JP3196006B2 - Method of manufacturing non-heat treated steel for hot forging, hot forged non-heat treated product, and hot forged non-heat treated product - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a steel material processed into a machine part such as an automobile or an industrial machine, which is processed by hot forging or rolling, and then hot worked or The present invention relates to a method for producing a hot-forged non-heat-treated steel material for hot forging, a hot-forged non-heat-treated product, and a hot-forged non-heat-treated product, which is a mechanical component after being subjected to aging treatment. It is characterized by high strength and high toughness as it is, and capable of induction hardening.

[0002]

2. Description of the Related Art Most of automotive and industrial machine parts are used after hot working bar stock, then quenching and tempering treatment (tempering treatment) to refine the structure and increase strength and toughness. In recent years, in order to reduce costs, mechanical parts used without the tempering treatment, that is, non-heat-treated steel for hot forging (hereinafter simply referred to as non-heat-treated steel) parts have rapidly become widespread.

Recently, it has been required to reduce the fuel consumption of automobiles in order to protect the global environment. One of the effective methods for achieving fuel economy of automobiles is to reduce vehicle weight. There is a need to reduce the size and weight of parts by improving strength.

[0004] To meet these demands, about 1000M
Research has been conducted on high-strength non-heat treated steels of Pa or higher, but especially for automobile parts and the like whose shape is becoming more complicated in recent years.
Bainite type non-heat treated steel having no so-called quenching distortion, which is used after hot forging and air cooling, is suitable.

[0005]

However, it is difficult to say that sufficient research has been made on non-heat treated steel having a bainite structure. For example, in general, a hot-forged bainite type non-refined product is not sufficiently timed to pass through the bainite transformation temperature range during air cooling after hot forging.
Austenite that cannot be transformed is stored to room temperature, and the austenite structure remains in the bainite structure.

[0006] Due to the presence of this soft retained austenite, the yield strength and toughness of hot forged steel are low, which is a disadvantage of bainite non-heat treated steel. Conventionally, tempering is performed to decompose the austenite structure. However, there was no effective means for improving the yield strength and toughness in the hot forged state.

In the case of ferritic / pearlite type non-heat treated steel, it is known that the strength increases and the toughness decreases as the carbon content increases. I have been. for that reason,
There are few inventions of bainite non-heat treated steel containing high carbon exceeding 0.30%, and even if the carbon content is 0.3
Even if steel in the range of 0% or more is included, there are few descriptions of such high carbon steel in the examples.

On the other hand, shafts are used after hardening the surface layer by induction hardening, and in order to obtain sufficient hardness by induction hardening, 0.3% or more of carbon is required.
If a bainite type non-heat treated steel containing more than 0.30% of carbon and requiring no tempering is completed, its application range to various parts is greatly expanded.

As an invention of a bainite type non-heat treated steel containing more than 0.30% of carbon and requiring no tempering, there is, for example, JP-A-3-260010. The present invention not only improves the quality of hot forged parts by controlling the casting conditions, but also adjusts the amount of Al added to improve the castability of a non-heat treated steel, and This is a method for producing a non-heat treated steel part.

In the embodiments of the present invention, tensile strength, yield strength and impact value are shown, but the maximum tensile strength is less than 1000 MPa (102 kgf / mm ² ). The low-temperature impact value is a value that does not cause any practical inconvenience,
If the tensile strength is 93.8 kgf / mm ² or more, 3 kgf
m / cm ² (29.4 J / cm ² ), which is not very good.

Japanese Patent Application Laid-Open No. 4-285118 discloses a method for improving the yield strength by hot-forging steel of a specific component and then gradually cooling the steel from 200 to 500 ° C. to room temperature. In the case of slow cooling, which takes up to 800 minutes, cost is a problem, such as installation of slow cooling equipment.

Japanese Patent Application Laid-Open No. Hei 6-88162 discloses a tensile strength having a high durability limit ratio of 90 kgf / mm ² (882).
MPa) or higher strength, high toughness non-heat treated steel, but does not mention the yield point, and still has a low yield point (or low yield ratio) which is a weak point of bainite non-heat treated steel as hot forged. ) Has not been resolved. Further, in the present invention, in order to increase the toughness, the amount of carbon is preferably 0.3% or less.

