JP2004190049A - Pb-FREE STEEL WITH EXCELLENT BREAKING AND SPLITTING PROPERTY AND MACHINABILITY - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide Pb-free steel having excellent internal cracking suppressing property in continuous casting and also having superior breaking and splitting property and machinability. <P>SOLUTION: The Pb-free steel has a composition which contains 0.2 to 0.6% C, ≥0.1% V, 0.5 to 2% Mn, 0.03 to 0.1% S, ≥0.0003% Ca, ≥0.0003% Mg, and 0.01 to 0.3% Bi and in which the mass ratio between the above Mn and S, Mn/S, is made to ≥17.5. It is desirable that this steel contains ≤0.6% (not including 0%) Si, ≤0.1% (not including 0%) P, ≤0.05% (not including 0%) Al, and ≤0.02% (not including 0%) N, and further contains 0.01 to 1% Cr. Moreover, it is desirable that the carbon equivalent Ceq defined by equation Ceq=[C]+0.28×[Mn]-1.03×[S]+0.323×[Cr]+1.69×[V] is made to ≥0.8. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００５５】
表１及び図１より明らかなように、Ｐｂ添加鋼などのようにＢｉ無添加では、割れが生じない（Ｎｏ．１６，１７参照）。一方、Ｐｂ無添加としＢｉを添加したときには、比率Ｍｎ／Ｓが小さいと、割れ長さが長くなる（Ｎｏ．１０〜１５参照）。そこで比率Ｍｎ／Ｓを大きくすると、急激に割れが短くなった（図１、Ｎｏ．１〜９参照）。またこのＮｏ．１〜９は、Ｎｏ．１６〜１７のＰｂ無添加鋼と同等のドリル寿命を示す。
【００５６】
さらに図２より明らかなように、Ｃｅｑを大きくすれば引張強さも大きくできる。
【００５７】
【発明の効果】
本発明のＰｂフリー鋼は、Ｃ及びＶを含有しておりかつＭｇ及びＣａによってＭｎＳが球状化されているため破断分割性に優れており、Ｓ、Ｃａ、Ｍｇ及びＢｉを含有しているため被削性に優れており、Ｍｎ／Ｓ比が所定の範囲に制御されてＭｎＳ比率が高められているために連続鋳造によって製造しても内部割れが抑制されている。
【図面の簡単な説明】
【図１】図１はＭｎ／Ｓ比と割れ長さとの関係を示すグラフである。
【図２】図２はＣｅｑと引張強さとの関係を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to a Pb-free steel (Pb-free steel) useful for manufacturing a steel part requiring break splittability and machinability such as a split-split type connecting rod (commonly referred to as a split connecting rod). It is.
[0002]
[Prior art]
The connecting rod (abbreviation: connecting rod) is an automobile part that transmits the movement of the piston to the crank, and is composed of two parts, a connecting rod body (abbreviation: connecting rod body) and a connecting rod cap (abbreviation: connecting rod cap). The connecting rod is attached to the crank by fitting the connecting rod body and the connecting rod cap across the crank.
[0003]
Conventionally, the connecting rod body and the connecting rod cap are separately manufactured by forging or the like. However, the connecting rod body and the connecting rod cap manufactured in this manner require precise machining of the fitting surface in order to enhance the fitting property. In addition, pin processing is often performed so that the mating surfaces do not shift, and the manufacturing is complicated.
[0004]
2. Description of the Related Art In recent years, a connecting rod has been manufactured by integrally forming a connecting rod body and a connecting rod cap by forging or the like, applying an impact to the integrally formed body, and dividing (breaking) the connecting rod body and the connecting rod cap. (For example, see Patent Document 1), in this specification, the connecting rod obtained in this manner is referred to as a split-split type connecting rod (commonly referred to as a split connecting rod).
[0005]
[Patent Document 1]
JP 2000-73141 A (Prior Art)
[0006]
[Problems to be solved by the invention]
It is important for the split connecting rod to enhance the fracture splitting property [fitting property of the fracture splitting surface (fracture surface), low deformability (roundness) at the time of fracture splitting]. In the split connecting rod, a portion (a hole portion) sandwiching the crank is formed by cutting after forging and before splitting. Therefore, it is also important to have excellent machinability. Conventionally, Pb-added steel has been frequently used to improve machinability, but Pb-free steel has recently been demanded from the viewpoint of environmental considerations.
[0007]
Therefore, the present inventors have started studies to improve the fracture splitting property and machinability of Pb-free steel.
