JP6860265B2 - Steel, crankshafts and automotive parts - Google Patents

Description

The present invention relates to steel materials, crankshafts and automobile parts.

Generally, automobile parts such as crankshafts are manufactured by casting or forging, but when strength and rigidity are important, carbon steel or low alloy steel that has been hot forged is used. There is. Then, when further increasing the strength is required, induction hardening treatment and soft nitriding treatment are performed, but after that, bending straightening processing needs to be performed.
Therefore, steel materials used for manufacturing automobile parts such as crankshafts are required to have excellent straightness as well as strength.

For example, Patent Document 1 contains C, Si, Mn, S, Al, Ti, and N in a specific range, the balance is composed of Fe and impurities, and the content of P and Cr in the impurities is in a specific range. A non-healing material for soft nitriding is described in which a specific component satisfies a specific formula. The soft nitriding component of the present invention stably has a high bending fatigue strength of 600 MPa or more and excellent bending correctability even when water-cooled under various soft nitriding conditions, particularly in a cooling step after soft nitriding. It is described that it is suitable as a crankshaft for automobiles and the like.

Further, for example, in Patent Document 2, the steel cross-sectional structure excluding the soft nitriding layer is made of non-microalloyed steel having a ferrite + pearlite structure, and the composition of the steel is described. Described is a soft nitrided non-tempered steel member characterized by containing Fe as a main component, C, Si, Mn, S, Cu, Ni, Cr in a specific range, and the specific component satisfying a specific formula. ing. It is described that such a soft nitrided non-tempered steel member is a steel member having sufficient bending straightness while ensuring a certain level of fatigue strength.

International Publication No. 2012/070349 Pamphlet JP-A-2007-197812

For example, the conventional steel materials described in Patent Documents 1 and 2 may lack strength (fatigue strength) or manufacturability (bending straightness). Further, it is expected to find a steel material having more excellent strength (fatigue strength) and manufacturability (bending straightening property) as compared with the conventional one.

An object of the present invention is to solve the above problems.
That is, an object of the present invention is that both strength (particularly fatigue strength) and manufacturability (particularly bending straightness) are excellent, and after molding into a part shape, a high strength treatment such as soft nitriding treatment is performed to correct bending. This makes it possible to provide a steel material that can be used as an automobile part such as a crankshaft. Further, it is to provide automobile parts such as a crankshaft made of the steel material.

The present inventor has made diligent studies to solve the above problems and completed the present invention.
The present invention is the following [1] to [6].
[1] By mass%,
C: 0.25 to 0.35%,
Si: 0.01-0.35%,
Mn: 0.55 to 1.50%,
Cu: 0.60% or less,
Ni: 0.30% or less,
S: 0.01 to 0.15%,
Cr: 0.01 to 0.20%,
Mo: 0.01-0.05%,
s-Al: 0.020% or less,
Ti: 0.040% or less,
N: 0.003 to 0.030%,
Ca: 0.0003 to 0.0060%,
O: Contained at 0.010% or less, the balance consists of Fe and unavoidable impurities.
Equation 1 = C + 0.07 x Si + 0.16 x Mn + 0.19 x Cu + 0.17 x Ni + 0.20 x Cr + 0.40 x Mo
When defined as, the equation 1 ≧ 0.43 is satisfied, and
Equation 2 = 0.55 × C + 0.06 × Si + 0.18 × Mn + 0.09 × Cu + 0.47 × Mo + Cr
When defined as, the equation 2 ≧ 0.38 is satisfied, and
Equation 3 = 11 × Ca / (N + 1.5 × O)
A steel material satisfying 1.77 ≦ Equation 3 ≦ 3.20 when defined as.
[2] S0: Total number of sulfides having a diameter of 1.5 μm or more in ^{1 mm 2 of} a cross section parallel to the rolling direction ^{(pieces / mm 2} ),
^{S1: The number of sulfides (pieces / mm 2} ) having a diameter of 1.5 μm or more and containing 2.0% or more of Ca in ^{1 mm 2 of} a cross section parallel to the rolling direction.
Equation 4 = S1 / S0
The steel material according to the above [1], which satisfies the formula 4 ≧ 0.15 when defined as.
[3] Further, in% by mass,
V: 0.5% or less,
The steel material according to the above [1] or [2], which contains at least one selected from the group consisting of Nb: 0.5% or less and W: 2.0% or less.
[4] Further, in% by mass,
Bi: 0.3% or less,
Pb: 0.3% or less,
Mg: 0.020% or less,
Se: 0.30% or less,
Te: 0.15% or less,
Sn: 0.04% or less,
Sb: 0.04% or less,
REM: 0.05% or less,
Zr: 0.5% or less,
B: The steel material according to any one of the above [1] to [3], which contains one or more of 0.015% or less.
[5] An automobile part obtained by hot forging the steel material according to any one of the above [1] to [4] and soft nitriding it.
[6] A crankshaft obtained by hot forging the steel material according to any one of the above [1] to [4] and soft nitriding it.

