WO2023162571A1

WO2023162571A1 - Steel plate and method for manufacturing same

Info

Publication number: WO2023162571A1
Application number: PCT/JP2023/002491
Authority: WO
Inventors: 恭野安田; 和彦塩谷
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-02-24
Filing date: 2023-01-26
Publication date: 2023-08-31
Also published as: JPWO2023162571A1; TW202336240A; TWI826257B

Abstract

Provided is a high-strength steel plate which has excellent ammonia SCC resistance and low-temperature toughness, and is used for storage tanks for storing liquefied gas in energy transport ships. The steel plate: has a predetermined composition; has hardness properties in which the average hardness is 230 HV0.1 or less and the hardness variation is 30 HV0.1 or less at a depth of 0.5 mm from the surface of the steel plate, the maximum hardness in the plate thickness direction is at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface of the steel plate, and the hardness variation in the plate thickness direction is 70 HV1 or less; and has a metal structure in which the volume fraction of a bainite structure at a depth of 0.5 mm from the surface of the steel plate is 90% or more.

Description

Steel plate and its manufacturing method

The present invention relates to a high-strength steel sheet with excellent toughness and corrosion resistance, particularly suitable for structural members such as tanks used in a low-temperature, liquid-ammonia environment. The present invention relates to a steel sheet for low temperature use and a method for manufacturing the same.

With the recent increase in energy demand, liquefied gas is being actively transported by energy transport ships. For efficient operation of energy carriers, tanks may carry liquid ammonia as well as LPG.
Recently, the use of hydrogen carriers for liquid ammonia and liquid ammonia fuel has been promoted, and thus the size of tanks for transportation and storage of liquefied ammonia has been increased.

Here, in carbon steel pipes, storage tanks, tank cars, line pipes, etc. that handle liquefied ammonia, stress corrosion cracking (hereinafter referred to as ammonia SCC (Stress Corrosion Cracking)) due to liquid ammonia is known to occur. Therefore, for steel materials used in a liquid ammonia environment, application of steel materials with low ammonia SCC susceptibility and engineering measures to suppress ammonia SCC have been taken.

For example, it is known that the occurrence of ammonia SCC is correlated with the strength of the material. Avoidance is being attempted. On the other hand, from the standpoint of increasing the size of tanks and reducing the amount of steel used in recent years, there is an increasing demand for higher strength steel sheets.

In addition, since liquefied gases such as LPG and liquid ammonia are transported and stored at low temperatures, the steel plates used in storage tanks for these liquefied gases are required to have excellent low temperature toughness.

Patent Documents 1 and 2 disclose techniques for satisfying the low-temperature toughness and the predetermined strength range required for liquefied gas storage tanks as described above. In the techniques described in these documents, high low-temperature toughness and predetermined strength properties are achieved by heat-treating a steel plate that has been cooled after hot-rolling, or by heat-treating a steel plate that has been water-cooled after hot-rolling several times. is realized.

JP-A-10-140235 JP-A-10-168516

However, the methods described in Patent Literatures 1 and 2 above had the economic problem of requiring multiple heat treatments, which required high equipment and energy costs.

The present invention solves the above problems and provides a high-strength steel sheet with excellent ammonia SCC resistance and low-temperature toughness, which is used for storage tanks used for storing liquefied gas in energy transport ships, and a method for producing the same. for the purpose.

In order to achieve the above objectives, the present inventors used the TMCP process and an online induction heating device to extensively study various factors affecting the low-temperature toughness and strength characteristics of steel sheets. As a result, the steel sheet contains a predetermined amount of elements such as C, Si, Mn, and Al, and the metal structure is controlled so that the volume fraction of the bainite structure at a position 0.5 mm from the surface of the steel sheet is 90% or more. At a depth of 0.5 mm from the surface of the steel sheet, the average hardness is 230 HV0.1 or less, the variation in hardness is 30 HV0.1 or less, and the maximum hardness in the thickness direction is the surface of the steel sheet By making it exist at a position of 1.0 mm or more and 1/4 or less of the plate thickness, and the hardness variation in the plate thickness direction is 70 HV1 or less, the SCC resistance in the liquid ammonia environment is effectively It has been found that multiple heat treatments, which are obtained and costly, can be omitted.

That is, the present invention was made based on the above findings, and the gist of the present invention is as follows.
1. in % by mass,
C: 0.010 to 0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.50%,
Al: 0.010 to 0.060%,
N: 0.0010% or more and 0.0100% or less,
P: 0.020% or less,
A steel sheet having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities,
At a depth of 0.5 mm from the surface of the steel sheet, the average hardness is 230 HV0.1 or less, the hardness variation is 30 HV0.1 or less, and the maximum hardness in the thickness direction is the steel sheet. A hardness characteristic that is located at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface and has a hardness variation of 70 HV1 or less in the plate thickness direction;
A steel sheet having a metal structure in which the volume fraction of the bainite structure at a position 0.5 mm deep from the surface of the steel sheet is 90% or more.

