Hot-stamped plated steel sheet excellent in hydrogen embrittlement resistance and impact resistance, hot-stamped part, and method for manufacturing the same

Abstract

This invention provides a coated steel sheet for hot forming, hot-formed parts manufactured therefrom, and methods for manufacturing the same. The coated steel sheet for hot forming comprises: a base steel sheet, which, by weight percent, contains: C: 0.14-0.5%, Si: 0.001-1%, Mn: 0.3-4%, P: 0.001-0.015%, S: 0.0001-0.02%, Al: 0.001-0.1%, Cr: 0.001-1%. N: 0.001-0.02%, Ti: less than 0.1%, B: less than 0.01%, Sb: 0.005-0.1%, balance Fe and other unavoidable impurities; an aluminum or aluminum alloy coating disposed on at least one side of the base steel plate; and an Sb enrichment layer disposed between the base steel plate and the coating, wherein the hot-formed coated steel plate satisfies the following relations 1-1 and 1-2. In relations 1-1 and 1-2, the Sb... 镀层 This indicates the average Sb content in the coating, expressed in weight %. The Sb 最大 The value represents the maximum Sb content in the Sb-enriched layer, expressed in weight %. The value Δt represents the distance from the boundary between the coating and the Sb-enriched layer to the point where the Sb content is measured. 最大 The straight-line distance between the positions is expressed in μm.

Description

Technical Field

[0001] This invention relates to a coated steel sheet for hot forming with excellent resistance to hydrogen embrittlement and impact resistance, hot-formed parts, and methods for manufacturing the same. Background Technology

[0002] In recent years, thermoformed parts have been widely used in automotive structural components to improve fuel efficiency and protect passengers through vehicle lightweighting. In particular, they can be used in bumpers, doors, pillar reinforcements, etc., where ultra-high strength or energy absorption is required, and a representative example of this thermoforming technology is U.S. Patent Publication No. 6296805 (hereinafter, Patent Document 1).

[0003] In Patent Document 1, by heating an Al-Si based coated steel sheet to above 850°C and then hot-forming and rapidly cooling it using a stamping press, the microstructure of the component is formed into martensite, thereby ensuring ultra-high strength with high tensile strength. When using this ultra-high strength steel for hot forming, complex shapes can be easily formed due to the high temperature, and the increased strength from rapid cooling in the mold can lead to a lightweight effect. However, martensitic microstructures are known to have low resistance to hydrogen embrittlement, especially since hot-formed parts exhibit residual stresses generated by rapid cooling after heating. Therefore, an increase in the diffusible hydrogen content in the steel can lead to delayed fracture due to hydrogen embrittlement, thus limiting the application of the component.

[0004] Furthermore, variations in process parameters can cause changes in the overall or local mechanical properties of the sheet. Therefore, in order to manufacture coated steel sheets and hot-formed parts with good mechanical properties and uniformity, a steel composition with low sensitivity to changes in manufacturing parameters is required, and delayed fracture caused by hydrogen embrittlement must be prevented. However, to date, no technology has been developed that can meet all these requirements.

[0005] (Patent Document 1) U.S. Patent Publication No. 6296805 Summary of the Invention

[0006] Technical problems to be solved

[0007] The present invention is intended to solve the problems described above, and the object of the present invention is to provide a hot-formed coated steel sheet, hot-formed parts, and a method for manufacturing the same, which have excellent resistance to hydrogen embrittlement and impact resistance.

[0008] The technical problem addressed by this invention is not limited to the above-described contents. Those skilled in the art can readily understand the additional technical problems of this invention based on the entirety of this specification.

[0009] Technical solution

[0010] One aspect of the present invention provides a coated steel sheet for hot forming, the coated steel sheet for hot forming comprising:

[0011] The base steel plate, by weight percent, comprises: C: 0.14-0.5%, Si: 0.001-1%, Mn: 0.3-4%, P: 0.001-0.015%, S: 0.0001-0.02%, Al: 0.001-0.1%, Cr: 0.001-1%, N: 0.001-0.02%, Ti: less than 0.1%, B: less than 0.01%, Sb: 0.005-0.1%, with the balance being Fe and other unavoidable impurities;

[0012] An aluminum or aluminum alloy coating, said aluminum or aluminum alloy coating being disposed on at least one side of the base steel plate; and

[0013] An Sb enrichment layer is disposed between the base steel plate and the coating, and the hot-formed coated steel plate satisfies the following relations 1-1 and 1-2.

[0014] [Relation 1-1]

[0015]

[0016] [Relationship 1-2]

[0017]

[0018] In relation 1-1 and relation 1-2, the Sb 镀层(coat) This indicates the average Sb content in the coating, expressed in weight %. The Sb 最大(max) The value represents the maximum Sb content in the Sb-enriched layer, expressed in weight %. The value Δt represents the distance from the boundary between the coating and the Sb-enriched layer to the point where the Sb content is measured. 最大 The straight-line distance between the locations is expressed in μm.

[0019] Another aspect of the present invention provides a method for manufacturing a coated steel sheet for thermoforming, comprising the following steps:

[0020] The steel billet is reheated to 1050-1300℃, and the steel billet contains, by weight%,: C: 0.14-0.5%, Si: 0.001-1%, Mn: 0.3-4%, P: 0.001-0.015%, S: 0.0001-0.02%, Al: 0.001-0.1%, Cr: 0.001-1%, N: 0.001-0.02%, Ti: less than 0.1%, B: less than 0.01%, Sb: 0.005-0.1%, with the balance being Fe and other unavoidable impurities;

[0021] Hot-rolled steel plates are obtained by precision rolling heated steel billets at 800-950℃.

[0022] The hot-rolled steel sheet is coiled at 500-700℃;

[0023] The coiled hot-rolled steel sheet is pickled to achieve a product of acid concentration and pickling time of 800-10000 g / L. s;

[0024] In an annealing furnace, pickled steel plates are annealed at 700-900℃ under dew point temperatures ranging from -75℃ to +20℃; and

[0025] After annealing, the steel sheet is plated by a plating bath composed of aluminum or an aluminum alloy.

[0026] Another aspect of the present invention provides a thermoformed component, the thermoformed component comprising:

[0027] The base steel plate, by weight percent, comprises: C: 0.14-0.5%, Si: 0.001-1%, Mn: 0.3-4%, P: 0.001-0.015%, S: 0.0001-0.02%, Al: 0.001-0.1%, Cr: 0.001-1%, N: 0.001-0.02%, Ti: less than 0.1%, B: less than 0.01%, Sb: 0.005-0.1%, with the balance being Fe and other unavoidable impurities;

[0028] An aluminum or aluminum alloy coating, said aluminum or aluminum alloy coating being disposed on at least one side of the base steel plate; and

[0029] An Sb enrichment layer is disposed between the base steel plate and the coating, and the hot-formed component satisfies the following relations 2-1 and 2-2.

[0030] [Relation 2-1]

[0031]

[0032] [Relation 2-2]

[0033]

[0034] In relation 2-1 and relation 2-2, the Sb 镀层 This indicates the average Sb content in the coating, expressed in weight %. The Sb 最大The value represents the maximum Sb content in the Sb-enriched layer, expressed in weight %. The value Δt represents the distance from the boundary between the coating and the Sb-enriched layer to the point where the Sb content is measured. 最大 The straight-line distance between the locations is expressed in μm.

[0035] Another aspect of the present invention provides a method for manufacturing a thermoformed part, comprising:

[0036] Within a temperature range of Ac3 to 950°C, the hot-formed coated steel sheet manufactured by the above-described method for manufacturing hot-formed coated steel sheet is heat-treated for 1-1000 seconds and then hot-pressed.

[0037] Beneficial effects

[0038] According to one aspect of the present invention, by manufacturing a coated steel sheet in which an Sb enrichment layer is retained between a base steel sheet and a coating, the amount of diffusible hydrogen in the steel can be reduced, thereby providing a coated steel sheet for hot forming with excellent resistance to hydrogen embrittlement and impact resistance, hot-formed parts, and a method for manufacturing the same.

[0039] The numerous advantages and effects of the present invention are not limited to the above description, and may be more readily understood in the process of describing specific embodiments of the present invention. Attached Figure Description

[0040] Figure 1 This is a schematic diagram showing the gradient concentration of the Sb-enriched layer at the interface between the base steel plate and the coating.

