WO2017213166A1

WO2017213166A1 - Rolled steel bar for hot forging

Info

Publication number: WO2017213166A1
Application number: PCT/JP2017/021072
Authority: WO
Inventors: 幹高須賀; 有祐宮越; 長谷川　達也
Original assignee: 新日鐵住金株式会社
Priority date: 2016-06-07
Filing date: 2017-06-07
Publication date: 2017-12-14
Also published as: CN109312434A; CN109312434B; JP6673475B2; JPWO2017213166A1

Abstract

This rolled steel bar for hot forging includes, in mass%, 0.39-0.55% of C, 0.10-1.0% of Si, 0.50-1.50% of Mn, 0.05-0.50% of Cr, 0.01-0.10% of Mo, 0.05-0.40% of V, 0.150-0.250% of Ti, 0.005-0.050% of Al, and 0.0020-0.020% of N, and satisfies formulae (1) and (2), wherein: the total area ratio of polygonal ferrite and pearlite is 90% or more; the total content (mass%) of Mo included in precipitates accounts for 50% or more of the total Mo content (mass%) in the steel; and the total number of precipitates having a circle-equivalent diameter of 5-100 nm in the polygonal ferrite accounts for 80% or more of the total number precipitates having a circle-equivalent diameter of 3-1000 nm. 0.60≤C+0.2Mn+0.25Cr+0.75V+0.81Mo≤1.00 (1) 0.12C+0.35Mn+0.42Cr+Mo-0.08Si≤0.70 (2)

Description

Rolled steel bar for hot forging

The present invention relates to a steel bar, and more particularly to a rolled steel bar for hot forging.

Connecting rods (hereinafter referred to as connecting rods) used in automobile engines and the like are engine parts that connect a piston and a crankshaft, and convert the reciprocating motion of the piston into the rotational motion of the crank.

Fig. 1 is a front view of a general connecting rod. As shown in FIG. 1, the connecting rod 1 includes a large end portion 100, a flange portion 200, and a small end portion 300. The large end portion 100 is disposed at one end of the flange portion 200, and the small end portion 300 is disposed at the other end of the flange portion 200. The large end 100 is connected to the crankpin. The small end portion 300 is connected to the piston via a piston pin.

The connecting rod 1 has two parts (cap 2 and rod 3). These parts are usually manufactured by hot forging. One end portions of the cap 2 and the rod 3 correspond to the large end portion 100. Other parts than the one end of the rod 3 correspond to the flange 200 and the small end 300. The large end portion 100 and the small end portion 300 are formed by cutting. For this reason, the connecting rod 1 is required to have high machinability.

The connecting rod 1 receives a load from surrounding members during engine operation. Recently, in order to save fuel, the connecting rod 1 is required to be reduced in weight and size. Therefore, the connecting rod 1 is required to have an excellent yield strength that can cope with a load transmitted from the piston even if the flange portion 200 is made thin. Furthermore, since the compressive load and the tensile load are repeatedly applied to the connecting rod, excellent fatigue strength is also required.

¡Forging at ultra-high temperatures is effective to reduce the weight of the connecting rod. Specifically, if forging is performed at a temperature of 1330 ° C. or higher, molding is facilitated, and the thickness of parts unnecessary for functions can be reduced. Thereby, a connecting rod can be reduced in weight.

¡Forging at a very low temperature is also effective in order to reduce the size of the connecting rod. Specifically, forging is performed at a temperature of 850 ° C. or less, the crystal grains are refined, and the strength is increased. Thereby, a connecting rod can be reduced in size.

By the way, in the prior art, the cap 2 and the rod 3 of the connecting rod 1 were manufactured separately. Therefore, knock pin processing or the like is necessary for positioning the cap 2 and the rod 3. Further, a cutting process is required for the mating surface of the cap 2 and the rod 3. In recent years, cracking connecting rods that can omit these steps have begun to spread.

In the cracking connecting rod, after integrally forming the connecting rod, a jig is inserted into the hole of the large end portion 100, and the large end portion is broken by applying a stress to form two parts (corresponding to the cap 2 and the rod 3). To divide. Then, the two parts divided when attached to the crankshaft are joined. If the fracture surface of the large end 100 is a brittle fracture surface without deformation, the fracture surfaces of the cap 2 and the rod 3 can be combined and connected with bolts. Therefore, in this case, the knock pin machining process and the cutting process are omitted. As a result, the manufacturing cost is reduced.

US Pat. No. 5,135,587 (Patent Document 1), JP-A-2005-68460 (Patent Document 2), JP-A-2005-29825 (Patent Document 3), JP-A-2011-195862 (Patent Document 4), Japanese Patent Laying-Open No. 2014-77200 (Patent Document 5) proposes a steel material for cracking connecting rods and a manufacturing method for cracking connecting rods.

The forging steel for cracking connecting rod disclosed in Patent Document 1 is C: 0.6 to 0.75%, Mn: 0.25 to 0.5%, S: 0.04 to 0.12% by weight%. , Mn / S> 3.0, the balance being Fe and impurities: having a chemical composition of about 1.2% or less, and the structure is a pearlite structure. Further, the particle size number according to ASTM standard E112-88 is 3-8. This describes that excellent machinability can be obtained in Patent Document 1.

The hot forged non-tempered steel disclosed in Patent Document 2 has a ferrite single phase structure, and fine precipitates having a particle size of less than 10 nm are dispersed and precipitated in the ferrite phase. The yield stress is 600 N / mm ² or more and the yield ratio is 0.85 or more, and the fracture surface due to tensile fracture is a brittle fracture surface. This describes that excellent machinability can be obtained in Patent Document 2.

The manufacturing method of a connecting rod disclosed in Patent Document 3 uses hot forged non-heat treated steel whose final material state is a precipitation strengthened state based on V-based precipitates as a steel material constituting the connecting rod. By hot forging this steel material in a temperature range of 1100 ° C. or more and 1300 ° C. or less, a hot forging step for obtaining a forged body having a base shape of a connecting rod, and after completion of the hot forging step, the forged body is 800 ° C. Intermediate cooling step for intermediate cooling so that the average cooling rate in the first temperature range from 1 to 500 ° C. is 1 ° C./second or more, and after the intermediate cooling step, And an aging heat treatment step for aging precipitation of V-based precipitates in two temperature ranges. Thus, Patent Document 3 describes that the strength of the connecting rod can be increased.

The non-heat treated steel for hot forging disclosed in Patent Document 4 is C: 0.35-0.55%, Si: 0.15-0.40%, Mn: 0.50-1. 00%, P: 0.100% or less, S: 0.040 to 0.100%, Cr: 1.00% or less, V: 0.20 to 0.50%, Ca: 0.0005 to 0.0100 %, N: 0.0150% or less, with the balance being Fe and inevitable impurities. 2Mn + 5Mo + Cr ≦ 3.1, C + Si / 5 + Mn / 10 + 10P + 5V ≧ 1.8, and Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + V is 0.90 to 1.10. Furthermore, the hardness is HV330 or more, the yield ratio is 0.73 or more, and the structure is a ferrite pearlite structure having bainite of 10% or less. In this document, it is described in Patent Document 4 that the formation of bainite is suppressed by satisfying 2Mn + 5Mo + Cr ≦ 3.1, and excellent cracking property is obtained by satisfying C + Si / 5 + Mn / 10 + 10P + 5V ≧ 1.8. Yes.

The cracking connecting rod disclosed in Patent Document 5 is made of ferritic pearlite-type non-heat treated steel containing 0.20 to 0.60% C in mass%, and at least a large engagement with the crankshaft and piston respectively. An end portion and a small end portion are connected to each other, and a collar portion that is connected between them and subjected to coining processing is provided. The essential additive elements are C, N, Ti, Si, Mn, P, S, and Cr, and the optional additive elements are V, Pb, Te, Ca, and Bi. In the essential additive elements, by mass%, Si is in the range of 0.05 to 2.0%, Mn is in the range of 0.30 to 1.50%, and P is in the range of 0.01 to 0.2%. , S in the range of 0.060 to 0.2% and Cr in the range of 0.05 to 1.00%, N in the range of 0.005 to 0.030% and Ti in the range of 0.0. Within a range of 20% or less (excluding 0.154% or more), it is included so as to satisfy Ti ≧ 3.4N + 0.02. Arbitrary addition element, in mass%, V is 0.14% or less, Pb is 0.30% or less, Te is 0.3% or less, Ca is 0.01% or less, and Bi is 0.30% or less. And made of steel composed of the balance Fe and inevitable impurities. The 0.2% yield strength at the large end is less than 650 MPa, and the 0.2% yield strength at the coined heel is greater than 700 MPa. Thus, Patent Document 5 describes that the strength and machinability can be improved.

US Pat. No. 5,135,587 JP 2005-68460 A JP 2005-29825 A JP 2011-195862 A JP 2014-77200 A

In the conventional techniques described so far, there is a problem that the temperature condition range of hot forging for producing a connecting rod exhibiting sufficient performance is narrow.

For example, in Patent Document 1, hot forging is performed at 1037 to 1260 ° C. (1900 to 2300 ° F.), and hot forging at 850 ° C. or lower or 1330 ° C. or higher is not assumed. Furthermore, the connecting rod of Patent Document 1 lacks fatigue strength and yield strength as compared with a connecting rod obtained by tempering conventional carbon steel for mechanical structures.

In Patent Documents 2 to 5, sufficient strength can be obtained. However, even in the inventions disclosed in these documents, forging at an ultrahigh temperature of 1330 ° C. or higher and forging at an extremely low temperature of 850 ° C. or lower are not assumed. Therefore, when it manufactures on such conditions, sufficient cracking property, machinability, yield strength, and fatigue strength may not be obtained.

On the other hand, as described above, it is possible to reduce the weight and size of the connecting rod by hot forging at an ultrahigh temperature of 1330 ° C. or higher or an extremely low temperature of 850 ° C. or lower. Furthermore, rolled steel bars that can realize products with sufficient performance under various production conditions can be used in many production factories. Therefore, a steel material (rolled steel bar) capable of producing a product with sufficient performance in a wide forging temperature range is desired.

The object of the present invention is to realize a connecting rod having high yield strength and fatigue strength even when forged at an ultra-high temperature of 1330 ° C. or higher, or at an extremely low temperature of 850 ° C. or lower. Another object of the present invention is to provide a rolled steel bar for hot forging that can realize the machinability and crackability.

The rolled steel bar for hot forging according to this embodiment has a chemical composition of mass%, C: 0.39 to 0.55%, Si: 0.10 to 1.0%, Mn: 0.50 to 1. 50%, P: 0.010 to 0.100%, S: 0.040 to 0.130%, Cr: 0.05 to 0.50%, Mo: 0.01 to 0.10%, V: 0 0.05 to 0.40%, Ti: 0.150 to 0.250%, Al: 0.005 to 0.050%, N: 0.0020 to 0.020%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Nb: 0 to 0.20%, Pb: 0 to 0.30%, Zr: 0 to 0.1000%, Te: 0 to 0.3000%, Ca: 0 to 0 0.0100% and Bi: 0 to 0.3000%, with the balance being Fe and impurities, satisfying formulas (1) and (2). In the microstructure of the rolled steel bar for hot forging, the total area ratio of polygonal ferrite and pearlite is 90% or more. The total content (% by mass) of Mo in the precipitate is 50.0% or more of the total Mo content (% by mass) in the steel. The total number of precipitates having an equivalent circle diameter of 5 to 100 nm in polygonal ferrite is 80.0% or more of the total number of precipitates having an equivalent circle diameter of 3 to 1000 nm in polygonal ferrite.
0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1)
0.12C + 0.35Mn + 0.42Cr + Mo−0.08Si ≦ 0.70 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2).

