JP3489655B2 - High-strength, high-toughness free-cut non-heat treated steel - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength and high-toughness free-cutting non-heat treated steel material. More specifically, it relates to a non-heat treated steel material suitable as a material for machine structural parts and the like, which has an excellent strength-toughness balance and is also excellent in machinability even without performing a tempering treatment after hot working. is there.

[0002]

2. Description of the Related Art Conventionally, mechanical structural parts, etc., which require high tensile strength and toughness, are roughly hot-worked into a predetermined shape, and then cut into a desired shape.
It was general to perform tempering treatment such as quenching and tempering. However, this conditioning process consumes a lot of energy and cost. Therefore, in recent years, non-heat treated steel that can be used as it is without hot working has been actively developed in order to meet the social demand for energy saving and at the same time reduce cost.

Further, for the purpose of facilitating the cutting work after the hot working, the demand for free-cutting steel excellent in machinability is increasing more and more.

Generally, the machinability of steel largely depends on the metal structure, and in the case of a steel having a ferrite / pearlite structure, the machinability is good, and the ferrite / bainite structure and the single-phase structure of bainite or martensite are formed. It is known that steel has poor machinability. Also, Pb, T
It is also well known that the machinability is improved by adding free-cutting elements such as e, Bi, Ca and S alone or in combination. Therefore, conventionally, a method of adding the above-mentioned free-cutting element to non-heat treated steel to improve the machinability (machinability) after hot working has been adopted. However, when the free-cutting element is simply added to the non-heat treated steel, it is often impossible to secure desired mechanical properties, especially toughness.

JP-A-6-145890 discloses "high-strength and high-toughness free-cutting steel". However, in order to impart the desired high strength and high toughness to the steel proposed in this publication, as is clear from the description in the examples, carburizing and quenching, induction hardening and quenching low temperature tempering are essential treatments. Therefore, the heat treatment cost increases. In addition, when the steel is hardened to undergo martensitic transformation, a large transformation strain is generated, which causes a problem of large bending. Furthermore, when quenching steel with a particularly high C content, it is necessary to consider quench cracking.

Japanese Unexamined Patent Publication No. 7-166235 discloses a steel material having a specific chemical composition in which 80% or more of the structure cooled after hot forging is ferrite-bainite.
A "method for producing a sub-hot forging steel having excellent toughness, durability ratio, yield ratio and machinability" which is aged at 700 ° C is disclosed. However, the steel proposed in this publication contains V in an amount of 0.30 to 0.70% by weight. Therefore, as is clear from the description of the example, for example, the tensile strength is 1100 MPa (112.2 Kgf / m).
The impact value at the m ² ) level is 40 J / cm ² (4.1 kg
f / m / cm ² ) and the toughness is low. Therefore, it is not necessarily applicable when high strength and good toughness are required. Furthermore, 800-1050 ° C
Since it is necessary to perform hot forging at a low temperature range of, the steel material may be cracked during forging.

In other words, all of the techniques proposed in the above publications meet the demands of the industrial world for achieving both "non-tempering" and "improving machinability of high strength and high toughness steel". It was something I couldn't do.

Japanese Unexamined Patent Publication (Kokai) No. 7-54100 discloses "a ferrite + bainite type high strength non-heat treated steel for hot forging having excellent machinability" having a specific chemical composition.
However, the steel proposed in this publication has a Si content of 3.
Since it also includes 0 to 5.0, it has low toughness even if high strength can be achieved, and is not necessarily applicable when high strength and good toughness are required.

Japanese Unexamined Patent Publication (Kokai) No. 7-268538 discloses that after hot forging, it has a two-phase structure of bainite + ferrite or a bainite structure in which it is naturally cooled and has a tensile strength of 883MP.
Not less than a, 2mm U Notch Charpy impact value is 59J / c
A "high toughness non-heat treated steel excellent in machinability" of m ² or more is disclosed. However, since the steel proposed in this publication has a high N content of 0.01 to 0.03%, it is not always satisfactory in terms of strength-toughness balance. That is, in the case of the steel proposed in the above publication, as is clear from the relationship between the tensile strength and the impact value described in the examples,
Especially when the tensile strength is 1100 MPa or more, the impact value is 6
It is about 0 J / cm ² , which is not necessarily applicable when high toughness is required.

Iron and Steel (vol. 57 (1971) S4
84) has reported that machinability may be enhanced by adding Ti to the deoxidized controlled free-cutting steel. But T
It is also stated that the addition of a large amount of i increases tool wear due to the large amount of TiN produced, which is not preferable in terms of machinability. For example, C: 0.45%, S
i: 0.29%, Mn: 0.78%, P: 0.017
%, S: 0.041%, Al: 0.006%, N: 0.
Steel containing 0087%, Ti: 0.228%, O: 0.004% and Ca: 0.001% conversely has a shortened drill life and poor machinability. As described above, the machinability is not improved simply by adding Ti to steel.

Zr may be added for the purpose of controlling the sulfide morphology of free-cutting sulfur steel. For example, iron and steel (vo
l. 62 (1976) p. 885), Zr has little effect on machinability. That is, the machinability cannot be improved simply by adding Zr to the steel.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned current situation, and under the conditions of normal hot working and cooling,
It also aims to provide at low cost a steel material that has high strength and excellent toughness without being heat treated and that is suitable as a material for machine structural parts and the like that has good machinability at each strength level. And

[0013]

The gist of the present invention resides in a high-strength, high-toughness, free-cutting, non-heat treated steel material having the following (1) and (2).

