JP2011042812A - Method for manufacturing forged steel article superior in toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a forged steel article which has higher toughness and lower temper embrittlement sensitivity than those of an NiCrMoV-containing steel that are used in a steam turbine rotor material, a gas turbine disk material and the like. <P>SOLUTION: A forged steel member comprises, by mass%, 0.25-0.35% C, 0.01-0.20% Si, 0.01-0.50% Mn, 4.10-4.70% Ni, 1.6-2.0% Cr, 0.30-0.50% Mo, 0.07-0.15% V and the balance Fe with unavoidable impurities. The method for manufacturing the forged steel article includes quenching the above forged steel member at 800-1,000&deg;C, and tempering the quenched member at 500-700&deg;C. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention is a steel material that has high strength and toughness after heat treatment and does not cause temper embrittlement even after long-term use, and is used for steam turbine rotor materials, gas turbine disk materials, high-temperature pressure vessel materials, etc. The present invention relates to a method for producing a NiCrMoV-containing forged steel product.

  In the power plant business, in order to increase the thermal efficiency from the viewpoint of energy saving problems, means for increasing the steam temperature of the steam turbine or the gasification temperature of the gas turbine, and increasing the capacity of the power plant are required. The members for large turbines used in these plants are generally divided into high-temperature members and low-temperature members, and CrMoV steel, high Cr heat-resisting steel, Ni-based alloy, etc. excellent in high-temperature strength are used for the high-temperature members, For low temperature turbine rotor materials and gas turbine disk materials, NiCrMoV forged steel (3.25 to 4.00% Ni-1.25 to 2.2.5) specified in ASTM-A469 / 470 having excellent strength and toughness. 00% Cr-0.25 to 0.60% Mo-0.05 to 0.15% V steel) is generally used.

  Many NiCrMoV steels with improved steel quality have been developed. For example, in Patent Document 1, C: 0.10 to 0.35% by mass, Si: 0.35% or less, Mn: 1.0% or less, Ni: 0.3 to 2.5%, Cr : 1.5 to 3.5%, Mo: 0.3 to 1.5%, V: 0.05 to 0.30, W: 0.1 to 2.0%, Nb, if necessary A turbine rotor shaped body of an Fe-based alloy containing a small amount of N or B is heated to 1000 to 1150 ° C. and then annealed to 900 to 970 ° C., and further 600 to 700 ° C. A method of manufacturing a turbine rotor excellent in strength and toughness that has been tempered by heating is disclosed (see claims of Patent Document 1).

  Further, in Patent Document 2, C: 0.15 to 0.35% by mass, Si: 0.15% or less, Mn: 1.2% or less, Ni: 0.5 to 2.0%, Cr : 2.0 to 2.5%, Mo: 0.5 to 1.5%, W: 0.5 to 1.0%, V: 0.2 to 0.45%, Nb, Ta , A high toughness low-alloy steel excellent in high-temperature strength used for turbine rotor materials and disk materials, which contains one or two or more small amounts of N, the balance being Fe and inevitable impurities is disclosed ( (See the claims of Patent Document 2).

In the paper entitled “Effects of Impurity Elements and Si and Mn Content on Long-Time Isothermal Embrittlement of 3.5% NiCrMoV Steel for Low-Pressure Turbine Rotors” in Non-Patent Document 1, impurities such as P and Sb It is disclosed that elements, Si, and Mn are effective to reduce in order to embrittle the steel.
Alternatively, in a paper entitled “Effects of chemical composition and tempering heat treatment on strength and impact properties of NiCrMoV steel” in Non-Patent Document 2, the strength is increased by increasing the amount of Ni using a conventional rotor steel composition as a test material. There is a description that an improvement in the transition of the impact fracture surface transition temperature (50% FATT) to the low temperature side is observed due to the extremely low amount of impurities such as S.

  In addition to the above documents, the forging material for turbine rotors is a tough turbine rotor material (see Patent Document 3) containing 8 to 13% by mass of Cr in a low alloy steel containing a small amount of NiMoV, or Ni. There is disclosed a turbine rotor material (see Patent Documents 4 and 5) that is a low alloy system that does not contain and has excellent toughness.

