JP2020117779A - Steel plate and method for manufacturing steel plate - Google Patents

Abstract

To provide a steel plate that has an excellent arrestability and a high strength.SOLUTION: Provided is a steel plate, having a predetermined chemical composition and, in the front and back layers, containing the processed-ferrite phase by 30% to 90% (area fraction), and in which the processed-ferrite phase has an aspect ratio of 2.0 or more, the average particle size (diameter) of all phases including the processed-ferrite phase measured by electron beam backscattering diffraction method is 25 μm or less and the average value of aggregate texture intensity ratio I, Iand Iis 3.0 or more, and, at the 1/2 position in the plate thickness direction from the steel plate surface, the average particle size (diameter) of all phases including the processed-ferrite phase measured by electron beam backscattering diffraction method is 30 μm or less and the average value of aggregate texture intensity ratio I, Iand Iis 2.0 or more.SELECTED DRAWING: None

Description

The present invention relates to a steel plate and a method for manufacturing a steel plate.

Examples of applications of the steel sheet include ships, buildings, bridges, marine structures, LNG storage tanks, other large tanks, and line pipes (see, for example, Patent Documents 1 to 4).
Steel sheets applied to these applications are required to have good weld heat affected zone (HAZ) toughness in order to suppress brittle fracture, and in the unlikely event that a brittle crack occurs at the weld joint site. However, brittle crack propagation stopping characteristics (BCA: Brittle Crack Arrest; hereinafter sometimes referred to as "arrestability") for stopping the brittle crack in the base material are required.

Examples of steel plates applied to ships include important members of container ships (for example, hatchside combing and upper deck). Since the container ship has a structure in which the upper deck has a large opening, a steel plate having a large thickness and high strength is used as an important member in order to secure the cross-sectional rigidity of the hull.

In recent years, container ships have been increasing in size in order to reduce environmental load and operating costs. Recently, a super large ship (mega container ship) of 14000 to 20000 TEU (Twenty-foot Equivalent Unit) class has been constructed. Therefore, from the viewpoint of suppressing large-scale destruction of the hull, steel sheets used as important members of container ships are required to have further improved arrestability. Further, in order to stably mass-produce steel sheets having high arrestability, new technological development for improving arrestability is desired.

JP, 2009-221585, A JP, 2009-235458, A JP, 2011-214116, A JP, 2002-020835, A

The present invention has been made in view of such circumstances, and provides a steel plate having excellent arrestability and high strength.

Means for solving the above problems include the following aspects.

<1>
In mass %,
C: 0.040% to 0.160%,
Si: 0.01% to 0.50%,
Mn: 0.70% to 2.50%,
P: 0.030% or less,
S: 0.020% or less,
Nb: 0.003% to 0.050%,
Ti: 0.003% to 0.050%,
Al: 0.001% to 0.100%,
N: 0.0010% to 0.0080%,
With a chemical composition of Fe and impurities as the balance,
Between the position of 1 mm from the surface of the steel plate and the position of 5 mm from the surface of the steel plate, the metallographic structure of the section in the plate thickness direction parallel to the rolling direction has an area fraction of worked ferrite phase of 30% to 90% and bainite phase of 10%. -60%, a total of 30% or less of a polygonal ferrite phase, a pearlite phase, and a martensite/austenite mixed phase,
The aspect ratio of the worked ferrite phase in the sheet thickness direction cross section parallel to the rolling direction between the position 1 mm from the steel plate surface and the position 5 mm from the steel plate surface is 2.0 or more,
Between the position of 1 mm from the surface of the steel sheet and the position of 5 mm from the surface of the steel sheet, the processed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite-austenite mixed phase in a cross section perpendicular to the rolling direction The average particle size (diameter) is 25 μm or less when measured by electron beam backscattering diffractometry,
Of the {hkl} plane with respect to the rolling surface and the rolling direction and the texture strength ratio I _{{hkl}<uvw> in} the <uvw> direction between the position 1 mm from the steel plate surface and the position 5 mm from the steel plate surface _, I _{ The average value of _001}<110> , I _{113}<110> , and I _{112}<110> is 3.0 or more,
Electron beam backscatter diffraction of a work ferrite phase, a bainite phase, a polygonal ferrite phase, a pearlite phase, and a martensite-austenite mixed phase in a cross section perpendicular to the rolling direction at the 1/2 position in the plate thickness direction from the steel plate surface. Has an average particle diameter (diameter) of 30 μm or less when measured by the method,
Of the rolling surface, the {hkl} plane with respect to the rolling direction, and the texture strength ratio I _{{hkl}<uvw> in} the _<uvw> direction at the 1/2 position in the plate thickness direction from the steel plate surface, I _{001}<110> , I _{112}<110> , and I _{{332}<113> having} an average value of 2.0 or more.
<2>
The chemical composition is
Cu: 0.01% to 1.50%,
Ni: 0.01% to 2.50%,
Cr: 0.01% to 1.00%,
Mo: 0.01% to 1.00%,
V: 0.001% to 0.150%
B: 0.0001% to 0.0050%,
The steel sheet according to <1>, containing at least one selected from the group consisting of:
<3>
The chemical composition is
Mg: 0.0001% to 0.0100%,
Ca: 0.0001% to 0.0100%,
REM: 0.0001% to 0.0100%,
<1> or <2> which contains at least 1 sort(s) selected from the group which consists of.
<4>
The chemical composition is
Zr: 0.0001% to 0.0100%,
Te: 0.0001% to 0.0100%,
The steel sheet according to any one of <1> to <3>, which includes at least one selected from the group consisting of:
<5>
Carbon equivalent Ceq. represented by the following formula (1). Is a steel plate given in any 1 paragraph of <1>-<4> which is 0.30%-0.55%.
Formula (1) Ceq. =C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
(However, C, Mn, Cr, Mo, V, Cu, and Ni in the formula (1) represent the content (mass %) of each element contained in the steel sheet.)
<6>
A method for manufacturing the steel sheet according to any one of <1> to <5>, comprising:
A step of heating a steel slab having the chemical composition according to any one of <1> to <5> in a temperature range of 950°C to 1150°C.
A step of performing rough rolling on the heated billet in a temperature range where the surface temperature of the steel sheet is the recrystallization temperature _{Trex to} 1050°C and the cumulative reduction is 10% to 75%;
The temperature of the steel sheet after the rough rolling at a position of 1 mm from the steel sheet surface is 35° C. from the cooling start temperature of Ar ₃ point to 1050° C. to the cooling stop temperature of 500° C. to (Ar ₃ point −30° C.). /Sec to 100°C/sec, a primary cooling step at an average cooling rate;
A step of finish rolling the steel sheet after the primary cooling at a surface temperature of the steel sheet in a temperature range of 600° C. to 800° C. and a cumulative rolling reduction of 50% to 75%;
The temperature of the steel sheet after the finish rolling at the 1/4 position in the thickness direction from the surface of the steel sheet is 1°C/sec from the cooling start temperature of 600°C to 800°C to the cooling stop temperature of 0°C to 550°C. Secondary cooling at an average cooling rate of ~20°C/sec;
And a method for manufacturing a steel sheet.
<7>
Furthermore, the manufacturing method of the steel plate as described in <6> which has the process of tempering the steel plate after said secondary cooling in a 350-650 degreeC temperature range.

According to this embodiment, a steel plate having excellent arrestability and high strength is provided.

It is a schematic diagram which shows the relationship between the crystal orientation and material axis illustrated on ODF. It is a graph which shows the influence of the average value of texture strength ratio I _{001}<110> , I _{113}<110> , and I _{112}<110> which acts on arrestability. It is a graph which shows the influence of the average value of the texture intensity ratio I _{001}<110> , I _{112}<110> , and I _{{332}<113> which} affects arrestability.

Hereinafter, an example of a preferred embodiment of the present invention will be described in detail.
In addition, in this specification, the numerical range represented using "-" means the range which includes the numerical values described before and after "-" as a lower limit and an upper limit.
In the present specification, regarding the content of the component (element), for example, in the case of the content of C (carbon), it may be expressed as “C amount”. In addition, the contents of other elements may be similarly expressed.
In the present specification, the term “process” is used not only as an independent process, but also in the case where the intended purpose of the process is achieved even when it cannot be clearly distinguished from other processes. included.

