JP2010121191A - High-strength thick steel plate having superior delayed fracture resistance and weldability, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength thick steel plate having superior delayed fracture resistance and weldability, and to provide a method for manufacturing the same. <P>SOLUTION: The high-strength thick steel plate having superior delayed fracture resistance and weldability has a component composition including, by mass%, 0.20-0.26% C, 0.03-0.5% Si, 0.1% or more but less than 0.8% Mn, 0.020% or less P, 0.010% or less S, 0.5-2.0% Cr, 0.1-0.5% Mo, more than 0.10% and equal to or less than 0.30% V, 0.01-0.15% Al, 0.0003-0.0030% B, 0.006% or less N while satisfying Pcm so as to be 0.39% or less; has a martensite structure which occupies 90% or more by a fraction; has a yield strength of 1,300 MPa or higher; and has a tensile strength of 1,400-1,650 MPa. The method for manufacturing the same is also disclosed. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention has a yield strength of 1300 MPa or more and excellent tensile strength of 1400 MPa or more, and a plate thickness of 4.5 mm or more and 25 mm or less, which is excellent in delayed fracture resistance and weldability used for structural members of construction machinery and industrial machinery. The present invention relates to a certain high-strength thick steel plate and a manufacturing method thereof.

  In recent years, against the backdrop of global construction demand, production of construction machines such as cranes and concrete pump cars has continued to grow, and at the same time, the size of these construction machines has been increasing. In order to suppress an increase in weight associated with an increase in size of a machine, there is an increasing need for weight reduction of a structural member, and a shift to a high strength steel having a yield strength of 900 MPa or 1100 MPa is progressing. Recently, there is an increasing demand for a thick steel plate for structural members having a higher strength, yield strength of 1300 MPa or more (tensile strength of 1400 MPa or more).

  In general, when the tensile strength exceeds 1200 MPa, delayed cracking due to hydrogen may occur. Therefore, the yield strength 1300 MPa grade steel sheet is first required to have high delayed fracture resistance. In addition, the higher the strength, the more disadvantageous the weldability, but this is also required not to be significantly reduced as compared with the conventional 1100 MPa class high strength steel.

Regarding the technical disclosure regarding the thick steel plate for structural members having a yield strength of 1300 MPa, for example, Patent Document 1 discloses a method for producing a steel plate having a tensile strength of 1370 to 1960 N / mm ² and excellent hydrogen embrittlement resistance. Yes. However, the technique of Patent Document 1 relates to a 1.8 mm cold-rolled plate, and is premised on a high cooling speed of 70 ° C./sec or higher, and no consideration is given to weldability.

  As a technique for improving the delayed fracture resistance of high-strength steel, a technique for reducing the crystal grain size is conventionally known. Examples thereof are Patent Document 2 and Patent Document 3. However, in these examples, in order to improve the delayed fracture resistance, the prior austenite crystal grain size is set to 5 μm or less or 7 μm or less, and it is normal to make a thick steel plate finer to such a level. The manufacturing process is not easy. Both Patent Document 2 and Patent Document 3 show techniques for refining the prior austenite crystal grain size by rapid heating during quenching heating, but special heating is required to rapidly heat thick steel plates. Because equipment is required, it is difficult to realize. In addition, since hardenability decreases as the crystal grain is refined, an extra alloy element is required to ensure strength. Therefore, excessive grain refinement is preferable from the viewpoint of weldability and economy. Absent.

  High-strength steel materials corresponding to a yield strength of 1300 MPa are widely used for applications requiring wear resistance, and there are examples of steel materials that take into account delayed fracture resistance. For example, Patent Document 4 relates to a wear-resistant steel having a tensile strength of 1450 MPa to 1600 MPa, in which a fine martensite structure is obtained by quenching heat treatment by rapid reheating to improve delayed fracture characteristics. However, in patent document 4, there is no description about the yield strength of steel materials. Hardness is an important factor for wear resistance, and tensile strength has a great influence on wear resistance, but yield strength does not have much influence, so yield strength is not usually considered in wear resistant steel. Therefore, it is thought that it is not suitable as a structural member for construction machinery or industrial machinery. Furthermore, rapid reheating is a major limitation in ordinary thick plate heat treatment equipment and is difficult to apply to thick steel plates.

