WO2017077967A1

WO2017077967A1 - Steel member and steel plate, and production processes therefor

Info

Publication number: WO2017077967A1
Application number: PCT/JP2016/082223
Authority: WO
Inventors: 亮太宮田; 克壮小林
Original assignee: 株式会社神戸製鋼所
Priority date: 2015-11-06
Filing date: 2016-10-31
Publication date: 2017-05-11
Also published as: KR102106766B1; JP6735082B2; KR20180075659A; CN108350539A; EP3372702A1; CN108350539B; EP3372702A4; EP3372702B1; JP2017088938A

Abstract

Provided is a steel member that includes a plate-thickness central part which has high strength and highly excellent toughness even when PWHT after welding was conducted for a long time in a production step for the steel member. The steel member has a thickness of 100 mm or less, and contains C, Si, Mn, P, S, Al, Cu, Ni, Cr, Mo, N, B, and V respectively within the delimited ranges and has an Nb content reduced to 0.005% or less, a Ti content reduced to 0.001% or less, and a total content of Ca, Mg, REM, and Zr reduced to 0.0010% or less, with the remainder comprising iron and unavoidable impurities. The steel member is characterized in that the plate-thickness central part has a structure which satisfies both of the following (a) and (b) and the steel member has a Charpy absorption energy at -38ºC of 100 J or greater. (a) The structure is tempered bainite and/or tempered martensite. (b) When the average equivalent circular diameter of crystal grains each surrounded by high-angle grain boundaries where the two adjoining crystals have a difference in orientation of 15º or greater is expressed by D and the maximum diameter of grain-boundary carbides is expressed by d, then the value represented by D/d is 54 or less.

Description

Steel member, steel plate and production method thereof

The present invention relates to a steel member, a steel plate, and a manufacturing method thereof. More specifically, the present invention relates to a steel member obtained by subjecting a steel plate to welding and a post-weld heat treatment (hereinafter sometimes referred to as “PWHT”), in particular, the PWHT at a high temperature for a long time. Even if it exists, it is related with the steel member excellent in the intensity | strength and low-temperature toughness of a plate | board thickness center part, the steel plate used for manufacture of this steel member, and these manufacturing methods. Hereinafter, the low temperature toughness is sometimes simply referred to as “toughness”.

Middle and high temperature pressure vessels used in the chemical industry including petroleum refining tend to require further high temperature and pressure resistance for the purpose of improving the efficiency of operation. Therefore, the steel plate used for the steel member such as the pressure vessel is required to have high strength. From the viewpoint of safety, the steel member is also required to have a high level of low temperature toughness.

In order to increase the strength, the steel sheet is subjected to normalization and quenching. However, if the plate thickness of the steel plate is thick, there is a problem that the cooling rate is low in the steel plate during normalization or quenching, particularly in the central portion of the plate thickness, and high strength and the like are difficult to obtain. By the way, the steel member such as the pressure vessel is obtained by performing stress-relieving annealing for removing strain, that is, PWHT after welding the steel plate. Although PWHT is performed for a long time to remove the strain, a steel member subjected to PWHT for a long time has a problem that low-temperature toughness and the like are lowered.

Further, as a method for ensuring high toughness, increasing the amount of alloy elements can be mentioned. For the steel member such as the pressure vessel, Cr—Mo steel containing Cr and Mo as alloy elements is used. For example, when 2.25Cr-1.0Mo steel is used as the Cr—Mo steel, it is known that good toughness can be obtained even in the central portion of the thick steel plate where it is difficult to ensure toughness. In recent years, however, the desire to save resources and reduce costs has increased. Therefore, it is strongly sought to realize a steel member that is superior in strength and toughness at the center 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. It has been.

In response to the above problems, a technique for achieving high strength and high toughness by appropriately adjusting chemical components while suppressing the amount of alloying elements has been proposed. For example, Patent Documents 1 and 2 disclose techniques for improving low-temperature toughness for steel having a component composition of 1.25Cr-0.5Mo level, which is difficult to ensure toughness.

