Cold-rolled steel sheet for hot forming with excellent bendability, hot-formed member, and manufacturing methods therefor

Abstract

An aspect of the present invention aims to provide a steel material for hot forming, a hot-formed member using same, and a manufacturing method therefor, wherein the steel material is suitable for automotive parts requiring impact resistance and possesses a strength of as high as or higher than 1,800 MPa on the basis of tensile strength standard while securing high bendability.

Description

Technical Field

[0001] The present disclosure relates to a cold-rolled steel sheet for hot forming having excellent bendability, a hot-formed member, and a method for manufacturing the same.Background Art

[0002] A hot-formed ultra-high-strength member has recently been widely applied to a structural member of automobiles for the purpose of improving fuel efficiency and protecting passenger through the weight reduction of automobiles.

[0003] Patent Document 1 has proposed a representative technology for such hot forming. Patent Document 1 proposes a technology for securing ultra-high strength having a tensile strength exceeding 1600 MPa by heating a steel sheet to a temperature of 850°C or higher, and then forming a structure of the member into martensite through hot forming and rapid cooling using a press. In the case of the technology proposed in Patent Document 1, since the steel sheet is formed at high temperatures, even a complex shape thereof can be easily formed, and a weight reduction effect due to high strength may be expected through the increased strength due to rapid cooling within a mold.

[0004] Bendability is considered to be a representative indicator for evaluating collision resistance properties of an HPF formed member used for the purpose such as passenger protection, or the like. For example, in the case of an automobile B-pillar, when the HPF formed member is bent due to a side collision with the vehicle, the property (bendability) able to withstand bending of a certain distance (angle) without fracturing is required.

[0005] Accordingly, various studies have been conducted to improve collision resistance properties of an HPF steel material and member, such as improving impact energy absorption capacity by partially improving bendability by forming HPFs with different strength of steel types through a Tailor Welded Blank (TWB) as in Patent Document 2.

[0006] However, even in improving collision resistance properties through a TWB, limitations in improving the properties of parts requiring collision resistance properties, such as deterioration in a weld zone resulting in rather poor bendability, may be exhibited.(Prior art document)

[0007] (Patent document 1) US Reexamination Certificate No. 6296805 (Patent document 2) Korean Patent Publication No. 10-2021-0080239 Summary of InventionTechnical Problem

[0008] An aspect of the present disclosure is to provide a steel material for hot forming that can have high strength and impart excellent bendability to a member, a hot-formed member, and a method for manufacturing the same.

[0009] An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire contents of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.Solution to Problem

[0010] According to an aspect of the present disclosure, provided is a cold-rolled steel sheet for hot forming, the cold-rolled steel sheet for hot forming including, by weight%: 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities, wherein a value of a structure ratio represented by the following [Relational Expression 1] is 0.2 or more and 1.3 or less. = Structure ratio V p S + V θ S / V p C + V θ C wherein V p S and V θ S represent an area ratio of pearlite and cementite in a surface layer portion, respectively, and V p C and V θ C represent an area ratio of pearlite and cementite in a central portion, respectively.

[0011] The cold-rolled steel sheet for hot forming may further include at least one selected from the following a) to f). a) 0.001% to 0.4 wt% of total contents of Ti, Nb, Zr and V b) 0.0001 to 0.01 wt% of B c) 0.001 to 1.0 wt% of total contents of Mo and W d) 0.005 to 2.0 wt% of total contents of Cu and Ni e) 0.001 to 1.0 wt% of total contents of Sb and Sn f) REM: 0.0001 to 0.02 wt%

[0012] According to another aspect of the present disclosure, provided is a method for manufacturing a hot-formed member, the method including operations of: heating a slab, including by weight%, 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities, to a temperature within a range of 1000 to 1300°C; hot rolling the heated slab at a finishing rolling temperature of Ar3 to 1000°C to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet at a cooling rate of 400°C / s or more and 750°C / s or less; coiling the hot-rolled steel sheet at a temperature within a range of more than Ms to 750°C or lower; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and continuously annealing the cold-rolled steel sheet.

[0013] The steel slab further comprises at least one selected from the following a) to f): a) 0.001% to 0.4 wt% of total contents of Ti, Nb, Zr and V b) 0.0001 to 0.01 wt% of B c) 0.001 to 1.0 wt% of total contents of Mo and W d) 0.005 to 2.0 wt% of total contents of Cu and Ni e) 0.001 to 1.0 wt% of total contents of Sb and Sn f) 0.0001 to 0.02 wt% of REM

[0014] The cold rolling may be performed at a cumulative reduction ratio of 30 to 80%.

[0015] The continuous annealing may be performed at a temperature within a range of 700 to 900°C for 1 to 1000 seconds.

