JPH1171615A - Production of thick steel plate excellent in low temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a thick steel plate having a graded superfine-grained structure in which the average grain size is regulated to <= specified value and the degree of the mixing of the grains is small without requiring a complicated hot working or heat treating stage by subjecting a slab in which the chemical compsn. and the contents of P and S as impurities are specified to heating in a specified temp. range, starting finish rolling in hot rolling in a specified temp. range and executing rolling under specified conditions. SOLUTION: A slab having a chemical compsn. contg., by weight, 0.01 to 0.20% C, 0.03 to 1.0% Si, 0.30 to 2.0% Mn, 0.005 to 0.1% Al and 0.001 to 0.01% N, in which T1( deg.C) shown by formula I satisfies >=700 deg.C, the content of P is regulated to <=0.02% and that of S to <=0.01%, and the balance Fe is used. This slab is heated in the range of T2( deg.C)-100 deg.C to 1200 deg.C and at the time of hot rolling, finish rolling is started at 650 deg.C to T1( deg.C)+100 deg.C, rolling in which the draft per pass is regulated to 30 to 70% is included by one or more passes, and also, the rolling stage in which the cumulative draft is regulated to 50 to 95% is included.

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 used for a structural member requiring low-temperature toughness. The steel material produced by this method can be used, for example, in general for welded steel structures such as offshore structures, pressure vessels, shipbuilding, bridges, buildings, and line pipes. In particular, offshore structures requiring low-temperature toughness It is useful as a steel plate for structures such as shipbuilding. In addition, the present invention is applicable to a steel pipe material or a shaped steel which is used as a structural member and requires low-temperature toughness.

[0002]

2. Description of the Related Art Thick steel plates are often used as structural steels, and are required to have low-temperature toughness from the viewpoint of ensuring the safety of structures. Various methods for improving low-temperature toughness of steel plates have been proposed, but as a method of improving low-temperature toughness without using other expensive alloy elements such as Ni and causing other characteristic deterioration, ferrite ( (Hereinafter referred to as α).

Various methods have been conventionally proposed as a method for reducing the α particle size. Representative methods include, for example, JP-B-49-7291 and JP-B-57-210.
No. 07, JP-B-59-14535, etc., controlled rolling is performed in a non-recrystallization temperature region of austenite (hereinafter referred to as γ), and subsequently, accelerated cooling is performed to convert γ to α. There is a method of reducing α during transformation to α.

In the method utilizing the transformation from γ to α as described above, when γ is coarse, the γ / α conversion ratio (γ particle size before transformation / α particle after transformation Particle size) can be increased to make α finer, but when the γ particle size is fine, the γ / α conversion ratio approaches 1, so α
The miniaturization becomes saturated. Therefore, in the method based on the refinement of α through the transformation from γ to α, the degree is controlled by the degree of refinement of γ.

In order to solve this problem, there has been proposed a technique for improving the strength and toughness by so-called two-phase rolling, in which the temperature range of controlled rolling is expanded to a γ / α two-phase range. For example, Japanese Patent Publication No. 58-5967 proposes a technique for improving the toughness by two-phase rolling by devising components and rolling conditions. However, in these conventional two-phase rolling technologies, the α grain size is almost the same as the α grain size obtained by controlled rolling, and is substantially applied to the steel sheet surface at the time of fracture mainly due to texture called separation. Toughness is improved by using the effect of reducing triaxial stress due to the occurrence of layered cracks occurring in parallel. However, although separation is effective in lowering the fracture transition temperature in the Charpy test, it is limited in its degree, and furthermore, it causes a reduction in absorbed energy, so its use is limited.

There has also been proposed a method of reducing the α grain size by heat treatment without using hot working such as rolling.
For example, [Iron and Steel, 77th year, No. 1, 1991, No. 1
71 to 178], V and N are added in a larger amount than usual to reduce γ and increase the γ / α conversion ratio during transformation to normalize. A method of forming a fine α-structure by processing has been developed.

Further, [Materials and Processes, 3rd year, 6th
No. 1990, pp. 1796], the particle size was 3 μm due to complicated working heat treatment including repetition of γ / α transformation.
A method for obtaining ultrafine-grained steel having a diameter of m or less has been proposed. In this method, after controlled rolling, accelerated cooling is performed, accelerated cooling is stopped at about 500 ° C., and then 900 ° C. without cooling to room temperature.
C., and hot-rolled at a predetermined temperature to obtain ultrafine-grained steel.

[0008]

As described above, various attempts have been made to reduce the grain size. [Iron and Steel, 77th year, No. 1, 1991, pp. 171-178] As shown in the above, V and N are added in larger amounts than usual to achieve a finer γ, and at the time of transformation γ / α
In order to increase the conversion ratio and obtain a fine α-structure by a normalizing process to obtain a fine α-structure, V is 0.1%
As described above, N must be added in an amount of 0.01% or more, and the attainable α particle size is about 5 μm.

