EP1805340B1 - Creep-resistant, martensitically hardenable, heat-treated steel - Google Patents

Abstract

A maraging heat-treatment steel includes 8.5 to 9.5% by weight of Cr, 0.15 to 0.25% by weight of Mn, 2 to 2.7% by weight of Ni, 0.5 to 2.5% by weight of Mo, 0.4 to 0.8% by weight of V, 0.001 to 0.15% by weight of Si, 0.06 to 0.1% by weight of C, 0.11 to 0.15% by weight of N, 0.02 to 0.04% by weight of Nb, maximum 0.007% by weight of P, maximum 0.005% by weight of S, maximum 0.01% by weight of Al, iron and standard impurities, wherein a weight ratio of vanadium to nitrogen V/N is in a range between 4.3 and 5.5.

Description

Technical area

The invention relates to martensitic-hardenable steels with increased nitrogen contents, which are characterized by a very good combination of properties, in particular by a high resistance to creep and good ductility.

State of the art

Martensite-hardenable steels based on 9-12% chromium are widely used materials in power plant technology. It is known that the addition of chromium in the abovementioned range not only provides good resistance to atmospheric corrosion but also complete through-hardenability of thick-walled forgings, for example as monobloc rotors or as rotor disks in gas and steam turbines. Proven alloys of this type usually contain about 0.08 to 0.2% carbon, which in solution allows the setting of a hard martensitic structure. A good combination of heat resistance and ductility of martensitic steels is made possible by a tempering treatment in which Forming a particle-stabilized subgrain structure by the precipitation of carbon in the form of carbides with simultaneous recovery of the dislocation substructure. The tempering behavior and the resulting properties can be effectively influenced by the choice and proportionate tuning of specific carbide formers such as Mo, W, V, Nb and Ta.

Strengths above 850 MPa for 9-12% chromium steels can be adjusted by keeping the tempering temperature low, typically in the range 600 to 650 ° C. However, the use of low tempering temperatures leads to high transition temperatures from the brittle to the ductile state (above 0 ° C), with which the material exhibits brittle fracture behavior at room temperature. Significantly improved ductilities can be achieved if the tempered strength is lowered below 700 MPa. This is achieved by raising the tempering temperature to over 700 ° C. The use of increased tempering temperatures has the advantage that the set structural states are stable for longer times at elevated temperatures. A typical representative who has found widespread use in steam power plants, in particular as rotor steel, is the German steel X20CrMoV12.1 known under DIN.

It is also known that the ductility at a strength level of 850 MPa can be significantly improved by alloying nickel. For example, it is known that by alloying about 2 to 3% nickel, even after a tempering treatment at temperatures of 600 to 650 ° C, the transition temperature from brittle to ductile state is still below 0 ° C, resulting in a significantly improved overall combination of strength and ductility. Such alloys are therefore widely used where significantly higher demands are placed on both strength and ductility, typically as disk materials for gas turbine rotors. A typical representative of such alloys, which in the gas turbine technology, in particular as a material for rotor disks width Has found use is known under DIN German steel X12CrNiMo12.

In recent years, various efforts have been made to improve specific properties of these steels. For example, in the publication of Kern et al .: High Temperature Forged Components for Advanced Steam Power Plants, Materials for Advanced Power Engineering 1998, Proceedings of the 6th Liege Conference, ed. By J. Lecomte-Becker et. al , describing the development of novel rotor steels for steam turbine applications. In such alloys, the contents of Cr, Mo, W were further optimized, taking into account about 0.03 to 0.07% N, 0.03 to 0.07% Nb and / or 50 to 100 ppm B, to creep and creep strengths for 600 ° C applications to improve.

On the other hand, especially for gas turbine applications, efforts have been made either to improve the creep rupture strengths in the range of 450 to 500 ° C at high ductility level or to reduce the embrittlement tendency at temperatures between 425 and 500 ° C. This is how the European patent application describes EP 0 931 845 A1 a nickel-containing 12% chromium steel similar in structure to the German steel X12CrNiMo12, in which the element molybdenum is reduced compared to the known steel X12 CrNiMo12, but an increased content of tungsten was added. In DE 198 32 430 A1 is a further optimization of the X12CrNiMo12 similar steel with the name M152 disclosed in which by the addition of rare earth elements, the embrittlement tendency in the temperature range between 425 and 500 ° C is limited.

