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, has high hardenability. Therefore, if these alloy steels were produced hot-rolled to wire rods, hard tissue of bainite or the like is formed. Since the wire is stiff including hard tissue, hardly forged wire drawing and cold. Therefore, when forming the bolt with a wire of alloy steel, usually, before the wire drawing and cold forging, performing multiple rounds of softening heat treatment. A plurality of softening heat treatment, raising the manufacturing cost of the bolt. Thus, while suppressing the manufacturing cost, a bolt that can achieve high strength and excellent hydrogen embrittlement resistance is required.
[0005]
　To suppress the formation of bainite at the time of wire production, it may be reduced alloy elements such as Mo and V in the steel. In this case, since the formation of bainite is suppressed, softening heat treatment can be omitted or simplified. However, it is difficult to bolt to a high strength, further, is also reduced hydrogen embrittlement resistance.
[0006]
　Bolt having high strength, for example, it has been proposed in a plurality of the following patent documents. It has been bolt proposed in these patent documents, to enhance the hardenability by containing boron, to strengthen grain boundaries, to enhance the strength.
[0007]
　Specifically, by bolts disclosed in JP-A 10-53834 (Patent Document 1), B mass%: 0.0008 ~ 0.004%, C : 0.4% or less, Ti: 0. 025 ~ 0.06%, N: containing 0.006% or less. In this bolt, 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. Furthermore, it is the austenite grain size number is 5 or more. Accordingly, tensile strength 785N / mm 2 exceeding, that described in Patent Document 1.
[0008]
　However, the bolt in Patent Document 1, a high Mn content, when Cr content is low, there are cases hydrogen embrittlement resistance is low.
[0009]
　Been bolt disclosed in Kohyo 2009-521600 (Patent Document 2), in weight percent, carbon 0.35 to 0.55%, silicon 0.05 to 2.0% manganese 0.1 to 0.8% boron 0.001 to 0.004%, chromium 0.3 to 1.5% total oxygen (T.O) is 0.005% or less, phosphorus of 0.015% or less, sulfur containing 0.010% or less, more vanadium 0.05 to 0.5% niobium 0.05 to 0.5% nickel 0.1 to 0.5% molybdenum 0.1 to 1.5% and titanium contains at least one element selected from the group consisting 0.01 - 0.1 percent, with the balance consisting of Fe and impurities. The bolt has an internal structure made of ferrite and tempered martensite, in the internal organization, the content of ferrite is 3 to 10% by area ratio. The bolt has excellent resistance to delayed and Re realizing fracture characteristics and high strength, and are described in the patent document 2.
[0010]
　However, the bolt proposed in Patent Document 2 is a composite structure steel with the area ratio as an internal organization of the bolt 3-10% of soft ferrite and tempered martensite. Therefore, as compared with the case of steel having a structure of tempered martensite single phase, the bolt strength tends to decrease. Therefore, in order to adjust the desired intensity level as compared with the steel of tempered martensite single phase, the more necessary tempering treatment at a low temperature. As a result, the resistance to hydrogen embrittlement in the desired strength is decreased. Further, in the manufacturing process, re-hardening and tempering, etc., it is adjustment processing ferrite structure is required. Therefore, manufacturing cost is increased.
[0011]
　JP 2008-156678 discloses a high-strength bolts disclosed in (Patent Document 3), in mass%, C: 0.15% than 0.30% or less, Si: 1.0% or less, Mn: 1. 5%, Ti: 0.1% or less, Mo: 0.3% to 0.5% or less, B: 0.0005% to 0.01% or less, the steel balance being Fe and impurities after quenching, to implement the tempering at 100 ~ 400 ° C., an average prior austenite grain size after quenching is to 10μm following steel structure. Thus, high-strength bolts bolt strength range with excellent delayed fracture resistance and corrosion resistance of about 1200 ~ 1800 MPa is obtained, and is described in Patent Document 3.
[0012]
　However, in the bolt Patent Document 3, since Mo is 0.3 to 0.5% is 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 hydrogen embrittlement resistance is low.
[0013]
　JP 2012-162798 discloses a high-strength bolt steel disclosed in (Patent Document 4), in 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 ~ a 0.010% respectively contain, Cu, 0.10 ~ 3.0% in total of one or more selected from the group consisting of Ni and Cr contained, the balance being iron and unavoidable impurities. This steel In addition, the ratio of the Si content [Si] and the C content [C] ([Si] / [C]) is 1.0 or more. Thus, without addition of a large amount of expensive alloy elements such as Cr and Mo, a high strength of at least 1100MPa delayed excellent boron-added high-strength bolts fracture resistance can be obtained even with the patent document 4 It has been described.
