Plated steel sheet exhibiting excellent paint adhesion and corrosion resistance after hot press forming, method for manufacturing plated steel sheet, and hot press formed member

Abstract

The present invention relates to a plated steel sheet for hot press forming that exhibits excellent paint adhesion and corrosion resistance after hot press forming, a method for producing the plated steel sheet, and a hot press formed member. A plated steel sheet according to one embodiment of the present invention includes a base steel sheet and a plating layer made of an Al-Fe alloy formed on the base steel sheet, in which a total content of Al and Fe in the plating layer is 80% or more by weight, an average content of Fe in the plating layer is 20% or more by weight, and a product of Ra and Rpc of a surface of the plating layer may be 60 to 150 μm / cm. Here, Ra means the arithmetic mean roughness, and its unit is μm, and RPc means the number of peaks per unit length, and its unit is / cm.

Description

[Technical Field]

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

[0002] Recently, with the depletion of petroleum energy resources and growing concern about the environment, regulations for improving automobile fuel efficiency are becoming stricter day by day. From a materials perspective, one method for improving automobile fuel efficiency is to reduce the thickness of the steel plate used, but reducing the thickness can pose a risk to automobile safety, so it is necessary to ensure that the strength of the steel plate is improved.

[0003] For these reasons, there has been a continuous demand for high-strength steel sheets, and various types of steel sheets have been developed. However, such steel sheets have a problem of poor workability due to their high strength. That is, since the product of strength and elongation tends to always have a constant value depending on the grade of steel sheet, there has been a problem that as the strength of steel sheet increases, the elongation, which is an indicator of workability, decreases.

[0004] To solve these problems, a hot press forming method has been proposed. In the hot press forming method, a steel sheet is processed at a high temperature where it is easy to process, and then rapidly cooled at a low temperature to form a low-temperature structure such as martensite in the steel sheet, thereby increasing the strength of the final product. This method has the advantage of minimizing workability problems when manufacturing high-strength components.

[0005] However, the above-mentioned hot press forming method has the problem that the surface of the steel sheet is oxidized because the steel sheet is heated at a high temperature, and therefore an additional process of removing the oxides on the steel sheet surface is required after press forming. Patent Document 1 has proposed a method to solve this problem. Patent Document 1 uses a process of heating and quenching an aluminum-plated steel sheet after hot press forming or room temperature forming (i.e., "post-heat treatment"), and because the aluminum-plated layer is present on the steel sheet surface, the steel sheet does not oxidize during heating. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] U.S. Patent Publication No. 6,296,805 Summary of the Invention [Problem to be solved by the invention]

[0007] According to one embodiment of the present invention, there are provided a plated steel sheet for hot press forming that can not only improve the paint adhesion of hot press-formed parts but also ensure corrosion resistance, and a method for manufacturing the plated steel sheet.

[0008] According to another embodiment of the present invention, a hot press-formed part having excellent paint adhesion and corrosion resistance is provided.

[0009] The object of the present invention is not limited to the above-mentioned scope, and anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the overall matters described in this specification. [Means for solving the problem]

[0010] A plated steel sheet according to one embodiment of the present invention includes a base steel sheet and a coating layer made of an Al-Fe alloy formed on the base steel sheet, wherein the total content of Al and Fe in the coating layer is 80% or more by weight, the average content of Fe in the coating layer is 20% or more by weight, and the product of Ra and Rpc on the surface of the coating layer may be 60 to 150 μm / cm.

[0011] Here, Ra means the arithmetic mean roughness, and its unit is μm, and RPc means the number of peaks per unit length, and its unit is / cm.

[0012] According to another embodiment of the present invention, there is provided a plated steel sheet comprising a base steel sheet and a plating layer made of an Al-Fe alloy formed on the base steel sheet, wherein the total content of Al and Fe in the plating layer is 80% or more by weight, the average content of Fe in the plating layer is 20% or more by weight, and the number of cracks present in each region obtained by dividing a field of view obtained by observing the surface of the plating layer at 100x magnification with a scanning electron microscope is 10 equal parts horizontally and vertically, and the number of cracks present in each region is 1 mm or less. 2 The number of indentations may be 10 to 200 per one indentation, and the proportion of the area occupied by the indentations on the surface of the plating layer may be 5 to 50%.

[0013] Here, the indented portion means a region having a brightness of 70% or more of the highest brightness measured in a region observed at 100x magnification with an optical microscope.

[0014] A method for producing a plated steel sheet according to yet another embodiment of the present invention includes the steps of obtaining an Al-Fe alloy-plated steel sheet in which a plating layer made of an alloy of Al and Fe is formed on a base steel sheet; and skin-pass rolling the Al-Fe alloy-plated steel sheet under conditions such that the SPMI, represented by the following Relational Formula 1, is 5000 to 8500.

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[0015] A hot press-formed member according to yet another embodiment of the present invention includes a base steel sheet and a plating layer made of an Al-Fe alloy formed on the base steel sheet, wherein the total content of Al and Fe in the plating layer is 70% or more by weight, the content of Fe in the plating layer is 30% or more by weight, and the product of Ra and RPc on the surface of the plating layer may be 60 to 150 μm / cm.

[0016] Here, Ra means the arithmetic mean roughness, and its unit is μm, and RPc means the number of peaks per unit length, and its unit is / cm.

[0017] According to yet another embodiment of the present invention, there is provided a hot press-formed member comprising a base steel sheet and a coating layer made of an Al-Fe alloy formed on the base steel sheet, wherein the total content of Al and Fe in the coating layer is 70% or more by weight, the average content of Fe in the coating layer is 30% or more by weight, and the number of cracks present in each region obtained by dividing a field of view obtained by observing the surface of the coating layer at 100x magnification with a scanning electron microscope is 10 equal parts horizontally and vertically, is 1 mm or less. 2 The number of indentations may be 15 to 220 per indentation, and the proportion of the area occupied by the indentations on the surface of the plating layer may be 5 to 50%.

