KR20230155516A - Steel plate and its manufacturing method - Google Patents

Abstract

The chemical composition of the steel sheet, in mass%, is: C: 0.030 to 0.200%, Si: 0.050 to 0.500%, Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.001 to 0.100%, N: 0.0005~0.0080%, O: 0.0005~0.0080%, Ti: 0.001~0.050%, Nb: 0.001~0.050%, Cu: 0.01~0.50%, Mo: 0.01~0.10%, Sn: 0.01~0.30%, remainder : Fe and impurities, the total content of dissolved Mo and dissolved Sn in the surface layer of the steel sheet is 0.005% or more, and the metal structure at the 1/4t position is pearlite: 5 to 30%, bainite: 10% or less, and the balance: ferrite. , The metal structure at the 1/10t position is pearlite: 1 to 20%, bainite: 5% or less, the balance: ferrite, and the average grain size of the ferrite at the 1/10t position is 5 to 50 μm, pearlite at the 1/10t position. Steel plate with an average particle diameter of 30μm or less.

Description

Steel plate and its manufacturing method

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

Welded structural steel with excellent strength and weldability is used in steel oil tanks (hereinafter collectively referred to as “crude oil tanks”) that transport or store crude oil, such as crude oil tankers or above-ground or underground crude oil tanks. In addition, steel used as a crude oil tank is required to have excellent corrosion resistance against corrosive gas components, salts, etc. contained in crude oil (for example, see Patent Document 1).

Patent Document 1 shows that, with respect to crude oil corrosion occurring in a forced oil tank, it is excellent in suppressing general corrosion that corrodes uniformly on the surface of the steel sheet and local corrosion that occurs concentrated in local areas of the steel sheet surface, and also suppresses corrosion products (sludge) containing solid S. ) Steel for a crude oil tank for a welded structure that can suppress the production of crude oil tank, a method for manufacturing steel for a crude oil tank, a crude oil tank, and a method for protecting the crude oil tank are disclosed.

Japanese Patent Publication No. 2004-204344

The steel for crude oil tanks described in Patent Document 1 has excellent corrosion resistance because it contains a predetermined amount or more of Mo and W in a solid solution state. However, as a result of examination conducted by the present inventors, it was found that there is room for further improvement in corrosion resistance.

The purpose of the present invention is to solve the above problems and provide a steel sheet with excellent corrosion resistance to corrosive gas components, salts, etc. contained in crude oil, and a method for manufacturing the same.

As a result of detailed examination of the above problem, the present inventors have arrived at the following findings.

A possible method of improving the corrosion resistance of a steel sheet is to include Cu, Sn, and Mo. However, in the case where steel containing these elements contains a pearlite structure, which is a mixed structure of ferrite and cementite, and a bainite structure, a local cell is formed between ferrite and cementite due to a difference in C concentration, leading to corrosion. There is a problem that occurs.

If the metal structure of the steel is ferrite single phase, the above problem does not occur, but there is a problem that sufficient strength cannot be secured.

Accordingly, as a result of investigation, the present inventors found that both corrosion resistance and strength can be achieved by increasing the area ratio of ferrite in the surface layer region of the steel and forming a double phase structure containing ferrite and pearlite in the inner layer region of the steel.

The present invention has been made based on the above-mentioned knowledge, and has as its summary the following steel plate and its manufacturing method.

(1) The chemical composition of the steel sheet in mass%,

C: 0.030~0.200%,

Si: 0.050~0.500%,

Mn: 0.50~2.00%,

P: 0.030% or less,

S: 0.010% or less,

Al: 0.001~0.100%,

N: 0.0005~0.0080%,

O: 0.0005~0.0080%,

Ti: 0.001~0.050%,

Nb: 0.001~0.050%,

Cu: 0.01~0.50%,

Mo: 0.01~0.10%,

Sn: 0.01~0.30%,

Rest: Fe and impurities,

The total content of solid solution Mo and solid solution Sn in the surface layer portion of the steel sheet is 0.005% or more in mass%,

In the cross section of the steel sheet in the rolling direction, when the thickness of the steel sheet is t,

The metal structure at a position of 1/4t from the surface of the steel plate is expressed in area%,

Perlite: 5-30%,

Bainite: 10% or less,

The remainder: ferrite,

The metal structure at a position of 1/10t from the surface of the steel plate is expressed in area%,

Perlite: 1~20%,

Bainite: 5% or less,

The remainder: ferrite,

The average grain size of ferrite at a position of 1/10t from the surface of the steel sheet is 5 to 50 μm,

The average particle size of pearlite at a position of 1/10t from the surface of the steel plate is 30 μm or less,

Steel plate.

(2) The chemical composition of the steel plate in mass%,

C: 0.030~0.200%,

Si: 0.050~0.500%,

Mn: 0.50~2.00%,

P: 0.030% or less,

S: 0.010% or less,

Al: 0.001~0.100%,

N: 0.0005~0.0080%,

O: 0.0005~0.0080%,

Ti: 0.001~0.050%,

Nb: 0.001~0.050%,

Cu: 0.01~0.50%,

Mo: 0.01~0.10%,

Sn: 0.01~0.30%,

W: 0~0.20%,

Sb: 0~0.30%,

Pb: 0~0.30%,

As: 0~0.30%,

Bi: 0~0.30%,

Ni: 0~0.50%,

Cr: 0~0.10%,

V: 0~0.100%,

B: 0~0.0050%,

Ta: 0~0.50%,

Zr: 0~0.50%,

Ca: 0~0.0080%,

Mg: 0~0.0080%,

REM: 0~0.0080%;

Rest: Fe and impurities,

The total content of solid solution Mo and solid solution Sn in the surface layer portion of the steel sheet is 0.005% or more in mass%,

In the cross section of the steel sheet in the rolling direction, when the thickness of the steel sheet is t,

The metal structure at a position of 1/4t from the surface of the steel plate is expressed in area%,

Perlite: 5-30%,

Bainite: 10% or less,

The remainder: ferrite,

The metal structure at a position of 1/10t from the surface of the steel plate is expressed in area%,

Perlite: 1~20%,

Bainite: 5% or less,

The remainder: ferrite,

The average grain size of ferrite at a position of 1/10t from the surface of the steel sheet is 5 to 50 μm,

The average particle size of pearlite at a position of 1/10t from the surface of the steel plate is 30 μm or less,

Steel plate.

