Technical field
[0001]
　The present invention relates to a cold forging age hardened steel.
Background technique
[0002]
　As automobile parts, industrial machine parts and construction machine parts, such as structural steel as a material for machine structural parts, mechanical structural carbon steel and machine structural alloy steel is used.
[0003]
　To produce parts from these steels, conventional primarily - been "hot forging cutting" process employed. In recent years, in order to improve productivity - to switch to the "cold forging cutting" process it is oriented. Thus - by adopting "cold forging cutting" process, near-net shaping is achieved by cold forging, and because the amount of cutting is reduced, the productivity is improved.
[0004]
　However, in general, cold forging, because the working ratio is large, the processing load is high, mold life is short, a problem cracking component is likely to occur arises. Therefore, increasing cold forging of steel as the material (the cold forgeability), that is, to reduce the load during cold forging, is the most important challenge is to suppress the cracking.
[0005]
　On the other hand, auto parts, the mechanical structural parts, such as industrial machine parts and construction machine parts, high fatigue strength is required. In order to achieve a high fatigue strength, it is effective to increase the hardness after cold forging. However, by increasing the hardness of the steel material which is a material, when trying to increase the hardness after cold forging, lowering the cold forgeability. That is, in the steel material, it has been difficult to achieve both the fatigue strength and cold forgeability.
[0006]
　To solve such a problem, in order to increase the fatigue strength of the cold forging, after cold forging, Ac 3 by heating to a temperature above a heat treatment of quenching and tempering or induction hardening, the entire or it has been conducted to cure the surface.
[0007]
　However, in this method, in order to part the hardness after heat treatment becomes higher, lowering of machinability it can not be avoided, the benefits of improved productivity by cold forging has a problem that can not be enjoyed.
[0008]
　Therefore, at the time of cutting without increasing unnecessarily the hardness is applied to the application to increase the hardness by heat treatment after machining, it is the so-called age hardening steel materials.
[0009]
　Patent Document 1, a chemical component is mass%, C: 0.01 ~ 0.15%, Si: 0.05% or less, Mn: 0.10 ~ 0.90%, P: 0.030% or less , S: 0.030% or less, Cr: 0.50 ~ 2.0%, V: 0.10 ~ 0.50%, Al: 0.01 ~ 0.10%, N: 0.00080% or less and O: containing 0.0030% or less, the balance being Fe and impurities, 399 × C + 26 × Si + 123 × Mn + 30 × Cr + 32 × Mo + 19 × V ≦ 160 or less, 20 ≦ (669.3 × logC-1959.3 × logN -6983.3) × a (0.067 × Mo + 0.147 × V) ≦ 80,160 ≦ 140 × Cr + 125 × Al + 235 × V, 90 ≦ 511 × C + 33 × Mn + 56 × Cu + 15 × Ni + 36 × Cr + 5 × Mo + 134 × V ≦ 170 meet, Woven ferrite-pearlite structure, a ferrite-bainite structure or ferrite-pearlite-bainite structure, and it is the area ratio of ferrite is 70% or more, V content of precipitates by extraction residue analysis 0.10% or less , and the core hardness is Vickers hardness at 220 or more, cold 鍛窒 of steel, characterized in that it is the effective case depth 0,20mm above, a technique relating to steel and cold 鍛窒 of parts for a cold 鍛窒 of It has been disclosed.
[0010]
　Patent Document 2, the chemical components are mass%, C: 0.06 ~ 0.50%, Si: 0.05% or less, Mn: 0.5 ~ 1.0% or less, V: 0.10 ~ comprises 0.60% and the total amount of area ratio of the pro-eutectoid ferrite and pearlite 90%, and the pro-eutectoid ferrite amount is represented by the formula f = 100-125 [C] +22.5 [V] is the area% of the above f values, technology relating to good cold heading steel cold workability that VC is precipitated in the eutectoid ferrite is disclosed.
