JP2004183094A - Nonheat-treated steel easily fractured and separated at low temperature, and engaged member fractured and separated at low temperature - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonheat-treated steel which has suitable toughness within a use temperature range, and is easily fractured and separated not only in the conventional low temperature region including a cryogenic region for liquid nitrogen cooling or the like required by the object to be fractured, but also in an inexpensively reachable low temperature region, and to provide an engaged member fractured and separated at low temperatures. <P>SOLUTION: The nonheat-treated steel easily fractured and separated at low temperatures has a composition comprising, by mass, 0.15 to 0.35% C, 0.5 to 2.0% Si, 0.5 to 1.5% Mn, 0.03 to 0.15% P, 0.01 to 0.15% S, 0.01 to 0.5% Cu, 0.01 to 0.5% Ni, 0.01 to 1.0% Cr, 0.001 to 0.01% s-Al, 0.005 to 0.035% N, 0.0001 to 0.01% Ca and 0.001 to 0.01% O, and satisfying 0.6≤Ceq≤0.85 and 0≤T<SB>Tr</SB>≤1.5, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a non-heat-treated steel that is easy to break and separate at low temperature and is suitable for a joint member used for break-separation into two or more parts after forging and a connecting rod for an engine made of this non-heat-treated steel that breaks and separates at low temperature. And the like.

  Conventionally, for mating members such as parts that are separated into two parts after forging, such as connecting rods for engines (connecting rods), and are connected to the crankshaft, after forging into the final shape, finish machining is performed. After that, it was divided into two parts by machining. However, this manufacturing method requires an extra material as a cutting allowance in a cut portion, and it is necessary to cut the separation surface after cutting and finish it by polishing or the like, which causes an increase in cost.

  In order to solve these problems, in the case of a connecting rod, a method has been proposed in which the connecting rod is processed into a final shape and then divided by fracture separation. This fracture separation is performed by forming a cutout groove 4 in the large end 2 of the connecting rod 1 as shown in FIG. 1 (A) and then applying a load at room temperature to form the large end as shown in FIG. 1 (B). This is a method in which the cap part 5 and the rod part 6 are broken and divided. In order to carry out this method, a material having low ductility at room temperature is required in order to suppress deformation at the time of fracture separation and to enable easy division. In order to satisfy this requirement, a material in which the contents of Si, V and P are adjusted to suppress the toughness at room temperature (see steel that breaks and separates at room temperature in FIG. 2) has been developed (for example, Patent Document 1). And Patent Document 2.).

  However, in general, when a component such as a connecting rod is designed and machined with steel that can be easily broken without being deformed only by forming the above-described notch groove, the influence of existing defects such as slight notches can be sufficiently reduced. There is a problem that the weight must be taken into consideration as a result. Further, since a large amount of expensive V needs to be added, there is a problem that the merit of reducing the cost is reduced.

Therefore, a method has been proposed in which fracture separation is performed at a low temperature by utilizing the low-temperature brittleness phenomenon of steel irrespective of the component composition of the alloy (for example, see Patent Document 3). According to this method, the connecting rod has sufficient toughness at the operating temperature, and the connecting rod can be embrittled only at the time of fracture separation. However, in the case of ordinary steel materials, it is necessary to cool to −130 ° C. or less in order to perform fracture separation (see FIG. 2), and it is necessary to use liquid nitrogen (−196 ° C.) as a cooling medium for the cooling. Therefore, there is a problem that the cost for cooling is very high.
JP-A-9-111412 JP-A-10-219389 JP 2001-3924 A

  The present invention has an appropriate toughness within the operating temperature range, and breaks apart not only in the cryogenic region by liquid nitrogen cooling and the like that was required for conventional ones that break at low temperatures, but also in the low temperature region that can be reached at low cost. It is an object of the present invention to provide a non-heat treated steel which can be easily formed and a fitting member which breaks and separates at a low temperature.

