STEEL MATERIAL

The present invention relates to a steel material containing, in terms of mass %: 0.30%≤C≤0.45%, 0.10%≤Si≤1.00%, 0.60%≤Mn≤1.20%, 0.20%≤Cr≤0.70%, 0.30%≤V≤0.47%, Ti≤0.015%, P≤0.100%, and S≤0.080%, with the balance being Fe and inevitable impurities, and has a P0 value defined by P0=P0′×V/P1, satisfying P0≥0.30, here, P0′=Mn+0.49Cu+0.89Ni+0.40Cr−0.30Si, and P1=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.17Ni+0.2Cr+V, in the formulae, each element symbol indicates a content of each element in units of mass %.

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Description
CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2021-199356 filed on Dec. 8, 2021 and Japanese Patent Application No. 2022-182428 filed on Nov. 15, 2022, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a steel material, and more particularly to a steel material that can be used for manufacturing automobile parts and the like without heat treatment such as quenching and tempering.

BACKGROUND ART

A steel material can be enhanced its material strength by being subjected to heat treatment such as quenching and tempering. However, in automobile engine parts such as a crank shaft and a connecting rod, from the viewpoint of simplifying a manufacturing process and the like, a non-heat-treated steel whose composition is designed so as to obtain high strength even in an as-hot-forged state where heat treatment is omitted has been widely used. A component composition of this type of non-heat-treated steel is disclosed in, for example, Patent Literature 1 below.

  • Patent Literature 1: WO 2019/203348A1

SUMMARY OF INVENTION

In recent years, it has been studied to apply a non-heat-treated steel also to high power automobile engines. With an increase in power output of the engine, the non-heat-treated steel is also required to have increased strength. In order to increase strength of the non-heat-treated steel, adding V is a widely used technique. Patent Literature 1 also discloses that “precipitation strengthening of steel due to fine V carbides by adding a large amount of V has been utilized,” and “among elements generating alloy carbides, V has a large solid solution amount in the steel material and a large precipitation strengthening amount through heating (around 1,250° C.) before hot forging.” On the other hand, Patent Literature 1 discloses that “the solid solution amount of V in the steel material has a limitation, and it is difficult to further increase strength only by increasing the content of V.” Therefore, in Patent Literature 1, Ti is contained in the steel in addition to V to further increase strength. Patent Literature 1 describes that Ti carbides are precipitated first and the Ti carbides serve as nuclei of the V carbides, so that the V carbides are precipitated more finely and in a larger amount than the case where the V carbides are precipitated alone. However, the Ti carbide has a higher solid solution temperature than the V carbide, and there is a concern of coarse precipitation in the austenite phase. Therefore, it is difficult to increase the addition amount of Ti in order to further increase strength of the non-heat-treated steel.

An object of the present invention is to provide a steel material having high strength without heat treatment.

In order to achieve the above-described object, the steel material according to the present invention contains, in terms of mass %:


0.30%≤C≤0.45%,


0.10%≤Si≤1.00%,


0.60%≤Mn≤1.20%,


0.20%≤Cr≤0.70%,


0.30% V≤0.47%,


Ti≤0.015%,


P≤0.100%, and


S≤0.080%,

    • with the balance being Fe and inevitable impurities, and

has a P0 value defined by the following formula (1), satisfying P0≤0.30:


P0=P0′×V/P1  (1)


here,


P0′=Mn+0.49Cu+0.89Ni+0.40Cr−0.30Si  (2)


P1=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.17Ni+0.2Cr+V  (3)

    • in the formulae (1) to (3), each element symbol indicates a content of each element in units of mass %.

The steel material may be used in a state where heat treatment is not performed.

The steel material may further contain, in terms of mass %, at least one selected from:


0%≤Cu≤0.50%,


0%≤Ni≤0.50%,


0%≤Nb≤0.010%,


0%≤Ti≤0.015%,


0%≤Pb≤0.30%,


0%≤Bi≤0.20%,


0%≤Ca≤0.0100%,


0%≤Zr≤0.010%,


0%≤Mg≤0.010%, and


0%≤Te≤0.010%.

The steel material may satisfy in terms of mass %:


Mo≤0.10%,


Al≤0.050%, and


N≤0.030%.

The P1 value defined by the formula (3) may satisfy 1.04≤P1≤1.15.

The steel material may has a P2 value defined by the following formula (4) and a P3 value defined by the following formula (5), satisfying P2/P3≥1.4:


P2=417−242C+30Si−25Mn−17Cu−22Ni−14Cr−35Mo  (4)


P3=10∧(1.35−0.54C+0.02Si+0.77Mn+0.47Cu+0.42Ni+0.52Cr+4.84Mo)   (5)

in the formulae (4) and (5), each element symbol indicates a content of each element in units of mass %.

In a cross section of the steel material in a state after hot forging, the number of precipitates satisfying (V+Ti)≥30% and having an area of 1 μm2 or more may be 10 or less per 1 mm2.

The steel material may have, in a state after hot forging:

a yield point of 900 MPa or more, and

a yield ratio of 0.80 or more.

The steel material according to the present invention has a component composition in which C, Si, Mn, Cr, and V are contained in predetermined ranges, contents of Ti, P, and S are limited to predetermined upper limits or less, and the P0 value obtained based on the contents of component elements satisfies P0≥0.30, so that the steel material has high strength without heat treatment. In particular, the contents of Mn and Cr are set in the ranges in which a high effect is obtained in improving strength by fine precipitation of V-based carbides while preventing generation of bainite that causes a decrease in yield ratio of the steel material, so that the steel material has high strength without adding a large amount of V and Ti.

Here, in the case where the steel material is used without heat treatment, the process of heat treatment is omitted, and thus a process of manufacturing various members by using the steel material is simplified. As described above, the steel material according to the present invention has a predetermined component composition, so that the steel material has high strength without being subjected to heat treatment.

In addition, in the case where the steel material further contains at least one selected from predetermined amounts of Cu, Ni, Nb, Ti, Pb, Bi, Ca, Zr, Mg, and Te, an effect of increasing strength by Cu, Ni, Nb, and Ti and improving machinability by Pb, Bi, Ca, Zr, Mg, and Te can be obtained.

In the steel material, in the case where the contents of Mo, Al, and N are limited to the predetermined upper limits or less, generation of bainite due to an excessive content of Mo and a decrease in fatigue strength due to excessive contents of Al and N are prevented.

In the case where the P1 value satisfies 1.04≤P1≤1.15, in the steel material, a high effect of improving hardness and strength can be obtained, and machinability can be maintained high.

In addition, in the case where the P2 value and the P3 value satisfy P2/P3≥1.4, in the steel material, the generation of bainite that causes a decrease in yield ratio is prevented, and a high effect of increasing strength is obtained.

