JP2000017374A - Age hardening type high strength bainitic steel and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the strength and machinability of steel by subjecting steel having a specified compsn. to hot rolling or hot forging and thereafter executing cooling under specified conditions to control the hardness, structure and grain size therein to specified ones and thereafter executing machining or plastic working and aging treatment. SOLUTION: Steel contains, by weight, 0.06 to 0.20% C, 0.03 to 1.00% Si, 1.50 to 3.00% Mn, 0.50 to 2.00% Cr, 0.05 to 1.00% Mo, 0.002 to 0.100% Al, 0.51 to 1.00% V and 0.0080 to 0.0200% N. This steel is subjected to hot rolling or hot forging at 1,150 to 1,300 deg.C. Then, it is cooled to <=200 deg.C in such a manner that the average cooling rate in the range of 800 to 500 deg.C: CV ( deg.C/min) is controlled to 40/(Mn%+0.8Cr%+1.2Mo%)<=CV<=500/(Mn%+0.8Cr%+1.2Mo%) to control the structure to >=70% bainitic ratio and the old austenite grain size to <=80 μm. After that, machining or plastic working is executed at need, and, moreover, aging treatment is executed at 550 to 700 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an age hardening type bainite steel for hot forging, which is most suitable for parts requiring high strength and excellent machinability, such as a crankshaft and a connecting rod of an automobile engine. It is intended to provide a method for manufacturing a forged product, and to reduce the weight of an automobile engine component.

[0002]

2. Description of the Related Art Engine parts such as crankshafts and connecting rods of automobile engines are required to have high strength and excellent machinability.
Steel to which free-cutting elements such as Pb and Pb are added is hot forged, subjected to heat treatment of quenching and tempering, and then machined for use. In recent years, due to the need for cost reduction, so-called non-heat treated steel, which omits the heat treatment of quenching and tempering, has been actively used in the above-mentioned parts. , Bainite type,
Alternatively, various non-heat treated steels such as martensite type have been developed,
Has been put to practical use.

[0003] On the other hand, recently, in response to the trend of fuel efficiency of automobiles and the trend toward higher output of engines, there is a strong need to reduce the weight of automobile engine parts, and there is an active movement to reduce the thickness and weight of parts using high-strength steel. Has become. Therefore, as a method of satisfying both the need for cost reduction and the need for weight reduction, application of high-strength non-heat treated steel is considered.
In the ferrite pearlite type non-heat treated steel, by controlling the chemical composition and controlling the forging heating conditions, the invention of improving the fatigue strength of the parts used with the surface as black forged skin is disclosed in Japanese Patent Application Laid-Open No. 5-302116. Japanese Patent Application Laid-Open No. 10-140285 discloses an invention in which the yield ratio and the durability ratio are improved by subjecting bainite or bainite + martensite type non-heat treated steel to tempering after being forged and allowed to cool. The invention in which machinability and high strength are achieved at the same time by properly combining the cooling rate dependency of the above and the shape and dimensions of the forged part is disclosed.

[0004] However, when steel is to be strengthened before obtaining a weight-reducing effect comparable to that of a titanium alloy or an aluminum alloy, a significant increase in hardness is unavoidable, leading to a reduction in machinability. Occurs. With respect to this problem, it is impossible to achieve the target weight reduction effect in the above-mentioned Japanese Patent Application Laid-Open No. 4-193931. Therefore, the cost of the material increases, and the increase in the amount of inclusions due to the increase in the content of free-cutting elements causes problems such as deterioration in plastic workability such as rolling and forging and deterioration in fatigue strength. Also, in Japanese Patent Application Laid-Open No. 10-140285, by reducing the thickness of the non-cutting portion and increasing the thickness of the cutting portion, both increasing strength and ensuring machinability can be achieved without increasing the amount of free-cutting elements added. Although it is ensured to some extent, its effect is limited, and there is a problem that it is restricted by the above-mentioned component shape.

[0005]

As described above, the strength of steel is increased until a weight reduction effect comparable to that of a titanium alloy or an aluminum alloy is obtained, and the content of free-cutting elements is not greatly increased. In addition, the inventors have conducted various studies on measures for ensuring machinability without restricting the design shape of parts, and as a result, have reached the following idea. That is, V
It has been found that in a steel having a high content and a bainite structure as a main component, the hardness before and after the aging treatment changes greatly, and the hardness before the aging can be reduced and the hardness after the aging can be increased. Therefore, by performing the aging treatment after performing the rough cutting before the aging, it is possible to achieve both high strength and ensuring machinability.

