JP5068087B2 - Steel for fracture split type connecting rods with excellent fracture splitability and machinability - Google Patents

Description

  The present invention relates to a steel suitably used for the manufacture of a connecting rod (hereinafter may be abbreviated as “connecting rod”) used as a part of an automobile engine or the like.

  An internal combustion engine such as a gasoline engine or a diesel engine uses a connecting rod as a component that connects a piston and a crankshaft and transmits the reciprocating motion of the piston to the crankshaft to convert it into a rotational motion. This connecting rod is a part with a substantially circular through-hole for assembling to the crankshaft, and the through-hole part is separated (divided) into two substantially semicircles in order to facilitate removal during assembly and maintenance. Is configured to do. Of the separated connecting rods, the side directly connected to the piston is called a connecting rod body, and the rest is called a connecting rod cap.

  Such a connecting rod can be manufactured, for example, by subjecting the connecting rod body and the connecting rod cap to hot forging separately and then processing the mating surfaces by cutting. In this case, knock pin processing may be performed as necessary to prevent displacement. However, when such processing is performed, there is a problem that the yield of the material is reduced and the cost is increased due to a large number of steps.

  Therefore, after the connecting rod is integrally hot forged and subjected to machining (through hole formation processing (drilling processing), bolt hole processing, etc. for assembling to the crankshaft), the through hole portion becomes two substantially semicircles. As described above, a method is used in which breakage is divided in the cold (splitting processing), and finally the fractured surface is fitted with the crankshaft interposed therebetween and fastened with bolts for assembly. According to this method, it is not necessary to process the mating surface by cutting the fracture surface.

  Further, there is an increasing demand for improving machinability for steel for connecting rods. However, it is generally difficult to satisfy both machinability and fracture splitting property. In order to improve the machinability, it is conceivable to decrease the hardness of the steel by reducing the alloy component, but if the alloy component is decreased, the ductility of the steel increases and the fracture splitting property decreases. These are in a trade-off relationship, and it is difficult to achieve both.

For example, Patent Documents 1 to 3 are known as steels for connecting rods excellent in fracture splitting property. Patent Document 1 proposes to promote brittle fracture by controlling the amount of Si, V, P, N, Al, Ti, Nb, N, B, etc., and Patent Document 2 describes Si, V, It is proposed to control the amount of P and the like to promote brittle fracture, and Patent Document 3 proposes to control the amount of Al, N and the like to promote brittle fracture. Furthermore, these patent documents 1 to 3 say that brittle fracture can be promoted by adding Ti. However, these connecting rod steels have poor machinability. For example, with respect to Patent Document 1, C is 0.35% or more, or 0. In most examples, the alloy components are used excessively, such as Mo being 0.38% or more. On the other hand, when the content of C or the like is suppressed, the fracture splitting property is secured by using Ti in excess of 0.10. Also, in Patent Examples 2 and 3, Ti is used in an amount exceeding 0.10%, and the machinability is inferior.
JP-A-9-176785 Japanese Patent Laid-Open No. 11-199967 JP-A-11-286746

  The present invention has been made paying attention to the above-described circumstances, and an object of the present invention is to provide a fracture split type connecting rod steel that can achieve both fracture splitability and machinability.

  In steels with a C content of 0.3% or less, the fracture splitting property becomes extremely poor unless an alloying element such as Ti is added (see the Ti-free addition example in FIG. 1). In order to increase the brittleness of steel and increase the fracture splitting property, it is important to add an alloying element that can replace a low C content. For that reason, in Patent Documents 1 to 3, Ti is used in excess of 0.10% (see Ti 0.150% addition example in FIG. 1). However, when Ti is added, the machinability decreases. Considering from the prior art, the fracture splitting property and the machinability are in a trade-off relationship, and no specific measures for achieving both have been shown.

  However, as a result of extensive research conducted by the present inventors in order to solve the above problems, when the amount of Ti is arranged from the viewpoint of effective Ti (Ti not forming a nitride), it breaks with a very small amount of effective Ti. The splitting property increases rapidly, and then the effect immediately saturates. On the other hand, the machinability is moderately lowered and the machinability is hardly lowered when the effective Ti amount is very small (see FIG. 2). Therefore, it was found that controlling the Ti amount from the viewpoint of the effective Ti amount can achieve both fracture splitting property and machinability (see FIG. 1), and the present invention has been completed.

