KR101491228B1 - High-strength thick steel plate with excellent drop weight characteristics - Google Patents

Abstract

In the microstructure of the high strength steel sheet according to the present invention in a microstructure at a position satisfying a predetermined chemical composition and having a depth of t / 4 to t / 2 (t represents a plate thickness, hereinafter the same) from the surface, The area fraction of the nitrite is 90% or more, the average value of the lath width of the bainite is 3.5 탆 or less, the maximum value of the circle equivalent diameter of the island-shaped martensite in the bainite is 3.0 탆 or less, And is useful as a material for pressure vessels of nuclear power plants as well as structural materials such as offshore structures, ships and bridges.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high strength steel plate,

TECHNICAL FIELD [0001] The present invention relates to a high strength steel sheet used as a material for a pressure vessel of a nuclear power plant in addition to structural materials such as an offshore structure, ship, bridge, etc., .

(Hereinafter sometimes referred to as " QT steel plate ") used for quenching and tempering has high strength and toughness and has good weldability, it has been conventionally used for a bridge, high-rise structure, ship, tank Has been widely used as a welded structure. Such a QT steel sheet tends to require a higher strength (for example, a yield strength of 415 MPa or more and a tensile strength of 620 MPa or more) along with the recent design of a welded structure to be larger.

It is necessary for the steel sheet to have not only high strength but also excellent throwing power, which is an index of brittle fracture characteristics. However, these characteristics are difficult to meet due to high strength and thickening.

For example, a technique similar to Patent Document 1 has been proposed as a technique for improving the dropping characteristics. In this technique, the grain boundary is strengthened by reducing the P content as much as possible, and a fine grain effect is obtained by adding a predetermined amount of N, and the effect of improving the toughness by adding Cr is intended. However, the steel sheet obtained by this technique has a non-smell transition temperature (NDT), which is an index of the drop weight characteristics, is only about -50 DEG C, and thus it can not meet the recent required characteristics.

Patent Document 2 proposes a technique for producing fine ferrite by performing low temperature rolling to achieve good drop weight characteristics. However, in this technique, it is difficult to increase the strength, so that it is not possible to secure good dropping characteristics with high strength.

Further, in Patent Document 3, there is proposed a technique of achieving favorable dropping characteristics by producing fine ferrite while suppressing the production of bainite by roller-quenching quenching. However, even in this technique, it is difficult to increase the strength, so that it is difficult to secure a good dropout characteristic with high strength.

Japanese Patent Application Laid-Open No. Hei 02-93045 Japanese Patent Application Laid-Open No. 55-79828 Japanese Patent Application Laid-Open No. 60-155620

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of producing a steel material for a nuclear power plant, And to provide a useful high strength steel sheet.

The steel sheet according to the present invention, which has been able to solve the above problems, has a composition of C: 0.03 to 0.150% (meaning "mass%", , S: not more than 0.01% (not including 0%), Al: 0.005 to 0.06%, Cr: 0.10% or less, (Not including 0%), N: 0.0020 to 0.010%, and O: 0.010% or less (not including 0%), The area fraction of bainite in the microstructure at the position of depth t / 4 to t / 2 (t represents the plate thickness, hereinafter the same) from the surface made of iron and inevitable impurities is 90 % Or more, and the average value of the lath width of bainite is 3.5 占 퐉 or less and the average value of the circle equivalent diameter of the island-shaped martensite in the bainite Daegap to have a base to 3.0㎛ or less points.

In the post-steel sheet of the present invention, it is preferable that the average circle-equivalent diameter of the island-shaped martensite is 1.0 탆 or less, whereby the drop weight property becomes better. In the present invention, the " circle equivalent diameter " refers to the size of the island-shaped martensite (hereinafter may be abbreviated as " MA ") and refers to the diameter do.

(A) Cu: not more than 2% (not including 0%) and / or Ni: not more than 2% (not including 0%), (b) Nb: 0.05 (Inclusive of 0%) (inclusive of 0%) and / or B of 0.005% or less (inclusive of 0% (D) Zr: not more than 0.1% (not including 0%) and / or Hf: not more than 0.05% (not including 0%), (e) Ca: 0.0035% (F) Co: not more than 2.5% (not including 0%) and / or W: not more than 2.5% (not including 0%), (g) rare earth element: 0.01 % Or less (it does not include 0%), and the like. Further, by containing such an element, the properties of the steel sheet are further improved according to the kind thereof.

