Patent Application
20240200161
The present invention belongs to the technical field of metal materials, and relates to a heat-resistant steel for steel pipes and castings.
Among pressure vessels, a boiler is an energy conversion device. Energy, including chemical energy in the fuel and electrical energy, is input to the boiler, and the boiler outputs steam, high-temperature water or organic heat carrier with certain thermal energy. A steam turbine in power machinery, also known as a steam turbine engine, is a rotary steam power unit. At the steam turbine, high-temperature high-pressure steam passes through a fixed nozzle to become an accelerated steam flow which is then directed onto blades, so that the rotor with a row of blades rotates and does external work. Boilers and steam turbines are the main equipment in a modern thermal power plant.
Improving the steam temperature parameters of a coal-fired unit in the thermal power plant can increase the unit efficiency, reduce the consumption of fossil fuels and achieve energy conservation and emission reduction. The operating temperatures of the boilers and the steam turbines are limited by the maximum operating temperatures of the materials of key components, for example, steel pipes such as boiler pipes, and castings such as cylinders and valves in the steam turbine.
High-temperature materials used for boiler pipes and cylinders, valves and other parts in the steam turbine have been developed from Cr—Mo steel to various 9%-12% Cr ferritic steels. Among the existing high-temperature materials for steel pipes such as boiler pipes, T92/P92 is currently an available option. Among the existing high-temperature materials for castings such as cylinders and valves in the steam turbine, ZG13Cr9Mo2Co1NiVNbNB is currently an available option. However, the maximum working temperatures of these steel grades cannot exceed 630° C. As a result, there is currently no heat-resistant steel for steel pipes and castings that can satisfy the working temperature of 650° C.
In view of the above defects in the prior art, an objective of the present invention is to provide a heat-resistant steel for steel pipes and castings, which can be made into a boiler pipe and a steam turbine casting and can satisfy the use requirements of pressure vessels or parts of power machinery with a working temperature of 650° C. and below 650° C.
In order to achieve the above objective and other related objectives, a first aspect of the present invention provides a heat-resistant steel for steel pipes and castings, including the following elements in mass percentage:
Preferably, in the heat-resistant steel for steel pipes and castings, a Cr (chromium) equivalent is <8.5% based on Cr+6Si+4Mo+1.5W+11V+5Nb-40C-2Mn-4Ni-2Co-30N, and a mass ratio of B to N is 0.65-2.40:1.
Preferably, the heat-resistant steel for steel pipes and castings includes the following elements in mass percentage:
More preferably, in the heat-resistant steel for steel pipes and castings, a Cr (chromium) equivalent is 8.0% based on Cr+6Si+4Mo+1.5W+11V+5Nb-40C-2Mn-4Ni-2Co-30N, and a mass ratio of B to N is 0.75-2.10:1.
