JP2003147478A - Non-heattreated steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide ferrite-pearlite or ferrite-pearlite-bainite non-heattreated steel which has a high endurance ratio. SOLUTION: To provide non-heattreated steel containing, by mass, 0.25 to 0.55% C, 0.15 to 2.0% Si, 0.6 to 2.0% Mn, 0.05 to 0.3% V, 0.01 to 0.05% Al, 0.006 to 0.015% N, <=0.05% P, <=0.10% S, 0 to 0.2% Cu, 0 to 0.2% Ni, 0 to 0.7% Cr, 0 to 0.2% Mo and 0 to 0.4% W so as to satisfy Mo+0.5W: 0 to 0.2%, 0 to 0.06% Ti and 0 to 0.1% Zr so as to satisfy Ti+0.5Zr: 0 to 0.06%, 0 to 0.03% Nb, 0 to 0.30% Pb and <=0.002% O (oxygen), and the balance Fe with impurities, and in which the value of fn1=C+0.1Si+0.2Mn+1.6V-0.2Cr-0.75 is 0.85 to 1.15%, and the total of the cleanliness of B based inclusions and C based inclusions is <=0.03%. The steel has a ferrite-pearlite structure, or a ferrite-pearlite-bainite structure.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is suitable for applications such as mechanical structural members without performing a so-called "tempering treatment" of quenching and tempering after hot working. For non-heat treated steel. More specifically, a yield strength (0.2%) suitable as a material for undercarriage parts of automobiles, industrial machines, civil engineering construction machines, etc., particularly as a material for wheel hubs, a material for engine parts, and particularly as a material for crankshafts and connecting rods. % Proof stress 560M
Pa or more, tensile strength 800 MPa or more, yield ratio (yield strength / tensile strength) 0.7 or more, fatigue strength 440 MP
a, an endurance ratio (fatigue strength / tensile strength) of 0.54 or more, and an impact value at room temperature in an impact test using a 2 mm U notch Charpy test piece described in JIS Z 2202 is 10 J / c.
any one of a mixed structure of ferrite and pearlite (hereinafter, referred to as “ferrite-pearlite structure”) or a mixed structure of ferrite, pearlite, and bainite (hereinafter, referred to as “ferrite-pearlite bainite structure”) having m ² or more , That is, a non-heat treated steel of ferrite pearlite type or ferrite pearlite bainite type. 2. Description of the Related Art Conventionally, a wheel hub as an undercarriage part of a machine structural part, particularly an automobile, an industrial machine, a civil engineering construction machine, and the like, and a crankshaft and a connecting rod as an engine part are conventionally made of carbon steel for machine structure. (S45C, S50C
) Or alloy steel (such as SCM440), and then hot-worked, then machined and tempered to ensure the desired shape and performance. [0003] However, the above-mentioned refining process requires a large amount of heat energy, so that the production cost increases. Therefore, from the viewpoint of energy saving and cost reduction, non-heat treated steels having properties equivalent to those of tempered steels while being hot worked have been developed. [0004] As non-heat treated steels, bainite, martensite, ferrite perlite, and ferrite perlite bainite non heat treated steels are known. Of these, bainite and martensitic non-heat treated steels provide high strength but low machinability. Therefore, when high machinability is required, ferrite
Non-heat treated steels of the pearlite and ferrite perlite bainite types have been used. [0005] However, the non-heat treated steels of the ferrite pearlite type and the ferrite pearlite bainite type generally have a low durability ratio of about 0.45 to 0.50. For this reason, there is an increasing demand in the industry for ferritic pearlite and ferrite pearlite bainite non-heat treated steels having a high durability ratio. SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has as its object to provide materials for undercarriage parts and engine parts of automobiles, industrial machines, civil engineering construction machines, and the like. However, it is suitable as a material for wheel hubs, connecting rods, crankshafts, etc., and is a non-heat treated ferrite-pearlite or ferrite-pearlite bainite type steel with a high durability ratio. Among them, the yield strength (0.2% proof stress) is high. 560 MPa or more, tensile strength of 800 MPa or more, yield ratio (yield strength / tensile strength) of 0.7 or more, fatigue strength of 440 MPa or more, durability ratio (fatigue strength / tensile strength) of 0.54 or more, and JIS Z 2202 2mm described in
An object of the present invention is to provide a ferrite perlite type or ferrite perlite bainite type non-heat treated steel having an impact value at room temperature of an impact test using a U-notch Charpy test piece of 10 J / cm ² or more. [0007] The gist of the present invention resides in the following non-heat treated steel. That is, “in mass%, C: 0.25 to
0.55%, Si: 0.15 to 2.0%, Mn: 0.6
To 2.0%, V: 0.05 to 0.3%, Al: 0.01
-0.05%, N: 0.006-0.015%, P:
0.05% or less, S: 0.10% or less, Cu: 0-0.
