Member for hot forming

Abstract

To provide a member for hot forming which has high strength and excellent bendability.SOLUTION: There is provided a member for hot forming manufactured using a steel material for hot forming comprising, by weight: 0.04 to 0.45% of C; 1.5% or less of Si (excluding 0%); 0.2 to 2.5% of Mn; 0.05% or less of P; 0.02% or less of S; 0.01 to 0.1% of Al; 0.01 to 5.0% of Cr; 0.02% or less of N, and a balance of Fe and inevitable impurities, wherein a surface roughness factor calculated by the following [Relational Expression 1] is 1.8 μm or less. [Relational Expression 1] where, Rt is defined as a vertical distance between the highest peak and the deepest valley in a random measurement section on a surface of a steel sheet, and Rdq is a root mean square of a slope of the peak in a random measurement section on the surface of the steel sheet.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot-formed steel material used in automobiles, etc., a hot-formed member, and a method for manufacturing the same. Regarding the method. [Background technology]

[0002] Recently, efforts have been made to improve fuel efficiency by reducing the weight of automobiles. However, reducing the thickness may cause problems with the stability of the vehicle. Therefore, the strength of the steel material must be improved. The demand for steel continues to grow, and various types of steel are being developed. Such steel materials have high strength, but have the problem of poor workability.

[0003] In order to solve these problems, a hot forming method has been proposed. After processing at a high temperature where it is easy to process, it is rapidly cooled to a low temperature to create martensite in the steel. This is a method to form low-temperature structures such as tensile strength, which increases the strength of the final product. In this case, workability problems when manufacturing high strength components can be minimized. .

[0004] Patent Document 1 describes a technique related to such hot forming. i After heating the coated steel sheet to 850°C or higher, the parts are assembled by hot forming using a press and rapid cooling. By forming martensite in the weave, ultra-high strength with a tensile strength of over 1600 MPa is achieved. We are proposing technologies to ensure this.

[0005] On the other hand, the hot-formed parts used for the purpose of protecting automobile occupants have excellent crashworthiness. The bending property is often used as a typical index for evaluating such crashworthiness. For example, in the case of components such as the B-pillar of an automobile, When a hot-formed part is bent due to a side impact, it can withstand a certain distance (angle) without breaking. The required property is bendability.

[0006] Patent Document 2 proposes a method for controlling the ferrite structure in the surface layer of a hot-formed member. In addition, to compensate for the relatively poor energy absorption capacity, different materials or Blanks with different thickness combinations (TWB, Tailor welded blanks) Technologies incorporating nk) into hot forming have been proposed and various research projects are being conducted.

[0007] However, the structure of the surface ferrite can be controlled by optimizing the hot forming conditions, which leads to poor bendability. Furthermore, there is a limit to the improvement of crashworthiness through TWB. This actually reduces bending properties, and there are limits to how much can be improved in parts that require crash resistance. there were. [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-formed member having high strength and excellent bendability. Hot forming steel material that can be used for hot forming, hot forming parts manufactured using the same, and their manufacturing methods The present invention aims to provide a manufacturing method.

[0010] The object of the present invention is not limited to the above. A further object of the present invention is to The present invention is described in detail and should not be construed as limiting the scope of the present invention. If there is any, it is not necessary to understand the further object of the present invention from the contents described in the specification of the present invention. There are no difficulties. [Means for solving the problem]

[0011] One aspect of the present invention is a composition containing, by weight, C: 0.04 to 0.45%, Si: 1.5% or less (0% excluding the above), Mn: 0.2 to 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, balance Fe and non-containing Contains unavoidable impurities, The surface roughness index (Surface Roughness Index) is calculated using the following [Equation 1]. To provide a hot forming steel material having a grain size (Gradient Factor) of 1.8 μm or less.

[0012] [Equation 1]

number

[0013] Another aspect of the present invention is a composition containing, by weight, C: 0.04 to 0.45%, Si: 1.5% or less ( 0%, Mn: 0.2 to 2.5%, P: 0.05% or less, S: 0.02% or less, A l:0.01~0.1%, Cr:0.01~5.0%, N:0.02% or less, balance Fe and obtaining a cold-rolled steel sheet using a steel slab containing the steel and inevitable impurities; and temper rolling the cold-rolled steel sheet so as to satisfy the following [Relationship 2]. The present invention provides a method for manufacturing steel for steelmaking.

[0014] [Equation 2]

number

[0015] Yet another embodiment of the present invention is a composition comprising, 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 Bottom, Al: 0.01~0.1%, Cr: 0.01~5.0%, N: 0.02% or less, balance Contains Fe and inevitable 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 the temperature for 1 to 1000 seconds; and, and cooling the heated and held blank after hot forming. A method for manufacturing a component is provided. [Effects of the Invention]

[0017] According to the present invention, the steel sheet has high strength after hot forming, excellent bendability, and crash resistance. Hot forming steel materials and the like for this purpose can be manufactured into hot formed parts with excellent properties. It is possible to provide a hot-formed part manufactured by the above method, as well as a method for manufacturing the same.

