JP5657026B2 - High-strength steel sheet with excellent post-weld heat treatment resistance and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel sheet used in a crude oil refining facility, a storage tank, a heat exchanger, a reaction furnace, a condenser, etc. in a wet hydrogen sulfide environment, and more specifically, it is strong even after a post-weld heat treatment (PWHT). And a steel plate excellent in toughness and a method for producing the same.

  Recently, with the trend of oil shortage and high oil prices, oil fields with poor environment have been actively developed, and thickening has been performed on steel for refinement and storage of crude oil.

  In addition to the thickening of steel materials as described above, welding is performed to remove stress generated during welding for the purpose of preventing deformation of the structure after welding and stabilizing the shape and dimensions when welding steel materials. Post heat treatment (PWHT, Post Weld Heat Treatment) is performed. However, a steel sheet that has been subjected to a long-term PWHT process has a problem that the tensile strength of the steel sheet decreases due to the coarsening of the structure.

  That is, after PWHT for a long period of time, a phenomenon in which strength and toughness simultaneously decrease due to softening of the matrix structure (Matrix) and crystal grain boundaries, crystal grain growth, coarsening of carbides, and the like.

  As means for preventing deterioration of physical properties due to the above-mentioned long-term PWHT heat treatment, in Patent Document 1, the weight percentage is C: 0.05 to 0.20%, Si: 0.02 to 0.5%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.10%, if necessary, one of Cu, Ni, Cr, Mo, V, Nb, Ti, B, Ca, rare earth elements or After heating and hot rolling a slab containing two or more types, the balance being iron and inevitable impurities, air cooling at room temperature, and maintaining at PWHT up to 16 hours by heating at the Ac1 to Ac3 transformation points and gradually cooling I was able to do it.

  However, the PWHT holding time shown in the above technique is very short when the conditions for thickening and welding are severe, and there is a problem that it is impossible to apply PWHT for a longer period of time.

  Accordingly, as the steel material becomes thicker and the conditions of the weld zone become severe, there is a demand for a steel material having high resistance to PWHT in which strength and toughness do not decrease even after long-term PWHT.

Japanese Patent Laid-Open No. 1997-256037

  One aspect of the present invention provides a high-strength steel sheet having excellent post-weld heat treatment (PWHT) resistance that does not decrease strength and toughness even after long-term post-weld heat treatment (PWHT) and a method for manufacturing the same. .

The present invention is mass %, C: 0.1-0.3%, Si: 0.15-0.50%, Mn: 0.6-1.2%, P: 0.08 % or less, S: 0.020% or less, Al: 0.001 to 0.05%, Cr: 0.01 to 0.35%, Mo: 0.005 to 0.2%, V: 0.005 to 0.05%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, Ca: 0.0005-0.005%, Ni: 0.05-0.5%, Cu: 0.00 005~0.5%, Co: 0.005~0.2% and W: 0.005 to 0.2% is selected from the group consisting of the one or more, and the remainder of Fe and unavoidable impurities, the The composition provides a high-strength steel sheet excellent in post-weld heat treatment resistance that satisfies the following relational expression.
Cu + Ni + Cr + Mo: 1.5% or less Cr + Mo: 0.4% or less V + Nb: 0.1% or less Ca / S: 1.0 or more

Further, step present invention to hot rolling at the stage and a temperature range of the re-heated steel slab _T nr _~T nr + 100 ℃ reheating the steel slab satisfying the above composition range in a temperature range of from 1,050 to 1250 ° C. And the above hot-rolled hot-rolled steel sheet is held at a temperature range of 850 to 950 ° C. for 1.3 × t + (10 to 30 minutes) (where t means the thickness (mm) of the steel material). Provided is a method for producing a high-strength steel plate having excellent post-weld heat treatment resistance, including a heat treatment step and a step of cooling the heat-treated steel plate at a cooling rate of 0.1 to 10 ° C./sec.

  According to the present invention, it is possible to provide a steel plate for a pressure vessel that has a strength of 500 MPa class or more, does not deteriorate in strength and toughness even after 100 hours of PWHT, and has excellent resistance to hydrogen-induced cracking.

  Hereinafter, the present invention will be described in detail.

First, the composition range of this invention is demonstrated in detail (henceforth, mass %).

