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

Abstract

One embodiment of the present invention aims to provide a hot-formed steel material that has a high tensile strength of 1800 MPa or more, suitable for automotive parts that require crashworthiness, while also ensuring high bendability, a hot-formed part using the same, and a manufacturing method for the same.

Description

[Technical Field]

[0001] The present invention 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 technology]

[0002] Recently, hot-formed ultra-high strength members have been widely applied to structural members of automobiles for the purposes of improving fuel efficiency by reducing the weight of automobiles and protecting passengers.

[0003] A representative technique related to such hot forming is proposed in Patent Document 1. Patent Document 1 proposes a technique for ensuring ultra-high strength with a tensile strength exceeding 1600 MPa by heating a steel plate to 850°C or higher, hot forming it using a press, and then rapidly cooling it to form the structure of the part into martensite. In the case of the technique proposed in Patent Document 1, since forming is performed at high temperatures, complex shapes can be easily formed, and the increased strength due to rapid cooling in a mold can be expected to result in weight reduction due to the increased strength.

[0004] Bendability is a typical indicator for evaluating the crashworthiness of HPF formed materials used for passenger protection, etc. For example, in the case of an automobile B-pillar, when an HPF formed material bends in a side collision with the vehicle, it is required to have the property (bendability) to withstand a certain distance (angle) or more without fracture.

[0005] Therefore, various studies have been conducted to improve the crashworthiness of HPF steel materials and components, such as improving crash energy absorption capacity by improving partial bendability by HPFing steel types with different strengths through a TWB (Tailor Welded Blank) as in Patent Document 2.

[0006] However, even when crashworthiness was improved through TWB, the bendability actually decreased due to deterioration of the welded joints, showing limitations in improving the properties of parts that require crashworthiness. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Reexamination Certificate No. 6296805 [Patent Document 2] Korean Patent Publication No. 10-2021-0080239 Summary of the Invention [Problem to be solved by the invention]

[0008] An embodiment of the present invention provides a hot-forming steel material that can impart high strength to a part while also providing excellent bendability, a hot-formed part, and a method for manufacturing the same.

[0009] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the entire content of the specification of the present invention. [Means for solving the problem]

[0010] One embodiment of the present invention contains, in weight percent, C: 0.25 to 0.45%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.1% or more but less than 5.0%, N: 0.001 to 0.02%, with the remainder being Fe and other unavoidable impurities, The present invention provides a cold-rolled steel sheet for hot forming, in which the value of the texture ratio represented by the following [Relational Formula 1] is 0.2 or more and 1.3 or less.

[0011]

number

[0012] The cold-rolled steel sheet for hot forming may further include one or more selected from the following a) and f): a) the total content of Ti, Nb, Zr and V: 0.001% to 0.4% by weight; b) B: 0.0001~0.01% by weight, c) total content of Mo and W: 0.001 to 1.0 wt%; d) Total content of Cu and Ni: 0.005 to 2.0 wt%; e) Total content of Sb and Sn: 0.001 to 1.0 wt% f)REM:0.0001~0.02wt%

[0013] Another embodiment of the present invention provides a method for manufacturing a cold-rolled steel sheet for hot forming, comprising the steps of: heating a steel slab having the above-described alloy composition to 1000 to 1300°C; hot-rolling the heated slab to a finish rolling temperature of Ar3 to 1000°C to obtain a hot-rolled steel sheet; cooling the hot-rolled 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 range exceeding Ms and 750°C or less; cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and continuous annealing the cold-rolled steel sheet.

[0014] The steel slab may further include one or more selected from the following a) to f): a) the total content of Ti, Nb, Zr and V: 0.001% to 0.4% by weight; b) B: 0.0001~0.01% by weight, c) total content of Mo and W: 0.001 to 1.0 wt%; d) Total content of Cu and Ni: 0.005 to 2.0 wt%; e) Total content of Sb and Sn: 0.001 to 1.0 wt% f)REM:0.0001~0.02wt%

[0015] The cold rolling can be carried out at a cumulative reduction rate of 30 to 80%.

[0016] The continuous annealing can be carried out at a temperature range of 700 to 900° C. for 1 to 1000 seconds.

[0017] Yet another embodiment of the present invention provides a method for manufacturing a hot-formed member, including the steps of: manufacturing a cold-rolled steel sheet according to the method for manufacturing a 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.

[0018] The cooling can be performed by setting the cooling stop temperature to Mf (martensitic transformation finish temperature) or lower. However, the cooling can also be performed by setting the cooling stop temperature to Mf (martensitic transformation finish temperature) or higher and Ms (martensitic transformation start temperature) or lower. In this case, the cooling can further include a step of maintaining the temperature at the cooling finish temperature after cooling or reheating to Ac1 or lower.

[0019] Yet another embodiment of the present invention comprises, in weight percent, C: 0.25 to 0.45%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.1% or more but less than 5.0%, N: 0.001 to 0.02%, with the remainder being Fe and other unavoidable impurities, A hot-formed member is provided having a hardness ratio expressed by the following [Relational Formula 2] of 0.1 or more and 10 or less.

[0020]

number

[0021] The hot-formed member may further include one or more selected from the following a) and f): a) the total content of Ti, Nb, Zr and V: 0.001% to 0.4% by weight; b) B: 0.0001~0.01% by weight, c) total content of Mo and W: 0.001 to 1.0 wt%; d) Total content of Cu and Ni: 0.005 to 2.0 wt%; e) Total content of Sb and Sn: 0.001 to 1.0 wt% f)REM:0.0001~0.02wt%

[0022] The hot-formed part may have a tensile strength of 1800 MPa or more and a yield strength of 1200 MPa or more.

[0023] The above-described solutions to the problems do not encompass all of the features of the present invention. The various features of the present invention and their associated advantages and effects can be more fully understood with reference to the following specific embodiments. [Effects of the Invention]

[0024] According to one embodiment of the present invention, it is possible to provide a hot-forming steel material that has a high strength of 1800 MPa or more in tensile strength standard while ensuring high bendability, a hot-formed part using the same, and a method for manufacturing the same.

[0025] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into several other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0027] Unless otherwise specified in the specification of the present invention, the unit of content of each element is based on weight, and the unit of ratio of the structure is based on area.

[0028] The present inventors recognized that in the case of ungalvanized ultra-high strength cold rolled steel sheets for hot forming, there is a problem in that bendability after a hot forming process is low under normal hot rolling conditions, making it difficult to ensure excellent bendability, and conducted in-depth research to solve this problem.

[0029] As a result, it was confirmed that by controlling the cooling rate in the cooling section during the hot rolling process, it is possible to adjust the area ratio of pearlite / cementite in the surface layer and center of the cold-rolled steel sheet after annealing, and as a result, it is possible to reduce the hardness deviation between the martensite in the surface layer and center after hot forming, and ensure excellent bendability, which led to the completion of the present invention.

[0030] Hereinafter, a cold-rolled steel sheet for hot forming according to one embodiment of the present invention will be described in detail.

[0031] A cold-rolled steel sheet for hot forming with excellent surface quality according to one embodiment of the present invention may contain, in weight percent, C: 0.25 to 0.45%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.1% or more but less than 5.0%, N: 0.001 to 0.02%, with the remainder being Fe and other unavoidable impurities.

[0032] First, the alloy composition of the cold-rolled steel sheet for hot forming having excellent surface quality according to one embodiment of the present invention will be described in detail.

[0033] C: 0.25 to 0.45% C is an essential element that must be added in an appropriate amount to increase the strength of the heat-treated member.

[0034] If the C content is less than 0.25%, it is difficult to ensure sufficient strength, so it is preferable to add 0.25% or more. A more preferable lower limit is 0.26%, and an even more preferable lower limit is 0.27%. On the other hand, if the C content exceeds 0.45%, the strength of the hot-rolled material becomes too high when cold-rolling the hot-rolled material, which not only significantly deteriorates the cold-rollability but also significantly reduces the spot weldability, so it is preferable that the C content be 0.45% or less. A more preferable upper limit is 0.42%, and an even more preferable upper limit is 0.40%.

[0035] Si: 0.01 to 3.0% Si not only plays an important role in forming an Si-based amorphous oxide layer by concentrating on the surface of cold-rolled steel sheets during annealing in a continuous annealing line, but also in suppressing the formation of (Fe, Mn, Cr) oxide layers during the hot forming process, ensuring the spot weldability of the parts.

[0036] If the Si content is less than 0.01%, the above-mentioned effects are insufficient, so the lower limit is preferably 0.01%. A more preferable lower limit is 0.1%. On the other hand, if the Si content exceeds 3.0%, a problem occurs in that an excessively thick Si-based amorphous oxide layer is formed, which actually deteriorates spot weldability. A more preferable upper limit is 2.8%, and an even more preferable upper limit is 2.5%.

[0037] Cr: 0.1% or more and less than 5.0% Cr not only improves the hardenability of the steel sheet, but also plays a role in stably assisting in the formation of a surface Si-based amorphous oxide layer through an appropriate reaction with Si.

[0038] If the Cr content is less than 0.1%, the above-mentioned effects are insufficient. A more preferable lower limit is 0.15%, and an even more preferable lower limit is 0.2%. On the other hand, if the Cr content is 5.0% or more, the effects saturate, and there is a problem of increased manufacturing costs. A more preferable upper limit is 4.5%, and an even more preferable upper limit is 4.0%.

[0039] Mn: 0.01 to 4.0% Mn needs to be added not only to ensure the solid solution strengthening effect but also to lower the critical cooling rate for securing martensite in hot-formed parts.

[0040] If the Mn content is less than 0.01%, the above-mentioned effects are insufficient. A more preferable lower limit is 0.05%, and an even more preferable lower limit is 0.1%. On the other hand, if the Mn content exceeds 4.0%, the strength of the steel sheet before the hot forming process is too high, making the blanking operation difficult, and there are drawbacks such as an increase in cost due to the addition of excessive alloy ferroalloys and deterioration of spot weldability. A more preferable upper limit is 3.0%, and an even more preferable upper limit is 2.5%.

[0041] Al: 0.001 to 0.4% Al, together with Si, acts as a deoxidizer during steelmaking and can increase the cleanliness of steel.

[0042] If the Al content is less than 0.001%, the above-mentioned effects are insufficient. A more preferable lower limit is 0.002%, and an even more preferable lower limit is 0.003%. If the Al content exceeds 0.4%, the Ac3 temperature rises excessively, necessitating an increase in the heating temperature. A more preferable upper limit is 0.3%, and an even more preferable upper limit is 0.2%.

[0043] P: 0.001 to 0.05% P is an impurity, and controlling its content to less than 0.001% requires significant production costs. If its content exceeds 0.05%, the weldability of hot-formed parts may be significantly reduced. A more preferable upper limit is 0.03%.

[0044] S: 0.0001 to 0.02% S is an impurity, and controlling its content to less than 0.0001% requires significant production costs. If its content exceeds 0.02%, it impairs the ductility, impact properties, and weldability of the component. A more preferable upper limit is 0.01%.

[0045] N: 0.001 to 0.02% N is an impurity, and controlling its content to less than 0.001% requires significant production costs. If its content exceeds 0.02%, not only will the slab be susceptible to cracking during continuous casting, but impact properties may also deteriorate. A more preferable upper limit is 0.01%.

[0046] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, the contents of all of them will not be specifically mentioned in this specification.

[0047] In addition to the above-mentioned component composition, the present invention may further contain one or more elements selected from the following a) to f). By adding these elements arbitrarily, properties such as surface quality and hot formability can be further improved.

[0048] a) Total content of Ti, Nb, Zr and V: 0.001% to 0.4% Ti, Nb, Zr, and V are effective in improving the strength of heat-treated components by forming fine precipitates, and in stabilizing retained austenite and improving impact toughness by refining crystal grains. If their contents (meaning the total if two or more elements are added) are less than 0.001%, the above-mentioned effects may be insufficient. A more preferred lower limit is 0.005%, and an even more preferred lower limit is 0.008%. If their contents exceed 0.4%, not only will the effects saturate, but the addition of excessive ferroalloys may also result in increased costs. A more preferred upper limit is 0.38%, and an even more preferred upper limit is 0.35%.

[0049] b) B: 0.0001 to 0.01% B is an element that can improve hardening ability even with a small amount of addition, and can also segregate to the prior austenite grain boundaries to suppress embrittlement of hot-formed parts due to grain boundary segregation of P and / or S.

[0050] If the B content is less than 0.0001%, the above-mentioned effects are insufficient. A more preferable lower limit is 0.00012%, and an even more preferable lower limit is 0.00015%. If the B content exceeds 0.01%, not only will the effects saturate, but hot embrittlement may occur during hot rolling. A more preferable upper limit is 0.005%.

[0051] c) Total content of Mo and W: 0.001 to 1.0 wt% Mo and W can be added to improve hardenability, improve strength through precipitation strengthening, and refine grains. If their content (meaning the total content when both Mo and W are added) is less than 0.001%, the above-mentioned effects are insufficient, so the lower limit is more preferably 0.0015%, and even more preferably 0.002%. If their content exceeds 1.0%, not only will the effects saturate, but costs will also increase. The upper limit is more preferably 0.95%, and even more preferably 0.9%.

[0052] d) Total content of Cu and Ni: 0.005 to 2.0 wt% Cu can be added as an element to form fine precipitates and improve strength. Ni is added as needed because adding Cu alone can cause hot embrittlement. However, if the total content of these elements is less than 0.005%, the above-mentioned effects are insufficient. A more preferred lower limit is 0.006%, and an even more preferred lower limit is 0.007%. Exceeding 2.0% can result in excessive cost increases. A more preferred upper limit is 1.95%, and an even more preferred upper limit is 1.9%.

[0053] e) Total content of Sb and Sn: 0.001 to 1.0 wt% The Sb and Sn elements have the effect of suppressing the formation of oxides that can form at the surface grain boundaries of hot-rolled steel containing Si, and can suppress dent defects caused by the detachment of the surface grain boundaries during annealing of the cold-rolled steel. To achieve this effect, it is preferable to add 0.001% or more. The more preferable lower limit is 0.002%, and the even more preferable lower limit is 0.03%.

[0054] On the other hand, if its content (when both Sb and Sn are added, this means the total of the two) exceeds 1.0%, not only may costs increase excessively, but it may also dissolve in the slab grain boundaries and induce coil edge cracks during hot rolling. A more preferred upper limit is 0.95%, and an even more preferred upper limit is 0.9%.

[0055] f) REM: 0.0001~0.02% REM elements can control the activity of Fe in steel and thereby control the thickness of the surface Fe scale formed during hot forming. To achieve this effect, the addition of 0.0001% or more of REM elements is necessary. A more preferable lower limit is 0.00015%, and an even more preferable lower limit is 0.0002%. On the other hand, if the REM content exceeds 0.02%, the ability to control Fe activity is lost, and surface quality may deteriorate. Therefore, it is preferable to control the REM content to 0.02% or less, and more preferably to 0.01% or less.

[0056] A cold-rolled steel sheet for hot forming according to one embodiment of the present invention may not only satisfy the above-mentioned alloy composition, but also have a structure ratio represented by the following [Relational Formula 1] of 0.2 or more and 1.3 or less.

[0057]

number

[0058] In the present invention, the surface layer portion can refer to a region within 100 μm from the surface in the thickness direction, and the center portion can refer to a region 1 / 2t±50 μm from the surface in the thickness direction (where t means the thickness of the steel material (mm)).

[0059] If the texture ratio expressed by [Relationship 1] exceeds 1.3, the hardness deviation between martensites formed in the surface layer after hot forming becomes large, and bending stress may concentrate on the stronger martensite. The difference in hardness deviation across the thickness increases stress non-uniformity across the thickness, resulting in poor bendability. More preferably, the texture ratio may be 1.15 or less, and even more preferably, 0.95 or less. On the other hand, if the texture ratio is less than 0.2 as proposed in this patent, the strength after hot forming cannot be ensured, and the tensile strength may be 1800 MPa or less. A more preferred lower limit is 0.25, and an even more preferred lower limit is 0.3.

[0060] The microstructure of the cold-rolled steel sheet according to the present invention may contain ferrite and cementite. There is no particular need to limit the area ratio, but for example, the total area ratio of ferrite and cementite may be 5% or more.

[0061] When cold-rolled steel sheets are made into hot-formed parts, excessive strength can lead to die wear, so the above microstructure must be maintained. If this is not taken into consideration, bainite, martensite, etc. can be included, but this is not excluded.

[0062] The components of the present invention will be described in detail below.

[0063] The member of the present invention can satisfy the condition that the value of the hardness ratio represented by the following [Relational Formula 2] is 0.1 or more and 10 or less.

[0064]

number

[0065] When the standard deviation of the surface hardness becomes large and the hardness ratio expressed by [Relationship 2] exceeds 10, the bendability may deteriorate due to stress concentration caused by the hardness deviation between martensites and stress imbalance in the thickness direction, while when the hardness ratio is 10 or less, the hardness deviation is good and the bendability can be improved. However, when the hardness ratio is less than 0.1, the hardness deviation in the center part in the thickness direction becomes relatively large, which may deteriorate the bendability.

[0066] The composition of the base steel sheet of the member according to the present invention is the same as the composition of the cold-rolled steel sheet described above, and therefore will not be described separately.

[0067] The microstructure of the component of the present invention will now be described in detail.

[0068] A hot-formed part according to an embodiment of the present invention may include martensite or bainite as a primary phase to ensure high strength. The primary phase in the present invention may refer to the phase having the largest area ratio among various phases constituting the microstructure. While the area ratio is not particularly limited, it may preferably be 50% or more.

[0069] Hereinafter, a method for producing a cold-rolled steel sheet for hot forming, which is another embodiment of the present invention, will be described in detail.

[0070] A method for producing a cold-rolled steel sheet for hot forming, which is another embodiment of the present invention, includes the steps of heating a slab having the above-described alloy composition to 1000 to 1300°C; hot-rolling the heated slab to a finish rolling temperature of Ar3 to 1000°C to obtain a hot-rolled steel sheet; cooling the hot-rolled hot-rolled steel sheet at a cooling rate of 400°C / s to 750°C / s; coiling the hot-rolled steel sheet at a temperature range exceeding Ms and 750°C or less; cold-rolling the coiled hot-rolled steel sheet at a cumulative reduction of 30 to 80% to obtain a cold-rolled steel sheet; and continuous annealing the cold-rolled steel sheet at a temperature range of 700 to 900°C for 1 to 1000 seconds.

[0071] Slab heating stage A slab having the above-mentioned alloy composition is heated to 1000 to 1300°C.

[0072] If the heating temperature is less than 1000°C, it is difficult to homogenize the slab structure, and if it exceeds 1300°C, excessive oxide formation and increased manufacturing costs may occur.

[0073] Hot rolling stage The heated slab is hot rolled at a finish rolling temperature of Ar3 to 1000°C to obtain a hot rolled steel sheet.

[0074] If the finish rolling temperature is below the Ar3 temperature, rolling tends to occur in the two-phase region, resulting in the formation of a duplex structure in the surface layer, making it difficult to control the shape of the hot-rolled steel sheet.If the finish rolling temperature exceeds 1000°C, the crystal grains of the hot-rolled steel sheet tend to become coarse.

[0075] Cooling stage of hot rolling The hot-rolled steel sheet is cooled at a cooling rate of 400°C / s or more and 750°C / s or less.

[0076] If the cooling rate during hot rolling exceeds the upper limit, an excessive area ratio of pearlite and cementite is formed in the surface layer after annealing, resulting in a large hardness deviation between the surface martensite after hot forming, making it impossible to ensure good bendability.If the cooling rate is below the lower limit, only a small amount of pearlite and cementite is formed in the surface layer, making it impossible to ensure sufficient strength after hot forming of the cold steel sheet.

[0077] Winding stage The hot-rolled steel sheet is coiled at a temperature in the range of Ms or higher and 750°C or lower.

[0078] If the coiling temperature is below Ms (martensitic transformation start temperature), the strength of the hot-rolled steel sheet will be too high, reducing its cold rolling properties. If the coiling temperature exceeds 750°C, an increase in the thickness of the oxide layer and surface grain boundary oxidation will occur, resulting in poor pickling properties and possibly the detachment of surface grain boundaries during annealing in a continuous annealing furnace.

[0079] Cold rolling stage 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 can be performed before cold rolling.

[0080] At this time, the reduction rate of the cold rolling does not need to be particularly limited, but it can be carried out at a reduction rate of 30 to 80% in order to ensure a predetermined target thickness.

[0081] Continuous annealing stage The continuous annealing of the cold rolled steel sheet is carried out in a temperature range of 700 to 900°C.

[0082] If the annealing temperature is less than 700°C, the rolled structure formed by cold rolling is unlikely to recover and recrystallize, and if it exceeds 900°C, the annealing equipment may be deteriorated, which may result in frequent equipment replacement and increase process costs.

[0083] The annealing time may be 1 to 1000 seconds. If the annealing time is less than 1 second, it is difficult to obtain the annealing effect, and if the annealing time exceeds 1000 seconds, productivity may decrease.

[0084] A method for producing a hot-formed member according to yet another embodiment of the present invention will now be described in detail.

[0085] A method for manufacturing a hot-formed member according to yet another embodiment of the present invention includes the steps of heating a cold-rolled steel sheet manufactured by the method for manufacturing a cold-rolled steel sheet according to the present invention 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.

[0086] Heating stage The cold-rolled steel sheet manufactured by the method for manufacturing a cold-rolled steel sheet according to the present invention described above is heated to a temperature of 700° C. or higher at a heating rate of 1 to 1000° C. / sec.

[0087] If the heating temperature is less than 700°C, the recrystallization of ferrite may be insufficient, which may result in a problem of increased anisotropy in bending after hot forming.

[0088] If the temperature rise rate is less than 1°C / sec, it is difficult to ensure sufficient productivity, and if the temperature rise rate exceeds 1000°C / sec, excessively expensive equipment is required.

[0089] Hot forming and cooling stages The heated cold-rolled steel sheet is hot-formed and then cooled at a cooling rate of 10 to 1000°C / sec.

[0090] If the cooling rate is less than 10°C / sec, undesirable ferrite and pearlite are formed, making it difficult to ensure tensile strength. On the other hand, in order to control the cooling rate to more than 1000°C / sec, expensive special cooling equipment is required.

[0091] At this time, the cooling stop temperature of the cooling step is M f (martensitic transformation finish temperature) or lower. f This is because if cooling is stopped due to excess and then cooled again to room temperature, it may be difficult to ensure shape fixability of the hot-formed part.

[0092] However, to ensure better elongation and impact properties in hot-formed parts, M f(Martensitic transformation finish temperature) and M s After stopping cooling between the temperature at which the martensite transformation begins and the temperature at which the cooling is completed, the material can be maintained at the cooling end temperature or reheated to below Ac1 to temper the martensite and stabilize the retained austenite.

[0093] In addition, the hot-formed member may have martensite or bainite as the main phase to ensure high strength. Here, the main phase refers to the phase that has the largest area ratio among the various phases that make up the microstructure. There is no particular need to limit the area ratio, but it may be, for example, 50% or more.

[0094] On the other hand, the hot-formed part can have a tensile strength of 1800 MPa or more. By ensuring a high strength of 1800 MPa or more, it can be preferably used as an automobile structural member or reinforcing material that requires crash resistance.

[0095] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. [Example]

[0096] (Example) A 40 mm thick slab having the chemical composition shown in Table 1 below was vacuum melted, heated in a heating furnace at 1200°C for 1 hour, and then hot rolled at a finish rolling temperature of 930°C to produce a hot-rolled steel sheet with a final thickness of 3 mm. The hot-rolled steel sheet was cooled at the cooling rate shown in Table 2 below and coiled at 640°C. The hot-rolled steel sheet was then pickled and cold-rolled at a cold reduction of 50%. After the cold rolling, continuous annealing was performed at 800°C for 80 seconds to produce a cold-rolled steel sheet for hot forming.

[0097] The cold-rolled steel sheet was then heated at a heating rate of 20°C / s and heat-treated at 900°C for 6 minutes, and the heated cold-rolled steel sheet was hot-formed. The hot-formed steel sheet was then cooled to room temperature at a cooling rate of 20°C / s to produce a hot-formed part.

[0098] [Table 1]

[0099] The area ratios of the surface and center microstructures of the manufactured cold-rolled steel sheets for hot forming and the microstructure ratios of [Relationship 1] are shown in Table 2. To measure the area ratio of the microstructure at each position in the thickness direction, an optical microscope (OM) was used to observe the cross section of the microstructure after nital etching at 500x magnification. After optical photograph measurement, the area ratios of the surface and center microstructures were measured three times each using CLEMEX Vision PE software, and the average values ​​are shown in Table 2.

[0100] The hardness ratio, which is the ratio of the standard deviation of the surface and center hardness of the hot-formed parts manufactured after hot forming, is also shown in Table 2 based on [Relationship 2]. Tensile strength and maximum bending angle are also shown. Hardness was measured using a Vickers hardness tester (DuraScan 80G5) at a minimum of 10 points at 1 mm intervals under a 10 kgf load. Tensile strength was measured using a JIS No. 5 test piece in a room-temperature tensile test in accordance with ISO 6892. The maximum bending angle was reported as the bending angle converted from the maximum bending strength specified in the VDA 238-100 standard, in accordance with the bendability evaluation method. The bending angle variation rate indicates the bending angle deviation ratio between the bending angle of a test piece manufactured under the manufacturing conditions proposed in this invention and a test piece manufactured outside the proposed manufacturing conditions.

[0101] [Table 2]

[0102] As shown in Table 2, in the case of Comparative Examples 1 and 3 to 6, which were hot-rolled at a cooling rate exceeding the upper limit of the present invention, the texture ratio expressed by [Relationship 1] exceeded 1.3 after annealing, and the standard deviation of the surface hardness after hot forming became larger than the standard deviation of the center hardness. As a result, the hardness ratio expressed by [Relationship 2] exceeded 10, and the bending angle deteriorated.

[0103] In the case of Comparative Examples 2 and 7, in which the cooling rate during hot rolling did not satisfy the range proposed by the present patent and did not satisfy the lower limit, the maximum bending angle after hot forming of the cold-rolled steel sheet was improved, but sufficient strength could not be ensured due to excessive formation of soft martensite in the surface layer.

[0104] In the case of Examples 1 to 5 of the present invention, the cooling rate after hot rolling was controlled within the range limited by the present invention, so that the texture ratio of the cold-rolled steel sheet was in the range of 0.2 to 1.3, and at the same time, the hardness ratio was 10 or less, and the hot-formed parts thus manufactured exhibited good bendability.

Claims

1. In weight percent, C: 0.25 to 0.45%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.1% or more but less than 5.0%, N: 0.001 to 0.02%, with the remainder being Fe and other unavoidable impurities, A cold-rolled steel sheet for hot forming, wherein the value of the texture ratio represented by the following [Relational Formula 1] is 0.2 or more and 1.3 or less. [Equation 1]

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

3. a step of heating a slab containing, in weight percent, C: 0.25 to 0.45%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.1% or more but less than 5.0%, N: 0.001 to 0.02%, with the remainder being Fe and other unavoidable impurities, to 1000 to 1300°C; hot rolling the heated slab at a finish 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 to 750°C / s; coiling the hot-rolled steel sheet at a temperature in the range of more than Ms and not more than 750°C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and A method for manufacturing a cold-rolled steel sheet for hot forming, comprising the step of continuously annealing the cold-rolled steel sheet.

4. The method for producing a cold-rolled steel sheet for hot forming according to claim 3, wherein the steel slab further comprises one or more selected from the following a) and f): a) the total content of Ti, Nb, Zr and V: 0.001% to 0.4% by weight; b) B: 0.0001 to 0.01% by weight, c) Sum of Mo and W content: 0.001 to 1.0 wt.%; d) Total content of Cu and Ni: 0.005 to 2.0 wt.%; e) Sum of Sb and Sn content: 0.001 to 1.0 wt% f) REM: 0.0001-0.02% by weight

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

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

7. Producing the cold rolled steel sheet according to claim 3 or 4; heating the cold-rolled steel sheet to a temperature of 700°C or more at a heating rate of 1 to 1000°C / sec; hot forming the heated cold-rolled steel sheet; and The method for manufacturing a hot-formed member includes a step of cooling the hot-formed steel plate at a cooling rate of 10 to 1000°C / sec.

8. The method for producing a hot-formed part according to claim 7, wherein the cooling is performed at a cooling stop temperature equal to or lower than Mf (martensitic transformation finish temperature).

9. The cooling is performed at a cooling stop temperature of not less than Mf (martensitic transformation finish temperature) but not more than Ms (martensitic transformation start temperature), The method for producing a hot-formed part according to claim 7, further comprising, after the cooling, maintaining the temperature at the cooling end temperature or reheating to Ac1 or below.

10. In weight percent, C: 0.25 to 0.45%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.1% or more but less than 5.0%, N: 0.001 to 0.02%, with the remainder being Fe and other unavoidable impurities, A hot-formed member having a hardness ratio represented by the following [Relationship 2] of 0.1 or more and 10 or less. [Equation 2]

11. The hot-formed part according to claim 10, further comprising one or more selected from the following a) and f): a) the total content of Ti, Nb, Zr and V: 0.001% to 0.4% by weight; b) B: 0.0001 to 0.01% by weight, c) Sum of Mo and W content: 0.001 to 1.0 wt.%; d) Total content of Cu and Ni: 0.005 to 2.0 wt.%; e) Sum of Sb and Sn content: 0.001 to 1.0 wt% f) REM: 0.0001-0.02% by weight

12. 12. The hot-formed part according to claim 10 or 11, having a tensile strength of 1800 MPa or more and a yield strength of 1200 MPa or more.

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