JPH08295982A - Thick steel plate excellent in toughness at low temperature and its production - Google Patents

Abstract

PURPOSE: To form a structure composed of fine ferritic grains and to provide excellent toughness at low temp. by applying controlled rolling and controlled cooling to a low Si-Mn steel. CONSTITUTION: This steel has a composition consisting of, by weight ratio, 0.01-0.20% C, 0.30-1.0% Si, 0.0.-2.00% Mn, 0.005-0.1% Al, 0.001-0.1%, and the balance essentially Fe, and further, over the whole steel plate thickness, average grain size of ferrite is regulated to <=3μm. It is preferable to add Ni, Cr, Mo, V, Nb, REM, etc., to the composition, if necessary. In order to obtain this structure of fine ferritic grains, a steel slab is heated to a temp. between the Ac3 transformation point and 1200 deg.C, roughed in the austenitic region, and finish- rolled from the state of <=15μm average ferritic grain size and 50-90% ferrite ratio, and then, rolling in two-phase region at 50-90% cumulative rolling rate is finished at 650-750 deg.C and successively accelerated cooling is done at (5 to 50) deg.C/sec cooling rate down to 550-20 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate having excellent low temperature toughness and a method for producing the steel plate.

[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 improving low temperature toughness in thick steel sheets have been proposed, as a method for improving low temperature toughness without causing deterioration of other characteristics without using an expensive alloying element such as Ni, ferrite ( α) The refinement of crystal grain size is typical.

Various methods have heretofore been proposed as a method for reducing the α particle size. Typical methods include, for example, Japanese Patent Publication No. 49-7291 and Japanese Patent Publication No. 57-210.
No. 07, JP-B-59-14535, etc., the transformation from γ to α by performing controlled rolling in the unrecrystallized temperature range of austenite (γ) and subsequently performing accelerated cooling. At times, a method of making α fine is proposed. In such a method utilizing the transformation from γ to α, when γ is coarse, the γ / α conversion ratio (γ grain size before transformation / α grain size after transformation) can be obtained by effectively utilizing the unrecrystallized region rolling. However, as the γ particle size becomes finer, the γ / α conversion ratio approaches 1, so that the degree of α refinement becomes saturated. Therefore, in the method of refining α through the transformation from γ to α, the degree thereof is regulated by the degree of refining of γ, and thus the drastic miniaturization of the α particle size cannot be expected.

A technique for improving strength and toughness by so-called two-phase region rolling, which expands the temperature range of controlled rolling to the γ / α two-phase region, has also been proposed. For example, Japanese Examined Patent Publication No. 58-5967 discloses a technique for improving the toughness by suppressing the occurrence of the separation characteristic of the two-phase region rolling by devising the components and the rolling conditions. However, in the conventional two-phase region rolling technique, the α grain size is about the same as the α grain size obtained by 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.

Further, a method of refining the α-grain size by heat treatment is also shown, instead of hot working such as rolling.
For example, [Iron and Steel, 1977, No. 1, 1991, No. 1
71 to 178], V and N are added in a larger amount than usual to refine γ and increase the γ / α conversion ratio during transformation to normalize. A method of making a fine α-structure by treatment has been developed. However, in order to obtain a fine α structure by this method, V is set to 0.
It is necessary to add 01% or more and N to 0.01% or more,
The reachable α particle size is about 5 μm.

Furthermore, [Materials and Processes, Third Year, Sixth Year]
No. 1990, p. 1796], there is disclosed a method for obtaining ultrafine grain steel having a grain size of 3 μm or less by a complicated thermomechanical treatment including repeated γ / α transformation. This method performs accelerated cooling after controlled rolling, stops accelerated cooling at about 500 ° C, then reheats it to 900 ° C without cooling to room temperature, and performs hot rolling at a predetermined temperature to obtain ultra-fine rolling. Although grain steel is obtained, the α grain size is strongly influenced by the cooling stop temperature, and ultrafine grains α having a grain size of 3 μm or less have not been obtained except in the vicinity of the cooling stop temperature of 500 ° C., It is considered to be difficult to manufacture industrially stably.

Therefore, in any of the above-mentioned conventional methods, cost increase is unavoidable due to deterioration of productivity, increase of heat treatment steps, and increase of alloy elements. In addition, 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 complicated process that is strictly controlled, and it is significantly reduced by further miniaturization of α. Toughness cannot be expected.

[0008]

According to the present invention, the productivity is high and the average α particle diameter is 3 μm or less without adding a large amount of expensive alloying elements, a process with poor productivity or a complicated process. An object of the present invention is to provide a thick steel sheet having an ultrafine-grained α structure with a uniform mixed grain size and excellent low-temperature toughness, and a manufacturing method thereof.

[0009]

The inventors of the present invention utilize α recovery / recrystallization by hot working of α, because the γ / α transformation, which is a conventional typical grain refining method, has a limit. By paying attention to the method described above, and by conducting a detailed investigation of the hot working behavior of α, a means for ultrafine graining of α was found, and the present invention was completed.

That is, the gist of the present invention is that
The invention resides in the following first to seventh inventions. [First invention] C: 0.01 to 0.20% by weight,
Si: 0.03 to 1.0%, Mn: 0.30 to 2.0
%, Al: 0.005-0.1%, N: 0.001-
0.01%, with the balance Fe and unavoidable impurities, and having an average ferrite grain size of 3μ over the entire thickness of the steel sheet.
A thick steel sheet excellent in low temperature toughness, which is characterized by being m or less.

[Second invention] Further, Cr: 0.01 to
0.50%, Ni: 0.01 to 3.0%, Mo: 0.0
1 to 0.50%, Cu: 0.01 to 1.5%, Ti:
0.003-0.10%, V: 0.005-0.20
%, Nb: 0.003 to 0.05%, B: 0.0003
~ 0.0020%, Ca: 0.0005 to 0.005
%, REM: 0.0005 to 0.01% of 1 or 2
A thick steel sheet excellent in low temperature toughness according to the first invention, characterized by containing at least one kind.

[Third invention] C: 0.01-
0.20%, Si: 0.03 to 1.0%, Mn: 0.3
0-2.0%, Al: 0.005-0.1%, N: 0.
A steel slab containing 001 to 0.01% and 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., and after rough rolling in the austenite region, the average ferrite grain size is 15 μm. From the state in which the proportion of ferrite is 50 to 90% below, 650 to 750 ° C. is performed as finish rolling in the two-phase region rolling in which the cumulative reduction is 50 to 90%.
The method for producing a thick steel sheet excellent in low temperature toughness, characterized in that the method is finished at the temperature of.

[Fourth Invention] As the rough rolling, rolling having a cumulative rolling reduction of 20 to 70% is completed at a temperature of 900 to 800 ° C., which is excellent in low temperature toughness as described in the third invention. Manufacturing method of thick steel plate. [Fifth invention] Further, Cr: 0.01 to 0.50%,
Ni: 0.01 to 3.0%, Mo: 0.01 to 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 to 0.0020
%, Ca: 0.0005 to 0.005%, REM: 0.
The method for producing a thick steel sheet having excellent low temperature toughness according to the third or fourth invention, characterized in that it contains one or more of 0005 to 0.01%.

[Sixth invention] Any one of the third to fifth inventions, characterized in that after finishing rolling, the accelerated cooling is continued to 550 to 20 ° C at a cooling rate of 5 to 50 ° C / sec. The method for producing a thick steel sheet excellent in low temperature toughness as described in 1. [Seventh invention] After finishing rolling, continuously 5 to 50 ° C
After accelerated cooling to 550 to 20 ° C at a cooling rate of / sec,
Third to fifth, characterized by tempering at 600 to 400 ° C
The method for manufacturing a thick steel sheet having excellent low temperature toughness according to any one of the above-mentioned inventions.

[0015]

The present invention will be described in detail below based on experimental results. The present invention is characterized in that a method by recovery / recrystallization of processed α is used as a means for refining α, which exceeds the conventional achievement level. That is, γ / α
The α obtained by processing in the two-phase region is recovered and recrystallized after the processing, and the actual ultrafine graining of α is measured. In that case, in order to obtain uniform fine grains without impairing the productivity, the recovery and recrystallization of α after processing is not a method such as reheating heat treatment, and is preferably performed during cooling after rolling, preferably during rolling. It is advantageous to generate it during or immediately after. However, in general, a steel material that has undergone two-phase rolling has a high dislocation density in the α matrix, and therefore the toughness of the α matrix deteriorates. In other words, there is a limit to the improvement in toughness because they offset each other with the effect of improving toughness through the introduction of separation by two-phase rolling. Therefore, in the case of the manufacturing method based on the two-phase rolling, α
It is considered that it is necessary to reduce the dislocation density of the matrix at the same time as making the grains ultrafine. However, since the grain size is reduced by the direct processing of α, it is inevitable that the processing be performed in a temperature range where α is the main structure, and α is generated in a certain amount or more in the cooling process such as normal rolling. If this is done, the temperature will inevitably decrease, so it is not easy to achieve a significant reduction in dislocations during rolling or subsequent cooling.

In order to make α into ultrafine grains in two-phase rolling, it is premised that a certain amount or more of dislocations are introduced into α to give a driving force for recovery / recrystallization. On the other hand, finally, it is necessary to reduce dislocations remaining in the matrix as much as possible. As a result of studying means for satisfying both of these conditions at the same time, it has been found that it is the most important point to reduce the α grain size before the two-phase rolling. That is, if the α-grain size before processing is fine, recovery / recrystallization easily occurs even with the same dislocation density, and the residual dislocation density is reduced accordingly, and the achieved α-grain size has the same processing rate. In that case, it was found that the α particle size before processing was in a substantially proportional relationship. Therefore, the manufacturing conditions for obtaining an ultrafine-grained α-structure with a low dislocation density in the matrix, which is effective for improving toughness, are as follows: Of the α-grain size before processing, which has a large effect on the dislocation density and the final α-grain size, and the proportion of α formed by the two-phase zone rolling in the final structure. It is achieved by properly combining the ratios before entering.

In FIG. 1, the chemical composition is C: 0.15%, Si:
0.2%, Mn: 1.2%, Nb: 0.006%, T
i: 0.01%, N: 0.0032% steel at 1150 ° C
And the rolling conditions (reduction rate 10 to 80%, reduction temperature 1000 to 800 ° C.) and cooling conditions (cooling rate 0.1.
The state of α after the two-phase region rolling when the α grain size before the two-phase region rolling is variously changed by 5 ℃ / sec), the cumulative rolling reduction of the two-phase region rolling and the two-phase region before the two-phase region rolling It is the figure which investigated the relationship with alpha particle size. The α fraction before rolling in the two-phase region is 60 to
It is controlled in the range of 75% and the cooling rate is about 2 after rolling in the two-phase region
The temperature was adjusted to 0 ° C./sec, accelerated cooling from about 650 ° C. to room temperature was performed, and the structure in the center of the plate thickness was observed.
The morphology of α was determined by observing the Nital corrosion structure with an optical microscope, and the α particle size was 35 times with a scanning electron microscope.
The measurement was carried out using a 00-time photograph.

According to FIG. 1, as the α grain size before the start of the two-phase region rolling becomes finer and as the cumulative reduction ratio of the two-phase region rolling increases, the final value after the two-phase region rolling is increased. The α structure becomes uniform and fine. This is α before the start of two-phase rolling
This is because the finer the particle size, the easier the recovery and recrystallization even with the same cumulative rolling reduction. Separately, according to the relationship with the low temperature toughness and the result of the examination of the electron microscopic structure, the observed α-grains are elongated grains (elongation degree 1.5 or more) or mixed grains (elongation grains with elongation degree 1.5 or more). Is 30% or more), α
Since the dislocations in the matrix are not sufficiently recovered, the improvement in low temperature toughness does not appear clearly even with the same crystal grain size.

Therefore, in order to remarkably improve the low temperature toughness, it is necessary to make α very fine and at the same time reduce the proportion of elongated grains as much as possible. From FIG. 1, in order to make the final α after the two-phase region rolling a fine-grained grain structure that does not include elongated grains, both the α-grain size before the start of the two-phase region rolling and the cumulative rolling reduction of the two-phase region rolling are performed. It became clear that it was necessary to optimize If the cumulative reduction is increased, the average grain size becomes finer, but even if the cumulative reduction is 70% or more, the α grain size before the two-phase rolling is 17 μm.
If it is coarser than a certain degree, a mixed grain structure is formed, and significant improvement in toughness cannot be expected.

In the case where the α fraction before the α two-phase rolling is 60% or more, it is expected that the α will be made into ultra-fine grains as shown in FIG. Since α of γ is generated by the transformation from γ, it is not possible to expect uniform ultrafine graining of the entire structure, no matter how part of α is ultrafine grained by two-phase rolling. The present invention has been made on the basis of the above research results, and according to the present invention, the average grain size α of 3 μm or less is uniformly stable over the entire thickness of the steel sheet.
By making the structure of the grain size, the low temperature toughness is −100 ° C. or less at the fracture surface transition temperature of the Charpy impact test and 8 of the DWTT test.
It became clear that even at a 5% fracture surface transition temperature, a very good low temperature toughness of -90 ° C or less can be achieved.

The reasons for limiting each manufacturing condition will be described in detail below. First, in the present invention, the heating temperature of the billet is set to Ac.
_The temperature was set to ₃ transformation points or more and 1200 ° C. or less. When the heating temperature is below the Ac ₃ transformation point, solution treatment is not sufficiently performed,
This is because the heating γ grain size becomes extremely coarse at a high heating temperature exceeding 1200 ° C., and it may be difficult to make α grains fine before starting the two-phase region rolling by subsequent rolling.

One of the most important points of the present invention is to make the α grain size fine before starting the two-phase rolling. Based on FIG. 1, in consideration of the range of realistic reduction ratios in the two-phase region, the α grain size before starting the two-phase region rolling is set to 15 μm or less. As a means for refining α before entering the two-phase region rolling, decrease of the reheating temperature of the billet, optimization of the conditions for the γ region rolling,
There are various possibilities such as accelerated cooling and devising of chemical components, so that it is not particularly limited, but in the present invention, in order to achieve a desired α grain size, rolling with a cumulative rolling reduction of 20 to 70% is performed.
Finish at a temperature of 00 to 800 ° C., and if necessary,
After the rough rolling is finished, it is preferable to cool until the start of two-phase region rolling as finish rolling at a cooling rate of 0.1 to 5 ° C / sec. This is because the rolling reduction in the γ range in the components of the present invention, the heating condition range, the cumulative rolling reduction is 20 to 70%, and the rolling end temperature is in the range of 900 to 800 ° C. This is because the effect of rolling in the non-recrystallized region is also superimposed, and if the subsequent cooling rate up to the two-phase region rolling temperature is in the range of 0.1 to 5 ° C./second, the α particle size is 15 μm or less.

Further, the α fraction at the stage of entering the two-phase region rolling is also important, but if the induced α transformation during the two-phase region rolling is also taken into consideration, if the α fraction before the start of rolling can be secured at 50% or more, it will be final. Α can be uniformly fine-grained in tissue. The upper limit of the α fraction is specified to 90%, but this is because the slightly remaining hard γ phase is effective in uniformizing the α processing and finely sizing, and exerts its effect. This is because γ is preferably 10% or more. Also, incidentally, when α is cooled to 90% or more, α is substantially recovered.
It will be below the lower limit temperature for recrystallization.

The cumulative reduction ratio of the two-phase rolling is shown in FIG.
Based on the above, on the premise that the α grain size before the two-phase region rolling is 15 μm or less, the necessary condition for stably obtaining ultrafine grains is 50% or more. In the case of two-phase rolling, the rolling temperature is inevitably low and recrystallization does not occur in a short time during rolling.Therefore, it is sufficient to stipulate only the cumulative rolling reduction. It does not matter the time interval. The larger the cumulative rolling reduction of the two-phase rolling, the more effective it is for grain refinement. However, even if the rolling reduction exceeds 90%, the effect saturates and the rolling time becomes long, making it difficult to secure the finishing temperature. Therefore, the upper limit is 90% in consideration of economic efficiency.

However, if the end temperature of the two-phase region rolling is too low, no matter how the α grain size before the two-phase region rolling is refined, the recovery and recrystallization of α after the rolling will not proceed sufficiently, and the ultra-high temperature will not be exceeded. Fine grain and α
Since the reduction of dislocations in the matrix becomes insufficient, the lower limit temperature at which the recovery / recrystallization proceeds under the condition that the α grain size is 15 μm or less and the cumulative rolling reduction is 50% or more is based on the experimental temperature. The end temperature of phase rolling is 650 ° C. or higher. The upper limit of the end temperature of the two-phase region rolling is 750 ° C. In the two-phase rolling, the end temperature may be higher than the start of rolling due to heat generated during processing. In this case, if the temperature rises excessively, the obtained ultrafine particles α grow and become coarse and mixed grains. Therefore, the upper limit of the end temperature is set to 750 based on the experimental results, as the end temperature sufficient to prevent this. ℃ was made.

Further, as described below, as the cooling condition after the two-phase region rolling, it is possible to perform accelerated cooling according to the desired characteristics. If the time until the start of cooling is extremely short, there is concern that recovery and recrystallization will not proceed sufficiently. According to the actual manufacturing results, recovery and recrystallization sufficiently proceed within the transportation time from the end of rolling in the actual steel plate manufacturing to the cooling equipment for accelerated cooling. It is preferable to secure a time for this recovery / recrystallization of 20 seconds or more.

As for the heat history of the steel sheet after the completion of the two-phase rolling, various heat histories can be received in order to obtain desired mechanical properties within the range in which the structure generated at the end of rolling is preserved. Is. That is, after rolling, it may be allowed to cool as it is, or after rolling, it may be subjected to accelerated cooling, or after accelerated cooling, it may be subjected to tempering treatment. However, in the case of accelerated cooling, the cooling rate is required to be 5 ° C./second or more in order to exert the effect of accelerated cooling. On the other hand, even if the cooling rate exceeds 50 ° C./sec, the effect of improving the structure control and mechanical properties is saturated, so the range of the cooling rate in accelerated cooling is 5
~ 50 ° C / sec.

For the same reason, accelerated cooling is 550 ° C.
Although it is necessary to perform the following steps, the metallurgical factor that affects the mechanical properties changes substantially at room temperature, so the lower limit of the cooling stop temperature is 20 ° C. When tempering is performed after accelerated cooling for adjusting strength, improving toughness, etc., the tempering temperature needs to be limited to 600 ° C. or lower because it is necessary to preserve the ultrafine grain structure obtained by rolling. is there. However, within the components and structure range of the present invention, improvement of mechanical properties by tempering is expected from 400 ° C. or higher, so the tempering temperature range is 400 to 600 ° C.

The above are the reasons for limiting the present invention regarding the production conditions. However, in order to secure desired strength and low temperature toughness, not only the production method but also the chemical composition must be within an appropriate range. The reasons for limiting the chemical components in the present invention will be described below. First, C is added as an effective component for improving 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%, Since the toughness and the weld cracking resistance are remarkably reduced, the content is set to 0.01 to 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 set to 0.03 to 1.0%. Mn is the strength of the base metal,
It is an element necessary for ensuring toughness, and it is necessary to add at least 0.30% or more, but the upper limit was set to 2.0% within the allowable range of the material such as toughness and crackability of the welded portion.

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 to exert the effect, but if it exceeds 0.1%, it is excessive. 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 it is necessary to contain N in an amount of 0.001% or more in order to clarify the effect. 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 for the purpose of increasing the strength of the base metal and ensuring toughness in accordance with the desired strength level, Cr, Ni, Mo, Cu, Ti, and
One or more of V, Nb, B, Ca and REM can 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 0.
If added over 50%, the toughness tends to deteriorate, so the content is made 0.01 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 Ar ₃ transformation point is extremely lowered. Since it becomes impossible to simultaneously satisfy the α amount of 50% or more before zone rolling and the two-phase zone rolling end temperature of 650 ° C. or more at the same time, the upper limit is set to 3.0% in consideration of economic efficiency.

Next, Cu has almost the same effect as Ni, but if added over 1.5%, problems occur in hot workability, so the range is limited to 0.01 to 1.5%. Ti contributes to the improvement of the base metal strength by precipitation strengthening, and TiN
Although it is an element which is also effective for making the γ grains fine by the formation of γ, 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 in excess, they deteriorate toughness. 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 in increasing the hardenability of steel materials and increasing the strength, but if added in excess, BN
On the contrary, the hardenability is deteriorated and the toughness is greatly deteriorated, so the upper limit is made 0.0020%.

Both Ca and REM are effective elements for improving the anisotropy of mechanical properties and improving the lamella tear resistance. In the case of Ca, if the content is less than 0.0005%, the effect is not clear, and if it exceeds 0.005%, inclusions become coarse and the toughness and ductility may be adversely affected.
The range is from 05 to 0.005%. In the case of REM,
If less than 0.0005%, the effect is not clear and 0.01
%, As with Ca, inclusions may become coarse and adversely affect toughness and ductility.
The range is 0.01%.

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

[0038]

[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 15 satisfy the chemical composition range of the present invention, and Steel Nos. 16 to 18 are outside the chemical composition range of the present invention.

