JP7248885B2 - Steel plate and steel plate manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel sheet and a method for manufacturing a steel sheet.

Uses of steel plates include, for example, ships, buildings, bridges, offshore structures, LNG storage tanks, other large tanks, line pipes, and the like (see Patent Documents 1 to 4, for example).
In order to suppress brittle fracture, steel sheets applied to these applications are required to have good weld heat affected zone (HAZ) toughness, and if a brittle crack occurs at a welded joint, However, a brittle crack arrest property (BCA: Brittle Crack Arrest; hereinafter sometimes referred to as "arrestability") for arresting brittle cracks in the base material is required.

Steel plates applied to ships include, for example, important members of container ships (eg, hatch side coamings and upper decks). Since a container ship has a structure with a wide open upper deck, thick and high-strength steel plates are used for important members in order to ensure the cross-sectional rigidity of the hull.

In recent years, the size of container ships has been increasing in order to reduce environmental impact and operating costs. Recently, 14,000 to 20,000 TEU (Twenty-foot Equivalent Unit) class mega-container ships have been built. Therefore, from the viewpoint of suppressing large-scale destruction of the hull, steel plates used for important members of container ships are required to have further improved arrestability. In addition, in order to stably mass-produce steel sheets having high arrestability, it is desired to develop new technologies for improving arrestability.

JP 2009-221585 A JP 2009-235458 A Japanese Unexamined Patent Application Publication No. 2011-214116 Japanese Patent Application Laid-Open No. 2002-020835

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

Means for solving the above problems include the following aspects.

<1>
in % by 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 having a chemical composition consisting of Fe and impurities as the balance,
The metal structure of the plate 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 has an area fraction of 30% to 90% deformed ferrite phase and 10% bainite phase. ~ 60%, composed of 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 deformed ferrite phase in the plate 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,
A deformed 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 between a position 1 mm from the steel plate surface and a position 5 mm from the steel plate surface, An average particle size (diameter) of 25 μm or less as measured by an electron beam backscatter diffraction method,
Between the position 1 mm from the steel plate surface and the position 5 mm from the steel plate surface, the texture intensity ratio I of the {hkl} plane and <uvw> direction with respect to the rolling surface and the rolling direction I _{{hkl} <uvw> ,} I _{{ 001} <110>} , I _{{113} <110>} and I _{{112} <110>} have an average value of 3.0 or more,
Electron beam backscatter diffraction of the deformed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite-austenite mixed phase in the 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) when measured by the method is 30 μm or less,
I _{{001} <110>} out of the texture intensity ratio I _{{hkl} <uvw>} of the {hkl} plane and <uvw> direction with respect to the rolled surface and rolling 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.
<2>
The chemical composition further comprises
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 further comprises
Mg: 0.0001% to 0.0100%,
Ca: 0.0001% to 0.0100%,
REM: 0.0001% to 0.0100%,
The steel sheet according to <1> or <2>, containing at least one selected from the group consisting of:
<4>
The chemical composition further comprises
Zr: 0.0001% to 0.0100%,
Te: 0.0001% to 0.0100%,
The steel sheet according to any one of <1> to <3>, containing at least one selected from the group consisting of
<5>
Carbon equivalent Ceq. represented by the following formula (1). is 0.30% to 0.55%, the steel sheet according to any one of <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 formula (1) represent the content (mass%) of each element contained in the steel sheet.)
<6>
A method for producing a steel plate according to any one of <1> to <5>,
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 rough rolling the heated steel slab in a temperature range where the surface temperature of the steel plate is the recrystallization temperature T _rex to 1050 ° C. and the cumulative rolling reduction is 10% to 75%;
The steel plate after the rough rolling is heated at a position 1 mm from the steel plate surface 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.), 35 ° C. A step of primary cooling at an average cooling rate of / sec to 100 ° C. / sec;
A step of finish rolling the steel plate after the primary cooling in a temperature range of the surface temperature of the steel plate of 600° C. to 800° C. and a cumulative rolling reduction of 50% to 75%;
The steel plate after the finish rolling is heated so that the temperature at the quarter position in the plate thickness direction from the surface of the steel plate 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;
A method for manufacturing a steel plate.
<7>
The method for producing a steel sheet according to <6>, further comprising a step of subjecting the steel sheet after the secondary cooling to a tempering heat treatment in a temperature range of 350°C to 650°C.

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

FIG. 3 is a schematic diagram showing the relationship between crystal orientations and material axes illustrated on the ODF. 10 is a graph showing the effect of average texture intensity ratios I _{001}<110> , I _{113}<110> , and I _{112}<110> on arrestability. Fig. 10 is a graph showing the effect of the mean values of texture intensity ratios I _{001}<110> , I _{112}<110> and I _{332}<113> on arrestability.

An example of preferred embodiments of the present invention will be described in detail below.
In this specification, a numerical range represented by "-" means a range including the numerical values before and after "-" as lower and upper limits.
In this specification, the content of a component (element) may be expressed as "amount of C", for example, in the case of the content of C (carbon). Contents of other elements may also be expressed similarly.
In this specification, the term "step" is used not only for independent steps, but also for the intended purpose of the step even if it cannot be clearly distinguished from other steps. included.

