JPH06240355A - Production of high toughness thick tmcp steel plate - Google Patents

Abstract

PURPOSE:To produce a thick steel plate in which the transition temp. in the central part in the direction of the plate thickness, vTrs, is regulated to <=-70 deg.C and excellent in toughness in the direction of the plate thickness, more specifically, suitable for steel for a massive structure such as a marine structure. CONSTITUTION:A bloom contg. <=0.10% C is heated to the temp. range of 950 to 1150 deg.C, is subjected to rough rolling at >=0.50 average shape ratio in the temp. range of 900 to 1100 deg.C, is moreover subjected to finish rolling at >=0.70 average shape ratio in the temp. range of <=780 deg.C and is immediately water-cooled to the temp. range of <=500 deg.C at <=10 deg.C/sec cooling rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high toughness thick TMCP steel sheet.

[0002]

2. Description of the Related Art As is well known, toughness is a qualitative concept indicating the tenacity of a material, and is evaluated by factors such as the strength of the material and the resistance to fracture. The toughness of steel materials has been evaluated by elongation in a tensile test, drawing, absorbed energy in an impact test, and fracture toughness.

By the way, in recent years, structures used in harsh environments such as offshore structures have been constructed, but some of the steel materials constituting such structures have notches. The crack opening displacement test (COD test, see JIS G 0202: 1604), which shows the fracture resistance of steel materials in some cases, has also become important.

The crack opening displacement test is a test for investigating the fracture characteristics of a material, in which an in-plane bending load is applied to a flat plate test piece with a crack on one side, and the clip gauge allows the relationship between the crack opening displacement and the load. In the normal case, 5% compared to the elastic line
This is a test in which a load intersecting a curve having a gentle slope is obtained and a stress intensity factor corresponding to the load is obtained as a COD value.

Conventionally, high-strength steel sheets of 50 kg class and 60 kg class have been frequently used for the above-mentioned applications, for example, the construction of offshore structures.
Alternatively, it is manufactured by performing normalizing or quenching and tempering after performing hot rolling.

However, the COD value of these steel sheets has not been particularly studied because it has not been a problem so far. In addition, until then, the COD regarding the rolling direction of the steel sheet and the direction orthogonal to the rolling direction in the same plane
The method for increasing the COD value has been slightly studied, but the method for increasing the COD value in the plate thickness direction has not been studied at all.

Therefore, in JP-A-58-58225, in order to increase the COD value in the plate thickness direction, C: 0.3% or less (hereinafter, "%" means "in this specification unless otherwise specified". Weight%), Si: 1.0
% Or less, Mn: 2.5% or less, T.Al: 0.005 to 0.1%, balance
A steel slab consisting of Fe and inevitable impurities is heated at 950 to 1150 ℃
It is heated to the temperature range of 850 to 1000 ℃ and the rolling reduction is 40
% Or more, the rolling reduction per pass is 10% or more, and the rolling reduction is 60% or less in the temperature range of 850 ° C or less, and the finishing temperature T (° C) = (918-369 ・ C + 24.6 ・ Si- 68.1 ・ Mn−
(36.1 ・ Ni-20.7 ・ Cu-24.5 ・ Cr + 29.6 ・ Mo) After reducing to the product thickness as above, immediately by cooling to a temperature range of 400 to 600 ℃ at a cooling rate of 5 ℃ / sec or more,
A method for producing a high yield point steel plate having an excellent COD value in the plate thickness direction has been proposed.

[0008]

Generally, in order to improve the toughness value in the plate thickness direction, a Zaku flaw in the central portion in the plate thickness direction (a void caused by solidification shrinkage of a steel slab remains after rolling) It is effective to reduce the
In order to achieve these, the method proposed by Japanese Patent Laid-Open No. 58-58225 intends to carry out sufficient rolling down to the central portion in the sheet thickness direction. Therefore, the rolling reduction in the temperature range of 850 ° C or lower: It is specified as 60% or less.

The "reduction rate" in this proposal is
((Plate thickness before rolling-plate thickness after rolling) / plate thickness before rolling) x 100
(%) Is meant. However, the rolling reduction in the temperature range of 850 ℃ or less: 60% or less, by limiting the rolling reduction, it is possible to sufficiently roll down to the central portion in the plate thickness direction to reduce the scratch marks and to reduce the grain size. This is possible to some extent in the case of a steel plate having a plate thickness of less than 50 mm, but in the case of a thick steel plate having a plate thickness of 50 mm or more, it is difficult to perform sufficient reduction to the central portion in the plate thickness direction. Therefore, it is not easy to reduce the scratches on the thick steel plate and to reduce the grain size even by the method proposed in Japanese Patent Laid-Open No. 58-82525, and it is a thick material with excellent toughness in the thickness direction. Steel sheets cannot be manufactured stably.

