JP2013249497A - Steel for forgings excellent in hydrogen-crack resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique with which a hydrogen-crack resistance of a steel for forgings, which is composed of a carbon steel, can be improved by controlling structural formation without depending on a means such as the addition of expensive alloy elements.SOLUTION: A steel for forgings satisfies the prescribed component composition, and when the steel cross-sectional surface at the position of D/4 depth (D; the diameter corresponding to the circle in the cross-sectional surface of the steel for forgings) is observed in the measuring range of 5 cm×5 cm, the steel for forgings is composed of a sound part formed with a bainitic structure and the remainder (hereinafter, described as "macrosegregation part"). The ratio of the sound part to the steel cross-sectional surface is ≥90 area%, and the ratio of the bainitic structure to the sound part is ≥90 area%, and the ratio of the bainitic structure to the macrosegregation part is ≥90 area%. A sulphide-series inclusion having 1 μm or more of major axis of the macrosegregation part has ≤200 μm of an average circle equivalent diameter, ≤50 of an average aspect ratio and ≤300 pieces/cmof a numeral density. An oxide-series inclusion having 1 μm or more of the major axis of the macrosegregation part has ≤100 μm of an average circle equivalent diameter, ≤20 of an average aspect ratio and ≤300 pieces/cmof a numeral density.

Description

  The present invention relates to a forged steel material used as a member constituting equipment such as a pressure vessel and a steam generator used in a nuclear power generation facility.

  Forged steel materials have excellent strength and toughness, and thus have been suitably used as assembly members for devices such as pressure vessels and steam generators used in nuclear power generation facilities. On the other hand, the degree of increase in energy demand is more remarkable in recent years than in the past, and the above devices tend to be larger. For this reason, forged steel materials used as assembly members for the above-mentioned devices tend to increase in size, and are further required to be more excellent in strength and toughness, and also excellent in hydrogen cracking resistance.

  Regarding hydrogen cracking resistance, measures are being studied from both aspects of (a) steel refining technology and (b) steel component composition.

  (A) From the aspect of refining technology, an upper limit value of the amount of hydrogen at the time of refining molten steel is specified, and dehydrogenation treatment is carried out in actual operation when the upper limit value is exceeded. However, this dehydrogenation treatment is limited in reducing the amount of hydrogen in terms of treatment time and treatment cost. The dehydrogenation treatment is generally managed with a hydrogen amount of 1 to several ppm level. However, since hydrogen cracking occurs with a smaller amount of hydrogen, hydrogen cracking cannot be completely prevented with this level of control.

  (B) From the viewpoint of the steel component composition, by increasing the S content in the steel, MnS inclusions are actively introduced into the steel, and the MnS inclusions are trapped in diffusible hydrogen. Patent Document 1 proposes a method for improving the hydrogen cracking resistance by utilizing the above.

JP 2003-268438 A

  By the method disclosed in Patent Document 1, the hydrogen cracking resistance could be improved to some extent. However, when the S content is increased, coarse sulfide inclusions are formed, and there is a concern that the coarse sulfide inclusions do not become a hydrogen trap site but instead become a hydrogen cracking origin.

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is not to add an expensive alloy element, but to control the resistance of a forged steel material made of carbon steel by controlling the form of the structure. The object is to provide a technology capable of improving the hydrogen cracking property.

The forged steel material excellent in hydrogen cracking resistance according to the present invention that has solved the above problems is C: 0.15 to 0.24% (meaning mass%; the same applies to the component composition hereinafter), Si: 0.15 to 0.3%, Mn: 1 to 1.6%, P: 0.015% or less (excluding 0%), S: 0.0002 to 0.010%, Ni: 0.7 to 1.1%, Cr: 0.05-0.3%, Mo: 0.4-0.6%, Al: 0.015-0.03%, O: 0.003% or less (including 0%) N), 0.005 to 0.015%, and the balance: iron and inevitable impurities. And the said forged steel material is a healthy part comprised by a bainite structure, when the steel cross section in the position of depth D / 4 (D: circle equivalent diameter of a forged steel material cross section) is observed in the measurement range of 5 cm x 5 cm. And the balance (hereinafter referred to as “macro-segregation part”), the ratio of the healthy part to the steel cross section is 90% by area or more, and the ratio of the bainite structure to the healthy part is 90% by area or more, The ratio of the bainite structure to the macro segregation part is 90 area% or more, and the sulfide inclusions having a major axis of 1 μm or more in the macro segregation part have an average equivalent circle diameter of 200 μm or less, an average aspect ratio of 50 or less, and a number density. There 300 / cm ² or less, oxide inclusions major diameter of more than 1μm in the macro segregation is the average equivalent circular diameter of 100μm or less, an average aspect ratio of 20 or less, the number dense There has a gist in that it is 300 / cm ² or less.

  In the macro-segregation part, the number density of the oxide inclusions having a major axis of 1 μm or more is preferably equal to or higher than the number density of sulfide inclusions having a major axis of 1 μm or more.

The forged steel material, as another element,
(A) Cu: 0.1% or less (excluding 0%),
(B) V: 0.05% or less (excluding 0%),
(C) Ca: 0.01% or less (excluding 0%),
(D) Any one or more elements selected from the group consisting of Ti, Zr, and Hf: 0.01% or less in total (excluding 0%),
Etc. may be contained.

  The forged steel material can be used, for example, for nuclear power generation equipment.

  According to the present invention, by appropriately controlling the shape and number density of sulfide inclusions and oxide inclusions in the macrosegregation portion of the forged steel material, the starting point of cracks can be reduced. Forged steel with excellent hydrogen cracking ability can be provided.

FIG. 1 is a drawing-substituting photograph in which a macrosegregation portion appearing in a cross section of a forged steel material is photographed. FIG. 2 is a drawing-substituting photograph in which (a) a macro-segregation part and (b) a healthy part are photographed in a cross-section of the forged steel material. FIG. 3 is a schematic explanatory diagram showing a state in which a low strain rate test is performed.

