WO1998023784A1

WO1998023784A1 - Steel having excellent machinability and machined component

Info

Publication number: WO1998023784A1
Application number: PCT/JP1997/004297
Authority: WO
Inventors: Koji Watari; Yasutaka Okada
Original assignee: Sumitomo Metal Industries, Ltd.
Priority date: 1996-11-25
Filing date: 1997-11-25
Publication date: 1998-06-04
Also published as: CA2243123A1; EP0903418B1; CN1205036A; KR19990076784A; CA2243123C; EP0903418A4; EP0903418A1; CN1095503C; KR100268536B1; DE69718784D1; DE69718784T2

Abstract

A steel with excellent machinability containing 0.05 - 0.6 wt.% of C, 0.002 - 0.2 wt.% of S, 0.04 - 1.0 wt.% of Ti, not more than 0.008 wt.% of N, 0 - 0.1 wt.% of Nd, 0 - 0.5 wt.% of Se, 0 - 0.05 wt.% of Te, 0 - 0.01 wt.% of Ca, 0 - 0.5 wt.% of Pb and 0 - 0.4 wt.% of Bi. The maximum diameter of titanium carbide/sulfide in this steel is not larger than 10 νm and its content is not less than 0.05 % in terms of cleanness. The steel is a suitable material for structural components of various machines such as automobiles and other transportation machines, industrial machines, building machines, etc., for instance, crankshafts, connecting rods, gears, etc. which are subjected to machining.

Description

Description: Steel with excellent machinability and machined parts

The present invention relates to a steel material excellent in machinability and a part processed by cutting. More specifically, steel materials that have good machinability and are suitable as materials for structural parts of various machines such as automobiles and other transport machinery, industrial machinery, construction machinery, etc. Shafts, connecting rods, gears, etc., related to various machine structural parts machined by cutting. Background art

Conventionally, various mechanical structural parts such as transport machines, industrial machines, and construction machines are: (a) rough-worked to a predetermined shape by hot working, and then finished to a desired shape by cutting. In general, quenching and tempering were performed, or (b) hot working and quenching and tempering were performed, followed by cutting to obtain a desired shape.

However, as the strength of mechanical structural parts has increased, the cost of cutting has increased. Therefore, there is an increasing demand for free-cutting steel with excellent machinability in order to facilitate cutting and reduce costs.

It is well known that machinability can be improved by adding free-cutting elements (machinability improving elements) such as Pb, Te, Bi, Ca and S, singly or in combination, to steel. For this reason, conventionally, there has been adopted a method of improving the machinability by adding the above-mentioned free-cutting elements to steels for machine structural use and the like. However, simply adding a free-cutting element to steel for machine structural use and the like often fails to ensure desired mechanical properties (for example, toughness / fatigue strength).

Under these circumstances, after the hot working of (a) above, Techniques for performing a tempering process are disclosed in, for example, JP-A-2-111842 and JP-A-6-279849. In other words, C in steel is present as graphite, and the machinability is improved by utilizing the notch and lubrication effects of this graphite. “Hot rolled steel with excellent machinability and hardenability” ”And“ Method for producing machine structural steel with excellent machinability ”.

However, in the steel material proposed in Japanese Patent Application Laid-Open No. 2-111842, the addition of B is essential because B is added and B nitride (BN) is used as graphite precipitation nuclei to promote graphitization. However, there is a problem that cracks are likely to occur during solidification. On the other hand, the method described in Japanese Patent Application Laid-Open No. 6-279849 promotes graphitization in the hot-rolled state by controlling the content of steel (oxygen) to be low together with the addition of A1. For this reason, it is necessary to perform a graphitizing annealing treatment for 5 hours or more after hot rolling, which is not necessarily economical.

On the other hand, a technique of performing the hot working and the quenching / tempering treatment (b) and then performing a cutting work is disclosed in, for example, JP-A-6-212347. This is "a hot forged product having high fatigue strength and a method for producing the same", in which steel having a specific chemical composition is quenched immediately after hot forging and then tempered to precipitate TiC. However, the hot forgings described in this publication merely specify the amount of N as the chemical composition of steel as less than 0.1 in N / Ti, which is the ratio to the amount of Ti, so that it is not necessarily a good coating. In some cases, it may not be possible to secure machinability. In other words, if the N content of steel containing 0.01 to 0.20% Ti in weight% is specified as less than 0.1 in NZTi, a large amount of hard TiN is formed and the machinability increases. In some cases, and also in toughness.

For iron and steel (Vo 1.57 (1971) S484), it has been reported that machinability may be enhanced by adding Ti to deoxidized adjusted free-cutting steel ₍ but It is also stated that the addition of a large amount of Ti increases tool wear due to the large amount of TiN generated, which is not desirable from the viewpoint of machinability ₍ For example, C: 0.45%, S i: 0.29%, Mn: 0.78%, P: 0.017%, S: 0.041%, A1: 0.006%, N : 0.00

In the case of I containing 87%, Ti 0.228%, 〇: 0.004% and Ca: 0.001%, the drill life is rather shortened and the machinability is poor. Thus, the mere addition of Ti to I does not improve machinability. Disclosure of the invention

An object of the present invention is to provide a steel material which has good machinability and is suitable as a material for structural parts of various machines such as automobiles, transportation machines, industrial machines, construction machines, and the like. We provide a variety of machine structural parts that have been machined by cutting, such as crankshafts, connecting ports, and gears.

The gist of the present invention is as follows.

(I) By weight%, C: 0.05 to 0.6%, S: 0.002 to 0.2%, Ti: 0.04 to; L. 0%, N: 0.008% or less, Nd : 0 to 0.1%, Se: 0 to 0.5%, Te: 0 to 0.05%, Ca: 0 to 0.01%, Pb: 0 to 0.5%, Bi : With a chemical composition containing 0 to 0.4%, the maximum diameter of Ti carbosulfide in steel is 10m or less and its amount is 0.05% or more in cleanliness. Excellent machinability Steel.

(Π) C: 0.2 to 0.6%, S i: 0.05 to 1.5%, Mn: 0.1 to 2.0%, P: 0.07% or less, S: 0.0 1 to 0.2%, A1: 0.002 to 0.05%, Cu: 0 to 1.0%, Ni: 0 to 2.0%, Cr: 0 to 2.0%, Mo: 0 ~ 0.5%, V: 0 ~ 0.3%, Nb: 0 ~ 0.1%, with the balance being Fe and unavoidable impurities. The composition is more than 90% The non-heat treated steel material according to the above (I), which is a parent light. (ΠΙ) C: 0.05-0.3%, Si: 0.05-; L. 5%, Al: 0.002-0.05%, Cu: 0-1.0%, Mo: 0 to 0.5%, V: 0 to 0.30%, Nb: 0 to 0.1%, B: 0 to 0.02%, the value of fn 3 expressed by the following formula is 2. 5 to 4.5%, the balance being Fe and unavoidable impurities, and 90% or more of the tissue is bainite or ferrite and bainite. Non-heat treated steel.

f n 3 = 0.5 S i (%) + Mn (%) +1.13 C r (%) +1.98 N i (%)

(IV) C: 0.1 to 0.6%, Si: 0.05 to 1.5%, Mn: 0.4 to 2.0%, A1: 0.002 to 0.05%, C u: 0 to 1.0%, Ni: 0 to 2.0%, Cr: 0 to 2.0%, Mo: 0 to 0.5%, V: 0 to 0.3%, Nb: 0 0.1%, B: 0 to 0.02%, the balance being a chemical composition composed of Fe and unavoidable impurities, and 50% or more of the tissue is martensite, as described in (I) above. Tempered steel material.

(V) The steel material described in (I) above is used as the material, and is processed by cutting.

(I) Parts made from the non-heat treated steel material described in (i) above and processed by cutting.

() Parts made from the non-heat treated steel described in (m) above and processed by cutting.

() Parts made from the tempered steel described in (IV) above and processed by cutting.

Note that the “Ti carbosulfide” in the present invention includes simple Ti sulfide.

As used herein, "maximum diameter (of Ti carbosulfide)" refers to "the longest diameter of an individual Ti carbosulfide." The cleanliness of Ti carbosulfide is a value obtained by measuring 60 fields of view using the “microscopic test method for nonmetallic inclusions in steel” specified in JIS G 0555, with the magnification of an optical microscope set to 400 times.

In this specification, the term "non-heat treated steel" refers to a steel material in which "quenching and tempering" as a so-called "temper treatment" is omitted. In addition to “available steel material”, it includes “a steel material that has been subjected to aging treatment equivalent to tempering after cooling after hot working”. “Tempered steel” refers to steel that has been “quenched and tempered”.

