JP5194807B2 - Manufacturing method of high yield strength and high toughness thick steel plate - Google Patents

Description

  The present invention relates to a method for producing a high yield strength / tough steel plate having excellent brittle crack propagation stopping performance and weld heat affected zone toughness, and particularly suitable as a line pipe material for transportation of natural gas or crude oil. About.

  Line pipes used for the transportation of natural gas and crude oil are becoming stronger and thicker year by year in order to increase pressure and improve transportation efficiency, but they are required to satisfy the characteristics according to the installation environment and application. The

  In the submarine pipeline system that transports natural gas, thicker line pipes are used from the viewpoint of buckling strength during installation, water pressure resistance during operation and safety against tidal currents as the installation depth increases. Is required.

  In addition, the development of gas and oil fields has a tendency to extend to extremely cold areas such as Russia and Alaska, and cold sea areas such as the North Sea. And the base metal is required to have excellent brittle crack propagation characteristics.

  Various prior arts are disclosed for such a request. For example, Patent Document 1 describes a steel pipe having a large tolerance for deformation of a pipeline due to ground fluctuation and a method for manufacturing the material.

  In order to realize high deformability, a dual phase structure is used, and in order to compensate for the decrease in Charpy absorbed energy and the decrease in productivity due to the multiphase structure, two stages of accelerated cooling are performed after hot rolling, and the second stage cooling is stopped. An APIX 60-100 grade high-strength steel sheet that achieves high strength and high deformability with a structure in which a ferrite phase is finely dispersed in a low-temperature transformation structure mainly composed of a bainite phase by setting the temperature to 300 ° C. or less. It is disclosed.

Patent Document 2 discloses Ar ₃ as a method for producing a steel sheet for APIX 70 grade high strength and high toughness line pipes, which is excellent in unstable ductile fracture propagation stopping characteristics required for gas transportation line pipes such as natural gas. It is disclosed that the hot rolling is immediately accelerated to 400 ° C. or less immediately after the hot rolling is finished at a point or more, and the main structure becomes a high-strength bainite structure.
Japanese Patent No. 3869747 Japanese Patent Laid-Open No. 62-4826

  By the way, increasing the strength of line pipes has progressed, and in the case of line pipes laid in cold regions, the deformation performance as a pipeline, excellent brittle crack propagation characteristics and low temperature toughness of the base metal / welding heat affected zone are impaired. It is required to ensure the necessary strength. In particular, in a natural gas pipeline or the like, since the operation pressure is determined based on the yield strength, it is necessary to sufficiently ensure not only the tensile strength but also the yield strength in order to increase the transportation efficiency.

  However, as described in Patent Document 1, a dual-phase structure steel composed of a soft ferrite structure obtained from weak acceleration cooling and a hard bainite or martensite structure obtained by lowering the stop temperature of strong acceleration cooling has a tensile strength. The yield strength is significantly reduced compared to

  In order to ensure sufficient yield strength, when the base material is composed of a high component, there is a concern not only about the increase in alloy cost but also the deterioration of the base material and the weld heat affected zone toughness.

Further, according Patent Document 2, a single-phase structure consisting resulting hard bainite structure by strong cooling from above the Ar ₃ point is not caused a decrease in the yield strength found in duplex structure of Patent Document 1 The texture does not develop because it is difficult to perform hot rolling in the vicinity of Ar ₃ points.

  Therefore, separation does not occur sufficiently even in various brittle crack propagation stop performance evaluation tests, and it is difficult to secure excellent brittle crack propagation stop performance.

  Therefore, the present invention has an excellent brittle crack propagation stopping performance and weld heat-affected zone toughness without impairing deformation performance as a pipeline, suitable for use in cold regions, and has a yield strength of 470 Mpa or more. An object of the present invention is to provide a method for producing a high-tensile steel plate having a tensile strength of 600 MPa or more and a thickness of 6 mm or more.

Based on the premise of obtaining a tensile strength of 600 MPa or more, the present inventors have obtained a multiphase structure steel capable of obtaining excellent brittle crack propagation stopping performance by separation. 1. Improved toughness of weld heat affected zone. The earnest examination about the high yield point was carried out, and the following knowledge was acquired.
[Improved toughness of weld heat affected zone]
1. When welding a thick steel plate for line pipes having a tensile strength of 600 MPa or more from two inner and outer surfaces, the weld heat affected zone near the fusion line is a rough upper bainite structure containing island martensite as the hard second phase. In addition, reducing the amount of island martensite is effective in improving toughness.

