JP5916553B2 - Steel for connecting rod and connecting rod - Google Patents

Description

  The present invention relates to a steel for a connecting rod, and more particularly to a steel for a connecting rod used for a connecting rod constituting an engine.

  Recently, in order to prevent global warming, regulations for reducing carbon dioxide emissions from transportation equipment such as automobiles and ships are becoming stricter worldwide. In order to comply with such regulations, technological development relating to weight reduction of parts used in transportation equipment (hereinafter referred to as machine structural parts) and engine efficiency has been promoted.

  Reflecting the recent economic situation, the movement to reduce the manufacturing costs of various automobile parts has become active, and this movement is no longer an exception for connecting rods (hereinafter referred to as connecting rods) which are engine parts. For this reason, there is an increasing demand for a connecting rod that does not undergo quenching-tempering tempering, which is expensive to manufacture, that is, it is non-tempered and can provide the same fatigue strength as when tempering carbon steel for mechanical structures. It is used in some car models.

  With regard to weight reduction and non-heat-treated steel for machine structural parts, methods for increasing the tensile strength and fatigue strength of materials for machine structural parts have been studied. Japanese Patent Application Laid-Open No. 2010-53430 (Patent Document 1) discloses a steel for hot forging having high strength even if it is not tempered. In patent document 1, Si is 0.5 mass% or less, and the non-tempered steel containing V is heated at the heating temperature according to C and V content, and is hot forged. And after hot forging, it cools to 720 degreeC-550 degreeC with the cooling rate of 1.5 degree-C / s or more, and cools to 400 degreeC with the cooling rate of 0.1 degree-C / s or more. The steel manufactured by the above process has a ferrite pearlite structure, and fine V carbide is generated in ferrite and lamellar ferrite in the pearlite. Therefore, it is described in Patent Document 1 that excellent strength can be obtained.

  Recently, in addition to non-tempered steel, in order to further reduce manufacturing costs, both the connecting rod body and connecting rod cap are integrally formed by hot forging, and then cracked to the connecting rod body and connecting rod cap at the large end. So-called “cracking connecting rods” are beginning to be adopted.

  If the cracking connecting rod has a brittle fracture surface with a smooth fracture surface, it is not necessary to cut the mating surface that sandwiches the crankshaft, and the manufacturing cost can be reduced. Further, since the connection is performed at the fracture surface, the fastening rigidity, that is, the strength is excellent.

  Japanese Patent Laid-Open No. 2002-256394 (Patent Document 2) discloses a fracture separation type connecting rod (non-hot forging for non-hot forging) that does not cause plastic deformation at the fracture surface when fractured and has a high fatigue strength and proof stress. Tempered steel). In patent document 2, the production | generation amount of AlN in steel is adjusted based on the relational expression of Al content and oxygen (O) content, and the relational expression of N content and O content. It is described that crystal grains can be refined by AlN. In Patent Document 2, fatigue strength is evaluated by a rotating bending test piece. Patent Document 2 further describes that, by positively containing phosphorus (P), deformation of the fracture surface at the time of fracture is suppressed, and adhesion of the fracture surface is increased.

  Japanese Patent Application Laid-Open No. 2004-277841 (Patent Document 3) discloses a steel that is excellent in machinability, fracture splitting property and fatigue resistance and is suitable as a material for a cracking connecting rod. In Patent Document 3, the machinability is improved by increasing the proportion of ferrite in the ferrite pearlite structure, and the ferrite is greatly strengthened by adding Ti and V in combination, and the fracture splitting property and fatigue resistance are increased. It is described that the characteristics can be secured.

JP 2010-53430 A JP 2002-256394 A JP 2004-277841 A

  As described above, the steels disclosed in Patent Documents 1 to 3 have high strength. Accordingly, these documents disclose steels that can contribute to weight reduction of machine structural components. However, these documents do not propose steel that contributes to higher engine efficiency.

  One way to increase engine efficiency (fuel consumption performance) is to reduce friction loss inside the engine. A crankpin of a crankshaft is rotatably inserted into a large end portion of the connecting rod via a slide bearing. Engine oil is supplied between the connecting rod and the crankpin. The engine oil forms an oil film between the connecting rod and the crank pin, and the friction loss between the connecting rod and the crank pin is reduced by the slide bearing and the oil film.

  Engine oil viscosity generally decreases with increasing temperature. If the viscosity is low, the fluidity of the engine oil increases and the coefficient of friction between the connecting rod and the crankpin decreases. Therefore, if the temperature of the engine oil rises rapidly after the engine is started, the viscosity of the engine oil is quickly lowered, and the friction loss between the connecting rod and the crankpin is reduced. As a result, engine efficiency (fuel consumption performance) is increased.

  In order to quickly reduce the viscosity of the engine oil after the engine is started, it is only necessary to suppress the heat generated at the time of starting the engine from being dissipated outside the engine. Most of the frictional heat generated between the connecting rod and the crankpin is transmitted to the outside through the connecting rod. If the thermal conductivity of the connecting rod is low, heat radiation generated in the engine can be suppressed, and the viscosity of the engine oil can be quickly reduced when the engine is started. As a result, the friction loss between the connecting rod and the crankpin is reduced, and the engine efficiency (fuel consumption performance) is increased. Therefore, it is preferable that the thermal conductivity of the connecting rod steel is low.

  Furthermore, as described above, the connecting rod is required to have high strength (yield strength, tensile strength, and fatigue strength). Patent Document 2 evaluates the fatigue strength of a cracking connecting rod by a rotating bending fatigue test. However, the connecting rods are primarily subjected to repeated axial tensile and compressive stresses during operation rather than rotational bending stresses. Therefore, the connecting rod is required to have fatigue strength against repeated tensile and compressive stress in the axial direction rather than rotational bending fatigue strength.

  In the cracking connecting rod, excellent cracking property (breaking separation property) is further required. The steel disclosed in Patent Document 2 cannot be said to have sufficient cracking properties.

  An object of the present invention is to provide a steel for connecting rods having low thermal conductivity, high yield strength, high tensile strength, high fatigue strength against repeated tensile and compressive stresses in the axial direction, and excellent cracking properties.

The steel for connecting rods according to the present invention is, in mass%, C: 0.20 to 0.50%, Si: more than 0.90% to 2.0% or less, Mn: 0.50 to 1.20%, P : 0.020 to 0.15%, S: 0.010 to 0.12%, Cr: 0.05 to 0.40%, Al: 0.005 to 0.08%, Ti: 0.050 to 0 .30%, V: 0.050 to 0.35%, N: 0.0020 to 0.020%, O: 0.0050% or less, Cu: 0.30% or less (including 0), Ni: 0 .50% or less (including 0) and Mo: 0.50% or less (including 0), with the balance being Fe and impurities, and fn1 defined by formula (1) being 57.1 or more Fn2 defined by equation (2) is 0.45 or more, fn3 defined by equation (3) is 0.16 or more, and defined by equation (4) n4 is 1.20 or less.
fn1 = 6.7 × (42 [Si%] + 25 [Mn%] + 14 [Cu%] + 12 [Ni%] + 16 [Cr%] + 12 [Mo%] + 42 [Al%] + 18 [Ti%] + 14 [V %]) ^0.5 ... (1)
fn2 = [C%] + [Si%] / 7+ [Mn%] / 5+ [Cr%] / 9+ [V%] / 2-5 [S%] / 7 (2)
fn3 = [Ti%] + [V%] (3)
fn4 = [Mn%] + [Cr%] (4)
Here, in [element symbol%] in the formulas (1) to (4), the content of the corresponding element (mass%)
) Is substituted. If the corresponding element is not contained, “0” is assigned to [element symbol%].

  The connecting rod steel according to the present invention has low thermal conductivity, high yield strength, high tensile strength, high fatigue strength against repeated tensile and compressive stresses in the axial direction, and excellent cracking properties.

  The connecting rod according to the present invention is manufactured by hot forging using the above-described connecting rod steel.

FIG. 1 is a diagram showing the relationship between fn1 defined by equation (1) and the thermal conductivity of steel. FIG. 2 is a side view of the connecting rod. FIG. 3 is a plan view and a side view of the heat conduction test piece used in the example. FIG. 4 is a side view of the fatigue test piece used in the examples.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, “%” of the element content means mass%.

  The present inventors examined the yield strength, tensile strength, fatigue strength, cracking property and thermal conductivity of steel for connecting rods. As a result, the present inventors obtained the following knowledge.

  (A) As described above, if the thermal conductivity of the connecting rod is low, the heat radiation generated in the engine can be prevented from being released, and the viscosity of the engine oil can be quickly reduced when the engine is started. As a result, the friction loss between the connecting rod and the crankshaft is reduced, and the engine efficiency (fuel consumption performance) is increased.

Si, Mn, Cu, Ni, Cr, Mo, Al, and V lower the thermal conductivity of steel, and in particular, Si, Mn, and Al significantly lower the thermal conductivity of steel. Specifically, if fn1 defined by the formula (1) is 50.0 or more, the thermal conductivity of the connecting rod steel can be kept low.
fn1 = 6.7 × (42 [Si%] + 25 [Mn%] + 14 [Cu%] + 12 [Ni%] + 16 [Cr%] + 12 [Mo%] + 42 [Al%] + 18 [Ti%] + 14 [V %]) ^0.5 ... (1)

  FIG. 1 is a diagram showing the relationship between fn1 and thermal conductivity. The horizontal axis in FIG. 1 indicates the value of fn1. The vertical axis represents the thermal conductivity (W / (m · K)). FIG. 1 was obtained by conducting a thermal conductivity measurement test described later.

  Referring to FIG. 1, as fn1 increases, the thermal conductivity decreases in proportion to fn1. And if fn1 becomes 50.0 or more, the thermal conductivity of steel will be 38 W / (m * K) or less, and will become lower than the conventional steel for connecting rods.

  (B) Steel for connecting rods is further required to have high tensile strength and high fatigue strength. The fatigue strength required for steel for connecting rods is the fatigue strength against repeated tensile and compressive stresses in the axial direction. Unlike a crankshaft that is subject to rotational bending stress mainly in the circumferential direction, the connecting rod is subject to repeated tensile and compressive stresses mainly in the axial direction. In the stress distribution due to rotational bending like a crankshaft, the stress on the surface layer of the steel material (crankshaft) is maximized. On the other hand, in the connecting rod 1 shown in FIG. 2, the stress distribution in the cross section (cross section perpendicular to the axial direction) of the rail portion 30 connecting the large end portion 10 and the small end portion 20 is almost uniform. Therefore, the connecting rod is required to have higher fatigue strength against repeated tensile and compressive stresses in the axial direction than rotational bending fatigue strength.

The tensile strength of the connecting rod steel has a correlation with fn2 defined by the equation (2). fn2 is a carbon equivalent. As fn2 increases, the tensile strength increases. As tensile strength increases, fatigue strength also increases. Moreover, if the tensile strength is increased, the cracking property is also improved. Specifically, if fn2 is 0.45 or more, high tensile strength is obtained, and thereby high fatigue strength and good cracking properties are obtained.
fn2 = [C%] + [Si%] / 7+ [Mn%] / 5+ [Cr%] / 9+ [V%] / 2-5 [S%] / 7 (2)

(C) In order to ensure cracking properties, it is necessary to further strengthen the ferrite by containing Ti and V in combination and precipitating carbonitride in the ferrite. For that purpose, fn3 defined by equation (3) may be set to 0.16 or more.
fn3 = [Ti%] + [V%] (3)

(D) When Ti and V are contained in order to obtain cracking properties, bainite is easily generated during cooling after hot forging. When bainite is generated, yield strength and tensile strength are reduced, and therefore fatigue strength is also reduced. In order to suppress the formation of bainite, the content of the element that enhances the hardenability may be reduced. Specifically, fn4 defined by the formula (4) may be set to 1.20 or less.
fn4 = [Mn%] + [Cr%] (4)

  Based on the above findings, the present inventors have completed the present invention. The connecting rod steel according to the present invention will be described below.

[Chemical composition]
The steel for connecting rods according to the present invention has the following chemical composition.

C: 0.20 to 0.50%
Carbon (C) increases the tensile strength of steel. On the other hand, if C is contained excessively, the tensile strength becomes excessively high and the machinability of the steel decreases. Therefore, the C content is 0.20 to 0.50%. The lower limit of the preferable C content is higher than 0.20%, more preferably higher than 0.25%, and still more preferably 0.27% or more. The upper limit of the preferable C content is less than 0.50%, more preferably 0.48% or less, and still more preferably 0.45% or less.

Si: More than 0.90% and 2.0% or less Silicon (Si) reduces the thermal conductivity of steel. On the other hand, if Si is contained excessively, the hot workability of steel is lowered. Accordingly, the Si content is more than 0.90% and not more than 2.0%. The minimum of preferable Si content is 1.0% or more, More preferably, it is 1.2% or more. The upper limit of the Si content is preferably less than 2.0%, more preferably 1.8% or less, and still more preferably 1.7% or less.

Mn: 0.50 to 1.20%
Manganese (Mn) reduces the thermal conductivity of steel. Mn further increases the tensile strength of the steel. On the other hand, if Mn is contained excessively, the hot workability of the steel decreases. If Mn is contained excessively, the tensile strength becomes excessively high and the machinability of the steel decreases. Therefore, the Mn content is 0.50 to 1.20%. The minimum of preferable Mn content is higher than 0.50%, More preferably, it is 0.60% or more, More preferably, it is 0.70% or more. The upper limit of the preferable Mn content is less than 1.20%, more preferably 1.10% or less, and still more preferably 1.00% or less.

P: 0.020 to 0.15%
Phosphorus (P) is effective for making the fracture surface of the connecting rod cracked into a smooth brittle fracture surface. On the other hand, if P is contained excessively, the hot workability of the steel is lowered. Therefore, the P content is 0.020 to 0.15%. The minimum of preferable P content is higher than 0.020%, More preferably, it is 0.030% or more, More preferably, it is 0.040% or more. The upper limit of the preferable P content is less than 0.15%, more preferably 0.13% or less, and still more preferably 0.10% or less.

S: 0.010 to 0.12%
Sulfur (S) combines with Mn to form MnS and enhances the machinability of the steel. On the other hand, if S is contained excessively, the hot workability of the steel is lowered. Therefore, the S content is 0.010 to 0.12%. The minimum of preferable S content is higher than 0.010%, More preferably, it is 0.015% or more, More preferably, it is 0.040% or more. The upper limit of the preferable S content is less than 0.12%, more preferably 0.11% or less, and still more preferably 0.10% or less.

Cr: 0.05-0.40%
Chromium (Cr) reduces the thermal conductivity of steel. Cr further strengthens the cementite in the pearlite and increases the yield strength of the steel. On the other hand, if Cr is excessively contained, the tensile strength becomes excessively high and the machinability of the steel is lowered. Therefore, the Cr content is 0.05 to 0.40%. The lower limit of the preferable Cr content is higher than 0.05%, more preferably 0.10% or more, and further preferably 0.15% or more. The upper limit of the Cr content is preferably less than 0.40%, more preferably 0.35% or less, and still more preferably 0.30% or less.

Al: 0.005 to 0.08%
Aluminum (Al) reduces the thermal conductivity of steel. On the other hand, even if Al is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the Al content is 0.005 to 0.08%. The Al content in the present invention is a so-called acid-soluble Al content (sol. Al). The minimum of preferable Al content is higher than 0.005%, More preferably, it is 0.006% or more, More preferably, it is 0.007% or more. The upper limit of the preferable Al content is less than 0.08%, more preferably 0.07% or less, and further preferably 0.06% or less.

Ti: 0.050 to 0.30%
Titanium (Ti) reduces the thermal conductivity of steel. Furthermore, Ti precipitates in the ferrite as a carbonitride to increase the strength. Ti is significantly strengthened by adding it in combination with V. Excellent cracking properties can be obtained by strengthening ferrite. Furthermore, strengthening ferrite increases yield strength and tensile strength and further suppresses the occurrence of fatigue cracks, so that excellent fatigue strength can be obtained. Moreover, Ti precipitates as a nitride and suppresses coarsening of crystal grains. Furthermore, Ti forms sulfides and improves the machinability of steel. On the other hand, if Ti is excessively contained, coarse Ti nitride is generated. Coarse Ti nitride serves as a starting point for fatigue failure, so that the fatigue strength of the steel decreases. Therefore, the Ti content is 0.050 to 0.30%. The minimum of preferable Ti content is higher than 0.050%, More preferably, it is 0.070% or more. The upper limit of the preferable Ti content is less than 0.30%, more preferably 0.25% or less, and still more preferably 0.20% or less.

V: 0.050 to 0.35%
Vanadium (V) decreases the thermal conductivity of steel. Further, V precipitates in the ferrite as carbides and / or carbonitrides and increases the strength. V strengthens ferrite significantly by containing it in combination with Ti. Excellent cracking properties can be obtained by strengthening ferrite. Furthermore, strengthening ferrite increases yield strength and tensile strength and further suppresses the occurrence of fatigue cracks, so that excellent fatigue strength can be obtained. On the other hand, if V is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the V content is 0.050 to 0.35%. The minimum of preferable V content is higher than 0.050%, More preferably, it is 0.080% or more, More preferably, it is 0.10% or more. The upper limit of the preferable V content is less than 0.35%, more preferably 0.25% or less, and further preferably 0.20% or less.

N: 0.0020 to 0.020%
As described above, nitrogen (N) combines with C, Ti, and V to precipitate carbonitride in the ferrite, thereby increasing tensile strength and fatigue strength. Moreover, N precipitates as a nitride and suppresses coarsening of crystal grains. On the other hand, if N is contained excessively, the effect is saturated. Therefore, the N content is 0.0020 to 0.020%. The minimum of preferable N content is higher than 0.0020%, More preferably, it is 0.0040% or more, More preferably, it is 0.0050% or more. The upper limit of the preferable N content is less than 0.020%, more preferably 0.015% or less, and still more preferably 0.012% or less.

O: 0.0050% or less Oxygen (O) is an impurity. O forms oxide inclusions in the steel and reduces the fatigue strength of the steel. Therefore, it is preferable that the O content is small. The O content is 0.0050% or less. Preferable O content is less than 0.0050%, More preferably, it is 0.0040% or less.

  The balance of the connecting rod steel according to the present invention is Fe and impurities. Impurities are ores and scraps used as raw materials for steel, or elements mixed in from the environment of the manufacturing process.

  The connecting rod steel according to the present invention may further contain one or more selected from the group consisting of Cu, Ni and Mo. That is, these elements are selective elements. All of these elements reduce the thermal conductivity of steel.

Cu: 0.30% or less (including 0)
Copper (Cu) is a selective element and may not be contained. Cu may be contained in order to reduce the thermal conductivity of the steel. However, if Cu is contained excessively, the hot workability of steel is lowered. Therefore, cracks may occur in the steel during hot rolling or hot forging. Therefore, the Cu content is 0.30% or less (including 0). The upper limit of the preferable Cu content is less than 0.30%, more preferably 0.25% or less, and still more preferably 0.20% or less. When Cu is contained to reduce the thermal conductivity, the Cu content is preferably 0.05% or more.

Ni: 0.50% or less (including 0)
Nickel (Ni) is a selective element and may not be contained. Ni may be contained in order to reduce the thermal conductivity of the steel. However, if Ni is contained excessively, bainite is generated, yield strength and tensile strength are lowered, and therefore fatigue strength is also lowered. Therefore, the Ni content is 0.50% or less (including 0). The upper limit of the Ni content is preferably less than 0.50%, more preferably 0.30% or less, and still more preferably 0.20% or less. When Ni is contained to reduce the thermal conductivity, the Ni content is preferably 0.05% or more.

Mo: 0.50% or less (including 0)
Molybdenum (Mo) is a selective element and may not be contained. Mo may be included to reduce the thermal conductivity of the steel. However, if Mo is contained excessively, the tensile strength becomes excessively high, and the machinability of the steel decreases. Therefore, the Mo content is 0.50% or less (including 0). The upper limit of the preferable Mo content is less than 0.50%, more preferably 0.30% or less, and still more preferably 0.20% or less. When Mo is contained to reduce the thermal conductivity, the Mo content is preferably 0.03% or more.

[About fn1 to fn4]
Furthermore, in the steel for connecting rods by this invention, fn1-fn4 defined by Formula (1)-(4) satisfy | fills the following conditions.

[About fn1]
Fn1 defined by Formula (1) is 57.1 or more.
fn1 = 6.7 × (42 [Si%] + 25 [Mn%] + 14 [Cu%] + 12 [Ni%] + 16 [Cr%] + 12 [Mo%] + 42 [Al%] + 18 [Ti%] + 14 [V %]) ^0.5 ... (1)
The content (mass%) of the corresponding element is substituted into [element symbol%] in the formula (1). If the corresponding element is not contained, “0” is assigned to [element symbol%].

For example, when the selective elements Cu, Ni, and Mo are not contained, in other words, when Cu, Ni, and Mo are at the impurity level, [Cu%], [Ni%] in Formula (1) and “0” is assigned to [Mo%]. Therefore, when Cu, Ni, and Mo are not contained, fn1 is as follows.
fn1 = 6.7 × (42 [Si%] + 25 [Mn%] + 16 [Cr%] + 42 [Al%] + 18 [Ti%] + 14 [V%]) ^0.5

  fn1 is an index related to the thermal conductivity of connecting rod steel. All of Si, Mn, Cu, Ni, Cr, Mo, Al, and V in fn1 lower the thermal conductivity of the steel. As shown in FIG. 1, the thermal conductivity decreases in proportion to fn1. If fn1 is 50.0 or more, the thermal conductivity of the connecting rod steel will be sufficiently low, and specifically the thermal conductivity will be 38 W / (m · K) or less.

[About fn2]
Fn2 defined by the formula (2) is 0.45 or more.
fn2 = [C%] + [Si%] / 7+ [Mn%] / 5+ [Cr%] / 9+ [V%] / 2-5 [S%] / 7 (2)
The content (mass%) of the corresponding element is substituted into [element symbol%] in the formula (2).

  fn2 is a carbon equivalent. fn2 has a correlation with the tensile strength, and as fn2 increases, the tensile strength increases. As tensile strength increases, fatigue strength also increases. Moreover, if the tensile strength is increased, the cracking property is also improved. Therefore, fn2 is 0.45 or more. On the other hand, when fn2 becomes excessively large, the tensile strength becomes excessively high, and the machinability decreases. Therefore, the upper limit of fn2 is preferably 1.00 or less.

[About fn3]
Fn3 defined by Formula (3) is 0.16 or more.
fn3 = [Ti%] + [V%] (3)
The content (mass%) of the corresponding element is substituted into [element symbol%] in the formula (3).

  In order to ensure the cracking property, it is necessary to contain Ti and V in combination, and the total content of Ti and V needs to be a certain level or more. Therefore, fn3 is 0.16 or more.

[About fn4]
Fn4 defined by the formula (4) is 1.20 or less.
fn4 = [Mn%] + [Cr%] (4)
The content (mass%) of the corresponding element is substituted into [element symbol%] in the formula (4).

  fn4 is an index for suppressing the formation of bainite during cooling after hot forging. In the present invention, Ti and V are contained in combination in order to obtain cracking properties. However, when Ti and V are contained in combination, bainite is likely to be generated. When bainite is generated, the yield strength, tensile strength, and fatigue strength of the connecting rod are reduced. By limiting the total content of Mn and Cr, which enhances the hardenability of steel, the formation of bainite can be suppressed. Therefore, fn4 is 1.20 or less.

In the connecting rod steel according to the present invention, the above-described fn1 is 57.1 or more, fn2 is 0.45 or more, fn3 is 0.16 or more, and fn4 is 1.20 or less. is there. Therefore, the connecting rod steel according to the present invention has low thermal conductivity, high yield strength, high tensile strength, high fatigue strength, and excellent cracking properties.

[Production method]
An example of the connecting rod steel and connecting rod manufacturing method according to the present invention will be described. First, the configuration of the connecting rod will be described. FIG. 2 is a side view of the connecting rod. Referring to FIG. 2, the connecting rod 1 generally includes a rod 40 and a cap 50. The rod 40 includes a small end portion 20, a rail portion 30, and an upper half of the large end portion 10. The cap 50 corresponds to the lower half of the large end portion 10 and is fastened to the rod 40 by a pair of connecting rod bolts 60 after sandwiching the crank pin of the crankshaft.

  An example of the steel for a connecting rod used for manufacture of the above-mentioned connecting rod and a manufacturing method of the connecting rod is as follows.

  The molten steel which is in the range of the above-mentioned chemical composition and fn1-fn4 satisfy | fills the above-mentioned conditions is manufactured. The molten steel is bloomed by continuous casting. You may make molten steel into an ingot (steel ingot) by the ingot-making method. The steel for connecting rods is manufactured by the above process. The produced bloom or ingot is hot-worked to produce a billet (steel piece). A billet is hot-rolled to produce a steel bar. The manufactured steel bar is heated. Then, hot forging is performed on the heated steel bar to produce a connecting rod. A preferable heating temperature at the time of hot forging is 1150 to 1300 ° C, and a preferable finishing temperature is 900 to 1150 ° C.

  The connecting rod after hot forging is allowed to cool to room temperature or air cooled. Machining is performed on the connecting rod after cooling as necessary. The connecting rod is manufactured through the above steps. The rod 40 and the cap 50 shown in FIG. 2 are integrally formed by hot forging. As described above, the connecting rod steel according to the present invention is used as a cracking connecting rod. Therefore, the rod 40 and the cap 50 are cracked at the large end 10. For the cracking, a method is applied in which a jig is inserted into a hole (for example, the N portion in FIG. 2) of the large end portion 10 which is a portion to be divided of the integrally molded material, and stress is applied to break.

  In the manufacturing method described above, a connecting rod is manufactured using a steel bar. However, the ingot may be hot forged to produce a connecting rod. In short, as long as the connecting rod can be manufactured by hot forging, the manufacturing process of the connecting rod steel before hot forging is not particularly limited.

  The connecting rod according to the present invention can obtain high tensile strength, yield strength and fatigue strength without performing tempering treatment (quenching and tempering) after hot forging.

  Several connecting rod steels with different chemical compositions were produced. Then, the thermal conductivity, yield strength, tensile strength, fatigue strength, and Charpy impact value of each manufactured steel were investigated.

[Investigation method]
Steels of numbers 1 to 18 having the chemical composition shown in Table 1 were melted in a vacuum melting furnace to produce ingots.

  In each element symbol column in Table 1 (C, Si, Mn, P, S, Cr, Al, Ti, V, N, O, Cu, Ni, Mo), the corresponding element in the steel of each steel type number The content (mass%) of is entered. The balance other than the elements described in Table 1 of the chemical composition of each steel type number is an impurity other than Fe and O. “-” In the table indicates that the corresponding element content is at the impurity level.

  In the “fn1” column in Table 1, the value of fn1 defined by Equation (1) is entered. In the “fn2” column, the value of fn2 defined by equation (2) is entered. In the “fn3” column, the value of fn3 defined by equation (3) is entered.

The chemical compositions of Nos. 1 to 13 were all within the range of the chemical composition of the present invention. Further, fn1 of numbers 1 to 5, 7 to 11 and 13 was 57.1 or more, fn2 was 0.45 or more, fn3 was 0.16 or more, and fn4 was 1.20 or less. .

On the other hand, the Si content of No. 14 was less than the lower limit of the Si content of the present invention, and fn1 of No. 14 was less than the lower limit ( 57.1 ) of fn1 of the present invention.

The chemical compositions numbered 15 to 18 were within the chemical composition range of the present invention. However, in any of the numbers 15 to 18, any of fn1 to fn4 is out of the scope of the present invention. Specifically, fn1 of number 15 was less than the lower limit ( 57.1 ) of fn1 of the present invention. Fn2 of No. 16 was less than the lower limit (0.45) of fn2 of the present invention. Fn3 of No. 17 was less than the lower limit (0.16) of fn3 of the present invention. No. 18 fn4 exceeded the upper limit (1.20) of fn4 of the present invention.

  Each manufactured ingot was heated to 1200 ° C. Thereafter, each ingot was hot forged (forged) so that the finishing temperature was 1000 ° C. to produce a round bar having a diameter of 20 mm. The round bar after hot forging was allowed to cool in the atmosphere.

  The manufactured round bar is heated to 1250 ° C. and held for 30 minutes to eliminate non-uniformity in temperature distribution during forging. The tissue was homogenized. The following tests were carried out using the round bar after normalization.

[Thermal conductivity measurement test]
A disc-shaped test of the shape shown in FIG. 3 from the R / 2 part of each numbered round bar (including the part that bisects the center point of the cut surface (circular shape) of the round bar and the outer circumference). Pieces were collected. The numerical value in FIG. 3 shows the dimension (a unit is mm) of a corresponding part. The thermal conductivity (W / (m · K)) of each test piece was measured at room temperature (25 ° C.) by a laser flash method specified in JIS H 7801 (2005). The thermal conductivity was 38 W / (m · K) as an evaluation standard.

[Tensile test]
A 14A test piece defined in JIS Z 2201 (1998) was collected from R / 2 part of each numbered round bar. The transverse shape of the parallel part of the test piece was circular and the diameter was 5 mm. A tensile test was performed in the air at normal temperature (25 ° C.) using the collected test pieces, and yield strength (MPa) and tensile strength (MPa) were obtained. The yield strength was 550 MPa, and the tensile strength was 800 MPa.

[Fatigue test]
A test piece having the shape shown in FIG. 4 was collected from the R / 2 portion of each numbered round bar along the longitudinal direction of the round bar. The numerical value in FIG. 4 shows the dimension (a unit is mm) of a corresponding part. Using a hydraulic servo type testing machine, a fatigue test was repeatedly performed on the test piece repeatedly in tension and compression in the axial direction. The test was performed in a normal temperature (25 ° C.) atmosphere, and the tension and compression were repeated with a complete double swing at a frequency of 30 Hz. In each number, among the test pieces that did not break up to 1.0 × 10 ⁷ times, the highest amplitude stress was defined as the fatigue strength (MPa) of that number. The fatigue strength was 350 MPa as an evaluation standard.

[Shock test]
For impact characteristics, a V-notch standard impact test piece with a width of 10 mm (notch depth 2 mm) specified in JIS Z 2242 (2005) is cut out from the center of each round bar, and a Charpy impact test is performed at room temperature by a normal method. The impact value was measured. The Charpy impact value at room temperature using a V-notch test piece having a width of 10 mm can be an index for evaluating cracking properties. The smaller the value, the better the cracking property. The Charpy impact value was 7.0 J / cm ² as an evaluation standard.

[Test results]
The test results are shown in Table 1. In the “thermal conductivity” column in Table 1, the thermal conductivity (W / (m · K)) of each numbered round bar is entered. In the “yield strength” column, the 0.2% yield strength (MPa) of each numbered round bar is entered as the yield strength. In the “Tensile Strength” column, the tensile strength (MPa) of each numbered round bar is entered. In the “fatigue strength” column, the fatigue strength (MPa) of each number is entered. In the “impact value” column, the Charpy impact value (J / cm ² ) of each number is entered.

Referring to Table 1, the chemical compositions of No. 1 to No. 5, No. 7 to No. 11, and No. 13 and fn1 to fn4 were all within the scope of the present invention. Therefore, the thermal conductivities of the round bars of No. 1 to No. 5, No. 7 to No. 11, and No. 13 were low, and all were 38 W / (m · K) or less. Furthermore, the yield strengths of No. 1 to No. 5, No. 7 to No. 11, and No. 13 were all 550 MPa or higher, and the tensile strength was 800 MPa or higher. Furthermore, the fatigue strengths of No. 1 to No. 5, No. 7 to No. 11, and No. 13 were all 350 MPa or more. Furthermore, the Charpy impact values of No. 1 to No. 5, No. 7 to No. 11, and No. 13 were all 7.0 J / cm ² or less. Therefore, the round bars of No. 1 to No. 5, No. 7 to No. 11, and No. 13 all had low thermal conductivity, high yield strength, high tensile strength, high fatigue strength, and excellent cracking properties. .

  On the other hand, the Si content of No. 14 was less than the lower limit of the Si content of the present invention. Fn1 was also less than the lower limit of the present invention. Therefore, the thermal conductivity of No. 14 exceeded 38 W / (m · K).

  Although the chemical composition number 15, fn2, fn3, and fn4 were within the scope of the present invention, fn1 was less than the lower limit of the present invention. Therefore, the thermal conductivity of No. 15 exceeded 38 W / (m · K).

  Although the chemical composition No. 16, fn1, fn3, and fn4 were within the scope of the present invention, fn2 was less than the lower limit of the present invention. Therefore, the tensile strength of No. 16 was less than 800 MPa, and the fatigue strength was less than 350 Mpa.

Although the chemical composition of No. 17, fn1, fn2, and fn4 were within the scope of the present invention, fn3 was less than the lower limit of the present invention. Therefore, the Charpy impact value of No. 17 exceeded 7.0 J / cm ² and was inferior in cracking properties.

  Although the chemical composition of number 18, fn1, fn2, and fn3 were within the scope of the present invention, fn4 exceeded the upper limit of the present invention. Therefore, No. 18 produced bainite, yield strength was less than 550 MPa, tensile strength was less than 800 MPa, and fatigue strength was less than 350 MPa.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  The present invention can be widely applied to machine structural parts, and is preferably applicable to machine structural parts used in engines. In particular, it is applicable to cracking connecting rods.

1 Connecting rod 10 Large end 20 Small end 30 Rail 40 Rod 50 Cap 60 Bolt

Claims (2)

% By mass
C: 0.20 to 0.50%,
Si: more than 0.90% and 2.0% or less,
Mn: 0.50 to 1.20%,
P: 0.020 to 0.15%,
S: 0.010 to 0.12%,
Cr: 0.05 to 0.40%,
Al: 0.005 to 0.08%,
Ti: 0.050 to 0.30%,
V: 0.050 to 0.35%,
N: 0.0020 to 0.020%,
O: 0.0050% or less,
Cu: 0.30% or less (including 0),
Ni: 0.50% or less (including 0),
Mo: 0.50% or less (including 0),
And the balance consists of Fe and impurities,
Fn1 defined by the formula (1) is 57.1 or more,
Fn2 defined by the formula (2) is 0.45 or more,
Fn3 defined by the formula (3) is 0.16 or more,
Steel for a connecting rod, wherein fn4 defined by formula (4) is 1.20 or less.
fn1 = 6.7 × (42 [Si%] + 25 [Mn%] + 14 [Cu%] + 12 [Ni%]
] +16 [Cr%] + 12 [Mo%] + 42 [Al%] + 18 [Ti%] + 14 [V%]
^0.5 ... (1)
fn2 = [C%] + [Si%] / 7+ [Mn%] / 5+ [Cr%] / 9+ [V%] / 2
-5 [S%] / 7 (2)
fn3 = [Ti%] + [V%] (3)
fn4 = [Mn%] + [Cr%] (4)
Here, in [element symbol%] in the formulas (1) to (4), the content of the corresponding element (mass%)
) Is substituted. If the corresponding element is not contained, “0” is assigned to [element symbol%]. A connecting rod produced by hot forging the connecting rod steel according to claim 1.

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