That is, the fracture split type connecting rod steel excellent in fracture splitability and machinability according to the present invention has C: 0.15 to 0.3% (meaning mass%, the same applies hereinafter), Si: 2% or less. (Excluding 0%), Mn: 0.6-2%, Al: 0.06% or less (not including 0%), P: 0.15% or less (not including 0%), S: 0 .15% or less (excluding 0%), Cr: 2% or less (excluding 0%), V: more than 0.20%, 0.5% or less, Ti: 0.01 to 0.10%, And N: 0.007% or less (not including 0%), the balance is made of iron and inevitable impurities, and the f value represented by the following formula (1) is 0.000 or more, from the steel surface In the longitudinal section at the position of D / 4 (D is the thickness or diameter of the steel), there are 100 or more sulfide inclusions per 1 mm ^{2 with} a width of 1 μm or more. In addition, the gist is that the average aspect ratio (length / width) of the sulfide inclusions having a width of 1 μm or more is 15 or less.
f = [Ti] − [N] × 48/14 (1)
[In the formula, [Ti] and [N] indicate the contents (mass%) of Ti and N in the steel, respectively. ]

  The steel is further Ca: 0.005% or less (not including 0%), Mg: 0.005% or less (not including 0%), and Zr: 0.1% or less (not including 0%) Nb: 0.05% or less (not including 0%), Mo: 0.05% or less (not including 0%), Cu: 0.2% or less (not including 0%) , And Ni: one or more of 0.2% or less (not including 0%) may be contained. When Ca is contained, Al is recommended to be 0.01% or less.

  In the most preferred embodiment of the steel of the present invention, Ti is 0.03% or less and the f value is 0.010 or less.

  According to the present invention, in steel with a C content of about 0.3% or less, Ti content, N content, effective Ti content, etc. are appropriately controlled, so both the fracture splitting property and machinability of steel for connecting rods. The characteristics of can be enhanced.

  The steel of the present invention will be described first from its chemical composition. The chemical component composition of the steel of the present invention is as follows.

C: 0.15-0.3%
C is an element necessary for securing strength and improving break splitting property. Therefore, the C amount is set to 0.15% or more. The amount of C is preferably 0.17% or more, more preferably 0.18% or more. However, if the amount of C is excessive, machinability deteriorates. Therefore, the C amount is set to 0.3% or less. The amount of C is preferably 0.28% or less, more preferably 0.25% or less.

Si: 2% or less (excluding 0%)
Si is useful as a deoxidizing element when melting steel. In order to sufficiently exhibit this effect, the Si amount is preferably 0.01% or more, preferably 0.05% or more, more preferably 0.10% or more. However, if the amount of Si is excessive, machinability deteriorates. Therefore, the amount of Si is set to 2% or less. The amount of Si is preferably 1% or less, more preferably 0.5% or less.

Mn: 0.6-2%
Mn is an element that acts as a deoxidation and desulfurization element during melting and prevents cracking during casting. Furthermore, Mn combines with S to form sulfide inclusions (for example, MnS, etc.), thereby improving fracture splitting. In order to fully exhibit these effects, the amount of Mn was determined to be 0.6% or more. The amount of Mn is preferably 0.7% or more, more preferably 0.8% or more. However, if the amount of Mn is excessive, bainite is generated in the metal structure, and the machinability and fracture splitting property are lowered. Therefore, the amount of Mn is set to 2% or less. The amount of Mn is preferably 1.8% or less, more preferably 1.5% or less.

Al: 0.06% or less (excluding 0%)
Al is an element useful for deoxidation and grain refinement. In order to sufficiently exhibit these effects, the Al content is preferably 0.001% or more, more preferably 0.003% or more. However, the effect is saturated even if there is too much Al. Moreover, when the amount of Al is excessive, nozzle clogging of the melting pot may occur during melting. Therefore, the Al content is determined to be 0.06% or less. The amount of Al is preferably 0.03% or less. In the present invention, Ca may be added to the steel as described later. When Ca is added, the nozzle is easily clogged, so the Al content is preferably 0.01% or less, more preferably 0.005% or less.

P: 0.15% or less (excluding 0%)
P is an impurity inevitably contained in the steel material. If the amount is large, the hot workability of the steel is lowered. Therefore, the P content is set to 0.15% or less. The amount of P is preferably 0.10% or less, more preferably 0.07% or less. On the other hand, P segregates at the grain boundary and lowers the toughness and ductility, so that it has the effect of improving fracture splitting. In order to sufficiently exhibit this effect, P may be contained in an amount of 0.001% or more. The amount of P is preferably 0.01% or more, more preferably 0.02% or more.