The rolled steel bar for hot forging according to the present embodiment can realize a connecting rod having high yield strength and fatigue strength even when forged at an ultrahigh temperature of 1330 ° C. or higher, or at an extremely low temperature of 850 ° C. or lower, Furthermore, excellent machinability and cracking properties can be realized when manufacturing the connecting rod.

FIG. 1 is a front view of a conventional connecting rod. FIG. 2 is a front view of a tensile test piece used in the tensile test. FIG. 3 is a front view of a fatigue strength test piece used in the fatigue strength test.

The inventor investigated and examined the strength (yield strength and fatigue strength), machinability, and crackability after hot forging of rolled steel bars for hot forging. As a result, the present inventor obtained the following knowledge.

(A) Yield strength, fatigue strength, and machinability are contradictory mechanical properties. If the chemical components can be adjusted appropriately, these mechanical properties can be achieved.

In the rolled steel bar for hot forging of the present embodiment, C: 0.39 to 0.55%, Si: 0.10 to 1.0%, Mn: 0.50 to 1.50%, P in mass%. : 0.010 to 0.100%, S: 0.040 to 0.130%, Cr: 0.05 to 0.50%, Mo: 0.01 to 0.10%, V: 0.05 to 0 .40%, Ti: 0.150 to 0.250%, Al: 0.005 to 0.050%, N: 0.0020 to 0.020%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Nb: 0 to 0.20%, Pb: 0 to 0.30%, Zr: 0 to 0.1000%, Te: 0 to 0.3000%, Ca: 0 to 0.0100%, And Bi: 0 to 0.3000%, with the balance being a chemical composition comprising Fe and impurities, and in the microstructure, polygonal ferrite and The total area ratio of the 90.0% or more.

In the rolled steel bar for hot forging of this embodiment, the chemical composition further satisfies the formula (1).
0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

Defined as fn1 = C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo. fn1 is an index of strength (yield strength, fatigue strength) and machinability. fn1 shows a positive correlation with the intensity. If fn1 is higher than 1.00, the strength of the steel becomes too high, and the machinability of the steel decreases. If fn1 is less than 0.60, the strength of the steel is too low. If fn1 is 0.60 to 1.00, the strength and machinability can be improved.

(B) Bainite has higher toughness than ferrite and pearlite. Therefore, when two parts (a cap and a rod) are manufactured by breaking the large end portion of the cracking connecting rod, the broken portion is plastically deformed, and a ductile fracture surface is generated on the fracture surface. That is, cracking properties are reduced. If the generation of bainite is suppressed, the cracking property can be improved.

Therefore, in the rolled steel bar for hot forging of the present embodiment, the chemical composition further satisfies the formula (1) as well as the formula (2).
0.12C + 0.35Mn + 0.42Cr + Mo−0.08Si ≦ 0.70 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (2).

Defined as fn2 = 0.12C + 0.35Mn + 0.42Cr + Mo−0.08Si. fn2 is an index of the amount of bainite generated after hot forging. When fn2 exceeds 0.70, bainite is likely to be generated particularly in forging at an ultrahigh temperature of 1330 ° C. or higher. As a result, precipitation strengthening due to precipitates generated in ferrite due to phase interface precipitation cannot be used, and yield strength and fatigue strength are reduced. Furthermore, since bainite has higher toughness than ferrite, cracking properties also deteriorate. If fn2 is 0.70 or less, even after forging at an ultrahigh temperature of 1330 ° C. or higher, the microstructure of the steel material tends to be a ferrite-pearlite structure, and bainite is not easily generated. Therefore, excellent cracking properties can be obtained together with sufficient yield strength and fatigue strength.

(C) However, sufficient yield strength, fatigue strength and cracking properties cannot be obtained if the chemical composition only satisfies the formulas (1) and (2). Therefore, in the rolled steel bar for hot forging of this embodiment, among the precipitates precipitated in polygonal ferrite by phase interface precipitation, V-Ti-Mo composite carbide which is a composite carbide of V carbide, Ti carbide and Mo carbide. Pay attention to. Here, the V—Ti—Mo composite carbide is a carbide in which Ti and Mo are contained in the V carbide.

Precipitates generated in polygonal ferrite may include carbides such as V carbide, Ti carbide, Mo carbide, and V-Ti-Mo composite carbide, as well as other precipitates different from carbides such as TiS. is there. However, the equivalent circle diameter of other precipitates different from carbides such as TiS is larger than 1 μm (1000 nm). On the other hand, when attention is paid to carbides in the polygonal ferrite of the steel having the above-mentioned chemical composition satisfying the formulas (1) and (2), Ti carbides and Mo carbides are hardly formed, and most of the carbides are V carbides and V carbides. -Ti-Mo composite carbide. And the equivalent circle diameter of these carbides is 1000 nm or less.

In the present embodiment, among the precipitates having an equivalent circle diameter of 3 to 1000 nm in the polygonal ferrite, the ratio of the precipitates having an equivalent circle diameter of 5 to 100 nm is increased. The precipitate having an equivalent circle diameter of 5 to 100 nm is substantially either V carbide or V—Ti—Mo composite carbide. By increasing the proportion of precipitates having an equivalent circle diameter of 5 to 100 nm, excellent yield strength and fatigue strength can be obtained even after hot forging, and excellent cracking properties can also be obtained. Hereinafter, this point will be described in detail.

When a steel bar having the above chemical composition and satisfying the formulas (1) and (2) is hot forged, precipitates are generated in polygonal ferrite by phase interface precipitation in the cooling step after hot forging. . In the above chemical composition, the Mo content is as low as 0.01 to 0.10%. Therefore, Mo hardly precipitates as Mo ₂ C, and is easily dissolved in V carbide (VC) generated by phase interface precipitation. Furthermore, Ti is easily dissolved in V carbide (VC). In the case of steel having the above-described chemical composition, Mo is either contained in the precipitate or is dissolved in the matrix.

In this embodiment, there are mainly V carbide, Ti carbide, and Mo carbide as precipitates generated by phase interface precipitation, and V-Ti-Mo composite carbide in which Ti and Mo are dissolved in V carbide. Exists. V-Ti-Mo composite carbide has a different precipitation form from V carbide (VC). Specifically, the V—Ti—Mo composite carbide can dramatically increase the yield strength and fatigue strength of the steel material after hot forging by dissolving Mo in solid solution. In order to precipitate fine V—Ti—Mo composite carbide in the steel material after hot forging, it is preferable that V, Ti and Mo are dissolved in austenite in the steel material before hot forging.

However, when the rolled steel bar for hot forging is heated in the hot forging process and its microstructure becomes austenite, if Mo is excessively dissolved in austenite, the hardenability of the steel is excessively increased, The generation of bainite is promoted. In particular, in forging at an ultrahigh temperature of 1330 ° C. or higher, the tendency (promotion of bainite generation) becomes remarkable. In this case, since the formation of ferrite is suppressed, the amount of VC and V—Ti—Mo composite carbide produced by phase interface precipitation becomes insufficient. Therefore, sufficient yield strength and fatigue strength cannot be obtained. Furthermore, since the toughness is increased by the generation of bainite, sufficient cracking properties cannot be obtained.

On the other hand, the above-mentioned V—Ti—Mo composite carbide has a feature that it does not easily dissolve at an ultra-high temperature of 1330 ° C. or higher as compared with VC. The reason for this is not clear, but it is considered that the V—Ti—Mo composite carbide contains Ti, so that it is less likely to dissolve at a higher temperature than VC even at high temperatures.

Therefore, the present inventors have the above-mentioned chemical composition and in a rolled steel bar for hot forging satisfying the formulas (1) and (2), a predetermined amount of carbides precipitated in polygonal ferrite by phase interface precipitation. If the proportion of V-Ti-Mo composite carbide of an appropriate size (equivalent circle diameter of 5 to 100 nm) is increased, each V-Ti-Mo composite carbide dissolves slightly during heating in the hot forging process. However, it was considered that the V—Ti—Mo composite carbide was not completely dissolved but remained. In this case, since Mo can be prevented from excessively dissolving in the steel, the V—Ti—Mo composite carbide precipitated before hot forging is suppressed after hot forging while suppressing the formation of bainite. It can also remain in steel. Furthermore, fine V—Ti—Mo composite precipitates newly generated by phase interface precipitation during cooling in the hot forging process can also be included in the steel material after hot forging. As a result, it was considered that excellent yield strength, fatigue strength, and cracking properties were obtained. As a result of further investigation and examination by the present inventors based on the above examination results, the following knowledge was obtained.

As described above, the equivalent circle diameter of carbides generated in polygonal ferrite is 1000 nm or less, and the equivalent circle diameter generated in polygonal ferrite is 1000 nm in a chemical composition satisfying the formulas (1) and (2). The deposit is substantially carbide. When there are many precipitates with an equivalent circle diameter of less than 5 nm in the polygonal ferrite of a rolled steel bar for hot forging, the V-Ti-Mo composite carbide is too solid before hot forging, that is, Mo is excessive. It means that it is in solid solution. In this case, since the hardenability becomes excessively high during hot forging, the generation of bainite is promoted after hot forging. In particular, in hot forging at an ultrahigh temperature of 1330 ° C. or higher, bainite is excessively generated, and as a result, sufficient yield strength, fatigue strength, and crackability cannot be obtained.

On the other hand, when there are many precipitates with an equivalent circle diameter exceeding 100 nm in the polygonal ferrite of the rolled steel bar for hot forging, the coarse V-Ti present in the polygonal ferrite of the steel bar during heating in the hot forging process. -Most of the Mo composite carbide is not completely dissolved but remains coarse, and Ti, V and Mo are not sufficiently dissolved in the matrix. In this case, new fine V—Ti—Mo composite precipitates are difficult to precipitate during cooling in the hot forging process. As a result, the strength of the steel material after hot forging decreases. In particular, in hot forging at 850 ° C. or lower, the solid solution amount of Mo decreases, and sufficient strength (yield strength, fatigue strength) cannot be obtained.

The total number of precipitates having a circle equivalent diameter of 5 to 100 nm in the polygonal ferrite of the rolled steel bar for hot forging is 80% of the total number of precipitates having a circle equivalent diameter of 3 to 1000 nm in the polygonal ferrite. If it is 0% or more, the yield is excellent in the steel material after hot forging, provided that the chemical composition satisfies the formulas (1) and (2) and further satisfies the requirements of the item (D) described later. Strength, fatigue strength and cracking properties are obtained.

(D) Furthermore, the total content of Mo in the precipitate in the hot forging rolled steel bar (Mo content in the precipitate CP _-Mo ) is the total Mo content in the hot forging rolled steel bar (in the steel). 50.0% or more of the total Mo amount C _T-Mo ) As described above, Mo is either contained in the carbide or dissolved in the matrix. If the amount of precipitated Mo is less than 50.0% of the total amount of Mo, Mo is excessively dissolved in the hot forging rolled steel bar. Therefore, bainite is generated in the steel material after hot forging, and sufficient cracking properties cannot be obtained. Furthermore, when bainite is generated, precipitation strengthening caused by precipitates generated in polygonal ferrite due to phase interface precipitation cannot be used, so that sufficient yield strength and fatigue strength cannot be obtained. The amount of precipitated Mo is determined from the extract extracted by extraction residue analysis.

In a rolled steel bar for hot forging having a chemical composition satisfying the above formulas (1) and (2), the total number of precipitates having a circle equivalent diameter of 5 to 100 nm in polygonal ferrite is 3 to 3 in polygonal ferrite. 80.0% or more of the total number of precipitates having an equivalent circle diameter of 1000 nm, and the total content of Mo in the precipitates in the hot forging rolled steel bar is the total Mo content in the hot forging rolled steel bar When the amount is 50% or more, excellent yield strength, fatigue strength, cracking property and machinability can be obtained in the steel material after hot forging. In order to manufacture such a hot forging steel bar, for example, the following manufacturing method may be carried out.