[0041] Figure 2 This is a schematic diagram of the CIE graph used to display load-displacement curves and impact resistance during a three-point bending test.

[0042] Figure 3 This is a concentration gradient diagram showing the Sb content in the thickness direction from the coating to the base steel plate side, as measured by glow discharge optical emission spectrometry (GDS) according to embodiments and comparative examples of the present invention.

[0043] Figure 4 It is an exemplary thermoformed part according to the invention (Sb) 最大 -Sb 镀层 The graph shows the change in the diffusive hydrogen content of the parameter ) / 2×△t.

[0044] Figure 5 It is the thermoformed part according to the present invention (Sb) 最大 -Sb 镀层 CIE distribution diagram of parameter ) / 2×△t.

[0045] Figure 6 shows the Mn segregation bands of the thermoformed component used to confirm Embodiment 14 of the present invention. Figure 6a ) and Sb enrichment in Mn segregation bands ( Figure 6b EPMA component mapping results.

[0046] Figure 7 This schematically illustrates an Sb concentration gradient plot of an exemplary GDS according to the present invention.

[0047] Best practice

[0048] The preferred embodiments of the present invention will now be described. However, the embodiments of the present invention can be modified in various ways, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to provide a more complete explanation of the present invention to those skilled in the art.

[0049] Diffusible hydrogen in grain boundaries can promote the formation of grain boundary cracks when stress is generated, so there is a need for a method that can reduce the amount of diffusible hydrogen in steel after hot stamping.

[0050] Therefore, the inventors analyzed the diffusible hydrogen content in hot-formed steel, an indicator that effectively represents resistance to hydrogen embrittlement, and calculated the area reaching the maximum load (crack initiation energy, CIE) during the three-point bending test (VDA238-100), one of the indicators of impact resistance. They analyzed the effects of various components, including added Sb, manufacturing conditions, and microstructure. This revealed that the formation of an Sb-enriched layer reduces the diffusible hydrogen content, and led to the design of coated steel sheets, hot-formed parts, and their manufacturing methods for hot-formed steel exhibiting excellent resistance to hydrogen embrittlement and impact resistance.

[0051] Hereinafter, a detailed description will be given of a galvanized steel sheet and a thermoformed component for thermoforming according to one aspect of the present invention.

[0052] According to one aspect of the present invention, a coated steel sheet comprises: a base steel sheet, which, by weight percent, comprises: C: 0.14-0.5%, Si: 0.001-1%, Mn: 0.3-4%, P: 0.001-0.015%, S: 0.0001-0.02%, Al: 0.001-0.1%, Cr: 0.001-1%, N: 0.001-0.02%, Ti: less than 0.1%, B: less than 0.01%, Sb: 0.005-0.1%, with the balance being Fe and other unavoidable impurities; an aluminum or aluminum alloy coating disposed on at least one side of the base steel sheet; and an Sb enrichment layer disposed between the base steel sheet and the coating.

[0053] First, the alloy composition of the basic steel plate of this invention will be described in detail. It should be noted that, unless otherwise specifically defined, the content of each element in this invention is expressed as a percentage by weight.

[0054] Carbon (C): 0.14-0.5%

[0055] The carbon (C) is an element that improves the strength and hardenability of heat-treated parts, and it needs to be added appropriately to adjust the strength as an essential element. When the C content is less than 0.14%, hardenability is low, sufficient martensite cannot be ensured when the cooling rate is reduced, and impact resistance deteriorates due to the formation of ferrite; therefore, it should be added at least 0.14%. On the other hand, when the C content exceeds 0.5%, the strength increases excessively, which may lead to brittleness and poor weldability; therefore, the upper limit of the C content is preferably 0.5% or less. Alternatively, the lower limit of the C content is more preferably 0.147%, and the upper limit of the C content is more preferably 0.335%.

[0056] Silicon (Si): 0.001-1%

[0057] Si is added as a deoxidizer in steelmaking, and it is also a solid solution strengthening element and a carbide formation inhibitor. Therefore, Si is effective in homogenizing the internal structure, helps improve the strength of hot-formed parts, and is effective in homogenizing the material. However, when the Si content is less than 0.001%, the above effects cannot be expected, and the manufacturing costs and process expenses for controlling the Si content increase, making it unsuitable. On the other hand, when the Si content exceeds 1%, the plating properties are significantly reduced due to the excessive Si oxides formed on the steel plate surface during annealing; therefore, adding less than 1% Si is preferable. Alternatively, the lower limit of the Si content is more preferably 0.11%, and the upper limit of the Si content is more preferably 0.81%.

[0058] Manganese (Mn): 0.3-4%

[0059] The Mn content not only ensures the required strength through solid solution strengthening, but also needs to be added to suppress ferrite formation during hot forming by improving hardenability. When the Mn content is less than 0.3%, it is difficult to obtain sufficient hardenability, and excessive amounts of other expensive alloying elements are required to achieve the insufficient hardenability, leading to a significant increase in manufacturing costs. When the Mn content exceeds 4%, the banded structure along the rolling direction in the fine microstructure deepens, which may cause inhomogeneity in the internal structure, thereby potentially reducing impact resistance. Therefore, Mn content below 4% is added. Alternatively, the lower limit of the Mn content is more preferably 0.5%, and the upper limit of the Mn content is more preferably 3.7%.

[0060] Phosphorus (P): 0.001-0.015%

[0061] Phosphorus (P) exists as an impurity in steel. Maintaining a minimum P content of less than 0.001% incurs significant manufacturing costs and is therefore undesirable. However, when the P content exceeds 0.015%, the physical properties of the material deteriorate due to reduced weldability of hot-formed parts and high-temperature grain boundary segregation. Therefore, an upper limit for the P content is set at 0.015%. Alternatively, a lower limit for the P content is more preferably 0.003%, and an upper limit for the P content is more preferably 0.013%.

[0062] Sulfur (S): 0.0001-0.02%

[0063] S is an impurity and an element that hinders the ductility, impact properties, and weldability of the component; therefore, the maximum S content is limited to 0.02%. Furthermore, when the minimum S content is less than 0.0001%, manufacturing costs increase significantly, which is therefore undesirable. Alternatively, the lower limit of the S content can be 0.001%, and the upper limit of the S content is more preferably 0.007%.

[0064] Aluminum (Al): 0.001-0.1%

[0065] Al, along with Si, is an element that, in steelmaking, acts as a deoxidizer to improve the cleanliness of the steel. When the Al content is less than 0.01%, it is difficult to achieve the above effect. When the Al content exceeds 0.1%, excessive AlN precipitates formed during the continuous casting process reduce high-temperature ductility, leading to cracks in the slab and potentially causing manufacturing problems. Therefore, the upper limit of the Al content is set at 0.1%. Alternatively, the lower limit of the Al content can more preferably be 0.011%, and the upper limit of the Al content can more preferably be 0.071%.

[0066] Cr: 0.001-1%

[0067] Similar to Mn, Cr is added as an element to suppress the formation of ferrite after hot forming by ensuring the hardenability of the steel. When the Cr content is less than 0.001%, it is difficult to ensure the above effect. On the other hand, when the Cr content exceeds 1%, the effect of improving hardenability is negligible compared to the amount added, and excessive coarse iron carbides are formed, which may cause cracks under stress, resulting in material deterioration. Therefore, the upper limit of the Cr content is set to 1%. Alternatively, the lower limit of the Cr content is more preferably 0.011%, and the upper limit of the Cr content is more preferably 0.50%.

[0068] Nitrogen (N): 0.001-0.02%

[0069] The nitrogen (N) is included in the steel as an impurity. To keep the N content below 0.001%, which results in excessively high manufacturing costs, and because when the N content exceeds 0.02%, it easily leads to slab cracking due to the formation of AlN with added Al, the upper limit of the N content is set at 0.02%. Alternatively, the lower limit of the N content is more preferably 0.0026%, and the upper limit is more preferably 0.0077%.

[0070] Ti: 0.1% or less (including 0)

[0071] The Ti is a selectively added element in this invention, and it combines with nitrogen, which remains as an impurity in the steel, to form TiN, thereby protecting B, which is used to ensure hardenability, from becoming a compound. Furthermore, precipitation strengthening and grain refinement effects are expected through the formation of TiC precipitates. However, when the Ti content exceeds 0.1%, a large amount of coarse TiN is formed, leading to a deterioration in the steel's material properties; therefore, the upper limit for the Ti content is set at 0.1%. Additionally, since Ti is a selective element, it also includes cases where it is not added; therefore, the lower limit for the Ti content can be 0%.

