JP2021507099A - Steel plate for pressure vessels with excellent tensile strength and low temperature impact toughness and its manufacturing method - Google Patents

Abstract

Provided are a steel sheet for a pressure vessel having excellent tensile strength and low temperature impact toughness, and a method for manufacturing the same. By weight%, C: 0.12 to 0.20%, Si: 0.30 to 0.40%, Mn: 1.50 to 1.70%, Mo: 0.03 to 0.10%, Cu: 0.05 to 0.30%, V: 0.03 to 0.10%, Ni: 0.03 to 0.25%, Cr: 0.03 to 0.25%, Al: 0.005 to 0. 06%, Ca: 0.0005 to 0.0030%, P: 0.025% or less, S: 0.025% or less, Ti: 0.003 to 0.015%, Nb: 0.005 to 0 Includes two or more selected from the group consisting of .025% and Ta: 0.002 to 0.050%, the balance consisting of Fe and other unavoidable impurities, ferrite, tempered bainite, pearlite, and digenerated. It consists of a mixed structure containing one or two of pearlite, and the tempered bainite fraction is 5 to 50 area%. [Selection diagram] None

Description

The present invention relates to a steel sheet for a pressure vessel having excellent tensile strength and low temperature impact toughness and a method for producing the same, and more specifically, the tensile strength and low temperature impact which can be preferably applied as a material for _{a CO 2 storage tank and a pressure vessel.} The present invention relates to a steel plate for a pressure vessel having excellent toughness and a method for manufacturing the same.

Recently, the demand for CO2 storage tanks and pressure vessels that store and transport carbon gas, which is the main cause of greenhouse gases associated with global environmental policy, is increasing. It is an important issue to increase the strength of such steel materials and to secure low temperature toughness.

In addition to increasing the strength of steel materials and ensuring low-temperature toughness as described above, when steel materials are welded, they occur during welding for the purpose of preventing deformation of the structure after welding and stabilizing the shape and dimensions. Post-weld heat treatment (PWHT, Post Weld Heat Treatment) is performed in order to remove the stress. However, the steel sheet that has been subjected to the PWHT process for a long time has a problem that the tensile strength of the steel sheet is lowered due to the coarsening of the structure.

That is, after a long period of PWHT, the strength and toughness both decrease due to softening of the matrix structure and grain boundaries, grain growth, and coarsening of carbides.

Conventionally, like the ASTM A612 steel of Non-Patent Document 1, in terms of weight%, C: 0.25% or less, Si: 0.15 to 0.50%, Mn: 1.00 to 1.50%, Mo and V: 0.08% or less, Cu: 0.3% or less, Ni: 0.25% or less, Cr: 0.25% or less, P: 0.025% or less, S: 0.025% or less. It was manufactured by applying As lolled or normalizing or normalizing + SR (Stress Relief) heat treatment pattern by utilizing the steel sheet material. When the steel produced in this way is used, welding that is indispensable for manufacturing a structure comes to be performed. Here, in order to prevent deformation of the structure after welding and to stabilize the shape and dimensions, post-welding heat treatment (PWHT, Post Weld Heat Treatment) is performed in order to remove the stress generated during welding. .. However, the steel sheet that has been subjected to the PWHT process for a long time has a problem that the tensile strength and the low temperature impact toughness of the steel sheet are significantly lowered due to the coarsening of the structure.

A612 / A612M-12: Standard Specialization for Pressure Vessel Plates, Carbon Steel, High Strength, for Moderate and Lower Temperature Service

An object of the present invention is to provide a steel sheet for a pressure vessel having excellent tensile strength and low-temperature impact toughness even after PWHT heat treatment, and a method for producing the same.

