Hot press formed member and method for manufacturing same

Abstract

The present invention provides a hot press formed member, particularly a hot press formed member obtained from a non-plated steel sheet, and a method for manufacturing same.

Description

Hot-formed part and method of manufacturing the same

[0001] The present invention relates to a hot-formed part and a method for manufacturing the same.

[0002] Recently, as regulations regarding passenger protection and fuel efficiency improvement through vehicle weight reduction have become stricter, the application of hot-formed parts as materials for automotive structural components is increasing to achieve these goals. Hot-formed parts are obtained by applying a hot-forming process to steel sheets, which enables them to possess high strength. Accordingly, hot-formed parts are particularly suitable for applications such as bumpers, doors, and pillar reinforcements that require ultra-high strength or significant energy absorption capabilities.

[0003] Patent Document 1 proposes a technology regarding the hot forming of steel sheets as described above. This document describes a method for securing ultra-high strength with high tensile strength by heating an Al-Si plated steel sheet to 850°C or higher, and then forming the microstructure of a member manufactured by hot forming and rapid cooling using a press into martensite. As such, since the hot-formed member (part) is obtained by forming the steel sheet at a high temperature, it has the advantage of being easily formed when manufacturing parts with complex shapes. Furthermore, since rapid cooling within the form can increase strength, a lightweighting effect resulting from increased strength can be expected.

[0004] Meanwhile, a technology for manufacturing parts by hot-forming hot-rolled or cold-rolled (annealed) steel sheets, rather than plated steel sheets, as hot-forming steel sheets, is being proposed; that is, a technology for manufacturing hot-formed parts from non-plated steel sheets that are not plated. When heating these non-plated steel sheets for hot forming, carbon on the surface of the steel sheet reacts with oxygen present in the heating equipment, causing decarburization to occur on the surface of the steel sheet. In other words, a decarburized layer is formed on the surface of the non-plated steel sheet, which results in the surface of the hot-formed part becoming uneven, leading to problems such as inferior surface quality and deterioration of physical properties like bendability.

[0005] Therefore, there is a need to develop technology capable of solving problems such as reduced surface quality, reduced bendability, and reduced fatigue properties when manufacturing hot-formed parts by hot-forming non-plated steel sheets.

[0006] (Patent Document 1) U.S. Patent No. 6296805

[0007] (Patent Document 2) Korean Registered Patent No. 10-1129370

[0008] (Patent Document 3) Korean Registered Patent No. 10-0373280

[0009] One aspect of the present invention is to provide a hot-formed part, in particular a hot-formed part obtained from an unplated steel sheet. Specifically, according to one aspect of the present invention, a hot-formed part with improved surface quality, bendability, and fatigue characteristics, and a method for manufacturing the same are provided.

[0010] The problems of the present invention are not limited to those described above. A person skilled in the art will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0011] According to one aspect of the present invention, the invention comprises: a base steel plate; and an Fe oxide region formed on the surface of the base steel plate, wherein the minimum carbon content (C) existing within 6 μm from a point where the Fe content is 80% when measured by GDS in the thickness direction of the base steel plate on the surface of the base steel plate min ) and the ratio (C) of the nominal carbon content (C0), which is the carbon content at the 45㎛ point when GDS measured in the thickness direction of the base steel plate on the surface of the base steel plate. min Provides a hot-formed part with / C0) less than 0.60.

[0012] In one embodiment of the present invention, the maximum carbon content (C) existing within a 9㎛ point from the point where the Fe content is 80% when measured by GDS in the thickness direction of the base steel plate on the surface of the base steel plate max ) and the ratio (C) of the above nominal carbon content (C0) max / C0) can be 0.60 or higher.

[0013] In one embodiment of the present invention, the maximum carbon content (C max ) and the ratio (C) of the above nominal carbon content (C0) max / C0) may be 1.50 or less.

[0014] As described above, a hot-formed part according to one aspect of the present invention has excellent surface quality and, together with this, can have the effect of improved fatigue characteristics and bendability.

[0015] In one embodiment of the present invention, the minimum Mn content (Mn) existing within 1 μm from the point where the Fe content is 80% when measured by GDS on the surface of the base steel plate in the thickness direction of the base steel plate min ) and minimum Cr content (Cr min Sum of )(Mn min +Cr min) and, when GDS measured in the thickness direction of the base steel plate on the surface of the base steel plate, the ratio (Mn) of the sum of the Mn content (Mn0) and Cr content (Cr0) at the 45㎛ point (Mn0+Cr0) min +Cr min / Mn0+Cr0) may be 0.95 or less.

[0016] In one embodiment of the present invention, the above (Mn min +Cr min The region where / Mn0+Cr0) is 0.95 or less can be composed of ferrite.

