[0004]

Claims

[1] Ultra-high-strength cold-rolled steel sheet comprising: Carbon (C): 0.1 wt% to 0.3 wt%, Silicon (Si): 1.0 wt% to 2.0 wt%, Manganese (Mn): 1.5 wt% to 3.0 wt%, Aluminum (Al): more than 0 wt% and up to 0.05 wt%, a combination of one or more selected from Titanium (Ti), Niobium (Nb) and Vanadium (V): more than 0 wt% and up to 0.05 wt%, Phosphorus (P): more than 0 wt% and up to 0.02 wt%, Sulfur (S): more than 0 wt% and up to 0.005 wt%, Nitrogen (N): more than 0 wt% and up to 0.006 wt%, and the balance Iron (Fe) and other unavoidable impurities, wherein the ultra-high-strength cold-rolled steel sheet satisfies: a Yield Strength (YS): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, an elongation (EL): 14% or more, a hole expansion ratio (HER): 25% or more and TS×EL×HER / 1000: 500 or more. [2] Ultra-high-strength cold-rolled steel sheet according to claim 1, wherein the ultra-high-strength cold-rolled steel sheet has a mixed structure of ferrite, retained austenite, bainite, fresh martensite and tempered martensite, where the area fraction of ferrite is in a range of 10% to 20%, where the area fraction of retained austenite is in a range of 5% to 20%, where the area fraction of bainite is in a range of 5% to 20%, and where a sum of area fractions of fresh martensite and tempered martensite is a remaining area fraction. [3] The ultra-high strength cold-rolled steel sheet according to claim 2, wherein a ratio (FM / TM) of fresh martensite (FM) to tempered martensite (TM) is 0.1 to 0.

6. [4] Ultra-high strength cold-rolled steel sheet according to claim 2, wherein a density of iron carbide particles in the tempered martensite is 1.0 × 10 6 particles / mm 2 or more. [5] The ultra-high strength cold-rolled steel sheet according to claim 2, wherein a grain size of the tempered martensite is 5 µm or less. [6] Ultra-high strength cold-rolled steel sheet according to claim 1, further comprising a combination of chromium (Cr) and molybdenum (Mo): more than 0 wt% and up to 1.0 wt%. [7] A method of manufacturing an ultra-high-strength cold-rolled steel sheet, the method comprising: Producing a hot-rolled steel sheet having an alloy composition of carbon (C): 0.1 wt% to 0.3 wt%, silicon (Si): 1.0 wt% to 2.0 wt%, manganese (Mn): 1.5 wt% to 3.0 wt%, aluminum (Al): more than 0 wt% and up to 0.05 wt%, a combination of one or more selected from titanium (Ti), niobium (Nb) and vanadium (V): more than 0 wt% and up to 0.05 wt%, phosphorus (P): more than 0 wt% and up to 0.02 wt%, sulfur (S): more than 0 wt% and up to 0.005 wt%, nitrogen (N): more than 0 wt% and up to 0.006 wt%, and the balance iron (Fe) and other unavoidable impurities (S110), Producing a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet (S120), Primary soaking of the cold-rolled steel sheet at a primary soaking temperature of Ac3-30 °C to 900 °C for 30 seconds to 200 seconds (S130), Primary cooling of the primarily soaked cold-rolled steel sheet at a cooling rate of 5 °C / s to 15 °C / s to a primary cooling temperature of 620 °C to 720 °C (S140), Secondary cooling of the primary cooled cold-rolled steel sheet at a cooling rate of 15 °C / s to 100 °C / s to a secondary cooling temperature of 250 °C to 480 °C (S150), Secondary soaking of the secondary cooled cold-rolled steel sheet at a secondary soaking temperature of 250 °C to 480 °C for 50 seconds to 300 seconds (S160), Tertiary cooling of the secondary soaked cold-rolled steel sheet to a tertiary cooling temperature of 150 °C or less (S170), and Tertiary soaking of the tertiary cooled cold rolled steel sheet at a tertiary soaking temperature of 150 °C to 300 °C for 100 seconds to 30000 seconds (S180). [8] The method according to claim 7, wherein the manufacturing of the hot-rolled steel sheet (S110) comprises: Reheating a steel material with the alloy composition at a slab reheating temperature of 1150 °C to 1250 °C, Hot rolling of the reheated steel material, Cooling the hot-rolled steel material at a cooling rate of 10 °C / s to 50 °C / s, and Coiling the cooled steel material at a coiling temperature of 500 °C to 700 °C, and wherein hot rolling comprises: a rough rolling process at 1000 °C to 1150 °C with a reduction ratio of 40% to 50% in a final pass, and a finish rolling operation at a finish rolling temperature of 880 °C to 980 °C, wherein rolling is carried out by a final three-roll stand at a temperature of 1020 °C or less and a total reduction ratio of 40% or more and a reduction ratio of 40% to 60% in a first pass. [9] The method according to claim 8, wherein a time required to pass the steel sheet through the final three-roll stand during the finish rolling process is not longer than 2.0 seconds (and longer than 0 seconds). [10] The method according to claim 8, wherein a time required from the end of the finish rolling operation to the start of cooling of the hot-rolled steel is not longer than 1.5 seconds. [11] The method according to claim 7, further comprising softening the hot-rolled steel sheet at a temperature in a range of 500°C to 650°C after the hot-rolled steel sheet is produced. [12] The method according to claim 7, further comprising hot-dip galvanizing the cold-rolled steel sheet (S161) after the cold-rolled steel sheet has been secondarily soaked. [13] The method according to claim 12, wherein the secondary soaking (S160) is hot-dip galvanizing (S161) of the cold-rolled steel sheet. [14] The method according to claim 12 or 13, further comprising alloying the cold-rolled steel sheet (S162) after the cold-rolled steel sheet has been hot-dip galvanized.

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