(1) C: 0.05 to 0.3 by weight%
%, Si : 0.05 to 1.5 %, Mn: 0.4 to 3.5
% , S: 0.002-0.2%, Ti: 0.04-1.
0%, Al: 0.005-0.05%, N: 0.008
% Or less, Cr: 0 to 3.0%, Ni: 0 to 2.0% ,
V: 0-0.3%, Nb: 0-0.1%, Mo: 0-
0.5%, Cu: 0 to 1.0%, B: 0 to 0.02%,
Nd: 0-0.1%, Pb: 0-0.5%, Ca: 0-
0.01%, Se: 0 to 0.5%, Te: 0 to 0.05
%, Bi : 0 to 0.4%, fn1 represented by the following formula (1)
Exceeds 0% and fn2 represented by the following formula (2) is 2.5 to
4.5%, the balance is the chemical composition of Fe and unavoidable impurities,
High strength and high toughness in which the maximum diameter of Ti carbosulfide in steel is 10 μm or less, the amount is 0.05% or more in cleanliness, and 90% or more of the structure is bainite or ferrite bainite structure. Free-cutting non-heat treated steel.

Fn1 = Ti (%)-1.2S (%)
... (1) fn2 = 0.5Si (%) + Mn (%) + 1.13Cr
(%) + 1.98Ni (%) (2) (2)% by weight, C: 0.05-0.3%, Si: 0.
05-1.5%, Mn: 0.4-3.5% , S: 0.0
02~0.2%, Ti: 0~1.0%, Zr: 1.0%
Below, and Ti (%) + Zr (%): 0.04 ~
1.0%, Al: 0.005-0.05%, N: 0.0
08% or less, Cr: 0 to 3.0%, Ni: 0 to 2.0
% , V: 0 to 0.3%, Nb: 0 to 0.1%, Mo: 0
~ 0.5%, W: 0-0.8%, Cu: 0-1.0%,
B: 0 to 0.02%, Nd: 0 to 0.1%, Pb: 0
0.5%, Ca: 0-0.01%, Se: 0-0.5
%, Te: 0 to 0.05%, Bi: 0 to 0.4%, the following
Fn3 represented by the formula (3) exceeds 0%, fn2 represented by the following formula (2) is 2.5 to 4.5%, and the balance is the chemical composition of Fe and unavoidable impurities. The maximum diameter of carbosulfide and Zr carbosulfide is 10 μm or less, and the sum of the amounts is 0.05% or more in cleanliness, and 90% or more of the structure is bainite or ferrite bainite structure. High-strength, tough, free-cutting, non-heat treated steel.

Fn3 = Ti (%) + Zr (%)-1.
2S (%) ・ ・ ・ ・ ・(3) fn2 = 0.5Si (%) + Mn (%) + 1.13Cr
(%) + 1.98Ni (%) (2) In the present invention, "Ti carbosulfide" is simply Ti sulfide, and "Zr carbosulfide" is simply Zr sulfide. It includes each thing. In addition, the "maximum diameter (of Ti and Zr carbosulfide)" refers to "the longest diameter in each Ti and Zr carbosulfide". The cleanliness of Ti carbosulfide and Zr carbosulfide was measured in 60 fields of view by the "microscopic test method for non-metallic inclusions of steel" defined in JIS G 0555, with a magnification of 400 of an optical microscope. Says the value.

The proportion of tissue means the proportion of tissue when observed under a microscope. It should be noted that "90% or more of bainite" means that when the structure does not contain ferrite,
Bainite occupies 0% or more, and "90% or more of ferrite / bainite" means that the sum of the proportions of ferrite and bainite in the structure when bainite and ferrite are mixed is 90% or more. Say.

As used herein, the term "non-heat treated steel material" refers to a steel material in which "quenching / tempering" as so-called "heat treatment" is omitted, "used after being hot-worked and in a cooled state". In addition to "a steel material that can be formed", "a steel material that has been subjected to an aging treatment corresponding to tempering after cooling after hot working" is included.

The following items (1) and (2) are respectively referred to as inventions (1) and (2).

The inventors of the present invention have conducted extensive research on the chemical composition and structure of non-heat treated steel materials, and as a result, after hot working the steel to which at least one of Ti and Zr is added, the steel is cooled at an appropriate cooling rate. Then, it was found that the machinability of steel material was dramatically improved. Therefore, as a result of further research, the following matters were found.

(A) In consideration of the balance with S, Ti is added to steel.
And Zr are positively added, Ti carbosulfide or Zr carbosulfide is formed in the steel, and Ti and Zr
When Ti is added, Ti carbosulfide and Zr carbosulfide are formed in the steel.

(B) The above-mentioned Ti carbosulfide and Zr in steel
The production of carbosulfide reduces the amount of MnS produced.

(C) If the S content in the steel is the same, T
i carbosulfide and Zr carbosulfide have a greater machinability improving effect than MnS. This is based on the fact that the melting point of Ti carbosulfide and Zr carbosulfide is lower than that of MnS, so that the lubricating action on the rake face of the tool is increased during cutting.

(D) In order to fully exert the effects of Ti carbosulfide and Zr carbosulfide, it is important to limit the N content to a low level. This is because when N content is high, TiN and Z
This is because Ti and Zr are fixed as rN and the generation of Ti carbosulfide and Zr carbosulfide is suppressed.

(E) In order to improve the machinability and secure the toughness by using Ti carbosulfide or Zr carbosulfide, T
It is important to optimize the size of i carbosulfide or Zr carbosulfide and the amount represented by the cleanliness (hereinafter, simply referred to as "cleanliness").