[0018] The various beneficial advantages and effects of the present invention are not limited to those described above, but may be varied by the specific embodiments of the present invention. This can be more easily understood in the course of describing the various embodiments. [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 terms used herein are for the purpose of describing the present invention and are not intended to limit the present invention. Also, as used herein, the singular forms "a," "the," and "the" are used unless the associated definition clearly indicates otherwise. Unless otherwise indicated, plural forms are included.

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

[0022] Unless otherwise specified, all terms, including technical and scientific terms, used herein are The same meaning as that generally understood by a person having ordinary skill in the technical field to which the present invention pertains. The dictionary-defined terms are consistent with the relevant technical literature and the present disclosure. It is interpreted as having meaning.

[0023] First, an embodiment of the hot forming steel material of the present invention will be described in detail. By weight, C: 0.04-0.45%, Si: 1.5% or less (excluding 0%), Mn: 0. 2~2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01~0.1%, Cr: 0.01 to 5.0%, N: 0.02% or less, balance Fe and unavoidable impurities Each alloy composition will be explained in detail below, with % meaning % by weight.

[0024] Carbon (C): 0.04~0.45% The above C is an essential element added to improve the strength of the component. If it is less than 0.04%, it is difficult to secure sufficient strength, and ultimately the bending property is high. However, the impact energy absorption capacity is rather reduced, so it is preferable to add 0.04% or more. On the other hand, if the C content exceeds 0.45%, the strength increases but the bendability decreases. However, the impact energy absorption capacity decreases, so it is preferably 0.45% or less.

[0025] Silicon (Si): 1.5% or less (excluding 0%) The above-mentioned Si is not only necessary to be added as a deoxidizer in steelmaking, but also as a solid solution strengthening element. It is also an element that inhibits the formation of carbides, contributing to the increase in strength of hot-formed parts and the uniformity of the material. If its content exceeds 1.5%, the Silicon oxides formed on the surface of steel sheets can reduce plating properties. The Si content is preferably 1.5% or less (excluding 0%).

[0026] Manganese (Mn): 0.2 to 2.5% The above Mn not only ensures the solid solution strengthening effect but also improves the hardening ability and thermal It is also necessary to add Mn to suppress the formation of ferrite during cold forming. If the content is less than 0.2%, there is a limit to the effect described above, and the insufficient hardening ability is improved. This requires excessive amounts of other expensive alloying elements, which significantly increases the manufacturing cost. On the other hand, if the Mn content exceeds 2.5%, the problem of The strength of the steel sheet increases and the cold rolling ability decreases, resulting in the rolling of the fine structure phase. Band-like structures arranged in the direction of the bone become deeper, reducing the ability to absorb impact energy. Therefore, the Mn content is preferably 0.2 to 2.5%.

[0027] Phosphorus (P): 0.05% or less The above-mentioned P exists as an impurity in steel, and if its content exceeds 0.05%, it On the other hand, the above P can cause defects during the manufacturing of steel materials. The lower limit of the unavoidable impurities is not particularly limited, but the P content should be 0.00 Controlling it to less than 1% can require significant manufacturing costs, so it is 0.001% It can be more than that.

[0028] Sulfur (S): 0.02% or less The above-mentioned S exists as an impurity in steel and inhibits the ductility, impact properties and weldability of hot-formed parts. It is preferable to limit the amount of S to a maximum of 0.02%. Impurities, the lower limit of which does not need to be specifically limited, but should be less than 0.0001% To control this, it may require a lot of manufacturing costs, so it is recommended to limit it to 0.0001% or more. It is possible.

[0029] Aluminum (Al): 0.01 to 0.1% The above-mentioned Al, together with Si, is an element that acts as a deoxidizer in steelmaking and increases the cleanliness of steel. If the Al content is less than 0.01%, it is difficult to obtain the above effect. In this case, excessive AlN formed during the continuous casting process reduces the high temperature ductility and Therefore, the Al content is set to 0.01 to 0.1%. It is preferable that:

[0030] Chromium (Cr): 0.01 to 5.0% The Cr mentioned above ensures the hardening ability of steel as well as a beautiful surface during the HPF process, just like Mn. If the Cr content is less than 0.01%, it is difficult to ensure sufficient hardening ability. On the other hand, if the content exceeds 5.0%, the improvement in hardening ability may be smaller than the amount added. The effect is slight, and it promotes the formation of coarse Cr-based carbides, reducing the impact energy absorption capacity. Therefore, it is preferable that the content does not exceed 5.0%.

