JP6149368B2 - Manufacturing method of high-tensile steel plate with excellent delayed fracture resistance - Google Patents

Description

  The present invention relates to a method for producing a high-strength steel sheet having excellent delayed fracture resistance, and particularly relates to a suitable high-tensile steel material having a tensile strength of 780 MPa or more.

  In recent years, in the field of using steel materials such as construction industrial machines, tanks, penstocks, line pipes, etc., against the backdrop of increasing the size and weight of structures, the strength of steel materials to be used has been increasing and the amount of steel materials used has rapidly increased. It has increased.

  In response to such an increase in demand, Patent Documents 1, 2, and 3 propose a method of manufacturing a 780 MPa class high-tensile thick steel plate that omits the tempering treatment.

  In order to further meet the demand for high-strength steel, Patent Document 4 proposes a method for manufacturing a high-strength steel sheet having a yield strength of 885 MPa or more, in which tempering treatment is omitted.

JP 2007-277622 A JP 2007-277623 A JP 2009-263774 A JP 2011-012315 A

  In Patent Documents 1 to 3, “as a method for producing a high strength steel sheet having a tensile strength of 780 MPa class without tempering treatment, an acceleration at which the cooling rate is 8 to 80 ° C./s from 700 ° C. or higher after completion of non-recrystallization zone rolling. It is described that cooling is started and accelerated cooling is stopped at room temperature to 350 ° C., and this production method is liable to generate non-uniform carbides by self-annealing, and there is a problem from the viewpoint of delayed fracture resistance. In addition, it is necessary to reduce C in order to adjust the as-quenched strength, and in order to satisfy the necessary characteristics, it is necessary to add a large amount of alloying elements, and there is a problem that economical efficiency is lowered. .

In Patent Document 4, “as a method for producing a high-strength steel sheet having a yield strength of 885 MPa or more without tempering treatment, accelerated cooling is performed at an average cooling rate of 10 ° C./s or more from a temperature of Ar ₃ or more after completing non-recrystallization zone rolling. The cooling is stopped at 200 to 400 ° C. and then cooled at 20 ° C./min or less ”, and this manufacturing method is similar to Patent Documents 1 to 3 in that non-uniform carbides by self-annealing are described. It tends to occur and is problematic in terms of delayed fracture resistance.
Moreover, since the strength is secured by auto-aging, it is difficult to say that the low temperature toughness is sufficient.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for producing a high-tensile steel sheet having a tensile strength of 780 MPa or more and superior in delayed fracture resistance to conventional steel materials.

  In order to solve the above-mentioned problems, the present inventors have conducted extensive research on a steel sheet having a strength of 780 MPa or more and delayed fracture resistance, and as a result, the rate of temperature rise at the center of the steel sheet in the thickness direction during tempering, By defining the tempering temperature and holding time, the formation and coarsening of carbides due to tempering can be effectively suppressed, and accumulation of diffusible hydrogen into carbides can be suppressed. It has been found that by reducing the amount of diffusible hydrogen, delayed fracture during cutting and during use can be suppressed, and a high-tensile steel sheet having better delayed fracture resistance than conventional steel can be obtained.

  The present invention has been made on the basis of the above-described findings and has been further studied. The summary of the present invention is as follows.

[1] By mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.8%, Mn: 0.5 to 2%, P: 0.010% or less, S: 0.0. A steel containing 0.003% or less, Al: 0.005 to 0.1%, N: 0.0005 to 0.008%, with the balance being Fe and unavoidable impurities, is heated to the Ac ₃ transformation point or higher, subjected to hot rolling to a cumulative reduction ratio is 80% or less at the recrystallization temperature region, Ar ₃ hot rolling finished by transformation point or higher, subsequently Ar ₃ transformation point at 10 ° C. / s or more cooling rate from above After cooling to a temperature of 250 ° C. or lower, reheat at an average temperature increase rate of 1 ° C./s or higher, and perform a tempering treatment with a maximum temperature of 100 to 400 ° C. An excellent method for producing high-tensile steel sheets.

