JP7398559B2 - Steel plate for pressure vessels with excellent heat treatment resistance after high temperature welding and method for manufacturing the same - Google Patents

Description

The present invention relates to a steel plate for pressure vessels that has excellent resistance to heat treatment after high-temperature welding, and a method for manufacturing the same, and more specifically relates to steel materials used in boilers, pressure vessels, etc. of power plants and chemical plants, The present invention relates to a steel material for pressure vessels that has excellent resistance to heat treatment after high-temperature welding, and a method for manufacturing the same.

In recent years, demand for materials for high-temperature pressure vessels has continued to increase in environments of about 350 to 600°C, such as boilers and pressure vessels in power plants and chemical plants.
Additionally, as equipment becomes more sophisticated, lifespan increases, and steel becomes thicker, there is a need to supply steel that is highly resistant to tempering heat treatment at high temperatures.

On the other hand, in addition to thickening steel materials, when welding steel materials, post-weld heat treatment is used to prevent the deformation of the structure after welding and to stabilize the shape and dimensions, and to remove the stress generated during welding. (Post Weld Heat Treatment, PWHT) is being performed. The post-weld heat treatment process is carried out for a long time, and the steel plate subjected to this process has a problem in that the tensile strength of the steel plate decreases due to coarsening of the microstructure particles.
That is, after a long period of PWHT, strength and toughness simultaneously decrease due to softening of matrix and grain boundaries, grain growth, coarsening of carbides, etc.

In order to solve these problems, the following techniques have been proposed.
According to Patent Document 1, a tempering heat treatment pattern is applied to a thick steel plate material containing appropriate amounts of C, Si, Mn, Cr, Mo, Ni, Cu, etc., that is, a tempering heat treatment pattern is applied after high temperature heat treatment (high temperature tempering). We are proposing a method that utilizes the precipitation strengthening effect generated by low-temperature tempering to counteract the decrease in strength by reducing the dislocation density during high-temperature tempering. However, even if such a method is applied, there is a problem that the resistance due to long-term PWHT deteriorates significantly.
Therefore, there is a need to develop a steel material that can minimize the deterioration of physical properties even after long-term PWHT and can be used suitably in medium and high temperature environments.

Korean Published Patent No. 2012-0073448

The object of the present invention is to provide a steel plate for pressure vessels with excellent resistance to high-temperature post-weld heat treatment, which can minimize deterioration of strength and toughness despite the application of long-term post-weld heat treatment (PWHT) at high temperatures. The object of the present invention is to provide a manufacturing method thereof.
The object of the present invention is not limited to the above-mentioned content. The problems to be solved by the present invention can be understood from the entire contents of this specification, and a person having ordinary knowledge in the technical field to which the present invention pertains will not need to understand the additional problems to be solved by the present invention. There are no difficulties.

The steel material for pressure vessels of the present invention having excellent resistance to heat treatment after high temperature welding has, in weight percent, carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, Manganese (Mn): 0.4 to 0.6%, Chromium (Cr): 6.5 to 7.5%, Molybdenum (Mo): 0.7 to 0.9%, Aluminum (Al): 0.005 ~0.05%, Phosphorus (P): 0.015% or less, Sulfur (S): 0.020% or less, Niobium (Nb): 0.002-0.025%, Vanadium (V): 0.25 ~0.35%, the balance being Fe and other unavoidable impurities, and is characterized by containing a mixed structure of tempered martensite and tempered bainite as a fine structure.

The method of manufacturing a steel material for pressure vessels with excellent resistance to heat treatment after high-temperature welding according to the present invention includes the steps of preparing a steel slab having the above-mentioned alloy composition system, and heating the steel slab in a temperature range of 1050 to 1250°C. a step of hot rolling the heated steel slab in a temperature range of 800 to 1000°C to produce a hot rolled steel plate; and a step of hot rolling the heated steel slab in a temperature range of 1000 to 1050°C { ×t)+(10~30)} minutes (here, t means the steel material thickness (mm)), and the heat-treated hot-rolled steel plate is cooled at 1~30°C/s. and a tempering heat treatment step of holding the cooled hot rolled steel sheet in a temperature range of 800 to 825° C. for {(1.6×t)+(10 to 30)} minutes. Features.