An example of the manufacturing method of the hot forged rolled steel bar according to the present embodiment includes a casting process and a hot working process. The hot working step includes a rough rolling step typified by split rolling and a finish rolling step using a continuous rolling mill in which a plurality of rolling stands are arranged in a row. In the rough rolling process, in cooling the steel material after the rough rolling, the cooling time until the steel material temperature reaches 800 ° C. to 500 ° C. is set to 20 minutes or more. The steel material after the rough rolling step is cooled to 400 ° C. or lower, preferably room temperature (25 ° C.). In the finish rolling step, the heating temperature T1 is set to 1100 ° C. or less, and the heating time t1 is set to 30 minutes or less. Furthermore, the steel material temperature T2 during finish rolling is controlled to 1200 ° C. or lower, and the finishing temperature is set to 1000 ° C. or lower. In the cooling of the steel material after finish rolling, the cooling time until the steel material temperature reaches 800 ° C. to 500 ° C. is set to 5 minutes or less. In this case, since the cooling rate can be set slower in the rough rolling process, V-Ti-Mo composite carbide having a certain size is generated in the polygonal ferrite. In finish rolling, since the material temperature during rolling is set to be low, those V—Ti—Mo composite carbides are not completely dissolved and remain even during finish rolling. Further, since the cooling rate after finish rolling is set to be high, the V-Ti-Mo composite carbide is prevented from becoming coarse again. Through the above steps, in the steel having the chemical composition satisfying the formulas (1) and (2), the total number of precipitates having a circle equivalent diameter of 5 to 100 nm in the polygonal ferrite is 3 to 1000 nm in the polygonal ferrite. The total content of Mo in the precipitates in the steel can be 50% or more of the total Mo content in the steel. In addition, the said manufacturing method is an example of the manufacturing method of the rolled steel bar for hot forging of this embodiment.

The rolled steel bar for hot forging according to the present embodiment completed based on the above knowledge is, in mass%, C: 0.39 to 0.55%, Si: 0.10 to 1.0%, Mn: 0.50 to 1.50%, P: 0.010 to 0.100%, S: 0.040 to 0.130%, Cr: 0.05 to 0.50%, Mo: 0.01 to 0.10%, V : 0.05 to 0.40%, Ti: 0.150 to 0.250%, Al: 0.005 to 0.050%, N: 0.0020 to 0.020%, Cu: 0 to 0.40 %, Ni: 0 to 0.30%, Nb: 0 to 0.20%, Pb: 0 to 0.30%, Zr: 0 to 0.1000%, Te: 0 to 0.3000%, Ca: 0 Containing 0.0 to 100% and Bi: 0 to 0.3000%, with the balance being Fe and impurities, and having a chemical composition satisfying formulas (1) and (2) . In the microstructure of the rolled steel bar for hot forging, the total area ratio of polygonal ferrite and pearlite is 90% or more. The total content (% by mass) of Mo in the precipitate is 50.0% or more of the total Mo content (% by mass) in the steel. The total number of precipitates having an equivalent circle diameter of 5 to 100 nm in polygonal ferrite is 80.0% or more of the total number of precipitates having an equivalent circle diameter of 3 to 1000 nm in polygonal ferrite.
0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1)
0.12C + 0.35Mn + 0.42Cr + Mo−0.08Si ≦ 0.70 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2).

The chemical composition is one or two selected from the group consisting of Cu: 0.01 to 0.40%, Ni: 0.005 to 0.30%, and Nb: 0.001 to 0.20%. It may contain seeds or more. The chemical composition is Pb: 0.05 to 0.30%, Zr: 0.0003 to 0.1000%, Te: 0.0003 to 0.3000%, Ca: 0.0003 to 0.0100%, and Bi: One or more selected from the group consisting of 0.0003 to 0.3000% may be contained.

Hereinafter, the rolled steel bar for hot forging of this embodiment will be described in detail. “%” Of the content of each element means “mass%”.

[Chemical composition]
The chemical composition of the hot forging rolled steel bar according to the present embodiment contains the following elements.

[About essential elements]
C: 0.39 to 0.55%
Carbon (C) increases the strength of the steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, the hardness of the steel material increases, and the machinability decreases. Therefore, the C content is 0.39 to 0.55%. The minimum with preferable C content is more than 0.39%, More preferably, it is 0.40%, More preferably, it is 0.42%. The upper limit with preferable C content is less than 0.55%, More preferably, it is 0.53%, More preferably, it is 0.51%.

Si: 0.10 to 1.0%
Silicon (Si) deoxidizes steel. Si further dissolves in the steel to increase the fatigue strength of the steel. If the Si content is too low, these effects cannot be obtained. On the other hand, if the Si content is too high, the above effect is saturated. If the Si content is too high, the hot workability of the steel further decreases, and the manufacturing cost of the steel bar increases. Therefore, the Si content is 0.10 to 1.0%. The minimum with preferable Si content is more than 0.10%, More preferably, it is 0.12%, More preferably, it is 0.15%. The upper limit with preferable Si content is less than 1.00%, More preferably, it is 0.95%, More preferably, it is 0.90%.

Mn: 0.50 to 1.50%
Manganese (Mn) deoxidizes steel. Mn further increases the strength of the steel. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, the hot workability of the steel decreases. If the Mn content is too high, the hardenability is further increased, and bainite is generated in the steel structure. In this case, the yield strength, fatigue strength, and crackability of the steel material after hot forging are reduced. Therefore, the Mn content is 0.50 to 1.50%. The minimum with preferable Mn content is more than 0.50%, More preferably, it is 0.55%, More preferably, it is 0.60%. The upper limit with preferable Mn content is less than 1.50%, More preferably, it is 1.45%, More preferably, it is 1.40%.

P: 0.010 to 0.100%
Phosphorus (P) segregates at the grain boundaries and embrittles the steel. Therefore, the fracture surface of the cracking connecting rod after the fracture split becomes smooth. As a result, the cracking property of the steel material after hot forging is increased, and the accuracy of assembling the cracking connecting rod after the fracture split is increased. If the P content is too low, this effect cannot be obtained. On the other hand, if P content is too high, the hot workability of steel will fall. Therefore, the P content is 0.010 to 0.100%. The minimum with preferable P content is more than 0.010%, More preferably, it is 0.015%, More preferably, it is 0.020%. The upper limit with preferable P content is less than 0.100%, More preferably, it is 0.090%, More preferably, it is 0.07%.

S: 0.040 to 0.130%
Sulfur (S) combines with Mn and Ti to form sulfides and enhances the machinability of steel. If the S content is too low, this effect cannot be obtained. On the other hand, if the S content is too high, the fatigue strength decreases. If the S content is too high, the hot workability of the steel further decreases. Therefore, the S content is 0.040 to 0.130%. The minimum with preferable S content is more than 0.040%, More preferably, it is 0.045%, More preferably, it is 0.050%. The upper limit with preferable S content is less than 0.130%, More preferably, it is 0.125%, More preferably, it is 0.120%.

Cr: 0.05 to 0.50%
Chromium (Cr) increases the strength of the steel. If the Cr content is too low, this effect cannot be obtained. On the other hand, if the Cr content is too high, the hardenability of the steel increases and bainite is generated in the steel structure. In this case, the yield strength, fatigue strength, and crackability of the steel material after hot forging are reduced. If the Cr content is too high, the production cost further increases. Therefore, the Cr content is 0.05 to 0.50%. The minimum with preferable Cr content is 0.10%, More preferably, it is 0.12%, More preferably, it is 0.15%. The upper limit with preferable Cr content is less than 0.50%, More preferably, it is 0.45%, More preferably, it is 0.40%.

Mo: 0.01 to 0.10%
Molybdenum (Mo) increases the strength of steel by solid solution strengthening. Mo further dissolves in VC formed in polygonal ferrite by phase interface precipitation to increase the strength (yield strength and fatigue strength) of the steel. More specifically, since the Mo content is low in the chemical composition of the present embodiment, Mo is difficult to precipitate as Mo ₂ C, and is dissolved in VC formed in polygonal ferrite by phase interface precipitation. This forms a V—Ti—Mo composite carbide in which Ti also forms a solid solution. V-Ti-Mo composite carbide has a different precipitation form from VC, and significantly increases the yield strength and fatigue strength of steel. If the Mo content is too low, these effects cannot be obtained. On the other hand, if the Mo content is too high, the solid solution Mo amount increases, so that the hardenability becomes too high. In this case, since the formation of bainite in the steel structure before hot forging or after hot forging is promoted, the yield strength, fatigue strength, and cracking properties of the steel material after hot forging are reduced. Therefore, the Mo content is 0.01 to 0.10%. The upper limit with preferable Mo content is less than 0.10%, More preferably, it is 0.09%, More preferably, it is 0.08%.

V: 0.05 to 0.40%
As described above, vanadium (V) forms V-Ti-Mo composite carbide in polygonal ferrite by phase interface precipitation, and increases the yield strength and fatigue strength of the steel material after hot forging. Furthermore, by containing with Ti, the V—Ti—Mo composite carbide is refined. Therefore, toughness falls and the cracking property of the steel material after hot forging increases. If the V content is too low, these effects cannot be obtained. On the other hand, if the V content is too high, not only the production cost of steel becomes extremely high, but also the machinability decreases. Therefore, the V content is 0.05 to 0.40%. The minimum with preferable V content is more than 0.05%, More preferably, it is 0.06%, More preferably, it is 0.10%. The upper limit with preferable V content is less than 0.40%, More preferably, it is 0.35%, More preferably, it is 0.32%.

Ti: 0.150 to 0.250%
Titanium (Ti) forms Ti nitride in polygonal ferrite by phase interface precipitation, or dissolves in VC to form V-Ti-Mo composite carbide to yield steel after hot forging. Increase strength and fatigue strength. Ti further produces sulfides or carbosulfides to enhance the machinability of the steel. Ti further refines the V—Ti—Mo composite carbide to increase the cracking property of the steel by reducing the toughness of the steel. If the Ti content is too low, these effects cannot be obtained. On the other hand, if the Ti content is too high, the amount of Ti carbide will be too much. In this case, the tensile strength of the steel becomes too high and the machinability of the steel decreases. Therefore, the Ti content is 0.150 to 0.250%. The minimum with preferable Ti content is 0.151%, More preferably, it is 0.155%. The upper limit with preferable Ti content is less than 0.250%, More preferably, it is 0.220%.

Al: 0.005 to 0.050%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, Al forms hard oxide inclusions and lowers fatigue strength. Therefore, the Al content is 0.005 to 0.050%. The minimum with preferable Al content is 0.020%. The upper limit with preferable Al content is 0.040%. In the rolled steel bar for hot forging of the present embodiment, the Al content means acid-soluble Al (so-called “sol. Al”).

N: 0.0020 to 0.020%
Nitrogen (N) combines with V or Ti to form nitrides and increases the strength of the steel. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, this effect is saturated. Therefore, the N content is 0.0020 to 0.020%. The minimum with preferable N content is more than 0.0020%, More preferably, it is 0.0030%, More preferably, it is 0.0040%. The upper limit with preferable N content is less than 0.020%, More preferably, it is 0.019%, More preferably, it is 0.018%.