[0018] Here, the indented portion means a region having a brightness of 70% or more of the highest brightness measured in a region observed at 100x magnification with an optical microscope. [Effects of the Invention]

[0019] As described above, the plated steel sheet of the present invention has surface Ra and RPc controlled to appropriate levels, so that sufficient paint adhesion of the part can be ensured even if a significant increase in illuminance does not occur during the hot press forming process. [Brief explanation of the drawings]

[0020] [Figure 1] 1 shows photographs of the surfaces of aluminum-iron-based plated steel sheets produced by Comparative Example 1(a) and Inventive Example 2(b), observed with a scanning electron microscope (SEM). [Figure 2] 1 shows photographs of the surfaces of the aluminum-iron-based plated steel sheets produced in Comparative Example 1(a) and Inventive Example 2(b) observed with an optical microscope, and the images were processed accordingly. [Figure 3] 1 is a graph showing the relationship between the number of cracks generated on the surface of an aluminum-iron-plated steel sheet and skin-pass rolling conditions. DETAILED DESCRIPTION OF THE INVENTION

[0021] The terminology used herein is for the purpose of referring to particular embodiments only and is not intended to limit the invention. As used herein, the singular forms "a," "an," and "the" include the plural unless the context clearly dictates otherwise.

[0022] The meaning of "comprising" as used in the specification embodies certain properties, regions, integers, steps, operations, elements and / or components, and does not exclude the presence or addition of other certain properties, regions, integers, steps, operations, elements, components and / or groups.

[0023] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Commonly used, predefined terms are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted as having an ideal or very formal meaning unless defined.

[0024] In the present invention, the term "steel sheet" refers to a coil or sheet material that has not yet been processed into a specific shape, and the term "part" refers to a material that has been processed into a non-plate shape through a forming process. Furthermore, the term "plated layer" refers to a layer of a metal, alloy, or intermetallic compound that is formed in contact with the base steel sheet.

[0025] In the present invention, it should be noted that the content of each element is based on weight unless otherwise specified, the proportion of crystals and structures is based on area unless otherwise specified, and the content of gas is based on volume unless otherwise specified.

[0026] The present invention will be described in detail below.

[0027] As described above, when an aluminum-plated steel sheet is heated for hot press forming, problems such as melting of the aluminum plating layer due to the heating rate and contamination of the equipment may occur. Furthermore, in the case of high-strength components, the problem of so-called hydrogen-induced delayed fracture may occur, in which hydrogen trapped in the base steel sheet accumulates and even leads to destruction of the component.

[0028] One approach to solving these problems is to heat an aluminum-plated steel sheet before heating for hot press forming to form an aluminum-iron alloy layer on the steel sheet, and then use the steel sheet with the aluminum-iron alloy plating layer formed thereon for hot press forming. That is, by alloying the plating layer at a relatively low temperature range before heating for hot press forming, problems caused by aluminum melting can be prevented even if the steel sheet is heated at a relatively high rate, even if it is heated to a temperature higher than the melting point of aluminum, because aluminum has already been alloyed. Furthermore, in the case of a pre-alloyed plated steel sheet, an alloy layer with a structure that facilitates hydrogen release can be provided on the surface, which can reduce the possibility of hydrogen-induced delayed fracture.

[0029] On the other hand, when painting a typical coated steel sheet, there is no anchor to secure the paint layer, which can lead to poor adhesion between the paint and the steel sheet surface. To solve this problem, phosphate treatment is typically performed, but phosphate treatment increases the roughness of the steel sheet surface, which leads to better adhesion between the steel sheet and the paint.

[0030] However, when a plated steel sheet having an aluminum-iron plating layer formed on its surface is hot-press-formed, there is a problem in that the surface roughness of the part is difficult to increase further. The plating layer formed on the surface of a hot-press-formed part is formed by an alloying reaction between aluminum and iron and is relatively chemically stable. Because the surface of the hot-press-formed part is thus chemically stable, it is difficult to further improve the roughness even with phosphate treatment. However, when a conventional aluminum-plated steel sheet is heated and hot-press-formed, the surface roughness increases during the heating process, so sufficient roughness can be ensured even without phosphate treatment, and there may be no significant problems with paint adhesion.

[0031] On the other hand, in the case of an aluminum-iron alloy-plated steel sheet on which an aluminum-iron alloy layer is formed by a pre-alloying treatment, the increase in surface roughness is not significant when heated for subsequent hot press forming. Therefore, even if the roughness increases partially during the pre-alloying heat treatment, the increase in roughness during the subsequent hot press forming is not significant. Therefore, the surface roughness of a member obtained by hot press forming an aluminum-iron alloy-plated steel sheet is insufficient compared to the surface roughness of a member obtained by hot press forming an aluminum-plated steel sheet directly without the alloying heat treatment, resulting in a problem of insufficient paint adhesion.

[0032] Furthermore, the aluminum alloy plating layer is not as excellent in sacrificial corrosion protection as the zinc-based plating layer, and when the steel plate is exposed due to cracks or the like, corrosion occurs together with bristle formation.

[0033] Generally, increasing the surface roughness (Ra) improves paint adhesion, which in turn improves corrosion resistance after painting. However, the inventors have confirmed through their research results that simply increasing the surface roughness, as well as increasing the product (Ra x RPc) of the number of peaks per unit length (RPc) and the roughness (Ra), is effective in improving paint adhesion and corrosion resistance.

[0034] Furthermore, while investigating the paintability of hot press-formed parts manufactured using so-called aluminum-iron alloy-plated steel sheets that have already been alloyed before hot press forming, the inventors of the present invention found that, since a considerable amount of iron has already diffused into the plating layer in the case of alloy-plated steel sheets as described above, the increases in surface roughness (Ra) and peaks per unit length (RPc) due to additional iron diffusion are not significant. Therefore, ultimately, it is effective to increase the roughness (Ra) or peaks per unit length (RPc) of the plated steel sheet before hot press forming, thereby increasing the Ra×RPc value.

