JP6051735B2 - Method for producing high-tensile steel sheet with excellent weldability and delayed fracture resistance - Google Patents

Description

  The present invention relates to a method for producing a high-strength steel sheet having excellent weldability and delayed fracture resistance, and particularly relates to a suitable high-tensile steel material having a tensile strength of 1180 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 order to cope with the increase in strength of such steel materials, Patent Document 1 proposes a method of manufacturing a steel sheet by quenching and tempering with an increased alloy component.
In response to the increase in demand, Patent Documents 2, 3, and 4 propose 780 MPa class high-tensile thick steel plates that omit the tempering treatment.
It is known that increasing the strength of steel materials increases the hydrogen embrittlement susceptibility of steel materials. For example, in the field of high-strength bolts, F11T class bolts (tensile strength of 1100 to 1300 MPa) are not used as much as possible in JISB1186. The use of high-strength steel is limited, as described. For this reason, Patent Documents 5 and 6 disclose a method of manufacturing a steel sheet having excellent hydrogen embrittlement resistance using various techniques such as fine dispersion of carbides.

Japanese Patent No. 4174401 JP 2007-277622 A JP 2007-277623 A JP 2009-263774 A JP 2006-206942 A JP 2007-009324 A

  The technique described in Patent Document 1 is a method of manufacturing a steel sheet that is hardened and tempered by increasing the alloy components. Since many alloys are added, there is a problem that the manufacturing cost is high and the weldability is low.

  The high-strength steel sheets that omit the tempering treatment described in Patent Documents 2, 3, and 4 have a problem that the tensile strength is about 780 MPa, and the demand for higher-strength steel materials cannot be met.

  Moreover, when trying to obtain a high-strength steel sheet having a tensile strength of 1180 MPa or more by the methods described in Patent Documents 5 and 6, the tempering temperature is high, the productivity is inferior, and many alloys need to be added. There is a problem that weldability deteriorates.

  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 plate having a tensile strength of 1180 MPa or more and superior in weldability and 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 1180 MPa or more, weldability, and delayed fracture resistance, and as a result, ascended at the center in the thickness direction of the steel during tempering. By defining the temperature rate, tempering temperature, and holding time, the formation and coarsening of carbides can be effectively suppressed, and the accumulation of diffusible hydrogen into carbides can be suppressed. It has been found that it is possible to obtain a high-tensile steel sheet having excellent fracture characteristics.

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

[1] By mass%, C: 0.10 to 0.20%, Si: 0.01 to 0.6%, Mn: 0.5 to 2%, P: 0.02% or less, S: 0.00. 003% or less, Al: 0.005-0.1%, N: 0.0005-0.008%, the carbon equivalent Ceq shown in the following formula (1) is 0.60% or less, the following formula (2 ) weld cracking susceptibility parameter Pcm shown in is not more than 0.30%, the steel having a component composition and the balance being Fe and unavoidable impurities was heated to above Ac ₃ transformation point, a cumulative reduction in the pre-recrystallization temperature region After performing hot rolling at a rate of 30% or more, finishing hot rolling at or above the Ar ₃ transformation point, and subsequently cooling from the Ar ₃ transformation point to a temperature of 250 ° C. or less at a cooling rate of 10 ° C./s or more. Re-heating at an average temperature increase rate of 1 ° C./s or more, and tempering the highest temperature within the range of 100 to 400 ° C. Method for producing a high tensile steel sheet having excellent weldability and delayed fracture resistance, characterized in that performing management.

  [2] In addition to the steel, Mo: 1% or less, Nb: 0.1% or less, V: 0.5% or less, Ti: 0.03% or less, Cu: 2% or less, Ni : High tensile strength excellent in weldability and delayed fracture resistance according to [1] above, comprising at least one selected from: 4% or less, Cr: 2% or less, W: 2% or less A method of manufacturing a steel sheet.

  [3] One or more kinds selected from B: 0.0003% to 0.003%, Ca: 0.01% or less, REM: 0.02% or less are further added to the steel in mass%. The method for producing a high-strength steel sheet having excellent weldability and delayed fracture resistance as described in [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 maximum temperature in the tempering process is set to 60 s or less. The method for producing a high-tensile steel plate excellent in weldability and delayed fracture resistance according to any one of [1] to [3].

  [5] The metal structure of the high-tensile steel sheet is mainly composed of martensite, the equivalent circle diameter of carbide at the lath interface in the metal structure is 100 nm or less, and the carbide coverage is 20% or less. [1] A method for producing a high-tensile steel sheet excellent in weldability and delayed fracture resistance according to any one of [4].