In order to solve the above-mentioned problems, the present inventors satisfy the toughness value required for mechanical parts to be used by breaking and separating such as connecting rods, toughness values that can be easily broken and separated without deformation, and these toughness values, In addition, as a result of extensive research on the composition of steel, which easily breaks and separates even when cooled with a refrigerant at a temperature higher than that of liquid nitrogen, if it can be easily broken and separated at a temperature of 60 ° C or less without deformation, dry ice + Since ethanol cryogen can be used, the cost is low and the cooling is easy, and the toughness required for mechanical parts used by breaking and separating such as connecting rods has Charpy impact value (2 mm V notch test piece, the same.) it is 10J / cm ² or more, toughness can be easily broken separated without deformation, der 5 J / cm ² or less in a Charpy impact value The composition of steel that satisfies these impact values and can be easily broken and separated without deformation at ─60 ° C. or less is patented for the contents of C, Si, P, Mn, Cr, Cu and Ni. The inventor has obtained knowledge that it can be achieved by making it appropriate as described in the claims.
The present invention has been made based on these findings.

That is, in the non-heat treated steel according to the present invention, which can be easily fracture-separated at a low temperature, C: 0.15 to 0.35%, Si: 0.5 to 2.0%, Mn: 0.5 to 1.5%. %, P: 0.03-0.15%, S: 0.01-0.15%, Cu: 0.01-0.5%, Ni: 0.01-0.5%, Cr: 0. 01-1.0%, s-Al: 0.001-0.01%, N: 0.005-0.035%, Ca: 0.0001-0.01%, and O: 0.001-0. 01%, and if necessary, one or two of the following: Ti: 0.02% or less, Zr: 0.02% or less, Pb: 0.3% or less, and Bi: 0.3% or less. It is intended to contain the above and satisfy the following expressions 1 and 2, with the balance being Fe and unavoidable impurities.
Formula 1 ... 0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 × Si + 0.16 × Mn + 0.61 × P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Equation 2 ... 0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni)

In the joining member of the present invention which breaks and separates at a low temperature, the non-heat treated steel used is C: 0.15 to 0.35%, Si: 0.5 to 2.0%, and Mn: 0.5 to 0.5%. 1.5%, P: 0.03 to 0.15%, S: 0.01 to 0.15%, Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Cr : 0.01 to 1.0%, s-Al: 0.001 to 0.01%, N: 0.005 to 0.035%, Ca: 0.0001 to 0.01%, and O: 0.001 0.1% or less, Zr: 0.02% or less, Pb: 0.3% or less, and Bi: 0.3% or less. Or, two or more kinds are contained, and the following formulas 1 and 2 are satisfied, with the balance being Fe and unavoidable impurities.
Formula 1 ... 0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 × Si + 0.16 × Mn + 0.61 × P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Equation 2 ... 0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni)

The non-heat-treated steel of the present invention, which is easy to break and separate at low temperature, and the fitting member that breaks and separates at low temperature, have the following excellent effects by adopting the above configuration.
(1) As shown in the steel of the present invention in FIG. 2, within the normal operating temperature range, the toughness (10 J / cm ^{2 in} Charpy impact value) required for mechanical parts such as connecting rods is increased, and cooling is performed to break and separate. If the temperature is lower than −60 ° C., the toughness (but Charpy impact value is 5 J / cm ² ) which can be easily broken and separated without deformation becomes lower.
(2) Breaking separation can be easily performed without deformation at a temperature of -60 ° C or lower, which is higher than that conventionally proposed.
(3) It is inexpensive because it does not contain expensive elements.

Next, the reason why the component composition, Ceq, and T _Tr of the non-heat-treated steel of the present invention, which can be easily fracture-separated at low temperature, and the joining member that fracture-separates at low temperature, are specified as described above will be described.
C: 0.15 to 0.35%
C is an element necessary for increasing strength and obtaining an optimal impact transition curve. In a steel having a low C content, as shown in FIG. 3, the difference between the upper shelf energy and the lower shelf energy is large, and the transition is sharp, but the transition temperature is low. On the other hand, in steel having a high C content, the difference between the upper shelf energy and the lower shelf energy is small and the transition is gradual, but the transition temperature rises. When the fracture separation is performed by cooling to -60 ° C or lower as in the present invention, the energy of the upper shelf is as high as possible, and the impact value sharply decreases at -10 to -60 ° C, and the lower shelf energy decreases at -60 ° C or lower. It needs to be energy. Therefore, in the present invention, the upper limit of the C content is set to 0.35%. On the other hand, if the C content is too low, sufficient strength cannot be obtained, so the lower limit is made 0.15%.

