JP5281413B2 - High strength bolt excellent in delayed fracture resistance and method for manufacturing the same - Google Patents

Description

  The present invention relates to bolts used for fastening steel structures and automotive parts and a method for manufacturing the same, and more specifically, by appropriately controlling the microstructure of steel, it has excellent delayed fracture resistance and can be strengthened. Related to steel bolts.

  Recent building structures tend to change from rebar-concrete structures to steel structures with excellent safety. One of the important things in ensuring the safety of these steel structures is a joining technique of members. Methods for joining the members include welding joining and fastening with bolts. Bolt fastening does not require skilled techniques as compared to welded joints, and has the advantage of increasing the safety of steel structures in place of brittle welds. Increasing the strength of the bolt has the advantage of reducing the number of fastening bolts and increasing the bolt fastening force, shortening the work period, and reducing the area of the joint and pursuing the soundness of the joint. Therefore, recently, efforts have been made to increase the strength of bolts for fastening steel structures in order to construct more efficient steel structures.

  Conventional bolts ensure strength by a quenching process after processing the wire into a bolt shape. The process of processing the wire into a bolt shape is mostly performed by a cold forging process in consideration of productivity. Accordingly, the wire for bolt processing should have physical properties suitable for cold forging, that is, good CHQ (cold rollability) characteristics, but the most necessary condition for securing the CHQ characteristics is processing. This means that it is necessary to reduce the toughness of the wire to an appropriate level so that it is easy.

  In order to manufacture a wire having good CHQ properties as described above, the wire is manufactured in a state having as low a strength as possible, and after undergoing wire drawing for the purpose of sizing, the strength before bolting is increased. In order to reduce more, a spheroidizing heat treatment is performed. The spheroidizing heat treatment is a heat treatment that further reduces the strength of the wire by precipitating the carbon in a spheroidized carbide form because the carbon dissolved in the wire increases the strength of the wire by solid solution strengthening. Say. After the spheroidizing heat treatment, it is processed into a bolt shape as described above and then subjected to a quenching heat treatment. However, since a martensite structure is formed inside the quenched bolt, the toughness of the bolt is drastically reduced. Accordingly, a tempering process is performed to prevent a decrease in the toughness of the bolt due to the martensitic structure. The bolt manufactured by such a process has a so-called tempered martensitic structure inside.

  It is known that the addition of alloying elements, especially carbon, is the most effective for increasing the strength of steel materials having a tempered martensite structure. However, the addition of carbon increases the strength from the wire stage. Further, cold working is difficult, and the ductility-brittle transition temperature (DBTT) of the product is rapidly increased, and the delayed fracture resistance caused by hydrogen is greatly reduced. Moreover, work hardening increases at the time of processing, and it is disadvantageous for bolt forming, and a separate softening heat treatment is required.

  In addition, the tempered martensite has a structure in which Fe-based precipitates are distributed in crystal grain boundaries and precipitates are easily distributed in the base material of lath martensite. When such tempered martensite is applied to steel parts with high tension (high strength) such as bolts, it will be exposed to higher stress depending on the situation used, but such stress causes hydrogen migration. In addition, since a large amount of hydrogen accumulates in the precipitate, delayed fracture is likely to occur. Therefore, the tempered martensite structure as described above has a limit for use in manufacturing high-strength parts.

  As described above, it is very important to develop a bolt having both strength and delayed fracture resistance because the bolt strength and delayed fracture resistance are difficult to achieve at the same time. When a steel for bolts having excellent delayed fracture resistance and capable of high strength is developed, the expected advantages are as follows. On the side of steel structures, bolt tightening does not require skilled techniques compared to welded joints, and considering that it replaces weak welds, first of all, when tightening bolts, tightening force is strengthened and the area of the fastening part is reduced Therefore, the safety of the steel structure can be improved. Next, the amount of steel used can be reduced and the construction period can be shortened by reducing the number of bolts. Further, from the aspect of automobile parts, there is an advantage that it contributes to weight reduction of parts, and finally, it is possible to diversify and downsize the design of an automobile assembly apparatus by weight reduction of parts.