High-strength steel corresponding to a yield strength of 1300 MPa class is also widely used for bolts, and many examples have been proposed in which delayed fracture resistance is taken into account by hydrogen trap sites. For example, Patent Document 5 is a steel for bolts having a yield strength of 130 kgf / mm ² or more and a tensile strength of 140 kgf / mm ² or more, and Patent Document 6 is a steel for bolts having a tensile strength of 1350 MPa or more. All of them are subjected to tempering heat treatment at a high temperature by adding V, Mo and Cr, thereby precipitating relatively large carbides serving as hydrogen trap sites and improving delayed fracture resistance. However, in steel for bolts, weldability is not normally considered, and the steel of Patent Document 5 has a C content of 0.35% or more, and the steel of Patent Document 6 has a C content of 0.30% or more. The nature is not considered high.

Thus, in order to economically obtain a high-strength thick steel sheet steel for structural members having a yield strength of 1300 MPa or more and a tensile strength of 1400 MPa or more and further having use performance such as delayed fracture resistance and weldability, Conventional technology was not enough.
JP-A-7-90488 Japanese Patent Laid-Open No. 11-80903 JP 2007-302974 A Japanese Patent Laid-Open No. 11-229075 JP-A-6-158170 Japanese Laid-Open Patent Publication No. 2003-27186

  An object of the present invention is to provide a high-strength thick steel sheet for a structural member having a yield strength of 1300 MPa or higher and a tensile strength of 1400 MPa or higher, which is excellent in delayed fracture resistance and weldability, used for a structural member of a construction machine or an industrial machine, and a manufacturing method thereof. There is to do.

  The most economical means for obtaining a high strength with a yield strength of 1300 MPa or more is to make the steel structure martensite by quenching heat treatment from a certain temperature. In order to obtain a martensitic structure, the hardenability and cooling rate of the steel must be appropriate. The thickness of thick steel plates used as structural members for construction machines and industrial machines is almost 25 mm or less. In the case of a plate thickness of 25 mm, the cooling rate at the center of the plate thickness during quenching heat treatment by water cooling is usually about 20 ° C./sec or higher, so that quenching sufficient to form a martensite structure even at a cooling rate of 20 ° C./sec. The steel composition which has the property is required.

  In order to increase the hardenability and increase the strength, a large amount of alloy element may be added. However, when the alloy element increases, the weldability decreases. The inventor conducted a y-type weld crack test specified in JIS Z 3158 on various steel sheets having a plate thickness of 25 mm, a yield strength of 1300 MPa or more, and a tensile strength of 1400 MPa or more. The weld crack sensitivity index Pcm, the preheating temperature, The relationship was investigated. The result is shown in FIG. In order to reduce the welding load, it is desirable that the preheating temperature is as low as possible. Here, the crack stop preheating temperature at a plate thickness of 25 mm, that is, the preheating temperature at which the root crack rate becomes 0, was set to 175 ° C. or less as a target in weldability. From FIG. 1, the Pcm for the root crack rate to be 0 at a preheating temperature of 175 ° C. is 0.39% or less, and this is considered to be an approximate upper limit of the alloy addition amount.

  As means for improving delayed fracture resistance, many methods for utilizing carbides such as Mo and V as hydrogen trap sites have been proposed for bolt steel as described above. Since it is advantageous for the trap site to have a large amount of precipitates, the tempering temperature is high, and a high amount of C and many alloy elements are required to ensure the strength. As a result of investigating the strength and delayed fracture characteristics by varying the amount of alloy and manufacturing conditions in various ways, (1) In order to obtain a yield strength of 1300 MPa or more while suppressing Pcm to 0.39% or less, It is one effective means to temper the martensitic steel at a low temperature of 400 ° C. to 500 ° C. (2) At this time, the amount of C added must be at least 0.20%. When C is relatively high, the hardenability is ensured with Cr, Mo, etc. as much as possible, and the balance of strength and toughness is improved by reducing Mn as much as possible. (3) V and Cr are 400 ° C to 500 ° C. It has been found that carbides or carbonitrides are easily formed even at a low tempering temperature.

