JP6164171B2 - Low yield ratio high strength steel sheet with excellent high temperature strength and weldability and method for producing the same - Google Patents

Description

  The present invention relates to a low-yield-ratio high-tensile steel plate excellent in fire resistance used for various buildings in the field of construction and civil engineering, and a method for producing the same.

  For high-rise buildings with steel structures, use to ensure seismic design with the transition from elastic design (allowable stress design) to ultimate strength design based on the new seismic design standards implemented in June 1981 From the viewpoint of steel materials, a low yield ratio and a narrow YP range are required. For fires, the fireproof coating applied to the structural members to ensure safety in the event of a fire reduces the fireproof coating from the viewpoint of cost reduction and aesthetics of the building structure. Omission of coating (no coating) is desired, and a steel material having a high-temperature strength of 2/3 or more of the normal temperature strength standard value at 600 ° C. is required as a steel material for welded structures that does not require a fireproof coating.

  As a steel material for construction having such a low yield ratio and fire resistance, Patent Document 1 discloses that "in mass%, C: 0.05 to 0.15%, Si: 0.6% or less, Mn: 0 0.8% or less, P: 0.02% or less, S: 0.01% or less, Mo: 0.7 to 1.2%, Al: 0.06% or less, N: 0.006% or less, the balance being Consisting of iron and unavoidable impurities, the microstructure has an area fraction of 80% or more other than polygonal or pseudopolygonal ferrite, the average equivalent circle diameter of the old γ grains is 150 μm or less, and specific amounts of Cu, Ni, Cr , Nb, V, B, Ti, Mg, Ca, REM and high strength steel excellent in high temperature strength ”.

  Patent Document 2 states, “By weight, C: 0.04 to 0.15%, Si: 0.6% or less, Mn 0.5 to 1.6%, Nb: 0.005 to 0.04%, Mo: 0.4 to 0.7%, Al: 0.1% or less, N: 0.001 to 0.006%, the steel slab consisting of iron and unavoidable impurities in the temperature range of 1100 to 1300 ° C. "The manufacturing method of the low yield ratio steel sheet for construction with excellent fire resistance, in which hot rolling is finished in a temperature range of 800 to 1000 ° C after heating" is described.

Japanese Patent Laid-Open No. 2002-3985 Japanese Patent Laid-Open No. 2-77523

  However, in the techniques described in Patent Documents 1 and 2, it is necessary to contain Mo in order to obtain high-temperature strength, and it is necessary to increase the Mo content as the required fire resistance temperature increases. However, the addition of a large amount of Mo has a problem that the toughness of the base metal and the welded portion is significantly deteriorated in addition to the remarkable deterioration of the weldability.

  The present invention has been made in view of such a conventional technique, and has a high temperature strength and weldability in which a tensile strength at normal temperature is 490 MPa or more, a yield ratio is 80% or less, and a yield strength at 600 ° C. is 217 MPa or more. The goal is to provide an excellent low yield ratio high strength steel sheet.

  The inventor found that the balance between the soft phase (usually ferrite) governing yield and the hard phase (pearlite, bainite) to ensure tensile strength and the base material and welded part by adding a large amount of Mo to increase the high temperature strength As a result of various studies on toughness degradation of the present invention, the present invention has been achieved.

[1] Component composition is mass%, C: 0.04 to 0.12%, Si: 0.50% or less, Mn: 0.50 to 1.50%, P: 0.02% or less, S : 0.0050% or less, Al: 0.050% or less, Mo: 0.40 to 0.70%, V: 0.070 to 0.120%, Cr: 0.05 to 0.40% Further, Ceq defined by the following formula (1) satisfies 0.46 or less, the balance is Fe and unavoidable impurities, and ferrite is a main phase with an area ratio of 60% or more. Further, the average crystal of ferrite A low-yield ratio high-tensile steel sheet excellent in high-temperature strength and weldability, characterized in that the grain size is 5 to 70 μm and the yield strength at 600 ° C. is 217 MPa or more.
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14
(1)
Here, C, Si, Mn, Cr, Ni, Mo, and V represent the content (% by mass) of each element, and 0 when not contained.

