The present invention can be used as materials for cold forging, rod or linear rolled steel (hereinafter, referred to as "rolled wire rod") relates.
BACKGROUND
[0002]
　Cold parts produced by forging (cold forging parts) is excellent in surface texture and dimensional accuracy, hot parts produced by forging (hot forged part) as compared to lower manufacturing costs, even more yield it is good. Therefore, cold forging parts, parts for machine structure used in automobiles and various industrial machines, etc. (gear, shaft, bolt or the like) and are widely used as components for building structures.
[0003]
　Recently, in the component for machine structural progress in size and weight, because the size is progressing in part for building structure, the cold forging components regardless of their size, even higher strength It is desired.
[0004]
　These cold forging, and mechanical structural carbon steel material of JIS G 4051, and the like machine structural alloy steels of JIS G 4053. These steels are rolled into a rod or linear shape by hot, then spheroidizing annealing, after further repeated drawing processing and cold wire drawing, formed into the part shape by cold forging, quenching and tempering, etc. the heat treatment is generally being adjusted to a predetermined strength or hardness.
[0005]
　Steel material for machine structure, because it contains a relatively large amount of carbon (about 0.20 to 0.40 wt%), can be used as high-strength components by thermal refining. However, the mechanical structural steel or the like, because of the high strength of the forging material, unless a steel softened by spheroidizing annealing and cold wire drawing, wear and cracking of the mold occurs during cold forging parts molded easy, also, cracks in parts occurs. Therefore, since the problems on such production is concerned, the machine structural steel and the like, the steel material by softening is performed to adjust the strength and the like.
[0006]
　In recent years, along with a tendency that the component is a high strength, there is a tendency that the part shape complicated. Therefore, the steel high strength can be obtained by quenching and tempering for the purpose of softening before cold forging, prolonged or spheroidizing annealing, etc. repeat the spheroidizing annealing and cold wire drawing process, measures of It has been taken.
[0007]
　However, in the case of adopting these measures, not only costly, such as labor and equipment costs, energy loss is also increased. Therefore, omitting the spheroidizing annealing (and cold wire drawing), or the development of the steel obtained by a short time of them has been demanded.
[0008]
　Against this background, omit spheroidizing annealing, or, for the purpose of short reduction, reduced C, Cr, the strength of the rolled wire material reduced to the forging material content of alloy elements such as Mn and, reduced are boron-added steels proposed supplemented with decrease in hardenability by B (boron) added by the alloying elements, a number of improvements have been added.
[0009]
　Boron added steel showed high hardenability, without addition of alloying elements such as Cr and Mo can be ensured sufficient hardenability suppressed further cost low. Thus, boron-added steel have been widely in recent years, molded into part shapes by cold forging, for tensile strength after quenching and tempering to obtain a high strength part comprising the above 1000MPa, the hydrogen embrittlement problem also there is a need to overcome.
[0010]
　For example, Japanese Patent No. 3443285, Japanese Patent No. 5486634, and Japanese Laid-9-104945 and JP respectively, "grain coarsening prevention properties and cold forging excellent in cold forging for hot rolled steel a manufacturing method and a high strength of the manufacturing process "," cold working for machine structural steel and a manufacturing method thereof ", and" cold workability and delayed fracture resistance excellent high strength bolts steel, high strength bolts bolt "it is disclosed.
[0011]
　That is, Patent No. 3443285 discloses, C: 0.10 ~ 0.60%, Si: 0.50% or less, Mn: 0.30 ~ 2.00%, P: 0.025% or less, S: 0.025% or less, Cr: 0.25% or less, B: 0.0003 ~ 0.0050%, N: 0.0050% or less, Ti: comprises from 0.020 to 0.100 percent, the balance being Fe, and unavoidable impurities consists, and in the matrix follows a diameter 0.2 [mu] m TiC or Ti a (CN) 20 pieces / 100 [mu] m of steel 2 coarsening prevention properties and having a higher and cold forgeability excellent cold forging for hot rolled steel manufacturing method thereof are disclosed.
[0012]
　Further, Japanese Patent No. 5486634, in mass%, C: 0.2 ~ 0.6%, Si: 0.01 ~ 0.5%, Mn: 0.2 ~ 1.5%, P: 0 .03% or less, S: 0.01 ~ 0.05%, Al: 0.01 ~ 0.1%, N: 0.015% or less, and Cr: 0.5% greater, 2.0% or less contained, the balance is iron and inevitable impurities, the metal structure has a pearlite and pro-eutectoid ferrite, with a total area ratio of pearlite and pro-eutectoid ferrite to the total structure is 90% or more, the area of ​​the pro-eutectoid ferrite rate a is, Ae = (0.8-Ceq) × 96.75 (where a Ceq = [C] + 0.1 × [Si] + 0.06 × [Mn] + 0.11 × [Cr], [ (element name)] is the Ae represented by the content of each element means (wt%)), it has a relationship of a> Ae, Hatsu析Fu And wherein the average particle size of the ferrite of the light and in the pearlite is 15 ~ 25 [mu] m, by applying the usual spheroidization treatment, the cold working steel for machine structural use that can achieve sufficient softening, its production method is disclosed.
[0013]
　Further, JP-A-9-104945, by mass%, C: 0.15 ~ 0.35%, Si: 0.1% or less, Mn: 0.3 ~ 1.3%, P: 0. 01% or less, S: 0.01% or less, Cr: less than 0.5%, Ti: 0.01 ~ 0.10%, Al: 0.01 ~ 0.05%, B: 0.0005 ~ 0. 003%, and the balance together with Fe and unavoidable impurities, which satisfies the following equation 0.50 ≦ [C] +0.15 [Si] +0.2 [Mn] +0.11 [Cr] ≦ 0.60 cold workability and delayed fracture resistance excellent high strength bolts steel, wherein, there is disclosed.
Summary of the Invention
Problems that the Invention is to Solve
[0014]
　According to the technique disclosed in Japanese Patent No. 3443285, comprising since the hardness of the steel material can be reduced, but may be cold forging at low cost, also the coarsening prevention properties during heating for quenching can do. However, due to the low Cr content in the steel, low hardenability, it is to increase the strength of the part is limited, as the high-strength parts over 1000 MPa, there is a problem in resistance to hydrogen embrittlement.
[0015]
　Moreover, steel for cold working machinery structure disclosed in Japanese Patent No. 5486634, by subjecting it to usual spheroidization annealing, it is possible to soften is applicable to high-strength components. However, the addition amount balance of chemical components of the steel is not optimized, also ferrite fraction of tissue rolled steel is substantially less. Therefore, while products rolling or a steel of the state which has been subjected to brief spheroidizing annealing, when used in the cold forging of the part, cracked, it may not be possible to produce parts at a low cost.
[0016]
　Further, in the technique disclosed in JP-A-9-104945, C, Si, and defines the lower limit and upper limit of the total amount of Mn, and Cr, and the strength of the rolled material which does not adversely affect cold workability , to obtain the intensity of the rolled material that desired strength is obtained after the thermal refining. However, because of low hardenability low Cr content, the high strength parts over 1000MPa there is a problem in resistance to hydrogen embrittlement.
[0017]
　The present invention was made in view of the above circumstances, even without applying a spheroidizing annealing before cold forging, or even with a short time of spheroidizing annealing, the cracking during cold forging effectively suppressed, and an object thereof is to provide a rolled wire rod having excellent hydrogen embrittlement resistance after quenching and tempering subsequent to spheroidizing annealing.