2%, Ni: 0 to 0.2%, Cr: 0 to 0.7%, M
o: 0 to 0.2% and W: 0 to 0.4%, and Mo
(%) + 0.5 W (%): 0-0.2%, Ti: 0
0.06% and Zr: 0 to 0.1% and Ti
(%) + 0.5Zr (%): 0 to 0.06%, Nb: 0
0.03%, Pb: 0 to 0.30% and O (oxygen):
0.002% or less, the balance being Fe and impurities, and fn1 represented by the following formula is 0.85 to
1.15%, and the sum of cleanliness of B-based inclusions and C-based inclusions is 0.03% or less, and the structure is a ferrite-pearlite structure or a ferrite-pearlite bainite structure. steel. Here, fn1 = C + 0.1Si
+ 0.2Mn + 1.6V + 0.2Cr-0.7S ...
In the formula, the element symbol in the formula represents the content of the element in mass%. ". Here, B-based inclusions and C-based inclusions are JIS
Refers to inclusions classified by G 0555.
Refers to the value measured by the method specified in JIS G 0555. As described above, the term "ferrite-pearlite structure" refers to a mixed structure of ferrite and pearlite, and the term "ferrite-pearlite bainite structure" refers to a mixed structure of ferrite, pearlite, and bainite. . The present inventors have conducted various studies on the chemical composition and inclusions in order to increase the durability ratio of the ferritic pearlite non-heat treated steel and the ferrite pearlite bainite non heat treated steel. The following findings (a) and (b) were obtained. (A) Fatigue strength has a correlation with tensile strength. For steel having a specific chemical composition, its structure is ferrite-pearlite structure or ferrite-pearlite structure.
In the case of the pearlite bainite structure, by controlling the inclusions, the fatigue strength can be reduced by 10 even at the same tensile strength.
% Can be increased. (B) C of 0.25% or more by mass%;
When the structure of the steel containing 0.15% or more of Si and 0.6% or more of Mn is a ferrite-pearlite structure or a ferrite-pearlite bainite structure, the tensile strength of the steel is represented by the above formula. fn1. If the value of fn1 is 0.85% or more, 800MP
a tensile strength of not less than a can be obtained and a fatigue strength of not less than 440 MPa can be stably ensured, and a yield strength of not less than 560 MPa, a yield ratio of not less than 0.7 and a durability ratio of not less than 0.54 can be obtained. However, if the value of fn1 exceeds 1.15%, the toughness is greatly reduced, so that the impact value at room temperature in the impact test using the 2 mm U notch Charpy test piece falls below 10 J / cm ² . The present invention has been completed based on the above findings. Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the component content is “% by mass”.
Means (A) Chemical composition C of steel: C is an element effective for ensuring strength. However, if the content is less than 0.25%, the desired strength cannot be obtained in the case of a ferrite-pearlite type or a ferrite-pearlite bainite type non-heat treated steel. On the other hand, 0.
If the content exceeds 55%, the volume fraction of ferrite in the ferrite / pearlite structure or ferrite / pearlite / bainite structure is reduced, and the effect of ferrite strengthening is weakened, thereby reducing the fatigue strength and cutting with hard pearlite or bainite. The toughness also deteriorates, and the toughness also decreases. Therefore, the content of C is 0.25 to
0.55%. Si: Si promotes deoxidation,
It has the effect of strengthening ferrite by forming a solid solution in ferrite and increasing static strength and fatigue strength. Further, Si also has an effect of increasing the yield ratio. In order to sufficiently exhibit the above effects, the Si content needs to be 0.15% or more. On the other hand, even if the content of Si exceeds 2.0%, the above effect is saturated and the cost is increased. Therefore, the content of Si is set to 0.15 to 2.0%. Mn: Mn has a deoxidizing effect and an effect of increasing strength. In order to secure the effect, a content of 0.6% or more is required. However, when Mn is contained in excess of 2.0%, the effect is not only saturated and the cost is increased, but also the hardenability becomes too high to promote the formation of bainite and island-like martensite, and the yield ratio and The machinability decreases. Therefore, the content of Mn is set to 0.1.