[0017] In one embodiment of the present invention, the base steel sheet comprises, in weight%, carbon (C): 0.02~0.45%, silicon (Si): 0.50~2.00%, aluminum (Al): 0.001~1.000%, manganese (Mn): 0.4~3.0%, chromium (Cr): 1.0~5.0%, phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, nitrogen (N): 0.0100% or less, titanium (Ti): 0~1.00%, niobium (Nb): 0~0.10%, vanadium (V): 0~0.50%, boron (B): 0~0.0200%, molybdenum (Mo): 0~1.00%, tungsten (W): 0~1.00%, copper (Cu): 0~1.0%, Nickel (Ni): 0~1.0%, Tin (Sn): 0~1.00%, Antimony (Sb): 0~0.100%, Calcium (Ca): 0~0.10%, Magnesium (Mg): 0~0.10%, Cobalt (Co): 0~1.00%, Arsenic (As): 0~1.00%, Zirconium (Zr): 0~1.00%, Bismuth (Bi): 0~1.00%, Rare Earth Elements (REM): 0~0.3%, and the remainder may contain Fe and other unavoidable impurities.

[0018] According to another aspect of the present invention, a method for manufacturing a hot-formed part is provided, comprising the steps of preparing a base steel plate and heating the base steel plate, wherein the step of heating the base steel plate includes a first heating section and a second heating section.

[0019] In one embodiment of the present invention, the first heating section may be a temperature range of 300℃ to Ae1-50℃, and the second heating section may be a temperature range greater than Ae1-50℃.

[0020] In one embodiment of the present invention, the first heating section can be controlled to an oxygen concentration of 3.00 to 19.00%, and the second heating section can be controlled to an oxygen concentration of 1.00 to 10.00%.

[0021] In one embodiment of the present invention, the oxygen concentration of the second heating section may be lower than the oxygen concentration of the first heating section.

[0022] In this way, by dividing the temperature ranges during the process of heating the base steel plate and controlling the oxygen concentration in each temperature range, a target hot-formed part can be obtained.

[0023] As an example, the above hot-formed part may have excellent surface quality and excellent bendability and fatigue characteristics.

[0024] In one embodiment of the present invention, the base steel sheet is a non-plated steel sheet that has not undergone plating, and may be a hot-rolled steel sheet or a cold-rolled steel sheet, wherein the cold-rolled steel sheet may be either an un-annealed steel sheet or an annealed steel sheet.

[0025] According to the present invention, a hot-formed part obtained by hot-forming a non-plated steel sheet can be provided, and said hot-formed part has the effect of improved surface quality, fatigue characteristics, and bendability.

[0026] The various and beneficial advantages and effects of the present invention are not limited to those described above and may be more easily understood in the process of explaining specific embodiments of the present invention.

[0027] Figure 1 schematically illustrates the cross-sectional structure of a hot-formed part according to one embodiment of the present invention.

[0028] FIG. 2 schematically illustrates the cross-sectional structure of a hot-formed part according to another embodiment of the present invention.

[0029] FIG. 3 is a graph showing the GDS measurement results of a hot-formed part (Invention Example 1) according to one embodiment of the present invention, at the point where the Fe content is 80% from the surface, C min The branch and C max This shows an example of a point corresponding to.

[0030] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0031] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0032] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0033] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0034] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0035] In addition, in the present invention, the term "steel plate" refers to a coil or sheet material that has not yet been processed into a specific shape, and the term "part" refers to a part that has been processed into a non-plate shape through a forming process.

[0036] It should be noted that in the present invention, when expressing the content of each element, the basis is weight (weight%) unless specifically otherwise specified. Furthermore, the proportion of crystals or structures is based on area (area%) unless specifically otherwise expressed, and the gas content is based on volume unless specifically otherwise expressed.

[0037] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0038] The present invention will be described in detail below.

[0039] The present invention is designed to solve the aforementioned problems and has technical significance in providing a hot-formed part obtained using a non-plated steel sheet, and in providing a hot-formed part with improved surface quality as well as bendability and fatigue characteristics, and a method for manufacturing the same.

[0040] According to one aspect of the present invention, a hot-formed part is provided.

[0041] In one embodiment of the present invention, the hot-formed part may include a base steel plate and an Fe oxide region formed on the surface of the base steel plate.

[0042] In one embodiment of the present invention, the minimum carbon content (C) existing within 6㎛ from the point where the Fe content is 80% when measured by GDS on the surface of the base steel plate in the thickness direction of the base steel plate min ) and the ratio (C) of the nominal carbon content (C0), which is the carbon content at the 45㎛ point when GDS measured in the thickness direction of the base steel plate on the surface of the base steel plate. min / C0) may be less than 0.60.