2. The component composition further, in mass %,
Cu: 0.01-0.50%,
Ni: 0.01 to 2.00%,
Cr: 0.01 to 1.00%,
Sn: 0.01 to 0.50%,
Sb: 0.01 to 0.50%,
Mo: 0.01-0.50% and W: 0.01-1.00%
2. The steel sheet according to 1 above, containing one or more selected from.

3. The component composition further, in mass %,
V: 0.01 to 1.00%,
Ti: 0.005 to 0.100%,
Co: 0.01 to 1.00%,
Nb: 0.005 to 0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
3. The steel sheet according to 1 or 2 above, containing one or more selected from.

4. in % by mass,
C: 0.010 to 0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.50%,
Al: 0.010 to 0.060%,
N: 0.0010% or more and 0.0100% or less,
P: 0.020% or less,
A steel material having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities, is subjected to hot rolling at a rolling end temperature of Ar ₃ transformation point or higher. A method for producing a steel sheet, _comprising :
In the accelerated cooling, the cooling stop temperature is in the range of 200 to 600 ° C., and the cooling rate at the 1/4 position of the plate thickness of the steel plate is 20 to 120 ° C./s,
The reheating is performed until the temperature reached at a position 1/4 of the plate thickness of the steel plate is 500 ° C. or less, and the temperature reached at a depth of 0.5 mm from the surface of the steel plate is in the range of 400 to 680 ° C. The steel plate. manufacturing method.

5. The chemical composition of the steel material is further, in mass%,
Cu: 0.01-0.50%,
Ni: 0.01 to 2.00%,
Cr: 0.01 to 1.00%,
Sn: 0.01 to 0.50%,
Sb: 0.01 to 0.50%,
Mo: 0.01-0.50% and W: 0.01-1.00%
4. The method for producing a steel sheet according to 4 above, containing one or more selected from.

6. The chemical composition of the steel material is further, in mass%,
V: 0.01 to 1.00%,
Ti: 0.005 to 0.100%,
Co: 0.01 to 1.00%,
Nb: 0.005 to 0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
6. The method for producing a steel sheet according to 4 or 5 above, containing one or more selected from Mg: 0.0005 to 0.0200% and REM: 0.0005 to 0.0200%.

According to the present invention, a steel sheet having excellent low-temperature toughness, that is, low-temperature impact resistance and ammonia SCC resistance, and having high strength suitable for structural members such as tanks used in a low-temperature and liquid ammonia environment. , can be provided in an inexpensive process.

An embodiment of the present invention will be described below. In addition, "%" representing the content of the following components (elements) means "% by mass" unless otherwise specified.

(1) Regarding chemical composition The chemical composition (chemical composition) of the steel sheet will be described below.

C: 0.010-0.200%
C is the most effective element for increasing the strength of steel sheets produced by cooling according to the present invention. In order to obtain such effects, the C content is specified to be 0.010% or more. Furthermore, the C content is preferably 0.013% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost. On the other hand, if the C content exceeds 0.200%, the toughness and weldability of the steel sheet deteriorate. Therefore, the C content is specified at 0.200% or less. Furthermore, the C content is preferably 0.170% or less from the viewpoint of toughness and weldability.

Si: 0.01-0.50%
Si is added for deoxidation. In order to obtain such effects, the Si content is specified to be 0.01% or more. Furthermore, it is preferable to make it 0.03% or more. On the other hand, if the Si content exceeds 0.50%, the toughness and weldability of the steel sheet are deteriorated. Therefore, the Si content is specified to be 0.50% or less. Furthermore, the Si content is preferably 0.40% or less from the viewpoint of toughness and weldability.

Mn: 0.50-2.50%
Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that need to be added in order to achieve high strength as in the present invention. In order to obtain such effects, the Mn content is specified to be 0.50% or more. Furthermore, the content of Mn is preferably 0.70% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost. On the other hand, if the Mn content exceeds 2.50%, the toughness and weldability of the steel sheet deteriorate, and the alloy cost becomes excessively high. Therefore, the Mn content is specified at 2.50% or less. Furthermore, the Mn content is preferably 2.30% or less from the viewpoint of suppressing deterioration of toughness and weldability.