(3) The chemical composition is expressed in mass%, replacing part of the Fe,

W: 0.01~0.20%,

Sb: 0.03~0.30%,

Pb: 0.01~0.30%,

As: 0.01~0.30%, and

Bi: 0.01~0.30%

Containing one or two species selected from the group consisting of,

The steel plate described in (2) above.

(4) The chemical composition is expressed in mass%, replacing part of the Fe,

Ni: 0.05~0.50%,

Cr: 0.01~0.10%,

V: 0.010~0.100%,

B: 0.0003~0.0050%,

Ta: 0.005~0.50%, and

Zr: 0.005~0.50%

Containing at least one type selected from the group consisting of,

The steel plate according to (2) or (3) above.

(5) The chemical composition is expressed in mass%, replacing part of the Fe,

Containing a total of 0.0005 to 0.0080% of at least one type selected from the group consisting of Ca, Mg and REM,

The steel plate according to any one of (2) to (4) above.

(6) a refining process to produce molten steel,

A continuous casting process of continuously casting the molten steel to produce a steel piece having the chemical composition according to any one of (1) to (5) above,

A heating process of heating the obtained steel piece,

A hot rolling process to make a steel sheet by hot rolling the heated steel piece,

A standing cooling process of allowing the steel sheet after hot rolling to cool,

Provided with an accelerated cooling process of water cooling the steel sheet after standing to cool,

In the heating process, the steel piece is heated to a heating temperature of 950 to 1300°C,

In the hot rolling process, rolling is completed when the surface temperature of the steel piece is in the temperature range of Ar ₃ to T _rex ,

In the above standing cooling process, the surface temperature of the steel piece is left to cool to an end cooling temperature of Ar ₃ -100 to Ar ₃ -30°C, under the condition that the average cooling rate from the start of standing cooling to the end of standing cooling is 3°C/sec or less,

In the accelerated cooling process, the surface temperature of the steel piece is accelerated to 350 to 650°C under the condition that the average cooling rate from the start of accelerated cooling to the end of accelerated cooling exceeds 3°C/sec and is 30°C/sec or less. Water-cooled to temperature,

Manufacturing method of steel plate.

However, Ar ₃ is obtained from the formula (i) below, and T _rex is obtained from the formula (ii) below. In addition, the element symbol in the following formula represents the content (% by mass) of each element.

Ar ₃ =910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo… (i)

T _rex =-91900[Nb*] ² +9400[Nb*]+770 … (ii)

However, when the amount of dissolved Nb (mass%) obtained by equation (iii) below is taken as sol.Nb,

If Nb≥sol.Nb, [Nb*]=sol.Nb

If Nb<sol.Nb, [Nb*]=Nb

Do this.

sol.Nb=(10 ^{(-6770/(T+273)+2.26)} )/(C+12×N/14) … (iii)

In addition, T in the above formula represents the heating temperature (°C) of the steel piece.

(7) After the accelerated cooling process, a tempering process of heating to a temperature range of 350 to 650 ° C is additionally performed.

The method for manufacturing a steel plate according to (6) above.

According to the present invention, it becomes possible to obtain a steel sheet excellent in corrosion resistance to corrosive gas components, salts, etc. contained in crude oil.

Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition

The reasons for limitation of each element are as follows. In addition, in the following description, “%” relative to content means “mass%.” In addition, in this specification, "~" indicating a numerical range is used to mean that the numerical values described before and after it are included as the lower limit and upper limit, unless otherwise specified.

C: 0.030~0.200%

C is an element effective in forming pearlite and increasing strength. On the other hand, if the C content is excessive, it becomes difficult to secure weldability and joint toughness. Therefore, the C content is set to 0.030 to 0.200%. The C content is preferably 0.050% or more, 0.070% or more, or 0.100% or more, and is preferably 0.180% or less or 0.160% or less.

Si: 0.050~0.500%

Si is effective as an inexpensive deoxidizing element and reinforcing element. On the other hand, if the Si content is excessive, weldability and joint toughness deteriorate. Therefore, the Si content is set to 0.050 to 0.500%. The Si content is preferably 0.100% or more, and more preferably 0.150% or more. Moreover, the Si content is preferably 0.450% or less, and is preferably 0.400% or less.

Mn: 0.50~2.00%

Mn is effective as an element that improves the strength and toughness of the base material. On the other hand, if the Mn content is excessive, weldability and joint toughness deteriorate. Therefore, the Mn content is set to 0.50 to 2.00%. The Mn content is preferably 0.80% or more, and more preferably 0.90% or more. Moreover, the Mn content is preferably 1.60% or less, and more preferably 1.50% or less.

P: 0.030% or less

P is an element contained in steel as an impurity, and is set to 0.030% or less to ensure corrosion resistance. Additionally, in order to ensure toughness, the smaller the P content, the more preferable it is, preferably 0.015% or less. Additionally, there is no need to set a lower limit on the P content, and it may be 0%, but since excessive reduction leads to an increase in cost, it may be 0.003% or more.

S: 0.010% or less

S is an element contained in steel as an impurity, and in order to ensure corrosion resistance, the content is set to 0.010% or less. Additionally, in order to ensure toughness, a smaller S content is preferable, and the S content is preferably 0.003% or less. Additionally, there is no need to set a lower limit on the S content, and it may be 0%, but since excessive reduction leads to an increase in cost, it may be 0.001% or more.

Al: 0.001~0.100%

Al is an important deoxidizing element. On the other hand, if the Al content is excessive, the surface quality of the steel piece is impaired and inclusions harmful to toughness are formed. Therefore, the Al content is set to 0.001 to 0.100%. The Al content is preferably 0.005% or more or 0.010% or more, and is preferably 0.080% or less or 0.050% or less.

N: 0.0005~0.0080%

N forms nitrides with Al and improves joint toughness. On the other hand, if the N content is excessive, embrittlement due to dissolved N occurs. Therefore, the N content is set to 0.0005 to 0.0080%. The N content is preferably 0.0010% or more or 0.0020% or more, preferably 0.0070% or less, and more preferably 0.0060% or less.

O: 0.0005~0.0080%

O forms an oxide together with Ca, Mg, and REM, which will be described later. If the O content is excessive, the oxide becomes coarse and the toughness decreases. On the other hand, the lower the O content, the better, but excessively reducing it is not realistic because, for example, the refluxing operation to the RH vacuum degassing device takes a long time. Therefore, the O content is set to 0.0005 to 0.0080%.