[0043] As mentioned above, a hot-formed part according to one embodiment of the present invention can be obtained by hot-forming a non-plated steel sheet, for example, a hot-rolled steel sheet manufactured through a series of hot-rolling processes or a cold-rolled steel sheet manufactured through a series of cold-rolling processes on the hot-rolled steel sheet.

[0044] Unlike hot-formed parts obtained by hot-forming conventional non-plated steel sheets, which have an uneven surface and low bendability, a hot-formed part according to one embodiment of the present invention can relatively lower the strength of the surface layer by including a region with a low carbon content (also known as a carbon-deficient layer) in a certain area in the thickness direction from the surface of the base steel sheet. This contributes to improving the bendability of the hot-formed part.

[0045] According to one embodiment of the present invention, the above (Cmin If / C0) is 0.60 or higher, the reduction in strength of the surface layer is insufficient, so improvement in bendability cannot be achieved. In one embodiment of the present invention, the above (C min The lower limit of / C0) is not specifically limited, and if the value obtained by the carbon content in the base steel sheet and the process conditions for manufacturing hot-formed parts is less than 0.60, it may be considered to fall within the scope of the present invention. However, the above (C min / C0) can be greater than 0.00.

[0046] A hot-formed part according to one embodiment of the present invention has (C on the surface of a base steel plate min By including a region (carbon-deficient layer) where / CO) is less than 0.60, it can possess excellent bendability characteristics. As one example, during a bending test, the maximum bending angle (C min Compared to cases where the criteria of / C0) are not met, an effect of improvement of more than 15% can be obtained.

[0047] In one embodiment of the present invention, the ratio of carbon content (C min In order to measure / CO), GDS may be used as a non-limiting example, and this applies equally below. As one example, the above-mentioned minimum carbon content (C min ) is the C at the point with the lowest C content within 6㎛ in the thickness direction of the base steel plate, starting from the point where the Fe content is 80% during GDS measurement from the surface of the base steel plate in the thickness direction of the base steel plate. min The value can be taken, and the nominal C content (C0) of the base steel plate can be taken as the carbon (C) value at the 45㎛ thickness point when GDS measuring in the thickness direction of the base steel plate.

[0048] A hot-formed part according to one embodiment of the present invention has a maximum carbon content (C) existing within a 9㎛ point from the point where the Fe content is 80% when measured by GDS on the surface of the base steel plate in the thickness direction of the base steel plate.max ) and the ratio (C) of the above nominal carbon content (C0) max / C0) can be 0.60 or higher.

[0049] That is, a hot-formed part according to one embodiment of the present invention has, based on the thickness direction, the ratio of the carbon content (C min By having a certain carbon-enriched region (also known as a carbon enrichment layer) below a region where / C0) is less than 0.60 (excluding 0.00), defects such as cracks formed on the surface during the manufacturing process of the part or during additional processing of the part can be suppressed from spreading into the interior of the base steel sheet. Accordingly, a hot-formed part according to one embodiment of the present invention can have excellent fatigue characteristics in addition to the aforementioned bendability.

[0050] In one embodiment of the present invention, the ratio of carbon content (C) in the carbon-enriched region max If / CO) is less than 0.60, the carbon enrichment layer area is insufficient, so the effect of suppressing the propagation of cracks is insufficient, and consequently, the improvement of fatigue properties cannot be achieved.

[0051] In one embodiment of the present invention, the ratio of carbon content (C max / C0) may be 1.50 or less, and if the above value exceeds 1.50, the problem of fatigue properties becoming inferior may occur as carbon becomes excessively enriched.

[0052] In one embodiment of the present invention, the ratio of carbon content (C max In order to measure / CO), GDS can be used as described above. As one example, the above maximum carbon content (C max) can take the C value at the point where the C content is highest within 9㎛ in the thickness direction of the steel sheet from the point where the Fe content is 80% when GDS measuring in the thickness direction of the steel sheet on the surface of the steel sheet, and the C content (C0) of the steel sheet can take the C value at the point where the thickness is 45㎛ when GDS measuring in the thickness direction of the steel sheet.

[0053] As such, a hot-formed part according to one embodiment of the present invention may have excellent fatigue characteristics by including a region in which carbon is deficient in a certain area in the thickness direction on the surface of a base steel sheet, along with a region in which carbon is enriched. As one example, the ratio of carbon content (C) for which a fatigue limit is presented max You can achieve an improvement of more than 10% compared to parts that do not meet the standards of / C0).

