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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations

Definitions

• the present disclosure relates to an austenitic stainless steel.
• the present disclosure has been made to solve the above-mentioned problems and the gist thereof is the following austenitic stainless steel and steel tube.
• Figure 1 shows a micrograph of an observed Nb compound.
• the present inventors investigated methods to improve the creep strength, SCC resistance, and steam oxidation resistance of austenitic stainless steel and made the following findings.
• Nb promotes the formation of Cr oxides, which contribute to the improvement of steam oxidation resistance, although the formation rate is slow, by suppressing the excessive penetration of O into the steel in the service environment. Therefore, it is effective to leave Nb in a solid solution state in advance, not only from the viewpoint of creep strength, but also from the viewpoint of steam oxidation resistance. Then, it is preferable to perform shot processing, etc. afterwards to form a machined layer near the surface.
• One embodiment of the present invention is based on the above findings. The following is a detailed description of each requirement of the austenitic stainless steel and steel tube of the present embodiment.
• the C content is an element necessary to ensure high temperature strength, especially creep strength. Therefore, the C content is to be 0.002% or more.
• the C content is preferably 0.003% or more, and more preferably 0.004% or more. However, if C is contained in excess, the SCC resistance will be reduced. Therefore, the C content is to be 0.020% or less.
• the C content is preferably 0.015% or less, and more preferably 0.010% or less.
• Mn manganese
• MnS metal-organic compound
• the Mn content is to be 0.2% or more.
• the Mn content is preferably 0.4% or more, and more preferably 0.6% or more.
• the Mn content is to be 2.0% or less; the Mn content is preferably 1.5% or less, and more preferably 1.3% or less.
• P phosphorus
• the P content is to be 0.035% or less.
• the P content is preferably 0.030% or less, and more preferably 0.025% or less. It is preferable to reduce the P content as much as possible, but if the content is reduced excessively, production cost will increase. Therefore, the P content is preferably 0.010% or more.
• S sulfur is an element contained in steel as an impurity that reduces SCC resistance. S also reduces the hot workability and creep ductility of the steel. Therefore, the S content is to be 0.010% or less.
• the S content is preferably 0.009% or less, and more preferably 0.008% or less. It is preferable to reduce the content as much as possible, but an excessive reduction of the content causes an increase in production cost. Therefore, it is preferable that the S content be 0.0001% or more.
• the Cu content is to be 2.50% or more;
• the Cu content is preferably 2.70% or more, and more preferably 2.90% or more.
• an excessive Cu content reduces the hot workability and weldability. Therefore, the Cu content is to be 4.50% or less; the Cu content is preferably 4.00% or less, and more preferably 3.50% or less.
• Ni nickel is an element that stabilizes the austenite structure and has an effect of improving SCC resistance and corrosion resistance. Ni also has the effect of increasing creep strength. Therefore, the Ni content is to be 9.00% or more. The Ni content is preferably 10.00% or more, and more preferably 10.50% or more. However, an excessive Ni content will increase production cost, and in addition, the creep strength will be adversely reduced. Therefore, the Ni content is to be 16.00% or less; the Ni content is preferably 15.00% or less, and more preferably 14.00% or less.
• Mo mobdenum
• Mo has the effect of increasing creep strength. Therefore, the Mo content is to be 0.20% or more.
• the Mo content is preferably 0.35% or more, and more preferably 0.50% or more.
• an excessive Mo content reduces the stability of the austenite structure. Therefore, the Mo content is to be 1.50% or less.
• the Mo content is preferably 1.25% or less, and more preferably 1.00% or less.
• Nb 0.15 to 0.60%.
• Nb niobium
• the Nb content is to be 0.15% or more.
• the Nb content is preferably 0.20% or more, and more preferably 0.25% or more.
• an excessive Nb content will cause significantly coarse precipitates, resulting in a decrease in creep strength. Therefore, the Nb content is to be 0.60% or less.
• the Nb content is preferably 0.55% or less, and more preferably 0.50% or less Nb.
• N nitrogen
• Nb carbonitride has an effect of increasing strength through solid solution strengthening, and precipitation strengthening by Nb carbonitride. Therefore, the N content is to be 0.05% or more.
• the N content is preferably 0.06% or more, and more preferably 0.07% or more.
• an excessive N content will result in the formation of lumpy nitrides, which deteriorate the steel quality. As a result, the strength may be reduced. Therefore, the N content is to be 0.15% or less.
• the N content is preferably 0.13% or less, and more preferably 0.12% or less.
• the B content is to be 0.0010% or more.
• the B content is preferably 0.0015% or more, and more preferably 0.0020% or more.
