0001]The present invention relates to a bolt, more particularly, to a bolt of a high strength.
Background technique
[0002]Recently, in order to cope with the environmental problems and the like, automobile, industrial machinery, against members used in construction, etc., light weight and high strength are required. In particular, the engine cylinder head bolt, typified by the connecting rod bolts in the automotive bolt, tensile strength of not less than 1000MPa is required.
[0003]
　However, the tensile strength of the bolt if the high strength of at least 1000 MPa, increased susceptibility to hydrogen embrittlement, hydrogen embrittlement resistance (delayed fracture) characteristic is lowered. As the material for the high-strength bolts, SCM steel containing alloying elements such as Mo in a large amount (JIS standard), and an alloy steel containing expensive alloy element V or the like is used. These alloy steels are manufactured in the wire is produced to the bolt is further forged wire drawing and cold.
[0004]
　When using the above-mentioned alloy steel as bolts, it increases the resistance to hydrogen embrittlement. However, these steel alloys for containing a large amount of alloy elements, causing an increase in steel cost. In recent years, has been soaring prices of alloying elements, supply and demand environment also tends to fluctuate. Therefore, reducing these alloy elements, or while omitting to suppress the steel costs, bolts can realize high strength and excellent hydrogen embrittlement resistance is required.
[0005]
　In order to suppress the steel costs may be reduced alloy elements such as Mo and V in the steel. If reducing the alloying elements, reduces the hardenability of the steel material, when producing a hot-rolled to a wire, generation of hard tissue of bainite or the like can be suppressed. Accordingly, a softening heat treatment omitted or be simplified, manufacturing cost can be reduced. However, it is difficult to bolt to a high strength, further, is also reduced hydrogen embrittlement resistance.
[0006]
　Therefore, instead of the alloying elements such as Mo and V, high-strength bolts containing boron (B) has been studied. B, like the alloy elements such as Mo and V, increasing the hardenability of steel. However, if the B containing steel tensile strength is used as the above high-strength bolts 1000 MPa, there is a case hydrogen embrittlement resistance is low.
[0007]
　Bolt for overcoming this problem is, JP 2012-162798 (Patent Document 1), JP-A-11-293401 (Patent Document 2), Japanese Patent 10-53834 (Patent Document 3), and JP 2008-156678 JP has been proposed (Patent Document 4). The bolts of these documents, enhancing hardenability by containing B, increasing the strength by strengthening the grain boundary, further hydrogen embrittlement resistance is also enhanced.
[0008]
　Specifically, high-strength bolts for steel disclosed in Patent Document 1, by mass%, C: less than 0.20 ~ 0.40%, Si: 0.20 ~ 1.50%, Mn: 0. 30 ~ 2.0%, P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%), Ni: 0.05 ~ 1.0%, Cr: 0.01 ~ 1.50%, Cu: 1.0% or less (including 0%), Al: 0.01 ~ 0.10%, Ti: 0.01 ~ 0.1%, B: 0.0003 to 0.0050% and N: 0.002 ~ 0.010% and containing respectively, Cu, contains 0.10 to 3.0% in total of one or more selected from the group consisting of Ni and Cr, the balance There consisting of iron and inevitable impurities. This steel furthermore, the ratio of the content of Si and [Si] content of C and [C] ([Si] / [C]), together with less than 1.0, a ferrite and pearlite structure. Thus, the delayed excellent B added high-strength bolts fracture resistance can be obtained, and is described in Patent Document 1.
[0009]
　In the B added high-strength bolts Patent Document 1, the Si content was higher than the C content, increasing the strength of the matrix by Si, thereby improving the delayed fracture resistance. However, since the expensive Ni is contained as an essential element, the steel cost increases.
[0010]
　Steel disclosed in Patent Document 2 volts, in mass%, C: 0.10 ~ 0.45% , B: 0.0003 ~ 0.0050%, Ti: 0.01 ~ 0.1%, N : includes 0.0025 to 0.010%, still another component, Si: 0.03 ~ 0.5%, Mn: 0.3 ~ 1.5% and Al: 0.01 ~ 0.10% containing the balance consisting of Fe and unavoidable impurities. The steel further satisfies at least one of the following requirements (1) and (2). (1) extraction residue method particle size extracted by: Ti amount included in the 0.1μm than precipitates, less than 60% of the total Ti content in the steel. (2) particle size is observed by electron microscopy by the extraction replica method: average number of Ti-based precipitates of 0.01 ~ 0.2 [mu] m is, 25 [mu] m 2 is 10 to 500 during the observation field of. Thus, the bolt consisting of cold workability and delayed fracture resistance excellent B-containing steel is obtained, and is described in Patent Document 2.
[0011]
　However, in the bolt Patent Document 2, low Si content, further, the weight ratio of Si and Mn is less than 1.0. Therefore, control of the inclusions is difficult, in some cases hydrogen embrittlement resistance is low.
[0012]
　High-strength bolt steel disclosed in Patent Document 3, by mass%, B: 0.0008 ~ 0.004% , C: 0.4% or less (not including 0%), Ti: 0.025 ~ 0.06%, N: containing 0.006% or less (not including 0%), the balance being Fe and inevitable impurities. This steel In addition, the relationship between the Ti compound excluding ferrite grain size FGC and TiN during hot rolling, [Ti compound content / FGC except TiN 1/2 meet] × 1000 ≧ 3. Thus, the austenite grain size number is 5 or more, a tensile strength of 785N / mm 2 high strength bolts more than is obtained, and is described in Patent Document 3.
