Patent Application
20220025477
The present disclosure relates to manufacturing of a chromium-molybdenum steel plate having excellent creep properties, and more particularly, to a chromium-molybdenum steel plate which may have an excellent creep strength by forming only fine carbonitrides in the inside and the grain boundary of a martensite matrix of a composed phase of a steel material to interrupt dislocation movement at a high temperature and secure stability of sub-crystal grains, and a method for manufacturing the same.
Power generation and oil refinery/refining industries should consider the construction of environmentally friendly facilities and greater efficiency in energy use.
First, an increase in temperature and pressure of steam supplied to a turbine for increasing power generation efficiency is required, and thus, improvement of thermal resistance of boiler materials to produce steam at a higher temperature is essential.
In addition, in the oil refinery/refining industries, steel materials having excellent properties at raised temperature and pressure are being developed for higher efficiency due to recent strengthening of environmental regulation.
Since an austenite stainless steel contains a large amount of high-priced alloying elements, it is expensive and its use is limited due to poor physical properties (low thermal conductivity and high coefficient of thermal expansion) and a difficulty in manufacture of large parts. However, a chromium steel is often used due to its excellent creep strength, weldability, corrosion resistance, oxidation resistance, and the like.
For maintaining the creep strength at a high temperature of the thermal resistant chromium steel for a long time, solid solution strengthening and precipitation strengthening methods are applied. For this, molybdenum and vanadium, niobium, and titanium which are elements forming M(C,N) carbonitrides (M=metal element, C=carbon, N=nitrogen) are mainly alloyed. Also, a thermal resistant steel in which formation of (Fe,Cr)23C6 carbides which are thermodynamically unstable and easily coarsened to deteriorate creep properties is suppressed by extremely decreasing a carbon content to 0.002 wt %, and fine carbonitrides are precipitated to greatly improve creep properties, was suggested, but it is currently almost impossible to commercially mass-produce the thermal resistant steel having a lowered carbon content as described above.
An aspect of the present disclosure is to provide a chromium-molybdenum steel plate having excellent creep properties by completely suppressing formation of coarse precipitates such as (Fe,Cr)23C6 carbides and forming only fine carbonitrides without extremely lowering a carbon content, unlike the conventional technologies described above, using alloy design and heat treatment, and a method for manufacturing the same.
However, the object of the present disclosure is not limited to the object described above, and other objects which are not described above may be clearly understood by those skilled in the art from the following description.
According to an aspect of the present disclosure, a chromium-molybdenum steel plate having excellent creep strength includes, by weight: 0.11 to 0.15% of C, 0.10% or less (exclusive of 0%) of Si, 0.3 to 0.6% of Mn, 0.010% or less (exclusive of 0%) of S, 0.015% or less (exclusive of 0%) of P, 2.0 to 2.5% of Cr, 0.9 to 1.1% of Mo, 0.65 to 1.0% of V, 0.25% or less (exclusive of 0%) of Ni, 0.20% or less (exclusive of 0%) of Cu, 0.07% or less (exclusive of 0%) of Nb, 0.03% or less (exclusive of 0%) of Ti, 0.015% or less (exclusive of 0%) of N, 0.025% or less (exclusive of 0%) of Al, and 0.002% or less (exclusive of 0%) of B, with a balance of Fe and unavoidable impurities.
The steel plate may have a microstructure including tempered martensite.
It is preferable that the number of precipitates having a diameter of 200 nm or more including (Fe,Cr)23C6 is in a range of one/μm2 or less in the microstructure of the steel plate.
It is preferable that the number of precipitates having a diameter of 20 nm or less is in a range of 20/μm2 or more in the microstructure of the steel plate.
The precipitates having a diameter of 20 nm or less may be (V,Mo,Nb,Ti) (C,N).
