JP6299864B2 - Non-tempered crankshaft steel and non-tempered crankshaft - Google Patents

Description

  The present invention relates to a steel for a crankshaft and a crankshaft, and more particularly to a non-tempered steel for a crankshaft and a non-tempered crankshaft in which tempering treatment (quenching and tempering) after hot forging is omitted.

  Conventionally, forged crankshafts in which tempering treatment is omitted have been provided. The tempering treatment means a quenching and tempering treatment that improves the mechanical properties of the steel such as strength. Crankshaft steel from which tempering treatment is omitted is referred to as non-tempered crankshaft steel. With non-tempered crankshaft steel, the tempering treatment is not performed, so that the manufacturing cost can be reduced.

  FIG. 1 is a front view of the main part of the crankshaft. With reference to FIG. 1, the crankshaft 1 includes a crankpin 2, a crank journal 3, and a crank arm 4. The crank arm 4 is disposed between the crankpin 2 and the crank journal 3 and is connected to the crankpin 2 and the crank journal 3. The crankshaft 1 further includes a pin fillet portion 21 and a journal fillet portion 31. The pin fillet portion 21 is disposed between the crankpin 2 and the crank arm 4 and corresponds to a joint portion between the crankpin 2 and the crank arm 4. The journal fillet portion 31 is disposed between the crank journal 3 and the crank arm 4 and corresponds to a joint portion between the crank journal 3 and the crank arm 4.

  The crankpin 2 is rotatably attached to a connecting rod (not shown). The crank pin 2 is arranged so as to be shifted from the rotation axis of the crank shaft 1. The crank journal 3 is disposed coaxially with the rotation axis of the crankshaft 1. The crankpin 2 rotates with respect to the inner surface of the large end portion of the connecting rod (not shown) via a slide bearing. Therefore, wear resistance is required on the surface of the crankpin 2. Similar to the crankpin 2, the surface of the crank journal 3 is also required to have wear resistance.

  In order to improve the wear resistance of such a crankshaft, a surface hardening heat treatment may be performed on a steel material (hereinafter referred to as an intermediate product) after hot forging and machining (for example, cutting). The surface hardening heat treatment is, for example, induction hardening. In induction hardening, electromagnetic induction by high frequency electromagnetic waves is caused to heat and quench the surface of the intermediate product. In this case, a hardened hardened layer is formed on the surface of the intermediate product. The hardened layer increases the wear resistance of the crankshaft.

  The pin fillet portion 21 and the journal fillet portion 31 of the crankshaft 1 are collectively referred to as a fillet portion. Fillet roll processing is performed on the fillet portion. Fillet roll processing is performed to increase the bending fatigue strength of the fillet portion. Specifically, the fillet portion is plastically processed by roll processing, and the surface layer of the fillet portion is work-hardened.

  In order to increase the bending fatigue strength of the crankshaft, it is preferable not only to perform fillet roll processing but also to increase the strength of the base material. However, as described above, the crankshaft is manufactured by cutting an intermediate product after hot forging. Therefore, if the strength of the base material is too high, the machinability of the intermediate product is lowered. Furthermore, if the alloy element is excessively contained in order to increase the strength of the base material, when the induction hardening is performed on the crankshaft, the cracking is likely to occur. Therefore, crankshaft steel subjected to fillet roll processing is required to have high bending fatigue strength and machinability, and further to suppress occurrence of quenching cracks due to induction hardening.

  Recently, based on the demand for environmental considerations, there is a demand for lighter and smaller crankshafts. Therefore, for example, high bending fatigue strength of 750 MPa or more is required for the crankshaft after the fillet roll processing.

  Japanese Patent Application Laid-Open No. 2005-171311 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2005-179753 (Patent Document 2) propose crankshaft steel having high strength and high machinability.

  The non-tempered steel for crankshaft disclosed in Patent Document 1 is C: 0.27 to 0.43%, Si: 0.80 to 2.00%, Mn: 0.30 to 1 by weight%. 20%, P: 0.035% or less, S: 0.04% or less, Cr: 0.20 to 1.00%, Cu: 0.30% or less, Ni: 0.25% or less, Mo: 0 0.05% or less, the balance being substantially made of Fe, and satisfying the formula 0.78 ≦ C + 1/6 × Si + 1 / 4.5 × Mn + 1/15 × Ni + 1/4 × Cr ≦ 0.84, It is a ferrite + pearlite mixed structure after forging and air cooling. Patent Document 1 describes that the crankshaft steel does not contain V, has a tensile strength of 800 MPa or more, and is excellent in machinability.