[0013]
　However, the high strength bolts of Patent Document 3, a high Mn content, when Cr content is low, there are cases hydrogen embrittlement resistance is low.
[0014]
　High-strength bolt steel disclosed in Patent Document 4, by mass%, C: 0.15% than 0.30% or less, Si: 1.0% or less, Mn: 1.5% or less, Ti: 0 .1% or less, Mo: 0.3% or more, 0.5% or less, B: 0.0005% or more, containing 0.01%, the balance being Fe and impurities. The steel, after quenching, tempering is performed at 100 ~ 400 ° C., an average prior austenite grain size after quenching becomes 10μm or less of the steel structure. Thus, high-strength bolts bolt strength range with excellent delayed fracture resistance and corrosion resistance of about 1200 ~ 1600 MPa is obtained, and is described in Patent Document 4.
[0015]
　However, the bolt in Patent Document 4, since Mo is 0.3 to 0.5 mass%, becomes too high hardenability. Therefore, it is necessary to carry out long-softening heat treatment before between drawing and cold forging. In this case, there is a case where the production cost is greatly increased.
CITATION
Patent Document
[0016]
Patent Document 1: JP 2012-162798 Patent Publication
Patent Document 2: JP-A 11-293401 JP-
Patent Document 3: JP-A 10-53834 discloses
Patent Document 4: JP 2008-156678 JP
Summary of the Invention
Problems that the Invention is to Solve
[0017]
　An object of the present invention has a high strength, and is to provide a bolt having excellent hydrogen embrittlement resistance.
Means for Solving the Problems
[0018]
　High-strength bolt according to the invention, in mass%, C: 0.22 ~ 0.40% , Si: 0.10 ~ 1.50%, Mn: less than 0.20 ~ 0.40%, P: 0 . 020% or less, S: 0.020% or less, Cr: 0.70 ~ 1.45%, Al: 0.005 ~ 0.060%, Ti: 0.010 ~ 0.045%, B: 0.0003 ~ 0.0040%, N: 0.0015 ~ 0.0080%, O: 0.0020% or less, Cu: 0 ~ 0.50%, Ni: 0 ~ 0.30%, Mo: 0 ~ 0.04 % V: 0 ~ 0.05% and, Nb: 0 contained to 0.050% the balance being Fe and impurities, having a chemical composition satisfying the formula (1) and (2), It has a tensile strength of 1000 ~ 1300MPa.
　0.50 ≦ C + Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85
　(1) Si / Mn> 1.0 (2)
　wherein each element symbol of the formula (1) and (2), the corresponding element the content (mass%) is substituted.
[0019]
　High-strength bolt according to the invention has a high strength, and has excellent hydrogen embrittlement resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020]
[1] Figure 1 is a diagram illustrating the critical diffusable hydrogen amount, the relationship between the Si / Mn in the bolt.
FIG. 2 is a side view of a test piece with an annular V-notch.
FIG. 3 is a side view of the screw manufactured in Example.
DESCRIPTION OF THE INVENTION
[0021]
　We used Mo, not high content of expensive alloy elements V, etc., C, Si, Mn, and B-containing steel containing Cr and B, etc., the tensile strength of the bolt, water Motomoro conducted a survey examined the components and organization on the properties. As a result, the present inventors have obtained the following findings.
[0022]
　Tensile Strength of the bolt]
　tensile strength of the bolt to the high strength of 1000 ~ 1300 MPa, it is necessary to sufficiently hardenability. However, if too high hardenability, cold workability is deteriorated. In this case, before carrying out the cold working such as forging between wire drawing and cold against steel wire, etc., it must be a long softening heat treatment for the purpose of softening of the steel material was performed multiple times. Therefore, Mo, even without containing a large amount of alloy elements V, etc., the production cost is high. Thus, even without a plurality of times for a long period of time softening heat treatment it is capable of cold working, and steel having hardenability the tensile strength can be obtained is preferable.
[0023]
　If the bolt chemical composition satisfies the formula (1), obtained excellent cold workability and hardenability.
　0.50 ≦ C + Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85 (1)
　wherein each element symbol in the formula (1), the content of the corresponding element (mass%) is substituted.
[0024]
　fn1 = is defined as C + Si / 10 + Mn / 5 + 5Cr / 22. C, Si, Mn, and Cr are both an element that enhances the hardenability. Therefore, fn1 is a hardenability and cold workability index.
[0025]
　If fn1 is too low, no sufficient hardenability can not be obtained. On the other hand, if fn1 is too high, too high hardenability. In this case, when the bolt steel is rolled into wire rod, bainite is generated, it increases the strength and hardness. Therefore, drawing process of the next step and, prior to cold forging process, to be performed multiple times for a long period of time softening heat treatment, sufficient cold workability can not be obtained. fn1 is satisfies the equation (1), while obtaining excellent hardenability, without a plurality of times for a long period of time softening heat treatment, sufficient cold workability are obtained.
[0026]
　[Water for hydrogen
　embrittlement] [Relationship between the Si / Mn and hydrogen embrittlement resistance]
　be a high strength of 1000 ~ 1300 MPa tensile strength bolts, it satisfies the equation (2), excellent hydrogen embrittlement characteristics can be obtained.
