JPH06212349A - High toughness non-heat-treated high strength steel having high machinability and it production - Google Patents

Abstract

PURPOSE:To develop high toughness non-heat-treated high strength steel whose structure is formed into a bainitic one small in pro-eutectoid ferrite and excellent in machinability by subjecting carbon steel having a specified compsn. to hot forging and tempering treatment under specified temp. conditions. CONSTITUTION:An ingot having a compsn. contg., by weight, 0.07 to 0.20% C, 0.05 to 1.00% Si, 0.50 to 2.50% Mn, 0.005 to 0.150% S, 0.50 to 2.50% Cr, 0.05 to 0.80% Mo, 0.005 to 0.30% Bi, 0.005 to 0.20% V, 0.005 to 0.080% Nb, 0.005 to 0.080% Ti and 0.005 to 0.050% Sol.Al is heated to 950 to 1250 deg.C, is subjected to hot forging at >=750 deg.C finishing temp. and is worked to form its shape into a prescribed one. Or, after that, it is furthermore subjected to tempering treatment at 300 to 650 deg.C. The structure is formed into a bainitic one in which the area ratio of pro-eutectoid ferrite is regulated to <=10%, and A series inclusions are dispersed at the abundance of to 1000 pieces/mm<2> into a fine shape of 1 to 100mum average length and 5 to 50mum average width by the presence of Bi, by which the high toughness non-heat-treated high strength steel excellent in machinability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high machinability, high toughness non-heat treated high strength steel which is hot forged or is hot forged and then tempered.

[0002]

2. Description of the Related Art Many parts for mechanical structures such as automobile parts are formed into a predetermined shape by hot forging such as hot forging press, then quenched and tempered, and further subjected to machining such as cutting and polishing. Being manufactured. Such heat treatment has been carried out more often than ever since it is a very useful means for bringing the mechanical properties of the parts to the required values. Steel subjected to or obtained by such heat treatment is called heat-treated steel.

On the other hand, in recent years, there has been a strong demand for rationalization of manufacturing lines and improvement of productivity, and a proposal for a non-heat treated steel in which quenching and tempering treatment is omitted (for example, JP-A-61-238941 and JP-A-62-202054).
No. 62-207821). However, these proposals are mainly aimed at increasing the strength by using non-heat treated steel, and although the purpose has been achieved, the problem of machinability has recently become conspicuous in recent years.

For this reason, attempts have been made to improve the machinability by adding S or Pb, but if such an element is contained in an amount that contributes to the improvement of the machinability, the toughness is significantly deteriorated and it cannot be put to practical use. There was a problem. Therefore,
Even in the past, there has been a demand for the development of a high toughness non-heat treated high strength steel that has good machinability.

[0005]

Thus, the object of the present invention is to achieve high cutting strength which can ensure high strength and high toughness and can significantly improve the machinability by hot forging or by performing tempering treatment thereafter. A high toughness non-heat treated high strength steel and its manufacturing method.

[0006]

Means for Solving the Problems As a result of various studies to solve such problems, the present inventors have found that in order to improve the machinability, toughness, and strength of non-heat treated high strength steel, I have found that the means shown are effective.

(1) In order to satisfy both high strength and high toughness at the same time, the area ratio of proeutectoid ferrite was suppressed to 10% or less,
It is necessary to have a bainite structure. (2) In this case, the decrease in machinability becomes a problem, but Bi is 0.00
Addition of 5 to 0.300% exerts a remarkable improvement effect on machinability.

(3) Even if 0.005 to 0.300% of Bi is added, the toughness does not deteriorate. (4) This is the result of the addition of Bi finely dispersing the A-type inclusions (JIS-G0202), which resulted in the subdivision of the bainite structure.

Thus, the gist of the present invention is as follows.
% By mass, C: 0.07 to 0.20%, Si: 0.05 to 1.00
%, Mn: 0.50 to 2.50%, S: 0.005 to 0.150%, Cr:
0.50 to 2.50%, Mo: 0.05 to 0.80%, Bi: 0.005 to 0.
30%, V: 0.005 to 0.200%, Nb: 0.005 to 0.080
%, Ti: 0.005 to 0.050%, sol.Al: 0.005 to 0.050
% Of steel having a bainite structure in which the area ratio of proeutectoid ferrite is 10% or less, and A
System inclusions (JIS G 0202) are present at a frequency of 50 to 1000 / m
m ² , average length 1 to 100 μm, average width 0.5 to 50 μm
It is a high machinability, high toughness, non-heat treated, high strength steel.

