KR20150073024A - Steel plate for pressure vessel having excellent strength and toughness after post welding heat treatment and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel sheet for a pressure vessel having excellent strength and toughness after post welding heat treatment (PWHT) and a manufacturing method thereof. The steel sheet for a pressure vessel having excellent strength and toughness after PWHT comprises: 0.05 to 0.2 wt% of C, 0.15 to 0.5 wt% of Si, more than 1.6 wt% to 2.0 wt% or less of Mn, 0.030 wt% or less of P, 0.030 wt% or less of S, 0.015 to 0.05 wt% of Al, 0.005 to 0.05 wt% of Cr, 0.005 to 0.1 wt% of Mo, 0.005 to 0.08 wt% of V, 0.001 to 0.05 wt% of Ti, 0.05 to 0.6 wt% of Ni, more than 0.35 wt% to 0.65 wt% or less of Cu, and the remainder consisting of Fe and other inevitable impurities, wherein the steel sheet includes a mixed structure of polygonal ferrite and bainitic ferrite in a pillar form, and has a fine structure including carbonitride including one or more kinds of Cr, Mo, Ti, and V, 0.02 volume% or more. According to the present invention, it is possible to provide a steel sheet for a pressure vessel capable of securing an excellent mechanical property since strength and toughness do not deteriorate even after PWHT for about 50 hours while having a strength of about 600 MPa or higher.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate for pressure vessels having excellent strength and toughness after PWHT,

The present invention relates to a steel sheet for a pressure vessel excellent in strength and toughness after PWHT and a method of manufacturing the same, and more particularly, to a steel sheet for a pressure vessel excellent in strength and toughness after PWHT, Which is excellent in strength and toughness after heat treatment (PWHT), and a method for producing the same.

In recent years, due to the scarcity of petroleum and the trend of oil development in harsh environments due to the high oil price era, refining and storage steel for crude oil are being reused.

In order to prevent the deformation of the post-weld structure and to stabilize the shape and the dimension in the case of welding the steel material in addition to the post-welding of the steel material as described above, post-welding heat treatment (PWHT, Post Weld Heat Treatment). However, the steel material subjected to the PWHT process for a long time has a problem that the tensile strength is lowered due to the coarsening of the microstructure.

That is, after the PWHT for a long time, strength and toughness are simultaneously lowered due to softening of matrix and grain boundaries, grain growth, coarsening of carbide, and the like.

As a typical technique for preventing deterioration of physical properties due to the above-described long-time PWHT heat treatment, JP-A-2001-328701 has been proposed. The above-described technique is characterized by comprising, by weight, 0.05 to 0.20% of C, 0.02 to 0.5% of Si, 0.2 to 2.0% of Mn, 0.005 to 0.10 of Al, balance Fe and unavoidable impurities, , A slab further comprising at least one of Cr, Mo, V, Nb, Ti, B, Ca and rare earth elements is heated and hot rolled, and then air-cooled at room temperature and heated at Ac1 to Ac3 transformation point To a PWHT guaranteed time of up to 16 hours by a slow cooling process.

However, the PWHT guarantee time shown in the above-mentioned technique is very insufficient when the post-welding and welding conditions are severe, and it is impossible to apply the PWHT for a longer period of time.

Accordingly, there is a demand for a steel material having a high resistance to PWHT which does not deteriorate in strength and toughness after prolonged PWHT even if the post-treatment of the steel material and the welding conditions are severe.

Japanese Patent Application Laid-Open No. 09-256037

An object of the present invention is to provide a steel sheet for a pressure vessel excellent in strength and toughness after PWHT which can prevent deterioration of strength and toughness even after a long post-welding heat treatment (PWHT), and a method of manufacturing the same.