One embodiment of the present invention is, in% weight, C: 0.12 to 0.20%, Si: 0.30 to 0.40%, Mn: 1.50 to 1.70%, Mo: 0.03. ~ 0.10%, Cu: 0.05 ~ 0.30%, V: 0.03 ~ 0.10%, Ni: 0.03 ~ 0.25%, Cr: 0.03 ~ 0.25%, Al: 0.005 to 0.06%, Ca: 0.0005 to 0.0030%, P: 0.025% or less, S: 0.025% or less, and additionally Ti: 0.003 to Contains two or more selected from the group consisting of 0.015%, Nb: 0.005 to 0.025%, and Ta: 0.002 to 0.050%, the balance consisting of Fe and other unavoidable impurities. The microstructure consists of a mixed structure containing one or two of ferrite, tempered bainite, pearlite, and digenerated pearlite, and the tempered bainite fraction is 5 to 50 area% tensile strength and low temperature impact. Provided is a steel plate for a pressure vessel having excellent toughness.

In another embodiment of the present invention, in% by weight, C: 0.12 to 0.20%, Si: 0.30 to 0.40%, Mn: 1.50 to 1.70%, Mo: 0. 03 to 0.10%, Cu: 0.05 to 0.30%, V: 0.03 to 0.10%, Ni: 0.03 to 0.25%, Cr: 0.03 to 0.25% , Al: 0.005 to 0.06%, Ca: 0.0005 to 0.0030%, P: 0.025% or less, S: 0.025% or less, and additionally Ti: 0.003. Includes two or more selected from the group consisting of ~ 0.015%, Nb: 0.005 to 0.025%, and Ta: 0.002 to 0.050%, with the balance from Fe and other unavoidable impurities. The steel slab is reheated at 950 to 1200 ° C., the reheated steel slab is hot-rolled at a reduction rate of 2.5 to 30% per pass to obtain a hot-rolled steel sheet, and the heat The step of normalizing the rolled steel sheet at 820 to 930 ° C. at 1.3 × t + (10 to 30 minutes) (where t is the thickness (mm) of the steel sheet) and the above-mentioned normalizing heat treatment are performed. In the temperature interval from the normalizing temperature range to 450 ° C., the hot-rolled steel sheet is 0.5 to 30 ° C./based on (1/4) t (where t is the thickness (mm) of the steel sheet). The stage of cooling at the cooling rate of s and the above-mentioned cooled hot-rolled steel sheet at 550 to 680 ° C. for 1.6 × t + (10 to 30 minutes) (where t is the thickness (mm) of the steel sheet). Provided is a method for producing a steel sheet for a pressure vessel, which is excellent in tensile strength and low-temperature impact toughness, including a step of normalizing heat treatment.

According to the implementation of the present invention, it is possible to provide a steel sheet for a pressure vessel having excellent tensile strength and low-temperature impact toughness even after PWHT heat treatment, and a method for producing the same.

Hereinafter, the present invention will be described in detail. First, the alloy composition of the present invention will be described. However, the alloy composition described below means% by weight unless otherwise specified.

C: 0.12 to 0.20%
C is an element that improves strength, and when its content is less than 0.12%, the strength of the matrix phase itself decreases, and when it exceeds 0.20%, it becomes tough due to an excessive increase in strength. And there is a problem that the weldability is lowered.

Si: 0.30 to 0.40%
Si is an element effective for deoxidizing and strengthening solid solution, and is an element accompanied by an increase in impact transition temperature. It is necessary to add 0.30% or more in order to achieve the target strength, but if it is added in excess of 0.40%, the weldability is lowered and the impact toughness is deteriorated.

Mn: 1.50 to 1.70%
Mn is an alloying element that has an important effect on the strength and low temperature toughness of steel. If the Mn content is too low, the strength and toughness may deteriorate, so it is preferable to add 1.50% or more. However, if the content is too high, the weldability may decrease and the manufacturing cost of steel may increase. Therefore, the upper limit is preferably limited to 1.70%.

Mo: 0.03 to 0.10%
Mo is an element that not only improves the hardenability of steel and prevents sulfide cracks, but also improves the strength of steel by precipitating fine carbides after quenching and tempering. In the present invention, it is preferable to add 0.03% or more in order to obtain such an effect. However, if the content is too high, the manufacturing cost of steel may increase, so the upper limit is preferably limited to 0.10%.

