JP6735082B2 - Steel member, steel plate, and manufacturing method thereof - Google Patents

Description

The present invention relates to a steel member, a steel plate, and manufacturing methods thereof. More specifically, the present invention relates to a steel member obtained by subjecting a steel sheet to welding and post-weld heat treatment (Post Weld Heat Treatment, hereinafter sometimes referred to as “PWHT”), particularly, the PWHT at high temperature and for a long time. Even if it exists, it relates to a steel member excellent in strength and low temperature toughness in the central portion of the plate thickness, a steel plate used for manufacturing the steel member, and a manufacturing method thereof. Hereinafter, the low temperature toughness may be simply referred to as “toughness”.

Medium- and high-temperature pressure vessels used in the petroleum refining and other chemical industries tend to be required to have further high temperature and high pressure resistance for the purpose of high efficiency operation. Therefore, the steel plate used for the steel member such as the pressure vessel is required to have high strength. Further, from the viewpoint of safety, a high level of low temperature toughness is required for the above steel members.

In order to increase the strength, the steel sheet is subjected to normalization and quenching. However, when the plate thickness of the above steel plate is thick, there is a problem that it is difficult to obtain high strength because the cooling rate of the inside of the steel plate during normalizing or quenching, especially the central part of the plate thickness, is small. Further, the steel member such as the pressure vessel is obtained by welding the above steel plates and then performing stress relief annealing for strain relief, that is, PWHT. Although PWHT is carried out for a long time to remove the strain, the steel member subjected to the PWHT for a long time has a problem that the low temperature toughness and the like are deteriorated.

As a method of ensuring high toughness, increasing the amount of alloying elements can be mentioned. Cr-Mo steel containing Cr and Mo as alloy elements is used for steel members such as the pressure vessel. It is known that, when 2.25Cr-1.0Mo steel, for example, is used as the Cr-Mo steel, it is difficult to secure the toughness, and good toughness can be obtained even in the central portion of the plate thickness of the thick steel plate. However, in recent years, there has been an increasing tendency to save resources and reduce costs. Therefore, it is strongly demanded to realize a steel member having excellent strength and toughness in the central portion of the plate thickness on the premise that Cr-Mo steel having a smaller amount of alloying elements than the 2.25Cr-1.0Mo steel is used. Has been.

With respect to the above problems, a technique has been proposed in which high strength and high toughness are achieved by appropriately adjusting chemical components while suppressing the amount of alloying elements. For example, Patent Documents 1 and 2 disclose a technique for improving low temperature toughness for a steel having a compositional composition of 1.25Cr-0.5Mo level where it is difficult to secure toughness.

Patent Document 1 discloses a technique in which by adding Nb and Ca, the hardenability is secured and the characteristic deterioration at the time of SR (Stress Relief, stress relief annealing) is suppressed. However, if this technique is applied to a thick steel plate that is mainly cast by the ingot-making method, the Ca forms coarse inclusions, which may adversely affect the toughness. Therefore, it seems difficult to stably secure the toughness in the central part of the plate thickness of a steel member having a large plate thickness.

Further, Patent Document 2 discloses a technique in which austenite grain size is refined and low temperature toughness is ensured by performing controlled rolling or controlled rolling+accelerated cooling before quenching in a manufacturing process. However, the above-described controlled rolling in this technique may cause a decrease in productivity of the rolling line, and is therefore not practical.

Japanese Patent Laid-Open No. 06-279919 JP, 2000-345281, A

The present invention has been made by paying attention to the above circumstances, and an object thereof is to provide a steel material even when the PWHT after welding is carried out for a long time, particularly at a high temperature and a long time in the manufacturing process of the steel member. It is to establish a steel member having high strength and high low temperature toughness inside, a steel sheet useful for manufacturing the steel member, and a manufacturing method thereof. The “inside of steel material” means especially “a central portion of plate thickness”. The same applies hereinafter.

The steel member of the present invention that can solve the above problems,
C: 0.110% (meaning mass%; the same applies to chemical components) 0.15% or less,
Si: 0.50% or more and 0.80% or less,
Mn: 0.40% or more and 0.65% or less,
P: more than 0% and 0.0070% or less,
S: more than 0% and 0.0070% or less,
Al: 0.030% or more and 0.080% or less,
Cu: 0.05% or more and 0.20% or less,
Ni: 0.05% or more and 0.30% or less,
Cr: 1.05% or more and 1.50% or less,
Mo: 0.45% or more and 0.65% or less,
N: 0.0030% or more and 0.0070% or less,
B: 0.0003% or more and 0.0010% or less, and V: 0% or more and 0.030% or less,
Nb is 0.005% or less, Ti is 0.001% or less, and the total amount of Ca, Mg, REM and Zr is suppressed to 0.0010% or less, and the balance is iron and inevitable impurities,
The plate thickness is 100 mm or less,
It is characterized in that the structure in the central part of the plate thickness satisfies all of the following (a) and (b) and the Charpy absorbed energy at -38°C is 100 J or more.
(A) The structure is at least one of tempered bainite and tempered martensite.
(B) When the average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries in which the orientation difference between two adjacent crystals is 15° or more is D and the maximum diameter of grain boundary carbides is d, it is expressed as D/d. The value to be displayed is 54 or less.