  The content of carbon (C) is preferably limited to 0.1 to 0.3%. C is an element that improves the strength. If the content is less than 0.1%, the strength of the matrix phase decreases, and if it exceeds 0.3%, segregation occurs in the structure and decreases the resistance to hydrogen-induced cracking. There is a problem.

  The silicon (Si) content is preferably limited to 0.15 to 0.50%. Si is an element effective for deoxidation and solid solution strengthening, and is an element added for increasing the impact transition temperature. In order to achieve such an effect, 0.15% or more must be added. If it exceeds 0.5%, the weldability deteriorates and an oxide film is severely formed on the steel sheet surface. There is a problem that.

  The manganese (Mn) content is preferably limited to 0.6 to 1.2%. Since Mn forms MnS which is a non-metallic inclusion stretched with S and lowers the room temperature elongation and the low temperature toughness, it is preferably controlled to 1.2% or less. However, if Mn is added in an amount of less than 0.6% due to the characteristics of the present invention, it is difficult to ensure an appropriate strength, so the content is limited to 0.6 to 1.2%.

  The aluminum (Al) content is preferably limited to 0.001 to 0.5%. Al is one of the powerful deoxidizers in the steel making process together with Si. If less than 0.001%, the deoxidation effect is slight, and if added over 0.05%, the deoxidation effect is There is a problem that the manufacturing cost increases due to saturation.

Phosphorus (P) is an element that inhibits low-temperature toughness, and it takes a lot of money to remove it in the steelmaking process. Therefore, it is preferable to manage it within a range of 0.08 % or less.

  Sulfur (S) is an element that adversely affects low-temperature toughness together with P, and it can be costly to remove in the steel making process in the same manner as P. Therefore, it is preferable to manage it within a range of 0.020% or less.

  The content of chromium (Cr) is preferably limited to 0.01 to 0.35%. Cr is an element that increases the strength, and in the present invention, it must be added 0.01% or more for the effect of increasing the strength, but since it is an expensive element, if added over 0.35%, It causes an increase in manufacturing cost and is preferably controlled to 0.35% or less.

  The molybdenum (Mo) content is preferably limited to 0.005 to 0.2%. Like Cr, Mo is an element effective for increasing the strength, and is an element that prevents the occurrence of cracks due to sulfides. For the above effect, 0.005% or more must be added. However, Mo is also an expensive element and causes an increase in manufacturing cost, so it is preferable to limit it to 0.2% or less.

  The vanadium (V) content is preferably limited to 0.005 to 0.05%. V is an element effective for increasing the strength, such as Cr and Mo. Accordingly, if 0.005% or more is not added, the effect of increasing the strength cannot be achieved, but it is expensive, so 0.05% or less is preferably added.

  The content of niobium (Nb) is preferably limited to 0.001 to 0.05%. Nb is an important element that increases the strength by being dissolved in austenite to increase the hardening ability of austenite and precipitated as carbonitride (Nb (C, N)) consistent with the matrix (Matrix). . The above effect cannot be obtained unless the content is 0.001% or more, but if added in a large amount, coarse precipitates may be formed in the continuous casting process, which may serve as sites for hydrogen-induced cracking. The content is preferably limited to 0.05% or less.

  The content of titanium (Ti) is preferably limited to 0.001 to 0.05%. Ti is an important element that increases the strength by being precipitated as carbonitride (Ti (C, N)) in the same manner as Nb. The above effect cannot be obtained unless the content is 0.001% or more. However, if added in a large amount, coarse precipitates may be formed in the continuous casting process, which may serve as a site for hydrogen-induced cracking. Is preferably limited to 0.05% or less.

  The calcium (Ca) content is preferably limited to 0.0005 to 0.005%. Ca is generated in CaS and added to suppress non-metallic inclusions of MnS. For this purpose, 0.0005% or more must be added. However, if its content exceeds 0.005%, it reacts with O contained in the steel to produce CaO which is a non-metallic inclusion, so the upper limit is preferably limited to 0.005%. .

  The nickel (Ni) content is preferably limited to 0.05 to 0.5%. Ni is the most effective element for improving low-temperature toughness, and the above effect cannot be obtained unless the content is 0.05% or more. However, since it is an expensive element and causes an increase in manufacturing cost, 0.5% It is preferable to add to below%.

  The present invention includes one or more selected from the group consisting of Cu, Co and W in the above composition.

