JP3478381B2 - Manufacturing method of non-heat treated forging with excellent machinability and fatigue strength after compression - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present technology relates to a non-heat treated forged product, and more specifically, it is suitable for a part requiring high fatigue strength and excellent machinability such as a crankshaft of an automobile engine. The present invention relates to a method for manufacturing a non-heat treated forged product having excellent fatigue strength.

[0002]

2. Description of the Related Art For example, mechanical structural parts such as crankshafts and connecting rods for automobile engines are generally S50.
Carbon steels for mechanical structures such as C, or steels obtained by adding free-cutting elements such as S and Pb to these, which have been subjected to quenching and tempering treatment after hot forging, have been used in the past. In recent years, mainly non-heat treated steels in which quenching and tempering treatments are omitted by adding a trace amount of V, or steels in which free-cutting elements such as S, Pb and Ca are added are mainly used.

In order to increase the fatigue strength of the above parts, after being machined into a predetermined shape, roll processing, induction hardening treatment, soft nitriding treatment and the like are performed. Among them, compression processing such as roll processing can be applied only to the part where strength is required, and it can be inlined into the cutting processing line. It is one of the comprehensively excellent strengthening methods, such as the good point.

[0004]

[Problems to be Solved] With increasing concern over environmental issues in recent years, there has been a strong demand for improved fuel efficiency and lower emissions in automobiles, and the demand for smaller and lighter automobile engine parts has become stronger. Automotive engine parts are generally designed for fatigue strength, and it is essential to improve fatigue strength in order to reduce weight and size. It is considered that hardness is improved with respect to improvement of fatigue strength, but in this case, machinability is deteriorated.

Therefore, the above-mentioned roll working after machining has been noted as a measure for improving the fatigue strength, but the fatigue strength of the rolled parts is almost determined by the load applied during the roll working. Fatigue strength increases as the load applied to roll processing increases, but an extreme increase in load rather deteriorates the surface quality, leading to a decrease in fatigue strength or an increase in the cost of roll processing equipment. It becomes a problem. Therefore, it has been desired to develop a steel material and a forged product which have a high improvement rate in fatigue strength when subjected to roll processing or other compression processing under the same applied load and have excellent machinability.

[0006]

[Means for Solving the Problems] As a result of earnest studies by the present inventors, a steel material and a forged product having a high strength improvement rate when compression processing such as roll processing and the like and excellent machinability are obtained.
We have found that the objectives can be achieved by doing the following. That is, in terms of weight ratio, C: 0.15 to 0.45
%, Si: 0.10 to 0.80%, Mn: 0.50
1.50%, S: 0.040 to 0.120%, Cr:
0.10 to 0.50%, Al: 0.005 to 0.030
%, V: 0.15 to 0.50%, N: 0.0060% or more, with the balance being steel composed of Fe and unavoidable impurities, heated at 1000 to 1300 ° C, and air-cooled after hot forging Alternatively, by cooling with a fan, the microstructure consists of ferrite and pearlite, V amount: V (%), ferrite fraction: Vf (%), ferrite hardness: Hα (Hv)
Satisfying the relations of V × Vf × Hα ≧ 700, Vf ≧ 15 (%) , and further, in the part where high strength is required,
Rolling, cold forging, cold coining, or non-heat treated forging excellent in fatigue strength after compression processing and machinability, characterized in that performing the compression process of any one of cold extrusion It is in the method of manufacturing products.

As the compression processing, there are various processing methods such as cold forging processing, cold coining processing and cold extrusion processing in addition to roll processing.

Next, the reasons for limitation of the present invention will be described.
All percentages of alloying elements are% by weight.

(1) C C is 0.1 in order to secure the strength required as structural steel.
The content must be 5% or more, preferably 0.25% or more. However, when a large amount of C is contained, the amount of ferrite decreases, Vf <15, and sufficient machinability and fatigue strength after compression processing cannot be obtained. Therefore, the upper limit is 0.45%, preferably 0. 43% or less.

(2) Si Si improves fatigue strength by solid solution hardening of the base material,
In addition, it is necessary to contain at least 0.10%, preferably 0.15% or more as a deoxidizing auxiliary agent. On the other hand, if a large amount of Si is contained, the machinability decreases, so the upper limit is set to 0.
80%, preferably 0.60%, more preferably 0.3
5% or less.

(3) Mn Mn is at least 0.50 in order to secure necessary strength and hardenability and to form MnS which is effective in improving machinability.
%, Preferably 0.80% or more is necessary. On the other hand, if it is contained in a large amount, bainite remains hot forged,
Martensite and the like are generated, and the hardness becomes too high, and conversely the machinability is deteriorated. Therefore, the upper limit is set to 1.50%, preferably 1.30% or less.