JP-A-1-230723 JP-A-6-256893 JP-A-7-118811 JP-A-8-260091 JP-A-9-105305

Iron and steel Vol. 79 (1993) No. 4 Pages 80-86 Iron and steel Vol. 89 (2003) No. 6 pages 83-88

Low-temperature turbine rotors and gas turbine disks are structurally large-sized members, and uniform and excellent strength and toughness are required from the surface layer to the center of the members. For this reason, a forged steel product of 3.5NiCrMoV steel, which has excellent hardenability among low alloy steels, is used for these large members for low temperature. Furthermore, since this type of forged steel is highly susceptible to temper embrittlement in which impact resistance decreases in the range of 380 to 500 ° C., in addition to reducing impurity elements such as P, Sb, and Sn that promote embrittlement, Steelmaking techniques for reducing the amount of Si and Mn that promote temper embrittlement have also been established.
However, when reducing Mn for suppressing temper embrittlement susceptibility, there is a problem that the quenching effect by Mn is reduced and the strength and toughness of the large forged steel product are lowered.
That is, the present invention solves the above-mentioned problem, and forged steel products having reduced temper embrittlement susceptibility while ensuring the toughness of NiCrMoV-containing steels used in steam turbine rotor materials and gas turbine disk materials. The purpose is to provide.

In order to produce a forged steel product having low temper embrittlement susceptibility and excellent toughness, the present inventors consider the current steelmaking technology, and the composition and content of NiCrMoV forged steel members, Various investigations were made from the heat treatment conditions after forging. And when the member which cast and forged NiCrMoV steel containing about 4.3% of Ni is cooled from a high temperature, the bainite transformation region shifts to a low speed side, and the bainite transformation start / end points are at a low temperature. In order to shift to the side, it has been found that a toughness is improved by generating a bainite structure denser than other steel types such as 3.5NiCrMoV steel and 5NiCrMoV steel.
In addition, the inventors reduced the contents of Si and Mn and, if necessary, managed and controlled the relationship with the contents of impurity elements such as P, Sn, and Sb to a certain amount or less. It was also found that the temper embrittlement susceptibility, which decreases the impact resistance, is reduced in long-term use in the range of ° C.

This invention is comprised based on the said knowledge, The summary is as follows.
(1) By mass%, C: 0.25 to 0.35%, Si: 0.01 to 0.20%, Mn: 0.01 to 0.50%, Ni: 4.10 to 4.70% Cr, 1.6 to 2.0%, Mo: 0.30 to 0.50%, V: 0.07 to 0.15%, the balance being a forged steel member made of Fe and inevitable impurities, A method for producing a forged steel product comprising quenching at 800 to 1000 ° C and tempering at 500 to 700 ° C.

(2) By mass%, C: 0.25 to 0.35%, Si: 0.01 to 0.20%, Mn: 0.01 to 0.50%, Ni: 4.10 to 4.70% , Cr: 1.6 to 2.0%, Mo: 0.30 to 0.50%, V: 0.07 to 0.15%, P, Sn and Sb which are unavoidable impurities are P : 0.01% or less, Sn: 0.005% or less, Sb: 0.003% or less, and the contents of Si, Mn, P, Sn, and Sb are (Si + Mn) (P + Sn + Sb) × 10 ⁴ ≦ A method for producing a wrought steel product, characterized by quenching a forged steel member satisfying the relationship of 10 consisting of Fe and other unavoidable impurities at 800 to 1000 ° C and tempering at 500 to 700 ° C.

  The NiCrMoV forged steel product according to the present invention produced by the above-described component composition and heat treatment exhibits high strength of 1000 MPa or more in tensile strength depending on the tempering temperature, and 50% FATT is also excellent at −100 ° C. or less. Showing toughness. This numerical value is higher than that of a conventional 3.5NiCrMoV forged steel product and exhibits excellent toughness.

It is the graph which plotted the relationship between tear ridge space | interval which is a unit of the cleavage surface surrounded by the ductile fracture part of the impact test piece fracture surface, and 50% FATT.

Hereinafter, the method for producing a forged steel product according to the present invention will be described in detail.
Forging steel forging and die forging from molten steel, which is refined to the target component composition through melting furnaces such as electric furnaces and vacuum induction melting furnaces, or vacuum carbon deoxidation method or electroslag remelting method. A steel ingot having a shape suitable for forging, such as for use, is made, the steel ingot is hot forged, and formed into a forged member having a predetermined shape. The component composition of the forged steel member of the present invention is limited from the range in which the target forged steel product is manufactured based on the test results and the examination results of the present invention. explain. In addition, content of each component is a mass% display.