Patent Document 1 discloses a technique that secures hardenability by adding Nb and Ca and suppresses deterioration in characteristics during SR (Stress Relief). However, when this technique is applied to thick steel plates mainly cast by the ingot-making method, there is a concern that the Ca forms coarse inclusions and adversely affects toughness. Therefore, it seems that it is difficult to stably ensure the toughness of the central part of the thick steel member.

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

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

The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to produce a steel material even when the PWHT after welding is set to a long time, particularly a high temperature and a long time, in the manufacturing process of the steel member. The object is to establish a steel member having high strength and high low temperature toughness inside, a steel plate useful for the production of the steel member, and a production method thereof. The above “steel material inside” particularly means a “plate thickness central portion”. The same applies hereinafter.

The steel member of the present invention that has solved the above problems has a component composition of
C: 0.110% (meaning mass%, the same applies to chemical components) to 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% to 0.0010% and V: 0% to 0.030%,
Nb is 0.005% or less, Ti is 0.001% or less, and the total 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) D / d, where D is the average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries whose orientation difference between adjacent two crystals is 15 ° or more, and d is the maximum diameter of grain boundary carbides. Value is 54 or less.

The steel sheet of the present invention that has solved the above problems is a steel sheet used for the production of the steel member, and the component composition is
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% to 0.0010% and V: 0% to 0.030%,
Nb is 0.005% or less, Ti is 0.001% or less, and 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 by being 100 mm or less.

Furthermore, in the method for producing a steel sheet that can solve the above-described problems, after hot rolling a steel piece that satisfies the above component composition, quenching is performed at a heating temperature of 910 ° C. or more and 940 ° C. or less, and a holding time at the heating temperature: performed under the following conditions 60 minutes more than 25 minutes, the tempering after the quenching, heating temperature: 620 ° C. or higher Ac ₁ point or less, and P _T value represented by the following formula (1) is 19.2 or more 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 tempering heating temperature (K), and t _T represents a tempering heating time (hr).

The manufacturing method of the said steel member is also contained in this invention. The method for producing the steel member is characterized in that welding is performed using the steel plate, and 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) is 20 or more. Have.
P _PWHT value = T _PWHT × (20 + logt _PWHT ) × 10 ⁻³ (2)
In 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.

If the steel plate of the present invention is used for the production of steel members, the steel material has a sufficiently high strength and toughness even when the PWHT during the production process of the steel members is set to a long time, particularly a high temperature for a long time. A member is obtained. As a result, it is possible to provide a medium / high temperature pressure vessel or the like exhibiting high strength and high toughness.

Furthermore, the steel member of the present invention contributes to resource saving and cost reduction because the amount of alloying elements is suppressed.

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

Based on the premise that a steel plate made of Cr—Mo steel whose alloy element amount is less than that of the 2.25Cr-1.0Mo steel is used, the present inventors give a particularly long PWHT to the steel plate. Even when a steel member was produced by applying the steel, intensive research was conducted to obtain a steel member having excellent low-temperature toughness and strength at the center of the plate thickness.

As a result, in order to obtain a steel member having a high toughness at the center of the plate thickness,
・ Aim to refine the grain boundary carbide that has a fine structure and is easy to be coarsened and is a starting point of fracture. Specifically, (a) the structure is at least one of tempered bainite and tempered martensite, and (b) a crystal grain surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystals is 15 ° or more. When the average equivalent circle diameter of D is D and the maximum diameter of the grain boundary carbide is d, the value represented by D / d is 54 or less; and the suppression of temper embrittlement susceptibility is described in detail. Satisfy the component composition;
Found that it was effective. Hereinafter, the “average equivalent circular diameter of crystal grains surrounded by large-angle grain boundaries whose orientation difference between two adjacent crystals is 15 ° or more” may be simply referred to as “large-angle grain size”. Further, the “suppression of temper embrittlement sensitivity” is also referred to as “suppression of temper embrittlement” and “suppression of intergranular cracking”.

