KR20240075904A - Heavy steel plate and manufacturing method thereof - Google Patents

Abstract

Provided is a thick steel plate with excellent internal properties that can be manufactured at low cost without requiring special equipment. The predetermined component composition is set, and the area ratio of void defects at the central position of the sheet thickness is set to 0.5% or less.

Description

Heavy steel plate and manufacturing method thereof

The present invention relates to thick steel plates and their manufacturing methods.

Recently, structures are becoming larger in fields such as ships, line pipes, buildings, bridges, offshore structures, wind power generators, construction machinery, and pressure vessels. Along with this, the thickness of steel plates used in the above fields is also increasing.

As a technology related to such a thick steel plate (hereinafter also referred to as thick steel plate) and its manufacturing method, for example, Patent Document 1,

“In the hot forging method of steel materials for extending forging steel materials of an axially symmetrical shape on an upper anvil and a lower anvil, between the start of forging and the end of forging, A hot forging method for steel materials characterized by forming a process of forming a cross-sectional shape perpendicular to the direction of forging of the steel material into a rectangular or approximately rectangular shape with a ratio of the length of the long side to the length of the short side at least 1.4.”

This is disclosed.

In Patent Document 2,

“A slab forging method comprising continuously applying reduction in the width direction and then in the thickness direction to a slab using an asymmetric anvil with different widths of the upper and lower anvils,

Assume that the above reduction in the width direction is performed from one end in the longitudinal direction of the slab. At that time, the amount of deviation of the end positions of the upper and lower anvils on the other end in the longitudinal direction of the slab is ΔL, and the difference between the slab and the upper and lower anvils among the upper and lower anvils A slab forging method characterized by limiting the ratio ΔL/B to 0.20 or less when the contact length of the shorter contact length is B.

This is disclosed.

In Patent Document 3,

“In the hot forging method of a slab, which consists of continuously applying reduction in the width direction and then in the thickness direction using a vertically asymmetric anvil to a slab manufactured by continuous casting,

The slab reduction in the width direction is performed in two stages in which the slab is flipped between the first and second stages, and at least two reductions are performed in each stage, and when the slab is reduced in the width direction in each stage, Using an anvil with a width of 400 to 1,200 mm as a short side anvil, and an anvil with a width of 800 to 1,500 mm as a long side anvil, The reduction position on the anvil is such that the deviation (ΔL) between the slab feed allowance boundary at the time of the first slab reduction and the center of the anvil contact length (B) at the next reduction satisfies ΔL≤0.20B. By acting out of phase,

Each reduction ratio in the slab reduction in the width direction is 4% or more, and

The total reduction ratio in the slab reduction in the thickness direction is 10% or more.

A hot forging method of a slab characterized by the following.

This is disclosed.

In Patent Document 4,

“A method of manufacturing an ultra-thick steel plate in which a continuous casting method is subjected to broad side pass rolling in a rough rolling process and additionally rolled to the product thickness in a finish rolling process. Because,

In the finish rolling process, a method of manufacturing an extremely thick steel plate with excellent internal properties, characterized in that multiple passes are rolled at a rolling speed of 200 to 350 mm/sec.

This is disclosed.

In Patent Document 5, in a slab

“Al: Continuously cast strands of aluminum killed steel of 0.07% by weight or less are cut to a predetermined length, then hot charged in a blooming soaking furnace as colonnade pieces, and heated at 1050∼ Cracking at a temperature of 1150°C and performing slab rolling so that the value of the aspect ratio R according to the following formula is 0.5 or more,

This slab is then subjected to a dehydrogenation treatment to reduce the diffusible hydrogen contained in the center of its thickness to 1.2 ppm or less,

Thereafter, the slab is reheated to 950-1050°C and then rolled to a finished plate thickness of 50 mm or more.

After completion of this heavy plate rolling, accelerated cooling is performed at a heat generation rate of 15°C or more per minute between Ar ₃ and 500°C to 350°C at a temperature not lower than 40°C or higher, which is a continuous process. “Method for manufacturing high-toughness thick steel plates with excellent internal quality by casting.”

This is disclosed.

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

However, since the plate thickness of the thick steel plate is thick, there is a high risk of occurrence of fractures such as ductile fracture, brittle furnace fracture, and fatigue fracture. Therefore, there is a demand for a thick steel plate that reduces the risk of such destruction and has excellent internal properties. However, in thick steel plates manufactured using the techniques of Patent Documents 1 to 5, the risk of such destruction cannot necessarily be sufficiently reduced, and excellent durability characteristics may not be obtained.

