KR100723186B1 - High-strength steel bolt having excellent resistance for delayed fracture and method for producing the same - Google Patents

Abstract

The present invention relates to a bolt for use in fastening steel structures and automobile parts, and a method of manufacturing the same. More particularly, the present invention relates to a steel bolt capable of high strength while being excellent in delayed fracture resistance by appropriate control of a microstructure.

The bolt of the present invention is carbon (C): 0.35 to 0.55%, silicon (Si): 0.05 to 2.0%, manganese (Mn): 0.1 to 0.8%, boron (B): 0.001 to 0.004%, chromium (Cr): 0.3-1.5%, oxygen (TO): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): 0.010% or less, balance iron and other unavoidable impurities, including vanadium (V): 0.05 ~ 0.5%, niobium (Nb): 0.05-0.5%, nickel (Ni): 0.1-0.5%, molybdenum (Mo): 0.1-1.5% and titanium (Ti): 0.01-0.1% Or a high-strength bolt having a composition further comprising two or more, it has an internal structure consisting of ferrite and temper martensite, the content of the ferrite in the internal structure is characterized in that 3 ~ 10% by area fraction.

According to the present invention, it is possible to provide a high-strength bolt that satisfies both the delayed fracture resistance and high strength without causing a large amount of alloying elements and does not cause the problem of lowering the notch toughness. It may provide a method.

Delayed fracture resistance, high strength, notch toughness, bolt, carbide

Description

High-strength bolt with excellent delay fracture resistance and manufacturing technology {HIGH-STRENGTH STEEL BOLT HAVING EXCELLENT RESISTANCE FOR DELAYED FRACTURE AND METHOD FOR PRODUCING THE SAME}

The present invention relates to a bolt for use in fastening steel structures and automobile parts, and a method of manufacturing the same. More particularly, the present invention relates to a steel bolt capable of high strength while being excellent in delayed fracture resistance by appropriate control of a microstructure.

In recent years, building structures have been shifting from steel-concrete structures to steel structures with high safety. One of the important things in securing the safety of these steel structures is the member joining technique, and the method of joining the members includes welding and bolting. Bolt fastening does not require skilled skills compared to welded joints, and also has the advantage of increasing the safety of steel structures by replacing weak welds. Strengthening the bolt has the advantage of reducing the number of fastening bolts and increasing the bolt fastening force to shorten the air and at the same time reduce the joint area to pursue the integrity of the joint. Therefore, in recent years, efforts have been made to increase the strength of steel structure fastening bolts for more efficient steel structure construction.

Conventional bolts are secured by hardening after wire rods are processed into bolts. The process of processing wire rod into bolt shape is mostly done by cold forging process in consideration of productivity. Therefore, bolt processing wire should have good properties of cold forging, that is, good CHQ (Cold Heading Quality) characteristics. The most necessary condition for securing CHQ characteristics is that the wire toughness needs to be lowered to an appropriate level for easy processing.

In order to manufacture the wire having good CHQ properties as described above, the wire is not only manufactured in a low strength state, but also subjected to spheroidization heat treatment in order to further reduce the strength before bolting after the wire is processed for sizing purposes. Suffer. Spheroidal heat treatment refers to a heat treatment that further reduces the strength of the wire rod by depositing carbon in the form of spheroidized carbide because carbon dissolved in the wire rod increases the wire rod strength due to solid solution strengthening. After the spheroidization heat treatment, as described above, after processing into a bolt shape, the hardening heat treatment is performed. By the way, the martensite structure is formed in the hardened bolt, and the toughness of a bolt falls rapidly. Therefore, in order to prevent the deterioration of the toughness of the bolt due to the martensite structure, go through a soaking process, the bolt manufactured by this process has a so-called tempered martensite structure inside.

It is known that the addition of alloying elements, especially carbon, is most effective for the high strength of steel with tempered martensitic structure.However, the addition of carbon increases the strength from the wire stage, making it difficult to cold work and the soft-brittle transition temperature of the product. Increases abruptly and significantly reduces hydrogen delayed fracture resistance. In addition, work hardening increases during processing, which is disadvantageous for bolt molding, and requires a separate softening heat treatment.

