Hot-forming plated steel sheet excellent in hydrogen embrittlement resistance and impact resistance, hot-forming member, and method for producing the same

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, 0.07-0.5% C, 0.05-1% Si, 0.5-5% Mn, 0.001-0.015% P, 0.0001-0.02% S, 0.01-0.1% Al, 0.01-1% Cr, 0.001-0.02% N, 0.1% or less Ti, 0.01% or less B, 0.01-0.1% Sn, 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 Sn-enriched layer provided between the base steel sheet and the plating layer, wherein the Sn-enriched layer satisfies a specific relationship.

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 the purpose of improving fuel efficiency and protecting passengers by reducing the weight of automobiles. In particular, they can be used for bumpers, doors, pillar reinforcements, etc. that require extremely high strength or high energy absorption capacity. As such a hot-forming technology, there is typically US Patent No. 6296805 (hereinafter, Patent Document 1).

[0003] Patent Document 1 discloses that after heating an Al-Si-based plated steel sheet to 850°C or higher, the structure of the member can be formed as martensite by hot-forming with a press and rapid cooling, thereby ensuring extremely 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 a weight reduction effect can be expected due to the increase in strength by rapid cooling in the mold. However, it is known that the martensite structure has low resistance to hydrogen embrittlement. In particular, residual stress due to rapid cooling after heating exists in hot-forming members, and the amount of diffusible hydrogen in the steel increases, raising concerns about delayed fracture due to hydrogen embrittlement, so there is a drawback that the application of the members is limited. Furthermore, since such an amount of diffusible hydrogen moves to crack generation sites such as grain boundaries when stress occurs, it may deteriorate the impact resistance, and various studies have been conducted to overcome this.

[0004] In addition, changes in process parameters during coil manufacturing may cause overall or local changes in mechanical properties within the sheet. Therefore, a steel composition with low sensitivity to changes in manufacturing parameters for producing a plated steel sheet and a hot-forming member having good mechanical properties and homogeneity is required.

Prior Art Documents

Patent Document

[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. For anyone with ordinary knowledge in the technical field to which the present invention pertains, it is not difficult to understand 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 includes a base steel sheet containing, by weight%, C: 0.07 to 0.5%, Si: 0.05 to 1%, Mn: 0.5 to 5%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, Cr: 0.01 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, Sn: 0.01 to 0.1%, the balance being Fe and other inevitable impurities, an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel sheet, and a Sn-enriched layer provided between the base steel sheet and the plating layer, and provides a hot-forming plated steel sheet satisfying the following Relational Expression 1-1 and the following Relational Expression 1-2.

[0009] [Relational Expression 1-1]

Number

[0010] [Relational Expression 1-2] [Number] [In the above Relational Expression 1-2, the above Sn(x) represents the Sn content along the x-axis direction from any point in the plating layer to the base steel plate side, x1 represents the x-axis point at the boundary between the plating layer and the Sn enrichment layer, and the above x2 represents the x-axis point at the maximum value of the Sn content in the Sn enrichment layer.]

[0011] Still another aspect of the present invention is, by weight %, C: 0.07 to 0.5%, Si: 0.05 to 1%, Mn: 0.5 to 5%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, Cr: 0.01 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, Sn: 0.01 to 0.1%, the balance being Fe and other inevitable impurities, reheating a steel slab to 1050 to 1300 °C, finish-rolling the heated steel slab at 750 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, and provides a method for manufacturing a plated steel sheet for hot forming, including the above steps.

[0012] Still another aspect of the present invention is a base steel sheet containing, by weight%, C: 0.07 to 0.5%, Si: 0.05 to 1%, Mn: 0.5 to 5%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, Cr: 0.01 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, Sn: 0.01 to 0.1%, the balance being Fe and other inevitable impurities, an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel sheet, and a Sn-enriched layer provided between the base steel sheet and the plating layer, and provides a hot-forming member satisfying the following relational expressions 2-1 and 2-2.

[0013] [Relational Expression 2-1] [In the above relational expression 2-1, Sn represents the average Sn content in the plating layer, and the unit is wt%. Also, Sn coat represents the maximum value of the Sn content in the Sn-enriched layer, and the unit is wt%.] max

[0014] [Relational Expression 2-2] [In the above relational expression 2-2, Sn(x) represents the Sn content along the x-axis direction from any point in the plating layer toward the base steel sheet side, x1 represents the x-axis point at the boundary between the plating layer and the Sn-enriched layer, and x2 represents the x-axis point at the maximum value of the Sn content in the Sn-enriched layer.]

[0015] Still another aspect of the present invention provides a method for manufacturing a hot-forming member, which comprises heat-treating the above-described plated steel sheet for hot forming at a temperature in the range of Ac3 to 950°C for 1 to 1000 seconds and then performing hot press forming. [Effects of the Invention]

[0016] ​​According to one aspect of the present invention, by forming an Sn-enriched layer between a base steel sheet and a plating layer, the amount of diffusible hydrogen in the steel can be reduced, and 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, can be provided.

[0017] The various and beneficial advantages and effects of the present invention are not limited to the above-described content, and can be more easily understood in the process of explaining the specific embodiments of the present invention.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Number

Figure 5

Figure 6

Modes for Carrying Out the Invention

[0019] 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 art.

[0020] Diffusive hydrogen at the grain boundaries promotes the generation of cracks at the grain boundaries during stress generation. Therefore, a method capable of reducing the amount of diffusive hydrogen in steel after hot stamping is required.

[0021] Therefore, the present inventors used methods of analyzing the amount of diffusive hydrogen in 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 impact resistance, in hot-forming steel materials to analyze the effects of various components, manufacturing conditions, structures, etc. including the addition of Sn. As a result, it was found that the amount of diffusive hydrogen can be reduced by forming an Sn-enriched layer, and thus a hot-forming plated steel sheet, a hot-forming member, and their manufacturing methods excellent in both hydrogen embrittlement resistance and impact resistance were devised.

[0022] First, the hot-forming plated steel sheet and the hot-forming member according to an embodiment of the present invention will be described in detail below.

[0023] The plated steel sheet according to an embodiment of the present invention contains, by weight%, a base steel sheet containing C: 0.07 to 0.5%, Si: 0.05 to 1%, Mn: 0.5 to 5%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, Cr: 0.01 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, Sn: 0.01 to 0.1%, the balance being 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 Sn-enriched layer provided between the base steel sheet and the plating layer.

[0024] First, the alloy composition of the base steel plate of the present invention will be described in detail. When expressing each element by content in the present invention, it should be noted that, unless otherwise specified, it means weight%.

