JP5655366B2 - Bainite steel - Google Patents

Description

  The present invention relates to bainite steel.

  Conventionally, various automobile parts and machine structural parts are manufactured by hot forging. When high strength and high toughness are required for these parts, tempering is performed after hot forging. In recent years, automobiles and the like have been strongly demanded to reduce the weight of vehicles in order to achieve low fuel consumption, and for this reason, miniaturization of parts is directed. Along with this, further improvement in the yield strength ratio and durability ratio is required for steel materials.

  In order to increase the yield strength ratio and durability ratio of a steel material, it is effective to perform a tempering treatment by quenching and tempering so that the steel material has a martensitic structure. However, since the martensite structure is very hard, the machinability of the steel material is lowered. Therefore, it is conceivable to perform machining after hot forging and then perform tempering by quenching and tempering. However, in this case, the desired shape cannot be maintained due to heat treatment distortion or expansion due to the tempering treatment, and eventually additional machining is required. Therefore, the problem of machinability still occurs.

  Therefore, attempts have been made to increase the strength of bainite steel, which is softer than martensite steel and is advantageous for machinability.

  For example, in Patent Document 1, C: 0.11 to 0.60 mass%, Si: 0.03 to 3.0 mass%, Mn: 0.01 to 2.5 mass%, Mo: 0.3 to 4.0% by mass, V: 0.05 to 0.5% by mass, Cr: 0.1 to 3.0% by mass, the balance is Fe and inevitable impurities, and between each component, 4C + Mn + 0.7Cr + 0.6Mo A relationship satisfying −0.2 V ≧ 2.5, C ≧ Mo / 16 + V / 5.7, V + 0.15Mo ≧ 0.4 is established, and after rolling, forging, or solution treatment, the temperature starts from 800 ° C. It is cooled at an average cooling rate of 0.05 to 10 ° C./sec during 300 ° C., and before the aging treatment, the area ratio of the bainite structure is 50% or more and the hardness is 40 HRC or less. Aged hardened steel whose hardness is higher by 7 HRC or more than the hardness before aging treatment It is shown.

JP 2006-37177 A

  However, the conventional bainite steel still has room for improvement. That is, the conventional bainite steel can be increased in hardness by aging treatment, but it is difficult to achieve both hot forgeability and machinability after hot forging.

  The present invention has been made in view of the above circumstances, and the problem to be solved by the present invention is excellent in hot forgeability and machinability after hot forging, and is improved in strength by age hardening after machining. It is to provide a bainite steel that can be achieved.

The bainite steel according to the present invention is mass%, C: 0.14 to 0.35%, Si: 0.05 to 0.70%, Mn: 1.10 to 2.30%, S: 0.003. ~0.120%, Cu: 0.0 5 ~0.40 %, Ni: 0.01~0.40%, Cr: 0.01~0.50%, Mo: 0.01~0.30% And V: 0.05 to 0.45%, with the balance being Fe and inevitable impurities, 13 [C] +8 [Si] +10 [Mn] +3 [Cu] +3 [Ni] +22 [Mo ] +11 [V] ≦ 30, 5 [C] + [Si] +2 [Mn] +3 [Cr] +2 [Mo] +4 [V] ≦ 7.3, 2.4 ≦ 0.3 [C] +1.1 [Mn] +0.2 [Cu] +0.2 [Ni] +1.2 [Cr] +1.1 [Mo] +0.2 [V] ≦ 3.1, 2.5 ≦ [C] + [Si] +4 Mo] +9 [V], and subject matter to meet the [C] ≧ [Mo] / 16 + [V] / 3.

  The bainite steel according to the present invention further contains one or more kinds selected from Ti: 0.001 to 0.100% and Ca: 0.0003 to 0.0100% in mass%. May be.