[0008]
The present inventors first designated C and V as essential addition elements for the above purpose. This is because C and V are important for reducing the toughness of the steel and increasing the fracture splitting ability. However, when C is added, hard cementite is increased, thereby reducing the machinability of steel. Therefore, S was added to compensate for the machinability. This is because S is important in increasing machinability instead of Pb. However, when S is added, Mn is combined with Mn normally present in steel to form MnS. This MnS is elongated in the rolling direction when producing the raw steel slab of the split connecting rod, and therefore extends in a direction perpendicular to the breaking direction (thickness direction of the integrally formed body) before the breaking split. . Therefore, even if an attempt is made to divide the fracture, the progress of the rupture is blocked by the MnS, and the fracturability is reduced. Therefore, even if S is added, the amount of use must be suppressed in order to maintain the fracture splitting property.
[0009]
Therefore, together with the addition of S, Ca and Mg were also added. Ca and Mg are themselves useful for improving machinability, and also have the effect of reducing the anisotropy (aspect ratio) of MnS. Therefore, it is also useful for improving the fracture splitting property. However, even if Ca and Mg are excessively added, the machinability improving effect is saturated, so that the addition of a further machinability improving element is required.
[0010]
Therefore, the present inventors focused on the addition of Bi. This is because Bi is also an element effective for improving machinability.
[0011]
However, it has been found that when Bi is added to steel containing predetermined amounts of C, Mn, V, S, Ca, and Mg, internal cracks occur when the Bi-added steel material is manufactured by continuous casting.
[0012]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a Pb-free steel excellent in continuous casting internal crack suppressing ability, and excellent in fracture splitting and machinability. It is in.
[0013]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, in steels containing a predetermined amount of C, Mn, V, S, Ca, Mg and Bi, the ratio of Mn to S (Mn / S The present inventors have found that there is a high correlation between S) and internal cracks, and that when the ratio Mn / S is set to 17.5 or more, the number of internal cracks sharply decreases, thereby completing the present invention.
[0014]
That is, the Pb-free steel of the present invention, which can achieve the above object and has excellent fracture splitting and machinability, is expressed by mass% (hereinafter the same),
C: 0.2-0.6%,
V: 0.1% or more,
Mn: 0.5-2%,
S: 0.03-0.1%,
Ca: 0.0003% or more,
Mg: 0.0003% or more,
Bi: contains 0.01 to 0.3%,
The gist is that the mass ratio (Mn / S) of Mn to S is 17.5 or more. The Pb-free steel desirably contains Cr in an amount of about 0.01 to 1% in order to increase proof stress and fatigue strength. The Pb-free steel usually contains Si, P, Al, and N in the following ranges.
[0015]
Si: 0.6% or less (excluding 0%)
P: 0.1% or less (excluding 0%)
Al: 0.05% or less (excluding 0%)
N: 0.02% or less (excluding 0%)
The Pb-free steel of the present invention preferably has a carbon equivalent Ceq defined by the following formula of 0.8 or more. The larger the Ceq is, the higher the tensile strength can be.
[0016]
Ceq = [C] + 0.28 × [Mn] −1.03 × [S] + 0.323 × [Cr] + 1.69 × [V]
[Wherein [C], [Mn], [S], [Cr], and [V] indicate the contents (% by mass) of C, Mn, S, Cr, and V in the steel, respectively]
The present invention also includes a fracture split type connecting rod obtained from the Pb-free steel.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
The Pb-free steel of the present invention has substantially no internal cracks even when manufactured by continuous casting, and is excellent in fracture splitting properties and machinability. Therefore, the Pb-free steel of the present invention can be used to produce steel products (fracture split type connecting rods, etc.) obtained by performing deformation processing (hot working processing such as hot forging), cutting processing, fracture splitting processing, and the like. Useful for
[0018]
Hereinafter, the present invention will be described in detail. First, the Pb-free steel of the present invention contains C, V, Mn, S, Ca, Mg, and Bi. The reasons for adding and the contents of these elements are as follows.
[0019]
C: 0.2 to 0.6% (meaning by mass%; hereinafter the same)
C is an element that is useful in securing the strength of a steel product (for example, a connecting rod) and enhancing fracture splitting properties. Therefore, the content of C is about 0.2% or more, preferably 0.25% or more, and more preferably 0.30% or more. On the other hand, when C is excessive, the machinability of steel decreases. Therefore, the content of C is about 0.6% or less, preferably about 0.5% or less, and more preferably about 0.4% or less.