[0031] Nitrogen (N): 0.02% or less The above-mentioned N is contained in steel as an impurity. If the N content exceeds 0.02%, the above-mentioned A As in the case of l, there is a problem that slab cracks are likely to occur due to the formation of AlN. The above-mentioned N is an impurity, and there is no need to particularly limit the lower limit of the N content. Controlling it to less than 0.001% can require significant manufacturing costs, so 0. It can be 0.01% or more.

[0032] On the other hand, the above steel material contains, in addition to the above alloying components, Mo: 0.5% or less, Ni: 0.5% or less Furthermore, one or more of Nb: 0.1% or less, Ti: 0.1% or less, and B: 0.01% or less It can also be included in:

[0033] Molybdenum (Mo): 0.5% or less Mo, like Cr and Mn, not only has the effect of improving the hardenability of steel, By forming fine precipitates, it is possible to obtain effects such as increased bendability due to the refinement of crystal grains. However, if the Mo content exceeds 0.5%, the cost of the ferroalloy is excessive compared to the effect. Therefore, it is preferable that its content does not exceed 0.5%. The Mo content is 0. It is more preferable that it is 45% or less, and even more preferable that it is 0.4% or less, and 0. It is even more preferable that it is 35% or less.

[0034] Nickel (Ni): 0.5% or less The above-mentioned Ni is an element that stabilizes austenite, and the addition of Ni improves the hardening ability of steel. However, since Ni is an expensive alloying element, it is difficult to improve the hardening ability. Considering the overall increase in manufacturing costs, it is preferable to set the upper limit at 0.5%. To fully obtain the hardening effect of adding Ni, it is necessary to include a minimum of 0.01%. It is preferable that the content is 0.03% or more, more preferable that the content is 0.05% or more. The upper limit of Ni is more preferably 0.45%, and more preferably 0.4%. It is more preferable that the content is 0.35%, and most preferable that the content is 0.35%.

[0035] Niobium (Nb): 0.1% or less The Nb is an element that can obtain a precipitation strengthening effect by forming fine precipitates, This provides the effect of increasing strength and improving bendability by making the crystal grains finer. In addition, excessive grain growth is suppressed during heating for hot forming, and fluctuations in heat treatment conditions are minimized. However, if the Nb content exceeds 0.1%, this effect is reduced. Not only does the effect saturate, but the increase in precipitation temperature also increases the number of relatively coarse precipitates, leading to cost Therefore, the Nb content is 0.1% or less. The lower limit of the Nb content is preferably 0.005%, and more preferably 0.01%. The Nb content is more preferably 0.015% and even more preferably 0.015%. The upper limit is more preferably 0.09%, and even more preferably 0.08%. Most preferably it is 0.07%.

[0036] Titanium (Ti): 0.1% or less The above Ti combines with nitrogen remaining as an impurity in the steel to form TiN. It is also an element that is added together with B to ensure hardening ability. Through the formation of C precipitates, precipitation strengthening and grain refinement effects can be expected. However, when the Ti content exceeds 0.1%, a large amount of coarse TiN is formed, and the collision energy The upper limit of Ti is preferably 0.1% because Ti reduces the absorption capacity. is preferably 0.005%, more preferably 0.01%, and The upper limit of Ti is more preferably 0.08%. It is more preferably 0.06%, and most preferably 0.05%.

[0037] Boron (B): 0.01% or less The above B can improve hardening ability even with a small amount of addition, and also improves the hardening ability of the old austenite. It segregates at the grain boundaries of the alloy and effectively suppresses the embrittlement of hot-formed parts caused by the grain boundary segregation of P and / or S. However, if its content exceeds 0.01%, Fe 23 C The upper limit is 0.01% because the formation of B6 complex compounds causes embrittlement during hot rolling. On the other hand, the lower limit of the B content is preferably 0.0001%. It is more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the B content is more preferably 0.009%, and more preferably 0.007%. It is more preferable that the content is 0.005%, and most preferable that the content is 0.005%.

[0038] The remainder of the material contains iron (Fe), which is intentionally removed from raw materials or the surrounding environment during normal manufacturing processes. These impurities cannot be excluded because they inevitably get mixed in. Since the product is something that anyone with ordinary skill in the manufacturing process would understand, So I won't go into detail about all of it.

[0039] The hot forming steel material of the present invention contains the above alloy composition and has a surface defined by the following [Relation 1]. The surface roughness factor is 1.8 μm or less. Surface roughness is expressed in various ways (Ra, Rt, Rsk, etc.), Simply changing the Ra, Rsk, etc. of steel materials is not enough to improve the bendability of hot-formed parts. The inventors of the present invention have studied how to improve the bendability of hot-formed members. As a result, when the surface roughness of the steel material is controlled to a constant value, the bendability of the hot-formed member can be improved. It was not just a matter of measuring Rt and Rdq, but also a matter of measuring hot In order to ensure the bendability of the formed part, the technical relationship between Rt and Rdq is derived and the following [relationship] is established. The surface roughness index of the hot-formed part is derived from the equation (1). In order to improve the bendability to increase the gas absorption capacity, the above surface roughness is The surface roughness factor is 1.8 μm or less. If the surface roughness index exceeds 1.8 μm, the slope of the peaks becomes large. However, the bendability may be reduced due to the maximized surface notch effect during bending.