Means for Solving the Problems
[0018]
　The present inventors have conducted various studies to solve the problems described above. As a result, the present inventors have obtained the following findings (a) ~ (e).
[0019]
　Omitted (a) spheroidizing annealing, or, even if a shorter time, so that it can ensure the cold forging to the extent moldable parts internal tissue excluding the surface layer portion are likely to produce a decarburized layer for a mixed structure of 95% or more by area ratio of ferrite and pearlite, and further it is necessary to ferrite fraction exceeds 40%.
[0020]
　(B) it is a mixed structure of the same ferrite and pearlite, in the vicinity of the surface of the rolled wire rod inclusions was fine, and by reducing the extended inclusions, to improve the cold forgeability It is possible, which makes it possible to form more complex components. Moreover, miniaturization and reduction of inclusions, thereby improving the hydrogen embrittlement resistance after quenching and tempering.
[0021]
　(C) C, Si, Mn, additional element such as Cr is mainly affects the strength of the rolled wire rod. Furthermore, Mn, Ti, N, additive elements S, etc., affect the composition and the form of inclusions which are inevitably contained in the rolled wire rod. And excellent cold forgeability, for having a hardenability and resistance to hydrogen embrittlement required to be used as cold forging components, unless the balance between these two types of additive elements due consideration not not. Then, it comprises the cold forging, etc. may further be performed rolling ratio 6 or more primary rolling after high-temperature heating of the pre-product rolling and, such as the temperature of the subsequent finish rolling, the production conditions of the steel there is a need to control. Thus, to obtain the premise of ensuring the hardenability at usable levels as cold forging parts omitted spheroidizing annealing, or a rolled wire rod which can realize a good cold forgeability be a shorter time be able to.
[0022]
　In (d) In particular, after producing a steel ingot or slab of the chemical components from the molten steel to a predetermined balance, and high-temperature heating to 1280 ° C. or more at the stage before than products rolling, at least 30min or more Hitoshi immediately after securing the thermal time, performing primary rolling or rolling ratio 6, it is cooled. Thus, coarse carbonitrides and carbides containing Ti generated during solidification, and some coarse sulfide containing Ti and Mn are, once with a solid solution in the steel, coarse sulfide by the primary rolling at a high temperature object is divided, finely reprecipitated in the subsequent cooling process. Accordingly, the coarse sulfides adversely affect cold forgeability is inhibited, fine carbonitrides and carbides reprecipitation, as pinning particles during subsequent to the time of product rolling in the hot performing heating act, contributes to the coarse growth prevention of austenite grains. As a result, the ferrite fraction is high ferrite precipitated at the time of after product roll cooling is miniaturized, it becomes possible to obtain a tissue described in turn (a).
[0023]
　(E) Primary rolled steel strip after high-temperature heating as described above is reheated, are products rolled between heat wire having the predetermined diameter. However, the final finish rolling of the product rolling is preferably performed at a temperature range of 750 ~ 850 ° C. at a processing rate of 5 ~ 15 / sec. By managing the machining speed and temperature range of the finish rolling, austenite grains before ferrite transformation becomes finer, since the ferrite fraction is high, it is possible to obtain a tissue according to (a). If the finish rolling temperature is lower than 750 ° C., increasing the strength of the rolled wire rod ferrite grains becomes too fine, while cold forgeability deteriorates, if the finish rolling temperature is 850 ° C. greater than the (a) It can not be obtained tissue according. The heating temperature at the time of product rolling is preferably set to 1050 ° C. or less.
[0024]
　Above (a) ~ rolled wire material obtained by the finding of (e) has an internal structure the sum of ferrite and pearlite is not more than 95% by area ratio ferrite fraction exceeds 40%. Further, in this rolled wire rod, the average area of the sulfide D / 8 (D is the. Representative of the diameter (mm) of the rolled wire rod) to the outermost layer is present in the range of 6 [mu] m 2 or less. Further, in this rolled wire rod, an average aspect ratio of the sulfide is 5 or less. Therefore, the rolled wire is the presence ratio is small wires of coarse and elongated sulfide.
[0025]
　As a result, the rolled wire is omitted spheroidizing annealing treatment, or be suitably used as a cold forging parts since even if a shorter time has sufficient cold forgeability, and the hardenability can be secured can be can be a good wire to hydrogen embrittlement resistance after quenching and tempering.
[0026]
　Incidentally, Ac for quenching after cold forging 3 is heated to a temperature above the point, sometimes abnormal grain growth portion of the austenite crystal grains grow abnormally large occurs, it causes the part strength varies. However, rolled wire rod in the present invention is excellent in耐粗particle reduction characteristics, Ac after cold forging 3 when heated to a temperature above the point also, it is possible to suppress abnormal grain growth of crystal grains.
[0027]
　The present invention has been completed based on the above findings and has as its gist lies in rolled wire material shown in the following (1) to (3).
[0028]
　(1) in
　　mass%, C: less than 0.20% or more
　　0.40%, Mn: less than 0.10% or more
　　0.40%, S: less than
　　0.020%, P: less than
　　0.020%, Cr 0.70% to 1.60% or
　　less, Al: 0.005% to 0.060% or
　　less, Ti: 0.010% to 0.080% or less
　　B: 0.0003% to 0.0040% or less, and
　　N: 0.0020% or more 0.0080% or less
contained, balance being Fe and
　impurities, Ti, N, and each content of S (mass%), respectively [Ti], [N], [ when S],
　　[in the case of S] ≦ 0.0010, [Ti] is (4.5 × [S] + 3.4 × [N]) or more, and, (0.008 + 3.4 × [N ] ) in which one or less
　　in the case of [S] ≧ 0.0010, [Ti ] is (4.5 × [S] + 3.4 × [N]) or more, and, (8.0 × [S] +3 . × [N]) or less,
　the internal tissue is ferrite fraction of 40% or more in area ratio, a mixed structure of ferrite and pearlite, in a cross-section of a plane including the axial, diameter D (mm ) and average area of sulfides present in a range from the outermost layer to D / 8 position when the is 6 [mu] m 2Or less, an average aspect ratio of the sulfide is 5 or less, and wherein the rolled wire material.
[0029]
　(2) instead of a part of the Fe, by mass%, Si: 0% 0.40% or more and less than Nb: 0% or more 0.050% containing at least one of the following, the above (1) It rolled wire according.
[0030]
　(3) instead of a part of the Fe, by mass%, Cu: 0.50% or less, Ni: 0.30% or less, Mo: 0.05% or less, and V: less of at least 0.05% containing one, above (1) or rolled wire rod according to any one of (2).
[0031]
　(4) instead of a part of the Fe, by mass%, Zr: 0.05% or less, Ca: 0.005% or less and Mg: containing 0.005% or less of at least one, the (1 ) from (3) rolled wire rod according to any one of.
The invention's effect
[0032]
　By using rolled wire rod of the present invention as a material, omitting the spheroidizing annealing treatment, or, even if a shorter time, can be molded into parts by cold forging, the crystal grains be heated into the austenite region during quenching is abnormal grain growth suppression, can be used as a cold forging components also excellent in hydrogen embrittlement resistance after quenching and tempering.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033]
FIG. 1 is a diagram showing the region satisfying the relation between [Ti] and [S] of the present embodiment.
Is a diagram illustrating a FIG. 2 annular V-notch test piece.
DESCRIPTION OF THE INVENTION
[0034]
　It will be described in detail below rolled wire rod of the present embodiment. Note that the rolled wire rod of the present embodiment, a rod-like or linear rolled steel, its diameter means of about 5 ~ 25 mm. Further, "%" of the content of each element below the display means "% by mass".
