JP4160103B1 - Steel ingot for forging - Google Patents

Abstract

A forging steel ingot excellent in fatigue characteristics and hydrogen cracking resistance is provided.
A forging steel ingot formed by a mold, wherein a density (D _BOT ) of inclusions having a major axis of 5 to 10 μm observed in a steel cross section at the bottom of the steel ingot is 10 to 80 pieces / cm ² . Yes, the density (D _TOP ) of inclusions having a major axis of 5 to 10 μm observed in the steel cross section at the upper part of the steel ingot is 20 to 90 / cm ² , and the inclusions of the major axis of 40 μm or more observed in the steel section A forging steel ingot having a density of 5 pieces / cm ² or less both in the lower portion of the steel ingot and the upper portion of the steel ingot and satisfying (D _TOP ) / (D _BOT ) ≧ [S (ppm)] / 18.
[Selection] Figure 8

Description

  The present invention relates to a forging steel ingot manufactured by an ingot-making method. Forged steel products manufactured using the forging steel ingot of the present invention are widely and effectively used in industrial fields such as machinery, ships, and generators, and particularly require high fatigue strength like rotary motion parts. Suitable for parts to be made.

In Patent Document 1, in order to improve hydrogen cracking resistance of a crankshaft of a ship, an average value of circularity of inclusions having a maximum chord length of 1 μm or more contained in steel (hereinafter referred to as average circularity) is disclosed. The number of inclusions of 0.5 or more, the maximum chord length of 20 μm or more is less than 40 per 100 mm ² , the average circularity is 0.25 or more, and the number of inclusions having a maximum chord length of 1 to 10 μm is 100 mm Steel for forging that is 100 or more per ² is described.

In Patent Document 2, C: 0.62 to 0.80%, Si: 0.60% or less, Mn: 0.30 to 1.80 for the purpose of improving the machinability and wear resistance of the crankshaft. %, S: 0.04 to 0.35%, Cr: 0.05 to 0.50%, Al: less than 0.005%, O: 0.0020% or less, remaining Fe and inevitable impurities, hot A steel material containing a sulfide-based inclusion whose main structure after forging is mainly pearlite having a pro-eutectoid ferrite fraction of 3% or less and a thickness of 20 μm or less is described.
JP 2006-336092 A JP 2002-194502 A

  In particular, in ship parts, hydrogen cracking due to hydrogen defects and a decrease in fatigue strength due to inclusion defects are particularly problematic in recent years. However, in the above conventional technology, a steel ingot for forging having excellent fatigue characteristics and sufficient hydrogen cracking resistance, which are excellent in machinability and wear resistance, but are not easily broken even under severe use environment. Has not yet been manufactured. This invention is made | formed in view of such a situation, Comprising: It aims at providing the steel ingot for forging excellent in both fatigue characteristics and hydrogen cracking resistance.

  In order to prevent hydrogen cracking, MnS-based inclusions that normally capture hydrogen in the steel material are distributed in the steel material. However, MnS-based inclusions improve the hydrogen cracking resistance, but reduce the fatigue strength of the steel material. Therefore, it is very difficult to simultaneously improve both the hydrogen cracking resistance and the fatigue strength in such a trade-off relationship.

  Under such circumstances, the present inventors have found that the hydrogen concentration in steel that causes hydrogen cracking is higher in the upper part of the steel ingot than in the lower part of the steel ingot in one steel ingot. When further research was carried out, the fatigue strength of the steel ingot was reduced when the ratio of inclusion density in the upper and lower parts of the steel ingot and the concentration of S (sulfur), which is closely related to the formation of inclusions, met a certain relationship. The present invention has been completed by ascertaining that hydrogen cracking of the steel ingot for forging can be prevented without causing it.

The forging steel ingot of the present invention that has achieved the above object is
A forging steel ingot formed by a mold, which has a major axis of 5 to 5 observed at a steel cross section at the bottom of the steel ingot (the end in the direction of gravity and within 20% of the total height of the ingot from the end). The density of inclusions (D _BOT ) of 10 μm is 10 to 80 pieces / cm ² , and the upper part of the steel ingot (the end opposite to the lower part of the steel ingot, which is 20% of the total height of the steel ingot) The density (D _TOP ) of inclusions having a major axis of 5 to 10 μm observed in the steel cross section at 20 to 90 / cm ² in the steel section and the density of inclusions having a major axis of 40 μm or more observed in the steel cross section Is a steel ingot for forging which is 5 pieces / cm ² or less in both the lower part of the steel ingot and the upper part of the steel ingot and satisfies the following formula (1).

However, [S] shows content (mass ppm) of S in steel.