[0040] [Equation 1]

number

[0041] Here, Rt is the perpendicular distance between the highest peak and the deepest valley in any measurement section on the steel plate surface. Rdq is the root mean square of the slope of the peaks in any measurement section on the steel plate surface. It means the square root (root mean square). Rt and An example of the calculation method for Rdq is shown in Figure 1, through which ordinary engineers can calculate the above Rt and Rd There is no difficulty in deriving q.

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

[0043] The ferrite is a soft phase, and when making a blank, it is hard to withstand the load of the blanking process of the steel material. It is an effective structure for reducing the amount of carbon dioxide, and for this reason, it is preferable that the amount is 50% by area or more. If the area percentage exceeds 0%, carbon is excessively dispersed in structures other than ferrite during blank preparation. Therefore, the carbon may be unevenly distributed even after hot forming. The area ratio of the particles is preferably 50 to 90%.

[0044] If the above pearlite exceeds 30% by area, cementite will be incompletely dissolved after hot forming. This can lead to a decrease in strength or to unevenness in the material. If the area percentage of ferrite or martensite exceeds 20%, the strength of the steel sheet increases excessively. However, problems such as die wear may occur when making the blank.

[0045] The steel material for hot forming of the present invention may include a plating layer on at least one surface, The plating layer is divided into types such as zinc (Zn) plating layer and aluminum (Al) plating layer. There are no particular limitations, and there are no particular limitations on the method, such as hot dip plating or electroplating. Preferably, for example, an Al-based plating layer may be formed. Although there is no particular limitation on the plating, as an example, the Al-based plating layer contains, in weight %, Si : 6 to 12%, Fe: 1 to 4%, and the remainder may contain Al and unavoidable impurities.

[0046] Next, an embodiment of the hot forming steel material of the present invention will be described in detail. The manufacturing method is only one embodiment of all possible embodiments, and the hot forming steel of the present invention This does not necessarily mean that the material must be manufactured only by the following manufacturing methods. do not have.

[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 The steel is produced by temper rolling so that the following [Relationship 2] is satisfied.

[0048] [Equation 2]

number

[0049] The cold rolled steel sheet is subjected to temper rolling to control the surface roughness of the steel material. The rolling force P and the arithmetic mean roughness Ra of the roll roll Considering the technical effects of The above [Relationship 2] was derived based on the recognition that the pressure during temper rolling can be optimized. Although the downward force P is an important factor, the present invention does not particularly limit the upper and lower limits of the downward force P. However, For example, if no rolling force is applied, problems such as poor winding may occur. It may be 100 tons or more, and more preferably 150 tons or more. In addition, if the rolling force is too high, cracks may occur in the surface coating layer, and the above [related If the coefficient exceeds 40, the upper limit can be restricted. For example, When the arithmetic mean roughness of the rolling roll is 4 μm, in order to satisfy the above [Relationship 2], The rolling force is preferably 400 tons or less.

[0050] The cold-rolled steel sheet is produced by heating the steel slab, hot rolling, coiling, cooling, cold rolling, annealing, etc. Each step will be explained below.

[0051] Steel slab heating The steel slab is heated at 1050 to 1300°C. If the thickness is less than 1000 μm, not only will it be difficult to homogenize the structure of the steel slab, but it will also be difficult to utilize the precipitated elements. On the other hand, if the heating temperature exceeds 1300°C, it may be difficult to redissolve the alloy. If the temperature is too high, an excessive oxide layer will be formed, which may cause surface defects after hot rolling. Therefore, the heating temperature of the steel slab is preferably 1050 to 1300°C. The lower limit of the steel slab heating temperature is more preferably 1070°C, and more preferably 1100°C. It is more preferable that the upper limit of the steel slab heating temperature is 1280°C. The temperature is preferably 1250°C, and more preferably 1250°C.

[0052] hot rolling The heated steel slab is hot rolled and finish hot rolled at 800 to 950°C to produce hot rolled steel. If the finish hot rolling temperature is less than 800°C, the surface layer of the steel sheet may be damaged due to the rolling in the two-phase region. On the other hand, the above-mentioned finish hot rolling may cause a duplex structure in the strip, making it difficult to control the strip shape. When the temperature exceeds 950°C, the problem of coarsening of 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 more preferably 820°C. The upper limit of the finish hot rolling temperature is more preferably 940°C. The temperature is preferably 930°C.

