WO2023106899A1

WO2023106899A1 - Material for hot stamping

Info

Publication number: WO2023106899A1
Application number: PCT/KR2022/020150
Authority: WO
Inventors: 신누리; 서용기; 손지희; 정상배
Original assignee: 현대제철 주식회사
Priority date: 2021-12-10
Filing date: 2022-12-12
Publication date: 2023-06-15
Also published as: KR20230088118A

Abstract

A material for hot stamping according to an embodiment of the present invention comprises: a steel sheet comprising 0.28 to 0.50 wt% of carbon (C), 0.15 to 0.70 wt% of silicon (Si), 0.5 to 2.0 wt% of manganese (Mn), 0.05 wt% or less of phosphorus (P), 0.01 wt% or less of sulfur (S), 0.1 to 0.5 wt% of chromium (Cr), 0.001 to 0.005 wt% of boron (B), 0.1 wt% or less of an additive, and the remainder of iron (Fe) and other inevitable impurities; and fine precipitates distributed in the steel sheet, wherein the additive includes at least one of titanium (Ti), niobium (Nb) and vanadium (V), the fine precipitates include a nitride or carbide of the at least one of titanium (Ti), niobium (Nb) and vanadium (V) and trap hydrogen, and the coefficient of variation of the average diameter, which is a value obtained by dividing the standard deviation of the average diameter of the fine precipitates by the average diameter of the fine precipitates, is 0.7 or less.

Description

Material for hot stamping

Embodiments of the present invention relate to materials for hot stamping, and more particularly, to materials for hot stamping capable of securing excellent mechanical properties and delayed hydrogen fracture characteristics of hot stamping parts.

High-strength steel for weight reduction and stability is applied to parts used in automobiles and the like. On the other hand, high-strength steel can secure high-strength characteristics in comparison to its weight, but as its strength increases, its press formability deteriorates, resulting in material breakage or spring back during processing, making it difficult to form products with complex and precise shapes. There are difficulties.

As a method to improve these problems, there is a hot stamping method as a representative method, and as interest in this method increases, research on materials for hot stamping is being actively conducted. For example, as disclosed in Korean Patent Laid-Open Publication No. 10-2017-0076009, the hot stamping method is a forming technique for manufacturing a high-strength part by heating a boron steel sheet to an appropriate temperature, forming it in a press mold, and then rapidly cooling it. According to the invention of Korean Unexamined Patent Publication No. 10-2017-0076009, it is possible to manufacture parts with good precision by suppressing problems such as crack generation or shape freezing defect during molding, which is a problem in high-strength steel sheets.

However, in the case of a hot stamping steel sheet, there is a problem in that delayed hydrogen fracture occurs due to hydrogen introduced in the hot stamping process and residual stress. In this regard, Korean Patent Laid-Open Publication No. 10-2020-0061922 discloses that preheating is performed before heating a hot stamping blank to a high temperature to form a thin oxide layer on the surface of the blank, thereby blocking the inflow of hydrogen in the high-temperature heating process and delaying hydrogen. Initiate minimizing destruction. However, since it is impossible to completely block the inflow of hydrogen, there is a concern that the inflow of hydrogen cannot be controlled, leading to delayed hydrogen destruction.

Embodiments of the present invention are intended to solve various problems including the above-mentioned problems, and a material for hot stamping capable of securing high strength, high toughness, excellent mechanical properties and improved hydrogen delayed fracture characteristics of hot stamping parts and a manufacturing method thereof can provide. However, these tasks are illustrative, and the scope of the present invention is not limited thereby.

A material for hot stamping according to an embodiment of the present invention contains carbon (C): 0.28 to 0.50% by weight, silicon (Si): 0.15 to 0.70% by weight, manganese (Mn): 0.5 to 2.0% by weight, phosphorus (P) : 0.05% by weight or less, sulfur (S): 0.01% by weight or less, chromium (Cr): 0.1 to 0.5% by weight, boron (B): 0.001 to 0.005% by weight, additives: 0.1% by weight or less and the rest iron (Fe) and other unavoidable impurities; and fine precipitates distributed in the steel sheet, wherein the additive includes at least one of titanium (Ti), niobium (Nb) and vanadium (V), and the fine precipitates are titanium (Ti) and niobium (Nb) and vanadium (V) containing at least one nitride or carbide, trapping hydrogen, and dividing the standard deviation of the average diameter of the microprecipitates by the average diameter of the microprecipitates, the diameter average coefficient of variation is 0.7 below

The material for hot stamping, after hot stamping, may exhibit a tensile strength of 1,680 MPa or more, a bending angle of 40 degrees or more, and an amount of activated hydrogen of 0.5 wppm or less.

An average diameter of the fine precipitates may be 0.006 μm or less.

More than 90% of the fine precipitates may have a diameter of 0.01 μm or less.

More than 60% of the fine precipitates may have a diameter of 0.005 μm or less.

The number of fine precipitates per unit area (100 μm ² ) may be 25,000 or more and 30,000 or less.

The number of fine precipitates having a diameter of 0.01 μm or less per unit area (100 μm ² ) may be 23,000 or more and 29,000 or less.

The number of fine precipitates having a diameter of 0.005 μm or less per unit area (100 μm 2 ) may be 15,200 or more and 29,000 or less.

A number coefficient of variation, which is a value obtained by dividing a standard deviation of an average number of the fine precipitates by an average number of the fine precipitates, may be 0.8 or less.

A material for hot stamping according to an embodiment of the present invention contains carbon (C): 0.28 to 0.50% by weight, silicon (Si): 0.15 to 0.70% by weight, manganese (Mn): 0.5 to 2.0% by weight, phosphorus (P) : 0.05% by weight or less, sulfur (S): 0.01% by weight or less, chromium (Cr): 0.1 to 0.5% by weight, boron (B): 0.001 to 0.005% by weight, additives: 0.1% by weight or less and the rest iron (Fe) and other unavoidable impurities; and fine precipitates distributed in the steel sheet, wherein the additive includes at least one of titanium (Ti), niobium (Nb) and vanadium (V), and the fine precipitates are titanium (Ti) and niobium It contains at least one nitride or carbide of (Nb) and vanadium (V), traps hydrogen, and has a number coefficient of variation, which is a value obtained by dividing the standard deviation of the average number of the fine precipitates by the average number of the fine precipitates, is 0.8 or less. am.

The material for hot stamping may exhibit a tensile strength of 1,680 MPa or more, a bending angle of 40 degrees or more, and an amount of activated hydrogen of 0.5 wppm or less after hot stamping.

The first number coefficient of variation, which is a value obtained by dividing the standard deviation of the average number of fine precipitates having a diameter of 0.01 μm or less among the fine precipitates by the average number of fine precipitates having a diameter of 0.01 μm or less, may be 0.8 or less.

The second number coefficient of variation, which is a value obtained by dividing the standard deviation of the average number of fine precipitates having a diameter of 0.005 μm or less among the fine precipitates by the average number of fine precipitates having a diameter of 0.005 μm or less, may be 0.8 or less.

The number of fine precipitates having a diameter of 0.005 μm or less per unit area (100 μm ² ) may be 15,200 or more and 29,000 or less.

An average diameter of the fine precipitates may be 0.006 μm or less.

More than 90% of the fine precipitates may have a diameter of 0.01 μm or less.

According to embodiments of the present invention, it is possible to implement a material for hot stamping and a manufacturing method thereof capable of securing high strength, high toughness, excellent mechanical properties, and improved hydrogen delayed fracture characteristics of hot stamping parts. Of course, the scope of the present invention is not limited by these effects.

1 is a TEM (Transmission Electron Microscopy) image showing a part of a material for hot stamping according to an embodiment of the present invention.

Figures 2a and 2b are views schematically showing a portion of the state in which fine precipitates are dispersed according to an embodiment of the present invention.

3A and 3B are exemplary views schematically showing a part of a state in which hydrogen is trapped in microprecipitates.

4 is a flowchart schematically illustrating a method for manufacturing a material for hot stamping according to an embodiment of the present invention.

Figure 5 is a graph showing the comparison of tensile strength and bending stress according to the coiling temperature of Examples and Comparative Examples of the present invention.

6A and 6B are images showing the results of a 4 point bending test according to the winding temperature of Examples and Comparative Examples.

Since the present invention can apply various transformations and have various embodiments, specific embodiments will be illustrated in the drawings and described in detail in the detailed description. Effects and features of the present invention, and methods for achieving them will become clear with reference to the embodiments described later in detail together with the drawings. However, the present invention is not limited to the embodiments disclosed below and may be implemented in various forms.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, and when describing with reference to the drawings, the same or corresponding components are assigned the same reference numerals, and overlapping descriptions thereof will be omitted. .

In the following embodiments, terms such as first and second are used for the purpose of distinguishing one component from another component without limiting meaning.

In the following examples, expressions in the singular number include plural expressions unless the context clearly dictates otherwise.

In the following embodiments, terms such as include or have mean that features or elements described in the specification exist, and do not preclude the possibility that one or more other features or elements may be added.

In the following embodiments, when a part such as a film, region, component, etc. is said to be on or on another part, not only when it is directly above the other part, but also when another film, region, component, etc. is interposed therebetween. Including if there is

In the drawings, the size of components may be exaggerated or reduced for convenience of description. For example, since the size and thickness of each component shown in the drawings are arbitrarily shown for convenience of description, the present invention is not necessarily limited to the illustrated bar.

When an embodiment is otherwise implementable, a specific process sequence may be performed differently from the described sequence. For example, two processes described in succession may be performed substantially simultaneously, or may be performed in an order reverse to the order described.

In this specification, "A and/or B" represents the case of A, B, or A and B. And, "at least one of A and B" represents the case of A, B, or A and B.

In the following embodiments, when films, regions, components, etc. are connected, when films, regions, and components are directly connected, or/and other films, regions, and components are interposed between the films, regions, and components. It also includes cases where they are interposed and indirectly connected. For example, when a film, region, component, etc. is electrically connected in this specification, when a film, region, component, etc. is directly electrically connected, and/or another film, region, component, etc. is interposed therebetween. This indicates an indirect electrical connection.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

As shown in FIG. 1 , the material 1 for hot stamping according to an embodiment of the present invention may include a steel plate 10 and fine precipitates 20 distributed in the steel plate 10 .

The material 1 for hot stamping may be controlled such that the content of alloy elements included in the steel sheet 10 and the precipitation behavior of the fine precipitates 20 satisfy preset conditions. Through this, the molded part after hot stamping can have excellent mechanical properties of high strength and high toughness, and improved hydrogen delayed fracture properties. In one embodiment, the material 1 for hot stamping may exhibit a tensile strength of 1,680 MPa or more after hot stamping, and preferably may exhibit a tensile strength of 1,680 MPa or more and less than 2,300 MPa. In addition, a bending angle of 40 degrees or more and an amount of activated hydrogen of 0.5 wppm or less may be indicated. Here, "bending angle" may mean a V-bending angle in a rolling direction (RD).

The steel sheet 10 may be a steel sheet manufactured by performing a hot rolling process and/or a cold rolling process on a slab cast to include a predetermined content of a predetermined alloy element. The steel sheet 10 contains carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), boron (B), and the balance of iron (Fe) and other unavoidable impurities. can include Also, in one embodiment, the steel sheet 10 may further include at least one of titanium (Ti), niobium (Nb), and vanadium (V) as an additive. Alternatively, the steel sheet 10 may further include a predetermined amount of calcium (Ca).

