KR102106766B1 - Steel members and steel plates, and methods for manufacturing them - Google Patents

Abstract

In the manufacturing process of the steel member, even when PWHT is performed for a long time after welding, a steel member having a high center of thickness and high toughness is provided. The steel member, C, Si, Mn, P, S, Al, Cu, Ni, Cr, Mo, N, B and V are within a prescribed range, Nb is 0.005% or less, Ti is 0.001% or less, and The sum of Ca, Mg, REM and Zr is suppressed to 0.0010% or less, the balance is iron and inevitable impurities, the plate thickness is 100 mm or less, and the structure at the center of the plate thickness satisfies both (a) and (b) below. It is also characterized in that the Charpy absorbed energy at -38 ° C is 100 J or more.
(a) The tissue is at least one of tempering bainite and tempering martensite.
(b) When the average circular equivalence diameter of grains surrounded by diagonal grain boundaries having an azimuth difference between two adjacent crystals of 15 ° or more is D and the maximum diameter of grain boundary carbide is d, the value expressed by D / d is 54 or less.

Description

Steel members and steel plates, and methods for manufacturing them

The present invention relates to steel members and steel plates, and methods for manufacturing them. Specifically, the present invention is a steel member obtained by performing welding and post-weld heat treatment (hereinafter sometimes referred to as "PWHT") for a steel sheet, particularly the strength of the central portion of the plate thickness even when the PWHT is high temperature for a long time. And steel members excellent in low-temperature toughness, steel sheets used for manufacturing the steel members, and methods of manufacturing the steel members. Hereinafter, low-temperature toughness may be simply referred to as "toughness".

The medium and high temperature pressure vessels used in the chemical industry including petroleum refining tend to require high temperature and high pressure resistance in a further layer for the purpose of high efficiency of operation. Therefore, it is required to increase the strength of the steel sheet used for the steel member such as the pressure vessel. In addition, from the viewpoint of safety, high-level low-temperature toughness is also required for the steel member.

In order to increase the strength, the steel sheet is subjected to soaking or quenching. However, if the thickness of the steel sheet is thick, there is a problem in that the cooling rate inside the steel sheet at the time of soaking or quenching, particularly the central portion of the thickness of the steel sheet is small, so that high strength and the like are difficult to obtain. By the way, the steel member such as the pressure vessel is obtained by welding the steel sheet, and then performing stress relief annealing, that is, PWHT for strain relief. Although the PWHT is performed for a long time to remove the deformation, the steel member subjected to the PWHT for a long time has a problem such as low temperature toughness and the like.

Moreover, as a method of ensuring high toughness, what raises the alloy element amount is mentioned. Cr-Mo steel containing Cr and Mo is used as an alloy element in the steel member such as the pressure vessel. It is known that, as the Cr-Mo steel, for example, if 2.25Cr-1.0Mo steel is used, good toughness can be obtained even in the central portion of the plate thickness of the thick steel plate, which is difficult to secure toughness. However, in recent years, the goal of saving resources and reducing costs has been increasing. Therefore, on the premise of using Cr-Mo steel with a reduced alloying element content than the above 2.25Cr-1.0Mo steel, it is strongly desired to realize a steel member having excellent strength and toughness at the center of the plate thickness.

Regarding the above-mentioned problems, a technique has been proposed that achieves high strength and high toughness by appropriately adjusting the chemical composition while suppressing the amount of alloying elements. For example, in Patent Documents 1 and 2, a technique for improving low-temperature toughness is shown for steels having a component composition of 1.25Cr-0.5Mo level, which is difficult to secure toughness.

Patent Literature 1 discloses a technique in which quenching properties are secured by adding Nb and Ca, and further suppressed deterioration of characteristics during SR (Stress Relief). However, when this technique is applied to a thick steel sheet, which is mainly cast by an agglomeration method, there is a fear that the Ca forms coarse inclusions and adversely affects toughness. Therefore, it is considered difficult to stably secure the toughness in the central portion of the plate thickness of the steel member having a thick plate thickness.

In addition, Patent Document 2 discloses a technique in which the austenite particle size is refined and the low-temperature toughness is secured by performing controlled rolling or controlled rolling + accelerated cooling before quenching in the manufacturing process. However, it is difficult to say that the above-mentioned control rolling in this technique may cause a decrease in productivity of the rolling line, so it is practical.

Japanese Patent Publication No. Hei06-279919 Japanese Patent Publication No. 2000-345281

The present invention has been made in view of the above circumstances, and the object thereof is that, in the manufacturing process of a steel member, even when PWHT after welding is performed for a long time, especially at a high temperature for a long time, the inside of the steel material is high strength and exhibits high low temperature toughness. It is to establish a steel member, a steel plate useful for the production of the steel member, and a method for manufacturing them. The above "inner steel" means "in the center of the plate thickness" in particular. It is the same below.

