JP7622839B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a high-strength steel plate having excellent toughness and corrosion resistance, in particular a high-strength steel plate having excellent low-temperature toughness and resistance to liquid ammonia stress corrosion cracking, which is suitable for structural components such as tanks used at low temperatures and in liquid ammonia environments, and a method for manufacturing the same.

With the increase in energy demand in recent years, the transportation of liquefied gas by energy carriers has become common. To ensure efficient operation of energy carriers, the tanks of some ships may carry not only LPG but also liquid ammonia.

It is known that liquid ammonia can cause stress corrosion cracking (hereinafter referred to as ammonia SCC) in carbon steel piping, storage tanks, tank cars, line pipes, etc. that handle liquefied ammonia. For this reason, steel materials with low susceptibility to ammonia SCC have been applied to steel materials used in liquid ammonia environments, and engineering measures have been taken to suppress ammonia SCC.

For example, it is known that the occurrence of ammonia SCC is correlated with the strength of the material, and when using carbon steel, efforts are made to avoid stress corrosion cracking caused by ammonia by controlling the yield strength (YS) to 440 MPa or less. On the other hand, in recent years, there has been an increasing demand for higher strength steel plates in view of the increasing size of tanks and the need to reduce the amount of steel used.

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

Technologies for satisfying the low-temperature toughness and strength range required for liquefied gas storage tanks as described above are disclosed in Patent Documents 1 and 2. The technologies described in these documents achieve high low-temperature toughness and specified strength characteristics by heat treating thick steel plate several times after it has been cooled after hot rolling, or by heat treating thick steel plate several times after it has been water-cooled after hot rolling.

Japanese Patent Application Publication No. 10-140235 Japanese Patent Application Publication No. 10-168516

However, the methods described in Patent Documents 1 and 2 above require multiple heat treatments, which poses economic problems due to the high costs of equipment and energy required for this.

The present invention aims to solve the above problems and provide a high-strength steel plate with excellent ammonia SCC resistance and low-temperature toughness, and a manufacturing method thereof, for use in storage tanks used to store liquefied gas on energy transport ships.

In order to achieve the above object, the inventors have conducted extensive research using the TMCP process into various factors affecting the low-temperature toughness and strength properties of steel plates. As a result, they have found that adding elements such as C, Si, Mn, and N to a steel plate in a predetermined amount or more and controlling the metal structure (microstructure) of the steel plate so that the total volume fraction of the ferrite structure and bainite structure at the 1/2 position of the plate thickness of the steel plate is 60% or more can effectively contribute to achieving the desired low-temperature toughness and strength properties.

Furthermore, it was discovered that by adding elements such as Cu, Cr, Sb, Sn, etc. in specified amounts or more and controlling the hardness at a depth of 1.0 mm from the surface of the steel plate to Hv300 or less, SCC resistance can be obtained in a liquid ammonia environment, and costly heat treatment as in conventional technology can be omitted.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. In mass percent,
C: 0.010-0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 2.50%,
Al: 0.060% or less,
N: 0.0010-0.0100%,
P: 0.020% or less,
S: 0.0100% or less and O: 0.0100% or less; and
Cu: 0.01 to 0.50%,
Cr: 0.01-1.00%,
Sb: 0.01 to 0.50% and Sn: 0.01 to 0.50%
A steel sheet having a component composition containing one or more of the following, a CR value calculated by the following formula (1) of 0.70 or more, and the balance being Fe and unavoidable impurities:
A hardness characteristic in which the hardness at a position 1.0 mm deep from the surface of the steel plate is Hv300 or less;
and a metal structure in which, at a half position in the plate thickness of the steel plate, a volume fraction of a bainite structure is 20% or more and a total volume fraction of a ferrite structure and a bainite structure is 60% or more.
CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn]...Formula (1)
Here, [X] indicates the content (mass%) of the X element in the steel.

