JPH0770698A - High fatigue strength free-cutting non-heat treated steel - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a steel that has high tensile strength and excellent fatigue strength in an untempered state and is also excellent in machinability. [Structure] C: 0.30 to 0.50% by weight, Si:
0.3-1.5%, Mn: 0.5-2.0%, P: 0.02
-0.10%, S: 0.15% or less, Cr: 0.7% or less, V: 0.05-0.3%, Cu: 0.5-if necessary
1.0%, the balance consisting of Fe and inevitable impurities,
And high fatigue strength non-heat treated steel that satisfies the following formula. [Equation 1] [C%] + 0.28 × ([Mn%] + [Cr
%]) <0.92 [Equation 2] 80 × [C%] + 8 × [Si%] + 15 × [M
n%] + 55 × [P%] + 18 × [Cr%] + 132 ×
[V%] + 3 × [Cu%]> 70 Furthermore, in order to improve machinability, Pb, Ca, Se,
A small amount of one or more elements selected from the group consisting of Te and Bi can be contained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel having high tensile strength and excellent fatigue strength in an untempered state, and in particular, a ratio of fatigue strength to tensile strength (durability ratio) in an untempered state. high,
The present invention also relates to steel having excellent machinability.

[0002]

2. Description of the Prior Art Conventionally, forged parts for automobiles such as connecting rods, crankshafts, knuckles, etc. are made of carbon steel or alloy steel for machine structure, and are given high fatigue strength after being formed into parts by hot forging. The tempering process (quenching and tempering process) was performed. However, due to the problems of cost, labor saving, heat treatment deformation, etc., it has been desired to develop a steel that can obtain high fatigue strength and tensile strength as-forged (non-heat treated) by omitting the heat treatment.

Further, since forged parts for automobiles are machined after forging, they are required to have excellent machinability. From such a background, in JP-A-4-202741 and JP-A-4-193931 entitled "Hot forged product having excellent fatigue strength", a hot work having excellent fatigue strength in a non-tempered state. The chemical composition of the forged product and heat treatment conditions are disclosed. In these non-heat treated steels, 0.15 to 0.60 wt% of V is added as a strengthening element, while the addition of Si is made as low as 0.30 wt% or less.

By the way, although the effect of strengthening by the addition of the precipitation strengthening element saturates at a certain amount or more, the strengthening by the addition of the solid solution element can be added as the solid solution strengthening. For example, the tensile strength and fatigue strength can be effectively increased by the combined addition of the solid solution element Si or P and the precipitation element V. However,
In the above-mentioned conventional non-heat treated steel, only precipitation strengthening by expensive V is used, and solid solution strengthening by inexpensive Si is not used at all. Furthermore, in the above-mentioned conventional non-heat treated steel, since the black skin forged skin is often used in the first place, an element effective for machinability such as S, Pb, or Ca is added if necessary, There was no intention to make the structure of the steel itself a ferrite + pearlite structure that is easy to cut, merely by improving the machinability.

Matsubara et al., "High fatigue strength free-cutting non-heat treated steel for connecting rods" (Honda R & D Technical Re
view) was developed. This is a description of the composition and heat treatment conditions of a hot forged product which has excellent fatigue strength and machinability while still being non-heat treated.

However, in the above-mentioned non-heat treated steel, the fatigue strength is drastically lowered by the addition of the free-machining property-imparting component. That is, optimal inclusion control is not performed. Further, P, which greatly contributes to the improvement of machinability and fatigue strength, was not positively added, and it seems that there is room for improvement of the chemical composition.

[0007]

According to the present invention, by appropriately selecting the chemical composition, the tensile strength (900 MPa) required for as-forged and a high durability ratio of 0.5% or more (fatigue strength / tensile strength) An object of the present invention is to provide a steel having the above characteristics and excellent machinability.

