JP6540131B2 - Ferritic heat resistant steel - Google Patents

Description

  The present invention relates to a ferritic heat resistant steel, and more particularly to a ferritic heat resistant steel excellent in high temperature strength, particularly high temperature creep strength.

  As structural steels used for a long time under high temperature and high load stress, there is a strong demand for ferritic heat resistant steels having high creep strength. Examples of steels used for this type of application include steels to which Cr such as JIS STBA 24 (2.25Cr-1Mo steel), SCMV 4 (1.25Cr-0.5Mo-0.3V steel) and the like are added. Be

  For the improvement of high temperature strength and creep strength, precipitation strengthening by carbide, solid solution strengthening by Mo or the like is used. However, when exposed to high temperatures for a long time, precipitates become coarse and solid solution elements form precipitates, so there is a limit to the improvement of creep strength in strengthening by these metallurgical factors.

  Several steels containing oxides and methods for producing the same have also been disclosed for the purpose of improving high temperature strength. Since oxides generally have high stability at high temperatures compared to carbides and nitrides, dispersion strengthening with oxides is likely to provide stable creep strength.

  As a method of dispersing the oxide, there is a mechanical alloying method (hereinafter referred to as a MA method). For example, JP-A-4-6244 describes an example in which an oxide with a particle diameter of 1 μm or less is dispersed in a high Cr heat resistant steel by MA method to improve high temperature creep strength.

  The MA method is the following process. Several kinds of metal or alloy powder and oxide particles mixed so as to obtain a desired alloy composition are mixed by a ball mill under an inert atmosphere to obtain an oxide-containing alloy powder (hereinafter referred to as MA powder). Next, the MA powder is vacuum-sealed in a metal can, and then made into one body by hot extrusion or high temperature isostatic pressing (HIP), and finally heat treated and processed to give a product.

  Further, an example in which tantalum oxide with a particle diameter of 1 μm or less is contained in Cr-based heat resistant steel by adding tantalum oxide to molten steel regardless of MA method, and an example in which high temperature creep strength is improved is disclosed in JP-A-6-65690. Is disclosed in

  Furthermore, in addition to the method using oxides directly, techniques for dispersing Ti-based oxides in steel by deoxidation with Ti have been developed for the purpose of improving weld zone toughness of low alloy steels.

  A technique has been developed to improve creep strength over conventional standard steels by using carbides, nitrides or their composite precipitates as a strengthening method. For example, in Japanese Patent Application Laid-Open No. 5-195061, it is disclosed that fine precipitates are precipitated by containing carbide which is a nitride-forming element V and further quenching directly after low-temperature rolling and then tempering. .

Unexamined-Japanese-Patent No. 4-6244 JP-A-6-65690 Unexamined-Japanese-Patent No. 5-195061

  The above-described prior art is common in that the number density of carbides, oxides, etc. is increased to improve creep strength. The above-mentioned prior art has the following problems.

  In the method of dispersing the oxide by the MA method, it is easy to disperse the oxide finely and uniformly, and the number density can be made relatively large. However, it is not suitable for structural steels due to the high cost of manufacturing and the difficulty in manufacturing the large size required for use in the structure.

  The method of adding tantalum oxide to molten steel has a poor casting yield, and the amount of addition is naturally large. In addition, the variation in number density of tantalum oxide in the thickness direction is large, and the material is easily unstable.

  The method of dispersing Ti-based oxides in steel by Ti deoxidation has not been applied to heat resistant steel. According to the results in low alloy steels, it is difficult to secure the number density of oxides necessary to enhance the high temperature creep strength, and it is considered that they can not be applied as they are to the production of heat resistant steels.

  The technique of Japanese Patent Application Laid-Open No. 5-195061 aims at the improvement of hydrogen corrosion resistance and the improvement of brittleness, and has not been developed from the viewpoint of the improvement of high temperature creep strength.

  An object of the present invention is to provide a ferritic heat resistant steel excellent in high temperature strength, particularly high temperature creep strength.

