JP7025950B2 - Cu-containing low alloy steel with high strength and high toughness and its manufacturing method - Google Patents

Description

The present invention relates to a Cu-containing low alloy forged steel having high strength and high toughness and a method for producing the same, and is suitable as a steel for offshore structures used for mooring equipment, risers, flow lines and the like. ..

Oil and natural gas are widely used as the center of energy. In recent years, their development is shifting from land to ocean, and in particular, marine resource development is becoming mainstream in mining deeper than the continental shelf. From the viewpoint of ensuring safety, the steel for offshore structures used for this ultra-deep water development has high low temperature toughness, high yield strength, and low yield ratio. Is required.

In order to achieve both strength and toughness, steel sheets containing, for example, 1.0 to 1.3% by mass of Cu specified in ASTM A710 are known as steels for offshore structures, and forged steel materials, for example, ASTM A707. Steels containing Cu of 0.43% by mass or less specified in the above are known.
The above-mentioned steel secures strength in a component system having low carbon and low carbon equivalent by precipitating Cu by aging treatment, and has both strength and low temperature toughness.
Non-Patent Document 1 describes the results of improving the component system based on ASTM A707 Grade L5, performing quenching and tempering, and evaluating the mechanical properties. In this non-patent document 1, the FATT is −60 ° C., and it is necessary to further improve the low temperature toughness from the viewpoint of ensuring safety.

In Patent Document 1, a steel sheet containing Cu: 0.7 to 1.5% by mass% is subjected to a reduction of 900 ° C. or lower and 30% or more at 700 ° C. or higher, and then Cu precipitation treatment is performed in the range of 500 to 650 ° C. A method for producing a low C—Cu precipitation type high-strength steel having excellent low-temperature toughness and weldability has been proposed. However, in Patent Document 1, since rolling is required in the manufacturing process, this manufacturing method cannot be applied to a large structure including a flange portion having a thick wall of 150 mm or more.

In addition, research results on the improvement of material properties by quenching in the two-phase region have also been reported. For example, in Patent Document 2, as a two-phase region quenching method for B-added steel, the addition amounts of B, N and Ti are specified, and the two-phase region quenching temperature is specified to stabilize a high-strength steel having a low yield ratio. We are proposing to manufacture it.

Further, Patent Document 3 proposes to produce a Ni-containing steel sheet having an excellent low-temperature toughness and strength-toughness balance by two-phase region quenching. However, neither patent document specifies the Cu content.

In Patent Document 4, two-phase region quenching is applied to a large structure containing C: 0.01 to 0.08% and Cu: 0.80 to 1.50%, and the two-phase region quenching temperature is specified. Has proposed a Cu-containing low alloy steel having an excellent strength-toughness balance and a method for producing the same. However, the 0.2% proof stress range of the invention example of Patent Document 4 is 525 MPa or more and 610 MPa or less, and the FATT is −70 ° C. or less.
Therefore, in Cu-containing low alloy steel whose strength changes due to aging treatment, it is characterized by high strength and high toughness having a 0.2% proof stress of 150 to 500 mm thick forged steel material of 620 MPa or more and a FATT of −90 ° C. or less. The steel to be used and the manufacturing method thereof have not been clarified.
The cooling rate of thick-walled forged steel is slow, and the metal structure of Cu-containing low-alloy steel is generally bainite. Since bainite transformed at each temperature during continuous cooling becomes complicated, granular bainite ferrite (αB) and bainite ferrite (α ^o B) described in Non-Patent Document 2 are used as detailed classifications of metal structures. Defined.

Japanese Unexamined Patent Publication No. 61-149430 Japanese Unexamined Patent Publication No. 5-171263 Japanese Unexamined Patent Publication No. 2008-81776 JP-A-2017-150041

Steel Frogings: Second Volume, ASTM STP 1259 p. 196 Bainite Photobook of Steel-1, edited by The Iron and Steel Institute of Japan Basic Research Group Bainite Research Group

Even in large structures using Cu-containing low alloy steel, which is widely used as steel for offshore structures, in addition to having excellent low-temperature toughness from the viewpoint of ensuring safety, high yield is achieved from the viewpoint of weight reduction. Steel with strength is needed. As described above, the strength of this Cu-containing low alloy steel can be improved by improving the tempering conditions by aging treatment, but since the strength and toughness are in a trade-off relationship, high strength and high toughness cannot be achieved. In addition, it is necessary to increase the C content in order to further increase the strength.

