Technical field
[0001]
　The present invention is suitable as a material for cold forging, to cold forgeability and耐粗particle reduction characteristics excellent rolling steel bars and rolled wire rod. The present invention is particularly, the abnormal grain growth during quenching can be suppressed with a HRC hardness 34 or more after quenching and tempering, suitable as a material for high strength cold forged parts, cold forging excellent in rolling steel bars and rolled wire rod on.
　The present application, on November 18, 2014, claiming priority based on Japanese Patent Application No. 2014-233973, filed in Japan, the contents of which are incorporated here.
Background technique
[0002]
　Cold forging surface skin part after forging, and excellent dimensional precision, also, parts produced by cold forging has a lower manufacturing cost compared to parts produced by hot forging, the yield was good is there. Therefore, cold forging, gears and shafts, are widely applied to the manufacture of components for various industrial machines and building construction, including the automobile, such as bolts.
[0003]
　Recently, automobile, in the mechanical structural components for use in industrial machinery progress in size and weight, size is progressing in building construction. Against this background, even higher strength in the parts produced by cold forging is desired.
[0004]
　These The cold forged parts, conventionally, carbon steel for machine structural use steel as defined in JIS G 4051, such as mechanical structural alloy steels as defined in JIS G 4053 have been used. These steels are generally the product rolled steel by hot to the shape of the steel bars and wire rods, after repeating spheroidizing annealing, the drawing and cold drawing to process, and molded into the part shape by cold forging It is adjusted to a predetermined intensity and hardness by heat treatment such as quenching and tempering.
[0005]
　Mechanical structural steel as described above are contained relatively high carbon content of about 0.20 to 0.40%, can be used as a high-strength part through a tempering process. On the other hand, mechanical structural steel as described above strength steel bars and wire rods are rolled steel as a forging material is high. Therefore, in the manufacturing process, if the steel softened by adding a cold drawing and subsequent spheroidizing annealing step, parts cold wear and cracking of the mold is likely to occur at the time of the forging for shaping, also, such as cracks in parts occurs, manufacturing problems arise.
[0006]
　In recent years, the parts are high strength, it tends to complicate part shapes. Since part shape is concerned the generation of complex I see cracks, a steel high strength can be obtained by quenching and tempering for the purpose of further soften before cold forging, or prolonged spheroidizing annealing, cold measures such as repeated or multiple times drawing process and spheroidizing annealing step is taken.
[0007]
　However, these measures are not only costly, such as labor and equipment costs, energy loss is also large. Therefore, steel can be omitted or briefly up this process has been desired.
[0008]
　Against this background, reduced for the purpose of omitted or briefly the spheroidizing annealing treatment, C, Cr, the strength of rolled steel by reducing the content becomes forging material alloying elements such as Mn in terms of the boron steel and the like it has been proposed, supplemented with decrease in hardenability by the reduction of alloying elements in the boron-added.
[0009]
　For example, Patent Document 1, intercrystalline grain coarsening prevention properties and the cold forgeability and the excellent cold forging hot rolled steel and a manufacturing method thereof are disclosed. Specifically, Patent Document 1, 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: includes 0.020 to 0.100%, and the steel diameter in a matrix 0.2μm following TiC or Ti a (CN) 20 pieces / 100 [mu] m 2 and having a higher, grain growth characteristics and cold forgeability and between excellent cold forging heat rolled steel and a manufacturing method thereof are disclosed.
[0010]
　Furthermore, cold working steel for machine structural use and a manufacturing method thereof is disclosed in Patent Document 2. Specifically, containing C, Si, Mn, P, S, Al, N, and Cr, the metal structure, and a pearlite and pro-eutectoid ferrite, the sum of the perlite and pro-eutectoid ferrite to the total structure with an area ratio of 90% or more, the area rate a of the pro-eutectoid ferrite, Ae = (0.8-Ceq) × 96.75 (where, Ceq = [C] + 0.1 × [Si] +0.06 × [Mn] + 0.11 × [Cr] ([(element name)] the content of each element (means mass%)) has a relationship of a> Ae between Ae represented by, pro-eutectoid ferrite and cold working for machine structural steel and a manufacturing method thereof, wherein the average particle size of the ferrite in the pearlite is 15 ~ 25 [mu] m is disclosed. in addition, the cold working of the patent document 2 in use steel for machine structural use, by applying the usual spheroidization treatment, sufficient soft It is disclosed that can realize structure formation.
[0011]
　According to the technique disclosed in Patent Document 1, it is possible to reduce the hardness of the steel material. Therefore, it is possible to cold forging at low cost and can be provided with a grain coarsening prevention properties during heating for quenching. However, the steel of Patent Document 1, has a lower Cr content of the steel, low hardenability, is to increase the strength of the components is limited.
[0012]
　Cold working for machine structural steel disclosed in Patent Document 2, by performing ordinary spheroidizing annealing treatment, are possible softening, it is applicable to high-strength components. However, the balance of the content of chemical components of the steel is not optimized, also ferrite fraction of tissue rolled steel is substantially less. Therefore the steel of the state which has been subjected to product rolled left and short spheroidizing annealing process, cracks when used in the cold forging of the parts occurs, there is a problem that can not be manufactured of parts at low cost.
CITATION
Patent Document
[0013]
Patent Document 1: Japanese Patent Publication No. 3443285
Patent Document 2: Japanese Laid-Open Patent Publication No. 2013-227602
Summary of the Invention
Problems that the Invention is to Solve
[0014]
　The present invention has been made in view of the above situation, provides hardenability, rolled steel in the form of cold forgeability and 耐粗 particle reduction characteristics excellent high strength cold forged parts for the bars or wire rods an object of the present invention is to. Here, the excellent hardenability, HRC hardness of the central portion after quenching and tempering says that the 34 or more. In addition, the excellent cold forgeability, refers to the generation of be omitted or briefly the spheroidizing annealing before cold forging cracking during cold forging is effectively suppressed. In addition, the excellent 耐粗 particle reduction characteristics, refers to abnormal grain coarsening is suppressed during heating of the quenching treatment.
Means for Solving the Problems
[0015]
　The present inventors have conducted various studies to solve the problems described above. As a result, we obtained the following findings.
[0016]
　Be omitted or briefly the (a) spheroidizing annealing process, to ensure the cold forging to the extent capable of molding parts, the as products rolled steel (rolled steel bar or rolled steel) Tensile strength the must be equal to or less than 750MPa. The internal organization except the surface layer portion where there is a possibility that the decarburized layer is produced is ferrite-pearlite structure, and it is necessary to ferrite fraction exceeds 40%.
[0017]
　To ensure a high part strength by (b) quenching and tempering is with by increasing the C content increase the quenching hardness (hardness after quenching), Mn, is contained alloying elements such as Cr quenching it is necessary to increase the sex. That is, for use as a high-strength cold forged parts must ensure sufficient hardening hardness and hardenability required therefor.