Cu: 0.05 to 0.30%
Cu is an element that is effective in increasing the strength, and the above effect cannot be obtained unless 0.05% or more is added. However, since it is expensive, the upper limit is preferably limited to 0.30%.

V: 0.03 to 0.10%
V is an element capable of easily forming fine carbides and nitrides, and the above effect cannot be obtained unless 0.03% or more is added. However, since it is expensive, the upper limit is preferably limited to 0.30%.

Ni: 0.03 to 0.25%
Ni is the most effective element for improving low temperature toughness, and the above effect cannot be obtained unless its content is 0.03% or more. However, since it is an expensive element and causes an increase in manufacturing cost, it is preferable to add 0.25% or less.

Cr: 0.03 to 0.25%
Cr is an element that increases the strength. In the present invention, it is necessary to add 0.03% or more for the effect of increasing the strength. However, since it is an expensive element, if it is added in an amount exceeding 0.25%, the production cost will increase.

Al: 0.005 to 0.06%
Al, along with Si, is one of the powerful antacids in the steelmaking process. If the content is less than 0.005%, the deoxidizing effect is slight, and if it is added in excess of 0.06%, the deoxidizing effect is saturated and the manufacturing cost rises. is there.

Ca: 0.0005 to 0.0030%
Ca is produced as CaS and plays a role of suppressing non-metallic inclusions of MnS, so 5 ppm or more is added. However, if the amount added is excessive, it reacts with O contained in the steel to form CaO, which is a non-metal inclusion, which is not good for the physical properties. Therefore, the upper limit is limited to 30 ppm.

P: 0.025% or less P is an impurity unavoidably added in steel and is an element that lowers low temperature toughness and increases temper embrittlement susceptibility. Therefore, it is preferable to control the content as low as possible, and in the present invention, the content of P is controlled to 0.025% or less.

S: 0.025% or less S is also an impurity inevitably contained in steel, and is an element that lowers low temperature toughness and forms MnS inclusions to inhibit the toughness of steel. Therefore, it is preferable to control the content as low as possible, and in the present invention, the content of S is controlled to 0.025% or less.

In addition to the alloy composition described above, the steel sheet of the present invention additionally has Ti: 0.003 to 0.015%, Nb: 0.005 to 0.025%, and Ta: 0.002 to 0.050. It is preferable to include two or more kinds selected from the group consisting of%.

Ti: 0.003 to 0.015%
Ti is an element effective in forming fine carbides or nitrides to prevent softening of the matrix structure. In the present invention, it is necessary to add 0.003% or more in order to obtain such an effect, but since it is an expensive element, it is preferable to limit the upper limit to 0.015%.

Nb: 0.005 to 0.025%
Nb is an element effective in forming fine carbides or nitrides to prevent softening of the matrix structure. In the present invention, it is necessary to add 0.005% or more in order to obtain such an effect, but since it is an expensive element, it is preferable to limit the upper limit to 0.025%.

Ta: 0.002 to 0.050%
Ta is an element that is effective in forming fine carbides or nitrides to prevent softening of the matrix structure. In the present invention, it is necessary to add 0.002% or more in order to obtain such an effect, but since it is an expensive element, it is preferable to limit the upper limit to 0.050%.

The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may be unavoidably mixed from the raw material or the surrounding environment, and this cannot be excluded. Since such impurities can be understood by any engineer in a normal manufacturing process, all the contents thereof are not specifically described in the present specification.

The fine structure of the steel sheet for a pressure vessel of the present invention is composed of a mixed structure containing one or two of ferrite, tempered bainite, pearlite, and digenerated pearlite, and by ensuring such a fine structure. Excellent strength and low temperature impact toughness can be ensured even after PWHT.

