JP4730088B2 - Low yield ratio high strength thick steel plate and method for producing the same - Google Patents

Description

  The present invention relates to a steel plate having characteristics of high strength, low yield ratio and high toughness, suitable for use in buildings, bridges, line pipes, offshore structures, etc., in particular, yield strength (YP) is 650 MPa or more, The present invention relates to a high strength thick steel sheet for construction having a yield ratio (YR) of 80% or less and a method for producing the same.

  In recent years, with the increase in size and span of building structures, it has been required to increase the thickness and strength of steel used. From the viewpoint of safety of steel structures, it has high allowable stress and yield. There is a need to reduce the ratio.

  When the yield ratio is reduced, even if a stress higher than the yield point is applied, the stress allowed until failure increases, and the uniform elongation increases, so that the steel material is excellent in plastic deformability.

  In the case of line pipes, there is a high demand for higher strength of steel materials used, and accordingly, springback, buckling, etc. during UO press processing become problems, so the yield ratio is low for the purpose of suppressing these A high strength steel sheet is desired.

  In particular, in a high-tensile steel sheet having a tensile strength exceeding 780 MPa, it is common to add a large amount of an alloy for securing the strength, so that the yield ratio tends to increase and the toughness also decreases.

  Conventionally, a multistage heat treatment including reheating and quenching into a ferrite + austenite two-phase region is generally used as a manufacturing process of a low yield ratio high strength thick steel plate, but the obtained microstructure is mainly composed of a ferrite phase and is hard. Bainite or martensite is dispersed as the second phase, and depending on the volume fraction of the ferrite phase, it is difficult to stably achieve a tensile strength of 780 MPa or more and a yield strength (YP) of 650 MPa or more. .

On the other hand, the manufacturing process of the 780 MPa class thick steel sheet without considering the yield ratio is quenched immediately after the rolling or after reheating and from the temperature range of Ac ₃ or higher to a relatively low temperature range, so that the microstructure is bainite phase or martensite. It becomes a site phase and it is difficult to obtain a yield ratio of 80% or less.

  Therefore, a thick steel plate having both high strength and a low yield ratio is desired, and various proposals have been made.

  In Patent Document 1 and Patent Document 2, after quenching the hot-rolled steel sheet, it is again heated to the two-phase region of ferrite + austenite for quenching to achieve high strength and low yield ratio. Are listed.

  In Patent Document 3, after rolling, by direct quenching immediately after quenching, the microstructure after quenching is changed to a bainite phase or a martensite phase, and then again heated to a ferrite + austenite two-phase region and subjected to normalization. It is described that high strength and low yield ratio are achieved.

In Patent Document 4, after a certain period of time has passed after rolling, ferrite is precipitated, and then a direct quenching method in which quenching is performed to obtain a two-phase structure of ferrite phase + martensite phase, thereby increasing strength and reducing yield ratio. It is described to achieve.
JP 2001-288512 A JP-A-6-248337 JP-A-5-230530 JP-A-7-97626

  However, the techniques described in Patent Document 1, Patent Document 2 and Patent Document 3 are concerned with an increase in manufacturing cost due to a complicated heat treatment process. In the technique described in Patent Document 4, the manufacturing conditions and the steel plate Depending on the inner position, the volume fraction of the ferrite and martensite phases is likely to change, and the operation load for adjusting the manufacturing conditions in order to stably obtain high strength and a low yield ratio is large.

  Therefore, an object of the present invention is to provide a low-yield ratio high-tensile steel plate that is easy to manufacture and has stable performance, and a method for manufacturing the same.

  In order to achieve the above-mentioned problems, the present inventors have hitherto been considered to be very brittle among various factors affecting the strength and yield ratio of a thick steel plate, and lower the ductility and toughness of the base metal. Focusing on island martensite, which has not been actively used so far, as a structural control factor for high strength steel plates. We conducted intensive research and obtained the following knowledge.

  In order to stably achieve a tensile strength of 1 780 MPa or higher, a yield strength of 650 MPa or higher, and a low yield ratio of 80% or lower, a carbon equivalent Ceq of 0.38 to 0.55 in an appropriately selected steel composition is achieved. It is important to appropriately control the area fraction, the particle size, and the aspect ratio of the island martensite in the microstructure.