The steel member of the present invention capable of solving the above problems has a component composition,

C: 0.110% (meaning% by mass. Same for chemical components or less) 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 less than 0.0070%,

S: more than 0% and less than 0.0070%,

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

Satisfy,

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,

The plate thickness is 100mm or less,

The structure in the central portion of the plate thickness satisfies both (a) and (b) below, and also has a characteristic that the Charpy absorption energy at -38 ° C is 100 J or more.

(a) The tissue is at least one of tempering bainite and tempering martensite.

(b) When the average circular equivalence diameter of grains surrounded by diagonal grain boundaries having an azimuth difference between two adjacent crystals of 15 ° or more is D and the maximum diameter of grain boundary carbide is d, the value expressed by D / d is 54 or less.

In addition, the steel sheet of the present invention capable of solving the above problems is a steel sheet used for manufacturing the steel member, and has a component composition,

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 less than 0.0070%,

S: more than 0% and less than 0.0070%,

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

Satisfy,

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 thickness of the plate is 100 mm or less.

In addition, in the method of manufacturing a steel sheet capable of solving the above problems, after hot rolling a steel piece satisfying the above-mentioned composition, quenching, heating temperature: 910 ° C or more and 940 ° C or less, and holding time at the heating temperature: 25 minutes or more Performed under the conditions of 60 minutes or less, and tempering after this quenching, heating temperature: 620 ° C or higher and Ac ₁ point or lower, and at a heating temperature and heating time in which the P _T value represented by the following formula (1) becomes 19.2 or more and 20.6 or less. It has a characteristic to do it.

P _T value = T _T × (20 + logt _T ) × 10 ^-3 … (One)

In Formula (1), T _T represents the heating temperature (K) of tempering, and t _T represents the heating time (hr) of tempering.

The present invention also includes a method for manufacturing the steel member. The manufacturing method of the steel member is characterized in that it is welded using the steel sheet and further heat-treated after welding at a heating temperature and a heating time at which the P _PWHT value represented by the following formula (2) becomes 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 heat treatment after welding, and t _PWHT represents the heating time (hr) of heat treatment after welding.

When the steel sheet of the present invention is used for the production of a steel member, even when PWHT in the manufacturing process of the steel member is set for a long time, particularly at a high temperature for a long time, a steel member having a high strength inside and sufficiently excellent toughness is obtained. As a result, it is possible to provide a medium-high temperature pressure vessel or the like having high strength and high toughness.

Moreover, since the amount of alloying elements is suppressed in the steel member of the present invention, it contributes to resource saving and cost reduction.

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

The present inventors, on the premise of using a steel sheet made of Cr-Mo steel with an alloy element content of less than the above 2.25Cr-1.0Mo steel, produced a steel member by subjecting the steel sheet to PWHT in particular for a long time. Yeodo, the steel member was repeatedly studied so that it was possible to obtain excellent low-temperature toughness and strength in the central portion of the plate thickness.

As a result, first of all, in order to obtain a steel member having a high toughness at the center of the plate thickness, in particular,

-It is made into a fine structure, and also aims at miniaturization of grain boundary carbides that are easily coarse and easily become a starting point for destruction. Specifically, (a) the structure of at least one of the tempering bainite and tempering martensite, and (b) the average circular equivalent diameter of the grains surrounded by a diagonal grain boundary having an azimuth difference of two adjacent crystals of 15 ° or more D, when the maximum diameter of the grain boundary carbide is d, the value indicated by D / d is 54 or less; And

-To suppress the tempering embrittlement susceptibility, specifically, to satisfy the component composition described later;

I found this to be valid. Hereinafter, the above-mentioned "average circular equivalent diameter of crystal grains surrounded by diagonal grain boundaries having an azimuth difference between two adjacent crystals of 15 ° or more" may be simply referred to as "diagonal grain size". Moreover, the said "suppression of tempering embrittlement susceptibility" is also referred to as "suppression of tempering embrittlement" and "suppression of intergranular cracking" below.

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

In addition, in the following description, "the structure of the center part of a plate thickness" is simply called "structure". In addition, the characteristics shown below, that is, strength and low-temperature toughness, shall refer to the characteristics of at least the central portion of the plate thickness after welding and PWHT are performed on the steel member, that is, the steel sheet.

(a) The tissue is at least one of tempering bainite and tempering martensite.

The tempering bainite or tempering martensite is a fine structure, and is particularly effective for securing the strength and toughness of the central portion of the plate thickness. In the steel member of the present invention, the structure is at least one of tempering bainite and tempering martensite. Other inevitable structures may include polygonal ferrite, residual austenite, pearlite, and the like, but these structures are suppressed to 5 area% or less in total, and most preferably, these structures are 0 area%. . Particularly, when the polygonal ferrite is present, the upper bainite structure having a coarse grain size is mainly used, and good toughness cannot be ensured.