Ti: 0.001~0.050%

Ti, when contained in trace amounts, contributes to improving toughness by refining the structure of the base material and weld zone. On the other hand, if the Ti content is excessive, the weld zone hardens and the toughness significantly deteriorates. Therefore, the Ti content is set to 0.001 to 0.050%. The Ti content is preferably 0.003% or more or 0.005% or more, and is preferably 0.040% or less or 0.030% or less.

Nb: 0.001~0.050%

Nb, when added in a small amount, contributes to the refinement of the structure and is an element effective in securing the strength of the base material. On the other hand, if the Nb content is excessive, the weld zone hardens and the toughness significantly deteriorates. Therefore, the Nb content is set to 0.001 to 0.050%. The Nb content is preferably 0.003% or more or 0.005% or more, and is preferably 0.040% or less or 0.030% or less.

Cu: 0.01~0.50%

Cu is an element effective in improving not only general corrosion resistance but also local corrosion resistance. In addition, there is an effect of suppressing the formation of S derived from corrosive gas components as solid S. On the other hand, if the Cu content is excessive, adverse effects such as promotion of surface cracks in the steel piece and deterioration of joint toughness also surface. Therefore, the Cu content is set to 0.01 to 0.50%. The Cu content is preferably 0.03% or more, preferably 0.40% or less, and more preferably less than 0.20%.

Mo: 0.01~0.10%

Mo is an element effective in improving local corrosion resistance. On the other hand, if the Mo content is excessive, the local corrosion resistance decreases and further deteriorates weldability and toughness. Therefore, the Mo content is set to 0.01 to 0.10%. The Mo content is preferably 0.02% or more, and more preferably 0.03% or more. Moreover, the Mo content is preferably 0.08% or less, and more preferably 0.07% or less.

Sn: 0.01~0.30%

Sn has the effect of further suppressing the progress of local corrosion. On the other hand, even if the Sn content exceeds 0.30%, the effect is saturated and there is a risk of adverse effects on other characteristics. Therefore, considering economic efficiency, the Sn content is set to 0.01 to 0.30%. The Sn content is preferably 0.03% or more or 0.05% or more, and is preferably 0.25% or less or 0.20% or less.

W: 0~0.20%

Since W is an element effective in improving local corrosion resistance, it may be contained as needed. On the other hand, if the W content is excessive, local corrosion resistance conversely decreases and weldability and toughness deteriorate. Therefore, the W content is set to 0.20% or less. The W content is preferably 0.15% or less, more preferably 0.10% or less, and even more preferably less than 0.05%. When it is desired to obtain the above effect more reliably, the W content is preferably 0.01% or more.

Sb: 0~0.30%

Since Sb has the effect of further suppressing the progress of local corrosion, it may be contained as needed. On the other hand, even if the Sb content exceeds 0.30%, the effect is saturated and there is a risk of adverse effects on other characteristics. Therefore, taking economic efficiency into consideration, the Sb content is set to 0.30% or less. The Sb content is preferably 0.25% or less or 0.20% or less. When it is desired to obtain the above effect more reliably, the Sb content is preferably 0.03% or more or 0.05% or more.

Pb: 0~0.30%

As: 0~0.30%

Bi: 0~0.30%

Since Pb, As, and Bi have the effect of further suppressing the progress of local corrosion, they may be contained as needed. On the other hand, even if the content of any one exceeds 0.30%, the effect is saturated and there is a risk of adverse effects on other characteristics. Therefore, taking economic efficiency into consideration, the contents of Pb, As, and Bi are all set to 0.30% or less. Moreover, it is preferable that the content of any element is 0.15% or less. When it is desired to obtain the above effect, it is preferable to contain at least one selected from Pb: 0.01% or more, As: 0.01% or more, and Bi: 0.01% or more.

Ni: 0~0.50%

Since Ni is effective in securing strength and improving toughness, it may be contained as needed. On the other hand, if the Ni content is excessive, the cost increases. Therefore, the Ni content is set to 0.50% or less. When it is desired to obtain the above effect more reliably, the Ni content is preferably 0.05% or more.

Cr: 0~0.10%

Cr improves hardenability and is effective in increasing strength, so it may be contained as needed. On the other hand, if the Cr content is excessive, the hardness of the joint may increase and the toughness may decrease. Therefore, the Cr content is set to 0.10% or less. When it is desired to obtain the above effect more reliably, the Cr content is preferably 0.01% or more.

V: 0~0.100%

Since V contributes to increasing strength through precipitation strengthening, it may be included as needed. On the other hand, if the V content is excessive, the joint toughness may be impaired. Therefore, the V content is set to 0.100% or less. When it is desired to obtain the above effect more reliably, the V content is preferably 0.010% or more.

B: 0~0.0050%

Since B contributes to improving the strength of the base material by increasing hardenability by adding a small amount, it may be included as needed. On the other hand, if the B content is excessive, the joint toughness deteriorates. Therefore, the B content is set to 0.0050% or less. When it is desired to obtain the above effect more reliably, the B content is preferably 0.0003% or more.

Ta: 0~0.50%

Zr: 0~0.50%

Ta and Zr are elements effective in increasing the strength of steel in trace amounts, and may be contained as needed, mainly for strength adjustment. On the other hand, when the content of either one exceeds 0.50%, toughness deterioration becomes significant. Therefore, the contents of Ta and Zr are both set to 0.50% or less. When it is desired to obtain the above effect, it is preferable to contain one or two types selected from Ta: 0.005% or more and Zr: 0.005% or more.

Ca: 0~0.0080%

Mg: 0~0.0080%

REM: 0~0.0080%

Ca, Mg, and REM all improve toughness by suppressing the generation of coarse inclusions (elongated MnS, etc.) by forming sulfides, so they may be contained as needed. On the other hand, even if the content of any one exceeds 0.0080%, the effect is saturated and coarse oxides or sulfides are formed to deteriorate toughness. Therefore, the contents of Ca, Mg, and REM are all set to 0.0080% or less.

When it is desired to obtain the above effects more reliably, it is desirable to set the total content of these elements to 0.0005% or more. Additionally, from the viewpoint of preventing deterioration of toughness characteristics due to coarse oxides or sulfides, it is preferable that the total content of these elements is 0.0080% or less. The total content is more preferably 0.0010% or more, and still more preferably 0.0015% or more. Moreover, the total content is more preferably 0.0060% or less, and even more preferably 0.0040% or less.