  The non-heat treated steel for crankshaft disclosed in Patent Document 2 is C: 0.35 to 0.55%, Si: 0.4 to 2%, Mn: 0.4 to 1.5% by weight. , P: 0.01 to 0.1%, Cu: 0.05 to 1%, Ni: 0.05 to 1%, Cr: 0.01 to 1%, s-Al: 0.005 to 0.05 %, N: 0.005 to 0.030%, the balance is substantially made of Fe, and the structure after hot forging is ferrite + pearlite. Patent Document 2 describes that the non-heat treated steel for crankshaft has high strength and high machinability without containing V.

JP 2005-171311 A JP 2005-179753 A

  However, the crankshaft steels of Patent Literature 1 and Patent Literature 2 have low bending fatigue strength. In any patent literature, the bending fatigue strength of the crankshaft is less than 750 MPa in any of the examples (Table 1 of Patent Literature 1). , See Table 2 of Patent Document 2).

  An object of the present invention is to provide a non-heat treated crankshaft steel and a non-heat treated steel that have high bending fatigue strength after fillet roll processing, excellent machinability, and suppress the occurrence of quench cracks even after induction hardening. It is to provide a crankshaft.

Non-tempered crankshaft steel according to the present embodiment is in mass%, C: 0.37 to 0.50%, Si: 0.15 to 0.79%, Mn: 1.15 to 1.40%, P: 0.040% or less, S: 0.040 to 0.10%, Cr: 0.10 to 0.30%, Al: 0.022 to 0.050%, Ti: 0.005 to 0.025 %, V: 0.20 to 0.30%, N: 0.0080 to 0.0180%, Mo: 0 to 0.30%, Cu: 0 to 0.8%, and Ni: 0 to 0.0. 50% is contained, and Formula (1)-Formula (3) is satisfy | filled.
0.86 ≦ C + Si / 7 + Mn / 5 + Cu / 15 + Ni / 15 + Cr / 9 + Mo / 4 + V ≦ 1.10 (1)
Ti / N ≦ 1.5 (2)
521-353C-22.0Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo ≧ 305 (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the expressions (1) to (3). When no element is contained, “0” is assigned to the corresponding element symbol.

  The non-tempered crankshaft steel may contain Mo: 0.05 to 0.30%. The non-tempered crankshaft steel may contain Cu: 0.05 to 1.0% and Ni: 0.01 to 0.50%.

  The non-tempered crankshaft according to this embodiment is obtained by hot forging the steel for non-tempered crankshaft, further performing cutting after hot forging, performing induction hardening after cutting, and fillet after induction hardening. Manufactured by rolling.

  In the non-heat treated crankshaft steel and the non-heat treated crankshaft according to the present embodiment, the bending fatigue strength after fillet roll processing is high, the machinability is excellent, and the occurrence of cracking after induction hardening is suppressed.

FIG. 1 is a front view of the main part of the crankshaft. FIG. 2 is a front view of an Ono type rotating bending fatigue test piece. FIG. 3 is a front view of a fillet roll processing apparatus for performing fillet roll processing for the Ono-type rotary bending fatigue test shown in FIG. 2. FIG. 4 is a side view of the fillet roll processing apparatus shown in FIG. 3.

  Hereinafter, embodiments of the present invention will be described in detail. The present inventors have made various studies in order to solve the above-described problems. As a result, the present inventors obtained the following knowledge.

  (A) If the hardness of steel is high, bending fatigue strength will also increase. If an appropriate amount of C, Si, Mn, Cu, Ni, Cr, Mo, and V, which are elements that increase the hardness of steel, is contained, the hardness of the steel is appropriately increased and the bending fatigue strength is increased. On the other hand, if the total amount of these element contents is too high, the hardness of the steel becomes too high. In this case, the machinability of steel decreases.

  It is defined as fn1 = C + Si / 7 + Mn / 5 + Cu / 15 + Ni / 15 + Cr / 9 + Mo / 4 + V. If it has the below-mentioned chemical composition and fn1 is 0.86 to 1.10, the hardness of suitable steel will be obtained and high bending fatigue strength will be obtained. Furthermore, the hardness of steel does not become too high, and high machinability is also obtained.