According to another aspect of the present disclosure, a method for manufacturing a steel plate includes:
hot-rolling a steel slab having the composition described above so that a finish rolling temperature is equivalent to or higher than Ar3 to manufacture a hot rolled steel plate, and then cooling the hot rolled steel plate;
reheating the cooled hot rolled steel plate in a temperature range of 900 to 1200° C. for 1 t to 3 t minutes [t (mm) is a thickness of the hot rolled steel plate] to austenitize the steel plate;
quenching the austenitized hot rolled steel plate to room temperature; and
tempering the quenched hot rolled steel plate in a temperature range of 675 to 800° C. for 30 minutes to 120 minutes.
As set forth above, according to an exemplary embodiment in the present disclosure, the chromium-molybdenum steel plate having excellent creep properties of the present disclosure having the configuration described above may have a longer creep life than an ASTM A387 Grade 91 steel containing chromium in a large amount of 9 wt %, with an excellent creep life at a high temperature, by quenching and tempering.
Hereinafter, the present disclosure will be described.
As described above, a conventional thermal resistant chromium steel mainly uses molybdenum and vanadium, niobium, and titanium which are elements forming M(C,N) carbonitrides (M=metal element, C=carbon, N=nitrogen), but the thermal resistant chromium steel as such is thermodynamically unstable and easily coarsened so that formation of (Fe,Cr)23C6 carbides which deteriorate creep properties may not be avoided, and thus, it is difficult to secure excellent creep properties.
In order to solve the problems of the conventional technology, the present inventors repeated studies and experiments, and as a result, confirmed that by optimizing an amount of vanadium added to a thermal resistant chromium steel alloy containing 2.0 to 2.5% of Cr and also properly controlling a tempering temperature, a thermal resistant chromium steel having excellent creep properties may be obtained, thereby suggesting the present disclosure.
The chromium-molybdenum steel plate having excellent creep strength of the present disclosure includes, by weight: 0.11 to 0.15% of C, 0.10% or less (exclusive of 0%) of Si, 0.3 to 0.6% of Mn, 0.010% or less (exclusive of 0%) of S, 0.015% or less (exclusive of 0%) of P, 2.0 to 2.5% of Cr, 0.9 to 1.1% of Mo, 0.65 to 1.0% of V, 0.25% or less (exclusive of 0%) of Ni, 0.20% or less (exclusive of 0%) of Cu, 0.07% or less (exclusive of 0%) of Nb, 0.03% or less (exclusive of 0%) of Ti, 0.015% or less (exclusive of 0%) of N, 0.025% or less (exclusive of 0%) of Al, and 0.002% or less (exclusive of 0%) of B, with a balance of Fe and unavoidable impurities.
Hereinafter, the reason for limiting the components of the chromium-molybdenum steel plate having excellent creep properties will be described, and “%” herein represents “wt %”, unless otherwise defined.
Carbon (C): 0.11 to 0.15%
Carbon is an element for austenite stabilization, which may adjust an Ae3 temperature and a martensite formation initiation temperature depending on the content and is very effective for applying asymmetric distortion as an interstitial element to a lattice structure of a martensite phase to secure high strength. However, when a carbon content in the steel is more than 0.15%, carbides are excessively formed and weldability is greatly deteriorated.
Therefore, it is preferable to limit the carbon content to a range of 0.11 to 0.15%, more preferably to a range of 0.11 to 0.14% in the present disclosure.
Silicon (Si): 0.10% or less (exclusive of 0%)
Silicon is added as a deoxidizer during casting as well as for strengthening of solid solution. However, while it is essential to form advantageous carbides such as fine carbides in the chromium-molybdenum steel plate having excellent creep properties according to an exemplary embodiment in the present disclosure, silicon serves to suppress carbide formation.
Therefore, it is preferable to limit a silicon content to 0.10% or less, more preferably to a range of 0.005 to 0.08% in the present disclosure.
Manganese (Mn): 0.3 to 0.6%
Manganese is an element for austenite stabilization, which greatly increases hardenability of a steel to allow a hard phase such as martensite to be formed. In addition, manganese reacts with sulfur so that MnS is precipitated, which is advantageous for preventing cracks at a high temperature by sulfur segregation. However, as a manganese content increases, an austenite stability degree is excessively increased.
Therefore, it is preferable to limit the manganese content to a range of 0.3 to 0.6%, more preferably to a range of 0.35 to 0.55% in the present disclosure.
