JP3576234B2 - Cast steel for steam turbine cabin or pressure vessel - Google Patents

Description

【００１６】
【表２】

【００１７】
【表３】

【００１８】
【発明の効果】
本発明のＣｒＭｏＶ鋳鋼材は、従来からの優れた高温強度、特にクリープ破断強度を損なうことなく、良好な延性、靭性を具備したものであることから、溶接補修性が改善されており、従来の材料よりも製造しやすいことが特徴となっており、従来材よりも安価に製造することができる。[0001]
[Industrial applications]
The present invention relates to a cast steel material for a steam turbine casing or pressure vessel for thermal power generation.
[0002]
[Prior art]
The cabin materials and pressure vessel materials used in steam turbine plants for thermal power, have been used casting material lot to accommodate complex shapes, as these materials primarily CrMoV cast steel, 2. 25% CrMo cast steel, and the like CrMo cast steel. These materials ensure the high temperature strength, it is necessary excellent weldability to a further cast steel products. Of these, 2.25% CrMo cast steel and CrMo cast steel, has excellent room-temperature impact properties, the result weldability is good. However, since V is not added, the creep rupture strength is not always sufficient, and it cannot meet the needs for steam turbine casing materials whose temperature increases year by year. On the other hand, CrMoV cast steel is excellent in creep rupture strength, poor weldability to impact properties are inferior, there is a weld repair is performed difficult problems during production.
[0003]
[Problems to be solved by the invention]
The present invention secures it stands an excellent creep rupture strength of CrMoV cast steel, by improving the toughness, can solve the above problems, a good CrMoV cast steel system pressure vessel weldability The purpose is to provide the material.
[0004]
[Means for Solving the Problems]
The above object can be achieved by a novel cast steel material having excellent toughness as described in the following (1) to ( 2 ) and having a creep rupture strength equivalent to that of a current CrMoV cast steel material.
(1) Carbon: 0.15 to 0.20%, preferably 0.16 to 0.18%, silicon: 0.15 to 0.35%, manganese: 0.5 to 0.8% by weight ratio, Chromium: 1.2-1.5%, preferably 1.3-1.5%, molybdenum: 0.8-1.2%, preferably 0.9-1.1%, vanadium: 0.1- 0.3%, preferably 0.15 to 0.25% and unavoidable impurities and iron, quenching temperature: maintained at 920 to 980 ° C., preferably 935 to 965 ° C. for 5 to 20 hours . the cooling rate to 600 ° C. and cooled in faster than 200 ° C. / time, further tempering Shi temperature six hundred and eighty to seven hundred thirty ° C., high temperature preferably characterized by being held 5-20 hours at six hundred ninety to seven hundred twenty ° C. Cast steel material for steam turbine casings or pressure vessels used in the environment .
[0005]
( 2 ) Carbon: 0.15 to 0.20%, preferably 0.16 to 0.18%, silicon: 0.15 to 0.35%, manganese: 0.5 to 0.8% by weight ratio, Chromium: 1.2-1.5%, preferably 1.3-1.5%, molybdenum: 0.8-1.2%, preferably 0.9-1.1%, vanadium: 0.1- 0.3%, preferably 0.15 to 0.25% , phosphorus as an impurity is 0.008% or less, preferably 0.004% or less, and sulfur as an impurity is 0.006% or less, preferably 0.1% or less. 002% or less, inevitable impurities and iron, quenching temperature: 920 to 980 ° C, preferably maintained at 935 to 965 ° C for 5 to 20 hours, and cooling rate of each part of the material to 600 ° C is 200 ° C / hour or more. It was cooled at a rate, further tempering Shi temperature six hundred eighty to seven hundred and thirty ° C., preferably Is a cast steel material for a steam turbine casing or pressure vessel used in a high-temperature environment, which is maintained at 690 to 720 ° C. for 5 to 20 hours .
[0006]
[Action]
The present inventors, as well as carefully identify the components for the composition of the CrMoV cast steel material, and found excellent toughness cast steel material having enough can not be obtained conventionally by heat treatment at a specific condition present invention Reached.
Describe reasons for content restrictions in the cast steel material of the present invention are described below.
C: C improves hardenability and forms carbides of Cr and Mo and contributes to improvement of high-temperature strength. However, if it is less than 0.15%, sufficient proof stress and creep rupture strength cannot be obtained, and if it exceeds 0.20%, excessive carbides are formed and the toughness is reduced, so that the content is made 0.15 to 0.20%. .
Si: Si is a useful element as a deoxidizing material and contributes to improving the flow of molten metal and sufficiently pouring the molten metal to the tip of the casting. However, Si promotes segregation and lowers toughness. If it is less than 0.15%, its function will not work sufficiently, and if it exceeds 0.35%, toughness will be reduced. For this reason, it is limited to 0.15 to 0.35%.
[0007]
Mn: Mn is useful as an element for improving hardenability and is effective in improving toughness. If it is less than 0.5%, the effect is not sufficient, and if it exceeds 0.8%, the creep rupture strength is reduced.
Cr: Cr contributes significantly to improving oxidation resistance and forming carbides to improve high-temperature strength. If it is less than 1.2%, the effect is not sufficient, and if it exceeds 1.5%, the creep rupture strength is conversely reduced.
[0008]
Mo: Mo forms carbides and is effective in improving high temperature creep rupture strength. It is also effective in improving hardenability and improving toughness. If it is less than 0.8, a sufficient effect cannot be obtained, and if it exceeds 1.2%, embrittlement occurs during use.
V: V forms carbides and strongly contributes to the improvement of creep rupture strength. However, if it is less than 0.1%, a sufficient effect cannot be obtained, and if it exceeds 0.3%, toughness is reduced. To 0.3%.
[0009]
P: P is an impurity and needs to be sufficiently refined and kept low in the melting stage. Particularly P reduces the toughness of the material during use undergo Shi embrittlement tempering. For this reason, it is desirable to make it 0.008% or less.