Further, in the present specification, between a position of 1 mm from the steel plate surface and a position of 5 mm from the steel plate surface (between the position of 1 mm depth from the steel plate surface and the position of 5 mm depth from the steel plate surface, and the back surface of the steel plate). Between the position 1 mm deep and the position 5 mm deep from the back surface of the steel plate) may be referred to as "front and back layers".
In the present specification, the 1/4 position in the plate thickness direction from the steel plate surface of the cross section perpendicular to the rolling direction is "1/4 position", and the 1/4 position in the plate thickness direction from the steel plate surface of the cross section perpendicular to the rolling direction. The position may be referred to as a "1/2 position".

<Steel plate>
The steel sheet according to the present embodiment is, in mass %, C: 0.040% to 0.160%, Si: 0.01% to 0.50%, Mn: 0.70% to 2.50%, P: 0.030% or less, S: 0.020% or less, Nb: 0.003% to 0.050%, Ti: 0.003% to 0.050%, Al: 0.001% to 0.100%, N: 0.0010% to 0.0080%, and the balance has a chemical composition of Fe and impurities.
In addition, between the position of 1 mm from the steel plate surface and the position of 5 mm from the steel plate surface, the metallographic structure of the cross section parallel to the rolling direction has a worked ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite. Composed of austenite mixed phase. The respective area fractions are 30% to 90% of the worked ferrite phase, 10% to 60% of the bainite phase, 30% or less of the total of the polygonal ferrite phase, the pearlite phase, and the mixed phase of martensite/austenite.
Further, the aspect ratio of the worked ferrite phase in the section parallel to the rolling direction between the position 1 mm from the steel plate surface and the position 5 mm from the steel plate surface is 2.0 or more.
Between the position of 1 mm from the surface of the steel sheet and the position of 5 mm from the surface of the steel sheet, the processed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite-austenite mixed phase in a cross section perpendicular to the rolling direction The average particle diameter (diameter) is 25 μm or less as measured by electron beam backscattering diffraction method.
Then, between the position 1 mm from the steel plate surface and the position 5 mm from the steel plate surface, among the {hkl} plane and the texture strength ratio I _{{hkl}<uvw> in} the _<uvw> direction with respect to the rolling surface and the rolling direction _, The average value of I _{001}<110> , I _{113}<110> , and I _{112}<110> is 3.0 or more.
Electron beam backscatter diffraction of a work ferrite phase, a bainite phase, a polygonal ferrite phase, a pearlite phase, and a martensite-austenite mixed phase in a cross section perpendicular to the rolling direction at the 1/2 position in the plate thickness direction from the steel plate surface. The average particle size (diameter) measured by the method is 30 μm or less.
I _{{001} <110>} in the rolling surface, the {hkl} plane relative to the rolling direction and the texture strength ratio I _{{hkl}<uvw> in} the _<uvw> direction at the 1/2 position in the plate thickness direction from the steel plate surface. , I _{112}<110> , and I _{{332}<113> have} an average value of 2.0 or more.

Conventionally, steel sheets applied to ships and the like have been desired to have further improved arrestability in order to be applied to, for example, important members of super-large mega container ships.

It is known that grain refinement is important as a means for improving arrestability. Furthermore, in recent years, studies have been conducted from the viewpoint of texture control. Brittle cracks propagate on the cleavage plane of steel. Therefore, the arrestability of the steel material can be improved by controlling the texture so that the cleavage plane of the steel sheet does not coincide with the crack propagation direction. Specifically, a steel sheet is proposed in which the temperature at the center of the plate thickness and the rolling conditions are controlled to increase the degree of integration of the (100) plane in the texture parallel to the rolled surface (for example, Patent Document 1). And Patent Document 2). These are manufactured by increasing the rolling reduction in a specific temperature range in the vicinity of Ar ₃ which is the ferrite transformation start temperature during cooling. However, in the steel sheets disclosed in Patent Document 1 and Patent Document 2, the grain size most effective for improving the arrestability is not considered. Further, only the two-dimensional texture based only on the surface strength ratio is specified, and the three-dimensional texture obtained by combining the surface and the orientation is not considered. Therefore, it is difficult to dramatically improve the arrestability.

Patent Document 3 discloses a steel sheet that considers the texture and the crystal grain size at the 1/10 position in the sheet thickness direction from the surface of the steel sheet and in the center portion of the sheet thickness. In this steel sheet, since both the 1/10 position and the center portion of the plate thickness are taken into consideration, excellent arrestability can be obtained. Furthermore, there is room for further improvement of arrestability (particularly NDT temperature) by considering the texture and crystal grain size in the vicinity of the surface layer.

The steel plate disclosed in Patent Document 4 does not consider the inside of the plate thickness. Arrestability is a characteristic of total plate thickness. Therefore, if the metal structure inside the plate thickness is inappropriate, the arrestability may not be sufficiently improved.

As described above, it is known that refinement of crystal grains is effective for improving arrestability. However, the thickness and strength of arrested steel have been remarkably increased, and it has become difficult to obtain sufficient arrestability by simply refining crystal grains. One of the causes is, for example, the bainite phase existing in the front and back layers of the steel sheet. In order to secure high strength even within the plate thickness of the steel sheet, it is effective to apply strong cooling (strong water cooling) after rolling. By applying this strong cooling, the steel sheet has high strength, but a high hardness coarse bainite phase is formed in the front and back layers of the steel sheet.

Therefore, in order to improve the arrestability while paying attention to a high strength steel sheet, attention was paid to the worked ferrite phase. Due to the dislocation strengthening, the worked ferrite phase has the same strength as the bainite phase, and the texture can be expected to have superior arrestability.

The worked ferrite phase is a structure obtained by performing rolling in the two-phase region of the austenite phase and the polygonal ferrite phase, so that the polygonal ferrite phase undergoes rolling processing and extends in the rolling direction. When the polygonal ferrite phase undergoes processing, its strength increases due to dislocation strengthening. Then, a rolling texture develops in the process in which the polygonal ferrite phase is processed. At this time, a texture called α-fiber is developed in which the {001} plane is mainly parallel to the rolled steel sheet surface and the <110> direction is mainly composed of a crystal orientation parallel to the rolling direction. Therefore, when the arrestability that causes brittle cracks in the direction orthogonal to the rolling direction is evaluated, the {100} plane that is the cleavage plane of the BCC structure is not arranged in the crack initiation direction, Since they are arranged in a direction inclined by 45° with respect to the crack initiation direction, the propagation of brittle cracks is skewed in the direction of 45° with respect to the crack initiation direction, which is a resistance to crack propagation, and therefore arrestability is increased. improves. For these reasons, it is considered that both strength and arrestability are improved by forming a worked ferrite phase.

Further description will be given with reference to FIG. FIG. 1 is a schematic diagram showing the relationship between the crystal orientation of an ODF (Oriented distribution function) and the crystal plane of a steel material. Specifically, it represents the main crystal orientation of the worked ferrite that appears on the ODF in the φ ₂ =45° cross section. The {001}<110> orientation, {113}<110> orientation, and {112}<110> orientation shown in FIG. 1 are typical α-fiber textures. In a conventional steel sheet in which no work ferrite exists or very little work ferrite exists, textures of {001}<010> orientation, {110}<110> orientation, and {110}<001> orientation have , May be arranged in a direction (TD) orthogonal to the rolling direction. Since the brittle cracks are generated in the direction (TD) orthogonal to the rolling direction, it is considered that the conventional steel sheet has brittle cracks that are likely to travel straight and propagate and that the arrestability is inferior.