CITATION
Patent Literature
[0011]
Patent Document 1: WO 2012/053541 Patent
Patent Document 2: JP 2000-273580 JP
Summary of the Invention
Problems that the Invention is to Solve
[0012]
　Technique disclosed in Patent Document 1, excellent while providing a steel and steel having a cold forgeability and machinability after cold forging, parts of the processing of cold forging and nitriding is performed, it can be provided a high core hardness, high surface hardness and deeper the effective hardened layer depth. However, no mention is made of the fatigue strength, it has not been studied improvement of the durability ratio (fatigue strength / tensile strength).
[0013]
　Technique disclosed in Patent Document 2, there is according to the cold heading steel which can be subjected to cold working while rolling in the hot rolling to precipitate VC, decreases the solid solution C there is provided a steel with improved cold forgeability by. However, Patent Document 2 described techniques do not considering fatigue strength. In the case of improving the strength, it is based on the assumption that the process refining, cutting is required in cured state after refining, lowering of machinability can not be avoided.
[0014]
　The present invention has been made in view of the above situation, more tensile strength 400 MPa, while ensuring the fatigue strength of at least 250 MPa, have high cold forgeability, and work hardening by cold forging, and cold by age hardening after forging, and an object thereof is to provide a highly durable cold ratio is obtained forging age hardened steel.
Means for Solving the Problems
[0015]
　The present inventors have conducted various studies to solve the above problems. As a result, it was found matters following (A) ~ (D).
[0016]
　For good cold forgeability (A), it is necessary to reduce the hardness of the material to be subjected to forging (steel). It can be reduced forging load by reducing the hardness of the material. Further, in order to suppress the cracking during cold forging, it is effective to reduce the C content of the steel as a material.
[0017]
　(B) in order to obtain a high fatigue strength after age hardening treatment, V carbonitrides, it utilizes precipitation hardening of Nb carbonitride, also, the pearlite area a microstructure on mainly composed of ferrite and pearlite it is effective to reduce the rate. Age hardening treatment is not only increase the fatigue strength, it has the effect of increasing the durability ratio (fatigue strength / tensile strength). When the durability ratio is high, while ensuring the fatigue strength required, since the tensile strength may be relatively low, the effect to prevent a reduction in cutting resistance. The high durability ratio in the present invention refers to is 0.600 or more.
[0018]
　Be (C) Nb is is contained alone, but it is impossible to obtain a sufficient durability ratio improvement after age-hardening, simultaneously to contain Nb and V, by complex carbonitride is precipitated, alone Nb in can of course when compared to steel which contains, obtain a large resistance ratio improving effect as compared with the steel which contains the V alone.
[0019]
　(D) also reduces the amount of C in order to exhibit excellent cold forgeability, if properly control the chemical composition of the steel as a material, sufficient aging precipitation is obtained, improving the durability ratio of steel to.
[0020]
　The present invention has been completed based on the findings of the above (A) ~ (D), its gist is as follows.
[0021]
　[1] chemical composition, in mass%, C: 0.02 ~ 0.13%, Si: 0.01 ~ 0.50%, Mn: 0.20 ~ 0.70%, P: 0.020% or less (including 0%), S: 0.005 ~ 0.020%, Al: 0.005 ~ 0.050%, Cr: 0.02 ~ 1.50%, V: 0.02 ~ 0.50 %, Nb: 0.005 ~ 0.050%, and N: containing 0.003 to 0.030%, balance being Fe and inevitable impurities, the content of the solute Nb (mass%) of the 25% or more with respect to the total content of nb, the content of dissolved V (wt%) 50% or more with respect to the total content of the V, fn1 is represented by the following formula (1) 0.03 above, there is fn2 represented by the following formula (2) is 13.5 or less, metal structure, an area ratio, ferrite: 85%, bainite and martensite Preparative Total: 5% cold forging age-hardening steel, which comprises (including 0%).
