Hot formed part and manufacturing method thereof

Abstract

The present invention relates to a hot formed part for use in components such as automotive structural members and reinforcements, and to a manufacturing method thereof.

Description

Hot-formed parts and their manufacturing methods

[0001] The present invention relates to a hot-formed member used in parts such as automobile structural members and reinforcing members, and a method for manufacturing the same.

[0002] With the increasing need for passenger safety regulations and weight reduction in automobiles, research is being conducted on increasing the strength of steel plates to improve the crashworthiness of vehicle bodies. However, increasing the strength of automotive steel plates significantly reduces their formability, limiting their potential for high strength.

[0003] To address these issues, a hot-formed component manufacturing technology exists that utilizes hot forming followed by rapid cooling to achieve high strength and formability. These hot-formed components have recently been widely used in automotive structural components for purposes such as improving fuel efficiency and passenger protection through lightweighting. In particular, they can be utilized in applications requiring ultra-high strength or high energy absorption, such as bumpers, doors, and pillar reinforcements. Patent Document 1 is a representative example of this hot-formed technology.

[0004] The above patent document 1 secures ultra-high strength with high tensile strength by heating an Al-Si plated steel sheet to 850°C or higher, hot forming it using a press, and then rapidly cooling it to form a martensite structure into the material. When such ultra-high strength steel for hot forming is applied, complex shapes can be easily formed because forming is done at high temperatures, and the increased strength due to rapid cooling within the mold can be expected to have a weight reduction effect due to high strength.

[0005] However, martensite structures are known to have low resistance to hydrogen embrittlement. In particular, parts manufactured after hot forming have residual stresses resulting from rapid cooling after heating, and increased diffusible hydrogen content in the steel raises concerns about delayed fracture due to hydrogen embrittlement, limiting their application. Various studies are being conducted to overcome these limitations.

[0006] In addition, since changes in process parameters during coil manufacturing can cause global or local changes in mechanical properties within the sheet, a steel composition that is less sensitive to changes in specific manufacturing parameters for manufacturing sheets with good mechanical properties and homogeneity is required, and delayed fracture due to hydrogen embrittlement must be prevented. In particular, since diffusible hydrogen in grain boundaries promotes grain boundary cracking when stress is generated, a method to reduce the amount of diffusible hydrogen in steel after hot press forming (or hot stamping) was considered.

[0007] (Patent Document 1) U.S. Patent No. 6,296,805

[0008] One aspect of the present invention is to provide a hot-formed member having excellent hydrogen embrittlement resistance and a method for manufacturing the same.

[0009] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0010] A hot-formed member according to one embodiment of the present invention is:

[0011] In weight %, C: 0.1~2.0% and B: 0.0030~0.0150%, the remainder containing Fe and other unavoidable impurities,

[0012] The highest B content value in the B content profile measured by 3D-APT (Three-Dimensional Atom Probe Tomography) (B max ) may be a hot-formed member including austenite grain boundaries of 0.15 to 0.35 wt.%.

[0013] The above hot-formed member may include Si+Al: 0.4 to 3.0%, Mn: 0.1 to 5.0%, P: 0.0001 to 0.1%, and S: 0.0001 to 0.03%.

[0014] The above hot-formed member may include one or more of a) to f).

[0015] a) One or more selected from the group consisting of Cr, Mo and W: 0.01 to 2.0%,

[0016] b) One or more elements selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%,

[0017] c) 0.005 to 2.0% of one or both of Cu and Ni,

[0018] d) B: 0.0001~0.01%,

[0019] e) Sb: 0.001~0.1%, and

[0020] f) As: 1.00% or less

[0021] In the above B content profile, the average B content (B) in the area up to 3 nm before and after the highest B content value av ) can be 0.1 to 0.3 wt.% / nm.

[0022] In the B content profile and C content profile measured by the above 3D-APT (Three-Dimensional Atom Probe Tomography), the ratio of the highest B content value to the highest C content value (B max / C max ) may contain austenite grain boundaries of 1.5 or less.

[0023] The tensile strength of the above hot-formed member may be 1200 to 1700 MPa.

[0024] The diffusible hydrogen content of the above hot-formed member may be 0.3 ppm or less.

[0025] The microstructure of the above hot-formed member may be martensite at 95 area% or more.

[0026] A method for manufacturing a hot-formed member according to another embodiment of the present invention is as follows:

[0027] A step for providing a steel material containing, by weight%, C: 0.1 to 2.0%, B: 0.0030 to 0.0150%, the remainder Fe and other unavoidable impurities;

[0028] A step of providing a steel for hot forming by annealing the above steel; and

[0029] It includes a step of manufacturing a hot-formed member by hot-forming the above hot-formed steel material,

[0030] When performing the above annealing heat treatment, it may be a method for manufacturing a hot-formed member that satisfies the value of the following [Relational Expression 1].

[0031] [Relationship 1]

[0032]

[0033] The above [B] is the content of the element in the steel (weight ppm), and the above α is derived from the following relationships 2 to 4.

