JPH0860239A - Production of thick steel plate excellent in low temperature toughness - Google Patents

Abstract

PURPOSE: To produce a thick steel plate in which ferritic grains are made superfine with good productivity without adding a large quantity of expensive alloy elements thereto by subjecting a slab having a specified compsn. to rough rolling, cooling and finish rolling under specified conditions, working ferrite and thereafter recovering and recrystallizing the same. CONSTITUTION: A slab contg., by weight, 0.01 to.20% C, 0.03 to l.0% Si, 0.30 to 2.0% Mn, 0.005 to 0.1% Al, 0.001 to 0.01% N, and the balance Fe is subjected to rough rolling under heating to the Ac3 transformation point to 1200 deg.C and finishing the same at the Ar3 transformation point or above. Thus, the average austenite grain size is regulated to <=50μm then, it is cooled to a temp. in which the ratio of ferrite is regulated to >=50% at a cooling rate of <=2 deg.C/sec, and thereafter, rolling at >=50% cumulative draft is finished 9 at >=650 deg.C. If required, it is subjected to accelerated cooling or is subjected to tempering after the accelerated cooling. Moreover, the steel may be added with prescribed amounts of one or more kinds among Ni, Mo, Cu, Ti, V, Nb, B and Cr.

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 sheet having excellent low temperature toughness.

[0002]

2. Description of the Related Art Since thick steel plates are used as structures,
From the viewpoint of ensuring the safety of structures, low temperature toughness is often required. Although various methods for obtaining low temperature toughness have been proposed for thick steel sheets, miniaturization of ferrite (α) crystal grain size is a typical method that does not cause deterioration of other properties without using expensive alloy elements. As a method of refining .alpha., .Alpha., Various methods have been proposed and put to practical use.

As a typical method, for example, Japanese Patent Publication No. Sho 49
As disclosed in Japanese Patent Publication No. 7291, Japanese Patent Publication No. 57-21007, Japanese Patent Publication No. 59-14535, etc., by performing controlled rolling in the unrecrystallized temperature range of austenite (γ) and subsequently performing accelerated cooling. A method of refining α during the transformation from γ to α has been proposed. In the method utilizing the transformation from γ to α, it is possible to increase the γ / α conversion ratio by effectively utilizing the unrecrystallized zone rolling when γ is coarse, but when γ becomes fine, the γ / α conversion Since the ratio approaches 1, the degree of miniaturization of α is saturated, and significant miniaturization of α cannot be expected.

A technique for improving the strength and toughness by so-called two-phase region rolling, in which the temperature range of controlled rolling is expanded to the α / γ two-phase region, has also been proposed. For example, as disclosed in Japanese Examined Patent Publication No. 58-5967, there is proposed a technique for improving the toughness by suppressing the occurrence of the separation characteristic of the two-phase region rolling by devising the composition and the rolling condition in the γ region. ing. However, in the conventional technique, the α-grain size is almost the same as that in the ordinary controlled rolling, and the toughness can be significantly improved only by using the effect of reducing the triaxial stress due to the occurrence of separation.

Also, a method of heat treatment has been proposed instead of hot working such as rolling. For example, [Iron and Steel, No. 77
, No. 1, 1991, pp. 171-178], by adding a large amount of V and N,
Has been developed, a method of increasing the γ / α conversion ratio during transformation and making a fine α structure by normalizing treatment is developed. However, in order to fully exert the grain-refining effect by normalizing, V is 0.01% or more and N is also 0.01%.
% Or more, and the attainable α particle size is 5 μm
It is a degree.

Furthermore, [Materials and Processes, Third Year, Sixth Year]
No., 1990, p. 1796], the grain size was 3 μm due to complicated thermomechanical treatment including repeated γ-α transformation.
A method for obtaining ultra-fine grain steel of m or less is disclosed. In this method, after controlled rolling, accelerated cooling is performed, and after accelerated cooling is stopped at about 500 ° C., 900 ° C. without cooling to room temperature.
Ultra-fine grain steel is obtained by reheating to ℃ and hot rolling at a prescribed temperature. The α grain size is strongly influenced by the cooling stop temperature, and the cooling stop temperature is very close to 500 ° C. Other than that, an α particle size of 3 μm or less has not been obtained, and it is considered difficult to industrially manufacture the α particle size.

Therefore, all of the above-mentioned conventional methods are accompanied by high costs such as deterioration of productivity, increase of heat treatment steps, increase of alloying elements, and the like. Except for the experimental method, the limit is about 10 μm, and the limit is about 5 μm even by a complicated process that is strictly controlled, and it is not possible to expect a significant improvement in toughness by further refinement of α.

