JP5974623B2 - Age-hardening bainite non-tempered steel - Google Patents

Description

  The present invention relates to an age-hardened bainite non-heat treated steel having a bainite structure after hot working and precipitation hardening by the subsequent age hardening treatment, and more specifically, an aging having higher toughness than conventional ones. The present invention relates to hardened bainite non-heat treated steel.

  Conventionally, tempered steel that has been used after being tempered (tempered) after hot working such as hot forging has been used for automotive parts and machine structural parts that require strength and toughness. It was.

  However, although tempered steel is excellent in strength and toughness, heat treatment strain due to martensitic transformation is high in addition to the problem of high heat treatment costs for quenching and tempering treatment (tempering treatment) after hot working when manufacturing parts. Large, the amount of machining for shape correction and dimension correction after heat treatment increases, resulting in poor yield, and the machinability (workability) is poor because the processing is performed in a hard martensite state. There is a problem that the time required for manufacturing the parts is long and the cost is high.

  Non-tempered steel that develops the required hardness as it is hot-worked (specifically as it is cooled by air cooling afterwards) and that can achieve the desired strength even if the quenching and tempering treatment after hot-working is omitted. As a substitute for tempered steel, it can be widely applied to machine structural parts, etc., as a solution to cost reduction.

As such non-tempered steel, there is ferritic / pearlite type non-tempered steel with a small amount of V added to medium carbon steel, but in ferritic / pearlite non-tempered steel, It is necessary to increase the area ratio of pearlite until it becomes almost pearlite single phase.
However, in this case, since the steel structure is a pearlite-based structure that is brittle compared to ferrite, the toughness is significantly reduced. Therefore, it is difficult to increase the strength beyond a certain level while ensuring toughness.

Non-tempered steel includes bainite non-tempered steel that exhibits a bainite structure as it is hot-worked. This has better toughness than ferritic and pearlite non-tempered steel, but has a problem of low yield strength. There is.
Further, if the hardness is simply increased in order to improve the proof stress, the machinability is deteriorated, and the load at the time of cutting is increased to deteriorate the workability.
As one solution, age hardened bainite non-tempered steel has been studied.

Age-hardened bainite non-tempered steel uses bainite as a hot-worked structure and then increases the hardness by age-hardening treatment. Machining can be performed in a soft state after inter-working, and the hardness can be increased to the required hardness by subsequent age hardening treatment.
For example, Patent Document 1 and Patent Document 2 listed below disclose this type of age-hardened bainite non-tempered steel.

However, the conventional age-hardened bainite non-tempered steel has better toughness than the ferrite-pearlite non-tempered steel, but still has insufficient toughness compared to the conventional tempered steel.
However, in the conventional age-hardened bainite non-tempered steel, the main focus of research is mainly directed to increasing the hardness and strength, and no steel with sufficiently enhanced toughness has yet been provided.
For example, even those described in Patent Document 1 and Patent Document 2 are intended to increase strength and contain a large amount of C in order to produce a large amount of precipitates during age hardening. Moreover, since it is not aimed at improving toughness in particular, the toughness value in Examples is not shown.
Under such circumstances, the age hardening type bainite non-tempered steel currently provided or proposed has been difficult to apply to parts that particularly require impact resistance.

  As another prior art to the present invention, the following Patent Document 3 discloses an invention relating to a “high toughness high tensile non-heat treated thick steel plate”, in which a bainite non-heat treated steel having a bainite structure fraction of 90% or more. However, this is not an age-hardening type and has a small amount of V as an alloy component, and further does not have the characteristic configuration of the present invention described later for controlling the formation and form of cementite. This is different from the present invention.

  As another prior art, the following Patent Document 4 discloses an invention relating to “a steel material for non-tempered reinforcing steel and a method for producing the same”, in which a bainite non-tempered steel having a metal structure substantially bainite is disclosed. However, this is also different from the present invention in that it is not age-hardened and does not have the characteristic configuration of the present invention for controlling the formation and form of cementite.

  As another prior art, the following Patent Document 5 discloses an invention relating to “precipitation hardening type nitrided steel”, in which age hardening type bainite non-tempered steel by nitriding is disclosed. Is different from the present invention in that it is not added and does not have the characteristic configuration of the present invention to be described later for controlling the formation and form of cementite.

  Furthermore, as another prior art, the following Patent Document 6 discloses an invention about “high strength-high toughness bainite-type non-heat treated steel and its production method”, but only chemical components are included as examples. Although what is shown is disclosed, the characteristics as a result of such chemical components are not disclosed, and there is no addition of Cu or Ni in the examples. This is different from the present invention.

