JPS61221355A - 12cr heat resisting steel - Google Patents

Abstract

PURPOSE:To manufacture 12Cr heat resisting steel showing superior creep rupture strength by incorporating specific percentage of C, Si, Mn, Ni, Cr, Mo, V, Nb, Ta, N, W and B to Fe and by forming a structure of tempered martensite. CONSTITUTION:The 12Cr heat resisting steel consisting of, by weight, 0.05-0.25% C, >=0.2-1.0% Si, <=1.0% Mn, >=1.0-2.0% Ni, 8.0-13.0% Cr, 0.5-2.0% Mo, 0.1-0.3% V, and further, >=0.3-0.5%, in total, of at least one kind among Nb and Ta, 0.01-0.2% N, >=1.0-2.0% W, >=0.01-0.05% B, and the balance basically Fe and having a ferrite-free structure essentially composed of tempered martensite is prepared. In this way, the 12Cr heat resisting steel having improved creep characteristics can be obtained without deteriorating ductility and toughness.

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to 12Cr heat-resistant steel that exhibits excellent creep rupture strength at high temperatures in the range of 550 to 600°C, and is particularly applicable to parts such as steam turbine blades and bolts. The present invention relates to 12Cr heat-resistant steel suitable for.

[Technical background of the invention and its problems] The maximum temperature of the steam used to drive current steam turbines is 566°C, and the maximum pressure is 246 ko/min.
CI' is expected to raise the temperature and pressure of the steam used in order to improve thermal efficiency. Such steam conditions depend on the high-temperature strength of the materials of the parts constituting the turbine, and therefore, in order to improve the steam conditions, materials with higher high-temperature strength are being developed. Such development is important not only for major large parts such as rotors and casings, but also for blades, bolts, etc.

Centrifugal force due to high-speed rotation is constantly acting on steam turbine blades, and if they lack high-temperature strength, the blades will creep and deform, floating up from the rotor and causing their tips to come into contact with stationary parts. It may come to that. Additionally, the bolts used to seal the casing are initially given a certain tightening pressure based on their elastic force, but since the steam pressure acting on the casing is always acting on the bolts, the bolts are Creep deformation occurs, and the tightening pressure gradually decreases, making it impossible to keep the casing sealed. If steam leaks or creep deformation accumulates, the bolt itself may break.

As described above, materials for blades and bolts used in the high-temperature parts of steam turbines are required to have excellent creep properties, and 12Cr heat-resistant steel has conventionally been used. 12
Or-based heat-resistant steels are generally cheaper than other heat-resistant steels with equivalent high-temperature strength, and they also have good toughness at room temperature, as well as excellent vibration damping ability, which is essential for blade materials. . 12C heat-resistant steel has these characteristics, so in order to further improve its high-temperature strength while retaining its basic characteristics, various alloying components are added to strengthen the martensitic structure and carbonitrides are added. Maintaining high-temperature strength and ensuring structural stability at high temperatures for long periods of time are achieved by stabilizing the If this occurs, ferrite will form in the vicinity, which is undesirable. Therefore, a remelting process has been introduced to prevent such segregation and make the structure uniform.

Currently, the material used for steam turbine blades and bolts is 12Or-Mo-V-
Nb steel and 12Cr-Mo-V-Nb called 422 are used, both of which are heated at 600°C and 30°C.
Creep rupture time at kg/as2 load is 200
It is about 300 to 300 hours, and cannot be said to have sufficient high temperature strength to cope with future increases in steam temperature.

[Object of the Invention] The present invention has been made from the above-mentioned viewpoint, and its object is to develop a new 12Cr heat-resistant steel II! 4.

Another object of the present invention is to provide a new 12Cr heat resistant steel that has superior creep rupture strength compared to conventional 12Cr steels.

Another object of the present invention is to provide a 12Cr heat-resistant steel suitable for steam turbine parts, particularly blades and bolts.

[Summary of the invention] In order to achieve such an object, 12Cr according to the present invention
Heat-resistant steel contains 0.05 to 0.25% carbon based on the amount of snow,
Silicon over 0.2 and up to 1.0% by weight, 1 by weight
．． Manganese up to 0%, nickel over 1.0 up to 2.0% by weight, chromium from 8.0 to 13.0% by weight, molybdenum from 0.5 to 2.0% by weight, by weight vanadium from 0.1 to 0.3%, and at least one selected from niobium and tantalum in a total amount exceeding 0.3 to 0.5% by weight, and from 0.01 to 0.5% by weight. Nitrogen up to 0.2%, tungsten over 1.0 up to 2.0% by weight, boron over 0.01 up to 0.05% by weight, the balance essentially consisting of iron;
It is characterized by a substantially tempered martensitic structure.