  The ferritic heat resistant steel according to one embodiment of the present invention has a chemical composition of, by mass%, C: 0.05 to 0.15%, Si: 0.70% or less, Mn: 0.30 to 0.70% Cr: 0.30 to 11.50%, Co: 0.05 to 3.00%, N: 0.02 to 0.05%, B: 0.0003 to 0.020%, P: 0.03 % Or less, S: 0.015% or less, Al: 0.03% or less, Cu: 0.10% or less, Ni: 0.10% or less, V: 0 to 0.30%, Nb: 0 to 0. 06%, Mo: 0 to 2.00%, W: 0 to 2.50%, Ti: 0 to 0.10%, Zr: 0 to 0.010%, balance: Fe and impurities, bainite and / or Or have a martensitic structure, and the average particle size of the precipitates deposited at the interface of the bainitic and / or martensitic structure is less than 100 nm Ri, the precipitates, the number of precipitates containing B is not less than 95% of the total number of the precipitates.

  According to the present invention, a ferritic heat resistant steel excellent in high temperature strength, particularly high temperature creep strength is obtained.

FIG. 1 is a graph showing the relationship between the ratio of B-containing precipitates and the creep strength at 600 ° C. × 10 ⁴ hours.

  The present inventors are to increase the substantial number density of precipitates such as carbides and intermetallic compounds (hereinafter referred to simply as precipitates) on the basis of solution process, that is, between precipitates. We considered solving the above-mentioned subject by shortening the interval.

  The present inventors focused on the places where precipitates are deposited in the steel. More specifically, we focused on grain boundaries such as bainite and martensite laths, blocks and packets.

  Grain boundaries such as martensite and bainite laths, blocks and packets are planar or linear defects. Therefore, if the precipitates are precipitated at these grain boundaries, the precipitates are distributed in a planar or linear manner, and the spacing between the precipitates in the grain interface and in the grain boundary line direction becomes short. This can dramatically increase the substantial number density.

The reason why the high temperature creep strength is improved by increasing the substantial number density is as follows. The strain rate of a high temperature creep phenomenon is about 10 ^-7 % / sec, while the strain rate of a general tensile test at room temperature is about 10 ^-2 % / sec, and the deformation rate is extremely slow. In the high temperature creep phenomenon, it is considered that the precipitates act as a resistance to deformation to determine the deformation. When considering strengthening by precipitates, it is not necessary for the number density to be uniform at any place in the steel, and if locations with high number density are periodically present, deformation is limited at that place. The improvement of creep strength can be expected. Grain boundaries such as martensite and bainite laths, blocks and packets periodically exist in the steel. Therefore, creep strength can be expected to be improved by increasing the number of precipitates deposited on these interfaces. Furthermore, it is expected that abnormal grain growth at grain boundaries can be suppressed by precipitating at the interface.

  However, when the precipitation density at the interface is increased, the diffusion distance of the elements constituting the precipitates becomes short, and thus coarsening of the precipitates may be promoted during high temperature holding. Therefore, in order to increase the initial number density, it is necessary to control the size of the precipitate to less than 100 nm.

  The inventors of the present invention produced a steel in which the addition amounts of various elements were adjusted, and investigated the precipitation state of the precipitate and the creep strength. Specifically, the amounts of precipitate-forming elements C, Cr, Mo, Nb, and V are adjusted, and 0.1% C-9% Cr-2% W steel and 0.1% C-9% Cr The 2% W-0.01% B steel was heat treated to investigate the precipitation state of the precipitates and the creep strength. In addition, analysis of a precipitate was performed by TOF-SIMS (time-of-flight type secondary ion mass spectrometry), and the creep test was performed on the conditions of 650 degreeC x 700 hours.

  As a result, in order to improve the creep strength in the temperature range of 500 to 700 ° C., it was found that the inclusion of B is effective as described below.

  By including B, the hardenability of the steel is enhanced and the structure can be made finer. This increases the boundary length per unit area of martensitic or bainitic lath, block or packet and increases the substantial number density of precipitates.

Furthermore, in the B-containing steel, B is present in the M ₂₃ C ₆ type compound and the Fe ₂ W (Laves) type intermetallic compound. B remains in the carbide even after a long time creep test, and enhances the high temperature stability of the carbide. B also remains in intermetallic compounds such as Fe ₂ W to enhance the high temperature stability of the intermetallic compounds.

