JPH10152722A - Manufacture of thick steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacture of a thick steel plate excellent in strength and toughness with high productivity while obviating the necessity of controlled rolling. SOLUTION: A steel, having a composition containing, by weight, 0.03-0.2% C, <=0.6% Si, 0.3-3% Mn, 0.02-0.15% Nb, 0.001-0.1% sol.Al, 0-0.05% Ti, 0-1% Cr, 0-1% Mo, 0-2% Cu, 0-3% Ni, 0-0.2% V, and 0-0.003% B, is heated to 1100-1250'C and rolled at >=30% cumulative draft between 900 and 1050 deg.C, and rolling is finished at >=900 deg.C. After rolling, the resultant plate is subjected to concurrent heating up to 900-1000 deg.C for >=3min and then cooled down to <=300 deg.C at a rate of >=2 deg.C/sec or cooled down to 400-600 deg.C at a rate of (2 to 50) deg.C/sec.

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 thick steel plate having a good balance of strength and toughness used in bridges, shipbuilding, pressure vessels, low temperature tanks for storing liquefied gas, line pipes, offshore structures and the like. More specifically, in hot rolling of a thick steel plate, a thick steel plate having good strength and toughness can be economically and efficiently produced without using controlled rolling in an unrecrystallized region of austenite (hereinafter referred to as “γ”). It relates to a method of manufacturing.

[0002]

2. Description of the Related Art Conventionally, when high toughness is required for a thick steel plate, a method of obtaining a fine structure by combining controlled rolling and controlled cooling has been used (JP-A-57-1313).
No. 20, JP-A-2-80516, etc.).

[0003] However, these methods which require controlled rolling have the following problems. In the controlled rolling, the rolling must be finished after the temperature of the steel sheet has dropped to around 800 ° C., and the efficiency is significantly reduced as compared with the ordinary rolling.

[0004] Because of the strong working at low temperature, the generation of rolled texture is inevitable, and the anisotropy appears in strength and toughness. Furthermore, after assembling this rolling texture into a structure, when performing ultrasonic flaw detection near the welded part, flaw detection is performed to produce a difference in ultrasonic speed depending on whether the rolling direction of the thick steel plate or the direction perpendicular thereto. A problem arises.

[0005] In order to reduce the waiting time for rolling, the steel plate is strongly worked at a low temperature while cooling with water, so that the flatness of the thick steel plate is remarkably deteriorated, and much time is required for leveler correction to correct the flatness. This flatness deterioration is further increased by performing cooling after controlled rolling. Thick steel plates that have undergone significant distortion due to controlled rolling, accelerated cooling, etc. cannot be straightened by a normal leveler and are straightened by press working, so it may take several hours to flatten a single thick steel plate. .

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a thick steel plate having excellent strength and toughness while maintaining high productivity without performing controlled rolling in the low temperature range of the γ phase. And

[0007]

Means for Solving the Problems The present inventor has proposed that various steels having a chemical composition containing Nb be rolled at a rolling finishing temperature of 900 ° C. or higher, and then water-cooled or water-cooled, and tempering conditions changed to change various conditions. The following items were confirmed as a result of manufacturing a thick steel plate of a strength level and examining the anisotropy of mechanical properties and the like.

(A) In a normal rolling method, when the rolling finishing temperature is 900 ° C. or higher, the toughness deteriorates as the finishing temperature increases. This is due to coarsening of the structure and hardening due to precipitation of Nb in the ferrite region.

(B) Even when the rolling finishing temperature is 900 ° C. or higher, anisotropy of the microstructure is observed. However, when the finishing temperature exceeds 1050 ° C., the anisotropy of the microstructure almost disappears. However, the old γ grains are coarse, and the final structure, such as the mixed structure of ferrite and bainite, is coarse accordingly.

(C) Even if the finishing temperature is in the temperature range of 900 to 1050 ° C., if the material is heated to the temperature range of 900 to 1000 ° C. for 3 minutes or more after rolling, recrystallization of the processed γ grains progresses and And a fine structure can be obtained.

[0011] In the following description, "900-1 after rolling"
Heating to 000 ° C temperature range for 3 minutes or more ”
That.