2. The composition further comprises, in mass%,
Ni: 0.01-2.00%,
Mo: 0.01 to 0.50% and W: 0.01 to 1.00%
2. The steel plate according to claim 1, further comprising one or more selected from the following:

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

In mass percent,
C: 0.010-0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 2.50%,
Al: 0.060% or less,
N: 0.0010-0.0100%,
P: 0.020% or less,
S: 0.0100% or less and O: 0.0100% or less; and
Cu: 0.01 to 0.50%,
Cr: 0.01-1.00%,
Sb: 0.01 to 0.50%; and
Sn: 0.01~0.50%
A method for producing a steel plate, comprising the steps of: hot rolling a steel material having a composition in which the CR value calculated by the following formula (1) is 0.70 or more, with the balance being Fe and unavoidable impurities, with the rolling end temperature being equal to or higher than the _Ar3 transformation point; and then cooling the steel material from a cooling start temperature equal to or higher than the _Ar3 transformation point to a cooling stop temperature of 600°C or less,
In the cooling, the cooling rate at a position 1.0 mm deep from the surface of the steel plate is 150° C./s or less, and the cooling rate at a position 1/2 of the plate thickness of the steel plate is 10° C./s or more.
CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn]...Formula (1)
Here, [X] indicates the content (mass%) of the X element in the steel.

5. The composition of the steel material is further, in mass%,
Ni: 0.01-2.00%,
Mo: 0.01 to 0.50% and W: 0.01 to 1.00%
5. The method for producing a steel sheet according to 4 above, further comprising the step of:

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

According to the present invention, it is possible to provide, through an inexpensive process, a steel plate having excellent low-temperature toughness, i.e., impact resistance at low temperatures, and ammonia SCC resistance, and having high strength suitable for structural components such as tanks used at low temperatures and in liquid ammonia environments.

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

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

C: 0.010-0.200%
C is the most effective element for increasing the strength of the steel plate produced by the cooling method according to the present invention. In order to obtain this effect, the C content is specified to be 0.010% or more. Furthermore, other alloy elements From the viewpoint of reducing the C content and manufacturing at a lower cost, the C content is preferably 0.013% or more. On the other hand, if the C content exceeds 0.200%, the toughness and This leads to deterioration of weldability. Therefore, the C content is specified to be 0.200% or less. Furthermore, from the viewpoints of toughness and weldability, the C content is preferably set to be 0.170% or less.

Si: 0.01~0.50%
Silicon is added for deoxidation. To obtain this effect, the silicon content is set to 0.01% or more. It is further preferable to set it to 0.03% or more. On the other hand, when the silicon content is 0. If the Si content exceeds 50%, it will cause a deterioration in the toughness and weldability of the steel plate. Therefore, the Si content is specified to be 0.50% or less. Furthermore, from the viewpoint of toughness and weldability, the Si content is specified to be 0.40% or less. It is preferable that:

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 must be added in order to achieve high strength as in the present invention. The Mn content is set to 0.50% or more. Furthermore, from the viewpoint of reducing the contents of other alloy elements and manufacturing at a lower cost, the Mn content is set to 0.70% or more. On the other hand, if the Mn content exceeds 2.50%, the toughness and weldability of the steel plate are reduced, and the alloy cost is excessively high. Therefore, the Mn content is set to 2.50%. The Mn content is preferably set to 2.30% or less from the viewpoint of further suppressing the deterioration of toughness and weldability.

Al: 0.060% or less Al is an element that acts as a deoxidizer and also has the effect of refining crystal grains. In order to obtain such an effect, it is preferable that the Al content is 0.001% or more. On the other hand, if the Al content exceeds 0.060%, the oxide-based inclusions increase, the cleanliness decreases, and there is a concern that the toughness may deteriorate. Therefore, the Al content is specified to be 0.060% or less. Furthermore, from the viewpoint of further preventing the deterioration of toughness, it is preferable that the Al content is 0.050% or less.

N: 0.0010-0.0100%
N contributes to making the structure finer and improving the toughness of the steel plate. In order to obtain such an effect, the N content is specified to be 0.0010% or more, and preferably 0.0020% or more. If the N content exceeds 0.0100%, it will actually result in a decrease in toughness. Therefore, the N content is specified to be 0.0100% or less. Furthermore, the N content is set to a value that further reduces the toughness and weldability. From the viewpoint of suppressing this, the N content is preferably 0.0080% or less. Note that, when Ti is present, N can combine with the Ti and precipitate as TiN.

P: 0.020% or less P has an adverse effect, such as reducing toughness and weldability by segregating at grain boundaries. Therefore, it is desirable to make the P content as low as possible, but it is acceptable if it is 0.020% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%, but excessive reduction leads to an increase in refining costs, so from the viewpoint of cost, it is preferable to make the P content 0.0005% or more.