It is an object of the present invention to provide a method for producing a thick steel sheet having excellent toughness in the sheet thickness direction, and more specifically, for use in a low temperature region such as an offshore structure. Another object of the present invention is to provide a method for producing a high toughness thick TMCP steel sheet having a transition temperature vTrs (T direction) at the center in the sheet thickness direction of −70 ° C. or less, which is suitable for use as a steel material.

[0011]

Means for Solving the Problems To improve the toughness of the center portion of the thick steel plate in the plate thickness direction, particularly, the low temperature toughness, the present inventors reduced the scratch marks and crystallized crystals in the center part of the plate thickness direction. Various studies have been repeated from the viewpoint that it is effective to reduce the grain size.

As a result, the inventors of the present invention have found that, in order to reduce the scratches in the central portion in the sheet thickness direction and to reduce the grain size to improve the toughness in the sheet thickness direction, the method disclosed in JP-A-58- In the method proposed by Japanese Patent No. 58225, (i) rough rolling is performed in a high temperature range of 900 to 1100 ° C with an average shape ratio of 0.50 or more, and (ii) a temperature range of 780 ° C or less, in other words, The inventors have found that it suffices to perform finish rolling with a high average shape ratio that secures an average shape ratio of 0.70 or more in the austenite unrecrystallized temperature range, and further conducted various studies to complete the present invention.

The average shape ratio is 2 [R (t ₀ −t ₁ )] ^{1 /} when the steel sheet is rolled down from the thickness t ₀ to t ₁ by a rolling roll having a radius R as shown in FIG. ^{It is} given by ² / (t ₀ + t ₁ ).

According to the knowledge of the present inventors, the rolling force to the central portion in the plate thickness direction is greatly affected not only by the rolling reduction but also by the radius of the rolling roll. Therefore, in order to sufficiently reduce the thickness of the thick steel sheet to the center in the sheet thickness direction to reduce the scratches and reduce the grain size, it is necessary to consider the radius of the rolling roll together with the reduction rate. The above-mentioned average shape ratio is a characteristic value that incorporates both the rolling reduction and the roll radius as control factors.

The gist of the present invention is as follows.
C: 0.10% or less, Si: 0.40% or less, Mn: 1.80% or less, so
l.Al: 0.080% or less, if necessary, Cu: 0.50%
Below, Ni: 1.0% or less, Nb: 0.030% or less, and Ti: 0.
A steel slab having a steel composition consisting of one or more selected from the group consisting of 030% or less and the balance Fe and unavoidable impurities is heated to a temperature range of 950 to 1150 ° C, and then 900
Coarse rolling is performed at an average shape ratio of 0.50 or more in the temperature range of up to 1100 ° C, and finish rolling is performed at an average shape ratio of 0.70 or more in the temperature range of 780 ° C or less, then immediately to a temperature range of 500 ° C or less.
A method of manufacturing a high toughness thick TMCP steel sheet, characterized in that cooling is performed by water cooling, for example, at a cooling rate of 10 ° C./sec or less.

The thick steel plate in the present invention has a plate thickness of 50 mm.
The above hot rolled steel sheet. Its main application is steel for offshore structures. Further, rough rolling refers to rolling from the start of controlled rolling, and finish rolling refers to rolling from the start to end of controlled rolling.

The temperatures of the steel slab and the steel plate specified in the present invention are all based on the surface temperature. This is because, in the thick steel plate targeted by the present invention, the temperature difference between the surface portion and the central portion in the plate thickness direction is usually about 50 ° C., and therefore the measurement site needs to be specified.

FIG. 1 shows an example of a heat cycle of a hot rolling process to which the manufacturing method according to the present invention is applied. In FIG. 1, a steel slab having the above steel composition and a thickness t of 300 mm is 95
After being heated to a temperature range of 0 to 1150 ° C, rough rolling is performed at an average shape ratio of 0.50 or more in a temperature range of 900 to 1100 ° C, and the thickness t is temporarily set.
Was rolled down to 180 mm, and finish rolling was performed at an average shape ratio of 0.70 or more in the temperature range of 780 ° C or less to obtain a hot rolled steel sheet with a thickness of 100 mm, and immediately to a temperature range of 500 ° C or less at 10 ° C / sec
Water cooling is performed at the following cooling rate to obtain a product.