[0022]
　fn1 = [Nb] / [V] · · · (1)
[0023]
　fn2 = 125 × C-13 × V-4 × Nb · · · (2)
[0024]
　Equation (1) in equation (2), [V] is the mass% of the solid solution V, [Nb] represents the mass% of the solid solution Nb, C is the mass% of C containing the steel, V is contained in the steel mass% of V, Nb indicates a mass% of Nb to be contained in the steel.
[0025]
　[2] In addition, the chemical composition, instead of a part of Fe, Cu: 0.20% or less, Ni: 0.20% or less and Mo: containing one or more of 0.20% or less characterized in that said [1] cold forging age-hardening steel for.
Effect of the invention
[0026]
　Cold forging age hardened steel of the present invention is excellent in cold forgeability, and, without performing heat treatment of quenching and tempering or induction hardening, machinability high durability ratio by age hardening can be ensured. Furthermore, by using age hardening steel of the present invention as a material, hitherto common was the "hot forging - cutting" in place of step, "cold forging - aging treatment - cutting" by the process, automotive parts, industrial machinery parts, it is possible to manufacture the mechanical structural components such as construction machinery parts, thereby improving the productivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027]
And fn1 calculated by [1] Equation (1) is a graph showing the relationship between the endurance ratio (fatigue strength / tensile strength).
DESCRIPTION OF THE INVENTION
[0028]
　It will be described in detail below each requirement of cold forging age hardened steel of the present invention (hereinafter referred to as "steel" or "steel"). Incidentally, "%" for the content of each element in the following description, in particular means "% by weight" unless otherwise specified.
[0029]
　First, the chemical composition will be described.
[0030]
　[C: 0.02 ~ 0.13%]
　C is an element necessary for enhancing the strength of the mechanical structural components. However, in the present invention, in order to suppress the cracking during cold forging, to reduce the C amount. Since the amount of C exceeds 0.13% cracking during cold forging occur, its content and 0.13% or less. 400MPa or more tensile strength after age hardening treatment in C content is less than 0.02% can not be secured fatigue strength of at least 250 MPa. Therefore, the content of C to 0.02% or more. The content of C is 0.03% or more, is preferably less than 0.10%.
[0031]
　[Si: 0.01 ~
　0.50%] Si is an element necessary for the deoxidation during melting, in order to obtain this effect is contained more than 0.01%. However, Si is to strengthen the solid solution of ferrite, the content of Si exceeds 0.50%, decreases the cold forgeability. Accordingly, the content of Si is 0.50% or less. The content of Si is 0.05% or more, it is desirable to 0.45% or less.
[0032]
　[Mn: 0.20 ～ 0.70
　Pasento] Mn increases the strength of the final parts as a solid solution strengthening element. The content of Mn is insufficient strength of the final component is less than 0.20%, to lower the cold forgeability exceeds 0.70%. Therefore, the amount of Mn is 0.20 to .70%. The content of Mn is 0.25% or more, it is desirable to 0.65% or less.
[0033]
　[P: 0.020% or less]
　P is an impurity which is inevitably contained in steel, easily segregated in steel, causing local ductility decreases. When the content of P exceeds 0.020% local ductility decreases it becomes remarkable. Therefore, to limit the content to 0.020% or less. The content is preferably limited to not more than 0.018%. The content of P may be 0.
[0034]
　[S: 0.005 ~ 0.020% or less]
　S is an element that improves machinability. In order to obtain the effect of improving machinability, it is necessary to contain 0.005% or more. When the content exceeds 0.020% to produce a coarse sulfides in steel, causing cracking during cold forging. Thus, the 0.005 to 0.020% of the content of S. The content of S is desirably 0.018% or less.
[0035]
　[Al: 0.005 ~
　0.050%] Al is a deoxidizing agent during steel refining. It is contained at least 0.005% in order to obtain the effect of deoxidation. When the content is more than 0.050% produces coarse Al inclusions in steel and causes cracking during cold forging. Accordingly, the content of Al to 0.050% or less. The content of Al is desirably 0.045% or less.