Alternatively, a complicated working heat treatment including repetition of γ / α transformation shown in [Materials and Processes, 3rd year, No. 6, 1990, p. The method of obtaining fine-grained steel is to perform accelerated cooling after controlled rolling, stop accelerated cooling at about 500 ° C, and then reheat to 900 ° C without cooling to room temperature.
This is a method of obtaining ultra-fine grained steel by performing hot rolling at a predetermined temperature, and the α grain size is strongly affected by the cooling stop temperature,
The particle size is 3μ except when the cooling stop temperature is very close to 500 ° C.
m or less is not obtained, and it is considered that it is difficult to stably produce it industrially.

[0010] Therefore, in the above-mentioned conventional methods, cost increases are unavoidable in all cases, such as deterioration in productivity, increase in heat treatment steps, and increase in alloy elements. Also, the α particle size that can be obtained stably is about 10 μm except for some experimental methods, and the limit is about 5 μm even by a strictly controlled complicated process. Has not been realized.

The present invention does not require the addition of expensive alloying elements or the complicated hot working or heat treatment steps with low productivity, and the sized ultrafine particles having an average α particle size of 2 μm or less and a small mixed particle size. It has a grain α structure and is excellent not only in 2 mm V notch Charpy impact characteristics but also in brittle crack propagation stopping characteristics,
It is an object of the present invention to provide a method for manufacturing a thick steel plate having excellent low-temperature toughness.

[0012]

Means for Solving the Problems The present inventors have found that there is a limit in the austenite (γ) / ferrite (α) transformation, which is a conventional typical grain refining method, and for the first time α Focus on the method of utilizing the recovery and recrystallization of
By investigating the hot working behavior of α in detail, a means for ultrafine graining of α was found, and the present invention was devised. The present inventors have already disclosed in Japanese Patent Application Laid-Open No. 8-60239 or Japanese Patent Application No. 8-291294 a γ region and γ /
α by optimizing the heating and hot rolling conditions in the two-phase region
Means for ultra-fine processing and recrystallization grain size is disclosed, but it is necessary to refine at least a certain level of microstructure before entering the dual phase rolling, heat treatment and rolling for it is essential It was something. The present invention further researched the metallurgical principle of the processing and recrystallization conditions of α, and as a result, the recrystallization behavior was different depending on the way of processing to α, and not only the cumulative draft, but also the processing of each rolling pass By optimizing the conditions,
Using as-cast steel slabs, it is possible to achieve ultra-fine graining even if the two-phase region to α-region processing is performed directly, without taking measures for refining the pre-structure, and further uniform ultra-fine graining is possible. The present inventors have found that granulation and reduction of anisotropy can be achieved, and have reached the invention. The gist of the invention is as follows.

(1) C: 0.01 to 0.20% by weight
%, Si: 0.03-1.0%, Mn: 0.30-2.
0%, Al: 0.005 to 0.1%, N: 0.001 to
T1 containing 0.01% and represented by the formula (1)
(C) has a chemical composition such that T1 (C) ≥ 700C,
When the contents of P and S as impurities are P: 0.02% or less and S: 0.01% or less, a steel slab composed of the balance of Fe and inevitable impurities is expressed by T2 (° C.) 10
When heating to a temperature in the range of 0 ° C to 1200 ° C and producing a steel sheet by hot rolling, finish rolling is performed at 650 ° C to T1.
(° C) + 100 ° C, reduction rate per pass is 3
A method for producing a thick steel sheet having excellent low-temperature toughness, characterized by including a rolling step in which rolling of 0 to 70% is included in one or more passes and a cumulative draft is 50 to 95%.

T1 (° C.) = 750.8−26.6 · C% + 17.6 · Si% −11.6 · Mn% −22.6 · Cu% −23.0 · Ni% + 24.1 · Cr% + 22.5.Mo% -39.7.V% -5.7.Ti% + 232.6.Nb% -169.4.A1% -894.7.B% (1) T2 (° C.) = 937.2−476.5 · C% + 56.0 · Si% −19.7 · Mn% −16.3 · Cu% −26.6 · Ni% −4.9 · Cr% + 38. 1 · Mo% + 124.8 · V% + 136.3 · Ti% −19.1 · Nb% + 198.4 · Al% + 3315.0 · B% (2) (2) By weight%, C: 0.01 to 0.20%, Si:
0.03 to 1.0%, Mn: 0.30 to 2.0%, A
l: 0.005 to 0.1%, N: 0.001 to 0.01
%, And T2 (° C.) represented by the formula (2) is 9%.
Having a chemical composition such that 00 ° C ≧ T2 (° C) ≧ 800 ° C,
When the contents of P and S as impurities are P: 0.02% or less and S: 0.01% or less, a steel slab composed of the balance of Fe and inevitable impurities is expressed by T2 (° C.) 50
When the steel sheet is manufactured by heating to a temperature in the range of 1 to 1200 ° C. and hot rolling, T2 (° C.)-100 is used as finish rolling.
C. to T2 (° C.) to start at -20 ° C., including a rolling step in which rolling at a rolling reduction of 30 to 70% per pass is included in one or more passes and cumulative rolling reduction is 50 to 95%. A method for producing a thick steel plate having excellent low-temperature toughness characterized by the following.