In order to reduce the weight by increasing the strength of the steel as described above, it is particularly important to improve the yield strength (0.2% proof stress). With the same hardness compared to high-quality steel and non-heat-treated steel
It was found that 0.2% proof stress was increased, that is, a high yield ratio was obtained. Therefore, not only cutting before aging as described above, but also when cutting after aging,
Since the bainite steel with a high V content can have a lower hardness at the same 0.2% proof stress than ordinary heat-treated steel and non-heat-treated steel, machinability can be improved.

In order to obtain the above-mentioned effects, it is necessary to make the V content 0.51% or more.
After hot rolling or hot forging at a heating temperature of 00 ° C, 800
Average cooling rate in the temperature range of ~ 500 ° C: CV (° C / min)
Of 40 / (Mn% + 0.8Cr% + 1.2Mo%) ≦ CV ≦ 500 /
(Mn% + 0.8Cr% + 1.2Mo%), it was found that it is necessary to make the hardness Hv400 or less and the microstructure 70% or more by cooling to a temperature of 200 ° C or less. In addition, in the above steel materials or forged products, V carbonitride precipitates during the thermomechanical treatment to refine the crystal grains.
The following austenite grain size is obtained, which is
It has been found that the aging treatment at the above temperature makes it possible to increase the yield point or the 0.2% proof stress to 900 MPa or more as a result, leading to the present invention.

In the first invention, the chemical composition is expressed by weight%, and C: 0.
06 to 0.20%, Si: 0.03 to 1.00%, Mn: 1.50 to 3.00%, Cr:
0.50 to 2.00%, Mo: 0.05 to 1.00%, Al: 0.002 to 0.100%,
V: 0.51-1.00%, N: 0.0080-0.0200%, balance Fe
After hot rolling or hot forging of steel consisting of unavoidable impurities at a heating temperature of 1150 to 1300 ° C, the average cooling rate in the temperature range of 800 to 500 ° C: CV (° C / min) is set to 40 / (Mn% +
0.8Cr% + 1.2Mo%) ≤ CV ≤ 500 / (Mn% + 0.8Cr% + 1.
2Mo%) and cooled to a temperature of 200 ° C or less to reduce the hardness to Hv400 or less, the microstructure to 70% or more of hainite and the grain size of old austenite to 80μm or less, and then apply cutting or plastic working as necessary. And an aging treatment at a temperature of 550 to 700 ° C. so that the yield point or 0.2% proof stress is 900 MPa or more.

In the second invention, the chemical composition is expressed by weight%, and C: 0.
06 to 0.20%, Si: 0.03 to 1.00%, Mn: 1.50 to 3.00%, Cr:
0.50 to 2.00%, Mo: 0.05 to 1.00%, Al: 0.002 to 0.100%,
V: 0.51-1.00%, N: 0.0080-0.0200%, balance Fe
After hot rolling or hot forging of steel consisting of unavoidable impurities at a heating temperature of 1150 to 1300 ° C, the average cooling rate in the temperature range of 800 to 500 ° C: CV (° C / min) is set to 40 / (Mn% +
0.8Cr% + 1.2Mo%) ≤ CV ≤ 500 / (Mn% + 0.8Cr% + 1.
2Mo%) and cooled to a temperature of 200 ° C or less to reduce the hardness to Hv400 or less, the microstructure to 70% or more of hainite and the grain size of old austenite to 80μm or less, and then apply cutting or plastic working as necessary. And a aging treatment at a temperature of 550 to 700 ° C. so that the yield point or 0.2% proof stress is 900 MPa or more.

A third aspect of the present invention is directed to a method for manufacturing a semiconductor device, comprising:
One or two selected from 0.01 to 0.10%, Nb: 0.01 to 0.10%
The age hardening type high-strength bainite steel according to claim 1, which contains a seed.

A fourth aspect of the present invention is directed to a method of the present invention, wherein the chemical composition is
One or two selected from 0.01 to 0.10%, Nb: 0.01 to 0.10%
The method for producing an age-hardened high-strength bainite steel according to claim 2, comprising a seed.

According to a fifth aspect of the present invention, the chemical composition is represented by weight% and S:
0.04 to 0.12%, Pb: 0.01 to 0.30%, Bi: 0.01 to 0.30%, C
a: 0.0005 to 0.01%, REM: 1 selected from 0.001 to 0.10%
2. The composition according to claim 1, wherein the composition contains at least one species.
The age hardening type high-strength bainite steel described in 1.

