KR20090071164A - High-strength steel bolt having excellent resistance for delayed fracture and notch toughness method for producing the same - Google Patents

Abstract

A delay-tolerant break down high-strength bolt and a manufacturing method thereof are provided to reduce the fracture phenomenon by making the ferrite uniformly distributed. A delay-tolerant break down high-strength bolt comprises: a complex tissue of the tempered martensite; a ferrite in which the area rate of ferrite is 10% through 3; and a tempered martensite single phase structure. The area fraction of sphere precipitates generated in the tempered martensite structure due to the spheroidizing annealing is 10% or less. The tensile strength of the high-strength bolt is 130kg / mm^2 or greater. The sphere precipitate is Cr, and the Fe system carbon nitride. And diameter is 3mm through 0.5. The boron is added for hardenability and delay destruction resistibility improvement. The vanadium is added in order to improve with the delayed fracture resistance and softening resistance.

Description

HIGH-STRENGTH STEEL BOLT HAVING EXCELLENT RESISTANCE FOR DELAYED FRACTURE AND NOTCH TOUGHNESS METHOD FOR PRODUCING THE SAME}

The present invention relates to bolts used for fastening steel structures, automobile parts, and the like, and more particularly, bolts having a composite structure or a single phase structure having high strength, excellent notch toughness, and excellent resistance to delayed fracture. It relates to a manufacturing method.

In recent years, the building structure has been shifting from iron-concrete structure to steel structure with high safety. One of the important things in securing the safety of these steel structures is the member joining technique. There are two methods of joining a member, a welding joining method and a fastening method using bolts, among which bolt fastening does not require more skilled skills than welding joining, and has the advantage of increasing safety of steel structures by replacing weak welds. .

On the other hand, in the construction of steel structures using bolt fastening, high strength bolts can be used to reduce the number of fastening bolts, increase the bolt fastening force, and reduce the joint area in order to shorten the construction air and improve the stability of the building. There is a need. The strength of the bolt is determined by the composition, quenching, and heat treatment process. However, in consideration of the fastening force of the bolt, the bolt produced is preferably high strength, but should be as low as possible in order to facilitate the bolt molding in the raw material state of the raw material. Therefore, in the case of high strength bolted material, the strength is increased to 80kg / mm ² using spheroidizing heat treatment to increase cold forging. Will be lowered below. At this time, not only Fe ₃ C but also Cr-based carbides are spheroidized, and if not heated to a sufficient temperature during heat treatment, cracks may start at the interface between the carbide and the base material, which may cause a decrease in toughness of the material.

In addition, when the bolt is strengthened, a new problem occurs that the corrosion resistance, delayed fracture resistance and toughness of the bolt is lowered. Therefore, in order to solve such a problem and to construct a steel structure more efficiently, it is required to develop a steel structure fastening bolt having high strength and excellent delayed fracture resistance and toughness.

In general, conventional bolts are manufactured through a low temperature annealing in a wire state, followed by drawing for sizing purposes, and then subjected to spheroidizing heat treatment, bolt forming, quenching, and tempering processes. The bolted steel finally has a single phase structure of Tempered Martensite.

The main causes of lowering the embrittlement or toughness of steels having a tempered martensite structure include embrittlement due to hydrogen intrusion and deterioration of toughness due to Fe and Cr carbides generated during the spheroidizing heat treatment process. Hydrogen infiltrates in steel is known to degrade grain boundary strength, while Fe and Cr carbides are known to reduce toughness. Therefore, in order to use it for the high strength bolt steel which has the existing tempered martensite structure, work for improving delayed fracture resistance is needed.

Improvements in delayed fracture resistance include: corrosion inhibition of steel, minimization of hydrogen penetration, suppression of diffusive hydrogen contributing to delayed fracture, use of steel with high critical diffusion hydrogen concentration, minimization of tensile stress, relaxation of stress concentration, or miniaturization of austenite grain size Or the like. Currently, as a means to achieve this, it is mainly used to pursue high alloying or to give surface coating or plating to prevent external hydrogen intrusion.