[0020]
V: 0.1% or more V is also a useful element in securing the strength of the steel product and in improving the fracture splitting property. Therefore, the content of V is about 0.1% or more, preferably 0.15% or more, and more preferably 0.20% or more. On the other hand, even if V is excessively added, the effect of the addition is saturated. The upper limit of V is not particularly limited as long as it is a common sense level, but is preferably about 0.5% or less, more preferably about 0.4% or less, and particularly about 0.35% or less.
[0021]
Mn: 0.5-2%
Mn is an element effective as a deoxidizing and desulfurizing element at the time of steel smelting, and also increases the pearlite hardenability of a forged product to increase the amount of pearlite, thereby reducing the lamellar interval in pearlite, thereby reducing the steel product. Since it is an element effective for increasing the proof stress and the fatigue strength, it is usually contained in steel. The content of Mn is about 0.5% or more, preferably about 0.6% or more, and more preferably about 0.65% or more. On the other hand, when Mn is excessively added, bainite is generated in the metal structure after the steel is hot forged and cooled, so that the machinability and fracture splitting properties of the steel are reduced. Therefore, the content of Mn is set to about 2% or less, preferably about 1.8% or less, and more preferably about 1.7% or less.
[0022]
S: 0.03 to 0.1%
S is an element particularly useful for improving the machinability of Pb-free steel. Therefore, the content of S is 0.03% or more, preferably 0.035% or more, and more preferably 0.038% or more. On the other hand, when a large amount of S is added, the amount of MnS generated increases. This MnS is elongated in the rolling direction when producing a billet. Accordingly, the steel material is hot-forged into the outer shape of the connecting rod (approximately a club shape), and a hole for passing a crank is formed in the large diameter portion to form a rough connecting rod. When manufacturing the connecting rod by breaking the hole part of the rough connecting rod along the orthogonal plane, the MnS is elongated in a direction substantially perpendicular to the breaking direction. Lower. Therefore, the content of S is set to about 0.1% or less, preferably about 0.09% or less, and more preferably about 0.085% or less.
[0023]
Ca: 0.0003% or more
Mg: 0.0003% or more Ca and Mg form sulfide-based inclusions and form a composite oxide with Al, Si, and the like, and are useful for improving machinability. Is an essential element in order to further improve the machinability of the steel of the present invention, which cannot be added to steel. In addition, Ca and Mg reduce the anisotropy of MnS (spheroidize), and are therefore useful for restoring the fracture splitting property that is reduced by the addition of S. In addition, when Ca or Mg is added alone, large MnS cannot be spheroidized, whereas when Ca and Mg are added in combination, MnS is finely dispersed by the action of Mg, and Ca is extremely spheroidized. Since it effectively contributes, even large MnS can be made spherical, and the fracture splitting property can be remarkably recovered. Therefore, the Ca content is 0.0003% or more, preferably 0.0004% or more, and more preferably about 0.0005% or more, and the Mg content is 0.0003% or more, preferably 0.0004%. Above, more preferably 0.0005% or more. On the other hand, even if Ca and Mg are added in large amounts, the effect of the addition is saturated. The upper limit of the content of Ca and Mg is not particularly limited as long as it is a common sense level, but preferably the content of Ca is about 0.01% or less (for example, about 0.005% or less, particularly about 0.003% or less). ), And the Mg content is about 0.01% or less (for example, about 0.005% or less, particularly about 0.003% or less).
[0024]
Bi: 0.01 to 0.3%
Bi is also useful for improving machinability. In particular, in the present invention, in which Ca and Mg are added, in which a large amount of the free-cutting element S cannot be added from the viewpoint of fracture splitting, and the effect of improving the machinability is saturated, the addition of Bi It is important to further increase The Bi content is about 0.01% or more, preferably about 0.02% or more, more preferably about 0.025% or more, and particularly about 0.03% or more (for example, about 0.04% or more). On the other hand, when Bi is added excessively, the hot workability of the steel material is deteriorated. Therefore, the content of Bi is set to about 0.3% or less, preferably about 0.2% or less, and more preferably about 0.15% or less.
[0025]
The steel of the present invention does not substantially contain Pb, but usually contains Si, P, Al, and N. The contents of Si, P, Al and N are not particularly limited as long as they are at a common sense level, but are preferably as follows.