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

<Steel plate>
The steel sheet according to the present embodiment has, 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 the balance being Fe and impurities.
In addition, the metal structure of the cross section parallel to the rolling direction between the position of 1 mm from the steel plate surface and the position of 5 mm from the steel plate surface is the deformed ferrite phase, the bainite phase, the polygonal ferrite phase, the pearlite phase, and the martensite phase. It consists of an austenite mixed phase. The area fractions of each are 30% to 90% of the deformed ferrite phase, 10% to 60% of the bainite phase, the polygonal ferrite phase, the pearlite phase, and the martensite/austenite mixed phase in total of 30% or less.
Furthermore, the aspect ratio of the deformed ferrite phase in the 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.
A deformed 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 between a position 1 mm from the steel plate surface and a position 5 mm from the steel plate surface, The average particle size (diameter) measured by an electron beam backscatter diffraction method is 25 μm or less.
Then, between the position 1 mm from the steel plate surface and the position 5 mm from the steel plate surface, among the texture intensity ratios I _{{hkl} <uvw> in the} {hkl} plane and <uvw> direction with respect to the rolling surface and the rolling direction _, The average values of I _{001}<110> , I _{113}<110> and I _{112}<110> are 3.0 or more.
Electron beam backscatter diffraction of the deformed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite-austenite mixed phase in the 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>} out of the texture intensity ratio I _{{hkl} <uvw>} of the {hkl} plane and <uvw> direction with respect to the rolled surface and rolling direction at the 1/2 position in the plate thickness direction from the steel plate surface , I _{112}<110> , and I _{332}<113> are 2.0 or more.

Conventionally, it has been desired to further improve the arrestability of steel plates applied to ships and the like in order to apply them, for example, to important members of ultra-large mega-container ships.

It is known that refinement of crystal grains is important as a means for improving arrestability. Furthermore, in recent years, studies have also been conducted from the viewpoint of texture control. Brittle cracks propagate along the cleavage planes of steel. Therefore, it is possible to improve arrestability of the steel material 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 has been proposed in which the temperature and rolling conditions at the center of the sheet thickness are controlled to increase the degree of accumulation of the (100) plane in the texture parallel to the rolled surface (for example, Patent Document 1 and Patent Document 2). These are produced by increasing the rolling reduction in a specific temperature range near Ar ₃ , which is the temperature at which ferrite transformation starts during cooling. However, in the steel sheets disclosed in Patent Documents 1 and 2, no consideration is given to the most effective grain size for improving arrestability. In addition, only two-dimensional textures based on plane intensity ratios are specified, and three-dimensional textures combining planes and orientations are not considered. Therefore, it is difficult to dramatically improve arrestability.

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

The steel plate disclosed in Patent Document 4 does not consider the inside of the plate thickness. Crest resistance is a property of the total plate thickness. Therefore, if the metallographic 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, arrest steels have been remarkably thickened and strengthened, and it has become difficult to obtain sufficient arrestability simply by refining grains. One of the reasons for this is, for example, the bainite phase present in the front and back layers of the steel sheet. It is effective to apply strong cooling (strong water cooling) after rolling in order to secure high strength to the inside of the plate thickness of the steel plate. By applying this intense cooling, the steel sheet has high strength, but a coarse bainite phase with high hardness is formed in the front and back layers of the steel sheet.

Therefore, attention was paid to the deformed ferrite phase in order to improve arrestability while making a high-strength steel sheet. The deformed ferrite phase can be expected to have a strength equivalent to that of the bainite phase due to dislocation strengthening, and excellent arrestability due to the texture.

The deformed ferrite phase is a structure obtained by performing finish rolling in a two-phase region of an austenite phase and a polygonal ferrite phase so that the polygonal ferrite phase is subjected to rolling and elongated in the rolling direction. When the polygonal ferrite phase undergoes deformation, its strength increases due to dislocation strengthening. A rolling texture develops during the process of working the polygonal ferrite phase. At this time, a texture called α-fiber develops, which mainly has a crystal orientation in which the {001} plane is parallel to the steel sheet rolling surface and the <110> direction is parallel to the rolling direction. Therefore, when evaluating the arrestability for generating brittle cracks in the direction perpendicular to the rolling direction, the {100} plane, which is the cleavage plane of the BCC structure, is not arranged in the crack generation direction. Since it is arranged in a direction inclined at 45° to the crack generation direction, the brittle crack propagates obliquely at 45° to the crack generation direction, which acts as a resistance to crack propagation, so arrestability is improved. improves. For these reasons, it is considered that both the strength and arrestability are improved by forming the deformed ferrite phase.