S: 0.15% or less (excluding 0%)
S is an element that forms sulfide inclusions (for example, MnS or the like) to improve fracture splitting and improve machinability. In order to sufficiently exhibit these effects, the amount of S is preferably 0.01% or more, more preferably 0.03% or more, and further preferably 0.05% or more. However, when the amount of S becomes excessive, hot workability deteriorates. Therefore, the S amount is set to 0.15% or less. The amount of S is preferably 0.13% or less, more preferably 0.12% or less.

Cr: 2% or less (excluding 0%)
Cr, like Mn, is an element that contributes to an increase in strength. In order to sufficiently exhibit this effect, the Cr content is preferably 0.01% or more, more preferably 0.05% or more, and further preferably 0.10% or more. However, when the amount of Cr becomes excessive, the machinability of steel decreases. Therefore, the C amount is set to 2% or less. The amount of Cr is preferably 1% or less, more preferably 0.7% or less.

V: More than 0.20%, 0.5% or less V is an element useful for securing the strength of steel and improving break splitting property. Therefore, the V amount is determined to be more than 0.20%. V amount is preferably 0.21% or more, more preferably 0.23% or more. However, if the amount of V is too large, the effect is saturated, and excessive addition causes an increase in cost. Therefore, the V amount is set to 0.5% or less. The amount of V is preferably 0.4% or less, more preferably 0.3% or less.

Ti: 0.01-0.10%
Ti is an important element for improving the fracture splitting property of steel. In order to fully exhibit this effect, the Ti content was set to 0.01% or more. The amount of Ti is preferably 0.018% or more, more preferably 0.020% or more. However, if the Ti amount is increased, the machinability of the steel is lowered. Further, if the amount of effective Ti described later is increased, the fracture splitting property is rapidly improved by adding a little Ti, and the fracture splitting property is not improved even if the addition amount is further increased. Therefore, it is desirable to reduce Ti as much as possible as long as an effective Ti amount described later can be secured. Therefore, Ti is determined to be 0.10% or less. The amount of Ti is preferably 0.08% or less, more preferably 0.05% or less, and particularly 0.03% or less.

  The addition amount that can achieve both fracture splitting property and machinability is a phenomenon peculiar to the case where Ti is added while securing the effective Ti amount. When the present inventors conducted a separate experiment, the effect of improving the break splitting property when adding a small amount of about 0.01% (reduced value of the dimensional change described later) , P, V, Ti, etc.) was added to about 0.010 to 0.020 mm. However, when Ti was added while securing an effective amount of Ti, the reduction value of the dimensional change reached 0.060 mm even when added in a small amount of about 0.01%, which was improved to more than three times the conventional value. As a result, the amount of Ti added can be reduced, and the machinability can be improved.

N: 0.007% or less (excluding 0%)
The present invention is intended to reduce the amount of Ti added to improve machinability, while effectively utilizing the small amount of Ti to effectively improve the fracture splitting property. By limiting the amount of N in the steel, the formation of TiN can be suppressed, and a small amount of Ti can be effectively utilized. Therefore, the N amount is set to 0.007% or less. The N amount is preferably 0.006% or less, more preferably 0.005% or less. N is an element inevitably mixed in steel, and the amount cannot be industrially reduced to 0%.

  The basic component composition of the steel for connecting rods of the present invention is as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel. Furthermore, the steel for connecting rods of this invention may contain the following arbitrary elements as needed.

Ca: 0.005% or less (excluding 0%),
Mg: 0.005% or less (not including 0%), or Zr: 0.1% or less (not including 0%)
Ca, Mg and Zr are elements useful for improving the fracture splitting property by spheroidizing sulfide inclusions (MnS and the like), and may be contained in steel as necessary. In particular, as the amount of Mn increases, the fracture splitting property tends to decrease. Therefore, in order to avoid this influence as much as possible, it is recommended to add Ca, Mg, Zr or the like. In order to sufficiently exhibit this effect, the Ca content is preferably 0.0001% or more, more preferably 0.001% or more, and the Mg content is preferably 0.0001% or more, more preferably 0.001% or more. The amount of Zr is preferably 0.001% or more, and more preferably 0.01% or more. However, if these amounts are too large, the effect is saturated. If the amount of Ca or Mg is excessive, oxide inclusions increase and the fatigue strength of the steel decreases. On the other hand, if the amount of Zr is excessive, the machinability deteriorates. Therefore, when these elements are contained, the upper limit is determined as described above. The Ca amount is preferably 0.01% or less (particularly 0.005% or less), the Mg amount is preferably 0.01% or less (particularly 0.005% or less), and the Zr amount is preferably It is 0.1% or less (particularly 0.07% or less). Ca, Mg, and Zr may be added alone or in combination.