  [2] Component composition is further mass%, Cu: 0.05-0.30%, Ni: 0.05-0.30%, Ti: 0.005-0.020%, Nb: 0.005 The low yield ratio high-tensile steel sheet excellent in high-temperature strength and weldability according to [1], which contains at least one of ˜0.030% and B: 0.0003 to 0.0030%.

  [3] The component composition further includes one or more of Ca: 0.0005 to 0.0050% and REM: 0.0010 to 0.0050% by mass% [1] or [1] [2] A low-yield ratio high-tensile steel plate excellent in high-temperature strength and weldability.

[4] After heating the steel material having the composition described in any one of [1] to [3] to 1050 to 1200 ° C., the cumulative reduction amount in the temperature range of 950 ° C. or less at the surface temperature is 30% or more and 50 subjected to hot rolling of less than%, and the rolling end temperature is at 800 ° C. or less than _Ar 3 -100 ° C., then air-cooled to 300 ° C. or less, further, performing tempering at a temperature of 700 ° C. 400 ° C. or higher A method for producing a low-yield ratio high-strength steel sheet excellent in high-temperature strength and weldability.

  According to the present invention, a steel sheet having a tensile strength of 490 MPa or higher, a yield ratio of 80% or lower, a yield strength of 217 MPa or higher at 600 ° C., and a root crack can be obtained in an oblique y-type weld crack test at normal temperature.

  Hereinafter, the reasons for limiting the constituent requirements of the present invention will be described.

1. About component composition Hereinafter, the reason for limitation of the component in this invention is demonstrated. In addition, unless otherwise indicated, the% display of each element means the mass%.

C: 0.04 to 0.12%
C is an element useful for increasing the strength of steel and ensuring the strength at room temperature necessary as a structural steel material. Further, C is useful for forming a fine carbide with Mo or the like in a temperature range near 600 ° C. and ensuring the strength at 600 ° C. Furthermore, C has an effect of increasing the volume fraction of a hard phase such as bainite and lowering the yield ratio. In order to obtain such an effect, a content of 0.04% or more is required. On the other hand, if the content exceeds 0.12%, weldability and toughness are significantly reduced. For this reason, C amount is taken as 0.04 to 0.12% of range. Preferably it is 0.07 to 0.10% of range.

Si: 0.50% or less Si acts as a deoxidizer and dissolves in steel to increase the strength of the steel material. If the content exceeds 0.50%, the toughness of the base metal is reduced and the weld heat affected zone (also referred to as HAZ) toughness is remarkably reduced, so the Si content is 0.50% or less. In addition, it aims at giving desired intensity | strength to a steel plate, Preferably it is 0.20 to 0.40% of range.

Mn: 0.50 to 1.50%
Mn is an element that has the effect of increasing the strength of steel by solid solution and is inexpensive, and in the present invention, which aims to minimize the content of other expensive alloy elements, the desired high strength In order to ensure (normal temperature tensile strength 490 MPa or more), 0.50% or more of content is required. On the other hand, if the content exceeds 1.50%, the toughness and the HAZ toughness of the base material are significantly reduced. For this reason, the amount of Mn shall be 0.50 to 1.50% of range. In addition, Preferably it is 0.60 to 1.00% of range.

P: 0.02% or less P is an element having an action of increasing the strength of steel, but is an element that lowers toughness, particularly toughness of a welded portion. If the content exceeds 0.02%, the above-described adverse effects become significant, so the P content is 0.02% or less. Preferably it is 0.01% or less.

S: 0.0050% or less S is present in steel as sulfide inclusions such as MnS, which deteriorates the toughness of the base metal and the welded portion, and is unevenly distributed in the center segregated portion of the slab. It makes it easier to generate defects in pieces. Such a tendency becomes remarkable when the content exceeds 0.0050%. For this reason, the amount of S is made into 0.0050% or less. Preferably it is 0.0030% or less.

Al: 0.050% or less Al is an element that acts as a deoxidizer, and is most commonly used as a deoxidizer in a molten steel deoxidation process for high-strength steel. In order to obtain such an effect, it is desirable to contain 0.010% or more. However, if the content exceeds 0.050%, the toughness of the base material decreases, and the weld metal is mixed into the weld metal during welding. Reduce toughness. For this reason, Al is made into 0.050% or less. Preferably it is 0.010 to 0.045% of range.

