Hot-forming steel materials, hot-forming members, and methods for manufacturing the same.

Abstract

The present invention relates to a hot-forming steel material used in automobiles and the like, a hot-formed member, and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to hot-forming steel materials, hot-forming members, and methods for manufacturing them, used in automobiles and the like. [Background technology]

[0002] Recently, efforts have been made to improve fuel efficiency by reducing the weight of automobiles. Reducing the thickness of steel materials is an effective way to achieve this, but reducing thickness can cause problems with the stability of the automobile, so the strength of the steel must also be increased. For this reason, there is a continuous demand for high-strength steel plates, and various types of steel materials are being developed. However, because these steel materials have high strength, they have the problem of being difficult to process.

[0003] To solve these problems, a hot forming method called hot forming or hot press forming (HPF) has been proposed. Hot forming is a method in which steel materials are processed at a high temperature where processing is easy, and then rapidly cooled to a low temperature, thereby forming a low-temperature structure such as martensite within the steel material and increasing the strength of the final product. By using this method, the problems of processability when manufacturing high-strength components can be minimized.

[0004] Patent Document 1 is an example of a technology related to such hot forming. Patent Document 1 proposes a technology that ensures ultra-high strength of 1600 MPa or more by heating an Al-Si plated steel sheet to 850°C or higher, then hot forming by pressing and rapid cooling to form the structure of the member as martensite. With the technology proposed in Patent Document 1, complex shapes can be easily formed because the forming is done at high temperatures, and a weight reduction effect due to increased strength from rapid cooling in the mold can be expected.

[0005] On the other hand, hot-formed components used for passenger protection require excellent durability, and a key indicator is superior fatigue properties. For example, in the case of a B-pillar in an automobile, repeated stress and deformation cycles over long periods can cause fracture at stresses far lower than the yield strength or tensile strength. Therefore, the component must possess fatigue properties that allow it to withstand stresses without fracture for a certain number of cycles or more.

[0006] Fatigue properties, the main factor determining the durability of a material, have traditionally been considered a requirement for steels used primarily in bearings and vehicle chassis. However, with the recent increase in the strength of hot-formed steel, the importance of fatigue properties is constantly growing in hot-formed steel materials or their components used in automotive pillars.

[0007] Various methods have been proposed to improve the fatigue characteristics described above. For example, Patent Document 2 presents a method to improve the durability of a component by ensuring surface hardness through heat treatment after surface carburizing and nitriding treatment of the product. Patent Document 3 proposes a method to improve the fatigue life of a material by applying shot peening treatment to the surface of the product to form compressive residual stress on the surface.

[0008] However, the above-mentioned methods are applied after the product has been molded, and they have limitations in that they cannot be applied if plating or other processes have been performed, as they may impair the surface quality. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] U.S. Registered Patent No. 6296805 [Patent Document 2] Korean Registered Patent No. 10-1129370 [Patent Document 3] Korean Registered Patent No. 10-0373280

Summary of the Invention

Problems to be Solved by the Invention

[0010] The problem of the present invention is to provide a hot-forming steel material in which a hot-forming member has high strength, excellent surface quality and fatigue characteristics, a hot-forming member manufactured using the same, and methods for manufacturing them.

[0011] The problems of the present invention are not limited to the above matters. Further problems of the present invention are described in the overall content of the specification, and those having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding further problems of the present invention from the content described in the specification of the present invention.

Means for Solving the Problems

[0012] One embodiment of the present invention includes a base steel plate and a plating layer formed on the base steel plate. The base steel plate contains, in weight %, C: 0.04 to 0.45%, Si: 1.5% or less (excluding 0%), Mn: 0.2 to 2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 5.0%, N: 0.02% or less, and the balance Fe and unavoidable impurities, and relates to a hot-forming steel material that satisfies the following [Relational Expression 1].

[0013] [Relational Expression 1] Carbon enrichment index before hot forming: C (peak、HPF前) / C (nom、HPF前) ≧1.5 (In Relational Expression 1, C (peak、HPF前) is the highest carbon value of the first carbon peak (Peak) in the carbon profile in the direction of the base steel plate from the 1 / 3 point of the thickness of the plating layer on the surface, as determined by GDS analysis, and C (nom、HPF前) is the nominal carbon value of the steel.)

[0014] Another embodiment of the present invention involves obtaining a plated steel sheet using a steel slab containing, by weight %, C: 0.04~0.45%, Si: 1.5% or less (excluding 0%), Mn: 0.2~2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01~0.1%, Cr: 0.01~5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities. This invention relates to a method for manufacturing steel materials for hot forming, which includes the step of box annealing the above-mentioned plated steel sheet in a one-step method as described in [Box Annealing Condition 1] below.

[0015] [Box annealing condition 1] Temperature range (T1): 500~800℃ Retention time (t1): 1 minute or more (this is the retention time at the target temperature) Heating rate (H1): 20-160°C / hr (This is the rate at which the temperature rises to the target temperature.)

[0016] A further embodiment of the present invention is a step of obtaining a plated steel sheet using a steel slab containing, by weight %, C: 0.04~0.45%, Si: 1.5% or less (excluding 0%), Mn: 0.2~2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01~0.1%, Cr: 0.01~5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities, The present invention relates to a method for manufacturing steel materials for hot forming, which includes the step of box annealing the above-mentioned plated steel sheet in a two-stage method as described in [Box Annealing Condition 2] below.

[0017] [Box annealing condition 2] Temperature range for one interval (T2-1): 500~780℃ Holding time for one interval (t2-1): 1 minute or more (this is the holding time at the target temperature). Heating rate per section (H2-1): 20-160°C / hr (This is the heating rate to reach the target temperature) Temperature range for two sections (T2-2): 600~800℃ Holding time for the two sections (t2-2): 50 minutes or more (this is the holding time at the target temperature). Heating rate in section 2 (H2-2): 0.25~160°C / hr (This is the heating rate to reach the target temperature)

[0018] A further embodiment of the present invention comprises a base material containing, by weight %, C: 0.04-0.45%, Si: 1.5% or less (excluding 0%), Mn: 0.2-2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01-0.1%, Cr: 0.01-5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities, and a plating layer formed on the base material. This relates to a hot-formed member that satisfies the conditions of [Relational Equation 2] below.

[0019] [Relationship 2] C (peak、HPF後) / C (nom、HPF後) ≥0.1 (In relational equation 2, C (peak、HPF後) This is the highest carbon value of the first carbon peak (Peak) that appears in the carbon profile, as determined by GDS analysis results, starting from 1 / 3 of the plating layer thickness on the surface in the direction of the base steel sheet, and C (nom、HPF後) This is the nominal carbon value of steel.

[0020] Another embodiment of the present invention is the step of obtaining a blank using the hot-forming steel material described above, The above blank is heated to a temperature of Ac3 to 980°C, and then held for 1 to 1000 seconds. The present invention relates to a method for manufacturing a hot-formed member, which includes the step of hot-forming the heated and held blank described above, and then cooling it. [Effects of the Invention]

[0021] According to the present invention, it is possible to manufacture hot-formed members that have high strength after hot forming, as well as excellent surface quality and fatigue characteristics, and are highly durable. The present invention provides hot-forming steel materials, hot-formed members manufactured therefrom, and methods for manufacturing them.

[0022] The diverse and beneficial advantages and effects of the present invention are not limited to those described above and can be more easily understood in the process of describing specific embodiments of the present invention. [Brief explanation of the drawing]

[0023] [Figure 1] This graph shows the cross-sectional structure of the steel (a), the GDS (Glow Discharge Spectrometer) analysis results performed to measure the Cpeak value before and after hot forming, and the carbon profile (b). [Figure 2] (a) and (b) are TEM images showing the cross-sections of Invention Example 1 and Comparative Example 1 after box annealing heat treatment, respectively. [Figure 3] (a) and (b) are optical microscope images of the surfaces of Invention Example 2 and Comparative Example 7 after box annealing heat treatment, respectively. [Modes for carrying out the invention]

[0024] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Furthermore, the singular forms used herein also include plural forms unless the relevant definition clearly indicates otherwise.

[0025] As used herein, "includes" means to specify a configuration and does not exclude the existence or addition of other configurations.

[0026] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as those generally understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries shall be interpreted as having the meaning consistent with the relevant technical documents and the content currently disclosed.

[0027] First, one embodiment of the hot-forming steel material of the present invention will be described in detail. The steel material of the present invention may contain, by weight %, C: 0.04~0.45%, Si: 1.5% or less (excluding 0%), Mn: 0.2~2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01~0.1%, Cr: 0.01~5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities. The composition of each alloy will be described in detail below, where % means weight %.

[0028] Carbon (C): 0.04~0.45% The above-mentioned carbon (C) is an essential element added to improve the strength of the material. If the C content is less than 0.04%, it is difficult to ensure sufficient strength, and even if the flexibility is ultimately high, the collision energy absorption capacity will actually decrease. Therefore, it is effective to add 0.04% or more. On the other hand, if the C content exceeds 0.45%, although the strength increases, the flexibility decreases and the collision energy absorption capacity decreases. Therefore, it is effective to keep it below 0.45%.

[0029] Silicon (Si): 1.5% or less (excluding 0%) The above-mentioned Si should be added as a deoxidizing agent in steelmaking. Furthermore, it is a solid solution strengthening element and a carbide formation suppressing element, contributing to increased strength of hot-formed members and being an effective element for material uniformity. If its content exceeds 1.5%, the plating properties may decrease due to Si oxides formed on the surface of the steel sheet during annealing. Therefore, it is effective for the above-mentioned Si to be included at a concentration of 1.5% or less (excluding 0%).

[0030] Manganese (Mn): 0.2-2.5% The above-mentioned Mn is necessary not only to ensure a solid solution strengthening effect, but also to suppress ferrite formation during hot forming by improving hardening ability. If the Mn content is less than 0.2%, there are limitations to obtaining the above effect, and it may become necessary to excessively require other expensive alloying elements to improve the insufficient hardening ability, potentially leading to a significant increase in manufacturing costs. On the other hand, if the Mn content exceeds 2.5%, the cold rolling properties may decrease due to the increased strength of the steel sheet before the hot forming process, and the band structure aligned in the rolling direction of the microstructure phase may become more pronounced, potentially resulting in poor collision energy absorption. Therefore, an effective Mn content is between 0.2% and 2.5%.

[0031] Phosphorus (P): 0.05% or less The above-mentioned P exists as an impurity in steel, and if its content exceeds 0.05%, it can significantly weaken the weldability of hot-formed members. On the other hand, the above-mentioned P is an unavoidable impurity during steel manufacturing, and there is no particular limit to its lower limit. However, controlling the P content to less than 0.001% may require high manufacturing costs, so it may be 0.001% or higher.

[0032] Sulfur (S): 0.02% or less The above-mentioned S is an impurity in steel that inhibits the ductility, impact properties, and weldability of hot-formed members, and therefore it is effective to limit its content to a maximum of 0.02%. On the other hand, the above-mentioned S is an unavoidable impurity, and there is no particular need to limit its lower limit, however, controlling it to less than 0.0001% may require high manufacturing costs, so it may be 0.0001% or more.

[0033] Aluminum (Al): 0.01-0.1% The above-mentioned Al, along with Si, is an element that deoxidizes in steelmaking and improves the cleanliness of the steel. If the Al content is less than 0.01%, the above effect is difficult to obtain, and if the content exceeds 0.1%, there is a problem that the high-temperature ductility decreases due to the excess AlN formed during the continuous casting process, causing slab cracks. Therefore, an Al content of 0.01 to 0.1% is effective.

[0034] Chromium (Cr): 0.01-5.0% The above-mentioned Cr is added, like Mn, to ensure the hardening ability of the steel and to ensure a beautiful surface during the HPF process. If the Cr content is less than 0.01%, it may be difficult to ensure sufficient hardening ability. On the other hand, if the content exceeds 5.0%, the effect of improving hardening ability is small compared to the amount added, and it may promote the formation of coarse Cr-based carbides, which may reduce the impact energy absorption ability. Therefore, it is effective to keep the content below 5.0%.

[0035] Nitrogen (N): 0.02% or less The above-mentioned N is present in the steel as an impurity. If the above-mentioned N content exceeds 0.02%, a problem arises where slab cracks are more likely to occur due to the formation of AlN, similar to the case of Al mentioned above. Although the above-mentioned N is an impurity and there is no particular limit on its lower limit, controlling the N content to less than 0.001% may require high manufacturing costs, so it may be 0.001% or higher.

[0036] On the other hand, the above-mentioned steel material may further contain one or more of the following in addition to the alloy components described above: Mo: 0.5% or less, Ni: 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, and B: 0.01% or less.

[0037] Molybdenum (Mo): 0.5% or less The above-mentioned Mo not only improves the hardening ability of steel, like Cr and Mn, but also provides effects such as increased bendability due to grain refinement associated with the formation of fine precipitates. However, if the above-mentioned Mo content exceeds 0.5%, it leads to an excessive increase in the cost of ferroalloys compared to the effect, so it is effective to keep the content below 0.5%. The above-mentioned Mo content is more effective at 0.45% or less, even more effective at 0.4% or less, and even more effective at 0.35% or less.

[0038] Nickel (Ni): 0.5% or less The above-mentioned Ni is an austenite-stabilizing element, and its addition can improve the hardening ability of steel. However, since Ni is an expensive alloying element, considering the increase in manufacturing costs relative to the hardening ability improvement effect, it is effective to set the upper limit at 0.5%. On the other hand, in order to obtain a sufficient hardening ability improvement effect from the addition of Ni, it is effective to include at least 0.01%, more effective at 0.03%, and even more effective at 0.05%. The upper limit for Ni is more effective at 0.45%, even more effective at 0.4%, and most effective at 0.35%.

[0039] Niobium (Nb): 0.1% or less The above Nb is an element capable of obtaining a precipitation strengthening effect by forming fine precipitates, thereby obtaining an effect of improving the bending property due to strength increase and grain refinement. Furthermore, during heating for hot forming, excessive grain growth can be suppressed, and robustness against fluctuations in heat treatment conditions can be achieved. However, when the Nb content exceeds 0.1%, not only does its effect saturate, but relatively coarse precipitates increase due to an increase in the precipitation temperature, and there is a possibility that the efficiency decreases compared to the cost. Therefore, it is effective that the Nb content is 0.1% or less. The lower limit of the Nb content is effectively 0.005%, more effectively 0.01%, and even more effectively 0.015%. The upper limit of the Nb content is more effectively 0.09%, even more effectively 0.08%, and most effectively 0.07%.

[0040] Titanium (Ti): 0.1% or less The above Ti is an element that may also be added in combination when adding B to ensure hardenability by combining with nitrogen remaining as an impurity in the steel to form TiN. Also, precipitation strengthening and grain refinement effects can be expected due to the formation of TiC precipitates. However, when the Ti content exceeds 0.1%, rather a large amount of coarse TiN is formed, deteriorating the impact energy absorption ability. Therefore, it is effective that the upper limit is 0.1%. The lower limit of the above Ti is effectively 0.005%, more effectively 0.01%, and even more effectively 0.015%. The upper limit of the above Ti is more effectively 0.08%, even more effectively 0.06%, and most effectively 0.05%.

[0041] Boron (B): 0.01% or less The above B is an element that can not only improve the hardenability even with a small addition, but also effectively suppress the brittleness of the hot formed member due to grain boundary segregation of P and / or S by segregating at the prior austenite grain boundaries. However, when its content exceeds 0.01%, Fe 23The formation of CB6 composite compounds causes brittleness during hot rolling, so an upper limit of 0.01% is effective. On the other hand, an effective lower limit for the B content is 0.0001%, more effective is 0.0003%, and even more effective is 0.0005%. An upper limit for the B content is more effective is 0.009%, even more effective is 0.007%, and most effective is 0.005%.

[0042] The remainder contains iron (Fe), and it is impossible to eliminate these impurities because unintended contamination from raw materials or the surrounding environment is unavoidable during the normal manufacturing process. Since these impurities are easily recognizable to any ordinary technician in the manufacturing process, this specification will not specifically mention all of them.

[0043] The above-mentioned hot-forming steel material includes a plating layer on at least one surface. The type of the plating layer is not particularly limited, such as a zinc (Zn)-based plating layer or an aluminum (Al)-based plating layer, nor is the method of forming the plating layer particularly limited, such as hot-dip plating or electroplating. As a preferred example, an Al-based plating layer may be formed. The above-mentioned Al-based plating is not particularly limited, but as an example, the above-mentioned Al-based plating layer may be a plating layer formed after plating with a plating bath containing, by weight %, Si: 6-12%, Fe: 1-4%, with the remainder being Al and unavoidable impurities, and then alloyed by subsequent box annealing. In other words, it is difficult to consider the plating layer in the above-mentioned steel material as a pure plating layer plated by a plating bath, and it is preferable to consider it as a plating layer in which the base steel sheet and the plating layer have been alloyed.

[0044] The above-mentioned steel material for hot forming is effective if its surface carbon segregation factor before hot forming, as defined by [Relationship Formula 1] below, is 1.5 or higher.

[0045] [Relationship 1] C (peak、HPF前) / C (nom、HPF前)≥1.5 (In relational equation 1, C (peak、HPF前) This is the highest carbon value of the first carbon peak (Peak) that appears in the carbon profile, as determined by GDS analysis results, starting from 1 / 3 of the plating layer thickness on the surface in the direction of the base steel sheet, and C (nom、HPF前) This indicates the nominal carbon value of the steel, and usually represents the target carbon content within the steel.

[0046] The inventors of the present invention applied box annealing to hot-forming steel sheets, observed the surface properties after box annealing, and observed the surface properties and fatigue properties of the hot-formed members. As a result, they came to realize that by controlling the box annealing conditions and the surface properties of the steel sheets after box annealing to a constant level, it is possible not only to ensure the surface quality of the hot-formed members but also to improve the fatigue properties.

[0047] Specifically, in order to improve the fatigue characteristics for increasing the durability of the hot-formed member described above, it is effective for the surface carbon segregation factor of the hot-formed steel material, as defined in [Relationship Formula 1] above, to be 1.5 or higher.

[0048] The microstructure of the above-mentioned hot-forming steel material may contain 50-90 area% ferrite and one or more of the following: 30 area% or less pearlite, 20 area% or less bainite, and 20 area% or less martensite.

[0049] The ferrite mentioned above is a soft phase and is an effective microstructure for reducing the load during the blanking process of steel materials during blank production. For this purpose, it is effective when it accounts for 50% or more of the area. However, if it exceeds 90% of the area, carbon may be excessively distributed to microstructures other than ferrite during blank production, and carbon may be unevenly distributed even after hot forming. Therefore, it is effective when the ferrite accounts for 50-90% of the area.

[0050] If the amount of pearlite exceeds 30% of the surface area, the cementite may incompletely dissolve after hot forming, reducing strength or causing material inhomogeneity. On the other hand, if the amount of bainite or martensite exceeds 20% of the surface area, the strength of the steel sheet may increase excessively, potentially leading to problems such as mold wear during blank production.

[0051] On the other hand, the whiteness of the above-mentioned hot-forming steel material may be 60 or higher. If the heat treatment of box annealing for alloying (pre-alloying) of the plating layer of the hot-forming steel material is performed excessively, excessive surface oxides may be formed, resulting in poorer whiteness and potentially falling below 60. When surface quality deteriorates, problems such as roll contamination may occur during hot forming, so it is effective to use the box annealing conditions described later.

[0052] Next, one embodiment of the method for manufacturing hot-formable steel materials of the present invention will be described in detail. The manufacturing method described below is only one embodiment among all possible embodiments, and does not mean that the hot-formable steel materials should necessarily be manufactured only by the manufacturing method described below.

[0053] A plated steel sheet is manufactured using a steel slab that satisfies the alloy composition described above, and the resulting plated steel sheet is box-annealed.

[0054] Box annealing can be performed to ensure that the surface carbon concentration index of the steel material before hot forming is 1.5 or higher, as defined by [Relationship Formula 1] above, or that the surface carbon concentration index of the member after hot forming is 0.1 or higher, as defined by [Relationship Formula 2] described later. The above box annealing can be performed in one step or two steps, and the box annealing conditions will differ accordingly. These will be described in detail as [Box Annealing Condition 1] and [Box Annealing Condition 2], respectively.

[0055] [Box annealing condition 1] Temperature range (T1): 500~800℃ Retention time (t1): 1 minute or more (this is the retention time at the target temperature) Heating rate (H1): 20-160°C / hr (This is the rate at which the temperature rises to the target temperature.)

[0056] If the heating rate (H1) is less than 20°C / hr, the surface quality may deteriorate due to oxide formation on the surface layer caused by the prolonged heat treatment time. If the heating rate (H1) exceeds 160°C / hr, the surface quality may deteriorate due to melting of the plating layer caused by overheating of the edge due to temperature deviation in the width direction. If box annealing is performed at a temperature below 500°C within the target temperature range (T1), the surface quality and fatigue properties may deteriorate not only due to poor alloying of the plating layer but also due to poor surface carbon concentration index. On the other hand, if it exceeds 800°C, the surface quality may deteriorate due to excessive alloying. Furthermore, it is effective to hold the target temperature for 1 minute or more, and if the holding time is less than 1 minute, the entire plating layer may not be alloyed. However, if the holding time exceeds 100 hours, the surface quality may deteriorate due to excessive alloying.

[0057] [Box annealing condition 2] Temperature range for one interval (T2-1): 500~780℃ Holding time for one interval (t2-1): 1 minute or more (this is the holding time at the target temperature). Heating rate per section (H2-1): 20-160°C / hr (This is the heating rate to reach the target temperature) Temperature range for two sections (T2-2): 600~800℃ Holding time for the two sections (t2-2): 50 minutes or more (this is the holding time at the target temperature). Heating rate in section 2 (H2-2): 0.25~160°C / hr (This is the heating rate to reach the target temperature)

[0058] If the heating rate in section 1 is less than 20°C / hr, the heat treatment time may be excessive, potentially degrading the surface quality. If it exceeds 160°C / hr, melting may occur at the edges, potentially degrading the surface quality. If the heating rate in section 2 is less than 0.25°C / hr, excessive heat treatment may increase the surface oxide, potentially degrading the surface quality. If it exceeds 160°C / hr, melting may occur in the plating layer at the edges, potentially degrading the surface quality.

[0059] If the holding time at the target temperature in section 1 is less than 1 minute, it will be difficult to meet the surface carbon concentration index. However, there is no particular upper limit, but if the holding time exceeds 100 hours, there is a possibility that the surface quality will deteriorate due to excessive alloying, and the process cost may increase excessively. On the other hand, if the holding time at the target temperature in section 2 is less than 50 minutes, it will be difficult to meet the surface carbon concentration index. However, there is no particular upper limit, but if the holding time exceeds 100 hours, the process cost may increase excessively, and the surface oxide may increase, potentially degrading the surface quality.

[0060] On the other hand, it is effective that the target temperatures for sections 1 and 2 do not exceed 780°C and 800°C, respectively. If the target temperatures are exceeded, the surface quality may deteriorate, with the surface whiteness falling below 60 due to over-alloying, and the process cost may increase excessively due to the increased amount of heat input. On the other hand, if the target temperatures for sections 1 and 2 are below 500°C and 600°C, respectively, it is necessary to maintain heat treatment for more than 100 hours for complete alloying during box annealing, which may result in a deterioration of surface quality and an excessive increase in process cost due to the increased amount of heat input. The target temperature for section 2 is higher than the target temperature for section 1.

[0061] The purge gas in the above-mentioned box annealing furnace should effectively be one of the following: hydrogen (H2), nitrogen (N2), or a mixture thereof, and the purge volume should be 0.1 to 100 m³. 3 A rate of / hr is effective. The above purge amount is 0.1m 3If the rate is less than / hr, the atmosphere inside the box annealing furnace may not be controlled, potentially leading to a decrease in surface quality, and the purge amount should be 100m 3 If the rate exceeds [number] hours, the amount of heat required to maintain the temperature inside the furnace increases, which could lead to excessive manufacturing costs.

[0062] Furthermore, a circulation fan can be operated during box annealing, and in this case, it is effective for the circulation fan to operate at a rate of 10 rpm or more. If the circulation fan operates at a rate of less than 10 rpm, melting may occur due to temperature deviation in the width direction of the coil and overheating of the edges, potentially degrading the surface quality. The higher the circulation fan operating rate, the better, and since this is determined according to the capacity of the circulation fan motor, there is no particular upper limit.

[0063] There are various methods for manufacturing the above-mentioned plated steel sheets. As an example, a steel slab that meets the composition range described above can be obtained through processes such as heating, hot rolling, coiling, cooling, cold rolling, annealing, and plating. Each process will be explained below.

[0064] Heating of steel slabs The steel slab described above is heated to 1050-1300°C. If the heating temperature of the steel slab is below 1050°C, not only is it difficult to homogenize the structure of the steel slab, but it may also be difficult to redissolve precipitated elements if they are to be utilized. On the other hand, if the heating temperature exceeds 1300°C, an excessive oxide layer may be formed, which may increase the likelihood of inducing surface defects after hot rolling. Therefore, the heating temperature of the steel slab described above is effective at 1050-1300°C. The lower limit of the heating temperature of the steel slab described above is more effective at 1070°C, and even more effective at 1100°C. The upper limit of the heating temperature of the steel slab described above is more effective at 1280°C, and even more effective at 1250°C.

[0065] Hot rolling The heated steel slab described above is hot-rolled and then finish-hot-rolled at 800-950°C to obtain a hot-rolled steel sheet. If the finish-hot-rolling temperature is below 800°C, a mixed grain structure may develop in the surface layer of the steel sheet due to two-phase rolling, making it difficult to control the sheet shape. On the other hand, if the finish-hot-rolling temperature exceeds 950°C, there is a problem that grain coarsening due to hot rolling is likely to occur. Therefore, the finish-hot-rolling temperature should be between 800 and 950°C for optimal performance. A lower limit of 810°C is more effective for the finish-hot-rolling temperature, and 820°C is even more effective. A higher limit of 940°C is more effective for the finish-hot-rolling temperature, and 930°C is even more effective.

[0066] Winding The hot-rolled steel sheet described above is wound at 500-700°C. If the winding temperature is below 500°C, martensite may form on the entire or partial steel sheet, making it difficult to control the sheet shape. Furthermore, the increased strength of the hot-rolled steel sheet may lead to a decrease in rollability in subsequent cold-rolling processes. On the other hand, if the winding temperature exceeds 700°C, coarse carbides may form, potentially reducing the collision energy absorption capacity of the hot-formed member. Therefore, a winding temperature of 500-700°C is effective. A lower limit of 520°C is more effective, and 550°C is even more effective. A higher limit of 680°C is more effective, and 650°C is even more effective.

[0067] cooling The hot-rolled steel sheet wound up as described above is cooled from the winding temperature to 400°C at a cooling rate of 10°C / hr or more (hot-rolling cooling). However, if the cooling rate is less than 10°C / hr, a drawback may occur in which a large number of coarse carbides are formed during the cooling of the hot-rolled coil, giving the coils sufficient time to grow carbides. Therefore, a cooling rate of 10°C / hr or more is effective, 12°C / hr or more is more effective, and 15°C / hr or more is even more effective. On the other hand, as long as the cooling rate is 10°C / hr or more, the effects to be obtained through the present invention can be achieved, so there is no particular upper limit.

[0068] On the other hand, a pickling step can be added after the cooling process and before cold rolling. This pickling step removes scale formed on the surface of the steel sheet, thereby improving the surface quality of the product.

[0069] cold rolling After the above process, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. There are no particular limitations on the reduction ratio during the cold rolling process, but a reduction ratio of 30-80% can be applied to obtain the target thickness of the steel material.

[0070] Annealing and Cooling Annealing is performed on the cold-rolled steel sheet described above. To do this, the cold-rolled steel sheet is heated, and it is effective to heat it at a rate of 20°C / s or less within the temperature range from 400°C to the annealing temperature. If the heating rate from 400°C to the annealing temperature exceeds 20°C / s, there is insufficient time for the carbides precipitated during the hot-rolling stage to be redissolved, which may result in the retention of coarse carbides and a decrease in the impact energy absorption capacity of the final hot-formed member. Therefore, it is effective for the heating rate from 400°C to the annealing temperature to be 20°C / s or less. A heating rate of 18°C / s or less is more effective, and 15°C / s or less is even more effective. On the other hand, in this invention, the effects to be obtained can be achieved as long as the heating rate is 20°C / s or less, so the lower limit of the heating rate is not particularly limited. However, considering annealing productivity, the heating rate may be 0.5°C / s or more, more effectively 1°C / s or more, and even more effectively 1.5°C / s or more. On the other hand, in the present invention, the heating rate is not particularly limited in the temperature range from the cold rolling temperature to less than 400°C. This is because controlling the heating rate has only a slight effect on the redissolution of carbides.

[0071] It is effective to anneal the heated cold-rolled steel sheet at an annealing temperature of 740 to 860°C. If the annealing temperature is below 740°C, the recrystallization of the cold-rolled structure will not occur sufficiently, which may result in a poor sheet shape or excessively high strength after plating, potentially inducing die wear during the blanking process. On the other hand, if the annealing temperature exceeds 860°C, surface oxides such as Si and Mn may form during the annealing process, leading to a poor plated surface. Therefore, an annealing temperature of 740 to 860°C is effective. A lower limit of 750°C is more effective, and 760°C is even more effective. An upper limit of 850°C is more effective, and 840°C is even more effective.

[0072] It is effective to use a non-oxidizing atmosphere during the annealing process described above. For example, a hydrogen-nitrogen mixed gas can be used, in which case the dew point temperature of the atmospheric gas can be set to -70 to -30°C. Lowering the dew point temperature below -70°C requires additional control equipment, which increases manufacturing costs. If the dew point exceeds -30°C, excessive annealing oxides may form on the steel sheet surface during annealing, potentially causing defects such as unplated surfaces. Therefore, a dew point temperature of -70 to -30°C is effective for the atmospheric gas during continuous annealing. A lower limit of the dew point temperature of the atmospheric gas is more effective at -65°C, and even more effective at -60°C. An upper limit of the dew point temperature of the atmospheric gas is more effective at -35°C, and even more effective at -40°C.

[0073] 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 (annealing cooling). If the cooling rate is less than 1°C / s, a large amount of coarse carbides may be formed, which may reduce the impact energy absorption capacity of the final hot-formed member. Therefore, a cooling rate of 1°C / s or more is effective. A cooling rate of 1.5°C / s or more is more effective, and 2°C / s or more is even more effective. There is no particular upper limit to the cooling rate. However, from the viewpoint of suppressing shape defects in the steel sheet, the cooling rate may be 50°C / s or less, more effectively 45°C / s or less, and even more effectively 40°C / s or less.

[0074] Plating The annealed cold-rolled steel sheet can be further plated. While the present invention does not particularly limit the type and method of plating, an example of Al-based plating will be described. This plating involves cooling the annealed cold-rolled steel sheet and immersing it in an Al-based plating bath to form an aluminum-based plating layer. The composition of the Al-based plating bath and the plating conditions are not particularly limited.

[0075] However, as a non-limiting example, the composition of the plating bath may include, by weight %, Si: 6-12%, Fe: 1-4%, with the remainder being Al and other unavoidable impurities, and the plating amount may be 30-130 g / m², based on the single-sided standard commonly applied in the art. 2 It is possible that the above plating bath composition contains the following: If the Si content is less than 6% by weight, the plating bath temperature will rise excessively, which will degrade the equipment. If it exceeds 12% by weight, the alloying will be excessively delayed, requiring a longer heating time for hot forming. If the Fe content is less than 1% by weight, the plating adhesion and spot weldability may be poor. If it exceeds 4% by weight, excessive dross formation in the plating bath may occur, leading to poor surface quality. Plating adhesion amount is 30 g / m² on one side. 2 If the value is less than 130 g / m², it may be difficult to ensure the desired corrosion resistance of the hot-formed component. 2 If the amount exceeds this limit, not only will the manufacturing cost increase due to excessive plating, but it may also become difficult to apply a uniform amount of plating to the steel sheet in the entire width and length direction of the coil.

[0076] On the other hand, according to another embodiment of the present invention, continuous annealing and plating can be performed on cold-rolled steel sheets as described above, but plating can also be performed immediately after pickling on cooled hot-rolled steel sheets.

[0077] Next, one embodiment of the hot-formed member of the present invention will be described in detail. The hot-formed member of the present invention can be manufactured by hot-press forming the hot-forming steel material described above.

[0078] The above hot-formed member includes a base material and a plating layer formed on the base material, and it is effective that the surface carbon segregation factor after hot forming, as defined by [Relational Equation 2] below, is 0.1 or higher.

[0079] [Relationship 2] C (peak、HPF後) / C (nom、HPF後 )≧0.1 (In relational equation 2, C (peak、HPF後) This is the highest carbon value of the first carbon peak (Peak) that appears in the carbon profile, as determined by GDS analysis results, starting from 1 / 3 of the plating layer thickness on the surface in the direction of the base steel sheet, and C (nom、HPF後) This is the nominal carbon value of steel.

[0080] The hot-formed member described above satisfies a surface carbon segregation factor of 0.1 or higher after hot forming, ensuring excellent surface quality and fatigue resistance. It is effective for the surface carbon segregation factor after hot forming, as defined by [Relationship Formula 2] above, to not exceed 1.0.

[0081] It is effective for the surface carbide concentration index before hot forming, as defined by relation 1 above, to be greater than the surface carbide concentration index after hot forming, as defined by relation 2 ([Relation 1]>[Relation 2]).

[0082] On the other hand, C (nom、HPF後) This is the nominal carbon value of the steel, and it can be considered that there is almost no difference before and after hot forming. Therefore, in some descriptions, C is used without distinguishing between before and after HPF. nom It is sometimes written as follows.

[0083] The base material of the hot-formed member described above satisfies the alloy composition described above. On the other hand, the microstructure of the base material may have a single-phase martensite structure or a mixed structure containing martensite and bainite in an area of ​​40% or less. Since martensite is an effective structure for securing the strength targeted by the present invention, the microstructure of the member may be a single-phase martensite structure. On the other hand, although bainite is a structure with slightly lower strength than martensite, when formed within a martensite matrix, it is advantageous for securing strength without significantly reducing bendability. Therefore, in the present invention, a mixed structure containing bainite in an area of ​​40% or less together with martensite is also possible. However, if the bainite fraction exceeds 40% area, it may be difficult to secure the strength targeted by the present invention.

[0084] On the other hand, the above microstructure may further contain one or more of ferrite (10 area % or less) and retained austenite (5% or less). The above ferrite and retained austenite may be formed inevitably during the manufacturing process. If the ferrite structure exceeds 10 area %, not only will the strength decrease, but the bending properties may also be significantly worse. If the retained austenite structure exceeds 5 area %, the strength may decrease, and there is a higher possibility that hydrogen embrittlement will occur due to increased hydrogen inflow from the atmospheric gas during hot forming.

[0085] The plating layer of the hot-formed member mentioned above refers to the plating layer obtained after hot-forming, in addition to the plating layer of the steel material described above.

[0086] The above hot-formed member may exhibit a fatigue limit improvement of 5% or more. This improvement in fatigue limit refers to the improvement rate compared to a material without surface carbon concentration due to the absence of pre-alloying. This improvement in fatigue limit can be confirmed by tensile-compression fatigue testing. An improvement of 5% or more in fatigue limit indicates improved durability even with similar tensile properties.

[0087] Next, one embodiment of the method for manufacturing a hot-formed member of the present invention will be described in detail. The manufacturing method described below is only one embodiment of all possible embodiments, and does not mean that the hot-formed member should necessarily be manufactured only by the manufacturing method described below.

[0088] Prepare the hot-formed steel material described above or a hot-formed steel material manufactured by the method described above, and use it to manufacture a blank. Heat the blank to a temperature above the austenite single-phase region, more specifically to a temperature of Ac3 to 980°C, and then hold it for 1 to 1000 seconds.

[0089] If the blank heating temperature is below the Ac3 temperature, it may be difficult to secure the required strength due to the presence of untransformed ferrite. On the other hand, if the heating temperature exceeds 980°C, excessive oxides may be generated on the surface of the component, making it difficult to secure spot weldability. Therefore, the blank heating temperature should be between Ac3 and 980°C for optimal performance. A lower limit of the blank heating temperature of Ac3 + 5°C is more effective, and Ac3 + 10°C is even more effective. A higher limit of the blank heating temperature of 970°C is more effective, and 960°C is even more effective.

[0090] If the holding time is less than 1 second, the temperature will not be uniform throughout the blank, which may induce material differences between parts. If the holding time exceeds 1000 seconds, similar to excessive heating temperature, excessive oxides may be generated on the surface of the material, making it difficult to ensure spot weldability. Therefore, a holding time of 1 to 1000 seconds is effective. A lower limit of 30 seconds is more effective, and 60 seconds is even more effective. A higher limit of 900 seconds is more effective, and 800 seconds is even more effective.

[0091] Subsequently, the heated and held blank is hot-formed and then cooled to room temperature (molding cooling) to finally produce a hot-formed member. The specific conditions during the hot-forming process are not particularly limited, and any hot-forming method commonly known in the art to which this invention belongs can be applied. As a preferred example, a mold cooling method can be used. [Examples]

[0092] Next, embodiments of the present invention will be described.

[0093] It goes without saying that the following embodiments can be modified in various ways without departing from the scope of the present invention by anyone with ordinary skill in the art to which the present invention pertains. The following embodiments are for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following embodiments, but should be defined not only by the claims described below, but also by equivalents thereof.

[0094] (Examples) A 100 mm thick steel slab having the composition shown in Table 1 below (weight %), with the remainder being Fe and unavoidable impurities, was manufactured by vacuum melting.

[0095] After heating the above steel slab to 1250°C, it was hot-rolled at a finish hot-rolling temperature of 900°C and wound at a winding temperature of 640°C to produce a hot-rolled steel sheet with a final thickness of 2.5 mm. After pickling the hot-rolled steel sheet, it was cold-rolled with a cold reduction ratio of 45% to produce a cold-rolled steel sheet. After annealing to a normal annealing temperature of 780°C in a 5% hydrogen-95% nitrogen atmosphere, the cold-rolled steel sheet was cooled and then plated with an aluminum-based plating.

[0096] In this case, the composition of the Al-based plating bath was Al-9%Si-2%Fe with the remainder being unavoidable impurities, and the plating adhesion amount was 70 g / m² on a single-sided basis. 2 That's what I decided.

[0097] The surface layer of the plated steel sheet described above was subjected to a box annealing heat treatment for carbon enrichment. Depending on the type of process, the box annealing heat treatment was carried out using either a one-step or two-step method, and the specific conditions are described in Table 2 below.

[0098] [Table 1]

[0099] [Table 2]

[0100] In Table 2 above, H1 represents the heating rate for the 1-stage box annealing, T1 represents the target temperature for the 1-stage box annealing, t1 represents the holding time for the 1-stage box annealing, H2-1 represents the heating rate for one section of the 2-stage box annealing, T2-1 represents the target temperature for one section of the 2-stage box annealing, t2-1 represents the holding time for one section of the 2-stage box annealing, H2-2 represents the heating rate for two sections of the 2-stage box annealing, T2-2 represents the target temperature for two sections of the 2-stage box annealing, and t2-2 represents the holding time for two sections of the 2-stage box annealing.

[0101] After creating blanks from the steel plates manufactured in this manner, hot-formed members were produced by hot-forming them using a hot-forming die. At this time, the heating temperature of the blanks was 900°C, the holding time was 5 minutes, and the transport time from the heating furnace to the forming stage was consistently 10 seconds.

[0102] The surface carbon segregation factor (SCA) of the hot-formed steel material and hot-formed member was measured and is shown in Table 3 below. The SCA was determined by measuring the carbon distribution in the depth direction using a GDS analyzer, and the detailed analysis method is shown in Figure 1. Among the GDS carbon distribution from one-third of the surface including the plating layer, the highest carbon amount of the first carbon peak is C peak C nom This represents the average carbon content for each steel grade. On the other hand, in the graph in Figure 1(b), the solid line shows the carbon distribution before HPF, and the dotted line shows the carbon distribution after HPF.

[0103] [Table 3]

[0104] In Table 3 above, relational equations 1 and 2 are, respectively, C (peak、HPF前) / C (nom、HPF前) and C (peak、HPF後 ) / C (nom 、HPF後) This is the calculated value.

[0105] On the other hand, whiteness was measured using a colorimeter on tape that had been attached to and then peeled off the surface of the material. In the case of fatigue limit, the value was derived by repeatedly performing compression-tensile tests under specific load conditions, and the improvement rate of the fatigue limit for a material that had not undergone pre-alloying heat treatment and had no surface thickening was shown as the fatigue limit improvement percentage (%). The edges of the pre-alloyed material were observed with an electron microscope to determine whether or not melting occurred at the edges, and this was expressed as O / X.

[0106] As can be seen from Tables 1 and 2 above, it was confirmed that in the case of Invention Examples 1 to 7, which satisfy all of the alloy compositions and box annealing conditions proposed in this invention, excellent surface quality and fatigue properties can be ensured.

[0107] Comparative Examples 1, 5, 10, and 15, while satisfying the alloy composition proposed in the present invention, did not undergo box annealing heat treatment, and therefore could not alloy the plated layer, nor could they secure a surface carbon concentration index.

[0108] In Comparative Examples 2 and 12, although the heating rate in the box annealing process was the heating rate proposed in the present invention, the heat treatment temperature was less than 500°C, resulting in insufficient alloying. The surface carbon concentration index of the steel material before hot forming fell outside the range of the present invention, and the improvement in the fatigue limit was only slight. Figures 2(a) and 2(b) are photographs observing the cross-section of the plating layer after box annealing heat treatment for Invention Example 1 and Comparative Example 2, respectively. As can be seen in Figure 2(b), in Comparative Example 2, although the holding time met the conditions presented in the present invention, sufficient alloying was not achieved because the target temperature was not met. On the other hand, in Invention Example 1, which met all the pre-alloying heat treatment conditions, it can be seen that alloying was completed after the alloying heat treatment.

[0109] Comparative Example 3 was a material that underwent a single-stage box annealing treatment. While the purge gas flow rate and circulation fan drive speed met the requirements, it was confirmed that the heating rate was less than 20°C / hr, resulting in inferior whiteness of less than 60 due to the formation of surface oxides. On the other hand, in Comparative Example 11, the heating rate exceeded the range of the present invention, and the surface quality deteriorated due to the melting of the plating layer caused by overheating at the edges.

[0110] Comparative Examples 7 and 14 applied a two-stage box annealing heat treatment, but the heating rate in one of the two stages exceeded the range of the present invention. Due to excessive heat treatment, the surface quality deteriorated due to the melting of the plating layer caused by overheating at the edges. Figures 3(a) and 3(b) are photographs of Invention Example 2 and Comparative Example 7 observed with an optical microscope. Compared to Invention Example 2 in Figure 3(a), it was confirmed that in Comparative Example 7 in Figure 3(b), melting of the plating layer occurred at the edges before solidification, resulting in an uneven surface quality (box portion).

[0111] Comparative Examples 8 and 13 involved a two-stage box annealing process where the heating rate in two sections exceeded or fell short of the range of the present invention. In each case, melting occurred in the plating layer at the edges, or the surface quality deteriorated to less than 60 in whiteness.

[0112] Although Comparative Examples 4 and 6 met the requirements for heating rate and holding time, the flow rate of the purge gas did not meet the scope of the present invention, and therefore sufficient surface whiteness could not be secured. In Comparative Examples 9 and 16, during the box annealing heat treatment, the operation of the circulation fan inside the furnace was less than 10 rpm, and it was confirmed that the plating layer melted due to overheating of the edges caused by uneven temperature inside the furnace, resulting in a decrease in surface quality.

Claims

1. The material includes a base steel sheet and a plating layer formed on the base steel sheet. The aforementioned base steel sheet contains, by weight percent, C: 0.04 to 0.45%, Si: 1.5% or less (excluding 0%), Mn: 0.2 to 2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities. The following [Relationship 1] is satisfied, The whiteness is 60 or higher. There is no melting at the edges. A hot-forming steel material that improves the fatigue limit of hot-formed members by 5% or more. [Relationship 1] Carbon concentration index before hot forming: C （ｐｅａｋ、ＨＰＦ前） / C （ｎｏｍ、ＨＰＦ前） ≥ 1.5 (In relational equation 1, C （ｐｅａｋ、ＨＰＦ前） This is the highest carbon value of the first carbon peak (Peak) in the carbon profile, as determined by GDS analysis results, starting from a point 1 / 3 of the thickness of the plating layer on the surface in the direction of the base steel sheet, C （ｎｏｍ、ＨＰＦ前） (This is the nominal carbon value of steel.)

2. The steel material for hot forming according to claim 1, wherein the base steel sheet further comprises one or more of the following: Mo: 0.5% or less, 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 according to claim 1, wherein the microstructure of the base steel sheet contains 50-90% ferrite by area fraction, and one or more of pearlite (30% or less), bainite (20% or less), and martensite (20% or less).

4. A step of obtaining a plated steel sheet using a steel slab containing, by weight percent, C: 0.04 to 0.45%, Si: 1.5% or less (excluding 0%), Mn: 0.2 to 2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities. The process includes the step of box annealing the plated steel sheet using the two-step method described in [Box Annealing Condition 2] below, During the aforementioned box annealing, the purge gas inside the furnace is hydrogen (H ２ ), nitrogen (N ２ ) and any one of these mixed gases, with a purge volume of 0.1 to 100 m ３ The method for manufacturing a steel material for hot forming according to claim 1, wherein the rate of rotation is / hr, a circulation fan can be operated during the box annealing, and the operating rate of the circulation fan is 10 rpm or more. [Box annealing condition 2] Temperature range for one interval (T2-1): 500 to 780°C Holding time for one section (t2-1): 1 minute or more (this is the holding time at the target temperature). Heating rate per section (H2-1): 20-160°C / hr (This is the heating rate to reach the target temperature) Temperature range for two intervals (T2-2): 600-800°C Holding time for the two sections (t2-2): 50 minutes or more (this is the holding time at the target temperature). Heating rate in two sections (H2-2): 0.25–160°C / hr (This is the heating rate to reach the target temperature) [Relationship 1] Carbon enrichment index before hot forming: C (peak, before HPF) / C (nom, before HPF) ≧1.5 (In relational equation 1, C (peak, before HPF) is the highest carbon value of the first carbon peak (Peak) that appears in the carbon profile of the GDS analysis results, starting from a point one-third of the thickness of the plating layer on the surface in the direction of the base steel sheet, and C (nom, before HPF) is the nominal carbon value of the steel.)

5. The method for manufacturing a steel material for hot forming according to claim 4, wherein the steel slab further comprises one or more of the following: Mo: 0.5% or less, Ni: 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, and B: 0.01% or less.

6. The step of obtaining the aforementioned plated steel sheet is: The steps include heating the steel slab to 1050-1300°C, The process involves finishing hot-rolling the heated steel slab at 800 to 950°C to obtain a hot-rolled steel sheet, The steps include winding the hot-rolled steel sheet at 500 to 700°C, The steps include: cooling the wound hot-rolled steel sheet from the winding temperature to 400°C at a cooling rate of 10°C / hr or more; The steps include: cold rolling the cooled hot-rolled steel sheet at a reduction ratio of 30 to 80% to obtain a cold-rolled steel sheet; The steps include: heating the cold-rolled steel sheet in a temperature range from 400°C to the annealing temperature at a rate of 20°C / s or less; The steps include annealing the heated cold-rolled steel sheet at an annealing temperature of 740 to 860°C, The steps include: cooling the annealed cold-rolled steel sheet from the annealing temperature to 660°C at a cooling rate of 1°C / s or more; A method for manufacturing a steel material for hot forming according to claim 4, comprising the step of performing plating after annealing.

7. The method for manufacturing a steel material for hot forming according to claim 6, wherein the dew point temperature of the atmospheric gas during the annealing process is -70 to -30°C.

8. The method for producing a hot-formable steel material according to claim 6, wherein the plating is performed by immersing the material in an Al-based plating bath containing, by weight, 6-12% Si, 1-4% Fe, and the remainder being Al and unavoidable impurities to form an aluminum plating layer.

9. The material comprises, by weight percent, C: 0.04 to 0.45%, Si: 1.5% or less (excluding 0%), Mn: 0.2 to 2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.01 to 5.0%, N: 0.02% or less, with the remainder being Fe and unavoidable impurities, and a plating layer formed on the base material. The following conditions in [Relationship Equation 2] are met, The improvement in fatigue limits was 5% or more. The whiteness of the steel material for hot forming is 60 or higher. A hot-formed component in which the edges of the hot-formed steel material do not melt. [Relationship Equation 2] C （ｐｅａｋ、ＨＰＦ後） / C （ｎｏｍ、ＨＰＦ後） ≧0.1 (In relational equation 2, C （ｐｅａｋ、ＨＰＦ後） This is the highest carbon value of the first carbon peak (Peak) in the carbon profile, as determined by GDS analysis results, starting from 1 / 3 of the plating layer thickness on the surface and moving in the direction of the base steel sheet. （ｎｏｍ、ＨＰＦ後） (This is the nominal carbon value of steel.)

10. The hot-formable member according to claim 9, wherein the base material further comprises one or more of the following: Mo: 0.5% or less, Ni: 0.5% or less, Nb: 0.1% or less, Ti: 0.1% or less, and B: 0.01% or less.

11. The hot-formed member according to claim 9, wherein the microstructure of the hot-formed member is a single-phase martensite structure or a mixed structure containing martensite and bainite in an area of ​​40% or less.

12. The hot-formed member according to claim 9, wherein the value of [relational formula 2] is 1.0 or less.

13. The hot-formed member according to claim 9, wherein the hot-formed member is manufactured using a hot-forming steel material as described in any one of claims 1 to 3.

14. A step of obtaining a blank using a hot-forming steel material according to any one of claims 1 to 3, The steps include heating the blank to a temperature of Ac3 to 980°C and holding it for 1 to 1000 seconds, The process includes the step of hot-forming the heated and held blank and then cooling it, A method for manufacturing a hot-formed member that satisfies the conditions of the following [Relational Equation 2]. [Relationship Equation 2] C (peak, after HPF) / C (nom, after HPF) ≧0.1 (In relational equation 2, C (peak, after HPF) is the highest carbon value of the first carbon peak (Peak) in the carbon profile obtained from the GDS analysis results, starting from one-third of the plating layer thickness on the surface in the direction of the base steel sheet, and C (nom, after HPF) is the nominal carbon value of the steel.)

15. The method for manufacturing a hot-formed member according to claim 14, wherein the cooling is performed by a mold cooling method.

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