RU2558738C1 - Refractory martensitic steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel contains in wt %: carbon - 0.08-0.12, silicon not over 0.13, manganese 0.4-0.6, chromium 9.0-9.5, nickel from over 0.1 to 0.3, tungsten 1.2-1.7, molybdenum 0.5-0.8, vanadium 0.18-0.25, niobium 0.04-0.07, nitrogen not over 0.005, boron 0.01-0.014, cobalt from over 3.0 to 3.5, sulphur not over 0.006, phosphorus not over 0.01, aluminium not over 0.01, copper not over 0.03, titanium minimum 0.01, iron - the rest.

EFFECT: steel has increased creep resistance.

4 tbl, 1 ex

Description

Heat Resistant Martensitic Steel

The invention relates to the field of metallurgy, in particular, to heat-resistant chromium steels of the martensitic class, containing 9-12% chromium. The proposed steel can be used in the energy industry as structural materials for the production of boilers, rotors and other equipment of new generation thermal power plants operating at temperatures up to 640 ° C.

Currently, for the manufacture of elements of thermal power plants in Japan and America, P92 grade steel is used according to ASTM A 335 classification - American Society for Testing and Materials. Steel contains, mass. %

carbon 0,070-0,130 silicon no more than 0,500 manganese 0,300-0,600 chromium 8,900-9,500 nickel no more than 0,400 tungsten 1,500-2,000 molybdenum 0,300-0,600 vanadium 0.150-0.250 niobium 0,040-0,090 nitrogen 0,030-0,070 boron 0.001-0.006 sulfur no more than 0,010 phosphorus no more than 0,020 aluminum no more than 0,040 iron rest

P92 steel has a high level of strength and creep resistance to a temperature of 620 ° C. As a result of special heat treatment, a troostomartensitic structure is formed and secondary phase particles are released, which explains the increased creep resistance of this steel and allows it to be used as structural materials of boilers, rotors and other equipment for thermal power plants. Dispersion hardening of steel is achieved due to the precipitation of carbides of type M ₂₃ C ₆ and nanosized carbonitrides of type (VNb) (C, N). This steel retains high creep resistance until the dislocation structure of tempering martensite (troostomartensite) is stable.

The disadvantage of P92 steel is the intensive coagulation of M ₂₃ C ₆ carbides and particles of the Laves phase at temperatures above 620 ° C, which contributes to a significant decrease in the creep resistance of this steel and makes it impossible to use it for parts of power plants operating at super supercritical steam parameters (30 MPa, 630-650 ° C).

Closest to the alloying principle and the achieved result to the proposed invention is the heat-resistant steel of the martensitic class, disclosed in patent No. RU 2447184 C22C38 / 54. Steel contains, mass. %

carbon 0.080 - 0.120 silicon no more than 0,100 manganese 0,050-0,100 chromium 9,500-10,000 nickel no more than 0,200 tungsten 1,800-2,200 molybdenum 0.600-0.800 vanadium 0.180-0.250 niobium 0,040-0,070 nitrogen no more than 0,003 boron 0.008-0.010 cobalt 2,500-3,500 sulfur no more than 0,006 phosphorus no more than 0,010 aluminum no more than 0,010 copper no more than 0,010 titanium no more than 0,010 iron rest

In this steel, in comparison with P92 steel, the content of molybdenum and boron is increased, the nitrogen content is reduced, and copper and titanium are additionally introduced. Due to the increase in the molybdenum content to 0.6-0.8%, solid solution hardening occurs, as well as a decrease in the coagulation rate of carbides of the type M ₂₃ C ₆ , which increases the heat-resistant properties of steel. A molybdenum content of less than 0.6% does not provide the strength of steel at elevated temperatures, more than 0.8% - contributes to the formation of delta ferrite and the Laves phase. An additional increase in deformation resistance during creep, as well as an increase in stress corrosion resistance is achieved by doping with boron in an amount of 0.008-0.01%. Boron segregates along grain boundaries, mainly former austenitic, which suppresses grain-boundary slippage and thereby increases the time to failure. When the boron content is more than 0.01%, the weldability and ductility of steel is reduced. With a high boron content (up to 0.01%), it is advisable to reduce the nitrogen content (0.003% or less) in order to prevent the formation of large boron nitrides, which are the reason for the low toughness of steel. Less than 0.01% copper is introduced to prevent the formation of delta ferrite. Titanium in an amount of not more than 0.01% promotes the formation and stabilization of nanosized carbonitrides of the MX type. When the titanium content exceeds 0.01%, large carbonitrides are formed, which reduces the creep resistance.

The disadvantage of this steel is the impossibility of its use for parts of power plants operating at temperatures above 630 ° C, due to insufficiently high values of strength and toughness, which negatively affects the creep resistance of steel.

The objective of the invention is the development of steel with increased creep resistance and is operable at a temperature of 640 ° C, which is 10-20 ° C higher compared to existing analogues.