Nb: 0.05% or less (excluding 0%),
Mo: 0.05% or less (excluding 0%),
Cu: 0.2% or less (not including 0%), or Ni: 0.2% or less (not including 0%)
Nb, Mo, Cu and Ni are elements that contribute to improving the strength of the steel, and may be contained in the steel as necessary. In order to sufficiently exhibit this effect, the Nb amount is preferably 0.0001% or more, more preferably 0.0005% or more, and the Mo amount is preferably 0.01% or more, more preferably 0.00. The amount of Cu is preferably 0.01% or more, more preferably 0.03% or more, and the amount of Ni is preferably 0.01% or more, more preferably 0.03% or more. is there. However, if the amount of Nb or Mo is excessive, the machinability of the steel is reduced. If the amount of Cu is excessive, soot is generated on the steel surface during production. Furthermore, even if the amount of Ni is too large, the effect is saturated, and excessive addition causes an increase in cost. Therefore, when these elements are contained, the upper limit is determined as described above. The Nb amount is preferably 0.01% or less, the Mo amount is preferably 0.03% or less, the Cu amount is preferably 0.1% or less, and the Ni amount is preferably 0.1% or less. .

  And the characteristic of this invention exists in the point which controls an effective Ti amount appropriately, after adjusting steel composition to the said range. The effective Ti amount means the remaining Ti amount obtained by subtracting TiN from the Ti amount in the steel, and may be referred to as an f value in this specification. When the fracture splitting property is arranged from the viewpoint of the effective Ti amount, the fracture splitting property is rapidly increased with a very small amount of effective Ti, and thereafter the effect is saturated immediately. On the other hand, the machinability is moderately lowered, and when the effective Ti amount is extremely small, the machinability is hardly lowered. Therefore, by using Ti that can secure an effective Ti amount necessary for sharply increasing the fracture splitting ability within the minimum necessary range, both the fracture splitting ability and the machinability can be improved.

The effective Ti amount (f value) is given by the following formula (1). In order to reliably improve the fracture splitting property, the effective Ti amount (f value) is 0.000 or more, preferably 0.001 or more, and more preferably 0.002 or more. However, when the effective Ti amount (f value) increases, the amount of Ti added increases, and the machinability tends to decrease. Therefore, the effective Ti amount (f value) is preferably 0.1 or less, more preferably 0.05 or less, and particularly 0.03 or less.
f = [Ti] − [N] × 48/14 (1)
[In the formula, [Ti] and [N] indicate the contents (mass%) of Ti and N in the steel, respectively. ]

  In the most preferable aspect, the upper limit of the effective Ti amount (f value) and the upper limit of the Ti content in the steel are possible after ensuring the above-mentioned lower limit value of the effective Ti amount (f value) to ensure fracture splitting. As much as possible. By doing in this way, machinability can be most enhanced while surely improving break splitting property. When the effective Ti amount (f value) and the Ti content in steel are most narrowed down, the effective Ti amount (f value) is 0.010 or less (particularly 0.008 or less), and the Ti content in steel is 0.03% or less. (Especially 0.025% or less).

  Further, in the steel for connecting rods of the present invention, it is preferable to reduce the aspect ratio of sulfide inclusions (for example, MnS). Sulfide inclusions are stretched in the rolling direction or the forging direction by rolling or hot forging. If this stretched sulfide inclusion is present in the longitudinal direction (stretched in the direction perpendicular to the fracture split surface) during the fracture splitting of the steel, as the crack progresses, the sulfide inclusions are placed between the sulfide inclusion and the metal matrix. Peels and stress relaxation occurs. As a result, brittle fracture is inhibited, the toughness ductility value is improved, and the fracture splitting property is lowered. In contrast, when the sulfide inclusions are restrained from being stretched and the aspect ratio is reduced to a spheroidized shape, the cracks at the tip of the cracks generated around the sulfide inclusions are divided when the fracture is broken at the longitudinal direction. Stress increases and brittle fracture is promoted. As a result, the amount of plastic deformation can be reduced, and the fracture splitting property of steel is improved. Further, the effect of improving the breakability by the spheroidization of the sulfide inclusions is exhibited when the width of the sulfide inclusions is 1 μm or more. If the width of the sulfide inclusion is too small, the sulfide inclusion itself breaks, and it becomes impossible to promote brittle fracture of the steel.

