JPH09176786A - High strength / low ductility non-heat treated steel - Google Patents

Abstract

(57) Abstract: [PROBLEMS] To provide a steel suitable for a connecting rod, which has high strength, low ductility, can be split at room temperature, and has a brittle fracture surface with a flat fracture surface. SOLUTION: C: 0.20 to 1.20%, Si ≦ 1.5
0%, Mn: 0.30 to 2.00%, P ≦ 0.15%,
S ≦ 0.10%, Cu ≦ 0.20%, Ni ≦ 0.50
%, Cr: 0.02 to 2.00%, Mo ≦ 0.50%,
V ≦ 0.50%, Nb ≦ 0.17%, Ti ≦ 0.20
%, B ≦ 0.0100%, Al ≦ 0.100%, N ≦
0.030%, Pb ≦ 0.30%, and As ≦ 0.0
5%, Sb ≤ 0.05% and Sn ≤ 0.05% in a total amount of 0.03% or more, with the balance being Fe and inevitable impurities, and C + (Si / 10) + ( Mn
/5)+(5Cr/22)+1.65V-(5S/7)
High-strength, low-ductility non-heat treated steel with -0.8≥0. The symbol of the element in the formula represents the content of the element in% by weight.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength, low-ductility non-heat treated steel, and more specifically, it requires high strength but does not require ductility, and rather can be cold split at room temperature. The present invention relates to a high-strength, low-ductility non-heat treated steel which has a flat brittle fracture surface and is suitable as a steel for connecting rods and connecting rod caps of automobile engines.

[0002]

2. Description of the Related Art A connecting rod (commonly known as connecting rod) body 1 and a connecting rod cap (commonly known as connecting rod cap) 2 shown in FIG. 1 which are parts of an automobile engine or the like are conventionally hardened after hot forging in another process. After being tempered, it was subjected to bolt hole machining and finish shaping by machining, and then joined and assembled by a bolt 3 to a crankshaft having a complicated shape.

However, recently, reflecting the severe economic situation, the movement for reducing the manufacturing cost of various automobile parts has become active, and this movement is no exception in engine parts.

For this reason, the connecting rod body 1 and the connecting rod cap 2 are integrally formed by hot forging and subjected to heat treatment such as quenching and tempering, or hot forging, as a measure for reducing the manufacturing cost. After cooling, it is divided into a connecting rod body 1 and a connecting rod cap 2, and the joints (joining surfaces) are not machined for finishing shaping, and are assembled to the crankshaft with bolts 3 for assembly. That method is being considered. in this way,
The processing of the bolt holes is performed before or after dividing the integrally molded material.

As a method of dividing the integrally formed connecting rod body 1 and the connecting rod cap 2 described above, for example, a jig is inserted to give a force acting in the direction shown by the arrow in FIG. 1 to divide the connecting rod body 1 and the connecting rod cap 2. Can be considered. In this method, it is extremely important to flatten the dividing surface divided into the connecting rod body 1 and the connecting rod cap 2.

However, the steel (J
If it is integrally formed by hot forging using IS standard S45C or S48C equivalent steel etc. as it is, and then is divided into the connecting rod body 1 and the connecting rod cap 2 at room temperature, the dividing surface looks like a candy or gum is cut into pieces. There is a problem that a so-called "ductile fracture surface" cannot be obtained and a flat "brittle fracture surface" cannot be obtained, and a finish shaping process by machining must be performed. If the above division is performed at a low temperature (for example, liquid nitrogen temperature), brittle fracture occurs and a flat brittle fracture surface can be easily obtained, but it is technically easy to keep the temperature low in an actual operation line where a large amount of products flow. However, there is a problem that the cost for constructing and maintaining the equipment will increase, which does not necessarily lead to cost reduction.

On the other hand, since the heat treatment after integrally forming by hot forging is costly, there is a demand for a new type of steel in which the heat treatment can be omitted.

