Hot-Forming Plated Steel Sheet Excellent in Hydrogen Embrittlement Resistance and Impact Resistance, Hot-Forming Member, and Manufacturing Method Thereof

Abstract

Provided are a plated steel sheet for hot forming, a hot-formed member produced from the plated steel sheet, and methods for producing the same, which include a base steel sheet containing, by weight, 0.14 to 0.5% C, 0.001 to 1% Si, 0.3 to 4% Mn, 0.001 to 0.015% P, 0.0001 to 0.02% S, 0.001 to 0.1% Al, 0.001 to 1% Cr, 0.001 to 0.02% N, 0.001 to 0.02% Ti, 0.1% or less B, 0.01% or less Sb, and the balance being Fe and other unavoidable impurities; an aluminum or aluminum alloy plating layer provided on at least one side of the base steel sheet; and an Sb-enriched layer provided between the base steel sheet and the plating layer, and which satisfy the following Relational Expressions 1-1 and 1-2. [Equation 1] TIFF2023536487000020.tif50155 [In the above relational expressions 1-1 and 1-2, the above Sb coat indicates the average Sb content in the plating layer, expressed in weight percent. max indicates the maximum Sb content in the Sb-enriched layer, and its unit is weight percent. The Δt is the distance from the boundary between the plating layer and the Sb-enriched layer to the Sb max The unit is μm.

Description

Technical Field

[0001] The present invention relates to a hot - forming plated steel sheet, a hot - forming member, and a method for manufacturing the same, which are excellent in hydrogen embrittlement resistance and impact resistance.

Background Art

[0002] In recent years, hot - forming members have been widely applied to automotive structural members for purposes such as improving fuel efficiency by reducing the weight of automobiles and protecting passengers. In particular, they can be used for bumpers, doors, pillar reinforcements, etc. that require high ultra - strength or high energy absorption capacity. As such a hot - forming technology, typically, there is U.S. Patent Publication No. 6296805 (hereinafter, Patent Document 1).

[0003] Patent Document 1 discloses that after heating an Al - Si - based plated steel sheet at 850°C or higher, the structure of the member is formed as martensite by hot - forming by pressing and rapid cooling, thereby ensuring ultra - high strength with high tensile strength. When applying such a hot - forming ultra - high - strength steel, since it is formed at a high temperature, complex shapes can be easily formed, and due to the increase in strength caused by rapid cooling in the mold, a weight - reduction effect can be expected. However, the martensite structure is known to have low resistance to hydrogen embrittlement. In particular, there are residual stresses in the hot - forming member due to rapid cooling after heating. Therefore, when the amount of diffusible hydrogen in the steel increases, there is a concern about delayed fracture due to hydrogen embrittlement, and thus the application of the member is limited.

[0004] In addition, changes in process parameters can cause changes in the overall or local mechanical properties within the sheet. Therefore, a steel composition that is less sensitive to changes in manufacturing parameters for producing a plated steel sheet and a hot - forming member having good mechanical properties and homogeneity is required, and it is necessary to prevent delayed fracture associated with hydrogen embrittlement. However, a technology that can still fully meet such demands has not been developed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention is for solving the above problems, and provides a hot-forming plated steel sheet, a hot-forming member, and methods for manufacturing them, which are excellent in hydrogen embrittlement resistance and impact resistance.

[0007] The problems of the present invention are not limited to the above content. Any person having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding further problems of the present invention from the content throughout the specification of the present invention.

Means for Solving the Problems

[0008] One aspect of the present invention is a base steel sheet containing, by weight%, C: 0.14 to 0.5%, Si: 0.001 to 1%, Mn: 0.3 to 4%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.001 to 0.1%, Cr: 0.001 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, Sb: 0.005 to 0.1%, and the balance Fe and other inevitable impurities, an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel sheet, and an Sb-enriched layer provided between the base steel sheet and the plating layer, and provides a hot-forming plated steel sheet satisfying the following relational expressions 1-1 and 1-2.

[0009]

Number

[0010] Still another aspect of the present invention is by weight, C: 0.14 to 0.5%, Si: 0.001 to 1%, Mn: 0.3 to 4%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.001 to 0.1%, Cr: 0.001 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, Sb: 0.005 to 0.1%, and the balance being Fe and other inevitable impurities, reheating the steel slab at 1050 to 1300 °C; finish rolling the heated steel slab at 800 to 950 °C to obtain a hot-rolled steel sheet; coiling the above hot-rolled steel sheet at 500 to 700 °C; pickling the coiled hot-rolled steel sheet such that the product of the acid concentration and the pickling time is 800 to 10,000 g / L*s; annealing the pickled steel sheet at 700 to 900 °C under the dew point temperature condition of -75 to +20 °C in an annealing furnace; after annealing, passing the above steel sheet through a plating bath made of aluminum or an aluminum alloy for plating; A method for manufacturing a plated steel sheet for hot forming is provided, including

[0011] Still another aspect of the present invention is by weight, a base steel sheet containing C: 0.14 to 0.5%, Si: 0.001 to 1%, Mn: 0.3 to 4%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.001 to 0.1%, Cr: 0.001 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, Sb: 0.005 to 0.1%, and the balance being Fe and other inevitable impurities; an aluminum or aluminum alloy plating layer provided on at least one surface of the above base steel sheet; ​It includes an Sb enrichment layer provided between the base steel sheet and the plating layer, A hot-forming member is provided that satisfies the following relational expressions 2-1 and 2-2.

[0012]

Number

[0013] Still another aspect of the present invention is A method for manufacturing a hot-forming member is provided, in which a hot-forming plated steel sheet manufactured by the method for manufacturing a hot-forming plated steel sheet described above is heat-treated at a temperature in the range of Ac3 to 950 °C for 1 to 1000 seconds and then hot press formed.

Advantages of the Invention

[0014] According to one aspect of the present invention, by manufacturing a plated steel sheet in which an Sb enrichment layer is preserved between the base steel sheet and the plating layer, the amount of diffusible hydrogen in the steel is reduced, and a hot-forming plated steel sheet, a hot-forming member, and a method for manufacturing them, which are excellent in hydrogen embrittlement resistance and impact resistance, can be provided.

[0015] The various and beneficial advantages and effects of the present invention are not limited to the above description and can be more easily understood in the process of explaining the specific embodiments of the present invention.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Mode for Carrying Out the Invention

[0017] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Also, the embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the technical field.

[0018] Diffusible hydrogen at the grain boundaries promotes the generation of grain boundary cracks during stress generation. Therefore, a solution is needed to reduce the amount of diffusible hydrogen in the steel after hot stamping.

[0019] Therefore, the present inventors analyzed the effects of various components, manufacturing conditions, structures, etc. including Sb addition by calculating the area (CIE: Crack initiation Energy) until the maximum load is reached during the diffusible hydrogen amount analysis in the steel, which is an index clearly showing the hydrogen embrittlement resistance effect, and the three-point bending test (VDA238 - 100), which is one of the indices capable of showing the impact resistance, in the hot-forming steel material. As a result, it was found that the amount of diffusible hydrogen can be reduced by the formation of the Sb enrichment layer, and thus, a hot-forming plated steel sheet, a hot-forming member, and a manufacturing method thereof, which are excellent in impact resistance in addition to hydrogen embrittlement resistance, were devised.

[0020] Hereinafter, first, the hot-forming plated steel sheet and the hot-forming member according to one aspect of the present invention will be described in detail.

[0021] The plated steel sheet according to one aspect of the present invention includes a base steel sheet containing, by weight%, C: 0.14 to 0.5%, Si: 0.001 to 1%, Mn: 0.3 to 4%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.001 to 0.1%, Cr: 0.001 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, Sb: 0.005 to 0.1%, and the balance Fe and other inevitable impurities, an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel sheet, and an Sb enrichment layer provided between the base steel sheet and the plating layer.