[0014]
　However, Patent Document 4, a high Ni content. Therefore, there is a case in which hardenability becomes too high. 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 hydrogen embrittlement resistance is low.
[0015]
　JP-11-92868 (Patent Document 5) discloses a cold forging steel, by mass%, C: 0.10 ~ 0.40% , Si: 0.15% or less, Mn: 0. 30 ~ 1.00%, Cr: 0.50 ~ 1.20%, B: 0.0003 ~ 0.0050%, Ti: containing 0.020 ~ 0.100%, P: 0.015 % or less (including 0%), S: 0.015% or less (including 0%), N: respectively limited to 0.0100% or less (including 0%), the balance being Fe and unavoidable impurities. Furthermore, the total number of one or two particles of diameter 0.2μm or less of TiC in a matrix of the steel, Ti (CN) is 20/100 [mu] m 2 is at least. This allows improving the delayed fracture resistance by preventing coarsening of crystal grains, and are described in the patent document 5.
[0016]
　However, Patent Document 5 not technology specific to the bolt, when manufactured bolts, in some cases hydrogen embrittlement resistance is low.
CITATION
Patent Literature
[0017]
Patent Document 1: JP-A-10-53834 Publication
Patent Document 2: JP-T 2009-521600 Patent Publication
Patent Document 3: JP 2008-156678 Patent Publication
Patent Document 4: JP 2012-162798 Patent Publication
Patent Document 5: JP 11-92868 JP
Summary of the Invention
[0018]
　An object of the present invention has a high strength, and is to provide a bolt having excellent hydrogen embrittlement resistance.
[0019]
　Bolt according to an embodiment of the present invention, in mass%, C: 0.32 ~ 0.39% , Si: 0.15% or less, Mn: 0.40 ~ 0.65%, P: 0.020% or less , S: 0.020% or less, Cr: 0.85 ~ 1.25%, Al: 0.005 ~ 0.060%, Ti: 0.010 ~ 0.050%, B: 0.0010 ~ 0. 0030%, N: 0.0015 ~ 0.0080 %, O: 0.0015% or less, Mo: 0 ~ 0.05%, V: 0 ~ 0.05%, and, Cu: 0 ~ 0.50% , Ni: 0 - 0.30% and, Nb: 0 contains ~ 0.05%, the balance being Fe and impurities, having a chemical composition satisfying the formula (1) and (2), 1000 has a tensile strength of ~ 1300 MPa, satisfying the equation (3).
　≦ 10C + 4.9 Si + 2Mn + Cr + 4Mo + 5V ≦ 6.1
　(1) Mn / Cr ≦ 0.55 (2)
　[solid solution Cr] /Cr≧0.70 (3)
　, where each element of the formula (1) to (3) the symbols, 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) and (2) is assigned a "0". The [solute Cr] of formula (3), dissolved Cr content in the bolt (mass%) is substituted.
Effect of the invention
[0020]
　Bolt according to an embodiment of the present invention has a high strength, and has excellent hydrogen embrittlement resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
[1] Figure 1 is a diagram illustrating the critical diffusable hydrogen amount, the relationship between the Mn / Cr in the bolt.
FIG. 2 is a side view of a test piece with an annular V-notch.
FIG. 3 is a side view and a front view of the screw manufactured in Example.
DESCRIPTION OF THE INVENTION
[0022]
　The present inventors have, Mo, not high content of expensive alloy elements V, etc., C, Mn, with the boron-containing steel containing Cr and B, etc., the tensile strength of the bolt, hydrogen embrittlement resistance conducted a survey examined the components and organization on. As a result, the present inventors have obtained the following findings.
[0023]
　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, the hardenability is too high, before the cold working such as forging between wire drawing and cold against steel wire or the like, a long time softening heat treatment for the purpose of softening of the steel material to a plurality of times There must be. In this case, Mo, even without containing a large amount of alloy elements V, etc., the production cost is high. Thus, even without implementing the long softening heat treatment it is capable of cold working, and steel having hardenability the tensile strength can be obtained is preferable.
[0024]
　If the bolt chemical composition satisfies the formula (1), obtained excellent cold workability and hardenability.