T2 (° C.) = 937.2−476.5 · C% + 56.0 · Si% −19.7 · Mn% −16.3 · Cu% −26.6 · Ni% −4.9 · Cr% + 38.1 · Mo% + 124.8 · V% + 136.3 · Ti% −19.1 · Nb% + 198.4 · Al% + 3315.0 · B% (2) (3) Rolling completed The method for producing a thick steel sheet excellent in low-temperature toughness according to the above (1) or (2), wherein the subsequent steel sheet is accelerated and cooled at a cooling rate of 5 to 40 ° C./s to 20 to 600 ° C.

(4) The method for producing a thick steel sheet excellent in low-temperature toughness according to any one of the above (1) to (3), wherein tempering is performed at 450 ° C. or more and T1 (° C.) or less. (5) Cr: 0.01-1.0%, Ni:
0.01-3.0%, Mo: 0.01-1.00%, C
u: group consisting of 0.01 to 1.5%, Ti: 0.003
-0.10%, V: 0.005-0.50%, Nb:
0.003-0.10%, Zr: 0.003-0.10
%, Ta: 0.005 to 0.20%, W: 0.01 to
A group consisting of 2.0%, and B: 0.0003 to 0.0
The production of a thick steel sheet excellent in low-temperature toughness according to any one of the above (1) to (4), further comprising one or more of at least one of a group consisting of 020%. Method.

(6) Mg: 0.0005 to 5% by weight
0.01%, Ca: 0.0005 to 0.01%, RE
M: one or more of 0.005 to 0.10% of the above (1) to (5).
The method for producing a thick steel sheet having excellent low-temperature toughness according to any one of the above.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The present invention is characterized in that a method of recovering and recrystallizing the processed α is used for the first time as a means of refining α that exceeds the conventionally achieved level. In other words, in the conventional two-phase zone rolling technology, the strain introduced into α by two-phase zone rolling acts on the development and strengthening of the texture, but is actively used for the grain refinement of α. On the other hand, according to the present invention, the recovery and recrystallization of the processed α is extremely limited by the processing strain introduced in the γ / α dual-phase rolling, thereby achieving ultra-fine graining of α. . From the viewpoint of not hindering the productivity, and from the viewpoint of uniform fine granulation, the recovery and recrystallization of α after processing is not a method such as the conventional reheating treatment, but during cooling after rolling, preferably during rolling. Alternatively, it is more advantageous to generate it immediately after rolling.

The present inventors have previously disclosed in Japanese Patent Application Laid-Open No. Hei 8-6023
As shown in Japanese Patent Publication No. 9, the conditions for ultra-fine graining of α by rolling are examined, the γ grain size before transformation is reduced to 50 μm or less, and the ratio of α during two-phase region rolling is secured to 50% or more. As a result, it has been found that an ultrafine grain structure having an average particle size of 3 μm or less can be achieved. Further, Japanese Patent Application No. 8-291294 proposes a means for further refining the solidified structure and then performing reheating and rolling in the two-phase region to enable further uniform ultrafine graining. ing.

A common requirement of these inventions is that it is essential to refine the structure represented by the α grain size before processing α. In the case of Japanese Patent Application Laid-Open No. Hei 8-60239, the structure after transformation is refined by optimizing the rolling in the γ range. In the case of Japanese Patent Application No. 8-291294, the two-phase region heating is performed. The microstructure of the previous billet structure has been applied. Therefore, in the former case, the production efficiency (T / H) is slightly decreased due to the γ-range rolling, and in the latter case, the production process is inevitably increased.

The present invention is basically the same as the previously proposed one in utilizing the ultrafine graining of the α particle size by directly processing α, but not only the cumulative rolling reduction, By optimizing the processing conditions of each rolling pass, it is possible to achieve ultra-fine graining of α without considering the microstructural refinement before processing α. It can be realized with a simple process, and its industrial advantages are obvious. Further, according to the present invention, it is possible to achieve ultrafine graining more than the method proposed above, and it is possible to reduce anisotropy.

The basic requirements of the present invention are the components of the present invention,
There are the following two methods based on two temperatures, T1 and T2, which are estimated values obtained from experiments of the heating transformation point, the Ac ₁ and Ac ₃ transformation points in the range of the manufacturing process. T1 (° C.) = 750.8−26.6 · C% + 17.6 · Si% −11.6 · Mn% −22.6 · Cu% −23.0 · Ni% + 24.1 · Cr% + 22 5.5 · Mo% -39.7 · V% -5.7 · Ti% + 232.6 · Nb% −169.4 · Al% −894.7 · B% (1) T2 (° C.) = 937.2-476.5.C% + 56.0.Si% -19.7.Mn% -16.3.Cu% -26.6.Ni% -4.9.Cr% + 38.1.M o% + 124.8 · V% + 136.3 · Ti% −19.1 · Nb% + 198.4 · Al% + 3315.0 · B% (2) That is, the method of (1) is based on (1) T1 shown by the equation)
A steel slab having a chemical composition such that (° C.) is 700 ° C. or more is converted into T2 based on T2 (° C.) represented by the equation (2).
(° C.) Heating to −100 ° C. or more and 1200 ° C. or less, and when producing a steel sheet by hot rolling, rolling as finish rolling is started at 650 ° C. or more and T1 (° C.) + 100 ° C. or less.
Rolling is performed in such a manner that rolling including a rolling reduction of 30 to 70% per pass is included in one or more passes and the cumulative rolling reduction is 50 to 95%.

