JP4346404B2 - Non-heat treated steel for fracture separation at low temperature and fitting member made of this non-heat treated steel - Google Patents

Description

The present invention relates to a non-tempered steel for fracture separation having a Charpy impact value of −60 ° C. or lower and 5 J / cm ² or lower, which is suitable for a fitting member used by breaking and separating into two or more parts after forging. The present invention relates to a fitting member such as a connecting rod for an engine, which is made of quality steel and breaks and separates at −60 ° C. or less .

  Conventionally, for fitting 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, the final shape is integrally forged and then subjected to finishing machining. After that, it was divided into two parts by machining. However, this manufacturing method requires an extra material as a cutting allowance at the 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. As shown in FIG. 1 (A), the fracture end is formed by forming a notch groove 4 in the large end 2 of the connecting rod 1 and then applying a load at room temperature to form the large end as shown in FIG. 1 (B). In this method, 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 break 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 part such as a connecting rod is designed and processed with steel that can be easily broken without deforming just by forming the above-mentioned notch groove, the effect of defects such as slight notches is sufficient. There is a problem that it has to be taken into account and results in an increase in weight. Further, since it is necessary to add a large amount of expensive V, there is a problem that the merit of reducing the cost is reduced.

Therefore, a method of breaking and separating at a low temperature using a low temperature brittleness phenomenon of steel has been proposed regardless of the component composition of the alloy (see, for example, Patent Document 3). According to this method, the connecting rod has sufficient toughness at the operating temperature of the connecting rod, and the connecting rod can be embrittled only at the time of fracture separation. However, in order to perform fracture separation with ordinary steel materials, it is necessary to cool to −130 ° C. or lower (see FIG. 2), and it is necessary to use liquid nitrogen (−196 ° C.) as a refrigerant for the cooling. Since there exists, there exists a problem that the cost for cooling becomes very high.
Japanese Patent Laid-Open No. 9-111412 JP-A-10-219389 Japanese Patent Laid-Open No. 2001-3924

  The present invention has moderate toughness within the working temperature range, and breaks separation in a low temperature region that can be reached inexpensively, as well as a cryogenic region such as liquid nitrogen cooling that is required for conventional ones that break at low temperatures. It is an object of the present invention to provide a non-heat treated steel that is easy to break and a fitting member that breaks and separates at low temperatures.

In order to solve the above-mentioned problems, the present inventors satisfy the toughness value necessary for mechanical parts such as connecting rods to be used after breaking and separation, the toughness value that can be easily broken and separated without deformation, and satisfy these toughness values, In addition, as a result of diligent research on the component composition of steel that can be easily separated even when cooled with a refrigerant having a temperature higher than that of liquid nitrogen, if it can be easily separated without deformation at -60 ° C or lower, dry ice + Since ethanol cryogen can be used, the cost is low and the cooling is easy, and the toughness required for mechanical parts such as connecting rods to be used after breaking and separating is determined by Charpy impact value (2 mmV notch test piece, below 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 component composition of steel that satisfies these impact values and can be easily fractured and separated without deformation at -60 ° C or lower is patented for C, Si, P, Mn, Cr, Cu and Ni contents. The knowledge that it can achieve by making it appropriate as described in the claim was acquired.
The present invention has been made based on these findings.

That is, in the non-heat treated steel for fracture separation having a Charpy impact value of −60 ° C. or less of the present invention of 5 J / cm ² or less , 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 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 to 0.01%, further Ti: 0.02% or less, Zr: 0.02% or less, Pb: 0.3% or less, and Bi: 0 if necessary. It is intended to contain one or more of 3% or less of back, satisfy the following formulas 1 and 2, and the balance is composed of Fe and inevitable impurities.
Formula 1 ... 0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 x Si + 0.16 x Mn + 0.61 x P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Formula 2 ... 0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni)

