Thermoformed parts and methods of making same

Abstract

The present invention provides a hot stamped plated steel sheet having excellent impact properties after hot stamping, a hot stamped part manufactured using the hot stamped plated steel sheet, and a method of manufacturing the same, the hot stamped plated steel sheet including: a base steel sheet including, in terms of weight %, C: 0.15-0.4 %, Si: 0.1-1 %, 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 %, the balance of Fe and other impurities; and a plated layer formed on a surface of the base steel sheet and formed of zinc, aluminum, or an alloy including the same, wherein a ratio (C S / C B ) of a C content (C S ) of a surface layer portion to a C content (C B ) of the base steel sheet is 0.6 or less, and a ratio ((Mn S +Cr S ) / (Mn B +Cr B )) of a sum (Mn S +Cr S ) of Mn and Cr contents of the surface layer portion to a sum (Mn B +Cr B ) of Mn and Cr contents of the base steel sheet is 0.8 or more.

Description

[0001] This application is a divisional application of the Chinese patent application with the application date of December 19, 2019, the Chinese patent application number 201980084549.4 and the invention name “Hot-formed coated steel sheet with excellent impact properties after hot forming, hot-formed parts and their manufacturing methods”, and this application claims priority to the Korean application with application number 10-2018-0164822. Technical Field

[0002] The present invention relates to a plated steel sheet for hot forming having excellent impact properties after hot forming, which can be preferably used for automobile parts requiring impact resistance, a hot formed part, and methods for producing the same. Background Art

[0003] In recent years, regulations to improve automobile fuel efficiency have become increasingly stringent due to the depletion of petroleum resources and heightened environmental concerns. One method for improving automobile fuel efficiency is to reduce the thickness of the steel sheets used. However, this reduction in thickness can pose safety risks, necessitating an increase in steel sheet strength.

[0004] For the reasons described above, there is a continuous demand for high-strength steel sheets, and various types of steel sheets have been developed. However, these steel sheets inherently possess high strength, leading to poor workability. Specifically, the product of strength and elongation tends to remain constant across all grades of steel sheet. Consequently, as the strength of the steel sheet increases, the elongation, an indicator of workability, decreases.

[0005] To address this issue, hot press forming has been proposed. This method involves forming a steel sheet at a high temperature suitable for processing, followed by rapid cooling to a low temperature. This creates a low-temperature structure, such as martensite, within the steel sheet, thereby improving the strength of the final product. This approach offers the advantage of minimizing workability issues when producing high-strength components.

[0006] A representative technology for this type of hot-formed parts is Patent Document 1. In Patent Document 1, Al-Si-coated steel sheets are heated to above 850°C, then hot-formed by stamping and rapidly cooled, resulting in a martensite structure in the part, thereby achieving ultra-high tensile strength exceeding 1600 MPa. This ultra-high strength offers the advantage of easily achieving lightweight automobiles. However, according to Patent Document 1, the high strength results in relatively poor impact resistance during a collision, and depending on the hot-forming conditions, some parts may exhibit abnormally low impact resistance.

[0007] Therefore, Patent Document 2 proposes a technique for improving the impact properties after hot forming by adjusting the Ca / S ratio in a hot-forming steel sheet to spheroidize inclusions and adding alloying elements such as Nb to refine the grains. However, Patent Document 2 focuses on improving the impact properties of ordinary iron and steel materials by controlling inclusion and grain size, and is considered difficult to apply as a method for improving the low impact properties that occur during actual hot forming in the field of hot press forming.

[0008] Therefore, there is a need to develop a hot-forming plated steel sheet, a hot-formed component, and a method for producing the same that has excellent impact properties after hot forming.

[0009] (Patent Document 1) U.S. Patent Publication No. 6296805

[0010] (Patent Document 2) Korean Patent Publication No. 10-2010-0047011 Summary of the Invention

[0011] Technical problems to be solved

[0012] An object of the present invention is to provide a plated steel sheet for hot forming having excellent impact properties after hot forming, a hot formed component, and a method for producing the same.

[0013] The technical problems of the present invention are not limited to the above contents. Those skilled in the art in the art to which the present invention belongs can easily understand the additional technical problems of the present invention based on the full text of the present invention.

[0014] Technical Solution

[0015] One aspect of the present invention is a hot-forming plated steel sheet having excellent impact properties after hot forming, the hot-forming plated steel sheet comprising: a base steel sheet containing, by weight, C: 0.15-0.4%, Si: 0.1-1%, 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%, and the balance being Fe and other impurities; and a plated layer formed on the surface of the base steel sheet, the plated layer being formed of zinc, aluminum, or an alloy containing these, wherein the C content (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is less than 0.6, and the sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +CrB ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8 or more (wherein the surface layer portion refers to a region from the surface of the base steel sheet excluding the coating layer to a depth of 15 μm).

[0016] In terms of weight %, the base steel plate may further include one or more of B: 0.0005-0.01% and Ti: 0.01-0.05%.

