Cold-rolled steel sheet and manufacturing method thereof

Abstract

One aspect of the present invention provides an ultra-high strength cold-rolled steel sheet having excellent elongation and a manufacturing method thereof.

Description

Cold rolled steel sheet and manufacturing method thereof

[0001] The present invention relates to a cold rolled steel sheet and a method for manufacturing the same.

[0002] With the gradual increase in carbon dioxide regulations, the automotive industry is increasingly focused on improving fuel efficiency, body stability for passenger protection, and reducing vehicle weight. To achieve these goals, the use and development of ultra-high-strength cold stamping steel sheets with tensile strengths exceeding 1.5 GPa, compared to the high-strength steels used in conventional automotive components, is increasing.

[0003] However, cold-rolled steel sheets have a problem in that as the strength increases, the elongation decreases (i.e., an inverse relationship), so their formability is poor, and their application as a material for cold stamping is generally very limited.

[0004] To solve the above problems and secure excellent strength and elongation, high-strength steels such as dual steel (pre-steel) and transformation-induced plasticity steep (TRIP steel) that introduce low-temperature transformation phases into a soft matrix are being used as automobile body components.

[0005] Meanwhile, with the goal of weight reduction for improved fuel efficiency, demand is growing for cold-rolled steel sheets that offer relatively high elongation and formability for vehicle components, while also requiring increased strength relative to thickness reduction. However, high-strength steels with a strength of 1 GPa or higher suffer from poor elongation and hole expansion ratio (HER) due to localized interphase hardness differences.

[0006] To overcome these shortcomings, ultra-high strength steels (UHSS), composed solely of the low-temperature transformation phases bainite and / or martensite, have been developed. While these UHSSs generally have low bendability, they are used as parts that can be formed by roll forming.

[0007] As the use of high-strength steels in components increases, the need for ultra-high-strength steels with high ductility and their manufacturing methods to satisfy the characteristics of various components is growing. However, technology and manufacturing methods capable of meeting the demand for steel plates with a tensile strength of 1.5 GPa or higher and high ductility have not yet been developed.

[0008] (Patent Document 1) Korean Patent Publication No. 10-2017-7022118

[0009] (Patent Document 2) Japanese Patent Publication No. 2017-145469

[0010] One aspect of the present invention is to provide an ultra-high strength cold rolled steel sheet having excellent elongation and a method for manufacturing the same.

[0011] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0012] One aspect of the present invention provides a cold-rolled steel sheet. The cold-rolled steel sheet contains, in wt%, C: 0.2 to 0.4%, Si: 1.0 to 2.0%, Mn: 2.5 to 5.0%, P: 0.1% or less (excluding 0%), S: 0.03% or less (excluding 0%), Al: 0.01 to 0.1%, N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), Ti 0.01 to 0.04%, the remainder Fe, and other unavoidable impurities, and as a microstructure, it contains, in area%, 10 to 30% of retained austenite and 10 to 90% of the sum of bainite and martensite, and the value of the following relational expression 1 may be 0.25 to 1.5.

[0013] [Relationship 1]

[0014] (Mn_high - Mn_low) / Mn_base

[0015] (In the above relational expression 1, Mn_high represents the average concentration of Mn in a region where the content of Mn in the microstructure is 1% or more higher than the average content of Mn at the point t / 4 in the thickness direction of the cold-rolled steel sheet, Mn_low represents the average concentration of Mn in a region where the content of Mn in the microstructure is 1% or more lower than the average content of Mn at the point t / 4 in the thickness direction of the cold-rolled steel sheet, and Mn_base represents the average content of Mn at the point t / 4 in the thickness direction of the cold-rolled steel sheet.)

[0016] Additionally, in the cold rolled steel sheet described above, the area fraction of the bainite may be 30 to 60% based on the total fraction of the microstructure.

[0017] Additionally, in one of the cold rolled steel sheets described above, the area fraction of martensite may be 10 to 60% based on the total fraction of the microstructure.

[0018] In addition, in one of the cold-rolled steel sheets described above, one or more selected from Cr: 0.1% or less, Mo: 0.1% or less, and Nb: 0.1% or less may be further included in weight %.

[0019] Additionally, in one of the cold rolled steel sheets mentioned above, the tensile strength may be 1500 MPa or more and the elongation may be 10% or more.

[0020] Additionally, in one of the cold rolled steel sheets mentioned above, the yield strength may be 1000 MPa or more.

[0021] Another aspect of the present invention provides a method for manufacturing a cold-rolled steel sheet. The method comprises the steps of: reheating a slab containing, in wt%, C: 0.2 to 0.4%, Si: 1.0 to 2.0%, Mn: 2.5 to 5.0%, P: 0.1% or less (excluding 0%), S: 0.03% or less (excluding 0%), Al: 0.01 to 0.1%, N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), Ti 0.01 to 0.04%, the remainder Fe, and other unavoidable impurities; hot-rolling the reheated slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet; cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; The method may include a first annealing step of maintaining the cold-rolled steel sheet at a first annealing temperature of 700 to 800°C for 10 minutes to 24 hours; a second annealing step of maintaining the first annealed cold-rolled steel sheet at a second annealing temperature higher than the austenite single-phase region for a time of more than 0 seconds and less than 10 seconds; a step of cooling the second annealed cold-rolled steel sheet to a cooling end temperature of 280 to 480°C; and a step of maintaining the cooled cold-rolled steel sheet at the cooling end temperature for 5 minutes to 5 hours.

[0022] In addition, in the method described above, in the second annealing step, heating can be performed at an average heating rate of 2°C / s or more to the second annealing temperature.

[0023] Additionally, in one of the above-described methods, in the cooling step, cooling can be performed at an average cooling rate of 10°C / s or more to the cooling end temperature.

[0024] In addition, in one of the above-described methods, the reheating step may be performed at 1,100 to 1,300°C, and the step of obtaining the hot-rolled steel sheet may be performed by hot-rolling the reheated slab at 800 to 1,000°C.