In addition, the tempered martensite is a structure in which Fe-based precipitates are distributed at grain boundaries, and precipitates are easily distributed in the base material of lath martensite. When the tempered martensite is applied to a high tensile strength (high strength) steel component such as a bolt, it is exposed to high stress depending on the used situation. Due to this stress, hydrogen is more smoothly moved, and Since a large amount of hydrogen accumulates, it is a condition that is likely to cause delayed destruction. Therefore, such a tempered martensite structure is limited to use in the manufacture of high strength parts.

As described above, it is very important to develop bolts having both strength and delayed fracture resistance as properties of bolts that are incompatible with delayed fracture resistance. The followings are expected benefits when the steel for bolts, which have high delay fracture resistance and high strength, are developed. In the aspect of steel structure, bolt fastening does not require more skilled technique than welding joint, and considering the substitution of weak welds, firstly, it is possible to increase the safety of steel structure by strengthening the tightening force and reducing the joint's publicity. Reducing the number of fasteners can reduce steel usage and shorten construction air. In addition, in terms of automotive parts, it contributes to the lightening of parts. Fourth, there is an advantage in that the design diversification and compactness of the vehicle assembly apparatus according to the lighter parts are possible.

Conventional techniques for improving delayed fracture resistance include: 1) corrosion inhibition of steel, 2) minimization of hydrogen intrusion, 3) suppression of diffusive hydrogen contributing to delayed destruction, and 4) steel with high critical diffusion hydrogen concentrations. Use, 5) minimizing tensile stress, 6) mitigating stress concentration, and 7) miniaturizing austenite grain boundaries. As a means to achieve this, high alloying or surface coating or plating to prevent external hydrogen intrusion is mainly used. In addition, P and S which embrittle austenite grain boundaries are suppressed to the maximum while specific The addition of an element produces a precipitate that can trap diffusive hydrogen, or a method of controlling the microstructure.

An example of the technique developed for improving the delayed fracture resistance is the technique described in Japanese Patent Laid-Open Publication No. 2003-321743. The technique relates to a method for manufacturing a high strength bolt having excellent delayed fracture resistance, and has a weight ratio of C: 0.35. % Or less, Si: 0.50% or less, Mn: 0.1 to 2.0%, Mo: 0.05 to 0.6%, Nb: 0.08% or less, V: 0.15% or less, W: 1.5% or less Cu , Ni, Cr, B or two or more kinds, 0.5 ≤ (C / 12) / {(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51) + (W / 192 )} Satisfies ≤ 5 and is characterized by a tempered martensite single-phase steel composed of the remaining Fe and impurities. However, the Japanese Laid-Open Patent Publication No. 2003-321743 adds a large amount of expensive alloy elements to obtain delayed fracture properties, and has a problem in that it is applied to actual production due to its high tempering temperature.

In addition, Japanese Patent Application Laid-Open No. 7-173531 has a weight% of 0.05 to 0.3% C, 0.05 to 2.0% Si, 0.3 to 5.0% Mn, 1.0 to 3.0% Cr, 0.01 to 0.5% Nb, and 0.01 to 0.06% Al. The present invention relates to a method of manufacturing bainite + martensite abnormal composite tissue steel by hot-forming a steel having a phosphorous composition and continuously cooling it above a critical cooling rate at which no cornerstone ferrite is deposited, but having many heat treatment processes in the manufacturing process, and thus applying it to actual production. There is a problem that is difficult.

In addition, Korean Patent Laid-Open Publication No. 2000-0033852 uses ferrite and tempered martensitic composite steel as the basic structure in terms of weight% C: 0.4-0.6%, Si: 2.0-4.0%, Mn: 0.2-0.8%, Cr: 0.25 ~ 0.8%, P≤0.01%, S≤0.01%, N: 0.005 ~ 0.01%, O≤0.005%, where V: 0.05 ~ 0.2%, Nb: 0.05 ~ 0.2%, Ni: 0.3 ~ 2.0% , B: 0.001 ~ 0.003%, Mo: 0.01 ~ 0.5%, characterized in that the heat treatment method of a high-strength bolt manufacturing method optionally containing one or two or more of the group consisting of Ti, Cu, Co. However, the patent publication has a problem in that spheroidized carbides remain in the bolt due to low quenching temperature, thereby lowering the notch toughness.