  By using the present invention, it is possible to stably produce a high-tensile steel having a high tensile strength of 1180 MPa or more and excellent weldability and delayed fracture resistance 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: 0.10 to 0.20%
C is an indispensable element for obtaining the strength required for structural steel. However, if it is less than 0.10%, sufficient strength cannot be obtained, and a large amount of alloying element is required, so that weldability is required. Decreases. If the addition exceeds 0.20%, the amount of martensite produced in the weld heat affected zone increases and the toughness is reduced, so the C content is in the range of 0.10 to 0.20%. Preferably it is 0.12 to 0.18% of range. More preferably, it is 0.12 to 0.17% of range.

Si: 0.01 to 0.6%
Si is added for deoxidation, but if it is added less than 0.01%, the deoxidation effect is not sufficient, and if added over 0.6%, 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 0.6%. 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 1.4% of range.

P: 0.02% or less When P exceeds 0.02%, the toughness of the base metal and the weld heat-affected zone is remarkably reduced, so the P content is 0.02% or less. Preferably it is 0.01% 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. Preferably it is 0.001% 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 0.0050% of range.

Ceq: 0.60% or less In the present invention, the upper limit of the carbon equivalent Ceq is set to 0.60% or less from the viewpoint of achieving both strength and weldability. Preferably it is 0.45 to 0.60% of range, more preferably 0.50 to 0.60% of range.
Here, Ceq is obtained by the following equation (1).

Pcm: 0.30% or less The weld crack susceptibility index Pcm value is set to 0.30% or less in order to reduce the preheating temperature during welding and to make the steel material easy to construct. Preferably it is 0.20 to 0.30% of range, and more preferably is 0.25 to 0.30% of range.
Here, Pcm is obtained by the following formula (2).

  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: 1% or less Mo is an element effective for increasing the strength of the base material, but if added over 1%, it causes an increase in hardness due to precipitation of alloy carbide and decreases toughness, so Mo is added. In this case, the Mo amount is preferably 1% or less. More preferably, it is 0.2 to 0.8% of range.

Nb: 0.1% or less Nb is an element effective for strengthening steel, but addition exceeding 0.1% significantly reduces the toughness of the base metal. It is preferable to make it 1% or less. More preferably, it is 0.04% or less.

V: 0.5% or less 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, but if added over 0.5% Since toughness is reduced by the precipitation of hard VC, when V is added, the V content is preferably 0.5% or less.

Ti: 0.03% or less Ti generates TiN during rolling heating or welding, effectively suppressing austenite coarsening, and improves the toughness of the base material and the weld heat affected zone. However, if added over 0.03%, the nitride becomes coarse and lowers the toughness of the base metal and the weld heat affected zone. Therefore, when adding Ti, the Ti content may be 0.03% or less. preferable. 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%, the steel plate surface is cracked during hot rolling. The amount 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.

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 0.4 to 1.2% 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 metal structure in this invention is demonstrated.

  In order to increase the tensile strength of 1180 MPa or more, the metal structure needs to be a structure mainly composed of martensite. In order to achieve both strength and toughness, the martensite + lower bainite structure fraction is 95% or more. It is preferable that 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 produced at the lath interface of martensite to 100 nm or less, the amount of carbide in the lath interface (hereinafter referred to as carbide coverage) should be 20% 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 carbide formed at the lath interface of martensite, after etching with an appropriate corrosive solution such as nital, take a structure photograph with 10 fields of view with a scanning electron microscope. Determined using analysis. 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 30% or more, and terminates the 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 30% or more and preferably 70% or less. This is because if it exceeds 70%, it is difficult to finish the hot rolling at the Ar ₃ transformation point or higher.

  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%.

As the Ar ₃ transformation point, a value calculated by the following formula (3) is used.

As the Ac ₃ transformation point, a value calculated by the following equation (4) 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 After hot rolling is completed, forced cooling is performed from a temperature above the Ar ₃ transformation point to a temperature below 250 ° C. at a cooling rate of 10 ° C./s or more in order to ensure the base metal strength and toughness.

  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 range is a temperature range in which a soft phase such as ferrite or pearlite is likely to appear, and in order to obtain a high-strength martensite structure, it is necessary to cool this temperature range quickly.

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. By making the average heating rate 1 ° C / s or more and tempering temperature 400 ° C or less, the size of carbides generated between the laths is effectively refined, strength, toughness and delayed fracture resistance compared to conventional steel It becomes possible to obtain a steel plate having excellent characteristics.

  On the other hand, when the heating temperature is less than 100 ° C., it is difficult to ensure toughness stably, so the lower limit heating temperature is set to 100 ° C. or higher.

  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. In addition, an average temperature increase rate is the value which divided the temperature increase amount required for reheating to reheating temperature (100-400 degreeC) after cooling by the time required for reheating.