Further description will be made with reference to FIG. FIG. 1 is a schematic diagram showing the relationship between the crystal orientation shown on an ODF (Oriented Distribution Function) and the crystal plane of a steel material. Specifically, it represents the main crystal orientations of deformed ferrite appearing on the ODF of the φ ₂ =45° cross section. The {001}<110> orientation, {113}<110> orientation, and {112}<110> orientation shown in FIG. 1 are representative textures of α-fiber. In a conventional steel sheet that does not have deformed ferrite or that has very little deformed ferrite, the textures of {001}<010> orientation, {110}<110> orientation, and {110}<001> orientation are , in the direction perpendicular to the rolling direction (TD). Since the direction of brittle crack initiation is the direction (TD) perpendicular to the rolling direction, conventional steel sheets are considered to have inferior arrestability because brittle cracks tend to propagate straight.

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> are 3.0 in the front and back layers. 0 or more. That is, in the steel sheet according to the present embodiment, the texture of α-fiber is developed in the front and back layers, so the brittle crack generated in the direction perpendicular to the rolling direction is It is easy to propagate in the 45° direction, and the brittle crack propagates straight and is difficult to propagate. As a result, the steel plate according to the present embodiment is considered to have improved arrestability in evaluation of arrestability because the {100} plane, which is the cleavage plane of the BCC structure, is not arranged in the crack propagation direction. Furthermore, at the 1/2 position, the average values of texture intensity ratios I _{001}<110> , I _{112}<110> , and I _{332}<113> are 2.0 or more. That is, at the 1/2 position, {001}<110>, {112}<110>, and {332}<113> It is thought that brittle cracks are less likely to propagate straight in the direction of propagation due to the well-developed texture of the orientation, and arrestability is improved.

As used herein, deformed ferrite means grains with a GOS value greater than 4°. The deformed ferrite fraction means the area ratio of the deformed ferrite in the metal structure. The processed ferrite fraction is quantified by the parameter GOS value (Grain Orientation Spread) measured by EBSP (Electron Back Scattering Pattern). The GOS value is a parameter that indicates the average value of misorientation between a certain measurement point and another measurement point within the same crystal grain. It is thought that when dislocations are introduced into grains by two-phase rolling, the GOS value increases due to the difference in crystal orientation caused by the dislocations.

Furthermore, in utilizing the deformed ferrite phase mainly in the front and back layers, the method of manufacturing the steel sheet according to the present embodiment was studied. 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, by performing finish rolling in the two-phase region of the austenite phase and the polygonal ferrite phase, rolling is applied to the polygonal ferrite phase. Furthermore, by controlling the cooling conditions (secondary cooling conditions) after finish rolling, the generation of coarse bainite phase is suppressed as much as possible. As a result, it was found that a steel sheet having a predetermined metal structure containing deformed ferrite phases in the front and back layers could be obtained.

As described above, the steel sheet according to the present embodiment provides a steel sheet having excellent arrestability and high strength.

First, reasons for limiting the chemical composition of the steel sheet according to the present embodiment will be described.
In the following explanation, "%" in the explanation of each element means "% by mass".

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

(Si: 0.01% to 0.50%)
Si is a deoxidizing element and 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, when the amount of Si exceeds 0.50%, the HAZ toughness becomes inferior. Therefore, the amount of Si is set to 0.01% to 0.50%. A preferable lower limit of the amount of Si is 0.03%, and a more preferable lower limit is 0.05%. A preferable upper limit of the amount of Si is 0.40%, and a 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 Mn content 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 set to 0.70% to 2.50%. A preferred lower limit for the Mn content is 0.90%, and a more preferred lower limit is 1.20%. A preferred upper limit for the amount of Mn is 2.20%, and a more preferred upper limit is 2.00%.

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

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

(Nb: 0.003% to 0.050%)
Nb is an element that suppresses recrystallization, contributes to refinement of the structure when added in a small amount, and is effective in ensuring the strength and arrestability of the base material. If the Nb content is less than 0.003%, the effect of containing Nb cannot be obtained. If the Nb content exceeds 0.050%, the HAZ toughness becomes inferior. Therefore, the Nb content is set to 0.003% to 0.050%. A preferable lower limit of the Nb content is 0.005%, and a more preferable lower limit is 0.008%. A preferable upper limit of the Nb content is 0.035%, and a 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 through refinement of the structure of the base metal and the weld zone by addition of a small amount. Ti also functions as a deoxidizing element. On the other hand, if Ti is added excessively, the weld zone is hardened and the toughness is significantly deteriorated, resulting in inferior HAZ toughness. Therefore, the Ti content is set to 0.003% to 0.050%. A preferred lower limit for the Ti content is 0.006%, and a more preferred lower limit is 0.010%. A preferable upper limit of the Ti amount is 0.035%, and a more preferable upper limit is 0.020%.

(Al: 0.001% to 0.100%)
Since Al is a deoxidizing element, the amount of Al is made 0.001% or more. On the other hand, excessive addition of Al impairs the surface quality of the steel slab and forms inclusions harmful to arrestability and HAZ toughness, so the upper limit of the Al content is made 0.100%. Therefore, the Al content is set to 0.001% to 0.100%. A preferable lower limit of the Al content is 0.010%, and a more preferable lower limit is 0.015%. A preferable upper limit of the Al content is 0.080%, and a more preferable upper limit is 0.050%.

