KR20090080888A - Forging steel, and forged products obtainable therefrom - Google Patents

Abstract

A forging steel, and a forged product manufactured by the same are provided to produce a large forging product like a crank shaft used in a ship and control the form of inclusion by controlling amount of solid solution Mg and solid solution Ca. A forged product manufactured by a forging steel satisfies one expression among: 1 expression consisting of solid solution Ca 2~500 mass ppb, and solid solution Mg 0.04~5 mass ppm; 2 expression consisting of solid solution Ca 2~100 mass ppb, and solid solution Mg 5~10 mass ppm; 3 expression consisting of solid solution Ca 2 mass ppb or less, and solid solution Mg 0.04~5 mass ppm; and 4 expression consisting of the solid solution Ca 2~500 mass ppb, and solid solution Mg 0.04 mass ppm or less. The forged product has improved fatigue property by using the forging steel.

Description

Forging steels and forgings obtained by using them {FORGING STEEL, AND FORGED PRODUCTS OBTAINABLE THEREFROM}

The present invention relates to a forging steel and a forging obtained by using the same. Forging products forged using the forging steel of the present invention is widely used in industrial fields such as machinery, ships, electricity, etc., but is applied to the crankshaft used as a transmission member of the drive source for ships as a typical use example below The explanation proceeds mainly.

Large crankshafts, for example, manufactured using forged steel, which are transmission members for ship drive sources, require excellent fatigue properties that are less likely to cause fatigue failure even in harsh use environments.

As a method of improving the fatigue characteristics of the crankshaft, Non-Patent Document 1 describes the improvement of the fatigue characteristics by utilizing the technique in the working surface, and specifically, manufactured by the free forging method by employing the RR (Roedere Ruget) method. The thing which markedly improved the fatigue characteristic compared with the crankshaft, the thing which improved the fatigue strength by performing cold roll processing, etc. are described.

In addition, Non-Patent Document 2 examines the improvement of the fatigue characteristics of low-alloy steels employed in ship crankshafts. (1) Inclusions tend to be a starting point of fatigue failure, and the tendency becomes remarkable with increasing the strength of steel. (2) The larger the inclusion size, the lower the fatigue strength, and (3) the steel containing the elongated inclusions tends to exhibit anisotropy of fatigue strength. And in order to improve the fatigue characteristic of a forging material, it is concluded that making an inclusion shape spherical and reducing a dimension is effective.

However, these documents do not describe specific means for spheroidizing the inclusion shape and reducing the size, and do not reveal the kind or size of the inclusion to be controlled. Therefore, in order to implement | achieve the shape control of the inclusion effective for the improvement of a fatigue characteristic, it is thought that repeated examination is needed.

By the way, various methods have been proposed as a form control method of steel inclusions, and for example, Patent Document 1 reduces both sulfides and oxides as a means for obtaining structural low-alloy steels excellent in lamellae resistance and hydrogen-resistant organic cracking resistance. A method for controlling the shape of inclusions is also proposed. Specifically, in order to suppress the production of Mn sulfide that inhibits lamellar tear resistance and hydrogen organic crack resistance, it is proposed to reduce the amount of S and the amount of O and to add Ca or Mg.

In addition, Patent Literature 2 discloses inclusions such as suppressing the production of Al ₂ O ₃ based inclusions connected in the form of MnS or clusters which are easily elongated by hot rolling by addition of Mg and Ca, and changing the shape to achieve miniaturization. It is described to perform shape control.

Patent Document 3 and Patent Document 4 describe the increase in the surface fatigue strength and the gear bending fatigue strength as the gear material by achieving ultra miniaturization of the oxide inclusions, and specifically, it is difficult to aggregate and aggregate as the oxide inclusions. it has been proposed to produce a MgO or MgO · Al ₂ O _3. In addition, when a part of MnS which is a sulfide is made into (Mn * Mg) S, it is made clear that the elongation of an interference | inclusion is suppressed and the anisotropy of mechanical strength is reduced.

Patent Document 5 discloses that MnS, CaS, MgS, (Ca, Mn) S, (Ca, Mg, Mn) S are present as sulfides in order to obtain mechanical structural steel having excellent machinability, and in particular, REM, Ca And controlling the form of the sulfide by containing Mg, it is made clear that the anisotropy of the mechanical properties can be suppressed and the machinability can be improved more than that of the S-containing free cutting steel.

However, the shape control technique of these inclusions is not intended for forgings used under harsh environments such as transmission members of ship driving sources. Therefore, in order to raise the fatigue characteristic of a forged article, it is calculated | required to examine and establish the original inclusion control method for the forging steel used for manufacture of a forged article.

Patent document 6 is mentioned as the object for forging steel used for forgings, such as a transmission member of the said drive source for ships. This patent document 6 stipulates the content of S, Ca, Mg, Al, and O, and also states that fatigue properties can be improved by making the content of Ca and Mg satisfy Equation (1). However, in order to grasp the shape of the inclusions present in the large ingot in more detail and to reliably increase the fatigue characteristics, it is considered that repeated examination is necessary.