The tissue ratio means the tissue ratio when observed under a microscope, that is, the area ratio.

Regarding (iv) above, “90% or more of ferrite and perlite” means that the sum of the proportions of frite and perlite is 90% or more in an organization where ferrite and perlite are mixed. It means something.

Regarding (上記) above, “90% or more of bainite” refers to a state in which bainite occupies 90% or more of the organization when no ferrite is included in the organization. “Bainite is 90% or more” means that the sum of the proportions of ferrite and bayite is 90% or more in an organization where vanite and ferrite are mixed.

Regarding (IV) above, “50% or more of martensite” means that 50% or more of the organization is martensite. The above (IV) relates to “tempered steel material” which has been quenched and tempered. Therefore, the above martensite refers to a tempered martensite, that is, “tempered martensite”. , Simply referred to as “martensite”. BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors have found that the chemical composition and microstructure of steel affect machinability and mechanical properties. The experiment was repeated to examine the effects.

As a result, first, (a) an appropriate amount of Ti was added to the steel, (b) sulfide was changed to Ti carbosulfide to control inclusions in the steel, and (c) the Ti carbosulfide was refined. It has been found that if dispersed, the machinability of steel materials will be dramatically improved.

Therefore, as a result of further research, the following items (d) to (p) were found. _C (d) When Ti was actively added to steel containing an appropriate amount of S, Ti coal was added to the steel. Sulfides are formed.

(e) When the above-mentioned Ti carbosulfide is formed in the steel, the amount of MnS generated decreases.

(f) For the same S content in steel, Ti carbosulfide has a greater machinability improvement effect than MnS. This is based on the fact that the melting point of Ti carbosulfide is lower than that of MnS, so that the lubrication of the rake face of the tool during cutting is increased.

(g) In order to sufficiently exhibit the machinability improving effect of Ti carbosulfide, it is important to limit the N content to as low as 008% or less to suppress the precipitation of TiN.

(h) Controlling the N content leads to a decrease in TiN in the steel, which can also increase the toughness of the mechanical properties.

(i) In order to enhance machinability with Ti carbosulfide, it is necessary to optimize the size of Ti carbosulfide and the amount expressed by its cleanliness (hereinafter simply referred to as “cleanliness”). is important.

(j) Ti carbosulfide generated during steelmaking does not form a solid solution in the matrix at the normal heating temperature for hot working and the normal heating temperature for tempering. Therefore, Ti carbosulfide exhibits a so-called “pinning effect” in the austenite region, and is effective in preventing austenite grains from becoming coarse. Of course, Ti carbosulfide is used for normal tempering in tempering. Does not dissolve in the matrix even at the heating temperature or the heating temperature for aging treatment equivalent to tempering.

(k) In steel materials in which 90% or more of the microstructure is frit and parlite, bending and residual stress due to transformation strain are extremely small.

(1) The balance between strength and toughness is good for steel materials in which 9% or more of the structure is bainite, or steel with bainite and bainite.

(m) The balance between strength and toughness of a steel material in which 50% or more of the structure is martensite is extremely good.

(n) Non-heat treated steel with a specific chemical composition and 90% or more of the structure is made of graphite and pearlite.The proportion of the fluoride is 20 to 70% in area ratio and the particle size of ferrite. If the JIS particle size number is 5 or more, the average value of the pearlite lamellar spacing is 0.2 or less, and at least one of the conditions is satisfied, a good balance between strength and toughness can be obtained.

(o) When the value of fn 1 represented by the following formula (1) is greater than 0% and / or when the value of fn 2 represented by the following formula (2) is greater than 2, The effect of improving the performance increases. When the value of f n 2 represented by the formula (2) is larger than 2, the pinning effect of Ti carbosulfide is increased, and a large strength and excellent toughness can be obtained.

f n 1 = Ti (%) — 1.2 S (%) (1)

f n 2 = Ti (%) / S (%) (2)

() The value of fn 3 expressed by the following formula (3) correlates with the structure and toughness of non-heat treated steel having a specific chemical composition. If this value is within a specific range, 90% of the structure The above is bainite, or ferrite and bainite. f n 3 = 0.5 S i (%) + n () +1.13 C r (%) +1.98 Ni (%) (3)

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

Hereinafter, each requirement of the present invention will be described in detail. The content of each element The “%” indication of the amount means “% by weight”.

(A) Chemical composition of steel

C:

C combines with S with Ti to form Ti carbosulfide and has an effect of enhancing machinability. Further, C is an effective element for securing strength. However, if the content is less than 0.05%, these effects cannot be obtained. On the other hand, if C is contained in excess of 0.6%, the toughness will decrease. Therefore, the content of C is set to 0.05 to 0.6%.

Note that the C content of non-heat treated steel materials (hereinafter referred to as “condition X steels” for simplicity) in which 90% or more of the structure is fly and pearlite shall be 0.2 to 0.6%. It is more preferably 0.25 to 0.5%.

The C content of non-heat treated steel materials (90% or more of the structure is bainite or fly and bainite (hereinafter referred to as “condition Y steel” for simplicity)) is 0.05 to 0.3. %, Preferably 0.:! More preferably, it is set to 0.24%.

It is preferable that the C content of a tempered steel material in which 50% or more of the structure is martensite (hereinafter referred to as “condition Z steel” for simplicity) is 0.1 to 0.6%.

S:

S combines with C and Ti with Ti to form Ti carbosulfide and has the effect of improving machinability. However, if the content is less than 0.002%, the effect cannot be obtained.

Conventionally, the purpose of adding S to free-cutting steel has been to improve the machinability by forming MnS. However, according to the study by the present inventors, it has been found that the above-described action of improving the machinability of MnS is based on a function of enhancing lubricity between chips and the tool surface during cutting. Moreover, Mn S has become huge, The flaws may be enlarged, resulting in defects.

The machinability improving effect of S in the present invention can be obtained for the first time by forming Ti carbosulfide by complex addition of an appropriate amount of C and Ti. For this purpose, an S content of 0.002% or more is required as described above. On the other hand, the effect on machinability does not change even if S is contained in excess of 0.2%, but coarse MnS is regenerated in the steel, causing problems such as ground flaws. I will. In addition, the hot workability is significantly deteriorated, making hot plastic working difficult and toughness may be reduced. Therefore, the content of S is set to 0.002 to 0.2%.

The S content of the “steel material under the condition X” is preferably 0.01 to 0.2%, more preferably 0.02 to 0.17%.

The S content of the “steel material of condition Y” is preferably set to 0.005 to 0.17%.

Ti:

Ti is an important alloying element for controlling inclusions in the present invention. If the content is less than 0.04%, S cannot be sufficiently converted to Ti carbosulfide, so that machinability cannot be enhanced. On the other hand, if the content exceeds 1.0%, not only the machinability improvement effect is saturated but the cost is increased, but also the toughness and hot workability are significantly deteriorated. Therefore, the Ti content is set to 0.04 to: L. 0%.

The Ti content of the “steel under condition X” is preferably in the range of 08 to 0.8%.

It is preferable that the Ti content of the “steel material with condition γ” be 0.06 to 0.8%.

It is preferable that the Ti content of the “steel material of condition Z” is also 0.06 to 0.8%.

N: 0.008% or less In the present invention, it is extremely important to control the N content low. That is, since N has a large affinity for Ti, it easily bonds to Ti to form Ti N and fixes Ti, so that when N is contained in a large amount, the above-mentioned Ti carbosulfide is used. The effect of improving machinability cannot be fully exhibited. Further, coarse TiN reduces toughness and machinability. Therefore, the N content was set to 0.008% or less. In order to enhance the effect of Ti carbosulfide, the upper limit of the N content is preferably set to ◦ 0.006%.

Nd:

Nd need not be added. When added, Nd ₂ S ₃ has the effect of tip shake and has the effect of improving machinability. Furthermore, as Nd ₂ S ₃ is finely dispersed and generated in the relatively high temperature range of the molten steel, the growth of austenite grains during heating for hot working and quenching in the subsequent process is suppressed. It also has the effect of making the structure finer and increasing the strength and toughness of the steel. In order to surely obtain the above-mentioned effects, the content of Nd is preferably 0.005% or more. However, if the content exceeds 0.1%, Nd ₂ S ₃ itself becomes coarse, resulting in a decrease in toughness. Therefore, the content of Nd was set to 0 to 0.1%. The preferred upper limit of the Nd content is 0.08%. s ^e:

Se need not be added. If added, it has the effect of further improving the machinability of the steel. In order to surely obtain this effect, it is preferable that the content of ≤6 is 0.1% or more. However, if the content exceeds 0.5%, the above-mentioned effect is only saturated, and instead, coarse inclusions are formed and the fatigue strength and / or toughness are reduced. Therefore, the content of Se was set to 0 to 0.5%.

