JPH0621319B2 - Non-heat treated steel for hot forging - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a non-heat treated steel for hot forging for machine structure, which is manufactured after hot forging without performing heat treatment such as quenching and tempering. Is. INDUSTRIAL APPLICABILITY The present invention is particularly suitable for mechanical structural parts such as automobile steering knuckles and knuckle arms that require high strength and high toughness.

[Prior Art] Steel conventionally used for undercarriage parts of automobiles such as steering knuckles is required to have high strength and high toughness. Therefore, carbon steel (S45C) or Cr steel or C
Medium carbon low alloy steel such as r-Mo steel (SCr440, SCM
(440, etc.) is hot worked by a press, a hammer or the like, and thereafter, heat treatment such as quenching and tempering is performed in order to have high strength and high toughness. However, if these heat treatments are not performed, the cost can be significantly reduced and the social demands such as energy saving can be met. From this point of view, there has recently been a strong demand for the development of a non-heat treated steel that can be used as hot forged without requiring heat treatment such as quenching and tempering.

Therefore, researches on non-heat treated steel for hot forging which can be used as hot forging without performing the above heat treatment are being actively attempted. For example, a non-heat treated steel in which V is added to a medium carbon steel containing 0.30 to 0.50% of C has been proposed. In this non-heat treated steel, when it is cooled after being hot forged, V carbonitride precipitates, and the V carbonitride strengthens the ferrite material. Due to this strengthening action, strength can be provided only by hot forging without performing the heat treatment.

[Problems to be Solved by the Invention] However, in the above-proposed non-heat treated steel to be used as hot forging, hot forging is performed at a high temperature of 1110 ° C. or higher, and after hot forging, Since the cooling rate may be slow, a coarse ferrite and pearlite mixed structure is formed, and strength is sufficiently secured, but toughness is low, so both high strength and high toughness are required, such as in automobile steering knuckles. It was not a satisfactory machine structural part.

[Means for Solving Problems] The present invention has been made in view of the above circumstances, and an object thereof is to perform quenching and tempering treatment after hot forging without performing quenching and tempering treatment as hot forging. It is to provide a non-heat treated steel for hot forging which can secure strength and toughness equivalent to or higher than that of the above-mentioned medium carbon low alloy steel.

As a result of earnest research on the non-heat treated steel for hot forging under the above-mentioned objects, the present inventors have found that firstly, the carbon content is reduced to improve the toughness, and secondly, the hardenability is improved to improve baitite. Increasing the amount of Mn having a forming action and adding Mo to improve the hardenability, and the coarse weave of the non-heat treated steel for hot forging to have a mixed structure of ferrite and bainite or a bainite single structure. Third, since precipitation hardening can be expected by precipitating fine V carbonitrides in the above-mentioned structure, it has higher toughness than conventional non-heat treated steels, and as described above in hot forging. It has been found that strength and toughness equivalent to or higher than that of conventional medium carbon low alloy steel can be secured.

The present invention is based on this finding.

That is, the non-heat treated steel for hot forging according to the present invention, in% by weight,
C: 0.10 to 0.25%, Si: 0.10 to 0.50
%, Mn: 1.0 to 2.5%, Mo: 0.10 to 0.5
0%, V: 0.05 to 0.30%, Al: 0.05% or less, and the balance is steel substantially consisting of Fe. By hot forging the steel, it is cooled to obtain ferrite. And a bainite mixed structure or a bainite single structure is formed.

Further, the non-heat treated steel for hot forging according to the present invention has S: 0.
It is also preferable to contain at least one selected from the group consisting of 04 to 0.10% and Pb: 0.05 to 0.30%. If contained in this manner, the machinability of the non-heat treated steel for hot forging is improved, and it is suitable as a machine structural steel that is easily cut.

Next, the reason for limiting the composition of the non-heat treated steel for hot forging according to the present invention will be described.

C is an element necessary to secure the strength and is 0.10%
If it is less than this, the strength is insufficient. If C exceeds 0.25%, the toughness decreases. Si is added as a deoxidizing auxiliary agent, 0.10% is necessary, and 0.5
If it exceeds 0%, the toughness decreases. Mn is necessary to improve hardenability and bainite the structure.
If it is less than%, the hardenability is insufficient and it becomes difficult to form bainite, and if it exceeds 2.5%, the hardenability is excessively improved and martensite is generated, and the toughness is lowered. Mo is necessary to bainite the structure,
Alternatively, it is also necessary for age hardening, and if it is less than 0.10%, bainization becomes insufficient, and if Mo exceeds 0.50%, the cost becomes high. V is a basic element of non-heat treated steel for hot forging, and is 0.05 in order to secure the strength by forming carbon nitride of V.
% Is necessary, and if it exceeds 0.30%, the cost becomes high. Al is necessary as a deoxidizer, but 0.0
If it is more than 5%, the toughness is lowered. Since S improves machinability, it is preferable to contain 0.04 to 0.10% as mechanical structural steel. If it is less than 0.04%, the machinability is insufficient, and if it exceeds 0.10%, the toughness decreases. Further, in order to improve the machinability, Pb is preferably contained in an amount of 0.05 to 0.30%.
This is because if it exceeds 30%, the improvement effect is little improved.