<Non-Patent Document 1> "Improvement of Crankshaft Progress", Japanese Society of Marine Engineers, Oct. 1973, Vol. 8, No. 10, p.54-59

<Non-Patent Document 2> "A Study on the Fatigue Strength Characteristics of High Strength Crankshaft Material", Journal of the JIME, 2001, vol. 36, No. 6, p.385-390

<Patent Document 1> Japanese Patent Publication No. 58-35255

Patent Document 2: Japanese Patent Publication No. 57-59295

Patent Document 3: Japanese Patent Laid-Open No. 7-188853

Patent Document 4: Japanese Unexamined Patent Application Publication No. 7-238342

Patent Document 5: Japanese Unexamined Patent Application Publication No. 2000-87179

Patent Document 6: Japanese Unexamined Patent Application Publication No. 2004-225128

The present invention has been made in view of the above circumstances, and an object thereof is to provide forged steel for inclusion of a forged product exhibiting excellent fatigue characteristics, and a forged product capable of exhibiting good fatigue characteristics obtained by using such forged steel (especially , Crankshaft).

Forging steel according to the present invention,

C: 0.2 to 0.6% (mean of mass%. The same below),

Si: 0.05 to 0.5%,

Mn: 0.2 to 1.5%,

Ni: 0.1 to 3.5%,

Cr: 0.9 to 4%,

Mo: 0.1 to 0.7%,

Al: 0.005 to 0.1%,

S: 0.008% or less (does not contain 0%),

O: 0.0025% or less (does not contain 0%),

Total Ca: 0.0030% or less (does not contain 0%),

Total Mg: 0.0015% or less (does not include 0%)

At the same time, the solid solution Ca and the solid solution Mg satisfy any one of the following (I) to (IV).

(I) solid solution Ca: 2 to 500 ppb (mean of mass ppb. The same applies hereinafter)

Solid solution Mg: 0.04 to 5 ppm (mean of ppm by mass.

(II) Solid solution Ca: 2 to 100 ppb, and further solid solution Mg: 5 to 10 ppm

(III) Ca dissolved: 2 ppb or less (not including 0 ppb), and

Solid Mg: 0.04-5ppm

(IV) solid solution Ca: 2 to 500 ppb, and also

Solid solution Mg: 0.04ppm or less (does not include 0ppm)

Forging steel of the present invention, as another element,

(a) 0.005 to 0.2% of at least one selected from the group consisting of V, Nb, Ta, and Hf,

(b) Ti: 0.05% or less (does not contain 0%),

(c) Cu: 1.0% or less (it does not contain 0%) may be included.

The forging steel of the present invention may have a circle equivalent diameter of the largest inclusion in the steel of less than 100 μm.

The invention also includes forgings (particularly crankshafts) manufactured using the forging steel.

The present invention is configured as described above, and in particular, it is possible to control the form of the inclusions formed by adjusting the amount of solid solution Ca and the amount of solid solution Mg in the steel, and it is possible to provide a refined forging steel of the inclusions. The forged product obtained by using such forged steel can be expected to have excellent fatigue characteristics, and is particularly useful as a large forged product such as a crankshaft used in ships.

The present inventors examined from various angles with the final goal of improving the fatigue characteristics of forgings used under the harsh environment as described above. In particular, since the target fatigue strength is hard to be obtained in a large ingot (for example, 20 tons or more) having a low solidification temperature, studies from a different viewpoint have been conducted.

As a result, in particular, the amount of solid solution Ca and the amount of dissolved Mg in the steel is in the range of any one of the above (I) to (IV), and the total amount of Ca, the total Mg, and the amount of S in the steel are controlled. It was found that the size of the maximum inclusions was significantly smaller, and the fatigue characteristics could be sufficiently increased. Hereinafter, the present invention will be described in detail.

First, in this invention, the solid solution Ca amount and solid solution Mg amount in steel shall be in the range in any one of said (I)-(IV). Each range is demonstrated below.

(I) solid solution Ca: 2 to 500ppb, and also solid solution Mg: 0.04 to 5ppm

The oxide containing all the elements in (Ca, Al, Mg) O {() of low melting | fusing point as an oxide by making content of solid solution Ca and solid solution Mg in steel into the said range is mentioned. Same as below} is generated. Since the oxide is easily deformed at the time of forging, the size of inclusions in the forging can be made fine. Moreover, the sulfide which contains all the elements in (Ca, Mg, Mn) S {() of low melting | fusing point as a sulfide. The same applies to the following}, or (Ca, Mg) S is produced. Since the sulfide is more likely to be finely dispersed than MnS, the inclusion size in the forged product can be made fine.

(II) Solid solution Ca: 2 to 100ppb, and further solid solution Mg: 5 to 10ppm

By bringing content of solid solution Ca and solid solution Mg in steel into the said range, an oxide turns into high melting point MgO from low melting point (Ca, Al, Mg) O. In addition, the sulfide becomes (Ca, Mg) S having a low melting point from MgS having a high melting point. Since (Ca, Al, Mg) O is easily deformed at the time of forging, and (Ca, Mg) S is more likely to be finely dispersed than MgS, the inclusion size in the forging can be made fine.

