Cold-rolled steel sheet and manufacturing method thereof

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof and, more specifically, to an ultra-high strength cold-rolled steel sheet having excellent bendability and hydrogen embrittlement resistance, and a manufacturing method thereof.

Description

Cold rolled steel sheet and its manufacturing method

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to an ultra-high-strength cold-rolled steel sheet having excellent bendability and hydrogen embrittlement resistance and a method for manufacturing the same.

[0002] Recently, demand for higher strength steel sheets used in automotive frame components has been increasing. Furthermore, ultra-high-strength steel sheets with tensile strengths exceeding 1500 MPa are being applied to some components.

[0003] To achieve this ultra-high strength, it's effective to use a steel structure dominated by hard structures like martensite or bainite. However, these structures have lower elongation than composite structures like ferrite and martensite, and due to the occurrence of cracks during forming, they are limited to parts formed through relatively simple bending processes.

[0004] While the formation of this hardened structure is essential for enhancing the strength of steel plates, its application as a robust automotive component has been limited due to the risk of cracking during molding and the deterioration of impact properties during collisions. Therefore, to achieve high strength in automotive components with a martensite-based steel structure, in addition to securing the desired strength, development of steels with superior hydrogen embrittlement and fatigue properties is essential. This, in turn, ensures crashworthiness during vehicle collisions. This will ultimately lead to widespread adoption as a high-strength steel material for automotive components.

[0005] Meanwhile, HPF steel is a material with superior formability and crashworthiness. HPF (Hot Press Forming) is a forming technique that involves forming a material at high temperatures, followed by water cooling between the die and the material to achieve the required strength. This process is then press-formed in a hot state. Therefore, it not only achieves high strength per thickness, but also offers superior formability, making it widely used.

[0006] However, because there are problems in application due to excessive facility investment and increased process costs, research is actively being conducted on the development of materials for cold stamping with a tensile strength of 1500 MPa or higher.

[0007] Meanwhile, patent document 1 discloses a technology for securing strength by adding hardenable elements such as C and Mn to secure strength and then securing martensite at room temperature through water cooling after regular annealing.

[0008] In addition, patent document 2 discloses a technology for securing high-strength steel by optimizing cooling conditions during cooling.

[0009] However, since the strength of Patent Documents 1 and 2 is primarily achieved through the use of martensite, they suffer from numerous processing issues, including limitations in formability and poor shape due to rapid cooling, limiting the production of steel plates with superior shapes. Furthermore, there are limitations in securing crashworthiness and fatigue properties.

[0010] (Patent Document 1) Japanese Patent Publication No. 2010-215958

[0011] (Patent Document 2) Japanese Patent Publication No. 2013-227657

[0012] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same are provided.

[0013] According to one embodiment of the present invention, an object is to provide an ultra-high strength cold rolled steel sheet having excellent bendability and hydrogen embrittlement resistance and a method for manufacturing the same.

[0014] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0015] According to one embodiment of the present invention, it contains, in wt%, carbon (C): 0.20 to 0.30%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 3.5%, boron (B): 0.0005 to 0.0030%, phosphorus (P): 0.010% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 0.20% or less, molybdenum (Mo): 0.10% or less, niobium (Nb): 0.010 to 0.100%, titanium (Ti): 0.010 to 0.100%, and the remainder includes iron (Fe) and other unavoidable impurities.

[0016] The ratio a / b*100 of the residual stress (a) at the 1 / 2 point in the direction of the thickness of the steel plate and the residual stress (b) in the area from the surface to the center of the thickness of 30 μm may be 95.0% or more.

[0017] The above cold rolled steel sheet may further include at least one of vanadium (V): 0.01 to 0.10%, and antimony (Sb): 0.10% or less.

[0018] The above cold-rolled steel sheet has a microstructure in a region of 30 μm from the surface toward the center of the thickness, including at least one type of martensite and tempered martensite as the main phase, and may include 5.0% or less of ferrite and bainite in total in area %.

[0019] The above cold-rolled steel sheet may have a tensile strength of 1500 MPa or more and a bendability (R / t) of 3.5 or less.

[0020] According to one embodiment of the present invention, there is provided a step of reheating a steel slab containing, in wt%, carbon (C): 0.20 to 0.30%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 3.5%, boron (B): 0.0005 to 0.0030%, phosphorus (P): 0.010% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 0.20% or less, molybdenum (Mo): 0.10% or less, niobium (Nb): 0.010 to 0.100%, titanium (Ti): 0.010 to 0.100%, and the remainder iron (Fe) and other unavoidable impurities;

[0021] A step of hot rolling the above reheated steel slab;

[0022] A step of coiling the hot-rolled steel sheet at a temperature range of 400 to 600°C;

[0023] A step of cold rolling the above-mentioned rolled steel plate;

[0024] A step of annealing the above cold-rolled steel sheet in a temperature range of Ac3+10℃ to Ac3+80℃;

[0025] A step of first cooling the above-mentioned annealed steel plate to a temperature range of 680 to 750°C at an average cooling rate of 20°C / s or less;

[0026] A step of secondary cooling the above-mentioned primary cooled steel plate at an average cooling rate of 70 to 200°C / s to a temperature range of 100°C to Mf;

[0027] A step of heat-treating the secondarily cooled steel plate at a temperature range of 150 to 240°C; and

[0028] It may be a method for manufacturing a cold rolled steel sheet, including a step of subjecting the above-mentioned over-aging heat-treated steel sheet to temper rolling or tension leveling at a reduction ratio of 0.01 to 0.20%.

