JP3570379B2 - Low alloy heat resistant steel - Google Patents

Description

【表２】

【発明の効果】
実施例の試験結果から明らかなように、本発明の鋼は、合金成分の適切な含有によって、耐熱材料として優れた総合特性を持っている。その上で、高温強度、長時間クリープ強度または／および低温靱性が特に大きく改善されている。
【００９１】
本発明鋼はＣｒ含有量が１．５〜３％の安価な低合金鋼でありながら、上記の優れた特性を有するので、オーステナイト系耐熱鋼や高Ｃｒ耐熱鋼に代えて使用することができる。
【図面の簡単な説明】
【図１】ラス幅（および亜結晶粒径）の平均値と、高温強度との関係を示す図である。
【図２】固溶［Ｗ＋Ｍｏ］量と、クリープ強度との関係を示す図である。
【図３】Ｍｎ含有量とクリープ強度との関係を示す図である。
【図４】Ｍｎ含有量と固溶［Ｗ＋Ｍｏ］量との関係を示す図である。
【図５】平均結晶粒径とクリープ強度との関係を示す図である。
【図６】平均結晶粒径とシャルピー衝撃特性との関係を示す図である。
【図７】Ｎ含有量と平均結晶粒径との関係を示す図である。
【図８】粒界の固溶［Ｗ＋Ｍｏ］濃度と衝撃値の関係を示す図である。
【図９】結晶粒界のＷ、Ｍｏの濃度分布を模式的に示す図である。
【図１０】本発明鋼の金属組織の一例を説明するミクロ組織図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a heat-resistant steel suitable for use at a high temperature of 500 ° C. or more, particularly in the fields of boilers, nuclear power, and chemical industries. The heat-resisting steel of the present invention has all the properties required for the above-mentioned applications, and is characterized in that at least one of high-temperature strength, long-time creep strength and low-temperature toughness is remarkably excellent. The heat-resistant steel of the present invention is particularly suitable for use in, for example, heat exchanger tubes, piping tubes, and the like.
[0002]
[Prior art]
Austenitic stainless steel, high Cr ferrite steel with a Cr content of 9 to 12%, low Cr-Mo ferrite with a Cr content of 3.5% or less are used as high-temperature and heat-resistant members for boilers, chemical industries, nuclear power, and the like. Steel and carbon steel are used. Among them, low-alloy heat-resistant steel represented by 2.25% Cr-1% Mo steel is inexpensive and is used in large amounts according to the use environment (all percentages in the present specification are mass%. Is).
[0003]
The low alloy heat-resistant steel containing several% of Cr generally has a so-called ferritic structure such as tempered bainite and tempered martensite. In recent years, low-alloy heat-resisting steels which have high strength at high temperature and improved toughness by adding Mo, W, V, Nb, etc., based on this low-alloy steel have been disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 4-268040 and Japanese Patent Publication No. Many proposals have been made in, for example, JP-A-2926, JP-A-6-6771, JP-A-8-325669, and JP-A-10-46290.
[0004]
In the boiler design standard, for example, in a temperature range where creep phenomenon hardly causes a problem even in a high temperature range, tensile strength (tensile strength at use temperature: referred to as high temperature strength) is emphasized, and a temperature range where creep phenomenon becomes a problem, for example, At 520 ° C. or higher, long-term creep strength is emphasized.
[0005]
It is conventionally known that the addition of precipitation strengthening elements such as V and Nb improves the high-temperature strength and the long-term creep strength. However, since the high-temperature strength and the creep strength have different strengthening mechanisms, these two properties are not always improved simultaneously. Further, since these precipitation strengthening elements cause lowering of the low-temperature toughness, the use is restricted depending on the shape and thickness of the product.
[0006]
In a ferritic heat-resistant steel, even if the high-temperature strength or the short-time creep strength is high, the creep strength may be significantly reduced when exposed to high temperature for a long time (for example, a period exceeding 10,000 hours) with a low load stress. . Such a long-term decrease in creep strength cannot be predicted from the results of a short-term creep test (for example, a test under conditions where the rupture time is about 3,000 hours). That is, even if the material has sufficient strength in an unused state or a state used for a short time, the strength may be reduced in a long-time use. Therefore, unless long-term creep strength exceeding 10,000 hours is high, its use as a heat-resistant structural material is limited.
[0007]
The conventional low-alloy heat-resistant steel proposed in the above-mentioned publication is insufficient in stability of the long-term creep strength, and there is room for improvement in high-temperature strength and low-temperature toughness. In particular, a low-alloy heat-resistant steel having all of high-temperature strength, long-time creep strength and low-temperature toughness has not yet been developed.
[0008]
[Problems to be solved by the invention]
The present invention has been made to provide an inexpensive heat-resistant steel that can be used in place of austenitic stainless steel and high Cr ferritic steel in a field where the use of low-alloy heat-resistant steel has been limited. Takes advantage of low-alloy steel with a Cr content of 3% or less, has the basic characteristics of a heat-resistant material used in a temperature range of 500 ° C. to 600 ° C., and further has high-temperature strength and long-term creep. One or more of the strength and low-temperature toughness is to provide a heat-resistant steel that is particularly excellent, and a method for producing the same.