[0036]
　[Cr: 0.02 ~
　1.50%] Cr has the effect of increasing the fatigue strength of the forged as a solid solution strengthening element. However, when the content exceeds 1.50%, excessively cold forgeability by increasing material hardness is lowered. Therefore, the content of Cr and 0.02 to 1.50 percent. The content of Cr is 0.03% or more, it is desirable to 1.30% or less.
[0037]
　[V: 0.02% ~ 0.50%]
　V, by forming a complex carbonitride of V and Nb during the age hardening process, increasing the fatigue strength and durability ratio. To obtain this effect, the inclusion of V 0.02% or more. In terms of alloy cost, the upper limit is made 0.50%. Incidentally, V content is desirably 0.03% or more.
[0038]
　[Nb: 0.005% ~
　0.050%] Nb, by adding at the same time V, compositely form a carbonitride and V during the age hardening process, enhance the durability ratio. To obtain this effect, it is contained more than 0.005%. In terms of alloy cost, the upper limit is 0.050%. The content of Nb is preferably not less than 0.010%.
[0039]
　[N: 0.003 ~ 0.030% or less]
　N binds V, and Nb in age hardening treatment after cold forging, thereby improving the durability ratio by precipitation as a composite carbonitride. To obtain this effect, it is contained more than 0.003%. However, if excessively contained to become a cause of cold forgeability decreases, its content 0.030% or less. The content of N is preferably 0.025% or less.
[0040]
　Cold forging age-hardening steel of the present invention, in addition to the above elements, those having a chemical composition and the balance of Fe and unavoidable impurities. The unavoidable impurities, say when the industrial production of steel materials, ores as raw materials, those mixed etc. Scrap or manufacturing environments.
[0041]
　The chemical composition of the cold forging age-hardening steel of the present invention, in addition to the above elements, instead of a part of Fe, Cu, may contain one or more elements of Ni, and Mo.
[0042]
　Or less, Cu is any element, Ni, and the effects of the Mo, reasons for limiting the content will be described.
[0043]
　[Cu: 0.20% or
　less] Cu is because it has the effect of increasing the fatigue strength of steel, may be contained 0.20% or less. Exceeds 0.20%, cold forgeability is degraded. From the viewpoint of cold forgeability secured, the amount of Cu in the case of containing is preferably set to 0.15% or less.
[0044]
　[Ni: 0.20% or
　less] Ni is because it has the effect of increasing the fatigue strength of steel, it may be contained 0.20% or less. Exceeds 0.20%, cold forgeability is degraded. From the viewpoint of cold forgeability secured, the amount of Ni in the case of containing is preferably set to 0.15% or less.
[0045]
　[Mo: 0.20% or
　less] Mo is because it has the effect of increasing the fatigue strength of steel, may be contained 0.20% or less. Exceeds 0.20%, cold forgeability is degraded. From the viewpoint of cold forgeability securing the amount of Mo in the case of containing it is preferably set to 0.15% or less.
[0046]
　The content of the solid solution Nb (wt%) 25% or more with respect to the total content of the Nb, required content of dissolved V (mass%) is 50% or more relative to the total content of the V there is.
[0047]
　Solute V amount and, among the V contained in the steel is the mass% of V which are not precipitated as carbonitride, solid solution Nb amount and, among the Nb contained in the steel, carbonitrides % by mass of Nb that it does not precipitate as.
[0048]
　As described above, by simultaneously adding Nb and V in the steel, compositely to form a carbonitride and V during the aging treatment, it is possible to increase the durability ratio. To form a composite manner carbonitride and V during the age hardening process, in age hardening before the steel, it is necessary to the proper amount of solid solution Nb, solid solution V exists.
[0049]
　Specifically, components of the cold forging age hardened steel of the present invention, fn1 defined by formula (1) is must be 0.03 or more. This is because, when the aging treatment is to obtain a composite carbonitride of appropriate amounts of Nb and V in order to enhance the durability ratio. The upper limit of the fn1 is not particularly limited, it may be 0.90 or less.