[0035] Therefore, in one embodiment of the present invention, taking into account the surface Ra×RPc of a part obtained by hot press forming to ensure high paint adhesion, the Ra×RPc of the surface of the coating layer of the coated steel sheet may be 60 μm / cm or greater. In this formula, Ra is the arithmetic mean roughness measured in μm, and RPc is the peak count measured in inverse cm ( / cm). If Ra×RPc is insufficient, sufficient paint adhesion is unlikely to be achieved, so the lower limit of Ra×RPc may be set to 60 μm / cm. In some cases, the lower limit of Ra×RPc may be set to 70 μm / cm. While a higher Ra×RPc value is advantageous for improving paint adhesion, if the value is too high, excessive cracks may be introduced into the surface of the coating layer during processing to increase Ra and RPc, resulting in reduced corrosion resistance. Therefore, in one embodiment of the present invention, the upper limit of Ra×RPc may be set to 150 μm / cm, and in some cases, the upper limit of Ra×RPc may be set to 140 μm / cm.

[0036] In addition, the plated steel sheet according to another embodiment of the present invention can improve the paint adhesion and corrosion resistance of a member obtained by a subsequent hot press forming process by appropriately adjusting the number of cracks formed on the surface and the area ratio of the indentations. To this end, the plated steel sheet according to one embodiment of the present invention can improve the paint adhesion and corrosion resistance of a member obtained by a subsequent hot press forming process by appropriately adjusting the number of cracks formed on the surface and the area ratio of the indentations. 2 There may be 10 to 200 cracks per indentation, and the proportion of the area occupied by the indentations on the surface of the plating layer may be 5 to 50%.

[0037] The cracks can act as anchoring points for fixing the coating layer on the surface of the hot press-formed part. In one embodiment of the present invention, the cracks are formed in a unit area of ​​1 mm 2 The number of cracks is calculated by dividing the field of view of a microscope (magnification: 100x) into 10 equal parts vertically and horizontally, and counting the number of cracks observed in 100 areas per observation area of ​​1 mm. 2 The measurement is converted into the value observed in a 1 mm area. In one embodiment of the present invention, the microscope may be a ZEISS SUPRA 55VP model scanning electron microscope. In this case, if a single crack is observed in multiple areas, the number of cracks may be counted as the number of areas in which the crack is observed. If multiple cracks are observed in one area, the number of cracks may be counted as the number of cracks. This is a concept that takes into account the total length of the cracks within the observation area, because the total length of the cracks affects the fixing effect of the paint layer. However, unlike zinc-based plated steel sheets, aluminum-based plated steel sheets do not have a sacrificial corrosion protection function, so if cracks are present, corrosion may occur through the cracks. Therefore, since an excessive number of cracks may impair the corrosion resistance of the steel sheet, the 1 mm calculated using the above method may be reduced. 2 The maximum number of cracks per hit can be limited to 200, or in some cases, 180.

[0038] In one embodiment of the present invention, a large number of indentations can be formed on the surface of the plating layer to increase the contact area with the paint layer. The presence of these indentations can increase the Ra and RPc of the surface of the plating layer. In one embodiment of the present invention, the proportion of indentations on the surface of the plating layer can be 5% or more, and in some cases, 8% or more. In one embodiment of the present invention, the indentations can refer to areas having a brightness of 70% or more of the highest brightness measured in an area observed at 100x magnification with an optical microscope. Although not necessarily limited thereto, in one embodiment of the present invention, the results of observing a surface image at 100x magnification using a Leica DM6000M model optical microscope can be divided into 256 color brightness levels using Clemex Vision PE software, and areas with a brightness of 70% or more of the highest brightness value can be identified as the indentations, and their area percentage can be calculated. If the proportion of the indentations is too high, the load applied to the steel sheet to form the indentations is excessive, which may increase surface cracks, so the upper limit of the proportion of the indentations can be set to 50%, or can also be set to 45%. In one embodiment of the present invention, the indentations can be formed by skin-pass rolling, but the method is not necessarily limited to this.

[0039] Furthermore, since the present invention is directed to aluminum-iron alloy-plated steel sheets, the total content of Al and Fe must be 80% or more by weight. There is no particular upper limit to the total content of these elements, and a plating layer consisting of 100% Al and Fe alone is also included.

[0040] Furthermore, one embodiment of the present invention is directed to a fully alloyed plated steel sheet, and therefore has an average Fe content of 20% or more by weight. If the Fe content in the plated layer is less than 20%, it may not be very useful in solving problems such as melting of the aluminum plated layer during heating and hydrogen embrittlement. Therefore, the present invention is directed to an Al-Fe alloy plated steel sheet containing 20% ​​or more Fe by weight. In some cases, the Fe content may be 30% or more, or even 40% or more.

[0041] While there is no particular upper limit for the Fe content, considering the Fe content in typical alloy-plated steel sheets, the upper limit for the Fe content can be set at 90%, or even 80% or less in some cases. Here, the average Fe content refers to the average Fe content in the entire coating layer. While several methods for measuring this can be used, in this example, the Fe content curve corresponding to the depth (thickness) obtained by analyzing from the surface of the coating layer to the interface with the steel sheet using glow discharge spectrometry (GDS) can be integrated, and then divided by the thickness of the coating layer (i.e., the distance from the surface to the interface with the steel sheet). While various criteria can be used to determine the interface between the coating layer and the steel sheet, in one embodiment, the point where the curves for Al and Fe content intersect based on the GDS results, i.e., where the contents of the two elements are similar, can be defined as the interface between the coating layer and the steel sheet.