The toughness of the thick steel sheet produced according to the present invention,
Particularly, the low temperature toughness of the central portion in the plate thickness direction is excellent, and the transition temperature vTrs (T direction) is −70 ° C. or lower.

[0020]

The operation of the present invention will be described in detail below. (1) First, the reason for limiting the composition of the steel slab used in the present invention will be described.

C: 0.10% or less C is contained to secure the strength of the product, but 0.10%
If contained too much, the toughness deteriorates, making it unsuitable as a steel material for offshore structures used in low temperature regions. Therefore, in the present invention, the C content is limited to 0.10% or less. Desirably 0.
It is from 03% to 0.10%.

Si: 0.40% or less Si is contained in order to secure the strength, but if it is contained more than 0.40%, the improvement of toughness is suppressed and it becomes unsuitable as a steel material for offshore structures. Therefore, in the present invention, the Si content is 0.
Limited to 40% or less. Desirably, it is 0.05% or more and 0.40% or less.

Mn: 1.80% or less Mn is contained in order to secure the strength of the product, but if the Mn content exceeds 1.80%, the improvement of toughness is suppressed and it becomes unsuitable as a steel material for offshore structures. . Therefore, in the present invention, Mn
The content is limited to 1.80% or less. Desirably, it is 1.00% or more and 1.80% or less.

Sol.Al : 0.080% or less Al is an element that deoxidizes the steel and forms a nitride to have a grain refining effect, but the sol.Al content is 0.080%.
If it exceeds%, inclusions are formed to impair the cleanliness of molten steel. Therefore, in the present invention, the sol.Al content is limited to 0.080% or less. Desirably, it is 0.010% or more and 0.080% or less.

Further, the steel slab used in the present invention has Cu: 0.50% or less, Ni: 1.0% or less, Nb: 0.03, if necessary.
One or more selected from the group consisting of 0% or less and Ti: 0.030% or less may be added for improving strength. Hereinafter, these optional additional elements will be described.

Cu: 0.50% or less Cu is an element that forms a solid solution in steel to enhance strength and weather resistance, but if added in excess of 0.50%, hot brittleness occurs and weldability deteriorates. Not suitable as structural steel. Therefore, when Cu is added, its content is preferably limited to 0.50% or less.

Ni: 1.0% or less Ni is an element that improves the low temperature toughness and prevents hot embrittlement that occurs when Cu is added, and contributes to the strength increase. However, if added over 1.0%, the cost increases significantly. . Therefore,
When Ni is added, its content is preferably limited to 1.0% or less. More preferably, 0.20% or more 1.
It is 0% or less.

Nb: 0.030% or less Nb is an element useful for increasing the tensile strength and improving the toughness by refining the crystal grains, but if added in excess of 0.030%, the toughness of the welded joint is significantly deteriorated. Therefore, when Nb is added, its content is preferably limited to 0.030% or less. More preferably, it is 0.005% or more and 0.030% or less.

Ti: 0.030% or less Ti is an element that increases the strength of the product by precipitation strengthening, but if added over 0.030%, toughness deteriorates. Therefore, when Ti is added, its content is preferably limited to 0.030% or less. More preferably 0.005
% To 0.030% or less. Compositions other than the above are Fe and inevitable impurities. As the inevitable impurities, for example, N can be exemplified, and the content thereof is 0.009% or less.

(2) In the present invention, a steel slab having the above composition is first heated to a temperature range of 950 to 1150 ° C. In the present invention, the heating temperature before rolling is set to 950 ° C. or higher and 1150 ° C. or lower by refining the initial austenite crystal grains, thereby refining the ferrite crystal grains after the austenite / ferrite transformation, and finally making the finished product. This is to improve the toughness of.

FIG. 3 is a graph showing a general relationship between the heating temperature of the billet and the size of the initial austenite grain size. From the figure, it can be seen that the lower the heating temperature of the billet, the finer the grain size of the initial austenite grains can be.