  [2] In addition to the steel, by mass%, Mo: 0.01-1%, Nb: 0.001-0.1%, V: 0.001-0.5%, Ti: 0.001- It contains at least one selected from 0.1%, Cu: 2% or less, Ni: 4% or less, Cr: 2% or less, W: 2% or less. A method for manufacturing high-tensile steel sheets with excellent delayed fracture resistance.

  [3] The steel further contains at least one kind selected from B: 0.0003 to 0.003%, Ca: 0.01% or less, and REM: 0.02% or less in mass%. The method for producing a high-tensile steel sheet having excellent delayed fracture resistance according to [1] or [2] above.

  [4] Reheating after rolling is performed using a heating device installed on the same production line as the rolling mill and the cooling device, and the holding time at the highest temperature in the tempering process is set to 60 s or less. The method for producing a high-tensile steel sheet having excellent delayed fracture resistance according to any one of [1] to [3].

  [5] The steel contains, by mass%, Nb: 0.001 to 0.1%, a cumulative rolling reduction in the non-recrystallization temperature range of 40% or more, and a hot rolling end temperature of 860 ° C. or less. The method for producing a high-tensile steel plate having excellent delayed fracture resistance according to any one of claims 1 to 4.

  [6] The metal structure of the high-strength steel sheet is mainly composed of martensite and lower bainite, the equivalent circle diameter of carbide at the lath interface in the metal structure is 100 nm or less, and the carbide coverage is 30% or less. The method for producing a high-tensile steel sheet excellent in weldability and delayed fracture resistance according to any one of [1] to [5].

  According to the present invention, a high-tensile steel having a high tensile strength of 780 MPa or more and excellent in delayed fracture resistance can be stably produced at a low cost.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. About component composition First, the reason which prescribed | regulated the component composition of the steel of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.02-0.25%
C is an indispensable element for obtaining the strength required for structural steel, but if it is added less than 0.02%, sufficient strength cannot be obtained, and a large amount of alloying element is required, so that weldability is achieved. Decreases. If added over 0.25%, the amount of martensite produced in the weld heat affected zone increases and the toughness decreases, so the C content is in the range of 0.02 to 0.25%. Preferably it is 0.12 to 0.22% of range. More preferably, it is 0.12 to 0.18% of range.

Si: 0.01 to 0.8%
Si is added for deoxidation, but if less than 0.01% is added, the deoxidation effect is not sufficient, and if added over 0.8%, the toughness of the base metal and the weld heat-affected zone significantly decreases. At the same time, the weldability is remarkably lowered, so the Si content is in the range of 0.01% to 0.8%. Preferably it is 0.1 to 0.5% of range. More preferably, it is 0.2 to 0.5% of range.

Mn: 0.5-2%
Mn is added from the viewpoint of securing the strength of the base metal. However, if it is added in an amount of less than 0.5%, the effect is not sufficient. Since the toughness is remarkably lowered, the amount of Mn is set in the range of 0.5 to 2%. Preferably it is 0.6 to 1.6% of range. More preferably, it is 0.6 to 1.3% of range.

P: 0.010% or less When P exceeds 0.010%, the toughness of the base metal and the weld heat-affected zone is remarkably reduced, so the P content is 0.010% or less.

S: 0.003% or less If S is contained in excess of 0.003%, the toughness of the base metal and the weld heat-affected zone is remarkably reduced. Therefore, the amount of S is made 0.003% or less.

Al: 0.005 to 0.1%
Al is added in order to sufficiently deoxidize molten steel, but if it is added less than 0.005%, the deoxidation effect is not sufficient, and if added over 0.1%, the amount of Al that dissolves in the base metal And the base metal toughness is reduced, so the Al content is in the range of 0.005 to 0.1%. Preferably, it is 0.01 to 0.06% of range. More preferably, it is 0.01 to 0.04% of range.