[0054] Meanwhile, a hot-formed part according to one embodiment of the present invention has a minimum Mn content (Mn) existing within a 1㎛ point from the point where the Fe content is 80% when measured by GDS on the surface of the base steel plate in the thickness direction of the base steel plate. min ) and minimum Cr content (Cr min Sum of )(Mn min +Cr min ) and, when GDS measured in the thickness direction of the base steel plate on the surface of the base steel plate, the ratio (Mn) of the sum of the Mn content (Mn0) and Cr content (Cr0) (Mn0+Cr0) at the 45㎛ point min +Cr min / Mn0+Cr0) may be 0.95 or less. In one embodiment of the present invention, the above (Mn min +Cr min The lower bound of / Mn0+Cr0) is not specifically limited, but 0.00 is excluded.

[0055] Mn and Cr are elements that inhibit the formation of ferrite in steel. In one embodiment of the present invention, by forming a region in which the content of Mn and Cr is relatively low, the formation of ferrite can be induced in that region. Accordingly, the region in which the content of Mn and Cr is low may contain ferrite. In this way, by including a region composed of ferrite in the thickness direction on the surface of a hot-formed part, the bendability of the part can be improved more advantageously. As one example, the region in which the content of Mn and Cr is low may be composed of a single ferrite phase, but ferrite and martensite may be mixed. In the latter case, the martensite phase may be included in an area fraction of 15% or less (including 0%).

[0056] According to one embodiment of the present invention, the ratio of Mn to Cr (Mn min +Cr min Regions where / Mn0+Cr0) is 0.95 or less may exist in the form of a band.

[0057] In one embodiment of the present invention, GDS can be used as described above to measure the ratio of Mn and Cr content. As one example, the minimum content of Mn and Cr (each Mn min , Cr min ) can take the Mn and Cr values ​​at the point where the Fe content is 80% when measuring in the thickness direction of the base steel plate from the surface of the part, and the Mn and Cr content of the base steel plate (nominal Mn and nominal Cr content) can take the Mn and Cr values ​​at the point where the thickness is 45㎛ when measuring in the thickness direction of the base steel plate with GDS.

[0058] Meanwhile, a hot-formed part according to one embodiment of the present invention may include an oxide layer on its surface, and the region where the oxide layer exists may be defined as extending from the surface of the base steel plate to a point where the Fe content is 80% when measured via GDS in the thickness direction of the base steel plate. As one example, the oxide layer may include one or more of Mn, Cr, and Si.

[0059] In one embodiment of the present invention, the base steel sheet for obtaining a hot-formed part is not specifically limited, and any steel that is hot-formable and is an unplated steel sheet that has not been plated may be used.

[0060] In one embodiment of the present invention, the base steel sheet may include elements that can typically be added to steel, and the types and contents thereof are not specifically limited. However, non-limiting examples of elements that may be added to a base steel sheet according to one embodiment of the present invention include, in weight%, carbon (C): 0.02~0.45%, silicon (Si): 0.50~2.00%, aluminum (Al): 0.001~1.000%, manganese (Mn): 0.4~3.0%, chromium (Cr): 1.0~5.0%, phosphorus (P): 0.050% or less, sulfur (S): 0.0200% or less, nitrogen (N): 0.0100% or less, titanium (Ti): 0~1.00%, niobium (Nb): 0~0.10%, vanadium (V): 0~0.50%, boron (B): 0~0.0200%, molybdenum (Mo): 0~1.00%, tungsten (W): 0~1.00%, Copper (Cu): 0~1.0%, Nickel (Ni): 0~1.0%, Tin (Sn): 0~1.00%, Antimony (Sb): 0~0.100%, Calcium (Ca): 0~0.10%, Magnesium (Mg): 0~0.10%, Cobalt (Co): 0~1.00%, Arsenic (As): 0~1.00%, Zirconium (Zr): 0~1.00%, Bismuth (Bi): 0~1.00%, Rare Earth Elements (REM): 0~0.3%, and the remainder may contain Fe and other unavoidable impurities.

[0061] Among the alloy compositions described above, C, Mn, etc., can be added to ensure the strength of the steel; Si is effective not only for deoxidation but also for reducing the segregation of Mn and Cr within the base steel sheet, and Al has a deoxidation effect. Mn and Cr can increase the hardenability of the material, making them effective for ensuring the strength of the material. Meanwhile, the above Si and Cr are oxygen-affinity elements and are effective in improving the surface quality of non-plated hot-formed steel. It should be noted that P, S, N, etc., may be elements inevitably introduced during the steel manufacturing process, but are not limited to these. Furthermore, it will be obvious to a person skilled in the art that, in addition to the compositions described above, Ti, B, Cu, Mo, Ni, V, Ca, Nb, Sn, W, Sb, Mg, Co, As, Zr, Bi, REM, etc., may be additionally included in consideration of the target physical properties of the final product.