Here, in the present invention, REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM means the total content of these elements. Additionally, lanthanoid is industrially added in the form of misch metal.

In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities. Here, “impurities” are components that are mixed when industrially manufacturing steel sheets due to raw materials such as ore and scrap and various factors in the manufacturing process, and are allowed as long as they do not adversely affect the present invention.

Total content of dissolved Mo and dissolved Sn in the surface layer of the steel sheet: 0.005% or more

Since it is preferable for corrosion resistance that Mo and Sn exist in a solid solution state, the amounts of dissolved Mo and dissolved Sn in the surface layer portion of the steel sheet are secured to be above a predetermined value. Specifically, the total content of dissolved Mo and dissolved Sn in the surface layer portion of the steel sheet, in mass%, is set to 0.005% or more. The total content of solid solution Mo and solid solution Sn in the surface layer portion of the steel sheet is preferably 0.010% or more, and more preferably 0.020% or more. There is no need to set an upper limit on the total content of solid solution Mo and solid solution Sn, but the practical upper limit is 0.40%, which is the upper limit of the total content of Mo and Sn contained in the steel.

In addition, in the present invention, the steel sheet surface layer refers to the area from the surface of the steel sheet to a position of 1 mm in the depth direction. In addition, the total content (mass %) of solid solution Mo and solid solution Sn is measured according to the following procedure. First, two test pieces with a thickness of 1 mm are cut from the surface of the steel plate. Then, for one of the test pieces, the contents of Mo and Sn in the test piece are measured by using a known chemical analysis method (for example, ICP emission spectrometry).

Additionally, on the other side, 0.4 g of electrolyzed electrolyte was carried out with 10% by mass acetylacetone and 1% by mass tetramethylammonium chloride/methanol at a current density of 20mA/cm ² . The solution used in the electrolysis was filtered through a filter with a pore diameter of 0.2 μm, and a known chemical analysis method (e.g., ICP emission spectroscopy) was used for the extraction residue collected on the filter to determine Mo and Sn in the extraction residue. Measure the content of

Mo and Sn in the test piece are considered to be Mo precipitates, Sn precipitates, and dissolved Mo and dissolved Sn, and Mo and Sn in the extraction residue are considered to be Mo precipitates and Sn precipitates. Then, the contents of dissolved Mo and dissolved Sn are determined by determining the difference between the contents of Mo and Sn in the extraction residue from the contents of Mo and Sn in the test piece.

In addition, in the chemical composition of the steel sheet according to the present invention, Ceq defined by the formula (iv) below may be within the range of 0.20 to 0.50%.

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15... (iv)

However, the element symbol in the above formula represents the content (mass %) of each element contained in the steel sheet, and if not contained, 0 is substituted.

By setting the value of Ceq to 0.20% or more, it becomes easy to secure the strength required for the steel sheet. On the other hand, by setting Ceq to 0.50% or less, excellent toughness can be secured. Ceq is preferably 0.22% or more, more preferably 0.24% or more, and even more preferably 0.26% or more. Also, Ceq is preferably 0.48% or less, more preferably 0.46% or less, and even more preferably 0.45% or less.

(B) Metal structure

In the present invention, the steel sheet has a metal structure shown below at the inner layer position and the surface layer position, respectively. The metal structure of the inner layer position and the surface layer position of the steel sheet will be explained respectively.

In the following description, “%” means “area %.” In addition, the metal structure at the inner layer position of the steel sheet refers to the structure at a position 1/4t from the surface of the steel sheet when the thickness of the steel sheet is t. In addition, the metal structure at the surface layer of the steel sheet refers to the structure at a position of 1/10t from the surface of the steel sheet.

(B-1) Metal structure at the inner layer of the steel plate

Perlite: 5~30%

In order to secure yield stress and tensile strength, which are strength characteristics, the area ratio of pearlite is set to 5 to 30%. The area ratio of pearlite is preferably 10 to 20%.

Bainite: 10% or less

In the present invention, the metal structure is mainly composed of ferrite and includes a predetermined amount of pearlite. Even if 10% or less of bainite is contained, the above-mentioned effects are not impaired, but if the area ratio of bainite is excessive, toughness deteriorates. Therefore, the area ratio of bainite is preferably 10% or less, and is preferably 5% or less. Bainite does not have to be included, that is, the area ratio of bainite may be 0%.

Remaining: Ferrite

Ferrite is a structure with excellent toughness. Structures other than pearlite and bainite are ferrite. That is, the area ratio of ferrite is 60% or more. Meanwhile, from the viewpoint of securing strength characteristics, the area ratio of ferrite is preferably 90% or less, and more preferably less than 80%.

(B-2) Metal structure at the surface layer of the steel plate

Perlite: 1~20%

Bainite: 5% or less

Remaining: Ferrite

Pearlite is inevitably included in the metal structure. Additionally, there is a possibility that bainite may also be mixed in. However, as described above, in a corrosive environment, when the surface region of the steel sheet contains a large amount of pearlite structure and bainite structure, a local battery is formed between ferrite and cementite, and corrosion occurs. Therefore, it is necessary to reduce the area ratio of pearlite and bainite at the surface layer position. From that perspective, the area ratio of pearlite is set to 1 to 20%, and the area ratio of bainite is set to 5% or less.

It is desirable that the area ratio of pearlite and bainite be as low as possible. Specifically, the area ratio of pearlite is preferably 10% or less, and more preferably 5% or less. Moreover, the area ratio of bainite is preferably 3% or less, and more preferably 1% or less. Bainite does not have to be included, that is, the area ratio of bainite may be 0%.

In the metal structure at the surface layer, the remainder is ferrite. That is, the area ratio of ferrite is 75% or more. The area ratio of ferrite is preferably greater than 85%, and is preferably greater than 95%. The practical upper limit of the area ratio of ferrite is 99%.

The average particle size of ferrite is 5 to 50 μm.

In the metal structure at the surface layer, it becomes possible to improve toughness by refining the ferrite particles. Therefore, the average grain size of ferrite is set to 50 μm or less. In addition, the finer the ferrite particles, the more desirable it is, but since it is difficult to industrially realize particles less than 5 μm, the lower limit was set at 5 μm. The average particle diameter of ferrite is preferably 40 μm or less, and more preferably 30 μm or less.

The average particle size of pearlite is 30μm or less.