S: 0.0100% or less S is an element that exists in steel as sulfide-based inclusions such as MnS, and has adverse effects such as becoming the starting point of fracture and reducing the toughness of the steel plate. Therefore, it is desirable to make the S content as low as possible, but it is acceptable if it is 0.0100% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%, but excessive reduction leads to an increase in refining costs, so from the viewpoint of cost, it is preferable to make the S content 0.0005% or more.

O: 0.0100% or less O is an element that forms oxides, becomes the starting point of fracture, and has adverse effects such as reducing the toughness of the steel plate, so it is limited to 0.0100% or less. The O content is preferably 0.0050% or less, and 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%, but excessive reduction leads to an increase in refining costs, so from the viewpoint of cost, it is preferable to set the O content to 0.0010% or more.

One or more of Cu: 0.01-0.50%, Cr: 0.01-1.00%, Sb: 0.01-0.50%, and Sn: 0.01-0.50%, and the CR value calculated by formula (1) is 0.70 or more. Cu, Cr, Sb, and Sn are particularly important elements in the present invention for improving ammonia SCC resistance. Therefore, in the present invention, it is necessary that one or more of these elements are contained in the above amounts, and that the CR value calculated by the following formula (1) is 0.70 or more.
CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn]...Formula (1)
Here, [X] indicates the content (mass%) of the X element in the steel.
That is, Cu, Cr, Sb, and Sn rapidly form protective corrosion products in a liquid ammonia environment, suppressing stress corrosion cracking. In order to obtain such an effect, when Cu is added, the Cu content must be limited to 0.01% or more, when Cr is added, the Cr content must be limited to 0.01% or more, when Sb is added, the Sb content must be limited to 0.01% or more, and when Sn is added, the Sn content must be limited to 0.01% or more.
The formula for calculating the CR value was devised to estimate ammonia SCC resistance from the content of each element, and the higher the CR value, the better the ammonia SCC resistance. By making the CR value 0.70 or more, it becomes possible to suppress stress corrosion cracking in a liquid ammonia environment.
On the other hand, excessive addition of Cu, Cr, Sb and Sn deteriorates weldability and toughness, and is also disadvantageous from the viewpoint of alloy cost. Therefore, the Cu content is limited to 0.50% or less, the Cr content to 1.00% or less, the Sb content to 0.50% or less, and the Sn content to 0.50% or less. Preferably, the Cu content is 0.40% or less, the Cr content is 0.80% or less, the Sb content is 0.40% or less, and the Sn content is 0.40% or less. The upper limit of the CR value is not particularly limited, but when the CR value exceeds 7.00, the effect is saturated and the excessive addition of the above elements leads to a rise in price, so that it is preferably about 7.00.

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

At least one selected from Ni: 0.01-2.00%, Mo: 0.01-0.50%, and W: 0.01-1.00% Ni, Mo, and W are elements that further improve ammonia SCC resistance, and at least one of them can be contained. In order to obtain such an effect, it is preferable to adjust the Ni content to 0.01% or more when Ni is contained, the Mo content to 0.01% or more when Mo is contained, and the W content to 0.01% or more when W is contained. On the other hand, excessive Ni content leads to deterioration of weldability and increase in alloy cost. In addition, excessive addition of Mo and W deteriorates weldability and toughness, which is also disadvantageous from the viewpoint of alloy cost. Therefore, it is preferable to adjust the Ni content to 2.00% or less, the Mo content to 0.50% or less, and the W content to 1.00% or less, respectively. More preferably, the Ni content is adjusted to 1.50% or less, the Mo content is adjusted to 0.40% or less, and the W content is adjusted 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 added as desired. In order to obtain such an effect, when V is added, the V content is set to 0.01% or more. On the other hand, if the V content exceeds 1.00%, it leads to deterioration of weldability and an increase in alloy cost. Therefore, when V is added, the V content is set to 1.00% or less. More preferably, the lower limit of the 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 effect of fixing N and reducing the amount of solute N, and can be added as desired. In addition, Ti improves the toughness of the base material and welded parts. In order to obtain these effects, when Ti is added, the Ti content is preferably 0.005% or more, and more preferably 0.007% or more. On the other hand, if the Ti content exceeds 0.100%, the toughness is rather reduced. Therefore, when Ti is added, the Ti content is preferably 0.100% or less. It is more preferable that the content of Si is 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 added as desired. In order to obtain such an effect, when Co is added, the Co content is set to 0.01% or more. On the other hand, if the Co content exceeds 1.00%, it leads to deterioration of weldability and an increase in alloy cost. Therefore, when Co is added, the Co content is set to 1.00% or less. More preferably, the lower limit of the Co content is 0.05% and the upper limit is 0.50%.

