WO2007074984A9

WO2007074984A9 - High-strength steel bolt having excellent resistance for delayed fracture and method for producing the same

Info

Publication number: WO2007074984A9
Application number: PCT/KR2006/005457
Authority: WO
Inventors: Sang-Yoon Lee; Duk-Lak Lee; Sang-Woo Choi
Original assignee: Posco
Priority date: 2005-12-26
Filing date: 2006-12-14
Publication date: 2010-07-08
Also published as: JP2009521600A; CN101346481A; CN101346481B; WO2007074984A1; JP5281413B2; KR100723186B1

Abstract

A bolt for use in connection of steel structures and for use as automotive parts and components, and a method for producing the same are disclosed. More specifically, provided is a steel bolt which is capable of achieving high strength simultaneously with an excellent delayed fracture resistance, by appropriate control of the steel microstructure. The bolt has a composition comprising (i) 0.35-0.55 wt% of carbon, 0.05-2.0 wt% of silicon, 0.1-0.8 wt% of manganese, 0.001-0.004 wt% of boron, 0.3-1.5 wt% of chromium, 0.005 wt% and less of oxygen (T.O), 0.015 wt% and less of phosphorous, 0.010 wt% and less of sulfur, and the balance of Fe with inevitable impurities; and further comprising (ii) at least one selected from the group consisting of 0.05-0.5 wt% of vanadium, 0.05-0.5 wt% of niobium, 0.1-0.5 wt% of nickel, 0.1-1.5 wt% of molybdenum and 0.01-0.1 wt% of titanium, wherein the bolt has an internal structure composed of ferrite and tempered martensite, and a content of ferrite in the internal structure is 3 to 10% in terms of an area fraction. The present invention can provide a high-strength bolt which is capable of simultaneously achieving an excellent delayed fracture resistance and a high strength without addition of large amounts of alloying elements, and also exhibits no deterioration of notch toughness. Further, the present invention can provide a method for preparing such a bolt which is simply and conveniently carried out without complicated heat treatment processes.

Description

HIGH-STRENGTH STEEL BOLT HAVING EXCELLENT RESISTANCE FOR DELAYED FRACTURE AND METHOD FOR

PRODUCING THE SAME

Technical Field

[1] The present invention relates to a bolt which is used in connection of steel structures and is used as automotive parts and components, and a method for producing the same. More specifically, the present invention relates to a steel bolt which is capable of achieving high strength simultaneously with an excellent delayed fracture resistance, by appropriate control of the steel microstructure.

[2]

Background Art

[3] The recently growing trend in the construction of buildings and structures has been toward steel structures having excellent safety from reinforced concrete structures. One of the important factors that should be considered in securing of the safety of the steel structures is a member-member joining technology. As examples of member-member joining methods, mention may be made of welding and bolt fastening. The bolt fastening does not require sophisticated skills as compared to the welding process, and advantageously provides enhanced safety of the steel structures by replacement of weak welding parts. Realization of high strength in the bolts advantageously leads to decreased numbers of bolts used in member-member connection and increased bolt clamping force, which consequently reduces a construction period while simultaneously decreasing the joint area to thereby pursue wholesomeness and soundness of the joint parts. Therefore, for construction of more efficient steel structures, many efforts have recently been made to achieve high strength of the bolts for connection of the steel structures.

[4] Conventional bolts are designed to secure the bolt strength by processing a steel wire rod into a bolt shape and then subjecting the shaped wire rod to a quenching process to thereby obtain a desired strength. Processing of the steel wire rod into a bolt shape is primarily carried out by a cold forging process, taking into consideration the productivity. Therefore, steel wire rods for bolt processing should have a physical property suitable for cold forging, i.e. good cold heading quality (CHQ). Therefore, the most important thing necessary for obtaining the desired CHQ is to lower the toughness of the wire rod to a proper level, such that processing of the wire rod can be easily carried out.

[5] In order to manufacture such a steel wire rod having good CHQ, the wire rod is made to have the strength as low as possible. In addition, the wire rod is subjected to a wiredrawing process for sizing, and then spheroidizing heat treatment for further decreasing the strength prior to the bolt processing. The spheroidizing heat treatment refers to heat treatment of further decreasing the strength of the wire rod by precipitating carbon in the form of spheroidized carbide, because the carbon solid-solution in the wire rod enhances the strength of the wire rod by solid solution strengthening. As discussed hereinbefore, the spheroidizing heat treatment is followed by processing of the wire rod into the bolt shape and quenching heat treatment thereof. However, formation of the martensite structure inside the thus-quenched bolt leads to sharp deterioration in the toughness of the bolt. Therefore, in order to prevent toughness degradation of the bolt due to the presence of the martensite structure, the bolt is subjected to a tempering process. As a result, the thus-prepared bolt will have the so- called tempered martensite structure inside thereof.