Al: 0.010-0.060%
Al acts as a deoxidizing agent. In order to obtain such effects, the Al content is specified to be 0.010% or more. On the other hand, when the Al content exceeds 0.060%, the oxide inclusions increase to lower the cleanliness and toughness. Therefore, the Al content is specified at 0.060% or less. Furthermore, the Al content is preferably 0.050% or less from the viewpoint of further preventing toughness deterioration.

N: 0.0010 to 0.0100%
N contributes to the refinement of the structure and improves the toughness of the steel sheet. In order to obtain such effects, the N content is specified to be 0.0010% or more. Preferably, it is 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, the toughness is rather lowered. Therefore, the N content is specified at 0.0100% or less. Furthermore, the N content is preferably 0.0080% or less from the viewpoint of further suppressing deterioration of toughness and weldability. Incidentally, when Ti is present, N can bond with Ti and precipitate as TiN.

P: 0.020% or less P has an adverse effect, such as lowering toughness and weldability, by segregating at grain boundaries. Therefore, it is desirable to make the P content as low as possible, but a P content of 0.020% or less is acceptable. The lower limit of the P content is not particularly limited, and may be 0%. However, since P is an element that can industrially remain in steel, it may exceed 0%. Moreover, since excessive reduction causes a rise in refining cost, it is preferable to set the P content to 0.0005% or more from the viewpoint of cost.

S: 0.0100% or less S is present in steel as sulfide-based inclusions such as MnS, and is an element that exerts adverse effects, such as deteriorating the toughness of the steel sheet by becoming the origin of fracture. Therefore, it is desirable that the S content be as low as possible, but a content of 0.0100% or less is permissible. The lower limit of the S content is not particularly limited, and may be 0%. However, since S is an element that can industrially remain in steel, it may exceed 0%. Moreover, since an excessive reduction causes a rise in refining cost, it is preferable to set the S content to 0.0005% or more from the viewpoint of cost.

O: 0.0100% or less O is an element that forms an oxide, becomes a starting point of fracture, and has an adverse effect such as lowering the toughness of the steel sheet. The O content is preferably 0.0050% or less, more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited, and may be 0%. However, since O is an element that can industrially remain in steel, it may exceed 0%. Moreover, since an excessive reduction causes a rise in refining cost, it is preferable to set the O content to 0.0010% or more from the viewpoint of cost.

In the chemical composition of the steel sheet of the present invention, the balance other than the above components is Fe and unavoidable impurities. However, the above component composition can contain the elements described below, if necessary.

Cu: 0.01-0.50%, Ni: 0.01-2.00%, Cr: 0.01-1.00%, Sn: 0.01-0.50%, Sb: 0.01- 0.50%, Mo: 0.01 to 0.50%, and W: one or more selected from 0.01 to 1.00% Cu, Ni, Cr, Sn, Sb, Mo and W are It is an element that improves strength and ammonia SCC resistance, and one or more of these elements can be contained. In order to obtain such an effect, when Cu is contained, the Cu content is 0.01% or more, when Ni is contained, the Ni content is 0.01% or more, and when Cr is contained, When the Cr content is 0.01% or more, the Sn content is 0.01% or more when Sn is contained, and the Sb content is 0.01% or more when Sb is contained. When Mo is contained, the Mo content is preferably adjusted to 0.01% or more, and when W is contained, the W content is preferably adjusted to 0.01% or more. On the other hand, an excessive Ni content causes deterioration of weldability and an increase in alloy cost. Also, if Cu, Cr, Sn, Sb, Mo and W are contained excessively, weldability and toughness deteriorate, which is disadvantageous from the viewpoint of alloy cost. Therefore, the Cu content is 0.50% or less, the Ni content is 2.00% or less, the Cr content is 1.00% or less, the Sn content is 0.50% or less, and the Sb content is It is preferable to adjust the Mo content to 0.50% or less, the W content to 0.50% or less, and the W content to 1.00% or less. More preferably, the Cu content is 0.40% or less, the Ni content is 1.50% or less, the Cr content is 0.80% or less, the Sn content is 0.40% or less, and the Sb content is The amount is adjusted to 0.40% or less, the Mo content to 0.40% or less, and the W content to 0.80% or less.

V: 0.01-1.00%
V is an element that has the effect of improving the strength of the steel sheet, and can be optionally added. In order to obtain such an effect, when V is added, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 1.00%, the weldability deteriorates and the alloy cost increases. Therefore, when V is added, the V content is preferably 1.00% or less. More preferably, the lower limit of V content is 0.05% and the upper limit is 0.50%.

