[0001]The present invention relates to a rolling bars are steel bar or wire rod remains hot rolled (rolling Mom material), more particularly, is cold forging, and cold forging tone is refining the treated part on rolling bars for quality goods.
Background technique
[0002]Recently, various industrial machines, shafts and parts such as bolts are used as automobile parts and building structure may dimensional accuracy and yield, from the viewpoint of manufacturing cost, are produced by cold forging. Such cold forging, but also or used while cold working, if even higher strength is desired, refining (quenching and tempering process) the conducted by high strength Figure It is. Among such cold forging, those treated temper, herein referred cold forged microalloyed products.
[0003]
　As the steel to achieve high strength by performing the tempering treatment, there are JIS G4053 (2008) a defined mechanical structural alloy steel. Machine structural alloy steel, for example, chromium steel, chromium molybdenum steel, and a nickel-chromium-molybdenum steel. These steels are, in order to increase mainly hardenability and temper softening resistance, a high content of expensive Mo, Ni.
[0004]
　In recent years, Mo, has been soaring prices of alloying elements such as Ni, which is supply and demand environment is also likely to change. Therefore, reducing these alloy elements, or while omitting to suppress the steel cost, required for cold forging, the steel is excellent in mechanical properties such as fatigue properties and hydrogen embrittlement resistance are required.
[0005]
　Therefore, instead of the alloying elements such as Mo and V, steels containing boron (B) (B-containing steel) is widely used. B, like the alloy elements such as Mo and V, increasing the hardenability of steel. However, B-containing steel is treated cold forging and tempering, if the tensile strength is used as the above cold forging microalloyed article 1000 MPa (e.g. bolts), there is a case hydrogen embrittlement resistance is low. Therefore, excellent B containing steel hydrogen embrittlement resistance has been demanded. B-containing steel further, since the component (such as bolts) by cold forging as described above, obtained excellent cold workability.
[0006]
　Fatigue strength, hydrogen embrittlement resistance or cold workability excellent B-containing steel, JP 2012-162798 (Patent Document 1), JP-A-9-104945 (Patent Document 2), Japanese open 2013-227602 (Patent Document 3), and has been proposed in JP 2001-234277 (Patent Document 4).
[0007]
　Patent Document 1 proposes an excellent bolt steel delayed fracture resistance (hydrogen embrittlement resistance). Bolt steel disclosed in this document, C: less than 0.20 ~ 0.40%, Si: 0.20 ~ 1.50%, Mn: 0.30 ~ 2.0%, P: 0.03 % or less, S: 0.03% or less, Ni: 0.05 ~ 1.0%, Cr: 0.01 ~ 1.50%, Cu: 1.0% or less, Al: 0.01 ~ 0.10 %, Ti: 0.01 ~ 0.1%, B: 0.0003 ~ 0.0050%, and N: containing 0.002-.010%, Furthermore, Cu, selected from the group consisting of Ni and Cr contains from 0.10 to 3.0% of one or more in total that the balance being iron and unavoidable impurities, further, the content of Si [Si] and the content of C [C] ratio ([Si ] / [C]) is not less than 1.0, characterized in that it is a ferrite-pearlite structure.
[0008]
　Patent Document 2 proposes the cold workability and delayed fracture resistance excellent bolt steel. Bolt steel disclosed in this document, 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, and satisfies the following equation 0.50 ≦ [C] +0.15 [Si] +0.2 [Mn] +0.11 [Cr] ≦ 0.60.
[0009]
　In Patent Document 3, by performing the spheroidizing annealing treatment, we propose a cold working steel for machine structural use that can achieve sufficient softening. Mechanical structural steel disclosed in this document, C: 0.2 ~ 0.6%, Si: 0.01 ~ 0.5%, Mn: 0.2 ~ 1.5%, P: 0.03 % or less, S: 0.001 ~ 0.05%, Al: 0.01 ~ 0.1%, N: 0.015% or less, and Cr: 0.5% greater, containing 2.0% , balance being 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, of the pro-eutectoid ferrite area ratio a but, Ae = a (0.8-Ceq) × 96.75 (However, Ceq = [C] + 0.1 × [Si] + 0.06 × [Mn] + 0.11 × [Cr]), a> Ae has a relationship, the average particle size of the ferrite of the pro-eutectoid ferrite and in pearlite is 15 ~ 25 [mu] m.
[0010]
　Patent Document 4 proposes a high-strength steel excellent in fatigue characteristics. High strength steel disclosed in this document, C: 0.2 ~ 1.3%, Si: 0.01 ~ 3.0%, Mn: contains 0.2 to 3.0% the balance being Fe and becomes unavoidable impurities, the carbon equivalent Ceq (However, Ceq = [C] + [Si] / 15 + [Mn] / 10 + [Cr] / 11 + [Mo] / 7 + [V] / 5 + [Ni] / 45 + [Cu] / 45) is not less 0.8% or more, and the amount of hydrogen released upon heating from room temperature to 500 ° C. is not more than 0.3 ppm.
CITATION
Patent Document
[0011]
Patent Document 1: JP 2012-162798 Patent Publication
Patent Document 2: JP-A 9-104945 Patent Publication
Patent Document 3: JP 2013-227602 Patent Publication
Patent Document 4: JP 2001-234277 JP
Non-Patent Document
[0012]
非特許文献1 : “Ｅｌｅｍｅｎｔｓ　ｏｆ　Ｍｅｔａｌｌｕｒｇｙ　ａｎｄ　Ｅｎｇｉｎｅｅｒｉｎｇ　Ａｌｌｏｙｓ”，　ｅｄ．ｂｙ　Ｆ．Ｃ．Ｃａｍｐｂｅｌｌ，ＡＳＭ　Ｉｎｔｅｒｎａｔｉｏｎａｌ，Ｍａｔｅｒｉａｌｓ　Ｐａｒｋ，２００８，ｐｐ１８５－１９１．
Summary of the Invention
Problems that the Invention is to Solve
[0013]
　The disclosed in Patent Document 1 steel, the Si content is increased than C content, while ensuring the strength of the matrix in a solid solution strengthening of Si, to enhance the resistance to delayed fracture. However, since the expensive Ni has become an essential element, steel cost increases.
[0014]
　The disclosed in Patent Document 2 steel, C, Si, and defines the lower limit and the upper limit of the total amount of Mn, and Cr, the desired intensity and strength of the rolled material, after refining can maintain the cold workability defining the strength of the rolled material which is obtained. However, due to the low Cr content and Si content of the steel, hardenability and, in some cases the softening resistance after the tempering is low.
[0015]
　The disclosed in Patent Document 3 steel, assuming equilibrium estimates the area ratio of the eutectoid ferrite, it is to be equal to or greater than the predetermined value, improve the cold workability. However, the actual production process is a continuous cooling, also variously changed by production conditions cooling rates. Therefore, in actual operation, there is a case where cold workability is not sufficiently obtained.
