JP7273298B2 - Steel plates for pressure vessels with excellent low-temperature toughness - Google Patents

Description

TECHNICAL FIELD The present invention relates to a pressure vessel steel plate having excellent low temperature toughness.

The present invention is used for large structures such as pressure vessels, bridges, buildings, and shipbuilding, and the average value of JIS 4 Charpy impact absorption energy at -20 ° C after post-welding heat treatment at 650 ° C for 30 hours is 30 J or more. The present invention relates to a high-strength steel plate having a yield stress of 485 MPa or more, a tensile strength of 620 MPa or more and 795 MPa or less, and a thickness of 120 mm or more, and particularly to a steel plate having excellent low temperature toughness.

The steel plate used is required to have low temperature toughness in addition to strength. Toughness after post-weld heat treatment (hereinafter referred to as PWHT) for removing residual stress during construction welding is required. In recent years, as pressure vessels and structures have increased in size, there has been a demand for higher strength and thicker steel sheets, making it difficult to ensure toughness after PWHT.

In the production of this type of pressure vessel steel, reheating, quenching, and tempering heat treatment are applied after hot rolling, and a large amount of alloying elements are added to ensure the strength and toughness of the plate as the thickness increases. Patent Document 1 describes a thick high-toughness high-strength steel sheet with a thickness of 100 mm or more having an average prior austenite grain size of 50 μm or less in the microstructure and a martensite and/or bainite structure having an area fraction of 80% or more. There is

These days, the PWHT conditions are getting stricter due to the harsher usage environment. For example, this time "650°C x 30 hours" corresponds to it. However, the low-temperature toughness conditions after PWHT, which are stricter for plate thicknesses of 120 mm or more (650 ° C. × 30 hours after welding after heat treatment, the average of JIS 4 Charpy impact absorption energy (vE-20) at -20 ° C. is 30 J or more. ) are not mentioned.

Patent No. 5928654

In general, brittle fracture due to low-temperature toughness reduction is a fracture mode that should be avoided because it instantly collapses the entire structure. After PWHT, the low-temperature toughness is lowered due to the concentration of impurity elements, the coarsening of alloy carbides, and the like. In order to secure the low temperature toughness after PWHT, it is necessary to take measures to suppress the deterioration of low temperature toughness due to PWHT.
This type of high-strength steel sheet is manufactured by reheating quenching and tempering heat treatment after hot rolling, and it is necessary to add a large amount of alloying elements in order to ensure the strength and toughness in the sheet due to the increase in thickness. On the other hand, it is desired to suppress the addition of alloying elements from the viewpoint of suppressing steel material alloy costs and ensuring the soundness of welded joints.

The present invention solves such conventional problems, and suppresses the alloying elements, and ensures the strength and low temperature toughness after stricter PWHT at 650 ° C. for 30 hours. Pressure excellent in low temperature toughness A steel plate for containers is provided.

The present inventors have studied high-strength steel plates having a tensile strength of 620 MPa or more and a plate thickness of 120 mm or more for large structures such as bridges, buildings, ships, and pressure vessels. As a result, it was found that the above object can be achieved by controlling the size of coarse carbides by adjusting the addition of C, Cr, and Mo within appropriate ranges.

The present invention has been completed based on such findings. The gist of the present invention is as follows.

(1) In mass %, C: 0.201 to 0.300%, Si: 0.10 to 0.50%, Mn: 1.00 to 1.80%, P: 0.0200% or less, S: 0.0100% or less, Ni: 0.10 to 0.80%, Cr: 0.10 to 0.80%, Mo: 0.10 to 0.80%, Al: 0.010 to 0.050%, N: contains 0.0080% or less, the balance is Fe and impurities, and the content of C [C], the content of Cr [Cr], and the content of Mo [Mo] is H = 1000 × ([C ]−0.3×[Mo]) ² /(1+10×[Cr])+[Cr] ² <5.0 is satisfied, and coarse carbide after heat treatment after welding at 650° C. for 30 hours is 2.0 μm or less. , The average value of JIS4 Charpy impact absorption energy at -20 ° C. is 30 J or more, the yield stress is 485 MPa or more, the tensile strength is 620 MPa or more and 795 MPa or less, and the plate thickness is 120 mm or more. steel plate.

(2) Furthermore, in mass%, Cu: 0.05 to 0.50%, Nb: 0.01 to 0.10%, V: 0.005 to 0.100%, Ti: 0.005 to 0.100 %, Ca: 0.0003 to 0.0050%, Mg: 0.0003 to 0.0050%, and REM: 0.0003 to 0.0100%. , The steel plate for pressure vessel according to claim 1.

