JP2003221619A - Method for manufacturing thick steel plate superior in arresting characteristics and ductile-fracture property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a thick steel plate inexpensively even with the satisfactory productivity, which is superior not only in arresting characteristics but also in ductile-fracture properties. <P>SOLUTION: This manufacturing method comprises giving an equivalent plastic strain ε onto the surface region of the steel plate so as to make ε≥0.5 in a non-recrystallizing-temperature region, then cooling the surface region to a temperature range of 450-650°C at a cooling rate of 2-15°C/s, within a period during which the amount of residual accumulated equivalent plastic strain εr in the surface region satisfies εr≥0.5, while keeping a temperature of the inner region of the steel plate to the Ar<SB>3</SB>transformation point or higher, then restarting rolling so as to give the residual accumulated equivalent plastic strain εr in a range of 0.35≤εr<0.55 to the above inner region, finishing the rolling at the Ar<SB>3</SB>transformation point or higher, to recuperate heat in the above surface region to the Ar<SB>3</SB>transformation point or lower due to the working heat and the inner sensible heat, and then cooling it with the average cooling rate kept to 1-10°C/s. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention is excellent in arrest property (brittle crack propagation stopping property) which is one of important properties for ensuring the safety of a structure, and is ductile as a fracture mode at room temperature. The present invention relates to a method capable of inexpensively manufacturing a thick steel plate having excellent fracture characteristics without adding a large amount of an expensive element such as Ni.

[0002]

2. Description of the Related Art From the viewpoint of ensuring the safety of building structures, etc., the steel has excellent arresting characteristics to prevent (stop) the propagation of a brittle crack once it has occurred. Is an important requirement for steel sheets used as materials for building structures and the like.

In order to improve the arrest characteristics of a steel sheet,
It has been conventionally known that miniaturization of equiaxed ferrite grains (α grains) in the surface layer portion of the plate thickness is effective. From this point of view, the conventional techniques for improving arrest characteristics are:
The mainstream is to reduce the equiaxed ferrite grain size in the surface layer. Equiaxial ferrite is the aspect ratio
It means a ferrite grain whose (major axis / minor axis) is about 1, that is, a ferrite grain which is not elongated in the rolling direction by rolling.

As such a technique, for example, Japanese Patent Laid-Open No. 61-
In No. 235534, recrystallization of ferrite grains and Ac caused by cooling during rolling and subsequent rolling in a recuperative process.
It has been proposed to make ferrite crystal grains finer by utilizing the α → γ reverse transformation by raising the temperature to ₃ transformation points or higher. However, such a technique has a problem that the productivity is lowered because the recuperation at the Ac ₃ transformation point or higher is an essential step.

On the other hand, as a method for alleviating the decrease in productivity, a technique such as Japanese Patent Laid-Open No. 4-141517 has been proposed. In this technique, the reheat temperature is set to Ac ₃ by cooling during the rolling and rolling during the subsequent temperature rise.
Utilizing recrystallization of ferrite grains by controlling below the transformation point,
This is intended to reduce the size of ferrite grains in the surface layer portion. Moreover, in this technique, not only the surface layer portion of the steel sheet, but also the difference in deformation resistance between the surface layer portion and the inside of the sheet thickness is utilized, and the aim is to miniaturize the steel sheet internal structure of the steel sheet.

However, in the method of obtaining fine ferrite by utilizing recrystallization of ferrite crystal grains, a considerably large reduction amount is required. Therefore, when the thickness and width of the steel sheet are increased, the crystal grain level is rolled. As the result depends on the capability of the machine, there is a possibility that stable and high quality cannot be maintained.