　4.9 ≦ 10C + Si + 2Mn + Cr + 4Mo + 5V ≦ 6.1 (1)
　where the element symbol in the formula (1), the content of the corresponding element (mass%) is substituted. For the Mo and V, if these elements are impurity levels, the corresponding element symbol in the formula (1) is assigned a "0".
[0025]
　fn1 = 10C + Si + 2Mn + Cr + 4Mo + 5V and defined. C, Si, Mn, Cr, Both Mo and V, which is an element to enhance hardenability. Therefore, fn1 is a hardenability and cold workability index.
[0026]
　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 is not obtained cold workability. fn1 is satisfies the equation (1), while obtaining excellent hardenability, be omitted softening heat treatment, or even without carrying out long softening heat treatment, sufficient cold workability are obtained.
[0027]
　[Water for hydrogen
　embrittlement] [Relationship between Mn / Cr 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.
　Mn / Cr ≦ 0.55 (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 = Mn / Cr. fn2 corresponds to the left-hand side of equation (2). The following describes the formula (2).
[0028]
　Figure 1 is a critical diffusable hydrogen amount is a diagram showing the relationship between the fn2. Figure 1 was obtained by the following method.
[0029]
　Steel A ~ M having the chemical compositions shown in Table 1 was vacuum melted to produce an ingot of 50 kg.
[0030]
[Table 1]

[0031]
　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.
[0032]
　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, hydrogen in the specimen is prevented from leaking to the outside. 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 (ppm).
[0033]
　Further, based on the critical diffusible hydrogen amount Href (ppm) of the steel M having a chemical composition corresponding to SCM435 of JIS G4053 (2008), the critical diffusible hydrogen amount ratio HR (hereinafter simply referred to as a ratio HR) to the next defined in formula (a).
　HR = Hc / Href (A)
[0034]
　The ratio HR is an indication of resistance to hydrogen embrittlement. Based on the obtained ratio and HR and fn2 of each steel, it was prepared to FIG.
[0035]
　Referring to FIG. 1, as fn2 is reduced, i.e., the larger the ratio of Mn content relative Cr content is small, the ratio HR is significantly enhanced. Then, if the fn2 is 0.55, the ratio HR is higher than 1.00, the resulting excellent hydrogen embrittlement resistance.
[0036]
　[Relationship between solute Cr and hydrogen embrittlement resistance]
　Furthermore the hydrogen embrittlement resistance, dissolved Cr content in the bolt affects. Bolt satisfies the equation (3), increases the resistance to hydrogen embrittlement.
　[Solute Cr] /Cr≧0.70 (3)
　where the [solute Cr] in the formula (3), dissolved Cr content in the bolt (mass%) is substituted, the Cr, Cr content in volts chemical composition (i.e., the total Cr content, the unit is mass%) is substituted.
[0037]
　The present inventors have found that a solid solution Cr was first found that increasing the strength against hydrogen embrittlement of tempered martensite. Bolt matrix tissue of the present embodiment is a tempered martensite single phase. Therefore, if dissolved Cr content in the bolt Takamare, since the Cr solid solution amount of the tempered martensite single phase increases, increasing resistance to hydrogen embrittlement of the bolt. fn3 = it is defined as [solid solution Cr] / Cr. If fn3 is 0.70 or more, since the solid solution amount of Cr to enhance the strength of the tempered martensite it is sufficient, resulting excellent hydrogen embrittlement resistance.
[0038]
　As is conventional, after a steel (e.g. wire) by hot working, by a plurality of times for a long period of time softening heat treatment, the bolt after manufacture, a large number of Cr carbonitrides are formed. In this case, dissolved Cr content in the bolt is reduced. Therefore, fn3 does not satisfy the equation (3).
[0039]
　Therefore, in this embodiment, with respect to the steel after hot working, no heat treatment is performed for the purpose of softening before drawing before and cold forging, or, even when carrying out the heat treatment, the steel the 700 ° C. or more holding time for is less than 40 minutes. In this case, if, even when the heat treatment is performed, generation of carbides containing Cr is suppressed. As a result, it is possible fn3 to maintain sufficient dissolved Cr amount enough to satisfy the expression (3).