The 12Cr heat-resistant steel of the present invention is 12Or-MO-V-N, which is a conventionally used 12Cr heat-resistant steel.
This was developed as a result of a systematic study of b-W steel.

During the development process of the present invention, detailed investigations were conducted into the effects of alloying elements such as carbon, silicon, manganese, nickel, chromium, molybdenum, vanadium, niobium, tantalum, nitrogen, tungsten, and boron on creep rupture strength.・A test was conducted.

Also, regarding the metal structure, the ductility and toughness are lower than that of conventional 12Cr.
Research was conducted with the aim of ensuring that the strength of the steel was not lower than that of heat-resistant steel.

The results can be summarized as follows.

(1) Carbon Carbon stabilizes the austenite phase during quenching,
Furthermore, it increases creep rupture strength by forming carbides, but
For that purpose, 0.05% or more is necessary. However, 0
．． If it exceeds 25%, the carbide content becomes excessive, which actually reduces the creep rupture strength. Therefore, the amount of carbon should be 0.05 to 0.25%, preferably 0.08 to 0.1%.
It is 5%.

■Silicon Silicon is an element necessary as a deoxidizing agent during melting, and 0.2
If it is less than 1%, the purpose cannot be fully achieved, and if it is less than 1%, 0%
Exceeding this causes the formation of δ ferrite phase. Therefore, the amount of silicon should be more than 0.2% and up to 1.0%, preferably from 0.21 to 0.6%.

(3) Manganese Manganese is an element added as a deoxidizing/desulfurizing agent during melting, like silicon, but if added in a large amount, the creep rupture strength decreases, so it is limited to 1.0% or less. Preferably it is 0.3 to 0.8%.

(3) Nickel Nickel is an austenite-forming element and is effective in stabilizing the austenite phase during quenching and preventing the formation of the δ ferrite phase, but for this purpose it is necessary to add more than 1.0%. . However, if it is added in an amount exceeding 2.0%, the creep rupture strength will be extremely reduced, and the AC11 degree will also be lowered, which is undesirable.
．． 0%, preferably 1. It exceeds O and is up to 1.5%.

■Chromium Chromium is an element necessary to prevent oxidation in high-temperature environments and to improve creep rupture strength. For this purpose, it is necessary to add 8.0% or more, but 1
If it exceeds 3.0%, δ ferrite phase will be generated, so 8
．． The range is from 0 to 13.0%, preferably 9.5%.
12.0%.

(2) Molybdenum Molybdenum is an effective element for improving creep rupture strength and preventing deoxidization during tempering, but for this purpose it is necessary to add 0.5% or more. However, 2.0%
If the content exceeds δ, ferrite phase is formed, resulting in a decrease in creep rupture strength and toughness.

(7) Vanadium Vanadium is an effective element for improving creep rupture strength. To achieve this purpose, it is necessary to add 0.1% or more, but if it exceeds 0.3%, δ ferrite is likely to be produced, so it is preferably added from 0.1 to 0.3%. , 0.15 to 0.29%.

■Niobium, Tantalum Niobium and tantalum both have the effect of refining crystal grains and increasing ductility and toughness. Furthermore, these niobium and tantalum form carbides and carbonitrides! ! It is dispersed and precipitated in the ground and significantly improves creep characteristics. In order to obtain these effects, the sum of at least one type must be 0.
．． It is necessary to add more than 3%, but if this sum exceeds 0.5%, δ ferrite will be formed and carbides will precipitate coarsely, which is not preferable. Therefore, the amount added should be more than 0.3 and up to 0.5%, but preferably 0.3%.
exceeding 0.45%.

(9) Nitrogen Nitrogen is effective in suppressing the formation of ferrite phase,
It is also an element necessary to form carbonitrides of niobium and tantalum. For this purpose, it is necessary to add 0.01% or more, but if it exceeds 0.2%, pinholes and blowholes may be formed, which is undesirable.
It is set to 0.01 to 0.2%. Preferably it is 0.03 to 0.08%.