  (B) Fillet roll processing was performed on a fatigue test piece simulating a crankshaft to form a fillet portion. A bending fatigue test was performed on the fatigue test piece in which the fillet portion was formed. Microscopic observation was performed on the fillet portion of the fatigue test piece that was endured in the bending fatigue test (that is, not broken in the bending fatigue test). As a result, although a fine fatigue crack occurred in the fillet portion, the progress of the fatigue crack stopped halfway. The surface layer of the fillet part of the fatigue test piece was further observed. As a result, much of the fatigue crack propagation stopped at the grain boundaries.

  From the above results, in the case of a crankshaft that has been subjected to fillet roll processing, if there are many crystal grain boundaries, even if a fatigue crack occurs, its progress can be suppressed. As a result, the fatigue strength of the crankshaft is increased. If the crystal grains are refined, the crystal grain boundaries increase. Therefore, an appropriate amount of fine Ti nitride (TiN) is produced in the steel. Ti nitride crystallizes in the molten steel during casting or ingot forming. Ti nitride is not melted by heating in subsequent hot working such as hot forging and is present in the steel. Therefore, if Ti nitride is formed in the steel, crystal grains in the steel can be refined without depending on the heating conditions in the hot working.

  On the other hand, if the Ti content is too high relative to the N content, coarse Ti nitrides are formed or coarse Ti carbonitrides are formed. In this case, coarse Ti nitride and Ti carbonitride are the starting points of fatigue cracks. For this reason, the fatigue strength of steel is rather lowered.

  It is defined as fn2 = Ti / N. If it has the chemical composition mentioned later and fn2 is 1.5 or less, the formation of coarse Ti nitride and Ti carbonitride is suppressed. Therefore, the occurrence of crack starting points is suppressed. Furthermore, the crystal grains are refined by the fine Ti nitride, and the propagation of cracks is suppressed. Therefore, bending fatigue strength is increased in the fillet portion.

  (C) Of the elements contained in fn1, C, Si, Mn, Cu, Ni, Cr, and Mo all increase the hardenability of the steel and increase the hardness of the steel. Therefore, if the total content of these elements is excessively high, quench cracking may occur due to induction hardening.

  It is defined as fn3 = 521-353C-22.0Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo. If it has the chemical composition mentioned later and fn3 is 305 or more, the content of these elements is appropriate, and the occurrence of quench cracks due to induction hardening is suppressed.

  The non-heat treated crankshaft steel and the non-heat treated crankshaft according to the present embodiment completed based on the above knowledge will be described in detail.

[Chemical composition]
The chemical composition of the non-tempered crankshaft steel of this embodiment contains the following elements.

C: 0.37 to 0.50%
Carbon (C) increases the hardness of steel (that is, the base material of the crankshaft) and increases the bending fatigue strength of the crankshaft. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, the crankshaft steel will be too hard and the machinability will be reduced. Therefore, the C content is 0.37 to 0.50%. The minimum with preferable C content is higher than 0.37%, More preferably, it is 0.39%, More preferably, it is 0.40%. The upper limit with preferable C content is less than 0.50%, More preferably, it is 0.45%, More preferably, it is 0.44%.

Si: 0.15 to 0.79%
Silicon (Si) deoxidizes steel. Si further dissolves to increase the hardness of ferrite in the steel and increase the hardness of the steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the hardness of the steel becomes too high and the machinability decreases. Therefore, the Si content is 0.15 to 0.79%. The minimum with preferable Si content is higher than 0.15%, More preferably, it is 0.20%, More preferably, it is 0.24%. The upper limit with preferable Si content is less than 0.79%, More preferably, it is 0.70%, More preferably, it is 0.67%.

Mn: 1.15% to 1.40%
Manganese (Mn) deoxidizes steel in the same way as Si. Mn further increases the hardness and strength of the steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the hardness of the crankshaft steel becomes too high, and the machinability deteriorates. Therefore, the Mn content is 1.15% to 1.40%. The minimum with preferable Mn content is higher than 1.15%, More preferably, it is 1.18%. The upper limit with preferable Mn content is less than 1.40%, More preferably, it is 1.35%, More preferably, it is 1.30%.