[0018]
　It improves the forgeability (c) cooling to ensure the hardness after quenching by improving hardenability, in order to further satisfy all of the 耐粗 particle reduction characteristics, C, Si, Mn, Cr, Ti , the balance of the element content and the Nb content or the like with due consideration, it is necessary to control also the internal tissue.
[0019]
　The present invention has been completed based on the above findings and has as its gist is as follows.
[0020]
　(1) cold forging for rolling steel bars or rolled wire rod according to an embodiment of the present invention, the chemical composition, in mass%, C: 0.24 ~ 0.36% , Si: less than 0.40%, Mn : 0.20 ~ 0.45%, S: less than 0.020%, P: less than 0.020%, Cr: 0.70 ~ 1.45 %, Al: 0.005 ~ 0.060%, Ti: 0.010% than 0.050% or less, Nb: 0.003 ~ 0.050%, B: 0.0003 ~ 0.0040%, N: 0.0020 ~ 0.0080%, Cu: 0 ~ 0 .50%, Ni: 0 ~ 0.30 %, Mo: 0 ~ 0.050%, V: 0 ~ 0.050%, Zr: 0 ~ 0.050%, Ca: 0 ~ 0.0050%, and mg: 0 ~ 0.0050%, contain, the balance being Fe and impurities, the following formula <1>, Y1, Y2 is the following formula represented by <2> Satisfy the relationship represented by 3>, a tensile strength of not more than 750 MPa, and an internal tissue ferrite-pearlite structure, in the internal tissue is ferrite fraction of 40% or more.
　　= Y1 [Mn] × [Cr]
　　· · · <1> Y2 = 0.134 × (D / 25.4- (0.50 × √ [C])) / (0.50 × √ [C]) · · ·
　　<2> Y1> Y2 · · · <3>
　However, [C] in the formula, [Mn], [Cr] represents the content by mass percent of the respective elements, D is rolled steel bar or it represents the diameter of a unit-mm rolled wire rod.
[0021]
　(2) above (1) cold forging for rolling steel bars or rolled wire rod according to the chemical composition of the steel material, by
　mass%, Cu: 0.03
　~ 0.50%, Ni: 0.01 ~
　% 0.30, Mo: 0.005 ~ 0.050%, and
　V: 0.0050.050%
containing at least one member selected from the group consisting of
[0022]
　(3) above (1) or cold forged parts for rolling steel bars or rolled wire rod according to (2), the chemical composition, in mass%, Zr: 0.003 ~ 0.050%, Ca: 0. 0,005 to 0.0050%, and Mg: it may contain one or more selected from the group consisting of 0.0005 to 0.0050%.
[0023]
　The "impurities" in "Fe and impurities" as the balance, a component contained in the steel during unintentionally, when the industrial production of steel materials, ores as raw material, scrap, or manufacturing environment It refers to something which is mixed and the like.
[0024]
　The rolled steel bar or rolled wire rod, refers to a rolled steel having the shape of a product rolled left bars and wire rods in hot. Hereinafter, the description of the present invention, it may be expressed as collectively "rolled steel bar or rolled wire rod" "rolled bars" or "steel material". Moreover, the products rolling at hot, sometimes expressed as "hot rolling".
Effect of the Invention
[0025]
　Cold forged parts for rolling bars of the above embodiment of the present invention (rolled steel bar or rolled wire rod), the tensile strength is less 750 MPa, the internal metal structure is ferrite fraction of 40% or more of ferrite-pearlite structure, and, since the content of each element is controlled, cold forgeability is excellent in hardenability and 耐粗 particle properties. Therefore, by using the rolling bars of the present invention as a material, be omitted or briefly the spheroidizing annealing treatment, can be formed into parts by cold forging, HRC hardness 34 or more through the quenching and tempering it can be obtained in high strength cold forged parts. Further, the rolling bars of the present invention, since the abnormal grain growth of crystal grains even when heated to the austenite region during quenching is suppressed, the resulting high strength cold forged parts, to suppress the variation in component strength can.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026]
FIG. 1 is a view showing the shape of a bolt and forging in the examples.
And FIG. 2] Cr content and the Mn content is a diagram showing the relationship between the hardenability.
DESCRIPTION OF THE INVENTION
[0027]
　Hereinafter, (sometimes referred to as rolling bars according to the present embodiment) the present invention cold forging for rolling steel bars or rolled wire rod according to an embodiment with be described in detail. "%" For the content of each element in the following description means "mass%".
[0028]
　(A) Chemical Composition (chemical composition):
[0029]
　C: 0.24 ~ 0.36%
　C increases the hardenability of steel, an element which contributes to improvement of strength. To obtain this effect, the C content is 0.24% or more. Further, when desired to increase the quenched hardness of cold forging components, it is preferable that the content of C 0.26% or more. On the other hand, when the C content exceeds 0.36%, cold forgeability is degraded. Therefore, the C content to 0.36% or less. If further desired to increase the cold forgeability, it is preferable that the C content is 0.33% or less.
[0030]
　Si: 0.40% less
　after hot rolling to reduce the tensile strength of the rolled steel (rolled left), since Si content is as low as possible preferably, the Si content may be 0%. Meanwhile, Si because strengthening ferrite by solid solution strengthening, for the purpose of obtaining an effect of increasing the temper hardness of cold forging components, may contain Si. However, since Si content is significantly reduced cold forgeability 0.40% or more, even if to be contained, the Si content should be less than 0.40%. From the viewpoint of cold forgeability, it is preferable that the Si content is less than 0.30%, even more preferably less than 0.20%, when also considering the tensile strength of the rolled steel, 0.10 % is even more preferred or less.
[0031]
　Mn: 0.20 ~
　0.45% Mn is an element increasing the hardenability of the steel, to obtain this effect, the Mn content is set to 0.20% or more. To enhance the hardenability, Mn is preferably not less than 0.25%. On the other hand, when the Mn content exceeds 0.45%, by starting temperature of ferrite transformation is reduced during cooling after finish rolling, bainite is generated with ferrite fraction is reduced, resulting in inter-steel cold forging is lowered. Therefore, the Mn content is set to 0.45% or less. More preferably to less 0.42% of Mn content If you want to improve cold forgeability, and more preferably to 0.40% or less, it is even more preferred that 0.35% or less .
[0032]
　S: less than 0.020%
　S is contained as an impurity. S is an element lowering the cold forgeability, the content thereof is preferably small. In particular, when the S content is more than 0.020%, MnS becomes stretched coarse form, cold forgeability is remarkably lowered. Therefore, to limit the S content to less than 0.020% or. Preferably, less than 0.010%.
[0033]
　P: less than 0.020%
　P is an impurity contained. P not only lowers the cold forgeability, is an element which causes the cracking during quenching was segregated at the grain boundaries during heating to austenite temperature region. Therefore, P content is preferably small. In particular, the occurrence of reduction and cracking of cold forgeability becomes significant when P content is 0.020% or more. Therefore, the P content less than 0.020% or. Preferably, less than 0.010%.