A sixth aspect of the present invention is a method of the present invention, wherein the chemical composition is expressed by weight% and S:
0.04 to 0.12%, Pb: 0.01 to 0.30%, Bi: 0.01 to 0.30%, C
a: 0.0005 to 0.01%, REM: 1 selected from 0.001 to 0.10%
3. The composition according to claim 2, wherein the composition contains at least two species.
3. A method for producing an age hardening type high-strength bainite steel according to (1).

According to a seventh aspect of the present invention, the chemical composition has a weight percentage of Ti:
One or two selected from 0.01 to 0.10%, Nb: 0.01 to 0.10%
Contains seeds, S: 0.04-0.12%, Pb: 0.01-0.30
%, Bi: 0.01 to 0.30%, Ca: 0.0005 to 0.01%, REM: 0.001
The age-hardened high-strength bainite steel according to claim 1, comprising one or more selected from 0.10%.

[0015] An eighth invention provides a method for manufacturing a semiconductor device, comprising:
One or two selected from 0.01 to 0.10%, Nb: 0.01 to 0.10%
Contains seeds, S: 0.04-0.12%, Pb: 0.01-0.30
%, Bi: 0.01 to 0.30%, Ca: 0.0005 to 0.01%, REM: 0.001
The method for producing an age-hardenable high-strength bainite steel according to claim 2, wherein one or two or more kinds selected from 0.1% to 0.10% are contained.

Next, the reasons for limiting the components in the present invention will be described. C: 0.06 to 0.20% C is an essential element for securing strength as steel for machine structural use, and is required to be 0.06% or more, preferably 0.08% or more. However, if the content is too large, the machinability is degraded due to the increase in hardness, so the upper limit is set to 0.20%, preferably 0.18% or less. Si: 0.03 to 1.00% Since Si is indispensable as a deoxidizing material during steelmaking, the lower limit is set to 0.
03%, preferably 0.10% or more. However, if added excessively, SiO ₂ which is a high-hardness inclusion is generated in the steel to deteriorate the machinability, so the upper limit is made 1.00%, preferably 0.80% or less.

Mn: 1.50% to 3.00% Mn is an important element for securing hardenability for obtaining a hainite structure in a cooling process after hot rolling or hot forging. Requires at least 1.50%, preferably at least 1.80%. However, if the content is too large, the structure becomes mainly martensite, the hardness before aging treatment increases, and the machinability deteriorates, so the upper limit is 3.00%, desirably.
2.70% or less. Cr: 0.50 to 2.00% Like Mn, Cr is an important element in securing hardenability to obtain a hainite structure in a cooling process after hot rolling or hot forging, and is required to obtain a hainite structure. At least
0.50% or more, desirably 0.70% or more is required. But,
If the content is too large, the structure becomes mainly martensite, the hardness before the aging treatment increases, and the machinability deteriorates. Therefore, the upper limit is made 2.00%, preferably 1.60% or less.

Mo: 0.05 to 1.00% Mo, like Mn and Cr, is an element necessary for stably obtaining a hainite structure in a cooling process after hot rolling or hot forging, and also refines the hainite structure. To increase the toughness,
Further, it has a function of precipitating Mo ₂ C during aging treatment and effecting age hardening, and at least 0.05% or more, desirably 0.10% or more is required to obtain the above effect. However, even if it is added more than necessary, the effect is saturated and the cost increases, so the upper limit is made 1.00%, preferably 0.60% or less. Al: 0.002 to 0.100% Al is an indispensable element for deoxidation and needs to be 0.002% or more, preferably 0.005% or more. However, if added more than necessary, the machinability is deteriorated due to the formation of Al ₂ O _3. Therefore, the upper limit is set to 0.100%, preferably 0.060% or less.

