JP4427522B2 - Method for producing high strength thick steel plate with tensile strength of 780 MPa excellent in weldability and low temperature toughness - Google Patents

Description

  The present invention is a high-tensile steel plate having a tensile strength of 780 MPa that is excellent in preheating-free high weldability and joint low temperature toughness, does not use expensive Ni, and does not require reheating and tempering after rolling. The present invention relates to a method for manufacturing with low productivity and low cost. The steel of the present invention is used in the form of a thick steel plate having a thickness of 12 to 40 mm as a structural member of a welded structure such as a construction machine, an industrial machine, a bridge, a building, or a shipbuilding.

  High tensile strength steel sheets with a tensile strength of 780MPa used as welded structural members for construction machinery, industrial machinery, bridges, architecture, shipbuilding, etc., in addition to the high strength and toughness of the base material, the high strength of the structural members 780MPa class heavy steel sheet that satisfies all of these characteristics, is inexpensive, and can be manufactured in a short construction period. The plate thickness has been required to be about 40 mm. That is, (a) high strength and high toughness of base metal, (b) free preheating during small heat input welding, and (c) low temperature toughness of joints, all with the low cost component system, short construction period + low cost manufacturing Need to be satisfied with the process.

  For example, as disclosed in Patent Documents 1 to 3, a conventional method of manufacturing a high-strength thick steel plate of 780 MPa class imparted with high weldability is directly quenched immediately after rolling of the steel plate, and then tempered. There are methods by direct quenching and tempering.

  Regarding the method for producing a high-strength thick steel plate of 780 MPa class with non-tempering, for example, there are disclosures in Patent Documents 4 to 8, both of which are excellent in terms of production period and productivity in that reheating and tempering heat treatment can be omitted. It is. Among these, patent documents 4-7 are the manufacturing methods by the accelerated cooling-intermediate stop process which stops the accelerated cooling after rolling of a steel plate on the way, and patent document 8 is a manufacturing method which cools to room temperature by air cooling after rolling. is there.

Japanese Patent Laid-Open No. 03-232923 JP 09-263828 A JP 2000-160281 A JP 2000-319726 A JP 2005-15859 A JP 2004-052063 A JP 2001-226740 A Japanese Patent Laid-Open No. 08-188823

  However, since the conventional techniques disclosed in Patent Documents 1 to 3 require reheating and tempering heat treatment, and there are problems in the manufacturing period, productivity, and manufacturing cost, a so-called non-tempered manufacturing method in which the reheating and tempering heat treatment can be omitted. The demand for is strong. Moreover, in the manufacturing method disclosed in Patent Document 4, preheating at 50 ° C. or higher is necessary at the time of welding as described in the examples, and high weldability without preheating cannot be satisfied. Furthermore, since the manufacturing method disclosed in Patent Document 5 requires addition of 0.6% or more of Ni, it is an expensive component system and has a problem in manufacturing cost. In the manufacturing method disclosed in Patent Document 6, it is possible to manufacture only a plate thickness of 15 mm described in the examples, and it is not possible to satisfy a plate thickness requirement up to a plate thickness of 40 mm. Furthermore, even when the plate thickness is 15 mm, there is a problem that the C content is small and the microstructure of the joint becomes coarse and sufficient joint low temperature toughness cannot be obtained. In the manufacturing method disclosed in Patent Document 7, since about 1.0% of Ni addition is necessary as described in the examples, it becomes an expensive component system and has a problem in manufacturing cost. The manufacturing method disclosed in Patent Document 8 can only manufacture up to a plate thickness of 12 mm described in the examples, and cannot satisfy the plate thickness requirement up to a plate thickness of 40 mm. Furthermore, as a feature of the rolling conditions, rolling is performed at a cumulative reduction ratio of 16 to 30% in the two-phase temperature range of ferrite and austenite, so that the ferrite grains are likely to be coarsened and the strength and toughness are likely to be reduced even in the production of a sheet thickness of 12 mm. There is also a problem.