In the case of containing Ti, the content of Ti is set to 0.005 to 0.030%, the Ti-based dispersed particles present in the steel sheet have an average circle equivalent diameter of 40 nm or less, or the Ti- Is preferably 10 nm or more. By satisfying these requirements, the toughness of the weld heat affected zone (HAZ) can be further enhanced in addition to the favorable drop weight characteristics. Also, the Ti-based dispersed particles means dispersed particles such as carbides, nitrides, oxides containing Ti, or carbonitride composites thereof.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to realize a post-steel sheet capable of exhibiting favorable drop-weight characteristics with high strength by strictly regulating chemical composition and precisely defining micro-structure. Such post- In addition to structural materials such as ships and bridges, it is extremely useful as a material for pressure vessels in nuclear power plants.

1 is a plan view showing the shape of a test piece used in the drop test.

The inventors of the present invention have studied various means for realizing a steel sheet capable of securing high strength and favorable drop weight characteristics. As a result, by selecting bainite as the main body (the area fraction of bainite is 90% or more) as the microstructure of the steel sheet, it is possible to secure high strength, and at the same time, the lass width of bainite Width) of 3.5 mu m or less and the size of MA in the bainite (maximum value of the circle-equivalent diameter) is 3.0 mu m or less, the present invention has been completed.

Further, in the post-steel sheet of the present invention, the evaluation position of the microstructure is set to a position of t / 4 to t / 2 (t: sheet thickness) from the surface, I selected it as a location.

In the post-steel sheet of the present invention, it is important to define the width of the bainite. The particle width affects the drop weight characteristics, and when the average value is 3.5 mu m or less, the drop weight characteristics can be realized. This is considered to be because the number of lasers that suppress the progress of the destruction is increased. Further, the lath width of bainite is preferably 3 m or less, and more preferably 2 m or less.

In the bainite, the island-like martensite (MA) is present as a plate or granular form between the lattices of the bainite, but the maximum value of the circle-equivalent diameter affects the drop weight characteristics and the maximum value ) Is 3.0 m or less, it is extremely effective for improving the drop weight characteristics. This is thought to be due to the fact that it is difficult to become a starting point of destruction. In addition, it is preferable that the average value (average circle equivalent diameter) of the size of MA is 1.0 占 퐉 or less. By satisfying these requirements, it is possible to further improve the dropout characteristics because the energy for breakage is improved.

In the post-steel sheet of the present invention, the microstructure is made of bainite as the main body (the area fraction of bainite is 90% or more, preferably 95% or more), and when the whole is bainite (total area ratio of bainite (For example, one or more of ferrite, weedman stent, ferrite, pearlite, martensite, cementite, etc.) may be added to a part (that is, an area fraction of 10% or less) Species) may be included.

The knowledge that the size (average circle equivalent diameter) of MA is correlated with the A value defined by the following equation (1) based on the contents of C, Si and Al is experimentally determined for the addition amount of the alloy element and the MA size , And the size of MA (average circle equivalent diameter) can be controlled to 1.0 탆 or less by setting the value of A to a value smaller than 1.0 (%). In the following formula (1), Si included as occasion demands is included. However, when Si is not contained, the value of A is calculated without the Si, and when Si is included, Value.

Note that [C], [Si] and [Al] represent the contents (mass%) of C, Si and Al, respectively.

Next, the basic composition of the steel sheet of the present invention will be described. It is also necessary that the base steel (C, Si, Mn, P, S, Al, Cr, Mo, V, N and O) serving as the steel sheet in the steel sheet of the present invention is within the appropriate range as shown below. The reason for limiting the range of these components is as follows.

[C: 0.03 to 0.150%]

C is an element necessary for securing the strength of the steel sheet, and it is necessary to contain 0.03% or more in order to secure the desired strength. However, if C is contained excessively, the dropout characteristic is rather lowered. In this respect, the upper limit should be 0.150%. The lower limit of the C content is preferably 0.05%, and the upper limit is preferably 0.13%.

[Si: 0.5% or less (including 0%)]

Si is an effective element for securing the strength of the steel sheet, and is contained if necessary. However, if it is contained in excess, it causes coarsening of the island-shaped martensite (MA) in the steel material (base material) and degrades the dropping characteristics. In this respect, the upper limit was set at 0.5%. Further, the lower limit of the Si content is preferably 0.05%, and the upper limit is preferably 0.25%.