The forging steel ingot is
C: 0.2 to 0.6% (meaning mass%, hereinafter the same)
Si: 0.05-0.5%
Mn: 0.2 to 1.2%
Ni: 0.1 to 3.5%
Cr: 0.9 to 2.5%
Mo: 0.1 to 0.7%
V: 0.005-0.2%
Al: 0.01 to 0.1%
S: 0.005% or less (excluding 0%)
Ti: 0.005% or less (excluding 0%)
O: 0.0015% or less (excluding 0%)
It is recommended that the balance consists of iron and inevitable impurities.

  According to the present invention, the density of the fine inclusions in the lower part of the steel ingot, the density of the fine inclusions in the upper part of the steel ingot, the density of the coarse inclusions in the lower part of the steel ingot and the upper part of the steel ingot, The forging steel ingot excellent in fatigue characteristics and hydrogen cracking resistance can be manufactured by satisfying a certain relationship between the ratio of inclusion density in the lower portion and the S concentration in the steel.

  For example, the current crankshaft is assumed to have a load corresponding to an output of 2000 kW per cylinder, but in future crankshafts for large ships, the engine will be made smaller and lighter to improve fuel efficiency. In order to meet this demand, it is necessary to have fatigue characteristics that can withstand this. For this purpose, the endurance limit ratio (fatigue strength / tensile strength) is required to be 0.45 or more regardless of the size of the crankshaft. According to the present invention, a crankshaft satisfying this requirement can be provided.

  As shown in FIG. 1, the steel ingot produced by the ingot-making method has a high inclusion density in the steel ingot lower portion that is a precipitation crystal zone and the steel ingot upper portion that is the final solidification portion. Therefore, the steel ingot lower part and the steel ingot upper part are portions in which the hydrogen cracking resistance and fatigue characteristics of the steel ingot are remarkably reflected, and are suitable as parts for specifying the characteristics of the steel ingot.

In the present invention, as shown in FIG.
Steel ingot lower part: the end of the steel ingot in the direction of gravity and within 20% of the total ingot height from the end. Steel ingot upper part: the end opposite to the lower part of the steel ingot, from the end Each part is defined as within 20% of the total height of the steel ingot.

(Density of fine inclusions at the bottom of the steel ingot (D _BOT ): 10-80 pieces / cm ² )
As described above, hydrogen cracking resistance can be improved by dispersing minute inclusions in the steel, but in order to exert this effect effectively, it is observed in the steel cross section at the bottom of the steel ingot. It is necessary that the fine inclusions (major axis 5 to 10 μm) be 10 / cm ² or more (more preferably 20 / cm ² or more, and further preferably 30 / cm ² or more). On the other hand, even if there are too many fine inclusions, as shown in the scanning electron micrographs of FIGS. 3 to 5, inclusion groups are formed and become the starting point of fatigue fracture as in the case of coarse inclusions. End up. Therefore, the fine inclusions observed in the steel cross section need to be 80 pieces / cm ² or less (more preferably 70 pieces / cm ² or less, more preferably 60 pieces / cm ² or less).

  In practice, inclusions of less than 5 μm also have hydrogen cracking resistance, so inclusions of less than 5 μm may be counted as minute inclusions. However, inclusions less than 5 μm have almost the same distribution characteristics as inclusions of 5 to 10 μm, so counting the number of inclusions of 5 to 10 μm is sufficient to determine hydrogen cracking resistance. It is. Therefore, the convenience of the follow-up test was improved by excluding inclusions of less than 5 μm from the count target.

(Density of fine inclusions at the top of the steel ingot (D _TOP ): 20 to 90 pieces / cm ² )
In the upper part of the steel ingot, the fine inclusions (major axis 5 to 10 μm) observed in the steel cross section are 20 pieces / cm ² or more (more preferably 30 pieces / cm ² or more, more preferably 40 pieces / cm ² or more). There is a need. In addition, as described above, if too much fine inclusions are included, an inclusion group is formed, which is the starting point of fatigue failure as in the case of coarse inclusions. Therefore, the fine inclusions observed in the steel cross section need to be 90 pieces / cm ² or less (more preferably 80 pieces / cm ² or less, more preferably 70 pieces / cm ² or less).

(Density of coarse inclusions: 5 / cm ² or less)
Coarse inclusions since become a starting point of fatigue fracture, in both of the lower steel ingot upper portion and the steel ingot, the coarse inclusions observed in the steel section (over diameter 40 [mu] m) 5 / cm ² or less (more preferably 4 pieces / cm ² or less, more preferably 3 pieces / cm ² or less.