In connection with such a technique, for example, Japanese Patent Laid-Open No.
No. 301540 proposes a method of refining ferrite grains using the heat generated during processing during rolling and utilizing the α → γ reverse transformation by recuperation above the Ac ₃ transformation point. However, this technology is intended for wire rods and hot rolled steel sheets capable of high-speed continuous rolling, and although it is feasible for these, it was not applicable to thick steel sheets.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 8-295982 proposes a method in which the entire cross section of the plate thickness is cooled to a two-phase region of ferrite-austenite and then rolled. However, when rolling is performed in a state in which ferrite and austenite having different deformation resistances are mixed, there is a possibility that the plate thickness accuracy and flatness required as a steel plate may be disturbed, so that there is a drawback that it cannot be carried out easily.

By the way, in the steel materials as described above, even if the ductile fracture is seen in a collision of a vessel such as a large tanker, the energy at the collision is absorbed by the ductile fracture of the steel sheet to minimize the damage. It is also necessary to have excellent properties that can be retained (ductile fracture property).

[0010]

The present invention has been made under these circumstances, and an object thereof is to inexpensively produce a thick steel plate excellent not only in the arrest characteristics as described above but also in the ductile fracture characteristics. It is to provide a method that can be manufactured with high productivity.

[0011]

Means for Solving the Problems The method of the present invention capable of achieving the above-mentioned object is that when the thickness during rolling is t, the surface layer is 0.05t or more and 0.15t or less from both surfaces in the plate thickness direction. An equivalent plastic strain ε of ε ≧ 0.5 is applied to the region in the non-recrystallization temperature range of Ar ₃ transformation point or more and 900 ° C. or less, and then the residual cumulative equivalent plastic strain amount εr of the surface layer region is εr ≧ Within a time that satisfies 0.5, the plate thickness t /
While maintaining than 4 positions the temperature of the inner area of the core portion side than the Ar ₃ transformation point, the surface layer region is cooled to a temperature range of 450 to 650 ° C. at a cooling rate of 2 to 15 ° C. / s,
Then, the rolling is restarted, and in this rolling, the residual cumulative equivalent plastic strain ε of 0.35 ≦ εr <0.55 with respect to the internal region is satisfied.
Rolling is performed to impart r, and the rolling is completed at the Ar ₃ transformation point or higher, and the surface layer region is double-heated to the Ar ₃ transformation point or lower by the process heat generation and the internal sensible heat, and then the average cooling rate is 1 to 10 The point is that the cooling is performed so that the temperature becomes ° C / s.

As the thick steel plate used in the method of the present invention,
As basic components, C: 0.03 to 0.2%, Si: 0.
5% or less (not including 0%), Mn: 1.8% or less (0
%), Al: 0.01-0.1% and N:
Thick steel plates each containing 0.001 to 0.01% are included, and the thick steel plates may include (1) Ti: 0.
02% or less (not including 0%), Nb: 0.03% or less (not including 0%) and V: 0.05% or less (not including 0%), 1 or more selected from the group consisting of: (2) C
r: 0.5% or less (not including 0%) and / or M
It is also effective to further contain o: 0.5% or less (not including 0%), etc., and the characteristics of the thick steel sheet are further improved depending on the type of the element to be contained.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION With respect to ferrite-pearlite steel, the present inventors have examined from various angles a technique for improving arrest characteristics in place of fine equiaxed ferrite grains, which are said to be difficult to realize. As a result, if a structure extending in the rolling direction, generally called work ferrite, is formed in the surface layer of the steel sheet, even if a brittle fracture crack should occur, its propagation will be suppressed and it will be observed during normal collisions. Absorbs the energy at the time of collision even for ductile destruction
It was found that the effect of being able to minimize the damage caused by the collision is exhibited, and its technical significance was recognized, so the application was filed first (Japanese Patent Laid-Open No. 2000-309851).
issue).

In the thick steel plate described above, the arrest property is improved by forming a structure called work ferrite which extends in the rolling direction in the surface layer of the steel plate.
It is not only necessary to form the worked ferrite, and its distribution must satisfy the requirements (a) to (c) below, and the arrest characteristics of the steel sheet are improved only when these requirements are satisfied. You can do it.