Japanese Laid-Open Patent Publication No. 2003-321743 relates to a method for manufacturing high strength bolt steel having excellent delayed fracture resistance, and has a weight% of C≤0.35%, Si≤0.50%, Mn: 0.1 to 2.0%, and Mo: 0.05 to 0.6%. And one or two or more of Nb ≦ 0.08%, V ≦ 0.15%, W ≦ 1.5%, one or two or more of Cu, Ni, Cr, and B and the remaining Fe and impurities, and C ≦ (C A tempered martensitic single phase tissue steel satisfying / 12) / {(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 192)} ≦ 5 is described. have. However, in order to obtain the delayed fracture properties by the method of Japanese Patent Laid-Open Publication No. 2003-321743, a large amount of expensive elements must be added, and the tempering temperature is high, which makes it difficult to apply to the actual process.

On the other hand, Japanese Patent Application Laid-Open No. 7-173531 discloses 0.05-0.3% C, 0.05-2.0% Si, 0.3-5.0% Mn, 1.0-3.0% Cr, 0.01-0.5% Nb, and 0.01-0.06% Al by weight. A method of producing bainite + martensite abnormal composite tissue steel by hot forming a steel having a chemical composition and then continuously cooling it above a critical cooling rate at which no cornerstone ferrite is deposited is described. However, this also has a lot of heat treatment process is difficult to apply to the actual process.

Korean Patent Laid-Open Publication No. 2000-0033852 discloses 0.4-0.6% C, 2.0-4.0% Si, 0.2-0.8% Mn, 0.25-0.8 based on ferritic and tempered martensite composite steel as the basic structure. % Cr, P≤0.01%, S≤0.01%, 0.005-0.01% N, O≤0.005, including V 0.05-0.2%, Nb 0.05-0.2%, Ni 0.3-2.0%, B 0.001 A method for producing a high strength bolt is described which optionally contains one or two or more of the group consisting of -0.003%, 0.01-0.5% Mo, Ti, Cu, Co. However, in the case of the present invention, due to the low quenching temperature, there is a problem in that spheroidized carbide remains in the bolt, thereby lowering the notch toughness.

Therefore, the present invention improves the notch toughness of bolts by controlling Fe and Cr-based carbides, which can reduce the toughness of the material, and has an area of less than 10% of the tempered martensite single phase structure or ferrite having excellent delay fracture resistance even at high strength. It is an object of the present invention to provide a bolt steel having a ferrite-tempered martensite composite structure uniformly distributed in fractions and a method of manufacturing the same.

As a result of repeated studies to achieve the above object, the inventors of the present invention have found that high strength bolts having excellent notch toughness can be manufactured by hardening and treating steel having a specific range of composition under specific conditions. In addition, by adding a heating and quenching step under specific conditions between the quenching and soaking process, a composite high-strength bolt steel having a ferrite area fraction of 10% or less and a uniform distribution of ferrite can be produced with excellent delayed fracture resistance. Figured out.

In one aspect, the present invention, the composition is by weight,

Carbon 0.35-0.55%, Silicon 0.05-2.0%, Manganese 0.1-0.8%, Phosphorus 0.015% or less (0% not included), Sulfur 0.01% or less (0% not included), Oxygen 0.005% or less (0% not included), Boron 0.001 Contains -0.004%, titanium 0.01-0.1%, chromium 0.3-1.5%, 1 selected from the group consisting of Mo 0.1-1.5%, Ni 0.01-0.5%, Nb 0.01-0.5% and V 0.05-0.5% Species or two or more and the remaining Fe and other impurities,

It has a complex structure of ferrite and tempered martensite or tempered martensite single phase structure with an area ratio of ferrite of 3-10%,

It provides a high-strength bolt having excellent notch toughness, characterized in that the area fraction of the spherical precipitates generated by the spheroidizing heat treatment in the tempered martensite structure is 10% or less.

In this case, the bolt is a high strength bolt having a tensile strength of 130kg / mm ² or more.