  2 After hot rolling the steel material having the above composition, it is subjected to a cooling process in which the cooling rate and the cooling stop temperature are optimized, and further, the heating rate after the cooling stop and the reheating temperature are optimized. By performing the heat treatment, a desired microstructure can be obtained.

The present invention has been made by further studying the obtained knowledge, that is, the present invention
1. Steel composition is mass%,
C: 0.03 to 0.20%
Si: 0.05 to 0.50%,
Mn: 0.8 to 3.0%,
P: 0.02% or less,
S: 0.0050% or less,
Al: 0.005 to 0.1%,
N: 0.0070% or less,
Ceq defined by the following formula (1) satisfies 0.38 to 0.55%, the balance is composed of Fe and unavoidable impurities, and the microstructure is a parent phase mainly composed of a ferrite phase and a bainite phase. The yield strength is characterized in that, as the hard phase, an island-shaped martensite having an average equivalent circle diameter of 1 to 10 μm and an average aspect ratio of 4.0 or less is contained in an area fraction of 3 to 30%. Low yield ratio high strength thick steel plate with (YP) of 650 MPa or more, tensile strength of 780 MPa or more , and yield ratio (YR) of 80% or less.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
However, C, Mn, Si, Ni, Cr, Mo, V: The element not contained in the content (mass%) of each element is set to 0.

The steel slab having the steel composition described in 2.1 is heated to 1000 to 1250 ° C., and after the hot rolling is finished in the temperature range of 800 ° C. or higher, 5 to 60 ° C./s from the temperature range of Ar ₃ points or higher. After cooling to a temperature range of Ar ₃ -400 ° C. to Ar ₃ -100 ° C. at a cooling rate of ₁ , the cooling is temporarily interrupted, and then the temperature is increased to 0.5 ° C./s or more to a temperature range of ₁ point or less of Ac. After reheating at a temperature rate, air cooling, and further tempering at 400 ° C. or more and Ac ₁ point or less, yield strength (YP) is 650 MPa or more, tensile strength is 780 MPa or more , yield ratio (YR ) Is a method for producing a low yield ratio high strength thick steel plate having a strength of 80% or less.

  According to the present invention, it is possible to stably produce a thick steel plate having a tensile strength of 780 MPa or more and a yield strength (YP) of 650 MPa or more and a low yield ratio of 80% or less without complicated heat treatment. It can greatly contribute to increasing the size of steel structures, improving the earthquake resistance of steel structures and improving construction efficiency, and has a remarkable industrial effect.

In the present invention, the microstructure and component composition are defined.
The microstructure contains an island-shaped martensite having an average equivalent circle diameter of 1 to 10 μm and an average aspect ratio of 4.0 or less in an area fraction of 3 to 30%.
In the present invention, the microstructure is a parent phase mainly composed of a ferrite phase and a bainite phase, and includes island-like martensite as a hard phase.

  Island-like martensite has a very high dislocation density, and due to the concentration of C, it is a very hard phase compared to the parent phase, so TS is improved and a large amount of movable dislocations increases YS. Suppression is effective for achieving both high strength and a low yield ratio.

  If the volume fraction of island-like martensite is less than 3%, the effects of increasing the strength and reducing the yield ratio as described above cannot be obtained. On the other hand, if it exceeds 30%, the ductility and toughness of the base material deteriorate. . For this reason, the area fraction is limited to a range of 3 to 30%. In addition, Preferably, it is 5 to 20%.

  If the average equivalent circle diameter of the island-like martensite is less than 1 μm, the effects of high strength and low yield ratio as described above cannot be obtained, while if it exceeds 10 μm, the toughness of the base material deteriorates. For this reason, an average equivalent circle diameter is limited to the range of 1-10 micrometers. In addition, Preferably, it is 3-8 micrometers.

  The average aspect ratio of the island martensite is preferably as low as possible because the base material toughness deteriorates as the average aspect ratio increases. When the average aspect ratio exceeds 4.0, this tendency becomes remarkable, so the upper limit is set.

  In addition, island martensite is identified by observing with an optical microscope having a magnification of 1000 times after carrying out repelling corrosion (JOURNAL OF METALS, March, 1980, p.38-39) on the sample. The aspect ratio was obtained by using an image analysis device for an image taken with an optical microscope having a magnification of 1000 times.