[0029] The above steel slab may further contain at least one of vanadium (V): 0.01 to 0.1%, and antimony (Sb): 0.1% or less.

[0030] The above reheating step is performed at a temperature range of 1100 to 1300°C,

[0031] The above hot rolling step is performed at a temperature range of Ar3 or higher,

[0032] The above cold rolling step can be performed at a reduction ratio of 30 to 80%.

[0033] The above annealing step is performed for more than 30 seconds,

[0034] The above-mentioned heat treatment step can be performed for 200 seconds or longer.

[0035] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0036] According to one embodiment of the present invention, an ultra-high strength cold-rolled steel sheet having excellent bendability and hydrogen embrittlement resistance and a method for manufacturing the same can be provided.

[0037] According to one embodiment of the present invention, an ultra-high strength cold-rolled steel sheet, a plated steel sheet, and a manufacturing method thereof can be provided, which have excellent hydrogen embrittlement resistance, excellent component performance, and excellent crash resistance characteristics in the event of a collision, and can be used as an automotive steel sheet.

[0038] The various advantageous and beneficial 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.

[0039] Figure 1 is a graph showing the bendability values ​​according to the sum of the ferrite and bainite fractions (F+B) in the area from the surface to the center of thickness, up to 30 μm.

[0040] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0041] Hereinafter, the present invention will be described in detail.

[0042] Below, the steel composition of the present invention is described in detail.

[0043] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0044] A cold rolled steel sheet according to an embodiment of the present invention may include, in wt%, carbon (C): 0.20 to 0.30%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 3.5%, boron (B): 0.0005 to 0.0030%, phosphorus (P): 0.010% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 0.20% or less, molybdenum (Mo): 0.10% or less, niobium (Nb): 0.010 to 0.100%, and titanium (Ti): 0.010 to 0.100%.

[0045] Carbon (C): 0.20~0.30%

[0046] Carbon (C) is necessary to improve hardenability and obtain a steel structure in which the martensite area ratio at the 1 / 4 point in the thickness direction is 95% or more. In addition, carbon (C) is necessary from the viewpoint of increasing martensite strength and securing the desired tensile strength. If the carbon (C) content is less than 0.20%, the desired strength cannot be obtained. Therefore, according to one embodiment of the present invention, the carbon (C) content may be 0.20% or more. According to one embodiment of the present invention, the carbon (C) content may be 0.22% or more. On the other hand, if the content exceeds 0.30%, it may become difficult to obtain good weldability or delayed fracture resistance. Therefore, according to one embodiment of the present invention, the carbon (C) content may be 0.30% or less. According to one embodiment of the present invention, it may be 0.25% or less.

[0047] Silicon (Si): 0.5% or less

[0048] Silicon (Si) is a strengthening element through solid solution strengthening, and can be included to improve bendability by suppressing the formation of film-like carbides during tempering at high temperatures. From the perspective of obtaining the above effect, a higher silicon (Si) content is advantageous; however, if the content is excessive, weldability and LME (Liquid Metal Embrittlement) may deteriorate, and therefore the upper limit may be limited to 0.5%. According to one embodiment of the present invention, it may be 0.32% or less. However, the lower limit of the silicon (Si) content is not particularly limited, and 0% is excluded because it is added as an impurity element during the manufacturing process. Meanwhile, according to one embodiment of the present invention, the lower limit of the silicon (Si) content may be 0.001%.

[0049] Manganese (Mn): 1.5~3.5%

[0050] Manganese (Mn) contributes to the improvement of strength by increasing the martensite area ratio through increased hardenability and by solid solution strengthening. In addition, manganese (Mn) may be included to fix sulfur (S) in steel as manganese (MnS) and reduce hot brittleness. If the manganese (Mn) content is less than 1.5%, the possibility of forming ferrite or bainite rather than martensite during cooling increases, so the lower limit may be limited to 1.5%. According to one embodiment of the present invention, it may be 1.7% or more. Meanwhile, the manganese (Mn) content may be limited to 3.5% or less from the viewpoint of welding stability. According to one embodiment of the present invention, it may be 3.0% or less. According to one embodiment of the present invention, it may be 2% or less.

[0051] Boron (B): 0.0005~0.0030%

[0052] Boron (B) is an element that suppresses ferrite formation, and therefore, the present invention has the advantage of suppressing the formation of ferrite during cooling after annealing. However, if the content of boron (B) exceeds 0.0030%, ductility may be significantly reduced. According to one embodiment of the present invention, it may be 0.0020% or less. On the other hand, if the content of boron (B) is less than 0.0005%, not only may the hardenability effect be absent at all, but also ferrite may be formed on the surface, which may tend to result in poor bendability.