According to the present invention, both strength (particularly fatigue strength) and manufacturability (particularly bending straightness) are excellent, and after molding into a part shape, a high strength treatment such as soft nitriding treatment is performed to correct bending. , A steel material that can be used as an automobile part such as a crankshaft can be provided. Further, it is possible to provide automobile parts such as a crankshaft made of the steel material.

It is a schematic side view for demonstrating the shape of the steel material used in an Example and a test piece obtained by heat-treating this.

The present invention will be described.
In the present invention, in terms of mass%, C: 0.25 to 0.35%, Si: 0.01 to 0.35%, Mn: 0.55 to 1.50%, Cu: 0.60% or less, Ni : 0.30% or less, S: 0.01 to 0.15%, Cr: 0.01 to 0.20%, Mo: 0.01 to 0.05%, s-Al: 0.020% or less, Ti: 0.040% or less, N: 0.003 to 0.030%, Ca: 0.0003 to 0.0060%, O: 0.010% or less, and the balance consists of Fe and unavoidable impurities. , Equation 1 = C + 0.07 × Si + 0.16 × Mn + 0.19 × Cu + 0.17 × Ni + 0.20 × Cr + 0.40 × Mo, equation 1 ≧ 0.43 is satisfied, and equation 2 = 0.55. When defined as × C + 0.06 × Si + 0.18 × Mn + 0.09 × Cu + 0.47 × Mo + Cr, equation 2 ≧ 0.38 was satisfied, and equation 3 = 11 × Ca / (N + 1.5 × O) was defined. In this case, it is a steel material satisfying 1.77 ≦ Equation 3 ≦ 3.20.
Such a steel material is also referred to as "the steel material of the present invention" below.

The composition of the steel material of the present invention will be described.

The content of the C component is 0.25 to 0.35% by mass, preferably 0.32% by mass or less.
When the range of the C component content is in the above range, the fatigue strength of the steel material of the present invention and the automobile parts (including the crankshaft; the same applies hereinafter) made of the steel material is high, and the correctability is also excellent. .. If this content is too high, the correctability and machinability tend to decrease.

In the present invention, the term "correctability" means that the deformation (bending) of a steel material generally formed by performing a soft nitriding treatment is straightened (processed to return to the original shape) easily and strictly. It shall mean that it can be done. If the straightness is low or inferior, the steel may crack or not return to its original state when the deformation of the steel is corrected.

The content of the Si component is 0.01 to 0.35% by mass, preferably 0.05% by mass or more. The content of the Si component is preferably 0.30% by mass or less, and more preferably 0.25% by mass or less.
Si is used as a deoxidizer during steel melting, and when this content is within the above range, it acts to increase the fatigue strength of the steel material of the present invention and the automobile parts made of the same.
When the range of the content of the Si component is within the above range, the fatigue strength of the steel material of the present invention and the automobile parts made of this steel material is high, and the correctability is also excellent. If this content is too high, the correctability tends to decrease.

The content of the Mn component is 0.55 to 1.50% by mass, preferably 0.70% by mass or more. The content of the Mn component is preferably 1.20% by mass or less.
Mn is an essential element for forming Mn-based sulfides that contribute to improving machinability.
When the range of the Mn component content is in the above range, the fatigue strength of the steel material of the present invention and the automobile parts made of this steel material is high, and the correctability is also excellent. If this content is too high, the volume fraction of pearlite tends to be high and the correctability tends to be lowered. If this content is too low, the fatigue strength tends to decrease.

