JP5092498B2 - Low yield ratio high strength high toughness steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a low-yield-ratio, high-strength, high-toughness steel sheet suitable for use mainly in the field of line pipes and a method for producing the same.

  In recent years, steel materials for welded structures are required to have a low yield ratio and high uniform elongation from the viewpoint of earthquake resistance in addition to high strength and high toughness. In general, by making the metal structure of steel a structure in which hard phases such as bainite and martensite are moderately dispersed in a soft phase like ferrite, low yield ratio and high uniform elongation of steel are achieved. It is known to be possible.

  As a production method for obtaining a structure in which a hard phase is appropriately dispersed in the soft phase as described above, quenching from a two-phase region of ferrite and austenite (Q ′) between quenching (Q) and tempering (T). There is known a heat treatment method for applying (see, for example, Patent Document 1).

  In this heat treatment method, a low yield ratio can be achieved by appropriately selecting the Q 'temperature, but the number of heat treatment steps increases, resulting in a decrease in productivity and an increase in manufacturing cost.

As a method that does not increase the number of manufacturing steps, a method is disclosed in which after the end of rolling at an Ar ₃ temperature or higher, the start of accelerated cooling is delayed until the temperature of the steel material becomes equal to or lower than the Ar ₃ transformation point at which ferrite forms (for example, , See Patent Document 2).

  However, since it is necessary to cool the temperature range from the end of rolling to the start of accelerated cooling at a cooling rate that is about the ability to cool, productivity is extremely reduced.

As a technique for achieving a low yield ratio without performing a complex heat treatment as disclosed in Patent Document 1 and Patent Document 2, the rolling of the steel material is completed at the Ar ₃ transformation point or higher, and the subsequent accelerated cooling rate and A method is known in which a two-phase structure of acicular ferrite and martensite is achieved by controlling the cooling stop temperature to achieve a low yield ratio (see, for example, Patent Document 3).
JP-A-55-97425 JP 55-41927 A Japanese Patent Laid-Open No. 1-176027

However, in the technique described in Patent Document 3, as shown in the examples, in order to obtain a steel material having a tensile strength of 490 N / mm ² (50 kg / mm ² ) or more, the carbon content of the steel material is increased, Or since it is necessary to set it as the component composition which increased the additional amount of other alloy elements, not only the cost of a raw material will be raised, but the deterioration of toughness of a welding heat affected zone becomes a problem.

  In this way, the conventional technology produces low yield ratio, high strength, high toughness steel sheets with high uniform elongation with excellent weld heat affected zone toughness without lowering productivity or raising material costs. It is difficult to do.

  Therefore, the present invention solves such problems of the prior art, and can be manufactured at high production efficiency and low cost, and has a low yield ratio and high strength and high toughness steel plate with high uniform elongation characteristics of API 5L X70 grade or less. And it aims at providing the manufacturing method.

  In order to solve the above-mentioned problems, the present inventors have earnestly studied a manufacturing process of a steel sheet, particularly a manufacturing process of accelerated cooling after controlled rolling and subsequent reheating, and as a result, has obtained the following knowledge.

  (A) During the accelerated cooling process, during the bainite transformation, that is, in the temperature region where untransformed austenite exists, cooling is stopped, and then reheating is performed from the bainite transformation finish temperature (hereinafter referred to as Bf point) or higher. The metal structure is made to have a structure in which hard island martensite (hereinafter referred to as MA) is uniformly formed in the mixed phase of ferrite and bainite, and a low yield ratio can be achieved.

  MA can be easily identified by, for example, etching with a 3% nital solution (nital: nitrate alcohol solution), followed by electrolytic etching and observing. When the microstructure of the steel sheet is observed with a scanning electron microscope (SEM), MA is observed as a white floating part.

  (B) Since an untransformed austenite is stabilized by adding an appropriate amount of an austenite stabilizing element such as Cu or Ni, hard MA can be generated without adding a large amount of a hardenability improving element such as C or Mn. Is possible.