[0034] [Relationship 2]

[0035]

[0036] [Relationship 3]

[0037]

[0038]

[0039] tn: Unit time (sec.) in the temperature range above Ac1 during annealing heat treatment

[0040] Tn: Temperature (℃) per unit time in the temperature range above Ac1 during annealing heat treatment

[0041] [Relationship 4]

[0042]

[0043]

[0044] tn: Unit time (sec.) in the temperature range above Ac1 during annealing heat treatment

[0045] The above [Relationship 1] may be 12.0 or less.

[0046] It may include providing a hot-forming steel material including a plating layer by performing plating after the above annealing heat treatment.

[0047] The above hot forming steel has 10 / mm of unplated defects with a diameter of 0.5mm or more. 2 It may be less than.

[0048] The step of providing the above steel material is:

[0049] Step of heating the steel slab at 1000~1300℃;

[0050] A step of hot rolling the above-mentioned heated steel slab at a temperature of Ar3 temperature or higher and 1000℃ or lower to obtain a hot-rolled steel sheet;

[0051] A step of coiling the hot-rolled steel sheet at a temperature exceeding Ms but below 750°C; and

[0052] The step of obtaining a cold-rolled steel sheet by pickling and cold rolling after the above-mentioned winding may be included.

[0053] The above hot forming step is:

[0054] A step of heat treating the above hot forming steel in a temperature range of Ac3 or higher and 975°C or lower for 1 to 1,000 seconds; and

[0055] It may include a step of forming and quenching the above heat-treated steel.

[0056] According to one aspect of the present invention, in a hot-formed member mainly used in automobile body parts, a hot-pressed member having excellent hydrogen embrittlement resistance and excellent processability after hot press forming can be provided by reducing the amount of diffusible hydrogen remaining in the steel after hot forming, and a method for manufacturing the same can be provided.

[0057] Figure 1 is a graph showing the component profile of B and C contents after analyzing the old austenite grain boundaries of specimen 4 in Example 1 using 3D-APT.

[0058] Figure 2 is a schematic diagram showing the 3D-APT analysis evaluation method of Figure 1 above.

[0059] Figure 3 is a schematic diagram showing the relationship of relational expression 2.

[0060] Hereinafter, embodiments of the present invention will be described in detail. These embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the art. The shapes and sizes of elements in the drawings may be exaggerated for clarity.

[0061] In describing the embodiments of the present invention, if a detailed description of a known technology related to the present invention is judged to unnecessarily obscure the gist of the present invention, such detailed description will be omitted. In addition, the terms described below are terms defined in consideration of their functions in the present invention, and these may vary depending on the intention or custom of the user or operator. Therefore, the definitions should be made based on the contents throughout this specification. The terminology used in the detailed description is only for the purpose of describing the embodiments of the present invention and should in no way be limiting. Unless clearly defined otherwise, expressions in the singular form include plural meanings.

[0062] In this description, expressions such as "including" or "having" are intended to indicate certain features, numbers, steps, operations, elements, portions or combinations thereof, and should not be construed to exclude the presence or possibility of one or more other features, numbers, steps, operations, elements, portions or combinations thereof other than those described.

[0063] Below, the present invention will be described in detail through each embodiment or example of the present 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 be combined with other embodiments or examples. Therefore, the citation of a claim in the patent claims is only an example of an embodiment, and the technical concept of the present invention should not be interpreted solely as a combination with the cited claim, and combinations with various claims also fall within the scope of the technical concept of the present invention.

[0064] The present inventors have discovered that during hot forming, when the forming steel is heated, water vapor present in the heat treatment furnace is adsorbed on the surface of the steel, and the hydrogen generated when the adsorbed water vapor dissociates is absorbed into the steel while it has an austenite phase with high hydrogen solubility at high temperatures. In addition, they have discovered that when rapid cooling occurs after forming and changes to a martensite phase, the hydrogen solubility drops sharply, causing defects due to hydrogen delayed fracture. In particular, when the steel includes a plating layer, they have discovered that the alloy plating layer further suppresses hydrogen release as it becomes alloyed.

[0065] In particular, it was found that the absorbed hydrogen mainly diffuses into the old austenite grain boundaries. The old austenite grain boundaries may be formed by the austenite transformation of the steel during the heating process for hot forming. Accordingly, the inventors of the present invention confirmed that it is important to suppress the amount of diffusible hydrogen in order to reduce the possibility of crack formation due to delayed hydrogen fracture, and in particular, by trapping the hydrogen that diffuses into the old austenite grain boundaries during hot forming, the diffusible hydrogen that diffuses into the steel can be effectively blocked, thereby reducing its content, and ultimately, it was judged that the effect of improving hydrogen embrittlement resistance can be expected.

[0066] Accordingly, the inventors of the present invention have developed a method to secure hydrogen embrittlement resistance by reducing the amount of diffusible hydrogen in a hot-formed part to 0.30 ppm or less by using an appropriate level of boron (B), and have achieved the present invention.

[0067] First, a hot-formed member, which is one aspect of the present invention, will be described in detail.