((D _TOP ) / (D _BOT ) ≧ [S] / 18)
When the present inventors investigated the hydrogen concentration of the steel ingot, it was found that the hydrogen concentration was higher in the upper portion of the steel ingot than in the lower portion of the steel ingot, as shown in FIG. We also investigated the hydrogen cracking resistance and durability limit ratio at the top of the steel ingot. The result is shown in FIG. In FIG. 7, (D _TOP ) / (D _BOT ) on the vertical axis and [S] on the horizontal axis, the hydrogen cracking resistance and durability limit ratio satisfying the predetermined criteria (●), not satisfied Each is plotted as (x). The determination criterion (• / ×) is the same as the determination criterion (• / ×) in “Comprehensive evaluation” in Tables 1 to 3 described later.

[S] indicates the concentration (mass ppm) of S in the steel. From FIG. 7, (D _TOP ) / (D _BOT ) = [S] / 18, the case indicated by (●) appears above the straight line and the case indicated by (×) appears below the straight line. I understand that.

In FIG. 7, in the region where the S concentration in the steel is high, the value of (D _TOP ) / (D _BOT ) is not high, that is, the concentration of fine inclusions at the top of the steel ingot must be higher than that at the bottom of the steel ingot. For example, this indicates that hydrogen cracking occurs in the upper part of the steel ingot. However, it should be noted that hydrogen cracking does not occur in the region where the S concentration in the steel is low, even if the value of (D _TOP ) / (D _BOT ) is not high. For example, even when the value of (D _TOP ) / (D _BOT ) is less than 1, hydrogen cracking does not occur.

  For example, when the S concentration in the steel is 0.003%, the allowable hydrogen value in the steel ingot is 1.5 ppm, whereas when the S concentration is 0.001%, the allowable hydrogen is The value is as low as 1.0 ppm. Usually, when manufacturing one crankshaft from one steel ingot, the range of hydrogen value is about 0.5-1.8 ppm.

  As will be described later, since the present inventors also enabled a process for suppressing the hydrogen value to 1.2 ppm or less, even if the S concentration is 0.003% or less, a steel ingot for forging is produced without causing hydrogen cracking. It became possible to do. This created room for further reducing the S concentration.

Normally, hydrogen cracking is likely to occur when the S concentration is lowered in order to improve the fatigue characteristics. However, from FIG. 7, even if the S concentration is lowered, (D _TOP ) / (D _BOT ) ≧ [S] / 18 If this condition is satisfied, the hydrogen cracking resistance and fatigue characteristics are maintained. Thereby, it is considered that the balance between the fatigue characteristics of the steel ingot and the hydrogen cracking resistance can be improved as compared with the conventional case.

(Chemical composition of steel ingot)
As described above, the present invention is characterized in that the size and density of inclusions present in the steel material are controlled, and the basic composition of the steel is not particularly limited, but is required, for example, as a crankshaft or the like. In order to satisfy the strength, toughness, and fatigue characteristics, it is desirable to satisfy the following basic composition in light of the general technical level of steel materials.

(C: 0.2-0.6%)
C is an element that contributes to strength improvement. In order to ensure sufficient strength for the crankshaft, for example, 0.2% or more, more preferably 0.25% or more, and further preferably 0.3% or more is contained. Is good. However, if the amount of C is too large, the toughness of the crankshaft is deteriorated. For example, it is suppressed to 0.6% or less, more preferably 0.55% or less, and further preferably 0.5% or less.

(Si: 0.05-0.5%)
Si acts as a strength improving element, and in order to ensure sufficient strength for the crankshaft, for example, it is 0.05% or more, more preferably 0.1% or more, and still more preferably 0.15% or more. However, if the amount is too large, the reverse V segregation becomes remarkable and it becomes difficult to obtain a clean steel ingot. For example, it is 0.5% or less, more preferably 0.45% or less, and still more preferably 0.4% or less. .

(Mn: 0.2-1.2%)
Mn is also an element that enhances hardenability and contributes to improvement in strength, and in order to ensure sufficient strength and hardenability, for example, 0.2% or more, more preferably 0.5% or more, and still more preferably 0.8. However, if it is too much, reverse V segregation may be promoted. For example, it is 1.2% or less, preferably 1.1% or less, more preferably 1% or less.

(Ni: 0.1-3.5%)
Ni is an element useful as a toughness-improving element. For example, it is recommended to contain 0.1% or more, preferably 0.2% or more. However, if the amount of Ni is excessive, the cost increases. 5% or less, preferably 3% or less.

(Cr: 0.9-2.5%)
Cr is an effective element that enhances hardenability and improves toughness. Their action is, for example, 0.9% or more, preferably 1.1% or more, and more preferably 1.3% or more. However, if it is too much, reverse V segregation may be promoted and it may be difficult to produce highly clean steel. For example, it is 2.5% or less, preferably 2.3% or less, more preferably 2.1% or less. To do.

