Cold-rolled steel sheet for hot forming having excellent bendability, hot-formed member, and method for producing same

Abstract

The purpose of one aspect of the present invention is to provide a steel material for hot forming, a hot-formed member using the steel material for hot forming, and a method for manufacturing the steel material and the hot-formed member. The steel material for hot forming can ensure high bendability while having a high strength of 1800 MPa or more on the basis of tensile strength, the steel material being suitable for use in automotive members requiring collision resistance characteristics.

Description

Technical Field

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

[0002] In recent years, for the purpose of improving fuel efficiency by reducing the weight of automobiles and protecting passengers, etc., hot formed ultra-high strength components have been widely used for the structural components of automobiles.

[0003] As a representative technique regarding such hot forming, Patent Document 1 has been proposed. In Patent Document 1, a technique has been proposed in which a steel sheet is heated to 850°C or higher and then the structure of the component is formed into martensite by hot forming and rapid cooling according to a stamping machine, so as to ensure an ultra-high strength with a tensile strength exceeding 1600 MPa. In the case of the technique proposed in Patent Document 1, since forming is performed at a high temperature, even complex shapes can be easily formed, and due to the strength increase by rapid cooling in the mold, a weight reduction effect due to high strength can be expected.

[0004] A representative index for evaluating the collision resistance characteristics of HPF formed components used for the purpose of protecting passengers, etc. is regarded as bendability. For example, in the case of an automotive B-pillar, etc., when the vehicle side is collided and the HPF formed component is bent, a characteristic (bendability) of being able to support up to a specific distance (angle) or more without breaking is required.

[0005] Therefore, various studies have been conducted to improve the collision resistance characteristics of HPF steel materials and components. For example, as in Patent Document 2, HPF of different strength steel types is formed by a tailor welded blank (TWB), thereby improving the collision energy absorption ability, etc. by improving partial bendability.

[0006] However, in terms of improving the collision resistance characteristics by TWB, there are limitations in improving the characteristics of components that require collision resistance characteristics. For example, the bendability may deteriorate due to the deterioration of the welded part.

[0007] (Prior Art Documents)

[0008] (Patent Document 1) U.S. Reissue Patent No. 6296805

[0009] (Patent Document 2) Korean Patent Publication No. 10-2021-0080239 Summary of the Invention

[0010] (I) Technical Problems to be Solved

[0011] An object of one aspect of the present invention is to provide a hot-forming steel sheet, a hot-formed part, and a manufacturing method thereof that can impart high strength and excellent bendability to a part.

[0012] The technical problems of the present invention are not limited to the above. Those skilled in the art to which the present invention pertains can easily understand the additional technical problems of the present invention from the overall content of the specification of the present invention.

[0013] (II) Technical solution

[0014] One aspect of the present invention provides a cold-rolled steel sheet for hot forming, which, by weight %, comprises: C: 0.25 - 0.45%, Si: 0.01 - 3.0%, Mn: 0.01 - 4.0%, Al: 0.001 - 0.4%, P: 0.001 - 0.05%, S: 0.0001 - 0.02%, Cr: 0.1% or more and less than 5.0%, N: 0.001 - 0.02%, the balance being Fe and other inevitable impurities, and the value of the structure ratio represented by the following [Relationship 1] is 0.2 or more and 1.3 or less.

[0015] [Relationship 1] Structure ratio =

[0016] (In the formula, and respectively represent the area ratios of pearlite and cementite in the surface layer part, and respectively represent the area ratios of pearlite and cementite in the central part.)

[0017] The cold-rolled steel sheet for hot forming may further contain one or more selected from the following a) and f).

[0018] a) The sum of the contents of Ti, Nb, Zr, and V: 0.001 - 0.4% by weight,

[0019] b) B: 0.0001 - 0.01% by weight,

[0020] c) The sum of the contents of Mo and W: 0.001 - 1.0% by weight,

[0021] d) The sum of the contents of Cu and Ni: 0.005 - 2.0% by weight,

[0022] e) The sum of the contents of Sb and Sn: 0.001 - 1.0% by weight,

[0023] f) REM: 0.0001 - 0.02% by weight.

[0024] Another aspect of the present invention provides a method for manufacturing a cold-rolled steel sheet for hot forming, the manufacturing method comprising the following steps: heating a steel billet having the above alloy composition to 1000 - 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 / second (S) or more and 750 °C / second or less; coiling the hot-rolled steel sheet within a temperature range exceeding Ms and below 750 °C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and continuously annealing the cold-rolled steel sheet.

[0025] The steel billet and the slab may further contain 1 or more selected from the following a) to f).

[0026] a) Sum of Ti, Nb, Zr and V contents: 0.001 - 0.4 wt%,

[0027] b) B: 0.0001 - 0.01 wt%,

[0028] c) Sum of Mo and W contents: 0.001 - 1.0 wt%,

[0029] d) Sum of Cu and Ni contents: 0.005 - 2.0 wt%,

[0030] e) Sum of Sb and Sn contents: 0.001 - 1.0 wt%,

[0031] f) REM: 0.0001 - 0.02 wt%.

[0032] The cold rolling may be performed at a cumulative reduction ratio of 30 - 80%.

[0033] The continuous annealing may be performed within a temperature range of 700 - 900 °C for 1 - 1000 seconds.

[0034] Another aspect of the present invention provides a method for manufacturing a hot-formed component, the manufacturing method comprising the following steps: manufacturing a cold-rolled steel sheet according to the above method for manufacturing a cold-rolled steel sheet for hot forming; heating the cold-rolled steel sheet to a temperature above 700 °C at a heating rate of 1 - 1000 °C / second; hot forming the heated cold-rolled steel sheet; and cooling the hot-formed steel sheet at a cooling rate of 10 - 1000 °C / second.

[0035] The cooling can be performed by setting the cooling termination temperature below the martensitic transformation termination temperature (Mf). However, the cooling can be performed by setting the cooling termination temperature above the martensitic transformation termination temperature (Mf) and below the martensitic transformation start temperature (Ms). In this case, after cooling, it may further include a step of maintaining the temperature at the cooling termination temperature or reheating it below Ac1.

[0036] Another aspect of the present invention provides a hot - formed part which, by weight percentage, contains: C: 0.25 - 0.45%, Si: 0.01 - 3.0%, Mn: 0.01 - 4.0%, Al: 0.001 - 0.4%, P: 0.001 - 0.05%, S: 0.0001 - 0.02%, Cr: more than 0.1% and less than 5.0%, N: 0.001 - 0.02%, the balance being Fe and other inevitable impurities, and the value of the hardness ratio represented by the following [Relationship 2] is 0.1 or more and 10 or less.

[0037] [Relationship 2] Hardness ratio =

[0038] (In the above formula 1, represents the standard deviation of the hardness of the surface layer part, is the standard deviation of the hardness of the central part.)

[0039] The hot - formed part may further contain one or more selected from the following a) and f).

[0040] a) The sum of the contents of Ti, Nb, Zr and V: 0.001 - 0.4% by weight,

[0041] b) B: 0.0001 - 0.01% by weight,

[0042] c) The sum of the contents of Mo and W: 0.001 - 1.0% by weight,

[0043] d) The sum of the contents of Cu and Ni: 0.005 - 2.0% by weight,

[0044] e) The sum of the contents of Sb and Sn: 0.001 - 1.0% by weight,

[0045] f) REM: 0.0001 - 0.02% by weight.

[0046] The tensile strength of the hot - formed part can be 1800 MPa or more, and the yield strength can be 1200 MPa or more.

[0047] In addition, the above technical solution does not list all the features of the present invention. The various features of the present invention and the resulting advantages and effects can be understood in more detail by referring to the following specific embodiments.

[0048] (III) Beneficial Effects

[0049] According to one aspect of the present invention, there can be provided a hot-forming steel having a high strength of 1800 MPa or more based on the tensile strength and capable of ensuring high bendability, a hot-forming part using the hot-forming steel, and a manufacturing method thereof.

[0050] The various beneficial advantages and effects of the present invention are not limited to the above, and can be more easily understood during the description of the specific embodiments of the present invention. Best Mode for Carrying Out the Invention

[0051] Hereinafter, preferred specific embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other embodiments, and the scope of the present invention is not limited to the following embodiments. In addition, the embodiments of the present invention are provided to more completely describe the present invention to those skilled in the art.

[0052] In addition, unless otherwise specifically specified, the content unit of each element in the description of the present invention is based on weight, and the unit of the ratio of the structure is based on area.

[0053] The present inventors recognized that in the case of a non-coated ultra-high strength cold-rolled steel sheet for hot forming, under normal hot rolling conditions, the bendability after the hot forming process is reduced, and there is a problem that it is difficult to ensure excellent bendability. To solve this problem, in-depth research was conducted.

[0054] As a result, it was confirmed that when controlling the cooling rate in the cooling section during the hot rolling process, the area ratio of pearlite / cementite in the surface layer and the central part of the cold-rolled steel sheet after annealing can be adjusted. Thereby, the hardness deviation between the surface layer and the central part of the martensite after hot forming can be reduced, and thus excellent bendability can be ensured, and the present invention was completed.

[0055] Hereinafter, a cold-rolled steel sheet for hot forming according to one aspect of the present invention will be described in detail.

[0056] The cold-rolled steel sheet for hot forming with excellent surface quality according to one aspect of the present invention may contain, by weight%: C: 0.25 - 0.45%, Si: 0.01 - 3.0%, Mn: 0.01 - 4.0%, Al: 0.001 - 0.4%, P: 0.001 - 0.05%, S: 0.0001 - 0.02%, Cr: 0.1% or more and less than 5.0%, N: 0.001 - 0.02%, the balance being Fe and other inevitable impurities.

[0057] First, the alloy composition of the cold-rolled steel sheet for thermoforming with excellent surface quality according to one aspect of the present invention will be described in detail.

[0058] C: 0.25 - 0.45%

[0059] C is an essential element for increasing the strength of heat-treated components and should be added appropriately.

[0060] When the C content is less than 0.25%, it is difficult to ensure sufficient strength. Therefore, it is preferred to add C of 0.25% or more. The more preferred lower limit is 0.26%, and the further preferred lower limit is 0.27%. On the other hand, when the C content exceeds 0.45%, when cold-rolling the hot-rolled material, the strength of the hot-rolled material is too high, not only the cold-rollability deteriorates significantly, but also the spot weldability decreases significantly. Therefore, it is preferably 0.45% or less. The more preferred upper limit is 0.42%, and the further preferred upper limit is 0.40%.

[0061] Si: 0.01 - 3.0%

[0062] Si plays an important role in forming a Si-based amorphous oxide layer by enriching on the surface when annealing the cold-rolled steel sheet in a continuous annealing production line, and inhibits the formation of (Fe, Mn, Cr) oxide layers in the thermoforming process, thereby playing a role in ensuring the spot weldability of components.

[0063] When the Si content is less than 0.01%, the above effects are insufficient. Therefore, the lower limit of the Si content is preferably 0.01%. The more preferred lower limit is 0.1%. On the other hand, when the Si content exceeds 3.0%, an overly thick Si-based amorphous oxide layer is formed, and there is a problem that the spot weldability decreases instead. The more preferred upper limit is 2.8%, and the further preferred upper limit is 2.5%.

[0064] Cr: 0.1% or more and less than 5.0%

[0065] Cr not only improves the hardenability of the steel sheet but also, through an appropriate reaction with Si, can play a role in stably assisting the formation of the surface Si-based amorphous oxide layer.

[0066] When the Cr content is less than 0.1%, the above effects are insufficient. The more preferred lower limit is 0.15%, and the further preferred lower limit is 0.2%. On the other hand, when the Cr content is 5.0% or more, its effects saturate, and there is a problem of increased manufacturing cost. The more preferred upper limit is 4.5%, and the further preferred upper limit is 4.0%.

[0067] Mn: 0.01 - 4.0%

[0068] Mn can not only ensure the solution strengthening effect, but also needs to be added in hot-formed parts to reduce the critical cooling rate required to ensure martensite.

[0069] When the Mn content is less than 0.01%, the above effects are insufficient. The more preferable lower limit is 0.05%, and the further preferable lower limit is 0.1%. On the other hand, when the Mn content exceeds 4.0%, the strength of the steel sheet before the hot-forming process becomes too high, so the blanking operation becomes difficult, and there are disadvantages such as increased cost due to excessive addition of alloying iron and poor spot weldability. The more preferable upper limit is 3.0%, and the further preferable upper limit is 2.5%.

[0070] Al: 0.001 - 0.4%

[0071] Al, together with Si, plays a role in deoxidation during the steelmaking process, thereby improving the cleanliness of the steel.

[0072] When the Al content is less than 0.001%, the above effects are insufficient. The more preferable lower limit is 0.002%, and the further preferable lower limit is 0.003%. When the Al content exceeds 0.4%, the Ac3 temperature rises excessively, and there is a problem of needing to increase the heating temperature. The more preferable upper limit is 0.3%, and the further preferable upper limit is 0.2%.

[0073] P: 0.001 - 0.05%

[0074] P is an impurity. To control the P content to less than 0.001%, a large amount of manufacturing cost is required. When the P content exceeds 0.05%, the weldability of the hot-formed parts may be significantly reduced. The more preferable upper limit is 0.03%.

[0075] S: 0.0001 - 0.02%

[0076] S is an impurity. To control the S content to less than 0.0001%, a large amount of manufacturing cost is required. When the S content exceeds 0.02%, it hinders the ductility, impact properties and weldability of the parts. The more preferable upper limit is 0.01%.

[0077] N: 0.001 - 0.02%

[0078] N is an impurity. To control the N content to less than 0.001%, a large amount of manufacturing cost is required. When the N content exceeds 0.02%, it is not only sensitive to crack generation during slab continuous casting, but also the impact properties may become poor. The more preferable upper limit is 0.01%.

[0079] The remaining components of the present invention are iron (Fe). However, during the normal manufacturing process, undesirable impurities may inevitably be mixed in from raw materials or the surrounding environment, so these impurities cannot be excluded. These impurities are well-known to those skilled in the normal manufacturing process, so all of their contents are not particularly mentioned in this specification.

[0080] In addition to the above component composition, in the present invention, one or more selected from the following a) to f) may be further included. By arbitrarily adding these elements, characteristics such as surface quality and hot formability can be further improved.

[0081] a) The sum of the contents of Ti, Nb, Zr, and V: 0.001 - 0.4%

[0082] Ti, Nb, Zr, and V form fine precipitates, thereby improving the strength of heat-treated components, and having the effect of stabilizing retained austenite and improving impact toughness by refining grains. When their content (when two or more are added, it represents the sum of them) is less than 0.001%, the above effects may be insufficient, and the more preferred lower limit is 0.005%, and the further preferred lower limit is 0.008%. When their content exceeds 0.4%, not only do their effects saturate, but the cost may also increase due to excessive addition of alloy iron. The more preferred upper limit is 0.38%, and the further preferred upper limit is 0.35%.

[0083] b) B: 0.0001 - 0.01%

[0084] B is an element that can improve hardenability even when added in a small amount, and can suppress the brittleness of hot-formed components caused by grain boundary segregation of P and / or S by segregating at the prior austenite grain boundaries.

[0085] When the B content is less than 0.0001%, the above effects are insufficient. The more preferred lower limit is 0.00012%, and the further preferred lower limit is 0.00015%. When the B content exceeds 0.01%, not only do its effects saturate, but hot brittleness may occur during hot rolling. The more preferred upper limit is 0.005%.

[0086] c) The sum of the contents of Mo and W: 0.001 - 1.0 wt%

[0087] Mo and W may be added to improve hardenability, improve strength through precipitation strengthening effect, and refine grains. When their content (when both Mo and W are added, it represents the sum of their contents) is less than 0.001%, the above effects are not sufficient, and the more preferred lower limit is 0.0015%, and the further preferred lower limit is 0.002%. When their content exceeds 1.0%, not only do their effects saturate, but there is a problem of increased cost. The more preferred upper limit is 0.95%, and the further preferred upper limit is 0.9%.

[0088] d) Sum of the contents of Cu and Ni: 0.005 - 2.0% by weight

[0089] Cu can be added as an element to increase strength by forming fine precipitates. In addition, when Cu is added alone, it may cause hot brittleness, so Ni is added as needed. However, when the sum of these components is less than 0.005%, the above effects may be insufficient. The more preferable lower limit is 0.006%, and the further preferable lower limit is 0.007%. When the sum of these components exceeds 2.0%, it may lead to an excessive increase in cost. The more preferable upper limit is 1.95%, and the further preferable upper limit is 1.9%.

[0090] e) Sum of the contents of Sb and Sn: 0.001 - 1.0% by weight

[0091] Sb and Sn have the effect of suppressing the formation of oxides that may be generated at the grain boundaries on the surface layer of the hot-rolled material of the steel added with Si, and can suppress the dent defects caused by the shedding of the surface layer grain boundaries during annealing of the cold-rolled material. To obtain this effect, it is preferably added in an amount of 0.001% or more. The more preferable lower limit is 0.002%, and the further preferable lower limit is 0.03%.

[0092] On the other hand, when their content (when both Sb and Sn are added, it means the sum of Sb and Sn) exceeds 1.0%, not only may the cost increase excessively, but also it may be dissolved in the grain boundaries of the slab and may cause edge cracks during hot rolling. The more preferable upper limit is 0.95%, and the further preferable upper limit is 0.9%.

[0093] f) REM: 0.0001 - 0.02%

[0094] The REM element controls the activity of Fe in the steel and can control the formation thickness of the surface Fe oxide scale during hot forming. To obtain this effect, it is necessary to add more than 0.0001% of the REM element. The more preferable lower limit is 0.00015%, and the further preferable lower limit is 0.0002%. On the other hand, when the REM content exceeds 0.02%, the control ability of the Fe activity is lost, and thus the surface quality may deteriorate. Therefore, it is preferably controlled below 0.02%, and more preferably controlled below 0.01%.

[0095] The cold-rolled steel sheet for hot forming according to one aspect of the present invention not only satisfies the above alloy composition, but also the value of the tissue ratio represented by the following [Relationship 1] can be 0.2 or more and 1.3 or less.

[0096] [Relationship 1] Tissue ratio =

[0097] (The and respectively represent the area ratios of pearlite and cementite in the surface layer part, and respectively represent the area ratios of pearlite and cementite in the central part.)

[0098] In the present invention, the surface layer part may refer to the region within 100 μm from the surface in the thickness direction, and the central part in the present invention may refer to the region at 1 / 2t ± 50 μm from the surface in the thickness direction (where t represents the thickness (mm) of the steel material).

[0099] When the structure ratio represented by [Relationship 1] exceeds 1.3, the deviation of hardness between martensites formed on the surface layer after hot forming increases, and the stress caused by bending may concentrate in the stronger martensites. Due to the difference in hardness deviation in the thickness direction, the stress non-uniformity in the thickness direction increases, and thus the bendability may deteriorate. More preferably, the structure ratio may be 1.15 or less, and further preferably may be 0.95 or less. On the other hand, when the structure ratio is less than 0.2 proposed in the present invention, the strength cannot be ensured after hot forming, and the tensile strength may be 1800 MPa or less. The more preferable lower limit is 0.25, and the further preferable lower limit is 0.3.

[0100] In addition, the fine structure of the cold-rolled steel sheet according to the present invention may include ferrite and cementite. There is no particular limitation on their area ratio. However, for example, in terms of area ratio, the sum of ferrite and cementite may be 5% or more.

[0101] When making a blank for manufacturing the cold-rolled steel sheet into a hot-formed part, when its strength is too high, die wear may easily occur, so it is necessary to ensure the above-mentioned fine structure. If this is not considered, it may include bainite, martensite, etc., and such a situation is not excluded.

[0102] Hereinafter, the components of the present invention will be described in detail.

[0103] The value of the hardness ratio represented by the following [Relationship 2] of the component of the present invention may satisfy 0.1 or more and 10 or less.

[0104] [Relationship 2] Hardness ratio =

[0105] (In the above formula 1, represents the standard deviation of the hardness of the surface layer part, is the standard deviation of the hardness of the central part.)

[0106] When the standard deviation of the surface layer hardness increases and the hardness ratio represented by [Relationship 2] exceeds 10, due to the stress concentration phenomenon caused by the hardness deviation between martensites and the stress imbalance in the thickness direction, the bendability may deteriorate. On the other hand, when the hardness ratio is 10 or less, the hardness deviation is good, and thus the bendability can be improved. However, when the hardness ratio is less than 0.1, the hardness deviation at the center in the thickness direction is relatively aggravated, and thus the bendability may deteriorate.

[0107] The base steel plate of the component according to the present invention has the same composition as the above cold-rolled steel plate, and thus will not be described separately.

[0108] Hereinafter, the microstructure of the component of the present invention will be described in detail.

[0109] In order to ensure high strength, the hot-formed component according to one aspect of the present invention may contain martensite or bainite as the main phase. In the present invention, the main phase may refer to the phase having the largest area ratio among the plurality of phases constituting the microstructure. The area ratio thereof is not particularly limited, but a more preferable area ratio may be 50% or more.

[0110] Hereinafter, the manufacturing method of the cold-rolled steel plate for hot forming according to another aspect of the present invention will be described in detail.

[0111] The manufacturing method of the cold-rolled steel plate for hot forming according to another aspect of the present invention may include the following steps: heating a slab satisfying the above alloy composition to 1000 - 1300 °C; performing hot rolling on the heated slab at a finish rolling temperature of Ar3 to 1000 °C to obtain a hot-rolled steel plate; cooling the hot-rolled steel plate at a cooling rate of 400 °C / second or more and 750 °C / second or less; coiling the hot-rolled steel plate in a temperature range exceeding Ms and 750 °C or less; performing cold rolling on the coiled hot-rolled steel plate at a cumulative reduction rate of 30 - 80% to obtain a cold-rolled steel plate; and continuously annealing the cold-rolled steel plate in a temperature range of 700 - 900 °C for 1 - 1000 seconds.

[0112] Heating step of the slab

[0113] Heat the slab satisfying the above alloy composition to 1000 - 1300 °C.

[0114] When the heating temperature is lower than 1000 °C, it is difficult to homogenize the slab structure. When the heating temperature exceeds 1300 °C, excessive oxides may be formed, and the manufacturing cost may increase.

[0115] Hot rolling step

[0116] Perform hot rolling on the heated slab at a finish rolling temperature of Ar3 to 1000 °C to obtain a hot-rolled steel plate.

[0117] When the finish rolling temperature is lower than the Ar3 temperature, two-phase zone rolling is likely to occur, resulting in a mixed grain structure on the surface layer and making it difficult to control the shape of the hot-rolled steel sheet. When the finish rolling temperature exceeds 1000 °C, the grains of the hot-rolled steel sheet tend to coarsen.

[0118] Hot rolling cooling step

[0119] Cool the hot-rolled steel sheet obtained by the said hot rolling at a cooling rate of 400 °C / second or more and 750 °C / second or less.

[0120] When cooling is carried out exceeding the upper limit of the cooling rate during hot rolling, a pearlite and cementite with an excessive area ratio are formed on the surface layer after annealing, resulting in an increased hardness deviation between the surface martensites after hot forming, and thus excellent bendability cannot be ensured. When cooling is carried out not reaching the lower limit of the cooling rate, the formation of pearlite and cementite on the surface layer is extremely small, and sufficient strength cannot be ensured for the cold steel sheet after hot forming.

[0121] Coiling step

[0122] Coil the said hot-rolled steel sheet within a temperature range exceeding Ms and 750 °C or less.

[0123] When the coiling temperature is below the martensite transformation start temperature (Ms), the strength of the hot-rolled steel sheet is too high and the cold rolling property is reduced. When the coiling temperature exceeds 750 °C, the thickness of the oxide layer increases and surface grain boundary oxidation is caused, not only making the pickling property worse, but also problems such as surface grain boundary peeling may occur during annealing in a continuous annealing furnace.

[0124] Cold rolling step

[0125] Cold roll the hot-rolled steel sheet obtained by the said coiling to obtain a cold-rolled steel sheet. This is to more precisely control the thickness of the steel sheet, and pickling can be carried out before cold rolling.

[0126] At this time, there is no need to specifically limit the reduction ratio of the said cold rolling, but in order to ensure the specified target thickness, the reduction ratio can be 30 - 80%.

[0127] Continuous annealing step

[0128] The continuous annealing of the said cold-rolled steel sheet can be carried out within a temperature range of 700 - 900 °C.

[0129] When the annealing temperature is lower than 700 °C, the rolling structure generated by cold rolling is difficult to recover and recrystallize. When the annealing temperature exceeds 900 °C, it may deteriorate the annealing equipment, and thus may become a factor increasing the process cost due to frequent replacement of equipment, etc.

[0130] In addition, the annealing time can be 1 - 1000 seconds. When the annealing time is less than 1 second, it is difficult to obtain an annealing effect. When the annealing time exceeds 1000 seconds, productivity may be reduced.

[0131] Hereinafter, a method for manufacturing a thermoformed part according to another aspect of the present invention will be described in detail.

[0132] The method for manufacturing a thermoformed part according to another aspect of the present invention includes the following steps: heating a cold-rolled steel sheet manufactured by the above-described method for manufacturing a cold-rolled steel sheet according to the present invention to a temperature of 700°C or higher at a heating rate of 1 - 1000°C / second; thermoforming the heated cold-rolled steel sheet; and cooling the thermoformed steel sheet at a cooling rate of 10 - 1000°C / second.

[0133] Heating step

[0134] A cold-rolled steel sheet manufactured by the method for manufacturing a cold-rolled steel sheet according to the present invention is heated to a temperature of 700°C or higher at a heating rate of 1 - 1000°C / second.

[0135] When the heating temperature is lower than 700°C, recrystallization of ferrite is insufficient, and there may be a problem of increased anisotropy in bending after thermoforming.

[0136] When the heating rate is less than 1°C / second, it is difficult to sufficiently ensure productivity. When the heating rate exceeds 1000°C / second, equipment with too high costs is required.

[0137] Thermoforming and cooling steps

[0138] The heated cold-rolled steel sheet is thermoformed and then cooled at a cooling rate of 10 - 1000°C / second.

[0139] When the cooling rate is less than 10°C / second, undesired ferrite and pearlite are formed, making it difficult to ensure tensile strength. On the other hand, in order to control the cooling rate to exceed 1000°C / second, expensive special cooling equipment is required.

[0140] At this time, the cooling termination temperature of the cooling step can be below the martensite transformation termination temperature (M f ). This is because when cooling is terminated above M f and then cooled to room temperature again, it is difficult to ensure the shape freezing property of the thermoformed part.

[0141] However, in order to ensure more excellent elongation and impact properties in the thermoformed part, it can be between the martensite transformation termination temperature (M f ) and the martensite transformation start temperature (M s) After termination of cooling between them, it is maintained at the cooling termination temperature or reheated below Ac1 to temper the martensite and stabilize the retained austenite.

[0142] In addition, in order to ensure high strength, the hot-formed component may have martensite or bainite as the main phase. Here, the main phase refers to the phase having the largest area ratio among multiple phases constituting the fine structure. There is no particular limitation on its area ratio, but for example, the area ratio may be 50% or more.

[0143] Furthermore, the hot-formed component may have a tensile strength of 1800 MPa or more. By ensuring a high strength of 1800 MPa or more, it can preferably be applied to automotive structural components or reinforcements that require collision resistance, etc.

[0144] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention for more detailed description and are not used to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the content recorded in the claims and the content reasonably deduced therefrom. Detailed Description of the Invention

[0145] (Example)

[0146] A slab with a thickness of 40 mm having the composition shown in Table 1 below was subjected to vacuum melting, heated in a heating furnace at 1200 °C for 1 hour, and then hot-rolled at a finish rolling temperature of 930 °C to manufacture a hot-rolled steel sheet with a final thickness of 3 mm. After cooling the hot-rolled steel sheet at the cooling rate shown in Table 2 below, it was coiled at 640 °C. Then, the hot-rolled steel sheet was pickled and cold-rolled with a cold reduction rate of 50%. In addition, after the cold rolling, continuous annealing was performed at 800 °C for 80 seconds to manufacture a cold-rolled steel sheet for hot forming.

[0147] After that, the manufactured cold-rolled steel sheet was heated at a heating rate of 20 °C / second, then heat-treated at 900 °C for 6 minutes, and the heated cold-rolled steel sheet was hot-formed. Then, the hot-formed steel sheet was cooled to room temperature at a cooling rate of 20 °C / second to manufacture a hot-formed component.

[0148] [Table 1]

[0149]

[0150] Table 2 shows the area ratio of the surface layer and center layer structures of the cold-rolled steel sheet for thermoforming and the structure ratio of [Relationship 1]. To measure the area ratio of the structures at each position in the thickness direction, the cross-section of the structure after nitric acid ethanol etching was observed at a magnification of 500 times using an optical microscope (OM, Optical Microscopy). After measuring the optical photograph, the area ratio of the surface layer and center layer structures was measured using CLEMEX Vision PE software three times each, and their average values are shown in Table 2.

[0151] In addition, according to [Relationship 2], the hardness ratio, which is the ratio of the standard deviations of the hardness of the surface layer and center layer of the thermoformed part manufactured after thermoforming, is shown in Table 2. The tensile strength and maximum bending angle are shown. The hardness was measured using a Vickers hardness tester (Vikckers Hardness tester, Dura Scan 80G5) with a load of 10 kgf at intervals of 1 mm at at least 10 points or more. The tensile strength value was measured using a JIS-5 test piece according to the ISO6892 standard through a normal temperature tensile test. The maximum bending angle was calculated by converting the maximum bending strength clearly shown in the standard into the value of the bending outer angle according to the bending property evaluation method of the VDA238-100 standard and recorded. In addition, the bending angle change rate represents the ratio of the bending angle deviation between the test piece manufactured under the manufacturing conditions proposed in the present invention and the test piece manufactured under the manufacturing conditions deviating from the proposed manufacturing conditions.

[0152] [Table 2]

[0153]

[0154] As shown in Table 2, in Comparative Examples 1 and 3 to 6 where hot rolling was performed at a cooling rate exceeding the upper limit proposed in the present invention, the structure ratio represented by [Relationship 1] exceeded 1.3 after annealing, resulting in a larger standard deviation of the surface layer hardness than that of the center layer hardness after thermoforming. Therefore, the hardness ratio represented by [Relationship 2] exceeded 10, and thus the bending angle was poor.

[0155] In the case of Comparative Examples 2 and 7 where the cooling rate during hot rolling did not meet the range proposed in the present invention and did not reach the lower limit, the maximum bending angle after thermoforming of the cold-rolled steel sheet was improved, but due to the formation of excessive soft martensite in the surface layer, sufficient strength could not be ensured.

[0156] In the case of Invention Examples 1 to 5, the cooling rate after hot rolling was controlled within the range limited in the present invention, so that the structure ratio of the cold-rolled steel sheet satisfied the range of 0.2 or more and 1.3 or less, and the hardness ratio was 10 or less. Therefore, the thermoformed parts manufactured showed good bendability.

Claims

1. A cold-rolled steel sheet for hot forming, by weight%, the cold-rolled steel sheet for hot forming contains: C: 0.25 - 0.45%, Si: 0.01 - 3.0%, Mn: 0.01 - 4.0%, Al: 0.001 - 0.4%, P: 0.001 - 0.05%, S: 0.0001 - 0.02%, Cr: more than 0.1% and less than 5.0%, N: 0.001 - 0.02%, the balance Fe and other inevitable impurities, The value of the structure ratio represented by the following [Relationship 1] is 0.2 or more and 1.3 or less, [Relationship 1] Tissue ratio = The and respectively represent the area ratios of pearlite and cementite in the surface layer, and respectively represent the area ratios of pearlite and cementite in the central part.

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

3. A method for manufacturing a cold-rolled steel sheet for hot forming, which includes the following steps: Heating a slab to 1000 - 1300 °C, by weight%, the slab contains: C: 0.25 - 0.45%, Si: 0.01 - 3.0%, Mn: 0.01 - 4.0%, Al: 0.001 - 0.4%, P: 0.001 - 0.05%, S: 0.0001 - 0.02%, Cr: more than 0.1% and less than 5.0%, N: 0.001 - 0.02%, the balance Fe and other inevitable impurities; 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 obtained by the hot rolling at a cooling rate of 400 °C / second or more and 750 °C / second or less; Coiling the hot-rolled steel sheet in a temperature range exceeding Ms and below 750 °C; Cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; And Performing continuous annealing on the cold-rolled steel sheet.

4. The manufacturing method of the cold-rolled steel sheet for thermoforming according to claim 3, wherein, The steel billet further contains one or more selected from the following a) and f): a) The sum of the contents of Ti, Nb, Zr and V: 0.001 - 0.4% by weight, b) B: 0.0001 - 0.01% by weight, c) The sum of the contents of Mo and W: 0.001 - 1.0% by weight, d) The sum of the contents of Cu and Ni: 0.005 - 2.0% by weight, e) The sum of the contents of Sb and Sn: 0.001 - 1.0% by weight, f) REM: 0.0001 - 0.02% by weight.

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

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

7. A method for manufacturing a thermoformed part, wherein Including the following steps: Manufacturing a cold-rolled steel sheet according to claim 3 or 4; Heating the cold-rolled steel sheet to a temperature above 700 °C at a heating rate of 1 - 1000 °C / second; Hot form the heated cold-rolled steel sheet; and Cool the hot-formed steel sheet at a cooling rate of 10 - 1000 °C / second.

8. The manufacturing method of the thermoformed part according to claim 7, wherein, The cooling is performed by setting the cooling termination temperature below the martensite transformation termination temperature (Mf).

9. The manufacturing method of the thermoformed part according to claim 7, wherein, The cooling is performed by setting the cooling termination temperature above the martensite transformation termination temperature (Mf) and below the martensite transformation start temperature (Ms). After the cooling, it further includes a step of maintaining the temperature at the cooling termination temperature or reheating to below Ac1.

10. A hot-formed component, by weight %, the hot-formed component contains: C: 0.25 - 0.45%, Si: 0.01 - 3.0%, Mn: 0.01 - 4.0%, Al: 0.001 - 0.4%, P: 0.001 - 0.05%, S: 0.0001 - 0.02%, Cr: 0.1% or more and less than 5.0%, N: 0.001 - 0.02%, the balance Fe and other inevitable impurities, The value of the hardness ratio represented by the following [Relationship 2] is 0.1 or more and 10 or less, [Relationship 2] Hardness ratio = In the formula 1, represents the standard deviation of the surface layer hardness, is the standard deviation of the hardness of the central part.

11. The thermoformed part according to claim 10, wherein, The hot-formed component further contains one or more selected from the following a) and f): a) The sum of the contents of Ti, Nb, Zr, and V: 0.001 - 0.4 wt%, b) B: 0.0001 - 0.01 wt%, c) The sum of the contents of Mo and W: 0.001 - 1.0 wt%, d) The sum of the contents of Cu and Ni: 0.005 - 2.0 wt%, e) The sum of the contents of Sb and Sn: 0.001 - 1.0 wt%, f) REM: 0.0001 - 0.02 wt%.

12. The thermoformed part according to claim 10 or 11, wherein, The tensile strength is 1800 MPa or more, and the yield strength is 1200 MPa or more.

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