  The bainite steel according to the present invention contains the above-described specific element in a specific range and satisfies a specific formula. Therefore, it is excellent in hot forgeability and excellent in machinability after hot forging and before aging treatment. And if an aging treatment is performed after cutting, it becomes hard by age hardening, yield strength ratio and durability ratio comparable to martensitic steel can be obtained, and high strength can be achieved. Moreover, since the bainite steel according to the present invention hardly undergoes the structural transformation by the aging treatment, the heat treatment strain does not occur, and the deterioration of the dimensional accuracy can be suppressed.

  Hereinafter, bainite steel according to the present invention (hereinafter sometimes referred to as “main bainite steel”) will be described in detail. This bainite steel contains the following elements, and the balance consists of Fe and inevitable impurities. Moreover, a specific relational expression is satisfied.

  The types, contents, reasons for limitation, and technical significance of each relational expression of each additive element in the bainite steel are as follows. In addition, the unit of content is mass%.

C: 0.14-0.35%
C is an element necessary for ensuring strength, and precipitates carbides of Mo and V by aging treatment to increase the strength of the steel core. In order to obtain the effect, the lower limit of the C content is 0.14% or more. The lower limit of the C content is preferably 0.15% or more, more preferably 0.16% or more.

  However, excessive addition of C impairs hot forgeability and excessively increases the hardness after hot forging (the hardness of the material before cutting), thereby degrading the machinability. Therefore, the upper limit of the C content is set to 0.35% or less. The upper limit of the C content is preferably 0.30% or less, more preferably 0.28% or less.

Si: 0.05 to 0.70%
Si functions as a deoxidizer during steel melting, and enhances age hardening characteristics. In order to obtain the effect, the lower limit of the Si content is set to 0.05% or more. The lower limit of the Si content is preferably 0.07% or more, more preferably 0.10% or more.

  However, excessive addition of Si impairs hot forgeability and decreases manufacturability. Moreover, the hardness after hot forging becomes excessive, and the machinability is deteriorated. Therefore, the upper limit of the Si content is set to 0.70% or less. The upper limit of the Si content is preferably 0.65% or less, more preferably 0.60% or less.

Mn: 1.10 to 2.30%
Mn is an element that plays an important role in the present invention, and is an indispensable element for generating a bainite structure in the structure after hot forging. Mn is also an essential element for forming Mn-based sulfides that contribute to improved machinability. In order to obtain the effect, the lower limit of the Mn content is set to 1.10% or more. The lower limit of the Mn content is preferably 1.30% or more, more preferably 1.40% or more.

  However, excessive addition of Mn facilitates the appearance of a martensite structure, increases the hardness after hot forging, and causes a decrease in machinability. Furthermore, hot forgeability is also impaired. Therefore, the upper limit of the Mn content is 2.30% or less. The upper limit of the Mn content is preferably 2.10% or less, more preferably 2.00% or less.

S: 0.003-0.120%
S is an element necessary for producing Mn-based sulfides that contribute to improvement of machinability together with Mn. In order to obtain the effect, the lower limit of the S content is set to 0.003% or more. The lower limit of the S content is preferably 0.005% or more, more preferably 0.010% or more.

  However, excessive addition of S not only impairs the toughness and ductility of the steel, but also tends to cause cracks during hot forging. In addition, in high-strength steel, inclusions become the starting point of fatigue failure, which deteriorates fatigue characteristics. Therefore, the upper limit of the S content is 0.120% or less. The upper limit of the S content is preferably 0.100% or less, more preferably 0.07% or less.

Cu: 0.01 to 0.40%
Cu, like Mn, is an indispensable element for generating a bainite structure. In order to obtain the effect, the lower limit of the Cu content is set to 0.01% or more. The lower limit of the Cu content is preferably 0.05% or more, more preferably 0.10% or more.

  However, excessive addition of Cu not only increases the hardness after hot forging and lowers machinability, but also impairs hot forgeability. Furthermore, it leads to an increase in cost. Therefore, the upper limit of the Cu content is set to 0.40% or less. The upper limit of the Cu content is preferably 0.35% or less, more preferably 0.30% or less.