[6] It is known that the addition of an alloying element, particularly carbon is most effective for strengthening of steel materials having the tempered martensite structure. However, the addition of carbon leads to an increase of the strength from the early stage of the steel, i.e. a wire rod, which thereby results in a difficulty to perform cold working, a sharp increase in a ductile-brittle transition temperature (DBTT) of the product and a significant decrease in a resistance to hydrogen-induced delayed fracture. In addition, increased work hardening during processing is disadvantageous to bolt forming, thereby resulting in a need for an additional softening heat treatment.

[7] In addition, the tempered martensite exhibits distribution of Fe-based precipitates in the grain boundary due to intrinsic properties thereof, and the matrix of lath martensite is also susceptible to distribution of precipitates. Where such tempered martensite is applied to high-tension (high- strength) steel components such as bolts, the tempered martensite is exposed to high stress depending upon circumstances under use thereof. Such high stress leads to facilitated migration of hydrogen and large amounts of hydrogen also accumulate in the precipitates, consequently resulting in conditions susceptible to the occurrence of delayed fracture. As a result, the tempered martensite structure suffers from limitations in application thereof to manufacture of high-strength components.

[8] As discussed above, the bolt strength and the delayed fracture resistance are incompatible physical properties to each other and it is therefore very important to develop bolts having both the desired strength and delayed fracture resistance. Development of bolts which are capable of achieving high strength in conjunction with an excellent delayed fracture resistance is expected to provide the various advantages as follows. In the aspect of the steel structure, bolt-fastening does not require elaborate skills, as compared to weld-joining. Further, upon considering replacement of weak welding parts, the bolt- fastening provides the following advantages: strengthened clamping force upon fastening of members by a bolt and enhanced safety of the steel structures due to a decreased area of joining parts; reduced amounts of steel materials to be used due to decreased numbers of bolts used in fastening of members and a reduced construction period; contribution to reduction of the weight of parts in terms of automotive parts and components; and capability to realize diverse designs and compactness of automotive assembly facilities, due to feasibility of realizing weight reduction of parts.

[9] Conventional arts to improve the delayed fracture resistance may include 1) inhibiting corrosion of steel materials, 2) minimizing penetration of hydrogen into the steel material, 3) reducing a concentration of diffusible hydrogen attributable to delayed fracture, 4) using steel materials having a high critical diffusible hydrogen content, 5) minimizing tensile stress, 6) alleviating stress concentration, 7) reducing the austenite grain-boundary size, and the like. For this purpose, high alloying has been pursued, or surface coating or plating methods for preventing penetration of external hydrogen have been primarily used. In addition, there are also methods of forming precipitates capable of trapping diffusible hydrogen or controlling a micro structure of steel by addition of certain elements to steel materials, while minimizing contents of phosphorous (P) and sulfur (S) causing embrittlement of the austenite grain boundary.

[10] As a technique developed for improvement of the delayed fracture resistance,

Japanese Patent Publication Laid-open No. 2003-321743 discloses a method for producing a high-strength bolt having an excellent delayed fracture resistance. According to this patent, the high-strength bolt has a tempered martensite single phase structure comprised of 0.35 wt% and less of carbon (C), 0.50 wt% and less of silicon (Si), 0.1 to 2.0 wt% of manganese (Mn) and 0.05 to 0.6 wt% of molybdenum (Mo), further one or more metals selected from 0.08 wt% and less of niobium (Nb), 0.15 wt% and less of vanadium (V) and 1.5 wt% and less of tungsten (W), one or more metals selected from copper (Cu), nickel (Ni), chromium (Cr) and boron (B), and the balance of Fe with inevitable impurities, wherein 0.5 <

(C/12)/{(Ti/48)+(Mo/96)+(Nb/93)+(V/51)+(W/192)} <5 is satisfied. However, this Japanese Patent suffers from disadvantages in that large amounts of expensive alloying elements are added to obtain the delayed fracture resistance and a tempering temperature is high, thus presenting a difficulty of application thereof to practical production.

[11] In addition, Japanese Patent Publication Laid-open No. Hei 7-173531 discloses a method for production of bainite+martensite dual phase steel by hot forming a steel having a composition of 0.05 to 0.3 wt% of carbon (C), 0.05 to 2.0 wt% of silicon (Si), 0.3 to 5.0 wt% of manganese (Mn), 1.0 to 3.0 wt% of chromium (Cr), 0.01 to 0.5 wt% of niobium (Nb) and 0.01 to 0.06 wt% of aluminum (Al) and continuously cooling the steel at a critical cooling rate or higher, such that pro-eutectoid ferrite is not precipitated. However, this method also suffers from a difficulty associated with application thereof to practical production, due to large numbers of heat treatment processes.