[0050]
　fn1 = [Nb] / [V ] ··· (1)
　However, [V] is the mass% of the solid solution V, [Nb] represents the mass% of the solid solution Nb.
[0051]
　Solute V amount, solute Nb amount is, for example, obtained by the following extraction residue analysis.
[0052]
　From the position of the radius × 0.5 of molded into a round bar age hardened steel, cut a sample of 10 mm × 10 mm × 10 mm, and extraction residue analytical sample. The sample, 10% AA-based solution constant current electrolysis of by (tetramethylammonium chloride, acetylacetone, methanol 1: 10 100 in a mixed liquid) inside.
[0053]
　At this time, in order to remove deposits on the surface, firstly, current: 1000 mA, time: After the pre-electrolysis at 28 minutes of conditions, removing from the sample the deposition of the sample surface to ultrasonic cleaning in alcohol and, measuring the mass of deposits were removed sample, the mass of the sample before electrolysis to do next.
[0054]
　Then, current: 173MA, Time: 142 minutes, electrolysis of the sample under the conditions of room temperature. Removed electrolysis sample, deposit the sample surface (residue) was ultrasonically cleaned in alcohol to remove from the sample. Thereafter, the solution used for solution and ultrasonic cleaning after the electrolysis, collecting the residue was suction filtered through a filter with a mesh size of 0.2 [mu] m. Deposits (residue) measuring the mass of the sample removed from the difference between the measured value of the mass of the sample before and after the electrolysis, determine the "weight of electrolyzed sample".
[0055]
　The residue was collected on the filters were dried and transferred into a petri dish, after measuring the mass, conforming to JIS G 1258, and analyzed by ICP emission spectrometer (Inductively Coupled Plasma Emission Spectrometer), "the residue Request mass "of V and Nb in.
　Then, obtained as described above "mass in the residue of V and Nb," divided by "the mass of the electrolyzed sample", those percentages displayed, solid solution V amount of "extraction residue analysis and it is a solid solution Nb amount ".
[0056]
　fn1 relates explain the basis of the derivation of the above equation (1).
[0057]
　The present inventors have, C: 0.02 ~ 0.13%, Si: 0.01 ~ 0.50%, Mn: 0.20 ~ 0.70%, P: 0.020% or less (0% including), S: 0.005 ~ 0.020%, Al: 0.005 ~ 0.050%, Cr: 0.02 ~ 1.50%, V: 0.02 ~ 0.50%, Nb: 0 .005 to 0.050%, and N: 0.003 contains to 0.030% for the balance being Fe and unavoidable impurities steel, various and perform tests for 30min ~ 60min kept below point A3 to prepare a test steel having a Do solute V amount and solute Nb amount. Then, while measuring the solid solution V amount and solute Nb amount in the manner described above, tensile test with respect to the sample steel (JIS Z 2241 compliant), perform tests Ono-type rotating bending (JIS Z 2274 compliant) to determine the durability ratio.
[0058]
　From the results obtained, we determined the percentage of the solute Nb content relative to the solid solution amount of V sample steels shown in Figure 1 the results of investigating the relationship between the durability ratio.
[0059]
　Than 1, by a ratio of the solid solution Nb amount with respect to the solid solution amount of V test steel to 0.03 or more, it became clear that it is possible to endurance ratio less than 0.60. The value of fn1 is less than 0.03 as defined by equation (1), since no precipitation of composite carbonitride, not to obtain the effect of improving the durability ratio. Therefore, to limit the value of fn1 to 0.03 or more.
[0060]
　Microstructure of cold forging age hardened steel of the present invention are those mainly composed of a mixed structure of ferrite and pearlite, and the area ratio of ferrite is 85% or more. Area ratio of pearlite is often even less, it may be 0. As ferrite and non-pearlite (remaining structure), there is a case where the bainite and martensite are generated, in such a case, necessary to total area ratio of bainite and martensite is limited to not more than 5% there is.