At this time, the fraction of the tempered bainite is preferably 5 to 50 area%. PWHT resistance can be improved by securing 5 area% or more of tempered bainite in the mixed structure. However, if it exceeds 50 area%, there may be a problem that the strength increases too much. Therefore, the fraction of the tempered bainite is more preferably in the range of 5 to 50 area%.

Further, the steel sheet of the present invention contains MX [(M = Ti, Nb, Ta), [X = N, C]] type precipitates having an average size of 5 to 80 nm inside the crystal grains at a volume fraction of 0. It preferably contains 003 to 0.15%. PWHT resistance can be further improved through the control of precipitates as described above. If the size of the precipitate is less than 5 nm, it may be difficult to secure the target strength. On the other hand, if it exceeds 80 nm, there is a possibility that the impact toughness is lowered. Further, when the fraction of the precipitate is less than 0.003% by volume, the effect of improving the strength may not be sufficient, and when it exceeds 0.15% by volume, there is a drawback that the impact toughness is lowered. There is.

On the other hand, the size of the precipitate means the equivalent circular diameter of the particles detected by observing the cross section of the steel sheet in the thickness direction.

The steel sheet for a pressure vessel of the present invention described above can be manufactured by various methods, and the manufacturing method is not particularly limited. However, as a preferable example, the following method can be applied.

Hereinafter, an embodiment of the method for manufacturing a steel sheet for a pressure vessel of the present invention will be described.

First, the steel slab having the above-mentioned alloy composition is reheated at 950 to 1200 ° C. When the reheating temperature is less than 950 ° C., it is difficult to dissolve the solute atom. On the other hand, if the temperature exceeds 1200 ° C., the austenite crystal grain size may become excessively coarse and impair the properties of the steel sheet.

The reheated steel slab is hot-rolled at a reduction rate of 2.5 to 30% per pass to obtain a hot-rolled steel sheet. If the reduction rate per pass is less than 2.5%, the reduction amount may be insufficient and internal defects may occur, and if it exceeds 30%, the reduction capacity of the equipment may be exceeded.

The hot-rolled steel sheet is subjected to normalizing heat treatment at 820 to 930 ° C. for 1.3 × t + (10 to 30 minutes) (where t is the thickness (mm) of the steel sheet). When the normalizing heat treatment temperature is less than 820 ° C., it becomes difficult to re-solidify the solid solute element, and it becomes difficult to secure the strength. On the other hand, when the temperature exceeds 930 ° C., the growth of crystal grains occurs and the low temperature toughness is inhibited. Further, if the maintenance time is less than 1.3 × t + 10 minutes), the homogenization of the tissue may not be sufficient, and if it exceeds 1.3 × t + 30 minutes, productivity may be impaired.

In the temperature interval from the normalizing temperature range to 450 ° C., (1/4) t (where t is the thickness (mm) of the steel sheet) of the hot-rolled steel sheet that has undergone the normalizing heat treatment is defined as 0. Cool at a cooling rate of 5 to 30 ° C./s. When the cooling rate is less than 0.5 ° C./s, proper bainite transformation becomes difficult and it becomes difficult to secure the strength. On the other hand, when the temperature exceeds 20 ° C./s, a fine structure having an excessive bainite fraction is obtained, so that an excessive tensile strength can be obtained. In addition, the low temperature toughness may decrease.

The cooled hot-rolled steel sheet is tempered at 550 to 680 ° C. for 1.6 × t + (10 to 30 minutes) (where t is the thickness (mm) of the steel sheet). When the tempering heat treatment temperature is less than 550 ° C., it becomes difficult to precipitate fine precipitates, and it becomes difficult to secure the strength. On the other hand, above 680 ° C., precipitation growth occurs, which inhibits strength and low temperature toughness. Further, if the maintenance time during the tempering heat treatment is less than 1.6 × t + 10 minutes, the homogenization of the structure may not be sufficient, and if it exceeds 1.6 × t + 30 minutes, productivity may be impaired. There is.