[0017] In the microstructure of the base steel plate, in terms of area %, the surface portion may contain 40-100% ferrite, and the balance may contain 0-60% pearlite, bainite or martensite; the central portion may contain 30-90% ferrite, and the balance may contain 10-70% pearlite, bainite or martensite.

[0018] Another aspect of the present invention is a hot-formed component with excellent impact properties, the hot-formed component comprising: a base steel plate containing, by weight, the following: C: 0.15-0.4%, Si: 0.1-1%, 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%, and the balance being Fe and other impurities; and an alloy plating layer formed on a surface of the base steel plate, the alloy plating layer being formed of an alloy containing zinc or aluminum, wherein the C content (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is less than 1.2, and the sum of the Mn and Cr contents in the surface layer of the component (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn PS +Cr PS ) / (Mn B +Cr B )) is 0.8 or more (wherein the component surface layer portion refers to a region from the surface of the base steel sheet excluding the alloy plating layer to a depth of 25 μm).

[0019] The ferrite coverage in the martensite grain boundaries of the surface layer portion of the component may be 30% or less.

[0020] Another aspect of the present invention is a method for producing a hot-forming plated steel sheet having excellent impact properties after hot forming, the method comprising the steps of preparing a slab and heating the slab to a temperature of 1050-1300° C., wherein the slab comprises, in terms of weight %, C: 0.15-0.4%, Si: 0.1-1%, 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. 0.01-0.5% and the balance Fe and other impurities; hot rolling the heated slab within a hot finishing rolling temperature range of 800-950° C. to obtain a hot rolled steel sheet; after the hot finishing rolling, coiling the hot rolled steel sheet at 450-750° C.; heating the coiled hot rolled steel sheet to 740-860° C. and annealing for 10-600 seconds in an atmosphere with a dew point temperature of -10° C. to 30° C.; and after annealing, immersing the hot rolled steel sheet in a plating bath for plating, the plating bath consisting of zinc, aluminum or an alloy containing them.

[0021] After the hot rolling and before the coiling, a step of obtaining a cold-rolled steel sheet by cold rolling may be further included.

[0022] In terms of weight %, the slab may further include one or more of B: 0.00005-0.01% and Ti: 0.01-0.05%.

[0023] Another aspect of the present invention is a method for manufacturing a hot-formed part with excellent impact properties, wherein the hot-formed plated steel sheet manufactured by the above-mentioned method for manufacturing a hot-formed plated steel sheet with excellent impact properties after hot forming is heat-treated in a temperature range of Ac3 to 950°C for 1 to 15 minutes and then hot press-formed.

[0024] Beneficial effects

[0025] According to the present invention, there is an effect of providing a plated steel sheet for hot forming having excellent impact properties after hot forming and a method for producing the same.

[0026] The hot-formed component manufactured by hot press-forming the hot-forming plated steel sheet of the present invention has a bending angle of 60° or more measured by the VDA238-100 bending test at a tensile strength level of 1500 MPa, thereby ensuring excellent impact properties.

[0027] The various advantages and effects of the present invention are not limited to the above contents and can be more easily understood in the process of describing specific embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1This is a graph showing the results of analyzing the concentrations of carbon (C), manganese (Mn), and chromium (Cr) in the depth direction from the surface layer using GDS before hot press forming of the plated steel sheet for hot forming according to Inventive Example 1.

[0029] Figure 2 This is an optical microscope photograph showing the surface structure of the component after hot forming in Inventive Example 1.

[0030] Figure 3 This is a graph showing the results of GDS analysis of the concentrations of carbon (C), manganese (Mn), and chromium (Cr) in the depth direction from the surface layer of the plated steel sheet for hot forming of Comparative Example 1 before hot press forming.

[0031] Figure 4 This is an optical microscope photograph showing the surface structure of the component of Comparative Example 3 after hot forming.

[0032] Best Practice

[0033] The following describes preferred embodiments of the present invention. However, the embodiments of the present invention may be modified into various other embodiments, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more fully illustrate the present invention to those skilled in the art.

[0034] The present inventors have noted that unplated materials exhibit significantly better bend angles after hot forming compared to plated materials. Further research has revealed that in the case of unplated materials, decarburization occurs in the surface portion of the steel sheet during the heating process for hot forming, resulting in the formation of a soft ferrite layer in the surface portion and improved bendability.

[0035] Therefore, the inventors of the present invention considered that the bendability of hot-formed parts could be improved by reducing the carbon content in the surface layer of the base steel sheet in the plated material. However, they discovered that, unlike unplated materials, plated materials do not undergo sufficient decarburization during the heating process for hot forming. Therefore, it is difficult to form a soft ferrite layer as in unplated materials. Furthermore, if the ferrite layer is not formed continuously and sufficiently, the bendability is reduced.