[0025] In addition, in one of the above-described methods, the coiling step may be performed at 400 to 700°C, and the step of obtaining the cold-rolled steel sheet may be performed by cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 20 to 75%.

[0026] According to the present invention, it is possible to secure an elongation of 10% or more while having an ultra-high strength of 1.5 GPa or more.

[0027] In addition, according to the present invention, an ultra-high strength cold rolled steel sheet having excellent elongation can be suitably used for cold stamping.

[0028] In addition, according to the present invention, by distributing the concentration of Mn in the microstructure, transformation of the bainite phase is promoted, thereby securing austenite stability, thereby securing retained austenite at room temperature, thereby providing an ultra-high strength steel having excellent elongation.

[0029] The various advantageous and beneficial advantages and effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0030] In order to more fully understand the drawings cited in the detailed description of the present invention, a brief description of each drawing is provided.

[0031] Figure 1 is a schematic diagram showing a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention.

[0032] Figure 2 is a photograph showing the microstructure of a cold-rolled steel sheet of Invention Example 3 according to one embodiment of the present invention.

[0033] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified in various other forms, and the terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include plural forms, unless the relevant definition clearly indicates a contrary meaning.

[0034] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0035] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0036] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0037] First, the alloy composition of the cold-rolled steel sheet according to one embodiment of the present invention will be described. The content of the alloy composition refers to weight percent unless otherwise specified.

[0038] Carbon (C): 0.2 to 0.4%

[0039] Carbon (C) may be included to secure strength and hardenability. If the C content is less than 0.2%, it may be difficult to secure a tensile strength of 1.5 GPa or higher. In addition, if the C content exceeds 0.4%, the strength may increase excessively compared to the target level, which may result in a decrease in elongation, an increased potential for brittle fracture, and inferior spot weldability. Since the higher the C content, the greater the degree of carbide generation during martensite formation, the upper limit of the C content can be appropriately controlled. That is, the C content may be 0.2 to 0.4%, more specifically, 0.20 to 0.40%, and even more specifically, 0.22 to 0.35%.

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

[0041] Silicon (Si) is added as a deoxidizer in the steelmaking process and can be included to strengthen solid solutions and suppress carbide formation. Additionally, Si can help uniformly distribute the microstructure during annealing and increase the stability of austenite during cooling, thereby securing retained austenite at room temperature. If the Si content is less than 1.0%, it is difficult to sufficiently achieve the aforementioned effects, making it difficult to secure the desired elongation. If the Si content exceeds 2.0%, excessive Si-based oxides may form on the surface of the steel sheet during hot rolling, which may cause surface defects during cold rolling. Furthermore, if the Si content exceeds 2.0%, the resistivity of the final cold-rolled steel sheet may increase excessively, resulting in reduced spot weldability. That is, the content of Si may be 1.0 to 2.0%, specifically 1.00 to 2.00%, more specifically 1.05 to 1.90%, and even more specifically 1.10 to 1.75%.

[0042] Manganese (Mn): 2.5 to 5.0%

[0043] Manganese (Mn) is a stabilizing element of austenite, which can ensure the hardenability of martensite and suppress the formation of ferrite during annealing after cold rolling. When the Mn content is less than 2.5%, martensite hardenability can be secured, but it is difficult to form a gradient of Mn concentration in the cold-rolled steel sheet, so it may be difficult to secure a microstructure according to the Mn concentration difference aimed at in the present invention. In addition, when the Mn content exceeds 5.0%, the strength may increase excessively, or Mn bands may be formed in the thickness direction in the base steel from the steelmaking and casting stages, thereby reducing the crash resistance. That is, the Mn content may be 2.5 to 5.0%, more specifically, 2.7 to 4.5%, and even more specifically, 3.0 to 4.12%.

[0044] Phosphorus (P): 0.1% or less (excluding 0%)

[0045] Phosphorus (P) is an element included as an impurity in steel. When the P content exceeds 0.1%, not only may the weldability deteriorate due to P segregation, but the potential for steel embrittlement may also increase. That is, the P content may be 0.1% or less, more specifically 0.03% or less, and even more specifically 0.02% or less. Meanwhile, the lower limit of the P content may exclude 0% (i.e., exceeding 0%) in consideration of cases where it is unavoidably included. More specifically, the lower limit of the P content may be 0.003%, and even more specifically 0.005%.

[0046] Sulfur (S): 0.03% or less (excluding 0%)

[0047] Sulfur (S) is an impurity element that is inevitably added to steel. When the S content exceeds 0.03%, the precipitates formed due to MnS precipitation during the hot rolling process may not be completely decomposed during annealing, which may lower the ductility of the final cold rolled steel sheet or cause problems such as hindering weldability. That is, the S content may be 0.03% or less, more specifically 0.02% or less, and even more specifically 0.005% or less. Meanwhile, the lower limit of the S content may exclude 0% (i.e., exceeding 0%) in consideration of cases where it is unavoidably included. More specifically, the lower limit of the S content may be 0.001%, and even more specifically 0.002%.

[0048] Aluminum (Al): 0.01 to 0.1%

[0049] Aluminum (Al) can be added to remove oxygen in molten steel during the steelmaking process and can remove impurity elements in the steel. In addition, Al can play a role in suppressing carbide formation and can improve the elongation of the steel sheet because it contributes to the C stabilization of austenite. If the Al content is less than 0.01%, it may be difficult to sufficiently secure the aforementioned effect. In addition, if the Al content exceeds 0.1%, excessive precipitation of AlN may cause peripheral cracks to occur, which may cause defects in the product. That is, the Al content may be 0.01 to 0.1%, specifically 0.01 to 0.10%, more specifically 0.01 to 0.08%, and even more specifically 0.02 to 0.06%.

