JP2002180184A - Nonlead steel for machine structural use, having excellent machinability and small strength anisotropy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for machine structural use in which toughness and machinability can be improved by using nonlead steel and using sulfide inclusions as free-cutting substances in consideration for environmental problems and forming an optimum form of sulfide dispersion and, further, excellent machinability can be provided over a wide range of cutting methods and cutting conditions without causing much deterioration in the strength characteristics of the steel material and, particularly, excellent chip disposability and excellent resistance to cemented carbide tool wear can be provided. SOLUTION: Low alloy steel for machine structural use containing sulfide inclusions as free-cutting substances, e.g. chromium steel prepared by adding Mg, Ca and S are free-cutting components to chromium steel specified as JIS SCR is melted. A stock of this chromium steel is hot-rolled or hot-forged to regulate the ratio between the major axis and interparticle spacing of sulfide particles of >=1.5 μm major axis existing in the steel stock to <0.5 and also regulate the average area of the sulfide particles of >=1.5 μm major axis to >=9 μm2 as shown in Fig. 1. By applying quench- and-temper treatment capable of providing excellent drilling machinability, the nonlead free-cutting steel product can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a steel material having excellent machinability in a wide range of cutting methods and cutting conditions without significantly deteriorating the strength characteristics of the steel material. The present invention relates to a steel for machine structural use having excellent heat resistance.

[0002]

2. Description of the Related Art With the recent increase in speed and automation of cutting, the machinability of steel materials used for machine structural parts is regarded as important, and the demand for free-cutting steel is increasing. However, the required strength of steel materials is becoming stricter, and machinability usually deteriorates when steel materials are increased in strength. In other words, there is a demand for improvement of contradictory characteristics such as high strength of steel and machinability. At present, Pb, S,
There is a steel material containing Ca. However, these free-cutting steels do not show any free-cutting property or have a problem of deterioration of the material depending on the cutting method, and therefore, their use and the amount of the free-cutting substance are limited.

[0003] That is, Pb free-cutting steel shows less deterioration in mechanical properties than basic steel and exhibits improved chip disposability in general turning, and is used for tools such as drilling, reaming, boring and the like. It is very useful for prolonging the service life and facilitating the discharge of chips during deep hole drilling when (hole depth / drill diameter) ≧ 3 is set as a deep hole, and preventing breakage of the tool due to sudden chipping. It is an effective element. However, regarding the tool life during turning, the effectiveness of adding Pb is small for both high-speed steel and cemented carbide tools. Furthermore, toxicity of Pb has been regarded as a problem due to an increase in environmental problems in recent years, and the amount of Pb used will be reduced in the future.

[0004] S free-cutting steel has the effect of extending the tool life for relatively wide range of cutting work, but has a poorer chipping property than Pb free-cutting steel, and has an improvement effect especially in the high-speed cutting region. Since MnS existing as inclusions is small during the hot rolling or hot forging, and in the strength aspect of the steel material,
There is a problem in that mechanical properties such as impact strength are reduced (anisotropic) as approaching the direction perpendicular to the rolling direction.

[0005] The conventional Ca free-cutting steel in which oxide-based inclusions in the steel have been reduced in melting point by Ca deoxidation has almost no effect on the strength characteristics of the steel material, and significantly extends the life of the cemented carbide tool in the high-speed cutting region. Show the effect. However, since Ca deoxidized free-cutting steel hardly shows any machinability improving effect other than the carbide tool life, it may be used in combination with S or Pb in order to obtain all-round machinability. General.

Unlike conventional Ca-deoxidized steel, the anisotropy, which is a disadvantage of free-cutting steel, is improved by uniformly dispersing and distributing inclusions in the steel by adding Ca, and at the same time, machinability is improved. As an example of the improvement, there is an invention described in Japanese Patent Publication No. 5-15777. In this case, there is no disadvantage such as Ca deoxidized free-cutting steel, but it is necessary to add a large amount of S to obtain sufficient machinability, and in that case, it is necessary and sufficient to control the sulfide form. Including an amount of Ca in steel
Production is extremely difficult as mass-produced steel due to low yield.