[0034]
　Cr: 0.70 ~
　1.45% Cr, like Mn, is an element which enhances the hardenability of the steel. To obtain this effect, the Cr content is 0.70% or more. To obtain a stable and high hardenability, it is preferred that the Cr content 0.80% or more, and more preferably 0.90% or more. On the other hand, when the Cr content exceeds 1.45%, and the increase hardenability decreases starting temperature of ferrite transformation reduces the ferrite fraction at the time of cooling after finish rolling, bainite is generated. As a result, it lowers the cold forgeability of the steel. Therefore, the Cr content is 1.45% or less. Further, when desired to increase the cold forgeability is preferably a Cr content 1.30% or less, and more preferably 1.20% or less.
[0035]
　Al: 0.005 ～ 0.060
　Pasento Al is an element having a deoxidizing effect. Further, Al is, AlN was formed by combining the N, the austenite grains during hot rolling fine by pinning effect, which is an element having an effect of inhibiting the formation of bainite. To obtain these effects, the Al content is 0.005% or more. If you want to more reliably suppress the generation of bainite is desirably the content of Al of 0.015% or more, and more preferably 0.020% or more. On the other hand, when the Al content exceeds 0.060%, the effect is not only saturated, cold forgeability coarse AlN is generated is reduced. Therefore, the Al content is 0.060% or less. From the viewpoint of enhancing the cold forgeability, it is preferable that Al content is 0.050% or less, more preferably at most 0.045%.
[0036]
　Ti: 0.010% than 0.050% or less
　Ti combines with N, C, or carbide, to form a nitride or carbo-nitrides, fine austenite grains during hot rolling by their pinning effect is an element which has the effect of. Refinement of austenite grains, and suppress the formation of bainite in the cooling step after finish rolling, which contributes to the improvement of the ferrite fraction. Further, Ti, since the N forming a solid solution in the steel by fixing as TiN to suppress the formation of BN, also has the effect of increasing the effect of improving hardenability by B. To obtain these effects, the Ti content is 0.010 percent. Ti content is preferably 0.020% or more, and more preferably 0.025 percent. On the other hand, if the Ti content exceeds 0.050%, greater precipitated carbides and carbonitrides of fine Ti during finish rolling, the ferrite is enhanced tensile strength becomes excessively high. Therefore, the Ti content is 0.050% or less. Preferably the Ti content is 0.040% or less, more preferably 0.035% or less.
[0037]
　Nb: 0.003 ~
　0.050% Nb combines with C and N, carbides, to form a nitride or carbo-nitrides, or to form a complex carbonitride with Ti, their pinning is an element having the effect of refining the austenite grains during hot rolling due to the effect. Refinement of austenite grains to suppress the bainite in the cooling step after finish rolling, which contributes to the improvement of 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. To obtain these effects, the Nb content is 0.003% or more. Nb content is preferably, and more preferably to 0.010% or more Nb content when it is desired to obtain these effects more stably 0.005% or more. On the other hand, when the Nb content exceeds 0.050%, not only these effects are saturated, cold forgeability is degraded. Therefore, the Nb content is 0.050% or less. Preferably Nb content is 0.040% or less, more preferably 0.030% or less.
[0038]
　B: 0.0003 ~ 0.0040%
　B is an element effective for enhancing the hardenability by containing trace amounts. To obtain this effect, B content of 0.0003% or more. If it is desired to further enhance the hardenability, it is preferable that the content of B 0.0005% or more, and more preferably 0.0010% or more. On the other hand, B content exceeds 0.0040%, the hardenability improving effect is saturated, cold forgeability is degraded. When improving the cold forgeability Further, it is preferable to the B content is 0.0030% or less, and more preferably 0.0025% or less.
[0039]
　N: 0.0020 ~ 0.0080%
　N combines with Al, Ti or Nb to produce a nitride or carbo-nitride, when quenching the austenite grains finer and cold forging components during hot rolling It has an effect of suppressing abnormal grain growth during heating. To obtain this effect, the N content is 0.0020% or more. Preferably it is greater than or equal to 0.0030% or. On the other hand, when the N content is excessive, the effect is not only saturated, nitrides are produced by bonding the N and B, it weakens the effect of improving hardenability by B. Therefore, the N content to less 0.0080%. To stably improve the hardenability, it is preferred that the N content is less than 0.0070%, and more preferably set to 0.0060% or less.
[0040]
　The bars of the present embodiment, in addition to the content of each element, it is necessary to control the balance of the content of the element. Specifically, the Y1 represented by the following formula <1>, Y2 of the formula <2> is, satisfies the relationship represented by the formula <3>.
Y1 = [Mn] × [Cr ] formula
<1> Y2 = 0.134 × (D / 25.4- (0.50 × √ [C])) / (0.50 × √ [C]) Formula <
2> Y1> Y2 formula <3>
　wherein, [C] in the formula, [Mn], [Cr] represents the content by mass percent of the respective elements, D is the rolling bars diameter (mm ) represents the.
[0041]
　If Y1> Y2, general quenching, tempering (after heating to a temperature range of, for example, 880 ~ 900 ° C., subjected to quenching by oil cooling, 400 ° C. ~ out the tempering at 600 ° C.) after by refining, center with the hardenability to be 34 or more HRC hardness in part.
[0042]
　Equation <1> to formula <3> will be described.
　Y1, as described above, is a value represented by Mn, weight percent of the product of Cr contained in the steel, a hardenability parameter required for high strength cold forged parts for rolling bars.
　Y2 in diameter to heat the rolled bars is D (mm) to a temperature above Ac3 point, in the case where the quenching by oil cooling, rolling bars D / 2 from the surface is a center of (mm) It affects the fraction of martensitic structure obtained in position, is a parameter indicating the relation between D and [C]. The cooling rate of quenching by oil cooling can also vary the diameter D of the rolled bars, but is generally 10 ~ 40 ℃ / sec approximately.
　Ac3 point, based on the chemical composition, the known formulas, such Ac3 = 912.0-230.5 × C + 31.6 × Si-20.4 × Mn-39.8 × Cu-18.1 × Ni-14 it can be calculated from .8 × Cr + 16.8 × Mo. Or, experimentally, the expansion rate of the steel during heating heating measures, can be estimated from the change of the expansion ratio.
[0043]
　Quenching after tempering treatment by tempering, to obtain a HRC hardness 34 or more at the center, the quenching hardness prior to the tempering of the rolled bars center (D / 2 parts) 45 or more HRC hardness it is necessary to control so as to. Then, quenching hardness of the order of 45 or more HRC hardness has to be adjusted greatly affects C, Mn, the content of Cr in the quenching hardness.
　If tissue is martensite, the hardness, while being substantially determined by the C content, C content is 45 or more if HRC hardness within a range of rolling bars according to the present embodiment. Therefore, in order to ensure 45 or more quenching hardness in HRC hardness, (90% or more tissue fraction) tissue mainly after quenching it may be martensite.