In recent years, there has been an increasing demand for low fuel consumption of automobiles, and not only is demand for higher strength of parts, especially higher yield point and higher toughness, but also cost reduction by low alloy saving process. However, in each of the above-mentioned inventions, the fact is that it is still not enough.

The present invention solves the above-mentioned problems, and provides a bainite-type hot forging method which has high strength without tempering as hot forging, particularly high yield strength and high toughness, and which can be induction hardened and has a low alloy cost. Provided are a material for a tempered steel part, that is, a non-heat treated steel for hot forging, a method for producing a hot forged non-heat treated product using this steel, and a hot forged non-heat treated product.

[0015]

SUMMARY OF THE INVENTION Tempering has heretofore been performed to increase the yield strength of bainite non-refined products and toughness as measured primarily by impact value. Tempering is considered to have the effect of decomposing the retained austenite to strengthen the steel. Therefore, a method of increasing the yield strength and toughness is considered to combine a component that does not generate retained austenite and a cooling method. However, any component may be 1000 MPa or more as hot forged. When the components are adjusted to the above values, it is difficult to eliminate the residual austenite from the structure as hot forged.

Further, the reduction of the retained austenite by the controlled cooling causes a significant increase in cost even if it is possible. However, when the present inventors examined in detail, the yield strength and toughness were greatly affected by not only the total amount of retained austenite but also its distribution, and even if the total amount of austenite was equal to that of conventional steel, it was finely dispersed in the bainite structure. If so, the yield strength and toughness were found to be improved.

The refinement of the structure, particularly the refinement of the reheated austenite structure during hot forging and the refinement of the bainite lath when cooled and transformed into a bainite structure, greatly contribute to the improvement in yield strength and toughness. I do.

Conventional research, for example, CAMP-ISIJ
Vol. 5 (1992) -801, it is reported that the microstructure obtained by reproducing the hot forged state is refined by the addition of V and Mo, and the ductility is improved. However, improvement of the yield strength is not recognized. Has been described. It is considered that the steel components reported in this report are due to insufficient fine dispersion of the reheated austenite structure, bainite lath structure and retained austenite structure.

The method of refining the structure by adjusting the steel composition found by the present inventors is based on the high carbon and main alloying elements Si, M
Combination of n, Cr, V, and dispersion of nitride of Ti, Al. Conventionally, it has been thought that an increase in the amount of added carbon lowers the yield strength and toughness.
By increasing the content to more than 3%, the austenite remaining near the prior austenite grain boundary becomes fine, and the bainite lath structure can be easily and very finely divided.

As a result, the yield strength and the toughness are improved despite the increase in carbide. Still further, TiN and A
Fine dispersion of 1N is very effective for miniaturization of the bainite packet through miniaturization of the austenite structure at the time of reheating, and is essential for obtaining a high yield strength and impact value. The present invention completed based on the above findings is as shown in the following (1) to (3).

(1) In the first invention, C: over 0.30 to 0.60%, Si: 0.20 to 0.52 %, Mn: over 1.60 to 3.00% by weight. %, Cr: 0.05 to 2.00%, Ti: 0.005 to 0.050%, Al: 0.005 to 0.050%, N: 0.0098 to 0.0146 %, and V: 0.01 to 0.30%, Nb: 0.01 to
0.30% of one or two kinds, the balance being Fe and unavoidable impurities, and the carbon equivalent Ceq. Ri Der but more than 0.82%, hot forging was allowed to cool
A non-heat treated steel for hot forging having a toughness in which 80% or more of the volume of the subsequent metal structure is a bainite structure .

(2) In the second invention, C: over 0.30 to 0.60%, Si: 0.20 to 0.52 %, Mn: over 1.60 to 3.00% by weight. %, Cr: 0.05 to 2.00%, Ti: 0.005 to 0.050%, Al: 0.005 to 0.050%, N: 0.0098 to 0.0146 %, and V: 0.01 to 0.30%, Nb: 0.01 to
0.30% of one or two kinds, the balance being Fe and unavoidable impurities, and the carbon equivalent Ceq. Hot forging microalloyed steel, more than 80% of the volume of the metal structure and bainite is but 0.82% or more
So as processed at temperatures above 1270K, cool to it, or processed at temperatures above 1270K, cool after 550
A method for producing a hot-forged non-refined product that can be induction hardened and is characterized by aging from K to 900K.