P: 0.040% or less Phosphorus (P) is an impurity. P reduces the bending fatigue strength of steel. Therefore, the P content is 0.040% or less. The upper limit with preferable P content is less than 0.040%, More preferably, it is 0.030%. The P content is preferably as low as possible.

S: 0.040 to 0.10%
Sulfur (S) increases the machinability of steel. If the S content is too low, this effect cannot be obtained. On the other hand, if the S content is too high, the bending fatigue strength of the steel decreases. Therefore, the S content is 0.040 to 0.10%. The minimum with preferable S content is higher than 0.040%, More preferably, it is 0.050%, More preferably, it is 0.055%, More preferably, it exceeds 0.060%. The upper limit with preferable S content is less than 0.10%, More preferably, it is 0.080%, More preferably, it is 0.070%.

Cr: 0.10 to 0.30%
Chromium (Cr) increases the hardness of steel and increases the bending fatigue strength. If the Cr content is too low, this effect cannot be obtained. On the other hand, if the Cr content is too high, the hardness of the steel becomes too high and the machinability decreases. Therefore, the Cr content is 0.10 to 0.30%. The minimum with preferable Cr content is higher than 0.10%, More preferably, it is 0.20%, More preferably, it is 0.22%. The upper limit with preferable Cr content is less than 0.30%, More preferably, it is 0.28%, More preferably, it is 0.27%.

Al: 0.022 to 0.050%
Aluminum (Al) deoxidizes steel in the same way as Si. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, coarse oxides are formed in the steel, and the bending fatigue strength of the steel decreases. Therefore, the Al content is 0.022 to 0.050%. The minimum with preferable Al content is higher than 0.022%, More preferably, it is 0.025%, More preferably, it is 0.030%. The upper limit with preferable Al content is less than 0.050%, More preferably, it is 0.040%, More preferably, it is 0.038%. Al content in this specification is sol. It means the content of Al (acid-soluble Al).

Ti: 0.005-0.025%
Titanium (Ti) combines with nitrogen to form Ti nitride. Ti nitride suppresses the coarsening of crystal grains and refines the crystal grains. If fatigue cracks occur near the surface of the fillet part (pin fillet part 21 and journal fillet part 31) that has undergone fillet roll processing, if the crystal grains in the fillet part are fine, the fatigue crack Is prevented by the grain boundaries. Therefore, Ti increases the bending fatigue strength of the crankshaft. If the Ti content is too low, the above effect cannot be obtained. On the other hand, if the Ti content is too high, coarse Ti nitride or coarse carbonitride is formed. Coarse Ti nitride and Ti carbonitride serve as starting points for fracture and lower the bending fatigue strength of the steel. Therefore, the Ti content is 0.005 to 0.025%. A preferable lower limit of the Ti content is higher than 0.005%, more preferably 0.007%, and further preferably 0.008%. The upper limit with preferable Ti content is less than 0.025%, More preferably, it is 0.020%, More preferably, it is 0.015%.

V: 0.20 to 0.30%
Vanadium (V) forms V carbide or V carbonitride to increase the hardness of the steel and increase the bending fatigue strength. If the V content is too low, this effect cannot be obtained. On the other hand, if the V content is too high, the hardness of the steel becomes too high, and the machinability of the steel decreases. Therefore, the V content is 0.20 to 0.30%. The minimum with preferable V content is higher than 0.20%, More preferably, it is 0.22%, More preferably, it is 0.24%. The upper limit with preferable V content is less than 0.30%, More preferably, it is 0.28%, More preferably, it is 0.26%.

N: 0.0080 to 0.0180%
Nitrogen (N) combines with Ti to form Ti nitride and refines the crystal grains. Therefore, N increases the bending fatigue strength of steel. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, the above effect is saturated. Therefore, the N content is 0.0080 to 0.0180%. The minimum with preferable N content is higher than 0.0080%, More preferably, it is 0.085%, More preferably, it is 0.0090%. The upper limit with preferable N content is less than 0.0180%, More preferably, it is 0.0160%, More preferably, it is 0.0150%.

  The balance of the chemical composition of the non-tempered crankshaft steel of this embodiment consists of Fe and impurities. The impurities referred to here are ores and scraps used as raw materials for steel, or elements mixed in from the environment of the manufacturing process.

  The chemical composition of the non-tempered crankshaft steel of this embodiment may further contain Mo instead of a part of Fe.