Ti: 0.005-0.100%
Ti is an element that has a strong tendency to form nitrides and has the action of fixing N and reducing solid solution N, and can be added arbitrarily. In addition, Ti can improve the toughness of the base material and the weld zone. In order to obtain such an effect, when adding Ti, the Ti content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more. On the other hand, when the Ti content exceeds 0.100%, the toughness rather decreases. Therefore, when adding Ti, the Ti content is preferably 0.100% or less. Furthermore, the Ti content is more preferably 0.090% or less.

Co: 0.01-1.00%
Co is an element that has the effect of improving the strength of the steel sheet, and can be optionally added. In order to obtain such an effect, when Co is added, the Co content is preferably 0.01% or more. On the other hand, if the Co content exceeds 1.00%, the weldability deteriorates and the alloy cost increases. Therefore, when Co is added, the Co content is preferably 1.00% or less. More preferably, the Co content has a lower limit of 0.05% and an upper limit of 0.50%.

Nb: 0.005-0.100%
Nb is an element that has the effect of reducing the grain size of prior austenite and improving the toughness by precipitating as a carbonitride. In order to obtain such an effect, when Nb is added, the Nb content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more. On the other hand, when the Nb content exceeds 0.100%, a large amount of NbC precipitates, resulting in a decrease in toughness. Therefore, when Nb is added, the Nb content is preferably 0.100% or less. Furthermore, it is more preferable to make it 0.060% or less.

B: 0.0001 to 0.0100%
B is an element that has the effect of significantly improving hardenability even when added in a very small amount. That is, the strength of the steel sheet can be improved. In order to obtain such an effect, when B is added, the B content is preferably 0.0001% or more. On the other hand, when the B content exceeds 0.0100%, the weldability deteriorates. Therefore, when B is added, the B content is preferably 0.0100% or less. More preferably, the B content has a lower limit of 0.0010% and an upper limit of 0.0030%.

Ca: 0.0005-0.0200%
Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Ca, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such effects, when Ca is added, the Ca content is preferably 0.0005% or more. On the other hand, when the Ca content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is added, the Ca content is preferably 0.0200% or less. More preferably, the Ca content has a lower limit of 0.0020% and an upper limit of 0.0100%.

Mg: 0.0005-0.0200%
Mg, like Ca, is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Mg, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such an effect, when Mg is added, the Mg content is preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. More preferably, the Mg content has a lower limit of 0.0020% and an upper limit of 0.0100%.

REM: 0.0005-0.0200%
Like Ca and Mg, REM (rare earth metal) is an element that binds to S and suppresses the formation of MnS or the like elongated in the rolling direction. That is, by adding REM, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such an effect, when REM is added, the REM content is preferably 0.0005% or more. On the other hand, when the REM content exceeds 0.0200%, the cleanliness of the steel deteriorates. A decrease in cleanliness leads to a decrease in toughness. Therefore, when REM is added, the REM content is preferably 0.0200% or less. More preferably, the REM content has a lower limit of 0.0020% and an upper limit of 0.0100%.

(2) Hardness characteristics and metal structure The steel sheet of the present invention has the above chemical composition, and in addition, the average The hardness is 230 HV0.1 or less, the variation in hardness at the 0.5 mm position is 30 HV0.1 or less, and the maximum hardness in the thickness direction is 1.0 mm or more from the surface of the steel sheet. 4 or less, and has a hardness characteristic in which the variation in hardness in the plate thickness direction is 70HV1 or less.
Further, the steel sheet of the present invention has a metal structure in which the volume fraction of bainite structure (hereinafter also simply referred to as bainite) at the 0.5 mm position is 90% or more.
The reasons for limiting the hardness properties and metallographic structure of the steel sheet as described above will be explained below.

[At the 0.5 mm position, the average hardness is 230HV0.1 or less, and the variation in hardness is 30HV0.1 or less]
At the 0.5 mm position, the average hardness is set to 230 HV0.1 or less, and the variation in hardness is set to 30 HV0.1 or less. If a high-hardness region exists in the extreme surface layer of the steel sheet, specifically, at a position of 0.5 mm from the surface of the steel sheet, stress corrosion cracking in a liquid ammonia environment is promoted. Moreover, when a local high-hardness region exists, stress concentration occurs when stress is applied to the steel sheet, and stress corrosion cracking is promoted. Therefore, in the steel sheet of the present invention, the average hardness at the 0.5 mm position is 230 HV0.1 or less, and the hardness variation is 30 HV0.1 or less by adjusting the hardness characteristics. can ensure the integrity of the Although the lower limit of the average hardness at the 0.5 mm position is not particularly limited, it is preferably about 130HV0.1. The lower limit of the variation in hardness at the 0.5 mm position may be 0HV0.1, but industrially it is about 10HV0.1.
Here, the average hardness can be calculated by measuring Vickers hardness at a plurality of points (for example, 100 points) at a position of 0.5 mm. Further, the variation in hardness means the standard deviation of the Vickers hardness measured to obtain the average hardness.