(A) Existence over a range of 5% or more of the plate thickness from the surface layer of the plate thickness (b) The area of the processed ferrite grains is 5 μm in equivalent circle diameter.
Being the following crystal grains (c) Aspect ratio of 2 or more and 4 or less Here, "circle equivalent diameter" is focused on individual grains of the corresponding tissue and is assumed to have the same area. This is the diameter of the circle when

Further, it has also been found that by appropriately controlling the microstructure inside the steel sheet, the elongation characteristic, which is important for ductile fracture at room temperature, is improved as compared with the ordinary steel sheet. That is,
In order to improve the elongation property of the steel sheet, it is necessary to satisfy the requirement (d) below. (D) From the center part in the plate thickness direction to the surface layer part, the plate thickness direction t /
In the range of 4 or more (that is, the inner region on the core side from the t / 4 position in the plate thickness direction from both surfaces), the ferrite has a circle-equivalent grain size of 4 μm or more and 10 μm or less and an aspect ratio of 2 or less.

As described above, by forming processed ferrite grains in the surface layer of the steel sheet and making the microstructure inside the steel sheet uniform equiaxed ferrite grains, even if brittle cracks should occur, the cracks should occur. It was possible to realize a steel sheet that has excellent properties of stopping the propagation and is also excellent in the ductile fracture that is observed when a ship such as a large tanker collides.

As a method for forming fine ferrite in the surface layer portion, rolling has been generally carried out so as to increase the cumulative rolling reduction in the austenite (γ) non-recrystallization temperature range.
Such a method aims to increase ferrite nucleation sites by accumulating a large amount of strain in austenite. However, in such a method, fine grains of about 5 to 6 μm are the limit, and fine grains of 5 μm or less are limited. Since a larger reduction amount is required to achieve the crystal grains, the thickness size of the steel sheet to be applied is limited.

In order to efficiently realize the above-described structure, the inventors of the present invention have used conventional controlled rolling and accelerated cooling that is subsequently performed [ie, using equipment such as a shower and allowing to cool. [Cooling forcibly lowering the temperature of the steel sheet to be cooled by the cooling speed above (air cooling)] was examined to clarify the refinement of the crystal grains. As a result, the fact that the crystal grains are refined as the cooling rate increases confirmed.
However, when accelerated cooling of about 5 ° C./s or more is applied, the microstructure changes from the ferro-pearlite structure to the more brittle bainite structure and, at the same time, the ductile fracture property deteriorates.

Further, even when the hardenability is lowered by reducing the austenite unrecrystallized temperature range to increase the ferrite transformation nucleation site in order to suppress the formation of the bainite structure, it is about 10 ° C. When cooled at a cooling rate of not less than 1 sec / sec, a bainite structure is generated, so a ferrite-bainite two-phase mixed structure is formed, and the ferrite crystal grains obtained at that time are 6 μm.
The degree was the limit.

The inventors of the present invention have made further detailed studies on the bainite formation conditions, and as a result, have found that the bainite structure is not generated instantaneously but starts with a certain incubation period. . By utilizing such a phenomenon and performing a temperature raising process after cooling, even when a higher cooling rate (about 15 ° C./s) than usual is applied, the formation of bainite structure is suppressed and Also succeeded in obtaining a fine bainite structure. Based on these findings, the inventors of the present invention further studied, and found that a thick steel sheet having the above-described structure can be obtained with good productivity by operating under the above-mentioned manufacturing conditions, and completed the present invention. . The manufacturing conditions specified in the present invention will be described below.

In the method of the present invention, the two-stage rolling is performed as described above, and the cooling conditions after each rolling are appropriately adjusted, but the first cooling (hereinafter referred to as "primary cooling").
As the condition of, the cooling stop temperature of the surface layer region is set to 650 to 4
It is necessary to set the temperature range to 50 ° C. When the cooling stop temperature is higher than 650 ° C., ferrite nucleation mainly occurs from the austenite grain boundaries, so that the resulting ferrite grains become coarse. When the cooling stop temperature is 500 ° C. or lower, the bainite structure starts to be formed even when the hardenability is reduced by rolling in the austenite non-recrystallization temperature range.
The ductile fracture characteristic begins to deteriorate, but when the fraction is less than 5% and finely dispersed, it effectively acts on the arrest characteristic. However, when the cooling stop temperature is lower than 450 ° C., the bainite structure is mainly formed, and the ductile fracture characteristics start to largely decrease.
From this viewpoint, the cooling stop temperature needs to be 450 ° C. or higher.