Additionally, the techniques of Patent Documents 1 to 3 involve hot forging a slab. However, the manufacturing efficiency of hot forging is very low compared to 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 perform hot rolling rather than hot forging on the slab, but it is necessary to apply a reduction with a large rolling aspect ratio. However, in order to apply reduction with a large rolling aspect ratio in the stage where the slab plate thickness is thick, 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 upper limit of load resistance and a high upper limit of torque.

The present invention was developed in consideration of the above phenomenon, and its purpose is to provide a thick steel plate with excellent internal properties that can be manufactured at low cost (in other words, under high productivity) without requiring special equipment. Additionally, the present invention aims to provide a method for manufacturing the above-mentioned thick steel plate.

As a result of intensive studies to solve the above problems, the present inventors have obtained the following findings.

·Slabs, which are rolled or forged materials of thick steel plates, are generally manufactured by continuous casting or ingot casting. Therefore, usually, the final solidification position is near the center of the plate thickness of the slab. When molten steel solidifies, volumetric contraction occurs. Therefore, void defects inevitably occur near the center of the plate thickness of the slab. And, these void defects become the starting point of fractures such as ductile fracture, brittle fracture, and fatigue fracture, and as the amount of void defects increases, the frequency of occurrence of fracture increases.

· To reduce the amount of void defects generated near the center position of the plate thickness of the slab, it is effective to increase the amount of strain introduced during hot rolling near the position. However, the distribution of strain introduced into the slab by hot rolling in the sheet thickness direction is largest near the surface of the slab in contact with the rolling roll, and becomes smaller as it approaches the center of the sheet thickness. Therefore, at the center of the slab's plate thickness, the amount of deformation is the smallest and the ability to compress void defects is also the lowest.

Therefore, the present inventors conducted various studies to increase the amount of deformation near the center position of the plate thickness of the slab during hot rolling without using special equipment.

As a result, the present inventors obtained the following recognition.

· By performing reduction with the temperature difference between the surface of the slab and the center of plate thickness above a certain level, the deformation resistance near the surface of the slab is made relatively high with respect to the center of plate thickness, and the amount of deformation applied near the surface of the slab is reduced. It can be reduced. And, by reducing the amount of strain applied near the surface of the slab, there is an effect of increasing the amount of strain applied near the center position of the plate thickness.

·In addition, by performing rolling reduction at a temperature at the center of the plate thickness of the slab at a certain level or higher, specifically at 700°C or higher, void defects can more advantageously be closed by rolling deformation and compressed by metal bonding. there is.

Then, the present inventors conducted additional studies based on the above recognition, and in particular, increased the reduction ratio in the rolling pass that satisfies the following (a) and (b), thereby reducing the thickness of the slab near the center position. It was recognized that the amount of void defects could be significantly reduced.

(a) Temperature at the center of the slab thickness: 700°C or more

(b) Temperature difference between the surface of the slab and the center of the plate thickness: 100℃ or more

Furthermore, the present inventors conducted additional studies and obtained the following knowledge.

· By setting the area ratio of void defects at the center of the plate thickness of a thick steel plate to 0.5% or less, excellent internal properties are obtained with a sufficiently reduced risk of fracture.

To keep the area ratio of void defects at the center of the thickness of a thick steel plate to 0.5% or less, the total reduction ratio in the rolling passes that satisfy the above (a) and (b) in the hot rolling process must be 30%. It is valid to exceed it.

The present invention was completed based on the above recognition and further investigation.

That is, the main structure of the present invention is as follows.

[1] In mass%,

C: 0.03 to 0.18%,

Si: 0.03 to 0.70%,

Mn: 0.30 to 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

and has a component composition where the balance consists of Fe and inevitable impurities,

A thick steel plate in which the area ratio of void defects at the center of the plate thickness is 0.5% or less.

[2] The above component composition is further expressed 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,

Mg: 0.0200% or less and

REM: 0.0500% or less

The thick steel plate according to [1] above, comprising one or two or more types selected from the group consisting of.

[3] A method for manufacturing the thick steel plate according to [1] or [2] above,

A preparation process of preparing a slab having the component composition described in [1] or [2] above,

A hot rolling process is provided for hot rolling the slab,

A method for producing a thick steel plate, wherein the total reduction ratio in rolling passes that satisfies the following (a) and (b) in the hot rolling process is more than 30%.