[0035]
　(A) Chemical composition
　C: less than 0.20% to 0.40%
　C is an element to strengthen steel, it must be contained 0.20% or more. On the other hand, when the content of C is 0.40% or more, cold forgeability is degraded. Therefore, the C content was less than 0.40% to 0.20%. If further desired to increase the quenched hardness of cold forging components, preferably it is 0.24% or more the content and C, further For greater cold forgeability, it is 0.35% or less It is preferred.
[0036]
　Mn: less than 0.10% to 0.40%
　Mn are the elements necessary for enhancing the hardenability, and the lower limit value 0.10%. However, when the content of Mn is 0.40%, the starting temperature of ferrite transformation is reduced by lowering the ferrite fraction at the time of cooling after finish rolling, further generates bainite, cold forgeability descend. Therefore, the Mn content is required to be less than 0.40%. In order to increase the hardenability, Mn is preferably not less than 0.20%.
[0037]
　S: less than 0.020%
　S is contained as an impurity. However, when the content of S is 0.020% or more, be in a form sulfides contained in steel is extended coarse, lowering the cold forgeability. The content of S is preferably less than 0.010%. Moreover, to obtain the form and size of the outstanding sulfide cold forgeability, S is also the same content range must contain in consideration of the balance of Ti and N.
[0038]
　P: less than 0.020%
　P is an impurity contained. However, when the content of P is 0.020% or more, not only the cold forgeability is degraded, become a cause of cracking during quenching P during heating to austenite is segregated at the grain boundaries, yet hardened - to reduce the resistance to hydrogen embrittlement after tempering. Therefore, the P content must be less than 0.020% or. The content of P is preferably set to less than 0.010%.
[0039]
　Cr: 0.70% or more 1.60% or less
　Cr, like Mn, is an element necessary for enhancing the hardenability must be contained 0.70% in the present invention. However, when the content of Cr is more than 1.60% and hardenability is enhanced, because the starting temperature of ferrite transformation is reduced by lowering the ferrite fraction, and further generates bainite during cooling after finish rolling , cold forging is lowered. To stably obtain a high hardenability, it is preferable that the content of Cr 0.80% or more, further preferably contained 0.90%. On the other hand, if desired to increase further the cold forgeability is preferably the content of Cr and 1.50% or less, still more preferably 1.40% or less.
[0040]
　Al: 0.005% or more 0.060% or less
　of Al not only has deoxidizing effect, AlN was formed by combining the N, by its pinning effect, austenite grains during hot rolling finer and bainite It has the effect of inhibiting the production of. Therefore, Al must be allowed to contain more than 0.005%. On the other hand, when the content of Al exceeds 0.060%, not only the effect is saturated, decrease the cold forgeability because coarse AlN is generated. If you want to further suppress the formation of bainite, preferably the content of Al is 0.015% or more, more preferably 0.020% or more. Further, in view of enhancing the cold forgeability, the content of Al is preferably at 0.050% or less, more preferably not more than 0.045%.
[0041]
　Ti: 0.010% or more 0.080% or less
　Ti combines with N, C, or carbide, to form a nitride or carbonitride by their pinning effect, austenite grains finer during hot rolling Te, and suppress the formation of bainite in the cooling step after finish rolling, has the effect of improving the ferrite fraction. Further, Ti is, Ac for quenching after cold forging 3 when heated to a temperature above the point also has the effect of suppressing the abnormal grain growth. Furthermore, Ti, in order to suppress the formation of BN to reduce the N forming a solid solution in the steel also has the effect of increasing the effect of improving hardenability by B. In addition, Ti may change the composition of the sulfide reacts with S to fine sulfides, since also have the effect of increasing the cold forgeability and resistance to hydrogen embrittlement, in consideration of the balance between the N and S It must be added Te.
[0042]
　To obtain these effects, Ti must be contained 0.010% or more. Further To obtain these effects, the content of Ti is 0.030% or more, still more preferably 0.060% or more. On the other hand, when it is contained exceeding 0.080%, and would be fine Ti carbides are precipitated during finish rolling, so to strengthen the ferrite phase worsens the cold forgeability, the content of Ti 0 is less than or equal to .070%. Incidentally, Ti gives carbides, thereby forming a nitride or carbonitride, a solid solution sulfide, an effect on the form and size of the sulfides. Thus, inhibition or cold forging of abnormal grain growth during quenching, contributing to the improvement of hydrogen embrittlement resistance. Therefore, the Ti content be in the above range, it must contain in consideration of the balance between S and N.
[0043]
　B: 0.0003% or more 0.0040% or less
　B is effective in enhancing the hardenability of steel by adding small amount must be contained 0.0003% or more. However, not only the effect when the content exceeds 0.0040% of saturated, cold forgeability deteriorates. If it is desired to further enhance the hardenability, the content of B is preferably 0.0005% or more, still more preferably 0.0010% or more. On the other hand, if further improving cold forgeability, the content of B is preferably set to 0.0030% or less, still more preferably 0.0025% or less.
[0044]
　N: 0.0020% or more 0.0080% or less
　N produces a nitride or carbo-nitride combines with Ti or Al, effects and of refining the austenite grains during hot rolling, cold forging It has an effect of suppressing abnormal grain growth during heating at the time of quenching. However, N content must be determined by considering the balance between the Ti that affects the composition and the form of sulfide. To obtain these effects, N is the must be contained or 0.0020%, it is preferable to contain more than 0.0030% or. However, even if excessively contain a N not only these effects are saturated, and generates nitrides bonded by B, since weaken the effect of improving hardenability by B, the content of N is 0 there is a need to .0080% or less. N content to improve the hardenability and stable is preferably less than 0.0070%, further preferably set to 0.0060% or less. Incidentally, N is the forming nitrides or carbonitrides in combination with Ti. Therefore, N is the influence of Ti amount to influence the morphology and size of the sulfides. Therefore, N must contain in consideration of the balance of Ti and S.
[0045]
　In the present invention, as described so far, Ti, N, the balance of each element of S is important. In particular, ([Ti] -3.4 × [ N]) if too small in ratio of [S], Ti is not obtained the effect of refining the sulfide dissolved in the sulfide coarse sulfide is likely to exist Do not. This is, in the present invention a low content of Mn in order to obtain a ferrite-pearlite structure which is suitable for cold forgeability, behind may easily coarsened sulfide as a solid solution of Fe in the sulfide.
　On the other hand, if ([Ti] -3.4 × [N ]) is too large in the ratio of the [S], fine Ti carbides are precipitated in ferrite to increase the strength of the ferrite, the cold forgeability decrease.
　([Ti] -3.4 × [N ]) is that an amount suitable in ratio of [S], sulfides contained in the composition Ti is solid-solved. As a result, it sulfides finer and cold forgeability of the base material is improved. The crystal grains of abnormal grain growth even if heated to the austenite region is suppressed at the time of quenching, it can be used as a cold forging components also excellent in hydrogen embrittlement resistance after quenching and tempering.
[0046]
　Based on this, the content of Ti, N, and S in the rolled wire rod of the present embodiment (mass%), respectively [Ti], [N], when the [S], these,
　　[S] ≦ 0. for 0010, [Ti] is (4.5 × [S] + 3.4 × [N]) or more, and, (0.008 + 3.4 × [N ]) one or less
　　[S] ≧ 0. for 0010, [Ti] is (4.5 × [S] + 3.4 × [N]) or more and is not more than (8.0 × [S] + 3.4 × [N]),
and It satisfies the condition of. In this definition, a formula that defines the upper limit of [Ti], has changed the boundaries [S] = 0.0010. The reason for this will be described later.
