JP2001262268A - High strength low alloy heat resistant steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce steel excellent in tempering embrittlement resistance and SR cracking resistance even in the case of being high strength low alloy steel high in creep strength at high temperatures of >=400 deg.C. SOLUTION: This high strength low alloy heat resistant steel has a composition containing, by mass, 0.01 to 0.25% C, 0.1 to 2.5% Mo, 0.01 to 0.5% V, 0.05 to 3% Cr and <=0.01% N, and has an index M expressed by the following formula of >=0.1: M=(0.08Cr-0.1W+0.03/C+0.04+0.855V)×(Mo+0.5√Mo)×0.65, wherein, the element symbols denote the content (mass%) of each element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant steel having excellent resistance to temper embrittlement and stress relieving annealing cracking (hereinafter referred to as SR cracking resistance) and high creep strength at a high temperature of 400 ° C. or higher. More specifically, low alloys suitable for applications such as heat exchangers and pipes, heat-resistant valves and connecting joints in the fields of boilers, chemical industry and nuclear power, especially those requiring stress relaxation heat treatment after processing and heat treatment after welding Heat resistant steel.

[0002]

2. Description of the Related Art Heat-resistant steels used at a high temperature of 400 ° C. or higher include (1) low Cr ferrite steels having a Cr content of several percent,
(2) High Cr ferritic steel having a Cr content of 9 to 12%,
(3) It is roughly classified into austenitic steel and the like. The type of heat-resistant steel to be used is appropriately determined in consideration of the use environment such as temperature and pressure and the economic efficiency.

[0003] Among these heat-resistant steels, carbon steel and low Cr
Ferritic steels are characterized by being significantly less expensive, having a lower coefficient of thermal expansion, and having better thermal conductivity than high Cr ferritic steels and austenitic steels. STBA12 (0.45 / 0.65Mo) and STBA22 (0.8 standardized by JIS as typical examples of such carbon steel and low alloy steel).
/1.25Cr-0.45/0.65Mo), STBA23 (1 / 1.5Cr-0.45 / 0.
65Mo) and STBA24 (1.9 / 2.6Cr-0.87 / 1.13Mo) are known.

[0004] High temperature strength is extremely important in designing a pressure-resistant member, and it is desirable that the strength be high regardless of the operating temperature. In particular, in a heat-resistant and pressure-resistant steel pipe used for boilers, chemical industries and nuclear power, the wall thickness of the pipe is determined according to the high-temperature strength of the material.

In order to obtain a desired high strength as described above, low C
In r-ferritic steels, precipitation strengthening is often used. That is, high temperature strength can be obtained by adding a precipitation strengthening element such as V, Nb and Ti to precipitate fine carbonitrides.

[0006] Such low Cr ferritic steel is, for example,
JP-A-57-131349, JP-A-57-13135
0, JP-A-59-226152, JP-A-8-158
A number of proposals have been made in each gazette such as No. 022 and the like, and practical applications have been made.

However, in the case of high-strength steel, since the strength within the grains is high, the grain boundary strength is relatively weak, and tempering embrittlement is likely to occur, and SR cracking accompanied by grain boundary cracking during stress relief annealing. May occur. For this reason, in order to ensure the safety of the structure, there are often detailed restrictions on the structure control of the base metal, the welding method, the heating and cooling rates during heat treatment, the heat treatment temperature and the heat treatment time, etc. It is not always easy to handle.

As a method of improving temper embrittlement of low alloy steel, for example, as disclosed in Japanese Patent Application Laid-Open No. 55-6458, a method of limiting the amount of impurity elements such as Sb, As and P in steel, As disclosed in JP-A-61-16419, a method has been proposed in which a nitride such as AlN is used to suppress coarsening of a base material.

[0009] However, even now that steelmaking technology has been improved and the amount of impurity elements can be sufficiently reduced,
In addition, since the problem has not been solved, and nitrides cause deterioration of low-temperature toughness and corrosion resistance, and the amount of precipitation is limited, a more essential measure is desired. This problem is particularly serious in low alloy steels having a low Cr content.

[0010]

The problem to be solved by the present invention is that
An object of the present invention is to provide a steel excellent in temper embrittlement resistance and SR crack resistance even in a high-strength low-alloy steel having a high creep strength at a high temperature of 0 ° C. or higher.