(a) Temperature at the center of the slab thickness: 700°C or more

(b) Temperature difference between the surface of the slab and the center of the plate thickness: 100℃ or more

According to the present invention, it is possible to obtain a thick steel plate with excellent internal properties that can be manufactured at low cost without requiring special equipment.

In addition, the use of the thick steel plate of the present invention is not particularly limited, and can be used in a wide range of fields where thick steel plates are generally applied, such as ships, line pipes, buildings, bridges, marine structures, wind power generators, drying machines, and pressure vessels. Applicable.

(Form for carrying out the invention)

The thick steel plate of the present invention will be described based on the following embodiments.

First, the component composition of the thick steel plate according to one embodiment of the present invention will be described. In addition, the unit of element content in the component composition is all “mass %”, and hereinafter, unless otherwise specified, it is simply expressed as “%”.

C: 0.03 to 0.18%

C is an element that improves the strength of steel at the lowest cost. Additionally, C is an element that contributes to strengthening austenite grain boundaries. If the C content is less than 0.03%, the grain boundary strength of austenite decreases, causing hot cracking of the slab. Therefore, manufacturability significantly decreases. Additionally, sufficient strength is not obtained. On the other hand, when the C content exceeds 0.18%, weldability decreases. Additionally, toughness also decreases. Therefore, the C content is set to 0.03 to 0.18%. Additionally, the C content is preferably 0.05% or more. Additionally, the C content is preferably 0.17% or less.

Si: 0.03 to 0.70%

Si is an element effective in deoxidation. If the Si content is less than 0.03%, sufficient effects cannot be obtained. However, when the Si content exceeds 0.70%, weldability decreases. Therefore, the Si content is set to 0.03 to 0.70%. Additionally, the Si content is preferably 0.04% or more. Additionally, the Si content is preferably 0.60% or less.

Mn: 0.30 to 2.50%

Mn is an element that improves the hardenability of steel at low cost and improves strength. From the viewpoint of obtaining these effects, the Mn content is set to 0.30% or more. On the other hand, when the Mn content exceeds 2.50%, weldability decreases. Therefore, the Mn content is set to 0.30 to 2.50%. Additionally, the Mn content is preferably 0.50% or more. Additionally, the Mn content is preferably 2.20% or less.

P: 0.030% or less

P is an element that has a large effect of embrittlement of grain boundaries. Therefore, when P is contained in a large amount, the toughness of the steel decreases. Therefore, the P content is set to 0.030% or less. The P content is preferably 0.025% or less. On the other hand, since the smaller the amount of P, the more preferable it is, 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 excessive reduction of P results in an increase in refining time and an increase in cost. Therefore, the P content is preferably 0.001% or more.

S: 0.0200% or less

S reduces the toughness of steel. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less. On the other hand, since the smaller the amount of S, the more preferable it is, the lower limit of the S content is not particularly limited and may be 0%. However, S is an element that is inevitably contained in steel as an impurity, and excessive reduction of S leads to 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 in deoxidation. Additionally, Al is an element that has the effect of forming nitrides and reducing the austenite grain size. To obtain this effect, the Al content is set to 0.001% or more. On the other hand, when the Al content exceeds 0.100%, the cleanliness of the steel decreases. As a result, ductility and toughness decrease. Therefore, the Al content is set to 0.001 to 0.100%. Additionally, the Al content is preferably 0.005% or more. Additionally, 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, since the smaller the O, the more preferable, 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 of 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, since the smaller the N, the more preferable, the lower limit of the N content is not particularly limited and may be 0%. However, since N is an element that is inevitably contained in steel as an impurity, it may be more than 0% industrially. Additionally, excessive N reduction causes an increase in refining time and an increase in cost. Therefore, the N content is preferably 0.0005% or more.

Above, the basic component composition of the thick steel plate according to one embodiment of the present invention has been described, but in addition, from the viewpoint of further improvement in strength and weldability (toughness of the weld zone, welding workability, etc.), the following optional additions are made as appropriate. It may contain one or two or more types of elements.

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 cracking caused by the Cu-enriched layer formed immediately below the scale becomes a problem. Therefore, when containing Cu, it is preferable that the Cu content is 2.00% or less. Moreover, the Cu content is more preferably 0.01% or more. Moreover, the Cu content is more preferably 1.50% or less.