(3) In the third invention, C: over 0.30 to 0.60%, Si: 0.20 to 0.52 %, Mn: over 1.60 to 3.00% by weight. %, Cr: 0.05 to 2.00%, Ti: 0.005 to 0.050%, Al: 0.005 to 0.050%, N: 0.0098 to 0.0146 %, and V: 0.01 to 0.30%, Nb: 0.01 to
0.30% of one or two kinds, the balance being Fe and unavoidable impurities, and the carbon equivalent Ceq. Hot forged non-heat-treated having a toughness capable of induction hardening having a component of 0.82% or more, a bainite structure of 80% or more of the structure volume, and a tensile strength of 1000 MPa or more. Goods.

[0024]

## EQU4 ## Ceq. (%) = C (%) + 0.10 (% S
i) +0.18 (% Mn) +0.21 (% Cr) +0.
328 (% V +% Nb)

[0025]

The reason for limiting the present invention together with the operation will be described below.

C: C is a toughening element of steel. 0.30
% Or less, the structure as hot forged and the fine dispersion of retained austenite become insufficient, and the yield strength and toughness decrease. Also, at 0.30% or less, 1000M
In order to achieve a tensile strength of Pa or more, a large amount of alloy is required, which increases the cost and shortens the life of the forging die to increase deformation resistance during hot forging. Further, if it is 0.30% or less, the surface hardness after induction hardening is lower than normally required, and is not practical for induction hardening. On the other hand, when the content exceeds 0.60%, the yield strength is rather lowered, and the ferrite and the pearlite structure are liable to be transformed during forged cooling.

[0027] Si: Si is a solid solution strengthening element, 0.2
Addition of 0% or more is effective. However, if it exceeds 0.52 %, ferrite or pearlite structure is easily transformed and mechanical properties are deteriorated.

Mn: Mn enhances the hardenability and transforms the as-cooled structure into bainite. Also, an appropriate addition increases the toughness of the steel. If the content is 1.60% or less, the structure while hot forging and cooling is not easily transformed into bainite.
If it exceeds 00%, the martensite is transformed and becomes harder than necessary, thereby reducing toughness and machinability.

Cr: Cr has the same function as Mn,
In order to reduce the bainite transformation start temperature and refine the structure, it is effective to use Mn and V together. If a large amount of Cr is added, martensite is transformed.
00% or less. Also, it is limited to 0.05% or more that can be guaranteed by the process capability in steel production.

Ti, Al: Ti, Al precipitates and disperses in the steel as nitrides, thereby preventing the austenite structure from being coarsened during forging reheating and increasing the yield strength and toughness. The amount added for preventing coarsening is Ti: 0.002%
As described above, Al is 0.005% or more, but if added in a large amount, the precipitates become coarse and the steel becomes brittle, so the upper limit is set to T.
i: 0.050%, Al: 0.050%.

N: N forms various nitrides to prevent the austenite structure from becoming coarse during hot forging.
It is an element that refines bainite packets and increases the yield strength, and requires at least 0.0098 % or more. However
The effect saturates even if it exceeds 0.0146 %.

V, Nb: V and Nb lower the bainite transformation temperature during forging to make the bainite structure as-forged and cooled to a fine structure to increase the toughness, and precipitate upon cooling after forging to strengthen the steel. In addition, V, N in solid solution in steel
b has the effect of precipitating during aging and further strengthening the steel. In order to exhibit these effects, it is necessary to add 0.01% or more of both V and Nb. However, the upper limit is set to 0.30% in order to suppress costs.

In order to adjust the tensile strength of the steel after tempering in a hot forged non-tempered state, the present inventors heated and allowed the steel of various components to be heated to a high temperature, assuming hot forging. Tensile properties and microstructure were examined. The test was performed by heating the steel at 1500 K for 300 seconds and then cooling it at a cooling rate of 1.0 K / S. When the relationship between the tensile strength and the alloy component was examined from the test results, the following Expressions 5 and 6 were obtained.