In the metal structure at the surface layer, the finer the average particle size of pearlite is, the finer the cementite that becomes the cathode site is, and the local corrosion is reduced. Therefore, the average particle size of pearlite is set to 30 μm or less.

(B-3) Relationship between metal structure at inner layer position and surface layer position

As described above, in the present invention, corrosion resistance and strength are achieved by increasing the area ratio of ferrite at the surface layer of the steel sheet and forming a double-phase structure containing ferrite and pearlite at the inner layer position of the steel sheet. If the metal structure at the inner layer position and the surface layer position of the steel sheet satisfies the above-mentioned conditions, both corrosion resistance and strength are achieved. Therefore, there is no need to particularly restrict the relationship between the metal structure at the inner layer position and the surface layer position, but in order to further improve both corrosion resistance and strength, the area ratio of ferrite at the surface layer position is changed to that of the ferrite at the inner layer position. It is preferable that it is higher than the area ratio.

(B-4) Method for measuring metal structure

In the present invention, the area ratio of the metal structure is calculated as follows. As described above, samples are first collected from positions 1/4t and 1/10t from the surface of the steel plate. Then, the cross section in the rolling direction (so-called L direction cross section) of the sample is observed. In addition, the above “rolling direction” means the rolling direction in finish rolling.

Specifically, the sample is nital etched, and after etching, observation is performed using an optical microscope at a magnification of 500 times and a field of view of 300 μm x 300 μm. Then, image analysis is performed on the obtained tissue photograph, and the area ratio is determined by assuming that the white color is ferrite and the black color is pearlite. In addition, in the present invention, pearlite also includes pseudo-perlite. In the steel sheet of the present invention, since bainite is used except for ferrite and pearlite, the area ratio of bainite is determined from the area ratio of the remainder. In addition, when observed under the above conditions, bainite appears gray.

In addition, the average particle diameter of ferrite and pearlite at the surface layer position is measured in the above-described microscope observation. Specifically, the area of each particle of ferrite and pearlite included in the field of view is determined through image analysis, and the diameter of a circle equivalent to this area is determined to determine the crystal grain size of ferrite and pearlite. Then, the average particle diameter of ferrite and pearlite is obtained by calculating the average value of the diameters of all ferrite and all pearlite in the field of view, respectively. Additionally, when calculating the average particle size of ferrite and pearlite, the minimum particle size subject to analysis is set to 1 μm.

(C) Mechanical properties

There are no particular restrictions on mechanical properties, but it is desirable that the steel sheet according to the present invention has the strength necessary for use as a crude oil tank, for example. Specifically, it is preferable that the yield stress (YS) is 235 MPa or more and the tensile strength (TS) is 400 to 620 MPa. In addition, the reason that the upper limit is set in the range of the appropriate tensile strength is that if the tensile strength is excessive, the toughness may deteriorate.

In addition, the tensile strength (TS) and yield stress (YS) were measured based on JIS Z 2241:2011 using a No. 1B tensile test piece taken in a direction perpendicular to the rolling direction. In detail, the yield stress (YS) is the proof stress of the permanent elongation method at a permanent elongation rate of 0.2%.

(D) Manufacturing method

There are no particular restrictions on the manufacturing conditions for the steel sheet according to the present invention, but it can be manufactured by sequentially performing the refining process, continuous casting process, heating process, hot rolling process, standing cooling process, and accelerated cooling process described later. Each process is explained.

(a) Refining process

Molten steel is manufactured in the refining process. For the refining process, any known method may be adopted, and there are no particular restrictions.

(b) Continuous casting process

In the continuous casting process, molten steel is continuously cast to produce a steel piece having the above-mentioned chemical composition. A known method may be adopted for the continuous casting process, and there are no particular restrictions.

(c) heating process

In order to perform hot rolling on a steel piece, the steel piece is heated. In the heating process, the steel piece having the above-mentioned chemical composition is heated to a heating temperature of 950 to 1300°C. The heating process may be performed in a heating furnace. In addition, heating the steel piece to 950 to 1300°C means heating it so that the average temperature of the entire thickness of the steel piece when extracted from the heating furnace is in the range of 950 to 1300°C. In this specification, the average temperature of the entire thickness of the steel piece is in the range of 950 to 1300°C. The temperature is called the heating temperature of the steel piece. In addition, the overall thickness average temperature can be calculated by calculation from the temperature in the heating furnace, the heating time, and the surface temperature of the steel piece.

If the heating temperature is less than 950°C, hot rolling becomes difficult. On the other hand, by setting the heating temperature to 1300°C or lower, coarsening of ferrite particles and pearlite particles at the surface layer position can be suppressed, and the area ratio of ferrite and pearlite at the inner layer position can be optimized. The heating temperature is preferably 1200°C or lower, and more preferably 1100°C or lower.

Additionally, there is no particular limitation on the holding time when heating the steel piece, and for example, it can be 120 minutes or less. The holding time is preferably 80 minutes or less or 60 minutes or less. In addition, the holding time in this specification refers to the man hour when the heating temperature of the steel piece is in the temperature range of 950 to 1300°C.

(d) hot rolling process

In the hot rolling process, hot rolling is performed on a steel piece to make a steel plate. At this time, rolling is completed when the surface temperature of the steel piece is in the temperature range of Ar ₃ to _Trex . By completing rolling at Ar ₃ or higher, the formation of stretched ferrite can be suppressed. In addition, by ending rolling in the non-recrystallized region of T _rex or less, it becomes possible to suppress coarsening of ferrite and pearlite particles at the surface layer position and optimize the area ratio of pearlite at the inner layer position.

Here, Ar ₃ is the ferrite transformation start temperature when cooling the steel, and is obtained by equation (i) below.

Ar ₃ =910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo… (i)

However, the element symbol in the above formula represents the content (% by mass) of each element.

In addition, T _rex means the recrystallization start temperature at which the generation and growth of new austenite grains begins, and is obtained by equation (ii) below. (ii) The formula is an empirical formula. Since some Nb is not dissolved in solid solution due to low-temperature heating, [Nb*] in the formula (ii) is the theoretical amount of dissolved Nb (mass %) calculated using the amount of Nb in the steel and the heating temperature, and the amount of Nb in the steel. It is corrected by taking into account, and T _rex is calculated using this [Nb*].