Si: 0.5 to 2.0%
Si has a deoxidizing effect at the time of steel smelting, and forms a solid solution in ferrite as an alternative element to V, and the strength of ferrite, which is a soft phase that is a main cause of plastic deformation at the time of fracture separation, It is an element that improves proof stress and fatigue strength, suppresses deformation during fracture separation, and improves the adhesion of fracture surfaces. It is also an element that raises the transition temperature to improve the fracture separation characteristics at low temperatures. To obtain these effects, it is necessary to contain 0.5% or more. However, if the content is too large, the hardness is remarkably increased and the machinability is reduced. Therefore, the upper limit is set to 2%.

Mn: 0.5-1.5%
Mn is an element to be contained because it dissolves in the matrix to increase the strength and lowers the impact transition temperature to improve the toughness at room temperature. In the present invention, it has a function of suppressing a large rise in the impact transition temperature due to Si and P. In order to obtain these effects, it is necessary to contain 0.5% or more. However, if the content is too large, bainite is formed after forging, and the hardness is remarkably increased to reduce machinability. 1.5%.

P: 0.03 to 0.15%
P is an unavoidable impurity and segregates at the grain boundary to lower toughness. Therefore, it is general to keep P as low as possible. However, in the present invention that performs fracture separation, deformation at fracture is suppressed, and Since it is very effective for improving the adhesion, it is an element to be positively contained. Further, P is effective as a substitute element for V as a substitute element for Si in solid solution in ferrite to improve the strength of ferrite, thereby improving proof stress and fatigue strength, and also greatly increases impact transition temperature. , Also for these elements. In order to obtain these effects, it is necessary to contain 0.03% or more. However, if the content is too large, the impact value at room temperature is significantly reduced. Therefore, the upper limit is set to 0.15%.

S: 0.01-0.15%
S is an element contained for producing sulfides with Mn to improve machinability. In order to obtain the function and effect, it is necessary to contain 0.01% or more. However, if the content is too large, the hot workability is deteriorated. Therefore, the upper limit is made 0.15% or less.

Cu: 0.01-0.5%, Ni: 0.01-0.5%
Cu and Ni, like Mn and Cr, improve the impact value at room temperature and lower the transition temperature, so that they are elements to be contained for them. In order to obtain these effects, it is necessary to contain 0.01% or more. However, if the content increases, the cost increases (because it is expensive compared to Mn and Cr), so the upper limit is made 0.5%. . In addition, since the electric furnace melting material mainly made of scrap contains 0.05 to 0.2% of Cu and Ni, it is advantageous in terms of cost to use it in this range.

Cr: 0.01 to 1.0%
Cr is an element contained for Cr because it dissolves in the matrix to increase the strength and lowers the impact transition temperature to increase the toughness at room temperature. The present invention has a function of suppressing a large increase in the impact transition temperature due to Si and P. In order to obtain these effects, it is necessary to contain 0.01% or more. However, when the content is too large, bainite is formed after forging, and the hardness is remarkably increased to reduce machinability. 1.0%.

s-Al: 0.001 to 0.01%
s-Al (acid-soluble Al) has a deoxidizing effect at the time of smelting steel, forms fine nitrides, suppresses coarsening of crystal grains during hot forging, and improves strength. Elements to be contained for them. In order to obtain these effects, it is necessary to contain 0.001% or more. However, if the content is too large, the effect is saturated, so the upper limit is made 0.01%.

N: 0.005 to 0.035%
Although N is an unavoidable impurity, it is an element that suppresses coarsening of crystal grains during hot forging by combining with Al to form fine nitrides and dispersing them in steel. Although this effect can be 0.005% or less, the lower limit is made 0.005% because it is not economical to make it 0.005% or less. Further, if contained in a large amount, it causes casting defects, so the upper limit is made 0.035%.

