JP2861698B2 - Manufacturing method of high yield ratio high toughness non-heat treated high strength steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated high-strength steel, in particular, a high yield ratio characterized by having excellent strength, toughness and a high yield ratio by performing a tempering treatment after hot forging. This is a method for producing a high toughness non-heat treated high strength steel.

[0002]

2. Description of the Related Art Many mechanical parts, such as automobile parts, are formed into a predetermined shape by hot forging such as a hot forging press, then quenched and tempered, and further subjected to machining such as cutting and polishing. I have. Since such a heat treatment is a very useful means for obtaining the required mechanical properties of a part, it has been widely used in the past. Steels subjected to or obtained by such a heat treatment are called tempered steels.

On the other hand, in recent years, there has been a strong demand for rationalization of production lines, improvement of productivity and elimination of heat energy of heat treatment, and a proposal has been made on a non-heat treated high-strength steel in which quenching and tempering treatments are omitted (Japanese Patent Laid-Open No. 61-1986) No. 238941, No. 62-202054,
No. 62-207821).

However, these proposals have sufficient tensile strength.
(TS) can be secured, but considering the yield strength as a structural component, the yield ratio (YP / TS) is often less than 0.75, so even if the TS is high, weaving it to the design strength There was a contradiction that it could not be included. Especially TS is 90kgf / mm
^{In the case of} a non-heat-treated high-strength steel having a bainite structure exceeding ² and exhibiting high toughness (see JP-A-61-279656), a low yield ratio becomes apparent.

[0005]

However, conventional non-heat-treated steel parts are inferior in toughness as compared with conventional heat-treated forged steel parts, so that a limited number of parts which do not require toughness are required. However, it has not been possible to generally commercialize even important parts requiring high strength and high toughness.

[0006] However, today, as described above, when the rationalization of the production line is strongly demanded, application to important parts by improving the toughness of the non-heat treated steel is also demanded. ing.

Thus, an object of the present invention is to perform tempering after hot forging to improve not only toughness but also to secure a high yield ratio and to be applied to parts requiring high strength and high toughness. It is an object of the present invention to provide a method for producing a high yield ratio, high toughness, non-heat treated, high-strength steel that can be reduced in strength.

[0008]

Means for Solving the Problems The present inventors have conducted various studies in order to solve the above problems, and as a result, after hot forging, tempered to obtain a high yield ratio and high toughness non-heat treated high strength steel. It has been found that the following means are effective for the production of

(1) In order to obtain stable high strength regardless of product dimensions, the area ratio of proeutectoid ferrite is suppressed to 10% or less. (2) For that purpose, it is effective to exhibit the effect of improving the hardenability of B, and

[0010]

(Equation 2)

However, (B), (N) and (Ti) represent B,
A component system that satisfies the content of N and Ti (ppm).
In the following, the above equation may be simply referred to as equation (1).

(3) In order to improve YR, 300 to 650
Tempering is preferably carried out in the temperature range of 500C, particularly in the range of 500 to 600C, but softening tends to occur. (4) On the other hand, in order to maintain high TS and achieve high YR by tempering, the addition of microalloys such as Nb and V is effective, but significant softening resistance is obtained unless Nb and V are combined. Since it does not work, it is added in combination.

Here, the present invention has been completed based on such findings, and the gist of the present invention is
C: 0.10 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%, M
o: 0.05 to 0.50%, Nb: 0.005 to 0.080%, V: 0.005 to 0.2
00%, Ti: 0.005 to 0.050%, B: 0.0001 to 0.0030%,
sol.Al: 0.005 to 0.050%, and if necessary, Pb: 0.
005 to 0.50%, Ca: 0.001 to 0.050%, Se: 0.01 to 0.50
% And Te: one or more of 0.005 to 0.050%, and the balance being Fe and unavoidable impurities, and

[0014]

(Equation 3)

However, (B), (N) and (Ti) represent B,
N, the content of Ti to steel having a steel composition which satisfies shows a (ppm), after heating to 950 ° C. or higher 1250 ° C. or less, and molded into a predetermined shape at a temperature above 750 ° C. by hot forging, release Cold
Ri after pro-eutectoid ferrite area ratio of the bainite structure is less than 10%, from 300 to 650 and performing tempering in a temperature range of ° C., the method of producing a high yield ratio high-toughness non heat-treated high-strength steel It is.

[0016]

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 function of the present invention.

C: 0.10% to 0.20% C is an element necessary for increasing the strength, and unless 0.10% or more is contained, the predetermined strength cannot be satisfied. On the other hand, if the content of C exceeds 0.20%, the effect of increasing the strength can be obtained, but on the other hand, remarkable deterioration of toughness is caused, so that C: 0.10 to 0.1.
20%.

Si: 0.05-1.00% Si is an element added at the time of steel making as a deoxidizing agent.
For that purpose, it is necessary to contain 0.05% or more. on the other hand,
Even if the content exceeds 1.00%, the deoxidizing effect is not only saturated, but also causes a remarkable decrease in toughness.