Ni: 0.01-0.40%
Ni, like Mn, is an indispensable element for generating a bainite structure. In order to obtain the effect, the lower limit of the Ni content is set to 0.01% or more. The lower limit of the Ni content is preferably 0.05% or more, more preferably 0.10% or more.

  However, excessive addition of Ni not only increases the hardness after hot forging and decreases machinability, but also deteriorates hot forgeability. Furthermore, it leads to an increase in cost. Therefore, the upper limit of the Ni content is set to 0.40% or less. The upper limit of the Ni content is preferably 0.35% or less, more preferably 0.30% or less.

Cr: 0.01 to 0.50%
Cr, like Mn, is an essential element for generating a bainite structure. In order to stably generate a bainite structure, the lower limit of the Cr content is set to 0.01% or more. The lower limit of the Cr content is preferably 0.02% or more, more preferably 0.05% or more.

  However, excessive addition of Cr increases the hardness after hot forging and reduces the machinability. Moreover, hot forgeability is also impaired. Therefore, the upper limit of the Cr content is 0.50% or less. The upper limit of the Cr content is preferably less than 0.50%.

Mo: 0.01-0.30%
Mo is an element that plays an important role in the present invention, and is an indispensable element for increasing the hardness by age hardening and for generating a bainite structure. In addition, precipitation of Mo and V carbonitrides by aging treatment contributes to improvement of the yield strength ratio and durability ratio, and is therefore an important element for increasing the strength after machining. In order to obtain the effect, the lower limit of the Mo content is set to 0.01% or more. The lower limit of the Mo content is preferably 0.02% or more, more preferably 0.05% or more.

  However, excessive addition of Mo not only increases the hardness after hot forging and decreases machinability, but also deteriorates hot forgeability. Furthermore, it leads to an increase in cost. Therefore, the upper limit of the Mo content is set to 0.30% or less. The upper limit of the Mo content is preferably less than 0.30%.

V: 0.05 to 0.45%
V is an element that plays an important role in the present invention, and is an essential element for increasing the hardness by aging treatment and generating a bainite structure. In addition, precipitation of Mo and V carbonitrides by aging treatment contributes to improvement of the yield strength ratio and durability ratio, and is therefore an important element for increasing the strength after machining. In order to obtain the effect, the lower limit of the V content is set to 0.05% or more. The lower limit of the V content is preferably 0.07% or more, more preferably 0.10% or more.

  However, excessive addition of V not only increases the hardness after hot forging and lowers machinability, but also impairs hot forgeability. Furthermore, it leads to an increase in cost. Therefore, the upper limit of V content is 0.45% or less. The upper limit of the V content is preferably less than 0.45%.

  In addition to the essential elements described above, the bainite steel may contain one or more of the following elements as necessary.

Ti: 0.001 to 0.100%
Ti combines with O in the steel to form a fine oxide. This acts as a nucleus for the precipitation of Mn-based sulfides. Therefore, it helps to finely disperse the Mn-based sulfide. Ti also combines with C and N in the steel to form fine nitrides or carbonitrides. This contributes to preventing austenite crystal grains from coarsening during hot forging and improving strength. In order to obtain the effect, the lower limit of the Ti content is set to 0.001% or more. The lower limit of the Ti content is preferably 0.002% or more, more preferably 0.003% or more.

  However, since Ti can become a core of ferrite formation during forging cooling, it is desirable to reduce it when it is assumed that a bainite structure is obtained. Excessive addition of Ti produces coarse Ti nitride, which becomes a stress concentration source and leads to a decrease in the fatigue strength of the part. Moreover, since it becomes easy to produce | generate a ferrite, an age hardening characteristic is reduced and a strength fall is caused. Therefore, the upper limit of Ti content is 0.100% or less. The upper limit of the Ti content is preferably 0.060% or less, more preferably 0.020% or less.

