JP5659758B2 - TMCP-Temper type high-strength steel sheet with excellent drop weight characteristics after PWHT that combines excellent productivity and weldability - Google Patents

Description

  The present invention relates to a method for producing 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, particularly in the slab production stage. The present invention relates to a method for producing a TMCP-Temper type high-strength thick steel plate having a tensile strength (TS) of over 580 MPa, which has both excellent productivity and excellent weldability in local construction. The present invention is directed to a method for manufacturing a steel plate having a thickness of 6 mm to 80 mm.

  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 for 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, the steel material component is a so-called medium carbon component having a carbon content of about 0.08 to 0.16% by mass%, so slab surface cracks due to peritectic solidification during continuous casting occur. However, the slab maintenance load is high, and it is becoming difficult to meet vigorous steel sheet needs accompanying the rapid increase in the number of nuclear power 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. Is essential, but from the viewpoint of safety and welding technology at the time of assembly in the field, PWHT is being carried out under extremely severe conditions at high temperatures and for a long time compared to conventional conditions. .

  Although the strength is reduced by such PWHT, the addition of an alloying element that suppresses temper softening is effective for the suppression, but the addition of an excessive alloying element causes a decrease in weldability and is strict in field construction. It is necessary to control the preheating temperature.

On the other hand, in high-strength steel, cracks occur in the heat affected zone due to PWHT. Therefore, an SR cracking sensitivity index P _SR , ΔG that limits the amount of precipitation hardening elements such as Cr, Mo, V has been proposed (for example, Patent Document 1). In the case of ΔG, ΔG = Cr + 3.3Mo + 8.1V-2 ( However, each element symbol is designed so that the content (mass%) is negative.

  Patent Document 2 relates to a high-strength steel sheet excellent in stress-relieving annealing characteristics and weldability, by suppressing the coarsening of cementite by optimizing the addition amount of Cr, Mn, and V, and the strength decrease before and after PWHT is small. Moreover, a thick steel plate excellent in weldability has been proposed.

  Patent document 3 relates to a method for producing a 60 kg class high-strength steel excellent in weldability and toughness after strain aging. As a strength compensation for reducing the amount of C for the purpose of improving weldability, Nb carbonitride is used. A thick steel plate that uses precipitation strengthening and has excellent weldability and strain aging characteristics has been proposed.

  In addition, for thick steel plates used in parts where safety is important, such as reactor containment vessels and steam generators, NDT temperature is an index related to the brittle fracture resistance of materials (according to ASTM E208 regulation NRL drop weight test). However, when PWHT is applied, the segregation of P to the grain boundaries during the cooling process significantly lowers the low temperature toughness, particularly the drop weight characteristic.

  Patent Document 4 relates to a steel for a reactor pressure vessel excellent in drop weight characteristics, and discloses a technique for improving the drop weight characteristics by reducing the amount of C and P in the component composition and increasing the amount of N. .

JP-A 61-126978 JP 2008-150656 A JP 2001-64724 A JP-A-2-93044

  However, the steel sheet described in Patent Document 2 has a strength level after PWHT of not more than 580 MPa, and even after PWHT, a tensile strength exceeding 580 MPa cannot be obtained.

  In the steel sheet described in Patent Document 3, Nb once dissolved when subjected to welding heat history is precipitated as Nb carbonitride when PWHT is applied at a high temperature for a long time, resulting in a decrease in HAZ toughness. There is concern. Since the steel sheet described in Patent Document 4 increases the N amount, when manufacturing 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.

  In addition, most of the conventional techniques involve reheating the steel sheet obtained by hot rolling, followed by quenching. Accelerated cooling is performed following direct quenching and rolling, which are more advantageous in terms of manufacturing time, and rolling. Development of a manufacturing method utilizing the method to be performed is desired. Here, Patent Document 2 describes a technique for accelerating and cooling a steel sheet at a predetermined cooling rate after rolling, but as described above, the tensile strength exceeding 580 MPa targeted in the present invention has been achieved. Therefore, the development of new manufacturing technology has been desired.