  Conventional techniques for improving delayed fracture resistance include 1) inhibition of steel corrosion, 2) minimization of hydrogen penetration, 3) suppression of diffusible hydrogen contributing to delayed fracture, and 4) critical diffusible hydrogen. Use of a steel material having a high concentration, 5) minimization of tensile stress, 6) relaxation of stress concentration, 7) refinement of austenite grain boundary size, and the like. In order to achieve this, high alloying is pursued, or surface coating or plating is mainly used to prevent external hydrogen intrusion. In addition, austenite grain boundaries are brittle. For example, there is a method of adding a specific element while suppressing phosphorus (P) and sulfur (S) to be converted to the maximum, thereby generating a precipitate capable of trapping diffusible hydrogen, or controlling a microstructure.

As an example of a technique developed for improving such delayed fracture resistance, the technique described in Patent Document 1 can be cited. However, Patent Document 1 describes a high-strength bolt excellent in delayed fracture resistance. It relates to a production method, carbon (C) is 0.35 mass % or less, silicon (Si) is 0.50 mass % or less, manganese (Mn) is 0.1 to 2.0 mass %, molybdenum (Mo) Is 0.05 to 0.6% by mass , niobium (Nb) is 0.08% by mass or less, vanadium (V) is 0.15% by mass or less, and tungsten (W) is 1.5% by mass or less. 1 type or 2 types or more, 1 type or 2 types or more selected from copper (Cu), nickel (Ni), chromium (Cr), boron (B), and 0.5 ≦ (C / 12 ) / {(Ti / 48) + (Mo / 96) + (Nb / 93) + V / 51) + (W / 192)} satisfies the ≦ 5, the balance being single phase steel tempered martensite consisting of Fe and impurities. However, in Patent Document 1, a large amount of an expensive alloy element is added to obtain delayed fracture resistance, and the tempering temperature is high.

In Patent Document 2, carbon (C) is 0.05 to 0.3 mass %, silicon (Si) is 0.05 to 2.0 mass %, and manganese (Mn) is 0.3 to 5.0 mass %. %, Chromium (Cr) has a composition of 1.0 to 3.0% by mass , niobium (Nb) has a composition of 0.01 to 0.5% by mass , and aluminum (Al) has a composition of 0.01 to 0.06% by mass. This is related to a method of manufacturing bainite + martensite dual-phase composite steel by hot forming and then continuously cooling at or above the critical cooling rate at which pro-eutectoid ferrite does not precipitate. There is a problem that it is difficult to apply to production.

Further, Patent Document 3, a ferrite and tempered martensite composite structure steel as a basic structure, (i) carbon 0.4 to 0.6 wt%, silicon 2.0 to 4.0 wt%, manganese 0 0.2 to 0.8 mass %, chromium is 0.25 to 0.8 mass %, phosphorus is 0.01 mass % or less, sulfur is 0.01 mass % or less, and nitrogen is 0.005 to 0.01 mass %. , Oxygen is 0.005 mass % or less, and (ii) vanadium is 0.05 to 0.2 mass %, niobium is 0.05 to 0.2 mass %, nickel is 0.3 to 2.0 mass % , boron 0.001 to 0.003 wt%, molybdenum 0.01 to 0.5 wt%, fabrication of titanium, copper, and high strength bolts having a composition comprising at least one selected from the group consisting of cobalt A method for heat treatment of the method is presented. However, the above-mentioned patent document has a problem that carbides spheroidized by a low quenching temperature remain in the bolt, and the notch toughness is lowered.

JP 2003-321743 A Japanese Patent Laid-Open No. 7-173531 Korean Patent Publication 2000-0033852 Specification

  Accordingly, the present invention has been made in consideration of the above-mentioned problems, and its purpose is to realize high strength together with delayed fracture resistance without adding a large amount of alloy elements, and to reduce the notch toughness. It is to provide a high-strength bolt that does not occur.

  Another object of the present invention is to provide a method for manufacturing the bolt that can be carried out easily and conveniently without complicated heat treatment steps.