The present invention has been invented against the background of the above circumstances, and an object of the present invention is to provide a Cu-containing low alloy steel having high strength and high toughness and a method for producing the same.

Therefore, the present invention makes it possible to provide a Cu-containing low alloy steel having high strength and high toughness by applying an appropriate composition range and a two-phase region quenching treatment.
First, the proper composition range in the present invention is clarified. Secondly, in the production method of the present invention, appropriate tempering conditions including a two-phase region quenching treatment for producing a Cu-containing low alloy steel having high strength and high toughness are sought.
The configurations of these inventions are optimized in consideration of high strength and high toughness based on the composition of the forged steel material specified by ASTM A707, which is widely applied as steel for offshore structures. Furthermore, as a result of investigating a method for producing Cu-containing low alloy steel having the desired material properties by optimizing the tempering conditions, especially the two-phase region quenching conditions, the results are high even in the manufactured products for large structures. We have found a method capable of producing a Cu-containing low alloy steel having high strength and high toughness, and have reached the present invention.

That is, among the Cu-containing low alloy steels having high strength and high toughness of the present invention, the first form is by mass%, C: 0.09 to 0.20%, Si: 0.10 to 0.40%. , Mn: 0.80 to 1.80%, Ni: 0.80 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 It contains ~ 0.60%, Al: 0.010 to 0.050%, Nb: 0.030 to 0.080%, N: 0.005 to 0.020%, and the balance consists of Fe and unavoidable impurities. It has a chemical composition, a 0.2% toughness of 620 MPa or more, a yield ratio of 0.82 or less, and a ductile brittle fracture surface transition temperature (FATT) of -90 ° C or less measured by a V-notch Charpy impact test. Have.

The invention of the Cu-containing low alloy steel having high strength and high toughness in other forms further contains Ca: 0.010% or less in% by mass in other forms.

The invention of the Cu-containing low alloy steel having high strength and high toughness in other forms has an absorption energy of 90 J or more at −100 ° C. in other forms.

The invention of Cu-containing low alloy steels having high strength and high toughness in other forms is characterized in that, in other forms, the metallographic structure after tempering is bainitic ferrite.

The first aspect of the invention of the method for producing a Cu-containing low alloy steel having high strength and high toughness of the present invention is the production method for producing a Cu-containing low alloy steel according to any one of claims 1 to 4. Then, the material is heated to a temperature range of 850 to 950 ° C. for quenching, and then heated again to a temperature range of (AC ₃ transformation point -100 ° C.) or higher and (AC ₃ transformation point -50 ° C.) or lower. It is characterized by having a tempering condition in which a two-phase region quenching treatment is performed and a tempering treatment is further performed at 580 to 610 ° C.

The invention of another form of a method for producing a Cu-containing low alloy steel having high strength and high toughness comprises a hot forging step in another form.

The invention of another form of a method for producing a Cu-containing low alloy steel having high strength and high toughness is characterized in that it is applied to a large structural steel having a thick portion having a plate thickness of 150 mm to 500 mm in another form. do.

The contents specified in the present invention and the actions based on the specifications will be described below.
The following components are all shown in% by mass.

C: 0.09 to 0.20%
From the viewpoint of ensuring strength and hardenability, C is a necessary additive element, so 0.09% is the lower limit. However, if the content exceeds 0.20%, the absorption energy decreases due to the increase in strength, and the precipitation of the hard phase during quenching in the two-phase region occurs. Therefore, the upper limit is 0.20%. For the same reason, it is desirable that the lower limit is 0.10% and the upper limit is 0.15%, and further, it is more desirable that the upper limit is 0.13%.

Si: 0.10 to 0.40%
Si is used as a deoxidizing element when melting and refining alloys. Further, since it is an element necessary for ensuring strength, the lower limit is 0.10%. However, since excessive content causes a decrease in toughness and a decrease in weldability, the upper limit is 0.40%.