[0053] Winding The hot-rolled steel sheet is coiled at 500 to 700°C. When the coiling temperature is less than 500°C, , martensite is formed in the whole or part of the steel plate, making it difficult to control the plate shape. The increased strength of hot-rolled steel sheets leads to a problem of reduced rollability in the subsequent cold rolling process. On the other hand, if the coiling temperature exceeds 700°C, coarse carbides are formed. Therefore, the impact energy absorption capacity of the hot-formed part may be reduced. The temperature is preferably 500 to 700°C. The lower limit of the coiling temperature is 520°C. The upper limit of the coiling temperature is 6 A temperature of 80°C is more preferable, and a temperature of 650°C is even more preferable.

[0054] cooling The coiled hot-rolled steel sheet is cooled from the coiling temperature to 400°C at a rate of 10°C / Hr or more. However, if the cooling rate is less than 10°C / Hr, carbides may form. The reason is that a large number of coarse carbides are formed during the cooling of the hot rolled coils, because there is sufficient time for the carbides to grow. Therefore, the cooling rate should be 10°C / hr or more. It is preferable that the temperature is 12°C / Hr or more, and more preferable that the temperature is 15°C / Hr or more. On the other hand, the present invention can be obtained as long as the cooling rate is 10°C / Hr or more. Since the desired effect can be obtained, there is no particular restriction on the upper limit.

[0055] After the cooling, a pickling step may be added before the cold rolling. This removes the scale formed on the steel plate surface and improves the surface quality of the product. It can be done.

[0056] cold rolling After the above steps, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. There is no particular restriction on the rolling reduction ratio during rolling, but in order to obtain the target thickness of the steel material, it is recommended to use a rolling reduction ratio of 30 to 100%. A rolling reduction of 80% can be applied.

[0057] Annealing and cooling The cold-rolled steel sheet is annealed. First, the cold-rolled steel sheet is heated to 400°C. It is preferable to heat the temperature range from the temperature to the annealing temperature at a rate of 20°C / s or less. If the heating rate from 0°C to the annealing temperature exceeds 20°C / s, the carbides precipitated in the hot rolling stage will There may not be enough time for redissolution, and coarse carbides may remain, resulting in a loss of heat Therefore, the above 400℃ The heating rate up to the annealing temperature is preferably 20°C / s or less. ° C. / s or less, and more preferably 15° C. / s or less. On the other hand, in the present invention, the effect to be obtained by the present invention can be achieved as long as the heating rate is 20°C / s or less. Therefore, there is no particular restriction on the lower limit of the heating rate. Considering this, the heating rate may be 0.5°C / s or more, and more preferably 1°C / s or more. On the other hand, in the present invention, the cold rolling temperature is In the temperature range from 100 to less than 400°C, there is no particular limitation on the heating rate. This is because controlling the heating rate has only a small 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 less than 740°C, the cold rolling will not sufficiently recrystallize the structure, and the sheet shape will not be obtained. The shape may become poor, or the strength after plating may become too high, causing mold wear during the blanking process. On the other hand, if the annealing temperature exceeds 860°C, Si, M, etc. This can cause problems such as the formation of surface oxides such as silicon dioxide, resulting in poor plating surface quality. Therefore, the annealing temperature is preferably 740 to 860°C. The annealing temperature is more preferably 0°C, and even more preferably 760°C. The upper limit 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 mixture Gas can be used, and the dew point of the atmospheric gas is The dew point temperature can be between -70 and -30°C. In order to keep the dew point temperature below -70°C, This requires additional equipment, which increases production costs. If the temperature exceeds 0°C, excessive annealing oxides will form on the surface of the steel sheet during annealing, resulting in defects such as unplated surfaces. Therefore, during the continuous annealing, the dew point temperature of the atmospheric gas (Dew The dew point of the atmospheric gas is preferably −70 to −30° C. The lower limit is more preferably -65°C, and even more preferably -60°C. The upper limit of the dew point temperature of the atmospheric gas is preferably -35°C, and more preferably -40°C. is more preferable.

[0060] The above annealed cold-rolled steel sheet is cooled from the annealing temperature to 660°C at a cooling rate of 1°C / s or more. If the cooling rate is less than 1°C / s, a large amount of coarse carbides will be formed. This may result in a decrease in 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 preferably 2°C / s or more, and more preferably 2°C / s or more. However, from the viewpoint of suppressing defects in the shape of the steel sheet, the upper limit of the speed is not particularly limited. The cooling rate may be 50°C / s or less, more preferably 45°C / s or less, and even more preferably Alternatively, it may be 40°C / s or less.

