JP2016180165A - Molding component for fracture separation type connecting rod and fracture separation type connecting rod, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fracture separation type connecting rod good in fracture separation property and fitting characteristic of fractured faces, and a molding component using the same.SOLUTION: There is provided a molding component for fracture separation type connecting rod containing, by mass%, C:0.1 to 0.70%, Si:over 0% to 1.5%, Mn:0.01 to 2.0%, P:over 0% to 0.2%, S:over 0% to 0.2% and Cr:0.01 to 2.0%, and the balance iron with inevitable impurities and having a metallographic structure with area percentage of tempered martensite of 80% or more and prior austenite grain number of 6.0 or less.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a molded part for a fractured-separating type connecting rod and a fractured-separated type connecting rod obtained from the molded part.

  In an internal combustion engine such as an automobile engine, a connecting rod is used as a part 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. The connecting rod includes a connecting rod main body and a connecting rod cap, and these are attached to the crankshaft so as to sandwich the crankshaft.

  Conventionally, the connecting lot main body and the connecting rod cap are generally manufactured separately. However, in the connecting rod manufactured by such a method, it is necessary to precisely process the fitting surface by cutting etc. in order to improve the fitting property between the connecting rod body and the connecting rod cap, and the fitting surface is displaced. In order to avoid this, it is common to perform knock pin processing. Such a manufacturing method has a problem that the yield of the material is reduced and the manufacturing becomes complicated.

  Therefore, the connecting rod main body and the connecting rod cap are integrally formed by hot forging, and after processing the through hole for inserting the crankshaft, the hole for inserting the bolt, etc., the through hole portion has two approximately A method of breaking and separating the connecting rod main body and the connecting rod cap by cold working is adopted so as to form a semicircle. The rod body and the cap thus obtained are assembled by fitting the broken surfaces of the rod body and the cap with a crankshaft and fastening them with bolts to form a connecting rod. In such a method, it is not necessary to precisely process the fractured surface to be a fitting surface by cutting or the like, or to perform knock pin processing. The connecting rod manufactured in this way is hereinafter referred to as a fracture separation type connecting rod.

  For example, in Patent Document 1, a steel material having a predetermined composition is formed into a connecting rod shape, heated at 920 ° C. for about 10 seconds using an induced current, and subjected to tempering, thereby increasing weight reduction and fatigue strength. It is disclosed that a strength connecting rod can be manufactured.

  In addition, the above break separation type connecting rod is required to have good break separation property. The break separation property is evaluated by, for example, the dimensional change of the through hole before and after the break separation. The smaller the dimensional change is, the better the break separation property is. In order to satisfy these characteristics, it is common to adjust the chemical composition of the steel material, but this causes an increase in steel material cost and production cost. For example, Patent Document 2 discloses that, when manufacturing a connecting rod, if hot forging is performed in a temperature range that becomes an austenite region, quenching can be performed as it is after molding, regardless of control of chemical composition. Has been. According to Patent Document 1, it is described that it is possible to manufacture a connecting rod having a high break-separation property that simplifies the manufacturing process and reduces the manufacturing cost.

  The fracture separation type connecting rod has improved fitting property of the fracture surface compared to the case where the rod body and the cap are manufactured separately, but further improvement in fitting property is also required. However, in general, it is difficult to achieve both improvement in fracture separation and improvement in fitting property of the fracture surface, and a technique that has achieved both of these has not been proposed yet.

JP 2004-286196 A JP 2011-47002 A

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a fracture separation type connecting rod having a good fracture separation property and a good fracture surface fitting property, and a molded part used therefor. is there.

The present invention that has achieved the above problems
% By mass
C: 0.1 to 0.70%,
Si: more than 0%, 1.5% or less,
Mn: 0.01 to 2.0%,
P: more than 0%, 0.2% or less,
S: more than 0%, 0.2% or less,
Cr: 0.01 to 2.0% each, the balance is iron and inevitable impurities,
Among metal structures, the area ratio occupied by tempered martensite is 80% or more,
It is a molded part for a break-away connecting rod having a prior austenite grain size number of 6.0 or less. The present invention also includes a fracture separation type connecting rod obtained from the molded part.