Mn: 0.80 to 1.80%
Like Si, Mn is a useful element as a deoxidizing element and contributes to the improvement of hardenability. In order to exert its effect, the content of 0.80% or more is required. However, since excessive content causes a decrease in toughness, the upper limit is 1.80%. For the same reason, it is desirable that the lower limit is 1.00% and the upper limit is 1.5%.

Ni: 0.80 to 2.50%
Since Ni is an element necessary for ensuring strength by improving hardenability and ensuring low temperature toughness, the lower limit is 0.80%. However, excessive content stabilizes the residual γ and causes a decrease in toughness, so the upper limit is 2.50%. For the same reason, it is desirable to set the lower limit to 1.50% and the upper limit to 2.30%.

Cr: 0.50 to 1.00%
Since Cr is an important element for ensuring hardenability and ensuring strength and toughness, the lower limit is 0.50%. However, excessive content enhances hardenability, lowers toughness, and increases susceptibility to weld cracks. Therefore, the upper limit is 1.00%.

Cu: 0.80 to 1.50%
Cu precipitates during the aging process to improve the strength of the steel. In low carbon steel, it is very important to secure the strength by Cu precipitates. Further, since it is an important element for improving corrosion resistance, the lower limit is 0.80%. However, since excessive content causes a decrease in toughness and a decrease in hot workability, the upper limit is 1.50%. For the same reason, it is desirable that the lower limit is 1.10% and the upper limit is 1.30%.

Mo: 0.20 to 0.60%
Since Mo contributes to the improvement of hardenability and is an important element for ensuring strength and toughness, the lower limit is 0.20%. However, since excessive content causes a decrease in toughness and a decrease in weldability, the upper limit is 0.60%.

Al: 0.010 to 0.050%
Al combines with N to form AlN, which suppresses crystal grain growth. Finer grain size is essential for improving toughness, and the lower limit of Al content is 0.010%. However, since excessive content causes a decrease in toughness due to coarse AlN, the upper limit is 0.050%.

Nb: 0.020 to 0.080%
Since Nb is an element that suppresses crystal grain growth as a carbonitride and is important for refining the crystal grain size, the lower limit is 0.020%. However, the excessive content promotes the coagulation coarsening of the carbonitride and causes a decrease in toughness, so the upper limit is 0.080%.

N: 50-200ppm
As AlN and carbonitride, N suppresses crystal grain growth and refines the crystal grain size. Therefore, it is an important element for refining the crystal grain size, and the lower limit is 50 ppm. However, the excessive content promotes the precipitation and coagulation coarsening of a large amount of AlN and carbonitride, and causes a decrease in toughness, so the upper limit is 200 ppm. For the same reason, it is desirable to set the lower limit to 70 ppm and the upper limit to 120 ppm.

Ca: 0.010% or less Ca is used as a deoxidizing and desulfurizing element as desired because it forms oxides and sulfides as Ca—Si. However, excessive addition causes a decrease in toughness, so the content should be 0.010% or less. For the same reason, it is desirable to further set the upper limit to 0.005%.
Further, in order to obtain the above action, it is desirable to set the lower limit to 0.001% when the content is desired.

Metallic structure The metallographic structure after tempering needs to be a bainitic ferrite defined in Non-Patent Document 2 described above. As the hardenability decreases, it changes from bainitic ferrite to granular bainitic ferrite, and granular bainitic ferrite cannot achieve high strength and high toughness. Further, in martensite obtained when the hardenability is further increased, the increase in strength becomes excessive, and the toughness is lowered, particularly the absorbed energy is lowered.

Conditioning conditions In the case of quenching treatment, it is necessary to heat to a temperature at least AC ₃ transformation point or higher. Further, even if the heating temperature of the quenching treatment is AC ₃ points or more, the hardenability cannot be ensured when the temperature is low, so the lower limit temperature is set to 850 ° C. However, when the quenching treatment temperature is raised, the γ grain size becomes coarse during heating, which causes a decrease in toughness thereafter, so the upper limit is set to 950 ° C. This quenching process can be repeated a plurality of times as needed. Further, the heating means and the cooling means at the time of quenching are not particularly limited in this invention, and the means for obtaining the desired heating ability and cooling ability can be appropriately selected.