Ca: 0.0003 to 0.0100%
This bainite steel reduces Pb which has been positively added as a machinability improving element in the past, and specifically keeps it to 0.03% by mass or less of the inevitable impurity level. Ca is an element that can be added in order to compensate for the machinability deterioration caused thereby. Further, Ca is advantageous because it partially dissolves in MnS and suppresses the deformation of sulfide during hot forging, so that it improves machinability. In order to make the effect remarkable, the lower limit of the Ca content is set to 0.0003% or more. The lower limit of the Ca content is preferably 0.0005% or more, more preferably 0.0007% or more.

  However, excessive addition of Ca causes the formation of large inclusions of Ca oxide and CaS, leading to a decrease in strength. Therefore, the upper limit of the Ca content is 0.0100% or less. The upper limit of the Ca content is preferably 0.0070% or less, more preferably 0.0050% or less.

  Further, the present bainite steel may contain, for example, Al, O, P, N, and the like within a range in which the above effects are not impaired.

Al: 0.040% or less Al may be added as a deoxidizer, but is an unavoidable impurity in the steelmaking process. Excessive Al tends to lead to the formation of coarse oxides, which acts as a stress concentration source and reduces the fatigue strength of the part. Therefore, the upper limit of the Al content is preferably 0.040% or less, more preferably 0.035% or less.

O: 0.008% or less O is an unavoidable impurity in the steelmaking process. Further, O combines with Al to form an oxide, which becomes a stress concentration source and reduces the fatigue strength of the component. Therefore, the upper limit of the O content is preferably 0.008% or less, more preferably 0.006% or less.

  On the other hand, when Ti is added to improve machinability, it is useful to form an oxide of Ti. From the viewpoint of obtaining the effect, the lower limit of the O content is preferably 0.0003% or more, more preferably 0.0004% or more.

P: 0.04% or less P is an element that can be mixed as an inevitable impurity in the steelmaking process. Since P reduces the toughness of steel, its content is preferably 0.04% or less, more preferably 0.03% or less, and even more preferably 0.02% or less.

N: 0.003 to 0.025%
N combines with Al to form a nitride, and when this nitride precipitates finely, it suppresses crystal grain growth during hot forging and contributes to strength improvement. In order to obtain such an effect, the lower limit of the N content is preferably 0.003% or more. More preferably, it is 0.004% or more.

  However, even if N is added in a large amount, the effect is saturated. On the contrary, coarse carbonitride becomes a nucleus and it is easy to generate ferrite, and age hardening characteristics are lowered and strength is lowered. Therefore, the upper limit of the N content is preferably 0.025% or less. More preferably, it is 0.020% or less.

  Here, the bainite steel needs to satisfy the following relational expression. [X] represents mass% of the element X.

13 [C] +8 [Si] +10 [Mn] +3 [Cu] +3 [Ni] +22 [Mo] +11 [V] ≦ 30
The above relational expression is closely related to hot forgeability. That is, the bainitic steel is preferably 900 to 1300 ° C. (A1 transformation point or higher), more preferably from the viewpoint of sufficiently reducing deformation resistance while suppressing decarburization and efficiently processing into a desired part shape. Is preferably hot forged in a temperature range of 950 to 1200 ° C. If a large amount of Mo or V is added to obtain age-hardening characteristics, the hot deformation resistance during hot forging increases and the processing efficiency is lowered. Therefore, the device for obtaining the age hardening amount while reducing these elements is important. There is also a method for improving softening resistance by adding Si so as not to reduce the hardness of the matrix structure when an aging treatment at 500 to 700 ° C. is performed. However, although Si is not as much as Mo, hot deformation resistance is increased. Moreover, in this bainite steel, it is desirable that the structure after hot forging is a bainite single phase (details will be described later in Examples). For that purpose, C, Mn, Cu, Ni, Cr, Mo, V may be added, but these elements also increase the hot deformation resistance. Considering all of these, it is necessary to satisfy the above relational expression. As a result, the hot deformation resistance of the present bainite steel can be lower than 140 MPa, which is the hot deformation resistance of a general hot forged non-heat treated steel, and the significance of maintaining the working efficiency is significant. is there.

