JP2011001620A - High strength thick steel plate combining excellent productivity and weldability and having excellent drop weight characteristic after pwht, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength thick steel plate in a TS class of >580 MPa having excellent productivity and weldability, and having excellent drop weight characteristic after PWHT (Post Weld Heat Treatment), and to provide a method for producing the same.SOLUTION: The steel plate has a composition containing, by mass, 0.04 to 0.08% C, 0.05 to 0.6% Si, 1.2 to 2.0% Mn, ≤0.010% P, ≤0.003% S, 0.01 to 0.05% Al, 0.01 to 0.50% Cu, 0.05 to 0.60% Ni, 0.01 to 0.50% Cr, 0.05 to 0.40% Mo, 0.01 to 0.1% V, 0.0010 to 0.0040% N, has a Pcm of ≤0.22, a hardenability index (DI value) of 40 to 100 and a Y value of 1.20 to 1.50, and the balance Fe with inevitable impurities, and has a microstructure composed of tempered bainite and/or tempered martensite: hardenability index (DI value): DI=8√C×(1+0.64Si)×(1+4.1Mn)×(1+0.27Cu)×(1+0.52Ni)×(1+2.33Cr)×(1+3.14Mo); and Y value: Y=Cr+2Mo+10V.

Description

  The present invention relates to a high-strength thick steel plate suitable for applications requiring excellent drop weight characteristics after PWHT, such as a reactor containment vessel, a pressure vessel, a steam generator, or various reaction vessels, and a method for producing the same, particularly in the slab production stage. The present invention relates to a high-strength steel plate having a tensile strength (TS) of over 580 MPa, which has both excellent productivity and excellent weldability in local construction. In addition, the thick steel plate of this invention targets the steel plate of plate thickness 6mm-80mm.

  In recent years, from the viewpoint of expansion of energy demand and prevention of global warming, there is a strong demand for the construction of nuclear power plants worldwide, and the development of applicable steel materials is strongly demanded.

  Steel plates used in pressure vessels and reaction vessels in nuclear power plants and chemical plants are often thick, and are required to have good weldability in addition to high strength and toughness.

  Conventionally, as a steel plate for a pressure vessel suitable for this kind of application, steel materials such as SPV type of JIS G3155 mainly composed of C-Si-Mn type have been used. However, in these thick steel plates, since the steel material component is a so-called medium carbon component having a carbon content of about 0.08 to 0.16%, slab surface cracks due to peritectic solidification during continuous casting occur. The maintenance load is high, and it is becoming difficult to meet the vigorous needs for steel sheets due to the rapid increase in the number of nuclear plants built.

  By the way, in the manufacture of large reactor containment vessels, reaction vessels, etc., PWHT (Post Weld Heat Treatment: also called post-weld heat treatment, SR treatment or stress relief annealing) is applied after cutting, bending, and welding. However, from the viewpoint of the welding technique at the time of assembly at the site and the safety, the PWHT is frequently performed at a high temperature and for a long time as compared with the conventional PWHT conditions.

  In order to suppress such strength reduction due to high temperature and long time PWHT, it is effective to add an alloy element that suppresses temper softening. However, the addition of an excessive alloy element causes a decrease in weldability, and the locality. Strict preheating temperature management is required for construction.

On the other hand, in high-strength steel, cracks occur in the heat affected zone due to PWHT. Therefore, an SR cracking susceptibility index P _SR , ΔG that limits the amount of precipitation hardening elements such as Cr, Mo, V has already been proposed (for example, JP-A 61-126978). In the case of ΔG, ΔG = Cr + 3.3Mo + 8 The components are designed so that 1V-2 is negative.

  Patent Document 1 proposes a thick steel plate that suppresses coarsening of cementite by optimizing the addition amount of Cr, Mn, and V, has a small decrease in strength before and after PWHT, and is excellent in weldability.

  In Patent Document 2, there is a TS600 MPa class thick steel plate excellent in weldability and strain aging characteristics that uses precipitation strengthening of Nb carbonitride as a strength compensation for reducing the amount of C for the purpose of improving weldability. Proposed.