(Mo: 0.1-0.7%)
Mo is an element that effectively works to improve all of the hardenability, strength, and toughness, and in order to exert these actions effectively, for example, 0.1% or more, more preferably 0.2% or more, Preferably, 0.25% or more is contained. However, since Mo has a small equilibrium partition coefficient and easily causes microsegregation (normal segregation), for example, 0.7% or less, preferably 0.6% or less, more preferably 0.5% or less.

(V: 0.005-0.2%)
V has an effect of precipitation strengthening and refinement of structure, and is an element useful for increasing the strength of steel. In order to effectively exhibit such an action, it is recommended that V is contained, for example, 0.005% or more, preferably 0.01% or more. However, even if contained excessively, the above effect is saturated and is economically wasteful, so 0.2% or less, more preferably 0.15% or less.

(Al: 0.01 to 0.1%)
Al is effective as a deoxidizing element in the steel making process, and is also effective in cracking resistance of steel. Therefore, it is encouraged that the Al content is, for example, 0.01% or more, preferably 0.015% or more. On the other hand, Al fixes N in the form of AlN or the like, and inhibits the strengthening action of steel by the blending of N and V, etc., and also binds with various elements to produce non-metallic inclusions and intermetallic compounds, Since the toughness of the steel may be lowered, it is preferably 0.1% or less, more preferably 0.08% or less, for example.

(S: 0.005% or less (excluding 0%))
Since S tends to form coarse inclusions in the forging steel, it may reduce the fatigue strength of the forging steel ingot. Therefore, the S content in the steel is, for example, 0.005% or less, preferably 0.0045% or less, more preferably 0.004% or less, and still more preferably 0.0035% or less.

  On the other hand, when fine S-based inclusions are contained in the forging steel at a certain density or more, a large number of stress fields are formed in the steel, and it is easy to capture surplus hydrogen in the steel exceeding the solid solubility limit. It has the effect of improving hydrogen cracking resistance.

  In order to secure such S-based inclusions, the S content in the steel is preferably 0.0002% or more, more preferably 0.0004% or more, still more preferably 0.0006% or more, and still more preferably. Is 0.0008% or more.

S content can be adjusted by controlling the slag composition at the time of melting. Specifically, the S content in the steel is reduced by increasing the ratio of CaO concentration to SiO ₂ concentration in the slag (CaO / SiO ₂ : hereinafter sometimes referred to as “C / S”). be able to. In addition, as a supplementary measure, by increasing the ratio of CaO concentration to Al ₂ O ₃ concentration (CaO / Al ₂ O ₃ : hereinafter sometimes referred to as “C / A”), S content in steel is increased. The amount can be reduced. Conversely, when it is desired to increase the S content, the slag composition is adjusted so that C / S and / or C / A is reduced.

(Ti: 0.005% or less (excluding 0%))
Ti may form coarse nitrides in the steel and may reduce the fatigue strength of the forging steel. Therefore, the Ti content in the steel is, for example, 0.005% or less, preferably 0.004% or less, and more preferably 0.003% or less.
Ti constitutes fine inclusions such as TiN, TiC, Ti ₄ C ₂ S ₂ and is dispersed in the steel, occludes and captures excess hydrogen in the steel exceeding the solid solubility limit, and resists the resistance of the steel. It has the effect of improving hydrogen cracking properties. When securing such Ti-based inclusions, the Ti content in the steel is, for example, 0.0002% or more, preferably 0.0004% or more, more preferably 0.0006% or more.

  About Ti content, it can adjust by adjusting the usage-amount ratio of the alloy (low grade alloy) with many impurity Ti content in an auxiliary material, and the alloy (high grade alloy) with little impurity Ti content.

(O: 0.0015% or less (excluding 0%))
O (oxygen) is an element that forms oxides such as SiO ₂ , Al ₂ O ₃ , MgO, and CaO and becomes inclusions to reduce the fatigue strength of the steel ingot. Therefore, it is preferable to reduce O as much as possible, and the total oxygen amount is 0.0015% or less, more preferably 0.001% or less.

  The preferred basic components of the forging steel used in the present invention are as described above, and the remaining component is substantially Fe, but inevitable impurities are allowed to be contained in the forged steel. Examples of inevitable impurities include P and N. For example, P is preferably 0.03% or less, and more preferably 0.02% or less. It is also possible to use forging steel that further contains other elements within a range that does not adversely affect the operation of the present invention.