On the other hand, in the steel sheet according to the present embodiment, the average values of the texture strength ratios I _{001}<110> , I _{113}<110> , and I _{{112}<110> in} the front and back layers are 3. It is 0 or more. That is, in the steel sheet according to the present embodiment, since the α-fiber texture is developed in the front and back layers, the brittle crack generated in the direction orthogonal to the rolling direction is Easily propagates in the 45° direction and brittle cracks go straight and are difficult to propagate. As a result, in the evaluation of arrestability, the steel sheet according to the present embodiment is considered to have improved arrestability because the {100} plane that is the cleavage plane of the BCC structure is not arranged in the crack propagation direction. Further, at the 1/2 position, the average value of the texture intensity ratios I _{001}<110> , I _{112}<110> , and I _{332}<113> is 2.0 or more. That is, at the 1/2 position, {001}<110> orientation, {112}<110> orientation, and {332}<113> which are typical textures when the compression-processed austenite phase is transformed. It is considered that the brittle crack is difficult to go straight in the propagation direction because the texture of the orientation is developed, and the arrestability is improved.

In the present specification, processed ferrite means crystal grains having a GOS value of more than 4°. The work ferrite fraction means the area ratio of work ferrite in the metal structure. The processed ferrite fraction is quantified by a parameter GOS value (Grain Orientation Spread) measured by EBSP (Electron Back Scattering Pattern). The GOS value is a parameter indicating the average value of the orientation difference between a measurement point and another measurement point within the same crystal grain. When dislocations are introduced into the crystal grains by the two-phase rolling, it is considered that the GOS value increases because the crystal orientation difference is caused by the dislocations.

Furthermore, in utilizing the worked ferrite phase mainly in the front and back layers, the method for manufacturing the steel sheet according to the present embodiment was examined. By controlling the cooling conditions (primary cooling conditions) after rough rolling, the polygonal ferrite phase is sufficiently generated in the front and back layers. Then, the rolling process is applied to the polygonal ferrite phase by performing finish rolling in the two-phase region of the austenite phase and the polygonal ferrite phase. Further, by controlling the cooling conditions after the finish rolling (secondary cooling conditions), generation of coarse bainite phase is suppressed as much as possible. As a result, it was found that a steel sheet having a predetermined metallographic structure including the worked ferrite phase in the front and back layers was obtained.

From the above, with the steel sheet according to the present embodiment, a steel sheet having excellent arrestability and high strength can be obtained.

First, the reasons for limiting the chemical composition of the steel sheet according to this embodiment will be described.
In the following description, “%” in the description of each element means “mass %”.

(C: 0.040% to 0.160%)
C is an element necessary for ensuring strength. If the C content is less than 0.040%, the required strength of the steel sheet (hereinafter, also referred to as “base material”) cannot be secured. However, when the amount of C exceeds 0.160%, the arrestability and HAZ toughness are inferior. Therefore, the C content is 0.040% to 0.160%. The preferable lower limit of the amount of C is 0.050%, and the more preferable lower limit is 0.060%. The preferable upper limit of the amount of C is 0.140%, and the more preferable upper limit is 0.120%.

(Si: 0.01% to 0.50%)
Si is a deoxidizing element and is an element effective for solid solution strengthening. If the amount of Si is less than 0.01%, the effect of containing Si cannot be obtained. On the other hand, if the Si content exceeds 0.50%, the HAZ toughness becomes inferior. Therefore, the Si amount is set to 0.01% to 0.50%. The preferable lower limit of the amount of Si is 0.03%, and the more preferable lower limit is 0.05%. The preferable upper limit of the amount of Si is 0.40%, and the more preferable upper limit is 0.35%.

(Mn: 0.70% to 2.50%)
Mn is an effective element that improves the strength and arrestability of the base material. If the amount of Mn is less than 0.70%, the effect of containing Mn cannot be obtained. On the other hand, when the Mn content exceeds 2.50%, the HAZ toughness becomes inferior. Therefore, the Mn content is 0.70% to 2.50%. The preferable lower limit of the amount of Mn is 0.90%, and the more preferable lower limit is 1.20%. The preferable upper limit of the amount of Mn is 2.20%, and the more preferable upper limit is 2.00%.

(P: 0.030% or less)
P is present in the steel sheet as an impurity. However, when the amount of P becomes excessive, the arrestability and HAZ toughness become inferior. Therefore, the upper limit of the amount of P is set to 0.030%. The preferable upper limit of the amount of P is 0.020%, and the more preferable upper limit is 0.010%. Since the smaller the amount of P, the more preferable, the lower limit is not particularly limited, and may be 0.001% or more from the viewpoint of manufacturing cost.

(S: 0.020% or less)
S is present in the steel sheet as an impurity. However, if the amount of S becomes excessive, a large amount of inclusions of sulfides and oxysulfides will be formed, and the arrestability and HAZ toughness will be inferior. Therefore, the upper limit of the amount of S is set to 0.020%. The preferable upper limit of the amount of S is 0.010%, and the more preferable upper limit is 0.005%. Since the smaller the amount of S, the more preferable, the lower limit is not particularly limited, and may be 0.001% or more from the viewpoint of manufacturing cost.

(Nb: 0.003% to 0.050%)
Nb is an element that suppresses recrystallization, contributes to the refinement of the structure by adding a trace amount, and is effective in securing the strength and arrestability of the base material. If the amount of Nb is less than 0.003%, the effect of containing Nb cannot be obtained. If the amount of Nb exceeds 0.050%, the HAZ toughness becomes inferior. Therefore, the amount of Nb is set to 0.003% to 0.050%. A preferable lower limit of the Nb amount is 0.005%, and a more preferable lower limit thereof is 0.008%. The preferable upper limit of the amount of Nb is 0.035%, and the more preferable upper limit is 0.025%.

(Ti: 0.003% to 0.050%)
Ti is an element that contributes to the improvement of arrestability and HAZ toughness by refining the microstructures of the base material and the welded portion by adding a trace amount. Further, Ti also functions as a deoxidizing element. On the other hand, if Ti is excessively added, the welded portion is hardened, the toughness is remarkably deteriorated, and the HAZ toughness becomes inferior. Therefore, the Ti amount is 0.003% to 0.050%. A preferable lower limit of the Ti amount is 0.006%, and a more preferable lower limit thereof is 0.010%. The preferable upper limit of the Ti amount is 0.035%, and the more preferable upper limit is 0.020%.

(Al: 0.001% to 0.100%)
Since Al is a deoxidizing element, the Al content is 0.001% or more. On the other hand, if Al is added excessively, the surface quality of the steel slab is impaired and inclusions harmful to the arrestability and HAZ toughness are formed, so the upper limit of the Al content is made 0.100%. Therefore, the amount of Al is set to 0.001% to 0.100%. The preferable lower limit of the amount of Al is 0.010%, and the more preferable lower limit is 0.015%. The preferable upper limit of the amount of Al is 0.080%, and the more preferable upper limit is 0.050%.

(N: 0.0010% to 0.0080%)
N forms a nitride with Ti and Al to improve the joint toughness, so the lower limit of the N content is made 0.0010%. However, when the content of N is excessive, the arrestability due to the solid solution N and the elongation of the base material decrease, so the upper limit of the amount of N is set to 0.0080%. Therefore, the N amount is set to 0.0010% to 0.0080%. The preferable lower limit of the amount of N is 0.0015%, and the more preferable lower limit is 0.0020%. The preferable upper limit of the amount of N is 0.0070%, and the more preferable upper limit is 0.0060%.

Furthermore, the steel sheet according to the present embodiment may contain the following elements in mass %. The following elements are arbitrary elements and may not be contained (that is, the content may be 0%). Specifically, at least one selected from the group consisting of Cu, Ni, Cr, Mo, V, and B may be included. These elements are elements that contribute to high strength. It may contain at least one selected from the group consisting of Mg, Ca, and REM. These elements are elements that contribute to the effect of suppressing inclusion formation. It may contain at least one selected from the group consisting of Zr and Te. These elements are elements that contribute to the improvement of toughness.

(Cu 0.01% to 1.50%)
Cu is an element that improves the hardenability and is effective in increasing the strength of the base material. Therefore, Cu may be contained. However, if the Cu content exceeds 1.50%, the toughness decreases as the hardness of the joint increases. In order to obtain the effect of the above action more reliably, the lower limit of the amount of Cu is preferably set to 0.01%. Therefore, when Cu is contained, the Cu amount is 0.01% to 1.50%. A more preferable upper limit of the Cu amount is 0.80%, and a further preferable upper limit thereof is 0.50%. A more preferable lower limit of the Cu amount is 0.05%, and a further preferable lower limit thereof is 0.10%.