[0072] B: Less than 0.01% (including 0)

[0073] The element B is selectively added in this invention and is one that can effectively improve hardenability. When B is added, the brittleness of hot-formed parts caused by grain boundary segregation of P and / or S as impurities can be suppressed by segregation at the original austenite grain boundaries. However, when the B content exceeds 0.01%, due to Fe... 23 The formation of the CB6 composite compound may cause brittleness during hot rolling; therefore, the upper limit of the B content is set at 0.01%. Furthermore, since B is a selective element, it includes cases where it is not added; therefore, the lower limit of the B content can be 0%.

[0074] Sb: 0.005-0.1%

[0075] Sb is a core element in manufacturing the hot-formed parts of this invention, and Sb plays a role in reducing the hydrogen content absorbed during heat treatment and reducing the sensitivity to hydrogen-induced delayed fracture by forming an Sb-enriched layer at the interface between the base steel sheet and the coating. When the Sb content is less than 0.005%, a sufficient enriched layer is not formed at the interface between the coating and the base iron, so the above-mentioned effect is difficult to expect. On the other hand, when the Sb content exceeds 0.1%, excessive Sb precipitates in the grain boundaries, which can cause grain boundary fracture when stress is generated, resulting in a deterioration of the material. Therefore, the upper limit of the Sb content is preferably set to 0.1%. Alternatively, the lower limit of the Sb content is more preferably 0.006%, and the upper limit of the Sb content is more preferably 0.095%.

[0076] The balance, in addition to the components mentioned above, is iron (Fe). Further additions are not particularly restricted, provided they are components already included in the hot-pressed steel sheet. Furthermore, undesirable impurities may inevitably be introduced from the raw materials or the surrounding environment during conventional manufacturing processes, and therefore cannot be excluded. These impurities are well-known to those skilled in the art in conventional manufacturing processes, and therefore their contents are not specifically described in this specification.

[0077] Furthermore, the coating comprises an aluminum or aluminum alloy coating disposed on at least one side of the base steel plate. The coating imparts corrosion resistance in the final hot-formed part.

[0078] In this invention, the type of coating is not particularly limited. Any coating used for existing hot-formed coated steel sheets can be used without restriction. As an example, the coating can be an aluminum or aluminum alloy coating, and the coating preferably contains 6-12% Si, 1-4% Fe, the balance Al, and other unavoidable impurities.

[0079] According to one aspect of the invention, the coated steel sheet may include an Sb enrichment layer disposed between the base steel sheet and the coating. In this case, the Sb enrichment layer is a region disposed between the base steel sheet and the coating and enriched with Sb, defined by its Sb content.

[0080] Although not particularly limited, according to one specific embodiment of the present invention, such Sb-enriched layers and coatings can be distinguished by analyzing the change in Sb content along the thickness direction from a point in the coating to the base steel plate using glow discharge spectroscopy (GDS).

[0081] Specifically, according to one particular embodiment of the present invention, although not particularly limited, but as... Figure 7 As shown, the judgment is based on the following chart, where the x-axis represents the straight-line distance from any position inside the coating 1 to the thickness direction of the base steel plate 3, and the y-axis represents the Sb content measured using the GDS.

[0082] For example, to schematically illustrate the above GDS measurement results Figure 7 Based on this, in the Sb content increase interval 21 along the x-axis (+) direction between the coating 1 and the base steel plate 3, the Sb enrichment layer 2 is considered to start from the last contact point 11 along the x-axis (+) direction between the average Sb content line 10 of the coating and the Sb content line 100 measured by the GDS (along the thickness direction on the side of the base steel plate 3).

[0083] At this point, the average Sb content line 10 of the coating 1 can refer to the point 200 (Sb) in the Sb enrichment layer 2 where the Sb content is at its maximum. 最大The extension line of the average Sb content line from point 1 to point 20 μm away from the point 1 on the side of the coating.

[0084] Similarly, the Sb enrichment layer 2 and the base steel plate 3 are treated in the same way as described above. In the Sb content increase interval 22 along the x-axis (-) direction between the base steel plate 3 and the coating 1 as measured by the GDS, the Sb enrichment layer 2 is considered to start from the last contact point 31 along the x-axis (-) direction between the average Sb content line (30) of the base steel plate and the Sb content line 100 measured by the GDS (along the thickness direction on the side of the coating 1).

[0085] At this point, the average Sb content line 30 of the base steel plate 3 can refer to the point 200 (Sb) in the Sb enrichment layer 2 where the Sb content is at its maximum. 最大 The extension of the Sb average content line from point 3 to point 15μm away from the base steel plate and from point 20μm away.

[0086] Through in-depth and meticulous research, the inventors discovered that defects caused by hydrogen-induced delayed fracture can be suppressed during the heat treatment of hot forming by reducing the amount of diffusible hydrogen retained. Specifically, during the step of heating a blank with a coating composed of aluminum or aluminum alloy during hot forming, water vapor present in the heat treatment furnace is adsorbed onto the surface of the blank. Subsequently, hydrogen generated during the dissociation of water is retained in the steel as it exists in the austenitic phase, which has high hydrogen solubility at high temperatures. However, if rapid cooling occurs due to hot forming and the hydrogen transforms into the martensitic phase, the solubility of hydrogen decreases sharply, and the alloy coating formed during the alloying of the coating acts as an obstacle to hydrogen release. Therefore, a large amount of diffusible hydrogen remains in the steel, thus increasing the likelihood of crack formation due to hydrogen-induced delayed fracture. Therefore, reducing the amount of diffusible hydrogen retained during heat treatment is an important factor in suppressing defects.

[0087] Furthermore, the inventors' numerous studies confirmed that as the hydrogen content in steel decreases, impact resistance tends to increase. This causes diffusible hydrogen retained in the steel during heat treatment, especially diffusible hydrogen present at grain boundaries, to be stressed during bending processing, thus easily initiating and propagating grain boundary cracks. Therefore, properties such as bending resistance and impact resistance can be improved by reducing the diffusible hydrogen content in steel.

[0088] In particular, the inventors have discovered that this effect can be achieved by forming an Sb-enriched layer with appropriate concentration and thickness between the base steel sheet and the coating. This is because the Sb-enriched layer acts as an effective protective film that relatively reduces the amount of diffusible hydrogen adsorbed in the steel.

[0089] Specifically, in order to improve hydrogen embrittlement resistance while ensuring excellent impact resistance by effectively reducing the hydrogen content in the steel, the hot-formed coated steel sheet preferably satisfies the following relationships 1-1 and 1-2. Here, relationships 1-1 and 1-2 are empirically derived values, so the units are not particularly limited, as long as the units of the variables satisfy the following definitions.

[0090] [Relation 1-1]

[0091]

[0092] [Relationship 1-2]

[0093]

[0094] In relation 1-1 and relation 1-2, the Sb 镀层 This indicates the average Sb content in the coating, expressed in weight %. The Sb 最大 The value represents the maximum Sb content in the Sb-enriched layer, expressed in weight %. The value Δt represents the distance from the boundary between the coating and the Sb-enriched layer to the point where the Sb content is measured. 最大 The straight-line distance between the locations is expressed in μm.

[0095] That is, in order to achieve the desired effect of this invention, the Sb concentration and thickness of the Sb enrichment layer disposed between the base steel plate and the coating are formed to a certain level or above, satisfying the above-mentioned relationships 1-1 and 1-2. At this time, the Sb... 镀层 Sb 最大 The values ​​of Sb and Δt can be measured by analyzing data obtained from the glow discharge spectroscopy (GDS) method described above, which analyzes the change in Sb content along the thickness direction of the base steel plate from a certain point within the coating. In other words, the Sb... 镀层 This can refer to the invention. Figure 7 In the GDS curve measured by the method, the point 200 (Sb) in the Sb-enriched layer 2 is the point where the Sb content is at its maximum. 最大 The extension line of the average Sb content line from point 1 to point 20 μm away from the point 1 on the side of the coating.