[0016]
　The disclosed in Patent Document 4 steel, 0.3 ppm with defining the lower limit of carbon equivalent Ceq, steel is released when heated from room temperature to 500 ° C., the hydrogen content contained in the steel It is less than or equal to. Thus, to improve the fatigue properties. However, does not disclose how to ensure cold workability, further, there is no disclosure also hydrogen embrittlement resistance of cold forged microalloyed products.
[0017]
　An object of the present invention has an excellent cold workability, when treated cold forging and tempering, high strength and excellent cold forged microalloyed products for rolling bars with hydrogen embrittlement resistance it is to provide.
Means for Solving the Problems
[0018]
　Cold forged microalloyed products for rolling bars according to the invention, in mass%, C: 0.22 ~ 0.40% , Si: 0.35 ~ 1.5%, Mn: 0.20 ~ 0.40 %, P: less than 0.020%, S: less than 0.015%, Cr: 0.70 ~ 1.45 %, Al: 0.005 ~ 0.060%, Ti: 0.01 ~ 0.05% , B: 0.0003 ~ 0.0040%, N: 0.0020 ~ 0.0080%, O: 0.0020% or less, Cu: 0 ~ 0.50%, Ni: 0 ~ 0.30%, Mo : 0% to 0.05%, V: 0 to 0.05% and, Nb: 0 contains ~ 0.05%, the balance being Fe and impurities, satisfies the formula (1) and (2) having a chemical composition. In the matrix organization, the total area ratio of the pro-eutectoid ferrite and pearlite is 90% or more, the area ratio of the pro-eutectoid ferrite is 30% or more. Cold forged microalloyed products for rolling bars according to the invention has a tensile strength below 700 MPa.
　0.50 ≦ C + Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85
　(1) Si / Mn> 1.0 (2)
　wherein each element symbol in the above formula, the content of the corresponding element (mass%) of It is assigned.
[0019]
　Cold forged microalloyed products for rolling bars according to the invention, which has excellent cold workability, when treated cold forging and tempering, has high strength and excellent hydrogen embrittlement resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020]
[1] Figure 1 is used in the fatigue test Ono-type rotating bending embodiments, a side view of the test piece.
FIG. 2 is used in the hydrogen embrittlement resistance evaluation test of Example is a side view of an annular V-notch test piece.
DESCRIPTION OF THE INVENTION
[0021]
　The present inventors have conducted various studies to solve the above problems. As a result, the present inventors have obtained the following findings.
[0022]
　(A) while maintaining the cold workability, ensuring hardenability, and to reduce the steel costs, preferably chrome steel (JIS G4053 (symbol "SCr" defined in 2008)) to the base . However, in the compositional standard for machine structural alloy steel as defined above JIS standard, low hydrogen embrittlement resistance.
[0023]
　(B) in order to increase the resistance to hydrogen embrittlement of cold forged microalloyed products, it is effective to increase the tempering temperature in the manufacturing process. However, increasing the tempering temperature, tempering softening occurs. To enhance the temper softening resistance, the Si content is effective to increase than component standard alloy steel for the machine structure. In addition, Mn in the steel is conducive to grain boundary segregation of P, to promote the grain boundary fracture. Therefore, it is effective to a Mn content less than compositional standard alloy steel for the machine structure.
[0024]
　(C) the tensile strength of the cold forged microalloyed products and high strength such as 1000 ~ 1300 MPa, and, in order to obtain a high fatigue strength, it is necessary to sufficiently hardenability. However, if too high hardenability, cold workability of rolling bars to be cold forged microalloyed product material is reduced. In this case, before carrying out the cold working of the wire drawing and cold forging or the like to the rolling bars shall long softening heat treatment for the purpose of softening of the rolled bars was performed multiple times. Therefore, Mo, even without containing a large amount of alloy elements V, etc., the production cost is high. Therefore, it is possible cold working without a plurality of times for a long period of time softening heat treatment, and the high strength and high fatigue rolling bars strength having hardenability is obtained is preferable.
[0025]
　Therefore, the rolling bars according to the present invention, while ensuring the hardenability, in order to maintain the cold workability, satisfies the equation (1).
　0.50 ≦ C + Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85 (1)
　wherein each element symbol in the formula (1), the content of the corresponding element (mass%) is substituted.
[0026]
　fn1 = is defined as C + Si / 10 + Mn / 5 + 5Cr / 22. C, Si, Mn, and Cr are both an element that enhances the hardenability. Therefore, fn1 is a hardenability and cold workability index.
[0027]
　If fn1 is too low, no sufficient hardenability can not be obtained. In this case, the cold forging refining products, high tensile strength and high fatigue strength can not be obtained. On the other hand, if fn1 is too high, it is too high hardenability of the rolling bars. In this case, when manufacturing the rolled bars and rolled finishing a billet, and bainite is produced in the steel increases the strength and hardness, cold workability is deteriorated. In this case, drawing process of the next step and, prior to cold forging process, to be performed multiple times for a long period of time softening heat treatment, sufficient cold workability can not be obtained.
[0028]
　fn1 is satisfies the equation (1), while obtaining excellent hardenability and fatigue strength, without a plurality of times for a long period of time softening heat treatment, sufficient cold workability are obtained.
[0029]
　(D) in order to increase the fatigue strength and hydrogen embrittlement resistance of cold forged microalloyed products, to reduce the inclusions in the steel, or, it is effective to fine inclusions. If Cr content is about 1% of the steel, if greater than 1 the ratio of Mn content of the Si content in the steel, that is, if satisfied Equation (2) is, inclusions soft MnO- SiO 2 becomes. The inclusions vitrified is drawn and separated during rolling, it is miniaturized. Therefore, coarse inclusions to lower the fatigue strength is reduced, increased fatigue strength, and to improve the hydrogen embrittlement resistance.
　Si / Mn> 1.0 (2)
　wherein each element symbol in the formula (2), the content of the corresponding element (mass%) is substituted.
[0030]
　(E) cold workability of cold forging microalloyed products for rolling bars, in addition to the above matters, but also on the steel matrix tissue. Matrix organization is primarily mixed structure of pro-eutectoid ferrite and pearlite, and, the higher the area ratio of the pro-eutectoid ferrite, excellent cold workability. Specifically, inside the matrix tissue during cold forging microalloyed products for rolling bars, it is a total area ratio of the pro-eutectoid ferrite and pearlite total 90% or more, and the area ratio of the pro-eutectoid ferrite if but less than 30%, increased cold workability.
[0031]
　Or cold forging microalloyed products for rolling bars according to the present invention has been completed based on the findings, in mass%, C: 0.22 ~ 0.40% , Si: 0.35 ~ 1.5%, Mn : 0.20 ~ 0.40%, P: less than 0.020%, S: less than 0.015%, Cr: 0.70 ~ 1.45 %, Al: 0.005 ~ 0.060%, Ti: 0.01 ~ 0.05%, B: 0.0003 ~ 0.0040%, N: 0.0020 ~ 0.0080%, O: 0.0020% or less, Cu: 0 ~ 0.50%, Ni: 0 ~ 0.30%, Mo: 0 ~ 0.05%, V: 0 ~ 0.05%, and, Nb: 0 contains ~ 0.05%, the balance being Fe and impurities, formula (1 ) and having a chemical composition satisfying the equation (2). In the matrix organization, the total area ratio of the pro-eutectoid ferrite and pearlite is 90% or more, the area ratio of the pro-eutectoid ferrite is 30% or more. Cold forged microalloyed products for rolling bars according to the invention has a tensile strength below 700 MPa.