When the stop temperature of the primary cooling is set to a temperature range of 650 to 450 ° C., the strain introduced into the austenite grains by the austenite unrecrystallized temperature range rolling from the Ar ₃ transformation point to 900 ° C. The ferrite nucleation is activated, and by regenerating the heat by the subsequent processing heat generation or internal sensible heat, the generation of grain boundary ferrite and intragranular ferrite is activated, and finer ferrite is obtained compared with the conventional cooling only process. Is obtained. Also,
As for the surface layer region to be cooled, if the surface area is less than 0.05 t from both surfaces (t is the thickness during rolling), it is not possible to expect a significant improvement in arrest characteristics, so it is necessary to make the surface area 0.05 t or more from both surfaces. . However, when an excessively large area is cooled, it is difficult to recover the heat due to sensible heat inside the steel sheet, and the workability of the ferrite structure inside the steel sheet increases, making it impossible to improve the target ductile fracture characteristics. Therefore, the cooling range needs to be 0.15 t or less.

The cooling rate in the above primary cooling needs to be 2 ° C./s or more from the viewpoint of suppressing the γ → α transformation in which the grain boundary ferrite is the main. Since a bainite structure that is weaker than the structure is likely to be formed, it is necessary to set the temperature to 15 ° C./s or less. In addition, during the primary cooling, the temperature of the inner region on the core side from the plate thickness t / 4 position from both surfaces is Ar from the viewpoint of effectively achieving the subsequent heat recovery.
_It is necessary to maintain at least ₃ transformation points.

In the rolling before the primary cooling, in the unrecrystallized temperature range from the Ar ₃ transformation point to 900 ° C., ε is formed in the surface layer region.
It is necessary to give an equivalent plastic strain ε of ≧ 0.5,
This equivalent plastic strain ε is the residual cumulative equivalent plastic strain amount εr before primary cooling, but if the amount is small, the ferrite grains will not be sufficiently refined and the desired fine ferrite structure will be obtained. Indicates that the residual cumulative equivalent plastic strain amount εr is 0.5.
It is necessary to control the above. From this point of view, the time from the end of rolling to the start of primary cooling must be within a time period in which the residual cumulative equivalent plastic strain amount εr in the surface layer region satisfies εr ≧ 0.5.

The residual cumulative equivalent plastic strain amount εr, unlike the conventional cumulative rolling reduction, indicates the strain amount substantially remaining inside the material before cooling. That is, since the strain introduced by rolling decreases with the passage of time, even when rolling with the same cumulative rolling reduction is applied, if the time from the completion of rolling to the start of cooling differs, The amount of residual residual strain varies. As a result, a predetermined strain amount may not be secured, and the desired fine ferrite may not be obtained, so it is necessary to secure the residual cumulative equivalent strain amount εr as described above.

When the fine ferrite grains obtained by the primary cooling and the reheating as described above are rolled at a temperature not higher than the Ar ₃ transformation point, the ferrite structure mainly recovers rather than recrystallizes. It was found that the refinement did not proceed, and the finely processed ferrite was elongated in the rolling direction. As a result, a micro-machined ferrite structure can be stably realized, and further, due to the cumulative equivalent plastic strain introduced during rolling before primary cooling and during rolling after primary cooling-recuperation, steel sheets other than the surface layer region concerned. The ferrite particles inside were made finer, and at the same time, the improvement of ductile fracture characteristics was also realized. At this time, as the cumulative equivalent strain introduced into the steel sheet increases, the ferrite crystal grains become finer, but at the same time, the workability (aspect ratio) of the ferrite grains in the steel sheet surface layer portion increases, and the ductile fracture characteristics are reduced. Since it causes deterioration, the residual cumulative equivalent plastic strain amount εr introduced into the steel plate by the subsequent rolling is 0.35.
It is necessary to satisfy ≦ εr <0.55. Further, in the subsequent rolling, from the viewpoint of weaving a ferrite set having a small aspect ratio (specifically, 2 or less), it is necessary to complete the rolling at the Ar ₃ transformation point or higher. That is, the rolling completion temperature is Ar
_{When the} temperature is less than ₃ transformation points, ferrite is generated, and this is expanded by rolling, resulting in a ferrite structure with a large aspect ratio.