[0022] First, the alloy composition of the base steel sheet of the present invention will be described in detail. It should be noted that when each element is expressed by content in the present invention, it means weight% unless otherwise specified.

[0023] Carbon (C): 0.14 to 0.5% The above C is an element that increases the strength of the heat-treated member and improves the hardenability, and should be appropriately added as an essential element for strength adjustment. When the C content is less than 0.14%, the hardenability is low. When the cooling rate decreases, sufficient martensite cannot be ensured, and due to the formation of ferrite, the impact resistance is poor. Therefore, it should be added in an amount of 0.14% or more. On the other hand, when the C content exceeds 0.5%, the strength may increase excessively and induce brittleness, and the weldability is poor. Therefore, the upper limit is preferably 0.5% or less. Alternatively, more preferably, the lower limit of the above C content may be 0.147%, and more preferably, the upper limit of the above C content may be 0.335%.

[0024] Silicon (Si): 0.001 - 1% The above Si should not only be added as a deoxidizer in steelmaking, but is also a solid solution strengthening element and an element that suppresses carbide formation. Therefore, Si is not only an element effective for homogenizing the internal structure, but also contributes to the increase in strength of the hot-formed member and is added as an element effective for material homogenization. However, when the Si content is less than 0.001%, the above effects cannot be expected, and furthermore, the manufacturing cost and process cost for controlling the Si content increase, so it is not appropriate. On the other hand, when the Si content exceeds 1%, the plating property significantly decreases due to excessive Si oxide generated on the surface of the steel sheet during annealing. Therefore, it is added in an amount of 1% or less. Alternatively, more preferably, the lower limit of the above Si content may be 0.11%, and more preferably, the upper limit of the above Si content may be 0.81%.

[0025] Manganese (Mn): 0.3 - 4% The above-mentioned Mn can not only ensure the desired strength due to the effect of solid solution strengthening, but also needs to be added to suppress the formation of ferrite during hot forming by improving the hardenability. When the Mn content is less than 0.3%, it is difficult to obtain the sufficient effect of hardenability, and due to the insufficient hardenability, other expensive alloying elements are required in excess, resulting in a significant increase in manufacturing cost. When the Mn content exceeds 4%, the banded structure arranged in the rolling direction of the fine tissue phase deepens, inducing the inhomogeneity of the internal structure, which may reduce the impact resistance, so the addition is limited to 4% or less. Alternatively, more preferably, the lower limit of the above Mn content may be 0.5%, and more preferably, the upper limit of the above Mn content may be 3.7%.

[0026] Phosphorus (P): 0.001 - 0.015% The above P exists as an impurity in the steel. Since a high manufacturing cost is required for its minimum content to be less than 0.001%, it is not preferable. However, when the P content exceeds 0.015%, the weldability of the hot-forming member and the material properties due to high-temperature grain boundary segregation deteriorate, so the upper limit is set at 0.015%. Alternatively, more preferably, the lower limit of the above P content may be 0.003%, and more preferably, the upper limit of the above P content may be 0.013%.

[0027] Sulfur (S): 0.0001 - 0.02% The above S is an impurity and an element that inhibits the ductility, impact properties and weldability of the member, so the maximum content is limited to 0.02%. Also, when the minimum content is less than 0.0001%, it is not preferable because it significantly increases the manufacturing cost. Alternatively, the lower limit of the above S content may be 0.001%, and more preferably, the upper limit of the above S content may be 0.007%.

[0028] Aluminum (Al): 0.001 - 0.1% The above Al, together with Si, is an element that deoxidizes in steelmaking to improve the cleanliness of steel. When the Al content is less than 0.001%, it is difficult to obtain the above effects. When the content exceeds 0.1%, the hot ductility decreases due to excessive AlN precipitates formed during the continuous casting process, and slab cracks may occur, inducing manufacturing problems. Therefore, the upper limit is set at 0.1%. Alternatively, more preferably, the lower limit of the above Al content may be 0.011%, and more preferably, the upper limit of the above Al content may be 0.071%.

[0029] Cr: 0.001 - 1% The above Cr is added as an element to ensure the hardenability of steel like Mn and suppress the formation of ferrite after hot forming. When the Cr content is less than 0.001%, it is difficult to ensure the above effects. On the other hand, when the Cr content exceeds 1%, not only is the improvement effect of hardenability slight compared to the addition amount, but coarse iron carbides may be excessively formed, inducing cracks during stress application, deteriorating the material quality. Therefore, the upper limit is set at 1%. Alternatively, more preferably, the lower limit of the above Cr content may be 0.011%, and more preferably, the upper limit of the above Cr content may be 0.50%.

[0030] Nitrogen (N): 0.001 - 0.02% The above N is contained as an impurity in the steel. To make the N content less than 0.001%, excessive manufacturing costs are involved. When the content exceeds 0.02%, slab cracks due to the formation of AlN, like the added Al, are likely to occur. Therefore, the upper limit is set at 0.02%. Alternatively, more preferably, the lower limit of the above N content may be 0.0026%, and more preferably, the upper limit of the above N content may be 0.0077%.

[0031] Ti: 0.1% or less (including 0) The above-mentioned Ti is an element selectively added in the present invention. By combining with nitrogen remaining as an impurity in the steel to form TiN, it can play a role in protecting B for ensuring hardenability from forming compounds. Also, the effects of precipitation strengthening and grain refinement can be expected due to the formation of TiC precipitates. However, if its content exceeds 0.1%, rather a large amount of coarse TiN is formed, deteriorating the quality of the steel, so its upper limit is set at 0.1%. On the other hand, since the above-mentioned Ti is a selective element and includes the case where it is not added, the lower limit of the above Ti content may be 0%.

[0032] B: 0.01% or less (including 0) The above-mentioned B is an element selectively added in the present invention and is an element that can effectively improve hardenability. When the above B is added, it segregates at the prior austenite grain boundaries and can suppress the brittleness of the hot-formed member due to the grain boundary segregation of impurities P or / and S. However, if it exceeds 0.01%, Fe 23 CB 6 There is a possibility of causing brittleness in hot rolling due to the formation of a composite compound, so its upper limit is set at 0.01%. On the other hand, since the above B is a selective element and includes the case where it is not added, the lower limit of the above B content may be 0%.

[0033] Sb: 0.005 - 0.1% Sb is a core element in manufacturing the present hot-formed member. It forms an Sb enrichment layer at the interface between the base steel plate and the plating layer, plays a role in reducing the hydrogen content absorbed during heat treatment, and reducing hydrogen-induced delayed fracture sensitivity. If the Sb content is less than 0.005%, a sufficient enrichment layer is not formed at the interface between the plating layer and the base iron, and the above-mentioned effects cannot be expected. On the other hand, if the Sb content exceeds 0.1%, Sb precipitates excessively at the grain boundaries, inducing grain boundary fracture when stress occurs and deteriorating the material quality. Therefore, it is preferable that its upper limit is 0.1%. Alternatively, more preferably, the lower limit of the above Sb content may be 0.006%, and more preferably, the upper limit of the above Sb content may be 0.095%.

[0034] The balance other than the above components is iron (Fe), and no particular additional addition is restricted as long as it is a component that can be included in the hot press forming steel sheet. Also, since unintended impurities may unavoidably mix in from the raw materials or the surrounding environment during the normal manufacturing process, it is impossible to eliminate them. Since these impurities are understandable to any technician in the normal manufacturing process, all of their details are not particularly mentioned in this specification.

[0035] Also, the plating layer includes an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel sheet. The plating layer imparts corrosion resistance in the final hot formed member.

[0036] In the present invention, the type of the plating layer is not particularly limited, and any plating layer applicable to the conventional hot press forming plated steel sheet can be applied without limitation in the present invention. As an example, the plating layer may be an aluminum or aluminum alloy plating layer, and preferably, the plating layer can contain Si: 6 to 12%, Fe: 1 to 4%, the balance being Al and other inevitable impurities.