(N: 0.0010% to 0.0080%)
N forms nitrides together with Ti and Al to improve joint toughness, so the lower limit of the N content is made 0.0010%. However, if the N content is excessive, the arrestability and the elongation of the base metal are lowered due to solid solution N, so the upper limit of the N content is made 0.0080%. Therefore, the amount of N is set to 0.0010% to 0.0080%. A preferable lower limit of the N content is 0.0015%, and a more preferable lower limit is 0.0020%. A preferable upper limit of the amount of N is 0.0070%, and a more preferable upper limit is 0.0060%.

Furthermore, the steel sheet according to the present embodiment may contain the following elements in terms of % by mass. The following elements are optional 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. At least one selected from the group consisting of Mg, Ca, and REM may be included. These elements are elements that contribute to the effect of suppressing formation of inclusions. At least one selected from the group consisting of Zr and Te may be included. These elements are elements that contribute to the improvement of toughness.

(Cu 0.01% to 1.50%)
Cu is an element that improves hardenability and is effective in increasing the strength of the base material. Therefore, Cu may be contained. However, when the amount of Cu exceeds 1.50%, toughness decreases as the hardness of the joint increases. In order to more reliably obtain the effect of the above action, the lower limit of the amount of Cu is preferably set to 0.01%. Therefore, when Cu is contained, the amount of Cu should be 0.01% to 1.50%. A more preferable upper limit of the amount of Cu is 0.80%, and a further preferable upper limit is 0.50%. A more preferable lower limit for the amount of Cu is 0.05%, and a further preferable lower limit 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 amount of Ni 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 amount is preferably 0.01% or more. Therefore, when Ni is contained, the amount of Ni should be 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 further preferable lower limit is 0.20%.

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

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

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

(B: 0.0001% to 0.0050%)
B is an element that enhances the hardenability and contributes to the strength improvement of the base metal when added in a very small amount. Therefore, B may be contained. When the amount of B exceeds 0.0050%, the arrestability and HAZ toughness are lowered. In order to more reliably obtain the effect of the above action, the lower limit of the B amount is preferably 0.0001%. Therefore, when B is contained, the amount of B should be 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 is 0.0010%.

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

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

(REM: 0.0001% to 0.0100%)
REM is a deoxidizing element that suppresses the formation of coarse inclusions by forming sulfides and suppresses the formation of harmful inclusions by forming fine oxides. Therefore, REM may be included. However, when the amount of REM exceeds 0.0100%, coarse oxides, sulfides and oxysulfides are likely to be formed, and arrestability and HAZ toughness are lowered. In order to more reliably obtain the effect of the above action, the lower limit of the REM content is preferably 0.0001%. Therefore, when REM is contained, the amount of REM should be 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 generic term for a total of 17 elements including Sc, Y, and lanthanoids. REM may contain one or more of the 17 elements in total. The REM content refers to the total content of these elements.

(Zr: 0.0001% to 0.0100%)
Zr is an element that, when added in a small amount, contributes to the improvement of toughness by refining the structure of the base metal and the weld zone. Zr also functions as a deoxidizing element. Therefore, Zr may be contained. If the Zr content exceeds 0.0100%, the HAZ toughness is lowered. The lower limit of the Zr amount is preferably set to 0.0001% in order to more reliably obtain the effect of the above action. Therefore, when Zr is contained, the amount of Zr should be 0.0001% to 0.0100%. A more preferable upper limit of the Zr amount is 0.0070%, and a further preferable upper limit is 0.0050%. A more preferable lower limit of the Zr content is 0.0005%, and a further preferable lower limit is 0.0010%.

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

(remainder)
The balance is Fe and impurities. Impurities refer to components contained in the raw material or components mixed in during the manufacturing process and not intentionally included in the steel sheet.

(Carbon equivalent Ceq.: 0.30% to 0.55%)
The steel sheet according to the present embodiment has a carbon equivalent Ceq. calculated by the following formula (1). 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 Formula (1) represent the contents (% by mass) of each element contained in the steel sheet.
If there is an element whose content is 0% by mass, 0% by mass is substituted for the content of the corresponding element in formula (1).