To solve this problem, a heat-resistant steel of martensitic class is proposed, containing carbon, silicon, manganese, chromium, nickel, tungsten, molybdenum, vanadium, niobium, nitrogen, boron, cobalt, sulfur, phosphorus, aluminum, copper, titanium and iron, and in it the tungsten content is reduced, the content of copper, boron and manganese is increased in the following ratio of components, mass. %: carbon 0.080-0.120; silicon not more than 0.130; manganese 0.400-0.600; chrome 9,000-9,500; nickel from more than 0.1 to 0.300; tungsten 1,200-1,700; molybdenum 0.500-0.800; vanadium 0.180-0.250; niobium 0.040-0.070; nitrogen to less than 0.005; boron 0.010-0.014; cobalt from more than 3.0 to 3,500; sulfur not more than 0.006; phosphorus not more than 0.010; aluminum no more than 0.010; copper no more than 0,030; titanium to less than 0.010; iron the rest.

In the presented steel, the tungsten content is reduced, the content of copper, boron and manganese is increased, compared with the steel of the prototype. Due to the increase in boron content, type M carbides are stabilized.₂₃FROM₆as well as martensitic microstructure due to a decrease in the coagulation rate of type M carbides₂₃FROM₆, which, in turn, increases the heat resistance of this steel (F. Abe et al. “Suppression of Type IV fracture and improvement of creep strength of 9Cr steel welded joints by boron addition). The content of Σ (W + Mo) in steel in the amount of 2.0-2.4% reduces the diffusion rate in solid solution [Vaillant J. et al. “New grades for advanced coal-fired power plants-Properties and experience”, Abe F. Et al. "Alloy design of creep resistant 9Cr steel using a dispersion of nanosizedcarbonitrides"] and, accordingly, suppresses the creep of dislocations, which is one of the main ways to increase the creep resistance of martensitic steel with 9% chromium content. Copper contributes to the expansion of the austenite region, and also forms precipitates that increase strength at elevated temperatures. Copper also plays the role of additional phase nuclei released during creep, due to which a finer dispersed phase distribution is formed, which increases the creep resistance of steel. Silicon in an amount of <0.15% and manganese in an amount of 0.4-0.6% are used for deoxidation of steel. When the silicon content is more than 0.15%, the tendency of steel to thermal brittleness is enhanced. With the introduction of manganese less than 0.4% - low deoxidizing ability of silicon, more than 0.6% - practically does not affect the deoxidizing ability, so the introduction of a high content of this element is impractical. When the nitrogen content is less than 0.008%, the formation of large boron nitrides, which are the cause of low toughness, does not occur in this steel.

An example implementation.

The alloy of the proposed chemical composition was cast (Table 1). The alloy was quenched from 1060 ° C and tempered at 750 ° C for 3 hours.

Table 1
The chemical composition of the proposed steel C Si Mn Cr Ni Co Mo W V Nb N B Al S Ti P Cu 0.1 0.12 0.4 9 0.24 2,8 0.57 1,5 0.2 0.05 0.007 0.012 0.01 0.006 0.002 0.008 0,027

Mechanical tensile tests were carried out according to GOST 1497-84 at room temperature and according to GOST 9651-84 at elevated temperatures (Table 2). Toughness tests were carried out according to GOST 9454-78 (table 3). Creep tests were carried out according to GOST 3248-81 (Table 4). As can be seen from tables 2, 3, 4, the mechanical properties of the proposed steel are higher in comparison with the prototype.

table 2
Mechanical properties of steel depending on test temperature Test temperature, ° С Prototype Steel offered at _0.2 , MPa y _in , MPa d,% at _0.2, MPa y _in , MPa d,% twenty 540 700 fifteen 590 810 16 450 450 540 12 430 630 12 500 400 500 13 375 590 fourteen 550 340 455 fourteen 370 530 17 600 365 390 twenty 420 455 twenty 650 300 320 25 345 370 23 700 215 240 33 250 270 25

In table 2: σ _0.2 - conditional yield strength; σ _in - ultimate strength; δ,% - elongation after rupture.

Table 3
Impact strength of steel at a temperature of 20 ° C Prototype Steel offered KCV, J / cm ² 237 248

Table 3: KCV — Impact Strength

Table 4
Creep tests at a temperature of 650 ° C Applied stress, MPa Time to sample destruction, h Prototype Steel offered 140 1425 3430 160 210 1035 180 eighteen 243

As can be seen from the tables, the properties of the proposed steel allow it to be used for the manufacture of boilers, rotors and other elements of power plants. The use of steel in the power industry will raise the operating temperature of thermal power plants to 640 ° C.

Claims (1)

Heat-resistant steel of the martensitic class containing carbon, silicon, manganese, chromium, nickel, tungsten, molybdenum, vanadium, niobium, nitrogen, boron, cobalt, sulfur, phosphorus, aluminum, copper, titanium and iron, characterized in that it contains components when the following ratio, wt.%:
carbon 0.08 - 0.12 silicon no more than 0,13 manganese 0.4 - 0.6 chromium 9.0 - 9.5 nickel from more than 0.1 to 0.3 tungsten 1.2 - 1.7 molybdenum 0.5 - 0.8 vanadium 0.18 - 0.25 niobium 0.04 - 0.07 nitrogen less than 0.005 boron 0.01 - 0.014 cobalt from more than 3.0 to 3.5 sulfur no more than 0,006 phosphorus no more than 0,01 aluminum no more than 0,01 copper no more than 0,03 titanium less than 0.01 iron rest