[0009]
[Means for Solving the Problems]
The present inventor has obtained the following findings as a result of repeated experiments for the purpose of clarifying the factors governing various properties of a low-alloy steel including the high-temperature strength.
(1) High temperature strength
(1) -1. The tensile strength of a heat-resistant ferritic steel made of bainite or martensite is greatly reduced at high temperatures. However, the high-temperature strength can be improved by increasing the tempering softening resistance.
[0010]
(1) -2. To improve the tempering softening resistance, it is effective to add a precipitation strengthening element such as V, Nb, Mo, W, B, or a solid solution strengthening element.
[0011]
(1) -3. Of these elements, Mo and W have conventionally been considered to have approximately the same effect. However, the effect of W is greater in improving the tempering softening resistance, and the higher the amount of W added, the higher the high-temperature strength.
[0012]
(1) -4. In the structure of the ferritic heat-resistant steel composed of bainite and martensite, small units such as laths or sub-crystal grains are present in the prior austenite grains. Temper softening resistance is improved by the refinement of lath or sub-crystal grains. The lath here is a small unit constituting martensite or bainite, and the sub-crystal grains are sub-crystal grains of ferrite formed by tempering (see FIG. 10 described later).
(2) Long-term creep strength
(2) -1. For example, heat-resistant steel used for a long time at a high temperature as a boiler tube requires a material design that further enhances the long-term creep strength. However, even when a large amount of the precipitation strengthening element or the solid solution strengthening element is added, the creep strength may decrease during long-time use.
[0013]
(2) -2. The decrease in the long-term creep strength can be prevented by suppressing the precipitation of W and Mo and maintaining the total amount of solid solution W and solid solution Mo (hereinafter, referred to as solid solution [W + Mo] amount). However, even if the amount of solid solution [W + Mo] is sufficient, if the crystal grains are fine, the creep strength decreases. That is, in order to improve the creep strength, it is necessary to design a component that suppresses the precipitation of W and Mo at a high temperature for a long time and reduces the crystal grain size.
[0014]
(2) -3. Since W has a lower deposition rate than Mo and can exist in a solid solution state for a longer time, W is more effective than Mo.
[0015]
(2) -4. The precipitation of W and Mo is suppressed by reducing the Mn content and adding V.
(3) Low temperature toughness
(3) -1. For example, in the case of a thick member or a steel material used for a part having a complicated shape, a material design particularly considering low-temperature toughness is required.
[0016]
(3) -2. In a material with low temperature toughness, the total concentration of solid solution W and solid solution Mo at the grain boundary decreases, that is, the total amount of solid solution W and solid solution Mo on the grain boundary (that is, the solid solution [W + Mo] amount). Deficiency is observed. Preventing a decrease in the amount of solid solution [W + Mo] on the grain boundaries can improve low-temperature toughness. The amount of solid solution [W + Mo] on the grain boundaries effective for improving low-temperature toughness is 0.4% or more.
The amount of solid solution [W + Mo] on the grain boundary is a concentration not containing a precipitate and means the amount of solid solution [W + Mo] within a range of ± 0.5 μm across the grain boundary.
[0017]
(3) -3. Low-temperature toughness can be further improved by increasing the nitrogen content (higher N content). This is because the precipitation amount of NbN increases and coarsening of crystal grains is suppressed.
[0018]
(3) -4. The particle size effective for improving the low-temperature toughness is 150 μm or less in average particle size.
[0019]
The present invention has been made based on the above findings, and the features of the heat-resistant steel of the present invention are as follows.
[0020]
(A) Chemical composition (% relating to component content is% by mass)
C: 0.03 to 0.15%, Si: 0.7% or less, Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5%, W: 1 -3%, V: 0.05-0.35%, Nb: 0.01-0.1%, N: 0.002-0.03%, B: 0.0001-0.02%, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0. 05% or less, Fe and impurities: balance.
[0021]
Among these components, Si, Mn, Al, Ti, Ni, Cu, Mg, Ca, and Zr do not always need to be positively added. The content when not added is the usual impurity level. However, since each has a specific function and effect, it may be contained in the range of not more than the above upper limit depending on the purpose of use of the steel. The effect and desired content in that case will be described later.
(B) Metal structure and others
(B) -1. In particular, in steels characterized by high high-temperature strength, the average lath width of bainite and martensite and the average grain size of subcrystal grains are 3 μm or less.
[0022]
(B) -2. Particularly, in a steel characterized by high long-term creep strength, the solid solution [W + Mo] exceeds 0.9% and the average crystal grain size is 5 μm or more.
(B) -3. In a steel characterized by particularly excellent low-temperature toughness, the solid solution [W + Mo] on the grain boundaries is 0.4% or more and the average crystal grain size is 150 μm or less.
[0023]
The steel of the present invention may include the above-mentioned features (B) -1 to (B) -3 independently, or may include a plurality of these in combination.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the chemical composition and metal structure of the steel of the present invention will be described in detail.
