KR20220027391A - Chromium steel having excellent creep strength and impact toughness and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to steel having excellent creep properties, and more particularly, to chromium steel having excellent creep strength and impact toughness and a manufacturing method thereof. Chromium steel of the present invention comprises 0.05 to 0.10 wt% of carbon (C), 0.5 wt% or less (excluding 0 wt%) of silicon (Si),: 0.1 to 0.6 wt% of manganese (Mn), 0.01 wt% or less (excluding 0 wt%) of sulfur (S), 0.02 wt% or less (excluding 0 wt%) of phosphorus (P), 4.0 to 6.0 wt% of chromium (Cr), 0.3 to 0.6 wt% of molybdenum (Mo), 1.4 to 2.0 wt% of tungsten (W), 0.7 to 1.1 wt% of vanadium (V), 0.4 wt% or less (excluding 0 wt%) of nickel (Ni), 0.10 wt% or less (excluding 0 wt%) of niobium (Nb), 0.10 wt% or less (excluding 0 wt%) of titanium (Ti), 0.005 to 0.04 wt% of nitrogen (N), 0.02 wt% or less (excluding 0 wt%) of aluminum (Al), 0.006 wt% or less (excluding 0 wt%) of boron (B), and the balance Fe and unavoidable impurity elements.

Description

Chromium steel with excellent creep strength and impact toughness and manufacturing method thereof

The present invention relates to steel having excellent creep properties, and more particularly, to chromium steel having excellent creep strength and impact toughness, and a method for manufacturing the same.

The considerations for thermal/nuclear power generation are the construction of environment-friendly facilities and high efficiency of energy use. In order to increase power generation efficiency, it is required to increase the temperature and pressure of steam supplied to the turbine, and it is important to improve the heat resistance of the related material.

In addition, the oil refining/refining industry is demanding high efficiency due to the recent tightening of environmental regulations, and is examining the application of steel materials with excellent high temperature characteristics.

On the other hand, austenitic stainless steel containing a large amount of expensive alloying elements such as nickel (Ni) among high-temperature steels has poor physical properties such as low thermal conductivity and high coefficient of thermal expansion, so its application to large parts is limited. .

On the other hand, chromium steel containing more than a certain amount of chromium (Cr) has high thermal conductivity and low coefficient of thermal expansion as well as excellent creep strength, weldability, corrosion resistance and oxidation resistance, and thus is being actively applied. In particular, chromium steel is suitable for application as a structural material for nuclear power generation, because the swelling phenomenon caused by neutron irradiation is suppressed compared to austenitic stainless steel. Therefore, chromium steel is a situation in which the application is expanding by replacing the austenitic stainless steel.

Among the various properties of chromium steel, in order to maintain the high temperature creep strength for a long time, the method of solid solution strengthening and precipitation strengthening of steel is applied.

Specifically, vanadium (V), niobium (Nb), titanium as solid solution strengthening elements and M(C,N) carbon nitride (where M is a metal element, C means carbon, and N means nitrogen) forming elements (Ti) and the like mainly form an alloy composition.

As an example, by reducing the carbon content to 0.002% by weight or less of the limit, the formation of (Fe,Cr) ₂₃ C ₆ carbides that are thermodynamically unstable and easily coarsen to deteriorate creep properties are suppressed, while fine carbon nitrides A heat-resistant steel with significantly improved creep properties by precipitating silver has been proposed. However, as such, heat-resistant steel of ultra-low carbon has a disadvantage in that it is very difficult to mass-produce commercially.

In addition to the high temperature creep strength, another physical property required for chromium steel is impact properties.

When chromium steel is applied to a blanket in a nuclear fusion reactor that produces energy by utilizing the energy generated by an accessory mass defect together with helium generated when hydrogen atoms are combined, embrittlement phenomenon occurs due to irradiation with high-energy neutrons. appears, and this may result in deterioration of impact properties.

Accordingly, there is an increasing demand for the development of steel materials having excellent creep strength considering the impact properties sufficiently.

Masaki Taneike, Fujio Abe and Kota Sawada, Creep-strengthening of steel at high temperatures using nano-sized carbonitrides dispersions, Nature, Vol.242, pp. 294-296, 2003

One aspect of the present invention is to suppress the formation of coarse precipitates without extremely lowering the carbon content by optimizing the alloy composition system and manufacturing conditions, while providing a steel material with excellent creep strength through the formation of fine carbon-nitrides. do. Furthermore, it is an object to provide chromium steel having structural reliability by ensuring excellent impact properties.

The subject of the present invention is not limited to the above. The subject of the present invention will be understood from the overall content of the present specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject of the present invention.