[0008]

DISCLOSURE OF THE INVENTION The present invention has good productivity and an average α particle size of about 3 μm or less without adding a large amount of expensive alloying elements, a process having poor productivity, and a complicated repeating process. It is an object of the present invention to provide a method for producing a thick steel sheet having excellent low temperature toughness, which can obtain the ultrafine grain α.

[0009]

In order to solve the above-mentioned problems, the present inventors have found a means for ultra-fine graining of α by investigating the hot working behavior of α in detail, and The invention has been completed. That is, the gist of the present invention is, by weight%, C: 0.01 to 0.20%,
Si: 0.03 to 1.0%, Mn: 0.30 to 2.0
%, Al: 0.005-0.1%, N: 0.001-
0.01%, and if necessary, Cr: 0.
01 to 0.50%, Ni: 0.01 to 3.0%, Mo:
0.01 to 0.50%, Cu: 0.01 to 1.5%, T
i: 0.003 to 0.10%, V: 0.005 to 0.2
0%, Nb: 0.003 to 0.05%, B: 0.000
Containing one or more 3 to 0.0020%, a slab containing the balance of Fe and inevitable impurities, A _c3 transformation point or higher, then heated to a temperature of 1200 ° C. or less, A _r3 transformation point or above the temperature After the average austenite grain size is reduced to 50 μm or less by the rough rolling ending in step 1, and after cooling to a temperature at which the proportion of ferrite is 50% or more at a cooling rate of 2 ° C./second or less, finish rolling with a cumulative reduction of 50% or more. Is finished at a temperature of 650 ° C. or higher, or after rolling is finished, it is subsequently accelerated cooled to a temperature of 550 ° C. or lower at a cooling rate of 5 ° C./sec or higher, and tempered at a heating temperature of 600 ° C. or lower, if necessary. A method for producing a thick steel sheet excellent in low temperature toughness, which is characterized by the above.

The present invention will be described in detail below based on experimental results. The present invention is characterized in that a method of recovery / recrystallization of processed α is used as a means for making α fine. That is, the α obtained by applying the processing in the γ / α two-phase region is recovered and recrystallized after the processing to substantially reduce the grain size of α. In this case, for the purpose of ultra-fine graining, it is advantageous that the recovery / recrystallization of α after working is not performed by a method such as reheating heat treatment, but is performed during cooling after rolling, preferably during rolling or immediately after rolling. Become. However, if it is attempted to generate a certain amount or more of α during cooling, the temperature will inevitably decrease, so it is generally difficult to cause proper recovery and recrystallization during rolling or during cooling thereafter. . The present inventors investigated the behavior of α in the processing during this cooling process by detailed experiments, and in order to achieve ultrafine graining by recovery / recrystallization of the processed α, the two-phase region processing conditions should be optimized together with the optimization. The inventors have found that it is necessary to define the structure before entering the phase region processing, and have completed the present invention. Further description will be given below based on experiments.

The chemical composition is C: 0.15%, Si: 0.2
%, Mn: 1.2%, Nb: 0.006%, Ti: 0.
01%, N: 0.0032% steel is heated to 1150 ° C., and the rolling conditions (reduction rate, reduction temperature) in the γ region are changed to change the γ grain size before the γ / α transformation to about 30 μm and 65 μm. Two
After selecting the type, the two-phase region one-pass processing was performed by changing the reduction rate and the reduction temperature. Changing the two-phase zone rolling temperature means changing the α fraction during processing. The initial slab thickness was changed so that the plate thickness is uniform when entering the two-phase processing. Therefore, the plate thickness after the two-phase region reduction depends on the reduction rate. After reducing the pressure in the two-phase region, the cooling rate was adjusted to about 20 ° C./sec, control cooling was performed to about room temperature, and the structure at the center of the plate thickness was observed. The form of α was determined by observing the Nital corrosion structure with an optical microscope, and the α particle size was measured with a scanning electron microscope using a photograph at a magnification of 3500 times. In a scanning electron micrograph, so-called high-angle grain boundaries and small-angle grain boundaries cannot be clearly distinguished, but in the present invention, both were regarded as grain boundaries for measurement.

