JP2010138421A - Thick steel plate with low yield ratio and high strength, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate with a low yield ratio and a high strength, which has a tensile strength (TS) of 1,000 MPa or more, a yield ratio (YR) of 85% or less and little anisotropy of the material, and to provide a method for manufacturing the same. <P>SOLUTION: The thick steel plate has a steel composition comprising a specific amount of C, Si, Mn, P, S, Al and N, one or more elements of Cu, Ni, Cr, Mo, Nb, V, Ti, B, Ca, REMs and Mg as needed, while controlling Ceq so as to satisfy 0.38-0.60%, and the balance Fe with unavoidable impurities; and has a constitution structure of a mixed structure of a martensite phase in which an average circle-equivalent diameter of former austenite grains is 30-100 μm and an average aspect ratio is 4 or less, and a bainite phase. Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14, wherein C, Si, Mn, Ni, Cr, Mo and V represent the contents (mass%) of respective elements; and the content of an element which is not contained is made zero. The manufacturing method includes hot-rolling the steel having the above composition, then acceleratingly cooling the plate and reheating the plate. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  INDUSTRIAL APPLICABILITY The present invention is suitable for construction, civil engineering, and the like, and has a tensile strength (TS) of 1000 MPa or more, a yield ratio (YR) of 85% or less, and a low yield ratio high strength thick steel plate with little material anisotropy and its manufacture. Regarding the method.

  In recent years, the demand for thick, high-strength steel has been increasing with the increase in the size and span of building structures. However, in application, it has a high allowable stress from the viewpoint of the safety of steel structures. , It is required to have a low yield ratio.

  Patent Documents 1 to 4 relate to a method of manufacturing a low yield ratio high strength thick steel sheet having a yield strength exceeding 650 MPa. Patent Documents 1 and 2 disclose that after the steel sheet after hot rolling is quenched, the ferrite + It is described that the two phases of austenite are heated and quenched to achieve high strength and low yield ratio.

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

In Patent Document 4, ferrite is precipitated by a direct quenching method that delays the start of quenching after rolling, and then rapidly cooled to obtain a two-phase structure of a ferrite phase and a martensite phase, thereby increasing the strength and reducing the yield ratio. It is described to achieve
JP 2001-288512 A JP-A-6-248337 JP-A-5-230530 JP-A-7-97626

  As described above, the multi-stage heat treatment including reheating and quenching in the ferrite + austenite two-phase region is common as a manufacturing process for low yield ratio and high strength thick steel plates. Since bainite or martensite is dispersed as two phases, it is difficult to stably achieve a high strength with a tensile strength of 1000 MPa or more.

On the other hand, high-strength thick steel plates with a tensile strength of 1000 MPa or more are rolled, immediately or after reheating, quenched from a temperature range of Ac ₃ points to a low temperature range, and then used in a heat treatment furnace at a temperature range of Ac ₁ points or less When manufactured by tempering for a long time, the structure becomes a tempered martensite phase, and a yield ratio of 85% or less cannot be obtained.

  Moreover, the steel sheet obtained by the direct quenching process after the controlled rolling in the low temperature range of austenite has a large difference in material properties between the rolling direction and the direction perpendicular to the rolling, and material anisotropy remarkably appears.

  Note that the techniques described in Patent Document 1, Patent Document 2 and Patent Document 3 require a complicated heat treatment process, resulting in low production efficiency and high production cost. In the technique described in Patent Document 4, the volume fraction of the ferrite and martensite phases changes depending on the manufacturing conditions and the position in the steel sheet, so that it is not possible to achieve high strength and low yield ratio stably. Isotropic.

  The present invention solves the above-mentioned problems, does not perform complicated heat treatment, has a tensile strength (TS) of 1000 MPa or more, a yield ratio (YR) of 85% or less, and a low yield ratio high strength thickness with a small material anisotropy. It aims at providing a steel plate and its manufacturing method.