V: 0.51 to 1.00% V is an element that plays the most important role in precipitating V (CN) after aging treatment to increase hardness and yield strength in the present invention. In order to obtain the above, at least 0.51% or more, preferably 0.53% or more is required. However, even if it is added more than necessary, the effect is saturated, and significant toughness deterioration and cost increase are caused.
The upper limit is set to 1.00%, preferably 0.70% or less. N: 0.0080 to 0.0200% N is an element having a high affinity for V, and in the present invention,
Precipitates as VN during hot rolling or hot forging, has the effect of reducing the austenite grain size to 80 μm or less by its pinning effect, and is indispensable for the increase in strength due to V (CN) precipitation after aging treatment And at least 0.0080%, preferably 0.0100% or more in order to obtain the above-mentioned effects sufficiently and sufficiently. However, even if added more than necessary, the effect is saturated, and significant toughness degradation and cost increase are incurred, so the upper limit is 0.0200%,
Desirably, it is 0.0180% or less.

Ti: 0.01 to 0.10%, Nb: 0.01 to 0.10% Ti and Nb are precipitated in steel as Ti (CN) and Nb (CN) in the same manner as V, and the austenite grain size is obtained by the pinning effect. Has a function of making the particles finer, and is added as necessary. To achieve the above effects, at least 0.01%
The above content is necessary. However, even if it is added more than necessary, the effect is saturated and the cost is increased. Therefore, the upper limit is set to 0.10%. S: 0.04-0.12%, Pb: 0.01-0.30%, Bi: 0.01-0.30
%, Ca: 0.0005 to 0.01%, REM: 0.001 to 0.10% S, Pb, Bi, Ca, and REM are effective elements for improving machinability, and are added as necessary. To obtain the above effects, 0.04%, 0.01%, 0.01%, 0.0005%,
0.001% content is required. However, if it is contained in a large amount, the cost increases, and the increase in the amount of inclusions causes problems such as rolling and forging, etc., which degrade plastic workability and decrease fatigue strength. Therefore, the upper limits are 0.12%, 0.30%, and 0.30%, respectively.
%, 0.01%, and 0.10%.

Next, the reasons for limiting the manufacturing conditions of the present invention will be described. Heating temperature during hot rolling or hot forging is 1150
When the heating temperature is lower than 1150 ° C, V is not sufficiently dissolved in the steel at the stage before the aging treatment, so that the age hardening cannot be sufficiently obtained. 1300
If the heating temperature exceeds ℃, the austenite grains in the heating step become coarse or mixed, and finally the prior austenite grain size of 80 μm or less cannot be achieved. Here, the heating temperature at the time of hot rolling or hot forging means that the process of controlling the heating temperature differs depending on the manufacturing process of the parts. The heating temperature during forging is limited to the above temperature range, and when hot forging is not performed, for example, when cutting and manufacturing a part directly from a rolled steel material, the heating temperature during hot rolling is set to the above temperature range. It needs to be limited.

The reason why the average cooling rate after hot rolling or hot forging: CV (° C./min) is limited in the temperature range of 800 to 500 ° C. is that the average cooling rate: CV (° C./min) is 40 / ( Mn% + 0.8
If the content is less than (Cr% + 1.2Mo%), proeutectoid ferrite or halite is generated, and it is difficult to secure a hainite ratio of 70% or more. CV (° C./min) is 500 / (Mn%). + 0.8Cr% + 1.
If it exceeds 2 Mo%), martensite is generated, and it is difficult to secure a hainite ratio of 70% or more. Here, the average cooling rate: CV (° C / min) is 800 ° C during cooling.
The value obtained by dividing 300 ° C (= 800 ° C-500 ° C) by the time (min) required to reach 500 ° C after the temperature reached 500 ° C.

The reason why the cooling is limited to a temperature of 200 ° C. or less is that the hainite transformation during cooling is sufficiently generated to reduce the hainite rate.
This is to secure at least 70%. The reason for limiting the hardness to Hv400 or less is to ensure the workability of the cutting or plastic working performed thereafter, and when the hardness exceeds Hv400, the machinability and plastic workability deteriorate rapidly. . In addition,
When the steel having the composition within the above-mentioned claims is hot-rolled or hot-forged at a heating temperature of 1150 to 1300 ° C. and then cooled under the above-mentioned limited conditions, the hardness becomes Hv400 or less.