The hardened steel material is then subjected to a two-phase region quenching treatment in which it is heated in a temperature range of (AC ₃ transformation point -100 ° C.) or higher and (AC ₃ transformation point -50 ° C.) or lower, and then cooled. To. The heating means and cooling means for quenching in the two-phase region are not particularly limited in this invention, and means for obtaining desired heating ability and cooling ability can be appropriately selected.
This heat treatment is the most important in the production method of the present invention.

The heating temperature in this heat treatment is defined in a temperature range of (AC ₃ transformation point −100 ° C.) or higher and (AC ₃ transformation point −50 ° C.) or lower. If the heating temperature is less than (AC3 transformation point _- 100 ° C.), the amount of transformation to the γ phase is insufficient, many α phases undergo high-temperature tempering, and the Cu precipitate becomes coarse, so 0.2%. It is not possible to secure the bearing capacity. In addition, the subsequent crystal grain size is not refined, and the components are concentrated in the transformed γ phase, and the γ phase remains even at room temperature, making it difficult to secure toughness. On the other hand, when the temperature is higher than (AC ₃ transformation point -50 ° C), the amount of transformation to the γ phase becomes excessive, the hardenability of the transformed γ phase deteriorates, and it becomes a granular vanitic ferrite, which is a good metal. I can't get the tissue. Moreover, the crystal grain size becomes coarse, and sufficient strength and low temperature toughness cannot be ensured. For this reason, the temperature range is defined as (AC ₃ transformation point −100 ° C.) or higher and (AC ₃ transformation point −50 ° C.) or lower.

Following the above-mentioned two-phase region quenching treatment, a tempering treatment is performed in a temperature range of 580 to 610 ° C. If the heating temperature is less than 580 ° C., the proof stress increases by 0.2% due to the aging effect of the Cu precipitate, which leads to a decrease in toughness. Further, at a low tempering temperature, the internal stress during tempering cannot be relaxed, which causes damage during operation. On the other hand, if the temperature exceeds 610 ° C., overaging occurs and 0.2% proof stress cannot be secured. Therefore, the temperature range of the tempering process is 580 to 610 ° C.

Thick part The present invention can be suitably applied to the production of a material having a thick part. For example, a thick portion having a maximum wall thickness of 150 mm or more and 500 mm or less is shown.
With a material having a wall thickness of 150 mm or more, temper rolling is difficult, and the effect of the present invention can be remarkably obtained. On the other hand, if the wall thickness exceeds 500 mm, the cooling rate decreases in the cooling process of quenching and two-phase region quenching, which causes a decrease in strength.

That is, according to the present invention, the following effects can be obtained.
(1) According to the Cu-containing low alloy steel of the present invention, a 0.2% proof stress of 620 MPa or more and a yield ratio of 0.82 or less are secured, and good low temperature toughness is ensured.
(2) According to the method for producing a Cu-containing low alloy steel of the present invention, it can be used for mooring equipment, risers, flow lines, etc. due to the regulation of composition range, fine graining of crystal grain size after tempering, and good metallographic structure. It enables the production of thick Cu-containing low-alloy forged steels having excellent low-temperature toughness and particularly high strength and high toughness, which are suitable as steels for marine structures to be used.

It is a figure which shows one embodiment of the manufacturing method of the Cu-containing low alloy steel of this invention, and shows the heat pattern example of the two-phase region quenching process. It is a figure which shows the map which showed the large-angle boundary of the boundary angle of 15 ° or more for some invention materials and some comparison materials in the Example of the Cu-containing low alloy steel of this invention. Similarly, it is a figure which shows the relationship between 0.2% proof stress and FATT in a part of an Example.

The Cu-containing low alloy steel used in the present invention can be melted by a conventional method if the composition specified in the present invention is targeted, and the method is not particularly limited in the present invention. The molten steel ingot can be hot forged to form any shape.
The content and method of hot training are not particularly limited, and the training ratio and the like are not particularly limited. The hot-forged material can be a thick-walled material, for example, a material having a thick-walled portion having a plate thickness of 150 mm to 500 mm. The Cu-containing low alloy steel of the present invention has a particularly suitable effect in the material applied to the large structural steel having the thick portion described above, but the plate thickness is not particularly limited in the present invention. It can also be used in applications where the thickness is thinner than the above.