One possible approach to improving the high temperature strength and high ductility has been proposed with the development of steels with increased nitrogen contents. In EP 0 866 145 A2 describes a new class of martensitic chromium steels with nitrogen contents in the range 0.12 to 0.25%. In this class of steel, the entire structure of the structure is determined by the Formation of Sonderitriden, in particular controlled by Vanadiumnitriden, which can be distributed by the forging treatment, by the austenitization, by a controlled cooling treatment or by a tempering treatment in a variety of ways. While strength is achieved through the hardening effect of the nitrides, the high ductility setting in this patent application is aimed at by the distribution and morphology of the nitrides, but especially by limiting grain coarsening during forging and solution heat treatment. This is achieved in the cited document by an increased volume fraction as well as by a high particle coarsening resistance of sparingly soluble nitrides, so that a dense dispersion of nitrides is still capable of effectively limiting grain growth even at austenitizing temperatures of 1150 to 1200 ° C. The essential benefit of in EP 0 866 145 A2 The alloys listed above have the possibility of optimally influencing the combination of strength and ductility solely by the formation of nitrides with regard to distribution and morphology by a suitable definition of the heat treatment.

However, an optimized formation state of nitrides is only one factor for achieving maximum ductility. Another factor is the effect of dissolved substitution elements such as nickel and manganese. Manganese is known from carbon steels to be embrittling rather than promoting ductility. In particular, it causes embrittlement when the alloy is subjected to long-term annealing at temperatures in the range of 350 to 500 ° C. It is also known that nickel in carbon steels improves ductility but tends to lower high temperature hot strength as well. This is related to reduced carbide stability in nickel-containing steels.

Out EP 1 158 067 A1 a martensitic-hardenable tempering steel having the following chemical composition is known (in% by weight): 9 to 12 Cr, 0.001 to 0.25 Mn, 2 to 7 Ni, 0.001 to 8 Co, at least one of W and Mo in the sum between 0.5 and 4, 0.5 to 0.8, at least one of Nb, Ta, Zr Hf in the Sum between 0.001 to 0.1, 0.001 to 0.05 Ti, 0.001 to 0.15 Si, 0.01 to 0.1 C, 0.12 to 0.18 N, max. 0.025 P, max. 0.015 S, max. 0.01 Al, max. 0.0012 Sb, max. 0.007 Sn, max. 0.012 As, balance Fe and common impurities, and the proviso that the weight ratio of vanadium to nitrogen V / N ranges between 3.5 and 4.2. These alloys are characterized by a very good combination of impact energy at room temperature and heat resistance at 550 ° C, especially at higher Cr contents. The relatively high N content increases the creep rupture strength. V and N are in the specified range in nearly stoichiometric proportions. This achieves optimum solubility and coarsening resistance of the vanadium nitrides. The high solubility is required to dissolve as much of the precipitation hardening vanadium nitride as possible, while a high resistance to coarsening of the nitrides is needed in order to have the finest possible fine structure in the EP 1 158 067 A1 To achieve the described heat treatment.

It is known that in steels with about 12% chromium and an increased proportion of N adversely affects the α'Cr phase in the temperature range of about 425 to 500 ° C, which leads to embrittlement of the steel. Although these precipitates increase the strength properties, the ductility, notched impact strength and corrosion resistance values decrease. Thus, such steels are only of limited use for use in compressors or turbines in the power plant area. The formation of VN in such steels further enhances the tendency to precipitate the α'Cr phase and hence the embrittlement tendency in said temperature range.

Presentation of the invention

The invention has for its object to provide a martensitic-hardenable tempering steel with high ductility in the temperature range between 350 and 500 ° C and good creep resistance in the temperature range up to 550 ° C.

Core of the invention is a martensitic-hardenable tempering steel having the following composition (in wt .-%): 8.5 to 9.5 Cr, 0.15 to 0.25 Mn, 2 to 2.7 Ni, 0.5 to 2.5 Mo, 0.4 to 0.8 V, 0.02 to 0.04 Nb , 0.001 to 0.15 Si, 0.06 to 0.1 C, 0.11 to 0.15 N, maximum 0.007 P, maximum 0.005 S, maximum 0.01 Al, balance iron and common impurities, and the requirement that the weight ratio of vanadium to nitrogen V / N in the range between 4.3 and 5.5.

Preferred ranges for the individual alloying elements of the inventive composition are contained in the subclaims.

The advantage of the invention is that in the said alloy, a compensation structure is set, which is characterized by a tough matrix and the presence of heat-resistant nitrides, at the same time a tendency to embrittlement in the range between 350 and 500 ° C is suppressed. The toughness of the base matrix is adjusted by the presence of substitution elements, preferably nickel. The contents of the substitution elements are determined so as to allow optimum unfolding of both martensite hardening and particle hardening by means of special nitrides, preferably vanadium nitrides, to provide high creep strength coupled with good ductility. The embrittlement tendency of the inventive steel in the temperature range of 350 to 500 ° C due to precipitation of the α'Cr phase is suppressed by the low compared to the prior art, the Cr content and moderate N content.

