JP5755965B2 - 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.

  Regarding weight reduction of machine structural parts, methods for increasing the tensile strength and fatigue strength of materials of 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.

  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.

JP 2010-53430 A JP 2002-256394 A

  As described above, the steels disclosed in Patent Documents 1 and 2 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 is to reduce engine internal friction losses. A crankpin of a crankshaft is rotatably inserted into a large end portion of a connecting rod (hereinafter referred to as a 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 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 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.

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

Steel for connecting rods according to the present invention is in mass%, C: 0.25 to 0.48%, Si: 0.50 to 2.00%, Mn: 0.50 to 1.50%, P: 0.060. % Or less, S: 0.010 to 0.120%, Cr: 0.05 to 0.40%, Al: 0.005 to 0.080%, Ti: 0.0010 to 0.0100%, V: 0 0.020 to 0.300%, N: 0.0050 to 0.0200%, O: 0.0050% or less, Cu: 0.30% or less (including 0), Ni: 0.50% or less (0 And Mo: 0.50% or less (including 0), with the balance being Fe and impurities, fn1 defined by formula (1) is 47 or more, and defined by formula (2) fn2 is 0.60 or more, and fn3 defined by the formula (3) is 0.0002 to 0.0080.
fn1 = 6.7 × (42 [Si %] + 25 [Mn%] + 14 [Cu%] + 12 [Ni%] + 16 [Cr%] + 12 [Mo%] + 42 [Al%] + 14 [V%]) 0. ⁵ ... (1)
fn2 = [C%] + [Si%] / 7+ [Mn%] / 5+ [Cr%] / 9+ [V%] / 2-5 [S%] / 7 (2)
fn3 = [Ti%] − 0.599 [O%] (3)
Here, the content (mass%) of the corresponding element is substituted for [element symbol%] in the expressions (1) to (3). If the corresponding element is not contained, “0” is assigned to [element symbol%].

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

  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, and thermal conductivity of connecting rod steel. 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 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 47 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%] + 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 47 or more, the heat conductivity of steel will be 40 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 strength (yield strength and tensile strength). The yield strength and tensile strength of the connecting rod steel are proportional to the value of fn2 defined by the equation (2). If fn2 is 0.60 or more, a yield strength of 550 MPa or more and a tensile strength of 750 MPa or more are obtained.
fn2 = [C%] + [Si%] / 7+ [Mn%] / 5+ [Cr%] / 9+ [V%] / 2-5 [S%] / 7 (2)
In this specification, the yield strength is defined as 0.2% yield strength.

  (C) A rotational bending stress is mainly applied to the crankshaft in the circumferential direction. On the other hand, the connecting rod is mainly subjected to repeated stresses of tension and compression in the axial direction. Therefore, the stress applied during operation differs between the crankshaft and the connecting rod. 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.

  In order to increase the fatigue strength, it is preferable to refine the crystal grains. In order to refine crystal grains, Ti nitride and / or Ti carbonitride are used as pinning particles. The connecting rod is manufactured by hot forging. Since Ti nitride and / or Ti carbonitride remains in the steel without being dissolved during hot forging, the fine Ti nitride and / or Ti carbonitride is connected to the connecting rod by a so-called pinning effect. Refine crystal grains.

  Ti preferentially bonds with oxygen (O) over nitrogen (N) and carbon (C). Therefore, in order to produce fine Ti nitride and / or Ti carbonitride, it is necessary to consider the Ti content and the O content.

  On the other hand, coarse Ti nitride can be a starting point for fatigue failure. As described above, in the stress distribution in rotational bending fatigue, the stress in the surface layer portion of the steel material is maximized. Therefore, even if coarse Ti nitride exists in the steel material, the influence on the rotational bending fatigue strength is small. On the other hand, as described above, the stress distribution in the cross section (cross section perpendicular to the axial direction) of the steel material due to repeated tensile and compressive stresses in the axial direction is substantially constant. Therefore, if coarse Ti nitride exists in the steel material, it can be a starting point for fatigue failure. As mentioned above, in order to produce | generate fine Ti nitride and / or Ti carbonitride, it is necessary to consider so that Ti content may not increase excessively with respect to O content.

If fn3 defined by Formula (3) is 0.0002 to 0.0080, fine Ti nitrides and Ti carbonitrides are produced while suppressing the production of coarse Ti nitrides. As a result, crystal grains are refined by the pinning effect, and a connecting rod excellent in fatigue strength can be obtained.
fn3 = [Ti%] − 0.599 [O%] (3)

  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: 0.25 to 0.48%
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 and toughness of the steel decrease. Therefore, the C content is 0.25 to 0.48%. The minimum of preferable C content is higher than 0.25%, More preferably, it is 0.30% or more, More preferably, it is 0.38% or more. The upper limit of the preferable C content is less than 0.48%, more preferably 0.46% or less, and further preferably 0.45% or less.