[0061]
　Furthermore, cold forging age hardened steel of the present invention, fn2, defined by equation (2) it must be 13.5 or less. Note that preferably the lower the the fn2 value, the lower limit is not particularly limited, from the upper and lower limits of the contents of the elements, and 0.80 or more.
[0062]
　fn2 = 125 × C-13 × V-4 × Nb ··· (2)
　However, C is C% by weight of the contained steel, V is the mass% of V containing the steel, Nb is contained in the steel Nb It shows the mass%.
[0063]
　fn2 relates explain the basis of the derivation of the above equation (2).
[0064]
　In order to improve the durability ratio, it is necessary to make the area ratio of ferrite and 85% or more. And, further, it is important to strengthen the ferrite. V and Nb are precipitated carbonitrides during aging treatment, is an element to strengthen the ferrite. The value of fn2 is 13.6 or more, which is defined by equation (2), the ferrite can not be enhanced sufficiently. Further, there is a case where the ferrite area ratio is not less than 85%. Therefore, it is impossible to obtain 0.60 more durable ratio. Therefore, the fn2 in order to obtain a durable ratio obtained by the present invention is 13.5 or less.
[0065]
　Bainite and martensite is a tissue inferior as compared with ferrite-pearlite structure in cold deformability, it becomes a cause of cracking during cold forging. Accordingly, bainite and martensite in total, must be limited to 5% or less in area ratio. From the viewpoint of suppressing cracking during cold forging, bainite structure, martensite structure may be that the amount is 0.
[0066]
　Next, a method for manufacturing a cold forging age hardened steel of the present invention.
[0067]
　To obtain a cold forging age hardened steel of the present invention, for example, a cast slab or steel strip having a chemical composition described above as a material to be rolled was rolled by hot rolling, and rolling in the final rolling step after completion, it may be cooled to room temperature.
[0068]
　Slab or a method for obtaining a steel strip is not particularly limited, and may be carried out in a usual manner. Hot rolling, in order to obtain the fn1 value rules in equation (1) ([Nb] / [V]), rolling temperature in the final rolling step should be implemented as more than 900 ° C..
[0069]
　Also, upon cooling to room temperature after the end of hot rolling, in order to obtain a microstructure of the regulations, it is necessary to perform martensite, the method is not a large cooling rate such as bainite is produced, for example, by cooling or the like. More specifically, it is necessary that the average cooling rate below 0.6 ° C. / s.
[0070]
　
　steel age hardening of the present invention, for example, can be used to manufacture mechanical structural parts. When manufacturing a machine structural part, the steel for age hardening of the present invention is subjected cold forging, the age hardening process in order, then, subjected to processing steps such as cutting.
[0071]
　While as much as possible inhibit hardening after age hardening treatment subsequent to cold forging, in order to obtain a component having a high fatigue strength, after carrying out the cold forging to obtain the desired component shape, for example, 200 ° C. ~ Ac3 the following temperature range point may be carried 30min or more reheating (age hardening treatment).
[0072]
　The heating temperature is lower than 200 ° C., since the precipitation of carbonitrides does not occur, there is a possibility that high durability ratio can not be obtained. Further, when heated above the Ac3 point, not only does the durability ratio is obtained higher by coarsening of the precipitates, to transform to austenite, heat treatment distortion can not be avoided.
[0073]
　Is less than the heating time is 30min, it does not occur the precipitation of carbonitrides, high durability ratio may not be obtained. Even longer heating time, a similar effect can be obtained, to enhance the the production cost becomes too long, preferably less than or equal 180 min.
[0074]
　Incidentally, Ac3 points can be calculated by the following equation.
[0075]
　Ac3 (℃) = - 230.5 × C + 31.6 × Si-20.4 × Mn-39.8 × Cu-18.1 × Ni-14.8 × Cr + 16.8 × Mo + 912
　element symbol in the formula is indicating the content of the element in the steel (mass%).
[0076]
　It has been described above age-hardening steel for the present invention. The shape of age-hardening steel of the present invention is not particularly limited, applicable steel, steel pipes, steel bars (shape steel, steel bars, wire rods, rails, etc.) and the like, in any shape.