(III) Solid solution Ca: 2 ppb or less (not including 0 ppb), and solid solution Mg: 0.04 to 5 ppm

By setting the content of solid solution Ca and solid solution Mg in steel within the above range, spinel (Al, Mg) O, which is harder to aggregate than Al ₂ O ₃ as an oxide, is formed, and is more susceptible to fine dispersion than MnS as a sulfide (Mg, Mn) Since S is produced, generation of coarse inclusions is suppressed as a result, and fatigue characteristics can be improved.

(IV) Solid solution Ca: 2 to 500ppb, and solid solution Mg: 0.04ppm or less (does not include 0ppm)

By setting the content of solid solution Ca and solid solution Mg in the steel within the above range, it is more difficult to aggregate than Al ₂ O ₃ as an oxide, and (Al, Ca) O of low melting point is formed, and is more susceptible to fine dispersion than MnS as a sulfide ( Since Ca and Mn) S are produced, generation of coarse inclusions is suppressed as a result and fatigue characteristics can be improved.

When content of solid solution Ca and solid solution Mg in steel exists in the range other than said (I)-(IV), since coarse inclusions become easy to produce | generate, it is unpreferable. For example, when the amount of solid solution Mg is more than 10 ppm, a high melting point of MgS or Mg0 is generated during solidification, regardless of the amount of solid solution Ca, and becomes coarse inclusions, which is not preferable. In addition, even when the amount of solid solution Mg is 10 ppm or less, when the amount of solid solution Ca is less than 2 ppb, MgS and MgO are produced, and these are aggregated and coarse. In addition, since MgS and Mg0 have high melting points as described above, they are hardly deformed during forging, and remain as coarse inclusions in the forging. On the other hand, when the amount of solid solution Mg is less than 0.04 ppm and the amount of solid solution Ca is less than ₂ ppb, coarse Al ₂ O ₃ is produced as an oxide and coarse MnS is produced as a sulfide.

In addition, content of the solid solution Ca and solid solution Mg in the said steel is measured by SIMS (Secondary Ionization Mass Spectrometer) as shown in the Example mentioned later.

Next, the reason for defining the total Ca amount, Total Mg amount and S amount in the present invention will be described.

[Total Ca: 0.0030% or less (not including 0%)]

When the total amount of Ca in steel exceeds 0.0030%, coarse Ca containing oxides (CaO etc.), Ca containing sulfides (CaS), and these composite inclusions will generate easily. Therefore, in the present invention, the total Ca amount is suppressed to 0.0030% or less. Preferably it is 0.0020% or less, More preferably, it is 0.0015% or less.

[Total Mg: 0.0015% or less (does not include 0%)]

When the total Mg amount in steel exceeds 0.0015%, coarse Mg containing oxides (Mg0 etc.), Mg containing sulfides (MgS), and these composite inclusions will generate easily. Therefore, in the present invention, the total Mg amount is suppressed to 0.0015% or less. Preferably it is 0.0010% or less, More preferably, it is 0.0008% or less.

1 is a graph showing the ranges of the Total Ca amount and Total Mg amount defined in the present invention, which are summarized using data of Examples described later.

[S = 0.008% or less (does not include 0%)]

Since S is easy to form coarse sulfides (MnS, CaS, MgS) in the steel, S is an element that causes the fatigue strength of the forging steel ingot. Therefore, the amount of S in steel is 0.008% or less, Preferably it is 0.005% or less, More preferably, it is 0.003% or less, More preferably, you may be 0.001% or less.

The present invention is characterized in that the above-described components are adjusted in order to refine the inclusions. However, for example, the strength and toughness required for forged products such as crankshafts, and the improvement of the fatigue characteristics aimed at the present invention are improved. In order to make sure, it is good to make steel satisfy | fill the following component composition.

[C: 0.2 to 0.6%]

C is an element contributing to strength improvement, and in order to secure sufficient strength, it is preferable to contain C at least 0.2%, preferably at least 0.25%, more preferably at least 0.3%. However, if the amount of C is too large, the toughness of the crankshaft deteriorates, so it is suppressed to 0.6% or less, preferably 0.55% or less, and more preferably 0.5% or less.

[Si: 0.05 to 0.5%]

Si is an element which acts on strength improvement and deoxidation. In order to fully exhibit both effects, the amount of Si is preferably 0.05% or more, preferably 0.1% or more, and more preferably 0.15% or more. However, when too much, the reverse V segregation becomes remarkable, so it is 0.5% or less, preferably 0.45% or less, and more preferably 0.4% or less.

[Mn: 0.2 to 1.5%]

Mn is also an element that improves the hardenability and contributes to the strength improvement. In order to secure sufficient strength and hardenability, it is preferable to contain Mn of 0.2% or more, preferably 0.5% or more, and more preferably 0.8% or more. However, when too large, it may promote reverse V segregation, so it is 1.5% or less, Preferably it is 1.2% or less.

[Ni: 0.1 to 3.5%]

Ni is an element useful as a toughness improving element and contains 0.1% or more. Preferably it is 0.2% or more. However, if the amount of Ni is too large, the cost rises, so it is 3.5% or less, preferably 3.0% or less.