The high-strength bolt excellent in delayed fracture resistance of the present invention for achieving the above object is 0.35-0.55% by mass of carbon, 0.05-2.0% by mass of silicon, and 0.8% of manganese. 1 to 0.8 mass%, boron is 0.001 to 0.004 mass%, chromium is 0.3 to 1.5 mass%, total oxygen (TO) is 0.005 mass% or less, and phosphorus is 0 0.015 mass% or less, sulfur 0.010 mass% or less, vanadium 0.05-0.5 mass%, molybdenum 0.1-1.5 mass%, titanium 0.01-0.1 mass% , And the bolt-shaped wire of the composition consisting of iron and other inevitable impurities in the balance ,
A quenching step of heating at a temperature of Ae3 + 80 ° C or higher and then rapidly cooling, a re-quenching step of rapidly cooling the rapidly cooled wire at a temperature of Ae3-10 ° C to Ae3 + 10 ° C, and a re-quenched wire at a temperature of 450 ° C to 500 ° C. A high-strength bolt manufactured by heating and tempering at a temperature of ℃ or less,
The high-strength bolt is composed of an internal structure of ferrite 3% or more and less than 10% by area ratio and tempered martensite 90 to 97%, and carbide is contained within 10% or less by area ratio, and the carbide is equivalent to a circle. The diameter is 5 μm or less.

According to another aspect of the present invention for the production of high strength bolts have excellent delayed fracture resistance as described above, carbon-containing is 0.35-0.55 wt%, silicon 0.05 to 2. 0 mass%, manganese 0.1-0.8 mass%, boron 0.001-0.004 mass%, chromium 0.3-1.5 mass%, total oxygen (TO) is 0.00. 005 wt% or less, phosphorus of 0.015% by mass or less, sulfur 0.010 wt% or less, vanadium 0.05 to 0.5 wt%, molybdenum 0.1 to 1.5 wt%, titanium Is a bolt-shaped wire having a composition of 0.01 to 0.1% by mass and the balance of iron and other inevitable impurities,
A quenching step of heating at a temperature of Ae3 + 80 ° C. or higher and then rapidly cooling;
A re-quenching step in which the rapidly cooled wire is again heated at a temperature of Ae3-10 ° C to Ae3 + 10 ° C and rapidly cooled;
Heating and tempering the re-quenched wire at a temperature of 450 ° C. to 500 ° C .;
It is characterized by comprising.

  According to the present invention, it is possible to provide a high-strength bolt having high delayed strength with excellent delayed fracture resistance without causing a problem of notch toughness reduction without adding a large amount of alloy elements. It is possible to provide a method for manufacturing a simple high-strength bolt easily and conveniently without a complicated heat treatment process.

  Hereinafter, the present invention will be described in detail.

  The inventors of the present invention have solved the problems of the prior art and repeated various extensive and intensive studies and experiments together with detailed experiments on bolts and methods of manufacturing the bolts with excellent strength and delayed fracture resistance. Reached the following conclusion.

  That is, compared to the conventional single-phase structure of martensite, a composite structure in which ferrite and tempered martensite coexist, and when the ferrite fraction is limited to a certain level, the ferrite is uniformly distributed and hydrogen atoms are present in the prior austenite grain boundaries. Delayed fracture resistance can be improved by preventing intrusion, and since the ferrite is a soft phase compared to tempered martensite, crack propagation can be hindered by the effect of blunting the crack tip during crack propagation. This is effective in ensuring delayed fracture resistance. Along with this, coarse carbides such as iron carbide (Fe) and chromium carbide (Cr) are reduced as much as possible to prevent delayed fracture due to hydrogen traps, and existing carbides are distributed and distributed in the smallest possible size. Providing a large amount of fine hydrogen trap sites is effective in improving delayed fracture resistance. Further, for the structure and carbide distribution advantageous for improving the delayed fracture resistance as described above, it is important to control the steel composition within an appropriate range as described below.

