WO2023089951A1

WO2023089951A1 - Thick steel sheet and manufacturing method therefor

Info

Publication number: WO2023089951A1
Application number: PCT/JP2022/035525
Authority: WO
Inventors: 祐介寺澤
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2021-11-19
Filing date: 2022-09-22
Publication date: 2023-05-25
Also published as: CN118119725A; JPWO2023089951A1; KR20240075905A; EP4403661A1; JP7508026B2

Abstract

Provided is a thick steel sheet that has excellent internal quality properties and high strength, and can be manufactured at a low cost without requiring special equipment. The thick steel sheet has a prescribed component composition and satisfies the relationship Ceq/t ≥ 0.0015, and the area ratio of gap defects in a central position of the sheet thickness is 0.5% or less.

Description

Thick steel plate and its manufacturing method

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

In recent years, structures have become larger in the fields of ships, line pipes, buildings, bridges, offshore structures, wind power generators, construction machinery, and pressure vessels. Along with this, the plate thickness of steel plates used in the above fields is also increasing.

As a technique related to such a thick steel plate (hereinafter also referred to as a thick steel plate) and a method for manufacturing the same, for example, Patent Document 1 includes:
"In a hot forging method for steel materials that forges and stretches axially symmetrical steel materials in the upper anvil and the lower metal spread, between the start of forging and the end of forging, A method for hot forging a steel material, comprising a step of forming a rectangular or substantially rectangular cross-sectional shape with a ratio of the length of the long side to the length of the short side of at least 1.4.
is disclosed.

In Patent Document 2,
"A slab forging method in which asymmetrical anvils with different widths for the upper and lower anvils are used to continuously apply reduction in the width direction and then in the thickness direction,
The above-mentioned reduction in the width direction is performed from one end in the longitudinal direction of the slab. A slab forging method characterized by limiting the ratio ΔL/B to 0.20 or less, where B is the contact length of the shorter contact length. ”
is disclosed.

In Patent Document 3,
"In a hot forging method for slabs produced by continuous casting, using an asymmetrical anvil, continuous reduction is applied in the width direction and then in the thickness direction,
The slab reduction in the width direction is performed in two stages, in which the slab is reversed between the first stage and the second stage, and the reduction is performed at least twice in each stage, and the slab reduction in the width direction in each stage In this case, an anvil with a width of 400 to 1200 mm is used as an anvil on the short side, and an anvil with a width of 800 to 1500 mm is used as an anvil on the long side, and the pressing position of the anvil on the short side is the first slab. The rolling phase is shifted so that the deviation (ΔL) between the slab feed allowance boundary at the time of rolling and the center of the anvil contact length (B) at the time of the next rolling satisfies ΔL≦0.20B, and
A hot forging method for a slab, characterized in that each rolling reduction in the slab rolling in the width direction is 4% or more, and the total rolling reduction in the slab rolling in the thickness direction is 10% or more. ”
is disclosed.

In Patent Document 4,
"In the manufacturing method of extra-thick steel plate, the slab produced by the continuous casting method is tentered in the rough rolling process and further rolled to the product thickness in the finish rolling process.
A method for producing an extra-thick steel plate having excellent internal properties, characterized in that in the finish rolling step, multiple passes are rolled at a rolling speed of 200 to 350 mm/sec. ”
is disclosed.

In Patent Document 5, a slab is described as "Al: 0.07% by weight or less aluminum killed continuous cast strand is cut into a predetermined length and then hot-charged in a blooming soaking furnace as it is. Soaking at a temperature of 1150 ° C. and performing slab rolling so that the value of the shape ratio R according to the following formula is 0.5 or more,
Then, the slab is subjected to a dehydrogenation treatment to reduce the diffusible hydrogen contained in the central part of the thickness to 1.2 ppm or less,
After that, the slab is reheated to 950 to 1050° C., and then thick plate rolling is performed to a required thickness of 50 mm or more.
After the plate rolling is completed, accelerated cooling is performed at a heat removal rate of 15°C or more per minute from Ar ₃ or a temperature not lower than 40°C or less to 500 to 350°C.
A method for producing a high toughness steel plate with excellent internal quality by continuous casting, characterized by the order bonding of ”
is disclosed.

JP-A-58-167045 Patent No. 6137080 Patent No. 6156321 Japanese Patent Publication No. 6-69569 JP-A-59-74220

By the way, since thick steel plates are thick, there is a high risk of fractures such as ductile fractures, brittle fractures, and fatigue fractures. Therefore, there is a demand for a steel plate having excellent internal properties with reduced risk of such fractures. However, in the thick steel plates manufactured by the techniques of Patent Documents 1 to 5, the risk of occurrence of such fracture cannot necessarily be sufficiently reduced, and excellent internal properties may not be obtained.

In addition, the techniques of Patent Documents 1 to 3 apply hot forging to the slab. However, the production efficiency of hot forging is much lower than that of hot rolling. Therefore, there is a problem that the production capacity is low and the manufacturing cost is high.