Further, the steel sheet of the present invention that has solved the above problems is a steel sheet used for manufacturing the steel member,
C: 0.110% or more and 0.15% or less,
Si: 0.50% or more and 0.80% or less,
Mn: 0.40% or more and 0.65% or less,
P: more than 0% and 0.0070% or less,
S: more than 0% and 0.0070% or less,
Al: 0.030% or more and 0.080% or less,
Cu: 0.05% or more and 0.20% or less,
Ni: 0.05% or more and 0.30% or less,
Cr: 1.05% or more and 1.50% or less,
Mo: 0.45% or more and 0.65% or less,
N: 0.0030% or more and 0.0070% or less,
B: 0.0003% or more and 0.0010% or less, and V: 0% or more and 0.030% or less,
Nb is 0.005% or less, Ti is 0.001% or less, the total of Ca, Mg, REM, and Zr is suppressed to 0.0010% or less, the balance is iron and inevitable impurities, and the plate thickness is It is characterized in that it is 100 mm or less.

Further, in the method for manufacturing a steel sheet that has solved the above-mentioned problems, after hot rolling a steel slab satisfying the above-mentioned composition, quenching is performed at a heating temperature of 910°C or higher and 940°C or lower, and a holding time at the heating temperature: The heating is performed at a temperature of 620° C. or higher and Ac ₁ point or lower, and a P _T value represented by the following formula (1) is 19.2 or more and 20.6 or less. It is characterized in that it is performed at a heating temperature and a heating time.
P _T value=T _T ×(20+logt _T )×10 ⁻³ (1)
In the formula (1), T _T represents a heating temperature for tempering (K), and t _T represents a heating time for tempering (hr).

The present invention also includes a method for manufacturing the steel member. The method for producing the steel member is characterized in that the steel sheet is welded, and post-weld heat treatment is performed at a heating temperature and a heating time at which a P _PWHT value represented by the following formula (2) is 20 or more. Have.
P _PWHT value=T _PWHT ×(20+logt _PWHT )×10 ⁻³ (2)
In the formula (2), T _PWHT represents the heating temperature (K) of the heat treatment after welding, and t _PWHT represents the heating time (hr) of the heat treatment after welding.

When the steel sheet of the present invention is used for manufacturing a steel member, even if the PWHT during the manufacturing process of the steel member is set for a long time, especially at a high temperature for a long time, the steel having high strength and sufficiently excellent toughness inside the steel material The member is obtained. As a result, it is possible to provide a medium/high temperature pressure vessel having high strength and high toughness.

Further, the steel member of the present invention has a reduced amount of alloying elements, which contributes to resource saving and cost reduction.

FIG. 1 is a graph showing the relationship between D/d and Charpy absorbed energy at −38° C. in the example.

The present inventors presuppose that a steel plate made of Cr-Mo steel in which the amount of alloying elements is suppressed more than that of the 2.25Cr-1.0Mo steel is used, and PWHT for a particularly long time is performed on the steel plate. Even when the steel member was manufactured by applying the steel member, intensive research was conducted in order to obtain a steel member having excellent low temperature toughness and strength in the central portion of the plate thickness.

As a result, first, in order to obtain a steel member having a high toughness in the central part of the plate thickness
-A fine structure is used, and the grain boundary carbides that tend to become coarse and easily become the starting point of fracture are refined. Specifically, (a) at least one of tempered bainite and tempered martensite, and (b) the average circle of crystal grains surrounded by large-angle grain boundaries in which the orientation difference between two adjacent crystals is 15° or more. When the equivalent diameter (hereinafter sometimes simply referred to as "large-angle grain boundary size") is D and the maximum diameter of grain boundary carbide is d, the value represented by D/d is 54 or less; and temper embrittlement To suppress sensitivity, in particular, to satisfy the component composition described later;
Have found that is effective. The above "suppression of temper embrittlement susceptibility" is also referred to as "suppression of temper embrittlement" and "suppression of grain boundary cracking".

Below, the said (a) and (b) regarding the microstructure of the plate|board thickness center part of the steel member of this invention are demonstrated first.

In the following description, the "structure in the central part of the plate thickness" is simply referred to as "structure". Further, the properties shown below, that is, strength and low temperature toughness, refer to each property of at least the central portion of the plate thickness after welding and PWHT are performed on a steel member, that is, a steel plate.

(A) The structure is at least one of tempered bainite and tempered martensite.
The tempered bainite and the tempered martensite have a fine structure, and are structures particularly effective for ensuring the strength and toughness of the central portion of the plate thickness. The structure of the steel member of the present invention is at least one of tempered bainite and tempered martensite. Other structures that can be inevitably included include polygonal ferrite, retained austenite, and pearlite, but these structures can be suppressed to 5 area% or less in total, and most preferably these structures are 0 area%. is there. In particular, when the polygonal ferrite is present, the coarse upper bainite structure having a crystal grain size is the main constituent, and good toughness cannot be secured.

(B) When the average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries in which the orientation difference between two adjacent crystals is 15° or more is D and the maximum diameter of grain boundary carbides is d, it is expressed as D/d. The value to be displayed is 54 or less.

The structure of the central portion of the plate thickness, as described above, by at least one of tempered bainite and tempered martensite, it is possible to refine the structure, in the present invention, by reliable refinement of the structure In order to obtain high toughness, the above (b) is specified.

In the case of structures of tempered bainite and tempered martensite, generally, a so-called large-angle grain boundary in which the orientation difference (crystal orientation difference) between two adjacent crystals is 15° or more is a difference between two adjacent crystal orientations. Is large, the progress of brittle fracture is curved, the fracture surface unit of brittle fracture becomes small, and it contributes to the improvement of toughness. On the other hand, the steel member of the present invention has undergone PWHT, in particular, PWHT for a long time, and further PWHT for a long time at high temperature, as described above. When Cr-Mo steel constituting a steel member is subjected to PWHT, M ₂₃ C ₆ grain boundary carbides are generally formed. When the PWHT conditions are severe such as high temperature and long time, the grain boundary carbides become coarse and easily become a starting point of fracture, resulting in deterioration of toughness.