In the heat-resistant steel for steel pipes and castings provided by the present invention, C ensures hardenability. In the tempering process, C is combined with other elements to form M23C6 carbides at the grain boundaries and the martensite lath boundaries and form MX carbonitrides inside the martensite laths, which can increase the high-temperature strength. In addition to ensuring strength and toughness, C is an indispensable element to inhibit the formation of harmful phases 6-ferrite and BN. However, an excessive addition will reduce the toughness and strength of the steel and impair the long-term creep rupture strength of the steel. Therefore, the C content should be limited to 0.08%-0.14%. Further, the optimal content of C should be limited to 0.08-0.13%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, Si, as a deoxidizer for molten steel, can improve the oxidation resistance of the steel together with Cr. However, if Si is added excessively, the deoxidation product SiO2 will remain in the steel, reducing the purity and toughness of the molten steel. In addition, Si also promotes the precipitation of the intermetallic compound Laves phase, which reduces creep plasticity. Si will increase temper brittleness when used at a high temperature. Therefore, the Si content should be limited to 0.20-0.40%. Further, the optimal content of Si should be limited to 0.20-0.30%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, Mn can remove oxygen and sulfur in molten steel, improve the hardenability and strength of steel, inhibit the formation of 6-ferrite and BN and promote the precipitation M23C6 carbides. However, with the increase of the Mn content, the creep rupture strength decreases. Therefore, the Mn content should be limited to 0.30-0.60%. Further, the optimal content of Mn should be limited to 0.40-0.50%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, Ni can increase the hardenability of the steel, inhibit the formation of 6-ferrite and BN, and improve the strength and toughness at room temperature. However, the addition of Ni is not conducive to the high-temperature creep properties of the steel and may increase the temper brittleness of the steel. In order to ensure the desired high-temperature creep strength of the heat-resistant steel of the present invention, the Ni content should be as low as possible. The Ni content is expected not to exceed 0.20%, and optimally not to exceed 0.10%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, Cr can improve the oxidation resistance and corrosion resistance, and increase the high-temperature strength by precipitating M23C6 carbides. In order to achieve the above effects, the minimum Cr content in the heat-resistant steel of the present invention is 9.00%. However, if the Cr content exceeds 10.00%, it is easy to form 6-ferrite, which decreases the high-temperature strength and toughness. Therefore, the Cr content should be limited to 9.00-10.00%. Further, the optimal content of Cr should be limited to 9.00-9.60%. At the same time, the Cr equivalent (Cr+6Si+4Mo+1.5W+11V+5Nb-40C-2Mn-4Ni-2Co-30N) of the heat-resistant steel of the present invention is limited to 8.5% or below, more preferably limited to 8.0% or below. In this way, the precipitation of 6-ferrite can be avoided.
In the heat-resistant steel for steel pipes and castings provided by the present invention, Mo can improve the hardenability, inhibit the temper brittleness, promote the dispersion and precipitation of the M23C6 carbides and increase the tensile strength and creep rupture strength of the steel. However, an excess of Mo promotes the precipitation of the 6-ferrite and intermetallic compound Laves phase, which obviously decreases the toughness. Therefore, the Mo content should be limited to 0.55-0.80%. Further, the optimal content of Mo should be limited to 0.60-0.75%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, W is effective in inhibiting the coarsening of M23C6 carbides, and its effect exceeds that of Mo, so W can significantly increase the creep rupture strength. The addition of W to replace part of Mo ensures the Mo equivalent (Mo+½W) to be about 1.5%, the strengthening effect to be the most obvious and no excess of 6-ferrite or intermetallic compound Laves phase to be formed. If the amount of W added exceeds 1.90%, the plasticity, toughness and creep rupture strength will be impaired, and segregation may easily occur in the steel. Therefore, the W content should be limited to 1.65-1.90%. Further, the optimal content of W should be limited to 1.70-1.85%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, Co is capable of solid solution strengthening and inhibiting the precipitation of 6-ferrite. Co can interact with Mo and W, which obviously improves the high-temperature strength and increases the toughness of the steel. However, in order to control the cost, the Co content should not be too high. The Co content should be limited to 2.80-3.30%. Further, the optimal content of Co should be limited to 2.90-3.20%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, V can increase the tensile strength. Moreover, V can form fine carbonitrides of V inside the martensite laths, thereby increasing the creep rupture strength. The addition of a certain amount of V can refine the grains and improve the toughness. However, if the amount of V added is too large, the toughness will be reduced, and the carbon will be fixed excessively, resulting in a decrease in the precipitation of the M23C6 carbides. Therefore, the V content is 0.15-0.25%, and expected to be 0.18-0.25%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, like V, Nb can improve the tensile strength and creep rupture strength. Nb may form fine NbC with C, which can refine the grains and increase the toughness. Moreover, the MX carbonitrides formed from Nb and V have the effect of increasing the high-temperature strength. The minimum Nb content should be 0.03%. However, when the Nb content is 0.08% or above, as in the case with V, the carbon will be fixed excessively, resulting in a decrease in the precipitation of the M23C6 carbides and thus a decrease in the high-temperature strength. Therefore, the Nb content should be limited to 0.03-0.08%, and expected to be 0.04-0.07%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, N may precipitate VN (vanadium nitride) with V, which combines in the solid solution state with Mo and W to improve the high-temperature strength. The minimum N content should be 0.005%. However, if the amount exceeds 0.015%, the plasticity will be impaired. Moreover, when N coexists with B, it is easy to form eutectic Fe2B and BN, which impairs the creep properties and toughness of the steel. Therefore, the N content should be limited to 0.006-0.015%. Further, the optimal content of N should be limited to 0.007-0.014%.