  2. Generation of island-like martensite can be suppressed by reducing the Si, Nb, and V of the steel material, and by reducing to a level that does not substantially contain Si, the proportion of island-like martensite contained in the upper bainite is greatly increased. It is possible to reduce it.

  3. By setting the ratio of island martensite contained in the upper bainite structure of the weld heat affected zone to 3% or less, the weld heat affected zone toughness is greatly improved.

  4). Moreover, the hardness of a welding heat affected zone reduces by reducing the amount of P in steel, and the weld heat affected zone toughness improves. In particular, when the P content is 0.006 mass% or less, the weld heat affected zone toughness is remarkably improved.

[High yield point]
5. After the two-phase rolling, the duplex structure steel made of hard ferrite and soft ferrite obtained by stopping accelerated cooling at a low temperature has yield strength changed from Luders type to round house type, resulting in increased tensile strength. The yield strength is lower than that.

  6). However, even multi-phase steels exhibiting round-house type yield behavior will exhibit Luders-type yield behavior due to strain aging treatment such as low-temperature reheating, and yield strength will be improved.

  7). When reheating treatment is performed by rapid heating immediately after accelerated cooling, the same phenomenon occurs. In particular, when the temperature is raised to 150 ° C. or higher, the transition to the Luders type occurs and the yield strength is improved.

  8). In the case of a multiphase steel obtained by reheating by rapid heating immediately after accelerated cooling, it shows a Luders-type yielding behavior after reheating, but plastic processing is added when making a UOE line pipe. Therefore, it becomes a round house type yielding behavior that improves the buckling resistance of the line pipe.

  9. However, when the reheating treatment after accelerated cooling is performed after the UOE process, a high yield strength is achieved, but the yielding behavior is a Luders type, so that sufficient buckling resistance cannot be obtained. It is known that the yield strength of the line pipe base material is highly correlated with the nominal stress (0.5% YS) at the nominal strain of 0.5% regardless of the difference in yield behavior at the steel plate stage. Yes.

The present invention has been made by further study based on the above knowledge, that is, the present invention,
(1) Component composition is mass% C: 0.03-0.08%
Si: 0.05% or less Mn: 1.0 to 2.0%
P: 0.006% or less S: 0.005% or less Al: 0.02-0.05%
Nb: 0.005 to 0.025%
Ti: 0.005-0.030%
N: 0.001 to 0.010%
Furthermore, Cu: 0.10 to 0.60%
Ni: 0.10 to 1.20%
Cr: 0.05-0.40%
Mo: 0.05-0.40%
Containing one or more of
0.30 ≦ Ceq ≦ 0.45
After the steel composed of the remaining Fe and inevitable impurities is heated to 1000 to 1200 ° C., the cumulative reduction ratio is 50% or more in the austenite non-recrystallization temperature range of 900 ° C. or less, and the cumulative reduction ratio is 10 in the two-phase range. After hot rolling with a rolling end temperature of 660 ° C. or more at ˜50%, immediately start cooling at a cooling stop temperature of 400 ° C. or less at a cooling rate of 10-80 ° C./s, and immediately after cooling stops A method for producing a high yield strength and high toughness thick steel plate, characterized in that the steel sheet is reheated to a temperature range exceeding the stop temperature and not lower than 150 ° C and lower than 450 ° C.
However, Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14,
Each element is the content (% by mass), and the element not contained is 0.
(2) In addition to the component composition, Zr: 0.0005 to 0.0300% by mass%
Ca: 0.0005 to 0.0100%
Mg: 0.0005 to 0.0100%
REM: 0.0005 to 0.0200%
1 type or 2 types or more of these are included, The manufacturing method of the high yield strength and the toughness thick steel plate as described in (1) characterized by the above-mentioned.

  According to the present invention, suitable for transportation of natural gas and crude oil, brittle crack propagation stopping performance and base material at low temperature, tensile strength excellent in weld heat affected zone toughness is 600 MPa or more, yield strength is 470 MPa or more. This makes it possible to produce high-strength and high-tough steel plate for line pipes, which is extremely effective in industry.