[0047]
　Hatched portion in FIG. 1 shows a region satisfying the relation of the [Ti] and [S] and [N]. In Figure 1, the value A represented by the vertical axis is a value that depends on the [N] (3.4 times the value of [N]), in particular from 0.0068 (wt%) 0.0272 is a value that varies in a range of up to (% by weight). Since the present invention as defined [N] is less 0.0080% or more 0.0020%, the value A is 0.0068 or more, the 0.0272 or less.
[0048]
　For the [S] ≧ 0.0010, that is [Ti] is (4.5 × [S] + 3.4 × [N]) or more, sulfides contained becomes composition Ti is solid-solved It is improved cold forgeability for miniaturization.
　In the case of [S] ≧ 0.0010, that is less than or equal to [Ti] is (8.0 × [S] + 3.4 × [N]), reducing the amount of precipitation of fine Ti carbide, ferrite strength does not become excessively excessively high, it is possible to prevent deterioration of the cold forgeability.
[0049]
　For the case of [S] ≦ 0.0010, as in the case of [S] ≧ 0.0010, that is [Ti] is (4.5 × [S] + 3.4 × [N]) or , sulfides contained becomes composition Ti is solid-solved are improving cold forgeability for miniaturization.
　In contrast, the upper limit of [Ti] in the case of [S] ≦ 0.0010 is defined as (0.008 + 3.4 × [N] ). If [Ti] is in this range, less amounts of fine Ti carbides precipitated inside the wire, the strength of ferrite is not excessively high, it is possible to prevent deterioration of the cold forgeability.
[0050]
　Here, the upper limit of [Ti], explaining why the divided formulas before and after the [S] = 0.0010. As described above, it is limited in order to make proper the upper limit value is to suppress the precipitation amount of fine Ti carbides, strength of the wire of [Ti]. [Ti] is less space, especially in [Ti] is (0.008 + 3.4 × [N] ) the following areas, ([S] regardless of the content) fine Ti carbides to be generated there in trace amounts , a small effect on the wire hardness. Intersection of the formula (8.0 × [S] + 3.4 × [N]) that defines the upper limit of [Ti] in [S] is relatively large area, and (0.008 + 3.4 × [N] ) in a [S] = 0.0010.
　That is, in the range of [S] ≦ 0.0010, even [Ti] is (8.0 × [S] + 3.4 × [N]) above, (0.008 + 3.4 × [N ]) if less, it can be produced a rolled wire material capable of achieving the object of the present invention. Therefore, the region of [S] ≦ 0.0010, and introducing different provisions and [S] ≧ 0.0010 region.
[0051]
　Further, it rolled wire rod according to the present embodiment mainly after cold forging, it is often used in part to provide strength by quenching and tempering. Therefore, in order to ensure the hardenability as part, C contained in the rolled wire rod, Mn, Cr preferably satisfies the following formula <1>.
[Mn] × [Cr]> 0.134 × (D / 25.4- (0.50 × √ [C])) / (0.50 × √ [C]) ···· <1>
　, where in the above formula, [Mn], [Cr] , [C] represents the content by mass percent of the respective elements, D is represents the diameter (mm) of the rolled wire material.
[0052]
　Here, the left side of the equation <1>, Mn contained in the steel, a value represented by mass% of the product of Cr, necessary to ensure the hardenability required as a high strength cold forged parts it is a parameter.
[0053]
　In contrast, the right side of the expression <1> has a diameter D of the rolled wire material is (mm) Ac 3 was heated to a temperature above points, in the case where the quenching by oil cooling, is the heart of the rolled wire rod affects the fraction of martensite obtained in D / 2 (mm) from the surface, is a parameter indicating the relation between D and [C].
[0054]
　Then, in order to ensure sufficient hardenability as a high strength cold forged parts, in formula <1> that the value of the left-hand side is greater than the value of the right side preferred.
[0055]
　Note that the balance in the rolled wire rod according to the present embodiment is "Fe and impurities". Here, the "impurities", is a component contained in the rolled wire rod unintentionally, when the industrial production of steel material, which is mixed from ores and scraps as raw materials, or manufacturing environment It refers to those mixed depending on. For example, oxygen is an impurity to suppress the formation of coarse oxides, in order to avoid that the cold forgeability is degraded, preferably be suppressed to 0.0030% or less, be kept below 0.0020% more preferably, it is highly preferred to keep the 0.0015% or less.
[0056]
　Then, the wire according to the present embodiment, at least instead of a part of Fe as the balance, if necessary, Si, Nb, Cu, Ni, Mo, V, Zr, selected from Ca and Mg one or more elements may be contained. Hereinafter, Nb is an optional additive element, Cu, Ni, Mo, V, Zr, and the content of Ca and Mg, will be described in detail setting reason of the content.
[0057]
　Si: 0% or more and less than 0.40%
　Si, in order to reduce the tensile strength of the rolled wire rod of the hot rolled state, its content is preferably as low. However, Si is to strengthen the ferrite by solid solution strengthening, if desired to increase the temper hardness of cold forging components may be contained. In this case, the content of Si should be less than 0.40%. The content of Si is cold forgeability decreases 0.40% or more. If want to improve cold forgeability, the content of Si is preferably less than 0.30%, more preferably less than 0.20%.
[0058]
　Nb: 0% or more 0.050% or less
　Nb is combined with C and N, carbides, and forming a nitride or carbonitride by their pinning effect, to refine the austenite grains during hot rolling Therefore, to suppress the bainite in the cooling step after finish rolling, it has the effect of improving the ferrite fraction. Also, the carbide of Nb, nitrides or carbonitrides, suppresses grain abnormal grain growth during heating at the time of quenching cold forged parts. In the present embodiment, even without the addition of Nb, it is possible to realize the suppression of the ferrite fraction increased and grain abnormal grain growth. However, in case of realizing these effects reliably, it is effective to add Nb. That is, in order to obtain these effects reliably, Nb is preferably contained 0.003% or more, more preferably be contained 0.005% or more, it is highly preferable to contain 0.010% or more. On the other hand, the Nb, the case where the content exceeds 0.050%, not only these effects are saturated, there is a fear that reducing the cold forging of the rolled wire material. Therefore, the content of Nb is 0.040% or less, more preferably 0.030% or less.
[0059]
　Cu: 0.50% or less
　Cu is an element enhancing the hardenability, may be contained. However, when the Cu content exceeds 0.50%, excessively high hardenability, will be bainite is generated after finish rolling, deteriorating the cold forgeability of the rolled wire material. Accordingly, the content of Cu is preferably 0.50 or less%, it is highly preferred further preferably 0.30% or less, 0.20% or less. In order to stably obtain the effect of adding Cu described above, the content of Cu is 0.03% or more, more preferably 0.05% or more.
[0060]
　Ni: 0.30% or less
　Ni is an element to increase hardenability, may be contained. However, if the content of Ni exceeds 0.30%, not only the effect is saturated, too high hardenability, will be generated by the bainite after finish rolling, deteriorating the cold forgeability. Therefore, the Ni content is preferably 0.30% or less, it is highly preferred further preferably 0.20% or less, 0.10% or less. In order to stably obtain the effect of Ni described above, the content of Ni is preferably 0.01% or more, further preferably in the range from 0.03% or more.