Steel pieces having the chemical composition shown in Table 1 are shown in Tables 2 and 3 (Table 2).
No. 1), a steel plate was manufactured under the conditions shown in Table 4 (No. 2 of Table 2), and the strength, Charpy impact characteristics, and DW were measured.
The TT characteristics were investigated. All the test pieces were sampled at right angles to the rolling direction (C direction) from the center of the plate thickness. The Charpy impact property was evaluated at 50% fracture transition temperature (vTrs), and the DWTT property was evaluated at 85% ductile fracture transition temperature (85% FATT). The test results of strength and toughness are also shown in Tables 2 to 4.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

[0043]

[Table 4]

In Tables 2 to 4, the test No. A1-A
All of 20 are steel sheets produced according to the present invention, and all α structures finally obtained are sized and the average grain size is 2.
It is 5 μm or less, and an ultrafine grain structure having an average grain size of 3 μm or less is stably obtained. Toughness value is vTrs -1
10 ° C or less, -90 ° C in 80% FATT of DWTT test
The following have been achieved:

On the other hand, the test No. B1 to B7 are comparative examples, and any one of the conditions is out of the limited range of the present invention, and thus both the Charpy impact characteristic and the DWTT characteristic are far inferior to the examples of the present invention. That is, the test No. In B1, the heating temperature of the steel slab is too high, and the α grain size before the two-phase region rolling is coarse, so that sufficient ultrafine graining cannot be achieved.

Test No. B2 also has a coarse ferrite grain size before rolling in the two-phase region, so sufficient ultrafine graining has not been achieved and the toughness is poor. Test No. Since B3 has a small α fraction before processing in the two-phase region, the ratio of α transformed from γ increases,
Ultra-fine grain is not measured. Test No. Since B4 has a small cumulative rolling reduction in two-phase rolling, α is not sufficiently recovered and recrystallized, and the toughness is not improved.

Test No. Since the chemical components of B5 to B7 are out of the range of the present invention, ultra-fine graining cannot be achieved, or the low temperature toughness is poor due to other toughness deterioration factors. From the above examples, it is clear that the present invention stably achieves an ultrafine grain structure, thereby obtaining a very good low temperature toughness.

[0048]

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 form of α, and a grain size.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuichi Shuichi Oita City 1st Nishinosu Oita City Nippon Steel Stock Company Oita Works

Claims (7)

[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%, balance Fe and unavoidable impurities, and an average ferrite grain size of 3 μm or less over the entire thickness of the steel sheet. steel sheet. 2. Cr: 0.01 to 0.50%, Ni: 0.01 to 3.0%, Mo: 0.01 to 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 to 0.0020%, Ca: 0.0005 . 0.005%, REM: 0.0005 to 0.01%, and one or more kinds are contained, and the thick steel sheet excellent in low temperature toughness according to claim 1. 3. 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, and a steel slab composed of the balance Fe and unavoidable impurities is converted to an Ac ₃ transformation point or more, 120
After heating to a temperature of 0 ° C. or lower and rough rolling in the austenite region, from the state where the average ferrite grain size is 15 μm or less and the proportion of ferrite is 50 to 90%, the cumulative rolling reduction as finish rolling is 50 to 90%. The two-phase region rolling is 650
A method for producing a thick steel sheet excellent in low-temperature toughness, which is characterized by ending at a temperature of 750 ° C. 4. The rough rolling has a cumulative rolling reduction of 20 to 20.
The method for producing a thick steel sheet excellent in low temperature toughness according to claim 3, wherein 70% rolling is finished at a temperature of 900 to 800 ° C. 5. Further, Cr: 0.01 to 0.50%, Ni: 0.01 to 3.0%, Mo: 0.01 to 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 to 0.0020%, Ca: 0.0005 ~ 0.005%, REM: 0.0005-0.01% of 1 type or 2 types or more are contained, The manufacturing method of the thick steel plate excellent in the low temperature toughness of Claim 3 or 4 characterized by the above-mentioned. 6. After finishing rolling is completed, the temperature is continuously 5 to 50 ° C.
The method for producing a thick steel sheet having excellent low-temperature toughness according to any one of claims 3 to 5, wherein accelerated cooling is performed at a cooling rate of / sec to 550 to 20 ° C. 7. After finishing rolling, 5 to 50 ° C. continuously.
After accelerated cooling to 550 to 20 ° C at a cooling rate of / sec,
It tempers at 600-400 degreeC, It is characterized by the above-mentioned.
5. The method for producing a thick steel sheet having excellent low temperature toughness according to any one of 5 above.