[0096] In the relation 1-1, when Sb 最大 / Sb 镀层 The value is less than 1.2, or in the relation 1-2, when (Sb 最大 -Sb 镀层 When the value of ) / 2×Δt is less than 0.008, the Sb concentration or the thickness formed in the Sb-enriched layer is insufficient, making it difficult to expect a protective film that relatively reduces the amount of diffusible hydrogen adsorbed in the steel. Furthermore, Figure 1The area corresponding to the aforementioned relation 1-2 is represented by the slashed portion, and the area represented by the slashed portion is based on Sb. 镀层 Sb 最大 The Sb concentration gradient is represented by Δt, where Δt represents the measured Sb concentration gradient. 镀层 Points and measurements Sb 最大 The distance between points.

[0097] Alternatively, according to a specific embodiment of the present invention, Sb is defined by the relation 1-1. 最大 / Sb 镀层 The lower limit of the value can more preferably be 1.20, as defined by the relation 1-1. 最大 / Sb 镀层 The upper limit of the value can preferably be 5.11.

[0098] Furthermore, according to a specific embodiment of the present invention, (Sb) defined by the relation 1-2 最大 -Sb 镀层 The lower limit of the value of ) / 2×△t can more preferably be 0.0080, as defined by the relationship 1-2. 最大 -Sb 镀层 The upper limit of the value of ) / 2×△t can preferably be 0.1438.

[0099] Furthermore, although not specifically limited, according to one specific embodiment of the present invention, the thickness of the Sb-enriched layer in the clad steel sheet can range from 1 μm to 20 μm. In the clad steel sheet, when the thickness of the Sb-enriched layer is less than 1 μm, even with subsequent heat treatment for hot forming, a sufficiently rich Sb layer will not form, thus making it difficult to expect the desired improvement in hydrogen embrittlement resistance and impact resistance. Moreover, when the thickness of the Sb-enriched layer exceeds 15 μm, excessive precipitation of Sb at grain boundaries after hot forming can create crack sites due to stress, thus potentially worsening impact resistance.

[0100] To maximize the aforementioned effects, more preferably, in the clad steel sheet, the lower limit of the thickness of the Sb-enriched layer can be 3 μm, and the upper limit of the thickness of the Sb-enriched layer can be in the range of 15 μm. Alternatively, in the clad steel sheet, the thickness of the Sb-enriched layer can be in the range of 3-15 μm.

[0101] Furthermore, through in-depth and meticulous research to further improve the physical properties of the coated steel sheet and components, the inventors discovered that appropriate enrichment of Sb within the manganese segregation band present in the base steel sheet of the coated steel sheet improves performance. Specifically, the inventors' research under various conditions confirmed that this effect is further enhanced when the Sb enrichment within the segregation band reaches a certain level or above, or when the thickness of the Mn segregation band is below a certain level.

[0102] Specifically, according to one embodiment of the present invention, although not particularly limited, in the clad steel sheet, the base steel sheet includes Mn segregation bands, and in the Mn segregation bands, the area where the average Sb content, in terms of area fraction, is more than 1.015 times the average Sb content in the base steel sheet can be 60% or more (or, more preferably, more than 70%). When this condition is met, the formation of inclusions such as MnS, which are mainly generated in the Mn segregation bands, can be reduced, which can suppress the formation of cracks and the formation of crack propagation sites when stress is generated, thereby ensuring excellent impact resistance. In addition, when too much MnS is formed, there may be a problem of overdeveloped brittle fracture surfaces. Therefore, when Sb is enriched, MnS can be reduced by satisfying the above composition, thereby reducing brittle fracture surfaces and thus further improving bending performance.

[0103] At this point, while there is no particular upper limit on the average Sb content in the Mn segregation zone, as an example, it can be less than 5 times the average Sb content in the region of the base steel plate outside the Mn segregation zone. Furthermore, although not particularly limited, the upper limit for the area of ​​the portion in the Mn segregation zone where the average Sb content is more than 1.015 times the average Sb content in the base steel plate can be 90%.

[0104] Figure 6a As shown, the Mn segregation bands can be distinguished using the EPMA composition mapping results of Mn. Specifically, after heat-treating the coated steel sheet at a temperature above 1200°C for several hours and then rapidly cooling it to remove the Mn segregation bands, the average value of the Mn intensity measured according to EPMA is set as Mn0. Then, among the points measured in EPMA, a 0.4 μm² area is plotted around a specific point, extending horizontally and vertically. 2 When constructing a square, if more than 50% of the points within the square have an Mn intensity greater than 1.015 times that of Mn0, then that specific point is defined as an Mn segregation point; if the area within the square is less than 50%, then it is defined as not an Mn segregation point. These Mn segregation points are collected, and the region formed by connecting the outermost Mn segregation points with straight lines is designated as the Mn segregation zone.

[0105] Furthermore, the ratio (Sb2 / Sb1) of the average Sb content (Sb2) to the average Sb content (Sb1) of the Mn segregation zone of the base steel plate can be measured by applying the ratio of the intensity based on the compositional mapping results of Sb using EPMA.

[0106] Furthermore, according to a specific embodiment of the present invention, although not particularly limited, the thickness of the Mn segregation band in the hot-formed coated steel sheet can be 20 μm or less. When this condition is met, impact resistance and bending properties can be further improved. As the thickness of the Mn segregation band decreases, impact resistance and bending properties can be improved; therefore, a lower limit for the thickness of the Mn segregation band does not need to be separately limited. However, as an example, the lower limit for the thickness of the Mn segregation band can exceed 0 μm or 1 μm. In this case, the thickness of the Mn segregation band can be defined as the value of the average thickness measured in the thickness direction (the direction perpendicular to the rolling direction of the steel sheet) from an image of the Mn segregation band determined by the above method. In addition, in the hot-formed coated steel sheet, to maximize the above-mentioned effects, more preferably, the upper limit for the thickness of the Mn segregation band can be 18.9 μm, or the lower limit for the thickness of the Mn segregation band can be 6.9 μm.

[0107] Furthermore, for the hot-formed coated steel sheet having the above composition, hot-formed parts with excellent resistance to hydrogen embrittlement and impact resistance can be manufactured by the hot pressing method described later.

[0108] A hot-formed component according to one aspect of the present invention comprises: a base steel plate having the same alloy composition as the base steel plate described above for coated steel plates; an aluminum or aluminum alloy coating disposed on at least one side of the base steel plate; and an Sb enrichment layer disposed between the base steel plate and the coating, wherein the hot-formed component satisfies the following relations 2-1 and 2-2. In this case, the descriptions of the base steel plate, the coating, and the Sb enrichment layer can be the same as described above. In this case, relations 2-1 and 2-2 are empirically derived values, therefore, the units are not particularly limited, as long as the units of each variable are satisfied.

[0109] [Relation 2-1]

[0110]

[0111] [Relation 2-2]

[0112]

[0113] In relation 2-1 and relation 2-2, the Sb 镀层This indicates the average Sb content in the coating, expressed in weight %. The Sb 最大 The value represents the maximum Sb content in the Sb-enriched layer, expressed in weight %. The value Δt represents the distance from the boundary between the coating and the Sb-enriched layer to the point where the Sb content is measured. 最大 The straight-line distance between the locations is expressed in μm.

[0114] In this invention, when the coated steel sheet is heated for hot forming, the Sb enrichment degree of the Sb-enriched layer is further aggravated. Therefore, in the hot-formed part according to the invention, the hydrogen content in the steel is effectively reduced by satisfying the aforementioned relations 2-1 and 2-2, thereby improving hydrogen embrittlement resistance and impact resistance. In this case, the method for distinguishing the boundary between the coating and the Sb-enriched layer, and the boundary between the base steel sheet and the Sb-enriched layer, in the hot-formed part can be the same as the method used in the coated steel sheet described above.

[0115] Alternatively, according to a specific embodiment of the present invention, Sb is defined by the relation 2-1. 最大 / Sb 镀层 The lower limit of the value can more preferably be 1.57, as defined by the relation 2-1. 最大 / Sb 镀层 The upper limit of the value can preferably be 7.39.

[0116] Furthermore, according to a specific embodiment of the present invention, (Sb) is defined by the relation 2-2. 最大 -Sb 镀层 The lower limit of the value of ) / 2×△t can more preferably be 0.0148, as defined by the relation 2-2 (Sb 最大 -Sb 镀层 The upper limit of the value of ) / 2×△t can preferably be 0.1940.

[0117] Alternatively, although not particularly limited, according to a specific embodiment of the present invention, it is more preferable to satisfy the following relations 2-3, thereby further improving the resistance to hydrogen embrittlement and impact resistance.