[0016] According to another aspect of the present disclosure, provided is a method for manufacturing a hot-formed member, the method including operations of: manufacturing a cold-rolled steel sheet according to the method for manufacturing the cold-rolled steel sheet for hot forming; heating the cold-rolled steel sheet to a temperature of 700°C or higher at a heating rate of 1 to 1000°C / sec; hot forming the heated cold-rolled steel sheet; and cooling the hot-formed steel sheet at a cooling rate of 10 to 1000°C / sec.

[0017] The cooling may be performed by setting a cooling stop temperature to a martensite transformation finish temperature (Mf) or lower. However, the cooling may be performed by setting the cooling stop temperature to a martensite transformation finish temperature (Mf) or higher and a martensite transformation start temperature (Ms) or lower, and in this case, after the cooling, maintaining the temperature at a cooling end temperature or reheating the same to an Ac1 or lower, may be further included.

[0018] According to another aspect of the present disclosure, provided is a hot-formed member, the hot-formed member including by weight%: 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities, wherein a value of a hardness ratio represented by the following [Relational Expression 2] is 0.1 or more and 10 or less. Hardness ratio = μ Hν S / μ Hν C wherein, μ Hν S represents a standard deviation of hardness in a surface layer portion, and μ Hν C represents a standard deviation of hardness in a central portion.

[0019] The hot-formed member may further include at least one selected from the following a) to f). a) 0.001% to 0.4 wt% of total contents of Ti, Nb, Zr and V, b) 0.0001 to 0.01 wt% of B, c) 0.001 to 1.0 wt% of total contents of Mo and W, d) 0.005 to 2.0 wt% of total contents of Cu and Ni, e) 0.001 to 1.0 wt% of total contents of Sb and Sn, f) 0.0001 to 0.02 wt% of REM.

[0020] The hot-formed member may have a tensile strength of 1800 MPa or more and a yield strength of 1200 MPa or more.

[0021] In addition, the solution to the above-mentioned problem does not enumerate all features of the present invention. The various features of the present disclosure and the advantages and effects thereof may be understood in more detail by referring to the specific embodiments below.Advantageous Effects of Invention

[0022] As set forth above, according to an aspect of the present disclosure, a steel material for hot forming having a high tensile strength of 1800MPa or more, while ensuring high bendability, a hot-formed member using the same, and a method for manufacturing the same, may be provided.

[0023] The various and beneficial advantages and effects of the present disclosure are not limited to the above-described contents, and may be more easily understood through descriptions of specific embodiments of the present disclosure.Best Mode for Invention

[0024] Hereinafter, preferred embodiments of the present disclosure will be described. However, the following embodiments of the present disclosure may be modified to have various other forms, and the scope of the present disclosure is not limited thereto. In addition, the embodiments of the present disclosure are provided to more completely explain the present disclosure to a person having average knowledge in the relevant technical field.

[0025] In addition, unless otherwise specifically provided in the specification of the present disclosure, a unit of content of each element is based on weight, and a unit of a ratio of structure is based on an area.

[0026] The inventors of the present disclosure have recognized that in the case of a unplated ultra-high strength cold-rolled steel sheet for hot forming, there is a problem that it is difficult to secure excellent bendability because the bendability is reduced after the hot forming process under normal hot rolling conditions, and have conducted in-depth research to solve the problem.

[0027] As a result, it was confirmed that if a cooling rate in a cooling section during the hot rolling process is controlled, an area ratio of pearlite / cementite between a surface layer portion and a central portion of the cold-rolled steel sheet after annealing may be adjusted, and thus a difference in hardness between martensites in the surface layer portion and in the central portion after hot forming may be reduced, thereby ensuring excellent bendability, thereby completing the present disclosure.

[0028] Hereinafter, a cold-rolled steel sheet for hot forming according to an aspect of the present disclosure will be described in detail.

[0029] According to an aspect of the present disclosure, a cold-rolled steel sheet for hot forming having excellent surface quality may include by weight %, 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities.

[0030] First, an alloy composition of a cold-rolled steel sheet for hot forming having excellent surface quality according to an aspect of the present disclosure will be described in detail.Carbon (C): 0.25 to 0.45%

[0031] Carbon (C) is essential element to increase the strength of a heat-treated member and should be added appropriately.

[0032] Since it is difficult to secure sufficient strength when a content of C is less than 0.25%, it is preferable that 0.25% or more of C is added. A more preferable lower limit of the content of C is 0.26%, and an even more preferable lower limit of the content of C is 0.27%. On the other hand, when the content of C exceeds 0.45%, the strength of a hot-rolled material becomes too high when the hot-rolled material is cold rolled, which greatly reduces the cold-rollability, and also greatly reduces the spot weldability, so it is preferable that the content of C is 0.45% or less. A more preferable upper limit of the content of C is 0.42%, and an even more preferable upper limit of the content of C is 0.40%.Silicon (Si): 0.01 to 3.0%

[0033] Silicon (Si) plays an important role in forming a Si-based amorphous oxide layer by being concentrated on the surface during annealing of a cold-rolled steel sheet in a continuous annealing line, and also plays a role in securing spot weldability of a member by suppressing the formation of (Fe, Mn, Cr) oxide layers during the hot forming process.