[0011]

Means for Solving the Problems The present inventor has obtained the following findings as a result of repeating experiments for the purpose of clarifying the causes of temper embrittlement and SR cracking of low alloy steel.

A) Temper embrittlement and SR cracking of low alloy steel are reduced by maintaining the amount of solid solution Mo in the matrix at 0.1% or more.

B) When Mo is added to steel, Mo takes the following forms. That is, Mo in steel is (1) M ₂₃ C
₆ type carbide, M ₇ C ₃ type carbide, precipitated as a part of M ₆ C type carbide, (2) solid solution in cementite, (3) Mo ₂ C
(4) precipitated as a part of MC type precipitate, and (5) solid solution in the matrix without being precipitated. Here, M is Fe, Cr, Mo, W,
It is a general term for metal elements such as V and Nb, and "precipitates as a part of each carbide" means that a part of M is replaced by Mo atom. Therefore, the form of Mo changes depending on the difference in the chemical composition, so that the total content of Mo is 0.1%.
Even if it is 1% or more, if the amount of precipitation as a carbide is large, the amount of solid solution in the matrix will be less than 0.1%.

C) In low alloy steel, the amount of dissolved Mo increases with the addition of Cr and V, and decreases with the increase of the amount of C and the addition of W.

D) In order to keep the amount of solid solution Mo at 0.1% or more, it is necessary to design the components so that the index M represented by the following formula becomes 0.1 or more.

M = (0.08Cr-0.1W + 0.03 / C + 0.04 + 0.855V) ×
(Mo + 0.5√Mo) × 0.65 Based on the above findings, among the elements constituting low alloy steel, C
Arranged in relation to the amount of r, V, W and C and the amount of solid solution Mo,
In addition, by designing components for enhancing performance such as high-temperature strength and low-temperature toughness, a high-strength low-alloy steel excellent in temper embrittlement resistance and SR crack resistance has been obtained. The gist of the present invention is as follows. (1) In mass%, C: 0.01 to 0.25% Mo: 0.1 to 2.5% V: 0.01 to 0.5% Cr: 0.05 to 3% N: 0.01 %, And an index M represented by the following formula (1) is 0.1 or more.

M = (0.08Cr + 0.03 / C + 0.04 + 0.855V) × (Mo + 0.5√Mo) × 0.65 (1) Here, the element symbol indicates the content (% by mass) of each element. Show. (2) The high-strength low alloy according to the above (1), further containing W: 0.05 to 3% by mass% and having an index M represented by the following formula (2) of 0.1 or more. Heat resistant steel.

M = (0.08Cr-0.1W + 0.03 / C + 0.04 + 0.855V) × (Mo + 0.5√Mo) × 0.65 (2) (3) In mass%, Ti: 0.001 to 0.001 0.05%, N
b: The high-strength low-alloy heat-resistant steel according to the above (1) or (2), containing one or two kinds of 0.005 to 0.1%.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of the heat-resistant steel of the present invention will be described in detail below. In the following description, all percentages of the content of the chemical composition mean mass%.

C: 0.01 to 0.25% C stabilizes the structure as an austenite stabilizing element. Further, the steel of the present invention is martensite, bainite, ferrite or a mixed structure containing two or more thereof, and the C content is also important for controlling the balance of these structures. Further, when precipitation strengthening elements such as V, Nb, Ti, and Zr are contained, these elements form MX-type fine carbides and contribute to improvement in high-temperature strength. However, if the C content is less than 0.01%, the above effects cannot be obtained, and the hardenability is reduced, and the strength and toughness are impaired. On the other hand, if it exceeds 0.25%, the amount of carbide precipitation increases and M
The solid solution amount of o cannot be secured, and tempering embrittlement and SR cracking are likely to occur. Therefore, the C content is 0.01
０．２0.25%. Preferably, 0.03 to 0.20
%, More preferably 0.05 to 0.2%.

Mo: 0.1 to 2.5% Mo is the most important element in the steel of the present invention. By securing the solid solution amount of Mo in the matrix of 0.1% or more, the resistance to tempering embrittlement and SR cracking is improved. Is improved. further,
Mo has an effect of solid solution strengthening, and contributes to improvement in strength. Further, since fine precipitates such as Mo ₂ C carbide and MX type carbide are formed, they also have a precipitation strengthening effect. However, when the content is less than 0.1%, the solid solution amount of Mo in the matrix is insufficient, and does not contribute to improvement in temper embrittlement resistance and SR crack resistance. On the other hand, if the content exceeds 2.5%, the amount of coarse carbides such as M ₂₃ C ₆ and M ₆ C increases, which adversely affects toughness and creep strength. Therefore, the content of Mo is set to 0.1 to 2.5%. Preferably, it is 0.15 to 1%, more preferably 0.25 to 0.1%.
75%.