  The content of copper (Cu) is preferably 0.005 to 0.5%. Cu prevents strength deterioration even after PWHT heat treatment by strengthening the matrix structure by solid solution strengthening or e-Cu precipitation, and prevents strength and toughness deterioration by strengthening the base and suppressing recovery. However, since it is expensive, it is preferable to add the content within the range of 0.005 to 0.5%.

  The content of cobalt (Co) is preferably 0.005 to 0.2%. Co is an effective element for preventing the softening of the matrix structure, but is expensive, so it is preferably added in the range of 0.005 to 0.2%.

  The content of tungsten (W) is preferably 0.005 to 0.2%. W has the characteristics that it can form WC, decrease the cementite precipitation rate, prevent the growth / aggregation of cementite and prevent deterioration of strength and toughness, so 0.005% It is preferable to add more. However, since W is expensive, it is more preferable to add it in the range of 0.005 to 0.2%.

The steel material of the present invention can be used as a steel material for pressure vessels. Considering this, it is preferable that the content of elements such as Cu, Ni, Cr, Mo, V, and Nb below satisfy the following relationship.
Cu + Ni + Cr + Mo: 1.5% or less Cr + Mo: 0.4% or less V + Nb: 0.1% or less Ca / S: 1.0 or more

  That is, the relationship of Cu + Ni + Cr + Mo, Cr + Mo, and V + Nb is a numerical value restricted respectively in the basic standard (ASTM A20) of steel materials for pressure vessels, whereby the Cu + Ni + Cr + Mo content is 1.5% or less and the Cr + Mo content is 0.4%. Hereinafter, the V + Nb content is limited to 0.1% or less. However, alloy elements not included in the embodiment of the present invention can be calculated as zero.

Further, the Ca / S ratio is an essential component ratio for improving the resistance of hydrogen-induced cracking by spheroidizing MnS inclusions, and when the ratio is less than 1.0, the effect is difficult to expect. Adjust to 1.0 or higher .

  The remainder consists of Fe and inevitable impurities.

  Hereinafter, the microstructure of the present invention will be described in detail.

  When the steel having the above composition is subjected to controlled rolling and heat treatment more suitable for the process described later, the microstructure can be composed of ferrite or a mixed structure of ferrite and pearlite, and a low temperature structure can be formed in the structure. Is preferably not included, but up to 10% may include bainite. The reason why the structure is controlled to the above-described form is that it has excellent resistance to hydrogen-induced cracking, which is a target of the present invention, and has appropriate strength and toughness.

  Further, in order to ensure the resistance of hydrogen-induced cracking, a banding index (measured by ASTM E-1268) indicating how much a band structure vulnerable to hydrogen-induced cracking has been formed is 0. .25 or less is preferable. When the banding index value exceeds 0.25, the resistance of hydrogen-induced cracking rapidly decreases in the microstructure.

  The average size of the ferrite crystal grains in the central portion (3/8 to 5 / 8t, t: thickness of the steel plate) in the thickness direction of the steel plate is preferably 50 μm or less. This is because if the size of the ferrite crystal grains is too large, the strength and toughness may decrease. Although there is no lower limit to the size of the crystal grains, it is difficult to obtain crystal grains of less than 5 μm with the steel material targeted by the present invention, so the crystal grain size may be 5 μm or more.

  Below, the manufacturing method of this invention is demonstrated in detail.

  In the present invention, a steel slab satisfying the above composition range is reheated in a temperature range of 1050 to 1250 ° C. When the reheating temperature is lower than 1050 ° C., it is difficult to dissolve the solute atoms. When the reheating temperature is higher than 1250 ° C., the size of the austenite crystal grains becomes excessively large, which impairs the properties of the steel sheet.

  After the reheating, the present invention has a ferrite + pearlite two-phase composite structure for resistance to hydrogen-induced cracking, and a banding index (measured by ASTM E-1268) is 0.25 or less. Recrystallization controlled rolling, heat treatment, and PWHT heat treatment are required.

Recrystallization controlled rolling is performed by subjecting the reheated steel slab to hot rolling at a temperature higher than non-recrystallization. T _nr which is the non-recrystallization temperature can be calculated from the following equation.