[Mn: 1.0 to 2.0%]

Mn is an element effective in improving the quenching and securing the strength of the steel sheet. In order to exhibit such an effect, it is necessary to contain 1.0% or more of Mn. However, if Mn is added excessively, the dropout characteristics of the steel sheet deteriorate, so the upper limit is set to 2.0%. The lower limit of the Mn content is 1.2%, and the upper limit is 1.6%.

[P: not more than 0.015% (not including 0%)]

P is an impurity that is inevitably mixed and adversely affects the weight dropping characteristics of the steel sheet, so that it is preferable that P is as small as possible. From this viewpoint, it is preferable to suppress P to 0.015% or less. The preferable upper limit of the P content is 0.010%.

[S: not more than 0.01% (not including 0%)]

S is an impurity which combines with an alloy element in the steel sheet to form various inclusions and acts harmfully to the weight drop characteristics of the steel sheet. Therefore, it is preferable that S be as small as possible, and in consideration of the degree of cleanliness of practical steel, Is 0.005% or less). Further, S is an impurity inevitably contained in the steel, and it is difficult to make the amount of S to 0% on industrial production.

[Al: 0.005 to 0.06%]

Al is an effective element as a deoxidizing agent and also exhibits an effect of improving the steel sheet strength by microstructure of the steel sheet. In order to exhibit such an effect, the Al content needs to be 0.005% or more. However, if it is contained in excess, it causes coarsening of the island-shaped martensite (MA), which degrades the dropping characteristics. In this respect, the upper limit was set at 0.06%. The lower limit of the Al content is preferably 0.01%, and the upper limit is preferably 0.04%.

[Cr: 0.10 to 0.5%]

Cr is an element effective for increasing the hardenability of the steel sheet and improving the strength. In order to exhibit such an effect, the Cr content needs to be 0.10% or more. However, when the Cr content is excessive, the dropout characteristics are deteriorated. In view of this, the Cr content needs to be 0.5% or less. The lower limit of the Cr content is preferably 0.2%, and the upper limit is preferably 0.4%.

[Mo: 0.05 to 0.5%]

Mo is an element effective for forming a fine carbide and improving the strength of a steel sheet. In order to exhibit such an effect, the Mo content needs to be 0.05% or more. However, when the content is excessive, carbide coarsening is promoted, and the drop weight characteristics are rather lowered. In view of this, the Mo content needs to be 0.5% or less. Further, the lower limit of the Mo content is preferably 0.15%, and the upper limit is preferably 0.3%.

[V: not more than 0.10% (not including 0%)]

V has an effect of improving the hardenability and improving the strength of the steel sheet. V also has the effect of increasing the temper softening resistance. However, if it is contained in a large amount, the dropout characteristic deteriorates, so that the content is preferably 0.10% or less (more preferably 0.05% or less). In addition, the V content for effectively exhibiting the effect is 0.02% or more.

[N: 0.0020 to 0.010%]

N has an effect of bonding with Al or the like to form a nitride to make the steel sheet structure finer and improve the drop weight characteristics. In order to exhibit such effects, it is necessary to contain N in an amount of 0.0020% or more. However, when the N content is excessive, the dropout characteristics deteriorate rather than being 0.010%. The lower limit of the N content is preferably 0.004%, and the upper limit is preferably 0.008%.

[O: 0.010% or less (not including 0%)]

O is contained as an inevitable impurity, but exists as an oxide in the steel. However, when the content exceeds 0.010%, a coarse oxide is produced and the dropout characteristics are deteriorated. In this respect, the upper limit of the O content is set at 0.010%. The preferred upper limit of the O content is 0.003%.

The contained elements specified in the present invention are as described above, and the remaining parts are iron and inevitable impurities. As the inevitable impurities, incorporation of the elements to be brought in may be permitted depending on the conditions of the raw materials, materials, manufacturing facilities, and the like. (A) Cu: not more than 2% (not including 0%) and / or Ni: not more than 2% (not including 0%), (b) Nb: And Ti: not more than 0.05% (not including 0%) and / or B: not more than 0.005% (excluding 0%), (c) (D) Zr: not more than 0.1% (not including 0%) and / or Hf: not more than 0.05% (not including 0%), (e) Ca: 0.0035 Or less of W (not including 0%), (g) rare earth element: not more than 2.5% (not including 0%), 0.01% or less (it does not include 0%), and the like, and the inclusion of such an element further improves the properties of the steel sheet depending on the type thereof.