Nb: 0.005-0.100%
Nb is an element that has the effect of reducing the prior austenite grain size and improving toughness by precipitating as carbonitrides. When Nb is added to obtain such an effect, the Nb content is set to 0. The Nb content is set to 0.005% or more. It is further preferably set to 0.007% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of NbC precipitates, and the toughness decreases. Therefore, when Nb is added, In this case, the Nb content is preferably 0.100% or less, and more preferably 0.060% or less.

B: 0.0001-0.0100%
B is an element that has the effect of significantly improving hardenability even when added in a small amount. In other words, it can improve the strength of the steel sheet. When B is added to obtain this effect, the B content is It is preferable that the B content is 0.0001% or more. On the other hand, if the B content exceeds 0.0100%, the weldability decreases. Therefore, when B is added, the B content should be 0.0100% or less. More preferably, the lower limit of the B content is 0.0010% and the upper limit is 0.0030%.

Ca: 0.0005-0.0200%
Ca is an element that combines with S and has the effect of suppressing the formation of MnS and the like that elongates in the rolling direction. In other words, by adding Ca, the morphology of the sulfide-based inclusions is controlled so that they are spherical. In order to obtain such an effect, when Ca is added, the Ca content is preferably 0.0005% or more. If the Ca content exceeds 0.0200%, the cleanliness of the steel decreases. The 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 lower limit of the Ca content is 0.0020% and the upper limit is 0.0100%.

Mg: 0.0005-0.0200%
Mg, like Ca, is an element that combines with S and suppresses the formation of MnS, etc., which elongates in the rolling direction. In other words, by adding Mg, the sulfide-based inclusions become spherical. In order to obtain such an effect, when Mg is added, the Mg content is preferably 0.0005% or more. If the content exceeds 0.0200%, the cleanliness of the steel decreases. The decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is added, the Mg content should be 0.0200% or less. More preferably, the lower limit of the Mg content is 0.0020% and the upper limit is 0.0100%.

REM: 0.0005-0.0200%
Like Ca and Mg, REM (rare earth metal) is an element that combines with S and suppresses the formation of MnS, etc., which elongates in the rolling direction. It is possible to control the shape of the steel so that the steel has a spherical shape, thereby improving the toughness of the welded portion, etc. When REM is added to obtain such an effect, the REM content is preferably 0.0005% or more. On the other hand, if the REM content exceeds 0.0200%, the cleanliness of the steel decreases. The decrease in cleanliness leads to a decrease in toughness. Therefore, when REM is added, the REM content is set to 0.0200% or less. % or less. More preferably, the lower limit of the REM content is 0.0020% and the upper limit is 0.0100%.

(2) Hardness Characteristics and Metal Structure In addition to having the above-mentioned composition, the steel sheet of the present invention has a hardness characteristic of Hv300 or less at a position 1.0 mm deep from the surface of the steel sheet (also referred to as the 1.0 mm position in the present invention).
Furthermore, the steel plate of the present invention has a metal structure in which, at a 1/2 position of the plate thickness of the steel plate (which in the present invention means a position at a depth of 1/2 of the plate thickness; hereinafter, this is also simply referred to as the 1/2 position or the plate thickness center portion), the volume fraction of a bainite structure (hereinafter, this is also simply referred to as bainite) is 20% or more, and the total volume fraction of a ferrite structure (hereinafter, this is also simply referred to as ferrite) and bainite is 60% or more.
The reasons for limiting the hardness characteristics and metal structure of the steel sheet as described above will be explained below.

[Hardness at 1.0 mm position is Hv300 or less]
The hardness at the 1.0 mm position is Hv300 or less. If a high hardness region exists at the very surface layer of the steel sheet, specifically at the 1.0 mm position from the surface of the steel sheet, stress corrosion cracking in a liquid ammonia environment is promoted. Therefore, in the steel sheet of the present invention, the hardness at the 1.0 mm position is adjusted to Hv300 or less to ensure excellent ammonia SCC resistance. The lower limit of the hardness at the 1.0 mm position is not particularly limited, but is preferably about Hv130.
Here, the hardness can be calculated by measuring the Vickers hardness at multiple points (for example, 100 points) at 0.5 mm positions.