　0.50 ≦ C + Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85
　(1) Si / Mn> 1.0 (2)
　wherein each element symbol in the formula (1) and (2), the corresponding element the content (mass%) is substituted.
[0032]
　Here, the cold forged microalloyed product is cold forged, refining means (quenching and tempering) are parts. Further, the rolling bars, means a steel bar or wire rod leaving the hot rolling (so-called Azuroru material).
[0033]
　The chemical composition, Cu: 0.02 ~ 0.50%, Ni: 0.01 ~ 0.30%, Mo: 0.005 ~ 0.05%, and, V: 0.005 ~ 0.05% it may contain one or more members selected from the group consisting of. Further, the chemical composition, Nb: 0.0015 may contain ~ 0.05%.
[0034]
　Method for producing a cold forged microalloyed products for rolling bars described above, comprises a slabbing process, the finish rolling step. The slabbing process, after heating the material having the chemical composition described above to 1200 ° C. or higher temperatures, producing billets performed slabbing. The finish rolling step, after heating the billet to a temperature of 1050 ° C. or less, to produce a rolled bars implemented finish rolling. Finish rolling step, the temperature of between 750 ~ 850 ° C. billet, the step of producing a rolled bars and rolled finished billet machining speed Z defined in the formula (3) as 5 to 15 / sec, the cooling rate from immediately after rolling completion to 500 ° C. as 0.2 ~ 5.0 ° C. / sec, and a step of rolling bars cooling.
　Z = -ln (1-R) / t (3)
　where the R in the formula (3) is a cross-sectional reduction rate at finish rolling (%), defined by equation (4). t is the finish rolling time (in seconds).
　= R (A 0 -A) / A 0　(4)
　Equation (4) A in 0 , the cross-sectional area of the billet before finish rolling (mm 2 are), A is rolled bars after the finish rolling cross-sectional area (mm 2 is).
[0035]
　It described in detail below cold forged microalloyed products for rolling bars according to the invention. "%" Related elements, unless otherwise specified, it means mass%.
[0036]
　[Chemical composition]
　The chemical composition of cold forged microalloyed products for rolling bars of the present invention contains the following elements.
[0037]
　C: 0.22 ~ 0.40%
　carbon (C) increases the strength of steel. If C content is less than 0.22%, this effect can not be obtained. On the other hand, C content if it exceeds 0.40% the cold workability of the steel is lowered. Therefore, C content is 0.22 to 0.40%. A preferred lower limit of the C content in the case of further enhancing the hardenability is 0.26%. The preferable lower limit of C content is 0.24%, more preferably 0.26%. The preferable upper limit of the C content in the case of further increasing the cold workability is 0.35%.
[0038]
　Si: 0.35 ~ 1.5%
　silicon (Si), the deoxidizing steel. Si further, to strengthen the steel material in solid solution in the ferrite. Si further suppresses the precipitation of cementite increases the temper softening resistance of the steel.
[0039]
　Si also has the following advantages. MnO-SiO deoxidation product 2 The melting point of as low as 1250 ° C.. Therefore, MnO-SiO 2 is liquid at the melt prior to solidification, after solidification, the inclusions soft vitrified. MnO-SiO 2 is miniaturized be stretched and separated during hot rolling. Therefore, fatigue strength and reduces the coarse inclusions which inhibit the hydrogen embrittlement resistance, enhanced fatigue strength and hydrogen embrittlement resistance.
[0040]
　Proper MnO-SiO 2 generates and, in order to enhance the temper softening resistance after tempering treatment, the Si content defined in JIS standard chrome steel (from 0.15 to 0.35%) it is insufficient. Specifically, Si content is less than 0.35%, not the effect. On the other hand, if it exceeds Si content of 1.5%, the strength of the steel material becomes too high, the cold workability is deteriorated. Therefore, Si content is 0.35 to 1.5%. A preferable lower limit of Si content is 0.36%, more preferably 0.38%. If further enhance the cold workability, the preferred upper limit of the Si content is less than 1.0%.
[0041]
　Mn: 0.20 ~ 0.40%
　manganese (Mn) is to enhance the hardenability of steel and increases the toughness. When Mn content is less than 0.20%, these effects can not be obtained. On the other hand, if the Mn content is too high, when the refining was conducted, Mn is segregated at the grain boundaries sometimes promote grain boundary fracture. In addition, proper-SiO MnO 2 , it is difficult to obtain. Therefore, hydrogen embrittlement resistance is deteriorated. In the Mn content specified in JIS standard chrome steel (0.60 to 0.85%), difficult to suppress grain boundary fracture due to segregation, proper MnO-SiO 2 are difficult to obtain. Therefore, Mn content is from 0.20 to .40%. The preferable lower limit of the Mn content is 0.22% and a preferable upper limit is 0.35%.
[0042]
　P: less than 0.020% or
　phosphorus (P) is an impurity. P tends to segregate in the austenite grain boundaries, after the refining process, cause of grilled cracking and intergranular fracture. Accordingly, P content is less than 0.020% or. The preferable upper limit of the P content is less than 0.010%. P content is preferably as small as possible.
[0043]
　S: less than 0.015%
　sulfur (S) is an impurity. S lowers the cold workability to form a sulfide. Thus, S content is less than 0.015%. S content is preferably as small as possible.
[0044]
　Cr: 0.70 ~ 1.45%
　chromium (Cr) increases the hardenability of steel. If Cr content is less than 0.70%, this effect can not be obtained. On the other hand, if it exceeds 1.45 percent Cr content, bainite is generated in too high hardenability after hot rolling cooling. In this case, the strength of the steel is excessively high, the cold workability of rolled bars decreases. Therefore, Cr content is from 0.70 to 1.45%. A preferable lower limit of Cr content is 0.90%, and the preferable upper limit is 1.20%.
[0045]
　Al: 0.005 ~ 0.060%
　of aluminum (Al) is deoxidized steel. Al further, AlN was formed by combining the N, to secure the N. Al further by pinning effect of AlN particles and suppresses austenite grains coarsening during heating. If Al content is less than 0.005%, these effects can not be obtained. On the other hand, if the Al content exceeds the 0.060%, Al 2 O 3 is excessively generated cold workability is deteriorated. Therefore, Al content is 0.005 to 0.060%. When increasing the cold workability, the preferred upper limit of Al content is 0.060%, more preferably 0.050%, still more preferably 0.045%. In the chemical composition of the cold forged microalloyed products for rolling bars according to the present invention, Al content is meant the total amount of Al contained in the steel.