An embodiment of the present invention is a ferritic stainless steel comprising 0.05 to 0.2% of C, 0.15 to 0.5% of Si, more than 1.6 to 2.0% of Mn, not more than 0.030% of P, not more than 0.030% of S, 0.001 to 0.05% of Cr, 0.005 to 0.05% of Cr, 0.005 to 0.1% of Mo, 0.005 to 0.08% of V, 0.001 to 0.05% of Ti, 0.05 to 0.6% of Ni, 0.35 to 0.65% of Cu, Fe and other unavoidable impurities, and contains a polygonal ferrite + baynitic ferrite mixed structure as a main phase and a carbonitride containing at least 0.02% by volume of carbonitride containing at least one of Cr, Mo, Ti and V A steel sheet for a pressure vessel excellent in strength and toughness after PWHT having a microstructure is provided.

Another embodiment of the present invention is a ferritic stainless steel comprising 0.05 to 0.2% of C, 0.15 to 0.5% of Si, more than 1.6 to 2.0% of Mn, not more than 0.030% of P, not more than 0.030% of S, 0.001 to 0.05% of Cr, 0.005 to 0.05% of Cr, 0.005 to 0.1% of Mo, 0.005 to 0.08% of V, 0.001 to 0.05% of Ti, 0.05 to 0.6% of Ni, 0.35 to 0.65% of Cu, Reheating the steel slab containing Fe and other unavoidable impurities at 1050 to 1250 占 폚; Hot-rolling the reheated steel slab to a cumulative rolling reduction of 30% or more in a temperature range of Ar3 to Tnr to obtain a hot-rolled steel sheet; Cooling the hot-rolled steel sheet to 450 to 550 ° C at a cooling rate of 2 to 30 ° C / s based on 1/4 thickness of the steel sheet; Reheating the cooled hot-rolled steel sheet to (Ac1 + 30 占 폚) to (Ac3-50 占 폚); And secondarily cooling the reheated hot-rolled steel sheet to a temperature of 400 ° C or less at a cooling rate of 5 to 100 ° C / s. The present invention also provides a method for manufacturing a steel sheet for a pressure vessel excellent in strength and toughness after PWHT.

According to the present invention, it is possible to provide a steel sheet for a pressure vessel that has strength of 600 MPa or more and does not deteriorate strength and toughness even after 50 hours of post-weld heat treatment (PWHT), thereby securing excellent mechanical properties. In addition, the normalizing heat treatment included in the conventional manufacturing method can be omitted, and a steel sheet for a pressure vessel can be produced more economically.

Hereinafter, the present invention will be described. % Of the alloy composition described below means% by weight.

C: 0.05 to 0.2%

C is an element for improving the strength. When the content is less than 0.05%, the strength of the matrix is deteriorated. When the content exceeds 0.20%, there is a problem of deterioration in weldability. Therefore, C is preferably in the range of 0.05 to 0.2%.

Si: 0.15-0.5%

Si is an effective element for deoxidation and solid solution strengthening, and is an element added for the effect of increasing the impact transition temperature. In order to achieve such an effect, 0.15% or more should be added, but when it is added in excess of 0.5%, the weldability is lowered and the oxide film is formed on the surface of the steel sheet. Therefore, the Si content is preferably 0.15 to 0.50%.

Mn: not less than 1.6% and not more than 2.0%

Mn is an element which is advantageous for securing strength, and for the above effect, it is preferable that Mn is added in a range exceeding 1.6%. However, if it exceeds 2.0%, there is a problem that MnS which is a non-metallic inclusion drawn together with S is formed to lower the room temperature elongation and the low temperature toughness. Therefore, it is preferable that the Mn is satisfied in a range of more than 1.6% to 2.0% or less.

P: not more than 0.030%

P is an element that is inevitably contained in the production process and deteriorates the low-temperature toughness, so it is preferable to control the content of P to be as low as possible. However, since excessive cost is required to remove the P in a very low content in the steelmaking process, it is preferable to control the P within a range of 0.030% or less.