[0007] As an example aiming at the same effect as Ca in this case, Mg, B described in JP-B-52-7405.
a or less of 0.1% or less of the first group element of a and S,
0.03% of one or more of the second group elements consisting of Se and Te
A free-cutting steel containing up to 0.5% and having an atomic ratio of (first group element / second group element) of 0.01 or more has been proposed. However, Se and Te are highly toxic and have a large environmental load. Also, there is an invention described in JP-A-51-63312, in which Zr is added to tool steel, and the amount of O + N and the amount of coexistence with the sulfide of the Zr compound are adjusted to a predetermined ratio, whereby free cutting tool steel is obtained. Have gained. These use Mg, Ba, Sr, etc., but all have the same problems as Ca.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional problems.
b The sulfide-based inclusions are made into free-cutting materials as the steel, and the toughness and machinability are improved by making the sulfide dispersion form the most suitable, without significantly deteriorating the strength characteristics of the steel material.
Provided is a steel for machine structural use having excellent machinability in a wide range of cutting methods and cutting conditions.

[0009]

Means for Solving the Problems To solve the above problems,
According to the first aspect of the present invention, there is provided a free-cutting substance.
Low alloy steel for machine structural use containing sulfide inclusions
The sulfide particles of the low alloy steel for machine structural use have a long diameter.
The ratio of the long diameter of 1.5 μm or more to the distance between particles is not more than 0.5.
Of sulfide particles that are full and have a major axis of 1.5 μm or more
Average area is 9μm ^TwoMachining characterized by the above
Lead-free mechanical structural steel with excellent heat resistance and low strength anisotropy
It is.

According to the second aspect of the present invention, the low alloy steel for a machine structure containing a sulfide-based inclusion as a free-cutting substance has a mass% of
, C: 0.10-0.65%, Si: 0.03-1.
00%, Mn: 0.30 to 2.50%, S: 0.03 to
0.35%, Al: less than 0.020%, O: 20 ppm
And Ca: 0.0005 to 0.002
2. Means according to claim 1, characterized in that it contains one or two selected from 0% and Mg: 0.0003 to 0.020%, and the balance is Fe and inevitable impurities. And lead-free mechanical structural steel with low strength anisotropy.

According to the third aspect of the present invention, the low-alloy steel for machine structural use containing a sulfide-based inclusion as a free-cutting substance is added to the steel component of the second aspect, further comprising: 1 to
2.0%, Mo: 0.05 to 1.00%, Ni: 0.1
To 3.5%, V: 0.01 to 0.50%, Nb: 0.0
1 to 0.10%, Ti: 0.01 to 0.10%, B:
2. A lead-free mechanical structure having excellent machinability and small strength anisotropy according to claim 1, wherein the material contains one or two selected from 0.0005 to 0.0100%. It is steel.

Next, the reasons for limiting the steel components in the present invention will be described. In addition,% is shown by mass%. C: 0.10 to 0.65% C is an essential element for securing strength as steel for machine structural use, and is added in an amount of 0.10% or more. However, if the content is too large, the hardness increases, so that the toughness and the machinability deteriorate. Therefore, the upper limit is set to 0.65%. In particular, in the case of a non-tempered tough steel, the content is preferably 0.10 to 0.55%, and more preferably 0.35 to 0.50%. In case of case hardening steel, it is preferably 0.10 to 0.30%, more preferably 0.12 to 0.28%.

Si: 0.03 to 1.00% Since Si is indispensable as a deoxidizing agent in steel making, the lower limit is made 0.03%. However, an excessive addition lowers the ductility and also generates SiO ₂ , which is a high-hardness inclusion in the steel, thereby deteriorating the machinability. Therefore, the upper limit is set to 1.00%. Si is preferably 0.10 to 0.50%, and more preferably 0.15 to 0.35%, in any of the above three steel types.