[0046]
　Ti: 0.01 ~ 0.05%
　titanium (Ti) is, TiN was formed by combining the N, to secure the N. Ti further by pinning effect of TiN particles, suppress austenite grain coarsening during heating. If the Ti content is less than 0.01%, these effects can not be obtained. On the other hand, if Ti content exceeds the 0.05%, Ti (C, N ) and many precipitates, the strength of the steel material becomes excessively high. In this case, it decreases the cold workability of the steel. Therefore, Ti content is 0.01 to 0.05%. A preferable lower limit of the Ti content is 0.05%. The preferable upper limit of the Ti content is 0.045%.
[0047]
　B: 0.0003 ~ 0.0040%
　boron (B) increases the hardenability of steel. B further suppress the grain boundary segregation of P, enhances hydrogen embrittlement resistance of the steel. If B content is less than 0.0003%, these effects can not be obtained. On the other hand, B content if it exceeds 0.0040%, the effect of improving hardenability is saturated. Further, coarse BN is generated workability and toughness cold reduced. Therefore, B content is 0.0003 to 0.0040 percent. If B is to form a BN combined with solid solution N, since the amount of solid solution B is lowered, hardenability decreases. The solid solute B amount is sufficient, the preferred lower limit of the B content to further increase the hardenability is 0.0005%, more preferably 0.0010%. The preferable upper limit of the B content to further suppress a decrease in cold workability and toughness are 0.0030%, more preferably 0.0025%.
[0048]
　N: 0.0020 ~ 0.0080%
　nitrogen (N) combines with Al or Ti in the steel to produce nitrides, inhibit austenite grain coarsening during heating. If N content is less than 0.0020%, this effect can not be obtained. On the other hand, N content if it exceeds 0.0080%, BN is generated excessively, the amount of solute B decreases. In this case, hardenability of the steel is lowered. Therefore, N content is from 0.0020 to 0.0080 percent. The preferable lower limit of the N content is 0.0022%. The preferable upper limit of the N content to further increase the hardenability is less than 0.0070%, more preferably 0.0060%.
[0049]
　O: 0.0020% or less
　oxygen (O) is an impurity. O decreases the cold workability by forming an oxide. If O content exceeds 0.0020%, the oxides are produced in large quantities, MnS is coarse, cold workability is significantly decreased. Therefore, O content is 0.0020%. The preferable upper limit of the O content is 0.0018%. O content is preferably as small as possible.
[0050]
　The remainder of the chemical composition of the cold forged microalloyed products for rolling bars according to the invention consists of Fe and impurities. Here, the impurity, when the industrial production of the rolling bars, ore as a raw material, there is to be mixed etc. Scrap or manufacturing environment, the allowable range of the present invention does not adversely affect It is the means something.
[0051]
　[For any element]
　cold forged microalloyed products for rolling The bar described above further, in place of part of Fe, Cu, Ni, Mo, and one selected from the group consisting of V or two or more it may contain. All of these elements are also optional element, enhances the hardenability of steel.
[0052]
　Cu: 0 ~ 0.50%
　copper (Cu) is an optional element and may not be contained. If contained, Cu increases the hardenability of steel. However if it exceeds 0.50% of Cu content, it tends to generate bainite too high hardenability. In this case, the cold workability is deteriorated. Therefore, Cu content is 0 to 0.50%. The preferable lower limit of the Cu content to obtain the above effect more effectively is 0.02%, more preferably 0.05%. When increasing the cold workability, the preferred upper limit of the Cu content is 0.30%, more preferably 0.20%.
[0053]
　Ni: 0 ~ 0.30%
　nickel (Ni) is an optional element and may not be contained. If contained, Ni increases the hardenability of steel, further enhances also grain boundary strength. However, if Ni content exceeds 0.30%, its effect is saturated, steel cost increases. Therefore, Ni content is from 0 to 0.30%. A preferable lower limit of the Ni content in order to obtain the above effect more effectively is 0.01%, more preferably 0.03%. When increasing the cold workability, the preferred upper limit of the Ni content is 0.20%, more preferably from 0.10%.
[0054]
　Mo: 0 ~ 0.05%
　of molybdenum (Mo) is an optional element and may not be contained. If contained, Mo enhances the hardenability of steel. However, if it exceeds 0.05% is Mo content becomes too high hardenability, bainite and martensite is easily generated. In this case, it decreases the cold workability of the steel. Therefore, Mo content is 0 to 0.05%. A preferable lower limit of Mo content in order to obtain the above effect more effectively is 0.005%. The preferable upper limit of the Mo content is 0.03%, more preferably 0.02%.
[0055]
　V: 0 ~ 0.05%
　vanadium (V) are optional elements may not be contained. If contained, V is increasing the hardenability of the steel. V further carbides, to form a nitride or carbo-nitrides increase the strength of steel. However, V content if it exceeds 0.05%, carbides lowers the cold workability coarsened. Therefore, V content is 0 to 0.05%. The preferable lower limit of V content to obtain the above effect more effectively is 0.005%. When increasing the cold workability, the preferred upper limit of the V content is 0.03%, more preferably 0.02%.
[0056]
　The chemical composition of the cold forged microalloyed products for rolling bars according to the invention further, instead of a part of Fe, and may contain Nb.
[0057]
　Nb: 0 ~ 0.05%
　niobium (Nb) is an optional element and may not be contained. If contained, Nb combines with C and N, carbide, to form a nitride or carbo-nitrides, refining the crystal grains. Nb further enhances hydrogen embrittlement resistance of cold forged microalloyed articles manufactured using the rolling bars of the present invention. However, if it exceeds the Nb content is 0.05%, the cold workability of rolled bars decreases generate coarse carbides. Therefore, Nb content is 0 to 0.05%. The preferable lower limit of Nb content to obtain the above effect more effectively is 0.0015%. The preferable upper limit of Nb content is 0.04%, more preferably from 0.03%.
[0058]
　[Equation (1)]
　The present invention further chemical composition of the cold forged microalloyed products for rolling bars by, satisfying the equation (1).
　0.50 ≦ C + Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85 (1)
　The formula (1) each element symbol in the content of the corresponding element (mass%) is substituted.
[0059]
　fn1 = C + Si / 10 + Mn / 5 + 5Cr / 22 is indicative of the strength and cold workability of the steel. fn1 denotes the carbon equivalent of the steel. If fn1 is too low, sufficient hardenability can not be obtained, no tensile strength and fatigue strength. On the other hand, if fn1 is too high, too high hardenability. In this case, when the rolling bars are rolled, it is generated bainite and / or martensite, the strength and hardness of the steel becomes too high, the cold workability is deteriorated. In this case, drawing process of the next step by using the rolling bars, and, before the cold forging process, to the rolling bars, to be performed multiple times for a long period of time softening heat treatment, sufficient not obtained workability cold. If fn1 is 0.50 to 0.85 while obtaining excellent hardenability, without carrying out long softening heat treatment, sufficient cold workability are obtained. A preferred lower limit of the fn1 is 0.55. A preferred upper limit of the fn1 is 0.80.
[0060]
　[Equation (2)]
　The present invention further chemical composition of the cold forged microalloyed products for rolling bars by, satisfying the equation (2).
　Si / Mn> 1.0 (2)
　wherein each element symbol in the formula (2), the content of the corresponding element (mass%) is substituted.