Accordingly, an object of the present invention is to solve the problems of the prior art, to provide a high-strength bolt that satisfies both the delayed fracture resistance and high strength without causing the addition of a large amount of alloying elements and does not cause the problem of lowering the notch toughness.

Still another object of the present invention is to provide a method of manufacturing the bolt, which is a method of manufacturing the bolt simply without a complicated heat treatment process.

Bolt of the present invention for achieving the above object is carbon (C): 0.35 ~ 0.55%, silicon (Si): 0.05 ~ 2.0%, manganese (Mn): 0.1 ~ 0.8%, boron (B): 0.001 ~ 0.004% , Chromium (Cr): 0.3-1.5%, oxygen (TO): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): 0.010% or less, balance iron and other unavoidable impurities Vanadium (V): 0.05 ~ 0.5%, Niobium (Nb): 0.05 ~ 0.5%, Nickel (Ni): 0.1 ~ 0.5%, Molybdenum (Mo): 0.1 ~ 1.5% and Titanium (Ti): 0.01 ~ 0.1% A high-strength bolt having a composition further comprising one or two or more selected from the group consisting of, having an internal structure consisting of ferrite and tempered martensite, the content of ferrite in the internal structure is 3 to 10% by area fraction It is characterized by.

At this time, it is preferable that carbide is contained 10% or less in area fraction inside.

The carbide may have a maximum equivalent diameter of 5 µm or less.

The manufacturing method of the present invention for producing a high strength bolt having excellent delayed fracture resistance as described above is carbon (C): 0.35 ~ 0.55%, silicon (Si): 0.05 ~ 2.0%, manganese (Mn): 0.1 ~ 0.8%, Boron (B): 0.001 ~ 0.004%, Chromium (Cr): 0.3 ~ 1.5%, Oxygen (TO): 0.005% or less, Phosphorus (P): 0.015% or less, Sulfur (S): 0.010% or less, balance iron and It consists of other unavoidable impurities, including vanadium (V): 0.05-0.5%, niobium (Nb): 0.05-0.5%, nickel (Ni): 0.1-0.5%, molybdenum (Mo): 0.1-1.5% and titanium (Ti): quenching step of quenching after heating the bolt-shaped wire rod having a composition further comprising one or two or more selected from the group consisting of 0.01 ~ 0.1% to a temperature of Ae3 + 80 ℃ or more; Re-heating step of quenching the quenched wire again to Ae3-10 ℃ ~ Ae3 + 10 ℃ temperature; And heating the re-sintered wire rod to a temperature of 450 ° C. or more.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have solved the above-mentioned problems of the prior art, and at the same time, have studied the bolts excellent in strength and delayed fracture resistance and the method of manufacturing the same.

That is, when the ferrite and the tempered martensite coexist in the composite structure compared with the conventional martensite single phase structure, and the ferrite fraction is limited to a certain level, the ferrite is uniformly distributed and hydrogen atoms invade into the old austenite grain boundary. In addition, the ferrite is softer than the tempered martensite, so that the crack propagation may be interrupted by the effect of the crack tip cracking during crack propagation, thereby securing the delay fracture resistance. Effective in In addition, by reducing the coarse carbides such as Fe and Cr as much as possible to prevent delayed destruction by hydrogen traps, the existing carbides are dispersed and distributed in the finest possible size to provide a large amount of fine hydrogen trap sites. It is effective for improving delayed fracture resistance. In addition, it is important to control within the appropriate range as shown below to emphasize the texture and carbide distribution for the improvement of the delayed fracture resistance as described above.

Hereinafter, the emphasis, structure and precipitate distribution of the preferred bolt provided by the present invention will be described in detail.