The present invention further includes a method for producing a molded part for a breakable connecting rod as described above, and specifically, the production method includes:
% By mass
C: 0.1 to 0.70%,
Si: more than 0%, 1.5% or less,
Mn: 0.01 to 2.0%,
P: more than 0%, 0.2% or less,
S: more than 0%, 0.2% or less,
Cr: each containing 0.01 to 2.0%, the balance is steel and steel unavoidable impurities,
Reheat after hot forging, quench at a quenching temperature of 1100-1300 ° C,
Next, tempering is performed. The present invention also includes a method for producing a break-separating connecting rod that obtains a break-separable connecting rod molded part by the above-described method for producing a molded part and breaks and separates the molded part.

  According to the molded part of the present invention, the ratio of tempered martensite in the metal structure is not less than a predetermined value and the prior austenite grain size is not more than a predetermined value, so that it has excellent break separation and fit after break separation. Are better.

FIG. 1 is a view showing a sampling position of a tissue observation test piece. FIG. 2 is a schematic diagram showing the shape of a test piece used for evaluation of separation strain. FIG. 3 is a schematic view of an apparatus used for the fracture separation test. FIG. 4 is a diagram illustrating a method for evaluating break separation. FIG. 5 is a diagram for explaining the evaluation position of the fitting property of the fracture surface. FIG. 6 is a graph plotting the results of the examples.

  As a result of investigation by the present inventors, it was possible to cause grain boundary embrittlement cracking with the majority of the structure as tempered martensite during fracture separation of the connecting rod and to increase the prior austenite grain size, thereby maintaining fracture separation. It has been clarified that the swell of the fracture surface becomes large and the fitting property can be improved.

  The molded part for a fracture separation type connecting rod according to the present invention (hereinafter simply referred to as “molded part”) has a prior austenite grain size number of 6.0 or less. The smaller the prior austenite grain size number, that is, the larger the prior austenite grain size, the greater the waviness of the fracture surface when intergranular embrittlement cracking occurs, resulting in better fit of the fracture surface. Become. Further, when the particle size number is small, the hardenability is improved, so that the brittleness of the tempered martensite is promoted and the break separation property is also improved. The breaking separation property can be evaluated by, for example, a dimensional change in the pore diameter before and after breaking separation (hereinafter referred to as strain at the time of separation) as described later in Examples. The particle size number is preferably 5.5 or less, more preferably 5.0 or less. Although the minimum of a particle size number is not specifically limited, For example, it is 0.

  In the molded part of the present invention, the ratio of tempered martensite is 80% or more in terms of area ratio. By doing in this way, a grain boundary embrittlement crack can be caused at the time of fracture separation of a connecting rod. The ratio of tempered martensite is preferably 90% or more, more preferably 95% or more, and the upper limit may be 100%.

  Next, the chemical composition of the molded part of the present invention will be described. In this specification, all chemical compositions mean mass%.

C: 0.1 to 0.70%
C is an element necessary for securing strength and reducing strain at break separation. Further, if the amount of C is small, the martensitic transformation start temperature rises and it becomes difficult for the steel material to be burned. From such a viewpoint, the C amount needs to be 0.1% or more. A preferable lower limit is 0.2% or more, and a more preferable lower limit is 0.3% or more. However, if the amount of C becomes excessive, machinability deteriorates. From such a viewpoint, the C amount needs to be 0.70% or less. A preferable upper limit is 0.60% or less, and a more preferable upper limit is 0.55% or less.