Te: Te need not be added. If added, it has the effect of further increasing the machinability of steel. To ensure this effect, the content of Te is preferably 0.005% or more. However, if the content exceeds 0.05%, not only the above-mentioned effect is saturated, but rather coarse inclusions are formed, resulting in a decrease in fatigue strength and Z or toughness. Furthermore, the addition of a large amount of Te causes deterioration of hot workability, and particularly when the content exceeds 0.05%, flaws are generated on the surface of the hot-worked steel material. Therefore, the content of Te was set to 0 to 0.5%.

C a:

Ca need not be added. If added, it has the effect of greatly enhancing the machinability of steel. To ensure this effect, it is preferable that Ca has a content of 0.001% or more. However, if the content exceeds 0.01%, not only the above-mentioned effect is saturated, but rather coarse inclusions are formed and the fatigue strength and Z or toughness are reduced. Therefore, the content of Ca was set to 0 to 0.01%.

P b:

Pb need not be added. If added, it has the effect of further increasing the machinability of the steel. To ensure this effect, the content of Pb is preferably set to 0.05% or more. However, when the content exceeds 0.5%, not only the above-mentioned effect is saturated, but rather, coarse inclusions are formed and the fatigue strength and the Z or toughness are reduced. Furthermore, the addition of a large amount of Pb causes deterioration of hot workability, and particularly when the content exceeds 0.5%, flaws are generated on the surface of the hot-worked steel material. Therefore, the content of Pb was set to 0 to 0.5%.

B i ：

B i may not be added. When added, it has the effect of greatly improving the machinability of steel. To ensure this effect, the content of Bi is preferably 0.05% or more. However, its content exceeds 0.4% As a result, the above effects are saturated, and instead the force is generated, and coarse inclusions are generated, resulting in a decrease in fatigue strength and Z or toughness. Further, the hot workability is deteriorated, so that the surface of the hot worked steel material is flawed. Therefore, the Bi content was set to 0 to 0.4%.

As far as the machinability is concerned, the “steel material excellent in machinability” of the present invention has already been described (: S, Ti, N, Nd, Se, Te, Ca, Pb and elements other than Bi) However, there is no need to impose any special restrictions on steel, but steel materials often require other properties in addition to machinability, such as reducing the occurrence of bending and residual stress due to transformation strain. In such a case, it is necessary to improve the balance between strength and toughness.In such a case, C, S, Ti, N, Nd, Se, Te, Ca, What is necessary is just to determine the chemical composition of elements other than Pb and Bi.

Hereinafter, regarding the chemical composition of elements other than C, S, Ti, N, Nd, Se, Te, Ca, Pb, and Bi, the above-mentioned "steel material of condition X", "steel material of condition Y" and The explanation is made separately for the case of “steel material of condition Z”.

(A-1) Non-heat treated steel material (“Condition X steel”) in which 90% or more of the structure is made of fly and powder

S i:

Si has the effect of deoxidizing steel and strengthening the frit phase. In addition, the increase in the Si content enhances the lubricating effect on the chip surface during cutting and extends the tool life, thereby improving the machinability. However, if the content is less than 0.05%, the effect of addition is poor. On the other hand, if the content exceeds 1.5%, not only the above effect is saturated, but also the toughness deteriorates. Therefore, the content of S i is preferably set to 0.05 to 1.5%. The content of S i is preferably 0.3 to 1.3%, and more preferably 0.5 to 1.3%.

Mn: Mn has an effect of improving fatigue strength by solid solution strengthening. However, if the content is less than 0.1%, it is difficult to obtain the effect. On the other hand, if the Mn content exceeds 2.0%, the durability ratio (fatigue strength Z tensile strength) and the yield ratio (yield strength Z tensile strength) may be reduced in the case of the “steel material of condition X”. . Therefore, the content of Mn is preferably set to 0.1 to 2.0%. The Mn content is preferably set to 0.4 to 2.7%, and more preferably set to 0.5 to 1.7%.

P:

P may be added intentionally. This is because “Steel under condition X” has the effect of increasing the tensile strength and fatigue strength. In order to ensure this effect, it is preferable that the content of P be 0.01% or more. However, if the content exceeds 0.07%, the toughness is remarkably deteriorated, and the hot workability is further reduced. Therefore, the content of P is preferably set to 0.07% or less. In addition, when P is added positively, the content is preferably set to 0.015 to 0.05%.

A1:

A1 is an effective element for deoxidizing steel. However, if the content is less than 0.002%, it is difficult to obtain the desired effect. If the content exceeds 0.05%, the effect is saturated and the machinability may be reduced. Therefore, the content of A1 is preferably set to 0.002 to 0.05%. Preferably, the content of A1 is 0.005 to 0.03%.

C u:

Cu need not be added. If added, it has the effect of improving the strength of the steel, especially the fatigue strength, by precipitation strengthening. To ensure this effect, it is preferable that the content of Cu be 0.2% or more. However, if the content exceeds 1.0%, in addition to the deterioration of hot workability, precipitates may be coarsened and the above-mentioned effects may be saturated or, on the contrary, may be reduced. Furthermore, The costs are only increasing. Therefore, the content of Cu is preferably set to 0 to 1.0%.

Ni:

Ni need not be added. If added, it has the effect of increasing the strength. _C To ensure this effect, the Ni content is preferably at least 0.02%. However, if the content exceeds 2.0%, this effect is saturated and the cost increases. Therefore, the content of Ni is preferably set to 0 to 2.0%.

C r:

Cr need not be added. If added, it has the effect of improving fatigue strength by solid solution strengthening. To ensure this effect, the content of Cr is preferably set to 0.02% or more. However, if the content exceeds 2.0%, in the case of “steel material of condition X”, the durability ratio and the yield ratio may decrease. Therefore, the content of Cr is preferably set to 0 to 2.0%. In addition, when Cr is added, its content is preferably in the range of 0.05 to 1.5%.

Mo:

Mo may not be added. If added, it has the effect of refining the microstructure of ferrite and pearlite and improving the strength of the steel, especially the fatigue strength. To ensure this effect, the Mo content is preferably set to 0.05% or more. However, if the content exceeds ◦ .5%, the structure after hot working is rather abnormally coarsened and the fatigue strength is reduced. Therefore, the content of Mo is preferably set to 0 to 0.5%.

V:

V need not be added. If added, it precipitates as fine nitrides and carbonitrides, and has the effect of improving the strength of steel, especially fatigue strength. To ensure this effect, V should be contained at a content of 0.05% or more. No. However, if the content exceeds 0.3%, the precipitates become coarse, so that the above-mentioned effects may be saturated or may be rather reduced. In addition, raw material costs are only increasing. Therefore, the content of V is preferably set to 0 to 0.3%.

b:

N b need not be added. If added, it precipitates as fine nitrides or carbonitrides, preventing the austenite grains from coarsening and has the effect of improving the strength of the steel, particularly the fatigue strength. In order to surely obtain this effect, the content of Nb is preferably set to 0.05% or more. However, if the content exceeds 0.1%, the above-mentioned effects are only saturated, and coarse hard carbonitrides are produced, which may damage the tool and cause a decrease in machinability. Therefore, the content of Nb is preferably set to 0 to 0.1%. The upper limit of the Nb content is preferably set to 0.05%.

f n 1, f n 2:

As described above, when the value of fn 1 represented by the above equation (1) is much larger than 0% and when the value of Z or fn 2 represented by the above equation (2) is larger than 2, T i The effect of carbosulfide on machinability improvement is great. Further, when the value of f n 2 represented by the equation (2) is larger than 2, the pinning effect of the Ti carbosulfide increases, and the tensile strength and fatigue strength increase. Therefore, it is preferred that the value of f n1 be greater than 0% or that the value of f n 2 be greater than 2. The upper limits of the values of f n1 and n 2 are not particularly defined, but may be the upper limits from the component plane.