(High strength)

Carbon (C): 0.35 to 0.55 wt%

Carbon is an element added to secure the strength of the product. However, when the carbon content exceeds 0.55% by weight, a large number of carbides in the form of film precipitate at the austenite grain boundary, thereby degrading hydrogen delayed fracture resistance. Since the bolt tensile strength is not sufficient, the carbon content is preferably 0.35 to 0.55% by weight.

Silicon (Si): 0.05 ~ 2.0 wt%

Silicon is not only useful for deoxidation of steel but also effective for securing strength. However, when the silicon content exceeds 2.0% by weight, the work hardening phenomenon occurs suddenly during cold forging work in which the wire is bolted, and the workability is poor. The silicon content is preferably limited to 0.05 to 2.0% by weight.

Manganese (Mn): 0.1 to 0.8 wt%

The manganese (Mn) is an element which forms a solid solution to form a solid solution to strengthen the solid solution, and is very useful for high tensile bolt characteristics. The content of manganese is preferably in the range of 0.1 to 0.8% by weight. That is, when the manganese is added in excess of 0.8% by weight, the tissue heterogeneity due to the manganese segregation has a more harmful effect on the bolt characteristics than the solid solution strengthening effect. When the steel solidifies, macro segregation and micro segregation are easy to occur due to the segregation mechanism. Manganese segregation promotes segregation due to the relatively low diffusion coefficient compared to other elements, and the improvement of hardenability results in the core martensite. This is the main reason for generating. In addition, when the manganese is added less than 0.1%, the effect of segregation due to manganese segregation is hardly expected, but stress relaxation improvement effect due to solid solution strengthening is difficult to expect. 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 exceeds 0.8%, tissue anisotropy is increased due to increased local quenchability due to manganese segregation and formation of segregation zone. The deepening, i.e., the unevenness of the tissue, causes the bolt characteristics to deteriorate.

Boron (B): 0.001 to 0.004 wt%

The boron (B) has a major function as a grain boundary strengthening element added to improve the hardenability and delayed fracture resistance in the present invention. The lower limit of the boron content is preferably 0.0010% by weight, because when the boron content is less than 0.0010% by weight, the effect of improving grain boundary strength or hardenability due to grain boundary segregation during heat treatment is insufficient. On the contrary, when the boron content exceeds 0.004% by weight, the effect is saturated, and boron nitride precipitates at the grain boundary, thereby lowering the grain boundary strength.

Chromium (Cr): 0.3 ~ 1.5 wt%

Chromium is an effective element for improving the hardenability during hardening and thinning heat treatment. If the content of chromium is less than 0.3% by weight, it is difficult to secure sufficient quenchability during the hardening and heat treatment, so the chromium content needs to be 0.3% by weight or more. In addition, according to the results of the present inventors, the quenchability improvement of chromium itself is insignificant, but it is known that the addition of chromium is necessary because the effect is greatly increased when put together with boron. On the contrary, when chromium is added in excess of 1.5% by weight, it is not preferable because carbides in the form of films are formed in the steel. This is because such a film-type carbide is known to reduce hydrogen delayed fracture resistance when present in the austenite grain boundary.

Oxygen (T.O): 0.005% by weight or less

Oxygen is analyzed in the form of total oxygen (T.O), the content of which is limited to 0.005% by weight or less. This is because when the oxygen content exceeds 0.005% by weight, fatigue life decrease due to oxide-based nonmetallic inclusions is feared.

Phosphorus (P): 0.015 wt% or less

The content of phosphorus (P) is limited to 0.015% by weight or less. The phosphorus is segregated at the grain boundary, and the upper limit is limited to 0.015% by weight because it is a major cause of lowering toughness and reducing delayed fracture resistance.

Sulfur (S): 0.010 wt% or less

Sulfur is a low melting point grain boundary segregation to lower the toughness and form an emulsion, which has a detrimental effect on the delayed fracture resistance and stress relaxation characteristics, it is preferable to limit the upper limit to 0.010% by weight.

In addition to the above composition, it is preferable to further add one or two or more of vanadium (V), niobium (Nb), molybdenum (Mo), and nickel (Ni) in the content ranges defined below.