[CT: 0.9 to 4%]

Cr is an effective element that increases the hardenability and improves toughness, and their effect is effectively exhibited by containing 0.9% or more, preferably 1.1% or more, and more preferably 1.3% or more. However, when too much, it may promote reverse V segregation, so 4% or less, preferably 3% or less, and more preferably 2% or less.

[Mo: 0.1 to 0.7%]

Mo is an element that effectively acts to improve all the hardenability, strength, and toughness. In order to effectively exhibit these effects, Mo is contained in an amount of 0.1% or more, preferably 0.20% or more, and more preferably 0.25% or more. However, Mo is an element having a small equilibrium distribution coefficient and a tendency to cause micro segregation (normal segregation), so it is 0.7% or less, preferably 0.6% or less, and more preferably 0.5% or less.

[Al: 0.005 to 0.1%]

Al is effective as a deoxidation element in a steelmaking process, and is effective also in crack resistance of steel. Therefore, Al amount (total Al amount. The same below) is contained 0.005% or more, Preferably it is 0.010% or more. On the other hand, Al fixes N in the form of AlN, inhibits the reinforcing action of the steel by the combination of N and V, and combines with various elements to form nonmetallic inclusions or intermetallic compounds to lower the toughness of the steel. Since Al may be 0.1% or less, Preferably you may be 0.08% or less.

[O: 0.0025% or less (does not include 0%)]

O (oxygen) is an element for forming an oxide such as _{SiO 2, Al 2 O 3,} MgO, CaO , and the inclusions are reduced fatigue strength of the steel ingot. Therefore, it is preferable to reduce O as much as possible, and O amount (Total O amount) is 0.0025% or less, Preferably you may be 0.0015% or less.

The composition of the forging steel used in the present invention is as described above, the remainder being iron and unavoidable impurities. As an unavoidable impurity, P, N, etc. are mentioned, for example, It is preferable that P is 0.03% or less, and it is more preferable that it is 0.02% or less. Moreover, it is preferable that N is 0.01% or less, More preferably, it is 0.008% or less.

It is also possible to use forging steel which actively contains another element as follows in the range which does not adversely affect the action of the present invention.

[1 or more types selected from the group consisting of V, Nb, Ta, and Hf: 0.005 to 0.2% in total]

V, Nb, Ta, and Hf have elements of strengthening precipitation and miniaturizing the structure, and are useful elements for high strength of steel materials. In order to exhibit such an effect effectively, it is preferable to contain 1 or more types chosen from the group which consists of V, Nb, Ta, and Hf 0.005% or more in total, More preferably, it is 0.01% or more in total. However, even if it contains too much, since the said effect is saturated and it is economically useless, it is preferable to set it as 0.2% or less in total, More preferably, it is 0.15% or less in total.

[Ti: 0.05% or less (does not include 0%)]

Ti is inevitably included as an impurity or an element that is expected to contain an effect of improving hydrogen cracking resistance of steel. Ti-based inclusions are composed of fine inclusions such as TiN, TiC, Ti ₄ C ₂ S ₂ and dispersed in the steel, and occlude and capture excess hydrogen in the steel exceeding the solid solution limit to improve hydrogen cracking resistance of the steel. have. In order to express such an effect, it is preferable to make Ti amount in steel into 0.0002% or more, More preferably, it is 0.0004% or more, More preferably, it is 0.0006% or more. However, even when included as an unavoidable impurity or when Ti is contained to express the above effect, when the Ti amount exceeds 0.05%, coarse nitride may be formed in the steel to degrade fatigue characteristics. Therefore, it is preferable to make Ti amount in steel into 0.05% or less. More preferably, it is 0.03% or less, More preferably, it is 0.01% or less.

[Cu: 1.0% or less (not including 0%)]

Cu is an element which is inevitably contained as an impurity or may be added as a toughness improving element. (In addition, when Cu is contained as a toughness improving element, it is preferable to make Cu amount 0.05% or more, More preferably, 0.1% or more). However, when Cu amount exceeds 1.0%, it may cause a cost increase and there exists a possibility that hot crack may generate | occur | produce. Therefore, Cu amount is 1.0% or less, Preferably it is 0.5% or less.

In addition, examples of other elements that can be positively added include B having a hardenability improving effect, and solid solution strengthening elements or precipitation strengthening elements such as W, Ce, La, Zr, Te, and the like. Although the above can be added complex, it is preferable to suppress about 0.1% or less in total amount.

Although the following method is recommended as a means for keeping S amount, total Ca amount, total Mg amount, solid solution Ca amount and solid solution Mg amount in steel within the scope of the above provisions, the present invention also defines a method for producing forged steel. It is not limited to, and is not limited to the process mentioned later.

S content can be adjusted by controlling the top slag composition at the time of secondary refining. Specifically, the S content in the steel is lowered by increasing the ratio of the CaO concentration in the top slag and the SiO ₂ concentration (CaO / SiO ₂ : below, sometimes referred to as “C / S”) to 3.0 or more. You can. In addition, by increasing the ratio (CaO / Al ₂ O ₃ ) between the CaO concentration and the Al ₂ O ₃ concentration as a supplementary means, the S content in the steel can be lowered.