[0061]
　fn2 = is defined as Si / Mn. Si and Mn, MnO-SiO in the process of deoxidation 2 generates a. MnO-SiO 2 is a melting point of about 1250 ° C.. Therefore, in the molten metal prior to solidification is a liquid, it is solid at slab in after solidification, the inclusions soft vitrified. The inclusions are fine be stretched and separated during hot rolling. Therefore, fatigue strength is improved, thereby improving the hydrogen embrittlement resistance. Fine MnO-SiO 2 in order to obtain, it is necessary to properly control the ratio Mn of Si. This index is fn2.
[0062]
　As fn2 increases, fatigue strength and hydrogen embrittlement resistance of cold forged microalloyed article produced by using the rolling bars increases. When the fn2 exceeds 1.0, excellent in significantly hydrogen embrittlement resistance than SCM435 of JIS standard. Therefore, as shown in equation (2), a fn2> 1.0. A preferred lower limit of the fn2 is 1.2.
[0063]
　[Matrix tissue and tensile strength]
　matrix tissue cold forging microalloyed products for rolling bars according to the invention consists mainly pro-eutectoid ferrite and pearlite. The term "predominantly matrix organization consisting of pro-eutectoid ferrite and pearlite" as used herein, in a matrix organization, means that the total area ratio of the pro-eutectoid ferrite and pearlite is 90% or more. The remainder of the matrix organization cold forging microalloyed products for rolling bars according to the invention are, for example, bainite and martensite. Furthermore, the tensile strength of the cold forged microalloyed products for rolling bars according to the invention is less than 700 MPa.
[0064]
　Pro-eutectoid ferrite and pearlite are softer than bainite, excellent cold workability. In addition, the pro-eutectoid ferrite is excellent in cold workability than pearlite. The total area ratio of the pro-eutectoid ferrite and pearlite is less than 90 percent, or, if pro-eutectoid ferrite area ratio is less than 30%, no sufficient cold workability can not be obtained. And the pro-eutectoid ferrite and the total area ratio of pearlite of 90% or more, and, if the area ratio of the pro-eutectoid ferrite is 30% or more, resulting excellent cold workability.
[0065]
　The total area ratio of the pro-eutectoid ferrite and pearlite is preferably 92% or more, still more preferably 95% or more. A preferable lower limit of the area ratio of the pro-eutectoid ferrite is 35% or more, more preferably 40% or more.
[0066]
　By the matrix tissue and tensile strength, cold forged microalloyed products for rolling bars of the present invention has excellent cold workability. The lower limit of the tensile strength is not particularly limited, for example, 500 MPa.
[0067]
　Matrix organization is measured by the following methods. R / 2 parts of rolled bars (R is the distance to the outer surface from the central axis of The bar) obtaining a sample from. Of the samples taken of the surface, and the observation plane surface perpendicular to the rolling direction of the rolling bars. After polishing the observation surface, etched with a 3% nitric acid alcohol (nital corrosion solution). The etched observed surface was observed at 500-power optical microscope, to produce a photographic image of any five field.
[0068]
　In each field, the pro-eutectoid ferrite, pearlite, bainite, each phase such as martensite, contrast differs for each phase. Therefore, on the basis of the contrast, to identify the phases. Of the identified phase, the area of proeutectoid ferrite at each field ([mu] m 2 and), the area of perlite ([mu] m 2 obtains a). The sum of the areas of the pro-eutectoid ferrite area and perlite at all viewing, the ratio of total area of all field, defined as the total area ratio of the pro-eutectoid ferrite and pearlite (%). Similarly, the total area of the pro-eutectoid ferrite in all viewing, the ratio of total area of all field, is defined as a pro-eutectoid ferrite area ratio (%).
[0069]
　[Preferred outer diameters of the rolling bars]
　Preferably, the diameter Dc of the rolling bars (critical diameter, the unit is mm) satisfies the following formula (A).
　D C = 4 × (0.70Si + 1.0) × (3.33Mn + 1.0) × (2.16Cr + 1.0) (A)
　wherein each element symbol, the content of the corresponding element (mass%) There is assigned.
[0070]
　The hardenability of the prediction, for example, described in Non-Patent Document 1. Assume an example of the steel consistent with the present invention. C content is 0.3%, the grain size number of austenite grains No. Reference diameter D of 8 round bar B is about 0.165 inches (about 4 mm). Here, reference diameter D B and the critical diameter obtained in C content only means (round bar central portion diameter is 50% martensite). Reference diameter D B by using the critical diameter D C that defines the.
[0071]
　Critical diameter D took into consideration the effect of improving the hardenability due to alloying elements (hardenability multiples) C is, D C = D B × f (Si) × f (Mn) × f (Cr), is expressed by the formula. Here, f (X) is a function of the amount X of alloying elements X, called specific hardenability multiples each alloy element. In the present invention, by using the numerical values described in Non-Patent Document 1, it is defined by the following equation.
　f
　(Si) = 0.70Si + 1.0
　f (Mn) = 3.33Mn + 1.0 f (Cr) = 2.16Cr + 1.0
　Thus, in the present invention, the critical diameter D of the rolled bars which hardenability is ensured C It is defined by the following formula (a).
　D C = 4 × (0.70Si + 1.0) × (3.33Mn + 1.0) × (2.16Cr + 1.0) (A)
[0072]
　The diameter of the rolled bars are critical diameter D C as long or less, more sufficient hardenability can be obtained.
[0073]
　[Manufacturing Method]
　[Cold Forging microalloyed products for rolling bars of Production Method]
　An example of the production method of the present invention cold forged microalloyed products for rolling bars by will be described. Method for producing a rolled bars of the present embodiment includes a step of producing a billet (slabbing step), a step of rolling the manufactured billet bars (finish-rolling step). Below, it will be described in detail each of the steps.
[0074]
　[Slabbing step]
　First, prepare a material having the above chemical composition. For example, material can be prepared in the following manner. Molten steel having the chemical composition described above, produced using a converter and an electric furnace or the like. Producing slab by continuous casting method using a molten steel. Or, to produce an ingot by ingot method using the molten steel.
[0075]
　After heating the prepared material (slab, an ingot), and slabbing with blooming mill, if necessary, be further rolled in a continuous rolling mill after slabbing to prepare a billet. In continuous rolling mills, horizontal stand, vertical stand are arranged in a row alternately, by rolling the material with a grooved formed in the rolling roll in each stand, to the billet.
[0076]
　Preferred heating temperature of slabbing material before is 1200 ° C. or higher. If the heating temperature is 1200 ° C. or higher, Ti (C, N) generated during solidification of the material and TiC such coarse carbonitrides, carbides are dissolved upon heating. Since the coarse carbonitrides is inhibited, increases cold workability of the rolling bars.
[0077]
　Finishing rolling step]
　was further carried out hot rolling with respect to the billet produced by blooming rolling process, to produce a cold forged microalloyed products for rolling bars.