  Then, this invention provides the manufacturing method of the TMCP-Temper type high-strength thick steel plate which was excellent in the drop weight characteristic after PWHT which has the outstanding productivity and weldability which solves the said subject. Here, TMCP is an abbreviation for thermo-mechanical control process (processing heat treatment, thermal processing control). In the present invention, a TMCP-Temper type high-strength thick steel plate is a steel plate manufactured by utilizing control rolling technology and controlled cooling (accelerated cooling technology), which are two major element technologies of TMCP, for example, controlled rolling. A high-strength thick steel plate manufactured by tempering a steel plate obtained by accelerating cooling of the steel plate as it is is meant.

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 0.22% or less Is effective.
2. TMCP- is a steel whose Cr, Mo, V content is adjusted so that the value specified by the parameter formula: Cr + 2Mo + 10V (where each element symbol is the content (mass%)) is 1.00-1.50. When Temper is applied, strength exceeding TS580MPa is secured even after high-temperature and long-time PWHT, and slab cracking due to precipitation of V carbonitride during continuous cast slab manufacturing can be reduced, improving productivity. It is.
3. By reducing the austenite refinement by adjusting the rolling conditions and introducing processing strain, if the bainite or martensite packet or block size that is transformed during the accelerated cooling that follows the rolling process is refined, excellent weight loss after PWHT is achieved. It is possible to ensure the characteristics.

  The present invention has been made by further study based on the obtained knowledge, and the gist thereof is as follows.

1. Component composition is mass%, C: 0.04-0.08%, Si: 0.05-0.6%, Mn: 1.2-2.0%, P: 0.003-0.020 %, S: 0.003% or less, 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: 0.05-0.40%, V: 0.01-0.1%, N: 0.0010-0.0040%, Pcm: 0.22% or less, hardenability index (DI Value): 40 to 100, the remaining cast iron and inevitable impurities continuously cast slabs are reheated to 1000 to 1250 ° C. without surface care, and the cumulative rolling reduction at 900 ° C. or less is 50% or more, rolling finish Temperature: Hot rolling at less than 900 ° C. to a sheet thickness of 80 mm or less, followed by a cooling rate with an average cooling rate of 3 ° C./s or more. Accelerated cooling to a temperature of 500 ° C or lower, and then tempering in the temperature range of 600 to 750 ° C, with excellent productivity and weldability, and excellent weight loss characteristics after PWHT TMCP-Temper type high strength thick steel plate manufacturing method.
Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (Each element symbol is a content (mass%).)
Hardenability index (DI) = 8√C × (1 + 0.64Si) × (1 + 4.1Mn) × (1 + 0.27Cu) × (1 + 0.52Ni) × (1 + 2.33Cr) × (1 + 3.14Mo) (however, (Each element symbol is the content (% by mass).)
2. Furthermore, the component composition satisfies 1.00 to 1.50 as a value of the following formula (Y value), and the drop weight characteristic after PWHT having excellent productivity and weldability according to 1. Method of TMCP-Temper type high-strength thick steel plate with excellent resistance.
Y = Cr + 2Mo + 10V
However, each element symbol is a content (% by mass).
3. Furthermore, the component composition is 1% or more of Ti: 0.004-0.010%, Ca: 0.0005-0.0015%, REM: 0.001-0.010% in mass%. A method for producing a TMCP-Temper type high-strength thick steel plate having excellent productivity and weldability according to 1 or 2 and having excellent drop weight characteristics after PWHT.
4). Further, Nb and B, which are inevitable impurities in the component composition, are mass%, Nb: 0.003% or less, and B: 0.0003% or less, any one of 1 to 3 The TMCP-Temper type high-strength thick steel sheet having excellent drop weight characteristics after PWHT, which has excellent productivity and weldability.

  According to the present invention, TS: 580 MPa, which combines excellent productivity at the slab manufacturing stage and excellent weldability in field construction, suitable for applications requiring excellent drop weight characteristics after PWHT, such as a reactor containment vessel. Super-class TMCP-Temper type high-strength thick steel sheets can be manufactured, which is extremely useful industrially.