5 [C] + [Si] +2 [Mn] +3 [Cr] +2 [Mo] +4 [V] ≦ 7.3
The above relational expression is closely related to the hardness after hot forging (before aging treatment). That is, the hardness of the steel having the above-described chemical composition after heating, hot forging, and air cooling greatly affects the machinability in the subsequent machining process. In general, it is said that the hardness that can be machined is about 300 HV. As a result of intensive studies, the present inventors have satisfied the above relational expression, thereby reducing the hardness after hot forging (before aging treatment). It has been found that it can be made 300 HV or less.

2.4 ≦ 0.3 [C] +1.1 [Mn] +0.2 [Cu] +0.2 [Ni] +1.2 [Cr] +1.1 [Mo] +0.2 [V] ≦ 3.1
The above relational expression is closely related to obtaining a stable bainite structure. That is, the hot forged part has a complicated shape, and the cooling rate varies depending on the part. Therefore, it is desirable to obtain a bainite structure in a wide cooling range. By satisfying the above relational expression, it becomes possible to stably obtain a bainite structure. The bainite steel preferably has a bainite single phase (details will be described later in Examples).

2.5 ≦ [C] + [Si] +4 [Mo] +9 [V], [C] ≧ [Mo] / 16 + [V] / 3
The above relational expression is closely related to age hardening characteristics. That is, in order to increase the fatigue strength, it is necessary to increase the age hardening amount. Therefore, this bainite steel needs to satisfy the above relational expression. In order to obtain sufficient age-hardening properties, the temperature is preferably 500 to 700 ° C, more preferably 575 to 675 ° C, preferably 0.5 to 10 hours, more preferably 1 to 5 hours. Aging treatment is desirable. Most preferably, an aging treatment is performed at 625 ° C. for 1 to 4 hours from the viewpoint of efficiently obtaining the effect.

  Specifically, the bainite steel described above can be suitably used for applications such as hot forged parts, particularly automobile parts such as connecting rods, knuckle arms, and common rails.

Hereinafter, the present invention will be described more specifically with reference to examples.
50 kg of steel ingots of various chemical components shown in Tables 1 and 2 were melted in a vacuum induction furnace and hot forged into φ70 mm steel bars. This was further forged by 1 heat at 1100 ° C. or more to φ10, 20, 50, 60 mm, and air-cooled at an appropriate interval to make the cooling rate constant, and used as a test material.

(Hot forgeability)
A specimen of φ10 mm is machined, a specimen of φ8 mm × 12 mm is taken, and heated at a temperature of 1100 ° C. using a hot working reproduction test device (for example, “Processing For Master” manufactured by Fuji Electric Koki). The inter-deformation resistance was measured. The case where the hot deformation resistance was 140 MPa or less was considered to be excellent in hot forgeability. The reason why the “hot deformation resistance is 140 MPa or less” is that the capacity range of a general hot forging machine.

(Machinability)
Hardness specimens and machinability specimens were sampled using a specimen having a diameter of 20 mm, and the test was performed. The hardness was measured at a half of the radius of the test piece. Moreover, drilling workability was evaluated using a carbide tool. Specifically, the cutting speed is 200 m / min, the feed is 0.1 mm / rev, the hole depth is 60 mm, and the machining time until the side flank average tool wear width reaches 0.2 mm is measured. The tool life ratio when the processing time of steel was 100 was defined as the machinability index, and an object having a value less than 100 was judged to be inferior in machinability. Those having 300 HV or less have a machinability index exceeding 100 and are excellent in machinability. However, even if the hardness after forging is less than 300 HV, if S is low, MnS contributing to free machinability is reduced and machinability is inferior.

(Bainite single phase)
The metal structure of the entire axial cross section of the specimens with φ10 mm, 20 mm, 50 mm, and 60 mm was observed. “O” when the area ratio of the bainite structure is 90% or more, “F” when the area ratio of the bainite structure and the ferrite structure (the area ratio of the ferrite structure is 10% or more), and the bainite structure and the martensite structure (M is a case where the area ratio of martensite structure is 10% or more). Then, when both the φ50 mm round bar and the φ10 mm round bar were “◯”, it was determined that the bainite was made into a single phase.

  A specimen having a diameter of 20 mm was subjected to aging treatment at a temperature of 625 ° C. ± 10 ° C. for 2 hours. Thereafter, a hardness test piece of φ20 mm × 10 mm plate was collected, and the Vickers hardness after aging treatment was measured. The hardness was measured at a half of the radius of the test piece. When it was 300 HV or more, it was judged that the hardness was sufficiently increased by the aging treatment.

(Tensile strength, 0.2% yield strength, yield strength ratio)
A JIS Z 2201 No. 4 tensile test piece was collected from the specimen after the aging treatment, and the tensile strength, 0.2% proof stress, and proof stress ratio (0.2% proof stress / tensile strength) were measured using the specimen. When the tensile strength was 950 MPa or more and the proof stress ratio was 0.80 or more, it was judged that the material had high proof stress.

(Fatigue strength, durability ratio)
A JIS Z 2274 No. 1 rotating bending fatigue test specimen was collected from the specimen after the above aging treatment, and the rotating bending fatigue strength and durability ratio (fatigue strength / tensile strength) were measured using this specimen. When the fatigue strength was 510 MPa or more and the durability ratio was 0.50 or more, it was judged to have high durability.

  Tables 1 and 2 show chemical components of steel materials according to Examples and Comparative Examples, and Tables 3 and 4 show test results.

  When the results in Tables 3 and 4 are evaluated relative to each other, the following can be understood.

  In Comparative Example 1, the C content is below the amount specified in the present application. Therefore, Mo and V carbides cannot be sufficiently precipitated by the aging treatment, and sufficient hardness cannot be obtained, so that the strength cannot be increased.

  In Comparative Example 2, the C content exceeds the amount specified in the present application. Therefore, the hot forgeability is inferior and the productivity is poor. Also, the hardness after hot forging (the hardness of the material before cutting) is relatively high.

  In Comparative Example 3, the Si content exceeds the amount specified in the present application. Therefore, the hot forgeability is inferior and the productivity is poor.

  In Comparative Example 4, the Mn content is below the amount specified in the present application. Therefore, a large amount of other alloy elements are added to achieve a single phase of bainite. As a result, hot forgeability and machinability are impaired. This is thought to be due to the following reasons. That is, by adding all of Mn, Cr, Mo, C, and V, bainite can be easily formed, but the hardness is increased and hot forgeability and machinability are deteriorated. However, when an equivalent amount of other elements required for the bainite single phase is added, the hardness does not increase equally. Since Cr, Mo, C, and V have a greater adverse effect on hot forgeability and machinability by increasing the hardness than the contribution to the bainite single phase, it is considered that the above results were obtained. It is done.

  In Comparative Example 5, the Mn content exceeds the amount specified in the present application. Therefore, a martensite structure is likely to appear, the hardness after hot forging becomes high, and the machinability is inferior. Moreover, it is inferior to hot forgeability.

  In Comparative Example 6, the S content is below the amount specified in the present application. Therefore, the amount of sulfide produced is insufficient and the machinability is poor.

  In Comparative Example 7, the S content exceeds the amount specified in the present application. For this reason, the inclusions become coarse, which becomes the starting point of fatigue fracture and is inferior in fatigue strength.

  In Comparative Example 8, the Cu and Ni contents exceed the amounts specified in the present application. Therefore, the hot forgeability is inferior and the productivity is poor. It also leads to increased costs.

  In Comparative Example 9, the Cr content exceeds the amount specified in the present application. Therefore, the hardness after hot forging becomes high and the machinability is inferior. Moreover, it is inferior to hot forgeability, and its productivity is also poor.

  In Comparative Example 10, the Mo content is below the amount specified in the present application. For this reason, the increase in hardness due to age hardening becomes insufficient, and high strength cannot be achieved.

  In Comparative Example 11, the Mo content exceeds the amount specified in the present application. Therefore, the hot forgeability is inferior and the productivity is poor. It also leads to increased costs.

  In Comparative Example 12, the V content is below the amount specified in the present application. For this reason, the increase in hardness due to age hardening becomes insufficient, and high strength cannot be achieved.

  In Comparative Example 13, the V content exceeds the amount specified in the present application. Therefore, the hot forgeability is inferior and the productivity is poor. Also, the hardness after hot forging is relatively high.

  In Comparative Example 14, the value of Formula 1 exceeds 30. Therefore, the hot forgeability is inferior and the productivity is poor.

  In Comparative Example 15, the value of Expression 2 exceeds 7.3. Therefore, the hardness after hot forging becomes high and the machinability is inferior.

  In Comparative Example 16, the value of Equation 3 is less than 2.4. Therefore, it is difficult to achieve a bainite single phase, and it is difficult to increase the strength.

  In Comparative Example 17, the value of Expression 4 (1) is less than 2.5. Therefore, it is difficult to increase the strength by age hardening.

  In Comparative Example 18, the value of Formula 4 (2) exceeds the C content. Therefore, Mo and V carbides cannot be sufficiently precipitated by the aging treatment, and sufficient hardness cannot be obtained, so that the strength cannot be increased.

  In Comparative Example 19, the Ti content exceeds the amount specified in the present application. For this reason, coarse inclusions are generated, which becomes a stress concentration source and causes a decrease in fatigue strength.

  In Comparative Example 20, the Ca content exceeds the amount specified in the present application. For this reason, coarse inclusions are generated, which becomes a stress concentration source and causes a decrease in fatigue strength.

  On the other hand, Examples 1-21 which satisfy the conditions prescribed in the present application are excellent in hot forgeability and machinability after hot forging, and can achieve high strength by age hardening after machining. I understand that it is possible. Therefore, it can be suitably used as a material for automobile parts, machine structural parts and the like that are required to be miniaturized.

  The bainite steel according to the present invention has been described above. However, the present invention is not limited to the above-described embodiments and examples, and various modifications can be made without departing from the scope of the present invention.

Claims (3)

% By mass
C: 0.14-0.35%,
Si: 0.05-0.70%
Mn: 1.10 to 2.30%,
S: 0.003 to 0.120%,
Cu: 0.0 5 ~0.40%,
Ni: 0.01-0.40%,
Cr: 0.01 to 0.50%,
Mo: 0.01 to 0.30%, and
V: 0.05 to 0.45%, the balance is made of Fe and inevitable impurities,
13 [C] +8 [Si] +10 [Mn] +3 [Cu] +3 [Ni] +22 [Mo] +11 [V] ≦ 30,
5 [C] + [Si] +2 [Mn] +3 [Cr] +2 [Mo] +4 [V] ≦ 7.3,
2.4 ≦ 0.3 [C] +1.1 [Mn] +0.2 [Cu] +0.2 [Ni] +1.2 [Cr] +1.1 [Mo] +0.2 [V] ≦ 3.1 ,
2.5 ≦ [C] + [Si] +4 [Mo] +9 [V], [C] ≧ [Mo] / 16 + [V] / 3
Bainitic steel characterized by satisfying % By mass
Ti: 0.001 to 0.100%, and
The bainite steel according to claim 1, comprising one or more selected from Ca: 0.0003 to 0.0100%.   The bainite steel according to claim 1 or 2, wherein the Ni content is 0.05 mass% or more.