In the present invention, regarding the relationship between the average circle equivalent diameter D of these large-angle grain boundary sizes and the maximum diameter d of the grain boundary carbides, if the value represented by D/d as described above in (b) satisfies 54 or less, It has been found that a sufficiently excellent toughness can be secured even after PWHT. The D/d is preferably 50 or less, more preferably 48 or less. The lower limit of D/d is about 12 in consideration of the component composition and manufacturing conditions specified in the present invention.

In the present invention, it suffices that D/d be 54 or less, and the individual values of the average circle equivalent diameter D of the large angle grain boundary and the maximum diameter d of the grain boundary carbide are not particularly limited. The average circle equivalent diameter D of the large-angle grain boundary size can be, for example, 45 μm or less, further 35 μm or less, further 30 μm or less, further 25 μm or less, further 15 μm or less. The lower limit of the large-angle grain boundary size is about 10 μm in manufacturing. Further, the maximum diameter d of the grain boundary carbide can be set to, for example, 0.8 μm or less. The maximum diameter of the grain boundary carbide can be 0.70 μm or less, and further 0.60 μm or less. The lower limit of the maximum diameter of the grain boundary carbide is about 0.20 μm within the range of the component composition and manufacturing conditions specified in the present invention.

In the present invention, it is necessary to control the structure of the central part of the plate thickness as described above, but the structure of other parts such as the surface layer part of the plate thickness is not particularly limited. In addition, since the cooling rate at the time of quenching is generally higher in the surface layer side part than the plate thickness center part, it is easy to obtain a finer structure than in the plate thickness center part, and both strength and toughness It tends to be better than the thick center part.

In order to obtain the above-mentioned fine structures (a) and (b) in the central part of the plate thickness, it is necessary that the composition of the steel sheet used for manufacturing the steel member be as follows. That is, it is necessary to improve the hardenability by including B in an amount described below and making it exist as free B (solid solution B) in order to make the average circle equivalent diameter D fine. For that purpose, in order to secure free B, it is important to fix N, which is easy to combine with B to form BN, as AlN by adding the amount of Al described later. This AlN is useful for suppressing coarsening of prior austenite (γ) grains during quenching and obtaining a fine structure.

As described above, for the refinement of D, it is effective to add an alloying element to improve the hardenability. However, excessive C, excessive Cu and Ni increase the strength more than necessary to improve the toughness. Cause decline. Therefore, from the viewpoint of ensuring toughness, it is necessary to set the upper limits of C, Cu and Ni.

Further, in the present invention, the contents of Nb and Ti are suppressed. This is because if the content of these elements is large, it becomes difficult to achieve the D/d in the above range. Further, these elements increase the strength more than necessary, resulting in deterioration of workability. Further, the contents of Ca, Mg, REM and Zr are also suppressed. This is because these elements increase inclusions and lead to a decrease in toughness. Further, in order to control the size of the grain boundary carbides, it is necessary to control the Cr content in addition to the above C. Furthermore, in order to suppress temper embrittlement susceptibility and ensure toughness, it is also necessary to control the content of Si and the like.

Further, as the manufacturing conditions, as will be described in detail later, it is important to appropriately control the conditions of quenching and tempering at the time of manufacturing the steel sheet to be used for welding.

In the following, first, the component compositions of the steel plate and the steel member necessary for ensuring the above-mentioned structure and characteristics will be described.

C: 0.110% or more and 0.15% or less When quenching a steel sheet, C obtains at least one of tempered bainite and tempered martensite and increases hardenability even in the central portion of the thickness where the cooling rate is small. This is an element necessary for making the average crystal grain size D fine and setting D/d within the above range. Further, it is an element necessary for securing grain boundary carbides and obtaining sufficient base metal strength. In order to fully exert these effects, the C content is set to 0.110% or more. The C content is preferably 0.120% or more, more preferably 0.130% or more. However, when the amount of C is excessive, after a long PWHT, the grain boundary carbides are coarsened and the toughness deteriorates. In addition, weld cracking tends to occur during welding of steel sheets. Therefore, the C content is 0.15% or less. The C content is preferably 0.145% or less.

Si: 0.50% or more and 0.80% or less Si is an element effective for improving the base metal strength of the steel member, that is, the strength of the central portion of the plate thickness. It is also an element used as a deoxidizer. Furthermore, it is an element that is also useful for securing toughness by suppressing temper embrittlement susceptibility. In order to exert these effects, the Si content is 0.50% or more. The amount of Si is preferably 0.55% or more, more preferably 0.60% or more. However, if the Si amount becomes excessive, the temper embrittlement susceptibility increases and the toughness deteriorates, so the content is made 0.80% or less. The amount of Si is preferably 0.75% or less, more preferably 0.70% or less.

Mn: 0.40% or more and 0.65% or less Mn stabilizes austenite and lowers the transformation temperature to improve hardenability and obtain a fine structure, resulting in strength and toughness. It is an effective element for securing. In order to exert such effects, Mn is contained by 0.40% or more. The Mn amount is preferably 0.45% or more, more preferably 0.46% or more. However, if Mn is excessively contained, temper embrittlement susceptibility increases and toughness deteriorates. Therefore, the Mn content is 0.65% or less, preferably 0.60% or less, more preferably 0.55% or less, still more preferably 0.50% or less.