Hereinafter, the above (a) and (b) relating to the microstructure of the central portion of the steel member of the present invention will be described first.

In the following description, the “structure at the center of the plate thickness” is simply referred to as “structure”. Further, the characteristics shown below, that is, strength and low temperature toughness, refer to each characteristic of at least the center of the plate thickness after welding and PWHT to the steel member, that is, the steel plate.

(A) The structure is at least one of tempered bainite and tempered martensite.
The tempered bainite and tempered martensite are fine structures, and are particularly effective structures 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 unavoidably included include polygonal ferrite, retained austenite, pearlite, etc., but these structures are limited to 5% by area or less in total, and most preferably these structures are 0% by area. is there. In particular, when the polygonal ferrite is present, the upper bainite structure having a coarse crystal grain size is the main component, and good toughness cannot be ensured.

(B) D / d, where D is the average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries whose orientation difference between adjacent two crystals is 15 ° or more, and d is the maximum diameter of grain boundary carbides. Value is 54 or less.

As described above, the structure of the central part of the plate thickness can be made at least one of tempered bainite and tempered martensite, so that the structure can be refined. In order to obtain high toughness, the above (b) is specified.

In the case of the structure of tempered bainite and tempered martensite, generally, the so-called large-angle grain boundary in which the orientation difference (crystal orientation difference) between two adjacent crystals is 15 ° or more is the difference between the two adjacent crystal orientation differences. Therefore, the progress of brittle fracture is curved, the fracture surface unit of brittle fracture is reduced, and contributes to the improvement of toughness. On the other hand, as described above, the steel member of the present invention is subjected to PWHT, in particular, PWHT for a long time, and further PWHT for a long time at a high temperature. When Cr—Mo steel constituting a steel member is subjected to PWHT, M ₂₃ C ₆ grain boundary carbide is generally generated. When the PWHT conditions are severe such as high temperature and long time, the grain boundary carbides are coarsened and tend to be the starting point of fracture, leading to deterioration of toughness.

In the present invention, regarding the relationship between the average equivalent circle 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 shown in (b) above is 54 or less, It has been found that sufficiently excellent toughness can be ensured even after PWHT. The D / d is preferably 50 or less, more preferably 48 or less. Note that the lower limit of the D / d is about 12 in consideration of the component composition and manufacturing conditions defined in the present invention.

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

In the present invention, it is necessary to control the structure in the central portion of the plate thickness as described above, but the other portions, for example, the structure of the plate thickness surface layer portion and the like are not particularly limited. In addition, since the cooling rate during quenching is generally higher in the portion on the surface layer side than the plate thickness center portion, the finer structure is easier to obtain than the plate thickness center portion, and both strength and toughness are obtained. It tends to be better than the thickness center.

In order to obtain the fine structures of (a) and (b) in the central part of the plate thickness, the component composition of the steel plate used for the production of the steel member needs to be particularly as follows. That is, in order to reduce the average equivalent circle diameter D so as to satisfy the above D / d of 54 or less, the amount of B described later is contained and hardened by being present as free B (solid solution B). It is necessary to increase. For that purpose, in order to secure free B, it is important to fix N which easily binds to B and forms BN by adding an amount of Al described later. This AlN is useful for obtaining a fine structure by suppressing coarsening of prior austenite (γ) grains during quenching.

In order to reduce the average equivalent circle diameter D, it is effective to improve the hardenability by adding an alloy element as described above. However, excessive C, excessive Cu, and Ni have higher strength than necessary. To increase the toughness. Therefore, it is necessary to set the upper limit of C, Cu and Ni from the viewpoint of securing toughness.

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

Furthermore, as described in detail later, it is important to appropriately control the quenching and tempering conditions when manufacturing the steel sheet to be welded, as will be described in detail later.