Ni: 2.50% or less

Ni is an element that improves the quenching properties of steel. Additionally, Ni is also an element that has the effect of improving toughness. However, if the Ni content exceeds 2.50%, an increase in manufacturing cost becomes a problem. Therefore, when containing Ni, it is preferable that the Ni content is 2.50% or less. Moreover, the Ni content is more preferably 0.01% or more. Moreover, 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 the steel. However, when the Cr content exceeds 1.50%, weldability decreases. Therefore, when Cr is included, it is preferable that the Cr content is 1.50% or less. Moreover, the Cr content is more preferably 0.01% or more. Moreover, 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 the steel. However, when the Mo content exceeds 1.00%, weldability decreases. Therefore, when Mo is included, it is preferable that the Mo content is 1.00% or less. Moreover, the Mo content is more preferably 0.01% or more. Moreover, 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 austenite structure by dissolved Nb or finely precipitated NbC. Additionally, Nb is also an element that has the effect of raising the temperature of the non-recrystallization temperature range. However, when the Nb content exceeds 0.100%, weldability decreases. Therefore, when containing Nb, it is preferable that the Nb content is 0.100% or less. Moreover, the Nb content is more preferably 0.001% or more, and even more preferably 0.005% or more. Moreover, the Nb content is more preferably 0.075% or less, and even more preferably 0.050% or less.

Ti: 0.100% or less

Ti is an element that has the effect of suppressing grain growth by pinning the movement of grain boundaries by precipitating as TiN. However, when the Ti content exceeds 0.100%, the cleanliness of the steel decreases. As a result, ductility and toughness decrease. Therefore, when containing Ti, it is preferable that the Ti content is 0.100% or less. Moreover, the Ti content is more preferably 0.001% or more. Moreover, 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 the steel and generating carbonitrides. However, when the V content exceeds 0.30%, weldability decreases. Therefore, when V is included, it is preferable that the V content is 0.30% or less. Moreover, the V content is more preferably 0.01% or more. Moreover, 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 the steel. However, when the B content exceeds 0.0100%, weldability decreases. Therefore, when B is included, it is preferable that the B content is 0.0100% or less. Moreover, the B content is more preferably 0.0001% or more. Moreover, 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 the steel. However, when the W content exceeds 0.50%, weldability decreases. Therefore, when containing W, it is preferable that the W content is 0.50% or less. Moreover, 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 an acid sulfide with high stability at high temperatures. However, when the Ca content exceeds 0.0200%, the cleanliness of the steel decreases and the toughness of the steel decreases. Therefore, when Ca is included, it is preferable that the Ca content is 0.0200% or less. Moreover, the Ca content is more preferably 0.0001% or more. Moreover, the Ca content is more preferably 0.0180% or less.

Mg: 0.0200% or less

Mg is an element that improves weldability by forming an acid sulfide with high stability at high temperatures. However, if the Mg content exceeds 0.0200%, the effect of adding Mg is saturated and an effect corresponding to the content cannot be expected, which becomes economically disadvantageous. Therefore, when Mg is included, it is preferable that the Mg content is 0.0200% or less. Moreover, 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 oxysulfides with high stability at high temperatures. However, if the REM content exceeds 0.0500%, the effect of adding REM is saturated and an effect corresponding to the content cannot be expected, which becomes economically disadvantageous. Therefore, when REM is included, it is preferable that the REM content is 0.0500% or less. Moreover, the REM content is more preferably 0.0001% or more. Moreover, the REM content is more preferably 0.0450% or less.

In the component composition of the thick steel plate according to one embodiment of the present invention, the remainder other than the above elements is Fe and inevitable impurities. In addition, with respect to elements according to the above-mentioned optional addition components, if the content thereof is less than each appropriate lower limit, the elements are treated as inevitable impurities.

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

Area ratio of void defects at the center of plate thickness: 0.5% or less

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

Here, the area ratio of void defects at the central position of the plate thickness is measured according to the method described in the Examples described later. In addition, excellent internal properties (excellent internal properties) mean that the area reduction ratio in the thickness direction of the thick steel plate measured by a tensile test based on ASTM A370 (2010) is 35% or more. it means. In addition, the detailed test conditions are the same as those described in [Plate Thickness Direction Tensile Test] in the Examples described later.

In addition, the 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 even more preferably 101 mm or more. In addition, 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 an embodiment of the present invention will be described.

A method for manufacturing a thick steel plate according to an embodiment of the present invention,

A preparation process of preparing a slab (steel material) having the above-mentioned composition,

A hot rolling process is provided for hot rolling the slab,

The total reduction ratio in the rolling passes satisfying the following (a) and (b) in the hot rolling process is more than 30%.