On the other hand, the steel sheet for a pressure vessel of the present invention manufactured through the above heat treatment step requires a PWHT treatment for removing residual stress or the like by a welding step added at the time of manufacturing the pressure vessel. Generally, the strength and toughness deteriorate after a long period of PWHT heat treatment. On the other hand, the steel sheet produced by the present invention has an advantage that it can be welded without a significant decrease in strength and toughness even after heat treatment in a temperature range of 580 to 650 ° C., which is a normal PWHT condition. is there. As an example, the steel sheet for a pressure vessel of the present invention has a tensile strength of 590 MPa or more even after 10 hours of post-welding heat treatment (Post Weld Heat Treatment, PWHT) in a temperature range of 580 to 650 ° C, and has a Charpy impact at −50 ° C. The energy value can be 150 J or more.

Hereinafter, the present invention will be described in more detail with reference to examples. However, it should be noted that the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of rights of the present invention. This is because the scope of rights of the present invention is determined by the matters stated in the claims and the matters reasonably inferred from the matters.

(Example)
After reheating the steel slab having the alloy composition shown in Table 1 below at 1140 ° C. for 300 minutes and hot rolling in the recrystallization region under the condition of a reduction rate of 10 to 15% per pass, a hot-rolled steel sheet is obtained. The hot-rolled steel sheet is subjected to normalizing heat treatment at 890 ° C. for 1.3 × t + 20 minutes (where t is the thickness (mm) of the steel sheet). Next, the hot-rolled steel sheet that has undergone the normalizing heat treatment is based on (1/4) t (where t is the thickness (mm) of the steel sheet) in the temperature interval from the normalizing temperature range to 450 ° C. After cooling under the conditions shown in Table 2 below, a steel sheet for a pressure vessel was manufactured by tempering heat treatment at 650 ° C. for 1.6 × t + 20 minutes (where t is the thickness (mm) of the steel sheet).

After observing the microstructure of the manufactured steel plate and performing PHWT heat treatment, the yield strength, tensile strength, elongation, and low temperature impact toughness were measured, and the results are shown in Table 2 below. For reference, the remaining microstructures other than tempered bainite in Table 2 below are ferrite and pearlite. The fraction of the precipitate is MX [(M = Ti, Nb, Ta), [X =) located inside the crystal grains of the mixed structure of ferrite, pearlite, and tempered bainite and having an average size of 5 to 80 nm. N, C]] means the volume fraction of type precipitates. The low temperature impact toughness is a Charpy impact energy value obtained by performing a Charpy impact test on a test piece having a V notch at −50 ° C.

As shown in Tables 1 and 2 above, in the case of the invention steels 1 to 3 satisfying the alloy composition and production conditions of the present invention, even if the PWHT time reaches 10 hours, the mechanical strength and low temperature impact toughness are mechanical. It can be seen that the level is excellent in physical properties.

On the other hand, in the case of the comparative steel 1 which does not satisfy the alloy composition of the present invention, even if the production conditions of the present invention are satisfied, the tensile strength is about 70 MPa and the low temperature impact toughness is about 150 J as compared with the invention steels 1 to 3. It can be seen that the level is lower than that.

V: 0.03 to 0.10%
V is an element capable of easily forming fine carbides and nitrides, and the above effect cannot be obtained unless 0.03% or more is added. However, since it is expensive, the upper limit is preferably limited to 0.10%.

In the temperature interval from the normalizing temperature range to 450 ° C., (1/4) t (where t is the thickness (mm) of the steel sheet) of the hot-rolled steel sheet that has undergone the normalizing heat treatment is defined as 0. Cool at a cooling rate of 5 to 30 ° C./s. When the cooling rate is less than 0.5 ° C./s, proper bainite transformation becomes difficult and it becomes difficult to secure the strength. On the other hand, when the temperature exceeds 30 ° C./s, a fine structure having an excessive bainite fraction is obtained, so that an excessive tensile strength can be obtained. In addition, the low temperature toughness may decrease.