[0078]
　First, they were charged a billet in a heating furnace and heated. Preferred heating temperature is 1050 ° C. or less. If the heating temperature during product rolling is too high, fine carbides and carbonitrides precipitated after slabbing process forms a solid solution again. In this case, when the ferrite transformation during cooling after finish rolling, carbides and carbonitrides are precipitated aligned. The precipitated carbonitrides and carbides enhances the strength of the steel after product rolling, to reduce the cold workability. If the heating temperature is 1050 ° C. or less, since excessive dissolution of carbides and carbonitrides is suppressed during heating, it is possible to further enhance the cold workability.
[0079]
　Using a heated billet, finish rolling at a finish rolling mill train (hot rolling) to make the bars of a predetermined diameter. Finish rolling mill train includes a plurality of stands arranged in a row. Each stand includes a plurality of rolls arranged around a pass line. And rolling the billet by using a grooved formed in the rolling roll in each stand, to produce a rolled bars.
[0080]
　Manufacturing conditions in the finish rolling using the finish rolling mill train is as follows.
[0081]
　Finishing temperature: 750 ~ 850 ° C.
　finishing temperatures among the plurality of stands of the finishing rolling mill train, the last stand of rolling the billet (hereinafter, referred to as the finishing stand) means a surface temperature of the billet at the exit side of (℃) to. Finishing temperature is determined by measuring the surface temperature of the billet by using an infrared radiation thermometer disposed on the exit side of the finishing stands.
[0082]
　If the finishing temperature is below 750 ° C., ferrite transformation begins austenite grains non-recrystallized matrix structure after cooling is too fine. In this case, the tensile strength of the steel exceeds 700 MPa, cold workability is deteriorated. On the other hand, if the finishing temperature exceeds 850 ° C., the austenite grains after recrystallization coarse, starting temperature of ferrite transformation becomes low. Therefore, the area ratio of the pro-eutectoid ferrite after cooling is reduced. Furthermore, there is a case where the hard tissue of bainite or the like is generated. As a result, the tensile strength of the steel exceeds 700 MPa, cold workability is deteriorated.
[0083]
　If the finishing temperature is 750 ~ 850 ° C., under the condition that later cooling condition is satisfied, the matrix organization consists of pro-eutectoid ferrite and pearlite. Specifically, the total area of ​​the pro-eutectoid ferrite and pearlite in the matrix structure is 90% or more.
[0084]
　Processing rate Z: 5 ~ 15 / sec
　processing rate Z (/ sec) is defined by equation (3).
　Z = -ln (1-R) / t (3)
　R in formula (3) is a cross-sectional reduction rate in the finish rolling by the finishing mill train (%). t is the finish rolling time (in seconds).
[0085]
　Reduction of area R is defined by Equation (4).
　= R (A 0 -A) / A 0　(4)
　Equation (4) A in 0 , the cross-sectional area of the billet before finish rolling (mm 2 are), A is the cross-sectional bars after the finish rolling area (mm 2 is).
[0086]
　Finish rolling time t is the time to pass through the rolling mill train finish billet, finishing mill first stand average from (mill) distance to the last stand (rolling mill) (m) to billets conveying column is a value obtained by dividing (s) at a speed (m / sec).
[0087]
　If processing speed Z is 5-15 / sec, matrix organization less likely to be fine after cooling, it is possible to the tensile strength of bars than 700 MPa. As a result, it increased cold workability. If it is less than the processing rate Z is 5 / sec, recrystallization does not occur sufficiently, hardenability austenite grains becomes rough to increase. This reduces the area ratio of the pro-eutectoid ferrite, increases hard bainite or martensite structure area ratio of the tensile strength of the rolled bars exceeds 700 MPa. If processing speed Z is exceeds 15 / s, the austenite grains finer by recrystallization, although hardenability is increased eutectoid ferrite area ratio decreases, the ferrite crystal grains bars too fine the tensile strength is more than 700MPa.
[0088]
　Cooling rate to finish rolling after 500 ℃: 0.2 ~ 5.0 ℃ / sec
　after finish rolling, cooling rate until the surface temperature of the rolled bars is 500 ° C. is 0.2 ~ 5.0 ° C. / is seconds. If the cooling rate is 0.2 ~ 5.0 ° C. / sec, the total area of the pro-eutectoid ferrite and pearlite in the matrix structure is 90% or more, the area ratio of the pro-eutectoid ferrite can be 30% or more. If it exceeds the cooling rate is 5.0 ° C. / sec, bainite or the like of the hard becomes easily generated in the steel, the tensile strength of the rolled bars exceeds 700 MPa. The lower limit of the cooling rate is not particularly limited, considering the actual production operations, the lower limit of the cooling rate is, for example, 0.2 ° C. / sec.
[0089]
　With the above-described manufacturing process, cold forging for rolling bars of the present invention is produced. That is, cold forging for rolling the bars, the so-called rolling Mom material (Azuroru material). Matrix organization of cold-forged products for rolling bars consists of pro-eutectoid ferrite and pearlite, the tensile strength is less than or equal to 700MPa. Therefore, excellent cold workability.
[0090]
　[Cold Forging microalloyed article Manufacturing Method
　An example of a method for manufacturing the cold with rolling bars above forged microalloyed products, the manufacturing method of the bolt. This manufacturing method includes wire drawing step, the cold forging process, and, refining step (quenching and tempering process). The following describes each step.
[0091]
　[Drawing process]
　First, by performing the wire drawing to produce steel wire to the rolling bars described above. Wire drawing, may be only a primary wire drawing, secondary wire drawing, etc., may be performed a plurality of times of drawing. During wire drawing, to form a lubricating coating on the surface of the wire. Lubricating coating, for example, a phosphate coating or a non-phosphorus-based lubricating coating.
[0092]
　[Cold forging step]
　The steel wire after drawing and cut to a predetermined length, to implement the cold forging to produce a (volts in this case) cold forging to the cutting steel wire.
[0093]
　[Softening heat treatment for]
　In the manufacturing method of the conventional cold forging, for the purpose of softening the tensile strength too high bars, before before wire drawing and cold forging, has a softening heat treatment was performed multiple times. However, the rolling bars according to the invention, by satisfying the equation (1), excellent cold workability. Therefore, omitting the softening heat treatment, or can be simplified.
[0094]
　[Refining process (quenching and tempering process)
　with respect to cold forging, to implement the refining in a known condition (hardening and tempering), to produce a cold forged microalloyed products. Since the high hardenability of the rolling bars described above, by performing the thermal refining, cold forged microalloyed products high strength can be obtained. Specifically, by appropriately adjusting the quenching temperature and tempering temperature, it can produce cold forged microalloyed article having a tensile strength of 1000 ~ 1300 MPa.
[0095]
　Cold forged microalloyed articles produced by the above production process has a high strength, and has excellent hydrogen embrittlement resistance.
Example 1
[0096]
　Chemical composition of Table 1 was prepared molten steel having.
[0097]
[Table 1]

[0098]
　Referring to Table 1, steel type l had a chemical composition corresponding to SCM435 of JIS G4053 (2008).