After hot forging, quenching (Q), two-phase region quenching (L), and tempering (T) are performed.
Further, a heat treatment such as normalizing (N) can be performed between the hot forging and the quenching treatment. As the normalizing condition, for example, a heating condition of 950 to 1000 ° C. can be shown.
In the tempering treatment, the Cu-containing low alloy steel is heated to a temperature range of 850 to 950 ° C. and quenched. Then, a two-phase region quenching treatment is performed in a temperature range of (AC ₃ transformation point −100 ° C.) or higher and (AC ₃ transformation point −50 ° C.) or lower, and further tempering treatment is performed at 580 to 610 ° C.
A heat treatment such as normalizing (N) can also be performed between the hot forging and the tempering treatment. As the normalizing condition, for example, a heating condition of 950 to 1000 ° C. can be shown.

The heat pattern of the above tempering conditions is shown in FIG.
In the first quenching, it is heated to a temperature of Ac ₃ or higher, and heat treatment is performed so that the heating temperature at the time of two-phase quenching falls within a specified range. Further, in the tempering treatment, heat treatment is performed by heating to a temperature of Ac ₁ or less.

According to the composition range specified above and the manufacturing method thereof, the thickness is suitable for marine structural steel used for mooring equipment, risers, flow lines, etc., and has excellent low-temperature toughness, especially strength-low-temperature toughness balance. It enables the production of low alloy forged steel containing meat Cu.
The Cu-containing low alloy steel obtained above has a strength characteristic of 0.2% proof stress of 620 MPa or more and a yield ratio of 0.82. Further, the ductile brittle fracture surface transition temperature (FATT) measured by a 2 mm V notch Charpy impact test has a characteristic of −90 ° C. or lower.

Hereinafter, examples of the present invention will be described in comparison with comparative examples.
The test material having the composition shown in Table 1 was melted into a 50 kg steel ingot by a vacuum induction melting furnace. Each molten steel ingot is hot forged at 1250 ° C. to a thickness of 45 mm × width of 130 mm (forging ratio: 3.1 s or more), and after normalizing (960 ° C.), the adjustments shown in Table 2 are performed. Normalizing was performed under the quality conditions (Q treatment, L treatment, T treatment).
The Q treatment (quenching) temperature of the examples is all 900 ° C., but the quenching temperature is not particularly limited as long as it is in the range of 850 to 950 ° C. for the above-mentioned reason. Further, the cooling of the Q treatment and the L treatment (two-phase region quenching treatment) was set to a cooling rate (10 ° C./min) simulating the equivalent of water cooling with a plate thickness of 450 mm.
The conditions for T treatment (tempering) of each test material are shown in Table 2.

From the obtained test material, test pieces were collected and subjected to a tensile test and a Charpy impact test to evaluate their strength and low temperature toughness. The test method was as follows.
In the tensile test, a round bar tensile test piece (parallel part diameter: 12.5 mm, GL: 50 mm) is collected from the obtained test material, and the tensile test is performed at room temperature in accordance with JIS Z 2241. It was carried out and 0.2% proof stress (YS) and tensile strength (TS) were determined. In addition, the yield ratio (YS / TS; YR) was calculated from these results.

In the impact test, a 2 mm V notch Charpy impact test piece was collected from the obtained test material and complied with JIS Z 2242. The Charpy impact test was carried out at -100 ° C to determine the absorbed energy vE- 100 ° C (J) at -100 ° C. In addition, three tests were carried out for each of the test materials, the obtained absorbed energy was arithmetically averaged, and the average value was taken as the absorbed energy value of the steel material. Furthermore, FATT performed a Charpy impact test at an arbitrary test temperature, and FATT was collected from the transition curve.

In addition, samples were taken from these test materials, and EBSD (OTS manufactured by TSL) measurement was also performed. The measurement pitch of the EBSD measurement was 0.07 μm, and the range of 100 × 100 μm was measured. From the obtained results, a large-angle boundary map of 15 ° or more was prepared, and the microstructures of granular bainitic ferrite (αB), bainitic ferrite (α ^o B), and martensite (M) were discriminated. The criteria for the tissue discrimination method are the presence or absence of a substructure (block, packet) having a large-angle boundary inside the old γ grain and its frequency, and αB is a tissue that does not have a block inside and looks like a macroscopic mass. α ^o B has a block having a large-angle boundary inside the tissue, and becomes a macroscopic needle-like tissue. M also has a large-angle boundary corresponding to the substructure, but there are packets in addition to the block, and the intervals between the large-angle boundaries are close. In addition, α ^o B and M can be distinguished by measuring the transformation temperature.
The results obtained by each test are shown in Table 3.