The present invention further provides a steel having the above steel composition at 950 ° C.
A method for producing the above-mentioned high machinability, high toughness non-heat treated high strength steel by heating above 1250 ° C or lower and then hot forging to form it into a predetermined shape at a temperature of 750 ° C or higher. According to another aspect of the present invention, after hot forging, further 300 ~
You may temper in the temperature range of 650 degreeC.

[0011]

Next, the reason why the steel composition and the processing conditions are defined in the present invention as described above will be explained together with the operation of the present invention. In this specification, unless otherwise specified,
"%" Is% by mass.

C: 0.07 to 0.20% C is an element necessary for increasing the strength, and if 0.07% or more is not contained, the predetermined strength cannot be satisfied. On the other hand, if the content of C exceeds 0.20%, the strength increasing effect is obtained, but conversely, the toughness is remarkably deteriorated, so 0.07 to 0.20%
Was defined as the component range.

Si: 0.05-1.00% Si is an element added as a deoxidizing agent during steelmaking.
Therefore, 0.05% or more is contained. On the other hand, if the content exceeds 1.00%, not only the deoxidizing effect is saturated, but also the toughness is remarkably lowered, so the content was made 0.05 to 1.00%.

Mn: 0.50 to 2.50% Mn is an element that improves the hardenability and exerts an effect of making the metal structure of the steel sheet after hot forging into a bainite structure. Therefore, 0.50% or more is contained. Let On the other hand, 2.50%
If the content is exceeded, not only will the effect be saturated,
In order to increase the manufacturing cost, it was set to 0.50 to 2.50%, preferably 1.50 to 2.20%.

S: 0.005 to 0.150% S has the function of improving the machinability as MnS and, at the same time, having the function of refining the crystal grains and improving the toughness, so 0.005% or more is contained. On the other hand, if it is contained in excess of 0.150%, huge MnS will be formed and the fatigue properties will be deteriorated. Therefore, the range of 0.005 to 0.150% was set as the component range.

Cr: 0.50 to 2.50% Cr is an element effective for imparting a predetermined hardenability to steel and improving static strength. 0.50% or more is contained in order to fully exhibit the effect. On the other hand, if the content exceeds 2.50%, the toughness decreases. Therefore, 0.50 to 2.50% was set as the Cr component range. It is preferably 1.0 to 2.0%.

Mo: 0.05 to 0.80% Mo is an element effective for imparting a predetermined hardenability to steel and improving static strength and toughness. It also has the effect of improving the temper softening resistance. 0 to make full use of its effect.
Include at least 05%. On the other hand, if the content exceeds 0.80%, the effect is saturated and the economic efficiency is impaired.
% Was defined as the component range.

Bi: 0.005 to 0.30% Bi is a finely dispersed A type inclusion (JIS-G0202),
It is an element that contributes to the subdivision of the bainite structure and is effective in significantly improving the machinability. It is contained in an amount of 0.005% or more in order to fully exert its machinability improving effect. On the other hand, if the content exceeds 0.30%, the effect is saturated and the economy is impaired. By adding in the range of 0.005 to 0.30%, the presence frequency of A type inclusions is 50 to 1000 pieces / mm ² , and the average length is 1 to 100.
μm, 0.5 to 50 μm in average width, greatly improving machinability. Therefore, the composition range is set to 0.005 to 0.30%.

Nb: 0.005 to 0.080, V: 0.005 to 0.200 These elements have a function of making the structure after hot forging into a uniform fine bainite structure and improving the strength and toughness of bainite. In addition, the temper softening resistance is increased, which greatly contributes to high YR. In order to fully bring out the effect, it is necessary to contain 0.005% or more of both. On the other hand, Nb0.080
%, V0.200%, the effect is saturated and the economical efficiency is impaired, so 0.005 to 0.080% and 0.
The composition range was 005 to 0.200%. In addition, the combined addition of Nb and V exerts a remarkable softening resistance rather than the addition of Nb and V alone.

Ti: 0.005 to 0.050% Ti has the effect of forming nitrides as TiN and refining the crystal grains. Therefore, the minimum content is 0.005%. On the other hand, if the content exceeds 0.050%, a huge Ti nitride is formed and the toughness is reduced. Therefore, Ti: 0.005 ~
It was set to 0.050%.