As a non-heat treated steel in which heat treatment as a heat treatment performed after hot rolling or hot forging can be omitted, for example, "non-heat treated high strength steel" is proposed in Japanese Patent Laid-Open No. 5-195140. . However, the non-heat treated steel described in this publication is a high-strength non-heat treated steel of the type that prevents cracks on the bloom surface during continuous casting. Therefore, when the above-mentioned proposed steel is used as the steel for the connecting rod body 1 and the connecting rod cap 2, the desired strength can be obtained, but the connecting rod body 1 and the connecting rod cap 2 can be formed on the connecting rod body 1 and the connecting rod cap 2 at room temperature after being integrally molded as described above. With respect to the method of dividing, the ductility is too great to obtain a brittle fracture surface. Therefore, it is necessary to perform finish shaping by machining.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has a tensile strength equal to or higher than that of conventional steel, and a hot forged integrally formed material is obtained at room temperature by the method as described above. It is an object to provide a high-strength, low-ductility non-heat treated steel in which the fracture surface when divided by is a flat brittle fracture surface.

[0010]

Means for Solving the Problems The present inventor has obtained the following findings as a result of various studies to solve the above problems.

The fracture mode of non-heat treated steel having a specific chemical composition at room temperature has a correlation with the contents of As, Sb and Sn, and the value of the following fn1 which is the total amount of one or more of these is 0.03. In the above cases, brittle fracture is promoted. In addition,
The symbol of element in fn1 represents the content of the element in% by weight.

Fn1 = As + Sb + Sn When the above-mentioned fn1 ≧ 0.03 is satisfied and the elongation value of the steel material at the normal temperature tensile test is 10% or less, the hot forged integrally formed material has a flat brittleness at the room temperature dividing surface. It becomes a fractured surface.

In the case of the non-heat treated steel having the above-mentioned specific chemical composition, the tensile strength can be arranged by the following fn2, and this value is 0.
In the above cases, a tensile strength of 800 MPa or more can be obtained.
The symbol of an element in fn2 also means the content of the element in% by weight.

Fn2 = C + (Si / 10) + (Mn /
5) + (5Cr / 22) + 1.65V- (5S / 7)-
Strictly controlling the chemical composition of 0.8 steel,
≧ 0.03, elongation value of steel material at room temperature tensile test ≦ 10
%, And if the conditions of fn2 ≧ 0 are satisfied, a flat brittle fracture surface can be obtained by division at room temperature and high strength can be obtained. Therefore, the connecting rod having a tensile strength of 800 MPa or more, which is the desired strength, can be obtained by the new process described above. The body 1 and the connecting rod cap 2 can be manufactured.

The gist of the present invention based on the above findings is the following high-strength, low-ductility non-heat treated steel.

"% By weight, C: 0.20 to 1.20%,
Si: 1.50% or less, Mn: 0.30 to 2.00%,
P: 0.15% or less, S: 0.10% or less, Cu: 0.
20% or less, Ni: 0.50% or less, Cr: 0.02-
2.00%, Mo: 0.50% or less, V: 0.50% or less, Nb: 0.17% or less, Ti: 0.20% or less,
B: 0.0100% or less, Al: 0.100% or less,
N: 0.030% or less, Pb: 0.30% or less, As: 0.05% or less, Sb: 0.05% or less and S
n: 0.03 or less out of 0.05% in total amount of 0.03
% Or more, the balance consists of Fe and unavoidable impurities,
In addition, a high-strength, low-ductility non-heat treated steel characterized by satisfying the above-mentioned fn2 ≧ 0. "

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of steel in the present invention as described above will be explained below. In addition, "%" means "weight%."

C: C is an element necessary for imparting a desired static strength to steel, but on the other hand, it increases the deformation resistance during hot working,
It is also an element that reduces hot workability. To obtain the minimum static strength (tensile strength of 800 MPa or more),
0.20% or more is required. On the other hand, when the content exceeds 1.20%, the deformation resistance during hot becomes large and a large processing facility is required, and further the hot workability of steel is deteriorated, and at the time of hot working. It tends to crack. Therefore,
The C content was 0.20 to 1.20%. Note that C
The content of is preferably 0.30 to 1.10%.