[0118] [Relationship 2-3]

[0119]

[0120] Although not specifically limited, according to one embodiment of the present invention, in the hot-formed component, the microstructure of the base steel plate may contain less than 5% ferrite and the balance martensite. Further, it may also contain less than 1% of other phases such as upper bainite, retained austenite, cementite, and pearlite.

[0121] According to one specific embodiment of the invention, the base steel plate may contain less than 5% ferrite by area fraction. During hot forming, the ferrite fraction can be controlled to be less than 5% by ensuring hardenability adjusted according to the composition and a sufficient cooling rate. Furthermore, in hot-formed parts, when the ferrite fraction of the base steel plate exceeds 5%, not only does the strength decrease, but localized stresses are relatively concentrated on the soft ferrite, promoting crack propagation and resulting in a significant decrease in impact resistance.

[0122] Furthermore, according to a specific embodiment of the present invention, although not particularly limited, the thickness of the Sb-enriched layer in the hot-formed component can be 2-30 μm. In the hot-formed component, when the thickness of the Sb-enriched layer is less than 2 μm, hydrogen permeation into the steel cannot be effectively suppressed during hot forming, thus potentially failing to fully realize the desired improvement in hydrogen embrittlement resistance and impact resistance. Furthermore, in the hot-formed component, when the thickness of the Sb-enriched layer exceeds 30 μm, not only does Sb form an enriched layer, but excessive precipitation may also occur at the grain boundaries of the surface layer of the base iron, thereby promoting crack initiation and propagation during bending, potentially leading to a decrease in impact resistance.

[0123] To maximize the aforementioned effects, more preferably, in the thermoformed component, the lower limit of the thickness of the Sb-enriched layer can be 3 μm, or the upper limit of the thickness of the Sb-enriched layer can be 25 μm. Alternatively, in the thermoformed component, the thickness of the Sb-enriched layer can be in the range of 3-25 μm.

[0124] Furthermore, although not specifically limited, according to one embodiment of the invention, the diffusible hydrogen content of the thermoformed part can be below 0.2 ppm, thereby ensuring excellent resistance to hydrogen embrittlement. This is because with such a diffusible hydrogen content of below 0.2 ppm, even if the specimen is subjected to stress by bending at the same yield stress of the material for 120 minutes, no cracks will form in the part.

[0125] Furthermore, although not specifically limited, according to one specific embodiment of the present invention, in the hot-formed component, the base steel plate includes Mn segregation bands, and in these Mn segregation bands, the area where the average Sb content, in terms of area fraction, is more than 1.015 times the average Sb content in the base steel plate can be 60% or more (or, more preferably, more than 70%). When this condition is met, by reducing the formation of inclusions such as MnS, which mainly occur in the Mn segregation bands, the generation and propagation sites of cracks are suppressed when stress is generated, thus ensuring excellent impact resistance. In addition, when too much MnS is formed, there may be a problem of overdeveloped brittle fracture surfaces. Therefore, by satisfying the above composition when Sb is enriched, MnS can be reduced, thereby reducing brittle fracture surfaces and further improving bending performance.

[0126] At this point, although there is no particular upper limit on the average Sb content in the Mn segregation zone, as an example, the average Sb content in the Mn segregation zone can be less than 5 times the average Sb content in the region of the base steel plate excluding the Mn segregation zone. Furthermore, although not particularly limited, the upper limit for the area of ​​the portion in the Mn segregation zone where the average Sb content is more than 1.015 times the average Sb content in the base steel plate can be 95%.

[0127] Furthermore, although not specifically limited, according to one embodiment of the present invention, in the thermoformed part, the thickness of the Mn segregation band can be 15 μm (or 15.0 μm) or less. By satisfying this condition, impact resistance and flexural properties can be further improved. As the thickness of the Mn segregation band decreases, impact resistance and flexural properties can be improved, so a lower limit for the thickness of the Mn segregation band is not required. However, as an example, the lower limit for the thickness of the Mn segregation band can exceed 0 μm, or can be 1.5 μm. Additionally, to maximize the above-mentioned effects, preferably, the upper limit for the thickness of the Mn segregation band can be 12.0 μm, or the lower limit for the thickness of the Mn segregation band can be 6.0 μm.

[0128] At this point, in the hot-formed part, 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 plate, and the measurement of the thickness of the Mn segregation band can be the same as the measurement methods and standards used in the above-mentioned coated steel plate.

[0129] Next, a method for manufacturing thermoformed steel sheets will be described as another aspect of the present invention.

[0130] A method for manufacturing hot-formed coated steel sheet according to one aspect of the present invention may include the following steps: reheating a steel billet to 1050-1300°C, wherein the steel billet has the above-mentioned alloy composition by weight; precision rolling the heated steel billet at 800-950°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 500-700°C; and pickling the coiled hot-rolled steel sheet so that the product of acid concentration and pickling time is 800-10000 g / L. s; In an annealing furnace, the pickled hot-rolled steel sheet is annealed at 700-860°C under dew point temperature conditions of -75°C to +20°C; and after annealing, the steel sheet is plated by a plating bath composed of aluminum or aluminum alloy.

[0131] Slab reheating steps

[0132] First, the steel billet is reheated to 1050-1300°C. When the reheating temperature is below 1050°C, the slab structure cannot be sufficiently homogenized, making it difficult to resolution using precipitated elements. On the other hand, when the reheating temperature exceeds 1300°C, excessive oxide layers are formed, leading to increased manufacturing costs for oxide layer removal and an increased likelihood of surface defects after finishing rolling.

[0133] Finishing rolling steps

[0134] Finish rolling should be performed at 800-950℃. When the finish rolling temperature is below 800℃, ferrite is introduced into the surface layer of the steel plate due to rolling in the two-phase region, and it is difficult to control the plate shape. On the other hand, when the finish rolling temperature exceeds 950℃, coarsening of hot-rolled grains may occur.

[0135] Rolling up steps

[0136] After finishing rolling, the hot-rolled steel sheet is coiled and cooled at 500-700°C to produce hot-rolled coils. When the coiling temperature is below 500°C, martensite forms in the entirety or locally of the steel sheet, making it difficult to control the shape of the coil. Furthermore, due to the excessive increase in strength of the hot-rolled steel sheet, there is a problem of reduced cold-rollability in subsequent applications. On the other hand, when the coiling temperature exceeds 700°C, coarse carbides form, and stress in the hot-formed parts promotes crack formation, thus reducing impact resistance.

[0137] Pickling process

[0138] The coiled hot-rolled steel sheet is pickled to achieve a product of acid concentration and pickling time of 800-10000 g / L. s. An Sb-enriched layer is formed on the steel plate after the above-mentioned reheating, finishing rolling and coiling steps, when the product of the acid concentration and pickling time in the pickling process is 800-10000 g / L. When the value of S is within the range of s, the Sb enrichment layer, which is the core of this invention, can be effectively protected, thereby reducing the amount of diffusible hydrogen in the steel.

[0139] Specifically, when the product of acid concentration and pickling time is less than 800 g / L If the oxide scale generated during the finishing rolling process is not sufficiently removed, it may cause product quality problems. On the other hand, when the product of acid concentration and pickling time exceeds 10000 g / L... When the acid concentration is low, either the entire or a portion of the Sb-enriched layer is lost during pickling, thus failing to achieve the desired effect and potentially increasing manufacturing costs. Therefore, the upper limit of the product of acid concentration and pickling time is set at 10000 g / L. However, if there is one or more pickling tanks and the acid concentration and pickling time of each tank are different, the above value can be represented by adding the products of the acid concentration and pickling time of each tank separately.

[0140] Furthermore, to maximize the above-mentioned effects, more preferably, the lower limit of the product of the acid concentration and the pickling time can be 3000 g / L. s, or the upper limit of the product of the acid concentration and the pickling time can be 5000 g / L. s.

[0141] While not specifically limited, according to one particular embodiment of the invention, the acid that can be used in the pickling step can be any acid commonly used in the art. Representative examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), etc. Particularly in the present invention, hydrochloric acid (HCl) offers superior pickling capability and economical process costs compared to the use of other acids, and the likelihood of foreign matter forming on the pickled surface is low, thus easily ensuring surface quality.