(b) When the average circular equivalence diameter of grains surrounded by diagonal grain boundaries having an azimuth difference between two adjacent crystals of 15 ° or more is D and the maximum diameter of grain boundary carbide is d, the value expressed by D / d is 54 or less.

The structure of the central portion of the plate thickness can be refined by setting at least one of tempering bainite and tempering martensite as described above, but in the present invention, in order to obtain high toughness by reliable miniaturization of the structure, The above (b) is defined.

In the case of the structure of tempering bainite and tempering martensite, in general, the so-called diagonal grain boundary, in which the azimuth of two adjacent crystals (crystal azimuth) is 15 ° or more, is because the azimuth of two adjacent crystals is large, The progress of brittle fracture is curved, and the fracture surface unit of brittle fracture becomes small, contributing to the improvement of toughness. On the other hand, the steel member of the present invention, as described above, is PWHT, in particular, PWHT for a long time, and also PWHT for a long time at a high temperature. When Cr-Mo steel constituting a steel member receives PWHT, grain boundary carbides of M ₂₃ C ₆ are generally produced. When the conditions of the PWHT become stringent conditions such as high temperature and long time, the grain boundary carbides become coarse and easily become a starting point for destruction, leading to deterioration of toughness.

In the present invention, with respect to the relationship between the average circular equivalent diameter D of these diagonal grain sizes and the maximum diameter d in the grain boundary carbide, if the value indicated by D / d satisfies 54 or less as in (b) above, even after PWHT is sufficient. It has been found that excellent toughness can be secured. The D / d is preferably 50 or less, and more preferably 48 or less. On the other hand, when considering the composition of ingredients or manufacturing conditions prescribed in the present invention, the lower limit of the D / d is about 12.

In the present invention, the D / d need only satisfy 54 or less, and the individual values of the average circular equivalent diameter D of the diagonal grain boundaries and the maximum diameter d of the grain boundary carbide are not particularly limited. The average circular equivalent diameter D of the diagonal grain 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 circular equivalent diameter D of the diagonal grain size is approximately 10 μm in production. In addition, the maximum diameter d of the grain boundary carbide can be, for example, 0.8 μm or less. The maximum diameter d of the grain boundary carbide can be further set to 0.70 μm or less, and further to 0.60 μm or less. On the other hand, the lower limit of the maximum diameter d of the grain boundary carbide is approximately 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 of the central portion of the plate thickness as described above, but the structure of other portions, for example, the surface portion of the plate thickness is not particularly limited. On the other hand, since the cooling rate at the time of quenching is generally greater in the portion on the surface layer side than in the center portion of the plate thickness, a microscopic structure is easily obtained than in the center portion of the plate thickness, and both strength and toughness tend to be better than in the center portion of the plate thickness. .

In the central portion of the plate thickness, in order to obtain the fine structures of (a) and (b), it is necessary to make the component composition of the steel sheet used in the production of the steel member, particularly as follows. That is, in order to refine the average circular equivalent diameter D so that the D / d satisfies 54 or less, the quenching property is achieved by containing B in an amount to be described later and present as free B (employment B). It needs to be raised. For that purpose, it is important to fix N, which is easy to form BN by combining with B, as an AlN by adding an amount of Al described later to secure the free B. This AlN is useful for suppressing the coarsening of old austenite (γ) grains during quenching and obtaining a fine structure.

For the refinement of the average circular equivalent diameter D, it is effective to improve the hardenability by adding an alloying element as described above, but excessive C, excessive Cu or Ni increases the strength more than necessary, leading to a decrease in toughness. do. Therefore, from the viewpoint of securing toughness, it is necessary to set the upper limits of C, Cu, and Ni.

Moreover, in this invention, content of Nb and Ti is suppressed. It is because it becomes difficult to achieve D / d of the said range when a lot of these elements are contained. Moreover, it is because these elements raise the strength more than necessary and cause a decrease in workability. In addition, the contents of Ca, Mg, REM and Zr are also suppressed. This is because these elements increase inclusions, leading to a decrease in toughness. In addition, in order to control the size of the grain boundary carbide, it is necessary to control the content of Cr in addition to C. Moreover, in order to suppress tempering embrittlement susceptibility and secure toughness, it is also necessary to control the content of Si or the like.

Moreover, as manufacturing conditions, it is important to appropriately control the conditions of quenching and tempering at the time of manufacturing the steel sheet to be provided for welding, as described later.

Hereinafter, the composition of the components of the steel sheet and the steel member, which are necessary for securing the structure and properties, will be described first.