By the way, 〇 (oxygen) as an impurity element forms hard oxide-based inclusions, which may damage the cutting tool during cutting and reduce machinability. In particular, when the 〇 content exceeds 0.015%, the machinability may be significantly reduced. Therefore, in order to maintain good machinability, the content of 〇 as an impurity element is preferably set to 0.015% or less. No. The content of 〇 is more preferably not more than 0.01%. (A -2) Non-heat treated steel material (“Condition Y steel”) in which 90% or more of the organization is bainite or ferrite and payite

S i:

Si has the effect of increasing the deoxidizing and hardenability of steel. Further, in the case of “steel material of condition Y”, the machinability is improved because the lubrication of the chip surface during cutting is increased and the tool life is extended with the increase of the Si content. However, if the content is less than 0.05%, the effect of addition is poor. On the other hand, if it exceeds 1.5%, not only the above effect is saturated but also the toughness is deteriorated. Therefore, the content of Si is preferably set to 0.05 to 1.5%. In addition, the Si content is preferably set to 0.5 to 1.3%.

A1:

A1 is a strong deoxidizing element. In order to secure the effect, the content is preferably 0.002% or more. However, if the content exceeds 0.05%, the effect is saturated and the cost is increased. Therefore, the content of A1 should be 0.002 to 0.05%. Preferably, the A1 content is 0.005 to 0.04%.

C u:

Cu need not be added. If added, it has the effect of increasing the strength of the steel without lowering the toughness and further enhancing the machinability. In order to surely obtain this effect, the content of Cu is preferably set to 0.2% or more. However, if the content exceeds 1.0%, in addition to deterioration of hot workability, precipitates may become coarse and the above-mentioned effects may be saturated, or toughness may be reduced. In addition, costs are only increasing. Therefore, the content of Cu is preferably set to 0 to 1.0%.

Mo:

Mo may not be added. If added, it will enhance hardenability and It has the effect of refining the structure and improving the strength of the steel. To ensure this effect, the content of Mo is preferably set to 0.05% or more. However, if its content exceeds 0.5%, the structure after hot working is rather abnormally coarsened and the toughness is reduced. For this reason, the content of Mo is preferably set to 0 to 0.5%.

V:

V need not be added. If added, it precipitates as fine nitrides and carbonitrides, and has the effect of increasing the strength of steel and enhancing the lubricity of chips during cutting to improve machinability. In order to surely obtain such an effect, it is preferable that the content of V is ◦ .05% or more. However, if the content exceeds 0.30%, the precipitates become coarse, so that the above effects may be saturated or the toughness may be reduced. In addition, raw material costs are only increasing. Therefore, the content of V is preferably set to 0 to 0.30%.

Nb:

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

B:

B need not be added. If added, it has the effect of improving the hardenability and increasing the strength and toughness of the steel. To ensure this effect, the B content is preferably 0.0003% or more. However, if the content exceeds 0.02%, the above effect may be saturated or, on the contrary, the toughness may be reduced. For this reason, the content of B is preferably set to 0 to 0.02%. fn 3:

As described above, the value of fn3 represented by the above equation (3) has a correlation with the structure and toughness of a non-heat treated steel having a specific chemical composition, and this value is 2.5 to 4.5. %, The main structure of the non-heat treated steel is bainite or ferrite and bainite, and a good balance of strength and toughness can be obtained. Si, Mn, Cr and Ni related to fn 3 have the effect of increasing the hardenability of steel.However, if the value of fn 3 is less than 2.5%, the desired effect of improving the hardenability is not obtained, and the toughness is reduced. May drop. On the other hand, if the value of fn3 exceeds 4.5%, the hardenability becomes too high, and on the contrary, the toughness may decrease. Therefore, the value of f n 3 represented by the equation (3) is preferably set to 2.5 to 4.5%. Note that the content of each element other than Si described above need not be particularly limited, since the above fn3 may satisfy 2.5 to 4.5%. However, the contents of Mn, Cr and Ni are preferably 0.4 to 3.5%, 3.0% or less, and 2.0% or less, respectively.

f n 1, f n 2:

As described above, also in the case of “steel material of condition Y”, when the value of fn 1 represented by the above equation (1) is larger than 0%, and when Z or fn 2 represented by the above equation (2), When the value of is larger than 2, the effect of improving the machinability of Ti carbosulfide increases. Further, when the value of f n 2 represented by the formula (2) is larger than 2, the pinning effect of Ti carbosulfide increases, and the tensile strength and fatigue strength increase. Therefore, it is preferred that the value of f nl be greater than 0% or that the value of f n 2 be greater than 2. The upper limits of the values of f n1 and f n2 are not particularly limited, and may be the upper limits from the component plane.

By the way, 〇 (oxygen) as an impurity element forms hard oxide-based inclusions, which may damage the cutting tool during cutting and reduce machinability. In particular, when the content exceeds 0.015%, In some cases, it may cause a significant decrease. Therefore, even in the case of “steel material of condition Y”, the content of 〇 as an impurity element is preferably set to 0.015% or less in order to maintain good machinability. The content of 〇 is more preferably not more than 0.01%.

Further, the content of P as an impurity element is preferably set to 0.05% or less from the viewpoint of securing the toughness of the steel.

(A -3) In the case of tempered steel material (“Steel with condition Z”) in which 50% or more of the structure is martensite

S i:

Si has the effect of increasing the deoxidizing and hardenability of steel. Furthermore, in the case of “steel material of condition Z”, the machinability is improved because the lubrication of the chip surface during cutting is increased and the tool life is prolonged as the Si content increases. However, if the content is less than ..05%, the effect of addition is poor, while if it exceeds 1.5%, not only the above effect is saturated, but also the toughness is deteriorated. Therefore, the content of S i is preferably set to 0.05 to 1.5%.

Mn:

Mn has the effect of improving the hardenability of steel and improving the fatigue strength by solid solution strengthening. However, if the content is less than 0.4%, the effect cannot be obtained. If the content exceeds 2.0%, not only this effect is saturated, but also the hardness becomes too hard to lower the toughness. Therefore, the content of Mn is preferably set to 0.4 to 2.0%.

A1:

A1 is a strong deoxidizing element. In order to secure the effect, the content is preferably 0.002% or more. However, if the content exceeds 0.05%, the effect is saturated and the cost is increased. Therefore, the content of A1 should be 0.002 to 0.05%. The A1 content is preferably 0.005 to 0.04%. C u:

Cu need not be added. If added, it has the effect of increasing the strength of the steel without lowering the toughness and further enhancing the machinability. To ensure this effect, it is preferable that the content of (11 is 0.2% or more. However, if the content exceeds 1.0%, the hot workability is deteriorated. However, the above-mentioned effect may be saturated or may be rather deteriorated due to coarsening of the precipitates, and the cost may be increased only, so that the content of Cu should be 0 to 1.0%. Is good.

N i:

Ni need not be added. If added, it has the effect of improving the hardenability of steel. To ensure this effect, the Ni content is preferably set to 0.02% or more. However, if the content exceeds 2.0%, this effect is saturated and the cost increases. Therefore, the content of Ni is preferably set to 0 to 2.0%.

C r:

Cr need not be added. If added, it has the effect of improving the hardenability of the steel and improving the fatigue strength by solid solution strengthening. To ensure these effects, it is preferable that the content of Cr is 0.03% or more. However, if the content exceeds 2.0%, not only the above-mentioned effects are saturated, but also the steel becomes too hard and the toughness is reduced. For this reason, the content of Cr is preferably set to 0 to 2.0%.

Mo:

Mo may not be added. If added, it has the effect of increasing the hardenability of steel. To ensure this effect, the content of Mo is preferably set to 0.05% or more. However, if the content exceeds 0.5%, not only this effect is saturated, but also the hardness becomes too high, the toughness is reduced, and the cost is increased. For this reason, the content of Mo is 0-0. A good value is 5%.

V:

V need not be added. If added, it precipitates as fine nitrides and carbonitrides, and has the effect of improving the strength of steel, especially fatigue strength. To ensure this effect, it is preferable that V has a content of 0.05% or more. However, when the content exceeds 0.3%, the precipitates are coarsened, so that the above-mentioned effects may be saturated or may be reduced. In addition, raw material costs are only increasing. Therefore, the content of V is preferably set to 0 to 0.3%.

Nb:

Nb need not be added. If added, it precipitates as fine nitrides or carbonitrides, prevents austenite grains from coarsening, and has the effect of improving the strength of steel, particularly fatigue strength and toughness. To ensure this effect, it is preferable that the content of Nb be 0.005% or more. However, if the content exceeds 0.1%, not only the above effect is saturated, but also coarse hard carbonitrides are generated, which may damage the tool and lower the machinability. Therefore, the content of Nb is preferably set to 0 to 0.1%. The upper limit of the Nb content is preferably 0.05%.