In the heat-resistant steel for steel pipes and castings provided by the present invention, B has the grain boundary strengthening effect, has the effect of inhibiting the coarsening of the M23C6 carbides, and can improve the high-temperature strength. However, if the B content is 0.015% or above, it is unfavorable for forgeability and weldability. Therefore, the B content should be limited to 0.009-0.015%. Further, the optimal content of B should be limited to 0.010-0.015%. In order to prevent B from combining with N to form BN, the mass ratio of B to N should be controlled to 0.65-2.40:1, more preferably 0.75-2.10:1.
The above-mentioned inevitable impurities are inclusion elements that are inevitably contaminated in the steel smelting process. The contents of these elements should be as low as possible. If steelmaking raw materials are strictly selected, the cost will rise. Therefore, the P content should be limited to be not higher than 0.020%, the S content to be not higher than 0.010%, and the Cu content to be not higher than 0.15%. Meanwhile, the other inclusion elements, including Al, Ti, Zr, Sn, As and Sb, have adverse effects on the mechanical properties of the heat-resistant steel, and their contents should be as low as possible.
A second aspect of the present invention provides a preparation method of a steel pipe. Raw materials are mixed according to element proportioning of the heat-resistant steel and then smelted, a pipe blank is made first by any one of continuous casting, die casting, hot rolling or hot forging, then the pipe blank is made into a steel pipe by any one of hot rolling, hot extrusion, hot expansion, cold drawing, cold rolling, or forging and boring, and the steel pipe is normalized or quenched, and then tempered.
Preferably, the normalizing or quenching is carried out at a temperature of 1070-1160° C.
Preferably, the tempering is carried out at least one time, and the tempering is carried out at a temperature of 740-790° C.
The above continuous casting, die casting, hot rolling, hot forging, hot extrusion, hot expansion, cold drawing, cold rolling, or forging and boring are all well-known technical processes in the field of steel manufacturing.
The above technical process of the steel pipe conforms to the national standard GB5310.
A third aspect of the present invention provides a preparation method of a casting. Raw materials are mixed according to element proportioning of the heat-resistant steel, then smelted and poured to obtain a casting, and the casting is normalized or quenched, and then tempered.
Preferably, the normalizing or quenching is carried out at a temperature of 1070-1160° C.
Preferably, the tempering is carried out at least one time, and the tempering is carried out at a temperature of 730-780° C.
The above pouring is a well-known technical process in the field of steel manufacturing.
A fourth aspect of the present invention provides use of the above heat-resistant steel or the steel pipe in a pressure vessel.
Preferably, the pressure vessel is a boiler pipe.
A fifth aspect of the present invention provides use of the heat-resistant steel or casting in power machinery.
Preferably, the power machinery is a steam turbine.
In the above preparation method of a steel pipe and/or casting, the smelting includes alloy melting and refining. The above alloy melting and refining in the smelting are both well-known technical processes in the field of steel manufacturing.