In the method (2), a slab having a chemical composition such that T2 (° C.) is in a range of 800 to 900 ° C. is heated to T2 (° C.) of -50 ° C. or more and 1200 ° C. or less, When producing a steel sheet by hot rolling, rolling is performed as finish rolling at T2 (° C.)-100 ° C. or more and T2 (° C.)-2.
Starting at 0 ° C or less, reduction rate per pass is 30-70
% Rolling in one or more passes and the cumulative rolling reduction is 50-9.
Rolling that is 5%.

First, in the first method, in order to make ultra-fine grains irrespective of the microstructure before working, in hot rolling, rolling at a rolling reduction of 30 to 70% per pass is performed for one pass or more. It is essential to perform rolling so that the rolling rate is included and the cumulative draft is 50 to 95%. In this case, the temperature for processing α is preferably higher. For that purpose, first, α /
It is necessary to reheat to the two-phase region of γ, or to generate α during cooling after heating to the γ region. In the hot rolling, if the temperature range for processing α is low, ultra-fine graining by recovery and recrystallization of α cannot be sufficiently achieved.

From the above, in order to stably secure α at a high temperature, consideration must be given to both the composition and the manufacturing conditions, and the requirement obtained by a detailed experiment is as follows. A steel slab having a chemical composition such that T1 (° C.) shown is 700 ° C. or more is heated to T2 (° C.)-100 ° C. or more and 1200 ° C. or less based on T2 (° C.) shown in Formula (2). That is. In the case of steel having a low T1 (° C.) lower than 700 ° C. and having a low transformation point, when a desired α / γ ratio is to be obtained by heating in the two-phase region, or α is generated in the γ region during cooling after heating. In both cases, it is difficult to secure α at a high temperature, so that uniform ultrafine graining cannot be expected. When the slab before rolling is heated to the two-phase region as the heating temperature of the slab, A
When the T2 corresponding to c ₃ transformation point is 100 ° C. cryogenic, since it is difficult to sufficient temperature secured to restore and recrystallization of the alpha, the lower limit of the heating temperature is required to be T2 (° C.) -100 ° C. There is. Further, after heating to the γ single phase region, it is possible to make ultra-fine grains by processing of α generated during cooling, as in the present invention, when optimizing the rolling reduction of each pass of rolling, but heating is possible. If the γ particle size is too large, it is disadvantageous for ultrafine graining, and the waiting time until rolling may be unnecessarily long. Therefore, the upper limit of the heating temperature is set to 1200 ° C.

As described above, in order to ultra-fine-grain α after defining the chemical composition and the heating conditions, rolling at a rolling reduction of 30 to 70% per pass includes at least one pass, and Rolling at a rolling reduction of 50 to 95% at 650 ° C to T
It is necessary to start at 1 (° C.) + 100 ° C. If the prior structure is refined as in the prior art, as rolling conditions, only the cumulative draft may be secured to a certain level or more. , It is necessary to include one or more passes in which the rolling reduction per pass is 30% or more. By performing such a large reduction, a coarse structure is refined. The lowering rate of reduction for that is 30%. In this case, the recovery and recrystallization of α are promoted by increasing the temperature due to the large processing rate itself and the heat generated during processing,
The structure can be miniaturized. The higher the rolling reduction, the better the grain refinement, but the recrystallized α grains grow and become coarse due to excessive processing heat. For example, the upper limit of the rolling reduction per pass is set to 70% because of the following three reasons: excessive application of heat and bad influence on the shape of the steel sheet.

Rolling with a rolling reduction of 30 to 70% in one pass is α
From the purpose of reducing the grain size, the effect is greater in the early stage of rolling, and it is preferable.However, if the rolling reduction is within the range of the present invention, it is necessary to perform the rolling at any stage. An ultrafine grain structure is obtained. Further, it is preferable that there are many passes under 30-70% pressure, but if at least one pass is included, there is no problem in ultra-fine graining.

[0028] In addition to the above-mentioned pass including the pass of which the rolling reduction is 30 to 70%, the cumulative rolling reduction is 50 to 95%, and the rolling start temperature is in the range of 650 ° C to T1 (° C) + 100 ° C. Ultra-fine graining for the first time. The rolling start temperature needs to be higher than a certain value in order to recover and recrystallize α, and was set to 650 ° C. or higher based on experiments. On the other hand, if the rolling temperature is too high, the main component of the structure to be processed will not be α, and the average grain size will not be reduced, and the mixed grains will include some coarse grains. α
The temperature regarded as the main component is constant irrespective of the composition based on T1 corresponding to the Ac ₁ transformation point. In the present invention, based on experiments, T1 (° C.) + 100 ° C. is set as the temperature at which α is the main component. The upper limit of the starting temperature was set.