It is recommended that the MgO concentration in the top slag be 5% or more, and the CaO concentration be 30% or more. On the other hand, if the Mg0 concentration and the Ca0 concentration in the top slag are too high, the slag is solidified and the refining operation is difficult. Therefore, it is recommended that the MgO concentration in the top slag is 25% or less, and the CaO concentration in the top slag is 65% or less. do.

In addition, the dissolved Al concentration in molten steel at the time of refining is recommended to be in the range of 50 to 900 ppm. If the dissolved Al concentration in the molten steel is less than 50 ppm, the amount of dissolved oxygen increases, the number of oxides crystallized during solidification increases, and thus the cleanliness is deteriorated. On the other hand, when the dissolved Al concentration exceeds 900 ppm, the dissolved oxygen concentration decreases, and the amount of dissolved Ca in the steel and the amount of dissolved Mg in the steel are too large, which is not preferable.

By employing such a method, the amount of solid solution Ca in steel and the amount of solid solution Mg in steel can be made within the prescribed range.

In the present invention, molten steel withdrawn from the converter or electric furnace is subjected to the first heating and component adjustment, degassing treatment is performed on the molten steel after the completion of the first heating and component adjustment, and the molten steel after the degassing treatment. It is effective to refine | purify by the process containing (heating, component adjustment → degassing process → heating, component adjustment), such as performing 2nd heating and a component adjustment with respect to the said process.

The first heating and component adjustment is a treatment in which the molten steel component is within a predetermined range, and the degassing treatment is a treatment in which gas components such as hydrogen present in the molten steel are removed. It is necessary to increase the stirring power density while suppressing the mixing of the slag as much as possible.

On the other hand, in the second heating and component adjustment, the degassing treatment is mainly responsible for the function of floating and separating the top slag once mixed in the molten steel, and finely adjusting the component and temperature, and adjusting the molten steel temperature to the temperature according to the casting conditions. It is good to stir at low stirring power density so that mixing of new saw slag does not occur.

Specifically, during degassing treatment after component adjustment (including Al amount adjustment), the stirring power density (?) Is calculated by the following formula (1). Stirring at 50 to 200 W / ton is recommended. Thus, the flow rate of the injection gas is adjusted so that the stirring power density is preferably 50 W / ton or more, more preferably 60 W / ton or more, preferably 200 W / ton or less, more preferably 180 W / ton or less, The subsequent degassing treatment (after medium period) is recommended to adjust the flow rate of the injection gas such that the stirring power density is 140 W / ton or less, preferably 120 W / ton or less (excluding 0 W / ton).

In the second heating and component adjustment, the stirring power density is preferably 25 W / ton or less, more preferably 20 W / ton or less, preferably adjusting the flow rate of the injection gas so as to be 2.0 W / ton or more. Recommended.

In more detail, it is based on the following procedure. First, the molten steel outgoing to the ladle from a converter or an electric furnace is conveyed to a secondary refining apparatus, and the first heating and component adjustment (hereinafter, referred to as LF-I) is performed. Specifically, by generating an arc discharge, flux is inputted using a flux supply means while heating molten steel to T _L = 1600 ° C., and further, Ar gas is injected from the gas injection means to stir the molten steel.

In the LF-I, the type and amount of the flux is the composition of the top slag at the end of the vacuum degassing treatment described later (in other words, at the start of the second heating and component adjustment),

(i) the mass of CaO becomes 3.0 times or more with respect to the weight of SiO _2,

(Ii) the mass of CaO is 1.5 to 3.5 times the mass of Al ₂ O ₃ ;

(Iii) The total sum of the mass of T.Fe and the mass of Ddn0 in the top slag composition is 1.0% or less of the total mass of the top slag,

It is recommended to control the heating temperature or to adjust the input of the subsidiary materials (flux) so as to satisfy all three conditions of.

The molten steel having completed the first heating and component adjustment is conveyed to the vacuum degassing apparatus through the ladle, and vacuum degassing treatment (hereinafter, sometimes referred to as VD) is performed on the molten steel.

Specifically, the exhaust device is operated to exhaust the gas in the ladle through the exhaust pipe and above the molten steel to bring the atmospheric pressure P in the ladle to a vacuum of about 0.5 Torr. In addition, Ar gas is injected from the gas injection means to stir the molten steel. By the method as mentioned above, the process of removing hydrogen from the molten steel which component adjustment is almost completed is performed.

In this treatment, it is preferable to employ a stirring power density ε that is compatible with prevention of top slag mixing into molten steel and dehydrogenation. Therefore, in the first half of the VD, if the flow rate Qg of the bottom-blown gas is adjusted so that the stirring power density F is 50 to 200 W / ton, dehydrogenation can be efficiently performed while suppressing the mixing of the saw slag to the minimum amount. have. In the latter half of the VD, it is preferable to adjust the flow rate Qg of the low blowing gas so that the stirring power density ε is 140 W / ton or less (excluding 0 W / ton), since the floating separation of the mixed top slag is promoted.

In addition, in calculation of the stirring power density (epsilon), the temperature before injection of the low blowing gas (T _o ) (the temperature before injection of Ar gas) shall be normal temperature (298K), and the temperature after injection of the low blowing gas (T _g) ) (after injection temperature of Ar gas) are in the molten steel temperature (T _L).