This invention prescribes | regulates the component composition and manufacturing condition of a steel plate. The reason for limitation will be described below.
[Component Composition] In the following description, “%” means “mass%”.
C: 0.03-0.08%
C contributes to an increase in strength by dissolving in supersaturation in a low temperature transformation structure. In order to obtain this effect, addition of 0.03% or more is necessary. However, if the addition exceeds 0.08%, the hardness of the heat-affected zone with high heat input is increased and island martensite is generated in the structure. Since the toughness deteriorates, the upper limit is made 0.08%.

Si: 0.05% or less Si is an element that acts as a deoxidizer and further increases the strength of the steel by solid solution strengthening. When the structure of the weld heat affected zone is upper bainite, island martensite The generation of heat and significantly deteriorates the weld heat-affected zone toughness. In the present invention, it has been found that the toughness of the weld heat affected zone when the weld heat affected zone is upper bainite is remarkably improved by substantially not containing Si. Therefore, it is desirable to reduce Si as much as possible, but 0.05% is allowed. Preferably it is less than 0.04%.

Mn: 1.0-2.0%
Mn acts as a hardenability improving element, and its effect can be obtained by addition of 1.0% or more. However, when a continuous casting process is applied, the concentration of the central segregation part is remarkably increased, and addition exceeding 2.0% If done, the toughness of the segregation part deteriorates, so the upper limit is made 2.0%.

P: 0.006% or less P is an element that increases the strength by solid solution strengthening. However, in order to deteriorate the toughness and weldability of the base metal and the weld heat affected zone, the content can be generally reduced. desired. In the present invention, by reducing P, the hardness of the weld heat affected zone is reduced and the weld heat affected zone toughness is improved. In particular, P is made 0.006% or less in order to significantly improve the weld heat affected zone toughness by making it 0.006% or less.

S: 0.005% or less S is present as an inevitable impurity in steel. In particular, the segregation at the center segregation part is an element that promotes toughness deterioration due to the segregation part of the base material. Therefore, it is desirable to reduce S as much as possible, but up to 0.005% is allowed due to restrictions in the steelmaking process.

Al: 0.02 to 0.05%
Al acts as a deoxidizing element. Sufficient deoxidation effect can be obtained with addition of 0.02% or more, but if added over 0.05%, the cleanliness in the steel is lowered and toughness deteriorates, so the upper limit is 0.05%. To do.

Nb: 0.005 to 0.025%
Nb has the effect of expanding the austenite non-recrystallized region at the time of hot rolling, and in order to make the non-recrystallized region up to 900 ° C., addition of 0.005% or more is necessary. On the other hand, when the amount of Nb added is increased, island martensite is generated in the weld heat affected zone, particularly in the heat affected zone of high heat input welding, and further, precipitation embrittlement occurs in the reheat weld heat affected zone during multi-layer welding. And the toughness deteriorates significantly, so the upper limit is made 0.025%. The amount of Nb added is preferably as low as possible from the viewpoint of weld heat affected zone toughness.

Ti: 0.005-0.030%
Ti forms nitrides and is effective in reducing the amount of solute N in steel. The precipitated TiN suppresses the coarsening of the austenite grains in the base metal and the weld heat affected zone during slab heating before hot rolling, especially the weld heat affected zone in high heat input welding due to the pinning effect, and the base metal and welding heat Contributes to improved toughness of the affected area. In order to obtain this effect, addition of 0.005% or more is necessary, but if added over 0.030%, the base material and weld heat affected zone toughness deteriorate due to precipitation of coarse TiN and carbides. Therefore, the upper limit is made 0.030%.

N: 0.001 to 0.010%
N is usually present as an inevitable impurity in steel, but is defined to form TiN that suppresses austenite coarsening by adding Ti as described above. In order to obtain the required pinning effect, it is necessary to be present in the steel in an amount of 0.001% or more. Therefore, the upper limit is made 0.010%.

  In the present invention, one or more of Cu, Ni, Cr, and Mo are further added. Cu, Ni, Cr, and Mo all act as a hardenability improving element, and by adding one or more of these elements, it is possible to increase the strength of a thick steel plate having a thickness of 6 mm or more.