[0068] The above hot-formed member contains boron (B) in an amount of 0.0030 to 0.0150 wt%. If the B content is less than 0.0030 wt%, precipitation of B at the austenite grain boundaries is insufficient, making it difficult to exhibit the effect of improving hydrogen embrittlement. If it exceeds 0.0150 wt%, Fe3(C,B) or Fe may be present at the grain boundaries. 23 There is a problem that the austenite grain boundaries are weakened due to excessive production of precipitates such as (C, B)6, making it easy for cracks to occur when stress is generated. The B content is preferably 0.0040 to 0.0130 wt%.

[0069] Other alloy compositions of the above-mentioned thermoformed member are not particularly limited, and as a preferred example, it may include carbon (C): 0.1 to 2.0 wt%, silicon (Si) + aluminum (Al): 0.4 to 3.0 wt%, manganese (Mn): 0.1 to 5.0 wt%, phosphorus (P): 0.0001 to 0.1 wt%, and sulfur (S): 0.0001 to 0.03 wt%. In addition, it may further include at least one selected from the group consisting of Cr, Mo, and W at 0.01 to 2.0 wt%, at least one selected from the group consisting of Ti, Nb, Zr, and V at 0.001 to 0.4 wt%, one or two selected from Cu or Ni at 0.005 to 2.0 wt%, and one or two selected from Sb: 0.001 to 0.1 wt%.

[0070] Carbon (C): 0.1~2.0% (% below means weight%)

[0071] C is not only an essential element for increasing the strength of steel plates, but also needs to be added appropriately to secure the retained austenite desired in the present invention. If the C content is less than 0.1%, sufficient strength cannot be obtained even if heat treatment is performed in the austenite single-phase region, and it is also difficult to secure more than 5% of retained austenite when heat treating a member after hot forming or cold forming. In addition, if the C content exceeds 2.0%, the toughness and weldability are likely to deteriorate, and not only does it make welding of the steel plate difficult during the pickling and rolling processes of the hot-rolled steel plate during the manufacturing process, but also the strength of the steel plate is too high during the annealing and plating processes, making cold forming difficult. Accordingly, the C content range is preferably set to be 0.1 to 2.0%.

[0072] Silicon (Si) + Aluminum (Al): 0.4~3.0%

[0073] Si and Al play a very important role in the present invention. Si+Al refers to the total sum of one or more of Si and Al. When the steel sheet is rapidly cooled to between Ms and Mf and then maintained below the Ac1 temperature, it is an element that prevents carbon from precipitating in martensite, thereby concentrating a large amount of carbon into untransformed retained austenite and securing stable retained austenite in the final member. When the total content of Si and Al is less than 0.4%, it is difficult to expect this effect, and when it exceeds 3%, it is difficult to remove the surface scale of the hot-rolled steel sheet and the heat treatment temperature for manufacturing the member is increased, which causes an increase in manufacturing cost. Therefore, the content is preferably set to 0.4 to 3.0%.

[0074] Manganese (Mn): 0.1~5.0%

[0075] Manganese (Mn) is a solid solution strengthening element that not only contributes to increasing strength, but also delays the transformation from austenite to ferrite and lowers the Ac3 temperature. When the Mn content is less than 0.1%, a high heat treatment temperature is required to heat treat the steel sheet in the austenite single-phase region, which accelerates oxidation of the steel sheet and deteriorates the heat resistance of the coated steel sheet even if it is used. In addition, the desired high strength cannot be secured by heat treatment in the two-phase region where ferrite and austenite coexist. When the Mn content exceeds 5.0%, problems such as weldability and hot-rollability arise, so the Mn content range is preferably 0.1 to 5.0%.

[0076] Phosphorus (P): 0.0001~0.1%

[0077] Similar to Si, P exhibits an effect of suppressing carbide formation during martensite heat treatment. However, excessive P content deteriorates weldability, so the upper limit is limited to 0.1%. However, controlling P to less than 0.0001% incurs significant manufacturing costs, so it is desirable to limit the lower limit to 0.0001%.

[0078] Sulfur (S): 0.0001~0.03%,

[0079] Sulfur (S) exists as an impurity in steel and is an element that impairs the ductility and weldability of steel plates. Since these adverse effects are minimal at S contents below 0.03%, it is desirable to set the upper limit at 0.03%. However, controlling the S content below 0.0001% incurs significant manufacturing costs, so it is desirable to set the lower limit at 0.0001%.

[0080] In addition to the steel composed as described above, at least one selected from the group consisting of Cr, Mo, and W, which are hardenability enhancing elements, may be additionally included in an amount of 0.01 to 2.0%, at least one selected from the group consisting of Ti, Nb, Zr, and V, which are precipitation strengthening elements, may be additionally included in an amount of 0.001 to 0.4%, one or two selected from Cu or Ni, which are strength enhancing elements, may be additionally included in an amount of 0.005 to 2.0%, and B: 0.0001 to 0.01% as a grain boundary strengthening and hardenability element or Sb: 0.001 to 0.1% for improving plating properties may be additionally included.

[0081] At least one of chromium (Cr), molybdenum (Mo), and tungsten (W): 0.01 to 2.0%

[0082] Cr, Mo, and W enhance hardenability, which significantly contributes to achieving high strength. Furthermore, because they enhance hardenability, they can maintain adequate strength even when cooling performance is compromised due to incomplete contact with the mold during high-temperature forming. When the Cr, Mo, or W content is below 0.01%, sufficient hardenability cannot be achieved, and when it exceeds 2.0%, the effect becomes saturated and manufacturing costs increase. Therefore, a content range of 0.01 to 2.0% is recommended.