Example
[0077]
　Hereinafter will be described in more detail the present invention through examples. The following examples are intended to show an example of the present invention in detail, the present invention is not limited to the conditions used in the following examples. Incidentally, in the table, the values ​​underlined indicate that the value is outside the present invention.
[0078]
　Steel A ~ P having the chemical compositions shown in Table 1 were melted to 150kg ingots in a vacuum melting, and after 1200 ° C. heating, forging molding into a round bar of φ42 at 1000 ° C. finishing (hot forging), in the air in was cooling. The test number 17, which will be described later, to start forging by heating 1050 ° C., was 780 ° C. finishing.
[0079]
　Among the steels A ~ P, steels A ~ J are steel chemical composition is within the range defined in the present invention. On the other hand, the steel K ~ P are steels of comparative examples in which the chemical composition is out of the range defined in the present invention.
[0080]
　Table 2 shows the hardness of the steel after hot forging, tissue, solid-solution amount of V, solute Nb content, the fn1, fn2. Table 2 "F" in the "microstructure" ferrite, "P" pearlite, "B" bainite, "M" denotes the martensite. Also, "B, M area ratio" in Table 2 and indicates the total area fraction of bainite and martensite.
[0081]
[Table 1]

[0082]
[Table 2]

[0083]
　Wherein (in φ same.. Or less representative of the diameter) .phi.14 × 21 mm from the round bar forged material cut out a cylindrical test piece, subjected to a compression test by cold pressing was performed cold forging evaluation.
[0084]
　Evaluation items are working ratio ((1-machining after the height / unmachined height) × 100) is presence of occurrence (crack 70% during processing) Crack at the time of 70%, and when the processing rate of 50% forging load (50% during processing of the load, (ton)) of the. Crack was observed using the 5-fold magnifying glass, when the above crack length 0.5mm is not observed in the 5 test pieces was determined that no crack. For forging load, the following 20ton, was determined to be low enough good.
[0085]
　Furthermore, the φ42mm round bar forged material, after implantation in a resin so as to observe its cross section, polished, with corrosion to observe the microstructure at nital, Vickers hardness with a load of 9.8N It was measured. Microstructure observation, Vickers hardness were all carried out in the vicinity of the center of the round bar forged material. Vickers hardness was measured an average of 3 points.
[0086]
　Subsequently, after peeling processing the round bar forged material Fai36mm, after performing the drawing processing to φ18mm simulating over 75% of the cold forging, 60min hold heated to 600 ° C. (age hardening treatment), and air cooling, a tensile test were taken rotary bending test piece Ono-type were subjected to each test.
[0087]
　Furthermore, 10 mm than the φ42mm rod forged material 3 cut out extraction residue specimens, solid solution amount of V by extraction residue analysis method described above, it was measured solute Nb amount.
[0088]
　Table 3, can 裂有 when working rate of 70% in the cold forging Evaluation of Test Nos. 1 to 17 using the steel A ~ Q Mu, forging load at working ratio of 50%, at 600 ° C. After φ18mm drawing 60min It indicates tensile strength after holding, fatigue strength, endurance ratio (fatigue strength / tensile strength). When the endurance ratio is 0.600 or more, it is determined as good, 400 MPa or more for tensile strength, the fatigue strength was judged as good when it is more than 250 MPa. Underlined in Table 3 means that it is not determined that good.
[0089]
　Incidentally, endurance ratio, the fatigue strength, determined to what all forging load at 50% processing was good "cold malleability × fatigue strength" is good, those not enjoy the effect of the present invention It was evaluated as.
[0090]
[table 3]

[0091]
　From Table 3, when the steel bar satisfies the test numbers 1 to 10 in the chemical composition and microstructure specified by the present invention, evaluation of the "cold forgeability × fatigue strength" "○", i.e., 70% of the target no crack in the processing, forging load at 50% working is not more than 20ton, desired cold forgeability was obtained. Moreover, the age hardening treatment after forging, the durability ratio is 0.60 or more, with reduced hardness, high fatigue strength was obtained.