[0037] According to one aspect of the present invention, the plated steel sheet can include an Sb enrichment layer provided between the base steel sheet and the plating layer. At this time, the Sb enrichment layer is a region provided between the base steel sheet and the plating layer, where Sb is concentrated and classified as an Sb content.

[0038] Although not particularly limited, according to one implementation example of the present invention, such an Sb enrichment layer and the plating layer can be distinguished by analyzing the change in the Sb content in the thickness direction from a certain point on the plating layer toward the base steel sheet side by utilizing glow discharge spectrometry (GDS).

[0039] Specifically, according to one implementation example of the present invention, although not particularly limited, as shown in FIG. 7, the x-axis indicates the linear distance in the thickness direction from an arbitrary position inside the plating layer 1 toward the base steel sheet 3 side, and the y-axis is determined based on a graph showing the Sb content measured by utilizing the above GDS.

[0040] For example, based on FIG. 7 schematically showing the above-described GDS measurement results, among the rising intervals 21 of the Sb content in the x-axis (+) direction provided between the plating layer 1 and the base steel plate 3, from the last contact point 11 in the x-axis (+) direction of the Sb average content line 10 of the plating layer and the Sb content line 100 measured using the above GDS, it is regarded as the Sb enrichment layer 2 (in the thickness direction on the side of the base steel plate 3).

[0041] At this time, the Sb average content line 10 of the plating layer 1 can mean the extension line of the Sb average content line for the interval from the point 15 μm away from the point 200 (the point of Sb) on the side of the plating layer 1 to the point 20 μm away from the point where the Sb content is the maximum value in the Sb enrichment layer 2. max of Sb) to the point 20 μm away from the point 15 μm away from the side of the plating layer 1.

[0042] Similarly, for the Sb enrichment layer 2 and the base steel plate 3 as well, in the same manner as the above method, among the rising intervals 22 of the Sb content in the x-axis (-) direction provided between the base steel plate 3 and the plating layer 1 measured using the above GDS, from the last contact point 31 in the x-axis (-) direction of the Sb average content line 30 of the base steel plate and the Sb content line 100 measured using the above GDS, it is regarded as the Sb enrichment layer 2 (in the thickness direction on the side of the plating layer 1).

[0043] At this time, the Sb average content line 30 of the base steel plate 3 can mean the extension line of the Sb average content line for the interval from the point 15 μm away from the point 200 (the point of Sb) on the side of the base steel plate 3 to the point 20 μm away from the point where the Sb content is the maximum value in the Sb enrichment layer 2. max of Sb) to the point 20 μm away from the point 15 μm away from the side of the base steel plate 3.

[0044] As a result of intensive studies, the inventors have found that reducing the amount of diffusible hydrogen absorbed during the heat treatment of hot forming can suppress the induction of defects due to hydrogen-induced delayed fracture. Specifically, during the stage of heating a blank having a plating layer made of aluminum or an aluminum alloy during hot forming, water vapor present in the heat treatment furnace is adsorbed on the surface of the blank. Subsequently, hydrogen generated by the dissociation of water is absorbed into the steel while it has an austenite phase with a high hydrogen solubility at high temperature. However, when rapid cooling occurs due to hot forming and the phase changes to a martensite phase, the solubility of hydrogen rapidly decreases, and the alloy plating layer formed by alloying the plating layer serves as an obstacle that makes it difficult for the plating layer to release hydrogen. Therefore, a significant amount of diffusible hydrogen content remains in the steel, thereby increasing the possibility of crack generation due to hydrogen-induced delayed fracture. Therefore, reducing the amount of diffusible hydrogen absorbed during heat treatment is an important factor for defect suppression.

[0045] Furthermore, as a result of several studies, the inventors have confirmed that the impact resistance tends to increase as the hydrogen content in the steel decreases. This is because diffusible hydrogen absorbed in the steel during heat treatment, particularly diffusible hydrogen present at grain boundaries, acts to receive stress during bending and facilitate the induction and propagation of cracks at grain boundaries. Therefore, by reducing the content of diffusible hydrogen in the steel, properties such as bendability and impact resistance can be improved.

[0046] In particular, the inventors have found that by forming an Sb-enriched layer having an appropriate concentration and thickness between the base steel plate and the plating layer, such an effect can be achieved. This is because the Sb-enriched layer serves as an effective protective film that relatively reduces the amount of diffusible hydrogen absorbed in the steel.

[0047] Specifically, in order to effectively reduce the hydrogen content in steel and improve hydrogen embrittlement resistance, while also ensuring excellent impact resistance, the hot-forming plated steel sheet preferably satisfies the following relational expressions 1-1 and 1-2. At this time, since the relational expressions 1-1 and 1-2 are empirically obtained values, it is not particularly necessary to define the units, and it is only necessary to satisfy the units of each variable defined below.

[0048] [Number] [In the above relational expressions 1-1 and 1-2, the above Sb coat represents the average Sb content in the plating layer, and the unit is wt%. The above Sb max represents the maximum value of the Sb content in the Sb enrichment layer, and the unit is wt%. The above Δt represents the linear distance between the points where Sb max is measured from the boundary between the plating layer and the Sb enrichment layer, and the unit is μm.]

[0049] That is, in order to exhibit the intended effects of the present invention, the Sb concentration and thickness of the Sb enrichment layer provided between the base steel sheet and the plating layer must be above a certain level and formed to satisfy both of the above relational expressions 1-1 and 1-2. At this time, the above Sb coat , Sb max and Δt can be measured from the data obtained when analyzing the change in the Sb content in the thickness direction of the base steel sheet at any point in the plating layer by utilizing the above-described glow discharge spectroscopy (GDS). In other words, the above Sb coat means the extension line of the Sb average content line for the section from a point 15 μm away from the plating layer 1 side to a point 20 μm away from the point 200 (the point of Sb max ) where the Sb content is the maximum value in the Sb enrichment layer 2 in the GDS profile graph measured by the method of FIG. 7 of the present application.

[0050] In the above relational expression 1-1, the value of Sb max / Sb coat is less than 1.2, or in the above relational expression 1-2, (Sb max-Sb coat ) / 2 × Δt is less than 0.008, the Sb concentration or the formed thickness in the Sb enrichment layer is insufficient, and it cannot be expected to serve as a protective film that relatively reduces the amount of diffusible hydrogen occluded in the steel. On the other hand, in FIG. 1, the area corresponding to the above relational expression 1-2 is shown by the comb-tooth portion, and the area of the comb-tooth portion described above is Sb coat at the measurement point and Sb max shows the Sb concentration gradient according to Δt indicating the distance between the measurement point and Sb

[0051] Alternatively, according to one embodiment of the present invention, more preferably, Sb defined from the above relational expression 1-1 max / Sb coat The lower limit of the value may be 1.20, and more preferably, Sb defined from the above relational expression 1-1 max / Sb coat The upper limit of the value may be 5.11.

[0052] Also, according to one embodiment of the present invention, more preferably, (Sb defined from the above relational expression 1-2 max / Sb coat ) / 2 × Δt The lower limit of the value may be 0.0080, and more preferably, (Sb defined from the above relational expression 1-2 max / Sb coat ) / 2 × Δt The upper limit of the value may be 0.1438.

[0053] On the other hand, although not particularly limited, according to one embodiment of the present invention, although not particularly limited, in the above electroplated steel sheet, the thickness of the Sb enrichment layer may be in the range of 1 μm or more and 20 μm or less. If the thickness of the Sb enrichment layer in the electroplated steel sheet is less than 1 μm, a sufficient Sb enrichment layer will not be generated even if heat treatment for hot forming is performed later, and it may be difficult to expect the effects of improving hydrogen embrittlement resistance and impact resistance. Further, when the thickness of the Sb enrichment layer exceeds 15 μm, Sb may excessively precipitate at the grain boundaries after hot forming, and may act as a crack site during stress generation, resulting in poor impact resistance.