6% to 2.0%. V: V has the effect of increasing the strength by forming nitrides and carbonitrides. To ensure that effect,
A content of 0.05% or more is required. But 0.3
%, The effect is saturated and the cost is increased. Therefore, the content of V is 0.05
-0.3%. Al: Al is an element having a strong deoxidizing effect. Further, Al has an effect of forming oxides or nitrides to make crystal grains finer and to increase strength and toughness. However, if the content is less than 0.01%, the effect of addition is poor. On the other hand, if the content exceeds 0.05%, the oxides and nitrides become coarse and the fatigue strength is reduced. Therefore, Al
Was 0.01 to 0.05%. Note that the Al content in the present invention is a so-called “sol. Al (acid-soluble A
l) Amount ”. N: N has the effect of forming nitrides to refine crystal grains and increase strength and toughness. However, if the content is less than 0.006%, it is difficult to obtain the above effects.
On the other hand, when the content exceeds 0.015%, the nitride aggregates to lower the fatigue strength. Therefore, the content of N is set to 0.006 to 0.015%. P: P is contained as an impurity in steel, and need not be added as an essential component. If added, it has the effect of increasing the yield strength. In order to surely obtain this effect, the content of P is preferably set to 0.005% or more. However, if the content exceeds 0.05%, the toughness is significantly reduced. Therefore, the content of P is set to 0.1.
05% or less. S: S is contained as an impurity in steel, and need not be added as an essential component. If added, it has the effect of improving machinability. To ensure this effect, the content of S is preferably set to 0.015% or more. However, if the content exceeds 0.10%, the toughness decreases. Therefore, the content of S is 0.10%
The following was set. Cu: Cu may not be added. If added, it has a precipitation strengthening effect. To ensure this effect, it is preferable that the content of Cu is 0.01% or more. However, even when Cu is contained in an amount exceeding 0.2%, the above-described effect is saturated and not only the economic efficiency is impaired, but also the toughness and the hot workability are reduced. Therefore, the content of Cu is set to 0 to 0.2%. Ni: Ni may not be added. Addition has the effect of increasing toughness. To ensure this effect, the content of Ni is preferably set to 0.05% or more. However, even if Ni is contained in an amount exceeding 0.2%, the above-described effect is saturated, impairing economic efficiency, and reducing machinability. Therefore, the content of Ni is set to 0 to 0.2%. Cr: Cr may not be added. If added, it has the effect of forming carbides and carbonitrides to increase the strength. To ensure this effect, the content of Cr is preferably set to 0.1% or more. However, even if the content exceeds 0.7%, the above effect is saturated and the cost is increased. Therefore, the content of Cr is set to 0 to 0.7%. Mo, W: Mo and W need not be added. When added, all have the effect of improving the toughness similarly to Ni, and also have the effect of increasing the hardenability of steel and the effect of strengthening the crystal grain boundaries. The above effect relates to the contents of Mo and W, and the value of Mo (%) + 0.5W (%) is 0.05.
% Is definitely obtained. However, Mo (%) +
Even if Mo and W are contained in an amount of more than 0.2% at a value of 0.5 W (%), the effect is saturated and the cost is increased. It is not necessary to add Mo and W in combination in order to obtain the above-mentioned effects, and only one of them may be added alone. When Mo is added alone, it is preferable that the content of Mo be 0.05% or more in order to surely obtain the above-mentioned effects. However, even if Mo is contained in excess of 0.2%, the above effects are saturated and the cost is increased. When W is added alone, it is preferable that the content of W be 0.1% or more in order to surely obtain the above-mentioned effects. However, even if W is contained in an amount exceeding 0.4%, the above effect is saturated, and the cost is increased. Therefore, for Mo and W, the content of Mo is 0 to
0.2%, W content is 0-0.4% and Mo
The value of (%) + 0.5 W (%) was set to 0 to 0.2%. Ti, Zr: Ti and Zr need not be added. When added, any of them has the effect of generating carbides and carbonitrides, making crystal grains finer, and increasing strength and toughness. The above effect relates to the contents of Ti and Zr,
(%) + 0.5Zr (%) is reliably obtained when the value is 0.01% or more. However, Ti (%) + 0.5Zr
If the content of Ti and Zr exceeds 0.06% in terms of (%), not only does the cost increase, but also the crystal grains are rather coarsened and the toughness is reduced. In order to obtain the above effect, Ti and Zr are used.