C: 0.110% or more and 0.15% or less

C is, when quenching the steel sheet, in order to obtain at least one of tempering bainite and tempering martensite, even in the center of the thickness of the plate having a small cooling rate, and by increasing the quenching property, minimizing the average grain size D, D It is an element necessary to make / d within the above range. In addition, it is an element necessary for securing grain boundary carbide and obtaining sufficient base material strength. In order to sufficiently exhibit these effects, the amount of C is made 0.110% or more. C amount is preferably 0.120% or more, and more preferably 0.130% or more. However, if the amount of C is excessive, coarse grain boundary carbides are caused after a long period of PWHT, and toughness deteriorates. In addition, welding cracks are likely to occur during welding of the steel sheet. Therefore, the C content is set to 0.15% or less. C amount 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. It is also an element that is useful for securing toughness by suppressing the tempering embrittlement susceptibility. In order to exhibit these effects, the Si content is 0.50% or more. Si amount is preferably 0.55% or more, and more preferably 0.60% or more. However, when the Si content is excessive, the tempering embrittlement susceptibility increases and the toughness deteriorates, so it is set to 0.80% or less. The Si content is preferably 0.75% or less, and more preferably 0.70% or less.

Mn: 0.40% or more and 0.65% or less

Mn is an effective element for stabilizing austenite and lowering the transformation temperature, thereby improving hardenability, obtaining a fine structure, and as a result, securing strength and toughness. In order to exert this effect, Mn is contained in an amount of 0.40% or more. The Mn content is preferably 0.45% or more, and more preferably 0.46% or more. However, when Mn is contained excessively, the tempering embrittlement susceptibility becomes high, and the toughness deteriorates. Therefore, the Mn content is 0.65% or less, preferably 0.60% or less, more preferably 0.55% or less, even more preferably 0.50% or less.

P: more than 0% and less than 0.0070%

The unavoidable impurity P adversely affects the toughness of the base material and the welded part, and segregates particularly at the grain boundary of the steel member, causing grain boundary cracking and deteriorating toughness. In order not to cause these problems, the P content is suppressed to 0.0070% or less. P amount is preferably 0.0060% or less, and more preferably 0.0050% or less.

S: more than 0% and less than 0.0070%

S is an element that forms MnS and is liable to cause welding cracks during welding of the steel sheet. Therefore, S is preferably as small as possible, and the amount of S 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, as described above, is a very important element in the present invention, and is an element necessary for securing the hardenability by free B by fixing N as AlN during quenching. In addition, AlN is useful for suppressing the coarsening of spherical γ grains during quenching and obtaining a fine structure. Al is also an element necessary for deoxidation. In order to exert these effects, the amount of Al is made 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 is excessive, a coarse inclusion of alumina system is formed and the toughness is lowered. Therefore, the Al content is set to 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 elements effective for increasing strength without significantly impairing toughness. To sufficiently 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 , 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, leading to a decrease in toughness. 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 preferably 0.18% or less, and more preferably 0.17% or less. Moreover, Ni amount is preferably 0.28% or less, and more preferably 0.26% or less.

Cr: 1.05% or more and 1.50% or less

Cr is an element effective for suppressing the coarsening of carbides by PWHT and securing the toughness of steel members. In addition, it is an element effective in securing strength in the mid and high temperature range and further improving corrosion resistance. In order to exert these effects, Cr is contained in 1.05% or more. Cr amount is preferably 1.10% or more, and more preferably 1.20% or more. On the other hand, when Cr is excessively contained, the tempering embrittlement susceptibility becomes high, and grain boundary cracks are likely to occur after PWHT, adversely affecting toughness. In addition, excessive Cr causes a decrease in workability and weldability, and further increases in manufacturing cost. Therefore, the Cr content is set to 1.50% or less. Cr amount is preferably 1.45% or less, and more preferably 1.40% or less.

Mo: 0.45% or more and 0.65% or less

Mo is an element effective in suppressing tempering embrittlement while increasing hardenability. In order to acquire these effects, it is necessary to contain Mo 0.45% or more. Mo amount is preferably 0.50% or more, and more preferably 0.55% or more. On the other hand, even if the amount of Mo exceeds 0.65%, the improvement in effect is small and leads to an increase in manufacturing cost, so the upper limit of the amount of Mo is set to 0.65% or less. Mo amount is preferably 0.62% or less.

N: 0.0030% or more and 0.0070% or less

N together with Al is an important element in the present invention. By producing AlN and fixing N at the time of quenching, the effect of improving the hardenability by pre-B can be maximized. In addition, AlN is useful for suppressing the coarsening of the spherical γ grains during quenching and obtaining a fine structure. When the amount of N is less than 0.0030%, AlN is insufficient, and the spherical γ grains become coarse, and as a result, fine structure is not obtained and toughness deteriorates. Therefore, the amount of N is made 0.0030% or more. It is preferably 0.0035% or more, and more preferably 0.0040% or more. On the other hand, when the amount of N exceeds 0.0070%, the effect of fixing N by Al is not obtained, BN is generated, the effect of improving hardenability by free B is inhibited, the structure becomes coarse, and toughness deteriorates. Therefore, the amount of N is made 0.0070% or less. 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