  In the forged steel materials (particularly large forged steel materials), the present inventors have found that the macrosegregated portion of the alloy element is unavoidably different from the continuous cast material, and that the macrosegregated portion is generally more than the healthy portion. Focusing on the fact that there are many coarse inclusions that can become crack initiation points, we investigated the relationship between inclusions and hydrogen cracking in the macrosegregation zone. As a result, it was found that hydrogen cracking tends to occur starting from coarse inclusions due to the concentration of hydrogen in the macro-segregation part.

  This phenomenon is estimated to occur as follows. That is, in the macro-segregation part, the alloy elements are concentrated more than the healthy part, so coarse inclusions (for example, MnS) are formed more easily than the healthy part. From the viewpoint, it tends to be a high hardness structure that works in a disadvantageous direction. Further, due to the transformation to bainite at the time of cooling, the healthy part becomes almost a bainite structure, but in the macrosegregation part, the austenite having high hydrogen solid solubility remains due to the difference in transformation rate. Here, between bainite and austenite, there is a difference in hydrogen solubility and hydrogen diffusion rate, so that hydrogen is easily concentrated in austenite. When austenite enriched with hydrogen is transformed into bainite or martensite, it is presumed that hydrogen is enriched around coarse inclusions and in strained portions accompanying transformation, resulting in hydrogen cracking. Therefore, in order to improve the hydrogen cracking resistance of the forged steel material, it is considered effective to suppress the formation of these macrosegregated portions.

  However, with large forged steel materials (especially steel ingots of several tens of tons or more), it is virtually impossible to make the cooling rate uniform in all parts, so it is impossible to eliminate macro segregation in the steel ingot. In addition, there is a limit to reducing the macro segregation part.

  Therefore, the present inventors paid attention to inclusions in the macrosegregation part. Specifically, it suppresses the generation of coarse sulfide inclusions (for example, MnS, etc.) that are the starting point of hydrogen cracking, refines sulfide inclusions, lowers the aspect ratio, and further increases the long diameter. However, the origin of hydrogen cracking is reduced by reducing the number density of sulfide inclusions of 1 μm or more.

In the present invention, in addition to appropriately controlling the form and number density of the sulfide inclusions, it is important to further control the form and number density of the oxide inclusions. Since oxide inclusions generally have a nearly spherical shape, they are unlikely to be the starting point of hydrogen cracking even if they are dispersed in steel. However, when coarsened or aggregated, the same behavior as coarse sulfide inclusions is exhibited. Specifically, when the S content increases, oxide inclusions (for example, Al ₂ O ₃ , MgO, composite oxides of MgO and Al ₂ O ₃ , etc.) serve as nuclei, and sulfides around them are formed. Inclusions (for example, MnS, etc.) are precipitated to form composite inclusions of oxide and sulfide. As a result of further growth of the sulfide inclusions, the oxide inclusions are coarsened and expanded to increase the aspect ratio. Therefore, not only the amount of S in the steel but also the amount of O and Al are appropriately controlled to suppress the formation of oxide inclusions, refine the oxide inclusions, and reduce the aspect ratio. Furthermore, it has been found that the hydrogen cracking resistance of the forged steel can be improved by reducing the number density of oxide inclusions having a major axis of 1 μm or more.

  For reference, FIG. 1 shows a drawing-substituting photograph in which a macrosegregation portion appearing in a cross section of the forged steel material is photographed. FIG. 1A is a drawing-substituting photograph showing a streak-like macro-segregation portion that can be observed without using a microscope, and FIG. 1B is an enlarged view of the dotted line portion in FIG. It is the drawing substitute photograph shown. The part that appears whitish other than the streak-like macro-segregation part is a healthy part. Moreover, in FIG. 2, the drawing substitute photograph which image | photographed (a) macrosegregation part and (b) healthy part in the cross section of a forged steel material is shown.

The forged steel material excellent in hydrogen cracking resistance according to the present invention has a predetermined component composition, and measures a steel cross section at a depth D / 4 (D: circle equivalent diameter of a forged steel material cross section) of 5 cm × 5 cm. When observed in a range, it is composed of a healthy part composed of a bainite structure and a remaining part (macro-segregation part), and the ratio of the healthy part to the steel cross section is 90% by area or more. The ratio is 90 area% or more, the ratio of the bainite structure to the macrosegregation part is 90 area% or more, and the sulfide inclusions having a major axis of 1 μm or more in the macrosegregation part have an average equivalent circle diameter of 200 μm or less, the average aspect ratio of 50 or less, the number density was 300 / cm ² or less, oxide inclusions major diameter of more than 1μm in the macro segregation is a mean circle equivalent diameter of 100μm or less, the average Ass The transfected ratio 20 or less, in which the number density of 300 / cm ² or less. Hereinafter, the basic configuration of the forged steel material according to the present invention will be described.

  First, the component composition of the forged steel material of the present invention will be described.

1. Component composition of forged steel [C: 0.15 to 0.24%]
C is an element that contributes to improving the strength of the forged steel material. In order to ensure sufficient strength in the forged steel material, it is necessary to contain 0.15% or more of C, and preferably 0.17% or more. However, if the amount of C is too large, the toughness of the forged steel is deteriorated, so it is necessary to make it 0.24% or less. The amount of C is preferably 0.22% or less, more preferably 0.20% or less.

[Si: 0.15-0.3%]
Si is an element that acts as a deoxidizing element and as an element for improving the strength of forged steel. Therefore, in this invention, it is necessary to contain 0.15% or more, Preferably it is 0.18% or more. However, if the amount of Si is too large, the reverse V segregation of the forged steel material becomes remarkable and coarse inclusions are formed in the macro segregation part, so that the hydrogen cracking resistance cannot be improved. Therefore, the amount of Si needs to be 0.3% or less, preferably 0.27% or less, more preferably 0.25% or less.