In one embodiment, the steel sheet 10 contains carbon (C): 0.28 to 0.50% by weight, silicon (Si): 0.15 to 0.70% by weight, manganese (Mn): 0.5 to 2.0% by weight, phosphorus (P): greater than 0 0.05% by weight or less, sulfur (S): greater than 0 and less than 0.01% by weight, chromium (Cr): 0.1 to 0.5% by weight, boron (B): 0.001 to 0.005% by weight, titanium (Ti) and niobium (Nb) as additives , And the sum of one or more of vanadium (V): more than 0 and 0.1 wt% or less, and the rest may include iron (Fe) and other unavoidable impurities.

Carbon (C) acts as an austenite stabilizing element in the steel sheet 10. Carbon is the main element that determines the strength and hardness of the steel sheet 10, and after the hot stamping process, the purpose of securing the tensile strength of the steel sheet 10 (eg, tensile strength of 1,680 MPa or more) and securing hardenability characteristics can be added as Such carbon may be included in an amount of about 0.28wt% to about 0.50wt% based on the total weight of the steel sheet 10 . When the carbon content is less than about 0.28wt%, it may be difficult to satisfy the mechanical strength of the steel sheet 10 because it is difficult to secure a hard phase (such as martensite). Conversely, when the carbon content exceeds about 0.50 wt%, brittleness of the steel sheet 10 or reduction in bending performance may be caused.

Silicon (Si) may act as a ferrite stabilizing element in the steel sheet 10 . Silicon (Si), as a solid-solution strengthening element, can improve the ductility of the steel sheet 10 and improve the carbon concentration in austenite by suppressing the formation of low-temperature carbides. In addition, silicon is a key element for hot rolling, cold rolling, hot press structure homogenization (perlite, manganese segregation zone control), and fine dispersion of ferrite. Silicon acts as a martensitic strength heterogeneity control element and serves to improve impact performance. Silicon may be included in an amount of about 0.15 wt % to about 0.70 wt % based on the total weight of the steel sheet 10 . If the content of silicon is less than 0.15wt%, it is difficult to obtain the above-mentioned effect, cementite formation and coarsening may occur in the final hot-stamped martensite structure, and the effect of equalizing the steel sheet 10 is insignificant and the V-bending angle can be secured. may become impossible On the contrary, when the content of silicon exceeds about 0.70 wt%, the hot rolling and cold rolling loads increase, the hot rolled red scale becomes excessive, and the plating characteristics of the steel sheet 10 may deteriorate.

Manganese (Mn) acts as an austenite stabilizing element in the steel sheet 10. Manganese may be added for the purpose of increasing hardenability and strength during heat treatment. Manganese may be included in an amount of about 0.5 wt % to about 2.0 wt % based on the total weight of the steel sheet 10 . When the content of manganese is less than about 0.5 wt%, the crystal grain refinement effect is not sufficient, and the hard phase fraction in the molded article after hot stamping may be insufficient due to insufficient hardenability. On the other hand, when the content of manganese exceeds about 2.0 wt%, ductility and toughness may be deteriorated due to segregation of manganese or pearlite band, and a deterioration in bending performance may occur and a heterogeneous microstructure may occur.

Phosphorus (P) may be included in an amount greater than 0 and less than or equal to about 0.05 wt% based on the total weight of the steel sheet 10 in order to prevent deterioration in toughness of the steel sheet 10 . When the content of phosphorus exceeds about 0.05 wt%, iron phosphide compounds are formed, resulting in deterioration in toughness and weldability, and cracks may be induced in the steel sheet 10 during the manufacturing process.

Sulfur (S) may be included in an amount greater than 0 and about 0.01 wt% or less based on the total weight of the steel sheet 10 . When the sulfur content exceeds about 0.01 wt%, hot workability, weldability, and impact properties are deteriorated, and surface defects such as cracks may occur due to the formation of large inclusions.

Chromium (Cr) may be added for the purpose of improving hardenability and strength of the steel sheet 10 . Chromium can make crystal grain refinement and strength secured through precipitation hardening. Chromium may be included in an amount of about 0.1 wt % to about 0.5 wt % based on the total weight of the steel sheet 10 . When the chromium content is less than about 0.1wt%, the precipitation hardening effect is poor, and on the contrary, when the chromium content exceeds 0.5wt%, the amount of Cr-based precipitates and matrix solids increases, resulting in lowered toughness and increased cost. Production costs may increase

Boron (B) may be added for the purpose of securing hardenability and strength of the steel sheet 10 by suppressing ferrite, pearlite, and bainite transformations to secure a martensitic structure. In addition, boron may be segregated at grain boundaries to lower grain boundary energy to increase hardenability, and may have an effect of grain refinement by increasing austenite grain growth temperature. Boron may be included in an amount of about 0.001 wt % to about 0.005 wt % based on the total weight of the steel sheet 10 . When boron is included in the above range, it is possible to prevent grain boundary brittleness in the hard phase and to secure high toughness and bendability. When the boron content is less than about 0.001wt%, the hardenability effect is insufficient, and on the contrary, when the boron content exceeds about 0.005wt%, the solid solubility is low and the hardenability is easily precipitated at the grain boundary depending on the heat treatment conditions. Deterioration or high-temperature embrittlement may occur, and toughness and bendability may be reduced due to grain boundary brittleness in the hard phase.

The additive is a nitride or carbide generating element that contributes to the formation of the fine precipitates 20 . Specifically, the additive may include at least one of titanium (Ti), niobium (Nb), and vanadium (V). Titanium (Ti), niobium (Nb), and vanadium (V) form nitride or carbide-type fine precipitates 20, thereby securing the strength of a member subjected to hot stamping and quenching. In addition, these are contained in the Fe-Mn-based composite oxide, function as a hydrogen trap site effective for improving delayed fracture resistance, and may be elements necessary for improving delayed fracture resistance. These additives may be included in an amount greater than 0 and less than or equal to about 0.1 wt% based on the total weight of the steel sheet 10 in total. If the content of the additive exceeds about 0.1 wt%, the increase in yield strength may be excessively increased.

Titanium (Ti) may be added for the purpose of strengthening hardenability and improving the material by forming precipitates after hot press heat treatment. In addition, by forming a precipitate phase such as Ti (C, N) at high temperature, it effectively contributes to the refinement of austenite crystal grains. Titanium may be included in an amount of about 0.01wt% to about 0.05wt% based on the total weight of the steel sheet 10 . When titanium is included in the above content range, it is possible to prevent poor performance and coarsening of precipitates, easily secure physical properties of steel materials, and prevent defects such as cracks on the surface of steel materials. On the other hand, when the content of titanium exceeds about 0.05wt%, the precipitate is coarsened and elongation and bendability may decrease.

Niobium (Nb) and vanadium (V) may be added for the purpose of increasing strength and toughness according to a decrease in martensite packet size. Each of niobium and vanadium may be included in an amount of about 0.01 wt % to about 0.05 wt % based on the total weight of the steel sheet 10 . When niobium and vanadium are included in the above range, the crystal grain refinement effect of steel is excellent in hot rolling and cold rolling processes, cracks in slabs during steelmaking/playing, and brittle fractures of products are prevented, and steelmaking coarse precipitates are generated can be minimized.

Meanwhile, when the additive includes titanium (Ti) and niobium (Nb), the total amount of titanium (Ti) and niobium (Nb) may be about 0.02wt% to about 0.09wt% based on the total weight of the steel sheet 10. It is not limited to this.

Calcium (Ca) may be added to control the inclusion shape. Calcium may be included in an amount of about 0.003 wt% or less based on the total weight of the steel sheet 10 .

The fine precipitates 20 may serve to trap hydrogen by being distributed in the steel sheet 10 . That is, the fine precipitates 20 can improve the hydrogen delayed fracture characteristics of the hot stamped product by providing a trap site for hydrogen introduced into the inside during or after the manufacturing process of the material 1 for hot stamping. In one embodiment, the microprecipitates 20 may include nitrides or carbides of additives. Specifically, the fine precipitates 20 may include at least one nitride or carbide of titanium (Ti), niobium (Nb), and vanadium (V).

Precipitation behavior of the fine precipitates 20 can be controlled by adjusting process conditions. For example, by controlling the coiling temperature (CT) range of process conditions, it is possible to control precipitation behavior such as the number of fine precipitates 20, the average distance between the fine precipitates 20, and the diameter of the fine precipitates 20. can A detailed description of process conditions will be described later with reference to FIG. 4 .

Precipitation behavior of the fine precipitates 20 may be controlled to satisfy preset conditions. In addition, the precipitation behavior of these fine precipitates 20 can be controlled through process conditions (eg, coiling temperature (CT) range). For example, the number of fine precipitates 20 per unit area, the number variation coefficient C1 representing the uniformity of the number distribution per unit area, the average distance between the fine precipitates 20, the diameter of the fine precipitates 20, and the uniformity of the diameter distribution per unit area. The diameter average coefficient of variation (C2, C21, C22), etc. representing the can be controlled to satisfy preset conditions. A detailed description of process conditions will be described later with reference to FIGS. 4 and 5 . Precipitation behavior conditions for the number coefficient of variation (C1) and average diameter coefficient of variation (C2, C21, C22) will be described later in more detail through Examples and Comparative Examples.

On the other hand, the precipitation behavior of such fine precipitates 20 can be measured by a method of analyzing a TEM (Transmission Electron Microscopy) image. Specifically, TEM images of arbitrary regions are acquired as many as a preset number of specimens. Fine precipitates 20 are extracted from the acquired images through an image analysis program, etc., and the number of fine precipitates 20 with respect to the extracted fine precipitates 20, the average distance between the fine precipitates 20, and the number of fine precipitates The diameter of (20) can be measured.

In one embodiment, in order to measure the precipitation behavior of the fine precipitates 20, a surface replication method may be applied as a pretreatment to the specimen to be measured. For example, a one-step replica method, a two-step replica method, an extraction replica method, and the like may be applied, but are not limited to the above examples.

Alternatively, when measuring the diameter of the fine precipitates 20, the diameter of the fine precipitates 20 may be calculated by converting the shape of the fine precipitates 20 into a circle in consideration of the non-uniformity of the shape of the fine precipitates 20. Specifically, the diameter of the microprecipitate 20 is measured by measuring the area of the microprecipitate 20 extracted using a unit pixel having a specific area, and converting the microprecipitate 20 into a circle having the same area as the measured area. can be calculated.

Alternatively, the average distance between the fine precipitates 20 may be measured through the above-described mean free path. Specifically, the average distance between the fine precipitates 20 can be calculated using the particle area fraction and the number of particles per unit length. For example, the average distance between the fine precipitates 20 may have a correlation as shown in Equation 1 below.

[Equation 1]

λ=(1-AA)/NL

(λ: average distance between particles, AA: particle area fraction, NL: number of particles per unit length)

A method of measuring the precipitation behavior of the fine precipitates 20 is not limited to the above-described example, and various methods may be applied.

The precipitation behavior of the fine precipitates 20 greatly affects the mechanical properties and delayed hydrogen fracture characteristics of the molded part after hot stamping. Hereinafter, differences in the improvement effect of the mechanical properties and delayed hydrogen fracture characteristics of a molded part after hot stamping according to the precipitation behavior of the fine precipitates 20 will be described with reference to FIGS. 2A to 3B.