[0025] Carbon (C): 0.07 - 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 0.07%, it is difficult to sufficiently ensure the hardenability, and sufficient martensite cannot be ensured when the cooling rate decreases, and there is a possibility of inferior impact resistance due to excessive ferrite formation. On the other hand, when the C content exceeds 0.5%, the strength may increase excessively and induce brittleness, and there is a possibility of inferior weldability. Therefore, it is preferable to control the C content in the range of 0.07 - 0.5%. However, from the viewpoint of further improving the above effects, the lower limit of the C content may be 0.078%, and the upper limit of the C content may be 0.348%.

[0026] Silicon (Si): 0.05 - 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 strength increase of the hot-forming member and is added as an element effective for material homogenization. However, when the Si content is less than 0.05%, the above effects cannot be expected, and it is not appropriate because the manufacturing cost and process cost for controlling the Si content increase. On the other hand, when the Si content exceeds 1%, the plating property is greatly reduced due to excessive Si oxide generated on the surface of the steel plate during annealing, so it is added at 1% or less. However, from the viewpoint of further improving the above effects, the lower limit of the Si content may be 0.17%, and the upper limit of the Si content may be 0.80%.

[0027] Manganese (Mn): 0.5 - 5% 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 0.5%, it is difficult to obtain a sufficient hardenability effect, and due to the insufficient hardenability, other expensive alloying elements are required in excess, resulting in a large increase in manufacturing cost. When the Mn content exceeds 5%, the banded structure arranged in the rolling direction of the fine tissue phase deepens, inducing inhomogeneity of the internal structure, which may reduce the impact resistance. Therefore, the addition is limited to 5% or less. However, from the perspective of further improving the above-mentioned effects, the lower limit of the Mn content may be 0.55%, and more preferably, the upper limit of the Mn content may be 4.4%.

[0028] Phosphorus (P): 0.001 - 0.015% The above-mentioned 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. Therefore, the upper limit is set at 0.015%. On the other hand, from the perspective of further improving the above-mentioned effects, the lower limit of the P content may be 0.005%, and more preferably, the upper limit of the P content may be 0.012%.

[0029] Sulfur (S): 0.0001 - 0.02% The above-mentioned S is an impurity and an element that inhibits the ductility, impact characteristics, and weldability of the member. Therefore, the maximum content is limited to 0.02%. Also, when its minimum content is less than 0.0001%, it is not preferable because it significantly increases the manufacturing cost. On the other hand, from the perspective of further improving the above-mentioned effects, the lower limit of the S content may be 0.001%, and the upper limit of the S content may be 0.01%.

[0030] Aluminum (Al): 0.01 - 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 0.01%, 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%. On the other hand, from the perspective of further improving the above effects, the lower limit of the above Al content may be 0.020%, and the upper limit of the above Al content may be 0.081%.

[0031] Cr: 0.01 - 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 0.01%, 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 are excessively formed, which may induce cracks during stress application, deteriorating the material quality. Therefore, the upper limit is set at 1%. On the other hand, from the perspective of further improving the above effects, the lower limit of the above Cr content may be 0.10%, and the upper limit of the above Cr content may be 0.61%.

[0032] 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 are likely to occur due to the formation of AlN like the added Al. Therefore, the upper limit is set at 0.02%. On the other hand, from the perspective of further improving the above effects, the lower limit of the above N content may be 0.003%, and more preferably, the upper limit of the above N content may be 0.005%.

[0033] 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 from forming compounds to ensure hardenability. 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 Ti content may be 0%, and from the perspective of further improving the above-mentioned effects, the lower limit of the Ti content may be 0.01%, and the upper limit of the Ti content may be 0.035%.

[0034] 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-mentioned B is added, it can segregate at the prior austenite grain boundaries and suppress the brittleness of the hot-forming member due to the grain boundary segregation of impurities P or / and S. However, if it exceeds 0.01%, there is a possibility of causing brittleness in hot rolling due to the formation of the Fe 23 CB6 composite compound, so its upper limit is set at 0.01%. On the other hand, since the above-mentioned B is a selective element and includes the case where it is not added, the lower limit of the B content may be 0%, and from the perspective of further improving the above-mentioned effects, the lower limit of the B content may be 0.002%, and the upper limit of the B content may be 0.008%.

[0035] Sn: 0.01 - 0.1% Sn is a core element in manufacturing this hot forming member. It forms an Sn 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 the hydrogen-induced delayed fracture sensitivity. When the Sn content is less than 0.01%, a sufficient enrichment layer cannot be formed at the interface between the plating layer and the base iron, and the above-mentioned effects cannot be expected. On the other hand, when the Sn content exceeds 0.1%, Sn precipitates excessively at the grain boundaries, inducing intergranular fracture during stress generation and deteriorating the material. Therefore, its upper limit is preferably 0.1%. However, from the perspective of further improving the above-mentioned effects, the lower limit of the Sn content may be 0.011%, and more preferably, the upper limit of the Sn content may be 0.097%.

[0036] The balance other than the above components is iron (Fe), and there is no particular restriction on additional addition as long as it is a component that can be contained in the hot press forming steel plate. Also, since unintended impurities may inevitably be mixed 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 contents are not particularly mentioned in this specification.

[0037] Also, the above plating layer includes an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel plate. The above plating layer imparts corrosion resistance to the final hot forming member. In the present invention, the type of the above plating layer is not particularly limited, and any plating layer applicable to conventional hot forming plated steel plates can be applied without limitation in the present invention. As an example, the above plating layer may be an aluminum or aluminum alloy plating layer, and preferably, the above plating layer can contain Si: 6 - 12%, Fe: 1 - 4%, and the balance Al and other inevitable impurities.

[0038] According to an embodiment of the present invention, the plated steel sheet may include a Sn-enriched layer provided between the base steel sheet and the plating layer. Such a Sn-enriched layer can be expressed in the concentration gradient layer in the boundary region between the base steel sheet and the plating layer. The concentration gradient layer is a layer in which the concentration changes in the boundary region where the base steel sheet and the plating layer are in contact, and means a layer in which Fe changes by 0.3 wt% / μm or more in the range of 20 to 99% based on the gravimetric method in the thickness direction of the steel sheet. On the other hand, the Sn-enriched layer is a region provided between the base steel sheet and the plating layer and is a region where Sn is concentrated and classified by the Sn content. Such a Sn-enriched layer and the plating layer can be distinguished by analyzing the change in the Sn content in the thickness direction from any point of the plating layer to the base steel sheet side by utilizing glow discharge spectroscopy (GDS).