Further, in FIG. 4, C: 0.06%, Si: 0.22%, Mn: 1.44%, P: 0.0, manufactured by the continuous casting method (CC).
007%, S: 0.002%, Cu: 0.27%, Ni: 0.74%, Nb:
A steel slab consisting of 0.012%, Ti: 0.012%, sol.Al: 0.021%, the balance Fe and unavoidable impurities is heated to a temperature range of 1000 to 1200 ° C, and then averaged in a temperature range of 900 to 1100 ° C. After rough rolling at a ratio of 0.51 to 0.60, and finish rolling at an average shape ratio of 0.83 to 0.84 in a temperature range of 780 ° C or lower,
Immediately after water cooling at a cooling rate of 4.5 ° C / sec to a temperature range of 500 ° C or less, the transition temperature vTrs (T direction) and heating temperature at the central part (1/2 t part) in the plate thickness direction were measured. The relationship is shown in the graph.

From the results shown in FIGS. 3 and 4, by limiting the heating temperature of the steel slab to 1150 ° C. or less, the initial austenite crystal grains are refined and the ferrite crystal grains after the austenite / ferrite transformation are refined. As shown in FIG. 4, it can be seen that if the heating temperature is 1150 ° C. or lower, the toughness in the central portion in the plate thickness direction becomes good. Therefore, in the present invention, the upper limit of the heating temperature of the billet is limited to 1150 ° C.

On the other hand, if the heating temperature is lower than 950 ° C., the temperature of the billet decreases by rolling and the deformation resistance increases, which causes a problem during hot rolling. Therefore, in the present invention, the heating temperature of the steel slab before hot rolling is limited to 950 ° C or higher and 1150 ° C or lower.

(3) In the present invention, the steel piece is heated to 950 ° C. or higher and 1150 ° C.
After heating below, rough rolling is performed in the temperature range of 900 to 1100 ° C. with an average shape ratio of 0.50 or more. Rough rolling temperature range 900 ℃
The reason for limiting the temperature to 1100 ° C or less is that if it is less than 900 ° C, the deformation resistance increases and rolling becomes impossible, while if it exceeds 1100 ° C, the initial austenite grain size becomes coarse and the center portion in the plate thickness direction becomes
This is because vTrs exceeds -70 ° C.

The average shape ratio of the rough rolling is limited to 0.50 or more in that the average shape ratio after heating is in the temperature range of 900 to 1100 ° C .:
This is because by performing rough rolling of 0.50 or more, the scratches are reduced and the crystal grains are refined. The larger the average shape ratio of rough rolling, the more desirable.

Table 1 shows a steel slab (thickness: 300 mm) having a composition satisfying the range specified in the present invention, which was obtained by rough rolling to a plate thickness of 180 mm with an average shape ratio of 0.63 and 0.32. Scratch defect reduction rate when a test piece is cut from thick steel plate
(%), RAZ (%) and ferrite grain size number are shown below. In Table 2, the scratch reduction rate (%) is

[0038]

[Equation 1]

RAZ (%) means that the cross-sectional area of the A-A portion of the test piece before the tensile test shown in FIG. 5 (a) is A, and that after the tensile test shown in FIG. 5 (b). When the cross-sectional area of the B-B portion of the test piece is B,

[0040]

[Equation 2]

It is calculated by

[0042]

[Table 1]

From Table 1, the average shape ratio in rough rolling is 0.
It can be seen that when it is 50 or more, the scratch defects are remarkably reduced and the ferrite crystal grains are refined. (4) After rough rolling, finish rolling is performed at a temperature of 780 ° C or lower, that is, in a non-recrystallized region at an average shape ratio of 0.70 or more to a product thickness.

By increasing the average shape ratio in the finish rolling to 0.70 or more, the scratch defects at the central portion in the sheet thickness direction are further reduced, and the low temperature toughness at the central portion in the sheet thickness direction is significantly improved. .

FIG. 6 shows C: 0.06%, Si: 0.22%, Mn: 1.
44%, sol.Al: 0.021%, Cu: 0.27%, Ni: 0.74%, N
b: 0.012%, Ti: 0.012%, balance Fe and unavoidable impurities, and a steel slab produced by continuous casting method (CC) is heated to 1120 ° C, and then in a temperature range of 900 to 1100 ° C.
Rough rolling is performed with an average shape ratio of 0.28 to 0.36 or 0.51 to 0.60, and finish rolling is performed at a temperature of 780 ° C or less so that the average shape ratio is 0.50 to 0.82, and immediately up to 500 ° C or less.
In the case of manufacturing with cooling at a cooling rate of 4.5 ° C / sec, the transition temperature vTrs (° C) in the center part of the plate thickness direction was measured, and an ultrasonic flaw detection test was conducted to check the presence or absence of internal defects. It is a graph shown.