  0.25% or more of C is contained as an effective component that ensures hardenability during heat treatment and combines with other components such as Cr to form double carbides and increase strength. However, the inclusion of a large amount of C exceeding 0.35% has a problem that the toughness is lowered because a large amount of Cr double carbide is generated and the strength becomes too high. Therefore, the C content is limited to 0.25 to 0.35%.

  Si has a deoxidizing effect when it is contained in an amount of 0.01% or more, and also has an effect of increasing the fluidity of the molten steel and the strength of the steel. However, if the content exceeds 0.20%, When it remains excessively, it impairs the cleanliness of steel and lowers strength and toughness. Accordingly, in the present invention, Si is limited to a content of 0.01 to 0.20% as an optimum range for increasing the toughness of the forged steel product.

  Mn has an effect of improving the deoxidation effect and hardenability of molten steel by containing 0.01% or more and improving the strength and toughness of the forged steel product, but hardenability with a large content exceeding 0.50%. However, there is a problem of excessively improving and decreasing toughness. Therefore, Mn is limited to a content of 0.01 to 0.50%.

  Ni is an effective component that improves the hardenability of the forged steel member and imparts excellent values of strength and toughness of the forged steel product, and is 4.10 to 4.70%, preferably 4.20 to 4.60. It is desirable. Thereby, even in a large forged steel product having a low cooling rate during quenching, a lower bainite structure is mainly exhibited after quenching, and excellent strength and toughness can be obtained after tempering. Further, when the Ni content is 4.70% or less, the toughness does not deteriorate due to long-term use due to temper embrittlement. However, when the Ni content is less than 4.10%, the hardenability is not sufficiently improved, and a structure mainly composed of upper bainite is exhibited, so that it is not effective in improving strength and toughness. Further, if the excessive content exceeds 4.70%, there is a problem that a structure mainly composed of a martensite structure is exhibited due to excessive improvement in hardenability, and the toughness is impaired although the strength is high. Therefore, the Ni content is limited to 4.10 to 4.70%, which ensures excellent strength and toughness in large forged steel products and does not cause temper embrittlement.

  Cr is an effective component that imparts oxidation resistance and increases hardenability to improve strength. However, if the Cr content is less than 1.6%, these effective effects cannot be obtained, and if it exceeds 2.0%, the strength becomes too high and the toughness is impaired. . Therefore, the Cr content is limited to 1.6 to 2.0%.

  Mo contains 0.30% or more and is an effective component that increases hardenability like Cr and increases softening resistance during tempering to increase strength. However, an excessive content of Mo exceeding 0.50% has an excessive hardenability, and the structure after quenching and tempering is mainly martensite, so that there is a problem that sufficient toughness cannot be obtained. Therefore, Mo is specified to be 0.30 to 0.50% in consideration of the balance between strength and toughness.

  V is required to have a content of 0.07% or more as an effective component for precipitating fine V carbonitrides in the matrix when tempering to increase the strength. On the other hand, if the V content exceeds 0.15%, high strength cannot be maintained and the toughness tends to decrease. Therefore, the V content is limited to 0.07 to 0.15% in consideration of the balance between strength and toughness of the forged steel product.

  A forged steel member containing the above component composition and the balance being Fe and inevitable impurities can produce a forged steel product having low temper embrittlement sensitivity and excellent toughness, which is the object of the present invention.

  P, Sn, and Sb contained as unavoidable impurities are elements that increase the susceptibility to temper embrittlement, and are desirably reduced as much as possible. The allowable content of these unavoidable impurities is 0.01% or less, Sn is 0.005% or less, and Sb is 0 in consideration of the amount inevitably mixed with the current refining technology or raw materials. Limited to 0.003% or less.

The present inventors, combined with Si and Mn contained as effective components for increasing the strength of steel, and P, Sn and Sb contained as inevitable impurities, increase susceptibility to temper embrittlement and significantly reduce toughness. In order to suppress the problem, the correlation between the contents of Si and Mn as strength increasing components and the contents of P, Sn and Sb as inevitable impurities on temper brittleness was investigated and arranged. As a result, it is the product of the sum of the contents of Si and Mn (unit: mass%), which is the temper embrittlement promoting component, and the sum of the contents of P, Sn, Sb (unit: mass%), which are unavoidable impurities. Furthermore, it was found that if the value obtained by multiplying 10 ⁴ satisfies 10 or less, the temper embrittlement susceptibility is suppressed. That is, the contents of Si, Mn, P, Sn, and Sb are susceptible to temper embrittlement by adjusting the components of the forged steel member at the time of steel making so as to satisfy the relationship of (Si + Mn) (P + Sn + Sb) × 10 ⁴ ≦ 10. Was found to be suppressed.