P: more than 0% and 0.0070% or less P, which is an unavoidable impurity, adversely affects the toughness of the base metal and the welded portion, and segregates particularly at the grain boundaries of the steel member, causing intergranular cracking and degrading the toughness. .. In order not to cause these inconveniences, the P amount is suppressed to 0.0070% or less. The P amount is preferably 0.0060% or less, more preferably 0.0050% or less.

S: more than 0% and 0.0070% or less S is an element that forms MnS and easily causes weld cracking during welding of steel sheets. Therefore, the S content is preferably as small as possible, and the S content is suppressed to 0.0070% or less, preferably 0.0050% or less, and more preferably 0.0030% or less.

Al: 0.030% or more and 0.080% or less Al is an extremely important element in the present invention as described above, and is an element necessary for securing N as AlN at the time of quenching and ensuring hardenability by free B. is there. Further, AlN is useful for suppressing coarsening of old γ grains during quenching and obtaining a fine structure. Further, Al is also an element necessary for deoxidation. In order to exert these effects, the amount of Al is set to 0.030% or more. The Al amount is preferably 0.040% or more, more preferably 0.045% or more, still more preferably 0.050% or more. On the other hand, when the amount of Al is excessive, coarse alumina-based inclusions are formed and the toughness is reduced. Therefore, the Al amount is 0.080% or less. The amount of Al is preferably 0.075% or less, more preferably 0.071% or less.

Cu: 0.05% or more and 0.20% or less, Ni: 0.05% or more and 0.30% or less Cu and Ni are effective elements for increasing the strength without significantly impairing the toughness. In order to sufficiently exert this effect, Cu is 0.05% or more, preferably 0.10% or more, more preferably 0.11% or more, and Ni is 0.05% or more, preferably 0.10% or more. , More preferably 0.15% or more, still more preferably 0.16% or more. However, the addition of a large amount of these elements causes the strength to be increased more than necessary and the toughness to be lowered as described above. Therefore, the upper limit of the amount of Cu is 0.20% or less and the upper limit of the amount of Ni is 0.30% or less. The amount of Cu is more preferably 0.18% or less, still more preferably 0.17% or less. The amount of Ni is more preferably 0.28% or less, still more preferably 0.26% or less.

Cr: 1.05% or more and 1.50% or less Cr is an element effective in suppressing the coarsening of carbide due to PWHT and ensuring the toughness of the steel member. In addition, it is an element that is effective for securing strength in the medium and high temperature range and for improving corrosion resistance. In order to exert these effects, Cr is contained at 1.05% or more. The Cr amount is preferably 1.10% or more, more preferably 1.20% or more. On the other hand, if Cr is contained excessively, temper embrittlement susceptibility is increased, grain boundary cracking easily occurs after PWHT, and toughness is adversely affected. Further, excessive Cr causes deterioration in workability and weldability, and further increases in manufacturing cost. Therefore, the Cr content is set to 1.50% or less. The Cr amount is preferably 1.45% or less, more preferably 1.40% or less.

Mo: 0.45% or more and 0.65% or less Mo is an element that enhances hardenability and is effective in suppressing temper embrittlement. To obtain these effects, it is necessary to contain Mo by 0.45% or more. The amount of Mo is preferably 0.50% or more, more preferably 0.55% or more. On the other hand, even if the amount of Mo exceeds 0.65%, the improvement of the effect is small, which leads to an increase in manufacturing cost. Therefore, the upper limit of the amount of Mo is made 0.65% or less. The amount of Mo is preferably 0.62% or less.

N: 0.0030% to 0.0070% N is an important element in the present invention together with Al. By generating AlN and fixing N during quenching, the hardenability improving effect of Free B can be maximized. AlN is also useful for suppressing coarsening of old γ grains during quenching and obtaining a fine structure. When the amount of N is less than 0.0030%, AlN becomes insufficient and the old γ grains become coarse, and as a result, a fine structure cannot be obtained and the toughness deteriorates. Therefore, the amount of N is 0.0030% or more. It is preferably 0.0035% or more, more preferably 0.0040% or more. On the other hand, when the amount of N exceeds 0.0070%, the N fixing effect of Al cannot be obtained and BN is generated, and the hardenability improving effect of free B is hindered, the structure becomes coarse, and the toughness deteriorates. To do. Therefore, the N content is 0.0070% or less. The N content is preferably 0.0060% or less, more preferably 0.0055% or less, still more preferably 0.0050% or less.

B: 0.0003% or more and 0.0010% or less As described above, B is present as free B (solid solution B) to enhance hardenability, and in particular, the cooling rate during quenching is slow and the plate thickness is thick. The average crystal grain size D can be refined also in the central part of the plate thickness of the steel plate. As a result, excellent toughness can be secured even in the central portion of the plate thickness. In order to obtain such an effect, B is required to be 0.0003% or more even on the premise that the above-mentioned Al and N contents and the quenching conditions described later are controlled. The amount of B is preferably 0.0005% or more, more preferably 0.0007% or more. On the other hand, if B is contained excessively, the hardenability may be rather deteriorated, or weld cracking may be caused. Therefore, the upper limit of the B content is 0.0010%. The amount of B is preferably 0.0009% or less, more preferably 0.0008% or less.

V: 0% or more and 0.030% or less V is an element that is effective in forming carbides and nitrides to contribute to the improvement of strength and enhancing hardenability to obtain a fine structure. In order to obtain these effects, the V content may be preferably 0.003% or more. The V amount is more preferably 0.005% or more. On the other hand, excessive addition of V causes an increase in cost, so the upper limit is made 0.030% or less. The V amount is preferably 0.027% or less, more preferably 0.020% or less, still more preferably 0.010% or less.

