KR102042061B1 - High-strength wire rod and steel with excellent hydrogen retardation resistance and manufacturing the same - Google Patents

Abstract

Disclosed is a high strength wire rod having excellent hydrogen delayed fracture resistance. According to the wire rod according to an embodiment of the present invention, in weight%, C: 0.4 to 0.6%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.0%, Cr: 1.0% or less (excluding 0), N: 0.005 % Or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% or less (excluding 0), balance Fe and inevitable impurities,
Alternatively Mo: 0.4-0.8% and V: 0.1-0.4%,
XmCn less than 30 nm (m = 0.1 to 0.9, n = 0.1 to 0.9, X = Fe + Cr + Mo) carbide or less than 30 nm YmCn (m = 0.1 to 0.9, n = 0.1 to 0.9, Y = Fe + Cr + V) carbides.

Description

High-strength wires, steels and methods of manufacturing them with excellent hydrogen delay fracture resistance {{HIGH-STRENGTH WIRE ROD AND STEEL WITH EXCELLENT HYDROGEN RETARDATION RESISTANCE AND MANUFACTURING THE SAME}

The present invention relates to a high-strength steel having excellent hydrogen delaying resistance and a method of manufacturing the same, and more particularly, to a steel material capable of securing high strength and excellent hydrogen delaying fracture characteristics through controlling alloy composition, microstructure, and carbide. It relates to a manufacturing method.

In recent years, steel materials have been continuously strengthened to meet the development of nonferrous materials such as Al and various environmental regulations. In the automotive market, the demand for high strength wire rods is steadily increasing to reduce the weight of the vehicle and the size of the engine. In the construction market, the demand for high strength bolts is increasing to shorten the construction period.

The problem is that since the strength of hydrogen delayed fracture is significantly reduced when the strength of steel is higher than 1.2 GPa, it is necessary to verify the resistance of hydrogen delayed fracture in the design. Most of the hardened products, in particular, are made of Tempered-Martensite steel. Tempered martensitic steel is particularly more susceptible to hydrogen delay failure than other steel-based structures, such as pearlite and austenite.

To improve hydrogen delayed fracture resistance in high-strength materials with such tempered martensitic structure, 1) improved corrosion characteristics of steel materials, 2) refinement of grain boundaries, 3) relaxation of stress concentrations, and 4) P and S embrittling austenite grain boundaries. There is a method such as minimizing.

One of the surest ways is to use carbides to control the movement of hydrogen inside the material. Elements that can form carbide in tempered martensitic steels include Cr, V, Mo, Ti, and Nb. However, since these carbides sometimes form complex carbides when they are added together, the carbides are formed in accordance with the addition of alloying elements. Therefore, it is difficult to use because it has not yet been identified which carbide is currently helpful in improving the resistance to hydrogen delayed fracture.

One aspect of the present invention is to provide a high-strength tempered martensitic steel having excellent hydrogen delay fracture resistance by controlling the formation of carbides through an alloy composition and a manufacturing method and a manufacturing method thereof.

According to the high-strength wire rod excellent in hydrogen delayed fracture resistance according to an aspect of the present invention, in weight%, C: 0.4 to 0.6%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.0%, Cr: 1.0% or less (0 N: 0.005% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% or less (excluding 0), balance Fe and inevitable impurities, Mo: 0.4-0.8% and V : Alternatively containing 0.1 to 0.4% of XmCn (m = 0.1 to 0.9, n = 0.1 to 0.9, X = Fe + Cr + Mo) carbide or 30 nm or less of YmCn (m = 0.1 to 0.9, n = 0.1 ~ 0.9, Y = Fe + Cr + V) carbides may be included.

In addition, according to an embodiment of the present invention, the high strength wire rod having excellent hydrogen delaying resistance may further include Al: 0.010 to 0.10%, Cu: 0.1 to 0.3%.

In addition, according to an embodiment of the present invention, the high strength wire having excellent hydrogen delaying resistance may satisfy C + Mo: 1.2% or less (excluding 0) or C + V: 0.8% or less (excluding 0). .

According to the method for producing a high strength wire rod having excellent hydrogen delaying resistance according to another aspect of the present invention, in weight%, C: 0.4 to 0.6%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.0%, Cr: 1.0 % Or less (excluding 0), Mo: 0.4 to 0.8% or V: 0.1 to 0.4%, N: 0.005% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% or less (excluding 0) Heating the steel strip including the balance Fe and unavoidable impurities to a temperature range of 1,000 to 1,200 ° C .; Finally hot rolling the heated upper piece in a temperature range of 800 to 1,100 ° C. to obtain a wire rod; And cooling the wire at a cooling rate of 0.15 to 1.5 ° C./s.