[0061] On the other hand, before the temper rolling, the annealed cold-rolled steel sheet may be further plated. In the present invention, there is no particular limitation on the type and method of plating, but Al-based plating is preferred. An example of the above-mentioned plating is described below. The above-mentioned annealed cold-rolled steel sheet is cooled and coated with an Al-based plating. The aluminum-based plating layer is formed by immersing the workpiece in a plating bath. There are no particular restrictions on the items.

[0062] However, as a non-limiting example, the composition of the plating bath is, in weight %, Si: 6 to 12%, Fe : 1 to 4%, the balance may contain Al and other unavoidable impurities, and the plating amount is determined by the technology The single-sided standard commonly applied in the medical field is 30 to 130 g / m 2 It can be. If the Si content in the plating bath composition is less than 6% by weight, the temperature of the plating bath may rise excessively. If the content exceeds 12% by weight, the alloying process becomes excessive. The disadvantage is that the heating time for hot forming must be extended due to the delay in the process. If the content is less than 1% by weight, the plating adhesion and spot weldability may be reduced. If it exceeds 4% by weight, excessive dross will be generated in the plating bath, resulting in poor surface quality. The plating weight is 30g / m on one side. 2 If it is less than It may be difficult to ensure the corrosion resistance of hot-formed parts of 130g / m 2 If it exceeds In addition to increasing the manufacturing cost due to excessive plating coverage, It may not be easy to coat the steel sheet uniformly across its width and length.

[0063] On the other hand, according to another aspect of the present invention, the cold-rolled steel sheet is subjected to continuous annealing and aluminum treatment as described above. Aluminum plating can be applied to cooled hot-rolled steel sheets immediately after pickling. Aluminum plating can also be performed.

[0064] Hereinafter, one embodiment of the hot-formed part of the present invention will be described in detail. The material can be produced by hot press forming the above-mentioned hot forming steel material.

[0065] The microstructure of the hot-formed parts is a martensite single phase structure or a martensite and 40-plane The martensite may have a mixed structure containing bainite of less than 50% by volume. The microstructure of the present invention is martensitic because it is an effective structure for ensuring the strength that the invention aims for. On the other hand, bainite is a structure with slightly lower strength than martensite. However, when forming in the martensite matrix, it is possible to increase the strength without significantly reducing the bendability. In the present invention, the 40-plane martensite is used together with the martensite. It is also possible to have a mixed structure containing bainite of less than 0.05% by volume. If the area ratio exceeds 40%, it may be difficult to achieve the strength targeted by the present invention. be.

[0066] On the other hand, the above microstructure is composed of 10% by area or less of ferrite and 5% by area or less of retained austenite. The ferrite and the residual austenite may further include one or more of the following: The ferrite structure is formed unavoidably in the manufacturing process. If this occurs, not only will the strength decrease, but the bending properties may also decrease significantly. If the retained austenite structure exceeds 5% by area, the strength will decrease or the hot forming process will be difficult. The inflow of hydrogen from the atmospheric gas into the mold may increase, increasing the possibility of hydrogen embrittlement. .

[0067] The above hot-formed parts have a yield strength (YS) of 800 MPa or more and a tensile strength (TS) of 10 00 MPa or more, and elongation (El): 3.5% or more.

[0068] The hot-formed member of the present invention may have a maximum bending angle variation of 5% or less. This can be verified by a three-point bending test according to the VDA standard (VDA238-100). If the above maximum bending angle change exceeds 5%, bending or collision may be impaired even if the physical properties are similar. The characteristics may be deteriorated.

[0069] Next, an embodiment of the method for producing a hot-formed part of the present invention will be described in detail. The manufacturing method described in is only one embodiment of all possible embodiments, and is not intended to be limiting of the present invention. This means that hot-formed parts must be manufactured only by the following manufacturing methods: It is not something that can be done.

[0070] The hot-formed steel material or the hot-formed steel material manufactured by the method described above is prepared, and the hot-formed steel material is used. A blank is manufactured, and the blank is heated to a temperature above the austenite single phase region temperature, more specifically, Ac3 After heating to a temperature of 980°C or less, the temperature is maintained for 1 to 1000 seconds.

[0071] If the blank heating temperature is lower than the Ac3 temperature, the presence of untransformed ferrite will cause the specified 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 ink heating temperature is more preferably Ac3+5°C, and more preferably Ac3+10°C. The upper limit of the blank heating temperature is more preferably 970°C. , 960°C is more preferable.

[0072] If the above-mentioned holding time is less than 1 second, the temperature will not be uniform throughout the blank, and some parts may If the holding time exceeds 1000 seconds, the heating temperature may become excessive. As in the case of other welding, excessive oxides are generated on the surface of the component, making it difficult to ensure spot weldability. Therefore, the retention time is preferably 1 to 1000 seconds. The lower limit of the retention time is more preferably 30 seconds, and even more preferably 60 seconds. The upper limit of the retention time is more preferably 900 seconds, and even more preferably 800 seconds. More preferable.