In the above-described one-pass rolling reduction and rolling temperature conditions, if the cumulative rolling reduction is less than 50%, recovery / recrystallization is not sufficient even if other conditions are satisfied, and recovery / recrystallization is not performed. Since the subsequent α particle size does not become 3 μm or less, the cumulative draft must be 50% or more. The greater the cumulative rolling reduction, the more advantageous for ultra-fine graining. However, even if rolling exceeds 95%, the degree of ultra-fine graining is saturated,
If the cumulative rolling reduction exceeds 95%, the range of the thickness of the slab and the final sheet thickness is significantly limited and is not practical. Therefore, in the present invention, the upper limit of the cumulative rolling reduction is limited to 95%.

Next, the method (2) is different from the first method in the range of the chemical composition capable of ultrafine graining, the heating temperature, and the rolling start temperature. That is, T2 (° C.) is 800 to 900 on the basis of T2 corresponding to the Ac ₃ transformation point.
Steel slab in the range of ℃ is T2 (℃) -50 ℃ or more 1200
When the steel sheet is manufactured by heating to a temperature of not more than 100 ° C. and hot rolling, the rolling is performed at T2 (° C.)-100 ° C. or more and T2 (° C.)-20
Start below ℃. However, the method (2) is a slight modification of the method (1), and has almost the same concept as the concept of ultrafine graining. That is, in the method (1), the production conditions for ultra-fine graining are defined on the assumption that an α-based structure is formed before finish rolling in which α is directly processed. Is to allow the case where the α ratio is small at the stage of finishing rolling, and the condition of ultra-fine graining in the case where α is generated during rolling and becomes α main structure in the middle stage of rolling. It is shown.

In the method (2), the chemical composition represented by T2 and satisfying 900 ° C. ≧ T2 ≧ 800 ° C. is a composition in which α can be easily generated during rolling.
If the composition is lower than 0 ° C., the generation of α during rolling is not sufficient. The higher the composition of T2, the easier the generation of α is. However, in practice, if the composition of T2 is 900 ° C. or less, the condition of ultrafine graining can be satisfied. And

In the method (2), the lower limit of the heating temperature is T2 (° C.)-50 ° C. This is because the method (2) is premised on the formation of α during rolling, so that it is not necessary to lower the temperature as in the two-phase zone heating temperature in the method (1). At a chemical composition of 800 ° C., the temperature at which ultra-fine graining is stably achieved by heating in the two-phase region is limited based on experimental results, assuming a rolling start temperature described later. 1200 ℃ upper limit
Is the same as the limitation reason of the first method, and the rolling start temperature is set to T2 (° C.)-100 ° C. to T2 (° C.)-2 by the method (2).
The reason why the temperature is set to 0 ° C. is that when the slab is heated under the above conditions and the rolling start temperature is lower than T2 (° C.)-100 ° C., the recovery and recrystallization of α do not sufficiently proceed, and therefore, ultrafine graining On the other hand, if T2 (° C.) is more than −20 ° C., the rolling temperature range becomes high, recrystallized α grains grow, and the average grain size becomes coarse, and the mixed grains become large. This is because there is a high possibility that the tissue will be revealed. Therefore, in the present invention, the rolling start temperature range is T2 (° C) -100 ° C to T2.
(° C.) −20 ° C.

Although only the rolling start temperature is specified in the present invention, if the rolling start temperature is regulated within the range of the heating condition and the rolling reduction condition of the present invention, the rolling end temperature can be reduced to an ultra-fine grain size. Inevitably falls within a temperature range that does not interfere. However, if the rolling end temperature is excessively increased due to uneven heating or the like, ultrafine graining may not be achieved in some cases. Therefore, preferably, the rolling end temperature should be limited to T2 (° C.)-20 ° C. or lower. .

The basic requirements for the rolling described above relate to finish rolling, and do not depend on the rolling conditions before finish rolling. That is, even if the rough rolling is performed for the purpose of adjusting the thickness before the finish rolling, the effect of the present invention is not impaired. As long as the requirements of the present invention are satisfied, since rolling itself before finish rolling leads to microstructural refinement, it may be preferable for ultrafine graining.

As the cooling after the rolling, depending on the desired strength and toughness level, it is allowed to cool as it is, or at 5 to 40 ° C.
The cooling rate may be accelerated to 20 to 600 ° C. at a cooling rate of / s. Furthermore, the steel plate after cooling or accelerated cooling is heated to 450 ° C.
Tempering may be performed in a range of T1 (° C.). The cooling rate in the case of accelerated cooling is limited to 5 to 40 ° C./s. However, when the cooling rate is less than 5 ° C./s, the structural change due to accelerated cooling is not clear, This is because no improvement can be expected, and if the temperature exceeds 40 ° C./s, the difference in the structure or characteristics between the surface layer and the inside is large, which is not preferable. The accelerated cooling at the cooling rate stops at 20 to 600 ° C. depending on the desired strength and toughness of the steel sheet. Setting the stop temperature of the accelerated cooling to less than 20 ° C. has no effect in controlling the material, and merely causes an increase in manufacturing cost. Conversely, if accelerated cooling is stopped at more than 600 ° C., the effect of improving strength and toughness by accelerated cooling cannot be clearly obtained.

