JPH0941077A - High-strength steel sheet excellent in crack propagation arresting property and method for producing the same - Google Patents

Abstract

(57) [Abstract] (Corrected) [Problem] Even if brittle fracture occurs due to an unexpected cause, it is possible to guarantee that the influence is limited to a certain range by preventing crack propagation. Suitable for use with
A high-tensile steel plate having a strength of 780 MPa or more and a method for manufacturing the same. SOLUTION: C, Si, Mn, Cu, Ni, Mo, N
Plate thickness 5% for each of the upper and lower surfaces of the steel plate with b, B, and Ti specified
The surface layer corresponding to the above suspends rolling of a steel sheet having a martensite structure or a martensite / bainite mixed structure having a prior austenite grain size with an average grain size of 10 μm or less, and the upper and lower surface temperatures are 350 ° C. or higher and Ar ₃ points. Than 2
After water cooling to a lower temperature of 5 ° C or more and stopping halfway, rolling is performed in the austenite unrecrystallized region in the surface layer part and at the same time in the austenite recrystallized region in the steel plate central part at a rolling reduction of 50% or more, and recuperating After finishing quenching and burning back.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength steel sheet excellent in low temperature toughness used for large-scale steel structures such as marine structures, tanks and pen stocks, and particularly excellent in crack propagation arresting property at low temperature, And a method for producing the steel sheet with high productivity.

[0002]

2. Description of the Related Art High-tensile steel plates used for large-scale steel structures such as offshore structures, tanks, and penstocks are required to have excellent low temperature toughness in order to ensure a high degree of safety. In recent years, in addition to the low temperature toughness obtained by a small Charpy test, etc., in consideration of an unexpected situation, the crack propagation stopping property that guarantees that even if a crack occurs due to force majeure, it does not propagate greatly is also included. Are often required. As a means for improving the low temperature toughness and the crack propagation arresting property, a method of increasing the Ni content is conventionally known. However, since an increase in the amount of Ni is accompanied by an increase in the cost of the alloy, there has been a demand for a method of improving the low temperature toughness, especially the crack propagation arresting property, without increasing the amount of Ni.

Japanese Unexamined Patent Publication (Kokai) No. 5-271760 discloses Ni.
A method of improving low temperature toughness without increasing the amount is disclosed. Here, a method is proposed in which the steel slab is once cooled during the rolling, the surface layer portion is cooled to the Ar ₃ point or lower, and then reheated to the Ac ₃ point or higher to restart the rolling in the unrecrystallized region. This method rapidly cools the steel sheet during rolling and adds a large amount of work strain to the center of the plate thickness due to the difference in deformation resistance due to the temperature difference between the surface and the center, in an attempt to reduce the ferrite grain size caused by transformation. To do. The steel targeted by the method disclosed herein has a tensile strength (TS: Tensile St
It is a low strength steel mainly composed of rength) 500 MPa and its structure is also limited to ferrite pearlite.

A method aimed at improving the crack propagation stopping property is disclosed in Japanese Patent Laid-Open No. 5-271757.
In this method, the surface layer part is cooled to less than Ar ₁ point during rolling to sufficiently generate ferrite, and then it is subjected to reduction, and then dislocations in ferrite are rearranged during the reheating to refine the structure of the surface layer part. It is what When the ferrite grains in the surface layer portion are made finer to improve the toughness, even if brittle fracture occurs in the central portion, the surface layer portion does not become brittle fracture but ductile fracture. Of the ductile fractures in the surface layer, the shear fractured portion in the surface layer is called shear lip. Shear lip not only exhibits a ductile fracture surface, but also has a large plastic deformation zone,
Make a constant angle with the steel plate surface. When a shear lip occurs, the energy required for crack propagation becomes high, and the crack propagation stopping property of the entire plate thickness improves. However, this method also does not substantially affect the crack propagation arrest property unless the size of the shear lip, that is, the depth is larger than a certain extent, and the steel to be targeted is also a ferrite-pearlite structure having a low strength level. Limited to The 780 to 950 MPa class high-strength steel targeted by the present invention is rarely accompanied by a clear shear fracture, that is, a shear lip, in a brittle fracture surface that is easily observable with the naked eye.