[0008]

Therefore, the present inventors have
After repeated studies to solve the above problems, we decided to consider a solution in three stages. That is, (1) formation of a ferrite-pearlite structure excellent in machinability, (2) improvement of durability ratio of steel having a ferrite-pearlite structure, (3) improvement of machinability by addition of an element imparting free machinability, Is.

(1) As described above, forged parts for automobiles are generally required to have excellent machinability as well as tensile strength and fatigue strength. The machinability depends strongly on the metal structure, and the ferrite-pearlite structure has good machinability.
It is known that the ferrite-bainite or bainite single phase structure has poor machinability. Therefore, first, a non-heat treated steel having a ferrite-pearlite structure containing no free-cutting property-imparting elements (Pb, Ca, Se, Te, Bi) was formed, and an attempt was made to improve the fatigue strength of the steel.

First, in order to understand the relationship between the added amount of each element of C, Mn, and Cr, which is considered to have a great influence on the hardenability in obtaining the ferrite-pearlite structure, and the structure, 9 is shown in Table 1. The types of materials were melted, prototyped, and the structure of the resulting steel was investigated.

[0011]

[Table 1]

A hot forging process of forging a slab at 900 ° C. or higher and then allowing it to cool is simulated to 1100 ° C.
It was replaced with a hot forging step by normalizing the material by holding it for a minute and then allowing it to cool. The results of the heat treatment are also shown in Table 1. Mn,
Since the hardenability multiples of Cr (JIS-G0561) are almost the same, the structure was arranged as a function of the weight% of C and the weight% of Mn + Cr, and the results are shown in FIG.

From FIG. 1,

## EQU7 ## If [C%] + 0.28 × ([Mn%] + [Cr%]) <0.92, it can be seen that a ferrite-pearlite structure is formed.

(2) The fatigue strength of a ferrite-pearlite structure steel is determined by the fatigue strength of the soft ferrite phase.
It is necessary to add a strengthening element. It is natural that the addition of the strengthening element improves the tensile strength.

Therefore, prior to the completion of the present invention, the influence of the additive elements other than the free machinability imparting element on the fatigue strength and the tensile strength was investigated. C 0.35%, Si 0.30%, M
n 1.20%, P 0.010%, S 0.050%, C
r 0.2%, V 0.01%, Cu 0.00% Based on No. 1 steel consisting of balance Fe and unavoidable impurities, C, Si, P, Cr, V, Cu Steel Nos. 2 to 13 with different addition amounts of No. 1100 ° C for simulating the hot forging process for each of these steels, 1
After normalizing for 5 minutes, the test piece (tensile test piece (JIS 14
No.) and an Ono-type rotary bending fatigue test piece (parallel portion 8φ)) and subjected to the test. Table 2 shows the chemical composition, tensile strength, and Ono-type rotary bending fatigue limit of each test material, and Fig. 2 shows the relationship between tensile strength and fatigue limit.

[0016]

[Table 2]

From the above results, C, Si, Mn, P, C
The addition of any of r, V, and Cu improves both the tensile strength and the fatigue limit of steel, but the ratio of the fatigue limit increase amount to the tensile strength increase amount (gradient of the graph in FIG. 2).
It turns out that is different. The numerical values of this ratio are shown in Table 3.

[0018]

[Table 3]

Generally, an increase in tensile strength also leads to a decrease in machinability and hot forgeability. Therefore, it is effective to add a reinforcing element that suppresses the increase in tensile strength and increases fatigue strength as much as possible. From the results of FIG. 2 and Table 3, it can be seen that other additive elements, particularly Si, V, and P, are more effective in improving fatigue strength than C.

Conventionally, Si promotes decarburization of the surface, so that addition of Si has been positively avoided. However, it is considered that the addition of an appropriate amount makes the plus side (solid solution strengthening by addition of Si) larger than the minus side (decrease in surface hardness due to decarburization) exerted on fatigue strength.

Further, from the results of Table 2, a relational expression between the amount of each element added and the tensile strength of the non-heat treated steel was obtained by multiple regression.