[Cu: not more than 2% (not including 0%) and / or Ni: not more than 2% (not including 0%)]

Cu and Ni are effective elements for increasing the hardeness and improving the strength, and are contained as needed. However, if the content of these elements is excessive, the dropout characteristics are rather lowered, and therefore, it is preferable that the content is 2% or less (more preferably 1% or less). The lower limit for achieving the above effect is preferably not less than 0.2% (more preferably not less than 0.3%).

[Nb: not more than 0.05% (not including 0%) and / or B: not more than 0.005% (not including 0%)]

Nb and B exhibit the effect of improving the hardenability and improving the strength of the steel sheet. However, if it is contained in a large amount, generation of carbide or nitride is increased, and the dropout characteristics are deteriorated. More preferably, Nb is 0.04% or less and B is 0.002% or less. In order to effectively exhibit these effects, the content of Nb is 0.01% or more and B is 0.0005% or more.

[Mg: not more than 0.005% (not including 0%) and / or Ti: not more than 0.030% (not including 0%)]

Mg and Ti form oxides or nitrides to suppress the coarsening of the austenite grains and have an effect of improving the characteristics of the HAZ, so that Mg and Ti are contained as needed. However, if the content is excessive, the inclusions are coarsened and the dropout characteristics are deteriorated. Therefore, Mg is 0.005% or less (more preferably 0.003% or less) and Ti is 0.030% or less (more preferably 0.02% or less) It is good to do.

In the case of containing Ti, the content of Ti is controlled to 0.005 to 0.030%, and the average size (average circle equivalent diameter) of Ti dispersed particles present in the steel sheet is controlled to be 40 nm or less, In addition, it is possible to further improve the toughness of the HAZ, which is preferable. The average size of the Ti-based dispersed particles is more preferably 30 nm or less, and as the average size is smaller, better characteristics are obtained.

On the other hand, controlling the minimum size (the minimum value of the circle-equivalent diameter) of the Ti-based dispersed particles to 10 nm or more is more preferable because the effect of improving the HAZ toughness is remarkable. The minimum size of the Ti-based dispersed particles is more preferably 15 nm or more.

[Zr: not more than 0.1% (not including 0%) and / or Hf: not more than 0.05% (not including 0%)]

Zr and Hf are effective elements for improving HAZ characteristics by forming nitrides with N and making the austenite grains finer. However, if it is contained in excess, the dropout characteristic is deteriorated rather. Therefore, when these elements are contained, Zr is 0.1% or less (more preferably 0.003% or less) and Hf is 0.05% or less (more preferably 0.01% or less).

[Ca: not more than 0.0035% (not including 0%)]

Ca is an element that contributes to the improvement of HAZ characteristics by controlling the form of sulfide. However, even if the content is over 0.0035%, the dropout characteristics are rather deteriorated. A more preferable upper limit of the Ca content is 0.0020% or less.

[Co: not more than 2.5% (not including 0%) and / or W: not more than 2.5% (not including 0%)]

Co and W have an effect of improving the hardenability and the strength of the steel sheet, so that Co and W are included as needed. However, if it is contained in excess, the HAZ toughness deteriorates, so the upper limit is set to 2.5%. A more preferable upper limit of the content thereof is 0.5% or less.

[Rare earth element (REM): not more than 0.01% (not including 0%)]

The rare earth element (REM) is an element which contributes to improvement of the toughness of the base material and the HAZ by refinement or spheroidizing the shape of the inclusions (oxides, sulfides, etc.) inevitably incorporated into the steel. Such an effect increases as the content thereof increases. However, if the content of REM becomes excessive, the inclusions become coarse and the dropout characteristics deteriorate. Therefore, it is desirable to suppress the effect to 0.01% or less. In the present invention, REM means that the lanthanoid element (15 elements from La to Lu) and Sc (scandium) and Y (yttrium) are included.

In the production of the post-steel sheet of the present invention, a steel material satisfying the above-mentioned chemical composition is sprayed by a conventional casting method, the molten steel is cooled to form a slab, and heated to 900 to 1300 ° C And then subjected to rough rolling so that the reduction ratio in the temperature range from 950 to 850 캜 is 10% or more. Thereafter, the reduction rate in the final rolling pass in the temperature range of 800 to 850 캜 3 to 10%, and thereafter directly cooled to 400 deg. C at an average cooling rate of 0.1 to 30 deg. C / sec and reheated to a temperature range of 900 to 1000 deg. C to perform quenching, Lt; 0 > C and at least two times of tempering may be performed. The reason for setting the range of each condition in this method is as follows. The temperature shown above is controlled by the temperature of the surface of the steel sheet.