[At the 1/2 position, the volume fraction of bainite is 20% or more, and the total volume fraction of ferrite and bainite is 60% or more]
The structure at the 1/2 position must have a bainite volume fraction of 20% or more and a total volume fraction of ferrite and bainite of 60% or more. If ferrite is generated excessively, it will lead to a decrease in strength or toughness. If the total volume fraction of ferrite and bainite is less than 60%, the volume fractions of other structures, i.e., island martensite structure, martensite structure, pearlite structure, and austenite structure, will increase, and sufficient strength or toughness will not be obtained, making it impossible to satisfy the mechanical properties. The total volume fraction of ferrite and bainite may be 100%.

Here, the ferrite refers to ferrite formed during the cooling process before tempering, and the bainite refers to bainite formed during the cooling process before tempering. The microstructure at the center of the plate thickness is specified because the microstructure at the center of the plate thickness affects the strength characteristics of the center of the plate thickness, and the strength characteristics at the center of the plate thickness affect the strength of the entire steel plate.

The remaining structure, which occupies 40% or less by volume, may include a martensite structure in addition to a pearlite structure and an austenite structure. The fraction of each structure in the remaining structure does not need to be particularly limited, but it is preferable that the remaining structure is a pearlite structure.
The volume fractions of various microstructures can be measured by the methods described in the Examples below.

(3) Manufacturing Conditions The manufacturing method in the present invention is to produce a steel sheet containing C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.060% or less, N: 0.0010 to 0.0100%, P: 0.020% or less, S: 0.0100% or less, and O: 0.0100% or less, and further containing one or more of Cu: 0.01 to 0.50%, Cr: 0.01 to 1.00%, Sb: 0.01 to 0.50%, and Sn: 0.01 to 0.50%, and the CR value calculated by the above formula (1) is 0.70 or more, and in addition, Ni: 0.01 to 2.00%, Mo: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Mo: 0.01 to 0.20%, and Ni: 0.01 to 2.00%. A steel material having a composition containing one or more selected from 0.01-0.50% and 0.01-1.00% W, and/or one or more selected from 0.01-1.00% V, 0.005-0.100% Ti, 0.01-1.00% Co, 0.005-0.100% Nb, 0.0001-0.0100% B, 0.0005-0.0200% Ca, 0.0005-0.0200% Mg, and 0.0005-0.0200% REM, with the balance being Fe and unavoidable impurities, is heated and hot-rolled, and then cooled as specified in accordance with the present invention. The reasons for limiting the manufacturing conditions of the steel plate are explained below.
First, the manufacturing conditions of the steel material do not need to be particularly limited, but it is preferable to melt molten steel having the above-mentioned composition by a known melting method such as a converter, and form the steel material into a slab or the like of a predetermined size by a known casting method such as a continuous casting method. There is no problem in forming the steel material into a slab or the like of a predetermined size by the ingot casting-separating rolling method.

The steel material thus obtained is either directly hot rolled without cooling, or reheated and then hot rolled. The hot rolling is performed with the rolling end temperature set to a temperature equal to or higher than the _Ar3 transformation point (hereinafter simply referred to as the _Ar3 transformation point). After the hot rolling, cooling is performed under predetermined conditions from a cooling start temperature equal to or higher than the _Ar3 transformation point to a cooling end temperature of 600°C or lower.

The heating temperature of the steel material (the temperature when subjected to hot rolling) is not particularly limited, but if the heating temperature is too low, the deformation resistance will be high, which will increase the load on the hot rolling machine and may make hot rolling difficult. On the other hand, if the temperature is too high and exceeds 1300°C, oxidation will become severe, increasing oxidation loss and increasing the risk of reduced yield. For these reasons, it is preferable that the heating temperature be between 950°C and 1300°C.