From the figure, it can be seen that the internal defects are completely eliminated by setting the average shape ratio of finish rolling to 0.70 or more. In the present invention, the finish rolling temperature is limited to 780 ° C. or lower because the surface temperature of the steel sheet during finish rolling is 780 ° C.
This is because by limiting the temperature to ℃ or less, the entire thick steel sheet can be controlled to a non-recrystallized region temperature (820 ℃ or less), and coarsening of crystal grains due to recrystallization is suppressed, so that the toughness is improved.

FIG. 7 shows C: 0.06%, Si: 0.22%, Mn: 1.
44%, sol.Al: 0.021%, Cu: 0.27%, Ni: 0.74%, N
b: 0.012%, Ti: 0.012%, balance Fe and unavoidable impurities.
After heating to 20 ℃, high shape ratio (average shape ratio in rough rolling at 900 to 1100 ℃: 0.6, average shape ratio in finish rolling at 770 to 730 ℃: 0.80) or low shape ratio (900 to 11)
Average shape ratio in rough rolling at 00 ° C: 0.4, 770 to 730
Average shape ratio in finish rolling at ℃: 0.6 or 900 〜
Average shape ratio in rough rolling at 1100 ° C: 0.4, 950-90
A plate at −30 ° C. when a thick steel plate is manufactured by hot rolling at an average shape ratio of 0.6) in finish rolling at 0 ° C. and immediately cooling to a temperature of 500 ° C. or less at a cooling rate of 4.5 ° C./sec. Charpy absorbed energy in the central part in the thickness direction 1 / 2t part vE
₃ is a graph showing the relationship between _-30 (J) and rolling conditions in finish rolling.

From the graph shown in the figure, it is possible to manufacture a thick steel sheet having a high toughness in the thickness direction by performing hot rolling in a non-recrystallized region with a high shape ratio as in the present invention. Recognize.

(5) As described above, in the present invention, the rolling is performed with the average shape ratio of rough rolling: 0.50 or more and the average shape ratio of finish rolling: 0.70 or more, and immediately after that, the temperature range of 500 ° C. or less is reached.
Water cooling is performed at a cooling rate of 10 ° C / sec or less. This is 10 ℃ / s
By cooling with water cooling at a cooling rate of ec or less, for example, coarsening of crystal grains due to recrystallization can be suppressed, and miniaturization of crystal grains by rolling with a high shape ratio can be achieved.

As described above, according to the present invention, (i) the initial austenite crystal grains can be refined by controlling the heating temperature of the steel slab to a low temperature of 950 to 1150 ° C.
It is possible to refine the crystal grains after ferrite transformation,
(ii) By performing rolling with a high average shape ratio by rough rolling and finish rolling, it is possible to reduce zigzag defects in the central portion in the plate thickness direction and reduce the grain size, and (iii) unrecrystallized region By rolling with, it is possible to suppress the coarsening of crystal grains due to recrystallization, and therefore, it is suitable for use as a thick steel plate with excellent toughness in the plate thickness direction, for example, as a steel material for huge structures such as marine structures. In addition, it is possible to manufacture a thick steel plate having a transition temperature vTrs of −70 ° C. or less at the center in the plate thickness direction.

Further, the present invention will be described with reference to examples, but this is an example of the present invention and the present invention is not limited thereby.

[0052]

EXAMPLES Steel pieces A to D having the compositions shown in Table 2
(Slab thickness: 300 mm or 260 mm) was manufactured by continuous casting method (CC) or ingot making method (I't).

[0053]

[Table 2]

As shown in Table 3, these steel pieces were treated with 1120
After heating to ℃ or 1000 ℃, rough rolling in the temperature range of 900 to 1100 ℃ with the rough shape ratio (average shape ratio) shown in Table 3, the same when the finish rolling start temperature shown in Table 3 is reached. Finishing rolling is started at the finishing shape ratio (average shape ratio) shown in Table 3, and hot rolling is finished at the finishing temperature shown in Table 3, and the product thickness is
Hot-rolled steel sheets of 100 mm were made, and these hot-rolled steel sheets were water-cooled at the water-cooling start temperature, water-cooling stop temperature and water-cooling rate shown in Table 3 to obtain products.

