Hot-formed member and manufacturing method thereof

Abstract

The present invention relates to a hot-formed member, particularly a hot-formed member obtained from a non-plated steel sheet and, more specifically, to a hot-formed member improved in surface quality and weldability, and a manufacturing method thereof.

Description

Hot-formed member and method of manufacturing the same

[0001] The present invention relates to a hot-formed member 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 components as materials for automotive structural parts is increasing to achieve these goals. Hot-formed components can be obtained by applying a hot-forming process to a specified steel plate, which enables them to possess high strength. Accordingly, hot-formed components can be advantageously utilized for bumpers, doors, and pillar reinforcements where ultra-high strength or high energy absorption capacity is required.

[0003] Patent Document 1 proposes a technology for manufacturing a component by hot forming a steel plate. Patent Document 1 enables the securing of ultra-high strength with high tensile strength by forming the microstructure of a component manufactured by heating an Al-Si plated steel plate to 850°C or higher, followed by hot forming and rapid cooling using a press, into martensite. As such, since the hot-formed component is obtained by forming the steel plate 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 has been proposed to manufacture components (parts) by hot forming the hot-rolled or cold-rolled (annealed) steel sheet itself, rather than a plated steel sheet, as a steel sheet for hot forming, that is, a technology to manufacture hot-formed components from a non-plated steel sheet that is not plated. However, when the non-plated steel sheet is heated for hot forming, it reacts with oxygen present in the heating equipment, causing an oxide layer to form on the surface of the steel sheet. As a result, the surface of the component obtained by hot forming the non-plated steel sheet becomes uneven, leading to poor surface quality, and there is a problem where weldability is reduced due to increased resistivity.

[0005] Therefore, there is a need to develop technology that can resolve the problem of surface quality degradation and improve weldability when manufacturing hot-formed components by hot-forming non-plated steel sheets.

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

[0007] One aspect of the present invention is to provide a hot-formed member, in particular a hot-formed member obtained from a non-plated steel sheet. Specifically, according to one aspect of the present invention, a hot-formed member with improved surface quality and weldability, and a method for manufacturing the same are provided.

[0008] 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.

[0009] According to one aspect of the present invention, a hot-formed member is provided, comprising a base steel plate and an oxide layer on the surface of the base steel plate, wherein the oxide layer has a thickness of more than 1 μm and a surface coverage ratio of more than 0.10 and less than or equal to 0.95.

[0010] In this way, by controlling the surface coverage ratio according to the thickness of the oxide layer formed on the surface of the hot-formed member, that is, on the surface of the base steel plate, it is possible to provide a hot-formed member with improved surface quality and weldability.

[0011] In one embodiment of the present invention, the ratio of the surface coverage may be greater than 0.15 and less than or equal to 0.85.

[0012] In one embodiment of the present invention, the ratio of the surface coverage may be greater than 0.20 and less than or equal to 0.80.

[0013] In one embodiment of the present invention, the member may satisfy that the internal oxide formed from the surface of the base steel plate to a depth of 1 μm or more in the thickness direction of the base steel plate is 0.20 or less per unit length (1 μm) based on cross-section.

[0014] In addition, in one embodiment of the present invention, the member may include an oxide layer having a thickness of more than 1 μm and a width of more than 5 μm.

[0015] Meanwhile, in one embodiment of the present invention, the base steel sheet comprises 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.

[0016] According to another aspect of the present invention, a method for manufacturing a hot-formed member is provided, comprising the steps of: preparing a base steel plate; heating and maintaining the base steel plate; and rapidly cooling and forming (pressing) the base steel plate after heating and maintaining.

[0017] In one embodiment of the present invention, the step of heating and maintaining the substrate steel plate is a process of passing through a first heating section and a second heating section, wherein the first heating section is a temperature range of 300 to Ae1-50℃ and the second heating section may be a temperature range greater than Ae1-50℃.

[0018] In one embodiment of the present invention, the first heating section can be controlled to an oxygen concentration of 19.00% or less.

[0019] In one embodiment of the present invention, the second heating section can be controlled to an oxygen concentration of 10.00% or less.

[0020] In one embodiment of the present invention, the second heating section may have a dew point temperature of -5.0℃ or lower.