　Si / Mn> 1.0 (2)
　wherein each element symbol in the formula (2), the content of the corresponding element (mass%) is substituted. In the following description, it is defined as fn2 = Si / Mn. fn2 corresponds to the left-hand side of equation (2). The following describes the formula (2).
[0027]
　Figure 1 is a critical diffusable hydrogen amount HR, is a diagram showing the relationship between the fn2. Figure 1 was obtained by the following method.
[0028]
　Steel a ~ m having the chemical compositions shown in Table 1 was vacuum melted to produce an ingot of 50 kg.
[0029]
[Table 1]

[0030]
　After heating the produced ingots 1200 ~ 1300 ° C., hot forging assuming hot rolling was performed to produce a round bar with a diameter of 15 mm. A round bar after hot forging and allowed to cool in the air. Subsequently, with respect to round bar, to implement the assumed quenching and tempering heat treatment after bolt molded to adjust the tensile strength of the rod to about 1200 MPa. Implemented machining against a round rod tensile strength is adjusted to produce a test piece with an annular V-notch shown in FIG. Numbers units are not shown in Figure 2, corresponding parts of the dimensions of the test piece (in mm) it shows a. "Φ Numbers" in the figure indicate the diameter of a portion that is specified (mm). "60 °" indicates that V-notch angle is 60 °. "0.175R" indicates that V-notch bottom radius of 0.175 mm.
[0031]
　Using an electrolytic charging method, hydrogen was introduced at various concentrations into the specimen of each steel a ~ m. Electrolytic charge method was performed as follows. It was immersed specimen into ammonium thiocyanate solution. While immersed specimens, taken hydrogen to the test piece in by generating an anode potential on the surface of the test piece. Thereafter, the galvanized coating is formed on the surface of each specimen, to prevent dissipation of hydrogen in the specimen. Subsequently, it was performed the constant load test to load the constant load as the tensile stress of the nominal stress 1080MPa is loaded against V-notch section of the test piece. Broken specimen during testing and for breaking that did not test piece was conducted heating analysis method using a gas chromatograph, to measure the amount of hydrogen in the test piece. After the measurement, each steel, and the maximum amount of hydrogen was not broken specimens is defined as the critical diffusible hydrogen amount Hc.
[0032]
　Further, JIS G4053 (2008) based on the critical diffusible hydrogen amount Href steel m having a chemical composition corresponding to SCM435 of, the critical diffusible hydrogen amount ratio HR (hereinafter simply referred to as HR) equation of the following (A) in the definition.
　HR = Hc / Href (A)
[0033]
　HR is an indication of resistance to hydrogen embrittlement. Based on the obtained HR and the fn2 of each steel, was prepared to FIG.
[0034]
　Referring to FIG. 1, as fn2 increases, that is, the larger the ratio of the Mn content of the Si content, HR is significantly enhanced. And, if the fn2 1.0 or more, HR becomes 1.20 or more, and obtained excellent resistance to hydrogen embrittlement. In the case of fn2 1.0 or more, even if the increase is fn2, HR does not change so much.
[0035]
　High-strength bolt according to the present invention has been completed based on the above findings, by mass%, C: 0.22 ~ 0.40% , Si: 0.10 ~ 1.50%, Mn: 0.20 ~ 0. less than 40%, P: 0.020% or less, S: 0.020% or less, Cr: 0.70 ~ 1.45%, Al: 0.005 ~ 0.060%, Ti: 0.010 ~ 0. 045%, B: 0.0003 ~ 0.0040 %, N: 0.0015 ~ 0.0080%, O: 0.0020% or less, Cu: 0 ~ 0.50%, Ni: 0 ~ 0.30% , Mo: 0 ~ 0.04%, V: 0 ~ 0.05%, and, Nb: 0 contained to 0.050% the balance being Fe and impurities, formula (1) and (2) It has a chemical composition satisfying, has a tensile strength of 1000 ~ 1300 MPa.
　0.50 ≦ C + Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85
　(1) Si / Mn> 1.0 (2)
　wherein each element symbol of the formula (1) and (2), the corresponding element the content (mass%) is substituted.
[0036]
　High-strength bolt according to the invention further contains, by mass%, Cu: 0.02 ~ 0.50%, Ni: 0.03 ~ 0.30%, Mo: 0.01 ~ 0.04%, and, V: it may contain one or more members selected from the group consisting of 0.005 to 0.05%.
[0037]
　High-strength bolt according to the invention further, Nb: 0.0015 may contain less to 0.050 mass%.
[0038]
　Preferably, the thread root of the high-strength bolts, the absolute value of the compressive residual stress in the surface layer of the thread root is 10 to 90% of the tensile strength of high strength bolts. In this case, further increases the resistance to hydrogen embrittlement.
[0039]
　Described in detail below high-strength bolt according to the present invention. "%" Related elements, unless otherwise specified, it means mass%.
[0040]
　[Chemical composition]
　chemical composition of the high strength bolt of the present embodiment contains the following elements.
[0041]
　C: 0.22 ~ 0.40%
　carbon (C) increases the hardenability of the bolt, increasing the tensile strength of the bolt after quenching and tempering above 1000 MPa. If C content is less than 0.22%, not the effect. On the other hand, C content if it exceeds 0.40% hardenability becomes too high. In this case, the strength of the bolt steel material after the hot working is too high, the cold workability is deteriorated. Therefore, drawing, and, with respect to the steel before carrying out the cold working such as cold forging, a softening must be performed multiple times for a long time heat treatment for the purpose of manufacturing cost increases. Further, when carrying out the heat treatment, hydrogen embrittlement resistance is deteriorated. Therefore, C content is 0.22 to 0.40%. The preferable lower limit of C content is 0.24%, more preferably 0.26%. The preferable upper limit of C content is 0.38%, more preferably 0.35%.