Mn: 0.50 to 2.50% Mn is an element which improves the hardenability and exerts an effect to make the metal structure of the steel material after hot forging into a bainite structure. Is required. On the other hand, if the content exceeds 2.50%, the effect is not only saturated, but also increases the production cost.
%, 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, has the function of refining crystal grains and improving the toughness.
% Or more. On the other hand, if the content exceeds 0.150%, huge MnS is generated, and the fatigue characteristics are reduced. Therefore, the component range was 0.005 to 0.150%.

Cr: 0.50 to 2.50% Cr is an element effective for imparting a predetermined hardenability to steel and improving static strength. In order to exert its effect sufficiently, the content must be 0.50% or more. On the other hand, if the content exceeds 2.50%, the toughness decreases. Therefore 0.50-2.50
% Is defined as the Cr component range. Preferably it is 1.0 to 2.0%.

Mo: 0.05 to 0.50% Mo is an element effective for imparting predetermined hardenability to steel and improving static strength and toughness. It also has the effect of improving the tempering softening resistance. To make full use of its effects
0.05% or more is required. On the other hand, if the content exceeds 0.50%, the effect will be saturated and the economic efficiency will be impaired.
The component range was set to 05 to 0.50%.

Nb: 0.005 to 0.080%, V: 0.005 to 0.20
0% These elements make the structure after hot forging a uniform and fine bainite structure, and improve the strength and toughness of bainite.
In addition, it increases the tempering softening resistance and greatly contributes to high YP. In order to fully demonstrate its effects, both 0.005%
The above content is necessary. On the other hand, Nb: 0.080%, V: 0.200
%, The effect is saturated and the economy is impaired. Therefore, the content ranges are 0.005 to 0.080% and 0.005 to 0.200%, respectively. In order to exert such an effect effectively, the combined softening exhibits remarkable softening resistance rather than the single addition of Nb and V.

Ti: 0.005 to 0.050% Ti has an effect of forming a nitride as TiN and refining crystal grains. Further, in order to improve the strength and toughness by adding B, which is an object of the present invention, it is necessary to add Ti, generate a nitride, and fix N. The minimum required for it
The Ti 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 to 0.050%.

B: 0.0001 to 0.0030% B has the effect of improving hardenability and at the same time improving toughness. 0.0001% to fully demonstrate its effect
It is necessary to contain the above. On the other hand, if the content exceeds 0.0030%, the crystal grains become coarse and the toughness is reduced.
成分 0.0030% was taken as the component range.

Sol. Al: 0.005 to 0.050% sol. Al is an element necessary for deoxidizing steel, like Si and Mn. In addition, there is an effect of generating Al nitride and refining crystal grains. Further, in order to improve the strength and toughness by adding B, which is the object of the present invention, it is necessary to add sol.Al in addition to Ti and fix N. The minimum required content is 0.005%. However, if the content exceeds 0.050%, a huge Al oxide is formed, and the fatigue strength is reduced. Further, the crystal grains become coarse and the toughness is reduced. Therefore, the content range is 0.005 to 0.050%.

N: N is an element necessary for generating TiN for refining crystal grains, and is added within a range satisfying the above formula (1).

[0028]

(Equation 4)

However, when (B), (N) and (Ti) indicate the contents (ppm) of B, N and Ti, the reason for limiting the value of equation (2) to 1 or more and 30 or less is B In order to sufficiently exhibit the effect of B as a solid solution B without generating nitrides of Ti, it is necessary to add Ti having a higher nitride forming ability than B. At that time, the expression
If the value of (2) is less than 1, the amount of solid solution B becomes insufficient, and if it exceeds 30, the effect of improving hardenability and toughness is lost in the state of excessive B.

Therefore, it is necessary to adjust the steel composition so as to satisfy the above equation (1). Pb, Ca, S to further improve the machinability as a steel composition targeted by the present invention
At least one of e and Te may be additionally blended.

Pb: 0.005 to 0.50% Pb is an element that improves machinability. In order to exert its effect sufficiently, the content must be at least 0.005% or more. On the other hand, if the content exceeds 0.50%, the fatigue properties are significantly reduced. Therefore, the component range is 0.005 to 0.50%.

Ca: 0.001 to 0.050% Ca, like Pb, is an element that improves machinability, but it is necessary to contain at least 0.001% in order to exert its effect sufficiently. On the other hand, if the content exceeds 0.050%, the toughness is significantly reduced. Therefore, 0.001 to 0.05
The component range was 0%.

Se: 0.01 to 0.50% Se is also an element that improves machinability. In order to exert its effect sufficiently, it is necessary to contain at least 0.01%. On the other hand, if the content exceeds 0.50%, the toughness decreases. Therefore, the component range was 0.01 to 0.50%.

Te: 0.005 to 0.050% Te is also an element that improves machinability. In order to exert its effect sufficiently, it is necessary to contain at least 0.005%. On the other hand, if the content exceeds 0.050%, the toughness decreases. Therefore, the content range is 0.005 to 0.050%. The steel whose composition has been adjusted in this way is subjected to hot forging and tempering to obtain a steel component having a high yield ratio and excellent toughness and strength.

Since the steel according to the present invention is generally used in the form of a steel bar in general, hot forging is performed by, for example, die forging with a press. However, you are not limited to a particular one. Here, the thermomechanical processing conditions in the present invention will be described.

Heating temperature: 950-1250 ° C. If the heating is lower than 950 ° C., the additives do not form a solid solution in the steel, and the desired strength and toughness cannot be obtained. On the other hand, when heating exceeds 1250 ° C., the additives become saturated, and the effects of various additives cannot be obtained. Therefore, it is limited to 950-1250 ° C.

Finishing temperature: 750 ° C. or higher After heating, the product is formed into a predetermined shape by hot forging. There is a need to. For that purpose, the finishing temperature must be 750 ° C or higher. Below 750 ° C, the area ratio of proeutectoid ferrite decreases.
It exceeds 10%, and the strength is significantly reduced. Therefore, 750 ° C
Above. Preferably, the temperature is 850 ° C. or higher. In other words, the amount of proeutectoid ferrite in the bainite structure can be adjusted by changing the finishing temperature. As described above, other forging means and conditions are not limited to specific ones.

Tempering temperature: 300 to 650 ° C. In the present invention, YR is improved by tempering. In order to exert such effects, a minimum tempering temperature of 300 ° C. is required. However, when the temperature exceeds 650 ° C., the strength is significantly reduced. Therefore, the temperature is set to 300 to 650 ° C. Preferably,
500-600 ° C. Next, the operation and effect of the present invention will be described more specifically with reference to examples.

[0039]

EXAMPLE Steels having the chemical compositions shown in Table 1 were melted in the air and 950 melted.
After heating to 1250 ° C or higher and 1250 ° C or lower, hot forging with a hot forging press is performed at a finishing temperature of 750 ° C
After forming a bainite structure in which the area ratio of proeutectoid ferrite was 10% or less, tempering was performed in a temperature range of 250 to 700 ° C.

A JIS No. 4 tensile test piece was manufactured from the center of the simulated hot forged material and a JIS No. 3 Charpy test piece was manufactured from the R / 2 part, and the mechanical properties were investigated. Table 2
Table 4 shows the test results.

As is clear from the results in Table 2, the steel obtained by the present invention at a tempering temperature of 550 ° C. has a tensile strength of
Both have excellent Charpy absorbed energy and high strength and high toughness. On the other hand, among the steel compositions listed as comparative examples, those in which C, Si, Mn, P, Cr, B, Ti, and sol.Al are higher than the specified values have inferior impact characteristics. Also,
If C, B, Ti, and sol.Al are lower than the specified values, the strength is reduced.

In general, strong carbide forming elements such as Mo, V, and Nb significantly suppress temper softening when added in a small amount.
The reason for this is that the upward movement of dislocations is suppressed in order to delay the diffusion of vacancies.

However, as shown in Table 3, at a tempering temperature lower than 300 ° C., the effect is not remarkably exhibited, and YR (yield ratio)
Does not improve much. On the other hand, as shown in Table 4, in the example of the present invention in which the tempering temperature was 550 ° C., a high YR was obtained regardless of the product size.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

As described above, according to the present invention, a bainite structure in which the area ratio of pro-eutectoid ferrite is 10% or less is obtained, and stable high strength and high toughness are obtained regardless of the product dimensions. Further, in the present invention, such a high TS is maintained, and a high YR by tempering (YP / T
S) can be achieved.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshihiko Kamada 1 Konomi-cho, Kokura-Kita-ku, Kitakyushu Sumitomo Metal Industries, Ltd. Inside the Kokura Works (72) Inventor Kazuhiko Nishida 1 Konomi-cho, Kita-ku, Kitakyushu Sumitomo Gold (72) Inventor Motohide Mori Motoyoshi 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Shigeru Yasuda 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (56) References JP-A-4-371547 (JP, A) JP-A-5-302117 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 8/00-8 / 08 C21D 6/00 B21J 5/00

Claims (2)

(57) [Claims] C. 0.10 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 0.50% by weight% , Nb: 0.005 to 0.080%, V: 0.005 to 0.200%, Ti: 0.005 to 0.050%, B: 0.0001 to 0.0030%, sol.Al: 0.005 to 0.050%, and the balance Fe and unavoidable impurities, and Equation 1 Where (B), (N) and (Ti) are the contents of B, N and Ti, respectively.
The steel having a steel composition which satisfies shows a (ppm), after heating to 950 ° C. or higher 1250 ° C. or less, the hot forging is formed into a predetermined shape at 750 ° C. or higher, the cooling of the pro-eutectoid ferrite After having a bainite structure with an area ratio of 10% or less,
A method for producing a high-yield-ratio, high-toughness, non-heat treated, high-strength steel, characterized by performing tempering in a temperature range of 300 to 650 ° C. 2. The steel composition further comprising, by weight%, Pb:
0.005 to 0.50%, Ca: 0.001 to 0.050%, Se: 0.01 to 0.50
The method for producing a high-yield-ratio, high-toughness, non-heat treated, high-strength steel according to claim 1, wherein the steel contains one or more of 0.005 to 0.050%.