Nb is 0.005% or less, Ti is 0.001% or less, and the total of Ca, Mg, REM and Zr is 0.0010% or less. In the present invention, Nb is 0.005% or less and Ti is 0.001%. Below, the sum of Ca, Mg, REM (Rare Earth Metal) and Zr is suppressed to 0.0010% or less. As described above, Nb and Ti make the old γ grains during quenching fine and reduce the hardenability. As a result, the large-angle grain boundary size is coarse, that is, the average equivalent circle diameter D becomes large, and D/d exceeds the specified range. In addition, Nb and Ti are elements that increase the strength more than necessary and cause deterioration of workability. Further, Ca, Mg, REM, and Zr increase inclusions, leading to a decrease in toughness. From the above, it is preferable to suppress these elements as much as possible, and all elements may be zero. In the present invention, the REM means a lanthanoid element, that is, 15 elements from La to Lu, and scandium and yttrium.

The steel sheet and steel member of the present invention contain the above chemical components, and the balance is iron and inevitable impurities.

Next, a method of manufacturing the steel sheet and the steel member of the present invention will be described. First, a method of manufacturing a steel sheet will be described.

A steel piece having the above-described composition is hot-rolled by a conventional method to obtain a steel sheet, and then the steel sheet is quenched and tempered. In order to obtain the fine structure specified in (a) and (b) of the steel member, it is necessary to perform quenching and tempering under the following conditions in the manufacturing process of the steel sheet.

Quenching heating temperature: 910°C or more and 940°C or less, and holding time at the heating temperature: 25 minutes or more and 60 minutes or less By setting the quenching heating temperature to 910 to 940°C and the heating holding time to 25 minutes or more, The old γ grains can be grown to some extent, and as a result, the hardenability is improved and a fine structure can be obtained.

When the heating temperature of quenching is lower than 910°C, the old γ grains during quenching remain fine, so that a fine structure cannot be obtained in a portion with a slow cooling rate, such as the central portion of the plate thickness, which is excellent. Toughness cannot be secured. Therefore, the heating temperature for quenching is 910° C. or higher. It is preferably 920° C. or higher. On the other hand, when the heating temperature exceeds 940° C., part of N fixed as AlN is solid-solved and combined with B to form BN, and the effect of improving the hardenability by free B cannot be obtained. As a result, a fine structure cannot be obtained and the toughness deteriorates. Therefore, the heating temperature for quenching is set to 940°C or lower. It is preferably 935° C. or lower.

Even if the heating temperature at the time of quenching is within the above range, if the holding time at the heating temperature (heating holding time) is shorter than 25 minutes, the old γ grains remain fine. Even if it is included, sufficient hardenability cannot be obtained, and as a result, the structure becomes coarse and the toughness deteriorates. Therefore, the heating and holding time is set to 25 minutes or more. It is preferably 30 minutes or more. The upper limit of the heating and holding time is 60 minutes or less, preferably 55 minutes, from the viewpoint of productivity and the like.

In addition, it is preferable to control the quenching conditions as described above so that the old γ grain size is within the range of about 50 to 100 μm because a fine structure can be easily obtained.

Following the quenching, tempering is performed at a temperature of 620° C. or higher and an Ac ₁ point or lower, and a heating temperature and a heating time at which the P _T value represented by the following formula (1) is 19.2 or more and 20.6 or less.
P _T value=T _T ×(20+logt _T )×10 ⁻³ (1)
In the formula (1), T _T represents a heating temperature for tempering (K), and t _T represents a heating time for tempering (hr).

Heating temperature for tempering (tempering temperature): 620°C or higher and Ac ₁ point or lower In the above-mentioned quenching, the cooling rate is high near the surface layer regardless of the plate thickness, and the hardness of the surface layer tends to become hard. Thus, workability such as bending of the steel sheet can be improved. Therefore, in the manufacturing process of the steel member, it is preferable to perform tempering in order to reduce the hardness of the surface layer from the viewpoint of improving the workability of the steel sheet. As a condition for tempering, it is preferable to set the tempering temperature to 620° C. or higher and Ac ₁ point or lower. By setting the tempering temperature to 620° C. or higher, the hardness of the surface layer is sufficiently reduced, and good workability can be secured. The tempering temperature is more preferably 700° C. or higher. On the other hand, when the tempering temperature exceeds the Ac ₁ point, a part of the structure undergoes reverse transformation and is subsequently air-cooled, so that polygonal ferrite becomes mixed. As a result, at least one of tempered bainite and tempered martensite, which are the desired structures, cannot be obtained, leading to a decrease in strength, and the reverse transformation portion also has a rough structure, leading to a decrease in toughness. Therefore, the upper limit of the tempering temperature is preferably Ac ₁ point or less. The tempering temperature is more preferably 750°C or lower. The Ac ₁ point is determined by the method described in Examples below.

The tempering is further carried out at a heating temperature and a heating time such that the P _T value represented by the prescribed formula (1) falls within the above range. If the P _T value is less than 19.2, the hardness becomes too high and the workability deteriorates. Therefore, the P _T value is 19.2 or more, preferably 19.3 or more, and more preferably 19.4 or more. On the other hand, when the P _T value exceeds 20.6, coarsening of carbides occurs and the characteristics such as toughness deteriorate. Therefore, the P _T value is 20.6 or less, preferably 20.3 or less, and more preferably 20.0 or less.