[12] Further, Korean Patent Publication Laid-open No. 2000-0033852 discloses a method for preparing a high-strength bolt which has a ferrite and tempered martensite dual phase steel as a basic structure and comprises (i) 0.4-0.6 wt% of carbon, 2.0-4.0 wt% of silicon, 0.2-0.8 wt% of manganese; 0.25-0.8 wt% of chromium, 0.01 wt% and less of phosphorous, 0.01 wt% and less of sulfur, 0.005-0.01 wt% of nitrogen and 0.005 wt% and less of oxygen; and optionally (ii) at least one selected from 0.05-0.2 wt% of vanadium, 0.05-0.2 wt% of niobium, 0.3-2.0 wt% of nickel, 0.001-0.003 wt% of boron, 0.01-0.5 wt% of molybdenum, titanium, copper and cobalt. However, this Korean Patent suffers from problems associated with deterioration of notch toughness due to the presence of the retained spheroidized carbide in the bolt, resulting from a low quenching temperature.

[13]

Disclosure of Invention Technical Problem

[14] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a high-strength bolt which is capable of achieving both an excellent delayed fracture resistance and a high strength without addition of large amounts of alloying elements, and also exhibits no deterioration of notch toughness.

[15] It is another object of the present invention to provide a method for preparing the above bolt which is simply and conveniently carried out without complicated heat treatment processes.

[16]

Technical Solution

[17] In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a high-strength bolt having a composition comprising (i) 0.35-0.55 wt% of carbon, 0.05-2.0 wt% of silicon, 0.1-0.8 wt% of manganese, 0.001-0.004 wt% of boron, 0.3-1.5 wt% of chromium, 0.005 wt% and less of oxygen (T.O), 0.015 wt% and less of phosphorous, 0.010 wt% and less of sulfur, and the balance of Fe with inevitable impurities; and further comprising (ii) at least one selected from the group consisting of 0.05-0.5 wt% of vanadium, 0.05-0.5 wt% of niobium, 0.1-0.5 wt% of nickel, 0.1-1.5 wt% of molybdenum and 0.01-0.1 wt% of titanium, wherein the bolt has an internal structure composed of ferrite and tempered martensite, and a content of ferrite in the internal structure is 3 to 10% in terms of an area fraction.

[18] At this time, an amount of carbide in the internal structure is preferably 10% and less in terms of an area fraction. In addition, the carbide has preferably an equivalent circle diameter (Heywood's diameter) of up to 5 μm.

[19] In accordance with another aspect of the present invention, there is provided a method for preparing a high-strength bolt having an excellent delayed fracture resistance, comprising heating a bolt-shaped wire rod, which has a composition comprising (i) 0.35-0.55 wt% of carbon, 0.05-2.0 wt% of silicon, 0.1-0.8 wt% of manganese, 0.001-0.004 wt% of boron, 0.3-1.5 wt% of chromium, 0.005 wt% and less of oxygen (T.O), 0.015 wt% and less of phosphorous, 0.010 wt% and less of sulfur, and the balance of Fe with inevitable impurities; and further comprising (ii) at least one selected from the group consisting of 0.05-0.5 wt% of vanadium, 0.05-0.5 wt% of niobium, 0.1-0.5 wt% of nickel, 0.1-1.5 wt% of molybdenum and 0.01-0.1 wt% of titanium, to a temperature of more than Ae3+80°C, followed by rapid cooling (quenching); heating the cooled wire rod to a temperature of Ae3-10°C to Ae3+10°C, followed by rapid cooling (re-quenching); and tempering the re-quenched wire rod at a temperature of 45O⁰C or higher.

[20]

Advantageous Effects

[21] As discussed hereinbefore, the present invention can provide a high-strength bolt which is capable of simultaneously achieving an excellent delayed fracture resistance and a high strength without addition of large amounts of alloying elements, and also exhibits no deterioration of notch toughness.

[22] Further, the present invention can provide a method for preparing such a bolt which is simply and conveniently carried out without complicated heat treatment processes.

[23]

Best Mode for Carrying out the Invention

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

[25] As a result of a variety of extensive and intensive studies and experiments to solve the problems suffered by conventional arts, in conjunction with scrupulous examination on a bolt having the excellent strength and delayed fracture resistance and a method for preparing the same, the inventors of the present invention reached the following conclusions.