N: 0.0005% to 0.008%
N is added in order to refine the structure by forming a nitride with Ti or the like and to improve the toughness of the base material and the weld heat affected zone. However, if the addition is less than 0.0005%, the effect of refining the structure is not sufficient. On the other hand, if the addition exceeds 0.008%, the amount of N dissolved in the base material increases and the base material toughness is remarkably increased. Further, a coarse carbonitride is formed also in the weld heat affected zone and the toughness is lowered. Therefore, the N content is set in the range of 0.0005% to 0.008%. Preferably, it is 0.0010 to 0.0060% of range. More preferably, it is 0.0010 to 0.0040% of range.

  The above is the basic chemical component of the present invention, and the balance consists of Fe and unavoidable impurities, but one kind selected from Mo, Nb, V, Ti, Cu, Ni, Cr, and W for the purpose of further increasing the strength. The above may be added as selective elements.

Mo: 0.01 to 1%
Mo is an element effective for increasing the strength of the base material, but its effect is insufficient if it is less than 0.01%, and if added over 1%, it causes an increase in hardness due to precipitation of alloy carbides, and toughness When Mo is added, the amount of Mo is preferably in the range of 0.01 to 1%. More preferably, it is 0.05 to 0.8% of range.

Nb: 0.001 to 0.1%
Nb is an element effective for strengthening steel and has an effect of refining carbides precipitated after tempering under appropriate rolling conditions. However, if it is less than 0.001%, the effect is insufficient. Since addition exceeding 1% remarkably lowers the toughness of the base material, when Nb is added, the Nb content is preferably in the range of 0.001 to 0.1%. More preferably, it is 0.001 to 0.05% of range.

V: 0.001 to 0.5%
V is effective in improving the strength and toughness of the base metal, and is effective in lowering the solid solution N by being precipitated as VN. However, if it is less than 0.001%, the effect is insufficient. If added over 5%, the toughness decreases due to precipitation of hard VC. Therefore, when V is added, the V content is preferably in the range of 0.001 to 0.5%. More preferably, it is 0.01 to 0.1% of range.

Ti: 0.001 to 0.1%
Ti generates TiN at the time of rolling heating or welding, effectively suppressing the coarsening of austenite, and improves the toughness of the base material and the weld heat affected zone. However, if added over 0.1%, the Ti nitride is coarsened and the toughness of the base metal and the weld heat affected zone is reduced. Therefore, when adding Ti, the Ti content is 0.001 to 0.1%. It is preferable to be in the range. More preferably, it is 0.005 to 0.020% of range.

Cu: 2% or less Cu can improve the strength of the steel without impairing the low-temperature toughness, but if added over 2%, it causes cracking on the surface of the steel sheet during hot rolling, so when adding Cu, The amount of Cu is preferably 2% or less. More preferably, it is 1% or less.

Ni: 4% or less Ni is a beneficial element that improves the strength of the steel and the toughness of the heat affected zone of the steel, but if added over 4%, the effect is saturated and the economy is inferior, so Ni is added. In this case, the amount of Ni is preferably 4% or less. More preferably, it is 2% or less.

Cr: 2% or less Cr is an element effective for improving the strength and toughness, but if added over 2%, the weldability deteriorates. Therefore, when Cr is added, the Cr amount is 2% or less. It is preferable to do. More preferably, it is 0.1 to 1% of range.

W: 2% or less W is an element that has the effect of improving the strength, but if added over 2%, the weldability deteriorates, so when adding W, the amount of W is 2% or less. It is preferable that More preferably, it is 0.05 to 2% of range.

  In addition to the above composition, the high-tensile steel of the present invention may be added with one or more selected from B, Ca, and REM as selective elements for the purpose of further improving the material.