Need not be added in combination, and only one of them may be added alone. When Ti is added alone, it is preferable that the content of Ti is 0.01% or more in order to surely obtain the above-mentioned effects. However, when Ti is contained in excess of 0.06%, carbonitrides are coarsened and coarsened, the effect of refining crystal grains is lost and economic efficiency is impaired, and moreover, the crystal grains are coarsened and the toughness is reduced. In addition, when Zr is added alone, in order to surely obtain the above-mentioned effect, Zr is added in an amount of 0.
The content is preferably at least 02%. But Z
If the content of r exceeds 0.1%, the crystal grains become coarse and the toughness is reduced. Therefore, for Ti and Zr, the content of Ti is 0 to 0.06% and the content of Zr is 0%.
0.1% and the value of Ti (%) + 0.5Zr (%) was set to 0 to 0.06%. Nb: Nb may not be added. If added, it produces nitrides and carbonitrides and has the effect of increasing the strength. In order to ensure this effect, it is preferable that the content of Nb is 0.01% or more. However, 0.03%
If the content exceeds the above range, the above effect is saturated and the cost is increased. Therefore, the content of Nb is set to 0 to 0.
03%. Pb: Pb may not be added. When added, it has the effect of enhancing machinability. To ensure this effect, it is preferable that the content of Pb is 0.05% or more. However, when the content exceeds 0.30%, the fatigue strength is remarkably reduced, and the fatigue resistance is deteriorated. Therefore, the content of Pb is set to 0 to 0.30%. O (oxygen): O (oxygen) forms oxide-based inclusions and causes deterioration of fatigue characteristics. In particular, when the content exceeds 0.002%, the fatigue characteristics are significantly reduced.
Therefore, the content of O is set to 0.002% or less. Fn1: 0.25% or more of C by mass%,
In the case where the steel containing 0.15% or more of Si and 0.6% or more of Mn has a ferrite-pearlite structure or a ferrite-pearlite bainite structure, the tensile strength in a state as it is hot worked Can be arranged by fn1 represented by the above equation. When the value of fn1 is 0.85% or more, a tensile strength of 800 MPa or more can be obtained, a fatigue strength of 440 MPa or more can be stably ensured, and a yield strength of 560 MPa or more and 0.
A yield ratio of 7 or more and a durability ratio of 0.54 or more are obtained.
On the other hand, if the value of fn1 exceeds 1.15%, the toughness is greatly reduced. Therefore, the impact value at room temperature in the impact test using the 2 mm U notch Charpy test piece is 10 J / cm ^2.
Below. Therefore, fn1 is set to 0.85
1.15%. (B) Inclusions In the steel having the chemical composition described in the above (A), when the structure is a ferrite-pearlite structure or a ferrite-pearlite bainite structure, the same tensile strength can be obtained by controlling the inclusions. Even in the case of strength, the fatigue strength can be increased by about 10%. That is, when the sum of the cleanliness of the B-based inclusions and the C-based inclusions is 0.03% or less, a large fatigue strength can be obtained and the durability ratio can be increased. Therefore, B
The sum of the cleanliness of the system inclusions and the C system inclusions was 0.03% or less. The steel having the chemical composition described in the above section (A) is, for example, melted in a vacuum furnace or so-called “VA”.
The sum of cleanliness of B-based inclusions and C-based inclusions is reduced to 0.03% or less by performing secondary refining using “D” or “RH” alone or in combination and performing melting. be able to. The steel after being smelted is subjected to hot rolling or forging, for example, by a usual method, and is further machined as necessary to finish it into a predetermined shape part such as a connecting rod, a crankshaft or a wheel hub. Can be In addition, cooling after performing hot rolling or forging by a normal method is performed at a cooling rate such as a ferrite-pearlite structure, or a ferrite-pearlite bainite structure, such as air cooling or cooling. Just do it. EXAMPLES 150 kg of steel having the chemical compositions shown in Tables 1 to 4 was melted using a test furnace. Steels 1 to 19 in Tables 1 and 2
Are steels of the examples of the present invention whose chemical composition is in the range specified by the present invention, and steels 20 to 35 in Tables 3 and 4 are steels of comparative examples in which any of the components is out of the range specified by the present invention. is there. In addition, steel 2 to 6, steel 8 to 15 and steel 17 to 19 among the steels of the present invention, and steel 20 to 22 and steel 2 of the steel of the comparative example.