  Examples of other elements that are allowed to be positively added include B having a hardenability improving effect, W, Nb, Ta, Cu, Ce, Zr, Te, etc., which are solid solution strengthening elements or precipitation strengthening elements, They can be added alone or in combination of two or more. These additive elements are preferably about 0.1% or less in total amount, for example.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

Increasing the cleanliness of the molten steel injected into the mold, the density of inclusions with a major axis of 5 to 10 μm observed at the steel cross section at the bottom of the steel ingot is about 10 to 80 / cm ² , and the steel cross section at the top of the steel ingot is observed at the steel cross section. In order to set the density (D _TOP ) of inclusions having a major axis of 5 to 10 μm to about 20 to 90 / cm ² and the density of inclusions having a major axis of 40 μm or more to about 5 / cm ² or less, the following will be described. It is recommended to refine the steel by the method.

  In this refining method, the first refining is performed on the molten steel produced from the converter or the electric furnace, the degassing treatment is performed on the molten steel after the second refining is completed, and the degassing is performed. Highly clean steel is manufactured by performing the second secondary refining on the treated molten steel.

  That is, in order to produce a highly clean steel with low inclusions due to slag entrainment and high cleanliness, secondary refining treatment → degassing treatment → secondary to molten steel produced by a converter It is effective to perform secondary refining twice in the order of refining treatment.

  The first secondary refining process is a process for making the molten steel components predetermined, and the degassing process is a process for removing gas components such as hydrogen present in the molten steel. It is necessary to increase the stirring power density while suppressing the entrainment of floating slag as much as possible.

  On the other hand, the second secondary refining process mainly has the function of floating and separating the slag once entrained in the molten steel by the degassing process, so that no new slag entrainment occurs while heating and holding the molten steel. In addition, it is necessary to perform stirring at a low stirring power density.

Specifically, in the first secondary refining process, the blowing gas density is set so that the stirring power density is 5 W / ton or more (preferably 10 W / ton or more) and 60 W / ton or less (preferably 50 W / ton or less). While adjusting the flow rate, the slag composition after the degassing treatment was such that CaO / SiO ₂ ≧ 3.5 and CaO / Al ₂ O ₃ = 1.5 to 3.5 and T.I. Slag adjustment is performed so that Fe + MnO ≦ 1.0 mass%. T. T. Fe means the total amount of iron atoms.

  In the degassing process, the blowing gas is used so that the stirring power density is 50 W / ton or more, preferably 60 W / ton or more, 200 W / ton or less, preferably 180 W / ton or less until the middle stage (intermediate) of the degassing process. In the subsequent degassing process (after the middle period), the flow rate of the blown gas is adjusted so that the stirring power density is 140 W / ton or less, preferably 120 W / ton or less (excluding 0 W / ton).

  In the second secondary refining treatment, the flow rate of the blown gas is adjusted so that the stirring power density is 25 W / ton or less, preferably 20 W / ton or less (excluding 0 W / ton).

  More specifically, according to the following procedure.

First, the molten steel discharged from the converter or electric furnace to the ladle is transported to the secondary refining apparatus and subjected to the first secondary refining treatment (hereinafter sometimes referred to as LF-I). . Specifically, the molten steel is heated to about T _L = 1600 ° C. by generating an arc discharge, and the flux is supplied using a flux supply unit, and Ar gas is blown from the gas blowing unit to stir the molten steel. To do. As the stirring strength of the molten steel, the flow rate of Ar gas is adjusted so that the stirring power density ε calculated by the following formula (2) is 5 to 60 W / ton.

In the calculation of the stirring power density ε, the temperature T _o before blowing the bottom blowing gas (temperature before blowing Ar gas) is room temperature (298 K), and the temperature T _g after blowing the bottom blowing gas (temperature after blowing Ar gas). ) Is the molten steel temperature _TL .

  In LF-I, which first refines a ladle received from a converter or an electric furnace, the heating of the molten steel and the component adjustment are the main components. Uniformity cannot be achieved. However, excessive molten steel agitation tends to entrain slag even if the components and temperature are uniform, and is likely to become a source of subsequent defects. Therefore, the stirring power density ε is 5 to 60 W / ton. This makes it possible to make the composition and temperature of the molten steel uniform while preventing slag entrainment.

ε: Stirring power density (W / ton)
T _o: of bottom-blown gas blowing before temperature (room temperature (298K))
T _L : Molten steel temperature (K)
M _L: amount of molten steel (ton)
ρ _L : Molten steel density (kg / m ³ )
Q _g : Bottom blowing gas flow rate (Nl / min)
T _g : Temperature after blowing the bottom blowing gas (K)
P: Atmospheric pressure (torr)
h _o : Molten steel depth (m)

For example, the first secondary refining process (LF-I), the size and the actual molten steel charging amount _{M L} like the ladle, although some conditions _differ, the Q g / _{M L} 0.30~3. By setting it to 75 Nl / min · ton, the stirring power density ε is 4.7 to 67.2 W / ton.