[0142] Furthermore, although not specifically limited, according to one embodiment of the present invention, the concentration of the acid can be in the range of 40-500 g / L. When the acid concentration is less than 40 g / L, the surface oxide scale generated during hot rolling is not sufficiently removed within the limited pickling time, which may lead to defects in the surface portion of the steel sheet. On the other hand, when the acid concentration exceeds 500 g / L, the loss of the Sb-enriched layer may make it difficult to achieve the desired effect of the present invention in the final hot-formed part, and may cause surface defects due to over-pickling. In addition, to maximize the above-mentioned effects, more preferably, the lower limit of the acid concentration can be 180 g / L, or the upper limit of the acid concentration can be 230 g / L.

[0143] Furthermore, although not specifically limited, according to one specific embodiment of the present invention, the pickling time can be 5-60 seconds (s). When the pickling time is less than 5 seconds, the surface oxide scale of the steel plate is not sufficiently removed, which may cause surface defects. When the pickling time exceeds 60 seconds, productivity is reduced due to the loss of the Sb-enriched layer, which may lead to an increase in process costs. More preferably, to maximize the above-mentioned effects, the lower limit of the pickling time can be 18 seconds, or the upper limit of the pickling time can be 50 seconds.

[0144] Furthermore, although not specifically limited, according to one embodiment of the present invention, the pickling temperature can be 40-120°C. When the pickling temperature is below 40°C, insufficient pickling capacity may negatively impact product quality. On the other hand, when the pickling temperature exceeds 120°C, maintaining the high temperature not only increases fixed costs but also increases the vaporization of the pickling solution, potentially leading to increased costs for replenishing lost pickling solution. More preferably, to maximize the above-mentioned effects, the lower limit of the pickling temperature can be 50°C, or the upper limit of the pickling temperature can be 100°C. Alternatively, the pickling temperature can be in the range of 50-100°C.

[0145] Annealing steps

[0146] The pickled hot-rolled steel sheet can be annealed in an annealing furnace at a dew point temperature of -75°C to +20°C at 700-900°C. When the annealing temperature is below 700°C, the recrystallization of the cold-rolled structure is not fully completed, so the shape of the sheet may be poor, and the excessive strength after plating may cause wear of the die during the blanking process. On the other hand, when the annealing temperature exceeds 900°C, the formation of surface oxides is promoted during the annealing process, thus causing defects on the Al-Si plating surface. Alternatively, in terms of suppressing die wear and plating surface defects during the blanking process, 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, and most preferably 860°C.

[0147] Furthermore, the atmosphere used during annealing is preferably a non-oxidizing atmosphere, and a hydrogen-nitrogen mixture can be used. In this case, the dew point temperature of the aforementioned atmosphere is between -75°C and +20°C. When the dew point temperature is below -75°C, additional equipment for controlling the dew point is required, thus increasing manufacturing costs. On the other hand, when the dew point temperature exceeds +20°C, annealing oxides form on the steel plate surface during annealing, which may cause surface quality defects such as incomplete plating. Furthermore, to maximize the above effects, the lower limit of the dew point temperature during annealing is more preferably -70°C, and most preferably -40°C. Alternatively, the upper limit of the dew point temperature during annealing is more preferably +15°C, and most preferably -20°C.

[0148] Plating steps

[0149] Immediately after the annealing process, Al-Si plating is performed. Specifically, after annealing, the hot-rolled steel sheet is passed through a plating bath composed of aluminum or an aluminum alloy to produce a coated steel sheet. At this time, the plating conditions can be applied to this invention without limitation, as long as they are those commonly used for hot-pressed steel sheets. However, as an example, the composition of the plating bath may contain 6-12% Si, 1-4% Fe, the balance Al, and other unavoidable impurities.

[0150] At this point, although not specifically limited, in the plating step, based on a typically manufactured single side, the plating amount is preferably 20-140 g / m². 2 When the coating weight on a single side is less than 20 g / m² 2 At times, it is difficult to ensure the required corrosion resistance of thermoformed parts, especially when the coating weight on a single side exceeds 140 g / m². 2 In this case, not only does excessive coating adhesion lead to increased manufacturing costs, but it is also difficult to uniformly coat the coating across the entire width and length of the roll.

[0151] Cold rolling process

[0152] A method for manufacturing a hot-formed coated steel sheet according to one aspect of the present invention further includes the following steps: after the above-mentioned pickling step, the hot-rolled steel sheet is cold-rolled to manufacture a cold-rolled steel sheet.

[0153] Continuous casting steps

[0154] The method for manufacturing coated steel sheets for hot forming according to the present invention may further include a step of continuous casting by light pressing before the slab heating step, but is not particularly limited thereto. The present invention reduces segregation and improves impact resistance by using light pressing during slab manufacturing in continuous casting. This is because when excessive segregation occurs in the slab, segregation bands thickly accumulate and form in the final hot-formed part, and impact resistance may be reduced due to the hardness difference between these segregation bands and the base steel sheet region other than the Mn segregation bands, and the formation of inclusions within the segregation bands.

[0155] Therefore, in order to produce the slab of the present invention, a light reduction should be performed during continuous casting before the final solidification position of the slab, and the total reduction rate during the light reduction in continuous casting is preferably controlled at 0.5-5%. When the total reduction rate during continuous casting is less than 0.5%, it is almost impossible to achieve reduction, and center segregation is not sufficiently removed, which may lead to a decrease in the impact resistance of the hot-formed part. On the other hand, when the total reduction rate during continuous casting exceeds 5%, the reduction roller equipment may be subjected to excessive load, which may promote equipment failure and aging. Furthermore, in order to maximize the above-mentioned effects, more preferably, the lower limit of the total reduction rate during the light reduction in continuous casting can be 0.52%, or the upper limit of the total reduction rate during the light reduction in continuous casting can be 4.10%.

[0156] By hot-pressing a hot-formed coated steel sheet manufactured using the above-described manufacturing method, hot-formed parts with excellent resistance to hydrogen embrittlement and impact resistance can be produced. Specifically, a method for manufacturing a final part with excellent resistance to hydrogen embrittlement and impact resistance using a coated steel sheet manufactured by the above method through hot forming and die hardening will be described. A blank for hot forming is manufactured using a coated steel sheet manufactured according to the above-described steel composition and manufacturing method. The blank is heated in a temperature range above the austenitic single-phase temperature, and more specifically, above the Ac3 temperature and below 975°C. At this time, when the heating temperature is below the Ac3 temperature, it is difficult to ensure strength and impact resistance due to the presence of untransformed ferrite in the two-phase region. On the other hand, when the heating temperature exceeds 975°C, excessive oxides form on the surface of the part, making it difficult to ensure spot weldability, and increasing the manufacturing cost of maintaining the high temperature.

[0157] The heated blank is preferably held within the aforementioned temperature range for 1-1000 seconds. When the holding time is less than 1 second, it is difficult to achieve uniform temperature distribution across the entire blank temperature range, which may cause material deviations at different locations. On the other hand, when the holding time exceeds 1000 seconds, similar to exceeding the heating temperature, excessive oxide formation on the component surface makes it difficult to ensure spot weldability and increases the manufacturing cost of the component.

[0158] The heated blank is transferred to a press and thermoformed and die-hardened at a cooling rate of -20°C / s or higher to manufacture the final part. During this cooling process, at a rate less than -20°C / s, ferrite phases are introduced and form at grain boundaries, potentially reducing strength and impact resistance. The transfer, thermoforming, and cooling steps of the blank are not particularly limited, and commonly used thermoforming processes can be directly applied.

[0159] In the hot-formed parts thus produced, an Sb-enriched layer is formed between the base steel sheet and the coating, thus enabling the manufacture of a hot-formed part with excellent resistance to hydrogen embrittlement and impact resistance due to the reduction of diffusible hydrogen in the steel. Detailed Implementation

[0160] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the contents of the claims and the contents reasonably deduced therefrom.

[0161] (Experimental Example 1)

[0162] The composition of the steel used for this invention is shown in Table 1 below (equivalent to the balance Fe and other impurities). A slab with a thickness of 40 mm and its respective composition was manufactured by vacuum melting. This slab was held at 1200°C for 1 hour, then hot-rolled at a hot-rolling finish temperature of 900°C, and coiled at a coiling temperature of 600°C. After pickling and cold rolling, it was annealed under the conditions in Table 2 below and then immersed in a plating bath consisting of 9% Al, 2% Si, Fe, and the balance trace impurities for plating. At this time, a pickling process was performed using an HCl concentration of 200 g / L and a pickling time of 20 seconds, with the product of the HCl concentration and pickling time uniformly applied as 4000 g / L·s within the scope of this invention.