The reason why the structure is limited to a hainite ratio of 70% or more is to obtain a necessary and sufficient age hardening characteristic by V (CN), and the structure of ferrite, barite or martensite is reduced when the hainite ratio becomes less than 70%. When the fraction increases, the necessary and sufficient age hardening characteristics cannot be obtained, that is, the hardness before the aging treatment increases or the hardness after the aging treatment decreases. In addition, steel having a composition within the above-mentioned claims was heated at a temperature of 1150 to
After hot rolling or hot forging at 1300 ° C., when cooled under the above-mentioned limited conditions, the hainite ratio becomes 70% or more. The reason why the prior austenite crystal grain size is limited to 80 μm or less is that it is necessary to achieve high yield strength and fatigue strength. If the old austenite crystal grain size exceeds 80 μm, the strength characteristics deteriorate.
In addition, steel having a composition within the above-mentioned claims was heated at a temperature of 1150 to 1300.
After hot rolling or hot forging at ℃, and cooled under the above-mentioned limited conditions, the prior austenite crystal grain size becomes 80 μm.

The reason why the aging treatment temperature is limited to 550 to 700 ° C. is that V (CN) is finely precipitated as necessary and sufficiently in steel having a structure mainly composed of hainite to age harden. Aging temperature
If the temperature is lower than 550 ° C, the amount of V (CN) deposited is small and sufficient age hardening cannot be obtained.If the aging temperature exceeds 700 ° C, the precipitated V (CN) becomes coarse and softens. Therefore, it is necessary to limit the aging temperature to 550 to 700 ° C. Yield point or 0.2% proof stress: 900MPa or more is a strength level necessary for obtaining a weight-reducing effect comparable to that of titanium alloys and aluminum alloys with steel. After hot rolling or hot forging, cooled under the above-mentioned limited conditions, then 550-7
It is achieved by aging at 00 ° C.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS To implement the first and second inventions,
% By weight, C: 0.06-0.20%, Si: 0.03-1.00%, Mn: 1.5
0 to 3.00%, Cr: 0.50 to 2.00%, Mo: 0.05 to 1.00%, Al: 0.
1150-13 containing steel containing 002-0.100%, V: 0.51-1.00%, N: 0.0080-0.0200%, the balance being Fe and unavoidable impurities
After hot rolling or hot forging at a heating temperature of 00 ° C, 800
Average cooling rate in the temperature range of ~ 500 ° C: CV (° C / min)
Of 40 / (Mn% + 0.8Cr% + 1.2Mo%) ≦ CV ≦ 500 /
(Mn% + 0.8Cr% + 1.2Mo%), cooling to a temperature of 200 ° C or less, the hardness becomes Hv400 or less, the structure becomes 70% or more of hainite and the grain size of old austenite is 80μm or less. Add cutting or plastic processing according to
The yield point or 0.2% proof stress is made 900 MPa or more by aging treatment at a temperature of ~ 700 ° C. The steel material and the forged product thus obtained can be strengthened before obtaining a weight-reducing effect comparable to that of a titanium alloy or an aluminum alloy, and greatly increase the content of free-cutting elements. Machinability can be ensured without any restrictions and without restricting the design shape of the part.

In order to carry out the third and fourth inventions, the first and second inventions are required.
In addition to the elements according to the invention, Ti: 0.01 to 0.1
0%, Nb: steel containing one or two selected from 0.01 to 0.10%, the balance consisting of Fe and unavoidable impurities is 1150 to 1300
800 ~ 5 after hot rolling or hot forging at heating temperature of ℃
Average cooling rate in the temperature range of 00 ° C: CV (° C / min)
/(Mn%+0.8Cr%+1.2Mo%) ≤ CV ≤ 500 / (Mn% +
(0.8Cr% + 1.2Mo%), cooling to a temperature of 200 ° C or less to reduce the hardness to Hv400 or less, the microstructure to 70% or more of hainite and the grain size of old austenite to 80μm or less. Or plastic working, then 550-700 ℃
The aging treatment at the temperature of
0.2% proof stress should be 900MPa or more. The steel material and the forged product obtained in this way can be strengthened before obtaining a weight-reducing effect comparable to that of a titanium alloy or an aluminum alloy, and greatly increase the free-cutting element content. Machinability can be ensured without any restrictions and without restricting the design shape of the part.