[0034] When a content of Si is less than 0.01%, the above-described effect is insufficient, so a lower limit of the content of Si is preferably 0.01%. A more preferable lower limit of the content of is 0.1%. On the other hand, when the content of Si exceeds 3.0%, there is a problem in that the spot weldability is rather reduced due to the formation of an Si-based amorphous oxide layer, which is too thick. A more preferable upper limit of the content of Si is 2.8%, and an even more preferable upper limit of the content of Si is 2.5%.Chromium (Cr): 0.1% or more and less than 5.0%

[0035] Chromium (Cr) not only improves hardenability of a steel sheet, but also plays a role in helping to form a stable surface Si-based amorphous oxide layer through an appropriate reaction with Si.

[0036] When a content of Cr is lower than 0.1%, the above-described effect is insufficient. A more preferable lower limit of the content of C is 0.15%, and an even more preferable lower limit of the content of is 0.2%. On the other hand, when the content of Cr is more than 5.0%, there is a problem that the effect is saturated and the manufacturing cost increases. A more preferable upper limit of the content of Cr is 4.5%, and an even more preferable upper limit of the content of Cr is 4.0%.Manganese (Mn): 0.01 to 4.0%

[0037] Manganese (Mn) may not only secure a solid solution strengthening effect, but also needs to be added to lower a critical cooling rate for securing martensite in a hot-formed member.

[0038] When a content of Mn is less than 0.01%, the above-described effect is insufficient. A more preferable lower limit of the content of Mn is 0.05%, and an even more preferable lower limit of the content of Mn is 0.1%. On the other hand, when the content of Mn exceeds 4.0%, since the strength of a steel sheet before the hot forming process increases too much, there are disadvantages in that not only may a blanking operation be difficult, but also the cost thereof may be increased due to the excessive addition of alloy iron and the spot weldability may be inferior. A more preferable upper limit of the content of Mn is 3.0%, and an even more preferable upper limit of the content of is 2.5%.Aluminum (Al): 0.001 to 0.4%

[0039] Aluminum (Al), along with Si, may act as a deoxidizer in steelmaking and improve the cleanliness of steel.

[0040] When a content of Al is less than 0.001%, the above-described effect is insufficient. A more preferable lower limit of the content of Al is 0.002%, and a more preferable lower limit of the content of Al is 0.003%. When the content of the content of Al exceeds 0.4%, there is a problem that an Ac3 temperature increases excessively and a heating temperature should be increased. A more preferable upper limit of the content of Al is 0.3%, and an even more preferable upper limit of the content of Al is 0.2%.Phosphorus (P): 0.001 to 0.05%

[0041] Phosphorus (P)is an impurity, and to control a content of P be less than 0.001%, a lot of manufacturing costs are required, and when the content of P exceeds 0.05%, the weldability of a hot-formed member may be significantly reduced. A more preferable upper limit of the content of P is 0.03%.Sulfur (S): 0.0001 to 0.02%

[0042] Sulfur (S) is an impurity, and to control a content of S be less than 0.0001%, a lot of manufacturing costs are required, and when the content of S exceeds 0.02%, the ductility, impact properties, and weldability of a member are deteriorated. A more preferable upper limit of the content of S is 0.01%.Nitrogen (N): 0.001 to 0.02%

[0043] Nitrogen (N) is an impurity, and to control a content of N be less than 0.001%, a lot of manufacturing costs are required, and when the content of N exceeds 0.02%, it will not only be susceptible to cracking during continuous casting of a slab, but also the impact characteristics may deteriorate. A more preferable upper limit of the content of N is 0.01%.

[0044] The remaining component of the present disclosure is iron (Fe). However, since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.

[0045] In addition to the composition of components described above, in the present disclosure, at least one selected from the following a) to f) may be further included. By arbitrarily adding these elements, characteristics such as surface quality and hot formability may be further improved.a) Total contents of Ti, Nb, Zr and V: 0.001% to 0.4%

[0046] Ti, Nb, Zr and V are effective in improving the strength of a heat-treated member by forming fine precipitates, stabilizing retained austenite and improving impact toughness by refining grains. When the content (meaning the total contents of two or more elements when two or more elements are added) is less than 0.001%, the above-described effect may be insufficient, and a more preferable lower limit of the contents thereof is 0.005%, and an even more preferable lower limit of the contents thereof is 0.008%. When the content of the contents thereof exceeds 0.4%, not only will the effect be saturated, but it may also cause an increase in costs due to excessive addition of alloy iron. A more preferable upper limit of the contents thereof is 0.38%, and an even more preferable upper limit of the contents thereof is 0.35%b) Boron (B): 0.0001 to 0.01%

[0047] Boron (B) is an element that can improve hardenability even with a small amount of addition, and can suppress brittleness of a hot-formed member due to grain boundary segregation of P and / or S by being segregated in grain boundaries of old austenite.

[0048] When the content of B is less than 0.0001%, the above-described effect is insufficient. A more preferable lower limit of the content of B is 0.00012%, and an even more preferable lower limit is 0.00015%. When the content of B exceeds 0.01%, not only will the effect be saturated, but it may also cause hot brittleness during hot rolling. A more preferable upper limit of the content of B is 0.005%.c) Total contents of Mo and W: 0.001 to 1.0 wt%

[0049] Mo and W may be added to improve hardenability, improve strength through precipitation strengthening effect, and refine grains. When the content (meaning the sum of Mo and W when both Mo and W are added) is less than 0.001%, the above-described effect is insufficient, and a more preferable lower limit of the contents of Mo and W is 0.0015%, and an even more preferable lower limit of the contents of Mo and W is 0.002%. When the contents of Mo and W exceeds 1.0%, not only is the effect saturated, but there is also the problem of increased costs. A more preferable upper limit of the contents of Mo and W is 0.95%, and an even more preferable upper limit the contents of Mo and W is 0.9%.d) Total contents of Cu and Ni: 0.005 to 2.0 wt%

[0050] Cu may be added as an element which improves strength by forming fine precipitates. In addition, hot brittleness can be caused when Cu is added alone, so Ni can be added as needed. However, if the sum of these components is less than 0.005%, the above-described effect may be insufficient, and a more preferable lower limit of the contents of Cu and Ni is 0.006%, and an even more preferable lower limit of the contents of Cu and Ni is 0.007%. If the sum of the contents of Cu and Ni exceeds 2.0%, a more preferable upper limit thereof is 1.95%, and an even more preferable upper limit thereof is 1.9%.e) Total contents of Sb and Sn: 0.001 to 1.0 wt%

[0051] Sb and Sn have the effect of suppressing the formation of oxides that may be formed at grain boundaries of a surface layer of a hot-rolled material of a steel material to which Si is added, and may suppress dent defects caused by the detachment of the grain boundaries of the surface layer when a cold-rolled material is annealed. To obtain such an effect, it is preferable that 0.001% or more of Sb and Sn are added. A more preferable lower limit of the contents of Sb and Sn is 0.002%, and an even more preferable lower limit of the contents of Sb and Sn is 0.03%.

[0052] On the other hand, when the content (meaning the total contents of Sb and Sn when both are added) exceeds 1.0%, not only may the cost increase excessively, but it may also be dissolved in the slab grain boundaries, causing coil edge cracks during hot rolling. A more preferable upper limit thereof is 0.95%, and an even more preferable upper limit thereof is 0.9%.f) REM: 0.0001 to 0.02%

[0053] An REM element may control the activity of Fe in steel to control a thickness of Fe scales formed in the surface layer during hot forming. To obtain such an effect, the addition of REM element of 0.0001% or more is required. A more preferable lower limit a content of an REM element is 0.00015%, and an even more preferable lower limit the content of the REM element of is 0.0002%. On the other hand, when the content of the REM element exceeds 0.02%, the controllability of Fe activity may be lost, and the surface quality may deteriorate. Therefore, it is preferable that the content of the REM element be controlled to 0.02% or less, and more preferable that the content of the REM element be controlled to 0.01% or less.

[0054] The cold-rolled steel sheet for hot forming according to an aspect of the present disclosure may not only satisfy the alloy composition described above, but also may have a value of a structure ratio represented by the following [Relational Expression 1] of 0.2 or more and 1.3 or less. Structure ratio = V p S + V θ S / V p C + V θ C wherein, V p S and V θ S represent an area ratio of pearlite and cementite in a surface layer portion, respectively, and V p C and V θ C represent an area ratio of pearlite and cementite in a central portion, respectively.

[0055] In the present disclosure, the surface layer portion may mean a region within 100 µm in a thickness direction from the surface, and the central portion in the present disclosure may mean a region of 1 / 2t±50 µm in the thickness direction from the surface, where t means a thickness of a steel material (mm).