Si: more than 0% and 1.5% or less Si is useful as a deoxidizing element during steel melting, and is effective in improving yield strength and fatigue strength by increasing hardenability and strength. It is an element. Further, it is effective for suppressing deformation evaluated by a change in roundness or the like at the time of fracture separation and improving the fitting property of the fracture surface. These effects increase as the Si amount increases, and in order to fully exhibit these effects, the Si amount is preferably 0.15% or more, more preferably 0.20% or more. . However, if the amount of Si becomes excessive, the hardness increases more than necessary and the machinability deteriorates, so it is necessary to make it 1.5% or less. A preferable upper limit is 1.0% or less, and a more preferable upper limit is 0.5% or less.

Mn: 0.01 to 2.0%
Mn is an element that can secure a stable strength of the steel material by improving the hardenability of the steel material. In order to exhibit such an effect, it is necessary to contain Mn 0.01% or more. The amount of Mn is preferably 0.05% or more, more preferably 0.1% or more. However, when the amount of Mn becomes excessive, since Mn is an austenite stabilizing element, retained austenite is generated after tempering, and the strength is decreased. From such a viewpoint, the amount of Mn needs to be 2.0% or less. A preferable upper limit is 1.5% or less, and a more preferable upper limit is 1.0% or less.

P: More than 0% and 0.2% or less P is an effective component for suppressing deformation of the steel material at the time of fracture separation and improving the fitting property of the fracture surface. These effects increase as the content increases. In order to exhibit the effect more effectively, P may be contained positively by 0.03% or more. The amount of P is more preferably 0.05% or more. However, excess P may induce casting defects during continuous casting. From such a viewpoint, the amount of P needs to be 0.2% or less. The upper limit with the preferable amount of P is 0.15% or less, and a more preferable upper limit is 0.08% or less.

S: more than 0%, 0.2% or less S is an element effective for improving the machinability of steel by reacting with Mn to produce MnS. In addition, when the MnS has a shape elongated and elongated by rolling or the like in the manufacturing process, this inhibits the progress of the fracture surface at the time of fracture separation and exhibits the effect of improving the fitting property. These effects increase as the content increases. In order to effectively demonstrate the effect, 0.01% or more of S may be positively included. The amount of S is more preferably 0.03% or more. However, excessive S generates excessive MnS and causes hot forging cracks and fatigue strength reduction, so the S amount needs to be 0.2% or less. The upper limit with the preferable amount of S is 0.15% or less, and a more preferable upper limit is 0.12% or less.

Cr: 0.01 to 2.0%
Cr is an element that can obtain a stable strength of the steel material by improving the hardenability of the steel material. In order to exert such effects, it is necessary to contain 0.01% or more of Cr. The amount of Cr is preferably 0.1% or more, and more preferably 0.3% or more. However, when the amount of Cr is excessive, Cr suppresses low-temperature temper embrittlement, so that good fracture separation cannot be obtained. From such a viewpoint, the Cr amount needs to be 2.0% or less. A preferable upper limit is 1.5% or less, and a more preferable upper limit is 1.1% or less.

  The basic components of the molded part of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that unavoidable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing facilities, and the like, and other components may be included as long as the action of the present invention is not hindered.

  In order to obtain the molded part of the present invention, the steel is melted, cast, ingot-rolled, hot-rolled, and subjected to a normal manufacturing method of forging the obtained rolled material, followed by reheating, predetermined It is necessary to quench at a temperature and further temper.

  In the method of quenching without performing reheating immediately after forging as is generally done with tempered forging members as in Patent Document 2 described above, the prior austenite crystal grains are deformed by forging pressure. Therefore, the crystal grains cannot be coarsened. In the present invention, it is necessary to reheat and set the quenching temperature to 1100 to 1300 ° C. The quenching temperature is preferably higher because the crystal grains are coarsened, and by setting the quenching temperature to 1100 ° C. or higher, the prior austenite grain size of the molded part can be 6.0 or lower. The quenching temperature is preferably 1150 ° C. or higher. On the other hand, if the quenching temperature is too high, burning occurs, that is, melting occurs, so the temperature is set to 1300 ° C. or lower. In addition, it is preferable to perform the soaking process which hold | maintains about 1-3 hours at the quenching temperature for the purpose of controlling the steel materials at the uniform temperature for the difference in heating steel materials. The quenching may be water quenching or oil quenching.