In addition, the present invention is a bolt manufacturing method comprising a spheroidizing heat treatment step, bolt forming step, quenching step, tempering step, the heat treatment temperature of the hardening step is 880 ℃ to 920 ℃, The heat treatment temperature of the step is 450 to 550 ℃ to provide a method for producing a high strength bolt excellent in notch toughness.

On the other hand, in the case of manufacturing a ferritic-tempered martensite composite steel having better delayed fracture resistance than the tempered martensite single phase structure, after the heat treatment step before heat treatment step at the temperature of Ae3-10 to Ae3-10 after Perform the step of quenching.

Bolted steel produced by the method of the present invention can significantly reduce the delayed fracture phenomenon caused by the hydrogen in the austenite grain boundary due to the uniform distribution of ferrite, and due to the nature of the soft structure according to the ferrite distribution, The blunting effect can reduce the rate of crack propagation.

In addition, the present invention was proposed to ensure the notch toughness of the bolt by suggesting the dissolution conditions of Fe, Cr carbide produced during the spheroidization process.

In addition, when the bolt steel of the present invention is used as an automotive part, it contributes to the weight reduction of the parts, and it is possible to diversify and compact the design of the vehicle assembly apparatus according to the weight reduction of the parts.

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

First, the bolt steel composition of the present invention will be described.

The composition of the bolted steel of the present invention is by weight, 0.35-0.55% carbon, 0.05-2.0% silicon, 0.1-0.8% manganese, 0.015% phosphorus or less (0% not included), sulfur 0.01% or less (0% not included), oxygen 0.005% or less (0% not included), boron 0.001-0.004%, titanium 0.01-0.1%, chromium 0.3-1.5%, including Mo 0.1-1.5%, Ni 0.01-0.5%, Nb 0.01-0.5% and It is preferred to include one or two or more selected from the group consisting of V 0.05-0.5% and the remaining Fe and other impurities.

0.35-0.55% carbon

The reason for limiting the carbon content to 0.35-0.55% is that when the content exceeds 0.55%, carbides in the form of films frequently precipitate at the austenite grain boundary, thereby degrading hydrogen delayed fracture resistance. This is because the bolt tensile strength due to quenching and heat treatment cannot be sufficiently secured.

Silicone 0.05-2.0%

The reason for limiting the content of silicon (Si) to 0.05-2.0% is as follows. If the Si content exceeds 2.0%, the work hardening phenomenon may suddenly occur during the cold forging process to make bolts, and may cause problems in workability. If the content is less than 0.05%, sufficient strength may not be secured and the cementation of cementite may be adversely affected. Crazy

Manganese 0.1-0.8%

Manganese (Mn) is an element that forms a solid solution to form a solid solution to strengthen the solid solution, a very useful element for high-strength bolt characteristics, the content is preferably 0.1-0.8%. When the content of manganese exceeds 0.8%, tissue heterogeneity due to manganese segregation may occur, deteriorating bolt characteristics. Macro segregation and micro segregation tend to occur depending on the segregation mechanism of steel, and manganese segregation promotes segregation due to relatively low diffusion coefficient compared to other elements, and the improvement of hardenability is caused by core martensite. ) Is the main reason for generating On the other hand, when the manganese is added less than 0.1% it is difficult to expect the effect of improving stress relaxation by solid solution strengthening.

In other words, if the content of manganese is less than 0.1%, the improvement of hardenability and permanent deformation resistance is insufficient due to insufficient solidification effect, and if it is more than 0.8%, local anisotropy is increased due to manganese segregation and the formation of segregation zone intensifies tissue anisotropy. That is, tissue nonuniformity occurs, and the bolt characteristic falls.

Phosphorus 0.015% or less (0% not included)

Since phosphorus is the main cause of segregation at grain boundaries to lower toughness and reduce delayed fracture resistance, the smaller the amount of phosphorous is, the more preferable it is. However, when it is included at 0.015% or less, the effect is insignificant. It is preferable to limit to 0.015%.