As described above, the gist of the present invention is to utilize the transformation incubation period in bainite transformation to realize a higher cooling rate and a higher degree of supercooling due to a lower cooling stop temperature than in the prior art. It was possible to easily obtain a fine ferrite structure that cannot be obtained, and by making not only the surface layer structure but also its internal structure finer, it was possible to exhibit excellent arrest characteristics that could not be exhibited by conventional steel sheets. .

The thick steel sheet obtained as described above has a structure mainly composed of pearlite and ferrite, but a bainite structure is mixed in a part thereof (for example, less than 5%). May be.

In the method of the present invention, the chemical composition of the thick steel plate to be used is not particularly limited, but the appropriate ranges and the reasons for the basic compositions such as C, Si, Mn, Al and N are as follows. On the street.

C: 0.03 to 0.2% From the viewpoint of required strength for use, the lower limit of the content of C is specified to be 0.03%. The C content is preferably 0.
It is preferable to set it to 05% or more, more preferably 0.08% or more, but if it is contained excessively, weldability and base material toughness are deteriorated, so 0.2% is made the upper limit.

Si: 0.5% or less (not including 0%) Si is an element that increases the strength of the base material and is effective as a deoxidizer for molten steel. The upper limit is 0.5% because it deteriorates the toughness of the base material. In order to exert the above effect, Si is 0.1
% Or more is preferably contained.

Mn: 1.8% or less (not including 0%) Mn is an element effective in increasing the strength of the base metal, but if contained in excess, the weldability and the base material toughness deteriorate. Therefore, the upper limit is 1.8%. In addition, in order to exert the above effect, Mn is preferably contained in an amount of 0.7% or more.

Al: 0.05 to 1.5% Al is an element that acts as a deoxidizing agent for molten steel and forms a nitride, which is effective for refining the base metal structure, and exerts such an effect. Therefore, it is necessary to contain 0.05% or more. However, if it is contained excessively, the toughness of the base material is deteriorated, so 1.5% is made the upper limit. Incidentally, A
The preferable lower limit of l is 0.10%, and the preferable upper limit is 1.0%.

N: 0.001 to 0.01% N forms an nitride with an additive element of Al (or Ti, Nb, V, etc.), and exerts an effect of refining the matrix structure. If the content is less than 0.001%, such effects are not exhibited. However, if the N content exceeds 0.01% and becomes excessive, it causes an increase in solute N and particularly deteriorates the toughness of the welded portion, so it is necessary to set it to 0.01% or less.

The basic chemical composition of the steel material used in the present invention is as described above, and the balance consists essentially of Fe. If necessary, (1) Ti: 0.02
% Or less (not including 0%), Nb: 0.03% or less (0
%) And V: at least one selected from the group consisting of 0.05% or less (not including 0%), (2) Cr:
0.5% or less (not including 0%) and / or Mo:
In addition to 0.5% or less (not including 0%), (3) Ca:
0.5% or less (not including 0%) and / or Zr:
0.5% or less (not including 0%), (4) B: 0.00
2% or less (not including 0%), (5) Cu: 0.5% or less (not including 0%), (6) Ni: 0.5% or less (0
%) Is also effective, and the properties of the steel material are further improved depending on the type of element to be included. The reason for limiting the range when these elements are contained is as follows.