  Tempering is performed after quenching. If tempering is not performed, tempered martensite cannot be secured, so that no intergranular embrittlement cracking occurs during fracture separation of the connecting rod, and the fitting property of the fracture surface deteriorates. Tempering at a low temperature embrittlement temperature is preferable because grain boundary embrittlement cracking is further promoted. The lower limit of the tempering temperature is preferably 200 ° C. or higher, more preferably 250 ° C. or higher. The upper limit of the tempering temperature is preferably 450 ° C. or lower, more preferably 400 ° C. or lower. The tempering time is not particularly limited, but is, for example, about 2 to 5 hours.

  In addition, although the conditions of hot rolling are not specifically limited, For example, what is necessary is just to set start temperature to about 1000-1100 degreeC, and end temperature to about 850-950 degreeC.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

Steel types A to D having the chemical composition shown in Table 1 below are melted according to a normal melting method, cast and divided, and then hot-rolled at a start temperature of 1050 ° C. and an end temperature of 900 ° C., and have a diameter of 50 mm. Got the steel bar. The obtained steel bar was cut perpendicularly to the longitudinal direction to obtain a steel bar having a length of 90 mm. The steel bar was heated to 1200 ° C. and then compressed by hot forging in the direction perpendicular to the longitudinal direction of the steel bar, processed into a plate shape having a length of 100 mm, a width of 70 mm, and a thickness of 25 mm, and allowed to cool. .
Steel type D shown in Table 1 below is a comparative steel with a large amount of C, and corresponds to C70S6 of DIN (Deutsche Industry Normen).

  Each plate-like member after hot forging is heated to the quenching temperature shown in Table 2, subjected to soaking for 2 hours at that temperature, then quenched with water at room temperature, and further tempered as shown in Table 2. And tempered for 3 hours to produce a tempered member. In Table 2, the tempering temperature “0” means that tempering is not performed.

  With respect to the tempered member thus obtained, the structure, separation strain at break separation, and fitability were evaluated by the following methods (1) to (3).

(1) Evaluation method of structure The longitudinal direction of the tempered member, that is, the center portion in the longitudinal direction and the center portion in the thickness direction, and 1 / A specimen was taken from a cross section perpendicular to the longitudinal direction so that a portion satisfying 4 parts could be observed. The sampling position of the test piece is shown in FIG. After the surface of this test piece was mirror-polished, the specimen was prepared by corroding with nital for structural observation and corroding with a hydrochloric acid + picric acid-based corrosive liquid for the prior austenite particle size measurement. Then, using an optical microscope, the photo size of one field of view was photographed at 400 × with a size of 9 cm × 7 cm, and the metal structure was judged from the obtained photo and the prior austenite particle size measurement according to JIS G0551 was performed. Table 2 below shows the martensite structure fraction observed with an optical microscope. The structure of the example tempered in the manufacturing process is judged to be tempered martensite, and tempering is not performed in the manufacturing process. The structure was judged to be quenched martensite.

(2) Evaluation method of separation distortion Each tempered member obtained above was cut and processed into a test piece shown in FIG. FIG. 2A is a schematic top view of a test piece used in the fracture separation test, and FIG. 2B is a schematic side view of the test piece. The test piece has a plate shape of 65 mm × 65 mm × thickness 22 mm, and a cylindrical hole having a diameter of 43 mm is extracted at the center. Two end portions of the central hole are provided with notches a having a curvature R of 0.2 mm, a depth of 1.0 mm, and an opening angle of 60 °. Further, the test piece is provided with bolt holes b having a diameter of 8.3 mm in two places along the rolling direction c.
FIG. 3 is a schematic view of an apparatus for explaining a method of a fracture separation test. As shown in FIG. 3, the holders 3a and 3b were passed through the center hole of the test piece 6 and set in a 1600t press test machine, and the test piece was broken and separated at a press speed of 270 mm / sec. In FIG. 3, reference numeral 2 denotes a support base. The test piece was broken and separated with a press 1. The breaking speed of the test piece 6 is calculated to be about 150 mm / second because the wedge angles of the wedges 4 and 5 are 30 °.
And as shown in FIG. 4, the hole diameter difference (L2-L1) before and after break separation, that is, before and after the test was measured as a dimensional change, and those having a dimensional change of 155 μm or less were evaluated as excellent in break separation. The standard for dimensional change of 155 μm or less is equivalent to that of DIN C70S6.