At the time of supplementary heat, precipitation of Nb carbonitride proceeds at the same time, but since precipitation occurs in the γ region before transformation, precipitation hardening is suppressed. Further, pinning of γ grains occurs due to Nb carbonitride precipitated during the supplementary heat, and fine recrystallized γ grains are obtained, and the final structure becomes fine.

(D) The effect of the supplementary heat is as follows.
This is most noticeable when the steel containing the above is rolled at a finishing temperature of 900 to 1000C and the auxiliary heat temperature is 920 to 980C.

(E) In the case of directly quenching to 300 ° C. or less after the supplementary heat, or in the case of stopping the water cooling in the temperature range of 400 to 600 ° C. (in the middle of accelerated cooling),
The effect of the supplementary heat is remarkable, and a thick steel plate with small anisotropy can be obtained. In addition, conventional reheating quenching and tempering (reheating Q
Higher strength is obtained as compared to the T) method. That is, the method of directly quenching or stopping during accelerated cooling after the supplementary heat enables to secure the strength and toughness superior to the reheating QT method.

FIG. 1 is a drawing showing the respective conditions of rolling, supplementary heat and cooling according to the present invention. FIG. 1 (a) shows a cooling method performed when strengthening a steel having insufficient hardenability or trying to increase the strength even at a slight rate.
Cool down to below 00 ° C. Thereafter, tempering may be performed if necessary. FIG. 1 (b) shows 400 to 600 ° C. after supplementary heat.
In this temperature range, cooling is stopped in that temperature range, and then allowed to cool. This method may be followed by tempering if necessary.

The method of the present invention combines the above items,
It was completed by confirming the remarkable effect in actual production, and the gist of the following method for producing a thick steel plate is shown (see FIG. 1).

(1) C: 0.03 to 0.2%, Si: 0.6% or less, Mn: 0.3 to 3%, Nb: 0.02 to 0.15%, sol.Al: 0.001 to 0.1%, Ti: 0 -0.05%, Cr: 0-1%, Mo: 0-1%, Cu: 0-2%, Ni: 0-3%, V: 0-0.2%, and B: 0-0.003% Is heated to 1100 to 1250 ° C, rolled at a cumulative draft of 30% or more in a temperature range of 900 to 1050 ° C, finished rolling at 900 ° C or more, and rolled to a temperature range of 900 to 1000 ° C for 3 minutes. A method of manufacturing a thick steel plate having excellent toughness, including a step of holding the above and then cooling it to 300 ° C. or less at a cooling rate of 2 ° C./sec or more (referred to as [Invention 1]).

(2) By weight ratio, C: 0.03 to 0.2%, Si: 0.6% or less, Mn: 0.3 to 3%, Nb: 0.02 to 0.15%, sol.Al: 0.001 to 0.1%, Ti: 0 ~ 0.05%, Cr: 0 ~ 1%, Mo: 0 ~ 1%, Cu: 0 ~ 2%, Ni: 0 ~ 3%, V: 0 ~ 0.2% and B: 0 ~ 0.003% Heating to 1100-1250 ° C, rolling at a rolling reduction of 30% or more in the temperature range of 900-1050 ° C, finishing rolling at 900 ° C or more, and rolling to 900-1000 ° C for 3 minutes or more after rolling A method of manufacturing a thick steel plate having excellent toughness including a step of holding and then accelerated cooling to 400 to 600 ° C. at a cooling rate of 2 to 50 ° C./sec (referred to as [Invention 2]).

In [Invention 1], “cooling to 300 ° C. or less at a cooling rate of 2 ° C./sec or more” is cooling mainly for increasing strength, and such cooling is sometimes referred to as direct quenching. is there. A method of performing tempering after direct quenching is naturally included in [Invention 1]. In the direct quenching, the flatness of a thick steel plate can be warm-corrected only by using the residual heat after tempering. Therefore, when the flatness requirement for building materials is severe, tempering is performed. However, when the demand for flatness is not so severe, it may be determined whether or not to perform tempering based on the necessity of increasing toughness or the like.