(F) Ti carbosulfide and Z produced during steelmaking
rCarbosulfide does not form a solid solution in the matrix at the heating temperature for normal hot working.

(G) To control the amount of N is TiN in steel.
It also leads to a decrease in ZrN, which can dramatically improve the toughness.

Furthermore, as a result of examining the relationship between the chemical composition and structure of the non-heat treated steel and the strength and toughness, the following (h)
The matter became clear.

(H) The structure and toughness of steel having a specific chemical composition have a correlation with the above formula (2) , and when this value is in a specific range, the main structure of non-heat treated steel is bainite. Alternatively, it becomes ferrite bainite and a good strength-toughness balance can be obtained.

The present invention has been completed based on the above findings.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION Each requirement of the present invention will be described in detail below. In addition, "%" of the content of a chemical component means "weight%."

(A) The chemical composition C: C of the steel material has a function of combining with S and Ti or Zr to form Ti carbosulfide or Zr carbosulfide and improving the machinability. C is an element effective for ensuring the strength of steel. In order to secure the above effects, the content of C is required to be 0.05% or more. However, if the content exceeds 0.3%, a pearlite structure is generated and the toughness decreases. Therefore, the content of C is set to 0.05 to 0.3%. The C content is preferably 0.10 to 0.24%.

Si: Si has a function of promoting deoxidation of steel and a function of enhancing hardenability. Furthermore, as the Si content increases, the lubricating effect on the chip surface during cutting increases and the tool life is extended, so that it also has the effect of improving machinability. But,
If the content is less than 0.05%, the effect of addition is poor, while if it exceeds 1.5%, the effect is saturated and the toughness rather deteriorates.
It was set to 05 to 1.5%. In addition, the preferable content of Si is 0.5 to 1.3%.

S: S, together with C, forms a Ti carbosulfide or a Zr carbosulfide by combining with Ti or Zr, and has the action of enhancing the machinability. However, if the content is less than 0.002%, the desired effect cannot be obtained. On the other hand, if it exceeds 0.2%, MnS is excessively generated, so that the effect of improving the machinability by Ti carbosulfide or Zr carbosulfide decreases. Therefore, the content of S is set to 0.002 to 0.2%.

Ti, Zr: Ti and Zr are important elements in the present invention, and are bonded to C and S, respectively, to form Ti.
It forms a carbosulfide or a Zr carbosulfide, and has the effect of enhancing machinability.

When Ti is added alone, the above effect can be reliably obtained when the content thereof is 0.04% or more. However, even if Ti exceeds 1.0%, Ti
Not only does the machinability improving effect of carbosulfide saturate and the cost increases, but also the carbosulfide becomes coarser, which rather leads to a decrease in toughness. Therefore, in the invention of (1), the content of Ti is set to 0.04 to 1.0%. In the case of the invention (1), in order to stably obtain good machinability and toughness, T
The content of i is preferably 0.06 to 0.8%. On the other hand, the effect of improving the machinability is that Ti and Zr
Value of Ti (%) + Zr (%) is 0.0
Even if it is 4% or more, it is surely obtained. However, Ti
(%) + Zr (%) value of Ti and Zr exceeding 1.0%
Even if it is included, the machinability improving effect is saturated, so the cost increases. Since it is only necessary that the value of Ti (%) + Zr (%) is 0.04 to 1.0%, it is not always necessary to combine Ti and Zr. When Zr is not added, the invention of the above (1) is obtained. In this case, if Ti is contained in an amount of more than 1.0%, the machinability improving effect of Ti carbosulfide is saturated and the cost increases. , Ti carbosulfide coarsens, and rather causes deterioration of toughness. When Ti is not added, that is, when Zr is added alone, if Zr exceeds 1.0%, Zr
Not only the machinability improving effect of carbon sulfide is saturated and the cost is increased, but also Zr carbosulfide is coarsened, which rather causes reduction in toughness. Therefore, in the invention of (2),
0 to 1.0% Ti content, 1.0% Zr content
And the value of Ti (%) + Zr (%) is 0.0 or less.
It was set to 4 to 1.0%. In the case of the invention (2), in order to stably obtain good machinability and toughness, Ti and Zr
The upper limit of the content of each is preferably 0.8%.

Al: Al is an element having a strong deoxidizing action. In order to secure the effect, the content of 0.005% or more is required. However, even if the content exceeds 0.05%, the effect is saturated and the cost only increases. Therefore, the content of Al is 0.005 to 0.05.
%. The Al content is 0.005-0.04%
It is preferable that

N: In the present invention, it is extremely important to control the N content to be low. That is, since N has a large affinity with Ti and Zr, it easily binds to Ti and Zr to form TiN and ZrN, and fixes Ti and Zr. Therefore, when N is contained in a large amount, Therefore, the effect of improving the machinability of the Ti carbosulfide and the Zr carbosulfide cannot be sufficiently exhibited. In particular, when the content of Ti or Zr is low, the influence of the N content becomes remarkable. Furthermore, coarse TiN and ZrN reduce toughness.

In the case of the invention of (1), the N content is 0.0
When the content is 08% or less and fn1 represented by the formula (1) is a positive value, the effect of the Ti carbosulfide described above is secured. In order to enhance the effect of Ti carbosulfide, (1)
The upper limit of the N content in the invention is preferably 0.006%.

In the case of the invention of (2), the N content is 0.0
The effect of the Ti carbosulfide and the Zr carbosulfide described above is ensured when the content is 08% or less and fn3 represented by the formula (3) is a positive value. In addition, in order to enhance the effects of Ti carbosulfide and Zr carbosulfide, the upper limit of the N content is preferably 0.006% also in the invention of (2).

