JPH08302445A - Boron-containing steel excellent in workability and strength, and production of forged parts made of the same - Google Patents

Abstract

PURPOSE: To efficiency produce a B-contg. steel excellent in workability and strength by preparing a steel contg. C, Si, Mn, Al, Ti, N, B and Cr under speci fied conditions. CONSTITUTION: This B-contg. steel having a compsn. contg., by weight, 0.15 to 0.35% C, <=0.25% Si (not including zero), 0.30 to 1.50% Mn, 0.02 to 0.04% Al, 0.015 to 0.08% Ti, 0.003 to 0.01% N, 0.0005 to 0.004% B and <=1.0% Cr (not including zero), in which P value prescribed by the formula of P value = [C]+0.60[S]+0.15[Mn]+0.15[Cr] is regulated to 0.35 to 0.60% and G value prescribed by the formula of G value = [Ti]-3.5[N] is regulated to 0.01 to 0.07%, and the balance Fe with inevitable impurities is prepd. Moreover, in the formulae, [C], [Si], [Mn], [Cr], [Ti] and ]N] respectively show their contents (%). Thus, the B-contg. steel having about >785N/mm<2> tensile strength and in which the austenite grain size number is regulated to >=5 can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing various forged products used in automobiles and various industrial machines. More specifically, the tensile strength exceeds 785 N / mm ² , and the austenite grain size number is 5 or more. The present invention relates to a method for efficiently manufacturing a certain B-containing forged part.

[0002]

2. Description of the Related Art Conventionally, after cold forging steel for forging parts,
Tensile strength of 785 when subjected to quenching and annealing
In manufacturing a forged part having a high strength level exceeding N / mm ² , it is necessary to perform a spheroidizing annealing process as a heat treatment before cold forging. However, since the spheroidizing annealing process requires a long-time heat treatment, simplification or omission of the heat treatment is desired from the viewpoint of cost reduction and energy saving.

Therefore, by using B-added steel as the steel material, the hardenability is maintained without deteriorating the cold forgeability, and the cost at the time of manufacturing the forged parts is reduced. However, this B-added steel has a problem that the crystal grain size of austenite tends to become coarse during quenching and heating. Therefore, for the purpose of suppressing coarsening of the austenite grain size, various methods have been proposed in which the heating rate is controlled particularly during quenching and heating.

For example, Japanese Patent Laid-Open No. 57-79116 discloses a method in which the heating rate during quenching is 3 to 50 ° C./sec.
Different from the heating rate (0.5 ° C./sec or less) at the time of quenching a normal component, rapid heat treatment is required, and there is a problem that the applicable range is limited.

Japanese Patent Publication No. 56-13768 discloses a method in which the heating rate at the time of quenching is 3 ° C./minute or less. However, the heating means is premised on the induction heating method, and the usual parts quenching / annealing. The disadvantage is that the electric furnace heating that is frequently used in processing is difficult to use. Further, according to this method, in order to adjust the crystal grain size of austenite, it is premised that hot working or heat treatment at 950 ° C. or higher and 1000 ° C. or lower is performed before quenching the steel, which complicates the process. There are also problems such as increased costs.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of these circumstances, and its purpose is to quench while maintaining the improvement of cold forgeability and hardenability by using B-added steel. B-containing steel having improved workability and strength by preventing coarsening of the austenite grain size during heating, and forged parts having good mechanical properties such as workability using this B-containing steel efficiently It is to provide a method that can be manufactured.

[0007]

The B-containing steel of the present invention capable of achieving the above object is C: 0.15 to 0.35%, Si: 0.25% or less (not including 0%), Mn: 0.30 to 1.50. %, Al: 0.
02 to 0.04%, Ti: 0.015 to 0.08%, N: 0.003 to 0.01
%, B: 0.0005 to 0.004%, Cr: 1.0% or less (not including 0%), and the P value defined by the following formula (1) is 0.35 to 0.60%, and the following formula (2) The G value defined by is satisfied with the range of 0.01% or more and 0.07% or less, and the balance is Fe and inevitable impurities. P value = [C] +0.60 [Si] +0.15 [Mn] +0.15 [Cr] ... (1) G value = [Ti] -3.5 [N] ... (2) In the formula, [C], [Si], [Mn], [Cr], [T
i] and [N] are C, Si, Mn, Cr and Ti, respectively.
And the content (%) of N are shown.