S: not more than 0.030%

S is also an element which is inevitably contained in the manufacturing process and adversely affects the low temperature toughness. However, as in the case of P, excessive cost is required to remove the S in a very low content in the steelmaking process. .

Al: 0.015 to 0.05%

Al is an element that is used as a strong deoxidizer in the steelmaking process together with the Si. However, when the content of Al is 0.015%, the effect of deoxidation is insignificant. When the content of Al exceeds 0.05%, the deoxidation effect is saturated and the manufacturing cost is increased. Therefore, it is preferable that the above Al satisfies a range of 0.015 to 0.05%.

Cr: 0.005 to 0.05%

Cr is an element which increases the strength by forming carbonitride. In the present invention, it is preferable that Cr is added in an amount of 0.005% or more for the purpose of increasing the strength. However, Cr is an expensive element, and when it is added in an amount exceeding 0.05%, the production cost is increased. Therefore, the Cr content is preferably in the range of 0.005 to 0.05%.

Mo: 0.005 to 0.1%

Like Cr, Mo is not only an element effective for increasing the strength, but also an element preventing the occurrence of cracks due to sulfides. For the above effect, the Mo content should be 0.005% or more, but Mo is also an expensive element, leading to an increase in production cost, and therefore, it is preferable that the Mo content is 0.1% or less. Therefore, the Mo content is preferably in the range of 0.005 to 0.1%.

V: 0.005 to 0.08%

V is a very favorable element for increasing the strength by forming carbonitride. For the above effect, the V should be added in an amount of 0.005% or more, but it is preferably added in an amount of 0.08% or less in view of the high cost. Therefore, it is preferable that V is in the range of 0.005 to 0.08%.

Ti: 0.001 to 0.05%

Ti, like V, is an important element for increasing the strength by forming carbonitride. For the above effect, the Ti should be added in an amount of 0.001% or more, but if it exceeds 0.05%, the Ti may appear as a coarse precipitate in the continuous casting process, resulting in a decrease in toughness. Therefore, the Ti content is preferably in the range of 0.001 to 0.05%.

Ni: 0.05 to 0.6%

Ni is the most effective element for improving the low-temperature toughness. For the above effect, it is preferable that Ni is added in an amount of 0.05% or more. However, since the Ni is an expensive element, when it exceeds 0.6%, the production cost is increased. Therefore, the Ni is preferably in the range of 0.05 to 0.6%.

Cu: more than 0.35% to less than 0.65%

Cu is an element effective for improving the strength and is preferably added in an amount exceeding 0.35% for the above effect. However, since Cu is an expensive element, it leads to an increase in production cost, and therefore it is preferable to add Cu at 0.65% or less. Therefore, it is preferable that the Cu has a range of more than 0.35% to 0.65% or less.

The remainder of the present invention is Fe. However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

On the other hand, the steel sheet provided by the present invention preferably has a microstructure containing the polygonal ferrite + bainite ferrite mixed structure as a main phase, satisfying the above-mentioned alloy composition, and thereby securing excellent strength and toughness even after PWHT can do. In this case, it is preferable that the polygonal ferrite is 40 to 70% by area and the bainitic ferrite is 25 to 60% by area. When the polygonal ferrite is 40% by area or less or the bainitic ferrite is more than 60% Toughness may be rapidly deteriorated. When the polygonal ferrite is more than 70% by area or the bainitic ferrite is less than 25% by area, excellent strength can not be secured. On the other hand, the bainitic ferrite may include an accicular ferrite and bainite. In addition, the microstructure of the present invention may inevitably be formed with a normal second phase structure, wherein the second phase structure is preferably controlled to 5% or less by area. The second phase structure may include microstructures such as pearlite, martensite, and graphite martensite.

In addition, the steel sheet of the present invention preferably contains 0.02% or more by volume of carbonitride containing at least one of Cr, Mo, Ti, and V, thereby securing excellent strength and low temperature toughness. The greater the amount of the carbonitride is, the more advantageous the effect to be obtained by the present invention is, but it is difficult to exceed 5% by volume of the carbonitrided product at the normal level.