Tungsten No.6 has the effect of improving creep rupture strength like molybdenum, and for this purpose it is necessary to add more than 1.0%, but if it exceeds 2.0%, it will not produce δ ferrite. reach. Therefore, the amount added exceeds 1゜260
%, preferably from 1.1 to 1.5%.

Boron is an element necessary to improve hardenability and creep rupture strength, and in order to obtain this effect it is necessary to add more than 0.01%, but on the contrary 0.05 to
If added in excess of 0.01% and up to 0.05%, forging cracks are likely to occur during manufacturing. Preferably it is 0.015 to 0.030%.

The 12Cr heat-resistant steel of the present invention has the above chemical composition, and has a 65%
At the same time, it exhibits excellent creep properties up to temperatures of about 0"C, and other mechanical properties are comparable to those of conventional 12Cr heat-resistant steel. Therefore, the 12Cr heat-resistant steel of the present invention can be used in steam turbine parts, etc. However, when used for such purposes, it is necessary to have sufficient fatigue strength and toughness as well as creep strength, and to meet these requirements, the 12Cr heat-resistant steel of the present invention is a material that does not contain ferrite. It consists of a tempered martensitic structure.

In order to prevent the formation of ferrite in the structure, the amount of alloying elements added may be adjusted within the above range, but as will be described later, in order to increase the quenching temperature and prevent the formation of ferrite, It is desirable that the chromium equivalent defined by the following formula be within the range of 6 to 11.

Margin below (chrome per fi) - 40x [%C] -30x [%
N]-2X[%Mn1-4X[%Nil+[%Cr]
+4X [%Mo]+6x[%Sil+11X [%
Vl +5X [%Nbl+2.5×[%Ta]+1
．． 5X [%Wl +40X [%B] The 12Cr heat-resistant steel of the present invention whose composition has been determined in this way has a
The material is heated and held at a temperature of 50° C. to austenite, then rapidly cooled and quenched, and then tempered at a temperature of 600 to 700° C., thereby substantially forming a tempered martensitic structure. In addition, before this tempering at 600 to 700°C, 50°C was heated to decompose the residual austenite.
Tempering at 0 to 600°C is permitted.

It is also possible to carry out the process twice at different temperatures within this range of m degrees.

As mentioned above, by austenitizing and quenching at a high temperature of 1,050 to 1,150°C, carbides, nitrides, or carbonitrides such as di-Album and tantalum can be precipitated more uniformly, finely, and in large quantities. In addition, if the austenitization temperature is in the range of 105O to 1150℃,
The austenite crystal grains do not become coarse, and if the chromium equivalent is within the above range, the formation of ferrite can also be prevented.

Here, the production of the 12Or heat-resistant steel according to the present invention and turbine parts such as blades and bolts made of the same will be briefly described.

First, raw materials blended to have a limited composition range are melted in air or vacuum using a suitable furnace, such as an electric furnace. After melting, the molten metal is agglomerated into ingots of appropriate size and shape. Note that arc melting or electroslag melting this ingot again is effective in homogenizing the components and reducing impurities.

Next, the ingot is heated to a temperature in the range of about 1150 to 1200°C in a heating furnace such as a heavy oil furnace, an electric furnace, or a gas furnace, and then subjected to conventional methods such as press forging and hammer forging. Forged by method.

The 12Cr heat-resistant steel thus forged is heated in a heating furnace to a temperature in the range of 1050 to 1150'C, held at this temperature until the whole becomes uniformly austenitized, and then placed in oil, water, or by impact. It is rapidly cooled and hardened using methods such as wind cooling.

Afterwards, this 12Cr heat-resistant steel is heated to 600 Cr in a heating furnace.
Tempered by heating and holding at a temperature in the range of 700°C,
The structure becomes tempered martensite. In addition, during tempering, in order to decompose the residual austenite phase during quenching, before tempering at 600 to 700 °C as described above, first heat to a lower temperature in the range of 500 to 600 °C. After holding, tempering may be performed at 600 to 700'C, or tempering may be performed twice at different temperatures within this range of 600 to 700'C.

The 12Cr heat-resistant steel thus obtained is cut and formed into a desired shape, such as a turbine component, for example. In addition,
In the case of blades, the forged billet is cut into appropriate sizes, heated to a temperature of about 1100 to 1200°C, die-forged to form the blade shape, and then quenched and tempered.
It may then be machined to final dimensions.