Moreover, in the fitting member which fractures and separates at −60 ° C. or less according to the present invention, the non-heat treated steel used is C: 0.15-0.35%, Si: 0.5-2.0%, Mn: 0 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: 0.01-1.0%, s-Al: 0.001-0.01%, N: 0.005-0.035%, Ca: 0.0001-0.01% and O: Containing 0.001 to 0.01%, and if necessary, Ti: 0.02% or less, Zr: 0.02% or less, Pb: 0.3% or less, and Bi: 0.3% or less 1 type or 2 types or more are included, the following formulas 1 and 2 are satisfied, and the balance is made of Fe and inevitable impurities.
Formula 1 ... 0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 x Si + 0.16 x Mn + 0.61 x P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Formula 2 ... 0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni)

The non-refined steel for fracture separation having a Charpy impact value of −60 ° C. or less of the present invention of 5 J / cm ² or less and the fitting member that fractures and separates at −60 ° C. or less have the following configuration. Excellent effect.
(1) As shown in the steel of the present invention in FIG. 2, the toughness required for mechanical parts such as connecting rods (Charpy impact value of 10 J / cm ² ) is increased within the normal operating temperature range, and it is cooled to break and separate. to lower than (5 J / cm ² in Charpy impact value) toughness which can be easily broken separated without deformation below -60 ° C. temperature.
(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 component composition, Ceq, and T _Tr of the non-heat treated steel for fracture separation having a Charpy impact value of −60 ° C. or less of the present invention of 5 J / cm ² or less and the fitting member that fractures and separates at −60 ° C. or less are described above. Explain why it is specified.
C: 0.15-0.35%
C is an element necessary for increasing the strength and obtaining an optimal impact transition curve. In steel with 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 rapid, but the transition temperature is low. On the other hand, in steel with a high C content, the difference between the upper shelf energy and the lower shelf energy is small and the transition is slow, but the transition temperature rises. When the fracture separation is performed by cooling to −60 ° C. or lower as in the present invention, the upper shelf energy is as high as possible, the impact value decreases rapidly at −10 to −60 ° C., and the lower shelf 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 action when steel is melted, dissolves in ferrite as an alternative element of V, and the strength of ferrite, which is a soft phase, which is the main cause of plastic deformation during fracture separation, It is an element that improves the yield strength and fatigue strength, suppresses deformation at break separation, and improves the adhesion of the fracture surface. It is also an element that raises the transition temperature and improves the fracture separation characteristics at low temperatures. In order to obtain these effects, it is necessary to contain 0.5% or more. However, if the amount is too large, the hardness is remarkably increased and the machinability is lowered, so the upper limit is made 2%.

Mn: 0.5 to 1.5%
Mn is an element to be contained because Mn is dissolved in the matrix to increase the strength and lower the impact transition temperature to improve the toughness at room temperature. In the present invention, there is a function of suppressing a large increase in impact transition temperature due to Si and P. In order to obtain these functions and effects, it is necessary to contain 0.5% or more. However, if the amount is too large, bainite is generated after forging, the hardness is remarkably increased and the machinability is lowered. 1.5%.

P: 0.03-0.15%
P is an inevitable impurity and segregates at the grain boundary to reduce toughness. Therefore, it is generally kept as low as possible. However, in the present invention in which fracture separation is performed, deformation at break is suppressed, and Since it is very effective for improving the adhesion, it is an element that is actively included. P, like Si, is effective in improving the strength and fatigue strength by dissolving in ferrite as a substitute element for V and improving the strength of the ferrite, and also greatly increases the impact transition temperature. It is also an element to be included for them. In order to obtain these effects, it is necessary to contain 0.03% or more, but if it is too much, the impact value at room temperature is also significantly reduced, so the upper limit is made 0.15%.

S: 0.01 to 0.15%
S is an element to be contained for generating Mn and sulfide to improve machinability. In order to acquire the effect, it is necessary to contain 0.01% or more, but if it increases too much, hot workability will deteriorate, Therefore The upper limit shall be 0.15% or less.

Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%
Cu and Ni are elements included for improving the impact value at room temperature and lowering the transition temperature in the same manner as Mn and Cr. In order to obtain these effects, it is necessary to contain 0.01% or more, but if it increases, the cost increases (because it is more expensive than Mn and Cr), so the upper limit is made 0.5%. . In addition, since the electric furnace melting material which uses a scrap as a main raw material contains 0.05 to 0.2% of Cu and Ni, it is advantageous in terms of cost to use within this range.

Cr: 0.01 to 1.0%
Cr is an element to be contained for Cr because it dissolves in the base to increase the strength and lowers the impact transition temperature to increase the toughness at room temperature. In the present invention, there is a function of suppressing a large increase in impact transition temperature due to Si and P. In order to obtain these functions and effects, it is necessary to contain 0.01% or more, but if it increases too much, bainite is generated after forging, the hardness increases remarkably and machinability decreases, so the upper limit is set. 1.0%.

s-Al: 0.001 to 0.01%
s-Al (acid-soluble Al) has a deoxidizing action during steel melting and forms fine nitrides to suppress coarsening of crystal grains during hot forging and improve strength. Therefore, it is an element to be contained for them. In order to obtain these functions and effects, it is necessary to contain 0.001% or more, but if the amount is too large, the effect is saturated, so the upper limit is made 0.01%.

N: 0.005 to 0.035%
N is an inevitable impurity, but is an element that suppresses coarsening of crystal grains during hot forging by combining with Al to form fine nitrides and dispersing in steel. Although this effect is 0.005% or less, since it is not economical to make it 0.005% or less, the lower limit is made 0.005%. Moreover, since it will cause a casting defect when it contains abundantly, the upper limit shall be 0.035%.

Ca: 0.0001 to 0.01%
Ca is part of Mn in MnS to form MnS in which Ca is dissolved, which adheres to the tool during cutting and improves the machinability. Therefore, Ca is an element to be contained. In order to obtain the effect, it is necessary to contain 0.0001% or more, but even if added in a large amount, the effect is saturated, 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 needs to be present adjacently. O is an inevitable impurity, but is an element necessary for producing the Ca oxide. In order to obtain the effect, it is necessary to contain 0.001% or more. However, if the amount is too large, oxide inclusions increase and cracking during hot working is likely to occur. 0.01%.

Ti: 0.02% or less, Zr: 0.02% or less Ti and Zr are elements to be contained for the purpose of refining the distribution state of MnS and improving chip crushability during machining. However, even if contained excessively, the effect is saturated and disadvantageous economically, 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, they are elements that are included as necessary when further improving machinability. However, since an excessive content reduces strength and hot workability, the upper limit is made 0.3% or less.

0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 x Si + 0.16 x Mn + 0.61 x P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Ceq is a value indicating the post-forging hardness of the non-tempered steel, and the post-forging hardness can be managed by adjusting this value. The reason why Ceq is 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 lowered. is there. Further, the upper limit is set to 0.85 because if Ceq is too high, the toughness at room temperature is lowered, and it is too hard and the machinability is also lowered.

0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni)
The impact transition temperature changes as a result of the influence of not only the hardness but also the alloy element as described above, and increases with an increase in the contents of C, Si and P, and increases in Mn, Cr, Cu and Ni. It is lowered by. The reason why T _Tr is made 0 or more is that if it is less than 0, the impact transition temperature is lowered and the fracture separation characteristic is lowered at −60 ° C. or less. 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 for fracture separation having a Charpy impact value of −60 ° C. or less of the present invention of 5 J / cm ² or less and the fitting member that breaks and separates at −60 ° C. or less are within the range of the above component composition for the above reasons. And the above two formulas are satisfied, and the balance is Fe and inevitable impurities.