Mo: 0.40 to 0.70%
Mo is an indispensable element for forming fine carbides in the temperature range near 600 ° C. and ensuring the high temperature strength of the steel. In particular, in order to stably obtain a yield strength of 217 MP or more at 600 ° C., a content of 0.40% or more is necessary. However, the content exceeding 0.70% deteriorates weldability and HIC resistance. Therefore, the Mo amount is set to a range of 0.40 to 0.70%. Preferably it is 0.45 to 0.65% of range. More preferably, it is 0.49 to 0.62% of range.

V: 0.070 to 0.120%
V is an element effective for increasing the strength by precipitation strengthening, and is important for forming a fine carbide in a temperature range near 600 ° C. and ensuring high temperature strength. In order to acquire such an effect, 0.070% or more of content is required. However, if it exceeds 0.120%, HAZ toughness and base metal toughness deteriorate. Therefore, the V amount is in the range of 0.070 to 0.120%. Preferably it is 0.070 to 0.110% of range. More preferably, it is 0.080 to 0.100% of range.

Cr: 0.05-0.40%
Cr is an element that increases the strength of the base material through improvement in hardenability, and is an element useful for increasing the strength of thick steel plates. In order to obtain such an effect, a content of 0.05% or more is necessary, but a content exceeding 0.40% causes an increase in alloy cost. For this reason, the Cr content is in the range of 0.05 to 0.40%. In addition, Preferably it is 0.10 to 0.40% of range. More preferably, it is 0.20 to 0.40% of range.

Ceq: 0.46 or less When Ceq exceeds 0.46, weldability decreases, and base metal toughness and HAZ toughness decrease. For this reason, Ceq is set to 0.46 or less. Ceq is based on the following equation.
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14
(1)
Here, C, Mn, Cr, Mo, V, Cu, and Ni represent the content (% by mass) of each element, and 0 when not contained.

  The above is the basic chemical component of the present invention, and the balance is composed of Fe and unavoidable impurities, but one or more selected from Cu, Ni, Ti, Nb, and B are added as selective elements for the purpose of further increasing the strength. May be.

Cu: 0.05-0.30%
Cu increases the strength of the steel sheet through solid solution strengthening and hardenability improvement, and contributes to increasing the strength of the thick steel sheet. In order to acquire such an effect, it is preferable to contain 0.05% or more, but inclusion exceeding 0.30% causes an increase in alloy cost and deterioration of surface properties due to hot brittleness. For this reason, when it contains, it is preferable to make Cu amount into the range of 0.05 to 0.30%. In addition, More preferably, it is 0.05 to 0.20% of range.

Ni: 0.05-0.30%
Ni is an element that increases the strength of the steel sheet with little deterioration in toughness, and has a small adverse effect on the HAZ toughness, and is an element useful for increasing the strength of thick steel sheets. In order to obtain such an effect, the content is preferably 0.05% or more. However, if the content exceeds 0.30%, Ni is an expensive element, which causes an increase in alloy costs. For this reason, when it contains, it is preferable to make Ni amount into the range of 0.05 to 0.30%. In addition, More preferably, it is 0.05 to 0.20% of range.

Ti: 0.005-0.020%
Ti is an element having a strong affinity for N, and precipitates as TiN during solidification, thereby reducing solid solution N in the steel and reducing the toughness deterioration due to strain aging of N after cold working. Ti also contributes to the improvement of HAZ toughness through the improvement of the HAZ structure. In order to acquire such an effect, it is preferable to contain 0.005% or more. On the other hand, if the content exceeds 0.020%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, when it contains, it is preferable to make Ti amount into the range of 0.005-0.020%. In addition, More preferably, it is 0.007 to 0.015% of range.

Nb: 0.005 to 0.030%
Nb is an element that increases hardenability and increases the strength of the base metal through the action of increasing the effect of controlled rolling and refining the microstructure, and is a useful element for increasing the strength. In order to acquire such an effect, it is necessary to contain 0.005% or more. On the other hand, the content exceeding 0.030% reduces the toughness of the base material and the HAZ. For this reason, when it contains, it is preferable to make Nb amount into 0.005 to 0.030% of range. In addition, More preferably, it is 0.007 to 0.025% of range.