Vanadium (V): 0.05-0.5 wt%

Vanadium is an element that forms a precipitate to improve delayed fracture resistance and softening resistance, and its content is limited to 0.05 to 0.5% by weight. If the content is less than 0.05% by weight, the distribution of vanadium-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 it is difficult to expect the precipitation strengthening. If the content is more than 0.5% by weight, the improvement effect on delayed fracture resistance and softening resistance due to precipitates is saturated, and coarse alloy carbide which is not dissolved in the base material during austenite heat treatment is obtained. This is because the increase of the same action as the non-metallic inclusions causes a decrease in the fatigue properties.

Niobium (Nb): 0.05-0.5 wt%

Niobium, like vanadium, forms a precipitate to improve delayed fracture resistance and softening resistance, and its content is limited to 0.05 to 0.5% by weight. If the content is less than 0.05% by weight, the distribution of niobium-based precipitates in the base material decreases, 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 it is difficult to expect the precipitation strengthening. If the content is more than 0.5% by weight, the improvement effect on delayed fracture resistance and softening resistance due to precipitates is saturated, and coarse alloy carbide which is not dissolved in the base material during austenite heat treatment is obtained. This is because the increase of the same action as the non-metallic inclusions causes a decrease in the fatigue properties.

Nickel (Ni): 0.1 ~ 0.5 wt%

The nickel (Ni) is an element that forms a nickel thickening layer on the surface during heat treatment to suppress permeation of external hydrogen to improve delayed fracture resistance. If the content is less than 0.1% by weight, it is difficult to expect the effect of improving the delayed fracture resistance due to the incomplete formation of the surface thickening layer, and there is no improvement effect of the cold forming during cold bolt processing, and if the content exceeds 0.5% by weight, the retained austenite There exists a possibility that impact toughness may fall that quantity increases.

Molybdenum (Mo): 0.1 ~ 1.5% by weight

Molybdenum (Mo) content is limited to 0.1 to 1.5% by weight. The reason is that at 0.1 wt% or less, cementite inhibits cementite growth when the cementite transitions from epsilon carbide during tempering to improve softening resistance or insufficient carbide production to improve delayed fracture resistance. When molybdenum is added more than 1.5% by weight, it is very effective for increasing softening resistance, but it is easy to form low temperature structure (martensite, bainite) during wire rod manufacturing.

Titanium (Ti): 0.01 ~ 0.1 wt%

When boron forms nitride, the effect of improving hardenability is remarkably reduced. Titanium is a useful element because it combines with nitrogen instead of boron to suppress the formation of nitride of boron. In the present invention, the content of titanium is limited to 0.01 to 0.1% by weight. If the content is less than 0.01% by weight, 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% by weight, the effect is saturated and coarse titanium-based. This is because nitride forms and is detrimental to fatigue properties.

(Lecture microstructure)

The microstructure of the bolt to be treated in the present invention is a composite structure containing ferrite and martensite. Preferably, the ferrite is uniformly distributed and distributed. The reason for this is that the ferrite prevents the intrusion of hydrogen atoms into the former austenite grain boundary, thereby increasing the delayed fracture resistance, and the ferrite is formed on the tempered martensite. This is because it is soft in comparison with the crack propagation, which can interfere with the propagation of cracks due to the effect of blunting the crack tip. In order to obtain the uniform dispersion effect of the ferrite, the area fraction of the ferrite is preferably limited to 3 to 10%. If the area fraction of the ferrite is less than 3%, it is difficult to expect the effect of improving the delayed fracture resistance due to the above-described ferrite. On the contrary, if it exceeds 10%, the ferrite is not uniformly dispersed and the tensile strength of the bolt is excessively low. This is because strength is difficult to obtain.

(Sediment Distribution)

As described above, the bolt is processed into a bolt shape after undergoing a spheroidizing heat treatment in the wire rod step. Since the spheroidization heat treatment is a process for depositing carbon to increase the strength of the wire as carbide, a large amount of coarse carbide is distributed in the wire after the spheroidization heat treatment. In particular, according to the composition of the bolt as the object of the present invention, carbides of iron and chromium are produced, and since they provide hydrogen trap sites, the delayed fracture resistance is reduced. Therefore, in the bolt step, their content needs to be minimized, and the conditions are as follows.