In order to achieve the above-mentioned problems, the present inventors diligently studied various factors affecting strength, yield ratio, and material anisotropy, and obtained the following knowledge.
1. In order to stably achieve both a tensile strength of 1000 MPa or more and a low yield ratio of 85% or less, and to achieve a small material anisotropy, a carbon equivalent Ceq of 0.38 to The component composition of 0.60% is essential.
2. Further, it is important to control the microstructure of the steel sheet to a mixed structure of martensite phase and bainite phase in which the average equivalent circle diameter of the prior austenite grains is 30 μm to 100 μm and the average aspect ratio is 4 or less.
3. The microstructure is formed by subjecting the steel material with the above components adjusted to hot rolling with an appropriate end temperature, and then an accelerated cooling process with an optimized cooling rate and cooling stop temperature.
4). Furthermore, when reheating treatment is performed to adjust the strength-toughness balance in the microstructure, when the heating rate, reheating temperature, and holding time are optimized, the tempered microstructure is 1000 MPa or more. The tensile strength of the steel and a low yield ratio of 85% or less can be stably achieved, and a small material anisotropy can be obtained.

The present invention has been completed on the basis of the above-described findings and has been completed. That is, the gist of the present invention is as follows.
1. Steel composition is mass%,
C: 0.03-0.2%,
Si: 0.05 to 0.5%,
Mn: 0.8 to 3%,
P: 0.02% or less S: 0.005% or less Al: 0.1% or less N: 0.007% or less, and Ceq defined by the following formula is 0.38 to 0.60% A martensitic phase and bainite satisfying and having a balance consisting of Fe and inevitable impurities, the composition of which is an austenite grain having an average equivalent circular diameter of 30 μm to 100 μm and an average aspect ratio of 4 or less A low-yield-strength, high-strength steel plate with a tensile strength (TS) of 1000 MPa or more and a yield ratio (YR) of 85% or less.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
Here, C, Si, Mn, Ni, Cr, Mo, V: Elements not included in the content (mass%) of each element are 0.
2. In mass%,
Cu: 0.1 to 1%
Ni: 0.1 to 3%
Cr: 2% or less Mo: 1% or less Nb: 0.1% or less V: 0.2% or less Ti: 0.03% or less B: 0.005% or less Ca: 0.005% or less REM: 0.02 2. The low yield ratio high strength thick steel plate according to 1, having a composition containing not more than 1% and Mg: not more than 0.005%.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
Here, C, Si, Mn, Ni, Cr, Mo, V: Content of each element (mass%)
3. After slab or steel slab having the steel composition described in 3.1 or 2 is heated to 1000 to 1250 ° C, it is hot-rolled in a temperature range of 880 ° C or higher, and subsequently from a temperature range of Ar ₃ or higher. A method for producing a low-yield-ratio high-strength thick steel sheet, characterized by performing accelerated cooling to a temperature range of 350 ° C. or lower at a cooling rate of 5 to 60 ° C./s.
4). After accelerated cooling, it was further reheated to a temperature range of 350 to 550 ° C. at a temperature rising rate of 2 ° C./s or more, and the reheating temperature was 10 min. 3. The method for producing a low yield ratio high strength thick steel sheet according to 3, wherein the steel sheet is cooled after being held.

  According to the present invention, a thick steel plate having a low material anisotropy having excellent base material toughness, a tensile strength of 1000 MPa or more and a low yield ratio of 85% or less can be stably produced without complicated heat treatment. It can be manufactured, greatly contributes 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 component composition and the microstructure are defined.

[Ingredient composition] Unless otherwise indicated, the "%" display regarding a component shall mean the mass%.
C: 0.03-0.2%
C is an element useful for increasing the strength of the steel and ensuring the strength necessary for structural steel, and requires a content of 0.03% or more. On the other hand, if the content exceeds 0.2%, the HAZ toughness and weld crack resistance deteriorate, and the toughness of the base material deteriorates. For this reason, C is limited to the range of 0.03 to 0.2%. In addition, Preferably, it is 0.05 to 0.15%.

Si: 0.05-0.5%
Si acts as a deoxidizing material, and at least 0.05% is necessary for steelmaking. However, if it exceeds 0.5%, the toughness of the base metal deteriorates and weldability and HAZ toughness are notable. to degrade. For this reason, Si is limited to the range of 0.05 to 0.5%. In addition, Preferably, it is 0.05 to 0.35%.

Mn: 0.8 to 3%
Mn has the effect of increasing the strength of the steel, and in order to ensure a tensile strength of 1000 MPa or more, it needs to be contained by 0.8% or more. On the other hand, if the content exceeds 3%, the toughness and the HAZ toughness of the base material are remarkably deteriorated. For this reason, Mn is limited to the range of 0.8 to 3%. In addition, 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, P should be reduced as much as possible. If P is contained in excess of 0.02%, this tendency becomes remarkable. In addition, excessive P reduction raises the refining cost and is economically disadvantageous, so 0.002% or more is desirable.

S: 0.005% 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. If S is contained in excess of 0.005%, 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, which contributes to improving the toughness of the base metal. Such an effect is recognized when Al: 0.005% or more is contained.

  On the other hand, if the content exceeds 0.1%, the toughness of the base metal decreases, and it is mixed into the weld metal part during welding to deteriorate the toughness. Therefore, the content is limited to 0.1% or less. In addition, Preferably, it is 0.01 to 0.07%.

N: 0.007% or less N is contained in steel as an unavoidable impurity, and if it exceeds 0.007%, the toughness of the base metal and welded portion is remarkably lowered. For this reason, it is limited to 0.007% or less.

Ceq: 0.38-0.60%
In the present invention, Ceq (= C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14, provided that C, Si, Mn, Ni, Cr, Mo, V are the contents of each element (mass%) and are not contained. The content of each component is adjusted so that Ceq is 0.38 to 0.60% including elements to be described later.

  If Ceq is less than 0.38%, the hardenability during rolling and accelerated cooling is insufficient, and a desired tensile strength of 1000 MPa or more cannot be ensured. On the other hand, if the Ceq exceeds 0.60%, the base material toughness decreases, so the Ceq is set to 0.38 to 0.60%.

  In the present invention, in order to further improve desired characteristics, the above-described basic component system is selected from Cu: 0.1 to 1%, Ni: 0.1 to 3% as a selective component, or 2 or more, and / or Cr: 2% or less, Mo: 1% or less, Nb: 0.1% or less, V: 0.2%, Ti: 0.03% or less, B: 0.005% or less 1 or 2 or more types selected from among these, and / or Ca: 0.005% or less, REM: 0.02% or less, and Mg: 0.005% or less More than seeds can be contained.

One or more selected from Cu: 0.1 to 1%, Ni: 0.1 to 3% Cu and Ni are elements that can increase strength while maintaining high toughness. , Because it has little influence on HAZ toughness, it is a useful element for increasing strength.

  Cu is preferably contained in an amount of 0.1% or more. However, if the content exceeds 1%, hot brittleness is caused to deteriorate the surface properties of the steel sheet. Therefore, Cu is limited to a range of 0.1 to 1%. To do. In addition, Preferably, it is 0.2 to 0.7%.

  Ni is preferably contained in an amount of 0.1% or more, but if it exceeds 3%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Ni is limited to 0.1 to 3%. In addition, Preferably it is 0.2 to 2.8%.

1 selected from Cr: 2% or less, Mo: 1% or less, Nb: 0.1% or less, V: 0.2% or less, Ti: 0.03% or less, B: 0.005% or less The seeds or two or more kinds of Cr, Mo, Nb, V, Ti, and B are elements that contribute to improving the strength of the steel.

  Cr is preferably contained in an amount of 0.05% or more, but if it exceeds 2%, the HAZ toughness is deteriorated. For this reason, it is desirable to limit Cr to 2% or less.

  Mo is preferably contained in an amount of 0.05% or more, but the inclusion exceeding 1% adversely affects the base metal toughness and the HAZ toughness. For this reason, it is desirable to limit Mo to 1% or less.

  Nb is preferably contained in an amount of 0.005% or more, but the content exceeding 0.1% degrades the base metal toughness and the HAZ toughness. For this reason, it is desirable to limit Nb to 0.1% or less.

  V is preferably contained in an amount of 0.01% or more, but if it exceeds 0.2%, the HAZ toughness is deteriorated. For this reason, it is desirable to limit V to 0.2% or less.

  Containing 0.005% or more of Ti contributes to strength improvement, and has strong affinity with N, precipitates as TiN during solidification, and suppresses the austenite grain coarsening in HAZ, thereby increasing the toughness of HAZ. Contribute to. On the other hand, if it exceeds 0.03%, the toughness of the base metal is deteriorated. For this reason, it is desirable to limit Ti to 0.03% or less.

  B has the effect of increasing the strength of the steel through improving hardenability. On the other hand, if the content exceeds 0.005%, the hardenability is remarkably increased and the toughness and ductility of the base material are deteriorated. For this reason, B is limited to 0.005% or less. In addition, Preferably, it is 0.0003 to 0.002%.

One or more selected from Ca: 0.005% or less, REM: 0.02% or less, and Mg: 0.005% or less Ca, REM, and Mg are all elements that contribute to toughness improvement It is.
Ca: 0.005% or less Ca 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 contained in excess of 0.005%. Since the effect is saturated, 0.005% is made the upper limit.

  REM is preferably contained in an amount of 0.002% or more, but even if contained over 0.02%, the effect is saturated, so 0.02% is 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 the effect is saturated even if contained over 0.005%. The upper limit is 0.005%. The balance other than the above components is Fe and inevitable impurities.

[Microstructure]
The microstructure is a mixed structure of martensite phase and bainite phase in which the average equivalent circle diameter of the prior austenite grains is 30 μm to 100 μm and the average aspect ratio is 4 or less.

  If the average equivalent circle diameter of the prior austenite grains is less than 30 μm, the hardenability is lowered and a mixed structure of martensite phase and bainite phase cannot be obtained, so that a tensile strength of 1000 MPa or more cannot be satisfied.

  On the other hand, when the average equivalent circle diameter is 100 μm or more, the toughness decreases. For this reason, the average equivalent circle diameter of the prior austenite grains is limited to a range of 30 to 100 μm.

  When the structural structure is not a mixed structure of martensite phase and bainite phase, a high strength with a tensile strength of 1000 MPa or more and a low yield ratio of 85% or less cannot be obtained. The martensite phase and the bainite phase, which are quenched immediately after rolling, have a very high dislocation density and a very hard phase. High strength and low yield ratio can be achieved by suppressing excessive rise.

  Furthermore, if the above microstructure is reheated to adjust the balance between strength and toughness, it becomes a tempered martensite phase and a bainite phase. Generally, the yield strength is significantly increased due to the disappearance of movable dislocations. Invite.

  Therefore, in the present invention, as will be described later, in the reheating treatment, the heating rate, the reheating temperature, and the holding time are optimized and the tempered microstructure has a tensile strength of 1000 MPa or more and 85%. It achieves the following low yield ratio stably and achieves small material anisotropy. Therefore, in the present invention, the martensite phase and the bainite phase include a tempered martensite phase and a bainite phase.

  In addition to the martensite phase and bainite phase, the strength decreases when microstructures such as ferrite, pearlite, and cementite are mixed, and the toughness decreases when island martensite is mixed. Less is better. However, the influence of the mixed organization is negligible when the area fraction is 10% or less.

  If the average aspect ratio of the prior austenite grains is larger than 4, the material anisotropy increases, so it is set to 4 or less. In addition, if the microstructure prescribed | regulated by this invention is satisfied in the 90% or more area | region of a plate | board thickness cross-section part, it can be set as the thick steel plate provided with the desired performance.

  The structure of the prior austenite grains was identified by observing with a 500 times optical microscope after corroding the sample with picric acid, and the average equivalent circle diameter and average aspect ratio were taken with an optical microscope with 500 times magnification. An image can be obtained using an image analyzer.

  The thick steel plate according to the present invention is preferably produced by the following production method. In the description, the temperature “° C.” means a temperature of 1/2 t part thickness unless otherwise specified.

  The molten steel having the above composition is melted by a generally known method such as a converter, electric furnace, vacuum melting furnace or the like and then cooled to obtain a steel material. The obtained steel material is hot-rolled after being reheated to 1000 ° C to 1250 ° C.