The balance of the chemical composition of the rolled steel bar for hot forging according to the present embodiment is composed of Fe and impurities. Here, the impurities are mixed from ore as a raw material, scrap, or production environment when industrially manufacturing rolled steel bars for hot forging, and are used for hot forging in this embodiment. It means that it is allowed as long as it does not adversely affect the rolled steel bar. The chemical composition of the rolled steel bar for hot forging of this embodiment can contain the following elements as impurities.
B: 0.0002% or less,
Sb: 0.05% or less,
Sn: 0.03% or less,
Co: 0.03% or less,
Rare earth element (REM): 0.03% or less,
O (oxygen): 0.0050% or less, and
H (hydrogen): 0.0005% or less

Note that REM in this specification contains at least one of Sc, Y, and lanthanoid (La of atomic number 57 to Lu of 71), and the REM content is the total content of these elements. Means.

[Arbitrary elements]
The chemical composition of the hot forging rolled steel bar according to the present embodiment may further include one or more selected from the group consisting of Cu, Ni, and Nb instead of part of Fe. These elements are arbitrary elements, and all increase the strength of steel.

Cu: 0 to 0.40%
Copper (Cu) is an optional element and may not be contained. When contained, Cu dissolves in the steel and increases the strength of the steel. However, if the Cu content is too high, the manufacturing cost of steel increases. Therefore, the Cu content is 0 to 0.40%. The minimum with preferable Cu content is 0.01%, More preferably, it is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Cu content is less than 0.40%, More preferably, it is 0.35%, More preferably, it is 0.30%.

Ni: 0 to 0.30%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni dissolves in the steel and increases the strength of the steel. However, if the Ni content is too high, the manufacturing cost increases. Therefore, the Ni content is 0 to 0.30%. The minimum with preferable Ni content is 0.005%, More preferably, it is 0.01%. The upper limit with preferable Ni content is less than 0.30%, More preferably, it is 0.28%, More preferably, it is 0.25%.

Nb: 0 to 0.20%
Niobium (Nb) is an optional element and may not be contained. When contained, Nb precipitates as carbide or nitride in the steel and increases the yield strength and fatigue strength of the steel after hot forging. However, if the Nb content is too high, not only the manufacturing cost of steel becomes very high, but also the machinability decreases. Therefore, the Nb content is 0 to 0.20%. The minimum with preferable Nb content is 0.001%, More preferably, it is 0.005%, More preferably, it is 0.01%. The upper limit with preferable Nb content is less than 0.20%, More preferably, it is 0.18%, More preferably, it is 0.15%.

The chemical composition of the rolled steel bar for hot forging according to the present embodiment further includes one or more selected from the group consisting of Pb, Zr, Te, Ca and Bi instead of a part of Fe. May be. These elements are arbitrary elements, and all enhance the machinability of steel.

Pb: 0 to 0.30%
Lead (Pb) is an optional element and may not be contained. When contained, Pb increases the machinability of the steel. However, if the Pb content is too high, the hot ductility of the steel decreases, and wrinkles are likely to occur in the steel after rolling. Therefore, the Pb content is 0 to 0.30%. The minimum with preferable Pb content is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Pb content is less than 0.30%, More preferably, it is 0.25%, More preferably, it is 0.20%.

Zr: 0 to 0.1000%
Zircon (Zr) is an optional element and may not be contained. When contained, Zr enhances the machinability of the steel. However, if the Zr content is too high, the hot ductility of the steel decreases, and wrinkles are likely to occur in the rolled steel bar. Therefore, the Zr content is 0 to 0.1000%. The minimum with preferable Zr content is 0.0003%, More preferably, it is 0.0005%, More preferably, it is 0.0010%. The upper limit with preferable Zr content is less than 0.10%, More preferably, it is 0.0800%, More preferably, it is 0.0500%.

Te: 0 to 0.3000%
Tellurium (Te) is an optional element and may not be contained. When contained, Te increases the machinability of the steel. However, if the Te content is too high, the productivity of the steel decreases, and wrinkles are likely to occur in the rolled steel bar. Therefore, the Te content is 0 to 0.3000%. The minimum with preferable Te content is 0.0003%, More preferably, it is 0.0005%, More preferably, it is 0.0010%. The upper limit with preferable Te content is less than 0.3000%, More preferably, it is 0.2500%, More preferably, it is 0.2000%.

Ca: 0 to 0.0100%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca increases the machinability of steel. However, if the Ca content is too high, the manufacturing cost increases. Therefore, the Ca content is 0 to 0.0100%. The minimum with preferable Ca content is 0.0003%, More preferably, it is 0.0005%, More preferably, it is 0.0010%. The upper limit with preferable Ca content is less than 0.0100%, More preferably, it is 0.0080%, More preferably, it is 0.0050%.

Bi: 0 to 0.3000%
Bismuth (Bi) is an optional element and may not be contained. When contained, Bi increases the machinability of the steel. However, if the Bi content is too high, the productivity of the steel decreases, and wrinkles are likely to occur in the rolled steel bar. Therefore, the Bi content is 0 to 0.3000%. The minimum with preferable Bi content is 0.0003%, More preferably, it is 0.0005%, More preferably, it is 0.0010%. The upper limit with preferable Bi content is less than 0.3000%, More preferably, it is 0.2000%, More preferably, it is 0.1000%.

[Method for chemical composition analysis]
Analysis of the chemical composition of the rolled steel bar for hot forging of this embodiment can be obtained by a well-known component analysis method. Specifically, the following method is used. A hot forging rolled steel bar is cut perpendicular to the longitudinal direction, and a sample having a length of 20 mm is taken. Chips are generated by drilling the R / 2 part of the sample in a direction parallel to the steel material longitudinal direction using a φ5 mm drill, and the chips are collected. The collected chips are dissolved in acid to obtain a solution. The R / 2 part is a part that bisects between the center and the outer periphery of the cross section (circular shape) of the steel bar. An IPC-OES (Inductively Coupled Plasma Optical Emission Spectrometry) is performed on the solution to perform an elemental analysis of the chemical composition. About C content and S content, it calculates | requires by the known high frequency combustion method. Specifically, the above solution is burned by high-frequency heating in an oxygen stream, and the generated carbon dioxide and sulfur dioxide are detected, and the C content and S content are determined.

[Regarding Formula (1)]
The chemical composition of the hot forged rolled steel bar of this embodiment further satisfies the formula (1).
0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

Defined as fn1 = C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo. fn1 is an index of strength (yield strength, fatigue strength) and machinability of steel after hot forging. If fn1 is higher than 1.00, the strength of the steel becomes too high, and the machinability of the steel decreases. If fn1 is less than 0.60, the strength of the steel is too low. When fn1 is 0.60 to 1.00, excellent strength and machinability after hot forging can be obtained in a rolled steel bar for hot forging. The minimum with preferable fn1 is 0.61, More preferably, it is 0.63, More preferably, it is 0.65. The upper limit with preferable fn1 is 0.99, More preferably, it is 0.98, More preferably, it is 0.95.

[Regarding Formula (2)]
The chemical composition of the hot forged rolled steel bar of this embodiment further satisfies the formula (2).
0.12C + 0.35Mn + 0.42Cr + Mo−0.08Si ≦ 0.70 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (2).

Defined as fn2 = 0.12C + 0.35Mn + 0.42Cr + Mo−0.08Si. fn2 is an index of bainite generation after hot forging. When fn2 exceeds 0.70, bainite is likely to be generated particularly in forging at an ultrahigh temperature of 1330 ° C. or higher. As a result, yield strength, fatigue strength, and cracking properties are reduced. If fn2 is 0.70 or less, a ferrite and pearlite structure can be obtained even in forging at an ultrahigh temperature of 1330 ° C. or higher. Therefore, excellent cracking properties can be obtained together with sufficient yield strength and fatigue strength. The upper limit with preferable fn2 is 0.67, More preferably, it is 0.65.

[Microstructure]
The microstructure of the rolled steel bar for hot forging of this embodiment is substantially a ferrite pearlite structure. More specifically, in the microstructure of the rolled steel bar for hot forging of the present embodiment, the total area ratio of polygonal ferrite and pearlite is 90.0% or more, more preferably 95.0% or more, More preferably, it is 100.0%. When the total area ratio of polygonal ferrite and pearlite is not 100.0%, the balance of the microstructure is bainite and / or retained austenite.

The total area ratio of polygonal ferrite and pearlite in the microstructure can be measured by the following method. Ten samples are taken from an arbitrary R / 2 part of the rolled steel bar for hot forging. The R / 2 part is a part that bisects between the center and the outer periphery of the cross section (circular shape) of the steel bar. Of the collected samples, the surface perpendicular to the central axis of the hot forged rolled steel bar is taken as the observation surface. After the observation surface is polished, it is etched with 3% nitric acid alcohol (nitral etchant). The etched observation surface is observed with a 200 × optical microscope, and photographic images with arbitrary five fields of view are generated. The area of each visual field is 0.302 mm ² .

In each visual field, each phase of polygonal ferrite, pearlite, bainite, and retained austenite has a different contrast for each phase. Therefore, each phase is specified based on the contrast. Among the identified phases, the total area A _{F + P} (μm ² ) of polygonal ferrite and pearlite in each visual field is obtained. The ratio of the total area A _{F + P} of polygonal ferrite and pearlite in all fields of view (5 fields x 10) to the total area A _TOTAL of all fields of vision (5 fields x 10) is the total of polygonal ferrite and pearlite. The area ratio is defined as RA _{F + P} (%). That is, the total area ratio RAF _{+ P} of polygonal ferrite and pearlite is defined by the following equation.
RA _{F + P} = A _{F + P} / A _TOTAL × 100

[Mo content ratio in precipitates in steel]
As described above, in the rolled steel bar for hot forging of this embodiment, precipitates are formed in polygonal ferrite by phase interface precipitation. In the pearlite, almost no precipitate is generated. Mo is either contained in the precipitate or is dissolved in the matrix.

In the present embodiment, the above-described V—Ti—Mo composite carbide is included to some extent among the precipitates generated in the polygonal ferrite by the phase interface precipitation. Specifically, in the microstructure of the rolled steel bar for hot forging according to the present embodiment, the total content of Mo in the precipitate (Mo content in the precipitate C _P-Mo ) is the total Mo content in the steel ( It is 50.0% or more of the total Mo amount in the steel (C _T-Mo ). That is, the precipitates Mo amount _{C P-Mo,} when defining the total amount of Mo _C precipitates the ratio _T-Mo Mo ratio _{RA Mo} in the steel, precipitates Mo ratio _RA Mo (%) is the following It is defined by an expression.
Mo amount ratio in precipitates RA _Mo = Mo amount in precipitates C _P-Mo / Total Mo amount in steel C _T-Mo × 100

If the precipitates Mo amount ratio _{RA Mo} is less than 50.0%, of the precipitates in polygonal ferrite, small proportion of V-Ti-Mo composite carbide. In this case, it means that Mo is excessively dissolved in the rolled steel bar for hot forging. Therefore, bainite is generated after hot forging, and sufficient yield strength, fatigue strength, and cracking properties cannot be obtained. If the precipitates Mo amount ratio _{RA Mo} is 50.0% or more, of the precipitates in polygonal ferrite, the ratio of V-Ti-Mo composite carbide is sufficiently high. Therefore, excellent yield strength and fatigue after hot forging, provided that the chemical composition satisfies the formulas (1) and (2) and the ratio of the number of precipitates of the specific size described later is 80.0% or more. Strength and cracking properties can be obtained. A preferred lower limit of the precipitates Mo amount ratio _{RA Mo} is 55.0%, still more preferably 60.0%.