[The maximum hardness in the plate thickness direction is located at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface of the steel plate]
When the maximum hardness of the steel sheet exists at a position distant from the surface to some extent, the hardness of only the surface layer can be reduced while maintaining the hardness of most of the steel sheet. That is, it is possible to ensure excellent ammonia SCC resistance while maintaining the strength of the steel sheet.
Specifically, if the maximum value is located at a position less than 1.0 mm from the surface of the steel sheet, the hardness at the 0.5 mm position cannot be sufficiently reduced. On the other hand, if the maximum value is at a position exceeding 1/4 of the plate thickness from the surface of the steel plate, the steel plate itself cannot ensure sufficient strength. Therefore, in the steel plate of the present invention, the maximum value of the hardness in the plate thickness direction (Vickers hardness (HV1)) is defined as being at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface of the steel plate. .

[Hardness variation in plate thickness direction is 70HV1 or less]
If the hardness variation in the sheet thickness direction is large, not only will the uniform elongation of the steel sheet decrease, but also the residual stress due to the internal stress introduced by accelerated cooling will increase, so there is concern that the ammonia SCC resistance will deteriorate. . Therefore, in the present invention, the variation in hardness in the plate thickness direction is specified to be 70HV1 or less.
Here, the variation is calculated by measuring the Vickers hardness (HV1) at a pitch of 0.5 mm in the plate thickness direction and obtaining the difference between the maximum value and the minimum value.

[Bainite volume ratio at 0.5 mm position is 90% or more]
In order to satisfy strength characteristics and ammonia SCC resistance, it is necessary to make the volume fraction of bainite 90% or more in the structure at the 0.5 mm position. When a hard phase such as a martensite structure or an island-shaped martensite (MA) structure is generated in the surface layer, the surface layer hardness increases, and the variation in hardness within the steel sheet increases, impairing the uniformity of the material quality. That is, when the volume fraction of bainite is less than 90%, the volume fractions of other structures, that is, ferrite, island-shaped martensite, martensite, pearlite, and austenite, increase. Strength/or ammonia SCC resistance is not obtained.

Here, bainite includes a structure called bainitic ferrite or granular ferrite that transforms during or after accelerated cooling that contributes to transformation strengthening, and a structure obtained by tempering them.

The remaining structure occupying 10% or less in volume fraction may include a martensite structure in addition to the ferrite, pearlite, and austenite structures. The fraction of each structure in the remaining structure is not particularly limited, but the remaining structure is preferably a pearlite structure.
In addition, the volume fraction of various metal structures can be measured by the method described in Examples below.

(3) Manufacturing conditions In the manufacturing method of the present invention, a steel material having the same chemical composition as that described above for the steel sheet is heated and hot-rolled, then accelerated cooling is performed, and then reheating is performed. is. Reasons for limiting the manufacturing conditions of the steel sheet will be described below.
First, the manufacturing conditions of the steel material need not be particularly limited. It is preferable to use a steel material such as a slab of predetermined dimensions in the method. It should be noted that there is no problem in making a steel material such as a slab having a predetermined size by the ingot casting-decomposition rolling method.

The steel material thus obtained is directly hot-rolled without cooling or hot-rolled after reheating. Hot rolling is performed with a rolling end temperature equal to or higher than the Ar ₃ transformation point, then accelerated cooling from a cooling start temperature equal to or higher than the Ar ₃ transformation point is performed under predetermined conditions, and then reheating is performed under predetermined conditions.

The heating temperature of the steel material is not particularly limited, but if the heating temperature is too low, the deformation resistance increases, the load on the hot rolling mill increases, and hot rolling may become difficult. On the other hand, if the temperature exceeds 1300° C., the oxidation becomes significant, the oxidation loss increases, and the yield increases. For these reasons, the heating temperature is preferably 950° C. or higher and 1300° C. or lower.