The content of the S component is 0.01 to 0.15% by mass, preferably 0.12% by mass or less.
The S component is an essential forming element of Mn-based sulfide that contributes to the improvement of machinability together with Mn. If the content of the S component is too low, the amount of sulfide produced is insufficient and the machinability becomes insufficient. On the contrary, if the content of the S component is too high, the toughness and ductility of the steel material of the present invention are impaired, inclusions become the starting point of fatigue fracture, and the fatigue strength characteristics tend to deteriorate.

The content of the Cu component is 0.60% by mass or less, preferably 0.45% by mass or less, and more preferably 0.30% by mass or less. Further, this content is preferably 0.01% by mass or more.
When the content of the Cu component is in the above range, the proof stress is increased, and the compound layer that can be formed when the soft nitriding treatment is performed becomes relatively thin, so that the steel material of the present invention and the automobile parts made of the present invention are fatigued. Increases strength.
If the content of the Cu component is too high, the hardness after hot forging becomes high, the machinability decreases, and the hot forging property may also be impaired. In this case, it leads to cost increase. Further, the addition of a large amount of Cu induces hot cracking due to segregation of Cu at the grain boundaries of steel.

The content of the Ni component is 0.30% by mass or less, preferably 0.25% by mass or less, and more preferably 0.20% by mass or less. Further, this content is preferably 0.01% by mass or more.
When the content of the Ni component is in the above range, the ductility of pearlite in the nitrided layer is improved, so that the correctability of the steel material of the present invention and the automobile parts made of the same is improved.
If the content of the Ni component is too high, the hardness after hot forging becomes high, the machinability decreases, and the hot forging property may also be impaired. In this case, it leads to cost increase.

The content of the Cr component is 0.01 to 0.20% by mass, preferably 0.15% by mass or less.
The Cr component enhances the fatigue strength and toughness of steel. If the content of this component is too high, a large amount of nitride may be generated when the soft nitriding treatment is performed, and the bending correctability may be deteriorated.

The content of the Mo component is 0.01 to 0.05% by mass.
When the range of the Mo component content is in the above range, the fatigue strength of the steel material of the present invention after the soft nitriding treatment and the automobile parts made of the steel material is high and hard. If this content is too high, the machinability may decrease, which leads to an increase in cost.

The content of s-Al is 0.020% by mass or less.
s—Al means acid-soluble Al.
When the range of the content of the s—Al component is within the above range, the fatigue strength of the steel material of the present invention and the automobile parts made of this steel material is high, and the correctability is also excellent.
If this content is too high, the hardness tends to vary. The higher the hardness, the lower the correctability tends to be.

The content of the Ti component is 0.040% by mass or less.
Ti forms a nitride with N, suppresses the growth of old austenite grains during hot forging by fine precipitation, refines the ferrite pearlite structure, and enhances bending straightness.
If this content is too high, coarse TiN may increase and machinability may decrease.

The content of the N component is 0.003 to 0.030% by mass, preferably 0.010% by mass or more.
N forms a nitride with Ti, suppresses the growth of old austenite grains during hot forging by fine precipitation, refines the ferrite pearlite structure, and enhances bending straightness.
If this content is too high, coarse carbonitride is generated, which causes cracks and the like, which tends to reduce fatigue strength.

The content of the Ca component is 0.0003 to 0.0060% by mass, preferably 0.0005% by mass or more, and more preferably 0.0015% by mass or more.
When the range of the Ca component content is in the above range, the machinability is enhanced. On the other hand, even if the Ca component is increased more than necessary, the machinability tends to be saturated and the hot workability tends to decrease.

The content of the O component (oxygen component) is 0.010% by mass or less.
If the content of the O component is too high, a coarse oxide is generated, which causes cracks and the like, so that the fatigue strength tends to decrease.

The steel material of the present invention is a steel material containing C, Si, Mn, Cu, Ni, S, Cr, Mo, s-Al, Ti, N, Ca, and O in a specific ratio as described above, and further V, It preferably contains at least one selected from the group consisting of Nb and W in the following range, and the balance is Fe and unavoidable impurities.