The plate thickness of the steel sheet of the present invention is 100 mm or less. The lower limit of the plate thickness is 6 mm or more, and further 10 mm or more. The steel member obtained by using the above steel plate also has the same plate thickness as the above steel plate.

The steel member of the present invention is welded to a steel sheet obtained by carrying out the above-mentioned quenching and tempering by a generally-used method, and further, a post-weld heat treatment (PWHT) for removing strain as described above. It is obtained by applying.

The method for manufacturing a steel member of the present invention is characterized in that the post-weld heat treatment is performed at a heating temperature and a heating time at which the P _PWHT value represented by the following formula (2) becomes 20 or more. This condition indicates a severe condition of high temperature and long time (for example, when the temperature is 680° C. or higher and the heating time is 20 hours or longer, the P _PWHT value is 20.3). According to the present invention, a steel member having sufficiently excellent toughness can be obtained even after the heat treatment under such severe conditions of high temperature and long time. Examples of the PWHT conditions include heating temperature: 600 to 690° C. and heating time: 5 hours to 22 hours.
P _PWHT value=T _PWHT ×(20+logt _PWHT )×10 ⁻³ (2)
In the formula (2), T _PWHT represents the heating temperature (K) of the heat treatment after welding, and t _PWHT represents the heating time (hr) of the heat treatment after welding.

The steel member of the present invention can be used, for example, as a medium/high temperature pressure vessel used in the chemical industry including petroleum refining.

Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to the following Examples, and may be appropriately modified within a range compatible with the gist of the preceding and the following. Of course, it is possible to carry out, and all of them are included in the technical scope of the present invention.

Steel pieces satisfying the component compositions shown in Table 1-1 and Table 1-2 were hot-rolled by a conventional method, and then quenched and tempered under the conditions shown in Table 2 to obtain the plate thicknesses shown in Table 2. A steel plate was obtained. The plate thickness is also the plate thickness of a test piece simulating a steel member. The Ac ₁ point shown in Table 2 was obtained by analyzing the expansion coefficient change when heating was performed at a temperature rising rate of 0.5° C./sec using steel sheets having the component compositions shown in Table 1-1 and Table 1-2. I asked. The heating temperature for quenching and tempering is the temperature at the center of the plate thickness of the steel sheet, and can be calculated by the difference method from the atmospheric temperature in the furnace of the heat treatment furnace and the time in the furnace, or when the experimental furnace is used, the plate thickness is the same. It is the temperature actually measured by inserting a thermocouple into the dummy material.

Further, after simulating PWHT after welding, heat treatment was performed in a trolley-type electric furnace in the air atmosphere under the conditions of heating temperature: 690° C. and heating holding time: 22 hours to obtain a test piece simulating a steel member. It was The above-mentioned condition is a remarkably severe condition among the currently practiced conditions, and in this case, the P _PWHT value is 20.6. The rate of temperature increase from room temperature to the heating temperature and the rate of temperature decrease from the heating temperature to room temperature were both 55° C./hr or less.

When manufacturing a steel member, the steel plate is welded and then PWHT is applied. After the welding, for example, multi-layer welding is performed, the welding is performed after the characteristics of the steel member (including the weld heat affected zone) ( In particular, the test piece was produced without performing the heat treatment for welding in this example because it does not adversely affect the toughness.

Using the test pieces obtained as described above, evaluation of the metal structure, tensile test, and Charpy impact test were carried out in the following manner. Further, in order to evaluate the workability of a steel sheet, which is a characteristic required in the manufacturing process of steel members, the surface hardness was measured using the steel sheet before the PWHT was performed.

[Observation of metal structure]
The observation of the metal structure was carried out as follows.
(1) A sample was taken from the above steel sheet so that a cross section of the sheet thickness including the front and back surfaces of the steel sheet, which was parallel to the rolling direction and perpendicular to the steel sheet surface, could be observed.
(2) The observation surface was mirror-finished by polishing with wet emery polishing paper (#150 to #1000) or by a polishing method having a function equivalent to that (polishing with an abrasive such as diamond slurry). ..
(3) The polished sample was corroded with a 3% nital solution to reveal grain boundaries.
(4) At the t (plate thickness)/2 site, the exposed tissue was photographed at a magnification of 400 times. In this example, a 6 cm×8 cm photograph was taken. Next, in the photograph taken, it was determined that polygonal ferrite was generated at the old austenite grain boundary, and it was painted black. Next, the photograph was taken into an image analysis device. The area of the photograph corresponds to 150 μm×200 μm at 400 times magnification. The image was taken into the image analysis device so that the total area was 1 mm×1 mm or more at any magnification. That is, at a magnification of 400, at least 35 of the above photographs were taken.
(5) In the image analysis device, the black area ratio was calculated for each photo, and the average value of all the photos was taken as the polygonal ferrite (F) fraction, which was subtracted from the whole to obtain tempered bainite and tempered martens. The fraction of at least one of the sites (B+M) was used.

The term "tempered bainite" used herein refers to a structure in which upper bainite, lower bainite, bainitic ferrite, etc. are tempered, but it is generally difficult to select these structures including tempered martensite. In addition, since the structure was sufficiently tempered after PWHT, the structure other than polygonal ferrite was defined as at least one of tempered bainite and tempered martensite (B+M). It was also confirmed that none of the test pieces used in this example contained a pearlite structure.