The techniques of Patent Documents 4 and 5 apply hot rolling to the slab instead of hot forging, but it is necessary to apply a large reduction in rolling shape ratio. However, in order to apply a reduction with a large rolling shape ratio when the thickness of the slab is large, it is necessary to increase the amount of reduction per pass. Therefore, there is a problem that it is necessary to introduce expensive rolling equipment with a high load-bearing upper limit and torque upper limit.

Furthermore, in recent years, there has been a desire to reduce the weight of large structures. Therefore, it is required to further increase the strength of the steel plate, particularly to improve the yield strength.

The present invention has been developed in view of the above-mentioned current situation, and can be manufactured at low cost (in other words, with high productivity) without requiring special equipment, and has excellent intrinsic properties. and a steel plate having high strength. Another object of the present invention is to provide a method for manufacturing the thick steel plate.

The present inventors have made intensive studies to solve the above problems, and as a result, have obtained the following findings.
・Slabs, which are materials for rolling or forging thick steel plates, are generally manufactured by continuous casting or ingot casting. Therefore, the final solidification position is usually the vicinity of the thickness center position of the slab. When molten steel solidifies, volumetric contraction occurs. Therefore, void defects inevitably occur in the vicinity of the thickness center position of the slab. These void defects become starting points for fractures such as ductile fractures, brittle fractures and fatigue fractures, and the more void defects there are, the more frequently fractures occur.
・In order to reduce the amount of void defects generated in the vicinity of the thickness center position of the slab, it is effective to increase the amount of strain introduced in the vicinity of the position during hot rolling. However, the distribution of the strain introduced into the slab by hot rolling in the thickness direction is greatest near the surface of the slab that is in contact with the rolling rolls, and decreases toward the center of the thickness. Therefore, the strain amount is the smallest at the thickness center position of the slab, and the void defect crimping capability is also the lowest.

Therefore, the present inventors conducted various studies to increase the amount of strain in the vicinity of the thickness center position of the slab in hot rolling without using special equipment.
As a result, the present inventors obtained the following findings.
・By reducing the temperature difference between the slab surface and the thickness center position to a certain level or more, the deformation resistance near the slab surface is increased relatively to the thickness center position and applied to the slab surface vicinity. Strain amount can be reduced. The reduced amount of strain applied near the surface of the slab has the effect of increasing the amount of strain applied near the plate thickness center position.
・In addition, by reducing the temperature at the center of the thickness of the slab at a certain level or higher, specifically 700° C. or higher, it is possible to more advantageously close void defects by rolling strain and crimp by metal bonding.

Then, the present inventors conducted further studies based on the above knowledge, and in particular, by increasing the rolling reduction in the rolling pass that satisfies the following (a) and (b), the thickness center position of the slab We have found that the amount of void defects in the vicinity can be greatly reduced.
(a) Temperature at the thickness center position of the slab: 700°C or more (b) Temperature difference between the surface of the slab and the thickness center position: 100°C or more

In addition, the inventors of the present invention made further studies and obtained the following findings.
・By setting the area ratio of void defects at the plate thickness center position of the thick steel plate to 0.5% or less, it is possible to obtain excellent internal properties with a sufficiently reduced risk of fracture occurrence.
・In order to make the area ratio of void defects at the thickness center position of a thick steel plate 0.5% or less, the total rolling reduction in the rolling passes that satisfy the above (a) and (b) in the hot rolling process is More than 30% is effective.

In addition, the inventors of the present invention have further studied, and have made further studies to achieve a higher strength, specifically, a yield strength of 325 MPa or more while ensuring excellent internal properties. I got some insight.
That is, it is effective to appropriately control Ceq defined by the following equation (1) according to the thickness of the steel plate, specifically to satisfy the relationship Ceq/t≧0.0015.
Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15 (1)

In addition, the present inventors found that in cooling after hot rolling, the cooling rate when passing through the transformation temperature range from the austenitic structure, specifically, the 700 to 1/4 position of the thickness of the hot rolled steel sheet. It has been found that it is effective to set the average cooling rate in the temperature range of 600° C. to 6000 t ^−1.8 or more depending on the thickness t (mm) of the hot rolled steel sheet.
The present invention has been completed based on the above findings and further studies.

That is, the gist and configuration of the present invention are as follows.

[1] % by mass,
C: 0.04 to 0.18%,
Si: 0.03 to 0.70%,
Mn: 0.30-2.50%,
P: 0.030% or less,
S: 0.0200% or less,
Al: 0.001 to 0.100%,
O: 0.0100% or less and N: 0.0100% or less, with the balance being Fe and inevitable impurities,
Ceq and plate thickness t (mm) defined by the following formula (1) satisfy the relationship of Ceq/t ≥ 0.0015,
The area ratio of void defects at the plate thickness center position is 0.5% or less,
A thick steel plate having a yield strength of 325 MPa or more.
Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15 (1)
Here, [element symbol] in formula (1) is the content (% by mass) of each element in the component composition, and is calculated as 0 when the element is not contained.