  The delayed fracture resistance was evaluated by the “limit diffusible hydrogen content” which is the upper limit of the hydrogen content that does not break in the delayed fracture test. This method is disclosed in Iron and Steel Vol. 83 (1997), p454. Specifically, the sample with notches having the shape shown in FIG. 2 was charged with various levels of diffusible hydrogen by round bar electrolytic hydrogen charging, and then the surface of the sample was subjected to plating treatment to produce hydrogen. Prevent dissipation. A predetermined load is applied and held in the atmosphere, and the time until delayed fracture occurs is measured. The load stress in the delayed fracture test was 0.8 times the tensile strength of each steel material. FIG. 3 is an example of the relationship between the amount of diffusible hydrogen and the fracture time until delayed fracture. The smaller the amount of diffusible hydrogen contained in the sample, the longer the time until delayed fracture occurs. When the amount of diffusible hydrogen is less than a certain value, delayed fracture does not occur. Immediately after the test, the test piece is collected, and the integral value of the amount of diffusible hydrogen measured with a gas chromatograph under a temperature rising condition of 100 ° C./hr is set to “diffusible hydrogen amount”. It is defined as “diffusible hydrogen amount Hc”.

On the other hand, the amount of hydrogen that enters the steel from the environment also varies depending on the metallurgical factors of the steel. In order to evaluate intrusion hydrogen from the environment, a corrosion acceleration test was conducted. This test is a test in which drying and wetting is repeated for 30 days in the cycle shown in FIG. 4 using a 5 mass% NaCl solution. After the test, the amount of hydrogen that had entered the steel material was measured by a gas chromatograph under the same temperature rise conditions, and this was defined as “the amount of diffusible hydrogen intruding from the environment HE”.
If the “limit diffusible hydrogen amount Hc” is relatively sufficiently higher than the “diffusible hydrogen amount HE entering from the environment”, the delayed fracture susceptibility is considered to be low. When Hc / HE was larger than 3, it was evaluated that delayed fracture susceptibility was low and delayed fracture resistance was good.
Further, if the tensile strength exceeds 1650 MPa, the bending workability is greatly reduced, so this is the upper limit of the tensile strength.
Based on these findings, a thick steel plate having a yield strength of 1300 MPa or more and a tensile strength of 1400 MPa or more and excellent in delayed fracture resistance, bending workability, and weldability can be obtained.

The gist of the present invention is as follows.
(1) By mass%, C: 0.20% or more, 0.26% or less, Si: 0.03% or more, 0.5% or less, Mn: 0.1% or more, less than 0.8%, P : 0.020% or less, S: 0.010% or less, Cr: 0.5% or more, 2.0% or less, Mo: 0.1% or more, 0.5% or less, V: more than 0.10% 0.30% or less, Al: 0.01% or more, 0.15% or less, B: 0.0003% or more, 0.0030% or less, N: 0.006% or less, other Fe and unavoidable It has a component composition that consists of impurities and satisfies the following Pcm of 0.39% or less, the martensite structure fraction is 90% or more, the yield strength is 1300 MPa or more, the tensile strength is 1400 MPa or more, Excellent delayed fracture resistance and weldability, characterized by being 1650 MPa or less Strength steel plate. Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B] where [C] , [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are C, Si, Mn, Cu, Ni, Cr, Mo, and V, respectively. , B mass%.

(2) By mass%, Cu: 0.1% or more, 0.5% or less, Ni: 0.1% or more, 2.0% or less, Nb: 0.003% or more, 0.10% or less Ti: A thick steel plate excellent in delayed fracture resistance and weldability according to (1), characterized by containing one or two of 0.005% or more and 0.05% or less.