[0044]
　Results of study of the present inventors, by the Mn content and the Cr content to a predetermined value or more, was found at the center of the rolling bars, that more than 90% of martensite after quenching is obtained . Specifically, Y1, expressed by the product of the content of Mn and Cr to enhance the hardenability, affects the fraction of martensitic structure obtained at the center of the rolling bars, the D and the [C] is greater than the parameter Y2 representing the relationship, tissue in the center of the rolling bars after quenching was found to contain 90% or more of martensite. Therefore, the rolling bars according to the present embodiment, and Y1> Y2. On the other hand, in the case of Y1 ≦ Y2 is incomplete hardened structure such as bainite or ferrite is generated during quenching, it can not be secured martensite 90% or more. In this case, it decreases the strength and hydrogen embrittlement resistance.
[0045]
　2, the diameter of the rolling bars is 15 mm, and the Cr content and the Mn content in the case C content is 0.30%, which is a diagram showing the relationship between the hardenability. In Figure 2, Mn content and the Cr content is, if the upper side of the boundary line B, Y1> is Y2, the rolling bars more than 90% martensite structure at the center of the after quenching Become.
[0046]
　Specific measure of hardenability, hardenability test method JIS G 0561 steel (end quenching method), in the so-called Jomini test, there in hardness J7mm at 7mm position of at least the quenching end HRC hardness 45 or more Bayoi.
[0047]
　Since the hardness of the rolling bars after quenching depends also on the diameter D of the rolled bars, from the viewpoint of hardenability, it is desirable diameter D of the rolled bars are small, applied to the high strength cold forged parts to case, it is more preferable as the rolling bars in the range of 8 ~ 16 mm preferably about diameter 6 ~ 35 mm.
[0048]
　Rolling bars according to the present embodiment contains the above chemical components, and the balance basically that the Fe and impurities. However, instead of a part of the remainder of Fe, optionally, Cu, Ni, Mo, V, Zr, may contain at least one element selected from Ca and Mg. However, since these elements do not necessarily have to be contained, the lower limit is 0%. Here, the "impurities", is a component contained in the steel during unintentionally, when the industrial production of steel materials, ores as raw material, scrap, or those mixed from such manufacturing environment points.
[0049]
　Hereinafter will be described the effects of the Cu, Ni, Mo, V, Zr, Ca and Mg are optional elements, the preferable content of the case to be contained.
[0050]
　Cu: 0.50% or less
　Cu is an element enhancing the hardenability, may be contained. To get this effect stably, it is preferable that the Cu content is 0.03% or more, more preferably not more than 0.05% or more. On the other hand, when the Cu content exceeds 0.50%, excessively high hardenability, and bainite is generated after finish rolling, deteriorating the cold forgeability. Therefore, even in the case of containing, the Cu content is set to 0.50% or less. From the viewpoint of improving the cold forgeability, it is preferred that the Cu content of the case of containing at most 0.30%, more preferably not more than 0.20%.
[0051]
　Ni: 0.30% or less
　Ni is an element to increase hardenability, may be contained. To get this effect stably, it is preferable that the Ni content is 0.01% or more, more preferably in the range from 0.03% or more. On the other hand, when the Ni content exceeds 0.30%, not only the effect is saturated, too high hardenability, and bainite is generated after finish rolling, deteriorating the cold forgeability. Therefore, even in the case of containing, the Ni content is set to 0.30% or less. Ni content in the case of incorporating in view of improving the cold forgeability is preferably not more than 0.20%, more preferably not more than 0.10%.
[0052]
　Mo: 0.050% or less
　Mo is an element to strengthen the steel by solid solution strengthening, thereby greatly improving the hardenability of the steel. To obtain this effect, it may contain Mo. To get this effect stable is, Mo content is preferably 0.005% or more. On the other hand, when the Mo content exceeds 0.050% bainite and martensite formed after the finish rolling, deteriorating the cold forgeability. Therefore, even in the case of containing, the Mo content is 0.050% or less. Mo content in the case of incorporating in view of improving the cold forgeability is preferably at 0.030% or less, and more preferably not more than 0.020% or.
[0053]
　V: 0.050% or less
　V combines with C and N, it is an element forming carbides, nitrides or carbo-nitrides. Also, V is also a element to improve the hardenability of steel when the content is small. Therefore, it may be contained V. To get these effects stable is, V content is preferably 0.005% or more. On the other hand, when the V content exceeds 0.050%, the strength of the steel material is increased by precipitation carbides or carbonitrides, deteriorating the cold forgeability. Therefore, the V content even in the case of containing 0.050% or less. V content case of containing in terms of improving the cold forgeability is preferably at 0.030% or less, and more preferably not more than 0.020% or.
[0054]
　Zr: 0.050% or less
　Zr is an element having an effect of improving the hardenability of the steel material containing a trace amount. It may contain Zr traces that purpose. To get this effect stable is, Zr content is preferably 0.003% or more. On the other hand, when the Zr content exceeds 0.050%, coarse nitrides are produced, cold forgeability is degraded. Therefore, the Zr content even in the case of containing 0.050% or less. Zr content of the case of containing in terms of improving the cold forgeability is preferably at 0.030% or less, and more preferably not more than 0.020% or.
[0055]
　Ca: 0.0050% or less
　Ca combines with S, to form a sulfide, act as nuclei for MnS. MnS which was nuclei of CaS is finely dispersed, since the nuclei for ferrite is precipitated during cooling after finish rolling, the presence of finely dispersed MnS, thereby improving the ferrite fraction. That is, by containing Ca, since it is possible to improve the ferrite fraction, may contain Ca. To get this effect stably, it is preferable that the Ca content is 0.0005% or more. On the other hand, even when Ca content exceeds 0.0050%, not only the effect is saturated by Ca to generate by reacting with oxygen in the steel coarse oxides with Al, cold forgeability There is reduced. Therefore, even in the case of containing, the Ca content is 0.0050% or less. From the viewpoint of improving the cold forgeability, it is preferred that the Ca content of the case of containing at 0.0030% or less, and more preferably in the range from 0.0020%.
[0056]
　Mg: 0.0050% or less
　Mg combines with S, to form a sulfide, an element acting as nuclei for MnS, has the effect of dispersing MnS finely. MnS that is dispersed finely, since ferrite is precipitated the MnS dispersed during cooling after finish rolling as nuclei, thereby improving the ferrite fraction. To obtain this effect, it may contain Mg. To get this effect stably, it is preferable that the Mg content is 0.0005% or more. On the other hand, Mg content be greater than 0.0050%, the effect is saturated. Further, Mg has a poor addition yield, since exacerbate manufacturing cost, it is preferred that the amount of Mg in the case of containing at 0.0030% or less, and more preferably in the range from 0.0020%.