[0039] Specifically, as shown in FIG. 2, the x-axis indicates the linear distance in the thickness direction from an arbitrary position inside the plating layer 1 to the base steel sheet 3 side, and the y-axis is determined based on a graph showing the Sn content measured by utilizing the above GDS.

[0040] For example, based on FIG. 2 schematically showing the above-described GDS measurement results, in the rising section 21 of the Sn content in the x-axis (+) direction provided between the plating layer 1 and the base steel sheet 3, from the last contact point 11 in the x-axis (+) direction of the Sn average content line 10 of the plating layer and the Sn content line 100 measured by utilizing the above GDS, the Sn-enriched layer 2 is defined (in the thickness direction of the base steel sheet 3 side).

[0041] At this time, the Sn average content line 10 of the plating layer 1 can mean an extension line of the Sn average content line for the section from a point 8 μm away from the plating layer 1 side to a point 15 μm away from the point 200 (the point of Sn) where the Sn content is the maximum value in the Sn-enriched layer 2. max of the point) to the point 15 μm away from the point 8 μm away from the plating layer 1 side.

[0042] Similarly, the Sn enrichment layer 2 and the base steel plate 3 are also measured using the above GDS in the same manner as the above-described method. In the rising interval 22 of the Sn content in the x-axis (-) direction provided between the base steel plate 3 and the plating layer 1, the Sn average content line 30 of the base steel plate and the last contact point 31 in the x-axis (-) direction of the Sn content line 100 measured using the GDS are used to define the Sn enrichment layer 2 (in the thickness direction on the plating layer 1 side).

[0043] At this time, the Sn average content line 30 of the base steel plate 3 can be defined as the extension line of the Sn average content line for the interval from the point 8 μm away from the point 200 (the point where Sn is at its maximum value in the Sn enrichment layer 2) on the base steel plate 3 side to the point 15 μm away from it. max The inventors have conducted intensive studies and found that reducing the amount of diffusible hydrogen occluded during the heat treatment of hot forming can suppress the induction of defects due to hydrogen 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 occluded in the steel while it has an austenite phase with a high hydrogen solubility at high temperatures. However, when rapid cooling occurs due to hot forming and the phase changes to martensite, the solubility of hydrogen decreases rapidly, 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, increasing the possibility of crack generation due to hydrogen delayed fracture. Thus, reducing the amount of diffusible hydrogen occluded during heat treatment is an important factor for defect suppression.

[0044] As a result of intensive studies by the present inventors, it has been found that reducing the amount of diffusible hydrogen occluded during the heat treatment of hot forming can suppress the induction of defects due to hydrogen 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. Next, hydrogen generated by the dissociation of water is occluded in the steel while it has an austenite phase with a high hydrogen solubility at high temperatures. However, when rapid cooling occurs due to hot forming and the phase changes to martensite, the solubility of hydrogen decreases rapidly, 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, increasing the possibility of crack generation due to hydrogen delayed fracture. Thus, reducing the amount of diffusible hydrogen occluded during heat treatment is an important factor for defect suppression.

[0045] Furthermore, as a result of several investigations, the inventors confirmed that the impact resistance tends to increase as the hydrogen content in the steel decreases. This is because diffusible hydrogen occluded 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 the formation of an Sn-enriched layer having an appropriate concentration and thickness between the base steel sheet and the plating layer enables the achievement of such an effect. This is because the Sn-enriched layer serves as an effective protective film that relatively reduces the amount of diffusible hydrogen occluded in the steel.

[0047] According to an embodiment of the present invention, in order to effectively reduce the hydrogen content in the steel, improve hydrogen embrittlement resistance, and ensure excellent impact resistance, the hot-forming plated steel sheet preferably satisfies the following relational expressions 1-1 and 1-2. At this time, the unit of the following relational expression 1-2 corresponds to [μm*wt%] (the same applies to relational expressions 2-2 and 2-3 described later).

[0048] [Relational expression 1-1] [Number] [In the above relational expression 1-1, the above Sn coat represents the average Sn content in the above plating layer, and the unit is weight%. Also, the above Sn max represents the maximum value of the Sn content in the above Sn-enriched layer, and the unit is weight%.]

[0049] [Relational expression 1-2] [Number] In the above relational expression 1-2, Sn(x) represents the Sn content along the x-axis direction from any point in the plating layer to the substrate steel sheet side, x1 represents the x-axis point at the boundary between the plating layer and the Sn enrichment layer, and the above x2 represents the x-axis point at the maximum value of the Sn content in the Sn enrichment layer.

[0050] That is, in order to exhibit the intended effects of the present invention, the Sn concentration and thickness of the Sn enrichment layer provided between the substrate 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 Sn coat Sn max Sn(x), x1, x2, etc. can be measured from the obtained data when analyzing the change in the Sn content in the thickness direction of the substrate steel sheet at any point in the plating layer by utilizing the above-described glow discharge spectroscopy (GDS). In other words, the above Sn coat can correspond to the Sn average content line 10 of the plating layer 1 described above in the GDS profile of FIG. 2 of the present application, and the above Sn max can correspond to the point 200 where the Sn content in the Sn enrichment layer 2 is the maximum value in the GDS profile of FIG. 2 of the present application.

[0051] In the above relational expression 1-1,

Number

Number

[0052] On the one hand, in FIG. 1, the area corresponding to the above relational expression 1-2 is represented by the hatched portion. That is, in FIG. 1 described above, the area of the hatched portion is the integral value with respect to Sn(x) which is a function showing the change in the Sn content along the x-axis direction from the base steel plate side from any one point within the plating layer from x1 (the x-axis point at the boundary 11 between the plating layer and the Sn enrichment layer) to x2 (the x-axis point at the maximum value 200 of the Sn content in the Sn enrichment layer) (that is, in the present invention, it can mean the area of the corresponding region obtained by utilizing the piecewise integration method). At this time, the above integral value can be obtained by calculation using the piecewise integration method per unit length (or unit depth) of 0.01 μm of the x-axis.

[0053] Alternatively, according to an embodiment of the present invention, more preferably, Sn defined by the above relational expression 1-1 max / Sn coat The lower limit of the value may be 1.53, and more preferably, Sn defined by the above relational expression 1-1 max / Sn coat The upper limit of the value may be 13.0.

[0054] Also, according to an embodiment of the present invention, more preferably, the

Number

Number

[0055] Also, according to an embodiment of the present invention, although not particularly limited, in the above plating steel plate, the thickness of the above Sn enrichment layer may be in the range of 1 to 20 μm (that is, 1 μm or more and 20 μm or less).