ε: Stirring power density (W / ton)

T _o : Temperature before injection of low blowing gas [room temperature (298K)]

T _L : molten steel temperature (K)

M _L : molten steel (ton)

ρ _L : molten steel density (kg / ㎥)

Q _g : Low blown gas flow rate (Nl / min)

T _g : Temperature after injection of low blowing gas (K)

P: atmosphere pressure (torr)

h _o : molten steel depth (m)

For example, in the first heating and component adjustment (LF-I), Q _g / M _L is 0.30 to 3.75 although some conditions such as ladle size and actual molten steel loading amount (M _L ) are different. By setting it as Nl / min * ton, stirring power density (epsilon) becomes 4.7-67.2 W / ton.

In addition, high-clean steel can be manufactured by performing 2nd heating and component adjustment (it may describe as LF-II hereafter) with respect to molten steel after VD.

That is, the molten steel after vacuum degassing process is conveyed to the secondary refining apparatus as a whole ladle, and a 2nd heating and component adjustment is performed with respect to molten steel. Specifically, for example, heating the molten steel to about T _L = 1600 ° C. by generating an arc discharge while injecting Ar gas from the gas injection means to stir the molten steel. As the stirring strength of the molten steel, it is recommended to adjust the flow rate (Q _g ) of the Ar gas so that the stirring power density ε calculated by the above equation (1) is 25 W / ton or less and 2.0 W / ton or more. By setting the stirring power density ε to 25 W / ton or less, new saw slag mixing can be prevented. In this LF-II, component analysis may be performed and component fine adjustment may be performed as needed.

Thus, by performing LF process (LF-II) again, "floating separation of the mixed top slag and deoxidation product" performed from the middle of VD can be further accelerated.

In addition, as described above, the top slag component in LF-II is

(i) basicity, ie CaO / SiO ₂ ≧ 3.0

(Ii) CaO / Al ₂ O ₃ = 1.5 to 3.5,

(Iii) T.Fe + MnO ≦ 1.0 mass%,

It is desirable to be able to reliably prevent reoxidation of the molten steel component by the oxide in the top slag.

As mentioned above, what is necessary is just to include the process of (heating, component adjustment → degassing process → heating, component adjustment) in a refining process, and is not limited about the process before and behind that. Therefore, for example, (degas treatment → heating · component adjustment) or (degas treatment → heating) under the above conditions or conditions other than the above after the (heating / component adjustment → degassing treatment → heating / component adjustment) process. Add a step to be performed once, or add a step of repeating one or both of the plurality of times, or degassing under the above conditions or other conditions after the above (heating / component adjustment → degas treatment → heating / component adjustment) step. You may provide the process of performing bay again, etc.

The present invention also includes a forged product obtained by using the forged steel, but the production method is not particularly limited. For example, a step of heating the forged steel and then forging a material, and heating the product after an intermediate inspection. What is necessary is just to manufacture by the process including the process of forging to a shape, the process of homogenizing by heat processing, the process of hardening by hardening and hardening, and the process of performing a finishing machining.

Forging products obtained by the above method are crank shafts (integrated crank shafts and assembled crank shafts), and because they exhibit excellent fatigue characteristics, slow, general mechanical components, pressure vessels, etc. High strength products such as hollow materials.

In the case of manufacturing the crankshaft as a forged product, it is preferable to manufacture the integrated crankshaft because the shaft surface layer side can be occupied by a portion having high cleanliness, and an excellent strength and fatigue property can be obtained. In this case, the manufacturing method of the integrated crankshaft is not particularly limited, but preferred is the RR and TR forging method (forging is performed such that the shaft center of the ingot is the center of the crankshaft axis, and the part which tends to cause deterioration of characteristics by the center segregation is It is produced by a method such as forging integrally so as to be the center of all the axes.

As another forging method, it may be produced by a free forging method (a method of forging by using a crank arm and a crank pin as a united block and finishing the crankshaft by gas cutting and machining).

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited by the following Examples, but may be carried out by appropriately modifying the present invention in a range that may be suitable for the purpose of the preceding and the following. It is also possible, and these are all included in the technical scope of the present invention.

<Example>

In the electric furnace, 20 to 100 tons of scrap raw materials were dissolved and tapped into the ladle. Then, the molten steel process was performed using the ladle heating type refinery apparatus provided with the low blowing stirring apparatus. In this molten steel treatment step, the first heating and component adjustment (LF-I) is performed on the molten steel withdrawn from the converter or the electric furnace, and the degassing treatment (VD) is performed on the molten steel after the first heating and component adjustment is completed. Second heating and component adjustment (LF-II) was performed on the molten steel after the degassing treatment.

When component adjustment in the heating and composition adjustment of the first time is added to jojae (造滓) agent such as CaO, Al ₂ O ₃ and Mg0 the molten steel surface, comprising an amount of Ca0 and Mg0 shown in Table 1 Formed top slag. Next, Al was added to deoxidize the molten steel, and dehydrogenation was performed by vacuum treatment by a cover degassing apparatus. During the molten steel treatment, the molten steel was properly sampled to measure the dissolved Al concentration in the molten steel, and further Al was added as necessary so that the dissolved Al concentration was within the recommended range. Table 1 shows the dissolved Al concentrations in molten steel. In addition, S containing alloy, Mg containing alloy, and Ca containing alloy shown in following Table 1 were added at the time of LF-II. The stirring power density up to the middle stage of the degassing treatment (VD) (first half of the VD), the stirring power density of the degassing treatment after the middle stage (after the middle stage, and the second half of the VD), and the stirring power density in the LF-II are shown in the table. As shown in 1.