[0054] From the perspective of further maximizing the above effects, more preferably, in the above electroplated steel sheet, the lower limit of the thickness of the Sb-enriched layer may be 3 μm, and the upper limit of the thickness of the Sb-enriched layer may be in the range of 15 μm. Alternatively, in the above electroplated steel sheet, the thickness of the Sb-enriched layer may be in the range of 3 to 15 μm.

[0055] On the other hand, as a result of intensive studies to further improve the physical properties of the electroplated steel sheet and the member, the inventors have found that the performance is improved by enriching an appropriate amount of Sb in the manganese segregation band present in the base steel sheet of the electroplated steel sheet. That is, as a result of examining various conditions, the inventors have confirmed that such an effect is further improved when the Sb enrichment amount in the segregation band satisfies a certain level or more, or in addition to this, when the thickness of the Mn segregation band satisfies a certain level or less.

[0056] Specifically, according to one embodiment of the present invention, although not particularly limited, in the above electroplated steel sheet, the base steel sheet includes a Mn segregation band, and among the above Mn segregation bands, the region where the average Sb content is 1.015 times or more the average Sb content in the base steel sheet may be 60% or more in terms of area fraction (alternatively, more preferably 70% or more). By satisfying this, it is possible to reduce the generation of inclusions such as MnS mainly generated in the Mn segregation band, play a role in suppressing the generation and propagation sites of cracks during stress generation, and ensure excellent impact resistance. In addition, when MnS is excessively generated, there may be a problem that brittle fracture surfaces are excessively developed. Therefore, by satisfying the above-described configuration during Sb enrichment, MnS can be reduced, thereby reducing brittle fracture surfaces and further improving bendability.

[0057] At this time, the upper limit of the average Sb content in the above Mn segregation band is not particularly limited, but as an example, it may be 5 times or less the average Sb content in the region of the base steel sheet other than the above Mn segregation band. Also, although not particularly limited, the upper limit of the area of the portion where the average Sb content in the above Mn segregation band is 1.015 times or more the average Sb content in the base steel sheet may be 90%.

[0058] As shown in Fig. 6a, the above Mn segregation band can be distinguished by utilizing the EPMA component mapping results for Mn. Specifically, after heat-treating the plated steel sheet at a temperature of 1200 °C or higher for several hours and then rapidly cooling to remove the Mn segregation band, the average value of the Mn intensity measured by EPMA is defined as Mn 0 Hereinafter, among the points measured by EPMA, when a square with an area of 0.4 μm 2 is drawn around a specific point in the up, down, left, and right directions, if the area where the Mn intensity is 1.015 times or more of Mn 0 among the points located within the above square is 50% or more, the specific point is defined as an Mn segregation point; if it is less than 50%, it is defined as not being an Mn segregation point. By collecting such Mn segregation points, the region formed by connecting the Mn segregation points located on the outermost contour with a straight line is defined as the Mn segregation band.

[0059] Also, the ratio (Sb2 / Sb1) of the average Sb content (Sb2) of the above Mn segregation band to the average Sb content (Sb1) of the above base steel sheet can be measured by utilizing the ratio of the intensity based on the component mapping results of Sb using EPMA.

[0060] Also, according to one embodiment of the present invention, although not particularly limited, in the above hot-forming plated steel sheet, the thickness of the above Mn segregation band may be 20 μm or less, and by satisfying this, the impact resistance and bendability can be further improved. Since the impact resistance and bendability can be improved as the thickness of the Mn segregation band becomes thinner, the lower limit of the thickness of the Mn segregation band does not need to be separately limited. However, as an example, the lower limit of the thickness of the Mn segregation band can be more than 0 μm or 1 μm or more. At this time, the thickness of the above Mn segregation band can be defined as the value obtained by measuring the average thickness in the thickness direction (the direction perpendicular to the rolling direction of the steel sheet) from the image of the Mn segregation band determined by the method described above. On the other hand, in the above hot-forming plated steel sheet, from the viewpoint of maximizing the above effects, more preferably, the upper limit of the thickness of the above Mn segregation band may be 18.9 μm, or the lower limit of the thickness of the above Mn segregation band may be 6.9 μm.

[0061] On the other hand, a hot-formed plated steel sheet having the above-described configuration can be used to produce a hot-formed member having excellent hydrogen embrittlement resistance and impact resistance by the hot press forming method described below.

[0062] The hot-formed member according to one aspect of the present invention includes a base steel sheet having the same alloy composition as the base steel sheet of the above-described plated steel sheet, an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel sheet, and an Sb-concentrated layer provided between the fired steel sheet and the plating layer. The hot-formed member satisfies the following relational expressions 2-1 and 2-2. At this time, the description of the base steel sheet, the plating layer, and the Sb-concentrated layer can be applied in the same manner as the above-described content. At this time, since the relational expressions 2-1 and 2-2 are empirically obtained values, it is not particularly necessary to define a unit, and it is only necessary to satisfy the units of each variable.

[0063]

Number

[0064] In the present invention, when the plated steel sheet is heated for hot forming, the degree of Sb concentration in the Sb-concentrated layer further deepens. Therefore, in the hot-formed member according to the present invention, by satisfying the above relational expressions 2-1 and 2-2, the hydrogen embrittlement resistance and impact resistance can be improved by effectively reducing the hydrogen content in the steel. At this time, in the hot-formed member, the method of distinguishing the boundary between the plating layer and the Sb-concentrated layer and the boundary between the base steel sheet and the Sb-concentrated layer can be applied in the same manner as the method of distinguishing in the above-described plated steel sheet.

[0065] Alternatively, according to one implementation example of the present invention, more preferably, Sb defined by the above relational expression 2-1 max / Sb coat The lower limit of the value may be 1.57, and more preferably, Sb defined by the above relational expression 2-1 max / Sb coat The upper limit of the value may be 7.39.

[0066] Also, according to one implementation example of the present invention, more preferably, (Sb defined by the above relational expression 2-2 max / Sb coat ) / 2×Δt The lower limit of the value may be 0.0148, and more preferably, (Sb defined by the above relational expression 2-2 max / Sb coat ) / 2×Δt The upper limit of the value may be 0.1940.

[0067] Alternatively, although not particularly limited, according to one implementation example of the present invention, more preferably, the following relational expression 2-3 can be satisfied, whereby hydrogen embrittlement resistance and impact resistance can be further improved.

[0068]

Equation

[0069] Although not particularly limited, according to one implementation example of the present invention, in the above hot-formed member, the microstructure of the above base steel sheet can include 5% or less of ferrite and the balance martensite. Further, it can further include other phases such as 1% or less of upper bainite, retained austenite, cementite, and pearlite.

[0070] According to one implementation example of the present invention, the base steel sheet can contain ferrite in an area fraction of 5% or less. This can manage the ferrite fraction to 5% or less in hot forming by ensuring the hardening ability through steel component adjustment and ensuring a sufficient cooling rate. On the other hand, in the hot formed member, if the ferrite fraction of the base steel sheet exceeds 5%, not only will the strength decrease, but relatively, local stress will concentrate on the relatively soft ferrite, promoting crack propagation and possibly significantly deteriorating the impact resistance.

[0071] Also, according to one implementation example of the present invention, although not particularly limited, in the hot formed member, the thickness of the Sb enrichment layer may be 2 to 30 μm. If the thickness of the Sb enrichment layer in the hot formed member is less than 2 μm, hydrogen penetrating into the steel during hot forming cannot be effectively suppressed, and the effects of improving hydrogen embrittlement resistance and impact resistance characteristics may not be fully exerted. Also, if the thickness of the Sb enrichment layer in the hot formed member exceeds 30 μm, Sb may not only form the enrichment layer but also precipitate excessively at the grain boundaries of the surface layer of the base iron, which may promote crack generation and propagation during bending and reduce the impact resistance characteristics.