[0061]
　Mo: 0.05% or less
　Mo is an element to strengthen the steel by solid-solution strengthening, greatly improve the hardenability of steel. It may contain Mo in this purpose. However, if the content of Mo exceeds 0.05%, bainite and martensite is formed after the finish rolling, deteriorating the cold forgeability. Accordingly, the content of Mo is 0.05% or less, more preferably 0.03% or less, it is highly preferably not more than 0.02%. In order to get a stable effect of Mo as described above, the content of Mo is preferably 0.005% or more.
[0062]
　V: 0.05% or less
　and V combines with C and N, carbides, forms a nitride or carbo-nitrides may also act to improve the hardenability of steel by adding a small amount. Therefore, it may be contained V. However, if the content of V exceeds 0.05%, the strength of the rolled wire rod is increased by precipitated carbides or carbonitrides, deteriorating the cold forgeability. Accordingly, the content of V is preferably 0.05% or less. The content of V from the viewpoint of improving the cold forgeability, it is highly preferred further preferably 0.03% or less, 0.02% or less. In order to get a stable effect of V mentioned above, the content of V is preferably 0.005% or more.
[0063]
　Zr: 0.05% or less
　Zr may also serve to improve the hardenability of steel by adding a small amount. The Zr traces may be added for that purpose. However, if the content of Zr exceeds 0.05%, coarse nitrides are produced, lowering the cold forgeability. Accordingly, the content of Zr is preferably 0.05% or less. More preferably an amount of Zr in terms of improving the cold forgeability is 0.03% or less, it is highly preferably not more than 0.02%. In order to get a stable effect of Zr described above, the content of Zr is preferably 0.003% or more.
[0064]
　Ca: 0.005% or less
　Ca combines with S to form sulfides, to act as nuclei for MnS, has the effect of dispersing finely the MnS in Ca. In this way, that the finely dispersed MnS, since ferrite is precipitated the MnS during cooling after finish rolling as nuclei, the Ca has the effect of improving the ferrite fraction. Therefore, it may be contained Ca. However, the effect is saturated when the content of Ca exceeds 0.005%, yet Ca oxide becomes coarse to reacts with the oxygen in steel with Al, lowering the cold forgeability. Accordingly, the content of Ca is rather preferred that 0.005% or less, more preferably 0.003% or less, it is highly preferred that not more than 0.002%. In order to get a stable effect of Ca described above, the content of Ca is preferably 0.0005% or more.
[0065]
　Mg: 0.005% or less
　Mg combines with S to form sulfides, to act as nuclei for MnS, the Mg is effective to disperse MnS finely. In this way, that the finely dispersed MnS, since ferrite is precipitated the MnS during cooling after finish rolling as nuclei, the Mg has an effect of improving the ferrite fraction. Therefore, it may contain Mg. However, the effect is saturated when the content of Mg exceeds 0.005%. Moreover, Mg is added yield is poor, since exacerbate manufacturing cost, are contained. Accordingly, the Mg content is preferably 0.005% or less, it is highly preferred further preferably 0.003% or less, 0.002% or less. In order to get a stable effect of Mg as described above, the content of Mg is preferably 0.0005% or more.
[0066]
　(B) Internal structure of the rolled wire rod
　rolled wire rod according to the present embodiment is excellent in cold forgeability, omitting the spheroidizing annealing process after product rolling has been required conventionally about 20 hours, or the process also a time as about half, and reduced die life during cold forging, does not occur such as cracking of the molded part. This not only chemical components of the adjusted steel, by controlling the production conditions of the rolled wire material, the metal structure of the rolled wire material is because it is controlled to a form suitable for cold forging.
[0067]
　Specifically, the rolled wire rod according to the present embodiment, the internal tissues except the surface layer portion that may decarburized layer is generated, 95% or more in area ratio is a mixed structure of ferrite and pearlite , the fraction of ferrite structure is 40% or more of the organization. Here, the ferrite in the present embodiment does not include ferrite between lamellar cementite contained in pearlite. Also it means that mixed structure of ferrite and pearlite and the entire 95% by area ratio, the sum of the martensite and bainite is less than 5%. For good cold forgeability, as described above, the mixed structure of ferrite and pearlite, it is necessary to be an area ratio of 95% or more, more preferably 100%.
[0068]
　When the ferrite fraction is less than 40%, good cold forgeability can not be secured, cracks in the component occurs during molding, and mold life is shortened due to high deformation resistance of the rolled wire, such problems It occurs. Preferably ferrite fraction is 45% or more, it is highly preferably 50% or more.
[0069]
　Also preferred for reasons that it ferrite fraction is 60% or less can suppress the forging defects due seizure during cold forging. Ferrite fraction is more preferably 55% or less.　
[0070]
　The form of the inclusions (C) wire rod
　rolled wire rod according to the present embodiment is excellent in cold forgeability, during cold forging, it does not occur such as cracking of the mold life reduction and molding parts. Moreover, even when heated to the austenite region for the purpose of quenching the wire, is suppressed abnormal grain growth of crystal grains, and further excellent in hydrogen embrittlement resistance after tempering. This is not only to control the metal structure of the chemical composition and rolled wire rod of the adjusted steel, further the form of a sulfide contained in the rolled wire rod near the surface to a fine, with fewer sulfide elongated in the rolling direction This is because you are.
[0071]
　Specifically, in the wire according to the present embodiment, the internal structure of the rolled wire rod by optimizing the chemical composition and rolling conditions, the ferrite fraction is 40% or more, a mixed structure of ferrite and pearlite, cold thereby improving the forgeability. In particular, in order to obtain a mixed structure of ferrite and pearlite suitable for cold forgeability, but limits the amount of Mn, the component of such a low Mn, sulfides contained in the slab since the sulfide solid solution of Fe, it tends to coarsen. Therefore, by containing well Ti, N, and S balance as described above, Ti is dissolved in the sulfide can be suppressed coarsening of the sulfide.
[0072]
　Furthermore, still for the remaining coarse sulfides are at the stage of cast piece, and high-temperature heating to 1280 ° C. or more at the stage before than products rolling, rolling ratio of 6 or more immediately after securing the at least 30min or more soaking time by performing the primary rolling, it is divided. Then, further portions of coarse sulfide was dissolved by high-temperature heating is finely reprecipitated in the subsequent cooling process. By these processes, it is possible to suppress the coarse sulfides adversely affect cold forgeability and resistance to hydrogen embrittlement. In particular, when forming a cold forging parts from rolled wire material, the surface layer from the D / 8: sulphide present in the range of (D diameter of the rolled wire rod) induces cracking and hydrogen embrittlement by cold forging. Therefore, in the present embodiment, in a cross section including the axial direction of the rolled wire material, the average area of the sulfide present in the range from the outermost surface of D / 8 6 [mu] m 2 average aspect ratio of not more than, more the sulfide the are 5 or less.
[0073]
　Average area of the sulfide 6 [mu] m 2 becomes larger than the stress is concentrated around the coarse sulfides during cold forging regardless of its form, as a starting point of cracking. The average area of sulfides 6 [mu] m 2 becomes larger than, also deteriorates hydrogen embrittlement resistance after quenching and tempering. Therefore, in the rolled wire rod according to the present embodiment, the average area of the sulfide present in the range from the outermost surface of D / 8 6 [mu] m 2 is less. The average area of the sulfide is preferably smaller.
[0074]
　Further, in the rolled wire rod according to the present embodiment, regardless of the size of the sulfides, it is less than 5 the average aspect ratio is the ratio of the maximum length and the maximum width of the sulfide. This suppresses to become a starting point of the extended sulfides cracking. The average aspect ratio of the sulfide is preferably as small as possible.