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

  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.

  Twenty kinds of steel types A to T having 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 manufacturing conditions, and the yield strength, tensile strength, fracture surface transition temperature (vTrs), preheating temperature, and delayed fracture safety index of the obtained steel sheet.

  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 at tempering temperature, 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.

  The toughness was evaluated by a fracture surface transition temperature (vTrs) obtained by a Charpy impact test using a test piece taken from 1/4 part of the plate thickness after collecting an impact test piece specified in JIS Z 2242.

  Weldability was evaluated by a preheating temperature necessary for preventing root cracking by performing coated arc welding with a heat input of 17 kJ / cm by a method prescribed in JIS Z 3158.

The delayed fracture safety index is obtained by charging a test specimen surface with zinc after charging hydrogen so that the amount of diffusible hydrogen in the test specimen is about 0.5 massppm using a cathodic hydrogen charging method using a rod-shaped specimen. 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 a test piece that is not charged with hydrogen at the same strain rate is used. A tensile test was also conducted 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 1100 MPa or higher, a tensile strength of 1180 MPa or higher, a vTrs of −40 ° C. or lower, and a necessary preheating temperature of 150 ° C. or lower. The target for the delayed fracture resistance index was set to 70% or more.

  The structure of the steel sheet was quantified by evaluating the average value of the measured values obtained by image analysis by revealing the structure with a nital corrosion solution 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 Nos. 14, 21 to 25, and 27 to 29, the equivalent circle diameter and the coverage of the carbide were not measured.
In addition, the structure of the steel sheet was quantified by evaluating the average value of the structure around the 1/4 thickness of the steel sheet, which was revealed with a nital etchant, observed with an optical microscope for five fields of view.

  As shown in Table 2, Invention Example No. manufactured by the method of the present invention was used. 1-No. No. 12 has a chemical composition and a production method within the scope of the present invention, and a steel sheet having good strength, weldability and delayed fracture resistance can be obtained.

  On the other hand, comparative example No. in which a component remove | deviates from this invention. 13-No. In No. 20, any one or more of the properties of strength, toughness, and preheating temperature does not reach the target value. In addition, Comparative Example No. in which the manufacturing conditions deviate from the present invention. 21-No. As for 30 steel plates, any one or more of strength, toughness, preheating temperature and delayed fracture resistance does not reach the target value.

Claims (5)

In mass%, C: 0.10 to 0.20%, Si: 0.01 to 0.6%, Mn: 0.5 to 2%, P: 0.02% or less, S: 0.003% or less , Al: 0.005 to 0.1%, N: 0.0005 to 0.008%, the carbon equivalent Ceq shown in the following formula (1) is 0.60% or less, and shown in the following formula (2) A steel having a weld crack susceptibility index Pcm of 0.24% or more and 0.30% or less, the balance of which is composed of Fe and inevitable impurities, is heated to the Ac ₃ transformation point or higher, and in a non-recrystallization temperature range. the cumulative rolling reduction subjected to hot rolling to 30% or more, and terminates the hot rolled at Ar ₃ transformation point or higher, subsequently to a temperature of 250 ° C. or less with a cooling rate higher than 10 ° C. / s from Ar ₃ transformation point or more after cooling, reheated at 1 ° C. / s or more an average heating rate, and the range of 100 to 400 ° C. the highest temperature The maximum retention time in the ultimate temperature and performing a tempering treatment to within 60s, the production method of the tensile high tensile steel strength with excellent weldability and delayed fracture resistance is not less than 1180 MPa.

In addition to the steel, by mass%, Mo: 1% or less, Nb: 0.1% or less, V: 0.5% or less, Ti: 0.03% or less, Cu: 2% or less, Ni: 4% The weld strength and delayed fracture resistance according to claim 1 , wherein the tensile strength is 1180 MPa or more, characterized by containing at least one selected from Cr: 2% or less and W: 2% or less. An excellent method for producing high-tensile steel sheets. One or more selected from B: 0.0003% to 0.003%, Ca: 0.01% or less, and REM: 0.02% or less are further added to the steel in mass%. The method for producing a high-tensile steel sheet excellent in weldability and delayed fracture resistance according to claim 1 , wherein the tensile strength is 1180 MPa or more . Characterized the TURMERIC line using a heating device installed reheating to the rolling machine and the cooling device and the same production line on after rolling, the tensile strength in any of claims 1 to 3 is not less than 1180MPa A method for producing a high-tensile steel sheet having excellent weldability and delayed fracture resistance. 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 20% or less. The method for producing a high-tensile steel sheet having excellent weldability and delayed fracture resistance according to any one of claims 1 to 4 , wherein the tensile strength is 1180 MPa or more .