[0083] At least one of titanium (Ti), niobium (Nb), zirconium (Zr), and vanadium (V): 0.001 to 0.4%

[0084] Ti, Nb, Zr, and V are elements that enhance the strength, grain refinement, and heat treatment properties of steel sheets. If the content of Ti, Nb, Zr, and V is less than 0.001%, it is difficult to expect the above-mentioned effects, and if the content exceeds 0.4%, excessive manufacturing costs will increase. Therefore, it is preferable to keep the content between 0.001 and 0.4%.

[0085] At least one of copper (Cu) and nickel (Ni): 0.005–2.0%

[0086] Copper (Cu) is an element that enhances strength by forming fine Cu precipitates. If the Cu content is less than 0.005%, the desired strength cannot be sufficiently achieved, and if it exceeds 2.0%, workability may be impaired. Meanwhile, Ni (Ni) is an effective element for increasing strength and improving heat-treatability. However, if the Cu content is less than 0.005%, the effect cannot be achieved, and if it exceeds 2.0%, manufacturing costs increase. Therefore, the Cu and Ni contents are preferably set at 0.005 to 2.0%.

[0087] Antimony (Sb): 0.001~0.1%

[0088] Sb is a surface-enriching element, and can suppress the formation of oxides due to surface enrichment of Si and Al added in the present invention during annealing, which deteriorates plating properties. However, this effect cannot be achieved when the content is less than 0.001%, and when it exceeds 0.1%, hot workability deteriorates. Therefore, it is preferable to set the content to 0.001 to 0.1%.

[0089] Arsenic (As): 1.00% or less

[0090] As may be additionally included considering the target properties of the final product, etc., and if its content exceeds 1.00%, it significantly increases manufacturing costs, so its content may be limited.

[0091] In addition to the aforementioned components, the remaining iron and other unavoidable impurities may be included. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during the normal manufacturing process, they cannot be completely excluded. Since these impurities are readily apparent to anyone skilled in the art, their full contents are not specifically mentioned in this specification.

[0092] In addition, the addition of additional effective ingredients other than the aforementioned ingredients is not completely ruled out.

[0093] The hot-formed member comprises at least 95 area% martensite and may comprise some other structures. The hot-formed member may comprise 100 area% martensite.

[0094] The above hot-formed member was measured using 3D-APT (Three-Dimensional Atom Probe Tomography) and the highest B content (B) in the B content profile near the old austenite grain boundary was max ) may contain 0.15 to 0.35 wt.% of old austenite grain boundaries. The 3D APT (Atom Probe Tomography) technology is the only material analysis technology that provides a wide range of capabilities for 3D imaging and chemical composition measurements at the atomic scale (approximately 0.1 to 0.3 nm in depth and 0.3 to 0.5 nm in side). The sample is prepared in the form of a very sharp tip. The cooled tip induces a high electrostatic field (tens of V / nm) at a high DC voltage (3 to 15 kV), so that atomic ions at the tip are evaporated from the surface and projected onto a position sensitive detector (PSD) with very high detection efficiency, and the atomic ratio projected onto the detector is calculated, enabling 3D imaging and chemical composition measurements.

[0095] As an example, Fig. 1 is a graph showing the B and C content profiles obtained by 3D-APT analysis of the vicinity of the old austenite grain boundary of sample 4 in Example 1 described below. Referring to Fig. 1, the maximum B content (B) at the grain boundary max ) and the highest C content (C max ) can be confirmed.

[0096] B above max If it is less than 0.15 wt.%, the hydrogen introduced during hot forming is easily segregated at the grain boundaries, making it difficult to expect an improvement in hydrogen embrittlement. If it exceeds 0.35 wt.%, Fe3(C, B) or Fe is formed at the grain boundaries. 23There is a problem that excessive precipitation of (C,B)6 is generated, making it easier for cracks to occur when stress is generated.

[0097] Meanwhile, in the above B content profile, the highest B content value (B max ) based on the average B content (B) in the area up to 3 nm before and after av ) can be 0.1 to 0.3 wt.% / nm. The above B amx The area up to 3 nm before and after the point can be seen as the width of the austenite grain boundary. The above B av If it is less than 0.1 wt.%, the hydrogen introduced during hot forming is easily segregated at the grain boundaries, making it difficult to expect an improvement in hydrogen embrittlement. If it exceeds 0.3 wt.%, Fe3(C, B) or Fe is formed at the grain boundaries. 23 There is a problem that excessive precipitation of (C,B)6 is generated, making it easier for cracks to occur when stress is generated.

[0098] In the B content and C content profiles measured by 3D-APT of the above austenite grain boundaries, the ratio of the highest B content to the highest C content (B max / C max ) may be less than or equal to 1.5. The above (B max / C max ) exceeds 1.5, Fe3(C, B) and Fe 23 (C, B)6 promotes the formation of precipitates, making it easy for cracks to occur.

[0099] The above hot-formed member may include an alloy layer formed by alloying a plating layer on the surface of the base iron during the hot-forming process.