[0034]

## EQU5 ## Ceq. (%) = C (%) + 0.10 (% S
i) +0.18 (% Mn) +0.21 (% Cr) +0.
328 (% V +% Nb)

[0035]

[Formula 6] TS (MPa) = 1046 × Ceq. +144

Here, the carbon equivalent Ceq. Is set to 0.82% or more, the tensile strength when the steel of the present invention is hot forged and allowed to cool can be set to 1000 MPa or more. At this time, in the case of the steel of the present invention, the structure becomes a bainite single phase or a bainite structure containing a small amount of ferrite, austenite or martensite.

The steel of the present invention is subjected to aging after hot forging to thereby improve the toughness measured by yield strength and impact value mainly through decomposition of soft retained austenite contained in bainite steel or tempering martensite. Can be further enhanced. In order to expect these effects of aging, it is efficient to age at 550 K or more after forging. If the aging temperature is lower than 550K, the yield strength cannot be increased or a long time is required.

In general, the tensile strength and hardness after aging at 900 K or lower are slightly lower than those at the time of cooling, and thus there is an advantage that cutting can be easily performed. However, when the temperature exceeds 900 K, the tensile strength is significantly reduced and is not practical. When a relatively high aging temperature is employed, a reduction in tensile strength can be prevented by adding an appropriate amount of the age hardening elements V and Nb.

At the time of working in the second manufacturing method of the present invention described in the above (2), the steel is made of an austenitic single phase, the deformation resistance during hot forging is reduced, and the life of the forging die is reduced to a practical length. Therefore, it is necessary to process at a temperature of 1270K or more. In addition, if the size of the automotive component is within the range, the steel of the present invention does not undergo any special cooling after processing, and becomes a structure mainly composed of a bainite structure while being left to cool, and has a tensile strength of 1000 MPa or more. Can be.

In the non-refined product according to the third aspect of the present invention as described in the above (3), high strength and high toughness can be obtained when at least 80% of the volume of the structure is a bainite structure. When the bainite structure becomes less than 80% due to cooling conditions,
Mechanical properties may be degraded due to other mixed tissues.

For example, when the mixed structure is ferrite or pearlite, the tensile strength decreases, and when the mixed structure of martensite and austenite is mixed, the tensile strength increases and the toughness decreases.

For improving machinability, S, Pb, Bi, T
Even if appropriate amounts of e, Se, and Ca are added, the material of the present invention does not deteriorate. To improve machinability, S: 0.02
-0.10%, Pb: 0.005-0.50%, Bi:
0.010 to 0.50%, Te: 0.001 to 0.20
%, Se: 0.010-0.50% is desirable,
Particularly, when the machinability in cutting using a carbide tool is regarded as important, the addition of Ca: 0.0004 to 0.0050% is effective.

S forms MnS to improve machinability, and also functions to prevent coarsening of prior austenite grains and improve toughness.

[0044]

EXAMPLES Steels of various compositions shown in Tables 1 and 2 were used for 150 times.
A steel melted in a kg vacuum melting furnace and molded to a thickness of 40 mm was used as a material. Immediately after heating these steels at 1,475 K for 1200 S, they were forged 50% and allowed to cool, and then left as they were, or 60% at 570 K, 830 K.
Tensile tests and impact tests were performed on the steels aged for minutes. The steel was processed into a rod having a diameter of 20 mm, induction hardened, and the hardness of the quenched portion below the surface layer was measured.

As shown in Tables 3 and 4, the steel of the present invention in a non-heat treated state in which the steel of the present invention was forged and allowed to cool was subjected to a tensile strength (T.
S. ) It has a high strength of 1000 MPa or more, a yield ratio (YR) of 0.71 or more, and a good impact value (uE) of 60 J / cm ² or more.

The steel aged after being allowed to cool for forging has further improved yield ratio and impact value. Furthermore, the steel of the present invention containing more than 0.30% of carbon has sufficient induction hardening hardness of Vickers hardness of 648 or more.