V: 0.01 to 0.5% V suppresses the precipitation of M ₂₃ C ₆ type carbide and increases the amount of Mo dissolved. Further, V forms MX-type fine carbonitrides and contributes to high strength. However, if the content is less than 0.01%, these effects cannot be obtained. On the other hand, when the content exceeds 0.5%, the MX-type carbonitride coarsens, and rather impairs the strength and toughness. Therefore, the V content is 0.1.
01 to 0.5%. Preferably, 0.03-0.2
%, More preferably 0.05 to 0.15%.

Cr: 0.05-3% Cr is an element that contributes to the increase of solid solution Mo and is indispensable for improving oxidation resistance and high-temperature corrosion resistance. If the Cr content is less than 0.05%, these effects cannot be obtained. On the other hand, if the content exceeds 3%, the economic efficiency is reduced and the advantage of the low alloy steel is reduced, so that the Cr content is less than 0.1%.
05 to 3%. A preferred lower limit of Cr is 0.5%,
More preferably, it is 1%.

In addition to the above alloy components, if necessary, one or two or more elements selected from the following groups (1) to (10) may be contained. The contents of P and S are respectively 0.0% by mass.
It is preferably at most 3% and at most 0.015%.

(1) W: 0.05 to 3% (2) N: 0.01% or less (3) Ti: 0.001 to 0.05%, Nb: 0.005
(1) Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%
5%, one or more selected from Co: 0.01 to 0.5% (5) Ta: 0.002 to 0.1%, Zr: 0.001 to
One or two kinds of 0.1% (6) B: 0.0001 to 0.01% (7) Al: 0.001 to 0.05% (8) Si: 0.01 to 1% (9) Mn: 0.01-1% (10) Ca: 0.0001-0.01%, Mg: 0.00
One or two kinds of 01 to 0.01% The above various elements will be described below.

W: 0.05 to 3% W is an element to be contained if necessary. If W is contained, it contributes to solid solution strengthening and is effective for improving the creep strength at higher temperatures. This effect can be obtained by adding 0.05% or more. However, when the content exceeds 3%, M
In addition to the decrease in the solid solution amount of o, coarse M ₆
It forms C-type precipitates and impairs creep strength and toughness. Therefore, when W is contained, 0.01 to 3%
It is preferred that More preferably, 0.05 to 2
%, More preferably 0.1 to 1.5%.

N: 0.01% or less N is an element to be contained if necessary. If N is contained, it forms a fine nitride and contributes to improvement in creep strength and improvement in toughness by grain refinement. If contained, 0.00
1% or more is preferable. On the other hand, if the content exceeds 0.01%, the nitride coarsens and the toughness is remarkably deteriorated. Preferably 0.
001 to 0.008%, more preferably 0.003 to
0.007%.

Ti: 0.001-0.05%, Nb:
0.005 to 0.1% Ti and Nb are one or more elements to be contained as necessary. If Ti is contained, it combines with N to form fine nitride Ti.
It forms N to prevent coarsening of crystal grains, and is effective for improving toughness, suppressing temper embrittlement and suppressing SR cracking. However,
If the content exceeds 0.05%, coarse nitrides are formed and the toughness is rather deteriorated.
It is preferably set to 001 to 0.05%. More preferably, it is 0.003 to 0.02%, and more preferably, 0.1 to 0.02%.
005 to 0.012%.

When Nb is contained, it combines with N and C to form fine carbonitrides. Further, if Nb is contained in a complex with Ti, complex precipitation (Nb, Ti) (N, C)
Is fine and stable over a wide temperature range, which is effective in preventing crystal grain coarsening. However, 0.005
%, These effects cannot be obtained. On the other hand, 0.1%
If the Nb is more than Nb, coarse carbonitrides are formed and the toughness is rather deteriorated. Therefore, the Nb content is preferably set to 0.005 to 0.1%. More preferably, 0.1 to 0.08
%, More preferably 0.02 to 0.06%.