(Hot rolling)
[Rolling end temperature: Ar ₃ transformation point or higher]
In the present invention, after heating to the above temperature, hot rolling is started and the hot rolling is terminated at the _Ar3 transformation point or higher.
If the rolling end temperature is lower than the _Ar3 transformation point, ferrite is generated, and the generated ferrite is affected by processing, resulting in a deterioration in toughness. Furthermore, the load on the hot rolling machine increases. Therefore, the rolling end temperature in hot rolling is set to be equal to or higher than the _Ar3 transformation point. Preferably, the rolling end temperature in hot rolling is a temperature equal to or higher than the _Ar3 transformation point + 10°C. On the other hand, if the rolling end temperature exceeds 950°C, the structure may become coarse and the toughness may deteriorate, so the rolling end temperature is preferably 950°C or lower.
Here, the _Ar3 transformation point can be calculated by the following formula.
Ar ₃ (℃)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo
Here, each element indicates the content (mass %) of the element in the steel.

(cooling)
[Cooling start temperature: Ar ₃ transformation point or higher]
Next, the steel sheet after hot rolling is cooled from a cooling start temperature equal to or higher than the _Ar3 transformation point. If the cooling start temperature is lower than the _Ar3 transformation point, ferrite is excessively generated, resulting in insufficient strength. Therefore, the cooling start temperature is set to be equal to or higher than the _Ar3 transformation point.

[Cooling stop temperature: 600℃ or less]
In the present invention, after the hot rolling is completed, the steel sheet is cooled under predetermined conditions to a cooling stop temperature of 600° C. or less, so that the ferrite and bainite in the center of the steel sheet thickness are formed at a predetermined volume ratio. Here, if the cooling stop temperature exceeds 600°C, ferrite structure and pearlite structure are excessively generated, which may lead to insufficient strength. Therefore, the cooling stop temperature is specified to be 600°C or less. The lower limit of the cooling stop temperature is not particularly limited, but if the cooling stop temperature is too low, the volume fraction of the island martensite structure becomes too large, and the toughness decreases. Therefore, the cooling stop temperature is set to 200° C. It is preferable that the amount is equal to or more than that.
The cooling stop temperature is the temperature at the 1/2 position of the steel plate.

[Cooling rate at 1.0 mm position: 150° C./s or less]
In the above cooling, if the cooling rate at the 1.0 mm position exceeds 150° C./s, the hardness at the 1.0 mm position exceeds Hv300, and the ammonia SCC resistance deteriorates. Therefore, the cooling rate at the 1.0 mm position is specified to be 150° C./s or less.
On the other hand, the lower limit of the cooling rate is not particularly limited, but if the cooling rate is too low, ferrite structure or pearlite structure may be generated excessively, which may lead to insufficient strength or deterioration of toughness. Therefore, from the viewpoint of more reliably preventing this, it is preferable to set the cooling rate to 50° C./s or more.
The cooling rate can be controlled by intermittent cooling including cooling stop periods. It is difficult to physically measure the temperature at the 1.0 mm position directly. However, the temperature distribution in the plate thickness cross section, particularly the temperature at the 1.0 mm position, can be calculated in real time by performing a differential calculation using, for example, a process computer based on the surface temperature at the start of cooling measured by a radiation thermometer and the surface temperature at the target time of cooling stop.

[Cooling rate at 1/2 position: 10° C./s or more]
Cooling at a 1/2 position at a cooling rate of 10°C/s or more is an essential process for obtaining a steel plate with high strength and high toughness, and cooling at a high cooling rate can achieve a strength increase effect due to transformation strengthening. In order to achieve this effect, the cooling rate at the 1/2 position during cooling according to the present invention is specified to be 10°C/s or more. If the cooling rate is less than 10°C/s, ferrite and pearlite are excessively generated, and sufficient strength cannot be obtained. Therefore, the cooling rate at the 1/2 position of the plate thickness is specified to be 10°C or more.
On the other hand, although there is no particular upper limit to the cooling rate, if the cooling rate is too high, the volume fraction of island martensite becomes too high, which may lead to deterioration of toughness. Therefore, the cooling rate at the 1/2 position is preferably 80° C./s or less.
The cooling rate can be controlled by intermittent cooling including cooling stop periods. It is difficult to physically measure the temperature at the 1/2 position directly. However, the temperature distribution in the plate thickness cross section, particularly the temperature at the 1/2 position, can be obtained in real time by performing a difference calculation using, for example, a process computer based on the surface temperature at the start of cooling measured by a radiation thermometer and the surface temperature at the target time of cooling stop.