(4) Since S S has the effect of improving machinability as MnS as described above, it is necessary to contain at least 0.040% or more. However, if contained in a large amount, the hot workability deteriorates, so the upper limit is 0.120%, preferably 0.07%.
0%

(5) Cr As with Mn, Cr must be added at least 0.10% for the purpose of ensuring the required strength and hardenability. However, if contained in a larger amount than necessary, bainite, martensite, etc. will be generated in the as-hot-forged state and machinability will be reduced due to increased hardness, so the upper limit is 0.50%, desirably 0.30% or less. And

(6) Al Al is added for deoxidation, but if it is less than 0.005%, its effect is insufficient, so the lower limit is made 0.005%. On the other hand, if a large amount of Al ₂ is contained, Al ₂
As the amount of O ₃ increases, AlN is also formed in association with N and traps N acting as an aging element during compression processing, so the upper limit is made 0.030%, preferably 0.020% or less.

(7) V V has an effect of improving the fatigue strength by forming carbonitrides in the ferrite structure during the cooling process after forging, as has been found in the past. In particular, when compression processing such as roll processing is performed, the effect of further improving fatigue characteristics can be obtained due to the interaction with the transition and residual stress in the base material.
In addition, since V carbonitride acts as tool surface protective lubrication, it is also effective for machinability. To obtain the above effect,
It is necessary to contain at least 0.15%, preferably 0.20% or more. The lower limit of the content may be limited to exceed 0.25%. However, even if added in a large amount, the effect is saturated and the cost becomes high, so the upper limit is made 0.50%, preferably 0.35%, and more preferably 0.30% or less.

(8) N N is an element effective in improving fatigue strength by so-called strain aging, which fixes dislocations introduced by compression working such as roll working, and therefore is at least 0.060.
%, Preferably 0.080% or more.

(9) Ca, Pb, Bi Ca, Pb, Bi are elements effective for further improving the machinability, and it is preferable to add one or two or more thereof according to need (claims). 2). In order to obtain the effect, each of Ca, Pb, and Bi is at least 0.
It is necessary to contain 0005%, 0.05% and 0.01%. However, even if added excessively, the effect is saturated, and the hot workability is deteriorated and the cost is increased. Therefore, the upper limits of Ca, Pb, and Bi are 0.0060 each.
%, 0.30% and 0.30%.

Next, the forging conditions and the reasons for limiting the structure, hardness, etc. of the forged product in the present invention will be explained.

(A) Heating temperature Since the characteristics of the forged product change depending on the heating temperature during hot forging, it is necessary to limit the range. When the heating temperature is low, V contained in steel does not sufficiently form a solid solution, and the effect on improving fatigue strength and machinability is not exhibited, so the lower limit was made 1000 ° C. However, if the heating temperature is too high, the grain size becomes coarse and the ferrite fraction is lowered, and the fatigue strength and machinability are rather lowered, so the upper limit was made 1300 ° C.

(B) Cooling after hot forging Since the characteristics of the forged product also change due to cooling after hot forging, it is necessary to limit it. In the present invention, in order to obtain good machinability and fatigue strength after compression working, it is necessary to form V carbonitride finely in a structure consisting of ferrite and pearlite. The subsequent cooling was air cooling or fan cooling.

(C) Microstructure As described above, in the present invention, in order to obtain good machinability and fatigue strength after compression processing such as roll processing, it is necessary to have a structure composed of ferrite and pearlite.

(D) V × Vf × Hα ≧ 700 In the earnest research until reaching the present invention, the relational expression V × Vf × Hα is the parameter that most affects the fatigue strength after compression processing such as roll processing. Led. Here, each variable is V amount: V (%), ferrite fraction: V
f (%), ferrite hardness: means Hα (Hv).
Then, as shown in FIG. 4, V × Vf × Hα is 700
It was found that the fatigue strength after compression processing such as roll processing shows a stable and high value if it is above, but the fatigue strength after compression processing sharply decreases below 700. Therefore, for the purpose of obtaining good fatigue strength after compression processing, V × Vf × Hα
It is necessary to be ≧ 700.

(E) Vf ≧ 15 (%) In the earnest research until reaching the present invention, as shown in FIG. 3, the ferrite fraction greatly affects the machinability (carbide turning life). It was found that the machinability improves as the ferrite fraction increases. In the present invention,
In order to secure the target machinability, it is necessary to satisfy Vf ≧ 15 (%).

[0024]

Embodiments Next, the features of the invention according to the present application will be described with reference to embodiments by comparison with comparative steels and conventional steels.

Table 1 shows the chemical composition of these test materials. In Table 1, A to G steels are the present invention, and A to C steels.
Steel is the first invention and steels D to G are the second invention. Also H ~ N
Steel is a comparative steel in which some elements do not satisfy the conditions of the present invention, O steel is a conventionally used ferrite / pearlite type non-heat treated steel, and P steel is JIS steel S55C.