The area ratio of the carbide needs to be controlled to 10% or less. When it exceeds 10%, not only the delayed fracture resistance is reduced but also the problem that the notch toughness due to carbide decreases. Therefore, the area ratio of the carbide is preferably 10% or less.

In addition, the diameter of the carbide which exists without being removed needs to be 5 micrometers or less. That is, when the size of the carbide is fine at the same carbide area ratio, the number of sites to which hydrogen is trapped increases and becomes fine, so that the size of the carbide may be lowered because it may eventually reduce the partial pressure of hydrogen accumulated. It is necessary to keep it at 5 micrometers or less.

Hereinafter, a method of manufacturing a high strength bolt having excellent delayed fracture resistance provided by the present invention will be described.

(Manufacturing method)

In order to manufacture the high-strength bolt as described above, it is necessary to carry out the process of hardening, re-quenching and reflecting (aka Q-Q'-T) on the wire rod processed into the bolt shape with the above-mentioned preferred emphasis.

Hardening (Q) process is to manage the area ratio of carbide to 10% or less by forming carbides such as iron and chromium and to form fine carbides. In this step, the heating temperature for solidifying the carbides is Ae3 + 80 ° C or higher. There is a need. When heating below the above temperature, carbides are not sufficiently dissolved into the matrix of bolts, causing coarse carbides to remain. The bolts heated to this temperature and reclaimed carbides are then quenched to prevent the carbides from re-precipitation.

Thereafter, a desired quenching process (Q ′) is followed by heating to the boundary of the austenite and ferrite + austenite abnormal zone section in order to obtain a uniform ferrite phase. It is preferable that the said heating temperature is Ae3 + 10-Ae3-10 degreeC. When the heating temperature exceeds Ae3 + 10 ° C., the ratio of ferrite decreases, so that the effect of improving delayed fracture resistance due to the uniform dispersion of ferrite intended in the present invention cannot be obtained. On the contrary, the heating temperature is less than Ae3-10 ° C. In this case, the ratio of ferrite is excessively high, which makes it difficult to uniformly disperse the ferrite and decreases the tensile strength of the bolt.

Next, the heat treatment T for securing the toughness of the bolt is followed. The heat treatment of the bolt according to the composition of the present invention should be carried out at a temperature of more than 450 ℃, if the treatment is less than the temperature there is a fear that brittle brittleness may occur as well as a problem that the film-shaped carbide precipitates on the austenite grain boundary This may occur. On the contrary, when the soaking treatment is performed at a high temperature exceeding 500 占 폚, the tensile strength of the bolt is not sufficient, so that the suitable soaking heat treatment temperature is in the range of 450 to 500 占 폚.

Hereinafter, the present invention will be described in more detail with reference to the following examples.

(Example)

Steel sheets having the composition described in Table 1 were hot rolled by homogenizing heat treatment at 1200 ° C. for 48 hours. When rolling, the rolling ratio was 80%, the finishing rolling temperature was 950 ° C., and hot-rolled air cooled to prepare a steel wire having a diameter of 13 mm.

In the following Table 1, Inventive Material 1 and Inventive Material 2 indicate the steel braidability satisfying the composition specified in the present invention, Comparative Material 1 indicates the steel braidability outside the composition range of the present invention.

division C Si Mn Cr Mo V Al Ti B Cu Ae3 (℃) Invention 1 0.404 0.204 0.701 1.2 0.297 0.098 0.032 0.02 0.0022 - 810 Invention 2 0.346 0.204 0.704 1.21 0.297 0.1 0.03 0.02 0.0021 0.098 810 Comparative Material 1 0.350 0.200 0.8 1.2 0.200 - 0.03 - - - 820

Specimens for evaluating mechanical properties (tensile and elongation) and delayed fracture characteristics were obtained from the rolled materials as described above in the rolling direction of the rolled material.