After the molten steel treatment was completed, a sample of top slag was taken and a steel ingot (20 to 100 tons) was cast by an under pouring pouring ingot-making method. After the solidification of the steel ingot was completed, the steel ingot was extracted from the mold and heated to 1150 ° C. or higher to perform hot forging to produce round rod-shaped forgings of various sizes. At this time, hot forging was performed on the 20 ton steel ingot and finished in a round bar shape having a diameter of 250 to 450 mm. Hot forging was performed on the 50 ton steel ingot and finished in a round bar shape having a diameter of 350 to 700 mm. The 100-ton ingot was hot forged and finished in a round rod shape having a diameter of 600 to 1200 mm. In addition, Ca0 concentration and Mg0 concentration in the top slag were examined from the sample of the top slag by ICP emission spectroscopy. The results are shown in Table 1.

In addition, Table 2 shows the results of investigating the chemical composition of each forging material by chemical analysis. In addition, the amount of solid solution Ca and solid solution Mg in the steel ingot was measured, and the inclusion composition analysis and the fatigue test in the forged product were conducted by the following method. In addition, the amount of Total Ca and Total Mg in steel in Table 2 were calculated | required by ICP-mass spectrometry (ICP-MS method).

[Measurement of solid solution Ca (Sol.Ca) and solid solution Mg (Sol.Mg) in steel]

The sample collected from the ingot was polished and loaded into a secondary ion mass spectrometer (manufactured by "ims5f" CAMECA), and the secondary ion phases of Ca and Mg were observed in the region of 500 x 500 (占 퐉 ² ) for each sample. Then, three places where Ca and Mg were not locally concentrated in the area were selected and analyzed in the depth direction. The primary ion source at this time is O ²⁺ . And when the density | concentration distribution of a depth direction is constant, the value was made into solid solution concentration. In the case of the presence of inclusions in the course of the depth direction analysis, the concentration distribution fluctuated greatly, but the analysis was carried out to the depth where the inclusions do not exist. In addition, regarding the quantification method of the concentration, the relative iron obtained by measuring ²⁴ Mg (150 keV, 1 × 10 ¹⁴ atoms / cm 2) and ²⁷ Al (200 keV, 1 × 10 ¹⁴ atoms / cm 2) ion-implanted as a standard sample was measured. Atomic concentration was measured using the sensitivity coefficient (RSF). These measurement results are written together in Table 2.

Inclusion Composition Analysis

In the round bar after forging, the sample was cut out from the center part of the ingot bottom equivalent position, and the composition analysis of the inclusion was performed by EPMA. At this time, 50 or more inclusions were randomly selected with respect to each sample, and composition analysis was performed. The results are shown in Table 3. Table 3 also shows the equivalent circle diameter of the largest inclusion among the 50 or more inclusions.

In addition, the "fine" in the item of the "inclusion in a forging" of Table 3 means the case where all the said inclusions are circle equivalent diameter: less than 100 micrometers, and when the inclusion composition shown is an oxide type, all the oxide systems which performed the said analysis The composition of the oxide which occupies 50% or more of the inclusions is shown, and in the case of a sulfide system, the composition of the sulfide which occupies 50% or more of the total sulfide inclusions in which the said analysis was performed is shown. In addition, "coarse" means the case where one or more coarse inclusions with a circle equivalent diameter: 100 micrometers or more are detected, and the inclusion composition shown has shown the composition of the said coarse inclusions. In addition, in the above-mentioned "inclusions in the forging", the oxide and the sulfide may take the form of a contiguous form or a complex form (for example, a form in which the sulfide is present around the oxide as an oxide). Although the case of a composite inclusion is included, even if it is a composite inclusion, the size of oxide and sulfide in the said composite inclusion is calculated | required separately and evaluated.

[Fatigue test and inclusion size measurement]

In the round bar after forging, the smoothing test piece of diameter: 10 mm x length: 30 mm was extract | collected radially from the center part of the ingot bottom equivalent position, and the fatigue test was done on condition of the following. In addition, the tensile test was performed at normal temperature using the test piece collected from the same position as the fatigue test piece. And the durability limit ratio (fatigue intensity (sigma) w / tensile strength (sigma) B) was calculated | required as an index of a fatigue limit. This test was done with five test pieces, the average value of the durability limit ratio was calculated | required, and it was evaluated that this fatigue limit ratio was more than 0.42 that the fatigue characteristic was excellent. The results are shown in Table 3.

Test method: Rotational bending fatigue test (stress ratio = -1, number of revolutions: 3600rpm)

Fatigue Strength Evaluation Method: Method

Dependent stress: 20MPa

Initial stress: 300 MPa

Number of test pieces: 5 each

Fatigue Strength for Each Test Piece = (Break Stress)-(Earth Stress)

It can consider as follows from Tables 1-3 (The following No. shows experiment No. in Tables 1-3). Since Nos. 1-17 satisfy | fill the component composition prescribed | regulated by this invention, it turns out that the maximum inclusion which exists in steel is small, and as a result, a high durability limit ratio is obtained.

On the other hand, in Nos. 18 to 28, since the chemical composition of the steel deviates from the requirements of the present invention, the maximum inclusions present in the steel become coarse as follows, and as a result, the durability limit ratio is lowered.

In detail, No. 18 is an example in which an S-containing alloy, an Mg-containing alloy, and a Ca-containing alloy were added, and since the stirring during refining was too strong, the amount of S, Total Ca, Total Mg, Solid Ca and solid Mg Both amounts were out of the upper limit, and as a result, both oxides and sulfides became coarse.

Since No. 19 had too strong agitation during refining, MgO and CaO in the top slag were mixed, and as a result, the total amount of Ca, the total amount of Mg, and the amount of solid solution Ca exceeded the upper limits, resulting in coarsening of both oxides and sulfides.

In No. 20, since the total Mg amount and the solid solution Mg amount exceeded the upper limit by adding the Mg-containing alloy, coarse Mg-containing inclusions were formed.

No. 21 added the Ca containing alloy, and since the total amount of Ca and the amount of solid solution Ca exceeded the upper limit, coarse Ca containing inclusions were formed.

No. 22 is an example in which an Mg-containing alloy is added and stirring in LF-II is weak. In this case, the amount of solid solution Mg was within the prescribed range but the total amount of Mg exceeded the upper limit, resulting in coarse Mg-containing inclusions.

No. 23 is an example in which a Ca-containing alloy is added and stirring in LF-II is weak. In this case, the amount of solid solution Ca was within the prescribed range, but coarse Ca-containing inclusions occurred because the total Ca amount exceeded the upper limit.

In No. 24, the amount of S exceeded the upper limit by the addition of the S-containing alloy, resulting in coarsening of sulfides.

No. 25 is an example in which the stirring intensity | strength of general VD is small, and stirring in LF-II is weak. In this case, since the amount of dissolved Ca and the amount of dissolved Mg did not reach the lower limit, coarse Al ₂ O ₃ and coarse MnS occurred as a result.

No. 26 contained coarse Ca-containing inclusions because the dissolved Al concentration in the molten steel exceeded the recommended range and the amount of solid solution Ca exceeded the upper limit.

In No. 27, coarse Al ₂ O ₃ or coarse MnS was formed because the dissolved Al concentration in the molten steel was less than the recommended range, and the amount of dissolved Ca and dissolved Mg did not fall below the prescribed lower limit.

No. 28 did not keep components in the top slag within the recommended range, and coarse Al ₂ O ₃ and coarse MnS were formed because the amount of solid solution Ca did not reach the lower limit.

Fig. 2 is a graph summarizing the relationship between the circle equivalent diameter of the maximum inclusion and the endurance limit ratio present in the steel detected by EPMA, but from this Fig. 2 the relationship between the endurance limit ratio and the circle equivalent diameter of the maximum inclusion is very good. You can see that you can see. In addition, it can be seen that when the circle equivalent diameter of the largest inclusion is less than 100 µm, excellent fatigue characteristics of more than 0.42 can be achieved.

1 is a graph showing the range of Total Ca amount and Total Mg amount defined in the present invention.

2 is a graph showing the relationship between the equivalent circle diameter of the largest inclusion in the steel and the endurance limit ratio.

Claims (5)

C: 0.2 to 0.6% (mean of mass%. The same below), Si: 0.05 to 0.5%, Mn: 0.2 to 1.5%, Ni: 0.1 to 3.5%, Cr: 0.9 to 4%, Mo: 0.1 to 0.7%, Al: 0.005 to 0.1%, S: 0.008% or less (does not contain 0%) O: 0.0025% or less (does not contain 0%), Total Ca: 0.0030% or less (does not contain 0%), Total Mg: 0.0015% or less (does not include 0%) And forging Ca and Mg for satisfying any one of the following (I) to (IV). (I) solid solution Ca: 2 to 500 ppb (mean of mass ppb. The same applies hereinafter) Solid solution Mg: 0.04 to 5 ppm (mean of ppm by mass. (II) solid solution Ca: 2 to 100 ppb, and also Solid Mg: 5-10ppm (III) Ca dissolved: 2 ppb or less (not including 0 ppb), and Solid Mg: 0.04-5ppm (IV) solid solution Ca: 2 to 500 ppb, Solid solution Mg: 0.04ppm or less (does not include 0ppm) The forging steel according to claim 1, further comprising at least one of the following groups as another element. (A) 0.005 to 0.2% of one or more selected from the group consisting of V, Nb, Ta and Hf (B) Ti: 0.05% or less (does not contain 0%) (C) Cu: 1.0% or less (does not contain 0%) 3. The forging steel according to claim 2, wherein the circle equivalent diameter of the largest inclusion present in the steel is less than 100 mu m. A forged product manufactured by using the forging steel according to claim 3. The forged product according to claim 4, which is a crankshaft.

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