[0073] After that, the heated and held blank is hot-formed and then cooled to room temperature (form cooling). ) and finally produce a hot-formed part. There is no particular limitation on the method, and the method may be a hot forming method commonly known in the technical field to which the present invention pertains. As a preferred example, a mold cooling method can be used. Cut. [Example]

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

[0075] The following examples are provided for illustrative purposes only and are not intended to be limiting of the present invention. It goes without saying that various modifications are possible without departing from the scope of the invention. The following examples are provided for the purpose of understanding the present invention, and the scope of the present invention is not limited to the following examples. The scope of the present invention is not limited to the examples, but includes not only the claims below but also equivalents thereto. should be determined by the

[0076] (Example) The composition of Table 1 below (wt %, the remainder is Fe and unavoidable impurities) The steel slab was produced by vacuum melting. The steel slab was heated to 1250°C and then cooled to 900°C. After hot rolling at a finishing hot rolling temperature of 640°C and coiling at a coiling temperature of 640°C, the steel sheet is cut to a final thickness of 2 After pickling the hot-rolled steel sheet, it was cold rolled at a reduction rate of 45%. The steel sheets were then cold-rolled at a normal annealing temperature in a 5% hydrogen-95% nitrogen atmosphere. After annealing at a temperature of 780°C, 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. The coating weight is 70g / m on one side. 2 The surface roughness of the steel sheet was Further temper rolling is carried out to provide deviation in roughness, and the roughness and pressure of the temper rolling roll are measured. The rolling force was varied. The roll roughness and rolling force applied to each test piece are shown in Table 2. Ta.

[0078] After a blank is made using the steel plate produced in this way, it is heated using a hot forming die. The blank was heated to a temperature of 900°C. The temperature was 30°C, the holding time was 5 minutes, and the transfer time from the heating furnace to molding was 10 seconds for all samples. was applied to.

[0079] Yield strength (YS), tensile strength (TS) and elongation (El) were measured using ASTM standard test pieces. After the steel plate was sampled in a direction perpendicular to the rolling direction, a tensile test was carried out to measure the thickness.

[0080] The bending property, which is an important index for the collision energy absorption capacity, is VDA standard (VDA238 A three-point bending test was carried out according to the above-mentioned three-point bending test. The load-displacement curve obtained from the three-point bending test Crack Initiation Energy (CIE) By calculating the energy absorption coefficient (rgy), the impact energy absorption capacity of a material can be evaluated. Figure 2 shows a simplified version of the CIE concept, which is the standard for evaluating the collision energy absorption capacity. It is something.

[0081] [Table 1]

[0082] [Table 2]

[0083] In Table 2 above, Relational Formula 1 is the surface roughness index (Surface Roughness Factor)

number

[0084] Relation 2 is

number

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

[0086] Specifically, when comparing Example 1 with Comparative Examples 1 and 2, both were manufactured using the same steel type A. The maximum bending angle of Example 1, which satisfies the conditions of the present invention, is 60.14°, and the CIE It shows that excellent bending and crash resistance properties are obtained. Comparative Examples 1 and 2 have similar strength after hot forming to Inventive Example 1, but the strength after hot forming is lower under temper rolling conditions. Since the value of [Relationship 2] exceeds the upper limit of 40, the surface roughness index falls outside the range of the present invention. As a result, due to the surface notch effect, the bending angle change was smaller than that of Example 1. It was confirmed that the amount exceeded 5%, and there was a decrease in bendability.

[0087] Inventive Examples 2 to 4 and Comparative Example 3 were all manufactured using the same B steel type. Inventive Example 2, which satisfies the conditions of the invention, exhibits excellent bending properties and crash resistance properties, and Inventive Examples 3 and 4, In both cases, the reduction in the maximum bending angle was 5% or less compared to Example 2. In the case of 3, the temper rolling condition [Relationship 2] exceeds the upper limit of 40, and the surface roughness index is within the scope of the present invention. It was confirmed that the bending property and the crash resistance property were significantly reduced compared to Example 3. .

[0088] Inventive Example 5 and Comparative Examples 4 to 5 were manufactured using the same C steel type. Inventive Example 5 was manufactured using a 42° However, the maximum bending angle of Comparative Example 4 was 39566 Nm. In the range of 5, the temper rolling condition [Relationship 2] exceeds the upper limit of 40, and the surface roughness index is within the range of the present invention. The bending angle change was found to be more than 5% compared to Example 5, and the bending property and A decrease in crashworthiness was confirmed.