[0042] In one embodiment of the present invention, the plating layer may further contain elements commonly contained in plating layers in addition to the above-mentioned Al and Fe. Examples of such elements include one or more selected from Mg, Zn, Mn, Cr, Mo, Si, and Ti, and these elements may be contained in the plating layer in a total amount of up to 20 wt %.

[0043] In one embodiment of the present invention, the Fe content in the surface region of the aluminum-iron alloy plated steel sheet for hot press forming may be 50% or more of the average Fe content in the plating layer. That is, by making the Fe content in the surface 50% or more of the average Fe content in the plating layer, a plated steel sheet can be obtained in which the plating layer is sufficiently alloyed up to its surface. In one embodiment of the present invention, the Fe content in the surface may be 15% or more by weight. In one embodiment of the present invention, the surface region may refer to a point 1 μm deep from the outermost surface. In one embodiment of the present invention, the Fe content in the surface region may be measured by EDS area analysis at a region magnified 100 times using a scanning electron microscope.

[0044] The steel sheet of the present invention is a steel sheet for hot press forming, and its composition is not particularly limited as long as it is used for hot press forming. However, in one embodiment of the present invention, the steel sheet may have a composition, in weight percent (it should be noted that hereinafter, unless otherwise specified, the compositions of the steel sheet and the coating layer of the present invention are based on weight), containing: C: 0.01 to 0.5%, Si: 2.0% or less (excluding 0%), Mn: 0.1 to 4.0%, P: 0.05% or less, S: 0.02% or less, Al: 0.001 to 1%, Cr: 5.0% or less (excluding 0%), N: 0.02% or less, Ti: 0.1% or less (excluding 0%), B: 0.0001 to 0.01%, with the balance being Fe and unavoidable impurities.

[0045] C: 0.01 to 0.5% C is an essential element for improving the strength of the heat-treated member and can be added in an appropriate amount. That is, to ensure sufficient strength of the heat-treated member, C may be added in an amount of 0.01% or more. In one embodiment, the lower limit of the C content may be 0.05%. However, if the C content is too high, when producing a cold-rolled material, the strength of the hot-rolled material becomes too high when cold-rolling the hot-rolled material, significantly deteriorating cold-rollability and significantly reducing spot weldability. Therefore, to ensure sufficient cold-rollability and spot weldability, C may be added in an amount of 0.5% or less. Alternatively, the C content may be limited to 0.45% or less or 0.4% or less.

[0046] Si: 2.0% or less (excluding 0%) Si is not only necessary as a deoxidizer during steelmaking, but also suppresses the formation of carbides, which have the greatest impact on the strength of hot-press-formed parts. It may also be added to steel to concentrate carbon at martensite lath grain boundaries after martensite formation during hot-press forming, thereby ensuring retained austenite. However, to ensure sufficient plating performance when aluminum plating steel sheets, the upper limit of the Si content can be set to 2% (excluding 0%). In one embodiment of the present invention, the Si content can be limited to 1.5% or less. In another embodiment of the present invention, the lower limit of the Si content can be set to 0.01%.

[0047] Mn: 0.1 to 4.0% The Mn content may be 0.1% or more to ensure the effect of solid solution strengthening and to lower the critical cooling rate for obtaining martensite in hot press-formed members. Furthermore, in order to maintain the strength of the steel sheet appropriately, thereby ensuring workability in the hot press-forming process, reducing manufacturing costs, and improving spot weldability, the Mn content may be 4% or less, and in one embodiment of the present invention, may be 3.5% or less, or 2.5% or less.

[0048] P:0.05% or less The above-mentioned P exists as an impurity in steel, and the lower its content, the more advantageous it is. Therefore, in one embodiment of the present invention, P may be contained at a content of 0.05% or less. In another embodiment of the present invention, P may be limited to 0.03% or less. Since P is an impurity element that is more advantageous when contained in small amounts, there is no need to specifically set an upper limit for its content. However, since excessively reducing the P content may increase manufacturing costs, taking this into consideration, the lower limit may be set at 0.001%.

[0049] S: 0.02% or less Since S is an element that impairs the ductility, impact properties, and weldability of steel members as an impurity, its maximum content is set to 0.02% (preferably 0.01% or less). Furthermore, if the minimum content is less than 0.0001%, the manufacturing costs may increase, so in one embodiment of the present invention, the lower limit of S content can be set to 0.0001%.

[0050] Al: 0.001 to 1% The Al content may be 0.001% or more because it acts as a deoxidizer together with Si during steelmaking and can improve the cleanliness of the steel. The Al content may be 1% or less so that the Ac3 temperature is not too high and the heating required for hot press forming can be performed within an appropriate temperature range.

[0051] Cr: 5.0% or less (excluding 0%) Cr is necessary to improve the hardenability of steel and the strength of hot press-formed parts. In some cases, the lower limit of the Cr content can be set at 0.001%. However, if the Cr content exceeds 5.0%, it is difficult to expect any further improvement in the effect, and costs may increase. Therefore, the upper limit of the Cr content can be set at 5.0%.

[0052] N: 0.02% or less The N element is contained as an impurity in steel, and since a lower N content is advantageous for reducing sensitivity to crack generation during continuous casting of slabs and ensuring impact properties, the N content may be 0.02% or less. Although there is no need to set a lower limit, in one embodiment, the N content may be set to 0.001% or more in consideration of increasing manufacturing costs.

[0053] Ti: 0.1% or less (excluding 0%) The Ti reacts with nitrogen to improve the hardening ability of B. Furthermore, Ti improves the strength of hot press-formed parts by forming fine precipitates, and refines crystal grains to improve impact toughness. Therefore, Ti can be added in an amount of 0.1% or less (excluding 0%). To ensure the above-mentioned effects, in one embodiment of the present invention, the lower limit of the Ti content can be set to 0.0005%.