According to another aspect of the present invention, there is provided a steel for bolts having excellent hydrogen delaying resistance, in weight%, C: 0.4 to 0.6%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.0%, Cr: 1.0% Or less (excluding 0), N: 0.005% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% or less (excluding 0), remainder Fe and inevitable impurities, Mo: 0.4 to 0.8 % And V: alternatively 0.1 to 0.4%, the microstructure comprises an area fraction, 90% or more of tempered martensite, 10% or less of retained austenite, XC ( X = Fe + Cr + Mo) carbide or plate-like Y ₄ C ₃ (Y = Fe + Cr + V) carbide of 10 nm or less.

In addition, according to an embodiment of the present invention, the distribution density per unit area of the carbide may be 1 to 40 / m ² .

In addition, according to an embodiment of the present invention, the tensile strength may be 1300MPa or more.

According to another aspect of the present invention, there is provided a method for manufacturing a bolt steel having excellent hydrogen delaying resistance, in weight%, C: 0.4 to 0.6%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.0%, Cr : 1.0% or less (excluding 0), Mo: 0.4 to 0.8% or V: 0.1 to 0.4%, N: 0.005% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% or less (0 Heating the wire including the remaining Fe and unavoidable impurities to a temperature range of 900 to 1,000 ° C. and maintaining the temperature within 1 hour; Quenching the heated wire to a temperature range of 60 ° C. and maintaining it for 1 to 15 minutes; Maintaining the steel for 30 to 90 minutes after reheating to a temperature range of 400 to 700; And cooling the steel material.

Steel according to the present invention is composed of a tempered martensite base structure and by controlling the carbide that can trap the hydrogen to excellent hydrogen delay fracture resistance, such as materials for fastening bolts for automobiles, structures exposed to various corrosion environments Application is possible.

FIG. 1 is a heat dissipation spectroscopy (hereinafter referred to as TDS) for each material after charging Inventive Example 2 according to an embodiment of the present invention at each tempering temperature and charging hydrogen for about 24 hours (Charging density = 10A / m ² ). This is a graph analyzed.
Figure 2 is a photograph of the plate-shaped Y ₄ C ₃ carbide of the invention example 2 with a transmission electron microscope (Transmission Electron Microscope, TEM). A portion denoted by a red circle shows a plate-shaped Y ₄ C ₃ carbide.
3 is a photograph of 4 nm fine XC carbide of Inventive Example 1 observed with 3DAP (3-Dimensional Atom Probe). The red circle indicates the circular XC carbide.
4 is a graph showing the results of comparative tests of hydrogen delayed fracture resistance of Inventive Examples 1 and 2 and Comparative Examples 4 and 5. FIG.

The following embodiments are presented to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein but may be embodied in other forms. The drawings may omit illustrations of parts not related to the description in order to clarify the present invention, and may be exaggerated to some extent in order to facilitate understanding.

Throughout the specification, when a part is said to "include" a certain component, it means that it can further include other components, without excluding other components unless specifically stated otherwise.

Singular expressions include plural expressions unless the context clearly indicates an exception.

High-strength wire rod excellent in hydrogen delayed fracture resistance according to an aspect of the present invention, C: 0.4 to 0.6%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.0%, Cr: 0.5 to 1%, Mo: 0.4 to 0.8% or V: 0.1 to 0.4%, N: 0.005% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% or less (excluding 0), residual Fe and inevitable impurities .

Hereinafter, the reason for the numerical limitation of the content of the alloying component in the embodiment of the present invention will be described. In the following, the unit is% by weight unless otherwise specified.

The content of C is 0.4 to 0.6%.

Carbon (C) is preferably added at least 0.4% to secure the strength of the product. However, if the content is excessive, the plate-shaped tempered martensite structure is formed, the hydrogen delayed fracture resistance is weak, the upper limit can be limited to 0.6%.

The content of Si is 0.2 to 2.0%.

Silicon (Si) is a necessary component as a deoxidizer, and it is preferable to add 0.2% or more to secure strength through solid solution strengthening. However, if the content is excessive, the surface decarburization occurs, and since the processing of the material is difficult, the upper limit can be limited to 2.0%.

The content of Mn is 0.2 to 1.0%.

