KR102698603B1 - Steel plate and method for manufacturing same - Google Patents

Abstract

Provided is a steel plate having excellent stress corrosion cracking resistance, which is desirable for use in low-temperature environments.
A steel sheet having a composition of components containing, in mass%, C: 0.01% or more and 0.15% or less, Si: 0.01% or more and 1.00% or less, Mn: 0.10% or more and 3.00% or less, Al: 0.002% or more and 0.100% or less, Ni: 5.0% or more and 10.0% or less, N: 0.0010% or more and 0.0080% or less, Co: more than 0% and 1.50% or less, P: 0.030% or less, and S: 0.0050% or less, with the remainder being Fe and unavoidable impurities, and a structure from the surface of the steel sheet to a position 1 mm deep has an average equivalent circle diameter of crystal grains surrounded by large-angle grain boundaries with an orientation difference of 15° or more of 5 ㎛ or less, and a maximum equivalent circle diameter of retained austenite grains of Make it 1 ㎛ or less.

Description

{STEEL PLATE AND METHOD FOR MANUFACTURING SAME}

The present invention relates to a steel plate having excellent stress corrosion cracking resistance, particularly suitable for use in structural steel used in extremely low-temperature environments, such as tanks for storing liquefied gas, and a method for producing the same.

When hot-rolled steel plates are used in structures for storing liquefied gas, since the environment of use becomes extremely low, not only the strength of the steel plates but also the toughness at extremely low temperatures are required. For example, when hot-rolled steel plates are used in the storage of liquefied natural gas, it is necessary to secure excellent toughness at -164℃ or lower, which is the boiling point of liquefied natural gas. If the low-temperature toughness of the steel is low, there is a concern that the safety of the structure for storing liquefied natural gas cannot be maintained, so there is a strong demand for improved low-temperature toughness of the steel plates to be applied. In response to this demand, 7% nickel steel plates or 9% nickel steel plates have been used conventionally.

For example, patent documents 1, 2 and 3 propose low-temperature steel sheets having performance equivalent to or higher than 9% Ni steel sheets with a Ni content lower than 9%.

International Publication No. 2007/034576 International Publication No. 2007/080646 Japanese Patent Publication No. 2011-241419

However, the Ni-containing steels described in Patent Documents 1, 2, and 3, although having excellent low-temperature toughness, do not mention stress corrosion cracking caused by hydrogen, and thus there is still room for review. That is, for example, in the case of LNG tanks for ships, since the usage environment includes sulfides and chlorides, there is a high possibility that stress corrosion cracking caused by hydrogen will occur, and therefore it is required to also have stress corrosion cracking resistance, which is durability against stress corrosion cracking.

The present invention has been made in consideration of these problems, and aims to provide a steel plate having excellent stress corrosion cracking resistance, which is particularly preferable for use in a low-temperature environment.

In order to solve the above problem, the inventors of the present invention conducted a thorough study on the composition and structure of steel plates and obtained the following findings.

(1) By adding Co, when corrosion progresses on the surface of the steel plate, Co is concentrated on the surface of the steel plate, reducing hydrogen intrusion into the steel and reducing crack propagation due to hydrogen embrittlement.

(2) By setting the cooling rate to 1°C/s or more during cooling after hot rolling or after heat treatment (quenching or two-phase quenching), the structure up to 1 mm below the surface of the steel sheet becomes a microstructure in which the grain diameter is 5 ㎛ or less and is surrounded by large-angle grain boundaries with an orientation difference of 15° or more. In addition, since this microstructure disperses hydrogen trap sites, crack propagation due to hydrogen embrittlement can be reduced.

(3) By setting the maximum equivalent diameter of retained austenite in the surface layer of the steel plate to 1 ㎛ or less, the local concentration of hydrogen traps in the retained austenite can be dispersed, thereby reducing crack propagation due to hydrogen embrittlement.

The present invention has been made by further examining the above findings, and its gist is as follows.

