JP4271311B2 - Ferritic heat resistant steel - Google Patents

Description

【００３１】
【表２】

【００３２】
表２から明らかなように本発明鋼は、高温クリープ特性および靱性がバランスよく、いずれも優れた特性を有している。
一方、比較鋼は、高温クリープ特性と靱性とのバランスが悪く、いずれか一方において明らかに劣っている。
【００３３】
【発明の効果】
以上説明したように、本願発明のフェライト系耐熱鋼は、重量％で、Ｃ：０．０７〜０．１４％、Ｓｉ：０．１０〜０．２５％、Ｎｉ：０．８％以下、Ｃｒ：９．５〜１３．０％、Ｍｏ：０．１〜０．６％、Ｖ：０．１４〜０．２４％、Ｎｂ：０．０３〜０．０８％、Ｗ：１．８〜２．８％、Ｃｏ：２．０〜３．５％、Ｂ：０．００２〜０．００８％、Ｎ：０．０２５％以下を含有し、さらに希土類元素：０．００１〜０．０３％、Ｃａ：０．００１〜０．０３％の一種以上を含有し、残部がＦｅおよび不可避的不純物からなり、希土類元素含有量： REM(%) 、Ｃａ含有量： Ca(%) 、Ｓｉ含有量： Si(%) 、Ｍｎ含有量： Mn(%) 、Ａｌ含有量： Al(%) が、（ 47 × REM(%) ＋ 33 × Ca(%) ）／（ 1.5 × Si(%) ＋ 0.9 × Mn(%) ＋ 8 × Al(%) ）≧１．０の関係式を満たす成分範囲にあるので、高温クリープ特性および靱性においてバランスよく優れた特性が得られるとともに高温腐食特性および水蒸気酸化特性においても優れた特性が得られ、より優れた高温特性が要求される火力発電システム等に好適な材料として提供することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ferritic heat resistant steel, and more particularly to a ferritic high Cr steel which is excellent in creep rupture characteristics at high temperatures, toughness and steam oxidation resistance, and suitable for boiler steel pipes and the like.
[0002]
[Background Art and Problems to be Solved by the Invention]
In applications that require high temperature characteristics such as boiler tubes of thermal power generation systems, ferritic high Cr steels that are relatively inexpensive and excellent in high temperature characteristics are often used as constituent materials.
By the way, in the above system, steam conditions have been increased in temperature in order to improve power generation efficiency. For example, a temperature condition of 630 ° C. or 650 ° C. (that is, 630 ° C. or higher) is assumed. With these trends, materials used for boiler tubes and the like are required to be further improved in terms of high-temperature creep strength, toughness, and steam oxidation resistance.
In recent years, in order to improve high temperature strength in accordance with the above requirements, heat resistant materials to which W or B is added have been developed. In addition, heat resistant steels with improved oxidation resistance have been proposed.
[0003]
However, even the conventional improved steel types cannot be said to have sufficient high temperature characteristics, and further improvements are desired.
The present invention has been made against the background of the above circumstances, and provides a ferritic heat resistant steel having further improved high temperature characteristics, particularly creep characteristics, toughness, high temperature corrosion resistance, and steam oxidation resistance compared to conventional materials. Objective.
[0004]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention optimizes the alloy elements, actively adds REM and Ca, and further reduces the amount of Si, Mn, and Al as deoxidizers and desulfurizers, thereby providing excellent acid resistance. It is an object of the present invention to obtain a steel having a tempering property and maintaining a high temperature strength and a high toughness.
[0005]
That is, in order to achieve the above-described object, the first invention of the ferritic heat resistant steel of the present invention is weight percent, C: 0.07 to 0.14%, Si: 0.10 to 0.25%. Ni: 0.8% or less, Cr: 9.5-13.0%, Mo: 0.1-0.6%, V: 0.14-0.24%, Nb: 0.03-0. Contains 0.8%, W: 1.8-2.8%, Co: 2.0-3.5%, B: 0.002-0.008%, N: 0.025% or less, and further rare earth elements : 0.001 to 0.03%, Ca: 0.001 to 0.03%, containing the balance of Fe and inevitable impurities , rare earth element content: REM (%) , Ca content : Ca (%) , Si content: Si (%) , Mn content: Mn (%) , Al content: Al (%) are in a component range satisfying the following relational expression.
( 47 x REM (%) + 33 x Ca (%) ) / ( 1.5 x Si (%) + 0.9 x Mn (%) + 8 x Al (%) ) ≥ 1.0
[0006]
The ferritic heat-resistant steel of the second invention is the ferritic heat-resistant steel of the first invention, and among the inevitable impurities, Mn: 0.25% or less, Al: 0.010% or less is allowed content It is characterized by.
[0007]
The ferritic heat resistant steel of the third invention is the ferritic heat resistant steel of the first or second invention, and among the inevitable impurities, S: 0.010% or less, O: 0.0050% or less allowable content It is characterized by.
[0009]
The reasons for limiting the components of the present invention will be described below.
C: 0.07 to 0.14%
C forms a carbide in combination with a carbide-forming element and improves the high-temperature strength. However, if it is less than 0.07%, the strength is insufficient. On the other hand, if it exceeds 0.14%, the carbide becomes coarse and the high-temperature properties are increased. Therefore, the range is made 0.07 to 0.14%. For the same reason, it is desirable that the lower limit is 0.08% and the upper limit is 0.11%.
[0010]
Si: 0.10 to 0.25%
Si, because an effect of improving the steam oxidation resistance, is contained. However, since the Si content is high segregation inside the steel ingot increases, toughness lowers, further carbides by high temperature for a long time holding, since the creep strength to promote aggregation and coarsening of the Laves phase is reduced, the content of The upper limit is 0.25%. For the same reason, the upper limit is preferably less than 0.15% . The lower limit is 0.10% .
When Si is positively contained, the oxidation equivalent ratio (described later) is strictly controlled so that Si that works effectively in the steam oxidation resistance is not used for deoxidation .
[0011]
Ni: 0.8% or less Ni improves the hardenability and suppresses the formation of delta ferrite and improves toughness. However, if it is excessively contained, the high temperature creep strength decreases, so the upper limit is 0.8. %. In addition, in order to acquire the said effect | action reliably, it is desirable to make it contain 0.20% or more.