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

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

(Processed ferrite phase: 30% to 90%)
The deformed ferrite phase contributes to arrestability. If the area of the deformed ferrite phase exceeds 90%, the strength of the base metal may be insufficient. On the other hand, when the area fraction of the deformed ferrite phase is less than 30%, arrestability is inferior. Therefore, the area fraction of the ferrite phase is set to 30% to 90%. A preferable upper limit of the area fraction of the deformed ferrite phase is 85%, and a more preferable upper limit is 80%. A preferred lower limit for the area fraction of the deformed ferrite phase is 35%, and a more preferred lower limit 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 metal is inferior. On the other hand, when the area fraction of the bainite phase exceeds 60%, arrestability becomes inferior. Therefore, the area fraction of the bainite phase is set to 10% to 60%. A preferred upper limit for the area fraction of the bainite phase is 55%, and a more preferred upper limit is 50%. A preferred lower limit for the area fraction of the bainite phase is 15%, and a more preferred lower limit 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 decreases or becomes embrittled due to the polygonal ferrite phase, which is a soft phase. The decrease in arrestability due to the pearlite phase and the MA phase, which are phases, becomes remarkable. Therefore, the total area fraction of these phases (polygonal ferrite phase, pearlite phase, and MA phase) is set to 30% or less from the viewpoint of ensuring both the strength and arrestability of the base material. A preferred upper limit for the total area fraction of these is 25% or less. The total area fraction of these is preferably as small as possible, and the lower limit is not particularly limited. For example, the total area fraction of each of these phases may be 0%. Moreover, it may be more than 0%, or it may be 1% or more.

(Aspect ratio of deformed ferrite phase: 2.0 or more)
When the aspect ratio of the deformed ferrite phase in the front and back layers is 2.0 or more, arrestability is improved. The aspect ratio of the deformed 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 deformed ferrite phase in the front and back layers is not particularly limited, and may be, for example, 10.0 or less. When the deformed 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, the arrestability is improved.
Here, in this specification, the aspect ratio of the deformed ferrite phase in the front and back layers is 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 in one deformed ferrite phase. (maximum major axis length/maximum minor axis length). Since the deformed ferrite phase exhibits 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 of 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. A sample of the front and back layers is etched with nital, and the GOS value is determined by EBSP for the portion that appears white due to the nital etching. Image analysis may be performed, for example, on 8 or more fields of view with an area of 0.05 mm ² or more (a total of 0.40 mm ^{2 or} more), and the average value may be obtained.

(Average grain size (diameter) of deformed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite/austenite mixed phase: 25 μm or less)
Refinement of the structure of all phases of the deformed ferrite phase, the bainite phase, the polygonal ferrite phase, the pearlite phase, and the MA phase contributes to the improvement of arrestability. The average grain size (diameter) of each phase (deformed 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. Arrest property is improved by being below. The average grain size of each phase in the front and back layers is preferably as small as possible, and the lower limit is not particularly limited. For example, the average grain size of each phase in the front and back layers may be 1 μm or more, or may be 5 μm or more. A method for measuring the average grain 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)
Of the texture intensity ratios I _{{hkl} <uvw>} in the {hkl} plane and <uvw> direction with respect to the rolled surface and 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 intensity ratio A") is 3.0 or more, arrestability is improved. The texture intensity ratio A represents the average value of the texture intensity ratios when the polygonal ferrite phase is processed, and represents three representative textures of α-fiber. A texture strength ratio A of 3.0 or more indicates an increase in α-fiber. As described above, α-fiber does not have the {100} plane, which is the cleavage plane of the BCC structure, arranged in the direction of crack propagation in the arrestability evaluation. 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, for example, 10.0 or less. A method for measuring the texture intensity ratio A will be described later.

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

(Average grain size (diameter) of deformed 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 the deformed 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 (deformed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite/austenite mixed phase) at the 1/2 position measured by electron beam backscattering diffraction method is 30 μm or less, arrestability is improved. The average grain size of each phase in the front and back layers is preferably as small as possible, and the lower limit is not particularly limited. For example, the average grain size of each phase in the front and back layers may be 1 μm or more, or may be 5 μm or more.

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

From the viewpoint of obtaining a steel sheet having high strength and excellent arrestability, it is desirable that the average grain size of each phase in the front and back layers is smaller than the average grain size of each phase at the 1/2 position. From the same point of view, it is desirable that the texture intensity ratio A in the front and back layers is larger than the texture intensity ratio B at the 1/2 position.

-Method for measuring the area fraction of each phase in the 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 from 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 sheet is photographed with an optical microscope, and the metal structure is obtained by image analysis.
Specifically, first, a sample is taken from a portion of the front and back layers of the cross section in the direction parallel to the rolling direction of the steel sheet, which is located at the 1/4 position from the end surface in the width direction of the steel sheet.
Next, the collected sample is etched with nital, and after the etching, an optical microscope is used to photograph 8 fields of view of the cross section in the L direction at a magnification of 500. Then, the obtained tissue photograph is subjected to binarization processing by image analysis software, and image analysis is performed. The phase that looks white is the deformed ferrite phase or polygonal ferrite phase, the phase that looks black is the pearlite phase, the phase that looks gray is the bainite phase, or the MA phase (martensite/austenite mixed phase), and the area ratio of each is calculated. .
Next, the nital-etched portion was measured by EBSP, and the GOS value was obtained for the portion that looked white due to the nital etching. The portion of 4° or less is defined as the area ratio of the polygonal ferrite phase.
Further, the nital-etched portion is repelled and image analysis is performed on the gray portion due to the nital etching.
Then, the area ratio of the bainite phase is obtained by subtracting the area ratio of the MA phase (martensite-austenite mixed phase) from the area ratio that appears gray after nital etching.
The total area fraction of the deformed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and MA phase is 100%.

