Plated steel sheet for hot press forming

Abstract

To provide a plated steel sheet for hot press forming having excellent impact properties after hot press forming.SOLUTION: There is provided a plated steel sheet for hot press forming comprising: a base steel sheet; and a plating layer composed of zinc, aluminum, or an alloy containing zinc and aluminum, formed on the surface of the base steel sheet, wherein the ratio (CS / CB) of the content (CS) of C in a surface layer portion to the content (CB) of C in the base steel sheet is 0.05 to 0.6, and the ratio ((MnS+CrS) / (MnB+CrB)) of the total content (MnS+CrS) of Mn and Cr in the surface layer portion to the total content (MnB+CrB) of Mn and Cr in the base steel sheet is 0.8 to 2, wherein the surface layer portion refers to a region to a depth of 15 μm from the surface of the base steel sheet excluding the plating layer.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention is a hot rolling method that can be suitably applied to automobile parts and the like that require impact resistance. Hot-formed plated steel sheets with excellent impact properties after forming, hot-formed components, and their manufacturing methods It is related to. [Background technology]

[0002] In recent years, with the depletion of petroleum energy resources and growing concern about the environment, automobile fuel efficiency has Regulations on fuel economy are becoming stronger every day. One way to achieve this is to reduce the thickness of the steel plate used. When reducing the thickness, it is necessary to reduce the thickness of the steel sheet, as this may cause problems with the safety of the automobile. Strength improvement must be ensured.

[0003] For these reasons, there is a continuous demand for high-strength steel plates, and various types of steel plates are being developed. However, such steel sheets have poor workability due to their high strength. In other words, the product of strength and elongation is always a constant value for each grade of steel sheet. Therefore, when the strength of the steel sheet increases, it becomes an index of workability. However, there was a problem in that the stretching ratio was reduced.

[0004] In order to solve this problem, a hot press forming method has been proposed. is a method of processing steel sheets at high temperatures that make them easier to process, and then quenching them at low temperatures to make the steel sheets This method forms low-temperature structures such as martensite within the steel, increasing the strength of the final product. In this case, workability problems can be minimized when manufacturing high strength components. This has the advantage of being able to

[0005] A representative technique relating to such hot-formed members is disclosed in Patent Document 1. In 1, Al-Si plated steel sheet is heated to 850°C or higher, and then hot-formed by pressing. By rapidly cooling the structure of the component to martensite, the tensile strength is increased to 1600 MPa. By ensuring such ultra-high strength, it is possible to reduce the weight of automobiles. However, in accordance with Patent Document 1, high strength is required. Depending on the temperature, the impact resistance during a collision may be relatively low. There was a problem in which a phenomenon occurred in which the material showed very low impact properties.

[0006] In Patent Document 2, the Ca / S ratio is adjusted in a steel sheet for hot forming to remove inclusions into spherical particles. The impact properties after hot forming are improved by grain refinement and the addition of alloying elements such as Nb. However, Patent Document 2 proposes a technology to improve the impact properties of general steel materials. As for the control of inclusions and grain size to improve the It is suitable as a means of improving the low impact properties that occur during actual hot forming in the forming field. It is considered difficult to use.

[0007] Therefore, hot-forming plated steel sheets, hot-forming members, and There is a need for development of these manufacturing methods. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Patent No. 6,296,805 [Patent Document 2] Korean Patent Publication No. 10-2010-0047011 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention relates to a plated steel sheet for hot forming, a hot formed member, and a method for manufacturing the same, which are excellent in impact resistance after hot forming. The object of the present invention is to provide a method for producing the same.

[0010] The object of the present invention is not limited to the above. Those skilled in the art will be able to understand the further object of the present invention from the overall contents of the specification of the present invention. There is no difficulty in understanding it. [Means for solving the problem]

[0011] One aspect of the present invention is a composition comprising, in weight percent, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6~8%, P:0.001~0.05%, S:0.0001~0.02%, Al:0 .01~0.1%, N:0.001~0.02%, Cr:0.01~0.5%, balance Fe and base steel sheets containing other impurities; and zinc, aluminum formed on the surface of the above base steel sheets. a plating layer made of aluminum or an alloy containing aluminum; and the C content of the base steel sheet (C B ) relative to the C content in the surface layer (C S ) ratio (C S / C B ) is 0.6 or less, and the above element The total content of Mn and Cr in the base steel sheet (Mn B +Cr B ) Mn and Cr in the surface layer The total content of (Mn S +Cr S ) ratio ((Mn S +Cr S ) / (Mn B+Cr B ))but The hot-forming coated steel sheet has excellent impact resistance after hot forming, with a modulus of 0.8 or more. The surface layer refers to the area from the surface of the base steel sheet excluding the coating layer to a depth of 15 μm. Taste it.)

[0012] The above-mentioned base steel sheet contains, by weight%, B: 0.0005 to 0.01% and Ti: 0.01 to 0. It may further contain one or more of the following:

[0013] The microstructure of the above-mentioned base steel sheet is, in terms of area percentage, ferrite in the surface layer 40 to 100%, and the remainder Contains 0-60% pearlite, bainite or martensite, with ferrite in the center 30-90%, the balance is pearlite, bainite or martensite 10-70% It is possible.