B: 0.0003 to 0.0030%
B is an element that contributes to an increase in the strength of steel through the improvement of hardenability. In order to acquire such an effect, 0.0003% or more is contained, but inclusion exceeding 0.0030% deteriorates the HAZ toughness of the base material. For this reason, when it contains, it is preferable to make B amount into the range of 0.0003 to 0.0030%. In addition, More preferably, it is 0.0006 to 0.0020% of range.

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

Ca: 0.0005 to 0.0050%, REM: 0.0010 to 0.0050%
Both Ca and REM (rare earth metal) contribute to improvement of the toughness and ductility of the base material through the form control of the sulfide. Moreover, Ca and REM (rare earth metal) produce fine sulfide particles in the steel. These sulfide particles contribute to the improvement of HAZ toughness by acting as ferrite transformation nuclei. In order to obtain these effects, Ca preferably contains at least 0.0005% and REM contains at least 0.0010%. However, when both Ca and REM contain more than 0.0050%, inclusions are excessively generated, and the toughness may be reduced. For this reason, when it contains, it is preferable to make Ca amount into the range of 0.0005 to 0.0050%, and REM into the range of 0.0010 to 0.0050%.

  The high-tensile steel sheet of the present invention has ferrite as a main phase and an average crystal grain size of ferrite of 5 to 70 μm.

Ferrite: main phase (area ratio 60% or more)
In order to satisfy the desired strength and yield ratio, ferrite is the main phase. The balance other than ferrite is one or two of bainite and pearlite. Bainite and pearlite have higher hardness than ferrite, and both are hard phases with respect to ferrite. When the area ratio of ferrite is less than 60%, the area ratio of the hard phase increases and the yield strength increases, so that a desired low yield ratio cannot be obtained. For this reason, the high-tensile steel sheet of the present invention uses ferrite as the main phase with an area ratio of 60% or more. Here, the main phase means a phase having an area ratio of 60% or more with respect to the entire structure of the steel sheet. In addition, when a low-temperature transformation phase such as bainite is used as the main phase instead of ferrite, a desired low yield ratio cannot be obtained and a desired high-temperature strength cannot be obtained.

  A low-temperature transformation phase such as bainite has a high dislocation density by transformation at a low temperature, and has a structure in which fine carbides (mainly cementite which is an iron carbide) are dispersed. For this reason, when the low temperature transformation phase is heated to a temperature range near 600 ° C., the strength of the low temperature transformation phase is significantly reduced due to recovery of dislocations and dissolution of carbides. For this reason, also from a viewpoint of ensuring high temperature strength, the high-tensile steel sheet of the present invention has ferrite as a main phase.

Average crystal grain size of ferrite: 5 to 70 μm
If the average crystal grain size of ferrite is less than 5 μm, the yield strength becomes too high due to the strengthening of crystal grains, and a desired low yield ratio cannot be obtained. On the other hand, if the average crystal grain size of ferrite exceeds 70 μm, the structure is coarse and the toughness deteriorates. Therefore, the average crystal grain size of ferrite is set to 5 to 70 μm.

2. Production Conditions Steel having the above-described component composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and is formed into a slab by a conventional method such as a continuous casting method or an ingot-making agglomeration method. Then, it heat-rolls, air-cools after rolling, and then performs tempering. The melting method and the casting method are not limited to the methods described above.

Heating temperature: 1050-1200 ° C
When the heating temperature is less than 1050 ° C., the carbide is not completely dissolved and the solid solution C is insufficient, so that the strength tends to decrease. On the other hand, when heating temperature exceeds 1200 degreeC, a structure will coarsen and the toughness of a steel plate will fall. Moreover, hardenability increases and surface hardness increases. For this reason, the heating temperature of a steel raw material shall be the range of 1050 degreeC-1200 degreeC. In addition, Preferably it is the range of 1080 to 1150 degreeC.