[0072] From the perspective of further maximizing the above effects, more preferably, in the hot formed member, the lower limit of the thickness of the Sb enrichment layer may be 3 μm, or the upper limit of the thickness of the Sb enrichment layer may be 25 μm. Alternatively, in the hot formed member, the thickness of the Sb enrichment layer may be in the range of 3 to 25 μm.

[0073] Also, although not particularly limited, according to one implementation example of the present invention, the diffusible hydrogen content of the hot formed member may be 0.2 ppm or less, thereby ensuring excellent hydrogen embrittlement resistance. Such a diffusible hydrogen content of 0.2 ppm or less is because even if the test piece is stressed by bending under the same yield stress of the material for 120 hours, no cracks will occur in the parts.

[0074] Further, although not particularly limited, according to one embodiment of the present invention, in the hot forming member, the base steel sheet includes an Mn segregation band, and among the Mn segregation bands, a region where the average Sb content is 1.015 times or more the average Sb content in the base steel sheet may be 60% or more (alternatively, more preferably 70% or more) in terms of area fraction. By satisfying this, it plays a role in reducing the generation of inclusions such as MnS mainly generated in the Mn segregation band, suppressing the generation and propagation sites of cracks during stress generation, and ensuring excellent impact resistance. Also, when MnS is excessively generated, there may be a problem that brittle fracture surfaces are excessively developed. Therefore, by satisfying the above-described configuration during Sb enrichment, MnS can be reduced, thereby reducing brittle fracture surfaces and further improving bendability.

[0075] At this time, the upper limit of the average Sb content in the Mn segregation band is not particularly limited, but as an example, it may be 5 times or less the average Sb content in the region of the base steel sheet other than the Mn segregation band. Note that, although not particularly limited, the upper limit of the area of the portion where the average Sb content in the Mn segregation band is 1.015 times or more the average Sb content in the base steel sheet may be 95%.

[0076] Further, although not particularly limited, according to one embodiment of the present invention, in the hot forming member, the thickness of the Mn segregation band may be 15 μm (alternatively, 15.0 μm) or less, and by satisfying this, impact resistance and bendability can be further improved. Since impact resistance and bendability can be improved as the thickness of the Mn segregation band becomes thinner, the lower limit of the thickness of the Mn segregation band does not need to be separately limited. However, as an example, the lower limit of the thickness of the Mn segregation band may exceed 0 μm, or may be 1.5 μm. On the other hand, from the viewpoint of maximizing the above-described effects, more preferably, the upper limit of the thickness of the Mn segregation band may be 12.0 μm, or the lower limit of the thickness of the Mn segregation band may be 6.0 μm.

[0077] At this time, in the hot-formed member, the definition of the Mn segregation band, the ratio of the average Sb content in the Mn segregation band to the average Sb content in the base steel sheet, and the measurement of the thickness of the Mn segregation band can be applied in the same manner as the measurement method and measurement criteria in the plated steel sheet described above.

[0078] Next, a method for manufacturing a hot-formed plated steel sheet, which is still another aspect of the present invention, will be described.

[0079] The hot-formed plated steel sheet according to one aspect of the present invention, in terms of weight %, includes the steps of reheating a steel slab having the above-described alloy composition to 1050 to 1300°C, finish rolling the heated steel slab at 800 to 950°C to obtain a hot-rolled steel sheet, coiling the hot-rolled steel sheet at 500 to 700°C, pickling the coiled hot-rolled steel sheet such that the product of the acid concentration and the pickling time is 800 to 10,000 g / L*s, annealing the pickled hot-rolled steel sheet at 700 to 860°C under a dew point temperature condition of -75 to -20°C in an annealing furnace, and plating the annealed hot-rolled steel sheet by passing it through a plating bath made of aluminum or an aluminum alloy.

[0080] Slab reheating step First, the slab having the above-described alloy composition is reheated to 1050 to 1300°C. If the reheating temperature is less than 1050°C, the slab structure is not sufficiently homogenized, so when utilizing precipitation elements, it is difficult to re-dissolve them. On the other hand, if the reheating temperature exceeds 1300°C, an excessive oxide layer is formed, leading to an increase in manufacturing cost for removing the oxide layer and a high possibility of surface defects occurring after finish rolling.

[0081] Finish rolling step Finish rolling should be performed at 800 to 950°C. If the finish rolling temperature is less than 800°C, two-phase region rolling progresses, ferrite is introduced into the surface layer portion of the steel sheet, and it is difficult to control the sheet shape. On the other hand, if the finish rolling temperature exceeds 950°C, coarsening of the hot-rolled crystal grains may occur.

[0082] Coiling step After finishing finish rolling, the hot-rolled steel sheet is coiled at 500 to 700 °C and then cooled to produce a hot-rolled coil. If the coiling temperature is less than 500 °C, martensite is formed entirely or partially in the steel sheet, making it difficult to control the shape of the coil. In addition, there is a problem that the subsequent cold rolling property deteriorates due to an excessive increase in the strength of the hot-rolled steel sheet. On the other hand, if the coiling temperature exceeds 700 °C, coarse carbides are excessively formed, and the generation of cracks is promoted when stress occurs in the hot-formed member, resulting in a problem of reduced impact resistance.

[0083] Pickling treatment stage The coiled hot-rolled steel sheet is pickled so that the product of the acid concentration and the pickling time is 800 to 10,000 g / L*s. An Sb enrichment layer is formed on the steel sheet that has undergone the above-mentioned reheating, finish rolling, and coiling steps. However, when the product of the acid concentration and the pickling time is applied within the range of 800 to 10,000 g / L*s in the pickling treatment process, the Sb enrichment layer, which is the core of the present invention, is effectively protected, and the effect of reducing the amount of diffusible hydrogen in the steel can be exerted.

[0084] Specifically, when the product of the acid concentration and the pickling time is less than 800 g / L*s, the scale generated during finish rolling may not be sufficiently removed, which may induce product quality problems. On the contrary, when the product of the acid concentration and the pickling time exceeds 10,000 g / L*s, all or part of the Sb enrichment layer is lost during pickling, not only failing to exert the expected effect but also possibly leading to an increase in manufacturing cost. Therefore, the upper limit is set to 10,000 g / L*s. However, if there are one or more pickling tanks and the corresponding acid concentration and pickling time are different, the above value can be represented by adding the product of the acid concentration and the pickling time for each tank.

[0085] On the other hand, from the perspective of maximizing the above-mentioned effect more, more preferably, the lower limit of the product of the acid concentration and the pickling time may be 3,000 g / L*s, or the upper limit of the product of the acid concentration and the pickling time may be 5,000 g / L*s.

[0086] Although not particularly limited, according to one implementation example of the present invention, as the acid that can be used in the pickling treatment step, those that can be normally used in the technical field can be applied. Typically, hydrochloric acid (HCl), sulfuric acid (H 2 SO 4 ) etc. are available. In particular, in the present invention, using hydrochloric acid (HCl) has excellent pickling ability compared to using other acids, the process cost is economical, the possibility of surface foreign matters occurring after pickling is small, and it is easy to ensure the surface quality.

[0087] On the other hand, although not particularly limited, according to one implementation example of the present invention, the concentration of the above acid may be in the range of 40 to 500 g / L. When the acid concentration is less than 40 g / L, during the limited pickling time, the surface scale generated during hot rolling cannot be sufficiently removed, and defects may occur in the surface layer part of the steel plate. In contrast, when the acid concentration exceeds 500 g / L, due to the loss of the Sb enrichment layer, it may be difficult to exert the intended effect in the final hot forming member, and surface layer defects may be induced due to over-pickling. On the other hand, from the perspective of further maximizing the above effects, more preferably, the lower limit of the concentration of the above acid may be 180 g / L, or the upper limit of the concentration of the above acid may be 230 g / L.