B:

B need not be added. If added, it has the effect of improving the hardenability and increasing the strength and toughness of the steel. To ensure this effect, the B content is preferably 0.0003% or more. However, if the content exceeds 0.02%, the above effect may be saturated or, on the contrary, the toughness may be reduced. For this reason, the content of B is preferably set to 0 to 0.02%.

f n 1, f n 2:

As described above, also in the case of the steel material of the condition Z, when the value of fn 1 represented by the above equation (1) is larger than 0%, and when the value of fn 1 is represented by the equation (2) or When the value of fn2 is larger than 2, the effect of improving the machinability of Ti carbosulfide becomes large. Further, when the value of fn 2 represented by the formula (2) is larger than 2, the pinning effect of Ti carbosulfide is increased, and the tensile strength and fatigue strength are increased. Therefore, it is preferable to make the value of fnl larger than 0% or make the value of fn 2 larger than 2. The upper limits of the values of n 1 and fn 2 are not particularly limited, and may be the upper limits from the component plane.

By the way, 〇 (oxygen) as an impurity element forms hard oxide-based inclusions, which may damage the cutting tool during cutting and reduce machinability. In particular, if the 〇 content exceeds 0.015%, the machinability may be significantly reduced. Therefore, even in the case of "steel material of condition Z", the content as an impurity element should be reduced in order to maintain good machinability.

It is good to be 0 15% or less. The content of 0 is more preferably set to 0.01% or less.

Furthermore, as an impurity element, the content of P is determined from the viewpoint of securing the toughness of steel.

It is better to make it 0.05% or less.

(B) Size and cleanliness of Ti carbosulfide

In order to improve the machinability of steel having the chemical composition described in (A) above with Ti carbosulfide, it is important to optimize the size and cleanliness of Ti carbosulfide. As described above, the term “Ti carbosulfide” in the present invention also includes a single Ti sulfide.

If the amount of Ti carbosulfide having a maximum diameter of 10 m or less is less than 0.05% in cleanliness, the effect of improving the machinability by Ti carbosulfide cannot be exhibited. The above-mentioned cleanliness is preferably set to 0.08% or more. If the cleanliness value of the above-mentioned Ti carbosulfide is too large, the fatigue strength may be reduced. Therefore, the upper limit of the cleanliness of the above-mentioned Ti carbosulfide is preferably about 2.0%. No. The reason for limiting the size of Ti carbosulfide to a maximum diameter of 10 m or less is that if it exceeds 10 m, the fatigue strength and Z or toughness are reduced. It is preferable that the maximum diameter of Ti carbosulfide be 7 ^ m or less. If the maximum diameter of Ti carbosulfide is too small, the effect of improving machinability will be reduced. Therefore, the lower limit of the maximum diameter of Ti carbosulfide is preferably about 0.5 μm.

The form of Ti carbosulfide is basically determined by the contents of Ti, S and N in the steel. However, to keep the size and cleanliness of Ti carbosulfide at specified values, it is important to prevent excessive formation of Ti oxides. For this purpose, it may not be sufficient if the steel has the chemical composition described in the above item (A). For example, a steelmaking method in which deoxidation is sufficiently performed with Si and A1 and finally Ti is added. It is desirable to adopt

Note that Ti carbosulfide can be easily distinguished from other inclusions by color and shape when the specimen taken from steel material is mirror-polished and the polished surface is used as a test surface and observed with an optical microscope at a magnification of 400 or more. it can. That is, when observed under an optical microscope under the above conditions, the “color” of Ti carbosulfide is extremely light gray, and its “shape” is recognized as a granular shape (spherical shape) corresponding to the B-based inclusion of JIS. The detailed determination of Ti carbosulfide can also be performed by observing the test surface with an electron microscope equipped with an analysis function such as an EDX (energy dispersive X-ray analyzer).

Here, as described above, the cleanliness of Ti carbosulfides was measured in 60 visual fields using the `` microscopic test method for steel nonmetallic inclusions '' specified in JIS G 0555, with the magnification of the optical microscope set to 400 times. It refers to the measured value.

(C) Structure of steel

As far as machinability is concerned, regarding the “steel material excellent in machinability” of the present invention, C, S, Ti, N, Nd, Se, Te, Ca, Pb and Bi of (A) Content and the size and cleanliness of Ti carbosulfide in (B) above Is enough. However, when other properties are required in addition to machinability of the steel, the structure of the steel may be specified together.

First, when 90% or more of the microstructure of the steel is made of fly and pearlite, bending and residual stress due to transformation strain do not pose a major problem. Therefore, if 90% or more of the structure of the steel material is made of fly and pearlite, for example, the bending (straightening process) as a finishing process becomes unnecessary, leading to cost reduction. Furthermore, when the above-mentioned steel material is a non-heat treated steel material, much energy and cost for the heat treatment can be reduced. In order to make 90% or more of the structure of the non-heat-treated steel material consist of flylite and pearlite, the steel slab having the chemical composition described in (Π) described above is used, for example, from 150 After heating to 130 ° C, hot working such as hot forging is performed, and after finishing at a temperature of 900 ° C or more, at a cooling rate of 60 ° C / min or less, at least 5 ° C ◦ Air cooling or cooling to 0 ° C is sufficient. The term “cooling rate” as used herein refers to a cooling rate of a steel material surface.

In the case of non-heat treated steel with the above structure, the percentage of ferrite is 20 to 70% in area ratio, the grain size of ferrite is 5 or more in JIS grain size number, and the average value of pearlite lamellar intervals is 0. A good balance between strength and toughness can be obtained if at least one condition of .2 m or less is satisfied.

Next, when 90% or more of the structure of the steel material is payite or ferrite and bainite, the balance between strength and toughness is improved. Therefore, if a good balance between strength and toughness is required, 90% or more of the structure should be bainite or X-lite and bainite. Further, when the above-mentioned steel material is a non-heat treated steel material, it is possible to reduce the cost and energy required for the heat treatment.

In order to make 90% or more of the structure of the non-heat treated steel material consist of bainite or a mixture of the fly and the bainite, the steel slab having the chemical composition described in (ΠΙ) described above must be used. For example, after heating to 150-130 ° C, hot forging After finishing at a temperature of 900 ° C or more, air cooling or cooling to at least 300 ° C at a cooling rate of 60 ° C / min or less is recommended.

In the case of non-heat treated steel, the higher the forming ratio during hot working, the finer the structure and the better the balance between strength and toughness. Therefore, it is preferable to set the molding ratio to 1.5 or more during the hot working. The “forming ratio” is A. (Α.ΖΑ) when is the cross-sectional area before processing and A is the cross-sectional area after processing.

If the grain size of old austenite grains in the organization is 4 or more in JIS grain size number, 90% or more of the structure is bainite or a non-heat treated steel material composed of ferrite and bainite (that is, A good balance between strength and toughness can be ensured in “condition Y steel”. Here, “old austenite grains” in non-heat treated steel materials refer to austenite grains immediately before transformation of bainite diplite or the like occurs due to heating and hot working. In the case of a non-heat treated steel material in which 9% or more of the structure is bainite or ferrite and bainite, the former austenite grains can be easily corroded with nickel and observed with an optical microscope. Judgment is completed.

If aging treatment is performed at a temperature of 200 to 700 ° C. for about 20 to 150 minutes after performing hot working and cooling, a particularly excellent balance between strength and toughness can be obtained.

Finally, if more than 50% of the steel structure is martensite, the balance between strength and toughness is better. Therefore, if a better balance between strength and toughness is required, 50% or more of the structure should be made of martensite. Further, when the above steel material is a tempered steel material, an extremely good balance between strength and toughness can be obtained.

In order to make 50% or more of the structure of the tempered steel material consist of martensite, a slab having the chemical composition described in (IV) described above is used, for example, by adding 105 After heating to 0 to 130 ° C, hot working such as hot forging is performed at a forming ratio of 1.5 or more, and after finishing at a temperature of 900 ° C or more, 60 ° CZ Air cooling or cooling to at least 300 ° C at a cooling rate of less than 1 minute, then heating to 800 ° C (: up to 950 ° C and holding for 20-150 minutes And then quenched using a cooling medium such as water or oil, and further heated to a temperature range of 400 to 700 ° C, held for 20 to 150 minutes, and then cooled for 2 ° CZ or more. It may be air-cooled, allowed to cool, or in some cases, water-cooled, oil-cooled, and tempered. , So-called "direct quenching" may be used

In order to more stably secure the balance between excellent strength and toughness in the tempered steel, it is preferable that 80% or more of the structure be martensite. The remaining part of the structure other than martensite is composed of two phases: austenite and frit, a structure transformed from austenite by quenching, a perlite and bainite tempered. This is the structure in which the ferrite when quenched from the region has been tempered, or the structure in which the austenite that remains without transformation after quenching (so-called “retained austenite”) has been tempered. Substantially 100% of the organization may be martensite.