The size and form of the preferred sulfide inclusions are quantitatively expressed as follows. That is, in the steel of the present invention, there are 100 or more sulfide inclusions per 1 mm ² in a longitudinal section at a position D / 4 (D is the thickness or diameter of the steel) from the steel surface. The arithmetic average value (average aspect ratio) of the aspect ratio (length / width) of the sulfide inclusions having a width of 1 μm or more is 15 or less.

  The average aspect ratio is preferably 10 or less, more preferably 8 or less, and particularly 6 or less. The average aspect ratio is preferably closer to 1, and the lower limit is not particularly limited, but may be 2 or more (or 3 or more).

The number of sulfide inclusions having a width of 1 μm or more is preferably 300 or more, more preferably 400 or more, per 1 mm ² . However, when the number of sulfide inclusions increases, harmful effects such as cracking are likely to occur during rolling or hot forging. Accordingly, it is recommended that the sulfide inclusions having a width of 1 μm or more be, for example, 4000 or less, preferably 3000 or less, more preferably 2500 or less per 1 mm ² .

The “sulfide inclusion” in the present invention mainly means MnS, but also includes other sulfides and composite sulfides. Further, the width and average aspect ratio (length / width) of the sulfide-based inclusions, and the value of the number thereof are 1 mm in the longitudinal section at a position of D / 4 (D is the thickness or diameter of the steel) from the steel surface. ^This is a value obtained by observing the observation field ² with an optical microscope at an observation magnification of 1000 times.

  The size and form of sulfide inclusions are within a predetermined range by appropriately setting rolling conditions according to the amount of Mn, S and inclusion spheroidizing elements (Ca, Mg, Zr, etc.) added. Can be controlled. Regarding the rolling conditions, it is recommended to select the rolling start temperature from a range of 1000 ° C. or higher and the rolling end temperature from a range of 850 ° C. or higher. The higher the rolling start temperature and the rolling end temperature, the smaller the aspect ratio of the sulfide inclusions, and the easier it is to satisfy the predetermined value.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental example 1
Steel having the chemical composition shown in Table 1 was melted in accordance with an ordinary melting method, cast and divided, and then rolled at a start temperature of 1050 ° C. and an end temperature of 900 ° C. to obtain a 50 mmφ bar steel.
The characteristics of the obtained steel bar were examined as follows.

(1) Sulfide inclusions A 1 mm ² field of view is observed with an optical microscope (1000 times) in a longitudinal section at a position D / 4 (D is the diameter) from the surface of the steel bar. I counted the number of objects. Further, the aspect ratio of sulfide inclusions having a width of 1 μm or more was measured, and the arithmetic average value thereof was obtained.

(2) Breakability (dimensional change)
The steel bar obtained in the experimental example was cut to an appropriate length, heated to 1200 ° C., flattened to a thickness of 25 mm, forged, and then air-cooled. The obtained flat plate was cut and processed into a test piece as shown in FIG. In FIG. 3, (a) is a top view of the test piece, (b) is a side view of the test piece, a is a notch, b is a bolt hole, and c is an arrow indicating the rolling direction. The test piece has a plate shape of 65 mm × 65 mm × thickness 22 mm, and a cylindrical hole of φ43 mm is extracted at the center. A notch a (R 0.2 mm, depth 0.5 mm) is provided at the end of the central hole. The test piece is provided with a bolt hole b (φ8.3 mm) along the rolling direction.

  As shown in FIG. 4, the test piece 6 was set in a press tester (1600 t press) through the holders 3 a and 3 b in the center hole of the test piece 6, and the test piece was broken and divided at a press speed of 270 mm / s. The breaking speed of the test piece is calculated to be about 150 mm / s because the wedge angles of the wedges 4 and 5 are 30 °. And as shown in FIG. 5, the hole diameter difference (L2-L1) before and after the fracture splitting was measured as a dimensional change, and those having a dimensional change of 0.15 mm or less were evaluated as having excellent split splitting ability. The standard of dimensional change of 0.15 mm or less is equivalent to that of C7056 of DIN standard used in Europe.