Manganese (Mn) is a hardenability improving element, and it is preferable to add 0.2% or more to make a high strength tempered martensitic steel. However, if the content is excessive, there is a problem in that the hydrogen delayed fracture resistance and toughness are deteriorated as the Mn content increases in the tempered martensitic steel. Therefore, the upper limit thereof may be limited to 1.0%.

The content of Cr is 0.5 to 1%.

Chromium (Cr) is effective for improving the hardenability together with Mn, and it is preferable to add 0.5% or more as an element to improve the corrosion resistance of steel in a corrosive environment. However, when the content is excessive, M ₇ C ₃ carbide may be formed, which may lower the hydrogen delayed fracture resistance, and the upper limit thereof may be limited to 1%.

The content of Mo is 0.4 to 0.8%.

Molybdenum (Mo) is an element effective in improving the hardenability and inhibiting grain boundary oxidation, and when XC (X = Fe + Cr + Mo) carbide is formed, it is preferable to add 0.4% or more. However, if the content is excessive, all of the austenitization heat treatment does not dissolve, and coarse molybdenum carbides may be formed, thereby deteriorating mechanical properties or other types of carbides, and the upper limit thereof may be limited to 0.8%. have.

With regard to carbide production, it is necessary to control the amount of Mo added depending on the C content. In this alloy composition, the content of C + Mo is limited not to exceed 1.2%.

The content of V is 0.1 to 0.4%.

Vanadium (V) is a representative carbide forming element that forms V _X C _{1 -X} carbides. Vanadium carbide is relatively easier to control than Ti and Nb because it is a carbide that can be dissolved in the material during austenitization heat treatment even at a high carbon relative to Ti and Nb carbides. However, like molybdenum, vanadium carbide is controlled so that the content of C + V does not exceed 0.8% in consideration of the amount of C added to prevent coarse carbide formation.

In addition, vanadium, when added together with molybdenum, is Y ₄ C _{3 in the} form of a plate. (Y = Fe + Cr + V) carbide is formed. Unlike the case in which vanadium is added alone, when it is added in combination, Mo and V should be added alternatively because it causes more hydrogen inflow compared to improvement in hydrogen embrittlement resistance. In the case where V is alternatively added, the austenitization temperature is set near 980 ° C. in order to completely dissolve vanadium carbide in the tissue during the austenitization heat treatment.

The content of N is 0.006% or less.

Nitrogen (N) is managed as a nitrogen impurity, in combination with vanadium to form coarse VN precipitates which do not dissolve during heat treatment to inhibit vanadium carbide formation, so the upper limit can be limited to 0.006%.

The content of P is 0.015% or less.

Phosphorus (P) is an impurity contained inevitably, and segregation at grain boundaries lowers toughness and lowers hydrogen delayed fracture resistance. Therefore, it is preferable to manage the content as low as possible. In theory, the content of P is advantageously controlled to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit can be limited to 0.015%.

The content of S is 0.010% or less.

Sulfur (S) is an impurity that is inevitably contained and, like P, it is segregated at grain boundaries to reduce toughness, and MnS is formed to reduce hydrogen delayed fracture resistance. Therefore, it is desirable to manage the content as low as possible. In theory, the content of S is advantageously controlled to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit can be limited to 0.010%.

In addition, the high-strength wire having excellent hydrogen delayed fracture resistance according to an embodiment of the present invention may further include Al: 0.010 to 0.10% and Cu: 0.1 to 0.3%.

The content of Al is 0.010 to 0.10%.

Aluminum (Al) is a strong deoxidation element that can remove oxygen in the steel to increase cleanliness, so it is preferably added at least 0.010%. However, if the content is excessive, there is a problem in that the fatigue resistance is lowered by forming Al ₂ O ₃ inclusions, and the upper limit thereof may be limited to 0.10%.

The content of Cu is 0.1 to 0.3%.

Copper (Cu) is preferably added at least 0.1% to improve the corrosion resistance in a special corrosive environment. However, if the content is excessive, defects due to high temperature brittleness may be formed during hot rolling, and the upper limit thereof may be limited to 0.3%.

The remaining component of the present invention is iron (Fe). However, in the conventional manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

According to another aspect of the present invention, a method for producing a high-strength wire rod having excellent resistance to hydrogen delayed fracture is C: 0.4 to 0.6%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.0%, and Cr: 1.0% or less by weight. (Excluding 0), Mo: 0.4 to 0.8% or V: 0.1 to 0.4%, Al: 0.010 to 0.10%, N: 0.005% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% Hereinafter (except 0), heating the steel strip including the remaining Fe and unavoidable impurities at 1000 to 1200 ° C; Finally hot rolling the heated steel strip in a temperature range of 800 to 1100 ° C. to obtain a wire rod; And cooling the hot rolled wire at a cooling rate of 0.15 to 1.5 ° C./sec. It may include.