Si: 0.50 to 2.00%
Silicon (Si) reduces the thermal conductivity of steel. On the other hand, if Si is contained excessively, the hot workability of steel is lowered. Therefore, the Si content is 0.50 to 2.00%. The minimum of preferable Si content is higher than 0.50%, More preferably, it is 0.70% or more, More preferably, it is 0.80% or more. The upper limit of the Si content is preferably less than 2.00%, more preferably 1.60% or less, and still more preferably 1.30% or less.

Mn: 0.50 to 1.50%
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 hardenability of the steel becomes too high and bainite is generated. As a result, the tensile strength of the steel becomes too high, and the machinability and toughness of the steel decrease. Therefore, the Mn content is 0.50 to 1.50%. The minimum of preferable Mn content is higher than 0.50%, More preferably, it is 0.70% or more, More preferably, it is 0.80% or more. The upper limit of the preferable Mn content is less than 1.50%, more preferably 1.30% or less, and still more preferably 1.20% or less.

P: 0.060% or less Phosphorus (P) is an impurity. P decreases the hot workability and toughness of steel. Therefore, it is preferable that the P content is small. The P content is 0.060% or less. A preferable P content is less than 0.060%, more preferably 0.050% or less, and still more preferably 0.030% or less.

S: 0.010 to 0.120%
(Sulfur) S combines with Mn to form MnS and enhances the machinability of steel. On the other hand, if S is contained excessively, the hot workability and toughness of the steel deteriorate. Therefore, the S content is 0.010 to 0.120%. 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 0.120%, more preferably 0.110% or less, and still more preferably 0.100% 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 contained excessively, bainite is generated and the yield strength and toughness of the steel are 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 0.30% or more, and further preferably 0.25% or less.

Al: 0.005-0.080%
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 0.080%. 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 0.080%, more preferably 0.070% or less, and further preferably 0.065% or less.

Ti: 0.0010 to 0.0100%
Titanium (Ti) combines with N in the steel to form Ti nitride (TiN) and / or Ti carbonitride (Ti (C, N)). Ti nitride and / or Ti carbonitride function as pinning particles, and refine crystal grains by a pinning effect. Specifically, even when the steel material is heated to a temperature exceeding 1200 ° C. in the hot forging step, Ti nitride and / or Ti carbonitride suppresses austenite grain coarsening. As a result, the fatigue strength of the steel is increased.

  On the other hand, if the Ti content exceeds 0.0100%, coarse Ti nitride is produced. 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 0.0010 to 0.0100%. The minimum of preferable Ti content is higher than 0.0010%, More preferably, it is 0.0016% or more. The upper limit of the preferable Ti content is less than 0.0100%, more preferably 0.0050% or less, and still more preferably 0.0040% or less.

V: 0.020-0.300%
Vanadium (V) decreases the thermal conductivity of steel. V further forms V carbide (VC) and / or V carbonitride (V (C, N)). V carbide and / or V carbonitride increase the yield strength and tensile strength of steel. On the other hand, if V is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the V content is 0.020 to 0.300%. The minimum of preferable V content is higher than 0.020%, More preferably, it is 0.050% or more, More preferably, it is 0.070% or more. The upper limit of the preferable V content is less than 0.300%, more preferably 0.150% or less, and further preferably 0.120% or less.

N: 0.0050 to 0.0200%
Nitrogen (N) combines with Ti to form Ti nitride and / or Ti carbonitride as described above, and austenite grains are refined. As a result, the fatigue strength of the steel after hot forging is improved. N further forms V carbonitrides to increase the yield strength and tensile strength of the steel. On the other hand, if N is contained excessively, the effect is saturated. Therefore, the N content is 0.0050 to 0.0200%. The minimum of preferable N content is higher than 0.0050%, More preferably, it is 0.0060% or more, More preferably, it is 0.0070% or more. The upper limit of the preferable N content is less than 0.0200%, more preferably 0.0150% or less, and still more preferably 0.0120% 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 decreases the thermal conductivity of steel. If Cu is contained even a little, the above effect can be obtained. On the other hand, if Cu is contained excessively, the hot workability of the 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.