[0050] Nitrogen (N): 0.01% or less (excluding 0%)

[0051] Nitrogen (N) is an impurity element in steel. When the N content exceeds 0.01%, the risk of cracks occurring during playing may excessively increase due to AlN formation. That is, the N content may be 0.01% or less, specifically 0.010% or less, more specifically 0.009% or less, and even more specifically 0.008% or less. Meanwhile, the lower limit of the N content may exclude 0% (i.e., exceeding 0%) in consideration of cases where it is unavoidably included. More specifically, the lower limit of the N content may be 0.004%, and even more specifically 0.007%.

[0052] Boron (B): 0.005% or less (excluding 0%)

[0053] Boron (B) is an element that is advantageous in suppressing ferrite phase transformation during annealing, and can improve the hardenability of martensite through grain boundary strengthening and solid solution strengthening. When the B content exceeds 0.005%, Fe, a B-based precipitate phase, 23 Since (B,C)6 is formed at the austenite grain boundaries, it can cause brittle fracture in a hot-rolled state. That is, the B content can be 0.005% or less, more specifically 0.004% or less, and even more specifically 0.003% or less. Meanwhile, the lower limit of the B content can exclude 0% (i.e., more than 0%) in consideration of cases where it is inevitably included. More specifically, the lower limit of the B content can be 0.001%.

[0054] Titanium (Ti): 0.01 to 0.04%

[0055] Titanium (Ti) can play a role in increasing the strength of steel plates and refining grain size. However, if the Ti content is less than 0.01%, it may be difficult to sufficiently secure the aforementioned effects. Furthermore, if the Ti content exceeds 0.04%, manufacturing costs may increase excessively, or ductility may be significantly reduced due to excessive precipitates. In other words, the Ti content may be 0.01 to 0.04%, more specifically 0.012 to 0.038%, and even more specifically 0.015 to 0.035%.

[0056] A cold-rolled steel sheet according to one embodiment of the present invention may contain the above components, as well as iron (Fe) as a remaining component. Furthermore, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during a typical manufacturing process, this cannot be ruled out. For example, the steel may further contain up to a total of 1.0% of the elements that can be included in the steel. Since these impurities are readily apparent to anyone skilled in the art, their full content is not specifically discussed herein.

[0057] For example, the cold rolled steel sheet may optionally further include one or more elements selected from Cr: 0.1% or less, Mo: 0.1% or less, and Niobium (Nb): 0.1% or less. The reasons for adding each element and the reasons for limiting the content are explained below.

[0058] Chromium (Cr): 0.1% or less (including 0%)

[0059] Chromium (Cr) can increase the hardenability of martensite and suppress ferrite transformation, so that martensite with appropriate strength is ultimately formed. When the content of manganese (Mn) is adjusted within a certain range, the effect of the content of chromium (Cr) on the hardenability is reduced, so that martensite strength can be secured without adding manganese (Mn), and a small amount of Cr can also be added. When the content of chromium (Mn) exceeds 0.1%, coarse chromium-based carbides are formed, so that cracks may be induced due to local deformation and stress generation at the boundary between the carbide and the microstructure of the steel during molding of the part. That is, the content of chromium (Cr) may be 0.1% or less, specifically 0.10% or less, more specifically 0.08% or less, and even more specifically 0.05% or less. Since cases where Cr is not added may also be included, the lower limit of the content of chromium (Cr) may be 0%. Additionally / alternatively, when Cr is added, the lower limit of the Cr content may be 0.001%, more specifically 0.003%, and even more specifically 0.005%.

[0060] Molybdenum (Mo): 0.1% or less (including 0%)

[0061] Molybdenum (Mo) is an element that is effective in increasing martensite hardenability and suppressing ferrite formation during the cooling section during annealing. When the Mo content exceeds 0.1%, the problem of increased cost due to excessive alloy input may occur. That is, the Mo content may be 0.1% or less, specifically 0.10% or less, more specifically 0.08% or less, and even more specifically 0.05% or less. Since cases where Mo is not added may be included, the lower limit of the Mo content may be 0%. Additionally / alternatively, when Mo is added, as a non-limiting example, the lower limit of the Mo content may be 0.01% or 0.05%.

[0062] Niobium (Nb): 0.1% or less (including 0%)

[0063] Niobium (Nb) can segregate at austenite grain boundaries and suppress coarsening of austenite grains during annealing heat treatment. If the Nb content exceeds 0.1%, the manufacturing cost may increase excessively, or coarse precipitates may be formed during the casting process, which may cause a decrease in toughness. That is, the Nb content may be 0.1% or less, more specifically 0.08% or less, and even more specifically 0.06% or less. Since Nb may also be present in a case where it is not added, the lower limit of the Nb content may be 0%.

[0064] For example, cold rolled steel sheet may include, as a microstructure, 10 to 30% of retained austenite and 10 to 90% of the sum of bainite and martensite in area %.

[0065] When the area fraction of retained austenite is less than 10%, a cold-rolled steel sheet having a matrix structure of martensite is manufactured, which may cause a problem of insufficient elongation. In addition, when the area fraction of retained austenite exceeds 30%, the carbon stability within the retained austenite is reduced, and martensitic transformation occurs due to strain-induced transformation during processing, which may cause a problem of deteriorated formability. That is, the area fraction of retained austenite may be 10 to 30%, more specifically 12 to 28%, and even more specifically 14 to 26%.

[0066] In one embodiment of the present invention, in the case of composite structure steel, since it is not easy to distinguish between bainite and martensite in the microstructure by SEM observation, the sum of the area fractions of bainite and martensite can be controlled.

[0067] For example, the microstructure may include both bainite and martensite, in which case the sum of the area fractions of bainite and martensite may be from 10 to 90% based on the total fraction of the microstructure. However, this is not limited thereto, and as a non-limiting example, the microstructure may include one of bainite and martensite, in which case the area fraction of bainite or martensite may be from 10 to 90% based on the total fraction of the microstructure.