In the case where the number of precipitates having the predetermined component composition and area is 10 or less per 1 mm2 in a cross section of the steel material in a state after hot forging, a non-heat-treated steel in which a decrease in strength due to generation of coarse precipitates is prevented can be obtained.

In addition, in the case where the steel material has a yield point of 900 MPa or more and a yield ratio of 0.80 or more in the state after hot forging, the steel material has sufficiently high strength as a non-heat-treated steel to be applied to automobile engine parts or the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A and FIG. 1B are experimental results showing an effect of improving strength of a steel material by adding Mn. For two contents of Mn, a relation of a content of V with a yield point is shown in FIG. 1A and a relation of the content of V with a yield ratio is shown in FIG. 1B.

FIG. 2A is an experimental result showing a relation of a P0 value with a yield ratio, and FIG. 2B is an experimental result showing a relation of a P1 value with a yield point.

 DESCRIPTION OF EMBODIMENTS

Hereinafter, a steel material according to an embodiment of the present invention will be described in detail.

The steel material according to an embodiment of the present invention contains the following elements, with the balance being Fe and inevitable impurities. The types, contents, reasons for limitation, and the like of component elements are as follows. A unit of the contents is mass %. Hereinafter, unless otherwise specified, each characteristic is a value evaluated at room temperature (approximately 25° C.). The steel material according to the present embodiment may be used after being subjected to heat treatment such as quenching and tempering. However, the steel material according to the present embodiment has high strength even in a non-heat-treated state because of satisfying the following component composition, and is preferably used in a non-heat-treated state, that is, in an as-hot-forged state where heat treatment is not performed.

[Content of Each Component Element]

0.30%≤C≤0.45%

C contributes to improvement of strength of the steel material by forming carbides with V and Cr. From the viewpoint of sufficiently obtaining the effect of improving strength, C is set to satisfy 0.30%≤C, and preferably 0.33%≤C.

On the other hand, in the case where the content of C is excessive, a generation amount of hard pearlite increases, which leads to a decrease in yield ratio and deterioration in machinability in the steel material. This also leads to generation of coarse precipitates containing carbides. From the viewpoint of preventing these phenomena, C is set to satisfy C≤0.45% and preferably C≤0.38%.

0.10%≤Si≤1.00%

Si has an effect of increasing strength of the steel material and an effect of improving machinability. From the viewpoint of sufficiently exhibiting these effects, Si is set to satisfy 0.10%≤Si and preferably 0.40%≤Si.

On the other hand, in the case where the content of Si is excessive, an excessive increase in hardness leads to a decrease in life of a mold used for hot forging. From the viewpoint of avoiding such a situation, Si is set to satisfy Si≤1.00% and preferably Si≤0.85%.

0.60%≤Mn≤1.20%

Mn has an effect of promoting fine precipitation of V carbides in the steel. The fine precipitation of the precipitates improves strength of the steel material. From the viewpoint of sufficiently obtaining these effects, Mn is set to satisfy 0.60%≤Mn and preferably 0.75%≤Mn.

On the other hand, Mn promotes the generation of bainite. In the case where a large amount of bainite is generated, strength is reduced due to a decrease in yield ratio in the steel material. From the viewpoint of avoiding these phenomena, Mn is set to satisfy Mn 1.20% and preferably Mn≤1.00%.

0.20%≤Cr≤0.70%

Similar to Mn, Cr also has an effect of improving strength of the steel material by promoting fine precipitation of V carbides. In the steel material, from the viewpoint of obtaining sufficiently high strength, Cr is set to satisfy 0.20%≤Cr and preferably 0.30%≤Cr.

On the other hand, similar to Mn, Cr also causes a reduction of strength due to a decrease in yield ratio of the steel material due to the generation of bainite. From the viewpoint of avoiding these phenomena, Cr is set to satisfy Cr≤0.70% and preferably Cr≤0.50%.

0.30%≤V≤0.47%

V has an effect of increasing strength of the steel material by precipitation of carbides. From the viewpoint of obtaining sufficiently high strength, V is set to satisfy 0.30%≤V and preferably 0.33%≤V.

On the other hand, in the case where the content of V is too large, the effect of improving strength of the steel material is saturated, and it is difficult to ensure sufficiently high strength due to generation of bainite. From the viewpoint of avoiding these phenomena, V is set to satisfy V≤0.47%, preferably V≤0.45%, and more preferably V≤0.40%.

The steel material according to the present embodiment contains the above-described predetermined amounts of C, Si, Mn, Cr, and V, and the balance is Fe and inevitable impurities. Here, the steel material according to the present embodiment may contain Ti, P, and S as the inevitable impurities, and the contents thereof are limited to the following ranges.

Ti≤0.015%

Ti can generate coarse carbides and carbonitrides. The coarse precipitates of these carbides and carbonitrides decrease strength of the steel material. In particular, in the case where a large amount of Ti is contained, coarse precipitation tends to occur in the austenite phase. Therefore, in the steel material according to the present embodiment, a decrease in strength due to the generation of the coarse precipitates is prevented by limiting Ti to satisfy Ti≤0.015% and preferably Ti≤0.010%.

P≤0.100%, and S≤0.080%

P and S are elements that cause embrittlement due to grain boundary segregation in the steel material. From the viewpoint of avoiding an influence of the grain boundary segregation, the content of P is limited within the range of P≤0.100% and preferably P 0.080%, and the content of S is limited within the range of S≤0.080% and preferably S 0.065%.

Further, the steel material according to the present embodiment may contain Mo, Al, and N as inevitable impurities other than Ti, P, and S, and contents of these elements are preferably limited to the following ranges.

Mo≤0.10%

Mo is derived from a raw material and can be inevitably mixed into the steel material, and promotes the generation of bainite. The generation of bainite leads to a decrease in strength of the steel material. Therefore, from the viewpoint of preventing the generation of bainite, Mo is preferably limited to satisfy Mo≤0.10% and more preferably Mo≤0.05%.

Al≤0.050%

Al generates coarse Al2O3 inclusions in the steel. The Al2O3 inclusions cause a decrease in fatigue strength of the steel material. From the viewpoint of preventing the generation of the Al2O3 inclusions, Al is preferably limited to satisfy Al≤0.050% and more preferably Al≤0.030%.

N≤0.030%

N generates coarse nitride inclusions in the steel. The nitride inclusions cause a decrease in fatigue strength of the steel material. From the viewpoint of preventing the generation of the nitride inclusions, N is preferably limited to satisfy N≤0.030% and more preferably N≤0.015%.

Examples of inevitable impurities that can be contained in the steel material according to the present embodiment include Co≤0.03%, As≤0.010%, Sn≤0.010%, Sb 0.010%, and the like, other than Ti, P, S, Mo, Al, and N. In addition, a total amount of the inevitable impurities is preferably limited to be 3.0% or less.