V: V may not be added. If added, it precipitates as fine nitrides or carbonitrides, which has the effects of increasing the strength of steel and improving the lubricity of chips during cutting to improve machinability. In order to surely obtain such effects, it is preferable that the content of V be 0.05% or more. However, if its content exceeds 0.3%, the precipitates become coarse, so that the above-mentioned effect is saturated or the toughness is lowered. Furthermore, the raw material cost only increases. Therefore, the content of V is set to 0 to 0.3%.

Nb: Nb may not be added. If added, it precipitates as fine carbonitrides, has the effects of preventing coarsening of austenite grains and improving the strength and toughness of steel. In order to reliably obtain this effect, the Nb content is preferably 0.005% or more. However, if the content exceeds 0.1%, not only the above effects are saturated, but also coarse hard carbonitrides are generated, which damages the tool and causes a decrease in machinability. Therefore, the content of Nb is set to 0 to 0.1%.

Mo: Mo may not be added. If added, it has the effects of facilitating the formation of bainite and refining the structure to improve the strength and toughness of steel. In order to reliably obtain this effect, the Mo content is preferably 0.05% or more. However, its content is 0.5%
If it exceeds, the structure after hot working rather becomes abnormally coarse,
The toughness will decrease. Therefore, the content of Mo is 0 to
It was set to 0.5%.

W: W may not be added. If added, it has the effects of facilitating the formation of bainite and refining the structure to improve the strength and toughness of steel. In order to surely obtain this effect, the W content is preferably 0.05% or more. However, if its content exceeds 0.8%, the structure after hot working will rather become coarser and the toughness will decrease. Therefore, in the invention of (2),
The content of W was 0 to 0.8%.

Cu: Cu may not be added. If added, it has the effect of increasing the strength of the steel without lowering the toughness and further increasing the machinability. In order to reliably obtain this effect, the content of Cu is preferably 0.2% or more. However, if its content exceeds 1.0%, not only the hot workability is deteriorated, but also the precipitates are coarsened to saturate the above-mentioned effects and the toughness is lowered. In addition, the cost is high. Therefore, the Cu content is 0 to
It was 1.0%.

B: B may not be added. If added, it has the effect of improving the hardenability and improving the strength and toughness of the steel. In order to surely obtain this effect, the B content is preferably 0.0003% or more. However, if the content exceeds 0.02%, the above effects are saturated, or rather, the toughness is reduced. Therefore, the content of B is set to 0 to 0.02%.

Nd: Nd need not be added. If added, Nd ₂ S ₃ acts as a chip breaker and has the effect of improving machinability. Further, Nd ₂ S ₃ is finely dispersed and formed in a relatively high temperature region of the molten steel, so that MnS is finely dispersed and precipitated, and the pinning effect of the finely dispersed and precipitated MnS causes heat in a later step. The growth of austenite grains during heating for hot working is suppressed and the structure is refined, which also has the effect of increasing the strength and toughness of the steel. To ensure the above effect, N
The content of d is preferably 0.005% or more.
However, if its content exceeds 0.1%, the Nd ₂ S ₃ itself becomes coarse and the toughness is rather deteriorated. Therefore,
The content of Nd was set to 0 to 0.1%. The preferable upper limit of the Nd content is 0.08%.

Pb: Pb may not be added. If added, it has the effect of further improving the machinability of steel, especially the chip disposability. To ensure this effect, Pb should be 0.
It is preferable to set the content to 05% or more. However, if the content exceeds 0.5%, not only the above effect is saturated, but rather, coarse inclusions are formed and the toughness deteriorates. Furthermore, addition of a large amount of Pb causes deterioration of hot workability, and particularly when the content exceeds 0.5%, a flaw is generated on the surface of the hot worked steel material. Therefore, the Pb content is set to 0 to 0.5%.

Ca: Ca may not be added. If added, it has the effect of greatly improving the machinability of steel. In order to reliably obtain this effect, the content of Ca is preferably 0.001% or more. However, its content is 0.01%
If it exceeds, not only the above effect is saturated, but rather, coarse inclusions are generated, resulting in a decrease in toughness. Therefore, C
The content of a was 0 to 0.01%.

Se: Se may not be added. If added, it has the effect of further improving the machinability of steel. In order to reliably obtain this effect, it is preferable that the content of Se is 0.1% or more. However, its content is 0.5%
If it exceeds, not only the above effect is saturated, but rather, coarse inclusions are generated, resulting in a decrease in toughness. Therefore, S
The content of e was 0 to 0.5%.

Te: Te may not be added. If added, it has the effect of further improving the machinability of steel. In order to reliably obtain this effect, the content of Te is preferably 0.005% or more. However, its content is 0.05
If it exceeds%, not only the above-mentioned effect is saturated, but rather, coarse inclusions are formed to cause a decrease in toughness. Furthermore, Te
Addition of a large amount of causes a remarkable deterioration in hot workability, and especially when the content exceeds 0.05%, a flaw occurs on the surface of the hot worked steel material. Therefore, the content of Te is 0 to
It was set to 0.05%.