[0068] If the sum of the area fractions of bainite and martensite is less than 10%, it may be difficult to secure sufficient retained austenite, or the carbon stability within the retained austenite may deteriorate. Furthermore, if the sum of the area fractions of bainite and martensite exceeds 90%, it may be difficult to secure sufficient strength and / or elongation. That is, the sum of the area fractions of bainite and martensite may be 10 to 90%, more specifically 15 to 89%, and even more specifically 20 to 88%.

[0069] For example, when the microstructure includes bainite and martensite, the area fraction of bainite may be 30 to 60%, and the area fraction of martensite may be 10 to 60%, based on the total fraction of the microstructure. For example, the area fractions of bainite and martensite can be calculated by inverse estimation based on the physical properties of the final product. In addition, for example, as described below, bainite and martensite can be distinguished based on a method such as electron backscatter diffraction (EBSD) and the respective area fractions can be calculated.

[0070] When the area fraction of bainite is less than 30%, carbon distribution into austenite is limited, which can lead to problems in securing sufficient retained austenite. Furthermore, when the area fraction of bainite exceeds 60%, problems in securing sufficient strength can arise. Specifically, the area fraction of bainite may be 30 to 60%, more specifically 35 to 55%, and even more specifically 40 to 50%.

[0071] When the area fraction of martensite is less than 10%, the carbon stability in the retained austenite is reduced, and martensite transformation occurs due to strain-induced transformation during processing, which may cause a problem of deterioration in formability. In addition, when the area fraction of martensite exceeds 60%, a cold-rolled steel sheet having a matrix structure of martensite is manufactured, which may cause a problem of insufficient elongation. That is, the area fraction of martensite may be 10 to 60%, more specifically 17 to 53%, and even more specifically 24 to 46%.

[0072] For example, the value of the following relationship 1 for cold rolled steel sheets may be 0.25 to 1.5.

[0073] [Relationship 1]

[0074] (Mn_high - Mn_low) / Mn_base

[0075] In the above relational expression 1, Mn_high represents the average concentration (average content) of Mn in a region where the content of Mn in the microstructure is 1% or more higher than the average content of Mn at the point t / 4 in the thickness direction of the cold-rolled steel sheet, Mn_low represents the average concentration (average content) of Mn in a region where the content of Mn in the microstructure is 1% or more lower than the average content of Mn at the point t / 4 in the thickness direction of the cold-rolled steel sheet, and Mn_base represents the average content of Mn at the point t / 4 in the thickness direction of the cold-rolled steel sheet. Here, t may mean the thickness of the steel sheet.

[0076] For example, there is no separate limitation on the method for measuring the above Mn_high and Mn_low. For example, as a non-limiting example, after mechanical polishing is performed from the surface of the steel sheet to a position of 1 / 4t based on the entire thickness t of the steel sheet, the Mn content can be quantitatively measured using EPMA surface analysis at an area magnification of 30㎛×30㎛. In this case, the average Mn content of each region can be measured by performing EPMA surface analysis on a region with a locally high Mn content (e.g., a region that is 1 wt% or more higher than the average Mn content of the cold-rolled steel sheet) and a region with a locally low Mn content (e.g., a region that is 1 wt% or more lower than the average Mn content of the cold-rolled steel sheet).

[0077] For example, there is no specific limitation on the method for measuring the Mn_base. For example, as a non-limiting example, when manufacturing cold-rolled steel sheets, the content of Mn added to the slab can be interpreted as the value of Mn_base.

[0078] When the value of the above relational expression 1 is less than 0.25, it is difficult to realize the additional strength effect resulting from the difference in the Mn content, and it may be difficult to secure sufficient elongation because it is difficult to secure retained austenite. In addition, when the value of the above relational expression 1 exceeds 1.5, the contents of C and Mn increase accordingly, which may cause a problem of deterioration in workability due to the formation of Mn bands and carbides in the cold-rolled steel sheet. That is, the value of the above relational expression 1 may be 0.25 to 1.5, specifically 0.25 to 1.50, more specifically 0.27 to 1.45, and even more specifically 0.29 to 1.40.

[0079] According to one embodiment of the present invention, a cold-rolled steel sheet can form a hard martensite phase and a soft bainite phase according to the concentration of Mn in the microstructure by distributing the concentration of Mn, and can induce additional strengthening in the hard phase from the resulting strain distribution. For example, the concentration distribution of Mn in the microstructure can be appropriately adjusted by controlling the annealing process. Through this gradient of Mn concentration, bainite transformation is promoted, thereby ensuring austenite stability, thereby securing retained austenite at room temperature, and thus providing an ultra-high strength steel with excellent elongation.

[0080] For example, cold-rolled steel sheets may have a tensile strength of 1500 MPa or greater. Additionally / alternatively, cold-rolled steel sheets may have an elongation of 10% or greater. By meeting the above-mentioned tensile strength and / or elongation of the cold-rolled steel sheets, the cold-rolled steel sheets are suitable for cold stamping.

[0081] For example, there is no need to separately limit the yield strength of the cold rolled steel sheet, but as a non-limiting example, the yield strength of the cold rolled steel sheet may be 1000 MPa or more.

[0082] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described in detail. However, this does not necessarily mean that the method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention must be manufactured using the following manufacturing method.

[0083] [Slab reheating stage]

[0084] The slab having the aforementioned composition can be reheated. Prior to hot rolling, the slab can be reheated and homogenized. At this time, the composition of the slab is identical to that of the aforementioned cold-rolled steel sheet, and the explanation for the aforementioned cold-rolled steel sheet can be applied equally to the reasons for adding each component and limiting its content in the slab.

[0085] For example, the reheating step may be performed at 1,100 to 1,300°C. If the reheating temperature is lower than 1,100°C, a problem of a rapid increase in load during subsequent hot rolling may occur. In addition, if the reheating temperature exceeds 1,300°C, the amount of surface scale may increase, which may lead to material loss. That is, the reheating temperature may be 1,100 to 1,300°C, more specifically 1,120 to 1,280°C, and even more specifically 1,140 to 1,260°C.

[0086] [Hot-rolled steel sheet production stage]

[0087] In the above-described step, the reheated slab can be hot-rolled to obtain a hot-rolled steel sheet. For example, the temperature of the hot rolling may be 800 to 1,000°C. If the temperature of the hot rolling is lower than 800°C, the rolling load may increase excessively due to the introduction of unrecrystallized ferrite. In addition, if the temperature of the hot rolling exceeds 1,000°C, the possibility of surface defects due to scale and increased rolling roll wear may increase. That is, the temperature of the hot rolling may be 800 to 1,000°C, more specifically 810 to 990°C, and even more specifically 820 to 980°C.

[0088] [Winding stage]

[0089] The hot-rolled steel sheet obtained in the above-described step can be coiled. For example, the coiling temperature may be 400 to 700°C. If the coiling temperature is lower than 400°C, the strength of the hot-rolled steel sheet becomes excessively high, which not only increases the rolling load in the cold rolling process, but also increases the number of control variables in the cold rolling process to control the load, which may result in poor productivity. In addition, if the coiling temperature exceeds 700°C, an excessive oxide film may form on the surface of the steel sheet, which may cause defects (e.g., surface defects). That is, the coiling temperature may be 400 to 700°C, more specifically 410 to 690°C, and even more specifically 420 to 680°C.

[0090] [Cold-rolled steel sheet production stage]

[0091] The hot rolled steel sheet coiled in the above-described step can be cold rolled to obtain a cold rolled steel sheet. For example, cold rolling can be performed at a reduction ratio of 20 to 75%. If the reduction ratio is less than 20%, not only is it difficult to secure the target thickness, but the residual grains generated during the hot rolling process can affect the austenite formation during the annealing process and the final physical properties of the steel sheet. In addition, if the reduction ratio exceeds 75%, the load on the cold rolling mill increases, which can deteriorate workability. That is, the reduction ratio during cold rolling can be 20 to 75%, more specifically 25 to 70%, and even more specifically 30 to 65%.

[0092] For example, coiled hot-rolled steel sheets can be pickled prior to cold rolling. Pickling can remove the oxide layer formed on the surface of the steel sheet. The pickling method is not particularly limited, and any method commonly used in the art (e.g., immersing the steel sheet in a hydrochloric acid bath) can be applied.

[0093] Fig. 1 is a schematic diagram illustrating a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention. Referring to Fig. 1, the method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention may include a two-stage annealing process including the processes of primary annealing, secondary annealing, primary cooling, secondary cooling, and constant temperature maintenance. Through this two-stage annealing process, a high-elongation cold-rolled steel sheet having mechanical properties of an elongation of 10% or more while securing a tensile strength of 1.5 GPa or more can be manufactured.

[0094] Each process is described in detail below.

[0095] [First Annealing Stage]

[0096] The cold-rolled steel sheet obtained in the aforementioned step can be subjected to a first annealing at a first annealing temperature. Through the first annealing step, a difference in the Mn concentration between the ferrite and austenite generated in the ideal region can be generated, and ferrite with a low Mn concentration and austenite with a high Mn concentration can be formed.

[0097] For example, the Mn concentration difference can be maximized by appropriately controlling the first annealing temperature and / or the holding time at the first annealing temperature. For example, the first annealing temperature can be 700 to 800°C. Here, the method for measuring the first annealing temperature is not particularly limited, but the surface temperature of the steel sheet can be measured as the first annealing temperature. If the first annealing temperature is less than 700°C, the austenite phase is not formed, so the distribution of C and Mn does not occur properly, and therefore, even after the second annealing, the Mn concentration difference does not occur, making it difficult to secure the target physical properties. In addition, if the first annealing temperature exceeds 800°C, the austenite phase is excessively formed, so that although there is an effect of C and Mn being distributed into austenite, the concentration difference of C and Mn becomes small, and therefore, it may be difficult to secure the fraction of retained austenite and sufficient elongation. That is, the first annealing temperature may be 700 to 800°C, more specifically 705 to 795°C, and even more specifically 710 to 790°C.

[0098] For example, the cold-rolled steel sheet can be first annealed by maintaining it at the first annealing temperature for 10 minutes to 24 hours. Specifically, based on the surface temperature of the steel sheet, the steel sheet is heated so that the highest temperature becomes the first annealing temperature (i.e., 700 to 800°C), and the steel sheet can be maintained at the first annealing temperature for 10 minutes to 24 hours from the time the highest temperature is reached. As the holding time at the first annealing temperature is adjusted within the above range, the Mn concentration difference can be further maximized.

[0099] If the holding time at the first annealing temperature is less than 10 minutes, the C and Mn distribution effect in the ideal temperature range is minimal, so that the C and Mn concentrations become uniform and the Mn concentration difference becomes small, and thus, it may be difficult to secure the fraction of retained austenite and sufficient elongation. In addition, if the holding time at the first annealing temperature exceeds 24 hours, the strength may decrease due to grain coarsening, or the heterogeneity of the C and Mn concentrations in ferrite and / or austenite may be weakened. That is, the holding time at the first annealing temperature may be 10 minutes to 24 hours, more specifically, 12 minutes to 23 hours, and even more specifically, 14 minutes to 22 hours.

[0100] [Second Annealing Stage]

[0101] In the aforementioned step, the first-annealed cold-rolled steel sheet can be subjected to a second annealing at a second annealing temperature. Through the second annealing step, the desired microstructure composition and area fraction can be appropriately secured while appropriately maintaining the Mn distribution obtained from the first annealing.

[0102] The second annealing temperature may be a temperature higher than the austenite single-phase region. Here, the method for measuring the second annealing temperature is not specifically limited, but the surface temperature of the steel sheet may be measured as the second annealing temperature. By adjusting the second annealing temperature within the above range, 10% or more of bainite and martensite can be secured from the austenite in which Mn is distributed. If the second annealing temperature is lower than the austenite single-phase region, it may be difficult to appropriately secure bainite and / or martensite. In addition, the upper limit of the second annealing temperature is not specifically limited, but as a non-limiting example, the upper limit of the second annealing temperature may be 950°C.

[0103] For example, heating can be performed at an average heating rate of 2°C / s or more to the second annealing temperature. By adjusting the average heating rate within the above range, the Mn distribution obtained from the first annealing step can be maintained. If the average heating rate is less than 2°C / s, the Mn is redistributed, which can cause a problem in that the Mn concentration difference is minimized and the Mn concentration becomes uniform overall. That is, the average heating rate can be 2°C / s or more, more specifically, 3°C / s or more, and even more specifically, 4°C / s or more. In addition, the upper limit of the average heating rate is not specifically limited, but as a non-limiting example, the upper limit of the average heating rate can be 500°C / s.

[0104] For example, a first-annealed cold-rolled steel sheet can be secondarily annealed by maintaining it at the second annealing temperature for a time exceeding 0 seconds and not exceeding 10 seconds. Specifically, based on the surface temperature of the steel sheet, the steel sheet can be heated so that the highest temperature becomes the second annealing temperature (i.e., a temperature above the austenite single-phase region), and the steel sheet can be maintained at the second annealing temperature for a time exceeding 0 seconds and not exceeding 10 seconds from the time the highest temperature is reached.

[0105] If the holding time at the second annealing temperature exceeds 10 seconds, Mn is redistributed, so the Mn concentration difference is minimized and the Mn concentration becomes uniform overall, making it difficult to secure the fraction of retained austenite. That is, the holding time at the second annealing temperature may be more than 0 seconds and less than or equal to 10 seconds, more specifically, 1 to 9 seconds, and even more specifically, 2 to 8 seconds.

[0106] [Cooling stage]

[0107] In the aforementioned step, the secondary annealed cold-rolled steel sheet can be cooled to a predetermined cooling end temperature. Through the cooling step, a cold-rolled steel sheet having a combined area fraction of martensite and bainite of at least 40% can be obtained. Additionally / alternatively, through the cooling step, a cold-rolled steel sheet having a residual austenite area fraction of at least 10% can be obtained.

[0108] The cooling end temperature may be the bainite transformation temperature. Specifically, the cooling end temperature may be between 280 and 480°C. If the cooling end temperature is below 280°C or above 480°C, the bainite phase cannot be sufficiently secured, and thus the retained austenite formed during the final cooling cannot be secured, which may cause problems in securing elongation. In other words, the cooling end temperature may be between 280 and 480°C, more specifically between 290 and 470°C, and even more specifically between 300 and 460°C.

[0109] For example, cooling may be performed at an average cooling rate of 10°C / s or more to the cooling end temperature. If the average cooling rate is less than 10°C / s, excessive introduction of pearlite or bainite phases during cooling may result in a decrease in tensile strength and / or elongation. That is, the average cooling rate may be 10°C / s or more, more specifically 12°C / s or more, and even more specifically 14°C / s or more. In addition, the upper limit of the average cooling rate is not specifically limited, but as a non-limiting example, the upper limit of the average cooling rate may be 50°C / s.

[0110] [Standard temperature maintenance stage]

[0111] The cold-rolled steel sheet cooled in the aforementioned step can be maintained at the cooling end temperature (e.g., 280 to 480°C, which is the temperature range of the bainite transformation zone). If the constant temperature maintenance temperature is less than 280°C or exceeds 480°C, the bainite phase cannot be sufficiently secured, and thus, the retained austenite formed during the final cooling cannot be secured, which may cause problems in securing elongation.

[0112] The cooled cold-rolled steel sheet can be maintained at the cooling end temperature for 5 minutes to 5 hours. If the constant temperature holding time is less than 5 minutes, it may be difficult to secure the fraction of retained austenite because the time required for bainite transformation is not sufficient. In addition, if the constant temperature holding time exceeds 5 hours, the problem of not being able to secure the fraction of retained austenite may occur due to the redistribution of manganese. That is, the constant temperature holding time may be 5 minutes to 5 hours, more specifically, 6 minutes to 4.5 hours, and even more specifically, 7 minutes to 4 hours.

[0113] According to a manufacturing method according to one embodiment of the present invention, a cold rolled steel sheet having mechanical properties such as a tensile strength of 1.5 GPa or higher and / or an elongation of 10% or higher can be obtained. More specifically, in a two-stage annealing process, the tensile strength and / or elongation at the desired level can be secured through precise control of the first annealing temperature, first annealing time, second annealing temperature, second annealing time, cooling end temperature, etc.

[0114] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0115] (Example)

[0116] A slab having the alloy composition shown in Table 1 below (the remaining components of the alloy composition: Fe and unavoidable impurities, in Table 1, N is expressed in ppm units and the remaining components are expressed in wt%) was vacuum-melted into an ingot, maintained at 1200°C for 1 hour, and finish-rolled at 900°C to obtain a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was charged into a furnace preheated to 600°C, maintained for 1 hour, and then coiled through furnace cooling. The coiled hot-rolled steel sheet was pickled, and then cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet.

[0117] The cold-rolled steel sheet obtained as described above was subjected to primary annealing by heating to various ideal temperature ranges under the conditions described in Table 2 below, and then secondary annealing was performed at a temperature above the austenite single-phase region. Subsequently, the secondary annealed cold-rolled steel sheet was cooled to the cooling temperature (average cooling rate: 10°C / s) under the conditions described in Table 2 below, and then the temperature was maintained at the cooling temperature.