[Ingredient composition] In the description, “%” means “mass%”. PWHT is 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.
C: 0.04 to 0.08%
C is an element necessary for ensuring a predetermined strength, and it is necessary to contain 0.04% or more in order to ensure a strength of 580 MPa or more. 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, it is made into 0.04 to 0.08%. In addition, Preferably, it is 0.04 to 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 necessary. However, if the content exceeds 0.6%, the toughness of the weld heat affected zone decreases, so 0.05 to 0.6%. In addition, Preferably it 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 content is set to 1.2 to 2.0%. In addition, Preferably, it is 1.2 to 1.8%.

P: 0.003 to 0.020% or less P is inevitably mixed in the steel and segregates at the prior austenite grain boundaries in the slow cooling process of post-weld stress relief annealing (PWHT), resulting in grain boundary embrittlement. To reduce the drop weight characteristics. Therefore, it is desirable that P is as low as possible. However, in the hot rolling-accelerated cooling process to be described later, the microstructure can be refined as compared with the tempered heat treatment material, so that it can be contained up to 0.020%. On the other hand, in order to make it less than 0.003%, since there are many problems in terms of manufacturing cost such as slag reforming treatment at the time of melting, careful selection of raw materials, and increase in de-P treatment time, the lower limit is set to 0.003. %. In addition, Preferably, it is 0.005-0.015%.

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, Preferably, it 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 crystal grain refinement with AlN during accelerated cooling. In order to exert these effects, the content is made 0.01 to 0.05%. In addition, Preferably, it 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 is increased, the content is made 0.01 to 0.50%. In addition, Preferably, it 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 cracks increases, and the productivity is hindered, so 0.05 to 0.60%. In addition, Preferably, it is 0.10 to 0.50%.

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 lowered significantly after PWHT, so the content is made 0.01 to 0.50%. In addition, Preferably, it 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%, the weldability is deteriorated and the cost of the steel material is increased due to the expensive elements, so the content is made 0.05 to 0.40%. In addition, Preferably, it is 0.10 to 0.30%.

V: 0.01 to 0.1%
V is an element that improves hardenability, forms carbonitrides with C and N, and is important for securing 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. 0.01 to 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. Also, AlN is formed during hot rolling or accelerated cooling, contributing to improved toughness through austenite refinement. In order to acquire the effect, it is necessary to contain 0.0010% or more, but when it contains more than 0.0040%, concern about a slab crack will increase and productivity will be inhibited, and toughness of a welded part Since it also causes a decrease, the content is made 0.0010 to 0.0040%.

Pcm: 0.22% or less Pcm is a weld cracking sensitive composition, and Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (each element symbol 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, the amount of C is reduced in order to make Pcm 0.22% or less, so a subperitectic region is avoided, slab surface cracks are reduced, and productivity can be improved.

Hardenability index (DI): 40-100
In order to secure a desired 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 alloying 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), ( However, each element symbol is a content (mass%).

Y value: 1.00-1.50
Y value is a parameter formula: Cr + 2Mo + 10V, and each element symbol is a content (mass%) value, which is an index indicating the degree of temper softening resistance during tempering. If the Y value is less than 1.00, the steel sheet strength after tempering and PWHT is greatly reduced, 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 1.50, 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 1.00 to 1.50. And

  The above is the basic component composition of the present invention. In the present invention, the following elements can be selectively contained in order 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%.

  The contents of Nb and B, which are inevitable impurities, are preferably Nb: 0.003% or less and B: 0.0003% or less.

  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. Even when contained as an inevitable impurity, the amount is preferably limited to 0.003% or less.

B is an element that contributes to improving the hardenability and improving the strength by adding a very small amount, but even if contained in a very small amount, 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 lowered. Moreover, even if it contains, it is preferable to restrict | limit the upper limit with 0.0003%.
[Production conditions]
The temperature condition of steel defined in the present invention refers to the average temperature in the plate thickness direction of a steel piece or a steel plate.