As shown in Table 3, the steel No. 1 and steel No. Steel grade A was used as the test material of 2. Steel No. 1 and 2 are comparative examples using the QT process, which is a general manufacturing process. Steel No. In 1, 0.2% Y. in the QT process alone. S. And T. S. Is high, and toughness cannot be ensured. In addition, steel No. Steel No. 2 as in 2. Even when the tempering temperature was raised to a higher temperature, the strength was lowered, and the toughness was improved by the same QT process as in No. 1, good low temperature toughness could not be obtained. It is clear that it is difficult to secure good toughness and low YR only by the QT process because the YR (yield ratio) does not decrease in the QT process because the tempering temperature becomes higher than this. Is.

Steel No. 3 to No. No. 8 is steel No. The same steel type as No. 1 is used, and the manufacturing process is the QLT process. Steel No. In 3 (comparative example), the L temperature is (AC _3-100 ° C.) or less, the amount of transformation to the γ phase is insufficient, many α phases undergo high-temperature tempering, and the Cu precipitate becomes coarse. Therefore, 0.2% proof stress cannot be secured. In addition, the partially transformed γ phase remains as residual γ, resulting in insufficient low temperature toughness.
Steel No. Steel grade A is used in 4 and 5 (example of the present invention), and a steel material having high strength and high toughness is obtained by applying the heat treatment process of the present invention.
Steel No. In No. 6 (Comparative Example), the treatment is performed under the condition that the L temperature exceeds ₍ AC 3-50 ° C.). In the case of this comparative example, the area ratio of the γ phase during L heating is large, and the transformed γ phase does not have a good metal structure due to the decrease in hardenability due to the decrease in quenching temperature, and the decrease in low temperature toughness is observed. Has been done.

In addition, steel No. 1 (QT process) and steel No. FIG. 2 shows a large-angle boundary map having a boundary angle of 15 ° or more obtained from the EBSD measurement results of 5 and 6 (QLT process).
Steel No. 1 and steel No. Comparing No. 5, by performing the L treatment, the large-angle boundary reflecting the metallographic structure becomes complicated, and suppression of coarse grains is recognized. This increase in the proportion of large-angle boundaries contributes to the improvement of low-temperature toughness. On the other hand, Steel No. When the L temperature becomes high as in No. 6, the hardenability of the transformed γ phase deteriorates, the metal structure becomes granular bainitic ferrite, the proportion of the overall large angle boundary decreases, and toughness cannot be ensured. ..
Therefore, it was clarified that by applying the QLT process of the present invention and optimizing the L temperature, high strength and high toughness that could not be obtained by the conventional QT process can be obtained.

Steel No. 7 (Invention Example) and Steel No. No. 8 (comparative example) is the steel No. The same L process as in No. 5 is applied, and a low tempering temperature is applied. The strength is improved as the tempering temperature decreases. On the other hand, the toughness decreased, and the steel No. Sufficient toughness was obtained in No. 7, but Steel No. In No. 8, the effect of age hardening is large due to Cu, and toughness cannot be ensured.
Steel No. 9 and No. Reference numeral 10 (invention example) is the result of using steel type B. By applying the heat treatment process of the present invention, a steel material having high strength and high toughness is obtained.
Steel No. 11 to 14 are the results using the steel grade C. Steel No. 11 and Steel No. In No. 12, a steel material having high strength and high toughness is obtained by applying the heat treatment process of the present invention.