[0014] Another aspect of the present invention is a composition comprising, in weight percent, C: 0.15 to 0.4%, Si: 0.1 to 1%, M n:0.6~8%, P:0.001~0.05%, S:0.0001~0.02%, Al :0.01~0.1%, N:0.001~0.02%, Cr:0.01~0.5%, balance Base steel sheet containing Fe and other impurities; and zinc or includes an alloy plating layer made of an alloy containing aluminum; and the C content (C B ) relative to the C content of the surface layer of the material (C PS ) ratio (C PS / C B ) is 1.2 or less , the total content of Mn and Cr in the base steel sheet (Mn B +Cr B ) of the surface of the material The total content of Mn and Cr (Mn PS +Cr PS ) ratio ((MnPS +Cr PS ) / ( Mn B +Cr B )) is 0.8 or more, the hot-formed part has excellent impact properties. The surface layer of the component is the area from the surface of the base steel sheet excluding the alloy plating layer to a depth of 25 μm. It means the area.)

[0015] The ferrite coverage rate at the martensite grain boundaries in the surface layer of the member is 30% or less. Good too.

[0016] Another aspect of the present invention is a composition comprising, in weight percent, C: 0.15 to 0.4%, Si: 0.1 to 1%, M n:0.6~8%, P:0.001~0.05%, S:0.0001~0.02%, Al :0.01~0.1%, N:0.001~0.02%, Cr:0.01~0.5%, balance A slab containing Fe and other impurities is prepared and heated at a temperature of 1050 to 1300°C. Step: The heated slab is hot rolled in the finish hot rolling temperature range of 800 to 950°C. After the finish hot rolling is completed, the hot rolled steel sheet is heated at 450 to 750°C. Coiling step: The coiled hot-rolled steel sheet is heated at 740 to 860°C, and the dew point temperature is -10 to Annealing the hot-rolled steel sheet in an atmosphere of 30°C for 10 to 600 seconds; and galvanizing the hot-rolled steel sheet after annealing. a step of plating the metal by immersing it in a plating bath made of aluminum or an alloy containing aluminum; The present invention relates to a method for producing a plated steel sheet for hot forming, which has excellent impact properties after hot forming.

[0017] The method may further include a step of cold rolling the steel sheet after the hot rolling and before the coiling to obtain a cold-rolled steel sheet. can.

[0018] The above slab contains, in weight percent, B: 0.00005 to 0.01% and Ti: 0.01 to 0. It may further contain one or more of the following:

[0019] Another aspect of the present invention is the above-mentioned plated steel sheet for hot forming which is excellent in impact properties after hot forming. The hot forming plated steel sheet manufactured by the manufacturing method is subjected to a temperature range of Ac3 to 950°C for 1 A manufacturing method for hot-formed parts with excellent impact resistance, which are heat-treated for 15 minutes and then hot-pressed. is. [Effects of the Invention]

[0020] According to the present invention, there is provided a plated steel sheet for hot forming which has excellent impact properties after hot forming, and a method for producing the same. It has the effect of providing a law.

[0021] The hot-formed portion of the plated steel sheet for hot forming according to the present invention is manufactured by hot press forming. The material has a tensile strength of 1500 MPa and a bending angle measured using the VDA238-100 bending test. The angle is 60° or more, which has the effect of ensuring excellent impact resistance.

[0022] The various yet significant advantages and effects of the present invention are not limited to those described above, but include: The present invention can be more easily understood in the course of describing specific embodiments thereof. [Brief explanation of the drawings]

[0023] [Figure 1] The graph shows the results of a concentration analysis of carbon (C), manganese (Mn), and chromium (Cr) conducted on the plated steel sheet for hot forming of Example 1 using GDS before hot press forming, from the surface layer in the depth direction. [Figure 2] 1 is an optical microscope photograph showing the structure of the surface layer portion of a member of Example 1 after hot forming. [Figure 3]1 shows the results of a concentration analysis of carbon (C), manganese (Mn), and chromium (Cr) conducted on the plated steel sheet for hot forming of Comparative Example 1 from the surface layer in the depth direction using GDS before hot press forming. [Figure 4] 1 is an optical microscope photograph showing the structure of the surface layer portion of the member after hot forming in Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0024] The following describes preferred embodiments of the present invention. However, the embodiments of the present invention may differ in some respects. The present invention may be modified into other forms, and the scope of the present invention is not limited to the following embodiments. Furthermore, the embodiments of the present invention are not intended to be limiting unless they are clearly understood by those skilled in the art. is provided to more fully explain the

[0025] The inventors have found that in the case of non-plated materials, the bending angle after hot forming is significantly larger than that in the case of plated materials. Further research into this matter revealed that in the case of non-plated materials, Decarburization occurs in the surface layer of the steel sheet during heating for hot forming, resulting in the formation of soft fibrous materials in the surface layer. It was confirmed that a ferrite layer was formed and that the bendability was excellent.