Mo amount ratio RA _Mo in the precipitate can be measured by the following method based on the extraction residue method. A cubic sample having a side of about 10 mm is taken from R / 2 part of the steel material. The surface layer from the sample surface to a depth of 200 μm is removed by electrolysis using an AA electrolyte solution (an electrolyte solution containing 10 vol% acetylacetone and 1 vol% tetramethylammonium chloride, with the balance being methanol). Remove adhering impurities. The electrolysis time is adjusted with a constant current. The electrolyte is replaced with a new AA-based electrolyte, and the sample is electrolyzed again. The electrolysis time is adjusted so that the volume of the test piece to be electrolyzed is 58 mm ³ with the current being constant at 1000 mA. The electrolytic solution after electrolysis is filtered using a filter having a mesh size of 200 nm to obtain a residue. The obtained residue is subjected to inductively coupled plasma (IPC) emission spectroscopic analysis to determine the total Mo content in the precipitate (Mo amount in the precipitate, C _P-Mo , unit is mass (g)). Furthermore, the total Mo content in the steel (total Mo amount in steel C _T-Mo , unit is mass (g)) is obtained by the following formula.
Total Mo amount in steel C _T-Mo = Total Mo content in steel (% by mass) × mass of electrolyzed specimen (g)

Based in the obtained precipitates Mo amount _{C P-Mo} and steel in the total amount of Mo _{C T-Mo,} obtaining the precipitates Mo amount ratio _{RA Mo} from the following equation.
Mo amount ratio in precipitates RA _Mo = Mo amount in precipitates C _P-Mo / Total Mo amount in steel C _T-Mo × 100

[Number ratio of precipitates having equivalent circle diameter of 5 to 100 nm in polygonal ferrite]
As described above, in the rolled steel bar for hot forging of this embodiment, precipitates containing carbides are formed in polygonal ferrite by phase interface precipitation. And almost no precipitate is generated in the pearlite.

In the present specification, precipitates generated in polygonal ferrite are precipitates other than carbides (such as V carbide, Ti carbide, Mo carbide and V-Ti-Mo composite carbide) and carbides represented by TiS and the like. is there. Among these, the equivalent-circle diameter of deposits other than carbide is larger than 1000 nm. On the other hand, the equivalent circle diameter of most carbides is 1000 nm or less. In addition, in observation of precipitates with a transmission electron microscope of 200,000 times described later, it is difficult to confirm precipitates having an equivalent circle diameter of less than 3 nm. Therefore, in this specification, attention is paid to precipitates having a circle-equivalent diameter of 3 to 1000 nm in polygonal ferrite. Precipitates having a circle equivalent diameter of 3 to 1000 nm in polygonal ferrite are substantially V carbide, Ti carbide, Mo carbide, and V—Ti—Mo composite carbide. Moreover, as above-mentioned, in the chemical composition which satisfy | fills Formula (1) and Formula (2), Ti carbide | carbonized_material and Mo carbide | carbonized_material are hardly produced | generated. Therefore, most of the precipitates having a circle equivalent diameter of 3 to 1000 nm in polygonal ferrite are V carbide and V—Ti—Mo composite carbide. The V—Ti—Mo composite carbide means a carbide in which Ti and Mo are contained in the V carbide.

In the microstructure of the hot forging rolled steel bar of this embodiment, the total number N _5-100 of precipitates having a circle equivalent diameter of 5 to 100 nm in polygonal ferrite is 3 to 1000 nm in polygonal ferrite. It is 80.0% or more of the total number N _TOTAL of precipitates having an equivalent diameter. That is, the ratio of the total number N _5-100 of precipitates having an equivalent circle diameter of 5 to 100 nm to the total number N _TOTAL of precipitates having an equivalent circle diameter of 3 to 1000 nm in polygonal ferrite is the number ratio RA _5-100 The number ratio RA _5-100 is defined by the following equation.
Number ratio RA _5-100 = Total number N _5-100 / Total number N _TOTAL × 100

As described above, in the total number N _TOTAL , the precipitates having an equivalent circle diameter of less than 3 nm are not counted because it is difficult to identify precipitates having an equivalent circle diameter of less than 3 nm.

As described above, when there is a large proportion of precipitates with an equivalent circle diameter of less than 5 nm in the polygonal ferrite of hot forged rolled steel bars, the V-Ti-Mo composite carbide is too fine or Mo is dissolved. Too much. In this case, since the hardenability is too high, the generation of bainite is promoted after hot forging. In particular, the formation of bainite is promoted after hot forging at an ultrahigh temperature of 1330 ° C. or higher. For this reason, the amount of V—Ti—Mo composite carbide produced by phase interface precipitation along with the formation of ferrite is insufficient, and sufficient yield strength and fatigue strength may not be obtained. Furthermore, since bainite has high toughness, sufficient cracking properties may not be obtained.

On the other hand, Ti, V, and Mo are not sufficiently dissolved in the case of a large proportion of precipitates having an equivalent circle diameter exceeding 100 nm in the polygonal ferrite of the rolled steel bar for hot forging. In this case, after hot forging, fine V—Ti—Mo composite carbide is not easily generated in the polygonal ferrite by phase interface precipitation, and the hardenability is low due to the insufficient amount of Mo solid solution. As a result, the yield strength and fatigue strength of the steel material after hot forging are lowered. In particular, in forging at 850 ° C. or lower, sufficient strength may not be obtained.

Of the precipitates in polygonal ferrite, the total number N _{5-100 of} precipitates having an equivalent circle diameter of 5 to 100 nm is the total number N of precipitates having an equivalent circle diameter of 3 to 1000 nm in polygonal ferrite. If it is 80.0% or more of _TOTAL , a V-Ti-Mo composite carbide of an appropriate size can be sufficiently secured. Therefore, the yield strength chemical composition satisfies the formula (1) and (2), and the precipitates Mo amount ratio RA _Mo is the condition that is 50.0% or more, excellent after hot forging, fatigue Strength and cracking properties can be obtained.

[Measurement method for number ratio RA _5-100 ]
The total number N _{5-100 of} precipitates having a circle equivalent diameter of 5 to 100 nm in the polygonal ferrite of the microstructure of the rolled steel bar for hot forging of this embodiment, and the circle of 3 to 1000 nm in the polygonal ferrite The total number N _TOTAL of precipitates of equivalent diameter can be measured by the following method. A plate-shaped intermediate material having a thickness of 1 mm is cut out from the R / 2 portion of the rolled steel bar for hot forging. The cut out intermediate material is mechanically polished in the thickness direction to produce a plate-shaped test piece having a thickness of 300 μm. The plate-shaped test piece is electropolished with a perchloric acid-methanol mixture and thinned to obtain a sample for observation. The observation surface of the sample is observed with a transmission electron microscope (TEM) with a magnification of 200,000, and the precipitate is specified in any five visual fields in the plurality of polygonal ferrites in the observation surface. Precipitates can be identified and distinguished from inclusions by contrast. The size of one visual field is 250 nm × 350 nm. The area of the identified precipitate is obtained, and the equivalent circle diameter of each precipitate is calculated from the obtained area. In addition, the number survey of precipitates is intended only for precipitates that are clearly recognized as precipitates and whose equivalent circle diameter is 3 nm or more. The total number of precipitates having a circle-equivalent diameter of 3 nm to 1000 nm in five fields of view is defined as the total number N _TOTAL . The total number of precipitates having an equivalent circle diameter of 5 to 100 nm in five fields of view is defined as the total number N _5-100 . Based on the following formula, the number ratio RA _5-100 (%) of precipitates having an equivalent circle diameter of 5 to 100 nm among the precipitates having an equivalent circle diameter of 3 to 1000 nm in polygonal ferrite is determined.
Number ratio RA _5-100 = Total number N _5-100 / Total number N _TOTAL × 100

[Production method]
An example of the manufacturing method of the above-mentioned hot forging rolled steel bar will be described. This manufacturing method includes a casting process and a hot rolling process.

[Casting process]
A molten steel satisfying the above-described chemical composition and formulas (1) and (2) is manufactured by a well-known method. Using molten steel, a slab (slab or bloom) is produced by a continuous casting method.

[Hot working process]
In the hot working process, hot working is performed on the slab produced in the casting process to produce a steel bar. The hot working process includes, for example, a rough rolling process and a finish rolling process.

[Rough rolling process]
A billet is manufactured by hot working a slab or an ingot. Hot working is, for example, hot rolling. For example, the hot rolling is performed using a block rolling mill and a continuous rolling mill in which a plurality of stands are arranged in a line and each stand has a plurality of rolls. The hot-rolled billet is cooled.

In order to bring the number ratio RA _5-100 of precipitates having an equivalent circle diameter of 5 to 100 nm and the Mo amount ratio RA _{Mo in} precipitates having an equivalent circle diameter of 3 to 1000 nm within the above range, cooling after the rough rolling step is performed. The billet is cooled to satisfy the following conditions.

Cooling time Ct0 until the billet temperature (° C.) reaches 800 ° C. to 500 ° C .: 20 minutes or more If the cooling time Ct0 is less than 20 minutes, the cooling rate is too high, and V—Ti—Mo in polygonal ferrite Complex carbides are difficult to form and do not become coarse enough. V-Ti-Mo composite carbide that has not been sufficiently coarsened is likely to be dissolved in the finish rolling of the next step. In this case, the total Mo content in the precipitates in the steel (Mo content in the precipitates C _P-Mo ) is 50.0% of the total Mo content in the steels (the total Mo content in the steels C _T-Mo ). The total number N _{5-100 of} precipitates having an equivalent circle diameter of 5 to 100 nm in polygonal ferrite is less than the total number of precipitates having an equivalent circle diameter of 3 to 1000 nm in polygonal ferrite. The number N is less than 80.0% of _TOTAL .

If the cooling time Ct0 is 20 minutes or more, V-Ti-Mo composite carbide is likely to be formed and is sufficiently coarsened. Therefore, on the condition that the manufacturing conditions in the finish rolling process described later are satisfied, the total content of Mo in the precipitates in the polygonal ferrite (Mo amount in the precipitates C _P-Mo ) is the total Mo in the steel. The total number N _{5-100 of} precipitates having a content equivalent to 50.0% or more of the total Mo content in the steel (C _T-Mo ) and having a circle equivalent diameter of 5 to 100 nm in the polygonal ferrite, The total number of precipitates of 3 to 1000 nm in polygonal ferrite is 80.0% or more of N _TOTAL . As a result, after hot forging, the formation of bainite is suppressed, and excellent yield strength, fatigue strength, and crackability are obtained. The upper limit with preferable cooling time Ct0 is 180 minutes, More preferably, it is 120 minutes.

The billet temperature here means the surface temperature of the billet. The billet surface temperature is measured by the following method. At the center in the longitudinal direction of each section when the billet after the rough rolling process is divided into three equal parts in the longitudinal direction (that is, at three locations), the surface temperature of the billet is measured with a radiation thermometer for a predetermined time. The average value of the three measured points is defined as the billet temperature (° C.) at that time.

In addition, the further cooling method of a billet at the billet temperature of 500 degrees C or less is not specifically limited. Preferably, the billet after rough rolling is cooled to 100 ° C. or lower, more preferably to room temperature (25 ° C.).

[Finishing rolling process]
A steel bar is manufactured using the billet after the rough rolling process. Specifically, the billet is heated in a heating furnace (heating process). After heating, the billet is hot-rolled (finish rolling) using a continuous rolling mill to produce a rolled steel bar for hot forging (finish rolling step). Hereinafter, each step will be described.

[Heating process]
In order to keep the number ratio RA _5-100 of precipitates having an equivalent circle diameter of 5 to 100 nm and the Mo amount ratio RA _{Mo in the} precipitates within the above range, the heating process of the finish rolling process should satisfy the following conditions: And heating. The heating conditions are as follows.