[0089] Example 2 Steel having the steel composition shown in Table 3 below was subjected to the same steelmaking, hot rolling, cold rolling and tempering as in Example 1. The steel was manufactured through an annealing process. No further plating was performed at this time. Temper rolling is performed to give the annealed steel sheet a roughness, and the temper-rolled annealed steel sheet is then subjected to a hot forming process. In order to prevent surface decarburization that may occur during the process, electroplating was further performed. A blank is made using the steel plate manufactured by the above method, and then hot-formed using a hot-forming die. The blank was heated to 900°C and held at this temperature for 10 minutes. The holding time was 6 minutes, and the transfer time from the heating furnace to molding 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] In Table 4, looking at invention examples 6 to 8 and comparative example 6, which were manufactured using steel type D in Table 3, The maximum bending angle of specimen No. 6 was 58.5°, demonstrating excellent bendability. Examples 7 and 8 also satisfy the conditions of the present invention, and there is a change in the bending angle compared to Example 6. However, all of these were below 5%, and it was confirmed that the material had good bending properties and crash resistance. Cut.

[0094] On the other hand, in the case of Comparative Example 6, the same D steel type was used and the strength after hot forming was Although the alloy composition satisfies the scope of the present invention, the value according to relation 2 The surface roughness index exceeds 40, which is beyond the scope of the present invention. As a result of this effect, it was confirmed that the bending angle change amount was ultimately greater than 5% compared to Example 6. .

Claims

1. In weight percent, C: 0.04 to 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 to 5.0%, N: 0.02% or less, the balance being Fe and inevitable impurities; The surface roughness index (Surface Roughness) calculated by the following [Relationship 1] A steel material for hot forming, having a grain size (Grade Factor) of 1.8 μm or less. [Relationship 1] [Equation 1] (The above Rt is the perpendicular distance between the highest peak and the deepest valley in any measurement section on the steel plate surface. Rdq is the root mean square of the slope of the peaks at any measurement section on the steel plate surface. (root mean square)

2. The steel material contains Mo: 0.5% or less, Ni: 0.5% or less, Nb: 0.1% or less, Ti:

2. The hot rolling method according to claim 1, further comprising one or more of: S: 0.1% or less; B: 0.01% or less. Steel material for forming.

3. The microstructure of the steel material contains, in terms of area fraction, ferrite: 50 to 90%, pearlite 3 Contains one or more of the following: 0% or less, bainite 20% or less, and martensite 20% or less The hot forming steel material according to claim 1.

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

5. The plating layer contains, in weight percent, Si: 6 to 12%, Fe: 1 to 4%, and the remainder being Al and other impurities. The steel material for hot forming according to claim 4, which contains unavoidable impurities.

6. In weight percent, C: 0.04 to 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 to 5.0%, N: 0.02% or less, balance Fe and unavoidable impurities obtaining a cold rolled steel sheet using the slab; and temper rolling the cold-rolled steel sheet so as to satisfy the following [Relationship 2]. Manufacturing method for structural steel. [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 is)

7. The cold rolled steel sheet contains Mo: 0.5% or less, Ni: 0.5% or less, Nb: 0.1% or less, T 7. The method according to claim 6, further comprising one or more of: i: 0.1% or less; and B: 0.01% or less. Manufacturing method for hot forming steel.

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. and, coiling the hot-rolled steel sheet at 500 to 700°C; 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. and cooling the cold-rolling the cooled hot-rolled steel sheet at a reduction rate of 30 to 80% to obtain a cold-rolled steel sheet; 、 The cold-rolled steel sheet is heated in the temperature range from 400°C to the annealing temperature at a rate of 20°C / s or less. and annealing the heated cold-rolled steel sheet at an annealing temperature of 740 to 860°C; The annealed cold-rolled steel sheet is cooled from the annealing temperature to 660°C at a cooling rate of 1°C / s or more. The method for producing a steel material for hot forming according to claim 6, comprising the steps of:

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

10. After cooling the annealed cold-rolled steel sheet, it is immersed in an Al-based plating bath to be aluminum-plated. The method of claim 8 further comprising forming a layer.

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

12. In weight percent, C: 0.04 to 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 to 5.0%, N: 0.02% or less, the balance being Fe and inevitable impurities; A hot-formed member having a maximum bend angle variation of 5% or less.

13. The hot-formed member has a yield strength (YS): 800 MPa or more and a tensile strength (TS): 10 13. The hot-formed part according to claim 12, wherein the strength of the hot-formed part is 00 MPa or more and the elongation percentage (El): 3.5% or more. Material.

14. The microstructure of the hot-formed member is a martensite single phase structure or a martensite and 40 area 13. The hot-formed part according to claim 12, which has a mixed structure containing bainite in an amount of 0.1% or less.

15. The hot-formed member is manufactured by using the hot-forming steel material according to any one of claims 1 to 5. The hot-formed part of claim 12 as produced.

16. 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, and cooling the heated and held blank after hot forming. Manufacturing method of shaped components.

17. The method for producing a hot-formed part according to claim 16, wherein the cooling is performed by a mold cooling method.

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