[0057]
　(B) For the tensile strength of steel
[0058]
　Rolling bars according to the present embodiment is excellent in cold forgeability. Therefore, even if treated omitted or in a short time the spheroidizing annealing process after product rolling, or mold life is shortened during cold forging, cracks in the component will not be or occur during molding. This not only chemical components of the adjusted steel as described above, by controlling the production conditions of the steel material, and controlled to suitable tissue or precipitates rolled steel to cold forging, the strength of the steel due to the fact that has declined. In the present embodiment, and is excellent in cold forgeability, for example, means that cracks do not occur even when processed with round bar φ10.5mm × 40mmL cut from rolled bars bolt shown in Figure 1.
[0059]
　If the tensile strength exceeds 750 MPa, possibility of cracking of the parts it occurs increases during cold forging. Therefore, in the rolling bars according to the present embodiment, after controlling the tissue as described below, it is necessary that the tensile strength 750MPa or less.
　Even tensile strength beyond 750 MPa, by performing the long spheroidizing annealing treatment for about 20 hours, or more times of spheroidizing annealing treatment (e.g. 10 hours × 2 times), cracking of parts occurs during cold forging It becomes Nikuku. However, the rolling bars according to the present embodiment is intended to be a shorter time to heat treatment spheroidizing annealing omitted or at least within 10 hours to complete can be secured cold forgeability. To this end, the rolling bars according to the present embodiment, an upper limit on the tensile strength. Preferably the tensile strength of the rolled bars are less 700 MPa, more preferably at most 650 MPa.
[0060]
　(C) the internal organization of the steel
[0061]
　Rolling bars according to the present embodiment is excellent in cold forgeability. Therefore, omitted spheroidizing annealing process after product rolling has been required conventionally about 20 hours, or is treated with about half of the time, or, equal to spheroidizing had done more than once annealing process to once even the, and it decreased mold life in cold forging, no failure, such as cracking of the molded part. This not only adjustment of the chemical composition of the steel, by controlling the production conditions of the rolled bars, the metal structure of the rolled bars is because that controls a form suitable for cold forging.
[0062]
　Specifically, the rolling bars according to the present embodiment, is likely to decarburized layer is generated, portions of the tissue excluding the surface layer portion ranges from the surface of the 100 [mu] m (internal organization) is a ferrite-pearlite structure a is, and the fraction of ferrite is 40% or more. Here, (tissue sum of the area ratio of ferrite and pearlite area ratio of not less than 95%) is a ferrite-pearlite structure, the tissue more than 95% of the total area ratio is a mixed structure of ferrite and pearlite you say. Further, in the measurement of ferrite fraction, the ferrite does not include a ferrite phase between lamellar cementite contained in pearlite. A mixed structure of ferrite and pearlite is overall 95% in area ratio means that the total area of ​​the tissue other than the ferrite and pearlite, such as martensite and bainite is less than 5%. For good cold forgeability, mixed structure of ferrite and pearlite should the overall 95% in area ratio, it is desirable that 100%.
[0063]
　In internal tissue, when the ferrite fraction is less than 40%, tensile strength can not be secured or less was also good cold forgeability 750 MPa, or cracked in the part during molding, mold life is short a problem made arise. It is desirable ferrite fraction is 45% or more, more preferably not more than 50% or more. The upper limit of the ferrite fraction is not particularly specified, in order to make the ferrite content to 80 percent while the hot rolling, the lamellar cementite forming the pearlite structure needs to be spheroidized, for its length after rolling since the soaking time required, cost Kasami, it is difficult to industrially realize. Thus the upper limit of the ferrite fraction may be 80%.
　Also, if the mixed structure of ferrite and pearlite is less than 95% of the total area ratio, the hard tissue, such as martensite and bainite, the tensile strength of the rolled bars may exceed the 750 MPa. Further, since the hard tissue as a starting point of fracture, the cold forgeability is concerned may be lowered.
[0064]
　Identification of the tissue, and calculating the area ratio, for example, as follows.
　After cutting the rolled bars to length of 10 mm, and fills the resin to cross-section is the test surface, performing mirror polishing. Then, corroded surface is revealing the microstructure with 3% nitric acid alcohol (nital corrosion solution). Then, D / 4 position of the rolled steel bar or rolled wire rod: by photographing a microstructure photograph of five fields by an optical microscope of 500 times magnification at a position corresponding to (D diameter of rolled steel) identifying "phase" , using image analysis software to measure the ferrite area ratio of each field as a ferrite fraction to obtain an average value. Furthermore, the fraction of the total of ferrite and pearlite are likewise determined pearlite fraction is obtained by summing the ferrite fraction and the pearlite fraction.
[0065]
　(D) Preferred manufacturing process
　according to the present embodiment the rolling bars not only chemical composition of the steel, it is important to control the rolling it remains tissue. Therefore, the chemical composition and tissue morphology as long as the scope of the present invention are included in the rolling bars according to the present embodiment regardless of the production method.
　However, the steel having a predetermined chemical composition, by applying the production process including the steps indicated by the following rolling remains tissue can be controlled stably and preferred ranges. Hereinafter, a preferred production conditions will be described in detail.
[0066]

　First, obtain C, Si, Mn, Cr, and adjust the chemical components such as Nb, by casting a molten steel smelted by a converter and ordinary electric furnace or the like a steel ingot or slab. The resulting steel ingot or slab, and slabbing to billet (material for products rolling). To obtain a rolled bars according to the present embodiment, at a stage before the pre-rolling heating step to be described later, it is cooled after securing the at least 30min or more soaking time in a high temperature heat above 1250 ° C., high temperature it is preferable to carry out the soaking treatment. This was generated during solidification Nb (C, N) and NbC, Ti (C, N) ), the coarse carbonitride or carbide TiC or the like, once form a solid solution in the steel, finely re in the cooling step in order to precipitate. Fine carbonitrides and carbides precipitated in the cooling process is to act as pinning particles during product rolling during heating between subsequent conduct heat, which contributes to the coarse growth prevention of austenite grains. The result, ferrite precipitated at the time of after product roll cooling ferrite fraction increases with miniaturization.
　Hot soaking may be carried out at the stage of heating at the time of blooming the steel ingot or slab, in slabbing a steel ingot or slab after heating to slabbing to a temperature below 1250 ° C. the slab manufactured, may be reheated to 1250 ° C.. Anyway, by heating below 1050 ° C. for later heated to high temperatures to 1250 ° C. or more at the stage before than products rolled in hot, it is effective to secure at least 30min or more soaking time.
[0067]

　Then, prior to the rolling, heating the steel slab. It is preferable to 1050 ° C. or less to the extent possible rolling heating temperature at this time. A too high heating temperature fine carbonitrides and carbides reprecipitation dissolved again by the aforementioned high-temperature soaking, the strength after the product rolling since coherent precipitation in accordance with the ferrite transformation at the time after product rolled and cooled increases, the cold forgeability is concerned may be lowered. Not dissolved by heating before rolling Nb (C, N) and NbC, Ti (C, N) , carbonitride or carbide of TiC does not affect the strength after the product rolling, deteriorating the cold forgeability let not. Further, carbonitride or carbide of Nb has the effect of inhibiting the abnormal grain growth of crystal grains even when heated above Ac3 point during quenching after cold forging.