FIG. 1 and FIG. 2 are views showing the relationship between the two-phase zone rolling condition, the form of α and the α grain size. FIG. 1 shows γ before transformation.
When the particle size is about 30 μm, FIG. 2 shows the result when the particle size is 65 μm. It can be seen from Fig. 1 that the α particle size tends to become finer as the rolling reduction increases.
It is understood that not only the rolling reduction but also the α fraction during the two-phase region processing needs to be defined in order to become the following.

That is, when the α fraction is small, the proportion of processed α is small. Therefore, the obtained structure mainly consists of the structure transformed from γ rather than the structure formed from processed α, and even if the rolling reduction is large. Since there is a limit to the particle size of α generated by the transformation from γ, there is also a limit to the refinement of the entire α particle size.
Also, it tends to have a mixed grain structure. On the other hand, even if the α fraction is large, if the reduction ratio is small, the recovery and recrystallization of α are difficult to proceed, so that α remains as-processed and α cannot be refined. Therefore, in order to achieve ultrafine graining, it is necessary to secure the α fraction above a certain level and then apply the reduction at a certain down rate. FIG.
As a result of conducting investigations on steels of various components, it was found that the range of reduction conditions for ultra-fine graining is almost constant regardless of the steel type if it is specified by the α fraction and the reduction rate before processing. From the condition that the α particle size is stable at 3 μm or less,
As a rolling condition at the two-phase region temperature following the rolling reduction, the α fraction during processing is 50% or more, and the cumulative rolling reduction is 50%.
The above is the scope of the present invention.

Above, the γ grain size before the start of transformation is about 30 μm
However, in order to make α into ultrafine grains, it is necessary to specify not only the above-mentioned two-phase region rolling condition but also the structure before processing. That is, FIG. 2 shows the case where the γ particle size before transformation is about 65 μm. In this case, however, the α particle size is almost 3 μm or less even within the same two-phase region rolling condition range as in FIG. This is because the anterior structure is coarse and α recovery and recrystallization are suppressed. The structure before processing in the two-phase region is represented by the cooling rate in the transformation region and the γ grain size before transformation as rolling conditions.Generally, if the γ grain size is constant, the larger the cooling rate, the more It becomes fine. On the other hand, since the generation of α is suppressed as the cooling rate increases, the same α
It becomes necessary to cool to a lower temperature to obtain the fraction,
As a result, the processing temperature for α also decreases. Since the recovery and recrystallization of α is suppressed when the processing temperature of α is lowered even if the anterior structure is made finer, the refinement of the anterior structure by increasing the cooling rate is rather disadvantageous for the grain refinement of α. . Rather, the cooling rate should be low to ensure the amount of α at higher temperatures. As a method of refining the front structure under such conditions, miniaturization of γ is effective. Separately, as a result of detailed examination, after processing in the γ single-phase region, the desired α
If the cooling rate until obtaining the fraction is 2 ° C./sec or less and the γ grain size is 50 μm or less under the cooling conditions, the pre-structure can be made finer, and as shown in FIG. It has been found that it becomes possible to stably obtain ultrafine particles α of about 3 μm or less under the condition that the α fraction is 50% or more and the cumulative rolling reduction is 50% or more. When the cooling rate to obtain the desired α fraction is 2 ° C./sec or less and the γ particle size is 50 μm or less under the cooling conditions, the condition under the two-phase region pressure is not determined. Even if it is adjusted to, ultrafine particles α cannot be obtained, or even if it is obtained, the rolling condition range of the two-phase region is very narrow, and it is difficult to industrially utilize it.

For the above reasons, in the present invention, a steel piece having a predetermined chemical composition is _treated at the A _c3 transformation point or more and 1200 ° C.
Temperature at which the proportion of ferrite becomes 50% or more at a cooling rate of 2 ° C./sec or less after the average austenite grain size is set to 50 μm or less by rough rolling which is heated to the following temperature and finished at a temperature of A _r3 transformation point or more. After cooling down to 50, the cumulative rolling reduction is 50
It is necessary to perform finish rolling of not less than%.

Here, the heating temperature is set to the A _c3 transformation point or more, 12
The temperature is set to 00 ° C. or lower because if it is less than the _{Ac 3} transformation point, the compaction is insufficient, and if the untransformed α is coarse, the α is refined before the two-phase region processing depending on the subsequent rolling and cooling. On the other hand, if the temperature exceeds 1200 ° C., γ is likely to cause mixed grains, and even if the average γ grain size is 50 μm or less, a part of α does not become ultrafine grain, and the toughness value is This is because improvement cannot be expected.