Cumulative rolling reduction in a temperature range of 950 ° C. or less at the surface temperature: 30% or more and less than 50% Control rolling is performed in a temperature range where the surface temperature of the steel sheet is 950 ° C. or less in order to refine the microstructure appropriately. In the temperature range exceeding 950 ° C., recrystallization and grain growth of the austenite crystal grains extended by rolling occur immediately after rolling, so that rolling in the temperature range exceeding 950 ° C. hardly contributes to the refinement of crystal grains. For this reason, the cumulative amount of reduction in the temperature range of 950 ° C. or lower is specified. When the cumulative rolling reduction in the temperature range is less than 30%, the structure becomes coarse, hardenability increases, and surface hardness increases. Moreover, desired toughness cannot be ensured. For this reason, the cumulative rolling reduction in the temperature range of 950 ° C. or less at the surface temperature is set to 30% or more. Further, when the cumulative rolling reduction in the temperature range of 950 ° C. or lower is 50% or more, the structure is excessively refined, and the yield strength is increased by strengthening the grain refinement, so that a desired low yield ratio cannot be ensured. For this reason, the cumulative rolling reduction is set to less than 50%. In addition, Preferably they are 35% or more and 45% or less.

Rolling end temperature (surface temperature): Ar ₃ −100 ° C. or more and 800 ° C. or less Rolling the steel plate, introducing dislocations into the steel plate, and imparting strain can increase the high temperature strength of the steel plate. Further, this effect becomes more remarkable as the rolling end temperature is lowered. Therefore, in order to ensure a desired high temperature strength, the rolling end temperature is set to 800 ° C. or less. In addition, since the rolling efficiency will be remarkably deteriorated if the rolling end temperature is too low, the temperature is set to Ar ₃ -100 ° C or higher.

Cooling after rolling: Air cooling Cooling after hot rolling is air cooling. This is because the structure of the steel sheet has ferrite as the main phase. Air cooling refers to cooling by leaving in the atmosphere, and forcibly cooling by blowing air onto a steel plate with a blower or the like is not included in the air cooling in this case. The cooling rate during air cooling is 1 ° C./s or less, usually about 0.5 ° C./s. When cooling at a cooling rate exceeding air cooling, phases other than ferrite such as bainite increase, and a steel sheet having ferrite as a main phase cannot be obtained. Moreover, since the steel plate of the present invention is rolled at a very low rolling end temperature, a high residual stress is generated in the steel plate after the end of rolling. For this reason, if cooling is performed by a cooling method other than air cooling (water cooling or the like), the stress distribution in the steel sheet may be further uneven, and the steel sheet may be deformed.

Steel plate temperature after air cooling: 300 ° C. or less Rolled and then cooled to 300 ° C. or less by air cooling. This is because, before tempering, it is necessary to complete the ferrite transformation and to have a structure in which ferrite is the main phase. When tempering is performed in a state where the ferrite transformation is not completed and austenite remains, C is concentrated in the austenite. Thus, when C concentrates in austenite, the austenite enriched in C becomes martensite by cooling after tempering, and the toughness of the steel sheet is remarkably lowered.

Tempering temperature: 400 ° C. or higher and lower than 700 ° C. As described above, a high residual stress is generated in the steel sheet after the rolling is completed, and this residual stress remains in the steel sheet after air cooling. For this reason, tempering is necessary to remove the distortion (residual stress) of the steel sheet. In order to acquire this effect, it is necessary to carry out at 400 degreeC or more, but since 700 degreeC or more causes the fall of intensity | strength and a raise of a yield ratio, it needs to be less than 700 degreeC.

  Steel sheets (steel Nos. A to K) having the composition shown in Table 1 were made into slabs by a continuous casting method to produce a thick steel plate (N 0.1 to 18) having a plate thickness of 30 mm or 40 mm. The Ar3 temperature of each steel was determined by measuring the thermal expansion curve.

  The heated slab was rolled by hot rolling, cooled to 300 ° C. or lower by air cooling, and tempered using an induction heating furnace.

  Table 2 shows the production conditions of each steel plate (N0.1-18). The heating temperature, rolling end temperature, cooling start, post-cooling temperature, and tempering temperature were the steel sheet surface temperature.

Test pieces were collected from the thick steel plates produced as described above, the microstructure was observed, and the tensile properties at room temperature and high temperature were measured. In addition, the base metal toughness and weldability of the steel sheet were evaluated.

Microstructure observation A 90 ° direction (C direction) cross section was cut from a thick steel plate, polished to a mirror surface, and corroded with nital (1% concentrated nitric acid methanol solution) to reveal a structure. At a 1/4 thickness position, a 400 times photograph was taken with an optical microscope, and the tissue was identified. Further, the area ratio of ferrite and the average crystal grain size were determined by image analysis of the structure photograph. The ferrite average crystal grain size is a circle equivalent grain size.