According to the present invention, it is possible to provide a steel plate for pressure vessels that has stable low-temperature toughness even after stricter PWHT, and the present invention makes an extremely significant contribution to industry.

4 is a graph showing the relationship between parameter H and coarse carbide size. 4 is a graph showing the relationship between toughness and coarse carbide size.

The present inventors conducted studies to improve the toughness of steel sheets after PWHT, and obtained the following findings (A) to (C). A steel sheet and a method for manufacturing the same according to the present invention will be described below. Hereinafter, the "%" display of the content of each chemical component means "% by mass".

(A) Chemical composition C: 0.201 to 0.300%
C is an element necessary for ensuring the strength of the base material, and the amount of C is made 0.201% or more in the present invention. On the other hand, if the amount of C is less than 0.201%, the hardenability deteriorates and the strength becomes insufficient. On the other hand, when the amount of C exceeds 0.300%, not only the base metal but also the HAZ, which is the weld heat affected zone, especially the HAZ near the FL deteriorates significantly in toughness. There is also a tendency for the strength to become excessive. Therefore, the upper limit of the amount of C is set to 0.300%. A preferable upper limit of the amount of C is 0.280%.

Si: 0.10-0.50%
Si is a deoxidizer and an element for securing strength, and the amount of Si is made 0.10% or more to obtain the effect. In addition, Si is also an element that suppresses the decomposition and precipitation reaction to cementite from the supersaturated solid solution martensite in the tempering process and post-welding heat treatment, and the Si content is preferably 0.20% or more. , more preferably 0.25% or more. On the other hand, if the amount of Si exceeds 0.50%, island-shaped martensite is formed and the toughness decreases, so the upper limit is made 0.50%. Preferably, the upper limit of the amount of Si is 0.40%, more preferably 0.35%.

Mn: 1.00-1.80%
Mn is a deoxidizer and an element that improves hardenability. In the present invention, the amount of Mn is set to 1.00% or more in order to ensure the strength of the base material and HAZ. The Mn content is preferably 1.25% or more, more preferably 1.35% or more. On the other hand, if the Mn content exceeds 1.80%, the segregation increases and the hardenability becomes excessive, resulting in an increase in strength and a decrease in toughness. The Mn content is preferably 1.70% or less, more preferably 1.65% or less.

P: 0.0200% or less P is an impurity that segregates at grain boundaries and lowers toughness, so the amount of P is made 0.0200% or less. Preferably, the P content is 0.0080% or less. Although the lower limit is not particularly defined because the amount of P is preferably as small as possible, it may be contained in an amount of 0.0010% or more from the viewpoint of manufacturing costs.

S: 0.0100% or less S is an impurity, promotes center segregation, and may cause the formation of elongated MnS that is the starting point of brittle fracture, so the S content is 0.0100% or less. . Preferably, the S content is 0.0050% or less. Although the lower limit is not particularly defined because the amount of S is preferably as small as possible, the amount of S may be 0.0010% or more from the viewpoint of manufacturing costs.

Ni: 0.10-0.80%
Ni is an important element for ensuring toughness, and should be contained in an amount of 0.10% or more. More preferably, it is 0.20% or more. On the other hand, if Ni is included excessively, the production cost rises and the hardenability becomes excessive and the toughness of the base material may deteriorate, so the upper limit of the Ni content is 0.80%. The Ni content is preferably 0.65% or less, more preferably 0.60% or less.

Cr: 0.10-0.80%
Cr is an element that contributes to improvement of hardenability and affects strength, and should be contained in an amount of 0.10% or more. More preferably, it is 0.15% or more. On the other hand, if Cr is contained excessively, the hardenability becomes excessive and the toughness of the base material may deteriorate, so the upper limit of the Ni content is made 0.80%. The Cr content is preferably 0.75% or less, more preferably 0.70% or less.

Mo: 0.10-0.80%
Mo is an element that improves the strength and toughness of the base material and should be contained in an amount of 0.10% or more. More preferably, the Mo content is 0.15% or more. On the other hand, if the amount of Mo is excessive, the strength of the base material may increase and the toughness may be impaired, so the amount of Mo should be 0.80%. Preferably, the Mo content is 0.75 or less. More preferably, the Mo content is 0.70 or less.