As other components, Mo: 0.5% or less (not including 0%), Ni: 1.0% or less (not including 0%) and V: 0.2% or less (not including 0%) Those containing at least one selected from are preferred embodiments of the present invention.

The method for producing a forged part made of B-containing steel using the above-mentioned steel of the present invention is to cold-forge this steel into a predetermined shape, and then at an Ac ₃ point or more (Ac ₃ + 50 ° C.) or less. The point is that the material is heated and held up to the temperature range of 4 ° C./minute at a heating rate of more than 4 ° C./minute and then quenched and annealed.

[0010]

The reason for limiting the composition of each component in the B-containing steel of the present invention is as follows. C: 0.15 to 0.35% C is a useful element for ensuring the hardenability and strength of steel.
Tensile strength of 785 is obtained by quenching and annealing as in the present invention.
To obtain a high strength level of N / mm ² or more, 0.15% or more must be added. The preferred lower limit is
0.20%, more preferably 0.22%. on the other hand,
Cold forgeability is hindered as the amount of C added increases,
Since it becomes difficult to simplify or omit the spheroidizing annealing treatment which is a heat treatment before forging, the upper limit was set to 0.35%. The preferred upper limit is 0.30%, and more preferred is
It is 0.27%.

Si: 0.25% or less (not including 0%) Si is an element useful as a deoxidizer, and the preferable lower limit is 0.05%. However, the deformation resistance during cold forging increases as the amount of addition increases. Therefore, the upper limit is set to 0.25%. A preferred upper limit value is 0.09%, and a more preferred value is 0.05%.

Mn: 0.30 to 1.50% Mn is used as a hardenability improving element, is useful for imparting high strength, and also acts as a deoxidizer. In order to exert such an effect effectively, it is necessary to add 0.30% or more, the preferable lower limit is 0.40%, and more preferable is 0.50%. However, if the amount of Mn added is too large, a supercooled structure is generated after rolling, which causes an increase in deformation resistance during cold forging, resulting in a shortened life of the forging tool. The preferred upper limit is 1.20%, and more preferred is
It is 1.00%.

Al: 0.02 to 0.04% Al is used as a deoxidizing agent. Further, N in the steel is fixed to form AlN, and crystal grains are refined to improve delayed fracture resistance. It is an element that contributes. Addition of 0.02% or more is necessary to effectively exhibit such an effect. The preferred lower limit is 0.025%, and more preferred is 0.027%. However, if too much, the deformability during cold forging decreases due to the formation of oxide inclusions, so the upper limit was made 0.04%. A preferable upper limit value is 0.035%, and a more preferable upper limit value is 0.032%.

Ti: 0.015 to 0.08% Ti is very useful for fixing N in the steel in the form of TiN and sufficiently exerting the action by adding B. Especially Ti
The formation of N is very useful for refining the crystal grains, and thereby the delayed fracture resistance, which is a required function of the bolt, can be improved. How to effectively bring out this kind of action
It is necessary to add 0.015% or more. The preferred lower limit is 0.
It is 03%, and more preferably 0.05%. However, if it is too large, it causes coarsening of the nitride, and it is not possible to prevent coarsening of the austenite crystal grains, and the cold forgeability, in particular, the upper limit thereof is 0.08 because it causes a decrease in deformability.
%. A preferred upper limit value is 0.07%, and a more preferred value is 0.06%.

N: 0.003 to 0.01% N is an element that contributes to the improvement of delayed fracture resistance and the like by refining the crystal grains by forming AlN or TiN. In order to exert such an effect effectively, it is necessary to add 0.003% or more. A preferable lower limit value is 0.0035%, and a more preferable lower limit value is 0.004%. However, if the amount is too large, not all can be captured even by adding Al or Ti, excess N forms BN, and the effect of improving quenching by B cannot be secured, and the amount of solid solution N increases. This will impair delayed fracture resistance. The preferred upper limit is 0.00
It is 6%, and more preferably 0.005%.

B: 0.0005 to 0.004% B is an element that improves the hardenability of steel by segregating at grain boundaries. In order to bring out the effect, 0.0005% or more must be added. The preferred lower limit is 0.0012%,
More preferred is 0.0015%. However, if added excessively, the toughness and delayed fracture resistance are rather adversely affected, so the upper limit was made 0.004%. The preferred upper limit is 0.
0025%, and more preferably 0.0020%.

Cr: 1.0% or less (not including 0%) Cr is a very useful element for improving the hardenability and ensuring high strength, and also for improving the delayed fracture resistance by improving the corrosion resistance. However, if added excessively, the cold forgeability will be adversely affected, so the upper limit was made 1.0%. The preferable upper limit value is 0.8%, and the more preferable upper limit value is 0.6%.