Si: Si may not be added. If added, it not only promotes deoxidation of steel, but also has the effect of improving hardenability. To ensure these effects,
Is preferably 0.05% or more. However, if its content exceeds 1.50%, the hot workability is extremely deteriorated and cracks are likely to occur during hot working. Therefore, the Si content is set to 1.50% or less. In addition, in order to secure further stable hot workability, it is preferable to set the upper limit of the Si content to 1.00%.

Mn: Mn is necessary for deoxidation, and
It has the effect of enhancing hardenability and static strength. However, if the content is less than 0.30%, the desired effect cannot be obtained, and if it exceeds 2.00%, the hot workability deteriorates, so the content is set to 0.30 to 2.00. %.

P: P may not be added. If added, it has the effect of causing grain boundary embrittlement and lowering ductility, so it is effective in obtaining a flat brittle fracture surface by the above-described division method at room temperature. In order to ensure this effect, it is preferable that the content of P be 0.005% or more. However, if the content exceeds 0.15%, the hot workability deteriorates significantly. Therefore, the P content is 0.15
% Or less. In order to secure stable hot workability, the P content is more preferably 0.10% or less.

S: S may not be added. If added, it has the effect of causing grain boundary embrittlement and lowering the ductility, so that it is effective in obtaining a flat brittle fracture surface by the above-described division method at room temperature, similar to P. Further, S has a function of improving the machinability at the time of drilling a bolt hole. In order to surely obtain these effects, the content of S is preferably 0.005% or more. However, its content is 0.10
If it exceeds%, the hot workability is significantly deteriorated. Therefore, S
The upper limit of the content is 0.10%.

Cu: Cu may not be added. If added, it has the effect of enhancing hardenability and improving static strength. In order to reliably obtain this effect, the content of Cu is preferably 0.01% or more. However, if the content exceeds 0.20%, the hot workability is deteriorated and cracks are generated during hot rolling or hot forging. Therefore, C
The content of u was 0.20% or less.

Ni: Ni may not be added. If added, it has the effect of enhancing hardenability and improving static strength. To ensure this effect, the Ni content is preferably 0.01% or more. However, if the content exceeds 0.50%, ductility and toughness increase, and a flat brittle fracture surface cannot be obtained. Therefore, the Ni content is set to 0.50% or less.

Cr: Cr has the effect of improving hardenability and static strength. However, the content is 0.02
If it is less than%, the desired effect cannot be obtained, and if it is contained in excess of 2.00%, the effect is saturated, and only the cost rises and the economic efficiency is impaired. ~ 2.
00%. The Cr content is preferably 0.10% or more.

Mo: Mo may not be added. If added, it has the effect of improving hardenability and strength.
In order to surely obtain this effect, the Mo content is preferably 0.01% or more. However, even if the content exceeds 0.50%, the above effect is saturated, and only the cost rises, and the economical efficiency is impaired. Therefore, the content of Mo is set to 0.50% or less. The Mo content is 0.05%
It is more preferable to make the above.

V: V may not be added. If added, it has the effect of increasing strength. In order to surely obtain this effect, it is preferable that the content of V is 0.005% or more. However, even if the content exceeds 0.50%, the above effect is saturated, only the cost rises, the economic efficiency is impaired, and the hot workability may be deteriorated. Therefore, V
Content was 0.50% or less.

Nb: Nb may not be added. If added, it has the effect of increasing strength. In order to reliably obtain this effect, the Nb content is preferably 0.003% or more. However, even if the content exceeds 0.17%, the above effect is saturated, only the cost increases and the economical efficiency is impaired. Furthermore, the hot workability is deteriorated. Therefore, the Nb content is set to 0.17% or less. In order to secure more stable hot workability, the Nb content is preferably 0.10% or less.