Specifically, FIG. 2A shows a case where the size and distribution of the fine precipitates 20 dispersed inside the steel plate 10 are relatively non-uniform, and in FIG. 2B, the fine precipitates dispersed inside the steel plate 10 ( 20) is shown where the size and distribution are relatively uniform.

Referring to FIG. 2A , fine precipitates 20 are non-uniform and biasedly distributed inside the steel sheet 10 . Accordingly, a region in which a relatively large number of fine precipitates 20 are aggregated may exist in some regions inside the steel plate 10 . In addition, relatively more defects (eg, dislocations, etc.) may be aggregated in a region where a relatively large number of fine precipitates 20 are aggregated. In addition to this, when the sizes of the fine precipitates 20 are not uniform as shown in FIG. 2A, relatively more defects (eg, dislocations, etc.) may be aggregated in the relatively large-sized fine precipitates 20. . That is, defects may also be biasedly distributed within the steel sheet 10 , not uniformly.

In this way, when relatively more defects (eg, dislocations, etc.) are aggregated in a specific region, the region acts as a fracture notch, causing non-uniform stress distribution and stress concentration, resulting in mechanical damage such as strength and bendability. There is a problem of degrading the characteristics.

On the other hand, referring to FIG. 2B , fine precipitates 20 having a relatively uniform size are evenly dispersed inside the steel sheet 10 . Accordingly, defects (eg, dislocations, etc.) can be relatively uniformly distributed inside the steel sheet 10, and there is an effect of minimizing or preventing the occurrence of a region acting as the aforementioned notch.

In one embodiment, the number per unit area of the fine precipitates 20 dispersed inside the steel sheet 10 may be controlled to satisfy a preset range. Specifically, the number of fine precipitates 20 dispersed inside the steel sheet 10 per unit area (100 μm ² ) may be 25,000 or more and 30,000 or less. That is, the fine precipitates 20 may be formed in an amount of 25,000/100 μm ² or more and 30,000/100 μm ^{2 or} less inside the steel plate 10 .

In addition, the number of fine precipitates 20 having a diameter of 0.01 μm or less among the fine precipitates 20 dispersed inside the steel sheet 10 per unit area (100 μm ² ) may be 23,000 or more and 29,000 or less. That is, the fine precipitates 20 may be formed in an amount of 23,000/100 μm ² or more and 29,000/100 μm ^{2 or} less inside the steel sheet 10 .

In addition, the number of fine precipitates 20 having a diameter of 0.005 μm or less among the fine precipitates 20 dispersed inside the steel sheet 10 per unit area (100 μm ² ) may be 15,200 or more and 29,000 or less. That is, the fine precipitates 20 may be formed in an amount of 15,200/100 μm ² or more and 29,000/100 μm ^{2 or} less inside the steel sheet 10 .

When the number of fine precipitates 20 per unit area satisfies the aforementioned range, it is possible to secure required tensile strength (eg, 1,680 MPa or more) after hot stamping and improve formability or bendability. For example, the number of fine precipitates 20 is less than 25,000 / 100 μm ² or the number of fine precipitates 20 having a diameter of 0.01 μm or less is less than 23,000 / 100 μm ² or the number of fine precipitates 20 having a diameter of 0.005 μm or less If the number is less than 15,200/100 μm ² , strength may decrease. On the other hand, the number of fine precipitates 20 exceeds 30,000 / 100 μm ² or the number of fine precipitates 20 having a diameter of 0.01 μm or less exceeds 29,000 / 100 μm ² or fine precipitates having a diameter of 0.005 μm or less ( 20) in excess of 29,000/100 μm ² , formability or bendability may deteriorate.

Hereinafter, the coefficient of variation of the number average representing the uniformity of the distribution of the total number of fine precipitates per unit area (100 μm ² ) and the size (diameter) of the fine precipitates may be referred to as the second coefficient of variation (C2). The second coefficient of variation C2 may be defined as a value obtained by dividing the standard deviation of the number average by the number average. In one embodiment, the second coefficient of variation C2 may satisfy about 0.8 or less. When the second coefficient of variation (C2) exceeds about 0.8, the bendability may be deteriorated. Specifically, when the number distribution of fine precipitates is non-uniform, the increase in the density of the block boundary through refinement of the prior austenite grain size (PAGS) in the area where the fine precipitates are small is reduced, and Accordingly, it may be difficult to secure a sufficient slip band. Therefore, when the second coefficient of variation (C2) exceeds about 0.8, formability and bendability may be deteriorated. The precipitation behavior related to the number distribution through the second coefficient of variation C2 will be described in more detail through comparative examples and examples to be described later.

In one embodiment, the average distance between adjacent fine precipitates 20 may be controlled to satisfy a preset range. Here, the "average distance" may mean a mean free path of the fine precipitates 20, and details of a method for measuring this will be described later.

Specifically, the average distance between the fine precipitates 20 may be about 0.15 μm or more and about 0.4 μm or less. When the average distance between the fine precipitates 20 is less than about 0.15 μm, formability or bendability may deteriorate, whereas when the average distance exceeds about 0.4 μm, strength may decrease.

The diameter of the above-described fine precipitates 20 can have a great effect on improving hydrogen delayed fracture characteristics. Hereinafter, with reference to FIGS. 3A and 3B , the difference in the hydrogen delayed fracture characteristic improvement effect according to the diameter of the fine precipitates 20 will be described.

3A and 3B are diagrams schematically showing a part of a state in which hydrogen is trapped in the fine precipitates 20.

Specifically, FIG. 3A shows a state in which hydrogen is trapped in fine precipitates 20 having a relatively large diameter, and FIG. 3B shows a state in which hydrogen is trapped in fine precipitates 20 having a relatively small diameter. this is shown

When the microprecipitates 20 have relatively large diameters as shown in FIG. 3A , the number of hydrogen atoms trapped in one microprecipitate 20 may increase. That is, hydrogen atoms introduced into the steel sheet 10 may not be evenly dispersed, and the probability of a plurality of hydrogen atoms being trapped at one hydrogen trap site may increase. A plurality of hydrogen atoms trapped in one hydrogen trap site may combine with each other to form hydrogen molecules (H ₂ ). The formed hydrogen molecules increase the probability of generating internal pressure, and as a result, the hydrogen delayed fracture characteristics of the hot stamped product may be deteriorated.

In contrast, when the microprecipitates 20 have a relatively small diameter, as shown in FIG. 3B, the probability of a plurality of hydrogen atoms being trapped in one microprecipitate 10 may decrease. That is, hydrogen atoms introduced into the steel sheet 10 may be relatively evenly dispersed by being trapped at different hydrogen trap sites. Accordingly, since the hydrogen atoms are prevented from bonding with each other, the probability of generating internal pressure due to the hydrogen molecules is reduced, and the hydrogen delayed fracture characteristics of the hot stamped product may be improved.

In the steel sheet 1 for hot stamping according to an embodiment of the present invention, the diameter average of the fine precipitates 20 dispersed inside the steel sheet 10 is controlled to satisfy a preset range in order to improve the hydrogen delayed fracture characteristics. can Specifically, the average diameter of the fine precipitates 20 dispersed inside the steel sheet 10 may be about 0.006 μm or less. In addition, about 90% or more of the fine precipitates 20 dispersed inside the steel sheet 10 may have a diameter of about 0.01 μm or less. In addition, about 60% or more of the fine precipitates 20 dispersed inside the steel sheet 10 may have a diameter of about 0.005 μm or less.

Hereinafter, the coefficient of variation of the size (diameter) of the entire fine precipitate per unit area (100 μm ² ) may be referred to as the first coefficient of variation (C1). The first coefficient of variation C1 may be defined as a value obtained by dividing the standard deviation of the average diameter of all fine precipitates by the average diameter. In one embodiment, the first coefficient of variation C1 may satisfy about 0.7 or less. When the first coefficient of variation (C1) exceeds about 0.7, tensile strength and delayed hydrogen fracture characteristics may be deteriorated. Specifically, when the size (diameter) distribution of the fine precipitates is non-uniform, the non-uniform diameter distribution acts as an obstacle to dislocation movement, and thus the strength may decrease. That is, when an external impact is applied to the steel sheet after hot stamping, a portion where fine precipitates having a large size (diameter) are concentrated may act as a fracture notch due to dislocation accumulation. In addition, since the amount of hydrogen trapped in the fine precipitates is non-uniform, hydrogen delayed fracture characteristics may be deteriorated due to a local increase in hydrogen partial pressure due to hydrogen bonds. Precipitation behavior related to the diameter distribution through the first coefficient of variation (C1) will be described in more detail through comparative examples and examples to be described later.

On the other hand, miniaturization of the fine precipitates 20 can improve the bending characteristics of the molded part after hot stamping. In one embodiment, the micronized microprecipitates 20 may act as an obstacle to crystal grain growth, thereby minimizing the prior austenite grain size (PAGS). As the prior austenite grain size (PAGS) is refined, the martensite packet size and martensite lath size may decrease. Accordingly, a block boundary interval within a packet may decrease and block density may increase. This block boundary can act as a slip band when the martensitic structure is deformed by external impact, and the refinement of the fine precipitates 20 can secure more slip bands and contribute to improving bending properties. there is.

As shown in FIG. 4, the method for manufacturing a material for hot stamping according to an embodiment of the present invention includes a reheating step (S100), a hot rolling step (S200), a cooling/coiling step (S300), and a cold rolling step (S400). ), an annealing heat treatment step (S500) and a plating step (S600) may be included.

For reference, although steps S100 to S600 are shown as independent steps in FIG. 4 , some of steps S100 to S600 may be performed in one process, and some may be omitted if necessary.

First, a semi-finished slab to be subjected to the process of forming the material 1 for hot stamping is prepared. The slab contains carbon (C): 0.28 to 0.50% by weight, silicon (Si): 0.15 to 0.70% by weight, manganese (Mn): 0.5 to 2.0% by weight, phosphorus (P): greater than 0 and less than 0.05% by weight, sulfur (S ): more than 0 and 0.01% by weight or less, chromium (Cr): 0.1 to 0.5% by weight, boron (B): 0.001 to 0.005% by weight, additives: more than 0 and 0.1% by weight or less, and the rest iron (Fe) and other unavoidable impurities can include In this case, the additive may include at least one of titanium (Ti), niobium (Nb), and vanadium (V). For example, each content of titanium (Ti), niobium (Nb), and/or vanadium (V) may be about 0.010 wt % to about 0.050 wt %.

The reheating step (S100) is a step of reheating the slab for hot rolling. In the reheating step (S100), components segregated during casting may be re-dissolved by reheating the slab obtained through the continuous casting process in a predetermined temperature range.

Slab reheating temperature (SRT) can be controlled within a preset temperature range to maximize austenite refinement and precipitation hardening effect. At this time, the slab reheating temperature (SRT) range is included in the temperature range (about 1,000 ° C or more) in which the additives (Ti, Nb and / or V) are fully employed based on the equilibrium precipitation amount of the fine precipitates 20 when the slab is reheated. can If the slab reheating temperature (SRT) does not reach the full solidification temperature range of the additives (Ti, Nb and/or V), the driving force required to control the microstructure during hot rolling is not sufficiently reflected, resulting in excellent mechanical properties through the required precipitation control. The securing effect cannot be obtained.