The present invention has been made by further studying the above findings, that is, the present invention
1. By mass%, C: 0.03~0.08%, Si : 0.01~0.5%, Mn: 1.2~ 1.61%, Mo: 0.05~0.4%, Cu + Ni: 0.3 to 0.67 %, Ti: 0.005 to 0.04 %, Nb: 0.005 to 0.07 %, Al: 0.08% or less, the remainder Fe and inevitable impurities, metal The structure is a three-phase structure of ferrite, bainite, and island martensite, which includes island martensite with a volume fraction of 3 to 15% and residual austenite with a volume fraction of 2% or more. A low yield ratio, high strength, high toughness steel sheet having a uniform elongation of 12% or more.
2. Further, the steel composition is in mass%, V: 0.005 to 0.1%, Cr: 0.5% or less, Ca: 0.0005 to 0.003%, B: 0.005% or less. 2. The low yield ratio high strength high toughness steel plate according to 1, which contains one or more selected.
3.1 After heating steel having the component composition described in either 1 or 2 to a temperature of 1000 to 1300 ° C. and hot rolling at a rolling finish temperature of Ar ₃ temperature or higher, cooling at 5 ° C./s or higher A low yield ratio high strength high toughness steel sheet characterized by performing accelerated cooling to 500 to 650 ° C. at a speed, and then immediately reheating to 550 to 750 ° C. at a temperature rising rate of 0.5 ° C./s or more. Production method.

  According to the present invention, a low yield ratio high strength high toughness steel sheet with high uniform elongation characteristics can be produced at low cost without degrading the weld heat affected zone toughness or adding a large amount of alloying elements. Can do. For this reason, the steel plate mainly used for a line pipe can be stably manufactured in a large amount at a low cost, and the productivity and economy can be remarkably improved, which is extremely useful industrially.

Hereinafter, the metal structure, component composition, and production conditions of the high-strength steel sheet of the present invention will be described in detail.
[Metal structure]
In the present invention, in addition to ferrite and bainite, a metal structure uniformly including island martensite (MA) having a volume fraction of 3 to 15% and residual austenite having a volume fraction of 2% or more is used.

  A three-phase structure in which MA is uniformly formed in ferrite and bainite, that is, a composite structure containing hard MA in soft ferrite and bainite achieves a low yield ratio and a high uniform elongation. Yes.

  It is desirable that the ferrite fraction is 5% or more from the viewpoint of securing strength, and the bainite fraction is 10% or more from the viewpoint of securing toughness of the base material.

  When applied to an earthquake zone subjected to large deformation, high uniform elongation performance may be required in addition to low yield ratio. In the multiphase structure of soft ferrite, bainite and hard MA as described above, since the soft phase bears deformation, a highly uniform elongation of 12% or more can be achieved.

  The proportion of MA in the structure is preferably 3 to 15% in terms of the volume fraction of MA (calculated from the proportion of the area of MA in an arbitrary cross section of the steel sheet in the rolling direction and the sheet width direction). If the volume fraction of MA is less than 3%, it may be insufficient to achieve a low yield ratio, and if it exceeds 15%, the base material toughness may be deteriorated.

  Further, from the viewpoint of low yield ratio, high uniform elongation, and base metal toughness, the volume fraction of MA is particularly preferably 5 to 15%. The volume fraction of MA can be obtained, for example, by calculating from the area ratio occupied by MA by performing image processing on a microstructure photograph of at least four fields of view obtained by SEM observation.

  The average particle size of MA is desirably 10 μm or less. The average particle diameter of MA can be obtained as an average value of the diameters obtained by subjecting the microstructure obtained by SEM observation to image processing, obtaining the diameter of a circle having the same area as each MA, and obtaining the diameter of each MA. .

  In the present invention, Cu and Ni are added to stabilize untransformed austenite in order to produce MA without adding a large amount of hardenability improving elements such as C and Mn, and reheating and subsequent air cooling. It is important to suppress pearlite transformation and cementite formation.

  Therefore, austenite stabilized by Cu and Ni remains in the produced MA, and the residual austenite volume fraction is required to be 2% or more.

  The mechanism of MA generation in the present invention is as follows. Detailed manufacturing conditions will be described later.

After heating the slab, the rolling is finished in the austenite region, and then accelerated cooling is started at the Ar ₃ transformation temperature or higher.

  Accelerated cooling is completed during bainite transformation, that is, in the temperature range where untransformed austenite exists, and then reheated at a temperature higher than the bainite transformation finish temperature (Bf point) and then cooled in the manufacturing process. is there.

