Steel for hot forming, hot-formed member, and manufacturing methods therefor

Abstract

To provide a steel for hot forming which has high strength on a member and may impart excellent collision energy absorption capacity, a hot-formed member, and a manufacturing method therefor.SOLUTION: A steel for hot forming comprises, by wt.%, C: 0.06-0.1%, Si: 0.05-0.6%, Mn: 0.6-2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01-0.1%, Cr: 0.01-0.8%, Mo: 0.01-0.5%, N: 0.02% or less, with the balance being Fe and unavoidable impurities, where an alloy factor represented by relational expression 1 is 7 or more, and the number of carbides having a circular equivalent diameter of 0.5 μm or greater is 105 / mm2 or less. Alloy factor=I(Mn)×I(Si)×I(Cr)×I(Mo), where the I values for the components in expression 1 are I(Mn)=3.34×Mn+1, I(Si)=0.7×Si+1, I(Cr)=2.16×Cr+1, I(Mo)=3×Mo+1, and the content of each component is expressed as wt.%.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a hot-forming steel material, a hot-formed member, and a method for producing the same. [Background technology]

[0002] In recent years, hot-formed ultra-high strength components have been used for the purpose of improving fuel efficiency by reducing the weight of automobiles and protecting passengers. This is a typical example of a hot forming technique that is widely used in the construction of automobile structural members. Patent Document 1 proposes a method for reducing the temperature of an Al-Si plated steel sheet by heating at 850°C or higher. After heating, the material is hot-formed by pressing and then rapidly cooled to change its structure to martensite. By forming a tensile strength exceeding 1600 MPa, we have provided a technology that ensures ultra-high strength. In the case of the technology proposed in Patent Document 1, since molding is done at high temperatures, complex shapes can be easily formed. The material can be molded easily, and the strength is increased by rapid cooling inside the mold, which is expected to result in weight reduction due to the high strength. I can wait.

[0003] However, in the case of hot-formed components manufactured by hot forming, the microstructure changes from martensite to Although it has high strength during a collision, it has poor bending property, so it is not suitable for energy absorption. Therefore, in order to compensate for the relatively poor energy absorption capacity, different materials or Tailor welded blanks (TWBs) with combinations of thicknesses k) has been proposed for hot forming, and various research projects are being conducted.

[0004] However, from the viewpoint of energy absorption capacity as a material for TWB hot forming, Although excellent low-strength hot forming steels have been developed and applied, the hot forming steels for normal TWB are In the case of forming steel, the low hardening ability makes it difficult to determine the cooling conditions for each position in the actual part. Due to the difference in the strength, it is not easy to ensure uniform physical properties, and the tensile strength is 500 to 60 0MPa level, there is a need to achieve further weight reduction by improving strength. This is the reality. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 6,296,805 Summary of the Invention [Problem to be solved by the invention]

[0006] One aspect of the present invention is to provide a member having high strength and excellent collision energy absorption capability. To provide a hot forming steel material, a hot forming part, and a manufacturing method thereof, which can provide a hot forming steel material, a hot forming part, and a manufacturing method thereof. It is.

[0007] The object of the present invention is not limited to the above. Those skilled in the art will appreciate that the present invention is not limited to the above-mentioned specific embodiments. There is no difficulty in understanding the subject. [Means for solving the problem]

[0008] One embodiment of the present invention is a composition comprising, by weight, C: 0.06 to 0.1%, Si: 0.05 to 0.6 %, Mn: 0.6~2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01~ 0.1%, Cr: 0.01-0.8%, Mo: 0.5% or less (excluding 0%), N: 0.0 2% or less, the balance being Fe and unavoidable impurities, and the alloy index (all The oy factor is 7 or more, and the number of carbides with a circle equivalent diameter of 0.5 μm or more is 10 5 pieces / mm 2 The present invention provides a hot forming steel material which is:

[0009] [Equation 1] Alloy index=I(Mn)×I(Si)×I(Cr)×I(Mo) (Note that in the above Relational Formula 1, the I value for each component is I(Mn) = 3.34 × Mn + 1, I(Si)=0.7×Si+1, I(Cr)=2.16×Cr+1, I(Mo)=3 × Mo+1, and the content of each component is in weight percent.)

[0010] Another embodiment of the present invention is, in weight percent, C: 0.06 to 0.1%, Si: 0.05 to 0. 6%, Mn: 0.6~2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 ~0.1%, Cr: 0.01~0.8%, Mo: 0.5% or less (except 0%), N: 0. 02% or less, the balance being Fe and unavoidable impurities, and the alloy index (al The loy factor is 7 or more, and the number of carbides with a circle equivalent diameter of 0.5 μm or more is 10 4 pieces / mm 2 A hot-formed part is provided, which is

[0011] [Equation 1] Alloy index=I(Mn)×I(Si)×I(Cr)×I(Mo) (Note that in the above Relational Formula 1, the I value for each component is I(Mn) = 3.34 × Mn + 1, I(Si)=0.7×Si+1, I(Cr)=2.16×Cr+1, I(Mo)=3 × Mo+1, and the content of each component is in weight percent.)

[0012] In still another embodiment of the present invention, the composition is, by weight percent, C: 0.06 to 0.1%, Si: 0.05 ~0.6%, Mn: 0.6~2%, P: 0.05% or less, S: 0.02% or less, Al: 0 .01~0.1%, Cr: 0.01~0.8%, Mo: 0.5% or less (excluding 0%), N 0.02% or less, balance Fe and unavoidable impurities, alloy index represented by the following formula 1 The stage where a steel slab with an alloy factor of 7 or more is heated at 1050 to 1300°C The heated steel slab is then finish hot-rolled at 800 to 950°C to obtain hot-rolled steel sheets. and winding the hot-rolled steel sheet at 500 to 700°C. a step of cooling the sheet from the coiling temperature to 400°C at a cooling rate of 10°C / Hr or more; A step of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; and a step of reducing the annealing temperature from 400°C to 100°C. and heating the heated cold-rolled steel sheet at a rate of 20° C. / s or less through a temperature range of 100° C. to 150° C. A step of annealing at 740 to 860 ° C. and a step of heating the annealed cold-rolled steel sheet from the annealing temperature to 660 ° C. and cooling the steel at a cooling rate of 1°C / s or more at 100°C / s. do.

[0013] [Equation 1] Alloy index=I(Mn)×I(Si)×I(Cr)×I(Mo) (Note that in the above Relational Formula 1, the I value for each component is I(Mn) = 3.34 × Mn + 1, I(Si)=0.7×Si+1, I(Cr)=2.16×Cr+1, I(Mo)=3 × Mo+1, and the content of each component is in weight percent.)

[0014] Yet another embodiment of the present invention is a method for producing a hot forming steel material comprising the steps of obtaining a blank using the hot forming steel material described above; and heating the blank at AC3 to 980° C. and then holding the blank for 1 to 1000 seconds. and cooling the heated and held blank to room temperature after hot forming. A method for making a molded part is provided. Effect of the Invention

[0015] According to one aspect of the present invention, a tensile strength of 1000 MPa or more is obtained. Even in the case of It is possible to provide a hot forming steel material, a hot forming part using the same, and a manufacturing method thereof. do.

[0016] The various and beneficial advantages and effects of the present invention are not limited to those described above, but may be varied by the specific This can be more easily understood in the course of explaining the embodiments. [Brief description of the drawings]

[0017] [Figure 1] 1 shows a simplified concept of CIE (Crack Initiation Energy), which is a criterion for evaluating the impact energy absorption capacity in the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing a hot-formed part manufactured according to an embodiment of the present invention. [Diagram 3] 1 is a graph showing collision energy absorption capacity depending on carbon content and alloy index for Examples 1 to 7 and Comparative Examples 3 to 9 according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The terminology used herein is merely to refer to particular embodiments of the present invention. The singular forms used herein are intended to limit the scope of the term unless expressly stated otherwise. Unless otherwise indicated, plural forms are included.

[0019] As used in the specification, "comprising" means any specific feature, region, integer, step, operation, element, and and / or components, and other specific properties, regions, integers, steps, operations, elements, components, and / or components. It does not exclude the presence or addition of groups.

[0020] All terms, including technical and scientific terms, used herein but not otherwise defined The meanings are the same as those generally understood by those having ordinary skill in the art to which the present invention pertains. Terms defined in commonly used dictionaries have the same meaning as those in the relevant technical literature and the present disclosure. Unless otherwise defined and construed, these terms shall have a meaning consistent with that contained in the It is not to be construed as being abstract or very formal in nature.

[0021] The present inventors have developed a method for improving the collision energy absorption capacity of a hot-formed part. Therefore, the present inventors have studied the impact energy absorption capacity of hot-formed parts. As an index that can be easily expressed, the maximum load during a three-point bending test (VDA238-100) is The effects of various ingredients, manufacturing conditions, structures, etc. are examined using the energy (area of ​​the load-displacement curve) value. Evaluated.

[0022] As a result, the range of carbon content, hardening ability of alloying elements, and after hot rolling were examined for hot forming steel. The cooling conditions during annealing, the heating and cooling conditions during annealing, and the number density of coarse carbides present in the structure are set at a constant level. It is said that controlling the impact energy to a minimum level can maximize the impact energy absorption capacity of hot-formed parts. The present invention was completed based on this conclusion.

[0023] Hereinafter, a hot forming steel material according to one aspect of the present invention will be described in detail. It should be noted that the content of each element is expressed in weight percent unless otherwise specified. The proportion of crystals or structures is based on the area unless otherwise specified.

[0024] First, the composition of the hot forming steel material according to one aspect of the present invention will be described. The hot forming steel material contains, by weight, C: 0.06-0.1%, Si: 0.05-0.6%, M n: 0.6~2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01~0.1 %, Cr:0.01~0.8%, Mo:0.01~0.5%, N:0.02% or less, balance It may contain Fe and inevitable impurities.

[0025] Carbon (C): 0.06~0.1% Carbon (C) is an essential element that is added to improve the strength of heat-treated components. In general, ultra-high strength steel for hot forming has a C content of 0.2% or more. However, in order to maximize the effect in terms of collision energy absorption, an appropriate C content is required. If the C content is less than 0.06%, it is difficult to ensure sufficient strength. Finally, even if the bending property is high, the impact energy absorption capacity is rather decreased, so the bending property is 0.06 On the other hand, if the C content exceeds 0.1%, the strength increases, but Since bending property is reduced and impact energy absorption capacity is reduced, it should be limited to 0.1% or less. Therefore, the C content is preferably in the range of 0.06 to 0.1%. The lower limit of the C content is preferably 0.065%, more preferably 0.07%. The upper limit of the C content is more preferably 0.095%, and even more preferably 0.09 % is even more preferable.

[0026] Silicon (Si): 0.05-0.6% Silicon (Si) is an element that should be added as a deoxidizer in steelmaking. It is a solid solution strengthening element that suppresses the formation of carbides and contributes to increasing the strength of hot-formed components. It is added as an effective element for material uniformity. If its content is less than 0.05%, On the other hand, if the Si content exceeds 0.6%, the steel sheet becomes The Si oxide formed on the surface significantly reduces plating properties, so the content should be 0.6% or less. Therefore, the Si content is limited to the range of 0.05 to 0.6%. The lower limit of the Si content is preferably 0.1%, more preferably 0.1%. The upper limit of the Si content is preferably 0.55%. It is preferably 0.5%.

[0027] Manganese (Mn): 0.6-2% Manganese (Mn) not only ensures the effect of solid solution strengthening, but also improves hardening ability. , must be added to suppress ferrite formation during hot forming. Mn content is 0. If the Mn content is less than 6%, not only is there a limit to the effect described above, but if the Mn content is too low, However, the insufficient hardening ability requires excessive amounts of other expensive alloying elements, which increases the manufacturing cost. On the other hand, if the Mn content exceeds 2%, the thermal Not only does the increase in strength of the steel sheet prior to the cold forming process reduce its cold rolling properties, but the microstructural phase is also affected by the compression process. The band structure arranged in the rolling direction may deepen, resulting in poor impact energy absorption. Therefore, the Mn content is preferably in the range of 0.6 to 2%. The lower limit of the Mn content is more preferably 0.7%, and even more preferably 0.8%. The upper limit of the Mn content is preferably 1.8%, and most preferably 0.9%. More preferably, the ratio is 1.6%, and most preferably, the ratio is 1.4%. preferable.

[0028] Phosphorus (P): 0.05% or less Phosphorus (P) exists as an impurity in steel. If its content exceeds 0.05%, it On the other hand, the above P is an impurity and its There is no particular need to limit the lower limit, but in order to control the P content to less than 0.001%, However, it can be limited to 0.001% or more since it may require high manufacturing costs.

[0029] Sulfur (S): 0.02% or less Sulfur (S) exists as an impurity in steel and affects the ductility, impact properties and weldability of hot-formed components. Since it is an element that inhibits oxidation, the maximum content can be limited to 0.02%. S is an impurity. There is no particular need to limit the lower limit, but the S content should be 0.000 To control it to less than 1%, it would require high manufacturing costs, so it is recommended to limit it to 0.0001%. % or more.

[0030] Aluminum (Al): 0.01-0.1% Aluminum (Al) acts as a deoxidizer together with silicon during steelmaking, increasing the cleanliness of steel. It is an element. If the Al content is less than 0.01%, it is difficult to obtain the above effects, and if the content is 0. If it exceeds 1%, the high temperature ductility of the slab decreases due to the excessive AlN formed during the continuous casting process. Therefore, the Al content is set to 0.01 to 1.05%. The lower limit of the Al content is preferably 0.015%. The upper limit of the Al content is more preferably 0.08%, and more preferably 0.07%. %, more preferably 0.06%.

[0031] Chromium (Cr): 0.01-0.8% Chromium (Cr) is added to ensure the hardening ability of steel, just like Mn. If the content is less than 0.01%, it may be difficult to ensure sufficient hardening ability. If the amount of added iron exceeds 0.8%, the effect of improving hardening ability is small compared to the amount of added iron, and coarse iron carbides are formed. This promotes the formation of voids and reduces the ability to absorb impact energy, so the upper limit is set at 0.8%. Therefore, the Cr content is preferably in the range of 0.01 to 0.8%. The lower limit of the Cr content is preferably 0.015%, more preferably 0.02%. It is more preferable that the upper limit of the Cr content is 0.75%. It is more preferably 0.7%.

[0032] Molybdenum (Mo): 0.5% or less (excluding 0%) Molybdenum (Mo) has the effect of improving the hardening ability of steel, just like Cr and Mn. However, the formation of fine precipitates leads to finer grains, which improves bendability. However, if the Mo content exceeds 0.5%, the cost of the ferroalloy increases excessively compared to the effect. Therefore, the upper limit of Mo can be set at 0.5%. The Mo content is preferably in the range of 0.5% or less (excluding 0%). More preferably, it is 0.4% or less, and even more preferably, it is 0.35% or less. Most preferably, it is:

[0033] Nitrogen (N): 0.02% or less The above-mentioned N is contained in steel as an impurity. If the N content exceeds 0.02%, the above-mentioned Al As in the case of GaN, there is a problem that slab cracks are likely to occur due to the formation of AlN. On the other hand, the above N is an impurity, and the lower limit is not particularly limited, but the N content To control the amount of Cr to less than 0.001%, high manufacturing costs may be required. It can be limited to no less than .001%.

[0034] Meanwhile, the hot forming steel material according to one aspect of the present invention selectively contains Ni in addition to the above-mentioned alloy components. : 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less It may further include one or more.

[0035] Nickel (Ni): 0.5% or less Nickel (Ni) is an austenite stabilizing element. The addition of Ni increases the hardness of steel. However, Ni is an expensive alloying element, and its hardening effect is not improved. Considering the increase in manufacturing costs for the above, it is preferable to set the upper limit at 0.5%. Therefore, the Ni content is preferably in the range of 0.5% or less. In order to fully obtain the hardening effect of the addition, the lower limit of the Ni content is 0.01%. More preferably, it is 0.03%, and most preferably, it is 0.05%. The upper limit of the Ni content is preferably 0.45%, more preferably 0.4%. It is more preferable that the content is 0.5% and most preferable that the content is 0.35%.

[0036] Niobium (Nb): 0.1% or less Niobium (Nb) is an element that provides a precipitation strengthening effect by forming fine precipitates. This has the effect of improving the strength and bending properties by refining the crystal grains. In addition, during heating for hot forming, excessive grain growth is suppressed to prevent fluctuations in heat treatment conditions. However, if the Nb content exceeds 0.1%, this effect is reduced. Not only does it saturate, but the increase in precipitation temperature also increases the number of relatively coarse precipitates, which is expensive compared to the cost. Therefore, the Nb content is set to 0.1% or less. It is preferable that the lower limit of the Nb content is 0.005%. More preferably, it is 0.01%, and most preferably, it is 0.015%. The upper limit of the Nb content is more preferably 0.09%, and even more preferably 0.08%. It is preferably 0.07%, and most preferably 0.07%.

[0037] Titanium (Ti): 0.1% or less Titanium (Ti) combines with nitrogen remaining as an impurity in steel to form TiN. It is an element that is sometimes added together with B to ensure hardening ability. In addition, the formation of TiC precipitates is expected to have the effects of precipitation strengthening and grain refinement. If the Ti content exceeds 0.1%, a large amount of coarse TiN is formed, and the impact energy is increased. The upper limit can be limited to 0.1% because it reduces the absorption capacity. The Ti content is preferably in the range of 0.1% or less. The lower limit of the Ti content is 0.0 0.05% is more preferable, 0.01% is even more preferable, and 0.015% is even more preferable. The upper limit of the Ti content is preferably 0.08%. It is preferably at most 0.06%, and more preferably at most 0.05%.

[0038] Boron (B): 0.01% or less Boron (B) not only improves hardening ability even with a small amount of addition, but also The grain boundary segregation of P and / or S effectively suppresses the embrittlement of hot-formed components. However, if its content exceeds 0.01%, F e 23 The formation of CB6 complex compounds causes embrittlement during hot rolling, so the upper limit Therefore, the above B content is limited to 0.01% or less. It is preferable that the lower limit of the B content is 0.0001%. It is preferably 0.0003%, more preferably 0.0005%. The upper limit of the B content is preferably 0.009%, more preferably 0.007%. It is more preferable that the content is 0.005%.

[0039] In addition to the above-mentioned steel composition, the remainder may include Fe and inevitable impurities. These substances can be unintentionally mixed in during normal steel manufacturing processes, and it is essential to completely eliminate them. Any ordinary steel manufacturing engineer can easily understand its meaning. In addition, the present invention does not completely exclude the addition of any other composition than the above-mentioned steel composition. It is not something to be removed.

[0040] The hot forming steel material according to one aspect of the present invention satisfies the above-mentioned composition system and is suitable for hot forming. In order to maximize the impact energy absorption capacity of the component and minimize the hardness deviation, the following relationship is satisfied: It is preferable that the alloy factor represented by formula 1 is controlled to 7 or more. If the alloy index is less than 7, it is difficult to ensure sufficient hardening ability, and the hot forming part may become In addition, the grain boundaries in the surface layer of the hot-formed parts may be large. Ferrite may be formed, and the impact energy absorption capacity may be significantly reduced. More preferably, it is 7.5 or more, and even more preferably, it is 8 or more. The effect of the present invention can be obtained simply by having the alloy index of 7 or more. However, for example, from the viewpoint of reducing manufacturing costs, The alloy index may be 40 or less, and more preferably 30 or less. The above formula 1 is based on the results of the experiments conducted by the inventors in which the contents of the main alloying elements were changed based on the same carbon content. Testing the final hardness change by cooling rate after heating to the austenite region using several ferroalloys This is an equation derived through linear regression analysis of the effect of each element on hardenability.

[0041] [Equation 1] Alloy index=I(Mn)×I(Si)×I(Cr)×I(Mo) (Note that in the above Relational Formula 1, the I value for each component is I(Mn) = 3.34 × Mn + 1, I(Si)=0.7×Si+1, I(Cr)=2.16×Cr+1, I(Mo)=3 × Mo+1, and the content of each component is in weight percent.)

[0042] For hot forming steel used for TWB, energy absorption capacity is one of the important properties. The energy absorption capacity is affected by the strength and bending properties. The higher the bending properties, the better the energy absorption capacity. After hot forming, the factor that has the greatest effect on strength is the martensite fraction, especially In particular, when martensite is the main structure, it is greatly affected by the carbon content. Although the bending properties vary somewhat depending on the structure, a single phase usually provides excellent bending properties. In the case of a structure consisting of two or more phases, the smaller the difference in strength between the phases, the better the properties. Considering these characteristics, the strength is low in the region of carbon content less than 0.06%. In addition, it is difficult to avoid the formation of soft ferrite during cooling, and the interphase strength of the final structure is rather As the content increases, the bending properties relative to strength deteriorate, and the impact energy absorption capacity decreases. In the region where the C content exceeds 0.1%, a high-strength martensite single phase structure is easily obtained. However, the increased strength reduces the bending properties, and ultimately reduces the ability to absorb impact energy. Even if the C content satisfies the range of the present invention, if the alloy index value is less than 7, In this case, the formation of soft phases such as ferrite during cooling is difficult to avoid due to insufficient hardening ability, and bending is difficult. The bending characteristics are reduced, and therefore the collision energy absorption capacity is reduced. In order to ensure a good level of collision energy absorption, both strength and bending properties must be taken into consideration. Must.

[0043] The hot forming steel material according to one aspect of the present invention has 10 carbides having a circle equivalent diameter of 0.5 μm or more. 5 pieces / mm 2 The present inventors have found that excellent impact strength can be obtained in hot-formed parts. In order to maximize the impact energy absorption capacity, it is important to ensure that the strength and flexibility of the material is adequate. It was concluded that it is important to measure the number density of coarse carbides from steel plates through various experiments. It was confirmed that bending properties could be ensured by controlling the equivalent circle diameter to 0. Coarse carbides measuring 0.5 μm or more are present in the 5 pieces / mm 2 Above this, the heating for hot forming is Even if some of the iron carbides are redissolved in the steel, the other part remains in the steel after hot forming. The remaining coarse carbides, which were not completely dissolved, act as the initiation points for cracks during bending deformation. This causes a decrease in bending ability, ultimately resulting in a decrease in the ability to absorb impact energy. The lower the number density of such coarse carbides, the more advantageous it is. There is no particular restriction on the limit value.

[0044] The hot forming steel material according to one aspect of the present invention has ferrite: 50 to 90 area % and pearlite. % or less, bainite: 20% or less, and martensite: 20% or less The ferrite may be a soft phase, and may include one or more of the following: It is an effective structure for reducing the load in the blanking process of steel plates, and in order to obtain the above effect, It is preferable to secure 50% or more of the area. However, if it exceeds 90%, During the fabrication of the alloy, carbon is distributed excessively in structures other than ferrite, and the carbon is unevenly distributed even after hot forming. Therefore, the ferrite may be distributed in the range of 50 to 90 area %. When the above-mentioned pearlite content exceeds 30% by area, the cement is preferably mixed with the mixture after hot forming. This may result in incomplete melting of the tite, reducing strength, or causing non-uniformity in the material. When the above bainite and martensite each exceed 20% by area, the strength of the steel plate becomes too high. This can cause problems such as die wear during blank production.

[0045] The hot forming steel material according to one aspect of the present invention has an aluminum-based plating layer on at least one surface. In the present invention, the aluminum-based plating layer is not particularly limited. However, as a non-limiting embodiment, the composition may be, in weight percent, 6 to 12% Si, 1 to 4% Fe, and the balance. It may contain Al and inevitable impurities.

[0046] Hereinafter, a hot-formed part according to one aspect of the present invention will be described in detail. The hot-formed parts according to the surface can be manufactured by hot press forming the above-mentioned hot forming steel material. According to one aspect of the present invention, a hot-formed part has the above-mentioned alloy composition and alloy index. It is preferable that the number of carbides having a circle equivalent diameter of 0.5 μm or more is 10. 4 pieces / mm 2 Below It is preferable that the carbides present in the steel sheet before hot forming are removed during the heating stage for hot forming. Since the carbides dissolve in the steel during hot forming, the number density and size of the carbides are reduced compared to the state of the steel sheet before hot forming. However, large carbides exceeding a certain size are completely removed during normal heating. Therefore, the hot-formed parts are not melted and remain in the hot-formed parts. The number of carbides with an equivalent diameter of 0.5 μm or more is 10 4 pieces / mm 2 Above this, the coarse Carbide acts as a starting point for cracks during bending deformation, reducing the impact energy absorption capacity. There is a possibility that this could happen.

[0047] The hot-formed member according to one aspect of the present invention has a microstructure having a martensite single phase structure or a martensite single phase structure. The above-mentioned alloy may have a mixed structure containing annealed carbide and 40% by area or less of bainite. Tensite is an effective structure for ensuring strength, which is the target of the present invention, and therefore the fine structure of the present invention is The structure may be a single martensite phase, whereas bainite is more strongly related to martensite. Although it is a low-strength structure, when it is formed in a martensite matrix, it significantly reduces the bendability. Since this structure is advantageous for ensuring strength without causing the above martensite, in the present invention, Both may have a mixed structure containing 40% by area or less of bainite. If the fraction of porosity is less than 40% by area, it may be difficult to achieve the strength targeted by the present invention. There is.

[0048] On the other hand, the above microstructure is composed of 10% by area or less of ferrite and 5% by area or less of retained austenite. The ferrite and the residual austenite may further include one or more of the above. The above ferrite structure is unavoidably contained in the steel during the manufacturing process. If it exceeds this limit, not only will the strength decrease, but the bending properties may also be significantly deteriorated. If the residual austenite structure exceeds 5% by area, the strength decreases and the steel may deteriorate during hot forming. The inflow of hydrogen from the atmospheric gas increases, which may increase the possibility of hydrogen embrittlement occurring.

[0049] The hot-formed part according to one aspect of the present invention is manufactured according to the VDA standard (VDA238-100). When a three-point bending test was performed, the load-displacement curve obtained from the three-point bending test was The area required to reach the crack initiation energy (CIE) is 25 FIG. 1 shows the structure for evaluating the collision energy absorption capacity in the present invention. The concept of CIE (Crack initiation Energy), which is the standard used for crack initiation, is simplified. The figure shows the results.

[0050] According to one aspect of the present invention, there is provided a hot-formed part, the hardness of which is measured at any point on the hot-formed part. The difference between the maximum and minimum values ​​is taken as the hardness deviation, and the hardness deviation is taken as the average hardness of the hot-formed part. When the value obtained by dividing the difference by 1 is taken as the deviation level, the deviation level may be 0.3 or less. In the present invention, the number of the arbitrary points is not particularly limited. For example, the number of arbitrary points is set to 9 points. The hardness can be measured.

[0051] The hot-formed member according to one aspect of the present invention has a yield strength (YS) of 800 MPa or more and a tensile strength The thermal expansion coefficient (TS) may be 1000 MPa or more, and the elongation coefficient (El) may be 5% or more.

[0052] Hereinafter, a method for producing a steel material for hot forming according to one aspect of the present invention will be described in detail. However, the manufacturing method described below is merely one embodiment among all possible embodiments. The hot forming steel material of the present invention should be manufactured only by the following manufacturing method. This does not mean that.

[0053] First, a steel slab satisfying the above-mentioned alloy composition and alloy index is heated at 1050 to 1300°C. If the heating temperature of the above steel slab is less than 1050℃, the structure of the slab is not homogenized easily. However, when using precipitated elements, it may be difficult to redissolve them. Above 300℃, excessive oxide layer may be formed, which may induce surface defects after hot rolling. Therefore, the heating temperature of the steel slab is set to 1050 to 1300°C. The lower limit of the steel slab heating temperature is preferably 1070°C. The upper limit of the heating temperature of the steel slab is 1280° C. It is more preferable that the temperature is 1250° C., and even more preferable that the temperature is 1250° C.

[0054] The heated steel slab is then finish hot-rolled at 800 to 950°C to obtain hot-rolled steel sheets. If the above finish hot rolling temperature is less than 800°C, the surface layer of the steel sheet may be deteriorated by rolling in the two-phase region. A duplex structure may occur, making it difficult to control the plate shape. If the temperature exceeds 50°C, there is a problem that the crystal grains are likely to become coarse due to hot rolling. Therefore, the finish hot rolling temperature is preferably 800 to 950°C. The lower limit of the hot rolling temperature is preferably 810°C, and more preferably 820°C. The upper limit of the finish hot rolling temperature is more preferably 940°C, and more preferably 93 It is more preferably 0°C.

[0055] Then, the hot-rolled steel sheet is coiled at 500 to 700°C. If the steel sheet is too thin, martensite will form in the whole or part of the steel sheet, making it difficult to control the sheet shape. Moreover, the increase in strength of hot-rolled steel sheets reduces the rollability in the subsequent cold rolling process. On the other hand, if the coiling temperature exceeds 700°C, coarse carbides are formed. This may result in a decrease in the impact energy absorption capacity of the hot-formed part. The coiling temperature is preferably 500 to 700° C. The lower limit of the coiling temperature is 520° C. The upper limit of the coiling temperature is preferably 6 A temperature of 80°C is more preferable, and a temperature of 650°C is even more preferable.

[0056] After that, the coiled hot-rolled steel sheet is cooled from the coiling temperature to 400°C at a rate of 10°C / Hr or more. If the cooling rate is less than 10℃ / hr, it takes time for carbides to grow. This is a sufficient time to prevent the formation of many coarse carbides during the cooling of the hot rolled coil. Therefore, the cooling rate is preferably 10°C / Hr or more. The cooling rate is preferably 12° C. / Hr or more, more preferably 15° C. / Hr or more. On the other hand, in the present invention, the cooling rate is preferably 10° C. / Hr or more. The effects of the present invention can be obtained even if the cooling rate is only 100%. However, for example, from the viewpoint of reducing costs due to capital investment, the above cooling rate is It may be 500° C. / Hr or less, more preferably 45° C. / Hr or less, and even more preferably may be 400° C. / Hr or less.

[0057] After the coiling and cooling, and before cold rolling, the cooled hot-rolled steel sheet is pickled. The pickling step may further include removing the scale formed on the surface of the steel sheet. This can improve the quality of the product surface.

[0058] Thereafter, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio is not particularly limited, but for example, it is set to 30 to 80 in order to obtain the target thickness of the steel material. A reduction of 0% may be applied.

[0059] According to one aspect of the present invention, the cold rolled steel sheet is subjected to continuous annealing and aluminum-based plating. The cooled hot-rolled steel sheet may be subjected to pickling and immediately thereafter to an aluminum-based plating. may be applied.

[0060] Thereafter, in order to anneal the cold-rolled steel sheet, the cold-rolled steel sheet is heated from 400°C to the annealing temperature. It is preferable to heat the temperature range from 400°C to the annealing temperature at a rate of 20°C / s or less. If the heating rate exceeds 20°C / s, the carbides precipitated during the hot rolling stage do not have enough time to redissolve. However, coarse carbides may remain, and the collision energy of the final hot-formed part may be Therefore, the above heating from 400°C to the annealing temperature may decrease the energy absorption capacity. The heating rate is preferably 20°C / s or less. The heating rate is preferably 18°C / s or less. More preferably, the rate is 15° C. / s or less, and even more preferably, 15° C. / s or less. The effect of the present invention can be obtained even if the heating rate is only 20°C / s or less. The lower limit of the heating rate is not particularly limited. However, for example, in consideration of annealing productivity, The heating rate may be 0.5° C. / s or more, more preferably 1° C. / s or more, and even more preferably 1° C. / s or more. On the other hand, in the present invention, the rolling rate is preferably 4°C / s or more from the cold rolling temperature. In the temperature range below 00°C, the heating rate is not particularly limited, and this is because the heating rate is controlled This is because even if it is added, the effect on the re-dissolution of carbides is small.

[0061] The heated cold-rolled steel sheet is preferably annealed at a temperature of 740 to 860°C. If the temperature is less than 740°C, the recrystallization of the cold-rolled structure will not be sufficient, resulting in poor sheet shape. Or the strength after plating becomes too high, which may cause die wear during the blanking process. On the other hand, if the annealing temperature exceeds 860°C, Si, Mn, etc. may be added during the annealing process. This can cause problems such as surface oxide formation, resulting in poor plating surface. Therefore, the annealing temperature is preferably 740 to 860° C. The lower limit of the annealing temperature is The annealing temperature is more preferably 750°C, and further preferably 760°C. The upper limit of the temperature is more preferably 850°C, and further preferably 840°C.

[0062] On the other hand, the atmosphere during the continuous annealing is preferably a non-oxidizing atmosphere. A hydrogen-nitrogen mixed gas can be used. In this case, the dew point temperature (Dew point The dew point temperature may be -70 to -30°C. In order for the dew point temperature to be less than -70°C, However, since additional equipment for control is required, there is a problem that the manufacturing cost increases. If the dew point exceeds -30°C, excessive annealing oxides will form on the surface of the steel sheet during annealing, resulting in the unannealed steel sheet. Therefore, the atmospheric gas during the continuous annealing process is The dew point temperature is preferably -70 to -30°C. The lower limit of the dew point temperature of the gas is preferably -65°C, and more preferably -60°C. The upper limit of the dew point temperature of the atmospheric gas is more preferably −35° C., More preferably, it is 40°C.

[0063] Then, the annealed cold-rolled steel sheet is cooled from the annealing temperature to 660°C at a cooling rate of 1°C / s or more. The number density of carbides with a circular equivalent diameter of 0.5 μm or more, which is the target of the present invention, is 1 0 5 pieces / mm 2 In order to achieve this, the cooling rate from the annealing temperature to 660°C must be 1°C / s or faster. If the cooling rate is less than 1°C / s, a large amount of coarse carbides will be formed. As a result, the impact energy absorption capacity of the final hot-formed part may be reduced. Therefore, the cooling rate is preferably 1°C / s or more. It is more preferable that the rate is ℃ / s or more, and even more preferable that the rate is 2℃ / s or more. In the present invention, the effect to be obtained by the present invention can be obtained simply by the cooling rate being 1° C. / s or more. Therefore, there is no particular upper limit to the cooling rate. From the viewpoint of suppressing defects in shape, the cooling rate may be 50° C. / s or less, and more preferably may be 45° C. / s or less, more preferably 40° C. / s or less.

[0064] After the step of cooling the annealed cold-rolled steel sheet, the cooled cold-rolled steel sheet is coated with an Al-based plating. The present invention may further include a step of immersing the substrate in a plating bath to form an aluminum-based plating layer. In the present specification, the composition of the Al-based plating bath and the plating conditions are not particularly limited. As a restrictive embodiment, the composition of the plating bath is: Si: 6 to 12%, Fe: 1 to 4%, and the balance: The plating amount may include Al and other unavoidable impurities. Applicable single-sided standard: 30~130g / m 2 In the above plating bath composition, S If the i content is less than 6%, the plating bath temperature will rise excessively, causing equipment deterioration. If it exceeds 12%, the alloying process is excessively delayed, resulting in a shortened heating time for hot forming. If the Fe content is less than 1%, the plating density is low. If the content exceeds 4%, the plating bath may become dirty. This can lead to excessive loss and poor surface quality. Semi-30g / m 2 If the temperature is less than 100° C., it may be difficult to ensure the desired corrosion resistance of the hot-formed parts. 130g / m 2 If the thickness exceeds 100 mm, the production cost will increase due to the excessive plating weight. In addition, the amount of plating on the steel sheet can be uniform across the entire width and length of the coil. It may not be easy.

[0065] Hereinafter, a method for producing a hot-formed part according to one aspect of the present invention will be described in detail. However, the manufacturing method described below is merely one embodiment among all possible embodiments. The hot-formed member of the present invention must be manufactured only by the following manufacturing method. This does not mean that the

[0066] First, a hot forming steel material manufactured by the above-mentioned manufacturing method is prepared, and the hot forming steel material is The blank for hot forming is manufactured using the material. The blank is then molded using austenitic The alloy is heated to a temperature in the single-phase region or higher, more specifically, in a temperature range of from the Ac3 temperature to 980°C. If the blank heating temperature is lower than the Ac3 temperature, untransformed ferrite is present, It may be difficult to secure the required strength. On the other hand, if the heating temperature exceeds 980℃, Excessive oxides are generated on the surface of the component, which may make it difficult to ensure spot weldability. Therefore, the blank heating temperature is preferably Ac3 to 980° C. The lower limit of the blank heating temperature is more preferably Ac3+5°C, and more preferably Ac3+10°C. It is more preferable that the upper limit of the blank heating temperature is 970° C. It is preferable that the temperature is 960°C.

[0067] The heated blank is preferably held at the above temperature range for 1 to 1000 seconds. If the holding time is less than 1 second, the temperature will not be uniform throughout the blank, and the temperature will vary from part to part. If the holding time exceeds 1000 seconds, the heating temperature may become excessively high. As with the higher temperature, excessive oxides are generated on the surface of the component, which makes it difficult to ensure spot weldability. Therefore, the retention time is preferably 1 to 1000 seconds. The lower limit of the retention time is more preferably 30 seconds, and even more preferably 60 seconds. The upper limit of the retention time is preferably 900 seconds, and more preferably 800 seconds. is more preferred.

[0068] The heated and held blank is then hot-formed, cooled to room temperature, and finally In the present invention, the specific conditions for the hot forming are not particularly limited. The present invention is not limited to the above, and may be applied to any hot forming method commonly known in the art. can be applied. EXAMPLES

[0069] The present invention will be described in more detail below through examples. However, the following examples are not intended to be limiting of the present invention. The following description is provided to illustrate and embody the invention, and is not intended to limit the scope of the invention. It should be noted that the scope of the invention is not limited to the scope of the claims. This is because it is determined by the matters stated and those that can be reasonably inferred from them.

[0070] (Example) A 40 mm thick steel slab having the alloy composition shown in Table 1 below was produced by vacuum melting. The steel slab was heated to 1250°C and then hot-rolled to a finishing hot rolling temperature of 900°C. After that, the coiling temperature and the cooling temperature from the coiling temperature to 400°C were determined for each steel type. The speed was determined by applying the conditions in Table 2 below using a replica heat treatment furnace, and the final hot rolling thickness was The hot-rolled steel sheet was then pickled and cold rolled to a thickness of 3 mm. Cold rolling was performed at a reduction ratio of 50% to obtain cold-rolled steel sheets. The cold-rolled steel sheet was heated at a controlled heating rate from 400°C to the annealing temperature in a 5% hydrogen-9 After annealing in a 5% nitrogen atmosphere, the cooling rate was controlled from the annealing temperature to 660°C. After that, the cold-rolled steel sheet was cooled and then subjected to Al-based plating. The composition of the Al-based plating bath is Al-9%Si-2%Fe, with the remainder consisting of unavoidable impurities. The coating weight is 80g / m on one side. 2 The steel plate thus manufactured was called a blank. After the mold is made as shown in Fig. 2, it is hot-formed using a hot forming die. The heating temperature of the blank was 900° C. The holding time was 6 minutes, and the transport time from the heating furnace to molding was 10 seconds for all cases. The cold-rolled steel sheets manufactured as described above were found to have carbides with a circular equivalent diameter of 0.5 μm or more. The number of defects was measured, and the results are shown in Table 2 below. The number of carbides with a circular equivalent diameter of 0.5 μm or more, the microstructure, and mechanical properties of the material are measured. The results are shown in Tables 3 and 4 below.

[0071] The number of carbides with a circle equivalent diameter of 0.5 μm or more was determined by preparing a thin foil specimen. The measurements were made by observing 10 fields of view at 10,000x magnification using a transmission electron microscope (TEM). Ta.

[0072] The microstructure was measured by etching the surface of the steel sheet with Nital and then scanning electron microscopy. Measurements were made using a microscope.

[0073] Yield strength (YS), tensile strength (TS) and elongation (El) were measured using ASTM standard specimens. After sampling in a direction parallel to the rolling direction of the plate, a tensile test was carried out to measure the thickness.

[0074] Impact energy absorption capacity is measured by a three-point bending test in accordance with the VDA standard (VDA238-100). After this, the area (CI) of the load-displacement curve obtained from this to the maximum load is E: Crack initiation Energy) was measured and evaluated, and the crack initiation energy was measured at 15% of the normal level. If it is higher than 25,000 Nm, which is the CIE value for 0.00 MPa-class hot forming steel, it is considered "good"; If it was less than this, it was rated as "poor".

[0075] In addition, to check the uniformity of the physical properties of the hot-formed parts, the top, left and right sides of the formed parts were After taking three specimens from each area, the hardness was measured using a Vickers hardness tester. The average hardness, hardness deviation (the difference between the maximum and minimum hardness values ​​measured at nine points) and deviation level were measured. The hardness deviation / average hardness is measured, and the physical property uniformity is determined when the deviation level is 0.3 or less. If it was above this level, it was marked as "good" and if it was above this level, it was marked as "bad".

[0076] [Table 1]

[0077] [Table 2]

[0078] [Table 3]

[0079] [Table 4]

[0080] As can be seen from Tables 1 to 4 above, the alloy composition, alloy index and manufacturing conditions proposed by the present invention are In the case of Examples 1 to 7, which all satisfy the above requirements, the collision energy absorption capacity is good, and the physical properties within the member are It can be confirmed that the is uniform.

[0081] In the case of Comparative Examples 1 and 2, the alloy composition and alloy index are within the range of the present invention, but the winding The manufacturing conditions of the temperature, the cooling rate after coiling, and the heating rate during annealing are outside the range of the present invention. As a result, the number density of coarse carbides increases, and the collision energy is reduced in the final hot-formed part. It is clear that the energy absorption capacity is poor.

[0082] In Comparative Examples 3 to 5, the alloy composition of the steel sheet is satisfied, but the alloy index does not meet the conditions of the present invention. In the absence of hardening, excessive hardness deviation occurs due to insufficient hardening ability, resulting in uneven physical properties within the component. It is clear that the quality is poor.

[0083] Comparative Examples 6 to 8 are cases where the C content exceeds the conditions of the present invention, and the strength due to the increase in C content is However, compared to the increase in strength, the bendability is greatly reduced, and the impact energy absorption capacity is also reduced. is found to be rather inferior.

[0084] Comparative Example 9 is a case where the C content does not satisfy the conditions of the present invention, and the target strength is Not only was it not possible to secure the structure, but it was also found that this resulted in a decrease in the collision energy absorption capacity. .

[0085] FIG. 3 shows the relationship between the carbon content and the alloy index and the collision energy of Examples 1 to 7 and Comparative Examples 3 to 9. As shown in FIG. 3, the carbon content and the alloy index are indicative of the absorption capacity of the hot-formed parts. The C content and alloy index proposed by the present invention are directly related to the collision energy absorption capacity in the It can be seen that good collision energy absorption capacity can be ensured only when

Claims

1. The steel sheet contains, by weight, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 0.8%, Mo: 0.5% or less (excluding 0%), N: 0.02% or less, and the balance being Fe and unavoidable impurities, The alloy factor represented by the following relational expression 1 is 7 or more and 40 or less, 10 carbides with a circle equivalent diameter of 0.5 μm or more ５ pieces / mm ２ The following are hot forming steel materials. [Relationship 1] Alloy index = I (Mn) x I (Si) x I (Cr) x I (Mo) (In the above Relational Formula 1, the I value for each component is I(Mn) = 3.34 × Mn + 1, I(Si) = 0.7 × Si + 1, I(Cr) = 2.16 × Cr + 1, and I(Mo) = 3 × Mo + 1, and the content of each component is in wt %.)

2. The steel material for hot forming according to claim 1, further comprising one or more of Ni: 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, and B: 0.01% or less.

3. The steel material for hot forming as described in claim 1 has a microstructure including ferrite: 50 to 90 area %, and one or more of pearlite: 30 area % or less, bainite: 20 area % or less, and martensite: 20 area % or less.

4. The steel material for hot forming according to claim 1, wherein the steel material has an aluminum-based plating layer formed on at least one surface thereof.

5. 5. The steel material for hot forming according to claim 4, wherein the aluminum-based plating layer contains, by weight %, 6 to 12% of Si, 1 to 4% of Fe, the balance being Al and inevitable impurities.

6. The steel sheet contains, by weight, C: 0.06 to 0.1%, Si: 0.05 to 0.6%, Mn: 0.6 to 2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 0.8%, Mo: 0.5% or less (excluding 0%), N: 0.02% or less, and the balance being Fe and unavoidable impurities, The alloy factor represented by the following relational expression 1 is 7 or more and 40 or less, 10 carbides with a circle equivalent diameter of 0.5 μm or more ４ pieces / mm ２ The hot-formed member is as follows: [Relationship 1] Alloy index = I (Mn) x I (Si) x I (Cr) x I (Mo) (In the above Relational Formula 1, the I value for each component is I(Mn) = 3.34 × Mn + 1, I(Si) = 0.7 × Si + 1, I(Cr) = 2.16 × Cr + 1, and I(Mo) = 3 × Mo + 1, and the content of each component is in wt %.)

7. The hot-formed member described in claim 6, wherein the microstructure is a martensite single-phase structure or a mixed structure containing martensite and bainite at 40% by area or less.

8. A hot-formed member as described in claim 7, wherein the microstructure further includes one or more of ferrite of 10 area % or less and retained austenite of 5 area % or less.

9. When the member is subjected to a three-point bending test according to the VDA standard (VDA238-100), the area from the load-displacement curve obtained in the three-point bending test to the maximum load (CIE: Crack initiation Energy) is 25,000 Nm or more. The hot-formed member according to claim 6.

10. 7. The hot formed part of claim 6, wherein the part has a hardness deviation level of 0.3 or less. (Note that the hardness deviation level is the value obtained by dividing the hardness deviation by the average hardness value of the hot-formed part, and the hardness deviation means the difference between the maximum and minimum values ​​obtained by measuring the hardness at any point.)

11. 7. The hot-formed part according to claim 6, wherein the part has a yield strength (YS): 800 MPa or more, a tensile strength (TS): 1000 MPa or more, and an elongation (El): 5% or more.

12. heating a steel slab containing, by weight, C: 0.06-0.1%, Si: 0.05-0.6%, Mn: 0.6-2%, P: 0.05% or less, S: 0.02% or less, Al: 0.01-0.1%, Cr: 0.01-0.8%, Mo: 0.5% or less (excluding 0%), N: 0.02% or less, the balance being Fe and inevitable impurities, and having an alloy factor of 7 or more and 40 or less, represented by the following relational expression 1, at 1050-1300°C; Finish hot rolling the heated steel slab at 800 to 950°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 500 to 700°C; cooling the coiled hot-rolled steel sheet from a coiling temperature to 400°C at a cooling rate of 10°C / Hr or more; cold-rolling the cooled hot-rolled steel sheet to obtain a cold-rolled steel sheet; Heating the cold-rolled steel sheet at a rate of 20°C / s or less through a temperature range from 400°C to an annealing temperature; Annealing the heated cold-rolled steel sheet at 740 to 860°C; and cooling the annealed cold-rolled steel sheet from the annealing temperature to 660°C at a cooling rate of 1°C / s or more. [Relationship 1] Alloy index = I (Mn) x I (Si) x I (Cr) x I (Mo) (In the above Relational Formula 1, the I value for each component is I(Mn) = 3.34 × Mn + 1, I(Si) = 0.7 × Si + 1, I(Cr) = 2.16 × Cr + 1, and I(Mo) = 3 × Mo + 1, and the content of each component is in wt %.)

13. The method for producing a steel material for hot forming according to claim 12, further comprising pickling the cooled hot-rolled steel sheet before the cold rolling.

14. The method for producing a steel material for hot forming according to claim 12, wherein the reduction rate during the cold rolling is 30 to 80%.

15. The method for producing a steel material for hot forming according to claim 12, wherein the dew point temperature of the atmospheric gas during the annealing is −70 to −30° C.

16. 13. The method for producing a steel material for hot forming according to claim 12, further comprising, after the step of cooling the annealed cold-rolled steel sheet, immersing the cooled cold-rolled steel sheet in an Al-based plating bath to form an aluminum-based plating layer.

17. The method for producing a steel material for hot forming according to claim 16, wherein the Al-based plating bath contains, by weight%, 6 to 12% of Si, 1 to 4% of Fe, the balance being Al and inevitable impurities.

18. When forming the plating layer, the plating coating weight is 30 to 130 g / m on one side. ２ The method for producing a steel material for hot forming according to claim 16,

19. A step of obtaining a blank using a hot forming steel material manufactured according to any one of claims 12 to 18; Heating the blank to Ac3 to 980°C and then holding for 1 to 1000 seconds; and cooling the heated and held blank to room temperature after hot forming the blank.

20. The hot forming steel material according to claim 1, wherein the alloy factor is 7.5 or more and 40 or less.

21. The hot forming steel material according to claim 1, wherein the alloy factor is 7.5 or more and 30 or less.

22. The hot forming steel material according to claim 1, wherein the alloy factor is 8 or more and 40 or less.

23. The hot forming steel material according to claim 1, wherein the alloy factor is 8 or more and 30 or less.

24. The hot-formed member of claim 6, wherein the alloy factor is greater than or equal to 7.5 and less than or equal to 40.

25. The hot-formed member of claim 6, wherein the alloy factor is greater than or equal to 7.5 and less than or equal to 30.

26. The hot-formed member of claim 6, wherein the alloy factor is greater than or equal to 8 and less than or equal to 40.

27. The hot-formed member of claim 6, wherein the alloy factor is greater than or equal to 8 and less than or equal to 30.

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