The wires produced are XmCn (m = 0.1-0.9, n = 0.1-0.9, X = Fe + Cr + Mo) carbides of 30 nm or less, or YmCn (m = 0.1-0.9, n = 0.1-0.9, Y = Fe + Cr + V) carbides.

 These are precipitated in the form of very small carbides of 10 nm or less while undergoing the Austenitizing & Tempering process described later. If coarse carbides are present, they may be present after tempering because they are not dissolved in the material during the Austenitizing process. In particular, coarse carbides of 100 nm or more may reduce hydrogen delayed fracture resistance.

Meanwhile, the precipitation behavior of carbides may vary depending on the addition of alloying elements. Therefore, in order to select the tempering temperature to maximize hydrogen embrittlement resistance, the material is heat-treated by varying the tempering temperature from 400 ℃ to 700 ℃, and then hydrogen is injected into the material through cathodic charging (Current density = -10A / m ² ). TDS analysis was performed.

As a result, it is possible to obtain a graph as shown in FIG.

As a result of the TDS analysis, when tempering in the temperature range exceeding 600 ° C, there is a problem that the degradation of the hydrogen delayed fracture resistance is lowered due to the decrease in the strength and the activation energy of the carbide trapping hydrogen due to the spheroidization and coarsening of the carbide. When the tempering temperature is lower than 500 ° C, the temperature is not sufficient for carbides to precipitate, and film-like cementite is formed and the hydrogen delayed fracture resistance is lowered. Therefore, the tempering temperature range is limited to 500 to 600 ° C. can do.

The shape of cementite can be confirmed by transmission electron microscopy (TEM). If film-like cementite is formed after heat treatment, the desired level of hydrogen delayed fracture resistance cannot be obtained.

According to another aspect of the present invention, a method for manufacturing a steel for bolts having excellent hydrogen delayed fracture resistance, in weight%, C: 0.4 to 0.6%, Si: 0.2 to 2.0%, Mn: 0.2 to 1.0%, Cr: 1.0% or less (excluding 0), Mo: 0.4 to 0.8% or V: 0.1 to 0.4%, N: 0.005% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% or less (excluding 0) ), Heating the wire containing the balance Fe and unavoidable impurities to a temperature range of 900 to 1,000 ℃ and maintained within 1 hour; Quenching the heated wire to a temperature range of 60 ° C. and maintaining it for 1 to 15 minutes; Maintaining the steel for 30 to 90 minutes after reheating to a temperature range of 500 to 600; And cooling the steel.

The steel for bolts manufactured according to the present invention may include, as an area fraction, 90% or more of temper martensite and 10% or less of retained austenite.

Meanwhile, the tempered martensite may be acicular. In the case of martensite structure in which the needle-like and plate-shaped mixtures are mixed, it is preferable that the martensite structure is made of acicular-type martensite.

In addition, the steel material for bolts excellent in hydrogen delayed fracture resistance according to an embodiment of the present invention is XC (X = Fe + Cr + Mo) or Y ₄ C ₃ (Y = Fe + Cr + V) carbide, wherein the XC carbide is a circular carbide having a size of 2 to 5 nm, and the Y ₄ C ₃ carbide is a plate-shaped carbide having a size of 10 nm or less.

The carbides are formed in the grains or grain boundaries of the microstructure, thereby trapping the intruded hydrogen to prevent the intruded hydrogen from deteriorating the strength of the grain boundary to improve the hydrogen delay fracture resistance.

In addition, the distribution density per unit area of the carbide may be 1 to 40 / m ² .

In addition, the tensile strength of the bolt steel may be 1300MPa or more.

Hereinafter, the preferred embodiment of the present invention will be described in more detail.

Example

Ingot was cast while changing the content of each component as shown in Table 1 below, and subjected to homogenization heat treatment at 1200 ° C.