In the rolling for the purpose of adjusting the γ grain, the finishing temperature is set to the _{Ar 3} transformation point or higher. this is,
This is because rolling below the transformation point is not effective for refining γ grains, and processing before sufficiently forming α is not effective for ultrafine graining of α. The finish rolling, which is a two-phase rolling, is 65
It is necessary to finish above 0 ° C. This is because if the processing temperature is low, the recovery and recrystallization of α is suppressed and the ultra-fine graining becomes insufficient, so that the recovery and recrystallization of α progresses in the whole process of the two-phase region rolling, and it is stable. This is a condition for obtaining an ultrafine grain structure.

The effect of the present invention is not impaired by cooling after rolling, by cooling alone, or by accelerated cooling such as water cooling. After the accelerated cooling, it is possible to perform tempering for the purpose of strength adjustment, residual stress removal, etc., but in that case, the tempering temperature is 600 ° C. or lower so as not to grow the obtained ultrafine particles α. Should be.

[0019]

The above are the reasons for limiting the present invention relating to the manufacturing method. However, in order to secure the low temperature toughness, not only the manufacturing method but also the chemical composition must be within the proper range. less than,
The reasons for limiting the chemical components in the present invention will be described. First, C
Is added as an effective component to improve the strength of steel. If it is less than 0.01%, it is difficult to secure the strength required for structural steel, and if it is added in excess of 0.20%, toughness and resistance are increased. Since it significantly reduces weld crackability, etc., 0.01
It was set to a range of 0.20%.

Next, Si is an element effective as a deoxidizing element and for ensuring the strength of the base material. Addition of less than 0.03% results in insufficient deoxidation and is disadvantageous in securing strength. On the contrary, excessive addition of more than 1.0% forms a coarse oxide and causes ductility and toughness deterioration. Therefore, the range of Si is 0.
It was set to 03 to 1.0%. Further, Mn is an element necessary to secure the strength and toughness of the base metal, and it is necessary to add at least 0.30%, but the upper limit is 2 within the allowable range of the material such as the toughness and crackability of the welded portion. It was set to 0.0%.

Al is an element effective in deoxidizing, refining the γ grain size, etc., and must be contained in an amount of 0.005% or more in order to exert its effect. When added to, a coarse oxide is formed and ductility is extremely deteriorated, so it is necessary to limit the content to 0.005 to 0.1%. N works effectively in γ grain refinement in combination with Al and Ti, but in order to clarify the effect, it is necessary to contain N by 0.001% or more. On the other hand, if added excessively, solid solution N
Increases and adversely affects toughness, so the upper limit is made 0.01% as an allowable range.

The above are the basic components of the steel of the present invention, but Cr, Ni, Mo, Cu, Ti, V, N, if necessary, for the purpose of increasing the strength of the base material according to the desired strength level.
One or more of b and B may be contained.
First, Cr and Mo are both effective elements for improving the strength of the base material, but 0.01% or more is necessary for producing a clear effect, while Cr is added in excess of 0.50%. Then, since the toughness tends to deteriorate, 0.0
The range is from 1 to 0.50%.

Ni is a very effective element because it can improve the strength and toughness of the base material at the same time, but it is necessary to contain Ni in an amount of 0.01% or more in order to exert the effect. Although the strength and toughness are improved when the content is increased, the effect is saturated even if the content exceeds 3.0%, and the _Ar3 transformation point is extremely lowered, which is a condition of the present invention. Previous α amount 50
% And the finish rolling start temperature of 650 ° C. or more cannot be satisfied at the same time, so the upper limit is set to 3.0% in consideration of economic efficiency.

Next, Cu has almost the same effect as Ni, but addition of more than 1.5% causes a problem in hot workability, so the range is limited to 0.01 to 1.5%. Ti contributes to the strength improvement of the base material by precipitation strengthening, and
Although it is an element effective for making γ grains fine by forming N, it is necessary to add 0.003% or more in order to exert the effect. On the other hand, if it exceeds 0.10%, similarly to Al, a coarse oxide is formed to deteriorate toughness and ductility, so the upper limit is made 0.10%.

Both V and Nb mainly contribute to improving the strength of the base material by precipitation strengthening, but if added excessively, the toughness deteriorates. Therefore, V is 0.005 to 0.20 as a range in which the effect can be exhibited without deteriorating the toughness.
% And Nb are 0.003 to 0.05%. B is 0.0
Addition of a very small amount of 003% or more is very effective for enhancing the hardenability of steel materials and increasing the strength, but if added in excess, it forms BN, which adversely reduces the hardenability and greatly deteriorates the toughness. Therefore, the upper limit is made 0.0020%.