[0163] In this embodiment, thermoforming is performed at various temperatures, with a heating time of 6 minutes and a mold transfer time of 10 seconds, followed by mold quenching.

[0164] Microstructure analysis was performed on the specimens manufactured as described above to observe ferrite. To confirm the Sb-enriched layer near the surface, a GDS850A (model, manufactured by LECO), DC, and FR instrument was used, and the Sb was detected by glow discharge spectroscopy (GDS). Figure 3 The Sb-rich layer at the interface between the iron substrate and the coating is shown. Based on this result, as... Figure 1 As shown, for parameter Sb 最大 Sb 镀层The parameters Δt were measured, and the parameter P1 value of the enriched layer described in the table below was calculated using these measurements. To confirm the amount of diffusible hydrogen, a Thermal Desorption Analysis (TDA) instrument (model: Bruker G8) was used for measurement. The temperature was increased to 400°C at a rate of 20°C / min, and held for a sufficient time to allow the diffusible hydrogen peak to appear, thus measuring the diffusible hydrogen curve. The total amount of diffusible hydrogen in the steel was obtained by integrating these curves.

[0165] In addition, to confirm impact resistance, flexural properties were evaluated one week after thermoforming heat treatment, and methods such as... Figure 2 The load-displacement curves obtained from the three-point bending test are used for evaluation, up to the area at which the maximum load is reached (crack initiation energy (CIE)). To determine whether the impact resistance is excellent, a CIE value of 35,000 N·m or higher is considered "good," and a CIE value less than 35,000 N·m is considered "poor." The results are shown in Tables 2 and 3.

[0166] [Table 1]

[0167]

[0168] [Table 2]

[0169]

[0170] [Table 3]

[0171]

[0172] Ac3=902-255×C+19×Si-11×Mn-5×Cr[℃]

[0173] As can be seen from Tables 1 to 3, in the cases of Comparative Examples 1 to 5, the Sb content in the steel did not reach the scope of the present invention. 最大 / Sb 镀层 The ratio and P1 value do not meet the scope of this invention, and therefore cannot effectively suppress the permeation of diffusible hydrogen in steel, resulting in poor impact resistance.

[0174] On the other hand, in the cases of Examples 1 to 8, since the values ​​of Relations 1-1, 1-2, 2-1 and 2-2 satisfy the scope of the present invention, the amount of diffusible hydrogen in the steel is reduced, and the CIE value, which is an indicator of impact resistance, is increased, thus confirming that it has excellent impact resistance.

[0175] In addition, aside from some results where ferrite microstructure is outside the scope of this invention, such as Figure 4 As indicated by the markings, the amount of diffusible hydrogen gradually decreases with increasing P1 value, which can be interpreted as the Sb enrichment layer acting as an effective protective film in reducing the amount of diffusible hydrogen in the steel. Due to this effect, such as... Figure 5 As shown, when an Sb-enriched layer is exhibited, the CIE value increases significantly while simultaneously ensuring excellent impact resistance.

[0176] (Experimental Example 2)

[0177] Except for changing the pickling concentration and pickling time as shown in Table 4 below, and applying the thermoforming temperature under the conditions in Table 4, test pieces were manufactured using the same method as in Experimental Example 1 above. In this case, the pickling temperature was uniformly applied at 80°C.

[0178] For each embodiment and comparative example in Table 4 below, the values ​​of Equations 1-1, 1-2, 2-1, and 2-2 were measured using the same method as in Experimental Example 1 described above, and are shown in Table 5 below. Furthermore, for the coated steel sheets (or, hot-formed parts) of each embodiment and comparative example, the thickness of the Sb-enriched layer was measured at 10 points on the coated surface, based on data obtained by analyzing the change in Sb content in the thickness direction of the base steel sheet using GDS analysis. This measurement was performed using the same method as described in the specification (i.e., for the schematic diagram). Figure 7 The distance along the thickness direction from the last contact point 11 of the Sb average content line of the coating and the Sb content line according to GDS along the x-axis (+) direction to the last contact point 31 of the Sb average content line of the base steel plate and the Sb content line according to GDS along the x-axis (-) direction is measured. The average thickness of the Sb-enriched layer at the 10 points is measured and shown in Table 5 below.

[0179] To evaluate the various embodiments and comparative examples, impact resistance was assessed using the same method as in Experimental Example 1 described above. To further evaluate surface characteristics (i.e., the presence of surface defects), the presence of residual hot oxide scale after pickling was evaluated. To confirm the presence of residual hot oxide scale after pickling, for each steel grade, adhesive tape was applied to the surface of the test piece and then peeled off. The oxide residue adhering to the tape was then attached to white paper, and the whiteness was measured based on color difference analysis. A whiteness of 95% or higher was considered "good," and a whiteness less than 95% was considered "poor."

[0180] [Table 4]

[0181]

[0182] [Table 5]

[0183]

[0184] [Table 6]

[0185]

[0186] As can be seen from Tables 4 to 6, the product of Sb content, acid concentration, and pickling time is less than 800 g / L, which does not meet the requirements of this invention. In Comparative Example 6, it was confirmed that not only could not the relations 1-1, 1-2, 2-1 and 2-2 be satisfied, but the thermal oxide scale was not completely removed, so the possibility of surface defects in subsequent processes was high.

[0187] Furthermore, the product of acid concentration and pickling time exceeds 10000 g / L. Comparative Examples 7 and 8 do not satisfy Relations 1-1, 1-2, 2-1, and 2-2. As a result, the steel has a high amount of diffusible hydrogen, which leads to poor resistance to hydrogen embrittlement. At the same time, the CIE value, which is an indicator of impact resistance, is also low, resulting in poor impact resistance.

[0188] On the other hand, in Examples 9 to 12 of the present invention, the product of the steel composition, acid concentration, and pickling time is 800-10000 g / L. The range of s is such that relations 1-1, 1-2, 2-1, and 2-2 satisfy the scope of this invention. Therefore, not only are the performance characteristics excellent, but the amount of diffusible hydrogen in the steel is also reduced, resulting in excellent resistance to hydrogen embrittlement. Furthermore, the CIE value, an indicator of impact resistance, increases simultaneously, thus exhibiting excellent impact resistance.

[0189] (Experimental Example 3)

[0190] Before reheating the slab, a light reduction was applied to produce the slab using the total reduction amounts listed in Table 7 below. Test pieces were manufactured using the same method as in Experimental Example 1, except that the conditions in Table 7 were applied. For these test pieces, each characteristic was evaluated using the same method as in Experimental Example 1, and the thickness of the Mn segregation bands and the Sb content within the Mn segregation bands in the coated steel sheet and the hot-formed component were further measured and are shown in Table 7 below.

[0191] Specifically, for the thickness of the Mn segregation band and the area of ​​the portion where the average Sb content in the Mn segregation band is more than 1.015 times the average Sb content in the base steel plate, the method described above in the instruction manual was applied and measured using an electron probe X-ray microanalyzer (EPMA). The element mapping results for these Mn and Sb components are shown below. Figure 6a and Figure 6b middle.

[0192] At this point, the surface properties, diffusible hydrogen content, and impact resistance are measured using the same method as described above.

[0193] Furthermore, to further evaluate the bending properties, bending tests were performed on the components manufactured in the various embodiments and comparative examples listed in Table 7 below. Specifically, when the proportion of the fracture surface from the surface to below 100 μm was measured using SEM to observe the fracture surface, ductile fracture and brittle fracture were observed. In this case, when the area of ​​the ductile fracture surface relative to the total measured area is 70% or more, it is indicated as "○", and when the proportion is less than 70%, it is indicated as "×".

[0194] [Table 7]

[0195]

[0196] [Table 8]

[0197]

[0198] [Table 9]

[0199]

[0200] Ma In galvanized steel sheets, the area of ​​the portion where the average Sb content in the Mn segregation band is more than 1.015 times the average Sb content in the base steel sheet.

[0201] Mb In hot-formed parts, the area of ​​the portion where the average Sb content in the Mn segregation band is more than 1.015 times the average Sb content in the base steel plate.