  The reason why the temper embrittlement susceptibility is suppressed by adjusting the contents of Si, Mn, P, Sn, and Sb so as to satisfy the above relationship is that impurity elements such as P, Sn, and Sb are caused by grain boundary segregation. It is considered that grain boundary embrittlement occurs, and Si and Mn have a function of accelerating the grain boundary embrittlement of the impurity elements.

And by these knowledge, C, Si, Mn, Ni, Cr, Mo and V which are essential components, and P, Sn and Sb which are unavoidable impurities contain the above component composition, and Si, Mn , P, Sn, and Sb, the forged steel member satisfying the relationship of (Si + Mn) (P + Sn + Sb) × 10 ⁴ ≦ 10, the balance being Fe and other inevitable impurities, It is possible to produce a forged steel product that suppresses the back embrittlement sensitivity to a lower level and is excellent in toughness. Other inevitable impurities refer to other inevitable impurities excluding P, Sn and Sb.

Forged steel members adjusted to the above composition and formed into product shape by forging process, various heat treatments operated by heating temperature and cooling operation are performed to improve the properties required for forged steel products such as toughness. Applied.
First, the steel structure that has been coarsened by high-temperature heat and severe forging pressure in the forging process is improved, and precipitates such as metal carbides such as Cr, Mo, and V, and intermetallic double carbides are dissolved into a single solid solution. After the forged steel member is heated to 800 to 1000 ° C. to perform the conversion treatment, and subsequently subjected to a quenching treatment in which the average cooling rate at the center of the forged steel member is cooled to the bainite transformation completion temperature at a rate of 1 ° C./min or more. And tempering at 500 to 700 ° C.

  The forged steel quenched member of the present invention that has been subjected to the quenching treatment mainly exhibits a lower bainite structure, has high strength after tempering, has higher toughness than conventional NiCrMoV forged steel quenched members, and has a temper embrittlement susceptibility as well. The steel quality equivalent to is obtained. This steel quality is an effect obtained by involving the composition of the forged steel member in the present invention at this quenching temperature. At a quenching temperature lower than 800 ° C., austenitization is not sufficient, and after quenching, the structure mainly composed of lower bainite is obtained. It is difficult to obtain, and at an excessive quenching temperature exceeding 1000 ° C., the crystal grains become coarse, the hardenability becomes excessive, and the structure after quenching and tempering is mainly martensite, so that sufficient toughness cannot be obtained.

  In the present invention, in order to improve the toughness of the quenched steel-forged member subjected to the quenching treatment and reduce the susceptibility to temper embrittlement, the steel sheet is subjected to extreme thermal strain after being heated to 500 to 700 ° C. A tempering process is performed to cool at a speed that does not leave any residual. The tempering process is a modification that takes into account the strength required for reduced toughness and increased temper embrittlement susceptibility due to the quenching process, and precipitates metal carbides and the like that have been solutionized by the quenching process. The material is improved by using curing. Such an effect in the tempering process has a problem that the precipitation of metal carbides is not sufficient at a low temperature of less than 500 ° C., and the quenching effect is lost at an excessive temperature exceeding 700 ° C.

  Further, in the present invention, a normalizing process to make the cast structure at the time of casting finer or to remove the internal distortion caused by high temperature or rapid cooling at the time of hot forging to obtain a standard state, if necessary. May be applied to a steel ingot before hot forging or a forged steel member before quenching.

  Forged steel products such as steam turbine rotor materials, gas turbine disc materials, and high-temperature pressure vessel materials manufactured with the above-described composition have high strength, excellent toughness, and low temper embrittlement sensitivity. Can be used for a long time in a wide range of applications from low to low temperatures.

Next, examples of the present invention will be described.
The steel ingot that was melted in a basic electric furnace and refined and then ingot was hot forged at 1000 ° C. or higher to produce a forged material assuming a gas turbine disk. Table 1 shows the composition of the forged material at that time, the calculated value of (Si + Mn) (P + Sn + Sb) × 10 ⁴ (hereinafter referred to as J * value), heat treatment conditions, mechanical properties, and temper embrittlement susceptibility evaluation results. Is published. Test numbers 6-8, 10-11, 13-16, 18-19, 22-23, 26-27, 29-30 show examples of the present invention, and other test numbers depart from the present invention. The component composition or heat treatment temperature to be used is listed as a comparative example.