The above is the test results using a W-containing steel, but it is effective to contain B even in a W-free steel or a Mo-free steel that is expected to have the same effect as W. . That is, even when the carbide containing Fe as a main component or the Fe ₂ X type compound is not precipitated, it is effective to contain B when the M ₂₃ C ₆ type compound is precipitated. B improves the creep strength, in particular, because it refines the precipitates deposited on the high angle boundary (the boundary where the angle difference between adjacent regions around the common rotation axis is 15 ° C. or more).

  The present inventors have completed the present invention on the basis of these non-conventional findings. Hereinafter, a heat resistant ferritic steel according to an embodiment of the present invention will be described in detail.

[Chemical composition]
The heat resistant ferritic steel according to the present embodiment has the chemical composition described below. In the following description, “%” of the content of the element means mass%.

C: 0.05 to 0.15%
Carbon (C) forms carbides, enhances high-temperature strength by precipitation strengthening, and suppresses the formation of ferrite. If the amount is less than 0.05%, the amount of precipitated carbides is insufficient and sufficient strength can not be obtained, and the amount of ferrite increases, so the content is made 0.05% or more. Preferably, it is 0.06% or more, more preferably 0.07% or more. On the other hand, if it exceeds 0.15%, the carbides coarsen at high temperatures and the high temperature strength decreases, so the content is made 0.15% or less. In the point which prevents the fall of the creep strength in 650 degreeC, 0.12% or less is preferable. More preferably, it is 0.10% or less.

Si: 0.70% or less Silicon (Si) is a deoxidizing agent, and the lower limit is not particularly required, but 0.05% or more is preferable in order to surely obtain the effect of deoxidation. More preferably, it is 0.10% or more. Since Si is also an element that enhances the steam oxidation resistance, 0.20% or more is preferable in this respect. On the other hand, if it exceeds 0.70%, the toughness and workability of the steel decrease, so the content is made 0.70% or less. Preferably it is 0.50% or less.

Mn: 0.30 to 0.70%
Manganese (Mn) enhances the hardenability of the steel. Mn also contributes to the formation of bainite and / or martensite. In order to obtain this effect, it is 0.30% or more. In addition, Mn fixes S to improve the hot workability and contributes to the stabilization of the structure, and in this respect, 0.40% or more is preferable. On the other hand, when the content exceeds 0.70%, the formability and the weldability decrease, so the content is made 0.70% or less. Since Mn is also an element that enhances temper embrittlement susceptibility, 0.60% or less is preferable.

Cr: 0.30 to 11.50%
Chromium (Cr) contributes to creep strength and oxidation resistance. In order to secure the performance necessary for heat resistant steel, the content should be 0.30% or more. From the point of oxidation resistance and corrosion resistance, 0.50% or more is preferable. On the other hand, when it exceeds 11.50%, coarsening of carbides mainly composed of Cr and precipitation of coarse nitrides are promoted, and creep strength and toughness decrease, so the content is made 11.50% or less. Preferably it is 10.00% or less.

Co: 0.05 to 3.00%
Cobalt (Co) stabilizes austenite without impairing creep strength, suppresses precipitation of δ-ferrite, and promotes formation of bainite and / or martensite. In order to obtain this effect, it is 0.05% or more. In order to increase the high temperature strength, 0.80% or more is preferable. On the other hand, if it exceeds 3.00%, the weldability and the formability will decrease, so the content is made 3.00% or less. Preferably it is 2.90% or less.

N: 0.02 to 0.05%
Nitrogen (N) forms an MX type compound and an M ₂ X type compound, and contributes to an increase in high temperature strength. In order to obtain this effect, it is made 0.02% or more. Preferably it is 0.025% or more. On the other hand, when N is contained in excess, coarse MX type compounds and M ₂ X type compounds are formed, and the toughness decreases, so the content is made 0.05% or less. Preferably it is 0.045% or less.

B: 0.0003 to 0.020%
Boron (B) is the most important element in the present embodiment, and significantly improves the hardenability of the steel even in a small amount. In order to obtain this effect, it is made 0.0003% or more. From the viewpoint of dispersing and stabilizing carbides and enhancing creep strength, 0.003% or more is preferable, and 0.005% or more is more preferable. On the other hand, if it is contained excessively, weldability and processability will be reduced. Furthermore, coarse BN is formed, and in order to impair creep strength, make it 0.020% or less. Preferably it is 0.018% or less, More preferably, it is 0.008% or less.