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

C: 0.110% or more and 0.15% or less C increases the hardenability by obtaining at least one of tempered bainite and tempered martensite even at the center of the plate thickness where the cooling rate is small during quenching of the steel plate. Thus, it is an element necessary for reducing the average crystal grain size D and making D / d within the above range. Moreover, it is an element necessary for securing grain boundary carbide and obtaining a sufficient base material strength. In order to sufficiently exhibit these effects, the C content is set to 0.110% or more. The amount of C is preferably 0.120% or more, more preferably 0.130% or more. However, if the amount of C is excessive, after the PWHT for a long time, grain boundary carbides are coarsened and the toughness is deteriorated. Moreover, it becomes easy to produce a weld crack at the time of welding of a steel plate. Therefore, the C content is 0.15% or less. The amount of C is preferably 0.145% or less.

Si: 0.50% or more and 0.80% or less Si is an element effective for improving the strength of the base material 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 useful for ensuring toughness by suppressing temper embrittlement sensitivity. In order to exert these effects, the Si amount is 0.50% or more. The amount of Si is preferably 0.55% or more, more preferably 0.60% or more. However, if the amount of Si 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, thereby improving hardenability and obtaining a fine structure. As a result, strength and toughness are improved. It is an effective element for securing. In order to exhibit such an effect, Mn is contained at 0.40% or more. The amount of Mn is preferably 0.45% or more, more preferably 0.46% or more. However, when Mn is contained excessively, susceptibility to temper embrittlement increases and toughness deteriorates. Therefore, the amount of Mn is 0.65% or less, preferably 0.60% or less, more preferably 0.55% or less, and still more preferably 0.50% or less.

P: more than 0% and 0.0070% or less P, which is an inevitable impurity, adversely affects the toughness of the base metal and the welded part, and in particular segregates at the grain boundaries of the steel member, causing intergranular cracking and degrading toughness. . In order not to cause these inconveniences, the P content is suppressed to 0.0070% or less. The amount of P 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 the steel sheet. Therefore, S is preferably as small as possible, and the S amount is 0.0070% or less, preferably 0.0050% or less, more preferably 0.0030% or less.

Al: 0.030% or more and 0.080% or less As described above, Al is an extremely important element in the present invention. N is fixed as AlN during quenching and is an element necessary for ensuring hardenability by free B. is there. Moreover, AlN is useful for suppressing the coarsening of old γ grains during quenching and obtaining a fine structure. Furthermore, Al is an element necessary for deoxidation. In order to exert these effects, the Al content is 0.030% or more. The amount of Al is preferably 0.040% or more, more preferably 0.045% or more, and still more preferably 0.050% or more. On the other hand, when the amount of Al becomes excessive, coarse alumina inclusions are formed and the toughness is lowered. Therefore, the Al content is 0.080% or less. The amount of Al is preferably 0.075% or less, and 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 fully exhibit 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 increases the strength more than necessary as described above and causes a decrease in toughness. Therefore, the upper limit of the Cu amount is 0.20% or less, and the upper limit of the Ni amount is 0.30% or less. The amount of Cu is preferably 0.18% or less, more preferably 0.17% or less. Further, the amount of Ni is preferably 0.28% or less, 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 the carbide by PWHT and ensuring the toughness of the steel member. In addition, it is an element that is effective in securing strength in the middle and high temperature ranges and also in improving corrosion resistance. In order to exert these effects, 1.05% or more of Cr is contained. The amount of Cr is preferably 1.10% or more, more preferably 1.20% or more. On the other hand, when Cr is excessively contained, the susceptibility to temper embrittlement is increased, and intergranular cracking is likely to occur after PWHT, which adversely affects toughness. In addition, excessive Cr causes a decrease in workability and weldability, and further increases in manufacturing costs. Therefore, the Cr content is 1.50% or less. The amount of Cr 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 effective in improving hardenability and suppressing temper embrittlement. In order to obtain these effects, it is necessary to contain 0.45% or more of Mo. The amount of Mo is preferably 0.50% or more, and more preferably 0.55% or more. On the other hand, even if the Mo amount exceeds 0.65%, the effect is small and the manufacturing cost increases. Therefore, the upper limit of the Mo amount is set to 0.65% or less. The amount of Mo is preferably 0.62% or less.