  Microstructures at the end of accelerated cooling are bainite and untransformed austenite, and reheating at the Bf point or higher causes ferrite transformation from untransformed austenite. However, since ferrite has a small amount of C solid solution, C is untransformed. Discharged into austenite.

  Therefore, the amount of C in untransformed austenite increases with the progress of ferrite transformation during reheating. At this time, if Cu, Ni or the like, which is an austenite stabilizing element, is contained in a certain amount or more, untransformed austenite in which C is concentrated remains even at the end of reheating, and is transformed into MA by cooling after reheating. Finally, it becomes a structure in which MA is formed in two phases of bainite and ferrite.

  In the present invention, after accelerated cooling, it is important to perform reheating from a temperature range in which untransformed austenite exists, and when the reheating start temperature falls below the Bf point, bainite transformation is completed and untransformed austenite does not exist. The reheating start needs to be higher than the Bf point.

  In addition, the cooling after reheating is not particularly specified because it does not affect the transformation of MA, but basically it is preferably air cooling. In the present invention, steel with a certain amount of Cu and Ni added is used, and accelerated cooling is stopped during the bainite transformation, followed by continuous reheating, thereby generating hard MA without reducing the production efficiency. be able to.

  In the steel according to the present invention, the metal structure is a structure that uniformly contains a certain amount of MA and retained austenite in the two phases of ferrite and bainite, but other structures as long as the effects of the present invention are not impaired. And those containing precipitates are also included in the scope of the present invention.

  Specifically, when one or more different metal structures such as pearlite are mixed, the strength decreases. However, when the fraction of the structure other than ferrite, bainite, retained austenite and MA is low, the influence can be ignored. Therefore, other metal structures of 3% or less in total fraction, that is, pearlite, cementite, etc. You may contain 2 or more types.

The metal structure described above can be obtained by manufacturing the following composition using steel having the following composition.
[Chemical component] In the following description, all units shown in% are% by mass.

C
C is set to 0.03 to 0.1%. C contributes to precipitation strengthening as a carbide and is an important element for MA formation. However, if it is less than 0.03%, it is insufficient for formation of MA, and sufficient strength cannot be secured. Since addition exceeding 0.1% deteriorates the HAZ toughness, the C content is specified to be 0.03 to 0.1%. More preferably, it is 0.03 to 0.08%.

Si
Si is set to 0.01 to 0.5%. Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.5%, the toughness and weldability are deteriorated, so the Si content is 0.01 to 0.00. Specify 5%. More preferably, it is 0.01 to 0.3%.

Mn
Mn is set to 1.2 to 2.0%. Mn is added to improve strength and toughness, further improve hardenability and promote MA formation. However, if it is less than 1.2%, its effect is not sufficient, and if it exceeds 2.5%, toughness and weldability deteriorate. Therefore, the Mn content is specified to be 1.2 to 2.0%. Addition of 1.5% or more is desirable in order to stably produce MA regardless of changes in components and production conditions.

Mo
Mo is set to 0.05 to 0.4%. Mo is an element that improves the hardenability, and is an element that contributes to an increase in strength by strengthening the MA formation and the bainite phase. However, if it exceeds 0.4%, the weld heat-affected zone toughness is deteriorated, so the Mo content is specified to be 0.05 to 0.4%. Furthermore, it is preferable to make Mo content into 0.1 to 0.3% from a viewpoint of weld heat affected zone toughness.

Cu + Ni
Cu + Ni is 0.1% or more. Cu and Ni are important elements in the present invention. In order to produce MA without adding a large amount of a hardenability improving element such as C and Mn, it is necessary to add 0.1% or more of Cu + Ni which is an element that stabilizes untransformed austenite. Furthermore, in order to produce MA more stably, addition of 0.3% or more is preferable.

Ti
Ti is 0.005 to 0.04%. Ti is an important element that suppresses austenite coarsening during slab heating and improves the toughness of the base metal due to the pinning effect of TiN. The effect is manifested by adding 0.005% or more.

  However, since addition exceeding 0.04% causes deterioration of the weld heat affected zone toughness, the Ti content is specified to be 0.005 to 0.04%. From the viewpoint of weld heat affected zone toughness, the Ti content is preferably 0.005% or more and less than 0.02%.