B: 0.0003 to 0.003%
B has the effect of suppressing the ferrite transformation from the grain boundary by segregating at the austenite grain boundary and improving the hardenability, but 0.0003% or more may be added in order to fully exhibit this effect. Preferably, if added over 0.003%, it precipitates as carbonitride, lowering the hardenability and lowering the toughness. Therefore, when adding B, the amount of B is 0.0003 to 0.003%. It is preferable to be in the range. More preferably, it is 0.0005 to 0.002% of range.

Ca: 0.01% or less Ca is an element useful for controlling the form of sulfide inclusions. However, if added over 0.01%, the cleanliness is lowered, so when adding Ca, the Ca content is preferably 0.01% or less. More preferably, it is 0.0005 to 0.0020% of range.

REM: 0.02% or less REM also has the effect of improving the quality of the material by forming oxides and sulfides in the steel, similar to Ca, but the effect is saturated even if added over 0.02%. Therefore, when REM is added, the amount of REM is preferably 0.02% or less. More preferably, it is 0.0005 to 0.0020% of range.

2. About Metal Structure The reason for limiting the metal structure in the present invention will be described.

  In order to increase the tensile strength of 780 MPa or more, the metal structure needs to be a structure mainly composed of martensite and lower bainite. In order to achieve both strength and toughness, the martensite + lower bainite structure fraction should be It is preferably 95% or more. Less than 5% ferrite, upper bainite, residual γ, etc. are allowed.

  The structure fraction is quantified by etching with an appropriate corrosive solution such as nital, taking five or more views of the structure photograph with an optical microscope, and measuring the area ratio of martensite and lower bainite by image analysis.

  Tempering treatment is performed from the viewpoint of improving weldability and delayed fracture resistance by reducing the amount of diffusible hydrogen in the steel as much as possible.

  In addition, fine dispersion of carbide is extremely important for improving delayed fracture resistance. Most preferably, in addition to reducing the equivalent circle diameter of the carbide formed at the lath interface of martensite and lower bainite to 100 nm or less, the amount of carbide in the lath interface (hereinafter referred to as carbide coverage) is 30% or less. Thus, accumulation of diffusible hydrogen in the carbide can be effectively suppressed, and the delayed fracture resistance can be further improved.

  About the measurement method of the equivalent circle diameter and carbide coverage of the carbide formed at the lath interface of martensite, after etching with an appropriate corrosive solution such as nital, the structure photograph was taken with a scanning electron microscope over 10 fields of view. Find using photos. For example, the equivalent circle diameter of carbide is converted into an equivalent circle diameter by analyzing the size of carbide generated at the interface of 50 or more laths by image analysis. In addition, the carbide coverage is measured by image analysis of the length along the lath interface of carbide generated at 50 or more lath interfaces, and the total length of the length along the lath interface of carbide. Divide by the sum of the lath interface lengths and multiply by 100.

3. About manufacturing conditions The manufacturing method of this invention is demonstrated below.

  In the present invention, the steel having the above-described composition is melted by a melting means such as a converter or an electric furnace, and is made into a steel material such as a slab by a conventional method such as a continuous casting method or an ingot-bundling method. However, it does not specify a method for melting or casting steel.

The steel slab having the composition described above for rolling conditions, heated to Ac ₃ transformation point or higher in a heating furnace. As a method of charging a steel slab into the heating furnace, a hot piece charging method in which the slab is charged into the heating furnace without being cooled below the Ar ₃ transformation point, or a slab once cooled is charged into the heating furnace. However, although there is a cold piece charging method in which reheating is performed to the Ac ₃ transformation point or higher, any method can be used in the present invention.

The reason why the steel is heated to the Ac ₃ transformation point or higher in the heating furnace is to make the steel uniform in one phase of the austenite structure, and the heating temperature is preferably 1100 ° C. or higher and 1250 ° C. or lower. In particular, when emphasizing toughness, the temperature is more preferably 1100 ° C. or more and 1200 ° C. or less.