In [Invention 2], "2 to 50 ° C."
The reason for performing "accelerated cooling to 400 to 600 [deg.] C. at a cooling rate of / sec" is to obtain a strengthened structure such as bainite while avoiding ferrite transformation. However, unlike the cooling of [Invention 1] whose main purpose is to strengthen, in [Invention 2] 400 to
It is also aimed at stopping accelerated cooling in a temperature range of 600 ° C. and thereafter automatically tempering during cooling. Of course,
After the accelerated cooling is stopped, the steel sheet is allowed to cool, and then reheated to a point less than Ac ₁ point to add tempering to further improve the toughness. As described above, [Invention 2] is premised on automatic tempering after cooling is stopped, and such cooling is distinguished from "direct quenching" of [Invention 1] as "accelerated cooling".

In both [Invention 1] and [Invention 2], the designated temperature is the temperature of all the thick portions of the thick steel plate. For example, in [Invention 1] and [Invention 2], "900
1 to 3 minutes or more in the temperature range of ~ 1000 ° C ”
In, all of the part from the plate thickness surface to the center,
The time is counted from the time when the temperature enters the temperature range of 900 to 1000 ° C. If the entire thickness of the thick steel plate is less than 900 ° C. immediately before the heating, it is the center of the thickness that finally enters the temperature range of 900 to 1000 ° C. On the other hand, the center of the sheet thickness is 900 to 10
It is in the temperature range of 00 ° C, and only 900-1
If it is lower than the temperature range of 000 ° C, finally 900-100
It is near the surface that enters the temperature range of 0 ° C. In addition, when the entire thickness is in the temperature range of 900 to 1000 ° C. immediately before the heat exchange, the time when the thick steel sheet is completely carried into the heat exchanger is the time when the heat exchange starts.

The end of the supplementary heat time corresponds to the time when the supplementary heat treatment is completed and the thick steel sheet is taken out of the heating furnace, and the like, and thereafter, the entire thickness of the thick steel sheet may be in the temperature range of 900 to 1000 ° C. However, this time is not included in the heating time.

When cooling to 300 ° C. or lower after supplementary heat or accelerated cooling to 400 to 600 ° C., the center of the sheet thickness usually enters the temperature range most lately. [Invention 1]
At a cooling rate of 2 ° C./sec or more at 80 ° C.
It means the average value from 0 ° C to 400 ° C. Also,
The cooling rate of 2 to 50 ° C./sec in [Invention 2] is an average value from 800 ° C. at the center of the sheet thickness to the cooling stop temperature.

As long as both [Invention 1] and [Invention 2] are within the range of low-alloy steel, the alloy components may include any alloy components other than the above.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION

1. Rolling, supplementary heat and cooling conditions In the present invention, the slab serving as the raw material must be heated to 1100 ° C. or higher. This is because it is necessary to ensure sufficient solid solution Nb at the time of slab heating using Nb as an essential element.

Normally, when the purpose is to produce a high toughness steel, the heating temperature should not be raised more than necessary to avoid coarsening of γ grains. However, in the present invention, at the time of supplementary heat after rolling, in order to promote the recrystallization of the processed γ grains and to refine the grains,
Even if the heating temperature is increased, the toughness does not deteriorate. Rather, it is preferable that the heating temperature be higher in order to sufficiently dissolve Nb.
It may be 1200 ° C. or higher. On the other hand, heating exceeding 1250 ° C. increases the cost of the heating furnace and increases the scale loss of the raw material, so the heating temperature is set to 1250 ° C. or less.

In rolling, the cumulative draft at 900 to 1050 ° C. is set to 30% or more. If the cumulative rolling reduction at 900 to 1050 ° C. is less than 30%, remarkable deterioration of toughness occurs. Therefore, the cumulative rolling reduction in this temperature range is 30%.
Above. Here, the cumulative draft at 900 to 1050 ° C. is (the thickness at 1050 ° C.−the thickness at 900 ° C.) /
(Plate thickness at 1050 ° C.). The reason for limiting the cumulative rolling reduction in the temperature range of 900 to 1050 ° C. as described above is 9
If the temperature is lower than 00 ° C., the rolling efficiency decreases and the anisotropy of mechanical properties increases. On the other hand, if the temperature exceeds 1050 ° C., the growth of γ grains after rolling is remarkable, and the rolling process does not contribute to the refinement of the structure. Because.