According to the present invention, it is possible to provide a steel material for a pressure vessel whose strength and toughness do not deteriorate even after high-temperature heat treatment, particularly after long-term high-temperature PWHT. In particular, the steel material for pressure vessels of the present invention has the advantage that it can be suitably applied as a material for pressure vessels for medium and high temperatures.

The present inventor has discovered that post-weld heat treatment is performed to minimize the residual stress generated by welding when manufacturing pressure vessel steel materials used as structural steel in environments such as power plants and plant industries. The present invention has been completed through extensive research into ways to greatly improve resistance to deterioration of strength and toughness even after long periods of PWHT.
In particular, the present invention provides a steel material with excellent resistance to strength and toughness deterioration even when subjected to high-temperature tempering heat treatment and long-time PWHT by optimizing the content of specific elements in the alloy composition. There are particularly technical features.
The present invention will be explained in detail below.

The steel material for pressure vessels having excellent resistance to heat treatment after high temperature welding of the present invention has, in weight percent, carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, Manganese (Mn): 0.4-0.6%, chromium (Cr): 6.5-7.5%, molybdenum (Mo): 0.7-0.9%, aluminum (Al): 0.005 ~0.05%, Phosphorus (P): 0.015% or less, Sulfur (S): 0.020% or less, Niobium (Nb): 0.002-0.025%, Vanadium (V): 0.25 Contains ~0.35%.
Below, the reason why the alloy composition of the steel material for pressure vessels provided by the present invention is limited as described above will be explained in detail. In the present invention, unless otherwise specified, the content of each element is based on weight, and the ratio of structure is based on area.

Carbon (C): 0.10-0.16%
Carbon (C) is an advantageous element for improving the strength of steel. If the C content is less than 0.10%, the strength of the matrix structure itself decreases. On the other hand, if the content exceeds 0.16%, the strength may increase excessively and the toughness may deteriorate. Therefore, the above C is preferably contained in an amount of 0.10 to 0.16%.

Silicon (Si): 0.20-0.35%
Silicon (Si) is an effective element for deoxidation and solid solution strengthening, and is an element that increases the shock transition temperature. In order to achieve the target strength, it is preferable that the Si content is 0.20% or more, but if the content exceeds 0.35%, there is a problem that weldability decreases and impact toughness is poor. Therefore, the Si content is preferably 0.20 to 0.35%.

Manganese (Mn): 0.4-0.6%
Manganese (Mn) inhibits room temperature elongation and low temperature toughness by combining with sulfur (S) in steel to form MnS, which is a nonmetallic inclusion that is stretched, so its content is reduced to 0. Limit to 6% or less. However, if the content is less than 0.4%, it becomes difficult to secure an appropriate level of strength. Therefore, the Mn content is preferably 0.40 to 0.6%.

Chromium (Cr): 6.5-7.5%
In the present invention, chromium (Cr) is advantageous in increasing strength in addition to enabling heat treatment at high temperatures (tempering, PWHT), and for this reason, it is preferable to add 6.5% or more. . Thereby, the steel material of the present invention can ensure excellent resistance to high-temperature heat treatment. However, the above-mentioned Cr is an expensive element, and if its content exceeds 7.5%, the manufacturing cost increases significantly. Therefore, the content of Cr is preferably 6.5 to 7.5%.

Molybdenum (Mo): 0.7-0.9%
Molybdenum (Mo), like the above-mentioned Cr, is not only an effective element for increasing high-temperature strength, but is also advantageous in preventing cracking caused by sulfides. In order to fully obtain the above-mentioned effects, it is preferable to contain Mo in an amount of 0.7% or more, but if the content exceeds 0.9%, the manufacturing cost will increase. Therefore, the Mo content is preferably 0.7 to 0.9%.

Aluminum (Al): 0.005-0.05%
Aluminum (Al), together with the above-mentioned Si, is a strong deoxidizing agent in the steel manufacturing process. In order to obtain a sufficient deoxidizing effect, it is preferable to contain 0.005% or more of the above-mentioned Al, but if the content exceeds 0.05%, the deoxidizing effect is saturated, but the manufacturing cost increases. There is a problem with rising. Therefore, the above-mentioned Al is preferably contained in an amount of 0.005 to 0.05%.