[Mn: 1 to 1.6%]
Mn is an element that enhances the hardenability of the forged steel material and contributes to improving the strength. In order to ensure sufficient strength and hardenability, Mn must be contained in an amount of 1% or more, preferably 1.2% or more. However, when the amount of Mn becomes excessive, reverse V segregation is promoted and coarse sulfide-based inclusions are formed in the macrosegregation part, so that the hydrogen cracking resistance cannot be improved. Therefore, the amount of Mn needs to be 1.6% or less, preferably 1.5% or less.

[P: 0.015% or less (excluding 0%)]
P is an impurity element that is inevitably mixed in, and is an element that adversely affects toughness. Therefore, its content is preferably as small as possible. From such a viewpoint, the P content needs to be suppressed to 0.015% or less, and preferably 0.01% or less. However, it is difficult to make P in steel 0% industrially.

[S: 0.0002 to 0.010%]
S is an element that forms sulfide-based inclusions by combining with Mn, Mg, Ca, etc. in steel, and the sulfide inclusions in the macro-segregation part form a large number of stress fields to generate surplus hydrogen. Captures and has the effect of improving hydrogen cracking resistance. In order to ensure such sulfide inclusions, the amount of S must be 0.0002% or more, preferably 0.0004% or more, and more preferably 0.001% or more. However, S promotes reverse V segregation to form sulfide-based inclusions, and in particular, sulfide-based inclusions having an elongated shape tend to be coarse inclusions having a large major axis, and become a starting point for hydrogen cracking. Therefore, in order to reduce coarse sulfide inclusions, the S amount needs to be 0.010% or less, preferably 0.009% or less, more preferably 0.0060% or less.

[Ni: 0.7 to 1.1%]
Ni is an element useful as an element for improving the toughness of the forged steel, and needs to be contained by 0.7% or more. The amount of Ni is preferably 0.80% or more. However, an excessive amount of Ni causes an excessive increase in strength and adversely affects toughness. Therefore, the amount of Ni needs to be 1.1% or less, preferably 1.05% or less, more preferably 1.00% or less.

[Cr: 0.05 to 0.3%]
Cr is an element that enhances the hardenability of the forged steel material and improves the toughness, and exerts its action by containing 0.05% or more. The amount of Cr is preferably 0.1% or more. However, when the amount of Cr is excessive, reverse V segregation is promoted and coarse carbides are formed in the macrosegregation part, so that the hydrogen cracking resistance cannot be improved. Therefore, the Cr amount needs to be 0.3% or less, preferably 0.27% or less, more preferably 0.25% or less.

[Mo: 0.4 to 0.6%]
Mo is an element that acts to improve the hardenability, strength, and toughness of the forged steel material. In order to exert these effects, it is necessary to contain 0.4% or more. The Mo amount is preferably 0.45% or more, more preferably 0.50% or more. However, since Mo has a small equilibrium distribution coefficient, when the amount of Mo becomes excessive, microsegregation (normal segregation) is likely to occur. Moreover, when the amount of Mo becomes excessive, it leads to a cost increase. Therefore, the Mo amount needs to be 0.6% or less, preferably 0.55% or less.

[Al: 0.015 to 0.03%]
Al acts as a deoxidizing element in the steel making process and is an element necessary for reducing the amount of oxygen. Moreover, Al also has the effect | action which improves the crack resistance of a forged steel material. Accordingly, Al needs to be contained in an amount of 0.015% or more, preferably 0.018% or more. However, when the amount of Al becomes excessive, the amount of Al ₂ O ₃ formed increases or the produced Al ₂ O ₃ tends to aggregate, which adversely affects hydrogen cracking resistance. Therefore, the Al amount needs to be 0.03% or less, preferably 0.028% or less, more preferably 0.026% or less.

[O: 0.003% or less (not including 0%)]
O (oxygen) is an element that forms oxide inclusions such as SiO ₂ , Al ₂ O ₃ , MgO, and CaO. By reducing O as much as possible, formation of coarse inclusions can be suppressed and fine inclusions can be precipitated. Therefore, the O amount needs to be 0.003% or less, preferably 0.0020% or less, more preferably 0.0015% or less. However, it is difficult to reduce O to 0% in industrial production.

[N: 0.005 to 0.015%]
N is an element that forms a nitride together with Al or Ti and V added as necessary, and has an effect of improving toughness and hydrogen cracking property. In order to exert the effect, it is necessary to contain 0.005% or more, preferably 0.008% or more. However, if its content is excessive, it causes strain aging as a solid solution N and adversely affects toughness. Therefore, the N amount needs to be 0.015% or less, preferably 0.013% or less.

  The component composition of the forged steel material according to the present invention is as described above, and the remaining component is substantially iron, but it is of course acceptable to mix inevitable impurities.

It is preferable that the amount of Mn, the amount of S, the amount of Al, and the amount of O (oxygen) contained in the forged steel material satisfy the following formula (1). In the following formula (1), [] indicates the content (% by mass) of each element.
([Mn] × [S]) / ([Al] × [O]) ≦ 600 (1)

The product of Mn and S means easy generation of sulfide inclusions, and the product of Al and O means easy generation of oxide inclusions. In the present invention, the value of ([Mn] × [S]) / ([Al] × [O]) is preferably controlled to 600 or less, thereby reducing the amount of sulfide inclusions and oxide inclusions produced. The balance can be controlled appropriately, and the sizes of sulfide inclusions and oxide inclusions can also be adjusted appropriately. It is more preferable that the amount of Mn, S, Al, and O (oxygen) contained in the forged steel material satisfy the following formula (1a), and satisfy the following formula (1b). Further preferred.
([Mn] × [S]) / ([Al] × [O]) ≦ 500 (1a)
([Mn] × [S]) / ([Al] × [O]) ≦ 400 (1b)

  The forged steel material of the present invention may further contain other elements (selective elements) shown below as long as the above-described effects of the present invention are not adversely affected.

[Cu: 0.1% or less (excluding 0%)]
Cu is an element having an effect of improving the toughness of a forged steel material and a refinement of the structure. In order to effectively exhibit such an effect, it is preferable to contain 0.01% or more of Cu. Cu is more preferably 0.05% or more. However, when the amount of Cu is excessive, it causes an excessive increase in strength and an increase in hard structures such as martensite, which causes deterioration of toughness. Therefore, the amount of Cu is preferably suppressed to 0.1% or less, and more preferably 0.05% or less.