[0099]
　It was produced bloom with molten steel of each steel type. After heating the bloom at 1250 ° C., to implement the slabbing, cross-section to produce a billet of 162 mm × 162 mm (slabbing step). The finish rolling was performed by heating the billets to 1030 ~ 1050 ° C., to produce a round bar having a diameter of 20 mm (finish rolling step). Finish rolling temperature is 750 ~ 780 ° C., the processing rate was 5-15 / sec. After finish rolling, the cooling rate until the surface temperature becomes 500 ° C. was 0.2 ~ 5.0 ° C. / sec. Relative round bar manufactured by the above manufacturing steps were carried out following evaluation tests.
[0100]
　[Microstructure observation test]
　a round bar of each test number was cut in the rolling direction and the direction perpendicular, samples from the 2 / R of the cutting plane (central portion of a line connecting the central axis and the outer peripheral surface of the round bar) It was collected. As the cutting surface is the viewing surface and filled resin was subjected to mirror polishing. Then conducted microstructure observed by the method described above, were determined pro-eutectoid ferrite and the total area ratio of pearlite (%) and pro-eutectoid ferrite area ratio of. The results obtained are shown in Table 2. Table 2 in the "F + P" is the matrix structure becomes a pro-eutectoid ferrite and pearlite, the total area ratio of the pro-eutectoid ferrite and pearlite is meant 90% or more. "F + B" means that the matrix organization consisting of pro-eutectoid ferrite and bainite. "F + P + B" is, matrix organization pro-eutectoid ferrite, pearlite, and, meaning that consists of bainite.
[0101]
[Table 2]

[0102]
　[Tensile Test]
　from the center of the round bar of each test numbers were taken 14A test piece No. defined in JIS Z2241 (2011). Longitudinal direction of the test piece is the rolling direction of the wire, the diameter of the parallel portion is 6 mm, gauge length was 30 mm. Relative collected specimen to a tensile test at room temperature (25 ° C.), was determined tensile strength TS (MPa). The resulting tensile strength TS is shown in Table 2. Tensile strength is not more than 700 MPa, were determined to have an excellent cold workability.
[0103]
　[Cold workability evaluation test]
　from the center of the round bar of each test numbers were taken cylindrical specimen. The diameter of the cylindrical specimen is 14 mm, length was 21 mm. Longitudinal cylindrical test specimens was parallel to the longitudinal direction of the round bar.
[0104]
　Using a cylindrical specimen in the atmosphere at room temperature (25 ° C.), (in terms of strain rate, 10 deformation speed 2 mm / min -3 / sec) was performed to a compression test with. Height direction calculated compression rate from variation of conduct compressed to 60% and then unloaded. The surface of the cylindrical test piece after the test was visually observed using a 10x loupe, to confirm the presence or absence of cracks. If the crack has not been confirmed, it is determined that there is a high cold workability ( "○" in Table 2). On the other hand, if a crack is confirmed, it is determined that there is a low cold workability (in Table 2 of "×").
[0105]
　[Vickers hardness test using a refining specimens]
　cold forged microalloyed products to produce a simulated test specimens to determine the Vickers hardness. Specifically, a round bar of each test numbers were taken cylindrical test pieces described above. The cylindrical specimen was heated at the quenching temperature shown in Table 2 (° C.), then quenched by immersion in 60 ° C. oil. Against hardened been cylindrical test pieces was carried out tempering at a tempering temperature shown in Table 2 (° C.). The retention time of the tempering temperature was 1 hour. After tempering, it was allowed to cool cylindrical specimen. Through the above steps, the refining treated cylindrical test pieces were prepared.
[0106]
　Temper-treated cylindrical test piece was cut in the axial direction perpendicular to the direction. In any five points of 2 / R of the cut surface was carried out Vickers hardness test according to JIS Z2244 (2011). Test force was 4.9N. The average value of a number of the resulting five points was defined as Vickers hardness (HV) of the test numbers. If the Vickers hardness is not less than 320HV, it was judged to exhibit a high strength after tempering.
[0107]
　[Fatigue Strength Test]
　using a specimen simulating the cold forged microalloyed products were carried out fatigue tests. Refining relative round bar of each test number was carried out (quenching and tempering), Vickers hardness of the surface was adjusted to 360 ~ 370 HV. From round bar after refining, to produce a fatigue test piece shown in FIG. Numerical values in Figure 1 shows the dimensions of the corresponding portion of the (mm). "R24" in Figure 1 indicates that the curvature radius of the curved portion corresponding is 24 mm, "φ8" indicates that a diameter of 8 mm. The central axis of the fatigue test specimens was central axis coaxial with the round bar.
[0108]
　Using fatigue test piece described above, at room temperature, in the air atmosphere was carried out with the Ono-type rotary bending fatigue test conforming to JIS Z2274 (1978). The rotational speed and 3400 rpm, stress loads repeatedly count 10 7 maximum stress that was not broken after cycles was fatigue strength .sigma.w (MPa). If the fatigue strength σw is greater than or equal to 550MPa, it was judged to be excellent in fatigue strength.
[0109]
　[Hydrogen embrittlement resistance evaluation test]
　using a specimen simulating the cold forged microalloyed products was carried hydrogen embrittlement resistance evaluation test. Refining relative round bar of each test number was carried out (quenching and tempering), Vickers hardness of the surface was adjusted to 360 ~ 370 HV. However, if the tempering temperature is not obtained over the surface hardness of 320HV at 435 ° C., it is determined that insufficient strength, hydrogen embrittlement resistance evaluation was not carried out, it is determined outside the scope of the present invention.
[0110]
　The annular V-notch test piece shown in FIG. 2 from a round bar after refining, a plurality prepared per round bar of each test number. Numbers units are not shown in Figure 2, corresponding parts of the dimensions of the test piece (in mm) it shows a. "Φ Numbers" in the figure indicate the diameter of a portion that is specified (mm). "60 °" indicates that V-notch angle is 60 °. "0.175R" indicates that V-notch bottom radius of 0.175 mm.
[0111]
　Using an electrolytic charging method, for each steel type, hydrogen was introduced at various concentrations to the test piece. Electrolytic charge method was performed as follows. It was immersed specimen into ammonium thiocyanate solution. While immersed specimens, taken hydrogen to the test piece in by generating an anode potential on the surface of the test piece.
[0112]
　After introducing hydrogen into test specimens in a zinc plating film formed on the surface of the specimen, to prevent dissipation of hydrogen in the specimen. Subsequently, it was performed the constant load test to load the constant weights as tensile stress of the nominal stress 1080MPa is loaded against V-notch section of the test piece. Broken specimen during testing and for breaking that did not test piece was conducted heating analysis method using a gas chromatograph, to measure the amount of hydrogen in the test piece. After the measurement, in each test number and the maximum hydrogen amount of not broken specimens is defined as the critical diffusible hydrogen amount Hc.
[0113]
　Furthermore, the critical diffusable hydrogen amount of steel l having a chemical composition corresponding to SCM435 of JIS G4053 (2008), was critical diffusible hydrogen amount ratio HR of reference (Href). Based on the critical diffusible hydrogen amount Href, it was determined critical diffusible hydrogen amount ratio HR by using the formula (B).