[0056] In the above-mentioned plated steel sheet, if the thickness of the Sn-enriched layer is less than 1 μm, even if heat treatment for subsequent hot forming is performed, a sufficient Sn-enriched layer cannot be formed, and it may be difficult to expect the effects of improving hydrogen embrittlement resistance and collision resistance. Further, if the thickness of the Sn-enriched layer exceeds 20 μm, Sn may excessively precipitate at the grain boundaries after hot forming, acting as a crack site during stress generation and potentially resulting in poor collision resistance. On the other hand, from the perspective of further improving the above-mentioned effects, in the above-mentioned plated steel sheet, the lower limit of the thickness of the Sn-enriched layer may be 3 μm, and the upper limit of the thickness of the Sn-enriched layer may be 15 μm.

[0057] On the other hand, as a result of intensive studies by the present inventors to further improve the physical properties of the plated steel sheet and the members, it has been found that the performance is improved by concentrating an appropriate amount of Sn in the manganese segregation band present in the base steel sheet of the plated steel sheet. That is, as a result of examining various conditions, the present inventors have confirmed that such an effect is further enhanced when the Sn concentration 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.

[0058] Specifically, according to one embodiment of the present invention, in the above-mentioned plated steel sheet, the base steel sheet includes a Mn segregation band, and in the Mn segregation band, the region where the average Sn content is 1.015 times or more the average Sn content in the base steel sheet may be 50% or more (alternatively, 60% or more) in terms of area fraction. By satisfying this, the generation of inclusions such as MnS mainly generated in the Mn segregation band can be reduced, and it plays a role in suppressing the generation and propagation sites of cracks during stress generation, ensuring excellent collision resistance. Further, when MnS is excessively generated, there may be a problem that the brittle fracture surface excessively develops. Therefore, by satisfying the above-mentioned configuration during Sn concentration, MnS can be reduced, thereby reducing the brittle fracture surface and further improving the bendability.

[0059] At this time, the upper limit of the average Sn content in the Mn segregation band is not particularly limited. As an example, it may be 5 times or less the average Sn content in the area of the base steel sheet other than the Mn segregation band. Also, although not particularly limited, the upper limit of the area of the portion of the Mn segregation band where the average Sn content is 1.015 times or more the average Sn content in the base steel sheet may be 90%.

[0060] As shown in FIG. 5, the 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 Mn0. Thereafter, among the points measured by EPMA, when a square with an area of 0.4 μm 2 is drawn centered on a specific point, if the area of the region where the Mn intensity is 1.015 times or more that of Mn0 among the points located within the square is 50% or more, the specific point is defined as an Mn segregation point, and if it is less than 50%, it is defined as not an Mn segregation point. The region formed by connecting the outermost Mn segregation points with a straight line by gathering such Mn segregation points is defined as the Mn segregation band.

[0061] In addition, the ratio (Sn2 / Sn1) of the average Sn content (Sn2) of the Mn segregation band to the average Sn content (Sn1) of the base steel sheet can be measured by utilizing the ratio of the intensity based on the component mapping results of Sn using EPMA.

[0062] Further, according to one embodiment of the present invention, the thickness of the Mn segregation band may be 20 μm or less. 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 may be 1 μm. At this time, the thickness of the 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. At this time, from the viewpoint of further improving the above-described effects, in the electroplated steel sheet, the lower limit of the thickness of the Mn segregation band may be 3.0 μm, or the upper limit of the thickness of the Mn segregation band may be 13.4 μm.

[0063] On the other hand, a hot-forming electroplated steel sheet having the above-described configuration can be used to manufacture a hot-formed member having excellent hydrogen embrittlement resistance and impact resistance by the hot press forming method described below.

[0064] A hot-formed member according to one embodiment of the present invention includes a base steel sheet having the same alloy composition as the base steel sheet of the electroplated steel sheet described above, an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel sheet, and a Sn-enriched 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 Sn-enriched 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 the units, and it is only necessary to satisfy the units of each variable.

[0065] [Relational Expression 2-1] [Number] [In the above relational expression 2-1, the above Sn coat represents the average Sn content in the plating layer, and the unit is wt%. Also, the above Sn maxrepresents the maximum value of the Sn content in the Sn enrichment layer, with the unit of weight%.

[0066] [Relational Expression 2-2] [Number] [In the above Relational Expression 2-2, Sn(x) represents the Sn content along the x-axis direction from any point in the plating layer to the base steel plate side, x1 represents the x-axis point at the boundary between the plating layer and the Sn enrichment layer, and x2 represents the x-axis point at the maximum value of the Sn content in the Sn enrichment layer.]

[0067] In the present invention, when the plated steel sheet is heated for hot forming, the degree of Sn enrichment in the Sn enrichment layer becomes deeper. Therefore, in the hot-formed member according to the present invention, by satisfying the above Relational Expressions 2-1 and 2-2, the hydrogen content in the steel can be effectively reduced, thereby improving the hydrogen embrittlement resistance and collision resistance. At this time, in the hot-formed member, the method for distinguishing the plating layer and the Sn enrichment layer and the method for distinguishing the base steel plate and the Sn enrichment layer can be similarly applied to the above-described distinguishing method for the plated steel sheet.

[0068] Alternatively, according to an embodiment of the present invention, more preferably, the Sn max / Sn coat value may have a lower limit of 1.83, and more preferably, the Sn max / Sn coat value may have an upper limit of 13.46.

[0069] Also, according to an embodiment of the present invention, more preferably, the [Number] value may have a lower limit of 0.024, and more preferably, the [Number] The upper limit of the value may be 0.352.

[0070] Alternatively, according to an embodiment 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.

[0071] [Relational Expression 2-3]

Number

[0072] According to an embodiment of the present invention, in the above hot-formed member, the microstructure of the base steel plate can include 5% or less (including 0%) of ferrite and the balance martensite. Alternatively, it can further include other phases such as 1% or less of upper bainite, retained austenite, cementite, and pearlite.

[0073] According to an embodiment of the present invention, the base steel plate can contain 5% or less (including 0%) of ferrite in terms of area fraction. This can manage the ferrite fraction to 5% or less by ensuring the hardenability through steel component adjustment and a sufficient cooling rate in hot forming. On the other hand, in the hot-formed member, if the ferrite fraction of the base steel plate exceeds 5%, not only does the strength decrease, but relatively, local stress concentrates in the soft ferrite, which may promote crack propagation and significantly deteriorate the impact resistance.

[0074] Also, according to an embodiment of the present invention, in the above hot-formed member, the thickness of the Sn-enriched layer may be 2 to 30 μm.