[V: 0.05% or less (excluding 0%)]
V has an effect of precipitation strengthening of a forged steel material and refinement of a structure, and is an element useful for increasing the strength of a forged steel material. In order to effectively exhibit such an action, V is preferably contained in an amount of 0.01% or more, and more preferably 0.02% or more. However, when V is contained excessively, the oxide inclusions are coarsened and the toughness is adversely affected. Therefore, the V amount is preferably 0.05% or less, more preferably 0.03% or less.

[Ca: 0.01% or less (excluding 0%)]
Ca is an element that contributes to suppressing the extension of sulfide inclusions in the forged steel material and improving the resistance to hydrogen cracking. In order to effectively exhibit this action, Ca is preferably contained in an amount of 0.0001% or more, and more preferably 0.001% or more. However, since this action is saturated even if the Ca amount is excessive, the Ca amount is preferably 0.01% or less, more preferably 0.008% or less.

[One or more elements selected from the group consisting of Ti, Zr, and Hf: 0.01% or less in total (excluding 0%)]
Ti, Zr, and Hf have the effect of improving the toughness of the forged steel material and refining the structure, and are useful elements for increasing the strength of the forged steel material. In order to effectively exhibit these actions, any one or two or more elements may be contained in the steel. In order to effectively exhibit these actions, the total (in the case of one kind, a single content) is preferably 0.001% or more, more preferably 0.003% or more. However, when the amounts of Ti, Zr, and Hf are excessive, coarse carbides are precipitated, and mechanical properties and hydrogen cracking resistance may be deteriorated. Therefore, the total of the above elements (in the case of one kind, a single content) is preferably 0.01% or less, more preferably 0.008% or less.

  Examples of other elements that allow positive addition include (1) B (boron) having an effect of improving hardenability, (2) W, Nb, Ta, Ce, which are solid solution strengthening elements or precipitation strengthening elements, and Te etc. are mentioned. These additive elements can be contained alone or in combination of two or more. These additive elements are desirably about 0.1% or less in total.

2. Structure ratio of forged steel (1) Basic structure When the steel cross section at the position of depth D / 4 is observed in a measurement range of 5 cm x 5 cm, the steel structure is from the viewpoint of strength, toughness, and resistance to hydrogen cracking. It must be composed of a bainite structure. For example, in a ferrite structure, the strength is insufficient, and if there are many untransformed austenite structures and martensite structures, not only does the toughness decrease, but those structures tend to concentrate hydrogen, so Hydrogen cracking occurs at low hydrogen concentrations. In the steel structure, a portion where the alloy components such as Si, Mn, Ni, Cr, and Mo are not segregated is referred to as a healthy part in the present specification, and the alloy components such as Si, Mn, Ni, Cr, and Mo are present. The segregated portion is referred to as a macro segregation portion in this specification.

  In the forged steel according to the present invention, from the viewpoint of strength, toughness, and hydrogen cracking resistance, the ratio of the healthy part to the steel cross section needs to be 90 area% or more, preferably 95 area% or more, more preferably It is 97 area% or more, and particularly preferably 99 area% or more.

  The ratio of the bainite structure to the healthy part is preferably 90 area% or more, more preferably 95 area% or more, still more preferably 97 area% or more, and particularly preferably 99 area% or more. In addition to the bainite structure, ferrite, martensite, residual γ, and the like may be mixed in the healthy portion.

  The ratio of the remaining part (macro-segregation part) other than the healthy part to the steel cross section needs to be 10 area% or less, preferably 5 area% or less, more preferably 3 area% or less, and particularly preferably 1 area%. It is as follows.

  The ratio of the bainite structure to the macrosegregation portion is preferably 90 area% or more, more preferably 95 area% or more, still more preferably 97 area% or more, and particularly preferably 99 area% or more. In addition to the bainite structure, ferrite, martensite, residual γ, and the like may be mixed in the macro segregation part.

  In the present invention, the position of the depth D / 4 refers to the distance from the side surface of the forged steel material. The “D” means a circle-equivalent diameter (diameter of a circle having the same area as the cross-sectional area) of a cross-section of the forged steel (cross-section perpendicular to the longitudinal direction of the forged steel). Therefore, if the forged steel material is cylindrical, D is the diameter of the cylindrical forged steel material.

(2) Discrimination between healthy part and macro-segregation part A method for discriminating between a healthy part and a macro-segregation part will be described. First, a test material is cut out from a steel ingot, and the cut cross section to be discriminated is polished and etched, and the macro segregation portion is clearly distributed and photographed. A macro-segregation part refers to a part where an alloy component such as Si, Mn, Ni, Cr, Mo is segregated, and is more than a healthy part where an alloy component such as Si, Mn, Ni, Cr, Mo is not segregated. It looks dark. Using this fact, the healthy part and the macro-segregated part are discriminated.

  Next, the photograph of the cut section is taken into an electronic device as gray scale data, and the ratio (area%) of the healthy portion to the steel cross section (entire metal structure) is determined by distinguishing between the healthy portion and the macro-segregated portion. In the present invention, the proportion of the healthy part is 90 area% or more as described above. On the other hand, the ratio (area%) of the macro segregation part is a value obtained by subtracting the ratio (area%) of the healthy part from 100 area%. What is necessary is just to measure the structure and area ratio of a steel structure in the procedure shown in the Example mentioned later.

3. Inclusions in Macro Segregation Part Inclusions (sulfide inclusions and oxide inclusions) in the macro segregation part serve as starting points and propagation paths for hydrogen cracking and adversely affect the hydrogen cracking resistance of the forged steel. Therefore, in the forged steel of the present invention, the form of sulfide inclusions and oxide inclusions (equivalent circle diameter and aspect ratio) in the macro-segregation part, and the number density of sulfide inclusions and oxide inclusions are appropriately set. I have control.