Next, examples of the present invention will be described.
Example 1
The steels of the present invention and comparative examples having the composition shown in Table 1 below are melted and then ingot, hot forged to give a 50 mm square forged material, which is heated at 1200 ° C. for 60 minutes and then has a diameter of 22 mm. The forged bar was hot forged and left on the floor at an appropriate interval so as not to overlap, and cooled to room temperature. From this round bar, a hardness test piece, an Ono type rotary bending fatigue test piece with a parallel part diameter of 8 mm, and a JIS No. 4 impact test piece were cut out and used for the test.

The hardness was measured at room temperature using a Rockwell hardness meter for the 1 / 2R part of a forged 22 mm round bar. The results are shown in Table 2.
The fatigue test was performed at room temperature using the Ono rotary bending fatigue tester using the above test piece. The results are shown in Table 2.
The impact test was performed at room temperature and −60 ° C. using a Charpy impact tester using the above test piece. The results are shown in Table 2.

According to the results in Table 2, the inventive example has a hardness of 97.3-103.3HRB, a fatigue limit of 411-559 MPa, and a Charpy impact value (hereinafter referred to as “impact value”) of 13-20 J at room temperature. / cm ^2, and 2~5J / cm ² at -60 ℃. All of these have a hardness of around 100 HRB, a fatigue limit of 410 MPa or more, an impact value of 10 J / cm ² or more, which is an impact value at room temperature required for a connecting rod, etc., and deformation at −60 ° C. And it was 5 J / cm ² or less that could be easily broken and separated.

In contrast, Comparative Examples A and C, which have a lower C or Si content than the present invention, have higher impact values at room temperature than the present invention examples, but have lower hardness and fatigue limits than the present invention examples, and -60 The impact value at 8 ° C. was 8 J / cm ² or 9 J / cm ² , which is higher than the impact value (5 J / cm ² ) that can be easily broken and separated without deformation.
Comparative Example B, which has a higher C content than the present invention and a higher Ceq than the present invention, has the same hardness and fatigue limit as the present invention example, and the impact value at -60 ° C. can be easily broken and separated without deformation. Although it was below the impact value, the impact value at room temperature was 9 J / cm ² below the impact value (10 J / cm ² ) required for the connecting rod and the like.

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

Comparative Example F, which has a lower P content than the present invention, has a hardness similar to that of the present invention example, 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 ° C. was 12 J / cm ² , which is higher than the impact value at which breakage and separation can be easily performed without deformation.
Comparative Example H, which has a higher S content than the present invention, has a hardness similar to that of the present invention example, and the impact value at room temperature is not less than the impact value required for the connecting rod, etc., and there is no deformation at −60 ° C. Although it was below the impact value at which it 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 having less s-Al or N content than the present invention have the same hardness as the present invention example, and the impact value at -60 ° C is less than the impact value at which it can be easily broken and separated without deformation. However, the fatigue limit was lower than that of the example of the present invention, and the impact value at room temperature was 9 J / cm ² or 8 J / cm ² , which is lower than the impact value required for the connecting rod or the like.
Comparative Example L having a higher T _Tr than that of the present invention has the same hardness and fatigue limit as those of the present invention, and the impact value at −60 ° C. was below the impact value at which it can be easily fractured and separated without deformation. The impact value at room temperature was 8 J / cm ² below the impact value required for connecting rods and the like.

In Comparative Example M in which T _Tr is lower than that of the present invention, the impact value at room temperature was higher than the impact value required for the connecting rod or the like, but the hardness and fatigue limit were lower than those of the present invention example, and the impact value at −60 ° C. Was 10 J / cm ² , which is more than the impact value at which it can be easily broken and separated without deformation.
Comparative Example N of conventional steel (JIS S45C), which has a higher C content, a lower O content than the present invention example, and does not contain Ca, has an impact value at room temperature that is higher than the impact value required for connecting rods, etc. 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 is equal to or greater than the impact value at which it can be easily fractured and separated without deformation.

Comparative Example O of conventional steel (S35VC), which has a lower T _Tr than that of the present invention, contains V, and does not contain Ca, has an impact value at room temperature that is higher than the impact value required for connecting rods, etc. The fatigue limit was lower than that of the invention example, and the impact value at −60 ° C. was 11 J / cm ² , which is equal to or higher than the impact value at which it can be easily fractured and separated without deformation.