As described above, B is present as free B (employment B), thereby increasing hardenability, and in particular, in the middle of the sheet thickness of a steel sheet having a thicker sheet thickness with a slow cooling rate during quenching, the average grain size D can be refined. have. As a result, excellent toughness can be secured even in the central portion of the plate thickness. In order to obtain such an effect, even if it is assumed that the above-mentioned Al and N contents and the quenching conditions described later are controlled, B is required to be 0.0003% or more. B amount is preferably 0.0005% or more, and more preferably 0.0007% or more. On the other hand, if B is excessively contained, the hardenability may decrease, or welding cracks may occur, so the upper limit of the amount of B is set to 0.0010%. B amount is preferably 0.0009% or less, and 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, contributing to strength improvement, and improving hardenability to obtain a fine structure. In order to obtain these effects, the amount of V may be preferably contained 0.003% or more. V amount is more preferably 0.005% or more. On the other hand, since the excessive addition of V causes an increase in cost, the upper limit is made 0.030% or less. V amount 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 sum of Ca, Mg, REM and Zr is 0.0010% or less

In the present invention, Nb is 0.005% or less, Ti is 0.001% or less, and 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 the spherical γ grains during quenching fine, thereby lowering the quenching properties. As a result, the diagonal grain size becomes coarse, that is, the average circular equivalent diameter D becomes large, and D / d exceeds the prescribed range. In addition, Nb and Ti are also elements that increase the strength more than necessary and cause deterioration of workability. In addition, 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 remainder are iron and unavoidable impurities.

Next, the manufacturing method of the steel plate and steel member of this invention is demonstrated. First, a method of manufacturing a steel sheet will be described.

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

Quenching heating temperature: 910 ° C or more and 940 ° C or less, and holding time at the corresponding heating temperature: 25 minutes or more and 60 minutes or less

By setting the heating temperature of the quenching to 910 to 940 ° C and the heating holding time to 25 minutes or more, the spherical gamma grains can be grown to some extent, and as a result, the quenching property is improved and a fine structure can be obtained.

When the heating temperature of the quenching is less than 910 ° C, since the spherical γ grains during quenching are fine, the microstructure is not obtained in a portion where the cooling rate is slow, such as the central portion of the sheet thickness of the steel sheet, so that excellent toughness can be secured. none. Therefore, the heating temperature of quenching is 910 ° C or higher. It is preferably 920 ° C or higher. On the other hand, when the heating temperature exceeds 940 ° C., N fixed as AlN is partially dissolved, becomes BN in combination with B, and an effect of improving hardenability by free B is not obtained. As a result, a fine structure is not obtained, and toughness deteriorates. Therefore, the heating temperature of quenching is 940 ° C or lower. It is preferably 935 ° C or lower.

Further, even if the heating temperature at the time of quenching is within the above range, if the holding time (heating holding time) at the heating temperature is shorter than 25 minutes, the sphere γ grains remain fine, so that sufficient quenching property is obtained even if a predetermined amount of B is contained. This is not obtained, and as a result, the structure becomes coarse and the toughness deteriorates. Therefore, the heating and holding time is 25 minutes or more. It is preferably 30 minutes or more. The upper limit of the heating holding time is 60 minutes or less from the viewpoint of productivity and the like, and preferably 55 minutes or less.

On the other hand, if the conditions at the time of quenching are controlled as described above and the particle size of the sphere γ is within a range of about 50 to 100 μm, it is preferable because a fine structure is easily obtained.

Following the quenching, tempering is carried out at a heating temperature and heating time 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) of 19.2 or higher and 20.6 or lower.

P _T value = T _T × (20 + logt _T ) × 10 ^-3 … (One)

In Formula (1), T _T represents the heating temperature (K) of tempering, and t _T represents the heating time (hr) of tempering.

Tempering heating temperature (tempering temperature): 620 ° C or higher and Ac ₁ point or lower

In the quenching, since the cooling rate is fast in the vicinity of the surface layer regardless of the thickness of the plate, and the hardness of the surface layer tends to be hard, it is possible to improve workability such as bending of a steel sheet by tempering after quenching. Therefore, in the manufacturing process of the steel member, from the viewpoint of improving the workability of the steel sheet, tempering is performed to reduce the hardness of the surface layer. As the 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 secured. The tempering temperature is preferably 700 ° C or higher. On the other hand, when the tempering temperature exceeds Ac ₁ point, a part of the tissue is reverse-transformed and then air-cooled, so that polygonal ferrite is mixed. As a result, at least one of tempering bainite and tempering martensite, which are desired structures, is not obtained, leading to a decrease in strength, and the reverse transformation part also results in a decrease in toughness because the structure is rough. Therefore, the upper limit of tempering temperature is made into Ac ₁ point or less. The tempering temperature is preferably 750 ° C or lower. On the other hand, the Ac ₁ point is obtained by the method described in Examples to be described later.

Tempering is also performed at a heating temperature and a heating time in which the P _T value represented by the formula (1) defined is within the above range. When the P _T value is less than 19.2, there arises a problem that the hardness is too high and the workability is lowered. 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, etc. occurs, leading to deterioration of properties such as toughness. Therefore, the P _T value is 20.6 or less, preferably 20.3 or less, and more preferably 20.0 or less.

The sheet 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, moreover 10 mm or more. The steel member obtained using the steel sheet also has the same sheet thickness as the steel sheet.

The steel member of the present invention is obtained by welding a steel sheet obtained by performing the quenching and tempering in a manner that is generally performed, and further performing heat treatment (PWHT) after welding to remove deformation as described above.

The manufacturing method of the steel member of the present invention is characterized in that the heat treatment after welding 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 stringent condition for a long time at a high temperature (for example, a temperature of 680 ° C. or more and a P _PWHT value of 20.3 for a heating time of 20 hours or more). In the present invention, even after heat treatment under severe conditions for a long time at a high temperature, a steel member having excellent toughness is obtained. The upper limit of the P _PWHT value is approximately 21. Examples of the conditions for the PWHT include heating temperature: 600 to 690 ° C, heating time: 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 heat treatment after welding, and t _PWHT represents the heating time (hr) of 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, is incorporated herein by reference.

Example

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is of course not limited by the following examples, and it is of course also carried out by appropriately changing it within a range that can be suitable for the purpose of before and after. It is possible, and they are all included in the technical scope of the present invention.

For the steel pieces satisfying the component compositions shown in Tables 1-1 and 1-2, hot rolling was performed by a conventional method, followed by quenching and tempering under the conditions shown in Tables 2-1 and 2-2, and then Table 2 -1 and the sheet thicknesses shown in Table 2-2 were obtained. The plate thickness is also the plate thickness of a test piece simulating a steel member. Ac ₁ point shown in Table 2-1 and Table 2-2 analyzes the change in expansion rate when heated at a heating rate of 0.5 ° C./sec. Using a steel sheet having the component composition shown in Table 1-1 and Table 1-2. I did it by doing it. On the other hand, the heating temperature of quenching and tempering is the temperature at the center of the sheet thickness of the steel sheet, calculated by the difference method from the atmosphere temperature in the furnace of the heat treatment furnace and the ash time, or in the case of using an experimental furnace, a thermocouple is applied to the dummy material of the same sheet thickness. It is measured temperature by plugging in.

Moreover, the PWHT after welding was simulated, and heat treatment was carried out in a stand-by electric atmosphere-type electric furnace at a heating temperature of 690 ° C under conditions of 22 hours to obtain a test piece simulating a steel member. The above condition is a remarkably strict condition among the currently implemented 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 drop from the heating temperature to room temperature were both 55 ° C / hr or less.

On the other hand, when manufacturing a steel member, PWHT is performed after welding the steel sheet, but, for example, after multi-layer welding is performed as the welding, the welding is characterized in that the steel member also includes a welding heat-affected zone, particularly toughness. Since there is little adverse effect on the material, in this example, a test piece was produced without performing heat treatment for welding.

Using the test piece obtained as described above, evaluation of the metal structure, tensile test, and Charpy impact test were performed in the following manner. In addition, in order to evaluate the workability of the steel sheet which is a property that may be required in the manufacturing process of the steel member, surface hardness was measured using the steel sheet before the PWHT.

[Observation of metal structure]

The metal structure was observed as follows.

(1) A sample was taken from the steel sheet so that a plate thickness cross section including the front and back surfaces of the steel sheet parallel to the rolling direction and perpendicular to the steel sheet surface could be observed.

(2) The surface of the observation surface was mirror-finished by a polishing method such as polishing with a wet emery polishing paper (# 150 to # 1000), or polishing using an abrasive such as diamond slurry, which has a function equivalent thereto.

(3) The polished sample was corroded using a 3% nitral solution to reveal grain boundaries.

(4) At the t / 2 area of the plate thickness, the resulting tissue was photographed at a magnification of 400 times. In this example, it was taken as a photograph of 6 cm x 8 cm. Next, it was determined that polygonal ferrite was formed in the old austenite grain boundaries in the photographed photograph, and all were painted black. Next, the photograph was introduced into an image analysis device. In the case of 400 times, the area of the photograph corresponds to 150 μm × 200 μm. The introduction to the image analysis apparatus was introduced so that the total of the regions was 1 mm x 1 mm or more in any magnification. That is, in the case of 400 times, at least 35 pictures were introduced.

(5) In the image analysis apparatus, at least one of the tempering bainite and the tempering martensite is calculated by calculating the area ratio of black for each photo, taking the average value of all the photographs as the fraction of polygonal ferrite (F), and subtracting it from the whole. B + M).

On the other hand, the tempering bainite referred to herein refers to a structure in which upper bainite, lower bainite, and bainitic ferrite are tempered, but it is generally difficult to select these tissues by including tempering martensite, and also after PWHT. Since this was sufficiently tempered, a structure other than polygonal ferrite was used as at least one of tempering bainite and tempering martensite (B + M). On the other hand, it was also confirmed that the pearlite structure was not included in any of the test pieces used in this example.

[Measurement of diagonal grain size by EBSP (Electron Back Scattering Pattern) method]

Using the EBSP method, the average circular equivalent diameter (diagonal grain size) of crystal grains surrounded by diagonal grain boundaries in which two adjacent crystals have an orientation difference (crystal orientation difference) of 15 ° or more was determined. The measurement point was as follows.

(1) A sample was taken from the steel sheet so that a plate thickness cross section including the front and back surfaces of the steel sheet parallel to the rolling direction and perpendicular to the steel sheet surface could be observed.

(2) The surface of the observation surface was mirror-finished by polishing with wet emery paper (# 150 to # 1000), or a polishing method having a function equivalent thereto (polishing using an abrasive such as diamond slurry).

(3) Using an EBSP apparatus manufactured by TexSEM Laboratories, the measurement range in the plate thickness t / 2 part in the plate thickness direction is 200 × 200 μm, 0.5 μm pitch, and a boundary having a crystal orientation difference of 15 ° or more is used as a grain boundary. The size of the crystal grains (large diameter grains) surrounded by the grain boundaries was measured. At this time, measurement points with a confidence index smaller than 0.1 indicating reliability of the measurement orientation were excluded from analysis.

(4) The average value of the size of the crystal grains surrounded by the diagonal grains thus obtained is calculated, and in the present invention, as the "average equivalent equivalent diameter of the grains surrounded by the diagonal grains having an azimuth difference of two adjacent crystals of 15 ° or more". did. On the other hand, when the size of the crystal grains surrounded by the diagonal grain boundaries was 1.0 μm or less, it was judged as measurement noise and was excluded from the object of calculating the average value.

[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 sheet so that a plate thickness cross section including the front and back surfaces of the steel sheet parallel to the rolling direction and perpendicular to the steel sheet surface could be observed.

(2) The surface of the observation surface was mirror-finished by polishing with wet emery paper (# 150 to # 1000), or a polishing method having a function equivalent thereto (polishing using an abrasive such as diamond slurry).

(3) The polished sample was corroded using a 3% nitral solution to reveal grain boundaries.

(4) In the t / 2 area of the plate thickness, the resulting tissue was photographed at a magnification of 1000 times. In this example, it was taken as a photograph of 6 cm x 8 cm. Next, the photograph was introduced into an image analysis device. In the case of 1000 times, the area of the picture corresponds to 60 μm × 80 μm. The introduction to the image analysis apparatus was introduced such that the total of the regions was 0.4 mm x 0.4 mm or more. That is, in the case of 1000 times, at least 35 pictures were introduced.

(5) In the image analysis apparatus, the shortened length was calculated as the size of the grain boundary carbide for each picture, and the maximum value of the grain boundary carbide size of all the pictures was calculated.

[Tensile test (evaluation of tensile properties)]

A round bar tensile test piece was taken from the area of the plate thickness t / 2 in the direction perpendicular to the rolling, and a tensile test was performed in accordance with ASTM A370 to measure yield strength and tensile strength. Then, the case where the yield strength YS was 310 MPa or more and the tensile strength TS was 515 MPa or more was evaluated as high strength.

[Charpy impact test (evaluation of impact properties)]

A V-notch test piece having a full size was taken from the region of the plate thickness t / 2 in the direction perpendicular to the rolling, and the Charpy impact test was conducted at a test temperature of -38 ° C in accordance with ASTM A370 to measure the Charpy absorbed energy. On the other hand, as the Charpy absorbed energy, the average value of three test pieces was adopted. And when Charpy absorbed energy vE _{-38 at -38 degreeC} was 100 J or more, it evaluated as being excellent in toughness, ie, excellent in shock characteristics.

[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 using the steel sheet before PWHT, at a position 1 mm deep from the surface, in accordance with ASTM A370. Then, when the average value of HBW was 200 or less was evaluated to be excellent in workability, and when the average value of the HBW was more than 200, processability was evaluated to be a normal level.

The results are shown in Table 3-1 and Table 3-2. In addition, the following No. shows the test numbers of Table 2-1, Table 2-2, Table 3-1, and Table 3-2.

[Table 1-1]

[Table 1-2]

[Table 2-1]

[Table 2-2]

[Table 3-1]

[Table 3-2]

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. Since 1 to 5, 7 to 9, and 12 to 36 use steel that satisfies the component composition specified in the present invention and is manufactured under the prescribed conditions, the steel sheet exhibits excellent workability, and the obtained steel member is desired. It had a structure and showed excellent strength and toughness at the center of the plate thickness.