[0040]
　The bolt according to the present embodiment has been completed based on the above findings, by mass%, C: 0.32 ~ 0.39% , Si: 0.15% or less, Mn: 0.40 ~ 0.65%, P 0.020% or less, S: 0.020% or less, Cr: 0.85 ~ 1.25%, Al: 0.005 ~ 0.060%, Ti: 0.010 ~ 0.050%, B: 0.0010 ~ 0.0030%, N: 0.0015 ~ 0.0080%, O: 0.0015% or less, Mo: 0 ~ 0.05%, V: 0 ~ 0.05%, and, Cu: 0 ~ 0.50%, Ni: 0 ~ 0.30%, and, Nb: 0 contains ~ 0.05%, the balance being Fe and impurities, satisfies the formula (1) and (2) chemically It has a composition having a tensile strength of 1000 ~ 1300 MPa, satisfying the equation (3).
　≦ 10C + 4.9 Si + 2Mn + Cr + 4Mo + 5V ≦ 6.1
　(1) Mn / Cr ≦ 0.55 (2)
　[solid solution Cr] /Cr≧0.70 (3)
　, where each element of the formula (1) to (3) the symbols, 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) and (2) is assigned a "0". The [solute Cr] of formula (3), dissolved Cr content in the bolt (mass%) is substituted.
[0041]
　The chemical composition, Mo: 0.01 ~ 0.05%, V: 0.005 ~ 0.05%, Cu: 0.03 ~ 0.50%, and, Ni: 0.03 ~ 0.30% it may contain one or more members selected from the group consisting of.
[0042]
　The chemical composition, Nb: 0.0015 may contain ~ 0.05%.
[0043]
　Preferably, the P content is set to Ps (mass%) in the surface layer from the surface of the bolt to 50μm depth, when the P content of the central axis of the bolt and the Pc (mass%), satisfies the equation (4).
　Ps / Pc ≦ 1.2 (4)
[0044]
　In this case, further increases the resistance to hydrogen embrittlement.
[0045]
　Described in detail below bolt according to the present embodiment. "%" Related elements, unless otherwise specified, it means mass%.
[0046]
　[Chemical composition]
　The chemical composition of the bolt of the present embodiment contains the following elements.
[0047]
　C: 0.32 ~ 0.39%
　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 0.32%, not the effect. On the other hand, if the C content is too high, it is too high hardenability. 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, has to be performed multiple times for a long period of time softening heat treatment for the purpose of softening, production cost increases . Further, when carrying out the softening heat treatment, hydrogen embrittlement resistance is deteriorated. Therefore, C content is from 0.32 to 0.39 percent. The preferable lower limit of C content is 0.33%. The preferable upper limit of C content is 0.38%.
[0048]
　Si: 0.15% or less
　silicon (Si), the deoxidizing steel. Si further enhance the strength of the bolt to increase the hardenability. However, Si content if it exceeds 0.15%, excessively high hardenability, cold workability of the steel is lowered. Therefore, Si content is 0.15% or less. A preferable lower limit of Si content is 0.01%, more preferably 0.02%, more preferably 0.05%. The preferable upper limit of the Si content is 0.12%, more preferably from 0.10%.
[0049]
　Mn: 0.40 ~ 0.65%
　manganese (Mn), the tensile strength of the bolt increases the hardenability and more 1000 MPa. If Mn content is less than 0.40%, this effect can not be obtained. On the other hand, Mn content if it exceeds 0.65%, excessively high hardenability, cold workability of the bolt steel material is reduced. Therefore, Mn content is 0.40 to 0.65%. The preferable lower limit of the Mn content is 0.45%. The preferable upper limit of the Mn content is 0.60%, more preferably 0.55%.
[0050]
　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.
[0051]
　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.
[0052]
　Cr: 0.85 ~ 1.25%
　Chromium (Cr), the tensile strength of the bolt increases the hardenability and more 1000 MPa. Cr further includes a solid solution in the tempered martensite in volts, enhances hydrogen embrittlement resistance of the bolt. If Cr content is less than 0.85%, these effects can not be obtained. On the other hand, Cr content if it exceeds 1.25%, excessively high hardenability, cold workability of the bolt steel material is reduced. Therefore, Cr content is from 0.85 to 1.25%. A preferable lower limit of Cr content is 0.90%. The upper limit of Cr content is 1.20%.
[0053]
　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 bolt according to the present invention, Al content is meant the total amount of Al contained in the steel material.
[0054]
　Ti: 0.010 ~ 0.050%
　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 0.050%, 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 0.050%. A preferable lower limit of the Ti content is 0.015%. The preferable upper limit of the Ti content is 0.045%.
[0055]
　B: 0.0010 ~ 0.0030%
　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 0.0010%, these effects can not be obtained. On the other hand, B content if it exceeds 0.0030%, the effect of improving hardenability is saturated. Further, the cold workability is deteriorated coarse BN is generated. Therefore, B content is 0.0010 to 0.0030 percent. The preferable lower limit of the B content is 0.0015%. The preferable upper limit of the B content is 0.0025%.
