JPH0925539A - Free cutting non-heat treated steel excellent in strength and toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a free cutting non-heat treated steel, having excellent strength and toughness and suitable for material, e.g. for parts for machine structural use, at a low cost while obviating the necessity of the addition of Pb adversely affecting environments. SOLUTION: This steel is a free cutting non-heat treated steel excellent in strength and toughness, having a composition consisting of, by weight, 0.2-0.6% C, 0.3-1.5% Si, 0.5-2.0% Mn, 0.01-0.07% P, 0.01-0.15% S, 0.1-2.0% Cr, 0.002-0.05% Al, 0.01-0.05% N, 0.005-0.1% Nd, one or more kinds among 0.05-0.3% V, 0.005-0.05% Ti, and 0.005-0.05% Nb, and the balance Fe with inevitable impurities and further having a ferrite-pearlite structure. Further, one or more elements among Ca, Se, Te, Bi, Mo, and Cu can be incorporated other than the above components.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a free-cutting non-heat treated steel excellent in strength and toughness. More specifically, Pb suitable as a material for machine structural parts, etc., which has excellent strength and toughness without being subjected to tempering treatment after hot working.
The present invention relates to a non-addition type free-cutting non-heat treated steel.

[0002]

2. Description of the Related Art Conventionally, high tensile strength and fatigue strength (hereinafter,
(Tensile strength and fatigue strength may be simply called "strength")
In addition, steel machine structural parts that require high toughness,
In general, after roughing into a predetermined shape by hot working, and then to a final desired shape by cutting, a tempering treatment such as quenching and tempering is generally performed. However, this conditioning process consumes a lot of energy and cost. Therefore, in recent years, non-heat-treated steels that can be used as they are in hot working have been actively developed in order to meet social demands for energy saving and reduce costs.

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.

In general, the machinability of steel largely depends on the metal structure. In the case of a steel having a ferrite-pearlite structure, the machinability is good, and the steel has a ferrite bainite structure or a single phase structure of bainite or martensite. It is known that machinability is poor in steel. 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 after hot working has been adopted.

For example, the inventor of the present invention also proposes a "free-cutting non-heat treated steel excellent in strength and toughness" having a specific chemical composition containing Pb and having a ferrite / pearlite structure in Japanese Patent Application No. 7-1.
Proposed in the 3261 application. However, even though the previous proposal can provide free-cutting non-heat treated steel with excellent strength and toughness, P
There was a problem in terms of adding b.

Even if Pb is added, it does not cause deterioration of the mechanical properties of steel, and it is cheaper than Te, Bi, etc., so that it has been commonly used as a free-cutting element. However, since high-concentration Pb vapor is harmful to the human body, it requires a large-scale facility for recovering Pb vapor in the manufacturing process of steel materials and mechanical structural parts, and also has a problem from the viewpoint of recycling steel materials. For this reason, some nations are trying to regulate the addition of Pb to steel. Under these circumstances, development of a steel material that is not Pb-added and has machinability equal to or higher than that of Pb-added steel is desired.

Under such circumstances, for example, Japanese Patent Laid-Open No. 2-11
1842 and JP-A-6-279849,
By adding C in the steel as graphite and utilizing the notch and the lubricating effect of this graphite, the machinability is improved with the Pb-free type "hot-rolled steel with excellent machinability and hardenability. And "a method for manufacturing a steel for machine structural use, which has excellent machinability".

However, the steel material proposed in Japanese Patent Application Laid-Open No. 2-111842 is one in which B is added to promote the graphitization using B nitride (BN) as a nucleus of graphitization, and the addition of B is essential. Therefore, there is a problem that cracks easily occur during solidification. On the other hand, the method described in Japanese Patent Application Laid-Open No. 6-279849 promotes graphitization as it is in hot rolling by restricting O (oxygen) in steel together with addition of Al. It is not always economical because it needs to be annealed.
Further, since both of the proposals in the above-mentioned two publications utilize graphitization, in order to impart desired mechanical properties to mechanical structural parts etc. processed into a predetermined shape, quenching and tempering must be performed. It has to undergo quality treatment,
It has not been possible to meet the demands of the industrial world to achieve both "non-heat treatment" and "improvement of machinability of high strength steel".

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and is a Pb-free steel which is heat-treated under ordinary hot working and cooling conditions including tempering. The purpose of the present invention is to provide, at low cost, steel for materials such as machine structural parts that have high strength and good toughness, and have excellent machinability, without being subjected to heat treatment. .