Next, the effects of the present invention will be described more specifically by way of examples.

[0027]

[Examples] Table 1 shows the chemical composition of the test steels used in the examples. Each of the test steels was made into a billet by slab rolling or continuous casting after ingot casting. In Table 1, Steel Nos. 1 to 10 satisfy the chemical composition range of the present invention, and Steel Nos. 11 to 13 are out of the chemical composition range of the present invention.

The steel pieces in Table 1 are shown in Tables 2 and 3 (continued from Table 2).
1), Table 4 (Continued-2 in Table 2) and Table 5 (Continued-3 in Table 2) were manufactured into steel sheets and the strength, Charpy impact characteristics and DWTT characteristics were investigated. All test pieces were sampled in the direction perpendicular to the rolling from the center of the plate thickness. Charpy impact property is 50% fracture surface transition temperature (vTrs), and also DWT
The T characteristics were evaluated at 85% ductile fracture transition temperature (85% FATT). The test results of strength and toughness are also shown in Table 2 to Table 5.
Shown in

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

In Tables 2 to 5, the test No. A1-A
All of 17 are steel sheets manufactured according to the present invention, all of which have an ultrafine grain structure with an average α grain size of about 3 μm or less, a toughness value of vTrs of −100 ° C. or less, and a DWTT of 80% F.
A temperature of -70 ° C or lower is achieved by ATT. On the other hand, test N
o. B1 to B9 are comparative examples, and any one of the conditions is out of the limited range of the present invention, so that the ultrafine graining of α is not measured, and the Charpy characteristics and the DWTT characteristics are far more than those of the present invention examples. Inferior to. That is, the test No. The heating temperature of B1 is too high, and therefore the fineness of the front structure is not sufficiently measured. In B2 and B5, the γ grain size before transformation is coarse, and the α fraction during processing in the two-phase region is outside the range of the present invention. In addition, the test No. For B3 and B6, the rolling reduction in finish rolling is insufficient, and thus ultrafine graining is insufficient. Further, since B4 has a large γ particle size, and B7 to B9 have a chemical composition range outside the range of the present invention, ultra-fine graining cannot be achieved, or low temperature toughness is poor due to other toughness deterioration factors. .

From the above, it is clear that according to the present invention, it is possible to manufacture a steel sheet having an ultra-fine grained structure of about 3 μm or less and exhibiting very good low temperature toughness.

[0036]

INDUSTRIAL APPLICABILITY The present invention uses expensive alloy elements,
It is an epoch-making method that can produce thick steel plate with good low temperature toughness without reducing productivity due to complicated heat history.
Industrial effects such as reduction of manufacturing cost and improvement of safety as a structure are extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a two-phase region rolling condition, a morphology of α, and a grain size when a γ grain size before transformation is 30 μm.

FIG. 2 is a diagram showing a relationship between a morphology of α and a grain size in a two-phase region rolling condition when a γ grain size before transformation is 65 μm.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C22C 38/54

Claims (6)