V, Nb, and W are elements that contribute to the improvement of the strength of the steel material by combining with C to form carbides or combining with N to form nitrides, and the steel materials of the present invention are these elements. It may contain a single element from the above, or two or more elements arbitrarily selected.
In order to effectively exert such an effect, the content of the V component is preferably 0.05% by mass or more, and more preferably 0.10% by mass or more.
The content of the Nb component is preferably 0.05% by mass or more, more preferably 0.08% by mass or more, and further preferably 0.10% by mass or more.
The content of the W component is preferably 0.2% by mass or more, more preferably 0.5% by mass or more, and further preferably 0.7% by mass or more.

However, if V, Nb, and W are excessively contained, the formed carbides and nitrides may increase the deformation resistance of the steel material and reduce the workability.
Therefore, the content of the V component is preferably 0.5% by mass or less, preferably 0.4% by mass or less, and more preferably 0.3% by mass or less.
The content of the Nb component is preferably 0.5% by mass or less, more preferably 0.4% by mass or less, and further preferably 0.3% by mass or less.
The content of the W component is preferably 2.0% by mass or less, more preferably 1.5% by mass or less, and further preferably 1.0% by mass or less.

The steel material of the present invention is a steel material containing C, Si, Mn, Cu, Ni, S, Cr, Mo, s-Al, Ti, N, Ca, and O in a specific ratio as described above, and further includes Bi, It is preferable that one or more of Pb, Mg, Se, Te, Sn, Sb, REM, Zr, and B are contained in the following range, and the balance is Fe and unavoidable impurities.
In addition, REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium). Among these, the steel material of the present invention preferably contains at least one element selected from the group consisting of La, Ce and Y, and contains at least one element selected from the group consisting of La and Ce. Is more preferable.

Here, the steel material of the present invention is a steel material containing C, Si, Mn, Cu, Ni, S, Cr, Mo, s-Al, Ti, N, Ca, and O in a specific ratio as described above. Further, at least one selected from the group consisting of V, Nb and W is included in the following range, and in addition, one of Bi, Pb, Mg, Se, Te, Sn, Sb, REM, Zr and B. Alternatively, it is more preferable that two or more kinds are contained in the following range, and the balance is Fe and unavoidable impurities.

Bi, Pb, Mg, Se, Te, Sn, Sb, REM, Zr, and B are elements that contribute to the improvement of machinability, and the steel material of the present invention is a single element or arbitrarily among these elements. It may contain two or more selected elements.
In order to effectively exert such an effect, the content of the Bi component is preferably 0.03% by mass or more, more preferably 0.08% by mass or more, and 0.10% by mass or more. It is more preferable to have.
The content of the Pb component is preferably 0.03% by mass or more, more preferably 0.1% by mass or more, and further preferably 0.15% by mass or more.
The content of the Mg component is preferably 0.0001% by mass or more, more preferably 0.0003% by mass or more, and further preferably 0.0004% by mass or more.
The content of the Se component is preferably 0.001% by mass or more, and more preferably 0.002% by mass or more.
The content of the Te component is preferably 0.001% by mass or more, and more preferably 0.002% by mass or more.
Further, the content of the Sn component is preferably 0.002% by mass or more, more preferably 0.005% by mass or more, and further preferably 0.010% by mass or more.
The content of the Sb component is preferably 0.002% by mass or more, more preferably 0.004% by mass or more, and further preferably 0.006% by mass or more.
The content of the REM component is preferably 0.001% by mass or more, more preferably 0.002% by mass or more, and further preferably 0.003% by mass or more.
The content of the Zr component is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and further preferably 0.04% by mass or more.
The content of the B component is preferably 0.0005% by mass or more, and more preferably 0.0010% by mass or more.

However, if Bi, Pb, Mg, Se, Te, Sn, Sb, REM, Zr, and B are excessively contained, the strength is lowered, or even if the excess is contained, the machinability addition effect is saturated and the cost is increased. May be high.
Therefore, the content of the Bi component is preferably 0.3% by mass or less, more preferably 0.25% by mass or less, and further preferably 0.20% by mass or less.
The content of the Pb component is preferably 0.3% by mass or less, preferably 0.28% by mass or less, and more preferably 0.25% by mass or less.
The content of the Mg component is preferably 0.02% by mass or less, more preferably 0.01% by mass or less, and further preferably 0.0090% by mass or less.
The content of the Se component is preferably 0.3% by mass or less, more preferably 0.25% by mass or less, and further preferably 0.20% by mass or less.
The content of the Te component is preferably 0.15% by mass or less, and more preferably 0.10% by mass or less.
Further, the content of the Sn component is preferably 0.020% by mass or less, more preferably 0.018% by mass or less, and further preferably 0.015% by mass or less.
The content of the Sb component is preferably 0.020% by mass or less, more preferably 0.015% by mass or less, and further preferably 0.010% by mass or less.
The content of the REM component is preferably 0.05% by mass or less, and more preferably 0.04% by mass or less.
The content of the Zr component is preferably 0.5% by mass or less, more preferably 0.45% by mass or less, and further preferably 0.40% by mass or less.
The content of the B component is preferably 0.0200% by mass or less, more preferably 0.0180% by mass or less, and further preferably 0.0150% by mass or less.