(Ni: 0.01% to 2.50%)
Ni is an element effective in improving the strength and arrestability of the base material. Therefore, Ni may be contained. However, even if the Ni content exceeds 2.50%, the effect of containing Ni is saturated and the cost increases. In order to more reliably obtain the effect of the above action, the lower limit of the Ni content is preferably 0.01% or more. Therefore, when Ni is contained, the Ni content is 0.01% to 2.50%. A more preferable upper limit of the Ni amount is 1.50%, and a further preferable upper limit is 1.00%. A more preferable lower limit of the Ni content is 0.10%, and a still more preferable lower limit thereof is 0.20%.

(Cr: 0.01% to 1.00%)
Cr is an element that improves the hardenability and is effective in increasing the strength of the base material. Therefore, Cr may be contained. However, if the Cr content exceeds 1.00%, the toughness decreases as the hardness of the joint increases. In order to obtain the effect of the above action more reliably, the lower limit of the Cr amount is preferably 0.01%. Therefore, when Cr is contained, the Cr amount is 0.01% to 1.00%. A more preferable upper limit of the amount of Cr is 0.80%, and a still more preferable upper limit thereof is 0.60%. A more preferable lower limit of the amount of Cr is 0.05%, and a further preferable lower limit thereof is 0.10%.

(Mo: 0.01% to 1.00%)
Mo is an element that improves the hardenability and is effective in increasing the strength of the base material. Therefore, Mo may be contained. However, if the Mo content exceeds 1.00%, the toughness decreases as the hardness of the joint increases. In order to obtain the effect of the above action more reliably, the lower limit of the amount of Mo is preferably 0.01%. Therefore, when Mo is contained, the Mo content is 0.01% to 1.00%. A more preferable upper limit of the Mo amount is 0.60%, and a further preferable upper limit is 0.40%. A more preferable lower limit of the Mo content is 0.05%, and a further preferable lower limit thereof is 0.10%.

(V: 0.001% to 0.150%)
V is an element that contributes to the strength increase of the base material by precipitation strengthening. Therefore, V may be contained. However, if the V content exceeds 0.150%, the joint toughness is impaired. In order to more reliably obtain the effect of the above action, the lower limit of the V amount is preferably 0.001%. Therefore, when V is contained, the V content is 0.001% to 0.150%. A more preferable upper limit of the V amount is 0.100%, and a further preferable upper limit is 0.080%. A more preferable lower limit of the V amount is 0.010%, and a still more preferable lower limit thereof is 0.020%.

(B: 0.0001% to 0.0050%)
B is an element that enhances the hardenability by adding a trace amount and contributes to the strength improvement of the base material. Therefore, B may be contained. If the B content exceeds 0.0050%, the arrestability and HAZ toughness deteriorate. In order to obtain the effect of the above action more reliably, the lower limit of the amount of B is preferably 0.0001%. Therefore, when B is contained, the B content is 0.0001% to 0.0050%. A more preferable upper limit of the amount of B is 0.0040%, and a further preferable upper limit is 0.0030%. A more preferable lower limit of the amount of B is 0.0005%, and a further preferable lower limit thereof is 0.0010%.

(Mg: 0.0001% to 0.0100%)
Mg is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions by forming sulfides and forms fine oxides to suppress the formation of harmful inclusions. Therefore, Mg may be contained. However, if the amount of Mg exceeds 0.0100%, coarse oxides, sulfides, and oxysulfides are easily formed, and the arrestability and HAZ toughness deteriorate. The lower limit of the amount of Mg is preferably 0.0001% in order to obtain the effect of the above action more reliably. Therefore, when Mg is contained, the amount of Mg is set to 0.0001% to 0.0100%. A more preferable upper limit of the amount of Mg is 0.0070%, and a further preferable upper limit thereof is 0.0050%. A more preferable lower limit of the amount of Mg is 0.0005%, and a further preferable lower limit thereof is 0.0010%.

(Ca: 0.0001% to 0.0100%)
Ca is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions by forming sulfides, forms fine oxides, and suppresses the formation of harmful inclusions. Therefore, Ca may be contained. However, when the Ca content exceeds 0.0100%, coarse oxides, sulfides, and oxysulfides are easily formed, and the arrestability and HAZ toughness deteriorate. In order to obtain the effect of the above action more reliably, the lower limit of the Ca amount is preferably 0.0001%. Therefore, when Ca is contained, the amount of Ca is 0.0001% to 0.0100%. A more preferable upper limit of the amount of Ca is 0.0070%, and a further preferable upper limit thereof is 0.0050%. A more preferable lower limit of the amount of Ca is 0.0005%, and a still more preferable lower limit thereof is 0.0010%.

(REM: 0.0001% to 0.0100%)
REM is a deoxidizing element, and is an element that suppresses the formation of coarse inclusions by forming sulfides and forms fine oxides to suppress the formation of harmful inclusions. Therefore, REM may be included. However, if the REM content exceeds 0.0100%, coarse oxides, sulfides, and oxysulfides are easily formed, and the arrestability and HAZ toughness deteriorate. In order to obtain the effect of the above action more reliably, the lower limit of the REM amount is preferably 0.0001%. Therefore, when REM is contained, the REM content is 0.0001% to 0.0100%. A more preferable upper limit of the REM amount is 0.0070%, and a further preferable upper limit is 0.0050%. A more preferable lower limit of the REM amount is 0.0005%, and a further preferable lower limit is 0.0010%.
Here, "REM" is a general term for a total of 17 elements of Sc, Y, and lanthanoid. The REM may include one or more elements out of a total of 17 elements. The content of REM refers to the total content of these elements.

(Zr: 0.0001% to 0.0100%)
Zr is an element that contributes to the improvement of toughness by refining the microstructures of the base material and the welded portion by adding a trace amount. Zr also functions as a deoxidizing element. Therefore, Zr may be contained. If the amount of Zr exceeds 0.0100%, the HAZ toughness will decrease. In order to obtain the effect of the above action more reliably, the lower limit of the Zr amount is preferably 0.0001%. Therefore, when Zr is contained, the Zr content is 0.0001% to 0.0100%. A more preferable upper limit of the Zr amount is 0.0070%, and a further preferable upper limit thereof is 0.0050%. A more preferable lower limit of the Zr amount is 0.0005%, and a further preferable lower limit thereof is 0.0010%.

(Te: 0.0001% to 0.0100%)
Te is an element that contributes to the improvement of toughness due to the refinement of the structure. Therefore, Te may be contained. Even if the Te amount exceeds 0.0100%, the effect due to the above-mentioned action is saturated. In order to obtain the effect of the above action more reliably, the lower limit of the Te amount is preferably 0.0001%. Therefore, when Te is contained, the Te content is 0.0001% to 0.0100%. A more preferable upper limit of the Te amount is 0.0070%, and a further preferable upper limit thereof is 0.0050%. The more preferable lower limit of the amount of Te is 0.0005%, and the more preferable lower limit thereof is 0.0010%.

(The rest)
The balance is Fe and impurities. Impurity refers to a component contained in a raw material or a component mixed in a manufacturing process and not intentionally contained in a steel sheet.

(Carbon equivalent Ceq.: 0.30% to 0.55%)
The steel sheet according to the present embodiment has a carbon equivalent Ceq. Is preferably 0.30% to 0.55%.
Formula (1) Ceq. =C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
However, C, Mn, Cr, Mo, V, Cu, and Ni in the formula (1) represent the content (mass %) of each element contained in the steel sheet.
In addition, when there is an element with a content of 0 mass %, 0 mass% is substituted as the content of the corresponding element in the formula (1) for calculation.

If the carbon equivalent is less than 0.30%, it becomes difficult to satisfy the required strength characteristics (tensile strength, yield stress) of the steel sheet as the base material. If the carbon equivalent exceeds 0.55%, it becomes difficult to improve the arrestability and HAZ toughness. The lower limit of the carbon equivalent is more preferably 0.35%, and further preferably 0.40%. The upper limit of the carbon equivalent is more preferably 0.52%, further preferably 0.50%.