  For thick steel plates used in areas where safety is important, such as nuclear reactor containment vessels and steam generators, NDT temperature (ASTL E208 regulation NRL) indicating material characteristics related to brittle crack propagation stopping performance when brittle fracture occurs It is required to have excellent drop characteristics using the drop weight test as an index. However, when PWHT is applied, the segregation of P to the grain boundaries during the cooling process significantly reduces low-temperature toughness, particularly drop characteristics. .

  In this regard, Patent Document 3 discloses a technique for improving the drop weight characteristic by reducing the C amount and the P amount and increasing the N amount.

JP 2008-150656 A JP 2001-64724 A JP-A-2-93044

  However, the strength level after PWHT of the steel sheet described in Patent Document 1 is not more than 580 MPa in tensile strength, and the effectiveness of high strength steel exceeding 580 MPa is unknown. The steel sheet described in Patent Document 2 receives a welding heat history. There is a concern that once dissolved Nb is subjected to high-temperature and long-time PWHT, it precipitates as Nb carbonitride and causes a reduction in HAZ toughness. Since the steel sheet described in Patent Document 3 increases the N amount, when producing a slab by a continuous casting process, a slab surface crack or the like is likely to occur, and there is a concern about a decrease in productivity.

  Therefore, the present invention provides a high-strength thick steel plate that has excellent productivity and weldability and has excellent drop weight characteristics after PWHT, and a method for producing the same, which solves the above problems.

Inventors repeated earnest research in order to solve the said subject, and acquired the following knowledge.
1. In order to combine excellent productivity at the slab manufacturing stage with excellent weldability at the site construction, in the component design, C content is reduced from the subperitectic region and Pcm is less than 0.22% Is effective.
2. By optimizing the amount of Cr, Mo, V added and limiting the Y value defined by Y = Cr + 2Mo + 10V within the range of 1.2 to 1.5, it exceeds TS 580 MPa even after high temperature and long time PWHT. In addition to ensuring strength, slab cracking due to precipitation of V carbonitride, particularly during slab production, can be reduced, and productivity can be improved.
3. By optimizing the quenching temperature, refine the old γ grain size during heating, refine the microstructure (packet and block size) after bainite or martensite transformation during subsequent quenching, and excellent after PWHT It is possible to ensure the falling weight characteristic.

The present invention has been made by further study based on the obtained knowledge, and the gist thereof is as follows.
1. In mass%, C: 0.04 to 0.08%, Si: 0.05 to 0.6%, Mn: 1.2 to 2.0%, P: 0.010% or less, S: 0.003 %: Al: 0.01 to 0.05%, Cu: 0.01 to 0.50%, Ni: 0.05 to 0.60%, Cr: 0.01 to 0.50%, Mo: 0 0.05 to 0.40%, V: 0.01 to 0.1%, N: 0.0010 to 0.0040%, Pcm: 0.22 or less, Hardenability index (DI value): 40 to 100, Y Value: satisfying 1.20 to 1.50, balance Fe and inevitable impurities, the microstructure is tempered bainite and / or tempered martensite structure, and has excellent productivity and weldability In addition, a high-strength thick steel plate with excellent drop weight characteristics after PWHT.