1. In mass%,

C: 0.01% or more and 0.15% or less,

Si: 0.01% or more and 1.00% or less,

Mn: 0.10% or more and 3.00% or less,

Al: 0.002% or more and 0.100% or less,

Ni: 5.0% or more and 10.0% or less,

N: 0.0010% or more and 0.0080% or less,

Co: 0% or more but less than or equal to 1.50%,

P: 0.030% or less and

S: 0.0050% or less

It contains a composition of elements that are Fe and inevitable impurities.

A steel plate having a structure from the surface of the steel plate to a depth of 1 mm, in which the average equivalent circle diameter of crystal grains surrounded by angled grain boundaries with an orientation difference of 15° or more is 5 ㎛ or less, and the maximum equivalent circle diameter of retained austenite grains is 1 ㎛ or less.

2. The above composition of ingredients is, additionally, in mass%,

Nb: 0.001% or more and 0.030% or less,

V: 0.01% or more and 0.10% or less,

Ti: 0.003% or more and 0.050% or less,

B: 0.0003% or more and 0.0100% or less,

Cu: 0.01% or more and 1.00% or less,

Cr: 0.01% or more and 1.50% or less,

Sn: 0.01% or more and 0.50% or less,

Sb: 0.01% or more and 0.50% or less,

Mo: 0.03% or more and 1.00% or less and

W: 0.05% or more and 2.00% or less

A steel plate as described in 1 above, containing one or more kinds selected from .

3. The above composition of ingredients is, additionally, in mass%,

Ca: 0.0005% or more and 0.0050% or less,

Zr: 0.0005% or more and 0.0050% or less,

Mg: 0.0005% or more and 0.0050% or less and

REM: 0.0010% or more and 0.0100% or less

A steel plate as described in 1 or 2 above, containing one or more kinds selected from .

4. A method for manufacturing a steel plate, comprising heating a steel material having a composition described in any one of 1 to 3 above, performing hot rolling, and then performing a cooling treatment, wherein the average cooling rate in a temperature range of 600°C or less and 200°C or more in the cooling treatment is set to 1°C/s or more.

5. A method for manufacturing a steel plate, comprising heating a steel material having a composition described in any one of 1 to 3 above, performing hot rolling, performing heat treatment again, and then performing a cooling treatment, wherein an average cooling rate in a temperature range of 600°C or less and 200°C or more in the cooling treatment is set to 1°C/s or more.

According to the present invention, a steel plate having high durability against stress corrosion cracking due to hydrogen can be provided. By providing this steel plate to a steel structure used in a low-temperature environment, such as a liquefied gas storage tank, the safety of the steel structure can be improved, thereby bringing about a remarkable industrial effect.

Hereinafter, embodiments of the present invention will be described. In addition, the present invention is not limited to the embodiments below.

[Ingredients]

First, the composition of the steel plate of the present invention and the reasons for its limitation are explained. In the present invention, in order to secure excellent corrosion resistance, the composition of the steel plate is stipulated as follows. In addition, the % indication in the composition of the steel plate means mass% unless specifically stated.

C: 0.01% or more and 0.15% or less

C is effective for high strength, and to obtain that effect, it is necessary to contain C at 0.01% or more. On the other hand, if it is contained in excess of 0.15%, the low-temperature toughness deteriorates. Therefore, C is set to 0.01% or more and 0.15% or less. It is preferably set to 0.03% or more. It is preferably set to 0.10% or less.

Si: 0.01% or more and 1.00% or less

Si acts as a deoxidizer and is necessary for steelmaking, and also has the effect of strengthening the steel sheet by solid solution strengthening when dissolved in steel. In order to obtain this effect, it is necessary to contain Si at 0.01% or more. On the other hand, if the content exceeds 1.00%, the low-temperature toughness deteriorates. Therefore, Si is set to 0.01% or more and 1.00% or less. It is preferably set to 0.03% or more. It is preferably set to 0.5% or less.

Mn: 0.10% or more and 3.00% or less

Manganese is an element that is effective in increasing the strength of steel plates and enhancing the hardness of steel sheets. In order to obtain the effect, manganese must be contained at 0.01% or more. On the other hand, if the content exceeds 3.00%, the corrosion resistance and cracking resistance deteriorates. Therefore, manganese is contained in a range of 0.10% or more and 3.00% or less. It is preferably 0.20% or more. It is preferably 2.00% or less, and more preferably 1.00% or less.