After observing the microstructure of the manufactured steel plate and performing PWHT heat treatment, the yield strength, tensile strength, elongation, and low temperature impact toughness were measured, and the results are shown in Table 2 below. For reference, the remaining microstructures other than tempered bainite in Table 2 below are ferrite and pearlite. The fraction of the precipitate is MX [(M = Ti, Nb, Ta), [X =) located inside the crystal grains of the mixed structure of ferrite, pearlite, and tempered bainite and having an average size of 5 to 80 nm. N, C]] means the volume fraction of type precipitates. The low temperature impact toughness is a Charpy impact energy value obtained by performing a Charpy impact test on a test piece having a V notch at −50 ° C.

Claims (4)

By weight%, C: 0.12 to 0.20%, Si: 0.30 to 0.40%, Mn: 1.50 to 1.70%, Mo: 0.03 to 0.10%, Cu: 0.05 to 0.30%, V: 0.03 to 0.10%, Ni: 0.03 to 0.25%, Cr: 0.03 to 0.25%, Al: 0.005 to 0. Includes 06%, Ca: 0.0005 to 0.0030%, P: 0.025% or less, S: 0.025% or less, and additionally Ti: 0.003 to 0.015%, Nb: 0 Contains two or more selected from the group consisting of .005 to 0.025% and Ta: 0.002 to 0.050%, the balance consisting of Fe and other unavoidable impurities
The microstructure consists of a mixed structure containing one or two of ferrite, tempered bainite, pearlite, and digenerated pearlite.
A steel sheet for a pressure vessel having excellent tensile strength and low-temperature impact toughness, wherein the tempered bainite has a fraction of 5 to 50 area%. The steel sheet for a pressure vessel contains 0.003 to 0.15% by volume of MX [(M = Ti, Nb, Ta), [X = N, C]] type precipitates having an average size of 5 to 80 nm. The steel sheet for a pressure vessel having excellent tensile strength and low-temperature impact toughness according to claim 1. The pressure vessel steel plate has a tensile strength of 590 MPa or more and a Charpy impact energy value of 150 J or more at −50 ° C. even after 10 hours of post-welding heat treatment (Post Weld Heat Treatment, PWHT) in a temperature range of 580 to 650 ° C. The steel plate for a pressure vessel having excellent tensile strength and low-temperature impact toughness according to claim 1, wherein the steel plate is characterized by the above. By weight%, C: 0.12 to 0.20%, Si: 0.30 to 0.40%, Mn: 1.50 to 1.70%, Mo: 0.03 to 0.10%, Cu: 0.05 to 0.30%, V: 0.03 to 0.10%, Ni: 0.03 to 0.25%, Cr: 0.03 to 0.25%, Al: 0.005 to 0. Includes 06%, Ca: 0.0005 to 0.0030%, P: 0.025% or less, S: 0.025% or less, and additionally Ti: 0.003 to 0.015%, Nb: 0 A steel slab containing two or more selected from the group consisting of .005 to 0.025% and Ta: 0.002 to 0.050%, with the balance consisting of Fe and other unavoidable impurities at 950-1200 ° C. The stage of reheating and
The step of hot-rolling the reheated steel slab at a reduction rate of 2.5 to 30% per pass to obtain a hot-rolled steel sheet, and
The step of normalizing and heat-treating the hot-rolled steel sheet at 820 to 930 ° C. at 1.3 × t + (10 to 30 minutes) (where t is the thickness (mm) of the steel sheet).
The hot-rolled steel sheet that has undergone the normalizing heat treatment is 0 in the temperature interval from the normalizing temperature range to 450 ° C. based on (1/4) t (where t is the thickness (mm) of the steel sheet). .The stage of cooling at a cooling rate of 5 to 30 ° C / s and
Includes a step of tempering the cooled hot-rolled steel sheet at 550 to 680 ° C. for 1.6 × t + (10 to 30 minutes) (where t is the thickness (mm) of the steel sheet). A method for producing a steel sheet for a pressure vessel, which is characterized by excellent tensile strength and low temperature impact toughness.