(hot rolling)
[Rolling end temperature: Ar ₃ transformation point or higher]
In the present invention, after heating the steel material to the above temperature, hot rolling is started, and the hot rolling is finished at the Ar ₃ transformation point or higher.
When the rolling end temperature is lower than the Ar ₃ transformation point, ferrite is generated, the uniformity of the material in the surface layer of the steel sheet is hindered, and the variation in hardness increases, thereby deteriorating the ammonia SCC resistance. In addition, since the generated ferrite is affected by working, the toughness deteriorates. Furthermore, the load on the hot rolling mill increases.
Therefore, the rolling end temperature in hot rolling in the present invention is set to the Ar ₃ transformation point or higher. The rolling end temperature is more preferably Ar ₃ transformation point +10°C or higher. On the other hand, if the rolling end temperature exceeds 950°C, the structure may coarsen and the toughness may deteriorate, so the rolling end temperature is preferably 950°C or less.
Here, the Ar ₃ transformation point (°C) can be obtained by the following formula.
Ar ₃ (° C.)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo
However, each element indicates the content of the element in steel (% by mass).

(accelerated cooling)
[Cooling start temperature: Ar ₃ transformation point or higher]
Next, the hot-rolled steel sheet is subjected to accelerated cooling from a cooling start temperature equal to or higher than the Ar ₃ transformation point. If the cooling start temperature is less than the Ar ₃ transformation point, ferrite is excessively formed and the cooling rate increases, so it coexists with the martensitic structure or bainite, which has a large difference in strength, resulting in insufficient strength and deterioration of toughness. is generated, and further the ammonia SCC resistance deteriorates. Therefore, the cooling start temperature should be the Ar ₃ transformation point or higher.

[Cooling rate at 1/4 position of steel plate thickness: 20 to 120 ° C./s]
Accelerated cooling in which the cooling rate at the position of 1/4 of the plate thickness of the steel plate is 20 ° C./s or more is an indispensable process for obtaining a high-strength and high-toughness steel plate. A strength increase effect can be obtained by strengthening. Therefore, in order to obtain such an effect, the cooling rate at the position of 1/4 of the plate thickness of the steel plate during accelerated cooling according to the present invention is specified to be 20° C./s or more. On the other hand, when the cooling rate exceeds 120° C./s, the volume fraction of martensite becomes too large, resulting in a decrease in toughness. Therefore, the cooling rate at the 1/4 position of the plate thickness of the steel plate is specified to be 120° C./s or less.
The cooling rate can be increased by active cooling operation such as water cooling, and can be controlled by intermittently performing the cooling operation as appropriate (providing a period in which the cooling operation is stopped). . Moreover, it is difficult to physically and directly measure the temperature at the 1/4 position of the plate thickness of the steel plate. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, by using a process computer to calculate the difference, the temperature in the thickness cross section The distribution, particularly the temperature at the 1/4 position of the plate thickness, can be obtained in real time.

[Cooling stop temperature: 200 to 600°C]
In the present invention, after hot rolling is completed, a predetermined accelerated cooling is performed to a cooling stop temperature arbitrarily set in the range of 200 to 600 ° C., so that ferrite and bainite are reduced to a predetermined volume ratio at the center of the plate thickness. It is possible to improve strength and toughness satisfactorily.
Here, if the cooling stop temperature is less than 200° C., the volume fraction of the island-shaped martensite structure becomes too large, resulting in a decrease in toughness. On the other hand, if the cooling stop temperature exceeds 600° C., ferrite and pearlite structures are excessively formed, resulting in insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is specified in the range of 200 to 600°C. Further, the cooling stop temperature in the present invention is the temperature at the 1/4 position of the plate thickness of the steel plate.

(reheat)
[Attainment temperature at 0.5 mm position from the surface is 400 to 680 ° C.]
In the present invention, it is necessary to reheat after the accelerated cooling. Accelerated cooling of a thick steel plate increases the cooling rate of the surface layer of the steel plate and cools the surface layer of the steel plate to a lower temperature than the inside of the steel plate. Therefore, a hard structure such as martensite is likely to form in the surface layer of the steel sheet, and the ammonia SCC resistance may be deteriorated. Therefore, in the present invention, the surface layer portion of the steel sheet is reheated after accelerated cooling. This is because the hardness of the surface layer portion can be reduced. Preferably, reheating is performed immediately after accelerated cooling.
Here, if the reheating temperature at the position 0.5 mm from the surface is less than 400°C, the hardness is not sufficiently reduced, while if it exceeds 680°C, the strength of the entire steel sheet is reduced. It becomes difficult to obtain the strength of
Therefore, the temperature reached at a position 0.5 mm from the surface during reheating after accelerated cooling is specified in the range of 400 to 680.degree.

[Attainment temperature at 1/4 position of steel plate thickness is 500 ° C. or less]
If the temperature reached at the position of 1/4 of the plate thickness of the steel plate during reheating exceeds 500° C., the strength and toughness are deteriorated. Therefore, the temperature reached at the position of 1/4 of the plate thickness of the steel plate during reheating is specified to be 500° C. or less.