• an excessive B content will reduce weldability and hot workability at high temperatures. Therefore, the B content is to be 0.0060% or less.
• the B content is preferably 0.0050% or less, and more preferably 0.0045% or less.
• one or more elements selected from V and Ti may be included within the ranges shown below. The reasons for the limitation of each element are explained below.
• V vanadium
• V vanadium
• the V content is to be 0.50% or less.
• the V content is preferably 0.40% or less, and more preferably 0.30% or less.
• the V content may be 0% or more, but to achieve the above effect, for example, the V content is preferably 0.01% or more, and more preferably 0.02% or more.
• Ti titanium
• Ti titanium
• the Ti content is preferably 0.400% or less, more preferably 0.100% or less, and even more preferably 0.050% or less.
• the Ti content may be 0% or more, but to achieve the above effect, for example, the Ti content is preferably 0.001% or more, and more preferably 0.002% or more.
• one or more elements selected from Co, W, Ta, Sn, Al, Ca, Mg, and REM may be included within the ranges shown below. The reasons for the limitation of each element are explained below.
• Co stabilizes the austenite structure and has the effect of increasing creep strength. Therefore, Co may be included as necessary. However, an excessive Co content will increase production cost. Therefore, the Co content is to be 1.00% or less.
• the Co content is preferably 0.50% or less, and more preferably 0.30% or less.
• the Co content may be 0% or more, but to achieve the above effect, for example, the Co content is preferably 0.02% or more.
• W tungsten
• W is dissolved in the matrix phase and has the effect of increasing the creep strength of the steel. Therefore, W can be included as necessary. However, if W is included in excess, the stability of the austenite phase will be reduced, resulting in a decrease in creep strength and toughness. Therefore, the W content is to be 1.00% or less.
• the W content is preferably 0.50% or less, and more preferably 0.30% or less.
• the W content may be 0% or more, but to achieve the above effect, for example, the W content is preferably 0.01% or more.
• Ta (tantalum) combines with C to form carbonitride and reduce dissolved C. As a result, Ta has the effect of increasing SCC resistance. Ta also has the effect of increasing creep strength. For this reason, Ta can be included as necessary. However, if Ta is contained in excess, ⁇ -ferrite will be formed and the creep strength, toughness, and weldability of the steel will be reduced. Therefore, the Ta content is 0.40% or less.
• the Ta content is preferably 0.30% or less, and more preferably 0.10% or less.
• the Ta content may be 0% or more, but to achieve the above effect, for example, the Ta content is preferably 0.01% or more.
• Sn (tin) has the effect of improving corrosion resistance and high temperature properties. Therefore, Sn may be included as necessary. However, an excessive Sn content will reduce weldability and producibility. Therefore, the Sn content is to be 0.0300% or less.
• the Sn content is preferably 0.0200% or less, and more preferably 0.0100% or less.
• the Sn content may be 0% or more, but to achieve the above effect, for example, the Sn content is preferably 0.0010% or more.
• Al is an element that has a deoxidizing effect and improves hot workability by fixing O as inclusions. For this reason, Al can be included as necessary. However, if Al is included in excess, an excessive amount of inclusions will be formed and the surface properties will deteriorate. In addition, the hot workability will also be reduced. Therefore, the Al content is 0.035% or less.
• the Al content is preferably 0.030% or less, and more preferably 0.025% or less.
• the Al content may be 0% or more, but to achieve the above effect, for example, the Al content is preferably 0.0005% or more.
• Mg manganesium
• Mg magnesium
• the Mg content is to be 0.0100% or less.
• the Mg content is preferably 0.0050% or less, and more preferably 0.0030% or less.
• the Mg content may be 0% or more, but to achieve the above effect, for example, the Mg content is preferably 0.0002% or more.
• the REM (rare earth element), like Ca and Mg, has the effect of fixing S and O as inclusions and enhancing the hot workability and creep ductility of steel. For this reason, REM may be included as necessary. However, if REM is contained in excess, the hot workability and long-term creep ductility will be reduced. Therefore, the REM content is to be 0.0800% or less.
• the REM content is preferably 0.0600% or less, and more preferably 0.0400% or less.
• the REM content may be 0% or more, but to achieve the above effect, for example, the REM content is preferably 0.0010% or more.
• one or more elements selected from the group consisting of the above-mentioned Group A and Group B may be included as necessary.
• the balance is Fe and impurities.
• impurities means constituents that are mixed in from ores and scrap as raw materials or from the production environment in the industrial production of the steel material and that are allowed within a range that does not adversely affect the austenitic stainless steel.
• the value on the right side of Inequality (i) is to be 0.010 or more.