[0075]
　The manufacturing process (D) wire
　in the present embodiment, not only the chemical composition of the rolled wire, by controlling the production conditions of the rolled wire, by controlling the form of product-rolled tissue or inclusions, cold forging it is possible to provide a rolled wire material which can be suitably used as a component. The following illustrates the preparation method for controlling tissue and after product roll, the form of inclusions. Incidentally, the form of the chemical composition and tissue morphology and inclusions of the rolled wire rod, does not impair the effects of the present invention as long as it is within the scope of the present invention described above. Incidentally, if, rolled wire form of chemical composition and structure are within the scope of the present invention, even if obtained by the production process other than the manufacturing process below, the rolled wire is included in the present invention.
[0076]
　Specifically, C, Mn, Cr, Ti, S, by adjusting the chemical components such as N, melting by a converter or an electric furnace or the like, the cast steel ingot or slab to slabbing, slab and products for rolling the material is. To obtain a rolled wire rod of the present invention, at the stage of slabbing a steel ingot or slab, and high-temperature heating to at least 1280 ° C. or higher, subjected to primary rolling or rolling ratio 6 immediately after the soaking state for more than 30min , then it is necessary to cool. This coarse sulfides was produced in the slab stage divided by the primary rolling, some of the more coarse sulfide is dissolved by high-temperature heating, in order to finely re-precipitated in the subsequent cooling process is there. Further, coarse Ti carbonitrides generated on the cast slab by solidifying, carbonitride or carbide, such as Ti carbide, once a solid solution in the steel by high temperature heating, in order to finely reprecipitated during cooling is there.
[0077]
　Then reheated steel slab obtained by blooming, although products rolled across heat to the wire of predetermined diameter, the heating temperature at the time of product rolling at this time to 1050 ° C. or less. This, too high a heating temperature during product rolling, fine carbonitrides and carbides reprecipitation dissolved again by high-temperature heat treatment described above, these along with the ferrite transformation at the time after product rolled and cooled nitrides and carbides is used to deposit matched. With such coherent precipitation occurs, will increase the strength of product after rolling, cold forgeability deteriorates. Incidentally, Ti carbonitride not dissolved by heating at the time of product rolling, carbonitrides and carbides such as Ti carbide, without affecting the strength after the product rolling, without deteriorating the cold forgeability, cold Ac during quenching after forging 3 be heated above points has an effect of suppressing abnormal grain growth of crystal grains.
[0078]
　Furthermore, the finish rolling product rolled, finally finish the wire of predetermined diameter. Finish rolling is a rolling to be carried out in the finishing rolling mill train in the final step of the product roll, the processing speed Z and 5 ~ 15 / sec, performed at rolling temperature range of 750 ~ 850 ° C.. Machining speed Z is a value calculated reduction of area of the wire by the final rolling and the finish rolling time by the following <2> equation. Further, the finish rolling temperature, the temperature of the finish rolling mill train delivery side may be measured using an infrared radiation thermometer.
　Z = -ln (1-R) / t ···· <2>
　where, R is the reduction of area of the wire by finish rolling, t refers to the finish rolling time (sec).
　The cross-sectional reduction rate R finish rolling prior to the cross-sectional area A of the rolled wire rod 0 R = (A from area A after the rolling and finishing 0 -A) / A 0 is determined by.
[0079]
　Finish rolling time t is the time to pass through the rolling mill train finish rolled wire rod, the distance from the first rolling mill of the finishing rolling mill train to the end of the rolling mill is determined by dividing the average transport speed of the rolled wire be able to.
[0080]
　And when the temperature of the finish rolling is lower than 750 ° C., if the processing speed of the finish rolling is 15 / sec, greater than for the ferrite transformation from the austenite grains of non-recrystallized begins structure after cooling becomes too fine strength Te is high, cold forging deteriorates. In contrast, and when the temperature of the finish rolling is 850 ° C. greater, if the processing speed is less than 5 / sec, the austenite grains after recrystallization coarse, because the starting temperature of ferrite transformation is reduced, the cooling ferrite fraction is reduced the tissue after, cold forgeability deteriorates. Incidentally, after the finish rolling is completed, the range of cooling rate until the surface temperature of the rolled wire material is 500 ° C. is preferable to be 0.2 ~ 5 ℃ / sec.
Example
[0081]
　Hereinafter, specific examples illustrate the present invention.
　In the present invention, it may not satisfy the requirements of the present invention by the manufacturing process of steel of the same chemical composition. Therefore, firstly, the chemical components with substantially the same steel manufactures rolled wire rod in different conditions was investigated the effects of the present invention. The chemical components using different steel manufactures rolled wire rod in the same conditions to investigate the effects of the present invention.
[0082]
　First, an example where the chemical components with substantially the same steel, by employing the components shown in Table 1, further the table shows the condition (primary rolling heating temperature, the primary rolling reduction ratio, the wire rolling heating temperature, and finish rolling temperature according) to give the steel piece after slabbing, and product rolled from the billet to a wire rod having the predetermined diameter, to obtain a rolled wire rod (invention examples A0 and Comparative examples A1 ~ A6). Incidentally, in Table 1, "-" notation, the content of the element is an impurity level, which means that it can be determined not to be substantially contained.
[0083]
[Table 1]

[0084]
　Next, an example of using the chemical composition different steel adopted The components shown in Table 2, at the stage of obtaining a steel strip from the slab, the primary rolling heating temperature was 1280 ° C. or higher, the primary rolling reduction ratio 6 It was slabbing or more. Then, using the obtained steel slab, product rolled (wire rod rolling heating temperature: 1030 ~ 1050 ° C., the finish rolling temperature: 750 ~ 850 ° C.) performing, rolled wire rod (invention examples 1 to 14 and Comparative Examples 15 to to obtain a 25). Incidentally, in Table 2, "-" notation, the content of the element is an impurity level, indicating that it can be determined not to be substantially contained. Incidentally, in Table 2, "-" notation, the content of the element is an impurity level, which means that it can be determined not to be substantially contained.
[0085]
　Further also shown an index Y1 of the following formula in Table 2.
　Y1 = ([Ti] -3.4 × [N]) / [S] ··· <1>
　where, [Ti], [N] , [S] , the content by mass percent of the respective elements representing the amount.
[0086]
　Y1 is an expression that represents the content balance of Ti, N, S contained in the steel, hardenability and can be used as a high-strength cold component forging, the form of sulfides present in the surface vicinity of the steel material to control the size is a parameter required to provide excellent control of abnormal grain growth during cold forgeability and quenching, and excellent hydrogen embrittlement resistance after quenching and tempering.
[0087]
　As described above, when a rolled wire rod of the embodiment of the [S] ≧ 0.0010 is, [Ti] is (4.5 × [S] + 3.4 × [N]) or more, (8.0 × [S] + 3.4 × [N]) it is a requirement less. This requirement, by using an index Y1, is expressed as 4.5 ≦ Y1 ≦ 8.0.
[0088]
　In the case of [S] ≦ 0.0010, the lower limit of [Ti] is (4.5 × [S] + 3.4 × [N]). This is expressed as 4.5 ≦ Y1. On the other hand, the upper limit of [Ti] in the range of [S] ≦ 0.0010 is represented as (0.008 + 3.4 × [N]), does not depend on Y1. In this region, [Ti] is allowed to be in the area to be Y1> 8.0.
[0089]
[Table 2]

[0090]
　Table 1 shows the test number of the inventive example chemical components A0, and Comparative Examples A1 ~ A6 was prepared rolled wire rod as follows.