Al: 0.002% or more and 0.100% or less

Al acts as a deoxidizer and is most commonly used in the deoxidation process of molten steel for steel plates. In addition, it has the effect of fixing the dissolved nitrogen in the steel to form AlN, thereby suppressing the deterioration of toughness due to the reduction of the dissolved N. On the other hand, if it is contained more than 0.100%, the toughness deteriorates, so it is set to 0.100% or less. Preferably it is set to 0.010% or more. Preferably it is set to 0.070% or less. More preferably, it is set to 0.020% or more. More preferably, it is set to 0.060% or less.

Ni: 5.0% or more and 10.0% or less

Ni is a very effective element for improving the low-temperature toughness of steel plates. On the other hand, since it is an expensive element, the cost of the steel plate increases as its content increases. Therefore, in the present invention, the Ni content is set to 10.0% or less. However, if the Ni content is less than 5.0%, not only is the strength of the steel plate reduced, but stable retained austenite is not obtained at low temperatures, resulting in reduced low-temperature toughness and strength of the steel plate. Therefore, the Ni content is set to 5.0% or more and 10.0% or less. Preferably, it is set to 9.5% or less. Preferably, it is set to 6.0% or more.

N: 0.0010% or more and 0.0080% or less

N forms precipitates in steel, and when its content exceeds 0.0080%, it causes a decrease in the toughness of the base metal and the welded heat-affected zone when the steel plate is welded to form a welded structure. However, N is also an element that contributes to the refinement of the base metal by forming AlN, and this effect is obtained by setting the N content to 0.0010% or more. Therefore, the N content is set to 0.0010% or more and 0.0080% or less. Preferably, it is set to 0.0020% or more. More preferably, it is set to 0.0060% or less.

Co: Over 0% but less than or equal to 1.50%

Co is an important element that contributes to suppressing corrosion cracking by concentrating on the surface layer of the steel plate under a corrosive environment and reducing the intrusion of hydrogen. Therefore, it is necessary to contain more than 0%. Preferably, the Co amount is 0.05% or more, more preferably 0.1% or more. However, even if it is contained more than 1.50%, the effect is saturated, and since Co is an expensive element, the maximum addition amount is 1.50%.

P: 0.030% or less

P, when contained in excess of 0.030%, reduces corrosion resistance and cracking resistance. Therefore, it is desirable to reduce P as much as possible, with 0.030% as the upper limit. Accordingly, P is set to 0.030% or less. Since the properties improve as the P content decreases, it is preferably set to 0.025% or less, and more preferably 0.020% or less. In addition, although it goes without saying that the P content may be 0%, since P removal requires a high cost, it is desirable to set it to 0.002% or more from the viewpoint of cost.

S: 0.0050% or less

Since S forms MnS in steel and significantly deteriorates low-temperature toughness, it is desirable to reduce it as much as possible, with an upper limit of 0.0050%. Preferably, it is 0.0020% or less. In addition, although the S content may be 0%, since S removal requires a high cost, it is desirable to set it to 0.0005% or more from the viewpoint of cost.

It is based on a composition containing each of the above elements, with the remainder being Fe and inevitable impurities.

In the present invention, in order to further improve strength and low-temperature toughness, in addition to the above essential elements, the following elements may be contained as necessary.

Nb: 0.001% or more and 0.030% or less

Nb is an element effective in improving the strength of steel plates. In order to obtain this effect, it is preferable to add Nb in an amount of 0.001% or more. On the other hand, if it exceeds 0.030%, coarse carbonitrides may precipitate, which may deteriorate the toughness of the base material. Therefore, when Nb is contained, it is set to 0.001% or more and 0.030% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.025% or less, more preferably 0.022% or less.

V: 0.01% or more and 0.10% or less

V is an element effective in improving the strength of steel plates. In order to obtain this effect, it is preferable to add V in an amount of 0.01% or more. On the other hand, if it exceeds 0.10%, coarse carbonitrides may precipitate, which may become a starting point for fracture. In addition, the precipitates may become coarser, which may deteriorate the toughness of the base material. Therefore, when V is contained, it is set to 0.01% or more and 0.10% or less. It is preferably 0.02% or more, more preferably 0.03% or more. It is preferably 0.09% or less, more preferably 0.08% or less.