Further, in the high-strength steel having a high strength level, which is the object of the present invention, there is a problem that the surface layer portion is over-quenched during quenching and the toughness is lowered. Since the surface layer portion has a high quenching cooling rate, the structure obtained by quenching becomes a single phase of martensite and is not divided by the effect of the mixed structure, so if the former austenite grain size is not fine, it becomes a substantially coarse structure. Because. For this reason, the ES
When the SO test is performed, brittle fracture precedes in the surface layer portion, and the crack propagation stop characteristics of the steel sheet may deteriorate due to the surface layer portion. However, since the surface is not constrained by stress in three directions, brittle fracture does not precede, and in reality, the inside slightly deteriorates the crack propagation stopping property.

In the following, the "surface" literally means the surface of the steel sheet, and the "surface layer" means a certain range including the surface and the inside of the steel sheet. Also, "surface temperature"
In this case, the surface temperature measured from the outside is meant.

[0007] In the hot rolling of steel slabs, the rolling performed in the process of cooling the steel to give a temperature difference between the surface layer portion and the central portion, resuming the cooling after the cooling is called secondary rolling, and rolling before the cooling is performed. It is called primary rolling. Sometimes there is no distinction between the two and each is simply called rolling.

[0008]

SUMMARY OF THE INVENTION The present invention utilizes existing equipment, and without the use of expensive alloying elements, 780-950 with excellent low temperature toughness and crack propagation arrest properties.
It is an object of the present invention to provide a high-strength steel sheet of MPa class and a manufacturing method thereof. A specific object of the present invention is to provide a high-strength steel sheet having the following characteristics (1), which are difficult to achieve by the conventional techniques.

Tensile strength (TS) of 780 MPa or more, 2 mmV notch Charpy test (JIS Z 224)
2) Absorbed energy at −80 ° C. of −200 J or more, fracture toughness value Kca = 6 in ESSO test described later
The temperature at which the pressure reaches 56 kgf / mm ^1.5 is -70 ° C or lower.

[0010]

[Means for Solving the Problems] The inventors of the present invention improve the low temperature toughness, especially the crack propagation arresting property at low temperature, by changing the microstructure in the thickness direction of high strength steel sheets having a TS level of 780 to 950 MPa. Considered. As a result, it was confirmed that the steel sheet having the following structure and the rolling and heat treatment method for giving the structure achieve the object.

Here, the gist of the present invention is a steel plate and a manufacturing method for manufacturing the steel plate shown below. (Refer to FIG. 1) (1) By weight%, C: 0.02 to 0.15%, Si:
0.30% or less, Mn: 0.4 to 1.5%, Cu: 1.
4% or less, Ni: 0.50 to 6.0%, Cr: 0.8%
Below, Mo: 0.8% or less, Nb: 0.05% or less,
V: 0.08% or less, B: 0.0025% or less, Ti:
0.03% or less, solAl: 0.001 to 0.08%
And N: 0.006% or less, with the balance being Fe and unavoidable impurities, and a surface layer portion corresponding to a plate thickness of 5% or more on each of the upper and lower surfaces of the steel sheet has an average grain size of 10 μm. A high-strength steel sheet excellent in crack propagation arrest properties at low temperatures, characterized by comprising a martensite structure having the following old austenite grains or a mixed structure of martensite and bainite.

(2) In the hot rolling of a steel slab having the composition in the range described in (1) above, after the primary rolling, the surface temperature is 950 ° C. or higher and the average cooling rate is 15 ° C./s or higher. After the surface temperature reaches a temperature range lower than the Ar ₃ point by 25 ° C or more and 350 ° C or more, the cooling is interrupted,
During the process in which the surface temperature is reheated to the intermediate temperature between Ac ₁ and Ac ₃ by an external heat source or the heat inside the billet, the cumulative rolling reduction is 50% between the stop of cooling and the end of reheating.
The above secondary rolling is performed, and the steel sheet surface layer portion is subjected to the temperature range of the austenite unrecrystallized region and simultaneously to the steel sheet central portion corresponding to a range of 20% or more above and below the center of the sheet thickness. On the other hand, a crack propagation stopping property at low temperature is characterized by applying a reduction in the temperature range of the austenite recrystallization region to form a steel plate having a predetermined thickness, quenching, and tempering at Ac ₁ point or less. Excellent manufacturing method for high strength steel sheet.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The composition, structure and rolling heat treatment conditions of the steel sheet according to the above invention will be described below separately.