[0042]
　Si: 0.10 ~ 1.50%
　silicon (Si) is to suppress the precipitation of cementite increases the temper softening resistance. Si further deoxidizing steel. MnO-SiO deoxidation products 2 are inclusions soft vitrification, it is miniaturized be stretched and separated during hot rolling. Therefore, it increases the resistance to hydrogen embrittlement. If Si content is less than 0.10%, the above effect can not be obtained. On the other hand, S content if it exceeds 1.50%, the strength becomes too high. In this case, it decreases the ductility and cold forgeability of the steel. Therefore, Si content is 0.10 to 1.50%. A preferable lower limit of Si content is 0.35 percent, more preferably 0.40%, more preferably from 0.45%, more preferably 0.50 percent. The preferable upper limit of the Si content is 1.20%, more preferably 1.00%.
[0043]
　Mn: less than 0.20 to 0.40 percent
　manganese (Mn), the tensile strength of the bolt increases the hardenability and more 1000 MPa. Mn is further bonded to Si inclusions (MnO-SiO 2 to form a). The inclusions are soft, because it is stretched and broken into during hot rolling is miniaturized, MnO-SiO 2 reduces the density of, increases resistance to hydrogen embrittlement resistance. If Mn content is less than 0.20%, this effect can not be obtained. On the other hand, Mn content is equal 0.40% or more, segregated in the grain boundary to promote grain boundary fracture. Therefore, Mn content is less than 0.20 to .40%. The preferable lower limit of the Mn content is 0.22%, more preferably 0.25%. The preferable upper limit of the Mn content is 0.38%, more preferably 0.35%.
[0044]
　P: 0.020% or less
　phosphorus (P) is an impurity. P is segregated in the grain boundary to lower the cold workability, it decreases the hydrogen embrittlement resistance of the bolt. Accordingly, P content is 0.020% or less. The preferable upper limit of the P content is 0.015%. P content is preferably as small as possible.
[0045]
　S: 0.020% or less
　sulfur (S) is an impurity. S decreases cold workability by forming a sulfide, reducing the hydrogen embrittlement resistance of the bolt. Thus, S content is 0.020% or less. The preferable upper limit of the S content is 0.010%, more preferably 0.008%. S content is preferably as small as possible.
[0046]
　Cr: 0.70 ~ 1.45%
　Chromium (Cr), the tensile strength of the bolt increases the hardenability and more 1000 MPa. Cr further enhances hydrogen embrittlement resistance of the bolt. If Cr content is less than 0.70%, these effects can not be obtained. On the other hand, Cr content if it exceeds 1.45% excessively high hardenability, cold workability of the bolt steel material is reduced. Therefore, Cr content is from 0.70 to 1.45%. A preferable lower limit of Cr content is 0.90%. The preferable upper limit of the Cr content is 1.20%.
[0047]
　Al: 0.005 ~ 0.060%
　of aluminum (Al) is deoxidized steel. If Al content is less than 0.005%, this effect can not be obtained. On the other hand, Al content if it exceeds 0.060% cold workability coarse oxides are generated based inclusions is reduced. Therefore, Al content is 0.005 to 0.060%. A preferable lower limit of Al content is 0.010%. The preferable upper limit of Al content is 0.055%. In the chemical composition of the high-strength bolt according to the present invention, Al content is meant the total amount of Al contained in the steel material.
[0048]
　Ti: 0.010 ~ 0.045%
　titanium (Ti) is formed nitrides by combining with N in the steel to (TiN). The generation of TiN, generation of BN is suppressed, solid solution B content is increased. As a result, it increases the hardenability of steel. Ti further refining the crystal grains bonded with C to form a carbide (TiC). This increases resistance to hydrogen embrittlement of bolts. If Ti content is less than 0.010%, these effects can not be obtained. On the other hand, Ti content if it exceeds 0.045%, coarse TiN is a large amount of produce. In this case, cold workability and hydrogen embrittlement resistance is deteriorated. Therefore, Ti content is 0.010 to 0.045%. A preferable lower limit of the Ti content is 0.015%. The preferable upper limit of the Ti content is 0.040%.
[0049]
　B: 0.0003 ~ 0.0040%
　boron (B) increases the hardenability of steel. B further suppress the grain boundary segregation of P, enhances hydrogen embrittlement resistance of the bolt. If B content is less than 0.0003%, these effects can not be obtained. On the other hand, B content if it exceeds 0.0040%, the effect of improving hardenability is saturated. Further, the cold workability is deteriorated coarse BN is generated. Therefore, B content is 0.0003 to 0.0040 percent. The preferable lower limit of the B content is 0.0005%. The preferable upper limit of the B content is 0.0025%.
[0050]
　N: 0.0015 ~ 0.0080%
　nitrogen (N) is combined with Ti in the steel to produce a nitride, refining the crystal grains. If N content is less than 0.0015%, this effect can not be obtained. On the other hand, N content if it exceeds 0.0080%, the effect is saturated. Further, N is bonded to B to produce a nitride, to reduce the solid solution B content. In this case, hardenability of the steel is lowered. Therefore, N content is from 0.0015 to 0.0080%. The preferable lower limit of the N content is 0.0020%. The preferable upper limit of the N content is 0.0070%.