[0075] If the thickness of the Sn-enriched layer in the hot-formed member is less than 2 μm, hydrogen that penetrates into the steel during hot forming cannot be effectively suppressed, and there is a possibility that the effects of improving hydrogen embrittlement resistance and collision resistance cannot be fully exerted. Further, if the thickness of the Sn-enriched layer in the hot-formed member exceeds 30 μm, Sn not only forms the enriched layer but may also precipitate excessively at the grain boundaries of the surface layer of the base iron, which may promote the generation and propagation of cracks during bending and reduce the collision resistance. On the other hand, from the viewpoint of further improving the above-described effects, in the hot-formed member, the lower limit of the thickness of the Sn-enriched layer may be 3.4 μm, and the upper limit of the thickness of the Sn-enriched layer may be 25 μm.

[0076] Further, according to an embodiment of the present invention, the diffusible hydrogen content of the hot-formed member may be 0.4 ppm or less, whereby excellent hydrogen embrittlement resistance can be ensured. Such a diffusible hydrogen content of 0.4 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 occur in the parts.

[0077] Further, according to an embodiment of the present invention, in the hot-formed member, the base steel plate includes an Mn segregation band, and in the Mn segregation band, the region where the average Sn content is 1.015 times or more the average Sn content in the base steel plate may be 50% or more in area fraction (alternatively, more preferably 60% or more). By satisfying this, the generation of inclusions such as MnS mainly generated in the Mn segregation band is reduced, and it plays a role in suppressing the generation and propagation sites of cracks during stress generation, and excellent collision resistance can be ensured. Further, if MnS is excessively generated, there may be a problem that a brittle fracture surface is excessively developed. Therefore, by satisfying the above-described configuration during Sn enrichment, MnS can be reduced, and thereby the brittle fracture surface is reduced and the bendability can be further improved.

[0078] At this time, the upper limit of the average Sn content in the Mn segregation band is not particularly limited. As an example, it may be 5 times or less than the average Sn content in the base steel plate region outside the Mn segregation band. Note that, although not particularly limited, the upper limit of the area of the portion in the Mn segregation band where the average Sn content is 1.015 times or more the average Sn content in the base steel plate may be 95%.

[0079] Further, according to one embodiment of the present invention, in the hot-formed member, the thickness of the Mn segregation band may be 15 μm or less. 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 necessarily need to be separately limited. However, as an example, the lower limit of the thickness of the Mn segregation band may be 1.5 μm.

[0080] On the other hand, from the viewpoint of further improving the above-described effects, in the hot-formed member, the lower limit of the thickness of the Mn segregation band may be 4.1 μm, or the upper limit of the thickness of the Mn segregation band may be 15.0 μm.

[0081] At this time, in the hot-formed member, the definition of the Mn segregation band, the ratio of the average Sn content in the Mn segregation band to the average Sn content in the base steel plate, and the measurement of the thickness of the Mn segregation band can be applied in the same manner as the measurement methods and measurement standards in the above-described plated steel plate.

[0082] Next, a method for manufacturing a hot-formed plated steel sheet, which is still another embodiment of the present invention, will be described.

[0083] The hot-formed plated steel sheet according to an embodiment 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 750 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 hot-rolled steel sheet at 700 to 860°C under the dew point temperature condition of -75 to -20°C in an annealing furnace, and after annealing, passing the hot-rolled steel sheet through a plating bath made of aluminum or an aluminum alloy for plating.

[0084] 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 costs for removing the oxide layer, and the possibility of surface defects occurring after finish rolling becomes high.

[0085] Finish rolling step Finish rolling should be performed at 750 to 950°C. If the finish rolling temperature is less than 750°C, two-phase region rolling progresses, ferrite is introduced into the surface layer 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, excessive hot scale is generated, and the manufacturing cost of the subsequent processes for this increases, so it is not appropriate.

[0086] Coiling step After finishing finish rolling, the hot-rolled steel sheet is coiled at 500 - 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 and not only that, but there is also a problem that the subsequent cold rolling property deteriorates due to excessive strength increase 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-forming member, so there is a problem that the impact resistance deteriorates.

[0087] The stage of pickling treatment The coiled hot-rolled steel sheet is pickled so that the product of the acid concentration and the pickling time is 800 - 10,000 g / L*s. Although a Sn-enriched layer is formed on the steel sheet that has undergone the above-described reheating, finish rolling, and coiling steps, when the product of the acid concentration and the pickling time is applied within the range of 800 - 10,000 g / L*s in the pickling treatment process, the Sn-enriched 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.

[0088] 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 is not 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 Sn-enriched layer is lost during pickling, not only failing to exhibit the expected effect but also potentially leading to an increase in manufacturing cost, so its upper limit is set to 10,000 g / L*s. However, when 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. On the other hand, from the perspective of further improving the above-described effect, the lower limit of the product of the acid concentration and the pickling time may be 900 g / L*s, or the upper limit of the product of the acid concentration and the pickling time may be 4,200 g / L*s.

[0089] According to an embodiment 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, there are hydrochloric acid (HCl), sulfuric acid (H2SO4), etc. 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.

[0090] On the other hand, although not particularly limited, according to an embodiment of the present invention, the concentration of the above acid may be in the range of 40 to 500 g / L. If the acid concentration is less than 40 g / L, the surface scale generated during hot rolling may not be sufficiently removed during the limited pickling time, and defects may occur in the surface layer portion of the steel plate. On the contrary, if the acid concentration exceeds 500 g / L, due to the loss of the Sn-enriched layer, it may be difficult to exert the intended effect in the final hot forming member, and defects in the surface layer portion may be induced due to over-pickling. On the other hand, from the perspective of further improving the above effects, the lower limit of the concentration of the above acid may be 150 g / L, or the upper limit of the concentration of the above acid may be 250 g / L.

[0091] Also, although not particularly limited, according to an embodiment of the present invention, the pickling time may be 5 to 60 seconds (s). If the pickling time is less than 5 seconds, the surface scale of the steel plate may not be sufficiently removed, and defects in the surface layer portion may be induced. If the pickling time exceeds 60 seconds, due to the loss of the Sn-enriched layer, it may reduce productivity and increase the process cost. On the other hand, from the perspective of further improving the above effects, the pickling time may be 5 to 25 seconds.

[0092] Further, although not particularly limited, according to one embodiment 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 other hand, if the pickling temperature exceeds 120°C, not only does the fixed cost increase to maintain the high temperature, but there may also be a problem that the vaporization amount of the pickling solution increases due to the high temperature, and the cost for replenishing the lost pickling solution increases. On the other hand, from the perspective of further improving the above-mentioned effects, the lower limit of the pickling temperature may be 50°C, or the upper limit of the pickling temperature may be 100°C.