Ti: 0.003% or more and 0.050% or less

Ti is an element that is effective in improving the strength of steel sheets by being precipitated as nitrides or carbonitrides. In order to obtain this effect, it is preferable to add Ti in an amount of 0.003% or more. On the other hand, if it exceeds 0.050%, the precipitates may become coarse, which may deteriorate the toughness of the base material. In addition, coarse carbonitrides may be precipitated, which may become a starting point for fracture. Therefore, when containing Ti, it is set to 0.003% or more and 0.050% or less. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.035% or less, more preferably 0.032% or less.

B: 0.0003% or more and 0.0100% or less

B is an element effective in improving the strength of steel plates. In order to obtain this effect, it is preferable to add B in an amount of 0.0003% or more. On the other hand, if it is contained in excess of 0.0100%, coarse B precipitates may be formed, which may lower the toughness. Therefore, the B amount is set in a range of 0.0003% or more and 0.0100% or less. It is preferably 0.0030% or less.

Cu: 0.01% or more and 1.00% or less

Cu is an effective element for increasing the strength of steel sheets by improving the quenching property, but if its content exceeds 1.00%, there is a concern that the low-temperature toughness of the steel sheet may deteriorate. Therefore, when Cu is added, it is preferable that the content be 1.00% or less. On the other hand, when it is less than 0.01%, the effect of increasing the strength is not obtained, so when adding it, it is preferable that it be 0.01% or more. More preferably, it is 0.10% or more and 0.30% or less.

Cr: 0.01% or more and 1.50% or less

Cr is an element that contributes to improving the low-temperature toughness and corrosion resistance of high-Mn steel. For this purpose, it is preferable that the Cr content be 0.01% or more. On the other hand, Cr may precipitate in the form of nitrides, carbides, carbonitrides, etc. during rolling, and the formation of such precipitates may become a starting point for corrosion or destruction, thereby lowering the low-temperature toughness. Therefore, it is preferable that the upper limit be 1.50%. More preferably, it is 1.00% or less.

Mo: 0.03% or more and 1.00% or less

Mo is an effective element for suppressing tempering embrittlement susceptibility of steel plates, and is also an element for increasing the strength of steel plates without damaging low-temperature toughness. In order to obtain this effect, it is preferable that the Mo content be 0.03% or more. On the other hand, if Mo exceeds 1.00%, there is a concern that low-temperature toughness may deteriorate. Therefore, when Mo is contained, the content is set to 0.03% or more and 1.00% or less. More preferably, it is set to 0.05% or more and 0.30% or less.

Sn: 0.01% or more and 0.50% or less

Sb: 0.01% or more and 0.50% or less

W: 0.05% or more and 2.00% or less

Sn, Sb and W are elements effective for improving corrosion resistance. These effects are expressed when Sn and Sb are 0.01% or more and W is 0.05% or more. However, if any one of the elements is contained in a large amount, there is a concern that weldability and toughness may deteriorate and it may also be disadvantageous from the standpoint of cost. Therefore, the Sn amount is set to be in the range of 0.01% or more and 0.50% or less, the Sb amount is set to be in the range of 0.01% or more and 0.50% or less, and the W amount is set to be in the range of 0.05% or more and 2.00% or less. Preferably, the Sn amount is set to be 0.02% or more and 0.25% or less, the Sb amount is set to be 0.02% or more and 0.25% or less, and the W amount is set to be 0.10% or more and 1.00% or less.

In addition, the present invention may contain the following elements as needed.