[Metal structure of front and back layers]
Next, the reasons for limiting the metal structure (microstructure) in the front and back layers of the steel sheet according to this embodiment will be described.

(Processed ferrite phase: 30% to 90%)
The worked ferrite phase contributes to arrestability. If the area of the worked ferrite phase exceeds 90%, the strength of the base material may be insufficient. On the other hand, if the area fraction of the worked ferrite phase is less than 30%, the arrestability becomes poor. Therefore, the area fraction of the ferrite phase is set to 30% to 90%. The preferable upper limit of the area fraction of the worked ferrite phase is 85%, and the more preferable upper limit thereof is 80%. The preferable lower limit of the area fraction of the worked ferrite phase is 35%, and the more preferable lower limit thereof is 40%.

(Bainite phase: 10% to 60%)
The bainite phase mainly contributes to the strength of the base material. If the area fraction of the bainite phase is less than 10%, the strength of the base material becomes poor. On the other hand, when the area fraction of the bainite phase exceeds 60%, the arrestability becomes poor. Therefore, the area fraction of the bainite phase is set to 10% to 60%. The preferable upper limit of the area fraction of the bainite phase is 55%, and the more preferable upper limit thereof is 50%. Moreover, the preferable lower limit of the area fraction of the bainite phase is 15%, and the more preferable lower limit thereof is 20%.

(Total of polygonal ferrite phase, pearlite phase, and martensite/austenite mixed phase: 30% or less)
When the total area fraction of the polygonal ferrite phase, the pearlite phase, and the martensite/austenite mixed phase (MA phase) exceeds 30%, the strength of the base material is reduced or the brittleness is caused by the polygonal ferrite phase which is a soft phase. The arrestability due to the pearlite phase and the MA phase, which are the phases, becomes remarkable. Therefore, the total area fraction of each of these phases (polygonal ferrite phase, pearlite phase, and MA phase) is set to 30% or less from the viewpoint of ensuring both strength and arrestability of the base material. The preferable upper limit of the total area fraction is 25% or less. The total area fraction of these is preferably as small as possible, and the lower limit value is not particularly limited. For example, the total area fraction of each of these phases may be 0%. Further, it may be more than 0% or 1% or more.

(Processed ferrite phase aspect ratio: 2.0 or more)
When the aspect ratio of the processed ferrite phase in the front and back layers is 2.0 or more, arrestability is improved. The aspect ratio of the processed ferrite phase in the front and back layers is preferably 2.5 or more, more preferably 3.0 or more. The upper limit of the aspect ratio of the processed ferrite phase in the front and back layers is not particularly limited, and may be 10.0 or less, for example. When the processed ferrite phase in the front and back layers has an area fraction of 30% to 90% and an aspect ratio of 2.0 or more, improvement in arrestability is improved.
Here, in the present specification, the aspect ratio of the processed ferrite phase in the front and back layers is, in one processed ferrite phase, the maximum length of the major axis and the maximum length of the minor axis in the minor axis direction orthogonal to the major axis direction. Is represented by the ratio (maximum major axis length/maximum minor axis length). Since the worked ferrite phase has a shape extending in the rolling direction, the length of the major axis is the length extending in the rolling direction, and the minor axis is the length in the plate thickness direction (so-called ND direction). The aspect ratio is measured using a sample having a cross section (so-called L cross section) parallel to the rolling direction of the steel sheet, which is measured when measuring the area fraction of each phase in the front and back layers. The samples in the front and back layers are subjected to nital etching, and the GOS value is obtained by EBSP for the part that appears white by the nital etching, and the part where the GOS value exceeds 4° is processed as a processed ferrite phase to be obtained by image analysis. The image analysis may be performed by, for example, performing 8 or more visual fields having an area of 0.05 mm ² or more (total 0.40 mm ² or more) and obtaining an average value.

(Average grain size (diameter) of the processed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite/austenite mixed phase: 25 μm or less)
The refinement of the structure of all phases of the worked ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and MA phase contributes to the improvement of arrestability. The average particle size (diameter) of each phase (processed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite/austenite mixed phase) in the front and back layers measured by electron beam backscattering diffraction method is 25 μm. By being the following, arrestability improves. The average particle diameter of each phase in the front and back layers is preferably small, and the lower limit value is not particularly limited. For example, the average particle size of each phase in the front and back layers may be 1 μm or more, or 5 μm or more. The method for measuring the average particle size of each phase in the front and back layers will be described later.

(Average value of I _{001}<110> , I _{113}<110> , and I _{112}<110> : 3.0 or more)
Among the {hkl} planes and the texture strength ratio I _{{hkl}<uvw> in} the _<uvw> direction with respect to the rolling surface and the rolling direction in the front and back layers, I _{001}<110> , I _{113}<110> , and When the average value of I _{112}<110> (hereinafter sometimes referred to as “texture strength ratio A”) is 3.0 or more, arrestability is improved. The texture strength ratio A represents the average value of the strength ratio of the texture when the polygonal ferrite phase is processed, and represents three representative textures of α-fiber. The texture strength ratio A of 3.0 or more indicates that the α-fiber is increasing. As described above, in the α-fiber evaluation of the arrestability, the {100} plane which is the cleavage plane of the BCC structure is not arranged in the crack propagation direction. Therefore, the higher the texture strength ratio A, the better the arrestability. The texture strength ratio A is preferably 3.3 or more, more preferably 3.5 or more. The upper limit of the texture intensity ratio A is not particularly limited, and may be 10.0 or less, for example. The method for measuring the texture strength ratio A will be described later.

[Metal structure at 1/2 position]
Next, the reason for limiting the metal structure (microstructure) at the 1/2 position in the plate thickness direction of the steel sheet according to this embodiment will be described.

(Average grain size (diameter) of the processed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite/austenite mixed phase: 30 μm or less)
Even at the 1/2 position, the refinement of the structure of all phases of the worked ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and MA phase contributes to the improvement of arrestability. Average particle size (diameter) of each phase at 1/2 position (worked ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite-austenite mixed phase) measured by electron backscatter diffraction Is 30 μm or less, the arrestability is improved. The average particle diameter of each phase in the front and back layers is preferably small, and the lower limit value is not particularly limited. For example, the average particle size of each phase in the front and back layers may be 1 μm or more, or 5 μm or more.

(Average value of I _{001}<110> , I _{112}<110> , and I _{332}<113> : 2.0 or more)
Of the {hkl} plane with respect to the rolling surface and the rolling direction at the 1/2 position and the texture strength ratio I _{{hkl}<uvw> in} the <uvw> direction, I _{001}<110> , I _{112}<110> , And the average value of I _{332}<113> (hereinafter sometimes referred to as “texture strength ratio B”) is 2.0 or more, the arrestability is improved. The texture strength ratio B represents an average value of strength ratios of typical transformation textures when the austenite phase subjected to compression processing is transformed. Not only does the texture in the front and back layers be 3.0 or more, but the texture strength ratio B at the 1/2 position is 2.0 or more, the arrestability is improved. The texture strength ratio B is preferably 2.3 or more, more preferably 2.5 or more. The upper limit of the texture strength ratio B is not particularly limited, and may be 10.0 or less, for example.

From the viewpoint of a steel plate having high strength and excellent arrestability, it is desirable that the above-mentioned average particle size of each phase in the front and back layers is smaller than the average particle size of each phase at the 1/2 position. From the same viewpoint, the texture strength ratio A in the front and back layers is preferably larger than the texture strength ratio B at the 1/2 position.