It is preferable to use induction heating as the reheating means after accelerated cooling. In particular, it is preferable to use high-frequency induction heating so that the heating concentrates on the surface layer of the steel sheet. Moreover, after reheating, cooling can be performed as appropriate. Cooling after reheating is not particularly limited, but in a thick steel plate having a thickness exceeding about 40 mm, the cooling rate becomes slow, and there is a concern that toughness is deteriorated due to agglomeration and coarsening of carbides. In such a case, water cooling or mist cooling may be performed after the reheating treatment.

By manufacturing a steel material having the above chemical composition according to the above manufacturing conditions, it is possible to obtain a steel plate having the chemical composition, hardness characteristics and metallographic structure according to the present invention. The steel sheet thus obtained has excellent strength characteristics and toughness, and is excellent in ammonia SCC resistance. Here, the excellent strength characteristics are yield strength YS (yield point YP when there is a yield point, 0.2% yield strength σ0.2 when there is no yield point): 450 MPa or more, tensile strength (TS): 570 MPa or more and uniform elongation (uEl): 10% or more. In addition, excellent toughness means that vTrs conforming to JIS Z 2241 is -30°C or less. A steel sheet having these properties is the steel sheet of the present invention having excellent ammonia SCC resistance.

In addition, in the manufacturing method according to the present invention, any item not described in this specification can be used by a conventional method.

A slab is formed by continuous casting of steel having the chemical composition shown in Table 1 (steel grades A to AI, the balance being Fe and unavoidable impurities), and hot rolling, accelerated cooling, and reheating are sequentially performed under the conditions shown in Table 2, Thick steel plates (No. 1 to 50) with a thickness of 30 mm were obtained. The obtained steel sheet was subjected to measurement of the metal structure fraction at a position of 0.5 mm from the surface of the steel sheet, evaluation of hardness characteristics, evaluation of strength characteristics and toughness, and evaluation of ammonia SCC resistance. Each test method is as follows. These results are also shown in Table 2.

[Structural fraction of metal structure at 0.5 mm position from steel plate surface]
A sample was taken from each steel plate so that the 0.5 mm position was the observation surface. Then, the sample was mirror-polished, and after being subjected to nital corrosion, a scanning electron microscope (SEM) was used to photograph an area of 10 mm×10 mm at a magnification of 500 to 3000. Then, the photographed image was analyzed using an image analyzer to determine the area fraction of the microstructure (structure fraction of the metal structure). When the anisotropy of the microstructure is small, the area fraction corresponds to the volume fraction, so in the present invention the area fraction is regarded as the volume fraction.

In addition, in the present example, the determination when obtaining the fraction of the metal structure of the sample was performed as follows.
That is, in the photographed images described above, the polygonal ferrite is discriminated as ferrite, and the bainite (Table 2 It was determined as B) in

[Hardness characteristics]
Regarding the cross section perpendicular to the rolling direction of each steel plate, the Vickers hardness (HV0.1) was measured at 100 points at a position of 0.5 mm according to JIS Z 2244, and the average value was obtained. Also, the standard deviation of the Vickers hardness of 100 points was obtained, and the variation in hardness at the 0.5 mm position was obtained. Here, the reason why HV0.1 is used instead of HV10, which is usually used for hardness measurement of a steel plate, is that the indentation becomes smaller by measuring with HV0.1, so that hardness information at a position closer to the surface can be obtained. , it is possible to obtain hardness information more sensitive to the microstructure.
Also, the Vickers hardness (HV1) was measured in the plate thickness direction, and the position in the plate thickness direction (distance from the surface) where the maximum value exists was measured. Further, the difference between the maximum value and the minimum value of Vickers hardness (HV1) in such measurement was calculated and used as variation in hardness in the plate thickness direction.

[Strength characteristics]
From the total thickness of each steel plate, a JIS Z 2201 No. 1B test piece is taken so that the direction perpendicular to the rolling direction is the longitudinal direction of the test piece, and a tensile test is performed according to the procedure described in JIS Z 2241 to obtain the yield strength. YS (yield point YP when there is a yield point, 0.2% yield strength σ0.2 when there is no yield point), tensile strength (TS) and uniform elongation (uEl) were measured. A steel sheet having a yield strength of 450 MPa or more, a tensile strength of 570 MPa or more, and a uniform elongation of 10% or more was evaluated as having excellent strength characteristics.

[Toughness]
A V-notch test piece of JIS Z 2202 was collected from a portion cut by 1 mm from the surface side of each steel plate so that the rolling direction was the longitudinal direction of the test piece, and a Charpy impact test was performed according to JIS Z 2242. fracture surface transition temperature) was measured. A steel sheet with such vTrs of -30°C or less was evaluated as a steel sheet having excellent toughness.