[2] The component composition further contains, in % by mass,
Cu: 2.00% or less,
Ni: 2.50% or less,
Cr: 1.50% or less,
Mo: 1.00% or less,
Nb: 0.100% or less,
Ti: 0.100% or less,
V: 0.30% or less,
B: 0.0100% or less,
W: 0.50% or less,
Ca: 0.0200% or less,
The steel plate according to [1] above, containing one or more selected from the group consisting of Mg: 0.0200% or less and REM: 0.0500% or less.

[3] A method for manufacturing a thick steel plate according to [1] or [2] above,
a preparation step of preparing a slab having the composition according to [1] or [2];
A hot rolling step of hot rolling the slab into a hot rolled steel sheet;
a cooling step of cooling the hot-rolled steel sheet;
with
The total rolling reduction in the rolling passes satisfying the following (a) and (b) in the hot rolling process is more than 30%,
A method for producing a thick steel plate, wherein in the cooling step, an average cooling rate (° C./s) in a temperature range of 700 to 600° C. at a position of 1/4 thickness of the hot rolled steel plate is 6000 t ^−1.8 or more.
(a) Temperature at the thickness center position of the slab: 700° C. or more (b) Temperature difference between the surface of the slab and the thickness center position: 100° C. or more Here, t is the thickness (mm) of the hot-rolled steel sheet.

[4] The method for manufacturing a thick steel plate according to [3], further comprising a tempering step in which the hot-rolled steel plate is tempered at a tempering temperature of 650°C or less after the cooling step.

According to the present invention, it is possible to obtain a thick steel plate that can be manufactured at low cost without requiring special equipment, has excellent internal properties, and has high strength.
The use of the thick steel plate of the present invention is not particularly limited, and it is generally used for thick steel plates such as ships, line pipes, buildings, bridges, offshore structures, wind power generators, construction machinery and pressure vessels. is applicable to a wide range of fields where

A thick steel plate according to the present invention will be described based on the following embodiments.
First, the chemical composition of the steel plate according to one embodiment of the present invention will be described. In addition, the unit of the content of the element in the component composition is "mass %", and hereinafter, unless otherwise specified, it is indicated simply as "%".

C: 0.04-0.18%
C is an element that can most inexpensively improve the strength of steel. If the C content is less than 0.04%, the desired strength cannot be obtained. On the other hand, when the C content exceeds 0.18%, the weldability deteriorates. Also, the toughness is reduced. Therefore, the C content should be 0.04 to 0.18%. In addition, 0.05% or more of C content is preferable. Also, the C content is preferably 0.17% or less.

Si: 0.03-0.70%
Si is an element effective for deoxidation. If the Si content is less than 0.03%, sufficient effects cannot be obtained. However, when the Si content exceeds 0.70%, the weldability deteriorates. Therefore, the Si content should be 0.03 to 0.70%. Incidentally, the Si content is preferably 0.04% or more. Also, the Si content is preferably 0.60% or less.

Mn: 0.30-2.50%
Mn is an element that improves the hardenability and strength of steel at low cost. From the viewpoint of obtaining such effects, the Mn content is set to 0.30% or more. On the other hand, when the Mn content exceeds 2.50%, the weldability deteriorates. Therefore, the Mn content should be 0.30 to 2.50%. Incidentally, the Mn content is preferably 0.50% or more. Moreover, the Mn content is preferably 2.20% or less.

P: 0.030% or less P is an element that greatly embrittles grain boundaries. Therefore, when P is contained in a large amount, the toughness of the steel is lowered. Therefore, the P content should be 0.030% or less. The P content is preferably 0.025% or less. On the other hand, the lower the P content, the better, so the lower limit of the P content is not particularly limited, and may be 0%. However, P is an element that is inevitably contained in steel as an impurity, and an excessive decrease in P leads to an increase in refining time and cost. Therefore, the P content is preferably 0.001% or more.

S: 0.0200% or less S lowers the toughness of steel. Therefore, the S content should be 0.0200% or less. The S content is preferably 0.0100% or less. On the other hand, the lower the S content, the better, so the lower limit of the S content is not particularly limited, and may be 0%. However, S is an element that is unavoidably contained in steel as an impurity, and excessive reduction of S causes an increase in refining time and an increase in cost. Therefore, the S content is preferably 0.0001% or more.

Al: 0.001-0.100%
Al is an element effective for deoxidation. Also, Al is an element that forms nitrides and has the effect of reducing the grain size of austenite. In order to obtain such effects, the Al content is made 0.001% or more. On the other hand, when the Al content exceeds 0.100%, the cleanliness of the steel is lowered. As a result, ductility and toughness are reduced. Therefore, the Al content is set to 0.001 to 0.100%. In addition, 0.005% or more of Al content is preferable. Also, the Al content is preferably 0.080% or less.