-Method for measuring average particle size-
The method for measuring the average grain size of each phase in the front and back layers and the 1/2 position is as follows.
At the front and back layers and at the 1/2 position, a sample for measurement of a cross section (so-called C cross section) perpendicular to the rolling direction of the steel plate is taken. The average grain size of all phases is measured by EBSP, which can accurately measure crystal orientation information over a wide field of view. By using EBSP, it is possible to measure the grain size of complex structures such as the bainite phase. Specifically, it is measured by the following method. By EBSP, a region of 500 μm×500 μm on the front and back layers is repeatedly measured while moving the measurement position by 1 μm. The 1/2 position is also measured in the same way. Here, a boundary with a crystal orientation difference of 15° or more with an adjacent grain is defined as a grain boundary, and the weighted average value of the circle equivalent diameter (diameter) surrounded by this grain boundary is obtained, and this is the average grain size. and
Specifically, the weighted average value is calculated by the following formulas (D1), (D2) and (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 indicates the maximum grain size among the equivalent circle diameters (diameters) of crystal grains, and D _MIN indicates the minimum grain size among the equivalent circle diameters (diameters) of crystal grains. be. N represents the number of divisions when calculating the weighted average, and a value of 10 or more is used to obtain the weighted average. By dividing the difference between the maximum value D _MAX and the minimum value D _MIN of the crystal grain size (the width of all crystal grain size distributions) by the number of divisions N, p, which is the width of the division range, is calculated by the formula (D1). can do.
_Dk is the median value of the crystal grain size in the k-th division range, and k is an integer equal to or more than 1 and equal to or less than the number of divisions N, and can be obtained by the formula (D2). S _k is the area of the crystal grain in the k-th division range, measured using the electron beam backscatter 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 the median value _Dk of the grain size in the k-th division range and the total area S of grains having grain sizes in the k-th division range, Calculate (DK × SK), the value multiplied by _k , and sum up ( _DK × _SK ) for k from 1 to the number of divisions N, 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 S _N .

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

[Physical properties of steel plate]
The plate thickness of the steel plate 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 is preferably 510 MPa or more (preferably 510 MPa to 720 MPa, more preferably 570 MPa to 720 MPa) in terms of high strength. Also, 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). Yield stress (YP) means the yield strength of the permanent elongation method at 0.2% permanent elongation of JIS Z 2241 (2011).

In addition, the steel sheet according to the present embodiment has a brittle crack arrest toughness value K _ca at a test temperature of −10 ° C. in a temperature gradient type ESSO test (hereinafter referred to as “arrest toughness value K _{ca−10 ° C.} ) is preferably 6000 N/mm ^1.5 or more (preferably 8000 N/mm ^1.5 or more). Satisfying this characteristic, the steel sheet has excellent arrestability.
The crack arrest toughness value Kca _-10°C is based on NK Classification Society Steel Ship Rules and Inspections, Part K, Appendix K3.12.2-1. (2016) in accordance with "Inspection Procedures for Temperature Gradient ESSO Test and Temperature Gradient Double Tensile Test".

Furthermore, the steel sheet according to the present embodiment preferably has a non-ductility transition temperature of −80° C. or less (preferably −85° C. or less, more preferably −100° C. or less) in the NRL drop weight test. Satisfying this characteristic, the steel sheet has excellent arrestability.
The non-ductile transition temperature (NDT temperature; Nil-Ductility-Transition Temperature) is determined by testing in accordance with the NRL (Naval Research Laboratory) drop weight test method specified in ASTM E208-06. The test piece is 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. A test piece is taken in the rolling direction (L direction), a weld bead is provided on the outermost surface of the test piece in the L direction, and a notch is provided in a direction perpendicular to the rolling direction (C direction) as a crack starter.

In the steel sheet according to the present embodiment, the chemical composition, the front and back layers, and the aspect of the metal structure at the 1/2 position are controlled so as to satisfy the above-mentioned conditions. For example, 50 mm to 100 mm), the tensile strength (TS), yield stress (YP), arrest toughness value K _{ca-10 ° C.} , and non-ductile transition temperature (NDT temperature) all have values in the above ranges. be satisfied.

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

The steel plate according to this embodiment can be obtained, for example, by a manufacturing method having the following steps.
a step of heating a steel billet having the chemical composition described above in a temperature range of 950° C. to 1150° C. (heating step);
A step of rough rolling the steel slab after heating at a cumulative rolling reduction of 10% to 75% in a temperature range where the surface temperature of the steel plate is the recrystallization temperature T _rex to 1050 ° C. (rough rolling step);
The steel plate after the rough rolling is heated at a position 1 mm from the steel plate surface 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.), 35 ° C. A step of primary cooling at an average cooling rate of / sec to 100 ° C. / sec (primary cooling step),
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% (finish rolling step);
The steel plate after the finish rolling is heated so that the temperature at the quarter position in the plate thickness direction from the surface of the steel plate 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/sec (secondary cooling step).