Phosphorus (P): 0.015% or less Phosphorus (P) is an element that inhibits the low-temperature toughness of steel and increases the susceptibility to temper embrittlement, and it is preferable to control its content as low as possible. However, the process for lowering the P content is complicated, and the additional process may increase production costs. Taking this into consideration, the above-mentioned P is preferably limited to 0.015% or less, and the present invention has clarified that even if the above-mentioned P is contained at a maximum of 0.015%, it is not unreasonable to ensure the intended physical properties. I'll keep it.

Sulfur (S): 0.020% or less Sulfur (S) is also an element that reduces the low-temperature toughness of steel, and inhibits the toughness of steel by forming MnS inclusions in steel, so it should be avoided as much as possible. It is preferable to control the content low. However, the steps to reduce the S content are difficult, and the additional steps may increase production costs. Taking this into consideration, the above-mentioned S should be limited to 0.020% or less, and the present invention has clarified that even if the above-mentioned S is contained at a maximum of 0.020%, it is not unreasonable to secure the intended physical properties. I'll keep it.

Niobium (Nb): 0.002-0.025%
Niobium (Nb) is an element effective in forming fine carbides or nitrides in steel to prevent softening of the base structure. In order to fully obtain such effects, it is preferable to contain 0.002% or more of Nb, but it is an expensive element, and if the content exceeds 0.025%, manufacturing costs may increase significantly. There is.
Therefore, the Nb content is preferably 0.002 to 0.025%.

Vanadium (V): 0.25-0.35%
Vanadium (V), like the above-mentioned Nb, is an element that easily forms fine carbides or nitrides in steel. In order to fully obtain such effects, it is preferable to contain 0.25% or more of the above-mentioned V, but since this is also an expensive element, taking this into consideration, the content should be reduced to 0.35% or less. It is better to limit Therefore, the above-mentioned V is preferably contained in an amount of 0.25 to 0.35%.

The remaining component of the present invention is iron (Fe). However, in normal manufacturing processes, unintended impurities may inevitably be mixed in from raw materials or the surrounding environment, and this cannot be eliminated. These impurities are known to anyone skilled in the art of normal manufacturing processes, and therefore, all details thereof will not be specifically mentioned in this specification.

The steel material of the present invention having the above alloy component system can include a mixed structure of tempered martensite and tempered bainite as a microstructure, and includes the above mixed structure throughout the entire thickness regardless of the thickness of the steel material. be able to.
In the mixed structure, the tempered martensitic phase preferably has an area fraction of 40% or more. If the fraction of the above-mentioned tempered martensitic phase is less than 40%, the target strength cannot be sufficiently ensured. Therefore, the tempered martensitic phase is contained in an area fraction of 40 to 90%.

Below, the method of manufacturing the steel material for pressure vessels having excellent resistance to heat treatment after high-temperature welding according to the present invention will be described in detail.
The steel material for pressure vessels according to the present invention can be manufactured through the steps of [heating - hot rolling - heat treatment - cooling - tempering heat treatment] to produce a steel slab that satisfies the alloy composition system proposed by the present invention. will be explained in detail.

[Steel slab heating]
After preparing a steel slab that satisfies the above alloy composition system, it is heated in a temperature range of 1050 to 1250°C. When the heating temperature is less than 1050°C, solute atoms are difficult to form a solid solution. On the other hand, if the temperature exceeds 1250°C, the size of austenite crystal grains becomes excessively coarse, which poses a problem of impairing the physical properties of the steel.

[Hot rolling]
The steel slab heated as described above is hot-rolled to produce a hot-rolled steel plate. The hot rolling is preferably carried out at a temperature range of 800 to 1000° C. and a reduction rate of 2.5 to 30% per pass.
If the temperature during the hot rolling is less than 800°C, there is a problem that the rolling load increases. On the other hand, if the temperature exceeds 1000° C., there is a problem that crystal grains become coarse. Further, when the rolling reduction per pass during the hot rolling is less than 2.5%, there is a problem that rolling productivity decreases and manufacturing cost increases. On the other hand, if the rolling reduction per pass exceeds 30%, excessive load may be generated on the rolling mill, which may have a fatal adverse effect on the equipment.
The hot-rolled steel sheet obtained by completing the above-mentioned hot rolling can be cooled to room temperature by air cooling, and then the subsequent process can be performed.