[0092]
　In contrast, in the case of test numbers 11 to 17 bars deviating from at least one of the conditions of chemical composition and microstructure specified by the present invention, evaluation of the "cold forgeability × fatigue strength" met "×" Te, the desired cold forgeability or fatigue strength is not obtained.
[0093]
　For Test No. 11, since the content of C exceeds the range defined in the present invention, high load during cold forging and crack also observed, not cold forgeability can be obtained to determine. Also, low ferrite area ratio of, further the value of fn2 is higher than the value specified in the present invention, not endurance ratio required by obtained.
[0094]
　For Test No. 12, since the content of C is below the range specified in the present invention, the forging of the cold forging although satisfactory tensile strength after age hardening treatment, the fatigue strength is low, seek performance It has not been obtained.
[0095]
　For Test No. 13, since the V is not added, the ferrite is not enhanced, also a low ferrite area ratio of, further the value of fn2 is higher than the value specified in the present invention, the durability ratio obtaining It has not been obtained.
[0096]
　For Test No. 14, since the amount of V is below the range specified in the present invention, ferrite is not enhanced sufficiently, also lower ferrite area ratio of, further specified in the present invention the value of fn2 since exceeds the value, the durability ratio obtaining is not obtained.
[0097]
　For Test No. 15, since Nb is not added, the ferrite is not strengthened, endurance ratio obtaining is not obtained.
[0098]
　For Test No. 16, since the amount of Nb is below the range specified in the present invention, since the ferrite is not enhanced sufficiently and the value of fn1 is higher than the value specified in the present invention, obtaining durability the ratio has not been obtained.
[0099]
　For Test No. 17, since the content of the content and solid solution V of solute Nb is below the value specified in the present invention, ferrite is not enhanced sufficiently, no durability ratio is obtained to determine.
Industrial Applicability
[0100]
　For for cold forging age hardening of the present invention, a high fatigue strength can be secured, it is excellent in cold forgeability, so far "hot forging - cutting" automobile parts has been prepared in step, industrial machines and , it can contribute to the near net shape of the machine structural parts, such as for construction machinery parts.

The scope of the claims
[Claim 1]
　Chemical composition, in
　　mass%, C:
　　0.02
　　~ 0.13%, Si: 0.01 ~ 0.50%, Mn: 0.20
　　~ 0.70%, P: 0.020% or less (0 % to
　　include),
　　S:
　　0.005 ~ 0.020%, Al: 0.005 ~
　　0.050%, Cr: 0.02 ~ 1.50%, V: 0.02 ~
　　0.50%, Nb : from 0.005 to 0.050%, and
　　N: 0.003 ~ 0.030%
containing the balance is Fe and inevitable impurities,
　the total content of dissolved Nb (mass%) of the Nb 25% or more relative to the content,
　the content of dissolved V (wt%) 50% or more with respect to the total content of the V,
　fn1 is 0.03 or more represented by the following formula (1),
　the following a is fn2 represented by the formula (2) is 13.5 or less,
　metal structure, an area ratio,
　　ferrite: 85%,
　　bainite and martensite Ito Total: 5% or less (including 0%)
containing
cold forging age hardened steel, characterized in that.
　　= fn1 [Nb] / [V] · · ·
　　(1) fn2 = 125 × C-13 × V-4 × Nb · · · (2)
　equation (1) in equation (2), [V] is a solid weight% of soluble V, [Nb] represents the mass% of the solid solution Nb, C is C% by weight of the contained steel, V is the mass% of V containing the steel, Nb is the mass% of Nb containing the steel show.
[Claim 2]
　Furthermore, the chemical composition, instead of a part of Fe, Cu: 0.20% or less, Ni: 0.20% or less and Mo: characterized by containing one or more of 0.20% or less cold forging age-hardening steel according to claim 1,.