  As described above, all of the high strength and high toughness of the base metal, high weldability, and low temperature toughness of the joint, without adding the expensive alloy element Ni, and omitting reheating and tempering heat treatment after rolling cooling In spite of the strong demand of consumers, the present situation is that the method for producing a high-tensile thick steel plate that can be satisfied with the present invention has not yet been invented. In the case of a thick steel plate having a base material strength of 780 MPa, the influence of the plate thickness on making the preheating free is very large. If the plate thickness is less than 12 mm, preheating-free can be achieved relatively easily. This is because if the sheet thickness is less than 12 mm, the cooling rate of the steel sheet during water cooling can be physically increased to 100 ° C./sec or more even at the center of the sheet thickness. In this case, a hard martensite or bainite structure can be obtained at a high cooling rate without adding a large amount of C or an alloy element, and a strength of 780 MPa can be obtained. With thin 780 MPa steel with a small amount of alloying element added, the hardness of the weld heat affected zone can be kept low without preheating, and weld cracking can be prevented even without preheating. On the other hand, as the plate thickness increases, the cooling rate during water cooling decreases. For this reason, with the same component as the thin steel plate, the strength of the thick steel plate decreases due to insufficient quenching, and the strength of the 780 MPa class cannot be satisfied. In particular, the strength reduction is remarkable at the center of the plate thickness (1/2 t portion) where the cooling rate is the smallest. When the steel plate is thicker than 40 mm and the cooling rate is less than 8 ° C./sec, it is essential to add a large amount of alloying elements to ensure the strength of the base material, and preheating becomes extremely difficult.

  Therefore, the present invention has all the high strength and toughness of the base metal, high weldability, and low temperature toughness of the joint, without adding the expensive alloy element Ni, and omitting the reheating and tempering heat treatment after rolling cooling. It is an object of the present invention to provide a satisfactory method for producing a high-tensile steel plate having a tensile strength of 780 MPa which is excellent in weldability and low-temperature toughness.

In addition, the characteristic of the specific steel plate which this invention makes object is as follows.
(A) At the center of the thickness of the base metal, the tensile strength is 780 MPa or more, the yield stress is 685 MPa or more, the Charpy absorbed energy at −80 ° C. is 100 J or more. (B) The necessary preheating temperature during the y-cracking test is 25 ° C. or less ( c) Charpy absorbed energy of the weld heat affected zone (HAZ zone) of submerged arc welding (SAW) joint at welding heat input of 3.0 kJ / mm is 60 J or more at −40 ° C.

  Moreover, the plate | board thickness of the steel plate which this invention makes object is 12-40 mm.

  In order to solve the above-described problems, the present inventors have made many studies on the base metal and the welded joint on the premise of manufacturing by direct quenching after rolling with a component system containing no Ni. There are two problems that have been difficult to solve, and one of them is to secure joint low temperature toughness without adding Ni. As a result of various investigations on the influence of the additive component on the heat affected zone (HAZ) toughness of the submerged arc welding (SAW) joint at a welding heat input of about 3.0 kJ / mm with respect to this problem, the amount of C added was set to be 0.1. Strictly regulated to 03% or more and 0.055% or less, and after adding B, the hardenability of steel that can be evaluated by the hardenability index DI value is set to an optimal range of 1.70 or more and 2.50 or less, Furthermore, it was newly found that good joint toughness can be obtained at −40 ° C. without addition of Ni only when the three elements of Mo, V, and Si are not added as much as possible.

  Based on this new knowledge, Ni, Mo, V, and Si are not added, but the B additive component system that satisfies the above-mentioned C content and DI value ranges is added to achieve preheating free during small heat input welding. As a result of examining the components, by limiting the weld cracking susceptibility index that can be evaluated by the Pcm value to 0.22% or less, the required preheating temperature during the y cracking test can be 25 ° C or less, and preheating is free. It became clear that it became possible.