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

(Steel composition)
Carbon (C): 0.35-0.55 mass %
Carbon (C) is an element added to ensure the strength of the product. However, if the content of carbon exceeds 0.55 mass% is deposited many carbide film form austenite grain boundaries is not preferable to reduce the hydrogen delayed fracture resistance, is less than 0.35 wt% Since the tensile strength of the bolt by quenching and tempering heat treatment is not sufficient, the carbon content is preferably 0.35 to 0.55 mass %.

Silicon (Si): 0.05 to 2.0 mass %
Silicon (Si) is an element that is useful for deoxidizing steel and is effective in securing strength. However, when the silicon content exceeds 2.0% by mass, during the cold forging operation in which the wire is processed into a bolt shape, the work hardening phenomenon occurs suddenly, resulting in poor workability and less than 0.05% by mass . Since it is difficult to ensure the bolt strength with the content, the silicon content is preferably limited to 0.05 to 2.0 mass %.

Manganese (Mn): 0.1-0.8% by mass
Manganese (Mn) is an element that forms a substitutional solid solution in the matrix structure and strengthens the solid solution, and is an extremely useful element for high tension bolt characteristics. The content of manganese is preferably in the range of 0.1 to 0.8% by mass . That is, when manganese is added in an amount exceeding 0.8% by mass, the heterogeneity of the structure due to manganese segregation has a more harmful effect on the bolt characteristics than the effect of solid solution strengthening. Macrosegregation and microsegregation are likely to occur due to the segregation mechanism during solidification of the steel, but manganese segregation facilitates the generation of segregation zone due to the relatively low diffusion coefficient compared to other elements, It serves as the main factor for generating a low temperature structure (core martensite) in the center. In addition, when manganese is added to less than 0.1% by mass, there is almost no influence of segregation due to manganese segregation, but it is difficult to expect an effect of improving stress relaxation by solid solution strengthening. That is, when the content of manganese is less than 0.1% by mass , the effect of solid solution strengthening is not sufficient, so the improvement in quenching curability and permanent deformation resistance is not sufficient, and exceeds 0.8% by mass. In this case, at the time of casting, the bolt characteristics are deteriorated due to the deepening of the structure anisotropy due to the increase in the local quenching hardenability due to the segregation of manganese and the formation of the segregation zone, that is, the uneven structure.

Boron (B): 0.001 to 0.004 mass %
In the present invention, boron (B) has a main function as a grain boundary strengthening element to be added for improving quenching hardenability and delayed fracture resistance. The lower limit of the boron content is preferably 0.0010% by mass. However, when the boron content is less than 0.0010% by mass , the grain boundary strength is improved by the grain boundary segregation during the heat treatment, and the quenching hardenability is improved. The effect is not enough. Conversely, when the boron content exceeds 0.004 mass %, the boron attachment effect is saturated, boron nitride precipitates at the crystal grain boundary, and the grain boundary strength decreases.

Chromium (Cr): 0.3 to 1.5% by mass
Chromium (Cr) is an element effective for improving quenching curability during quenching and tempering heat treatment. If the chromium content is less than 0.3% by mass, it is difficult to ensure sufficient quenching curability during quenching and tempering heat treatment, so the chromium content must be 0.3% by mass or more. Further, according to the results of research by the present inventor, the improvement in quenching hardenability of chromium itself is insignificant. However, when added together with boron, the improvement effect is greatly increased, so that addition of chromium is necessary. Are known. On the contrary, when adding chromium over 1.5 mass %, since the carbide | carbonized_material of a film form is produced | generated in steel materials, it is unpreferable. This is because it is known that when such a film-like carbide is present in the austenite grain boundary, the delayed fracture resistance caused by hydrogen is reduced.

Total oxygen (TO): 0.005 mass % or less Oxygen is analyzed as a form of total oxygen (TO), but its content is limited to 0.005 mass % or less. This is because, when the oxygen content exceeds 0.005% by mass , the oxide-based nonmetallic inclusions may reduce the life due to fatigue.

Phosphorus (P): 0.015 mass % or less The content of phosphorus (P) is limited to 0.015 mass % or less. Phosphorus is segregated at the grain boundaries and is the main factor for reducing toughness and reducing delayed fracture resistance, so its upper limit is limited to 0.015 mass %.