[Measurement of large-angle grain boundary size by EBSP method]
Using the EBSP method, the average equivalent circle diameter (large-angle grain boundary size) of crystal grains surrounded by large-angle grain boundaries in which the orientation difference (crystal orientation difference) between two adjacent crystals was 15° or more was determined. The measurement procedure is as follows.
(1) A sample was taken from the above steel plate so that a cross section of the plate thickness including the front and back surfaces of the steel plate, which was parallel to the rolling direction and perpendicular to the steel plate surface, could be observed.
(2) The observation surface was mirror-finished by polishing with wet emery polishing paper (#150 to #1000) or by a polishing method having a function equivalent to that (polishing with an abrasive such as diamond slurry). ..
(3) Using an EBSP (Electron BackScattering Pattern) device manufactured by TexSEM Laboratories, the measurement range at t/2 part in the plate thickness direction: 200×200 μm, 0.5 μm pitch, with a crystal orientation difference of 15° or more. Was used as the crystal grain boundary, and the size of the crystal grain (large tilt angle grain) surrounded by the crystal grain boundary was measured. At this time, the measurement points whose confidence index indicating the reliability of the measurement direction is smaller than 0.1 were excluded from the analysis target.
(4) The average value of the sizes of the crystal grains surrounded by the large-angle grain boundaries obtained in this way is calculated, and in the present invention, “the large-angle grain boundaries in which the orientation difference between two adjacent crystals is 15° or more is The average circle equivalent diameter of the formed crystal grains". When the size of the crystal grain surrounded by the large angle grain boundaries was 1.0 μm or less, it was determined as measurement noise and excluded from the target of average value calculation.

[Measurement of grain boundary carbide size]
The grain boundary carbide size was measured as follows.
(1) A sample was taken from the above steel sheet so that a cross section of the sheet thickness including the front and back surfaces of the steel sheet, which was parallel to the rolling direction and perpendicular to the steel sheet surface, could be observed.
(2) The observation surface was mirror-finished by polishing with wet emery polishing paper (#150 to #1000) or by a polishing method having a function equivalent to that (polishing with an abrasive such as diamond slurry). ..
(3) The polished sample was corroded with a 3% nital solution to reveal grain boundaries.
(4) At t (which refers to the plate thickness; the same applies hereinafter)/2 site, the exposed tissue was photographed at a magnification of 1000 times. In this example, a 6 cm×8 cm photograph was taken. Next, the photograph was taken into an image analysis device. The area of the photograph corresponds to 60 μm×80 μm when the magnification is 1000 times. The image was taken into the image analyzer so that the total area was 0.4 mm×0.4 mm or more. That is, in the case of 1000 times, at least 35 of the above photographs were taken.
(5) In the image analyzer, the minor axis length was calculated as the size of the grain boundary carbide for each photograph, and the maximum value of the grain boundary carbide size of all the photographs was calculated.

[Tensile test (evaluation of tensile properties)]
A round bar tensile test piece was sampled in the direction perpendicular to the rolling direction from the t/2 portion, and a tensile test was performed in the same manner as ASTM A370 to measure the yield strength and the tensile strength. Then, the case where YS which is the yield strength is 310 MPa or more and TS which is the tensile strength is 515 MPa or more was evaluated as high strength.

[Charpy impact test (evaluation of impact properties)]
A full-size V-notch test piece was sampled in the direction perpendicular to the rolling direction from the t/2 portion, and a Charpy impact test was performed at a test temperature of -38°C according to the procedure of ASTM A370 to measure the Charpy absorbed energy. The Charpy absorbed energy was the average value of three test pieces. Then, when the Charpy absorbed energy vE _{−38 at} −38° C. was 100 J or more, it was evaluated that the toughness was excellent, that is, the impact property was excellent.

[Measurement of surface hardness (evaluation of workability of steel sheet)]
In order to evaluate the workability of the steel sheet, a Brinell hardness test was performed according to ASTM 370 at a position 1 mm deep from the surface using the steel sheet before PWHT. Then, when the average value of HBW was 200 or less, it was evaluated as excellent in workability, and when the average value of HBW was more than 200, the workability was evaluated as normal level.

The results are shown in Tables 2 and 3. In addition, the following No. Is the test No. of Table 2 and Table 3. Indicates.

The following can be seen from Table 1-1, Table 1-2, Table 2 and Table 3. That is, No. Steel sheets 1 to 5, 7 to 9, and 12 to 36 were produced using the steel satisfying the prescribed composition in the present invention and under the prescribed conditions, so that the steel sheet exhibited excellent workability and was obtained. The steel member had a desired structure and exhibited excellent strength and toughness in the central portion of the plate thickness.

On the other hand, in the examples other than the above, the result is that the workability of the steel sheet cannot be secured or at least one of the tensile properties and impact properties in the central part of the plate thickness is inferior because either the component composition or the manufacturing conditions are out Became.

That is, No. Sample No. 6 satisfied the component composition, but the P _T value at the time of tempering was too low, so it was not sufficiently tempered, and the Brinell hardness was high, that is, the workability was poor. On the other hand, No. Although No. 11 satisfied the component composition, the P _T value at the time of tempering was too high, so that the carbides coarsened and the properties deteriorated.

No. Although No. 10 satisfied the component composition, since the heating time for quenching was too short, quenching was not sufficiently performed, and D/d exceeded the upper limit, resulting in poor toughness.

No. Since No. 37 had an excessive amount of C, the toughness was deteriorated, and the Brinell hardness was high and the workability was poor.

No. Since Nos. 38, 42 and 49 did not contain B, D/d was large and the toughness was poor. In addition, No. The sample No. 48, which did not contain B, had a large D/d and had an excessive amount of P, and thus was inferior in toughness.