(A) Chemical composition
C: 0.03 to 0.15%
C combines with Cr, Fe, W, Mo, V, Nb and Ti optionally added to contribute to high-temperature strength, and itself is an austenite stabilizing element, so that martensite or / and bainite can be used. Important to form an organization. If the C content is less than 0.03%, these effects cannot be obtained, and the crystal grains are coarsened to deteriorate the toughness. On the other hand, when the C content exceeds 0.15%, carbides are excessively precipitated, and the steel is hardened to impair workability and weldability. Further, precipitation of W or Mo added as a solid solution strengthening element is promoted, and the amount of these solid solutions is reduced. Therefore, the appropriate content of C is 0.03 to 0.15%. Desirable is 0.04 to 0.12%, and more preferable is 0.05 to 0.08%.
[0025]
Si: 0.7% or less
Si is effective as a deoxidizing agent for steel and has an effect of increasing steam oxidation resistance. However, when the Si content exceeds 0.7%, toughness and workability are reduced, and strength is reduced. Further, it promotes tempering embrittlement and impairs weldability.
[0026]
Since Si need not always be added, the lower limit of the content may be the impurity level. However, when the steam oxidation resistance is intended to be improved, the content is preferably 0.2% or more. Even in that case, the upper limit of the content is set to 0.7% for the above-mentioned reason. The desirable content is 0.5% or less, and the most desirable content range is 0.2 to 0.3%.
[0027]
Mn: 0.7% or less
Since Mn is an element effective for improving hardenability, Mn is added as necessary. However, if the Mn content exceeds 0.7%, the precipitation of W and Mo is promoted, and the solid solution amount thereof is reduced, so that the tempering softening resistance is reduced and the high-temperature strength is significantly reduced. Further, like Si, it promotes temper embrittlement and deterioration of weldability. Therefore, the appropriate content of Mn is 0.7% or less.
[0028]
In order to improve the long-term creep strength, the Mn content is desirably suppressed to 0.5% or less. This is because the amount of solid solution of W and Mo increases. To make the most of this effect, the Mn content is more desirably less than 0.35%.
[0029]
3 and 4 show some of the test data of the examples described later (steel No. is steel No. in Table 1 described later). As is clear from FIG. 3, the creep strength of steel with a higher Mn content is lower. On the other hand, in FIG. 4, the higher the Mn content of the steel, the smaller the amount of solid solution [W + Mo]. 3 and 4, it can be seen that the amount of solid solution [W + Mo] can be increased by reducing the Mn content, and thus the long-term creep strength can be increased.
[0030]
Cr: 1.5-3%
Cr is an essential element for improving the oxidation resistance and high-temperature corrosion resistance of low alloy steel. To increase the tensile strength or creep strength at a high temperature such as 500 to 600 ° C., a Cr content of 1.5% or more is required. On the other hand, if the Cr content exceeds 3%, toughness and weldability are deteriorated, and the material cost is increased, and the characteristics of the low alloy steel are impaired. Therefore, the proper content of Cr is 1.5 to 3%. The desirable Cr content is 2-3%, and the most desirable is 2.2-2.7%.
[0031]
Mo: 0.01 to 0.5%
Mo is an element having the functions of solid solution strengthening and precipitation strengthening like W. Although its effect is smaller than that of W, even a very small amount prevents grain boundary embrittlement and does not impair ductility even with a high-strength material. It is also effective in improving low-temperature toughness and weldability. However, if it is less than 0.01%, the effect cannot be expected. If it exceeds 0.5%, toughness and workability are impaired. Therefore, the appropriate content of Mo is 0.01 to 0.5%. More preferred is 0.03-0.3%, most preferred is 0.05-0.15%.
[0032]
W: 1-3%
W is an element having the functions of solid solution strengthening and precipitation strengthening, similar to Mo, but since diffusion is slower than that of Mo, the solid solution state continues for a long time, and the solid solution strengthening action is maintained for a long time. For this reason, it is effective in improving temper softening resistance and improving long-time creep strength. Further, coarsening of carbides containing Cr, Fe, Mo, W and the like is suppressed. This is also because the diffusion of W is slow. However, if the W content is less than 1%, the above effects cannot be obtained. On the other hand, when the W content exceeds 3%, the steel is hardened significantly, and toughness, workability, and weldability are impaired. Therefore, the proper content of W is 1 to 3%, preferably 1.3 to 2%, and more preferably 1.4 to 1.8%.
[0033]
The above contents of Mo and W are the average contents of the steel of the present invention. When the amount of solid solution [W + Mo] exceeds 0.9%, the creep strength is greatly improved. The presence of more than 1% of solid solution [W + Mo] is even more desirable. This will be described in detail later.
[0034]
V: 0.05-0.35%
V mainly combines with C to form MX (M represents W + Mo + V, X represents C, that is, carbon) type fine carbide, and contributes to improvement of creep strength. Further, V binds to C in preference to W and Mo, and therefore suppresses the formation of W and Mo carbides and suppresses a decrease in the amount of solid solution [W + Mo]. If it is less than 0.05%, this effect is not sufficient, and if it exceeds 0.35%, the high temperature strength and the creep strength are impaired, and the toughness and weldability are also reduced. Therefore, the appropriate content of V is 0.05 to 0.35%. Desirable is 0.1 to 0.28%, and more preferable is 0.18 to 0.25%.