[0021] In this way, by dividing the heating section during hot forming and controlling the oxygen concentration, dew point temperature, etc., in each section, the formation behavior of the oxide layer on the surface of the manufactured hot-formed member, i.e., the surface of the base steel sheet, can be controlled. From this, a hot-formed member with improved surface quality and weldability can be provided.

[0022] According to the present invention, a hot-formed member obtained by hot-forming a non-plated steel sheet can be provided, and said hot-formed member has the effect of improved surface quality and weldability.

[0023] 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.

[0024] Figure 1 is a schematic diagram of a method for calculating the coverage ratio of a surface oxide layer according to one embodiment of the present invention.

[0025] FIG. 2 shows a cross-section in the thickness direction of a hot-formed member (Invention Example 1 and Comparative Example 3) according to one embodiment of the present invention, observed using SEM.

[0026] FIG. 3 shows a cross-section in the thickness direction of a hot-formed member (Invention Example 6 and Comparative Example 9) according to one embodiment of the present invention, observed using SEM.

[0027] FIG. 4 illustrates a method for measuring the size of a nugget to derive a weldable current range according to one embodiment of the present invention.

[0028] 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.

[0029] 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.

[0030] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0031] 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.

[0032] 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.

[0033] 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 "member" refers to a material that has been processed into a non-plate shape through a forming process.

[0034] 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 ratio of crystals or structures is based on area (area%) unless specifically otherwise expressed, and the gas content is based on volume (volume%) unless specifically otherwise expressed.

[0035] 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.

[0036] The present invention will be described in detail below.

[0037] The present invention is designed to solve the aforementioned problems and has technical significance in providing a hot-formed member obtained using a non-plated steel sheet, and in providing a hot-formed member with improved surface quality as well as weldability, and a method for manufacturing the same.

[0038] According to one aspect of the present invention, a hot-formed member is provided.

[0039] In one embodiment of the present invention, the hot-formed member may include a base steel plate and an oxide layer on the surface of the base steel plate. Here, the oxide layer is a layer composed of oxides derived from the alloy compositions constituting the base steel plate, and as one example, it may be a layer composed of one or more oxides of Fe, Si, Cr, and Mn.

[0040] In one embodiment of the present invention, the value of the surface coverage ratio, which indicates the coverage ratio of the oxide layer covering the surface of the substrate steel plate, may be greater than 0.10 and less than or equal to 0.95.

[0041] In one embodiment of the present invention, the base steel sheet is a non-plated steel sheet. During the heating process for hot forming, oxides are generated on the surface of the non-plated steel sheet, which leads to a deterioration in the quality of the hot-formed member. Accordingly, reducing the amount of oxides or reducing the thickness of the oxide layer formed by the oxides is advantageous for securing the physical properties of the hot-formed member. Taking this into consideration, in one embodiment of the present invention, the surface coverage ratio of the oxide layer on the surface of the base steel sheet is limited.

[0042] Referring to the drawings, Figure 1 schematically illustrates examples of methods for calculating the ratio of an oxide layer covering the surface of a steel plate when an oxide layer exists on the surface of the steel plate. As one example, as shown in Figure 1 (a), when an oxide layer is formed with a certain thickness directly on the surface of the steel plate, the width of the location where the oxide layer is formed—that is, the length (l1, l2) in contact with the steel plate surface—can be expressed as a ratio to the total length (l0) of the observation portion (observation surface) of the steel plate, based on an oxide layer with a thickness exceeding 1 μm. As another example, as shown in Figure 2 (b), when the roughness of the steel plate surface is large and there is a difference between peaks and valleys, the width of the oxide layer area can be measured by applying a projection method. In other words, based on an oxide layer with a thickness greater than 1 μm, the length (l1, l2) drawn with a line parallel to the steel plate on the bottom surface of the oxide layer can be expressed as a ratio to the total length (l0) of the observation portion (observation surface) of the steel plate. In this case, if the oxide layer does not have a uniform thickness overall, only the width of the portion of the oxide layer with a thickness greater than 1 μm is taken.

[0043] Meanwhile, for measuring the oxide layer present on the surface of the steel plate, a scanning electron microscope (SEM) may be used as a non-limiting example. As one example, the surface coverage ratio of the aforementioned oxide layer can be derived by measuring the cross-section in the thickness direction of a hot-formed member using an SEM at a magnification of 1,000 times and then analyzing the observed image. At this time, three cross-sectional measurement images per member can be analyzed and expressed as an average value.