　That is, Invention Examples A0 shown in Table 1, insert the slab in a furnace of 1290 ° C., after soaking for 2 hours, and slabbing Immediately after removal out of the furnace, and the steel strip of 162mm square. At this time, the rolling ratio was 7.5.
[0091]
　On the other hand, Comparative Example A1 inserts a billet into a furnace of 1180 ° C., after soaking for 2 hours, and slabbing Immediately after removal out of the furnace, and the steel strip of 162mm square. At this time, the rolling ratio was the same 7.5 as A0.
　In Comparative Example A5 inserts a billet into a furnace of 1200 ° C., after soaking for 2 hours, and slabbing Immediately after removal out of the furnace, and the steel strip of 162mm square. At this time, the rolling ratio was the same 7.5 as A0.
[0092]
　In Comparative Example A2, A6 is a cross-sectional area is inserted the smaller slab than A0 and A1 in an oven at 1290 ° C., after soaking for 2 hours, and slabbing Immediately after removal out of the furnace, 162 mm It was a steel piece of the corner. At this time, the rolling ratio of Comparative Example A2 was 2.4, the rolling ratio of Comparative Example A6 was 5.3.
[0093]
　Next, a slab containing these for rolling material, after heating at 1040 ° C., respectively, as the finish rolling temperature becomes the predetermined diameter at 820 ° C. at product rolled to produce a rolled wire rod. At this time, a range of machining speed is 5 ~ 15 / sec by finish rolling, after the finish rolling completion, was adjusted cooling an average cooling rate until the transformation is complete as 0.4 ° C. / sec.
[0094]
　Comparative Example A3, A4 are the same chemical composition as the invention example A0, a slab of 162mm angle obtained by slabbing under the same conditions as A0 and products for rolling material, before the product rolling heating temperature and the finish rolling by changing the temperature, to produce a rolled wire rod. Specifically, Comparative examples A3, after heating the heating temperature of the product rolled as 1050 ° C., subjected to finish rolling to rolling temperature becomes the predetermined diameter at 950 ° C., to produce a rolled wire rod. At this time, the processing speed of the finish rolling is in the range of 5 ~ 15 / sec, after the finish rolling completion, the average cooling rate until the transformation is complete was 0.4 ° C. / sec.
[0095]
　Comparative Example A4, after heating the heating temperature of the product rolled as 1150 ° C., subjected to finish rolling to rolling temperature becomes the predetermined diameter at 830 ° C., to produce a rolled wire rod. At this time, the processing speed of the finish rolling is in the range of 5 ~ 15 / sec, after the finish rolling completion, the average cooling rate until the transformation is complete was 0.4 ° C. / sec.
[0096]
　Comparative Example A6 is a different chemical composition as invention example A0, a slab of 162mm angle obtained by slabbing under different conditions as A0 and products for rolling material, the temperature of the product before rolling heating temperature and finish rolling change to produce a rolled wire rod. Specifically, Comparative Example A6, the primary rolling temperature of 1290 ℃℃, an example in which 5.3 the primary rolling reduction ratio after heating the heating temperature of the product rolled as 1040 ° C., rolling temperature 820 perform finish rolling so as to have a predetermined diameter at ° C., to produce a rolled wire rod. At this time, the processing speed of the finish rolling is in the range of 5 ~ 15 / sec, after the finish rolling completion, the average cooling rate until the transformation is complete was 0.4 ° C. / sec.
[0097]
　Next, Test No. of the invention examples of chemical components shown in Table 2 1-14 and Comparative Examples 15 to 25 were produced rolled wire rod as follows.
　That is, the steel of the chemical compositions shown in Table 2, were melted in a vacuum melting furnace. Melting the slab is inserted into a furnace heated to 1290 ° C., after soaking for 2 hours, then the billet of 140mm square and slabbing immediately after removal out of the furnace, which a product for rolling materials did. At this time, the rolling ratio was 7.4. Then, after heating the material products rolled at 1030 ~ 1050 ° C., the finish rolling temperature is adjusted to be between 750 ~ 850 ° C., and carrying out the product rolled to a wire having a diameter of 14 mm. At this time, a range of machining speed Both 5 ~ 15 / sec by finish rolling, after the finish rolling completion, the average cooling rate until the transformation is complete was 0.4 ~ 2 ℃ / sec.
[0098]
　The fabricated rolled wire rod as described above (invention example A0 and Comparative Examples A1 ~ A6 as well as the invention examples 1 to 14 and Comparative Examples 15 to 25), the ferrite fraction (area%) in the form of inclusions (sulfide average area ([mu] m 2 ) and sulfide average aspect ratio), cold forgeability (deformation resistance and cracking), were investigated for the presence of hydrogen embrittlement resistance, and abnormal coarse grains occurs.
[0099]
　(Investigation of the microstructure of rolled wire rod (ferrite fraction))
　was cut to a length of 10mm the rolled wire rod, and fills the resin to cross-section (cross section perpendicular to the axis of the rolled wire rod) is the test surface, mirror the polishing was carried out. Then corroded surface with a 3% nitric acid alcohol (nital etchant) to appear microstructure. Then, D / 4 from the surface of the rolled wire rod: at the position (D diameter of the rolled wire rod), using an optical microscope to identify the "phase" to shoot a microstructure photograph of 5 fields at a magnification of 200 times. As a result, in any of the specimens of Examples and Comparative Examples, 95% or more in area ratio was confirmed to be a ferrite-pearlite. Further, using image analysis software to measure the ferrite area ratio in each field, and the ferrite fraction at each example seeking these average values.
[0100]
　(Embodiment (sulfide average area of inclusions ([mu] m 2 ) and sulfide average aspect ratio) of the study)
　after cutting the rolled wire rod length 12 mm, (the plane including the axis of the wire) vertical section of the rolled wire material to be so that the sample surface was filled resin was subjected to mirror polishing. Test surface is parallel to the longitudinal direction of the rolled wire rod, D / 8 from the surface of the rolled wire material: inclusions scanning electron microscope suspected of sulfides present in the range up to the position of (D diameter of the rolled wire rod) It was identified by (SEM). More specifically, in the range of D / 8 from the surface of the rolled wire rod, any observation area within the test surface, identified 100 places at 500-fold magnification. Area of each observation region is 254 micrometers × 190 .mu.m, the total area of the observation region is 4.8 mm 2 was. Then, based on the contrast, which is determined by a reflection electron image of the observation area, to identify the inclusions was measured area and aspect ratio of each inclusions identified. Finally, sulfide average area of each example seeking these mean values (area%), and was sulfide average aspect ratio. Note that inclusions identified was confirmed to be a sulfide by energy dispersive X-ray spectroscopy (EDS).
[0101]
　(Cold forgeability (deformation resistance, investigation of cracks))
　cold forgeability, and deformation resistance at the time of cold working, and evaluated by the presence or absence of cracking of the rolled wire material. More specifically, from the position corresponding to the center portion of the rolled wire rod, cut and machined round bar of φ10 × 15mmL, and measuring the deformation resistance by cold compression test, also investigated whether cracking during processing did. The specimen strain stepwise compressed until (epsilon = 2.2), the maximum load at the time of compression was measured to calculate the deformation resistance. Furthermore, as judged by visual observation whether a crack specimen surface occurs.
[0102]
　Deformation resistance, deformation resistance calculated from the maximum load 100 kgf / mm 2 while the case was less than (980 MPa) and "good", the deformation resistance is 100 kgf / mm 2 where was (980 MPa) or higher It was "not good". For cracks, while the case of the even cracking did not occur which part of the specimen as "good", and the case where cracks in at least one surface of the test piece caused by the "not good". Then, comprehensive evaluation of the case none of the evaluation of the deformation resistance and the cracking was "good" while an overall rating as "good", in the case of at least one of the failure of deformation resistance and cracking as "not good" did.