Al: 0.010-0.050%
Al is an element that suppresses deoxidation and cementite formation, and further refines the grains as pinning particles AlN. In order to obtain the effect, it is made 0.010% or more. If the Al content exceeds 0.050%, inclusions increase and the toughness may be lowered, so the Al content should be 0.050% or less.

N: 0.0080% or less N is an impurity and lowers toughness, so the N content is made 0.0080% or less. Preferably, the N content is 0.0060% or less, more preferably 0.0050% or less. N is preferably reduced as much as possible, but from the viewpoint of denitrification cost, the N content may be 0.0001% or more.

Furthermore, one or more of Cu, Nb, V, Ti, Ca, Mg, and REM may be contained as necessary.

Cu: 0.05-0.50%
Cu is an element that contributes to an increase in strength, and may be contained in an amount of 0.05% or more. More preferably, the amount of Cu is 0.15% or more. On the other hand, if Cu is contained excessively, the toughness of the base metal may be lowered, so the upper limit of the amount of Cu is made 0.50%. More preferably, the amount of Cu is 0.40% or less.

Nb: 0.01-0.10%
Nb is an element that refines the structure by a pinning effect and improves low-temperature toughness, and may be contained in an amount of 0.01% or more. More preferably, the Nb content is 0.02% or more. On the other hand, if an excessive amount of Nb is added, the pinning effect is saturated, and coarse carbides and nitrides precipitate, which may lead to deterioration of toughness. Preferably, the Nb content is 0.08% or less, more preferably 0.05% or less.

V: 0.005-0.100%
V is an element that contributes to improving the strength of the base material, and may be contained in an amount of 0.005% or more. More preferably, the V content is 0.020% or more. On the other hand, even if an excessive amount of V is added, the effect is saturated and the toughness may deteriorate, so the amount of V should be 0.100% or less. Preferably, the V content is 0.070% or less, more preferably 0.060% or less.

Ti: 0.005 to 0.100%
When Ti is used for deoxidization, it forms an oxide phase composed of Al, Ti, and Mn, which has the effect of refining the structure and affecting the strength. good too. More preferably, the Ti content is 0.007% or more, and still more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.100%, Ti oxides and Ti—Al oxides are formed, which may reduce the toughness. More preferably, the Ti content is 0.080% or less.

Ca: 0.0003-0.0050%
Ca is an element that controls the form of oxides and sulfides, and the amount of Ca is preferably 0.0003% or more. More preferably, the Ca content is 0.0005% or more, more preferably 0.0010% or more. When Ca is added excessively, the effect saturates and the toughness may be impaired by the formation of inclusions, so the amount of Ca is made 0.0050% or less.

Mg: 0.0003-0.0050%
Mg is an element that controls the form of oxides and sulfides, and the Mg content is preferably 0.0003% or more. More preferably, the Mg content is 0.0005% or more, more preferably 0.0010% or more. When Mg is added excessively, the effect saturates and the toughness may be impaired by the formation of inclusions, so the Mg content is made 0.0050% or less.

REM: 0.0003-0.0100%
REM is an element that controls the form of oxides and sulfides, and the total amount of REM is preferably 0.0003% or more. More preferably, the total amount of REM is 0.0005% or more, more preferably 0.0010% or more. When REM is added excessively, the effect saturates and the toughness may be impaired by the formation of inclusions, so the total amount of REM is made 0.0100% or less. In addition, it is preferable to use one or more of Y, La, Ce, Nd, Pr, and Sm for REM in view of availability.

In order to suppress toughness deterioration due to PWHT, it is important ^to suppress the size of coarse carbides. ] The amount of C, Cr, and Mo added must be within an appropriate range so that the parameter H specified as ² is H<5.0, preferably H<4.5. FIG. 1 shows the relationship between parameter H and coarse carbide size. [C], [Cr], and [Mo] in the formula are the contents (% by mass) of each element.

The steel plate for pressure vessels of the present invention contains the above components, and the balance is Fe and impurities. The term "impurities" as used herein refers to components mixed in with various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel plate for pressure vessels is industrially manufactured, which adversely affects the present invention. It means what is allowed as long as it does not give

(B) Metallographic structure Coarse carbides after heat treatment after welding at 650°C for 30 hours are 2.0 µm or less Coarse carbides in the steel sheet serve as starting points for brittle fracture and reduce toughness. If coarse carbides exceed 2.0 μm, toughness is adversely affected. FIG. 2 shows the relationship between toughness and coarse carbide size. Here, the coarse carbide size was obtained by observing 10 or more fields of view (field size: 20 µm × 15 µm) of 5000x scanning electron micrographs after electropolishing, and taking the average value of the top 10 shorter diameters of the carbides.