P: 0.03% or less Phosphorus (P) is an impurity and impairs the toughness, workability, and weldability of the steel, so it is limited to 0.03% or less. Preferably it is 0.01% or less, More preferably, it is 0.005% or less. The lower limit is not particularly defined, but from the viewpoint of the cost of practical steel, 0.001% is a practical lower limit.

S: 0.015% or less Sulfur (S) is an impurity and impairs the toughness, workability, and weldability of the steel, so the content is limited to 0.015% or less. Preferably it is 0.003% or less, More preferably, it is 0.001% or less. The lower limit is not particularly defined, but in view of the cost of practical steel, 0.0001% is a practical lower limit.

Al: 0.03% or less Aluminum (Al) functions as a deoxidizer. However, if it exceeds 0.03%, the creep strength decreases, so the content is made 0.03% or less. Preferably it is 0.02% or less. The lower limit is not particularly defined, but from the viewpoint of the cost of practical steel, 0.001% is a practical lower limit.

Cu: 0.10% or less Copper (Cu) is mixed from raw materials such as scrap and reduces the creep strength of the steel. If the Cu content exceeds 0.10%, the creep strength decreases, so the content is made 0.10% or less. Preferably it is 0.05% or less, More preferably, it is 0.01% or less. The lower limit includes 0%.

Ni: 0.10% or less Nickel (Ni) is mixed from raw materials such as scrap and reduces the creep strength of the steel. If the Ni content exceeds 0.10%, the creep strength decreases, so the content is made 0.10% or less. Preferably it is 0.05% or less, More preferably, it is 0.01% or less. The lower limit includes 0%.

  The balance of the chemical composition of the heat resistant ferritic steel according to the present embodiment consists of Fe and impurities. Here, the term "impurity" means an element mixed from ore or scrap used as a raw material when industrially manufacturing steel, or an element mixed from an environment of a manufacturing process or the like.

  The ferritic heat resistant steel according to the present embodiment is selected from the group consisting of Ti and Zr, and one or more selected from the group consisting of V, Nb, Mo, and W instead of part of the above Fe. It may contain one or two elements. V, Nb, Mo, and W all improve the creep strength of the steel by precipitation strengthening. Both Ti and Zr form carbides and nitrides to improve the creep strength of the steel.

  V, Nb, Mo, W, Ti, and Zr are all selective elements, and the heat resistant ferritic steel according to the present embodiment may not contain these elements. Even if the ferritic heat resistant steel according to the present embodiment does not contain these relatively expensive alloy elements, high creep strength can be obtained. Therefore, according to this embodiment, compared with the conventional ferritic heat resistant steel which makes Mo, W, etc. essential, creep strength equivalent to or more than before can be obtained, achieving alloy saving. Moreover, higher creep strength can be obtained by containing these alloying elements.

V: 0 to 0.30%
Vanadium (V) precipitates fine MX type compounds to enhance creep strength. Although this effect can be obtained even with a small amount, it is preferable to be 0.15% or more in order to obtain it more reliably. More preferably, it is 0.18% or more, further preferably 0.20% or more. On the other hand, when V is contained excessively, the toughness is lowered by precipitates, so the content is made 0.30% or less. Preferably it is 0.25% or less.

Nb: 0 to 0.06%
Niobium (Nb) forms a fine MX type compound to enhance creep strength. Although this effect can be obtained even with a small amount, it is preferable to be 0.03% or more in order to obtain it more reliably. More preferably, it is 0.045% or more. On the other hand, when Nb is excessively contained, coarse carbides are formed and the toughness is lowered, so the content is made 0.06% or less. Preferably it is 0.055% or less.

Mo: 0 to 2.00%
Molybdenum (Mo) significantly improves creep strength by solid solution strengthening and precipitation strengthening. Although this effect can be obtained even with a small amount, it is preferable to be 0.01% or more in order to obtain it more reliably. More preferably, it is 0.30% or more. On the other hand, when Mo is excessively contained, the carbide containing Mo as a main component is coarsened to lower the toughness and the creep strength, so the content is made 2.00% or less. Preferably it is 1.50% or less.