[26] That is, as compared to a conventional martensite single phase structure, when a dual phase structure of co-existing ferrite and tempered martensite is established in steel of interest and a fraction of ferrite in the internal structure is limited to a given level, this leads to uniform dispersion and distribution of the ferrite which in turn prevents penetration of hydrogen atoms into the prior austenite grain boundaries, thus enhancing a delayed fracture resistance. Further, the ferrite is relatively soft as compared to the tempered martensite and can therefore interfere with crack propagation as a result of blunting effects at the crack tip upon the occurrence of crack propagation, which is thus effective for securing of the delayed fracture resistance. In addition, delayed fracture due to hydrogen trapping can be prevented by minimizing amounts of coarse carbides such as iron (Fe) carbide, chromium (Cr) carbide and the like. Further, where large numbers of fine hydrogen trapping sites are provided by dispersing and distributing the remaining carbides to finer sizes, the delayed fracture resistance can be effectively improved. Further, in order to achieve the structure and carbide distribution advantageous for improvement of the delayed fracture resistance, it is important to control the steel composition to within the proper range as follows.

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

[28]

[29] Steel Composition

[30]

[31] Carbon (C): 0.35-0.55 wt%

[32] Carbon (C) is an element added to secure strength of the product. However, where the carbon content is higher than 0.55 wt%, large amounts of film-like carbides are undesirably precipitated at the austenite grain boundaries, thereby lowering a resistance to hydrogen-induced delayed fracture. On the other hand, where the carbon content is lower than 0.35 wt%, it is difficult to obtain sufficient tensile strength of a bolt by quenching and tempering heat treatment. Therefore, the carbon content is preferably in the range of 0.35 to 0.55 wt%.

[33]

[34] Silicon (Si): 0.05-2.0 wt%

[35] Silicon (Si) is an element which is useful for deoxidization of the steel and is also effective to secure desired strength of the steel. However, where the silicon content is higher than 2.0 wt%, work hardening rapidly takes place upon cold forging of processing a steel wire rod into a bolt shape, thereby resulting in poor processability. On the other hand, where the silicon content is lower than 0.05 wt%, it is difficult to secure desired bolt strength. Therefore, the silicon content is preferably limited to the range of 0.05 to 2.0 wt%.

[36]

[37] Manganese (Mn): 0.1-0.8 wt% [38] Manganese (Mn) is an element providing solid solution strengthening effects by the formation of a substitutional solid solution in the matrix structure and is very useful for characteristics of high-tension bolts. A content of manganese is preferably in the range of 0.1 to 0.8 wt%. That is, where manganese is added in an amount exceeding 0.8 wt%, detrimental effects of structural inhomogeneities due to manganese segregation on the bolt properties are greater than solid solution strengthening effects. Upon solidification of the steel, the steel is susceptible to macrosegregation and microsegregation depending upon segregation mechanisms. Due to a relatively low diffusion coefficient of manganese as compared to that of other elements, the manganese segregation facilitates formation of segregation zones and as a result, improvement of hardenability serves as a primary cause to produce a low temperature structure (core martensite). On the other hand, if manganese is added in an amount of 0.1 wt% and less, it is difficult to obtain desired improvements in stress relaxation via solid solution strengthening, even though there are substantially no effects of the manganese segregation on segregation zones. That is, where the manganese content is 0.1% and less , improvements in the hardenability and permanent deformation resistance are not sufficient due to poor solid solution strengthening effects. On the other hand, where the manganese content exceeds 0.8%, locally increased hardenability and the formation of segregation zones due to the manganese segregation upon casting of the steel material lead to severe structural anisotropy, i.e. deterioration of bolt properties due to structural inhomogeneities.

[39]

[40] Boron (B): 0.001-0.004 wt%

[41] Boron (B) primarily serves as a grain boundary- strengthening element added to improve the hardenability and delayed fracture resistance in the present invention. The lower limit of boron content is preferably 0.0010 wt%. If the boron content is lower than 0.0010 wt%, improvements in the grain boundary- strength and hardenability are insufficient due to the grain boundary segregation which takes place upon heat treatment. On the other hand, if the boron content is higher than 0.004 wt%, effects of boron addition are saturated and boron nitrides are precipitated at the grain boundary, thereby lowering the grain boundary-strength.

[42]

[43] Chromium (Cr): 0.3-1.5 wt%

[44] Chromium (Cr) is an element effective for improvement of the hardenability upon quenching and tempering heat treatment. Where the chromium content is 0.3 wt% and less, it is difficult to secure sufficient hardenability upon quenching and tempering treatment. Therefore, it is necessary to set the chromium content to a range of 0.3 wt% or higher. In addition, according to research results of the present inventors, it was found that improvement of the hardenability exerted by chromium per se is trivial, but co-addition of chromium with boron exhibits significantly increased effects on improvement of the hardenability, thus representing that addition of chromium is necessary. On the other hand, where the chromium content exceeds 1.5 wt%, this may undesirably result in formation of film- like carbides inside the steel material. This is because the presence of film-like carbides at austenite grain boundaries was known to lower a resistance to hydrogen-induced delayed fracture.