If the grain size of the former austenite grains is 5 or more in the JIS grain size number, extremely good strength and toughness can be obtained for tempered steel materials (50% or more of the tissue is made of martensite) A stable balance can be ensured. Here, "old austenite grains" in the tempered steel material means austenite grains immediately before quenching. In the case of a tempered steel material in which 50% or more of the structure is martensite, for example, after quenching the steel material, or after quenching and tempering the steel material, cut out a sample and add a surfactant Corrosion with an acid-based aqueous solution and observation with an optical microscope Thus, the old austenite grains can be easily determined.

( Example)

Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

(Example 1)

Steels with the chemical compositions shown in Tables 1 to 4 were melted using a 150-kg vacuum melting furnace or a 3-ton vacuum melting furnace. Steel 1, steel 6 and steels 36 to 40 were melted in a 3-ton vacuum melting furnace, and all others were melted in a 150 kg vacuum melting furnace. Except for steels 36 and 38, in order to prevent the formation of Ti oxides, deoxidize sufficiently with Si and A1 and add various elements. And the cleanliness was adjusted. For steels 36 and 38, Ti was added at the same time as deoxidation at Si and A1.

Steels 1 to 36 in Tables 1 to 3 are steels of the examples of the present invention whose chemical composition is within the range specified in the present invention. On the other hand, steels 37 to 46 in Table 4 are steels of comparative examples in which any of the elements is out of the range of the content specified in the present invention ₍

Table 2

fn 1 = T i (%)-1.2 S (%), f n2 = Ti (%) / S (%) f n3 = 0.5 S i (%) + Mn (%) + 1.13Cr (% ) + 1.98Ni (%)

Table 4

f n3 = 0.5 S i (%) + Mn (%) + 1.13Cr () +1.98 N i (%) ^

Next, these steels were heated to 1250 ° C and then hot forged to finish at 1 000 ° C to produce round bars with a diameter of 60 mm. The cooling conditions after hot forging were air-cooled or allowed to cool to 300 ° C so that the cooling rate was 5 to 35 ° CZ, and the structure of the round bar was adjusted to achieve a tensile strength of approximately 845 to 35 ° C. 8 The range was set to 70 MPa. Steel 6, Steel 7, Steel 9, Steel 11 Steel 29 to 36, Steel 40, Steel 45 and Steel 46 were cooled to 770 to 900 ° C for 1 hour after cooling after hot forging. Heating, water quenching, and tempering at 550 to 560 (air cooling after tempering) were performed to adjust the structure and strength level. '' From the position of 15 mm from the surface of the round bar thus obtained (RZ2 position, R is the radius of the round bar), JIS 14A tensile test specimen, Ono type rotary bending test specimen (parallel part Samples of 8 mm in diameter and 18.4 mm in length and JIS No. 3 impact test pieces (2 mm U notch sharpie test pieces) were taken out at room temperature for tensile strength, fatigue strength (fatigue limit) and toughness (impact). Values) were investigated.

From the surface of the round bar, a test specimen was sampled in accordance with JIS G 0555, Fig. 3 centering on the R / 2 part, and a mirror-polished surface with a width of 15 mm and a height of 20 mm was taken. By observing 60 visual fields with an optical microscope having a magnification of 400 times, the cleanliness of the Ti carbosulfide was measured while separating it from other inclusions. The maximum diameter of Ti carbosulfide was also investigated by observing 60 visual fields with a 400-fold optical microscope. After this, the mirror-polished test surface was further corroded with nails and observed with an optical microscope with a magnification of 100 times to observe the structure at the R / 2 part, and the ratio (area ratio) of each structure was determined. investigated.

Machinability was also evaluated by drill drilling tests. That is, a hole with a depth of 5 O mm was drilled in the length direction using a round bar with a length of 55 mm and a round bar with a diameter of 60 mm, and the hole immediately before it became impossible to drill due to abrasion of the cutting edge The machinability was investigated using the number of as the machinability evaluation index. The drilling conditions were 6 mm straight shank drills of JIS high speed tool steel S KH59. Using water-soluble lubricant, feed 0.20 mm / rev, speed 980 r

Pττ at P m.

Tables 5 to 8 show the results of the above various tests. Tables 5 to 8 also show the conditions of quenching and tempering for steel 6, steel 9, steel 9, steel 11, steel 29 to 36, steel 40, steel 45, and steel 46.

Five

In the organization column, F indicates flight, P indicates perlite, B indicates bainite, and M indicates martensite.

Table 6

7

In organization II, F indicates the flight, P indicates the perlite, B indicates the bainite, and M indicates the martensite.

Table 8

In the organization column, F indicates a light, P indicates a perlite, B indicates a bainite, and M indicates a martensite.

The asterisk indicates that the condition is outside the conditions specified in the present invention. The * mark in steel indicates that the chemical composition is out of condition.

From Tables 5 to 8, it is found that the steel contains C, S, Ti and N within the range specified in the present invention, and the maximum diameter of Ti carbosulfide in steel is 10 m or less and its cleanliness is 0.05%. In the case of Test Nos. 1 to 35, the machinability evaluation index exceeds 200. On the other hand, in the case of Test No. 36, although the contents of C, S, Ti and N of steel 36 as the test steel are within the range specified in the present invention, the cleanliness of the Ti carbosulfide is low. Since it is less than 0.05%, the machinability index is as low as 51. In the case of Test Nos. 37, 39 and 40, any one of the C, Ti and N contents of the test steels Steel 37, Steel 39 and Steel 40 was out of the range specified in the present invention. The gender index is low at 58, 40 and 45 respectively. In the case of Test No. 38, the S content of the test steel, Steel 38, was outside the range specified in the present invention, and the cleanliness of Ti carbosulfide was below 0.05%. Is as low as 3 1.

As described above, when the machinability was evaluated with the tensile strength levels almost equal, it is clear that the machinability of the steel material according to the present invention is excellent.

On the other hand, steels 41 to 46 whose Nd, Se, Te, Ca, Pb, and Bi contents were out of the ranges specified in the present invention were used as test steels. In this case, although the machinability is good, compared to the case of Test Nos. 2 to 7 where steels 2 to 7 in which the contents of each of the above elements are within the range specified in the present invention are used as test steels, It is clear that the strength and / or toughness is poor.

In addition, from Tables 5 to 8, in the case of the present invention, when the maximum diameter of Ti carbosulfide is 0.5 to 7 and the cleanliness is 0.08 to 2.0%, the machinability and fatigue strength are reduced. It is clear that the balance is excellent. Furthermore, if 90% or more of the organization is bainite or a combination of fly and bainite, the balance between strength and toughness is good, and if 50% or more of the organization is martensite, It is also clear that the balance between strength and toughness is very good. (Example 2)

Steels 47 to 54 having the chemical compositions shown in Table 9 were melted using a 150 kg vacuum melting furnace or a 3-ton vacuum melting furnace. Steels 47-49 were melted in a 3-ton vacuum melting furnace, and all others were melted in a 150 kg vacuum melting furnace. In order to prevent the formation of Ti oxides, the size and cleanliness of Ti carbosulfide were adjusted by adding Ti at the end of the deoxidation with Si and A1 and adding various elements. Steels 47 to 54 in Table 9 are all steels according to the present invention whose chemical composition is within the range specified in the present invention.

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Next, these steels were heated to 1250 ° C and then hot forged to finish at 1 000 ° C to produce round bars with a diameter of 60 mm. The cooling conditions after hot forging were air-cooled or allowed to cool to 400 ° C so that the cooling rate was 5 to 35 ° CZ, and the structure was mainly composed of ferrite and perlite. The tensile strength was adjusted.

Various test pieces were collected from the round bar obtained in the same manner as in Example 1 and investigated. In other words, from the R / 2 part position from the surface of the round bar, a JIS 14A tensile test specimen, an Ono-type rotating bending test specimen (parallel part having a diameter of 8 mm and a length of 18.4 mm) and JIS No. 3 impact test specimens (2 mm U notch test specimens) were sampled, and their tensile strength, fatigue strength (fatigue limit) and toughness (impact value) at room temperature were investigated.