[0051]
　O: 0.0020% or less
　oxygen (O) is an impurity. O decreases the cold workability by forming an oxide. If O content exceeds 0.0020%, the oxides are produced in large quantities, MnS is coarse, cold workability is significantly decreased. Therefore, O content is 0.0020%. The preferable upper limit of the O content is 0.0018%. O content is preferably as small as possible.
[0052]
　The remainder of the chemical composition of the high-strength bolt according to the invention consists of Fe and impurities. Here, the impurities, in producing the high strength bolt industrially, ore as a raw material, there is to be mixed etc. Scrap or manufacturing environment, is allowed to the extent that the present invention does not adversely affect means shall.
[0053]
　[For any element]
　above high strength bolts further, in place of part of Fe, Cu, Ni, may contain one or more selected from the group consisting of Mo, and V. All of these elements are also optional element, enhances the hardenability of steel.
[0054]
　Cu: 0 ~ 0.50%
　copper (Cu) is an optional element and may not be contained. If contained, Cu increases the hardenability of steel. However if Cu content exceeds 0.50%, the cold workability too high hardenability decreases. Therefore, Cu content is 0 to 0.50%. The preferable lower limit of the Cu content to obtain the above effect more effectively is 0.02%, more preferably 0.05%. The preferable upper limit of Cu content is 0.30%, more preferably 0.20%.
[0055]
　Ni: 0 ~ 0.30%
　nickel (Ni) is an optional element and may not be contained. If contained, Ni increases the hardenability of steel, further enhances the toughness of the steel after quenching. However, if it exceeds 0.30% of Ni content, the cold workability too high hardenability decreases. Therefore, Ni content is from 0 to 0.30%. A preferable lower limit of the Ni content in order to obtain the above effect more effectively is 0.03%, more preferably 0.05%. The preferable upper limit of the Ni content is 0.20%, more preferably from 0.10%.
[0056]
　Mo: 0 ~ 0.04%
　of molybdenum (Mo) is an optional element and may not be contained. If contained, Mo enhances the hardenability of steel. However, Mo content if it exceeds 0.04%, too high hardenability, cold workability of high strength bolts for steel is lowered. Therefore, Mo content is 0 to 0.04%. A preferable lower limit of Mo content in order to obtain the above effect more effectively is 0.01%, more preferably 0.015%. The preferable upper limit of the Mo content is 0.03%, more preferably 0.025%.
[0057]
　V: 0 ~ 0.05%
　vanadium (V) are optional elements may not be contained. If contained, V is increasing the hardenability of the steel. V further carbides, refining the crystal grains to form a nitride or carbo-nitrides. However, V content if it exceeds 0.05%, carbides lowers the cold workability coarsened. Therefore, V content is 0 to 0.05%. The preferable lower limit of V content to obtain the above effect more effectively is 0.005%. The preferable upper limit of the V content is 0.03%, more preferably 0.02%.
[0058]
　Chemical composition of the high-strength bolt according to the present invention further includes, in place of part of Fe, and may contain Nb.
[0059]
　Nb: 0 ~ 0.050%
　niobium (Nb) is an optional element and may not be contained. If contained, Nb combines with C and N, carbide, to form a nitride or carbo-nitrides, refining the crystal grains. Nb further enhances hydrogen embrittlement resistance of the bolt. However, if it exceeds 0.050% by Nb content, and generate coarse carbides lowers the cold workability of the steel. Therefore, Nb content is 0 to 0.050%. The preferable lower limit of Nb content to obtain the above effect more effectively is 0.0015%. The preferable upper limit of Nb content is 0.040%, more preferably 0.030%.
[0060]
　[Equation (1)]
　the chemical composition of the high-strength bolt according to the present invention further satisfies Expression (1).
　0.50 ≦ C + Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85 (1)
　The formula (1) each element symbol in the content of the corresponding element (mass%) is substituted. If the corresponding element is an impurity level, the corresponding element symbol of the formula (1) is assigned a "0".
[0061]
　fn1 = C + Si / 10 + Mn / 5 + 5Cr / 22 is indicative of the strength and cold workability of the steel. fn1 denotes the carbon equivalent of the steel. If fn1 is too low, sufficient hardenability can not be obtained, not strength. On the other hand, if fn1 is too high, hardenability is too high. In this case, when the high-strength bolts for steel is rolled into wire rod, bainite is generated, the strength and hardness of the steel material becomes too high. Therefore, drawing process of the next step and, prior to cold forging process, to be performed multiple times for a long period of time softening heat treatment, sufficient cold workability can not be obtained. If fn1 is 0.50 to 0.85 while obtaining excellent hardenability, without carrying out long softening heat treatment, sufficient cold workability are obtained. A preferred lower limit of the fn1 is 0.53. A preferred upper limit of the fn1 is 0.83.
[0062]
　[Equation (2)]
　the chemical composition of the high-strength bolt according to the present invention further satisfies the formula (2).
　Si / Mn> 1.0 (2)
　wherein each element symbol in the formula (2), the content of the corresponding element (mass%) is substituted. If the corresponding element is an impurity level, the corresponding element symbol of the formula (2) is assigned a "0".