Sulfur 0.01% or less (0% not included)

Sulfur is a low melting point element segregated to lower the toughness and form an emulsion has a harmful effect on the delayed fracture resistance and stress relaxation characteristics, so less is included, but less than 0.01%, the effect is minimal, In the present invention, the upper limit is preferably limited to 0.01% in consideration of economical efficiency.

Oxygen 0.005% or less (0% not included)

The content of oxygen (O) is limited to 0.005% or less. This is because if the content is more than 0.005%, fatigue life is lowered due to oxide non-rapid inclusions.

Boron 0.001-0.004%,

Boron (B) is a grain boundary strengthening element added to improve the hardenability and delayed fracture resistance, and the content thereof is preferably about 0.001% to 0.009%. If the content of boron is less than 0.001%, boron atoms have insufficient effect of improving grain strength or hardenability due to grain boundary segregation during heat treatment.If the content of boron is greater than 0.009%, no further increase is observed, and boron nitride precipitates at grain boundaries. This is because the grain boundary strength decreases. When added only for quenchability, it is added at 0.005% or less.

Titanium 0.01-0.1% ,

The content of titanium is preferably 0.01-0.1%. If the content is less than 0.01%, the effect of improving the corrosion resistance is insufficient, and it is difficult to produce titanium nitride which prevents the formation of boron nitride to improve the hardenability of boron. If the content exceeds 0.1%, the effect is saturated and coarse titanium nitride. This is because the formation is harmful to the fatigue characteristics.

Chromium 0.3-1.5%

It is known that chromium (Cr) by itself is insignificant in improving the hardenability, but when added with boron, the effect is greatly increased. In the present invention, the content of chromium is preferably 0.3-1.5%. If the content is less than 0.3%, it is difficult to secure sufficient quenchability during heat treatment and annealing, and when it is added more than 1.5%, carbides in the form of films are formed in the steel, and the carbides in the form of films exist at the austenite grain boundaries. This is because it is known to lower the hydrogen delayed fracture resistance.

Mo 0.1-1.5% ,

The molybdenum (Mo) content is limited to 0.1-1.5%. The reason for this is that less than 0.1% of cementite inhibits the growth of cementite when it grows on epsilon carbide during tempering, thereby improving softening resistance or insufficient carbide formation to improve delayed fracture resistance. It is very effective in increasing softening resistance, but it is easy to form low temperature structure (martensite, bainite) during wire rod manufacturing.

Ni 0.01-0.5% ,

The nickel (Ni) is an element that improves the delayed fracture resistance by forming a nickel thickened layer on the surface during heat treatment to suppress permeation of external hydrogen, and the content thereof is preferably 0.01 -0.5%. If the content is less than 0.1%, the surface thickening layer formation is incomplete, so it is difficult to expect the effect of improving the delayed fracture resistance, and there is no improvement effect of the cold forming during cold bolt processing, and if it exceeds 0.5%, the amount of residual austenite increases due to the impact. Toughness may fall.

Nb  0.01-0.5%

Niobium (Nb) combines with carbon in steel to form NbC-based carbides. These carbides act to fix the austenite grain boundaries in the high temperature range, thereby limiting the former austenite grain size. The former austenite grain size also affects the toughness, strength, and delayed fracture resistance of the steel. In this patent, niobium is used to improve the delayed fracture resistance. P, S are the elements that have a great influence on the delayed fracture resistance of the steel, and when these elements deepen segregation to the former austenite grain boundary, the grain boundary strength is lowered and the delayed fracture resistance is lowered. The presence of niobium carbide reduces the size of the old austenite in steel, which increases grain boundary area, resulting in P and S segregation falling per unit area. When the Nb content is 0.01% or less, it is difficult to limit the size of the old austenite due to the precipitation of carbides. When the amount of Nb is 0.5% or more, the size of the old austenite grows because the carbides are excessively grown and the austenite grains cannot be fixed.