[0026]

[Table 1]

The test material used as an example is a 90 mm diameter round bar manufactured by hot rolling, heated to 1200 ° C., then forged into a 60 mm diameter round bar at 1150 to 1100 ° C., and naturally cooled to room temperature. It was air-cooled, and only P steel, which is a conventional steel, was air-cooled, reheated at 880 ° C., oil-quenched, and tempered at 580 ° C.

A test piece was cut out from each of these test materials, and microstructure observation, ferrite fraction measurement, ferrite hardness measurement, tensile test, Ono-type rotary bending fatigue test, and turning tool life test with a cemented carbide tool were performed. It was

For microstructure observation, a sample corroded by Nital was observed with an optical microscope at a magnification of 100 times, and the ferrite fraction was measured by the point counting method for 50 fields of view similar to the above, and the average thereof was used. The value was used as the measured value. For the measurement of ferrite hardness, the measuring load is 10 gf
The hardness of 50 ferrite parts for each sample was measured with the Micro Vickers hardness meter of No. 2 and the average value thereof was used as the measured value.

As shown in FIG. 1, the Ono-type rotary bending fatigue test is shown in an enlarged view A in FIG. 1 in a round bar in which the diameter d ₁ of both ends 11 is φ12 mm and the diameter d ₂ of the central part 12 is φ10 mm. As shown, the radius of curvature R ₁ is 1.8 mm at the center.
The test piece 1 having the annular notched portion 13 was used.

Then, in the test apparatus 2 shown in FIG.
After rolling the test piece 1 on the receiving roller 22 under the condition of a load of 400 kgf using the roller 21 of R1.6 in the annular cutout portion 13, the test piece 1 is subjected to a fatigue test to determine the fatigue limit. I asked. Then, a value obtained by dividing the fatigue limit by the tensile strength obtained by the tensile test using the JIS No. 4 test piece separately cut from each test material, that is, the durability ratio was obtained.

For the turning tool life test, 60 mm round bars of each test material were peeled to a diameter of 55 mm to make test pieces, and a P10 cemented carbide tip was used on a lathe, cutting speed: 250 m /
min, feed: 0.2 mm / rev, depth of cut: 1.5 m
m, dry turning, lateral flank wear width 0.2 mm
The cutting time until it became to be measured as the life. The results are shown as an index with the life of conventional P steel being 100. Table 2 shows the results obtained by performing the above tests.

[0033]

[Table 2]

As is clear from Table 2, when comparing the forged product (round bar having a diameter of 60 mm) produced using the comparative steel and the conventional steel with the steel according to the present invention, the H steel has a high C content. Ferrite fraction as low as 9% and V × Vf × Hα
Is less than 700, the turning tool life and durability ratio are inferior. Since the I steel has a high Si content and a low S content, it has a particularly short tool life.
Since the J steel and the K steel have a high Mn content and a high Cr content, respectively, they have a mixed bainite structure and are inferior in both the turning tool life and the durability ratio. L steel has a low V content, and V ×
Since Vf × Hα is less than 700, both the turning tool life and the durability ratio are inferior. Since M steel and N steel each have a high Al content and a low N content, the strain aging effect after roll processing cannot be obtained and the durability ratio is poor.

Further, the conventional steel, O steel, is a non-heat treated steel of the ferrite / parrat type, but its V content is low and V × Vf
Since xHα is less than 700, both the life of the turning tool and the durability ratio are inferior. Although P steel is a tempered carbon steel, it still has poor turning tool life and durability ratio.

On the other hand, the forged products (round bars having a diameter of 60 mm) produced from the steels A to C of the present invention are mainly
V x Al by containing appropriate amounts of V, Al and N, controlling other chemical components, and controlling the heat treatment temperature and cooling rate.
Vf × Hα is 700 or more, and ferrite fraction: Vf
It was confirmed that by limiting the ratio to 15% or more, excellent characteristics can be obtained in both the turning tool life and the durability ratio. Further, it was confirmed that the forged products of the D to G steels to which the free-cutting element was added had a significantly improved life especially in the turning tool life.

Next, examples for investigating the effects of changes in the heating temperature during forging and the cooling rate after forging are shown below. Among the steels shown in Table 1, for the C steel which is the steel of the present invention, a round bar with a diameter of 90 mm manufactured by hot rolling is used.
After heating to the temperature of 3 conditions of 0, 1200, 1350 ℃,
(Heating temperature)-(50 to 100) ° C, forged into a round bar with a diameter of 60 mm, and then forged by four conditions of slow cooling by stacking in a pallet to room temperature, natural air cooling, fan cooling, oil quenching. It was manufactured and the same test as in the above-mentioned example was conducted. Table 3 shows the evaluation results.