[0053] Phosphorus (P): 0.010% or less

[0054] Phosphorus (P) is an impurity element contained in steel. While a lower amount added to steel is advantageous, a content of 0% is excluded in consideration of cases where it is unavoidably included during the manufacturing process. However, if the content exceeds 0.010%, weldability deteriorates and there is a risk of steel becoming brittle, so the upper limit may be limited to 0.010%. According to one embodiment of the present invention, it may be 0.003% or less.

[0055] Sulfur (S): 0.010% or less

[0056] Sulfur (S), like P, is an impurity that is inevitably included in steel, and is an element that impairs the ductility and weldability of steel sheets. Therefore, it is desirable to control the content as low as possible. Therefore, in the present invention, the content of sulfur (S) may be limited to 0.010% or less. According to one embodiment of the present invention, it may be 0.005% or less. According to one embodiment of the present invention, in order to minimize MnS precipitation in the steel and further contribute to improving bendability, it may be controlled to 0.002% or less. Meanwhile, 0% is excluded in consideration of cases where it is inevitably included during the manufacturing process.

[0057] Nitrogen (N): 0.010% or less

[0058] Nitrogen (N) is an impurity element, and if its content exceeds 0.010%, the risk of cracking during performance, such as AlN formation, increases significantly. Therefore, the upper limit may be limited to 0.010%. According to one embodiment of the present invention, it may be 0.008% or less. According to one embodiment of the present invention, it may be 0.006% or less. Meanwhile, 0% is excluded in consideration of cases where it is unavoidably included during the manufacturing process.

[0059] Aluminum (Al): 0.010~0.100%

[0060] Aluminum (Al) can be added to remove oxygen in molten steel, and like Si, it is an element that is effective in stabilizing residual austenite by suppressing precipitation of cementite during reheating and overaging. If the content of aluminum (Al) is less than 0.010%, deoxidation of the steel is not sufficient, and the cleanliness of the steel may be impaired. On the other hand, if the content of aluminum (Al) exceeds 0.100%, not only the castability of the slab deteriorates, but also the temperature required for single-phase heating during annealing increases, which may cause production and facility problems. According to one embodiment of the present invention, it may be 0.050% or less.

[0061] Chromium (Cr): 0.20% or less

[0062] Chromium (Cr) can be added to improve the hardenability of steel. However, since alloying elements have a higher cost than other hardenable elements, it is advantageous to control the content as low as possible. Therefore, the lower limit is not specifically set, but 0% is excluded considering the level of impurities that inevitably accompany it. On the other hand, if the chromium (Cr) content exceeds 0.20%, the solid solution speed of cementite during annealing is delayed, and the undissolved cementite remains, thereby deteriorating the bendability. Therefore, according to one embodiment of the present invention, the chromium (Cr) content can be controlled to 0.20% or less. According to one embodiment of the present invention, it can be 0.10% or less.

[0063] Molybdenum (Mo): 0.10% or less

[0064] Molybdenum (Mo) has the effect of improving the hardenability of steel, creating fine carbides containing molybdenum (Mo) that serve as hydrogen trap sites, and improving delayed fracture resistance by refining martensite. However, when the molybdenum (Mo) content exceeds 0.10%, the effect is not significant compared to the increase in cost due to the addition of high-value alloying elements, so the upper limit can be limited to 0.10%. Since the cost is high compared to other hardenable elements, it is advantageous to manage it as low as possible, so the lower limit is not greatly restricted; however, 0% is excluded considering the level of impurities that inevitably accompany it.

[0065] Niobium (Nb): 0.010~0.100%

[0066] Niobium (Nb) is an element that contributes to increasing strength by segregating at austenite grain boundaries and suppressing the growth of austenite grains during annealing heat treatment through a precipitation strengthening effect. However, when the content of niobium (Nb) exceeds 0.100%, precipitation of carbon and nitrides increases, lowering the workability of the base material, and increasing the cost due to excessive alloy input. According to one embodiment of the present invention, it may be 0.080% or less. On the other hand, when the content is less than 0.010%, it may not contribute to increasing strength at all. According to one embodiment of the present invention, it may be 0.020% or more.

[0067] Titanium (Ti): 0.010~0.100%

[0068] Titanium (Ti) is a nitride-forming element that scavenges nitrogen in steel by precipitating it as TiN. If titanium (Ti) is not added, cracks may occur during continuous casting due to the formation of AlN. In addition, when the Ti content is less than 0.010%, it does not contribute to increasing strength at all, similar to the Nb element. Therefore, the lower limit of the Ti content in the present invention may be limited to 0.010%. According to one embodiment of the present invention, it may be 0.020% or more. However, when the titanium (Ti) content exceeds 0.100%, in addition to the removal of dissolved nitrogen, the strength of martensite may be reduced due to additional carbide precipitation, and hole expandability and bending workability may be deteriorated due to the formation of carbon and nitrides such as TiC and TiN. According to one embodiment of the present invention, it may be 0.080% or less.

[0069] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0070] A cold-rolled steel sheet according to one embodiment of the present invention may further include at least one of vanadium (V): 0.01 to 0.10%, and antimony (Sb): 0.10% or less.