Thereafter, the temperature was lowered from 12001 ° C to 1000 ° C while hot rolling to a final thickness of 13 mm, followed by air cooling to produce a wire rod.

division Composition (wt%) C Si Mn Cr Mo V N P S Cu Example 1 0.59 2.0 0.21 1.01 0.39 0.005 0.003 0.003 Example 2 0.60 1.9 0.20 0.99 0.20 0.005 0.003 0.003 Example 3 0.51 0.2 0.51 1.00 0.50 0.005 0.003 0.003 0.21 Example 4 0.49 0.2 0.49 1.02 0.19 0.005 0.003 0.003 0.20 Comparative Example 1 0.60 2.0 0.20 0.98 1.01 0.005 0.003 0.003 Comparative Example 2 0.59 1.9 0.21 0.99 0.41 0.09 0.005 0.003 0.003 Comparative Example 3 0.59 1.0 0.20 1.02 0.1 0.005 0.003 0.003 Comparative Example 4 0.59 2.0 0.21 0.98 0.005 0.003 0.003 Comparative Example 5 0.60 1.9 0.20 1.98 0.005 0.003 0.003

Austenitizing / Tempering heat treatment was performed using these steels under the conditions of Table 2, and the physical properties were measured by ASTM E-8M subsize standard. The carbide formed using TEM and 3DAP was measured, and the Thermal Desorption Spectroscopy (hereinafter referred to as TDS) graph was obtained by changing the tempering temperature. Residual austenite volume fraction was measured using a Cu-Kα X-ray diffractometer.

TDS graphs were obtained using quadrupole mass spectrometry, and microstructures were observed using SEM.

Delayed destruction test uses Cathodic Charging to inject hydrogen above [H _E ] (maximum amount of hydrogen in the Cyclic Corrosion Test) level and apply Zn coating to prevent hydrogen from escaping. , CLT) was used to give a load of 90% of the tensile stress and the break time was measured.

Hydrogen delayed fracture specimens were made with a specimen diameter of 6mm, a notch diameter of 4mm, and a notch radius of 0.1mm, and five or more specimens were made of the same steel to reduce the variation of the test. Further cathodic was measured using the TDS analysis method the amount of hydrogen to be introduced during charging, the test specimens was found to be both more [HE] If the line walking cathodic charging of 10A / m ² of hydrogen are injected. [HE] is the maximum amount of hydrogen introduced in the Cyclic Corrosion Test (hereafter CCT).

In addition, after 100 cycles of the Cyclic Corrosion Test on the same specimens to confirm the brittle fracture resistance due to corrosion, the specimen was subjected to a load of 90% of tensile stress and then measured for fracture for 100 hours. In this test, notch specimens of the same size were used in the same manner as the above hydrogen delayed fracture test, and five specimens of each steel were made and tested.

Residual austenite volume fraction and tensile strength were measured and shown in Table 2.

Austenitizing Tempering Residual Austenite (%)
Tensile Strength (MPa) 100 hours break Carbide distribution Temperature
(℃) Holding time (min) Temperature
(℃) Holding time (min) Hydrogen injection After CCT shape Size (nm) Distribution Density (piece / μm ² ) Inventive Example 1 920 60 570 60 Less than 5 1668 X X circle R: 1-4 2 Inventive Example 2 980 60 570 60 Less than 5 1638 X X Plate t: 1-4
r: 20-40 3 Inventive Example 3 920 60 550 60 Less than 5 1355 X X circle 1-4 4 Inventive Example 4 980 60 550 60 Less than 5 1302 X X Plate t: 1-4
r: 20-40 3 Comparative Example 1 980 60 570 60 Less than 5 1685 O O Plate Type + Spherical Spherical: 20-150 5/40 Comparative Example 2 980 60 570 60 Less than 5 1712 O X Plate t: 1-4
r: 20-40 4 Comparative Example 3 920 60 460 60 Less than 5 1688 O O none Comparative Example 4 920 60 500 60 Less than 5 1589 X X none Comparative Example 5 920 60 550 60 Less than 5 1612 X O Oval (M ₇ C ₃ ) R: 20-80 8

Referring to Tables 1 and 2, Examples 1 to 4, which satisfy both the composition and the preparation conditions proposed by the present invention, have a residual austenite fraction of less than 5%, and have a strength of 1,300 MPa or more and excellent hydrogen delayed fracture resistance. I could confirm that.

On the contrary, in Comparative Example 1, since vanadium was added to the carbon content, hydrogen delayed fracture resistance was lowered compared to the same strength due to the coarse vanadium carbide that was not dissolved during the austenitization heat treatment.

In Comparative Example 2, the strength was improved as vanadium and molybdenum were added together, but the hydrogen delayed fracture resistance was lowered as a large amount of hydrogen was injected in a relatively same hydrogen injection environment.

In Comparative Example 3, the tempering temperature was relatively low at 460 ° C., so that carbide precipitation was not easy, and as the film-like cementite was formed, the hydrogen delayed fracture resistance decreased.