Mn: 0.30 to 2.50% Mn is an important element in general in securing the strength, toughness, hot workability and hardenability of steel.
Since it is an indispensable element for the formation of sulfide-based inclusions, 0.3
Add 0% or more. However, if the content is too large, the machinability deteriorates due to the increase in hardness. Therefore, the upper limit is set to 2.50%. Mn is preferably 0.42 in any of the above three steel types.
2.00%, more preferably 0.60-1.5%
0% is good.

S: 0.03-0.35% S is an element forming sulfide-based inclusions for improving machinability. To obtain machinability improving effect, S is at least 0.03%.
It is necessary to add 3% or more, and the machinability improves with an increase in S. However, if the amount is too large, sulfide form control becomes difficult and impact anisotropy deteriorates.
%. S is preferably 0.04 to 0.30%, and more preferably 0.08 to 0.20%, in any of the above three steel types.

Al: less than 0.020% When the Al content is 0.020% or more, high hardness Al ₂
Since inclusions made of O ₃ are generated, which deteriorates machinability and lowers fatigue strength, the content is set to less than 0.020%. As for Al, there is almost no difference in the preferable range among the above three steel types.

O: less than 20 ppm O is desirably reduced as much as possible from the viewpoint of suppressing the formation of oxide-based hard inclusions harmful to machinability. O is 20
If the content is not less than ppm, the amount of oxide-based hard inclusions increases and the machinability is impaired, and the fatigue strength is reduced. Therefore, it is necessary to make O less than 20 ppm.

Ca: 0.0005 to 0.020% Ca is a sulfide-forming element together with Mn and Mg, and also forms a composite oxide with Al and Si, thereby improving machinability and controlling sulfide morphology. Has the effect of improving the anisotropy of the mechanical properties. Ca may be added alone, but desirably is added in combination with Mg. To obtain the effect, at least 0.0005% is required. In addition, the yield of Ca in the production stage is very poor, and the effect is saturated even if it is contained more than necessary. Therefore, the upper limit of Ca is set to 0.020%. Ca is preferably 0.0005 to 0.0060%, and more preferably 0.0005 to 0.0040% in any of the above three steel types.

Mg: 0.0003% to 0.020% Mg exhibits the same effect as Ca, and the effect can be obtained by adding it alone. However, when present in combination with Ca, the machinability is greatly improved. An effect and an effect of improving anisotropy of mechanical properties can be obtained. At least 0.000 to achieve that effect
3% or more is required. On the other hand, even if contained more than necessary,
Since the effect becomes saturated and is useless, the upper limit of Mg is set to 0.020%. Mg is preferably 0.0003 to 0.0060% in any of the above three steel types.
And more preferably 0.0005 to 0.0040%
Is good.

According to a third aspect of the present invention, in the lead-free steel for machine structural use, Cr: 0.1-2.
0%, Mo: 0.05 to 1.00%, Ni: 0.1 to
3.5%, V: 0.01 to 0.50%, Nb: 0.01
0.10%, Ti: 0.01-0.10%, B: 0.
It is desirable to contain one or two selected from 0005 to 0.0100%.

The reasons for limiting the range of these desirable components are shown below. Cr: 0.1 to 2.0%, Mo: 0.05 to 1.00
%, Ni: 0.1 to 3.5% Cr, Mo, and Ni are elements that improve the hardenability and toughness of steel and are added when necessary. In order to obtain the effect, Cr is 0.1% or more, Mo is 0.05% or more, Ni
Must be added in an amount of 0.1% or more. When added in large amounts, the hardness of the work material increases, so that Cr is 2.0% or less and Mo is 1.00% in order to ensure machinability.
Hereinafter, Ni needs to be 3.5% or less. C
r is preferably 0.10 to 1.50%, more preferably 0.15% in any of the above three steel types.
~ 1.20% is good. Mo is preferably 0.10 to 0.40%, and more preferably 0.15 to 0.30%, in any of the above three steel types. Also, Ni
Is preferably 0.40 to 3.00%, more preferably 0.42% in any of the above three steel types.
~ 2.00% is good.