[0093] 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 after plating may be too high, which may induce die wear 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, inducing defects on the Al-Si plated surface. Or, from the perspective of further improving the effect of suppressing die wear during the blanking process and plating surface defects, the annealing temperature may be 750 to 860°C, or may be 800 to 860°C.

[0094] Also, the atmosphere during annealing 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-mentioned 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 is required to control the dew point, 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 defects in surface quality such as non-plated. On the other hand, from the perspective of further improving the above-mentioned effects, during annealing, the lower limit of the dew point temperature may be -55°C, or the upper limit of the dew point temperature may be 0°C.

[0095] Plating step After the annealing process, immediately perform Al-Si plating. 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 steel sheet for hot press forming without limitation as long as they are the plating conditions usually applied. As an example, the composition of the plating bath can contain Si: 6 - 12%, Fe: 1 - 4%, and the balance Al and other inevitable impurities.

[0096] At this time, although not particularly limited, in the plating step, the plating amount is usually 15 - 140 g / m based on one side 2 which is preferably set. If it is less than 15 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.

[0097] Cold rolling step The method for manufacturing a plated steel sheet for hot press forming according to an embodiment of the present invention can further include a step of cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet after the pickling treatment step described above.

[0098] Continuous casting step The method for manufacturing a plated steel sheet for hot press forming according to the present invention can further include a step of performing light 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 applying light reduction during continuous casting to produce a slab. This is because when slab segregation occurs excessively, a thick and concentrated segregation band is formed up to the final hot-formed 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 within the segregation band.

[0099] 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 ratio during soft reduction by continuous casting to 0.5% to 5%. If the total reduction ratio during continuous casting is less than 0.5%, almost no reduction is carried out and the central segregation cannot be sufficiently removed, so the impact resistance in the hot-formed member may be inferior. On the contrary, if the total reduction ratio during continuous casting exceeds 5%, there may be a problem with the reduction roll equipment, and there is a risk of promoting equipment failure and aging.

[0100] The hot-formed plated steel sheet manufactured by the above manufacturing method can be hot-press formed to manufacture a hot-formed member excellent in hydrogen embrittlement resistance and impact resistance. Specifically, a method for manufacturing a final member excellent in hydrogen embrittlement resistance and impact resistance by hot forming and die quenching using the plated steel sheet manufactured by the above method will be described. A blank for hot forming is manufactured using the plated steel sheet manufactured by the above steel composition and manufacturing method. The 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 is difficult to ensure strength and impact resistance due to the presence of undissolved ferrite associated with the two-phase region. On the contrary, if the heating temperature exceeds 975°C, excessive oxides are generated on the member surface, it is difficult to ensure spot weldability, and the manufacturing cost for maintaining a high temperature increases. On the other hand, in the present invention, the temperature of the Ac3 transformation point can be calculated by the following relational expression 3.

[0101] [Relational expression 3] Ac3 [°C] = 881 - 206×[C] + 53×[Si] - 15×[Mn] - 1×[Cr] + 41×[Mo] [In the above relational expression 3, [C], [Si], [Mn], [Cr], and [Mo] each independently represent the average weight% content of each element in parentheses contained in the base iron. Among the above elements, the content of the element not contained is calculated as "0".]

[0102] Thereafter, it is preferable to hold the heated blank within the above temperature range for 1 to 1000 seconds. If the holding time is less than 1 second, it becomes difficult to achieve 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 excessive oxide formation on the member surface, but it also induces an increase in the manufacturing cost of the member.

[0103] Transfer the blank heated in this way to a press, and perform hot forming and die quenching at a cooling rate of -20°C / s or more to manufacture the final member. At this time, if the cooling rate is 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 impact resistance. There are no particular limitations on the above-described transfer, hot forming, and cooling steps of the blank, and the commonly used hot forming methods can be directly applied.

[0104] In the hot-formed member manufactured in this way, a Sn-enriched layer in which Sn is concentrated is formed in the concentration gradient layer existing between the base steel plate and the plating layer, thereby reducing the diffusible hydrogen content in the steel and manufacturing a hot-formed member excellent in hydrogen embrittlement resistance and impact resistance.

Example

[0105] 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.

[0106] (Experimental Example 1) The composition of the steel used for the present invention is shown in Table 1 below (corresponding to the balance Fe and other impurities), and slabs having respective compositions were manufactured by vacuum melting at a thickness of 60 mm. After holding such slabs at 1200 °C for 1 hour, hot rolling was performed to a finish temperature of hot rolling of 900 °C and coiling was performed at a coiling temperature of 580 °C. Thereafter, as a pickling step, pickling was performed at a HCl concentration of 200 g / L and a pickling time of 20 seconds, and after batch application at 4,000 g / L·s where the product of the HCl concentration and the pickling time is within the range of the present invention, annealing was performed under the conditions shown in Table 2 below, and immersion was performed in a plating bath composed of Al-9% Si-2% Fe and the balance being trace impurities for plating. In this example, hot forming was performed at various temperatures, the heating time was applied as 6 minutes, the transfer time to the mold was applied as 10 seconds, and then die quenching was applied.

[0107] For the test piece manufactured as described above, a tissue analysis for ferrite observation was performed, and in order to confirm the Sn-enriched layer near the surface, using GDS850A (model name, manufactured by LECO Corporation), DC and FR equipment, by glow discharge spectrometry (GDS), as shown in FIG. 3, an Sn-enriched layer at the interface between the base steel plate and the plating layer was detected. At this time, from the above GDS experimental results, it was confirmed that there exists a concentration gradient layer in the region where Fe changes by 0.3 wt% / μm or more within the range of 20 to 99% based on the gravimetric method in the thickness direction (meaning the direction perpendicular to the rolling direction) between the base steel plate and the plating layer. Further, it was confirmed that an Sn-enriched layer exists within such a concentration gradient layer.

[0108] Based on this result, as shown in FIG. 1, the parameters

Number

Number

[0109] Also, for each example and comparative example, measurements were taken using a TDA (Thermal Desorption Analysis) device (Bruker G8; model name) to confirm the amount of diffusible hydrogen. At this time, the temperature was raised to 400 °C at a rate of 20 °C / min, and the time was held so that the diffusible hydrogen peak would fully appear to measure the diffusible hydrogen curve. The total amount of diffusible hydrogen in the steel was obtained by integrating such a curve.