V 0.05-0.5%

The vanadium (V) is an element that improves delayed fracture resistance and softening resistance, and its content is preferably 0.05-0.5%. If the content of vanadium is less than 0.05%, the distribution of vanadium or niobium-based precipitates in the base material becomes less, and thus the role of the non-diffusible hydrogen trap site is insufficient. Therefore, it is difficult to expect the effect of improving the delayed fracture resistance and expecting the precipitation strengthening. It is difficult to improve the softening resistance, and if the content exceeds 0.5%, the coarse alloy carbides which are saturated and delayed by the precipitates and the softening resistance are saturated and do not dissolve in the base metal during the austenitic heat treatment. It increases and acts like a non-metallic inclusion, leading to a decrease in fatigue properties.

Next, look at the steel structure of the present invention.

The present invention has a tempered martensite single phase structure or a complex structure of ferrite and tempered martensite.

At this time, the area fraction of Fe and Cr carbide in the tempered martensite is 10% or less, and the diameter is maintained at about 2㎛. When the area fraction of Fe and Cr carbides is maintained at 10% or less as in the present invention, crack formation generated from the carbides after bolt manufacturing can be suppressed, and as a result, the notch toughness of the bolt is improved.

In the present invention, in order to maintain the area fraction of Fe and Cr carbide in the tempered martensite to 10% or less, the quenching temperature was raised to 880 ° C to 920 ° C so that the carbides could be dissolved in the quenching step. Details thereof will be described later in the manufacturing method.

On the other hand, when the bolt steel of the present invention has a composite structure of ferrite and tempered martensite, the ferrite fraction is preferably 10% or less, preferably about 3-10%. Ferrite plays an important role in improving hydrogen delaying resistance. To obtain sufficient hydrogen delaying resistance improvement effect, ferrite fraction should be 10% or less and ferrite should be uniformly distributed. If the ferrite fraction exceeds 10%, it is difficult to secure sufficient tensile strength, and the ferrite phase may also not be uniformly distributed.

In the present invention, in order to produce a ferrite-tempered martensite composite tissue steel including a uniformly distributed ferrite phase of 10% area fraction or less, heat treatment at a temperature of Ae3-10 to Ae3-10 between the hardening step and the soaking step After the quenching step is additionally performed. Details thereof will be described later.

Hereinafter, the method of manufacturing the steel for bolts using the steel which has the above composition is demonstrated.

As described above, in order for the high strength bolted steel of the present invention to have excellent notch toughness and delayed fracture resistance, i) the area fraction of Fe and Cr carbides produced in the spheroidizing heat treatment step should be maintained at 10% or less, In the case of ferrite-tempered martensite composite tissue, ii) the area fraction of the ferrite phase is 10% or less, and ii) the ferrite phase must be uniformly distributed in the tempered martensite tissue.

In order to manufacture the bolt steel as described above, the present invention includes a spheroidizing heat treatment step, a bolt forming step, a quenching step, a tempering step, in the bolt manufacturing method, the heat treatment temperature of the hardening step is 880 ℃ to 920 ℃, the heat treatment temperature of the consideration step provides a method for producing a high strength bolt excellent in notch toughness, characterized in that 450 to 550 ℃.

In general, the quenching temperature in conventional bolt steel production is about 870 ℃, when quenched at 870 ℃, Fe, Cr-based precipitates produced in the spheroidizing step does not dissolve, to form a granular precipitate. In the case of steels formed with these precipitates, there is no noticeable difference in mechanical properties in the general tensile test, but when bolt products such as threads are used, the notch toughness is significantly lower than that of steels without precipitates.

Therefore, in the present invention, the quenching temperature was raised to 880 ° C. to 920 ° C., so that Fe and Cr carbides produced in the spheroidizing step were mostly dissolved in the quenching step. Since the Fe and Cr carbide phases are mostly dissolved at Ae 3 + 80 ° C. or higher, when the heat treatment is performed at the above quenching temperature, the Fe and Cr carbide phases are dissolved to maintain an area fraction of the Fe and Cr carbide phases in the bolt steel at 10% or less.