[0044] In one embodiment of the present invention, if the surface coverage ratio of an oxide layer with a thickness exceeding 1 μm on the surface of the base steel plate exceeds 0.95, the contact resistance increases during spot welding of the hot-formed member, which may result in poor weldability. Meanwhile, in order to keep the surface coverage ratio of the oxide layer 0.10 or less, the atmosphere inside the furnace must be strictly controlled during high-temperature heating for hot forming of the base steel plate; however, in this case, process costs may increase excessively, which is disadvantageous in terms of productivity and economy.

[0045] In another embodiment of the present invention, the surface coverage ratio of the oxide layer may be greater than 0.15 and less than or equal to 0.85, and according to yet another embodiment, it may be greater than 0.20 and less than or equal to 0.80.

[0046] Meanwhile, a hot-formed member according to one embodiment of the present invention may satisfy the condition that internal oxides formed from the surface of the base steel plate to a depth of 1 μm or more in the thickness direction of the base steel plate have 0.20 or fewer per unit length (1 μm) based on cross-section. A hot-formed member according to one embodiment of the present invention is obtained by hot-forming a non-plated steel plate, which is the base steel plate, and internal oxides may be formed below the surface during the process of heating the non-plated steel plate to a high temperature to hot-form it.

[0047] Referring to the drawings, as shown in FIGS. 3(a) and (b), it can be seen that an internal oxide formed below the surface of the base steel plate is formed to a certain depth in the thickness direction of the base steel plate (the part circled in FIG. 3).

[0048] In this way, if the number of oxides formed by extending to a depth of 1 μm or more in the thickness direction of the above-mentioned steel plate exceeds 0.20 per unit length (1 μm) based on the cross-section in the thickness direction, the contact resistance on the surface of the member increases, and weldability may be inferior.

[0049] In one embodiment of the present invention, the lower limit of the number of oxides per unit length (1 μm) based on the aforementioned cross-section is not specifically limited, and since the physical properties targeted in the present invention will naturally be secured when oxides are not present, the number may be 0.00.

[0050] In one embodiment of the present invention, as previously mentioned, an SEM can be used as a method to measure the number of specific internal oxides formed extending in the thickness direction of the base steel plate. As one example, the number of the aforementioned oxides can be measured by analyzing the observed image after measuring the cross-section of a hot-formed member obtained by hot forming at a magnification of 1000x using an SEM. At this time, the value can be derived by drawing a straight line in the width direction of the observation surface from a point 1㎛ deep from the surface based on the cross-section and counting the number of intersection points between the said straight line and the oxides. At this time, the total length of the cross-section (width of the observation surface) for observing the oxides may be at least 500㎛, and while the upper limit is not specifically limited, as one example, it may be 1000㎛.

[0051] In one embodiment of the present invention, the hot-formed member may include an oxide layer having a thickness of more than 1 μm and a width of more than 5 μm. This oxide layer may be formed on the surface of a base steel plate constituting the member.

[0052] The base steel sheet constituting the hot-formed member according to one embodiment of the present invention is not specifically limited, but any steel sheet capable of hot-forming that is a non-plated steel sheet that has not been plated may be used. As one example, the non-plated steel sheet may be a hot-rolled steel sheet obtained by hot rolling, or a cold-rolled steel sheet obtained by cold rolling the hot-rolled steel sheet. In this case, the cold-rolled steel sheet may be either an annealed steel sheet or an unanesthetized steel sheet.

[0053] 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 amounts thereof are not specifically limited. However, in one embodiment of the present invention, if giving limited examples of elements that may be added to the base steel sheet, 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.

[0054] 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.

[0055] Hereinafter, a method for manufacturing a hot-formed member 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 member, and that a hot-formed member 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.

[0056] According to one embodiment of the present invention, a hot-formed member can be manufactured by including the steps of: preparing a base steel plate; heating and maintaining the base steel plate; and rapidly cooling and forming (pressing) after heating and maintaining.

[0057] In one embodiment of the present invention, the base steel plate for obtaining a hot-formed member may be the base steel plate mentioned above, and it is noted that the composition is not particularly limited and is replaced by the aforementioned details.

[0058] 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.

[0059] 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.

[0060] The steel slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0061] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0062] In the electric furnace process, molten steel can be obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of including a large amount of scrap in this manner, elements not intended in the present invention (e.g., Cr, Cu, Mo, Ni, Sn, etc.) may be included in the molten steel within permissible limits.