[0056]
　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%.
[0057]
　O: 0.0015% or less
　oxygen (O) is an impurity. O decreases the cold workability by forming an oxide. If O content exceeds 0.0015%, the oxides are produced in large quantities, MnS is coarse, cold workability is significantly decreased. Therefore, O content is 0.0015% or less. The preferable upper limit of the O content is 0.0013%. O content is preferably as small as possible.
[0058]
　The remainder of the bolt chemical composition according to the present embodiment is composed of Fe and impurities. Here, the impurities, in producing the bolts industrially, ore as a raw material, there is to be mixed etc. Scrap or manufacturing environment, which is acceptable to the extent that the present invention does not adversely affect It means.
[0059]
　[For any element]
　above bolts further, in place of part of Fe, Mo, V, it may contain one or more selected from the group consisting of Cu and Ni. All of these elements are also optional element, enhances the hardenability of steel.
[0060]
　Mo: 0 ~ 0.05%
　of molybdenum (Mo) is an optional element and may not be contained. If contained, Mo enhances hardenability. However, Mo content if it exceeds 0.05%, too high hardenability, cold workability of the bolt steel material is reduced. Therefore, Mo content is 0 to 0.05%. 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%.
[0061]
　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%.
[0062]
　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 0.03%, more preferably 0.05%. The preferable upper limit of Cu content is 0.30%, more preferably 0.20%.
[0063]
　Ni: 0 ~ 0.30%
　nickel (Ni) is an optional element and may not be contained. If contained, Ni enhances the hardenability of steel. Ni further enhance 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%.
[0064]
　The chemical composition of the bolt according to the present embodiment further includes, in place of part of Fe, and may contain Nb.
[0065]
　Nb: 0 ~ 0.05%
　and niobium (Nb) optional elements 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 0.05% is Nb content, the cold workability of the steel material to produce coarse carbides is reduced. Therefore, Nb content is 0 to 0.05%. 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 0.04%, more preferably from 0.03%.
[0066]
　[Equation (1)]
　The present invention according to bolt the chemical composition further satisfying the equation (1).
　4.9 ≦ 10C + Si + 2Mn + Cr + 4Mo + 5V ≦ 6.1 (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".
[0067]
　fn1 = 10C + Si + 2Mn + Cr + 4Mo + 5V is indicative of hardenability. If fn1 is too low, no sufficient hardenability can not be obtained. On the other hand, if fn1 is too high, hardenability is too high. In this case, when the bolt steel is rolled into wire rod, bainite is generated, increases the strength and hardness of the steel material. 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 is not obtained cold workability. If fn1 is a 4.9 to 6.1, it is possible to obtain excellent hardenability. Further, omitting the softening heat treatment, or even without carrying out long softening heat treatment, sufficient cold workability are obtained. A preferred lower limit of the fn1 is 4.95. A preferred upper limit of the fn1 is 6.0.
[0068]
　[Equation (2)]
　The present invention according to bolt the chemical composition further satisfying the equation (2).
　Mn / Cr ≦ 0.55 (2)
　wherein each element symbol in the formula (2), the content of the corresponding element (mass%) is substituted.
[0069]
　fn2 = is defined as Mn / Cr. Referring to FIG. 1, as fn2 is reduced, the ratio HR is significantly enhanced. Then, if the fn2 is 0.55, the ratio HR is higher than 1.00, the resulting excellent hydrogen embrittlement resistance. A preferred upper limit of the fn2 is 0.50.
[0070]
　[For formula (3)]
　bolt according to the present embodiment further satisfies the Equation (3).
　[Solute Cr] /Cr≧0.70 (3)
　in the solid solution Cr] of formula (3) is substituted into dissolved Cr content in the bolt (mass%) of the bolt chemical composition to Cr Cr content (wt%) is substituted.
[0071]
　fn3 = [solute Cr] / Cr shows for the Cr content in the chemical composition, the solid solution amount of Cr ratio in the bolt. If fn3 is high, more and more of Cr is a solid solution. Therefore, the strength is increased with respect to hydrogen embrittlement of the tempered martensite, increases the resistance to hydrogen embrittlement. Satisfies Expression (2), and, if satisfying the formula (3), the ratio of the bolt HR is higher than 1.00, the resulting excellent hydrogen embrittlement resistance. A preferred lower limit of the fn3 is 0.75.