1. Composition of steel sheet (hereinafter, "%" means% by weight) C: 0.02 to 0.15% C is an element effective for increasing strength, and for that purpose, 0.0
2% or more is required. However, if the content exceeds 0.15%, the toughness and weldability deteriorate, so the upper limit is 0.1.
5%.

Si: 0.30% or less Si is an element effective for deoxidation, but if it is contained in excess of 0.30%, not only the toughness of the weld heat affected zone deteriorates, but also
The workability also deteriorates, so the upper limit is made 0.30%. In terms of the performance of the steel sheet, the content may be 0, but when Si is set to 0, the loss of Al becomes large at the time of deoxidation. Therefore, normally, the Si content that remains after deoxidation by adding Si, For example, about 0.01% is preferable as the lower limit.

Mn: 0.4 to 1.5% Mn is an element effective in increasing strength, and for that purpose, it is 0.1
4% or more is required. However, if it exceeds 1.5%, the toughness deteriorates, the transformation point such as Ar ₃ point decreases, the rolling temperature required in the present invention becomes low, and the deformation resistance increases, so the upper limit is 1.5%. And

Cu: 1.4% or less Cu is an element effective for increasing the strength, but if it exceeds 1.4%, the toughness deteriorates, so the upper limit is made 1.4%. Cu
Does not have to be added, but when it is added, it is preferably 0.2% or more. This is because the effect does not appear clearly unless the content is 0.2% or more.

Ni: 0.50 to 6.0% Ni improves the propagation stopping property of brittle cracks and the low temperature toughness, so 0.50% or more must be contained. Further, since it has an effect of preventing cracks on the surface of the steel material due to addition of Cu, it is desirable to contain at least half of the Cu content. However, even if it exceeds 6.0%, the increase in strength and the improvement in toughness commensurate with the cost increase cannot be obtained, so the upper limit is made 6.0%.

Cr: 0.8% or less Cr may be added without addition. When it is added to secure the strength, it is desirable to set it to 0.3 to 0.7%. If it exceeds 0.8%, the toughness deteriorates, the transformation point such as Ar ₃ point decreases, and rolling at low temperature becomes necessary, and the rolling efficiency decreases as the deformation resistance increases. Therefore, the upper limit is 0.8%. To do.

Mo: 0.8% or less Mo may be added without addition. When it is added to secure the strength, it is preferably 0.3 to 0.8%. If it exceeds 0.8%, the toughness deteriorates, the transformation point lowers, and rolling at low temperature is required, so the upper limit is made 0.8%.

Nb: 0.05% or less Nb may not be added. However, it is an element effective for refining crystal grains, and for that purpose, 0.005% or more is necessary. If it exceeds 0.05%, not only the toughness deteriorates but also the austenite recrystallization temperature range is significantly limited, so the desirable content is 0.005 to 0.05%.

V: 0.08% or less V may be added without addition. However, in order to secure strength,
It is preferable to contain about 0.01 to 0.06%.
This is because if the content is not 0.01% or more, the strength is not clearly improved, and the content is preferably 0.06% or less in order to secure extremely excellent toughness. The reason for setting it to 0.08% or less is that if it exceeds 0.08%, the toughness deteriorates.

B: 0.0025% or less B may be added without addition. However, in order to secure the strength, 0.
It is desirable to contain 0003 to 0.0020%.
This is because the effect is not exhibited unless the content is 0.0003%, and 0.0020% or less is desirable in order to secure extremely excellent toughness. The reason why the content is 0.0025% or less is that the toughness deteriorates if the content is exceeded.

Ti: 0.03% or less Ti may be added without addition. However, Ti is an element effective for refining the crystal grains during heating of the slab, and if added, it is desirable to contain 0.005% or more. Also, Nb
When Ti is added, a small amount of Ti is effective for suppressing the cracking of the surface of the continuously cast slab promoted by Nb, and such an effect is exhibited at 0.005% or more.
If it exceeds 0.03%, the toughness deteriorates significantly, so the upper limit is made 0.03%.

SolAl: 0.001 to 0.08% Al is an effective element as a deoxidizer, and if it is not contained in an amount of 0.001% or more, pinholes are generated during solidification, so s
The olAl content is 0.001% or more. But so
If the Al content exceeds 0.08%, the toughness deteriorates, so the upper limit is made 0.08%.

N: 0.006% or less N is not added as long as it is mixed in the steel as an impurity. However, if it exceeds 0.006%, the toughness of the welded portion deteriorates, so the content is made 0.006% or less.