Ti: Ti may not be added. If added, it has the effect of increasing strength. In order to reliably obtain this effect, the Ti content is preferably 0.005% or more. However, even if the content exceeds 0.20%, the above effect is saturated and only the cost increases, impairing the economical efficiency, and also causing the deterioration of the hot workability. Therefore,
The content of Ti is set to 0.20% or less.

B: B may not be added. If added, it has the effect of improving hardenability and increasing strength. In order to surely obtain this effect, the content of B is preferably 0.0003% or more. However, its content is 0.01
If it exceeds 00%, not only the effect of improving the hardenability is saturated, but also the hot workability is significantly deteriorated. Therefore,
The content of B was set to 0.0100% or less.

Al: Al may not be added. If added, it stabilizes the deoxidation of the steel and homogenizes it, and also has the effect of forming nitrides to refine the crystal grains and increase the strength. To ensure these effects, Al is 0.00
It is desirable to set the content to 5% or more. However, 0.
Even if the content exceeds 100%, the above effect is saturated, only the cost rises, the economic efficiency is impaired, and the hot workability may be deteriorated.

Therefore, the Al content is set to 0.100% or less.

N: N may not be contained. If it is contained, it has the effect of forming nitrides and carbonitrides to refine the crystal grains and increase the strength. To ensure this effect,
The N content is preferably 0.0030% or more. However, in general, N is 0.030 in the melting of steel.
Since a special raw material, equipment, and technology are required to contain more than 100%, the cost is increased, the economic efficiency is impaired, and the hot workability is sometimes deteriorated. Therefore, the content of N is set to 0.030% or less.

Pb: Pb may not be contained. If contained, it has the effect of improving the machinability during bolt hole processing. In order to reliably obtain this effect, the Pb content is preferably 0.01% or more. However, if Pb is 0.
If the content exceeds 30%, the hot workability deteriorates and cracks occur during hot rolling or hot forging. Therefore, Pb
Content was 0.30% or less.

As, Sb, Sn: As, Sb and Sn are important elements in the present invention. That is, these elements have the action of promoting brittle fracture at room temperature of non-heat treated steel containing 0.20% or more C, 0.30% or more Mn and 0.02% or more Cr. In order to obtain this effect, the total amount of the three elements of As, Sb and Sn is fn
The value of 1 must be 0.03% or more. Therefore, two elements of As, Sb, and Sn may be 0 (need not be added), but at least one must be added. On the other hand, if the contents of As, Sb and Sn exceed 0.05% each, the hot workability is significantly deteriorated. Therefore, the content of each of As, Sb and Sn is set to 0.
05% or less.

Fn1: As described above, 0.20% or more C, 0.30% or more Mn and 0.02% or more C
The fracture mode of non-heat treated steel containing r is As,
Brittle fracture is promoted when the value of fn1, which is the total amount of one or more of these, has a correlation with the contents of Sb and Sn, and is 0.03 or more. Then, when fn1 ≧ 0.03 and the elongation value of the steel material at the room temperature tensile test is 10% or less, the room temperature split fracture surface of the hot forged integrally formed material becomes a flat brittle fracture surface, which is described above. By such a new process, it is possible to manufacture a connecting rod body and a connecting rod cap having a tensile strength of 800 MPa or more, which is a desired strength. Therefore, fn1 ≧
Set to 0.03. The upper limit of this value is not particularly limited, and As, Sb, and S which are the elements constituting fn1.
It may be 0.15 which is the sum of the upper limits of the content of n.

Fn2: Only when the chemical composition of the steel is strictly controlled and the value of fn2 is set to 0 or more, the tensile strength of 800 MPa or more required for the connecting rod body and the connecting rod cap is changed to non-heat treated steel. Can be given. Therefore, fn2 ≧ 0. There is no particular upper limit to this value, and the maximum value (2.
It may be a value close to 23).

The steel having the above chemical composition is melted by a usual method, and then, for example, by hot rolling and forging by a usual method, the connecting rod body 1 and the connecting rod cap 2 are connected to each other. After being molded into, it is subjected to bolt hole processing. Then, the connecting rod body 1 and the connecting rod cap 2 are divided at room temperature by the method described above, and the bolts 3
It is assembled by being combined with the crankshaft.

[0039]

EXAMPLES Steels having the chemical compositions shown in Tables 1 and 2 were vacuum-melted in a test furnace by a usual method. Steels 1 to 15 in Table 1 are steels of the present invention, and steels 16 to 2 in Table 2
No. 8 is a comparative steel in which any of the components is out of the range of the content specified in the present invention.

Next, these invented steels and comparative steels were formed into billets by a usual method, heated to 1250 ° C., and then hot forged into a round bar having a diameter of 30 mm at a temperature of 1200 to 950 ° C., Then, it was cooled to room temperature by air.

A JIS No. 4 test piece was cut out from the thus-obtained hot-forged round bar and a tensile test was conducted at room temperature. Furthermore, the state of the fracture surface after the room temperature tensile test was observed with a scanning electron microscope (SEM).

The surface of the round bar hot forged to 30 mm was visually observed to confirm the presence of forging cracks.

Table 3 shows the results of the normal temperature tensile test, the fracture surface observation results, and the forging crack confirmation results.

In the steels 1 to 15 of the present invention, forging cracks did not occur, and the desired 800MP
A tensile strength of a or more and an elongation of 10% or less were obtained, and the fracture surfaces after the normal temperature tensile test were all flat brittle fracture surfaces.

On the other hand, among the comparative steels in which any of the components was out of the range of the content specified in the present invention, the steels in which the C content, the fn2, the Mn content, and the fn2 were each lower than the specified values. In Nos. 16, 19 and 28, the tensile strength does not reach 800 MPa.

Further, C amount, Si amount, Mn amount, P amount, Ti
Of steel, Nb, V, and B, which are higher than the specified values, 17, 18, 20, 21, 22, 23, 2
Hot forging cracks were observed in Nos. 4 and 25.

In Steels 26 and 27, the value of fn1 exceeded 0, and the room temperature elongation exceeded 10%. Therefore, all fracture surfaces after the room temperature tensile test were ductile fracture surfaces.

Then, the connecting rod body 1 and the connecting rod cap 2 are connected integrally by a normal hot forging method using the steels 1, 5, 10 and 15 which are the steels of the present invention shown in Table 1 above. 20 each
Each body was hot formed. Then, a split test was performed on the connecting rod body 1 and the connecting rod cap 2 at room temperature by the method described above. As a result, it was found that a flat brittle fracture surface was obtained for all of the 20 steels and that each steel could be used without finish shaping by machining.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

EFFECTS OF THE INVENTION By using the high-strength, low-ductility non-heat treated steel according to the present invention, it is possible to manufacture the connecting rod body and the connecting rod cap by a new process at a low cost, and the industrial effect is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing details of a connecting rod.

Claims (1)

[Claims] 1. C: 0.20 to 1.20% by weight, S
i: 1.50% or less, Mn: 0.30 to 2.00%,
P: 0.15% or less, S: 0.10% or less, Cu: 0.
20% or less, Ni: 0.50% or less, Cr: 0.02-
2.00%, Mo: 0.50% or less, V: 0.50% or less, Nb: 0.17% or less, Ti: 0.20% or less,
B: 0.0100% or less, Al: 0.100% or less,
N: 0.030% or less, Pb: 0.30% or less, As:
0.05% or less, Sb: 0.05% or less, Sn: 0.0
A high-strength, low-ductility non-heat treated steel characterized by containing 5% or less, the balance being Fe and unavoidable impurities, and having fn1 ≧ 0.03 and fn2 ≧ 0. However, fn1 = A
s + Sb + Sn, fn2 = C + (Si / 10) + (Mn
/5)+(5Cr/22)+1.65V-(5S/7)
-0.8, where the symbol of the element in the formula represents the content of the element in% by weight.