[0063] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0064] In one embodiment of the present invention, a process may be performed to heat the hot-rolled steel sheet or cold-rolled steel sheet, i.e., the non-plated steel sheet, manufactured according to the above to a high temperature and then maintain it.

[0065] 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 a temperature range of 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.

[0066] 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 second temperature range may be 850°C or higher and 1000°C or lower.

[0067] In order to improve the weldability of the hot-formed member targeted in the present invention, the oxygen concentration (volume%) in the second heating section can be controlled to 10.00% or less. If the oxygen concentration in this section exceeds 10.00%, an excessive amount of oxide may be formed on the surface of the member, i.e., on the surface of the base steel plate, thereby degrading weldability. By controlling the dew point temperature together with the oxygen concentration in the second heating section as described above, the thickness and number of oxides formed on the surface of the base steel plate can be controlled more strictly. In one embodiment of the present invention, the dew point temperature in the second heating section may be -5.0℃ or less. If the dew point temperature in the second heating section exceeds -5.0℃, there is a risk that the number of internal oxides formed from the surface of the base steel plate to a depth of 1㎛ or more in the thickness direction of the base steel plate will become excessive. As one example, internal oxides formed from the surface of the above-mentioned steel sheet to a depth of 1 μm or more in the thickness direction may exceed 0.2 per unit length (1 μm) based on cross-section.

[0068] Meanwhile, in order to obtain the aforementioned effect, it is advantageous for the oxygen concentration and dew point temperature of the second heating section to be as low as possible. Accordingly, the lower limit of the oxygen concentration and dew point temperature is not specifically limited. However, the oxygen concentration may exceed 0.00%, and the dew point temperature may be -70°C or higher. At this time, since excessive energy costs may be incurred to lower the dew point temperature in the second heating section to below -70°C, the lower limit of the dew point temperature may be set to -70°C in consideration of this.

[0069] In addition, the oxygen concentration in the first heating section can be controlled to 19.00% or less. If the oxygen concentration in the first heating section exceeds 19.00%, the number of oxide nucleation sites capable of inducing oxide formation increases excessively, and even if the oxygen concentration in the subsequent second heating section is managed within the aforementioned range, oxide formation on the surface of the substrate steel plate becomes excessive, which may result in poor weldability. Since it is advantageous to have the oxygen concentration as low as possible in the first heating section, the lower limit of the oxygen concentration is not specifically limited, but may exceed 0.00%.

[0070] 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.

[0071] In this way, by dividing the heating section according to temperature during the heating process for hot forming and controlling the oxygen concentration in each section, a target hot-formed member can be obtained. As one example, by controlling the number of oxides present on the surface of the member obtained by hot forming, that is, on the surface of the base steel sheet, a hot-formed member with improved weldability can be obtained.

[0072] In one embodiment of the present invention, when heating is performed through the first heating section and the second heating section, a process of maintaining the temperature for a certain period of time may be performed. At this time, the time of the maintaining process is not specifically limited and may be maintained for a few minutes at a level typically applied. As one example, it may be maintained for 3 to 15 minutes.

[0073] In one embodiment of the present invention, while forming (pressing) the non-plated steel sheet that has undergone the heating and holding 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.

[0074] As described above, the base steel sheet of a hot-formed member 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 30% 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 high strength, and for example, may have a tensile strength of 500 MPa or more. At this time, the hard structure may be represented by the structure measured at a point 1 / 4t (where t means thickness (mm)) in the thickness direction of the base steel sheet.

[0075] 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.

[0076] (Example)

[0077] 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 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 with a thickness of 1.5 mm.

[0078] 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 a hot-formed member was manufactured by pressing with a flat plate mold and cooling. At this time, the total heat treatment time 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℃. Also, the cooling rate during cooling was 20℃ / s or higher.

[0079] Meanwhile, the oxygen concentration according to each of the above heating sections and the dew point temperature of the second heating section are shown in Tables 2 and 3 below.

[0080] Subsequently, the tensile properties (tensile strength) and weldability were evaluated for each of the above-mentioned hot-formed members. At this time, the tensile properties were measured using a universal tensile testing machine in accordance with ISO 6892 standards with JIS-5 specimens, and the weldable current range was measured for the same specimens in accordance with the SEP1220-2 test method. The weldable current range is defined as the difference between the maximum and minimum current values ​​at which the nugget size after welding becomes greater than or equal to 4√t of the material thickness (where t represents the material thickness in mm) (see Fig. 4).