  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 surface treatment of the slab is no longer necessary, thick plate rolling can be performed by hot charging (rolling the high-temperature slab after continuous casting as it is, or rolling it by reheating from a temperature of several hundred degrees Celsius). It can be done and contributes greatly to improving production efficiency and energy efficiency.
[Slab heating temperature]
About the raw material (slab) obtained in this way, after reheating to 1000 to 1250 ° C., hot rolling is performed. However, when the reheating temperature is less than 1000 ° C., solutionization is insufficient and the strength decreases, Since heating exceeding 1250 ° C. causes a decrease in toughness (falling weight characteristics), the reheating temperature is set to 1000 to 1250 ° C. In addition, among the cases where the above-described hot charge is applied, the reheating temperature in the case of rolling by reheating from a temperature of several hundred degrees Celsius may also follow the above conditions. Moreover, when rolling the slab of the high temperature state after continuous casting as it is, the said conditions regarding reheating may be disregarded.
[Hot rolling conditions]
Next, the rolling conditions in the case of forming a thick steel plate having a thickness of 80 mm or less in hot rolling will be described. The cumulative rolling reduction at 900 ° C. or less is 50% or more, and the rolling finishing temperature is less than 900 ° C. In tempered bainite and / or martensite, it is an indispensable condition for making the average particle size of the packets surrounded by the large tilt grain boundaries of 15 ° or more measured by EBSP 15 μm or less, and drastically reduces falling weight characteristics. It is necessary to improve. Since it is preferable cooling start temperature below is Ar ₃ temperature or above and, correspondingly, it is preferable finishing rolling temperature is Ar ₃ temperature or above.
[Cooling conditions]
After rolling, the steel sheet needs to be subjected to accelerated cooling subsequent to rolling to a temperature of 500 ° C. or lower at an average cooling rate of 3 ° C./s or higher. When the cooling rate is less than 3 ° C./s, the bainite and / or martensite transformation is not sufficient, and the strength is lowered by precipitation of ferrite.

  When the cooling stop temperature exceeds 500 ° C., Mo and V carbonitride precipitate in the air cooling process after accelerated cooling, so that the tempering softening resistance after PWHT is lowered. Therefore, the cooling rate is 3 ° C./s or more, and the cooling stop temperature is 500 ° C. or less.

Although the cooling start temperature is not particularly defined, it is desirable that the cooling is not less than the Ar ₃ temperature and the cooling is started immediately after the rolling is completed. The steel sheet accelerated and cooled following the rolling is then subjected to tempering treatment in a temperature range of 600 to 750 ° C. to adjust toughening.

  When the tempering temperature is less than 600 ° C., the material change after PWHT becomes large. On the other hand, tempering exceeding 750 ° C. decreases in strength and cannot exceed TS580 MPa. Preferably, it is 650 degreeC to 700 degreeC.

  According to the above-described component composition and production conditions, the microstructure of the steel of the present invention is tempered bainite and / or tempered martensite, and is an average grain of crystal grains (packets) surrounded by a large-angle grain boundary of 15 degrees or more measured by EBSP. The diameter is 15 μm or less.

  When the microstructure after accelerated cooling is bainite and / or martensite, and after tempering, tempering bainite and / or tempered martensite, strength exceeding TS: 580 MPa is ensured after tempering and after PWHT at high temperature for a long time. The

  In tempered bainite and / or tempered martensite, the smaller the average grain size of the packets surrounded by the large-angle grain boundaries of 15 ° or more measured by EBSP, the lower the stress concentration at the crystal grain boundaries, and the greater the resistance to fracture. Increases low temperature toughness and drop weight characteristics.

As a result, it is possible to mitigate the decrease in P for preventing grain boundary embrittlement due to PWHT. By setting the average particle size of the packets to 15 μm or less, excellent drop weight characteristics (T _NDT <−25 ° C.) can be obtained even at 0.020% where the amount of P is relatively high. On the other hand, if it exceeds 15 μm, P must be reduced. T _NDT means the non-ductile transition temperature defined when performing the NRL drop weight test described in ASTM E208-95a.

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

  The TMCP-Temper type thick steel plate manufactured in this way is subjected to PWHT of 10 hours or more at a temperature exceeding 600 ° C. after welding. In the manufacturing process of pressure vessels, etc., the PWHT is often applied multiple times for the purpose of reducing residual stress after welding. The cumulative time of 10 hours or more is the total time when the PWHT is divided into multiple times. Means 10 hours or more.