JP 2011-236451 A JP 2006-37177 A JP-A-2005-350691 JP 2006-137990 A JP-A-4-154936 Japanese Patent Laid-Open No. 9-170047

  The present invention has been made for the purpose of providing an age-hardened bainite non-tempered steel having excellent toughness as compared with the background as described above.

Thus, the content of claim 1 is C: 0.06 to 0.18%, Si: 0.01 to 0.60%, Mn: 0.10 to 3.00%, S: 0.001 to 0.030%, Cu: 0.001 to 0.40%, Ni: 0.001 to 0.40 %, Cr: 0.10 to 2.00%, Mo: 0.15 to 1.00%, V: 0.20 to 0.40 %, balance Fe and inevitable impurities, and the value of the following formula (1) is 1.4 or less, formula (2 value of) 16.5 or less, the value of the expression (3) have a composition satisfying the 20.0 to 35.0, the area ratio of bainite structure and hardness at 85% or more is equal to or less than 36HRC.
15 × [C] − [Mo] −2 × [V] ・ ・ Formula (1)
4 x [C] + [Si] + 4 x [Mn] + [Cu] + [Ni] + 5 x [Cr] + 4 x [Mo] + 5 x [V] · Equation (2)
3 x [C] + 10 x [Mn] + 2 x [Cu] + 2 x [Ni] + 12 x [Cr] + 9 x [Mo] + 2 x [V]-Formula (3)
(However, the element symbols in the formulas (1) to (3) represent the mass% of the corresponding elements)

Of those claims 2, Oite to claim 1, 100 [mu] m and a square region an observation area to one side, the short dimension 5μm or more large size of cementite in the observation region 10 field of view than 5 in the average In addition, the ratio of the total cementite regardless of size is an average area ratio of 10% or less.

  In the present invention as described above, the cementite in the structure is greatly involved in the toughness of the age-hardened bainite non-tempered steel. Specifically, when the amount of cementite generated increases, A large amount of large cementite is generated, and there are many large cementites in the form of long and long aspect ratios in terms of shape, and cracks are likely to occur starting from these cementites, especially elongated shapes. The structure is divided by the cementite and cracks occur due to stress concentration at the end of the cementite, and the generated crack easily propagates along the elongated cementite, which greatly deteriorates toughness. It was made based on knowledge.

  Based on this finding, the present invention suppresses the formation of cementite by reducing the content of C, which is an important component for increasing the hardness by forming carbide during the age hardening treatment, and The decrease in hardness due to the reduction in quantity is compensated by other alloy elements.

  Further, in the present invention, not only the C content in the steel is simply reduced, but also the steel content is appropriately balanced by the relationship between the C content and the contents of Mo and V which are carbide forming elements. The amount of C that forms cementite by combining with Fe in the C contained in the steel is reduced, and the content of elements that control the hardness after hot working is controlled appropriately, and the bainite single phase The essential point is that the content of the bainite-forming element necessary for crystallization is appropriately controlled to enhance the properties of the steel.

  According to the present invention, the problem of insufficient toughness, which has been a problem in the past, can be solved, and an age-hardened bainite non-tempered steel excellent in toughness can be obtained.

In the age-hardened bainite non-tempered steel of the present invention, the structure before the age-hardening treatment is substantially a bainite single-phase structure. Specifically, the area ratio of the bainite structure is 85% or more . More preferably, it is 90% or more.

  When a ferrite structure is mixed in the structure before the age hardening treatment, not only the age hardening characteristics are deteriorated, but also the yield strength ratio and the durability ratio are lowered, and there is a concern that the fatigue strength is lowered. Therefore, it is desirable that the structure before age hardening is a bainite single phase structure.

Additionally the hardness of the pre-age hardening treatment Ru der below 36HRC.
When the hardness before the age hardening treatment is 36 HRC or less, the workability such as machinability at the time of working prior to the age hardening treatment is improved.
The hardness before age hardening is more preferably 33 HRC or less.
On the other hand, the hardness before age hardening is desirably 25 HRC or more, and more desirably 27 HRC or more.
If the hardness before the age hardening treatment is low, the machinability such as machinability is improved, but even if the hardness is increased by the age hardening treatment (precipitation hardening), the hardness can be sufficiently increased. It becomes difficult.

Age hardening type bainite non-thermal refined steel of the present invention, the aging treatment at a temperature of 500 to 700 ° C., have to desirable to keep as the hardness is higher than 4HRC than the hardness of the pre-aging treatment. More preferably, it should be higher than 5 HRC.

  The difference between the hardness before age-hardening treatment and the hardness after age-hardening treatment ensures a good machinability before age-hardening treatment, while the required hardness is obtained by subsequent age-hardening treatment. It becomes easy to be done.

  In the present invention, the size and amount of cementite in the steel structure are set to the following sizes and amounts. Specifically, a square region having one side of 100 μm is set as an observation region, and a short dimension in a 10 field of observation region. It is preferable that the average size of cementite having a large size of 5 μm or more is 5 or less, and the ratio of the total cementite regardless of size is 10% or less on the average.

  By controlling the size and amount of cementite in this way, it was confirmed that the toughness of the age-hardened bainite non-tempered steel can be effectively enhanced as will be clarified later.

The age-hardened bainite non-tempered steel of the present invention can be produced, for example, as follows.
That is, it can be manufactured by hot cooling such as rolling, forging, etc. or after solution heat treatment at a temperature between 800 ° C. and 300 ° C. at an average cooling rate of 0.05 to 10 ° C./second, usually by air cooling. it can.

  Then, if necessary, it is subjected to processing such as cutting and plastic working, and then subjected to aging treatment at a temperature of 500 to 700 ° C. for 1.5 to 4 hours, thereby achieving an object having excellent toughness. Hard parts can be obtained.

Next, the reasons for limiting each chemical component in the present invention will be described in detail below.
C: 0.06-0.18%
C is an element necessary for ensuring the strength, and Mo and V carbides are precipitated by age hardening to increase the hardness of the steel. For its function, 0.06% or more is necessary, and if it is less than 0.06%, the required hardness and strength cannot be secured.
On the other hand, when the content exceeds 0.18%, the amount of cementite increases, and at the same time, large cementite with a large aspect ratio and elongated shape increases, and these become crack initiation points and propagation paths, which deteriorates fatigue strength characteristics. Therefore, the C content is 0.18% or less. A desirable content range of C is 0.08 to less than 0.14%.

Si: 0.01-0.60%
Si is added as a deoxidizer during steel melting. It also works to increase the fatigue strength of steel. It is necessary to make it contain 0.01% or more for the function.
On the other hand, when it is contained in a large amount exceeding 0.60%, hot workability such as hot forging is impaired and productivity is lowered. In addition, the upper limit is set to 0.60% because the hardness of the material during processing such as cutting after hot processing becomes excessive and deteriorates workability. A desirable range is in the range of 0.05 to 0.55%.

Mn: 0.10 to 3.00%
Mn is an element that plays an important role in the present invention, and is an indispensable element for making the structure after hot working a bainite structure. Further, Mn is an essential element for forming Mn-based sulfides that contribute to improvement of machinability. Due to its function, the present invention contains 0.10% or more.
On the other hand, if it is contained in a large amount exceeding 3.00%, a martensite structure is likely to appear, the hardness after hot working is increased, and not only the machinability is deteriorated but also hot workability is deteriorated. Is set to 3.00%. Preferably, it is within the range of 0.50 to 2.20%.

S: 0.001 to 0.030%
S is an essential element of Mn sulfide that contributes to improvement of machinability together with Mn. If the content is less than 0.001%, the amount of sulfide produced is insufficient and the machinability becomes insufficient. Therefore, in the present invention, 0.001% or more is contained.
On the other hand, if over 0.030% is contained, the toughness and ductility of the steel are impaired, and the inclusions become the starting point of fatigue fracture, and the fatigue strength characteristics are deteriorated, so the upper limit is made 0.030%. A desirable range is in the range of 0.015 to 0.030%.

Cu: 0.001 to 0.40%
Ni: 0.001 to 0.40%
Cu and Ni are elements that move the ferrite-pearlite transformation start curve to the long time side and make it relatively easy to generate bainite, and contribute to improving the hardness of the bainite substrate by solid solution strengthening. In the present invention, 0.001% or more of Cu and Ni are contained for the function.
However, if it is added excessively over 0.40%, it becomes too hard, so the content is made 0.40% or less.
A desirable range is in the range of 0.10 to 0.30%.

Cr: 0.10 to 2.00%
Cr is an indispensable element for generating a bainite structure, and is contained in an amount of 0.10% or more in order to stably generate a bainite structure.
On the other hand, if it is contained in a large amount exceeding 2.00%, the hardness after hot working increases, leading to a decrease in machinability, and the hot workability is also impaired, so the upper limit is made 2.00%.
A desirable range is in the range of 0.20 to 1.80%.

Mo: 0.15 ~1.00%
Mo is an element that plays an important role in the present invention, and increases the hardness of steel by age hardening. Mo is also an indispensable element for forming a bainite structure. Furthermore, Mo is an important element in increasing the strength because Mo carbonitride is precipitated by age hardening to improve the yield ratio and increase the durability ratio. For these functions, the present invention contains 0.15 % or more.
On the other hand, if the content exceeds 1.00%, the hardness after hot working is increased, the workability of the steel is impaired, and the cost is increased, so the upper limit is made 1.00%.
A desirable range is in the range of 0.15 to 0.30%.

V: 0.20 to 0.40 %
V is an element that plays an important role in the present invention, and increases hardness by age hardening.
V is an indispensable element for forming a bainite structure together with Mo. Further, V is important in increasing strength in order to improve the yield strength ratio and increase the durability ratio when carbonitride is precipitated by age hardening. Element. Due to its function, the present invention contains 0.20% or more.
On the other hand, the inclusion in the over-over - increases the hardness after hot working, not only to reduce the processability of the machinability and the like, also impair hot workability such as hot forging, and increase in cost Therefore, the upper limit is set to 0.40 %. A desirable range is 0.20 to 0.35%, and more desirably within a range of 0.20 to 0.30%.

In the present invention, the following components such as P, N, and Al may be included within a range that does not affect the effect (characteristic) of the alloy.
P: An element that can be mixed as an inevitable impurity in the steelmaking process, but P lowers the toughness of the steel, so its content is preferably 0.04% by mass or less.

  N: Combines with Al to form nitrides, and the nitrides are finely precipitated to suppress crystal grain growth during hot forging and contribute to strength improvement. In order to acquire such an effect, it is necessary to contain 0.005% or more. Even if it is contained in a large amount, the effect is saturated. On the contrary, coarse carbonitrides become nuclei, and it is easy to generate ferrite, which deteriorates the age hardening characteristics and causes a decrease in strength. Furthermore, since blowholes are generated during casting and the soundness of the steel ingot is impaired, the content is preferably 0.025% or less.

  Al: contained as a deoxidizer during melting, but in that case, 0.001% or more is required. If it is contained in a large amount, coarse oxides and carbonitrides are produced, and this serves as a starting point to reduce the fatigue strength. Therefore, the content is preferably 0.10% or less.

Value of Formula (1): ≤1.4 (Formula (1) .. 15 x [C]-[Mo]-2 x [V])
Formula (1) is an index representing the amount of cementite produced. The larger the value of formula (1), the easier it is to produce cementite, and the smaller the value, the more the cementite production is suppressed. Specifically, if the amount of Mo and V increases, they combine with C to form carbides, and combine with Fe to reduce the amount of C that forms cementite.
Therefore, in order to reduce the production of cementite, it is necessary to reduce the amount of C and to reduce the value of the formula (1), and by reducing the production of cementite, the coarse and aspect ratio can be reduced. Generation of large and long large cementite can be suppressed, and it is possible to effectively suppress a decrease in toughness that the cementite serves as a crack starting point and propagation path.

In particular, the amount and size of cementite produced in the steel can be regulated as follows by setting the value of the formula (1) to 1.4 or less after making C low as described above.
That is, the observation area is a square area having one side of 100 μm, the average size of cementite in the short direction direction of 5 μm or more in the 10 viewing fields is 5 or less, and the total cementite regardless of the size. It is easy to regulate the ratio so that the average is 10% or less in terms of area ratio.

Value of Formula (2): ≦ 16.5 (Formula (2) .. 4 × [C] + [Si] + 4 × [Mn] + [Cu] + [Ni] + 5 × [Cr] + 4 × [Mo] + 5 × [V])
The value of this formula (2) is an index that represents the hardness after hot working such as hot forging (hereinafter simply referred to as hot forging), and more specifically, the hardness before age hardening treatment. The larger the value is, the harder the hardness after hot forging is, and the smaller the value is, the lower the hardness is after hot forging.

In the present invention, by setting the value of the formula (2) to 16.5 or less, the target post-hot forging hardness can be 36 HRC or less. Thereby, workability such as machinability prior to age hardening can be enhanced.
In addition, the lower the hardness before age hardening treatment, the better the workability, but on the other hand, when the age hardening treatment is performed to increase the hardness, the hardness should be sufficiently hardened. Becomes difficult. Therefore, in this sense, it is desirable that the hardness after hot forging before age hardening is higher than a certain level. Specifically, it is desirable that the hardness is 25 HRC or more, preferably 27 HRC or more. In order to achieve this, it is desirable to set the value of equation (2) to 11 or more.

Value of Formula (3): 20.0 to 35.0 (Formula (3) ... 3 x [C] + 10 x [Mn] + 2 x [Cu] + 2 x [Ni] + 12 x [Cr] + 9 x [Mo] + 2 x [ V])
Formula (3) is an index for stably forming bainite. In the present invention, the steel structure before age hardening treatment is substantially made into a bainite single-phase structure. In order to make the rate 85% or more, it is necessary to set the value of the formula (3) within the range of 20.0 to 35.0.

  For example, when 15% or more of the ferrite structure is mixed in the steel structure, not only the age hardening characteristic is lowered, but also the yield strength ratio and the durability ratio are lowered, and there is a concern that this leads to a decrease in fatigue strength. Therefore, it is desirable that the structure after hot forging is a bainite single phase.

Formula (3) for the bainite single phase has the following meaning.
In the present invention, Mo and V carbides are precipitated by age hardening treatment, and the steel is hardened and strengthened by precipitation strengthening. In this way, carbides such as Mo and V are secondarily precipitated. The technique itself is a technique that is generally used in quenching and tempering. However, when the material state is a martensite structure as in the quenching and tempering treatment, it is impossible to obtain a hardness higher than the quenching hardness by precipitation of carbides.

  However, when the steel structure is a bainite structure according to the formula (3), the hardness can be lowered under the material state before the age hardening treatment, and at the subsequent temperature of about 625 ° C. By the age-hardening treatment by heating and heating, the hardness of the steel can be made harder than the hardness in the initial bainite structure state before the secondary precipitation of carbides, thereby providing an excellent balance between strength and machinability. It becomes possible to provide the material.

It is the figure which compared and showed the scanning electron microscope (SEM) photograph of the invention steel 6 and the comparative steel 12 in an Example.

150 kg of steel having the chemical composition shown in Table 1 was melted in a vacuum induction melting furnace and forged into a round bar having a diameter of 45 mm at 1250 ° C. Thereafter, the φ45 mm round bar was forged into a φ30 mm round bar under the conditions of 1250 ° C. heating and 1100 ° C. forging, and then air-cooled to room temperature.
Thereafter, an age hardening treatment was performed at 625 ° C. for 2 hours, and subjected to a tensile test, a Charpy impact test, a hardness test, and a microstructure observation.
In addition, drill tests and hardness tests were performed even in a state in which air-cooled after forging and without age hardening.
Here, a tensile test, a Charpy impact test, a hardness test, and a microstructure observation were performed as follows.

<Tensile test>
For the tensile test, a JIS Z 2201 No. 14A test piece was prepared, and the tensile rate was 1 mm / sec, and a 0.2% yield strength ratio (0.2% yield strength / tensile strength) was obtained. The evaluation is shown in Table 2 with a target value of 0.80 or more as ◯ and a lower value as ×. Table 2 shows the numerical values of the proof stress ratios together with the evaluations of ◯ and ×.

<Charpy impact test>
In the Charpy impact test, a JIS Z 2202 No. 2 mmU notch test piece was prepared, the test was performed at room temperature, and the impact value was measured. The evaluation was made with ◯ when the impact value satisfied the target value of 20 J / cm ² , and x when the impact value did not satisfy the target value.
In Table 2, the actual impact measurement values are also shown in parentheses together with the evaluations of ○ and ×.

<Hardness test>
The hardness test was carried out in accordance with JIS Z 2245, using a Rockwell hardness tester with a 150 kgf diamond conical indenter.
The hardness was measured at a half of the radius of the test piece.

<Microstructure observation>
About the microstructure observation, after nitrite corrosion, it observed with the optical microscope (400-times multiplication factor), and measured the bainite rate. The amount (ratio) of cementite produced and the number of coarse cementite produced were observed and measured with a scanning electron microscope (SEM) (5000 times magnification).
Here, regarding cementite, a square region having one side of 100 μm is used as an observation region, and the average number of large-sized cementites having a short-side dimension of 5 μm or more in the observation region 10 field of view and the ratio of the total cementite regardless of size. (Area ratio) was measured.
As for the bainite ratio, a case where the area ratio of the bainite structure was 85% or more was evaluated as ◯, and a case where the bainite structure and the ferrite structure were mixed (area ratio of the ferrite structure was 15% or more) was × F. And a martensitic structure (a martensite structure area ratio of 15% or more) was evaluated as xM.
In Table 2, together with these evaluations of ○ and ×, the area ratio of bainite actually measured in parentheses is also shown.

<Drill test>
The drill test was performed using a straight shank with a diameter of 5 mm and a drill with a drill material of JIS SKH51, with a feed of 0.10 mm / rev and no lubricating oil, and evaluated at a speed that reached a machining distance of 5000 mm. .
When satisfying the target machining speed of 20 m / min, the results are shown in Table 2. Table 2 also shows specific numerical values (m / min) of the processing speed in parentheses together with the evaluations of ○ and ×.

  In the results of Table 1, the comparative steel 12 has a C amount larger than the upper limit value of the present invention, and the value of the formula (1) is higher than the upper limit value of the present invention. As a result, the lateral dimension is 5 μm or more. The number of cementite with a large size is large, the area ratio of cementite is high, and the impact value (absorbed energy) is lower than the target value.

The comparative steel 13 has a large amount of V exceeding the upper limit of the present invention, and the value of the formula (2), which is an index of hardness after forging, is higher than the upper limit of the present invention. Further, the value of the formula (3) which is an index of bainite single phase formation is higher than the upper limit value of the present invention.
As a result, the steel structure is a mixed structure with martensite and the machinability is poor. The impact value is also low.

  The comparative steel 14 has a large amount of Mo exceeding the upper limit of the present invention, and the values of the formula (2) and the formula (3) are higher than the upper limit of the present invention. It has a mixed structure with the site, has poor machinability and low impact value.

In the comparative steel 15, the value of the formula (3), which is an index of bainite single phase formation, is lower than the lower limit of the present invention, and the steel structure is a ferrite mixed structure. As a result, the degree of increase in hardness due to age hardening is low, and sufficient hardness is not obtained by age hardening. As a result, the fatigue strength is insufficient.
On the other hand, the invention steels 1 to 11 satisfying the conditions of the present invention have good characteristics.

FIG. 1 shows a comparison of scanning electron microscope (SEM) photographs (5000 magnifications) of Invention Steel 6 and Comparative Steel 12. In the figure, (A) shows a case where forging is performed under heating conditions of 1100 ° C., and (B) shows a case where forging is performed under heating conditions of 1300 ° C.
In the photograph, the whitened portion is formed cementite. As is clear from these comparisons, in Invention Steel 6, the formation of cementite is effectively suppressed, and the size of the cementite existing there is also round. small.

  On the other hand, in the comparative steel 12, a large amount of cementite is generated, and a large amount of large cementite having a large size and an elongated shape is generated there. In steels having such a structure, cementite becomes the starting point of cracks, and the cracks that are generated propagate along the cementite and cause a significant loss of toughness.

  Although the embodiments of the present invention have been described in detail above, these are merely examples, and the present invention can be implemented in variously modified forms without departing from the spirit of the present invention.

Claims (2)

By mass% C: 0.06-0.18%
Si: 0.01-0.60%
Mn: 0.10 to 3.00%
S: 0.001 to 0.030%
Cu: 0.001 to 0.40%
Ni: 0.001 to 0.40%
Cr: 0.10 to 2.00%
Mo: 0.15 ~1.00%
V: 0.20 to 0.40 %
And a balance of Fe and unavoidable impurities, and the value of the following formula (1) is 1.4 or less, the value of the expression (2) is 16.5 or less, have a composition the value of the expression (3) satisfies 20.0 to 35.0, bainite An age-hardened bainite non- tempered steel characterized by having an area ratio of 85% or more and a hardness of 36 HRC or less .
15 × [C] − [Mo] −2 × [V] ・ ・ Formula (1)
4 x [C] + [Si] + 4 x [Mn] + [Cu] + [Ni] + 5 x [Cr] + 4 x [Mo] + 5 x [V] · Equation (2)
3 x [C] + 10 x [Mn] + 2 x [Cu] + 2 x [Ni] + 12 x [Cr] + 9 x [Mo] + 2 x [V]-Formula (3)
(However, the element symbols in the formulas (1) to (3) represent the mass% of the corresponding elements) Oite to claim 1, 100 [mu] m and a square region an observation area to one side, the short dimension 5μm or more large size of cementite in the observation region 10 field is not more than 5 in the average, and regardless of the size An age-hardened bainite non-tempered steel characterized in that the ratio of total cementite is 10% or less on average in terms of area ratio.