O: 0.0100% or less O is an element that reduces ductility and toughness. Therefore, the O content is set to 0.0100% or less. On the other hand, the lower the O content, the better, so the lower limit of the O content is not particularly limited, and may be 0%. However, O is an element that is inevitably contained in steel as an impurity, and excessive reduction in O causes an increase in refining time and an increase in cost. Therefore, the O content is preferably 0.0005% or more.

N: 0.0100% or less N is an element that reduces ductility and toughness. Therefore, the N content is set to 0.0100% or less. On the other hand, the lower the N content, the better, so the lower limit of the N content is not particularly limited, and may be 0%. However, since N is an element that is unavoidably contained in steel as an impurity, it may exceed 0% industrially. In addition, an excessive reduction in N causes an increase in refining time and an increase in cost. Therefore, the N content is preferably 0.0005% or more.

The basic chemical composition of the steel plate according to one embodiment of the present invention has been described above, but from the viewpoint of further improving strength and weldability (toughness of weld zone, welding workability, etc.), the following optional additions are made as appropriate. One or more of the elements can be contained.
Cu: 2.00% or less,
Ni: 2.50% or less,
Cr: 1.50% or less,
Mo: 1.00% or less,
Nb: 0.100% or less,
Ti: 0.100% or less,
V: 0.30% or less,
B: 0.0100% or less,
W: 0.50% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less and REM: 0.0500% or less

Cu: 2.00% or less Cu is an element that improves the strength of steel without significantly deteriorating toughness. However, when the Cu content exceeds 2.00%, hot tearing due to the Cu-enriched layer formed directly under the scale becomes a problem. Therefore, when Cu is contained, the Cu content is preferably 2.00% or less. Incidentally, the Cu content is more preferably 0.01% or more. Also, the Cu content is more preferably 1.50% or less.

Ni: 2.50% or less Ni is an element that enhances the hardenability of steel. Ni is also an element that has the effect of improving toughness. However, when the Ni content exceeds 2.50%, an increase in manufacturing cost becomes a problem. Therefore, when Ni is contained, the Ni content is preferably 2.50% or less. Incidentally, the Ni content is more preferably 0.01% or more. Also, the Ni content is more preferably 2.00% or less.

Cr: 1.50% or less Cr is an element that improves the strength of steel by improving the hardenability of steel. However, when the Cr content exceeds 1.50%, the weldability deteriorates. Therefore, when Cr is contained, the Cr content is preferably 1.50% or less. Incidentally, the Cr content is more preferably 0.01% or more. Also, the Cr content is more preferably 1.20% or less.

Mo: 1.00% or less Mo is an element that improves the strength of steel by improving the hardenability of steel. However, when the Mo content exceeds 1.00%, the weldability deteriorates. Therefore, when Mo is contained, the Mo content is preferably 1.00% or less. Incidentally, the Mo content is more preferably 0.01% or more. Also, the Mo content is more preferably 0.80% or less.

Nb: 0.100% or less Nb is an element that suppresses recrystallization when strain is applied to the austenitic structure due to solid solution Nb and finely precipitated NbC. Nb is also an element that has the effect of increasing the temperature of the non-recrystallization temperature range. However, when the Nb content exceeds 0.100%, the weldability deteriorates. Therefore, when Nb is contained, the Nb content is preferably 0.100% or less. The Nb content is more preferably 0.001% or more, and still more preferably 0.005% or more. Also, the Nb content is more preferably 0.075% or less, and still more preferably 0.050% or less.

Ti: 0.100% or less Ti is an element that has the effect of pinning movement of grain boundaries and suppressing grain growth by precipitating as TiN. However, when the Ti content exceeds 0.100%, the cleanliness of the steel deteriorates. As a result, ductility and toughness are reduced. Therefore, when Ti is contained, the Ti content is preferably 0.100% or less. Incidentally, the Ti content is more preferably 0.001% or more. Also, the Ti content is more preferably 0.080% or less.

V: 0.30% or less V is an element that improves the strength of steel by improving the hardenability of steel and forming carbonitrides. However, when the V content exceeds 0.30%, the weldability deteriorates. Therefore, when V is contained, the V content is preferably 0.30% or less. Incidentally, the V content is more preferably 0.01% or more. Also, the V content is more preferably 0.25% or less.

B: 0.0100% or less B is an element that improves the strength of steel by improving the hardenability of steel. However, when the B content exceeds 0.0100%, the weldability deteriorates. Therefore, when B is contained, the B content is preferably 0.0100% or less. Incidentally, the B content is more preferably 0.0001% or more. Also, the B content is more preferably 0.0070% or less.

W: 0.50% or less W is an element that improves the strength of steel by improving the hardenability of steel. However, when the W content exceeds 0.50%, the weldability deteriorates. Therefore, when W is contained, the W content is preferably 0.50% or less. Incidentally, the W content is more preferably 0.01% or more. Moreover, the W content is more preferably 0.40% or less.