Sulfur (S): 0.010% or less (exclusive of 0%)
Sulfur is an impurity element and when the content is more than 0.010%, ductility and weldability of a steel are deteriorated.
Therefore, it is preferable to limit a sulfur content to 0.010% or less.
Phosphorus (P): 0.015% or less (exclusive of 0%)
Phosphorus is an element having a solid solution strengthening effect, but an impurity element like sulfur, when the content is more than 0.015%, a steel has brittleness and decreased weldability.
Therefore, it is preferable to limit a phosphorus content to 0.015% or less.
Chromium (Cr): 2.0 to 2.5%
Chromium is a ferrite stabilization element and an element increasing hardenability, and adjusts an Ae3 temperature and a delta ferrite forming temperature range depending on the amount. In addition, chromium reacts with oxygen to form a dense and stable protective film of Cr2O3 to increase oxidation resistance and corrosion resistance at a high temperature, but increases a delta ferrite forming temperature range. In a process of casting a steel having a high chromium content, delta ferrite may be formed, and remains even after heat treatment to adversely affect steel characteristics.
Therefore, it is preferable to limit the chromium content to a range of 2.0 to 2.5%, more preferably to a range of 2.1 to 2.4% in the present disclosure.
Molybdenum (Mo): 0.9 to 1.1%
Molybdenum is known as an element which increases hardenability and stabilizes ferrite. Molybdenum increases a creep life at a high temperature by strong solid solution strengthening, participates as a metal element forming M(C,N) carbonitrides to stabilize carbonitrides, and greatly reduces a coarsening speed. However, when a molybdenum content is increased, a delta ferrite forming temperature range may be widened and delta ferrite may be formed and remain in a process of casting a steel. Remaining delta ferrite adversely affects steel characteristics.
Therefore, it is preferable to limit the molybdenum content to a range of 0.9 to 1.1%, more preferably to a range of 0.95 to 1.05%.
Vanadium (V): 0.65 to 1.0%
Vanadium is one of elements forming M(C,N) carbonitrides, and when a vanadium content is increased, (Fe,Cr)23C6 carbide formation driving force is decreased, resulting in complete suppression of (Fe,Cr)23C6 carbide formation. In order to suppress (Fe,Cr)23C6 carbide formation in a chromium steel having a chromium content of 2.0 to 2.5%, 0.65% or more of a vanadium alloy is needed. However, when the vanadium content is more than 1.0%, there is a difficulty in a production process of materials.
Therefore, it is preferable to limit the vanadium content to a range of 0.65 to 1.0%, more preferably to a range of 0.67 to 0.98%.
Nickel (Ni): 0.25% or less (exclusive of 0%)
Nickel is an element for improving toughness of a steel and is added for increasing steel strength without deterioration of toughness at a low temperature. When nickel is added at the content of more than 0.25%, a price increase due to nickel addition is caused.
Therefore, it is preferable to limit the nickel content to a range of 0.25% or less, more preferably to a range of 0.005 to 0.24%.
Copper (Cu): 0.20% or less (exclusive of 0%)
Copper is an element for improving hardenability of materials and is added so that a steel plate has a homogeneous structure after heat treatment. However, the amount added is more than 0.20%, a possibility of crack occurrence for steel plate may be increased.
Therefore, it is preferable to limit a copper content to 0.20% or less, more preferably to a range of 0.005 to 0.18%.
Niobium (Nb): 0.07% or less (exclusive of 0%)
Niobium is one of elements forming M(C,N) carbonitrides. In addition, it is solid-solubilized when reheating a slab and suppresses austenite crystal grain growth during hot rolling, and then is precipitated to improve steel strength. However, when niobium is excessively added at more than 0.07%, weldability may be deteriorated and crystal grains may be fined than necessary.
Therefore, it is preferable to limit a niobium content to 0.07% or less, more preferably to a range of 0.005 to 0.06%.