[0071] Vanadium (V): 0.01–0.10%

[0072] Vanadium (V) is a carbonitride-forming element, but it also has a precipitation strengthening effect that increases the strength of steel, which is beneficial for increasing strength. However, when added excessively, not only does it increase the cost, but it is highly likely that brittle fracture will occur due to the generation of steel inclusions. If the content of vanadium (V) exceeds 0.10%, the hardenability may be weakened due to the removal of solid solution carbon, which may reduce the strength of martensite. According to one embodiment of the present invention, it may be included at 0.08% or less. On the other hand, when the content of vanadium (V) is less than 0.01%, it does not contribute to the increase in strength at all, similar to Nb, and therefore, in the present invention, the lower limit of the content may be limited to 0.01%. According to one embodiment of the present invention, it may be included at 0.02% or more.

[0073] Antimony (Sb): 0.10% or less

[0074] Antimony (Sb) can induce a uniform oxidation reaction in the surface layer of the steel plate and selectively suppress the formation of C or B oxides in the surface layer, thereby improving the bending properties. From the viewpoint of obtaining this effect, if the antimony (Sb) content exceeds 0.10%, castability deteriorates, and antimony (Sb) can segregate at the old austenite grain boundaries, thereby deteriorating the bending properties. Therefore, according to one embodiment of the present invention, the content can be limited to 0.10% or less. According to one embodiment of the present invention, it can be 0.06% or less. However, since a lower content is advantageous in terms of reducing alloy cost, the lower limit is not particularly limited as long as the required properties of the steel plate are not impaired.

[0075] Below, the steel microstructure of the present invention is described in detail.

[0076] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0077] According to one embodiment of the present invention, the microstructure of a region extending from the surface of the cold-rolled steel sheet to 30 μm in the direction of the center of the thickness includes at least one of martensite and tempered martensite as a main phase, and may include ferrite and bainite in an area % of 5.0% or less in total.

[0078] In the present invention, in order to strictly limit the bendability, the microstructure can be limited to a region of up to 30 μm from the surface of the steel plate toward the center of the thickness.

[0079] According to one embodiment of the present invention, in order to improve bendability and suppress hydrogen embrittlement, at least one of martensite and tempered martensite may be included as the main phase.

[0080] In addition, the more ferrite or bainite phases are mixed in the surface layer of the steel plate in addition to martensite or tempered martensite, the higher the residual stress between the phases, which may increase the possibility of crack occurrence. In particular, since hydrogen adsorbed at the interphase interface does not easily escape to the outside, which is disadvantageous for hydrogen embrittlement, according to one embodiment of the present invention, the main phase may be at least one of martensite and tempered martensite. According to one embodiment of the present invention, the main phase may be 90.0% or more.

[0081] Meanwhile, if the sum of the above ferrite and bainite fractions exceeds 5.0%, there is a concern that the bendability may deteriorate rapidly, and it is also disadvantageous from the perspective of hydrogen embrittlement, so the sum of the fractions may be limited to 5% or less. According to one embodiment of the present invention, it may be limited to 3% or less. In the present invention, a lower sum of the above ferrite and bainite fractions is advantageous, and thus the lower limit is not specifically limited.

[0082] According to one embodiment of the present invention, if necessary, the cold rolled steel sheet may further include a plating layer on at least one surface.

[0083] There are no specific limitations on the above-mentioned plating layer, and thus, not only the type thereof, such as zinc-based plating or aluminum-based plating, but also the method thereof, such as hot-dip plating or electroplating, is not limited. In other words, anything that can be used in the technical field to which the present invention pertains is sufficient. According to one embodiment of the present invention, the plating layer may be a zinc-based plating layer.

[0084] According to one embodiment of the present invention, a cold-rolled steel sheet may have a tensile strength of 1500 MPa or more, a bendability (R / t) of 3.5 or less, and a ratio a / b*100 of residual stress (a) at a point 1 / 2 in the direction of the thickness of the steel sheet and residual stress (b) in a region from the surface to the center of the thickness of 30 μm may be 95.0% or more.

[0085] The above bendability (R / t, where R is the bending die radius and t is the steel plate thickness) was performed using a jig with a tip inner angle of 90°. By changing the radius of curvature of the tip inner angle of the jig, the minimum jig tip inner angle at which no cracks were observed on the surface of the test piece was obtained, and the limit bending radius (R / t) was calculated by dividing the obtained radius (R) by the plate thickness (t).

[0086] When the residual stress ratio a / b*100 is less than 95.0%, the work hardening rate of the steel plate surface increases, resulting in poor bendability, and the hardening ability of the surface layer increases, increasing the risk of crack occurrence during part forming. According to one embodiment of the present invention, it may be 97.0% or more. According to one embodiment of the present invention, the ratio may be 100.0%. A ratio of 100.0% may mean that the work hardening of the surface layer is almost at the same level as that of the center.

[0087] Below, the steel manufacturing method of the present invention is described in detail.

[0088] A cold-rolled steel sheet according to one embodiment of the present invention can be manufactured by reheating, hot rolling, coiling, cold rolling, annealing, primary cooling, secondary cooling, and over-aging heat treatment of a steel slab satisfying the above-described alloy composition.