Further, as an unavoidable impurity that can be contained in Fe, for example, P can be mentioned.
Since P may reduce the toughness of steel, its content is preferably 0.04% by mass or less.

When each component in the steel material of the present invention is defined as the formula 1 = C + 0.07 × Si + 0.16 × Mn + 0.19 × Cu + 0.17 × Ni + 0.20 × Cr + 0.40 × Mo, the equation 1 ≧ 0.43 Fulfill.
Equation 1 is preferably 0.70 or less, more preferably 0.65 or less.
The present inventor has found that the value of the formula 1 is related to the hardness of the core portion (the portion other than the surface layer) of the steel material of the present invention.
When the value of Equation 1 is in the above range, the hardness and fatigue strength of the steel material of the present invention tend to increase.

Here, C + 0.07 × Si + 0.16 × Mn + 0.19 × Cu + 0.17 × Ni + 0.20 × Cr + 0.40 × Mo is C content + 0.07 × Si content + 0.16 × Mn content + 0. It means .19 × Cu content + 0.17 × Ni content + 0.20 × Cr content + 0.40 × Mo content.
The same applies to the equations 2 and 3 described later.

Each component in the steel material of the present invention satisfies the formula 2 ≧ 0.38 when defined as the formula 2 = 0.55 × C + 0.06 × Si + 0.18 × Mn + 0.09 × Cu + 0.47 × Mo + Cr.
Equation 2 is preferably 0.70 or less, and more preferably 0.60 or less.
The present inventor has found that the value of Equation 2 is related to the hardness of the surface layer (part other than the core) of the steel material of the present invention.
When the value of Equation 2 is in the above range, the hardness and fatigue strength of the steel material of the present invention tend to increase.

Each component in the steel material of the present invention satisfies 1.77 ≦ formula 3 ≦ 3.20 when defined as the formula 3 = 11 × Ca / (N + 1.5 × O).
Equation 3 is preferably 3.0 or less, and more preferably 2.8 or less.
The present inventor has found that the value of Equation 3 is related to the size of inclusions such as TiN that can be contained in the steel material of the present invention.
If the value of Equation 3 is too low, inclusions such as TiN become coarse, the structure becomes coarse, and the correctability of the steel material of the present invention tends to decrease. On the contrary, if the value of Equation 3 is too high, coarse oxides are generated and the fatigue strength tends to decrease.

The steel material of the present invention has the above composition, and when hot forged (immediately after hot forging and not subjected to tempering treatment), the steel cross-sectional structure is ferrite + pearlite (ferrite and pearlite). Is a mixed organization).

Steel of the present invention, the S0, the total number of sulfides or more in diameter 1.5μm in 1mm ² in a cross section parallel to the rolling direction (pieces / mm ^2), S1 a, 1mm in cross section parallel to the rolling direction ² The number of sulfides having a diameter of 1.5 μm or more and containing 2.0% or more of Ca (pieces / mm ² ), and when the formula 4 = S1 / S0 is defined, the formula 4 ≧ 0.15 is satisfied. Is preferable. It is more preferable that the formula 4 ≧ 0.25 is satisfied.
When the formula 4 is satisfied, the steel material of the present invention has a higher fatigue strength.
In addition, S0 and S1 shall be obtained as follows.
A distance of about 1/4 of the cross-sectional diameter of the round bar-shaped piece obtained by rolling or forging after melting, or a distance of about 1/4 of one side from the center of the cross-section of the square bar-shaped piece to one side. A 10 mm square piece is cut out from the portion to obtain a sample. Next, the cross section (cross section parallel to the longitudinal direction) of the obtained sample is mirror-polished, observed with a scanning electron microscope at a magnification of about 450 times in ^{a field of view of 100 mm 2} or more, and a photograph is taken. Then, the diameter of the sulfide (inclusion) is measured from the obtained photograph. Furthermore, EDX analysis will be carried out to understand the type and composition.