One aspect of the present invention, by weight, carbon (C): 0.05 to 0.10%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.1 to 0.6%, sulfur (S): 0.01% or less (excluding 0%), Phosphorus (P): 0.02% or less (excluding 0%), Chromium (Cr): 4.0 to 6.0%, Molybdenum (Mo): 0.3 to 0.6%, Tungsten (W): 1.4 to 2.0%, Vanadium (V): 0.7 to 1.1%, Nickel (Ni): 0.4% or less (excluding 0%), Niobium (Nb): 0.10% or less (excluding 0%), Titanium (Ti): 0.10% or less ( 0%), nitrogen (N): 0.005~0.04%, aluminum (Al): 0.02% or less (excluding 0%), boron (B): 0.006% or less (excluding 0%), balance Fe and unavoidable impurity elements including,

The relationship between V and the sum of the contents of impurity elements (SUM) satisfies the following Relational Equation 1, and the LMP value defined by Equation 2 is 19,500 or more at an applied stress of 225 MPa. Provide a chromium steel excellent in creep strength and impact toughness.

[Relational Expression 1]

0.5 ≤ (V - (10×SUM)) ≤ 1

(Where SUM is the total content of specific impurity elements, it means the total content (wt%) of [Cu + Co + La + Y + Ce + Zr + Ta + Hf + Re + Pt + Ir + Pd + Sb] .)

[Relational Expression 2]

LMP = T × (20 + log(tr))

(Here, T is the absolute temperature in Kelvin, and tr is the fracture time in time.)

Another aspect of the present invention is to prepare a steel slab that satisfies the above-described alloy composition and Relation 1 and heating it in a temperature range of 1000 to 1200°C; manufacturing a hot-rolled steel sheet by hot-rolling the heated steel slab to a finish rolling temperature Ar3 or higher; cooling the hot-rolled steel sheet to room temperature; an austenitization step of reheating the cooled hot-rolled steel sheet to a temperature range of 1000 to 1100° C. and then maintaining the hot-rolled steel sheet for 1t to 3t (minutes) based on the thickness (t, mm) of the hot-rolled steel sheet; cooling the austenitized hot-rolled steel sheet to room temperature at a cooling rate of 2.5° C./s or more; And it provides a method for producing chromium steel excellent in creep strength and impact toughness, including a tempering step of heat treatment for 30 minutes or more in a temperature range of 700 ~ 800 ℃ after the cooling.

According to the present invention, it is possible to provide a chromium steel with improved creep strength and impact properties.

The chromium steel of the present invention has an excellent creep life at high temperature, and has an effect of having a longer creep life compared to the existing steel (ex, ASTM A213 92 grade) containing a large amount of chromium (Cr).

1 shows the creep test results for steel grades 1 to 3 (invention examples and comparative examples) and conventional materials according to an embodiment of the present invention.
FIG. 2 shows photographs of steel grades 1 to 3 observed with a scanning electron microscope (SEM) according to an embodiment of the present invention.
3a to 3c show the dilatometer test results of steel grades 1 to 3 according to an embodiment of the present invention.

In the case of conventional heat-resistant chromium steel mainly using elements such as V, Nb, and Ti, which are carbon nitride-forming elements, the formation of (Fe,Cr) ₂₃ C ₆ carbide, which is thermodynamically unstable and easily coarsens, and deteriorates creep properties, is avoided. Therefore, there was a limit in securing excellent creep properties.

As a result of in-depth research to solve the above problems, the inventors of the present invention have optimized the types and amounts of carbon-nitride forming elements and their addition amount along with the Cr content of chromium steel, while optimizing the process conditions as well as creep properties as well as impact properties. It was confirmed that chromium steel having excellent

Hereinafter, the present invention will be described in detail.

Chromium steel having excellent creep strength and impact toughness according to an aspect of the present invention is, by weight, carbon (C): 0.05 to 0.10%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.1 ~0.6%, Sulfur (S): 0.01% or less (excluding 0%), Phosphorus (P): 0.02% or less (excluding 0%), Chromium (Cr): 4.0 to 6.0%, Molybdenum (Mo): 0.3 to 0.6 %, Tungsten (W): 1.4 to 2.0%, Vanadium (V): 0.7 to 1.1%, Nickel (Ni): 0.4% or less (excluding 0%), Niobium (Nb): 0.10% or less (excluding 0%), Titanium (Ti): 0.10% or less (excluding 0%), Nitrogen (N): 0.005 to 0.04%, Aluminum (Al): 0.02% or less (excluding 0%), Boron (B): 0.006% or less (excluding 0%) ) may be included.

Hereinafter, the reason for limiting the alloy composition of the chromium steel provided in the present invention as above will be described in detail.

Meanwhile, unless otherwise specified in the present invention, the content of each element is based on weight, and the ratio of the tissue is based on area.

Carbon (C): 0.05-0.10%

Carbon (C) is an austenite stabilizing element, and is an element capable of controlling the Ae3 temperature and the initiation temperature of martensite formation according to its content. In addition, as an interstitial element, it is an element effective in securing high strength by applying asymmetric distortion to the lattice structure of martensite.