Next, when the quenching process is finished, a tempering process is carried out. In the present invention, the heat treatment temperature is preferably about 450 ° C to 550 ° C. In the case of the boron-added steel, heat treatment may occur at 430 ° C. or lower, and may be brittle. In order to prevent film-shaped carbide precipitation at the austenite grain boundary, it is desirable to consider it at a temperature as high as possible. However, if the soaking temperature is too high, it is difficult to ensure sufficient bolt tensile strength, so it is preferable to consider it at a temperature between 450 ° C and 500 ° C under the compositional conditions of the present invention.

On the other hand, in the case of manufacturing a steel having a ferrite-tempered martensite composite structure other than the tempered martensite single phase structure, the step of quenching after heat treatment at the temperature of Ae3-10 to Ae3-10 before the soaking step after the hardening step Additionally. At this time, it is preferable that the area fraction of the ferrite phase is 10% or less, and the ferrite phase is uniformly distributed in the tempered martensite structure.

When a uniform ferrite phase is included, the effect of improving hydrogen delayed fracture resistance can be obtained. In order to obtain a uniform ferrite phase, it is most effective to perform heat treatment at the boundary between the austenite and ferrite + austenite abnormal zones. Therefore, in the present invention, by adding a step of quenching after the heat treatment at the temperature of Ae3-10 to Ae3-10 after the quenching step before the soaking step, to obtain a ferrite-tempered martensite composite steel with evenly distributed ferrite phase It was.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the following examples are merely examples for explaining the present invention, and the present invention is not limited by the following examples.

Example

Steels having a composition as shown in Table 1 were prepared. At this time, the invention material satisfies the component range of the present invention, the comparative material is a JIS standard SCM435 steel currently used in the F10T class deviating from the component of the present invention.

TABLE 1

C Si Mn Cr Mo V Al Ti B Cu Invention 0.404 0.204 0.701 1.2 0.297 0.098 0.032 0.02 0.0022 - Commercial 0.345 0.2 0.5 0.8 0.22 - 0.03 - - -

50 kg of the prepared steels were sampled, ingot cast, and then homogenized and heat-treated at 1200 ° C. for 48 hours to hot rolling to a thickness of 13 mm. At this time, the finish temperature was 950 ℃ or more and hot-rolled after hot rolling, the rolling ratio was 80% or more. Specimens for evaluating the mechanical properties (tensile and elongation) characteristics from the rolled materials as described above were taken in the rolling direction of the rolled material.

Then, the steels of the prepared invention and commercial materials were heated for several hours at an A1 temperature or more, and then subjected to a no-cool heat treatment, followed by a spheroidization heat treatment of about 20 to 30 hr.

In order to investigate the effect of the quenching / tempering (Q / T) heat treatment condition on the properties of the steel after the spheroidization step, the hardening step and the soaking step were performed under the conditions as shown in Table 2.

TABLE 2

Quenching Tenpering Temperature time Cooling Temperature time Example 1 870 30 Oil-cooled 480 90 Example 2 880 30 Oil-cooled 500 90 Example 3 900 30 Oil-cooled 500 90

Experimental Example  One - Hardening  Hardness change with temperature

In order to investigate the effect of Fe and Cr carbides on the toughness of steel under Q / T (Quenching / Tempering) heat treatment conditions, the hardness distribution of steels with different quenching temperatures was investigated. The result is shown in FIG. 1, it can be seen that the hardness change according to the quenching temperature is not large between 800 and 900 ° C.

Experimental Example  2-tensile strength and Elongation  Measure

Tensile strength and elongation of the steel materials of Examples 1 to 3 were measured. Tensile strength and heat treatment were performed according to KS B 0802 standard using the general ASTM sub size.

The measurement results are shown in Table 3. As shown in [Table 3], it can be seen that a constant tensile strength and elongation value are shown regardless of the quenching temperature in the general tensile test.