[0036] As a result of further in-depth research conducted by the present inventors to overcome the above-mentioned problems, they confirmed that by controlling the annealing conditions, the C content in the surface layer portion of the base steel plate can be controlled to a certain level or less relative to the C content in the center portion, and the sum of the Mn and Cr contents in the surface layer portion of the base steel plate can be controlled to a certain level or more relative to the sum of the Mn and Cr contents in the center portion. This allows for providing a hot-forming plated steel sheet, a hot-formed component, and a method for manufacturing the same that exhibit excellent impact properties after hot forming, thereby completing the present invention.

[0037] First, a hot-forming plated steel sheet and a hot-formed component having excellent impact properties after hot forming according to one aspect of the present invention will be described in detail below.

[0038] Hot-forming plated steel sheet with excellent impact properties after hot forming

[0039] A hot-forming plated steel sheet having excellent impact properties after hot forming according to one aspect of the present invention comprises: a base steel sheet containing, in weight percent, C: 0.15-0.4%, Si: 0.1-1%, 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%, and the balance Fe and other impurities; and a plated layer formed on a surface of the base steel sheet, the plated layer being composed of zinc, aluminum, or an alloy thereof, wherein the C content (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is less than 0.6, and the sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is above 0.8.

[0040] First, the alloy composition of the base steel plate of the present invention will be described in detail. In the present invention, it should be noted that the content of each element is expressed in weight % unless otherwise defined.

[0041] C: 0.15-0.4%

[0042] Carbon is an essential element for increasing the strength of hot-formed parts. A carbon content of less than 0.15% makes it difficult to ensure sufficient strength. On the other hand, a carbon content exceeding 0.4% results in excessively high strength during cold rolling of the hot-rolled material, significantly degrading cold rollability and potentially significantly reducing spot weldability. Therefore, the carbon content in the present invention is preferably limited to 0.15-0.4%.

[0043] Si: 0.1-1%

[0044] Si is added during steelmaking as a deoxidizer. It is a solid solution strengthening element and an element that inhibits carbide formation, contributing to increased strength in hot-formed parts. It is also an effective element for homogenizing the material. However, when the Si content is less than 0.1%, these effects are insufficient. On the other hand, when the Si content exceeds 1%, Al coating properties may be significantly reduced due to Si oxides formed on the steel sheet surface during annealing. Therefore, the Si content in the present invention can be limited to 0.1-1%.

[0045] Mn: 0.6-8%

[0046] Mn ensures a solid solution strengthening effect and is an element added to hot-formed parts to reduce the critical cooling rate required to maintain martensite. To achieve this effect, the Mn content must be at least 0.6%. On the other hand, if the Mn content exceeds 8%, the strength of the steel sheet increases before the hot forming process, which not only reduces cold rolling properties but also increases the cost of the alloy iron and deteriorates spot weldability. Therefore, the Mn content in the present invention can be limited to 0.6-8%.

[0047] P: 0.001-0.05%

[0048] Phosphorus (P) exists as an impurity in steel, and the lower the P content, the better. Therefore, the P content in the present invention can be limited to 0.05% or less, and preferably, to 0.03% or less. P is an impurity element, and the lower the P content, the better, so there's no need to specifically set a lower limit for the P content. However, excessively reducing the P content can increase manufacturing costs. Therefore, taking this into account, the lower limit for the P content can be set at 0.001%.

[0049] S: 0.0001-0.02%

[0050] S is an impurity in steel and an element that impairs the ductility, impact properties, and weldability of components. Therefore, the maximum S content is limited to 0.02%, and preferably to 0.01% or less. However, if the minimum S content is less than 0.0001%, manufacturing costs may increase, so the lower limit of the S content can be set to 0.0001%.

[0051] Al: 0.01-0.1%

[0052] Al, along with Si, acts as a deoxidizer during steelmaking, thereby improving steel cleanliness. To achieve this effect, Al can be added at a level of 0.01% or higher. However, if the Al content exceeds 0.1%, excessive AlN is formed during the continuous casting process, reducing high-temperature ductility and slab cracking. Therefore, the upper limit of the Al content should be set below 0.1%. Therefore, the Al content in the present invention is preferably between 0.01% and 0.1%.

[0053] N: 0.001-0.02%

[0054] Nitrogen is an element contained in steel as an impurity. When the N content exceeds 0.02%, excessive AlN is formed during the continuous casting process, reducing high-temperature ductility and slab cracking. Therefore, to reduce susceptibility to cracking during continuous casting of the slab and ensure impact resistance, N content of 0.02% or less may be used. While there is no need to specify a lower limit for the N content, it may be set to 0.001% or higher to account for factors such as increased manufacturing costs. Therefore, the N content in the present invention is preferably between 0.001% and 0.02%.

[0055] Cr: 0.01-0.5%

[0056] Similar to Mn, Cr is an element added to enhance solid solution strengthening and hardenability during hot forming. To achieve these effects, 0.01% or more of Cr can be added. However, if the Cr content exceeds 0.5%, while sufficient hardenability can be achieved, the properties become saturated, and the cost of manufacturing the steel sheet may increase. Therefore, the Cr content in the present invention is preferably 0.01-0.5%.