Ti: 0.02% or less (not including 0%
B), Nb: 0.03% or less (not including 0%) and
V: from a group consisting of 0.05% or less (not including 0%)
At least one selected from Ti has an effect of suppressing coarsening of austenite grains at the time of heating a billet and an effect of promoting ferrite transformation nucleation after completion of rolling through the formation of nitride, but more than 0.02%. Then, the toughness of the base material is deteriorated. still,
In order to exert the above effects, the content of Ti is 0.0
It is preferably at least 04%.

Nb has an effect of suppressing coarsening and recrystallization of austenite grains during rolling through the formation of carbides, and is an element effective for refining ferrite grains after rolling is completed, but it is more than 0.03%. If contained in excess, the weldability will be reduced. In order to exert the above effects, the Nb content is preferably 0.002% or more.

V has an effect of suppressing coarsening and recrystallization of austenite grains during rolling through the formation of carbides like Nb, so V is an element effective for refining ferrite grains after the rolling is completed. If it is contained in excess of 05%, the weldability will be reduced. In order to exert the above effects, the V content is preferably 0.002% or more.

Cr: 0.5% or less (not including 0%)
And / or Mo: 0.5% or less (not including 0%) Cr and Mo both are elements that precipitate carbonitrides and contribute to the strength increase. Since the material toughness is deteriorated, the upper limit is 0.5% in both cases. In order to exert the above effects, it is preferable that the content of each of them be 0.1% or more.

Ca: 0.005-0.5% and / or
Zr: 0.003 to 0.5% Ca and Zr are both effective elements for improving the toughness of the base material by spheroidizing the morphology of inclusions in the steel. If the content of the element becomes excessive, the toughness of the base material is rather deteriorated. Therefore, the content of each element should be 0.5% or less. In order to bring out the above effects, it is preferable to contain 0.005% or more of Ca and 0.003% or more of Zr.

B: 0.002% or less (not including 0%) B is an element effective for improving the toughness of the weld heat affected zone (HAZ), but if contained in excess, hardenability increases. As a result, the low temperature toughness of the base material deteriorates.
It is better to be less than or equal to%. In order to exert the above effects, the content of B is preferably 0.0002% or more.

Cu: 0.5% or less (not including 0%) Cu is an element effective for refining crystal grains, but if contained in a large amount, it deteriorates the weldability of the base metal, so 0.5 % Or less is preferable. The Cu content is preferably 0.1% or more in order to exert the above effects.

Ni: 0.5% or less (not including 0%) Ni is an element effective in improving the low temperature toughness, but since it is expensive, its upper limit is preferably 0.5%. The Ni content is preferably 0.1% or more in order to exert the above effects.

The thick steel plate used in the present invention may contain, in addition to the above-mentioned various components, trace amounts of components that do not impair the properties of spring steel, and such steel wire rods are also included in the scope of the present invention. is there. The trace components include impurities, especially inevitable impurities such as P, S, As, Sb and Sn.

Hereinafter, the present invention will be described in more detail with reference to Examples. The following Examples are not intended to limit the scope of the present invention, and any modification of the design of the present invention can be made without departing from the spirit of the preceding and the following. Are included in the technical scope of.

[0047]

Examples Various steel materials having the chemical composition shown in Table 1 below (steel types A to I) were used and treated under the conditions shown in Tables 2 and 3 below to obtain various thick steel plates. In Tables 2 and 3, "region A" means 0.05t or more and 0.1 from the surface layer in the plate thickness direction.
It means a surface layer region of 5t or less, and "region B" means an inner region on both sides from the plate thickness t / 4 position on the core side.

[0048]

[Table 1]

[0049]

[Table 2]

[0050]

[Table 3]

With respect to the obtained thick steel sheet, the ferrite grain size and the aspect ratio in each of the regions A and B were measured, and the arrest characteristic (brittle crack propagation stopping characteristic) at -80 ° C. was determined by WES. In addition to the measurement by the heavy tensile test, the elongation at room temperature tensile was measured by the tensile test described in JIS1B.