[0063]
　fn2 = is defined as Si / Mn. Si and Mn, MnO-SiO in the process of deoxidation 2 generates a. MnO-SiO 2 is a melting point of about 1250 ° C.. Therefore, in the molten metal prior to solidification is a liquid, it is solid at slab in after solidification, the inclusions soft vitrified. The inclusions are fine be stretched and separated during hot rolling. Therefore, fatigue strength is improved, thereby improving the hydrogen embrittlement resistance. Fine MnO-SiO 2 in order to obtain, it is necessary to properly control the ratio Mn of Si. This index is fn2.
[0064]
　Referring to FIG. 1, as fn2 increases, HR is significantly enhanced. If fn2 exceeds 1.0, HR becomes 1.2 or higher, obtained excellent hydrogen embrittlement resistance. In the case of fn2 1.0 or more, even if the increase is fn2, HR is not significantly increased. In other words, HR for fn2 has an inflection point in the fn2 = 1.0 near. Therefore, as shown in equation (2), a fn2> 1.0. A preferred lower limit of the fn2 is 1.1.
[0065]
　[For compressive residual stress in the thread root]
　Preferably, the thread root of the high-strength bolt according to the present invention, the absolute value of the compressive residual stress in the surface layer of the thread root is 10 to 90% of the tensile strength of high strength bolts.
[0066]
　In this case, the tensile stress exerted on the thread root at the time of bolt fastening is canceled out with the compressive residual stress. Therefore, the stress state of the origin section is relaxed, breaking is less likely to occur due to hydrogen embrittlement. If less than 10% of the compressive residual stress (absolute value of) the tensile strength (absolute value of) the tensile by compressive residual stress stress cancellation effect becomes insufficient, not obtained excellent hydrogen embrittlement resistance. On the other hand, if it exceeds 90% of the compressive residual stress is tensile strength (absolute value of) (absolute value of) the effect is saturated. Thus, compressive residual stress is 10 to 90% of the tensile strength.
[0067]
　Here, the "surface layer" refers to the range of up to 50μm depth toward the central axis from the surface of the high-strength bolts. Compressive residual stress is measured by a known X-ray method. Specifically, in conformity with JIS B2711 (2013), using X-ray stress measuring method using X-ray diffraction. Measurements characteristic X-ray Type: MnKarufa line, Cr filter, the reference diffraction angle 2 [Theta] 0 : 152.0 °, eta angle 14.0 °, X-ray stress constant K: carried out using -336MPa / °. The measurement site is centered on the center position of the thread root. The tensile strength is determined in accordance with the JIS Z2241 (2011).
[0068]
　[Production Method]
　An example of a method of manufacturing the high-strength bolt according to the present invention will be described. First, the production of high strength bolts for steel by a known production method (material production step). Then, using the high-strength bolts for steel for producing high strength bolts (high strength bolts manufacturing process). Hereinafter, the respective steps will be described.
[0069]
　[Material manufacturing process]
　to produce a molten steel having the chemical composition described above. Producing slab by continuous casting method using a molten steel. Or, to produce an ingot by ingot method using the molten steel. And slabbing cast slab or ingot manufactured to billet. The billet was hot working, a high strength bolt steel material (wire rod). Hot working example, a hot rolling.
[0070]
　High strength bolts manufacturing process]
　In the high-strength bolts manufacturing process, the production of high strength bolts with high strength bolts for steel. High strength bolts manufacturing process includes wire drawing step, the cold forging process, threading step, and the hardening and tempering processes. The following describes each step.
[0071]
　[Drawing process]
　First, by performing the wire drawing against the wire to produce a steel wire. Wire drawing, may be only a primary wire drawing, secondary wire drawing, etc., may be performed a plurality of times of drawing. During wire drawing, to form a lubricating coating on the surface of the wire. Lubricating coating, for example, a phosphate coating or a non-phosphorus-based lubricating coating.
[0072]
　Preferably, a lubricating coating which does not contain P. Or, if using a phosphate coating, in quenching process prior to the later, the steel (steel wire) surface cleaning or pickled to remove the phosphate coating from the surface. Cleaning is a well-known alkali cleaning, for example.
[0073]
　[Cold forging step]
　The steel after drawing was cut into a predetermined length to produce a high-strength bolts by implementing cold forging against cut steel.
[0074]
　About Softening Heat Treatment]
　In the manufacturing method of the conventional high strength bolts, for the purpose of softening the bolt steel material strength is too high (wire) before prior to wire drawing and cold forging, and a softening heat treatment was performed multiple times there. However, a high-strength bolt according to the present invention, by satisfying the equation (1), to simplify such softening heat treatment. This can suppress an increase in manufacturing cost due to the implementation of the softening heat treatment, further, it is possible to improve the hydrogen embrittlement resistance of high strength bolts.
[0075]
　[Screw processing step]
　for high strength bolts produced by cold forging, to implement the rolling process at conditions well known to form threads.
[0076]
　[Quenching and tempering process]
　for high strength bolts after threading, to implement the quenching and tempering in a known condition, adjusting the tensile strength of high strength bolts 1000 ~ 1300 MPa. The tensile strength is less than 1000MPa, the strength of the bolt is insufficient. On the other hand, the tensile strengths exceed the 1300 MPa, increased hydrogen sensitivity, hydrogen embrittlement resistance is deteriorated. Therefore, the tensile strength of the high-strength bolts is 1000 ~ 1300MPa. When utilizing a lubricating coating containing P typified by phosphate coating during drawing process, as described above, preferably, before the hardening, to alkali cleaning of the surface of the steel material (steel wire).