W: 0 to 2.50%
Tungsten (W) is an element having the same effect as Mo. By containing W, creep strength can be remarkably improved. Although this effect can be obtained even with a small amount, it is preferable to be 1.50% or more in order to obtain it more reliably. More preferably, it is 1.70% or more. On the other hand, if W is contained excessively, the weldability and the workability will decrease, so the content is made 2.50% or less. In order to secure the creep strength at 650 ° C. or more, the content is preferably 2.40% or less.

Ti: 0 to 0.10%
Titanium (Ti) functions as a deoxidizer. In addition, Ti is a strong carbide and nitride forming element, and contributes to the refinement of the structure and the stabilization of carbides and nitrides. Although these effects can be obtained even with a small amount, it is preferable to be 0.015% or more in order to obtain these more reliably. More preferably, it is 0.040% or more. On the other hand, when Ti is excessively contained, coarse TiN is formed and the toughness is reduced, so the content is made 0.10% or less. Preferably it is 0.09% or less.

Zr: 0 to 0.010%
Zirconium (Zr), like Ti, is a strong carbide and nitride forming element and contributes to the refinement of the structure and the stabilization of carbides and nitrides. Although this effect can be obtained even with a small amount, it is preferable to be 0.001% or more in order to obtain it more reliably. On the other hand, when Zr is excessively contained, coarse ZrN is formed and the toughness is reduced, so the content is made 0.010% or less. Preferably it is 0.005% or less.

[Organization]
The heat resistant ferritic steel according to the present embodiment has a bainite and / or martensitic structure. Here, the term "bainite and / or martensitic structure" includes bainitic structure, martensitic structure, and mixed structure of bainite and martensite. Also, the term "bainitic structure" includes tempered bainite, and the term "martensitic structure" includes tempered martensite.

  In the heat resistant ferritic steel according to the present embodiment, the volume fraction of bainite and / or martensite is preferably 95% or more. In other words, a structure other than bainite and martensite, for example, a structure such as ferrite or retained austenite may be contained by about 5%.

  The structure is preferably fine, and specifically, the prior austenite particle size is preferably 50 μm or less. The lath width of bainite and / or martensite is preferably 0.3 μm or less. The finer the texture, the greater the substantial number density of precipitates and the better the creep strength.

  The identification of the tissue can be performed, for example, by observing the tissue etched with nital or picral with a light microscope. The area ratio of bainite and / or martensite can be measured, for example, by image analysis. The lath width can be measured by, for example, a transmission electron microscope (TEM).

[Precipitate]
In the ferritic heat resistant steel according to the present embodiment, the average particle size of precipitates deposited on the interface between the bainite and / or martensite structure is less than 100 nm. The interface between bainite and / or martensite structures is grain boundaries such as laths, blocks and packets.

The precipitates are, for example, MX type compounds, M ₂ X type compounds, M ₂₃ C ₆ type compounds, Fe ₂ X type compounds, and Fe ₃ C (cementite), and precipitates combining these. Here, M in the MX type compound represents one or more elements of Nb, V, Ti, and Zr, and X represents C and N. M of the M ₂ X type compound represents one or more elements of Cr, Mo, Nb, and V, and X represents C and N. M of M ₂₃ C ₆ type compounds, Cr, as main constituent elements Fe, there is a case where Mn, Mo, W, Nb, V, Ti, include one or more elements of Zr. X of the Fe ₂ X type compound is mainly W or Mo.

  If the average particle size of the precipitates deposited on the boundary surface of the bainite and / or martensite structure is 100 nm or more, the precipitates become coarse during high temperature holding, and the high temperature creep strength decreases. If the average particle size of these precipitates is less than 100 nm, the initial number density of the precipitates can be increased, that is, the spacing between the precipitates can be reduced, and therefore the coarsening of the precipitates during high temperature holding is suppressed. can do. The average particle size of these precipitates is preferably 90 nm or less.

  The "average particle diameter" here is a number average diameter. That is, (sum of diameters of all precipitates) / (number of all precipitates) in a certain observation plane. More specifically, the average particle size is determined as follows. From the observation surface of steel, a specimen for TEM observation is collected by the extraction replica method to obtain a TEM image. The field of view of the TEM image is, for example, 0.73 μm × 0.95 μm. The TEM image is image-analyzed, the area of the precipitate is determined by elliptic approximation, and the equivalent circle diameter (diameter) of each precipitate is determined from the area. At this time, as for the number of precipitates, those having a circle equivalent diameter of 10 nm or more are counted.