[45]

[46] Oxygen (T.O): 0.005 wt% and less

[47] Oxygen (O) is analyzed in terms of total oxygen (T.O) and a content of oxygen is limited to a range of 0.005 wt% and less. This is because deterioration of a fatigue life may occur due to oxide-based nonmetallic inclusions, if the content of oxygen exceeds 0.005 wt%.

[48]

[49] Phosphorus (P): 0.015 wt% and less

[50] A content of phosphorus (P) is limited to a range of 0.015 wt% and less. Phosphorus is a main cause for deterioration of toughness and decrease of the delayed fracture resistance by segregation thereof in grain boundaries. Therefore, the upper limit of phosphorus content is limited to a range of 0.015 wt% and less.

[51]

[52] Sulfur (S): 0.010 wt% and less

[53] Sulfur (S) is an element having a low boiling point, and undergoes grain boundary segregation, thereby resulting in deterioration of toughness and exhibits adverse side effects on the delayed fracture resistance and stress relaxation properties by formation of sulfides. Therefore, the upper limit of sulfur content is preferably limited to 0.010 wt%.

[54]

[55] In addition to the above-mentioned composition components, it is preferred to further add one or more elements selected from vanadium (V), niobium (Nb), molybdenum (Mo) and nickel (Ni) in an amount defined as follows.

[56]

[57] Vanadium (V): 0.05-0.5 wt%

[58] Vanadium (V) is an element which improves the delayed fracture resistance and softening resistance via the formation of precipitates, and a content of vanadium is limited to a range of 0.05 to 0.5 wt%. Where the content of vanadium is 0.05 wt% and less, decreased distribution of vanadium-based precipitates in the matrix leads to an insufficient role of vanadium as non-diffusible hydrogen trapping sites and it is therefore difficult to achieve improvements of the delayed fracture resistance. In addition, it is also difficult to achieve desired precipitation strengthening effects, and therefore improvement of the softening resistance is not sufficient. On the other hand, where the content of vanadium exceeds 0.5 wt%, improvements of the delayed fracture resistance and softening resistance by the precipitates are saturated. In addition, increased amounts of coarse alloying carbides, which were not dissolved into the matrix upon heat treatment of austenite, serve as nonmetallic inclusions, thereby leading to deterioration of fatigue properties.

[59]

[60] Niobium (Nb): 0.05-0.5 wt%

[61] Similar to vanadium, niobium (Nb) is also an element improving the delayed fracture resistance and softening resistance via the formation of precipitates, and a content of niobium is limited to a range of 0.05 to 0.5 wt%. Where the content of niobium is 0.05 wt% and less, decreased distribution of niobium-based precipitates in the matrix leads to an insufficient role of niobium as non-diffusible hydrogen trapping sites and it is therefore difficult to achieve improvements of the delayed fracture resistance. In addition, it is also difficult to achieve desired precipitation strengthening effects, and therefore improving effects on the softening resistance are not sufficient. On the other hand, where the content of niobium exceeds 0.5 wt%, improvements of the delayed fracture resistance and softening resistance by the precipitates are saturated. In addition, increased amounts of coarse alloying carbides, which were not dissolved into the matrix upon heat treatment of austenite, serve as nonmetallic inclusions, thereby leading to deterioration of fatigue properties.

[62]

[63] Nickel (Ni): 0.1-0.5 wt%

[64] Nickel (Ni) is an element of improving the delayed fracture resistance by inhibiting permeation of external hydrogen via the formation of a nickel-enriched layer on the surface of the steel material upon heat treatment thereof. Where the content of nickel is 0.1 wt% and less, it is difficult to achieve desired improvements of the delayed fracture resistance due to incomplete formation of the surface-enriched layer of nickel, and there is no improvement in the cold formability upon cold working of bolts. On the other hand, where the content of nickel exceeds 0.5 wt%, increased amounts of the remaining austenite may lead to the risk of lowering impact toughness.

[65]

[66] Molybdenum (Mo): 0.1-1.5 wt%

[67] A content of molybdenum (Mo) is limited to a range of 0.1 to 1.5 wt%. This is because 0.1 wt% and less of molybdenum results in poor formation of carbides for improving the softening resistance or delayed fracture resistance by inhibiting the growth of cementite upon transition and growth of cementite from epsilon-carbide during a tempering process. On the other hand, addition of molybdenum in an amount of more than 1.5 wt% is highly effective to increase the softening resistance, but is liable to produce low temperature structures (such as martensite and bainite) upon making of a wire rod.