[0010]

In order to solve the above-mentioned problems, the present inventor has studied the chemical composition and structure of non-heat treated steel by controlling the morphology of MnS based on S free-cutting steel. The research was repeated about improving the sex. As a result, the following findings were obtained.

(A) By adding Nd to the steel, Nd ₂ S ₃ is finely dispersed and produced in a relatively high temperature region of the molten steel, and MnS is finely dispersed and precipitated with this as a nucleus.

(B) Due to the fine dispersion of MnS, the density of ferrite-producing nuclei increases, and the ferrite grains become finer as the amount of ferrite increases.

(C) Since MnS is produced at a relatively high temperature as described in (a) above, the morphology of MnS becomes spherical.

(D) Since MnS is produced at a relatively high temperature, the solid solution amount of Fe in MnS decreases, and the hot deformability of MnS decreases. The mechanism is considered as follows.

Usually, MnS is formed in the supersaturated region at the final stage of solidification, so that it exists along the austenite grain boundaries and segregates easily. Furthermore, in such formation, Fe in MnS
Is dissolved in a large amount, and the hot deformability is larger than that of MnS in which Fe is not dissolved. This is M in the cutting shear zone
This means that the deformation of nS becomes large, and the effect as a chip breaker becomes small. However,
When MnS is generated in a relatively high temperature region during solidification due to the addition of Nd, a substitution reaction between Fe that is a solid solution in MnS and Mn in molten steel during solidification cooling occurs, and as a result, Fe in MnS at the end of solidification The amount of solid solution of is reduced.

(E) Nd ₂ S ₃ itself also functions as a chip breaker.

Regarding the cost increase of the steel material, which is considered to be the most problematic problem when Nd is contained, it is possible to reduce the material cost by utilizing a relatively inexpensive misch metal. Further, since Nd has been widely used for high-performance magnets in recent years, in order to prevent the cost from increasing so much, this residual material may be utilized as a steel material.

The present invention based on the above findings is described in the following (1)-
The gist is a free-cutting non-heat treated steel having the chemical composition and structure shown in (4) and excellent in strength and toughness.

(1)% by weight, C: 0.2 to 0.6%,
Si: 0.3 to 1.5%, Mn: 0.5 to 2.0%,
P: 0.01 to 0.07%, S: 0.01 to 0.15
%, Cr: 0.1 to 2.0%, Al: 0.002 to 0.
05%, N: 0.01 to 0.05% and Nd: 0.0
05-0.1%, and V: 0.05-0.3%,
Ti: 0.005-0.05% and Nb: 0.005
A free-cutting non-heat treated steel containing at least one of 0.05 to 0.05%, the balance being Fe and unavoidable impurities, and having a ferrite-pearlite structure and having excellent strength and toughness.

(2) In addition to the components described in (1) above, further, by weight%, Ca: 0.001 to 0.01%, Se:
0.1 to 0.5%, Te: 0.005 to 0.05%, and Bi: 0.05 to 0.4%, and at least one of them, and a strength having a ferrite-pearlite structure. Free-cutting non-heat treated steel with excellent toughness.

(3) In addition to the components described in (1) above, further, in weight%, Mo: 0.05-0.5% and Cu:
Contains at least one of 0.5 to 1.0%, and the balance is F
Free cutting non-heat treated steel consisting of e and inevitable impurities, and having a ferrite / pearlite structure and excellent strength and toughness.

(4) In addition to the components described in (1) above, further, by weight%, Ca: 0.001 to 0.01%, Se:
0.1 to 0.5%, Te: 0.005 to 0.05% and Bi: 0.05 to 0.4%, and Mo: 0.05 to 0.5% and Cu. : 0.5 to 1.0
%, A balance of Fe and unavoidable impurities in the balance, and a free-cutting non-heat treated steel excellent in strength and toughness having a ferrite / pearlite structure.

[0023]

The reason for limiting the chemical composition and structure of steel in the present invention as described above will be explained below. In addition, “%” of the component content means “% by weight”.

(A) Chemical composition of steel C: C is added to secure the strength, but if the content is less than 0.2%, the effect of addition is poor, while 0.6% is added.
, The toughness deteriorates, so its content is 0.2-
It was set to 0.6%. The C content is 0.25 to 0.5%
It is preferable that

Si: Si is added for deoxidation of steel and strengthening of ferrite, but if it is less than 0.3%, these effects are insufficient, while if it exceeds 1.5%, the above effect is saturated. Not only does this cause a decrease in toughness, but its content is set to 0.3 to 1.5%. The preferable content of Si is 0.5 to 1.3%.