With respect to the steel sheet after cooling or accelerated cooling,
For the purpose of adjusting the strength, improving the toughness, and improving the shape, it is possible to further perform a tempering treatment. In that case, it is an essential requirement that the formed ultrafine grain structure is not damaged. In the present invention, the tempering temperature is preferably in the range of 450 ° C. to T1 (° C.). However, when the tempering temperature is lower than 450 ° C., the effect of the tempering is not clear and the T1 temperature corresponding to the Ac ₁ transformation point is low.
If the temperature exceeds (° C.), the form of the ultrafine grain structure in the surface layer may be impaired. Although the structure formed by the method of the present invention is a recovered and recrystallized structure of α, since it is a structure formed almost dynamically during rolling, it is considerably stable, and A
Even if the tempering temperature is increased to the c ₁ transformation point, the form of the ultrafine grains α is preserved. In the tempering temperature range, the heating and holding time for tempering is arbitrary as long as it is in an industrial range.
From the viewpoint of preserving the ultrafine grain structure of the surface layer, the holding time is 5 hours.
It is preferably within the range.

The above is a description of the requirements relating to the heating and rolling of the present invention. In order to secure the necessary strength as a structural member and to ensure low-temperature toughness, the above-mentioned heating and rolling must be performed.
In addition to conditions such as rolling, it is necessary that the chemical composition be within an appropriate range. That is, in the method (1), T1
(° C.) is equal to or higher than 700 ° C.
In the method (1), it is necessary to adjust the combination of chemical compositions so that T2 (° C.) is in the range of 800 to 900 ° C., and for each element, the composition range is adjusted for the following reason. There is a need.

First, C is added as an effective component for improving the strength of steel, and if it is less than 0.01%, it is difficult to secure the strength required for structural steel, and more than 0.20%. Excessive addition significantly reduces uniform elongation and toughness, as well as weld cracking resistance.
The range was 0%. Next, since Si is an element effective as a deoxidizing element and ensuring the strength of the base material,
% Or more. Conversely, an excessive addition exceeding 1.0% forms a coarse oxide, which leads to deterioration of ductility and toughness. Therefore, the range of Si is 0.03 to 1.0.
%.

Mn is an element necessary for securing the strength and toughness of the base material, and it is necessary to add at least 0.30%. Was set to 2.0%. Al is an element effective for deoxidation, refining of γ particle size, etc.
It is necessary to contain 5% or more, but if added in excess of 0.1%, a coarse oxide is formed and the ductility is extremely deteriorated, so it is limited to the range of 0.005 to 0.1%. There is a need to.

N works effectively with Al and Ti to refine γ grains and is effective in improving strength and toughness. However, it is necessary to contain 0.001% or more to exhibit the effects. On the other hand, if added excessively, solute N increases and adversely affects toughness, so the upper limit is made 0.01% as an acceptable range. P and S are preferably reduced as impurity elements as much as possible. However, since unnecessary reduction imposes a load on the steel making process, P and S are defined as allowable amounts that do not cause a decrease in toughness, ductility or weldability. Is limited to 0.02% or less, and S is limited to 0.01% or less.

The above are the basic components of the steel of the present invention. However, according to the desired strength level, Cr, Ni, Mo, Cu, Ti, V, Nb, Z
One or more of r, Ta, W, and B, and one or more of Mg, Ca, and REM can be further contained for the purpose of improving ductility. First, Cr and Mo
Are effective elements for improving the strength of the base material, but in order to obtain a clear effect, it is necessary to add 0.01% or more, respectively.
Since the toughness tends to deteriorate, the amount of each of these elements is set in the range of 0.01 to 1.0%.

Ni is a very effective element that can improve the strength and toughness of the base material at the same time, but it is necessary to contain 0.01% or more in order to exert the effect. When the content is increased, the strength and toughness are improved. However, if the content exceeds 3.0%, the effect is saturated, but the weldability is deteriorated. Therefore, the upper limit is set to 3.0%. Next, Cu also has substantially the same effect as Ni, but if it exceeds 1.5%, there is a problem in hot workability. Therefore, it is limited to the range of 0.01 to 1.5%.

Ti is an element that contributes to the improvement of the strength of the base material by precipitation strengthening and is also effective for the refinement of γ grains by the formation of TiN.
It is necessary to add 3% or more. On the other hand, if it exceeds 0.10%, as in the case of 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 the improvement of the strength of the base material by precipitation strengthening, but when added excessively, it deteriorates ductility and toughness. Therefore, V is 0.005 to 0.50% as a range in which the effect can be exhibited without deteriorating ductility and toughness.
Nb is made 0.003 to 0.10%.

Like V and Nb, Zr and Ta also contribute to improvement of the strength of the base material mainly by precipitation strengthening, but when added excessively, the ductility and toughness are deteriorated. Therefore, as a range in which the effect can be exhibited without causing deterioration of ductility and toughness, Zr
Is 0.003 to 0.10%, and Ta is 0.005 to 0.2%.
0%. W is effective for increasing the strength of the base metal by solid solution strengthening and precipitation strengthening, but it is necessary to add 0.01% or more to exhibit the effect. On the other hand, if the content exceeds 2.0%, the toughness deteriorates remarkably.
The upper limit is set to 2.0%.