N: 0.0030% or more and 0.0070% or less N is an element important for the present invention together with Al. By generating AlN and fixing N at the time of quenching, the effect of improving hardenability by free B can be exhibited to the maximum. AlN is useful for suppressing the coarsening of the old γ grains during quenching and obtaining a fine structure. If the N content is less than 0.0030%, AlN is insufficient, and the old γ grains become coarse. As a result, a fine structure cannot be obtained and toughness deteriorates. Therefore, the N amount is set to 0.0030% or more. Preferably it is 0.0035% or more, More preferably, it is 0.0040% or more. On the other hand, if the N amount exceeds 0.0070%, the N fixing effect by Al cannot be obtained, BN is generated, the effect of improving hardenability by free B is inhibited, the structure becomes coarse, and the toughness deteriorates. To do. Therefore, the N amount is 0.0070% or less. The N amount is preferably 0.0060% or less, more preferably 0.0055% or less, and still more preferably 0.0050% or less.

B: 0.0003% or more and 0.0010% or less B is present as free B (solid solution B) as described above, thereby improving hardenability, and in particular, increasing the plate thickness with a slow cooling rate during quenching. The average crystal grain size D can be refined also in the central portion of the steel plate thickness. As a result, excellent toughness can be ensured 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 assumption that the above-described contents of Al and N and 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 excessively contained, the hardenability may be deteriorated or weld cracking may be caused. Therefore, the upper limit of the B amount is set to 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 contributes to improving strength by forming carbides and nitrides, and is also effective in increasing the hardenability and obtaining a fine structure. In order to obtain these effects, the V content may be preferably 0.003% or more. The amount of V 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 amount of V is preferably 0.027% or less, more preferably 0.020% or less, and 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%. The total of Ca, Mg, REM (Rare Earth Metal) and Zr is suppressed to 0.0010% or less. As described above, Nb and Ti make old γ grains fine during quenching and reduce 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. Nb and Ti are also elements that increase the strength more than necessary and cause a decrease in workability. Further, Ca, Mg, REM, and Zr increase inclusions and cause a decrease in toughness. From the above, it is preferable to suppress these elements as much as possible, and any element 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 plate and steel member of the present invention contain the above chemical components, and the balance is iron and inevitable impurities.

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

The steel slab having the above-described component composition is hot-rolled by a conventional method to obtain a steel plate, and then the steel plate is quenched and tempered. In order to obtain the fine structure defined in the above (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.

Heating temperature of quenching: 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 heating temperature of quenching to 910 to 940 ° C. and the heating holding time of 25 minutes or more, The old γ grains can be grown to some extent. As a result, the hardenability is improved and a fine structure can be obtained.

If the heating temperature of quenching is lower than 910 ° C, the old γ grains during quenching remain fine, so a fine structure cannot be obtained at a portion with a slow cooling rate, such as the central portion of the plate thickness of the steel sheet. High toughness cannot be ensured. Therefore, the heating temperature for quenching is 910 ° C. or higher. Preferably it is 920 degreeC or more. On the other hand, when the heating temperature exceeds 940 ° C., a part of N fixed as AlN is solid-solved and combined with B to become BN, and the effect of improving 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 940 ° C. or lower. Preferably it is 935 degrees C or less.

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, so a predetermined amount of B is added. Even if included, sufficient hardenability cannot be obtained, and as a result, the structure becomes coarse and toughness deteriorates. Therefore, the heating and holding time is 25 minutes or more. Preferably it is 30 minutes or more. The upper limit of the heating and holding time is 60 minutes or less, preferably 55 minutes or less from the viewpoint of productivity and the like.

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

Subsequent to the quenching, tempering is performed at a temperature of 620 ° C. or more and Ac ₁ point or less, and at a heating temperature and a heating time at which the _PT 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 tempering heating temperature (K), and t _T represents a tempering heating time (hr).