• the value on the right side of Inequality (i) is preferably 0.012 or more, more preferably 0.015 or more, and even more preferably 0.020 or more.
• the value on the right side of Inequality (i) is preferably 0.120 or less. From the viewpoint of strength reduction due to excessive N fixation, it is preferable that the value on the right side of Inequality (i) be 0.100 or less.
• the amount of Ni, Cr, Mo, and Nb dissolved in the matrix phase is controlled. This is because the improvement in creep strength can be achieved by ensuring the amount of the above elements dissolved in the matrix while reducing C, which is effective for creep strength.
• the amount of each element dissolved in the matrix can be calculated from the difference between the content (mass%) of each element and the content (mass%) of each element in the precipitates obtained by the extraction residue analysis.
• the austenitic stainless steel of the present embodiment must satisfy Inequality (ii). 27.0 ⁇ 1.13 (Ni - Ni ER ) + (Cr - Cr ER ) + 1.85 (Mo - Mo ER ) + 1.79 (Nb - Nb ER ) ⁇ 40.5
• each symbol in the above Inequality is defined as follows, and each element symbol in the above Inequality represents the content (mass %) of each element in the steel, or zero if not contained.
• the value on the middle side of Inequality (ii) is less than 27.0, Ni, Cr, Mo, and Nb will not be sufficiently dissolved, and the creep strength will not be improved. Therefore, the value on the middle side of Inequality (ii) is to be 27.0 or more.
• the value on the middle side of Inequality (ii) is preferably 29.0 or more, and more preferably 31.0 or more.
• the value on the middle side of Inequality (ii) is 40.5 or more, Nb will be too much dissolved among the above elements, and the amount of Nb that combines with dissolved C to form precipitates cannot be sufficiently ensured, and Inequality (i) may not be satisfied. Therefore, the value on the middle side of Inequality (ii) is to be less than 40.5.
• the value on the middle side of Inequality (ii) is preferably 39.0 or less, more preferably 37.0 or less, and even more preferably 35.0 or less.
• Nb ER Nb content in precipitates obtained by extraction residue analysis (mass %)
• Nb ER which is the amount of Nb in a precipitated state
• the dissolved Nb promotes the formation of Cr oxide in the service environment. Cr oxide grows slowly.
• the dissolved Nb suppresses excessive penetration of O into the steel, thereby suppressing the formation of other oxides. Therefore, it is considered that sufficient time can be provided for the growth of Cr oxide. Therefore, in order to improve the steam oxidation resistance, it is effective to provide a machined layer by shot processing or the like after ensuring the amount of dissolved Nb.
• Nb is precipitated as Nb carbonitride with Ti, while a certain amount of Nb is present as dissolved Nb.
• Figure 1 shows a micrograph of the Nb compound collected and observed from precipitates obtained by extractive residue analysis of the austenitic stainless steel of the present embodiment. Analysis of this micrograph reveals that the above Nb compound is an Nb carbonitride with Ti.
• Nb may combine with N, Cr, etc. in steel to form fine NbCr nitrides.
• the Nb ER value increases and the amount of dissolved Cr decreases.
• the austenitic stainless steels are less likely to satisfy Inequality (ii). Therefore, Nb should be controlled so that Nb becomes Nb carbonitride as described above instead of NbCr nitride.
• the content (mass %) of each element in the precipitates obtained by extraction residue analysis can be determined by the following procedure. Specifically, approximately 0.4 g of the sample is electrolyzed with 10% acetylacetone-1% tetramethylammonium chloride/methanol at a current value of 20 mA/cm 2 . The electrolyzed solution of the sample is then filtrated through a 0.2 ⁇ m filter, and the residue is acid-decomposed. The amount (mass %) analyzed as electrolytic extraction residue is then calculated for the above elements using an ICP optical emission spectrometer.
• a machined layer is provided near the surface by shot processing or the like.
• providing a machined layer causes a recrystallized microstructure to be formed during use at high temperatures.
• This recrystallized microstructure has a very fine grain size, which facilitates grain boundary diffusion of Cr.
• the machined layer is a structure the surface vicinity of which is hardened. Therefore, the following Inequality (iv) is satisfied. 0.5 ⁇ Hv 40 ⁇ Hv t / 2 / Hv t / 2 wherein, each symbol in the above Inequality is defined as follows:
• the value on the right side of Inequality (iv) is called the hardness increase rate, and the larger this value is, the more processing has been introduced. If the value on the right side of Inequality (iv) is 0.5 or more, recrystallization will occur in the machined layer formed in the range from the surface to 40 ⁇ m in the thickness direction during use at high temperatures. As a result, even if peeling occurs on the scale during use at high temperatures, the peeled area can be repaired. Therefore, the value on the right side of Inequality (iv) is to be 0.5 or more.
• the fine crystalline structure also facilitates the supply of Cr to the surface at high temperatures. This prevents abnormal oxidation, especially in the initial stage of use, and contributes to the uniform formation of Cr oxide scale.
• the value on the right side of Inequality (iv) be 0.6 or more, more preferably 0.7 or more.
• the upper limit of the right side of Inequality (iv) is not particularly limited. However, the larger the value on the right side of Inequality (iv) becomes, the harder the surface becomes, which may cause problems in forming, welding, and other processes. For this reason, it is preferable that the value on the right side of Inequality (iv) be 2.0 or less.
• Hv 40 and Hv t/2 can be measured by the following procedure.
• a square test specimen of 15 mm is cut out, embedded in resin, and the cross section is cut and mirror-polished.
• the test specimen is measured for Vickers hardness with a load of 10 gf at a position 40 ⁇ m from the surface in the thickness direction and at a position t/2 from the surface in the thickness direction (when the total thickness is t) so that they are on the same straight line in the thickness direction.
• This measurement is repeated at different locations where the measurement at other locations on the specimen is not affected, so that the number of measurements at each position is five.
• the average of the five measurements at 40 ⁇ m in the thickness direction is then taken as Hv 40 .
• the average of the five measurement results at a position t/2 in the thickness direction is Hv t/2 .
• measurements are taken at a position 40 ⁇ m from the surface in the thickness direction and at a position t/2 from the surface in the thickness direction so that they are on the same straight line in the thickness direction, and these measurements are also taken at other locations so that the total number of measurements at each position is 5 times.
• the above measurements are taken at a position 40 ⁇ m from the inner surface of the tube in the direction of the wall thickness and at a position t/2 from the inner surface of the tube in the direction of the wall thickness (when the total thickness is t).
• the reason for measuring the hardness of the inner surface of the steel tube is that when the tube is used as a boiler heat transfer tube, high-temperature steam passes through the inner surface of the tube; therefore, the inner surface of the tube is required to be especially resistant to steam oxidation.
• a method for manufacturing the austenitic stainless steel of the present embodiment is described below. As a result of research conducted to date, the present inventors have confirmed that the austenitic stainless steel of the present embodiment can be manufactured by the following method.
• the resulting billet is then hot worked.
• the hot working conditions there are no restrictions on the hot working conditions, but in order to prevent harmful defects from occurring during tube making, for example, the billet is heated to a temperature in a range of 900 to 1300°C before hot working.
• the type of hot working there are no restrictions on the type of hot working. Hot rolling can be used to manufacture steel plates, and hot extrusion can be used to manufacture steel tubes.
• the hot-worked steel is quenched under the following conditions.
• the time (minutes) between the completion of hot working and the start of quenching is called “leaving time.”
• the “leaving time” refers to the time from the completion of hot working to the time when the steel is transported to a water cooling device and water cooling is started. If the leaving time exceeds 5.0 minutes, coarse precipitates will be formed. It is also more likely to form NbCr nitrides rather than the desired Nb carbonitrides.
• the leaving time should be 5.0 minutes or less.
• the leaving time is preferably 4.5 minutes or less, more preferably 4.0 minutes or less, and even more preferably 3.5 minutes or less.
• softening treatment temperature T 1 The holding temperature in the softening treatment (hereinafter referred to as "softening treatment temperature T 1 ") is above the recrystallization temperature and below the grain boundary melting temperature. In the softening process, it is recommended to perform soaking at 1040 to 1300°C.
• the time of softening treatment is not limited, but from the viewpoint of recrystallization in the thickness direction and production cost, 1 to 10 minutes is preferred. If the softening treatment time is less than 1 minute, the SCC resistance may be reduced. On the other hand, if the softening treatment time exceeds 10 minutes, the crystal grains may become coarse.
• the steel is subjected to solution heat treatment at 1100 to 1200°C.
• the soaking temperature in the solution heat treatment is referred to as the solution treatment temperature T 2 .
• the solution treatment temperature T 2 is less than 1100°C, the dissolution of elements effective for improving creep strength will be insufficient, and Inequality (ii) will not be satisfied, resulting in lower creep strength.
• the solution treatment temperature T 2 exceeds 1200°C, the elements to be precipitated may not satisfy Inequality (i) and/or Inequality (ii) due to excessive dissolution.
• the solution treatment temperature T 2 is preferably 1170°C or less.
• the softening treatment temperature T 1 is to be controlled to be higher than the solution treatment temperature T 2 .
• the softening temperature T 1 is controlled to satisfy the following Inequality (a). If the solution treatment temperature T 2 is higher than the softening temperature T 1 , it will be difficult to improve the SCC resistance sufficiently. T 1 > T 2
• the difference between the softening temperature T 1 and the solution temperature T 2 should be 150°C or less. That is, the temperature should be controlled so as to satisfy the following Inequality (b). T 1 ⁇ T 2 ⁇ 150
• the solution heat treatment time is preferably less than 10 minutes. If the solution heat-treatment time is 10 minutes or more, the elements to be precipitated will be excessively dissolved, and Inequality (i) and/or Inequality (ii) may not be satisfied.
• the lower limit of the solution heat treatment time is not particularly limited, but is usually 1 minute.
• the steel After solution heat treatment, the steel is cooled. This cooling is referred to as second cooling.
• the steel In the second cooling, the steel is cooled with water or at a cooling rate equal to or faster than water cooling, as in the first cooling. This is because if the steel is not cooled with water or at a cooling rate equal to or faster than water cooling, the creep strength will be reduced and product quality may not be consistent due to the formation and growth of precipitates. It is usually preferable to cool the steel to 600°C or less.
• a machined layer is formed in the vicinity of the surface of the steel that has undergone the second cooling.
• the method of forming a machined layer is not limited.
• various known spraying methods such as shot peening, shot blasting, shot processing, sand blasting, sanding, air blasting, water jetting, etc. can be used.
• steel, cast steel, stainless steel, glass, silica sand, alumina, amorphous, etc. can be used as materials.
• the shape of the above particles can be, for example, spherical, cut wire, grit, etc.
• the particles may be blown by compressed air, centrifugal force using an impeller (impeller type), high-pressure water, ultrasonic waves, etc.
• Particles can also be mixed with liquid and sprayed with compressed air, etc. (also called “liquid honing").
• a machined layer by polishing, ball milling, grinding, honing, or ultrasonic impact processing.
• a machined layer by particle spraying, which facilitates uniform processing over the entire surface.
• the machined layer is formed on a pickled surface. In other words, the machined layer is formed after pickling. This is because pickling improves the surface condition and facilitates surface deformation.
• Solution heat treatment was then performed under the conditions shown in Table 2, followed by water cooling to obtain a steel tube.
• shot processing was performed on the inner surface of the tubes to form a machined layer.
• conditions such as injection pressure, injection amount, injection angle, and nozzle shape were controlled and adjusted.
• the surface of the steel tube when shot processing was performed was the surface after pickling, except for No. 33.
• the obtained steel tubes were subjected to electrolytic extraction residue measurement, hardness measurement (Hv 40 and Hv t/2 ), SCC test, creep test, and steam oxidation test according to the procedures described below.
• a creep test was performed to evaluate creep strength. Specifically, a round bar creep test specimen was taken and a creep rupture test was performed. The creep properties were rated as very good (A) if the rupture strength was 210 MPa or more in a creep test at 600°C for 10,000 hours and also the rupture strength was 150 MPa or more in a creep test at 650°C for 10,000 hours.
• the creep properties were rated as good (B) if only one of the following was satisfied: either the ultimate tensile strength was 210 MPa or higher in a creep test at 600°C for 10,000 hours, or the ultimate tensile strength was 150 MPa or higher in a creep test at 650°C for 10,000 hours.
• the creep properties were rated as poor (D) except for the cases of very good creep properties (A) and good creep properties (B) as described above.
• Test Nos. 19, 24, and 26 to 28 were rated “D” in the creep test. In these cases, the value on the middle side of Inequality (ii) was less than 27.0, and it is considered that sufficient creep strength was not obtained due to insufficient dissolution of Ni, Cr, Mo, and Nb, which improves creep strength. Among these, Test Nos. 24, 26, and 27 did not satisfy Inequality (iii), which is considered to have resulted in the formation of NbCr nitrides and a decrease in creep strength.
• Test Nos. 1 to 23, 25, and 28 to 30 were rated "B" in the steam oxidation test. These examples satisfied Inequalities (iii) and (iv), and are therefore considered to have good steam oxidation resistance.
• Test Nos. 24, 26, 27, and 31 were rated "C” in the steam oxidation test. These examples satisfied Inequality (iv), but did not satisfy Inequality (iii), and are therefore considered to have failed to form a good machined layer, resulting in poor steam oxidation resistance.
• Test Nos. 32 and 33 were rated "D" in the steam oxidation test. These examples were considered to have failed to form a sufficient machined layer because the conditions for forming the machined layer were not within the preferred range.

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