The rolling finishing temperature is 900 ° C. or higher. When the rolling finishing temperature is lower than 900 ° C., the toughness can be ensured, but the rolling efficiency is greatly reduced as described above.
In addition, the anisotropy of the mechanical properties of the steel sheet tends to increase. Therefore, in the present invention, the rolling finish temperature is 900
C or higher. The upper limit of the finishing temperature is not particularly limited as long as the above-mentioned limitation of the cumulative rolling reduction is satisfied, but is preferably set to 1000 ° C. or lower in order to secure good toughness.

The heating temperature by the supplementary heat is (hereinafter referred to as "the supplementary heat temperature").
) And 900-1000 ° C. If the temperature is lower than 900 ° C., the recrystallization does not proceed in a short time and the productivity is reduced. If the temperature exceeds 1000 ° C., the growth of the γ particle size cannot be ignored after recrystallization and the structure becomes coarse. .

The heating time is 3 minutes or more. Heating time is 3
If it is less than minutes, recrystallization does not proceed sufficiently, and anisotropy of mechanical properties cannot be eliminated and the structure cannot be refined. Even if the supplementary heat time exceeds 20 minutes, the performance is not greatly adversely affected, but the productivity is lowered, so that it is preferable that the supplementary heat time be 20 minutes or less.

The supplementary heat is usually carried out in a heating gas furnace provided with a combustion gas heating furnace beside the rolling line or on a bypass. But,
Heating may be performed by energizing heating or high-frequency heating, and the heating method does not matter.

In order to obtain a greater effect by the method of the present invention, more preferably, the rolling is performed at a cumulative reduction of 50% or more in a temperature range of 900 to 1050 ° C., and after the rolling is completed, 920 to 980 ° C. is obtained by supplementary heat. It is desirable that the heating be performed for 5 minutes or more.

After the completion of the supplementary heat, cooling is performed. The cooling rates at the time of cooling in [Invention 1] and [Invention 2] are each an average value in the temperature range as described above. The difference between the two is a result of reflecting the difference in the cooling lower limit temperature in the temperature range when the average value is obtained. Although the difference between the two is not large and can be regarded as almost the same, the two should be distinguished, and the cooling rates of [Invention 1] and [Invention 2] mean the cooling rates as defined above. I do.

The cooling rate must be 2 ° C./sec or more at the cooling rate defined by [Invention 1] and [Invention 2].
This is because if both [Invention 1] and [Invention 2] are less than 2 ° C./sec, the structure becomes coarse and the toughness deteriorates. In particular, in [Invention 1], the reinforcement is insufficient, and there is no point in directly hardening.

The upper limit of the cooling rate is 50 ° C./sec or less in [Invention 2]. If it exceeds 50 ° C / sec, (a) 4
This is because the two conditions of stopping halfway at 00 to 600 ° C. and (b) not generating a large distortion in the entire steel plate cannot be satisfied. Regarding [Invention 1],
A cooling rate as high as possible may be employed,
There is no adverse effect if cooling is performed at a cooling rate exceeding 1 / sec.

In the case of [Invention 1], water is cooled to 300 ° C. or less without expecting automatic tempering after cooling is stopped halfway. When the cooling is stopped at a temperature exceeding 300 ° C., temperature unevenness is generated, and the unevenness of the strength is greatly increased. Thereafter, when a high degree of toughness is required or the requirement for flatness is severe, tempering may be performed at less than the Ac ₁ point as described above.

In the case of [Invention 2], after supplementary heat, 400 to 60
Accelerate cooling to a temperature range of 0 ° C. and stop. The reason why the accelerated cooling stop temperature is set to 400 to 600 ° C. is that if accelerated cooling is performed only to a temperature exceeding 600 ° C., ferrite transformation occurs and is not sufficiently strengthened. On the other hand, when accelerated cooling to less than 400 ° C., automatic tempering is not sufficiently performed during the subsequent cooling.
In addition, the temperature difference between the surface portion of the thick steel plate and the center of the thickness becomes large, and it becomes difficult to stop accelerated cooling of the entire thickness portion while maintaining good flatness. Variations occur. After allowing to cool after stopping the accelerated cooling, tempering may be performed at a temperature lower than the Ac ₁ point, if necessary, for the purpose of reducing hardness and residual strain and adjusting toughness.