[0026] Here, the inventors have also tried to reduce the C content in the surface layer of the plated steel sheet to make the surface layer of the base steel sheet soft. If a layer of the phase can be formed, the bendability of the hot-formed part can be improved. However, in the case of plated materials, the hot forming process is not as easy as with non-plated materials. During heating for this purpose, decarburization does not occur sufficiently, so the soft ferrite It is difficult to form a ferrite layer continuously and sufficiently. If the thickness is not sufficient, the bending property will be deteriorated.

[0027] The inventors have conducted further research to overcome these problems, and as a result, By controlling the conditions, the C content in the surface layer of the base steel sheet is kept at a certain level relative to the C content in the center. The total Mn and Cr content in the surface layer of the base steel sheet is controlled to be less than that of the Mn and Cr content in the center. By controlling the total content to a certain level or more, it has excellent impact properties after hot forming. To provide plated steel sheets for hot forming, hot formed components, and manufacturing methods thereof The present invention has been completed based on the above findings.

[0028] First, a hot forming plating having excellent impact properties after hot forming according to one aspect of the present invention will be described below. The hot-formed steel plate and hot-formed member will now be described in detail.

[0029] Hot-forming galvanized steel sheet with excellent impact resistance after hot forming The plated steel sheet for hot forming, which has excellent impact properties after hot forming according to one aspect of the present invention, comprises: So, C: 0.15~0.4%, Si: 0.1~1%, Mn: 0.6~8%, P: 0.00 1~0.05%, S:0.0001~0.02%, Al:0.01~0.1%, N:0. 0.01-0.02%, Cr: 0.01-0.5%, balance Fe and other impurities Base steel sheet; and zinc, aluminum or their alloys formed on the surface of the base steel sheet a coating layer consisting of the above-mentioned base steel sheet; and B ) relative to the C content in the surface layer ( C S ) ratio (C S / C B ) is 0.6 or less, and the Mn and Cr contents of the base steel sheet are Total (Mn B +Cr B ) the total content of Mn and Cr in the surface layer (Mn S +Cr S ) ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8 or more.

[0030] First, the alloy composition of the base steel sheet of the present invention will be described in detail. It should be noted that when indicating the content, it means weight percent unless otherwise specified. .

[0031] C: 0.15 to 0.4% C is an essential element for improving the strength of hot-formed parts. C content is 0.15% If the C content is less than 0.4%, it is difficult to ensure sufficient strength. In this case, when the hot-rolled material is cold-rolled, the strength of the hot-rolled material is too high, and the cold-rollability is significantly reduced. Not only does this cause a problem, but it can also significantly reduce spot weldability. The C content in the steel sheet is preferably limited to 0.15 to 0.4%.

[0032] Si: 0.1 to 1% Si is added as a deoxidizer in steelmaking, is a solid solution strengthening element, and suppresses the formation of carbides. As a controlling element, Si contributes to increasing the strength of hot-formed parts and is an effective element for uniformity of material properties. If the content is less than 0.1%, the above-mentioned effects are insufficient. If the content exceeds 1%, the Al content is increased by the Si oxides that form on the steel surface during annealing. Therefore, the Si content in the present invention is It can be limited to 0.1 to 1%.

[0033] Mn: 0.6 to 8% Mn ensures the effect of solid solution strengthening and ensures martensite in hot-formed parts. Mn is an element added to lower the critical cooling rate for the above effect. The content must be 0.6% or more. On the other hand, if the Mn content exceeds 8%, , the increase in the strength of the steel sheet before the hot forming process not only reduces the cold rolling ability, but also There are problems of increased cost and reduced spot weldability. The Mn content can be limited to 0.6-8%.

[0034] P: 0.001 to 0.05% P exists as an impurity in steel, and the lower its content, the more advantageous it is. Therefore, the P content in the present invention can be limited to 0.05% or less, and 0.03% It is also preferable to limit the content of P to the following: The less P there is, the more advantageous it is as an impurity element. However, excessively lowering the P content may increase production costs. Taking this into consideration, the lower limit can be set at 0.001%. can.

[0035] S: 0.0001 to 0.02% S is an element that acts as an impurity in steel and inhibits the ductility, impact properties, and weldability of components. The maximum content is limited to 0.02%, and it is more preferable to limit it to 0.01% or less. However, if the minimum content is less than 0.0001%, there is a risk that the manufacturing cost will increase. Therefore, the lower limit of the content can be set at 0.0001%.

[0036] Al: 0.01 to 0.1% Al, together with Si, acts as a deoxidizer during steelmaking, and can increase the cleanliness of steel. To obtain the above effect, it can be added at a content of 0.01% or more. If the temperature exceeds 100°C, the excess AlN formed during the continuous casting process reduces the high-temperature ductility and Because of the problem of the tendency for rub cracks to occur, the upper limit of its content is set at 0.1% or less. Therefore, the Al content in the present invention is 0.01 to 0.1%. is preferred.

[0037] N: 0.001 to 0.02% N is an element contained in steel as an impurity, and when the N content exceeds 0.02%, In the case of slab cracks, the high-temperature ductility is reduced due to excessive AlN formed during the continuous casting process. Therefore, the sensitivity to cracks during continuous casting of slabs is low. To reduce the strength and ensure the impact properties, N may be contained in an amount of 0.02% or less. There is no need to specifically set a limit for N content, but taking into consideration factors such as rising manufacturing costs, the lower limit for N content should be set at 0. Therefore, the N content in the present invention can be set to 0.001% or more. It is preferably at most 0.02%.