[0081] Steel Grade Alloy Composition (Wt%) CsiMnPSAlCrMoNTiBA 0.048 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

[0082] Steel Type Specimen No. Ae1 - 50℃ 1st Heating Section 2nd Heating Section Tensile Properties Number of Internal Oxides (pieces / ㎛) Oxide Layer Coverage Ratio Weldability (kA) Classification Oxygen Concentration (%) Oxygen Concentration (%) Dew Point Temperature (°C) YS (MPa) TS (MPa) El (%) A18 10 1.95 12.10 -12.24 566 72 14.3 0.27 0.97 10.5 Comparative Example 128 10 20.3 0.69 -12.34 2566 516.8 0.15 0.96 0.8 Comparative Example 238 10 6.07 3.40 -12.14 3966 416.0 0.0 10.17 82.8 Inventive Example 1B477217.0014.10-13.2111815468.10.1951.0000.4 Comparative Example 3577219.200.76-13.0110415438.00.150.9710.6 Comparative Example 467727.314.12-12.8107815127.70.130.6931.4 Inventive Example 277725.238.98-12.9108715346.80.180.5711.6 Inventive Example 3C87463.0011.80-15.4143817876.90.220.9620.2 Comparative Example 5974611.892.12-15.2143817876.90.140.2111.1 Invention Example 4D10741.821.000.55-14.2134620257.40.170.9620.2 Comparative Example 611741.85.7510.80-14.3138020197.00.210.9730.3 Comparative Example 712741.85.202.30-13.9136220207.80.100.2301.2 Invention Example 5- The number of internal oxides refers to the number of points of internal oxides that intersect a straight line drawn in the width direction of the observation surface at a depth of 1 μm when observing a cross-section at an SEM magnification of 1000x. At this time, the total width for measuring the number of internal oxides was standardized to 500㎛. - The oxide layer coverage ratio refers to the ratio of the width (length) of the location where an oxide layer exceeding 1㎛ in thickness is generated on the surface of the substrate steel plate to the total width of the observation surface.

[0083]

[0084] As shown in Table 2, Invention Examples 1 to 6, in which the oxygen concentration in the first heating section and the second heating section during the heating process satisfies the range according to one embodiment of the present invention, the oxide characteristics were formed exactly as intended, and as a result, the weldable current range was 1.0 kA or higher, and the weldability was excellent.

[0085] On the other hand, in comparative examples where the oxygen concentration in the first heating section or the second heating section falls outside the range of one embodiment of the present invention, an excessive amount of oxide was formed on the surface of the member or directly below the surface. As a result, the weldable current range was less than 1.0 kA, and the weldability was inferior.

[0086] Steel Type Specimen No. Ae1 - 50℃ Number of Oxides Inside 1st Heating Section 2nd Heating Section (pieces / ㎛) Oxide Layer Coverage Ratio Weldability (kA) Classification Oxygen Concentration (%) Oxygen Concentration (%) Dew Point Temperature (℃) A1 38 10 13.20 7.90 3.20.22 0.96 20.7 Comparative Example 8 1 48 10 13.20 7.90 10.20.30 1.00 0.4 Comparative Example 9 1 58 10 13.10 7.19 -13.20.00 90.05 42.9 Inventive Example 6 B 1 67 72 15.00 14.10 10.8 0.34 0.97 10.5 Comparative Example 1 1 77 72 16.80 14.30 18.10.48 0.98 30.2 Comparative Example 111777216.803.20-6.50.140.5671.9 Invention Example 7D19741.812.108.1012.50.380.9870.2 Comparative Example 1220741.812.008.70-15.10.080.3471.1 Invention Example 8 - The number of internal oxides refers to the number of points of internal oxides that intersect a straight line drawn in the width direction of the observation surface at a depth of 1 μm when observing a cross-section at a SEM magnification of 1000x. At this time, the total width for measuring the number of internal oxides was standardized to 500 μm. - The oxide layer coverage ratio refers to the ratio of the width (length) of the location where an oxide layer exceeding 1 μm in thickness is generated on the surface of the substrate steel plate to the total width of the observation surface.