S: S is also an impurity like P, and segregates during solidification, and the concentrated portion becomes a defect because the bonding strength of the material is weak. For this reason, it is necessary to keep it low, and it is desired that it is 0.006% or less.
[0010]
Cast steel material of the present invention having the above composition is a material for a steam turbine casing or pressure vessel for use in a high temperature environment, high-temperature strength, the weld repair of since it is especially the creep rupture strength and cast material It is generally required to have good toughness for securing. For this reason, it is preferable to perform a process for sufficiently obtaining the required characteristics even in performing the heat treatment.
Quenching temperature: Quenching temperature (liquefaction temperature) greatly affects the crystal grain size of a material. If the quenching temperature is high, crystal grains become coarse, and toughness and ductility decrease. On the other hand, if the quenching temperature is too low, the crystal grains become too fine, and the creep rupture strength decreases. For this reason, optimal temperature management is required. In the case of the material of the present invention, if the quenching treatment (solution treatment) is performed at a temperature exceeding 980 ° C., the crystal grains become coarse, and sufficient toughness and ductility cannot be obtained. If the heat treatment is performed at a temperature lower than 920 ° C., sufficient creep rupture strength cannot be obtained. For this reason, the quenching temperature is limited to 920 to 980 ° C.
[0011]
Quenching time: The quenching time is determined by the time required to sufficiently exert the above-described quenching effect. If the time is less than 5 hours, there arises a problem that the alloy element contained in the material cannot be sufficiently dissolved in the iron matrix and that the concentration segregation of the alloy element cannot be sufficiently eliminated. On the other hand, when the time is maintained for more than 20 hours, there is no difference in the effect of quenching as compared with the treatment within 20 hours, and conversely, the crystal grains are coarsened more than necessary, leading to a decrease in ductility and toughness. . For this reason, the quenching time is limited to 5 to 12 hours.
[0012]
Tempering Shi temperature, tempering Shi Time: eliminate defects introduced during the hardening in the teeth tempering is performed in order to in a tough material, the mechanical material by the heat treatment temperature and the retention time Changes in strength, ductility, and toughness. In tempering Shi treatment temperature is high, the longer the retention time tempering Shi processing proceeds material strength is low, the ductility and toughness are improved instead. On the other hand, if the holding time yet tempering Shi temperature is low is short, the material strength becomes higher becomes as low ductility, and toughness. Therefore, it is necessary to strictly control the temperature and time of Shi tempering. Doing Shi tempering at a temperature higher than 730 ° C., ductility and toughness becomes a sufficiently high mechanical strength is not sufficient. Further, when tempering at a temperature lower than 680 ° C., a sufficiently high mechanical strength can be obtained, but ductility and toughness are not sufficient. For this reason, tempering Shi temperature is set to 680~730 ℃. Also, if the time Shi tempering is less than 5 hours, without causing solid solution and diffusion of adequate alloy elements, sufficient creep rupture strength and ductility, not toughness obtained. Further, even when subjected to time tempering Shi processing exceeding 20 hours, it occurs only in about 20 hours Shi tempering time and little more than the solid solution and diffusion. In addition, when the aging treatment is performed for an unnecessarily long time, the mechanical strength is reduced. For this reason, tempering Shi time is 5 to 20 hours.
[0013]
Quenching speed: When the quenching speed is low, sufficient mechanical strength cannot be obtained because a ferrite + pearlite structure is formed during quenching. Therefore, it is necessary to increase the quenching speed. When actually quenching a large material, the quenching speed is limited and cannot be extremely increased. However, by cooling at a cooling rate of 200 ° C / hour or more from the quenching temperature to 600 ° C, Stable mechanical strength can be obtained. Therefore, the cooling rate to 600 ° C. during quenching is desirably 200 ° C./hour or more.
[0014]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.
The test materials shown in Table 1 were melted using a 50 kg vacuum melting furnace, and ingots were formed using a sand mold. Various heat treatments were performed on the test material manufactured as described above to obtain a test material. A room temperature tensile test, an impact test, and a creep rupture test were performed on the test material thus obtained, and the material properties were evaluated.
Table 2 shows the mechanical properties of each of the test materials. According to the results, the material of the present invention has better mechanical strength, tensile ductility and impact properties (50% FATT shows an impact transition) than the comparative material. It indicates the temperature, and it can be said that the lower the temperature, the better the impact properties) and the creep rupture strength (in the creep rupture test, the temperature and stress are constant as test conditions, so that the longer the rupture time is, It can be said that the creep rupture strength is strong.)
Table 3 summarizes the effects of the heat treatment on the mechanical properties. According to this, the heat treatment of the present invention has a high balance of ductility, toughness and high creep rupture strength as compared with the comparative heat treatment. You can see that it is.
[0015]
[Table 1]