Heating temperature T1: 1100 ° C. or less Heating time t1: Less than 30 minutes If the heating temperature T1 is too high and / or if the heating time t1 is too long, the V—Ti—Mo composite carbide in the polygonal ferrite in the billet is excessive. The precipitate is excessively refined. In this case, the fine V—Ti—Mo composite carbide increases in the polygonal ferrite in the steel material after finish rolling. As a result, the number of precipitates having an equivalent circle diameter of less than 5 nm increases in polygonal ferrite, and the total number N _{5-100 of} precipitates having an equivalent circle diameter of 5 to 100 nm is 3 to 1000 nm in polygonal ferrite. The total number of precipitates having an equivalent circle diameter of N is less than 80.0% of N _TOTAL . Furthermore, since the V—Ti—Mo composite carbide in the billet is excessively dissolved, the total Mo content in the precipitates (Mo amount in the precipitates C _P-Mo ) is equal to the total Mo content in the steel (steel The total amount of intermediate Mo (C _T-Mo ) is less than 50.0%. In this case, the generation of bainite is promoted after hot forging. In particular, in forging at an ultrahigh temperature of 1330 ° C. or higher, sufficient yield strength, fatigue strength, and crackability may not be obtained.

When the heating temperature T1 is 1100 ° C. or less and the heating time t1 is 30 minutes or less, the V—Ti—Mo composite carbide maintains an appropriate size. Therefore, in the steel bar after the finish rolling process, the total content of Mo in the precipitate (Mo amount in the precipitate C _P-Mo ) is the total Mo content in the steel (total Mo amount in the steel C _T-Mo ). The total number N _{5-100 of} precipitates having a circle equivalent diameter of 5 to 100 nm in the polygonal ferrite has a circle equivalent diameter of 3 to 1000 nm in the polygonal ferrite. It becomes 80.0% or more of the total number N _TOTAL of the deposits to have. As a result, generation of bainite can be suppressed, and sufficient yield strength, fatigue strength, and excellent cracking properties can be obtained even in hot forging at an ultrahigh temperature of 1330 ° C. or higher. The minimum with preferable heating temperature T1 is 900 degreeC, More preferably, it is 950 degreeC. The minimum with the preferable heating time t1 is 5 minutes, More preferably, it is 10 minutes. The upper limit with preferable heating time t1 is 29 minutes, More preferably, it is 25 minutes.

[Hot rolling process]
Using a finish rolling mill, the billet after heating is finish-rolled (hot-rolled) by a well-known method to produce hot-forged rolled steel bars. The finish rolling mill has a plurality of rolling stands arranged in a row. Each stand has a plurality of rolls (roll groups) arranged around the pass line. The roll group of each stand forms a hole mold, and when the billet passes through the hole mold, it is rolled down to produce a steel bar.

In order to keep the number ratio RA _5-100 of precipitates having an equivalent circle diameter of 5 to 100 nm and the Mo amount ratio RA _{Mo in the} precipitates within the above range, the following conditions are satisfied in the hot rolling step of the finish rolling step: Thus, finish rolling is carried out.

Rolling temperature T2: 1200 ° C. or lower Finishing temperature T3: 1000 ° C. or lower In a plurality of rolling stands arranged in a row in the above-described finish rolling mill, three zones (arranged zone, intermediate row zone, finish in order from the top) are arranged. Divided into column zones). The number of rolling stands in each zone is in the range of N ± 2 (N is a natural number). The rolling temperature T2 is defined by the average value (° C.) of the billet temperature measured on the exit side of any two stands of the rolling stands belonging to the middle row zone. The finishing temperature T3 is defined by the average value (° C.) of the billet temperature measured on the exit side of the rolling stand that has finally reduced the billet in the finishing row zone. The billet temperature at the rolling temperature T2 and the finish rolling temperature T3 is measured by the following method. When the billet is divided into three equal parts in the longitudinal direction, the surface temperature of the billet is measured with a radiation thermometer at the center in the longitudinal direction of each section (that is, at three locations), and the average value is taken as the billet temperature (° C). .

If the rolling temperature T2 and / or the finishing temperature T3 is too high, the V—Ti—Mo composite carbide in the polygonal ferrite of the billet will be excessively dissolved, so the V—Ti—Mo composite in the steel after finish rolling. Carbide becomes finer. As a result, the number of precipitates having an equivalent circle diameter of less than 5 nm increases, and the total number N _{5-100 of} precipitates having an equivalent circle diameter of 5 to 100 nm has an equivalent circle diameter of 3 to 1000 nm in polygonal ferrite. The total number of precipitates is less than 80.0% of N _TOTAL . Furthermore, since the V—Ti—Mo composite carbide in the polygonal ferrite in the billet is excessively dissolved, the total content of Mo in the precipitate (Mo content in the precipitate, C _P—Mo ) It becomes less than 50.0% of the total Mo content (total Mo amount in steel C _T-Mo ). In this case, the generation of bainite is promoted after hot forging. In particular, in forging at an ultrahigh temperature of 1330 ° C. or higher, sufficient cracking properties may not be obtained.

If the rolling temperature T2 is 1200 ° C. or lower and the finish rolling temperature T3 is 1000 ° C. or lower, a moderately sized precipitate is generated in the polygonal ferrite of the steel bar after the finish rolling, and the precipitation in the polygonal ferrite The total content of Mo in the product (Mo amount in precipitates C _P-Mo ) is 50.0% or more of the total Mo content in steel (total Mo amount in steel C _T-Mo ), and In the polygonal ferrite, the total number N _{5-100 of} precipitates having an equivalent circle diameter of 5 to 100 nm is 80 _% of the total number N _TOTAL of precipitates having an equivalent circle diameter of 3 to 1000 nm in the polygonal ferrite. 0% or more. As a result, generation of bainite can be suppressed, and sufficient yield strength, fatigue strength, and excellent cracking properties can be obtained even in hot forging at an ultrahigh temperature of 1330 ° C. or higher. The minimum with preferable rolling temperature T2 is 900 degreeC, More preferably, it is 950 degreeC. The minimum with preferable finishing temperature T3 is 850 degreeC, More preferably, it is 900 degreeC.

Cooling time Ct1 from 800 ° C. to 500 ° C. 1: 5 minutes or less If the cooling time Ct1 of the steel bar after finish rolling exceeds 5 minutes, the cooling rate is too slow. In this case, the V—Ti—Mo composite carbide is excessively coarsened in the polygonal ferrite. As a result, the number of precipitates having an equivalent circle diameter exceeding 100 nm increases in polygonal ferrite, and the total number N _{5-100 of} precipitates having an equivalent circle diameter of 5 to 100 nm is 3 to 1000 nm in polygonal ferrite. The total number of precipitates having a circle-equivalent diameter is less than 80.0% of N _TOTAL . In this case, the elements (V, Ti, Mo, C) forming the V—Ti—Mo composite carbide in the hot forging rolled steel bar are not sufficiently dissolved. Therefore, after hot forging, fine V—Ti—Mo composite carbide is hardly generated in polygonal ferrite due to phase interface precipitation. Furthermore, since the Mo solid solution amount is low, the hardenability may be lowered. Therefore, the yield strength and fatigue strength are reduced in the steel material after hot forging. In particular, in forging at 850 ° C. or lower, the hardenability is lowered and sufficient strength may not be obtained.

When the cooling time Ct1 is 5 minutes or less, the V—Ti—Mo composite carbide is maintained at an appropriate size. Therefore, the number of precipitates having an equivalent circle diameter exceeding 100 nm is not excessively increased in the polygonal ferrite, and the total number N _{5-100 of} precipitates having an equivalent circle diameter of 5 to 100 nm is 3 to 3 in the polygonal ferrite. It becomes 80.0% or more of the total number N _TOTAL of 1000 nm precipitates. As a result, sufficient yield strength and fatigue strength can be obtained in the steel material after hot forging. A preferable lower limit of the cooling time Ct1 is 1 minute, and more preferably 2 minutes.

In addition, it is preferable that the cumulative area reduction rate in the finish rolling mill in the finish rolling process is 70% or more. Here, the cumulative area reduction is defined by the following equation.
Cumulative area reduction ratio = (cross-sectional area of billet before finish rolling−cross-sectional area of rolled steel bar for hot forging after finish rolling) / cross-sectional area of billet before finish rolling × 100

The above-described rolled steel bar for hot forging is manufactured by the above manufacturing process.

[Method for manufacturing hot forged products]
A cracking connecting rod manufacturing method will be described as an example of a method for manufacturing a hot forged product using the above-described hot forging rolled steel bar. First, a steel material is heated in a high frequency induction heating furnace. In this case, the preferred heating temperature is 1100-1300 ° C. A preferred heating time in the high frequency induction heating furnace is 1 to 15 minutes. After heating in the high frequency induction heating furnace, the surface temperature of the rolled steel bar for hot forging becomes equal to the heating temperature. If the rolled steel bar for hot forging of this embodiment is used, excellent cracking property, machinability, yield strength, and fatigue strength can be obtained even in forging at ultrahigh temperatures of 1330 ° C. or higher and extremely low temperatures of 850 ° C. or lower. be able to. As described above, the V—Ti—Mo composite carbide in the polygonal ferrite of the rolled steel bar for hot forging contains Ti, so that it does not easily dissolve at high temperatures. Therefore, even when heated at an ultrahigh temperature of 1330 ° C. or higher for 1 to 15 minutes, most of the V—Ti—Mo composite carbide in the polygonal ferrite remains without being completely dissolved. As a result, during hot forging, it is possible to suppress the formation of bainite resulting from an excessively large amount of solute Mo, and it is possible to obtain excellent cracking properties while having high yield strength and fatigue strength.

¡Hot forging is performed on the heated steel bars to produce cracking connecting rods. Preferably, the degree of processing during hot forging is 0.22 or more. Here, the working degree is the maximum value of the logarithmic strain generated in the portion excluding burrs in the forging process.

クラ Allow the cracking connecting rod after hot forging to cool to room temperature. Machining is performed as necessary on the cracking connecting rod after cooling. The cracking connecting rod is manufactured by the above process.

[Microstructure of hot forged products]
The microstructure of the manufactured hot forged product (cracking connecting rod) is mainly composed of polygonal ferrite and pearlite. Preferably, in the microstructure, the total area ratio of polygonal ferrite and pearlite is 90% or more. The total area ratio of polygonal ferrite and pearlite in the microstructure is preferably 95.0% or more, and more preferably 100.0%. When the total area ratio of polygonal ferrite and pearlite is not 100.0%, the balance of the microstructure is bainite / and / or retained austenite. If the rolled steel bar for hot forging of this embodiment is used, even if forging is performed at an ultrahigh temperature of 1330 ° C. or higher or an extremely low temperature of 850 ° C. or lower, the microstructure of bainite having a microstructure of a hot forged product (for example, cracking connecting rod) Generation is suppressed.

When bainite is included in the microstructure, when the large end is fractured and divided into two parts (cap and rod), the fractured part is plastically deformed and a part of the fracture surface tends to become a ductile fracture surface, and cracking properties Is prone to decline. However, the rolled steel bar for hot forging of this embodiment can suppress the generation of bainite and maintain excellent cracking properties.

The area ratio of bainite in the microstructure in the hot forged product can be measured by the same method as the microstructure observation in the rolled steel bar for hot forging.

In the above description, a cracking connecting rod has been described as an example of a method for manufacturing a hot forged product. However, the rolled steel bar for hot forging of this embodiment is not limited to cracking connecting rod applications. The rolled steel bar for hot forging of this embodiment can be widely applied to hot forged products.

Moreover, the manufacturing method of the rolled steel bar for hot forging of this embodiment is not limited to the said manufacturing method. It has a chemical composition satisfying the formulas (1) and (2), the total area ratio of polygonal ferrite and pearlite is 90.0% or more in the microstructure, and the total content of Mo in the precipitate (precipitate Medium Mo amount C _P-Mo ) becomes 50.0% or more of the total Mo content in steel (total Mo amount in steel C _T-Mo ), and 5 to 100 nm in polygonal ferrite The production method is particularly limited if the total number N _5-100 of precipitates having an equivalent diameter is 80.0% or more of the total number N _TOTAL of precipitates having a circle equivalent diameter of 3 to 1000 nm in polygonal ferrite. Not.