Hot rolling, the cumulative reduction rate in the pre-recrystallization temperature region is 80% or less, and finished hot rolled at Ar ₃ transformation point or higher, the accelerated cooling start temperature is made to be than the Ar ₃ transformation point followed by .

  The cumulative rolling reduction is 80% or less, but a preferable rolling reduction range is 10 to 80%. The non-recrystallization temperature range is a temperature range where recrystallization does not occur during rolling, and is 930 ° C. or less in the steel of the present invention. The cumulative rolling reduction is expressed by (original thickness−finished thickness) / original thickness × 100%.

Incidentally, Ar ₃ transformation point, a value that is calculated by the following equation (1).

As the Ac ₃ transformation point, a value calculated by the following equation (2) is used.

  Moreover, about the steel which added Nb among this invention steel, when the unrecrystallized zone reduction of the said hot rolling shall be 40% or more, and the hot rolling completion temperature shall be 860 degrees C or less, in a structure | tissue after tempering Since the average equivalent circle diameter of the carbides observed in 1 is 80 nm or less, the delayed fracture resistance is further improved.

Cooling conditions after rolling In order to ensure the strength of the base metal and the toughness after the hot rolling is completed, it is necessary to perform forced cooling from a temperature equal to or higher than the Ar ₃ transformation point to 250 ° C. or lower.

  The reason for cooling until the cooling stop temperature is 250 ° C. or lower is to complete the transformation from austenite to martensite and strengthen the base material.

  In the present invention, in order to achieve both strength and toughness, the martensite + lower bainite structure fraction is set to 95% or more.

  The cooling rate during forced cooling is 10 ° C./s or more. This is because if it is less than 10 ° C./s, ferrite and pearlite are likely to be partially formed during cooling, and the desired strength and toughness cannot be secured stably.

As a cooling method, techniques such as direct quenching and accelerated cooling are used, but the cooling method is not specified if a cooling rate of 10 ° C./s or more and a cooling stop temperature of 250 ° C. or less are obtained.
In addition, a cooling rate is prescribed | regulated by the average cooling rate in 700-500 degreeC. This is because this temperature region is a temperature region in which a soft phase such as ferrite or pearlite is likely to be produced, and it is necessary to cool this temperature region quickly in order to obtain high strength.

Tempering conditions Tempering is preferably performed using a heating device installed directly on the same production line as the rolling mill and the direct cooling or accelerated cooling device. This is because the time required from the rolling / cooling process to the tempering process can be shortened by the direct connection, thereby improving the productivity and reducing the heat energy.

  As the heating method for tempering, any method such as induction heating, current heating, infrared radiation heating, atmosphere heating, or the like may be used as long as the average temperature rising rate can be achieved and the upper and lower limits of the heating temperature can be managed.

  The temperature condition for tempering is such that the average rate of temperature rise during heating is 1 ° C./s or more, and the upper limit of the heating temperature is 400 ° C. A steel plate that has an average temperature increase rate of 1 ° C./s or more and a tempering temperature of 400 ° C. or less, effectively reducing the size of carbides generated between the laths, and is superior in strength and toughness compared to conventional steel. Can be obtained.

  On the other hand, when the heating temperature is less than 100 ° C., the dehydrogenation effect is insufficient, and the effect of delayed fracture resistance is not sufficiently obtained.

  The average rate of temperature increase is a value obtained by dividing the amount of temperature increase required for reheating up to the reheating temperature (100 to 400 ° C.) by the time required for reheating after cooling.

  The temperature raising process at the time of tempering is not limited as long as a predetermined average temperature rising rate is obtained, and a linear temperature history may be taken or a temperature history that stays at an intermediate temperature may be taken.

  The above-described temperatures of the present invention are all temperatures at the center of the plate thickness unless otherwise specified, and are managed by calculation from the surface measured temperature.

  The holding time at the tempering temperature is preferably 60 s or less in order to prevent deterioration of toughness due to productivity / manufacturing cost and coarsening of precipitates.