-Method of measuring area fraction of each phase in front and back layers-
The method for measuring the area fraction of each phase in the front and back layers is as follows.
Samples for measurement are taken on the front and back layers. For each of the collected samples, the metal structure of a cross section (so-called L cross section) parallel to the rolling direction of the steel plate is photographed by an optical microscope, and the image is analyzed.
Specifically, first, a sample is sampled in the front and back layers of the cross section in the direction parallel to the rolling direction of the steel sheet, which is the quarter position from the end surface in the width direction of the steel sheet.
Next, the collected sample is subjected to nital etching, and after etching, 8 fields of view of the cross section in the L direction are photographed at a magnification of 500 using an optical microscope. Then, the obtained tissue photograph is binarized by image analysis software to perform image analysis. Obtain the area ratio of the white-looking phase as the processed ferrite phase or polygonal ferrite phase, the black-looking phase as the pearlite phase, the gray-looking phase as the bainite phase, or the MA phase (martensite-austenite mixed phase). ..
Next, the portion subjected to nital etching was measured by EBSP, the GOS value was obtained for the portion that appeared white by nital etching, and the portion where the GOS value exceeded 4° was defined as the area ratio of the processed ferrite phase. The area of 4° or less is the area ratio of the polygonal ferrite phase.
Further, the portion subjected to the nital etching is repeller-etched, and the portion that appears gray by the nital etching is subjected to image analysis, and the white portion is determined as the MA phase (martensite/austenite mixed phase) to determine the area ratio.
Then, the area ratio of the bainite phase is obtained by subtracting the area ratio of the MA phase (mixed phase of martensite/austenite) from the area ratio of gray that appears after nital etching.
The total area fraction of the processed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and MA phase is 100%.

-Measurement method of average particle size-
The method for measuring the average particle size of each phase at the front and back layers and at the 1/2 position is as follows.
At the front and back layers and the 1/2 position, samples for measurement of a cross section (so-called C cross section) perpendicular to the rolling direction of the steel sheet are taken. The average grain size of all phases is measured by EBSP, which enables accurate measurement of crystal orientation information in a wide field of view. If EBSP is used, it is also possible to measure the crystal grain size of a complex structure such as a bainite phase. Specifically, it is measured by the following method. The EBSP is used to repeatedly measure a 500 μm×500 μm region of the front and back layers while moving the measurement position by 1 μm. The 1/2 position is also measured in the same manner. Here, a boundary having a crystal orientation difference of 15° or more with an adjacent grain is defined as a crystal grain boundary, and a weighted average value of equivalent circle diameters (diameters) surrounded by the crystal grain boundary is obtained. And
Specifically, the weighted average value is calculated by the following formula (D1), formula (D2) and formula (D3).
Formula _{(D1) p = (D MAX} -D MIN) / N
Formula (D2) D _k =D _MIN +p×(k−1/2)
Formula (D3) D _AVE =(Σ[k=1, N](D _k ×S _k ))/(Σ[k=1, N]S _k )

Here, D _MAX is the maximum grain size in the equivalent circle diameter (diameter) of crystal grains, and D _MIN is the minimum grain size in the equivalent circle diameter (diameter) of crystal grains. is there. N represents the number of divisions when calculating the weighted average, and the weighted average is obtained using a value of 10 or more. The difference between the maximum grain size D _MAX and the minimum grain size D _MIN (width of all grain size distributions) is divided by the number N of divisions to calculate p, which is the width of the division range, by the formula (D1). can do.
D _k is the median value of the crystal grain size in the k-th division range, and k can be obtained by the equation (D2) using an integer of 1 or more and the division number N or less. S _k is the area of the crystal grains in the k-th divided range, measured using an electron backscattering diffraction method (EBSP method), (D _MIN +p×(k−1)) μm or more, (D _MIN +p Xk) represents the total area of crystal grains having an equivalent circle diameter of less than μm.
The weighted average value D _AVE is, as in equation (D3), the median value D _k of the crystal grain sizes in the k-th divided range and the total area S of the crystal grains having the crystal grain size in the k-th divided range. a value obtained by multiplying a _k, (DK × SK) calculates, between what k is the total of (D _K × S _K) between up to division number N from 1, k from 1 to the number of divisions N It is divided by the sum of SK. That is, the weighted average value can be calculated by dividing the sum of D ₁ ×S _{1 to} D _N ×S _N by the sum of S _{1 to SN} .

-Method of measuring texture strength ratio-
The method for measuring the texture strength ratio A in the front and back layers and the texture strength ratio B at the 1/2 position is as follows.
The texture strength ratio is analyzed by EBSP in order to clarify the influence of the texture of the steel sheet on the arrestability using the steel sheets manufactured by various processes. The measurement surface is a section (so-called C section) perpendicular to the rolling direction of the steel sheet with the rolling direction (RD) as the normal, and the measurement positions are the front and back layers and the 1/2 position. The measurement was performed in a region of 1.5 mm×1 mm at 3 μm intervals, a crystal orientation distribution function (Crystallite Orientation Distribution Function; ODF) was created, and then the ratio of a specific texture intensity to a random intensity was read. In the steel sheet according to this embodiment, software "OIM Analysis (ver. 7)" manufactured by TSL Solutions Co., Ltd. was used for analysis of texture.

[Physical properties of steel sheet]
The plate thickness of the steel sheet according to the present embodiment is not particularly limited and may be, for example, 50 mm or more, and further may be 50 mm to 100 mm.

The tensile strength (TS) of the steel sheet according to the present embodiment is not particularly limited and may be 510 MPa or more (preferably 510 MPa to 720 MPa, more preferably 570 MPa to 720 MPa) in terms of high strength. Further, the yield stress (YP) is preferably 390 MPa or more (preferably 390 MPa to 650 MPa, more preferably 460 MPa to 650 MPa).
Here, the tensile strength (TS) of the steel sheet according to this embodiment is measured using a No. 1B tensile test piece of JIS Z 2241 (2011). The yield stress (YP) means the proof stress of the permanent elongation method when the permanent elongation of JIS Z 2241 (2011) is 0.2%.

Further, the steel sheet according to the present embodiment is a temperature gradient type ESSO test in which the brittle crack propagation arrest toughness value K _ca at a test temperature of −10° C. (hereinafter referred to as “arrest toughness value K _ca −10° _C. ”) there.) is, 6000 N ^{/ mm 1.5} or more (preferably, may be a 8000 N ^{/ mm 1.5} or higher). By satisfying this property, the steel sheet has excellent arrestability.
The arrest toughness value K _{ca −10° C.} is determined by the NK Classification Society Steel Ship Regulations Inspection Guide, K Edition, Appendix K 3.12.2-1. (2016) The measurement is performed in accordance with "Inspection procedure for temperature gradient type ESSO test and temperature gradient type double tensile test".

Further, the steel sheet according to the present embodiment may have a non-ductile transition temperature in the NRL drop weight test of −80° C. or lower (preferably −85° C. or lower, more preferably −100° C. or lower). By satisfying this property, the steel sheet has excellent arrestability.
The non-ductile transition temperature (NDT temperature; Nil-Ductility-Transition Temperature) is determined by performing a test in accordance with the NRL (Naval Research Laboratory) drop weight test method defined by ASTM E208-06. The test piece is a P-3 type (T: 16 mm, L: 130 mm, W: 50 mm), and is sampled up to a position of 16 mm in the plate thickness direction so as to include the outermost surface of the steel plate. The test piece is taken in the rolling direction (L direction), a welding bead is provided in the L direction on the outermost surface of the test piece, and a notch is provided as a crack starter in the direction perpendicular to the rolling direction (C direction).

In the steel sheet according to the present embodiment, the chemical composition, the front and back layers, and the aspect of the metallographic structure at the 1/2 position are controlled so as to satisfy the above-mentioned conditions, and for example, the sheet thickness is 50 mm or more ( For example, even if it is 50 mm to 100 mm), the tensile strength (TS), the yield stress (YP), the arrest toughness value K _{ca-10° C.} , and the non-ductile transition temperature (NDT temperature) are all within the above range. Can be satisfied.

<Steel plate manufacturing method>
Next, an example of a preferable manufacturing method for obtaining the steel sheet according to the present embodiment will be described.