[0054] B: 0.0001 to 0.01% The addition of B in small amounts not only improves hardenability, but also segregates at prior austenite grain boundaries, suppressing brittleness of hot press-formed parts due to grain boundary segregation of P and / or S. Therefore, B may be added in an amount of 0.0001% or more. However, if the B content exceeds 0.01%, not only does the effect saturate but also embrittlement occurs during hot rolling. Therefore, the upper limit can be set to 0.01%, and in one embodiment, the B content can be set to 0.005% or less.

[0055] In one embodiment of the present invention, the alloy may further contain one or more elements selected from Nb: 0.1% or less, Mo: 0.5% or less, Ni: 1% or less, Cu: 1% or less, and V: 0.5% or less, as needed.

[0056] Nb: 0.1% or less Nb can be added to steel because it improves the quality of heat-treated steel sheets by forming fine precipitates and improves the stability of retained austenite and impact toughness by refining grains. However, if the amount of Nb added exceeds 0.1%, not only will the effect be saturated, but the addition of excessive ferroalloys may also increase costs. In one embodiment of the present invention, Nb can be added in an amount of 0.001% or more.

[0057] Mo: 0.5% or less Mo is an element that can improve hardenability and ensure strength and grain refinement through the effect of precipitation strengthening. However, if added in excess, weldability may be impaired, so in consideration of this, it can be added in an amount of 0.5% or less. In one embodiment of the present invention, when adding Mo, the lower limit of the amount of addition can be set to 0.001%.

[0058] Ni: 1% or less Ni is an element that forms fine precipitates to improve strength. However, if its content exceeds 1.0%, costs will increase excessively, so the upper limit is set to 1%. In one embodiment of the present invention, the amount of Ni added can be set to 0.005% or more to ensure the above-mentioned effects.

[0059] Cu: 1% or less Like Ni, Cu is an element that forms fine precipitates to improve strength. However, if its content exceeds 1.0%, costs will increase excessively, so the upper limit is set at 1%. To ensure the above-mentioned effects, the Cu content can be set to 0.005% or more.

[0060] V: 0.5% or less V can be added to steel because it improves the quality of heat-treated steel sheets by forming fine precipitates and improves the stability of retained austenite and impact toughness by refining grains. However, if the amount of V added exceeds 0.5%, not only will the effect saturate, but the addition of excessive ferroalloys may also increase costs. In one embodiment of the present invention, V can be added in an amount of 0.001% or more to ensure the effects of the V addition.

[0061] The balance other than the above-mentioned components includes iron and inevitable impurities, but is not particularly limited as long as it is a component that can be contained in a steel sheet for hot forming.

[0062] An example of a method for manufacturing a steel sheet for hot press forming according to an embodiment of the present invention will be described below. However, it should be noted that the method for manufacturing a steel sheet for hot press forming described below is merely an example, and the steel sheet for hot press forming of the present invention does not necessarily have to be manufactured by this manufacturing method, and that any manufacturing method can be used to realize each embodiment of the present invention as long as it satisfies the scope of the claims of the present invention.

[0063] According to one embodiment of the present invention, the steel sheet may be manufactured by the steps of obtaining an aluminum-iron (Al-Fe) alloy-plated steel sheet in which an aluminum-iron alloy plating layer is formed on a base steel sheet; and performing skin-pass rolling on the aluminum-iron (Al-Fe) alloy-plated steel sheet.

[0064] In one embodiment of the present invention, the aluminum-iron alloy plated steel sheet may be obtained by a process including the steps of obtaining an aluminum-plated steel sheet plated with aluminum or an aluminum alloy; and heating the aluminum-plated steel sheet to alloy it.

[0065] Any aluminum-plated steel sheet industrially considered aluminum-based can be used. In one embodiment of the present invention, an aluminum-plated steel sheet containing 70% or more Al by weight can be used. The remaining elements in the coating layer other than Al include Si and one or more elements selected from Mg, Zn, Mn, Cr, Mo, Ti, and Fe, which are commonly added to aluminum-based coating layers, and / or other impurity elements. Si may be contained in a proportion of 0.01 to 20%. Controlling the Si content to 0.01% or less requires high-purity raw materials, significantly increasing production costs. If the Si content exceeds 20%, the melting temperature of the coating bath increases, making equipment maintenance difficult, and the alloying rate decreases, making it difficult to achieve sufficient alloying. Therefore, in the present invention, the Si content in the coating bath can be limited to 0.01 to 20%. The total content of one or more elements selected from Mg, Zn, Mn, Cr, Mo, and Ti in the coating layer may be 20% or less by weight.

[0066] The aluminum-based plating layer can be formed by a hot-dip aluminum plating method in which a hot-rolled or cold-rolled and annealed steel sheet is immersed in a hot-dip aluminum plating bath.

[0067] In one embodiment of the present invention, the amount of aluminum plating is 10 to 100 g / m per side. 2 The plating amount may be 10 g / m 2 If the plating amount is less than 100 g / m, the corrosion resistance decreases. 2 If the amount exceeds 100 g / m, the weldability will be reduced. Therefore, in the present invention, the amount of aluminum plating is set to 10 to 100 g / m per side. 2 On the other hand, in another embodiment of the present invention, the plating amount in the aluminum plating is 20 to 90 g / m per side. 2 It can also be.

[0068] In one embodiment of the present invention, the step of heating and alloying the aluminum-plated steel sheet may be performed by online heating, which is directly connected to a line for plating steel sheets with aluminum or an aluminum alloy and heats the plated steel sheet while it is traveling. In one embodiment of the present invention, the heating temperature during the alloying may range from 670 to 900°C, and the maintenance time may be 1 to 20 seconds. In another embodiment of the present invention, the heating temperature may range from 680 to 880°C, and the maintenance time may be 1 to 10 seconds.