[0088] Also, although not particularly limited, according to one implementation example of the present invention, the pickling time may be 5 to 60 seconds (s). When the pickling time is less than 5 seconds, the surface scale of the steel plate cannot be sufficiently removed, and defects in the surface layer part may be induced. When the pickling time exceeds 60 seconds, due to the loss of the Sb enrichment layer, it may reduce productivity and increase the process cost. From the perspective of maximizing the above effects, more preferably, the lower limit of the pickling time may be 18 seconds (s), or the upper limit of the pickling time may be 50 seconds (s).

[0089] Also, although not particularly limited, according to one implementation example of the present invention, the pickling temperature may be 40 to 120°C. If the pickling temperature is less than 40°C, the pickling power may not be sufficient, which may have an adverse effect on the quality of the product. On the contrary, if the pickling temperature exceeds 120°C, not only will the fixed cost increase to maintain a high temperature, but also the vaporization amount of the pickling solution due to the high temperature will increase, and there may be a problem of increased cost for replenishing the lost pickling solution. From the perspective of maximizing the above-mentioned effects, more preferably, the lower limit of the pickling temperature may be 50°C, or the upper limit of the pickling temperature may be 100°C. Alternatively, the pickling temperature may be in the range of 50 to 100°C.

[0090] Annealing stage The pickled hot-rolled steel sheet described above can be annealed at 700 to 900°C under the dew point temperature condition of -75 to +20°C in an annealing furnace. When the annealing temperature is less than 700°C, the recrystallization of the cold-rolled structure may not be completed sufficiently, so the sheet shape may become defective, and the strength may be too high after plating, which may induce die wear during the blanking process. On the contrary, when the annealing temperature exceeds 900°C, the formation of surface oxides is promoted during the annealing process, inducing defects on the Al-Si plated surface. Alternatively, from the perspective of suppressing die wear during the blanking process and surface defects of the plating, the lower limit of the annealing temperature is more preferably 750°C, and most preferably 800°C. Similarly, the upper limit of the annealing temperature is more preferably 860°C.

[0091] Also, the annealing atmosphere is preferably a non-oxidizing atmosphere, and a hydrogen-nitrogen mixed gas or the like can be used. At this time, the dew point temperature of the above-described atmosphere gas is carried out at -75°C or higher and +20°C or lower. If the dew point temperature is less than -75°C, there is a problem that additional equipment for controlling the dew point is required, resulting in an increase in manufacturing cost. On the other hand, if the dew point temperature exceeds +20°C, annealing oxides may be formed on the surface of the steel sheet during annealing, which may cause poor surface quality such as unplated areas. From the viewpoint of maximizing the above-described effects, the lower limit of the dew point temperature during annealing may be more preferably -70°C, and most preferably -40°C. Alternatively, the upper limit of the dew point temperature during annealing may be more preferably +15°C, and most preferably -20°C.

[0092] Plating step After the annealing process, Al-Si plating is immediately performed. Specifically, after annealing, the hot-rolled steel sheet can be passed through a plating bath made of aluminum or an aluminum alloy to produce a plated steel sheet. At this time, the plating conditions can be applied to the present invention without limitation as long as they are the plating conditions usually applied to steel sheets for hot press forming. As an example, the composition of the plating bath can contain Si: 6 to 12%, Fe: 1 to 4%, and the remaining Al and other inevitable impurities.

[0093] At this time, although not particularly limited, in the plating step, the plating amount is usually 20 to 140 g / m based on one side 2 and it is preferably so. If it is less than 20 g / m based on one side 2 it is difficult to ensure the corrosion resistance of the desired hot-formed member, and if it exceeds 140 g / m 2 not only does the manufacturing cost increase due to the excessive plating adhesion amount, but it is not easy to uniformly plate the plating amount in the entire width and length directions of the coil.

[0094] Cold rolling step The method for manufacturing a hot-forming plated steel sheet according to one aspect of the present invention may further include a step of cold-rolling a hot-rolled steel sheet to produce a cold-rolled steel sheet after the pickling treatment step described above.

[0095] Continuous casting step The method for manufacturing a hot-forming plated steel sheet according to the present invention may further include a step of performing soft reduction and continuous casting before the slab heating step, but is not particularly limited thereto. The present invention can reduce segregation and improve impact resistance by producing a slab by applying soft reduction during continuous casting. This is because when slab segregation occurs excessively, a segregation band is formed thickly and concentrated up to the final hot-forming member, and the impact resistance may be reduced due to the hardness difference generated between such a segregation band and the base steel sheet region other than the Mn segregation band and the formation of inclusions in the segregation band.

[0096] Therefore, in order to produce this slab, soft reduction must be carried out before the final solidification position of the slab during continuous casting, and it is preferable to control the total reduction rate during soft reduction by continuous casting to 0.5 to 5%. If the total reduction rate during continuous casting is less than 0.5%, there is almost no reduction, and central segregation is not sufficiently removed, so the impact resistance in the hot-forming member may be inferior. On the other hand, if the total reduction rate during continuous casting exceeds 5%, there may be a problem with the reduction roll equipment, which may accelerate equipment failure and aging. On the other hand, from the viewpoint of maximizing the above-described effects, more preferably, the lower limit of the total reduction rate during soft reduction by the above continuous casting may be 0.52%, or the upper limit of the total reduction rate during soft reduction by the above continuous casting may be 4.10%.

[0097] The hot-formed plated steel sheet produced by the above manufacturing method is hot press formed to produce a hot-formed member excellent in hydrogen embrittlement resistance and collision resistance. Specifically, a method for producing a final member excellent in hydrogen embrittlement resistance and collision resistance by hot forming and die quenching using the plated steel sheet produced by the above method will be described. A blank for hot forming is produced using the plated steel sheet produced by the above steel composition and manufacturing method. The above blank is heated within a temperature range of not less than the austenite single-phase region temperature, more specifically, not less than the Ac3 temperature and not more than 975 °C. At this time, if the heating temperature is less than the Ac3 temperature, it becomes difficult to ensure strength and collision resistance due to the presence of untransformed ferrite associated with the two-phase region interval. On the other hand, if the heating temperature exceeds 975 °C, excessive oxides are generated on the member surface, making it difficult to ensure spot weldability and increasing the manufacturing cost for maintaining a high temperature.

[0098] Thereafter, it is preferable to hold the heated blank for 1 to 1000 seconds within the above temperature range. If the holding time is less than 1 second, it becomes difficult to obtain a uniform temperature distribution throughout the blank temperature, which may induce material variations by position. On the other hand, if the holding time exceeds 1000 seconds, similar to the case of excessive heating temperature, not only is it difficult to ensure spot weldability due to the generation of excessive oxides on the member surface, but it also induces an increase in the manufacturing cost of the member.

[0099] The blank heated in this way is transferred to a press, and hot forming and die quenching are performed at a cooling rate of -20 °C / s or more to produce a final member. At this time, at a cooling rate of less than -20 °C / s, a ferrite phase may be introduced during cooling and formed at the grain boundaries, which may reduce the strength and collision resistance. The above-described blank transfer, hot forming, and cooling steps are not particularly limited, and the commonly used hot forming processes can be directly applied.

[0100] The hot-formed member produced in this way forms an Sb-enriched layer in which Sb is enriched between the base steel sheet and the plating layer, whereby a hot-formed member excellent in hydrogen embrittlement resistance and collision resistance can be produced due to a decrease in the amount of diffusible hydrogen in the steel.

Example

[0101] Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are only for exemplifying and embodying the present invention, and are not for limiting the scope of rights of the present invention. The scope of rights of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0102] (Experimental Example 1) The composition of the steel used for the present invention is shown in Table 1 below (the balance corresponds to Fe and other impurities), and slabs having respective compositions were produced by vacuum melting at a thickness of 40 mm. After holding the temperature of such slabs at 1200 °C for 1 hour, hot rolling was performed to a finish hot rolling temperature of 900 °C and coiling was performed at a coiling temperature of 600 °C. Then, after pickling and cold rolling, annealing was performed under the conditions shown in Table 2 below, and plating was performed by immersing in a plating bath composed of Al-9% Si-2% Fe and the balance being trace impurities. At this time, the pickling process was performed at a HCl concentration of 200 g / L and a pickling time of 20 seconds, and the product of the HCl concentration and the pickling time was applied all at once within the range of 4,000 g / L·s of the present invention.