[Ammonia SCC resistance]
Ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was performed in a test solution and constant potential anodic electrolysis was performed to promote corrosion.
Specifically, we performed the following steps:
A test piece having a thickness of 5 mm×15 mm×115 mm was taken from the surface of the steel plate, subjected to ultrasonic degreasing in acetone for 5 minutes, and stress equal to the yield strength of each steel plate was applied by four-point bending. After filling a test cell in which such a 4-point bending test piece was installed with a mixed solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, a potentiostat applied a voltage of +2.0 V vs Pt to the test piece. and immersed at room temperature (25°C). After immersion for 168 hours, the ammonia SCC resistance was determined to be "good" when cracks were not observed, and the ammonia SCC resistance was determined to be "poor" when cracks occurred.

As can be seen from Tables 1 and 2, the invention examples all have a yield strength YS of 450 MPa or more, a tensile strength TS of 570 MPa or more, and a uniform elongation uEl of 10% or more, vTrs is -30 ° C. or less, A steel sheet excellent in low-temperature toughness and ammonia SCC resistance is obtained.

On the other hand, No. In Nos. 31 to 39, although the component composition is within the scope of the present invention, the manufacturing method is outside the scope of the present invention, so the desired metallographic structure and/or hardness properties are not obtained. As a result, the yield strength YS, tensile strength TS, toughness at low temperature, or resistance to ammonia SCC is inferior.

Also, No. In Nos. 40 to 50, the chemical composition of the steel is outside the range of the present invention, so the yield strength YS, tensile strength TS, toughness at low temperatures, or ammonia SCC resistance are inferior. In addition, in the present invention, the chemical composition of the steel may be considered as the chemical composition of the steel sheet.

Claims

in % by mass,
C: 0.010 to 0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.50%,
Al: 0.010-0.060%
N: 0.0010% or more and 0.0100% or less,
P: 0.020% or less,
A steel sheet having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities,
At a depth of 0.5 mm from the surface of the steel sheet, the average hardness is 230 HV0.1 or less, the hardness variation is 30 HV0.1 or less, and the maximum hardness in the thickness direction is the steel sheet. A hardness characteristic that is located at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface and has a hardness variation of 70 HV1 or less in the plate thickness direction;
A steel sheet having a metal structure in which the volume fraction of the bainite structure at a position 0.5 mm deep from the surface of the steel sheet is 90% or more.
The component composition further, in mass %,
Cu: 0.01-0.50%,
Ni: 0.01 to 2.00%,
Cr: 0.01 to 1.00%,
Sn: 0.01 to 0.50%,
Sb: 0.01 to 0.50%,
Mo: 0.01-0.50% and W: 0.01-1.00%
The steel sheet according to claim 1, containing one or more selected from.
The component composition further, in mass %,
V: 0.01 to 1.00%,
Ti: 0.005 to 0.100%,
Co: 0.01 to 1.00%,
Nb: 0.005 to 0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
The steel sheet according to claim 1 or 2, containing one or more selected from.
in % by mass,
C: 0.010 to 0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.50%,
Al: 0.010 to 0.060%,
N: 0.0010% or more and 0.0100% or less,
P: 0.020% or less,
A steel material having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities, is subjected to hot rolling at a rolling end temperature of Ar 3 transformation point or higher. A method for producing a steel sheet, comprising :
In the accelerated cooling, the cooling stop temperature is in the range of 200 to 600 ° C., and the cooling rate at the 1/4 position of the plate thickness of the steel plate is 20 to 120 ° C./s,
The reheating is performed until the temperature reached at a position 1/4 of the plate thickness of the steel plate is 500 ° C. or less, and the temperature reached at a depth of 0.5 mm from the surface of the steel plate is in the range of 400 to 680 ° C. The steel plate. manufacturing method.
The chemical composition of the steel material is further, in mass%,
Cu: 0.01-0.50%,
Ni: 0.01 to 2.00%,
Cr: 0.01 to 1.00%,
Sn: 0.01 to 0.50%,
Sb: 0.01 to 0.50%,
Mo: 0.01-0.50% and W: 0.01-1.00%
The method for producing a steel sheet according to claim 4, containing one or more selected from.
The chemical composition of the steel material is further, in mass%,
V: 0.01 to 1.00%,
Ti: 0.005 to 0.100%,
Co: 0.01 to 1.00%,
Nb: 0.005 to 0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
The method for producing a steel sheet according to claim 4 or 5, containing one or more selected from.