  However, another problem that has been difficult to solve is the compatibility of the base material strength and toughness over the entire thickness in the thickness direction up to a thickness of 40 mm, assuming a Pcm value of 0.22% or less. It was. On the other hand, as a result of various investigations on the Nb addition amount and rolling conditions in the B-added steel, the Nb addition amount is set to 0.02% or more and 0.05% or less, and the rolling conditions are within the austenite recrystallization temperature range. 930 ° C. or higher, and the unrecrystallized temperature range of 760 ° C. or higher and 900 ° C. or lower, the cumulative rolling reduction in two temperature ranges are strictly regulated, and Mn is 1.2% or higher, and After adding 0.6% or more of Cr, Mn and Cr are added so that the Pcm value satisfies 0.19% or more. Immediately after rolling, the cooling rate is 8O 0 C from 700 ° C or more to room temperature to 350 ° C or less. For the first time by cooling at 80 ° C./sec or more and 80 ° C./sec or less, both the strength and toughness of the base material over the entire thickness in the thickness direction up to a thickness of 40 mm, specifically, tensile strength of 780 MPa or more, yield Stress 685 MPa or more, at -80 ° C Charpy absorbed energy has knowledge to the new to be a possible satisfaction more than 100J.

The present invention has been made on the basis of the above novel findings, and the gist thereof is as follows.
(1) By mass%, C: 0.03-0.055%, Mn: 1.2-2.3%, Cr: 0.6-1.5%, Nb: 0.02-0.05% Ti: 0.005-0.015%, Al: 0.003-0.08%, B: 0.0005-0.0020%, N: 0.0015-0.0060%, P: 0.02% or less, S: 0.0050% or less, Mo: 0.09% or less, Si: 0.09% or less, V: 0.03% or less, and weld crack sensitivity shown below Steel having an index Pcm value of 0.19 to 0.22%, a hardenability index DI value shown below of 1.70 to 2.50, and a composition composed of the balance Fe and inevitable impurities The piece or slab is heated to 1000 to 1170 ° C, and the cumulative rolling reduction in the temperature range of 930 ° C or higher is 40 to 80. After the rough rolling, the finish rolling is performed at 760 ° C. or higher, and the cooling rate is from 8 ° C. to 80 ° C. from 700 ° C. or higher. A method for producing a high-tensile steel plate having a tensile strength of 780 MPa, which is excellent in weldability and joint low-temperature toughness, characterized by starting accelerated cooling at / sec and stopping the accelerated cooling at room temperature to 350 ° C.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
DI = 0.367 ([C] ^1/2 ) (1 + 0.7 [Si]) (1 + 3.33 [Mn]) (1 + 0.35 [Cu]) (1 + 0.36 [Ni]) (1 + 2.16 [ Cr]) (1 + 3.0 [Mo]) (1 + 1.75 [V]) (1 + 1.77 [Al])
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Al], and [B] are C, Si, and Mn, respectively. , Cu, Ni, Cr, Mo, V, Al, and the content expressed by mass% of B.
(2) Furthermore, Cu: 0.20% or less in mass%, the high strength thick steel plate having a tensile strength of 780 MPa and excellent weldability and joint low temperature toughness according to (1) above Manufacturing method.
(3) The composition further comprises one or two of Mg: 0.0005 to 0.01% and Ca: 0.0005 to 0.01% by mass%. The manufacturing method of the high strength thick steel plate of 780 MPa class which is excellent in the weldability and joint low temperature toughness as described in 1 above.

  According to the present invention, tensile strength excellent in preheating-free high weldability and joint low-temperature toughness suitable as a structural member for welded structures such as construction machines, industrial machines, bridges, buildings, shipbuilding and the like with strong needs for high strength. It is possible to manufacture a high-strength steel plate having a thickness of 12 to 40 mm of 780 MPa class with high productivity and low cost without using expensive Ni and requiring no reheating and tempering after rolling. It has a great effect on the industry.

  Below, the reason for limitation of manufacturing methods, such as each component and rolling conditions in this invention, is demonstrated.

  In order to improve the low temperature toughness of the joint, C is added in an amount of 0.03% or more and 0.055% or less.