The steel according to the present invention has a maximum heating temperature of 600 ° C. or higher, a high temperature of cumulative 10 hours or more, and PWHT for a long time, and TS: 580 MPa or more, drop weight characteristics (T _NDT <−25 ° C.) are obtained. It is suitable for a reactor containment vessel, a pressure vessel, a steam generator, or various reaction vessels that require characteristics.

  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 subjected to rolling, cooling and tempering treatment to produce a thick steel plate. 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./hr to prepare a steel plate for collecting specimens. .

  The microstructure of the steel plate was corroded with 3% nital using a sample taken from a 1/4 thickness of the steel plate after PWHT, and the structure was observed. The crystal grain size is based on the orientation difference mapping measured by EBSP (Electron Back Scattering Pattern), and the grain boundaries of crystal grains having an orientation difference of 15 degrees or more between adjacent grains are traced, and the average crystal is analyzed by image analysis. The particle size (equivalent circle diameter) was calculated.

  A tensile test of JIS No. 4 was taken from 1/2 part of the plate thickness of the steel plate after the PWHT so that the tensile direction was a direction perpendicular to the rolling direction, and the tensile test was in accordance with the provisions of JIS Z 2241. A test was conducted to determine a yield strength of 0.2% YS and a tensile strength TS.

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

In addition, the drop weight test was carried out by collecting P-3 test pieces (thickness 15.9 mm, width 51 mm, length 127 mm) from a quarter thickness of the steel sheet in accordance with ASTM E208-95a. . 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 crack prevention temperature was determined with the lowest preheating temperature at which the root crack rate was 0%.

Table 2 shows the occurrence of cracks in the continuously cast material, the presence or absence of care, manufacturing conditions (hot rolling conditions, cooling conditions, tempering conditions, and PWHT conditions), and the microstructure of the obtained steel sheet. Table 3 shows the test results. Steel sheets produced with components and production conditions within the scope of the present invention have a tensile strength after PWHT exceeding 580 MPa and a drop weight characteristic of T _NDT <−25 ° C. despite a relatively high amount of P. Has good low temperature toughness. 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 (3)

Component composition is mass%, C: 0.04-0.08%, Si: 0.05-0.6%, Mn: 1.2-2.0%, P: 0.003-0.020 %, S: 0.003% or less, 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: 0.05-0.40%, V: 0.01-0.1%, N: 0.0010-0.0040%, Pcm: 0.22% or less, the value of the following formula ( Y value) satisfying 1.00 to 1.50, hardenability index (DI value): 40 to 100, continuous cast slab of remaining Fe and unavoidable impurities, 1000 to 1250 ° C. without surface care To a thickness of 80 mm or less by performing hot rolling at a rolling reduction temperature of less than 900 ° C. Then, after performing accelerated cooling to a temperature of 500 ° C. or less at a cooling rate of 3 ° C./s or more at an average cooling rate, excellent production characterized by performing a tempering treatment in a temperature range of 600 to 750 ° C. Of TMCP-Temper type high-strength thick steel plate having excellent weight drop characteristics after PWHT, which has both heat resistance and weldability.
Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (Each element symbol is a content (mass%).)
Y = Cr + 2Mo + 10V (however, each element symbol is a content (mass%))
Hardenability index (DI) = 8√C × (1 + 0.64Si) × (1 + 4.1Mn) × (1 + 0.27Cu) × (1 + 0.52Ni) × (1 + 2.33Cr) × (1 + 3.14Mo) (however, (Each element symbol is the content (% by mass).) Furthermore, the component composition is 1% or more of Ti: 0.004-0.010%, Ca: 0.0005-0.0015%, REM: 0.001-0.010% in mass%. characterized by, according to claim 1 Symbol placement, excellent productivity and combines weldability, TMCP-Temper type high strength steel plate manufacturing method excellent in drop weight characteristics after PWHT. The excellent production according to claim 1 or 2 , wherein Nb and B, which are inevitable impurities in the component composition, are Nb: 0.003% or less and B: 0.0003% or less. Of TMCP-Temper type high-strength thick steel plate having excellent weight drop characteristics after PWHT, which has both heat resistance and weldability.