(3) A steel slab or cast slab having the composition described in (1) or (2) is heated to 1100 ° C. or higher and hot rolled to obtain a steel plate having a thickness of 4.5 mm or more and 25 mm or less. After accelerated cooling to 200 ° C. or lower under cooling conditions where the average cooling rate at the center of the plate thickness from 600 ° C. to 300 ° C. is 20 ° C./sec or higher, or after reheating to a temperature of Ac 3 transformation point + 20 ° C. or higher , Accelerated cooling to 200 ° C. or lower under cooling conditions where the average cooling rate at the center of the plate thickness from 600 ° C. to 300 ° C. is 20 ° C./sec or higher, and then tempering in a temperature range of 400 ° C. or higher and 500 ° C. or lower. A method for producing a high-strength thick steel plate excellent in delayed fracture resistance and weldability, characterized by performing heat treatment.

  According to the present invention, it is possible to economically provide a thick steel plate having a yield strength of 1300 MPa or more and a tensile strength of 1400 MPa or more, which is excellent in delayed fracture resistance and weldability, which is used for structural members of construction machines and industrial machines.

Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the steel components of the present invention will be described.
C is an important element that greatly affects the strength of the martensite structure. In order to obtain a yield strength of 1300 MPa or more and a tensile strength of 1400 MPa or more when the C content is in the range from 4.5 mm to 25 mm in the present invention and the martensite structure fraction is 90% or more. As a necessary amount, 0.20% or more is added. Moreover, since weldability may fall by excess C addition, and tensile strength may exceed 1650 Mpa, an addition amount shall be 0.26% or less.

  Si acts as a deoxidizing material and a strengthening element, and its effect is recognized when added in an amount of 0.03% or more. However, if added in a large amount, the toughness may be inhibited, so the upper limit is made 0.5%.

  Mn is an element effective for enhancing hardenability, but when martensitic steel having a relatively high C of 0.20% or more is tempered at 400 ° C. to 500 ° C. as in the steel of the present invention. Rather, the balance of strength and toughness can be improved by reducing Mn. Therefore, Mn is added in an amount of 0.1% or more for ensuring hardenability, but the upper limit of addition is less than 0.8%, preferably 0.5% or less.

  P is a harmful element that deteriorates the weldability of high-strength steel as an inevitable impurity. Therefore, the content is suppressed to 0.020% or less.

  S is also an inevitable impurity and is a harmful element that lowers the delayed fracture resistance. Therefore, the content is suppressed to 0.010% or less.

  Cr improves hardenability and is effective for precipitation strengthening and hydrogen trap site generation when tempering at 400 ° C. to 500 ° C. is performed, so 0.5% or more is added. However, if added excessively, the toughness may be lowered, so the addition is made 2.0% or less.

  Mo improves hardenability and also has a precipitation strengthening effect by tempering at 400 ° C. to 500 ° C., so 0.1% or more is added. However, if a large amount is added, weldability may be lowered, so the addition is made 0.5% or less.

  Al is added for the purpose of fixing N in order to secure a deoxidizer or free B necessary for improving hardenability, but excessive addition may reduce toughness. For this reason, the addition amount of Al shall be 0.01% or more and 0.15% or less.

  V improves the hardenability and has a precipitation strengthening effect when tempering at 400 ° C. to 500 ° C. Further, since carbide or nitride is effective as a hydrogen trap site, it is 0 in the present invention. • Addition of more than 10% is essential. However, if added in a large amount, the weldability is lowered, so the addition is made 0.30% or less.

  B is an essential element effective for enhancing the hardenability. In order to exert the effect, 0.0003% or more is necessary, but if added over 0.0030%, the weldability and toughness may be lowered, so the B content is 0.0003% or more, 0 0030% or less.

  If N is contained excessively, it lowers toughness and inhibits the effect of improving the hardenability of B by forming BN, so the content is suppressed to 0.006% or less.

The above are the basic components of steel in the present invention, but in the present invention, one or more of Cu, Ni, Nb and Ti can be added in addition to the above components.
Cu may be added for strengthening, but its effect is limited, and since it is an expensive element, the addition amount is 0.1% or more and 0.5% or less.

  Ni has the effect of improving hardenability and improving toughness, and for these purposes, 0.1% or more is added. However, since Ni is an expensive element, even when it is added, the upper limit is made 2.0%.

  Nb is effective in improving toughness because it produces fine carbide steel during rolling and expands the non-recrystallization temperature range to increase the controlled rolling effect and suppresses austenite coarsening during quenching heating by pinning. It is. When added for this purpose, 0.003% or more is added. However, if added excessively, weldability may be hindered, so the added amount is made 0.10% or less.