[0026]
Si: 0.6% or less (excluding 0%)
Si is an element that remains in steel because it is added for deoxidation during steelmaking. When the Si content is excessive, the workability of the steel material is deteriorated. Therefore, the Si content is, for example, about 0.6% or less, preferably about 0.5% or less, and more preferably about 0.4% or less. And Note that Si is useful for strengthening a steel product by forming a solid solution in a ferrite ground of a steel material, and thus may be positively left. In this case, the content of Si is, for example, about 0.01% or more, preferably 0.05% or more, and more preferably 0.1% or more.
[0027]
P: 0.1% or less (excluding 0%)
P is an impurity inevitably remaining in the steel material, and if its content is large, the workability of the steel material is reduced. Therefore, the content of P is, for example, about 0.1% or less, preferably about 0.07% or less, and more preferably about 0.05% or less. Note that P segregates at the grain boundary to reduce toughness, and is therefore useful for improving fracture splitting properties. From this viewpoint, P may be left in a predetermined amount or more. For example, the content of P is about 0.01% or more, preferably about 0.015% or more, and more preferably about 0.018% or more.
[0028]
Al: 0.05% or less (excluding 0%)
Al is an element that remains in steel because it is added for deoxidation during steelmaking. If the Al content is excessive, the workability of the steel material is deteriorated. Therefore, the Al content is, for example, about 0.05% or less, preferably about 0.03% or less, and more preferably about 0.01% or less. And In addition, Al is useful not only for stabilization and homogenization of steel deoxidation, but also for generating nitrides to refine crystal grains and increasing the strength of steel products. It may be left. In this case, the Al content is, for example, about 0.0005% or more, preferably about 0.001% or more, and more preferably about 0.003% or more.
[0029]
N: 0.02% or less (excluding 0%)
N is an unavoidable impurity in the steel material, and if its content is large, the workability of the steel material may be adversely affected. Therefore, the content of N is, for example, about 0.02% or less, preferably about 0.015% or less, and more preferably about 0.010% or less. N forms a nitride with V and Al, and is effective in securing ferrite strength and preventing crystal grains from becoming coarse. From this viewpoint, N may be left in a predetermined amount or more. For example, the content of N is preferably about 0.001% or more, preferably about 0.002% or more, and more preferably about 0.0025% or more. desirable.
[0030]
The steel of the present invention is preferably further added with Cr, and the balance is desirably Fe and inevitable impurities. If necessary, at least one of Nb, Ti, and rare earth metal (REM) may be used. May be added, and at least one of Zr, Te, and other elements may be added within a range that does not adversely affect the present invention.
[0031]
Cr, like Mn, is an element effective in improving pearlite hardenability and improving the proof stress and fatigue strength of steel products. When Cr is added (when it exceeds 0%), the preferable content is, for example, about 0.01% or more, more preferably 0.02% or more, and particularly about 0.025% or more. On the other hand, when Cr is added excessively, bainite is generated in the metal structure, which adversely affects the machinability. Therefore, the amount of Cr added is about 1% or less, preferably about 0.5% or less, and more preferably about 0.2% or less.
[0032]
Nb is an element that forms fine carbides and nitrides and precipitates on ferrite ground, and is effective in improving the proof stress and fatigue strength of steel products. When Nb is added, its content is, for example, about 0.15% or less (excluding 0%).
[0033]
Ti is an element effective for preventing cracking during processing. When Ti is added, its content is, for example, about 0.1% or less (not including 0%).
[0034]
REM is an effective element for further improving machinability. When REM is added, its content is, for example, about 0.3% or less (not including 0%).
[0035]
Zr is desirably not added because it has a strong affinity with N and lowers the precipitation strengthening ability of nitrides such as VN. However, Zr is an element effective for preventing cracking during processing and is also effective for spheroidizing MnS. Therefore, Zr may be added to the extent that the above-mentioned disadvantage does not occur.
[0036]
Te lowers the hot ductility at a temperature of 900 to 1100 ° C., so that surface defects occur when a continuously cast piece is hot-rolled to produce a steel slab. Therefore, it is desirable not to add Te. However, Te is effective in improving machinability and is also effective in spheroidizing MnS. Therefore, Te may be added in a range that does not cause the inconvenience.
[0037]
The steel of the present invention described above is characterized in that it contains C, V, Mn, S, Ca, Mg, and Bi in order to achieve both fracture splitting and machinability in a Pb-free steel. However, in a steel containing predetermined amounts of C, V, Mn, S, Ca, and Mg, if Bi, which is an element for ensuring sufficient machinability, is added, internal cracking occurs when steel is manufactured by continuous casting. May occur.
[0038]
Therefore, in the present invention, the contents of Mn and S are selected from the above range so that the mass ratio of Mn to S (Mn / S) in the steel is 17.5 or more. In steels containing predetermined amounts of C, V, Mn, S, Ca, Mg, and Bi, there is an excellent correlation between the occurrence of internal cracks during continuous casting and the Mn / S ratio, and When the Mn / S ratio is set to 17.5 or more, internal cracks rapidly decrease.
[0039]
It is considered that the reason why the internal crack can be suppressed by increasing the Mn / S ratio is as follows. That S is FeS, MnS, FeS-MnS, forming sulfide inclusions such as FeS-MnS-SiO _2, in these inclusions may have low melting point, also thickening precipitated at grain boundaries Therefore, it has a tendency to easily cause internal cracks during continuous casting. When Bi is further added to steel containing predetermined amounts of C, V, Mn, S, Ca, and Mg, it is considered that internal cracks actually occurred. However, among the sulfide-based inclusions, FeS and FeS—MnS—SiO ₂ are particularly prone to generate internal cracks, whereas MnS is relatively hard to generate internal cracks. It is considered that increasing the MnS ratio in the sulfide-based inclusions can suppress internal cracks. When the MnS ratio is simply increased, as described above, MnS causes a decrease in fracture splitting properties.However, in the present invention, MnS is made spherical by adding Mg and Ca in combination, and when the MnS ratio is increased. Is also prevented.
[0040]
The Mn / S ratio is preferably at least 18, more preferably at least 19, especially at least 20. As the Mn / S ratio is increased, variation in the results can be eliminated, and internal cracks can be reliably reduced. The upper limit of the Mn / S ratio is naturally determined by the maximum content of Mn and the minimum content of S, but is usually 50 or less, for example, 45 or less, and particularly about 40 or less.
[0041]
In addition, the steel of the present invention has C, Mn, S, Cr, and V so that the carbon equivalent Ceq defined by the following formula is 0.8 or more, preferably 0.9 or more, and more preferably 0.95 or more. Is selected from the above range. As described above, Cr may not be added in some cases. The higher the Ceq, the higher the tensile strength of the steel product. On the other hand, if the Ceq is too high, the machinability tends to decrease. Therefore, the Ceq is, for example, about 1.7 or less, preferably 1.6 or less, more preferably 1.5 or less, and especially about 1.4 or less. Is desirable.
[0042]
Ceq = [C] + 0.28 × [Mn] −1.03 × [S] + 0.323 × [Cr] + 1.69 × [V]
[Wherein [C], [Mn], [S], [Cr], and [V] indicate the contents (% by mass) of C, Mn, S, Cr, and V in the steel, respectively]
When Ceq is controlled in the above range, the strength of the Pb-free steel is, for example, 800 MPa or more (preferably about 800 to 1400 MPa).
[0043]
The Pb-free steel of the present invention can be obtained in the form of a slab, a slab, or a bar. Such Pb-free steel is prepared by continuously casting molten steel whose composition has been adjusted by a conventional method (acquisition of a slab) and, if necessary, hot slab rolling (acquisition of a slab) and further rolling (acquisition of a steel bar). ). The continuous casting conditions can be appropriately selected from conventional conditions. For example, the continuous casting is often performed under the following conditions.
[0044]
Casting speed: 0.5 to 1.2 m / min Specific water amount (= cooling water amount / total slab amount): 0.15 to 0.7 L / kg
The Pb-free steel of the present invention obtained as described above has excellent fracture splitting properties because it contains C and V, and has excellent machinability because it contains S, Ca, Mg and Bi. Because the Mn / S ratio is controlled within a predetermined range and the MnS ratio is increased, internal cracks are suppressed even when manufactured by continuous casting, and Ca and Mg are added in a complex manner. Even when the MnS ratio is increased, the fracture splitting property is not impaired. Therefore, it is extremely useful for manufacturing a steel product (for example, a split connecting rod or the like) that requires high machinability and high fracture splitting property.
[0045]
The split connecting rod (fracture split type connecting rod) can be manufactured, for example, as follows. That is, after the Pb-free steel is made into a steel bar, the outer shape of the connecting rod is formed by hot working (hot forging or the like), and a hole for passing a crank is formed by cutting to obtain a coarse connecting rod. By breaking and dividing the hole portion of the rough connecting rod, a split connecting rod can be obtained.
[0046]
The split connecting rod thus obtained is excellent in break splitting properties. In addition, since it contains C and V, it is excellent in strength, and particularly when Ceq is controlled in a predetermined range, it has high strength.
[0047]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to the following Examples, and may be appropriately modified within a range that can conform to the purpose of the preceding and the following. It is, of course, possible to implement them, and all of them are included in the technical scope of the present invention.
[0048]
Experimental example 1
Steel having the chemical components shown in Table 1 was melted in a converter according to a conventional method, and then cast by a continuous casting machine under the following conditions. The obtained slab was hot slab-rolled and then hot-rolled to produce a steel bar having a diameter of 50 mm.
[0049]
[Continuous casting conditions]
Casting speed: 0.90 m / min Specific water volume: 0.38 L / kg
The length of the crack generated inside the slab, and the drill life and tensile strength of the steel bar were measured as follows.
[0050]
[Crack length]
In order to observe the cross section (430 mm x 300 mm) of the slab, the slab was slice-cut to a thickness of 10 mm to obtain a test piece. A macro test (macro corrosion) of the cross section of this test piece was performed according to the hydrochloric acid method specified in JIS G0553. The portion corroded black in the cross section was regarded as a flaw (crack), and its length (if there are a plurality of flaws, the length of the longest one) was measured.
[0051]
[Drill life]
The bar was cut into a length of 50 mm, heated to a temperature of 1200 ° C., held for 30 minutes, and air-cooled. The obtained test steel was drilled with a drill, and the processing distance until the tool was broken or melted was measured. The cutting conditions are as shown below.
[0052]
Tool: SKH51 (φ10sd)
Cutting speed: 30m / min Feed: 0.21mm
Lubrication: dry type [tensile strength]
The bar was cut to prepare a No. 4 test piece specified in JIS Z2201, and the tensile strength was measured in accordance with JIS Z2241.
[0053]
The results are shown in Table 1 and FIGS.
[0054]
[Table 1]

[0055]
As is clear from Table 1 and FIG. 1, cracking does not occur when Bi is not added, such as Pb-added steel (see Nos. 16 and 17). On the other hand, when Bi is added without adding Pb, if the ratio Mn / S is small, the crack length becomes longer (see Nos. 10 to 15). Therefore, when the ratio Mn / S was increased, the cracks were sharply shortened (see FIG. 1, Nos. 1 to 9). This No. Nos. 1 to 9 are Nos. It shows the same drill life as Pb-free steel of No. 16-17.
[0056]
Further, as is clear from FIG. 2, the tensile strength can be increased by increasing Ceq.
[0057]
【The invention's effect】
Since the Pb-free steel of the present invention contains C and V and MnS is spheroidized by Mg and Ca, it is excellent in fracture splitting property and contains S, Ca, Mg and Bi. It has excellent machinability, and the Mn / S ratio is controlled within a predetermined range to increase the MnS ratio, so that internal cracks are suppressed even when manufactured by continuous casting.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the Mn / S ratio and the crack length.
FIG. 2 is a graph showing the relationship between Ceq and tensile strength.

Claims (5)

Mass% (hereinafter the same),
C: 0.2-0.6%,
V: 0.1% or more,
Mn: 0.5-2%,
S: 0.03-0.1%,
Ca: 0.0003% or more,
Mg: 0.0003% or more,
Bi: contains 0.01 to 0.3%,
A Pb-free steel excellent in fracture splitting and machinability, wherein the mass ratio of Mn to S (Mn / S) is 17.5 or more. The Pb-free steel according to claim 1, further containing Cr: 0.01 to 1%. Si: 0.6% or less (excluding 0%),
P: 0.1% or less (excluding 0%),
Al: 0.05% or less (excluding 0%),
The Pb-free steel according to claim 1 or 2, wherein N: contains 0.02% or less (excluding 0%). The Pb-free steel according to any one of claims 1 to 3, wherein a carbon equivalent Ceq defined by the following formula is 0.8 or more.
Ceq = [C] + 0.28 × [Mn] −1.03 × [S] + 0.323 × [Cr] + 1.69 × [V]
[In the formula, [C], [Mn], [S], [Cr], and [V] indicate the contents (% by mass) of C, Mn, S, Cr, and V in steel, respectively] A breaking split type connecting rod obtained from the Pb-free steel according to claim 1.

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