[0103] In this example, hot forming was performed at various temperatures. After applying a heating time of 6 minutes and a transfer time to the mold of 10 seconds, die quenching was applied.

[0104] For the test piece manufactured as described above, tissue analysis for ferrite observation was performed. In order to confirm the Sb enrichment layer near the surface, by using GDS850A (model name, manufactured by LECO Corporation), DC and FR equipment, a Sb enrichment layer at the interface between the base iron and the plating layer was detected by glow discharge optical emission spectrometry (GDS) as shown in FIG. 3. Based on this result, as shown in FIG. 1, the parameter Sb max , Sb coat, Δt was measured, and using this, the enrichment layer parameter P1 value described in the following table was calculated. To confirm the amount of diffusible hydrogen thereby, measurement was carried out using a TDA (Thermal Desorption Analysis) device (Bruker G8; model name). At this time, the temperature was raised to 400 °C at a rate of 20 °C / min, and the diffusion hydrogen curve was measured while holding the time so that a diffusible hydrogen peak would sufficiently appear. The total amount of diffusible hydrogen in the steel was obtained by integrating such a curve.

[0105] Also, to confirm the impact resistance, after one week had passed since the hot forming heat treatment, the bendability was evaluated, and it was evaluated using the area (CIE: Crack initiation Energy) index up to the maximum load from the load-displacement curve obtained in a three-point bending test as shown in Fig. 2. When the measured CIE value was 35,000 N·m or more, it was expressed as "good", and when it was less than 35,000 N·m, it was expressed as "poor" in order to determine the presence or absence of excellent impact resistance. The results are shown in Tables 2 and 3.

[0106]

Table 1

[0107]

Table 2

[0108]

Table 3

[0109] As shown in Tables 1 to 3 above, in the case of Comparative Examples 1 to 5, the Sb content in the steel did not reach the range of the present invention, and the ratio of Sb max / Sb coat and the P1 value did not satisfy the range of the present invention. Therefore, the penetration of diffusible hydrogen in the steel could not be effectively suppressed, and the impact resistance was poor.

[0110] In contrast, in the cases of Examples 1 to 8, since the values of Relational Expressions 1-1, 1-2, 2-1, and 2-2 satisfy the scope of the present invention, the amount of diffusible hydrogen in the steel decreases, and the CIE value, which is an index of impact resistance, increases, confirming that the impact resistance is excellent.

[0111] On the other hand, except for some results where the ferrite structure is outside the scope of the present invention, as shown in FIG. 4, it can be seen that as the P1 value increases, the amount of diffusible hydrogen gradually decreases, and it can be understood that the Sb enrichment layer serves as an effective protective film for reducing the amount of diffusible hydrogen in the steel. Due to such an effect, as shown in FIG. 5, it can be seen that when the Sb enrichment layer appears, the CIE value increases significantly while excellent impact resistance can be ensured at the same time.

[0112] (Experimental Example 2) The pickling concentration and pickling time were changed as shown in Table 4 below, and test pieces were manufactured in the same manner as in Experimental Example 1 described above, except that the hot forming temperature was applied under the conditions of Table 4. At this time, the pickling temperature was uniformly applied at 80°C.

[0113] For each of the examples and comparative examples shown in Table 4 below, the values of Relational Expressions 1-1, 1-2, 2-1, and 2-2 were measured in the same manner as in Experimental Example 1 described above and shown in Table 5 below. Also, for the plated steel sheets (or hot formed members) for each of the examples and comparative examples, based on the data obtained by analyzing the change in the Sb content in the thickness direction of the base steel sheet at 10 points on the surface of the plating layer with GDS, the method described in the specification was applied in the same manner, and the thickness of the Sb enrichment layer was measured (that is, from the last contact point 11 in the x-axis (+) direction of the Sb average content line of the plating layer and the Sb content line by GDS in the schematic diagram of FIG. 7 to the last contact point 31 in the x-axis (-) direction of the Sb average content line of the base steel sheet and the Sb content line by GDS, the linear distance in the thickness direction was measured). The average thickness of the Sb enrichment layer for the above 10 points was measured and shown in Table 5 below.

[0114] To evaluate each example and comparative example, the impact resistance was evaluated by the same method as in Experimental Example 1 described above. Further, to evaluate the surface characteristics (i.e., the presence or absence of surface defects), the presence or absence of residual hot scale after pickling was evaluated. To confirm the presence or absence of residual hot scale after pickling, after pickling, a tape was used to attach to and peel off the surface of the test piece for each steel type, and the oxide attached here was pasted onto white paper, and then the whiteness was measured by color difference analysis. At this time, when the whiteness was 95% or more, it was expressed as "good", and when it was less than 95%, it was expressed as "bad".

[0115]

Table 4

[0116]

Table 5

[0117]

Table 6

[0118] As shown in Tables 4 to 6 above, Comparative Example 6, which does not satisfy the Sb content of the present invention and in which the product of the acid concentration and pickling time is less than 800 g / L*s, not only does not satisfy the relational expressions 1-1, 1-2, 2-1, and 2-2, but also the hot scale is not completely removed, and it was confirmed that there is a high possibility of causing surface defects during the subsequent process.

[0119] Further, Comparative Examples 7 and 8 in which the product of the acid concentration and pickling time exceeds 10,000 g / L*s do not satisfy any of the relational expressions 1-1, 1-2, 2-1, and 2-2. As a result, the amount of diffusible hydrogen in the steel increases, resulting in poor hydrogen embrittlement resistance, and the CIE value, which is an index of impact resistance, is also low, and the impact resistance characteristics are also poor.

[0120] On the one hand, in Examples 9 to 12 of the present application, since the product of the steel composition of the present invention and the acid concentration and pickling time is in the range of 800 to 10,000 g / L*s, the relational expressions 1-1, 1-2, 2-1 and 2-2 satisfy the scope of the present invention. Thereby, not only is the surface property excellent, but also the amount of diffusible hydrogen in the steel is reduced and the hydrogen embrittlement resistance is excellent. At the same time, the CIE value, which is an index of impact resistance, increases, and the impact resistance property is excellent.

[0121] (Experimental Example 3) Before reheating the slab, the slab was produced by lightly pressing it with the total under-pressure amount described in Table 7 below, and test pieces were produced in the same manner as in Experimental Example 1 except that the conditions in Table 7 below were applied. For such test pieces, each property was evaluated in the same manner as in Experimental Example 1 above, and further, the thickness of the Mn segregation band in the steel sheet and the Sb content in the Mn segregation band for the plated steel sheet and the hot-formed member were measured and shown in Table 7 below.

[0122] In particular, for the thickness of the Mn segregation band and the area of the portion in the Mn segregation band where the average Sb content is 1.015 times or more the average Sb content in the base steel sheet, the EPMA (Electron Probe X-ray Micro Analyzer) method was utilized by applying the method described above in the specification, and the component mapping results for each Mn and Sb are shown in FIGS. 6a and 6b.

[0123] At this time, the surface property, the amount of diffusible hydrogen, and the impact resistance were measured in the same manner as the above-described method.

[0124] In addition, in order to further evaluate the bendability, a bending test was performed on the members produced from each of the examples and comparative examples in Table 7 below. Specifically, when the ratio of the fracture surface was observed by measuring 100 μm below the surface with SEM, a ductile fracture and a cleavage fracture were generated. At this time, when the area ratio of the ductile fracture surface to the total measurement area satisfied 70% or more, it was represented by "○", and when it was less than that, it was represented by "×".