Mo: 0 to 0.30%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo dissolves and increases the hardness of ferrite in the steel. In this case, the hardness of the steel increases and the bending fatigue strength increases. However, if the Mo content is too high, the above effect is saturated and the manufacturing cost increases. Therefore, the Mo content is 0 to 0.30%. The minimum with preferable Mo content is 0.05%, More preferably, it is 0.07%, More preferably, it is 0.1%. The upper limit with preferable Mo content is less than 0.30%, More preferably, it is 0.25%, More preferably, it is 0.20%.

  The chemical composition of the non-tempered crankshaft steel of this embodiment may further contain Cu and Ni instead of part of Fe.

Cu: 0 to 0.8%
Copper (Cu) is an optional element and may not be contained. When contained, Cu increases the hardness of the steel and increases the bending fatigue strength of the steel. However, if the Cu content is too high, the hot workability of the steel decreases. Therefore, the Cu content is 0 to 0.8%. The minimum with preferable Cu content is 0.05%, More preferably, it is 0.07%, More preferably, it is 0.1%. The upper limit with preferable Cu content is less than 0.8%, More preferably, it is 0.4%, More preferably, it is 0.3%.

Ni: 0 to 0.50%
Nickel (Ni) is an optional element and may not be contained. When Cu is contained, hot cracking called “Cu checking” is likely to occur. If Ni is contained, the occurrence of Cu checking is suppressed. However, this effect is saturated if the Ni content is too high. Therefore, the Ni content is 0 to 0.50%. The minimum with preferable Ni content is 0.01%, More preferably, it is 0.05%, More preferably,. 08%. The upper limit with preferable Ni content is less than 0.50%, More preferably, it is 0.20%, More preferably, it is 0.15%. The Ni content preferably further satisfies Ni / Cu ≧ 0.5.

[Regarding Formula (1) to Formula (3)]
The non-heat treated crankshaft steel according to the present invention further satisfies the formulas (1) to (3).
0.86 ≦ C + Si / 7 + Mn / 5 + Cu / 15 + Ni / 15 + Cr / 9 + Mo / 4 + V ≦ 1.10 (1)
Ti / N ≦ 1.5 (2)
521-353C-22.0Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo ≧ 305 (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the expressions (1) to (3). If no element is contained, “0” is assigned to the corresponding element symbol.

[Regarding Formula (1)]
It is defined as fn1 = C + Si / 7 + Mn / 5 + Cu / 15 + Ni / 15 + Cr / 9 + Mo / 4 + V. fn1 is an index related to the hardness of the non-heat treated crankshaft steel. As described above, C, Si, Mn, Cu, Ni, Cr, Mo, and V all increase the hardness of the steel. If fn1 is too low, the hardness of the steel is low and the bending fatigue strength is low. On the other hand, if fn1 is too high, the hardness of the steel becomes too high, and the machinability deteriorates.

  If fn1 is 0.86 to 1.10, the hardness of the steel is high and high bending fatigue strength can be obtained. Furthermore, the hardness of steel does not become too high, and high machinability is also obtained. The preferable lower limit of fn1 is higher than 0.86, more preferably 0.90, and further preferably 0.94. The upper limit with preferable fn1 is less than 1.10, More preferably, it is 1.07, More preferably, it is 1.05.

[Regarding Formula (2)]
It is defined as fn2 = Ti / N. If fn2 is too high, the Ti content is too high relative to the N content. In this case, coarse Ti nitride and coarse Ti carbonitride are formed. Therefore, the bending fatigue strength of the steel is reduced. If fn2 is 1.5 or less, generation of coarse Ti nitride and coarse Ti carbonitride is suppressed. Therefore, the occurrence of the fatigue crack starting point is suppressed. Furthermore, the crystal grains are refined by the fine Ti nitride, and the propagation of fatigue cracks is prevented by the crystal grain boundaries. Therefore, the bending fatigue strength of the crankshaft that has been subjected to fillet roll processing is increased. The upper limit with preferable fn2 is less than 1.5, More preferably, it is 1.40, More preferably, it is 1.30.