Nb
Nb is set to 0.005 to 0.07%. Nb is an element that improves toughness by refining the structure and contributes to an increase in strength by improving the hardenability of solid solution Nb. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.07%, the toughness of the weld heat-affected zone deteriorates, so the Nb content is specified to be 0.005 to 0.07%.

Al
Al is made 0.08% or less. Al is added as a deoxidizer, but if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is specified to be 0.08% or less. Preferably, the content is 0.01 to 0.08%.

N
N is preferably 0.007% or less. N is treated as an unavoidable impurity, but if over 0.007%, the weld heat affected zone toughness deteriorates, so the content is preferably made 0.007% or less.

  Furthermore, by optimizing Ti / N, which is the ratio of Ti amount to N amount, the austenite coarsening of the weld heat affected zone can be suppressed by TiN particles, and good weld heat affected zone toughness can be obtained. Therefore, Ti / N is preferably 2 to 8, and more preferably 2 to 5.

  The above is the basic component of the present invention. For the purpose of further improving the strength and toughness of the steel sheet and improving the hardenability and promoting the formation of MA, one or two of V, Cr, B, and Ca shown below are used. It may contain seeds or more.

V
V is set to 0.005 to 0.1%. It is an element that enhances hardenability and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.1%, the toughness of the weld heat affected zone deteriorates. Therefore, when added, the V content is specified to be 0.005 to 0.1%. .

Cr
Cr is 0.5% or less. Cr, like Mn, is an element effective for obtaining sufficient strength even at low C. In order to acquire the effect, it is preferable to add 0.1% or more. However, if it is added in a large amount, weldability deteriorates.

B
B is 0.005% or less. B is an element contributing to strength increase and HAZ toughness improvement. In order to obtain the effect, it is preferable to add 0.0005% or more, but if added over 0.005%, the weldability is deteriorated, so when added, the content is made 0.005% or less.

Ca
Ca is 0.0005 to 0.003%. Ca improves the toughness by controlling the form of sulfide inclusions. The effect appears at 0.0005% or more, and when it exceeds 0.003%, the effect is saturated, and conversely, the cleanliness is lowered and the toughness is deteriorated. And

  The remainder other than the above is Fe and inevitable impurities, and Mg and REM may be added to the inevitable impurities in an amount of 0.02% or less.

Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
[Production conditions]
In the description, temperatures such as heating temperature, rolling end temperature, cooling end temperature, and reheating temperature are the average temperatures of the steel plates. The average temperature is obtained by calculation based on the surface temperature of the slab or steel plate, taking into account parameters such as plate thickness and thermal conductivity. The cooling rate is an average cooling rate obtained by dividing the temperature difference required for cooling to the cooling end temperature (500 to 650 ° C.) by the time required for the cooling after the hot rolling is completed.

  The temperature increase rate is an average temperature increase rate obtained by dividing the temperature difference required for reheating up to the reheating temperature (550 to 750 ° C.) by the time required for reheating after cooling. Hereinafter, each manufacturing condition will be described in detail.

Heating temperature Heating temperature shall be 1000-1300 degreeC. If the heating temperature is less than 1000 ° C., the solid solution of the carbide is insufficient and the required strength cannot be obtained, and if it exceeds 1300 ° C., the base material toughness deteriorates, so the temperature is set to 1000 to 1300 ° C.

Rolling end temperature The rolling end temperature is Ar ₃ temperature or higher. If the rolling end temperature is not more than Ar ₃ temperature, the subsequent ferrite transformation rate is lowered, so that the concentration of C into untransformed austenite at the time of reheating becomes insufficient and MA is not generated. Therefore, the rolling end temperature is set to Ar ₃ temperature or higher.

Cooling conditions after hot rolling Immediately after the rolling, cooling is performed at a cooling rate of 5 ° C / s or more. When the cooling rate is less than 5 ° C./s, pearlite is generated at the time of cooling, so that strengthening by bainite cannot be obtained, so that sufficient strength cannot be obtained. Therefore, the cooling rate after the end of rolling is specified to be 5 ° C./s or more.

  About the cooling method at this time, it is possible to use arbitrary cooling equipment by a manufacturing process. In the present invention, the ferrite transformation can be completed without maintaining the temperature during the subsequent reheating by supercooling to the bainite transformation region by accelerated cooling.