The results are shown in Tables 4 and 5 below for the finished plate thickness, and it can be seen that the thick steel plate obtained by the method of the present invention is excellent in both arrest characteristics and ductile fracture characteristics. On the other hand, it can be seen that the thick steel sheet obtained by the manufacturing method that does not satisfy the requirements specified in the present invention is deteriorated at least in the brittle crack arrest property.

[0053]

[Table 4]

[0054]

[Table 5]

Based on the above results, the relationship between the cumulative equivalent plastic strain amount εr in the internal region and the average ferrite grain size when steel type A (Table 1) is used is shown in FIG. 1, but the cumulative equivalent plastic strain amount increases. As the ferrite grain size decreases, it is understood that the cumulative equivalent plastic strain amount should be controlled to 0.35 to 0.55 in order to control the ferrite grain size to 4.0 to 10.0 μm. .

FIG. 2 shows the relationship between the primary cooling stop temperature and the average ferrite grain size in the surface layer region when steel type A is used based on the above results. In addition, in FIG.
The mark indicates a cooling rate of 1.0 ° C / s, the ○ mark and the ● mark indicate a cooling rate of 3.0 ° C / s, and the □ and ■ marks indicate a cooling rate of 1
At 2.0 ° C / s, △ and ▲ marks have a cooling rate of 2
It shows the case of 0.0 ℃ / s, respectively, of these, white (▽,
◯, □, and Δ indicate that the bainite structure is less than 5%, and the filled (●, ■, and ▲) indicate that the bainite structure is 10% or more. As is clear from this result, it is understood that the ferrite grain size can be controlled in an appropriate range by controlling the primary cooling stop temperature in an appropriate temperature range under a predetermined cooling rate.

FIG. 3 shows the relationship between the structure of the surface layer of the steel sheet (worked ferrite layer thickness) and the arrest characteristic at -80 ° C. in the double tensile test based on the above results. is there. In FIG. 3, when the mark ■ is the aspect ratio: 4.1 to 4.7 and the particle size is 3.2 to 4.3 μm, the mark ○ is the aspect ratio: 3.1 to 3.8, the particle size: 3.
When the thickness is 3 to 4.3 μm, the Δ mark indicates the aspect ratio: 2.1 to
2.9, particle size: 3.3 to 4.4 μm, □ indicates aspect ratio: 1.3 to 1.9, particle size: 3.5 to 4.7 μm
Each time is shown. As is clear from this result,
It was found that even if the ferrite crystal grain size was the same, the arrest property at −80 ° C. improved as the aspect ratio increased, and if the layer thickness was 30 mm,
600 when 1.5 mm or more (0.05 t or more)
It can be seen that kgf / mm ^3/2 or more can be stably realized.

Based on the above results, FIG. 4 shows the relationship between the average ferrite grain size in the surface layer of the steel sheet and the arrest characteristics at -80 ° C. in the double tensile test when Steel Type A was used. The results shown in FIG. 4 show that the surface layer processed grain layer thickness is 2
.About.2.5 mm and aspect ratio of 2.5 to 3.2. As is clear from this result, the ferrite grain size is 5
It can be seen that excellent arrest characteristics (reference value: 600 kgf / mm ^3/2 ) are exhibited by setting the thickness to μm or less.

FIG. 5 shows the relationship between the structure (aspect ratio) of the steel sheet surface layer portion when steel type A is used and the elongation characteristics during the normal temperature tensile test based on the above results. In addition, this result shows that the average grain size of ferrite in the surface layer of the steel sheet is 4.1 to 4.8 μm, the average grain size of ferrite in the central part is 6.0 to 7.2 μm, and the aspect ratio is 1.3 to
It is the case of 1.8. As is clear from these results, the elongation characteristics deteriorate as the aspect of the ferrite grains in the surface layer of the steel sheet increases, but good elongation characteristics (target value: 27
% Or more).