In order to sufficiently obtain the above-described effect, it is advantageous to add C in an amount of 0.05% or more. However, when the content exceeds 0.10%, it may be difficult to secure the target tissue in the present invention, and there is a risk that the impact properties may be deteriorated due to excessive carbide formation.

Accordingly, the C may be included in an amount of 0.05 to 0.10%.

Silicon (Si): 0.5% or less (excluding 0%)

Silicon (Si) is an element added as a deoxidizer during casting as well as solid solution strengthening of steel.

While Si suppresses the formation of carbides, the chromium steel of the present invention requires the formation of beneficial precipitates such as fine carbon-nitrides. Considering this, Si can be limited to 0.5% or less. However, 0% may be excluded in consideration of the level that is unavoidably added during the steel manufacturing process.

Manganese (Mn): 0.1~0.6%

Manganese (Mn) is an austenite stabilizing element and is advantageous in inducing the formation of a hard phase such as martensite by greatly increasing the hardenability of steel. In addition, it combines with sulfur (S) in steel to precipitate MnS, which is effective in preventing high-temperature cracking due to segregation of S.

In order to sufficiently obtain the above-described effect, Mn may be included in an amount of 0.1% or more, but when the content exceeds 0.6%, there is a problem in that the stability of austenite is excessively increased.

Accordingly, the Mn may be included in an amount of 0.1 to 0.6%.

Sulfur (S): 0.01% or less (excluding 0%)

Sulfur (S) is an element that is unavoidably contained in steel, and when the content exceeds 0.01%, the impact properties and weldability of the steel are deteriorated.

Accordingly, the S may be limited to 0.01% or less, but 0% may be excluded in consideration of the unavoidably contained level.

Phosphorus (P): 0.02% or less (excluding 0%)

Phosphorus (P) is an element exhibiting a solid solution strengthening effect of steel, but when its content exceeds 0.02%, brittleness occurs in the steel, and there is a problem of impairing weldability.

Therefore, the P may be limited to 0.02% or less, but 0% may be excluded in consideration of the unavoidably contained level.

Chromium (Cr): 4.0~6.0%

Chromium (Cr) is a ferrite stabilizing element and is advantageous for improving hardenability. Such Cr controls the Ae3 temperature and the temperature of the region where delta ferrite is formed according to its content.

Cr reacts with oxygen (O) to form a dense and stable protective film of Cr ₂ O ₃ , so it is advantageous to improve high-temperature oxidation resistance and corrosion resistance as its content increases, while reducing the temperature region for forming delta ferrite. By widening, delta ferrite can be formed in the steel casting process. In this case, the formed delta ferrite remains after heat treatment to adversely affect the physical properties of the steel.

Accordingly, in order to sufficiently obtain high-temperature oxidation resistance and corrosion resistance of steel, Cr may be included in an amount of 4.0% or more, but when the content exceeds 6.0%, there is a risk that the delta ferrite phase is excessively formed.

Accordingly, the Cr may be included in an amount of 4.0 to 6.0%.

Molybdenum (Mo): 0.3~0.6%

Since molybdenum (Mo) improves the hardenability of steel, it is possible to effectively prevent a problem in which matrix strength is greatly reduced due to the formation of ferrite and pearlite structures. In addition, Mo increases the high-temperature creep life through strong solid solution strengthening and participates as a metal element forming M(C,N) carbon-nitride to stabilize the carbon-nitride and significantly lower the coarsening rate. In addition, Mo may contribute to the improvement of impact toughness as a grain boundary strengthening element.

In order to sufficiently obtain the above-described effect, it is advantageous to contain Mo in an amount of 0.3% or more. In consideration of this, the Mo may be included in an amount of 0.6% or less.

Accordingly, the Mo may be included in an amount of 0.3 to 0.6%.

Tungsten (W): 1.4~2.0%

Tungsten (W) affects solid solution strengthening to improve the high-temperature creep life of steel, contributes as a metal element for carbon-nitride formation, stabilizes carbon-nitride, and significantly lowers the coarsening rate.

In order to sufficiently obtain the above-described effects, it is advantageous to include W in an amount of 1.4% or more. However, when the content of W is excessive, there is a risk that delta ferrite may be formed in the process of casting steel by widening the formation temperature range of delta ferrite, which remains without being removed even after heat treatment and adversely affects creep characteristics. there is. In consideration of this, W may be included in an amount of 2.0% or less.

Accordingly, the W may be included in an amount of 1.4 to 2.0%.

Vanadium (V): 0.7-1.1%

Vanadium (V) is an element that increases the hardenability of steel and forms M(C,N) carbon-nitride. As the content of V increases, the driving force for forming the (Fe,Cr) ₂₃ C ₆ carbide becomes smaller, and as a result, the formation of the (Fe,Cr) ₂₃ C ₆ carbide can be significantly suppressed.