2. Structure The features of the steel sheet according to the present invention are the following structures in the upper and lower surface layer portions.

Average fine austenite grains (specifically 10 μm or less. The measuring method will be described later) Martensite structure or mixed structure of martensite and bainite Structure of the surface layer formed by quenching is martensite It must be a structure or a mixed structure of martensite and bainite. This is because high strength cannot be secured without these structures. Since martensite or bainite transforms while preserving the former austenite grain boundaries, the former austenite grain size immediately affects the toughness. This is because in the case of a martensite structure or a mixed structure of martensite and bainite, in addition to the size of each structure, these grain boundaries also substantially divide the structure. Further, since quenching is performed from flattened austenite that retains rolling distortion, the hardenability is reduced and the region where a martensite and bainite mixed structure suitable for toughness is generated extends to near the surface. Even if the surface layer near the surface has a martensite single-phase structure, the structure is substantially fine. This is because the flat grains have a higher ratio of the grain boundary area to the intragranular volume and the nucleation density of martensite generated from the grain boundaries is improved, and the grains are finely divided accordingly. Here, the flat shape refers to a shape in which austenite grains are pressed in the plate thickness direction and crushed. The former austenite grain size of the surface layer portion is an average value measured in lengths of 1 mm in 15 random directions in a cross section perpendicular to the steel sheet width direction.

This old austenite grain size is 1 in average grain size.
The plate thickness ratio of the surface layer portion having a thickness of 0 μm or less must be 5% or more on one side and 10% or more on the upper and lower surfaces. This is because if the upper and lower surfaces are each less than 5%, sufficient crack propagation stopping properties cannot be obtained as a steel sheet. Further, the plate thickness ratio of the surface layer portion is preferably 20% or less on one side and 40% or less on the upper and lower surfaces. More than 20% on one side, 40% on top and bottom
If it exceeds, even if it is reheated, the temperature does not become sufficiently high over the entire plate thickness, and the ratio of austenitization decreases, resulting in a lower strength.

Here, the central portion of the steel sheet is an isotropic fine ohmic contact.
It consists of martensite or bainite transformed from stenite grains or a mixed structure thereof. A high TS level high strength steel with good toughness is usually the structure.

When tested as a steel sheet, if the surface layer portion has the above-mentioned structure, brittle fracture does not precede at the surface layer portion, but rather the surface layer portion prevents brittle crack propagation in the central portion of the steel sheet that occurs at low temperature. Fulfill. As a result, the crack propagation stopping property of the steel sheet is improved. At this time, shear fracture does not occur in the surface layer portion, but even under this temperature condition, a ductile fracture surface is exhibited unlike the central portion. That is, even in the case of high-strength steel having a high TS level, even if a shear lip does not occur, if the toughness of the surface layer portion is improved, the crack propagation stopping property of the steel sheet can be improved.

In the upper and lower surface layers of the steel sheet, if the average grain size is 10 μm or less, sufficient toughness can be obtained even at a low temperature of about −80 ° C., and the crack propagation stopping property can be improved. The martensite or the mixed structure of martensite and bainite is usually used in a tempered state, but a structure in a state not subjected to tempering is also included in the scope of the present invention.

A steel sheet having such a structure in the surface layer portion can be manufactured by the rolling and heat treatment methods described below.

3. Rolling and heat treatment conditions Steel slab heating temperature: The steel slab heating temperature is 950 ° C. or more in order to completely austenite and to make the temperature of the central portion of the steel sheet a recrystallization region in rolling during post-cooling reheat described later, Desirably, the temperature is 1000 ° C. or higher in order to improve rolling efficiency. However, in order to prevent coarsening of austenite crystal grains, it is desirable to set the upper limit to 1250 ° C.

Rolling and cooling: After heating, the billets are subjected to primary rolling, after which the billets are cooled. The surface temperature before the start of cooling is not so different from the central part. FIG. 1 is a drawing showing a surface temperature during manufacturing of a steel sheet, a surface layer portion temperature A (a thickness position of 5% of the plate thickness from the surface) and a center portion temperature B (a sheet thickness center position). In the figure, the forced cooling start temperature must be performed from the surface temperature of 950 ° C. or higher. The cooling rate at this time is such that the temperature difference between the surface layer portion and the central portion of the plate thickness is sufficient, and the cooling rate at the surface is 15 ° C./s or more so that the central portion of the plate thickness is kept at a high temperature. There is a need. By cooling from a surface temperature of 950 ° C. or higher at a cooling rate of 15 ° C./s or higher, during secondary rolling during recuperation, which will be described later,
It appears that the surface layer part is in the non-recrystallized region and the central part is in the recrystallized region. The average cooling rate is preferably 25 ° C./s or less in order to avoid the cooling rate becoming too high and the cooling not stopping in an appropriate temperature range. These cooling rates are within a range that can be controlled by a normal control cooling device.