[0100] The type of the above-mentioned plating layer is not particularly limited, and any plating layer that is applied to a conventional hot-forming plating steel sheet can be applied to the present invention without limitation. As an example, the plating layer may be an aluminum or aluminum alloy plating layer, and preferably, the plating layer may include Si: 6 to 12%, Fe: 4% or less (including 0%), the remainder Al, and other unavoidable impurities.

[0101] Or, as another example, in the case of a hot-dip plating layer, a hot-dip aluminum plating layer, a hot-dip Al-Si plating layer, a hot-dip Al-Si-Mg plating layer, a hot-dip Al-Zn plating layer, a hot-dip Al-Mg plating layer, etc. can be applied. As an alloyed hot-dip plating layer, an alloyed hot-dip aluminum plating layer, an alloyed hot-dip Al-Si plating layer, an alloyed hot-dip Al-Si-Mg plating layer, an alloyed hot-dip Al-Zn plating layer, an alloyed hot-dip Al-Mg plating layer, etc. are exemplified. The plating layer may also contain at least one or more of Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, Sr, Mg, Be, Li, and Na. The adhesion amount of the plating layer is not particularly limited, and may be, for example, an adhesion amount within a general range.

[0102] The tensile strength of the hot-formed member may be 1200 to 1700 MPa, and the diffusible hydrogen content of the hot-formed member may be 0.3 ppm or less.

[0103] Next, a method for manufacturing a hot-formed member, which is another aspect of the present invention, will be described in detail.

[0104] In order to manufacture the above hot-formed member, a steel material having the alloy composition described above is provided, and the steel material is annealed and heat-treated to provide a steel material for hot forming. Thereafter, the steel material for hot forming is hot-formed to manufacture a hot-formed member.

[0105] Steel provided

[0106] The step of providing the above steel is not particularly limited to the present invention, and may be a hot-rolled steel sheet, a cold-rolled steel sheet, etc. As an example of a method for manufacturing the above steel, a steel slab having the above composition is heated at 1000 to 1300°C, and then hot-rolled. If the heating temperature is lower than 1000°C, homogenization of the cast structure is not sufficiently achieved, and if it exceeds 1300°C, there is a high possibility that the manufacturing cost will increase. Thereafter, hot finishing rolling is completed at a temperature higher than the Ar3 temperature and lower than 1000°C. If the hot finishing rolling temperature is lower than the Ar3 temperature, abnormal rolling may occur, which may cause hot-rolled mixed grains, and deteriorate the operability. If it exceeds 1000°C, it may cause grain coarsening. Then, coiling is performed at a temperature higher than the Ms temperature and lower than 750°C. Below the Ms temperature, there is a disadvantage that martensitic transformation occurs, resulting in an excessive increase in the strength of the hot-rolled steel sheet, and above 750°C, there is a disadvantage that the thickness of the oxide layer of the hot-rolled steel sheet increases. The hot-rolled steel sheet can be provided as a steel material for hot forming.

[0107] In addition, the hot-rolled steel sheet manufactured as described above is subjected to pickling and cold rolling. There are no particular restrictions on the reduction ratio during the cold rolling, and it can be performed under normal conditions.

[0108] Annealing heat treatment

[0109] The steel provided above is subjected to annealing heat treatment, and as an example, the annealing heat treatment can be performed at a temperature of Ac1+50℃ to 850℃ for 50 to 200 seconds. During the annealing heat treatment process, the steel is heated, maintained, and cooled, and during the temperature change process of the steel over time, in the section where the temperature of the steel is Ac1 or higher, the value of [Relational Expression 1] below can be satisfied. Meanwhile, the expression of [Relational Expression 1] can be 12.0 or less.

[0110] [Relationship 1]

[0111]

[0112] The above [B] is the content of the element in the steel (weight ppm), and the above α is derived from the following relationships 2 to 4.

[0113] [Relationship 2]

[0114]

[0115] [Relationship 3]

[0116]

[0117]

[0118] tn: Unit time (sec.) in the temperature range above Ac1 during annealing heat treatment

[0119] Tn: Temperature (℃) per unit time in the temperature range above Ac1 during annealing heat treatment

[0120] [Relationship 4]

[0121]

[0122]

[0123] tn: Unit time (sec.) in the temperature range above Ac1 during annealing heat treatment

[0124] Optionally, a plating layer may be formed on the surface of the annealed heat-treated hot-forming steel. The type of the plating layer is not particularly limited, and any plating layer that is applied to a conventional hot-forming plating steel sheet may be applied to the present invention without limitation. As an example, the plating layer may be an aluminum or aluminum alloy plating layer, and preferably, the plating layer may include Si: 6 to 12%, Fe: 4% or less (including 0%), the remainder Al, and other unavoidable impurities.