Ca: 0.0200% or less Ca is an element that improves weldability by forming oxysulfides that are highly stable at high temperatures. However, when the Ca content exceeds 0.0200%, the cleanliness of the steel is lowered and the toughness of the steel is lowered. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. Incidentally, the Ca content is more preferably 0.0001% or more. Also, the Ca content is more preferably 0.0180% or less.

Mg: 0.0200% or less Mg is an element that improves weldability by forming oxysulfides that are highly stable at high temperatures. However, if the Mg content exceeds 0.0200%, the effect of adding Mg is saturated and the effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less. Incidentally, the Mg content is more preferably 0.0001% or more. Moreover, the Mg content is more preferably 0.0180% or less.

REM: 0.0500% or less REM (rare earth metal) is an element that improves weldability by forming an oxysulfide that is highly stable at high temperatures. However, if the REM content exceeds 0.0500%, the effect of adding REM is saturated, and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when REM is contained, the REM content is preferably 0.0500% or less. The REM content is more preferably 0.0001% or more. Also, the REM content is more preferably 0.0450% or less.

The balance other than the above elements in the chemical composition of the steel plate according to one embodiment of the present invention is Fe and unavoidable impurities. Incidentally, if the content of the element related to the optional additive component is less than each preferred lower limit, the element is treated as an unavoidable impurity.

Further, in the thick steel plate according to one embodiment of the present invention, Ceq defined by the following formula (1) and the plate thickness t (mm) of the thick steel plate satisfy the relationship of Ceq/t≧0.0015. is important.
Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15 (1)
Here, [element symbol] in formula (1) is the content (% by mass) of each element in the component composition, and is calculated as 0 when the element is not contained.

Ceq/t≧0.0015
Ceq is a parameter correlated with the strength of the steel structure. A desired yield strength can be obtained by appropriately controlling the ratio of Ceq to the plate thickness t (mm) of the thick steel plate, specifically by setting Ceq/t to 0.0015 or more. Therefore, Ceq/t is set to 0.0015 or more. In addition, Ceq/t is preferably 0.0018 or more. Although the upper limit of Ceq/t is not particularly limited, Ceq/t is preferably 0.0200 or less, for example.

In addition, in the steel plate according to one embodiment of the present invention, it is extremely important that the area ratio of void defects at the plate thickness center position is 0.5% or less.

Area ratio of void defects at center position of sheet thickness: 0.5% or less Void defects inside the steel plate become starting points of fractures such as ductile fracture, brittle fracture and fatigue fracture. In particular, when a large amount of void defects remain at the plate thickness center position of the thick steel plate, specifically, when the area ratio of the void defects at the plate thickness center position exceeds 0.5%, the frequency of such fractures decreases. As a result, a thick steel plate with excellent internal properties cannot be obtained. Therefore, the area ratio of void defects at the plate thickness center position is set to 0.5% or less. The area ratio of void defects at the thickness center position is preferably 0.3% or less. In addition, the lower limit of the area ratio of the void defects at the plate thickness center position is not particularly limited, and may be 0%.

Here, the area ratio of void defects at the sheet thickness center position is measured in accordance with the procedure described in Examples described later. In addition, excellent internal properties (excellent internal properties) means that the reduction ratio in the plate thickness direction of the thick steel plate measured by a tensile test in accordance with ASTM A370 (2010) is 35% or more. means. In addition, the detailed test conditions are as described in [Plate thickness direction tensile test] in Examples described later.

Also, high strength means that the yield strength measured by a tensile test conforming to JIS Z2241 (2011) is 325 MPa or more. In addition, detailed test conditions are as described in [Panel width direction tensile test] in Examples to be described later.

In addition, the steel structure of the steel plate according to one embodiment of the present invention is not particularly limited, but from the viewpoint of obtaining the desired strength more advantageously, it is a structure mainly composed of fine structures with an average grain size of 15 μm or less. preferably.
Here, the microstructure-based structure having an average grain size of 15 μm or less means that the total area ratio of ferrite and bainite to the entire steel structure is 60% or more, and the average grain size of ferrite and bainite (large angle Equivalent circle diameter of the grain boundary) is 15 μm or less. Note that the residual structure other than ferrite and bainite includes pearlite, martensite, etc., and the total area ratio of the residual structure to the entire steel structure is preferably 40% or less.
The area ratio and average crystal grain size of each structure may be measured by a conventional method, and can be measured, for example, by optical micrographs or scanning electron micrographs. In addition, the measurement position of the area ratio of each structure and the average crystal grain size shall be the plate thickness center position.

Further, the plate thickness of the thick steel plate according to one embodiment of the present invention is preferably 30 to 240 mm. The plate thickness of the thick steel plate according to one embodiment of the present invention is more preferably 50 mm or more, and still more preferably 101 mm or more. Further, the plate thickness of the thick steel plate according to one embodiment of the present invention is more preferably 230 mm or less.