In LF-I, the type and amount of the flux is determined by the composition of slag after the vacuum degassing process described later (in other words, at the start of the second secondary refining process),
(I) The mass of CaO is 3.5 times or more with respect to the mass of SiO ₂ .
(Ii) The mass of CaO is 1.5 to 3.5 times the mass of Al ₂ O ₃ .
(Iii) T. in slag composition The sum of the mass of Fe and the mass of MnO is 1.0% or less of the total mass of the slag,
The heating temperature is controlled and the input amount of the auxiliary material (flux) is adjusted so as to satisfy the three conditions.

The molten steel that has undergone the first secondary refining process is transported to the vacuum degassing apparatus together with the ladle, and subjected to a vacuum degassing process (hereinafter sometimes referred to as VD).
Specifically, the exhaust device is operated, and the gas in the ladle and above the molten steel is exhausted through the exhaust pipe, whereby the atmospheric pressure P in the ladle is brought close to a vacuum state of about 0.5 Torr. In addition, the molten steel is stirred by blowing Ar gas from the gas blowing means. The gas components such as hydrogen existing in the molten steel are removed by the method as described above.

The VD time is about 20 minutes as a whole, and in the first half (before the middle of the processing time, the first half 10 minutes), the flow rate Q _g of the bottom blowing gas is set so that the stirring power density ε is 50 to 200 W / ton. The bottom blowing gas flow rate _Qg is adjusted so that the stirring power density ε is 140 W / ton or less (excluding 0 W / ton) in the second half (after the middle of the processing time and in the second half 10 minutes).

  In VD, hydrogen is removed from the molten steel whose component adjustment has been almost completed. At this time, the stirring power density ε that enables both prevention of slag entrainment in the molten steel and dehydrogenation should be adopted. Is preferred. Therefore, by setting the stirring power density ε to 50 to 200 W / ton in the first half of the VD processing time, dehydrogenation can be performed efficiently while minimizing slag entrainment. In addition, in the latter half of VD, when the stirring power density ε is suppressed to 140 W / ton or less, the floating separation of the slag involved is promoted.

Furthermore, in the case of the present embodiment, the second secondary refining (hereinafter sometimes referred to as LF-II) is performed on the molten steel after VD, thereby making it possible to produce highly clean steel.
That is, the molten steel that has been vacuum degassed is transferred to the secondary refining treatment apparatus together with the ladle, and the second refining treatment is performed on the molten steel. Specifically, the molten steel is agitated by blowing Ar gas from the gas blowing means while heating the molten steel to about T _L = 1600 ° C. by generating arc discharge. The stirring intensity of the molten steel, adjusting the flow rate _{Q g} of Ar gas as the agitation power density ε calculated in equation (2) is 25W / ton or less (0 W / ton are excluded).

In this way, by performing LF treatment (LF-II) again, “floating separation of entrained slag and deoxidized product” performed from the middle of VD can be further promoted. At this time, the stirring power density ε in LF-II needs to be 25 W / ton or less in order to prevent new slag entrainment. By heating and holding the molten steel at this stirring power density ε, the slag and the deoxidized product can be reliably separated by floating.
As described above, the slag component in LF-II is
(I) basicity, ie CaO / SiO ₂ ≧ 3.5,
(Ii) CaO / Al ₂ O ₃ = 1.5 to 3.5,
(Iii) T. Fe + MnO ≦ 1.0 mass%,
Therefore, reoxidation of the molten steel component by the oxide in the slag can be surely prevented.

  By adopting the manufacturing method of the high clean steel described above, it becomes possible to manufacture a high clean steel with few inclusions due to slag entrainment.

  The obtained highly clean molten steel was poured into a 10 to 90 ton class (total height of 2 to 4 m) mold by a bottom pouring ingot method to produce a steel ingot. After the solidified steel ingot was demolded, it was heated to about 1300 degrees and subjected to hot forging to finish a forged material having a cross-sectional diameter of 150 to 700 mm. Hot forging was performed by extending the steel ingot body with a press and then forming it into a round cross section using a dedicated tool.

Tables 1 to 3 show conditions in which the stirring power density ε in LF-I, the stirring power density ε in the first half of VD, the stirring power density ε in the second half of VD, and the stirring power density ε in LF-II are variously changed (conditions 1 to 20). ), Basicity (CaO / SiO ₂ ), CaO / Al ₂ O ₃ , T. The various conditions of the test (test numbers 1 to 59) conducted by changing the value of Fe + MnO (%) and the physical property data of the test pieces cut out from the upper and lower parts of the obtained steel ingot are shown.