In order to suppress the formation of (Fe,Cr) ₂₃ C ₆ carbide in the steel containing Cr, W, and Mo as described above, it is advantageous to include V in an amount of 0.7% or more. However, when the content exceeds 1.1%, there is a problem in that the manufacturing cost is increased as an expensive element and productivity is lowered.

Accordingly, the V may be included in the range of 0.7 to 1.1%.

Nickel (Ni): 0.4% or less (excluding 0%)

Nickel (Ni) improves the toughness of the steel, and can be added to increase the strength of the steel without deterioration of the low-temperature toughness. In addition, when Ni is added, the hardenability of steel is improved, thereby effectively preventing a problem in which matrix strength is greatly reduced due to formation of ferrite and pearlite structures.

Ni is an expensive element, and when its content is excessive, it causes an increase in manufacturing cost. Considering this, Ni may be contained in an amount of 0.4% or less, and 0% is excluded.

Niobium (Nb): 0.10% or less (excluding 0%)

Niobium (Nb) is an element that forms M(C,N) carbon-nitride. In addition, Nb is dissolved during reheating of the slab, inhibits the growth of austenite grains during hot rolling, and then precipitates, which is useful for improving the strength of steel.

When the content of Nb exceeds 0.10%, there is a fear that the weldability of the steel may be deteriorated, and there is a risk that the strength may be excessively increased due to the refinement of the crystal grains more than necessary.

In consideration of this, the Nb may be included in an amount of 0.10% or less, and 0% is excluded.

Titanium (Ti): 0.10% or less (excluding 0%)

Titanium (Ti) is an effective element for inhibiting the growth of austenite grains by precipitation in the form of TiN in steel.

When the content of Ti exceeds 0.10%, there is a problem that rather coarse Ti-based precipitates are formed, making it difficult to weld the material. do.

Nitrogen (N): 0.005-0.04%

Nitrogen (N) is an austenite stabilizing element, and is advantageous for forming M(C,N) carbon-nitride. Compared to MC carbide, when M(C,N) carbonitride is formed, the high temperature stability is greatly increased, so it plays a role in effectively increasing the creep strength of steel.

In order to form sufficient M(C,N) carbon-nitride in steel, it is advantageous to contain N in an amount of 0.005% or more. However, when the content exceeds 0.04%, there is a problem of increasing the risk of generating defects in the physical properties of the steel by combining with boron (B) in the steel to form BN.

Accordingly, the N may be included in an amount of 0.005 to 0.04%.

Aluminum (Al): 0.02% or less (excluding 0%)

Aluminum (Al) is an element that expands the ferrite region, and may be added to be used as a deoxidizer during casting.

When the content of Al exceeds 0.02% in the component system of the present invention as described above, a large amount of oxide-based inclusions may be formed, and in this case, there is a problem of impairing the physical properties of the material. In addition, since the chromium steel of the present invention contains ferrite stabilizing elements other than Al, there is a possibility that the Ae3 temperature is excessively increased as the content of Al increases.

In consideration of this, the Al may be included in an amount of 0.02% or less, and 0% may be excluded in consideration of an unavoidable level.

Boron (B): 0.006% or less (excluding 0%)

Boron (B) is a ferrite stabilizing element, and even a very small amount contributes greatly to improving the hardenability of steel. In addition, it is easily segregated at the grain boundary, which is effective for the grain boundary strengthening effect.

When the content of B exceeds 0.006%, there is a high possibility of forming BN, and in this case, it is not preferable because it adversely affects the physical properties of the steel.

In consideration of this, B may be included in an amount of 0.006% or less, and 0% may be excluded.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

However, it is preferable that the chromium steel of the present invention satisfies the following relation 1 for specific impurity elements.

[Relational Expression 1]

0.5 ≤ (V - (10×SUM)) ≤ 1

(Where SUM is the total content of specific impurity elements, it means the total content (wt%) of [Cu + Co + La + Y + Ce + Zr + Ta + Hf + Re + Pt + Ir + Pd + Sb] .)

In the chromium steel provided in the present invention, the content of V in the steel satisfies 0.7 to 1.1% in satisfying the aforementioned alloy composition, and impurity elements that may inhibit the beneficial effect of V are not contained in the steel of the present invention. You need to control it so it doesn't happen.

Specifically, after multiplying the defined 'SUM' by the number 10 and weighting it, when the value obtained by subtracting 10SUM from the content (weight%) of vanadium (V) in steel is 0.5% or more and 1.0% or less, described in the present invention You can get the effect of vanadium.