[Heating temperature of slab: 900 to 1300 DEG C]

From the viewpoint of austenitizing the structure of the steel sheet all at once, it is necessary to set the temperature to 900 DEG C or more. However, if the heating temperature exceeds 1300 DEG C, the austenite grains become coarse and it becomes difficult to obtain a desired structure in the subsequent steps.

[Rough rolling is carried out so that the reduction rate in the temperature range from 950 to 850 캜 is 10% or more]

The reduction rate in this temperature range affects the lass width of the bainite. By setting the reduction rate to 10% or more, the average value of the lass width of the bainite can be made 3.5 mu m or less have. When the reduction rate is less than 10%, the average value of the lath width of bainite can not be made 3.5 탆 or less.

[Finish rolling is performed so that the reduction ratio in the final rolling pass is 3 to 10% in the temperature range of 800 to 850 占 폚]

The reduction rate in this temperature range affects the lass width and MA size of the bainite. When the temperature exceeds 850 DEG C or the reduction rate is less than 3%, the lass width and MA size (maximum value) Exceeds the specified value. The rolling at this time in which the reduction rate exceeds 10% is not usually performed in the finish rolling.

[Direct cooling to 400 DEG C at an average cooling rate of 0.1 to 30 DEG C / sec]

After finishing rolling, it is necessary to directly cool the steel up to 400 占 폚 at an average cooling rate of 0.1 to 30 占 폚 / sec. When the average cooling rate during cooling is less than 0.1 占 폚 / sec or exceeds 30 占 폚 / sec, it can not be a bainite-based material. The reason why the cooling at this time is set to 400 DEG C is that no further tissue transformation occurs. The direct cooling is due to the fact that the structure before quenching is made finer and the quenching is finer.

[Reheating temperature of quenching: 900 to 1000 < 0 > C]

From the viewpoint of austenitization, the reheating temperature needs to be 900 DEG C or higher, but when the reheating temperature exceeds 1000 DEG C, it becomes coarse austenite. Further, in order to obtain a desired texture (a structure mainly composed of bainite) by exerting the effect of quenching, it is necessary to perform reheating in the above-mentioned temperature range, followed by quenching by cooling at an average cooling rate of 0.5 to 20 DEG C / have. That is, when the average cooling rate during quenching cooling is less than 0.5 占 폚 / sec, the structure of the ferrite-pearlite main body does not become the structure of the bainite-based material and the cooling of more than 20 占 폚 / .

[Performing at least two times of tempering in a temperature range of 550 to 700 占 폚]

After the quenching as described above, the tempering is performed, but it is important to appropriately control the tempering conditions at this time. The tempering condition affects the lath width and MA size (maximum value) of the bainite, and when the tempering temperature is less than 550 占 폚 or when the number of times of tempering is once, the MA size (maximum value) exceeds the specified value. Further, when the tempering temperature exceeds 700 DEG C, the lase width of the bainite exceeds the specified value.

In addition, when controlling the size of the Ti-based dispersed particles present in the steel sheet and containing 0.005 to 0.030% of Ti, by controlling the following conditions based on the above-described method of producing the steel sheet of the present invention do.

First, the slab heating temperature must be 1150 DEG C or higher. By making the heating temperature relatively higher in this manner, it is possible to melt already existing Ti-based dispersed particles at the time of heating the slab, thereby making it possible to reduce the average size. In addition, by making the temperature to be relatively high, the Ti-based dispersed particles produced in the subsequent steps can be promoted to a certain degree of growth, and as a result, the finest remaining ultimately can be reduced. Preferably 1200 占 폚 or more and 1200 占 폚 or more, the minimum size can be made 10 nm or more.

It can also be seen that the size of the Ti-based dispersed particles is also influenced by the content of elements such as C, Si, Mn, Nb, Cu, Ni, Cr, Mo and V. As a result of a study by the present inventors, it has been found that, in order to make the average size of the Ti-based dispersed particles to 40 nm or less, in addition to the control of the slab heating temperature, It is experimentally required to adjust the content of the element. The X value is preferably not less than 45 (%), but more preferably not less than 50 (%), and conversely, from the viewpoint of toughness deterioration, it is preferably not more than 150 (% ].

In addition, in the following expression (2), elements (Si, Nb, Cu, Ni and the like) contained as necessary are also included, but when these elements are not included, When an element is included, an X value may be calculated from the following equation (2).