Sulfur (S): 0.010% by mass or less Sulfur (S) is a low melting point element that segregates at the grain boundaries, lowers toughness, and forms sulfides, which has a detrimental effect on delayed fracture resistance and stress relaxation properties. Therefore, it is preferable to limit the upper limit to 0.010% by mass .

The above composition, further vanadium (V), Mo Ribuden (Mo), and further it is preferably added in an amount of content that limits the element titanium (Ti), respectively below.

Vanadium (V): 0.05 to 0.5% by mass
Vanadium (V) 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 mass %. If the content is less than 0.05% by mass , the distribution of vanadium-based precipitates in the base metal is poor, and the role as a hydrogen trap site that cannot be diffused is not sufficient. In addition, since the desired precipitation strengthening is not expected, the effect of improving the softening resistance is not sufficient. On the other hand, if the content exceeds 0.5% by mass, the effect of improving the delayed fracture resistance and softening resistance due to precipitates is saturated, and during the austenite heat treatment, coarse alloy carbides that are not dissolved in the base metal increase and non-metallic inclusions It acts like a thing and brings about a decrease in fatigue characteristics.

Niobium (Nb): 0.05 to 0.5 mass %
Niobium (Nb) is an element that forms precipitates and improves delayed fracture resistance and softening resistance in the same manner as vanadium, and its content is limited to 0.05 to 0.5 mass %. If its content is 0.05% by mass or less, the distribution of niobium-based precipitates in the base metal is not sufficient, and it does not serve as a hydrogen trap site that cannot be diffused, and is expected to improve delayed fracture resistance. The effect of improving the softening resistance is not sufficient because precipitation strengthening is difficult to expect. On the other hand, if the content exceeds 0.5% by mass, the effect of improving the delayed fracture resistance and softening resistance due to precipitates is saturated, and during the austenite heat treatment, coarse alloy carbides that are not dissolved in the base metal increase and non-metallic inclusions Since it acts like a thing, it brings about a decrease in fatigue characteristics.

Nickel (Ni): 0.1-0.5% by mass
Nickel (Ni) is an element that improves the delayed fracture resistance by forming a nickel concentrated layer on the surface during heat treatment to suppress permeation of external hydrogen. When the content is 0.1% by mass or less, the formation of the surface concentrated layer is incomplete, and it is difficult to expect the effect of improving the delayed fracture resistance, and the cold formability is improved during cold working of the bolt. If there is no effect and the content exceeds 0.5% by mass , the amount of retained austenite may increase and impact toughness may decrease.

Molybdenum (Mo): 0.1 to 1.5% by mass
The molybdenum (Mo) content is limited to 0.1 to 1.5% by mass . The reason for this is that when the content is 0.1% by mass or less, the transition from epsilon carbide to cementite grows during the tempering process, and the growth of cementite on the way is suppressed to improve softening resistance or delayed fracture resistance. This is because the generation of carbide is reduced. On the other hand, when molybdenum is added in an amount of 1.5% by mass or more, it is very effective in increasing the softening resistance, but a low-temperature structure (martensite, bainite) is easily generated during production of the wire.

Titanium (Ti): 0.01 to 0.1% by mass
When boron forms boron nitride, the quench hardening improvement is significantly reduced. Titanium (Ti) is an element useful for binding to nitrogen instead of boron and suppressing the formation of boron nitride. In the present invention, the titanium content is limited to 0.01 to 0.1% by mass . If the content is less than 0.01% by mass, the effect of improving the corrosion resistance is insufficient, and it is difficult to produce titanium nitride that prevents the formation of boron nitride for improving the quenching hardenability of boron. On the other hand, if it exceeds 0.1% by mass , the titanium attachment effect is saturated and coarse titanium-based nitrides are formed, which is harmful to fatigue characteristics.