  The parent phase excluding island martensite is essentially a mixed structure of bainite phase and ferrite phase, and the strength decreases when a structure such as pearlite and cementite is mixed, so the area fraction should be smaller. good.

  However, when the structure of pearlite, cementite or the like is 15% or less in area fraction, the influence can be ignored, so it may be contained. From the viewpoint of securing strength, the area fraction of the bainite phase is preferably 60% or more.

In the description of the component composition,% means mass%.
C: 0.03-0.20%
C is an element useful for increasing the strength of steel and ensuring the necessary strength as a structural steel material. In order to obtain the above-described island-shaped martensite, the content of C is 0.03% or more. I need.

  On the other hand, if the content exceeds 0.20%, the HAZ toughness and weld crack resistance are deteriorated and the toughness of the base material is deteriorated, so the content is limited to the range of 0.03 to 0.20%. Preferably, it is 0.05 to 0.15%.

Si: 0.05 to 0.50%
Si acts as a deoxidizing material, and at least 0.05% is necessary for steelmaking. However, when it exceeds 0.50%, the toughness of the base material deteriorates and the weldability and HAZ toughness are remarkable. In order to deteriorate, it limits to 0.05 to 0.50% of range. Preferably, it is 0.05 to 0.35%.

Mn: 0.8 to 3.0%
Mn has the effect of increasing the strength of steel, and in the present invention, it is necessary to contain 0.8% or more in order to ensure a tensile strength of 780 MPa or more. On the other hand, if the content exceeds 3.0%, the toughness of the base material and the HAZ toughness are remarkably deteriorated, so the content is limited to the range of 0.8 to 3.0%. Preferably, it is 1.0 to 2.5%.

P: 0.02% or less P is an element that increases the strength of steel and deteriorates toughness, and particularly deteriorates the toughness of welds. Therefore, it is desirable to reduce it as much as possible. When the content exceeds 0.02%, this tendency becomes remarkable, so the upper limit is set. In addition, since excessive P reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.005% or more.

S: 0.0050% or less S is an element that deteriorates the toughness of the base metal and the welded portion, and is desirably reduced as much as possible. When the content exceeds 0.0050%, this tendency becomes remarkable, so the upper limit is set.

Al: 0.1% or less Al acts as a deoxidizer, and is most commonly used in the molten steel deoxidation process of high-strength steel. In addition, N in the steel is fixed as AlN and contributes to the improvement of the toughness of the base metal. However, if the content exceeds 0.1%, the toughness of the base material decreases, and the weld metal part is mixed during welding, In order to deteriorate toughness, the content is limited to 0.1% or less. In addition, such an effect is recognized by containing Al: 0.005% or more, Preferably, it is 0.01 to 0.07%.

N: 0.0070% or less N is contained in steel as an inevitable impurity, and if it exceeds 0.0070%, the toughness of the base metal and the welded portion is remarkably lowered, so the content is limited to 0.0070% or less.

Ceq: 0.38 to 0.55%
In the present invention, the content is adjusted within the above-described component composition range so that the carbon equivalent Ceq defined by the formula (1) is 0.38 to 0.55% within the above-described component range.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
However, C, Mn, Si, Ni, Cr, Mo, V: The content (mass%) of each element, and 0 is not included.

  If Ceq is less than 0.38%, the hardenability at the time of rolling and accelerated cooling is insufficient, the above structural requirements are not satisfied, and a desired tensile strength of 780 MPa class or higher and a yield strength (YP) of 650 MPa or higher are ensured. Otherwise, island martensite is not formed after accelerated cooling and reheating, and a desired yield ratio of 80% or less cannot be ensured.

  On the other hand, when Ceq exceeds 0.55%, the base metal toughness after rolling and cooling, and the welded portion toughness deteriorate significantly, so the range is limited to 0.38 to 0.55%.

  In the present invention, in addition to the basic component system described above, one or more of Cu, Ni, Cr, Mo, Nb, V, Ti, B, Ca, REM, and Mg may be contained as necessary. it can.

One or more of Cu: 0.1 to 1.0%, Ni: 0.1 to 2.0% Cu and Ni are elements that can increase strength while maintaining high toughness, Since the influence on the HAZ toughness is small, it is an element useful for increasing the strength, and can be selected and contained as necessary.