(a) Temperature at the center of the slab thickness: 700°C or more

(b) Temperature difference between the surface of the slab and the center of the plate thickness: 100℃ or more

Accordingly, the thick steel plate according to one embodiment of the present invention described above can be suitably manufactured. Hereinafter, each process will be described.

Additionally, the surface temperature of the slab can be measured using, for example, a radiation thermometer. In addition, the temperature at the center of the slab's plate thickness can be measured, for example, by attaching a thermocouple to the center of the slab's plate thickness, or by calculating the temperature distribution within the slab's cross section by electrothermal analysis and calculating the result on the surface of the slab. It can be obtained by correcting by temperature. Hereinafter, unless otherwise specified, the temperature of the slab and steel plate shall mean the surface temperature. In addition, here, for convenience, the material to be rolled during the hot rolling process will be referred to as a slab rather than a steel plate (hot rolled steel plate or thick steel plate).

[Preparation process]

In the preparation process, a slab having the above-described component composition is prepared. The preparation method is not limited. For example, molten steel is melted by known melting methods such as a converter, electric furnace, and vacuum melting furnace. Optionally, secondary refining such as ladle refining may be performed. Next, the melted molten steel is formed into a slab by, for example, a continuous casting method or an ingot method, and a slab having the above-mentioned composition is prepared. In addition, for each condition, it is good to follow a normal method.

[Hot rolling process]

Next, the slab prepared in the preparation process is heated as needed and hot rolled to obtain a thick steel sheet (hot rolled steel sheet). And at this time, it is very important to satisfy the following conditions.

Total reduction ratio in rolling passes satisfying (a) and (b) (hereinafter also referred to as rolling passes with predetermined conditions): exceeding 30%

(a) Temperature at the center of the slab thickness: 700°C or more

(b) Temperature difference between the surface of the slab and the center of the plate thickness: 100℃ or more

In order to close void defects that exist near the center of the slab's plate thickness and compress them by metal bonding, it is effective to apply strain at a temperature of 700°C or higher at the center of the slab's plate thickness. Additionally, in order to increase the amount of strain applied near the center of the plate thickness of the slab, it is necessary to perform rolling with the temperature difference between the surface of the slab and the center of the plate thickness being 100°C or more. From the viewpoint of securing the amount of deformation necessary for closing and compressing void defects existing in the vicinity of the center position of the plate thickness of such a slab, the total reduction ratio in rolling passes under predetermined conditions is set to exceed 30%. The total reduction ratio in rolling passes under predetermined conditions is preferably 40% or more. Additionally, the upper limit of the total reduction ratio in rolling passes under predetermined conditions is not particularly limited, but it is preferable that the total reduction ratio in rolling passes under predetermined conditions is 65% or less.

In addition, the total reduction ratio in rolling passes under predetermined conditions is calculated using the following equation (1).

_{r t = 100 ​ ​ ​ ​ ​ ​ ​ ​} /t _iN ｝···(1)

From here,

r _t is the total reduction ratio (%) in rolling passes under predetermined conditions

t _iN is the plate thickness (mm) of the slab at the start of rolling of the Nth rolling pass among rolling passes under predetermined conditions,

t _fN is the plate thickness (mm) of the slab at the end of rolling of the Nth rolling pass among rolling passes under predetermined conditions,

N is the number of rolling passes under predetermined conditions.

In addition, whether or not the temperature conditions specified in (a) and (b) above are satisfied is determined based on the surface temperature of the slab at the start of rolling in the rolling pass and the temperature at the center of the plate thickness.

Additionally, the method of adjusting the temperature difference between the surface of the slab and the center position of the plate thickness 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 center position of the plate thickness can be adjusted to the above range.

There is no limitation to conditions other than the above, and normal methods may be used.

For example, the slab heating temperature is preferably 950 to 1300°C. It is preferable that the total number of rolling passes in hot rolling is 5 to 60 passes. It is preferable that N (the number of rolling passes under predetermined conditions) is 5 to 50 passes. Reduction ratio of hot rolling (=[thickness of slab at the start of hot rolling (start of first rolling pass) (mm)]/[thickness of steel sheet obtained after completion of hot rolling (end of final rolling pass)]) is preferably set to 1.6 to 16. The finish rolling temperature (exit temperature of the final pass) is preferably set to 650 to 1000°C.

In addition, after the above hot rolling process, optional cooling treatment may be further performed. Additionally, arbitrary heat treatment such as quenching, annealing, tempering, etc. may be performed. There is no particular limitation on these cooling treatment conditions and heat treatment conditions, and normal methods may be used.

Example

Molten steel having the composition shown in Table 1 was melted, and a slab with a plate thickness of 260 to 600 mm was prepared by continuous casting or ingot method. In addition, blank positions in the element column of Table 1 indicate that the element is not added intentionally, and include not only cases where it is not contained (0%) but also cases where it is unavoidably contained.

Next, hot rolling was performed on the prepared slab under the conditions shown in Table 2 to obtain a thick steel plate with a plate thickness (mm) shown in Table 2. Additionally, the reduction ratio of hot rolling was set to be in the range of 2.5 to 3.5, and N (the number of rolling passes under predetermined conditions) was set to be 5 to 37 passes. Additionally, the surface temperature of the slab was measured using a radiation thermometer, and the temperature at the center of the slab's plate thickness was measured using a thermocouple. In addition, the temperature difference between the surface of the slab and the center of the plate thickness was adjusted by forcibly cooling the surface of the slab by air cooling or water cooling. For conditions other than those mentioned above, it was decided to follow the usual method.

For each thick steel plate obtained in this way, the area ratio of void defects at the center of the plate thickness was measured in the following manner. The measurement results are listed in Table 2.

[Measurement of the area ratio of void defects at the center of the plate thickness]

From each obtained thick steel plate, at the center position in the longitudinal direction (rolling direction) of the thick steel plate, the cross section in the width direction (direction perpendicular to rolling) of the thick steel plate at the center of the plate thickness of the thick steel plate serves as the evaluation surface, corresponding to the entire width of the steel plate. samples were collected. Subsequently, each obtained sample was mirror polished with alumina buffing for finish. Next, the evaluation area for each sample was set as sheet thickness direction: sheet thickness center position ±3 mm × width direction: entire sheet width, and the area ratio of void defects in the evaluation area was measured by image analysis. . And, the measured value was taken as the area ratio of void defects at the center of the plate thickness.

In addition, for each of the obtained thick steel plates, a tensile test in the thickness direction was performed in the following manner to evaluate the internal properties. The evaluation results are listed in Table 2.

[Tensile test in plate thickness direction]

From each obtained thick steel plate, a tensile test piece was taken at a 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 collected so that the longitudinal center position of the tensile test piece was the center position of the plate thickness of the thick steel plate (1/2 plate thickness position). In addition, the sampling pitch in the sheet width direction was set to 100 mm, and the tensile test specimen was sampled over the entire width of the sheet. The shape of the tensile test specimen was ASTM A770 (2007) Type3 shape. Next, using each of the collected tensile test specimens, a tensile test based on ASTM A370 (2010) was performed to measure the draw rate. And, among the drawing rates measured with each tensile test piece taken across the entire width of the thick steel plate, the minimum value was taken as the drawing rate of the thick steel plate. And, when the value was 35% or more, it was evaluated that excellent resistance characteristics were obtained.

As shown in Table 2, excellent internal properties were obtained in all of the thick steel plates of the examples of the present invention. In addition, all of the thick steel plates of the examples of the present invention could be manufactured using general hot rolling equipment, and could be manufactured at low cost (under high productivity) without requiring special equipment.

On the other hand, in all of the thick steel plates of the comparative examples, sufficient internal properties were not obtained.

Claims (3)

In mass%,
C: 0.03 to 0.18%,
Si: 0.03 to 0.70%,
Mn: 0.30 to 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
and has a component composition where the balance consists of Fe and inevitable impurities,
A thick steel plate in which the area ratio of void defects at the center of the plate thickness is 0.5% or less. According to paragraph 1,
The above component composition is further expressed 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,
Mg: 0.0200% or less and
REM: 0.0500% or less
A thick steel plate containing one or two or more types selected from the group consisting of. A method for manufacturing the thick steel plate according to claim 1 or 2,
A preparation process of preparing a slab having the component composition according to claim 1 or 2,
A hot rolling process is provided for hot rolling the slab,
A method for producing a thick steel plate, wherein the total reduction ratio in rolling passes that satisfies the following (a) and (b) in the hot rolling process is more than 30%.
(a) Temperature at the center of the slab thickness: 700°C or more
(b) Temperature difference between the surface of the slab and the center of the plate thickness: 100℃ or more