T _rex =-91900[Nb*] ² +9400[Nb*]+770 … (ii)

Here, [Nb*] is when the amount of dissolved Nb obtained by equation (iii) below is taken as sol.Nb,

If Nb≥sol.Nb, [Nb*]=sol.Nb

If Nb<sol.Nb, [Nb*]=Nb

Do this.

sol.Nb=(10 ^{(-6770/(T+273)+2.26)} )/(C+12×N/14) … (iii)

In addition, T in the above formula represents the heating temperature (°C) of the steel piece.

(e) Air cooling process

In the standing cooling process, the steel sheet after rolling is left standing to cool. At this time, the surface temperature of the steel piece is allowed to cool to an end cooling temperature of Ar ₃ -100 to Ar ₃ -30°C under the condition that the average cooling rate from the start of cooling to the end of cooling is 3°C/sec or less. By setting the average cooling rate to 3°C/sec or less, it becomes possible to suppress pearlite transformation and bainite transformation at the surface layer of the steel sheet. In addition, in the present invention, the surface temperature of the steel piece at the end of standing cooling is managed as the standing cooling end temperature.

Additionally, by allowing the steel piece to cool until the surface temperature of Ar ₃ -30°C or lower, it becomes possible to sufficiently secure the area ratio of ferrite in the metal structure at the surface layer position. On the other hand, by setting the standing cooling end temperature in the standing cooling process to Ar ₃ -100°C or higher, the temperature at the inner layer position of the steel sheet during standing cooling is prevented from falling below Ar ₃ and a predetermined pearlite is generated in the metal structure at the inner layer position. You can.

(f) accelerated cooling process

In the accelerated cooling process, the steel sheet after standing to cool is water cooled. At this time, water cooling is performed to an accelerated cooling end temperature of 350 to 650°C under the condition that the average cooling rate from the start of accelerated cooling to the end of accelerated cooling exceeds 3°C/sec and is below 30°C/sec. By water cooling to an accelerated cooling end temperature of 350 to 650°C at an average cooling rate exceeding 3°C/sec and not exceeding 30°C/sec, pearlite of a predetermined area% can be generated in the metal structure at the inner layer position. In addition, in the present invention, the surface temperature of the steel slab when water cooling is completed and reheating of the surface temperature of the steel slab is completed is managed as the accelerated cooling end temperature.

(g) Tempering process

After the accelerated cooling process, a tempering process of heating to a temperature range of 350 to 650°C may be additionally performed. Additionally, when the cooling stop temperature in the accelerated cooling process is high, a self-tempering effect is obtained, so the tempering process does not need to be performed.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

Example

Using steel pieces with the chemical composition in Table 1, steel sheets with a thickness of 5 to 50 mm were produced under the manufacturing conditions in Table 2.

The metal structure of the obtained steel plate was observed, and the area ratio of each structure was measured. Specifically, first, in the cross section of the steel sheet in the rolling direction, when the thickness of the steel sheet was set to t, test pieces for metal structure observation were cut from positions 1/4t and 1/10t from the surface of the steel sheet.

Then, the cross-section in the rolling direction (so-called L-direction cross-section) of the above-described test piece was subjected to nital etching, and after etching, it was observed using an optical microscope at a magnification of 500 times and with a field of view of 300 μm × 300 μm. By performing image analysis on the obtained structure photograph, the area ratios of each of ferrite, pearlite, and bainite were determined. More specifically, the area ratio of each was determined by considering the white color as ferrite and the black color as pearlite, and the area ratio of bainite was determined from the area ratio of the remainder.

In addition, the average particle diameter of ferrite and pearlite at the surface layer position was measured in the following procedure. The area of each particle of ferrite and pearlite included in the field of view was determined through image analysis, and the diameter of a circle equivalent to this area was determined to determine the crystal grain size of ferrite and pearlite. Then, the average particle diameter of ferrite and pearlite was obtained by calculating the average value of the diameters of all ferrite and all pearlite in the field of view, respectively. Additionally, when calculating the average particle size of ferrite and pearlite, the minimum particle size subject to analysis was set to 1 μm.

In addition, the total content (mass %) of solid solution Mo and solid solution Sn in the surface layer portion of the steel sheet was measured according to the following procedures. First, two test pieces with a thickness of 1 mm were cut from the surface of the steel plate, and for one of them, the contents of Mo and Sn in the test piece were measured by using ICP emission spectroscopy.

On the other hand, 0.4 g was electrolyzed with 10% acetylacetone-1% tetramethylammonium chloride/methanol at a current density of 20 mA/cm ² , and the solution used for the electrolysis was filtered through a filter with a pore size of 0.2 μm. With respect to the extraction residue collected on the filter, the contents of Mo and Sn in the extraction residue were measured by using ICP emission spectrometry.

Then, the contents of dissolved Mo and dissolved Sn were determined by determining the difference between the contents of Mo and Sn in the extraction residue from the contents of Mo and Sn in the test piece.

In addition, the tensile strength (TS) and yield stress (YS) were measured based on JIS Z 2241:2011. The test piece was measured using the No. 1B tensile test piece, which was sampled with the longitudinal direction perpendicular to the rolling direction (width direction). Yield stress (YS) is the proof stress of the permanent elongation method at a permanent elongation rate of 0.2%.

Then, in order to evaluate the corrosion resistance of the steel plate, three types of corrosion tests shown below were performed. Additionally, for corrosion tests 1 and 2, the Resolution MSC of the International Maritime Organization IMO. It was conducted in accordance with 289(87).

<Corrosion test 1>

A test piece with a length of 60 mm in the rolling direction, a length of 25 mm in the width direction, and a length of 5 mm in the thickness direction was taken from the surface of the steel sheet. All six surfaces of the test piece were polished with Emery polishing paper No. 600, creating a test piece with base iron exposed on all surfaces. The test piece was immersed in a 10% by mass NaCl aqueous solution whose pH was adjusted to 0.85 with hydrochloric acid. The immersion conditions were a liquid temperature of 30°C and an immersion time of 72 hours. Additionally, the test liquid was replaced with a new one every 24 hours. The volume of the test liquid was set to 25 cc/cm ² based on the surface area ratio of the test piece.

The corrosion loss was measured and the corrosion rate was evaluated. The composition of the corrosion solution simulates the environmental conditions when local corrosion occurs in an actual steel structure, and as the corrosion rate in the corrosion test is reduced, the rate of development of local corrosion in the actual environment is reduced. In addition, the corrosion loss was obtained by subtracting the mass of the test piece after the corrosion product was removed by pickling after the corrosion test from the mass of the test piece before the corrosion test.