Cu: 0.10 to 0.60%
Cu contributes to the hardenability improvement of steel by adding 0.10% or more. On the other hand, if added in excess, the toughness of the base metal and the weld heat affected zone is deteriorated, so when added, the upper limit is made 0.60%.

Ni: 0.10 to 1.20%
Ni contributes to improving the hardenability of steel by adding 0.10% or more. In particular, even when added in a large amount, deterioration in toughness is small compared to other elements, and it is an effective element for toughening. However, since it is an expensive element and is added in excess of 1.20%, the hardenability is excessively increased and the weld heat affected zone toughness deteriorates. Therefore, when added, the upper limit is made 1.20%.

Cr: 0.05-0.40%
Cr contributes to the improvement of hardenability of steel by adding 0.05% or more. On the other hand, if added in excess, the toughness of the base metal and the weld heat affected zone is deteriorated. Therefore, when added, the upper limit is made 0.40%.

Mo: 0.05-0.40%
Mo contributes to improving the hardenability of steel by adding 0.05% or more. On the other hand, when the amount of Mo added is increased, the toughness of the high heat input welded portion is deteriorated. Moreover, since precipitation embrittlement is caused in the reheat welding heat-affected zone during multi-layer welding, and the toughness deteriorates, the upper limit is made 0.40% when added. The amount of Mo added is preferably as low as possible from the viewpoint of weld heat affected zone toughness.

Ceq: 0.30 to 0.45
Ceq (= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14. Each element is a content (mass%), and an element not contained is 0) is an estimate of the effect of a hardenable element such as C and Mn. It can be used as an index and is preferably controlled to 0.30 or more from the viewpoint of securing strength. On the other hand, if it exceeds 0.45, toughness and weldability will be impaired, so the upper limit is made 0.45%.

  The basic component composition of the present invention is as described above, but when further improving toughness, one or more of Zr, Ca, Mg, and REM can be added.

  Zr, Ca, Mg, and REM all control the morphology of MnS, which is a non-metallic inclusion in steel, or form oxides or nitrides, mainly pinning the austenite grain coarsening in the weld heat affected zone Suppress with.

Zr: 0.0005 to 0.0300%
Zr forms carbonitrides in steel and brings about a pinning effect that suppresses coarsening of austenite grains, particularly in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary, but when adding over 0.0300%, the cleanliness in the steel is remarkably lowered, and the toughness is lowered. When adding, it is 0.0005 to 0.0300%.

Ca: 0.0005 to 0.0100%
Ca is an element effective for controlling the form of sulfide in steel, and the addition of 0.0005% or more suppresses the generation of MnS harmful to toughness. However, if added over 0.0100%, a CaO-CaS cluster is formed and the toughness is deteriorated. Therefore, when added, the content is made 0.0005 to 0.0100%.

Mg: 0.0005 to 0.0100%
Mg is produced as fine oxides in the steel during the steel making process, and has a pinning effect that suppresses the coarsening of austenite grains, particularly in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary, but if added over 0.0100%, the cleanliness in the steel decreases, and the toughness decreases, When adding, it is made into 0.0005 to 0.0100%.

REM: 0.0005 to 0.0200%
REM is an element effective for controlling the form of sulfide in steel, and by adding 0.0005% or more, it suppresses the generation of MnS harmful to toughness. However, since it is an expensive element and the effect is saturated even if added over 0.0200%, when added, the content is made 0.0005 to 0.0200%.

  The steel according to the present invention contains the above-described component composition, and the balance is Fe and inevitable impurities. In the present invention, V is treated as an inevitable impurity, and the upper limit is controlled to 0.005%. When V is added in the same manner as Nb, V forms island-like martensite in the heat-affected zone of high heat input welds, and the reheat-welded heat-affected zone during multi-layer welding causes precipitation embrittlement, resulting in remarkable toughness. In order to deteriorate, the upper limit as an inevitable impurity is made 0.005%.

  V has the effect of expanding the austenite non-recrystallized region during hot rolling, but the effect is smaller than that of Nb, and in the present invention containing 0.005% or more of Nb, it is not necessary to add V.

[Production conditions]
The reason for limiting the manufacturing method will be described.
Slab heating temperature: 1000-1200 ° C
The minimum temperature for obtaining the minimum solid solution amount of Nb while converting the slab to austenite is 1000 ° C. On the other hand, when the slab is heated to a temperature exceeding 1200 ° C., the austenite grain growth is remarkable even in the pinning by TiN, and the base material toughness is deteriorated, so the upper limit is set to 1200 ° C.