  Mn needs to be added in an amount of 1.2% or more in order to achieve both the strength and toughness of the base material. If added over 2.3%, coarse MnS that is harmful to toughness tends to be generated in the center segregation portion, which may lead to a decrease in the base metal toughness in the center portion of the plate thickness, so the upper limit is 2.3%. And

  Cr needs to be added in an amount of 0.6% or more in order to achieve both the strength and toughness of the base material. If added over 1.5%, the base material toughness of the central portion of the plate thickness may be lowered, so the upper limit is made 1.5%.

  Nb needs to be added in an amount of 0.02% or more in order to achieve both strength and toughness of the base material. If added over 0.05%, the amount of precipitated C as Nb (C, N) increases, and the hardenability decreases due to the decrease in the amount of solid solution C. Since the base material strength may not be obtained, the upper limit is made 0.05%.

  B is required to be added in an amount of 0.0005% or more in order to improve hardenability and to obtain good joint low temperature toughness and high strength and high toughness of the base material. If added over 0.0020%, the hardenability decreases, and good joint low-temperature toughness and sufficient base metal high strength and high toughness may not be obtained, so the upper limit is made 0.0020%.

  Al is contained in a range of 0.003% to 0.08% of the range usually added as a deoxidizing element.

  Ti and N are effective for forming fine TiN particles and improving joint toughness through preventing austenite grain coarsening in the weld heat affected zone. Further, Ti has a certain effect on stabilizing the hardenability of the B-added steel. This is because solid solution N causes a decrease in hardenability by reducing the amount of solid solution B due to BN formation, but the effect of improving the hardenability of solid solution B is stable by adding Ti and precipitating solid solution N as TiN. It depends on what is obtained. In order to obtain both of these effects, it is necessary to add 0.005% or more of Ti and 0.0015% or more of N. In order to suppress both the joint low-temperature toughness drop due to coarse TiN particles and the hardenability drop due to excess N, the upper limit is 0.015% for Ti addition and 0.0060% for N addition.

  It is desirable not to contain P in order to lower the low temperature toughness of the base material and the joint. The allowable value as an impurity element inevitably mixed is 0.02% or less.

  It is desirable not to contain S in order to reduce the low temperature toughness of the base material and the joint. The allowable value as an impurity element inevitably mixed is 0.0050% or less.

  Mo, Si, and V are elements having a particularly important meaning in the present invention, and only when these three elements are not contained as much as possible, good joint toughness can be obtained at −40 ° C. without adding Ni. . When one of these three elements is added, island martensite, which is an embrittled structure, easily forms in the HAZ part in a fine form of about 1 μm or less, whereas this is fine when all three elements are not added. Island martensite is not generated. This is considered to be the reason why the low temperature toughness of the joint is good only when all three elements are not added. These three elements are preferably not contained as much as possible, but the allowable upper limit of addition of trace amounts as impurity elements inevitably mixed or for optimization of Pcm value, DI value, etc. is 0.09 for Mo and Si. % Or less and V is 0.03% or less. More preferably, Mo and Si are 0.05% or less, and V is 0.01% or less. In addition, since Mo and V are expensive elements like Ni, the present invention in which good characteristics can be obtained without adding Ni, Mo and V is more than simply adding Ni from the viewpoint of alloy cost. Has a great advantage.

  Cu may be added within the regulation range of the Pcm value and the DI value. However, if Ni is not added and Cu is added in an amount of 0.2% or more, there is a concern that the manufacturing period, productivity, and manufacturing cost due to the occurrence of surface cracks in steel slabs and steel plates may cause problems. The following.

  By adding one or two of Mg and Ca, sulfides and oxides can be formed to increase the base metal toughness and joint low temperature toughness. In order to obtain this effect, Mg or Ca needs to be added in an amount of 0.0005% or more. However, excessive addition over 0.01% may produce coarse sulfides and oxides, which may reduce toughness. Therefore, the addition amount is set to 0.0005% or more and 0.01% or less, respectively.

  In the present invention, Ni is not added. However, when Ni is inevitably mixed from scrap raw materials or the like, it is within the scope of the present invention because it is not expensive even if Ni is contained.