Example 2
After hot forging the connecting rod using the non-heat treated steel of Invention Example 1 and Comparative Example O, the product was finished by machining and the depth R0. Table 3 shows the change in roundness measured before and after performing break separation at a liquid nitrogen temperature, −60 ° C. and room temperature with a notch having a notch angle of 2 mm and a notch angle of 60 °.

  In Example 11 of the present invention, not only the liquid nitrogen temperature but also the rupture separation at −60 ° C. showed very little change in roundness. Furthermore, since the toughness was improved at room temperature, it was not possible to separate easily with the notches provided this time, and the change in roundness was large. On the other hand, Comparative Example P had a large change in roundness after fracture separation even when cooled to liquid nitrogen temperature, and did not fracture at room temperature.

  The non-heat treated steel and fitting member of the present invention can be cooled using inexpensive dry ice + ethanol cryogen as a refrigerant. And can be used industrially.

It is a perspective view of the connecting rod for demonstrating the shape of a connecting rod and the method of manufacturing by breaking and separating. It is a graph which shows the relationship between the toughness of the non- heat treated steel of this invention, general steel, and the invention steel of the said patent document 1, and temperature. It is a graph which shows the relationship between the toughness and temperature of steel with low C content and high steel.

1 Hot Forged Connecting Rod 2 Large End 3 Small End 4 Notch Groove 5 Cap Part 6 Rod Part

Claims (7)

% By mass (hereinafter the same), C: 0.15-0.35%, Si: 0.5-2.0%, Mn: 0.5-1.5%, P: 0.03-0.15 %, 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 the following formula 1 and A non-tempered steel for fracture separation having a Charpy impact value of −60 ° C. or less of 5 J / cm ² or less, characterized by satisfying Equation 2 and the balance being Fe and inevitable impurities.
Formula 1 ... 0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 x Si + 0.16 x Mn + 0.61 x P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Formula 2 ... 0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni) It replaces with a part of said balance Fe, and contains 1 type or 2 types of back of Ti: 0.02% or less and Zr: 0.02% or less, -60 degrees C or less of Claim 1 characterized by the above-mentioned. Non-tempered steel for fracture separation having a Charpy impact value of 5 J / cm ² or less . 3. Instead of a part of the remaining Fe, Pb: 0.3% or less and Bi: 0.3% or less of one or two kinds of backs are contained. Non-heat treated steel for fracture separation having a Charpy impact value of −60 ° C. or less of 5 J / cm ² or less . C: 0.15-0.35%, Si: 0.5-2.0%, Mn: 0.5-1.5%, 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 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 fitting member that breaks and separates at −60 ° C. or less , characterized by comprising inevitable impurities.
Formula 1 ... 0.6 ≦ Ceq ≦ 0.85
However, Ceq = C + 0.07 x Si + 0.16 x Mn + 0.61 x P
+ 0.19 × Cu + 0.17 × Ni + 0.2 × Cr
Formula 2 ... 0 ≦ T _Tr ≦ 1.5
However, T _Tr = (C + 0.8 × Si + 5 × P) −0.5 × (Mn + Cr + Cu + Ni) 5. -60 ° C. or less according to claim 4, characterized in that it contains one or two of Ti: 0.02% or less and Zr: 0.02% or less instead of a part of the remaining Fe. Fitting member that breaks and separates at 6. It replaces with a part of said remainder Fe, and contains 1 type or 2 types of back of Pb: 0.3% or less and Bi: 0.3% or less, The Claim 4 or Claim 5 characterized by the above-mentioned. A fitting member that breaks and separates at -60 ° C or lower . 7. The fitting member that breaks and separates at -60 [deg .] C. or lower according to any one of claims 4 to 6, wherein the fitting member is a connecting rod for an engine.

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