JPH0931594A - Non-heat treated steel with high strength and low ductility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel having high strength combined with low ductility, capable of division working at ordinary temp., having a division fracture surface of flat brittle fracture surface, and suitable for a steel for connecting rod. SOLUTION: This steel is a non-heat treated steel with high strength and low ductility, which has a composition consisting of 0.25-0.70% C, 0.1-1.5% Si, 0.3-2.0% Mn, 0.01-0.10% P, 0.01-0.10% S, 0.02-1.50% Cr, 0.05-0.50% V, <=0.10% Nb, <=0.20% Ti, <=0.100% Al, <=0.030% N, and the balance Fe with inevitable inpurities and satisfying Si(%)+2V(%)-0.5>=0 and C(%)+Si(%)/10+Mn(%)/5+5 Cr(%)/22+1.65V-5S(%)/7-0.8>=0. This steel can contain one or more elements among Cu, Ni, Mo, B, and Pb.

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 brittle fracture surface with a flat fracture surface and is suitable as a material for connecting rods and connecting rod caps of engines such as automobiles.

[0002]

2. Description of the Related Art FIG. 1 is a part of an engine of an automobile or the like.
Connecting rod (commonly called connecting rod) body 1 shown in
And, the connecting rod cap (commonly called connecting rod cap) 2 is conventionally hot-forged in a separate process, then tempered by quenching and tempering, and then machined to form bolt holes and finish shaping. A method has been adopted in which they are manufactured, and then they are connected and assembled to a crankshaft having a complicated shape by bolts 3.

However, recently, due to the severe economic situation, the demand for reduction of manufacturing cost of various automobile parts has become extremely active, and this move 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 for quenching and tempering, or hot forging, as a measure for reducing the manufacturing cost. After cooling, it is divided into connecting rod body 1 and connecting rod cap 2 and then bolt holes are machined, that is, the joints (joining surfaces) are not machined for finish shaping. A method of assembling by assembling the crankshaft with the bolt 3 is being studied.

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 effects of Si, which is known to strengthen ferrite by solid solution strengthening, and V, which is known to strengthen ferrite by precipitation strengthening, on the fracture morphology of steel when added individually. Is small. However, if appropriate amounts of Si and V are added at the same time, the ductility is greatly reduced, and brittle fracture of steel is remarkably promoted.

In order to realize the above-mentioned combined addition effect of Si and V, 0.1% or more of Si and 0.05% or more of V are contained, and fn1 = Si (%) + 2V (%)-0. It is necessary to set the value of 5 to 0 or more.

When the above-mentioned fn1 ≧ 0 is satisfied and the elongation value of the steel material at the room temperature tensile test is 10% or less, the room temperature split surface of the hot forged integrally formed material becomes a flat brittle fracture surface.

When the chemical composition of steel is within a specific condition range, the tensile strength of non-heat treated steel can be arranged by the following fn2. When this value is 0 or more, a tensile strength of 800 MPa or more can be obtained.

Fn2 = C (%) + Si (%) / 10 + M
n (%) / 5 + 5Cr (%) / 22 + 1.65V-5S
(%) / 7-0.8 After strictly controlling the chemical composition of steel, the above fn1
If the conditions of ≧ 0, elongation value of steel at room temperature tensile test ≦ 10%, and fn2 ≧ 0 are satisfied, a flat brittle fracture surface and high strength can be obtained by division at room temperature. The connecting rod body 1 and the connecting rod cap 2 having a tensile strength of 800 MPa or more, which is a desired strength, can be manufactured by the new process.

The present invention based on the above findings is described in the following (1)-
The high-strength, low-ductility non-heat treated steel of (4) is the gist.

(1) C: 0.25 to 0.70 by weight%
%, Si: 0.1 to 1.5%, Mn: 0.3 to 2.0
%, P: 0.01 to 0.10%, S: 0.01 to 0.1
0%, Cr: 0.02 to 1.50%, V: 0.05 to
0.50%, Nb: 0.10% or less, Ti: 0.20%
Hereinafter, high strength characterized by containing Al: 0.100% or less, N: 0.030% or less, the balance being Fe and unavoidable impurities, and having the above-mentioned fn1 ≧ 0 and fn2 ≧ 0. Low ductility non-heat treated steel.