B is very effective for increasing the strength by increasing the hardenability of steel by adding a very small amount of 0.0003% or more. However, when B is added excessively, it forms BN and conversely reduces the hardenability. In order to greatly deteriorate toughness, the upper limit is set to 0.
0020%. Mg, Ca, and REM are all effective in suppressing ductility of sulfide during hot rolling and improving ductility. In addition, the oxides are effectively refined to effectively improve joint toughness. The lower limit contents for exhibiting the effect are 0.0005% for Mg and Ca and 0.005% for REM.
05%. On the other hand, if it is contained excessively, sulfides and oxides are coarsened and ductility and toughness are deteriorated. Therefore, the upper limits are respectively 0.01% for Mg and Ca and 0.10% for REM.
And

Next, the effects of the present invention will be described more specifically with reference to examples.

[0047]

EXAMPLES The chemical compositions of the test steels used in the examples are shown in Tables 1 and 2 (continuation-1 in Table 1) and Table 3 (continuation-2 in Table 1). Each of the test steels was made into a slab by ingot slab rolling or continuous casting. Of Tables 1-3,
Steel numbers 1 to 14 satisfy the chemical composition range of the present invention,
Steel numbers 15 to 20 do not satisfy the chemical composition range of the present invention.

The steel slabs having the chemical compositions shown in Tables 1 to 3 were used in Tables 4 and 5
(Continued in Table 4-1), Table 6 (Continued in Table 4-2), Table 7 (Continued in Table 4-3), Table 8, Table 9 (Continued in Table 8-
1), Table 10 (continuation of Table 8-2), Table 11 (continuation of Table 8-3), Table 12, Table 13 (continuation of Table 12-1),
The steel plate was manufactured under the conditions shown in Table 14 (continuation-2 of Table 12) and Table 15 (continuation-3 of Table 12), and the strength at room temperature was determined.
The 2 mm V notch Charpy impact characteristics and the ESSO characteristics as brittle crack propagation stopping characteristics were investigated. Table 4-
7 shows what was manufactured by the method according to claim 1 of the present invention, and Tables 8 to 11 show what was manufactured by the method according to claim 2. Tables 12 to 15 show comparative examples.

All the test pieces were taken at right angles (C direction) to the rolling direction from the center of the sheet thickness. 50% Charpy impact properties
Fracture surface transition temperature (vTrs) and brittle crack propagation arrestability were measured by an ESSO test, and the Kca value was 400 kg.
They were evaluated respectively f · mm ^-3/2 become temperature (TKca400). Tables 4 to 15 also show the test results of strength and toughness.
The average α particle size was determined by a cutting method using a scanning electron microscope photograph at a magnification of 2000 times, and the average value in 20 visual fields was shown.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

[0053]

[Table 4]

[0054]

[Table 5]

[0055]

[Table 6]

[0056]

[Table 7]

[0057]

[Table 8]

[0058]

[Table 9]

[0059]

[Table 10]

[0060]

[Table 11]

[0061]

[Table 12]

[0062]

[Table 13]

[0063]

[Table 14]

[0064]

[Table 15] In Tables 4 to 7 and Tables 8 to 11, Test No. A1 to A1
7 is a steel plate in which a steel slab of the chemical composition of the present invention is manufactured in accordance with the requirements of the present invention, and the finally obtained α structure is sized and the average particle size is 2 μm or less. As a result, the toughness value was about −120 ° C. or less in vTrs, and TKc
At about -110 ° C or less at a400, it is clear that the present invention can provide extremely excellent low-temperature toughness in addition to brittle fracture initiation properties as well as brittle crack propagation arrest properties.

On the other hand, Test Nos. B1
-B10 are comparative examples, and any of the conditions are out of the limited range of the present invention, so that both Charpy impact characteristics and ESSO characteristics are much inferior to those of the present invention examples. That is, the test No. B1 has too much C, so it has Charpy impact properties,
Both ESSO characteristics are inferior.

Test No. B2 does not have good Charpy characteristics and ESSO characteristics because of an excessive amount of Mn.
Test No. Both B3 and B4 have excessive amounts of Ni, and T1 and T2 obtained from the chemical composition do not satisfy the conditions of the present invention. As a result, ultrafine graining is insufficient. Did not obtain good values for both Charpy impact characteristics and ESSO characteristics.

Test No. B5 has poor Charpy impact characteristics and ESSO characteristics due to excessive P as an impurity. Test No. In B6, since both Cr and Mo are excessive, both Charpy impact characteristics and ESSO characteristics are significantly deteriorated. Test No. Although B7 to B10 satisfy the chemical composition of the present invention, good Charpy impact characteristics and ESSO characteristics are not obtained because the production conditions are not in accordance with the present invention.