[Examples of the Invention] The present invention will be more clearly understood from the test results of Examples and Comparative Examples described below.

Example 1 Raw materials were blended to have the alloy composition shown in Example 1 in Table 1, melted in a high frequency vacuum melting furnace, and then cast into a mold to obtain an ingot. In addition, when blending the raw materials, nitrogen is added instead of Fe.
-Cr-N base alloy was used. Next, the surface of this ingot was shaved off by machining, then charged into a heavy oil furnace, heated to 1200° C., hammer forged, and forged into a round bar with a diameter of 3 Q mm.

The round bar thus obtained was cut to a length that would allow specimens to be collected for each test described later, and each of them was placed in an electric furnace at 110°C.
It was heated and held at 0°C for 2 hours, then put into oil at room temperature for quenching, and then heated and held at 650°C for 3 hours in an electric furnace for tempering.

Each material that has been heat treated is machined to create a test piece.
A tensile test and a creep rupture test were conducted using each test piece. The results of these tests are shown in Table 2.

The tensile test was conducted at room temperature, and Table 2 shows the tensile strength, elongation after cutting, and reduction of area. The creep rupture test was conducted under two conditions, varying the temperature and load, and the time required to reach rupture under each condition is shown in the table.

Example 2-4 For Examples 2-4, test pieces were prepared and tested in the same manner as in Example 1. These Examples 2-4
The alloy composition is shown in Table 1, and the test results are shown in Table 2.

Comparative Example For Comparative Examples 1 and 2, test pieces were prepared and tested in the same manner as above. However, as shown in Table 1, these comparative examples do not contain boron, tungsten, niobium, or tantalum, which are essential elements for the 12Or heat-resistant steel of the present invention, in their alloy compositions. Here, Comparative Example 1 corresponds to conventionally used 846, and Comparative Example 2 corresponds to a heat-resistant steel called 422.

The test results of these comparative examples are shown in the lower part of Table 2.

From the test results shown in Table 2, Example 1-4, which is the 12Cr heat-resistant steel according to the present invention, exhibits significantly superior creep rupture properties compared to the comparative example at both temperatures of 600°C and 650°C. is clear. Moreover, in a tensile test at room temperature, Example 1-4 and Comparative Example 1.2 showed similar tensile strength, and Example 1-4 had somewhat better values for elongation and reduction of area after cutting. What is being shown is understood.

[Effects of the Invention] As described above, the 12Cr heat-resistant steel of the present invention has improved creep properties without impairing ductility and toughness, and has extremely high value as a material for turbine parts, etc. be.

Claims (4)

[Claims] (1) Carbon from 0.05 to 0.25% by weight, silicon more than 0.2 to 1.0% by weight, 1.0% by weight
Manganese up to and including 1.0 to 2.0% by weight of nickel, 8.0 to 13.0% by weight of chromium, 0.5 to 2.0% by weight of molybdenum, 0 by weight ．．
1 to 0.3% vanadium, and at least one selected from niobium and tantalum, totaling more than 0.3% by weight and up to 0.5%, and 0.01% by weight
Nitrogen from up to 0.2%, over 1.0 to 2.0% by weight
Tungsten up to 0.01% by weight and 0.05%
12C, characterized in that up to boron and the remainder are essentially iron, and the structure is substantially tempered martensitic.
rHeat-resistant steel. (2) 0.08 to 0.15% carbon by weight, 0.21 to 0.6% silicon by weight, 0.3% by weight
Manganese from up to 0.8%, exceeding 1.0 by weight and 1.
up to 5% nickel, chromium from 9.5 to 12.0% by weight, molybdenum from 0.7 to 1.5% by weight, vanadium from 0.15 to 0.29% by weight, and also niobium. and at least one selected from tantalum in a total amount exceeding 0.3 to 0.45% by weight, nitrogen from 0.03 to 0.08% by weight, and nitrogen from 1.1 to 1.5% by weight Tungsten up to %, 0.0 by weight
12Cr heat-resistant steel according to claim 1, comprising from 15 to 0.030% boron, the balance essentially iron. (3) The 12Cr heat-resistant steel according to claim 1, which is applied to blades of steam turbines. (4) The 12Cr heat-resistant steel according to claim 1, which is applied to a bolt for a steam turbine.