2. Chemical composition Next, reasons for limiting each alloy component will be described. “%” Of the alloy component indicates “% by weight”.

C : 0.03 to 0.2% C is an element necessary for securing the strength, and unless 0.03% is contained, a steel having practical strength cannot be produced. Also, C combines with Nb to form a precipitate,
Contributes to the refinement of the structure, but for that reason, 0.03%
Is necessary.

However, if the content exceeds 0.2%, the toughness of the weld heat affected zone (hereinafter referred to as “HAZ”) is particularly deteriorated, so the upper limit is 0.2%.

Si : 0.6% or less Si is an element that can increase strength relatively inexpensively, but if it exceeds 0.6%, the toughness of HAZ is impaired.
This value is the upper limit. On the other hand, Si is known as a deoxidizing element, but since it can be sufficiently deoxidized with Mn, Al, or the like, even if it is 0.01% or less, deterioration of steel quality due to insufficient deoxidation does not occur. However, in actual production, 0.01
%, The economical efficiency such as a decrease in the yield of Al deteriorates. Therefore, the content is preferably 0.01% or more.

Mn : 0.3 to 3% Since Mn is an element necessary for securing strength and is also an important element for preliminary deoxidation, it must be contained in an amount of 0.3% or more. However, when the content exceeds 3%, the HAZ toughness is greatly deteriorated. For this reason, the upper limit is set to 3%.

Nb : 0.02 to 0.15% Nb is in a solid solution state and increases hardenability to increase strength. Further, it is an element effective for improving the balance between strength and toughness in order to suppress the recrystallization of γ grains by fine precipitation during rolling and to exert a controlled rolling effect. In the present invention, since the rolling finish temperature is set to a high temperature of 900 ° C. or more, if the driving force for precipitation of Nb carbonitride is not high, Nb
No carbonitride precipitation occurs. For this reason, Nb is set to 0.1.
Must contain at least 02%.

On the other hand, if the content exceeds 0.15%,
In addition to increasing the amount of undissolved coarse Nb precipitates during heating, the HAZ causes a significant deterioration in toughness.
5% or less.

Sol.Al : 0.001 to 0.1% Since Al is an important deoxidizing element, good steel quality can be ensured unless 0.001% is contained in the form of sol.Al. difficult. On the other hand, when Ti is not added to fix N as a nitride, Al forms AlN and also contributes to the refinement of crystal grains, so that it should be positively included for purposes other than deoxidation. Is also good. But even in that case, 0.1%
If the content exceeds 0.1%, the adverse effect on the toughness of the steel increases, so this value is made the upper limit.

Ti : 0 to 0.05% Ti may not be contained. However, Ti forms TiN and particularly suppresses the γ grain growth of HAZ. Further, since it is effective for ensuring the strength, it may be contained in order to obtain these effects. When the content is less than 0.002%, these effects are not clearly exhibited.
% Is desirable. On the other hand, Ti is an element harmful to toughness, and if it exceeds 0.05%, remarkable deterioration of toughness occurs.

Cr : 0 to 1% Cr may not be contained. However, Cr is effective in enhancing hardenability and improving strength, and therefore may be included when increasing strength. When it is contained, if it is less than 0.01%, a sufficient effect cannot be obtained, so it is desirable to make it 0.01% or more. On the other hand, if it exceeds 1%, the toughness of the base material and the HAZ is deteriorated.

Mo : 0 to 1% Mo may not be contained. However, Mo enhances hardenability and has high tempering softening resistance, so Mo is included when these effects are obtained and high strength is secured. 0.01% content
If the amount is less than 0.01%, these effects cannot be obtained. On the other hand, if it exceeds 1%, not only the base material toughness is deteriorated, but also the toughness of the HAZ is significantly deteriorated.

Cu : 0 to 2% Cu may not be contained. However, since the strength can be increased by containing Cu, it is included when high strength is required. If the content is less than 0.01%, the effect will not be apparent, so it is desirable to set the content to 0.01% or more. On the other hand, if the content exceeds 2%, low-temperature cracking of the welded portion is liable to occur.