[0069] In another embodiment of the present invention, the step of heating and alloying the aluminum-plated steel sheet may be performed by box annealing, in which a coiled aluminum-plated steel sheet is heated in a box annealing furnace. In this case, the coil cooled to room temperature after aluminum plating may be heated in a box annealing furnace in a hydrogen or hydrogen and nitrogen atmosphere with a dew point temperature of less than −10° C. at a temperature in the range of 600 to 800° C. for 0.1 to 100 hours to perform alloying heat treatment (in the present invention, the highest temperature reached by the furnace atmosphere within the above temperature range is defined as the heating temperature).

[0070] The maintenance time in each embodiment means the time from when the ambient temperature reaches the target temperature until when cooling starts.

[0071] In one embodiment of the present invention, the skin pass rolling can be carried out under conditions in which the SPMI, represented by the following relational expression 1, is 5000 to 8500.

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[0072] That is, the SPMI is a condition devised by the present inventors that can control the surface condition of a steel sheet. The Ra and RPc of the steel sheet surface are affected not only by the Ra and RPc of the roll surface but also by the pressing force applied by the roll. A quantitative analysis of the degree of influence of these factors revealed a relationship expressed by the above Relational Formula 1. In order for the product of Ra and RPc of the steel sheet surface to have a sufficient value, the SPMI value must be 5000 or more, and in some cases, the SPMI value may be limited to 5500 or more. However, if the SPMI value is too high, the corrosion resistance of the part obtained after hot press forming may be reduced, and therefore the value may be limited to 8500 or less, and in some cases, may be limited to 8000 or less.

[0073] Hereinafter, a hot press-formed part according to one embodiment of the present invention will be described. However, since a method for producing the hot press-formed part is a conventionally known method that involves heating a steel sheet to a temperature equal to or higher than the austenitizing temperature, maintaining the temperature, and then quenching and forming the steel sheet simultaneously, the method is not particularly limited in the present invention.

[0074] A hot press-formed member according to one embodiment of the present invention includes a base steel sheet and a plating layer made of an Al-Fe alloy formed on the base steel sheet, and the product of Ra and RPc on the surface of the plating layer can be adjusted to provide both paint adhesion and corrosion resistance.

[0075] In one embodiment of the present invention, to ensure high paint adhesion, the Ra×RPc of the surface of the plating layer of a member obtained by hot press forming may be 60 μm / cm or more. In this formula, Ra is the arithmetic mean roughness measured in μm, and RPc is the peak count measured in cm ( / cm). If Ra×RPc is insufficient, sufficient paint adhesion may be difficult to achieve. Therefore, the lower limit of Ra×RPc may be set to 60 μm / cm. In some cases, the lower limit of Ra×RPc may be set to 70 μm / cm. While a higher Ra×RPc value is advantageous for improving paint adhesion, if the value is too high, excessive cracks may be introduced into the surface of the plating layer during processing of the plated steel sheet to increase the Ra and RPc of the member according to one embodiment of the present invention, thereby reducing corrosion resistance. Therefore, in one embodiment of the present invention, the upper limit of Ra×RPc may be set to 150 μm / cm, and in some cases, the upper limit of Ra×RPc may be set to 140 μm / cm.

[0076] In addition, in the hot press-formed member according to another embodiment of the present invention, the number of cracks formed on the surface of the Al-Fe alloy plating layer and the area ratio of the indentations can be appropriately adjusted, thereby improving the paint adhesion and corrosion resistance of the member obtained by the subsequent hot press-forming process. 2 There may be 15 to 220 cracks per indentation, and the proportion of the area occupied by the indentations on the surface of the plating layer may be 5 to 50%.

[0077] The cracks can act as anchoring points for fixing the coating layer on the surface of the hot press-formed part, so in one embodiment of the present invention, the cracks are formed on the surface of the hot press-formed part by 1 mm2 of the coating layer. 2 The number of cracks can be determined by dividing the field of view of a microscope (magnification 100x) into 10 equal parts vertically and horizontally, and counting the number of cracks observed within 100 areas per observation area of ​​1mm. 2The measurement is converted into the value observed in a 1000-millimeter film. In one embodiment of the present invention, the microscope may be a ZEISS SUPRA 55VP model scanning electron microscope. In this case, if a single crack is observed in multiple regions, the number of cracks may be counted as the number of regions in which the crack is observed. Of course, if multiple cracks are observed in one region, the number of cracks will be counted as that number. This is a concept that takes into account the total length of the cracks within the observation area, because the total length of the cracks affects the fixing effect of the paint layer. However, unlike zinc-based plated steel sheets, aluminum-based plated steel sheets (members) do not have a sacrificial corrosion protection function, so if cracks are present, corrosion may occur through the cracks. Therefore, an excessive number of cracks may impair the corrosion resistance of the member, so a 1000-millimeter film is not required. 2 The maximum number of cracks per hit can be limited to 220, or in some cases, 200.

[0078] In one embodiment of the present invention, a large number of indentations can be formed on the surface of the coating layer of the steel sheet to increase the contact area with the paint layer. These indentations can remain on the component, improving paint adhesion. The presence of these indentations can increase the Ra and RPc of the surface of the coating layer. For this reason, in one embodiment of the present invention, the proportion of indentations on the surface of the coating layer can be 5% or more, and in some cases, 8% or more. Although not necessarily limited thereto, in one embodiment of the present invention, surface images were observed using a Leica DM6000M model optical microscope at 100x magnification. The results were then divided into 256 color brightness levels using Clemex Vision PE software, and areas with a brightness above 70% of the highest brightness level were identified as the indentations, and their area percentage was calculated. If the proportion of indentations is too high, excessive stress is applied to the coating layer to form the indentations, which can lead to increased surface cracking. Therefore, the upper limit of the proportion of indentations can be set to 50% or 45%.