[C], [Si], [Mn], [Nb], [Cu], [Ni], [Cr], [Mo] (% By mass) of Ni, Cr, Mo, and V, respectively.

The steel sheet to be used in the present invention basically assumes a steel sheet having a thickness of 50 mm or more. However, the steel sheet is equivalent in thickness to a steel sheet having a thickness of 50 mm or more, and is included in the scope of the present invention.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention but may be appropriately changed within a range that is suitable for the purpose of the preceding and following paragraphs, All of which are included in the technical scope of the present invention.

[First Embodiment]

Various types of molten steel showing the chemical composition are shown in Tables 1 and 2 below by a conventional casting method and the molten steel is cooled to form a slab (thickness: 300 mm), and hot rolled under the conditions shown in Tables 3 and 4 , Cooling and tempering were performed to obtain various steel sheets (thickness: 100 mm). In the following Tables 1 and 2, REM was added in the form of mischmetal containing about 50% of Ce and about 25% of La. In the following Tables 1 and 2, the column " - " indicates that no element is added.

The mechanical properties (yield strength YS of the post-warp steel sheet, tensile strength TS, tensile strength TS, and the like) were measured for each steel sheet obtained, The drop weight characteristic NDT) was measured by the following method.

[Measurement of area fraction of bainite]

The resulting steel sheet was observed under an optical microscope at a position of t / 4 (t: sheet thickness), and a color was applied to a portion other than the bainite and irradiated onto a transparent film. Thereafter, an image analyzer (manufactured by Media Cybernetics, Image-Pro Plus) was used to calculate the area ratio of the portion to which the color was applied and subtracted from 100% of the entire area, to obtain the bainite area fraction. At this time, the microscopic observation was taken at 3 times the visual field of 100 times, and the average value thereof was calculated.

[Measurement of lath width of bainite]

The obtained steel sheet was observed by a scanning type microscope (SEM) at a magnification of 1000 times using a sample taken from a position of t / 4 (t: plate thickness), and the average value of the three fields was divided by the width of the steel grade Respectively.

[Evaluation of tensile properties of steel sheet after backing]

Each steel sheet of t / 4 after: by the width direction from a position of (t plate thickness) collecting NK U14 test specimen, and carrying out a tensile test according to JIS Z2241, the yield stress YS (the yield point YP or 0.2% proof stress σ _0.2 ) And tensile strength TS were measured. The acceptance criteria is an average value at three times, a yield strength YS of 415 MPa or more, and a tensile strength TS of 620 MPa or more.

[Measurement of size (circle equivalent diameter) of island-shaped martensite (MA)] [

The samples were taken from the position of t / 4 (t: plate thickness) of each steel sheet after each step, and the steel was corroded by referee, and the structure was observed with an optical microscope, and the magnification: 1000 times and 5 fields were observed MA). The size (average circle equivalent diameter, maximum circle equivalent diameter) of MA was measured by image analysis by an image analyzer (Image Cybernetics: Image-Pro Plus).

[Assessment of dropout characteristics]

A drop test was conducted in accordance with ASTM E208 (2006), and the transition temperature NDT of each steel sheet was measured. The specimen used was P-3 type and was taken along the direction C (direction perpendicular to the rolling direction) from the position of t / 4 (t: sheet thickness) of the post-steel sheet. The beads formed on the surface of the test piece were formed into straight beads by using an electrode (NRL-S, manufactured by Kobe Steel Co., Ltd., diameter: 5 mm). The shape of the test piece used at this time is shown in Fig. 1 (average drawing) (L: 50 mm, W: 130 mm). The NDT passed below -70 ℃.

The results of these measurements are shown in Tables 5 and 6 (Test Nos. 1 to 55). Further, the column "-" (Test Nos. 51 and 52) in the item of organization in the following Table 6 means that there is no bainite structure (Test No. 51 is a ferrite-pearlite structure, No. 52 is a martensitic structure).

From these results, it can be considered as follows (the following No. indicates the test No. of Tables 1 to 6). Nos. 1 to 24 satisfy the requirements specified in the present invention, and it can be seen that the chemical composition and the structure are appropriately controlled, and the favorable drop weight characteristics are obtained along with the high strength.