Cumulative rolling reduction in a temperature range of 900 ° C. or lower: 50% or more 900 ° C. or lower is an austenite non-recrystallization temperature range due to Nb addition. The austenite grains are expanded by cumulatively reducing the temperature below this temperature range, and in particular, the base material toughness is improved by forming fine grains in the plate thickness direction.

Cumulative rolling reduction in two-phase temperature range: 10-50%
Austenite refined by austenite non-recrystallization zone rolling is further refined by performing hot rolling in the ferrite-austenite two-phase temperature range of Ar _{3 to} Ar ₁ .

  In addition, the ferrite is strengthened to increase the strength by strengthening the ferrite, and the brittle crack propagation stopping performance evaluation test such as DWTT causes separation on the fracture surface of the test piece to provide excellent brittle crack propagation stopping performance. It becomes possible.

  If the cumulative reduction amount in the two-phase region is less than 10%, the occurrence of separation is not sufficient and the improvement of brittle crack propagation stopping characteristics cannot be obtained. On the other hand, if the cumulative rolling reduction exceeds 50%, the ferrite becomes brittle due to excessive processing to ferrite and the base metal toughness deteriorates, so the upper limit is made 50%.

Rolling end temperature: 660 ° C. or higher When the rolling end temperature is lower than 660 ° C., the ferrite transformation proceeds and the effect of accelerated cooling is reduced, and the toughness of the base material deteriorates due to coarsening of the ferrite. And

Cooling rate: 10-80 ° C / s
Since ferrite produced after rolling is not processed, it is harmful from the viewpoint of securing strength and toughness. Accordingly, immediately after the rolling is completed, accelerated cooling is performed at a cooling rate of 10 ° C./s or more to transform the untransformed austenite into a bainite structure, thereby preventing the generation of ferrite and improving the strength without impairing the base material toughness. On the other hand, at a cooling rate exceeding 80 ° C./s, martensitic transformation occurs near the steel plate surface, and the strength of the steel plate increases, but the toughness deterioration, particularly the decrease in Charpy absorbed energy, is significant, so the upper limit is set to 80 ° C./s. .

Cooling stop temperature: 400 ° C. or less In order to obtain a tensile strength of 600 MPa or more, the cooling stop temperature is set to 400 ° C. or less, and the microstructure of the steel sheet is made to be a bainite or martensite structure. If the cooling stop temperature exceeds 400 ° C, the transformation temperature is high and the steel sheet cannot be sufficiently strengthened, so the upper limit is set to 400 ° C.

Reheating treatment In the present invention, the reheating treatment is an important heat treatment, and has a multiphase structure, so as to improve the yield strength without greatly reducing the tensile strength from the steel sheet with accelerated cooling, immediately after accelerated cooling. Reheat by rapid heating.

  When steel that has been rolled in a two-phase region is reheated immediately after accelerated cooling by rapid heating, the same effect as when subjected to strain aging treatment such as low-temperature reheating to double-phase structure steel is obtained. From the behavior, it shows the Luders type yielding behavior and the yield strength is improved.

  Since the steel according to the present invention is subjected to plastic working during UOE line pipe production after reheating treatment, a round house type yielding behavior that is preferable for a line pipe is obtained. Therefore, the reheating treatment is performed before applying the UOE process, which is desirable from the viewpoint of labor saving and productivity. When the reheating treatment after accelerated cooling is performed after the UOE process, high yield strength is achieved, but because the yield behavior is Luders type, sufficient buckling resistance is obtained as a line pipe. Absent.

  In order to improve the yield strength of the steel sheet, it is necessary to reheat to 150 ° C. or higher in order to obtain the same effect as strain aging. On the other hand, when the reheating temperature at the center of the plate thickness exceeds 450 ° C., the tensile strength is significantly lowered due to the tempering effect, so the upper limit is made less than 450 ° C. A heating condition in which the thickness center is 430 ° C. or lower is more preferable.

  Molten steel of the chemical composition shown in Table 1 is melted in a vacuum melting furnace, made into a slab of 250 mm thickness by a continuous casting method, reheated, hot rolled, accelerated cooled, then immediately reheated by air cooling or induction heating To obtain a 33 mm thick steel plate. Table 2 shows the manufacturing conditions of the obtained steel sheet.