　HR = Hc / Href (B)
[0114]
　If HR is higher than 1.00, it is determined that the excellent resistance to hydrogen embrittlement.
[0115]
　[Test Results]
　The test results are shown in Table 2.
[0116]
　The chemical composition of the rolled bars of Test No. 1-6 are suitable, fn1 satisfies Equation (1), fn2 satisfied the equation (2). Furthermore, production conditions were appropriate. Therefore, in the matrix tissue, and the total area ratio of the pro-eutectoid ferrite and pearlite is 90% or more, the area ratio of the pro-eutectoid ferrite was 30% or more. The tensile strength was less than or equal to 700MPa. As a result, these rolling bars had an excellent cold workability.
[0117]
　Furthermore, simulating cold forged microalloyed products, the refining specimens of these rolling bars, Vickers hardness after tempering is at any 320HV or more, corresponding to a tensile strength of at least 1000 MPa. Furthermore, the fatigue strength is 550MPa or more, HR is greater than 1.00, exhibited excellent fatigue strength and hydrogen embrittlement resistance.
[0118]
　On the other hand, Mn content of Test No. 7 was too high. Therefore, HR is 1.00 or less, had lower hydrogen embrittlement resistance.
[0119]
　In Test No. 8, fn1 is less than the lower limit of the formula (1). Therefore, even the Vickers hardness was performed tempering is less than 320HV at a tempering temperature of 435 ° C.. Therefore, fatigue strength was less than 550MPa.
[0120]
　In Test No. 9, fn1 exceeds the upper limit of the formula (1). Therefore, the matrix organization is composed of ferrite and bainite, the tensile strength of the rolling bars exceeds 900MPa. As a result, it lowers the cold workability of the rolling bars.
[0121]
　In Test No. 10 and 11, fn2 did not meet Equation (2). Therefore, fatigue strength was less than 550MPa. Further, HR becomes 1.00 or less, had lower hydrogen embrittlement resistance.
Example 2
[0122]
　[Test method]
　and the Bloom steels a and steel type d plurality prepared. After heating the bloom at a heating temperature shown in Table 3 T1 (° C.), to implement the slabbing, cross-section to produce a billet of 162 mm × 162 mm. Production condition indicating billet in Table 3 (heating temperature T2, finishing temperature T3, the processing speed Z, the cooling rate) was prepared round bar having a diameter of 20mm at.
[0123]
[table 3]

[0124]
　Respect manufactured round bar, in the same manner as in Example 1, microstructure observation test, tensile test, and was performed cold workability evaluation test.
[0125]
　[Evaluation Result]
　The evaluation results are shown in Table 3. In Test No. 13 and Test No. 19, the production conditions of the blooming step and the finish rolling step (heating temperature T1, the heating temperature T2, finishing temperature T3, the processing speed Z, the cooling rate) was both suitable. Therefore, in the matrix tissue, and the total area ratio of the pro-eutectoid ferrite and pearlite is 90% or more, the area ratio of the pro-eutectoid ferrite was 30% or more. Tensile strength TS was less than 700MPa. As a result, an excellent cold workability.
[0126]
　On the other hand, in Test No. 14 and Test No. 20, the heating temperature T1 at the time of blooming was too low. As a result, cold workability is low. The heating temperature T1 is too low, coarse inclusions is considered because that could not be dissolved.
[0127]
　In Test No. 15 and Test No. 21, the heating temperature T2 is too high. Therefore, the tensile strength exceeds 700 MPa, cold workability is low.
[0128]
　In Test No. 16 and Test No. 22, the processing speed Z too late. Therefore, the tensile strength exceeds 700 MPa, cold workability is low.
[0129]
　In Test No. 17 and Test No. 23, finishing temperature T3 was too high. Therefore, the tensile strength exceeds 700 MPa, cold workability is low.
[0130]
　In Test No. 18 and Test No. 24, the cooling rate is too fast. Therefore, the matrix organization includes bainite, tensile strength exceeds 700 MPa. As a result, cold workability is low.
[0131]
　It has been described an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by changing the above-described embodiments without departing from the scope and spirit thereof as appropriate.

claims

By mass%,
　C: 0.22
　~ 0.40%, Si: 0.35 ~ 1.5%, Mn: 0.20
　~ 0.40%, P: less than
　0.020%, S: 0.015 %
　less,
　Cr:
　0.70 ~ 1.45%, Al: 0.005 ~
　0.060%, Ti: 0.01 ~ 0.05%, B: 0.0003 ~ 0.0040%,
　N: 0
　~ 0.0080% .0020, O: 0.0020% or
　less,
　Cu: 0 ~
　0.50%, Ni: 0 ~ 0.30%, Mo: 0
　~ 0.05%, V: 0 ~ 0.05 %,
　and, Nb: 0 contains ~ 0.05, the balance being Fe and impurities, having a chemical composition satisfying the formula (1) and (2),
　in the matrix tissue, the pro-eutectoid ferrite and pearlite the total area ratio is 90% or more, the area ratio of the pro-eutectoid ferrite is 30% or
　more, less 700MPa With Zhang strength, cold-forged tone quality goods for rolling bars.
　≦ C + 0.50 Si / 10 + Mn / 5 + 5Cr / 22 ≦ 0.85
　(1) Si / Mn> 1.0 (2)
　Here, atomic symbols in the above formula, the content of the corresponding element (mass%) is substituted.
[Requested item 2]
　A cold forged microalloyed products for rolling bars according to claim 1,
　wherein the chemical
　composition,
　Cu: 0.02 ~ 0.50%, Ni: 0.01 ~
　0.30%, Mo: 0 .005% to 0.05%,
　and, V: 0.005 1 kind selected from the group consisting of 0.05% or containing two or more, cold forged microalloyed products for rolling bars.
[Requested item 3]
　A cold forged microalloyed products for rolling bars according to claim 1 or claim 2,
　wherein the chemical composition,
　Nb: 0.0015 contains ~ 0.05%, for cold forging refining products rolling bars.
[Requested item 4]
　After heating to a temperature above 1200 ° C. The material having a chemical composition according to any one of claims 1 to 3, the slabbing step of producing a billet performed slabbing,
　the billet after heating to a temperature of 1050 ° C. or less, and a finishing rolling process for producing rolled bars implemented finish rolling,
　the finish rolling step is
　in a range the surface temperature is 750 ~ 850 ° C. of the billet, a step of manufacturing the rolling bars by rolling finishing the billet machining speed Z defined in the formula (3) as 5 to 15 / sec,
　the cooling rate from to 500 ° C. immediately after the rolling completion 0.2 as ~ 5.0 ° C. / sec, and a step of cooling the rolled bars, manufacturing method of a cold forged microalloyed products for rolling bars.
　Z = -ln (1-R) / t (3)
　where the R in the formula (3) is a cross-sectional reduction rate at finish rolling (%), defined by equation (4). t is the finish rolling time (in seconds).
　= R (A 0 -A) / A 0　(4)
　A in the formula (4) 0 is the cross-sectional area of the billet before finish rolling (mm 2 are), A is the rolling rod after finish rolling cross-sectional area of the wire (mm 2 is).