Tnr (° C.) = 887 + 464 × C + 890 × Ti + 363 × Al-357 × Si + (6446 × Nb−644 × Nb ^1/2 ) + (732 × V−230 × V ^1/2 )

In order to reduce the banding index (measured by ASTM E-1268) value to 0.25 or less, the recrystallization controlled rolling is the most important variable, and the recrystallization controlled rolling is performed between T _{nr and} T _nr + 100 ° C. It is preferable that a rolling reduction rate of 10% or more is added to each rolling pass in the temperature range to give a cumulative rolling reduction rate of 30% or more. This is because when the cumulative rolling reduction is less than 30%, it is not possible to expect a banding index value of 0.25 or less. Moreover, the reason for limiting the temperature of the recrystallization controlled rolling is to control the banding index and to suppress the band structure in a state where the crystal grains are not coarsened. More specifically, if the temperature is lower than the recrystallization zone reference temperature (T _nr ), austenite is pancakeed and the banding index becomes high, which is not preferable. Conversely, if the temperature is too high, the size of the crystal grains becomes large. Since it becomes too much, it is not preferable.

  Thereafter, the hot rolling is performed, and the cooled hot-rolled steel sheet is heat-treated. The heat treatment is held in the temperature range of 850 to 950 ° C. under the conditions of 1.3 × t + (10 to 30 minutes) (where t means the thickness (mm) of the steel material). If the heat treatment temperature is less than 850 ° C., it is difficult to re-dissolve the solid solution solute element and it is difficult to ensure the strength. If the heat treatment temperature exceeds 950 ° C., crystal grain growth occurs and inhibits low temperature toughness. To do.

  The reason for limiting the heat treatment holding time is that when the holding time is shorter than 1.3 × t + 10 minutes (t means the thickness (mm) of the steel material), it is difficult to homogenize the structure. This is because when t + 30 minutes (t means the thickness (mm) of the steel material) is exceeded, productivity is hindered.

The held steel sheet is cooled at 0.1 to 120 ° C./sec with reference to the cooling rate at the center. If the cooling rate is lower than that, ferrite crystal grains may be coarsened during cooling, and if the cooling rate is higher than that, an excessive second phase (more than 10% bainite fraction) is likely to occur. is there.

  The cooling rate is to adjust the average crystal grain size of ferrite in the center of the steel sheet to 50 μm or less.

  The steel sheet of the present invention manufactured through the above heat treatment process needs PWHT treatment in order to remove residual stress by a welding process added at the time of manufacturing the pressure vessel. Generally, deterioration of strength and toughness occurs after long-term PWHT heat treatment, but the steel sheet produced according to the present invention has a normal PWHT temperature condition of 600 to 640 ° C. and a long period (up to 100 hours). Even if it is carried out, there is an advantage that welding can be performed without a great decrease in strength and toughness. In particular, the steel sheet of the present invention has a tensile strength of 450 MPa or more even after 100 hours of PWHT, and satisfies a Charpy impact energy value at −50 ° C. of 50 J or more.

  Examples of the present invention will be described in detail below. However, the present invention is not limited to the following examples.

  Table 1 below shows the chemical components of the inventive steel and the comparative steel. Steel slabs having the composition shown in Table 1 were manufactured by manufacturing the steel plate thickness, reheating temperature, rolling, heat treatment and cooling shown in Table 2.

  After the steel sheet manufactured under the above conditions is subjected to PWHT etc. under the conditions shown in Table 2 below, the yield strength, tensile strength, low temperature toughness, and crack length ratio (CLR, Crack Length Ratio,%) are examined. The results are shown in Table 2 below.

  However, the low temperature toughness was evaluated by a Charpy impact energy value obtained by conducting a Charpy impact test on a specimen having a V notch at −50 ° C., and the crack length ratio (Crack Length Ratio,%) in Table 2 below. ) Is measured according to NACE TM0277 standard.

  As can be seen from the results in Tables 1 and 2 above, the strength and toughness of the invention steel satisfying the composition and production conditions of the present invention does not decrease even when the PWHT time is 50 hours or more and 100 hours. The comparative steel deviates from the composition and production conditions of the present invention. Compared with the inventive steel, when the PWHT time is short, it shows substantially the same strength and toughness as the inventive steel, but the PWHT time is as long as 50 hours or more. It can be seen that the strength and toughness of the steel according to the invention are significantly deteriorated.