Titanium (Ti): 0.03% or less (exclusive of 0%)
Titanium is also an element effective for suppressing austenite crystal grain growth in a TiN form. However, when titanium is added at more than 0.03%, coarse Ti-based precipitates are formed and there is a difficulty in welding of materials.
Therefore, it is preferable to limit a titanium content to 0.03% or less, more preferably to a range of 0.005 to 0.025%.
Nitrogen (N): 0.015% or less (exclusive of 0%)
Since it is difficult to industrially completely remove nitrogen from steel, the upper limit of N is 0.015% which is a range allowable in a manufacturing process. Nitrogen is known as an austenite stabilization element, and stability at a high temperature is greatly increased when forming M(C,N) carbonitrides as compared with simple MC carbides, thereby effectively increasing creep strength of a steel material. However, when the content is more than 0.015%, nitrogen is bonded to boron to form BN, thereby increasing a risk of defect occurrence.
Therefore, it is preferable to limit a nitrogen content to 0.015% or less.
Alumina (Al): 0.025% or less (exclusive of 0%)
Aluminum enlarges a ferrite area, and is added as a deoxidizer during casting. Since in a chromium steel, other ferrite stabilization elements are alloyed much, when an aluminum content is increased, an Ae3 temperature may be excessively raised. In addition, when the amount added is more than 0.025%, an oxide-based inclusion is formed in a large amount to deteriorate the physical properties of a material.
Therefore, it is preferable to limit the aluminum content to 0.025% or less, more preferably to a range of 0.005 to 0.025%.
Boron (B): 0.002% or less (exclusive of 0%)
Boron is a ferrite stabilization element and contributes much to a hardenability increase only with a little amount. In addition, it is easily segregated in a crystal grain boundary to give a crystal grain boundary strengthening effect. However, when boron is added at more than 0.002%, BN may be formed, which may adversely affect the mechanical properties of materials.
Therefore, it is preferable to limit a boron content to 0.002% or less.
Other than that, a balance of Fe and unavoidable impurities are included. However, since in a common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or a surrounding environment, the impurities may not be excluded. Since these impurities are known to any person with ordinary knowledge in the art, the entire contents thereof are not particularly mentioned herein.
Hereinafter, a microstructure and precipitates of the chromium-molybdenum steel plate of the present disclosure having excellent creep properties will be described in detail.
First, the steel plate of the present disclosure includes a tempered martensite structure as the matrix microstructure. However, a tempered bainite structure may be partly included depending on heat treatment conditions.
It is preferable that the number of precipitates having a diameter of 200 nm or more including (Fe,Cr)23C6 is in a range of one/μm2 or less in the steel plate microstructure of the present disclosure. When the number of precipitates having a diameter of 200 nm or more is more than one/μm2, deteriorated creep properties may be caused by coarse carbides.
It is preferable that the number of precipitates having a diameter of 20 nm or less is in a range of 20/μm2 or more in the steel plate microstructure of the present disclosure. When the number of precipitates having a diameter of 20 nm or less is less than 20/μm2, a distance between fine carbonitrides is significantly increased. Therefore, since dislocation movement at a high temperature and movement of sub-crystal grains are not effectively prevented, an effect of improving creep properties may not be large.
The precipitates having a diameter of 20 nm or less in the present disclosure may include (V,Mo,Nb,Ti) (C,N).
Next, the method for manufacturing a precipitation hardening type chromium-molybdenum steel plate having excellent creep strength according to an exemplary embodiment in the present disclosure will be described.
The method for manufacturing a precipitation hardening type chromium-molybdenum steel plate having excellent creep strength of the present disclosure includes: hot-rolling a steel slab having the composition described above so that a finish rolling temperature is equivalent to or higher than Ar3 to manufacture a hot rolled steel plate, and then cooling the hot rolled steel plate; reheating the cooled hot rolled steel plate in a temperature range of 900 to 1200° C. for 1 t to 3 t minutes [t (mm) is a thickness of the hot rolled steel plate] to austenitize the steel plate; quenching the austenitized hot rolled steel plate to room temperature; and tempering the quenched hot rolled steel plate in a temperature range of 675 to 800° C. for 30 minutes to 120 minutes.