Further, the steel material of the present invention may be a non-microalloyed steel capable of omitting tempering treatment (quenching / tempering) after hot working. Since the steel material of the present invention is excellent in both strength (particularly fatigue strength) and manufacturability (particularly bending straightness), it is formed into a part shape by, for example, hot working (hot forging, etc.) and machining, and then soft nitriding treatment is performed. It can be used as an automobile part such as a crankshaft by performing a strength-enhancing treatment such as.

Further, the present invention is an automobile part made of the steel material of the present invention as described above.
Moreover, a crankshaft is mentioned as a preferable example as this automobile part.

<Manufacturing of test pieces>
For each of the invention steels 1 to 25 and the comparative steels A to F shown in Table 1, 150 kg of raw materials were mixed so as to have the composition shown in Table 1, melted in a vacuum melting furnace, and cross-sectional diameter was 1250 ° C. Was forged into a 70 mm round bar. Here, two identical round bars were obtained for each of the invention steels 1 to 25 and the comparative steels A to F.

Next, forging and the like were performed under two conditions.
Under condition 1, the obtained round bar is heated at 1250 ° C. for 2 hours, hot forged into a bar-shaped piece having a cross section of 45 mm square at a finishing temperature of 1050 ° C., and then cooled to room temperature at 0.3 ° C./s. Cooled at speed.
Under condition 2, the obtained round bar was heated at 1300 ° C. for 2 hours, hot forged into a round bar-shaped piece having a cross section of 30 mm at a finishing temperature of 1050 ° C., and then heated to room temperature at 0.6 ° C./s. It was cooled at the cooling rate.

Next, a round bar (length 210 mm) having a cross-sectional diameter of 15 mm is formed from a portion having a distance of about 1/4 of one side from the center of the 45 mm square rod-shaped piece obtained under the above condition 1 to one side. A steel material 10 having a parallel portion 3 having a diameter of φ15 mm, a grip portion 5 having a diameter of φ16 mm, and two test portions 7 provided at the center was obtained.
Further, from the round bar-shaped piece having a diameter of 30 mm obtained under the above condition 2, the diameter of the parallel portion 3 is φ15 mm, the diameter of the grip portion 5 is φ16 mm, and 2 in the center, as shown in FIG. A steel material 10 provided with a test section 7 was obtained.
Here, the test unit 7 is provided with a stress concentration unit having a radius of curvature of R2. Further, the diameter of the parallel portion 71 in the test portion 7 is φ10 mm, and the length of the parallel portion 71 is about 20 mm.

Next, the steel material provided with these test portions 7 was subjected to soft nitriding treatment at 600 ° C. for 2 hours to obtain a test piece. That is, the test piece is the steel material shown in FIG. 1 which has been subjected to soft nitriding treatment. The soft nitriding treatment used in the examples is a gas _{containing RX gas (a type of modified gas) and NH 3} gas in a volume ratio of 1: 1 (or NH ₃ gas: N ₂ gas: CO ₂ gas = 50). It is a process of arranging a steel material in a gas (gas contained in: 45: 5) and heating it. The soft nitriding treatment that can be used in the present invention does not have to be such a specific soft nitriding treatment, and may be a treatment in an atmosphere in which a compound layer is sufficiently formed on the surface layer of the object to be treated.

By performing such an operation, test pieces (two types each) relating to each of the invention steels 1 to 25 and the comparative steels A to F shown in Table 1 were obtained.

<Fatigue strength>
Each of the test pieces obtained as described above is subjected to precision processing so that the diameter of the grip portion is φ15 mm, and then in the air at room temperature, a fatigue tester (device name: Ono type rotary bending fatigue tester). ) Was used to perform a fatigue test under the condition of 2500 rpm, and the fatigue intensity of each test piece was measured.
The results are shown in Table 1.
When the fatigue strength was 430 MPa or more and the nominal stress was 430 MPa or more, the judgment was ◯, and when it was less than 430 MPa, the judgment was x.