(Heating process)
This is the step of heating the steel slab having the chemical composition described above at a predetermined temperature. This step contributes to the control of the structure of the austenitic phase by heating the steel slab. A steel slab 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 slab to 950° C. or higher, austenitization becomes sufficient and a fine austenite phase is 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 steel billet after heating is subjected to a surface temperature range of the recrystallization temperature T _rex to 1050° C. and a cumulative rolling reduction of 10% to 75%.
Under the above conditions, the austenite phase is recrystallized by the energy of dislocations generated by rolling, so that the austenite phase is refined and a steel sheet with improved arrestability is obtained.

If the rough rolling temperature is lower than _Trex , recrystallization of the austenite phase does not occur, so refinement of the austenite phase cannot be expected. On the other hand, if the rough rolling temperature exceeds 1050° C., the grain size of the austenite phase after recrystallization becomes large and the grain growth after recrystallization is promoted, so a fine austenite phase cannot be obtained.

In other words, the austenite phase is finely controlled by performing rough rolling at a relatively low temperature while ensuring the rolling reduction at which the austenite phase recrystallizes as in the above conditions. This improves arrestability of the steel sheet.

Here, the cumulative rolling reduction in rough rolling is obtained by the following formula (R1).
Formula (R1) Cumulative reduction rate of rough rolling = {(plate thickness before rough rolling - plate thickness after rough rolling) / plate thickness before rough rolling} × 100
The plate thickness before rough rolling refers to the plate before rolling when the billet (steel plate) is extracted from the heating furnace and then rolled in a temperature range where the surface temperature of the billet (steel plate) is the recrystallization temperature T _rex to 1050 ° C. Maximum thickness.
The plate thickness after rough rolling is the minimum value of the plate thickness after rolling when the steel slab (steel plate) is rolled in a temperature range where the surface temperature is the recrystallization temperature T _rex to 1050°C.

(Primary cooling process)
In the primary cooling, the steel plate after rough rolling is cooled 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.) at a position 1 mm from the steel plate surface. ,Cooling. And the average cooling rate during cooling is 35° C./sec to 100° C./sec. As the conditions 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. Securing a sufficient polygonal ferrite phase in the primary cooling step is an important step in forming a deformed ferrite phase in the 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 steel plate surface. Cooling rate is the average cooling rate during cooling.

(Finish rolling process)
The finish rolling is carried out so 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 a temperature range in which the surface temperature of the steel sheet is 600° C. to 800° C., the polygonal ferrite phase generated in the primary cooling process is subjected to rolling, and a deformed ferrite phase elongated in the rolling direction is generated.

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

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

Here, in this 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]
Formula (T2) T _rex =−91900 [Nb*] ² +9400 [Nb*]+770
(However, in the formula (T2), [Nb*] is represented by the following formulas (T3-1) and (T3-2).
Formula (T3-1) [Nb]≧[Sol. Nb], then [Nb*]=[Sol. Nb]
Formula (T3-2) [Nb]<[Sol. Nb], then [Nb*]=[Nb]
Here, [Nb] represents the Nb content (% by mass), and [Sol. Nb] is the Sol. represents Nb (solid solution Nb) (% by 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 (% by mass), respectively. )

(Secondary cooling process)
Next, the steel sheet after finish rolling is cooled at a predetermined cooling rate to a predetermined temperature (secondary cooling). Cooling under these conditions refines the metal structure and controls the structure morphology (area fraction of all phases described above). This step affects the strength and arrestability of the base material.

In the secondary cooling step, the steel plate after finish rolling is cooled from a cooling start temperature of 600 to 800 ° C. to a cooling stop temperature of 0 to 550 ° C. at a quarter position in the thickness direction of the steel plate. do. The average cooling rate at the quarter position is then between 1° C./s and 20° C./s.

The cooling method is not particularly limited, and examples thereof include methods such as 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 from a temperature of 600° C. or higher at the position of the 1/4 part of the steel plate and then cooling 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. Cooling rate is the average cooling rate during cooling.

A preferred manufacturing method for obtaining the steel sheet according to the present embodiment further includes a step (heat treatment step) of subjecting the steel plate after the cooling step to temper heat treatment 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 a 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 arrestability by strain removal increases. On the other hand, if the heat treatment temperature exceeds 650°C, the strength may decrease.

In addition, the manufacturing method of the steel plate which concerns on this embodiment is not limited to the above-mentioned manufacturing method. Even if the steel sheet is manufactured by a manufacturing method other than the above-described one, if the steel sheet is within the specified range, the steel sheet is considered to be included in the scope of the steel sheet according to the present embodiment.

EXAMPLES The present invention will be described in more detail below with reference to examples, but the following examples are not intended to limit the present invention. It is also possible to change and implement within the range compatible with the above or later described gist, and all of them are included in the technical scope of the present invention.