Hardenability index (DI value): DI = 8√C × (1 + 0.64Si) × (1 + 4.1Mn) × (1 + 0.27Cu) × (1 + 0.52Ni) × (1 + 2.33Cr) × (1 + 3.14Mo) Y value: Y = Cr + 2Mo + 10V, where each element symbol is a content (mass%). Furthermore, it contains one or more of Ti: 0.004 to 0.010%, Ca: 0.0005 to 0.0015%, REM: 0.001 to 0.010% by mass%. The high-strength thick steel plate having excellent drop weight characteristics after PWHT, which is characterized by having both excellent productivity and weldability, according to 1.
3. Further, the mass ratio of Nb: 0.003% or less and B: 0.0003% or less, characterized by combining the excellent productivity and weldability according to 1 or 2, after PWHT High-strength thick steel plate with excellent drop weight characteristics.
4). Furthermore, in the microstructure of tempered bainite and / or tempered martensite, the average value of the equivalent circle diameters of crystal grains surrounded by a large tilt grain boundary of 15 ° or more measured by EBSP is 20 μm or less. A high-strength thick steel plate that has excellent productivity and weldability according to any one of 1 to 3, and has excellent drop weight characteristics after PWHT.
After the steel material made of continuous casting having the component composition according to any one of 5.1 to 3 is made into a sheet thickness of 80 mm or less by hot rolling without performing surface maintenance, Ac _{3 to} Ac ₃ + 70 ° C. After the PWHT, which has excellent productivity and weldability, it is characterized in that it is subjected to a process of quenching after reheating in the temperature range of at least once, followed by a tempering process in a temperature range of 650 to 700 ° C. A method for manufacturing high-strength thick steel plates with excellent drop weight characteristics.
6). 6. The method for producing a high-strength thick steel plate having excellent drop weight characteristics after PWHT having excellent productivity and weldability, wherein the hot rolling is direct feed rolling or hot charge rolling.

  According to the present invention, TS: 580 MPa, which combines excellent productivity at the slab manufacturing stage and excellent weldability in local construction, suitable for applications requiring excellent drop weight characteristics after PWHT such as a reactor containment vessel. A super-grade high-strength thick steel plate and a method for producing the same are obtained, and are extremely useful in industry.

In the present invention, the component composition and the microstructure are defined. In the following description, PWHT is assumed to be performed by heating at a temperature exceeding the maximum heating temperature of 600 ° C. and exceeding 600 ° C. for a cumulative period of 10 hours or more.
[Ingredient composition] In the description, “%” means “mass%”.
C: 0.04 to 0.08%
C is an element necessary for securing a predetermined strength, and it is necessary to contain 0.04% or more in order to secure a strength exceeding 580 MPa. On the other hand, when the content exceeds 0.08%, peritectic solidification is accompanied, so that the surface of the material is easily cracked when melted and cast by continuous casting. If cracks occur on the surface of the material, it is necessary to remove the cracked part of the surface of the material by scarfing such as a hot scarf or cold scarf in order to prevent deterioration of the surface quality of the product after rolling. Is extremely disturbed. For this reason, the C content is set to 0.04 to 0.08%. In addition, preferable content is 0.04-0.07%.

Si: 0.05-0.6%
Si is an element that not only contributes to deoxidation of steel, but also dissolves in steel and is effective in increasing the strength of steel. To obtain the effect, Si should be contained in an amount of 0.05% or more. is required. However, if the content exceeds 0.6%, the toughness of the weld heat-affected zone decreases, so the Si content is 0.05 to 0.6%. In addition, preferable content is 0.1 to 0.5%.

Mn: 1.2 to 2.0%
Mn not only contributes to deoxidation of steel, but is a useful element that improves hardenability, and in order to obtain high strength, it is necessary to contain 1.2% or more. On the other hand, if the content exceeds 2.0%, weldability and weld heat-affected zone toughness are lowered, so the Mn content is set to 1.2 to 2.0%. In addition, preferable content is 1.2 to 1.8%.

P: 0.010% or less P is inevitably mixed in the steel, segregates in the prior austenite grain boundaries in the slow cooling process of stress relief annealing (PWHT) after welding, and promotes grain boundary embrittlement. Reduces falling weight characteristics. Therefore, it is desirable that the amount of P is as low as possible, but if it is 0.010% or less, grain boundary embrittlement can be prevented, so the upper limit of the content is made 0.010%. In addition, preferable content is 0.008% or less.

S: 0.003% or less S is an element that exists as inclusions such as MnS in steel and lowers toughness, and is desirably as low as possible. If the content exceeds 0.003%, the falling weight characteristic is lowered, so the upper limit is made 0.003%. In addition, preferable content is 0.002% or less.