[68]

[69] Titanium (Ti): 0.01-0.1 wt%

[70] When boron forms boron nitrides, improvement of hardenability is significantly decreased. Titanium (Ti) is a useful element in the present invention. That is, titanium combines with nitrogen in place of boron to thereby inhibit formation of boron nitrides. The content of titanium is limited to a range of 0.01 to 0.1 wt%. If the titanium content is 0.01 wt% and less, improving effects of titanium on the corrosion resistance are not sufficient and it is difficult to form titanium nitrides which prevent the formation of boron nitrides in order to improve the boron hardenability. On the other hand, if the titanium content is higher than 0.1 wt%, the addition effects of titanium are saturated and formation of coarse titanium-based nitrides may have detrimental effects on fatigue properties.

[71]

[72] Microstructure of steel

[73] Microstructure of a bolt, which will be addressed by the present invention, is a multiphase structure including ferrite and martensite. Preferably, the ferrite is uniformly dispersed and distributed. As described hereinbefore, this is because the ferrite prevents penetration of hydrogen atoms into the prior austenite grain boundaries, thereby enhancing the delayed fracture resistance; further, the ferrite is relatively soft as compared to the tempered martensite and can therefore interfere with crack propagation as a result of blunting effects at the crack tip upon the occurrence of crack propagation, which is thus effective for securing of the delayed fracture resistance. In order to obtain uniform dispersion of the ferrite, the area fraction of ferrite is preferably limited to a range of 3 to 10%. If the area fraction of ferrite is 3% and less, it is difficult to achieve improvement of the delayed fracture resistance by addition of the ferrite. On the other hand, if the area fraction of ferrite is higher than 10%, uniform dispersion of the ferrite is not achieved, and it is difficult to obtain desired strength of the bolt due to excessively lowered tensile strength thereof.

[74]

[75] Distribution of precipitates

[76] As discussed hereinbefore, a wire rod for making a bolt is subjected to spheroidizing heat treatment and then processed into a bolt shape. As the spheroidizing heat treatment is a process for precipitation of carbon increasing the strength of the wire rod, in the form of carbide, large amounts of coarse carbides are distributed in the wire rod after the spheroidizing heat treatment process. In particular, according to the composition of the bolt intended for the present invention, carbides of iron and chromium are formed, and they provide hydrogen trapping sites, thereby decreasing the delayed fracture resistance. Therefore, it is necessary to minimize the content of carbides upon entering a bolt processing stage. The conditions for this purpose are as follows.

[77] It is necessary to control an area ratio of the carbide to the range of 10% and less. If the area ratio of the carbide exceeds 10%, this may lead to a decrease in the delayed fracture resistance as well as a decrease in the notch toughness due to the carbide. Therefore, the area ratio of the carbide is preferably in the range of 10% and less.

[78] In addition, it is also necessary to ensure that a diameter of the retained carbide, not removed, is 5 μm and less. That is, when the size of the carbide is finer at the same area ratio of the carbide, the number of hydrogen trapping sites increases and those site become finer, consequently serving to lower the partial pressure of the accumulated hydrogen. Therefore, it is necessary to maintain the size of the carbide below 5 μm.

[79]

[80] Hereinafter, a method for preparing the high-strength bolt having an excellent delayed fracture resistance provided by the present invention will be described in more detail.

[81]

[82] Preparation of high-strength bolt

[83] In order to prepare the high-strength bolt, it is necessary to perform a quenching, re- quenching and tempering (so-called Q-Q'-T) process on the steel wire rod having the above-mentioned preferred steel composition and processed into a bolt-shape.

[84] The quenching (Q) process is intended to render carbides such as iron carbide and chromium carbide a solid-solution state, thereby controlling the area ratio of the carbide to 10% and less, and to form fine carbides. At this step, a temperature of solid solution treatment of the carbides is necessary to be a temperature of Ae3+80°C or higher. Where the heating temperature of the steel wire rod is Ae3+80°C and less, this leads to insufficient dissolution of the carbides into the matrix structure of the bolt, thereby presenting the problem associated with retention of coarse carbides. The bolt, where internal carbides were re-dissolved by heating it to the above-specified temperature, is then rapidly cooled to prevent re-precipitation of the carbides.

[85] Thereafter, in order to obtain a uniform ferrite phase, the wire rod is subjected to in- tercritical heat treatment in two-phase region of austenite and ferrite+austenite, followed by rapid cooling (Q'). The heating temperature is preferably in the range of Ae3+10°C to Ae3-10°C. If the heating temperature exceeds Ae3+10°C, the ratio of ferrite is decreased and it is therefore difficult to achieve improvements of the delayed fracture resistance via the uniform dispersion/distribution of the ferrite, as intended by the present invention. On the other hand, if the heating temperature is Ae3-10°C and less, the ratio of ferrite is excessively increased, which consequently results in a difficulty to achieve uniform distribution of the ferrite, as well as the risk of decreasing the tensile strength of the bolt.