The cooling rate at the 1.0 mm position and the cooling rate at the 1/2 position can each be changed by, for example, adjusting the cooling start temperature, water amount, etc. in a complex manner.

By manufacturing a steel material having the above-mentioned composition according to the above-mentioned manufacturing conditions, a steel plate having the composition, hardness characteristics, and metal structure according to the present invention can be obtained. The steel plate thus obtained has excellent strength characteristics and toughness. Here, excellent strength characteristics are a yield strength YS (yield point YP when there is a yield point, 0.2% proof stress σ0.2 when there is no yield point): 360 MPa or more and a tensile strength (TS): 490 MPa or more. In addition, excellent toughness is a vTrs according to JIS Z 2241 of -30°C or less.

In the manufacturing method according to the present invention, any items not described in this specification can be performed using conventional methods.

Steels with the chemical composition shown in Table 1 (steel types A to AH, the remainder being Fe and unavoidable impurities) were formed into slabs by continuous casting, which were then used to produce thick steel plates (Nos. 1 to 44) with a plate thickness of 30 mm. Next, hot rolling and cooling were performed in sequence under the conditions shown in Table 2 to obtain steel plates. For the obtained steel plates, the metal structure fraction at 1/2 the plate thickness was measured, the hardness was measured at a position 1.0 mm from the steel plate surface, and the strength characteristics and toughness were evaluated, as well as the ammonia SCC resistance was evaluated. The test methods are as follows. The results are also shown in Table 2.

[Structure fraction of metal structure at 1/2 position]
A sample was taken from each steel plate so that the 1/2 position (center of plate thickness) was the observation surface. The sample was then mirror-polished and further subjected to nital etching, after which an image of a 10 mm x 10 mm area was taken at a magnification of 500 to 3000 times using a scanning electron microscope (SEM). The image was then analyzed using an image analyzer to determine the areal fraction of the microstructure (structural fraction of the metal structure). When the anisotropy of the microstructure is small, the areal fraction corresponds to the volume fraction, and therefore in the present invention, the areal fraction was regarded as the volume fraction.

In this example, the discrimination for determining the fraction of the metal structure of the sample was performed as follows: In the above-mentioned photographed image, polygonal ferrite was discriminated as ferrite (F in Table 2), and the structure having elongated, lath-shaped ferrite and containing carbides with a circle equivalent diameter of 0.05 μm or more was discriminated as bainite (B in Table 2).

[Hardness at 1.0 mm position]
For a cross section perpendicular to the rolling direction of each steel plate, Vickers hardness (HV10) was measured at 100 points at 1.0 mm positions in accordance with JIS Z 2244, and the maximum value was determined.

[Strength characteristics]
A JIS Z 2201 No. 1B test piece was taken from the entire thickness of each steel plate in a direction perpendicular to the rolling direction and perpendicular to the plate thickness direction, and a tensile test was performed in the manner described in JIS Z 2241 to measure the yield strength YS (yield point YP when there is a yield point, and 0.2% proof stress σ0.2 when there is no yield point) and tensile strength (TS). Steel plates with a yield strength of 360 MPa or more and a tensile strength of 490 MPa or more were evaluated as having excellent strength properties.

[Toughness]
A V-notch test piece according to JIS Z 2202 was taken from a portion 1 mm removed from the surface side of each steel plate in the rolling direction, and a Charpy impact test was carried out in accordance with JIS Z 2242 to measure vTrs (fracture transition temperature). Steel plates with a vTrs of -30°C or less were evaluated as having excellent toughness.

[Ammonia SCC resistance]
The ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was carried out in a test solution and constant-potential anodic electrolysis was carried out to accelerate corrosion.
Specifically, the following steps were performed:
A test piece of 5 mm thickness x 15 mm x 115 mm was taken from the surface of the steel plate, ultrasonically degreased in acetone for 5 minutes, and a stress of 100% YS of the actual yield strength of each steel plate was applied by four-point bending. The four-point bending test piece was placed in a test cell, filled with a solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, and then controlled by a potentiostat so that +2.0 V vs Pt flowed through the test piece, and immersed at room temperature (25 ° C.). If no cracks were observed after 168 hours of immersion, the ammonia SCC resistance was judged to be "good", and if cracks occurred, the ammonia SCC resistance was judged to be "poor".