Tempering heating temperature (tempering temperature): 620 ° C. or higher and Ac ₁ point or lower In the quenching, regardless of the thickness of the surface layer, the vicinity of the surface layer has a high cooling rate and the surface layer tends to harden. As a result, workability such as bending of the steel sheet can be improved. Therefore, in the manufacturing process of the steel member, tempering is performed to reduce the hardness of the surface layer from the viewpoint of improving the workability of the steel plate. As conditions for tempering, the tempering temperature is set 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 ensured. The tempering temperature is preferably 700 ° C. or higher. On the other hand, when the tempering temperature exceeds the Ac ₁ point, a part of the structure is reversely transformed and then air-cooled, so that polygonal ferrite is mixed. As a result, at least one of tempered bainite and tempered martensite, which are desired structures, is not obtained, resulting in a decrease in strength, and the reverse transformation portion is coarse in structure, resulting in a decrease in toughness. Therefore, the upper limit of the tempering temperature is set to Ac ₁ point or less. The tempering temperature is preferably 750 ° C. or lower. The Ac ₁ point is obtained by the method described in Examples described later.

Tempering is further performed at a heating temperature and a heating time at which the _PT value represented by the prescribed formula (1) falls within the above range. When the P _T value is less than 19.2, there is a problem that the hardness becomes too high and the workability is lowered. Therefore, the P _T value is 19.2 or more, preferably 19.3 or more, more preferably 19.4 or more. On the other hand, when the P _T value exceeds 20.6, coarsening of the carbide occurs and the characteristics such as toughness are deteriorated. Therefore, the P _T value is 20.6 or less, preferably 20.3 or less, more preferably 20.0 or less.

The plate thickness of the steel plate 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 using the said steel plate is also the same board thickness as the said steel plate.

The steel member of the present invention is welded to a steel plate obtained by performing the above quenching and tempering by a generally performed method, and further, as described above, post-weld heat treatment (PWHT) to remove strain. To obtain.

The method for producing 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) is 20 or more. This condition indicates severe conditions of high temperature and long time (for example, when the temperature is 680 ° C. or more and the heating time is 20 hours or more, the P _PWHT value is 20.3). In the present invention, a steel member having sufficiently excellent toughness can be obtained even after heat treatment under such severe conditions of high temperature and long time. The upper limit of the P _PWHT value is approximately 21. Examples of the PWHT conditions include a heating temperature of 600 to 690 ° C. and a heating time of 5 to 22 hours.
P _PWHT value = T _PWHT × (20 + logt _PWHT ) × 10 ⁻³ (2)
In 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.

This application claims the benefit of priority based on Japanese Patent Application No. 2015-218435 filed on November 6, 2015. The entire contents of the specification of Japanese Patent Application No. 2015-218435 filed on November 6, 2015 are incorporated herein by reference.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all 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 subjected to hot rolling by a conventional method, and then quenched and tempered under the conditions shown in Table 2-1 and Table 2-2. Steel plates having thicknesses shown in Table 2-1 and Table 2-2 were obtained. The said plate | board thickness is also the plate | board thickness of the test piece which simulated the steel member. The Ac ₁ point shown in Table 2-1 and Table 2-2 indicates the expansion when the steel sheet having the component composition shown in Table 1-1 and Table 1-2 is used and heated at a temperature rising rate of 0.5 ° C./second. It was obtained by analyzing the rate change. The heating temperature for quenching and tempering is the temperature at the center of the plate thickness of the steel sheet, and it is calculated by the difference method from the furnace atmosphere temperature and furnace time of the heat treatment furnace, or when using an experimental furnace, the same plate thickness. The temperature was measured by inserting a thermocouple into the dummy material.

Furthermore, PWHT after welding was simulated, and heat treatment was performed under conditions of heating temperature: 690 ° C. and heating and holding time: 22 hours in a dolly-type electric furnace in an air atmosphere to obtain a test piece simulating a steel member. It was. The above conditions are extremely severe conditions among the currently implemented conditions. In this case, the P _PWHT value is 20.6. The heating rate from room temperature to the heating temperature and the cooling rate from the heating temperature to room temperature were both 55 ° C./hr or less.