P value: 0.35 to 0.60% (however, P value = [C] +0.60 [Si] +0.15 [Mn] +
The 0.15 [Cr]) P value is particularly specified in the present invention, and is set as an index for obtaining excellent cold forgeability. That is, even when the respective component ranges of C, Si, Mn, and Cr satisfy the ranges defined above, when the total amount of them increases, the deformation resistance during cold forging increases and the tool life becomes longer. Since it will decrease, the upper limit was defined as 0.60%. A preferable upper limit value is 0.55%, and a more preferable upper limit value is 0.50%.

On the other hand, when the lower limit of the P value is less than 0.35%, it is impossible to obtain a part having a tensile strength of more than 785 N / mm ² desired in the present invention in the quenching / annealing process after forming the forged part. A preferred lower limit value is 0.40%, and a more preferred value is 0.45%.

G value: 0.01% or more and 0.07% or less (where G value = [Ti] -3.5 [N]) Like the P value, the G value is also specified in the present invention and is austenite crystal grains. It is set as an index for preventing the coarsening of. That is, in order to secure the pinning effect for preventing coarsening of austenite crystal grains by the Ti compound, it is necessary to set the lower limit of the G value to 0.01%. A preferred lower limit value is 0.03%, and a more preferred value is 0.04%.

On the other hand, the upper limit value is necessarily determined by the Ti amount and the N amount specified in the present invention, but the preferable upper limit value is 0.06%, more preferably 0.05%. The above-mentioned elements include B according to the present invention.
Although it is an essential component in steel, the following elements may be added if necessary.

Mo: 0.5% or less, Ni: 1.0% or less and / or V: 0.2% or less (all of these elements do not include 0%) All of these contribute to improvement of mechanical properties (strength, etc.). It is an element. Of these, Mo is an element that enhances hardenability and contributes to strength improvement, and is also useful for improving delayed fracture resistance by improving corrosion resistance. However, excessive addition adversely affects the cold forgeability, so the upper limit is 0.5.
%. A preferable upper limit value is 0.3%.

Ni: 1.0% or less Ni also enhances hardenability and contributes to high strength, and
It is an element useful for improving delayed fracture resistance by increasing notch toughness. However, if added excessively, cold forgeability is impaired, so the upper limit was made 1.0% or less. A preferable upper limit value is 0.8%.

V: 0.2% or less V is an element that contributes to the improvement of delayed fracture resistance by achieving the refinement of crystal grains by forming carbon / nitride in steel. In order to effectively exhibit such an effect, it is preferable to add 0.05% or more. However, if added excessively, the carbon / nitride will be coarsened and it will not be possible to prevent coarsening of the austenite crystal grains, so the upper limit was made 0.2%. A preferable upper limit value is 0.15%.

Although the B-containing steel of the present invention has been described above, in order to manufacture a forged part after cold forging this B steel into a predetermined shape, before quenching, Ac ₃ points or more (Ac ₃ +
It is necessary to heat and hold up to a temperature range of 50 ° C.) or lower at a heating rate exceeding 4 ° C./min. For example, the holding time at the time of quenching at the time of manufacturing the bolt is about 1 hour at the maximum, and by performing such heating and holding, the bolt core can be sufficiently heated. By specifying the heating rate in a certain temperature range before quenching in this way, coarsening of austenite crystal grains during quenching can be prevented. A preferable heating rate is 8 ° C./min or more, more preferably 15 ° C./min or more. The manufacturing method of the present invention is characterized in that the heat treatment conditions before quenching are controlled in this manner. The cold forging conditions before the heat treatment and the heating / holding at the heating rate are followed by quenching / annealing. Conditions and the like are not particularly controlled, and preferable conditions can be appropriately selected within a range that does not impair the action of the present invention.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification or implementation is within the scope of the present invention without departing from the spirit thereof. It is included in the technical scope of.

[0027]

【Example】

Example 1 Various steels having the chemical compositions shown in Table 1 and Table 2
After being melted in a vacuum melting furnace of kg, it was rolled into a wire rod of 10.3 mmφ.