[0087]

[0088] Meanwhile, during the heating process, the oxygen concentration in the first heating section and the second heating section was controlled to satisfy the range according to one embodiment of the present invention, and the oxide characteristics were checked after changing the dew point temperature of the second heating section, and the results are shown in Table 3.

[0089] As shown in Table 3 above, Invention Examples 6 to 8, which satisfy a dew point temperature of -5.0℃ or lower in the second heating section, had the number of internal oxides and the oxide layer coverage ratio formed as intended, and as a result, the weldable current range was 1.0kA or higher, and the weldability was excellent.

[0090] On the other hand, in the case of comparative examples where the dew point temperature in the second heating section was higher than -5.0℃, the number of internal oxides was excessive in all comparative examples, and the oxide layer coverage ratio was not satisfied in some comparative examples. As a result, the weldable current range was less than 1.0kA, and the weldability was inferior. In particular, comparative examples 11 and 12 had even worse weldability because the dew point temperature was controlled to be higher than that of comparative examples 8 to 10.

[0091] FIG. 2 shows SEM images of the thickness direction cross-sections of Invention Example 1(a) and Comparative Example 3(b) according to one embodiment of the present invention.

[0092] As shown in FIG. 2, in Invention Example 1(a), even if oxide is generated, the coverage ratio of the oxide layer exceeding 1 μm in thickness on the surface of the steel sheet is significantly low at 0.178, whereas in Comparative Example 3(b), the oxide layer is formed to cover almost the entire surface of the steel sheet, and the coverage ratio is also significantly high at 0.971.

[0093] FIG. 3 shows SEM images of the thickness direction cross-sections of Invention Example 6(a) and Comparative Example 9(b) according to one embodiment of the present invention.

[0094] As shown in FIG. 3, in the image, in Invention Example 6(a), about two internal oxides are confirmed to be formed in the thickness direction from the surface of the steel sheet to a depth of 1 μm or more. On the other hand, in Comparative Example 9(b), it can be seen that more than 30 oxides are counted.

Claims

1. A base steel plate and an oxide layer on the surface of the base steel plate, and A hot-formed member having an oxide layer with a thickness exceeding 1 μm and a surface coverage ratio of greater than 0.10 and less than or equal to 0.

95.

2. In Paragraph 1, A hot-formed member having a surface coverage ratio greater than 0.15 and less than or equal to 0.

85.

3. In Paragraph 1 or 2, A hot-formed member having a surface coverage ratio greater than 0.20 and less than or equal to 0.

80.

4. In any one of paragraphs 1 to 3, The above-described member is a hot-formed member in which internal oxides formed from the surface of the base steel plate to a depth of 1 μm or more in the thickness direction of the base steel plate satisfy 0.20 or fewer per unit length (1 μm) based on cross-section.

5. In any one of paragraphs 1 through 4, The above-mentioned member is a hot-formed member comprising an oxide layer having a thickness of more than 1 μm and a width of more than 5 μm.

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 member 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 Element (REM): 0~0.3%, and the remainder being Fe and other unavoidable impurities.

7. Step of preparing the base steel plate; Steps for heating and maintaining the above-mentioned steel plate; and It includes the step of rapid cooling and molding (pressing) after the above heating and holding, The step of heating and maintaining the above-mentioned steel sheet is a process of passing through a first heating section and a second heating section, and A method for manufacturing a hot-formed member, wherein the first heating section is a temperature range of 300 to Ae1-50℃ and the second heating section is a temperature range greater than Ae1-50℃.

8. In Paragraph 7, A method for manufacturing a hot-formed member, wherein the first heating section is controlled to an oxygen concentration of 19.00% or less.

9. In Paragraph 7 or Paragraph 8, A method for manufacturing a hot-formed member, wherein the second heating section is controlled to an oxygen concentration of 10.00% or less.

10. In any one of paragraphs 7 through 9, The above second heating section is a method for manufacturing a hot-formed member in which the dew point temperature is -5.0℃ or lower.

11. In any one of paragraphs 7 through 10, The above base steel sheet comprises: Carbon (C): 0.02~0.45%, Silicon (Si): 0.5~2.000%, 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): 0.01% or less, Titanium (Ti): 0~0.1%, Niobium (Nb): 0~0.1%, Vanadium (V): 0~0.5%, 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%, A method for manufacturing a hot-formed member comprising 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 element (REM): 0~0.3%, and the remainder being Fe and other unavoidable impurities.

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