[0072]
　Dissolved Cr content is measured by the following method. Collecting a specimen containing the central axis of the bolt. The specimen is electrolyzed at 10% AA electrolytic solution. The 10% AA-based electrolyte solution, 10% acetylacetone-1% tetramethylammonium chloride - methanol solution. After the electrolysis, the electrolytic solution to recover the residue was filtered through a filter of 0.1μm having a pore size. Cr amount of recovered residue (mass%), analyzed using an inductively coupled plasma (ICP) mass spectrometer. Cr content in the residue is meant the amount of Cr than the amount dissolved Cr (i.e., the amount of Cr Cr precipitates such as Cr carbonitride). Thus, by using the Cr content in the residue, determining the amount of solid solution Cr (wt%) by the following equation (B).
　Dissolved Cr content = V total Cr content - Cr content in the residue (B)
　by using the obtained solid solution Cr content, obtaining the fn3.
[0073]
　[For formula (4)]
　is preferably, if the surface from 50μm depth of the bolt (coating of the plating layer or the like on the surface of the bolt is formed from the surface of the bolt itself to remove coating material (base metal) the P content in the surface layer of up to 50μm depth) as Ps (wt%), when the P content of the central axis of the bolt and the Pc (mass%), satisfies the equation (4).
　Ps / Pc ≦ 1.2 (4)
[0074]
　In this case, the P content of the bolt surface is not excessively higher than the P content of the inner bolt. Therefore, excess P in the surface layer can be prevented from segregating at the grain boundaries, further enhanced resistance to hydrogen embrittlement.
[0075]
　The surface layer of the P content Ps is obtained by the following method. In any one position of the bolt, P content in the range from the surface to 50μm depth (surface layer) (mass%) determined. Specifically, by using the electron probe microanalyzer (EPMA) apparatus, from the surface of the bolt to 50μm depth, measuring the P content in 1μm pitch. The average of the measured P concentration is defined as the surface layer of the P concentration Ps. Using the obtained P content Ps, seek fn4 = Ps / Pc. Here, Pc is the P content in the chemical composition of the bolt and (mass percent).
[0076]
　To produce the bolt satisfies the formula (4) may be applied lubricating coating (lubricant) containing no P during drawing. Or, after performing the cold working by using a lubricating coating containing P, it may be removed and the lubricating coating from the bolt surface before quenching process described below.
[0077]
　[Volt Organization
　matrix tissue of the bolt of the present embodiment is a tempered martensite single phase. That is, in the matrix tissue, an area ratio of tempered martensite is 100%.
[0078]
　[Production Method]
　An example of a bolt manufacturing method of the present invention will be described. First, to produce a bolt steel material by a known production method (material production step). Then, by using the bolt steel material, to produce a bolt (bolt manufacturing process). Hereinafter, the respective steps will be described.
[0079]
　[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, the bolt steel material (wire rod). Hot working example, a hot rolling.
[0080]
　[Volts manufacturing process]
　In the bolt manufacturing process, to produce a bolt with a bolt steel material. Bolt manufacturing process includes wire drawing step, the cold forging process, and the quenching and tempering process. The following describes each step.
[0081]
　[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.
[0082]
　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. In this case, the manufactured bolt satisfies the equation (4).
[0083]
　[Cold forging step]
　The steel after drawing was cut into a predetermined length to produce a bolt carried out cold forging against cut steel.
[0084]
　[Softening heat treatment for]
　In the manufacturing method of the high strength of previously bolts, for the purpose of softening the bolt steel material strength is too high (wire) before prior to wire drawing and cold forging, a softening heat treatment was performed multiple times ing. However, a bolt according to the invention, by satisfying the equation (1), be omitted or simplified 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 the bolt.
[0085]
　When carrying out the softened heat treatment Simplified, the 700 ° C. or more holding time is less than 40 minutes for the steel. In this case, it is possible to suppress the carbide containing Cr is excessively formed, dissolved Cr content in the bolt can be sufficiently ensured, satisfies the equation (3). Therefore, with the manufactured bolt excellent hydrogen embrittlement resistance.
[0086]
　[Quenching and tempering process]
　with respect to the bolt which is produced by cold forging, to implement the quenching and tempering in a known condition, adjusting the tensile strength of the bolt to 1000 ~ 1300 MPa. 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). Thus, P of the surface are removed, the bolts after tempering satisfies the equation (4).