[0028]

[Table 1]

[0029]

[Table 2]

After that, an annealing treatment of heating at 680 ° C. for 3 hours and then air cooling was performed, and wire drawing was performed to 9.85 mmφ. At that time, in order to evaluate the cold forgeability, a part of the wire rod after the cold forging was used to examine the deformation resistance at the time of 60% compression processing by the end face restraint compression test method. On the other hand, using the wire rod after wire drawing, M by the bolt header machine
10 volt was made. Then, after heating to 900 ° C. at a heating rate shown in Table 1 and Table 2 and holding for 1 hour, J
The austenite grain size was measured according to the quenching and tempering method of IS G0551. The results are shown in Tables 3 and 4.

[0031]

[Table 3]

[0032]

[Table 4]

The results shown in the table can be considered as follows. Examples satisfying the requirements specified by the present invention (No.
1 to 12) all have an austenite grain size of 5 or more, deformation resistance during cold forging of 665 N / mm ² or less, excellent cold forgeability, and tensile strength of 785 n / mm.
It was found to have a high strength above ² . Further, in Reference Examples 24 to 26, the contents of Ni, Mo and V, which are selectively acceptable components, exceeded the preferred upper limits, and the strength and the grain size of austenite were good, but the deformation resistance was slightly increased. . On the other hand, the comparative examples (Nos. 13 to 23) which do not satisfy the constituent requirements of the present invention have the following disadvantages.

Since No. 13 had a small amount of C, the tensile strength was lowered. Since No. 14 had a large amount of C, the P value exceeded the upper limit of the present invention, and therefore the deformation resistance increased. No.15
Since the amount of Si was large, the P value exceeded the upper limit of the present invention, and therefore the deformation resistance increased. Since No. 16 had a small amount of Mn, the tensile strength was lowered. Since No. 17 had a large amount of Mn, the deformation resistance increased.

In No. 18, the P value exceeded the upper limit of the present invention due to the large amount of Cr, and therefore the deformation resistance increased. No.
In No. 19, since the amount of Ti was small, the G value was below the lower limit of the present invention, so that the austenite grain size was coarsened.
Since No. 20 had a large amount of Ti, the austenite grain size became coarse. In No. 21, the chemical composition of steel satisfies the requirements of the present invention, but the heating rate is outside the range of the present invention, so that the austenite grain size becomes coarse. No.22
Indicates that the chemical composition of the steel satisfies the requirements of the present invention, but the P value is below the lower limit of the present invention, so the tensile strength is reduced. No.23
Shows that the chemical composition of steel satisfies the requirements of the present invention, but the P value exceeds the upper limit of the present invention, so the deformation resistance increases.

[0036]

EFFECTS OF THE INVENTION The B-containing steel of the present invention is constituted as described above, has excellent workability and strength, and does not cause coarsening of austenite crystal grains even after heat treatment, and has a homogeneous and fine crystal structure. It is a thing. The forged parts obtained by using this steel are very excellent in terms of workability, strength and cold forgeability.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshinori Yamamoto 2 Nadahamahigashi-cho, Nada-ku, Kobe-shi, Hyogo Stock Company Kobe Steel Works Kobe Steel Works

Claims (3)

[Claims] 1. C: 0.15 to 0.35% (meaning% by weight; the same applies hereinafter), Si: 0.25% or less (not including 0%), Mn:
0.30 to 1.50%, Al: 0.02 to 0.04%, Ti: 0.015 to 0.
08%, N: 0.003 to 0.01%, B: 0.0005 to 0.004%, C
r: 1.0% or less (not including 0%) is included, and the P value defined by the following formula (1) is 0.35 to 0.60%, and the G value defined by the following formula (2) is 0.01%. A B-containing steel excellent in workability and strength, characterized by satisfying the above range of 0.07% or less, and the balance being Fe and inevitable impurities. P value = [C] +0.60 [Si] +0.15 [Mn] +0.15 [Cr] ... (1) G value = [Ti] -3.5 [N] ... (2) In the formula, [C], [Si], [Mn], [Cr], [T
i] and [N] represent the contents (%) of C, Si, Mn, Cr, Ti and N, respectively. 2. As other components, Mo: 0.5% or less (not including 0%), Ni: 1.0% or less (not including 0%) and V: 0.2% or less (not including 0%). The B-containing steel according to claim 1, containing at least one selected from the group consisting of: 3. After cold forging the steel according to claim 1 or 2 into a predetermined shape, at an Ac ₃ point or more (Ac ₃ + 50 ° C.)
A method for producing a forged B-steel forged part having excellent workability and strength, which comprises heating and holding the following temperature range at a heating rate of more than 4 ° C./minute, followed by quenching and annealing.