Al: 0.01 to 0.05%
Al is an element useful as a deoxidizing element, and is useful for improving toughness through grain refinement with AlN during quenching. In order to exert these effects, the Al content is set to 0.01 to 0.05%. In addition, preferable content is 0.015-0.04%.

Cu: 0.01 to 0.50%
Cu is an element useful as a solid solution strengthening element by dissolving in steel, and it is necessary to contain 0.01% or more in order to obtain high strength, but it contains more than 0.50%. Then, since concern about Cu cracking at the time of hot rolling increases, the Cu content is set to 0.01 to 0.50%. In addition, preferable content is 0.01 to 0.40% or less.

Ni: 0.05-0.60%
Ni, like Cu, is a solid solution in steel, and is an element useful as a solid solution strengthening element and also for improving low temperature toughness. In order to acquire the effect, it is necessary to make it contain 0.05% or more. However, if the content exceeds 0.60%, the steel material cost increases, the frequency of occurrence of slab cracking increases, and the productivity is hindered. Therefore, the Ni content is 0.05 to 0.60. %. In addition, preferable content is 0.10 to 0.50% or less.

Cr: 0.01 to 0.50%
Cr is a useful element that improves hardenability and is an important element for securing strength after PWHT. In order to obtain the effect, it is necessary to contain 0.01% or more. However, if the content exceeds 0.50%, the weldability is deteriorated and the toughness is significantly lowered after PWHT. Therefore, the Cr content is set to 0.01 to 0.50%. A preferable content is 0.10 to 0.50%.

Mo: 0.05-0.40%
Mo is an element that improves hardenability, increases strength, and is useful for securing toughness. Moreover, like Cr, it is an element important for ensuring the strength after PWHT, and in order to obtain the effect, it is necessary to contain 0.05% or more. However, if the content exceeds 0.40%, weldability is deteriorated and Mo is an expensive element, which causes an increase in steel material cost. Therefore, the Mo content is set to 0.05 to 0.40%. A preferable content is 0.10 to 0.30%.

V: 0.01 to 0.1% or less V is an element that improves hardenability, forms carbonitrides with C and N, and is important for ensuring strength after PWHT. In order to acquire the effect, it is necessary to make it contain 0.01% or more. However, if it contains more than 0.1%, the weldability is deteriorated and the toughness is reduced due to precipitation of carbonitride. , V content is 0.01-0.1%. In addition, preferable content is 0.01 to 0.07%.

N: 0.0010 to 0.0040%
N, like C, forms a carbonitride with V and is an element useful for increasing the strength. Moreover, AlN is formed at the time of hot rolling or quenching, and contributes to improvement of toughness through austenite refinement. In order to obtain the effect, it is necessary to contain 0.0010% or more. However, if it contains more than 0.0040%, the concern about slab cracking increases, and the productivity is hindered, and the toughness of the welded portion. Since the reduction is also caused, the N content is set to 0.0010 to 0.0040%.

Pcm: 0.22% or less Pcm is a weld cracking sensitivity index, and Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (each element is a content (mass%)). If Pcm exceeds 0.22%, cold cracking occurs when welding without preheating, so strict preheating temperature control is required, so the upper limit is made 0.22%. Thereby, preheating temperature can be made into room temperature or less. In the present invention, since the Pcm is 0.22% or less, the amount of C is reduced, so the subperitectic region is avoided, slab surface cracks are reduced, and productivity can be improved.

Hardenability index (DI): 40-100
In order to ensure a sufficient strength up to a plate thickness of about 80 mm, it is necessary to limit the hardenability index (DI) to an appropriate range. When the hardenability index (DI) is less than 40, there is a concern about insufficient strength at the center of the plate thickness with a thick material having a plate thickness of 40 mm or more. On the other hand, when it exceeds 100, the alloy element addition amount increases and weldability increases. Since it falls remarkably, it is set to 40-100. The hardenability index (DI) is 8√C × (1 + 0.64Si) × (1 + 4.1Mn) × (1 + 0.27Cu) × (1 + 0.52Ni) × (1 + 2.33Cr) × (1 + 3.14Mo), each The element is the content (% by mass).