For the same reason as described above, it is desirable to set the lower limit to 0.25% and the upper limit to 0.45%.
[0012]
Cr: 9.5 to 13.0%
Cr is a basic alloy component that enhances hardenability and high-temperature strength in this steel type. However, if excessively contained, δ ferrite crystallizes out, and promotes precipitation of coarse Laves phases, which deteriorates high-temperature properties and toughness. Let From these viewpoints, the lower limit is 9.5% and the upper limit is 13.0%, preferably the lower limit is 11.0% and the upper limit is 12.5%.
[0013]
Mo: 0.1-0.6%
Mo needs to be contained in an amount of 0.1% or more in order to increase the temper softening resistance and improve the high temperature strength. However, even if it exceeds 0.6%, further effects can be expected. Moreover, since harmful δ ferrite is generated and the creep rupture strength is lowered, the content is limited to the range of 0.1 to 0.6%. For the same reason, it is desirable to set the lower limit to 0.2% and the upper limit to 0.4%.
[0014]
V: 0.14-0.24%
V has the effect of forming stable carbides and improving the creep strength. To obtain these effects, V is contained in an amount of 0.14% or more. On the other hand, if it is contained excessively, the ductility decreases, so the upper limit is made 0.24%. For the same reason, it is desirable to set the lower limit to 0.16% and the upper limit to 0.2%.
[0015]
Nb: 0.03 to 0.08%
Nb is included because it forms fine carbonitrides to improve the high temperature strength, and further added in combination with B to further improve the creep strength. However, if the content is less than 0.03%, there is no effect. On the other hand, if the content exceeds 0.08%, carbonitrides increase and ductility decreases, so the range is 0.03 to 0.08. %.
[0016]
W: 1.8-2.8%
W precipitates as a solid solution strengthening or in the form of a carbide or a Laves phase, and contributes to an improvement in high temperature strength. Furthermore, the high temperature creep strength is improved by adding B together. However, when it contains excessively, the segregation tendency will increase and ductility will fall.
In consideration of the above action, the lower limit is 1.8% and the upper limit is 2.8%, and more preferably the lower limit is 2.0% and the upper limit is 2.5%.
[0017]
Co: 2.0-3.5%
Co is contained in order to improve the impact properties by suppressing the precipitation of δ ferrite and to improve the creep rupture strength.
In consideration of the above action, Co is contained by 2.0% or more. However, since the effect is saturated even if it contains over 3.5%, the upper limit is made 3.5%. For the same reason, it is desirable to set the lower limit to 2.5% and the upper limit to 3.0%.
[0018]
B: 0.002 to 0.008%
B contains a trace amount to increase hardenability, improve toughness, suppress precipitation and aggregation of carbides in grain boundaries and grains, and increase high-temperature creep strength. Furthermore, the composite addition of an appropriate amount of Nb, W, and N contributes to the improvement of the high temperature creep strength. However, if the content is less than 0.002%, the above effect is insufficient. Further, if the content exceeds 0.008%, the high temperature creep ductility is lowered and the weldability is further deteriorated, so the content is limited to 0.002 to 0.008%. For the same reason, it is desirable that the lower limit is 0.004% and the upper limit is 0.006%.
[0019]
N: 0.025% or less N strengthens the base, forms carbonitrides with V or Nb, and effectively acts to improve creep strength by the combined addition effect with B. However, if it is excessively contained, coarse nitrides are formed, and ductility and high temperature creep strength are lowered. Therefore, the upper limit is made 0.025%. For the same reason, it is desirable to set the upper limit to 0.021%. Moreover, in order to acquire the said effect | action reliably, it is desirable to make it contain 0.014% or more.
[0020]
Rare earth elements: 0.001 to 0.03%
Ca: 0.001 to 0.03% of one or more REMs (rare earth elements) and Ca have deoxidation and desulfurization actions, and are added to a molten metal alone or in combination, thereby adding non-metallic inclusions inherent in steel. Can be reduced, refined, and uniformly dispersed, which contributes to improved toughness and creep ductility at high temperatures and for a long time. Furthermore, it suppresses the preferential oxidation of the grain boundaries and the diffusion of oxygen into the grains, promotes the formation of an oxide film that is stable at high temperatures, and contributes to the improvement of the high temperature corrosion resistance and the steam oxidation resistance.
In order to obtain the above effect, 0.001% or more of each element is required to be added. However, if the content exceeds 0.03%, an oxide is excessively generated, and the toughness is lowered. And Ca content was limited to the said range. For the same reason, it is desirable to set the lower limit to 0.003% and the upper limit to 0.025%, respectively, and it is more desirable to set the lower limit to 0.01% and the upper limit to 0.015%.
[0021]
(Inevitable impurities)
Mn: 0.25% or less Mn is used as a deoxidizing and desulfurizing agent in general steel production, but Mn combines with S to form coarse non-metallic inclusions to reduce toughness and toughness. In order to promote the deterioration with time and further lower the creep strength, the present invention does not actively add them but treats them as impurities. Therefore, it is desirable to reduce the content as much as possible, but it is desirable to set the upper limit to 0.25% in consideration of the limit of the refining technology. More preferably, it is limited to less than 0.10%.
[0022]
Al: 0.010% or less Al is generally used as a deoxidizing agent, but Al decreases toughness and further reduces creep of V and Nb carbonitrides that combine with nitrogen and contribute to high temperature strengthening. Since the strength is lowered, in the present invention, it is not positively added but is handled as an impurity. Therefore, it is desirable to reduce the content as much as possible, but considering the limit of the refining technology, it is desirable to limit it to 0.010% or less. More preferably, it is limited to 0.005% or less.
[0023]
S: 0.010% or less S promotes the generation of macrosegregation, forms sulfides with Mn, Fe, Nb, V, etc., and deteriorates toughness. Also, REM, Ca necessary for desulfurization It is preferable to reduce the S content as much as possible from the viewpoint of not containing excessive amount of. However, considering the limit of refining technology, the allowable content is preferably 0.010% or less.
[0024]
O: 0.0050% or less O is an oxide that forms oxides with Si, Mn, Al, etc. to deteriorate the ductility, and is not O in view of not containing excessive REM and Ca necessary for deoxidation. The content is preferably reduced as much as possible. However, considering the limitation of refining technology, the allowable content is preferably 0.0050% or less. More preferably, it is 0.0030% or less.
[0025]
Oxidation equivalent ratio ≧ 1.0
The present invention steel is characterized in that deoxidation and desulfurization are performed by REM and Ca without using Si, Mn, and Al. As for the deoxidation and desulfurization effect, when the oxidation equivalent ratio is less than 1.0, the deoxidation and desulfurization effect due to Si, Mn, and Al becomes relatively large, and the above-mentioned adverse effects due to Si, Mn, and Al occur and REM, Ca The above effect cannot be obtained. Moreover, since REM and Ca have the property of preferentially contributing to deoxidation and desulfurization over Si and the like, there is an effect of suppressing the above-described adverse effects such as Si by increasing the relative amount of REM and Ca. . For this reason, content of the said component is adjusted so that the said ratio may be 1.0 or more . In addition, since the effect | action of REM and Ca becomes comparatively large, so that the said ratio is large, 2.0 or more are further more desirable, 3.0 or more are still more desirable, and 5.0 or more are still more desirable.
However, the oxidation equivalent ratio is represented by (47 × REM (%) + 33 × Ca (%)) / (1.5 × Si (%) + 0.9 × Mn (%) + 8 × Al (%)) .
[0026]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, melting, refining, casting, and the like are performed according to the above-described components. In refining, deoxidation and desulfurization are performed using REM and Ca without using Si, Mn, and Al. Conventional methods can be employed in other processes.
The cast steel is processed into a desired shape by hot forging or rolling and then subjected to an appropriate heat treatment. For example, annealing is performed at 1000 to 1150 ° C., heating to 1000 to 1200 ° C. and forced cooling is performed, and then tempering is performed at 700 to 800 ° C.
It is desirable that the annealing and normalizing temperatures be 1000 ° C. or higher in order to perform solid solution of carbonitride and decomposition of δ ferrite. If this temperature is too high, the crystal grains become coarse and retransformation into δ ferrite occurs, so the upper limit temperature is set to 1150 ° C or 1200 ° C. Further, by tempering, a uniform tempered martensite structure can be obtained, and further, carbonitride can be finely precipitated and the creep rupture strength can be improved.
[0027]
The steel of the present invention is also excellent in weldability, and can be welded as necessary. For example, after the above-described series of heat treatments, welding is performed, and then stress-relieving annealing at 650 ° C. to 760 ° C. is performed. Do.
The obtained heat-resistant steel is excellent in high-temperature creep rupture characteristics, toughness, high-temperature corrosion resistance and steam oxidation resistance, and can be used as a material suitable for a boiler tube of a thermal power generation system, for example.
The steel of the present invention is suitable for boiler tube materials exposed to high-temperature steam as described above, but the application of the present invention is not limited to this application, and all or part of the above characteristics are required. It can be applied to various uses, and the above-mentioned excellent characteristics can be obtained according to the use.
[0028]
【Example】
Alloys (Examples and Comparative Examples) having the compositions shown in Table 1 were prepared as test materials used in the Examples. These alloys are melted in a melting furnace, deoxidized and desulfurized using REM, Ca (Example or Comparative Example) or Si, Mn, Al (Comparative Example), and then the molten metal is cast into a mold. Each 25 kg steel ingot was prepared as a test material. These test materials were subjected to hot forging and predetermined heat treatment.
The heat treatment was carried out by annealing at 2070 ° C. for 20 hours, followed by annealing in furnace cooling, holding at 1070 ° C. for 10 hours, performing normal cooling, and further holding at 740 ° C. for 16 hours followed by furnace cooling.
[0029]
The obtained specimens were evaluated for mechanical properties and high temperature creep strength, and the results are shown in Table 2.
The creep property test was performed under the conditions of 630 ° C. and 196 MPa.
[0030]
[Table 1]

[0031]
[Table 2]

[0032]
As is apparent from Table 2, the steel of the present invention has a good balance between high temperature creep characteristics and toughness, and both have excellent characteristics.
On the other hand, the comparative steel has a poor balance between high temperature creep properties and toughness, and is clearly inferior in either one.
[0033]
【The invention's effect】
As described above, the ferritic heat resistant steel of the present invention is, by weight, C: 0.07 to 0.14%, Si: 0.10 to 0.25%, Ni: 0.8% or less, Cr : 9.5 to 13.0%, Mo: 0.1 to 0.6%, V: 0.14 to 0.24%, Nb: 0.03 to 0.08%, W: 1.8 to 2 0.8%, Co: 2.0 to 3.5%, B: 0.002 to 0.008%, N: 0.025% or less, and further rare earth elements: 0.001 to 0.03%, Ca: comprise one or more from 0.001 to 0.03%, Ri Do from the balance Fe and unavoidable impurities, rare earth elements content: REM (%), Ca content: Ca (%), Si content : Si (%) , Mn content: Mn (%) , Al content: Al (%) is ( 47 × REM (%) + 33 × Ca (%) ) / ( 1.5 × Si (%) + 0.9 × Mn (%) + 8 × Al (%)) formed satisfying ≧ 1.0 relations Since the range, excellent characteristics are obtained even in high-temperature corrosion properties and water vapor oxidation property with well-balanced excellent properties in high-temperature creep characteristics and toughness is obtained, thermal power systems such as more excellent high temperature properties are required It can be provided as a suitable material.

Claims (3)

By weight, C: 0.07 to 0.14%, Si: 0.10 to 0.25%, Ni: 0.8% or less, Cr: 9.5 to 13.0%, Mo: 0.1 -0.6%, V: 0.14-0.24%, Nb: 0.03-0.08%, W: 1.8-2.8%, Co: 2.0-3.5%, B: 0.002 to 0.008%, N: 0.025% or less, rare earth elements: 0.001 to 0.03%, Ca: 0.001 to 0.03% or more And the balance is Fe and inevitable impurities , rare earth element content: REM (%) , Ca content: Ca (%) , Si content: Si (%) , Mn content: Mn (%) , Al Content: A ferritic heat resistant steel characterized in that Al (%) is in a component range satisfying the following relational expression.
( 47 x REM (%) + 33 x Ca (%) ) / ( 1.5 x Si (%) + 0.9 x Mn (%) + 8 x Al (%) ) ≥ 1.0
Ferritic heat resistant steel   The ferritic heat resistant steel according to claim 1, characterized in that, among inevitable impurities, Mn: 0.25% or less and Al: 0.010% or less are allowed.   The ferritic heat resistant steel according to claim 1 or 2, wherein, among inevitable impurities, S: 0.010% or less and O: 0.0050% or less are allowed.