In Comparative Example 4, the hydrogen delayed fracture did not occur in the hydrogen delayed fracture test, but as shown in FIG. 4, the thermally delayed fracture resistance was inferior to that of the inventive examples 1 and 2 in which carbides exist.

Comparative Example 5 shows that the Cr content is 1.98%, more than 1.0% M ₇ C ₃ carbide is formed, the hydrogen delayed fracture resistance is reduced.

The wire rod for bolts according to the embodiment of the present invention may be formed of a matrix structure of tempered martensite, and may improve strength and hydrogen delayed fracture resistance by controlling carbides capable of trapping hydrogen.

As described above, the exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person skilled in the art does not depart from the spirit and scope of the following claims. It will be understood that various changes and modifications are possible in the following.

Claims (8)

By weight, C: 0.4-0.6%, Si: 0.2-2.0%, Mn: 0.2-1.0%, Cr: 0.5-1.0%, N: 0.006% or less (excluding 0), P: 0.015% or less (excluding 0) ), S: 0.010% or less (excluding 0), including residual Fe and inevitable impurities,
Alternatively Mo: 0.4-0.8% and V: 0.1-0.4%,
C + Mo: 1.2% or less (excluding 0) or C + V: 0.8% or less (excluding 0)
XmCn (m = 0.1 to 0.9, n = 0.1 to 0.9, X = Fe + Cr + Mo) carbide of 30 nm or less, or YmCn (m = 0.1 to 0.9, n = 0.1 to 0.9, Y = Fe + Cr + of 30 nm or less V) High strength wire with excellent hydrogen delay fracture resistance including carbide The method of claim 1,
A high strength wire rod having excellent hydrogen delayed fracture resistance, further comprising Al: 0.010 to 0.10% and Cu: 0.1 to 0.3%. delete By weight, C: 0.4-0.6%, Si: 0.2-2.0%, Mn: 0.2-1.0%, Cr: 0.5-1.0%, Mo: 0.4-0.8% or V: 0.1-0.4%, N: 0.006% Or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% or less (excluding 0), residual Fe and inevitable impurities,
Heating the slabs satisfying C + Mo: 1.2% or less (excluding 0) or C + V: 0.8% or less (excluding 0) to a temperature range of 1,000 to 1,200 ° C;
Finally hot rolling the heated steel strip in a temperature range of 800 to 1,100 ° C. to obtain a wire rod; And
And cooling the wire at a cooling rate of 0.15 to 1.5 ° C./s. By weight, C: 0.4-0.6%, Si: 0.2-2.0%, Mn: 0.2-1.0%, Cr: 0.5-1.0%, N: 0.006% or less (excluding 0), P: 0.015% or less (excluding 0) ), S: 0.010% or less (excluding 0), including residual Fe and inevitable impurities,
Alternatively Mo: 0.4-0.8% and V: 0.1-0.4%,
C + Mo: 1.2% or less (excluding 0) or C + V: 0.8% or less (excluding 0)
The microstructure comprises an area fraction, 90% or more of tempered martensite, 10% or less of retained austenite,
A steel for bolts having excellent hydrogen delay fracture resistance, including circular XC (X = Fe + Cr + Mo) carbides of 2 to 5 nm or plate-shaped Y ₄ C ₃ (Y = Fe + Cr + V) carbides of 10 nm or less. The method of claim 5,
Steel for bolts having a hydrogen delayed fracture resistance of 1 to 40 pieces / ㎛ ² distribution density per unit area of the carbide. The method of claim 5,
Steel bolts with excellent tensile strength of 1300 MPa or more. By weight, C: 0.4-0.6%, Si: 0.2-2.0%, Mn: 0.2-1.0%, Cr: 0.5-1.0%, Mo: 0.4-0.8% or V: 0.1-0.4%, N: 0.006% Or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.010% or less (excluding 0), residual Fe and inevitable impurities,
Heating the wire which satisfies C + Mo: 1.2% or less (excluding 0) or C + V: 0.8% or less (excluding 0) to a temperature range of 900 to 1,000 ° C. and maintained within 1 hour;
Quenching the heated wire to a temperature range of 60 ° C. and maintaining it for 1 to 15 minutes;
Maintaining the wire for 30 minutes to 90 minutes after reheating to a temperature range of 500 to 600 ° C .; And
Cooling the wire; manufacturing method of steel for bolts having excellent hydrogen delay fracture resistance.

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