In Tables 1 to 3, the column of “Component / Temperature Uniformity” has a C component variation (ΔC) and a temperature variation (ΔT) from the beginning to the end of casting of the steel ingot, and ΔC ≦ In the case of 0.01% and ΔT ≦ 20 ° C., (◯) is entered, and in other cases (×) is entered.
In the column of “removal of hydrogen”, the hydrogen concentration [H] is measured immediately before the refining is completed. If [H] ≦ 1.2 ppm, (○) is entered, and if [H]> 1.2 ppm, (×) is entered.
In the “Slag Entrainment” column, if the number of inclusions with a major axis of 5 μm or more and a Ca concentration of 5% or more per 1 cm ² of observation field is 30 or less in the microscopic observation of the molten steel sample, If the number exceeds 30, (x) is entered.

In Tables 1-3, the S concentration (mass ppm) in steel of each test piece, the density (D _TOP ) of the fine inclusions (major axis 5-10 μm) corresponding to the upper part of the steel ingot, and the lower part of the steel ingot The density (D _BOT ) of the minute inclusions (major axis 5-10 μm) of the part, the density of the coarse inclusions (major axis 40 μm or more) corresponding to the upper part of the steel ingot, the coarse inclusion (major axis) of the part corresponding to the lower part of the steel ingot A density of 40 μm or more). The number of inclusions was determined by EPMA (JXA-8900L, manufactured by JEOL) for the number of inclusions per 1 cm ^{2 of the} spectroscopic surface of the test piece.

  The chemical components in the steel of the test piece were: C: 0.3%, Si: 0.25%, Mn: 0.55%, Ni: 1.6%, Cr: 1.6%, Mo: 0.00. It was 25%, V: 0.01%, Al: 0.03%, S: 0.002%, Ti: 0.003%, O: 0.0013%, P: 0.01%.

In addition, (D _BOT ) / (D _TOP ) × S concentration in steel (ppm by mass) (when this value is 18 or less, the formula (1) is satisfied), the size of the maximum inclusion (◯ is , Maximum size (φ sphere diameter conversion) <0.5 mm, Δ indicates 0.5 mm <maximum size ≦ 1.0 mm, and x indicates maximum size> 1.0 mm. In addition, about the item in which the distinction of the upper part (T) of a steel ingot and the lower part (B) is not described, the test result regarding the steel ingot upper part is shown.
However, in the column of cleanliness, DIN 3 standard, DIN K (3) ≦ 15 is set to ○, and DIN K (3)> 15 is set to ×, and at the top of the steel ingot, and at the bottom of the steel ingot In the case of ○, the cleanliness column is indicated by ○, when either one is ○ and the other is ×, the cleanliness column is Δ, and when both are ×, the cleanliness column is ×.

  In Tables 1 to 3, the endurance limit ratio at the upper and lower parts of the steel ingot and the test results of hydrogen cracking at the upper and lower parts of the steel ingot are described.

(Durability limit ratio)
From the results of the tensile strength test and fatigue strength test described later, the endurance limit ratio = fatigue strength / tensile strength is obtained. When the endurance limit ratio ≧ 0.45, (◯), 0.40 ≦ endurance limit ratio <0.45 In the case of (△), in the case of the durability limit ratio <0.40, it was set as (×).

(Tensile strength)
A tensile test piece (JIS Z 2204, 2241) was taken at room temperature from φ6 mm × gauge length 30 mm (two each) tensile test specimens from the vicinity of the center of the steel material of the round bar after forging.

(Fatigue strength)
A rotating bending fatigue test was performed using the following test pieces.
Test piece: 10 mm diameter smooth test piece Test method: Rotating bending fatigue test (stress ratio = -1, rotation speed: 3600 rpm)
Fatigue strength evaluation method: Difference method Difference stress: 20 MPa
Number of test pieces: 5 each Fatigue strength of each test piece = (breaking stress)-(step stress)

(Hydrogen cracking resistance)
An ultrasonic flaw detection test (UT) was performed at a frequency of 4 MHz (more specifically, “defects in forged steel products”, Japan Casting and Forging Society, Forging Steel Research Group, P32-33). If a defect echo indicating hydrogen cracking is detected from the middle part (1/3 to 1 / 5R) of the steel ingot, it is inferior in hydrogen cracking resistance (x), and if not detected, it is excellent in hydrogen cracking resistance. (○). However, when the side surface (surface layer) in the steel ingot width direction is 0R and the center is 1 / 2R, the center portion (1/2 to 1 / 3R), the intermediate portion (1/3 to 1 / 5R), the surface layer portion Each part was defined as (0R to 1 / 5R).

  In the “Comprehensive evaluation” column, fill in (●) if the endurance limit ratio at the upper and lower parts of the steel ingot, and the hydrogen cracking test results at the upper and lower parts of the steel ingot are all (○). ×) is filled in.

  In addition, in the test numbers 41, 49, and 55 in Table 3, the endurance limit ratio (B) is (x) even though the number of coarse inclusions at the bottom of the steel ingot is lower than the standard. This is because hydrogen cracks occurred in these test pieces, and the fatigue strength decreased due to the cracks.

8A shows the density (D _TOP ) of the fine inclusions (major axis 5 to 10 μm) corresponding to the upper part of the steel ingot on the vertical axis and the coarse inclusions on the part corresponding to the upper part of the steel ingot on the horizontal axis. Take the density of (major axis 40μm or more), and the overall evaluation is (●) is indicated by (●), and (×) is (×) in either the endurance limit ratio or hydrogen cracking test results. Indicated. When (D _TOP ) is less than 20 pieces / cm ² , hydrogen cracking occurs and is (x).
In addition, when (D _TOP ) exceeds 90 pieces / cm ² and when the density of inclusions having a major axis of 40 μm or more exceeds 5 pieces / cm ² , a predetermined durability limit ratio is not obtained, and (× ).

In FIG. 8B, the vertical axis shows the density (D _BOT ) of the minute inclusions (major axis 5-10 μm) corresponding to the lower part of the steel ingot, and the horizontal axis shows the coarse inclusions of the part corresponding to the lower part of the steel ingot. Take the density of the product (major axis 40μm or more), and the overall evaluation is (●), it is indicated by (●), and the test result of the endurance ratio ratio or hydrogen cracking is (×) (×) It showed in. When (D _BOT ) is _less than 10 pieces / cm ² , hydrogen cracking occurs (x).
Further, when (D _BOT ) exceeds 80 / cm ² and when the density of inclusions having a major axis of 40 μm or more exceeds 5 / cm ² , a predetermined durability limit ratio is not obtained, and (× ).

FIG. 1 is a diagram showing a solidified state of a steel ingot manufactured by the ingot-making method. FIG. 2 is a view showing a steel ingot manufactured by the ingot-making method. FIG. 3 is an SEM photograph of a steel cross section observed at 2000 times. FIG. 4 is an SEM photograph of the steel cross section observed at 200 times. FIG. 5 is an SEM photograph of the steel cross section observed at 200 times. FIG. 6 is a diagram showing the hydrogen concentration in the upper part of the steel ingot and the lower part of the steel ingot. FIG. 7 is a diagram showing the evaluation results regarding the hydrogen cracking and endurance limit ratio of the steel ingot, where the vertical axis represents the ratio of the fine inclusion density at the upper and lower parts of the steel ingot, and the horizontal axis represents the S concentration in the steel. It is a thing. FIG. 8 is a diagram showing the endurance limit ratio of the steel ingot and the presence or absence of hydrogen cracking, where (a) relates to the upper portion of the steel ingot and (b) relates to the lower portion of the steel ingot.

Claims (1)

A forging steel ingot formed by a mold, which has a major axis of 5 to 5 observed at a steel cross section at the bottom of the steel ingot (the end in the direction of gravity and within 20% of the total height of the ingot from the end). The density of inclusions (D _BOT ) of 10 μm is 10 to 80 pieces / cm ² , and the upper part of the steel ingot (the end opposite to the lower part of the steel ingot, which is 20% of the total height of the steel ingot) The density (D _TOP ) of inclusions having a major axis of 5 to 10 μm observed in the steel cross section at 20 to 90 / cm ² in the steel section and the density of inclusions having a major axis of 40 μm or more observed in the steel cross section Is a steel ingot for forging which is 5 pieces / cm ² or less both in the lower part of the steel ingot and the upper part of the steel ingot and satisfies the chemical components of the following formula (1) and the following (2) .
However, [S] shows content (mass ppm) of S in steel.
(2) Chemical components
C: 0.2 to 0.6% (meaning mass%, hereinafter the same)
Si: 0.05-0.5%
Mn: 0.2 to 1.2%
Ni: 0.1 to 3.5%
Cr: 0.9 to 2.5%
Mo: 0.1 to 0.7%
V: 0.005-0.2%
Al: 0.01 to 0.1%
S: 0.005% or less (excluding 0%)
Ti: 0.005% or less (excluding 0%)
O: 0.0015% or less (excluding 0%)
The rest: iron and inevitable impurities