On the other hand, Nos. 25 to 55 are examples deviating from any one of the requirements specified in the present invention, and at least one of the characteristics is inferior. In the example of No. 25, the C content does not fall within the range specified in the present invention, and the drop weight property is good but the strength is lowered. In the case of No. 26, since the C content exceeds the range specified in the present invention, high strength is obtained but the drop weight property is deteriorated.

In the example of No. 27, the size of the MA (maximum circle equivalent diameter) is large and the dropout characteristics are deteriorated because the Si content exceeds the range specified by the present invention (the A value also becomes high). In the case of No. 28, since the Mn content is not within the range specified in the present invention, the necessary strength is not obtained and the drop weight characteristics are also slightly lowered. In the case of No. 29, the Mn content exceeds the range specified in the present invention, and the drop weight characteristics are deteriorated.

In the example of No. 30, the P content exceeds the range specified in the present invention, and high strength is obtained, but the drop weight characteristics are deteriorated. In the example of No. 31, the S content exceeds the range specified by the present invention, and high strength is obtained, but the drop weight characteristics are deteriorated.

In the case of No. 32, the strength is lowered because the Al content does not fall within the range specified in the present invention. In the case of No. 33, the Al content exceeds the range specified by the present invention, and the size (maximum circle equivalent diameter) of MA is increased, and the drop weight characteristics are deteriorated.

In the case of No. 34, the content of Cu, which is an arbitrary additive component, exceeds the preferable range, and the maximum size of MA becomes large, and the drop weight characteristics are deteriorated. In the example of No. 35, the content of Ni, which is an arbitrary additive component, exceeds the preferable range, and the size (maximum circle equivalent diameter) of MA is increased and the drop weight characteristics are deteriorated.

In the example of No. 36, the Cr content is not in the range specified in the present invention, and the strength is lowered and the drop weight characteristics are slightly lowered. In the case of No. 37, the Cr content exceeds the range specified by the present invention, and high strength is obtained, but the drop weight characteristics are deteriorated.

In the case of No. 38, the Mo content is not in the range specified in the present invention, and the strength is lowered and the drop weight characteristics are slightly lowered. In the case of No. 39, although the Mo content exceeds the range specified in the present invention, high strength is obtained, but the drop weight characteristics are deteriorated.

In the example of No. 40, the V content exceeds the range specified by the present invention, and high strength is obtained, but the drop weight characteristics are deteriorated. In the example of No. 41, the content of Nb which is an arbitrary additive component exceeds the preferable range, and the drop weight characteristics are deteriorated.

In the case of No. 42, the Ti content, which is an arbitrary additive component, exceeds the preferable range, so that the strength is lowered and the drop weight characteristics are deteriorated. In the example of No. 43, the B content, which is an arbitrary additive component, exceeds the preferable range, and the dropout characteristics are deteriorated.

In the example of No. 44, the N content is not in the range specified in the present invention, and the drop weight characteristics are deteriorated. In the example of No. 45, the N content exceeds the range specified by the present invention, and the drop weight characteristics are deteriorated. In the example of No. 46, the O content exceeds the range specified in the present invention, and the dropout characteristics are deteriorated.

In the example of No. 47, rolling was performed at a reduction ratio of 5% at 950 to 850 ° C, and the width of the bainite was large, and the drop weight characteristics were deteriorated. In the examples of Nos. 48 and 49, the reduction ratio in the final pass is small, the width of the bainite is large, the size of the MA (maximum circle equivalent diameter) exceeds the specified value, Is deteriorated.

In the example of No. 50, since the temperature of the final pass is high, the width of the bainite becomes large and the size (maximum circle equivalent diameter) of the MA exceeds the specified value, and the dropping characteristics are degraded. In the examples of Nos. 51 and 52, since the cooling rate at the time of quenching is not within the predetermined range, the microstructure does not consist mainly of bainite, and both high strength and good dropping characteristics can not be satisfied.

In the example of No. 53, the number of times of tempering is one, and the size (maximum circle equivalent diameter) of MA exceeds the stipulated value, and the dropping characteristic is deteriorated. In the examples of Nos. 54 and 55, the tempering temperature is outside the appropriate temperature range, and either one of the glass width of the bainite or the size of the MA (maximum circle equivalent diameter) exceeds the specified value, Is deteriorated.

[Example 2]

In Table 7, various kinds of molten steel showing the chemical composition were sprayed by a conventional casting method, the molten steel was cooled to form a slab (thickness: 300 mm), and then subjected to hot rolling, cooling and tempering To obtain various steel sheets (thickness: 100 mm). In Tables 7 and 8, the test No. 24 shown in Tables 1, 3, and 5 was also shown at the same time for reference.