V: 0.01 to 0.50% V is an element having a strong precipitation strengthening effect, and is added when the quenching and tempering treatment is omitted. To obtain this effect, it is desirable to add 0.01% or more. On the other hand,
Even if the content exceeds 0.50%, the effect is saturated, so the upper limit is preferably set to 0.50%. In the case of non-heat treated steel, the content is more preferably 0.05 to 0.35%, and even more preferably 0.05 to 0.30%.

Nb: 0.01 to 0.10%, Ti: 0.
01 to 0.10% Nb and Ti each generate carbonitride and have an effect of refining crystal grains by a pinning effect, and are added as necessary. To obtain this effect, 0.01% or more is necessary. However, if the content exceeds 0.10%, the effect is saturated, so the upper limit is preferably set to 0.10%. More preferably, it is 0.01 to 0.08%, and still more preferably, 0.01 to 0.06%.

B: 0.0005 to 0.0100% B contains a small amount of B and has the effect of improving the hardenability and improving the mechanical properties of steel, and is added as necessary. To obtain this effect, 0.0005% or more is required.
Even if the content exceeds 0.0100%, the effect is saturated, so the upper limit is preferably set to 0.0100%. More preferably, it is 0.0005 to 0.0060%, and still more preferably 0.0005 to 0.0040%.

The above lead-free steel for machine structural use contains MnS, (Ca, Mn) S, (M
g, Mn) S, (Ca, Mg) S, (Ca, Mg, M
n) It is preferable to contain one or more of S. There are various sulfides of S with Ca, Mg, and Mn. In particular, complex sulfides of Ca, Mg, and S (C
By containing at least one of a, Mg) S or a complex sulfide (Ca, Mg, Mn) S of Ca, Mg, Mn, machinability and mechanical properties can be significantly improved. it can. However, these sulfides have different effects on machinability and impact characteristics depending on their size and number. Since a sulfide having a major axis of less than 1.5 μm has almost no effect on machinability and impact properties, only sulfides having a diameter of 1.5 μm or more were measured. The T direction impact value is improved when the ratio of the major axis of the sulfide to the distance between the particles is smaller than 0.5, and the machinability is improved when the average sectional area of the sulfide is 9 μm ² or more. In order to control the sulfide to these states, it is necessary to add Ca and Mg in a complex manner to obtain the sulfide composition described above.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention according to the first aspect of the present invention relates to a free-cutting low-alloy steel for machine structures containing a sulfide-based inclusion as a free-cutting substance, for example, a chromium steel specified as JIS SCR. Chromium steel containing Mg, Ca, and S as components is melted, and the long diameter of the sulfide particles having a long diameter of 1.5 μm or more interposed in the steel material by hot rolling or hot forging of the chromium steel material is 1.5 μm or more. The ratio of the distance between the particles and the distance between particles is smaller than 0.5, and the average area of the sulfide particles having a major axis of 1.5 μm or more is 9 μm ² or more, and quenched and tempered to produce a lead-free free-cutting steel material What you get.

According to an embodiment of the present invention, the low-alloy steel for machine structure containing a sulfide-based inclusion as a free-cutting substance has a C content of 0.10 to 0.65% by mass%. , Si:
0.03 to 1.00%, Mn: 0.30 to 2.50%,
S: 0.03 to 0.35%, Al: less than 0.020%,
O: contains less than 20 ppm, and Ca: 0.000
5 to 0.020%, Mg: 0.0003 to 0.020%
A chromium steel containing one or two selected from the group consisting of Fe and unavoidable impurities is melted, and the steel material of the chromium steel is subjected to hot rolling or hot forging at a rolling ratio or a forging ratio of 4 to 625. The major axis interposed in the steel material is 1.5
The ratio of the major axis of sulfide particles having a major axis of not less than 0.5 μm to the distance between the particles is smaller than 0.5, and the average area of the sulfide particles having a major axis of not less than 1.5 μm is 9 μm ² or greater. A lead free-cutting steel material is obtained.