[0035]
Nb: 0.01 to 0.1%
Nb combines with N and C to form an MX (M represents Nb, X represents C + N) type fine carbonitride, suppresses grain growth, contributes to improvement in high-temperature strength, and improves low-temperature toughness. Let it. If the Nb content is less than 0.01%, this effect cannot be expected. On the other hand, if the Nb content exceeds 0.1%, the strength is reduced due to abnormal grain growth, and a coarse nitride is formed, and the toughness is rather deteriorated. Therefore, the proper content of Nb is 0.01 to 0.1%. Desirable is 0.02 to 0.07%, and more preferable is 0.04 to 0.06%.
[0036]
N: 0.002 to 0.03%
N combines with Nb and C to form an MX (M represents Nb, X represents C + N) type fine carbonitride, suppresses grain growth, and, together with solid solution strengthening by solid solution N, increases the high temperature. It contributes to improvement in strength and low-temperature toughness. However, when the N content is less than 0.002%, the precipitation amount of MX is small, and the effects of suppressing grain growth, improving toughness, and improving high-temperature strength cannot be sufficiently obtained. On the other hand, N exceeding 0.03% forms a coarse nitride and rather deteriorates toughness. Therefore, the appropriate content of N is 0.002 to 0.03%.
[0037]
In order to increase the low-temperature toughness, it is necessary to suppress the coarsening of crystal grains by NbN. In order to effectively precipitate NbN, it is desirable to contain 0.004% or more and up to 0.01% of N. Even more desirable is a range from more than 0.005 to 0.01%.
[0038]
FIG. 7 is a plot of the relationship between the N content and the average crystal grain size of the steels of the examples described later (the steel No. in Table 1 is shown in the figure). It can be seen that if the N content is 0.004% or more, a desirable average crystal grain size of 150 μm or less can be obtained.
[0039]
B: 0.0001 to 0.02%
B improves the hardenability of the steel and improves the high-temperature strength and low-temperature toughness. Further, it stabilizes carbides for a long time and contributes to improvement of long-term creep strength. However, if the B content is less than 0.0001%, the above effects cannot be obtained. On the other hand, when the B content exceeds 0.02%, coarse carbides are formed on the grain boundaries, and the lack of W and Mo at the grain boundaries is promoted, and the toughness and weldability are rather deteriorated. Therefore, the appropriate content of B is 0.0001 to 0.02%. A more desirable content is 0.001 to 0.01%. More preferably, it is 0.003 to 0.004%.
[0040]
Al: 0.03% or less
Al is added as a deoxidizing agent. In addition, since Al combines with N to form a nitride, Al is effective in preventing coarsening of steel. However, when the Al content exceeds 0.03%, creep strength and workability are impaired. Therefore, the suitable content of Al is 0.03% or less, preferably 0.02% or less. More desirable is less than 0.01%. The Al content may be at a normal impurity level.
[0041]
The steel of the present invention can optionally contain the following components in addition to the above-described components, if necessary. When these components are not positively added, the lower limits of the contents are all normal impurity levels.
[0042]
Ti: 0.05% or less (desirable content when added is 0.002 to 0.05%)
Ti combines with Nb together with N to form an MX (M is Ti + Nb, X is N, ie, nitrogen) type nitride, increasing the high temperature strength of the steel. In order to surely obtain such an effect, the content is preferably 0.002% or more. However, when the Ti content exceeds 0.05%, the nitride coarsens, and the segregation of TiC induces abnormal grain growth and formation of a mixed grain structure. Therefore, when adding Ti, the appropriate range of the content is 0.002 to 0.05%. More preferably, it is 0.003 to 0.02%, and still more preferably 0.005 to 0.01%.
[0043]
Ni: 0.8% or less (desirable content when added is 0.1 to 0.5%)
Ni is an austenite stabilizing element and contributes to improvement in toughness, and therefore may be added as necessary. However, if the Ni content exceeds 0.8%, the long-term creep strength decreases, so that the Ni content should be kept to 0.8% or less even when Ni is added. Desirable is 0.5% or less, and more desirable is 0.3% or less. When Ni is added in order to obtain the above effects, the lower limit of the content is preferably set to 0.1%.
[0044]
Cu: 0.5% or less (desirable content when added is 0.1-0.3%)
Cu is also an austenite stabilizing element like Ni, and contributes to improvement in toughness. Therefore, Cu can be added instead of Ni, or together with Ni, or each alone as needed. However, when the Cu content exceeds 0.5%, tempering embrittlement is likely to occur. Therefore, even if it is added positively, its content should be 0.5% or less. Desirable is 0.3% or less. When Cu is added to obtain the above effects, the lower limit of the content is preferably set to 0.1%. It is desirable that Cu be used in combination with Ni.