  With respect to the cooling rate after tempering, the average cooling rate at the central portion of the plate thickness up to 100 ° C. or lower is 0.05 ° C./s or higher, 20 to prevent deterioration of toughness due to coarsening of precipitates during cooling. It is desirable that the temperature is not more than ° C / s.

  24 types of steel types A to X of chemical components shown in Table 1 were melted to cast a slab, heated in a heating furnace, and then rolled to obtain a steel plate. After rolling, the steel was then directly quenched and then tempered using a solenoid type induction heating device.

  Table 2 shows the steel sheet production conditions and the yield strength, tensile strength, absorbed energy at -40 ° C. (vE-40), and delayed fracture resistance safety index.

  The average heating rate at the center of the plate thickness was controlled by the plate passing rate of the steel plate. In addition, when hold | maintaining, it hold | maintained in the range of +/- 5 degreeC by reciprocating and heating a steel plate.

  The cooling after heating was air cooling. The temperature at the center of the plate thickness, such as the tempering temperature and the quenching temperature, was obtained by heat transfer calculation from the results of temperature measurement at the surface in succession by a radiation thermometer.

  Tensile tests are performed in accordance with JIS Z 2241. Yield strength and tensile strength are measured with a JIS No. 5 test piece when the plate thickness is 20 mm or less, and with a JIS No. 4 test piece taken from 1/4 of the plate thickness when the plate thickness exceeds 20 mm. did.

  Toughness was evaluated by absorbing energy (−40 ° C.) at −40 ° C. obtained by a Charpy impact test using a test specimen taken from ¼ part of the plate thickness after collecting the impact test specimen specified in JIS Z 2242. .

The delayed fracture resistance index is obtained by charging hydrogen so that the amount of diffusible hydrogen in the test piece is about 0.5 massppm by the cathodic hydrogen charging method using a smooth round bar test piece, and then galvanizing the surface of the test piece. Hydrogen is sealed by applying, and then a tensile test is performed at a strain rate of 1 × 10 ⁻⁶ / s to obtain a squeeze of the fractured test piece, while no hydrogen charge is performed at the same strain rate. A tensile test of the piece was also performed and evaluated according to the following formula.
Delayed fracture resistance index (%) = 100 x (X1 / X0)
Here, X0: Drawing of test specimen substantially not containing diffusible hydrogen (%)
X1: Test specimen containing diffusible hydrogen (%)
The target values for each characteristic were a yield stress of 685 MPa or more, a tensile strength of 780 MPa or more, a vE-40 of 40 J or more, and a delayed fracture resistance index target of 80% or more.

  Quantification of the structure of the steel plate was evaluated with an average value of measured values obtained by image analysis by revealing the structure with a nital etchant in the vicinity of ¼ part of the plate thickness, observing 5 fields of view with an optical microscope.

  For the measurement of the equivalent circle diameter and the coverage of the carbide, the structure was revealed with a nital etchant in the vicinity of ¼ part of the plate thickness, 10 fields of view were observed with a scanning electron microscope, and the average of the measured values by image analysis was evaluated.

  However, the strength of the steel sheet was not obtained, No. For 18, 25, 28, 30, and 31, the equivalent circle diameter and coverage of the carbide were not measured.

  As shown in Table 2, Invention Example No. manufactured by the method of the present invention was used. 1-No. No. 16 has chemical components and production methods within the scope of the present invention, and a steel sheet having good strength and toughness and delayed fracture resistance can be obtained.

  Further, among the Nb-added invention examples, the steel manufactured with a rolling finishing temperature of 870 ° C. or less and an unrecrystallized zone reduction of 40% or more has an average equivalent circle diameter of carbide of 80 nm or less, and is further resistant to delayed fracture. Excellent characteristics.