  About the obtained thick steel plate, the round bar tensile test of the sheet thickness center L direction extraction of the steel plate was done, and the yield strength, the tensile strength, and the yield ratio (YR) were measured. The yield stress was 0.5% YS.

  The brittle crack propagation stopping property was evaluated by the DWTT test. The ductile fracture surface ratio of DWTT was obtained by collecting DWTT specimens reduced to 19 mm collected from the 1 / 2t position, performing two tests at -47 ° C., and obtaining the average value of the obtained ductile fracture surfaces. .

  Moreover, the groove process of the V groove was given to the steel plate, and it welded by 4 electrode submerged arc welding to the V groove, and produced the welding part. The welding heat input at that time was 8.0 kJ / mm for all the steel plates.

  A V-notch Charpy test piece conforming to the JIS Z2202 standard was sampled so that the ratio of the weld metal and the weld heat-affected zone occupying the bottom of the notch from the obtained welded part was 1: 1, and a Charpy test conforming to the JIS Z2242 standard was performed. Implemented and determined the HAZ toughness. The Charpy test was performed three by three at a test temperature of -30 degrees, and the average value and the minimum value were obtained.

  In the above tests, the scope of the present invention was set to yield strength: 470 Mpa or more, tensile strength: 600 MPa or more, DWTT fracture surface ratio (average value): 80% or more, and minimum HAZ toughness value: 60 J or more.

  Table 3 shows the test results. Examples of the present invention (steel plates No. 1, 2, 7, 8, 13, 14, 15, 16) all have a yield strength of 470 Mpa or more, a tensile strength of 600 MPa or more, a DWTT fracture surface ratio of 80% or more, and HAZ toughness. Comparative examples (steel plates No. 3 to 5, 10 to 12, and 18 to 22) that fall outside the scope of the present invention satisfy any of these characteristics. Absent.

  Steel plate No. No. 3 has a low tensile strength because the cooling stop temperature is high. Steel plate No. No. 17 has a low tensile strength because the cooling rate of accelerated cooling is low. Steel plate No. Since Nos. 6 and 12 are not subjected to the reheating treatment, the yield strength is low.

  Steel plate No. No. 4 has a high slab heating temperature. No. 5 has an insufficient rolling reduction at 900 ° C. or lower, so No. 11 has an excessive reduction ratio in the phase region, so that the DWTT performance is deteriorated. Steel plate No. In 18 to 22, since the base material component is out of the scope of the present invention, the HAZ toughness is deteriorated.

Claims (2)

Component composition is mass% C: 0.03-0.08%
Si: 0.05% or less Mn: 1.0 to 2.0%
P: 0.006% or less S: 0.005% or less Al: 0.02-0.05%
Nb: 0.005 to 0.025%
Ti: 0.005-0.030%
N: 0.001 to 0.010%
Furthermore, Cu: 0.10 to 0.60%
Ni: 0.10 to 1.20%
Cr: 0.05-0.40%
Mo: 0.05-0.40%
Containing one or more of
0.30 ≦ Ceq ≦ 0.45
After the steel composed of the remaining Fe and inevitable impurities is heated to 1000 to 1200 ° C., the cumulative reduction ratio is 50% or more in the austenite non-recrystallization temperature range of 900 ° C. or less, and the cumulative reduction ratio is 10 in the two-phase range. After hot rolling with a rolling end temperature of 660 ° C. or more at ˜50%, immediately start cooling at a cooling stop temperature of 400 ° C. or less at a cooling rate of 10-80 ° C./s, and immediately after cooling stops A method for producing a high yield strength and high toughness thick steel plate, characterized in that the steel sheet is reheated to a temperature range exceeding the stop temperature and not lower than 150 ° C and lower than 450 ° C.
However, Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14,
Each element is the content (% by mass), and the element not contained is 0. In addition to the component composition, Zr by mass%: 0.0005 to 0.0300%
Ca: 0.0005 to 0.0100%
Mg: 0.0005 to 0.0100%
REM: 0.0005 to 0.0200%
1 or 2 types of these are contained, The manufacturing method of the high yield strength and high toughness thick steel plate of Claim 1 characterized by the above-mentioned.