The steel sheet according to this embodiment can be obtained, for example, by a manufacturing method including the following steps.
A step of heating a steel slab having the aforementioned chemical composition in a temperature range of 950° C. to 1150° C. (heating step),
A step of performing rough rolling on the heated billet at a surface temperature of the steel sheet in a temperature range of recrystallization temperature _{Trex to} 1050°C and a cumulative reduction rate of 10% to 75% (rough rolling step);
The temperature of the steel sheet after the rough rolling at a position of 1 mm from the steel sheet surface is 35° C. from the cooling start temperature of Ar ₃ point to 1050° C. to the cooling stop temperature of 500° C. to (Ar ₃ point −30° C.). /Second to 100°C/second, a step of primary cooling at an average cooling rate (primary cooling step),
A step of finishing rolling the steel sheet after the primary cooling with a surface temperature of the steel sheet in a temperature range of 600° C. to 800° C. and a cumulative rolling reduction of 50% to 75% (finish rolling step);
The temperature of the steel sheet after the finish rolling at the 1/4 position in the plate thickness direction from the steel sheet surface is 1°C/sec from the cooling start temperature of 600°C to 800°C to the cooling stop temperature of 0°C to 550°C. A step of secondary cooling at an average cooling rate of -20°C/second (secondary cooling step).

(Heating process)
This is a step of heating a steel slab having the aforementioned chemical composition at a predetermined temperature. This step is a step that contributes to the structure control of the austenite phase by heating the steel slab. A steel piece having a predetermined chemical composition is heated in a temperature range of 950°C to 1150°C. By setting the heating temperature of the steel piece to 950° C. or higher, austenitization becomes sufficient, and a fine austenite phase can be obtained. On the other hand, by setting the heating temperature of the steel slab to 1150° C. or less, coarsening of the austenite phase is suppressed, and a fine austenite phase is obtained.

(Rough rolling process)
In the rough rolling, the heated steel slab is applied such that the surface temperature of the steel slab (steel plate) is in the temperature range of the recrystallization temperature _{Trex to} 1050°C and the cumulative rolling reduction is in the range of 10% to 75%.
Under the above conditions, the energy of dislocations generated by rolling causes the austenite phase to recrystallize, whereby the austenite phase becomes finer and a steel sheet with improved arrestability can be obtained.

If the temperature of the rough rolling is less than _Trex , recrystallization of the austenite phase does not occur, so that the austenite phase cannot be miniaturized. Further, if the temperature of the rough rolling exceeds 1050° C., the grain size of the austenite phase after recrystallization becomes large and the grain growth after recrystallization is promoted, so that a fine austenite phase cannot be obtained.

That is, as in the above conditions, the austenite phase is finely controlled by performing the rough rolling at a relatively low temperature while ensuring the rolling reduction for recrystallizing the austenite phase. This improves the arrestability of the steel sheet.

Here, the cumulative rolling reduction in rough rolling is obtained by the following equation (R1).
Formula (R1) Cumulative rolling reduction in rough rolling = {(plate thickness before rough rolling-plate thickness after rough rolling)/plate thickness before rough rolling} x 100
The plate thickness before rough rolling is a plate before rolling when the surface temperature of the steel plate (steel plate) is rolled in a temperature range of recrystallization temperature _{Trex to} 1050°C after the steel plate is extracted from the heating furnace. It is the maximum thickness.
The plate thickness after rough rolling is the minimum value of the plate thickness after rolling when the surface temperature of the steel slab (steel plate) is rolled in the temperature range of recrystallization temperature _{Trex to} 1050°C.

(Primary cooling process)
In the primary cooling, the temperature of the steel plate after rough rolling at a position of 1 mm from the steel plate surface is from the cooling start temperature of Ar ₃ point to 1050° C. to the cooling stop temperature of 500° C. to (Ar ₃ point −30° C.). ,Cooling. The average cooling speed during cooling is 35°C/sec to 100°C/sec. As a condition for the primary cooling, as described above, a high cooling rate and a high cooling stop temperature are used to sufficiently secure the polygonal ferrite phase before finish rolling. Ensuring a sufficient polygonal ferrite phase in the primary cooling step is an important step in producing a worked ferrite phase in finish rolling. The cooling rate and the cooling stop temperature are the calculated cooling rate and the calculated cooling stop temperature at a position 1 mm from the surface of the steel sheet. The cooling rate is an average cooling rate during cooling.

(Finishing rolling process)
The finish rolling is performed such that the surface temperature of the steel sheet is in the temperature range of 600°C to 800°C and the cumulative rolling reduction is 50% to 75%. By performing finish rolling in the temperature range where the surface temperature of the steel sheet is 600° C. to 800° C., the polygonal ferrite phase produced in the primary cooling step is subjected to rolling, and a worked ferrite phase elongated in the rolling direction is produced.

Further, when the cumulative rolling reduction is 50% or more, strain can be efficiently accumulated in the worked ferrite phase, which contributes to the development of texture and refinement of each phase of the metal structure. On the other hand, if the cumulative rolling reduction exceeds 75%, rolling may be difficult due to the facility capacity, or productivity may be reduced due to the rolling temperature adjustment time or the like. Therefore, the cumulative rolling reduction is 50% to 75% (preferably 55% to 70%).

Here, the cumulative reduction in finish rolling is represented by the following formula (R2).
Formula (R2) Cumulative reduction ratio of finish rolling = {(plate thickness after rough rolling-plate thickness after finish rolling)/plate thickness after rough rolling} x 100
The plate thickness after the rough rolling is the same as the plate thickness when the finish rolling is started.

Here, in the present specification, Ar ₃ is represented by the following formula (T1), and the recrystallization temperature T _rex is represented by the following formula (T2).
Formula _{(T1) Ar 3 = 910-310 [} C] +65 [Si] -80 [Mn] -20 [Cu] -55 [Ni] -15 [Cr] -80 [Mo]
Equation ^{_{(T2) T rex = -91900 [}} Nb *] 2 +9400 [Nb *] + 770
(However, in the formula (T2), [Nb*] is represented by the following formula (T3-1) and the following formula (T3-2).
Formula (T3-1) [Nb]≧[Sol. In the case of Nb], [Nb*]=[Sol. Nb]
Formula (T3-2) [Nb]<[Sol. In the case of Nb], [Nb*]=[Nb]
Here, [Nb] represents the Nb content (mass %), and [Sol. Nb] is Sol. It represents Nb (solid solution Nb) (mass %).
Formula (T4) Sol. Nb=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N)
In the formula (T4), T is the heating temperature of the steel slab, the unit is Celsius temperature (°C), and C and N are the contents of C and N (mass %), respectively. )

(Secondary cooling process)
Next, the steel sheet after the finish rolling is cooled at a predetermined cooling rate to a predetermined temperature (secondary cooling). By cooling under this condition, the metal structure is refined and the structure morphology (area fraction of all phases described above) is controlled. This process affects the strength and arrestability of the base material.

In the secondary cooling step, the steel plate after finish rolling is cooled from the cooling start temperature of 600°C to 800°C to the cooling stop temperature of 0°C to 550°C at the 1/4 position in the plate thickness direction of the steel plate. To do. At that time, the average cooling rate at the 1/4 position is 1° C./sec to 20° C./sec.

The cooling method is not particularly limited, and examples thereof include water cooling. From the cooling start temperature of 600°C to 800°C at the 1/4 position to the cooling stop temperature of 0°C to 550°C, the cooling rate at the 1/4 position is 1°C/sec to 20°C/sec. A predetermined amount of metallographic structure is obtained by starting the temperature at the 1/4 part position of the steel sheet from a temperature of 600°C or higher and cooling it to a temperature of 550°C or lower.
The cooling rate and the cooling stop temperature are the calculated cooling rate and the calculated cooling stop temperature at the 1/4 position. The cooling rate is an average cooling rate during cooling.

A preferable manufacturing method for obtaining the steel sheet according to the present embodiment further includes a step (heat treatment step) of performing tempering heat treatment on the steel sheet after the cooling step in a temperature range of 350°C to 650°C, if necessary. May be.

(Heat treatment process)
After cooling the steel sheet, if necessary, tempering heat treatment may be performed in the temperature range of 350°C to 650°C (preferably 450°C to 550°C) to adjust the strength and toughness of the steel sheet. When the tempering heat treatment is performed, if the temperature of the heat treatment is 350° C. or higher, the effect of improving the arrestability by removing the strain is enhanced. On the other hand, if the temperature of the heat treatment exceeds 650°C, the strength may decrease.

In addition, the manufacturing method of the steel plate according to the present embodiment is not limited to the above-described manufacturing method. Even if the manufacturing method of the steel plate is a manufacturing method other than the above, if the steel plate is within the specified range, the steel plate is considered to be included in the range of the steel plate according to the present embodiment.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are not intended to limit the present invention. Modifications can be made within a range that is compatible with the spirit described above or below, and all of them are included in the technical scope of the present invention.

<Example 1>
Tables 1 and 2 show the chemical composition of the steel sheet. Steel types A to Z and AA to AG having the chemical compositions shown in Tables 1 and 2 are heated under the conditions shown in Tables 3 to 6, rough rolling, primary cooling, finish rolling, secondary cooling, and if necessary. By manufacturing through each step of heat treatment to be performed, No. 1-No. 49 steel plates were obtained. Tables 7 and 8 show the results of measuring the following properties of the obtained steel sheets.

For each of the obtained steel sheets, the area ratio of the worked ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite-austenite mixed phase in the front and back layers was measured according to the method described above. The average particle diameter (diameter) of all these phases was measured according to the method described above. Further, the aspect ratio of the processed ferrite phase was measured according to the method described above. Then, the texture strength ratio A was measured according to the method described above.

For each of the obtained steel plates, the average grain size (diameter of all phases of the worked ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite-austenite mixed phase at the 1/2 position in the plate thickness direction from the steel plate surface ) Was measured according to the method described above. The texture strength ratio B was measured according to the method described above.

For each steel sheet thus obtained, the tensile strength of steel plate (TS), yield stress (YP), brittle crack arrest toughness value at test temperature -10 ° C. in a temperature gradient type ESSO test _{K ca} (arrest toughness value _{K ca -10°C} ) and the non-ductile transition temperature (NDT temperature) in the NRL drop weight test were measured according to the method described above.

In Tables 1 and 2, "-" indicates that the additive was not intentionally added.

In Tables 5 and 6, "-" in the heat treatment column indicates that no tempering treatment is performed (tempering treatment is not performed).

The abbreviations in Tables 7 and 8 are as follows.
"DF fraction" represents the area fraction of the worked ferrite phase, "B fraction" represents the area fraction of the bainite phase, and "PF fraction" represents the area fraction of the polygonal ferrite phase.
“PF+P+MA” represents the total area fraction of the polygonal ferrite phase, the pearlite phase, and the martensite/austenite mixed phase.
The “average particle size” represents the average particle size (diameter) of all phases of the processed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite/austenite mixed phase.
“DF aspect ratio” represents the aspect ratio of the processed ferrite phase.
The “texture strength ratio A” represents the average value of the texture strength ratios I _{001}<110> , I _{113}<110> , and I _{{112}<110> in} the front and back layers.
The “texture intensity ratio B” represents the average value of the texture intensity ratios I _{001}<110> , I _{112}<110> , and I _{{332}<113> at} the 1/2 position.
“K _ca −10° _C. ” represents an arrest toughness value at −10° C., and “NDT temperature” represents a non-ductile transition temperature.

In Comparative Examples that did not satisfy at least one of the conditions of the chemical composition and the microstructure of the steel sheet according to this embodiment, at least one of the arrestability and the high strength was inferior.
On the other hand, the invention examples satisfying the conditions of the chemical composition and microstructure of the steel sheet according to this embodiment were steel sheets having excellent arrestability and strength.

As described above, the steel sheet according to this embodiment has excellent arrestability and high strength. Therefore, the steel sheet according to the present embodiment can be preferably applied particularly to important members of a container ship (for example, hatchside combing and upper deck).

Claims (7)

In mass %,
C: 0.040% to 0.160%,
Si: 0.01% to 0.50%,
Mn: 0.70% to 2.50%,
P: 0.030% or less,
S: 0.020% or less,
Nb: 0.003% to 0.050%,
Ti: 0.003% to 0.050%,
Al: 0.001% to 0.100%,
N: 0.0010% to 0.0080%,
With a chemical composition of Fe and impurities as the balance,
Between the position of 1 mm from the surface of the steel plate and the position of 5 mm from the surface of the steel plate, the metal structure of the section in the plate thickness direction parallel to the rolling direction has an area fraction of worked ferrite phase of 30% to 90% and bainite phase of 10%. -60%, a total of 30% or less of a polygonal ferrite phase, a pearlite phase, and a martensite/austenite mixed phase,
The aspect ratio of the worked ferrite phase in the sheet thickness direction cross section parallel to the rolling direction between the position 1 mm from the steel plate surface and the position 5 mm from the steel plate surface is 2.0 or more,
Between the position of 1 mm from the surface of the steel sheet and the position of 5 mm from the surface of the steel sheet, the processed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite-austenite mixed phase in a cross section perpendicular to the rolling direction The average particle size (diameter) is 25 μm or less when measured by electron beam backscattering diffractometry,
Of the {hkl} plane with respect to the rolling surface and the rolling direction and the texture strength ratio I _{{hkl}<uvw> in} the <uvw> direction between the position 1 mm from the steel plate surface and the position 5 mm from the steel plate surface _, I _{ The average value of _001}<110> , I _{113}<110> , and I _{112}<110> is 3.0 or more,
Electron beam backscatter diffraction of a work ferrite phase, a bainite phase, a polygonal ferrite phase, a pearlite phase, and a martensite-austenite mixed phase in a cross section perpendicular to the rolling direction at the 1/2 position in the plate thickness direction from the steel plate surface. Has an average particle diameter (diameter) of 30 μm or less when measured by the method,
I _{{001} <110>} in the rolling surface, the {hkl} plane relative to the rolling direction and the texture strength ratio I _{{hkl}<uvw> in} the _<uvw> direction at the 1/2 position in the plate thickness direction from the steel plate surface. , I _{112}<110> , and I _{{332}<113> having} an average value of 2.0 or more. The chemical composition is
Cu: 0.01% to 1.50%,
Ni: 0.01% to 2.50%,
Cr: 0.01% to 1.00%,
Mo: 0.01% to 1.00%,
V: 0.001% to 0.150%
B: 0.0001% to 0.0050%,
The steel sheet according to claim 1, comprising at least one selected from the group consisting of: The chemical composition is
Mg: 0.0001% to 0.0100%,
Ca: 0.0001% to 0.0100%,
REM: 0.0001% to 0.0100%,
The steel sheet according to claim 1 or 2, containing at least one selected from the group consisting of: The chemical composition is
Zr: 0.0001% to 0.0100%,
Te: 0.0001% to 0.0100%,
The steel sheet according to any one of claims 1 to 3, comprising at least one selected from the group consisting of: Carbon equivalent Ceq. represented by the following formula (1). Is 0.30% to 0.55%, and the steel plate according to any one of claims 1 to 4.
Formula (1) Ceq. =C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
(However, C, Mn, Cr, Mo, V, Cu, and Ni in the formula (1) represent the content (% by mass) of each element contained in the steel sheet.) A method for manufacturing the steel sheet according to any one of claims 1 to 5, comprising:
Heating a steel slab having the chemical composition according to any one of claims 1 to 5 in a temperature range of 950°C to 1150°C.
A step of performing rough rolling on the heated billet at a surface temperature of the steel sheet in a temperature range of a recrystallization temperature _{Trex to} 1050°C and a cumulative rolling reduction of 10% to 75%;
The temperature of the steel sheet after the rough rolling at a position of 1 mm from the steel sheet surface is 35° C. from the cooling start temperature of Ar ₃ point to 1050° C. to the cooling stop temperature of 500° C. to (Ar ₃ point −30° C.). /Sec to 100°C/sec, the primary cooling at an average cooling rate;
A step of finish rolling the steel sheet after the primary cooling at a surface temperature of the steel sheet in a temperature range of 600° C. to 800° C. and a cumulative reduction rate of 50% to 75%;
The temperature of the steel sheet after the finish rolling at the 1/4 position in the plate thickness direction from the steel sheet surface is 1°C/sec from the cooling start temperature of 600°C to 800°C to the cooling stop temperature of 0°C to 550°C. Secondary cooling at an average cooling rate of ~20°C/sec;
And a method of manufacturing a steel sheet. The method for manufacturing a steel sheet according to claim 6, further comprising a step of performing tempering heat treatment on the steel sheet after the secondary cooling in a temperature range of 350°C to 650°C.