[0110] On the other hand, a three-point bending test was conducted applying the VDA238-100 standard, which is one of the indicators capable of showing the impact resistance of the hot-formed member. Also, as shown in the following relational expression 4, P2, which is the product of the maximum load value and the bending angle at the maximum load value, was calculated to confirm the effect of improving the bendability. At this time, when the P2 value was 35,000 kgf*° or more, it was indicated as "good", and when it was less than 35,000 kgf*°, it was indicated as "bad". It can be understood that when the maximum load value or the bending angle due to it increases during the above-described bending test, the occurrence of cracks is relatively reduced during a collision, thereby improving the impact resistance. Therefore, in order to analyze the effect of improving the impact resistance for various components including the addition of Sn, this was represented by P2.

[0111] [Relational Expression 4] P2 = Maximum load [kgf] × Maximum bending angle [°]

[0112]

Table 1

[0113]

Table 2

[0114]

Table 3

[0115] As shown in Tables 1 to 3 above, in Examples 1 to 6 that satisfy the alloy composition and manufacturing conditions of the present invention, 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 it was confirmed that the impact resistance is excellent.

[0116] On the other hand, in the case of Comparative Examples 1 to 3, since the Sn content in the steel does not reach the scope of the present invention, in the plated steel sheet and the hot-formed member

Number

Number

[0117] On the other hand, regarding the above-described Examples and Comparative Examples, the effect of improving the impact resistance by forming the Sn-enriched layer is shown in FIG. 4, and it can be confirmed that the tendency for the impact resistance to increase due to the formation of the Sn-enriched layer in the hot-formed member clearly appears. Thereby, it can be understood that the greater the increase in the P1 value, the more the Sn-enriched layer serves as an effective protective film for reducing the amount of diffusible hydrogen in the steel, ensuring excellent hydrogen embrittlement resistance and excellent impact resistance.

[0118] (Experimental Example 2) Test pieces were manufactured in the same manner as in Experimental Example 1 above, except that the pickling concentration and pickling time, and the hot-forming temperature were changed to the conditions shown in Table 4 below. At this time, the pickling temperature was uniformly applied at 80°C.

[0119] For each of the examples and comparative examples 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 Sn content in the thickness direction of the base steel sheet at 10 points on the surface of the plating layer by GDS, the method described in the specification was similarly applied to measure the thickness of the Sn-enriched layer (that is, in the schematic diagram of Figure 2, from the last contact point 11 in the x-axis (+) direction of the Sn average content line 10 of the plating layer in the Sn content increasing interval 21 in the x-axis (+) direction of the Sn-enriched layer 2 to the Sn average content line 30 of the base steel sheet in the Sn content increasing interval 22 in the x-axis (-) direction of the Sn-enriched layer 2, the linear distance in the thickness direction up to the last contact point 31 in the x-axis (-) direction of the Sn content line 100 by GDS was measured). The average thickness of the Sn-enriched layer for the above 10 points was measured and shown in Table 5 below.

[0120] For the evaluation of each of the examples and comparative examples, the diffusible hydrogen amount and impact resistance were evaluated in the same manner as in Experimental Example 1 described above. Further, in order to evaluate the surface characteristics (that is, the presence or absence of surface defects), the presence or absence of remaining hot-rolled scale after pickling was evaluated. To confirm the presence or absence of remaining hot-rolled 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".

[0121]

Table 4

[0122]

Table 5

[0123]

Table 6

[0124] As shown in Tables 4 to 6 above, Comparative Example 4, which does not satisfy the Sn content of the present invention and has a product of acid concentration and pickling time of less than 800 g / L*s, was confirmed to have incomplete removal of the hot-rolled scale and a high possibility of causing surface defects during the subsequent process.

[0125] In addition, Comparative Examples 5 and 6, in which the product of 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 is increased, resulting in poor hydrogen embrittlement resistance and poor impact resistance.

[0126] On the other hand, in Examples 7 to 10 of the present application, since the steel composition of the present invention and the product of acid concentration and pickling time are 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. As a result, not only are the surface characteristics excellent, but also the amount of diffusible hydrogen in the steel is reduced, resulting in excellent hydrogen embrittlement resistance and excellent impact resistance.

[0127] (Experimental Example 3) Before reheating the slab, the slab was produced by lightly pressing 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 characteristic was evaluated in the same manner as in Experimental Example 1 above, and further, the thickness of the Mn segregation band in the steel plate and the Sn content in the Mn segregation band for the plated steel plate and the hot-formed member were measured and shown in Table 7 below.

[0128] In particular, the thickness of the Mn segregation band and the area of the portion of the Mn segregation band where the average Sn content is 1.015 times or more the average Sn content in the base steel plate were measured by applying the method described above in the specification and utilizing the EPMA (Electron Probe X-ray Micro Analyzer) method. An example of the component mapping results for such Mn and Sn (for Invention Example 12) is shown in FIGS. 5 and 6, respectively.

[0129] At this time, the surface characteristics, the amount of diffusible hydrogen, and the impact resistance were measured in the same manner as in the above-described method.

[0130] Further, in order to evaluate the formability, a bending test was conducted 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 from the surface down to 100 μm using 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 "×".

[0131] [Table 7]

[0132] [Table 8]

[0133] [Table 9]

[0134] Ma*: In the plated steel sheet, among the Mn segregation bands, the area [%] of the portion where the average Sn content is 1.015 times or more the average Sn content in the base steel sheet Mb*: In the hot-formed member, among the Mn segregation bands, the area [%] of the portion where the average Sn content is 1.015 times or more the average Sn content in the base steel sheet

[0135] [Table 10]

[0136] As shown in Tables 7 to 10 above, in Comparative Example 7, since the added Sn content was less than the scope of the present invention and Sn was not sufficiently concentrated in the Mn segregation band, poor impact resistance occurred.

[0137] In the case of Comparative Example 8, the total reduction ratio during continuous casting exceeded 5%, a failure occurred in the continuous casting press machine, making continuous casting impossible, and thus experimental evaluation was not possible due to this.

[0138] On the other hand, Examples 11 to 14 have excellent surface characteristics, a small amount of diffusible hydrogen, and excellent hydrogen embrittlement resistance by satisfying the ranges of Relational Expressions 1-1, 1-2, 2-1, and 2-2 of the present application.

[0139] In particular, among the above-described examples, in the case of the plated steel sheet, when the thickness of the Mn segregation band is 20 μm or less (or in the case of the hot-formed member, when the thickness of the Mn segregation band is 15 μm or less), or in the plated steel sheet and the hot-formed member, when the area of the portion of the Mn segregation band where the average Sn content is 1.015 times or more the average Sn content in the base steel sheet is 60% or more, in the case of Examples 12 to 14, it was confirmed that the formability was further improved compared to Example 11 that does not satisfy one or more of the above-described conditions and Comparative Example 7 that does not satisfy all of the above-described conditions.