The surface temperature can be measured, but the internal temperature distribution is not easy to measure, so the surface temperature is estimated by calculation.

Surface temperature in cooling: The surface temperature in cooling is 25 ° C. or more lower than the Ar ₃ point and 35
It is necessary to set the temperature to 0 ° C or higher. As a result, not only at the surface but also at the position where the plate thickness is 5% or more inside the surface, Ar
Martensite or bainite with high dislocation density is formed by cooling below ₃ points. If a reduction is also applied to these and the dislocation density is further increased, it is possible to re-operate due to recuperation.
Extremely fine austenite grains are obtained when reversely transformed into austenite.

By this cooling, it becomes possible to effectively apply a reduction to the central portion of the plate thickness. In the case of steel within the composition range of the present invention, the Ar ₃ point is not the ferrite transformation start temperature,
It means the transformation start temperature of bainite or martensite, and is a temperature around 450 to 500 ° C. Ar to produce sufficient amounts of bainite or martensite
It is desirable to cool the temperature to 50 ° C. or more lower than the _three points. By cooling to a temperature lower than the Ar ₃ point by 50 ° C. or more, the surface layer portion cooled to the Ar ₃ point or less exceeds 5% of the plate thickness on each side of the surface, and thus exceeds 10% in total. As a result, the range of the fine structure is expanded, and the effect of rolling down the central portion is enhanced. However, if the temperature is lower than 350 ° C., the cooling effect is exerted on the central portion of the steel sheet, and the recuperation becomes insufficient. Therefore, it is necessary to avoid lowering the temperature below 350 ° C.

Rolling during recuperation: As shown in FIG. 1, after the surface temperature is rapidly cooled to a temperature lower than the Ar ₃ point by 25 ° C. or more, the cooling is stopped midway, and after heating by an external heat source, or steel. Secondary rolling is performed during heat recovery due to the heat inside the piece. The heating by the external heat source is only auxiliary heating, and the heating is not performed to the extent that the temperature of the central portion and the surface layer portion is reversed. This heating may be performed by a gas heating furnace, a high frequency heating device, a direct current heating device, or the like, and the heating method is not particularly limited. The heating from the outside may be performed on the rolling line, or may be performed by providing a bypass and while passing the bypass. From quench stop to Ac ₁ and Ac ₃
The secondary rolling with a cumulative rolling reduction of 50% or more is performed while the heat is recovered to the intermediate temperature or higher. This secondary rolling is in the temperature range of the austenite unrecrystallized region with respect to the steel sheet surface layer portion, and at the same time, 20% or more and 40% or more in total above and below the center of the sheet thickness (see FIG. 1). The reduction is applied to the central portion (shown) in the temperature range of the austenite recrystallization region. There is a temperature difference between the surface layer portion and the central portion, and the surface layer portion is work-hardened without eliminating the processing strain due to rolling in the unrecrystallized temperature range, so the reduction is effectively applied to the thickness center portion, The recrystallized austenite grain size inside is refined. The temperature distribution of the surface layer portion and the central portion of the steel sheet can be estimated by calculation as described above.

Here, the "non-recrystallization temperature range" refers to a temperature range in which the dislocation density introduced by the rolling remains 25% or more for about 15 seconds after the rolling and before the next rolling. The “recrystallization region” is a temperature region that cannot be satisfied.