(2) In addition to the components described in (1) above, 0.01 to 0.2% by weight of Cu and 0.01 to 0.2% by weight.
0.5% Ni, 0.01 to 0.50% Mo and 0.
A high-strength, low-ductility non-heat treated steel containing at least one of 0003 to 0.0100% B and satisfying the above-mentioned fn1 ≧ 0 and fn2 ≧ 0.

(3) In addition to the components described in (1) above, 0.01% to 0.30% by weight of Pb is further contained,
In addition, a high-strength, low-ductility non-heat treated steel characterized by satisfying the above-mentioned fn1 ≧ 0 and fn2 ≧ 0.

(4) In addition to the components described in (1) above, 0.01% to 0.2% by weight of Cu and 0.01% to 0.01% by weight.
0.5% Ni, 0.01 to 0.50% Mo and 0.
High strength characterized by containing at least one of B of 0003 to 0.0100% and Pb of 0.01 to 0.30% and satisfying the above-mentioned fn1 ≧ 0 and fn2 ≧ 0. Low ductility non-heat treated steel.

The reasons for limiting the chemical composition of the steel in the present invention as described above will be explained below. In addition,
“%” Means “% by weight”.

C: C is an element necessary for imparting a desired static strength to steel, but it is also an element that reduces hot forgeability. Minimum static strength (800MP in tensile strength
In order to obtain (a or more), 0.25% or more is required. On the other hand, if the content of Al exceeds 0.70%, the hot forgeability is deteriorated, and cracks are likely to occur during hot forging. Therefore,
The C content was 0.25% to 0.70%.

Si: Si has the functions of promoting deoxidation of steel and strengthening the ferrite by forming a solid solution in the ferrite. Further, if V is added together with an appropriate amount, the ductility is greatly reduced and brittle fracture is promoted. To ensure these effects, Si is 0.1
The content must be at least%. However, if the content exceeds 1.5%, the hot forgeability is deteriorated, and cracks are likely to occur during hot forging. Therefore, the Si content is set to 0.1 to 1.5%.

Mn: Mn is necessary for deoxidation, and
It has the effect of enhancing hardenability and static strength. However, if the content is less than 0.3%, the desired effect cannot be obtained, and if it exceeds 2.0%, the hot forgeability is deteriorated, so the content is 0.3 to 2.0. %.

P: P has the effect of causing grain boundary embrittlement and reducing ductility, so it is effective in obtaining a flat brittle fracture surface by the above-described division method at room temperature. In order to surely obtain this effect, the content of P needs to be 0.01% or more. However, if the content exceeds 0.10%, the hot forgeability is significantly deteriorated. Therefore, the content of P is set to 0.01 to 0.10%.

Since S: S also has the effect of causing grain boundary embrittlement and lowering ductility, it is effective in obtaining a flat brittle fracture surface by the above-described division method at room temperature as in the case of P. Further, S has an effect of improving the machinability at the time of drilling a bolt hole. However, if the content is less than 0.01%, the desired effect cannot be obtained, while if it exceeds 0.10%, the hot forgeability is significantly deteriorated. Therefore, the content of S is set to 0.01 to 0.10%.

Cr: Cr has the effects of improving the hardenability and the static strength. However, if the content is less than 0.02%, the desired effect cannot be obtained, and
Even if the content exceeds 50%, the effect is saturated, and only the cost increases and the economic efficiency is impaired, so the content was made 0.02 to 1.50%.

V: V has the effect of forming (carbon) nitride and refining the crystal grains, and when added together with Si as described above, it has the effect of significantly reducing ductility and promoting brittle fracture.

However, if its content is less than 0.05%, the effect of addition is poor, while if it exceeds 0.50%, the hot workability of steel deteriorates. Therefore, the content of V is set to 0.05 to 0.50%.

Nb: Nb may not be added. If added, it has the effect of producing (carbon) nitride and refining the crystal grains. To obtain this effect reliably, Nb is 0.003
It is desirable that the content be at least%. However, 0.1
If the content of Ni exceeds 0%, the hot workability of steel will be significantly reduced. Therefore, the Nb content is set to 0.10% or less.

Ti: Ti may not be added. If added, it has the effect of producing (carbon) nitride and refining the crystal grains. To ensure this effect, Ti is 0.005
% Is preferable. However, 0.2
If it is contained in excess of 0%, a huge TiN is generated and cracks are likely to occur during hot working. Therefore, the upper limit of the Ti content is set to 0.20%.

Al: Al may not be added. If added, it has the effects of stabilizing and homogenizing the deoxidation of the steel, and at the same time having the action of forming nitrides and refining the crystal grains. In order to reliably obtain these effects, the content of Al is preferably 0.005% or more. However, if the content exceeds 0.100%, oxide-based inclusions increase remarkably, and cracking tends to occur during hot working. Therefore, the Al content is set to 0.100% or less.

N: N may not be contained. If contained, it has a function of forming a nitride or carbonitride with the above Al, Ti, Nb, etc., and refining the crystal grains. Further, free solid solution N has an action of promoting brittle fracture. In order to surely obtain these effects, it is desirable that the content of N be 0.0030% or more. However, containing more than 0.030% is technically difficult and leads to cost increase. Therefore, the upper limit of the N content is set to 0.030%. Fn1: When 0.1% or more of Si and 0.05% or more of V are added in combination, the effect of greatly reducing the ductility is
It can be sorted by 1. If this value is 0 or more, brittle fracture is promoted. When fn1 ≧ 0 and the elongation value of the steel material at the normal 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,
By the new process as described above, the connecting rod body and the connecting rod cap having a desired tensile strength of 800 MPa or more can be manufactured. Therefore, fn1 ≧ 0.

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.

The high-strength, low-ductility non-heat treated steel of the present invention may further contain one or more of Cu, Ni, Mo and B and / or Pb in addition to the above components. The effects and desirable contents of these alloy elements are as follows.

Cu, Ni, Mo, B: Cu, Ni, Mo
And B have the effect of enhancing the hardenability of steel and improving the static strength. Therefore, one or more of Cu, Ni, Mo and B may be added if necessary. However, in the case of Cu, if the content is less than 0.01%, the desired effect cannot be obtained, and if it exceeds 0.2%, the hot workability deteriorates, and during hot rolling or hot forging. Sometimes it causes cracks. On the other hand, in the case of Ni, if the content is less than 0.01%, the desired effect cannot be obtained, and if it exceeds 0.5%, ductility and toughness increase, and a flat brittle fracture surface is obtained. I will not be able to. Further, in the case of Mo, if the content is less than 0.01%, the desired effect cannot be obtained, and if it exceeds 0.50%, the effect is saturated, and only the cost increases and the economical efficiency is improved. Will be damaged. Furthermore, in the case of B, 0.000
If the content is less than 3%, the desired effect cannot be obtained,
If the content exceeds 00%, not only the effect of improving the hardenability is saturated, but also the hot forgeability is significantly deteriorated. Therefore, when adding one or more of these alloy elements,
Cu: 0.01 to 0.2%, Ni: 0.01 to 0.5
%, Mo: 0.01 to 0.50% and B: 0.0003
It is preferable to set the content to be 0.0100%.

Pb: Pb has the effect of improving the machinability when machining a bolt hole. Therefore, Pb may be added if necessary. However, if the content is less than 0.01%, the desired effect cannot be obtained, and if it exceeds 0.30%, the hot workability is deteriorated and cracking occurs during hot rolling or hot forging. . Therefore, when Pb is added,
The content is preferably 0.01 to 0.30%.

[0038]

BEST MODE FOR CARRYING OUT THE INVENTION After the steel having the above-mentioned chemical composition is melted by a usual method, the connecting rod body 1 and the connecting rod cap 2 are separated by hot rolling and forging by a usual method. It is molded into a connected one piece. After that, the connecting rod body 1 and the connecting rod cap 2 are divided at room temperature by the method described above, then bolt holes are drilled, and the bolts 3 are assembled to the crankshaft.

[0039]

EXAMPLES Steels having the chemical compositions shown in Tables 1 to 4 were vacuum-melted in a test furnace by a usual method. Steels 1 to 15 in Tables 1 and 2 are steels of the present invention, and steels 16 to 34 in Tables 3 and 4 are comparative steels in which any of the components deviates from the content range 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.

Tables 5 and 6 show the results of the room 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 room temperature tensile test were all flat brittle fracture surfaces (see Table 5).

On the other hand, among the comparative steels in which any of the components deviates from the content range specified in the present invention, the C content,
The tensile strength is 800MP for steels 16, 20 and 33 and 34 in which the values of Mn and fn2 deviate from the specified values, respectively.
a has not been reached.

Further, C amount, Si amount, Mn amount, P amount, S amount,
Steels 17, 19 and 2 in which the Cu content, Al content, Nb content, Ti content, V content and Pb content deviate from the specified values by a large amount.
1, 22, 23, 24, 26, 27, 28, 30 and 3
In No. 1, hot forging cracks were recognized.

Further, the steels 18, 29, 32 and 34, and N, in which the Si content, the V content and the value of fn1 deviated from the stipulated values to a lower value,
Steel 25 whose i amount deviates from the specified value a little has a room temperature elongation of 10
%, The fracture surfaces after the normal temperature tensile test were all ductile fracture surfaces. (See Table 6 above).

The connecting rod body 1 and the connecting rod cap 2 were connected by the normal hot forging method using the steels 1, 5, 10 and 15 of the present invention described in Tables 1 and 2 above. 20 pieces 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]

[Table 4]

[0053]

[Table 5]

[0054]

[Table 6]

[0055]

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.

Since the steel of the present invention has a low ductility, it can be used as a bullet material for lighting bullets and the like.

[Brief description of drawings]

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

Claims (4)

[Claims] 1. By weight%, C: 0.25 to 0.70%, S
i: 0.1-1.5%, Mn: 0.3-2.0%, P:
0.01-0.10%, S: 0.01-0.10%, C
r: 0.02 to 1.50%, V: 0.05 to 0.50
%, Nb: 0.10% or less, Ti: 0.20% or less, A
1: 0.100% or less, N: 0.030% or less, the balance consisting of Fe and unavoidable impurities, and fn1
High-strength, low-ductility non-heat treated steel characterized in that ≧ 0 and fn2 ≧ 0. However, fn1 = Si (%) + 2V (%)-0.5, fn2 = C (%) + Si (%) / 10 + Mn (%) / 5
+ 5Cr (%) / 2 2 + 1.65V-5S (%) / 7-
0.8 2. In addition to the components of claim 1, 0.01% to 0.2% Cu, 0.01% to 0.5% by weight.
Ni, 0.01 to 0.50% Mo and 0.0003
A high-strength, low-ductility non-heat treated steel characterized by containing at least one of B of 0.0100% and fn1 ≧ 0 and fn2 ≧ 0. However, fn1 = Si (%) + 2V (%)-0.5, fn2 = C (%) + Si (%) / 10 + Mn (%) / 5
+ 5Cr (%) / 2 2 + 1.65V-5S (%) / 7-
0.8 3. In addition to the components according to claim 1, further comprising 0.01 to 0.30% by weight of Pb, and fn
High strength characterized by 1 ≧ 0 and fn2 ≧ 0
Low ductility non-heat treated steel. However, fn1 = Si (%) + 2V (%)-0.5, fn2 = C (%) + Si (%) / 10 + Mn (%) / 5
+ 5Cr (%) / 2 2 + 1.65V-5S (%) / 7-
0.8 4. In addition to the components of claim 1, 0.01% to 0.2% by weight of Cu, 0.01% to 0.5%.
Ni, 0.01 to 0.50% Mo and 0.0003
~ 0.0100% of one or more of B, and 0.0
1 to 0.30% Pb and fn1 ≧ 0 and f
A high-strength, low-ductility non-heat treated steel characterized by n2 ≧ 0. However, fn1 = Si (%) + 2V (%)-0.5, fn2 = C (%) + Si (%) / 10 + Mn (%) / 5
+ 5Cr (%) / 2 2 + 1.65V-5S (%) / 7-
0.8