<Bending correction test>
For each of the test pieces obtained as described above, a bending straightening test for three-point bending was performed by applying a concentrated load to the parallel portions 3 while supporting both ends at a distance of 182 mm between the fulcrums. In this test, a load was applied to the central test section until a crack was generated, and the maximum displacement (stroke) until the crack was generated was measured using software (WAVE logger).
The results are shown in Table 1.
When the stroke was 7 mm or more, the judgment was ◯, and when the stroke was less than 7 mm, the judgment was x.

As shown in Table 1, it was found that all of the invention steels 1 to 25 have high fatigue strength and excellent correctability.
Steel materials such as the invention steels 1 to 25 can be said to be non-microalloyed steels that can omit tempering treatment (quenching / tempering) after hot working. Since these steel materials are excellent in both strength (particularly fatigue strength) and manufacturability (particularly bending straightness), for example, after forming into a part shape by hot working (hot forging, etc.) and machining, soft nitriding treatment, etc. After performing the high-strength treatment of, it can be easily corrected and returned to the original shape. Therefore, it can be preferably used as an automobile part such as a crankshaft.

On the other hand, the comparative steel A is inferior in bending straightness. It is considered that this is because the value of Equation 3 is too low.
Further, the comparative steel B is inferior in fatigue strength. It is considered that this is because the value of the formula 3 of the comparative steel B is too high.
Further, the comparative steel C is inferior in fatigue strength. It is considered that this is because the value of the formula 2 of the comparative steel C is too low.
Further, the comparative steel D is inferior in fatigue strength under condition 1. It is considered that this is because the value of the formula 1 of the comparative steel D is too low.
Further, the comparative steel E is inferior in bending straightness under the condition 2. It is considered that this is because the s—Al content of the comparative steel E is too high.
Further, the comparative steel F is inferior in bending straightness. It is considered that this is because the Cr of the comparative steel F is too high.

Claims (6)

By mass%
C: 0.25 to 0.35%,
Si: 0.01-0.35%,
Mn: 0.55 to 1.50%,
Cu: 0.60% or less,
Ni: 0.30% or less,
S: 0.01 to 0.15%,
Cr: 0.01 to 0.20%,
Mo: 0.01-0.05%,
s-Al: 0.020% or less,
Ti: 0.040% or less,
N: 0.003 to 0.030%,
Ca: 0.0003 to 0.0060%,
O: Contained at 0.010% or less, the balance consists of Fe and unavoidable impurities.
Equation 1 = C + 0.07 x Si + 0.16 x Mn + 0.19 x Cu + 0.17 x Ni + 0.20 x Cr + 0.40 x Mo
When defined as, the equation 1 ≧ 0.43 is satisfied, and
Equation 2 = 0.55 × C + 0.06 × Si + 0.18 × Mn + 0.09 × Cu + 0.47 × Mo + Cr
When defined as, the equation 2 ≧ 0.38 is satisfied, and
Equation 3 = 11 × Ca / (N + 1.5 × O)
A steel material satisfying 1.77 ≦ Equation 3 ≦ 3.20 when defined as. S0: Total number of sulfides with a diameter of 1.5 μm or more in ^{1 mm 2 of} a cross section parallel to the rolling direction ^{(pieces / mm 2} ),
^{S1: The number of sulfides (pieces / mm 2} ) having a diameter of 1.5 μm or more and containing 2.0% or more of Ca in ^{1 mm 2 of} a cross section parallel to the rolling direction.
Equation 4 = S1 / S0
The steel material according to claim 1, which satisfies the formula 4 ≧ 0.15 when defined as. In addition, by mass%,
V: 0.5% or less, and Nb: 0.5% or less under
Or containing at least one selected from Ranaru group, steel according to claim 1 or 2. In addition, by mass%,
Bi: 0.3% or less,
Pb: 0.3% or less,
Mg: 0.020% or less,
Se: 0.30% or less,
Te: 0.15% or less,
Sn: 0.02% or less,
Sb: 0.02% or less,
REM: 0.05% or less,
Zr: 0.5% or less,
B: The steel material according to any one of claims 1 to 3, which contains one or more of 0.015% or less. An automobile part obtained by hot forging the steel material according to any one of claims 1 to 4 and soft nitriding it. A crankshaft obtained by hot forging the steel material according to any one of claims 1 to 4 and soft nitriding it.

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