In order to carry out the fifth and sixth aspects of the present invention, the first and second aspects are required.
In addition to the elements according to the invention, S: 0.04 to 0.12
%, Pb: 0.01 to 0.30%, Bi: 0.01 to 0.30%, Ca: 0.0005 to
0.01%, REM: One or more selected from 0.001 to 0.10%, steel containing the balance Fe and unavoidable impurities
After hot rolling or hot forging at a heating temperature of 1150 to 1300 ° C, an average cooling rate in a temperature range of 800 to 500 ° C: CV (° C / m
in), 40 / (Mn% + 0.8Cr% + 1.2Mo%) ≦ CV ≦ 500
/(Mn%+0.8Cr%+1.2Mo%) Cooling to a temperature of 200 ° C or less, the hardness is Hv400 or less, the structure is 70% or more of hainite and the grain size of old austenite is 80μm or less. Cutting or plastic working according to
The yield point or 0.2% proof stress is made 900 MPa or more by performing aging treatment at a temperature of 0 to 700 ° C. The steel material and the forged product thus obtained can be strengthened before obtaining a weight-reducing effect comparable to that of a titanium alloy or an aluminum alloy, and greatly increase the content of free-cutting elements. Machinability can be ensured without any restrictions and without restricting the design shape of the part.

In order to carry out the seventh and eighth inventions, first, second
In addition to the elements according to the invention, Ti: 0.01 to 0.1
0%, Nb: contains one or two selected from 0.01 to 0.10%, and S: 0.04 to 0.12%, Pb: 0.01 to 0.30%, Bi: 0.
01-0.30%, Ca: 0.0005-0.01%, REM: One or two or more selected from 0.001-0.10%, steel consisting of Fe and unavoidable impurities is heated at a heating temperature of 1150-1300 ° C. After cold rolling or hot forging, the average cooling rate in the temperature range of 800 to 500 ° C: CV (° C / min) is increased to 40 / (Mn% + 0.8Cr% +
1.2Mo%) ≤ CV ≤ 500 / (Mn% + 0.8Cr% + 1.2Mo%) By cooling to a temperature of 200 ° C or less, the hardness is Hv400 or less, the structure is 70% or more of hainite and the former austenite crystal. Particle size 80
The yield point or 0.2% proof stress is adjusted to 900 MPa or more by applying cutting or plastic working as necessary, and then performing aging treatment at a temperature of 550 to 700 ° C. The steel material and the forged product thus obtained can be strengthened before obtaining a weight-reducing effect comparable to that of a titanium alloy or an aluminum alloy, and greatly increase the content of free-cutting elements. Machinability can be ensured without any restrictions and without restricting the design shape of the part.

[0030]

EXAMPLES Examples of the present invention will be described below by comparison with comparative steels and conventional steels. Tables 1 and 2 show the chemical components of the test materials used in the examples.

[0031]

[Table 1]

[0032]

[Table 2]

A steel of the present invention having a component composition of Table 1 and a comparative steel (including conventional steel) of Table 2 were melted in a 30 kg vacuum melting furnace and forged at 1200 ° C. to φ30 mm. Then, φ30mm material,
After forging into a 15mm thick plate under the conditions of 1200 ° C heating and 1050 ° C forging, air-cooling treatment is performed to room temperature, then aging treatment at 600 ° C for A to W steel, and X, Y, Z steel Was quenched at 880 ° C. and then tempered at 580 ° C., and used for a tensile test, an Ono-type rotary bending fatigue test, a drilling test, and microstructure observation. In this case, the average cooling rate in the temperature range of 800 to 500 ° C during air cooling after forging is 72 ° C.
/ min. Regarding the A to W steels, in addition to the above, a drilling test and a hardness test were performed even after forging and in an air-cooled state without aging treatment.

A tensile test was performed on a JIS14A test piece under the conditions of a tensile speed of 1 mm / sec, and a 0.2% proof stress and a tensile strength were measured. Ono rotating bending fatigue test formulas were tested to produce a smooth test piece parallel portion ø8, it obtains a fatigue strength at 10 ⁷ times,
It was evaluated with in this and the tensile strength and durability ratio took the ratio of (= 10 ⁷ times fatigue strength / tensile strength). Before and after aging treatment, the drill piercing test was performed using a straight shank with a diameter of φ6 mm and a drill material of SKH51.
The drilling speed was 966 rpm, no lubricating oil was used, and the load was 75 kg. The measurement results showed that the drilling distance of the conventional steel, Z steel, was 10
It was set to 0 and each perforation distance was evaluated by an integer ratio.

The hardness test was performed with a Vickers hardness meter at a measurement load of 10 kgf using a sample for a drilling test performed before the aging treatment. For microstructure observation,
Using a sample obtained by cutting and polishing the grip portion of the tensile test piece after the test, the sample was observed with an optical microscope at a magnification of 400, and the hainite ratio and the prior austenite crystal grain size were measured.

Table 3 shows the results of various tests and evaluations for the steel of the present invention, and Table 4 shows the results of comparative steels (including conventional steels).