The steel material according to the present embodiment may optionally contain one or two or more elements selected from the following elements in addition to the above-described essential elements. Contents, reasons for limitation, and the like of respective elements are as follows.

0%≤Cu≤0.50%, and 0%≤Ni≤0.50%

Cu and Ni have an effect of finely precipitating V carbides in the steel. The fine precipitation of the precipitates improves strength of the steel material. Even a small addition of Cu and/or Ni exhibit a high effect in improving strength of the steel material by promoting the fine precipitation and thus, lower limits of the contents thereof are not particularly limited. From the viewpoint of obtaining a particularly high effect, Cu and Ni may be set to satisfy 0.06%≤Cu and/or 0.03%≤Ni. Incidentally, less than 0.06% of Cu and less than 0.03% of Ni can be regarded as inevitable impurities.

On the other hand, Cu and Ni promote the generation of bainite. In the case where a large amount of bainite is generated, strength is reduced due to a decrease in yield ratio of the steel material. In addition, since Cu and Ni are expensive elements, in the case where Cu and Ni are added in a large amount, the cost of the steel material increases. From the viewpoint of avoiding these phenomena, Cu is set to satisfy Cu≤0.50% and preferably Cu≤0.25%, and Ni is set to satisfy Ni≤0.50% and preferably Ni≤0.25%.

0%≤Nb≤0.010%, and 0%≤Ti≤0.015%

Nb and Ti contribute to an improvement of strength of the steel material by generating carbides and carbonitrides. Since Nb and Ti exhibit a high strength improving effect even in a small amount, lower limits of the contents thereof are not particularly limited. From the viewpoint of obtaining a particularly high effect, Nb and Ti may be set to satisfy 0.001%≤Nb and/or 0.001%≤Ti.

However, in the case where coarse precipitates are generated as the carbides or carbonitrides of Nb and/or Ti, strength of the steel material is rather reduced. From the viewpoint of avoiding a decrease in strength, Nb is set to satisfy Nb≤0.010% and preferably Nb≤0.007%, and Ti is set to satisfy Ti≤0.015% and preferably Ti≤0.010%. Incidentally, Ti is also described above as an inevitable impurity, and the content of Ti as an inevitable impurity is limited to satisfy Ti≤0.015%. However, the content of Ti as an inevitable impurity varies depending on the raw materials used for producing the steel material. In the case where the content of Ti as an inevitable impurity is small, for the purpose of utilizing the strength-improving effect of Ti, Ti may be added as long as Ti is within the range of Ti≤0.015%.

0%≤Pb≤0.30%, 0%≤Bi≤0.20%, 0%≤Ca≤0.0100%, 0%≤Zr≤0.010%, 0%<Mg≤0.010%, and 0%<Te≤0.010%

Any of Pb, Bi, Ca, Zr, Mg, and Te has an effect of enhancing machinability of the steel material. Since any of these elements exhibits a high effect in improving machinability even in a small amount, lower limits of the contents thereof are not particularly limited. From the viewpoint of obtaining a particularly high effect on machinability, the contents of Pb and/or Bi may be set to be 0.02% or more and more preferably 0.05% or more, the content of Ca may be set to be 0.0005% or more and more preferably 0.0010 or more, and the contents of Zr, Mg, and/or Te may be set to be 0.001% or more.

On the other hand, in the case where a large amount of Ph, Bi, Ca, Zr, Mg, or Te is contained in the steel material, hot workability and fatigue strength are likely to decrease. Therefore, from the viewpoint of ensuring high hot workability, the upper limit of the content of each element is determined as described above. In order to further enhance the effect, the content of Pb is preferably set to be 0.20% or less, the content of Bi is preferably set to be 0.15% or less, and the content of Ca is preferably set to be 0.0050% or less.

[Relation Between Contents of Component Elements]

Next, the relation between the contents of the component elements will be described. Hereinafter, in mathematical formulae defining the relations between the contents of the component elements, each element symbol indicates the content of each element in units of mass %. In the case where an element other than the essential elements is not contained in the steel, the content thereof in the formulae is regarded as zero.

In the steel material according to the present embodiment, the P0 value obtained based on the following formula (1) satisfies P0≥0.30.


P0=P0′×V/P1  (1)

Here, the P0′ value and the P1 value in the formula (1) are defined by the following formulae (2) and (3), respectively.


P0′=Mn+0.49Cu+0.89Ni+0.40Cr−0.30Si  (2)


P1=C+0.07Si+0.16Mn+0.61P+0.19Cu+0.17Ni+0.2Cr+V  (3)

Mn, Cu, Ni, and Cr included in the definitional formula (2) of P0′ promote the fine precipitation of the V precipitates by lowering a ferrite transformation temperature. On the other hand, Si prevents the fine precipitation of the V precipitates. The formula (2) represents a sum of the contents of respective elements in consideration of the degree of contribution, and P0′ serves as an index indicating the degree of promotion of the fine precipitation of the V precipitates. The greater the P0′ value is, the higher the effect of promoting the fine precipitation of the V precipitates is. In addition, the numerator in the formula (1) is obtained by multiplying the P0′ value by the content of V, and the greater the value of the numerator is, the more the precipitation of fine V carbides is promoted, and the higher the effect of improving strength of the steel material is exhibited.

On the other hand, each component element included in the definitional formula (3) of P1 improves hardness and tensile strength of the steel material. In the definitional formula (1) of P0, P1 is a denominator, and when the P0′ value becomes large with respect to the P1 value and the P0 value becomes larger, an effect of increasing yield point due to the promotion of the fine precipitation of the V precipitates is improved with respect to tensile strength of the steel material, and the yield ratio is improved.

As described above, as the P0 value defined by the formula (1) increases, the effect of improving strength due to the fine precipitation of the V carbides is enhanced. In particular, the effect of improving yield ratio becomes high. In the steel material according to the present embodiment, owing to P0≥0.30, a high yield ratio of 0.80 or more can be obtained. The P0 value is preferably P0≥0.35 and more preferably P0≥0.40. The higher the effect of improving yield ratio is, the more preferable. Therefore, no particular upper limit is determined for the P0 value.

As long as the P0 value satisfies P0≥0.30 as a whole, P0′ and P1 may individually take any values. However, it is preferable that PI satisfies 1.04≤P1≤1.15. As described above, each element included in the definitional formula (3) of P1 has an effect of improving strength (tensile strength) and hardness of the steel material. Therefore, in the case of 1.04≤P1, a high effect is obtained in improving strength of the steel material, particularly in improving yield point, and a high yield point such as 900 MPa or more is easily achieved. The P1 value is more preferably 1.05≤P1.