[0125] Or, as another example, in the case of a hot-dip plating layer, a hot-dip aluminum plating layer, a hot-dip Al-Si plating layer, a hot-dip Al-Si-Mg plating layer, a hot-dip Al-Zn plating layer, a hot-dip Al-Mg plating layer, etc. can be applied. As an alloyed hot-dip plating layer, an alloyed hot-dip aluminum plating layer, an alloyed hot-dip Al-Si plating layer, an alloyed hot-dip Al-Si-Mg plating layer, an alloyed hot-dip Al-Zn plating layer, an alloyed hot-dip Al-Mg plating layer, etc. are exemplified. The plating layer may also contain at least one or more of Mn, Cr, Cu, Mo, Ni, Sb, Sn, Ti, Ca, Sr, Mg, Be, Li, and Na. The adhesion amount of the plating layer is not particularly limited, and may be, for example, an adhesion amount within a general range.

[0126] The above-mentioned plated steel can secure excellent plating properties, and for example, the surface of the steel plate can be observed at 10 to 20 times magnification using an optical microscope, with a 1 mm 2 Visual observation of the area revealed that there were 10 / mm of unplated defects with a diameter of 0.5 mm or more. 2 It could be as follows:

[0127] hot forming

[0128] The above hot forming steel is heated, formed, and cooled to manufacture a hot forming member. The hot forming is not particularly limited in the present invention, and is not particularly limited if it is a conventional hot forming process. As an example, a blank for hot forming is manufactured using the above hot forming steel. The blank is heated within a temperature range higher than the austenite single-phase temperature, more specifically, higher than the Ac3 temperature and lower than 975°C. At this time, if the heating temperature is lower than the Ac3 temperature, it is difficult to secure strength and crash resistance due to the presence of untransformed ferrite in the ideal region. On the other hand, if the heating temperature exceeds 975°C, excessive oxides are generated on the surface of the member, making it difficult to secure spot weldability, and the manufacturing cost for maintaining a high temperature increases. Meanwhile, since the Ac3 temperature can be equally applied to a definition commonly known in the art, it is not separately defined in this specification.

[0129] It is preferable that the heated blank be maintained in the above temperature range for 1 to 1,000 seconds. If the holding time is less than 1 second, it is difficult to achieve a uniform temperature distribution across the blank temperature, which may cause material deviations at different locations. On the other hand, if the holding time exceeds 1,000 seconds, not only is it difficult to secure spot weldability due to excessive oxide formation on the surface of the component, as when the heating temperature is exceeded, but it also causes an increase in the manufacturing cost of the component.

[0130] Meanwhile, although not particularly limited, according to one aspect of the present invention, the heated blank described above may be transferred to a press and hot formed and die-quenched at a cooling rate of 20°C / s or more. At this time, at a cooling rate of less than 20°C / s, a ferrite phase may be introduced during cooling and formed at grain boundaries, thereby deteriorating strength and crash resistance. There are no particular limitations on the transfer, hot press forming, and cooling steps of the blank described above, and a commonly used hot press forming method may be applied as is.

[0131] Hereinafter, the present invention will be described in detail through examples. However, it should be noted that the examples described below are intended only to illustrate and concretize the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0132] (Example 1)

[0133] A cold-rolled steel sheet containing, by weight%, C: 0.22%, Mn: 1.25%, Si: 0.25%, Cr: 0.2%, and containing B contents as shown in Table 1 below, and the remainder containing Fe and inevitable impurities was manufactured. At this time, Ac1 can be derived from the following (Formula 1) and is 720.28℃. At this time, the manufacturing process of the cold-rolled steel sheet is as follows: After heating the steel slab having the above composition to 1250℃ for 1 hour, hot-rolling and coiling temperature of 600℃ were applied to obtain a hot-rolled steel sheet to a target thickness of 3 mm, and cold-rolling was performed by applying a cold reduction ratio of 50% to manufacture it. (Formula 1)

[0134] Ac1=723-10.7Mn-16.9Ni+29.1Si+16.9Cr+290As+6.38W (℃)

[0135] The above cold rolled steel sheet was subjected to a heating rate of 4°C / s, and the annealing conditions were applied at a target temperature of 780°C, a time of 80 seconds, and a cooling rate of 2°C / s. During the annealing heat treatment, conditions α and √[B] * α at a temperature higher than Ac1 3 was derived from [Relationship 1] to [Relationship 4] below.

[0136] [Relationship 1]

[0137]

[0138] The above [B] is the content of the element in the steel (weight ppm), and the above α is derived from the following relationships 2 to 4.

[0139] [Relationship 2]

[0140]

[0141] [Relationship 3]

[0142]

[0143]

[0144] tn: Unit time (sec.) in the temperature range above Ac1 during annealing heat treatment

[0145] Tn: Temperature (℃) per unit time in the temperature range above Ac1 during annealing heat treatment

[0146] [Relationship 4]

[0147]

[0148]

[0149] tn: Unit time (sec.) in the temperature range above Ac1 during annealing heat treatment

[0150] The above annealed heat-treated steel plate was subjected to flat hot forming, and the heat treatment temperature was 900°C for 5.5 minutes, after which the forming was cooled to manufacture a non-conforming specimen.