As described above, the heat-resistant steel for steel pipes and castings provided by the present invention according to preferred element composition can be used to prepare the steel pipe and the casting with excellent properties through the preparation steps thereof. Compared with the existing boiler pipe material T/P92, Co is added to the heat-resistant steel of the present invention, the ratio of B to N is adjusted, the contents of Cr, Mo and B are increased, the contents of Nb, N and Ni are decreased, the contents of Si and W are limited more strictly, and the limit to the impurity elements Cu, Sn, As and Sb is added. Compared with the existing casting material ZG13Cr9Mo2Co1NiVNbNB, W is added, the ratio of B to N is adjusted, the contents of Co and B are increased, the contents of Mn, Mo, N and Ni are reduced, and the limit to the contents of the impurity elements Ti, Zr, Cu, Sn, As and Sb is added.
According to the heat-resistant steel for steel pipes and castings provided by the present invention, the high-temperature creep rupture strength and oxidation resistance are improved. This will increase its operating temperature, thereby improving the thermal efficiency of the generating unit and reducing the coal consumption and carbon dioxide emissions. When this novel heat-resistant steel is used as a steel pipe material, the material designation is TB4 (small diameter pipe)/PB4 (large-diameter pipe) for short. When it is used as a casting material, the material designation is CB4 for short.
The heat-resistant steel for steel pipes and castings provided by the present invention can be used to prepare the pressure vessel and power machinery, especially boiler pipes and steam turbine castings. The obtained boiler pipe and steam turbine casting have good high-temperature creep rupture strength and oxidation resistance in a high-temperature environment of 650° C. and below 650° C., and can satisfy the use requirements of the boiler and steam turbine with a working temperature of 650° C. and below 650° C.
The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the protection scope of the present invention.
The implementations of the present invention are described below through specific examples. Those skilled in the art can easily understand the other advantages and effects of the present invention from the content disclosed in the specification. The present invention may also be implemented or applied through other different specific implementations, and various details in the specification may also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.
Proportioning of elements was determined. As shown in Table 1, the heat-resistant steel included the following elements in mass percentage:
Based on Cr+6Si+4Mo+1.5W+11V+5Nb-40C-2Mn-4Ni-2Co-30N, a Cr equivalent was 7.62%. A mass ratio of B to N was 1:1.
Raw materials were mixed according to the proportioning of elements and then smelted. That is, the raw materials were subjected to alloy melting and refining sequentially, and then made into a pipe blank by die casting, the pipe blank was made into a steel pipe by hot extrusion, and the steel pipe was normalized and tempered to obtain a steel pipe sample 1 #. The normalizing was carried out at a temperature of 1100° C. The tempering was carried out 1 time, and the tempering was carried out at a temperature of 780° C. The steel pipe sample 1 #was a boiler pipe.
Proportioning of elements was determined. As shown in Table 1, the heat-resistant steel included the following elements in mass percentage:
Based on Cr+6Si+4Mo+1.5W+11V+5Nb-40C-2Mn-4Ni-2Co-30N, a Cr equivalent was 6.7%. A mass ratio of B to N was 1.4:1. Raw materials were mixed according to the proportioning of elements, and then smelted. That is, the raw materials were subjected to alloy melting and refining sequentially, and then poured to obtain a casting, and the casting was normalized and tempered to obtain a casting sample 1*. The normalizing was carried out at a temperature of 1140° C. The tempering was carried out 2 times, and the tempering was carried out at a temperature of 755° C. The casting sample 1*was a valve casing casting of a steam turbine.
The existing steel pipe material T92/P92 and casting material ZG13Cr9Mo2Co1NiVNbNB are selected, and element compositions of the existing steel pipe material T92/P92 and casting material ZG13Cr9Mo2Co1NiVNbNB and the heat-resistant steel of the present invention are shown in Table 2.
According to the standards ASTM A213/A335 and JB/T 14047, the mechanical properties of the existing steel pipe material T92/P92 and casting material ZG13Cr9Mo2Co1NiVNbNB are listed in Table 3. In Table 3, Rp0.2 is the yield strength, Rm is the tensile strength, A is the elongation, Z is the reduction of area, and KV2 is the impact energy absorbed. Meanwhile, the steel pipe sample 1 #obtained in Embodiment 1 and the casting sample 1*obtained in Embodiment 2 were subjected to a room temperature tensile test according to the national standard GB/T228.1, and subjected to a room temperature impact test according to the national standard GB/T229. The test results are shown in Table 3.