Mn: Mn has the effect of improving fatigue strength by solid solution strengthening. However, the content is 0.5
If it is less than 2.0%, the desired effect cannot be obtained, and if it exceeds 2.0%, the hardenability becomes too high and the formation of bainite structure or island martensite structure is promoted, and the durability ratio (fatigue limit / tensile strength) and yield The ratio (proof strength / tensile strength) comes to decrease. Therefore, the Mn content is set to 0.5 to 2.0%. The Mn content is preferably 0.7 to 1.7%.

P: P is a solid solution strengthening element and has an effect of improving tensile strength and fatigue strength.

However, if the content is less than 0.01%, the effect of addition is poor, while if it exceeds 0.07%, the effect is saturated and the toughness deteriorates and ductility (workability).
Therefore, the content of 0.01 to 0.0
It was set to 7%. The preferred content of P is 0.015 to 0.0
5%.

S: S is an element effective for improving the free cutting property. Further, based on the fine dispersion precipitation of Nd ₂ S ₃ and MnS, it has the effect of increasing the ferrite formation nucleus density, increasing the amount of ferrite, and refining the ferrite grains. However, if the content is less than 0.01%, the desired effects cannot be obtained, while if it exceeds 0.15%, not only are those effects saturated, but rather the toughness deteriorates. The amount was 0.01 to 0.15%. The S content is preferably 0.02 to 0.13%.

Cr: Cr has the effect of improving fatigue strength by solid solution strengthening. However, its content is 0.1
If it is less than 2.0%, the desired effect cannot be obtained, and if it exceeds 2.0%, the hardenability becomes too high to promote the formation of bainite structure or island martensite structure, and the durability ratio and the yield ratio decrease. . Therefore, the Cr content is 0.1 to
2.0%. The preferable content of Cr is 0.15
1.5%.

Al: Al is an element effective for stabilizing and homogenizing deoxidation of steel. However, if the content is less than 0.002%, the desired effect cannot be obtained, and 0.05%
%, The effect is saturated and the machinability of the steel is rather deteriorated.
02-0.05%. The Al content is 0.00
It is preferably set to 5 to 0.02%.

N: N itself not only enhances strength by solid solution strengthening, but also V, Ti and Nb in steel.
And is finely precipitated as a nitride or a carbonitride, and austenite grains are refined to improve toughness. But,
If the content is less than 0.01%, the desired effect cannot be obtained, and if it exceeds 0.05%, the effect is saturated,
On the contrary, since the toughness is deteriorated, its content is set to 0.
It was set to 01 to 0.05%.

Nd: Nd is an element having a very important meaning in the present invention. That is, Nd acts as a chip breaker as Nd ₂ S ₃ and improves machinability. Further, Nd ₂ S ₃ is finely dispersed and formed in a relatively high temperature region of the molten steel to finely disperse and precipitate MnS to increase the density of ferrite-forming nuclei, increase the amount of ferrite, and refine ferrite grains. As a result, a fine ferrite-pearlite structure is formed, which has the effect of increasing the strength and toughness of the steel. In addition, MnS is made spherical and M
By reducing the solid solution amount of Fe in nS, the hot deformability of MnS is reduced and the machinability of steel is improved. However, if the content is less than 0.005%, it is difficult to obtain the above-mentioned effect, while if it exceeds 0.1%, the ferrite is excessively generated, so that the strength is lowered and the fatigue property is also deteriorated. Therefore, the content of Nd is set to 0.005 to 0.1%. The preferable content of Nd is 0.005 to 0.08%.

V, Ti and Nb: V, Ti and Nb
Both precipitate as fine nitrides and carbonitrides, and
It has the effect of refining the stenite grains to improve the toughness of the steel and also to improve the strength of the steel, especially the fatigue strength. Therefore, one or more of these elements are added. However, in the case of V, if the content is less than 0.05%, the desired effect cannot be obtained, and even if it exceeds 0.3%, the effect is saturated and the raw material cost only increases. On the other hand, in the case of Ti, if the content is less than 0.005%, the desired effect cannot be obtained, and if it exceeds 0.05%, coarse Ti carbonitrides are produced and the fatigue strength and toughness are rather increased. Cause a decrease in
Further, in the case of Nb as well, if the content is less than 0.005%, the desired effect cannot be obtained, and if it exceeds 0.05%, the effect is saturated and rather a coarse nitride is formed, resulting in toughness. And brings about a decrease in fatigue strength. Therefore, when one or more of these elements are added, the content is V:
0.05 to 0.3%, Ti: 0.005 to 0.05%, and Nb: 0.005 to 0.05%.