[Claims] 1. By weight%, C: 0.01 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.30 to 2.0%, Al: 0.005 to 0. 1%, N: 0.001 to 0.01%, and a steel slab composed of the balance Fe and unavoidable impurities is heated to a temperature not lower than the _{Ac 3} transformation point and not higher than 1200 ° C.,
After the average austenite grain size is reduced to 50 μm or less by rough rolling ending at a temperature of A _r3 transformation point or more, the ferrite is cooled to a temperature at which the proportion of ferrite is 50% or more at a cooling rate of 2 ° C./sec or less, and then the cumulative rolling reduction is performed. 6 finish rolling with a rate of 50% or more
A method for producing a thick steel sheet excellent in low-temperature toughness, which is characterized by ending at a temperature of 50 ° C. or higher. 2. C: 0.01 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.30 to 2.0%, Al: 0.005 to 0. 1%, N: 0.001-0.01% is contained, Cr: 0.01-0.50%, Ni: 0.01-3.0%, Mo: 0.01-0.50% Cu: 0.01 to 1.5%, Ti: 0.003 to 0.10%, V: 0.005 to 0.20%, Nb: 0.003 to 0.05%, B: 0.0003 contain one or more ～0.0020%, a slab containing the balance of Fe and inevitable impurities, a _c3 transformation point or higher, then heated to a temperature of 1200 ° C. or less, in a _r3 transformation point or higher temperatures By finishing rough rolling, the average austenite grain size is reduced to 50 μm or less, and the ferrite ratio is 5 at a cooling rate of 2 ° C./sec or less.
After cooling to a temperature of 0% or more, the cumulative rolling reduction is 50%
A method for producing a thick steel sheet excellent in low-temperature toughness, characterized in that the above finishing rolling is finished at a temperature of 650 ° C. or higher. 3. C .: 0.01 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.30 to 2.0%, Al: 0.005 to 0. 1%, N: 0.001 to 0.01%, and a steel slab composed of the balance Fe and unavoidable impurities is heated to a temperature not lower than the _{Ac 3} transformation point and not higher than 1200 ° C.,
After the average austenite grain size is reduced to 50 μm or less by rough rolling ending at a temperature of A _r3 transformation point or more, the ferrite is cooled to a temperature at which the proportion of ferrite is 50% or more at a cooling rate of 2 ° C./sec or less, and then the cumulative rolling reduction is performed. 6 finish rolling with a rate of 50% or more
A method for producing a thick steel sheet excellent in low-temperature toughness, which comprises terminating at a temperature of 50 ° C. or higher, and subsequently performing accelerated cooling to a temperature of 550 ° C. or lower at a cooling rate of 5 ° C./sec or higher. 4. C .: 0.01 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.30 to 2.0%, Al: 0.005 to 0. 1%, N: 0.001-0.01% is contained, Cr: 0.01-0.50%, Ni: 0.01-3.0%, Mo: 0.01-0.50% Cu: 0.01 to 1.5%, Ti: 0.003 to 0.10%, V: 0.005 to 0.20%, Nb: 0.003 to 0.05%, B: 0.0003 contain one or more ～0.0020%, a slab containing the balance of Fe and inevitable impurities, a _c3 transformation point or higher, then heated to a temperature of 1200 ° C. or less, in a _r3 transformation point or higher temperatures By finishing rough rolling, the average austenite grain size is reduced to 50 μm or less, and the ferrite ratio is 5 at a cooling rate of 2 ° C./sec or less.
After cooling to a temperature of 0% or more, the cumulative rolling reduction is 50%
A method for producing a thick steel sheet excellent in low-temperature toughness, which comprises finishing the above-mentioned finish rolling at a temperature of 650 ° C. or higher, and subsequently performing accelerated cooling to a temperature of 550 ° C. or lower at a cooling rate of 5 ° C./second or higher. 5. In weight%, C: 0.01 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.30 to 2.0%, Al: 0.005 to 0. 1%, N: 0.001 to 0.01%, and a steel slab composed of the balance Fe and unavoidable impurities is heated to a temperature not lower than the _{Ac 3} transformation point and not higher than 1200 ° C.,
After the average austenite grain size is reduced to 50 μm or less by rough rolling ending at a temperature of A _r3 transformation point or more, the ferrite is cooled to a temperature at which the proportion of ferrite is 50% or more at a cooling rate of 2 ° C./sec or less, and then the cumulative rolling reduction is performed. 6 finish rolling with a rate of 50% or more
After terminating at a temperature of 50 ° C. or higher, and subsequently accelerating cooling to a temperature of 550 ° C. or lower at a cooling rate of 5 ° C./sec or higher, 60
A method for producing a thick steel sheet excellent in low-temperature toughness, which comprises tempering at a temperature of 0 ° C or lower. 6. In% by weight, C: 0.01 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.30 to 2.0%, Al: 0.005 to 0. 1%, N: 0.001-0.01% is contained, Cr: 0.01-0.50%, Ni: 0.01-3.0%, Mo: 0.01-0.50% Cu: 0.01 to 1.5%, Ti: 0.003 to 0.10%, V: 0.005 to 0.20%, Nb: 0.003 to 0.05%, B: 0.0003 contain one or more ～0.0020%, a slab containing the balance of Fe and inevitable impurities, a _c3 transformation point or higher, then heated to a temperature of 1200 ° C. or less, in a _r3 transformation point or higher temperatures The average austenite grain size is reduced to 50 μm or less by rough rolling to be finished, and then 2 ° C. /
After cooling to a temperature at which the proportion of ferrite is 50% or more at a cooling rate of not more than seconds, finish rolling with a cumulative reduction of 50% or more is completed at a temperature of 650 ° C or more, and subsequently at a cooling rate of 5 ° C / second or more. After accelerated cooling to a temperature below 550 ° C,
A method for producing a thick steel sheet excellent in low temperature toughness, which comprises tempering at a heating temperature of 600 ° C. or lower.