[0062] Hereinafter, a method for manufacturing a hot-formed part according to another aspect of the present invention will be described. However, it should be noted that the following method is merely one example for manufacturing a hot-formed part, and that a hot-formed part according to one embodiment of the present invention must not necessarily be manufactured by this manufacturing method. Furthermore, any manufacturing method that satisfies the claims of the present invention may be used without issue to implement each embodiment of the present invention.

[0063] According to one embodiment of the present invention, a hot-formed part can be manufactured by including the steps of preparing a base steel plate and heating the base steel plate.

[0064] In one embodiment of the present invention, the base steel sheet for obtaining a hot-formed part may be the aforementioned base steel sheet, and it should be noted that the composition is not particularly limited and is replaced by the foregoing details.

[0065] Meanwhile, the above-mentioned base steel sheet may be a hot-rolled steel sheet manufactured through a series of hot-rolling processes, or a cold-rolled steel sheet manufactured through a series of cold-rolling processes with respect to the above-mentioned hot-rolled steel sheet. Here, the cold-rolled steel sheet may be an un-annealed steel sheet or an annealed steel sheet.

[0066] The cold rolling process, including the hot rolling and annealing processes mentioned above, is not specifically limited as it can be applied under normal conditions. However, to provide a non-limiting example of manufacturing hot-rolled steel sheets and cold-rolled steel sheets, the process may include the steps of: preparing a steel slab having the aforementioned alloy composition, then heating the steel slab in a temperature range of 1050 to 1300°C; finishing hot rolling the heated steel slab in a temperature range of 800 to 950°C; and coiling the steel sheet in a temperature range of 500 to 740°C after finishing hot rolling. Additionally, the hot-rolled steel sheet manufactured according to the above may be pickled to produce a pickled hot-rolled steel sheet, and furthermore, a cold-rolled steel sheet may be obtained by cold rolling the pickled hot-rolled steel sheet at a cold reduction rate of 30 to 80%. In addition, an annealed steel sheet may be obtained by annealing the above cold-rolled steel sheet in a temperature range of 750 to 860°C and then cooling it.

[0067] In one embodiment of the present invention, the hot-rolled steel sheet or cold-rolled steel sheet manufactured according to the above, i.e., the non-plated steel sheet, can be heated to a high temperature.

[0068] In one embodiment of the present invention, the process of heating a non-plated steel sheet to a high temperature can be performed by distinguishing temperature ranges. As an example, a first heating range defined as 300℃ to Ae1-50℃ and a second heating range having a temperature range exceeding Ae1-50℃ can be defined, and the oxygen concentration up to each heating range can be controlled. Here, Ae1 represents the equilibrium phase transformation temperature at which austenite transformation begins, and can be calculated using commercial software such as Thermo.Calc.

[0069] In one embodiment of the present invention, the upper limit temperature of the heating step, that is, the upper limit temperature of the second temperature range, is not specifically limited and may be a temperature generally set during hot forming. As one example, the upper limit of the second temperature range may be 900°C or higher and 1000°C or lower.

[0070] In order to improve the bendability and fatigue characteristics targeted in the present invention, the oxygen concentration (volume%) in the second heating section can be controlled to 1.00 to 10.00%. If the oxygen concentration in this section is less than 1.00%, decarburization may not occur smoothly on the surface of the steel, and the improvement in bendability may be insufficient. On the other hand, if the concentration exceeds 10.00%, excessive decarburization may occur, and there is a risk that the strength of the material will be significantly degraded.

[0071] In addition, the oxygen concentration in the first heating section can be controlled to 3.00 to 19.00%. By controlling the oxygen concentration in the first heating section in this way, a carbon enrichment layer can be advantageously formed directly below the carbon-deficient layer, thereby improving the fatigue properties of the material. If the oxygen concentration in the first heating section is less than 3.00%, it becomes difficult to form a carbon enrichment layer, whereas if the concentration exceeds 19.00%, an excessive carbon enrichment layer is formed on the surface layer, which increases the brittleness of the material and may lead to a decline in fatigue properties.

[0072] In one embodiment of the present invention, when controlling the oxygen concentration in the first heating section and the second heating section, the oxygen concentration in the second heating section can be controlled to a lower concentration than that in the first heating section.

[0073] In this way, by distinguishing heating sections according to temperature during the heating process for hot forming and controlling the oxygen concentration in each section, a target hot-formed part can be obtained. As one example, a hot-formed part can be obtained that includes a base steel plate and an Fe oxide region formed on the surface of the base steel plate, and, when GDS is measured on the surface of the base steel plate in the thickness direction of the base steel plate, includes a region where the C content is lower relative to the C content of the base steel plate, and a region below the lowered C content where the C content is higher relative to the C content of the base steel plate.

[0074] In one embodiment of the present invention, the oxygen concentration in the first heating section may be in the range of 5.00 to 19.00%, and the oxygen concentration in the second heating section may be in the range of 2.00 to 10.00%. In this way, by controlling the oxygen concentration in each heating section to a more optimized range, oxygen-affinity elements such as Mn and Cr present in the base steel sheet can be diffused to the surface (surface layer), and as these diffused elements form oxides, the hot-formed part may include an oxide layer. The hot-formed part having the oxide layer may have the effect of further improving surface quality and bendability.

[0075] In one embodiment of the present invention, while forming (pressing) the non-plated steel sheet that has undergone the heating process in a hot state, a step of cooling at a cooling rate greater than or equal to the critical cooling rate may be performed. As a non-limiting example, the cooling may be performed at a cooling rate of 30°C / s or more.

[0076] As described above, the base steel sheet of a hot-formed part manufactured through a series of hot-forming processes may have a hard structure. As one example, the base steel sheet may have a microstructure with an area fraction of 70% or more of the combined martensite and bainite phases, and may also include pearlite, ferrite, etc. as other structures. However, it is not limited thereto. Accordingly, a hot-formed part according to one embodiment of the present invention may have ultra-high strength, and as an example, may have a tensile strength of 1000 MPa or more. At this time, the hard structure may exist in the region excluding the surface layer of the base steel sheet, that is, in the region excluding the aforementioned oxide layer, carbon-deficient layer, and additionally the carbon-enriched layer. As one example, it may be a structure measured at a point 1 / 4t (where t means thickness (mm)) in the thickness direction of the base steel sheet.

[0077] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody 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 matters described in the patent claims and matters reasonably inferred therefrom.

[0078] (Example)

[0079] After preparing steel slabs having the alloy composition shown in Table 1 below, each steel slab was heated to 1200°C, then finished hot-rolled at 930°C, and then coiled at 600°C to obtain hot-rolled steel sheets. Each hot-rolled steel sheet was cold-rolled with a cold reduction rate of 60–80% to produce cold-rolled steel sheets.

[0080] Each cold-rolled steel sheet manufactured according to the above was made into a blank, and each blank was loaded into a connected furnace and heat-treated by passing it through a first heating section and a second heating section, and then hot-formed using a flat plate mold to manufacture a hot-formed part. At this time, the total heat treatment time for passing through the first heating section and the second heating section was standardized to 5 minutes, and the upper limit temperature of the second heating section was standardized to 930~950℃. Meanwhile, the oxygen concentration according to each heating section is shown in Tables 2 to 4 below.

[0081] Subsequently, tensile properties (tensile strength), bending properties, and fatigue properties were evaluated for each of the above hot-formed parts.

[0082] At this time, tensile properties were measured using a universal tensile testing machine in accordance with ISO 6892 standards using JIS-5 specimens, and bending properties were measured by determining the bending angle at maximum load in accordance with the VDA238-100 test method. In addition, fatigue properties were measured by conducting a fatigue test after taking the specimens to determine the fatigue life (cycles). The fatigue test was conducted in bending mode in accordance with JIS Z2275 standards.

[0083] Meanwhile, to verify the surface characteristics of each hot-formed part, the content of C, Fe, Mn, and Cr was measured using GDS from the surface of each hot-formed part (the surface of the base steel sheet) in the thickness direction of the base steel sheet. From the measurement results, the location of the peak (point) and the point where the Fe content reaches 80% were identified, and based on this, the content ratio of specific elements in each layer was calculated.

[0084] Steel Grade Alloy Composition (Wt%) CsiMnPSAlCrMoNTiBA 0.085 1.4200.6200.0090.0010.0401.830 -0.0040.0300.002B 0.2201.5600.8000.0100.0010.0362.300 -0.0040.0310.003C 0.3101.3900.8700.0080.0010.0322.2100.1500.0040.0260.002D 0.3381.1200.7200.0060.0010.0341.5300.0900.0040.0290.002

[0085] Steel Specimen No. Mechanical Properties Ae1-50 Second Heating Section C min / C0 Bending Characteristic Classification YS(MPa)TS(MPa)TE(%)(°C) Oxygen Concentration(%) Bending Angle(°) Improvement Rate(%) A 1865 1105 6.478 0.70.59 1.238 4.7 - Comparative Example 1 2855 1100 7.078 0.70.69 0.729 0.36.6 Comparative Example 2 3846 1095 6.978 0.73.40 0.131 00.118.2 Inventive Example 1 B 41118 1546 8.1772 0.871 0.249.97 - Comparative Example 3 51104 1543 8.0772 0.76 0.685 2.645.3 Comparative Example 46107815127.7772.04.120.1061.8923.9 Invention Example 27108715346.8772.08.980.0560.5221.1 Invention Example 3C8143817876.9746.00.770.9846.60-Comparative Example 59143817876.9746.02.120.1555.3018.7 Invention Example 4D10134620257.4741.80.550.7642.82-Comparative Example 611138020197.0741.89.120.6249.9716.7 Invention Example 512136220207.8741.88.880.0754.2026.6 Invention Example 6 [Note] The oxygen concentration in the first heating section was applied equally to all specimens and was controlled to 5.00%. The improvement rate of bending characteristics was expressed by quantifying the level of improvement compared to the bending angle of a specimen (control group) pressed with a flat plate mold after a general heating process (heating and holding at a temperature of Ac3 or higher (5 minutes)) for steel grades A, B, C, and D.

[0086] As shown in Table 2, Invention Examples 1 to 6, in which the oxygen concentration in the second heating section satisfies the range according to one embodiment of the present invention, have a minimum carbon content (C) existing within a 6㎛ point from the point where the Fe content is 80% when GDS is measured in the thickness direction of the base steel plate on the surface of the base steel plate. min ) and the ratio of the nominal carbon content (CO), which is the carbon content at the 45㎛ point (C min / C0) was secured at 0.60 or less, and the bending angle improvement rate was significantly increased. Specifically, it can be confirmed that it was improved by more than 15% compared to the comparative examples.

[0087] On the other hand, in the case of comparative examples where the oxygen concentration in the second heating section falls outside the range of one embodiment of the present invention, it can be seen that the improvement rate of the bending angle is insufficient, and there were even cases where it was lower.

[0088] Steel Specimen No. Ae1-50 1st Heating Section 2nd Heating Section C max / CO Fatigue Characteristic Classification (°C) Oxygen Concentration (%) Oxygen Concentration (%) Fatigue Strength (MPa) Improvement Rate (%) A 17 80.7 1.9 50.4 7 -23 10 Comparative Example 7 27 80.7 20.3 01.3 81.7 82 23 -3.4 Comparative Example 8 37 80.7 6.0 73.5 90.6 72 55 10.5 Inventive Example 7 B 47 72.0 2.3 90.7 4 -4 400 Comparative Example 9 57 72.0 19.2 00.1 21.9 14 01 -8.8 Comparative Example 106 772.0 7.3 19.1 21.1 54 95 12.6 Inventive Example 8 C 87 46.0 3.0 0.6 7 -5 660 Comparative Example [Note] The improvement rate of fatigue characteristics was expressed by quantifying the level of improvement compared to the fatigue strength (MPa) of a specimen (control group) pressed with a flat plate mold after a general heating process (heating and holding at a temperature of Ac3 or higher (5 minutes)) for steel grades A, B, C, and D.

[0089] As shown in Table 3, Invention Examples 7 to 11, in which the oxygen concentration in the first heating section and the second heating section satisfies the range according to one embodiment of the present invention, have a maximum carbon content (C) existing within a 9㎛ point from the point where the Fe content is 80% when measured by GDS in the thickness direction of the base steel plate on the surface of the base steel plate. max ) and the ratio of the nominal carbon content (CO), which is the carbon content at the 45㎛ point (C max The ratio of / CO) satisfied 0.60 to 1.50. These inventive examples satisfy the ratio of carbon content (C max It can be confirmed that the fatigue characteristics are improved by more than 10% compared to comparative examples where / C0) falls outside the range of 0.60 to 1.50.

[0090] On the other hand, in the case of comparative examples where the oxygen concentration in one or more of the first and second heating sections falls outside the range of the embodiment of the present invention, it can be confirmed that the fatigue characteristics are not improved or have actually become inferior.