[0045]
Mg, Ca, Zr: 0.05% or less for each (desirable contents when added are 0.0005 to 0.05%, respectively)
These elements combine with the impurities P, S and O (oxygen) to suppress their undesired effects and improve the toughness of the steel. In any case, if the content is less than 0.0005%, the effect is not remarkable. Therefore, it is preferable that the content is 0.0005% or more. However, if the content of each exceeds 0.05%, the number of inclusions increases, and on the contrary, the toughness is impaired. Therefore, even when they are added, their contents should be 0.05% or less, and when two or more of these elements are used, the total content is preferably 0.05% or less. Desirable is 0.01% or less. If no addition is made, the lower limit is the impurity level.
[0046]
The essential components and optional components of the steel of the present invention are as described above. The balance is Fe and impurities, but P and S among the impurities impair the toughness and creep ductility of steel in particular. The allowable upper limit of P is 0.03%, and that of S is 0.015%.
[0047]
(B) Metal structure and others
(B) -1. (Lath width or / and subcrystal grain size: 3 μm or less)
The structure of the low alloy heat-resistant steel of the present invention basically comprises bainite or martensite or a mixed structure thereof. It may also contain a small amount of ferrite generated during tempering.
[0048]
FIG. 10 is a diagram showing an example of the metallographic structure of the steel of the present invention (No. 1 steel shown in Table 1 below). As shown in the figure, lath or ferrite subcrystal is the minimum unit constituting bainite, martensite and their tempered structures. If the average of these is larger than 3 μm, the tempering softening resistance is reduced and the high temperature strength is significantly reduced. Therefore, when high-temperature strength is particularly important, the average lath width in the tempered structure of bainite and / or martensite is 3 μm or less, and when ferrite is included, the average of the subcrystal grain size is 3 μm or less. Need to be smaller. Desirably, each is 2 μm or less, and more preferably, 1 μm or less.
[0049]
FIG. 1 is a diagram showing the relationship between the average value of the lath width (and subcrystal grain size) and the high-temperature strength of the steel (the steel shown in Table 1) of Examples described later. As is clear from this figure, the smaller the lath width (and the sub-crystal grain size), the higher the high-temperature strength.
[0050]
The following methods are available for reducing the lath width of martensite and bainite and the grain size of sub-crystal grains of ferrite.
[0051]
{Circle around (1)} A method in which the contents of W and Mo are increased to make the dislocations less likely to move, thereby suppressing the movement of the lath interface and grain boundaries.
[0052]
{Circle around (2)} A method of suppressing grain growth by utilizing the pinning effect of MX carbonitride (M is Ti, Nb, X is C, N).
[0053]
Specifically, the steel of the present invention can be manufactured by the following manufacturing method. That is, after heating a raw material (a billet, a slab, or the like) to, for example, 1,100 ° C. or more, processing at 20% or more at an austenite temperature is performed, and the material is cooled to 300 ° C. or less, and thereafter, 1,000 ° C./h or less A predetermined austenitizing temperature (Ac₁And a temperature of 1,150 ° C. or less is desirable). Next, at the temperature rising rate of 1,000 ° C./h or less, “Ac₁Point -50 ° C ”to Ac₁A predetermined tempering is performed by heating to a tempering temperature up to the point.
[0054]
In order to obtain the structure defined by the present invention, it is particularly important to adjust the temperature increase rate during normalizing and tempering. This is because, in part, fine MX carbonitrides (M is Ti, Nb, X is C, N) are precipitated in the course of temperature rise during normalizing, and coarse austenite is formed. In order to suppress the formation of carbonitrides, in the second step, a carbonitride MX mainly composed of V (M is V, X is C, N) is precipitated during the temperature raising process during tempering, and martensite or / and bainite lath and ferrite are precipitated. This is for suppressing coarsening of the sub-crystal grains. The above effects cannot be obtained at a high temperature rising rate exceeding 1,000 ° C./h in both normalizing and tempering. A desirable heating rate is 500 to 700 ° C./h.
[0055]
The coarsening of laths and sub-crystal grains occurs particularly during tempering. Therefore, in order to obtain the above effects, it is important to suppress the precipitation of W and Mo during tempering as much as possible, to increase the amount of solid solution [W + Mo], and to make the MX carbonitride finer. . For this purpose, it is desirable to adjust the temperature rise rate during the heat treatment as described above together with the adjustment of the components.
[0056]
(3) Solid solution [W + Mo] amount: amount exceeding 0.9%
This is a condition to be provided especially when long-term creep strength is required. In a low alloy steel having a Cr content of 3% or less, W and Mo precipitate as carbides during long-term use. However, long-term creep strength is maintained by keeping a large amount of W and Mo in a solid solution state for as long as possible. And the creep ductility is improved. Specifically, within the normal life of a boiler (for example, 20 years), it is necessary to secure the amount of solid solution [W + Mo] to about 0.5% or more even during use. In order to secure the amount of solid solution [W + Mo] of steel in use to 0.5% or more, the amount of solid solution [W + Mo] in a product (for example, a boiler tube) before use is 0.9% (desirably). Must exceed 1%). If it is 0.9% or less, the amount of solid solution [W + Mo] is reduced to less than 0.5% during use, and the creep strength is greatly reduced.