  When added, Cu is preferably contained in an amount of 0.1% or more. However, if the content exceeds 1.0%, hot brittleness is caused and the surface properties of the steel sheet are deteriorated. 0%. In addition, Preferably, it is 0.2 to 0.7%.

  When Ni is added, it is preferably contained in an amount of 0.1% or more. However, even if Ni is contained in an amount exceeding 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected. Since it becomes disadvantageous, it limited to 0.1 to 2.0%. In addition, Preferably it is 0.2 to 1.7%.

Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.2% or less, Ti: 0.03% or less, B: 0.005% or less Or 2 or more types Cr, Mo, Nb, V, Ti, and B are all elements that contribute to improving the strength of steel, and can be appropriately contained depending on the desired strength.

  When Cr is added, the content is preferably 0.05% or more. However, the content exceeding 1.0% deteriorates the HAZ toughness, so it is desirable to limit the content to 1.0% or less.

  When Mo is added, the content is preferably 0.05% or more. However, since the content exceeding 1.0% adversely affects the base material toughness and the HAZ toughness, it may be limited to 1.0% or less. desirable.

  When Nb is added, it is preferably contained in an amount of 0.005% or more. However, if the content exceeds 0.1%, the base material toughness and the HAZ toughness are deteriorated, so it is desirable to limit the content to 0.1% or less. .

  When V is added, it is preferably contained in an amount of 0.01% or more. However, if the content exceeds 0.2%, the HAZ toughness is deteriorated, so it is desirable to limit it to 0.2% or less.

  When Ti is added, it contributes to strength improvement by containing 0.005% or more, and has a strong affinity for N and precipitates as TiN during solidification, and suppresses coarsening of austenite grains in HAZ. Contributes to higher toughness.

  On the other hand, if the content exceeds 0.03%, the base material toughness is deteriorated, so it is desirable to limit the content to 0.03% or less.

  B has the effect of increasing the strength of the steel through the improvement of hardenability, but the content exceeding 0.005% remarkably increases the hardenability and brings about deterioration of the toughness and ductility of the base material. When it is made to be limited to 0.005% or less. In addition, Preferably, it is 0.0003 to 0.002%.

One or more of Ca: 0.005% or less, REM: 0.02% or less, and Mg: 0.005% or less Ca, REM, and Mg are elements that contribute to toughness improvement, and are desired It can be selected depending on the characteristics.
Ca: 0.005% or less Ca is a useful element that improves toughness through refinement of crystal grains. When Ca is contained, it is preferably contained in an amount of 0.001% or more, but exceeds 0.005%. Even if contained, the effect is saturated, so 0.005% was made the upper limit.

  When REM is contained, it is preferably contained in an amount of 0.002% or more. However, even if contained over 0.02%, the effect is saturated, so 0.02% was made the upper limit.

  Mg is a useful element that improves toughness through refinement of crystal grains, and is preferably contained in an amount of 0.001% or more, but even if contained in excess of 0.005%, the effect is saturated, The upper limit was 0.005%.

  The balance other than the above components is Fe and inevitable impurities.

  Next, a manufacturing method will be described. The temperature is a temperature of 1/2 t part of the plate thickness.

1000 ° C. to 1250 ° C. Heating The molten steel having the above composition is melted by a conventional method such as a converter, electric furnace, vacuum melting furnace, etc., and the obtained steel material is reheated to 1000 ° C. to 1250 ° C.

  If the reheating temperature is less than 1000 ° C., the deformation resistance in hot rolling becomes high, and the amount of reduction per pass cannot be made large. Therefore, the number of rolling passes increases and the rolling efficiency decreases, and the steel material The casting defect in (slab) may not be crimped.

  On the other hand, when the reheating temperature exceeds 1250 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. For this reason, it is preferable to make the reheating temperature of a steel raw material into the range of 1000-1250 degreeC.

Hot rolling The reheated steel material is subjected to hot rolling with a rolling end temperature of 800 ° C. or higher until a predetermined thickness is reached. The hot rolling conditions are not particularly limited as long as a predetermined plate thickness and shape can be satisfied except that the rolling end temperature is 800 ° C. or higher.