[0057] The base steel sheet of the hot-forming plated steel sheet according to one aspect of the present invention may further contain one or more of B: 0.0005-0.01% and Ti: 0.01-0.05% in addition to the above-mentioned components.

[0058] B: 0.0005-0.01%

[0059] B is an element that can improve hardenability even when added in small amounts. It also segregates at prior austenite grain boundaries and can suppress the brittleness of hot-formed parts caused by grain boundary segregation of P and / or S. To achieve these effects, 0.0005% or more of B can be added. However, if the B content exceeds 0.01%, not only will this effect be saturated, but brittleness may also result during hot rolling. Therefore, the upper limit of the B content can be set to 0.01%, and preferably, the B content can be set to 0.005% or less. Therefore, the B content in the present invention is preferably between 0.0005% and 0.01%.

[0060] Ti: 0.01-0.05%

[0061] Ti is added to form TiN by combining with nitrogen remaining as an impurity in the steel, thereby retaining the solid-solution boron necessary for ensuring hardenability. A Ti content of less than 0.01% is unlikely to yield sufficient results, while a Ti content exceeding 0.05% may not only saturate the properties but also increase the cost of steel sheet production. Therefore, the Ti content in the present invention is preferably between 0.01% and 0.05%.

[0062] The balance after the above-mentioned components is iron (Fe), and there are no particular restrictions on the further addition of these components, as long as they are components that can be included in the hot press-formable steel sheet. Furthermore, during normal manufacturing processes, unwanted impurities may inevitably enter from the raw materials or the surrounding environment, and therefore cannot be eliminated. These impurities are well known to those skilled in the art of normal manufacturing processes, and therefore, all of them are not specifically mentioned in this specification.

[0063] A hot-forming plated steel sheet with excellent impact properties after hot forming, according to one aspect of the present invention, includes a coating layer formed on the surface of a base steel sheet, wherein the coating layer is composed of zinc, aluminum, or an alloy thereof. The coating layer imparts corrosion resistance to the final component and also suppresses decarburization and scale formation in the base steel sheet during heating for hot forming.

[0064] The present invention does not impose any particular restrictions on the type of coating; any coating used for conventional hot-formed steel sheets may be used without limitation. As a non-limiting embodiment, the coating may be formed of zinc, aluminum, or an alloy thereof. More specifically, the coating may be a hot-dip galvanized layer, an electroplated zinc layer, an alloyed zinc layer, an aluminum layer, or an aluminum alloy layer.

[0065] In addition, according to one aspect of the present invention, within the scope not impairing the purpose of the present invention, the coating layer may contain components that may be contained in the manufacturing process, in particular, may contain other unavoidable impurities.

[0066] Furthermore, the thickness of the coating layer may be 5-100 μm. If the coating layer is less than 5 μm, sufficient corrosion resistance may be difficult to achieve in a thermoformed part. On the other hand, if the coating layer is more than 100 μm, the heating time for thermoforming may be excessively increased, and the manufacturing cost may be excessively increased relative to the effect of improving corrosion resistance.

[0067] In addition, in the hot forming plated steel sheet of the present invention, the C content (C S ) and the C content of the base steel plate (C B ) of the ratio (CS / C B )(hereinafter also referred to as "ratio (C S / C B )”) satisfies 0.6 or less. The surface layer portion refers to the area from the surface of the base steel plate excluding the coating layer to a depth of 15 μm.

[0068] Furthermore, according to one aspect of the present invention, in the hot-forming plated steel sheet, the C content (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) can preferably be 0.5 or less, more preferably be 0.4 or less, and most preferably be 0.35 or less.

[0069] When the ratio (C S / C B When the ratio (C) is controlled to be as low as 0.6 or less, a relatively soft martensite phase is formed in the surface portion due to the low C content, unlike the hard martensite phase formed in the center portion of the base steel sheet after hot forming. Since the soft martensite phase is formed in the surface portion of the plated steel sheet, the hardness of the surface portion is reduced, thereby ensuring excellent bending properties. S / C B ) exceeds 0.6, it is difficult to achieve the effect of improving the bendability by softening the surface layer after thermoforming. S / C B However, when the C content in the surface layer is too low, the strength of the component after hot forming may be reduced or the fatigue properties may be deteriorated. Therefore, the ratio (C S / C B ) can be set to be greater than 0.05, but is not limited thereto.

[0070] In addition, in the hot-forming plated steel sheet according to one aspect of the present invention, the sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B ))(hereinafter, also referred to as "ratio ((Mn S +Cr S ) / (Mn B +Cr B))”) can be 0.8 or more. The surface layer portion refers to the area from the surface of the base steel plate excluding the coating layer to a depth of 15 μm.