Room temperature tensile test JIS No. 5 tensile test specimen was sampled in 90 ° direction (C direction) with respect to the rolling direction, and tensile test was conducted at a crosshead speed of 10 mm / min in accordance with the provisions of JIS Z2241, yield stress (YS) The tensile strength (TS) was measured. A tensile strength (TS) of 490 MPa or more and a yield ratio (YS / TS) of 80% or less can be evaluated as good.

High temperature tensile test Specimens were sampled in the 90 ° direction (C direction) with respect to the rolling direction, subjected to a high temperature tensile test at a test temperature of 600 ° C. in accordance with JIS G 0567, and yield stress (YS) was measured. . A yield stress (YS) of 217 MPa or more can be evaluated as good.

Base metal toughness A Charpy impact test was conducted in accordance with JIS Z 2242. A V-notch Charpy impact test piece (width 10 mm) was taken so that the 90 ° direction (C direction) was the longitudinal direction with respect to the rolling direction of the steel sheet, an impact test was performed at 0 ° C., and the absorbed energy was measured. When the absorbed energy vE0 at 0 ° C. was 47 J or more, the base material toughness was evaluated as good.

Weldability evaluation (y-type weld cracking test)
A y-type weld cracking test was performed in accordance with JIS Z 3158 to evaluate weldability. Preheating temperature and ambient temperature are 5 ° C., welding conditions: current: 240 A, voltage: 28 V, welding speed: CO ₂ welding at 24 cm / min. The presence or absence of cracks is observed, and if there are no cracks, it can be evaluated as good.

  The test results are shown in Table 2.

  In all of the inventive examples, the tensile strength (TS) at room temperature is 490 MPa or more, the yield ratio is 80% or less, the tensile strength (TS) at 600 ° C. is 217 MPa or more, and vE0 is 47 J or more. It exhibits strength and base metal toughness and achieves a low yield ratio. Furthermore, all of the inventive examples have excellent weldability without generating cracks in the y-type weld crack test. On the other hand, the comparative example does not satisfy any target performance of yield ratio, high temperature strength, base metal toughness, and weldability.

Claims (4)

The component composition is mass%, C: 0.04 to 0.12%, Si: 0.50% or less, Mn: 0.50 to 1.50%, P: 0.02% or less, S: 0.00. 0050% or less, Al: 0.050% or less, Mo: 0.40 to 0.70%, V: 0.070 to 0.120%, Cr: 0.05 to 0.40%, and further below Ceq defined by the formula (1) satisfies 0.46 or less, consists of the balance Fe and unavoidable impurities, ferrite is the main phase of 60% or more in area ratio, and the average crystal grain size of ferrite is A low-yield ratio high-strength steel sheet excellent in high-temperature strength and weldability, characterized by having a yield strength of 5 to 70 μm and a yield strength at 600 ° C. of 217 MPa or more.
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Ni / 40 + Mo / 4 + V / 14
(1)
Here, C, Si, Mn, Cr, Ni, Mo, and V represent the content (% by mass) of each element, and 0 when not contained.   The component composition is further mass%, Cu: 0.05-0.30%, Ni: 0.05-0.30%, Ti: 0.005-0.020%, Nb: 0.005-0. The low yield ratio high tensile strength steel sheet excellent in high temperature strength and weldability according to claim 1, comprising at least one of 030% and B: 0.0003 to 0.0030%.   The component composition further comprises one or more of Ca: 0.0005 to 0.0050% and REM: 0.0010 to 0.0050% in terms of mass%. Low-yield ratio high-tensile steel sheet with excellent high-temperature strength and weldability. A method for producing a low-yield ratio high-tensile steel sheet excellent in high-temperature strength and weldability according to any one of claims 1 to 3 ,
After heating a steel material to 1050 to 1200 ° C., cumulative reduction ratio in the temperature range of 950 ° C. or less at a surface temperature makes a hot rolling less than 30% 50%, and rolling end temperature is _Ar 3 -100 ° C. The low yield ratio high-tensile steel sheet excellent in high-temperature strength and weldability, characterized by being air-cooled to 800 ° C. or lower, then air-cooled to 300 ° C. or lower, and further tempered at a temperature of 400 ° C. to 700 ° C. Production method.