[0079] According to one embodiment of the present invention, the aluminum-iron (Al-Fe) alloy plating layer may contain a total of 70% or more of Al and Fe by weight. Since the plating layer may be formed using only these elements, there is no need to specifically set an upper limit for the total content, and the total content of these elements may be 100%.

[0080] In the plating layer, Fe can be diffused into the plating layer during hot press forming, so it may be contained in an amount of 30% or more by weight. If the Fe content in the plating layer is less than 30%, it may not be very useful in solving problems such as hydrogen embrittlement during storage. Therefore, in the present invention, the Fe content in the plating layer of the member may be 30% or more by weight, and in some cases, the Fe content may be 35% or more, or even 40% or more.

[0081] While there is no particular upper limit for the Fe content, when considering the Fe content in the coating layer of a typical hot-press-formed part, the upper limit for the Fe content can be set at 90%, or even 80% or less in some cases. Here, the average Fe content refers to the average Fe content in the entire coating layer. While various measurement methods are possible, in this embodiment, the average Fe content is calculated by integrating the Fe content curve as a function of depth (thickness) obtained by analyzing the coating layer from the surface to the interface with the steel sheet using glow discharge emission spectrometry (GDS), and then dividing this value by the thickness of the coating layer. There are various criteria for determining the interface between the coating layer and the steel sheet, but in this embodiment, the point where the Al and Fe content curves intersect in the GDS results, i.e., where the contents of the two elements are equal, can be defined as the interface between the coating layer and the steel sheet.

[0082] According to one embodiment of the present invention, the plating layer of the hot press-formed part may further contain elements commonly contained in plating layers in addition to the above-mentioned Al and Fe. Examples of such elements include one or more selected from Mg, Zn, Mn, Cr, Mo, Si, and Ti, and these elements may be contained in the plating layer in a total amount of up to 20 wt %.

[0083] The base steel sheet for the hot press-formed member of the present invention may have various microstructures depending on the strength: if the tensile strength is 400 to 800 MPa, the microstructure may be composed of 5 to 50% martensite on an area basis and the remaining one or more phases selected from ferrite, pearlite, bainite, and austenite; if the tensile strength is 800 to 1300 MPa, the microstructure may be composed of 90% or more martensite on an area basis and the remaining one or more phases selected from ferrite, pearlite, bainite, and austenite; and if the tensile strength is 1300 MPa or higher, the microstructure may be composed of 95% or more martensite on an area basis and the remaining one or more phases selected from ferrite, pearlite, bainite, and austenite. [Example]

[0084] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are merely for the purpose of illustrating and embodying the present invention, and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0085] (Example) A cold-rolled steel sheet for hot press forming having the composition shown in Table 1 below was prepared as the base steel sheet. The base steel sheet was annealed by a conventional method and then hot-dip aluminum-plated. The coating bath had a composition consisting essentially of 9.5% Si by weight and the balance Al, and the coating bath temperature was 660°C. After coating, an air knife was used to remove the coating weight of 40 g / m2 per side. 2 was adjusted to.

[0086] Thereafter, alloying was performed by online or box annealing for each of the inventive and comparative examples to obtain Al-Fe alloy-plated steel sheets. Online alloying was performed by reheating the steel sheet to 720°C, maintaining that temperature for 5 seconds, and then cooling to room temperature, while box annealing involved maintaining the coil in a box annealing furnace at 650°C for 10 hours.

[0087] After alloying, the plated steel sheet was subjected to skin-pass rolling at the rolling force shown in Table 2 using rolls having the surface roughness (Ra) and peak count (RPc) shown in Table 2, thereby adjusting the surface condition of the alloyed coating layer of the steel sheet.

[0088] In all of the inventive examples and comparative examples, the total Al and Fe content and the Fe content in the alloy coating layers obtained by each alloying method and skin-pass rolling were 90% and 43%, respectively, and no particular differences were observed between the examples. Furthermore, in all of the inventive examples and comparative examples, the Fe content at the surface of the coating layer was at the 77% level relative to the average Fe content in the coating layer, showing no significant difference. Here, the surface of the coating layer refers to a point 1 μm deep from the outermost surface of the coating layer.

[0089] The skin-pass rolled plated steel sheet was heated to 930°C in an air atmosphere for 6 minutes, followed by hot press forming and quenching to obtain a hot press-formed part. The internal structure of the obtained hot press-formed part was confirmed to be substantially 100% martensite, and to have a strength of 1500 MPa. However, the structure and strength of the steel can be modified as needed, and an ordinary engineer would have no difficulty in manufacturing a part with the desired structure and strength by changing manufacturing conditions, including the steel composition and cooling conditions.

[0090] In addition, in all of the hot press-formed parts obtained in the invention examples and comparative examples, the total content of Al and Fe in the alloy plating layer and the Fe content were at the levels of 83% and 44%, respectively, and no particular differences were observed between the examples.

[0091] The surface roughness (Ra), number of peaks (RPc), number of cracks per unit area, and proportion of indentations were measured for skin-pass rolled plated steel sheets and hot press-formed parts. Surface roughness and number of peaks were measured at five locations according to JIS B 0601 (2013) and the values ​​were averaged. The number of cracks was calculated by dividing the field of view of a microscope (magnification: 100x) into 10 equal parts vertically and horizontally, and then counting the total number of cracks observed in each of the 100 areas per observation area of ​​1mm. 2 The area was measured by converting it to that observed in a 3D image. A ZEISS SUPRA 55VP model scanning electron microscope was used for the measurements, and the average of measurements taken at five locations was calculated and used for analysis. The percentage of indentations was determined by observing the surface image at 100x magnification with a Leica DM6000M model optical microscope, dividing the results into 256 color brightness levels using Clemex Vision PE software, and identifying the areas with a brightness level of 70% or higher as the indentation. The area percentage was also calculated as the average of the results of observing five locations. Table 3 lists the measurement results for the plated steel sheet, and Table 4 lists the results for the hot-press-formed parts.