[0125]

Table 7

[0126]

Table 8

[0127]

Table 9

[0128]

Table 10

[0129] As shown in Tables 7 to 10 above, in Comparative Example 9, the added Sb content was less than the scope of the present invention, and Sb did not sufficiently concentrate in the Mn segregation band, resulting in poor impact resistance.

[0130] In the case of Comparative Example 10, the total reduction ratio during continuous casting exceeded 5%, obstacles occurred during continuous casting rolling, making continuous casting impossible, and thus experimental evaluation was impossible.

[0131] On the other hand, Examples 13 to 16 satisfy the ranges of Relational Expressions 1-1, 1-2, 2-1, and 2-2 of the present application, and thus have excellent surface characteristics, a small amount of diffusible hydrogen, and excellent hydrogen embrittlement resistance.

[0132] In particular, among the above-described examples, in the case of Examples 14 to 16 where the thickness of the Mn segregation band is 20 μm or less in the plated steel sheet (or 15 μm or less in the hot-formed member), or the ratio of the average Sb content in the Mn segregation band to the average Sb content of the base steel sheet is 60% or more in both the plated steel sheet and the hot-formed member, it was confirmed that the bendability was more improved as compared with Example 13 that does not satisfy one or more of the above-described conditions and Comparative Example 9 that does not satisfy all of the above-described conditions.

[0133] According to the present invention from the foregoing experimental examples, even with the addition of a small amount of Sb into steel, it is possible to manufacture a hot-formed product that not only has resistance to hydrogen delayed fracture due to a reduction in the amount of diffusible hydrogen but also has excellent impact resistance at the same time. Such components can be applied and utilized in various fields including the automotive manufacturing field as structural materials or reinforcing components.

Explanation of reference numerals

[0134] 1: plating layer 2: Sb enrichment layer 21: Range of increase in Sb content in the x-axis (+) direction in the Sb enrichment layer 22: Range of increase in Sb content in the x-axis (-) direction in the Sb enrichment layer 3: base steel sheet 10: Sb average content line of the plating layer 11: Last contact point in the x-axis (+) direction between the Sb average content line of the plating layer and the Sb content line by GDS 30: Sb average content line of the base steel sheet 31: Last contact point in the x-axis (-) direction between the Sb average content line of the base steel sheet and the Sb content line by GDS 100: Sb content line by GDS 200: Point where the Sb content is the maximum value in the Sb enrichment layer

Claims

1. In terms of mass%, C: 0.14 to 0.5%, Si: 0.001 to 1%, Mn: 0.3 to 4%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.001 to 0.1%, Cr: 0.001 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, and Sb: 0.005 to 0.1%, with the balance being Fe and other inevitable impurities, a base steel plate, and an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel plate, an Sb enrichment layer provided between the base steel plate and the plating layer, including, a hot-forming plated steel sheet satisfying the following relational expressions 1-1 and 1-2. 【Number 1】 [In the relational expressions 1-1 and 1-2, the Sb ｃｏａｔ represents the average Sb content in the plating layer, and the unit is mass%. The Sb ｍａｘ represents the maximum value of the Sb content in the Sb enrichment layer, and the unit is mass%. The Δt represents the straight-line distance between the points where the Sb ｍａｘ was measured from the boundary between the plating layer and the Sb enrichment layer, and the unit is μm. ]

2. The hot-forming plated steel sheet according to Claim 1, wherein the thickness of the Sb enrichment layer is 1 μm or more and 20 μm or less.

3. The base steel plate includes an Mn segregation band, among the Mn segregation bands, the area of the part where the average Sb content is 1.015 times or more the average Sb content in the base steel plate is 60% or more, the part is the part where the ratio (Sb2 / Sb1) of the average Sb content (Sb2) of the Mn segregation band to the average Sb content (Sb1) of the base steel plate measured by utilizing the intensity according to the component mapping result of Sb using EPMA is 1.015 times or more. The hot-forming plated steel sheet according to Claim 1 or Claim 2.

4. The hot-forming plated steel sheet according to Claim 3, wherein the thickness of the Mn segregation band is 20 μm or less.

5. In terms of mass%, C: 0.14 to 0.5%, Si: 0.001 to 1%, Mn: 0.3 to 4%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.001 to 0.1%, Cr: 0.001 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, and Sb: 0.005 to 0.1%, with the balance being Fe and other inevitable impurities. Reheating a steel slab to 1050 to 1300 °C, finish rolling the heated steel slab at 800 to 950 °C to obtain a hot-rolled steel sheet, coiling the hot-rolled steel sheet at 500 to 700 °C, pickling the coiled hot-rolled steel sheet so that the product of the acid concentration and the pickling time is 800 to 10,000 g / L*s, annealing the pickled steel sheet at 700 to 900 °C under the dew point temperature condition of -75 to +20 °C in an annealing furnace. After annealing, passing the steel sheet through a plating bath made of aluminum or an aluminum alloy for plating to obtain a plated steel sheet for hot forming, The plated steel sheet for hot forming is, a base steel sheet, an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel sheet, and an Sb-enriched layer provided between the base steel sheet and the plating layer, and includes A method for manufacturing a plated steel sheet for hot forming that satisfies the following relational expressions 1-1 and 1-2. 【Number 2】

6. The method for manufacturing a plated steel sheet for hot forming according to claim 5, wherein the concentration of the acid is 40 to 500 g / L.

7. The method for manufacturing a plated steel sheet for hot forming according to claim 5 or claim 6, wherein the pickling time is 5 to 60 seconds.

8. The method for manufacturing a plated steel sheet for hot forming according to any one of claims 5 to 7, wherein the pickling temperature is 40 to 120°C.

9. Before the reheating step, further including a step of performing light rolling with a reduction ratio of 0.5 to 5% and continuously casting, the method for manufacturing a plated steel sheet for hot forming according to any one of claims 5 to 8.

10. By mass%, C: 0.14 to 0.5%, Si: 0.001 to 1%, Mn: 0.3 to 4%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.001 to 0.1%, Cr: 0.001 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, and Sb: 0.005 to 0.1%, a base steel sheet composed of the balance Fe and other unavoidable impurities, an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel sheet, and an Sb-enriched layer provided between the base steel sheet and the plating layer, and includes A hot forming member that satisfies the following relational expressions 2-1 and 2-2. [Number 3] [In the relational expressions 2-1 and 2-2, the Sb ｃｏａｔ represents the average Sb content in the plating layer, and the unit is mass%. The Sb ｍａｘ represents the maximum value of the Sb content in the Sb enrichment layer, and the unit is mass%. Further, the Δt represents the linear distance between the points where the Sb ｍａｘ was measured from the boundary between the plating layer and the Sb enrichment layer, and the unit is μm. ]

11. The hot forming member according to claim 10, wherein the base steel sheet contains 5% or less ferrite by area fraction.

12. The hot forming member according to claim 10 or claim 11, wherein the thickness of the Sb-enriched layer is 2 to 30 μm.

13. The hot forming member according to any one of claims 10 to 12 that satisfies the following relational expression 2-3. 【Number 4】

14. The hot forming member according to any one of claims 10 to 13, wherein the diffusible hydrogen content is 0.2 ppm or less.

15. The base steel sheet includes an Mn segregation band, Among the Mn segregation bands, the area of the portion where the average Sb content is 1.015 times or more the average Sb content in the base steel sheet is 60% or more. The hot-formed member according to any one of claims 10 to 14, wherein the portion is a portion where the ratio (Sb2 / Sb1) of the average Sb content (Sb2) of the Mn segregation band to the average Sb content (Sb1) of the base steel sheet measured by utilizing the intensity obtained from the component mapping result of Sb using EPMA is 1.015 times or more. **Claim 16** The hot-formed member according to claim 15, wherein the thickness of the Mn segregation band is 15 μm or less. **Claim 17** A method for manufacturing a hot-formed member, comprising hot-press forming after heat-treating a hot-formed plated steel sheet manufactured according to any one of claims 5 to 9 in a temperature range of Ac3 to 950°C for 1 to 1000 seconds.

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