If the weld crack sensitivity index Pcm value is not 0.22% or less, preheating during welding cannot be made free, so the upper limit is made 0.22% or less. If the Pcm value is less than 0.19%, the high strength and high toughness of the base material cannot be satisfied, so the lower limit is made 0.19%.
Here, Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B] [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] in this formula are C, Si, Mn, Cu, It means the content expressed by mass% of Ni, Cr, Mo, V, B.

When the hardenability index DI value is less than 1.70, the hardenability of the HAZ part becomes insufficient, and a coarse bainite structure including island martensite, which is an embrittled structure, is generated and the joint low temperature toughness is lowered. For this reason, 1.70 is set as the lower limit. If the DI value exceeds 2.50, the structure of the HAZ part itself becomes martensite with low toughness and the joint low-temperature toughness decreases, so the upper limit is made 2.50.
Here, DI = 0.367 ([C] ^1/2 ) (1 + 0.7 [Si]) (1 + 3.33 [Mn]) (1 + 0.35 [Cu]) (1 + 0.36 [Ni]) (1 + 2 .16 [Cr]) (1 + 3.0 [Mo]) (1 + 1.75 [V]) (1 + 1.77 [Al])
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Al], and [B] are C, Si, and Mn, respectively. , Cu, Ni, Cr, Mo, V, Al, and the content expressed by mass% of B.

  Next, manufacturing methods other than the component composition will be described.

  The heating temperature of the steel slab or slab needs to be 1000 ° C. or higher in order to sufficiently dissolve Nb (C, N), which is Nb carbonitride. When the temperature exceeds 1170 ° C., austenite grains become coarse and cause toughness reduction. In particular, in a component system with a low C content in which B and Nb are added without adding Ni according to the present invention, good base material toughness cannot be obtained unless the initial austenite grains during heating are made fine. For this reason, the upper limit is set to 1170 ° C.

  The cumulative rolling reduction in the austenite recrystallization temperature range needs to be 40% or more in order to finely austenite grains and obtain high strength and toughness of the base material. The sufficient austenite recrystallization temperature range of the steel of the present invention is 930 ° C. or higher. For this reason, it is necessary to set the cumulative rolling reduction at 930 ° C. or higher to 40% or higher. Here, the cumulative rolling reduction is a percentage expressed by dividing the total rolling thickness of rough rolling at 930 ° C. or higher by the starting thickness of the rough rolling, that is, the thickness of the steel slab or slab. If the cumulative rolling reduction exceeds 80%, the rolling time becomes long and the productivity decreases, so the upper limit is made 80%.

  The cumulative reduction ratio in the austenite non-recrystallization temperature region needs to be 50% or more in order to sufficiently austenite grains in the plate thickness direction to obtain high strength and high toughness of the base material. The sufficient austenite non-recrystallization temperature range of the steel of the present invention is 900 ° C. or less. For this reason, it is necessary to make the cumulative rolling reduction at 900 ° C. or less 50% or more. Here, the cumulative rolling reduction is a percentage expressed by dividing the total rolling thickness of finish rolling at 900 ° C. or less by the finish rolling start thickness. If the cumulative rolling reduction exceeds 70%, ferrite is locally generated due to accumulation of excessive rolling strain, and the high strength and high toughness of the base material cannot be obtained. Therefore, the upper limit is set to 70%. Similarly, if the rolling temperature is lower than 760 ° C., ferrite is locally generated due to accumulation of excessive rolling strain, and the high strength and high toughness of the base material cannot be obtained, so the lower limit of the rolling temperature is restricted to 760 ° C. .

  When the starting temperature of accelerated cooling after rolling is less than 700 ° C., ferrite is locally generated and the high strength and high toughness of the base material cannot be obtained, so the lower limit temperature is set to 700 ° C.

  When the cooling rate of accelerated cooling is less than 8 ° C./sec, ferrite is locally generated and the high strength and high toughness of the base material cannot be obtained. Therefore, the lower limit is set to 8 ° C./sec. The upper limit is 80 ° C./sec, which can be stably realized by water cooling.

