Hot-forming steel material, method for manufacturing hot-forming steel material, and method for manufacturing hot-formed member

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 a hot-formed steel material used in automobiles and the like, a hot-formed member, and a method for producing the same. [Background technology]

[0002] Recently, efforts have been made to reduce the weight of automobiles to improve fuel efficiency. While one method for achieving this is to reduce the thickness of steel sheets, this reduction can potentially affect the stability of automobiles, so the strength of the steel must also be improved. For this reason, there has been a continuous demand for high-strength steel sheets, and various types of steel sheets have been developed. However, due to their high strength, these steel sheets suffer from poor workability.

[0003] To solve these problems, a hot forming method has been proposed. In the hot forming method, a steel material is processed at a high temperature where it is easy to process, and then rapidly cooled to a low temperature to form a low-temperature structure such as martensite in the steel material, thereby increasing the strength of the final product. In this way, workability problems when manufacturing high-strength components can be minimized.

[0004] A technology relating to such hot forming is disclosed in Patent Document 1. Patent Document 1 proposes a technology in which an Al-Si plated steel sheet is heated to 850°C or higher, and then hot formed by pressing and rapidly cooled to form martensite in the component structure, thereby ensuring an ultra-high strength of 1600 MPa or higher in tensile strength.

[0005] On the other hand, the hot-formed parts used to protect automobile occupants must have excellent crashworthiness, and bendability is often used as a typical index for evaluating such crashworthiness. For example, in the case of parts such as automobile B-pillars, when a hot-formed part is bent in response to a side collision with the vehicle, the part is required to have a bendability that allows it to withstand a specific distance (angle) or more without fracture.

[0006] Patent Document 2 proposes a method for controlling the ferrite structure in the surface layer of a hot-formed part. In addition, a technology has been proposed that incorporates a blank (TWB, tailor welded blank) with a combination of different materials or different thicknesses into hot forming to compensate for the relatively poor energy absorption capacity, and various research efforts are being conducted.

[0007] However, there is a limit to how much bendability can be improved by controlling the surface ferrite structure through optimization of hot forming conditions. Also, even when improving crashworthiness through TWB, there is a limit to how much improvement can be made to the properties of parts that require crashworthiness, as bendability actually decreases due to deterioration of the weld. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Patent No. 6,296,805 [Patent Document 2] Korean Patent No. 10-1569508 Summary of the Invention [Problem to be solved by the invention]

[0009] One aspect of the present invention is to provide a hot-forming steel material that enables hot-formed parts to have high strength and excellent bendability, a hot-formed part manufactured using the same, and a method for manufacturing the same.

[0010] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the entire content of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0011] One aspect of the present invention 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, the balance being Fe and unavoidable impurities, Provided is a steel material for hot forming having a surface roughness factor calculated by the following [Relationship 1] of 1.8 μm or less.

[0012] [Equation 1]

number

[0013] Another aspect of the present invention is a method for manufacturing a cold-rolled steel sheet using a steel slab containing, by 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, the balance being Fe and unavoidable impurities; and temper rolling the cold-rolled steel sheet so as to satisfy the following [Relationship 2].

[0014] [Equation 2]

number

[0015] Yet another embodiment of the present invention 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, the balance being Fe and unavoidable impurities, To provide a hot-formed member in which the maximum bending angle variation is within 5%.

[0016] Yet another aspect of the present invention is a method for manufacturing a steel sheet for hot forming, comprising the steps of: obtaining a blank using the above-mentioned steel sheet for hot forming; Heating the blank to a temperature of Ac3 to 980 ° C. and then holding it for 1 to 1000 seconds; and cooling the heated and held blank after hot forming. [Effects of the Invention]

[0017] According to the present invention, it is possible to manufacture a hot-formed part that has high strength after hot forming, excellent bendability, and excellent crash resistance. For this purpose, it is possible to provide a hot-formed steel material, a hot-formed part manufactured using the steel material, and a method for manufacturing the same.

[0018] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0019] [Figure 1] This is a simple illustration of the concept of the surface roughness factor of [Relationship 1] proposed in the present invention. [Figure 2] This is a simple illustration of the CIE (Crack Initiation Energy) concept, which is a standard for evaluating the impact energy absorption capacity in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The terminology used herein is for the purpose of describing the invention and is not intended to limit the invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the related definition clearly indicates otherwise.

[0021] The term "comprises" as used herein is intended to embody features and does not exclude the presence or addition of other features.

[0022] Unless otherwise specified, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content.

[0023] First, one embodiment of the steel material for hot forming of the present invention will be described in detail. The steel material of the present invention can contain, 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 balance being Fe and inevitable impurities. Each alloy composition will be described in detail below, where % means % by weight.

[0024] Carbon (C): 0.04~0.45% C is an essential element added to improve the strength of components. If the C content is less than 0.04%, it is difficult to ensure sufficient strength, and even if the ultimate bendability is high, the collision energy absorption capacity will actually decrease, so it is preferable to add 0.04% or more. On the other hand, if the C content exceeds 0.45%, although the strength increases, the bendability and collision energy absorption capacity will decrease, so it is preferable that the C content be 0.45% or less.

[0025] Silicon (Si): 1.5% or less (excluding 0%) The above-mentioned Si is not only necessary as a deoxidizer in steelmaking, but is also a solid-solution strengthening element. It contributes to increasing the strength of hot-formed parts by suppressing carbide formation, and is added as an element effective in homogenizing material properties. If the Si content exceeds 1.5%, Si oxides formed on the steel sheet surface during annealing may reduce galvanic properties. Therefore, it is preferable that the Si content be 1.5% or less (excluding 0%).

[0026] Manganese (Mn): 0.2 to 2.5% Mn is necessary not only to ensure the solid solution strengthening effect but also to improve hardenability and suppress the formation of ferrite during hot forming. If the Mn content is less than 0.2%, the above effects are limited, and other expensive alloy elements are required excessively to improve the insufficient hardenability, which can result in a significant increase in manufacturing costs. On the other hand, if the Mn content exceeds 2.5%, the strength of the steel sheet increases before the hot forming process, which can reduce cold rolling properties. As a result, the band structure aligned in the rolling direction of the microstructure phase deepens, which can reduce impact energy absorption. Therefore, the Mn content is preferably 0.2 to 2.5%.

[0027] Phosphorus (P): 0.05% or less The P exists as an impurity in steel, and if its content exceeds 0.05%, it can significantly embrittle the weldability of hot-formed parts. On the other hand, the P is an unavoidable impurity during the production of steel, and there is no particular need to set a lower limit for the P content. However, controlling the P content to less than 0.001% can require significant production costs, so the P content can be set to 0.001% or more.

[0028] Sulfur (S): 0.02% or less Since S is present as an impurity in steel and is an element that impairs the ductility, impact properties, and weldability of hot-formed parts, it is preferable to limit it to a maximum of 0.02%. On the other hand, S is an unavoidable impurity, and there is no particular need to limit its lower limit, but controlling it to less than 0.0001% may require significant production costs, so it can be set to 0.0001% or more.

[0029] Aluminum (Al): 0.01 to 0.1% The Al element, together with Si, acts as a deoxidizer during steelmaking, enhancing the cleanliness of the steel. If the Al content is less than 0.01%, the above effect is difficult to obtain, while if the Al content exceeds 0.1%, there is a problem that the high-temperature ductility is reduced due to the excess AlN formed during the continuous casting process, resulting in the occurrence of slab cracks. Therefore, the Al content is preferably 0.01 to 0.1%.

[0030] Chromium (Cr): 0.01 to 5.0% Like Mn, Cr is added to ensure the hardenability of 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 hardenability. On the other hand, if the Cr content exceeds 5.0%, the effect of improving hardenability is small compared to the amount added, and the content may promote the formation of coarse Cr-based carbides, which may reduce the impact energy absorption capacity. Therefore, it is preferable that the Cr content does not exceed 5.0%.

[0031] Nitrogen (N): 0.02% or less N is contained in steel as an impurity. If the N content exceeds 0.02%, there is a problem that slab cracks are likely to occur due to the formation of AlN, as in the case of Al described above. N is an impurity, and there is no particular need to set a lower limit. However, controlling the N content to less than 0.001% may require significant production costs, so the N content can be set to 0.001% or more.

[0032] On the other hand, the steel material may further contain one or more of 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, in addition to the above alloy components.

[0033] Molybdenum (Mo): 0.5% or less Mo, like Cr and Mn, not only improves the hardenability of steel, but also forms fine precipitates, thereby improving bendability by refining crystal grains. However, if the Mo content exceeds 0.5%, the cost of the ferroalloy increases excessively compared to the effect, so the Mo content is preferably not more than 0.5%. The Mo content is more preferably 0.45% or less, even more preferably 0.4% or less, and even more preferably 0.35% or less.

[0034] Nickel (Ni): 0.5% or less Ni is an austenite stabilizing element, and its addition can improve the hardenability of steel. However, because Ni is an expensive alloying element, considering the increase in manufacturing costs compared to the effect of improving hardenability, its upper limit is preferably set at 0.5%. On the other hand, to fully obtain the effect of improving hardenability by adding Ni, it is preferable that the content be at least 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of Ni is more preferably 0.45%, even more preferably 0.4%, and most preferably 0.35%.

[0035] Niobium (Nb): 0.1% or less Nb is an element that can obtain a precipitation strengthening effect by forming fine precipitates, thereby increasing strength and improving bendability by refining crystal grains. In addition, it can suppress excessive grain growth during heating for hot forming, thereby improving robustness against fluctuations in heat treatment conditions. However, if the Nb content exceeds 0.1%, not only will the effect saturate, but the increase in precipitation temperature will increase the number of relatively coarse precipitates, which may result in a decrease in cost-effectiveness. Therefore, the Nb content is preferably 0.1% or less. The lower limit of the Nb content is preferably 0.005%, more preferably 0.01%, and even more preferably 0.015%. The upper limit of the Nb content is more preferably 0.09%, more preferably 0.08%, and most preferably 0.07%.

[0036] Titanium (Ti): 0.1% or less Ti is an element that is added when B is added to ensure hardenability by combining with nitrogen remaining as an impurity in the steel to form TiN. It can also be expected to have precipitation strengthening and grain refinement effects through the formation of TiC precipitates. However, if the Ti content exceeds 0.1%, a large amount of coarse TiN is formed, reducing impact energy absorption capacity. Therefore, the upper limit of Ti is preferably 0.1%. The lower limit of Ti is preferably 0.005%, more preferably 0.01%, and even more preferably 0.015%. The upper limit of Ti is more preferably 0.08%, more preferably 0.06%, and most preferably 0.05%.

[0037] Boron (B): 0.01% or less The addition of B in small amounts not only improves hardening ability, but also segregates at prior austenite grain boundaries, effectively suppressing the embrittlement of hot-formed parts due to the grain boundary segregation of P and / or S. However, if the B content exceeds 0.01%, the Fe 23Since the formation of CB6 complex compounds causes embrittlement during hot rolling, the upper limit of B is preferably 0.01%. On the other hand, the lower limit of the B content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the B content is more preferably 0.009%, more preferably 0.007%, and most preferably 0.005%.

[0038] The remainder other than the above includes iron (Fe), and since unintentional impurities can be unavoidably mixed in from raw materials or the surrounding environment during normal manufacturing processes, these cannot be excluded. These impurities are known to anyone with ordinary skill in manufacturing processes, and therefore, the present specification does not specifically mention all of the contents thereof.

[0039] The hot-forming steel material of the present invention preferably has the above alloy composition and a surface roughness factor (SRF) defined by the following [Relationship 1] of 1.8 μm or less. Surface roughness is expressed in various ways (Ra, Rt, Rsk, etc.), but simply changing the Ra, Rsk, etc. of the steel material is difficult to improve the bendability of hot-formed parts. As a result of research into improving the bendability of hot-formed parts, the inventors of the present invention have come to recognize that the bendability of hot-formed parts can be improved by maintaining the surface roughness of the steel material at a constant value. Rather than simply measuring Rt and Rdq, the inventors derived the technical relationship between Rt and Rdq to ensure the bendability of hot-formed parts, and derived the surface roughness factor (SRF) of the following [Relationship 1]. Therefore, in order to improve the bendability of hot-formed parts and increase their impact energy absorption capacity, the surface roughness factor (SRF) of the hot-forming steel material is preferably 1.8 μm or less. If the surface roughness index exceeds 1.8 μm, the gradient of the peaks becomes large, which may result in a decrease in bendability due to the maximum surface notch effect during bending.

[0040] [Equation 1]

number

[0041] Here, Rt is defined as the vertical distance between the highest peak and the deepest valley in any measurement section on the steel sheet surface, and Rdq is the root mean square of the peak gradient in any measurement section on the steel sheet surface. An example of a method for calculating Rt and Rdq using the above Relation 1 is shown in Figure 1, and ordinary engineers will have no difficulty in deriving the above Rt and Rdq through this.

[0042] The microstructure of the steel material for hot forming of the present invention may contain, in area fraction, 50 to 90% ferrite and one or more of 30% or less pearlite, 20% or less bainite, and 20% or less martensite.

[0043] The ferrite is a soft phase and is an effective structure for reducing the load in the blanking process of the steel material when producing a blank, so it is preferable that the ferrite content be 50% by area or more. However, if it exceeds 90% by area, carbon may be excessively distributed in structures other than ferrite when producing the blank, which may result in non-uniform distribution of carbon even after hot forming. Therefore, the ferrite content is preferably 50 to 90% by area.

[0044] If the pearlite content exceeds 30% by area, the cementite may be incompletely dissolved after hot forming, resulting in a decrease in strength or in a non-uniform material. On the other hand, if the bainite and martensite content each exceed 20% by area, the strength of the steel sheet may increase excessively, which may cause problems such as die wear during blank production.

[0045] The steel material for hot forming according to the present invention may include a plating layer on at least one surface. The type of the plating layer is not particularly limited, and the plating layer may be a zinc (Zn)-based plating layer, an aluminum (Al)-based plating layer, or the like, and the plating method is also not particularly limited, and may be hot-dip plating, electroplating, or the like. Preferably, an Al-based plating layer may be formed. Although the Al-based plating is not particularly limited, the Al-based plating layer may contain, by weight, 6 to 12% Si, 1 to 4% Fe, and the remainder Al and inevitable impurities.

[0046] Next, one embodiment of the hot forming steel material of the present invention will be described in detail. The manufacturing method described below is merely one embodiment among all possible embodiments, and does not necessarily mean that the hot forming steel material of the present invention should be manufactured only by the manufacturing method described below.

[0047] A cold-rolled steel sheet is produced using a steel slab satisfying the above-mentioned alloy composition, and then the cold-rolled steel sheet is subjected to temper rolling so as to satisfy the following [Relational Formula 2] to produce a steel material.

[0048] [Equation 2]

number

[0049] The cold-rolled steel sheet is subjected to temper rolling to control the surface roughness of the steel material. In the present invention, the rolling force P during temper rolling and the arithmetic mean roughness Ra of the roll are used. rollThe above [Relational Formula 2] was derived based on the recognition that the steel surface can be optimized by taking into account the technical effects of [Relational Formula 2]. The rolling force P during temper rolling is an important factor, but the present invention does not particularly limit its upper or lower limits. However, for example, if no rolling force is applied, issues such as poor coiling may occur, so the rolling force may be 100 tons or more, more preferably 150 tons or more. Furthermore, if the rolling force is excessively high, cracking of the surface coating layer may occur, and if it exceeds 40 according to the above [Relational Formula 2], the upper limit can be limited. For example, if the arithmetic mean roughness of the temper rolling roll is 4 μm, the rolling force is preferably 400 tons or less to satisfy the above [Relational Formula 2].

[0050] The cold-rolled steel sheet can be obtained through processes such as heating the steel slab, hot rolling, coiling, cooling, cold rolling, annealing, etc. Each process will be described below.

[0051] Steel slab heating The steel slab is heated at 1050 to 1300°C. If the steel slab heating temperature is lower than 1050°C, not only is it difficult to homogenize the structure of the steel slab, but when using precipitated elements, it may be difficult to redissolve them. On the other hand, if the heating temperature exceeds 1300°C, an excessive oxide layer may be formed, which may increase the possibility of inducing surface defects after hot rolling. Therefore, the steel slab heating temperature is preferably 1050 to 1300°C. The lower limit of the steel slab heating temperature is more preferably 1070°C, and even more preferably 1100°C. The upper limit of the steel slab heating temperature is more preferably 1280°C, and even more preferably 1250°C.

[0052] hot rolling The heated steel slab is hot rolled and finish hot rolled at 800 to 950°C to obtain a hot-rolled steel sheet. If the finish hot rolling temperature is less than 800°C, a duplex grain structure occurs in the surface layer of the steel sheet due to dual-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 coarsening of crystal grains due to hot rolling easily occurs. Therefore, the finish hot rolling temperature is preferably 800 to 950°C. The lower limit of the finish hot rolling temperature is more preferably 810°C, and even more preferably 820°C. The upper limit of the finish hot rolling temperature is more preferably 940°C, and even more preferably 930°C.

[0053] Winding The hot-rolled steel sheet is coiled at 500 to 700°C. If the coiling temperature is less than 500°C, martensite is formed in the entire or part of the steel sheet, making it difficult to control the sheet shape. This may also result in problems such as an increase in the strength of the hot-rolled steel sheet, which reduces the rollability in the subsequent cold rolling process. On the other hand, if the coiling temperature exceeds 700°C, coarse carbides are formed, which may reduce the impact energy absorption capacity of the hot-formed part. Therefore, the coiling temperature is preferably 500 to 700°C. The lower limit of the coiling temperature is more preferably 520°C, and even more preferably 550°C. The upper limit of the coiling temperature is more preferably 680°C, and even more preferably 650°C.

[0054] cooling The coiled hot-rolled steel sheet is cooled from the coiling temperature to 400°C at a cooling rate of 10°C / Hr or more (hot-rolling cooling). If the cooling rate is less than 10°C / Hr, there is a possibility that the carbides will not have enough time to grow, resulting in 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, more preferably 12°C / Hr or more, and even more preferably 15°C / Hr or more. However, as long as the cooling rate is 10°C / Hr or more, the effects of the present invention can be obtained, and therefore there is no particular upper limit.

[0055] After the cooling step, a pickling step can be added before cold rolling. The pickling step can remove scale formed on the surface of the steel sheet, thereby improving the surface quality of the product.

[0056] cold rolling After the above steps, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. In the present invention, there is no particular limitation on the reduction ratio during the cold rolling, but a reduction ratio of 30 to 80% can be applied to obtain the target steel thickness.

[0057] Annealing and cooling The cold-rolled steel sheet is then annealed. First, the cold-rolled steel sheet is heated, preferably at a heating rate of 20°C / s or less in 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, the carbides precipitated during the hot rolling stage do not have enough time to redissolve, resulting in residual coarse carbides, which may reduce the impact energy absorption capacity of the final hot-formed part. Therefore, the heating rate from 400°C to the annealing temperature is preferably 20°C / s or less. The heating rate is more preferably 18°C / s or less, and even more preferably 15°C / s or less. However, in the present invention, the intended effects of the present invention can be obtained as long as the heating rate is 20°C / s or less, and therefore 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 preferably 1°C / s or more, and even more preferably 1.5°C / s or more. On the other hand, in the present invention, there are no particular limitations on the heating rate in the temperature range from the cold rolling temperature to less than 400° C. This is because controlling the heating rate has little effect on the re-dissolution of carbides.

[0058] The heated cold-rolled steel sheet is preferably annealed at an annealing temperature of 740 to 860°C. If the annealing temperature is lower than 740°C, the recrystallization of the structure due to cold rolling may be insufficient, resulting in a poor sheet shape, or the strength after plating may be too high, which may induce mold wear during the blanking process. On the other hand, if the annealing temperature exceeds 860°C, problems may arise in that surface oxides of Si, Mn, etc. are formed during the annealing process, resulting in a poor plating surface. Therefore, the annealing temperature is preferably 740 to 860°C. The lower limit of the annealing temperature is more preferably 750°C, and even more preferably 760°C. The upper limit of the annealing temperature is more preferably 850°C, and even more preferably 840°C.

[0059] The atmosphere during the annealing is preferably a non-oxidizing atmosphere. For example, a hydrogen-nitrogen mixed gas can be used, and in this case, the dew point temperature of the atmospheric gas can be −70 to −30°C. Setting the dew point temperature below −70°C requires additional control equipment, which increases production costs. If the dew point exceeds −30°C, excessive annealing oxides may be formed on the steel sheet surface during annealing, which can cause defects such as non-plating. Therefore, the dew point temperature of the atmospheric gas during the continuous annealing is preferably −70 to −30°C. The lower limit of the dew point temperature of the atmospheric gas is more preferably −65°C, and even more preferably −60°C. The upper limit of the dew point temperature of the atmospheric gas is more preferably −35°C, and even more preferably −40°C.

[0060] 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 part. Therefore, the cooling rate is preferably 1°C / s or more. The cooling rate is more preferably 1.5°C / s or more, and even more preferably 2°C / s or more. There is no particular upper limit to the cooling rate. However, from the viewpoint of suppressing defects in the steel sheet shape, the cooling rate may be 50°C / s or less, more preferably 45°C / s or less, and even more preferably 40°C / s or less.

[0061] Meanwhile, before the temper rolling, the annealed cold-rolled steel sheet may be further plated. In the present invention, the type and method of plating are not particularly limited, but an example of Al-based plating will be described. The 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 plating conditions are not particularly limited.

[0062] However, as a non-limiting example, the composition of the plating bath may contain, by weight, Si: 6 to 12%, Fe: 1 to 4%, the balance being Al and other unavoidable impurities, and the plating amount may be 30 to 130 g / m on one side, which is commonly applied in the technical field. 2 In the composition of the coating bath, if the Si content is less than 6 wt%, there is a drawback in that the temperature of the coating bath rises excessively, causing deterioration of the equipment, and if it exceeds 12 wt%, there is a drawback in that the alloying is excessively delayed, requiring a longer heating time for hot forming. If the Fe content is less than 1 wt%, there is a possibility that the coating adhesion and spot weldability may be reduced, and if it exceeds 4 wt%, there may be excessive dross generation in the coating bath, causing poor surface quality. The coating weight is 30 g / m on one side 2 If it is less than 130 g / m, it may be difficult to ensure the desired corrosion resistance of the hot-formed part. 2If the coating weight exceeds 100%, not only will the manufacturing cost increase due to an excessive coating weight, but it may not be easy to coat the steel sheet with a uniform coating weight in the entire width and length directions of the coil.

[0063] On the other hand, according to another aspect of the present invention, as described above, continuous annealing and aluminum-based plating can be performed on a cold-rolled steel sheet, but aluminum-based plating can also be performed on a cooled hot-rolled steel sheet immediately after pickling.

[0064] An embodiment of the hot-formed member of the present invention will be described in detail below. The hot-formed member of the present invention can be produced by hot press forming the above-described hot-forming steel material.

[0065] The microstructure of the hot-formed member can have a martensite single-phase structure or a mixed structure containing martensite and 40 area % or less of bainite. Because the martensite is an effective structure for ensuring the strength targeted by the present invention, the microstructure of the present invention can be a martensite single-phase structure. On the other hand, although bainite has a slightly lower strength than martensite, it is advantageous for ensuring strength without significantly reducing bendability when formed within a martensite matrix. Therefore, the present invention can also have a mixed structure containing 40 area % or less of bainite along with the martensite. However, if the bainite fraction exceeds 40 area %, it may be difficult to achieve the strength targeted by the present invention.

[0066] Meanwhile, the microstructure may further include one or more of 10% by area or less ferrite and 5% by area or less retained austenite. The ferrite and retained austenite are inevitably formed during the manufacturing process. If the ferrite structure exceeds 10% by area, not only the strength but also the bending properties may be significantly reduced. If the retained austenite structure exceeds 5% by area, the strength may be reduced or hydrogen embrittlement may occur due to increased inflow of hydrogen from the atmospheric gas during hot forming.

[0067] The hot-formed part may have a yield strength (YS) of 800 MPa or more, a tensile strength (TS) of 1000 MPa or more, and an elongation (El) of 3.5% or more.

[0068] The hot-formed part of the present invention may have a maximum bending angle variation of 5% or less. The maximum bending angle can be confirmed by a three-point bending test in accordance with VDA standard (VDA238-100). If the maximum bending angle variation exceeds 5%, the bending property or impact resistance may be deteriorated even if the physical properties are similar.

[0069] Next, a detailed description will be given of an example of a method for manufacturing a hot-formed part of the present invention. The manufacturing method described below is merely one embodiment among all possible embodiments, and does not necessarily mean that the hot-formed part of the present invention should be manufactured only by the manufacturing method described below.

[0070] The above-mentioned hot-formed steel material or the hot-formed steel material produced by the above-mentioned method is prepared, and a blank is produced using this. The blank is then heated to a temperature in the austenite single phase region or higher, more specifically, to a temperature of Ac3 to 980°C, and held for 1 to 1000 seconds.

[0071] If the blank heating temperature is lower than the Ac3 temperature, the presence of untransformed ferrite may make it difficult to ensure a predetermined strength. 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 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 even more preferably Ac3 + 10°C. The upper limit of the blank heating temperature is more preferably 970°C, and even more preferably 960°C.

[0072] If the holding time is less than 1 second, the temperature will not be uniform throughout the blank, which may lead to differences in material quality at different locations. If the holding time exceeds 1,000 seconds, excessive oxides may be generated on the surface of the component, as with excessive heating temperatures, making it difficult to ensure spot weldability. Therefore, the holding time is preferably 1 to 1,000 seconds. The lower limit of the holding time is more preferably 30 seconds, and even more preferably 60 seconds. The upper limit of the holding time is more preferably 900 seconds, and even more preferably 800 seconds.

[0073] The heated and held blank is then hot-formed and cooled to room temperature (forming cooling), finally producing a hot-formed part. The present invention does not particularly limit the specific conditions for the hot-forming, and any hot-forming method commonly known in the technical field to which the present invention pertains can be applied. A preferred example is a mold cooling method. [Example]

[0074] Next, an embodiment of the present invention will be described.

[0075] It goes without saying that various modifications of the following examples are possible within the scope of the present invention, as long as they are not deviated from the scope of the present invention, by a person having ordinary skill in the art to which the present invention pertains. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention should not be limited to the following examples, but should be defined by the claims below as well as equivalents thereof.

[0076] (Example) A 40 mm thick steel slab having the composition (wt %) shown in Table 1 below (the remainder being Fe and unavoidable impurities) was produced by vacuum melting. The steel slab was heated to 1250°C, hot rolled at a finish hot rolling temperature of 900°C, and coiled at a coiling temperature of 640°C to produce a hot-rolled steel sheet with a final thickness of 2.5 mm. The hot-rolled steel sheet was pickled and then cold-rolled at a cold reduction of 45% to produce a cold-rolled steel sheet. After annealing at 780°C, a normal annealing temperature, in a 5% hydrogen-95% nitrogen atmosphere, the cold-rolled steel sheet was cooled and then subjected to Al-based plating.

[0077] At this time, the composition of the Al-based plating bath is Al-9%Si-2%Fe and the rest is unavoidable impurities, and the plating weight is 70g / m on one side. 2 In order to impart a roughness to the surface of the steel sheet, temper rolling was further performed, and the roughness and rolling force of the temper rolling roll were changed to impart a deviation in the roughness. The roll roughness and rolling force imparted to each test piece are shown in Table 2.

[0078] The steel sheets thus produced were used to prepare blanks, which were then hot-formed using a hot-forming die to produce hot-formed parts. The blanks were heated to 930°C for 5 minutes, and the transfer time from the heating furnace to the forming stage was 10 seconds.

[0079] The yield strength (YS), tensile strength (TS) and elongation (El) were measured by taking ASTM standard test pieces in a direction perpendicular to the rolling direction of the steel sheet and then conducting a tensile test.

[0080] Bendability, an important indicator of crash energy absorption capacity, was measured by a three-point bending test in accordance with the VDA standard (VDA238-100). The crash energy absorption capacity of a material can be evaluated by calculating the area from the load-displacement curve obtained in the three-point bending test to the maximum load (Crack Initiation Energy, CIE). Figure 2 shows a simplified representation of the CIE concept, which is the standard for evaluating crash energy absorption capacity.

[0081] [Table 1]

[0082] [Table 2]

[0083] In Table 2 above, Relational Formula 1 is the Surface Roughness Factor.

number

[0084] Relation 2 is

number

[0085] As can be seen from Tables 1 and 2 above, it was confirmed that excellent bendability can be ensured when the alloy composition and temper rolling conditions proposed by the present invention are all met and a surface roughness index of 1.8 μm or less is ensured.

[0086] Specifically, comparing Example 1 with Comparative Examples 1 and 2, both of which were manufactured using the same steel type A, Example 1, which satisfies the conditions of the present invention, exhibits a maximum bending angle of 60.14° and a CIE strength of 29,692 Nm, demonstrating excellent bendability and crashworthiness. However, while Comparative Examples 1 and 2 have similar strength after hot forming to Example 1, the temper rolling condition [Relationship 2] exceeds the upper limit of 40, resulting in a surface roughness index outside the range of the present invention. As a result, due to the surface notch effect, the final bending angle change was confirmed to exceed 5% compared to Example 1, confirming a decrease in bendability.

[0087] Inventive Examples 2 to 4 and Comparative Example 3 were all manufactured using the same steel type B, but Inventive Example 2, which satisfies the conditions of the present invention, exhibited excellent bendability and crash resistance properties, while Inventive Examples 3 and 4 all showed a reduction in maximum bend angle of 5% or less compared to Inventive Example 2. However, in Comparative Example 3, the temper rolling condition [Relationship 2] exceeded the upper limit of 40, and the surface roughness index was outside the range of the present invention, and a significant decrease in bendability and crash resistance properties was confirmed compared to Inventive Example 3.

[0088] Inventive Example 5 and Comparative Examples 4 to 5 were manufactured using the same C steel type, and Inventive Example 5 had a maximum bending angle of 42° and achieved a CIE strength of 39566 Nm. However, in Comparative Examples 4 to 5, the temper rolling condition [Relationship 2] exceeded the upper limit of 40, and the surface roughness index fell outside the range of the present invention. Compared to Inventive Example 5, a change in bending angle exceeding 5% was confirmed, and a decrease in bendability and crash resistance was confirmed.

[0089] Example 2 Steels having the steel compositions listed in Table 3 below were produced using the same steelmaking, hot rolling, cold rolling, and annealing processes as in Example 1. No additional plating was performed during this process. Temper rolling was performed on the annealed steel sheets that had passed through the annealing process to impart roughness, and the temper-rolled annealed steel sheets were further electroplated to prevent surface decarburization that may occur during the hot forming process. Blanks were produced using the steel sheets produced in this manner, and then hot-formed using a hot forming die to produce hot-formed parts. The heating temperature of the blanks was 900°C, the holding time was 6 minutes, and the transfer time from the heating furnace to the forming was 10 seconds.

[0090] [Table 3]

[0091] [Table 4]

[0092] In Table 4 above, Relational Expressions 1 and 2 are the same as those in Table 2 of the first embodiment described above.

[0093] Looking at Table 4 for Invention Examples 6 to 8 and Comparative Example 6, which were manufactured using steel type D in Table 3, it was confirmed that Invention Example 6 had a maximum bending angle of 58.5°, and had very good bendability. Invention Examples 7 and 8 also satisfy the conditions of the present invention, and although there was a change in the bending angle compared to Invention Example 6, both were 5% or less, confirming that they had good bendability and crash resistance properties.

[0094] On the other hand, in the case of Comparative Example 6, which was manufactured using the same D steel type and exhibited strength after hot forming, the alloy composition satisfied the range of the present invention, but the value calculated by Relational Formula 2 exceeded 40. As a result, the surface roughness index exceeded the range of the present invention, and it was confirmed that the final bending angle change amount exceeded 5% compared to Invention Example 6 due to the surface notch effect.

Claims

1. In mass%, it contains 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, the balance being Fe and inevitable impurities, A steel material for hot forming, having a surface roughness factor calculated by the following [Relationship 1] of 1.8 μm or less. [Relationship 1] [Equation 1] (The Rt is defined as the vertical distance between the highest peak and the deepest valley in any measurement section on the steel sheet surface, and Rdq is the root mean square of the gradient of the peaks in any measurement section on the steel sheet surface.)

2. The steel material for hot forming according to claim 1, further containing one or more of 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. 2. The steel material for hot forming according to claim 1, wherein the microstructure of the steel material contains, in area fraction, 50 to 90% ferrite and one or more of 30% or less pearlite, 20% or less bainite, and 20% or less martensite.

4. The steel material for hot forming according to claim 1 , further comprising a plating layer.

5. 5. The steel material for hot forming according to claim 4, wherein the plating layer contains, in mass%, Si: 6 to 12%, Fe: 1 to 4%, and the remainder being Al and inevitable impurities.

6. obtaining a cold-rolled steel sheet using a steel slab containing, in mass%, 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, the balance being Fe and unavoidable impurities; and temper rolling the cold-rolled steel sheet so as to satisfy the following [Relationship 2]. [Relationship 2] [Equation 2] (The above P is the rolling force during temper rolling, Ra ｒｏｌｌ is the arithmetic mean roughness (Ra) of the temper roll.

7. 7. The method for producing a steel material for hot forming according to claim 6, wherein the cold-rolled steel sheet further contains one or more of 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.

8. The step of obtaining the cold-rolled steel sheet comprises: heating the steel slab to 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 at a reduction rate of 30 to 80% 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 an annealing temperature of 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.

9. The method for producing a steel material for hot forming according to claim 8, wherein the dew point temperature of the atmospheric gas during the annealing is −70 to −30° C.

10. The method for producing a steel material for hot forming according to claim 8, further comprising the step of immersing the annealed cold-rolled steel sheet in an Al-based plating bath to form an aluminum plating layer after cooling the annealed cold-rolled steel sheet.

11. The method for producing a steel material for hot forming according to claim 10, wherein the Al-based plating bath contains, in mass%, Si: 6 to 12%, Fe: 1 to 4%, the balance being Al and inevitable impurities.

12. Obtaining a blank using the hot forming steel material according to any one of claims 1 to 5; heating the blank to a temperature of Ac3 to 980°C and then holding the temperature for 1 to 1000 seconds; and cooling the heated and held blank after hot forming.

13. The method for producing a hot-formed part according to claim 12, wherein the cooling is performed by a mold cooling method.

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