As is clear from the results of 0.2% proof stress, tensile strength and 50% FATT in Table 1, the steels of the present invention (No. 4-5, 8-10, 12-13, 15-18, 20-21, 24-25, 28-29, 31-33) exhibit a steel structure mainly composed of lower bainite and have high strength and toughness. In particular, 50% FATT (° C.) of the steel of the present invention is a comparative steel (No. 1-3, 6, 19, 23, 27) exhibiting a steel structure mainly composed of an upper bainite structure and a comparison exhibiting a steel structure mainly composed of a martensite structure. It is considerably lower than steel (No. 14, 22, 26, 30, 34) and exhibits excellent low temperature toughness.
In addition, the steel of the present invention is compared with a comparative steel (No. 7) having a low tempering temperature and exhibiting high strength and low toughness, and a comparative steel (No. 11) having a high tempering temperature and exhibiting low strength and high toughness. Excellent balance between strength and toughness. That is, the steel of the present invention demonstrates a steel quality that is excellent in both strength and toughness even after tempering.

  Here, FIG. 1 is a graph plotting the relationship between the tear ridge interval, which is a unit of the cleavage plane surrounded by the ductile fracture portion of the impact test piece fracture surface, and 50% FATT. The steel ridge spacing of the present invention steel (No. 10, 12, 16, 18, 20, 21, 24, 25, 33) is 8 μm or less, which is smaller than the comparative steel (No. 1, 2, 26, 30). Can be confirmed. And it can confirm that this has achieved high toughness. As described above, in the present invention, a steel structure mainly composed of lower bainite is obtained by optimizing the component composition and heat treatment conditions, and the graph also exhibits such a structure during the impact test. This demonstrates that the tear ridge spacing (fracture surface unit) is reduced, thereby providing high toughness.

Furthermore, among the steels of the present invention shown in Table 1, the test numbers 8, 18, 20, 21, and 32 having a J * value of 10 or less are the degree of embrittlement ΔFATT (50% FATT (° C.) after aging at 400 ° C. for 3000 hours) -50% FATT (° C) after tempering treatment is zero, but this is because the temper embrittlement temperature range does not cause deterioration over time by adjusting the J * value to 10 or less in the steel of the present invention. This proves that it can be used for a long time.
As is clear from the above experimental results, the forged steel product produced by the present invention has low temper embrittlement susceptibility and maintains toughness without embrittlement even when used for a long time in the temper embrittlement temperature range. It was confirmed that

As described above, the forged steel product produced with the forged steel component of the present invention is excellent in toughness and low in temper embrittlement sensitivity, so it can be used as a member for various turbines and in a wide range of applications such as high-temperature pressure vessel materials. Since it can be used for a long time, it is likely to be used more and more as an inexpensive cost member in the future.

Claims (2)

% By mass
C: 0.25 to 0.35%,
Si: 0.01-0.20%,
Mn: 0.01 to 0.50%,
Ni: 4.10 to 4.70%,
Cr: 1.6-2.0%,
Mo: 0.30 to 0.50%,
V: 0.07 to 0.15% is contained,
A method for producing a forged steel product comprising quenching a forged steel member, the balance of which is Fe and inevitable impurities, at 800 to 1000 ° C and tempering at 500 to 700 ° C.
% By mass
C: 0.25 to 0.35%,
Si: 0.01-0.20%,
Mn: 0.01 to 0.50%,
Ni: 4.10 to 4.70%,
Cr: 1.6-2.0%,
Mo: 0.30 to 0.50%,
V: 0.07 to 0.15% is contained,
The inevitable impurities P, Sn and Sb are
P: 0.01% or less,
Sn: 0.005% or less,
Sb: 0.003% or less,
For the contents of Si, Mn, P, Sn, and Sb,
(Si + Mn) (P + Sn + Sb) × 10 ⁴ ≦ 10 is satisfied,
A method for producing a wrought steel product comprising quenching a forged steel member, the balance of which is Fe and other inevitable impurities, at 800 to 1000 ° C and tempering at 500 to 700 ° C.

Images