(3) Machinability (tool life)
After milling the cut surface of the steel bar obtained in the experimental example, the milled surface was drilled under the following conditions, and the distance (total length) worked until the tool broke or melted was measured. .
Cutting tool: SKH51 (φ10 straight drill)
Cutting speed: 30 m / min
Feed: 0.15mm / rev
Hole depth: 30mm
Lubrication state: Dry type Hole location: D / 4 (D is the diameter of the steel bar)

The working distance L of each steel bar was arranged as a relative value with reference to the working distance L _A1 of A1 steel in Table 1, and the tool life was evaluated.
Tool life = L / L _A1
The results are shown in Table 1 and FIGS.

  As is clear from Table 1, FIG. 1, and FIG. 2, both the fracture splitting property and machinability can be improved by reducing the Ti addition amount while securing the effective Ti amount.

Experimental example 2
The same procedure as in Experimental Example 1 was performed except that steels having chemical compositions shown in Tables 2 and 3 were used.
The results are shown in Tables 2 and 3.

  Steel type B1 has deteriorated fracture splitting property (dimensional change) because the amount of C is too small. However, since other steel components have appropriate components, both the fracture splitting property and machinability could be improved.

Experimental example 3
The same procedure as in Experimental Example 1 was conducted except that the steel type A5 shown in Table 1 was used and the rolling start temperature and rolling end temperature were changed as shown in Table 4 below.
The results are shown in Table 4.

  As is apparent from Table 4, the aspect ratio of the sulfide inclusions can be reduced as the rolling start temperature and the rolling end temperature are increased.

FIG. 1 is a graph showing the relationship between fracture splitting property and machinability when the effective Ti amount is changed. FIG. 2 is a graph showing the relationship between the effective Ti amount and fracture splitting property or machinability. FIG. 3 (a) is a schematic top view of a test piece used in the fracture splitting test, and FIG. 3 (b) is a schematic side view of the test piece. FIG. 4 is a schematic view of the apparatus for explaining the method of the fracture split test. FIG. 5 is a schematic top view of the test piece before and after the fracture split test.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Press 2 Support stand 3a, 3b Holder 4, 5 Wedge 6 Test piece

Claims (5)

C: 0.15-0.3% (meaning mass%, the same shall apply hereinafter)
Si: 2% or less (excluding 0%),
Mn: 0.6-2%
Al: 0.06% or less (excluding 0%),
P: 0.15% or less (excluding 0%),
S: 0.15% or less (excluding 0%),
Cr: 2% or less (excluding 0%),
V: more than 0.20%, 0.5% or less,
Ti: 0.01 to 0.10%, and N: 0.007% or less (excluding 0%)
The balance consists of iron and inevitable impurities,
F value represented by following formula (1) is 0.000 or more,
In the longitudinal section at a position D / 4 (D is the thickness or diameter of the steel) from the steel surface, there are 100 or more sulfide inclusions per 1 mm ² with a width of 1 μm or more, and this sulfide with a width of 1 μm or more. A fracture split type connecting rod steel excellent in fracture splitting property and machinability, characterized in that the average aspect ratio (length / width) of the system inclusions is 15 or less.
f = [Ti] − [N] × 48/14 (1)
[In the formula, [Ti] and [N] indicate the contents (mass%) of Ti and N in the steel, respectively. ]   The steel for fracture split type connecting rod according to claim 1, wherein Ti is 0.03% or less and f value is 0.010 or less.   Further, Ca: 0.005% or less (not including 0%), Mg: 0.005% or less (not including 0%), and Zr: 0.1% or less (not including 0%) The steel for fracture split type connecting rods according to claim 1 or 2, comprising at least one selected.   The fracture split type connecting rod according to any one of claims 1 to 3, comprising 0.005% or less (excluding 0%) of Ca and 0.01% or less (not including 0%) of Al. steel.   Furthermore, Nb: 0.05% or less (not including 0%), Mo: 0.05% or less (not including 0%), Cu: 0.2% or less (not including 0%), and Ni: 0 The fracture split type connecting rod steel according to any one of claims 1 to 4, comprising at least one selected from the group consisting of .2% or less (excluding 0%).

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