[Heat treatment]
The hot-rolled steel sheet manufactured as described above is heated to a specific temperature range to perform heat treatment. Specifically, the above hot rolled steel plate was heated in a temperature range of 1000 to 1050°C for {(1.3×t)+(10 to 30)} minutes (where t means steel thickness (mm)). It is preferable to perform a heat treatment for holding.
If the temperature during the heat treatment is less than 1000° C., it becomes difficult to re-dissolve the solid solute elements, making it difficult to secure the target strength. On the other hand, if the temperature exceeds 1050° C., there is a problem in that crystal grains grow and impair low-temperature toughness.
During heat treatment in the above temperature range, if the holding time is less than {(1.3×t)+10} minutes, it is difficult to homogenize the structure. On the other hand, if the time exceeds {(1.3×t)+30} minutes, this is not preferable because productivity will be hindered.

[cooling]
The hot-rolled steel sheet heat-treated as described above is cooled at a cooling rate of 1 to 30° C./s. At this time, cooling can be performed to room temperature. Here, the cooling rate is based on the center of the thickness of the hot rolled steel plate (for example, 1/2t (t: thickness (mm) point)).
If the cooling rate is less than 1° C./s during the above cooling, coarse ferrite crystal grains may be generated during cooling. On the other hand, if the speed exceeds 30° C./s, there is a problem that economical efficiency decreases due to excessive cooling equipment.

[Tempering heat treatment]
The hot-rolled steel sheet cooled as described above is subjected to a tempering treatment. Specifically, a tempering heat treatment step can be performed in which the temperature range is 800 to 825° C. for {(1.6×t)+(10 to 30)} minutes.
If the temperature during the above-mentioned tempering heat treatment is less than 800°C, precipitation of fine precipitates becomes difficult, making it difficult to secure the target strength. On the other hand, if the temperature exceeds 825° C., there is a problem in that precipitates grow, leading to a decrease in strength and low-temperature toughness.
During the tempering heat treatment in the above-mentioned temperature range, if the holding time is less than {(1.6×t)+10} minutes, it is difficult to homogenize the structure. On the other hand, if the time exceeds {(1.6×t)+30} minutes, this is not preferable because productivity will be hindered.

The steel material for a pressure vessel of the present invention manufactured through the above process requires a post-weld heat treatment (PWHT) process for the purpose of removing residual stress due to the welding process added when manufacturing the pressure vessel.
Generally, deterioration of strength and toughness occurs after long-term PWHT heat treatment, but the steel produced by the present invention has a maximum temperature in the temperature range of 760 to 780 °C, which is higher than the normal PWHT temperature. It has the advantage that even after 50 hours of heat treatment, welding work is possible without significant deterioration in strength and toughness. In particular, the steel material of the present invention has excellent strength and toughness, with a tensile strength of 600 MPa or more and a Charpy impact energy value of 100 J or more at -30° C. even after PWHT for up to 50 hours at high temperatures.

Hereinafter, the present invention will be explained in more detail through Examples. However, it should be noted that the following examples are exemplified to explain the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of rights in the present invention is determined by the matters stated in the claims and matters reasonably inferred therefrom.

After preparing a steel slab having the alloy composition shown in Table 1 below, the steel slab is heated at 1,150°C for 300 minutes, and hot rolled at 800-1,000°C with a rolling reduction of 5-20% per pass. Manufactured steel plates. Thereafter, the hot-rolled steel sheet was air-cooled to room temperature, and then heat-treated by heating and holding it at 1050° C. again. At this time, the heat treatment was maintained for 150 to 280 minutes depending on the thickness of the hot rolled steel sheet. Thereafter, water cooling was performed to room temperature at a cooling rate of 2.5 to 15° C./s based on the center of the thickness of the hot rolled steel sheet, and tempering heat treatment and PWHT heat treatment were performed under the conditions shown in Table 2 below.