Cu, Ni, Co: These elements are elements to be contained as necessary, and if contained, all of them are elements contributing to stabilization of austenite and have a solid solution strengthening action. It is effective in improving and preventing a decrease in creep strength during long-term use. These effects can be obtained at 0.01% for each element. However, when any of the elements is contained in excess of 0.5%, the high-temperature creep strength decreases. Further, from the viewpoint of economic efficiency, excessive addition is not preferable. Therefore, when these elements are contained, the content of each element is 0.01 to 0.5.
%. The preferred range of each element is 0.02 to 0.3%, more preferably 0.1 to 0.
25%.

These elements can be contained alone or in a combination of two or more. Also,
Ni also has an effect of improving toughness and Cu has an effect of improving thermal conductivity.

Ta, Zr These elements are elements to be contained as necessary, and if they are contained, they form carbonitrides at a high temperature and contribute to suppression of coarsening of crystal grains. For this reason, when heat treatment at a high temperature is required, it may be contained as necessary.
It becomes remarkable at 0.002% or more and Zr: 0.001% or more. However, when each element is contained in excess of 0.1%, coarse precipitates are formed and the toughness is rather deteriorated. Therefore, the content of these elements when contained is T
a: 0.002 to 0.1%. Zr: 0.001-0.1
%. A more preferred range is Ta: 0.005 to 0.5.
07%, Zr: 0.003 to 0.05%, more preferably, Ta: 0.01 to 0.02%, Zr: 0.00
5 to 0.012%.

B: B is an element to be contained as necessary, and if contained, is an element effective for securing stable strength by improving hardenability. This effect is obtained at 0.0001% or more. However, when the content exceeds 0.1%, the carbide is coarsened, which causes a decrease in strength and a decrease in toughness. Therefore, when contained, the amount of B is 0.0001 to 0.1.
%. The preferred range is from 0.0005 to 0.5.
015%, more preferably 0.002 to 0.00
5%.

Al: Al is an element remaining when used as a deoxidizing agent. If included, this effect is 0.1%.
001% or more. However, when the content exceeds 0.05%, creep strength and workability are impaired. Therefore, when the Al content is 0.001 to 0.0
It is good to make it 5%. The preferred range is 0.0015 to
0.02%, more preferably 0.002 to 0.01
5%. In the present invention, Al means acid-soluble Al
(Sol. Al).

Si: Si is a deoxidizing agent and an effective element,
It is also an element that enhances the steam oxidation resistance of steel when contained. These effects can be obtained at 0.01% or more. However, when the content exceeds 1%, the toughness is remarkably reduced, and is harmful to the creep strength. Therefore, the content of Si when it is contained is preferably 0.01 to 1%. A preferred range is 0.1 to 0.6%, and a more preferred range is 0.15 to 0.45%.

Mn: Mn is an element to be contained as required. If contained, Mn improves hot workability by the desulfurization and deoxidation effects at the time of melting, and also improves hardenability. These effects can be obtained at 0.01% or more. However, when the content exceeds 1%, the stability of fine carbides effective for creep strengthening is impaired, and the creep strength at high temperature and long time decreases. Therefore, when contained, the Mn content is 0.01 to
It is better to be 1%. Desirable range is 0.05 to 0.6
5%, more preferably 0.1 to 0.5%.

Ca, Mg: Ca and Mg are elements to be contained as necessary, and if contained, reduce inclusions and contribute to improvement of castability, as well as temper embrittlement and welding cracks.
And an element that also contributes to improving toughness. The effect becomes remarkable at 0.0001 or more. However, 0.01
If it is contained in excess of%, carbides and sulfides increase, and on the contrary, toughness and strength are impaired. Therefore, when contained, the amounts of Ca and Mg are both 0.0001 to 0.
It is better to be 01%. The preferred range is 0.0002 to
0.005%, a more preferred range is from 0.0005 to 0.5.
0035%.

Index M: 0.1 or more In order to maintain the Mo solid solution amount in the matrix at 0.1% or more, C, Cr, It is necessary to control V, Mo, and W contained as necessary according to the following formula.

M = (0.08Cr-0.1W + 0.03 / C + 0.04 + 0.855V) ×
(Mo + 0.5√Mo) × 0.65 Here, the element symbol indicates the content (% by mass) of each element.

When M is less than 0.1, 0.1% or more of the amount of solid solution Mo is not guaranteed, and no effect of improving temper embrittlement resistance and SR crack resistance is recognized. It is not necessary to particularly define the upper limit of M, and the higher the Mo content is in the range of 0.1 to 2.5%, the better. It is preferably at least 0.2%.

[0041]

EXAMPLES In a 150 kg vacuum induction melting furnace, 23 kinds of steels having the chemical compositions shown in Table 1 were melted, and the obtained ingots were hot forged and then hot rolled to form steel plates having a thickness of 30 mm. This steel sheet was subjected to a normalizing process in a range of 950 to 1050 ° C, and a tempering process in a range of 700 to 760 ° C. The tempering conditions were all adjusted so that the Vickers hardness was within the range of 200 ± 10.

[0042]

[Table 1] From the steel sheet after tempering, a Charpy impact test specimen, an iron-strength y-type restraint weld crack specimen and a creep rupture specimen were sampled, and a ductile-brittle transition temperature was measured by a Charpy impact test under the following conditions. Evaluation of SR crack resistance by y-type constrained welding crack test, and 500 ° C by creep test
× 7000 hours strength was measured.

(1) Creep rupture test Specimen diameter: 6 mm Distance between gauge points: 30 mm A test was conducted at 500 ° C. for a maximum of 10,000 hours, and
The average creep rupture strength at 0 ° C. × 7000 hours was determined.

(2) Charpy impact test Test temperature: -80 ° C to + 80 ° C (3) Iron laboratory type y-type restraint welding crack test Test piece: 30 mm thickness x 150 mm width x 200 mm length Slit length: 80 mm Welding method: coated arc welding After welding, heating rate is 100 After performing an annealing treatment under the conditions of ° C / h, holding temperature of 700 ° C, and holding time of 5 hours, JIS
The section cracking rate was measured in accordance with Z3158, and the SR resistance was measured.
The breakability was evaluated. Table 2 shows the test results.

[0045]

[Table 2] In Table 2, symbol A steel has C content and M value, steel B has Mo content and M value, steel C has Mo content, steel D has W content, steel E has Cr, V content and M value. , F steel is a comparative steel having V content and M value, and G steel is a comparative steel having V content outside the range of the present invention. Table 2
As is clear from the above, all of the comparative steels have poor toughness after tempering. Further, the A, B, D, E, F and G steels were inferior in SR crack resistance.

On the other hand, in the steels of the present invention, the M value was 0.2 or more, and the ductility-brittle transition temperature after tempering was -30 ° C. or less, indicating good toughness. In addition, no SR cracking occurred in the Tekken y-type restraint welding crack test.

The average creep strength at 500 ° C. for 7000 hours was 250 MPa, and the creep characteristics were also good.

FIGS. 1 and 2 show the relationship between the M value and the toughness and SR crack resistance based on the test results shown in Table 2.

[0049]

The heat-resistant alloy of the present invention has a high creep strength at a high temperature of 400 ° C. or higher without causing temper embrittlement and SR cracking after welding, and is a structural material exposed to a high temperature for a long time. Furthermore, it is suitable for structural material members requiring heat treatment for removing residual stress after welding and working, and has many advantages in terms of construction.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a solid solution Mo coefficient M and toughness after tempering.

FIG. 2 is a graph showing a relationship between a solid solution Mo coefficient M and susceptibility to SR cracking.

Claims (3)

[Claims] C: 0.01 to 0.25% Mo: 0.1 to 2.5% V: 0.01 to 0.5% Cr: 0.05 to 3% N: 0 0.11% or less, and the index M represented by the following formula (1) is 0.1 or more: M = (0.08Cr + 0.03 / C + 0.04 + 0.855V) × (Mo + 0.5√Mo) × 0.65 (1) Here, the element symbol indicates the content (% by mass) of each element. 2. The method according to claim 1, wherein the mass M further includes W: 0.05 to 3%, and an index M represented by the following formula (2) is 0.1 or more. High strength low alloy heat resistant steel as described. M = (0.08Cr-0.1W + 0.03 / C + 0.04 + 0.855V) × (Mo + 0.5√Mo) × 0.65 (2) (3) Ti: 0.001 to 0.05 in mass%.
The high-strength low-alloy heat-resistant steel according to claim 1 or 2, comprising one or two of Nb: 0.005 to 0.1%.