In the case of an extremely thick steel plate having a plate thickness exceeding 80 mm, it is desirable to secure at least one or more rolling passes with a rolling reduction per pass of 15% or more for zaku pressure bonding.
When the rolling end temperature is less than 800 ° C., the deformation resistance becomes too high, the rolling load increases, and the burden on the rolling mill increases.

  Further, in order to lower the rolling temperature of the thick material to less than 800 ° C., it is necessary to wait in the middle of rolling, which greatly hinders productivity. For this reason, the rolling end temperature was set to 800 ° C. or higher.

Cooling conditions After the end of rolling, the obtained thick steel plate is cooled from an Ar ₃ point or higher temperature range to Ar ₃ -400 ° C. to Ar ₃ -100 ° C. at an average cooling rate of 5 to 60 ° C./s.

The cooling stop temperature is a particularly important control factor in the production method of the present invention. When the cooling stop temperature becomes lower than Ar ₃ -400 ° C., the bainite transformation is completed and no residual austenite exists after the cooling stop, At the time of reheating and air cooling, there is no formation of island martensite from retained austenite, and the desired yield ratio of 80% or less cannot be satisfied.

On the other hand, if the cooling stop temperature after cooling is higher than the Ar ₃ -100 ° C., cooling the stop does not proceed bainite transformation, in order to diffuse the C into the retained austenite does not proceed, without generating the island martensite The desired tensile strength of 780 MPa or higher and the yield strength (YP) of 650 MPa or higher and the yield ratio of 80% or lower cannot be satisfied.

  In addition, when the cooling rate after rolling is less than 5 ° C./s, the microstructure after accelerated cooling becomes a ferrite main structure, so that a desired tensile strength of 780 MPa class or higher and a yield strength (YP) of 650 MPa or higher is ensured. become unable.

On the other hand, when the cooling rate exceeds 60 ° C./s, it becomes difficult to control the temperature depending on the position of the steel sheet, resulting in material variations.

After the accelerated cooling is finished, the thick steel plate is temporarily cooled and reheated to a temperature range of Ac ₁ point or less at a heating rate of 0.5 ° C./s or higher and then air-cooled.

  If the rate of temperature rise is less than 0.5 ° C / s, it takes a long time to reach the desired reheating temperature, resulting in a decrease in production efficiency, and because pearlite transformation occurs, no island-like martensite is generated and the desired yield is achieved. The ratio of 80% or less cannot be satisfied.

When the reheating temperature is Ac ₁ point or higher, the desired tensile strength of 780 MPa or higher and yield strength (YP) of 650 MPa or higher cannot be satisfied due to softening of bainite.

  The reheating temperature is desirably raised by 100 ° C. or more from the cooling stop temperature in order to promote diffusion of C into the retained austenite.

  The holding time after reheating is preferably 15 min. So as not to inhibit productivity. The following. As means for reheating, atmospheric furnace heating, gas flame, induction heating and the like can be used, but in consideration of economy, controllability and the like, induction heating is preferable.

When the cooling rate after rolling is within the average cooling rate range of 5 to 60 ° C./s and the accelerated cooling stop temperature satisfies the range of Ar _{3 to} 400 ° C. to Ar _{3 to} 100 ° C., immediately after the accelerated cooling. In addition, a microstructure in which retained austenite is finely dispersed in the bainite main structure is obtained.

  Furthermore, C is diffused into the finely dispersed residual austenite by the subsequent reheating up to the Ac1 point or less at a heating rate of 0.5 ° C./s or more and air cooling, and island martensite is generated. A microstructure is achieved, and a tensile strength of 780 MPa or higher and a yield strength (YP) of 650 MPa or higher and a low yield ratio of 80% or lower are compatible.

Incidentally, Ar ₃ points observed correlation with chemical composition, as an example (2) can be used.
Ar ₃ = 868-396C + 25Si-68Mn-21Cu-36Ni-25Cr-30Mo (2)
(However, C, Si, Mn, Cu, Ni, Cr, Mo: content of each alloy element (mass%)).

Further, a correlation with the chemical composition is also observed at Ac ₁ point, and the formula (3) can be used as an example.
_{Ac 1 = 751-27C + 18Si-12Mn} -23Cu-23Ni + 24Cr + 23Mo-40V-6Ti + 233Nb-169Al-895B (3)
(However, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, B: content of each alloy element (mass%)).