[0089] Reheating

[0090] A steel slab satisfying the alloy composition according to one embodiment of the present invention can be reheated in a temperature range of 1100 to 1300°C.

[0091] This process can be performed to facilitate the subsequent hot rolling process and sufficiently achieve the desired steel sheet properties. If the reheating temperature is below 1100°C, the hot rolling load may rapidly increase. Conversely, if the reheating temperature exceeds 1300°C, there is a risk of increased surface scale formation, which could reduce material yield.

[0092] hot rolling

[0093] The above reheated steel slab can be hot rolled at a temperature range of Ar3 or higher.

[0094] When the finishing rolling temperature is lower than Ar3 (the temperature at which ferrite begins to appear when austenite is cooled), a dual phase region of ferrite and austenite or a ferrite region may occur, resulting in the formation of a mixed grain structure, and there may be concerns about malfunction due to fluctuations in the hot rolling load. According to one embodiment of the present invention, the upper limit of the hot rolling temperature is not particularly limited, but may be limited to 1000°C or lower.

[0095] [ceremony]

[0096] Ar3 = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo]

[0097] (In the formula, [C], [Mn], [Cu], [Cr], [Ni] and [Mo] are the weight percent of each element.)

[0098] Winding

[0099] The above hot-rolled steel plate can be coiled at a temperature range of 400 to 600°C.

[0100] If the coiling temperature exceeds 600℃, an excessive oxide film is formed on the surface of the steel sheet, which may cause defects, and the surface properties of the plating material may be deteriorated. In order to form a single-phase structure rather than a composite structure as much as possible after hot rolling, and to secure material uniformity across the entire length and width, the coiling temperature may be limited to be maintained low. According to one embodiment of the present invention, the coiling temperature may be 500℃ or lower. However, as the temperature decreases, the strength of the hot-rolled steel sheet increases, which has the disadvantage of increasing the rolling load during cold rolling, which is a subsequent process, and there may be a factor that makes actual production impossible, so the lower limit may be limited to 400℃. According to one embodiment of the present invention, the coiling temperature may be 420℃ or higher.

[0101] According to one embodiment of the present invention, cooling can be performed by water cooling after coiling. Furthermore, according to one embodiment of the present invention, after coiling, a pickling process to remove an oxide layer formed on the surface of the steel sheet can be additionally performed, if necessary.

[0102] cold rolling

[0103] The above-mentioned rolled steel plate can be cold rolled at a reduction ratio of 30 to 80%.

[0104] When cold rolling, if the reduction ratio is less than 30%, it is not only difficult to secure the target thickness, but there is also a concern that the residual hot-rolled grains may affect austenite formation and final properties during annealing. On the other hand, when the reduction ratio exceeds 80%, there is a problem that the material deviation of the final steel sheet may occur due to the uneven rolling amount in the longitudinal and transverse directions from the work hardening that occurs during cold rolling. In addition, the rolling load may make it difficult to secure the target thickness.

[0105] Sodun

[0106] The above cold-rolled steel sheet can be annealed in a temperature range of Ac3+10℃ to Ac3+80℃.

[0107] In the present invention, the temperature of Ac3 varies depending on the component and can be determined by the following equation.

[0108] When the annealing temperature is lower than Ac3+10℃, a mixed grain structure may be formed due to two-phase annealing rather than single-phase annealing across the entire coil length, which may have a detrimental effect on the material. On the other hand, when the annealing temperature exceeds Ac3+80℃, equipment trouble may occur due to overload of the annealing furnace.

[0109] According to one embodiment of the present invention, annealing can be performed for 30 seconds or more in the above temperature range.

[0110] If the annealing time is less than 30 seconds, fine carbides formed during hot rolling may not be re-dissolved, resulting in poor bendability. Furthermore, during annealing, a single austenite phase may not fully transform back into a single phase, and a mixed phase may exist. This may prevent sufficient martensite formation during final cooling, potentially resulting in tensile strength not meeting the standard. Meanwhile, the present invention does not specifically limit the upper limit of the annealing time.

[0111] [ceremony]

[0112] Ac3 = 910-203√([C])-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]

[0113] (In the formula, [C], [Ni], [Si], [V], [Mo] and [W] are the weight percent of each element.)

[0114] Primary cooling

[0115] The above-mentioned annealed steel plate can be first cooled at an average cooling rate of 20°C / s or less to a temperature range of 680 to 750°C.

[0116] During the first cooling, if the cooling end temperature is less than 680°C, the bendability (R / t) evaluation value may exceed the suggested range, resulting in poor formability. According to one embodiment of the present invention, it may be 700°C or higher. On the other hand, if the temperature exceeds 750°C, not only may it not be reproducible due to the equipment configuration, but the structure may become coarsened, resulting in poor strength. According to one embodiment of the present invention, it may be 730°C or lower. According to one embodiment of the present invention, it may be 720°C or lower. According to one embodiment of the present invention, it may be 710°C or lower.

[0117] During primary cooling, if the average cooling rate exceeds 20°C / s, problems may arise with the shape of the plate. Meanwhile, the lower limit of the cooling rate may be any range within the equipment configuration, and thus may not be particularly limited. According to one embodiment of the present invention, during primary cooling, the lower limit of the average cooling rate may be 1°C / s.