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

[Table 3]

[0050]

[Table 4]

[0051]

As described above, the first non-heat-treated steel for hot forging of the present invention has a tensile strength of not less than 1000 MPa, a high yield strength and a high toughness, and is capable of induction hardening. Ideal as a material for non-heat treated steel parts.

Further, according to the second method for producing a hot-forged non-heat-treated product of the present invention, a hot-forged non-heat-treated product having a tensile strength of 1000 MPa or more, a high yield strength and a high toughness and capable of induction hardening is obtained. Can be manufactured.

Further, since the hot forged non-refined product of the third invention has a tensile strength of 1000 MPa or more and has high toughness, it is designed to be compact when used as a part for automobiles or industrial machines. And induction hardening enables the parts to be even stronger.
This contributes to vehicle weight reduction, fuel consumption reduction, and cost reduction.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Akio Ono 2-6-3 Otemachi, Chiyoda-ku, Tokyo Inside Nippon Steel Corporation (72) Inventor Akemi Nanjo 5-33 Shiba 5-chome, Minato-ku, Tokyo No. 8 Inside Mitsubishi Motors Corporation (72) Inventor Toru Sakurada 5-33-8 Shiba, Minato-ku, Tokyo Inside Mitsubishi Motors Corporation (56) References JP-A-6-287677 (JP, A) JP-A-6-287679 (JP, A) JP-A-3-260010 (JP, A) JP-A-6-88162 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00

Claims (3)

(57) [Claims] 1. In weight%, C: more than 0.30 to 0.60%, Si: 0.20 to 0.52 %, Mn: more than 1.60 to 3.00%, Cr: 0.05 to 2.00%, Ti: 0.005 to 0.050%, Al: 0.005 to 0.050%, N: 0.0098 to 0.0146 %, and V: 0.01 to 0.30 % , Nb: 0.01 to
0.30% of one or two kinds, the balance being Fe and unavoidable impurities, and the carbon equivalent Ceq. Ri Der but more than 0.82%, hot forging was allowed to cool
A non-heat treated steel for hot forging having a toughness in which the subsequent metal structure has a bainite structure at least 80% of the volume . ## EQU00001 ## Ceq. (%) = C (%) + 0.10 (% Si) +0.18 (% Mn) +0.21 (% Cr) +0.328 (% V +% Nb) 2. In% by weight, C: more than 0.30 to 0.60%, Si: 0.20 to 0.52 %, Mn: more than 1.60 to 3.00%, Cr: 0.05 to 2.00%, Ti: 0.005 to 0.050%, Al: 0.005 to 0.050%, N: 0.0098 to 0.0146 %, and V: 0.01 to 0.30 % , Nb: 0.01 to
0.30% of one or two kinds, the balance being Fe and unavoidable impurities, and a carbon equivalent Ceq. Hot forging microalloyed steel, more than 80% of the volume of the metal structure and bainite is but 0.82% or more
So as processed at temperatures above 1270K, cool to it, or processed at temperatures above 1270K, cool after 550
A method for producing a hot-forged non-refined product, which is capable of induction hardening, characterized by aging from K to 900K. ## EQU00002 ## Ceq. (%) = C (%) + 0.10 (% Si) +0.18 (% Mn) +0.21 (% Cr) +0.328 (% V +% Nb) 3. In% by weight, C: more than 0.30 to 0.60%, Si: 0.20 to 0.52 %, Mn: more than 1.60 to 3.00%, Cr: 0.05 to 2.00%, Ti: 0.005 to 0.050%, Al: 0.005 to 0.050%, N: 0.0098 to 0.0146 %, and V: 0.01 to 0.30 % , Nb: 0.01 to
0.30% of one or two kinds, the balance being Fe and unavoidable impurities, and the carbon equivalent Ceq. Hot forged non-heat-treated having a toughness capable of induction hardening having a component of 0.82% or more, a bainite structure of 80% or more of the structure volume, and a tensile strength of 1000 MPa or more. Goods. ## EQU00003 ## Ceq. (%) = C (%) + 0.10 (% Si) +0.18 (% Mn) +0.21 (% Cr) +0.328 (% V +% Nb)