[Regarding Formula (3)]
It is defined as fn3 = 521-353C-22.0Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo. fn3 is an index relating to the occurrence of cracking when induction hardening is performed on the crankpin 2 and the crank journal 3. All of C, Si, Mn, Cu, Ni, Cr and Mo are dissolved in steel to enhance the hardenability of the steel. If fn3 is too low, the hardenability of the steel becomes too high. In this case, if induction hardening is performed on portions corresponding to the crankpin and the crank journal, a crack is generated. If fn3 is 305 or more, the occurrence of burning cracks is suppressed. The preferable lower limit of fn3 is higher than 305, more preferably 315, and further preferably 320.

[Production method]
An example of the manufacturing method of the steel for crankshafts and the non-tempered crankshaft of this embodiment mentioned above is demonstrated.

  The molten steel which satisfy | fills the said chemical composition and Formula (1)-Formula (3) is manufactured. The molten steel is made into a slab by a continuous casting method. You may make molten steel into an ingot (steel ingot) by the ingot-making method. The slab or ingot may be hot-worked into a billet (steel piece) or a steel bar. The crankshaft steel of this embodiment is manufactured by the above manufacturing process.

  A slab, ingot, billet or steel bar is heated in a heating furnace. A hot slab, ingot, billet or steel bar is hot forged to produce an intermediate product. Cool the intermediate product after hot forging.

  No tempering (quenching and tempering) is performed on intermediate crankshaft products. That is, in this embodiment, the intermediate product is non-tempered.

  The intermediate product is cut into a final shape. Of the intermediate product after cutting, induction hardening is performed on a portion corresponding to a crankpin and / or a crank journal. Induction hardening increases the wear resistance of crankpins and crank journals.

  After performing induction hardening, a fillet roll process is implemented with respect to a fillet part (the pin fillet part 21 and the journal fillet part 31). In fillet roll processing, a roller is pressed against the surface of the fillet portion while rotating the intermediate product after cutting. Thereby, the surface of a fillet part is plastically processed and work-hardened.

  The non-heat treated crankshaft of this embodiment is manufactured by the above manufacturing process. The chemical composition of the manufactured non-tempered crankshaft is as described above, and satisfies the formulas (1) to (3). Therefore, the crystal grains are fine even in the fillet portion. Therefore, the non-heat treated crankshaft of this embodiment has high strength and high bending fatigue strength, and also has excellent machinability. Furthermore, it is difficult for cracking due to induction hardening to occur.

[Test method]
The molten steels having test numbers 1 to 26 having the chemical compositions shown in Table 1 were produced using a 70-ton converter.

  Bloom was produced by continuously casting molten steel. The bloom was divided and rolled to produce a billet having a cross-sectional dimension of 180 mm × 180 mm.

  The billet was heated to 1200 ° C. Hot forging was performed on the heated billet to produce a steel bar having a diameter of 90 mm (corresponding to the intermediate product described above). The finishing temperature in hot forging was 1000 to 1050 ° C. The steel bar after hot forging was allowed to cool in the air and cooled to room temperature (25 ° C.). In addition, tempering treatment (quenching and tempering) was not performed on the steel bars. In other words, the manufactured steel bar was not tempered.

[Machinability evaluation test]
Peeling processing was performed on the manufactured steel bar having a diameter of 90 mm to manufacture a test material having a diameter of 80 mm. Turning, which is a kind of cutting, was performed on the manufactured test material. As the cutting tool, a P-type carbide tool that was not coated was used. Turning was performed under the conditions of a cutting speed of 160 m / min, a feed speed of 0.25 mm / rev, a cutting depth of 2.0 mm, and no lubricating oil. Turning for 30 minutes was performed under the above conditions, and the amount of flank wear (mm) of the cutting tool after turning was measured.

[Sintering crack evaluation test]
In the cross section of the steel bar having a diameter of 90 mm, the position at which the distance R (radius) between the outer peripheral surface of the steel bar and the central axis is equally divided is defined as the R / 2 position. A round bar specimen having a diameter of 30 mm and a length of 70 mm centered on the R / 2 position of a 90 mm diameter steel bar was taken from a 90 mm diameter steel bar. The length direction of the round bar test piece was parallel to the axial direction of a 90 mm diameter steel bar. A groove (depth 5 mm, width 10 mm) extending in the circumferential direction was formed on the outer peripheral surface at the center position of the length of the round bar test piece.