Y value: 1.20-1.50
The Y value (= Cr + 2Mo + 10V, each element content (% by mass)) is an index indicating the degree of temper softening resistance during tempering. When the Y value is less than 1.20, the steel sheet strength decreases after tempering and PWHT. Is large and it is difficult to achieve a predetermined strength.

  On the other hand, when the Y value exceeds 1.50, Cr, Mo, V is excessively added, so that the weldability is significantly lowered, and precipitation strengthening due to precipitates containing Cr, Mo, V is caused. It becomes remarkable and the toughness after PWHT is remarkably lowered.

  In addition, when the Y value exceeds 1.5, slab cracking due to precipitation of V carbonitride particularly at the time of slab production becomes prominent and the productivity is hindered, so the Y value is 1.2 to 1.5. And

  The above is the basic component composition of the present invention. In the present invention, the following elements can be selectively added to further improve the characteristics.

One or more of Ti: 0.004 to 0.010%, Ca: 0.0005 to 0.0015%, REM: 0.001 to 0.010% Ti forms a nitride with N, It is a useful element that contributes to improved toughness through refinement of austenite grains during hot rolling and welding. If the content is less than 0.004%, the effect is not sufficient. On the other hand, if it exceeds 0.010%, the toughness after PWHT is remarkably lowered. .

  Ca and REM form S and sulfides in steel and contribute to the improvement of toughness through refinement of austenite grains during hot rolling and welding. Therefore, Ca: 0.0005% or more, REM: 0.001 % Or more can be contained. However, excessive content causes a decrease in cleanliness due to an increase in sulfides. Therefore, the upper limit of the content is set to Ca: 0.0015% and REM: 0.010%.

  In the present invention, it is preferable that neither Nb nor B is contained. However, even when contained, the contents of Nb and B should be Nb: 0.003% or less and B: 0.0003% or less. preferable.

  Nb, like V, is an element that forms carbonitrides with C and N and increases the strength. However, when performing PWHT at a high temperature and for a long time as the object of the present invention, the embrittlement due to precipitation of Nb carbonitride is remarkable, and particularly the toughness of the weld heat affected zone is greatly reduced. It is preferable not to contain. Moreover, even if it contains, it is preferable to restrict | limit the upper limit to 0.003% or less.

  B is an element that increases the hardenability and contributes to the improvement of strength by adding an extremely small amount, but even with a small amount of addition, the hardness of the weld heat-affected zone after welding is suddenly increased and weld cracking susceptibility is increased. The weldability is significantly deteriorated, and the toughness is also lowered. Moreover, even if it contains, it is preferable to restrict | limit the upper limit with 0.0003%. The steel of the present invention is the remaining Fe and unavoidable impurities in addition to the above-described component composition.

[Microstructure]
The microstructure is tempered bainite and / or tempered martensite, and is characterized in that the average value of the equivalent circle diameters of crystal grains surrounded by a large tilt grain boundary of 15 ° or more measured by EBSP is 20 μm or less. The steel of the present invention may contain a small amount of other structures in addition to tempered bainite and / or tempered martensite as long as the effects are not impaired.

  In order to ensure strength exceeding TS: 580 MPa even after tempering and after PWHT at a high temperature for a long time, the microstructure after quenching is bainite and / or martensite, and after tempering, tempered bainite and / or tempered martensite. .

  By tempering and PWHT, the strength is reduced, while the toughness is improved, but in tempered bainite and / or martensite, the smaller the grain surrounded by the large tilt grain boundaries of 15 degrees or more measured by EBSP, The stress concentration at the grain boundary is reduced, the resistance to fracture is increased, and the low-temperature toughness and drop weight characteristics are improved.