In one embodiment, the slab reheat temperature (SRT) may be controlled between about 1,200°C and about 1,250°C. When the slab reheating temperature (SRT) is less than about 1,200 ° C, there is a problem in that the components segregated during casting are not sufficiently re-dissolved, making it difficult to see a large homogenization effect of alloy elements and a large solid solution effect of titanium (Ti). On the other hand, the higher the slab reheating temperature (SRT), the higher the homogenization, but when it exceeds about 1,250 °C, the grain size of austenite increases, making it difficult to secure strength and the excessive heating process only increases the manufacturing cost of the steel sheet. can

The hot rolling step (S200) is a step of manufacturing a steel sheet by hot rolling the slab reheated in step S100 in a predetermined finishing delivery temperature (FDT) range. In one embodiment, the finish rolling temperature (FDT) range may be controlled from about 840°C to about 920°C. If the finish rolling temperature (FDT) is less than 840°C, it is difficult to secure the workability of the steel sheet due to the occurrence of a mixed structure due to abnormal rolling, and there is a problem of deterioration in workability due to the non-uniformity of the microstructure, as well as hot rolling due to rapid phase change. During rolling, a problem of sheetability may occur. Conversely, when the finish rolling temperature (FDT) exceeds 920 °C, the austenite grains may be coarsened. In addition, there is a risk that the TiC precipitate is coarsened and the performance of the final part is deteriorated.

On the other hand, in the reheating step (S100) and the hot rolling step (S200), some of the fine precipitates 20 may be precipitated at grain boundaries with unstable energy. At this time, the fine precipitates 20 precipitated at the grain boundary act as an element that hinders the grain growth of austenite, thereby providing an effect of improving strength through austenite refinement. On the other hand, the fine precipitates 20 precipitated in steps S100 and S200 may be about 0.007wt% based on the equilibrium precipitated amount, but are not limited thereto.

The cooling/coiling step (S300) is a step of cooling and winding the hot-rolled steel sheet in step S200 in a predetermined coiling temperature (Coiling Temperature: CT) range, and forming fine precipitates 20 in the steel sheet. That is, in step S300, fine precipitates 20 are formed by forming nitrides or carbides of additives (Ti, Nb, and/or V) included in the slab. Meanwhile, winding may be performed in a ferrite station so that the equilibrium precipitate amount of the fine precipitates 20 may reach a maximum value. In this way, when the structure is transformed into ferrite after the crystal grain recrystallization is completed, the particle size of the fine precipitates 20 may be uniformly precipitated not only at the grain boundary but also within the grain.

In one embodiment, the winding temperature (CT) may be about 700 °C to about 780 °C. The coiling temperature (CT) can affect the redistribution of carbon (C). When the coiling temperature (CT) is less than about 700 ° C, the low-temperature phase fraction due to overcooling increases, so there is a risk of increasing strength and intensifying rolling load during cold rolling, and there is a problem in that ductility rapidly decreases. Conversely, when the coiling temperature exceeds about 780 ° C, there is a problem in that moldability and strength deterioration occurs due to abnormal crystal grain growth or excessive crystal grain growth.

In this way, according to this embodiment, by controlling the coiling temperature (CT) range, it is possible to control the precipitation behavior of the fine precipitates (20). Experimental examples of changes in the characteristics of the material 1 for hot stamping according to the coiling temperature (CT) range will be described later with reference to FIGS. 5, 6a, and 6b.

In the cold rolling step (S400), the steel sheet wound in step S300 is uncoiled, pickled, and then cold rolled. At this time, pickling is performed for the purpose of removing the scale of the rolled steel sheet, that is, the hot-rolled coil manufactured through the hot-rolling process. On the other hand, in one embodiment, the reduction ratio during cold rolling may be controlled to 30% to 70%, but is not limited thereto.

Annealing heat treatment step (S500) is a step of annealing heat treatment at a temperature of about 700 ° C or more of the cold-rolled steel sheet in step S400. In one embodiment, the annealing heat treatment includes heating the cold-rolled sheet material and cooling the heated cold-rolled sheet material at a predetermined cooling rate.

The plating step (S600) is a step of forming a plating layer on the annealed and heat-treated steel sheet. In one embodiment, in the plating step (S600), an Al-Si plating layer may be formed on the steel sheet subjected to the annealing heat treatment in step S500.

Specifically, the plating step (S600) includes forming a hot-dip plating layer on the surface of the steel sheet by immersing the steel sheet in a plating bath having a temperature of about 650°C to about 700°C, and cooling the steel sheet on which the hot-dip plating layer is formed to form the plated layer. It may include a cooling step of forming. At this time, the plating bath may include Si, Fe, Al, Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, Bi, etc. as additive elements, but is not limited thereto.

By performing the hot stamping process on the hot stamping material 1 manufactured through steps S100 to S600 as described above, it is possible to satisfy the conditions for the precipitation behavior of the fine precipitates 20 described above, and thus required strength and bending. It is possible to manufacture hot stamping parts satisfying the properties. In one embodiment, the material for hot stamping (1) manufactured to satisfy the above-described content conditions and process conditions has a tensile strength of 1,680 MPa or more, a bendability of 50 degrees or more, and an activation number of 0.8 wppm or less after hot stamping. may have a small amount.

Hereinafter, the present invention will be described in more detail through Examples and Comparative Examples. However, the following Examples and Comparative Examples are intended to explain the present invention in more detail, and the scope of the present invention is not limited by the following Examples and Comparative Examples. The following Examples and Comparative Examples may be appropriately modified or changed by those skilled in the art within the scope of the present invention.

Figure 5 is a graph showing the comparison of tensile strength and bending stress according to the winding temperature of Examples and Comparative Examples of the present invention, Figures 6a and 6b is a 4-point bending test (4 point bending test) according to the winding temperature of Examples and Comparative Examples These are images showing the results of the test).

Example (CT700) and Comparative Example (CT800) are specimens manufactured by hot stamping the material for hot stamping (1) prepared by performing steps S100 to S600 on a slab having the composition shown in Table 1 below. At this time, Example (CT700) and Comparative Example (CT800) are specimens manufactured by applying the same content conditions and process conditions in the manufacturing process of the material 1 for hot stamping, but differentially applying only the winding temperature (CT) as a variable. . Specifically, Example (CT700) and Comparative Example (CT800) are slab reheating temperature (SRT): 1,230 ° C, finish rolling temperature (FDT): 900 ° C, rolling reduction during hot rolling: 95%, annealing heat treatment temperature: 780 ℃, plating immersion temperature: specimens prepared by hot stamping after heating the hot stamping material manufactured under the conditions of 660 ℃ at 950 ℃ for 270 seconds.

성분(wt%)Ingredients (wt%)
CC	SiSi	MnMn	PP	SS	CrCr	BB	TiTi
0.290.29	0.20.2	1.51.5	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.00250.0025	0.0350.035

Specifically, the embodiment (CT700) is a specimen manufactured by hot stamping the material for hot stamping (1) manufactured by applying a winding temperature (CT) of 700 ° C, and the comparative example (CT800) is a specimen manufactured by applying a winding temperature (CT) of 800 ° C. It is a specimen manufactured by hot stamping the material for hot stamping (1) manufactured by applying the temperature (CT).

Here, description will be made with reference to FIG. 5 . 5 is a graph showing measured tensile strength and bending stress of Example (CT700) and Comparative Example (CT800).

Referring to FIG. 5, in the case of tensile strength, the tensile strength of Example (CT700) is greater than that of Comparative Example (CT800), and the bending stress affecting impact properties is also comparable to that of Example (CT700). It can be seen that the improvement is compared to the bending stress of the example (CT800).

As can be seen in Table 2 below, this is because the amount of fine precipitates 20 increased in the case of the example (CT700) and the improved hydrogen scavenging ability as compared to the comparative example (CT800).

Table 2 below shows the measured values of the equilibrium precipitation amount and the amount of activated hydrogen of the Example (CT700) and Comparative Example (CT800) and the results of a bent-beam stress corrosion test. Here, the equilibrium precipitation amount means the maximum number of precipitates that can be precipitated when thermodynamically equilibrium is achieved, and the greater the equilibrium precipitation amount, the greater the number of precipitates. In addition, the amount of activated hydrogen refers to the amount of hydrogen excluding hydrogen trapped in the fine precipitates 20 among the hydrogen introduced into the steel sheet 10 .

The amount of activated hydrogen can be measured using a thermal desorption spectroscopy method. Specifically, while heating the specimen at a preset heating rate and raising the temperature, the amount of hydrogen released from the specimen below a specific temperature may be measured. At this time, hydrogen released from the specimen below a certain temperature can be understood as activated hydrogen that is not trapped among the hydrogen introduced into the specimen and affects delayed hydrogen destruction.

샘플명sample name	평형 석출량 (wt%)equilibrium precipitation amount (wt%)	4점 굴곡 시험(4 point bending test) 결과4 point bending test result	활성화 수소량 (wppm)amount of activated hydrogen (wppm)
CT700CT700	0.0400.040	비파단non-break	0.4530.453
CT800CT800	0.0290.029	파단fracture	0.5500.550

Table 2 shows the results of a 4-point bending test for each of the samples with different equilibrium precipitation amounts of fine precipitates and the amount of activated hydrogen measured using the thermal desorption spectroscopy method. indicate

Here, the 4 point bending test is a test to check whether stress corrosion cracking occurs by applying stress below the elastic limit to a specimen prepared by reproducing the state in which the specimen is exposed to a corrosive environment at a specific point way. At this time, stress corrosion cracking means a crack that occurs when corrosion and continuous tensile stress act simultaneously.

Specifically, the results of the 4 point bending test in Table 2 are the results of confirming whether or not breakage occurred by applying a stress of 1,000 MPa in air for 100 hours to each of the samples. In addition, the amount of activated hydrogen was measured using the thermal desorption spectroscopy method described above, and the temperature was raised from room temperature to 500 °C at a heating rate of 20 °C/min for each sample at 350 °C. Hereinafter, the amount of hydrogen released from the specimen is measured.

Referring to Table 2, in the case of the equilibrium precipitation amount of the fine precipitates 20, the equilibrium precipitation amount of Example (CT700) was 0.040wt%, and the equilibrium precipitation amount of Comparative Example (CT800) was measured as 0.029wt%. That is, it can be seen that the embodiment (CT700) can provide more hydrogen trap sites by forming more fine precipitates 20 compared to the comparative example (CT800).

On the other hand, in the case of the 4-point bending test result, the example (CT700) was not broken and the comparative example (CT800) was broken. In addition, in the case of the amount of activated hydrogen, the amount of activated hydrogen in Example (CT700) was about 0.453 wppm, and the amount of activated hydrogen in Comparative Example (CT800) was measured to be about 0.550 wppm. In this regard, it can be confirmed that the example (CT700) with a relatively lower amount of activated hydrogen was not broken, and the comparative example (CT800) with a relatively higher amount of activated hydrogen was broken. It can be understood that the hydrogen delayed fracture characteristics of the embodiment (CT700) are improved compared to the comparative example (CT800).