Further, when the cooling start temperature becomes Ar ₃ temperature or lower and ferrite is generated, the strength is lowered and MA is not generated, so the cooling start temperature is set to Ar ₃ temperature or higher. About the cooling method at this time, it is possible to use arbitrary cooling equipment by a manufacturing process.

  In the present invention, the ferrite transformation can be completed without maintaining the temperature during the subsequent reheating by supercooling to the bainite transformation region by accelerated cooling.

  Cooling stop temperature shall be 500-650 degreeC. This process is an important production condition in the present invention. In the present invention, C-concentrated untransformed austenite present after reheating is transformed into MA upon subsequent air cooling.

  That is, it is necessary to stop the cooling in a temperature range where untransformed austenite during the bainite transformation exists. If the cooling stop temperature is less than 500 ° C., the bainite transformation is completed, so MA is not generated during air cooling, and a low yield ratio cannot be achieved. If it exceeds 650 ° C., C is consumed in the pearlite that precipitates during cooling, and MA is not generated, so the accelerated cooling stop temperature is defined as 450 to 650 ° C. From a viewpoint of MA production | generation, Preferably it is 530-650 degreeC.

Heat treatment conditions after accelerated cooling Immediately after the accelerated cooling is stopped, reheating is performed to a temperature of 550 to 750 ° C. at a temperature rising rate of 0.5 ° C./s or more. This process is also an important production condition in the present invention. Due to the ferrite transformation from the untransformed austenite at the time of reheating and the accompanying discharge of C to the untransformed austenite, the untransformed austenite enriched with C during the air cooling after the reheating transforms to MA.

  In order to obtain MA, it is necessary to reheat from the temperature above the Bf point to a temperature range of 550 to 750 ° C. after accelerated cooling.

  If the rate of temperature rise is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency deteriorates, and pearlite transformation occurs, so MA cannot be obtained, and a sufficiently low yield ratio. Can't get.

  If the reheating temperature is less than 550 ° C., ferrite transformation does not occur sufficiently and C is not sufficiently discharged into untransformed austenite, MA is not generated, and a low yield ratio cannot be achieved. If the temperature exceeds 750 ° C., sufficient strength cannot be obtained due to softening of bainite, so the temperature range of reheating is specified to be 550 to 750 ° C.

  In the present invention, after accelerated cooling, it is important to perform reheating from a temperature range in which untransformed austenite exists, and when the reheating start temperature falls below the Bf point, bainite transformation is completed and untransformed austenite does not exist. The reheating start needs to be higher than the Bf point.

  In order to reliably concentrate C that undergoes ferrite transformation into untransformed austenite, it is desirable to raise the temperature by 50 ° C. or more from the reheating start temperature. There is no need to set the temperature holding time at the reheating temperature.

  If the production method of the present invention is used, even if it is cooled immediately after reheating, sufficient MA can be obtained, so a low yield ratio and a high uniform elongation can be achieved. However, in order to further promote the diffusion of C and secure the MA volume fraction, the temperature can be maintained within 30 minutes.

  If the temperature is maintained for more than 30 minutes, dislocation recovery of the bainite phase occurs and the strength may decrease. The cooling rate after reheating is preferably basically air cooling.

  As equipment for performing reheating after accelerated cooling, a heating device can be installed downstream of the cooling equipment for performing accelerated cooling. As the heating device, it is preferable to use a gas combustion furnace or induction heating device capable of rapid heating of the steel sheet.

  Steels (steel types A to K) having chemical components shown in Table 1 were made into slabs by a continuous casting method, and thick steel plates (Nos. 1 to 16) having a thickness of 18 and 26 mm were manufactured.

  After the heated slab was rolled by hot rolling, it was immediately cooled using a water-cooled accelerated cooling facility and reheated using an induction heating furnace or a gas combustion furnace. The induction furnace was installed on the same line as the accelerated cooling equipment.

  Table 2 shows the production conditions of each steel plate (No. 1 to 16). The heating temperature, rolling end temperature, cooling stop (end) temperature, reheating temperature, and other temperatures were the average temperature of the steel sheet. The average temperature was determined from the surface temperature of the slab or steel plate by parameters and calculations such as plate thickness and thermal conductivity.