[Effects of the Invention] The non-heat treated steel for hot forging according to the present invention has a low carbon content, and since the hardenability is further improved by Mo and Mn, it is left as natural cooling or wind cooling after hot forging. Then, the structure becomes a bainite single structure or a mixed structure of ferrite and bainite, and therefore high toughness can be secured.
In the case of a mixed structure of ferrite and bainite,
The more bainite, the better. The area ratio is 50 to 1
It is preferably 00%.

In the non-heat treated steel for hot forging according to the present invention, since carbonitrides of V (VN, VC, etc.) precipitate in the bainite single structure or the mixed structure of ferrite and bainite,
Since the structure is strengthened by precipitation hardening, high toughness and high strength can be secured.

Further, in the non-heat treated steel for hot forging according to the present invention, when the soft nitriding treatment is performed, the soft nitriding property is good and a deep soft nitriding layer can be obtained in a short time. The reason why the soft nitriding property is good is that V is N
This is because, because of the strong affinity with, it is possible to promote the penetration of N into the steel during the soft nitriding treatment, and to obtain a high surface hardness due to the formation of V nitride.

By the way, in general, the lower the hardness of steel, the better the machinability, but the drawback is that the strength is low. In this respect, in the non-heat treated steel for hot forging according to the present invention, the hardness of the core portion is increased and the strength is further improved by the aging treatment (for example, the soft nitriding treatment). Therefore, if mechanical processing such as cutting is performed before heat treatment such as soft nitriding treatment and age treatment is performed after heat treatment such as soft nitriding treatment, mechanical workability is improved and strength can be secured.

[Examples] Next, the features of the non-heat treated steel for hot forging according to the present invention will be clarified by examples compared with conventional steels and comparative steels. In this embodiment, a steel bar having a diameter of 50 mm is heated to 1250 ° C. and then hot forged at about 1100 ° C., thereby 30 m
An m diameter steel bar was formed and then naturally cooled. From this steel bar, tensile test pieces (JIS No. 4) and impact test pieces (JI
S3) was produced. And the steel of the present invention (NO. 1 to N
O. With respect to the test piece of 6), 0.2% proof stress, tensile strength, elongation, impact value, component composition and microstructure were examined, and shown in the table.

No. as shown in the table. 1-NO. Regarding the chemical composition of the steel of the present invention according to No. 6, C is in the range of 0.10 to 0.25%, low carbon is achieved, and Mn having a bainite promoting action is 1.0 to 2. It is in the range of 5%, and Mo, which also has a bainite promoting effect, is 0.10 to 0.50%.
Is the range.

Further, for the comparative steels (NO.7 to NO.10) and the conventional steels (NO.11 to NO.12), tensile test pieces and impact test pieces were similarly prepared, and 0.2% proof stress, tensile strength, Elongation, impact value and microstructure were investigated and shown in the table. It should be noted that the comparative steel, which has a different composition from the steel of the present invention, was heated to about 1250 ° C. and then heated to about 11
Hot forging was carried out at 00 ° C. to form a steel bar having a diameter of 30 mm, and then naturally cooled. NO. No. 7 does not contain Mo, and NO. 8 has a large C content of 0.31%, and NO. No. 9 has a high Mn of 2.75%, and NO. No. 10 is a non-heat treated steel in which V is added to the above-mentioned medium carbon steel, and the main difference is that C is as large as 0.45% and Mo is not contained. Conventional steel NO.
11 is the composition of carbon steel S45C, which was hot forged under the same conditions as the steel of the present invention to form a steel bar having a diameter of 30 mm,
It was formed by heating at 860 ° C. for 40 minutes, oil-cooling (quenching) from that state, further heating at 580 ° C. for 90 minutes for tempering, and then water-cooling from this state. NO. The structure 11 is an incompletely quenched and tempered structure. In addition, the NO. No. 12 is the composition of SCM440, which was hot forged under the same conditions as in the present invention to form a steel bar having a diameter of 30 mm, which was then heated at 850 ° C. for 40 minutes, oil-cooled (quenched) from that state, and further tempered. 90 at 650 ° C for
It was heated for a minute and then oil cooled from this state. NO.
The 12th structure is a completely quenched and tempered structure.