  In addition, since there are few sulfide inclusions and oxide inclusions in the healthy part, it is not necessary to consider the form and number density of the sulfide inclusions and oxide inclusions in the healthy part.

(Sulfide inclusions)
With respect to sulfide inclusions having a major axis of 1 μm or more in the macrosegregation portion, if the average equivalent circle diameter exceeds 200 μm, hydrogen cracking tends to occur at the interface between the sulfide inclusions and the steel structure. Accordingly, the sulfide inclusions having a major axis of 1 μm or more in the macrosegregation part have an average equivalent circle diameter of 200 μm or less, preferably 160 μm or less, more preferably 100 μm or less, and still more preferably 50 μm or less.

  For sulfide inclusions having a major axis of 1 μm or more in the macrosegregation part, when the average aspect ratio exceeds 50, due to the difference in linear expansion coefficient between the sulfide inclusions and the steel structure when cooling the steel ingot, The interface between the sulfide inclusions and the steel structure is largely separated, causing hydrogen to aggregate in the voids and causing hydrogen cracking. Therefore, the sulfide inclusions having a major axis of 1 μm or more in the macro segregation part have an average aspect ratio of 50 or less, preferably 40 or less, more preferably 30 or less. The lower limit of the average aspect ratio of the sulfide inclusions is usually about 3.

When the number density exceeds 300 / cm ² for the sulfide inclusions having a major axis of 1 μm or more in the macrosegregation part, mechanical properties such as toughness are adversely affected. Therefore, the number density of sulfide inclusions having a major axis of 1 μm or more in the macrosegregation portion needs to be 300 pieces / cm ² or less, preferably 200 pieces / cm ² or less, more preferably 100 pieces / cm ² or less. To do. The lower limit of the number density of the sulfide inclusions is not particularly limited, but when sulfide inclusions are formed in the macrosegregation part, a large number of stress fields are dispersed, so the surplus exceeding the solid solubility limit is formed. Hydrogen can be easily captured, hydrogen concentration in the strained part can be suppressed, and hydrogen cracking resistance can be improved. Therefore, the number density of the sulfide inclusions is preferably 5 pieces / cm ² or more, more preferably 10 pieces / cm ² or more, and further preferably 20 pieces / cm ² or more.

(Oxide inclusions)
Unlike the sulfide inclusions described above, oxide inclusions are usually fine and have a low aspect ratio (a shape close to a circle) and are therefore considered not to adversely affect hydrogen cracking resistance. Yes. However, for oxide inclusions having a major axis of 1 μm or more in the macrosegregation portion, when the average equivalent circle diameter exceeds 100 μm, the oxide inclusions are aggregated, and like the sulfide inclusions, the oxide inclusions Hydrogen cracking is likely to occur at the interface between the inclusion and the steel structure. Therefore, the oxide inclusions having a major axis of 1 μm or more in the macrosegregation part have an average equivalent circle diameter of 100 μm or less, preferably 50 μm or less, more preferably 15 μm or less.

  With respect to oxide inclusions having a major axis of 1 μm or more in the macrosegregation portion, when the average aspect ratio exceeds 20, the oxide inclusions and the steel structure are cooled when the steel ingot is cooled, similar to the sulfide inclusions. The interface between the oxide inclusions and the steel structure is largely separated due to the difference in the linear expansion coefficient between the two and hydrogen, causing hydrogen to aggregate in the voids and causing hydrogen cracking. Therefore, the average aspect ratio of the oxide inclusions is 20 or less, preferably 15 or less, more preferably 10 or less. The lower limit of the average aspect ratio of the oxide inclusions is usually 1.

When the number density of the oxide inclusions having a major axis of 1 μm or more in the macrosegregation portion exceeds 300 / cm ² , mechanical properties such as toughness are adversely affected. Therefore, the number density of oxide inclusions having a major axis of 1 μm or more in the macrosegregation portion needs to be 300 pieces / cm ² or less, preferably 200 pieces / cm ² or less, more preferably 100 pieces / cm ² or less. To do. The lower limit of the number density of the oxide inclusions is not particularly limited, but when the oxide inclusions are formed in the macrosegregation part, a large number of stress fields are dispersed, so surplus hydrogen exceeding the solid solubility limit is formed. Can be easily captured, hydrogen concentration in the strained portion can be suppressed, and resistance to hydrogen cracking can be improved. Therefore, the number density of the oxide inclusions is preferably 5 pieces / cm ² or more, more preferably 20 pieces / cm ² or more, and further preferably 40 pieces / cm ² or more.

  The form of the sulfide inclusions and oxide inclusions (equivalent circle diameter and aspect ratio) and the number density of sulfide inclusions and oxide inclusions in the macrosegregation portion can be measured by the following procedure. . That is, a test piece was collected from a macrosegregation part existing at a depth D / 4 position of the forged steel material, and the surface of the test piece was subjected to SEM-EPMA (for example, “JXA-8900RL”, “XM” manufactured by JEOL Ltd.). -Z0043T "," XM-87562 ")), and the component composition of the observed inclusions is analyzed by EDX. The analysis of the component composition is performed for inclusions having a major axis of 1 μm or more, and the analysis by EDX may be performed by automatically analyzing the center of gravity of the inclusions for about 10 seconds per point. Sulfide inclusions and oxide inclusions whose major axis is less than 1 μm also have the effect of improving the hydrogen cracking resistance. However, in order to improve the measurement efficiency, the present invention uses an inclusion whose major axis is less than 1 μm. Objects are excluded from measurement.

  Among the inclusions analyzed for the component composition, those having a sulfur (S) content of 30% by mass or more and an oxygen (O) content of less than 1% by mass are defined as sulfide inclusions in this specification. . Further, among the inclusions analyzed for the component composition, those having an oxygen (O) content of 1% by mass or more are defined as oxide inclusions in this specification. For the sulfide inclusions and oxide inclusions identified by analyzing the component composition, the equivalent circle diameter and the aspect ratio (major axis / minor axis) may be measured to determine the average value.

In addition, the number density of sulfide inclusions and oxide inclusions is the sulfide inclusions having a major axis of 1 μm or more among the sulfide inclusions and oxide inclusions identified by analyzing the component composition and The number of oxide inclusions is measured, and the number density (units / cm ² ) per unit observation visual field area is calculated. As described above, sulfide inclusions and oxide inclusions having a major axis of less than 1 μm also have an effect of improving hydrogen cracking resistance, but measurement efficiency decreases when measuring too fine inclusions. . Therefore, in the present invention, inclusions whose major axis is less than 1 μm are excluded from the measurement target.

  For example, the number of observation fields is preferably 5 or more.

(Number density of sulfide inclusions and oxide inclusions)
In the forged steel material of the present invention, the number density of oxide inclusions is preferably equal to or higher than the number density of sulfide inclusions in the macrosegregation portion. That is, the number density of oxide inclusions is the same as the number density of sulfide inclusions, or the number density of oxide inclusions exceeds the number density of sulfide inclusions, the starting point of cracking is reduced, Since hydrogen trap ability can be improved, hydrogen cracking resistance can be improved. Therefore, the value of the number density of oxide inclusions / number density of sulfide inclusions is preferably 1.0 or more, more preferably 1.1 or more. The upper limit of the value of the number density of the oxide inclusions / number density of the sulfide inclusions is not particularly limited.

4). Manufacturing Method (1) Control of Structure The forged steel material of the present invention can be manufactured by controlling the steel structure and inclusions as follows, for example.

  In order to change the metal structure of the forged steel material to a bainite structure, macro segregation is performed after hot forging a steel satisfying the above-described composition under normal conditions (for example, heating at 1000 to 1300 ° C., any amount of processing strain). What is necessary is just to control a cooling rate so that the structure | tissue of a part may carry out a bainite transformation, and to hold | maintain in the bainite transformation temperature vicinity. Steel that satisfies the above component composition can be made into a bainite structure by air cooling after hot forging, but the macrosegregation part is bainite, martensite, and bainite transformation is incomplete and the bainite area ratio is low. Since it tends to be a mixed structure of residual austenite, the macrosegregation part can also be formed into a bainite structure by maintaining it in the vicinity of the bainite transformation temperature (for example, 250 to 350 ° C.). The cooling rate, holding temperature, and holding time may be set by preparing in advance a steel material that is a component system corresponding to the macro-segregation part and investigating the continuous cooling transformation behavior.

  In order to control the form and number density of sulfide inclusions and oxide inclusions in the macrosegregation portion of the forged steel material within a predetermined range, when manufacturing the test material to be subjected to the hot forging, molten steel What is necessary is just to reduce the amount of S and O in the inside. That is, S in steel combines with Mn, Mg, Ca, etc. to form sulfide inclusions. Among sulfide inclusions, MnS that has been expanded and coarsened (high aspect ratio) has a particularly adverse effect on hydrogen cracking. Therefore, in order to suppress the coarsening of sulfide inclusions, it is only necessary to reduce the amount of S in the molten steel. By reducing the amount of S, the number of sulfide inclusions themselves is reduced, and further growth is suppressed. Therefore, it is miniaturized (low aspect ratio).

The method for reducing the amount of S in the molten steel is not particularly limited, and a known method can be adopted. The slag composition may be set higher basicity calculated based on the amount of CaO and SiO ₂ content in the slag (CaO / SiO _2), basicity, for example, set in the range of 5 to 10 It is recommended.

  In order to further promote the reaction between the molten steel and the slag, it is preferable to perform a vacuum degassing treatment. By performing the vacuum degassing treatment, slag can be actively wound in the molten steel, so that the reaction between the molten steel and slag can be promoted, and the amount of S in the molten steel can be reduced.

  Further, Ca may be added in order to reduce sulfide inclusions (for example, MnS) in the macro-segregation part and reduce the aspect ratio. Ca is an element having an action of spheroidizing sulfide inclusions.

In order to refine the sulfide inclusions in the macro segregation part, the final pass in the hot forging is performed at a temperature of Ac ₃ point + 30 ° C. or higher, or the test material after hot forging is Ac _3. It is preferable to heat and hold at a temperature of point + 30 ° C. or higher. By heating to a temperature of Ac ₃ point + 30 ° C. or higher, the element concentrated in the macro-segregation part can be diffused and made uniform.

The Ac ₃ point can be obtained using the following formula (a) [see, for example, “Leslie Steel Material Science” Maruzen, (1985)]. In the following formula, [] indicates the content (% by mass) of each element. In addition, when it does not contain the element shown in each term, it is calculated that there is no term.
Ac ₃ points (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V] −11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15.2 × [Ni] (a)

  Moreover, what is necessary is just to reduce the amount of O in molten steel, in order to reduce the production amount of an oxide inclusion. That is, O in steel combines with Al, Mg and the like to form oxide inclusions. Oxide inclusions are usually difficult to coarsen like the above sulfide inclusions, so it is difficult to adversely affect hydrogen cracking resistance, but when the amount of oxide inclusions increases and aggregates, Like the sulfide inclusions, the hydrogen cracking is adversely affected. Therefore, it is recommended that the amount of oxide inclusions be reduced by reducing the amount of O in the molten steel.

  By reducing the production amount of oxide inclusions, the production amount of inclusions in which oxide inclusions and sulfide inclusions are combined can be reduced, so that hydrogen cracking resistance can be further improved. This is because sulfides are easily formed around oxide inclusions as product nuclei, and inclusions in which oxide inclusions and sulfide inclusions are combined adversely affect hydrogen cracking.

  A method for reducing the amount of O in the molten steel is not particularly limited, and a known method can be employed. For example, vacuum degassing may be sufficiently performed during melting.

  Finally, after heating to 900 to 1000 ° C., quenching at an average cooling rate of 10 ° C./min or more, further heating to 550 to 700 ° C. and holding for 1 to 20 hours, natural cooling to room temperature (average) Tempering may be performed at a cooling rate of, for example, 5 ° C./min or less.

  The forged steel material of the present invention is particularly useful for nuclear power generation equipment.

  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, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Steel with the component composition shown in Table 1 below (with the balance being iron and unavoidable impurities) is melted by a molten steel processing facility equipped with an electrode arc heating function, and a 40 ton class (total height 3 m, diameter 1.5 m) mold is used. And cast. The amount of hydrogen at the molten metal stage was 3 ppm as measured by the Hydris. The slag composition at the time of melting was adjusted so that the basicity (CaO / SiO ₂ ) in the slag was 5-10. Moreover, the vacuum degassing process was also performed at the time of melting. Table 1 below shows the results of calculating the value of ([Mn] × [S]) / ([Al] × [O]) based on the steel component composition and the above formula (1).

The solidified steel ingot is demolded at around 1000 ° C and then heated to about 1200 ° C. The final pass is hot forged at a processing rate of 15% with a temperature of Ac ₃ point + 30 ° C or higher, and a forged product with a cross-section diameter of 150 mm. Finished. The obtained forged product is cooled with a fan to room temperature (average cooling rate is about 5 ° C./min) or gradually cooled (average cooling rate is about 1 ° C./min), and then to 200 ° C., 300 ° C. or 400 ° C. After heating and holding at this temperature for 24 hours, it was naturally cooled slowly to room temperature (average cooling rate was about 0.5 ° C./min) to produce a forged steel material.

  About the forged steel obtained by cooling to room temperature, the metal structure of the steel cross section in the position of depth D / 4 was observed. For observation of the metal structure, an EBSP (Electron Back Scatter Diffraction Pattern) detector was provided after taking a sample from a position of depth D / 4, performing electropolishing to prevent transformation of retained austenite. The type of metal structure and its area ratio were measured with an FE-SEM (Field Emission type Scanning Electron Microscope). The EBSP determines the crystal orientation of the electron beam incident position by making an electron beam incident on the sample surface and analyzing the Kikuchi pattern obtained from the reflected electrons generated at this time. The orientation distribution on the sample surface can be measured by scanning the sample surface in two dimensions with the electron beam and measuring the crystal orientation at every predetermined pitch.

Regarding the sample set in the above-mentioned FE-SEM column, in a 5 cm × 5 cm region of the steel cross section, among the portions where the brightness is relatively high (white) in the metal structure observation described above, a healthy part is possible in advance. The electron beam is irradiated at 1 μm intervals in the high measurement range of 500 μm × 500 μm, the EBSP image projected on the screen is taken with a high-sensitivity camera, captured as a computer image, and image analysis is performed with the computer. Each hue (each texture) was color-mapped by comparing with a pattern obtained by simulation using a known crystal system. The area ratio of each tissue (region) mapped in this way was determined. As the hardware and software related to the above analysis, TexSEM Laboratories Inc. OIM (Orientation Imaging Microscopy ^™ ) system. As a result, the ratio of the healthy part with respect to the steel cross section was 90 area% or more in any forged steel material.

  Table 2 below shows the ratio (area ratio) of the bainite structure to the healthy part and the ratio (area ratio) of the bainite structure to the macrosegregation part.

  Further, the sample collected from the position of the depth D / 4 was etched with a hydrochloric acid aqueous solution to observe the distribution of the macrosegregation part.

  The macro-segregation part is observed by automatic EPMA according to the procedure described above, and the equivalent circle diameter is determined for sulfide inclusions having a major axis of 1 μm or more observed in the macro-segregation part or oxide inclusions having a major axis of 1 μm or more. And the aspect ratio was measured, and the average value was obtained for each. Further, the number density of sulfide inclusions having a major axis of 1 μm or more with respect to the observation visual field area and the number density of oxide inclusions having a major axis of 1 μm or more were also calculated.

  Further, the number density ratio of oxide inclusions (number density of oxide inclusions / number density of sulfide inclusions) is calculated with respect to the number density of sulfide inclusions in the macrosegregation part, and is also shown in Table 2 below.

  Next, the hydrogen cracking resistance was evaluated by the following procedure for the sample collected from the position of depth D / 4.

  The evaluation of hydrogen cracking resistance was performed based on a comparative test method for hydrogen cracking sensitivity. The apparatus shown in FIG. 3 was used for the comparative test of hydrogen cracking sensitivity. In FIG. 3, 1 is a test piece, 2 is a container, 3 is a test aqueous solution, 4 is a counter electrode (platinum electrode), 5 is a current control device, 6 is a crosshead, 7 is a fixed base, 8 is a personal computer, 9 is 1 shows a stress-strain control device.

As shown in FIG. 3, the hydrogen cracking susceptibility comparison test has a length of 150 mm, the gripping part at both ends is 8 mm in diameter, a screw is provided over a length of 15 mm, and the distance between marked lines is processed into a dumbbell shape with a distance of 10 mm. The test was performed using a test piece 1 having a central portion having a diameter of 4 mm and a macro-segregation portion located in this portion. The test piece 1 is mounted on a test apparatus and completely immersed in the aqueous solution so as to be surrounded by the aqueous test solution 3 which is 0.1 mol / L H ₂ SO ₄ +0.01 mol / L KSCN. Cathodic electrolysis was performed at a density of 0.1 mA / mm ^{2 for} 4 hours to make the hydrogen distribution in the test piece uniform. Then, applying the current density as it is, that is, applying a tensile load in the long axis direction to the specimen by SSRT (low strain rate test: stress increase type test instead of constant or constant amplitude) while cathodic electrolysis. The stress (S ₁ ) was measured. At this time, the tensile speed of the crosshead 6 of the test apparatus was 2 × 10 ⁻³ mm / min.

On the other hand, the SSRT test was carried out in the state where the immersion in the aqueous solution was omitted, that is, in the atmosphere, under the same tensile conditions as described above, and the breaking stress (S ₀ ) of the test piece was measured.

Then, a hydrogen cracking sensitivity index (S ₁ / S ₀₎ from the above measured values were evaluated hydrogen cracking resistance of the steel. The results are shown in Table 2. In the present invention, a hydrogen cracking susceptibility index (S ₁ / S ₀ ) of 0.70 or more was evaluated as excellent in hydrogen cracking resistance.

The following Table 1 and Table 2 can be considered as follows. No. 2, 3, 5, 6, 8, 10-12, and 20 are examples satisfying the requirements defined in the present invention, and a healthy part composed of a bainite structure and sulfide inclusions And the oxide inclusions and the macrosegregation part in which the number density satisfies the predetermined range, the hydrogen cracking susceptibility index (S ₁ / S ₀ ) is 0.70 or more, and hydrogen resistance It turns out that it is excellent in cracking property.

On the other hand, no. 1, 4, 7, 9, 13 to 19 are examples that do not satisfy the requirements defined in the present invention, and the hydrogen cracking sensitivity index (S ₁ / S ₀ ) is less than 0.70, and hydrogen cracking resistance It turns out that it has not improved.

  That is, no. 1-4 are examples in which the same steel type A is used. Nos. 2 and 3 improved the hydrogen cracking resistance. Since 1 and 4 were insufficiently transformed into a bainite structure, the ratio of the bainite structure in the macro-segregation portion was less than 90 area%, and the hydrogen cracking resistance could not be improved. No. 3 and no. No. 4 is the same in that steel type A is used and the holding temperature is 400 ° C. No. 3, because the cooling rate after hot forging was reduced, it was held in the bainite transformation region, the transformation to the bainite structure proceeded sufficiently, and the hydrogen cracking resistance was improved, No. No. 4 has a high cooling rate after hot forging and insufficient transformation to a bainite structure, and therefore could not improve hydrogen cracking resistance.

  No. Nos. 7 to 9 are examples using the same steel type D. No. 8 was able to improve the resistance to hydrogen cracking. 7 and 9 were insufficiently transformed into a bainite structure, so that the ratio of the bainite structure in the macro-segregation portion was less than 90% by area, and the hydrogen cracking resistance could not be improved.

  No. 13 is an example containing excessive S, and a large number of coarse sulfide inclusions were precipitated in the macro-segregation part, and the average aspect ratio was also increased. In addition, the number density of sulfide inclusions also increased. Moreover, the average aspect ratio of the oxide inclusions also increased. Therefore, the hydrogen cracking resistance could not be improved.

  No. No. 14 is an example containing excessive Mn, and a large number of coarse sulfide inclusions were precipitated in the macro-segregation part, and the average aspect ratio was also increased. In addition, the number density of sulfide inclusions also increased. Moreover, the average aspect ratio of the oxide inclusions also increased. Therefore, the hydrogen cracking resistance could not be improved.

No. 15 is an example containing O excessively, and a large number of oxide inclusions (for example, Al ₂ O ₃ ) were precipitated. For this reason, the precipitated oxide inclusions aggregated and exerted an action similar to that of coarse sulfide inclusions, so that the hydrogen cracking resistance could not be improved.

  No. No. 16 is an example containing excessive Al, and a large number of coarse oxide inclusions precipitated in the macro-segregation part. Therefore, the hydrogen cracking resistance could not be improved.

  No. Nos. 17 to 19 are examples using the same steel type L, which is an example that contains S excessively and has too little Al content. In these examples, hydrogen cracking resistance could not be improved because any of the average equivalent circle diameter, average aspect ratio, and number density of the sulfide inclusions was out of the range defined in the present invention.

1 Test piece 2 Container 3 Test aqueous solution 4 Counter electrode (platinum electrode)
5 Current control device 6 Crosshead 7 Fixed base 8 Personal computer 9 Stress-strain control device

Claims (7)

C: 0.15-0.24% (meaning mass%; the same applies to the component composition hereinafter),
Si: 0.15-0.3%,
Mn: 1 to 1.6%
P: 0.015% or less (excluding 0%),
S: 0.0002 to 0.010%,
Ni: 0.7-1.1%
Cr: 0.05 to 0.3%,
Mo: 0.4 to 0.6%,
Al: 0.015-0.03%,
O: 0.003% or less (excluding 0%),
N: 0.005 to 0.015% is contained,
The rest: a forged steel material consisting of iron and inevitable impurities,
When the steel cross section at a position of depth D / 4 (D: equivalent circle diameter of the cross section of the forged steel material) was observed in a measurement range of 5 cm × 5 cm, a healthy part and a remaining part (hereinafter, “macro” Segregation part ”),
The ratio of the healthy part to the steel cross section is 90 area% or more,
The ratio of the bainite structure to the healthy part is 90 area% or more,
The ratio of the bainite structure to the macro-segregation part is 90 area% or more,
The sulfide inclusions having a major axis of 1 μm or more in the macrosegregation part have an average equivalent circle diameter of 200 μm or less, an average aspect ratio of 50 or less, and a number density of 300 pieces / cm ² or less.
The oxide inclusions having a major axis of 1 μm or more in the macrosegregation part have an average equivalent circle diameter of 100 μm or less, an average aspect ratio of 20 or less, and a number density of 300 / cm ² or less. Forged steel with excellent crackability.   2. The forged steel material according to claim 1, wherein, in the macrosegregation portion, the number density of oxide inclusions having a major axis of 1 μm or more is greater than or equal to the number density of sulfide inclusions having a major axis of 1 μm or more. As other elements,
Cu: 0.1% or less (excluding 0%)
The forged steel material according to claim 1 or 2, comprising: As other elements,
V: 0.05% or less (excluding 0%)
The forged steel material in any one of Claims 1-3 containing. As other elements,
Ca: 0.01% or less (excluding 0%)
The forged steel material in any one of Claims 1-4 containing. As other elements,
One or more elements selected from the group consisting of Ti, Zr, and Hf: 0.01% or less in total (excluding 0%)
The forged steel material according to claim 1, comprising:   It is for nuclear power generation equipment, The forged steel material in any one of Claims 1-6.