Meanwhile, in the present invention, copper (Cu), which is an element constituting the 'SUM', is highly likely to adversely affect the surface sporadic cracks of chromium steel. And, since cobalt (Co) reduces hardenability, when included in steel, the intended structure (preferably, martensitic structure) in the process of cooling to room temperature by soaking or quenching austenitized hot-rolled steel sheet under specific conditions by reheating. may not be able to obtain Among other residual impurities, if very expensive rare earths are included in the steel, the price may increase significantly and mechanical properties may deteriorate. Therefore, the sum of weight percent of alloying elements that should not be included in the chromium steel provided in the present invention is limited to SUM.

As such, the chromium steel of the present invention that satisfies Equation 1 along with the alloy composition described above may have an excellent creep strength with an LMP value defined by Equation 2 below 19,500 or more at an applied stress of 225 MPa.

[Relational Expression 2]

LMP = T × (20 + log(tr))

(Here, T is the absolute temperature in Kelvin, and tr is the time to break (Hr) in time.)

On the other hand, the chromium steel of the present invention can ensure excellent impact properties as well as creep strength by having the following microstructure and precipitate structure, which will be described in detail below.

In the chromium steel of the present invention, it is preferable that the matrix structure is composed of a tempered martensite phase, and in this structure, (Fe,Cr) ₂₃ C ₆ Precipitates with a diameter of 200 nm or more containing 1 / μm ² or less are preferably present. Do. If the number of precipitates having a diameter of 200 nm or more exceeds 1/μm ² , it may cause deterioration of creep properties and impact properties due to coarse carbides.

In addition, in the chromium steel of the present invention, it is preferable that precipitates having a diameter of 20 nm or less exist in the structure in an amount of 20/㎛ ² or more. If the number of precipitates having a diameter of 20 nm or less is less than 20/μm ² , the distance between the fine carbonitrides is significantly increased. For this reason, the effect of improving the creep characteristics may not be significant because the movement of dislocations and sub-crystal grains at high temperature cannot be effectively prevented.

In the present invention, the precipitates having a diameter of 20 nm or less may be M(C,N) carbonitrides, preferably (V,Mo,W,Nb,Ti)(C,N) precipitates.

The chromium steel of the present invention satisfying the above-described alloy composition, relational formula 1, and microstructure composition has excellent impact properties as well as creep properties, and specifically, the effect of having excellent low-temperature impact toughness with a Charpy impact energy value of 30 J or more at -20 ° C. there is

Hereinafter, a method for manufacturing chrome steel having excellent creep strength and impact toughness, which is another aspect of the present invention, will be described in detail.

Briefly, the present invention can manufacture the desired steel through the process of [steel slab heating - hot rolling - cooling - reheating (austenitization) - cooling - tempering], but it is not limited thereto.

The conditions for each step will be described in detail below.

First, after preparing a steel slab that satisfies the above-described alloy composition and relational expression 1, the steel slab can be heated. In this case, the heating process is to facilitate the subsequent hot rolling process, and the temperature thereof is not particularly limited, but may be performed in a temperature range of 1000 to 1200°C.

A hot-rolled steel sheet can be obtained by hot-rolling the steel slab heated according to the above. At this time, the hot rolling is preferably performed so that the finish rolling temperature is Ar3 or higher. As described above, the uniformity of the structure can be increased by performing hot rolling at a temperature that becomes the austenite single-phase region.

The upper limit of the finish rolling temperature is not particularly limited, but there is a problem in that the austenite grains become coarse when the temperature is excessively high.

After the hot-rolled steel sheet manufactured as described above is cooled to room temperature (air cooling), it can be reheated at a high temperature to make it austenitized.

At this time, the reheating is performed in a temperature range of 1000 to 1100° C., and it is preferable to maintain it for 1 t to 3 t (minutes) based on the thickness of the steel sheet (t, mm) at that temperature.

If the temperature at the time of reheating is less than 1000 ° C, unintended carbides formed in the cooling process after hot rolling may not be sufficiently re-dissolved, whereas if the temperature exceeds 1100 ° C, crystal grains are coarsened and the properties of the steel may be inferior. there is.

In addition, if the austenitization time, that is, the holding time at the reheated temperature is less than 1 t (minute) based on the thickness of the hot-rolled steel sheet, re-dissolution of unavoidable carbides formed during cooling after hot rolling may not be sufficiently achieved, whereas If the time exceeds 3t (minutes), there is a risk that the crystal grains are coarsened and the properties of the steel are inferior.

Here, 't' means the thickness of the steel sheet (unit mm), for example, if the thickness of the hot-rolled steel sheet is 20mm, it is revealed that it can be maintained for 20 to 60 minutes.

Thereafter, the austenitized hot-rolled steel sheet according to the above may be cooled to room temperature, and at this time, it may be carried out at a cooling rate of 2.5°C/s or more, preferably at a cooling rate of 2.5 to 20°C/s. The cooling process may be a normalizing or quenching process.