[0118] Secondary cooling

[0119] The above-mentioned primary cooled steel plate can be secondarily cooled at an average cooling rate of 70 to 200°C / s to a temperature range of 100°C to Mf.

[0120] The above Mf refers to the martensite transformation finish temperature, and can be obtained using a method commonly known in the art, so it is not specifically defined in the present invention and can be measured using a dilatometer.

[0121] When the cooling end temperature exceeds the Mf temperature during the secondary cooling, martensite transformation may not be sufficiently achieved, making it difficult to secure the microstructure desired by the present invention. According to one embodiment of the present invention, the cooling end temperature during the secondary cooling may be 160°C or lower. According to one embodiment of the present invention, the cooling end temperature during the secondary cooling may be 150°C or lower. On the other hand, when the temperature is lower than 100°C, not only may it be disadvantageous in terms of shape due to the too low temperature, but it may also exceed the manufacturing process range of the equipment. According to one embodiment of the present invention, the cooling end temperature during the secondary cooling may be 120°C or higher.

[0122] According to one embodiment of the present invention, in order to secure the target level of strength, rapid cooling conditions may be applied during the secondary cooling. If the average cooling rate is less than 70°C / s during the secondary cooling, some bainite may be formed during cooling, making it difficult to secure the target strength. According to one embodiment of the present invention, the average cooling rate may be 80°C / s or more during the secondary cooling. On the other hand, if the average cooling rate exceeds 200°C / s during the secondary cooling, problems such as shape degradation of the steel sheet and material deviation in the width direction may occur due to the rapid martensite transformation rate. According to one embodiment of the present invention, the average cooling rate may be 180°C / s or less during the secondary cooling.

[0123] Over-aging heat treatment

[0124] The above secondarily cooled steel plate can be subjected to over-aging heat treatment at a temperature range of 150 to 240°C.

[0125] According to one embodiment of the present invention, the yield strength can be increased by transforming martensite obtained through the rapid cooling process by secondary cooling described above into tempered martensite through this process.

[0126] If the overaging heat treatment temperature is lower than 150℃, tempering is not sufficiently performed, so not only is the yield strength low, but sufficient toughness may not be secured. In addition, although the lower the temperature, the better the bendability, the lower limit may be limited to 150℃ in consideration of the equipment characteristics. According to one embodiment of the present invention, the overaging heat treatment may be performed at 160℃ or higher. On the other hand, if the temperature exceeds 240℃, the bending workability may deteriorate due to the precipitation and coarsening of a large amount of carbide. According to one embodiment of the present invention, it may be 200℃ or lower. According to one embodiment of the present invention, it may be 180℃ or lower.

[0127] According to one embodiment of the present invention, the over-aging heat treatment can be performed for 200 seconds or more in the above temperature range.

[0128] If the above heat treatment time is less than 200 seconds, tempering may not be sufficiently performed, which may cause a problem of lowering the yield strength. According to one embodiment of the present invention, it may be 300 seconds or more. According to one embodiment of the present invention, it may be 400 seconds or more. Meanwhile, according to one embodiment of the present invention, the upper limit of the overaging heat treatment time is not particularly limited, but since it is difficult to exceed 1000 seconds due to the characteristics of the continuous annealing equipment, the upper limit may be 1000 seconds. According to one embodiment of the present invention, it may be 900 seconds or less. According to one embodiment of the present invention, it may be 800 seconds or less.

[0129] Temper rolling or tension leveling

[0130] The above-mentioned heat-treated steel sheet can be subjected to temper rolling or tension leveling at a reduction ratio of 0.01 to 0.20%.

[0131] According to one embodiment of the present invention, the temper rolling or tension leveling process may be performed to improve the plate shape.

[0132] Meanwhile, if the reduction ratio exceeds 0.20%, the work hardening of the surface layer may increase, resulting in significantly poor bendability. That is, when processing a part, cracks may occur due to notches, and the bendability (R / t, where R is the bending die radius and t is the steel plate thickness) may deteriorate. According to one embodiment of the present invention, it may be 0.15% or less. On the other hand, if the reduction ratio is less than 0.01%, it may be advantageous for poor bendability, but may not be helpful at all in terms of improving the plate shape. According to one embodiment of the present invention, it may be 0.03% or more.

[0133] According to one embodiment of the present invention, a plating layer can be formed on the cold-rolled steel sheet as needed.

[0134] According to one embodiment of the present invention, a plating layer can be formed using a hot-dip plating method in which the manufactured cold-rolled steel sheet is dipped in a molten plating solution and a method of electroplating in an electrolyte solution. The plating conditions are not particularly limited as long as they are commonly known in the technical field to which the present invention pertains.

[0135] 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 explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0136] (Example)

[0137] Molten steel having the alloy composition described in Table 1 below was cast into an ingot, and then sizing-rolled to produce a steel slab. The steel slab was reheated to a temperature of 1200°C, maintained for 1 hour, and then final hot-rolled at 900°C. The slab was then placed in a heating furnace set to various conditions, maintained for 1 hour, and then cooled in the furnace to simulate hot-rolling. Thereafter, the hot-rolled steel sheet was pickled, and then cold-rolled at a cold reduction ratio of 50%. Then, annealing, primary cooling, secondary cooling, overaging heat treatment, and tension leveling were performed under the conditions described in Table 2 below to produce a cold-rolled steel sheet.