Next, a method for manufacturing a thick steel plate according to one embodiment of the present invention will be described.
A method for manufacturing a thick steel plate according to one embodiment of the present invention includes:
a preparatory step of preparing a slab (steel material) having the above composition;
A hot rolling step of hot rolling the slab into a hot rolled steel sheet;
a cooling step of cooling the hot-rolled steel sheet;
with
The total rolling reduction in the rolling passes satisfying the following (a) and (b) in the hot rolling process is more than 30%,
In the cooling step, the average cooling rate (° C./s) in the temperature range of 700 to 600° C. at the position of 1/4 thickness of the hot rolled steel sheet is 6000 t ^{−1.8 or} more.
(a) Temperature at the thickness center position of the slab: 700°C or more (b) Temperature difference between the surface of the slab and the thickness center position: 100°C or more (where t is the thickness (mm) of the hot-rolled steel sheet) Note that the thickness of the thick steel plate (which is the final product) and the thickness of the hot-rolled steel plate are often the same, so here both thicknesses are represented by t.)
Thereby, the thick steel plate according to one embodiment of the present invention described above can be suitably manufactured. Each step will be described below.

The surface temperature of the slab and steel plate can be measured, for example, with a radiation thermometer. In addition, the temperature at the thickness center position of the slab is measured, for example, by attaching a thermocouple to the thickness center position of the slab, or the temperature distribution in the slab cross section is calculated by heat transfer analysis, and the result is used for the slab. It can be obtained by correcting with the surface temperature. The temperature at the position of 1/4 thickness of the hot-rolled steel sheet can also be obtained in the same manner. Hereinafter, unless otherwise specified, the temperature of the slab and steel plate means the surface temperature. In addition, here, for convenience, the material to be rolled during the hot rolling process is called a slab instead of a steel plate (hot rolled steel plate or thick steel plate), and the steel plate obtained through the hot rolling process is It is called rolled steel plate.

[Preparation process]
In the preparation step, a slab having the component composition described above is prepared. The method of preparation is not limited. For example, molten steel is melted by a known melting method such as a converter, an electric furnace, and a vacuum melting furnace. Optionally, secondary refining, such as ladle refining, may be performed. Then, the melted steel is made into a slab by, for example, a continuous casting method, an ingot casting method, or the like, and a slab having the chemical composition described above is prepared. In addition, what is necessary is just to follow a conventional method about each condition.

[Hot rolling process]
Next, the slab prepared in the preparatory step is heated as necessary and subjected to hot rolling to obtain a hot-rolled steel sheet. In this case, it is extremely important to satisfy the following conditions.

Total rolling reduction in rolling passes that satisfy (a) and (b) (hereinafter also referred to as rolling passes under predetermined conditions): more than 30% (a) Temperature at the thickness center position of the slab: 700 ° C. or higher (b ) Temperature difference between the surface of the slab and the thickness center position: 100°C or more In order to close the gap defects existing near the thickness center position of the slab and press it by metal bonding, the temperature at the thickness center position of the slab should be 700°C. It is effective to apply strain at ℃ or higher. Also, in order to increase the amount of strain applied to the slab near the thickness center position, it is necessary to perform rolling with a temperature difference of 100° C. or more between the surface of the slab and the thickness center position. From the viewpoint of closing void defects existing near the thickness center position of the slab and securing the amount of strain required for crimping, the total rolling reduction in rolling passes under predetermined conditions is set to more than 30%. The total rolling reduction in rolling passes under predetermined conditions is preferably 40% or more. The upper limit of the total rolling reduction in rolling passes under predetermined conditions is not particularly limited, but the total rolling reduction in rolling passes under predetermined conditions is preferably 65% or less.

The total rolling reduction in rolling passes under predetermined conditions is calculated by the following equation (1).
r _t =100×{(t _i1 −t _f1 )/t _i1 +(t _i2 −t _f2 )/t _i2 +(t _i3 −t _f3 )/t _i3 + . . . +(t _iN −t _fN ) /t _iN } (1)
here,
r _t is the total rolling reduction (%) in the rolling pass under the given conditions
t _iN is the thickness (mm) of the slab at the start of rolling in the N-th rolling pass among the rolling passes under predetermined conditions;
t _fN is the thickness (mm) of the slab at the end of rolling of the N-th rolling pass among the rolling passes under predetermined conditions,
N is the number of rolling passes under a predetermined condition.

Whether or not the temperature conditions specified in (a) and (b) above are satisfied is determined by the surface temperature of the slab at the start of rolling in the relevant rolling pass and the temperature at the thickness center position.

The method of adjusting the temperature difference between the surface of the slab and the thickness center position is not particularly limited. For example, by forcibly cooling the surface of the slab by air cooling or water cooling, the temperature difference between the surface of the slab and the thickness center position can be adjusted within the above range.

Conditions other than the above are not limited, and may be in accordance with conventional methods.
For example, the slab heating temperature is preferably 950-1300.degree. The total number of rolling passes in hot rolling is preferably 5 to 60 passes. N (the number of rolling passes under predetermined conditions) is preferably 5 to 50 passes. Hot rolling reduction ratio (= [slab thickness (mm) at the start of hot rolling (start of first rolling pass)] / [end of hot rolling (end of final rolling pass) of hot-rolled steel sheet obtained after Thickness (mm)]) is preferably 1.6 to 16. It is preferable that the finish rolling end temperature (delivery side temperature of the final pass) is 700 to 1000°C.