The mechanical properties (yield strength YS of the post-warp steel sheet, tensile strength TS, tensile strength TS, and the like) were measured for each steel sheet obtained, (Average particle size, minimum particle size) and HAZ toughness of the Ti-based dispersed particles were measured by the following methods. The results of these measurements are shown in the following Table 9 together with the results of Test No. 24 (Test Nos. 56 to 61).

[Measurement of size of Ti-based dispersed particles]

The obtained steel sheet was observed with a transmission electron microscope (TEM) at a magnification of 60000 times to observe the position of t / 4 (t: sheet thickness) at an observation field of 2.0 x 2.0 (mu m) , And the area of Ti dispersed particles in the respective fields of view was measured and the circle equivalent diameter of each particle was calculated from this area. Whether or not the Ti-based dispersed particles were dispersed was determined by whether EDX (energy dispersive X-ray detector) attached to the TEM contained each of the particles of Ti. In addition, particles smaller than 1 nm were excluded from the measurement. A value obtained by arithmetically averaging the circle-equivalent diameters of each of the obtained particles was taken as an average size, and the smallest value was taken as the minimum value.

[Measurement of HAZ toughness]

For HAZ toughness, the Charpy impact test piece (No. 4 specimen of JIS Z 2201) was sampled from the position of t / 4 (t: plate thickness) with respect to the steel sheet obtained, and the reproduced HAZ thermal cycle V notch Charpy test . The reproduced HAZ thermal cycling conditions simulated the heat history of the heat input: 100 kJ / mm. As for the HAZ toughness, the absorption energy (vE- ₁₅ ) at -15 캜 was measured for three specimens, and the average value thereof was determined.

From these results, it can be considered as follows (the following No. indicates the test No. in Table 9). In the steel sheets No. 56 to 61, since the average size of the Ti-based dispersed particles is 40 nm or less, it can be seen that the HAZ toughness is improved as compared with the steel sheet No. 24. In particular, in the case of No. 60 and 61, it is found that the average size of the Ti-based dispersed particles is 40 nm or less and the minimum size thereof is 10 nm or more, which indicates good HAZ toughness.

Although the present invention has been described in detail with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention.

The present application is based on Japanese Patent Application (Japanese Patent Application No. 2010-110509) filed on May 12, 2010, the content of which is incorporated herein by reference.

The high strength steel sheet of the present invention is useful as a material for pressure vessels of nuclear power plants in addition to structural materials such as marine structures, ships, bridges and the like.

Claims (11)

C: not more than 0.5% (inclusive of 0%), Mn: 1.0 to 2.0%, P: not more than 0.015%, C: 0.03 to 0.150% S: not more than 0.01% (not including 0%), Al: 0.005 to 0.06%, Cr: 0.10 to 0.5%, Mo: 0.05 to 0.5%, V: 0.10% or less (Not including 0%), N: 0.0020 to 0.010%, and O: 0.010% or less (not including 0%), the balance being iron and inevitable impurities, Of the bainite is 90% or more and the average value of the lath width of the bainite is 3.5 m or less in the microstructure at the position of t / 2 (t represents the plate thickness, hereinafter the same) , And the maximum value of the circle equivalent diameter of the island-shaped martensite in the bainite is 3.0 m or less. After the steel sheet strength. The steel plate according to claim 1, wherein the average circle-equivalent diameter of the island-shaped martensite is 1.0 占 퐉 or less, and excellent in drop weight characteristics. The steel plate according to claim 1 or 2, further comprising at least one of the following groups (a) to (g):
(a) at least one selected from the group consisting of Cu: not more than 2% (not including 0%), Ni: not more than 2% (not including 0%),
(b) at least one selected from the group consisting of Nb: not more than 0.05% (excluding 0%), B: not more than 0.005% (not including 0%),
(c) at least one member selected from the group consisting of 0.005% or less of Mg (not including 0%) and 0.030% or less of Ti (excluding 0%),
(d) at least one selected from the group consisting of Zr: not more than 0.1% (not including 0%) and Hf: not more than 0.05% (excluding 0%),
(e) Ca: 0.0035% or less (not including 0%),
(f) at least one selected from the group consisting of not more than 2.5% of Co (not including 0%) and not more than 2.5% of W (not including 0%),
(g) Rare earth elements: 0.01% or less (not including 0%) delete delete delete delete delete delete delete delete

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