In the present invention, the steel sheet may be cooled to room temperature and then reheated and tempered. In the tempering step, it is possible to improve toughness by tempering at 400 ° C. or higher and Ac ₁ point or lower.

  As the microstructure after tempering, the hard phase becomes tempered island martensite, but if the hardness is sufficiently higher than the parent phase, the effect of achieving both high strength and low yield ratio can be obtained. .

In order to obtain such an effect, it is necessary to set the tempering temperature to 400 ° C. or higher. However, if the temperature exceeds ₁ Ac, the strength is reduced, so the tempering treatment should be performed at 400 ° C. to ₁ Ac. Is desirable.

  By using the steel material having the above composition, by performing hot rolling, cooling and reheating, and air cooling under the above-described conditions, it is possible to disperse and generate island martensite and have a tensile strength of 780 MPa or more. Thus, it is possible to easily produce a low yield ratio high strength steel plate having a yield strength (YP) of 650 MPa or more and a yield ratio of 80% or less.

  The steel materials prepared by the converter-ladder refining-continuous casting method were made into thick steel plates having various thicknesses by hot rolling-accelerated cooling-reheating-air cooling and tempering.

  Table 1 shows the component composition of the steel material, and Table 2 shows the manufacturing conditions and the plate thickness of the steel sheet. A JIS No. 4 tensile test piece was sampled from the plate thickness ½ position of each thick steel plate obtained, and a tensile test was performed according to the default of JIS Z 2241 (1998) to examine the tensile properties.

In addition, a V-notch test piece was taken from the position of 1/2 of the thickness of each steel plate obtained in accordance with JIS Z 2202 (1998), and in accordance with JIS Z 2242 (1998). Then, a Charpy impact test was carried out, the absorbed energy (vE ₀ ) at 0 ° C. was determined, and the base material toughness was evaluated.

  The obtained results are shown in Table 3. The examples of the present invention all have a tensile strength of 780 MPa or more, a yield strength (YP) of 650 MPa or more, a yield ratio of 80% or less, an absorbed energy at 0 ° C., a high strength of vEo> 100 J, a low yield ratio, and high toughness. The base material characteristics are

  On the other hand, in a comparative example that is out of the scope of the present invention, one or a plurality of characteristics among the base material strength, the yield ratio, and the base material toughness do not satisfy the target value.

Claims (2)

Steel composition is mass%,
C: 0.03 to 0.20%
Si: 0.05 to 0.50%,
Mn: 0.8 to 3.0%,
P: 0.02% or less,
S: 0.0050% or less,
Al: 0.005 to 0.1%,
N: 0.0070% or less,
Ceq defined by the following formula (1) satisfies 0.38 to 0.55%, the balance is composed of Fe and inevitable impurities, and the microstructure is mainly composed of ferrite phase and bainite phase.
The hard phase includes an island-like martensite having an average equivalent circle diameter of 1 to 10 μm and an average aspect ratio of 4.0 or less in an area fraction of 3 to 30%. Low yield ratio high strength thick steel sheet with a yield strength (YP) of 650 MPa or more, a tensile strength of 780 MPa or more , and a yield ratio (YR) of 80% or less.
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
(1)
However, C, Mn, Si, Ni, Cr, Mo, V: Elements not contained in the content (mass%) of each element shall be 0. The steel slab comprising the steel composition according to claim 1 is heated to 1000 to 1250 ° C, and after hot rolling is finished in a temperature range of 800 ° C or higher, 5 to 60 ° C / s from a temperature range of Ar ₃ points or higher. After cooling to a temperature range of Ar ₃ -400 ° C. to Ar ₃ -100 ° C. at a cooling rate of ₁ , the cooling is temporarily interrupted, and then the temperature is increased to 0.5 ° C./s or more to a temperature range of Ac ₁ point or less After reheating at a temperature rate, air cooling, and further tempering at 400 ° C. or more and Ac ₁ point or less, yield strength (YP) is 650 MPa or more, tensile strength is 780 MPa or more , yield ratio (YR ) Is a method for producing a low yield ratio high strength thick steel plate having a strength of 80% or less.