As shown in the table, the invention steel No. 1-NO. No. 6 is a mixed structure of ferrite and bainite having a bainite ratio of 90% or more, or a bainite single structure, pearlite is not generated due to low carbon, and the tensile strength is 70.
It has a high strength with secured kgf / mm ² , and its impact value is 9.8 to 18.1 kgf / cm ² . Therefore, the steels of the present invention (NO. 1 to NO. 6) are comparative steels NO. 7-NO.
10 (impact value 0.7 to 9.8 kgfm / cm ² , It has a higher impact value than the tensile strength of 60 kgf / mm ² or more), and therefore it is understood that the steel of the present invention is superior in high strength and toughness. In addition, NO. No. 9 has a high tensile strength of 130.5 kgf / mm ² , but has an extremely low impact value of 0.7 kgfm / cm ² . Further, as a comparative steel, NO. No. 8 has a bainite independent structure and secures a tensile strength of 98.8 kgf / mm ² , but since the C% is high, the impact value is as low as 5.1 kgfm / cm ² . Further, as a comparative steel, NO. 7 has an impact value of 9.8 kgfm / c
m ² can be secured, but tensile strength is 65.1 kgf
It is as low as m / mm ² .

In addition, the NO. 11, the tensile strength is 7
8.0 kgf / mm ² , impact value 9.5 kgfm / cm ²
And also NO. 12, the tensile strength is 85.1 kgf
/ Mm ² , and the impact value is 17.3 kgfm / cm ² . Therefore, the steels of the present invention (NO.1 to NO.6) have the same tensile strength and impact value, that is, strength and toughness, as the conventional steels (NO.11 and NO.12) that have been subjected to heat treatment such as quenching and tempering. You can see that

Further, for the steel of the present invention and the conventional steel, N ₂ : H ₂ =
Under the condition of gas composition of 1: 1, soft nitriding treatment was performed at 570 ° C. for 5 hours, and the depth of the nitrided layer was measured.
As shown in the figure. As shown in FIG. 1, in the steel of the present invention, the nitride layer has a hardness (0.02 mm depth) of Hv500 to 540, which is harder than that of the conventional steels (NO. 11 and NO. 12). The depth is as deep as 0.1 to 0.2 mm. Therefore, the steel of the present invention is likely to be nitrided. Further, the hardness of the core portion after the soft nitriding treatment was measured, and the result is shown in FIG. As shown in FIG. 2, the steel of the present invention (NO. 1, N
O. 2, NO. In 4), the hardness of the core after soft nitriding is
Although it is improved compared to before the soft nitriding treatment, the conventional steel (NO.
11 and NO. In 12), the hardness of the core is almost the same before and after the soft nitriding treatment. Therefore, the strength of the steel of the present invention is further improved by the soft nitriding treatment. Further, the aging temperature (holding for 5 hours) was variously changed, the relationship between the aging temperature and the hardness was measured, and the results are shown in FIG. As shown in FIG. 3, the hardness is constant in the conventional steel, but in the steel of the present invention (NO. 1), the hardness rapidly increases at an aging temperature of about 600 ° C. The reason for this is that solid solution Mo and V precipitate as carbonitrides of delicate Mo and V due to aging, and the strength is improved.

Here, in general, the lower the hardness of steel, the better the machinability such as machinability, but the drawback is that the strength becomes low. For example, in FIG. 3, the steel of the present invention (NO. 1) is the conventional steel (N.
O. Since the hardness is lower than 12), machinability such as machinability is good. Further, in the steel of the present invention, the hardness of the core portion is increased by the aging treatment (for example, soft nitriding treatment), and the present invention has the strength equal to or higher than that of the conventional steel. Therefore, in the steel of the present invention, if machining such as cutting is performed before heat treatment such as soft nitriding, and if soft nitriding is performed after machining, the aging treatment is necessarily performed together with the soft nitriding treatment. Invention steel has good machinability and high strength.

[Brief description of drawings]

FIG. 1 is a graph showing the hardness distribution of the nitrided layer, FIG. 2 is a graph showing the change in the hardness of the core portion by soft nitriding treatment, and FIG. 3 is a graph showing the relationship between the aging temperature and the hardness.

Claims (2)

[Claims] 1. C: 0.10 to 0.25% by weight, S
i: 0.10 to 0.50%, Mn: 1.0 to 2.5%,
Mo: 0.10 to 0.50%, V: 0.05 to 0.30
%, Al: 0.05% or less, with the balance being substantially Fe
A non-heat treated steel for hot forging, characterized in that a mixed structure of ferrite and bainite or a bainite single structure is formed by cooling the steel after hot forging. 2. By weight%, C: 0.10 to 0.25%, S
i: 0.10 to 0.50%, Mn: 1.0 to 2.5%,
Mo: 0.10-0 / 50%, V: 0.05-0.30
%, Al: 0.05% or less, and S: 0.04
To 0.10%, Pb: at least one selected from 0.05 to 0.30%, and the balance is steel substantially consisting of Fe, which is obtained by hot forging the steel and then cooling the steel. By this, a non-heat treated steel for hot forging characterized in that a mixed structure of ferrite and bainite or a bainite single structure is formed.