[0071] According to one aspect of the present invention, in the hot-forming plated steel sheet, the sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) may preferably be 0.85 or more, and more preferably be 0.87 or more.

[0072] When the ratio ((Mn S +Cr S ) / (Mn B +Cr B When the ratio ((Mn)) is as low as less than 0.8, the hardenability of the surface layer is insufficient during hot forming, so ferrite may be partially formed on the surface of the component. Ferrite partially formed in the hard martensite grain boundary is a factor that greatly deteriorates the bendability. Therefore, the ratio ((Mn)) is S +Cr S ) / (Mn B +Cr B )) preferably satisfies 0.8 or more. There is no need to limit the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) of the upper limit, but when the content of Mn and Cr in the surface part is too high, the hardness of the surface part after hot forming becomes high, which may cause the problem of poor bendability. Therefore, the above ratio ((Mn S +Cr S ) / (Mn B +Cr B The upper limit of )) can be set to less than 2, but is not limited to this.

[0073] In addition, the microstructure of the base steel plate does not need to be particularly limited, but in terms of area fraction, the surface portion may contain 40-100% ferrite, and the balance may contain 0-60% pearlite, bainite or martensite, and the center portion may contain 30-90% ferrite, and the balance may contain 10-70% pearlite, bainite or martensite.

[0074] Thermoformed parts with excellent impact properties

[0075] Furthermore, by heat treating the plated steel sheet for hot forming having the above composition within the temperature range of Ac3 to 950° C. for 1 to 15 minutes and then hot press forming, a hot formed part having excellent impact properties can be produced.

[0076] A hot-formed component having excellent impact properties according to one aspect of the present invention comprises: a base steel sheet having the same alloy composition as that of a base steel sheet of a plated steel sheet; and an alloy plating layer formed on a surface of the base steel sheet, wherein the alloy plating layer is formed of an alloy containing zinc or aluminum, wherein the carbon content (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B )(hereinafter also referred to as "ratio (C PS / C B )”) can be 1.2 or less, and the sum of the Mn and Cr contents in the surface layer of the component (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn PS +Cr PS ) / (Mn B +Cr B ))(hereinafter, also referred to as "ratio ((Mn PS +Cr PS ) / (Mn B +Cr B ))”) can be 0.8 or more. The surface layer portion of the component refers to the area from the surface of the base steel plate excluding the alloy plating layer to a depth of 25 μm.

[0077] In addition, according to one aspect of the present invention, in the thermoformed component, the carbon content (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) can preferably be 1.1 or less, and more preferably be 1.05 or less.

[0078] Furthermore, according to one aspect of the present invention, in the hot-formed component, the sum of the Mn and Cr contents (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) ratio may preferably be 0.9 or more, and more preferably be 0.93 or more.

[0079] Generally, when a plated steel sheet is heated for hot forming, the coating and the base iron are alloyed and the coating becomes thicker. Since the solid solubility of C in the coating is extremely low, the undissolved C is enriched in the surface part during the alloying process, which increases the C content in the surface part. The high C content in the surface part increases the hardness of the surface part, thereby deteriorating the bendability.

[0080] On the other hand, when a hot-formed part is produced by hot press forming the hot-formed plated steel sheet according to one aspect of the present invention, even if C is concentrated in the surface layer of the part, the C content (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is less than 1.2, so that the excessive increase in the hardness of the surface layer of the component can be suppressed. In addition, the sum of the Mn and Cr contents in the surface layer of the component (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn PS +Cr PS ) / (Mn B +Cr B )) is above 0.8, so the formation of ferrite is suppressed by sufficient hardenability, so that the ferrite coverage of the martensite grain boundaries in the surface part of the component (the proportion of ferrite in the martensite grain boundaries when observing the cross section) can be below 30%, as a result of which sufficient strength and excellent bendability can be ensured.

[0081] As described above, the ratio (C S / C B ) is 1.2 or less, and the ratio ((Mn PS +Cr PS ) / (Mn B +Cr B )) satisfies 0.8 or higher, so the bend angle measured by the VDA 238-100 bend test at a tensile strength of 1500 MPa is 60° or higher, thereby ensuring excellent impact properties. However, as the tensile strength increases, for example, when the tensile strength of a thermoformed part is 1800 MPa or higher, the bend angle standard for determining excellent impact properties may be lower.

[0082] Next, a method for producing a hot-forming plated steel sheet and a hot-formed component having excellent impact properties after hot forming according to another aspect of the present invention will be described in detail.

[0083] Method for producing hot-forming plated steel sheet with excellent impact properties after hot forming

[0084] Another aspect of the present invention provides a method for producing a hot-forming plated steel sheet having excellent impact properties after hot forming, comprising the following steps: heating a slab satisfying the above-mentioned alloy composition to 1050-1300°C; performing hot finish rolling on the heated slab in a temperature range of 800-950°C to obtain a hot-rolled steel sheet; after completing the hot finish rolling, coiling the hot-rolled steel sheet at 450-750°C; heating the coiled hot-rolled steel sheet to 740-860°C and annealing for 10-600 seconds in an atmosphere with a dew point temperature of -10°C to 30°C; and immersing the annealed hot-rolled steel sheet in a plating bath for plating, the plating bath being composed of zinc, aluminum or an alloy thereof.