In addition to the above-mentioned components, the free-cutting non-heat treated steel excellent in strength and toughness of the present invention further contains Ca, Se, Te and B.
One or more of i and / or one or more of Mo and Cu may be contained. The effects and desirable contents of these alloy elements are as follows.

Ca, Se, Te and Bi: Ca, S
e, Te and Bi have a function of improving machinability. Therefore, Ca, Se, Te and Bi may be added when further machinability is required. However, Ca
In the case of, if the content is less than 0.001%, the desired effect cannot be obtained, and if the content exceeds 0.01%, the above effect is not only saturated but rather coarse inclusions are generated to cause fatigue. It causes a decrease in strength. Also in the case of Se, if the content is less than 0.1%, the desired effect is not obtained, and if it exceeds 0.5%, the above effect is not only saturated but rather coarse inclusions are formed. Result in a decrease in fatigue strength. In the case of Te, if the content is less than 0.005%, the desired effect cannot be obtained, and if it exceeds 0.05%, coarse inclusions are formed and the fatigue strength is reduced. Further, also in the case of Bi, if the content is less than 0.05%, the desired effect is not obtained, and if it exceeds 0.4%, the above effect is not only saturated but rather coarse inclusions are formed. Causes fatigue strength to decrease. Therefore, when one or more of these alloy elements are added, Ca: 0.001 to 0.01%, Se: 0.
1-0.5%, Te: 0.005-0.05% and B
i: The content is preferably 0.05 to 0.4%.

Mo and Cu: Mo and Cu have the effect of improving fatigue strength. Of these, Mo also has the effect of refining the ferrite / pearlite structure to improve toughness. Therefore, Mo and Cu may be added if necessary. However, in the case of Mo, if the content is less than 0.05%, the desired effect cannot be obtained, and if it exceeds 0.5%, the structure after hot forging abnormally coarsens and the toughness and fatigue strength deteriorate. Cause On the other hand, in the case of Cu, 0.5
If the content is less than 1.0%, the desired effect cannot be obtained, and if it exceeds 1.0%, the effect of improving fatigue strength is saturated, resulting in increased cost and deterioration of toughness. Therefore, when one or more of these alloy elements are added, Mo: 0.05
˜0.5%, Cu: 0.5 to 1.0% is preferable.

(B) Structure of Steel Even if the steel has the above-mentioned chemical composition, it does not consist of so-called "low temperature transformation product" such as bainite or martensite while being cooled to room temperature after hot working. In addition to the deterioration of machinability, bending due to transformation strain occurs, which requires a straightening step for removing the bending, resulting in an increase in cost. Therefore, in order to have good machinability and reduce transformation strain, the steel structure must be a ferrite-pearlite structure. As a manufacturing method therefor, for example, a steel slab is heated to 1050 to 1300 ° C., and then hot forged into a round bar having a diameter of about 100 mm, and 900
After finishing at a temperature of ℃ or higher, there is a process of air cooling or cooling.

When the volume fraction of ferrite in the ferrite / pearlite structure is 20 to 60% and the crystal grain size of ferrite is JIS grain size number 7 or more, the strength and toughness are particularly excellent.

[0040]

EXAMPLES Steels having the chemical compositions shown in Tables 1 to 4 were melted by a usual method using a 150 kg vacuum melting furnace. Tables 1 and 2
Steels 1 to 19 in the present invention are steels of the present invention, Steels 2 in Tables 3 and 4
0 to 37 are comparative steels in which any of the components is out of the range of the content specified in the present invention.

Then, these steels of the present invention and comparative steels are heated to a temperature of 1250 ° C. for 1 hour, and then hot forged for finishing at 1000 ° C. is carried out once or 2-3 times to obtain a diameter of 60.
mm round bar was prepared. In the final hot forging step for producing a round bar having a diameter of 60 mm, the cooling conditions after the hot forging finishing at 1000 ° C. are cooling rates of 5 to 30 ° C. /
Air cooling or cooling was carried out so that the steel had a min.