A test piece was taken from the surface of the round bar, centering on the RZ2 position, in accordance with Figure 3 of JIS G 0555, and a mirror-polished test surface with a width of 15 mm and a height of 20 mm was magnified. Observed 60 visual fields with a 400 × optical microscope, and measured the cleanliness of Ti carbosulfide while separating it from other inclusions. The maximum diameter of Ti carbosulfide was also investigated by observing 60 visual fields with a 400-fold optical microscope. Thereafter, the mirror-polished test surface was further corroded with sodium and observed with an optical microscope having a magnification of 100 times to observe the structure of the RZ2 portion, and the ratio (area ratio) of each structure was examined. For steels 51 to 53 of test numbers 51 to 53, the average particle diameter lamella spacing was measured by taking JIS fine grain size numbers and taking scanning electron micrographs. I asked.

Machinability was also evaluated by drill drilling tests. The test conditions and evaluation method are as described in Example 1.

Table 10 shows the results of the various tests described above. Table 10

Test Ti Carbosulfide Tissues Mechanical properties Quenching conditions Test steel 5 Diameter cleanliness F P B M Pulling Fatigue boat Durability ratio Charpy Machinability hardening

J IS grain TS Law at lamellar spacing wZTS Impact value Evaluation index Power (^ m) () (%) Degree number (%) Employment () (%) (%) (MP a) (MP a) ( J / cra ² ) CO CO

47 47 6.4 0.21 22 78 0 0 835 376 66 302

48 48 7.1 1 1.19 55 45 0 0 878 400 64 320

49 49 7.2 1.21 24 76 0 0 865 433 0.50 64 319

50 50 6.5 1.03 43 55 2 0 827 379 0.46 68 333

51 51 4.3 0.26 43 9 52 0.15 5 0 843 405

71 336

52 52 3.8 0.24 54 9 44 0.16 3 0 884 424 0.48 70 338

53 53 1.3 0.12 52 6 41 0.13 7 0 859 399 0.46 73 334

54 54 5.8 0.25 20 80 0 0 833 383 68 308

In the column, F stands for frilite, P stands for pearlite, B stands for bainite, and M stands for mancite.

JIS number of ferrite, no. The lamella spacing of one light (No measurement was made for items where the 7Η¾ ^ 項 term is "1".

o 〇

〇 ο ①

From Table 10, it can be seen from Table 10 that if 90% or more of the structure is non-heat treated steel consisting of xlite and pearlite, the percentage of the graphite is 20 to 70% by area and the particle size of the graphite is JIS particle number. It is clear that a good balance between strength and toughness can be obtained if at least one of the conditions is satisfied, ie, 5 or more and the average value of the pearlite lamella spacing is 0.2 m or less. Further, when the value of fn 1 represented by the above formula (1) is larger than 0% and / or when the value of ίη 2 represented by the above formula (2) is larger than 2, the machinability index becomes large. It is clear that When the value of f η 2 represented by equation (2) is larger than 2, the fatigue strength is also large.

(Example 3)

Steels 55 to 59 having the chemical compositions shown in Table 11 were melted using a 150 kg vacuum melting furnace or a 3-ton vacuum melting furnace. Steel 55 and steel 56 were melted in a 3-ton vacuum melting furnace, and all others were melted in a 150 kg vacuum melting furnace. In this example, too, in order to prevent the formation of Ti oxides, sufficient deoxidation was performed with Si and A1 and various elements were added. The degree was adjusted. Steels 55 to 59 in Table 11 are all steels of the present invention whose chemical composition is within the range specified in the present invention.

Next, these steels were heated to 1250 ° C and then hot forged to finish at 1 000 ° C to produce round bars with a diameter of 60 mm. The cooling conditions after hot forging were air-cooled or allowed to cool to 300 ° C so that the cooling rate was 5 to 35 ° CZ, and the structure was mainly composed of bainite or ferrite. The tensile strength was adjusted so as to be composed of paynight. Steel 57 and steel 58 were also subjected to aging treatment (Test Nos. 60 and 61) which were cooled after hot forging, then heated at 560 ° C for 1 hour and air-cooled.

Various test pieces were collected from the round bar obtained in the same manner as in Example 1 and investigated. JIS 14A tensile test specimen, Ono-type rotary bending test specimen (parallel part having a diameter of 8 mm and length of 18.4 mm) and JIS 3A No. 2 impact test specimens (2 mmU notch rupee test specimens) were sampled and their tensile strength, fatigue strength (fatigue limit) and toughness (impact value) at room temperature were investigated.

A test piece was taken from the surface of the round bar, centering on the RZ2 position, in accordance with Figure 3 of JIS G 0555, and a mirror-polished test surface with a width of 15 mm and a height of 20 mm was magnified. Observed 60 visual fields with a 400 × optical microscope, and measured the cleanliness of Ti carbosulfide while separating it from other inclusions. The maximum diameter of Ti carbosulfide was also investigated by observing 60 visual fields with a 400-fold optical microscope. Thereafter, the mirror-polished test surface was further corroded with nickel and observed with an optical microscope at a magnification of 1 × 0 to observe the structure at the RZ2 portion, and the ratio (area ratio) of each structure was investigated.

Table 12 shows the results of the various tests described above. Table 12 also shows the conditions of aging treatment performed on steel 57 and steel 58 in test numbers 60 and 61 ₍ 1 2

In the organization column, F indicates ferrite, Ρ indicates personal light, Β indicates bainite, and Μ indicates martensite. In Test Nos. 60 and 61, what is described in the column of `` Tempering heating temperature '' is `` Tempering temperature ''.

According to Table 12, if 90% or more of the organization is made of non-heat treated steel consisting of payinite or X-lite and bainite, good aging after hot working and cooling It is clear that a balance between strength and toughness can be obtained. Further, when the value of fnl represented by the above formula (1) is larger than 0%, and when the value of Z or fn2 represented by the above formula (2) is larger than 2, the machinability index also increases. Is evident. When the value of f n 2 represented by equation (2) is larger than 2, the fatigue strength is also large.

(Example 4)

Steels 60 to 64 with the chemical compositions shown in Table 13 were melted using a 150 kg vacuum melting furnace or a 3-ton vacuum melting furnace. Steel 60 and steel 61 were melted in a 3-ton vacuum melting furnace, and the others were melted in a 150 kg vacuum melting furnace. In this example, too, in order to prevent the formation of Ti oxide, Ti and A1 were sufficiently deoxidized and various elements were added. The cleanliness was adjusted. Steels 60 to 64 in Table 13 are all steels according to the present invention whose chemical compositions are within the range specified by the present invention.

8 Halla ο

Next, these steels were heated to 1250 ° C and then hot forged to finish at 1 000 ° C to produce round bars with a diameter of 60 mm. The cooling conditions after hot forging were air-cooled or allowed to cool to 300 ° C so that the cooling rate was 5 to 35 ° C / min. After that, it was heated to 850 to 900 ° C for 1 hour, water-quenched, and tempered at 550 ° C (cooling after tempering was air-cooled) to adjust the tissue and strength level.

Various test pieces were sampled from the thus obtained round bar in the same manner as in Example 1 and investigated. In other words, from the R / 2 part position from the surface of the round bar, a JIS 14A tensile test specimen, an Ono-type rotating bending test specimen (parallel part having a diameter of 8 mm and a length of 18.4 mm) and JIS No. 3 impact test specimens (2 mm U notch test specimens) were sampled, and the tensile strength, fatigue strength (fatigue limit) and toughness (impact value) at room temperature were investigated.

From the surface of the round bar, a test piece was sampled in accordance with Fig. 3 of J1S G0555, centering on the RZ2 position, and a mirror-polished test surface with a width of 15 mm and a height of 20 mm was magnified. Observed 60 visual fields with a 400 × optical microscope, and measured the cleanliness of Ti carbosulfide while separating it from other inclusions. The maximum diameter of Ti carbosulfide was also investigated by observing 60 visual fields with a 400-fold optical microscope. After this, the mirror-polished test surface was further corroded with sodium and observed with an optical microscope at a magnification of 100 to observe the structure at the R / 2 position, and the ratio (area ratio) of each structure was investigated. .

Table 14 shows the results of the various tests described above. Table 14 also shows the quenching and tempering conditions for steels 60 to 64. 14

In the organization column, F indicates X-lite, P indicates perlite, B indicates bainite, and M indicates martensite.

From Table 14, it is clear that an extremely good balance of strength and toughness can be obtained when the tempered steel is composed of at least 50% of martensite. Further, when the value of fn 1 represented by the above equation (1) is greater than 0% and / or when the value of fn 2 represented by the above equation (2) is greater than 2, the machinability index also increases. It is clear that When the value of f n 2 represented by equation (2) is larger than 2, the fatigue strength is also large.

(Example 5)

Steel 1, steel 6, steel 36 to 40, steel 47 to 49, steel 55, steel 56, steel 60, and steel 61 melted using a three-ton vacuum melting furnace in Examples 1 to 4 described above. Was heated to 1250 ° C and then finished at 1 000 ° C by hot forging and allowed to cool to room temperature to produce a 125 mm square bar.

Next, after heating these square bars to 1250 ° C, hot die forging is performed so that the finishing temperature becomes 1 000 ° C or more, and the cooling rate after hot die forging is set at a cooling rate of 5 to 35 ° C. / Min and air cooled or allowed to cool to 300 ° C to produce a crankshaft base material, which was cut and finished into the final crankshaft. For test numbers 68, 69, 73, 79, and 80, after cooling after hot die forging, heat to 890 to 9 ° C for 1 hour, then quench with water to 550 ° C. The steel was tempered with C (cooling after tempering was air-cooled) to produce a crankshaft base material, which was then cut and finished into the final crankshaft.

For cutting the surface to finish the final product shape, use a carbide chip coated with the shape of JIS nominal symbol CN MG12041N-UX, dry type, cutting speed of 100 mZ, cutting depth It was performed under the conditions of 1.5 mm, feed 0.25 mm / rev. Then, using a 6 mm straight shank drill of JIS high-speed tool steel S KH59, using a water-soluble lubricant, feed the oil.SO mmZr ev Drill the crankshaft oil hole at 980 rpm. went. When drilling oil holes with drills The machinability of the body was evaluated by the number of crankshafts produced immediately before drilling was impossible.

Specimens were sampled according to JIS G 0555, Fig. 3, centered at a position 15mm from the surface of the pin (diameter 70mm) of the cast material of the crankshaft, and the mirror-polished width was used. The surface to be inspected, having a height of 15 mm and a height of 20 mm, was observed in 60 visual fields with an optical microscope having a magnification of 400 times, and its cleanliness was measured while distinguishing Ti carbosulfide from other inclusions. The maximum diameter of Ti carbosulfide was also investigated by observing 60 fields of view with an optical microscope with a magnification of 400x. Thereafter, the mirror-polished test surface was further corroded with Na and observed under an optical microscope at a magnification of 100 to observe the structure, and the ratio (area ratio) of each structure was examined. In addition, the test specimen was collected in a direction parallel to the axial direction of the crankshaft, and the JIS 14A tensile test specimen and the Ono-type rotary bending test specimen (the diameter of the parallel part was 8 mm and its length was 18.4 mm) and JIS No. 3 impact test specimens (2 mm U notched Charby test specimens) were sampled and the tensile strength, fatigue strength (fatigue limit) and toughness (impact value) at room temperature were investigated.

Table 15 shows the results of the above various tests. Table 15 also shows the hardening and tempering conditions for test numbers 68, 69, 73, 79, and 80.

1 5

In the organization column, F indicates ferrite, P indicates private, B indicates bainite, and M indicates martensite.

The asterisk indicates that the condition is out of the conditions specified in the present invention. The * mark in steel indicates that the chemical composition is out of condition.

From Table 15, it is clear that the crankshaft base material manufactured from the steel material according to the present invention has excellent machinability. Further, it is apparent that the crankshaft using the steel material according to the present invention as a raw material has a better balance between strength and toughness than the crankshaft using a steel material of a comparative example as a material. Industrial applicability

Since the steel material of the present invention has excellent machinability and a good balance of strength and toughness, it is used as a material for structural parts of various machines such as automobiles, transportation machines, industrial machines, and construction machines. can do. By using this steel material as a material and performing cutting, various mechanical structural parts can be manufactured relatively easily.

Claims

The scope of the claims

1.0% by weight, C: 0.05 to 0.6%, S: 0.002 to 0.2%, Ti: 0.04 to: L. 0%, N: 0.008% or less, Nd: 0 to 0.1%, S e: 0 to 0.5%, Te: 0 to 0.05%, Ca: 0 to 0.05%, Pb: 0 to 0.5%, B i: With a chemical composition containing 0-0.4%, the maximum diameter of Ti carbosulfide in steel is 10 or less, and the amount is 0.3% in cleanliness.

Steel material with excellent machinability of more than 05%.

2. The steel material according to claim 1, wherein the maximum diameter of Ti carbosulfide is 0.5 to 7 m and the amount thereof is 0.08 to 2.0% in terms of cleanliness.

3. The steel material according to claim 1 or 2, wherein 90% or more of the structure is perlite and pearlite.

4. The steel material according to claim 1 or 2, wherein 90% or more of the structure is bainite, or ferrite and bainite.

5. The steel material according to claim 1 or 2, wherein 50% or more of the structure is martensite.

6. C: 0.2 to 0.6%, Si: 0.05 to 1.5%, Mn: 0.1 to 2.0%, P: 0.07% or less, S: 0.01 ~ 0.2%, A1: 0.002 to 0.05%, Cu: 0 to 1.0%, Ni: 0 to 2.0%, Cr: 0 to 2.0%, Mo: 0 to 0.5%, V: 0-0.3%, Nb: 0-0.1%, with the balance being Fe and unavoidable impurities, and more than 90% of the tissue is free 2. The non-heat treated steel material according to claim 1, wherein the steel is a steel and a pearlite.

7. The non-heat treated steel material according to claim 6, wherein the maximum diameter of Ti carbosulfide is 0.5 to 7 m and the amount thereof is 0.08 to 2.0% in terms of cleanliness.

8. The percentage of ferrite is 20-70% in area ratio, and the ferrite grain size is J

1 S particle size number of 5 or more, average lamella spacing of perlite is 0.2 μm The non-heat-treated non-heat treated steel material according to claim 6 or 7, which satisfies at least one of the following.

9. Claims 6 to 8 wherein the value of fn 1 represented by the following formula (1) is greater than 0% and the value of fn 2 represented by the following formula (2) satisfies at least one of 2 or more. Non-heat treated steel material according to any one of the above.

f n 1 -Ti ()-1.2 S (%) • (1)

f n 2 = Ti (%) / S (%)

1 0. C: 0.05 to 0.3%, S i 0.05 to L. 5%. Al: 0.002 to 0.05%, Cu: 0 to 10%, Mo: 0 to 0.5%, V: 0 to 0.30%, Nb: 0 to 0.1%, B: 0 to 0.02%, the value of fn 3 expressed by the following formula (3) is 2. The composition of claim 1 which satisfies 5-4.5% and the balance is a chemical composition consisting of Fe and unavoidable impurities, and 90% or more of the tissue is bainite or ferrite and bainite. Non-heat treated steel material.

f n 3 = 0.5 S i (%) + Mn (%) +1 .13 Cr (%) +1.98 Ni (%) (3)

11. The non-heat treated steel material according to claim 10, wherein the maximum diameter of Ti carbosulfide is 0.5 to 7 m and the amount thereof is 0.08 to 2.0% in purity.

1 2. Claims where the value of f n 1 represented by the following equation (1) is greater than 0% and the value of f n 2 represented by the following equation (2) satisfies at least 1

10. The non-heat treated steel material according to 10 or 11.

f n 1 = Ti (%) 1 1.2 S () (1)

f n 2 = Ti (%) / S (%) (2)

1 3. C: 0.1 to 0.6%, Si: 0.05 to; L. 5%, Mn: 0.4 to 2.0%, Al: 0.002 to 0.05%, C u: 0 to 1.0%,

Ni: 0 to 2.0%, Cr: 0 to 2.0%, Mo: 0 to 0.5%, V: 0 to 0.3%, Nb: 0 to 0.1%, B: 0 to Including 0.02%, the rest The tempered steel material according to claim 1, wherein is a chemical composition comprising Fe and unavoidable impurities, and 50% or more of the tissue is martensite.

14. The tempered steel material according to claim 13, wherein the maximum diameter of Ti carbosulfide is 0.5 to Am and the amount thereof is 0.08 to 2. ◦% in purity.

1 5. Claims wherein the value of fn 1 represented by the following formula (1) is greater than 0% and the value of fn 2 represented by the following formula (2) satisfies at least one of 2 or more. Or a tempered steel material according to 14.

f n 1 = Ti (%)-1.2 S (%) (1)

f n 2 = Ti (%) / S (%) (2)

1 6. ρβ roll made from the steel material according to claim 1 and processed by cutting.

1 7. A part made from the non-heat treated steel according to claim 6 and processed by cutting.

1 8. A part made from the non-heat treated steel described in Claim 10 and subjected to cutting.

1 9. Parts made from the tempered steel described in Claims 13 and processed by cutting.