At this time, the carbonitride preferably has a size of 50 nm or less. By forming the fine carbonitride in this way, excellent strength and toughness can be secured even after PWHT. If the carbonitride is formed of coarse carbonitride in excess of 50 nm, the strength and toughness may be lowered.

The steel sheet of the present invention provided as described above has an excellent tensile strength of 600 MPa or more after heat treatment at 590 to 640 ° C for 25 to 50 hours and has an impact toughness of 50 J or more at -49 ° C, And can be suitably applied to a petrochemical production facility, a storage tank, a heat exchanger, a reaction furnace, and a condenser in which the mechanical properties are required.

Hereinafter, the production method of the present invention will be described.

First, the steel slab satisfying the alloy composition described above is reheated at 1050 to 1250 占 폚. When the heating temperature is lower than 1050 DEG C, solid solute atoms are difficult to solidify, and when the heating temperature exceeds 1250 DEG C, the austenite grain size becomes too coarse, thereby deteriorating the physical properties of the steel sheet.

Thereafter, the reheated steel slab is hot-rolled in the Ar3 to Tnr temperature range to obtain a hot-rolled steel sheet. The austenite can be pancake-formed by hot rolling in the temperature range described above, and the resulting polygonal ferrite and bainitic ferrite can be made finer, thereby ensuring excellent strength and toughness at the same time. When the hot rolling temperature exceeds Tnr (boundary temperature between the austenite non-recrystallized zone and the recrystallization zone), the crystal grains become coarse due to the recrystallization of austenite, and coarse microstructure appears after cooling, It becomes difficult. On the other hand, when the hot rolling temperature is lower than Ar 3, the hot rolling property may become difficult due to the low temperature, and thus quality defects may occur. Therefore, it is preferable that the hot rolling temperature has a range of Ar3 to Tnr. On the other hand, the Tnr temperature can be calculated from the following equation.

Tnr (占 폚) = 887 + 464 占 C + 890 占 Ti + 363 占 Al-357 占 Si + (732 占 V-230 占 V1 ^{/ 2} )

Further, when the cumulative rolling reduction is less than 30%, the nucleation sites of the polygonal ferrite and the bainitic ferrite become insufficient and the size of the crystal grains becomes 25 Lt; RTI ID = 0.0 > um. ≪ / RTI >

In addition, it is preferable that a reduction ratio of 10% or more is applied per pass during the hot rolling. By this, the austenite structure is stretched in the rolling direction to form a deformation band inside to obtain fine polygonal ferrite and bainitic ferrite. If the reduction ratio per pass is less than 10%, the nucleation sites of the polygonal ferrite and the bainitic ferrite may be insufficient and the grain size may become large.

Thereafter, the hot-rolled steel sheet obtained as described above is first cooled to 450 to 550 占 폚 at a cooling rate of 2 to 30 占 폚 / s based on 1/4 thickness of the steel sheet. When the cooling rate is less than 2 DEG C / s, the crystal grains of the ferrite are coarse and the fraction of the polygonal ferrite is increased. When the cooling rate is more than 30 DEG C / s, the fraction of bainitic ferrite or the second phase structure is increased It may be difficult to secure the microstructure to be obtained in the present invention.

Subsequently, the primary cooled hot-rolled steel sheet is reheated to (Ac1 + 30 deg. C) to (Ac3-50 deg. C). When the reheating temperature is lower than (Ac1 + 30 deg. C), the toughness improving effect is not sufficient. When the reheating temperature is higher than (Ac3-50 deg. C), the toughness improving effect is sufficiently obtained but the strength is lowered.