[86] Subsequently, tempering heat treatment (T) is carried out to secure the toughness of the bolt. The heat treatment of the bolt having the steel composition of the present invention should be conducted at a temperature of more than 45O⁰C, because tempering at a temperature lower than the above-specified range may result in the occurrence of temper embrittlement, as well as problems associated with precipitation of film-like carbides at austenite grain boundaries. On the other hand, if tempering treatment is carried out at a high temperature exceeding 500⁰C, the tensile strength of the bolt is not sufficient. Therefore, the proper tempering temperature is in the range of 450 to 500⁰C.

[87]

Mode for the Invention

[88] Now, the present invention will be described in more detail with reference to the following examples. These examples are provided only for illustrating the present invention and should not be construed as limiting the scope and spirit of the present invention.

[89]

[90] EXAMPLES

[91] Steel slabs having a steel composition as set forth in Table 1 below were subjected to homogenization heat treatment at 1200⁰C for 48 hours, followed by hot rolling. The rolling reduction rate was set to 80% and the finish rolling temperature was set to 95O⁰C. The hot rolling was followed by air-cooling to prepare steel wire rods having a diameter of 13 mm.

[92] In Table 1, Inventive materials 1 and 2 represent steel slab compositions satisfying the steel composition specified in the present invention, and Comparative material 1 represents a steel slab composition outside the specified composition of the present invention.

[93]

[94] Table 1 [Table 1] [Table ]

[95] [96] For evaluation of mechanical properties (tensile strength and elongation ratio) and delayed fracture properties, samples were collected from the thus-rolled steel materials in the rolling direction.

[97] As described in Table 2 below, the above-mentioned different steel materials were respectively subjected to heat treatment by two methods. The first method is to evaluate physical properties of the steel when the internal structure of the bolt is a tempered martensite single phase structure, and is sequentially carried out by heat treatment of the steel at a quenching temperature of 900⁰C for 40 min, followed by rapid cooling, and then heat treatment of the steel at a tempering temperature given in Table 2 for 90 min (so-called Q-T process). The second method is to achieve uniform distribution of a ferrite phase after the quenching heat treatment, and is sequentially carried out by heat treatment of the steel at a re-quenching temperature given in Table 2 for 40 min, followed by rapid cooling, and then heat treatment of the steel at a tempering temperature given in Table 2 for 90 min (so-called Q-Q'-T process). The difference between the first heat treatment method and the second heat treatment method is whether the internal structure of the steel is a tempered martensite single phase structure or a ferrite (area fraction of 10% and less)+tempered martensite multiphase structure. Provided that even though it was prepared by the same Q-Q'-T process, Comparative Example 3 was carried out at a quenching temperature of Ae3+80°C and less, which does not meet quenching conditions of the present invention and also does not meet the steel composition suitable for the present invention.

[98] The evaluation results of physical properties for bolts prepared by the above methods are given in Table 2 below.

[99] Evaluation of the delayed fracture resistance in Table 2 below was carried out by application of a constant load method conventionally used in the art. This evaluation method is a method of measuring the delayed fracture resistance as the time taken to reach the fracture according to applied stress or under specified stress. As to test stress applied upon performing a delayed-fracture test, the applied stress was determined based on the notched tensile strength.

[100] The delayed fracture test was carried out using a constant loading type delayed fracture testing machine. As samples for the delayed fracture test, test specimens having a sample diameter of 6 mm, a notch diameter of 4 mm, a notch root radius of 0.1 mm were prepared. As an atmosphere solution of the test specimen, a solution of NaCl-I-CH₃CHOOH (pH 2) was prepared and used for the test at room temperature (25°C+5°C).

[101] The critical delayed fracture strength means a tensile strength at which steel slab samples do not undergo breakage for more than 150 hours until the fracture occurs under the same stress ratio (loaded stress/notched tensile strength). The notch strength was calculated as a value of (maximum load/sectional area of notched part) by subjecting notched specimens to a tensile test. The number of test specimens for establishment of the critical delayed fracture strength was a minimum of 15.

[102] Table 2

[Table 2] [Table ]

[103] [104] As can be confirmed from Table 2, Examples 1 and 2 according to the present invention exhibited the tensile strength and elongation ratio equal to or higher than those of Comparative Examples 1 and 2, respectively.

[105] Further, it can be confirmed through the results of Table 2 that the delayed fracture strength of Example 1-1 according to the present invention is about 100 MPa higher than that of Comparative Example 2-4, even though the tensile strength may be the same therebetween.

[106] That is, even with use of the same components, the process of the present invention enables production of steel having a superior delayed fracture resistance while securing the tensile strength and elongation ratio comparable to the conventional Q-T process. [107] Upon comparing the tensile strength of Examples 1 and 2 with that of comparative materials (Comparative Examples 3 and Examples 4) which have been conventionally and widely used as the delayed fracture-resistant steel materials, it can be seen that the results of Examples 1 and 2 show superior tensile strength to the conventional comparative materials.

[108] Further, upon considering the fact that it is enough that if the material stably exhibits the elongation ratio of more than 13%, Examples 1 and 2 showed good values of the elongation ratio which is in no way inferior to Comparative Examples 3 and Examples 4.

[109] As such, it can be confirmed that Examples 1 and 2 of the present invention exhibited the tensile strength and elongation ratio equal to or higher than those of the conventional comparative materials (Comparative Examples 3 and Examples 4), while showing the delayed fracture -resistance strength, about 400 MPa higher than that of Comparative Examples 3 and about 100 MPa higher than that of Comparative Examples 3 thus representing that the steel materials of Examples 1 and 2 exhibit superior values of the delayed fracture-resistance as compared to the conventional delayed fracture-resistant steel materials, as shown in Table 2.

[HO] In order to confirm the results of a tensile test depending upon various quenching temperatures, bolts were manufactured using steel materials having steel composition components similar to Example 1 of the present invention and were then subjected to the tensile test. The results thus obtained are given in Table 3 below.

[111] [112] Table 3 [Table 3] [Table ]

[113] [114] As can be confirmed from Table 3, a heating temperature of 87O⁰C upon quenching of steel resulted in a significantly low elongation ratio, as compared to a quenching temperature of 900⁰C. In order to support such results, effects of iron (Fe) and chromium (Cr) carbides, not dissolved upon quenching heat treatment, were observed with reference to FIGS. 1 and 2. [115] The iron and chromium carbides, which were produced by spheroidizing heat treatment, should be completely removed in a subsequent quenching process. However, upon the presence of iron and chromium carbides, not dissolved even after quenching, as shown in FIG. 1, these carbides are believed to serve as an initiation point of crack. In order to avoid such a phenomenon, it is necessary to ensure that the quenching temperature during the quenching process is sufficiently elevated to completely dissolve the carbides, as shown in FIG. 2. When the carbides are completely dissolved, the elongation ratio of the bolt is recovered.

[116] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

[1] A high-strength bolt having an excellent delayed fracture resistance, comprising:

(i) 0.35-0.55 wt% of carbon, 0.05-2.0 wt% of silicon, 0.1-0.8 wt% of manganese, 0.001-0.004 wt% of boron, 0.3-1.5 wt% of chromium, 0.005 wt% and less of oxygen (T.O), 0.015 wt% and less of phosphorous, 0.010 wt% and less of sulfur, and the balance of Fe with inevitable impurities; and further comprising:

(ii) at least one selected from the group consisting of 0.05-0.5 wt% of vanadium, 0.05-0.5 wt% of niobium, 0.1-0.5 wt% of nickel, 0.1-1.5 wt% of molybdenum, and 0.01-0.1 wt% of titanium, wherein the bolt has an internal structure composed of 3 to 10% of ferrite and 90 to 97% of tempered martensite, in terms of an area fraction.

[2] The bolt according to claim 1, wherein an amount of a carbide in the internal structure is 10% and less in terms of an area fraction.

[3] The bolt according to claim 1 or 2, wherein the carbide has an equivalent circle diameter of up to 5 μm.

[4] A method for preparing a bolt having an excellent delayed fracture resistance, comprising: heating a bolt-shaped wire rod, having a composition comprising (i) 0.35-0.55 wt% of carbon, 0.05-2.0 wt% of silicon, 0.1-0.8 wt% of manganese, 0.001-0.004 wt% of boron, 0.3-1.5 wt% of chromium, 0.005 wt% and less of oxygen (T.O), 0.015 wt% and less of phosphorous, 0.010 wt% and less of sulfur, and the balance of Fe with inevitable impurities; and further comprising (ii) at least one selected from the group consisting of 0.05-0.5 wt% of vanadium, 0.05-0.5 wt% of niobium, 0.1-0.5 wt% of nickel, 0.1-1.5 wt% of molybdenum and 0.01-0.1 wt% of titanium, to a temperature of more than Ae3+80°C, followed by rapid cooling (quenching); heating the cooled wire rod to a temperature of Ae3-10°C to Ae3+10°C, followed by rapid cooling (re-quenching); and tempering the re-quenched wire rod at a temperature of 45O⁰C or higher.