  On the other hand, comparative example No. in which a component remove | deviates from this invention. 17-No. In No. 24, any one or more of strength, toughness, and delayed fracture resistance does not reach the target value. In addition, Comparative Example No. in which the manufacturing conditions deviate from the present invention. 25-No. As for 33 steel plate, any one or more of strength, toughness and delayed fracture resistance does not reach the target value.

Claims (6)

In mass%, C: 0.02-0.25%, Si: 0.01-0.8%, Mn: 0.5-2%, P: 0.010% or less, S: 0.003% or less , Al: 0.005 to 0.1% N: containing 0.0005 to 0.008%, heating the steel and the balance being Fe and unavoidable impurities than Ac ₃ transformation point, non-recrystallized temperature subjected to hot rolling to a cumulative reduction rate at frequency 80% or less, Ar ₃ to exit the hot rolling at lower than the transformation point, continuing Ar ₃ below 250 ° C. at 10 ° C. / s or more from the above transformation point of cooling rate After cooling to a temperature of 5 ° C / s, reheating at an average temperature increase rate of 5 ° C / s or more, setting the maximum temperature to 100 to 400 ° C, and performing a tempering process with the holding time at the maximum temperature being within 60s. The yield stress is 685 MPa or more, and the tensile strength is 780 MPa or more. Delayed production method excellent high-tensile steel plate in fracture characteristics that. In mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.8%, Mn: 0.5 to 1.5%, P: 0.010% or less, S: 0.003 % or less, Al: 0.005 to 0.1%, N: containing from 0.0005 to 0.008%, further, M o: 0.01~1%, Nb : 0.001~0.1% V: 0.001 to 0.5%, Ti: 0.001 to 0.1%, Cu: 2% or less, Ni: 4% or less, Cr: 2% or less, W: 2% or less A steel containing the balance of Fe and inevitable impurities is heated to the Ac ₃ transformation point or higher, and hot rolling is performed so that the cumulative reduction in the non-recrystallization temperature range is 80% or less, and Ar ₃ Exit hot rolling at lower than the transformation point, subsequently cooled to a temperature of 250 ° C. or less than the Ar ₃ transformation point from 10 ° C. / s or more cooling rate, 5 ° C. / s or higher Reheating at an average rate of temperature increase of 100 to 400 ° C., and performing a tempering treatment with a holding time at the maximum temperature of 60 s or less . The yield stress is 685 MPa or more, method for producing a high tensile steel sheet tensile strength and excellent resistance to delayed fracture resistance Ru der least 780 MPa.   The steel further contains one or more selected from B: 0.0003% to 0.003%, Ca: 0.01% or less, and REM: 0.02% or less in mass%. The method for producing a high-tensile steel sheet having excellent delayed fracture resistance according to claim 1 or 2, wherein the yield stress is 685 MPa or more and the tensile strength is 780 MPa or more.   The yield stress is 685 MPa or more and the tensile strength is 780 MPa or more, wherein reheating after rolling is performed using a heating device installed on the same production line as the rolling mill and the cooling device. 4. A method for producing a high-tensile steel sheet excellent in delayed fracture resistance according to any one of 3 above.   The steel contains, by mass%, Nb: 0.001 to 0.1%, a cumulative rolling reduction in the non-recrystallization temperature range of 40% or more, and a hot rolling end temperature of 860 ° C. or less. The method for producing a high-tensile steel sheet having excellent delayed fracture resistance according to any one of claims 1 to 4, wherein the yield stress is 685 MPa or more and the tensile strength is 780 MPa or more.   The metal structure of the high-tensile steel sheet has a total structure fraction of martensite and lower bainite of 95 area% or more, the equivalent circle diameter of carbide at the lath interface in the metal structure is 100 nm or less, and the carbide coverage is 30% or less. The method for producing a high-strength steel sheet having excellent delayed fracture resistance, wherein the yield stress is 685 MPa or more and the tensile strength is 780 MPa or more, according to any one of claims 1 to 5.