  The cooling rate is an average cooling rate obtained by dividing the temperature difference required for cooling to the cooling stop (end) temperature (350 to 700 ° C.) by the time required for the cooling after the hot rolling is completed. The reheating rate (temperature increase rate) is the average temperature increase rate divided by the time required to reheat the temperature difference required for reheating to the reheating temperature (570 to 660 ° C.) after cooling. is there.

  The tensile properties of the steel sheet produced as described above were measured. The measurement results are also shown in Table 2. Tensile strength was evaluated by taking two full thickness tensile test pieces in the vertical direction of rolling, conducting a tensile test, and evaluating the average value.

  The tensile strength of 517 MPa or more (API 5L X60 or more) was determined as the strength required for the present invention. Yield ratio and uniform elongation were evaluated by taking two tensile test pieces of full thickness round bars in the rolling direction, conducting a tensile test, and measuring the average value. The yield ratio required for the present invention was a yield ratio of 80% or less and a uniform elongation of 12% or more.

  For base metal toughness, three full-size Charpy V-notch test pieces in the vertical direction of rolling were sampled, Charpy test was performed, the absorbed energy at −10 ° C. was measured, and the average value was obtained. The absorption energy at −10 ° C. was determined to be 200 J or more.

  The amount of retained austenite was quantified by X-ray diffraction, and 2% or more was considered good.

  For the weld heat affected zone (HAZ) toughness, three specimens with a thermal history corresponding to a heat input of 40 kJ / cm were collected by a reproducible thermal cycle apparatus and subjected to a Charpy test. And the absorbed energy in -10 degreeC was measured and the average value was calculated | required. Those having Charpy absorbed energy at −10 ° C. of 100 J or more were considered good.

  In Table 2, all of Nos. 1 to 8 which are examples of the present invention have chemical components and production methods within the scope of the present invention, high strength of tensile strength of 517 MPa or more, yield ratio of 80% or less, uniform elongation of 10 % Yield ratio and high uniform elongation, and the toughness of the base metal and the weld heat affected zone was good.

  The structure of the steel sheet is a structure in which island martensite is formed in a two-phase structure of ferrite and bainite. The volume fraction of island martensite is in the range of 3 to 20%, and the amount of retained austenite is 2 in volume fraction. % Or more. In addition, the volume fraction of island martensite was calculated | required by image processing from the microstructure observed with the scanning electron microscope (SEM).

  In Nos. 9 to 12, the chemical components are within the scope of the present invention, but the production method is outside the scope of the present invention, so the structure is ferrite and bainite, and the yield ratio and uniform elongation are insufficient or sufficient. A sufficient strength could not be obtained. Nos. 14 to 18 had chemical components outside the scope of the present invention, so that sufficient strength could not be obtained, yield ratio was high, uniform elongation was low, or toughness was inferior.

Claims (3)

By mass%, C: 0.03~0.08%, Si : 0.01~0.5%, Mn: 1.2~ 1.61%, Mo: 0.05~0.4%, Cu + Ni: 0.3 to 0.67 %, Ti: 0.005 to 0.04 %, Nb: 0.005 to 0.07 %, Al: 0.08% or less, the remainder Fe and inevitable impurities, metal The structure is a three-phase structure of ferrite, bainite, and island martensite, which includes island martensite with a volume fraction of 3 to 15% and residual austenite with a volume fraction of 2% or more. A low yield ratio, high strength, high toughness steel sheet having a uniform elongation of 12% or more.   Further, the steel composition is in mass%, V: 0.005 to 0.1%, Cr: 0.5% or less, Ca: 0.0005 to 0.003%, B: 0.005% or less. The low yield ratio high strength high toughness steel sheet according to claim 1, comprising one or more selected. The steel having the component composition according to claim 1 or 2 is heated to a temperature of 1000 to 1300 ° C and hot-rolled at a rolling end temperature of Ar ₃ temperature or higher, and then 5 ° C / s or higher. Accelerated cooling to 500 to 650 ° C. at a cooling rate of 5 ° C., and then immediately reheating to 550 to 750 ° C. at a temperature rising rate of 0.5 ° C./s or more, low yield ratio high strength high toughness A method of manufacturing a steel sheet.