15 mm from the surface of the round bar thus obtained
From the position (R / 2 position, R: radius of round bar) to JIS
Tensile test pieces of No. 14A, JIS No. 3 impact test pieces and Ono-type rotary bending test pieces (diameter of parallel part is 8 mm and its length is 18.4 mm) were sampled, and tensile properties (tensile strength, proof stress), impact properties (Absorbed energy) and fatigue strength were investigated. The machinability was judged by the tool life by a drilling test. That is, a round bar having a diameter of 60 mm was cut into 25 mm lengths to form through holes in the length direction, and the number of through holes when it was impossible to drill due to abrasion of the cutting edge was used as a criterion. The drilling conditions are as follows: JIS high speed tool steel SKH51 φ5 mm taper drill, water-soluble lubricant, feed 0.20 mm / rev, rotation speed 1
It was performed at 800 rpm.

The results are shown in Tables 5 and 6. From the comparison between Table 5 and Table 6, it is clear that the steels of the present invention have good machinability and are excellent in strength (tensile strength and fatigue strength) and toughness. That is, regarding the steels 1 to 11 of the present invention, desired mechanical properties (tensile strength (PS): 60)
Tensile strength (TS) at 0 MPa or more: 900 MPa or more,
Fatigue characteristics are fatigue strength (σw): 450 MPa or more, durability ratio (σw / TS): 0.5 or more, absorbed energy (UE
20): 40 J or more, the number of perforations: 40 or more). On the other hand, the comparative steel 20 is Nd which is important in the present invention.
Is less than the specified content range, the machinability and toughness are not obtained among the desired mechanical properties described above, and the durability ratio is low. On the other hand, the comparative steel 21 has a low strength and durability ratio because Nd deviates from the specified content range to a higher degree. Comparative steel 22
No. 27 to 27, the contents of C, Si, Mn, P, S, and Cr, and the comparative steel 28, the contents of V, Ti, and Nb are out of the specified ranges. Has not been obtained.

The steels 12 to 16 of the present invention are C which is a free cutting element.
Since a, Se, Te and Bi are contained within the specified range, the desired mechanical properties described above can be obtained and the machinability is improved. On the other hand, in Comparative Steels 29 to 33, the desired fatigue properties were not obtained because the content of each of the above free-cutting elements was out of the specified range.

The steels 17 to 19 of the present invention are Mo which is a strengthening element.
The desired mechanical performance is obtained because the alloys contain Cu and Cu within the specified range, and in particular, the fatigue strength is excellent.

On the other hand, in Comparative Steels 34 and 35, Mo,
Since the Cu content is out of the specified content range, the desired fatigue strength and toughness described above are not obtained. Also, comparative steel 3
In Nos. 6 and 37, Nd, which is important in the present invention, deviates from the specified content range, so that the toughness and machinability among the desired mechanical properties described above are not obtained.

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

[Table 3]

[0050]

[Table 4]

[0051]

[Table 5]

[0052]

[Table 6]

[0053]

As described above, the free-cutting non-heat treated steel of the present invention is excellent in strength and toughness and is a Pb-free steel that adversely affects the environment, and is therefore extremely useful industrially.

Claims (4)

[Claims] C .: 0.2 to 0.6% by weight, Si:
0.3-1.5%, Mn: 0.5-2.0%, P: 0.
01-0.07%, S: 0.01-0.15%, Cr:
0.1-2.0%, Al: 0.002-0.05%,
N: 0.01-0.05% and Nd: 0.005-
0.1% and V: 0.05-0.3%, Ti:
0.005-0.05% and Nb: 0.005-0.
A free-cutting non-heat treated steel containing at least one of 05%, the balance consisting of Fe and unavoidable impurities, and having a ferrite / pearlite structure and excellent strength and toughness. 2. In addition to the components according to claim 1, further, in weight%, Ca: 0.001-0.01%, Se: 0.1-.
0.5%, Te: 0.005-0.05% and Bi:
Contains one or more of 0.05-0.4%,
Free-cutting non-heat treated steel with ferrite / pearlite structure and excellent strength and toughness. 3. In addition to the components according to claim 1, in a weight percentage, Mo: 0.05-0.5% and Cu: 0.5-.
A free-cutting non-heat treated steel containing at least one of 1.0%, the balance being Fe and unavoidable impurities, and having a ferrite / pearlite structure and having excellent strength and toughness. 4. In addition to the components according to claim 1, further, in weight%, Ca: 0.001 to 0.01%, Se: 0.1.
0.5%, Te: 0.005-0.05% and Bi:
One or more of 0.05 to 0.4% and Mo:
0.05 to 0.5% and Cu: 0.5 to 1.0%, at least one of which is composed of Fe and inevitable impurities, and further has strength and toughness with a ferrite-pearlite structure. Excellent free-cutting non-heat treated steel.