The rolled steel bar for hot forging of this embodiment has the above chemical composition satisfying the formulas (1) and (2), and the total area ratio of polygonal ferrite and pearlite is 90.0% or more in the microstructure. The total Mo content in the precipitates in the polygonal ferrite (Mo amount in the precipitates C _P-Mo ) is 50.0 of the total Mo content in the steel (total Mo amount in the steels C _T-Mo ). %, And the total number N _{5-100 of} precipitates having an equivalent circle diameter of 5 to 100 nm in polygonal ferrite is equal to the total number of precipitates having an equivalent circle diameter of 3 to 1000 nm in polygonal ferrite. The number N is 80.0% or more of _TOTAL . Therefore, it has high yield strength and fatigue strength, and has excellent machinability and cracking properties.

More specifically, a hot forging simulated product obtained by carrying out hot forward extrusion with a cross-section reduction rate of 60% after heating the above-described rolled steel bar for hot forging at 1350 ° C. for 5 minutes is a 1350 ° C. product. When a hot forging simulated product obtained by carrying out hot forward extrusion with a cross-section reduction rate of 60% after heating the above-described hot forging rolled steel bar at 800 ° C. for 5 minutes is defined as an 800 ° C. product. In both 1350 ° C and 800 ° C products, the yield strength is 600 MPa or more, and the fatigue strength obtained by the double fatigue test with the minimum maximum stress ratio −1 and the frequency 30 Hz at 25 ° C in the atmosphere is 360 MPa or more. Yes, when drilling with a high-speed drill with a drill diameter of 10 mm and a spindle rotation speed of 1000 times / min and a drill hole depth of 30 mm, when drilling the 51st hole Cutting resistance is at 50 N · cm or more, JIS Z2242 Charpy impact value obtained by the Charpy impact test at 0 ℃ conforming to (2005) of 8 J / cm ² or less.

The molten steel which has the chemical composition shown in Table 1 and Table 2 was manufactured.

Referring to Tables 1 and 2, the chemical compositions of Test Nos. 1 to 53 are appropriate and satisfy Formula (1) and Formula (2). On the other hand, for test numbers 54 to 76, the content of any element in the chemical composition was inappropriate or did not satisfy formula (1) and / or formula (2). The test number 76 was used as a comparative material used as a reference value for mechanical characteristics described later.

The molten steel of each test number was manufactured in a 3 ton electric furnace to produce an ingot.

The manufactured ingot was hot-rolled to produce a steel bar. In hot rolling, the time Ct0 until the steel material temperature at the time of cooling after rough rolling reaches 800 ° C. to 500 ° C. is as shown in Tables 3 and 4, and test numbers 1 to 46 and 48 to 76 are 23 to 29 minutes and test number 47 was 15 minutes. The steel material temperature after rough rolling was obtained by measuring with a radiation thermometer by the above-described method. Thereafter, finish rolling was performed using a finish rolling mill to produce a steel bar having a diameter of 33 mm.

As shown in Tables 3 and 4, the rolling conditions of test numbers 1 to 49 and 54 to 76 in the finish rolling process are as follows: heating temperature T1: 1023-1078 ° C., heating time t1: 22-26 minutes, rolling temperature T2: 989 to 1011 ° C., finishing temperature T3: 929 to 962 ° C. The rolling conditions of test number 50 were heating temperature T1: 1148 ° C, heating time t1: 25 minutes, rolling temperature T2: 1098 ° C, and finishing temperature T3: 1052 ° C. The rolling conditions of test number 51 were heating temperature T1: 1023 to 1078 ° C., heating time t1: 30 minutes, rolling temperature T2: 989 to 1011 ° C., and finishing temperature T3: 929 to 962 ° C. The rolling conditions of test number 52 were heating temperature T1: 1023 to 1078 ° C., heating time t1: 22 to 26 minutes, rolling temperature T2: 1098 ° C., and finishing temperature T3: 1052 ° C. The rolling conditions of test number 53 were heating temperature T1: 1148 ° C, heating time t1: 25 minutes, rolling temperature T2: 1098 ° C, and finishing temperature T3: 1052 ° C.

After finishing rolling, the steel bar was cooled. As shown in Tables 3 and 4, the cooling time Ct1 from 800 ° C. to 500 ° C. in Test Nos. 1 to 47 and 50 to 76 was 3 to 4 minutes. The time Ct1 from 800 ° C. to 500 ° C. for Test No. 48 and Test No. 49 was 7 minutes. The heating temperature T1, the rolling temperature T2, and the finishing temperature T3 were measured by the method described above. The cooling time Ct1 was obtained in the same manner as the cooling time Ct0. The rolled steel bars for hot forging No. 1 to No. 76 were manufactured by the above manufacturing method.

[Manufacture of heat forged simulated products]
The steel bar was cut in a direction perpendicular to the longitudinal direction, and a specimen having a diameter of 33 mm and a length of 60 mm was collected. The test material was subjected to hot forward extrusion by simulating hot forging. Specifically, the heating temperature in the heating furnace before hot extrusion was set to two conditions of 1350 ° C. and 800 ° C., and both were held for 5 minutes. Immediately after heating, hot forward extrusion with a cross-section reduction rate of 60% was performed, and a hot forging simulated product was manufactured by forming into a round bar with a diameter of 24 mm. The hot forging simulated product after forming was allowed to cool in the air. Hereinafter, those having a heating temperature of 1350 ° C are referred to as 1350 ° C products, and those having a heating temperature of 800 ° C are referred to as 800 ° C products.

Test No. 76 was held at a heating temperature of 1200 ° C. before hot extrusion for 5 minutes. Immediately after heating, hot forward extrusion with a cross-section reduction rate of 60% was performed to form a round bar having a diameter of 24 mm. This was used as a comparative material (a steel material used as a reference value for each mechanical characteristic).

[Evaluation test]
The following evaluation test was carried out using a test material that is a steel bar before hot forward extrusion and a simulated hot forging product. In addition, the above-mentioned well-known component analysis method was implemented with respect to the test material of each test number, and the chemical composition was analyzed. As a result, the chemical composition of the test material of each test number was as shown in Tables 1 and 2.

[Microstructure observation]
A microstructural observation test was performed using a test material that is a steel bar before hot forward extrusion of each test number and a hot forging simulated product. Specifically, a sample containing R / 2 part was collected from the longitudinal sections of the test material and the hot simulated product. The surface perpendicular to the central axis of the rolled steel bar for hot forging was used as the observation surface. After the observation surface was polished, it was etched with 3% nitric alcohol (nitral corrosive solution). The etched observation surface was observed with a 200 × optical microscope, and the total area ratio RA _{F + P} (%) of polygonal ferrite and pearlite was determined by the method described above. A case where the total area ratio RAF _{+ P of} polygonal ferrite and pearlite was 90.0% or more was defined as “≧ 90”. Polygonal ferrite and pearlite having a total area ratio RAF _{+ P} of less than 90.0% was defined as “<90”. The results are shown in Tables 3 and 4.

[Measurement of number ratio RA _5-100 in polygonal ferrite]
A sample was taken from R / 2 part of the test material, which is a steel bar before hot forward extrusion for each test number. Of the surface of the sample, the surface corresponding to the cross section (longitudinal cross section) including the axial direction of the specimen was used as the observation surface. By the above-mentioned method, it observed with 200000 times TEM, and identified the deposit in arbitrary five visual fields. The area of one field of view was 250 nm × 350 nm. Precipitates were identified by the above-described method, and the total number N _{5-100 of} precipitates having a circle equivalent diameter of 5 to 100 nm in polygonal ferrite and precipitates having a circle equivalent diameter of 3 to 1000 nm in polygonal ferrite The total number N of _TOTAL was determined. Then, based on the total number N _5-100 and the total number N _TOTAL , the number ratio RA _5-100 (%) of precipitates having a circle equivalent diameter of 5 to 100 nm in polygonal ferrite is obtained by the following formula. It was.
Number ratio RA _5-100 = Total number N _5-100 / Total number N _TOTAL × 100

The obtained number ratio RA _5-100 is shown in Tables 3 and 4. In the “RA _5-100 ” column in Tables 3 and 4, “A” means that the number ratio RA _5-100 in the corresponding test number was 90.0% or more. “B” means that the number ratio RA _5-100 in the corresponding test number was 80.0 to less than 90.0%. “N” means that the number ratio RA _5-100 in the corresponding test number was less than 80.0%. “N-1” and “N-2” both mean that the number ratio RA _5-100 was less than 80.0%. Specifically, “N-1” has an excessively large number ratio of precipitates having an equivalent circle diameter of less than 5 nm in polygonal ferrite, resulting in a number ratio RA _5-100 of less than 80.0%. Means that “N-2” indicates that the number ratio of RA _5-100 was less than 80.0% as a result of the excessive number ratio of precipitates having an equivalent circle diameter exceeding 100 nm in polygonal ferrite. means.

[Measurement of Mo amount ratio RA _{Mo in} precipitate]
A cubic sample with a side of 10 mm was taken from R / 2 part of the test material, which is a steel bar before hot forward extrusion of each test number. The surface layer from the sample surface to a depth of 200 μm was removed by electrolysis using an AA-based electrolytic solution (an electrolytic solution containing 10 vol% acetylacetone and 1 vol% tetramethylammonium chloride, with the balance being methanol). The electrolysis time was 30 minutes. The electrolyte was replaced with a new AA electrolyte, and the sample was electrolyzed. The electrolysis time was 150 minutes. The electrolytic solution after electrolysis was filtered using a filter having a mesh size of 200 nm to obtain a residue. Based on the obtained residue, in the manner described above to determine the precipitates Mo amount ratio RA _Mo.

Table 3 and Table 4 show the obtained Mo amount ratio RA _Mo in the precipitate. In the “RA _Mo ” column in Tables 3 and 4, “A” means that the Mo amount ratio RA _Mo in the precipitates in the corresponding test number was 90.0% or more. “B” means that the Mo amount ratio RA _Mo in the precipitate in the corresponding test number was 70.0 to less than 90.0%. “C” means that the Mo amount ratio RA _Mo in the precipitate in the corresponding test number was 50.0 to less than 70.0%. “N” means that the Mo content ratio RA _Mo in the precipitate in the corresponding test number was less than 50.0%.

[Yield strength evaluation]
Two JIS 14A test pieces having a diameter of 5 mm shown in FIG. 2 were collected from the R / 2 part of each hot forging simulated product. Referring to FIG. 2, the fatigue test piece had a circular cross section and a parallel part length of 35 mm. The numerical value in which the unit in FIG. 2 is not shown shows the dimension (a unit is mm) of the corresponding site | part of a test piece. The “φ numerical value” in the figure indicates the diameter (mm) of the designated part. The “R value” in the figure indicates the radius (mm) of the shoulder. “M numerical value” in the figure indicates a nominal diameter (mm). Using the collected specimens, a tensile test was performed at room temperature (25 ° C.) in the atmosphere, and the average yield strength YS (MPa) of the two steels was determined at 0.2% proof stress of the steel material.

When the yield strength YS (MPa) is 125% or more with respect to the yield strength (600 MPa) of the comparative material of test number 76, the evaluation is “A”, and the evaluation strength is “B” when the yield strength is less than 110 to 125%. The case where the yield strength YS was less than 110% was evaluated as “N”.

In the case of evaluation “A” or “B”, it was judged that sufficient yield strength was obtained. In the case of evaluation “N”, it was determined that the yield strength was low.