When the average grain size of the crystal grains surrounded by the large inclination grain boundaries of 15 degrees or more exceeds 20 μm, the fracture occurs from coarse grains as a starting point, and the toughness is low. Heavy properties (T _NDT <−25 ° C.) are obtained. _Here, the _{T NDT,} means NDT temperature (no ductile transition temperature) defined in the case of performing the NRL drop weight test described in ASTM E208.

  Further, a large tilt grain boundary of 15 degrees or more means a grain boundary in which the grain boundary orientation difference between adjacent crystal grains is 15 degrees or more based on the orientation difference mapping measured by EBSP (Electron Back Scattering Pattern). Further, the grain boundary can be traced, and the average crystal grain size (equivalent circle diameter) can be calculated by image analysis.

  Preferred production conditions for the steel of the present invention are described below.

  First, a slab to be a hot rolled material is manufactured by continuous casting from a molten metal having a composition within the range of the present invention by a conventional method.

  In the present invention, by limiting the amount of C to 0.04 to 0.08%, subperitectic solidification is avoided, so that cracks on the slab surface are reduced. For this reason, slab surface maintenance such as a hot scarf and a cold scarf becomes unnecessary, and these operations can be omitted. Furthermore, when the surface maintenance of the slab is no longer necessary, direct feed rolling in which the slab in a high temperature state after continuous casting is rolled as it is, or so-called hot charge rolling in which reheating is performed from a temperature of several hundred degrees C. Yes, it greatly contributes to the improvement of production efficiency and energy efficiency.

After forming into a thick steel plate with a thickness of 80 mm or less by hot rolling, the steel plate is reheated to Ac ₃ to Ac ₃ + 70 ° C. and then quenched once, followed by tempering at 650 to 700 ° C. The rolling may be performed according to a conventional method, and is not particularly limited. For example, a slab reheated to 1000 to 1200 ° C. may be hot-rolled at a cumulative reduction rate of 60% or more, and the rolling may be finished at 900 ° C. or more. Although cooling after rolling is not particularly defined, air cooling may be performed.

  Further, quenching and tempering may be performed in accordance with ordinary methods, and after heating to a predetermined temperature in a heating furnace, quenching by water cooling may be performed in a cooling facility. Tempering may be performed by air cooling after heating to a predetermined temperature in a heating furnace.

Quenching temperature is less than Ac _3, on which is two-phase temperature region of austenite + ferrite during heating, the austenite grain size is small, quenching becomes insufficient, it can not achieve a predetermined strength. On the other hand, when heating is performed at a temperature exceeding Ac ₃ + 70 ° C., the austenite grain size becomes coarse, the toughness of the base material is remarkably lowered, and the drop weight characteristic is also lowered. Therefore, the temperature is set to Ac _{3 to} Ac ₃ + 70 ° C. In addition, although the cooling rate at the time of hardening changes according to board thickness, it is preferable that it is a cooling rate of 5 degrees C / s or more.

  It is preferable to perform tempering after performing quenching twice or more while sequentially lowering the quenching temperature because the drop weight characteristics are further improved. This is because the structure after quenching and tempering is sized and refined by quenching twice or more.

As the value of the Ac ₃ transformation point, other values actually measured, for example, may be a value that is calculated by the following equation.

_{Ac 3 (℃) = 961.6-311.9C +} 49.5Si-36.4Mn + 12.7Al-51Cu-29Ni-8.7Cr + 13.5Mo + 308.1Nb-140V + 318.9Ti + 311.2B. However, each element symbol is a content (% by mass).

  The tempering temperature is preferably tempered at a high temperature in order to suppress the strength reduction after PWHT as much as possible, and is set to 650 ° C to 700 ° C. When the tempering temperature is less than 650 ° C., when PWHT of 600 ° C. or higher is performed, the strength decrease at PWHT is large and a predetermined strength (TS: over 580 MPa) cannot be obtained, while at 700 ° C. or higher, a predetermined strength is obtained after tempering. Strength (TS: more than 580 MPa) cannot be obtained.