TABLE 3

Example 1 Example 2 Example 2 Tensile Strength (kg / mm ² ) 135.2 132.2 135.7 135.9 137.2 138.0 121.1 131.4 130.2 Elongation (%) 6.9 6.9 6.2 5.9 6.5 5.9 6.2 6.7 6.3

Experimental Example  3- Notch  Toughness measurement

2 and 3 are photographs showing the microstructure of Example 1 and the commercially available material which was hardened and treated under the same conditions. As shown in Figure 2, in Example 1 subjected to annealing heat treatment at 870 ℃ granular precipitates are observed. However, it can be seen that these precipitates were not observed in the commercially quenched heat treatment at 870 ° C.

In order to find out whether the notch toughness is reduced when the bolt is manufactured using the steel having precipitates formed therein, the first embodiment and the commercial material are cold forged, and then manufactured by bolts to measure tensile strength. The specimen holder was manufactured as a holder to fix the bolt, and was carried out according to the KS standard.

The measurement results are shown in Table 4. As shown in Table 4, in the case of the bolt manufactured by the steel of Example 1 in which the precipitates were observed, it can be seen that the elongation is significantly lower than the bolt manufactured by the commercial material in which no precipitates were observed. This is because precipitates propagate cracks as the starting point of cracks when notches occur.

On the other hand, the fracture surface observation result of Example 1 (Fig. 4), it was found that in the initial fracture, the form of ductile fracture, distinctive cleavage fracture as the fracture progresses.

TABLE 4

Experimental Example  4

In order to determine the tensile properties of the bolts according to the quenching temperature, the microstructures of Examples 1 and 3 were compared. As shown in FIG. 5, precipitate grains are observed in Example 1, whereas precipitate grains are not observed in Example 3. This shows that in the case of Example 3, Fe, Cr-based grains were dissolved in the hardening step.

Next, after the steel materials of Examples 1 and 3 were manufactured with bolts, the tensile strength was measured.

The measurement results are shown in FIG. It can be seen from FIG. 5 that the tensile properties of the bolts made of the steel of Example 3 are superior to the bolts made of the steel of Example 1.

1 is a graph showing the hardness distribution of steel according to the quenching temperature.

Figure 2 is a photograph showing the microstructure of Example 1.

3 is a photograph showing the microstructure of the commercial material.

Figure 4 is a photograph showing the fracture surface observation results of Example 1.

5 is a view for comparing the microstructure and bolt tensile strength of Example 1 and Example 3.

Claims (5)

The composition is in weight percent, Carbon 0.35-0.55%; Silicon 0.05-2.0%; Manganese 0.1-0.8%; Less than 0.015% phosphorus (0% not included); Sulfur 0.01% or less (0% not included); Oxygen 0.005% or less (0% not included); Boron 0.001-0.004%; Titanium 0.01-0.1%; Chromium 0.3-1.5%; One or two or more selected from the group consisting of Mo 0.1-1.5%, Ni 0.01-0.5%, Nb 0.01-0.5% and V 0.05-0.5%; And the remaining Fe and other impurities, It has a complex structure of ferrite and tempered martensite or tempered martensite single phase structure with an area ratio of ferrite of 3-10%, High-strength bolt having excellent notch toughness, characterized in that the area fraction of the spherical precipitates generated by the spheroidizing heat treatment in the tempered martensite structure is less than 10%. The method of claim 1, High strength bolts having excellent notch toughness, characterized in that the tensile strength of the high-strength bolt is 130kg / mm ² or more. The method of claim 1, The spherical precipitate is Cr, Fe-based carbonitride, a high strength bolt having excellent notch toughness, characterized in that the diameter is in the range of 0.5 ~ 3um. In the bolt manufacturing method comprising a spheroidizing heat treatment step, a bolt forming step, a quenching step, a tempering step, The heat treatment temperature of the hardening step is 880 ℃ to 920 ℃, The heat treatment temperature of the thinning step is a manufacturing method of high-strength bolt excellent notch toughness, characterized in that 450 ℃ to 550 ℃. The method of claim 4, wherein After the quenching step and before the soaking step, the step of heat treatment at a temperature of Ae3-10 to Ae3-10 and quenching step further comprising the step of producing a high-strength bolt having excellent notch toughness.

Images