That is, the test No. In B7, the heating temperature is too high, so that ultrafine graining is not sufficient, and the Charpy impact characteristics and ESSO characteristics are inferior. Test No. Since B8 has an excessively low rolling reduction, the ultrafine graining is not sufficient, and both Charpy impact characteristics and ESSO characteristics are inferior. On the other hand, test N
o. B9 satisfies the range of the present invention, but does not include those having a rolling reduction of 30% or more per pass. Has not been achieved.

Test No. In B10, since the rolling start temperature is too high and processing in the γ single phase region has been completed, it is not possible to achieve ultra-fine graining that can be obtained only when α is directly processed, so that the Charpy impact characteristics and ESSO characteristics are significantly increased. No significant improvement is observed. As described above, it is clear from the examples that the present invention stably achieves an ultrafine-grained structure, thereby obtaining very good low-temperature toughness.

[0070]

The present invention does not require the addition of expensive alloying elements or the need for complicated hot working or heat treatment steps with inferior productivity, and the average grain size is 2 μm or less and the mixed grain size is small. It is an epoch-making method that can produce thick steel plate with good low-temperature toughness by obtaining ultrafine-grained α-structure, and its industrial effects are extremely low, such as reduction of manufacturing cost and improvement of safety as a structure. large.

Claims (6)

[Claims] C .: 0.01 to 0.20%, Si: 0.03 to 1.0%, Mn: 0.30 to 2.0%, Al: 0.005 to 0.5% by weight. 1%, N: 0.001 to 0.01%, and T1 (° C.) represented by the formula (1) is T1
(° C.) Steel having a chemical composition of ≧ 700 ° C., the content of P and S as impurities is P: 0.02% or less, S: 0.01% or less, and the balance is Fe and unavoidable impurities. A piece (2)
When the steel sheet is manufactured by heating to a range of T2 (° C.)-100 ° C. to 1200 ° C. represented by the formula and hot rolling, finish rolling at 650 ° C. to T1 (° C.) + 100 ° C. A method for producing a thick steel plate excellent in low-temperature toughness, comprising a rolling step in which rolling at a rolling reduction of 30 to 70% is included in one or more passes and a cumulative rolling reduction is 50 to 95%. T1 (° C.) = 750.8−26.6 · C% + 17.6 · Si% −11.6 · Mn% −22.6 · Cu% −23.0 · Ni% + 24.1 · Cr% + 22 5.5 · Mo% -39.7 · V% -5.7 · Ti% + 232.6 · Nb% −169.4 · Al% −894.7 · B% (1) T2 (° C.) = 937.2-476.5.C% + 56.0.Si% -19.7.Mn% -16.3.Cu% -26.6.Ni% -4.9.Cr% + 38.1.M o% + 124.8 · V% + 136.3 · Ti% −19.1 · Nb% + 198.4 · Al% + 3315.0 · B% (2) 2. 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 0.5%. 1%, N: 0.001 to 0.01%, and T2 (° C.) represented by the formula (2) is 90%.
It has a chemical composition of 0 ° C. ≧ T2 (° C.) ≧ 800 ° C., the content of P and S as impurities is P: 0.02% or less, S: 0.01% or less, and the balance of Fe and unavoidable Slab made of impurities (2)
When the steel sheet is manufactured by heating to a range of T2 (° C) −50 ° C. to 1200 ° C. represented by the formula and hot rolling, T2 (° C.) −100 ° C. to T2 (° C.) −20 ° C. as finish rolling.
And a rolling process in which rolling at a rolling reduction of 30 to 70% per pass is included in one or more passes and a cumulative rolling reduction is 50 to 95%. Manufacturing method of thick steel plate. T2 (° C.) = 937.2−476.5 · C% + 56.0 · Si% −19.7 · Mn% −16.3 · Cu% −26.6 · Ni% −4.9 · Cr% + 38 0.1.Mo% + 124.8.V% + 136.3.Ti% -19.1.Nb% + 198.4.Al% + 3315.0.B% (2) 3. The production of a thick steel sheet excellent in low-temperature toughness according to claim 1 or 2, wherein the steel sheet after rolling is cooled at a cooling rate of 5 to 40 ° C./s to 20 to 600 ° C. Method. 4. The method for producing a thick steel sheet having excellent low-temperature toughness according to claim 1, wherein tempering is performed at a temperature of 450 ° C. or more and T1 (° C.) or less. 5. Cr: 0.01-1.0%, Ni: 0.01-3.0%, Mo: 0.01-1.00%, Cu: 0.01-1. Group consisting of 5%, Ti: 0.003 to 0.10%, V: 0.005 to 0.50%, Nb: 0.003 to 0.10%, Zr: 0.003 to 0.10%, One or two of at least one of a group consisting of Ta: 0.005 to 0.20%, W: 0.01 to 2.0%, and B: 0.0003 to 0.0020% The method for producing a thick steel sheet having excellent low-temperature toughness according to any one of claims 1 to 4, further comprising at least one of a kind and a kind. 6. One or more of Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and REM: 0.005 to 0.10% by weight%. The method for producing a thick steel plate having excellent low-temperature toughness according to any one of claims 1 to 5, further comprising: