JP2003147477A - OVER 700 MPa CLASS NON-HEATTREATED LOW YIELD RATIO THICK STEEL PLATE, AND PRODUCTION METHOD THEREFOR - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heattreated thick steel plate having high strength, a low yield ratio, and high toughness, and to provide a production method therefor. SOLUTION: The steel plate contains 0.005 to 0.03% C, 0.05 to 0.5% Si, 1.5 to 3.0% Mn, 0.01 to 0.08% Al, 0.05 to 1.00% Mo, and 0.005 to 0.08% Nb, and a structure containing a bainitic ferrite phase in >=80% by an area ratio, or further containing insular martensite in <=2% by an area ratio, and the packet size of the bainitic ferrite is controlled to <=15 μm. Thus, a tensile strength of >700 MPa, a yield ratio of <=80%, and a fracture appearance transition temperature vTrs of <=-20 deg.C can be obtained. The steel plate can further contain one or more kinds selected from Cu, Ni, Cr, Ti, V and B, and/or one or more kinds selected from Ca% and rare earth metals.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-heat treated thick steel plate, and more particularly to bridges, construction, shipbuilding, construction machinery, industrial machinery,
The present invention relates to a thick steel plate having high strength, low yield ratio and high toughness, which is suitable for use in offshore structures, pen stocks and the like.

[0002]

2. Description of the Related Art Conventionally, thick steel sheets with a tensile strength (TS) of 700 MPa or higher are well-balanced to secure high strength and high toughness.
It was manufactured after being subjected to a tempering treatment. The temper treatment has the advantage that a product having high strength and excellent characteristics of high toughness can be stably produced, but the steel sheet subjected to the temper treatment is
Compared with the as-rolled non-heat treated steel sheet, there is a problem that a longer production period is required and the production cost becomes higher.

[0003] In response to such problems, attempts have been made to manufacture thick steel plates having high strength of TS: 700 MPa or more as they are rolled. For example, in Japanese Patent Laid-Open No. 8-188823, C,
A method for producing a TS: 780MPa grade non-heat treated thick steel sheet has been proposed in which a material with an appropriate amount of Si, Mn, Mo, and Nb added is subjected to dual phase rolling. However, when hot rolling is performed in the two-phase region, separation easily occurs, and there is a problem that absorbed energy in the Charpy impact test decreases.

Further, in Japanese Patent Laid-Open No. 11-264107, a steel slab having an extremely low C and a high Mn and containing an appropriate amount of Cu, Nb and B is heated to 1000 to 1350 ° C. and then 950 to 1100 ° C. Hot rolling with a cumulative reduction in the temperature range of 50% or more and a rolling end temperature of 800 ° C or more, and 1 ° C /
cooling at a cooling rate of s or more and 30 ° C / s or less, (Ar ₃ -2
A method for producing a non-heat treated high-strength steel is proposed, in which cooling is stopped at a temperature of 0 ° C) or lower and 550 ° C or higher. According to this technique, T
S: It is said that high-strength steel materials of 700 MPa class can be manufactured without heat treatment. However, the steel sheet manufactured by the technique described in Japanese Patent Application Laid-Open No. 11-264107 has a high yield ratio of 80% or more, and there is a problem in applying it to structural members requiring seismic resistance.

[0005]

The present invention advantageously solves the problems of the prior art described above, and has a high tensile strength TS: 700 MPa or more, a low yield ratio of 80% or less, and a fracture surface transition temperature. vTr
s: An object of the present invention is to propose a non-heat treated thick steel plate having a high toughness of -20 ° C or less and a manufacturing method thereof.

[0006]

[Means for Solving the Problems] In order to achieve the above-mentioned objects, the present inventors diligently studied the influence of the structure on the toughness in a non-heat treated high strength steel sheet of TS: 700 MPa grade or higher. As a result, in order to secure high toughness of fracture surface transition temperature vTrs: -20 ° C or less, ultra low carbon is used to prevent the formation of island martensite, or even if the area ratio is 2%.
It has been found that it is essential to reduce the amount to less than 80% and to have a structure containing a bainitic ferrite phase with an area ratio of 80% or more. Since bainitic ferrite undergoes shear transformation, coarsening of the structure is unlikely to occur and high toughness is easily secured.

The "bainitic ferrite" referred to in the present invention is a bundle structure having laths but no carbides (however, initial austenite grain boundaries remain) (Sheaf-
likewith laths but no carbide: conserving the prio
rγ-grain boundary) (Steel Bainite Photo Book-I:
Japan Iron and Steel Institute, Bainite Research Group, (1992) 4).

Further, since the island martensite has a very hard structure, the interface between the matrix and the island martensite is liable to peel off, which easily becomes the starting point of fracture. Therefore, if the formation of island-shaped martensite is prevented or if the area ratio is reduced to 2% or less, the influence on the decrease in toughness is small. From this, the present inventors, in order to obtain a thick steel sheet having high strength and high toughness, the structure is a structure mainly composed of bainitic ferrite, and further, island martensite, It has been thought that it is preferable to have a structure in which the area ratio is reduced to 2% or less even if it exists.

The present invention has been completed by further studies based on the above findings. That is, the present invention is mass%, C: 0.005 to 0.03%, Si: 0.05 to 0.5
%, Mn: 1.5 to 3.0%, Al: 0.01 to 0.08%, Mo: 0.05 to
1.00%, Nb: 0.005 to 0.08%, the composition consisting of the balance Fe and unavoidable impurities, and the bainitic ferrite phase with an area ratio of 80% or more, or island martensite with an area ratio of 2 % Or less, and the packet size of the bainitic ferrite phase is 15 μm.
A 700 MPa super grade non-heat treated low yield ratio thick steel sheet characterized by having a tensile strength of more than 700 MPa, a yield ratio of 80% or less, and a fracture surface transition temperature vTrs of -20 ° C or less. Also,
In the present invention, in addition to the above composition, in addition, in mass%, Cu:
0.05 to 2.0%, Ni: 0.05 to 2.0%, Cr: 0.05 to 2.0%, T
i: 0.003 to 0.050%, V: 0.003 to 0.080%, B: 0.00
It is preferable that the composition contains one or more of 03 to 0.0030%, and / or one or more of Ca: 0.0003 to 0.0030% and REM: 0.0003 to 0.010%.

In the present invention, the mass% is C: 0.005 to.
0.03%, Si: 0.05 to 0.5%, Mn: 1.5 to 3.0%, Al: 0.
01-0.08%, Mo: 0.05-1.00%, Nb: 0.005-0.08%, or mass% Cu: 0.05-2.0%,
Ni: 0.05 to 2.0%, Cr: 0.05 to 2.0%, Ti: 0.003 to 0.
050%, V: 0.003 to 0.080%, B: 0.0003 to 0.0030%
1 type or 2 types or more, and / or Ca: 0.00
A steel material containing 03-0.0030% and one or more of REM: 0.0003-0.010% and having the composition of balance Fe and unavoidable impurities is heated to 1100 ° C to 1350 ° C, and then the steel is heated. The material is hot-rolled with a cumulative reduction of 30% or more in the temperature range of 1000 to 1250 ° C, a cumulative reduction of 30% or more in the austenite unrecrystallized region, and a rolling end temperature of Ar ₃ points or more. After completion of hot rolling, cooling rate: 0.1-20
A method for producing a non-heat treated high strength low yield ratio high toughness steel plate characterized by cooling at ℃ / s.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the composition of the thick steel plate of the present invention will be described. Hereinafter, mass% in the composition will be simply referred to as%. C: 0.005 to 0.03% C is an element that increases the strength of steel. In the present invention, the content of 0.005% or more is required to secure a predetermined strength, but if it exceeds 0.03%, island-shaped martens are contained. The area ratio of the site tends to exceed 2%, and the toughness tends to deteriorate. Therefore, C is limited to 0.005 to 0.03%. In addition, it is preferably 0.015 to 0.022%.

Si: 0.05 to 0.5% Si is an element that acts as a deoxidizing agent. In the present invention, it is necessary to contain 0.05% or more in steelmaking, but if it exceeds 0.5%, toughness is deteriorated. . Therefore, Si is 0.05 ~
It was limited to the range of 0.5%. Incidentally, preferably, 0.20-0.
35%. Mn: 1.5 to 3.0% Mn is an element that increases the strength of steel. To make the tensile strength of the base metal exceed 700 MPa and to make the structure a bainitic ferrite structure, 1.5% or more is required. Requires inclusion. On the other hand, if the content exceeds 3.0%, the toughness of the welded portion is significantly deteriorated. Therefore, Mn is 1.5
Limited to ~ 3.0% range.

Al: 0.01 to 0.08% Al acts as a deoxidizing agent, and for this purpose it is necessary to contain 0.01% or more, but if it exceeds 0.08%, the toughness of the base material is lowered and , The toughness of the weld metal part is deteriorated by dilution into the weld metal part. Therefore, Al is 0.
It was limited to the range of 01-0.08%. In addition, preferably, 0.02
~ 0.04%.

Mo: 0.05 to 1.00% Mo is an element that increases the strength of steel and effectively acts to increase the strength of the base material. To obtain this effect,
Need to contain more than 0.05%. Further, the inclusion of Mo in an amount of 0.05% or more also has the effect of reducing the yield ratio to 80% or less. On the other hand, if the content exceeds 1.00%, the toughness is adversely affected. Therefore, Mo is limited to the range of 0.05 to 1.00%. The content is preferably 0.20 to 0.50%.

Nb: 0.005 to 0.08% Nb has a function of making the structure a bainitic ferrite structure, and therefore 0.005% or more of Nb must be contained.
On the other hand, if the content exceeds 0.08%, the toughness of the weld heat affected zone deteriorates. Therefore, Nb is limited to the range of 0.005 to 0.08%. In addition, it is preferably 0.01 to 0.06%.

The above are the basic components, but in addition to these components, the following components can be selected and contained. Cu: 0.05 to 2.0%, Ni: 0.05 to 2.0%, Cr: 0.05 to 2.0
%, Ti: 0.003 to 0.050%, V: 0.003 to 0.080%,
B: One or more of 0.0003 to 0.0030% Cu, N
Each of i, Cr, Ti, V, and B is an element that contributes to an increase in strength of steel, and can be contained alone or in combination as necessary.

Cu is an effective element that increases the strength of steel by solid solution strengthening and precipitation strengthening. In the present invention, when it is contained, it is preferable to contain 0.05% or more, but 2.0%
If it is contained in excess of 1.0, the toughness deteriorates. Therefore, Cu
It is preferably limited to 0.05 to 2.0%. Ni is an effective element that can increase the strength while maintaining the toughness of the base material. When it is contained in the present invention, it is preferably contained in an amount of 0.05% or more, but even if it exceeds 2.0%, it is saturated and the effect commensurate with the content cannot be expected, resulting in a cost disadvantage. Therefore, Ni is preferably limited to the range of 0.05 to 2.0%.

Cr is an effective element for increasing the strength of steel, and also has the function of lowering the bainite transformation start temperature and promoting the formation of a bainitic ferrite structure. In the present invention, in order to obtain such an effect, it is preferable to contain 0.05% or more, but if it exceeds 2.0%, toughness deteriorates. Therefore, Cr is 0.05
It is preferable to limit the content to the range of 2.0%.

Ti is an element that increases the strength by precipitation strengthening as Ti (CN), and effectively acts to improve the toughness by making the initial austenite grain size fine. In the present invention, if contained, it is preferable to contain 0.003% or more in order to obtain such effects. On the other hand, if the content exceeds 0.050%, the Ti (CN) particles become coarse and the desired effect cannot be obtained. Therefore, Ti is preferably limited to the range of 0.003 to 0.050%.

V is an element which acts as V (CN) advantageously on the strength increase by precipitation strengthening, and in order to obtain such an effect, it is preferable to contain 0.003% or more. On the other hand, if the content exceeds 0.080%, the toughness decreases. Therefore, V is preferably limited to the range of 0.003 to 0.080%. B is an element that lowers the bainite transformation start temperature and contributes to increasing the strength of steel, and when it is contained in the present invention, it is preferably contained in 0.0003% or more. On the other hand, if the content exceeds 0.0025%, the steel may be significantly hardened and the toughness may be deteriorated. Therefore, B is 0.0003-
It is preferable to limit the range to 0.0025%.

Ca: 0.0003 to 0.0030%, REM: 0.0003 to 0.
Of 010%, one or more of Ca and REM all have the effect of improving the HAZ toughness through morphology control of inclusions. When Ca is added in an amount of 0.0003% or more, the HAZ toughness is improved by appropriately selecting the balance with S and O by controlling the morphology of inclusions. on the other hand,
Even if added over 0.0030%, the effect is saturated. Therefore, Ca is preferably limited to the range of 0.0003 to 0.0030%.

REM forms REM (O, S) to improve HAZ toughness. Such an effect is observed at a content of 0.0003% or more, but even if the content exceeds 0.010%, the effect is saturated. Therefore, REM is preferably limited to 0.0003 to 0.010%. Next, the reasons for limiting the structure of the thick steel plate of the present invention will be described. In the present invention, the structure of the thick steel plate has a bainitic ferrite phase having an area ratio of 80% or more, and the packet size of the bainitic ferrite phase is 15 μm or less. In the present invention, it is preferable to suppress the generation of granular bainitic ferrite and to make the structure mainly composed of bainitic ferrite phase. Since bainitic ferrite undergoes shear transformation, coarsening of the structure is unlikely to occur and high toughness is easily secured. On the other hand, in the granular bainitic ferrite, the diffusion transformation is mainly caused to cause the coarsening of the structure, which deteriorates the toughness.

The "granular bainitic ferrite" referred to in the present invention is a granular bainite structure having a substructure with a high dislocation density (however, it is recovered so that almost no lath remains) (granular bainiti).
c ferritic Zw structure: dislocated substructure bu
(fairly recovered like "lath-less") (Steel Bainite Photographs-I: The Iron and Steel Institute of Japan, Bainite Research Group, (1992) 4).

By using a structure having a bainitic ferrite phase as a main phase, a thick steel sheet having high strength and high toughness can be obtained even in a non-heat treated condition. If the area ratio of the bainitic ferrite phase is less than 80%, it becomes difficult to obtain a thick steel sheet having high strength and high toughness. In order to ensure high toughness, the packet size of bainitic ferrite phase is 15 μm or less,
Need to miniaturize. If the packet size exceeds 15 μm, a coarse structure results, making it difficult to obtain high toughness. A bainitic ferrite packet has a width of 0.2.
It refers to a tissue in which laths with a size of 10 μm and a length of 10 μm gather in parallel. The packet size is determined by image analysis by tracing the packet particle size obtained by observation with an optical microscope or a scanning electron microscope.

Further, since the island martensite has a very hard structure, the interface between the matrix phase and the island martensite is easily separated, and this is the starting point of fracture. No island martensite is generated in the structure, or the area ratio is reduced to 2% or less. When the island martensite content exceeds 2%, the toughness is significantly deteriorated.

In the present invention, the types and contents of the phases other than those mentioned above are not particularly limited, but in terms of area ratio.
Granular bainitic ferrite up to 20% and pseudo-polygonal ferrite up to 5% in area ratio are acceptable. Next, a method of manufacturing the thick steel plate of the present invention will be described. First, molten steel having the composition described above is melted by a commonly known melting method such as a converter, and made into a steel material by a commonly known casting method such as continuous casting.

Then, the steel material is heated to a temperature range of 1100 to 1350 ° C. to completely austenize the steel material. If the heating temperature is less than 1100 ° C, sufficient recrystallization rolling cannot be performed thereafter. On the other hand, when the heating temperature exceeds 1350 ° C., the crystal grains are coarsened, and the oxidation loss becomes remarkable, which lowers the yield. After heating, hot rolling is performed, but in the manufacturing method of the present invention, the cumulative rolling reduction in the temperature range of 1000 to 1250 ° C. is 30%.
As mentioned above, the cumulative rolling reduction in the austenite unrecrystallized region
It is preferable to perform hot rolling with a rolling end temperature of 30% or more and Ar ₃ point or more.

In the present invention, the cumulative rolling reduction in the austenite recrystallization temperature range of 1000 to 1250 ° C. is 30.
% Or more to sufficiently recrystallize austenite and refine crystal grains. Austenite recrystallization temperature range is
950 to 1250 ° C., but particularly in the case of an ultra-low carbon steel such as the present invention, rolling with a rolling reduction of 30% or more in a high temperature region is necessary in order to recrystallize and coarsen coarse γ grains during heating. Become. Of course, after this, rolling in the low temperature recrystallization region is advantageous for further refinement of the austenite grains. Therefore, it is preferable to set the cumulative rolling reduction in the temperature range of 1000 to 1250 ° C to 30% or more. If the cumulative reduction rate is less than 30%, the reduction amount is insufficient, and sufficient austenite grain refinement cannot be achieved. Making the austenite grains fine in advance is effective in making the structure produced by the subsequent transformation fine, and improves the toughness of the finally obtained steel sheet. Further, if Ti is contained and the composition is such that TiN can be dispersed, it is more advantageous for refining the austenite grains.

Further, in the present invention, hot rolling with a cumulative rolling reduction of 30% or more is carried out in the austenite unrecrystallized region (temperature region below 950 ° C.). By rolling at a cumulative rolling reduction of 30% or more in the austenite unrecrystallized region, the area of austenite crystal grain boundaries can be geometrically increased and strain energy due to rolling can be accumulated. This promotes bainite transformation from the austenite grain boundaries and within the austenite grains. Due to the synergistic effect of the strong working in the austenite non-recrystallized region and the above-described fine working of the austenite grains by the strong working in the austenite recrystallized region, the bainite produced becomes bainite having a small packet size. This makes it possible to obtain a thick steel plate having good base material toughness and a low yield ratio.

In the hot rolling, the rolling is finished at a rolling end temperature of Ar ₃ point or higher. The rolling end temperature of hot rolling is Ar
_If it is less than ₃ points, the ferrite will be rolled, separation will occur and the toughness will be significantly reduced. After the hot rolling is finished, the steel sheet is cooled at a cooling rate of 0.1 to 20 ° C / s, preferably 4
Cool to below 00 ℃.

When the cooling rate is less than 0.1 ° C./s, the amounts of granular bainitic ferrite and pseudopolygonal ferrite produced are large. In the granular bainitic ferrite, the packet size of bainite is coarsened, so that the toughness is deteriorated. The term "quasi-polygonal ferrite" as used herein means a ferrite structure in which the grain boundaries are angular.

On the other hand, if the cooling rate exceeds 20 ° C./s, the amount of island martensite produced increases, and the toughness deteriorates.
Therefore, the cooling rate after completion of hot rolling is 0.1
It is preferable to limit it to -20 ° C / s. Further, from the viewpoint of completing the transformation of the bainitic ferrite, the cooling stop temperature is preferably 400 ° C or lower.

[0033]

Example (Example 1) Molten steel having the composition shown in Table 1 was melted in a converter and used as a steel material (slab) by a continuous casting method. Using these slabs (steel materials), the initial plate thickness was variously changed, and hot rolling was performed under the heating, rolling, and cooling conditions shown in Table 2 to obtain steel plates having a plate thickness of 4 to 100 mm.

The microstructure, tensile properties and toughness of these steel sheets were investigated. (1) Microscopic structure A test piece is sampled from each steel plate, the structure is imaged with an optical microscope or a scanning electron microscope at the plate thickness 1 / 2T position in the C direction, and the type of structure is identified using an image analyzer. ,
And the tissue fraction was measured. (2) Tensile properties A JIS No. 5 tensile test piece was taken from the center of the thickness of each steel plate,
A tensile test was carried out in accordance with the regulations of JIS Z 2241, the yield stress YS, the tensile strength TS, and the elongation El were measured, and the yield ratio YR was calculated. (3) Toughness A JIS No. 4 impact test piece was taken from the center of the thickness of each steel plate,
A Charpy impact test was carried out in accordance with JIS Z 2242 to determine the fracture surface transition temperature vTrs.

The results obtained are shown in Table 3.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

In all the examples of the present invention, the packet size is 15 μm.
Bainitic ferrite of m or less has an area ratio of 80% or more, island martensite has a structure of 2% or less, and TS:
High strength of 700 MPa or more, low yield ratio of YR: 80% or less, vT
rs: A thick steel plate having high toughness of -20 ° C or less.
On the other hand, the reduction ratio of the austenite recrystallization temperature range or the reduction ratio of the austenite non-recrystallization temperature range is less than 30% and the comparative example is outside the scope of the present invention (steel sheet No. 9, No. 10, No. 11),
vTrs: When the temperature exceeds -20 ° C, the toughness decreases.

When the cooling rate is less than 0.1 ° C./s, in the comparative example (steel sheet No. 7) out of the range of the present invention, the structure is a mixed structure of granular bainitic ferrite and pseudopolygonal ferrite, and the strength is It is insufficient. Also,
When the cooling rate exceeds 20 ° C./s, the comparative example (Steel plate No. 8), which is out of the range of the present invention, has sufficient strength but lacks toughness. The comparative examples (steel plates No. 14, No. 16 to No. 18) in which any of C, Mo and Nb deviates from the range of the present invention are insufficient in strength. In the comparative example (steel sheet No. 15) in which the C content is more than 0.03%, a large amount of island martensite is formed and the toughness is deteriorated. (Example 2) Molten steel having the composition shown in Table 4 was melted in a converter,
A steel material (slab) was made by the continuous casting method. By using these slabs (steel materials), various initial plate thicknesses can be obtained.
Hot rolling was performed under the heating, rolling, and cooling conditions shown in to obtain a steel plate having a plate thickness of 4 to 100 mm.