  Ti is added for the purpose of fixing N in order to secure the free B necessary for improving the hardenability, particularly when accelerated cooling is performed immediately after the end of hot rolling. However, excessive addition reduces weldability and toughness. May decrease. For this reason, when adding Ti, the addition amount shall be 0.005% or more and 0.05% or less.

In addition to the above component range limitation, as described above, in the present invention, in order to ensure weldability, the component composition is limited so that the following Pcm is 0.39% or less.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B], where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are C, Si, Mn, Cu, Ni, Cr, It is the mass% of Mo, V, and B.

Next, a manufacturing method will be described.
First, hot rolling is performed by heating a steel slab or slab having the above steel composition. The heating temperature is set to 1100 ° C. or higher so that Nb and V are sufficiently dissolved.

  A steel sheet having a thickness of 4.5 mm or more and 25 mm or less by performing hot rolling, and a cooling condition in which the average cooling rate at the center of the thickness from 600 ° C. to 300 ° C. immediately after the hot rolling is 20 ° C./sec or more. Accelerated cooling to 200 ° C. or lower after cooling, or after cooling once after the hot rolling, and then reheating to a temperature of Ac3 transformation point + 20 ° C. or higher, then the average cooling rate at the center of the plate thickness from 600 ° C. to 300 ° C. Accelerated cooling is performed to 200 ° C. or lower under a cooling condition of 20 ° C./sec or higher. In the case of reheating and quenching, the quenching heating temperature must naturally be higher than the Ac3 temperature. However, if the heating temperature is just above the Ac3 temperature, the structure becomes mixed and the hardenability and toughness may be insufficient. Therefore, quenching heating temperature shall be Ac3 + 20 degreeC or more. The purpose of accelerated cooling is to obtain a martensite structure having a structure fraction of 90% or more in a steel sheet having a thickness of 4.5 mm to 25 mm.

  By performing tempering heat treatment on the steel sheet after accelerated cooling at a temperature range of 400 ° C. or higher and 500 ° C. or lower, the strength is improved by the tempering effect of martensite structure and the precipitation strengthening effect of carbides such as Cr, Mo, V, or carbonitrides. It improves the toughness balance and produces V and Cr carbides or carbonitrides that are particularly effective as hydrogen trap sites. The time for the tempering heat treatment may be about 30 minutes or more.

  The steel pieces obtained by melting the steels A to A F having the composition shown in Table 1 are obtained according to the production conditions of the present invention examples 1 to 16 and the comparative examples 17 to 42 shown in Table 2, respectively. Steel plates having a thickness of 4.5 to 25 mm were manufactured.

These steel plates were evaluated for yield strength, tensile strength, martensite structure fraction, weld cracking property, delayed fracture resistance, and toughness.
Yield strength and tensile strength were measured by taking a No. 1A tensile test piece specified in JIS Z 2201 and performing a tensile test specified in JIS Z 2241.
The weld cracking property was evaluated by a y-type weld cracking test specified in JIS Z 3158. The welding conditions were CO2 welding with a heat input of 15 kJ / cm, and the plate thicknesses of the steel plates used for the evaluation were all 25 mm. As a result of the test, if the root cracking rate was 0 at a preheating temperature of 175 ° C., it was evaluated as acceptable. In addition, for the steel plates of Examples 2, 4, 8, and 13 having a plate thickness of less than 25 mm, the weldability is considered to be the same as that of Examples 1, 3, 7, and 12 of the same component. Omitted.

  Delayed fracture resistance is evaluated by measuring the “limit diffusible hydrogen amount Hc” and “diffusible hydrogen amount HE entering from the environment” of each steel sheet. When Hc / HE is greater than 3, delayed fracture resistance Was evaluated as being good.

As for toughness, a JIS Z 2201 No. 4 Charpy test piece was sampled perpendicularly to the rolling direction from the center of the plate thickness, and the absorbed energy value of the impact test at −20 ° C. was evaluated by the average value of the three test pieces. The target value was used. In addition, about the steel plate whose plate | board thickness is 4.5 mm, it was set as the target value that it was set as the Charpy test piece of 3 mm subsize, and the absorbed energy value per 1 cm < ² > was 27J or more.