[0068]

　After heating, the product rolling comprising finish rolling, the steel bars or wire having the predetermined diameter. Finish rolling is rolled to be implemented in the finishing rolling mill train in the final step of product rolling. In the finish rolling, machining speed Z and 5 ~ 15 / sec, it is preferable to perform the rolling temperature range of 750 ~ 850 ° C.. Machining speed Z is a value calculated reduction of area of the steel material by the finish rolling and the finish rolling time by the following formula (i). Further, the finish rolling temperature to a temperature of finish rolling mill train delivery side may be measured using an infrared radiation thermometer. Temperature of finish rolling, austenite grains before the ferrite transformation by managing machining speed becomes finer, since the ferrite fraction is high, predetermined tensile strength, it can be obtained tissue.
[0069]
　Z = {- ln (1- R)} / t ···· (i)
　where, R is the reduction of area of the steel material by the finish rolling, t refers to the finish rolling time (in seconds).
[0070]
　Reduction of area R is finish rolling prior to the cross-sectional area A of the rolling bars 0 R = (A from area A after the rolling and finishing 0 -A) / A 0 is determined by.
[0071]
　The finish rolling time t is a time through the rolling mill train finish rolling bars (in seconds), divided by the distance from the first rolling mill of the finishing rolling mill train to the end of the mill at an average transport speed of the rolled bars it can be determined by.
[0072]
　Or below 750 ° C. temperature of the finish rolling, if the processing speed of the finish rolling is too high, the ferrite transformation begins austenite grains non-recrystallized. In this case, the tissue becomes too fine strength after cooling becomes excessively high, the cold forgeability is degraded. Conversely, or above the temperature of 850 ° C. of finish rolling, if the processing speed is low, the austenite grains after recrystallization coarse, starting temperature of ferrite transformation becomes low. In this case, the ferrite fraction of the structure after cooling is reduced, cold forgeability is degraded.
[0073]

　After the finish rolling is completed, the surface temperature of the rolled steel material it is preferable to cool the cooling rate until 500 ° C. as 0.2 ~ 5 ℃ / sec.
　If the average cooling rate to 500 ° C. is less than 0.2 ° C. / sec, that the time to transformation from austenite to ferrite increases, there is a concern that the surface layer of the steel material decarburization occurs. On the other hand, if the average cooling rate is 5 ° C. / sec greater, hard tissue such as martensite and bainite are feared to be formed.
[0074]
　If the manufacturing process, including the above-described manufacturing process, while ensuring the hardenability quenching hardness can be obtained in usable as a high-strength cold forging component level, it is omitted or briefly the spheroidizing annealing process good cold forgeability can be realized tensile strength can be obtained rolled bars with internal tissue stability.
　Further, the rolling steel bars or wire according to the present embodiment, and cold forging, by performing quenching and tempering, it is possible to obtain a high strength cold forged parts.
Example
[0075]
　Specifically examples illustrate the present invention are shown below, but the invention is not limited to these examples.
[0076]
　In steel of the same chemical composition, the tissue will vary by the manufacturing process. Therefore, even if satisfying the chemical composition of the present invention, it may not satisfy the requirements of the present invention. Therefore, the first individual steel chemical composition obtained by preparing the same steel at different production conditions were evaluated tissue and characteristics. Then smelted different steel ingot of chemical composition, to produce the rolled steel under the same conditions, it was evaluated tissue and characteristics of each steel was obtained.
[0077]
　Specifically, first, a steel having a composition shown in Table 1 was melted in an electric furnace, and heating the resulting steel ingot to 1200 ° C., was slabbing steel pieces 162mm square. In steel having a composition shown in Table 1, A0, A1, A2, A3 have the same chemical composition, B0, B1, B2, B3 have the same chemical composition. In Table 1, "-" notation, the content of the element is an impurity level, indicating that it can be determined not to be substantially contained.
[0078]
　These steels were the steps from billet after slabbing until product rolled to wire rods having a predetermined diameter by changing the production conditions to obtain a steel bar or wire rod.
　That is, the present invention example A0, B0 shown in Table 1, insert the steel strip 162mm angle in an oven at 1280 ° C., after heating 2hr soaking, subjected to high-temperature soaking treatment to cool to room temperature taken out of the furnace . Next, after heating the steel slab at 1040 ° C., subjected the product rolled to finish rolling temperature is a predetermined diameter at 820 ° C., to produce a rolled steel bar or 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, was cooled to the average cooling rate until 500 ° C. as 0.4 ° C. / sec.
[0079]
　Comparative Example A1, B1 are omitted hot soaking with billet 162mm angle is the same chemical components respectively A0, B0, were products rolling. Rolling conditions are the same as A0, B0, after heating at 1040 ° C., subjected the product rolled to finish rolling temperature is a predetermined diameter at 820 ° C., to produce a rolled steel. At this time, the processing speed of the finish rolling is in the range of 5 ~ 15 / sec, after the finish rolling completion, was adjusted cooling an average cooling rate until 500 ° C. as 0.4 ° C. / sec.
[0080]
　Comparative Example A2, A3, B2, B3 is a slab of 162mm angle is the same chemical composition as Working Example A0, B0 inserted into a furnace heated to 1280 ° C., after heating 2hr soaking, taken out to the outside of the furnace It was subjected to a high-temperature soaking treatment to cool to room temperature. Next, by setting the temperature of the product before rolling heating temperature and finish rolling, as shown in Table 1, to prepare a rolled steel bar or rolled wire rod.
[0081]
　Specifically, Comparative Example A2, B2 after heating the heating temperature of the product rolled at 1050 ° C., subjected to finish rolling to rolling temperature becomes the predetermined diameter at 920 ~ 940 ° C., to produce a rolled steel. At this time, the processing speed of the finish rolling is in the range of 5 ~ 15 / sec, after the finish rolling completion, was cooled to the average cooling rate until 500 ° C. as 0.4 ° C. / sec.
[0082]
　Comparative Example A3, B3 after heating the heating temperature of the product rolled at 1150 ° C., subjected to finish rolling to rolling temperature becomes the predetermined diameter at 830 ° C., to produce a rolled steel. At this time, the processing speed of the finish rolling is in the range of 5 ~ 15 / sec, after the finish rolling completion, was cooled to the average cooling rate until 500 ° C. as 0.4 ° C. / sec.
[0083]
　Then the steel No. chemical components shown in Table 2 About 1-29 were produced rolled steel in the following manner. Table 2 in the "-" notation, the content of the element is an impurity level, indicating that it can be determined not to be substantially contained.