  Ten round bar test pieces described above were prepared for each test number. Induction hardening was performed on ten round bar test pieces. Specifically, after placing a round bar test piece in a high-frequency induction furnace, high-frequency was performed under the conditions of a frequency of 40 kHz, a voltage of 6 kV, a heating time of 3.0 seconds, and a cooling time of 5.0 seconds (using water-soluble heat-treated oil). Quenching was performed.

  Magnetic powder flaw detection was performed on the 10 round bar test pieces subjected to induction hardening, and it was visually observed whether or not quench cracks occurred in the grooves of the round bar test piece. The number of round bar specimens that were visually confirmed to be cracked was counted.

[Bending fatigue strength evaluation test]
As shown in FIG. 2, an Ono type rotating bending fatigue test piece (hereinafter simply referred to as a fatigue test piece) having a R / 2 position of a 90 mm diameter steel bar as a central axis, the central axis being parallel to the forging axis. The sample was taken from a steel bar having a diameter of 90 mm. Referring to FIG. 2, the fatigue test piece had a circular cross section, and the fatigue test piece had a parallel portion having a diameter of 10 mm including an annular groove. The annular groove was formed at the center of the parallel part, and the cross section of the annular groove was a semicircular arc having a radius of 1.5 mm. The numerical values in FIG. 2 mean dimensions.

  Fillet roll processing was performed on the annular groove of the fatigue test piece using the fillet roll processing apparatus shown in FIGS. 3 and 4. With reference to FIG.3 and FIG.4, the fatigue test piece 20 was arrange | positioned between a pair of backup rollers 10. FIG. At this time, the fatigue test piece 20 was disposed on the pair of backup rollers 10. The diameter of the backup roller 10 was 28 mm. Furthermore, the large roller 30 having a chemical composition corresponding to SKH51 defined in JIS G4403 (2006) was pressed against the annular groove of the fatigue test piece 20 from above the fatigue test piece 20 with a load of 4.9 kN. The diameter of the large roller was 40 mm, and the radius of curvature of the convex portion on the outer peripheral surface was 1.3 mm. The fatigue test piece 20 was rotated while pressing the large roller 30 to perform fillet roll processing. The number of rotations was 10 times.

An Ono-type rotating bending fatigue test was performed on the fatigue test piece subjected to the fillet roll processing under the conditions of swinging at room temperature (25 ° C.), in the air, and at a rotational speed of 3000 rpm. The maximum stress among the test results that did not break even after 10 ⁷ cycles was defined as the bending fatigue strength (MPa) of that test number.

[Test results]
The test results are shown in Table 2.

  With reference to Table 2, in the test numbers 1-11, chemical composition was appropriate and fn1-fn3 satisfy | filled Formula (1)-Formula (3). Therefore, the flank wear amount of these test numbers was 0.40 mm or less, indicating high machinability. In addition, the number of cracks in these test numbers was zero, and the occurrence of cracks due to induction hardening was suppressed. Furthermore, the bending fatigue strengths of these test numbers were all 750 MPa or more and had excellent bending fatigue strength.

  On the other hand, the C content of test number 12 was too high. Therefore, the amount of flank wear exceeded 0.40 mm, and the machinability was low. This is probably because the C content was too high and the hardness of the base material was too high.

  The C content of test number 13 was too low. Therefore, the bending fatigue strength was less than 750 MPa.

  The Si content of test number 14 was too high. Therefore, the amount of flank wear exceeded 0.40 mm, and the machinability was low. This is probably because the Si content is too high and the hardness of the base material is too high.

  The Mn content of test number 15 was too high. Therefore, the amount of flank wear exceeded 0.40 mm, and the machinability was low. This is probably because the Mn content is too high and the hardness of the base material is too high.

  The S content of test number 16 was too low. Therefore, the amount of flank wear exceeded 0.40 mm, and the machinability was low.

  The Cr content of test number 17 was too high. Therefore, the amount of flank wear exceeded 0.40 mm, and the machinability was low. This is probably because the Cr content was too high and the hardness of the base material was too high.

  The Al content of test number 18 was too high. Therefore, the bending fatigue strength was less than 750 MPa. Since Al content was too high, the coarse oxide used as the starting point of a crack produced | generated, and it is thought that the bending fatigue strength was low by this.

  The Ti content of Test No. 19 was too low. Therefore, the bending fatigue strength was less than 750 MPa. Since the Ti content was too low, the effect of crystal grain refinement could not be obtained, and the effect of suppressing crack growth could not be obtained, so the bending fatigue strength was considered to be low.