[0056] When the structure ratio represented by [Relational Expression 1] exceeds 1.3, a deviation in hardness between the martensite formed on the surface layer after hot forming increases, so that stress due to bending may be concentrated in the stronger martensite, and the stress unevenness in the thickness direction may increase due to a difference in the deviation in hardness in the thickness direction, which may result in poor bendability. More preferably, the structure ratio may be 1.15 or less, and even more preferably, 0.95 or less. On the other hand, when the structure ratio is less than 0.2 as suggested by the present disclosure, strength after hot forming may not be secured, and the tensile strength may be 1800 MPa or less. A more preferable lower limit thereof is 0.25, and an even more preferable lower limit thereof is 0.3.

[0057] In addition, the microstructure of the cold-rolled steel sheet according to the present disclosure may include ferrite and cementite. The area ratio is not particularly limited, but, for example, the total contents of ferrite and cementite may be 5% or more, by area ratio.

[0058] When making a blank to manufacture a hot-formed member from a cold-rolled steel sheet, if the strength is excessive, wear of a mold can easily occur, so the microstructure should be secured. When this is not considered, bainite, martensite, or the like may be included, but this is not excluded.

[0059] Hereinafter, a member of the present disclosure is described in detail.

[0060] The member the present disclosure may satisfy a value of a hardness ratio represented by the following [Relational Expression 2] of 0.1 or more and 10 or less. Hardness ratio = μ Hν S / μ Hν C

[0061] Wherein, μ Hν S represents a standard deviation of hardness in a surface layer portion, and μ Hν C represents a standard deviation of hardness in a central portion.

[0062] When the standard deviation of the hardness in the surface layer portion increases and the hardness ratio represented by [Relational Expression 2] exceeds 10, the bendability may be deteriorated due to a stress concentration phenomenon and stress imbalance in the thickness direction caused by the deviation in hardness between martensites. On the other hand, when the hardness ratio is 10 or less, the deviation in hardness is good, so that the bendability may be improved. However, when the hardness ratio is less than 0.1, the deviation in hardness in the central portion in the thickness direction may be relatively severe, resulting in poor bendability.

[0063] A composition of a base steel sheet of a member according to the present disclosure is the same as the composition of the cold-rolled steel sheet described above, so it is not described separately.

[0064] Hereinafter, a microstructure of the member according to the present disclosure is described in detail.

[0065] A hot-formed member according to an aspect of the present disclosure may include martensite or bainite as a main phase to secure high strength. In the present disclosure, the main phase may mean a phase having the largest area ratio among several phases forming a microstructure. An area ratio thereof is not particularly limited, but more preferably, the area ratio may be 50% or more.

[0066] Hereinafter, a method for manufacturing a cold-rolled steel sheet for hot forming, which is another aspect of the present disclosure, is described in detail.

[0067] According to another aspect of the present disclosure, a method for manufacturing a cold-rolled steel sheet for hot forming, the method including operations of: heating a slab, including by weight%, 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities, to a temperature within a range of 1000 to 1300°C; hot rolling the heated slab at a finishing rolling temperature of Ar3 to 1000°C to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet at a cooling rate of 400°C / s or more and 750°C / s or less; coiling the hot-rolled steel sheet at a temperature within a range of more than Ms to 750°C or lower; cold rolling the coiled hot-rolled steel sheet at a cumulative reduction ratio of 30 to 80% to obtain a cold-rolled steel sheet; and continuously annealing the cold-rolled steel sheet at a temperature within a range of 700 to 900°C for 1 to 1000 seconds.Slab heating operation

[0068] A slab satisfying the alloy composition described above to a temperature within a range of 1000 to 1300°C.

[0069] When the heating temperature is lower than 1000°C, it is difficult to homogenize a structure of the slab, and when the heating temperature exceeds 1300°C, the formation of excessive oxides and increased manufacturing costs may occur.Hot rolling operation

[0070] The heated slab is hot rolled at a finishing rolling temperature of Ar3 to 1000°C to obtain a hot-rolled steel sheet.

[0071] When the finishing rolling temperature is lower than the Ar3 temperature, since rolling in two phase region is likely to occur, a mixed grain structure can be generated on the surface, and it is difficult to control the shape of the hot-rolled steel sheet. When the finishing rolling temperature exceeds 1000°C, the grains of the hot-rolled steel sheet are likely to be coarse.Hot-rolling cooling operation

[0072] The hot-rolled steel sheet having the hot rolling performed is cooled at a cooling rate of 400°C / s or more and 750°C / s or less.

[0073] When the hot-rolled steel sheet is cooled at a rate exceeding an upper limit of the cooling rate during hot rolling, an excessive area ratio of pearlite and cementite are formed in a surface layer after annealing, which increases a difference in hardness between martensite in the surface layer after hot forming, and thus excellent bendability may not be secured. When the hot-rolled steel sheet is cooled at a rate below a lower limit of the cooling rate, the formation of pearlite and cementite in the surface layer is insignificant, and sufficient strength may not be secured after hot forming of the cold-rolled steel sheet.Coiling operation

[0074] The hot-rolled steel sheet is coiled at a temperature within a range of more than Ms to 750°C or lower.

[0075] When the coiling temperature is a martensite transformation start temperature (Ms) or lower, the strength of the hot-rolled steel sheet becomes too high, which reduces cold-rollability. When the coiling temperature exceeds 750°C, it causes an increase in a thickness of an oxide layer and oxidation of grain boundaries on a surface layer, which may not only reduce the pickling property but also cause the problem of the grain boundaries on the surface layer being removed during annealing in a continuous annealing furnace.Cold rolling operation

[0076] The coiled hot-rolled steel sheet is cold rolled to obtain a cold-rolled steel sheet. This is to control the thickness of the steel sheet more precisely, and pickling may be performed before cold rolling.

[0077] In this case, a reduction ratio of the cold rolling does not need to be particularly limited, but cold rolling may be performed at a reduction ratio of 30 to 80% to secure a predetermined target thickness.Continuous annealing operation

[0078] The cold-rolled steel sheet is continuously annealed at a temperature within a range of 700 to 900°C.

[0079] When an annealing temperature is lower than 700°C, it is difficult for a rolled structure created by cold rolling to recover and recrystallize, and when the annealing temperature exceeds 900°C, it can deteriorate an annealing equipment, which can become a factor in increasing process costs due to frequent replacement of the equipment.

[0080] In addition, an annealing time may be 1 to 1000 seconds. When the annealing time is less than 1 second, it is difficult to obtain the annealing effect, and when the annealing time exceeds 1000 seconds, productivity may decrease.

[0081] Hereinafter, a method for manufacturing a hot-formed member, which is another aspect of the present disclosure, is described in detail.

[0082] According to another aspect of the present disclosure, a method for manufacturing a hot-formed member includes heating a cold-rolled steel sheet manufactured by the method for manufacturing a cold-rolled steel sheet according to the present disclosure described above to a temperature of 700°C or higher at a heating rate of 1 to 1000°C / sec; hot forming the heated cold-rolled steel sheet; and cooling the hot-formed steel sheet at a cooling rate of 10 to 1000°C / sec.Heating operation

[0083] The cold-rolled steel sheet manufactured by the method for manufacturing a cold-rolled steel sheet according to the present disclosure described above is heated to a temperature of 700°C or higher at a heating rate of 1 to 1000°C / sec.

[0084] When a heating temperature is lower than 700°C, recrystallization of ferrite is insufficient, which may cause a problem in that anisotropy of bending is increased after hot forming.

[0085] When a heating rate is less than 1°C / sec, it is difficult to secure sufficient productivity, and when the heating rate exceeds 1000°C / sec, expensive equipment may be required.Hot forming and cooling operation

[0086] After hot forming the heated cold-rolled steel sheet, the steel sheet is cooled at a cooling rate of 10 to 1000°C / sec.

[0087] When the cooling speed is less than 10°C / sec, unwanted ferrite and pearlite are formed, making it difficult to secure tensile strength. On the other hand, expensive special cooling equipment may be required to control the cooling rate to exceed 1000°C / sec.

[0088] In this case, a cooling stop temperature in the cooling operation may be lower than M f (martensite transformation end temperature). This is because when cooling is stopped at a temperature exceeding M f and then cooled to room temperature again, it may be difficult to secure the shape fixability of a hot-formed member.

[0089] However, in order to secure better elongation and impact properties in hot-formed member, cooling may be stopped between M f (martensite transformation finish temperature) and M s (martensite transformation start temperature), and then maintained at a cooling end temperature or reheated to Ac1 or lower to temper martensite and stabilize retained austenite.

[0090] In addition, the hot-formed member may have martensite or bainite as a main phase in order to secure high strength. Here, the main phase means a phase having the largest area ratio among various phases forming the microstructure. An area ratio thereof does not need to be specifically limited, but the area ratio may be, for example, 50% or more.

[0091] Meanwhile, the hot-formed member may have a tensile strength of 1800 MPa or more. By securing a high strength of 1800 MPa or more, it may be preferably applied to automobile structural members, reinforcing materials, or the like, requiring collision resistance properties.

[0092] Hereinafter, the present disclosure will be specifically described through the following Examples. However, it should be noted that the following Examples are only for describing the present disclosure in detail by illustration, and are not intended to limit the right scope of the present disclosure. The reason is that the right scope of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.Mode for Invention(Example)

[0093] A slab having a thickness of 40 mm having the composition illustrated in Table 1 below was vacuum melted, heated in a heating furnace at a temperature of 1200°C for 1 hour, and then hot rolled at a finishing rolling temperature of 930°C to manufacture a hot-rolled steel sheet having a final thickness of 3 mm. The hot-rolled steel sheet was cooled at the cooling rate illustrated in Table 2 below and then coiled at a temperature of 640°C. Subsequently, the hot-rolled steel sheet was pickled and then cold rolled at a cold reduction ratio of 50%. In addition, a cold-rolled steel sheet for hot forming was manufactured by performing continuous annealing at a temperature of 800°C for 80 seconds after the cold rolling.

[0094] Thereafter, the manufactured cold-rolled steel sheet was heated at a heating rate of 20°C / sec, heat treated at 900°C for 6 minutes, and the heated cold-rolled steel sheet was hot formed. Subsequently, the hot-formed steel sheet was cooled to room temperature at a cooling rate of 20°C / sec to manufacture a hot-formed member. [Table 1]Steel typeCSiMnCrMoTiBSbA0.4230.221.30.150.10.030.0025-B0.291.540.80.7-0.0150.00250.03C0.2541.490.7912.01-0.030.002-D0.2551.550.831.53-0.0310.0028-E0.310.60.90.20.0150.030.0025-

[0095] An area ratio of structures in a surface layer portion and a central portion of the manufactured cold-rolled steel sheet for hot forming, and a structure ratio of [Relational Expression 1] are shown in Table 2. To measure an area ratio of the structure by location in a thickness direction, a cross-section of the structure after nital etching at a magnification of 500x using an optical microscope (OM). The area ratio of the structures in the surface layer portion and central portion were measured three times, respectively, using CLEMEX Vision PE software after the measurement using an optical photograph, and average values thereof were shown in Table 2.

[0096] In addition, a hardness ratio, which is a ratio of the standard deviation in the hardness in the surface layer portion and central portion of the hot-formed member manufactured after hot forming, was shown in Table 2 based on [Relational Expression 2]. A tensile strength and maximum bending angle were shown. Hardness was measured at a minimum of 10 points at 1 mm intervals using a Vickers hardness tester (Dura Scan 80G5) with a load of 10 kgf, and a value of the tensile strength was measured through a room temperature tensile test using a JIS-5 specimen according to the ISO6892 standard. The maximum bending angle is described as a value of an outer bending angle converted from a maximum bending strength specified in the standard according to a bendability evaluation method according to the VDA238-100 standard. In addition, a change rate of the bending angle represents a deviation ratio of a bending angle between a specimen manufactured under the manufacturing conditions proposed in the present disclosure and a specimen manufactured outside the proposed manufacturing conditions. [Table 2]St ee 1 ty peReferen ceHot rolling cooling ratePearlite + CementiteYSTSHardn ess ratioMaximum bending angleBending angle change rate°C / sCentra 1Surfac eStruct ureMPaMPa○% portio nlayer portio nratioAInventi ve Example 1607.60.3220.3180.99138220105.359.7-ACompara tive Example 1880.70.2510.4151.651373201016.450.6-15ACompara tive Example 2312.20.3210.050.16118016706.665.3+9BInventi ve Example 2506.10.3440.3310.96137720096.456.7-BCompara tive Example 3861.00.2960.4341. 471377201110.750.8-10CInventi ve Example 3633.40.3510.4151. 18133819900.659.5-CCompara tive Example 4805.20.3180.4671. 471345199427.254.1-9DInventi ve Example 4685.20.3370.3250.96140320704.450.4-DCompara tive Example767.20.3180.4181.321396207924.443.9-13 5EInventi ve Example 5421.80.3630.3651. 01129419009.561.6-ECompara tive Example 6990.70.3060.4251.391366197212.853.3-13ECompara tive Example 7385.70.4600.080.17108017202.166.2+7

[0097] As shown in Table 2, in Comparative Examples 1 and 3 to 6, in which hot rolling was performed at a rate exceeding an upper limit of a cooling rate limited by the present disclosure, a structure ratio represented by [Relational Expression 1] exceeded 1.3 after annealing, so that a standard deviation of the hardness in a surface layer portion was increased more than a standard deviation of the hardness in a central layer portion after hot rolling, and thus a hardness ratio represented by [Relational Expression 2] exceeded 10, so that a bending angle was inferior.

[0098] In Comparative Examples 2 and 7, in which a cooling rate during hot rolling did not meet the range suggested by the present patent and thus did not reach a lower limit thereof, a maximum bending angle after hot rolling of the cold-rolled steel sheet was shown to be improved, but sufficient strength was not secured due to the excessive formation of soft martensite in the surface layer.

[0099] In Inventive Examples 1 to 5, a cooling rate after hot rolling was controlled within the range limited by the present disclosure, so that a structure ratio of the cold-rolled steel sheet was satisfied to have a range of 0.2 or more and 1.3 or less, and at the same time, a hardness ratio thereof was 10 or less, and the hot-formed member thus manufactured exhibited good bendability.