(Steel microstructure)
The microstructure of the bolt that is the subject of the present invention is a composite structure containing ferrite and martensite. The ferrite is preferably uniformly distributed and distributed as described above because the ferrite can prevent delayed penetration of hydrogen atoms into the prior austenite grain boundaries and enhance the delayed fracture resistance. Also, since ferrite is a soft phase compared to tempered martensite, crack propagation can be hindered by the effect of blunting the crack tip during crack propagation, which is effective in ensuring delayed fracture resistance. . In order to obtain such a uniform dispersion effect of ferrite, the area ratio of ferrite is preferably limited to 3 to 10%. If the area ratio of the ferrite is less than 3%, it is difficult to expect the effect of improving the delayed fracture resistance by the above-mentioned ferrite. Conversely, if it exceeds 10%, the ferrite is not uniformly dispersed and the tension of the bolt The strength becomes excessively low and it is difficult to obtain a desired strength.

(Distribution of precipitates)
As described above, the bolt is subjected to spheroidizing heat treatment at the stage of the wire, and then processed into a bolt shape. Since the spheroidizing heat treatment is a process for precipitating carbon that enhances the strength of the wire in the form of carbides, a large amount of coarse carbide is distributed inside the wire after the spheroidizing heat treatment. In particular, the intended bolt composition of the present invention produces iron carbide and chromium carbide, which provide hydrogen trap sites and thus reduce delayed fracture resistance. Therefore, in the bolt processing stage, these contents need to be minimized, and the conditions are as follows.

  It is necessary to control the area ratio of carbide to 10% or less. If it exceeds 10%, the delayed fracture resistance is reduced, and there is a possibility that notch toughness due to carbide may be reduced. Therefore, the area ratio of carbide is preferably 10% or less.

  Moreover, the diameter of the carbide | carbonized_material which exists without being removed needs to be 5 micrometers or less. In other words, when the carbide size is fine with the same area ratio of carbide, the number of sites where hydrogen is trapped increases and it becomes fine, so it serves to reduce the partial pressure of accumulated hydrogen. Can do. Therefore, the size of the carbide needs to be maintained at 5 μm or less.

  Hereinafter, the method for producing a high-strength bolt excellent in delayed fracture resistance provided by the present invention will be described in more detail.

(Production method)
In order to manufacture a high-strength bolt, a process of performing quenching, re-quenching and tempering (so-called QQ'-T) process on a wire having the above-described preferable steel composition and processed into a bolt shape is necessary.

  The quenching (Q) process is for making carbides such as iron carbide and chromium carbide into a solid solution state, controlling the area ratio of the carbides to 10% or less, and forming fine carbides. The heating temperature for making it necessary to be Ae3 + 80 ° C. or higher. When heating below this temperature, the carbides are not sufficiently dissolved in the base metal structure of the bolt, and there is a problem that coarse carbides remain. The bolt heated at the specific temperature and re-dissolved in the internal carbide is then rapidly cooled to prevent the carbide from re-depositing.

  Thereafter, in order to obtain a uniform ferrite phase, a two-phase critical heat treatment in two phase regions of austenite and ferrite + austenite is performed, and then rapid cooling (Q ') is performed. This heating temperature is preferably in the range of Ae3 + 10 to Ae3-10 ° C. When the heating temperature exceeds Ae3 + 10 ° C., the ferrite ratio decreases, and the effect of improving the delayed fracture resistance due to the uniform dispersion distribution of ferrite intended in the present invention cannot be obtained. Is less than Ae3-10 ° C., the ratio of ferrite becomes excessive, and it is difficult to uniformly disperse ferrite, and the tensile strength of the bolt may be reduced.

  Subsequently, a tempering heat treatment (T) for ensuring the toughness of the bolt is followed. The heat treatment of the bolt according to the composition of the present invention must be performed at a temperature of 450 ° C. or higher. However, if the tempering treatment is performed at a temperature lower than the above temperature, temper embrittlement may occur, and a film-like carbide is formed at the austenite grain boundary. A problem of precipitation may occur. However, if the tempering process is performed at a high temperature exceeding 500 ° C., the bolt tensile strength is not sufficient. Accordingly, a suitable tempering heat treatment temperature is in the range of 450-500 ° C.