  When the accelerated cooling stop temperature is higher than 350 ° C, the amount of ferrite, upper bainite, and island martensite generated due to insufficient quenching increases, especially in the thickness center of thick materials with a thickness of 30 mm or more. Since strength and high toughness cannot be obtained, the upper limit of the stop temperature is set to 350 ° C. The lower limit of the stop temperature is room temperature, but the more preferable stop temperature is 100 ° C. or higher in terms of dehydrogenation of the steel sheet.

  Steel pieces obtained by melting steels having the component compositions shown in Tables 1 to 3 were made into steel plates having a thickness of 12 to 40 mm under the manufacturing conditions shown in Tables 4 to 7. Of these, 1-A to 20-T in Table 4 are steels of the present invention, and 21-U to 74-Q in Tables 5 to 7 are comparative examples. In the table, underlined numbers and symbols indicate that manufacturing conditions such as components or rolling conditions deviate from the patent scope, or the characteristics do not satisfy the following target values. In addition, the amount of Ni in Tables 1 to 3 is the content as an unavoidable impurity element.

  Tables 4 to 7 show the evaluation results of the base material strength, toughness, weldability, and joint low temperature toughness for these steel plates. The base material strength was measured by taking a 1A full thickness tensile test piece or a No. 4 round bar tensile test piece specified in JIS Z 2201 and measuring it according to JIS Z 2241. Tensile test specimens are sampled from No. 1A full-thickness specimens with a thickness of 20 mm or less, and No. 4 round bar tensile specimens with a thickness of more than 20 mm and a thickness center of 1/4 (1/4 t). Part (1/2 t part). The base material toughness is obtained by taking an impact test piece specified in JIS Z 2202 in a direction perpendicular to the rolling direction from the center of the plate thickness, and by using the method specified in JIS Z 2242, Charpy absorbed energy (vE-80 at −80 ° C.). ) Was evaluated. Weldability was evaluated by obtaining a preheating temperature necessary for preventing root cracking by performing coated arc welding with a heat input of 1.7 kJ / mm by the method prescribed in JIS Z 3158. The weld heat-affected zone toughness was obtained by performing SAW welding (current 500 A, voltage 30 V, speed 30 cm / min) with a heat input of 3 kJ / mm using a V-shaped groove having a root gap and an angle of 20 °, JIS Z 2202 with an impact test piece specified in JIS Z 2202 so that the bottom of the notch contains as much melt line (fusion line) as possible from 1 / 2t part), and the absorbed energy at -40 ° C (vE-40) evaluated. The target values of each characteristic are a base material yield stress of 685 MPa or more, a base material tensile strength of 780 MPa or more, a base material of vE-80 of 100 J or more, a required preheating temperature of 25 ° C. or less, and a joint low temperature toughness of vE-40. 60 J or more.

  In Examples 1-A to 20-T, the base material yield stress is 685 MPa or more, the base material tensile strength is 780 MPa or more, the base material vE-80 is 100 J or more, the required preheating temperature is 25 ° C. or less, Low temperature toughness is 60 J or more at vE-40.

  On the other hand, the following comparative examples lack the yield stress and tensile strength of the base material. That is, since Comparative Example 21-U has a small amount of C added, 25-Y has a small amount of Mn added, 29-AC has a small amount of Cr added, and 34-AH has a large amount of Nb added. Since AR, 45-AS, and 48-AV have low Pcm values, and 53-A has low heating temperature, 59-D and 60-D have a cumulative rolling reduction of 760 ° C to 900 ° C exceeding 70%. Therefore, since 61-E, 62-E, and 69-Q have rolling end temperatures below 760 ° C, 63-F, 64-F, and 70-Q have water cooling start temperatures below 700 ° C, so 65-G, Since 66-G and 71-Q have cooling rates below 8 ° C / sec, 67-H, 68-H, 72-Q and 73-Q have cooling stop temperatures above 350 ° C. And the tensile strength is insufficient.