Ca: 0.0001 to 0.01%
Ca replaces a part of Mn in MnS to form MnS in which Ca forms a solid solution, which adheres to a tool at the time of cutting to improve machinability, and is an element to be contained for that purpose. To obtain the effect, the content must be 0.0001% or more, but the effect is saturated even if added in a large amount, so the upper limit is made 0.01%.

O: 0.001 to 0.01%
In order to obtain MnS in which Ca is dissolved, Ca oxide must be present adjacently. O is also an unavoidable impurity, but is an element necessary for generating the Ca oxide. In order to obtain the function and effect, it is necessary to contain 0.001% or more. However, if the content is too large, oxide-based inclusions increase and cracks are easily generated during hot working. 0.01%.

Ti: 0.02% or less, Zr: 0.02% or less Ti and Zr are elements to be included for minimizing the distribution of MnS and improving the friability of chips during machining. However, even if it is contained excessively, the effect is saturated and it is economically disadvantageous, so the upper limit is made 0.02%.

Pb: 0.3% or less, Bi: 0.3% or less
Since both Pb and Bi improve machinability, Pb and Bi are elements to be contained as necessary when further improving machinability. However, an excessive content lowers the strength and hot workability, so the upper limit is made 0.3% or less.

0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 × Si + 0.16 × Mn + 0.61 × P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Ceq is a value indicating the hardness of the non-heat treated steel after forging, and the hardness after forging can be controlled by adjusting this value. The reason why Ceq is set to 0.6 or more is that if it is less than 0.6, the hardness is too low and the strength is insufficient, and the impact transition temperature is lowered, and the fracture separation characteristics at −60 ° C. or less are deteriorated. is there. Further, the upper limit is set to 0.85 because if Ceq is too high, the toughness at room temperature decreases, and the hardness becomes too hard, and the machinability also decreases.

0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni)
As described above, the impact transition temperature changes under the influence of not only the hardness but also the alloying elements, and rises due to the increase in the contents of C, Si and P, and the increase in Mn, Cr, Cu and Ni. Is reduced by The reason why T _{Tr is set} to 0 or more is that if it is less than 0, the impact transition temperature decreases, and if it is less than or equal to 60 ° C., the fracture separation characteristics deteriorate. That is, the Charpy impact value does not become 5 J / cm ² or less. The upper limit is set to 1.5 because if T _Tr is too high, the impact transition temperature becomes too high, and the toughness at room temperature decreases. That is, the Charpy impact value does not become 10 J / cm ² or more.

  The non-heat treated steel which is easily fracture-separated at low temperature and the mating member which fracture-separates at low temperature according to the present invention are within the range of the above component composition for the above-mentioned reason, and satisfy the above two formulas, and the balance is Fe and inevitable impurities. That is.

Next, examples of the present invention will be described.
Example 1
After ingoting the steels of the present invention examples and comparative examples having the component compositions shown in Table 1 below, they were ingot, hot forged to form a 50 mm square forged material, which was heated at 1200 ° C. for 60 minutes and then 22 mm in diameter. Was subjected to hot forging and allowed to cool to room temperature by leaving it on the floor at appropriate intervals so as not to overlap. From this round bar, a hardness test piece, an Ono-type rotary bending fatigue test piece having a parallel part diameter of 8 mm, and a JIS No. 4 impact test piece were cut out and subjected to a test.

The hardness was measured at room temperature using a Rockwell hardness tester by measuring the hardness of a 1 / 2R part of a forged 22 mm round bar. Table 2 shows the results.
The fatigue test was performed at room temperature using the above-mentioned test piece using an Ono-type rotary bending fatigue tester. Table 2 shows the results.
The impact test was carried out at room temperature and at -60 ° C. using a Charpy impact tester using the test pieces. Table 2 shows the results.