The reheated hot-rolled steel sheet is secondarily cooled to a temperature of 400 ° C or less at a cooling rate of 5 to 100 ° C / s. If the cooling rate is less than 5 ° C / s, mechanical properties such as strength and toughness may be deteriorated. The faster the cooling rate, the better the effect to be obtained by the present invention, but it is difficult to exceed 100 ° C / s due to limitations in the production facility.

According to the manufacturing method of the present invention provided as described above, the normalizing heat treatment at a high temperature, which is conventionally used for manufacturing a steel material for a pressure vessel, can be omitted, and a steel sheet for a pressure vessel can be manufactured more economically .

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

(Example)

Hot-rolled steel sheets were prepared by using the steel slabs having the alloy compositions shown in the following Table 1 under the conditions shown in Table 2 below. The reheating temperature of the hot-rolled steel sheet was 740 ° C, the secondary cooling rate was 20 ° C / s, and the secondary cooling-stop temperature was 560 ° C. 350 ° C. However, in the case of Comparative Example 4, after the hot rolling, the steel sheet was subjected to a normalizing heat treatment at 910 ° C. for 1.3 × t + 20 minutes, followed by air cooling (where t is the thickness of the steel sheet). The microstructure and carbonitride fractions of the hot-rolled steel sheet thus prepared were observed, and the results are shown in Table 2 below. In addition, welding was performed under the conditions shown in Table 3, followed by heat treatment (PWHT), and mechanical properties were measured. The results are shown in Table 3 below.

division Alloy composition (% by weight) C Si Mn P S Al Cr Mo V Ti Ni Cu Inventive Steel 1 0.09 0.31 1.62 0.007 0.0015 0.0028 0.04 0.07 0.060 0.010 0.44 0.54 Invention river 2 0.10 0.35 1.71 0.008 0.0014 0.023 0.02 0.05 0.051 0.012 0.46 0.48 Invention steel 3 0.08 0.39 1.79 0.010 0.0012 0.021 0.01 0.06 0.039 0.017 0.47 0.36 Comparative River 1 0.10 0.24 1.21 0.007 0.0011 0.020 0.15 0.05 0.013 - 0.20 0.13

division Grade Nr. Reheat temperature
(° C) Cumulative reduction rate
(%) Primary cooling rate
(° C / s) Steel plate
thickness
(mm) Microstructure (area%) Carbonitride
Fraction
(volume%) PF BF Inventory 1 Inventive Steel 1 1150 75 7.5 50 49 50 0.073 Inventory 2 1100 70 6.5 60 52 46 0.064 Inventory 3 1180 55 4.8 80 46 52 0.056 Honorable 4 1200 40 3.5 100 50 48 0.059 Inventory 5 Invention river 2 1100 75 7.5 50 41 58 0.102 Inventory 6 1150 70 6.5 60 41 56 0.070 Honorable 7 1200 55 4.8 80 46 50 0.085 Honors 8 1200 40 3.5 100 43 55 0.096 Proposition 9 Invention steel 3 1100 75 7.5 50 55 43 0.082 Inventory 10 1150 70 6.5 60 56 42 0.067 Exhibit 11 1200 55 4.8 80 57 42 0.076 Inventory 12 1200 40 3.5 100 60 38 0.064 Comparative Example 1 Comparative River 1 1100 75 1.2 50 72 24 0.011 Comparative Example 2 1100 70 0.7 80 71 26 0.006 Comparative Example 3 1180 55 1.5 100 74 25 0.013 Comparative Example 4 1150 60 Air cooling 80 99 0 - However, PF is polygonal ferrite and BF is bainitic ferrite.

division PWHT temperature
(° C) PWHT time
(Hr) Yield strength
(MPa) The tensile strength
(MPa) Shock Toughness (@ -49 ℃)
(J) Inventory 1 620 25 524 626 180 Inventory 2 620 50 525 623 165 Inventory 3 610 25 519 627 179 Honorable 4 610 50 520 625 143 Inventory 5 620 25 522 631 171 Inventory 6 620 50 516 630 153 Honorable 7 610 25 517 627 166 Honors 8 610 50 519 628 185 Proposition 9 620 25 528 619 173 Inventory 10 620 50 519 616 181 Exhibit 11 610 25 523 612 177 Inventory 12 610 50 520 609 162 Comparative Example 1 620 20 504 563 37 Comparative Example 2 620 25 503 565 29 Comparative Example 3 620 50 501 557 21 Comparative Example 4 620 50 471 539 18