  The V content of test number 20 was too high. Therefore, the flank wear amount exceeded 0.40 mm, and the machinability was low. This is probably because the V content was too high and the hardness of the steel was too high.

  The V content of test number 21 was too low. Therefore, the bending fatigue strength was less than 750 MPa. It is considered that the hardness of the steel was too low because the V content was too low.

  In test number 22, fn1 was less than the lower limit of formula (1). Therefore, the bending fatigue strength was less than 750 MPa. Since fn1 was less than the lower limit of formula (1), it is considered that the hardness of the steel was too low.

  In test number 23, fn1 exceeded the upper limit of formula (1). Therefore, the amount of flank wear exceeded 0.40 mm, and the machinability was low. It is considered that fn1 exceeded the upper limit of the formula (1) and the hardness of the steel became too high.

  In test number 24, fn2 did not satisfy Formula (2). Therefore, the bending fatigue strength was less than 750 MPa. Since fn2 does not satisfy the formula (2) and coarse Ti nitride or coarse Ti carbonitride is generated, it is considered that the bending fatigue strength was low.

  In test number 25 and test number 26, fn3 did not satisfy Formula (3). As a result, burning cracks occurred. This is probably because fn3 did not satisfy the formula (3) and the hardenability of the steel became too high.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

  The non-heat treated crankshaft steel of the present embodiment has high bending fatigue strength and high machinability, and is hard to cause quench cracking during induction hardening. Therefore, the non-heat treated crankshaft steel of the present embodiment can be widely used as a crankshaft for automobiles, industrial machines, construction machines and the like. Furthermore, if the non-heat treated crankshaft steel of the present embodiment is used, it is possible to cope with a lighter and smaller crankshaft based on recent environmental considerations.

Claims (4)

% By mass
C: 0.37 to 0.50%,
Si: 0.15 to 0.79%,
Mn: 1.15 to 1.40%,
P: 0.040% or less,
S: 0.040-0.10%,
Cr: 0.10 to 0.30%,
Al: 0.022 to 0.050%,
Ti: 0.005 to 0.025%,
V: 0.20 to 0.30%,
N: 0.0080 to 0.0180%,
Mo: 0 to 0.30%,
Cu: 0 to 0.8%, and
Ni: 0 to 0.50% is contained, the balance consists of Fe and impurities,
Non-tempered crankshaft steel that satisfies formulas (1) to (3).
0.86 ≦ C + Si / 7 + Mn / 5 + Cu / 15 + Ni / 15 + Cr / 9 + Mo / 4 + V ≦ 1.10 (1)
Ti / N ≦ 1.5 (2)
521-353C-22.0Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo ≧ 305 (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the expressions (1) to (3). When no element is contained, “0” is assigned to the corresponding element symbol. The non-heat treated crankshaft steel according to claim 1,
Mo: Steel for non-tempered crankshaft containing 0.05 to 0.30%. The non-tempered crankshaft steel according to claim 1 or 2,
Cu: 0.05-0.8%, and
Non-tempered crankshaft steel containing Ni: 0.01 to 0.50%. % By mass
C: 0.37 to 0.50%,
Si: 0.15 to 0.79%,
Mn: 1.15 to 1.40%,
P: 0.040% or less,
S: 0.040-0.10%,
Cr: 0.10 to 0.30%,
Al: 0.022 to 0.050%,
Ti: 0.005 to 0.025%,
V: 0.20 to 0.30%,
N: 0.0080 to 0.0180%,
Mo: 0 to 0.30%,
Cu: 0 to 0.8%, and
Ni: 0 to 0.50% is contained, the balance consists of Fe and impurities,
A non-heat treated crankshaft having a chemical composition satisfying the formulas (1) to (3) and including a crankpin, a crank journal, a crank arm, a pin fillet portion, and a journal fillet portion .
0.86 ≦ C + Si / 7 + Mn / 5 + Cu / 15 + Ni / 15 + Cr / 9 + Mo / 4 + V ≦ 1.10 (1)
Ti / N ≦ 1.5 (2)
521-353C-22.0Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo ≧ 305 (3)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the expressions (1) to (3). When no element is contained, “0” is assigned to the corresponding element symbol.

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