The steels were heat treated in each of two heat treatment methods as shown in Table 2 below. The first method is for evaluating the physical properties when the internal structure of the bolt is tempered martensite single phase structure, which is subjected to a hardening treatment for 40 minutes at 900 ° C., which is a hardening heat treatment temperature. After the heat treatment for a minute (also called QT process), the second method was quenched after heating for 40 minutes at the re-heating temperature shown in Table 2 to uniformly distribute the ferrite phase after the above hardening heat treatment, and then the soaking temperature shown in Table 2 Heat treatment for 90 minutes (aka Q-Q'-T process) was made in the order. The difference between the first heat treatment method and the second heat treatment method is whether the internal structure is a tempered martensite single phase structure or a ferrite (area fraction 10% or less) + tempered martensite composite structure. However, even when manufactured by the same Q-Q'-T process, Comparative Example 3 has a quenching temperature of less than Ae3 + 80 ℃ as the hardening condition does not satisfy the conditions of the present invention and the composition of the steel is suitable for the present invention This is not the case.

Table 2 shows the results of evaluation of physical properties of the bolts produced by the respective production methods.

Evaluation of the delayed fracture resistance of Table 2 was applied to the constant load method commonly used. This evaluation method is a method of evaluating delayed fracture resistance by the time required for breaking apart under a specific stress or under a specific stress. In the delayed fracture test, the test stress was determined based on the notched tensile strength.

The delayed fracture tester used a constant loading type delayed fracture testing machine. The delayed fracture test specimen was prepared with a specimen diameter of 6 mm, a notch diameter of 4 mm, and a notch radius of 0.1 mm. The solution of the test piece atmosphere made the solution (NaCl + CH3CHOOH) which is PH2, and was performed at normal temperature 25 degreeC +/- 5 degreeC.

Critical delay fracture strength means tensile strength that is not broken by 150 hours or more at the same stress ratio (load stress / notch tensile strength) at break time.The notch strength is the tensile test of the notched specimen (maximum load / notch cross-sectional area). Was obtained by The number of test pieces for setting the critical delay fracture strength was calculated based on at least 15 pieces.

division Heat treatment method division Hardening temperature (℃) Reheating temperature (℃) Consideration temperature (℃) Tensile Strength (MPa) Delayed breaking strength (MPa) Elongation (%) Invention 1 Q-Q'-T (Invention Example 1) One 900 810 450 1544 1475 14.7 2 900 810 500 1381 - 13.7 Q-T (Comparative Example 1) 3 900 - 500 1280 - 13.2 4 900 - 450 1510 1370 12.7 5 900 - 500 1398 - 14.4 Invention 2 Q-Q'-T (Invention Example 2) 6 900 820 450 1339 - 13.3 7 900 800 500 1333 - 14.8 8 900 820 450 1500 - 13.1 9 900 820 500 1377 - 13.7 Q-T (Comparative Example 2) 10 900 - 450 1470 - 12.9 11 900 - 500 1346 - 13 12 900 - 450 1351 - 13.7 13 900 - 500 1315 - 13.5 Comparative Material 1 Q-T (Comparative Example 3) 14 870 - 450 1150 965 14.5 15 900 - 500 1050 1365 14.8

As can be seen in Table 2, the tensile strength and the elongation of Inventive Example 1 and Inventive Example 2 of the present invention, when compared with Comparative Example 1 and Comparative Example 2, it can be seen that they have a value of equal or more.

In addition, even in the case of having the same tensile strength as described above, it can be confirmed from the above table that the delayed breaking strength of Example 1 of Inventive Example 1 is higher than that of the delayed breaking strength of Example 4 of Comparative Example 2.

That is, the process of the present invention can produce a steel having excellent delayed fracture properties while securing the same or higher tensile strength and elongation than the conventional Q-T process even if using the same component.

When comparing the results of tensile strength of Comparative Example 1 and Comparative Example 2 and Comparative Materials (Comparative Example 3 and Comparative Example 4), which have been conventionally used as delayed fracture-breaking steels, the results of Inventive Example 1 and Example 2 are compared It can be seen that it has a much higher tensile strength than ash.

In addition, the elongation may be a stable value of 13% or more, and this value is also good enough that there is no significant deterioration in comparison with Comparative Example 3 and Comparative Example 4.