[0085] Slab heating steps

[0086] First, a slab having the above alloy composition is heated to 1050-1300° C. If the heating temperature is lower than 1050° C., homogenization of the slab structure may be difficult to achieve, and if the heating temperature exceeds 1300° C., an excessive oxide layer may form.

[0087] Hot rolling steps

[0088] The heated slab is subjected to hot finish rolling at a temperature range of 800-950°C to obtain a hot-rolled steel plate. When the hot finish rolling temperature is lower than 800°C, a mixed crystal structure is generated in the surface layer of the steel plate due to two-phase rolling, making it difficult to control the shape of the plate. When the hot finish rolling temperature exceeds 950°C, the problem of grain coarsening may occur.

[0089] Cooling and winding steps

[0090] After hot finish rolling, the hot-rolled steel sheet is coiled at 450-750°C. Coiling temperatures below 450°C increase material deviation across the width, potentially leading to sheet breakage and shape defects during cold rolling. On the other hand, coiling temperatures exceeding 750°C can lead to coarsening of carbides, resulting in poor bendability.

[0091] Cold rolling steps

[0092] If desired, the coiled hot-rolled steel sheet may be cold-rolled to obtain a cold-rolled steel sheet before annealing. Cold rolling is performed to more precisely control the thickness of the steel sheet, so annealing and plating can be performed directly without cold rolling. In this case, cold rolling can be performed at a reduction ratio of 30-80%.

[0093] Annealing step

[0094] The coiled hot-rolled steel sheet is heated to 740-860°C and annealed for 10-600 seconds in an atmosphere with a dew point temperature of -10°C to 30°C. When the annealing temperature is lower than 740°C or the annealing time is less than 10 seconds, the recrystallization of the structure is insufficient, resulting in a poor shape of the sheet, or the strength after plating is too high, which may cause wear of the die during the punching process. In addition, the diffusion of C is insufficient during the annealing process, making it difficult to ensure the C content (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.6 or less. On the other hand, when the annealing temperature exceeds 860°C or the annealing time exceeds 600 seconds, a large amount of annealing oxide is formed on the surface of the steel sheet during the annealing process, which may cause non-plating or reduce the plating adhesion. In addition, due to internal oxidation, Mn, Cr, etc. in the base iron are formed at the interface between the plating layer and the base iron or at the base iron grain boundary, etc., making it difficult to ensure that the sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8 or more, the hardenability of the surface portion may be insufficient, so ferrite is partially formed in the surface portion after hot forming, resulting in a problem that the bendability may be reduced.

[0095] Furthermore, in the present invention, controlling the dew point of the annealing atmosphere is crucial in order to control the ratio of the C, Mn, and Cr contents in the surface layer to the base steel plate's base metal components. If the dew point of the annealing atmosphere is below -10°C, the decarburization reaction is insufficient, resulting in a limited improvement in bendability. On the other hand, if the dew point exceeds 30°C, excessive internal oxidation reduces the hardenability of the surface layer, leading to partial formation of ferrite and potentially deteriorating bendability.

[0096] Furthermore, according to one aspect of the present invention, more preferably, the coiled hot-rolled steel sheet may be heated to 800-840° C., and the annealing may be performed in an atmosphere with a dew point temperature of 10° C. to 30° C. for 10-100 seconds.

[0097] Plating steps

[0098] After annealing, the coiled hot-rolled steel sheet is immersed in a plating bath for plating, and the plating bath is composed of zinc, aluminum or an alloy containing them. In the present invention, there is no particular limitation on the composition of the plating bath used when forming the coating. However, as a non-restrictive specific embodiment, the plating bath used in the present invention can be composed of zinc, zinc alloy, aluminum, and aluminum alloy. In addition, as long as the plating conditions are generally used for the plating conditions of steel sheets for hot press forming, they can be used in the present invention without restriction, and therefore are not specifically mentioned in this specification. In addition, according to one aspect of the present invention, the plating bath may contain other inevitable impurities, and the zinc alloy and aluminum alloy may also contain components that can be generally contained, in particular, other inevitable impurities, within the scope of not damaging the purpose of the present invention.

[0099] Method for producing thermoformed parts with excellent impact properties

[0100] Hot-formed components with excellent impact properties can be produced by hot press-forming the hot-formed plated steel sheet produced according to the manufacturing method of the present invention. The hot press-forming method commonly used in the art can be used for the hot press-forming process. However, as a non-limiting embodiment, the hot-formed plated steel sheet can be heat-treated at a temperature range of Ac3 to 950°C for 1-15 minutes before being hot-formed by stamping. DETAILED DESCRIPTION

[0101] The present invention is described in more detail below using examples. However, it should be noted that the following examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the contents of the claims and any reasonable deductions therefrom.

[0102] (Example)

[0103] First, slabs having the alloy composition shown in Table 1 below were prepared, and the slabs were heated, hot-rolled, and coiled according to the manufacturing conditions shown in Table 2 below to produce hot-rolled steel sheets. Subsequently, the slabs were annealed according to the annealing conditions shown in Table 2, and then immersed in a zinc plating bath for plating so that the coating weight per side was 70 g / m 2 , thereby producing plated steel sheets.

[0104] [Table 1]

[0105] category C Si Mn P S Al N Cr Ti B Steel Grade A 0.21 0.25 1.3 0.01 0.002 0.035 0.005 0.22 0.03 0.0022 Steel grade B 0.2 0.1 2.5 0.009 0.001 0.03 0.004 0.1 - -

[0106] [Table 2]

[0107]

[0108] For the plated steel sheets of the invention examples and comparative examples produced under the aforementioned manufacturing conditions, the concentrations of carbon (C), manganese (Mn), and chromium (Cr) were analyzed at a sufficient depth from the surface layer to the depth direction using a glow discharge spectrometer (GDS; a LECO GDS 850A was used), which enables quantitative analysis of various components in the depth direction. Based on the GDS analysis results, the average concentrations in the surface region were analyzed using an integration method, and the results are shown in Table 3 below. Typically, GDS analysis is performed in the depth direction on a circular area of ​​2 to 6 mm, making it difficult to accurately identify the coating / base steel sheet interface based on the depth-direction concentration distribution. However, based on various optical and SEM analysis results, the coating / base steel sheet interface is defined as the location where the Zn content is 1%.

[0109] [Table 3]

[0110]

[0111]

[0112] Furthermore, the plated steel sheets of each of the inventive examples and comparative examples were hot press-formed under the conditions listed in Table 4 below to produce hot-formed parts. Test pieces were collected from flat surfaces of the produced hot-formed parts and subjected to tensile and bend tests (VDA 238-100). The concentrations of carbon (C), manganese (Mn), and chromium (Cr) were analyzed in the depth direction by GDS analysis. Furthermore, cross-sections were observed using an optical microscope to evaluate the ferrite coverage of the martensite grain boundaries in the surface layer of the parts. The results are also shown in Table 4.

[0113] [Table 4]

[0114]

[0115] The ratio (C S / C B ) is less than 0.6, and the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) satisfies 0.8 or more. Therefore, the 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 above, so that the ferrite coverage in the martensite grain boundaries of the surface portion is 30% or less, and the bending angle at a tensile strength of 1500 MPa level is 60° or above, thus showing good bending properties.

[0116] Comparative Example 1 is a case where the dew point temperature during annealing is lower than -10°C, and Comparative Example 2 is a case where the heating temperature during annealing does not meet the conditions of the present invention. The ratio of the plated steel sheets of Comparative Example 1 and Comparative Example 2 (C S / C B ) are all greater than 0.6, so the ratio (C PS / C B ) also exceeds 1.2, thereby deteriorating the bending characteristics.

[0117] In addition, Comparative Example 3 is a case where the dew point temperature during annealing exceeds 30°C, and Comparative Example 4 is a case where annealing is excessively performed. The ratio (C S / C B ) all meet the conditions of the present invention, but are higher than ((Mn S +Cr S ) / (Mn B +Cr B )) is less than 0.8, and the ratio of the hot formed part ((Mn PS +Cr PS ) / (Mn B +Cr B )) is less than 0.8. Therefore, the ferrite coverage in the martensite grain boundaries of the surface layer of the component exceeds 30%, and compared with other examples, the tensile strength is relatively reduced and the bendability is also greatly reduced.

[0118] The above description is made with reference to the embodiments, but it will be understood by those skilled in the art that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as described in the claims.

Claims

1. A hot-forming coated steel sheet comprising: foundation steel plate; and a coating layer formed on the surface of the base steel sheet, wherein the coating layer is formed of zinc, aluminum or an alloy containing them, wherein the C content (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.05-0.6, and the sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8-2, wherein the surface layer portion refers to an area from the surface of the base steel plate excluding the coating layer to a depth of 15 μm.

2. The hot-forming plated steel sheet according to claim 1, wherein: The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.05-0.

5.

3. The hot-forming plated steel sheet according to claim 1, wherein: The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.05-0.

4.

4. The hot-forming plated steel sheet according to claim 1, wherein The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.05-0.

35.

5. The hot-forming plated steel sheet according to claim 1, wherein The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.05-0.

14.

6. The hot-forming plated steel sheet according to claim 1, wherein The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.05-0.

1.

7. The hot-forming plated steel sheet according to claim 1, wherein: The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.1-0.

6.

8. The hot-forming plated steel sheet according to claim 1, wherein The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.14-0.

6.