[0077]
　[Compressive residual stress application step]
　Preferably, to implement the known residual compressive stress applying step for high strength bolts after quenching and tempering, 10 surface layer of compressive residual stress in the thread root of the tensile strength of high strength bolts to ~ 90%. Known residual compressive stress applying step, for example, a shot peening process. By appropriately adjusting the conditions of the shot peening process, it is possible to the surface layer of compressive residual stress in the thread root 10 to 90% of the tensile strength of high strength bolts.
[0078]
　In the above-described manufacturing method, it has been carried out the threading process (pre-rolling step) before quenching and tempering, in place of the previous rolling step may be carried out threading process (post-rolling step) after quenching and tempering . In this case, the surface layer of the thread root, it is possible to impart a tensile strength of 10-90% of the compressive residual stress of the high-strength bolts. For post-rolling step may not be carried out shot peening.
[0079]
　With the above-described manufacturing process, a high-strength bolt of the present invention is produced.
Example
[0080]
　The chemical composition of Table 2 were prepared molten steel having.
[0081]
[Table 2]

[0082]
　Referring to Table 2, as described above, the steel L had a chemical composition corresponding to SCM435 of JIS G4053 (2008).
[0083]
　Cross section by continuous casting method using the molten steel to produce a billet of 162 mm × 162 mm. Billet hot working (hot rolling) and to produce a wire having a diameter of 11.5 mm.
[0084]
　Against the wire of each test number shown in Table 3 were prepared steel wire was performed wire drawing. In this case and heat-treated for the purpose of softening. The heat treatment temperature is 750 ° C., the heat treatment time is 60 minutes, after heat treatment was gradually cooled. Furthermore, after degreasing and pickling, zinc phosphate treatment (75 ° C., soaking time 600 sec), and a metal soap treatment (80 ° C., soaking time 180 sec) were carried out, from the zinc phosphate coating and metal soap film on the surface lubricated film made was formed. Then perform finishing wire drawing, to produce a steel wire having a diameter of 10.5 mm. This was a steel wire as a material for high-strength bolts forged.
[0085]
[table 3]

[0086]
　And producing a high strength bolt shown in Figure 3 by carrying out cold forging against steel wire of each test number. Specifically, cold forging was carried out in two steps. In one step eyes were molded push the shaft portion of the high-strength bolts. The second step first, to design a mold to perform a process of molding the head and the flange portion of the high-strength bolts, and attached to a hydraulic forging press machine and subjected to cold forging. Each number in the figure shows the dimensions of the corresponding site (mm). "Φ Numbers" in the figure indicate the diameter of a portion that is specified (mm). "Numerical °" in the figure indicates the angle (°) of the site that is specified. "R Numeric" indicates the radius of curvature of the part that is specified (mm). "M7 × 1.0" in the figure shows that the outer diameter of 7 mm, a pitch of 1.0 mm.
[0087]
　After forming the high strength bolts, it was investigated whether the occurrence of cracks was observed high strength bolts visually. Which cracks were observed was a bolt molding impossible.
[0088]
　For high strength bolts of cracks were observed test No. was carried out hardening and tempering at a temperature shown in Table 3. Before carrying out the quenching process, to remove the phosphate coating of high strength bolts surface by alkali washing.
[0089]
　In hardening process, after holding for 40 minutes at a quenching temperature shown in Table 3 (° C.), and oil cooling. Tempering treatment was held 70 minutes at the tempering temperature shown in Table 3. Through the above process, it was prepared high strength bolts. Note that if the tempering temperature for obtaining the desired bolt tensile strength (1000 ~ 1300 MPa) is less than 435 ° C., it is determined that insufficient strength, hydrogen embrittlement resistance evaluation was not conducted, the subject of the present invention it is determined that the outside.
[0090]
　Test Nos. 1 to 6, 9, 10, and relative to steel wire 12-14 is subjected to a rolling after quenching and tempering treatment to impart residual stress on the surface of the thread root with threading. For steel wire Test No. 13 and Test No. 14 was subjected to rolling before hardening and tempering. The surface layer of compressive residual stress in the thread bottom, in conformity with JIS B2711 (2013), was measured using an X-ray stress measuring method using X-ray diffraction. Measurements characteristic X-ray Type: MnKarufa line, Cr filter, the reference diffraction angle 2 [Theta] 0 : 152.0 °, eta angle 14.0 °, X-ray stress constant K: -336MPa / ° was performed using. The measurement site is centered on the center position of the thread root.
[0091]
　[Tensile
　Test] in compliance with JIS B1051 (2000), at room temperature (25 ° C.), measured hardening and tempering treatment of each test number in the atmosphere, or rolling the high strength bolt of the tensile strength after processing (MPa) did. The measurement results are shown in Table 3.
[0092]
　[Hydrogen embrittlement resistance evaluation test]
　hardening and tempering of the test numbers, or for rolling high strength bolts after processing, to produce a circular V-notch test piece shown in FIG. 2, by using the electrolytic charging method , hydrogen was introduced at various concentrations. Electrolytic charge method was performed as follows. It was immersed high strength bolts in ammonium thiocyanate solution. While immersed high strength bolts, incorporating hydrogen into high strength in volts by generating an anode potential on the surface of the high-strength bolts.