  After welding, PWHT is performed for at least 10 hours at a temperature exceeding 600 ° C. In the manufacturing process of a pressure vessel or the like, PWHT is often applied multiple times for the purpose of reducing residual stress after welding. The cumulative 10 hours or more means that the total time at a temperature exceeding 600 ° C. is 10 hours or more when PWHT is performed in two or more times.

The steel of the present invention has a maximum heating temperature of PWHT of 600 ° C. or higher, and even when PWHT is performed for a long time at a high temperature of 10 hours or more, TS: 580 MPa or more and drop weight characteristics (T _NDT <−25 ° C.) Can be achieved. However, since the lower limit of the tempering temperature is 650 ° C., the PWHT temperature is preferably 650 ° C. or less.

The steel of the present invention, after being subjected to PWHT at a high temperature for a long time, TS: more than 580 MPa, a drop weight characteristic (T _NDT <−25 ° C.) is obtained, and a reactor containment vessel in which the drop weight characteristic is required, Suitable for pressure vessels, steam generators or various reaction vessels.

  The molten steel having the chemical composition shown in Table 1 was continuously cast to produce a 310 mm thick continuous cast slab, which was then heated to 1150 ° C. and then hot rolled to obtain a steel plate having a predetermined thickness. In the hot rolling, the steel sheet was produced by air-cooling by rolling to a predetermined thickness by thick plate rolling.

  These steel sheets were heated to a predetermined temperature in a heating furnace under the conditions shown in Table 2, then quenched by water cooling, subsequently heated to a predetermined temperature in a heating furnace, and tempered by air cooling. . In addition, steel plate No. 19 is an example in which after the first quenching at a quenching temperature of 900 ° C., the second quenching at a quenching temperature of 890 ° C. was performed. Steel plate No. No. 20 is an example in which after the first quenching at a quenching temperature of 920 ° C., the second quenching at a quenching temperature of 900 ° C. was performed, and then the third quenching at a quenching temperature of 890 ° C. was performed. . Steel plate No. 21 is an example in which after the first quenching at a quenching temperature of 890 ° C., the second quenching at a quenching temperature of 885 ° C. was performed.

  Then, as a heat treatment simulating PWHT in the manufacturing process of a pressure vessel or the like, after heating and holding at a predetermined temperature and time, furnace cooling was performed at a cooling rate of 55 ° C./h to prepare a steel plate for collecting specimens. .

  The microstructure of the steel plate was corroded with 3% nital using a sample collected from the steel plate after PWHT, and the structure was observed. The crystal grain size is defined as the grain boundary of a crystal grain surrounded by a grain boundary where the grain boundary orientation difference between adjacent crystal grains is 15 degrees or more, based on the orientation difference mapping measured by EBSP (Electron Back Scattering Pattern). The average crystal grain size (equivalent circle diameter) was calculated by image analysis.

  From these steel sheets after PWHT, JIS No. 4 tensile test was taken from 1 / 2t so that the tensile direction was perpendicular to the rolling direction, and the tensile test in accordance with JIS Z 2241 was conducted. Yield strength: YS and tensile strength: TS were obtained.

  In addition, in order to evaluate the toughness of the steel plate after PWHT, a 2 mm V notch Charpy impact test piece conforming to the standard of JIS Z2242 was sampled so as to be perpendicular to the rolling direction from the 1/4 t portion, and the Charpy impact test was performed. The fracture surface transition temperature (vTrs) was determined.

  Reproduction HAZ toughness was evaluated by using high-frequency induction heating to obtain a heat input of 50 kJ / cm using a specimen having a thickness of 15 mm, a width of 80 mm, and a length of 75 mm, which was taken from a quarter of the steel sheet before PWHT. After applying the corresponding welding heat cycle, a predetermined PWHT was performed, a 2 mmV notch Charpy test piece was taken from the test piece, and evaluated by the absorbed energy when a Charpy impact test was conducted at a test piece temperature of 0 ° C.