  Hereinafter, the present invention will be described in more detail with reference to the following examples. These examples are for explaining the present invention, and do not limit the scope or concept of the present invention.

  Steel slabs having the composition described in Table 1 were subjected to a homogenization heat treatment at 1200 ° C. for 48 hours and then hot-rolled. During rolling, the rolling rate was 80%, the finish rolling temperature was 950 ° C., and hot-rolled and then air-cooled to produce a steel wire having a diameter of 13 mm.

  In Table 1 below, materials 1 and 2 of the examples of the present invention indicate steel slab compositions that satisfy the composition defined in the present invention, and Comparative Example and Material 1 indicate the composition of steel slabs outside the composition range of the present invention. Is.

  Specimens for evaluating mechanical properties (tensile and elongation) and delayed fracture resistance were taken in the rolling direction of the rolled material.

  As described in Table 2 below, the steel material was heat-treated by two heat treatment methods. The first method is for evaluating the physical properties when the internal structure of the bolt is a single-phase structure of tempered martensite. After the heat treatment at a quenching temperature of 900 ° C. for 40 minutes and quenching, The heat treatment was performed in the order of heat treatment (so-called Q-T) for 90 minutes at the tempering temperature shown in Table 2. In the second method, after quenching heat treatment, in order to uniformly distribute the ferrite phase, heating is performed at the re-quenching temperature shown in Table 2 for 40 minutes, followed by rapid cooling, and then at the tempering temperature shown in Table 2 for 90 minutes. It performed in order of heat processing (what is called QQ'-T process). The difference between the first heat treatment method and the second heat treatment method is that the internal structure is a single-phase structure of tempered martensite or a composite structure of ferrite (area ratio 10% or less) + tempered martensite. It is whether it is. However, even when manufactured by the same QQ'-T process, Comparative Example 3 has a quenching temperature of less than Ae3 + 80 ° C. and the quenching conditions do not satisfy the conditions of the present invention, and the steel composition satisfies the conditions suitable for the present invention. It is a case where it does not satisfy.

  Table 2 shows the results of the physical property evaluation for the bolts manufactured by the above-described respective manufacturing methods.

  The constant load method generally used in this industry was applied to the evaluation of the delayed fracture resistance in Table 2. This evaluation method is a method for evaluating delayed fracture resistance characteristics according to the time required for fracture under load stress or specific stress. During the delayed fracture resistance test, the test stress was determined based on the notch tensile strength.

As the delayed fracture resistance tester, a constant load type delayed fracture resistance tester was used. The delayed fracture resistance test piece was manufactured to have a diameter of 6 mm, a notch diameter of 4 mm, and a notch radius of 0.1 mm. The solution in the test piece atmosphere was a PH2 solution (NaCl + CH ₃ CHOOH), which was carried out at room temperature of 25 ° C. ± 5 ° C.

  Critical delayed fracture strength means the tensile strength that does not break for 150 hours or more until failure at the same stress ratio (load stress / notch tensile strength). Notch strength is a tensile test of a notched specimen (maximum The load / notch cross-sectional area) was obtained. The number of specimens for setting the critical delayed fracture strength was at least 15.

  As can be seen from Table 2, it can be seen that the tensile strength and elongation of Example 1 and Example 2 of the present invention have values equal to or higher than those of Comparative Example 1 and Comparative Example 2, respectively. .

  Moreover, from the results of Table 2, it can be seen that the delayed fracture resistance of Example 1-1 is higher than that of Examples of Comparative Examples 2 to 4 by 100 MPa or more, while the tensile strength is substantially the same level. Can be confirmed.

  That is, even if the same components are used, the process of the present invention produces steel having excellent delayed fracture resistance while securing a tensile strength and elongation rate equal to or higher than those of the conventional QT process. be able to.

  Comparing the tensile strength of Example 1 and Example 2 with the tensile strength of comparative materials (Comparative Example 3 and Comparative Example 4) that have been conventionally used in steel materials with delayed fracture resistance, Example 1 and Example 2 are conventional. It can be seen that it has a tensile strength superior to that of the comparative material.