[0118] Steel grade classificationAlloy compositionCSiMnPSAlCrTiNbBNMoA Inventive steel0.281.53.80.010.0020.02500.0200.002400B Inventive steel0.251.440.010.0020.0250.050.0200.002400C Inventive steel0.31.53.50.010.0020.0250.10.0250.0 40.002400.05D Comparison Steel 0.151.430.010.0020.0250.10.0250.040.002400.05E Comparison Steel 0.31.520.010.0020.0250.10.0250.040.002400.05F Comparison Steel 0.250.53.50.010.0020.02500.0200.002400

[0119] Classification Steel grade 1st annealing 2nd annealing Cooling Constant temperature Maintaining temperature (℃) Maintaining time (min) Temperature (℃) Maintaining time (min) Cooling end temperature (℃) Maintaining time (min) Comparative example 1A75059005350120 Invention example 1A750109005350120 Invention example 2A750309005300120 Comparative example 2A820309005350120 Invention example 3B73060900533060 Comparative example 3B75030900603003 Comparative example 4B660309005300120 Invention example 4C75020900533060 Comparative example 5D750309005300120 Comparative example 6D73060900533060Comparative Example 7E750309005300120Comparative Example 8E73060900533060Comparative Example 9E75020900533060Comparative Example 10F750309005300120

[0120]

[0121] For each cold-rolled steel sheet manufactured above, the value of equation 1, the fraction of microstructure, and mechanical properties (yield strength, tensile strength, and elongation) were measured, and the results are shown in Table 3.

[0122] The value of relational expression 1 was calculated by the formula [(Mn_high - Mn_low) / Mn_base]. Specifically, for each cold-rolled steel sheet, mechanical polishing was performed from the surface of the steel sheet to a position of 1 / 4t based on the total thickness t of the steel sheet, and then the Mn content was measured using EPMA surface analysis at a magnification of 1,000 times or more. At this time, the average concentration of Mn in the area higher by 1 wt% or more than the average Mn content of the cold-rolled steel sheet was measured as Mn_high, and the average concentration of Mn in the area lower by 1 wt% or more than the average Mn content of the cold-rolled steel sheet was measured as Mn_low. In addition, Mn_base was expressed as the content of Mn contained in the slab during the manufacture of each cold-rolled steel sheet.

[0123] The microstructural fractions were measured for each cold-rolled steel sheet using XRD analysis equipment and a magnetic induction method (Metis). Specifically, the fraction of retained austenite, including micro-austenite, within the steel sheet was measured, and the fractions of bainite and martensite were calculated using optical microscopy, scanning electron microscopy, and EBSD analysis equipment for the remaining fractions. Bainite and martensite have a BCC structure, making them difficult to distinguish. Considering this, martensite has a relatively higher dislocation density than bainite and lacks a specific crystal orientation. Therefore, the IQ distribution and KAM index were used to distinguish between bainite and martensite. First, because phases with dislocation density or lattice distortion have relatively low IQ (Image Quality) values, a Gaussian function was used to distinguish individual peaks in the IQ distribution. Furthermore, because the criteria for the overlapping portion and threshold of the IQ distribution vary depending on the specimen location or EBSD measurement conditions, the KAM index was used to compensate for this. Specifically, martensite has a high KAM index because compressive stress acts due to volume expansion caused by phase transformation and has a disordered crystal orientation. In this way, bainite and martensite were distinguished by combining the IQ distribution and the KAM index. In this experimental example, a structure with a low critical IQ value or a KAM threshold exceeding 2.5 was classified as martensite. In addition, a structure with a high critical IQ value and a KAM threshold of 2.5 or less was classified as bainite.

[0124] In order to measure the mechanical properties, each cold-rolled steel plate was processed perpendicular to the rolling direction according to the JIS standard specifications, and then a tensile tester and an extensometer were attached to measure the yield strength, tensile strength, and elongation.

[0125] Classification relationship 1 Microstructure (area %) Mechanical properties MBγ Yield strength (MPa) Tensile strength (MPa) Elongation (%) Comparative example 10.2 165 305 11 46 17 49 6.7 Invention example 10.5 9 30 48 22 10 5 4 16 9 4 13.5 Invention example 20.6 8 20 5 4 26 10 2 1 6 4 3 16.2 Comparative example 20.1 46 8 28 4 1 6 2 1 7 6 4 7.9 Invention example 30.7 5 43 38 19 10 8 6 1 7 8 0 12.3 Comparative example 30.1 26 33 5 21 2 4 3 1 9 0 5 5.8 Comparative example 40.0 9 6 9 29 1 1 9 7 1 8 8 6 4.9 Invention example 40.862851211107180512.8Comparative example 50.72275023968145214.6Comparative example 60.83433918979146713.1Comparative example 71.2542508117817548.2Comparative example 81.2139556118417987.3Comparative example 91.1844524117917886.9Comparative example 100.6950455108216577.5- M: Martensite- B: Bainite- γ: Retained austenite

[0126]

[0127] In the case of invention examples 1 to 4 that satisfy the alloy composition and manufacturing conditions proposed in the present invention, the value of relational expression 1 was derived at an appropriate level, and accordingly, it was confirmed that a tensile strength of 1500 MPa or more, a yield strength of 1000 MPa or more, and an elongation of 10% or more were secured.

[0128] As a representative example of the invention, a photograph of the microstructure of the cold-rolled steel sheet of invention example 3 is shown in Fig. 2. Specifically, Fig. 2 (a) is a photograph of the microstructure of invention example 3 observed using a scanning electron microscope (FE-SEM). In addition, Fig. 2 (b) is a photograph of the microstructure of invention example 3 observed using electron backscatter diffraction (EBSD), and is a photograph showing a phase distribution map that enlarges the region where retained austenite is distributed for invention example 3.

[0129] Referring to Figure 2, it was found that a large amount of retained austenite was secured not through microstructural control with a uniform chemical composition, but rather through a non-uniform Mn content gradient. Accordingly, it was confirmed that ultra-high strength of 1.5 GPa and excellent elongation were achieved.

[0130] More specifically, referring to (a) of Fig. 2, the relatively dark region represents bainite, and the relatively bright region represents martensite. That is, it was found that martensite, a hard phase, and bainite, a soft phase, were appropriately formed through the gradient of Mn.