[0038]

[Table 3]

As is clear from Table 3, even if the chemical composition is within the scope of the claims of the present invention, if the heating temperature during forging and the cooling after forging fall outside the scope of the claims of the present invention, the ferrite fraction or V Since xVfxHα is out of the scope of the claims of the present invention, it is understood that the turning tool life or the durability ratio is reduced.

[0040]

INDUSTRIAL APPLICABILITY The present invention optimizes the chemical composition of steel, controls the heating temperature during hot forging and the cooling conditions after forging to obtain a ferrite-pearlite structure, and the V content: V ( %), Ferrite fraction: Vf
(%), Ferrite hardness: V × V between Hα (Hv)
By satisfying the relationship of f × Hα ≧ 700 and Vf ≧ 15 (%), it is possible to obtain a non-heat treated forged product excellent in machinability and fatigue strength after compression processing. Specifically, the durability ratio was 0.60 or more, and the life of the tool to be machined was an index value with the life of the quenched and tempered material of JIS steel S55C being 100.
Excellent performance of 0 or more can be obtained. More preferably, the durability ratio is 0.65 or more, and the excellent tool life is 120 or more in an index value with the life of the quenching and tempering material of JIS steel S55C being 150 in the tool life of the work.

Therefore, this technology is particularly advantageous in that it can achieve both weight reduction and cost reduction for parts that require high fatigue strength and excellent machinability, such as crankshafts of automobile engines. The contribution to the above is extremely large.

[Brief description of drawings]

FIG. 1 is an explanatory view of an Ono-type rotary bending fatigue test piece.

FIG. 2 is an explanatory view showing a method of rolling on an Ono-type rotary bending fatigue test piece.

FIG. 3 is a diagram showing a relationship between a ferrite fraction and a turning life.

FIG. 4 is a view showing a relationship between V × Vf × Hα and a durability ratio of a rolled Ono-type rotary bending fatigue test.

[Explanation of symbols]

1. ． ． Test pieces, 13. ． ． Annular notch, 2. ． ． Test equipment,

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/60 C22C 38/60 (72) Inventor Susumu Owaki Wano Wari No. 1 Arao-cho, Tokai City, Aichi Prefecture Aichi Steel Co., Ltd. (72 ) Inventor Go Kawamoto 1 Toyota Town, Toyota City, Aichi Prefecture, Toyota Motor Corporation (72) Inventor Motohide Mori 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation, (72) Inventor Takayuki Hariko Aichi 1 Toyota-cho, Toyota-shi, Japan (56) References JP-A-9-143610 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 8/00 B21J 5/00 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. A weight ratio of C: 0.15 to 0.45.
%, Si: 0.10 to 0.80%, Mn: 0.50
1.50%, S: 0.040 to 0.120%, Cr:
0.10 to 0.50%, Al: 0.005 to 0.030
%, V: 0.15 to 0.50%, N: 0.0060% or more, with the balance being steel consisting of Fe and inevitable impurities, heated at 1000 to 1300 ° C., air-cooled after hot forging Alternatively, by cooling with a fan, the microstructure consists of ferrite and pearlite, and V amount: V (%), ferrite fraction: Vf (%), ferrite hardness: Hα (Hv), V × Vf × Hα ≧ 700, Vf ≧ 15 (%), satisfying the relations , and roll processing and cold forging to the parts that require high strength.
A manufacturing method of a non-heat treated forged product excellent in machinability and fatigue strength after compression processing, which is characterized by performing compression processing of cold working, cold coining, or cold extrusion. 2. A weight ratio of C: 0.15 to 0.45.
%, Si: 0.10 to 0.80%, Mn: 0.50
1.50%, S: 0.040 to 0.120%, Cr:
0.10 to 0.50%, Al: 0.005 to 0.030
%, V: 0.15-0.50%, N: 0.0060% or more, and further Ca: 0.0005-
0.0060%, Pb: 0.05-0.30%, Bi:
Steel containing 1 or 2 or more of 0.01 to 0.20% and the balance of Fe and unavoidable impurities is used. The steel is heated at 1000 to 1300 ° C., hot-forged and then air-cooled or fan-cooled. As a result, the microstructure is composed of ferrite and pearlite, and V amount: V (%), ferrite fraction: Vf (%), ferrite hardness: Hα (Hv), V × Vf × Hα ≧ 700, Vf Satisfies the relation of ≧ 15 (%) , and further roll processing and cold forging to the parts where high strength is required.
A manufacturing method of a non-heat treated forged product excellent in machinability and fatigue strength after compression processing, which is characterized by performing compression processing of cold working, cold coining, or cold extrusion.