[0151] The old austenite grain boundaries of the manufactured hot-formed member were analyzed by 3D-APT((Three-Dimensional Atom Probe Tomography), and the B and C contents near the grain boundaries were measured to obtain a profile. Fig. 1 is a profile showing the B and C contents graphically by analyzing the vicinity of the old austenite grain boundaries of the above specimen 4 by 3D-APT((Three-Dimensional Atom Probe Tomography). For the results of Fig. 1, the old austenite grain boundary area of ​​the specimen 4 was highlighted as in Fig. 2. Fig. 2(a) is a microstructure photograph of the above specimen 4, specifying the position for observing the old austenite grain boundaries. The grain size shown in Fig. 2(a) is the grain size of the old austenite, and this is the result of evaluation by applying an etching method. Meanwhile, in Fig. 2(a), the faintly visible thing in the old austenite system can be understood as a trace of the martensite phase. there is.

[0152] Meanwhile, Fig. 2(b) is a three-dimensional representation of the distribution of B and C atoms at a specific location. In this case, the concentrated region corresponds to the grain boundary. Fig. 2(b) shows the content profile of each component along the length along the reference line (90°±5° from the grain boundary), which corresponds to Fig. 1. Typically, grain boundaries are formed three-dimensionally, and when the location for measurement is specified, they appear in the form of lines on the order of several to several tens of nm, and when expanded, they can be understood as three-dimensional.

[0153] Through the above profile, the highest B content (B max ), the average B content (B) in the area up to 3 nm before and after the highest B content value above av ) and the ratio of the highest B content to the highest C content (B max / C max ) were derived and shown in Table 1.

[0154] For each specimen manufactured in this way, the diffusible hydrogen content and a 4-point bending test were performed to determine whether cracks occurred, as shown in Table 1. The 4-point bending test was performed by applying a stress of 1200 MPa and immersing in 0.1 N hydrochloric acid (HCl) for 120 hours to measure whether cracks occurred. To evaluate the diffusible hydrogen content, TDA (Thermal Desorption Analysis) equipment (Bruker G8: model name) was used to measure the diffusible hydrogen content. The diffusible hydrogen content was measured 3 days after the hot forming heat treatment. The conditions for analyzing the diffusible hydrogen content were as follows: the temperature was raised to 400°C at 20°C / min, and the diffusible hydrogen curve was measured by maintaining the temperature for a sufficient time for the diffusible hydrogen peak to appear, and the total diffusible hydrogen content in the steel was obtained by integrating this curve.

[0155] Classification B content (wt.%) α√[B]*α 3 B max (wt.%)B av (wt.%)B max / C maxDiffusible hydrogen content (ppm) Crack occurrence during 4-point bending Specimen 10.00200.5460.2301934310.1020.0800.5310.302 Occurrence O Specimen 20.00310.5460.286588510.1510.1060.6310.287 Occurrence X Specimen 30.00410.5460.3295868350.1650.1230.7180.253 Occurrence X Specimen 40.00530.5460.3747277560.1880.1370.8080.207 Occurrence X Specimen 50.00720.5460.4367613270.2290.1670.8900.187 Occurrence X Specimen 60.00950.5460.5016949510.2590.1860.9820.159 Occurrence X Specimen 70.01100.5460.5398514480.2790.1991.0890.140 Occurrence X Specimen 80.01250.5460.5754835770.3020.2351.2710.122 Occurrence X Specimen 90.01430.5460.6155253540.3330.2821.4710.105 Occurrence X Specimen 100.01560.5460.6428952650.3560.3011.6510.089 Occurrence O

[0156] From the results in Table 1 and Figure 1 above, when the conditions of the present invention are met, excellent hydrogen embrittlement resistance and excellent processability can be secured.

[0157] (Example 2)

[0158] A cold-rolled steel sheet was manufactured under the same conditions as Example 1, and annealed under the conditions of Table 2. Afterwards, plating was performed and hot forming was performed to manufacture a hot-formed member specimen. The plating was performed by immersing the steel sheet in an Al plating bath containing 8.5 wt.% Si to manufacture an aluminum-plated steel sheet. After plating, unplated defects on the plating surface were observed, which are shown in Table 2. The unplated defects were measured by using an optical microscope at a magnification of 10 to 20 times on a 200x200mm-sized specimen of the plated steel sheet specimen, and a 1mm-sized specimen was observed. 2Visual observation of the area revealed that there were 10 / mm of unplated defects with a diameter of 0.5 mm or more. 2 If it is abnormal, X, 10 / mm 2 If it is less than that, it is marked as O.

[0159] Meanwhile, the hydrogen embrittlement resistance of hot-formed parts was measured by applying a stress of 1200 MPa and immersing in 0.1 N hydrochloric acid (HCl) for 120 hours to determine whether cracks occurred. If no cracks occurred, they were marked as O, and if they occurred, they were marked as X. This is shown in Table 2.

[0160] Classification B content (wt.%) α√[B]*α 3 Hydrogen embrittlement-resistant plating characteristics specimen 110.00311.30112.26OX specimen 120.00310.8583.52OO specimen 130.00310.3060.16XO specimen 140.00531.27114.94OX specimen 150.00531.14410.89OO specimen 160.00530.9365.97OO specimen 170.00530.3210.24OO specimen 170.00530.2390.10XO specimen 180.01251.27122.95OX specimen 190.01251.01411.65OO specimen Psalm 200.01250.7494.69OO Psalm 210.01250.3180.36OO Psalm 220.01250.2210.12OO Psalm 230.01431.02913.02OX Psalm 240.01430.99811.87OO Psalm 250.01430.2600.21OO Psalm 260.01430.1880.08XO

[0161] According to the results in Table 2 above, it was confirmed that even if the B content of the steel is within the range proposed by the present invention, it is difficult to secure excellent hydrogen embrittlement resistance and plating characteristics when the annealing heat treatment process of the present invention is exceeded.