On the other hand, each element included in the definitional formula (3) of P1 reduces machinability of the steel material. Therefore, in the case of P1≤1.15, high machinability can be ensured in the steel material. The P1 value is more preferably P1 1.10.

Further, in the steel material according to the present embodiment, it is preferable that the P2 value obtained based on the following formula (4) and the P3 value obtained based on the following formula (5) satisfy P2/P3≥1.4.


P2=417−242C+30Si−25Mn−17Cu−22Ni−14Cr−35Mo  (4)


P3=10∧(1.35−0.54C+0.02Si+0.77Mn4−0.47Cu+0.42Ni+0.52Cr+4.84Mo)   (5)

P2 calculated based on the formula (4) substantially corresponds to the temperature range from a ferrite transformation point to 500° C. As the P2 value is greater, a ferrite-pearlite transformation is more likely to be completed, and the generation of bainite is prevented. On the other hand, P3 calculated based on the formula (5) substantially corresponds to the cooling time (critical cooling time) up to 500° C., which is required to complete a ferrite transformation. A value obtained by dividing the temperature range represented by P2 by the critical cooling time represented by P3 is a critical cooling rate.

Therefore, in the case of P2/P3≥1.4, a high effect in preventing the generation of bainite is obtained. P2/P3 is more preferably P2/P3≥1.7, and further preferably P2/P3≥1.9. The greater the P2/P3 value is, the more stable a ferrite-pearlite microstructure is, and therefore, no particular upper limit is provided for P2/P3.

[Characteristics of Steel Material]

The steel material according to the present embodiment has the component composition described above, so that a high strength is exhibited even in a non-heat-treated state. In particular, since the contents of the elements, such as Mn and Cr, which have a property of promoting the generation of bainite that is a microstructure leading to a decrease in strength of the steel material while promoting the fine precipitation of V precipitates are controlled, and a balance between contents of these elements and other elements is controlled, a high effect in improving strength of the steel material is obtained. Therefore, the steel material according to the present embodiment can be suitably used as a ferrite-pearlite non-heat-treated steel for manufacturing automobile engine parts and the like.

The steel material according to the present embodiment tends to have a yield point of 900 MPa or more and a yield ratio of 0.80 or more in a non-heat-treated state after hot forging, in correspondence with having the above-mentioned component composition, particularly, containing predetermined amounts of Mn and Cr together with V. More preferably, the steel material may have a yield point of 920 MPa or more and a yield ratio of 0.81 or more. The steel material having a high yield point and a high yield ratio indicates that the steel material has high strength. An increase in yield point and yield ratio due to the addition of Mn is also shown in Examples to be described later. Here, the yield point and the yield ratio may be evaluated by a tensile test of JIS Z, 2241:2011 as a 0.2% yield strength and a 0.2% yield strength ratio (ratio of 0.2% yield strength to tensile strength), respectively. The hot forging in the evaluation may be performed, for example, under conditions of a heating temperature of 1,250° C. and a reduction of area of 70%. Since a higher strength of the steel material is more preferable, no particular upper limit is specified for the strength of the steel material.

In the steel material according to the present embodiment, as described above, high strength is obtained by the fine precipitation of the V precipitates, and the size of the precipitates also reflects the strength. The smaller the generation amount of coarse precipitates as precipitates containing V, the higher the strength of the steel material. For example, in a cross section of the steel material in a non-heat-treated state after hot forging, the number of precipitates satisfying (V+Ti)≥30% and having an area of 1 μm2 or more (the square root value of the area is 1 μm or more) is preferably 10 or less per 1 mm2, more preferably 8 or less, and further preferably 5 or less. The hot forging in the evaluation may be performed, for example, under conditions of a heating temperature of 1,250° C. and a reduction of area of 70%.

The steel material according to the present embodiment is not prevented from being used after heat treatment, but as described above, the steel material has high strength even in a non-heat-treated state. Therefore, it is preferable to use the steel material as a non-heat-treated steel without heat treatment after hot forging from the viewpoint of simplifying of a manufacturing process of a product or the like. The conditions of the hot forging are not particularly limited, but a mode in which a forging heating temperature is 1,100° C. to 1,260° C. and the reduction of area is 50% to 95% can be preferably exemplified.

EXAMPLES

Hereinafter, the present invention will be described in more detail with reference to Examples.

[1] Improvement of Strength by Addition of Mn

First, the effect of improving strength by the addition of Mn was confirmed, and the mechanism thereof was examined.

[Preparation of Samples]

Non-heat-treated steel samples containing 0.4% or 0.9% of Mn and 0.1% to 0.4% of V were prepared. Component elements other than Mn and V and contents thereof were C: 0.35%, Si: 0.50%, P: 0.05%, Cu: 0.12%, Ni: 0.07%, and Cr: 0.30% with the balance being Fe and inevitable impurities. In the preparation of each of the non-heat-treated steel samples, a steel having a predetermined component composition was melted in a vacuum induction furnace, and then an ingot was cast. The obtained ingot was subjected to rough forging and finish forging to control mechanical properties, and then subjected to evaluation. As the final forging to control mechanical properties, a round bar steel material was forged from 40D to 22D at a heating temperature of 1,250° C. (reduction of area: 70%). The forging end temperature was set to 1,050° C. or higher, and the cooling rate was set to 1.4° C./s to 1.6° C./s on average between 600° C. to 800° C.

[Test Method]

A No. JIS 14A test piece was prepared from the obtained non-heat-treated steel, and subjected to a tensile test in accordance with JIS Z 2241:2011 at room temperature in the air, to evaluate yield point (0.2% yield strength) and tensile strength. Further, the yield ratio (0.2% yield strength ratio) was calculated as a ratio of the yield point to the tensile strength.

[Test Results]

FIG. 1A and FIG. 1B show measurement results of the yield point and the yield ratio, respectively. In the graphs, the horizontal axis represents the content of V, and the vertical axis represents a value of the yield point or the yield ratio. In addition, the cases where the content of Mn was 0.4% are indicated by circle marks, and the cases where the content of Mn was 0.9% are indicated by square marks. According to FIG. 1A, regardless of the content of Mn, the yield point linearly increased as the content of V increased. According to FIG. 1B, the yield ratio also increased as the content of V increased. These results indicate that the strength of the non-heat-treated steel is increased by adding V to the non-heat-treated steel and further increasing the addition amount of V. This is because the addition of V causes precipitation of fine V carbides and precipitation strengthening.

Next, the cases where the content of Mn was 0.4% and the cases where the content of Mn was 0.9% are compared. In the entire region of the content of V, the yield point and the yield ratio were higher in the cases where the content of Mn was 0.9%. From this, it is found that the strength of the non-heat-treated steel was increased by increasing the content of Mn. It is considered that this is because the addition of Mn lowers the ferrite transformation temperature and promotes the fine precipitation of V precipitates such as V carbides.