[Cooling process]
The hot-rolled steel sheet is then cooled. In this case, it is extremely important to satisfy the following conditions.

Average cooling rate (°C/s) in the temperature range of 700 to 600°C at the 1/4 position of the thickness of the hot-rolled steel sheet: 6000t ^-1.8 or more Specifically, in order to achieve a yield strength of 325 MPa or more, the transformation temperature range from the austenite structure, particularly the cooling rate when passing through the temperature range of 700 to 600 ° C. at the 1/4 position of the thickness of the hot rolled steel sheet can be accelerated according to the thickness t (mm) of the hot-rolled steel sheet. Therefore, the average cooling rate in the temperature range of 700 to 600° C. (hereinafter also referred to as the average cooling rate at 700 to 600° C.) at the position of 1/4 thickness of the hot-rolled steel sheet is set to 6000 t ^−1.8 or more. The average cooling rate at 700-600° C. is preferably 7000 t ^−1.8 or higher. The average cooling rate at 700-600° C. is preferably 30000 t ^−1.8 or less.

In addition, conditions other than the above are not limited, and may be in accordance with ordinary methods. For example, cooling methods include water cooling and gas cooling. Moreover, the cooling rate in temperature ranges other than the above is not particularly limited, and any cooling method may be used to cool to room temperature.

[Tempering process]
After the cooling process described above, the hot-rolled steel sheet may optionally be subjected to a tempering treatment in order to adjust strength, ductility, toughness, and the like. However, if the tempering temperature exceeds 650°C, the softening progresses excessively and the desired yield strength cannot be obtained. Therefore, when tempering is performed, the tempering temperature is set to 650° C. or lower. In addition, conditions other than the above are not particularly limited, and conventional methods may be followed. The tempering temperature is the temperature at the 1/4 thickness position of the hot-rolled steel sheet during soaking.

A molten steel having the chemical composition shown in Table 1 was melted, and a slab (slab) with a thickness of 260 to 600 mm was prepared by a continuous casting method, an ingot casting method, or the like. It should be noted that the blanks in the column of elements in Table 1 indicate that they are not intentionally added, and include not only the case of not containing (0%) but also the case of unavoidable containing. .

Next, the prepared slabs were hot-rolled and cooled under the conditions shown in Table 2, and partially tempered to obtain thick steel plates having a thickness t (mm) shown in Table 2. Since the values of the plate thickness of the thick steel plate and the plate thickness of the hot-rolled steel plate are the same, they are indicated as plate thickness t in Table 2. In Table 2, "-" in the column of tempering temperature means that no tempering treatment was performed. The rolling reduction ratio of hot rolling was in the range of 2.5 to 3.5, and N (the number of rolling passes under predetermined conditions) was in the range of 5 to 37 passes. The slab surface temperature was measured with a radiation thermometer, and the slab thickness center temperature and hot-rolled steel plate temperature at the 1/4 thickness position were measured with a thermocouple. The temperature difference between the surface of the slab and the central position of the plate thickness was adjusted by forcibly cooling the surface of the slab by air cooling or water cooling. For conditions other than the above, it was assumed that the ordinary method was followed.

For each thick steel plate thus obtained, the area ratio of void defects at the plate thickness center position was measured in the following manner. The measurement results are also shown in Table 2. In addition, in all the examples of the present invention, a microstructure-based structure (a ferrite- and bainite-based structure) having an average grain boundary of 15 μm or less was obtained.

[Measurement of Area Ratio of Void Defects at Plate Thickness Center Position]
From each thick steel plate obtained, the thickness is adjusted so that the cross section in the width direction (perpendicular to rolling direction) of the thick steel plate at the center position of the thickness of the thick steel plate is the evaluation surface at the center position in the longitudinal direction (rolling direction) of the thick steel plate. Samples for the full width of the steel plate were taken. Then, each of the obtained samples was mirror-polished with an alumina buffing finish. Next, in each sample, the evaluation area was defined as thickness direction: thickness center position ±3 mm x width direction: full width of the sheet, and the area ratio of void defects in the evaluation area was measured by image analysis. Then, the measured value was taken as the area ratio of the void defect at the plate thickness center position.

In addition, for each thick steel plate obtained, a tensile test in the thickness direction and a tensile test in the width direction were performed in the following manner to evaluate the internal properties and yield strength. The evaluation results are also shown in Table 2.