[0091] Steel Specimen No. Ae1-50 1st Heating Section 2nd Heating Section (Cr Min +Mn min / (CrO+MnO)Bending Characteristic Classification(°)Oxygen Concentration(%)Oxygen Concentration(%)Bending Angle(°) B 477 2.0 2.3 90.8 70.9 549.97Comparative Example 1367 72.0 5.6 73.6 80.9 181.80Inventive Example 12 C 87 46.0 2.8 90.7 70.9 746.60Comparative Example 1497 46.0 11.8 92.1 20.8 160.50Inventive Example 13 D 107 41.8 2.0 10.5 50.9 842.82Comparative Example 151 17 41.8 17.9 98.8 80.6 658.10Inventive Example 14

[0092] As shown in Table 4, Invention Examples 12 to 14, in which the oxygen concentration in the first heating section and the second heating section is more strictly controlled within the range according to one embodiment of the present invention, have a minimum Mn content (Mn) existing within a 1 μm point from the point where the Fe content is 80% when measured by GDS in the thickness direction of the base steel plate on the surface of the base steel plate. min ) and minimum Cr content (Cr min Sum of )(Mn min +Cr min ) and the ratio of the sum of the Mn content (Mn0) and Cr content (Cr0) at the 45㎛ point (Mn min +Cr min / Mn0+Cr0) was secured at 0.95 or less. As a result, the surface quality and bendability of the hot-formed part after hot forming were further improved, and it can be seen that the bendability was particularly significantly improved.

Claims

1. Base steel plate; and It includes an Fe oxide region formed on the surface of the above-mentioned base steel plate, and The minimum carbon content (C) existing within a 6㎛ point from the point where the Fe content is 80% when measured by GDS on the surface of the above steel plate in the thickness direction of the above steel plate min ) and the ratio (C) of the nominal carbon content (C0), which is the carbon content at the 45㎛ point when GDS measured in the thickness direction of the base steel plate on the surface of the base steel plate. min Hot-formed parts with / C0) less than 0.

60.

2. In Paragraph 1, The maximum carbon content (C) existing within a 9㎛ point from the point where the Fe content is 80% when measured by GDS on the surface of the above steel plate in the thickness direction of the above steel plate max ) and the ratio (C) of the above nominal carbon content (C0) max Hot-formed parts with / C0) of 0.60 or higher.

3. In any one of Paragraphs 1 to 2, The above maximum carbon content (C max ) and the ratio (C) of the above nominal carbon content (C0) max Hot-formed parts with / C0) of 1.50 or less.

4. In any one of paragraphs 1 to 3, The minimum Mn content (Mn) existing within a 1㎛ point from the point where the Fe content is 80% when measured by GDS on the surface of the above steel plate in the thickness direction of the above steel plate min ) and minimum Cr content (Cr min Sum of )(Mn min +Cr min ) and, when GDS measured in the thickness direction of the base steel plate on the surface of the base steel plate, the ratio (Mn) of the sum of the Mn content (Mn0) and Cr content (Cr0) at the 45㎛ point (Mn0+Cr0) min +Cr min Hot-formed parts in which / Mn0+Cr0) is 0.95 or less.

5. In any one of paragraphs 1 through 4, The above (Mn min +Cr min The region where / Mn0+Cr0) is 0.95 or less is a hot-formed part containing ferrite.

6. In any one of paragraphs 1 through 5, The above base steel sheet comprises, in weight%, Carbon (C): 0.02~0.45%, Silicon (Si): 0.50~2.00%, Aluminum (Al): 0.001~1.000%, Manganese (Mn): 0.4~3.0%, Chromium (Cr): 1.0~5.0%, Phosphorus (P): 0.050% or less, Sulfur (S): 0.0200% or less, Nitrogen (N): 0.0100% or less, Titanium (Ti): 0~1.00%, Niobium (Nb): 0~0.10%, Vanadium (V): 0~0.50%, Boron (B): 0~0.0200%, Molybdenum (Mo): 0~1.00%, Tungsten (W): 0~1.00%, Copper (Cu): 0~1.0%, Nickel (Ni): A hot-formed part comprising 0~1.0%, Tin (Sn): 0~1.00%, Antimony (Sb): 0~0.100%, Calcium (Ca): 0~0.10%, Magnesium (Mg): 0~0.10%, Cobalt (Co): 0~1.00%, Arsenic (As): 0~1.00%, Zirconium (Zr): 0~1.00%, Bismuth (Bi): 0~1.00%, Rare Earth Elements (REM): 0~0.3%, and the remainder being Fe and other unavoidable impurities.

7. A step of preparing a base steel plate and a step of heating the base steel plate, and The step of heating the above-mentioned steel plate includes a first heating section and a second heating section, and The first heating section is a temperature range of 300℃ to Ae1-50℃, and the second heating section is a temperature range above Ae1-50℃. A method for manufacturing a hot-formed part by controlling the oxygen concentration of the second heating section to 1.00~10.00%.

8. In Paragraph 7, A method for manufacturing a hot-formed part in which the oxygen concentration of the first heating section is controlled to 3.00~19.00%.

9. In any one of paragraphs 7 to 8, A method for manufacturing a hot-formed part in which the oxygen concentration in the second heating section is lower than the oxygen concentration in the first heating section.

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