Similar to Example 1, the microstructure, tensile properties and toughness of these steel sheets were investigated. The obtained results are shown in Table 6.

[0042]

[Table 4]

[0043]

[Table 5]

[0044]

[Table 6]

In all the examples of the present invention, the packet size is 15 μm.
Bainitic ferrite of m or less has an area ratio of 80% or more, island martensite has a structure of 2% or less, and TS:
High strength of 700 MPa or more, low yield ratio of YR: 80% or less, vT
rs: A thick steel plate having high toughness of -20 ° C or less.
On the other hand, Comparative Examples out of the scope of the present invention, the strength is insufficient, the yield ratio is high, or the toughness is reduced, high strength, high toughness, which is the object of the present invention, the thickness of the low yield ratio Not a steel plate.

[0046]

As described above, according to the present invention, tensile strength TS: high strength exceeding 700 MPa, low yield ratio of 80% or less,
Fracture transition temperature vTrs: Non-heat treated thick steel plate having high toughness of −20 ° C. or less can be easily and stably manufactured, and industrially remarkable effect is exhibited.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/58 C22C 38/58 (72) Inventor Shinichi Amano 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama None) Kawasaki Steel Co., Ltd. Mizushima Works F term (reference) 4E002 AD07 BC05 BD07 4K032 AA01 AA02 AA04 AA08 AA11 AA12 AA14 AA15 AA16 AA17 AA19 AA22 AA23 AA24 AA27 AA29 AA31 AA35 CCA03 CD02 CDA03

Claims (5)

[Claims] 1. In mass%, C: 0.005-0.03%, Si: 0.05-0.5%, Mn: 1.5-3.0%, Al: 0.01-0.08%, Mo: 0.05-1.00%, Nb: 0.005-0.08%. Containing a balance of Fe and unavoidable impurities, and having a structure containing a bainitic ferrite phase of 80% or more in area ratio, the packet size of the bainitic ferrite phase is 15 μm or less, Strength: 700MPa, yield ratio: 80% or less, fracture surface transition temperature vTrs: -20 ° C or less, 700MPa super grade non-heat treated low yield ratio thick steel plate. 2. The structure according to claim 1, wherein the structure contains island martensite in an area ratio of 2% or less.
700MPa super grade non-heat treated low yield specific thickness steel sheet. 3. In addition to the above composition, in mass%, C
u: 0.05 to 2.0%, Ni: 0.05 to 2.0%, Cr: 0.05 to 2.0
%, Ti: 0.003 to 0.050%, V: 0.003 to 0.080%,
B: 0.0003 to 0.0030%, one or more kinds, and / or Ca: 0.0003 to 0.0030%, REM: 0.0003 to
700M according to claim 1 or 2, wherein the composition contains one or more of 0.010%.
Pa super grade non-heat treated low yield ratio thick steel plate. 4. Mass%, C: 0.005-0.03%, Si: 0.05-0.5%, Mn: 1.5-3.0%, Al: 0.01-0.08%, Mo: 0.05-1.00%, Nb: 0.005-0.08% A steel material containing a balance Fe and unavoidable impurities, after heating to 1100 ℃ ~ 1350 ℃, the steel material, the cumulative reduction rate in the temperature range of 1000 ~ 1250 ℃ 30% or more, austenite The cumulative rolling reduction in the unrecrystallized region was 30
% Or more, the hot rolling with the rolling end temperature of Ar ₃ points or more is performed, and after the hot rolling is finished, cooling is performed at a cooling rate of 0.1 to 20 ° C./s. Method for manufacturing high specific toughness steel plate. 5. In addition to the above composition, in mass%, C
u: 0.05 to 2.0%, Ni: 0.05 to 2.0%, Cr: 0.05 to 2.0
%, Ti: 0.003 to 0.050%, V: 0.003 to 0.080%,
B: 0.0003 to 0.0030%, one or more kinds, and / or Ca: 0.0003 to 0.0030%, REM: 0.0003 to
The method for producing a 700 MPa super-grade non-heat treated low yield ratio thick steel sheet according to claim 4, wherein the composition contains one or more of 0.010%.