  In particular, it can be seen that the low-temperature toughness value of the invention steel is not significantly reduced even after 100 hours of PWHT, but the low-temperature toughness value of the comparative steel is severely reduced.

On the other hand, CLR (Crack Length Ratio,%) indicating the resistance of hydrogen-induced cracking in an H ₂ S (Sour Gas) gas atmosphere shows that the inventive steel is excellent. Thus, the reason why the inventive steel is superior in CLR is that the banding index indicating the degree of homogenization of the microstructure composed of the composite structure of ferrite and pearlite is controlled to be lower than 0.25. It can be seen from this example that this is true.

Claims (3)

In mass%, C: 0.1 to 0.3%, Si: 0.15 to 0.50%, Mn: 0.6 to 1.2%, P: 0.08% or less, S: 0.020 % Or less, Al: 0.001 to 0.05%, Cr: 0.01 to 0.35%, Mo: 0.005 to 0.2%, V: 0.005 to 0.05%, Nb: 0 0.001 to 0.05%, Ti: 0.001 to 0.05%, Ca: 0.0005 to 0.005%, Ni: 0.05 to 0.5%, Cu: 0.005 to 0 1 or more selected from the group consisting of 0.5%, Co: 0.005 to 0.2%, and W: 0.005 to 0.2%, the remainder consisting of Fe and inevitable impurities,
The above composition meets the following equation,
The fine structure of the steel sheet is composed of ferrite or a mixed structure of ferrite and pearlite. At this time, the average size of ferrite crystal grains in the central part of the steel sheet is 50 μm or less, and the banding index (ASTM E-1268) of the steel sheet. high-strength steel sheet measurement) is 0.25 or less, which is excellent in the heat treatment after welding resistance by.
Cu + Ni + Cr + Mo: 1.5% or less Cr + Mo: 0.4% or less V + Nb: 0.1% or less Ca / S: 1.0 or more   2. The post-weld heat treatment resistance according to claim 1, wherein the steel sheet has a tensile strength of 450 MPa or more and a Charpy impact energy value at −50 ° C. of 50 J or more even after 100 hours of post-weld heat treatment (PWHT). High strength steel plate with excellent properties. In mass%, C: 0.1 to 0.3%, Si: 0.15 to 0.50%, Mn: 0.6 to 1.2%, P: 0.08% or less, S: 0.020 % Or less, Al: 0.001 to 0.05%, Cr: 0.01 to 0.35%, Mo: 0.005 to 0.2%, V: 0.005 to 0.05%, Nb: 0 0.001 to 0.05%, Ti: 0.001 to 0.05%, Ca: 0.0005 to 0.005%, Ni: 0.05 to 0.5%, Cu: 0.005 to 0 1 or more selected from the group consisting of 0.5%, Co: 0.005 to 0.2%, and W: 0.005 to 0.2%, the remainder consisting of Fe and inevitable impurities,
The above composition is a step of reheating a steel slab satisfying the following relational expression in a temperature range of 1050 to 1250 ° C .;
Adding the rolling reduction of 10% or more to each rolling pass in the temperature range of T _{nr to} T _nr + 100 ° C. and hot rolling the reheated steel slab at a cumulative rolling reduction of 30% or more ;
Heat treatment stage for holding the hot-rolled hot-rolled steel sheet in the temperature range of 850 to 950 ° C. for 1.3 × t + (10 to 30 minutes) (where t means the thickness (mm) of the steel material). When,
Cooling the heat-treated steel sheet at a cooling rate of 0.1 to 120 ° C./sec, and said T _nr (° C.) = 887 + 464 × C + 890 × Ti + 363 × Al-357 × Si + (6446 × Nb−644 × Nb) ^1/2 ) + (732 × V−230 × V ^1/2 ) ,
The fine structure of the steel sheet is composed of ferrite or a mixed structure of ferrite and pearlite. At this time, the average size of ferrite crystal grains in the central part of the steel sheet is 50 μm or less, and the banding index (ASTM E-1268) of the steel sheet. (Measured by) is 0.25 or less, a method for producing a high-strength steel sheet having excellent heat resistance after welding.
Cu + Ni + Cr + Mo: 1.5% or less Cr + Mo: 0.4% or less V + Nb: 0.1% or less Ca / S: 1.0 or more