[0038] Cr: 0.01 to 0.5% Cr is added to improve the effect of solid solution strengthening and hardenability during hot forming, similar to Mn. It is an element that is added to the steel, and can be added in an amount of 0.01% or more to obtain the above effect. If the content exceeds 0.5%, the hardening ability can be sufficiently secured, but the properties will saturate. Therefore, the Cr content of the present invention is not so high, and the manufacturing cost of the steel sheet may increase. The content is preferably 0.01 to 0.5%.

[0039] The base steel sheet of the plated steel sheet for hot forming according to one aspect of the present invention contains, in addition to the above-mentioned components, B: Further containing one or more of: 0.0005 to 0.01% and Ti: 0.01 to 0.05% It is possible.

[0040] B: 0.0005 to 0.01% Even a small amount of B added not only improves hardening ability, but also promotes the formation of a crystalline structure that is biased toward the prior austenite grain boundaries. This can suppress the embrittlement of hot-formed parts caused by grain boundary segregation of P and / or S. It is an element that can be added in an amount of 0.0005% or more to obtain the above effect. However, if the content exceeds 0.01%, not only does the effect saturate, but also embrittlement occurs during hot rolling. Therefore, the upper limit can be set at 0.01%, and the B content can be set at 0.005% or less. Therefore, the B content in the present invention is 0.0005 to 0.01%. It is preferable that:

[0041] Ti: 0.01 to 0.05% Ti combines with nitrogen remaining as an impurity in steel to form TiN, which improves hardening ability. It is added to retain the solute B, which is essential for ensuring the strength. When the Ti content is less than 0.01%, In some cases, it is difficult to fully expect the effect, and if it exceeds 0.05%, the properties Not only is there a risk of saturation, but the manufacturing cost of the steel sheet may also increase. In the present invention, the Ti content is preferably 0.01 to 0.05%.

[0042] The balance other than the above-mentioned components is iron (Fe), which can be contained in the steel sheet for hot press forming. There are no particular restrictions on the addition of additional ingredients as long as they are present in the product. Unintended impurities may inevitably be introduced from the surrounding environment, so these should be excluded. These impurities are obvious to any engineer working in the normal manufacturing process. Therefore, not all of the contents thereof will be specifically mentioned in this specification.

[0043] The hot-forming plated steel sheet according to one aspect of the present invention, which is excellent in impact resistance after hot forming, is a base steel It includes a plating layer made of zinc, aluminum or an alloy thereof formed on the surface of the plate. The plating layer provides corrosion resistance to the final component and prevents corrosion of the base steel sheet during heating for hot forming. It plays a role in suppressing decarburization and scale formation.

[0044] In the present invention, the type of the coating layer is not particularly limited, and the coating layer may be any of the coating layers that are applicable to conventional hot forming steel sheets. Any plating layer that can be applied to the present invention can be applied without limitation. In one embodiment, the plating layer is made of zinc, aluminum, or an alloy containing these. More specifically, the plating layer may be a hot-dip galvanized layer, an electrogalvanized layer, a zinc alloy layer, or the like. It may be a lead plating layer, an aluminum plating layer or an aluminum alloy plating layer.

[0045] On the other hand, according to one aspect of the present invention, the plating layer is , including ingredients that may be included during the manufacturing process, and in particular other unavoidable impurities. Cut.

[0046] The thickness of the plating layer may be 5 to 100 μm. If the thickness is less than 5 μm, it is difficult to exhibit sufficient corrosion resistance in hot-formed parts. However, if the thickness exceeds 100 μm, the heating time for hot forming will increase excessively. Furthermore, there is a risk of the manufacturing cost increasing excessively in relation to the effect of improving corrosion resistance. There is a problem.

[0047] On the other hand, the plated steel sheet for hot forming according to the present invention has a C content (C B ) for C content of the layer (C S ) ratio (C S / C B )(hereinafter referred to as “ratio (C S / C B ) The surface layer is defined as the area from the surface of the base steel sheet excluding the coating layer. This refers to the area up to a depth of 5 μm.

[0048] According to one aspect of the present invention, in the plated steel sheet for hot forming, the C content of the sheet is Amount (C B ) relative to the C content in the surface layer (C S ) ratio (C S / C B ) is less than or equal to 0.5 It is preferable that the ratio is 0.4 or less, more preferable that the ratio is 0.35 or less. preferable.

[0049] The above ratio (C S / C B ) is controlled to a low value of 0.6 or less, the center of the base steel sheet after hot forming Unlike the hard martensite phase formed in the core, the surface layer has a relatively low C content. A soft martensite phase is formed in the surface layer of the plated steel sheet. By forming this and reducing the hardness of the surface layer, it is possible to ensure excellent bending properties. If the above ratio (C S / C B ) exceeds 0.6, bending deformation occurs due to softening of the surface layer after hot forming. It becomes difficult to realize the effect of improving the performance. S / C B ) The lower limit does not need to be limited. However, if the C content in the surface layer is too low, the strength of the part after hot forming may decrease or fatigue may occur. Therefore, the above ratio (C S / C B ) lower bound It can be 0.05 or more, but is not limited to this.

[0050] In addition, the plated steel sheet for hot forming according to one aspect of the present invention has a Mn and Cr content of the base steel sheet. The total amount (Mn B +Cr B ) the total content of Mn and Cr in the surface layer (Mn S +C r S ) ratio ((Mn S +Cr S ) / (Mn B +Cr B ))(hereinafter referred to as “ratio((Mn S +Cr S ) / (Mn B +Cr B ))) may be 0.8 or more. The surface layer refers to the region extending from the surface of the base steel sheet excluding the coating layer to a depth of 15 μm.

[0051] On the other hand, according to one aspect of the present invention, in the plated steel sheet for hot forming, The total content of Mn and Cr (Mn B +Cr B ) Mn and Cr content in the surface layer The sum of (Mn S +Cr S ) ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8 It is preferably 5 or more, and more preferably 0.87 or more.

[0052] The above ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is low, less than 0.8, The hardening ability of the surface layer during forming is insufficient, so ferrite is partially formed on the surface of the part. Ferrite partially formed at the hard martensite grain boundaries can reduce bending strength. This is a factor that significantly reduces the ratio ((Mn S +Cr S ) / (Mn B +Cr B ))teeth, It is preferable that the ratio (Mn S +Cr S ) / (Mn B +Cr B ))of There is no need to limit the upper limit, but if the Mn and Cr contents in the surface layer are too high, The hardness of the surface layer after hot forming increases, which can lead to a problem of reduced bendability. Therefore, the above ratio ((Mn S +Cr S ) / (Mn B +Cr B )) upper limit It can be 2 or less, but is not limited to this.

[0053] On the other hand, the microstructure of the base steel sheet does not need to be particularly limited, but in terms of area fraction, it is preferable that the surface layer is Ferrite 40-100%, balance pearlite, bainite or martensite 0-6 0%, ferrite 30-90% in the center, and the rest pearlite, bainite or magenta Rutensite can be contained in an amount of 10 to 70%.

[0054] Hot-formed components with excellent impact properties On the other hand, the plated steel sheet for hot forming having the above-mentioned constitution is subjected to a temperature range of Ac3 to 950°C, 1 After 15 minutes of heat treatment, hot press forming is performed to produce hot-formed parts with excellent impact properties. can be done.

[0055] The hot-formed member having excellent impact properties according to one aspect of the present invention has the same properties as the base steel sheet of the plated steel sheet. and a substrate steel sheet having an alloy composition of zinc or aluminum formed on the surface of the substrate steel sheet. an alloy plating layer made of an alloy containing B ) Bill of Materials C content of the layer (C PS ) ratio (C PS / C B )(hereinafter referred to as “ratio (C PS / C B )" The total content of Mn and Cr in the base steel sheet (Mn B +Cr B ) the total content of Mn and Cr in the surface layer of the material (Mn PS +Cr PS ) ratio (( Mn PS +Cr PS ) / (Mn B +Cr B ))(hereinafter referred to as “ratio((Mn PS +Cr PS ) / (Mn B +Cr B ))) may be 0.8 or more. The layer portion refers to the area from the surface of the base steel sheet to a depth of 25 μm, excluding the alloy plating layer. do.

[0056] On the other hand, according to one aspect of the present invention, in the hot-formed member, the C content of the base steel sheet is (C B ) relative to the C content of the surface layer of the material (C PS ) ratio (C PS / C B ) is 1.1 or less It is preferable that the ratio is 1.05 or less, and more preferable that the ratio is 1.05 or less.

[0057] According to another aspect of the present invention, in the hot-formed member, Mn and Total Cr content (Mn B +Cr B) The sum of the Mn and Cr contents in the surface layer of the material Total (Mn PS +Cr PS ) is preferably 0.9 or more, and more preferably 0.93 or more. It is more preferable to do so.

[0058] Normally, when plated steel sheets are heated for hot forming, the plated layer and the base steel are alloyed. However, the plating layer has a very low solid solubility of C, so the solidification The carbon that could not be dissolved is concentrated in the surface layer, increasing the carbon content in the surface layer. A high C content in the surface layer increases the hardness of the surface layer, resulting in a decrease in bendability.

[0059] Meanwhile, a hot-formed member is obtained by hot press-forming the plated steel sheet for hot forming according to one aspect of the present invention. When manufacturing a steel sheet, even if C is concentrated in the surface layer of the component, the C content of the base steel sheet (C B ) The carbon content of the surface layer of the material (C PS ) ratio (C PS / C B ) is 1.2 or less and the surface of the material In addition, the Mn and Cr content of the base steel sheet can be reduced. The total amount (Mn B +Cr B ) the total content of Mn and Cr in the surface layer of the material (Mn P S +Cr PS ) ratio ((Mn PS +Cr PS ) / (Mn B +Cr B )) is 0.8 or more This allows for sufficient hardening ability to suppress ferrite formation and strengthen the martensite grain boundaries at the surface of the component. The ferrite coverage rate (the ratio of ferrite to the martensite grain boundary when observing the cross section) 30% or less, ensuring sufficient strength and excellent bending properties. It is possible.

[0060] As described above, the hot-formed member according to one aspect of the present invention has the above ratio (C S / C B ) is 1. 2 or less, and the above ratio ((Mn PS +Cr PS ) / (Mn B +Cr B )) is 0.8 or more As the material is filled, the tensile strength reaches 1500 MPa, measured by the VDA238-100 bending test. The bending angle is 60° or more, ensuring excellent impact resistance. However, the tensile strength When the tensile strength of a hot-formed part is increased, for example, to 1800 MPa or more, The bend angle criteria for determining successful impact properties may be even lower.

[0061] Next, another aspect of the present invention is a plated steel sheet for hot forming having excellent impact properties after hot forming. The method for manufacturing the hot-formed part will be described in detail.

[0062] Manufacturing method for hot-forming plated steel sheet with excellent impact properties after hot forming Another aspect of the present invention is the manufacture of a plated steel sheet for hot forming that is excellent in impact properties after hot forming. The method includes the steps of heating a slab having the above-mentioned alloy composition at 1050 to 1300°C; The slab is finish hot-rolled in a temperature range of 800 to 950°C to obtain a hot-rolled steel sheet. After the finish hot rolling is completed, the hot rolled steel sheet is coiled at 450 to 750°C. The hot-rolled steel sheet is heated to 740 to 860°C and cooled in an atmosphere with a dew point of -10 to 30°C. Annealing the hot-rolled steel sheet for 10 to 600 seconds; and treating the annealed hot-rolled steel sheet with zinc, aluminum or The method includes the step of plating by immersing the material in a plating bath containing an alloy containing these materials.

[0063] Slab heating stage First, a slab having the above-mentioned alloy composition is heated at 1050 to 1300°C. If the heat temperature is less than 1050°C, it may be difficult to homogenize the slab structure. If the temperature exceeds 1300°C, an excessive oxide layer may be formed.

[0064] Hot rolling stage The heated slab is finish hot-rolled in the temperature range of 800 to 950°C to obtain hot-rolled steel sheets. If the finish hot rolling temperature is less than 800°C, the surface layer of the steel sheet is rolled in the two-phase region. When the temperature exceeds 950°C, the crystal structure becomes difficult to control. There is a risk of the problem of coarsening of the particles.

[0065] Cooling and winding stage After the finish hot rolling is completed, the hot rolled steel sheet is coiled at 450 to 750°C. If the temperature is below 0°C, the material variation in the width direction will increase, causing the sheet to break during cold rolling. On the other hand, if the coiling temperature exceeds 750°C, problems such as poor shape may occur. However, there is a problem in that the carbides become coarse and the bendability deteriorates.

[0066] Cold rolling stage If necessary, the coiled hot-rolled steel sheet is cold-rolled before annealing to obtain a cold-rolled steel sheet. The cold rolling may be carried out for more precise control of the thickness of the steel sheet. In this case, the cold rolling may be omitted and annealing and plating may be carried out immediately. The cold rolling can be carried out at a reduction ratio of 30 to 80%.

[0067] Annealing stage The coiled hot-rolled steel sheet is heated at 740 to 860°C, and the dew point temperature is -10 to 30°C. Anneal in an atmosphere containing the above for 10 to 600 seconds. If the annealing temperature is less than 740°C or the annealing time is 1 If it is less than 0 seconds, the recrystallization of the structure is insufficient, resulting in a poor plate shape or plating. The strength of the blanking die is too high, which can cause wear on the die during the blanking process. The diffusion of C during annealing is insufficient, so the C content of the base steel sheet (C B ) surface area C content (C S ) ratio (C S / C B ) to be 0.6 or less. However, if the annealing temperature exceeds 860°C or the annealing time exceeds 600 seconds, A large amount of oxides is formed on the surface of the steel sheet, which can cause uncoated areas or reduce the coating adhesion. In addition, Mn, Cr, etc. in the base steel due to internal oxidation may be lost to the plating layer and the base steel. The total Mn and Cr content of the base steel sheet (M n B +Cr B ) the total content of Mn and Cr in the surface layer (Mn S +Cr S ) ratio ( (Mn S +Cr S ) / (Mn B +Cr B )) is difficult to secure at least 0.8, The hardening ability of the surface layer may be insufficient, which may result in partial ferrite formation on the surface after hot forming. Wrights may be formed, causing problems with reduced bendability.

[0068] On the other hand, in the present invention, the content of C, Mn, and Cr in the surface layer of the base steel sheet relative to the base metal components is In order to control the ratio of the amounts, it is very important to control the dew point temperature of the annealing atmosphere. If the dew point temperature of the annealing atmosphere is less than -10°C, the decarburization reaction is insufficient, resulting in poor bendability. On the other hand, when the dew point temperature exceeds 30°C, excessive internal oxidation The hardening ability of the surface layer decreases, and ferrite is formed in parts, causing problems such as reduced bendability. There is a risk of this happening.

[0069] According to one aspect of the present invention, the annealing is performed by heating the coiled hot-rolled steel sheet at 800 to 840°C. It is more preferable to heat the mixture at a temperature of 10 to 30°C for 10 to 100 seconds. It's nice.

[0070] Plating stage After annealing, the hot-rolled steel sheet is coiled and coated with zinc, aluminum or an alloy containing these. In the present invention, the plating bath used for forming the plating layer is However, as a non-limiting embodiment, the components used in the present invention are The plating baths used can consist of zinc, zinc alloys, aluminum, and aluminum alloys. In addition, the plating conditions are the same as those normally applied to steel sheets for hot press forming. Since the above can be applied to the present invention without any limitations, it will not be specifically mentioned in this specification. In some respects, the plating bath may contain other unavoidable impurities, and the zinc alloy and Aluminum alloys also contain components that can be normally contained within the range that does not impair the object of the present invention. It may contain other unavoidable impurities, among others.

[0071] Manufacturing method for hot-formed parts with excellent impact properties The hot forming plated steel sheet manufactured by the manufacturing method of the present invention described above was subjected to hot forming. In this case, the hot forming can be performed to produce a hot-formed part having excellent impact properties. For press molding, a method generally used in the technical field can be applied. As a non-limiting embodiment, the hot forming plated steel sheet is subjected to a 1000-1500°C (1000-1500°C) treatment at a temperature of Ac3 to 950°C. After heat treatment for 15 minutes, it can be pressed to form the hot shape. [Example]

[0072] The present invention will be described in more detail below with reference to examples. However, the following examples are not intended to be limiting of the scope of the present invention. The following is for illustrative purposes only and is not intended to limit the scope of the present invention. It should be noted that the scope of the present invention is not limited to the matters described in the claims. , because it is determined by matters that can be reasonably inferred from it.

[0073] (Example) First, a slab having the alloy composition shown in Table 1 below was prepared, and the slab was subjected to the alloying treatment shown in Table 2 below. The hot-rolled steel sheets were produced by heating the slab, hot rolling, and coiling under the manufacturing conditions shown in Table 2. After annealing under the specified annealing conditions, the steel was immersed in a zinc plating bath to achieve a plating weight of 70 g / m 2 Plated steel sheets were manufactured by plating so that the thickness became as follows:

[0074] [Table 1]

[0075] [Table 2]

[0076] The plated steel sheets of the invention and comparative examples manufactured under the above manufacturing conditions were subjected to various tests in the depth direction. Glow Discharge Spectrometer (GDS) enables quantitative analysis of various components. Using the method (using LECO GDS 850A, USA), the surface was measured in the depth direction. Concentrations of carbon (C), manganese (Mn) and chromium (Cr) for a depth sufficient for The analysis was carried out, and the average content of the area corresponding to the surface layer was calculated from the GDS analysis results using the integration method. The results are shown in Table 3 below. In the case of a normal GDS analysis, the diameter of the circle is 2 to 6 mm. Since the depth direction analysis is performed for the shape area, accurate concentration profile in the depth direction is obtained. Although it is difficult to specify the interface between the coating layer and the base steel sheet, various optical and SEM analysis results Based on these factors, in this invention, the point where the Zn content is 1% is set as the base of the interface between the coating layer and the base steel sheet. It was set as standard.

[0077] [Table 3]

[0078] In addition, the plated steel sheets of each of the invention examples and comparative examples were hot-pressed under the conditions shown in Table 4 below. The hot-formed parts were fabricated by hot forming. Test specimens were placed on the flat surface of the fabricated hot-formed parts. The specimens were collected and subjected to tensile and bending tests (VDA238-100), and then analyzed in depth via GDS analysis. The concentration analysis for carbon (C), manganese (Mn) and chromium (Cr) was carried out in the depth direction. In addition, cross-sectional optical microscope observations were performed to identify the ferrite at the martensite grain boundaries in the surface layer of the component. The coverage rate was evaluated, and the results are shown in Table 4.

[0079] [Table 4]

[0080] The plated steel sheets of Examples 1 and 2 manufactured under the conditions of the present invention had a ratio (C S / C B )but 0.6 or less, and the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8 or more As a result, the hot-pressed steel sheets manufactured by hot press forming the plated steel sheets of the above-mentioned invention examples 1 and 2 were The molding material has a ratio (C PS / C B ) is 1.2 or less, and the ratio ((Mn PS +Cr PS ) / ( Mn B +Cr B )) satisfies 0.8 or more, and the fracture at the martensite grain boundary in the surface layer The coating rate is 30% or less, the tensile strength is 1500 MPa, and the bending angle is 60° or more. It showed good bending properties.

[0081] Comparative Example 1 is a case where the dew point temperature during annealing is less than -10°C, and Comparative Example 2 is a case where the dew point temperature during annealing is less than -10°C. In both Comparative Examples 1 and 2, the ratio (C S / C B ) exceeds 0.6, which results in the ratio (C PS / C B ) also exceeds 1.2 As a result, the bending properties deteriorated.

[0082] On the other hand, in Comparative Example 3, the dew point temperature during annealing exceeded 30°C, and in Comparative Example 4, the dew point temperature during annealing exceeded 30°C. In both Comparative Examples 3 and 4, the ratio (C S / C B ) satisfied the conditions of the present invention, but the ratio ((Mn S +Cr S ) / (Mn B +Cr B ))but The ratio of Mn in hot-formed parts is less than 0.8. PS +Cr PS ) / (Mn B +Cr B )) This resulted in the ferrite grains at the martensite grain boundaries in the surface layer of the component being The coverage rate exceeded 30%, and the tensile strength was relatively low compared to other examples, while the bending strength was also relatively low. Sexuality also declined significantly.

[0083] Although the present invention has been described with reference to the embodiments, it is understood by those skilled in the art that The present invention is not limited to the above and is not intended to be limiting unless it is expressly stated otherwise. It will be understood that various modifications and variations of the invention are possible.

Claims

1. In weight percent, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, N: 0.001 to 0.02%, Cr: 0.01 to 0.5%, balance Fe and other impurities and A metal sheet made of zinc, aluminum or an alloy containing these formed on the surface of the base steel sheet a plating layer; The C content of the base steel sheet (C Ｂ ) relative to the C content (C Ｓ ) ratio (C Ｓ / C Ｂ ) is 0.6 or less, The total content of Mn and Cr in the base steel sheet (Mn Ｂ +Cr Ｂ ) Mn in the surface layer and the total content of Cr (Mn Ｓ +Cr Ｓ ) ratio ((Mn Ｓ +Cr Ｓ ) / (Mn Ｂ +Cr Ｂ )) is 0.8 or more. (Here, the surface layer portion is the surface of the base steel sheet excluding the plating layer to a depth of 15 μm. )

2. The base steel sheet contains, in weight percent, B: 0.0005 to 0.01% and Ti: 0.01 to 0.

2. The heat-molded product according to claim 1, further comprising one or more of the following: 0.05% Galvanized steel sheet for cold forming.

3. The microstructure of the base steel sheet is, in area %, The surface layer is 40 to 100% ferrite, with the remainder being pearlite, bainite, or martensite. Includes 0-60% of the site, The center is 30-90% ferrite, the rest is pearlite, bainite or martensite The hot forming material according to claim 1, which contains 10 to 70% of silica and has excellent impact properties after hot forming. Plated steel.

4. In weight percent, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, N: 0.001 to 0.02%, Cr: 0.01 to 0.5%, balance Fe and other impurities and An alloy plating formed on the surface of the base steel sheet is made of an alloy containing zinc or aluminum. a layer; The C content of the base steel sheet (C Ｂ ) relative to the C content (C ＰＳ ) ratio (C Ｐ Ｓ / C Ｂ ) is 1.2 or less, The total content of Mn and Cr in the base steel sheet (Mn Ｂ +Cr Ｂ ) of the surface layer of the material The total content of Mn and Cr (Mn ＰＳ +Cr ＰＳ ) ratio ((Mn ＰＳ +Cr ＰＳ ) / ( Mn Ｂ +Cr Ｂ )) is 0.8 or more. (Here, the surface layer of the member is defined as a layer that is 25 μm thick from the surface of the base steel sheet excluding the alloy plating layer.) It means the area up to the depth.)

5. The base steel sheet contains, in weight percent, B: 0.0005 to 0.01% and Ti: 0.01 to 0.

5. The hot-formed part having excellent impact properties according to claim 4, further comprising one or more of the following: 0.05%.

6. The ferrite coverage rate at the martensite grain boundaries in the surface layer portion of the component is 30% or less. The hot-formed part having excellent impact properties according to claim 4.

7. In weight percent, C: 0.15 to 0.4%, Si: 0.1 to 1%, Mn: 0.6 to 8%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Al: 0.01 to 0.1%, N: 0.001 to 0.02%, Cr: 0.01 to 0.5%, balance Fe and other impurities providing a slab containing the compound and heating it at a temperature of 1050 to 1300°C; The heated slab is hot rolled in a finish hot rolling temperature range of 800 to 950 ° C. obtaining a hot rolled steel sheet; After the completion of finish hot rolling, the hot-rolled steel sheet is coiled at 450 to 750°C; The coiled hot-rolled steel sheet is heated at 740 to 860°C, and the dew point temperature is -10 to 30°C. annealing in an atmosphere containing the metal for 10 to 600 seconds; and After annealing, the hot-rolled steel sheet is immersed in a plating bath containing zinc, aluminum, or an alloy containing these. immersion in plating; The present invention relates to a method for producing a plated steel sheet for hot forming, which has excellent impact properties after hot forming, comprising:

8. The method further comprises a step of cold rolling the steel sheet after the hot rolling and before the coiling to obtain a cold-rolled steel sheet. Item 8. A method for producing a plated steel sheet for hot forming having excellent impact properties after hot forming according to Item 7.

9. The slab contains, in weight percent, B: 0.00005 to 0.01% and Ti: 0.01 to 0.

8. The heat-molded product according to claim 7, further comprising one or more of the following: 0.05% A method for manufacturing plated steel sheets for cold forming.

10. The plated steel sheet for hot forming manufactured according to any one of claims 7 to 9 is Ac3 to 95 After heat treatment at a temperature range of 0°C for 1 to 15 minutes, it is hot press molded. Method for manufacturing cold-formed parts.

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