Hereinafter, the preferred amounts in weight percent for each element and the reasons for the selected according to the invention Alloy areas shown in their context with the resulting possibilities of heat treatments.

Chrome:

A weight content of 8.5 to 9.5% chromium allows a reasonable hardenability of thick-walled components and ensures sufficient oxidation resistance up to a temperature of 550 ° C. A weight fraction below 8.5% impairs the through-hardenability. Contents above 9.5% lead to the accelerated formation of the α'Cr phase during the tempering process, which leads to embrittlement of the material.

Manganese and silicon:

These elements promote temper embrittlement and must therefore be limited to the smallest amounts. The range to be specified should be between 0.15 and 0.25% for manganese and between 0.001 and 0.15% for silicon, taking into account the possibilities of ladle metallurgy.

Nickel:

Nickel is used as an austenite stabilizing element to suppress delta ferrite. In addition, it is said to improve ductility as a dissolved element in the ferritic matrix. Nickel contents between 2 to 2.7 wt .-% are optimal, since on the one hand, the nickel is homogeneously dissolved in the matrix, on the other hand, there is still no increased proportion of retained austenite or tempering austenite in the tempered martensite.

Molybdenum:

This element improves creep strength by solid solution hardening as a partially dissolved element and precipitation hardening during a long-term stress. However, an excessively high proportion of this element leads to embrittlement during a long-term aging, which is due to the excretion and coarsening of the sigma phase. For this Reason, the maximum proportion of Mo must be limited to 2.5%. A preferred range is about 1.4 to 1.6%.

Vanadium and nitrogen:

Together, these two elements significantly control grain size formation and precipitation hardening. The microstructural forms are optimal when the elements vanadium and nitrogen are alloyed in a slightly more than stoichiometric V / N ratio. A slightly more than stoichiometric ratio also increases the stability of the vanadium nitride over that of the chromium nitride. Overall, a V / N ratio in the range between 4.3 to 5.5 is preferred. The specific content of nitrogen and vanadium nitrides depends on the optimum volume fraction of the vanadium nitrides, which are to remain as insoluble primary nitrides during the solution annealing. The greater the total amount of vanadium and nitrogen, the greater the proportion of vanadium nitrides which no longer dissolve and the greater the grain refining effect. However, the positive effect of grain refining on ductility is limited because with increasing volume fraction of primary nitrides, the primary nitrides themselves limit ductility. Since VN also increases the tendency to form the brittle α'Cr phase, the preferred content of nitrogen should be in the range of 0.11 to 0.12% by weight and that of vanadium should be in the range of 0.5 to 0.6% by weight. Conceivable ranges are 0.11 to 0.15% by weight for N and 0.4-0.8% by weight V.

Niobium:

Besides vanadium, niobium is a preferred element among the special nitride formers. The preferred range is 0.02 to 0.04% by weight. In these small admixtures, the grain coarsening resistance in solution annealing is increased and the stability of primary and excrete V8N, C) nitrides is increased by partial substitution of V.

Phosphorus and sulfur:

Together with silicon and manganese, these elements increase the embrittlement of long-term aging in the range between 350 and 500 ° C. These elements should therefore be limited to the minimum tolerable levels.

Aluminum:

This element is a strong nitride former, which already sets nitrogen in the melt and thus strongly affects the effectiveness of the added nitrogen. The aluminum nitrides formed in the melt are very coarse and reduce the ductility. Aluminum must therefore be limited to a weight fraction of 0.01%.

Carbon:

Carbon forms chromium carbides on tempering, which are conducive to improved creep resistance. At too high carbon contents, however, the resulting increased volume fraction of carbides leads to a ductility reduction, which comes into play in particular by the carbide coarsening during long-term storage. The carbon content should therefore be limited upwards to 0.1%. Another disadvantage is the fact that carbon increases the hardening during welding. The most preferred carbon content is in the range of 0.06 to 0.08% by weight.

Short description of the drawing

In the drawing, an embodiment of the invention is shown. The sole figure shows the dependence of the voltage on time for achieving a creep strain of 1% at 550 ° C for an alloy according to the invention and an alloy known from the prior art.

Ways to carry out the invention

The invention will be explained in more detail with reference to embodiments and FIG. 1.

Table 1 shows the chemical composition (in% by weight) of a preferred alloy according to the invention (DM13) and of comparative alloys: Table 1: Chemical composition DM13A-2 St13TNiEL alloy
Type "D" C 12:08 12:12 12:04 Cr 9.0 11.5 11.2 Mn 12:19 Max. 0.25 12:05 Ni 2.4 2.3 3:06 Co 4:02 Not a word 1.4 1.5 1.83 V 0.6 12:25 0.61 Nb 12:04 12:03 Si 12:13 12:25 <00:02 N 0117 0035 0156 al 0008 <00:02 P Max. 0.025 0004 S Max. 0.015 0002 V / N 5.13 7.24 3.91

• DM13A-2:
  1100°C/3h/schnelle Luftabkühlung (Ventilator) + 640°C/5h/Luftabkühlung
• St13TNiEL:
  1050-1080 °C/>0.5h/Öl + 630-650 °C/>2h/Luftabkühlung
• Legierung "D":
  1180°C/2h/Luftabkühlung + 640°C/2h/Luftabkühlung + 600°C/1h/Ofenabkühlung

10 kg of melt were melted in an induction furnace and then forged flat bars with dimensions of 20mmx80mm were produced. The following heat treatments were performed:

• DM13A-2:
  1100 ° C / 3h / fast air cooling (fan) + 640 ° C / 5h / air cooling
• St13TNiEL:
  1050-1080 ° C /> 0.5h / oil + 630-650 ° C /> 2h / air cooling
• Alloy "D":
  1180 ° C / 2h / air cooling + 640 ° C / 2h / air cooling + 600 ° C / 1h / furnace cooling

Table 2 contains experimental data for determining the notched impact energy at room temperature: Table 2: Notch energy for various differently treated alloys alloy conditions Notch energy in J DM13A-2 Initial state after the above heat treatment 76 Outsourced at 400 ° C / 1032h 90 Outsourced at 480 ° C / 1032h 58 St13TNiEL Initial state after the above heat treatment > 40 (required) Alloy "D" Initial state after the above heat treatment 106 Outsourced at 300 ° C / 5000h 57 Outsourced at 380 ° C / 5000h 36 Outsourced at 450 ° C / 5000h 21 Outsourced at 500 ° C / 5000h 54

It can be clearly seen the reduction of impact energy in the alloy "D" after a swapping of the samples in the range between 300 and 500 ° C. This is due to the excretion of the α'Cr phase. In the case of the alloy DM13A-2 according to the invention, the tendency to precipitate this phase is opposed reduced, so that the embrittlement in the temperature range is lower.

Tensile tests at room temperature and 550 ° C of the heat-treated samples described above (initial state) gave the results shown in Table 3: alloy T in ° C Streckgren. in MPa Tensile in MPa Elongation in% Necking. in % E-Mod. in GPa DM13A-2 20 928 1036 14.4 64 212 550 600 637 19.9 75.3 155 St13TNiEL 20 852 985 550 470 530 alloy 20 975 1068 15.2 67 "D" 550 714 750 15.0 72

The alloy according to the invention is distinguished by a high heat resistance at 550 ° C., as well as by a high ductility and a good modulus of elasticity.

In the single figure, the voltage for 1% creep at 550 ° C for the alloys DM13A-2 and St13TNiEL is shown as a function of time. The advantage of the alloy according to the invention comes into play at high removal times.
Of course, the invention is not limited to the embodiment described.

Claims (8)

1. Maraging heat-treatment steel, characterized by the following chemical composition (details in % by weight): 8.5 to 9.5 Cr, 0.15 to 0.25 Mn, 2 to 2.7 Ni, 0.5 to 2.5 Mo, 0.4 to 0.8 V, 0.001 to 0.15 Si, 0.06 to 0.1 C, 0.11 to 0.15 N, 0.2 to 0.4 Nb, max 0.007 P, max 0.005 S, max 0.01 Al, remainder iron and standard impurities, with the proviso that the vanadium to nitrogen weight ratio V/N is in the range between 4.3 and 5.5.
2. Maraging heat-treatment steel according to Claim 1, characterized by 8.5 to 9% Cr by weight.
3. Maraging heat-treatment steel according to Claim 1, characterized by 0.2% by weight Mn.
4. Maraging heat-treatment steel according to Claim 1, characterized by 2.3 to 2.6% by weight Ni.
5. Maraging heat-treatment steel according to Claim 1, characterized by 1.4 to 1.6% Mo by weight.
6. Maraging heat-treatment steel according to Claim 1, characterized by 0.5 to 0.6% by weight V.
7. Maraging heat-treatment steel according to Claim 1, characterized by 0.11 to 0.12% N by weight.
8. Maraging heat-treatment steel according to Claim 1, characterized by 0.06 to 0.08% C by weight.

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