Sol.Al: 0.005 to 0.050 sol.Al is an element necessary for deoxidizing steel, like Si and Mn.
It also has the effect of producing Al nitrides and refining the crystal grains. The minimum content required for this is 0.005%. However, if the content exceeds 0.050%, not only a huge Al oxide is generated and the fatigue strength is lowered, but also the crystal grains are coarsened and the toughness is lowered. Therefore 0.005-0.05
The composition range was 0%.

The high machinability, high toughness and non-heat treated high strength steel according to the present invention has a bainite structure, and the pro-eutectoid ferrite is limited to an area ratio of 10% or less. This is because if the area ratio of proeutectoid ferrite exceeds 10%, the strength is significantly reduced. Furthermore, in the present invention, the existence form of the A-type inclusions defined by JIS G 0202 is limited.

That is, in order to improve the machinability,
A-type inclusions (JIS G0202) are present at a frequency of 50 to 1000 / m
m ² , average length 1-100 μm, average width 0.5-50 μm
To To do so, the amount of Bi added should be 0.00
5 to 0.30%. If the added amount is less than 0.005%, A
Presence of system inclusions <50 / mm ² , average length 100 μ
Since it exceeds m and the average width exceeds 50 μm, the machinability cannot be improved. On the other hand, even if added over 0.30%, the effect is saturated. Therefore, the amount of Bi added is 0.005 to 0.3.
The morphology of A-type inclusions is controlled by setting it to 0%. Next, according to the present invention, the steel having the above-mentioned steel composition is heated to 950 to 1250 ° C and then hot forged such as a hot forging press at a finishing temperature of 750 ° C or higher.

Heating temperature: 950 to 1250 ° C. If heating is less than 950 ° C., the additives do not sufficiently form a solid solution in the steel, and the prescribed strength and toughness cannot be obtained. On the other hand, if heated above 1250 ° C., the additive becomes saturated and the effects of various additive elements cannot be obtained. Therefore, it is limited to 950 to 1250 ° C.

Finishing temperature: After heating to 750 ° C. or more , hot forging is performed to form a predetermined shape. At that time, in order to obtain predetermined strength and toughness, a bainite structure having an area ratio of proeutectoid ferrite of 10% or less is obtained. To do. For that purpose, hot forging is performed at a finishing temperature of 750 ° C or higher. When the finishing temperature is less than 750 ° C, the area ratio of pro-eutectoid ferrite is
If it exceeds 10%, the strength is significantly reduced. Therefore, 750 ° C
That is all. It is preferably 850 ° C. or higher. In other words, the amount of pro-eutectoid ferrite in the bainite structure can be adjusted by changing the finishing temperature. As described above, other forming means and conditions by forging are not limited to particular ones.

Tempering temperature: 300 to 650 ° C. In the present invention, if necessary, tempering is performed to improve YR. A tempering temperature of at least 300 ° C is required to exert such effects. But 65
If the temperature exceeds 0 ° C, the strength will be significantly reduced. Therefore, 300 ~
650 ℃.

[0027]

【Example】

(Example 1) 150 kg of each steel having the chemical composition shown in Table 1 was melted in the air, heated to 950 ° C or higher and 1250 ° C or lower, and then hot forged by a hot forging press at a predetermined finishing temperature of 750 ° C or higher. It was molded into a shape (bar material with a diameter of 20 to 60 mm) and allowed to cool. Mechanical properties were investigated by producing JIS No. 4 tensile test pieces from the center of these simulated hot forged materials and JIS No. 3 Charpy test pieces from the R / 2 section. In addition, as a machinability test, the tool life (crater wear) when turning with a cemented carbide tool was investigated. The test results are summarized in Table 2.

Regarding these data and Example 2 which will be described later, the relationship between the Bi content and the number of A-type inclusions, the relationship between the number of A-type inclusions and the machinability improvement index, and the number of A-type inclusions and uE _+. The relationship with the ₂₀ values is summarized and shown in FIGS. The machinability improvement index is the crater wear ratio A / B when turning under the following conditions. A: Tool life when cutting steel of the present invention B: Tool life when cutting Bi-free steel However, tool life is judged by crater wear
Determined by cutting time until wear of 0.2 mm.

Cutting conditions: Cutting tool material: P20 Depth of cut: 2.0 mm Cutting speed: 100 m / min (dry type) As shown in FIG. 1, as the Bi content increases, the number of A-type inclusions increases. You can see it increasing. this is,
This is because Bi makes A-type inclusions fine.

Further, as shown in FIG. 2, by increasing the number of A-type inclusions, the tool life (crater wear) when turning with a cemented carbide tool is improved as compared with the Bi-free level. . Similarly, as shown in FIG. 3, when the number of A-type inclusions increases, the impact characteristics at room temperature improve.

(Example 2) 15 steels having the chemical composition shown in Table 1 were used.
After melting in air at 0 kg and heating above 950 ℃ to 1250 ℃,
By hot forging, the desired shape (diameter 20
(~ 60 mm), allowed to cool, and then tempered in the temperature range of 250 ° C to 700 ° C. After that, from the center of the simulation hot forged material to the JIS No. 4 tensile test piece and from the R / 2 part to JI
The S3 Charpy test piece was manufactured to investigate the mechanical properties.

Further, as a machinability test, the tool life (crater wear) when turning with a cemented carbide tool was investigated. Table 3 shows the test results.

As is clear from Tables 2 and 3, the steels obtained according to the present invention are excellent in both tensile strength and Charpy absorbed energy value, and have high strength and high toughness. On the other hand, among the comparative steels, those in which C, Si, Mn, Cr, Ti and sol. Al deviate higher than the specified values have poor impact properties. Also,
If C, Ti, and sol. Al deviate below the specified values, the strength will decrease. Further, as shown in Table 3, by performing forging and cooling after cooling, high strength and high toughness can be maintained and a high YR (yield ratio) can be obtained.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

As described above, according to the present invention, a bainite structure having an area ratio of proeutectoid ferrite of 10% or less can be obtained, and stable high strength and high toughness can be obtained regardless of the product size. Further, in the present invention, high YR can be achieved by maintaining such high TS and performing tempering without degrading toughness. In particular, as the strength of hot-forged non-heat treated steel becomes higher, the problem of machinability has become apparent, so that the present invention has great practical significance.

[Brief description of drawings]

FIG. 1 is a graph showing the results of Examples.

FIG. 2 is a graph showing the results of Examples.

FIG. 3 is a graph showing the results of Examples.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuhiko Nishida 1 Kunomi-cho, Kokurakita-ku, Kitakyushu City Sumitomo Metal Industries, Ltd. Kokura Works

Claims (3)

[Claims] 1. In mass%, C: 0.07 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.
50 to 2.50%, S: 0.005 to 0.150%, Cr: 0.50 to 2.50%, Mo: 0.
05 to 0.80%, Bi: 0.005 to 0.30%, V: 0.005 to 0.200%, Nb: 0.
It has a steel composition consisting of 005 to 0.080%, Ti: 0.005 to 0.050%, sol.Al: 0.005 to 0.050%, and consists of a bainite structure with an area ratio of proeutectoid ferrite of 10% or less. G 020
2) Presence frequency is 50 to 1000 pieces / mm ² , average length is 1 to 100
μm, average width 0.5 to 50 μm, high machinability, high toughness non-heat treated high strength steel. 2. In mass%, C: 0.07 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.
50 to 2.50%, S: 0.005 to 0.150%, Cr: 0.50 to 2.50%, Mo: 0.
05 to 0.80%, Bi: 0.005 to 0.30%, V: 0.005 to 0.200%, Nb: 0.
Steel having a steel composition consisting of 005 to 0.080%, Ti: 0.005 to 0.050%, and sol.Al: 0.005 to 0.050% is heated to 950 ° C or higher and 1250 ° C or lower, and then hot forged at a temperature of 750 ° C or higher. The area ratio of pro-eutectoid ferrite is 10 by forming it into the specified shape.
% Or less of bainite structure and A type inclusions (JIS G
0202) has a frequency of existence of 50 to 1000 pieces / mm ² , and an average length of 1 to
A method for producing a high machinability, high toughness, non-heat treated high strength steel, characterized in that the average width is 100 μm and the average width is 0.5 to 50 μm. 3. The method for producing a high machinability, high toughness, non-heat treated high strength steel according to claim 1, wherein tempering is performed in a temperature range of 300 to 650 ° C. after hot forging.