7, steels 30 to 33 and steel 35 are melted in a vacuum furnace. On the other hand, among the steels of the present invention, steel 1, steel 7 and steel 16
And steels 23 to 26, steel 28, steel 29 and steel 34 among the steels of the comparative examples were obtained by melting in an atmospheric furnace. [Table 1] [Table 2] [Table 3] [Table 4] Next, these steels were made into billets by a usual method, and then heated to 1150 to 1200 ° C.
It was hot forged into a round bar having a diameter of 40 mm at a finishing temperature of 1000 ° C., and then air-cooled to room temperature. An Ono-type rotary bending fatigue test piece having a parallel part diameter of 6 mm was cut out from the center of the thus obtained round bar and subjected to a fatigue test at room temperature (room temperature) and in the atmosphere at 3000 rpm. A JIS No. 4 tensile test piece and a 2 mm U notch Charpy test piece described in JIS Z 2202 were cut out from the center of the round bar and subjected to a tensile test and an impact test at room temperature.
Further, a test piece having a diameter of 40 mm and a thickness of 20 mm was cut out, and the structure of the central portion was observed with an optical microscope. Furthermore, a test piece having a diameter of 40 mm and a thickness of 20 mm was subjected to JIS G
A test piece was cut out according to the method specified in 0555, and B-based inclusions and C
The sum of the cleanliness of the system inclusion was measured. The test results are shown in Tables 5 and 6. [Table 5] [Table 6] From Tables 5 and 6, among the steels 1 to 19 of the present invention, whose chemical compositions are within the range specified in the present invention, steels 2 to 6, 8 to 15 and 17 to 19 Each of them has a fatigue strength (fatigue limit) of 440 MPa or more, and has a desired yield strength (0.2% proof stress) of 560 MPa or more, a tensile strength of 800 MPa or more, and 0.
Yield ratio of 7 or more, durability ratio of 0.54 or more, and 10 J / c
A ² mm U notch Charpy impact value of m ² or more is obtained. On the other hand, the steel of the comparative example in which one of the components is out of the range of the content specified in the present invention, and the B-based inclusion and the C-containing material even when the chemical composition is in the range specified in the present invention.
Steel 1 and Steel 7 whose sum of cleanliness of system inclusions exceeds 0.03%
The steel 16 does not have at least one of fatigue strength, yield strength, tensile strength, yield ratio, durability ratio and 2 mm U notch Charpy impact value. When the ferrite-pearlite or ferrite-pearlite bainite type non-heat treated steel of the present invention is used, the fatigue strength of 440 MPa or more, the yield strength of 560 MPa or more, the tensile strength of 800 MPa or more, A yield ratio of 0.7 or more, a durability ratio of 0.54 or more, and 10 J / cm ²
Since the above 2mm U notch Charpy impact value can be easily obtained, materials for undercarriage parts and engine parts of automobiles, industrial machines, civil engineering machines, etc., especially wheel hubs,
It can be used as a material for connecting rods and crankshafts.

Claims (1)

Claims: 1. mass%, C: 0.25 to 0.55%, S
i: 0.15 to 2.0%, Mn: 0.6 to 2.0%,
V: 0.05-0.3%, Al: 0.01-0.05
%, N: 0.006 to 0.015%, P: 0.05% or less, S: 0.10% or less, Cu: 0 to 0.2%, Ni:
0 to 0.2%, Cr: 0 to 0.7%, Mo: 0 to 0.2
% And W: 0 to 0.4%, and Mo (%) + 0.5 W
(%): 0 to 0.2%, Ti: 0 to 0.06%, and Z
r: 0 to 0.1% and Ti (%) + 0.5Zr
(%): 0 to 0.06%, Nb: 0 to 0.03%, P
b: 0 to 0.30% and O (oxygen): 0.002% or less, the balance being Fe and impurities, and the value of fn1 represented by the following formula being 0.85 to 1.15% so,
Further, the sum of the cleanliness of the B-based inclusions and the C-based inclusions is 0.03.
% Or less, and the structure thereof is a ferrite-pearlite structure or a ferrite-pearlite bainite structure. Here, fn1 = C + 0.1Si + 0.2Mn + 1.6V + 0.2Cr-0.7S ... where the symbol of the element indicates the content of the element in mass%.