As shown in Table 3, the comparison between the existing steel pipe material T92/P92 and the steel pipe sample 1 #obtained in Embodiment 1 showed that the room temperature mechanical properties of the steel pipe sample 1 #satisfied the indicator requirements of T92/P92.
As shown in Table 3, the comparison between the existing casting material ZG13Cr9Mo2Co1NiVNbNB and the casting sample 1*obtained in Embodiment 2 showed that the room temperature mechanical properties of the casting sample 1*satisfied the indicator requirements of ZG13Cr9Mo2Co1NiVNbNB.
The steel pipe sample 1 #obtained in Embodiment 1 and the casting sample 1*obtained in Embodiment 2 were subjected to a creep rupture strength test according to the national standard GB/T 2039. Then, according to the extrapolation method specified in the national standard GB/T 2039, the creep rupture strength limit Ru100,000 h/650° C. at 650° C./100,000 h was deduced, and respectively compared with the creep rupture strength of T92/P92 and ZG13Cr9Mo2Co1NiVNbNB at 650° C./100,000 h. The results are shown in Table 4.
As can be seen from Table 3, the extrapolated value of the creep rupture strength of the steel pipe sample 1 #obtained in Embodiment 1 is increased by 50% or more as compared with that of the existing steel pipe material T92/P92. As a result, the steel pipe sample has an obvious strengthening effect and can satisfy the use requirements of the boiler pipe with a working temperature of 650° C.
As can be seen from Table 3, the extrapolated value of the creep rupture strength of the casting sample 1*obtained in Embodiment 2 is increased by 40% or above as compared with that of the existing casting material ZG13Cr9Mo2Co1NiVNbNB. As a result, the casting sample has an obvious strengthening effect and can satisfy the use requirements of the steam turbine casting with a working temperature of 650° C.
The steel pipe sample 1 #obtained in Embodiment 1, the casting sample 1*obtained in Embodiment 2 and the existing steel pipe material T92/P92 and casting material ZG13Cr9Mo2Co1NiVNbNB were respectively subjected to an oxidation weight gain test at 620° C. and 650° C. The samples were placed in a flowing steam environment of 620° C./650° C. and 27 MPa for a maximum time of 2,000 h. The weight gain of each sample was measured in this time period. The smaller the oxidation weight gain, the better the oxidation resistance of the material.
The test results showed that at the same temperature, the oxidation resistance of the steel pipe sample 1 #was significantly better than that of T92/P92, and the oxidation resistance of the casting sample 1*was significantly better than that of ZG13Cr9Mo2Co1NiVNbNB.
At a different temperature, such as 650° C., the oxidation weight gain of the steel pipe sample 1 #was close to that of T92/P92 at 620° C., and the oxidation weight gain of the casting sample 1*was close to that of ZG13Cr9Mo2Co1NiVNbNB at 620° C. This indicates that the steel pipe and casting prepared from the heat-resistant steel of the present invention can basically satisfy the needs of long-term use under the working condition of 650° C. without using a surface protective coating to resist oxidation.
Therefore, the present invention effectively overcomes various defects in the prior art and has high industrial value in use.
The above embodiments merely exemplarily illustrate the principles and effects of the present invention, but are not intended to limit the present invention. Any person skilled in the art can modify or change the above embodiment without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those of ordinary skill in the art without departing from the spirit and technical ideas disclosed in the present invention should still be covered by the claims of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202010749897.3 | Jul 2020 | CN | national |
This application is a U.S. national phase application claiming priority to and the benefit of PCT Application No. PCT/CN2021/072875, filed Jan. 20, 2021, which in turn claims priority to and the benefit of Chinese Patent Application No. CN 202010749897.3, filed Jul. 30, 2020, the contents of which are incorporated herein by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2021/072875 | 1/20/2021 | WO |