[0131] Also, referring to (b) of Fig. 2, the area appearing in a bright color (white) represents retained austenite. That is, it was found that a large amount of retained austenite was secured through the non-uniform Mn content gradient.

[0132] Meanwhile, in Comparative Example 1, where the primary annealing holding time was excessively short, Mn distribution did not occur sufficiently, resulting in a very small value of the value of Equation 1 (i.e., the Mn concentration difference). As a result, it was difficult to secure the fraction of retained austenite, and it was found that the elongation was less than 10%.

[0133] In addition, Comparative Example 2, in which the temperature of the first annealing was excessively high, had a minimal effect on the distribution of Mn, and thus the value of Equation 1 (i.e., the difference in Mn concentration) was calculated to be very small. Accordingly, it was found that it was difficult to secure the fraction of retained austenite, and the elongation was less than 10%.

[0134] In addition, Comparative Example 3, in which the secondary annealing holding time was excessively long and the constant temperature holding time was excessively short, had problems in that the Mn concentration difference was minimized and the Mn concentration became uniform overall because Mn was redistributed. Accordingly, it was found that Comparative Example 3 had difficulty in securing the fraction of retained austenite and the elongation was less than 10%.

[0135] In addition, Comparative Example 4, in which the temperature of the first annealing was excessively low, had a problem in that the cementite formed within the ferrite phase was not completely dissolved, and thus remained even after the second annealing. Accordingly, Comparative Example 4 was unable to secure an appropriate area fraction of the microstructure, and it was found that the elongation was less than 10%.

[0136] In addition, it was confirmed that in the case of comparative examples 5 to 10 that did not satisfy the alloy composition proposed by the present invention, the yield strength, tensile strength and / or elongation at the level targeted by the present invention were not secured.

[0137] The above embodiments are merely examples and the present invention is not limited thereto. Anything that has substantially the same configuration and achieves the same operational effects as the technical concepts described in the claims of the present invention is included within the technical scope of the present invention.

Claims

1. Contains, in wt%, C: 0.2 to 0.4%, Si: 1.0 to 2.0%, Mn: 2.5 to 5.0%, P: 0.1% or less (excluding 0%), S: 0.03% or less (excluding 0%), Al: 0.01 to 0.1%, N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), Ti 0.01 to 0.04%, the remainder being Fe and other unavoidable impurities. As a microstructure, it contains, in area %, retained austenite: 10 to 30%, and the sum of bainite and martensite: 10 to 90%, Cold rolled steel sheet, wherein the value of the following relational expression 1 is 0.25 to 1.

5. [Relationship 1] (Mn_high - Mn_low) / Mn_base (In the above relational expression 1, Mn_high represents an average concentration of Mn in a region where the content of Mn in the microstructure is 1% or more higher than the average content of Mn at the t / 4 point in the thickness direction of the cold rolled steel sheet, Mn_low represents an average concentration of Mn in a region where the content of Mn in the microstructure is 1% or more lower than the average content of Mn at the t / 4 point in the thickness direction of the cold rolled steel sheet, and Mn_base represents an average content of Mn at the t / 4 point in the thickness direction of the cold rolled steel sheet.) 2. In paragraph 1, A cold rolled steel sheet having an area fraction of bainite of 30 to 60% based on the total fraction of the above microstructure.

3. In paragraph 1, A cold rolled steel sheet having an area fraction of martensite of 10 to 60% based on the total fraction of the above microstructure.

4. In paragraph 1, Cold rolled steel sheet further comprising at least one selected from Cr: 0.1% or less, Mo: 0.1% or less, and Nb: 0.1% or less in weight%.

5. In paragraph 1, Cold rolled steel sheet having a tensile strength of 1500 MPa or more and an elongation of 10% or more.

6. In paragraph 1, Cold rolled steel sheet with a yield strength of 1000 MPa or more.

7. A step of reheating a slab containing, by weight%, C: 0.2 to 0.4%, Si: 1.0 to 2.0%, Mn: 2.5 to 5.0%, P: 0.1% or less (excluding 0%), S: 0.03% or less (excluding 0%), Al: 0.01 to 0.1%, N: 0.01% or less (excluding 0%), B: 0.005% or less (excluding 0%), Ti 0.01 to 0.04%, the remainder Fe and other unavoidable impurities; A step of hot rolling a reheated slab to obtain a hot-rolled steel sheet; A step of coiling the above hot-rolled steel plate; A step of cold rolling a coiled hot rolled steel sheet to obtain a cold rolled steel sheet; A first annealing step of maintaining the cold rolled steel sheet at a first annealing temperature of 700 to 800°C for 10 minutes to 24 hours; A second annealing step of maintaining the first-annealed cold-rolled steel sheet at a second annealing temperature higher than the austenite single-phase range for a time of more than 0 seconds and less than 10 seconds; A step of cooling the above second annealed cold rolled steel sheet to a cooling end temperature of 280 to 480°C; and A method for manufacturing a cold rolled steel sheet, comprising a step of maintaining the cooled cold rolled steel sheet at the cooling end temperature for 5 minutes to 5 hours.

8. In paragraph 7, A method for manufacturing a cold rolled steel sheet, wherein in the second annealing step, heating is performed at an average heating rate of 2°C / s or more to the second annealing temperature.

9. In paragraph 7, A method for manufacturing a cold rolled steel sheet, wherein in the cooling step, cooling is performed at an average cooling rate of 10°C / s or more to the cooling end temperature.

10. In paragraph 7, A method for manufacturing a cold rolled steel sheet, wherein the reheating step is performed at 1,100 to 1,300°C, and the step of obtaining the hot rolled steel sheet is performed by hot rolling the reheated slab at 800 to 1,000°C.

11. In paragraph 7, A method for manufacturing a cold rolled steel sheet, wherein the coiling step is performed at 400 to 700°C, and the step of obtaining the cold rolled steel sheet is performed by cold rolling the coiled hot rolled steel sheet at a reduction ratio of 20 to 75%.

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