As can be seen from Tables 1 to 3, in the case of Inventive Examples 1 to 12 satisfying the alloy composition and manufacturing conditions proposed by the present invention, by securing the microstructure and the fraction of the carbonitride to be obtained according to the present invention, It can be seen that strength and toughness are very good even after reaching ~ 50 hours.

On the other hand, in the case of Comparative Examples 1 to 3, since the alloy composition and the manufacturing conditions proposed by the present invention were not satisfied, the microstructure and the fraction of the carbonitride to be obtained in the present invention could not be secured, It can be seen that all of the humanities are lower than those of the present invention.

Comparative Example 4 shows that when the general rolling and cooling conditions are applied, the microstructure and the fraction of the carbonitride to be obtained by the present invention can not be ensured, and both the strength and impact toughness after PWHT are very low have.

Claims (6)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.2% of C, 0.15 to 0.5% of Si, more than 1.6 to 2.0% of Mn, 0.030 to less than 0.05% of S, 0.030 to less than 0.05% 0.001 to 0.05% of Mo, 0.005 to 0.1% of Mo, 0.005 to 0.08% of V, 0.001 to 0.05% of Ti, 0.05 to 0.6% of Ni, more than 0.35 to 0.65% of Cu and the balance Fe and other unavoidable impurities In addition,
A polygonal ferrite + bainite ferrite mixed structure as a main phase,
A steel plate for a pressure vessel excellent in strength and toughness after PWHT having a microstructure containing at least 0.02% by volume of carbonitride containing at least one of Cr, Mo, Ti and V. The method according to claim 1,
Wherein the polygonal ferrite is 40 to 70% by area and the bainitic ferrite is 25 to 60% by area. The method according to claim 1,
Wherein the carbonitride is excellent in strength and toughness after PWHT having a size of 50 nm or less. The method according to claim 1,
The steel sheet is excellent in strength and toughness after PWHT having a tensile strength of 600 MPa or more and an impact toughness of 50 J or more at -49 캜 after heat treatment at 590 to 640 캜 for 25 to 50 hours. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.2% of C, 0.15 to 0.5% of Si, more than 1.6 to 2.0% of Mn, 0.030 to less than 0.05% of S, 0.030 to less than 0.05% 0.001 to 0.05% of Mo, 0.005 to 0.1% of Mo, 0.005 to 0.08% of V, 0.001 to 0.05% of Ti, 0.05 to 0.6% of Ni, more than 0.35 to 0.65% of Cu and the balance Fe and other unavoidable impurities Reheating the steel slab at 1050 to 1250 占 폚;
Hot-rolling the reheated steel slab to a cumulative rolling reduction of 30% or more in a temperature range of Ar3 to Tnr to obtain a hot-rolled steel sheet;
Cooling the hot-rolled steel sheet to 450 to 550 ° C at a cooling rate of 2 to 30 ° C / s based on 1/4 thickness of the steel sheet;
Reheating the primary cooled hot-rolled steel sheet to (Ac1 + 30 占 폚) to (Ac3-50 占 폚); And
And secondarily cooling the reheated hot-rolled steel sheet to a temperature of 400 ° C or less at a cooling rate of 5 to 100 ° C / s. The method for producing a steel sheet for a pressure vessel excellent in strength and toughness after PWHT. The method of claim 5,
A method for producing a steel sheet for a pressure vessel, which is excellent in the strength and toughness after PWHT, which has a reduction ratio of 10% or more per pass in the hot rolling.