  Moreover, the following comparative examples lack base material toughness. That is, since Comparative Example 22-V has a large amount of C added, 26-Z has a large amount of Mn added, 28-AB has a large S content, and 30-AD has a large amount of Cr added. Since AG has a small Nb addition amount, 34-AH has a large Nb addition amount, and 41-AO, 42-AP, and 43-AQ have a large amount of N, Ca, and Mg, respectively. AS, 48-AV has a low Pcm value, 53-A has a low heating temperature, 54-A has a high heating temperature, and 55-B and 56-B have a cumulative rolling reduction of 40 at 930 ° C. or higher. 57-C, 58-C is less than 50% of cumulative rolling reduction at 760 ° C. to 900 ° C., and 59-D, 60-D is cumulative rolling reduction at 760 ° C. to 900 ° C. 61-E, 62-E, 69-Q are pressure Since the end temperature is below 760 ° C., the cooling rate of 63-F, 64-F, and 70-Q is below 700 ° C., and the cooling rate of 65-G, 66-G, and 71-Q is 8 ° C./sec. Therefore, since 67-H, 68-H, 72-Q, and 73-Q have a cooling stop temperature exceeding 350 ° C., the base material toughness is insufficient.

  Further, Comparative Examples 46-AT, 47-AU, and 52-AZ do not satisfy the necessary preheating temperature because of the high Pcm value.

  Moreover, the following comparative examples do not satisfy the joint low temperature toughness. That is, since Comparative Example 21-U has a small amount of added C, 22-V has a large amount of added C, 23-W and 24-X have a large amount of Si, 27-AA and 28-AB are respectively Since P- and S-contents are large, 31-AE and 32-AF have a large Mo addition amount, 35-AI and 36-AJ have a large V addition amount, and 37-AK has a small Ti addition amount. Since 38-AL has a large Ti addition amount and 39-AM has a small B addition amount, 40-AN, 41-AO, 42-AP and 43-AQ each have a large addition amount of B, N, Ca and Mg. Therefore, since 48-AV and 49-AW have low DI values, 46-AT, 50-AX, 51-AY, and 52-AZ have high DI values, so none of them satisfies the joint low temperature toughness.

Claims (3)

% By mass
C: 0.03-0.055%,
Mn: 1.2 to 2.3%
Cr: 0.6 to 1.5%
Nb: 0.02 to 0.05%,
Ti: 0.005 to 0.015%,
Al: 0.003 to 0.08%,
B: 0.0005 to 0.0020%,
N: 0.0015 to 0.0060%
In addition, P: 0.02% or less,
S: 0.0050% or less,
Mo: 0.09% or less,
Si: 0.09% or less,
V: limited to 0.03% or less, weld crack sensitivity index Pcm value shown below is 0.19 to 0.22%, and hardenability index DI value shown below is 1.70 to 2 The steel slab or slab having a composition of the balance Fe and the inevitable impurities is heated to 1000 to 1170 ° C., and the cumulative rolling reduction in the temperature range of 930 ° C. or higher is set to 40 to 80%. After rough rolling, finish rolling is performed at 760 ° C. or higher, with a cumulative reduction rate in the range of 760 to 900 ° C. being 50 to 70%, and subsequently, the cooling rate is 8 to 80 ° C./sec from 700 ° C. or higher. A method for producing a high-tensile thick steel plate having a tensile strength of 780 MPa that is excellent in weldability and joint low-temperature toughness, characterized in that the accelerated cooling is started and the accelerated cooling is stopped at room temperature to 350 ° C.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
DI = 0.367 ([C] ^1/2 ) (1 + 0.7 [Si]) (1 + 3.33 [Mn]) (1 + 0.35 [Cu]) (1 + 0.36 [Ni]) (1 + 2.16 [ Cr]) (1 + 3.0 [Mo]) (1 + 1.75 [V]) (1 + 1.77 [Al])
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Al], and [B] are C, Si, and Mn, respectively. , Cu, Ni, Cr, Mo, V, Al, and the content expressed by mass% of B. Furthermore, in mass%,
Cu: 0.20% or less is contained, The manufacturing method of the high strength thick steel plate of the tensile strength 780MPa class which is excellent in the weldability and joint low temperature toughness of Claim 1 characterized by the above-mentioned. Furthermore, in mass%,
Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%
The method for producing a high-tensile thick steel plate having a tensile strength of 780 MPa class excellent in weldability and joint low-temperature toughness according to claim 1 or 2, wherein