[0087]
　With the above-described manufacturing steps, the bolt of the present invention is produced.
Example
[0088]
　The chemical composition of Table 2 were prepared molten steel having.
[0089]
[Table 2]

[0090]
　Referring to Table 2, as described above, the steel M had a chemical composition corresponding to SCM435 of JIS G4053 (2008).
[0091]
　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.
[0092]
　Against the wire of each test number shown in Table 3 were prepared steel wire was performed wire drawing. At this time, to the test numbers 2,5,14 and 15 and heat-treated for the purpose of softening. Heat treatment conditions (heat treatment temperature, heat treatment time, the cooling method after the heat treatment) were as shown in Table 3. The time wire is held at 700 ° C. or higher in the heat treatment (min) were as shown in Table 3. Incidentally, before wire drawing, to implement the phosphating against wire of each test number, to form a phosphate coating on the wire surfaces.
[0093]
[table 3]

[0094]
　It was produced bolt shown in Figure 3 by carrying out cold forging against steel wire of each test number. Referring to FIG. 3, the shape of the bolt is a metric thread conforming to JIS B0205, and more specifically, the nominal diameter (M) was 12mm of fine thread (pitch 1.25 mm). Each number in the figure shows the dimensions of the corresponding site (mm).
[0095]
　After forming the bolt was investigated whether the occurrence of cracks was observed bolts visually.
[0096]
　With respect to the bolt of the cracks were not observed test number, carried out hardening and tempering, the tensile strength was adjusted to 1000 ~ 1300 MPa. Before carrying out the quenching process, to remove the phosphate coating bolts surface of the test numbers 1 to 6 and 8 to 15 with an alkaline washing. On the other hand, it was not carried out alkali washing with respect to the bolt of the test numbers 7. Therefore, with respect to the bolt of the test numbers 7, it was carried out hardening treatment in a state in which the phosphate coating is adhered.
[0097]
　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 volts.
[0098]
　Note that if the tempering temperature for obtaining the desired bolt tensile strength (1000 MPa ~ 1300 MPa) is less than 435 ° C., it is determined that insufficient strength of the bolt and the outside of the object of the present invention.
[0099]
　[Tensile
　Test] in compliance with JIS B1051 (2000), at room temperature (25 ° C.), to measure the tensile strength of the bolt of each test number (MPa) in the atmosphere. The measurement results are shown in Table 3.
[0100]
　[Hydrogen embrittlement resistance evaluation test]
　with respect to the bolt of each test number, using the electrolytic charging method, hydrogen was introduced at various concentrations. Electrolytic charge method was performed as follows. It was dipped bolts into ammonium thiocyanate solution. While immersed bolts, incorporating the hydrogen into the bolts by generating an anode potential on the surface of the bolt.
[0101]
　After introducing hydrogen into the bolts, the zinc plating film formed on the bolt surface to prevent leakage of the bolt outside of the hydrogen in the bolts. Subsequently, it was performed the constant load test loaded with 95% of the tensile strength of the tensile strength of the bolt. Bolts were broken during the test, and with respect to the bolt that was not broken, to implement a Atsushi Nobori analysis method using a gas chromatograph, to measure the amount of hydrogen in the bolt. 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.
[0102]
　Further, based on the critical diffusible hydrogen amount Href steel M having a chemical composition corresponding to SCM435, was determined critical diffusible hydrogen amount ratio HR by using the formula (A).
[0103]
　[Dissolved Cr amount measurement test]
　were taken test piece including the central axis of the bolt of each test number was determined amount of solid solution Cr (wt%) by the method described above. Using a solid solution Cr amount calculated to determine the fn3.
[0104]
　[P concentration measurement test of the surface layer]
　In the screw portion of the bolt of each test number, selects an arbitrary thread root one position, by the method described above, was determined surface layer of P content Ps (wt%). With P content Ps obtained was determined fn4 = Ps / Pc. Here, Pc was the P content in the chemical composition of the bolt (P content in Table 2).
[0105]
　[Test Results]
　The test results are shown in Table 3.
[0106]
　The chemical composition of the bolt of the test Nos. 1 to 7 was appropriate. Furthermore, fn1 satisfies Equation (1), fn2 satisfies equation (2), fn3 satisfied the equation (3). As a result, the bolt these test numbers, even though the tensile strength is high strength and 1000 ~ 1300 MPa, the critical diffusible hydrogen amount ratio HR is higher than 1.00, excellent in hydrogen embrittlement resistance .