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

The area ratios of the deformed ferrite phase, the bainite phase, the polygonal ferrite phase, the pearlite phase, and the martensite-austenite mixed phase in the front and back layers of each of the obtained steel sheets were measured according to the method described above. Also, the average particle size (diameter) of all these phases was measured according to the method described above. Furthermore, the aspect ratio of the deformed 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 steel sheet obtained, the average grain size (diameter ) was measured according to the method described above. Also, the texture strength ratio B was measured according to the method described above.

For each steel plate obtained, the tensile strength (TS), yield stress (YP), and brittle crack arrest toughness value K _ca at a test temperature of −10° C. in the temperature gradient type ESSO test (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, "-" means not intentionally added.

In Tables 5 and 6, "-" in the heat treatment column indicates no tempering treatment (no tempering treatment).

Abbreviations in Tables 7 and 8 are as follows.
The "DF fraction" represents the area fraction of the deformed ferrite phase, the "B fraction" represents the area fraction of the bainite phase, and the "PF fraction" represents the area fraction of the polygonal ferrite phase.
"PF+P+MA" represents the sum of the area fractions of the polygonal ferrite phase, the pearlite phase, and the martensite-austenite mixed phase.
"Average grain size" represents the average grain size (diameter) of all phases including the deformed ferrite phase, the bainite phase, the polygonal ferrite phase, the pearlite phase, and the martensite-austenite mixed phase.
"DF aspect ratio" represents the aspect ratio of the deformed ferrite phase.
“Texture intensity ratio A” represents the average value of the texture intensity ratios I _{001}<110> , I _{113}<110> , and I _{112}<110> in the front and back layers.
"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 ½ position.
"Kca _-10°C " represents the arrest toughness value at -10°C, and "NDT temperature" represents the non-ductile transition temperature.

Comparative examples, which did not satisfy at least one of the chemical composition and microstructure of the steel sheet according to the present embodiment, were inferior in at least one of arrestability and high strength.
On the other hand, the invention examples satisfying the chemical composition and microstructure conditions of the steel sheet according to the present embodiment were steel sheets excellent in both arrestability and strength.

As described above, the steel sheet according to the present embodiment has excellent arrestability and high strength. Therefore, the steel plate according to this embodiment can be suitably applied particularly to important members of container ships (for example, hatch side coamings and upper decks).

Claims (6)

in % by 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 having a chemical composition consisting of Fe and impurities as the balance,
Carbon equivalent Ceq. represented by the following formula (1). is 0.30% to 0.55%,
The metal structure of the plate 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 has an area fraction of 30% to 90% deformed ferrite phase and 10% bainite phase. ~ 60%, composed of 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 deformed ferrite phase in the plate 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,
A deformed 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 between a position 1 mm from the steel plate surface and a position 5 mm from the steel plate surface, An average particle size (diameter) of 25 μm or less as measured by an electron beam backscatter diffraction method,
Between the position 1 mm from the steel plate surface and the position 5 mm from the steel plate surface, the texture intensity ratio I _{hkl} of the {hkl} plane and <uvw> direction with respect to the rolling surface and the rolling direction, I _{{ 001} <110>} , I _{{113} <110>} and I _{{112} <110>} have an average value of 3.0 or more,
Electron beam backscatter diffraction of the deformed ferrite phase, bainite phase, polygonal ferrite phase, pearlite phase, and martensite-austenite mixed phase in the 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) when measured by the method is 30 μm or less,
I _{{001} <110>} out of the texture intensity ratio I {hkl} <uvw> in the {hkl} plane and <uvw> direction with respect to the rolling surface and rolling 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.
Formula (1) Ceq. =C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
(However, C, Mn, Cr, Mo, V, Cu, and Ni in formula (1) represent the content (mass%) of each element contained in the steel sheet.) The chemical composition further comprises
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 further comprises
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, comprising at least one selected from the group consisting of The chemical composition further comprises
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 A method for producing a steel plate according to any one of claims 1 to 4 ,
a step of heating a steel slab having the chemical composition according to any one of claims 1 to 4 in a temperature range of 950°C to 1150°C;
a step of rough rolling the steel slab after heating in a temperature range where the surface temperature of the steel plate is from the recrystallization temperature Trex to 1050 ° C. and the cumulative rolling reduction is from 10% to 75%;
The steel plate after the rough rolling is heated at a position 1 mm from the steel plate surface 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.), 35 ° C. A step of primary cooling at an average cooling rate of / sec to 100 ° C. / sec;
A step of finish rolling the steel plate after the primary cooling in a temperature range of the surface temperature of the steel plate of 600° C. to 800° C. and a cumulative rolling reduction of 50% to 75%;
The steel plate after the finish rolling is heated so that the temperature at the quarter position in the plate thickness direction from the surface of the steel plate 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;
A method for manufacturing a steel plate. The method for manufacturing a steel sheet according to claim 5 , further comprising a step of subjecting the steel sheet after secondary cooling to a tempering heat treatment in a temperature range of 350°C to 650°C.

Images