That is, the amount of fine precipitates 20 increased in Example (CT700) compared to Comparative Example (CT800), and accordingly, the amount of activated hydrogen decreased. This means that the amount of hydrogen trapped inside in the embodiment (CT700) increased compared to the comparative example (CT800), and as a result, it can be understood that the hydrogen delayed fracture characteristics are improved.

6A and 6B are images showing the results of a 4-point bending test for the Example (CT700) and Comparative Example (CT800), respectively. Specifically, FIG. 6a shows the result of the 4-point bending test for the example (CT700), and FIG. 6b shows the 4-point bending test for the comparative example (CT800) under the same conditions as the example (CT700). corresponds to a result.

As shown in FIGS. 6A and 6B , in the case of the embodiment (CT700), the specimen was not broken as a result of the 4-point bending test, whereas in the case of the comparative example (CT800), it was confirmed that the specimen was broken.

In the case of the embodiment (CT700) of FIG. 6A, this is a specimen manufactured by hot stamping the material 1 for hot stamping prepared by applying a coiling temperature (CT) of 700 ° C, and having a diameter of 0.01 μm or less. Fine precipitates 20 are formed in 23,000 or more and 29,000 or less per unit area (100 μm ² ), and the average distance between the fine precipitates 20 satisfies 0.15 μm or more and 0.4 μm or less. Therefore, it can be confirmed that the embodiment (CT700) efficiently disperses and traps hydrogen introduced into the steel sheet 10, thereby improving hydrogen delayed fracture characteristics and improving tensile strength and bending characteristics.

On the contrary, in the case of the comparative example (CT800) of FIG. 6B, as a specimen prepared by hot stamping the material for hot stamping (1) prepared by applying a coiling temperature (CT) of 800 ° C, the fine precipitates (20) The amount of precipitate is not sufficient, and the diameter of the fine precipitates 20 is coarsened, increasing the probability of occurrence of internal pressure due to hydrogen bonding. Therefore, it can be confirmed that the comparative example (CT800) cannot efficiently disperse and trap hydrogen introduced into the steel sheet 10, and the tensile strength, bending characteristics, and delayed hydrogen fracture characteristics are deteriorated.

That is, even though they are composed of the same components, due to the difference in coiling temperature (CT), the strength, bendability, and delayed hydrogen fracture characteristics of the hot stamping material 1 after the hot stamping process are different. This is because a difference occurs in the precipitation behavior of the fine precipitates 20 according to the coiling temperature CT. Therefore, when the content conditions and process conditions according to the above-described embodiments of the present invention are applied, high strength may be secured, and bendability and delayed hydrogen fracture characteristics may be improved.

Tables 3 to 6 below quantify tensile strength, bendability, and hydrogen delayed fracture characteristics according to differences in precipitation behavior of fine precipitates 20 for a plurality of specimens. Specifically, in Tables 3 to 6, for a plurality of specimens, measured values of precipitation behavior (number of fine precipitates, average distance between fine precipitates, diameter of fine precipitates, etc.) and characteristics after hot stamping ( Tensile strength, bendability and amount of activated hydrogen) are measured.

Meanwhile, the plurality of specimens were each heated to 950 ° C and cooled at a cooling rate of 30 ° C / s or more to 300 ° C or less, and then the tensile strength, bendability and amount of activated hydrogen were measured.

At this time, the tensile strength and the amount of activated hydrogen were measured based on the aforementioned 4-point bending test and thermal desorption spectroscopy, and the bendability was measured by the German Automobile Industry Association (VDA: Verband Der The V-bending angle was measured according to VDA238-100, a standard of Automobilindustrie.

In addition, the precipitation behavior of the microprecipitates (the number of microprecipitates, the average distance between the microprecipitates, the diameter of the microprecipitates, etc.) was measured through the above-described TEM image analysis. In addition, the precipitation behavior of the fine precipitates was measured for arbitrary regions having an area of 0.5 μm*0.5 μm and converted based on a unit area (100 μm ² ).

시편Psalter	성분(wt%)Ingredients (wt%)
시편Psalter	CC	SiSi	MnMn	PP	SS	CrCr	BB	TiTi
AA	0.300.30	0.190.19	1.51.5	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.030.03
BB	0.280.28	0.190.19	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.030.03
CC	0.290.29	0.200.20	1.61.6	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.030.03
DD	0.290.29	0.210.21	1.51.5	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.030.03
EE	0.280.28	0.200.20	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.0330.033
FF	0.300.30	0.250.25	1.31.3	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.0330.033
GG	0.310.31	0.250.25	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.0330.033
HH	0.300.30	0.200.20	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.0330.033
II	0.290.29	0.200.20	1.51.5	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.00250.0025	0.0330.033
JJ	0.290.29	0.300.30	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.250.25	0.00250.0025	0.030.03
KK	0.280.28	0.300.30	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.250.25	0.00250.0025	0.0330.033

시편Psalter	전체 미세석출물total fine precipitates			직경 0.01㎛ 이하의 미세석출물Fine precipitates with a diameter of 0.01 μm or less	직경 0.005㎛ 이하의 미세석출물Fine precipitates with a diameter of 0.005㎛ or less	핫 스탬핑 후 특징Features after hot stamping
시편Psalter	개수 (개/100㎛²)Count (pcs/100㎛ ² )	평균 거리 (㎛)average distance (μm)	평균 직경 (㎛)average diameter (μm)	개수 (개/100㎛²) / 비율(%)Count (pcs/100㎛ ² ) / ratio(%)	개수 (개/100㎛²) / 비율(%)Count (pcs/100㎛ ² ) / ratio(%)	인장 강도 (MPa)Seal robbery (MPa)	굽힘각 (º)bending angle (º)	활성화 수소량 (wppm)amount of activated hydrogen (wppm)
AA	25,51125,511	0.310.31	0.00570.0057	23,011 / 90.2%23,011 / 90.2%	15,400 / 60.4%15,400 / 60.4%	17081708	4242	0.4940.494
BB	25,33325,333	0.290.29	0.00560.0056	23,572 / 93.0%23,572 / 93.0%	15,200 / 60.0%15,200 / 60.0%	16991699	4343	0.4910.491
CC	25,01225,012	0.280.28	0.00410.0041	24,412 / 97.6%24,412 / 97.6%	23,236 / 92.9%23,236 / 92.9%	16951695	4242	0.4970.497
DD	28,16028,160	0.180.18	0.00440.0044	27,960 / 99.3%27,960 / 99.3%	19,800 / 70.3%19,800 / 70.3%	17411741	4646	0.4530.453
EE	28,87528,875	0.180.18	0.00420.0042	28,813 / 99.8%28,813 / 99.8%	24,438 / 84.6%24,438 / 84.6%	17501750	4444	0.4480.448
FF	27,58327,583	0.190.19	0.00470.0047	26,148 / 94.8%26,148 / 94.8%	20,715 / 75.1%20,715 / 75.1%	17451745	4848	0.4550.455
GG	29,99829,998	0.160.16	0.00530.0053	26,998 / 90%26,998 / 90%	18,269 / 60.9%18,269 / 60.9%	17561756	5050	0.4710.471
HH	29,92729,927	0.150.15	0.00460.0046	28,999 / 96.9%28,999 / 96.9%	18,255 / 61.0%18,255 / 61.0%	17571757	4646	0.4220.422
II	29,85529,855	0.150.15	0.00380.0038	28,393 / 95.1%28,393 / 95.1%	25,228 / 84.5%25,228 / 84.5%	17501750	4444	0.4250.425
JJ	26,17726,177	0.40.4	0.00560.0056	24,397 / 93.2%24,397 / 93.2%	15,994 / 61.1%15,994 / 61.1%	17151715	4242	0.4910.491
KK	27,80027,800	0.20.2	0.0060.006	27,355 / 98.4%27,355 / 98.4%	21,573 / 77.6%21,573 / 77.6%	17361736	5555	0.4580.458

시편Psalter	성분(wt%)Ingredients (wt%)
시편Psalter	CC	SiSi	MnMn	PP	SS	CrCr	BB	TiTi
LL	0.300.30	0.190.19	1.51.5	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.030.03
MM	0.280.28	0.190.19	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.030.03
NN	0.290.29	0.200.20	1.61.6	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.030.03
OO	0.290.29	0.210.21	1.51.5	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.030.03
PP	0.280.28	0.200.20	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.0330.033
QQ	0.300.30	0.250.25	1.31.3	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.0330.033
RR	0.310.31	0.250.25	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.0330.033
SS	0.300.30	0.200.20	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.0020.002	0.0330.033
TT	0.290.29	0.200.20	1.51.5	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.20.2	0.00250.0025	0.0330.033
UU	0.290.29	0.300.30	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.250.25	0.00250.0025	0.030.03
VV	0.280.28	0.300.30	1.41.4	0.02 이하0.02 or less	0.015 이하Less than 0.015	0.250.25	0.00250.0025	0.0330.033

시편Psalter	전체 미세석출물total fine precipitates			직경 0.01㎛ 이하의 미세석출물Fine precipitates with a diameter of 0.01 μm or less	직경 0.005㎛ 이하의 미세석출물Fine precipitates with a diameter of 0.005㎛ or less	핫 스탬핑 후 특징Features after hot stamping
시편Psalter	개수 (개/100㎛²)Count (pcs/100㎛ ² )	평균 거리 (㎛)average distance (μm)	평균 직경 (㎛)average diameter (μm)	개수 (개/100㎛²) / 비율(%)Count (pcs/100㎛ ² ) / ratio(%)	개수 (개/100㎛²) / 비율(%)Count (pcs/100㎛ ² ) / ratio(%)	인장 강도 (MPa)Seal robbery (MPa)	굽힘각 (º)bending angle (º)	활성화 수소량 (wppm)amount of activated hydrogen (wppm)
LL	25,52525,525	0.350.35	0.00550.0055	22,998 / 90.1%22,998 / 90.1%	15,545 / 60.9%15,545 / 60.9%	16711671	4545	0.4520.452
MM	24,99924,999	0.370.37	0.00510.0051	22,874 / 91.5%22,874 / 91.5%	15,200 / 60.8%15,200 / 60.8%	16641664	5050	0.4950.495
NN	29,29829,298	0.160.16	0.00390.0039	29,005 / 99%29,005 / 99%	21,446 / 73.2%21,446 / 73.2%	17521752	3737	0.4570.457
OO	30,00930,009	0.160.16	0.00490.0049	27,969 / 93.2%27,969 / 93.2%	18,909 / 63.0%18,909 / 63.0%	17641764	3535	0.4810.481
PP	27,48527,485	0.230.23	0.00710.0071	26,028 / 94.7%26,028 / 94.7%	16,903 / 61.5%16,903 / 61.5%	16781678	4242	0.5050.505
QQ	29,95729,957	0.170.17	0.00420.0042	26,901 / 89.8%26,901 / 89.8%	25,403 / 84.8%25,403 / 84.8%	17421742	4040	0.5140.514
RR	25,02325,023	0.320.32	0.00580.0058	23,997 / 95.9%23,997 / 95.9%	14,989 / 59.9%14,989 / 59.9%	17121712	5151	0.5020.502
SS	29,99929,999	0.370.37	0.00510.0051	15,691 / 52.3%15,691 / 52.3%	17,909 / 59.7%17,909 / 59.7%	17551755	4343	0.5040.504
TT	29,98529,985	0.140.14	0.00360.0036	28,996 / 96.7%28,996 / 96.7%	28,426 / 94.8%28,426 / 94.8%	17531753	3838	0.4380.438
UU	24,21524,215	0.410.41	0.00590.0059	21,939 / 90.6%21,939 / 90.6%	14,578 / 60.2%14,578 / 60.2%	16781678	4444	0.4980.498
VV	25,11025,110	0.330.33	0.00550.0055	22,624 / 90.1%22,624 / 90.1%	15,112 / 60.2%15,112 / 60.2%	16711671	5454	0.4920.492

Tables 3 to 6 show the measured values of the precipitation behavior of the microprecipitates (the number of microprecipitates, the average distance between the microprecipitates, the diameter of the microprecipitates, etc.) and the characteristics after hot stamping (tensile strength, bendability and amount of activated hydrogen).