According to a third aspect of the present invention, there is provided a low alloy steel for a machine structure containing a sulfide-based inclusion as a free-cutting substance, wherein the steel component according to the second aspect of the present invention is used. Further, in mass%, Cr: 0.1 to 2.0%, Mo:
0.05-1.00%, Ni: 0.1-3.5%, V:
0.01 to 0.50%, Nb: 0.01 to 0.10%,
Ti: 0.01-0.10%, B: 0.0005-0.
Chromium steel containing one or two types selected from 0100%, the balance being Fe and unavoidable impurities is melted, and the steel material of the chrome steel is hot-rolled or hot-rolled at a rolling ratio or a forging ratio of 4 to 625. By forging, the ratio of the long diameter of the sulfide particles having a long diameter interposed in the steel material of 1.5 μm or more to the distance between the particles is smaller than 0.5, and the long diameter of the sulfide particles of 1.5 μm or more is obtained. The average area is 9 μm ² or more, and quenching and tempering are performed to obtain a lead-free free-cutting steel material.

[0029]

Embodiments of the present invention will be described below. In addition to the steel with the chemical composition shown in Table 1, the steel with the addition of Ca or Mg was further melted in addition to the composite steel with the steel of the present invention and the free-cutting steel with the comparative example, and these steels were heat-treated at a forging ratio of 32.7. Forging and quenching and tempering were performed to obtain a steel material having a hardness of 32 HRC.

[0030]

[Table 1]

The obtained steel material was subjected to a drill wear test using a high speed drill SKH51 under the drilling conditions shown in Table 2. In FIG. 1, 0.5mm from the corner of the drill flank
The relationship between the wear amount at the position and the sulfide size (average area) is shown. As a result, in the Ca or Mg-containing steel, the wear amount decreases as the sulfide size increases, and the larger the sulfide, the larger the contribution to machinability. On the other hand, in S2 steel, the sulfide size is small but the number is large, and sulfide is unevenly distributed.
Therefore, the presence of many sulfides and the uneven distribution of sulfides are also effective for machinability. From the above, considering that the larger sulfide size is more effective in improving machinability, the role of sulfide during cutting is to concentrate stress and strain around sulfide and to concentrate deformation and fracture. It can be seen that the energy required for cutting (cutting resistance and heat) is reduced by this. If the average area of the sulfide having a major axis of 1.5 μm or more is 9 μm ² or more, the wear amount is represented by Pb:
It can be equal to or higher than 0.17% lead free-cutting steel.

[0032]

[Table 2]

In the same manner as described above, the steel types shown in Table 1 were melted, and the steel materials were hot forged at a forging ratio of 6.3 and quenched and tempered to obtain a steel material having a hardness of 32 HRC. The obtained steel material was subjected to a Charpy impact test. FIG. 2 shows the relationship between these steel types and the Charpy impact value (normal temperature) in the T direction in the forging direction. As a result, the impact values in the T direction of all the Ca and Mg-containing steels were higher than the S1 level (15 J / cm ² ). FIG. 3 shows micrographs of the sulfides of the S2 steel and the Ca and Mg-containing steel and the fracture surface after the test. As can be seen from these, in S2 steel, many sulfides are crystallized and unevenly distributed between dendrite trees in a solid-liquid coexistence state. Possible and evenly dispersed. By adding Ca and Mg individually or in combination, the sulfide length (major axis) in the rolling direction or the forging direction is shortened (aspect ratio is reduced), and spheroidization of the sulfide occurs. As can be seen from the fracture surface of the example of the composite added steel shown in FIG. 3, the sulfide has a ductile fracture surface due to a decrease in aspect ratio and dispersion of the sulfide. Note that VNb, MoC, Ti
The same evaluations as in the other examples were performed on the four steel types V and TiB, and the effects of improving the T-direction toughness and machinability were recognized.