[0016]
[Table 2]

[0017]
[Table 3]

[0018]
【The invention's effect】
CrMoV cast steel material of the present invention has excellent high-temperature strength of a conventional, in particular without impairing the creep rupture strength, good ductility, since those provided with the toughness, are improved weld repair properties, conventional It is characterized in that it is easier to manufacture than the above materials, and can be manufactured at lower cost than conventional materials.

Claims (2)

By weight, carbon: 0.15 to 0.20%, silicon: 0.15 to 0.35%, manganese: 0.5 to 0.8%, chromium: 1.2 to 1.5%, molybdenum: 0 0.8 to 1.2%, vanadium: 0.1 to 0.3%, inevitable impurities and iron, quenching temperature: maintained at 920 to 980 ° C for 5 to 20 hours, and up to 600 ° C for each part of the material. the cooling rate was cooled in faster than 200 ° C. / time, further tempering Shi temperature 680-730 steam turbine casing or pressure used in a high temperature environment characterized by comprising holding 5-20 hours at ° C. Cast steel for containers. By weight, carbon: 0.15 to 0.20%, silicon: 0.15 to 0.35%, manganese: 0.5 to 0.8%, chromium: 1.2 to 1.5%, molybdenum: 0 0.8 to 1.2%, vanadium: 0.1 to 0.3% , phosphorus as an impurity is 0.008% or less, sulfur as an impurity is 0.006% or less, and unavoidable impurities and iron are quenched. temperature: 920 to 980 and held 5-20 hours at ° C., the cooling rate to 600 ° C. material each part is cooled at 200 ° C. / time faster than 5-20 in further tempering Shi temperature six hundred eighty to seven hundred and thirty ° C. A cast steel material for a steam turbine casing or pressure vessel used in a high temperature environment, which is maintained for a time.