[0103]
　(Water Survey of hydrogen embrittlement)
　subjected to quenching and tempering in the rolled wire rod was adjusted tensile strength of the rolled wire material to approximately 1200 MPa. Then, the tensile strength by machining with respect to the wire adjusted to give the annular V-notched test piece shown in FIG. In Figure 2, numerical values are not shown unit, the corresponding parts of the dimensions of the test piece (in mm) shows a. Further, in the figure, "φ numeric" indicates a diameter (mm) of the designated sites, "60 °" indicates the V notch angle "0.175R" indicates a V notch bottom radius. Incidentally, the test piece for each invention example and comparative example, respectively, were prepared ten.
[0104]
　Next, for each of Invention Examples and Comparative Examples, using an electrolytic charging method, in a plurality with an annular V-notch test pieces, and hydrogen was introduced in various concentrations. Incidentally, the electrolytic charging method was performed as follows. That is, in a state of immersing the test piece in the ammonium thiocyanate solution, by generating the anode potential on the surface of the test piece, incorporating hydrogen into the test strip within. Then, by forming a zinc plating film on the surface of the specimen to prevent dissipation of hydrogen in the specimen.
[0105]
　Then, as tensile stress of the nominal stress 1080MPa is loaded against V-notch section of the test piece was performed a constant load test to load the constant load. Broken specimen during testing, and for both ruptured not test pieces were conducted heating analysis method using a gas chromatograph, to measure the amount of hydrogen in the test piece. After the measurement, for each of Invention Examples and Comparative Examples, and the maximum amount of hydrogen was not broken specimens is defined as the critical diffusible hydrogen amount Hc.
[0106]
　Further, as a reference JIS G4053 (2008) critical diffusible hydrogen amount of steel having a chemical composition corresponding to SCM435 of (0.40ppm), for each of Invention Examples and Comparative Examples, the critical diffusible hydrogen amount 0 in the case of more than .40ppm was evaluated as "good", it was evaluated in the case of less than 0.40ppm as "not good".
[0107]
　(Abnormal coarse grains occurs in the presence or absence of investigation)
　cold forgeability (deformation resistance, crack) reheating the processed specimens at survey to confirm the presence or absence of abnormality coarse grains occurs. Specifically, the cold worked test specimen, an inert gas atmosphere, was heated for 30 minutes in an oven at 880 ° C., subjected to quenching is immersed in an oil bath at 60 ° C., by observing the microstructure of the specimen to observe the presence or absence of abnormal coarse grains. As internal tissue can be observed in the test piece, the hardened specimens were cut parallel to the axial direction and filled resin. Then, the old austenite grain boundaries corroded surface to allow revealing, were observed microstructure by an optical microscope. Magnification was 500 times, the test piece before cold working D1 / 4: observing the position corresponding to (D1 the diameter of the test piece), the case where only the sizing was observed "good", the abnormal coarse grains There was a case that has been observed is determined to be "not good". Incidentally, the tissue only sizing was observed, which exhibited a prior austenite grains of about 5 ~ 30 [mu] m, the abnormal coarse grains are observed tissue, grown crystal grains were mixed beyond 100 [mu] m.
[0108]
　Described above, the ferrite fraction (area%) in the form of inclusions (sulfide mean area ([mu] m 2 ) and sulfide average aspect ratio), cold forgeability (deformation resistance and cracking), hydrogen embrittlement resistance , and the presence or absence of abnormality coarse grains occurs, the survey results for (for invention examples A0 and Comparative examples A1 ~ A6) Table 3 and Table 4 (for Inventive examples 1 to 14 and Comparative examples 15 to 25). Further, in Tables 3 and 4, it meets the relation between FIG. 1 [Ti] and [S], are also shown also.
[0109]
[table 3]

[0110]
[Table 4]

[0111]
　According to Tables 3 and 4, the invention example A0 and invention examples 1 to 14, which has a present predetermined amount for each element, the relationship between the Ti content and S content range of the shaded region in FIG. 1 filled and, furthermore, ferrite fraction, the average area of ​​the sulfide, and an average aspect ratio of the sulfide is in the range of the present application given. Therefore, the invention in Examples A0 and Inventive Examples 1 to 14, cold forgeability, it is understood that the presence or absence of hydrogen embrittlement resistance, and abnormal coarse grains occurs, favorable results for any is obtained.
[0112]
　In contrast, Comparative Examples A1 ~ A6 and Comparative Examples 15 to 25, either having no application predetermined amount for each element, the relationship between the Ti content and S content satisfy the range of the shaded region in FIG. 1 or not, or ferrite fraction, the average area of ​​the sulfide, and an average aspect ratio of the sulfide is not in the scope of the present application given. Therefore, in Comparative Examples A1 ~ A6 and Comparative Examples 15 to 25, cold forgeability, the presence or absence of hydrogen embrittlement resistance, and abnormal coarse grains occurs, it can be seen that no good results are obtained for either at least .
Industrial Applicability
[0113]
　According to the present invention, even without applying a spheroidizing annealing before cold forging, or even with a short time the spheroidizing annealing, and effectively suppress cracking during cold forging, the spheroidizing annealing it is possible to provide a rolled wire rod having excellent hydrogen embrittlement resistance after the subsequent quenching and tempering. Accordingly, the present invention is promising in that it can particularly be used as material for cold forging.

WE claims

[Requested item 1]
　By
　　mass%, C: less than 0.20% or more
　　0.40%, Mn: less than 0.10% or more
　　0.40%, S: less than
　　0.020%, P: less than%
　　0.020, Cr: 0. 70% 1.60% or
　　less, Al: 0.005% to 0.060% or
　　less, Ti: 0.010% to 0.080% or less
　　B: 0.0003% to 0.0040% or less, and
　　N: 0.0020% or more 0.0080% or less
contained, balance being Fe and
　impurities, Ti, N, and each content of S (mass%), respectively [Ti], [N], and [S] Then,
　　in the case of [S] ≦ 0.0010, [Ti ] is (4.5 × [S] + 3.4 × [N]) or more and below (0.008 + 3.4 × [N] ) some one
　　in the case of [S] ≧ 0.0010, [Ti ] is (4.5 × [S] + 3.4 × [N]) or more, and, (8.0 × [S] + 3.4 × [ N] Or less,
　the internal tissue is ferrite fraction of 40% or more in area ratio, a mixed structure of ferrite and pearlite,
　in a cross-section of a plane including the axial, diameter in the case of a D (mm) average area of the sulfide present in the range from the outermost layer to D / 8 position 6 [mu] m 2Or less, the average aspect ratio of the sulfide is 5 or less, and wherein the rolled wire material.
[Requested item 2]
　Instead of a part of the Fe, by mass%, Si: 0% 0.40% or more and less than Nb: 0% or more 0.050% containing at least one of the following, rolled wire rod according to claim 1 .
[Requested item 3]
　Instead of a part of the Fe, by mass%, Cu: 0.50% or less, Ni: 0.30% or less, Mo: 0.05% or less, and V: 0.05% or less of at least one of containing, rolled wire rod according to claim 1 or 2.
[Requested item 4]
　Instead of a part of the Fe, by mass%, Zr: 0.05% or less, Ca: 0.005% or less and Mg: containing 0.005% or less of at least one, of claims 1 to 3 It rolled wire rod according to any one.