From the above, it is considered that the deterioration of toughness due to sulfide is mainly governed by the major axis of sulfide and the spacing between sulfide particles. The distance between particles in the S2 steel was evaluated in a portion where sulfides are locally unevenly distributed, and in other steel types, when converted from the total number, the Charpy value in the T direction and the sulfide long diameter / interparticle distance in FIG. 7 were obtained. As can be seen from the relationship, the T direction impact value is better when the sulfide major diameter / interparticle distance is smaller, and the T direction impact value increases as the sulfide becomes more spherical and the interparticle distance increases. You can see that

[0035]

As described above, according to the present invention, lead-free steel is used in consideration of environmental issues, sulfide-based inclusions are used as free-cutting substances, and an optimum sulfide dispersion form is obtained. Since the sulfide particles having a diameter of 1.5 μm or more have a ratio of the major axis to the distance between the particles of less than 0.5, and the average area of the sulfide particles having a major axis of 1.5 μm or more is 9 μm ² or more, Improved toughness and machinability, with excellent machinability in a wide range of cutting methods and cutting conditions without significantly deteriorating the strength characteristics of steel materials, especially for carbide tool wear resistance and chip processing Excellent mechanical structural steel.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a wear amount and a sulfide size (average area).

FIG. 2 is a graph showing the relationship of the Charpy impact value in T direction (normal temperature) in the forging direction.

FIG. 3 is a photomicrograph of sulfides of S2 steel and Ca, Mg-containing steel and a fracture surface after the test.

FIG. 4 is a graph showing the relationship between the Charpy value in the T direction and the major diameter of sulfide / distance between particles.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Motohide Mori 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Co., Ltd. (72) Inventor Naoki Iwama 1 Wanowari Arao Town, Tokai City, Aichi Prefecture Steel Co., Ltd. Inside the company (72) Inventor Masao Uchiyama 1 Wanowari, Arao-cho, Tokai-shi, Aichi Prefecture Inside Aichi Steel Co., Ltd. 72) Inventor Norimasa Join 3007 Nakajima character, Shima, Himeji City, Hyogo Prefecture Sanyo Special Steel Co., Ltd.

Claims (3)

[Claims] 1. Sulfide particles comprising a low-alloy steel for machine structure containing a sulfide-based inclusion as a free-cutting substance, wherein the long diameter of the low-alloy steel for machine structure is 1.5 μm or more. The ratio of the distance is less than 0.5 and the major axis is 1.5 μm
The lead-free mechanical structural steel having excellent machinability and low strength anisotropy, wherein the average area of the sulfide particles is 9 μm ² or more. 2. The low-alloy steel for machine structure containing a sulfide-based inclusion as a free-cutting substance has a C content of 0.10 to 0.10% by mass.
0.65%, Si: 0.03 to 1.00%, Mn: 0.
30 to 2.50%, S: 0.03 to 0.35%, Al:
Less than 0.020%, O: less than 20 ppm, Ca: 0.0005-0.0020%, Mg: 0.0
The lead-free material having excellent machinability and small strength anisotropy according to claim 1, characterized in that it contains one or two kinds selected from 003 to 0.020%, and the balance is Fe and unavoidable impurities. Addition of mechanical structural steel. 3. The low-alloy steel for machine structural use containing a sulfide-based inclusion as a free-cutting substance, further comprises: Mo: 0.05
1.00%, Ni: 0.1-3.5%, V: 0.01
~ 0.50%, Nb: 0.01 ~ 0.10%, Ti:
0.01 to 0.10%, B: 0.0005 to 0.010
The lead-free mechanical structural steel according to claim 1, which contains one or two selected from 0%, and has excellent machinability and low strength anisotropy.