[0107]
　Furthermore, the bolt of the test numbers 1 to 6, for removal of the phosphate coating by alkaline washing, fn4 (= Ps / Pc) satisfies the equation (4). Therefore, fn4 are compared with test No. 7 did not satisfy the formula (4), the ratio HR was high.
[0108]
　On the other hand, Cr content of Test No. 8 was too low. Therefore, the ratio HR of 1.00 or less as low as had lower hydrogen embrittlement resistance.
[0109]
　Mn content of Test No. 9 was too high. Therefore, cold workability of the bolt steel material (wire rod) is low, cracks bolt after cold forging was observed.
[0110]
　The bolt of the test number 10, fn1 is less than the lower limit of the formula (1). Therefore, even by lowering the tempering temperature to 435 ° C., the tensile strength was less than 1000 MPa.
[0111]
　The bolt of the test number 11, fn1 exceeds the upper limit of the formula (1). Therefore, cold workability of the bolt steel material (wire rod) is low, cracks bolt after cold forging was observed.
[0112]
　In Test No. 12 and 13, fn2 (= Mn / Cr) does not satisfy the equation (2). Therefore, the ratio HR becomes less than 1.00, it had lower hydrogen embrittlement resistance.
[0113]
　Volt Test No. 14 has a chemical composition corresponding to SCM435 in JIS standard used in the conventional bolt, the critical diffusable hydrogen amount, the critical diffusible hydrogen amount ratio HR of reference (Href) did.
[0114]
　In Test No. 15, when the softening heat treatment, the holding time at 700 ° C. or higher was over 40 minutes. Therefore, fn3 (= [solute Cr] / Cr) does not satisfy the equation (3). Therefore, the ratio HR becomes less than 1.00, it had lower hydrogen embrittlement resistance. Softening heat treatment Cr carbonitride generated dissolved Cr is decreased by, As a result, it is considered that hydrogen embrittlement resistance is low.
[0115]
　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.

we claims

[Claim 1]でmass%, C: 0.32~ 0.39%, Si: 0.15% or
　less, Mn: 0.40
　~ 0.65%, P: 0.020% or
　less, S: 0.020% or less, 　cr:
　　0.85 ~ 1.25%, of Al: 　0.005 ~ 0.060%, of Ti: 0.010 ~ 0.050%, B: 0.0010 　~ 0.0030%, N: 0.0015 ~ 　% 0.0080, O: 0.0015% or 　less, 　of Mo: 　0 ~ 0.05%, V: 0 ~ 0.05%, a 　Cu: 0 ~ 0.50%, of Ni: 0 ~ 0.30%, andび, 　of Nb: 0 ~ 0.05% wo containing shi, remnants ga Fe andびimpure wo the Man ta su chemical composition wo composition kara na ri, of formula (1) andびformula (2) has 　shi, 1000 ~ of 1300 MPaのtensile strength wo have shi, 　formula ( 3) the Man wo su ta, Baldwin Hikaru Suites. 　1OC ≦ Si + + 4.9 + 2Mn + 4Mo + 5V of Cr ≦ 6.1 　(1) Mn / of Cr ≦ 0.55 (2) 　[solid solution of Cr] /Cr≧0.70 (3) 　ko koでformula (1) to (3) of each elementのsymbol ni wa, Applied Duiのsuru element content (mass%) substituting Connecticut DomNode- ga ru. Dui Applied suru ga impurity element Toray Bakelite HikaruののDui Applied case, formula (1) ni wa suru element symbol "0" is substituted into Connecticut DomNode- ga ru. Of formula (3)の[solid solution of Cr] ni wa, referred Baldwin Hikaru Suites solution before the amount of Cr (% by mass)のga ru DomNode- substituting Connecticut.

[Claim 2]
　A bolt according to claim
　1,
　Mo: 0.01 ~ 0.05%,
　V: 0.005 ~ 0.05%, Cu: 0.03 ~ 0.50%,
　and, Ni: 0. containing one or more members selected from the group consisting of from 03 to 0.30%, volts.
[Claim 3]
　A bolt according to claim 1 or claim
　2, Nb: 0.0015 contains ~ 0.05%, volts.
[Claim 4]
　A bolt according to any one of claims 1 to 3,
　the P content in the surface layer from the surface of the bolt to 50μm depth and Ps (mass%), P at the center axis of the bolt when the content was Pc (mass%), satisfies the equation (4), bolts.
　Ps / Pc ≦ 1.2 (4)