Ni: 0.50% or less (including 0)
Nickel (Ni) is a selective element and may not be contained. Ni decreases the thermal conductivity of steel. If Ni is contained even a little, the above effect can be obtained. On the other hand, if Ni is contained excessively, the toughness of the steel decreases. 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.

Mo: 0.50% or less (including 0)
Molybdenum (Mo) is a selective element and may not be contained. Mo reduces the thermal conductivity of the steel. If Mo is contained even a little, the above effect can be obtained. On the other hand, if Mo is contained excessively, the toughness 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.

[About fn1 to fn3]
Furthermore, in the steel for connecting rods by this invention, fn1-fn3 defined by Formula (1)-(3) satisfy | fills the following conditions.

[About fn1]
Fn1 defined by the formula (1) is 47 or more.
fn1 = 6.7 × (42 [Si %] + 25 [Mn%] + 14 [Cu%] + 12 [Ni%] + 16 [Cr%] + 12 [Mo%] + 42 [Al%] + 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%] + 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 47 or more, the thermal conductivity of the connecting rod steel will be sufficiently low, specifically, the thermal conductivity will be 40 W / (m · K) or less.

[About fn2]
Fn2 defined by Formula (2) is 0.60 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 an index of yield strength and tensile strength of connecting rod steel. When fn2 is 0.60 or more, high yield strength and tensile strength are obtained. Specifically, if fn2 is 0.60 or more, the yield strength of the connecting rod steel is 550 MPa or more, and the tensile strength is 750 MPa or more.

[About fn3]
Fn3 defined by the formula (3) is 0.0002 to 0.0080.
fn3 = [Ti%] − 0.599 [O%] (3)
The content (mass%) of the corresponding element is substituted into [element symbol%] in the formula (3).

  fn3 is an index indicating the amount of Ti nitride and / or Ti carbonitride that functions as pinning particles. Ti in steel tends to preferentially bond with O over N and C. If fn3 is excessively small, Ti preferentially binds to O, resulting in a deficiency of Ti bound to N and C. Therefore, the amount of fine Ti nitride and / or Ti carbonitride during hot forging is reduced. In this case, the pinning effect cannot be obtained, the crystal grains become coarse and the fatigue strength of the steel decreases. On the other hand, if fn3 is excessively large, coarse Ti nitride is generated. Coarse Ti nitrides do not function as pinning particles, but rather serve as starting points for fatigue failure and reduce the fatigue strength of steel.

  If fn3 is 0.0002 to 0.0080, an appropriate amount of fine Ti nitride and / or Ti carbonitride is generated. As a result, the fatigue strength of the connecting rod steel increases.

  In the steel for connecting rods according to the present invention, the above-mentioned fn1 is 47 or more, fn2 is 0.60 or more, and fn3 is 0.0002 to 0.0080. Therefore, the connecting rod steel according to the present invention has high tensile strength and yield strength, excellent fatigue strength, and low thermal conductivity.

[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 is manufactured within the range of the chemical composition described above, and fn1 to fn3 satisfy the above-described conditions. 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. When hot forging is performed in this temperature range, Al nitride is dissolved. Therefore, Al nitride hardly contributes to refinement of crystal grains during hot forging. On the other hand, fine Ti nitride and Ti carbonitride do not dissolve in hot forging in the above temperature range but remain in the steel. Therefore, fine Ti nitride and Ti carbonitride refine crystal grains.

  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. As described above, the steel for connecting rods according to the present invention is used for the production of ordinary connecting rods. Therefore, the rod 40 and the connecting rod cap 60 shown in FIG. 2 are manufactured separately by hot forging, and then the rod 40 and the connecting rod cap 60 are joined.

  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. And the thermal conductivity, yield strength, tensile strength, and fatigue strength of each manufactured steel were investigated.

[Investigation method]
Steels numbered 1 to 17 having the chemical compositions 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 Fe and impurities. “-” In the table indicates that the corresponding element content is an impurity.

  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 11 were all within the range of the chemical composition of the present invention. Furthermore, fn1 of numbers 1 to 11 was 47 or more, fn2 was 0.60 or more, and fn3 was in the range of 0.0002 to 0.0080. On the other hand, the Ti content of No. 12 was less than the lower limit of the Ti content of the present invention. Moreover, the Si content of No. 17 was less than the lower limit of the Si content of the present invention. Therefore, the fn1 value of No. 17 was less than the lower limit (47) of fn1 of the present invention.