[0057]
FIG. 2 shows the relationship between the solid solution [W + Mo] before use and the creep strength at 575 ° C. × 100,000 hours of the steel (the steel shown in Table 1) in Examples described later.
[0058]
The amount of solid solution [W + Mo] can be determined, for example, by extracting only the precipitate by the extraction residue method, performing quantitative chemical analysis, and subtracting the extraction residue amount from the total amount of W and Mo contained in the steel.
[0059]
The amount of solid solution [W + Mo] exceeding 0.9% can be ensured by the above production conditions. It is particularly important to set the tempering temperature to "Ac₁Point -50 ° C ”to Ac₁The range is up to the point. Even when the temperature is lower or higher than this temperature range, the amount of solid solution [W + Mo] decreases.
[0060]
(4) Average crystal grain size: 5 μm or more, or 150 μm or less
In particular, when long-term creep strength is required, it is necessary to ensure the amount of the solid solution [W + Mo] and to make the average crystal grain size 5 μm or more. In the low alloy heat-resistant steel, if the average crystal grain size is less than 5 μm, the creep strength decreases. Therefore, the average crystal grain size needs to be 5 μm or more. It is preferably at least 20 μm, more preferably at least 40 μm.
[0061]
FIG. 5 is a diagram in which test data of Examples described later are arranged in relation to the average particle size and the creep strength. As shown, steel having a crystal grain size of 5 μm or more, particularly 40 μm or more shows high creep strength (for details, see Table 2 in Examples).
[0062]
On the other hand, for improving low-temperature toughness, a fine-grained structure is better. If the average crystal grain size exceeds 150 μm, the toughness is remarkably deteriorated.
[0063]
FIG. 6 is a diagram in which the data of Examples described later are arranged in relation to the average crystal grain size and the impact value. It is clear that the impact value is significantly increased at a particle size of 150 μm or less, particularly at 80 μm or less. Therefore, when importance is placed on low-temperature toughness, the average crystal grain size should be 150 μm or less, preferably 100 μm or less. It is more preferable that the thickness be 80 μm or less.
[0064]
As described above, in order to increase the long-term creep strength, the average crystal grain size is desirably 5 μm or more. Therefore, in order to combine long-term creep strength and low-temperature toughness, the average crystal grain size should be 5 to 150 μm, preferably 20 to 100 μm, and more preferably 40 to 80 μm.
[0065]
(5) Amount of solid solution [W + Mo] on grain boundaries: 0.4% or more
This is a condition to be prepared especially when importance is attached to low-temperature toughness.
[0066]
As schematically shown in FIG. 9, in the case of a Cr-containing low alloy, a depletion layer in which the concentration of Cr, W, Mo, V, etc. is locally reduced may be formed at the grain boundary. The layers result in poor low temperature toughness.
[0067]
FIG. 8 summarizes the data of Examples described later in relation to the amount of solid solution [W + Mo] on the grain boundary and the impact value. When the solid solution [W + Mo] on the grain boundary is less than 0.4%, the impact value is remarkably low. Therefore, when low-temperature toughness is emphasized, the amount of solid solution [W + Mo] on the grain boundary is desirably 0.4% or more.
[0068]
The amount of solid solution [W + Mo] on the grain boundary means an average value of the amount of solid solution [W + Mo] within a range of ± 0.5 μm across the grain boundary. It can be measured using:
[0069]
The control of the amount of solid solution [W + Mo] on the grain boundaries as described above can be realized by adjusting the tempering time and temperature according to the steel composition. Desirable tempering conditions are “Ac₁Point -50 ° C ”to Ac₁Is maintained for 0.5 to 1 hour within the range up to the point.
[0070]
【Example】
1. Test method
Ingots obtained by melting and casting each steel having a chemical composition shown in Table 1 in a 50 kg vacuum melting furnace were forged at 1150 to 950 ° C., and processed by 75 to 85% to obtain plates having a thickness of 20 mm.
[0071]
The heat treatment conditions for each steel are as follows.
[0072]
Steel No. 1 to 18:
Normalization: heating to 1,000 to 1,100 ° C. at a heating rate of 500 ° C./h, holding for 30 minutes, and air cooling to room temperature.
[0073]
Tempering: heating to 750-790 ° C. at a heating rate of 500 ° C./h, holding for a predetermined time determined by the following tempering parameters, and then air cooling to room temperature.
[0074]
Steel 7-1:
Normalization: Heated at a rate of 1,000 ° C / h to 1,050 ° C, held for 30 minutes, and air-cooled to room temperature.
Tempering: 800 ° C (Ac) at a heating rate of 500 ° C / h₁(Point + 5 ° C.) and maintained for a predetermined time determined by the following tempering parameters, and then air-cooled to room temperature.
[0075]
Steels 2-2 and 16-1:
Normalization: heating to 10,000 deg. C. at a heating rate of 10,000 deg./h, holding for 30 minutes, and air cooling to room temperature.
[0076]
Tempering: 650 ° C at a heating rate of 500 ° C / h (Ac₁(Point -145 ° C.), hold for a predetermined time determined by the following tempering parameters, and air-cool to room temperature.