  In the precipitates deposited on the boundary surface of the bainite and / or martensite structure, the number of B-containing precipitates is 95% or more of the number of all precipitates. In other words, the ratio of the number of B-containing precipitates (hereinafter referred to as B-containing precipitates) is 95% or more among the precipitates deposited on the boundary surface of the bainite and / or martensite structure.

  Here, if the precipitate contains at least B, the precipitate is counted as a B-containing precipitate. On the other hand, if the precipitate does not contain B at all, the precipitate is counted as a precipitate containing no B.

  The measurement of the B-containing precipitate can be measured, for example, by TOF-SIMS. That is, from the elemental ion map in the measurement field of view, the elemental ion map of each element constituting the precipitate is prepared, and the precipitation location of the precipitate, the size and the number density of the precipitated particles, etc. are known from the individual or the synthesis map. By comparing this with the elemental ion map of B, the ratio of the number of B-containing precipitates can be calculated.

  The measurement of the B-containing precipitate can also be performed by an apparatus combining a field emission type transmission electron microscope with an electron energy loss spectrometer (EELS).

  If the proportion of the B-containing precipitates is 95% or more, the stability of the precipitates at high temperatures is enhanced, and the high temperature creep strength of the steel is improved. The proportion of the B-containing precipitate is preferably 97% or more.

[Production method]
Next, an example of a method of manufacturing the heat resistant ferritic steel according to the present embodiment will be described.

  The heat resistant ferritic steel according to the present embodiment is manufactured by hot rolling a steel piece obtained by melting and casting steel by a conventional method.

  The heating before hot rolling is a process of reducing the deformation resistance of the steel and causing the solid solution of precipitates formed in the steel during casting to form a solid solution. The heating temperature is preferably 1050 ° C. or higher in order to complete the hot rolling at a temperature higher than the temperature at which carbides precipitate. More preferably, it is 1100 ° C. or higher. On the other hand, when the heating temperature is too high, the structure becomes coarse, so it is preferable to set the temperature to 1250 ° C. or less.

In hot rolling, as the total rolling reduction is increased, the structure can be refined, which is preferable. The total rolling reduction in hot rolling is preferably 50% or more, and more preferably 65% or more. Hot rolling is performed at a temperature equal to or higher than the Ar ₃ transformation point, which is a temperature range in which the metal structure is austenite. When hot rolling is performed at a temperature below the Ar ₃ transformation point, processed ferrite is formed and the toughness is reduced.

  After the end of the hot rolling, it is cooled to room temperature. The cooling method is, for example, air cooling. The ferritic heat resistant steel according to the present embodiment can obtain a bainite and / or martensitic structure without particularly accelerated cooling. The preferred cooling rate is 1 ° C./s or more.

  After cooling, if necessary, normalizing and tempering may be performed. Both tempering and tempering are optional steps. That is, one or both of tempering and tempering may not be performed.

  Normalization is performed by heating the ferritic heat-resistant steel to 1000 ° C. or higher, holding it for a predetermined time, and then cooling it. The cooling may be air cooling or accelerated cooling such as water cooling. By performing the normalization, the tissue can be made more uniform. When the normalizing temperature is too high, the prior austenite grain size becomes large. The normalizing temperature is preferably less than 1200 ° C., preferably less than 1150 ° C.

The tempering is performed by heating the ferritic heat-resistant steel to a temperature lower than the Ac ₁ transformation point and holding it for a predetermined time, and then cooling it. The cooling may be air cooling or accelerated cooling such as water cooling. By performing tempering, toughness can be improved. When the tempering temperature reaches a temperature above the Ac ₁ transformation point, the precipitates become coarse and the high temperature strength decreases.

  During normalizing or tempering, precipitation of precipitates is promoted. However, even if the normalizing and tempering are not performed, the ferrite type heat resistant steel according to the present embodiment can obtain high creep strength because precipitates are precipitated during use at high temperature (for example, 600 ° C.).

  Heretofore, the heat resistant ferritic steel according to the embodiment of the present invention has been described. According to this embodiment, a ferritic heat-resistant steel excellent in high temperature strength, particularly high temperature creep strength can be obtained.

  Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted to confirm the practicability and effects of the present invention, and the present invention is not limited to the one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the scope of the present invention.

  Steels having the chemical compositions shown in Tables 1 and 2 (continuation of Table 1) were melted and cast, and the obtained steel pieces were hot-rolled and air-cooled under the conditions shown in Table 3. The balance of the chemical composition is Fe and impurities. Thereafter, tempering and / or tempering were performed under the conditions shown in Table 3. In addition, steel No. Neither 23 nor 24 performed tempering and / or tempering.

  The steel before the creep test was cut, polished and etched, and the metallographic structure was observed by an optical microscope. Furthermore, test pieces for TEM observation were collected by the extraction replica method. The average particle size of the precipitate deposited at the interface between the bainite and / or martensite structure was measured by the method described in the embodiment from the TEM image. The field of view of the TEM image was 0.73 μm × 0.95 μm. In addition, about the steel of a ferrite structure, the average particle diameter of the precipitate which precipitated to the ferrite grain boundary was measured.

  Furthermore, about the steel materials which carried out the tempering process, the steel materials before a creep test were cut | disconnected, and the test piece for TOF-SIMS measurement was produced as mirror finish by the wet mechanical polishing method of a conventional method. About the steel materials which are not tempered, the steel materials after a creep test were cut, and the test piece for TOF-SIMS measurement was similarly produced as mirror finish by the wet mechanical polishing method of a conventional method. The TOF-SIMS measurement was performed by irradiating 200 pulses per pixel using a gallium (Ga) ion beam with an irradiation diameter of 80 nm as a primary ion source. The field of view of TOF-SIMS measurement was 25 μm × 25 μm. The elemental ion map of the secondary ion was acquired, and the types of precipitates were separated to calculate the ratio of the number of B-containing precipitates.

For the creep strength, a creep test was conducted at 200 ° C. for 200 MPa load test, at 650 ° C. for 100 MPa and 150 MPa load test, and further at 70 ° C. for 70 MPa and 90 MPa load test to determine creep rupture strength. Using the results obtained, by Larson Miller parameter, 600 ° C., the estimated rupture strength of 10 ⁴ hours (hereinafter, referred to as 600 ° C. × 10 ⁴ h creep strength) was evaluated. The results are shown in Table 4.

As shown in Table 1, Table 2 and Table 4, steel no. The steels 1 to 24 satisfy the chemical composition of the present invention, and the proportion of B-containing precipitates is 95% or more. The creep strengths of these steels were approximately 20 MPa or more, respectively, at 600 ° C. × 10 ⁴ hours, as compared with the comparative examples containing the same degree of alloying elements such as Cr.

  Low Cr steels (Cr content less than 3% by mass), medium Cr steels (Cr content not less than 3% by mass and 7% or less) depending on the Cr content, as steels containing similar amounts of Cr The creep strengths were compared by dividing into less than mass%) and high Cr steels (Cr content is 7 mass% or more).

Steel No. 2, 7, 15 and 24 are low Cr steels satisfying the chemical composition of the present invention. These steels are steel No. 1 containing comparable levels of Cr. The creep strength was increased by 20 MPa or more at 600 ° C. × 10 ⁴ hours as compared with 26, 27, and 30-32.

Steel No. 8, 9, 13, 14, 16, 18 and 20 are medium Cr steels which satisfy the chemical composition of the present invention. These steels are steel No. 1 containing comparable levels of Cr. The creep strength was 10 MPa or more greater at 600 ° C. × 10 ⁴ hours as compared with 29 and 36.

Steel No. 1, 3, 4 to 6, 10 to 12, 19 and 21 to 23 are high Cr steels satisfying the chemical composition of the present invention. These steels are steel No. 1 containing comparable levels of Cr. 25,28,33～35, and 37 compared to, 600 ° C. × 10 ⁴ h creep strength greater than 15 MPa.

Steel No. 6 and 10 are steels which satisfy the chemical composition of the present invention, and are examples in which normalizing is not performed after completion of hot rolling. Steel No. 1 that was standardized after hot rolling Compared with 5, it had creep strength equivalent to 600 ° C. × 10 ⁴ hours. In addition, steel No. 1 containing alloy elements such as Cr and the like to the same extent. The creep strength was 20 MPa or more greater at 600 ° C. × 10 ⁴ hours as compared with 35 and 37.

Steel No. The steels 23 and 24 satisfy the chemical composition of the present invention, and are examples in which neither normalizing nor tempering is performed after the end of hot rolling. Steel No. 4 which is an example in which at least one of normalizing and tempering is carried out after hot rolling and which contains alloying elements such as Cr and the like to the same degree Compared with 1, 6 and 10, they had creep strength equal to or higher than 600 ° C. × 10 ⁴ hours.

  On the other hand, steel No. In 25-34, sufficient creep strength was not obtained. It is considered that this is because the chemical composition of the present invention was not satisfied. Furthermore, it is considered that the proportion of the B-containing precipitate was less than 95%.

  Steel No. No. 26 has too little C content, and steel No. In No. 30, the Cr content was too low, so the amount of precipitates was small, and furthermore, since the ratio of the precipitates containing B was small, the precipitates could not be stabilized, so it is considered that the creep strength was low. Steel No. No. 25 was considered to have low creep strength because the Cr content was too high and the proportion of the B-containing precipitate was too small to stabilize the precipitate.

  Steel No. In 26 and 30-32, since the bainite and / or martensite structure is not formed, there are few precipitation places of fine precipitates, and furthermore, since the ratio of B-containing precipitates is small, stabilization of the precipitates can not be achieved. It is considered that the creep strength was low.

  Steel No. In 35 to 37, sufficient creep strength was not obtained. This is considered to be because the proportion of the B-containing precipitate was less than 95%. The low proportion of B-containing precipitates is considered to be due to the fact that the balance between the content of alloy elements such as Ti, Nb, V, Mo, etc. and the B content was relatively strong in the carbide forming action in the steel. . This leads to coarsening of Cr carbides and nitrides present in the boundaries of the structure, that is, inhomogeneity in the grain size of the precipitate, resulting in differences in possible B content, and a reduction in the proportion of B-containing precipitates. It is thought that

FIG. 1 is a graph showing the relationship between the ratio of B-containing precipitates and the creep strength at 600 ° C. × 10 ⁴ hours. It can be seen from FIG. 1 that the creep strength is remarkably improved when the ratio of the B-containing precipitates is 95% or more.

  According to the present invention, even when a large amount of Cr, Mo, W and a rare element are not added, creep strength equal to or higher than that of the conventional one can be obtained. As a result, since it becomes possible to provide the heat-resistant steel used in a boiler, a chemical industry, etc. at low cost, the present invention has extremely high industrial applicability.

Claims (3)

The chemical composition is in mass%,
C: 0.05 to 0.15%,
Si: 0.70% or less,
Mn: 0.30 to 0.70%,
Cr: 0.30 % or more and less than 7.0% ,
Co: 0.05-3.00%,
N: 0.02 to 0.05%,
B: 0.0003 to 0.020%,
P: 0.03% or less,
S: 0.015% or less,
Al: 0.03% or less,
Cu: 0.10% or less,
Ni: 0.10% or less,
V: 0 to 0.30%,
Nb: 0 to 0.06%,
Mo: 0 to 2.00%,
W: 0 to 2.50%,
Ti: 0 to 0.10%,
Zr: 0 to 0.010%,
Remainder: Fe and impurities,
Has a bainite and / or martensitic structure,
The volume fraction of the bainite and / or martensite structure is 95% or more,
The average particle size of the precipitates deposited on the interface of the bainite and / or martensite structure is less than 100 nm,
The said precipitate is a ferritic heat-resistant steel whose number of objects of the precipitate containing B is 95% or more of the number of objects of all the precipitates. The heat resistant ferritic steel according to claim 1, wherein
The chemical composition is, in mass%,
V: 0.15 to 0.30%,
Nb: 0.03 to 0.06%,
Mo: 0.01 to 2.00%, and W: 1.50 to 2.50%,
A ferritic heat resistant steel containing one or more elements selected from the group consisting of It is a ferritic heat resistant steel according to claim 1 or 2,
The chemical composition is, in mass%,
Ti: 0.015 to 0.10%, and Zr: 0.001 to 0.010%,
A ferritic heat resistant steel containing one or two elements selected from the group consisting of

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