  The underlined chemical composition and Pcm values in Table 1 indicate that the values are outside the scope of the present invention, and the underlined numbers in Table 2 indicate that the manufacturing conditions are outside the scope of the present invention. This indicates that the characteristics are insufficient.

  In Invention Examples 1 to 16 in Table 2, all of the above-described target values of yield strength, tensile strength, martensite structure fraction, weld cracking property, delayed fracture resistance, and toughness are satisfied. On the other hand, in Comparative Examples 17 to 36 in which the chemical components indicated by the underline in the table deviate from the range limited by the present invention, the yield strength and tensile strength were determined even though the production method was the method of the present invention. , One or more of martensite structure fraction, weld cracking property, delayed fracture resistance, and toughness is less than the target value. Although the steel component composition is within the range of the present invention, Comparative Example 37 in which the Pcm value deviates from the range of the present invention has a poor weld cracking property. Even if the steel component composition and the Pcm value are both within the range of the present invention, the comparative example 38 having a low heating temperature has insufficient yield strength, and the comparative example 39 having a quenching heating temperature lower than Ac3 + 20 ° C is also a yield. In Comparative Example 40, the strength is insufficient and the cooling rate from 600 ° C. to 300 ° C. is small, and a martensite structure fraction of 90% or more cannot be obtained. In Comparative Example 42, the yield strength is low and the tempering temperature exceeds 500 ° C., the yield strength is low.

It is a graph which shows the relationship between Pcm and the crack stop preheating temperature in a y-type weld crack test. It is explanatory drawing of the notch test piece for hydrogen embrittlement resistance evaluation. It is a graph which shows an example of the relationship between the amount of diffusible hydrogen, and the fracture | rupture time until it leads to delayed fracture. It is a graph which shows the repetition conditions of wet and dry and temperature of a corrosion acceleration test.

Claims (3)

% By mass
C: 0.20% or more, 0.26% or less,
Si: 0.03% or more, 0.5% or less,
Mn: 0.1% or more, less than 0.8%,
P: 0.020% or less,
S: 0.010% or less,
Cr: 0.5% or more, 2.0% or less,
Mo: 0.1% or more, 0.5% or less,
V: more than 0.10%, 0.30% or less,
Al: 0.01% or more, 0.15% or less,
B: 0.0003% or more, 0.0030% or less,
N: 0.006% or less, composed of other Fe and unavoidable impurities, satisfying the following Pcm of 0.39% or less, martensite structure fraction is 90% or more A high strength thick steel plate excellent in delayed fracture resistance and weldability, characterized in that the yield strength is 1300 MPa or more and the tensile strength is 1400 MPa or more and 1650 MPa or less.
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are C, Si, Mn, Cu, and Ni, respectively. , Cr, Mo, V, B mass%. In mass%,
Cu: 0.1% or more, 0.5% or less,
Ni: 0.1% or more, 2.0% or less,
Nb: 0.003% or more, 0.10% or less,
The thick steel plate excellent in delayed fracture resistance and weldability according to claim 1, characterized in that it contains one or two of Ti: 0.005% or more and 0.05% or less. The steel slab or slab having the component composition according to claim 1 or 2 is heated to 1100 ° C or higher and hot rolled to obtain a steel plate having a thickness of 4.5 mm or more and 25 mm or less, and immediately 600 ° C. Accelerated cooling to 200 ° C. or lower under cooling conditions in which the average cooling rate at the center of the plate thickness from 20 to 300 ° C. is 20 ° C./sec or higher, or reheat the thick steel plate to a temperature of Ac 3 transformation point + 20 ° C. or higher. After that, accelerated cooling is performed to 200 ° C. or lower under cooling conditions in which the average cooling rate at the center of the plate thickness from 600 ° C. to 300 ° C. is 20 ° C./sec or higher, and then the temperature range is 400 ° C. or higher and 500 ° C. or lower. A method for producing a high-strength thick steel plate excellent in delayed fracture resistance and weldability, characterized by performing tempering heat treatment at

Images