Steel No. No. 13 is processed under the condition that the L temperature exceeds ₍ AC 3-50 ° C.). In the case of this comparative example, the area ratio of the γ phase during L heating is large, and the crystal grains are coarse grains. As a result, the toughness is lowered and the absorbed energy is found to be lowered.
Steel No. In 14 (comparative example), the QT process is examined using the steel grade C. Even in steel grade C, the metallographic structure becomes martensite in the QT process, and strength can be ensured, but good toughness, especially absorbed energy, cannot be ensured.
Steel No. 15 to 17 (comparative example) are the results of using the steel grade D described in Patent Document 4. Steel No. 15 and steel No. In No. 16, the L temperature of the QLT process is fluctuated, but since the amount of C is low under any of the conditions, the metal structure becomes granular bainitic ferrite, and strength and toughness cannot be ensured.
The composition of steel grade D described in Patent Document 4 (remaining Fe and unavoidable impurities) is shown below.
C: 0.05%, Si: 0.25%, Mn: 1.42%, Ni: 2.14%, Cr: 0.72%, Cu: 1.24%, Mo: 0.45%, Al : 0.026%, Nb: 0.047%, Ca: 0.003%, N: 65ppm

Steel No. In No. 17 (Comparative Example), the tempering temperature was lowered and the strength was secured by age hardening of Cu, but the toughness decreased with the increase in strength, and good toughness could not be secured. Therefore, in the composition of steel grade D, the amount of C added is small, and even if the QLT process is applied, good strength and toughness cannot be ensured.

The correlation between 0.2% YS and FATT in the results shown in Table 3 is shown in FIG. This figure shows the strength-toughness balance of each steel, and the material properties obtained by the present invention are hatched portions. In the steel to which the composition of the present invention and the heat treatment process are applied (▲, ▼, ◆ in the figure), a steel material having high strength and high toughness is obtained. On the other hand, in the steel (□ in the figure) to which the QT process was applied using the steel type A, the tempering temperature was changed to 0.2% Y. S. It is clear that good toughness cannot be obtained even if the amount is reduced (dashed line in the figure). Further, when the steel grade C is used, even if the condition is out of the heat treatment process range of the present invention (◇ in the figure), 0.2% Y. S. Although FATT is good, absorption energy cannot be obtained due to the above-mentioned reasons.

From the above results, excellent 0.2% proof stress, yield ratio and low temperature toughness can be obtained by applying an appropriate composition and manufacturing process, and Cu-containing low alloy steel with high strength and high toughness can be manufactured. Is.

Although the present invention has been described above based on the above-described embodiment and the above-described embodiment, appropriate modifications can be made to the above-described embodiment and the above-mentioned embodiment as long as the present invention is not deviated from the scope of the present invention.

The present invention is suitable as steel for offshore structures used in mooring equipment, risers, flow lines and the like. However, the intended use is not limited to this.

Claims (7)

By mass%, C: 0.09 to 0.20%, Si: 0.10 to 0.40%, Mn: 0.80 to 1.80%, Ni: 0.80 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050%, Nb: 0.030 to 0. It contains 080%, N: 0.005 to 0.020%, has a chemical composition with the balance consisting of Fe and unavoidable impurities, has a 0.2% ductility of 620 MPa or more, and has a yield ratio of 0.82 or less. A Cu-containing low alloy steel having high strength and high toughness having a ductile brittle fracture surface transition temperature (FATT) of −90 ° C. or lower as measured by a V-notch Charpy impact test. Further, the Cu-containing low alloy steel having high strength and high toughness according to claim 1, which contains Ca: 0.010% or less in mass%. The Cu-containing low alloy steel having high strength and high toughness according to claim 1 or 2, which has an absorption energy of 90 J or more at −100 ° C. The Cu-containing low alloy steel having high strength and high toughness according to any one of claims 1 to 3, wherein the metallographic structure after tempering is bainitic ferrite. The method for producing a Cu-containing low alloy steel according to any one of claims 1 to 4, wherein the quenching treatment is performed by heating to a temperature range of 850 to 950 ° C., and then (AC ₃ ) is performed again. The tempering conditions are such that the two-phase region quenching treatment is performed by heating to a temperature range of (transformation point -100 ° C) or higher and (AC ₃ transformation point -50 ° C) or lower, and further tempering treatment is performed at 580 to 610 ° C. A method for producing a Cu-containing low alloy steel having high strength and high toughness. The method for producing a Cu-containing low alloy steel having high strength and high toughness according to claim 5, which has a hot forging step. The method for producing a Cu-containing low alloy steel having high strength and high toughness according to claim 5 or 6, wherein the method is applied to a large structural steel having a thick portion having a plate thickness of 150 mm to 500 mm.

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