[0022]

## EQU00008 ## TS (MPa) = 785 × [C%] + 78 × [Si%] + 145 × [Mn%] + 540 × [P%] + 175 × [Cr%] + 1295 × [V%] + 30 × [Cu% ] +200

However, [%] is the weight% of each additive element. When TS> 900MPa, the coefficient is reduced to

80 × [C%] + 8 × [Si%] + 15 × [Mn%] + 55 × [P%] + 18 × [Cr%] + 132 × [V%] + 3 × [Cu%]> 70 can get.

(3) Next, the improvement of the machinability of steel by the addition of the free machinability imparting element will be examined. It is known that the addition of free-cutting property imparting elements (S, Pb, Ca, Se, Te, Bi) improves the machinability but decreases the fatigue strength. Therefore, the effect of the addition amount of the free machinability imparting element on the fatigue strength was investigated. That is, changes in fatigue limit were investigated when S, Pb, Ca, Se, Te, and Bi were added to the No. 10 component of Table 2 alone. The relationship between the additive amount and the fatigue limit is shown in FIG.

As can be seen from FIGS. 3 (c) and 3 (d), the fatigue limit of steel greatly decreases as the amounts of S and Pb added increase. However, as can be seen from FIGS. 3 (a), (b) and (d), the fatigue strength of the steel hardly changes with other elements even if the addition amount is increased. The upper limit of the amount added is the durability ratio of 0.
Assuming that the addition amount is 5, S is 0.15% and Pb is 0.5.
It is necessary to set 0% as the upper limit.

The upper limits of the amounts of other elements added cannot be determined from the viewpoint of fatigue strength. Lower limit of addition amount and S,
The upper limit of the amount of addition other than Pb will be examined from the viewpoint of machinability.
A cutting test was conducted to evaluate the machinability. The cutting conditions are
The speed is 200 m / min., The feed is 0.35 mm / rev., The depth of cut is 2.0 mm, the tool is CNMG-433MA-CSI manufactured by Mitsubishi Materials, and the cutting range of the test material is 50φ.

A tool wear curve was prepared from the test results, and the cutting time at which the amount of flank wear was 0.2 mm was determined and used as the tool life. FIG. 4 shows the relationship between the addition amount of each free-cutting property imparting element and the tool life.

Here, the tool life is 1 as the lower limit of the addition amount.
The addition amount is 0 min. Pb, Ca, Se, Te,
The lower limit values of Bi are 0.05%, 0.001%, and 0.0%, respectively.
They are 10%, 0.005% and 0.10%. Also, Ca,
Se, Te, and Bi are 0.01%, 0.5%, and 0.0%, respectively.
Since the effect is saturated with addition of 05% and 0.4%, this value is made the upper limit value of the addition amount. The reason for limiting the addition amount of the free-cutting property-imparting element is also described in the paragraphs [0040] to [0042] of the action section for each element.

Based on the above various findings, the present inventors have completed an invention having the following constitution as means for solving the above problems. That is, 1. % By weight, C: 0.30 to 0.50%, Si: 0.3
~ 1.5%, Mn: 0.5-2.0%, P: 0.02-0.
10%, S: 0.15% or less, Cr: 0.7% or less, V:
A high-fatigue strength free-cutting non-heat treated steel which comprises 0.05 to 0.30%, the balance being Fe and unavoidable impurities, and which satisfies the following formula.

## EQU10 ## [C%] + 0.28 × ([Mn%] + [Cr%]) <0.92

80 × [C%] + 8 × [Si%] + 15 × [Mn%] + 55 × [P%] + 18 × [Cr%] + 132 × [V%]> 70

2. Furthermore, in% by weight, Pb: 0.05-
0.50%, Ca: 0.001 to 0.010%, Se: 0.0.
10-0.50%, Te: 0.005-0.050%, B
i: One or more of 0.1 to 0.40%, and satisfying the following formula: 1
High fatigue strength free-cutting non-heat treated steel described.

[Equation 12] [C%] + 0.28 × ([Mn%] + [Cr%]) <0.92

80 × [C%] + 8 × [Si%] + 15 × [Mn%] + 55 × [P%] + 18 × [Cr%] + 132 × [V%]> 70

3. Furthermore, in weight%, Cu: 0.5-1.
1 or 2 containing 0% and satisfying the following formula
High fatigue strength free-cutting non-heat treated steel described.

## EQU14 ## [C%] + 0.28 × ([Mn%] + [Cr%]) <0.92

80 × [C%] + 8 × [Si%] + 15 × [Mn%] + 55 × [P%] + 18 × [Cr%] + 132 × [V%] + 3 × [Cu%]> 70

[0032]

[Operation] The chemical composition of the steel of the present invention will be described. C
Is the most basic element to secure the necessary tensile strength, and 0.30% or more is necessary to obtain high tensile strength. However, as shown in Table 3, the addition contributes little to the improvement of the durability ratio, and may rather decrease. Therefore, it is set to 0.30 to 0.50%. In addition, it is preferably 0.30 to 0.40%.

Si is a solid solution strengthening element and an alloying element effective for improving the durability ratio. At least 0.3% is necessary to obtain such an effect. However, excessive addition saturates the effect and promotes decarburization of the surface during forging, so the content was made 0.3 to 1.5%.

At least 0.5% of Mn is necessary for deoxidation during steelmaking and for reducing decarburization during forging.
However, excessive addition improves the hardenability and forms bainite. Machinability decreases due to the formation of bainite. Therefore, it is set to 0.5 to 2.0%.

P is a solid solution strengthening element and is an alloying element effective for improving the durability ratio. At least 0.02% is necessary to obtain the effect. However, excessive addition will saturate the effect and reduce the workability (ductility). It was set to 0.10% or less.

Cr is a solid solution strengthening element and an alloying element effective for improving the durability ratio. However, excessive addition improves the hardenability and forms bainite. Therefore, it is set to 0.7% or less.

The condition under which bainite is not formed by the air cooling treatment is expressed by the following equation (1) as shown in FIG.

[C%] + 0.28 × ([Mn%] + [Cr%]) <0.92 Therefore, it is necessary to satisfy this expression in addition to the above-mentioned addition amount conditions.

V is a precipitation strengthening element, which is an alloy element effective for improving the durability ratio. At least 0.05% is necessary to obtain such an effect. However, even if added excessively, the precipitation strengthening amount becomes saturated as shown in FIG. 2, for example. Therefore, it is set to 0.05 to 0.30%.

Cu is a solid solution strengthening element and an alloying element effective for improving the durability ratio. However, since it is extremely expensive as compared with other elements, it should be added after considering the purpose. At least 0.5% is necessary to obtain such an effect. Even if added excessively, the effect is saturated, so the content was made 0.5 to 1.0%.

Since S is an element effective in improving the machinability, it should be discussed in the same group as Pb, Ca, Se, Te and Bi below, but S is an unavoidable impurity of steel. Since there is no case of no addition under normal dissolution conditions, we decided to consider only S separately. S
Although the effect of improving the machinability of steel due to the steel appears even in a small amount, it is necessary to add 0.04% or more from the result of FIG. 4 (c) in order to obtain the machinability improving effect only by S. .
However, as shown in FIG. 3 (c), excessive addition significantly reduces the fatigue limit. Considering the result of Fig. 3 (c), the effect is saturated even if added in excess, so the upper limit is 0.15.
%. That is, the S content is in the range of 0.15% or less, but the preferred range is 0.04 to 0.15%.
In addition, when improvement of machinability is not sufficient only by S, it is necessary to add 1 type or 2 types or more of the following Pb, Ca, Se, Te, Bi.

Pb is a very effective element for improving the machinability, and in order to obtain such an effect, addition of 0.05% or more is necessary. However, excessive addition significantly lowers the fatigue limit, so considering the results in Fig. 3 (d), 0.5
It was set to 0%.

Ca, Se, Te and Bi are all very effective elements for improving machinability, and in order to obtain such effect, from the results of FIGS. 4 (a) and 4 (b), 0 for Ca.
001% or higher, Te 0.005% or higher, and FIG.
From the result of (d), 0.1% or more for Se and 0.10 for Bi
% Or more must be added. However, since excessive addition saturates the effect, from the results of FIGS. 4 (a), (b) and (d), Ca is 0.01% or less, Te is 0.050% or less, and Se is 0.50% or less. %, And for Bi, 0.40% or less.

The steel of the present invention is applied to hot forged parts, and in consideration of workability, high tensile strength (90%
The component is limited so that it becomes 0 MPa or more). As a result of the investigation already shown in Table 2, the effect of additional elements on the tensile strength of non-heat treated steel is basically investigated, and the following regression equation is obtained.

80 × [C%] + 8 × [Si%] + 15 × [Mn%] + 55 × [P%] + 18 × [Cr%] + 132 × [V%] + 3 × [Cu%]> 70

[0044]

【Example】

Example 1 Steel having a chemical composition shown in Table 4 was melted in a melting furnace in the atmosphere of 50 kg, and then forged into a φ30 mm round bar at 900 ° C. or higher. Simulated heating and air-cooling process of hot forging 11
Normalizing was carried out at 00 ° C. for 15 minutes. A JIS No. 14 tensile test piece was sampled from the center of the obtained round bar, and the mechanical properties were investigated. For the fatigue test, an Ono-type rotary bending test piece having a parallel portion φ8 mm was sampled, and the fatigue test was performed at a repetition rate of 50 Hz in air temperature and air. The fatigue limit was defined as the stress amplitude at which the number of repeated fractures was 10 ⁷ .

[0045]

[Table 4]

In Table 4, in particular, test numbers (hereinafter abbreviated as No.) 1 to 4 are C, No. 5 to 11 are Si, and No. 14 to 1
No. 6 is Cr, No. 17-19 is V, No. 20-21 is C
Nos. 24 to 26 of u are the effects of the addition amount near the upper limit of S. No. 22 and 23 do not satisfy the formula (Eq.2), and No. 23 does not satisfy the formula (Eq.1), although the individual chemical components satisfy the claims.

In Table 4, No. 16 and No. 23, in which the numerical value of Eq.1 does not satisfy the range of less than 0.92 claimed in the present invention, the metal structure is ferrite-bainite. Therefore, the machinability of No. 16 and No. 23 is poor.

On the other hand, in Table 4, the numerical values of Eq.1 satisfy No. 1 which is less than 0.92 as claimed in the present application.
In No. 15, No. 17 to No. 22 and No. 24, 25, the metal structure is ferrite-pearlite. Therefore,
Their machinability is good.

Among the steels according to claim 1 of the claims of the present application, those showing the present invention in Table 4 all have a tensile strength of 900 MPa or more, a high fatigue strength and a durability ratio of 0.5 or more. . However, regarding those outside the scope of the claims of the present application (comparative examples in Table 2), No. 1,
No. 22, which has low tensile strength, No. 4, 5, 10, 11, 1
6,17,23,26 has a low durability ratio and is less than 0.5.

From the above, it has been found that the fatigue strength / durability ratio of the non-heat treated steel can be improved without excess or deficiency in claim 1.

Example 2 A steel having the chemical composition shown in Table 5 was melted in the atmosphere in a melting furnace in the same manner as in [Example 1] (50 kg), and then forged into a φ30 mm round bar at 900 ° C. or higher. Stretched. Simulated heating and air-cooling process of hot forging 1
Normalizing was performed at 100 ° C. for 15 minutes. A JIS No. 14 tensile test was taken from the center of the obtained round bar to examine the mechanical properties. Further, an Ono-type rotary bending test piece having a parallel portion φ8 mm was sampled, and a fatigue test was performed at room temperature and in the atmosphere at a repetition rate of 50 Hz. The fatigue limit was defined as the stress amplitude at which the number of repeated fractures was 10 ⁷ .

[0052]

[Table 5]

In Table 5, No. 31 is a steel containing no free-machinability imparting element corresponding to the steel of the above [Example 1], and a tool when this steel is cut according to a predetermined standard. Life is set to 1.0 and other steel grades 32 to
The tool life ratio when 46 is cut is shown in Table 5 together with other mechanical properties. In order to clearly show the effect of the free-cutting property-providing element, the content of S, which itself has a free-cutting property-providing action, was suppressed to a relatively low level of about 0.04%.

Nos. 32 to 40 steels having compositions within the scope of claim 2 or 3 of the present invention all have remarkably excellent machinability as compared with the base steel 31, that is, a high tool life ratio, Moreover, it can be seen that other properties are also excellent.

On the other hand, Comparative Example 42 in which the content of the free-machining-providing element (Pb) was added in excess of the range defined in claim 2.
Machinability was remarkably improved, but the durability ratio did not reach the predetermined value.

Further, Comparative Examples 43 to 4 in which the content of the free-cutting property-providing element is low and does not reach the lower limit of claim 2 or 3.
It can be seen that in No. 6, the machinability was hardly improved in any of the cases, and the level remained at the same level as the machinability of the steel of claim 1.

[0057]

As described in detail above, the steel manufactured according to the present invention has a high durability ratio and machinability as a non-heat treated steel, so that it is a hot forged part such as a crankshaft, a connecting rod and a knuckle. The present invention, which is optimal for the above and has such an effect, is extremely significant.

[Brief description of drawings]

1] [C%] and ([Mn%] + [Cr%]) in steel
3 is a diagram showing a range in which a ferrite-pearlite structure or a ferrite-bainite (martensite) structure is generated in the relationship with.

FIG. 2 is a graph showing the effects of various additive elements on the fatigue strength and tensile strength of steel for comparison.

FIG. 3 is a graph showing the relationship between the addition amount of various free-machining imparting elements and the durability ratio of steel.

FIG. 4 is a graph showing the relationship between the addition amount of various free-machining imparting elements and the life of a tool cut from steel.

Claims (3)

[Claims] 1. C: 0.30 to 0.50%, S by weight%
i: 0.3 to 1.5%, Mn: 0.5 to 2.0%, P: 0.0.
02-0.10%, S: 0.15% or less, Cr: 0.7%
Hereinafter, V: 0.05 to 0.30%, a high fatigue strength free-cutting non-heat treated steel which comprises Fe and unavoidable impurities in the balance and satisfies the following formula. ## EQU1 ## [C%] + 0.28 × ([Mn%] + [Cr%]) <0.92 ## EQU2 ## 80 × [C%] + 8 × [Si%] + 15 × [Mn%] + 55 × [P%] + 18 × [Cr%] + 132 × [V%]> 70 2. Further, in% by weight, Pb: 0.05 to 0.5.
50%, Ca: 0.001 to 0.010%, Se: 0.1
0-0.50%, Te: 0.005-0.050%, B
The high fatigue strength free-cutting non-heat treated steel according to claim 1, characterized in that it contains one or more of i: 0.10 to 0.40%, and satisfies the following formula. ## EQU3 ## [C%] + 0.28 × ([Mn%] + [Cr%]) <0.92 ## EQU4 ## 80 × [C%] + 8 × [Si%] + 15 × [Mn%] + 55 × [P%] + 18 × [Cr%] + 132 × [V%]> 70 3. Further, in weight%, Cu: 0.5 to 1.0.
%, And satisfies the following formula:
High fatigue strength free-cutting non-heat treated steel described. ## EQU5 ## [C%] + 0.28 × ([Mn%] + [Cr%]) <0.92 ## EQU6 ## 80 × [C%] + 8 × [Si%] + 15 × [Mn%] + 55 × [P%] + 18 × [Cr%] + 132 × [V%] + 3 × [Cu%]> 70