Claims

1. Contains C: 0.1~2.0% and B: 0.0030~0.0150% by weight, the remainder being Fe and other unavoidable impurities. The highest B content value in the B content profile measured by 3D-APT (Three-Dimensional Atom Probe Tomography) (B max ) A hot-formed member containing austenite grain boundaries of 0.15 to 0.35 wt.%.

2. In paragraph 1, The above hot-formed member is a hot-formed member containing Si+Al: 0.4 to 3.0%, Mn: 0.1 to 5.0%, P: 0.0001 to 0.1%, and S: 0.0001 to 0.03%.

3. In paragraph 1 or 2, The above hot-formed member is a hot-formed member comprising at least one of a) to f). a) One or more selected from the group consisting of Cr, Mo and W: 0.01 to 2.0%, b) One or more elements selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%, c) 0.005 to 2.0% of one or both of Cu and Ni, d) B: 0.0001~0.01%, e) Sb: 0.001~0.1%, and f) As: 1.00% or less 4. In paragraph 1, In the above B content profile, the average B content (B) in the area up to 3 nm before and after the highest B content value av ) is a hot-formed member having a content of 0.1 to 0.3 wt.% / nm.

5. In paragraph 1, In the B content profile and C content profile measured by the above 3D-APT (Three-Dimensional Atom Probe Tomography), the ratio of the highest B content value to the highest C content value (B max / C max ) is a hot-formed member containing austenite grain boundaries of 1.5 or less.

6. In paragraph 1, A hot-formed member having a tensile strength of 1200 to 1700 MPa.

7. In paragraph 1, A hot-formed member having a diffusible hydrogen content of 0.3 ppm or less.

8. In paragraph 1, The microstructure of the above hot-formed member is a hot-formed member in which martensite accounts for 95 area% or more.

9. A step for providing a steel material containing, by weight%, C: 0.1 to 2.0%, B: 0.0030 to 0.0150%, the remainder Fe and other unavoidable impurities; A step of providing a steel for hot forming by annealing the above steel; and It includes a step of manufacturing a hot-formed member by hot-forming the above hot-formed steel material, A method for manufacturing a hot-formed member that satisfies the value of the following [Relational Expression 1] when performing the above annealing heat treatment. [Relationship 1] The above [B] is the content of the element in the steel (weight ppm), and the above α is derived from the following relationships 2 to 4. [Relationship 2] [Relationship 3] tn: Unit time (sec.) in the temperature range above Ac1 during annealing heat treatment Tn: Temperature (℃) per unit time in the temperature range above Ac1 during annealing heat treatment [Relationship 4] tn: Unit time (sec.) in the temperature range above Ac1 during annealing heat treatment 10. In paragraph 9, A method for manufacturing a hot-formed member, wherein the above steel contains Si+Al: 0.4 to 3.0%, Mn: 0.1 to 5.0%, P: 0.0001 to 0.1%, and S: 0.0001 to 0.03%.

11. In paragraph 9 or 10, A method for manufacturing a hot-formed member, wherein the above steel comprises at least one of a) to f). a) One or more selected from the group consisting of Cr, Mo and W: 0.01 to 2.0%, b) One or more elements selected from the group consisting of Ti, Nb, Zr and V: 0.001 to 0.4%, c) 0.005 to 2.0% of one or both of Cu and Ni, d) B: 0.0001~0.01%, e) Sb: 0.001~0.1%, and f) As: 1.00% or less 12. In paragraph 9, The above [Relationship Formula 1] is a method for manufacturing a hot-formed member having a value of 12.0 or less.

13. In paragraph 9, A method for manufacturing a hot-formed member, comprising providing a hot-formed steel material including a plating layer by performing plating after the above annealing heat treatment.

14. In paragraph 13, The above hot forming steel has 10 / mm of unplated defects with a diameter of 0.5mm or more. 2 A method for manufacturing a hot-formed member having a thickness less than 100mm.

15. In paragraph 9, The step of providing the above steel material is: Step of heating the steel slab at 1000~1300℃; A step of hot rolling the above-mentioned heated steel slab at a temperature of Ar3 or higher and 1000°C or lower to obtain a hot-rolled steel sheet; A step of coiling the hot-rolled steel sheet at a temperature exceeding Ms but not exceeding 750°C; and A method for manufacturing a hot-formed member, comprising the steps of obtaining a cold-rolled steel sheet by pickling and cold rolling after the above-mentioned coiling.

16. In paragraph 9, The above hot forming step is: A step of heat treating the above hot forming steel material in a temperature range of Ac3 or higher and 975°C or lower for 1 to 1,000 seconds; and A method for manufacturing a hot-formed member, comprising the steps of forming and quenching the above heat-treated steel.

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