Ni : 0 to 3% Ni may not be contained. However, both strength and toughness can be improved by containing Ni. However, if the content is less than 0.01%, the effect is not clearly obtained. desirable. On the other hand, even if the content exceeds 3%, the effect is saturated,
Rather, the weldability such as low-temperature cracking is deteriorated.

V : 0 to 0.2% V may not be contained. However, since V contributes to an increase in strength due to precipitation hardening during tempering, it is included when high strength is intended. If the content is less than 0.005%, the effect of increasing the strength cannot be sufficiently obtained. Therefore, when the content is included, the content is preferably set to 0.005% or more. On the other hand, 0.2
%, The toughness is significantly degraded.

B : 0 to 0.003% B may not be contained. However, B is effective for stabilizing the γ grain boundary to increase the hardenability and to increase the base material strength even in a trace amount. 0.
If the content is less than 0002%, the hardenability cannot be sufficiently increased. On the other hand, if the content exceeds 0.003%, the toughness of the HAZ is unavoidably deteriorated, so the content is set to 0.003% or less.

Steel plates containing alloys other than the above-mentioned alloying elements in the range of low alloy steels are also an object of the present invention. For example, C
Steel containing a small amount of a, Zr, or the like, or a rare earth element such as La, Ce, or the like may be used.

It is desirable that the impurity element is as low as possible. For example, P is in a range that does not deteriorate the toughness.
It is desirable that S is not more than 3%, and S is not more than 0.02%, which hardly causes lamella tear during welding.

[0055]

EXAMPLES Next, the effects of the present invention will be described using examples.

Table 1 shows the chemical components of the steel used in the examples.

[0057]

[Table 1]

The table shows the chemical compositions of nine examples of the present invention and three examples of steels not containing Nb which are not included in the present invention as comparative examples.

Table 2 shows the conditions of rolling, supplementary heat, cooling and tempering performed on these steels.

[0060]

[Table 2]

In Table 2, DQ means that quenching was performed to 300 ° C. or less after rolling. In the comparative example,
The method after water cooling including water cooling is the same as the example of the present invention,
The conditions from hot rolling to supplemental heat were taken out of the scope of the present invention. This is because [Invention 1] and [Invention 2]
This is because the method after the water cooling including the water cooling is not much different from the conventional method of manufacturing a general steel sheet. The cooling rate is the cooling rate at the center of the sheet thickness. The supplementary heat temperature is a set temperature of the heating furnace for supplementary heat treatment, and the supplementary heat time is 95% as described above.
In the case of 0 ° C. finish rolling, it is the time after the transfer to the heating furnace, and when the rolling finish temperature exceeds 1000 ° C., for example, in the case of 1040 ° C. finish rolling, the time from the completion of the transfer to the heating furnace is similarly set. Time.

A JIS No. 4 tensile test piece (parallel diameter of 8.) in two directions of a rolling direction (L direction) and a direction perpendicular to the rolling direction (T direction) from a position of 1/4 of the thickness of the steel sheet subjected to these treatments.
5 mm) and a JIS No. 4 impact test piece (10 mm square) were sampled and subjected to a tensile test and a Charpy impact test. The toughness was evaluated by the ductile brittle fracture transition temperature vTrs in the Charpy impact test. Table 2 shows the results of these tests.
Are shown together.

In Test Nos. 1 to 12, which are examples of the present invention, even though the rolling finish temperature is 900 ° C. or higher, a steel sheet having a relatively excellent balance of strength and toughness is obtained. Furthermore, due to the effect of the supplementary heat after the finish rolling, the difference in strength and toughness between the L direction and the T direction was very small, and uniform performance was obtained.

Test Nos. 13 to 17, which are comparative examples, are examples in which no supplementary heat was applied after rolling or only for a very short time. In this case, although the strength is increased due to strengthening by precipitation of Nb in the ferrite region, the toughness is remarkably deteriorated and cannot be used for applications requiring low-temperature toughness.

Also, the test numbers 18, 20, 22, 23
Since the rolling reduction at 1050 ° C. or less is small, the processing of γ grains and the precipitation of Nb carbonitride in the γ region are insufficient, and the amount of Nb carbonitride during or after transformation increases, and only the strength is increased. This is a steel with high toughness and poor toughness.

Test No. 19 shows that the rolling finishing temperature was 790
As a result, the toughness in the L direction was good, but the toughness in the T direction was not good. If you lower the slab heating temperature enough,
Although this tendency is alleviated, in this case, the use efficiency of the heating furnace is reduced, and the rolling efficiency is reduced due to an increase in deformation resistance during rolling, and the economic efficiency is greatly negative. Furthermore, the anisotropy, that is, the difference in strength and toughness in the L and T directions cannot be eliminated. The large anisotropy of the mechanical properties in Test No. 19 means that the formation of the rolling texture is remarkable. For this reason, ultrasonic inspection near the weld line after assembling into a structure will be hindered, but since it is well known, no particular investigation was conducted.

The examples in which the slab heating temperature was lowered are Test Nos. 21 and 24 of the comparative examples. As a result of finishing at 910 ° C. in order to avoid excessive reduction in rolling efficiency and anisotropy, the present invention was applied to the same steel. Compared to Test Nos. 9 and 12 to which the method was applied,
Only the same or lower performance was obtained. In operation, despite the fact that slab low-temperature heating with very difficult and cost-increasing factors was performed, no performance advantage was obtained compared to the present invention example, and conversely, the effectiveness of the present invention method was shown. It became a thing.

Table 3 shows that, in order to avoid strengthening due to Nb precipitation in the ferrite region which cannot be avoided when high-temperature rolling is performed, steel Nos. 10 to 12 without Nb were subjected to supplementary heat after rolling. It shows mechanical properties.

[0069]

[Table 3]

The test numbers 25, 26 and 27 correspond to the above Nb
Since the steel had no steel, an excessive decrease in toughness did not occur, but the results were particularly poor in toughness as compared with Examples 1, 3, and 10 of the present invention for steels having almost the same composition except for Nb.

[0071]

Industrial Applicability According to the present invention, a steel sheet having excellent strength and toughness and small anisotropy can be mass-produced with high efficiency without lowering the slab heating temperature or performing low-temperature finish rolling at the expense of lowering productivity. It becomes possible. This is related to the basics of thick steel plate production that overturns the conventional idea of the method of manufacturing a high toughness thick steel plate, and greatly contributes to the development of the industry.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 (a) is a drawing showing [invention 1] and FIG. 1 (b) is a drawing showing [rolling, heating and cooling] conditions of [invention 2].

Claims (2)

[Claims] (Claim 1) C: 0.03-0.2%, Si: 0.6% or less, Mn: 0.3-3%, Nb: 0.02-0.15%, sol.Al: 0.001-0.1%, Ti: 0: Steel containing 0.05%, Cr: 0-1%, Mo: 0-1%, Cu: 0-2%, Ni: 0-3%, V: 0-0.2%, and B: 0-0.003% Heating to 1100-1250 ° C, rolling at a rolling reduction of 30% or more in the temperature range of 900-1050 ° C, finishing rolling at 900 ° C or more, and rolling to 900-1000 ° C for 3 minutes or more after rolling A method for manufacturing a thick steel plate having excellent toughness, comprising a step of holding and then cooling to 300 ° C. or less at a cooling rate of 2 ° C./sec or more. (Claim 2) C: 0.03-0.2%, Si: 0.6% or less, Mn: 0.3-3%, Nb: 0.02-0.15%, sol.Al: 0.001-0.1%, Ti: 0- 1100 steel containing 0.05%, Cr: 0-1%, Mo: 0-1%, Cu: 0-2%, Ni: 0-3%, V: 0-0.2% and B: 0-0.003% Heated to ~ 1250 ° C, rolled at a cumulative draft of 30% or more in the temperature range of 900 to 1,050 ° C, finished rolling at 900 ° C or more, and kept in the temperature range of 900 to 1,000 ° C for at least 3 minutes after rolling And a method for producing a thick steel plate having excellent toughness, which comprises a step of thereafter cooling at a cooling rate of 2 to 50 ° C./sec to 400 to 600 ° C.