No. 39 and No. 39. Since No. 46 contained a certain amount or more of Nb, the old γ grains at the time of quenching became fine, sufficient hardenability was not obtained, D/d increased, and the toughness was poor. In addition, No. In No. 46, the workability also deteriorated.

No. Nos. 40 and 43 were not sufficient in hardenability due to lack of C content, D/d was large, and toughness was poor. In addition, No. In No. 41, since the amount of C was insufficient, a large amount of ferrite was generated and desired strength could not be ensured, and D/d became large, resulting in poor toughness. No. No. 44 was insufficient in the amount of C and did not contain B, so that it was not possible to secure sufficient hardenability, and as a result, the strength was low and the D/d became large and the toughness deteriorated. No. In No. 51, since the amount of C was insufficient, the carbide size was small and D/d was large, and it was not possible to secure particularly desired toughness.

No. Since No. 45 contained a certain amount or more of Ti, the old γ grains during quenching became fine, sufficient hardenability was not obtained, D/d increased, and toughness was poor.

No. No. 47 was inferior in toughness because the P amount was excessive.

No. In No. 50, the amount of B was insufficient and the hardenability was insufficient, so the toughness decreased.

No. No. 52 contained Cu and Ni in an excessive amount and contained an excessive amount of C, so that the toughness was deteriorated.

FIG. 1 is a graph showing the relationship between D/d and Charpy absorbed energy at −38° C., using the data in Tables 2 and 3 above. From this graph, it is understood that if D/d is 54 or less, sufficiently excellent toughness can be secured. No. 1 in FIG. As described above, 47 and 52 are examples in which the D/d satisfies the range of the present invention, but the toughness is lowered due to the deviation of the component composition.

Claims (4)

C: 0.110% (meaning mass%; the same applies to chemical components) 0.15% or less,
Si: 0.50% or more and 0.80% or less,
Mn: 0.40% or more and 0.65% or less,
P: more than 0% and 0.0070% or less,
S: more than 0% and 0.0070% or less,
Al: 0.030% or more and 0.080% or less,
Cu: 0.05% or more and 0.20% or less,
Ni: 0.05% or more and 0.30% or less,
Cr: 1.05% or more and 1.50% or less,
Mo: 0.45% or more and 0.65% or less,
N: 0.0030% or more and 0.0070% or less,
B: 0.0003% or more and 0.0010% or less, and V: 0% or more and 0.030% or less,
Nb is 0.005% or less, Ti is 0.001% or less, and the total amount of Ca, Mg, REM and Zr is suppressed to 0.0010% or less, and the balance is iron and inevitable impurities,
The plate thickness is 100 mm or less,
A steel member characterized in that the structure in the central portion of the plate thickness satisfies all of the following (a) and (b), and the Charpy absorbed energy at -38°C is 100 J or more.
(A) The structure is at least one of tempered bainite and tempered martensite.
(B) When the average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries in which the orientation difference between two adjacent crystals is 15° or more is D and the maximum diameter of grain boundary carbides is d, it is expressed as D/d. The value to be displayed is 54 or less. A steel plate used for manufacturing the steel member according to claim 1,
C: 0.110% or more and 0.15% or less,
Si: 0.50% or more and 0.80% or less,
Mn: 0.40% or more and 0.65% or less,
P: more than 0% and 0.0070% or less,
S: more than 0% and 0.0070% or less,
Al: 0.030% or more and 0.080% or less,
Cu: 0.05% or more and 0.20% or less,
Ni: 0.05% or more and 0.30% or less,
Cr: 1.05% or more and 1.50% or less,
Mo: 0.45% or more and 0.65% or less,
N: 0.0030% or more and 0.0070% or less,
B: 0.0003% or more and 0.0010% or less, and V: 0% or more and 0.030% or less,
Nb is 0.005% or less, Ti is 0.001% or less, the total of Ca, Mg, REM, and Zr is suppressed to 0.0010% or less, the balance is iron and inevitable impurities, and the plate thickness is 100mm Ri der below,
The structure is at least one of tempered bainite and tempered martensite,
When the heat treatment is performed under the conditions of heating temperature: 690° C. and heating holding time: 22 hours, the average equivalent circle diameter of the crystal grains surrounded by the large angle grain boundaries in which the orientation difference between two adjacent crystals is 15° or more is determined. D, where D is the maximum diameter of the grain boundary carbides, the value represented by D/d is 54 or less . It is a manufacturing method of the steel plate of Claim 2, Comprising: The steel piece which satisfy|fills the component composition of Claim 2 is hot-rolled, Then, quenching is carried out at heating temperature: 910 degreeC or more and 940 degreeC or less, and this heating temperature. Holding time: 25 minutes or more and 55 minutes or less, tempering after the quenching, heating temperature: 620° C. or more, Ac ₁ point or less, and a P _T value represented by the following formula (1) of 19.2 or more A method for producing a steel sheet, which is performed at a heating temperature and a heating time of 20.6 or less.
P _T value=T _T ×(20+logt _T )×10 ⁻³ (1)
In the formula (1), T _T represents a heating temperature for tempering (K), and t _T represents a heating time for tempering (hr). It is a manufacturing method of the steel member of Claim 1, Comprising: Welding using the steel plate of Claim 2, and also post-weld heat processing makes P _PWHT value represented by following formula (2) 20 or more. The method for manufacturing a steel member is characterized in that the heating is performed at different heating temperatures and heating times.
P _PWHT value=T _PWHT ×(20+logt _PWHT )×10 ⁻³ (2)
In the formula (2), T _PWHT represents the heating temperature (K) of the heat treatment after welding, and t _PWHT represents the heating time (hr) of the heat treatment after welding.