First, in the present disclosure, a steel slab having the composition component described above is hot-rolled so that a finish rolling temperature is equivalent to or higher than Ar3 to obtain a hot rolled steel plate. The reason for performing hot rolling in an austenite single phase region is to increase uniformity of a structure.
Then, in the present disclosure, the hot rolled steel plate manufactured was cooled to room temperature.
Subsequently, in the present disclosure, the cooled hot rolled steel plate is reheated to austenitize the steel plate. Here, it is preferable that a reheating temperature range is 900 to 1200° C. and a reheating time is in a range of 1 t minute to 3 t minutes depending on a thickness t (mm) of the hot rolled steel plate.
When the reheating temperature is lower than 900° C., it is difficult to properly redissolve undesired carbides formed in a process of cooling after hot rolling. However, when the reheating temperature is higher than 1200° C., the characteristics may be deteriorated due to crystal grain coarsening.
It is preferable that the reheating time is in a range of 1 t to 3 t, in which the thickness of the hot rolled steel plate is t (mm). For example, a hot rolled steel plate having a thickness of 20 mm is reheated to be austenitized, reheating may be performed for 20 to 60 minutes. When the reheating time is less than it minute, it is difficult to properly redissolve undesired carbides formed in a process of cooling after hot rolling, but when the reheating time is more than 3 t minutes, the characteristics may be deteriorated due to crystal grain coarsening.
Then, in the present disclosure, the hot rolled steel plate austenitized by the reheating is quenched to be cooled down to room temperature, thereby obtaining a martensite structure. Here, when cooling a matrix structure, care should be taken so that ferrite and pearlite structures are not formed to greatly decrease matrix strength.
Subsequently, in the present disclosure, the quenched hot rolled steel plate is tempered. Here, it is preferable that a tempering temperature is 675 to 800° C., a tempering time is 30 minutes to 120 minutes, and then air cooling is performed.
When the tempering temperature is lower than 675° C., precipitation of fine carbonitrides may not be induced in time due to the low temperature. However, when the tempering temperature is higher than 800° C., tempering causes softening of materials to greatly decrease a creep life.
More preferably, the tempering temperature is controlled to a range of 700 to 780° C.
Meanwhile, when the tempering time is less than 30 minutes, precipitates to be formed may not be formed. However, when the tempering time is more than 120 minutes, precipitate coarsening and material softening occur to greatly decrease a creep life.
Hereinafter, the present disclosure will be described in detail through the following examples.
Hot rolled steel plates having alloy compositions of the following Table 1 and a thickness of 20 mm were prepared. Then, the hot rolled steel plate was reheated at 1000° C. for 1 hour and quenched to cool down to room temperature. Subsequently, the cooled steel plate was tempered at 730° C. for 1 hour and then air-cooled to room temperature to manufacture a Cr—Mo alloy steel. Meanwhile, in the following Table 1, steel type 1 is a composition of ASTM A542D steel and steel types 2 to 4 are steel types satisfying the steel composition components of the present disclosure.
For Cr—Mo alloy steels manufactured as described above, an ASTM E139 standard was utilized in a hot rolling direction to manufacture each of creep specimens having a gauge length of 15 mm and a gauge diameter of 6 mm, and the creep life at a high temperature of these specimens was evaluated using ATS 2320 creep test equipment and the results are shown in
In addition, phase transformation depending on a cooling rate after austenitization was confirmed using a dilatometer, and the results are shown in
Then, for the alloy steel specimen manufactured, a scanning electron microscope (SEM) was utilized to observe a microstructure, and the results are shown in
As shown in
It is seen from
Meanwhile,
Then,
The present disclosure is not limited to the exemplary embodiments and the examples, but may be made in various forms different from each other, and those skilled in the art will understand that the present disclosure may be implemented in other specific forms without departing from the spirit or essential feature of the present disclosure. Therefore, it should be understood that the above-mentioned exemplary embodiments and examples are illustrative but not restrictive in all aspects.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2018-0150819 | Nov 2018 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2019/016694 | 11/29/2019 | WO | 00 |