The drop weight test was performed in accordance with ASTM E208 by taking a P-3 test piece (thickness 16 mm, width 50 mm, length 130 mm) from a 1/4 t portion of the steel sheet after PWHT was performed. After performing PWHT, a drop weight test was performed. From the result of the drop test, the non-ductile transition temperature T _NDT was determined.

  Furthermore, as a weldability evaluation test, a y-type weld cracking test was performed in accordance with JIS Z3158. As welding conditions, welding was performed using a coated arc welding rod (LB-62UL) in an atmosphere of a temperature of 30 ° C. and a relative humidity of 80%. The test piece was sampled in full thickness and welded so that the weld line was perpendicular to the rolling direction. The presence or absence of the occurrence of root cracks at a preheating temperature of 30 ° C. was evaluated, and the crack prevention temperature (the lowest preheating temperature at which the root crack rate was 0%) was measured for the steel sheets that were cracked at 30 ° C.

Table 3 shows the test results. Steel sheets produced with components and production conditions within the scope of the present invention have good low temperature toughness with a tensile strength after PWHT exceeding 580 MPa and satisfying T _NDT <−25 ° C., which is an indicator of excellent drop weight characteristics. have. Moreover, the result of the y-type crack test also showed that the crack stop temperature was 30 ° C. or lower, and it had excellent weldability.

Claims (6)

In mass%, C: 0.04 to 0.08%, Si: 0.05 to 0.6%, Mn: 1.2 to 2.0%, P: 0.010% or less, S: 0.003 %: Al: 0.01 to 0.05%, Cu: 0.01 to 0.50%, Ni: 0.05 to 0.60%, Cr: 0.01 to 0.50%, Mo: 0 0.05 to 0.40%, V: 0.01 to 0.1%, N: 0.0010 to 0.0040%, Pcm: 0.22 or less, Hardenability index (DI value): 40 to 100, Y Value: satisfying 1.20 to 1.50, balance Fe and inevitable impurities, the microstructure is tempered bainite and / or tempered martensite structure, and has excellent productivity and weldability In addition, a high-strength thick steel plate with excellent drop weight characteristics after PWHT.
Hardenability index (DI value): DI = 8√C × (1 + 0.64Si) × (1 + 4.1Mn) × (1 + 0.27Cu) × (1 + 0.52Ni) × (1 + 2.33Cr) × (1 + 3.14Mo) Y value: Y = Cr + 2Mo + 10V, where each element symbol is the content (% by mass)   Furthermore, it contains one or more of Ti: 0.004 to 0.010%, Ca: 0.0005 to 0.0015%, REM: 0.001 to 0.010% by mass%. The high-strength thick steel plate having excellent drop weight characteristics after PWHT, which is characterized by having both excellent productivity and weldability.   The PWHT having excellent productivity and weldability according to claim 1, further comprising Nb: 0.003% or less and B: 0.0003% or less in terms of mass%. A high-strength thick steel plate with excellent post-fall characteristics.   Furthermore, in the microstructure of tempered bainite or tempered martensite, the average value of the equivalent circle diameters of crystal grains surrounded by a large tilt grain boundary of 15 degrees or more measured by EBSP is 20 μm or less. Item 4. A high-strength thick steel plate having excellent drop weight characteristics after PWHT, which has excellent productivity and weldability according to any one of Items 1 to 3. The steel material made of continuous casting having the composition according to any one of claims 1 to 3 is subjected to hot rolling to a sheet thickness of 80 mm or less without performing surface treatment, and then Ac _{3 to} Ac ₃ + 70 ° C. After the PWHT, which has excellent productivity and weldability, it is characterized in that it is subjected to a process of quenching after reheating in the temperature range of at least once, followed by a tempering process in a temperature range of 650 to 700 ° C. A method for manufacturing high-strength thick steel plates with excellent drop weight characteristics. 6. The production of a high-strength thick steel plate having excellent drop weight characteristics after PWHT, which has excellent productivity and weldability, according to claim 5, wherein the hot rolling is direct feed rolling or hot charge rolling. Method.