[0037]

[Table 3]

[0038]

[Table 4]

As shown here, the steels A to N of the present invention
The hardness of all steels before aging treatment is Hv331 or less, and sufficiently satisfies Hv400 or less corresponding to the claims, the hainite ratio is 82% or more, and the prior austenite particle size is 54 μm or less,
70% or more of the hainite ratio falling within the scope of claims, and the former austenite grain size of 80
μm or less, and 0.2% after aging treatment
All have a proof stress of 958MPa or more and fall within the scope of claims.
We are fully satisfied with 900MPa or more. In addition, the durability ratio shows an excellent value of 0.54 or more, and the torill piercing property shows a particularly excellent value before the aging treatment, and both before and after the aging treatment are superior to the conventional steel Z. Was confirmed.

On the other hand, the O steel of the comparative steel is inferior in 0.2% proof stress because the C content is lower than the claimed range, and the P steel is conversely because the C content is higher than the claimed range. In addition, the hardness before aging exceeds Hv400, and the drilling performance of the drill is inferior to that of the conventional steel Z. Q of comparative steel
Regarding the steel, since the Si content is higher than the claimed range, the drill piercing property is inferior to that of the conventional steel Z, and the R steel has a lower Mn content than the claimed range, so that ferrite barite is formed. The hainite rate is less than 70% and the 0.2% proof stress is 900MPa
It is lower than less. Since the S steel has both a higher Mn content and a higher Cr content than the scope of the present invention, it has a martensite-based structure, a low hainite ratio, and a hardness before aging exceeding Hv400, which results in a high drilling performance. Is inferior to the conventional steel Z. Since the T steel has a lower Mo content than that of the present invention, ferrite and barite are formed and the hainite ratio is less than 70%, and the 0.2% proof stress is as low as less than 900 MPa. Since both U content and N content of U steel are lower than the claimed range, age hardening is not sufficient, and 0.2% proof stress is 900 MPa.
It is lower than less.

The conventional steels V and W are conventional quenched and omitted steels of the hainite + martensite type.
Although 0.2% proof stress of a or more is achieved, V steel is inferior in drilling ability, and W steel has the same drilling ability as Z steel by containing free cutting elements. Has a low durability ratio. X,
Y and Z steels are S48C specified by JIS, respectively.
It is a steel corresponding to SCr440 and SCM440, and the 0.2% proof stress does not reach 900 MPa even after quenching and tempering.

Next, there will be described an embodiment in which the influence of the manufacturing conditions, that is, the influence of changes in the forging heating temperature, the cooling conditions after forging, and the aging treatment temperature was investigated. Among the steels of the present invention shown in Table 1, for B, D, G and M steels, round bars of φ30 mm
After heating to each temperature of 0, 1200, 1270, and 1350 ° C, forging into a 15 mm thick plate material, then changing the cooling condition to furnace cooling, air cooling, weak fan cooling, strong fan cooling, and further aging treatment temperature Was performed under five conditions of 500, 570, 600, 650 and 750 ° C., and used for a tensile test, a drilling test (only before aging treatment), a hardness test (only before aging treatment), and microstructure observation.
In this case, the average cooling rate in the temperature range of 800 to 500 ° C. after forging is 5 ° C./min for furnace cooling, 72 ° C./min for air cooling, 103 ° C./min for weak fan cooling, and 131 ° C. for strong fan cooling. ° C / min. The test conditions are the same as those described above.

Table 5 shows the results of various test evaluations for the steel of the present invention, and Table 6 shows the results of comparative examples.

[0044]

[Table 5]

[0045]

[Table 6]

In the range of the present invention of Nos. 1 to 15, the hardness before aging treatment is Hv 400 or less, the hainite ratio is 70% or more, the prior austenite crystal grain size is 80 μm or less, 0.2% It was confirmed that the steel satisfies all the proof stresses of 900 MPa or more, and also has superior toroidal piercing property compared to the refining of the conventional steel SCM440 (the Z steel of the above-described example). No.16 ~ 27
According to the results of the comparative examples, even if the chemical components are within the scope of the present invention, if any one of the forging heating temperature, the cooling condition after forging, and the aging treatment temperature is not satisfied, It became clear that the above characteristics could not be obtained.