According to the results in Table 2, the inventive examples have a hardness of 97.3 to 103.3 HRB, a fatigue limit of 411 to 559 MPa, and a Charpy impact value (hereinafter referred to as "impact value") of 13 to 20 J at room temperature. / Cm ² , ² to 5 J / cm ² at -60 ° C. Each of them has a hardness of about 100 HRB, a fatigue limit of 410 MPa or more, an impact value of 10 J / cm ² or more which is a room temperature impact value required for connecting rods, etc., and is deformed at -60 ° C. It was 5 J / cm ² or less that could be easily broken and separated.

On the other hand, Comparative Examples A and C having a lower C or Si content than the present invention have a higher impact value at room temperature than the present invention, but have a lower hardness and a lower fatigue limit than the present invention, and have a -60 level. The impact value at 8 ° C. was 8 J / cm ² or 9 J / cm ² , which was higher than the impact value (5 J / cm ² ) at which break-off could be easily performed without deformation.
Comparative Example B, in which the C content is higher than the present invention and the Ceq is higher than the present invention, has the same hardness and fatigue limit as those of the present invention, and the impact value at −60 ° C. can be easily separated by breaking without deformation. Although the impact value was not more than the impact value, the impact value at room temperature was 9 J / cm ² which was less than the impact value (10 J / cm ² ) required for connecting rods and the like.

Comparative Examples D and G, which have a higher Si or P content than the present invention and higher Ceq and T _Tr than the present invention, have higher hardness and fatigue limit than the present invention, and the impact value at −60 ° C. is easy without deformation. The impact value at room temperature was 8 J / cm ² or 9 J / cm ² , which was less than the impact value required for a connecting rod or the like.
Comparative Examples E and I having a Mn or Cr content higher than that of the present invention have higher hardness than the present invention, and have a high hardness and a significantly reduced machinability because the structure is bainite. The fatigue limits and impact values were not measured because they were apparently not suitable for the mechanical parts.

Comparative Example F, which has a lower P content than the present invention, has the same hardness as the present invention example, and has a higher impact value at room temperature than the present invention example, but has a lower fatigue limit than the present invention example and -60. The impact value at 12 ° C. was 12 J / cm ² , which was higher than the impact value at which break-off could be easily performed without deformation.
Comparative Example H having an S content higher than that of the present invention has hardness similar to that of the present invention, and has an impact value at room temperature equal to or higher than the impact value required for connecting rods and the like, and has no deformation at -60 ° C. Although it was less than the impact value that can be easily broken and separated, the fatigue limit was lower than that of the examples of the present invention.

Comparative Examples J and K in which the s-Al or N content is smaller than that of the present invention have hardness similar to that of the present invention, and have an impact value at −60 ° C. which is equal to or less than an impact value at which break-off can be easily performed without deformation. However, the fatigue limit was lower than that of the present invention, and the impact value at room temperature was 9 J / cm ² or 8 J / cm ² , which was lower than the impact value required for connecting rods and the like.
Comparative Example L, whose T _Tr was higher than that of the present invention, had the same hardness and fatigue limit as those of the present invention, and the impact value at −60 ° C. was equal to or less than the impact value that could be easily broken and separated without deformation. The impact value at room temperature was 8 J / cm ² which was less than the impact value required for connecting rods and the like.

Comparative Example M, in which T _Tr was lower than that of the present invention, had an impact value at room temperature equal to or higher than the impact value required for a connecting rod or the like, but had a lower hardness and fatigue limit than those of the present invention, and an impact value at −60 ° C. Also, it was 10 J / cm ² or more, which is equal to or higher than the impact value that can be easily broken and separated without deformation.
Comparative Example N, which is a conventional steel (JIS S45C) having a high C content, a low O content and less Ca than the present invention, has an impact value at room temperature higher than the impact value required for connecting rods and the like. However, the fatigue limit was lower than that of the example of the present invention, and the impact value at −60 ° C. was 8 J / cm ² , which was equal to or more than the impact value at which the sample could be easily broken and separated without deformation.