[0138] Steel alloy composition (weight %) CSiMnCrMoBPSNS.AlNbTiSbVA0.240.11.70.100.080.00250.0080.0020.0030.0420.0300.031--B0.240.11.90.100.050.00180.0060.0030.0040.0310.0250.032-0.02C0.220.21.80.150.060.00230.0070.0050.0020.0250.0310.028--D0.250.22.50.120.060.00090.0060.0 040.0050.0400.0410.0270.05-E0.280.32.60.110.050.00180.0080.0 060.0060.0200.0290.0260.04-F0.260.32.80.160.080.00180.0060.00 40.0060.0300.0290.025--G0.240.83.50.050.060.00030.0050.0050. 0040.040----H0.250.93.60.060.080.00040.0090.0030.0050.030----

[0139] Specimen number Steel grade Coiling Ac3 (℃) Annealing 1st cooling 2nd cooling and aging Heat treatment Tension leveling Temperature (℃) Temperature (℃) End temperature (℃) Speed ​​(℃ / s) End temperature (℃) Speed ​​(℃ / s) Temperature (℃) Time (sec) Reduction ratio (%) 1A420818852692121221021855200.10 2A42381886269581201031925150.11 3B6708178567001712178275 5150.084B45081779065016123562105200.075C550826858703121221522055300.126C580 826856698111311452045150.117D45081985969581281312035050.158D490819862670735 0551955200.359E47081886370091251281874900.1310E46581881070222126851924850. 0911F49082285970313122861785120.0812F48082284970111121881825160.0713G550848 86570610145453525400.3214G5608488647008146503265350.3315H700851842703615112 01825800.3516H71585183570171421151985600.3617H47085185669581051231785230.11

[0140] [Formula]Ac3 = 910-203√([C])-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]

[0141] (In the formula, [C], [Ni], [Si], [V], [Mo] and [W] are the weight percent of each element.)

[0142] Table 3 below shows the microstructure and physical properties observed from the manufactured steel plate.

[0143] First, the fractions of ferrite and bainite in the microstructure of the area from the surface of the steel sheet to the center of the thickness were measured and expressed. The cross-section parallel to the rolling direction was mechanically polished and nital-etched, and the ferrite and bainite in the area from the surface of the steel sheet to the center of the thickness of 30 μm were observed at four fields of view each (i.e., measurements were made at four points in the thickness direction for the area from the surface to 30 μm). That is, the area fraction of each structure was obtained by image analysis of the SEM image at a magnification of 2000 times, and the average area fraction was obtained by calculating the arithmetic mean value of the area fractions obtained in each of the four fields of view. In addition to ferrite and bainite, martensite or tempered martensite was observed.

[0144] In addition, the yield strength (YS), tensile strength (TS), and elongation (El) were measured and expressed by a tensile test. The tensile test was performed by cutting a JIS No. 5 tensile test piece at a point 1 / 4 in the width direction of the steel plate, such that the direction perpendicular to the rolling direction in the steel plate surface was the longitudinal direction, and performing a tensile test (JIS Z2241).

[0145] The bendability (R / t) value was measured through a bending test. The bending test was performed by cutting a strip-shaped test piece of 100 mm in a direction perpendicular to the rolling direction and 35 mm in the rolling direction at a point 1 / 4 of the steel sheet width direction, and using a jig with a tip inner angle of 90°. The radius of curvature of the tip inner angle of the jig was varied to obtain the minimum jig tip inner angle at which no cracks were observed on the surface of the test piece. The limit bending radius (R / t) was calculated by dividing the obtained radius (R) by the plate thickness (t). The smaller this value, the better the bendability. Cracks were judged using a stereoscopic microscope at a maximum magnification of 20 times, and the crack lengths were measured. Since it is difficult to distinguish microcracks less than 0.1 mm from surface irregularities using a stereoscopic microscope, cracks of 0.1 mm or more were judged as failures.

[0146] Residual stress was measured and analyzed using the principle of measuring the lattice plane spacing by X-ray diffraction using characteristic X-rays incident on the surface of a polycrystalline material and calculating it using Bragg's law. The residual stress at a point halfway through the thickness of the steel sheet was measured by grinding it in the thickness direction to the halfway point, and then heat-treating it at 200°C for 1 hour to completely remove the residual stress, and then measuring it to obtain the representative value.

[0147] To evaluate the occurrence of hydrogen embrittlement, a test piece measuring 10 mm in the direction perpendicular to the rolling and 35 mm in the rolling direction was prepared, V-bending was performed, and after fixing both sides using a jig, the test piece was immersed in 0.1 N hydrochloric acid (HCl) for 300 hours to determine whether cracks occurred.