A martensite phase can be formed as a steel structure through the cooling process, and it is necessary to be careful not to generate ferrite and pearlite structures that greatly reduce matrix strength in this process.

Since the steel of the present invention contains elements advantageous for hardenability improvement, for example, V, Mo, W, etc., it is preferable to control the cooling rate to suppress the formation of ferrite and pearlite, and specifically, at 2.5°C/ It is advantageous to do it at a cooling rate of s or more. On the other hand, when the cooling rate exceeds 20 °C / s, there is a fear that defects such as thermal cracks may occur due to a cooling deviation by thickness (eg, surface/center in the thickness direction).

Subsequently, the hot-rolled steel sheet soaked or quenched (quenched) according to the above may be subjected to a tempering treatment. At this time, the tempering treatment can be performed by heat treatment for 30 minutes or more in a temperature range of 700 ~ 800 ℃. More advantageously, the heat treatment may be performed for 30 to 120 minutes.

If the temperature during the tempering is less than 700° C., precipitation of fine carbides may not be induced within the heat treatment time due to the too low temperature. On the other hand, when the temperature exceeds 800° C., there is a risk that the material will be softened and the creep life will be greatly reduced.

On the other hand, if the time is less than 30 minutes during tempering in the above temperature range, there is a fear that the intended precipitate may not be formed. On the other hand, if the time exceeds 120 minutes, there is a problem in that the creep life is greatly reduced as coarse precipitates are formed and the material softens.

After the tempering heat treatment is completed, it can be cooled to room temperature, and at this time, it is revealed that it can be performed by air cooling.

Through the above-described series of processes, the target chromium steel can be obtained in the present invention, and the steel structure is preferably composed of tempered martensite, and specific carbon nitrides are uniformly distributed therein, so that the impact along with creep strength is An improvement in properties can be achieved.

The chromium steel of the present invention may have a preferred thickness range of 5 to 100 mm.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

(Example)

After preparing a steel slab having an alloy composition shown in Table 1 below, it was heated at 1000 to 1200° C., and then finished hot rolled in Ar3 or higher to prepare a hot-rolled steel sheet having a thickness of 13 mm.

Thereafter, each hot-rolled steel sheet was reheated at various temperatures within the range of 1000 to 1100° C. for 30 minutes, and then cooled to room temperature by soaking or quenching. At this time, cooling by soaking or quenching was performed at a cooling rate within the range of 2.5 to 20 °C / s.

Each of the hot-rolled steel sheets cooled according to the above was tempered at various temperatures within the range of 700 to 800° C. for 30 minutes, and then air-cooled to room temperature to prepare final steel.

On the other hand, in Table 1 below, steel type 1 is a steel outside the alloy composition proposed in the present invention, and all other steels satisfy the alloy composition proposed in the present invention.

For each of the steels manufactured according to the above, creep specimens having a gauge length of 15 mm and a gauge diameter of 6 mm were respectively prepared using ASTM E139 standard in the rolling direction, The high-temperature creep life was evaluated, and the results are shown in FIG. 1 and Table 3.

At this time, for comparison, ASTM A213 grade 92 material (conventional material 1, 9Cr-1.5W-0.5Mo-V-Nb-Ti containing steel) provided by NIMS for comparison, M ₂₃ C ₆ carbide is formed in the structure ), ASTM A213 grahe 91 (conventional material 2, 9Cr-1Mo-V-Nb-Ti containing steel, M ₂₃ C ₆ carbide is formed in the structure), JIS STBA 25 material (conventional material 3, 5Cr -0.5Mo-containing steel and M ₂₃ C ₆ carbide is formed in the structure) creep results are also shown.

The microstructure was observed using a scanning electron microscope (SEM) for the specimen for measuring the creep strength of each steel prepared as described above, and the results are shown in FIG. 2 .

And, for the measurement of the impact properties of each steel, an impact test specimen having a V-notch in the middle of a specimen of 10 mm in width and length × 55 mm in length was prepared using the ASTM E23 standard, followed by Charpy impact It was evaluated using a tester. At this time, the impact toughness according to the test temperature was measured, and the results are shown in Table 3. At this time, after measuring a total of 3 times, the average value was obtained.

steel grade Alloy composition (wt%) Relation 1* C Si Mn Al Cr Ni Mo Ti Nb V B N W One 0.061 0.409 0.204 0.018 4.46 0.145 0.342 0.051 0.052 0.308 0.005 0.011 1.50 0.188 2 0.053 0.445 0.199 0.019 4.09 0.157 0.317 0.047 0.053 0.700 0.004 0.011 1.45 0.54 3 0.100 0.443 0.593 0.016 5.98 0.378 0.574 0.089 0.094 1.070 0.004 0.037 1.99 0.88 P and S of all steel grades were 30ppm or less, respectively, and each value was not entered as it did not affect the physical properties.