[0084]
　Specifically, a steel having a composition shown in Table 2 were melted in an electric furnace, and heated to give the steel ingot to 1200 ° C., and slabbing steel pieces 162mm square. Then, insert the steel strip 162mm angle in an oven at 1280 ° C., after heating 2hr soaking, subjected to high-temperature soaking treatment to cool to room temperature taken out of the furnace. Then after heating the material products rolled at 1030 ~ 1050 ° C., were products rolled adjusted so that the finish rolling temperature is between 750 ~ 850 ° C.. In this case, the range of finish rolling by the machining rate are both 5 ~ 15 / sec, after the finish rolling completion, was cooled to the average cooling rate until 500 ° C. at 0.4 ~ 2 ℃ / sec.
[0085]
[Table 1]

[0086]
[Table 2]

[0087]
　The method rolled steel bar or the diameter of the rolled wire material manufactured in the tensile strength, ferrite fraction, the hardness after quenching and tempering, cold forgeability, the results of investigating occurrence or non-occurrence of abnormal grain growth Table 3, Table 4 show.
[0088]
　Tensile strength of the rolled steel bar or rolled wire rod, ferrite fraction, the sum of the ferrite fraction and the pearlite fraction, hardness after quenching, hardness after quenching and tempering, cold forgeability, the occurrence or non-occurrence of abnormal grain growth , it was investigated by the methods described below.
[0089]
　<1> Investigation of tensile strength of the rolled steel bar or rolled wire rod:
　the position of the center of the rolling steel bars or rolled wire rod, so that the longitudinal direction of the test piece is the rolling direction of the steel, 14A No. test specified in JIS Z 2241 piece (however, parallel portion diameter: 6 mm) were collected. Then, conduct a tensile test at room temperature gage length as 30 mm, it was determined tensile strength.
[0090]
　<2> ferrite fraction of rolling steel bars or rolled wire rod, the pearlite fraction Survey:
　after cutting the rolled steel bar or rolled wire rod to a length of 10 mm, and fills the resin to cross-section is the test surface, mirror-polished It was carried out. Then corroded surface with a 3% nitric acid alcohol (nital etchant) to appear microstructure. Then, D / 4 position of the rolled steel bar or rolled wire rod: by photographing a microstructure photograph of five fields by an optical microscope of 500 times magnification at a position corresponding to (D diameter of the rolled steel bar or rolled wire rod) "phase" identified, the ferrite area ratio of each field was measured as the ferrite fraction using image analysis software, the average value was determined. Similarly, determine the pearlite fraction was determined also the sum of the ferrite fraction and the pearlite fraction.
[0091]
　<3> quenched hardness of investigation:
　after cutting a length of 200mmL rolling steel bars or rolled wire rod, 880 ° C. × was 60min heated in an Ar gas atmosphere, and quenching by immersion in oil bath 60 ° C.. Then, after collecting a 10mm length of specimen from the longitudinal center position of the quenching round bar subjected to polishing the cross section as a test surface was measured HRC hardness at the center of the cross section.
[0092]
　<4> tempered hardness of the study:
　The remaining hardened round bar removed out of the furnace after heating 425 ° C. × 60min in an air atmosphere for cooling (air cooling) in the process, was tempered. After harvesting the 10mm length of specimen from the center of the round bar after tempering, polishing is performed a cross section as the test surface was measured HRC hardness at the center of the cross section.
[0093]
　The abnormal grain growth after cold forgeability and cold forging was evaluated by cold forging actually bolt using the rolling steel bars or rolled wire rod.
[0094]
　<5> cold forgeability studies:
　from a position corresponding to the center portion of the rolling steel bars or rolled wire rod was cut by machining a round rod of φ10.5mm × 40mmL. Then, degreasing, after pickling, zinc phosphate treatment (75 ° C., soaking time 600 sec), and a metal soap treatment (80 ° C., soaking time 180 sec) performed, consisting of zinc phosphate coating and metal soap film on the surface with a lubricating processing film, and the material of the bolt forged. Bolt forging after molding pushing the shaft in one step th forging to allow forging to the shape shown in FIG. 1, a mold designed to perform the machining to shape the bolt head and the flange portion in two steps first , and attached to the hydraulic forging press, it was cold forging. Unit of the values in Figure 1 is in mm.
　Cold forgeability when bolts molded to determine whether cracks on the bolt surface is caused by visual. A case in which a case of cracking on the bolt surface has occurred NG, cracking any part did not occur was evaluated as OK. Cracks in the bolt surface was mainly generated at the tip of the bolt head flange.
[0095]
　<6> reheating during abnormal grain growth studies:
　To confirm the occurrence of abnormal grain growth during the reheating after cold forging, 880 ° C. × a molded bolts by cold forging in a furnace of an inert gas atmosphere after 60min heated, subjected to quenching is immersed in an oil bath at 60 ° C., to confirm presence or absence of generating the abnormal grain growth was observed bolts microstructure. More specifically, as the internal tissue can be observed in the flange and the shank base of the R portion of the bolt, a hardened bolt cut parallel to the axial direction, and filling a resin, after mirror polishing, the old austenite grain boundary microstructure near the surface of the bolt flange and the shank root R part to corrode the surface was observed by an optical microscope so that it can emerge. Magnification was 500 times, and observed from the bolt flange and the shank root R part surface to a position of a depth of 0.5 mm, a case were both granulated OK, abnormal grain grown crystal grains are observed the case was judged as NG. Incidentally, any tissue that is sized are exhibited prior austenite grains of about 5 ~ 30 [mu] m, the steel grains grown beyond 100μm was mixed, it is determined that there is abnormal grain growth.
[0096]
[table 3]

[0097]
[Table 4]

[0098]
　From Table 3, since a is the test number A0, B0 the present invention examples, both satisfy the chemical composition and the formula <1> to <3>, and steel manufacturing conditions are appropriate, the tensile strength both are at 750MPa or less, the ferrite fraction had a ferrite-pearlite structure is 40% or more. Further, quenching hardness of the steel center is also a HRC hardness 45 or more, cold forgeability also no problem, even abnormal grain growth reheated after cold forging is not generated.
[0099]
　In contrast, Test Nos A1 ~ A3, B1 ~ B3 tensile strength, not ferrite fraction reaches the target, also tissue rather than the ferrite-pearlite structure, cold forgeability, the occurrence of abnormal grain growth any one or more of the has not reached the target.
[0100]
　Although Test No. A1 is the same chemical composition as A0, since the omitted high-temperature soaking treatment before products rolling, ferrite fraction has a 40% or less, poor cold forgeability, and abnormal grain growth also of occurrence has not been suppressed.
[0101]
　Although Test No. A2 are the same chemical composition as A0, the temperature of the finish rolling was as high as 940 ° C., a tensile strength of more than 750 MPa, the ferrite fraction is 40% or less, as a result, poor cold forgeability.