"20% or more above and below the center of plate thickness for a total of 40
The reason why the austenite recrystallized region is the central portion of the steel sheet corresponding to the range of not less than 10% is as follows. When quenching from an unrecrystallized state, hardenability deteriorates, a structure suitable for mechanical properties such as a mixed structure of martensite and bainite cannot be obtained, and strength and toughness deteriorate. Specifically, this is to avoid the formation of a coarse bainite one-phase structure. The low temperature toughness of the steel sheet has a large effect on the central portion of the sheet thickness, and the reduction is effectively applied to that portion to improve the toughness through the refinement of the structure. It is the center of the steel plate that governs the properties of the steel plate, and the object cannot be achieved unless the range of 20% or more and 40% or more in total above and below the center of the plate thickness is taken. Further, it is desirable that the total is 40% or less from the center of the plate thickness and 80% or less in total. If it exceeds 40% in each of the upper and lower parts and exceeds 80% in total, the grain-refining effect due to rolling reduction to the central part becomes unclear, the structure becomes coarse, and the toughness of the central part deteriorates a little.

The reason for reducing the surface layer portion in the non-recrystallized region is as follows. Since the surface layer portion is in a non-recrystallized state, the work-hardened state is maintained and effective reduction is applied to the center portion of the plate thickness. Even if such consideration is made, more reduction is given to the surface layer portion, and as a result of rolling in the unrecrystallized region, the austenite grains in the surface layer portion are flattened by being crushed in the rolling direction. The grain boundaries are preserved even after transformation into sites or bainite. As a result, martensite or a mixed structure of martensite and bainite is formed from the flattened austenite.
The advantages when transforming from such flattened austenite are as described above.

The reason why the cumulative reduction rate of the secondary rolling is set to 50% or more is that the above-mentioned effects at the central portion and the surface layer portion of the steel sheet cannot be sufficiently obtained unless the cumulative reduction rate is 50% or more.
The larger the cumulative rolling reduction, the better.

Temperature of central portion of plate thickness: The central portion temperature in the above cooling needs to be kept in the recrystallization temperature range. The reason is as described above. The surface temperature at the start of cooling, the cooling rate at the surface and the surface temperature at which cooling stopped were limited as described above so that the core temperature remained in the recrystallization region.

Surface temperature after recuperation and before quenching: The reheated surface temperature must be equal to or higher than the temperature just between the Ac ₁ point and the Ac ₃ point in order to secure the strength and keep the austenite grains fine. There is. At lower temperatures, the plate thickness is 5 from the surface.
%, The tempered structure is mostly present in the surface layer portion, and a sufficient amount of austenite is not generated. Therefore, it is not possible to obtain a sufficient amount of martensite or bainite by quenching, and it is not possible to obtain the required strength. At the same time, the two-phase structure in which ferrite and austenite are intricately formed is not formed before quenching, and the structure cannot be effectively divided.

Quenching after recuperation: For quenching after recuperation, rapid cooling such as jetting high-pressure water from a band-shaped nozzle must be performed in order to secure a tensile strength of 780 to 950 MPa. . This is because the center of the steel sheet is completely quenched.

Tempering: Following the above treatment, usually A
Although tempering is performed at a point of c ₁ or less, depending on the application, not so high toughness may be required, and tempering may not be performed for such application.

Rolling is performed by heating the billet heating temperature to 950 ° C. or higher to a temperature as close as possible to 950 ° C., and rolling until cooling is shifted to the lowest temperature side to make the austenite grains as fine as possible. A method for further enhancing the effect of the invention is included in the scope of the present invention.

[0049]

EXAMPLES Next, examples of the present invention will be described.

Table 1 is a list showing the compositions of 5 kinds of invention steels having compositions within the composition range defined in the present invention and 7 kinds of comparative steels outside the composition range. Table 2 is a list showing Ac ₁ points and Ac ₃ points of these steels. Steels having these compositions were melted in a laboratory, and the steel pieces obtained by casting were subjected to various rolling and heat treatment conditions to obtain steel sheets having a plate thickness of 50 and 78 mm. Table 3 is a list showing these rolling, cooling and heat treatment conditions. Further, Table 4 is a list showing heat histories at the center portion of the plate thickness and the surface layer portion due to such rolling and cooling.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

[Table 3]

[0054]

[Table 4]