[0093]
　After introducing hydrogen into high strength in volts, to form a galvanized coating with high strength bolts surface to prevent dissipation of the hydrogen of the high strength bolts in. Subsequently, it was performed the constant load test loaded with 95% of the tensile strength of the tensile strength of high strength bolts. High strength bolts were broken during the test, and for high strength bolts have not broken, to implement a Atsushi Nobori analysis method using a gas chromatograph, to measure the hydrogen content of the high-strength bolts in. After the measurement, in each test number and the maximum amount of hydrogen was not broken specimens is defined as the critical diffusible hydrogen amount Hc.
[0094]
　Furthermore, the critical diffusable hydrogen amount of steel wire test No. 14 with a chemical composition corresponding to SCM435 in JIS standard used in conventional bolt, and a critical diffusable hydrogen amount ratio HR of reference (Href). Based on the critical diffusible hydrogen amount Href, it was determined critical diffusible hydrogen amount ratio HR by using the formula (A). As the hydrogen embrittlement resistance evaluation, HR is as acceptable less than 1.2 ( "○" in Table 3) was judged as unsatisfactory ones of less than 1.2 ( "×" in Table 3).
[0095]
　[Test Results]
　The test results are shown in Table 3.
[0096]
　The chemical composition of the high strength bolt of Test No. 1-6 was appropriate. Furthermore, fn1 satisfies Equation (1), fn2 satisfied the equation (2). The absolute value of the compressive residual stress in the thread root surfaces of the high strength bolt satisfies 10-90% of the tensile strength of high strength bolts. As a result, high strength bolts of these test numbers, despite a tensile strength of high strength and 1000 ~ 1300 MPa, the critical diffusible hydrogen amount ratio HR becomes 1.20 or more, excellent hydrogen embrittlement resistance It was.
[0097]
　The chemical composition of the high strength bolt of Test No. 7 and Test No. 8 was appropriate. Furthermore, fn1 satisfies Equation (1), fn2 satisfied the equation (2). As a result, high strength bolts of these test numbers, despite a tensile strength of high strength and 1200 MPa, the critical diffusible hydrogen amount ratio HR becomes 1.20 or more, excellent hydrogen embrittlement resistance. However, since the absolute value of the compressive residual stress in the thread root surfaces of the high-strength bolts is less than 10% of the tensile strength of the high-strength bolts, HR was lower than in Test No. 1-6.
[0098]
　On the other hand, Mn content of Test No. 9 was too high. Therefore, HR is as low as less than 1.20, had lower hydrogen embrittlement resistance.
[0099]
　The high strength bolt of the test number 10, fn1 is less than the lower limit of the formula (1). Therefore the tensile strength was less than 1000MPa.
[0100]
　The high strength bolt of the test number 11, fn1 exceeds the upper limit of the formula (1). Therefore, high-strength bolts for steel (wire rod) cold workability is low, and for cracking the high-strength bolt after cold forging is observed, the subsequent processing and testing was carried out.
[0101]
　In Test No. 12 and Test No. 13, fn2 did not meet Equation (2). Therefore, HR is less than 1.20, had lower hydrogen embrittlement resistance.
[0102]
　The high strength bolt of test No. 15, Ti content was too high. Therefore, high-strength bolts for steel (wire rod) cold workability is low, and for cracking the high-strength bolt after cold forging is observed, the subsequent processing and testing was carried out.
[0103]
　It has been described an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by changing the above-described embodiments without departing from the scope and spirit thereof as appropriate.

claims

By mass%,
　C: 0.22 ~ 0.40%,
　Si: 0.10 ~ 1.50%, Mn: less than ~ 0.40%
　0.20, P: 0.020% or
　less, S: 0. 020% or
　less,
　Cr:
　0.70 ~ 1.45%, Al: 0.005 ~
　0.060%, Ti: 0.010 ~ 0.045%, B: 0.0003 ~ 0.0040%,
　N:
　~ 0.0080% 0.0015, O: 0.0020% or
　less,
　Cu: 0 ~
　0.50%, Ni: 0 ~ 0.30%, Mo: 0 ~
　0.04%, V: 0 ~ 0. 0.05%,
　and, Nb: 0 contained to 0.050% the balance being Fe and impurities, having a chemical composition satisfying the formula (1) and
　(2), having a tensile strength of 1000 ~ 1300 MPa , high-strength bolts.
　0.50 ≦ C + Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85
　(1) Si / Mn> 1.0 (2)
　wherein each element symbol of the formula (1) and (2), the corresponding element the content (mass%) is substituted.
[Requested item 2]
　A high strength bolt according to claim 1, in
　mass%,
　Cu: 0.02 ~ 0.50%,
　Ni: 0.03 ~ 0.30%, Mo: 0.01 ~ 0.04%,
　and, V: containing one or more members selected from the group consisting of 0.005 to 0.05%, high strength bolts.
[Requested item 3]
　A high strength bolt according to claim 1 or claim 2, in
　mass% Nb: 0.0015 containing 0.050% high strength bolts.
[Requested item 4]
　A high strength bolt according to any one of claims 1 to 3, wherein the high in thread root of the strength bolts 10 to the tensile strength of the absolute value of the high strength bolt of the surface layer of compressive residual stress in the thread root 90%, high strength bolts.