[0077]
The holding time of the tempering process is a tempering parameter, TP [TP = (T + 273) logt + 20). Here, T is the temperature (° C.) and t is the time (h)] was adjusted in the range of 21,000 to 21,300.
From the steel sheets thus obtained, a test piece for extracting extraction residue, a creep rupture test piece, a Charpy impact test piece, and a constant load tensile test piece were sampled and subjected to the following tests.
[0078]
(1) Measurement of the amount of solid solution [W + Mo]:
Electrolyte solution: 10% acetylacetone-methanol solution
Current density: 20 mA / cm²
Dissolution amount: 0.4 g
The W content and the Mo content in the extraction residue collected under the above conditions were defined as the amount of precipitation, and the difference from the total amount of W and Mo in the steel was defined as the amount of dissolved W and the amount of dissolved Mo.
[0079]
(2) High temperature tensile test: JIS G0567
Test piece: φ6mm x GL 30mm
Test temperature: 575 ° C
(3) Creep rupture test: JIS Z2272
Test piece: φ6mm x GL 30mm
Test temperature: 500-650 ° C
Load stress: 50 to 350 MPa
Under the above conditions, data up to a maximum of 15,000 hours were collected and extrapolated from the average value of creep rupture strength at 575 ° C. × 100,000 hours. Further, the difference between the average strength at 10,000 hours and the average strength at 100 hours at 575 ° C. was determined to evaluate the stability of the long-term creep strength.
[0080]
(4) Charpy impact test:
Test piece: 10 mm x 10 mm x 55 mm, 2 mm V notch
Test temperature: 0 ° C
(5) Measurement of concentration of solid solution [W + Mo] at grain boundaries:
It was measured by EDX analysis (energy dispersive X-ray analysis) using a transmission electron microscope. Ten points were analyzed at a pitch of 0.1 μm in a range of ± 0.5 μm across the grain boundary, and the minimum value of the amount of [W + Mo] therein was defined as the concentration of solid solution [W + Mo].
[0081]
(6) Measurement of particle size:
The grain size of 100 prior austenite grains was measured for each steel and evaluated by the average value.
2. Test results
Table 2 shows the test results. The test results are summarized below.
[0082]
(1) The chemical compositions of all steels shown in Table 1 are within the range specified by the present invention. Some of them satisfy one or more of the following conditions (1) to (3).
[0083]
Condition (1): The average of the lath width of bainite, the lath width of martensite, and the grain size of ferrite subcrystals are 3 μm or less.
[0084]
Condition (2): Solid solution [W + Mo] exceeds 0.9% and average particle diameter is 5 μm or more.
[0085]
Condition (3): The solid solution [W + Mo] on the grain boundary is 0.4% or more and the average particle size is 150 μm or less.
[0086]
(2) Steel No. Since steels other than 2-2 and 16-1 all satisfy condition (1), they have particularly high high-temperature strength exceeding 400 MPa at 575 ° C.
[0087]
(3) Steels satisfying the condition (2) (Steel Nos. 1, 2, 3 to 7, 8 to 11, 14, 16, and 17) have an average strength at 575 ° C. × 100,000 hours exceeding 100 MPa. That is, it has a particularly high creep strength. The difference between the creep rupture strength at 575 ° C. × 100 hours and the creep rupture strength at 575 ° C. × 10,000 hours is 90 MPa or less, and the long-term creep strength is extremely stable.
[0088]
(4) Steels satisfying the condition (3) (steels other than Nos. 14, 16, and 17) are No.s that do not satisfy the condition (3). Impact properties superior to steels 14, 16, and 17 (250 J / cm²Impact value).
[0089]
(5) Steel No. satisfying all of conditions (1) to (3). 1, 2, 3 to 7, 8 to 12 and 15 are remarkably excellent in all of high-temperature strength, long-term creep strength and low-temperature toughness.
[0090]
[Table 1]

[Table 2]

【The invention's effect】
As is clear from the test results of the examples, the steel of the present invention has excellent overall properties as a heat-resistant material due to the proper content of alloy components. In addition, the high-temperature strength, long-term creep strength and / or low-temperature toughness are particularly greatly improved.
[0091]
The steel of the present invention is an inexpensive low-alloy steel having a Cr content of 1.5 to 3%, but has the above-mentioned excellent characteristics, and thus can be used in place of an austenitic heat-resistant steel or a high Cr heat-resistant steel. .
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between an average value of lath width (and subcrystal grain size) and high-temperature strength.
FIG. 2 is a graph showing the relationship between the amount of solid solution [W + Mo] and creep strength.
FIG. 3 is a diagram showing the relationship between Mn content and creep strength.
FIG. 4 is a diagram showing the relationship between the Mn content and the amount of solid solution [W + Mo].
FIG. 5 is a diagram showing a relationship between an average crystal grain size and creep strength.
FIG. 6 is a diagram showing a relationship between an average crystal grain size and a Charpy impact characteristic.
FIG. 7 is a diagram showing a relationship between N content and average crystal grain size.