[Fatigue strength evaluation]
The fatigue test piece shown in FIG. 3 was collected from R / 2 part of each hot forging simulated product. Referring to FIG. 3, the fatigue test piece had a circular cross section and a parallel part length of 42 mm. The numerical value in which the unit in FIG. 3 is not shown shows the dimension (a unit is mm) of the corresponding site | part of a test piece. The “φ numerical value” in the figure indicates the diameter (mm) of the designated part. The “R value” in the figure indicates the radius (mm) of the shoulder. Using the collected specimens, a swing fatigue test (Ono-type rotating bending fatigue test) with a minimum maximum stress ratio of −1 was performed at room temperature (25 ° C.) in the atmosphere. The maximum stress that did not break after 10 ⁷ repetitions was defined as fatigue strength (MPa). The frequency was 30 Hz.

The case where the fatigue strength is 125% or more with respect to the fatigue strength (360 MPa) of the comparative material of test number 76 was evaluated as “A”, and the case where the fatigue strength was 110 to less than 125% was evaluated as “B”. The case where the fatigue strength was less than 110% was evaluated as “N”.

In the case of evaluation “A” or “B”, it was judged that sufficient fatigue strength was obtained. In the case of evaluation “N”, it was judged that the fatigue strength was low.

[Machinability evaluation]
Five hot forging simulated products were prepared for each test number. End portions of the prepared five hot forging simulated products were cut, and the cut surfaces were thrust processed. A 30 mm deep drilling process was performed on the thrust-processed thermal forging simulated product at a position perpendicular to the thrust surface, and the cutting resistance (N · cm) at the time of drilling the 51st hole was measured. At this time, the drill diameter was 10 mm, and the rotation speed of the main shaft was 1000 times / min. The drill used was a high speed drill.

When the cutting resistance was 90% or less with respect to the cutting resistance (50 N · cm) of the comparative material of test number 76, the evaluation was “A”, and the case where the cutting resistance was 90 to 110% was evaluated as “B”. The case where the cutting resistance exceeded 110% was evaluated as “N”.

In the case of evaluation “A” or “B”, it was judged that sufficient machinability was obtained. In the case of evaluation “N”, it was determined that the machinability was low.

[Cracking evaluation]
A V-notch Charpy impact test piece in which a notch was machined was collected from the center of the hot forging simulated product of each test number. The test piece had a width of 10 mm, a height of 10 mm, a length of 55 mm, and a notch depth of 2 mm. Each specimen was subjected to a Charpy impact test in accordance with JIS Z2242 (2005) at 0 ° C. to obtain a Charpy impact value (J / cm ² ).

In the evaluation of cracking property, the evaluation was “A” when the Charpy impact value was 8 J / cm ² or less, and the evaluation “N” when the Charpy impact value exceeded 8 J / cm ² .

In the case of evaluation “A”, it was judged that sufficient cracking property was obtained. In the case of evaluation “N”, it was judged that the cracking property was low.

[Evaluation results]
The evaluation results are shown in Tables 3 and 4. Referring to Tables 3 and 4, the chemical compositions of Test Nos. 1 to 46 are appropriate, fn1 satisfies Formula (1), and fn2 satisfies Formula (2). Further, the number ratio RA _5-100 (%) of precipitates having an equivalent circle diameter of 5 to 100 nm in polygonal ferrite is 80.0% or more, and the Mo amount ratio RA _Mo in the precipitates is 50.0%. That was all. As a result, superior cracking properties were obtained as compared with the comparative material of test number 76. Furthermore, yield strength, fatigue strength, and machinability were also good.

On the other hand, in test number 47, the chemical composition was appropriate, fn1 satisfied Formula (1), and fn2 satisfied Formula (2). However, the cooling time Ct0 was less than 20 minutes. For this reason, the cooling rate was too high, and V—Ti—Mo composite carbide was not precipitated in polygonal ferrite. Therefore, the Mo content ratio RA _Mo in the precipitate was less than 50.0%. Mo dissolved too much, and the generation of bainite was promoted. As a result, in the 1350 ° C. product, the total area ratio of polygonal ferrite and pearlite was less than 90.0%, and bainite was excessively generated. Therefore, the cracking property was low. Furthermore, the yield strength and fatigue strength were also low.

Test No. 48 and Test No. 49 have an appropriate chemical composition, fn1 satisfies the formula (1), and fn2 satisfies the formula (2). However, in the cooling after finish rolling, the cooling time Ct1 is 5 minutes. Exceeded. Therefore, the cooling rate is too slow, the V—Ti—Mo composite carbide in the polygonal ferrite is coarsened, the total number of precipitates having an equivalent circle diameter exceeding 100 nm is increased, and the number ratio RA _5-100 (%) is 80 Less than 0.0%. As a result, the yield strength and fatigue strength of the 800 ° C. product were low.

In test number 50, the chemical composition was appropriate, fn1 satisfied Formula (1), and fn2 satisfied Formula (2). However, the heating temperature T1 exceeded 1100 ° C. Therefore, the finish rolling temperature T3 also exceeded 1000 ° C. As a result, the V—Ti—Mo composite carbide in the polygonal ferrite is not sufficiently coarsened, the number ratio of precipitates having a diameter of less than 5 nm is large, and the number ratio of precipitates having a circle equivalent diameter of 5 to 100 nm is RA _{5- 100} was less than 80.0%. As a result, the formation of bainite was promoted, and the yield strength, fatigue strength, and cracking properties were low at 1350 ° C.

In test number 51, the chemical composition was appropriate, fn1 satisfied equation (1), and fn2 satisfied equation (2). However, the heating time t1 in finish rolling was too long. As a result, the V—Ti—Mo composite carbide in the polygonal ferrite is not sufficiently coarsened, the number ratio of precipitates having a diameter of less than 5 nm is large, and the number ratio of precipitates having a circle equivalent diameter of 5 to 100 nm is RA _{5- 100} was less than 80.0%. As a result, the formation of bainite was promoted, and the yield strength, fatigue strength, and cracking properties were low at 1350 ° C.

In test number 52, the chemical composition was appropriate, fn1 satisfied equation (1), and fn2 satisfied equation (2). However, finishing temperature T3 in finish rolling was too high. As a result, the V—Ti—Mo composite carbide in the polygonal ferrite is not sufficiently coarsened, the number ratio of precipitates having a diameter of less than 5 nm is large, and the number ratio of precipitates having a circle equivalent diameter of 5 to 100 nm is RA _{5- 100} was less than 80.0%. As a result, the formation of bainite was promoted, and the yield strength, fatigue strength, and cracking properties were low at 1350 ° C.

In test number 53, the chemical composition was appropriate, fn1 satisfied equation (1), and fn2 satisfied equation (2). However, the heating temperature T1 exceeded 1100 ° C. Therefore, the finish rolling temperature T3 also exceeded 1000 ° C. As a result, the V—Ti—Mo composite carbide in the polygonal ferrite is not sufficiently coarsened, the number ratio of precipitates having a diameter of less than 5 nm is large, and the number ratio of precipitates having a circle equivalent diameter of 5 to 100 nm is RA _{5- 100} was less than 80.0%. As a result, the formation of bainite was promoted, and the yield strength, fatigue strength, and cracking properties were low at 1350 ° C.

The C content of test number 54 was too high. Therefore, machinability was low.

The C content of test number 55 was too low. Therefore, the yield strength and fatigue strength were low.

The Si content of test number 56 was too low. Therefore, the yield strength and fatigue strength were low.

The Mn content of test number 57 was too high. Therefore, bainite was generated in the 1350 ° C. product, and the total area ratio of polygonal ferrite and pearlite in the microstructure was less than 90.0%. Therefore, the cracking property was low. Furthermore, the yield strength and fatigue strength were also low.

The Mn content of test number 58 was too low. Therefore, the yield strength and fatigue strength were low.

The P content of test number 59 was too low. Therefore, the cracking property was low.

The S content of test number 60 was too high. Therefore, the fatigue strength was low.

The S content of test number 61 was too low. Therefore, machinability was low.

The Cr content of test number 62 was too high. Therefore, bainite was generated in the 1350 ° C. product, and the total area ratio of polygonal ferrite and pearlite in the microstructure was less than 90.0%. Therefore, the cracking property was low. Furthermore, the yield strength and fatigue strength were also low.

The Cr content of test number 63 was too low. Therefore, the yield strength and fatigue strength were low.

The Mo content of test number 64 was too high. Therefore, bainite was generated in the 1350 ° C. product, and the total area ratio of polygonal ferrite and pearlite in the microstructure was less than 90.0%. Therefore, the cracking property was low. Furthermore, the yield strength and fatigue strength were also low.

The Mo content of test number 65 was too low. Therefore, the yield strength and fatigue strength were low.

The V content of test number 66 was too high. Therefore, machinability was low.

The V content of test number 67 was too low. Therefore, the yield strength and fatigue strength were low.

The Ti content of test number 68 was too high. Therefore, machinability was low.

The Ti content of test number 69 was too low. Therefore, yield strength, fatigue strength, and cracking properties were low.

The Al content of test number 70 was too high. Therefore, the fatigue strength was low.

N content of test number 71 was low. Therefore, the yield strength and fatigue strength were low.

In test numbers 72 and 73, fn1 was too high. Therefore, machinability was low.

In test number 74, fn1 was too low. Therefore, the yield strength and fatigue strength were low.

In test number 75, fn2 was too high. As a result, bainite was generated, and the yield strength, fatigue strength, and cracking property of the 1350 ° C. product were low.

The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

1 connecting rod 2 cap 3 rod 100 large end 200 collar 300 small end

Claims

Chemical composition is mass%,
C: 0.39 to 0.55%,
Si: 0.10 to 1.0%,
Mn: 0.50 to 1.50%,
P: 0.010 to 0.100%,
S: 0.040 to 0.130%,
Cr: 0.05 to 0.50%,
Mo: 0.01 to 0.10%,
V: 0.05 to 0.40%,
Ti: 0.150 to 0.250%,
Al: 0.005 to 0.050%,
N: 0.0020 to 0.020%,
Cu: 0 to 0.40%,
Ni: 0 to 0.30%,
Nb: 0 to 0.20%,
Pb: 0 to 0.30%,
Zr: 0 to 0.1000%,
Te: 0 to 0.3000%,
Ca: 0 to 0.0100%, and
Bi: 0 to 0.3000%, with the balance being Fe and impurities, satisfying formula (1) and formula (2),
In the microstructure, the total area ratio of polygonal ferrite and pearlite is 90% or more,
The total content (% by mass) of Mo contained in the precipitate is 50.0% or more of the total Mo content (% by mass) in the steel,
The total number of precipitates having an equivalent circle diameter of 5 to 100 nm in the polygonal ferrite is 80.0% or more of the total number of precipitates having an equivalent circle diameter of 3 to 1000 nm in the polygonal ferrite. ,
Rolled steel bar for hot forging.
0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1)
0.12C + 0.35Mn + 0.42Cr + Mo−0.08Si ≦ 0.70 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2).
A rolled steel bar for hot forging according to claim 1,
The chemical composition is
Cu: 0.01 to 0.40%,
Ni: 0.005 to 0.30%, and
Nb: Rolled steel bar for hot forging containing one or more selected from the group consisting of 0.001 to 0.20%.
It is a rolled steel bar for hot forging according to claim 1 or claim 2,
The chemical composition is
Pb: 0.05 to 0.30%,
Zr: 0.0003 to 0.1000%,
Te: 0.0003 to 0.3000%,
Ca: 0.0003 to 0.0100%, and
Bi: Rolled steel bar for hot forging containing one or more selected from the group consisting of 0.0003 to 0.3000%.