Bi: Bi need not be added. If added, it has the effect of significantly improving the machinability of steel. In order to reliably obtain this effect, the Bi content is preferably 0.05% or more. However, its content is 0.4
If it exceeds 0.1%, not only the above effect is saturated, but rather, coarse inclusions are formed and the toughness is deteriorated. Further, since the hot workability is deteriorated, the surface of the hot-worked steel material is flawed. Therefore, the Bi content is 0 to 0.4%.
And

Fn1, fn3: In the invention of (1) , a value of fn1 represented by the above formula (1) exceeding 0% when the N content is 0.008% or less (fn1 = Ti (%) − In the case of 1.2S (%)> 0%), the above-described machinability improving effect of Ti carbosulfide can be secured. When fn1 is a value of 0% or less (fn1 ≦ 0%), the amount of S becomes excessive, so that MnS is excessively generated and the machinability improving effect of Ti carbosulfide decreases. Therefore, in the invention of (1), the value of fn1 represented by the equation (1 ) is defined as a value exceeding 0% (fn1> 0%). The upper limit of the value of fn1 is not particularly specified, and may be the value when Ti is 1.0% and S is 0.002%.

In the invention of (2), the N content is 0.
If 008% or less, fn3 represented by the above formula (3) is 0%
Value (fn3 = Ti (%) + Zr (%)-1.2)
When S (%)> 0%), the above-mentioned Ti carbosulfide and Zr
The machinability improving effect of carbosulfide can be secured. fn3 is 0%
In the case of the following value (fn3 ≦ 0%), the amount of S becomes excessive, so that MnS is excessively generated and Ti carbosulfide and Z
The effect of improving the machinability due to r carbosulfide decreases. Therefore, in the invention of (2), f expressed by equation (3)
A value exceeding 0% (fn3> 0%) was defined for n3. The upper limit of the value of fn3 is not particularly specified, and the value of Ti (%) + Zr (%) is 1.0% and S is 0.
It may be a value in the case of 002%.

Fn2: fn2 represented by the above formula (2) has a correlation with the microstructure and toughness of steel, and when this value is 2.5 to 4.5%, the main microstructure of non-heat treated steel is It becomes bainite or ferrite bainite, and a good strength-toughness balance can be obtained.

Si, Mn, Cr and N for fn2
i has the effect of enhancing the hardenability of steel, but this fn2
If the value is less than 2.5%, the desired effect of improving hardenability cannot be obtained, so the amount of pearlite produced increases and the toughness decreases. On the other hand, when the value of fn2 exceeds 4.5%, the hardenability becomes too high, which promotes the formation of the island martensite structure, and rather reduces the toughness. Therefore, in the present invention, fn2 represented by the equation (2) is 2.5 to 4 .
It was set to 5%. In addition, Mn is 0.4 to 3.5% and Cr is 0.
~ 3.0%, Ni content 0 ~ 2.0%,
Within the range, the above fn2 satisfies 2.5 to 4.5%.
You can adjust it like this .

Since P causes grain boundary segregation and significantly deteriorates toughness, P as an impurity element in the steel of the present invention.
Is preferably 0.05% or less from the viewpoint of ensuring the toughness of steel.

(B) Size and Amount of Ti Carbosulfide, Zr Carbosulfide The machinability of the non-heat treated steel material having the above chemical composition is enhanced by Ti carbosulfide and Zr carbosulfide, and good strength- To ensure toughness balance, the size and cleanliness of Ti carbosulfide and Zr carbosulfide (sum of cleanliness of Ti carbosulfide and Zr carbosulfide when Ti and Zr are added together) It is important to optimize the amount represented by.

If the maximum diameter of Ti carbosulfide and Zr carbosulfide in steel exceeds 10 μm, the toughness is reduced. The maximum diameters of both Ti carbosulfide and Zr carbosulfide are preferably 7 μm or less. Ti carbosulfide and Z
If the maximum diameter of r carbosulfide is too small, the machinability improving effect will be reduced. Therefore, the lower limit of the maximum diameter of Ti carbosulfide and Zr carbosulfide is preferably about 0.5 μm.

In the invention of (1), the maximum diameter is 10 μm.
When the amount of Ti carbosulfide of m or less is less than 0.05% in cleanliness, the effect of improving the machinability by Ti carbosulfide cannot be exhibited. Therefore, in the invention of (1), the cleanliness is 0.05% when the maximum diameter of Ti carbosulfide is 10 μm or less.
That's it. The cleanliness degree is preferably 0.08% or more. If the cleanliness value of the above Ti carbosulfide is too large, the toughness decreases, so the upper limit value of the cleanliness of the above Ti carbosulfide is preferably about 2.0%.

In the invention of (2), the maximum diameter is 10 μm.
When the sum of the amounts of Ti carbosulfide and Zr carbosulfide of m or less is less than 0.05% in terms of cleanliness, Ti carbosulfide and Zr
The machinability improving effect of carbosulfide cannot be exhibited. Therefore, in the invention of (2), Ti carbosulfide and Zr
The maximum diameter of carbosulfide was 10 μm or less, and the sum of the amounts was 0.05% or more in terms of cleanliness. The sum of the cleanliness is preferably 0.08% or more. T above
If the sum of the cleanliness of i carbosulfide and Zr carbosulfide is too large, the toughness decreases, so the upper limit of the above sum of cleanliness is preferably about 2.0%.

The forms of the Ti carbosulfide and the Zr carbosulfide as described above are basically determined by the contents of Ti, Zr, S and N. However, in order to set the size and cleanliness (sum of cleanliness) of Ti carbosulfide and Zr carbosulfide to the above-mentioned values, it is important to prevent excessive generation of oxides of Ti and Zr. is there. For this purpose, it may not be sufficient for the steel to have the chemical composition described in the above section (A). Therefore, for example, sufficient deoxidation with Si and Al, and finally T
A steelmaking method in which i or Zr is added may be adopted.

For Ti carbosulfide and Zr carbosulfide, a test piece taken from a steel material was mirror-polished, and the polished surface was observed under an optical microscope at a magnification of 400 times or more. It can be easily distinguished from other inclusions. That is,
When observed under an optical microscope under the above-mentioned conditions, the color of Ti carbosulfide and Zr carbosulfide is extremely light gray, and the “shape” is granular (spherical) corresponding to JIS B-type inclusions and C-type inclusions. ). Detailed determination of Ti carbosulfide and Zr carbosulfide can also be performed by observing the test surface with an electron microscope having an analysis function such as EDX (energy dispersive X-ray analyzer).

As described above, the cleanliness of Ti carbosulfide and Zr carbosulfide is defined by JIS G 0555, "Microscope of non-metallic inclusions in steel," when the magnification of the optical microscope is 400 times. "Test method" means the value measured in 60 fields of view.
The maximum diameters of Ti carbosulfide and Zr carbosulfide may also be examined by observing 60 fields of view with an optical microscope having a magnification of 400 times.

Further, as described above, Ti carbosulfide and Zr carbosulfide produced during steelmaking do not form a solid solution in the matrix at the heating temperature for ordinary hot working. Therefore, a so-called "pinning effect" is exhibited in the austenite region, which is also effective in preventing coarsening of austenite grains.

(C) Structure of Steel Material Non-conditioning having Ti carbosulfide and Zr carbosulfide of the chemical composition described in the above (A) and the size and amount (cleanliness) described in the above (B). In order to obtain an excellent strength-toughness balance even in a high quality steel material, 90% or more of the structure must be bainite or ferrite bainite structure. As a manufacturing method therefor, for example, steel slab 105
After heating to a temperature higher than 0 ° C and lower than or equal to 1300 ° C, hot working such as hot forging is performed and finished at a temperature higher than or equal to 900 ° C, and then at a cooling rate of 60 ° C / min or lower, at least 300 ° C. There is a process of air cooling or cooling. The chemical composition of (A) is designed so that a desired structure (90% or more of the structure is bainite or ferrite-bainite structure) is formed when the steel material is cooled under the above conditions after hot working. Is.

Since the structure becomes finer and the strength-toughness balance becomes better as the forming ratio during hot working increases, it is preferable to set the forming ratio to 1.5 or more during the hot working. The term "molding ratio" as used in the present invention means (A ₀ / A) where A ₀ is the cross-sectional area before processing and A is the cross-sectional area after processing.
Refers to. In addition, when the crystal grain size of the old austenite grains in the organization is JIS grain size number 4 or more, the strength-toughness balance becomes excellent. The above-mentioned "former austenite grains" refer to austenite grains immediately before transformation and formation of bainite, ferrite, etc. undergoes heating and hot working. In the case of the non-heat treated steel material having the structure defined in the present invention, the old austenite grains can be easily determined by performing the nital corrosion and observing with an optical microscope.

After performing the above hot working and cooling, 200
When the aging treatment is performed by heating at a temperature of ˜700 ° C. for about 20 to 150 minutes, the strength-toughness balance becomes particularly excellent.

[0069]

【Example】

(Example 1) 150 kg of steel having the chemical composition shown in Tables 1 to 4
It was melted using a vacuum melting furnace. In order to prevent formation of Ti oxide, Ti was added at the end after deoxidizing sufficiently with Si and Al and adding various elements to adjust the size and cleanliness of Ti carbosulfide.

Steels 1 to 24 in Tables 1 and 2 are steels of the examples of the present invention having a chemical composition within the range specified by the present invention.
Steels 25 to 50 in Tables 3 and 4 are steels of comparative examples in which any one of the components is out of the range of the content specified in the present invention.

[0071]

[Table 1]

[0072]

[Table 2]

[0073]

[Table 3]

[0074]

[Table 4]

Next, these steels are heated to 1250 ° C. and then hot forged at 1000 ° C. to finish them to a diameter of 20.
mm round bar and a steel plate having a plate thickness of 12 mm were produced. This is because the round bar having a diameter of 20 mm and the steel plate having a plate thickness of 12 mm have the same structure. The cooling conditions after hot forging were air-cooled or allowed to cool to a cooling rate of 5 to 35 ° C / min and cooled to 300 ° C to adjust the structures of the round bar and the steel sheet.

From the center of the round bar obtained as described above, the tensile test piece of JIS No. 14A and 2 m of JIS No. 3 were measured.
The mU notch Charpy impact test piece was sampled and the tensile strength and the Charpy absorbed energy at room temperature were investigated. Also, JI
A test piece was sampled according to FIG. 5 of SG 0555, and the 300 mm ² test surface that had been mirror-polished was observed in 60 fields of view with an optical microscope with a magnification of 400 times to distinguish Ti carbosulfide from other inclusions. While measuring the cleanliness. Further, the maximum diameter of Ti carbosulfide was examined by observing 60 fields with an optical microscope having a magnification of 400 times. Further, the test piece was corroded with nital and the tissue was observed.

Machinability was also evaluated by a drilling test. That is, the steel plate finished to the thickness of 12 mm is subjected to surface grinding to reduce the thickness to 10 mm, and a through hole is formed in the thickness direction as a test piece, and the through hole is used when it becomes impossible to punch due to the abrasion of the blade edge. The machinability was evaluated by counting the number of. Drilling conditions are JIS high speed tool steel SKH51 φ5
Using a mm taper drill and a water-soluble lubricant, the feed was 0.20 mm / rev and the rotation speed was 980 rpm.

Tables 5 and 6 show the results of the above various tests. The proportion (%) of the structure in the structure column of the table is the ratio of bainite when ferrite is not contained in the structure when observed under an optical microscope, or the ratio of ferrite and bainite in the structure in which bainite and ferrite are mixed. It means the sum of proportions.

[0079]

[Table 5]

[0080]

[Table 6]

FIG. 1 shows steels 1 to 24 which are steels of the present invention and steels 25, 31 to 33 and 35 which are steels of comparative examples.
The relationship between the tensile strength of 43 and steel 46-50 and an impact value is shown. Further, in FIG. 2, steels 1 to 24, which are steels of the present invention, and
Steels 26 to 30, steels 32, 34, and steels 4 as comparative examples.
4 shows the relationship between the tensile strength of 4 and the steel 45 and machinability.

From Tables 5 and 6 and FIGS. 1 and 2, the steels 1 to 24 of the present invention have good toughness (impact value) and machinability (number of through holes) at each strength level. It is clear.

On the other hand, the steels of the comparative examples are inferior in at least one of toughness and machinability at each strength level. That is, steel 25, steel 31, steel 33, steel 3
5 to 43 and steels 46 to 50 are inferior in toughness, steels 26 to 30, steel 34, steel 44 and steel 45 are inferior in machinability, and steel 32 is inferior in both toughness and machinability.

(Example 2) A steel rod having a diameter of 20 mm and a steel plate having a thickness of 12 mm of steels 1 to 5 and steels 11 to 17 of the present invention obtained in the above Example 1 were aged at the temperatures shown in Table 7, respectively. Processed.

From the center of the round bar thus obtained, a tensile test piece of JIS No. 14A and a 2 mm U notch Charpy impact test piece of JIS No. 3 were sampled in the same manner as in Example 1, and the tensile strength at room temperature was measured. And Charpy absorbed energy was investigated. Further, the steel plate having a thickness of 12 mm was subjected to surface grinding to reduce the thickness to 10 mm, and the machinability was evaluated by a drilling test as in the case of Example 1.

Table 7 also shows the results of various tests.

[0087]

[Table 7]

From Table 7, for the steel of the present invention, 2
It is clear that when the aging treatment is performed at 00 to 700 ° C., the strength-toughness balance becomes particularly excellent.

Example 3 Steels having the chemical compositions shown in Tables 8 to 11 were melted using a 150 kg vacuum melting furnace. In addition, T
In order to prevent the formation of i oxide and Zr oxide, Ti and Z were added after deoxidizing sufficiently with Si and Al and adding various elements.
r was added to adjust the size and cleanliness (sum of cleanliness) of Ti carbosulfide and Zr carbosulfide.

Steels 51 to 74 in Tables 8 and 9 are steels of the examples of the present invention whose chemical composition is within the range specified in the present invention, and steels 75 to c in Tables 10 and 11 are any of their components. Is a steel of a comparative example out of the range of the content specified in the present invention.

[0091]

[Table 8]

[0092]

[Table 9]

[0093]

[Table 10]

[0094]

[Table 11]

Then, these steels were heated to 1250 ° C. and then hot forged at 1000 ° C. to give a diameter of 20.
mm round bar and a steel plate having a plate thickness of 12 mm were produced. As mentioned above, this is a round bar with a diameter of 20 mm and a plate thickness of 12 m.
This is because the steel sheet of m exhibits the same structure. The cooling conditions after hot forging were air-cooled or allowed to cool to a cooling rate of 5 to 35 ° C / min and cooled to 300 ° C to adjust the structures of the round bar and the steel sheet.

From the center of the round bar obtained as described above, a tensile test piece of JIS No. 14A and 2 m of JIS No. 3 were obtained.
The mU notch Charpy impact test piece was sampled and the tensile strength and the Charpy absorbed energy at room temperature were investigated. Also, JI
A test piece was sampled according to Fig. 5 of SG 0555 and the 300 mm ² test surface that had been mirror-polished was observed with an optical microscope at a magnification of 400 times for 60 fields of view to determine Ti carbosulfide and Zr carbosulfide. The cleanliness (sum of cleanliness) was also measured while distinguishing from the inclusions. The maximum diameters of Ti carbosulfide and Zr carbosulfide were also examined by observing 60 fields with an optical microscope at a magnification of 400 times. Further, the test piece was corroded with nital and the tissue was observed.

Machinability was also evaluated by a drilling test. That is, the steel plate finished to the thickness of 12 mm is subjected to surface grinding to reduce the thickness to 10 mm, and a through hole is formed in the thickness direction as a test piece, and the through hole is used when it becomes impossible to punch due to the abrasion of the blade edge. The machinability was evaluated by counting the number of. Drilling conditions are JIS high speed tool steel SKH51 φ5
Using a mm taper drill and a water-soluble lubricant, the feed was 0.20 mm / rev and the rotation speed was 980 rpm.

Tables 12 and 13 show the results of the above various tests. The percentage of organizations in the organization column of the table is
It means the proportion of bainite in the case where ferrite is not contained in the structure when observed under an optical microscope, or the sum of the proportions of ferrite and bainite in the structure in which bainite and ferrite are mixed. Further, in the column of “Ti, Zr carbosulfide”, when Ti and Zr are added together, “maximum diameter” is the value of the larger carbosulfide, and the cleanliness is the cleanliness of the cleanliness. Means Japanese.

[0099]

[Table 12]

[0100]

[Table 13]

From Tables 12 and 13, steels 51 to 51 of the present invention are shown.
It is clear that 74 has good toughness (Charpy absorbed energy) and machinability (number of through holes) at each strength level.

On the other hand, the steels of the comparative examples are inferior in at least one of toughness and machinability at each strength level. That is, steel 75, steel 81, steel 82, steel 8
4, steel 86 to 95 and steel 98 to c have toughness of steel 76 to 8
0, steel 85, steel 96 and steel 97 are inferior in machinability, and steel 83 is inferior in both toughness and machinability.

(Example 4) Steels 51 to 55 and steels 61 to 67 of the present invention obtained in Example 3 above were round-aged at a temperature shown in Table 14 and a round bar having a diameter of 20 mm and a steel plate having a thickness of 12 mm, respectively. Processed.

From the center of the round bar thus obtained, a tensile test piece of JIS No. 14A and a 2 mm U notch Charpy impact test piece of JIS No. 3 were sampled in the same manner as in Example 3, and the tensile strength at room temperature was measured. And Charpy absorbed energy was investigated. Further, the steel plate having a thickness of 12 mm was subjected to surface grinding to reduce the thickness to 10 mm, and the machinability was evaluated by a drilling test as in the case of Example 3.

Table 14 also shows the results of various tests.

[0106]

[Table 14]

It is apparent from Table 14 that the steel of the present invention has a particularly excellent strength-toughness balance when subjected to aging treatment at 200 to 700 ° C.

[0108]

The high-strength, high-toughness, free-cutting, non-heat treated steel of the present invention has both excellent toughness and machinability, and thus can be used as a material for machine structural parts and the like. This high-strength and high-toughness free-cutting non-heat treated steel material can be relatively easily manufactured at low cost.

[Brief description of drawings]

1] Steels 1 to 24 which are steels of the present invention used in Examples
In addition, steel 25, steel 31 to 33, and steel 3 which are steels of comparative examples
It is the figure which showed the tensile strength of 5 to 43 and steel 46 to 50, and the relationship of an impact value.

FIG. 2 Steels 1 to 24 which are steels of the present invention used in Examples
Also, steels 26 to 30, steels 32 and 3 which are steels of comparative examples.
It is the figure which showed the tensile strength and machinability of 4, steel 44, and steel 45.

Continued front page       (56) References JP-A-50-20917 (JP, A)                 JP-A-5-302116 (JP, A)                 JP 64-52020 (JP, A)                 JP-A-6-212349 (JP, A)                 JP-A-6-256895 (JP, A)                 Japanese Patent Publication 5-27685 (JP, B2)                 Japanese Patent Publication Sho 34-2405 (JP, B1)                 "Materials and processes" vol. 7               (1994) No. 6, Japan Iron and Steel Institute, P.               1839                 "Materials and processes" vol. 7               (1994) No. 3, Japan Iron and Steel Institute, P.               819                 "Materials and Processes", vol. 9               (1996) No. 3, Japan Iron and Steel Institute, P.               378

Claims (2)

(57) [Claims] 1. C: 0.05 to 0.3% by weight, S
i: 0.05 to 1.5% , Mn: 0.4 to 3.5% ,
S: 0.002-0.2%, Ti: 0.04-1.0
%, Al: 0.005-0.05%, N: 0.008%
Hereinafter, Cr: 0 to 3.0%, Ni: 0 to 2.0% , V:
0-0.3%, Nb: 0-0.1%, Mo: 0-0.5
%, Cu: 0 to 1.0%, B: 0 to 0.02%, Nd:
0-0.1%, Pb: 0-0.5%, Ca: 0-0.0
1%, Se: 0 to 0.5%, Te: 0 to 0.05%, B
i : 0 to 0.4%, fn1 represented by the following formula (1) is 0%
And fn2 represented by the following formula (2) is 2.5 to 4.5.
%, The balance is the chemical composition of Fe and unavoidable impurities, the maximum diameter of Ti carbosulfide in the steel is 10 μm or less, and the amount is 0.05% or more in cleanliness and 90% or more of the structure. A high-strength, high-toughness, free-cutting, non-heat treated steel with a bainite or ferrite bainite structure. fn1 = Ti (%)-1.2S (%) (1) fn2 = 0.5Si (%) + Mn (%) + 1.13Cr
(%) + 1.98Ni (%) ・ ・ ・ ・ ・(2) 2. By weight%, C: 0.05-0.3%, S
i: 0.05 to 1.5% , Mn: 0.4 to 3.5% ,
S: 0.002~0.2%, Ti: 0~1.0 %, Z
r: 1.0% or less and Ti (%) + Zr (%):
0.04 to 1.0%, Al: 0.005 to 0.05%,
N: 0.008% or less, Cr: 0 to 3.0%, Ni: 0
~ 2.0% , V: 0 ~ 0.3%, Nb: 0 ~ 0.1%,
Mo: 0-0.5%, W: 0-0.8%, Cu: 0-
1.0%, B: 0 to 0.02%, Nd: 0 to 0.1%,
Pb: 0-0.5%, Ca: 0-0.01%, Se: 0
~ 0.5%, Te: 0 to 0.05%, Bi: 0 to 0.4
%, Fn3 represented by the following formula (3) exceeds 0%, and the following (2)
Fn2 represented by the formula is 2.5 to 4.5%, the balance is the chemical composition of Fe and unavoidable impurities, the maximum diameter of Ti carbosulfide and Zr carbosulfide in steel is 10 μm or less, and The sum of the amount of cleanliness is 0.05% or more,
A high-strength, high-toughness, free-cutting, non-heat treated steel with a bainite or ferrite-bainite structure of which% or more. fn3 = Ti (%) + Zr (%)-1.2S (%) ...
(3) fn2 = 0.5Si (%) + Mn (%) + 1.13Cr
(%) + 1.98Ni (%) ・ ・ ・ ・ ・(2)