[0140] From the above-described experimental examples, according to the present invention, despite adding a small amount of Sn to the 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. Such parts can be applied and utilized in various fields including the automotive manufacturing field as structural materials or strengthening parts.

Explanation of Reference Numerals

[0141] 1: Plating layer 2: Sn-enriched layer 21: Rising section of Sn content in the x-axis (+) direction in the Sn-enriched layer 22: Rising section of Sn content in the x-axis (-) direction in the Sn-enriched layer 3: Base steel sheet 10: Average Sn content line of the plating layer 11: Last contact point in the x-axis (+) direction between the average Sn content line of the plating layer and the Sn content line by GDS 30: Average Sn content line of the base steel sheet 31: The Sn average content line of the base steel plate and the last contact point in the x-axis (-) direction of the Sn content line by GDS 100: Sn content line by GDS 200: In the Sn enrichment layer, the point where the Sn content is the maximum value

Claims

1. By weight, C: 0.07 to 0.5%, Si: 0.05 to 1%, Mn: 0.5 to 5%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, Cr: 0.01 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, and Sn: 0.01 to 0.1%, consisting of a base steel plate composed of the balance Fe and other inevitable impurities, and an aluminum or aluminum alloy plating layer provided on at least one surface of the base steel plate, and a Sn enrichment layer provided between the base steel plate and the plating layer, and a plated steel sheet for hot forming that satisfies the following relational expressions 1-1 and 1-2. [Relational expression 1-1] 【Equation 1】 [In the relational expression 1-1, the Sn ｃｏａｔ represents the average Sn content in the plating layer, and the unit is % by weight. Also, the Sn ｍａｘ represents the maximum value of the Sn content in the Sn enrichment layer, and the unit is % by weight. ] [Relational expression 1-2] 【Equation 2】 [In the relational expression 1-2, the Sn(x) represents the Sn content (% by weight) along the x-axis direction from any point in the plating layer toward the base steel plate side, x1 represents the x-axis point (μm) at the boundary between the plating layer and the Sn enrichment layer, and the x2 represents the x-axis point (μm) at the maximum value of the Sn content in the Sn enrichment layer. ]

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

3. The base steel plate includes an Mn segregation band, and the Mn segregation band is a band structure formed inside the base steel plate with reference to the cross-section in the thickness direction, The hot-formed plated steel sheet according to claim 1 or claim 2, wherein, among the Mn segregation bands, the area of the portion where the average Sn content is 1.015 times or more the average Sn content in the base steel sheet is 60% or more.

4. The hot-formed plated steel sheet according to claim 3, wherein the thickness of the Mn segregation band is 20 μm or less based on the cross-section in the thickness direction of the base steel sheet.

5. By weight, C: 0.07 to 0.5%, Si: 0.05 to 1%, Mn: 0.5 to 5%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, Cr: 0.01 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, and Sn: 0.01 to 0.1%, reheating a steel slab composed of the balance Fe and other unavoidable impurities to 1050 to 1300 °C; Finishing rolling the heated steel slab at 750 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 acid concentration is 40 to 500 g / L and 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 dew point temperature conditions of -75 to +20 °C in an annealing furnace; After annealing, passing the steel sheet through a plating bath composed of aluminum or an aluminum alloy for plating; The method for manufacturing a hot-formed plated steel sheet according to claim 1, comprising:

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

7. The method for manufacturing a hot-formed plated steel sheet according to claim 5 or claim 6, wherein the pickling temperature is 40 to 120 °C.

8. The manufacturing method of the hot-forming plated steel sheet according to any one of claims 5 to 7, further comprising a step of continuous casting with light rolling at a reduction rate of 0.5 to 5% before the reheating step.

9. In terms of weight%, C: 0.07 to 0.5%, Si: 0.05 to 1%, Mn: 0.5 to 5%, P: 0.001 to 0.015%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, Cr: 0.01 to 1%, N: 0.001 to 0.02%, Ti: 0.1% or less, B: 0.01% or less, and Sn: 0.01 to 0.1%, consisting of a base steel sheet composed of 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 a Sn-enriched layer provided between the base steel sheet and the plating layer. A hot-forming member satisfying the following relational expressions 2-1 and 2-2. [Relational expression 2-1] 【Equation 3】 [In the relational expression 2-1, the Sn ｃｏａｔ represents the average Sn content in the plating layer, and the unit is weight%. Also, the Sn ｍａｘ represents the maximum value of the Sn content in the Sn-enriched layer, and the unit is weight%.] [Relational expression 2-2] 【Equation 4】 [In the relational expression 2-2, the Sn(x) represents the Sn content (weight%) along the x-axis direction from any point in the plating layer to the base steel sheet side, x1 represents the x-axis point (μm) at the boundary between the plating layer and the Sn-enriched layer, and the x2 represents the x-axis point (μm) at the maximum value of the Sn content in the Sn-enriched layer.]

10. The hot-forming member according to claim 9, wherein the base steel sheet contains ferrite in an area fraction of 5% or less.

11. The hot-forming member according to claim 9 or claim 10, wherein the thickness of the Sn-enriched layer is 2 to 30 μm.

12. The hot-forming member according to any one of claims 9 to 11, which satisfies the following relational expression 2-3. [Relational expression 2-3] [Formula 5] [In the relational expression 2-3, Sn(x) represents the Sn content (wt%) along the x-axis direction from any point in the plating layer to the base steel plate side, x1 represents the x-axis point (μm) at the boundary between the plating layer and the Sn-enriched layer, and x2 represents the x-axis point (μm) at the maximum value of the Sn content in the Sn-enriched layer.]

13. The hot-forming member according to any one of claims 9 to 12, wherein the diffusible hydrogen amount is 0.4 ppm or less.

14. The base steel plate includes an Mn segregation band, The Mn segregation band is a band structure formed inside the base steel plate with reference to the cross-section in the thickness direction, The hot-forming member according to any one of claims 9 to 13, wherein the area of the portion of the Mn segregation band where the average Sn content is 1.015 times or more the average Sn content in the base steel plate is 60% or more.

15. The hot-forming member according to claim 14, wherein the thickness of the Mn segregation band is 15 μm or less with reference to the cross-section in the thickness direction of the base steel plate.

16. A method for manufacturing a hot-forming member, wherein the hot-forming plated steel sheet according to any one of claims 1 to 4 is heat-treated at a temperature range of Ac3 to 950 °C for 1 to 1000 seconds and then hot press-formed.

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