  If the reheating temperature is less than 1000 ° C, the deformation resistance in hot rolling becomes high, the amount of reduction per pass cannot be increased, the number of rolling passes increases, the rolling efficiency decreases, and the steel material (slab ) Since the casting defect in the case may not be pressure-bonded, the temperature is set to 1000 ° C. or higher.

  On the other hand, when the reheating temperature exceeds 1250 ° C., surface flaws are likely to occur due to the scale during heating, and not only the maintenance load after rolling increases, but also the austenite grains before rolling become coarse, and after the rolling, prior austenite Since the average crystal grain size of the grains exceeds 100 μm and the toughness is lowered, the reheating temperature of the steel material is set to a range of 1000 to 1250 ° C.

  In hot rolling, the rolling end temperature is set to 880 ° C. or higher. When the rolling end temperature is less than 880 ° C., rolling recrystallization is caused and the average crystal grain size of the prior austenite grains cannot be made 30 μm or more. In addition, the average aspect ratio of the prior austenite grains increases, cannot satisfy 4 or less, and material anisotropy occurs.

  In hot rolling, it is only necessary to obtain a predetermined thickness and shape, and other conditions are not particularly specified. However, in the case of an extremely thick steel plate having a plate thickness exceeding 80 mm, it is preferable to secure at least one or more rolling passes with a reduction rate of 15% or more per pass for zaku pressure bonding.

After the end of rolling, accelerated cooling is performed from a temperature range of ₃ or more points of Ar to a temperature range of 350 ° C. or less at an average cooling rate of 5 to 60 ° C./s.

  When the cooling rate is less than 5 ° C./s, the area fraction of the ferrite phase and the pearlite phase increases in the microstructure after accelerated cooling, the desired microstructure cannot be satisfied, and a tensile strength of 1000 MPa or more cannot be ensured.

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

  If the cooling stop temperature of accelerated cooling exceeds 350 ° C, the desired tensile strength of 1000 MPa or more cannot be secured. On the other hand, the stop temperature may be room temperature or higher, but considering productivity and economy, it is 150 ° C or higher. It is preferable that

  When tempering is performed after completion of accelerated cooling, reheating is performed at a temperature increase rate of 2 ° C./s or more to a temperature range of 350 to 550 ° C. for 10 min. Cool after holding below.

  If the rate of temperature increase is less than 2 ° C./s, it takes a long time to reach the target reheating temperature, resulting in a decrease in production efficiency. When the reheating temperature is less than 350 ° C., the effect of improving the base material toughness and ductility is not exhibited.

  On the other hand, when the reheating temperature exceeds 550 ° C., the mobile dislocations in the martensite phase and the bainite phase disappear, leading to a significant increase in yield strength, and the yield ratio of 85% or less cannot be satisfied.

  The holding time after reheating is 10 min. So as not to inhibit productivity. Is the upper limit. If the entire cross section of the steel sheet is heated uniformly to the target reheating temperature, 10 min. Less than a short time may be used.

  Reheating means include atmospheric furnace heating, gas flame, induction heating, etc. In consideration of economy and controllability, induction heating is preferable. The cooling method after reheating is not particularly specified.

  Steel materials of various compositions were manufactured by a converter, ladle refining, and continuous casting method, and made into thick steel plates of various thicknesses by hot rolling, accelerated cooling, reheating, and cooling. Table 1 shows the composition of the test steel, and Table 2 shows the conditions of hot rolling-accelerated cooling-reheating-cooling.

A JIS No. 4 tensile test piece was sampled from the position of half the thickness of each thick steel plate obtained, and a tensile test was carried out according to JISZ 2241 to examine the tensile properties. In addition, three V-notch test specimens were collected from ½ position of each thick steel plate obtained in accordance with JISZ2202, and Charpy impact test was conducted in accordance with JISZ2242, and the absorbed energy at 0 ° C. The average value of (vE ₀ ) was determined and the base metal toughness was evaluated. The tensile test piece and the V-notch test piece were taken from the rolling longitudinal direction (L direction) and the rolling width direction (C direction), and the material anisotropy was evaluated.