9. The hot-forming plated steel sheet according to claim 1, wherein The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.35-0.

6.

10. The hot forming plated steel sheet according to claim 1, wherein The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.4-0.

6.

11. The hot-forming plated steel sheet according to claim 1, wherein The C content of the surface layer (C S ) and the C content of the base steel plate (C B ) of the ratio (C S / C B ) is 0.5-0.

6.

12. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8-0.

85.

13. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8-0.

87.

14. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8-0.

93.

15. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8-0.

98.

16. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.8-0.

99.

17. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.85-2.

18. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.87-2.

19. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.93-2.

20. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.98-2.

21. The hot forming plated steel sheet according to any one of claims 1 to 11, wherein: The sum of the Mn and Cr contents in the surface layer (Mn S +Cr S ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn S +Cr S ) / (Mn B +Cr B )) is 0.99-2.

22. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: The coating is a hot-dip galvanized layer, an electroplated galvanized layer, an alloyed zinc coating, an aluminum coating or an aluminum alloy coating.

23. The hot forming plated steel sheet according to any one of claims 1 to 11, wherein: The thickness of the coating is 5-100 μm.

24. The hot-forming plated steel sheet according to any one of claims 1 to 11, wherein: In the microstructure of the base steel plate, in terms of area %, the surface portion contains 40-100% ferrite, and the balance contains 0-60% pearlite, bainite or martensite; the central portion contains 30-90% ferrite, and the balance contains 10-70% pearlite, bainite or martensite.

25. The hot forming plated steel sheet according to any one of claims 1 to 11, wherein: The base steel plate comprises, in weight%, C: 0.15-0.4%, Si: 0.1-1%, 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%, B: 0.0005-0.01%, Ti: 0.01-0.05%, and the balance Fe and other impurities.

26. A thermoformed part comprising: foundation steel plate; and an alloy plating layer formed on the surface of the base steel sheet, wherein the alloy plating layer is formed of an alloy containing zinc or aluminum, wherein the C content (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is 0.88-1.2, and the sum of the Mn and Cr contents in the surface layer of the component (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) of the ratio ((Mn PS +Cr PS ) / (Mn B +Cr B )) is 0.8 or more, wherein the component surface layer portion refers to a region from the surface of the base steel plate excluding the alloy plating layer to a depth of 25 μm.

27. The thermoformed part according to claim 26, wherein C content of the surface layer of the component (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is 0.88-1.

1.

28. The thermoformed part of claim 26, wherein: C content of the surface layer of the component (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is 0.88-1.

05.

29. The thermoformed part of claim 26, wherein: C content of the surface layer of the component (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is 0.88-0.

95.

30. The thermoformed part of claim 26, wherein: C content of the surface layer of the component (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is 0.88-0.

9.

31. The thermoformed part of claim 26, wherein: C content of the surface layer of the component (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is 0.9-1.

2.

32. The thermoformed part of claim 26, wherein: C content of the surface layer of the component (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is 0.95-1.

2.

33. The thermoformed part of claim 26, wherein: C content of the surface layer of the component (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is 1.05-1.

2.

34. The thermoformed part of claim 26, wherein: C content of the surface layer of the component (C PS ) and the C content of the base steel plate (C B ) of the ratio (C PS / C B ) is 1.1-1.

2.

35. The thermoformed part according to any one of claims 26 to 34, wherein The sum of the Mn and Cr contents in the surface layer of the component (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) is 0.8-0.

99.

36. The thermoformed part according to any one of claims 26 to 34, wherein The sum of the Mn and Cr contents in the surface layer of the component (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) is 0.8-0.

98.

37. The thermoformed part according to any one of claims 26 to 34, wherein The sum of the Mn and Cr contents in the surface layer of the component (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) ratio is 0.8-0.

97.

38. The thermoformed part according to any one of claims 26 to 34, wherein The sum of the Mn and Cr contents in the surface layer of the component (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) ratio is 0.8-0.

93.

39. The thermoformed part according to any one of claims 26 to 34, wherein The sum of the Mn and Cr contents in the surface layer of the component (Mn PS +Cr PS ) and the sum of the Mn and Cr contents of the base steel plate (Mn B +Cr B ) is 0.8-0.

9.

40. The thermoformed part according to any one of claims 26 to 34, wherein The coating is an alloyed zinc coating or an aluminum alloy coating.

41. The thermoformed part according to any one of claims 26 to 34, wherein The ferrite coverage in the martensite grain boundaries in the surface layer portion of the component is 30% or less.

42. The thermoformed part according to any one of claims 26 to 34, wherein The thermoformed part has a bending angle of 60° or more as measured by a VDA 238-100 bending test.

43. The thermoformed part according to any one of claims 26 to 34, wherein The base steel plate comprises, in weight%, C: 0.15-0.4%, Si: 0.1-1%, 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%, B: 0.0005-0.01%, Ti: 0.01-0.05%, and the balance Fe and other impurities.

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