A tensile test was conducted on the hot rolled steel sheet that had gone through all of the above steps, and the yield strength (YS), tensile strength (TS), and elongation rate (El) were measured. In addition, a Charpy impact test was conducted to derive the impact energy value. The above tensile test was conducted based on ASTM standards A20 and A370&E8, and the impact test was conducted at -30° C. using a Charpy impact test on a specimen having a V-notch. The results are shown in Table 3 below.

As shown in Tables 1 to 3 above, invention examples 1 to 9 that satisfy both the alloy composition and manufacturing conditions proposed by the present invention have a tensile strength of 600 MPa or more even after the post-weld heat treatment (PWHT) time reaches a maximum of 50 hours. Strength and Charpy impact energy value at -30°C (CVN@-30°C (J)) of 100 J or more can be ensured.
On the other hand, it can be confirmed that Comparative Examples 1 to 3, in which the alloy composition deviates from the present invention, have a strength of about 150 MPa and a low-temperature toughness of about 200 J compared to the invention examples after long-term PWHT.

As mentioned above, the steel material obtained using the alloy composition system and manufacturing conditions according to the present invention has excellent resistance not only to high-temperature tempering heat treatment but also to long-term high-temperature post-weld heat treatment (PWHT).・It has the effect of being suitable as a steel material for high-temperature pressure vessels.

Claims (6)

In weight%, carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, manganese (Mn): 0.4 to 0.6%, chromium (Cr) : 6.5 to 7.5%, Molybdenum (Mo): 0.7 to 0.9%, Aluminum (Al): 0.005 to 0.05%, Phosphorus (P): 0.015% or less, Sulfur (S): 0.020% or less, niobium (Nb): 0.002 to 0.025%, vanadium (V): 0.25 to 0.35%, the balance consisting of Fe and other inevitable impurities,
A steel material for pressure vessels with excellent resistance to heat treatment after high-temperature welding, characterized by containing a mixed structure of tempered martensite and tempered bainite as a microstructure. The steel material for a pressure vessel having excellent resistance to heat treatment after high-temperature welding according to claim 1, wherein the tempered martensite has an area fraction of 40% or more. The steel material for pressure vessels having excellent resistance to heat treatment after high-temperature welding according to claim 1 , wherein the steel material has a tensile strength of 600 MPa or more and a Charpy impact energy value of 100 J or more at -30°C. In weight%, carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, manganese (Mn): 0.4 to 0.6%, chromium (Cr) : 6.5 to 7.5%, Molybdenum (Mo): 0.7 to 0.9%, Aluminum (Al): 0.005 to 0.05%, Phosphorus (P): 0.015% or less, Sulfur A steel slab consisting of (S): 0.020% or less, niobium (Nb): 0.002 to 0.025%, vanadium (V): 0.25 to 0.35%, and the balance Fe and other unavoidable impurities. The preparation stage and
heating the steel slab to a temperature range of 1050-1250°C;
hot rolling the heated steel slab at a temperature range of 800 to 1000°C to produce a hot rolled steel plate;
A heat treatment step of holding the hot-rolled steel sheet in a temperature range of 1000 to 1050°C for {(1.3×t)+(10 to 30)} minutes (here, t means steel thickness (mm)) and,
cooling the heat-treated hot-rolled steel sheet to room temperature at a cooling rate of 1 to 30°C/s;
a tempering heat treatment step of holding the cooled hot-rolled steel sheet in a temperature range of 800 to 825 ° C. for {(1.6 × t) + (10 to 30)} minutes;
The steel material for pressure vessels having excellent resistance to high-temperature post-weld heat treatment according to claim 1 , further comprising the step of post-weld heat treatment at a temperature range of 760 to 780° C. for up to 50 hours after the tempering heat treatment. Production method. 5. The method for producing a pressure vessel steel material having excellent resistance to heat treatment after high-temperature welding according to claim 4, wherein the hot rolling is performed at a reduction rate of 2.5 to 30% per pass. 5. The method of manufacturing a pressure vessel steel material having excellent resistance to heat treatment after high-temperature welding according to claim 4, further comprising the step of air-cooling the hot-rolled steel sheet to room temperature after the hot rolling.