  Tensile strength 1000 MPa or more, yield ratio 85% or less, absorbed energy at 0 ° C. vEo> 100 J, and difference in tensile strength between the rolling longitudinal direction (L direction) and the rolling width direction (C direction) is less than 30 MPa. Target characteristics were used. Further, a sample for observing the microstructure was taken from the position of the thickness ½ of each obtained steel plate, and the microstructure of the cross section parallel to the rolling direction was investigated.

  The obtained results are shown in Table 3. No. 1-1, 1-2, 2-1, 3, 4-1, 5-8, 9 are all examples of the present invention. Tensile strength of 1000 MPa or more, yield ratio of 85% or less, absorbed energy at 0 ° C. It has a high strength, low yield ratio and high toughness of the base material with vEo> 100J, and the difference in tensile strength between the rolling longitudinal direction (L direction) and the rolling width direction (C direction) is less than 30 MPa, and the material is anisotropic. The nature is small.

  On the other hand, no. 1-3, 1-4, 1-5, 2-2, 4-2, and 8-2 are comparative examples, and any of anisotropy of base material strength, yield ratio, base material toughness, and tensile strength Or several characteristics do not satisfy the target value.

  No. In 1-3, the accelerated cooling rate after rolling was as slow as 2 ° C., and the microstructure was outside the scope of the present invention. 1-4 has a rolling end temperature as low as 830 ° C., and the microstructure is outside the scope of the present invention. 1-5 has a high yield ratio because the reheating temperature after stopping accelerated cooling is as high as 650 ° C.

  No. In No. 2-2, the stop temperature of accelerated cooling after rolling is as high as 510 ° C., and the microstructure is outside the scope of the present invention. 4-2 has a long reheating temperature holding time after stopping accelerated cooling, and the tensile properties are outside the scope of the present invention. No. No. 8-2 has a rolling end temperature as low as 800 ° C., and the microstructure is outside the scope of the present invention. 10-14 have a component composition outside the scope of the present invention.

  No. 1-4, no. No. 8-2 has a rolling end temperature of 880 ° C. or lower and is low. No. 13 has a C content outside the scope of the present invention, and in both cases, the difference in tensile strength between the rolling longitudinal direction (L direction) and the rolling width direction (C direction) is 30 MPa or more, and the material anisotropy is large.

Claims (4)

Steel composition is mass%,
C: 0.03-0.2%,
Si: 0.05 to 0.5%,
Mn: 0.8 to 3%,
P: 0.02% or less S: 0.005% or less Al: 0.1% or less N: 0.007% or less, and Ceq defined by the following formula is 0.38 to 0.60% A martensitic phase and a bainite satisfying and having a balance composed of Fe and inevitable impurities, and having a structure with an average equivalent circle diameter of prior austenite grains of 30 μm to 100 μm and an average aspect ratio of 4 or less A low-yield-strength, high-strength steel plate with a tensile strength (TS) of 1000 MPa or more and a yield ratio (YR) of 85% or less.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
Here, C, Si, Mn, Ni, Cr, Mo, V: Elements not included in the content (mass%) of each element are 0. In mass%,
Cu: 0.1 to 1%
Ni: 0.1 to 3%
Cr: 2% or less Mo: 1% or less Nb: 0.1% or less V: 0.2% or less Ti: 0.03% or less B: 0.005% or less Ca: 0.005% or less REM: 0.02 The low yield ratio high-strength thick steel plate according to claim 1, having a composition containing at least one of Mg and 0.005% or less. A slab or steel slab comprising the steel composition according to claim 1 or 2 is heated to 1000 to 1250 ° C, and then hot-rolled in a temperature range of 880 ° C or higher, and subsequently from a temperature range of Ar ₃ or higher. A method for producing a high yield thick steel plate having a low yield ratio, characterized by performing accelerated cooling to a temperature range of 350 ° C. or lower at a cooling rate of 5 to 60 ° C./s.   After accelerated cooling, it was further reheated to a temperature range of 350 to 550 ° C. at a temperature rising rate of 2 ° C./s or more, and the reheating temperature was 10 min. The method for producing a low yield ratio high strength thick steel sheet according to claim 3, wherein the steel sheet is cooled after being held.