[Thickness direction tensile test]
From each thick steel plate thus obtained, a tensile test piece was taken at the central position in the longitudinal direction (rolling direction) of the thick steel plate so that the longitudinal direction of the tensile test piece was parallel to the thickness direction of the thick steel plate. Here, the tensile test piece was sampled so that the center position of the tensile test piece in the longitudinal direction coincided with the center position of the plate thickness of the thick steel plate (1/2 plate thickness position). Also, the sampling pitch in the sheet width direction was set to 100 mm, and the tensile test pieces were sampled over the full width of the sheet. The shape of the tensile test piece was ASTM A770 (2007) Type 3 shape. Next, using each tensile test piece thus obtained, a tensile test was performed according to ASTM A370 (2010) to measure the reduction ratio. Here, among the reduction ratios measured in each tensile test piece sampled over the full width of the thick steel plate, the minimum value was taken as the reduction ratio of the thick steel plate. When the value was 35% or more, it was evaluated that excellent internal properties were obtained.

[Sheet width direction tensile test]
A tensile test piece was prepared from each of the obtained thick steel plates so that the longitudinal direction of the tensile test piece was parallel to the plate width direction (perpendicular to the rolling direction) of the thick steel plate at the center position in the longitudinal direction (rolling direction) of the thick steel plate. was taken. Here, the tensile test piece was sampled so that the central position of the tensile test piece in the thickness direction was at the 1/4 position of the plate thickness of the thick steel plate. Also, the sampling pitch in the sheet width direction was set to 500 mm, and the tensile test pieces were sampled over the full width of the sheet. The shape of the tensile test piece was JIS No. 4 shape. Then, using each tensile test piece thus obtained, a tensile test was performed according to JIS Z2241 (2011) to measure the yield strength. Here, the yield strength of the thick steel plate was defined as the minimum value among the yield strengths measured in each tensile test piece sampled over the full width of the thick steel plate. Then, when the value was 325 MPa or more, it was evaluated that high strength was obtained.

As shown in Table 2, all the steel plates of the present invention had excellent internal properties and high strength. In addition, all of the thick steel plates of the present invention examples can be manufactured by general hot rolling equipment, and can be manufactured at low cost (with high productivity) without requiring special equipment. rice field.

On the other hand, in the thick steel plates of the comparative examples, at least one of the internal properties and the strength was insufficient.
That is, Comparative Example No. In Nos. 23 and 24, sufficient strength was not obtained because the C content was below the proper range.
Comparative example no. Nos. 25 and 26 did not satisfy the relationship of Ceq/t≧0.0015, so sufficient strength was not obtained.
Comparative example no. In Nos. 27 and 28, since the hot rolling conditions were unsuitable, the area ratio of void defects at the sheet thickness center position was large, and sufficient internal properties were not obtained.
Comparative example no. For Nos. 29 and 30, the average cooling rate in the temperature range of 700 to 600° C. was too slow, so sufficient strength could not be obtained.
Comparative example no. In Nos. 31 and 32, sufficient strength was not obtained because the tempering temperature was too high.

Claims

in % by mass,
C: 0.04 to 0.18%,
Si: 0.03 to 0.70%,
Mn: 0.30-2.50%,
P: 0.030% or less,
S: 0.0200% or less,
Al: 0.001 to 0.100%,
O: 0.0100% or less and N: 0.0100% or less, with the balance being Fe and inevitable impurities,
Ceq and plate thickness t (mm) defined by the following formula (1) satisfy the relationship of Ceq/t ≥ 0.0015,
The area ratio of void defects at the plate thickness center position is 0.5% or less,
A thick steel plate having a yield strength of 325 MPa or more.
Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15 (1)
Here, [element symbol] in formula (1) is the content (% by mass) of each element in the component composition, and is calculated as 0 when the element is not contained.
The component composition further, in mass %,
Cu: 2.00% or less,
Ni: 2.50% or less,
Cr: 1.50% or less,
Mo: 1.00% or less,
Nb: 0.100% or less,
Ti: 0.100% or less,
V: 0.30% or less,
B: 0.0100% or less,
W: 0.50% or less,
Ca: 0.0200% or less,
The steel plate according to claim 1, comprising one or more selected from the group consisting of Mg: 0.0200% or less and REM: 0.0500% or less.
A method for manufacturing a thick steel plate according to claim 1 or 2,
A preparation step of preparing a slab having the component composition according to claim 1 or 2;
A hot rolling step of hot rolling the slab into a hot rolled steel sheet;
a cooling step of cooling the hot-rolled steel sheet;
with
The total rolling reduction in the rolling passes satisfying the following (a) and (b) in the hot rolling process is more than 30%,
A method for producing a thick steel plate, wherein in the cooling step, an average cooling rate (° C./s) in a temperature range of 700 to 600° C. at a position of 1/4 thickness of the hot rolled steel plate is 6000 t −1.8 or more.
(a) Temperature at the thickness center position of the slab: 700° C. or more (b) Temperature difference between the surface of the slab and the thickness center position: 100° C. or more Here, t is the thickness (mm) of the hot-rolled steel sheet.
The method for manufacturing a thick steel plate according to claim 3, further comprising a tempering step in which the hot-rolled steel plate is tempered at a tempering temperature of 650°C or less after the cooling step.