Specimens A to K are specimens manufactured by hot stamping the material for hot stamping manufactured through steps S100 to S600 by applying the above-described process conditions to slabs satisfying the content conditions shown in Table 3. At this time, specimens A to K had a slab reheating temperature (SRT): 1230 ° C, a finish rolling temperature (FDT): 900 ° C, a reduction rate during hot rolling: 95%, a coiling temperature (CT): 780 ° C, annealing heat treatment temperature: 780 ℃, plating immersion temperature: specimens prepared by hot stamping after heating the hot stamping material manufactured under the condition of 660 ℃ at 950 ℃ for 270 seconds. That is, specimens A to K are specimens satisfying the above-described conditions for precipitation behavior of fine precipitates.

Specifically, in specimens A to K, fine precipitates were formed in the steel sheet in an amount of 25,000/100 μm ² or more and 30,000/100 μm ² or less, the average diameter of all the fine precipitates was 0.006 μm or less, and the average distance between all the fine precipitates was 0.15 μm or more and 0.4 μm or less are satisfied. In addition, more than 90% of the fine precipitates formed in the steel sheet have a diameter of 0.01 μm or less, and the number of fine precipitates having a diameter of 0.01 μm or less satisfies 23,000/100 μm ² or more and 29,000/100 μm ^{2 or} less. In addition, more than 60% of the fine precipitates formed in the steel sheet have a diameter of 0.005 μm or less, and the number of fine precipitates having a diameter of 0.005 μm or less satisfies 15,200/100 μm ² or more and 29,000/100 μm ^{2 or} less.

It can be confirmed that specimens A to K satisfying the precipitation behavior conditions of the present invention have improved tensile strength, bendability, and delayed hydrogen fracture characteristics. Specifically, samples A to K satisfy tensile strength of 1,680 MPa or more after hot stamping, bendability of 40 degrees or more after hot stamping, and an amount of activated hydrogen after hot stamping of 0.5 wppm or less. .

On the other hand, specimens L to V are specimens manufactured by hot stamping the material for hot stamping manufactured through steps S100 to S600 by applying the above-described process conditions to slabs satisfying the content conditions shown in Table 5. At this time, specimens L to V had a slab reheating temperature (SRT): 1230 ° C, a finish rolling temperature (FDT): 900 ° C, a reduction ratio during hot rolling: 95%, a coiling temperature (CT): 790 ° C, annealing heat treatment temperature: 780 ℃, plating immersion temperature: specimens prepared by hot stamping after heating the hot stamping material manufactured under the condition of 660 ℃ at 950 ℃ for 270 seconds. That is, specimens L to V are specimens that do not satisfy at least some of the above-described precipitation behavior conditions of fine precipitates, and it can be confirmed that the tensile strength, bendability, and/or delayed hydrogen fracture characteristics are inferior to those of specimens A to K. there is.

In the case of specimen L, the number of fine precipitates with a diameter of 0.01 μm or less is 22,998. This is less than the lower limit of the condition for the number of fine precipitates having a diameter of 0.01 μm or less. Accordingly, it can be confirmed that the tensile strength of specimen L is only 1,671 MPa, which is relatively low.

In the case of specimen M, the total number of fine precipitates was 24,999, and the number of fine precipitates having a diameter of 0.01 μm or less was 22,874. This falls short of the lower limit of the condition for the total number of fine precipitates and the lower limit of the condition for the number of fine precipitates having a diameter of 0.01 μm or less. Accordingly, it can be confirmed that the tensile strength of specimen M is only 1,664 MPa, which is relatively low.

In the case of specimen N, the number of fine precipitates with a diameter of 0.01 μm or less is 29,005. This exceeds the upper limit of the condition for the number of fine precipitates having a diameter of 0.01 μm or less. Accordingly, it can be confirmed that the bendability of specimen N is only 37 degrees, which is relatively low.

In the case of specimen O, the total number of fine precipitates is 30,009. This exceeds the upper limit of the condition for the total number of fine precipitates. Accordingly, it can be confirmed that the bendability of specimen O is only 35 degrees, which is relatively low.

In the case of specimen P, the average diameter of all microprecipitates is 0.0071 μm. This exceeds the upper limit of the condition of the average diameter of all fine precipitates. Accordingly, the amount of activated hydrogen of specimen P was measured as a relatively high 0.505 wppm, confirming that the hydrogen delayed fracture characteristics were relatively deteriorated.

In the case of specimen Q, the ratio of fine precipitates with a diameter of 0.01 μm or less is 89.8%. This falls short of the lower limit of the condition for the ratio of fine precipitates with a diameter of 0.005 μm or less. Accordingly, the amount of activated hydrogen of specimen Q was measured as a relatively high 0.514 wppm, confirming that the hydrogen delayed fracture characteristics were relatively deteriorated.

In the case of specimen R, the ratio of fine precipitates with a diameter of 0.005 μm or less is 59.9%. This falls short of the lower limit of the condition for the ratio of fine precipitates with a diameter of 0.005 μm or less. Accordingly, the amount of activated hydrogen of specimen R was measured as a relatively high 0.502 wppm, confirming that the hydrogen delayed fracture characteristics were relatively deteriorated.

In the case of specimen S, the ratio of fine precipitates with a diameter of 0.005 μm or less is 59.7%. This falls short of the lower limit of the condition for the ratio of fine precipitates with a diameter of 0.005 μm or less. Accordingly, the amount of activated hydrogen of specimen S was measured as a relatively high 0.504 wppm, confirming that the hydrogen delayed fracture characteristics were relatively deteriorated.

In the case of specimen T, the average distance between all the microprecipitates is 0.14 μm. This falls short of the lower limit of the average distance condition between all the fine precipitates. Accordingly, it can be confirmed that the bendability of specimen T is only 38 degrees, which is relatively low.

In the case of specimen U, the average distance of all microprecipitates is 0.41 μm. This exceeds the upper limit of the total fine precipitate average distance condition. Accordingly, it can be confirmed that the tensile strength of specimen U is only 1,678 MPa, which is relatively low.

In the case of specimen V, the number of fine precipitates with a diameter of 0.005 μm or less is 15,112. This is less than the lower limit of the condition for the number of fine precipitates having a diameter of 0.005 μm or less. Accordingly, it can be confirmed that the tensile strength is only 1,671 MPa, which is relatively low.

Hereinafter, differences in the improvement effect of the tensile strength and delayed hydrogen fracture characteristics of molded parts after hot stamping according to the precipitation behavior of the fine precipitates 20 will be described using Table 7. Precipitation behavior of the following fine precipitates was measured through the above-described TEM image analysis. Precipitation behavior of fine precipitates is measured for 10 random areas having an area of 0.5㎛*0.5㎛ and converted based on a unit area (100㎛ ² ), and hereinafter, 'average' refers to random areas means the average of the precipitation behavior values for The total number of precipitates in the arbitrary regions was calculated, and their average value was referred to as 'the average number of total precipitates'. Similarly, the average distance between the fine precipitates was calculated through the mean free path in the arbitrary regions, and the average value thereof was referred to as 'average distance between the precipitates'. After converting the number of fine precipitates having a diameter of 10 nm or less and micro precipitates having a diameter of 5 nm or less in the arbitrary regions based on a unit area (100 μm ² ) and calculating their average value, the ‘average of the total number of precipitates’ The ratios were referred to as 'average ratios of 10 nm or less' and 'average ratios of 5 nm or less'.

The average of the diameters of all the fine precipitates in the above arbitrary regions was referred to as 'the average diameter of all precipitates', and the standard deviation of these values was referred to as the 'standard deviation of the average diameter'. The diameter average coefficient of variation, that is, the first coefficient of variation (C1), was defined as a value obtained by dividing the standard deviation of the average diameter by the average diameter. In the present invention, the first coefficient of variation (C1) may be 0.8 or less.

시편Psalter	전체 석출물 개수 평균 (개/㎛²)whole precipitate count average (pcs/㎛ ² )	석출물 간 거리 평균 (㎛)precipitate liver distance average (μm)	10nm 이하 비율 평균 (%)10 nm below ratio average (%)	5nm 이하 비율 평균 (%)5nm below ratio average (%)	전체 석출물 직경 평균 (㎛)entire precipitate diameter average (μm)	직경 평균의 표준 편차diameter average standard Deviation	직경 평균 변동계수 (C1)diameter average coefficient of variation (C1)	H/S 후 인장강도 (MPa)After H/S tensile strength (MPa)	H/S 후 활성화 수소량 (wppm)After H/S amount of activated hydrogen (wppm)
X1X1	25,13925,139	0.40 0.40	91.5 91.5	62.7 62.7	0.0044 0.0044	0.0041 0.0041	0.70 0.70	17411741	0.3880.388
X2X2	25,71825,718	0.40 0.40	90.2 90.2	61.5 61.5	0.0029 0.0029	0.0016 0.0016	0.39 0.39	17471747	0.3910.391
X3X3	26,71226,712	0.26 0.26	91.6 91.6	61.2 61.2	0.0031 0.0031	0.0025 0.0025	0.67 0.67	17551755	0.3810.381
X4X4	27,23627,236	0.23 0.23	90.3 90.3	61.7 61.7	0.0059 0.0059	0.0053 0.0053	0.66 0.66	17561756	0.3720.372
X5X5	26,63126,631	0.26 0.26	99.5 99.5	81.8 81.8	0.0032 0.0032	0.0019 0.0019	0.44 0.44	17431743	0.3710.371
X6X6	28,30228,302	0.23 0.23	98.4 98.4	92.6 92.6	0.0026 0.0026	0.0009 0.0009	0.25 0.25	17801780	0.3650.365
X7X7	28,88928,889	0.21 0.21	97.6 97.6	76.2 76.2	0.0035 0.0035	0.0016 0.0016	0.31 0.31	17981798	0.3660.366
X8X8	29,68829,688	0.15 0.15	96.7 96.7	69.4 69.4	0.0058 0.0058	0.0051 0.0051	0.67 0.67	17891789	0.3670.367
X9X9	25,08225,082	0.39 0.39	90.7 90.7	63.4 63.4	0.0046 0.0046	0.0044 0.0044	0.71 0.71	16811681	0.4810.481
X10X10	27,12127,121	0.26 0.26	90.6 90.6	63.4 63.4	0.0050 0.0050	0.0047 0.0047	0.73 0.73	16851685	0.4790.479
X11X11	27,33427,334	0.24 0.24	90.1 90.1	60.5 60.5	0.0059 0.0059	0.0057 0.0057	0.72 0.72	16951695	0.4830.483
X12X12	29,42429,424	0.17 0.17	97.1 97.1	91.4 91.4	0.0026 0.0026	0.0026 0.0026	0.72 0.72	16971697	0.4540.454

Table 7 quantifies the tensile strength and hydrogen delayed fracture characteristics according to the precipitation behavior of fine precipitates for a plurality of specimens. Specifically, in Table 7, with respect to the plurality of specimens (X1 to X12), the precipitation behavior of the microprecipitates is the average number of all microprecipitates, the average distance between the microprecipitates, the average ratio of the microprecipitates of a specific diameter or less, and the total microprecipitates. Measured values for the average diameter of the precipitates, the standard deviation of the average diameter, the first coefficient of variation (C1), and the resulting properties after hot stamping, the measured values for the tensile strength and the amount of activated hydrogen are described. In particular, it will be described in terms of tensile strength and hydrogen delayed fracture characteristics according to the first coefficient of variation (C1), which is the coefficient of variation of the average diameter.