Various test pieces were cut out from these products,
The transition temperature at YP, TS, El and the center in the plate thickness direction and the ferrite grain size number at the center in the plate thickness direction were measured. The results are also shown in Table 3. In Table 3, the rolling reduction is also shown in order to show the difference from the method described in JP-A-58-58225.

[0056]

[Table 3]

Examples of the present invention (Sample No. 3, Sample No. 5, Sample No. 8
To Sample No. 10 and Sample No. 16) are YP: 411 to 456k
gf / mm ² , TS: 510 ~ 571kgf / mm ² , El: 26.0 ~ 33.6%, vT
rs (T direction): -102 to -72 ° C, vTrs (L direction): -10
We were able to manufacture a thick steel plate with a ferrite grain size number of 9.0 to 9.7 in the center of the plate thickness direction of 7 to -86 ° C and a transition temperature vTrs of -70 ° C or less in the center of the plate thickness direction. I understand.

In sample No. 1, the vTrs (T direction) was deteriorated because the finish shape ratio was smaller than the range of the present invention. In Sample No. 2 and Sample No. 6, both the rough thickness shape ratio and the finish shape ratio were smaller than the range of the present invention, so vTrs (T direction) was deteriorated.

In Sample No. 4 and Sample No. 7, the vTrs (T direction) was deteriorated because the rough shape ratio was smaller than the range of the present invention. In Sample No. 11, the finish rolling start temperature was above the range of the present invention, and therefore vTrs (T direction) was deteriorated.

In Sample No. 12, the vTrs (T direction) was deteriorated because the C content exceeded the range of the present invention. Sample No.1
In No. 3, vTrs (T direction) deteriorated because the C content exceeded the range of the present invention and the finish rolling start temperature and the finish temperature exceeded the upper limits of the present invention.

In the sample No. 14, the Ti content exceeds the range of the present invention, so that vTrs (T direction) is deteriorated. further,
In Sample No. 15, the water cooling rate exceeded the range of the present invention, so that vTrs (T direction) was deteriorated.

[0062]

As described in detail above, according to the present invention, a thick steel plate having excellent toughness in the plate thickness direction, more specifically, a steel plate used in a low temperature region such as an offshore structure, is used. A high toughness thick TMCP steel plate having a transition temperature vTrs in the center portion in the plate thickness direction of −70 ° C. or less, which is suitable for the above, could be manufactured.

[Brief description of drawings]

FIG. 1 is an example of a heat cycle of a hot rolling process to which the manufacturing method according to the present invention is applied.

[FIG. 2] A steel plate is rolled with a radius R to a thickness of t ₀ to t ₁
It is explanatory drawing at the time of rolling down.

FIG. 3 is a graph showing a general relationship between the heating temperature of the billet and the size of the initial austenite crystal grain size.

[Fig. 4] Transition temperature vTrs (℃) at the center in the plate thickness direction
2 is a graph showing the relationship between the heating temperature (° C.) and.

FIG. 5 is an explanatory diagram showing a state of implementation of a tensile test.
(a) shows before implementation, and FIG. 5 (b) shows after implementation.

[Fig. 6] Transition temperature vTrs (℃) at the center in the thickness direction
It is a graph which shows the result of having investigated the presence or absence of the internal defect in the relationship with the average shape ratio in finish rolling.

FIG. 7 is a graph showing the relationship between Charpy absorbed energy vE _-30 (J) at the central portion in the plate thickness direction at −30 ° C. and rolling conditions.

Claims (2)

[Claims] 1. A steel slab having a steel composition consisting of C: 0.10% or less, Si: 0.40% or less, Mn: 1.80% or less, sol.Al: 0.080% or less, and the balance Fe and unavoidable impurities. , 950 to 1150 ℃, and then 900 to 11
Rough rolling is performed in the temperature range of 00 ℃ with an average shape ratio of 0.50 or more.
Furthermore, after finish rolling at an average shape ratio of 0.70 or more in the temperature range of 780 ° C or lower, immediately after 10 ° C until the temperature range of 500 ° C or lower
A method of manufacturing a high toughness thick TMCP steel sheet, which comprises cooling at a cooling rate of not more than / sec. 2. The steel billet further comprises, by weight, one or two selected from the group consisting of Cu: 0.50% or less, Ni: 1.0% or less, Nb: 0.030% or less and Ti: 0.030% or less. The method for producing a high toughness thick TMCP steel sheet according to claim 1, which contains at least one kind.