FIG. 6 shows the relationship between the average ferrite grain size in the steel sheet internal region and the elongation property in the normal temperature tensile test when Steel Type A was used based on the above results. Incidentally, FIG.
In, the ◯ marks have an aspect ratio of 2.0 or less, the Δ marks have an aspect ratio of 2.1 to 3.0, and the □ marks have an aspect ratio:
Times of 3.1 to 4.0 are shown respectively. In addition, this result shows that the average grain size of ferrite in the surface layer of the steel sheet is 4.
3 to 4.7 μm and an aspect ratio of 2.5 to 2.7. As is clear from this result, when the aspect ratio of ferrite grains in the steel sheet inner region is 2.0 or less and the ferrite grain size is 4 to 10 μm, good elongation characteristics (target value: 27% or more) are exhibited. I understand.

[0061]

The present invention is constructed as described above, and has realized a method capable of manufacturing a thick steel plate excellent not only in arrest characteristics but also in ductile fracture characteristics at low cost and with high productivity.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a cumulative equivalent plastic strain amount εr in an inner region of a steel sheet and an average ferrite grain size when a steel type A (Table 1) is used.

FIG. 2 is a graph showing the relationship between the primary cooling stop temperature and the average ferrite grain size in the surface layer region when steel type A is used.

FIG. 3 is a graph showing the relationship between the structure of the steel sheet surface layer region when steel type A is used and the arrest property by a double tensile test.

FIG. 4 is a graph showing the relationship between the average ferrite grain size in the surface layer region of the steel sheet when steel type A is used and the arrest property at −80 ° C. by the double tensile test.

FIG. 5 is a graph showing the relationship between the structure (aspect ratio) of the surface layer of the steel sheet when Steel Type A is used and the elongation characteristics during a room temperature tensile test.

FIG. 6 is a graph showing the relationship between the average ferrite grain size in the steel plate internal region and the elongation characteristics during a normal temperature tensile test when steel type A is used.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/38 C22C 38/38 F term (reference) 4E002 AA07 AD07 BC05 BC07 BD07 CB01 CB08 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA35 AA36 AA39 BA01 CA01 CA02 CB02 CC03 CC04 CD02

Claims (4)

[Claims] 1. A non-recrystallization temperature of not less than Ar ₃ transformation point and not more than 900 ° C. for a surface layer region of not less than 0.05 t and not more than 0.15 t from both surfaces in the plate thickness direction, where t is a thickness during rolling. In the region, an equivalent plastic strain ε of ε ≧ 0.5 is applied, and thereafter, within a time period in which the residual cumulative equivalent plastic strain amount εr of the surface layer region satisfies εr ≧ 0.5, the plate thickness t /
While maintaining than 4 positions the temperature of the inner area of the core portion side than the Ar ₃ transformation point, the surface layer region is cooled to a temperature range of 450 to 650 ° C. at a cooling rate of 2 to 15 ° C. / s,
Then, the rolling is restarted, and in this rolling, the residual cumulative equivalent plastic strain ε of 0.35 ≦ εr <0.55 with respect to the internal region is satisfied.
Rolling is performed to impart r, and the rolling is completed at the Ar ₃ transformation point or higher, and the surface layer region is double-heated to the Ar ₃ transformation point or lower by the process heat generation and the internal sensible heat. A method for producing a thick steel sheet excellent in arrest characteristics and ductile fracture characteristics, characterized by cooling at a temperature of ° C / s. 2. C: 0.03 to 0.2% (meaning mass%; the same applies hereinafter), Si: 0.5% or less (not including 0%), Mn: 1.8% or less (0% Is not included), Al:
0.01-0.1% and N: 0.001-0.01%
The manufacturing method according to claim 1, wherein thick steel plates containing the respective are used. 3. Further, Ti: 0.02% or less (not including 0%), Nb: 0.03% or less (not including 0%) and V: 0.05% or less (not including 0%). The manufacturing method according to claim 2, wherein a thick steel plate containing at least one selected from the group consisting of: 4. A thick steel plate containing Cr: 0.5% or less (not including 0%) and / or Mo: 0.5% or less (not including 0%) is used. The manufacturing method described.