  The chemical composition of No. 13 to No. 16 was within the range of the chemical composition of the present invention. However, in any of the numbers 13 to 16, any of fn1 to fn3 is out of the scope of the present invention. Specifically, the fn1 value of No. 13 was less than the lower limit of fn1 of the present invention. The fn2 value of No. 14 was less than the lower limit (0.60) of fn2 of the present invention. The fn3 value of No. 15 exceeded the upper limit (0.0080) of fn3 of the present invention. The fn3 value of No. 16 was less than the lower limit (0.0002) of fn3 of the present invention.

  Each manufactured ingot was heated to 1200 ° C. Thereafter, each ingot was hot forged (forge forging) so that the finishing temperature was 1000 ° C., and a round bar having a diameter of 30 mm was produced. The round bar after hot forging was allowed to cool in the atmosphere.

  The manufactured round bar was heated to 1250 ° C. and held for 30 minutes, and then subjected to normalization that was allowed to cool in the atmosphere to homogenize the round bar structure. 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 of each test piece was measured at room temperature (25 ° C.) by the laser flash method specified in JIS H 7801 (2005).

[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. Yield strength was defined as 0.2% yield strength.

[Fatigue strength 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 strength 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 swing 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.

[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 the round bar of each number is entered. In the “yield strength” column, the 0.2% yield strength (MPa) of each numbered round bar is entered. 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.

  Referring to Table 1, the chemical compositions, fn1 value, fn2 value, and fn3 value of No. 1 to No. 11 were all within the scope of the present invention. Therefore, the thermal conductivity of the round bars of No. 1 to No. 11 was low, and all were 40 W / (m · K) or less. Furthermore, the yield strengths of No. 1 to No. 11 were all 550 MPa or more, and the tensile strengths were all 750 MPa or more. Furthermore, the fatigue strengths of No. 1 to No. 11 were all 320 MPa or more. Therefore, each of the round bars of No. 1 to No. 11 had high tensile strength and yield strength, excellent fatigue strength against repeated tensile and compressive stresses in the axial direction, and low thermal conductivity. .

  On the other hand, the Ti content of No. 12 was less than the lower limit of the Ti content of the present invention. Therefore, the fatigue strength was less than 320 MPa. It is considered that the amount of Ti nitride and / or Ti carbonitride that functions as pinning particles was small because the Ti content was too small.

  Although the chemical composition of No. 13, the fn2 value and the fn3 value were within the scope of the present invention, fn1 was less than the lower limit of the present invention. Therefore, the thermal conductivity exceeded 40 W / (m · K). Although the chemical composition of No. 14, the fn1 value and the fn3 value were within the scope of the present invention, the fn2 value was less than the lower limit of the present invention. Therefore, the yield strength of No. 14 was less than 550 MPa, and the tensile strength was less than 750 MPa.

  Although the chemical composition of No. 15, the fn1 value and the fn2 value were within the scope of the present invention, the fn3 value exceeded the upper limit of the present invention. Therefore, the fatigue strength of No. 15 was less than 320 MPa. Although the chemical composition of No. 16, the fn1 value and the fn2 value were within the scope of the present invention, the fn3 value was less than the lower limit of the present invention. Therefore, the fatigue strength of No. 16 was less than 320 MPa.

  The Si content of No. 17 was less than the lower limit of the present invention, and as a result, the fn1 value was also less than the lower limit of the present invention. Therefore, the thermal conductivity of No. 17 exceeded 40 W / (m · K).

  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 can be applied to a connecting rod.

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

Claims (2)

% By mass
C: 0.25 to 0.48%,
Si: 0.50 to 2.00%
Mn: 0.50 to 1.50%,
P: 0.060% or less,
S: 0.010 to 0.120%,
Cr: 0.05 to 0.40%,
Al: 0.005 to 0.080%,
Ti: 0.0010 to 0.0100%,
V: 0.020-0.300%
N: 0.0050 to 0.0200%,
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 47 or more,
Fn2 defined by the formula (2) is 0.60 or more,
Steel for a connecting rod, wherein fn3 defined by formula (3) is 0.0002 to 0.0080.
fn1 = 6.7 × (42 [Si %] + 25 [Mn%] + 14 [Cu%] + 12 [Ni%] + 16 [Cr%] + 12 [Mo%] + 42 [Al%] + 14 [V%]) 0. ⁵ ... (1)
fn2 = [C%] + [Si%] / 7+ [Mn%] / 5+ [Cr%] / 9+ [V%] / 2-5 [S%] / 7 (2)
fn3 = [Ti%] − 0.599 [O%] (3)
Here, the content (mass%) of the corresponding element is substituted for [element symbol%] in the expressions (1) to (3). 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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