As can be seen from Tables 1 and 2, all of the invention examples (Nos. 1 to 26) have a yield strength YS of 360 MPa or more and a tensile strength TS of 490 MPa or more, and have a vTrs of -30°C or less, resulting in steel plates with excellent toughness at low temperatures and excellent ammonia SCC resistance.

On the other hand, Nos. 27 to 31 have a composition within the scope of the present invention, but the manufacturing method is outside the scope of the present invention, so the desired metal structure or hardness characteristics are not obtained. As a result, the yield strength YS, tensile strength TS, low-temperature toughness, or ammonia SCC resistance are inferior.

In addition, Nos. 32 to 44 have steel compositions outside the scope of the present invention, and therefore are inferior in either yield strength YS, tensile strength TS, low-temperature toughness, or ammonia SCC resistance. In the present invention, the steel composition may be considered to be the steel plate composition as it is.

Claims (6)

In mass percent,
C: 0.010-0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 2.50%,
Al: 0.060% or less,
N: 0.0010-0.0100%,
P: 0.020% or less,
S: 0.0100% or less and O: 0.0100% or less; and
Cu: 0.01 to 0.50%,
Cr: 0.01-1.00%,
Sb: 0.01 to 0.50% and Sn: 0.01 to 0.50%
A steel sheet having a component composition containing one or more of the following, a CR value calculated by the following formula (1) being 0.70 or more, and the balance being Fe and unavoidable impurities,
A hardness characteristic in which the hardness at a position 1.0 mm deep from the surface of the steel plate is Hv300 or less;
and a metal structure in which, at a half position in the plate thickness of the steel plate, a volume fraction of a bainite structure is 20% or more and a total volume fraction of a ferrite structure and a bainite structure is 60% or more.
CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn]...Formula (1)
Here, [X] indicates the content (mass%) of the X element in the steel. The composition further comprises, in mass%,
Ni: 0.01-2.00%,
Mo: 0.01 to 0.50% and W: 0.01 to 1.00%
The steel sheet according to claim 1, comprising one or more selected from the following: The composition further comprises, in mass%,
V: 0.01-1.00%,
Co: 0.01 to 1.00%,
Nb: 0.005-0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005-0.0200%,
Mg: 0.0005 to 0.0200% and REM: 0.0005 to 0.0200%
The steel sheet according to claim 1 or 2, comprising one or more selected from the following: In mass percent,
C: 0.010-0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 2.50%,
Al: 0.060% or less,
N: 0.0010-0.0100%,
P: 0.020% or less,
S: 0.0100% or less and O: 0.0100% or less; and
Cu: 0.01 to 0.50%,
Cr: 0.01-1.00%,
Sb: 0.01 to 0.50%; and
Sn: 0.01~0.50%
A method for producing a steel plate, comprising the steps of: hot rolling a steel material having a composition in which the CR value calculated by the following formula (1) is 0.70 or more, with the balance being Fe and unavoidable impurities, with the rolling end temperature being equal to or higher than the _Ar3 transformation point; and then cooling the steel material from a cooling start temperature equal to or higher than the _Ar3 transformation point to a cooling stop temperature of 600°C or less,
the cooling is performed at a cooling rate of 150°C/s or less at a position 1.0 mm deep from the surface of the steel plate, and at a cooling rate of 10°C/s or more at a position 1/2 of the plate thickness of the steel plate; and the steel plate has a hardness characteristic of Hv300 or less at a position 1.0 mm deep from the surface of the steel plate, and a metal structure in which a volume fraction of bainite structure is 20% or more and a total volume fraction of ferrite structure and bainite structure is 60% or more at the position 1/2 of the plate thickness of the steel plate.
CR=2.3[Cu]+2.8[Cr]+7.3[Sb]+3.6[Sn]...Formula (1)
Here, [X] indicates the content (mass%) of the X element in the steel. The composition of the steel material further comprises, in mass%,
Ni: 0.01-2.00%,
Mo: 0.01 to 0.50% and W: 0.01 to 1.00%
The method for producing a steel sheet according to claim 4, further comprising: The composition of the steel material further comprises, in mass%,
V: 0.01-1.00%,
Co: 0.01 to 1.00%,
Nb: 0.005-0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005-0.0200%,
Mg: 0.0005 to 0.0200% and REM: 0.0005 to 0.0200%
The method for producing a steel sheet according to claim 4 or claim 5, further comprising at least one selected from the following: