Steel material for thermoforming, thermoformed part, and method for manufacturing the same

Abstract

The present invention relates to a hot-stamping steel material for automobiles and the like, a hot-stamping member, and a method for manufacturing the same. In the present invention, when the formed steel material is subjected to skin pass rolling, bendability can be improved by controlling the reduction force and the roughness of the skin pass roll.

Description

Technical Field

[0001] This invention relates to a hot-formed steel for automobiles and the like, hot-formed parts, and methods for manufacturing the same. Background Technology

[0002] In recent years, fuel efficiency has been improved through vehicle lightweighting. This can be achieved by reducing the thickness of steel, but reducing thickness can compromise vehicle stability; therefore, increasing steel strength is the primary focus. For this reason, there is a continuous demand for high-strength steel sheets, leading to the development of various types of steel. However, this high strength results in poor machinability.

[0003] To address this problem, hot forming was proposed. Hot forming involves processing steel at a suitable high temperature and then rapidly cooling it to a low temperature, thereby forming martensite and other low-temperature structures within the steel, thus increasing the strength of the final product. As described above, when manufacturing high-strength components, processability issues can be minimized.

[0004] Patent Document 1 relates to this thermoforming technology. Patent Document 1 proposes a technology that heats Al-Si coated steel sheet to above 850°C and then forms the microstructure of the part into martensite by thermoforming and rapid cooling using a stamping press, thereby ensuring an ultra-high strength of over 1600 MPa in tensile strength.

[0005] Furthermore, the thermoformed components used for passenger protection should possess excellent impact resistance characteristics, and flexural strength is widely used as a representative indicator for evaluating such impact resistance characteristics. For example, in the case of automotive B-pillars, when a thermoformed component bends due to a side impact on the vehicle, it needs to have the characteristic of not breaking and remaining supportive above a certain distance (angle) (flexural strength).

[0006] Patent document 2 proposes a method for controlling the ferrite structure of the surface layer of a thermoformed part. In addition, in order to compensate for the relatively poor energy absorption capacity, a technique for combining blanks (Tailor welded blanks, TWBs) with different materials or different thickness combinations into thermoforming is proposed, and various studies are underway.

[0007] However, controlling the ferrite microstructure of the surface layer by optimizing hot forming conditions has limitations in improving flexural properties. Furthermore, improving impact resistance through TWB has limitations in enhancing the properties of components requiring impact resistance; for example, deterioration of the weld joint can actually worsen flexural properties.

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

[0009] (Patent Document 2) Korean Patent No. 10-1569508 Summary of the Invention

[0010] Technical problems to be solved

[0011] One aspect of the present invention is to provide a hot-formed steel that can have high strength and excellent flexibility for hot-formed parts, as well as hot-formed parts made using the hot-formed steel and methods thereof.

[0012] The technical problems of this invention are not limited to those described above. Additional technical problems of this invention are described throughout the entire specification, and those skilled in the art can easily understand these additional technical problems from the content described in the specification.

[0013] Technical solution

[0014] One embodiment of the present invention provides a hot-forming steel, which, by weight percent, comprises: 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, and the balance being Fe and unavoidable impurities, and has a surface roughness factor of 1.8 μm or less calculated using the following [relationship 1].

[0015] [Relation 1]

[0016]

[0017] (Rt is defined as the vertical distance between the highest peak and the deepest valley in any measurement interval on the steel plate surface, and Rdq is the root mean square of the slope of the peak in any measurement interval on the steel plate surface.)

[0018] Another embodiment of the present invention provides a method for manufacturing steel for hot forming, comprising the following steps: obtaining a cold-rolled steel sheet from a steel billet, wherein the steel billet comprises, by weight %,: C: 0.04-0.45%, Si: less than 1.5% (excluding 0%), Mn: 0.2-2.5%, P: less than 0.05%, S: less than 0.02%, Al: 0.01-0.1%, Cr: 0.01-5.0%, N: less than 0.02%, and the balance being Fe and unavoidable impurities; and leveling the cold-rolled steel sheet to satisfy the following [relationship 2].

[0019] [Relationship 2]

[0020]

[0021] (P is the reduction force during leveling rolling, Ra) 轧辊 It is the arithmetic mean surface roughness (Ra) of the leveling rolls.

[0022] Another embodiment of the present invention provides a thermoformed component, which, by weight percent, comprises: C: 0.04-0.45%, Si: less than 1.5% (excluding 0%), Mn: 0.2-2.5%, P: less than 0.05%, S: less than 0.02%, Al: 0.01-0.1%, Cr: 0.01-5.0%, N: less than 0.02%, and the balance Fe and unavoidable impurities, wherein the maximum bending angle variation of the thermoformed component is less than 5%.

[0023] Another embodiment of the present invention provides a method for manufacturing a thermoformed part, comprising the steps of: obtaining a blank using the above-described thermoforming steel; heating the blank to a temperature of Ac3 to 980°C and holding it for 1 to 1000 seconds; and thermoforming the heated and held blank and then cooling it.

[0024] Beneficial effects

[0025] According to the present invention, a thermoformed component can be manufactured that, after thermoforming, possesses high strength and excellent flexibility, thereby exhibiting excellent impact resistance. The present invention also provides a thermoforming steel for the thermoformed component, a thermoformed component manufactured using the thermoforming steel, and a method for manufacturing the same.

[0026] The numerous advantages and effects of the present invention are not limited to the above description, and may be more readily understood in the process of describing the specific embodiments of the present invention. Attached Figure Description

[0027] Figure 1The concept of surface roughness factor involving [relation 1] proposed in this invention is simply illustrated below.

[0028] Figure 2 The concept of Crack Initiation Energy (CIE), which serves as a benchmark for evaluating collision energy absorption capacity, is simply illustrated in this invention.

[0029] Best practice

[0030] The terminology used in this specification is for illustrative purposes and is not intended to limit the invention. Furthermore, unless the relevant definitions explicitly state otherwise, the singular form used in this specification also includes the plural form.

[0031] The terms "comprising" or "including" as used in this specification are used to specifically describe the composition and do not exclude the presence or addition of other compositions.

[0032] Unless otherwise defined, all terms, including technical and scientific terms used herein, shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries are interpreted as having meanings consistent with relevant technical literature and currently disclosed content.

[0033] First, a specific embodiment of the hot-forming steel of the present invention will be described in detail. The steel of the present invention, by weight percent, may contain: 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, and the balance Fe and unavoidable impurities. The composition of each alloy will be described in detail below, where % represents weight percent.

[0034] Carbon (C): 0.04-0.45%

[0035] C is an essential element added to improve the strength of the component. When the C content is less than 0.04%, it is difficult to ensure sufficient strength, and ultimately, even with high flexibility, the impact energy absorption capacity will decrease. Therefore, it is preferable to add more than 0.04% C. On the other hand, when the C content exceeds 0.45%, the strength increases, but the flexibility decreases, thereby reducing the impact energy absorption capacity. Therefore, the C content is preferably less than 0.45%.

[0036] Silicon (Si): Less than 1.5% (except 0%)

[0037] The silicon (Si) is added as a deoxidizer in steelmaking, and it is also a solid solution strengthening element and a carbide formation inhibitor, which helps to improve the strength of hot-formed parts and serves as an effective material homogenizing element. When the Si content exceeds 1.5%, the plating properties may decrease due to the Si oxide formed on the surface of the steel sheet during annealing. Therefore, the Si content is preferably 1.5% or less (except 0%).

[0038] Manganese (Mn): 0.2-2.5%

[0039] The Mn content ensures solid solution strengthening, and its addition is necessary to suppress ferrite formation during hot forming by improving hardenability. When the Mn content is less than 0.2%, there are limitations in achieving the aforementioned effects; excessive amounts of other high-priced alloying elements are required to improve insufficient hardenability, potentially leading to a significant increase in manufacturing costs. On the other hand, when the Mn content exceeds 2.5%, the cold rollability may decrease before hot forming due to increased steel sheet strength, and the banded structure along the rolling direction in the microstructure deepens, potentially worsening impact energy absorption. Therefore, the preferred Mn content is 0.2-2.5%.

[0040] Phosphorus (P): less than 0.05%

[0041] P exists as an impurity in steel. When the P content exceeds 0.05%, it may significantly reduce the weldability of hot-formed parts. In addition, P is an unavoidable impurity in the manufacture of steel. There is no particular limit to the lower limit of P content, but controlling the P content to less than 0.001% may require a lot of manufacturing costs. Therefore, the P content can be above 0.001%.

[0042] Sulfur (S): less than 0.02%

[0043] Sulfur (S) exists as an impurity in steel, and it is an element that hinders the ductility, impact properties, and weldability of hot-formed parts. Therefore, it is preferable to limit the maximum content of S to 0.02%. In addition, since S is an unavoidable impurity, a lower limit for the S content may not be particularly limited. However, controlling the S content to less than 0.0001% may require significant manufacturing costs, so the S content can be 0.0001% or more.

[0044] Aluminum (Al): 0.01-0.1%

[0045] Al, along with Si, acts as a deoxidizer in steelmaking, improving the cleanliness of the steel. When the Al content is less than 0.01%, the above effect is difficult to achieve. When the Al content exceeds 0.1%, excessive AlN formed during continuous casting reduces high-temperature ductility, thus increasing the likelihood of slab cracking. Therefore, the preferred Al content is 0.01-0.1%.

[0046] Chromium (Cr): 0.01-5.0%

[0047] Similar to Mn, Cr is added to ensure the hardenability of the steel and a fine surface finish during the HPF process. When the Cr content is less than 0.01%, it may be difficult to ensure sufficient hardenability. On the other hand, when the Cr content exceeds 5.0%, the effect on improving hardenability is negligible compared to the amount added, and it promotes the formation of coarse Cr-based carbides, resulting in poor impact energy absorption capacity. Therefore, the Cr content is preferably no more than 5.0%.

[0048] Nitrogen (N): less than 0.02%

[0049] The nitrogen (N) is contained in the steel as an impurity. When the N content exceeds 0.02%, similar to the case of Al mentioned above, there is a problem that slab cracks are easily generated due to the formation of AlN. The lower limit of the N as an impurity is not particularly limited, but in order to control the N content to less than 0.001%, a lot of manufacturing costs are required, so the N content can be above 0.001%.

[0050] In addition to the alloy composition mentioned above, the steel may further contain one or more of the following: Mo: less than 0.5%, Ni: less than 0.5%, Nb: less than 0.1%, Ti: less than 0.1%, and B: less than 0.01%.

[0051] Molybdenum (Mo): less than 0.5%

[0052] Like Cr and Mn, Mo improves the hardenability of steel and, due to grain refinement through the formation of fine precipitates, increases flexibility and other properties. However, when the Mo content exceeds 0.5%, it leads to an excessive increase in the cost of the alloy iron compared to its benefits; therefore, the Mo content is preferably no more than 0.5%. More preferably, the Mo content is 0.45% or less, even more preferably 0.4% or less, and even more preferably 0.35% or less.

[0053] Nickel (Ni): less than 0.5%

[0054] Ni is an austenite stabilizing element, and its addition can improve the hardenability of steel. However, since Ni is a high-priced alloying element, considering the increase in manufacturing cost relative to the improvement in hardenability, it is preferable to set the upper limit of Ni content to 0.5%. Furthermore, to fully obtain the effect of improving hardenability from adding Ni, it is preferable to contain at least 0.01% Ni, more preferably at least 0.03%, and even more preferably at least 0.05%. The upper limit of Ni is more preferably 0.45%, even more preferably 0.4%, and most preferably 0.35%.

[0055] Niobium (Nb): less than 0.1%

[0056] The Nb is an element that can achieve precipitation strengthening by forming fine precipitates, thereby improving strength and improving flexibility due to grain refinement. Furthermore, during the heating process for thermoforming, robustness to changes in heat treatment conditions can be achieved by suppressing excessive grain growth. However, when the Nb content exceeds 0.1%, its effect saturates, and the increase in relatively coarse precipitates due to the increase in precipitation temperature may reduce 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%, even more preferably 0.08%, and most preferably 0.07%.

[0057] Titanium (Ti): less than 0.1%

[0058] The Ti is an element added along with B to ensure hardenability by combining with nitrogen, which remains as an impurity in the steel, to form TiN. Furthermore, precipitation strengthening and grain refinement effects are expected by forming TiC precipitates. However, when the Ti content exceeds 0.1%, a large amount of coarse TiN is formed, resulting in poor collision energy absorption capacity; therefore, the upper limit of the Ti content is preferably 0.1%. The lower limit of the Ti content is preferably 0.005%, more preferably 0.01%, and even more preferably 0.015%. The upper limit of the Ti content is more preferably 0.08%, even more preferably 0.06%, and most preferably 0.05%.

[0059] Boron (B): less than 0.01%

[0060] The B element is one that can improve hardenability even in small amounts, and can effectively suppress the brittleness of hot-formed parts caused by grain boundary segregation of P and / or S by segregating at the original austenite grain boundaries. However, when the B content exceeds 0.01%, due to Fe... 23The formation of the CB6 composite compound causes brittleness during hot rolling; therefore, the upper limit of the B content is preferably 0.01%. Furthermore, 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%, even more preferably 0.007%, and most preferably 0.005%.

[0061] The remainder contains iron (Fe), and may inevitably be introduced from raw materials or the surrounding environment during normal manufacturing processes, making it impossible to eliminate these impurities. These impurities are well known to those skilled in the art of iron and steel manufacturing, and therefore their contents are not specifically described in this specification.

[0062] The hot-forming steel of the present invention comprises the alloy composition described above, and the surface roughness coefficient defined by the following [Equation 1] is preferably 1.8 μm or less. Surface roughness can be expressed in various ways (Ra, Rt, and Rsk, etc.), but it is difficult to improve the bendability in hot-formed parts simply by changing the Ra and Rsk of the steel. The inventors of the present invention studied the results of improving the bendability of hot-formed parts and realized that the bendability of hot-formed parts can be improved when the surface roughness of the steel is constantly controlled. In order to ensure the bendability of hot-formed parts, the surface roughness coefficient of the following [Equation 1] is obtained by obtaining the technical correlation between Rt and Rdq, rather than simply measuring Rt and Rdq. Therefore, in order to improve the bendability for improving the impact energy absorption capacity in hot-formed parts, the surface roughness coefficient in the hot-formed steel is preferably 1.8 μm or less. When the surface roughness coefficient exceeds 1.8 μm, the slope of the peak increases, and the bendability may deteriorate due to the maximization of the surface notch effect during bending.

[0063] [Relation 1]

[0064]

[0065] Where Rt is defined as the vertical distance between the highest peak and the deepest valley in any measurement interval on the steel plate surface, and Rdq is the root mean square of the slope of the peak in any measurement interval on the steel plate surface. Figure 1 An example of how Rt and Rdq are calculated using the aforementioned relation 1 is shown. Those skilled in the art can easily obtain Rt and Rdq using the above method.

[0066] Based on area fraction, the microstructure of the hot-forming steel of the present invention may include one or more of the following: 50-90% ferrite, less than 30% pearlite, less than 20% bainite, and less than 20% martensite.

[0067] The ferrite is a soft phase and is a microstructure that effectively reduces the blanking process load on the steel during blank making. Therefore, the ferrite is preferably 50% or more by area. However, when the ferrite exceeds 90% by area, carbon may become unevenly distributed even after hot forming because carbon is excessively distributed to the microstructure outside the ferrite during blank making. Therefore, the ferrite is preferably 50-90% by area.

[0068] When the pearlite content exceeds 30% by area, the cementite will not completely dissolve after hot forming, which may cause a decrease in strength or uneven material composition. In addition, when the bainite or martensite content exceeds 20% by area, the strength of the steel sheet will increase excessively, which may cause problems such as mold wear during blank manufacturing.

[0069] Furthermore, at least one side of the hot-formed steel of the present invention may include a coating. The type of coating is not particularly limited; for example, it may be a zinc (Zn)-based coating, an aluminum (Al)-based coating, etc., and there are no particular limitations on methods such as hot-dip galvanizing and electroplating. As a preferred example, an Al-based coating may be formed. There are no particular limitations on the Al-based coating, but as an example, by weight percent, the Al-based coating may contain: Si: 6-12%, Fe: 1-4%, and the balance Al and unavoidable impurities.

[0070] Next, a specific embodiment of the hot-formed steel 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 imply that the hot-formed steel of the present invention must be manufactured solely by the following manufacturing method.

[0071] Steel is manufactured by using a steel billet that satisfies the above alloy composition to produce a cold-rolled steel sheet, and then flattening and rolling the cold-rolled steel sheet to satisfy the following [relationship 2].

[0072] [Relationship 2]

[0073]

[0074] (P is the reduction force during leveling rolling, Ra) 轧辊 It is the arithmetic mean surface roughness (Ra) of the leveling rolls.

[0075] The surface roughness of the cold-rolled steel sheet is controlled by performing a leveling roll. In this invention, the reduction force (P) during leveling roll and the arithmetic mean roughness (Ra) of the roll are taken into account. 轧辊The technology utilizes the ability to optimize the surface of steel, thereby obtaining the aforementioned [Relationship 2]. The reduction force (P) during leveling rolling is an important factor, but its upper or lower limit is not specifically limited in this invention. However, for example, problems such as coiling defects may occur when no reduction force is applied; therefore, the reduction force can be 100 tons or more, more preferably 150 tons or more. Furthermore, when the reduction force is too high, surface coating cracking may occur; when [Relationship 2] exceeds 40, its upper limit can be limited. For example, when the arithmetic mean roughness of the leveling roll is 4 μm, the reduction force is preferably 400 tons or less to satisfy [Relationship 2].

[0076] The cold-rolled steel sheet can be obtained by heating, hot rolling, coiling, cooling, cold rolling, and annealing the steel billet. The following describes each process.

[0077] Heating of steel billets

[0078] The steel billet is heated at 1050-1300°C. When the heating temperature of the steel billet is below 1050°C, the microstructure of the steel billet is difficult to homogenize, and resolution may be difficult when using precipitated elements. On the other hand, when the heating temperature exceeds 1300°C, the possibility of surface defects after hot rolling may increase due to the formation of excessive oxide layers. Therefore, the heating temperature of the steel billet is preferably 1050-1300°C. The lower limit of the heating temperature of the steel billet is more preferably 1070°C, and even more preferably 1100°C. The upper limit of the heating temperature of the steel billet is more preferably 1280°C, and even more preferably 1250°C.

[0079] Hot rolling

[0080] The heated steel billet is hot-rolled and then hot-finished at 800-950°C to obtain a hot-rolled steel sheet. When the hot-finishing temperature is below 800°C, a mixed grain structure in the surface layer of the steel sheet is generated due to rolling in a two-phase region, making it difficult to control the sheet shape. On the other hand, when the hot-finishing temperature exceeds 950°C, grain coarsening caused by hot rolling is likely to occur. Therefore, the hot-finishing temperature is preferably 800-950°C. The lower limit of the hot-finishing temperature is more preferably 810°C, and even more preferably 820°C. The upper limit of the hot-finishing temperature is more preferably 940°C, and even more preferably 930°C.

[0081] Collect

[0082] The hot-rolled steel sheet is coiled at 500-700°C. When the coiling temperature is below 500°C, martensite forms in all or part of the steel sheet, making it difficult to control the sheet shape. Furthermore, the increased strength of the hot-rolled steel sheet may lead to reduced rollability in subsequent cold-rolling processes. On the other hand, when the coiling temperature exceeds 700°C, coarse carbides form, potentially reducing the impact energy absorption capacity of the hot-formed part. Therefore, the coiling temperature is preferably 500-700°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.

[0083] cool down

[0084] The hot-rolled steel sheet is cooled from the coiling temperature to 400°C (hot-rolled cooling) at a cooling rate of 10°C / hour or higher. However, when the cooling rate is less than 10°C / hour, sufficient time is allowed for carbide growth, which may result in the formation of a large number of coarse carbides during the cooling of the hot-rolled coil. Therefore, the cooling rate is preferably 10°C / hour or higher, more preferably 12°C / hour or higher, and even more preferably 15°C / hour or higher. Furthermore, as long as the cooling rate is 10°C / hour or higher, the desired effect of the present invention can be obtained; therefore, the upper limit of the cooling rate is not particularly limited.

[0085] Additionally, after cooling, a pickling process can be added before cold rolling. This pickling process removes the oxide scale formed on the surface of the steel sheet, thereby improving the surface quality of the product.

[0086] Cold rolling

[0087] Following the above process, hot-rolled steel sheets can be cold-rolled to obtain cold-rolled steel sheets. In this invention, the reduction rate during cold rolling is not particularly limited, but a reduction rate of 30-80% can be applied to obtain the desired steel thickness.

[0088] Annealing and cooling

[0089] The cold-rolled steel sheet is annealed, and for this purpose, the cold-rolled steel sheet is heated. Preferably, the heating rate is 20°C / second or less within the temperature range of 400°C to the annealing temperature. When the heating rate from 400°C to the annealing temperature exceeds 20°C / second, the time for resolution of the carbides precipitated during the hot rolling step is insufficient, potentially resulting in the presence of coarse carbides and a decrease in 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 / second or less. More preferably, the heating rate is 18°C / second or less, and even more preferably 15°C / second or less. Furthermore, in this invention, as long as the heating rate is 20°C / second or less, the desired effects of this invention can be obtained; therefore, the lower limit of the heating rate is not particularly limited. However, considering annealing productivity, the heating rate can be 0.5°C / second or more, more preferably 1°C / second or more, and even more preferably 1.5°C / second or more. Furthermore, in this invention, the heating rate is not particularly limited within the temperature range from cold rolling temperature to below 400°C, because even if the heating rate is controlled, the effect on the resolution of carbides is negligible.

[0090] Preferably, the heated cold-rolled steel sheet is annealed at an annealing temperature of 740-860°C. When the annealing temperature is below 740°C, insufficient recrystallization of the cold-rolled structure leads to poor sheet shape or excessively high strength after plating, which may cause die wear during the stamping process. On the other hand, when the annealing temperature exceeds 860°C, surface oxides such as Si and Mn are formed during the annealing process, which may cause a deterioration of the plating surface. Therefore, the annealing temperature is preferably 740-860°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.

[0091] The atmosphere used during annealing is preferably a non-oxidizing atmosphere. For example, a mixture of hydrogen and nitrogen can be used, in which case the dew point temperature of the atmosphere can be from -70°C to -30°C. To keep the dew point temperature below -70°C, additional equipment for temperature control is required, thus increasing manufacturing costs. When the dew point temperature exceeds -30°C, excessive annealing oxides form on the steel plate surface during annealing, potentially causing defects such as incomplete plating. Therefore, the dew point temperature of the atmosphere during continuous annealing is preferably from -70°C to -30°C. The lower limit of the dew point temperature of the atmosphere is more preferably -65°C, and even more preferably -60°C. The upper limit of the dew point temperature of the atmosphere is more preferably -35°C, and even more preferably -40°C.

[0092] The annealed cold-rolled steel sheet is cooled from the annealing temperature to 660°C at a cooling rate of 1°C / second or higher (annealing cooling). When the cooling rate is less than 1°C / second, a large amount of coarse carbides are formed, which may reduce the impact energy absorption capacity of the final hot-formed part. Therefore, the cooling rate is preferably 1°C / second or higher. The cooling rate is more preferably 1.5°C / second or higher, and even more preferably 2°C / second or higher. There is no particular limitation on the upper limit of the cooling rate. However, in terms of suppressing shape defects in the steel sheet, the cooling rate can be 50°C / second or lower, more preferably 45°C / second or lower, and even more preferably 40°C / second or lower.

[0093] Furthermore, the annealed cold-rolled steel sheet can be further coated before the leveling rolling process. The present invention does not impose particular limitations on the type and method of coating, but an example of Al-based coating will be described. In this coating, the annealed cold-rolled steel sheet is cooled and immersed in an Al-based coating bath to form an aluminum-based coating. The composition of the Al-based coating bath and the coating conditions are not particularly limited.

[0094] However, as a non-limiting example, the composition of the plating bath, by weight percent, may include: Si: 6-12%, Fe: 1-4%, and the balance Al and other unavoidable impurities, and the plating amount may be 30-130 g / m² based on a single side, as commonly used in this art. 2 When the Si content in the plating bath composition is less than 6% by weight, the excessive increase in plating bath temperature degrades the equipment. When the Si content exceeds 12% by weight, alloying is excessively delayed, requiring extended heating time for hot forming. When the Fe content is less than 1% by weight, plating adhesion or spot weldability may deteriorate. When the Fe content exceeds 4% by weight, excessive slag is generated in the plating bath, potentially leading to surface quality defects. Based on a single side, when the plating adhesion is less than 30 g / m... 2 At times, it may be difficult to ensure the desired corrosion resistance of thermoformed parts when the coating adhesion exceeds 130 g / m². 2 At the same time, due to excessive coating, not only does the manufacturing cost increase, but it may also be difficult to coat the steel plate with a uniform coating amount throughout the entire width and length of the coil.

[0095] In addition, as with another aspect of the invention, cold-rolled steel sheets can be continuously annealed and coated with aluminum, but hot-rolled steel sheets can also be coated with aluminum immediately after pickling.

[0096] The following provides a detailed description of a specific embodiment of the thermoformed component of the present invention. The thermoformed component of the present invention can be manufactured by hot-pressing the aforementioned thermoforming steel.

[0097] The microstructure of the thermoformed component can have a single-phase martensitic structure or a mixed structure comprising martensite and less than 40% bainite by area. Martensite is a structure that effectively ensures the strength desired by the present invention; therefore, the microstructure of the present invention can be a single-phase martensitic structure. Furthermore, although bainite is a structure with slightly lower strength than martensite, when formed in a martensitic matrix, it does not significantly reduce flexibility and is beneficial for ensuring strength; therefore, the present invention can also have a mixed structure comprising the martensite and less than 40% bainite by area. However, when the bainite fraction exceeds 40% by area, it may be difficult to ensure the strength desired by the present invention.

[0098] Furthermore, the microstructure may further comprise one or more of ferrite (less than 10% by area) and retained austenite (less than 5% by area). The ferrite and retained austenite may inevitably form during the manufacturing process. When the ferrite microstructure exceeds 10% by area, not only does the strength decrease, but the bending properties may also decrease significantly. When the retained austenite microstructure exceeds 5% by area, the strength decreases or the introduction of hydrogen from the atmospheric gas during hot forming increases, thus increasing the likelihood of hydrogen embrittlement.

[0099] The yield strength (YS) of the thermoformed part can be above 800MPa, the tensile strength (TS) can be above 1000MPa, and the elongation (E1) can be above 3.5%.

[0100] The variation in the maximum bending angle of the thermoformed part of the present invention can be less than 5%. The maximum bending angle can be determined by a three-point bending test according to VDA standard (VDA238-100). When the variation in the maximum bending angle exceeds 5%, the bending and impact characteristics may deteriorate with similar physical properties.

[0101] Next, a specific embodiment of the method for manufacturing thermoformed parts according to 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 thermoformed parts of the present invention must be manufactured solely by the following manufacturing method.

[0102] Prepare the above-mentioned hot-formed steel or hot-formed steel manufactured by the above method, use the hot-formed steel to manufacture a blank, heat the blank to above the temperature of the austenitic single-phase region, more specifically to a temperature of Ac3-980°C, and then hold it for 1-1000 seconds.

[0103] When the heating temperature of the blank is below Ac3, the required strength may be difficult to ensure due to the presence of ferrite that has not undergone phase transformation. On the other hand, when the heating temperature exceeds 980°C, spot weldability may be difficult to ensure due to the formation of excessive oxides on the surface of the component. Therefore, the heating temperature of the blank is preferably between Ac3 and 980°C. The lower limit of the heating temperature of the blank is more preferably Ac3 + 5°C, and even more preferably Ac3 + 10°C. The upper limit of the heating temperature of the blank is more preferably 970°C, and even more preferably 960°C.

[0104] When the holding time is less than 1 second, uneven temperature distribution across the blank may lead to material variations in different parts. When the holding time exceeds 1000 seconds, it is similar to excessively high heating temperatures, resulting in excessive oxide formation on the component surface, which may make it difficult to ensure spot weldability. Therefore, the holding time is preferably 1-1000 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.

[0105] Subsequently, the heated and held blank is thermoformed and then cooled to room temperature (forming cooling) to finally manufacture the thermoformed part. In this invention, the specific conditions for thermoforming are not particularly limited, and thermoforming processes known in the technical field to which this invention pertains can still be applied. As a preferred example, a mold cooling method can be used. Detailed Implementation

[0106] Next, embodiments of the present invention will be described.

[0107] Those skilled in the art to which this invention pertains can make various modifications to the following embodiments without departing from the scope of this invention. The following embodiments are used to understand this invention, and the scope of this invention is not limited to the following embodiments, but should be defined by the scope of the claims and their equivalents.

[0108] (Example)

[0109] A steel billet with a thickness of 40 mm and the composition (wt%, balance being Fe and unavoidable impurities) shown in Table 1 below is manufactured by vacuum melting. After heating the billet to 1250°C, it is hot-rolled at a hot finishing 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. After pickling the hot-rolled steel sheet, it is cold-rolled with a cold-rolling reduction of 45% to produce a cold-rolled steel sheet. Annealing is performed at a temperature of 780°C, the conventional annealing temperature, in a 5% hydrogen-95% nitrogen atmosphere. The cold-rolled steel sheet is then cooled and subsequently subjected to Al-based plating.

[0110] At this point, the Al-based plating bath composition consists of Al-9%Si-2%Fe and the balance being unavoidable impurities. Based on a single-sided reference, the plating adhesion is set at 70 g / m². 2 To impart surface roughness to the aforementioned steel plates, further leveling rolling is performed, and to adjust the roughness deviation, the leveling roll roughness and pressing force are varied. Table 2 records the roll roughness and pressing force applied to each test piece.

[0111] The steel plate manufactured as described above is made into a blank, and then thermoformed using a thermoforming mold to manufacture a thermoformed part. At this time, the blank is heated to 930°C and held for 5 minutes. The transfer time from the heating furnace to the forming process is the same, both being 10 seconds.

[0112] Yield strength (YS), tensile strength (TS), and elongation (E1) are measured by performing tensile tests on ASTM standard specimens taken in a direction perpendicular to the rolling direction of the steel plate.

[0113] Bending capacity, an important indicator of impact energy absorption, is determined by a three-point bending test according to the VDA standard (VDA238-100). The impact energy absorption capacity of a material can be evaluated by calculating the area (crack initiation energy (CIE)) up to the maximum load from the load-displacement curves obtained from the three-point bending test. Figure 2 The concept of CIE, which serves as a benchmark for evaluating the energy absorption capacity of a collision, is simply illustrated in the text.

[0114] [Table 1]

[0115]

[0116] [Table 2]

[0117]

[0118] In Table 2, Equation 1 represents the surface roughness coefficient.

[0119]

[0120] (Rt is defined as the vertical distance between the highest peak and the deepest valley in any measurement interval on the steel plate surface, and Rdq is the root mean square of the slope of the peak in any measurement interval on the steel plate surface.)

[0121] Relation 2 represents

[0122] (P is the reduction force during leveling rolling, Ra) 轧辊 It is the arithmetic mean roughness (Ra) of the leveling roll.

[0123] As can be seen from Tables 1 and 2, when the surface roughness coefficient below 1.8 μm is ensured by satisfying the alloy composition and level rolling conditions proposed in this invention, excellent bending performance can be ensured.

[0124] Specifically, when comparing Invention Example 1 and Comparative Examples 1 to 2, all were manufactured using the same grade A steel. Invention Example 1, which met the conditions of the present invention, ensured a maximum bending angle of 60.14° and a CIE of 29692 Nm, thereby ensuring excellent bending and impact resistance. However, Comparative Examples 1 to 2 had similar strength after hot forming as Invention Example 1, but [Relationship 2], which is a condition for leveling rolling, exceeded the upper limit of 40, so the surface roughness coefficient was not within the range of the present invention. As a result, due to the surface notch effect, it was confirmed that the final bending angle change exceeded 5% compared to Invention Example 1, thus reducing bending performance.

[0125] Examples 2 to 4 and Comparative Example 3 were all manufactured using the same B steel grade. However, Example 2, which met the conditions of the present invention, ensured excellent bending and impact resistance. Compared with Example 2, the maximum bending angle of Examples 3 to 4 was reduced by less than 5%. However, in Comparative Example 3, [Relationship 2], which is a leveling rolling condition, exceeded the upper limit of 40, and the surface roughness coefficient was not within the range of the present invention. Compared with Example 3, a significant reduction in bending and impact resistance was confirmed.

[0126] Invention Example 5 and Comparative Examples 4 to 5 were manufactured from the same C steel grade. Invention Example 5 has a maximum bending angle of 42° and can ensure a CIE of 39566 Nm. However, the [Relationship 2] for the leveling rolling conditions of Comparative Examples 4 to 5 exceeds 40 as the upper limit, and the surface roughness coefficient is not within the range of the present invention. Compared with Invention Example 5, a bending angle change of more than 5% is confirmed, thus confirming a reduction in bending and impact resistance properties.

[0127] (Example 2)

[0128] Steel with the steel composition described in Table 3 below was manufactured using the same steelmaking, hot rolling, cold rolling, and annealing processes as in Example 1, without further plating. To impart roughness to the annealed steel sheet after the annealing process, a leveling rolling process was performed, and to prevent surface decarburization that might occur during the hot forming process of the leveled rolled annealed steel sheet, further electroplating was performed. After the steel sheet manufactured as described above was formed into a blank, it was hot-formed using a hot-forming die to manufacture a hot-formed part. At this time, the blank was heated to 900°C, held for 6 minutes, and the transfer time from the heating furnace to the forming process was the same, both being 10 seconds.

[0129] [Table 3]

[0130] steel grades C Si Mn P S Al Cr N Ti B D 0.222 0.26 2.15 0.013 0.0001 0.022 0.215 0.0036 0.028 0.0018

[0131] [Table 4]

[0132]

[0133] In Table 4, Relationship 1 and Relationship 2 are the same as those in Table 2 of Embodiment 1 above.

[0134] In Table 4, referring to Invention Examples 6 to 8 and Comparative Example 6 manufactured from Grade D steel in Table 3, it can be confirmed that the maximum bending angle of Invention Example 6 is 58.5°, exhibiting excellent bending performance. Invention Examples 7 and 8 also meet the conditions of the present invention. Compared with Invention Example 6, there is a change in bending angle, but both are less than 5%, confirming good bending performance and impact resistance.

[0135] On the other hand, in the case of Comparative Example 6, although the same D steel grade was used for manufacturing, it exhibited strength after hot forming and the alloy composition met the scope of the present invention, but the value of Relation 2 exceeded 40, resulting in a surface roughness coefficient exceeding the scope of the present invention. Due to the surface notch effect, it can finally be confirmed that it showed a bending angle change of more than 5% compared with Invention Example 6.

Claims

1. A hot-forming steel, by weight percent, comprising: C: 0.04-0.45%, Si: less than 1.5% and excluding 0%, Mn: 0.2-2.5%, P: less than 0.05%, S: less than 0.02%, Al: 0.01-0.1%, Cr: 0.01-5.0%, N: less than 0.02%, the balance being Fe and unavoidable impurities, and having a surface roughness coefficient of less than 1.8 μm calculated using the following [Equation 1]. [Relation 1] Rt is defined as the vertical distance between the highest peak and the deepest valley in any measurement interval on the steel plate surface, and Rdq is the root mean square of the slope of the peak in any measurement interval on the steel plate surface.

2. The hot-forming steel according to claim 1, wherein, The steel further comprises one or more of the following: Mo: less than 0.5%, Ni: less than 0.5%, Nb: less than 0.1%, Ti: less than 0.1%, and B: less than 0.01%.

3. The hot-forming steel according to claim 1, wherein, The fine microstructure of the steel, by area fraction, comprises 50-90% ferrite and one or more of pearlite (less than 30%), bainite (less than 20%), and martensite (less than 20%).

4. The hot-forming steel according to claim 1, wherein, The steel further includes a coating.

5. The hot-forming steel according to claim 4, wherein, The coating comprises, by weight percent: Si: 6-12%, Fe: 1-4%, and the balance Al and unavoidable impurities.

6. A method for manufacturing steel for hot forming, comprising the following steps: Cold-rolled steel sheets are obtained from steel billets, wherein the steel billets, by weight percent, comprise: C: 0.04-0.45%, Si: less than 1.5% and excluding 0%, Mn: 0.2-2.5%, P: less than 0.05%, S: less than 0.02%, Al: 0.01-0.1%, Cr: 0.01-5.0%, N: less than 0.02%, and the balance being Fe and unavoidable impurities; and The cold-rolled steel sheet is leveled and rolled to meet the following [relationship 2]. [Relation 2], , (tons·μm) 1 / 2 P is the reduction force during leveling rolling, Ra 轧辊 It is the arithmetic mean roughness Ra of the leveling roll.

7. The method for manufacturing hot-formed steel according to claim 6, wherein, The cold-rolled steel sheet further comprises one or more of the following: Mo: less than 0.5%, Ni: less than 0.5%, Nb: less than 0.1%, Ti: less than 0.1%, and B: less than 0.01%.

8. The method for manufacturing hot-formed steel according to claim 6, wherein, The steps to obtain the cold-rolled steel sheet include the following: The steel billet is heated at 1050-1300℃; The heated steel billet is hot-rolled at 800-950℃ to obtain hot-rolled steel plate; The hot-rolled steel sheet is coiled at 500-700℃; The hot-rolled steel sheet is cooled from the coiling temperature to 400°C at a cooling rate of 10°C / hour or higher. The cooled hot-rolled steel sheet is cold-rolled at a reduction rate of 30-80% to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is heated at a rate of less than 20°C / second within a temperature range of 400°C to annealing temperature. The heated cold-rolled steel sheet is annealed at an annealing temperature of 740-860℃; and The annealed cold-rolled steel sheet is cooled from the annealing temperature to 660°C at a cooling rate of 1°C / second or higher.

9. The method for manufacturing hot-formed steel according to claim 8, wherein, The dew point temperature of the atmosphere gas during annealing is -70°C to -30°C.

10. The method for manufacturing hot-formed steel according to claim 8, wherein, The method further includes the following steps: immersing the annealed cold-rolled steel sheet in an Al-based plating bath after cooling to form an aluminum coating.

11. The method for manufacturing hot-formed steel according to claim 10, wherein, The Al-based plating bath, by weight percent, comprises: Si: 6-12%, Fe: 1-4%, and the balance being Al and unavoidable impurities.

12. A thermoformed component, by weight percent, comprising: C: 0.04-0.45%, Si: less than 1.5% and excluding 0%, Mn: 0.2-2.5%, P: less than 0.05%, S: less than 0.02%, Al: 0.01-0.1%, Cr: 0.01-5.0%, N: less than 0.02%, and the balance Fe and unavoidable impurities, wherein the maximum bending angle variation of the thermoformed component is less than 5%, and the thermoformed component is a thermoformed component manufactured using thermoformed steel according to any one of claims 1 to 5.

13. The thermoformed part according to claim 12, wherein, The thermoformed part has a yield strength (YS) of 800 MPa or more, a tensile strength (TS) of 1000 MPa or more, and an elongation (E1) of 3.5% or more.

14. The thermoformed part according to claim 12, wherein, The microstructure of the thermoformed part is a single-phase martensitic structure or a mixed structure containing martensite and less than 40% bainite by area.

15. A method for manufacturing a thermoformed part, comprising the following steps: A blank is obtained using the hot-forming steel according to any one of claims 1 to 5; After heating the blank to a temperature of Ac3 to 980°C, hold it for 1-1000 seconds; and The heated and held blank is thermoformed and then cooled.

16. The method for manufacturing a thermoformed part according to claim 15, wherein, The cooling is performed using a mold cooling method.

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