On the other hand, in the examples other than the above, since one of the component composition and manufacturing conditions deviates, the workability of the steel sheet cannot be secured, or at least one of the tensile properties and the impact properties at the center of the sheet thickness is inferior Became.

That is, No. 6 satisfies the component composition, but because the P _T value at the time of tempering was too low, it was not sufficiently tempered, and the Brinell hardness was high, that is, workability was inferior. Meanwhile, No. Although 11 satisfies the composition of the component, the P _T value at the time of tempering was too high, so that the carbide became coarse and the characteristics deteriorated.

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

No. 37 is a result of excessive C content, deteriorating toughness and high brinell hardness, resulting in poor workability.

No. Since 38, 42 and 49 do not contain B, D / d becomes large and toughness is inferior. Also No. Since 48 does not contain B, D / d becomes large, and since P amount is excessive, toughness is inferior.

No. 39 and No. Since 46 contains more than a certain amount of Nb, the spherical γ grain at the time of quenching becomes fine, sufficient quenching property is not obtained, D / d becomes large, and the toughness is inferior. Also No. At 46, workability was also lowered.

No. Since 40 and 43 lack C amount, sufficient quenching property cannot be ensured, D / d becomes large, and toughness is inferior. Also No. Since the amount of C is insufficient in 41, a large amount of ferrite is generated, the desired strength cannot be secured, and the D / d becomes large, resulting in poor toughness. No. Since 44 is insufficient in the amount of C and does not contain B, sufficient quenching properties cannot be ensured, and as a result, the strength is low, and the D / d becomes large, leading to a decrease in toughness. No. Since 51 had a insufficient amount of C, the carbide size was small and the D / d was large, so that the desired toughness could not be particularly secured.

No. Since 45 contains more than a certain amount of Ti, the spherical γ grain at the time of quenching becomes fine, sufficient quenching property is not obtained, and D / d becomes large, resulting in poor toughness.

No. 47 had an excessive amount of P, so the toughness was inferior.

No. As for 50, the amount of B was insufficient and the hardenability was insufficient, so the toughness fell.

No. 52 contains Cu and Ni in excess, and since C content is excessive, toughness was lowered.

1 is a graph showing the relationship between D / d and Charpy absorption 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 when D / d is 54 or less, sufficiently good toughness can be secured. On the other hand, No. 1 in FIG. 1. 47 and 52, as described above, D / d satisfies the scope of the present invention, but is an example in which the toughness is lowered because the composition of the components deviates.

Claims (4)

Ingredient composition, in 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 less than 0.0070%,
S: more than 0% and less than 0.0070%,
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
Satisfy,
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,
The plate thickness is 100mm or less,
The steel member characterized in that the structure in the central portion of the plate thickness satisfies both (a) and (b) below, and the Charpy absorption energy at -38 ° C is 100 J or more.
(a) The tissue is at least one of tempering bainite and tempering martensite.
(b) When the average circular equivalence diameter of grains surrounded by diagonal grain boundaries having an azimuth difference between two adjacent crystals of 15 ° or more is D and the maximum diameter of grain boundary carbide is d, the value expressed by D / d is 54 or less. A steel sheet used for manufacturing the steel member according to claim 1, wherein the component composition is in 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 less than 0.0070%,
S: more than 0% and less than 0.0070%,
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
Satisfy,
Steel sheet characterized in that 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 100 mm or less. As a method for manufacturing the steel sheet according to claim 2, after hot rolling a steel piece satisfying the component composition according to claim 2, quenching is performed. Heating temperature: 910 ° C or more and 940 ° C or less, and holding time at the heating temperature: 25 minutes It is carried out under conditions of not less than 55 minutes, and tempering is performed after this quenching, heating temperature: 620 ° C or higher, Ac ₁ point or lower, and heating temperature and heating time in which the P _T value represented by the following formula (1) becomes 19.2 or higher and 20.6 or lower Method for producing a steel sheet, characterized in that performed in.
P _T value = T _T × (20 + logt _T ) × 10 ^-3 … (One)
In Formula (1), T _T represents the heating temperature (K) of tempering, and t _T represents the heating time (hr) of tempering. A method for manufacturing the steel member according to claim 1, wherein the steel sheet according to claim 2 is welded, and further heat treatment after welding is performed by heating temperature at which the P _PWHT value represented by the following formula (2) becomes 20 or more, and The manufacturing method of the steel member characterized by performing at heating time.
P _PWHT value = T _PWHT × (20 + logt _PWHT ) × 10 ^-3 … (2)
In Formula (2), T _PWHT represents the heating temperature (K) of heat treatment after welding, and t _PWHT represents the heating time (hr) of heat treatment after welding.

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