Specimens X1 to X12 contain carbon (C): 0.28 to 0.50% by weight, silicon (Si): 0.15 to 0.70% by weight, manganese (Mn): 0.5 to 2.0% by weight, phosphorus (P): greater than 0 and less than 0.05% by weight, Sulfur (S): more than 0 and 0.01% by weight or less, chromium (Cr): 0.1 to 0.5% by weight, boron (B): 0.001 to 0.005% by weight, titanium (Ti), niobium (Nb), and vanadium (V ) The sum of one or more of: 0 to 0.1 wt% or less, and to the slab containing the remaining iron (Fe) and other unavoidable impurities, the material for hot stamping manufactured through steps S100 to S600 by applying the above-described process conditions Specimens produced by hot stamping. At this time, specimens X1 to X12 had a slab reheating temperature (SRT): 1230 ° C, a finish rolling temperature (FDT): 900 ° C, a reduction ratio during hot rolling: 95%, a coiling temperature (CT): 780 ° C, annealing heat treatment temperature: 780 ℃, plating immersion temperature: specimens prepared by hot stamping after heating the hot stamping material manufactured under the condition of 660 ℃ at 950 ℃ for 270 seconds. Meanwhile, X1 to X12 may satisfy not only the precipitation behavior with respect to the first coefficient of variation C1 but also the precipitation behavior with respect to the aforementioned second coefficient of variation C2. That is, X1 to X12 are specimens satisfying the above-described precipitation behavior conditions of fine precipitates.

Specifically, the total number of fine precipitates in specimens X1 to X12 is 25,000 to 30,000/㎛ ² , the average distance between adjacent fine precipitates is 0.15 μm or more and 0.40 μm or less, and the ratio of fine precipitates with a diameter of 0.01 μm or less is 90, respectively. % or more, the ratio of fine precipitates having a diameter of 0.005 μm or less is 60% or more, and the average diameter of all fine precipitates satisfies 0.006 μm or less.

However, specimens X1 to X8 satisfy the condition that the first coefficient of variation is 0.7 or less, and specimens X9 to X12 have a first coefficient of variation greater than 0.7 and do not satisfy the precipitation behavior condition according to the first coefficient of variation of the present invention. It can be seen that specimens X1 to X8 having a first coefficient of variation of 0.7 or less have superior tensile strength and delayed hydrogen fracture characteristics to specimens X9 to X12 having a first coefficient of variation of more than 0.7. Specifically, specimens X1 to X8 have a tensile strength of 1700 MPa or more after hot stamping and an amount of activated hydrogen of 0.4 wppm or less, and specimens X9 to X12 have a tensile strength of 1,680 MPa to 1700 MPa (less than 1700 MPa) and an amount of activated hydrogen after hot stamping. Compared to 0.4 wppm to 0.5 wppm, specimens X1 to X8 have excellent tensile strength and delayed hydrogen fracture characteristics compared to specimens X9 to X12.

In particular, specimens X1 to X4 have an average number of total precipitates of 25,000 or more and 27,500 or less, of which the average ratio of fine precipitates with a diameter of 10 nm or less is 90% to 92%, and the average ratio of fine precipitates with a diameter of 5 nm or less is 60 % to 63%, since the total number of precipitates and the ratio of fine precipitates having a diameter of 10 nm or 5 nm or less are also small, it may be a very difficult condition to secure tensile strength or delayed hydrogen fracture characteristics after hot stamping. Nevertheless, since specimens X1 to X4 satisfy the precipitation behavior condition of the present invention that the first coefficient of variation is 0.7 or less, they have relatively high tensile strength and delayed hydrogen fracture characteristics compared to specimens X9 to X12.

On the other hand, specimen X12 has an average number of precipitates of 29,424, and the ratio of fine precipitates with diameters of 10 nm and 5 nm or less is also high at 97.1% and 91.4%, respectively, which is a relatively favorable condition for securing tensile strength and hydrogen delayed fracture characteristics. can Nevertheless, specimen X12 does not satisfy the precipitation behavior condition of the present invention that the first coefficient of variation is 0.7 or less, so it has relatively low tensile strength and hydrogen delayed fracture characteristics compared to specimens X1 to X8.

Hereinafter, differences in the effect of improving the bendability of molded parts after hot stamping according to the precipitation behavior of the fine precipitates 20 will be described using Table 8. Precipitation behavior of the following fine precipitates was measured through the above-described TEM image analysis. Precipitation behavior of fine precipitates is measured for 10 random areas having an area of 0.5㎛*0.5㎛ and converted based on a unit area (100㎛ ² ), and hereinafter, 'average' refers to random areas means the average of the precipitation behavior values for After measuring the total number of precipitates, the number of fine precipitates having a diameter of 10 nm or less, and the number of fine precipitates having a diameter of 5 nm or less in the arbitrary regions, they are converted based on the unit area (100 μm ² ), and the average value of each of these is ‘total They were referred to as 'average number of precipitates', 'average number of precipitates of 10 nm or less', and 'average number of precipitates of 5 nm or less', and standard deviations of each were measured. The average distance between the fine precipitates was calculated through the mean free path in the arbitrary regions, and the average value thereof was referred to as 'average distance between the precipitates'.

The number coefficient of variation, that is, the second coefficient of variation (C2) was defined as a value obtained by dividing the standard deviation of the number average by the number average. In the present invention, the second coefficient of variation (C2) may be 0.8 or less. The second coefficient of variation (C2) includes a 2-1 coefficient of variation (C21), which is a coefficient of variation in the number of fine precipitates having a diameter of 10 nm or less, and a 2-2 coefficient of variation (C22), which is a coefficient of variation in the number of fine precipitates having a diameter of 5 nm or less. .

The 2-1 coefficient of variation (C21) is defined as a value obtained by dividing the standard deviation of the number average of fine precipitates having a diameter of 0.01 μm or less among all fine precipitates by the average number of fine precipitates having a diameter of 0.01 μm or less, and the second- 1 The coefficient of variation C21 may be 0.8 or less. Hereinafter, the 2-1st coefficient of variation C21 may be referred to as a 'first number coefficient of variation'. The 2-2 coefficient of variation (C22) is defined as a value obtained by dividing the standard deviation of the average number of fine precipitates having a diameter of 0.005 μm or less among all the fine precipitates by the average number of fine precipitates having a diameter of 0.005 μm or less, and the second- 2 The coefficient of variation C22 may be 0.8 or less. Hereinafter, the 2-2 coefficient of variation C22 may be referred to as a 'second number coefficient of variation'.

시편Psalter	전체 석출물whole precipitate				10nm 이하 석출물Precipitates of 10 nm or less			5nm 이하 석출물Precipitates of 5 nm or less			H/S후 굽힘성 (°)After H/S Bendability (°)
시편Psalter	개수 평균 (개/㎛²)count average (pcs/㎛ ² )	개수 평균 표준 편차count average Standard Deviation	개수 변동계수(C2)Count Coefficient of variation (C2)	석출물간 거리 평균 (㎛)between precipitates distance average (μm)	개수 평균 (개/㎛²)count average (pcs/㎛ ² )	개수 평균 표준 편차count average Standard Deviation	개수 변동계수 (C21)Count coefficient of variation (C21)	개수 평균 (개/㎛²)count average (pcs/㎛ ² )	개수 평균 표준 편차count average Standard Deviation	개수 변동계수 (C22)Count coefficient of variation (C22)	H/S후 굽힘성 (°)After H/S Bendability (°)
Y1Y1	25,04925,049	19,75319,753	0.79 0.79	0.19 0.19	23,04423,044	18,43518,435	0.80 0.80	15,77015,770	12,07512,075	0.77 0.77	5353
Y2Y2	26,70826,708	14,65114,651	0.55 0.55	0.26 0.26	25,70225,702	14,39314,393	0.56 0.56	20,04720,047	13,05913,059	0.65 0.65	5454
Y3Y3	27,96927,969	13,10513,105	0.47 0.47	0.26 0.26	26,21326,213	17,07617,076	0.65 0.65	24,99024,990	19,70619,706	0.79 0.79	5353
Y4Y4	29,54229,542	17,21917,219	0.58 0.58	0.26 0.26	26,63026,630	20,69520,695	0.78 0.78	28,22428,224	22,57922,579	0.80 0.80	5252
Y5Y5	28,78128,781	23,02523,025	0.80 0.80	0.19 0.19	27,68427,684	20,24920,249	0.73 0.73	24,08724,087	17,89317,893	0.74 0.74	5555
Y6Y6	29,87029,870	21,50721,507	0.72 0.72	0.17 0.17	28,73628,736	21,34721,347	0.74 0.74	25,79025,790	20,04220,042	0.78 0.78	5555
Y7Y7	29,75829,758	22,10622,106	0.74 0.74	0.18 0.18	28,57928,579	21,55721,557	0.75 0.75	26,92126,921	21,53721,537	0.80 0.80	5555
Y8Y8	29,93329,933	23,60523,605	0.79 0.79	0.16 0.16	28,98828,988	22,85922,859	0.79 0.79	27,64327,643	22,11422,114	0.80 0.80	5353
Y9Y9	25,02725,027	20,59320,593	0.82 0.82	0.21 0.21	23,97623,976	19,45519,455	0.81 0.81	18,94018,940	15,36915,369	0.81 0.81	4444
Y10Y10	26,69326,693	21,65921,659	0.81 0.81	0.24 0.24	25,70925,709	17,33617,336	0.67 0.67	20,21320,213	9,4719,471	0.47 0.47	4545
Y11Y11	29,53429,534	24,97724,977	0.85 0.85	0.15 0.15	28,60428,604	14,05714,057	0.49 0.49	24,00124,001	13,98913,989	0.58 0.58	4242
Y12Y12	25,12925,129	20,39020,390	0.81 0.81	0.38 0.38	23,04223,042	17,38017,380	0.75 0.75	15,62615,626	11,60811,608	0.74 0.74	4343
Y13Y13	26,19926,199	19,16319,163	0.73 0.73	0.36 0.36	23,75623,756	19,54819,548	0.82 0.82	16,97316,973	13,19013,190	0.78 0.78	4343
Y14Y14	27,28627,286	19,95719,957	0.73 0.73	0.37 0.37	24,78424,784	18,97718,977	0.77 0.77	17,75417,754	14,60914,609	0.82 0.82	4141

Table 8 quantifies the bending angle according to the precipitation behavior of fine precipitates for a plurality of specimens. Specifically, in Table 8, with respect to the plurality of specimens (Y1 to Y14), as the precipitation behavior of the microprecipitates, the number average of all microprecipitates, microprecipitates with a diameter of 10 nm or less and fine precipitates with a diameter of 5 nm or less, respectively, the number average Measured values for the standard deviation, the second coefficient of variation (C2), which is the coefficient of variation of the number of fine precipitates, and the measured values for the bending angle as characteristics after hot stamping according thereto are described. In particular, it will be described from the viewpoint of bendability according to the second coefficient of variation C2, which is the coefficient of variation of the number average.