(3) Evaluation method of fitting property Fitting property was evaluated by the shape of the fracture surface after fracture separation. As for the shape of the fracture surface, as shown in FIG. 5, the roughness measuring device starts from the side surface 7 of the fracture surface and the bottom 11 of the notch a of the center line 10 at the end 9 of the bolt hole 8. Using the fracture end 12 as an end point, the probe was operated in the radial direction of the center hole of the test piece after fracture division, and the swell shape on the fracture surface was evaluated.
This swell shape was evaluated by obtaining the maximum height swell Wz based on JIS B 0601: 2001. The evaluation conditions for the maximum height waviness Wz were as follows: reference length: 2.5 mm, number of sections: 3, cut-off value λf: 25.0 mm, cut-off value λc: 0.25 mm, filter type: Gaussian.
For the two fracture surfaces of the test piece on the cap side after the fracture separation, the above measurement is performed at two measurement parts on both sides of the bolt hole of each fracture surface, and the average value of a total of four locations is the value of the waviness Wz It was. And when the waviness Wz was 280 micrometers or more, it evaluated as a pass. This acceptance criterion is equivalent to that of DIN C70S6.

  The results are shown in Table 2.

  No. satisfying the requirements of the present invention. 10-12, 21-23, 25-27, 33-35, 37-39, the dimensional change at the time of break separation is small, showing good break separation, large waviness, and fit to the fracture surface It was excellent.

On the other hand, no. Nos. 1, 5, 13, 16, and 28 had low quenching temperatures and were not tempered, so that the prior austenite grain size became fine and the tempered martensite structure caused small undulations.
No. Since 2-4, 6-8, 14, 15, 17-19, 29-31 had low quenching temperature, the prior austenite grain size became fine and the undulation was small.
No. Nos. 9, 20, 24, 32, and 36 had appropriate quenching temperatures, but were not tempered, and therefore had quenching martensite structures and small swells.

  The results shown in Table 2 are shown in a graph in FIG. According to the present invention, it is clear from FIG. 6 that both the break separation property and the fitting property of the fracture surface, which are conventionally difficult to achieve, can be achieved.

1 Press 2 Support 3a, 3b Holder 4, 5 Wedge 6 Test piece

Claims (4)

% By mass
C: 0.1 to 0.70%,
Si: more than 0%, 1.5% or less,
Mn: 0.01 to 2.0%,
P: more than 0%, 0.2% or less,
S: more than 0%, 0.2% or less,
Cr: 0.01 to 2.0% each, the balance is iron and inevitable impurities,
Among metal structures, the area ratio occupied by tempered martensite is 80% or more,
Molded part for fracture separating type connecting rod having a prior austenite grain size number of 6.0 or less.   A fracture separation type connecting rod obtained from the molded part according to claim 1. A method for producing a molded part for a fracture separation type connecting rod according to claim 1,
% By mass
C: 0.1 to 0.70%,
Si: more than 0%, 1.5% or less,
Mn: 0.01 to 2.0%,
P: more than 0%, 0.2% or less,
S: more than 0%, 0.2% or less,
Cr: each containing 0.01 to 2.0%, the balance is steel and steel unavoidable impurities,
Reheat after hot forging, quench at a quenching temperature of 1100-1300 ° C,
Next, a method for producing a molded part for a break-away connecting rod that is tempered.   A method for producing a break-separating connecting rod, comprising obtaining a molded part for a break-separating connecting rod by the production method according to claim 3 and breaking and separating the molded part.