Claims

1. A cold-rolled steel sheet for hot forming, comprising by weight%: 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities, wherein a value of a structure ratio represented by the following [Relational Expression 1] is 0.2 or more and 1.3 or less, Structure ratio = V p S + V θ S / V p C + V θ C wherein V p S and V θ S represent an area ratio of pearlite and cementite in a surface layer portion, respectively, and V p C and V θ C represent an area ratio of pearlite and cementite in a central portion, respectively.

2. The cold-rolled steel sheet for hot forming of claim 1, further comprising: at least one selected from the following a) to f), a) 0.001 to 0.4 wt% of total contents of Ti, Nb, Zr and V b) 0.0001 to 0.01 wt% of B c) 0.001 to 1.0 wt% of total contents of Mo and W d) 0.005 to 2.0 wt% of total contents of Cu and Ni e) 0.001 to 1.0 wt% of total contents of Sb and Sn f) 0.0001 to 0.02 wt% of REM.

3. A method for manufacturing a cold-rolled steel sheet for hot forming, comprising operations of: heating a slab, including by weight%, 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities, to a temperature within a range of 1000 to 1300°C; hot rolling the heated slab at a finishing rolling temperature of Ar3 to 1000°C to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet at a cooling rate of 400°C / s or more and 750°C / s or less; coiling the hot-rolled steel sheet at a temperature within a range of more than Ms to 750°C or lower; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and continuously annealing the cold-rolled steel sheet.

4. The method for manufacturing a cold-rolled steel sheet for hot forming of claim 3, wherein the steel slab further comprises at least one selected from the following a) to f), a) 0.001% to 0.4 wt% of total contents of Ti, Nb, Zr and V b) 0.0001 to 0.01 wt% of B, c) 0.001 to 1.0 wt% of total contents of Mo and W d) 0.005 to 2.0 wt% of total contents of Cu and Ni e) 0.001 to 1.0 wt% of total contents of Sb and Sn f) 0.0001 to 0.02 wt% of REM.

5. The method for manufacturing a cold-rolled steel sheet for hot forming of claim 3 or 4, wherein the cold rolling is performed at a cumulative reduction ratio of 30 to 80%.

6. The method for manufacturing a cold-rolled steel sheet for hot forming of claim 3 or 4, wherein the continuous annealing is performed at a temperature within a range of 700 to 900°C for 1 to 1000 seconds.

7. A method for manufacturing a hot-formed member, comprising operations of: manufacturing a cold-rolled steel sheet according to claim 3 or 4; heating the cold-rolled steel sheet to a temperature of 700°C or higher at a heating rate of 1 to 1000°C / sec; hot forming the heated cold-rolled steel sheet; and cooling the hot-formed steel sheet at a cooling rate of 10 to 1000°C / sec.

8. The method for manufacturing a hot-formed member of claim 7, wherein the cooling is performed by setting a cooling stop temperature to a martensite transformation finish temperature (Mf) or lower.

9. The method for manufacturing a hot-formed member of claim 7, wherein the cooling is performed by setting the cooling stop temperature to a martensite transformation finish temperature (Mf) or higher and a martensite transformation start temperature (Ms) or lower, and after the cooling, maintaining the temperature at the cooling stop temperature or reheating the same to an Ac1 or lower, is further included.

10. A hot-formed member, comprising by weight%: 0.25 to 0.45% of C, 0.01 to 3.0% of Si, 0.01 to 4.0% of Mn, 0.001 to 0.4% of Al, 0.001 to 0.05% of P, 0.0001 to 0.02% of S, 0.1% or more and less than 5.0% of Cr, 0.001 to 0.02% of N, with a remainder of Fe and other unavoidable impurities, wherein a value of a hardness ratio represented by the following [Relational Expression 2] is 0.1 or more and 10 or less, Hardness ratio = μ Hν S / μ Hν C wherein, μ Hν S represents a standard deviation of hardness in a surface layer portion, and μ Hν C represents a standard deviation of hardness in a central portion.

11. The hot-formed member of claim 10, further comprising: at least one selected from the following a) to f), a) 0.001% to 0.4 wt% of total contents of Ti, Nb, Zr and V b) 0.0001 to 0.01 wt% of B, c) 0.001 to 1.0 wt% of total contents of Mo and W, d) 0.005 to 2.0 wt% of total contents of Cu and Ni e) 0.001 to 1.0 wt% of total contents of Sb and Sn f) 0.0001 to 0.02 wt% of REM12. The hot-formed member of claim 10 or 11, wherein a tensile strength is 1800 MPa or more and a yield strength is 1200 MPa or more.

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