[0202] [Table 10]

[0203]

[0204] As can be seen from Tables 7 to 10, in Comparative Example 9, the added Sb content was less than the range of the present invention, and Sb was not sufficiently enriched in the Mn segregation band, thus resulting in poor impact resistance.

[0205] In Comparative Example 10, the continuous casting press malfunctioned and continuous casting could not be carried out because the total reduction rate exceeded 5% during continuous casting, thus making it impossible to conduct the experimental evaluation.

[0206] On the other hand, in Examples 13 to 16, since they satisfy the range of Relations 1-1, 1-2, 2-1 and 2-2 of the present invention, they have excellent surface properties and low diffusible hydrogen content, thus exhibiting excellent resistance to hydrogen embrittlement.

[0207] In particular, in the above embodiments, in Examples 14 to 16 where the thickness of the Mn segregation band in the clad steel sheet is 20 μm or less (or, in the hot-formed part, the thickness of the Mn segregation band is 15 μm or less) or the ratio of the average Sb content of the Mn segregation band in the clad steel sheet and the hot-formed part to the average Sb content of the base steel sheet is 60% or more, further improved bendability was confirmed compared with Example 13, which did not meet one or more of the above conditions, and Comparative Example 9, which did not meet any of the above conditions.

[0208] According to the present invention, the following thermoformed products can be manufactured from the above experimental examples: even with the addition of a small amount of Sb to steel, they have resistance to hydrogen-induced delayed fracture due to the reduction of diffusible hydrogen content, and at the same time have excellent impact resistance. Furthermore, these components can be used as structural materials or reinforcing components in various fields, including the automotive manufacturing industry.

[0209] Explanation of reference numerals in the attached figures

[0210] 1: Coating

[0211] 2: Sb enrichment layer

[0212] 21: In the Sb-enriched layer, the range of Sb content increasing along the x-axis (+) direction.

[0213] 22: In the Sb-enriched layer, the range of Sb content increase along the x-axis (-) direction.

[0214] 3: Foundation steel plate

[0215] 10: Average Sb content line of the coating

[0216] 11: The last point of contact between the average Sb content line of the coating and the Sb content line according to GDS along the x-axis (+) direction.

[0217] 30: Average Sb content line of the base steel plate

[0218] 31: The final contact point along the x-axis (-) direction between the average Sb content line of the base steel plate and the Sb content line according to GDS.

[0219] 100: Based on the Sb content line of GDS

[0220] 200: The point where the Sb content is at its maximum in the Sb-rich layer.

Claims

1. A galvanized steel sheet for hot forming, the galvanized steel sheet for hot forming comprising: The base steel plate, by weight percent, comprises: C: 0.14-0.5%, Si: 0.001-1%, Mn: 0.3-4%, P: 0.001-0.015%, S: 0.0001-0.02%, Al: 0.001-0.1%, Cr: 0.001-1%, N: 0.001-0.02%, Ti: less than 0.1%, B: less than 0.01%, Sb: 0.005-0.1%, with the balance being Fe and other unavoidable impurities; An aluminum or aluminum alloy coating, said aluminum or aluminum alloy coating being disposed on at least one side of the base steel plate; and An Sb enrichment layer is disposed between the base steel plate and the coating. The galvanized steel sheet for hot forming satisfies the following relations 1-1 and 1-2: [Relation 1-1] ， [Relationship 1-2] ， In relation 1-1 and relation 1-2, Sb 镀层 The Sb content in the coating is expressed as an average Sb content (by weight %). 最大 This represents the maximum Sb content in the Sb-enriched layer, expressed in weight %; Δt represents the distance from the boundary between the coating and the Sb-enriched layer to the point where the Sb content is measured. 最大 The straight-line distance between the positions is expressed in μm.

2. The galvanized steel sheet for hot forming according to claim 1, wherein, The thickness of the Sb enrichment layer is greater than 1 μm and less than 20 μm.

3. The galvanized steel sheet for hot forming according to claim 1, wherein, The base steel plate includes Mn segregation bands, in which the area of ​​the portion where the average Sb content is more than 1.015 times the average Sb content in the base steel plate is more than 60%.

4. The galvanized steel sheet for hot forming according to claim 3, wherein, The thickness of the Mn segregation band is less than 20 μm.

5. A method for manufacturing coated steel sheet for hot forming, comprising the following steps: The steel billet is reheated to 1050-1300℃, and the steel billet contains, by weight%,: C: 0.14-0.5%, Si: 0.001-1%, Mn: 0.3-4%, P: 0.001-0.015%, S: 0.0001-0.02%, Al: 0.001-0.1%, Cr: 0.001-1%, N: 0.001-0.02%, Ti: less than 0.1%, B: less than 0.01%, Sb: 0.005-0.1%, with the balance being Fe and other unavoidable impurities; Hot-rolled steel plates are obtained by precision rolling heated steel billets at 800-950℃. The hot-rolled steel sheet is coiled at 500-700℃; The coiled hot-rolled steel sheet is pickled to achieve a product of acid concentration and pickling time of 800-10000 g / L. s; In an annealing furnace, pickled steel plates are annealed at 700-900℃ under dew point temperatures ranging from -75℃ to +20℃; and After annealing, the steel plate is plated through a plating bath composed of aluminum or an aluminum alloy. Prior to the reheating step, the process includes a continuous casting step involving light reduction at a reduction rate of 0.5-5%. The hot-formed coated steel sheet comprises a base steel sheet, an aluminum or aluminum alloy coating, and an Sb enrichment layer disposed between the base steel sheet and the coating. The galvanized steel sheet for hot forming satisfies the following relations 1-1 and 1-2: [Relation 1-1] ， [Relationship 1-2] ， In relation 1-1 and relation 1-2, Sb 镀层 The Sb content in the coating is expressed as an average Sb content (by weight %). 最大 This represents the maximum Sb content in the Sb-enriched layer, expressed in weight %; Δt represents the distance from the boundary between the coating and the Sb-enriched layer to the point where the Sb content is measured. 最大 The straight-line distance between the positions is expressed in μm.

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

7. The method for manufacturing coated steel sheet for hot forming according to claim 5, wherein, The pickling time is 5-60 seconds.

8. The method for manufacturing coated steel sheet for hot forming according to claim 5, wherein, The pickling temperature is 40-120℃.

9. A thermoformed part, the thermoformed part comprising: The base steel plate, by weight percent, comprises: C: 0.14-0.5%, Si: 0.001-1%, Mn: 0.3-4%, P: 0.001-0.015%, S: 0.0001-0.02%, Al: 0.001-0.1%, Cr: 0.001-1%, N: 0.001-0.02%, Ti: less than 0.1%, B: less than 0.01%, Sb: 0.005-0.1%, with the balance being Fe and other unavoidable impurities; An aluminum or aluminum alloy coating, said aluminum or aluminum alloy coating being disposed on at least one side of the base steel plate; and An Sb enrichment layer is disposed between the base steel plate and the coating. The thermoformed part satisfies the following relations 2-1 and 2-2: [Relation 2-1] ， [Relation 2-2] ， In relation 2-1 and relation 2-2, Sb 镀层 The Sb content in the coating is expressed as an average Sb content (by weight %). 最大 This represents the maximum Sb content in the Sb-enriched layer, expressed in weight %; Δt represents the distance from the boundary between the coating and the Sb-enriched layer to the point where the Sb content is measured. 最大 The straight-line distance between the positions is expressed in μm.

10. The thermoformed part according to claim 9, wherein, The base steel plate contains less than 5% ferrite by area fraction.

11. The thermoformed part according to claim 9, wherein, The thickness of the Sb enrichment layer is 2-30 μm.

12. The thermoformed part according to claim 9, wherein, The thermoformed part satisfies the following relationship 2-3: [Relationship 2-3] 。 13. The thermoformed part according to claim 9, wherein, The diffusible hydrogen content is below 0.2 ppm.

14. The thermoformed part according to claim 9, wherein, The base steel plate includes Mn segregation bands, in which the area of ​​the portion where the average Sb content is more than 1.015 times the average Sb content in the base steel plate is more than 60%.

15. The thermoformed part according to claim 14, wherein, The thickness of the Mn segregation band is less than 15 μm.

16. A method for manufacturing a thermoformed part, wherein, Within a temperature range of Ac3 to 950°C, the hot-formed coated steel sheet manufactured by the method of claim 5 is heat-treated for 1-1000 seconds and then hot-pressed.

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