FIG. 8 is a diagram showing the relationship between the solid solution [W + Mo] concentration at the grain boundary and the impact value.
FIG. 9 is a diagram schematically showing a concentration distribution of W and Mo at a crystal grain boundary.
FIG. 10 is a microstructure diagram illustrating an example of a metal structure of the steel of the present invention.

Claims (8)

In mass%, C: 0.03 to 0.15%, Si: 0.7% or less, Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5% , W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1%, N: 0.002 to 0.03%, B: 0.0001 to 0.02 %, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, Mg: 0.05% or less, Ca: 0.05% or less, Zr: contains 0.05% or less, with the balance being Fe and impurities, wherein the average of the lath width of bainite, the lath width of martensite, and the grain size of ferrite sub-crystal grains is 3 μm or less. Excellent low alloy heat resistant steel. In mass%, C: 0.03 to 0.15%, Si: 0.7% or less, Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5% , W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1%, N: 0.002 to 0.03%, B: 0.0001 to 0.02 %, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, Mg: 0.05% or less, Ca: 0.05% or less, Zr: a low alloy containing 0.05% or less, the balance being Fe and impurities, having a solid solution [W + Mo] of more than 0.9%, and having an average crystal grain size of 5 μm or more and having excellent long-term creep strength. Heat resistant steel. In mass%, C: 0.03 to 0.15%, Si: 0.7% or less, Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5% , W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1%, N: 0.002 to 0.03%, B: 0.0001 to 0.02 %, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, Mg: 0.05% or less, Ca: 0.05% or less, Zr: contains 0.05% or less, the balance consists of Fe and impurities, and the solid solution [W + Mo] on the grain boundary is 0.4% by mass or more and the average crystal grain size is 150 μm or less, and has excellent low-temperature toughness. Low alloy heat resistant steel. In mass%, C: 0.03 to 0.15%, Si: 0.7% or less, Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5% , W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1%, N: 0.002 to 0.03%, B: 0.0001 to 0.02 %, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the balance being Fe and impurities, the average of the lath width of bainite, the lath width of martensite, and the grain size of ferrite sub-crystal grains is 3 μm or less, and solid solution [W + Mo Is excellent in high-temperature strength and long-time creep strength, characterized by having a mean crystal grain size of 0.9% or more and an average crystal grain size of 5 μm or more. Alloy heat-resistant steel. In mass%, C: 0.03 to 0.15%, Si: 0.7% or less, Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5% , W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1%, N: 0.002 to 0.03%, B: 0.0001 to 0.02 %, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the balance being Fe and impurities, the average of the lath width of bainite, the lath width of martensite and the grain size of ferrite sub-crystal grains is 3 μm or less, and Low alloy heat-resistant steel excellent in high-temperature strength and low-temperature toughness having a solid solution [W + Mo] of 0.4% by mass or more and an average crystal grain size of 150 μm or less. . In mass%, C: 0.03 to 0.15%, Si: 0.7% or less, Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5% , W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1%, N: 0.002 to 0.03%, B: 0.0001 to 0.02 %, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, Mg: 0.05% or less, Ca: 0.05% or less, Zr: contains 0.05% or less, the balance consists of Fe and impurities, the solid solution [W + Mo] exceeds 0.9%, and the average crystal grain size is 5 μm or more and 150 μm or less, and A low-alloy heat-resistant steel excellent in long-term creep strength and low-temperature toughness, characterized in that the solid solution [W + Mo] of the steel is 0.4% by mass or more. In mass%, C: 0.03 to 0.15%, Si: 0.7% or less, Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5% , W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1%, N: 0.002 to 0.03%, B: 0.0001 to 0.02 %, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, one or two kinds, Mg: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, the balance being Fe and impurities, the average of the lath width of bainite, the lath width of martensite, and the grain size of ferrite sub-crystal grains is 3 μm or less. The solid solution [W + Mo] exceeds 0.9%, the average crystal grain size is 5 μm or more and 150 μm or less, and the solid solution [W + Mo] o] is 0.4 mass% or more, and high-temperature strength, wherein the average crystal grain size is, low alloy heat-resistant steel having excellent long-term creep strength and low temperature toughness. In mass%, C: 0.03 to 0.15%, Si: 0.7% or less, Mn: 0.7% or less, Cr: 1.5 to 3%, Mo: 0.01 to 0.5% , W: 1 to 3%, V: 0.05 to 0.35%, Nb: 0.01 to 0.1%, N: 0.002 to 0.03%, B: 0.0001 to 0.02 %, Al: 0.03% or less, Ti: 0.05% or less, Ni: 0.8% or less, Cu: 0.5% or less, Mg: 0.05% or less, Ca: 0.05% or less, Zr: steel containing 0.05% or less, with the balance being Fe and impurities, processed in the austenitic temperature range, then cooled to 300 ° C or less, and then at an austenite temperature range of 1,000 ° C / h or less. until heated performed normalizing baked, then heated to a temperature range at 1,000 ° C. / h less heating rate from "Ac ₁ point -50 ° C." to Ac ₁ point Method for producing a heat-resistant steel according to any one of claims 1 to 7, characterized in that the tempering Te.

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