  The elongation percentage may be a stable value of 13% or more, but Examples 1 and 2 show good values that are not so inferior to those of Comparative Examples 3 and 4.

  As described above, the tensile strength and elongation of Examples 1 and 2 of the present invention are not inferior to those of conventional comparative materials (Comparative Example 3 or Comparative Example 4), or have good values beyond that. The strength of delayed fracture resistance is 400 MPa or more when compared with Comparative Example 3, and 100 MPa or more when compared with Comparative Example 3. Thus, as can be seen in Table 2, the steel materials of Examples 1 and 2 show significantly superior values compared to conventional delayed fracture resistant steels.

  In order to confirm that the result of the tensile test depending on the quenching temperature is changed, a bolt was manufactured from a steel material having the same composition as in Example 1 of the present invention, and a tensile test was performed. The results are shown in Table 3.

From Table 3, if the quenching during the heating temperature of 870 ° C., it can be confirmed that the much lower elongation compared with the case of 900 ° C..

The Fe and Cr carbides produced by the spheroidizing heat treatment must be completely dissolved in the subsequent quenching process . To avoid phenomena such as this, it is necessary to completely dissolve sufficiently on up carbides quenching temperature in quenching heat treatment step. When the carbide is completely dissolved, the bolt elongation recovers.

  While the preferred embodiments of the present invention have been disclosed for purposes of illustration, those skilled in the art will recognize that various modifications, additions and additions can be made without departing from the scope and spirit of the invention as described in the claims. Alternatives are possible.

The present invention can be applied to the field of manufacturing high-strength bolts capable of realizing high strength with excellent delayed fracture resistance without complicated heat treatment processes and without adding a large amount of alloy components.

Claims (2)

Carbon is 0.35-0.55 mass%, Silicon is 0.05-2.0 mass%, Manganese is 0.1-0.8 mass%, Boron is 0.001-0.004 mass%, Chromium is 0.3 to 1.5 mass%, total oxygen (TO) is 0.005 mass% or less, phosphorus is 0.015 mass% or less, sulfur is 0.010 mass% or less, and vanadium is 0.05 to 0 A bolt-shaped wire having a composition of 0.5% by mass, molybdenum of 0.1 to 1.5% by mass, titanium of 0.01 to 0.1% by mass, and the balance of iron and other inevitable impurities,
A quenching step of heating at a temperature of Ae3 + 80 ° C. or higher and then rapidly cooling;
And re-hardening step of quenching after heating at the quenching temperatures of the wire again Ae3-10 ℃ ~Ae3 + 10 ℃,
Heating and tempering the re-quenched wire at a temperature of 450 ° C. or higher and 500 ° C. or lower;
A method for producing a high-strength bolt excellent in delayed fracture resistance. Carbon is 0.35-0.55 mass%, Silicon is 0.05-2.0 mass%, Manganese is 0.1-0.8 mass%, Boron is 0.001-0.004 mass%, Chromium is 0.3 to 1.5 mass%, total oxygen (TO) is 0.005 mass% or less, phosphorus is 0.015 mass% or less, sulfur is 0.010 mass% or less, and vanadium is 0.05 to 0 A bolt-shaped wire having a composition of 0.5% by mass, molybdenum of 0.1 to 1.5% by mass, titanium of 0.01 to 0.1% by mass, and the balance of iron and other inevitable impurities ,
A quenching step of heating at a temperature of Ae3 + 80 ° C. or higher and then rapidly cooling;
A re-quenching step in which the quenched wire is heated again at a temperature of Ae3-10 ° C to Ae3 + 10 ° C and then rapidly cooled;
A high-strength bolt manufactured by heating and tempering the re-quenched wire at a temperature of 450 ° C. or higher and 500 ° C. or lower ,
The high-strength bolt is composed of an internal structure of ferrite 3% or more and less than 10% in area ratio and tempered martensite 90 to 97%, and carbide is contained in the area ratio 10% or less, and the carbide is equivalent to a circle. A high-strength bolt excellent in delayed fracture resistance characterized by a maximum diameter of 5 μm or less.