[0092] The paint adhesion and corrosion resistance of the hot press-formed parts obtained were evaluated by the following methods. The results are shown in Table 4.

[0093] First, the paint adhesion grade was determined by applying paint to the parts obtained by the GMW14829 method, creating a grid scratch with 1mm intervals, and then evaluating it with tape peeling. A grade of 1 or less is considered good.

[0094] Corrosion resistance was evaluated by phosphate treatment and painting of the above-mentioned components according to the GMW14872 standard, making crosscuts, and then conducting a cyclic corrosion test in a saltwater atmosphere 52 times, after which the blister width was measured. In this example, a width of 2 mm or less was considered to be good.

[0095] [Table 1]

[0096] [Table 2]

[0097] [Table 3]

[0098] [Table 4]

[0099] In Comparative Examples 1 to 3, the SPMI during skin-pass rolling was less than 5000. As a result, the Ra × RPc of the alloyed coated steel sheet surface was not sufficiently ensured, or the proportion of indentations and the number of cracks were insufficient. The hot-press-formed parts obtained by hot-pressing such steel sheets also had a low Ra × RPc, or the number of cracks or proportion of indentations was insufficient. In such cases, the anchoring effect was insufficient, making it difficult for the paint layer to bond firmly to the surface of the part. As a result, it was confirmed that the paint adhesion grades were all poor, at 2 or higher. On the other hand, in Comparative Examples 4 to 6, the SPMI values ​​were too high. In such cases, the Ra × RPc of the coating layer, the proportion of indentations, and the number of cracks may be sufficient to ensure the paint adhesion of the part, but damage to the coating layer may occur, resulting in poor corrosion resistance of the part, as shown in Table 4. That is, if the plating layer is damaged, the base steel sheet may be exposed through the damaged gaps in the aluminum alloy-based plating layer, which cannot provide the corrosion protection performance of the sacrificial anode method, and corrosion may occur. As a result, the width of the blister, which is an index of corrosion resistance, may exceed the allowable limit.

[0100] On the other hand, in Examples 1 to 7 of the present invention, which satisfied the conditions of the present invention, the surface properties of the plated steel sheet were ensured by appropriately controlling the skin-pass rolling conditions, and as a result, the paint adhesion and corrosion resistance of the final component were simultaneously ensured.

[0101] Figure 1 shows a plated steel sheet (a) produced according to Comparative Example 1 and a plated steel sheet (b) produced according to Inventive Example 2. As can be seen from the drawing, the plated steel sheet produced according to Comparative Example 1 does not have sufficient surface irregularities, whereas the plated steel sheet produced according to Inventive Example 2 has sufficient surface irregularities, making the surface of the part produced by subsequent hot press forming suitable for fixing a paint layer.

[0102] Figure 2 shows the results of observing the surfaces of the steel sheets manufactured in Comparative Example 1 and Inventive Example 2 with a 100x microscope (DM6000M) and processing the images using Clemex Vision PE software, with areas (indentations) with 70% or more of the maximum brightness displayed in white. As can be seen from the figure, Inventive Example 2, in which skin-pass rolling was performed by controlling the SPMI within the appropriate range according to the present invention, was able to form much higher indentations than Comparative Example 1, in which skin-pass rolling was performed under low SPMI conditions.

[0103] The relationship between the SPMI value and the number of cracks is shown in the graph in Figure 3. It can be seen from the graph that when the SPMI value is between 5000 and 8000√Ton·μm / cm, the number of cracks can be maintained within an appropriate range.

[0104] Therefore, the advantageous effects of the present invention were confirmed.

Claims

1. Base steel sheet and a plating layer made of an Al-Fe alloy formed on the base steel sheet, The total content of Al and Fe in the plating layer is 80% or more by weight, The average content of Fe in the plating layer is 20% or more by weight, The plated steel sheet for hot press forming has a product of Ra and RPc on the surface of the plated layer of 60 to 150 μm / cm. Here, Ra means the arithmetic mean roughness, and its unit is μm, and RPc means the number of peaks per unit length, and its unit is / cm.

2. The plated steel sheet for hot press forming according to claim 1, wherein the Fe content in the surface portion of the plated layer is 50% or more of the average Fe content in the plated layer. Here, the Fe content in the surface region is a value measured by EDS area analysis at a region magnified 100 times with a scanning electron microscope (SEM), and the average Fe content is a value obtained by integrating the Fe content curve obtained by GDS analysis from the surface layer to the depth where the Fe and Al contents intersect, and then dividing this by the distance from the surface layer to the depth where the Fe and Al contents intersect.

3. The plated steel sheet for hot press forming according to claim 2, wherein the surface portion of the plated layer has an Fe content of 15% or more by weight.

4. The plated steel sheet for hot press forming according to claim 1 , wherein an average content of Fe in the plated layer is 90% or less.

5. 4. The plated steel sheet for hot press forming according to claim 1, wherein the plating layer contains one or more elements selected from Mg, Zn, Mn, Cr, Mo, Si, and Ti in a total content of 20 wt. % or less.

6. 4. The plated steel sheet for hot press forming according to claim 1, wherein the base steel sheet has a composition containing, by weight, C: 0.01 to 0.5%, Si: 2.0% or less (excluding 0%), Mn: 0.1 to 4.0%, P: 0.05% or less, S: 0.02% or less, Al: 0.001 to 1%, Cr: 5.0% (excluding 0%), N: 0.02% or less, Ti: 0.1% or less (excluding 0%), B: 0.0001 to 0.01%, and the balance being Fe and unavoidable impurities.

7. 7. The plated steel sheet for hot press forming according to claim 6, wherein the base steel sheet further contains one or more elements selected from Nb: 0.1% or less, Mo: 0.5% or less, Ni: 1% or less, Cu: 1% or less, and V: 0.5% or less.

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