As described above, the tensile strength and the elongation are inferior or inferior to those of the conventional comparative material (Comparative Example 3 or Comparative Example 4), but the delayed fracture resistance strength can be compared with Comparative Example 3 as shown in Table 2 above. In the case of 400 MPa or more and in comparison with Comparative Example 3, it was confirmed that the value of 100 MPa or more was significantly superior to that of the conventional delayed fractureable steel.

Table 3 shows the results of the tensile test by fabricating a steel material of a similar component to Inventive Example 1 with a bolt to confirm the tensile test results according to the quenching temperature.

Hardening condition Heat 870 ℃, hold for 30 minutes 900 ℃, 30 minutes Tensile Strength (MPa) 1486 1477 1447 1397 1397 Elongation (%) 2.17 1.93 2.41 6.1 6.0

From the above Table 3, when the heating temperature at the time of quenching is 870 ℃, it can be confirmed that the elongation is significantly lower than in the case of 900 ℃. In order to support these results, the effect of undissolved Fe and Cr carbides during the hardening heat treatment was observed with reference to FIGS. 1 and 2.

The Fe and Cr carbides are produced by spheroidization heat treatment, and must be completely dissolved in the quenching process. However, when Fe and Cr carbides are present after the quenching, as shown in FIG. In order to avoid this phenomenon, it is necessary to sufficiently raise the quenching temperature in the quenching heat treatment process to completely dissolve the carbides as shown in FIG. 2. Elongation of the bolts has been shown to recover when the carbides are fully dissolved.

As described above, according to the present invention, it is possible to provide a high-strength bolt that satisfies both the delayed fracture resistance and the high strength without causing the addition of a large amount of alloying elements and does not cause the problem of lowering the notch toughness. It is possible to provide a method for producing easily without resorting to.

Claims (4)

By weight%, carbon (C): 0.35 to 0.55%, silicon (Si): 0.05 to 2.0%, manganese (Mn): 0.1 to 0.8%, boron (B): 0.001 to 0.004%, chromium (Cr): 0.3 ~ 1.5%, oxygen (TO): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): 0.010% or less, balance iron and other unavoidable impurities, Vanadium (V): 0.05-0.5%, niobium (Nb): 0.05-0.5%, nickel (Ni): 0.1-0.5%, molybdenum (Mo): 0.1-1.5% and titanium (Ti): 0.01-0.1 A high strength bolt having a composition further comprising one or two or more selected from the group consisting of%, The high-strength bolt is an excellent high-strength bolt, characterized in that composed of the internal structure of the ferrite 3 ~ 10% and the tempered martensite 90 ~ 97% by area fraction. The high-strength bolt having excellent delay fracture resistance according to claim 1, wherein carbides are contained in an area fraction of 10% or less. The high-strength bolt according to claim 1 or 2, wherein the carbide has a maximum equivalent diameter of 5 µm or less. Carbon (C): 0.35 ~ 0.55%, Silicon (Si): 0.05 ~ 2.0%, Manganese (Mn): 0.1 ~ 0.8%, Boron (B): 0.001 ~ 0.004%, Chromium (Cr): 0.3 ~ 1.5%, Oxygen (TO): 0.005% or less, phosphorus (P): 0.015% or less, sulfur (S): 0.010% or less, consisting of residual iron and other unavoidable impurities, vanadium (V): 0.05-0.5%, niobium (Nb): 0.05 to 0.5%, nickel (Ni): 0.1 to 0.5%, molybdenum (Mo): 0.1 to 1.5% and titanium (Ti): 0.01 to 0.1% selected from the group consisting of Bolt-shaped wire rod having composition to include further An quenching step of quenching after heating to a temperature of Ae 3 + 80 ° C. or higher; Re-heating step of quenching the quenched wire again to Ae3-10 ℃ ~ Ae3 + 10 ℃ temperature; And Heating the re-sintered wire rod to a temperature of 450 ° C. or higher; A method of manufacturing a bolt having excellent delayed fracture resistance, characterized in that consisting of.