Further, when the effect due to the increase in the content of Mn is compared between the cases where the content of V was different, as the content of V was increased, a range of the improvement of the yield point and the yield ratio when the content of Mn was increased from 0.4% to 0.9% was increased. For example, an improvement amount of the yield point was 55 MPa when the content of V was 0.1%, and was increased to 92 MPa when the content of V was 0.4%. In addition, an improvement amount of the yield ratio was 0.013 when the content of V was 0.1%, and was increased to 0.027 when the content of V was 0.4%. That is, it can be said that the effect of improving the strength by increasing the content of V is amplified by increasing the content of Mn. In particular, an amplification effect at the yield ratio is increased.

In this way, by adding V to the non-heat-treated steel, the strength of the non-heat-treated steel is improved due to the generation of fine precipitates, and by further adding Mn, the fine precipitation of the V precipitates is promoted and a further effect of improving the strength is obtained. It is confirmed that not only Mn but also Cr, Ni, and Cu achieve similar effects.

[2] Characteristics of Non-heat-treated Steels Having Various Component Compositions

Next, non-heat-treated steels having various component compositions were prepared, and the characteristics thereof were evaluated.

[Preparation of Samples]

Non-heat-treated steels according to respective Examples and Comparative Examples having the component compositions (unit: mass %) shown in Tables 1 to 3 (balance is Fe and inevitable impurities) below were prepared, respectively. A method of preparing the non-heat-treated steels and conditions of hot forging were the same as those in the test [1] described above.

[Test Method]

(1) Evaluation of Microstructure and Precipitates in Steel

The cross section of a sample after hot forging was observed with a scanning electron microscope (SEM). In this case, a region corresponding to ½ of the radius from the center of the cross section of the sample was observed. In this region, the type of a formed microstructure was determined. In the respective Examples and Comparative

Examples, the observed microstructure was ferrite-pearlite (F+P) or bainite in addition to ferrite-pearlite (F+P+B). Further, with respect to samples in which the microstructure of F+P was obtained, the number of precipitates (coarse precipitates) satisfying (V+Ti)≥30% and having an area of 1 μm′ or more was evaluated per 1 mm2 by automatic SEM analysis.

(2) Evaluation of Strength

The yield point and the yield ratio of each sample were evaluated in the same manner as in the test [1] described above.

[Test Results]

Tables 1, 2 and 3 below show the component compositions of the non-heat-treated steels according to Examples 1 to 43 and Comparative Examples 1 to 14. Further, Tables 4, 5 and 6 show values of P0 to P3 and P2/P3 calculated based on the component compositions by the above-mentioned formulae (1) to (5), and results of evaluation by each test. In addition, among the samples, with respect to the samples in which the individual content of each element satisfies the range of the content specified in the embodiment of the present invention described above, that is, with respect to each of the respective Examples and Comparative Examples 1 to 5, 10, 13, and 14, a relation between the P0 value and the yield ratio is shown in FIG. 2A, and a relation between the P1 value and the yield point is shown in FIG. 2B. FIG. 2A and FIG. 2B also show approximate straight lines.

TABLE 1 Sample Component Contents (mass %) Number C Si Mn P S Cu Ni Cr Mo V Ex. 1 0.35 0.81 0.87 0.048 0.050 0.10 0.04 0.45 0.02 0.35 2 0.35 0.50 0.90 0.051 0.049 0.12 0.07 0.30 0.02 0.40 3 0.35 0.49 0.72 0.049 0.047 0.12 0.30 0.31 0.02 0.39 4 0.34 0.88 0.81 0.072 0.046 0.12 0.15 0.36 0.02 0.37 5 0.36 0.85 0.97 0.045 0.043 0.13 0.09 0.40 0.02 0.35 6 0.34 0.71 0.82 0.066 0.062 0.13 0.21 0.35 0.02 0.39 7 0.34 0.80 0.79 0.052 0.051 0.15 0.10 0.56 0.02 0.36 8 0.37 0.52 0.92 0.018 0.034 0.12 0.07 0.32 0.02 0.39 9 0.36 0.72 0.85 0.028 0.038 0.12 0.07 0.35 0.02 0.38 10 0.38 0.52 0.91 0.052 0.048 0.12 0.10 0.31 0.02 0.40 11 0.38 0.22 0.79 0.042 0.051 0.07 0.03 0.52 0.02 0.39 12 0.36 0.69 0.93 0.015 0.021 0.05 0.02 0.26 0.02 0.41 13 0.37 0.24 0.83 0.017 0.052 0.15 0.08 0.37 0.02 0.40 14 0.36 0.62 0.89 0.049 0.046 0.10 0.07 0.40 0.02 0.41 15 0.38 0.50 0.90 0.015 0.021 0.15 0.07 0.22 0.02 0.40 16 0.32 0.75 0.88 0.061 0.053 0.11 0.08 0.39 0.02 0.38 17 0.36 0.57 0.95 0.089 0.047 0.12 0.09 0.35 0.03 0.33 18 0.43 0.43 0.91 0.043 0.046 0.06 0.08 0.47 0.03 0.34 19 0.36 0.47 1.13 0.053 0.041 0.02 0.03 0.26 0.01 0.38 Sample Component Contents (mass %) Number Al Ti Nb Ca Pb Bi Zr Mg Te N Ex. 1 0.007 0.006 — 0.0022 0.18 — — — — 0.009 2 0.007 0.005 — — — — — — — 0.012 3 0.006 0.005 — 0.0025 — — — — — 0.010 4 0.008 0.006 — 0.0031 — — — — — 0.010 5 0.008 0.008 — 0.0043 — 0.08 — — — 0.011 6 0.007 0.005 — 0.0022 — — — — — 0.012 7 0.006 0.008 — 0.0014 — — — — — 0.010 8 0.006 0.006 — 0.0017 — — — — — 0.010 9 0.007 0.007 — — — — — — — 0.010 10 0.006 0.006 — 0.0027 — — — — — 0.011 11 0.008 0.005 — — — — — 0.003 — 0.009 12 0.006 0.003 0.002 — — — 0.002 — — 0.011 13 0.006 0.004 — 0.0021 — — — — — 0.010 14 0.007 0.006 — — — — — — 0.003 0.012 15 0.005 — — — — — — — — 0.006 16 0.010 0.004 — — — — 0.004 — — 0.008 17 0.027 — — — — — — — — 0.008 18 0.018 — — — — — — — — 0.013 19 0.022 — — — — — — — — 0.015