<Corrosion test 2>

A test piece with a length of 60 mm in the rolling direction, a length of 25 mm in the width direction, and a length of 5 mm in the thickness direction was taken from the surface of the steel sheet. The surface of the test piece was polished with Emery polishing paper No. 600. The cut surface (other than the surface) was covered with paint, and a test piece was made from a 60 mm x 25 mm steel plate with only the surface of the base iron exposed. Additionally, test specimens were prepared for measurement after 21 cycles, 49 cycles, 77 cycles, and 98 cycles, respectively.

A glass container was prepared in which distilled water was placed in the lower third, and the upper end of the opening of the glass container was sealed with an acrylic cover having a gas supply port on the lower surface of the collected test piece. Next, the sealed glass container is placed in a constant temperature bath, and the temperature cycle of distilled water temperature is 30°C and test piece temperature is 50°C , were given 4 levels of 77 and 98 cycles. At that time, a gas of the following composition was blown into the gaseous phase part in the glass container from the above gas supply port. The composition of the gas used was CO ₂ : 13 volume%, H ₂ S : 500 ppm, O ₂ : 4 volume %, SO ₂ : 100 ppm, N ₂ : the balance.

Then, the corrosion loss after 21 cycles, 49 cycles, 77 cycles, and 98 cycles were measured, respectively, and the corrosion rate was evaluated from their relationship. The composition of the corrosion solution simulates the environmental conditions when general corrosion occurs in an actual steel structure, and as the corrosion rate in the corrosion test is reduced, the rate of progress of general corrosion in the actual environment is reduced. In addition, the corrosion loss was obtained by subtracting the mass of the test piece after the corrosion product was removed by pickling after the corrosion test from the mass of the test piece before the corrosion test.

<Corrosion test 3>

A test piece with a length of 40 mm in the rolling direction, a length of 40 mm in the width direction, and a length of 4 mm in the thickness direction was taken from the surface of the steel sheet. The cut surface (other than the surface) was covered with paint, and the surface was wet polished 600 times to remove iron oxide (scale) on the surface of the steel sheet, making a test piece with only the base iron exposed on the surface of the 40 mm x 40 mm steel sheet. Then, using the test piece, the corrosion rate and the rate of production of sludge mainly composed of solid S were evaluated in the following procedures.

First, before the corrosion test, an aqueous NaCl solution was applied to the surface of the test piece so that the NaCl adhesion amount was 1000 mg/m ² , dried, and installed horizontally on a constant temperature heater plate in the test chamber. After that, the gas adjusted to a certain dew point (30°C) was sent into the test chamber. The gas used had a composition of CO ₂ : 12 volume%, H ₂ S : 500 ppm, O ₂ : 5 volume%, and N ₂ : the balance.

Then, a temperature cycle of 20°C x 1 hour and 40°C x 1 hour for a total of 2 hours/cycle was applied to ensure that wet and dry cycles occurred on the surface of the test piece. After 720 cycles, the corrosion rate was evaluated from the corrosion loss, and the sludge generation rate was evaluated from the amount of material generated on the surface of the test piece. In addition, the product was confirmed by chemical analysis and X-ray analysis, and that it was iron oxyhydroxide (iron rust) and solid S through preliminary tests. In addition, the amount of produced substances was obtained from the difference in mass before and after removal of corrosion products by pickling. In addition, the corrosion loss was obtained by subtracting the mass of the test piece after pickling from the mass of the test piece before the corrosion test.

From the measurement results of these corrosion tests 1, 2, and 3, the corrosion rate and sludge production rate of test number 45 were set to 100, and the relative values of each test number were obtained for each corrosion test. in other words,

Relative corrosion rate = (corrosion rate of each test number/corrosion rate of test number 45) × 100

Relative sludge production rate = (sludge production rate of each test number/sludge production rate of test number 45) × 100

am.

Table 3 shows the relative corrosion rate and relative sludge production rate for each corrosion test. In addition, in this example, it is determined that corrosion resistance is excellent when both the relative corrosion rate and relative sludge production rate are 40% or less.

As can be seen from Table 3, the present invention examples (test numbers 1 to 26) that satisfy the provisions of the present invention had appropriate strength and showed excellent corrosion resistance in all corrosion tests.

In contrast to these, among the comparative examples, Test No. At values 28, 31 to 38, and 40, corrosion resistance was poor. Specifically, test no. In case 28, because the C content was excessive, the area ratio of pearlite exceeded the specified range, and corrosion resistance deteriorated. Test No. In 31 and 32, the corrosion resistance deteriorated because the P and S contents were excessive, respectively. Test No. In 33, corrosion resistance deteriorated because the Mo content was excessive. Test No. In 34, corrosion resistance was deteriorated because it did not contain Sn and Sb.

Test No. In 35, because the heating temperature in the heating process was too high, the ferrite particles and pearlite particles at the surface layer position became coarse, and the area ratio of ferrite and pearlite at the inner layer position deviated from the specified range. Test No. In No. 36, because the rolling completion temperature in the hot rolling process was too low, ferrite transformation occurred before dislocations were sufficiently introduced, and the ferrite particles and pearlite particles at the surface layer position could not be refined. Meanwhile, test no. In No. 37, because the rolling completion temperature in the hot rolling process was too high, the dislocations decreased due to recrystallization, sufficient dislocations were not secured during ferrite transformation, and the ferrite grains and pearlite grains at the surface layer became coarse.

Test No. In 38, the average cooling rate in the standing cooling process was too high, and Test No. At 40, because the cooling end temperature in the standing cooling process was too high, the area ratio of pearlite and bainite became excessive at the surface layer position.

Also, among the comparative examples, Test No. In samples 27, 29, and 30, the contents of C, Si, and Mn were less than the specified values, respectively. Also, test no. In 39, the cooling end temperature in the standing cooling process was too low, and Test No. In 41, the average cooling rate in the accelerated cooling process was too low, and Test No. In case 44, because the accelerated cooling end temperature in the accelerated cooling process was too high, the area ratio of pearlite became insufficient at the inner layer position. Therefore, in these examples, although the corrosion resistance, which is the subject of the present invention, was good, the tensile strength was low.

Meanwhile, test no. In 42, the average cooling rate in the accelerated cooling process was too high, and Test No. In case 43, because the accelerated cooling end temperature in the accelerated cooling process was too low, the area ratio of bainite became excessive at the inner layer position. Therefore, in these examples, although the corrosion resistance, which is the subject of the present invention, was good, the strength was excessive and did not satisfy the appropriate conditions.

According to the present invention, it becomes possible to obtain a steel sheet excellent in corrosion resistance to corrosive gas components, salts, etc. contained in crude oil. Therefore, the steel plate according to the present invention can be suitably used for crude oil tanks.

Claims (7)

The chemical composition of the steel plate is expressed in mass%,
C: 0.030~0.200%,
Si: 0.050~0.500%,
Mn: 0.50~2.00%,
P: 0.030% or less,
S: 0.010% or less,
Al: 0.001~0.100%,
N: 0.0005~0.0080%,
O: 0.0005~0.0080%,
Ti: 0.001~0.050%,
Nb: 0.001~0.050%,
Cu: 0.01~0.50%,
Mo: 0.01~0.10%,
Sn: 0.01~0.30%,
Rest: Fe and impurities,
The total content of solid solution Mo and solid solution Sn in the surface layer portion of the steel sheet is 0.005% or more in mass%,
In the cross section of the steel sheet in the rolling direction, when the thickness of the steel sheet is t,
The metal structure at a position of 1/4t from the surface of the steel plate is expressed in area%,
Perlite: 5-30%,
Bainite: 10% or less,
The remainder: ferrite,
The metal structure at a position of 1/10t from the surface of the steel plate is expressed in area%,
Perlite: 1~20%,
Bainite: 5% or less,
The remainder: ferrite,
The average grain size of ferrite at a position of 1/10t from the surface of the steel sheet is 5 to 50 μm,
A steel plate wherein the average particle size of pearlite at a position of 1/10t from the surface of the steel plate is 30 μm or less. The chemical composition of the steel plate is expressed in mass%,
C: 0.030~0.200%,
Si: 0.050~0.500%,
Mn: 0.50~2.00%,
P: 0.030% or less,
S: 0.010% or less,
Al: 0.001~0.100%,
N: 0.0005~0.0080%,
O: 0.0005~0.0080%,
Ti: 0.001~0.050%,
Nb: 0.001~0.050%,
Cu: 0.01~0.50%,
Mo: 0.01~0.10%,
Sn: 0.01~0.30%,
W: 0~0.20%,
Sb: 0~0.30%,
Pb: 0~0.30%,
As: 0~0.30%,
Bi: 0~0.30%,
Ni: 0~0.50%,
Cr: 0~0.10%,
V: 0~0.100%,
B: 0~0.0050%,
Ta: 0~0.50%,
Zr: 0~0.50%,
Ca: 0~0.0080%,
Mg: 0~0.0080%,
REM: 0~0.0080%,
Rest: Fe and impurities,
The total content of solid solution Mo and solid solution Sn in the surface layer portion of the steel sheet is 0.005% or more in mass%,
In the cross section of the steel sheet in the rolling direction, when the thickness of the steel sheet is t,
The metal structure at a position of 1/4t from the surface of the steel plate is expressed in area%,
Perlite: 5-30%,
Bainite: 10% or less,
The remainder: ferrite,
The metal structure at a position of 1/10t from the surface of the steel plate is expressed in area%,
Perlite: 1~20%,
Bainite: 5% or less,
The remainder: ferrite,
The average grain size of ferrite at a position of 1/10t from the surface of the steel sheet is 5 to 50 μm,
A steel plate wherein the average particle size of pearlite at a position of 1/10t from the surface of the steel plate is 30 μm or less. In claim 2,
The chemical composition, in mass%, replaces a part of the Fe,
W: 0.01~0.20%,
Sb: 0.03~0.30%,
Pb: 0.01~0.30%,
As: 0.01~0.30%, and
Bi: 0.01~0.30%
A steel plate containing one or two types selected from the group consisting of. In claim 2 or claim 3,
The chemical composition, in mass%, replaces a part of the Fe,
Ni: 0.05~0.50%,
Cr: 0.01~0.10%,
V: 0.010~0.100%,
B: 0.0003~0.0050%,
Ta: 0.005~0.50%, and
Zr: 0.005~0.50%
A steel plate containing at least one member selected from the group consisting of. The method of any one of claims 2 to 4,
The chemical composition, in mass%, replaces a part of the Fe,
A steel plate containing a total of 0.0005 to 0.0080% of at least one type selected from the group consisting of Ca, Mg, and REM. A refining process to manufacture molten steel,
A continuous casting process of continuously casting the molten steel to produce a steel piece having the chemical composition according to any one of claims 1 to 5,
A heating process of heating the obtained steel piece,
A hot rolling process to make a steel sheet by hot rolling the heated steel piece,
A standing cooling process of allowing the steel sheet after hot rolling to cool,
Provided with an accelerated cooling process of water cooling the steel sheet after standing to cool,
In the heating process, the steel piece is heated to a heating temperature of 950 to 1300°C,
In the hot rolling process, rolling is completed when the surface temperature of the steel piece is in the temperature range of Ar ₃ to T _rex ,
In the above standing cooling process, the surface temperature of the steel piece is left to cool to an end cooling temperature of Ar ₃ -100 to Ar ₃ -30°C, under the condition that the average cooling rate from the start of standing cooling to the end of standing cooling is 3°C/sec or less,
In the accelerated cooling process, the surface temperature of the steel piece is accelerated to 350 to 650°C under the condition that the average cooling rate from the start of accelerated cooling to the end of accelerated cooling exceeds 3°C/sec and is 30°C/sec or less. A method of manufacturing steel plates that involves water cooling to high temperature.
However, Ar ₃ is obtained from the formula (i) below, and T _rex is obtained from the formula (ii) below. In addition, the element symbol in the following formula represents the content (% by mass) of each element.
Ar ₃ =910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo… (i)
T _rex =-91900[Nb*] ² +9400[Nb*]+770 … (ii)
However, when the amount of dissolved Nb (mass%) obtained by equation (iii) below is taken as sol.Nb,
If Nb≥sol.Nb, [Nb*]=sol.Nb
If Nb<sol.Nb, [Nb*]=Nb
Do this.
sol.Nb=(10 ^{(-6770/(T+273)+2.26)} )/(C+12×N/14) … (iii)
In addition, T in the above formula represents the heating temperature (°C) of the steel piece. In claim 6,
A method of manufacturing a steel plate, in which, after the accelerated cooling process, a tempering process of heating to a temperature range of 350 to 650° C. is additionally performed.