C. Manufacturing Method An example of the manufacturing method of the steel plate for pressure vessels of the present invention will be described. The steel plate can be produced by rolling a steel ingot produced by an ingot casting method or a slab produced by a continuous casting method through a rolling process - a quenching treatment process - a tempering process (steps 1 to 3). explain. As for the steel slab to be hot-rolled, the casting conditions are not particularly specified as long as the composition is within the range of the present invention. Cast slabs may also be used. From the viewpoint of production efficiency, yield and energy saving, it is preferable to use a continuously cast slab.

C-1. Rolling process (process 1)
It is preferable that the steel slab is heated again to 1000 to 1250° C. and then hot rolled at a rolling reduction of 50% or more. The heating temperature in the heating step before hot rolling is preferably 1,250° C. or lower to suppress coarsening of the structure, and is preferably 1,000° C. or higher to reduce the rolling roll load. After rolling, there are no particular restrictions, and air cooling is performed.

C-2. Quenching treatment process (process 2)
After hot rolling, the material is once cooled to 150°C or lower, reheated to 800°C or higher, and then cooled to 200°C or lower at a cooling rate of 1°C/sec. or higher between 800-500°C. Cooling at less than 1°C/sec. or stopping cooling at a temperature higher than 200°C makes it difficult to obtain a sufficient quenched structure, and strength may not be ensured.

C-3. Tempering process (process 3)
After quenching, tempering is performed by heating the steel sheet to 650 to 730°C or less. There are no particular restrictions on the cooling rate after tempering, and air cooling is used.

EXAMPLES The present invention will be described in more detail below with reference to examples.

Using steel slabs of steels 1 to 48 having the chemical compositions shown in Table 1, under the manufacturing conditions shown in Table 2, steel materials for pressure vessels with plate thicknesses of 120 to 210 mm shown in Table 3 were produced. In addition, the hot rolling was performed at a rolling reduction of 50% or more. After that, post-welding heat treatment was performed at 650° C. for 30 hours. Then, the size of coarse carbides in the steel material was measured. Tensile properties (yield strength, tensile strength) were evaluated as base material properties, and 2 mm V notch Charpy impact absorption energy (vE _-20 ) at -20°C was evaluated as toughness. Table 3 shows the measured values obtained. Each test piece was taken from a plate thickness of 1/4t. In the evaluation, a yield strength (YP) of less than 485 MPa and a tensile strength (TS) of less than 620 MPa or more than 795 MPa were rejected. In addition, the JIS No. 4 Charpy impact absorption energy (vE-20) at -20°C was measured for three samples, and the average value of less than 30J was considered unacceptable.

From Table 3, it can be seen that the steel materials for pressure vessels according to the examples of the present invention are excellent in base material strength and toughness, and are excellent as materials for pressure vessels.

On the other hand, it can be seen that the comparative examples, which do not satisfy the conditions specified in the present invention, cannot achieve the target properties in terms of base material strength and toughness.

Claims (2)

% by mass, C: 0.201 to 0.300%, Si: 0.10 to 0.50%, Mn: 1.00 to 1.80%, P: 0.0200% or less, S: 0.0100 % or less, Ni: 0.10 to 0.80%, Cr: 0.10 to 0.80%, Mo: 0.10 to 0.80%, Al: 0.010 to 0.050%, N: 0 .0080% or less, the balance is Fe and impurities, and the content of C [C], the content of Cr [Cr], and the content of Mo [Mo] is H = 1000 × ([C]-0 .3 × [Mo]) ² / (1 + 10 × [Cr]) + [Cr] ² <5.0, coarse carbides after heat treatment after welding at 650 ° C. for 30 hours are 2.0 μm or less, −20 A steel plate for a pressure vessel, having an average value of JIS4 Charpy impact absorption energy at °C of 30 J or more, a yield stress of 485 MPa or more, a tensile strength of 620 MPa or more and 795 MPa or less, and a plate thickness of 120 mm or more. Furthermore, in mass%, Cu: 0.05 to 0.500%, Nb: 0.01 to 0.10%, V: 0.005 to 0.100%, Ti: 0.005 to 0.100%, Ca : 0.0003 to 0.0050%, Mg: 0.0003 to 0.0050%, and REM: 0.0003 to 0.0100%. 2. The steel plate for pressure vessels according to 1.

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