TABLE 2 Sample Component Contents (mass %) Number C Si Mn P S Cu Ni Cr Mo V Ex. 20 0.36 0.58 0.66 0.063 0.051 0.10 0.12 0.67 0.03 0.38 21 0.35 0.62 0.87 0.041 0.055 0.09 0.07 0.41 0.01 0.44 22 0.33 0.98 0.88 0.056 0.051 0.12 0.10 0.40 0.03 0.39 23 0.37 0.74 0.83 0.037 0.033 0.41 0.07 0.36 0.01 0.34 24 0.34 0.82 0.85 0.062 0.058 0.10 0.08 0.34 0.05 0.37 25 0.37 0.76 0.92 0.046 0.072 0.12 0.06 0.37 0.02 0.38 26 0.37 0.75 0.95 0.051 0.045 0.12 0.08 0.45 0.02 0.31 27 0.35 0.64 0.88 0.051 0.052 0.09 0.08 0.39 0.01 0.47 28 0.34 0.30 1.15 0.030 0.020 0.10 0.10 0.20 0.01 0.41 29 0.34 0.88 0.90 0.020 0.021 0.12 0.15 0.36 0.02 0.37 30 0.36 0.85 0.97 0.045 0.043 0.13 0.09 0.40 0.02 0.35 31 0.40 0.60 0.72 0.011 0.022 0.17 0.13 0.23 0.07 0.39 32 0.33 0.69 0.79 0.008 0.010 0.14 0.12 0.61 0.02 0.38 33 0.38 0.11 0.85 0.021 0.018 0.07 0.03 0.52 0.02 0.39 34 0.42 0.15 1.13 0.004 0.003 0.08 0.06 0.28 0.02 0.38 35 0.38 0.33 0.88 0.020 0.020 0.07 0.14 0.48 0.03 0.42 36 0.34 0.83 0.83 0.017 0.022 0.02 0.01 0.61 0.02 0.37 37 0.34 0.25 0.91 0.042 0.051 0.15 0.09 0.44 0.02 0.39 38 0.41 0.22 0.99 0.016 0.019 0.10 0.08 0.39 0.02 0.36 Sample Component Contents (mass %) Number Al Ti Nb Ca Pb Bi Zr Mg Te N Ex. 20 0.009 0.004 — 0.0062 — — — — — 0.019 21 0.013 0.005 — 0.0049 — — 0.006 — — 0.014 22 0.033 — — — — — — — 0.005 0.009 23 0.038 0.003 — — — 0.14 — — — 0.022 24 0.017 — 0.005 0.0069 — — — — — 0.008 25 0.023 0.002 — 0.0052 — — — — — 0.014 26 0.013 0.005 — — — — — — — 0.015 27 0.017 0.005 — 0.0034 — — 0.005 — — 0.015 28 0.022 0.007 — — — 0.05 — — — 0.008 29 0.008 0.006 — — 0.15 — — — — 0.010 30 0.008 0.008 — — — 0.02 — — — 0.011 31 0.006 0.014 — 0.0010 — — — — — 0.026 32 0.007 0.006 — 0.0020 — — — — — 0.003 33 0.008 0.005 — 0.0008 0.12 — — — — 0.005 34 0.010 0.013 — — — — — — — 0.008 35 0.012 0.005 — — — 0.10 — — — 0.013 36 0.008 0.008 — — 0.08 — — — — 0.012 37 0.006 0.006 — — — — — — — 0.008 38 0.008 0.005 — 0.0016 — — — — — 0.010

TABLE 3 Sample Component Contents (mass %) Number C Si Mn P S Cu Ni Cr Mo V Ex. 39 0.37 0.42 0.98 0.065 0.022 0.09 0.09 0.38 0.03 0.35 40 0.36 0.19 0.77 0.015 0.022 0.17 0.12 0.50 0.02 0.39 41 0.38 0.28 0.92 0.022 0.018 0.08 0.01 0.50 0.03 0.42 42 0.37 0.20 1.00 0.017 0.027 0.15 0.08 0.37 0.02 0.40 43 0.34 0.47 1.13 0.053 0.041 0.11 0.09 0.26 0.01 0.38 Comp. 1 0.36 0.84 0.79 0.054 0.049 0.14 0.09 0.38 0.02 0.34 Ex. 2 0.34 0.83 0.86 0.046 0.049 0.03 0.04 0.42 0.02 0.35 3 0.38 0.69 0.77 0.060 0.049 0.12 0.07 0.32 0.02 0.35 4 0.35 0.80 0.62 0.050 0.050 0.12 0.20 0.32 0.02 0.37 5 0.36 0.88 0.86 0.046 0.053 0.10 0.10 0.39 0.02 0.34 6 0.37 0.78 0.73 0.053 0.049 0.15 0.10 0.43 0.02 0.38 7 0.47 0.44 0.89 0.034 0.059 0.12 0.09 0.35 0.02 0.33 8 0.36 0.61 0.95 0.055 0.049 0.12 0.08 0.42 0.02 0.28 9 0.34 0.52 0.53 0.059 0.039 0.17 0.15 0.57 0.04 0.39 10 0.36 0.77 0.65 0.050 0.049 0.12 0.10 0.40 0.02 0.35 11 0.36 0.63 0.92 0.032 0.049 0.10 0.07 0.72 0.02 0.34 12 0.34 0.77 1.24 0.050 0.045 0.15 0.10 0.42 0.02 0.32 13 0.40 0.92 0.78 0.023 0.022 0.09 0.07 0.27 0.03 0.35 14 0.36 0.88 0.84 0.014 0.016 0.12 0.11 0.28 0.02 0.34 Sample Component Contents (mass %) Number Al Ti Nb Ca Pb Bi Zr Mg Te N Ex. 39 0.021 — — — — — — — — 0.007 40 0.021 0.009 — — — — — — — 0.008 41 0.010 0.006 — — — — — — — 0.012 42 0.006 0.004 — — 0.10 — — — — 0.010 43 0.022 — — — — — — — — 0.015 Comp. 1 0.007 0.010 — 0.0026 0.19 — — — — 0.010 Ex. 2 0.008 0.006 — 0.0024 — — — — — 0.013 3 0.007 0.005 — 0.0014 — — — — — 0.012 4 0.006 0.007 — — — — — — — 0.010 5 0.005 0.009 — 0.0028 — — — — — 0.012 6 0.010 0.016 — — — — 0.004 — — 0.013 7 0.007 — — — — — — — — 0.009 8 0.013 0.004 — — — — — — — 0.012 9 0.031 — — — — — — — 0.002 0.024 10 0.008 0.009 — 0.0032 — 0.07 — — — 0.012 11 0.010 0.008 — 0.0026 — — — — — 0.009 12 0.008 0.009 — 0.0028 — — — — — 0.010 13 0.012 0.006 — — — — — — — 0.013 14 0.009 — — — — — — — — 0.012

TABLE 4 Coarse Characteristics Sample Values of Formula Precipitate Yield Point Tensile Strength Yield Number P0 P1 P2 P3 P2/P3 (number/mm2) Microstructure (MPa) (MPa) Ratio Examples 1 0.302 1.04 325.05 176.16 1.85 3.8 F + P 901 1124 0.80 2 0.376 1.05 316.32 159.44 1.98 5.2 F + P 927 1126 0.82 3 0.378 1.06 315.32 146.42 2.15 3.5 F + P 929 1137 0.82 4 0.306 1.07 329.79 162.59 2.03 4.8 F + P 940 1164 0.81 5 0.333 1.07 320.64 210.47 1.52 8.7 F + P 974 1191 0.82 6 0.360 1.08 323.09 173.82 1.86 5.6 F + P 934 1148 0.81 7 0.315 1.07 325.68 195.52 1.67 8.9 F + P 931 1147 0.81 8 0.375 1.05 311.30 165.20 1.88 5.1 F + P 926 1132 0.82 9 0.325 1.05 321.05 154.56 2.08 7.3 F + P 920 1138 0.81 10 0.375 1.10 308.61 163.04 1.89 6.6 F + P 951 1164 0.82 11 0.365 1.06 302.06 147.98 2.04 4.2 F + P 935 1147 0.82 12 0.342 1.04 321.70 141.06 2.28 2.3 F + P 915 1132 0.81 13 0.402 1.05 303.72 153.96 1.97 2.6 F + P 926 1127 0.82 14 0.365 1.10 316.69 171.59 1.85 7.9 F + P 966 1182 0.82 15 0.370 1.05 309.67 144.18 2.15 0.6 F + P 939 1140 0.82 16 0.341 1.04 330.27 179.76 1.84 1.3 F + P 928 1137 0.82 17 0.334 1.04 313.26 208.88 1.50 0.4 F + P 919 1132 0.81 18 0.333 1.09 292.68 189.89 1.54 1.2 F + P 947 1181 0.80 19 0.410 1.05 310.74 174.06 1.79 3.9 F + P 932 1122 0.83

TABLE 5 Coarse Characteristics Sample Values of Formula Precipitate Yield Point Tensile Strength Yield Number P0 P1 P2 P3 P2/P3 (number/mm2) Microstructure (MPa) (MPa) Ratio Examples 20 0.315 1.10 316.01 184.67 1.71 9.1 F + P 949 1186 0.80 21 0.379 1.11 319.99 150.18 2.13 8.2 F + P 968 1189 0.81 22 0.322 1.08 333.65 209.22 1.59 1 F + P 926 1156 0.80 23 0.320 1.08 315.01 182.73 1.72 6.4 F + P 935 1162 0.80 24 0.305 1.04 328.10 217.12 1.51 0.7 F + P 916 1134 0.81 25 0.333 1.09 318.02 175.63 1.81 1.9 F + P 936 1166 0.80 26 0.308 1.04 315.41 207.73 1.52 7.2 F + P 906 1131 0.80 27 0.394 1.15 320.40 150.83 2.12 9.8 F + P 992 1222 0.81 28 0.499 1.05 307.92 199.20 1.55 0.7 F + P 965 1127 0.86 29 0.343 1.05 327.54 190.72 1.72 4.8 F + P 940 1164 0.81 30 0.333 1.07 320.64 210.47 1.52 5.2 F + P 974 1191 0.82 31 0.307 1.05 308.78 196.47 1.57 7.2 F + P 920 1148 0.80 32 0.360 1.06 323.83 210.91 1.54 3.1 F + P 939 1166 0.81 33 0.404 1.05 297.26 163.76 1.82 3.7 F + P 935 1147 0.82 34 0.456 1.08 284.31 200.22 1.42 4.2 F + P 979 1153 0.85 35 0.429 1.11 300.90 206.78 1.46 1.3 F + P 1030 1210 0.85 36 0.301 1.04 329.07 177.71 1.85 6.3 F + P 937 1152 0.81 37 0.432 1.05 308.08 202.30 1.52 2.1 F + P 952 1147 0.83 38 0.406 1.06 290.01 188.58 1.54 0.4 F + P 980 1155 0.85

TABLE 6 Coarse Characteristics Sample Values of Formula Precipitate Yield Point Tensile Strength Yield Number P0 P1 P2 P3 P2/P3 (number/mm2) Microstructure (MPa) (MPa) Ratio Examples 39 0.375 1.05 305.68 216.82 1.41 1.7 F + P 941 1125 0.84 40 0.410 1.05 303.10 173.54 1.75 1.1 F + P 980 1147 0.85 41 0.416 1.10 300.74 202.80 1.48 1.1 F + P 1016 1203 0.84 42 0.461 1.07 298.27 207.73 1.44 1.3 F + P 972 1168 0.83 43 0.442 1.05 312.73 208.45 1.50 0.9 F + P 945 1120 0.84 Comparative 1 0.275 1.04 324.95 150.90 2.15 12.9 F + P 841 1072 0.78 Examples 2 0.287 1.01 330.15 155.35 2.13 8.1 F + P 896 1108 0.81 3 0.274 1.04 317.73 126.04 2.52 5.6 F + P 869 1100 0.79 4 0.268 1.03 329.18 114.29 2.88 7.1 F + P 882 1109 0.80 5 0.291 1.04 324.72 167.49 1.94 12.4 F + P 882 1125 0.78 6 0.291 1.09 321.14 144.81 2.22 23.2 F + P 922 1176 0.78 7 0.310 1.10 284.59 145.65 1.95 0.6 F + P 937 1180 0.79 8 0.302 0.99 314.05 201.60 1.56 1.6 F + P 850 1071 0.79 9 0.302 1.06 321.50 165.12 1.95 1.2 F + P 907 1143 0.79 10 0.250 1.02 326.19 118.77 2.75 8.2 F + P 877 1113 0.79 11 0.354 1.09 311.76 265.58 1.17 — F + P + B 922 1213 0.76 12 0.400 1.07 315.49 366.69 0.86 — F + P + B 673 1117 0.60 13 0.243 1.04 320.40 128.91 2.49 29.3 F + P 865 1137 0.76 14 0.287 1.00 326.20 146.22 2.23 10.7 F + P 857 1089 0.79

According to FIG. 2A, a high correlation was observed between the P0 value and the yield ratio, and it is found that as the P0 value increased, the yield ratio increased and a higher material strength was obtained. As described above, the P0 value can be used as a good index of the strength of the non-heat-treated steel. It is found that when the P0 value is set to 0.30 or more, a yield ratio of about 0.80 or more can be obtained.

In addition, according to FIG. 2B, a high correlation was observed between the P1 value and the yield point, and it is found that as the P1 value increased, the yield point increased and a higher material strength was obtained. As described above, the P1 value can also be used as a good index of the strength of the non-heat-treated steel. It is found that when the P1 value is set to 1.04 or more, a yield point of about 900 MPa or more can be obtained.

According to Tables 1 to 4, in each of Examples in which the content of each component element satisfied the range of the content specified in the embodiment of the present invention described above and satisfied P0≥0.30, a ferrite-pearlite microstructure was obtained, and the number of coarse precipitates was limited to 10/mm2 or less. In addition, a yield point of 900 MPa or more and a yield ratio of 0.80 or more were obtained, and it is confirmed that the non-heat-treated steel had high strength. Among Examples, a sample having larger contents of Mn, Cu, Cr, and Ni and also having a greater P0 value and a greater P1 value, tends to have a higher yield point and higher yield ratio and higher material strength.

On the other hand, in each of Comparative Examples, the content of any of the component elements did not satisfy the range of the content specified in the embodiment of the present invention described above, and/or did not satisfy P0≥0.30. In Comparative Examples 1 to 5, 10, 13, and 14, the content of each component element satisfied a predetermined range, but P0≤0.30. Therefore, the yield point was less than 900 MPa, and the yield ratio was less than 0.80 except for Comparative Examples 2 and 4.

In Comparative Example 6, the content of Ti was more than 0.015%, and the P0 value was also less than 0.30. Therefore, the number of coarse precipitates exceeded 10 per 1 mm2, and the yield ratio did not reach 0.80. In Comparative Example 7, the content of C exceeded 0.45%. Therefore, the yield ratio did not reach 0.80. In Comparative Example 8, the V content did not reach 0.30%, and a yield point of less than 900 MPa and a yield ratio of less than 0.80 were obtained. It can be said that the effect of improving strength by the V precipitates was not sufficiently obtained. In Comparative Example 9, the content of Mn was less than 0.60%, and the yield ratio was less than 0.80. On the other hand, in Comparative Example 11 containing Cr in an amount of more than 0.70% and Comparative Example 12 containing Mn in an amount of more than 1.20%, the microstructure contained bainite, and the yield ratio was significantly less than 0.80. In Comparative Example 12, the yield point was also significantly lower than 900 MPa. Mn and Cr have an effect of improving strength by promoting the fine precipitation of the V precipitates, but at the same time, promote the generation of bainite leading to a decrease in strength. It was shown that high strength cannot be obtained in the non-heat-treated steel due to excessive addition of Mn and Cr.

The embodiment and examples of the present invention have been described above. The present invention is not particularly limited to these embodiments and examples, and various modifications can be made.

Claims

1. A steel material,

comprising, in terms of mass %: 0.30%≤C≤0.45%, 0.10%≤Si≤1.00%, 0.60%≤Mn≤1.20%, 0.20%≤Cr≤0.70%, 0.30%≤V≤0.47%, Ti≤0.015%, P≤0.100%, S≤0.080%, Cu≤0.50%, Ni≤0.50%, Nb≤0.010%, Ti≤0.015%, Pb≤0.30%, Bi≤0.20%, Ca≤0.0100%, Zr≤0.010%, Mg≤0.010%, Te 5 0.010%. Mo≤0.10%, Al≤0.050%, and N≤0.030%, with the balance being Fe and inevitable impurities, and
having a P0 value defined by the following formula (1), satisfying P0≥0.30: P0=P0′×V/P1  (1) here, P0′=Mn+0.49Cu4−0.89Ni+0.40Cr−0.30Si  (2) P1C+0.07Si+0.16Mn+0.61P+0.19Cu+0.17Ni+0.2Cr+V  (3) in the formulae (1) to (3), each element symbol indicates a content of each element in units of mass %.

2. The steel material according to claim 1, wherein the steel material is used in a state where heat treatment is not performed.

3. The steel material according to claim 1,

comprising, in terms of mass %, at least one selected from: 0%≤Cu≤0.50%, 0%≤Ni≤0.50%, 0%≤Nb≤0.010%, 0%≤Ti≤0.015%, 0%≤Pb≤0.30%, 0%≤Bi≤0.20%, 0%≤Ca 0.0100%, 0%≤Zr≤0.010%, 0%≤Mg≤0.010%, and 0%≤Te≤0.010%.

4. The steel material according to claim 1, wherein the P1 value defined by the formula (3) satisfies 1.04≤P1≤1.15.

5. The steel material according to claim 1, having a P2 value defined by the following formula (4) and a P3 value defined by the following formula (5), satisfying P2/P3≥1.4: in the formulae (4) and (5), each element symbol indicates a content of each element in units of mass %.

P2=417−242C+30Si−25Mn−17Cu−22Ni−14Cr−35Mo  (4)
P3=10∧(1.35−0.54C+0.02Si+0.77Mn+0.47Cu+0.42Ni+0.52Cr−4.84Mo)   (5)

6. The steel material according to claim 4, having a P2 value defined by the following formula (4) and a P3 value defined by the following formula (5), satisfying P2/P3≥1.4:

P2=417−242C+30Si−25Mn−17Cu−22Ni−14Cr−35Mo  (4)
P3=10∧(1.35−0.54C+0.02Si+0.77Mn+0.47Cu±0.42Ni+0.52Cr+4.84Mo)   (5)
in the formulae (4) and (5), each element symbol indicates a content of each element in units of mass %.

7. The steel material according to claim 1, wherein

in a cross section of the steel material in a state after hot forging,
the number of precipitates satisfying (V+Ti)≥30% and having an area of 1 μm2 or more is 10 or less per 1 mm2.

8. The steel material according to claim 4, wherein

in a cross section of the steel material in a state after hot forging,
the number of precipitates satisfying (V+Ti)≥30% and having an area of 1 μm2 or more is 10 or less per 1 mm2.

9. The steel material according to claim 5, wherein

in a cross section of the steel material in a state after hot forging,
the number of precipitates satisfying (V+Ti)≥30% and having an area of 1 μm2 or more is 10 or less per 1 mm2.

10. The steel material according to claim 6, wherein

in a cross section of the steel material in a state after hot forging,
the number of precipitates satisfying (V+Ti)≥30% and having an area of 1 μm2 or more is 10 or less per 1 mm2.

11. The steel material according to claim 1, having, in a state after hot forging:

a yield point of 900 MPa or more, and
a yield ratio of 0.80 or more.
Patent History
Publication number: 20230175106
Type: Application
Filed: Dec 7, 2022
Publication Date: Jun 8, 2023
Inventors: Hiromasa Okuda (Nagoya-shi), Ryosuke Ohashi (Nagoya-shi)
Application Number: 18/076,653
Classifications
International Classification: C22C 38/50 (20060101); C22C 38/02 (20060101); C22C 38/04 (20060101); C22C 38/00 (20060101); C22C 38/06 (20060101); C22C 38/42 (20060101); C22C 38/48 (20060101); B21J 1/06 (20060101);