In addition, when manufacturing a steel member, PWHT is applied after the steel plate is welded. After the multilayer welding is performed as the welding, for example, the welding is characterized by the characteristics of the steel member including the weld heat affected zone, particularly toughness. In this example, a test piece was prepared without performing heat treatment related to welding.

Using the test piece obtained as described above, the evaluation of the metal structure, the tensile test, and the Charpy impact test were performed as follows. Moreover, in order to evaluate the workability of the steel plate which is a characteristic that can be required in the manufacturing process of the steel member, the surface hardness was measured using the steel plate before the PWHT.

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

The tempered bainite here refers to a structure in which upper bainite, lower bainite, bainitic ferrite and the like are tempered, but it is generally difficult to sort out these structures including tempered martensite. In addition, since the structure was sufficiently tempered after PWHT, the structure other than polygonal ferrite was set to at least one of tempered bainite and tempered martensite (B + M). In addition, 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 (Electron Back Scattering Pattern) 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 was as follows.
(1) A sample was taken from the steel plate so that a plate thickness cross section including the steel plate front and back surfaces parallel to the rolling direction and perpendicular to the steel plate surface could be observed.
(2) Mirror surface finishing of the observation surface was performed by polishing with wet emery polishing paper (# 150 to # 1000) or a polishing method having the same function (polishing using an abrasive such as diamond slurry). .
(3) Using an EBSP apparatus manufactured by TexSEM Laboratories, at a thickness t / 2 part in the thickness direction, the measurement range is 200 × 200 μm, a pitch of 0.5 μm, and a boundary having a crystal orientation difference of 15 ° or more The size of crystal grains (large tilt grains) surrounded by the crystal grain boundaries was measured. At this time, measurement points having a confidence index indicating the reliability of the measurement direction smaller than 0.1 were excluded from the analysis target.
(4) The average value of the size of the crystal grains surrounded by the large-angle grain boundaries determined in this way is calculated, and “in the large-angle grain boundary where the orientation difference between two adjacent crystals is 15 ° or more” in the present invention. The average equivalent circle diameter of the crystal grains ”. In addition, when the size of the crystal grain surrounded by the large-angle grain boundary was 1.0 μm or less, it was judged as measurement noise and excluded from the average value calculation target.

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

[Tensile test (evaluation of tensile properties)]
A round bar tensile test piece was taken from the portion of the plate thickness t / 2 in the direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with ASTM A370, and yield strength and tensile strength were measured. And the case where YS which is yield strength was 310 MPa or more and TS which was tensile strength was 515 MPa or more was evaluated as high strength.

[Charpy impact test (evaluation of impact properties)]
A full-sized V-notch test piece was taken from the portion of the plate thickness t / 2 in the direction perpendicular to the rolling direction, and subjected to a Charpy impact test at a test temperature of −38 ° C. in accordance with ASTM A370, and Charpy absorbed energy was measured. For Charpy absorbed energy, an average value of three test pieces was adopted. 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 a steel sheet before PWHT. And when the average value of HBW was 200 or less, it was evaluated that the workability was excellent, and when the average value of HBW was more than 200, the workability was evaluated as a normal level.

These results are shown in Table 3-1 and Table 3-2. The following No. Are the test numbers of Table 2-1, Table 2-2, Table 3-1, and Table 3-2. Indicates.

Table 1-1, Table 1-2, Table 2-1, Table 2-2, Table 3-1, and Table 3-2 show the following. That is, no. Nos. 1 to 5, 7 to 9, and 12 to 36 are made of steel satisfying the specified component composition in the present invention and manufactured under the specified conditions, so that the steel sheet showed excellent workability and was obtained. The steel member had a desired structure and exhibited excellent strength and toughness at the center of the plate thickness.

On the other hand, in the examples other than the above, either of the component composition or manufacturing conditions is out, so the workability of the steel sheet cannot be secured, or at least one of the tensile characteristics and impact characteristics in the center part of the thickness is inferior It became.

That is, no. No. 6 satisfied the component composition, but the _PT value at the time of tempering was too low, so it was not tempered sufficiently, and the Brinell hardness was high, that is, the workability was inferior. On the other hand, no. 11 satisfies the component composition, but the _PT value at the time of tempering was too high, so the carbide coarsened and the characteristics deteriorated.

No. No. 10 satisfies the component composition, but because the heating time for quenching is too short, quenching is not sufficiently performed, and D / d exceeds the upper limit, resulting in poor toughness.

No. In No. 37, since the amount of C was excessive, the toughness deteriorated and the Brinell hardness was high and the workability was poor.

No. Since 38, 42, and 49 did not contain B, D / d increased and the toughness was inferior. No. No. 48 was inferior in toughness because D / d was large because it did not contain B, and the amount of P was excessive.

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

No. Nos. 40 and 43 could not secure sufficient hardenability due to insufficient amount of C, and D / d increased, resulting in poor toughness. No. In No. 41, since the amount of C was insufficient, a large amount of ferrite was generated, the desired strength could not be ensured, and D / d increased, resulting in poor toughness. No. No. 44 had insufficient C content and did not contain B, so sufficient hardenability could not be ensured. As a result, the strength was low and D / d was increased, resulting in a decrease in toughness. No. No. 51 was insufficient in the amount of C, so the carbide size was small and the D / d was large, and the desired toughness could not be ensured in particular.

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

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

No. In No. 50, the amount of B was insufficient, and the toughness was lowered due to insufficient hardenability.

No. Since No. 52 contains Cu and Ni excessively and the amount of C is excessive, the toughness is lowered.

FIG. 1 is a graph showing the relationship between D / d and Charpy absorbed energy at −38 ° C. using the data in Tables 2-1, 2-2, 3-1 and 3-2. . From this graph, it can be seen that if D / d is 54 or less, sufficiently excellent toughness can be secured. In FIG. As described above, 47 and 52 are examples in which the toughness was lowered because the component composition was removed, although D / d satisfied the scope of the present invention.

Claims

Ingredient composition is mass%,
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% to 0.0010% and V: 0% to 0.030%,
Nb is 0.005% or less, Ti is 0.001% or less, and the total 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) D / d, where D is the average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries whose orientation difference between adjacent two crystals is 15 ° or more, and d is the maximum diameter of grain boundary carbides. Value is 54 or less.
It is a steel plate used for manufacture of the steel member according to claim 1, wherein the component composition is mass%,
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% to 0.0010% and V: 0% to 0.030%,
Nb is 0.005% or less, Ti is 0.001% or less, and 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 A steel plate characterized by being 100 mm or less.
It is a manufacturing method of the steel plate of Claim 2, Comprising: After hot-rolling the steel piece which satisfy | fills the component composition of Claim 2, hardening is performed by heating temperature: 910 degreeC or more and 940 degrees C or less, and this heating temperature. Holding time: 25 minutes to 60 minutes or less, tempering after this quenching, heating temperature: 620 ° C. or higher and Ac 1 point or lower, and P T value represented by the following formula (1) is 19.2 or higher The manufacturing method of the steel plate characterized by performing by the heating temperature and heating time used as 20.6 or less.
P T value = T T × (20 + logt T ) × 10 −3 (1)
In the formula (1), T T represents a tempering heating temperature (K), and t T represents a tempering heating time (hr).
It is a manufacturing method of the steel member of Claim 1, Comprising: It welds using the steel plate of Claim 2, Furthermore, P PWHT value represented by following formula (2) is 20 or more by heat processing after welding. The manufacturing method of the steel member characterized by performing by the heating temperature and heating time which become.
P PWHT value = T PWHT × (20 + logt PWHT ) × 10 −3 (2)
In 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.