[0102]
　Although Test No. A3 are the same chemical composition as A0, the heating temperature of the product rolling was as high as 1150 ° C., the tensile strength becomes higher 750 MPa, as a result, poor cold forgeability.
[0103]
　Although Test No. B1 is the same chemical composition as B0, since the omitted high-temperature soaking treatment before products rolling, ferrite fraction is 40% or less, as a result, poor cold forgeability. Nor it is suppressed occurrence of abnormal grain growth.
[0104]
　Although Test No. B2 is the same chemical composition as B0, the temperature of the finish rolling was as high as 920 ° C., a tensile strength of more than 750 MPa, the ferrite fraction is 40% or less, poor cold forgeability.
[0105]
　Although Test No. B3 are the same chemical composition as B0, the heating temperature of the product rolling was as high as 1150 ° C., a tensile strength of more than 750 MPa, the ferrite fraction is 40% or less, as a result, poor cold forgeability .
[0106]
　From Table 4, rolled steel bar or rolled wire rod of test numbers 1 to 16 are the examples of the present invention, both satisfying the chemical composition and the formula <1> to the <3>, and steel manufacturing conditions are appropriate since, both the tensile strength is at 750MPa or less, tissue, ferrite fraction was ferrite-pearlite structure is 40% or more. Further, quenching hardness of the steel center portion HRC45 or more, temper hardness is at 34 or higher in HRC, cold forgeability was also no problem. In addition, by heating after cold forging quenching abnormal grain growth does not occur.
[0107]
　In contrast, either the rolled steel bar or rolled wire rod chemical components of the test numbers 17 to 29 or the formula <1>, satisfying the requirements of the present invention the value of Y1, Y2 indicated by <2> and yet not, steel center quenching hardness, cold forgeability, has not reached the target any one or more for the occurrence of abnormal grain growth.
[0108]
　Although Test No. 17 and 18 chemical components satisfy the specified range of the present invention, because the value of Y1 is Y2 or less, quenching the hardness of the steel center is less than HRC 45, it is insufficient hardenability. Further, as a result, temper hardness is less than HRC34.
[0109]
　For Test No. 19 in which the C content is below the specified range of the present invention, quenching the hardness of the steel center is less than HRC 45, quenching hardness is not sufficient. Further, as a result, temper hardness is less than HRC34.
[0110]
　Test No. 20 has exceeded the prescribed range of the content of the present invention and C, the tensile strength above 750 MPa, because the fraction of ferrite is 40% or less, poor cold forgeability.
[0111]
　Test No. 21 is the content of Mn is higher than the specified range of the present invention, since the starting temperature of ferrite transformation becomes low, ferrite fraction is 40% or less, poor cold forgeability.
[0112]
　Test No. 22 tensile strength 750MPa or less, although the ferrite fraction is 40% or more, the content of S exceeds the specified range of the present invention, MnS is coarse, poor cold forgeability.
[0113]
　Test No. 23 is the content of Cr is below the specified range of the present invention, quenching the hardness of the steel center is less than HRC 45, is insufficient hardenability.
[0114]
　Test No. 24 since Nb is not contained, the occurrence of abnormal grain growth is not suppressed.
[0115]
　Test No. 25 is the content of Ti is below the specified range of the present invention, quenching the hardness of the steel center is less than HRC 45, it is insufficient hardenability. Further, as a result, temper hardness is less than HRC34. This, B is considered that it is the cause precipitated as BN react with N.
[0116]
　Test No. 26 has exceeded the prescribed range of the content of the present invention of Ti, the tensile strength of not less than 750 MPa, poor cold forgeability.
[0117]
　Test No. 27 is the content of B is below the specified range of the present invention, quenching the hardness of the steel center is less than HRC 45, it is insufficient hardenability. Further, as a result, temper hardness is less than HRC34.
[0118]
　Test No. 28 has exceeded the prescribed range of the content of the present invention of Cr, since bainite is generated, the tensile strength above 750 MPa, and a ferrite fraction is less than 40%, poor cold forgeability .
[0119]
　Test No. 29 is the content of V exceeds the specified range of the present invention. Since the V precipitate as fine carbonitrides and carbides, but ferrite fraction is 40% or more, a tensile strength of not less than 750 MPa, poor cold forgeability.
Industrial Applicability
[0120]
　By using a high strength cold forged parts for rolling bars of the present invention as a material, be omitted or briefly the spheroidizing annealing treatment, can be molded by cold forging, heated abnormal grain growth of crystal grains even There is suppressed, it is possible to obtain a high strength cold forged parts having excellent hardenability.
DESCRIPTION OF SYMBOLS
[0121]
　Boundary line B

The scope of the claims
[Claim 1]
　Chemical composition, in
　　mass%, C: 0.24
　　~ 0.36%, Si: less than%
　　0.40, Mn: 0.20
　　~ 0.45%, S: less than
　　0.020%, P: 0. less than%
　　020,
　　Cr: 0.70 ~ 1.45%, Al: 0.005
　　~ 0.060%, Ti: 0.010% than 0.050% or
　　less, Nb: 0.003 ~ 0.050% , 　　B:
　　　　0.0003 ~ 0.0040%, N: 0.0020 　　~ 0.0080%, Cu: 0 ~ 0.50%, 　　Ni: 0 ~ 0.30%, Mo: 0 ~ 0.050%, 　　V:　　0 ~ 0.050%, 　　Zr: 0 ~ 0.050%, Ca: 0 ~ 0.0050%, and 　　Mg: 0 ~ 0.0050%, contain, the balance being Fe and impurities, 　the following equation <1>, satisfy the relationship Y1, Y2 represented by <2> is represented by the following formula <3>, 　a tensile strength of 750MP Less and, and 　an internal tissue ferrite-pearlite structure, 　the inside tissue, ferrite fraction is 40% or more cold forging for rolling steel bars or rolled wire rod, characterized in that. 　　Y1 = [Mn] × [Cr ] ··· <1>

　　= 0.134 × Y2 (D / 25.4- (0.50 × √ [C])) / (0.50 × √ [C])
　　· · · <2> Y1> Y2 · · · <3>
　However, [C] in the formula, [Mn], [Cr] represents the content by mass percent of the respective elements, D is representative of the diameter in mm of the rolled steel bar or rolled wire rod.
[Claim 2]
　The chemical composition, in
　mass%,
　Cu: 0.03 ~
　0.50%, Ni: 0.01 ~ 0.30%, Mo: 0.005 ~ 0.050%, and
　V: 0.0050.050 %
containing at least one member selected from the group consisting of
cold forging for rolling steel bars or rolled wire rod according to claim 1, characterized in that.
[Claim 3]
　The chemical composition, in
　mass%,
　Zr: 0.003 ~ 0.050%, Ca: 0.0005 ~ 0.0050%, and
　Mg: 0.0005 ~ 0.0050%
1 selected from the group consisting of containing more species
cold forging parts for rolling steel bars or rolled wire rod according to claim 1 or 2, characterized in that.