'SUM', which is the content of impurity elements used to calculate Relational Expression 1*, was formulated as follows for each steel type.
Steel grade 1: Sum of Cu (0.005%), Co (0.003%) and other rare earth elements (0.004%)
Steel Grade 2: Sum of Cu (0.004%), Co (0.005%) and other rare earth elements (0.007%)
Steel grade 3: Sum of Cu (0.006%), Co (0.008%) and other rare earth elements (0.005%)

steel grade Reheat temperature (℃) Cooling method* Cooling rate (℃/s) Tempering temperature (℃) division One 1000 N 2.5 700 Comparative Example 1 One 1000 Q 10 700 Comparative Example 2 One 1100 N 2.5 800 Comparative Example 3 One 1100 Q 20 800 Comparative Example 4 2 1000 N 2.5 700 Invention Example 1 2 1000 Q 20 700 Invention Example 2 2 1100 N 2.5 800 Invention example 3 2 1100 Q 10 800 Invention Example 4 3 1000 N 2.5 700 Invention Example 5 3 1000 Q 20 700 Invention example 6 3 1100 N 5 800 Invention Example 7 3 1100 Q 20 800 Invention Example 8 In the cooling method*, N means normalizing and Q means quenching.

division microstructure creep strength LMP
(Relational 2) CVN
(-20℃, J) Temperature (℃) Stress (MPa) Break time (Hr) Comparative Example 1 T-M* 650 225 0.44 18134 5.74 650 220 1.13 18512 Comparative Example 2 T-M 650 225 0.76 18353 10.87 650 220 1.31 18571 Comparative Example 3 T-M 650 225 2.87 18886 8.13 650 220 5.11 19117 Comparative Example 4 T-M 650 225 3.62 18979 20.8 650 220 5.47 19144 Invention Example 1 T-M 650 225 18.15 19625 37.8 650 220 27.53 19792 Invention Example 2 T-M 650 225 25.89 1976 45.4 650 220 33.87 19875 Invention example 3 T-M 650 225 127.91 20408 51.6 650 220 246.83 20672 Invention Example 4 T-M 650 225 166.21 20513 39.1 650 220 284.67 20729 Invention Example 5 T-M 650 225 87.53 20256 37.2 650 220 118.74 20378 Invention example 6 T-M 650 225 59.62 20102 43.5 650 220 100.4 20311 Invention Example 7 T-M 650 225 83.97 20239 60.7 650 220 121.91 20389 Invention Example 8 T-M 650 225 42.5 19966 41.2 650 220 86.48 20251 T-M* means tempered martensite.

It can be seen that the chromium steels (Inventive Examples 1 to 8) manufactured by the alloy composition system and manufacturing conditions proposed in Table 3 and the present invention have superior creep properties compared to Comparative Examples 1 to 4 that are out of the alloy composition system proposed in the present invention. .

In addition, as shown in FIG. 1, the chromium steel of the present invention has superior creep compared to conventional materials 1 and 2 containing 9% chromium (Cr) and conventional material 3 containing 5% chromium (Cr). characteristics can be seen.

2 is a microstructure photograph of each steel, (A) is Comparative Example 1, (B) is Inventive Example 1, (C) shows the tissue measurement results of Inventive Example 5.

As shown in FIG. 2, it can be confirmed that, in both Inventive Examples 1 and 5, which are chromium steels according to the present invention, fine carbon-nitrides were precipitated within grains and along sub-crystal grain boundaries. These fine carbon nitrides not only effectively inhibit dislocation movement at high temperatures, but also effectively suppress the movement of sub-crystal grains in the martensite-containing steel, thereby improving stability, resulting in significantly improved creep properties compared to conventional chromium steels. it can be

On the other hand, in the case of Comparative Example 1, coarse (Fe,Cr) ₂₃ C ₆ carbide is formed, and creep properties are inferior to those of the inventions.

In addition, as shown in Table 3, it can be seen that the impact properties of the inventive examples are superior to those of the comparative examples, which is (Fe,Cr) ₂₃ C in which vanadium (V) contained in a certain amount in steel is coarsely formed at grain boundaries. It is thought to be due to suppressing the formation of _hexacarbide .

As a result, the chromium steel of the present invention tends to have excellent high-temperature creep strength and impact properties due to the formation of fine carbon-nitrides while suppressing the formation of coarse carbides.

On the other hand, in order to confirm the change of the phase transformation according to the cooling rate after austenitization, a hot-rolled steel sheet having each alloy composition system (Table 1) was prepared, and then reheated to 1050° C. and then cooled at various rates (0.25, After 0.5, 1.0, 2.5, 5, 10 (℃/s)), the phase transformation of each steel was confirmed with a dilatometer. The results are shown in Figs. 3a to 3c.

As shown in Figs. 3a to 3c, it can be seen that all the steels exhibit martensitic transformation behavior.