The comparative example O of the conventional steel (S35VC) having a lower T _Tr than the present invention, containing V, and not containing Ca has an impact value at room temperature that is higher than the impact value required for connecting rods and the like. Although it was almost the same as the invention example, the fatigue limit was lower than that of the invention example, and the impact value at −60 ° C. was 11 J / cm ² , which was equal to or more than the impact value at which break-off could be easily performed without deformation.

Example 2
After hot-forging a connecting rod using the non-heat treated steels of Inventive Example 1 and Comparative Example O, the connecting rod is finished into a product by machining, and has a depth of 0.5 mm and a tip R0. A notch with a notch angle of 2 ° and a notch angle of 60 ° was provided to perform fracture separation at liquid nitrogen temperature, −60 ° C. and room temperature. Table 3 shows changes in roundness measured before and after the breakage.

  In Inventive Example 11, the change in roundness was very small not only at the temperature of liquid nitrogen but also at break at -60 ° C. Furthermore, at room temperature, the notch as applied this time could not be easily separated because the toughness was improved, and the change in roundness was large. On the other hand, in Comparative Example P, even when cooled to the temperature of liquid nitrogen, the roundness change after breaking and separating was large, and it did not break at room temperature.

  The non-heat treated steel and the mating member of the present invention can be cooled by using inexpensive dry ice + ethanol cryogen as a refrigerant. Since cooling and separation can be performed after cooling at 60 ° C or less, the cost of cooling is greatly reduced. And can be used industrially.

It is a perspective view of the connecting rod for explaining the shape of the connecting rod, and the manufacturing method which breaks and separates. 3 is a graph showing the relationship between toughness and temperature of a non-heat treated steel, a general steel, and a steel of the invention of Patent Document 1 which are easily broken and broken at a low temperature according to the present invention. It is a graph which shows the relationship of the toughness and temperature of the steel with a low C content and the steel with a high C content.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Hot forged connecting rod 2 Large end 3 Small end 4 Notch groove 5 Cap part 6 Rod part

Claims (7)

In mass% (the same applies hereinafter), C: 0.15 to 0.35%, Si: 0.5 to 2.0%, Mn: 0.5 to 1.5%, P: 0.03 to 0.15 %, S: 0.01 to 0.15%, Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Cr: 0.01 to 1.0%, s-Al: 0.001 to 0.01%, N: 0.005 to 0.035%, Ca: 0.0001 to 0.01%, and O: 0.001 to 0.01%. A non-heat-treated steel which satisfies Equation 2 and whose balance is composed of Fe and unavoidable impurities, and which is easily fracture-separated at a low temperature.
Formula 1 ... 0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 × Si + 0.16 × Mn + 0.61 × P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Equation 2 ... 0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni)   The low-temperature fracture separation according to claim 1, wherein one or two of Ti: 0.02% or less and Zr: 0.02% or less are contained instead of a part of the remaining Fe. Easy non-heat treated steel.   3. The method according to claim 1, wherein one or two of Pb: 0.3% or less and Bi: 0.3% or less are contained in place of a part of the remaining Fe. 4. Non-heat treated steel that can be easily broken at low temperatures. C: 0.15 to 0.35%, Si: 0.5 to 2.0%, Mn: 0.5 to 1.5%, P: 0.03 to 0.15%, S: 0.01 to 0.15%, Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Cr: 0.01 to 1.0%, s-Al: 0.001 to 0.01% , N: 0.005 to 0.035%, Ca: 0.0001 to 0.01%, and O: 0.001 to 0.01%, and satisfy the following formulas 1 and 2, with the balance being Fe And a mating member that breaks and separates at a low temperature, characterized by being made of unavoidable impurities.
Formula 1 ... 0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 × Si + 0.16 × Mn + 0.61 × P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Equation 2 ... 0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni)   The low-temperature fracture separation according to claim 1, wherein one or two of Ti: 0.02% or less and Zr: 0.02% or less are contained instead of a part of the remaining Fe. Mating member to do.   6. The method according to claim 4, wherein one or two of Pb: 0.3% or less and Bi: 0.3% or less are contained instead of a part of the remaining Fe. A mating member that breaks and separates at low temperatures. 7. The mating member according to claim 4, wherein the mating member is a connecting rod for an engine.

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