Specimens Y1 to Y14 contain carbon (C): 0.28 to 0.50% by weight, silicon (Si): 0.15 to 0.70% by weight, manganese (Mn): 0.5 to 2.0% by weight, phosphorus (P): greater than 0 and less than 0.05% by weight, Sulfur (S): more than 0 and 0.01% by weight or less, chromium (Cr): 0.1 to 0.5% by weight, boron (B): 0.001 to 0.005% by weight, titanium (Ti), niobium (Nb), and vanadium (V ) The sum of one or more of: 0 to 0.1 wt% or less, and to the slab containing the remaining iron (Fe) and other unavoidable impurities, the material for hot stamping manufactured through steps S100 to S600 by applying the above-described process conditions Specimens produced by hot stamping. At this time, specimens Y1 to Y14 had a slab reheating temperature (SRT): 1230 ° C, a finish rolling temperature (FDT): 900 ° C, a reduction ratio during hot rolling: 95%, a coiling temperature (CT): 780 ° C, annealing heat treatment temperature: 780 ℃, plating immersion temperature: specimens prepared by hot stamping after heating the hot stamping material manufactured under the condition of 660 ℃ at 950 ℃ for 270 seconds. On the other hand, specimens Y1 to Y14 may satisfy not only the precipitation behavior with respect to the second coefficient of variation (C2) described later, but also the precipitation behavior with respect to the aforementioned first coefficient of variation (C1). That is, Y1 to Y14 are specimens satisfying the above-described precipitation behavior conditions of fine precipitates.

Specifically, the total number of microprecipitates in specimens Y1 to Y14 is 25,000 to 30,000/μm ² , the average distance between adjacent microprecipitates is 0.15 μm or more and 0.40 μm or less, and the number of fine precipitates with a diameter of 10 nm or less is 23,000 to 29,000 / μm ² , and the number of fine precipitates having a diameter of 5 nm or less is 15,200 to 29,000/μm ² . Although not shown in Table 8, in specimens Y1 to Y14, the ratio of fine precipitates with a diameter of 0.01 μm or less was 90% or more, the ratio of fine precipitates with a diameter of 0.005 μm or less was 60% or more, and the average diameter of all fine precipitates was 0.006 ㎛ or less is satisfied.

However, specimens Y1 to Y8 satisfy the condition that the second coefficient of variation is 0.8 or less, and specimens Y9 to Y14 have a second coefficient of variation greater than 0.8, which does not satisfy the precipitation behavior condition according to the second coefficient of variation of the present invention. It can be seen that specimens Y1 to Y8 having a second coefficient of variation of 0.8 or less have better bendability than specimens Y9 to Y14 having a first coefficient of variation of more than 0.8. Specifically, specimens Y9 to Y14 have a bending angle (°) of 40 to 50 degrees after hot stamping, and specimens Y9 to Y14 have a bending angle of 50 degrees or more and less than 60 degrees after hot stamping, whereas specimens Y1 to Y8 have It is superior in bendability to specimens Y9 to Y14.

In the case of specimen Y9, the 2nd coefficient of variation (C2), the 2-1st coefficient of variation (C21) and the 2-2nd coefficient of variation (C22) are 0.82, 0.81 and 0.81, respectively, and the number of coefficients of variation (C2, C21, C22 ) exceeds the upper limit (0.8) of the condition. Accordingly, it can be confirmed that the bendability of specimen Y9 is only 44 degrees, which is relatively low.

In the case of specimens Y10 to Y12, the second coefficient of variation (C2) is 0.81 to 0.85, exceeding the upper limit of the condition of the second coefficient of variation (C2). Accordingly, it can be confirmed that the bendability of specimens Y10 to Y12 is only 42 degrees to 45 degrees, which are relatively low.

In the case of specimen Y13, the 2-1 coefficient of variation (C21) is 0.82, exceeding the upper limit of the 2-1 coefficient of variation (C21) condition. Accordingly, it can be confirmed that the bendability of specimen Y13 is only 43 degrees, which is relatively low. In the case of specimen Y14, the 2-2 coefficient of variation (C22) is 0.82, exceeding the upper limit of the 2-2 coefficient of variation (C22) condition. Accordingly, it can be confirmed that the bendability of specimen Y14 is only 41 degrees, which is relatively low.

In particular, specimens Y5 to Y8 have an average number of total precipitates per unit area (100 μm ² ) of 28,000 or more and 30,000 or less, and among them, the number of fine precipitates with a diameter of 10 nm or less and fine precipitates with a diameter of 5 nm or less are relatively high. In many ways, it may be a difficult condition to secure bendability. Nevertheless, specimens Y5 to Y8 satisfy the precipitation behavior condition of the present invention that the second coefficient of variation is 0.8 or less, and thus have relatively high bendability compared to specimens Y9 to Y14.

On the other hand, samples Y9 to Y14 have a large number of total precipitates and fine precipitates having diameters of 10 nm and 5 nm or less, which may be relatively favorable conditions for securing bendability. Nevertheless, specimens Y9 to Y14 do not satisfy the precipitation behavior condition of the present invention that the second coefficient of variation is 0.8 or less, so they have relatively low bendability compared to specimens Y1 to Y8.

As a result, the material for hot stamping manufactured by the material manufacturing method for hot stamping to which the above-described content conditions and process conditions of the present invention are applied satisfies the above-described precipitation behavior conditions of the fine precipitates after undergoing hot stamping, and such fine precipitates It was confirmed that the hot stamping products satisfying the precipitation behavior conditions of these improved tensile strength, bendability and hydrogen delayed fracture characteristics. In particular, the material for hot stamping according to embodiments of the present invention satisfies the diameter average coefficient of variation and/or the number coefficient of variation among precipitation behavior conditions and uniformly distributes the number or size (diameter) of fine precipitates, thereby forming after hot stamping. It can be seen that the tensile strength, bendability and delayed hydrogen fracture characteristics of the parts are further improved.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

Claims

Carbon (C): 0.28 to 0.50% by weight, silicon (Si): 0.15 to 0.70% by weight, manganese (Mn): 0.5 to 2.0% by weight, phosphorus (P): greater than 0 and less than 0.05% by weight, sulfur (S): More than 0 and less than 0.01% by weight, chromium (Cr): 0.1 to 0.5% by weight, boron (B): 0.001 to 0.005% by weight, additives: more than 0 and less than 0.1% by weight, and the rest containing iron (Fe) and other unavoidable impurities grater; and

Including; fine precipitates distributed in the steel sheet,

The additive includes at least one of titanium (Ti), niobium (Nb) and vanadium (V),

The fine precipitates include a nitride or carbide of at least one of the titanium (Ti), the niobium (Nb), and the vanadium (V), trap hydrogen,

The material for hot stamping, wherein the average diameter coefficient of variation, which is a value obtained by dividing the standard deviation of the average diameter of the fine precipitates by the average diameter of the fine precipitates, is 0.7 or less.
According to claim 1,

A material for hot stamping, which exhibits a tensile strength of 1,680 MPa or more, a bending angle of 40 degrees or more, and an amount of activated hydrogen of 0.5 wppm or less after hot stamping.
According to claim 1,

The average diameter of the fine precipitates is 0.006㎛ or less, material for hot stamping.
According to claim 3,

A material for hot stamping, wherein 90% or more of the fine precipitates have a diameter of 0.01 μm or less.
According to claim 4,

A material for hot stamping, wherein 60% or more of the fine precipitates have a diameter of 0.005 μm or less.
According to claim 1,

The number of fine precipitates per unit area (100㎛ 2 ) is 25,000 or more and 30,000 or less, a material for hot stamping.
According to claim 6,

A material for hot stamping in which the number of fine precipitates having a diameter of 0.01 μm or less per unit area (100 μm 2 ) is 23,000 or more and 29,000 or less.
According to claim 7,

A material for hot stamping in which the number of fine precipitates having a diameter of 0.005 μm or less per unit area (100 μm 2 ) is 15,200 or more and 29,000 or less.
According to claim 1,

The number coefficient of variation, which is a value obtained by dividing the standard deviation of the average number of the fine precipitates by the average number of the fine precipitates, is 0.8 or less, a material for hot stamping.
Carbon (C): 0.28 to 0.50% by weight, silicon (Si): 0.15 to 0.70% by weight, manganese (Mn): 0.5 to 2.0% by weight, phosphorus (P): greater than 0 and less than 0.05% by weight, sulfur (S): More than 0 and less than 0.01% by weight, chromium (Cr): 0.1 to 0.5% by weight, boron (B): 0.001 to 0.005% by weight, additives: more than 0 and less than 0.1% by weight, and the rest containing iron (Fe) and other unavoidable impurities grater; and

Including; fine precipitates distributed in the steel sheet,

The additive includes at least one of titanium (Ti), niobium (Nb) and vanadium (V),

The fine precipitates include a nitride or carbide of at least one of the titanium (Ti), the niobium (Nb), and the vanadium (V), trap hydrogen,

The number coefficient of variation, which is a value obtained by dividing the standard deviation of the average number of the fine precipitates by the average number of the fine precipitates, is 0.8 or less, a material for hot stamping.
According to claim 10,

A material for hot stamping, which exhibits a tensile strength of 1,680 MPa or more, a bending angle of 40 degrees or more, and an amount of activated hydrogen of 0.5 wppm or less after hot stamping.
According to claim 10,

The first number coefficient of variation, which is a value obtained by dividing the standard deviation of the average number of fine precipitates having a diameter of 0.01 μm or less among the fine precipitates by the average number of fine precipitates having a diameter of 0.01 μm or less, is 0.8 or less. Material for hot stamping.
According to claim 12,

The second number coefficient of variation, which is a value obtained by dividing the standard deviation of the number average of fine precipitates having a diameter of 0.005 μm or less among the fine precipitates by the average number of fine precipitates having a diameter of 0.005 μm or less, is 0.8 or less, material for hot stamping.
According to claim 10,

The number of fine precipitates per unit area (100㎛ 2 ) is 25,000 or more and 30,000 or less, a material for hot stamping.
According to claim 14,

A material for hot stamping in which the number of fine precipitates having a diameter of 0.01 μm or less per unit area (100 μm 2 ) is 23,000 or more and 29,000 or less.
According to claim 15,

A material for hot stamping in which the number of fine precipitates having a diameter of 0.005 μm or less per unit area (100 μm 2 ) is 15,200 or more and 29,000 or less.
According to claim 10,

The average diameter of the fine precipitates is 0.006㎛ or less, material for hot stamping.
According to claim 17,

A material for hot stamping, wherein 90% or more of the fine precipitates have a diameter of 0.01 μm or less.
According to claim 18,

A material for hot stamping, wherein 60% or more of the fine precipitates have a diameter of 0.005 μm or less.