Claims (9)

By weight%, carbon (C): 0.05 to 0.10%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.1 to 0.6%, sulfur (S): 0.01% or less (excluding 0%) ), phosphorus (P): 0.02% or less (excluding 0%), chromium (Cr): 4.0 to 6.0%, molybdenum (Mo): 0.3 to 0.6%, tungsten (W): 1.4 to 2.0%, vanadium (V) : 0.7~1.1%, Nickel (Ni): 0.4% or less (excluding 0%), Niobium (Nb): 0.10% or less (excluding 0%), Titanium (Ti): 0.10% or less (excluding 0%), Nitrogen ( N): 0.005 to 0.04%, aluminum (Al): 0.02% or less (excluding 0%), boron (B): 0.006% or less (excluding 0%), the balance contains Fe and unavoidable impurity elements;
The relationship between V and the sum of the contents of impurity elements (SUM) satisfies the following Relational Expression 1,
A chromium steel having excellent creep strength and impact toughness with an LMP value of 19,500 or more at an applied stress of 225 MPa, defined by the above equation (2).

[Relational Expression 1]
0.5 ≤ (V - (10×SUM)) ≤ 1
(Where SUM is the total content of specific impurity elements, it means the total content (wt%) of [Cu + Co + La + Y + Ce + Zr + Ta + Hf + Re + Pt + Ir + Pd + Sb] .)
[Relational Expression 2]
LMP = T × (20 + log(tr))
(Here, T is the absolute temperature in Kelvin units, and tr is the time to break (Hr) in time units.)
The method of claim 1,
The steel is chromium steel with excellent creep strength and impact toughness, in which the microstructure is composed of tempered martensite.
The method of claim 1,
The steel is chromium steel with excellent creep strength and impact toughness, in which precipitates with a diameter of 200 nm or more containing (Fe,Cr) ₂₃ C ₆ in the microstructure exist at 1/㎛ ² or less.
The method of claim 1,
The steel is chromium steel excellent in creep strength and impact toughness in which 20 nm or less in diameter precipitates exist in the microstructure at 20/μm ² or more.
5. The method of claim 4,
Chromium steel with excellent creep strength and impact toughness, wherein the precipitates with a diameter of 20 nm or less are (V,Mo,W,Nb,Ti)(C,N).
The method of claim 1,
The steel is chromium steel with excellent creep strength and impact toughness with a Charpy impact energy value of 30 J or more at -20 °C.
By weight%, carbon (C): 0.05 to 0.10%, silicon (Si): 0.5% or less (excluding 0%), manganese (Mn): 0.1 to 0.6%, sulfur (S): 0.01% or less (excluding 0%) ), phosphorus (P): 0.02% or less (excluding 0%), chromium (Cr): 4.0 to 6.0%, molybdenum (Mo): 0.3 to 0.6%, tungsten (W): 1.4 to 2.0%, vanadium (V) : 0.7~1.1%, Nickel (Ni): 0.4% or less (excluding 0%), Niobium (Nb): 0.10% or less (excluding 0%), Titanium (Ti): 0.10% or less (excluding 0%), Nitrogen ( N): 0.005 to 0.04%, aluminum (Al): 0.02% or less (excluding 0%), boron (B): 0.006% or less (excluding 0%), the remainder including Fe and unavoidable impurity elements, the above V and impurities Preparing a steel slab in which the relation of the sum of the contents of the elements (SUM) satisfies the following relation 1 and heating it in a temperature range of 1000 to 1200°C;
manufacturing a hot-rolled steel sheet by hot-rolling the heated steel slab to a finish rolling temperature Ar3 or higher;
cooling the hot-rolled steel sheet to room temperature;
an austenitization step of reheating the cooled hot-rolled steel sheet to a temperature range of 1000 to 1100° C. and then maintaining the hot-rolled steel sheet for 1t to 3t (minutes) based on the thickness (t, mm) of the hot-rolled steel sheet;
cooling the austenitized hot-rolled steel sheet to room temperature at a cooling rate of 2.5° C./s or more; and
Tempering step of heat treatment for 30 minutes or more in the temperature range of 700 ~ 800 ℃ after the cooling
A method for producing chromium steel having excellent creep strength and impact toughness, comprising:

[Relational Expression 1]
0.5 ≤ (V - (10×SUM)) ≤ 1
(Where SUM is the total content of specific impurity elements, it means the total content (wt%) of [Cu + Co + La + Y + Ce + Zr + Ta + Hf + Re + Pt + Ir + Pd + Sb] .)
8. The method of claim 7,
The step of cooling the austenitized hot-rolled steel sheet is a method for producing chromium steel having excellent creep strength and impact toughness, which is performed by a normalizing or quenching process.
8. The method of claim 7,
Method for producing chromium steel excellent in creep strength and impact toughness further comprising the step of air cooling to room temperature after the tempering step.

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