The microstructures of the surface layers of these steel sheets, including the surface, were all composed mainly of martensite and mixed with bainite. Surface layer, plate thickness 1 / 4t and 1 /
A test piece was taken from the 2t part (center part) and subjected to a tensile test, 2
A mmV notch Charpy impact test and a temperature-gradient ESSO test for evaluating crack propagation termination properties were performed. The result of the plate thickness 1 / 4t represents the property at a position slightly closer to the center than the middle between the surface layer part and the plate thickness center part. FIG. 2 is a drawing showing an outline of a test piece and a crack in a temperature gradient type ESSO test. In this test, uniform stress was applied to a large test piece with a temperature gradient, and the crack that was forcedly generated at the end of the test piece rushed into the test piece, and the temperature and crack length at the stopped point were measured. Ask. The fracture toughness value Kca can be obtained from these, but in the temperature gradient type ESSO test, Kca
It is possible to obtain Kca in a wide temperature range by a small number of tests by utilizing the constant temperature dependence of. Table 5 shows the former austenite grain size of the surface layer portion of the steel sheet manufactured by the manufacturing method described in Tables 3 and 4 above, 1 / 4t and 1
It is a list showing the strength and toughness of a / 2t part, and a crack propagation stop characteristic. In Table 5, fracture toughness value Kca = 656
The reason for adopting a temperature of kgf / mm ^1.5 is Kca
Is 656 kgf / mm ^1.5, crack propagation can be prevented, and the lower the temperature at which this value is, the better the crack propagation stopping property. In the examples of the present invention, it is understood that the tensile strength of the base material is 780 MPa or more, and the absorbed energy of 200 J or more is obtained in the Charpy impact test at −80 ° C. Further, in the ESSO test, in the present invention example, excellent crack propagation stopping performance was obtained in which the temperature at which Kca = 656 kgf / mm ^1.5 was −80 ° C. or less,
There is a clear difference from the comparative example. This temperature is 5 ~
Considering that about 1% addition of Ni is required to lower the temperature by 10 ° C., it can be seen how great the effect of the present invention is.

[0056]

[Table 5]

[0057]

EFFECTS OF THE INVENTION The steel sheet according to the present invention has a tensile strength of 780 MPa or more because of having the above composition and structure.
The toughness is also good. Especially in the surface layer, the structure is extremely fine, so that the steel sheet has high crack propagation stopping performance. By using the steel of the present invention for a tank in a densely populated area or an offshore structure or penstock in a cold region, it is possible to prevent crack propagation even if brittle fracture occurs due to an unexpected cause, leading to a major disaster. It can guarantee a certain level of safety. The steel of the present invention can be easily and inexpensively manufactured by the above-mentioned method.

[Brief description of drawings]

FIG. 1 is a drawing showing a surface temperature, a surface layer temperature and a center temperature during steel sheet production.

FIG. 2 is a diagram showing an outline of a test piece and a crack in a temperature gradient type ESSO test.

Claims (2)

[Claims] 1. By weight%, C: 0.02 to 0.15%, S
i: 0.30% or less, Mn: 0.4 to 1.5%, Cu:
1.4% or less, Ni: 0.50 to 6.0%, Cr: 0.
8% or less, Mo: 0.8% or less, Nb: 0.05% or less, V: 0.08% or less, B: 0.0025% or less, T
i: 0.03% or less, solAl: 0.001 to 0.0
8% and N: 0.006% or less, and a balance of Fe and unavoidable impurities in the balance, and a surface layer portion corresponding to a plate thickness of 5% or more on each of the upper and lower surfaces of the steel sheet has an average grain size. A high-strength steel sheet excellent in crack propagation arrest properties at low temperatures, characterized by comprising a martensite structure having a prior austenite grain with a diameter of 10 μm or less, or a mixed structure of martensite and bainite. 2. In the hot rolling of a steel slab having the composition within the range defined in claim 1, the surface temperature is 950 ° C. after the primary rolling.
From the above, cooling was performed at an average cooling rate of 15 ° C / s or more, and the surface temperature was 25 ° C or more lower than the Ar ₃ point and 350
After reaching the temperature range of ℃ or more, the cooling is interrupted and the surface temperature becomes Ac by the external heat source or the heat inside the steel slab.
_{In the} process of reheating above the intermediate temperature between ₁ and Ac ₃ , secondary rolling with a cumulative rolling reduction of 50% or more is performed from the stop of cooling to the end of reheating, and austenite is not re-applied to the surface layer of the steel sheet. In the temperature range of the crystallization region, at the same time, and to the center portion of the steel plate corresponding to a range of 20% or more above and below the center of the plate thickness, a reduction is applied in the temperature range of the austenite recrystallization region to a predetermined range. A method for producing a high-strength steel sheet having excellent crack propagation arresting properties at low temperatures, which is characterized by quenching after tempering a steel sheet having a thickness and tempering at an Ac ₁ point or less.