[0047]

As is apparent from the above description, the present invention is suitable for hot forging for parts which require high strength and excellent machinability, such as crankshafts and connecting rods of automobile engines. An object of the present invention is to provide a method for producing an age-hardened bainite steel and a forged product thereof, and to reduce the weight of automobile engine parts.

Claims (8)

[Claims] 1. The chemical composition in weight%, C: 0.06-0.20%, S
i: 0.03 to 1.00%, Mn: 1.50 to 3.00%, Cr: 0.50 to 2.00%,
Mo: 0.05-1.00%, Al: 0.002-0.100%, V: 0.51-1.00
%, N: 0.0080 to 0.0200%, the steel consisting of the balance Fe and unavoidable impurities is hot rolled or hot forged at a heating temperature of 1150 to 1300 ° C, and then the average cooling rate in the temperature range of 800 to 500 ° C : CV (° C / min) is 40 / (Mn% + 0.8Cr% + 1.2Mo)
%) ≤ CV ≤ 500 / (Mn% + 0.8Cr% + 1.2Mo%) 20
By cooling to a temperature of 0 ° C. or less, the hardness is Hv 400 or less, the structure is 70% or more of hainite and the prior austenite crystal grain size is 80 μm or less, and then cutting or plastic working is performed as necessary, and then 550 Age hardening type high-strength bainite steel characterized by having a yield point or 0.2% proof stress of 900 MPa or more by aging at a temperature of up to 700 ° C. 2. The chemical composition in weight%, C: 0.06-0.20%, S
i: 0.03 to 1.00%, Mn: 1.50 to 3.00%, Cr: 0.50 to 2.00%,
Mo: 0.05-1.00%, Al: 0.002-0.100%, V: 0.51-1.00
%, N: 0.0080 to 0.0200%, the steel consisting of the balance Fe and unavoidable impurities is hot rolled or hot forged at a heating temperature of 1150 to 1300 ° C, and then the average cooling rate in the temperature range of 800 to 500 ° C : CV (° C / min) is 40 / (Mn% + 0.8Cr% + 1.2Mo)
%) ≤ CV ≤ 500 / (Mn% + 0.8Cr% + 1.2Mo%) 20
By cooling to a temperature of 0 ° C. or less, the hardness is Hv 400 or less, the structure is 70% or more of hainite and the prior austenite crystal grain size is 80 μm or less, and then cutting or plastic working is performed as necessary, and then 550 A method for producing an age hardenable high-strength bainite steel, wherein the yield point or 0.2% proof stress is made 900 MPa or more by performing aging treatment at a temperature of up to 700 ° C. 3. The chemical composition in weight%, Ti: 0.01 to 0.10
%, Nb: 0.01 to 0.10%, one or two kinds selected from the group consisting of 0.01% to 0.10%. 4. The chemical composition in weight%, Ti: 0.01 to 0.10
%, Nb: One or two kinds selected from 0.01 to 0.10% are contained, the method for producing an age-hardened high-strength bainite steel according to claim 2, characterized in that: 5. The chemical composition in weight%, S: 0.04 to 0.12%,
Pb: 0.01 to 0.30%, Bi: 0.01 to 0.30%, Ca: 0.0005 to 0.0
The age-hardened high-strength bainite steel according to claim 1, comprising one or more selected from 1%, REM: 0.001 to 0.10%. 6. The chemical composition in weight%, S: 0.04-0.12%,
Pb: 0.01 to 0.30%, Bi: 0.01 to 0.30%, Ca: 0.0005 to 0.0
The method for producing an age-hardenable high-strength bainite steel according to claim 2, characterized in that it contains one or more kinds selected from 1% and REM: 0.001 to 0.10%. 7. The chemical composition has a Ti content of 0.01 to 0.10% by weight.
%, Nb: contains one or two selected from 0.01 to 0.10%, and S: 0.04 to 0.12%, Pb: 0.01 to 0.30%, Bi: 0.
The age-hardened high-strength bainite steel according to claim 1, comprising one or more selected from 01 to 0.30%, Ca: 0.0005 to 0.01%, REM: 0.001 to 0.10%. 8. The chemical composition has a Ti content of 0.01 to 0.10% by weight.
%, Nb: contains one or two selected from 0.01 to 0.10%, and S: 0.04 to 0.12%, Pb: 0.01 to 0.30%, Bi: 0.
The age-hardenable high-strength bainite steel according to claim 2, comprising one or more selected from 01 to 0.30%, Ca: 0.0005 to 0.01%, REM: 0.001 to 0.10%. Method.