[0148] Specimen number Steel grade Microstructure Property Classification F+B Fraction (area %) YS (MPa) TS (MPa) El (%) Bending strength (R / t) Residual stress ratio a / b*100 (%) Presence of hydrogen embrittlement 1A3.5121815328.23.196.5X Invention example 12A3.5121515268.13.195.6X Invention example 23B8.5122515267.84.585.0X Comparative example 14B9.2122015357.63.782.0 ○Comparative Example 25C2.8122515417.33.296.1XInvention Example 36C3.6122615397.23.296.2XInvention Example 47D3.7128515957.63.195.8XInvention Example 58D9.5128615967.54.588.0XComparative Example 39E3.5127 515856.83.396.3X Invention Example 610E4.8128115866.53.992.0○ Comparative Example 411F3.4127615936.83.295.8X Invention Example 712F2.9130116016.72.897.2X Invention Example 813G4.5131216025.94 .687.0XComparison Example 514G3.8130216125.84.588.0○Comparison Example 615H7.2128615965.94.978.0XComparison Example 716H7.6128315895.94.977.0○Comparison Example 817H2.8129516016.24.887.0○Comparison Example 9

[0149] * F: Ferrite, B: Bainite, Residual stress ratio a / b*100: Ratio a / b*100 of residual stress (a) at the 1 / 2 point in the direction of the steel sheet thickness and residual stress (b) in the area from the surface to the center of the thickness of 30 μm As shown in Table 3 above, in the case of the invention examples satisfying the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties targeted by the present invention could also be secured.

[0150] Figure 1 is a graph showing the bendability values ​​according to the sum of the ferrite and bainite fractions (F+B) in a region from the surface to the center of thickness, 30 μm. As shown in Figure 1, it was confirmed that the bendability was reduced when the ferrite and bainite fractions exceeded 5%.

[0151] On the other hand, Comparative Examples 1 to 9 are examples that do not satisfy the alloy composition or manufacturing conditions proposed in the present invention. As a result, the proposed microstructure was not secured in the area from the surface to the center of the thickness of the steel plate surface up to 30 μm, and all of the desired properties, such as bendability, residual stress, and hydrogen embrittlement resistance, were not secured.

[0152] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Contains, by weight%, carbon (C): 0.20 to 0.30%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 3.5%, boron (B): 0.0005 to 0.0030%, phosphorus (P): 0.010% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 0.20% or less, molybdenum (Mo): 0.10% or less, niobium (Nb): 0.010 to 0.100%, titanium (Ti): 0.010 to 0.100%, and the remainder includes iron (Fe) and other unavoidable impurities. Cold rolled steel sheet having a ratio a / b*100 of the residual stress (a) at the half point in the direction of the thickness of the steel sheet and the residual stress (b) in the area from the surface to the center of the thickness within 30 μm of the surface is 95.0% or more.

2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further containing at least one of vanadium (V): 0.01 to 0.10%, and antimony (Sb): 0.10% or less.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet in which the microstructure of a region extending from the surface to the center of the thickness within 30 μm includes at least one of martensite and tempered martensite as a main phase, and includes 5.0% or less of ferrite and bainite in total in area %.

4. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a tensile strength of 1500 MPa or more and a bendability (R / t) of 3.5 or less.

5. A step of reheating a steel slab containing, by weight%, carbon (C): 0.20 to 0.30%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 3.5%, boron (B): 0.0005 to 0.0030%, phosphorus (P): 0.010% or less, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, aluminum (Al): 0.010 to 0.100%, chromium (Cr): 0.20% or less, molybdenum (Mo): 0.10% or less, niobium (Nb): 0.010 to 0.100%, and titanium (Ti): 0.010 to 0.100%, with the remainder being iron (Fe) and other unavoidable impurities; A step of hot rolling the above reheated steel slab; A step of coiling the hot-rolled steel plate at a temperature range of 400 to 600°C; A step of cold rolling the above-mentioned rolled steel plate; A step of annealing the above cold-rolled steel sheet in a temperature range of Ac3+10℃ to Ac3+80℃; A step of first cooling the above-mentioned annealed steel plate to a temperature range of 680 to 750°C at an average cooling rate of 20°C / s or less; A step of secondarily cooling the above-mentioned first-cooled steel plate at an average cooling rate of 70 to 200°C / s to a temperature range of 100°C to Mf; A step of heat-treating the secondarily cooled steel plate in a temperature range of 150 to 240°C; and A method for manufacturing a cold rolled steel sheet, comprising: a step of subjecting the above-mentioned overheat-treated steel sheet to temper rolling or tension leveling at a reduction ratio of 0.01 to 0.20%.

6. In claim 5, A method for manufacturing a cold rolled steel sheet, wherein the above steel slab further contains at least one of vanadium (V): 0.01 to 0.1% and antimony (Sb): 0.1% or less.

7. In claim 5, The above reheating step is performed at a temperature range of 1100 to 1300°C. The above hot rolling step is performed at a temperature range of Ar3 or higher. A method for manufacturing cold rolled steel sheets, wherein the above cold rolling step is performed at a reduction ratio of 30 to 80%.

8. In claim 5, The above annealing step is performed for 30 seconds or more, A method for manufacturing a cold rolled steel sheet, wherein the above-mentioned heat treatment step is performed for 200 seconds or longer.

Images