Ca: 0.0005% or more and 0.0050% or less, Zr: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0100% or less, 1 or 2 or more

Ca, Zr, Mg and REM are elements useful for shape control of inclusions such as MnS, and can be added as needed. Here, shape control of the inclusion means changing an expanded sulfide-based inclusion into a granular inclusion. Through shape control of this inclusion, the toughness and sulfide stress corrosion cracking resistance are improved. In order to obtain such an effect, it is preferable to contain Ca, Zr and Mg at 0.0005% or more, and REM at 0.0010% or more. On the other hand, if any one of the elements is contained in a large amount, the amount of non-metallic inclusions increases, and there are cases where the low-temperature toughness rather decreases. Therefore, when Ca, Zr and Mg are contained, they are respectively 0.0005% or more and 0.0050% or less, and when REM is contained, they are respectively 0.0010% or more and 0.0100% or less. More preferably, the amount of Ca is 0.0010% or more and 0.0040% or less, the amount of Zr is 0.0010% or more and 0.0040% or less, the amount of Mg is 0.0010% or more and 0.0040% or less, and the amount of REM is 0.0020% or more and 0.0100% or less.

[Surface tissue]

Next, it is important that the structure from the surface of the steel plate to a depth of 1 mm (hereinafter also referred to as the surface structure) has an average equivalent circle diameter of 5 ㎛ or less of crystal grains surrounded by angled grain boundaries with an orientation difference of 15° or more, and that the maximum equivalent circle diameter of the retained austenite grains is 1 ㎛ or less.

First, it is necessary to make the average circle diameter of crystal grains surrounded by diagonal grain boundaries with an orientation difference of 15° or more 5 ㎛ or less. This is because the amount of crystal grain boundaries with an orientation difference of 15° or more, which serve as hydrogen trap sites, increases and is also dispersed, so that crack propagation due to hydrogen embrittlement can be reduced. In addition, the average circle diameter of the crystal grains is preferably 4 ㎛ or less, and more preferably 3 ㎛ or less.

In addition, the specification of crystal grains surrounded by diagonal grain boundaries with an orientation difference of 15° or more and the specification of the average equivalent circle diameter of the crystal grains can be performed by the measuring method in the examples described later.

In order to make the average equivalent circle diameter of the crystal grains surrounded by the angled grain boundaries of 15° or more 5 ㎛ or less, after hot rolling, or if heat treatment is performed after hot rolling, after the heat treatment, a cooling treatment is performed at an average cooling rate of 1 ℃/s or more in a predetermined temperature range.

In addition, in the surface structure, it is necessary to make the maximum circle diameter of the retained austenite grains 1 ㎛ or less. This is because by making the maximum circle diameter 1 ㎛ or less, hydrogen traps for the retained austenite are dispersed, and local concentration of hydrogen traps is avoided, thereby reducing crack propagation due to hydrogen embrittlement. In addition, the amount of retained austenite in the surface structure is preferably 15% or less in area ratio, and more preferably 10% or less.

In addition, it is preferable that the structure of the steel plate is martensite and/or bainite. At that time, it is preferable that the area ratio of martensite and/or bainite is 80% or more.

Next, the conditions for manufacturing the steel plate of the present invention will be described. That is, the steel material having the above-mentioned composition can be manufactured by heating, hot rolling, and cooling, or performing heat treatment again after hot rolling and cooling. At that time, in the case of cooling after hot rolling, or performing heat treatment after hot rolling, it is necessary to set the average cooling rate in a predetermined temperature range to 1°C/s or more in the cooling after the heat treatment in order to obtain the above-mentioned surface layer structure. Hereinafter, the manufacturing conditions will be described in the order of the processes. In the following description, temperature (°C) means the temperature at the center of the plate thickness.

First, it is desirable that the reheating temperature of the steel material in hot rolling be 1000℃ or higher and 1300℃ or lower.

[Reheating temperature of steel material: 1000℃ or higher and 1300℃ or lower]

Heating the steel material to 1000°C or higher is to dissolve precipitates in the structure and to uniformize the crystal grain size, etc., and the heating temperature is preferably 1000°C or higher and 1300°C or lower. That is, if the heating temperature is less than 900°C, there is a concern that the desired properties may not be obtained because the precipitates may not be dissolved sufficiently. On the other hand, if heating exceeds 1300°C, the material may deteriorate due to coarsening of the crystal grain size, and there is a concern that excessive energy may be required for manufacturing, which may lower productivity. More preferably, the temperature is 1050°C or higher and 1250°C or lower, and further, 1100°C or higher and 1250°C or lower.

[Cooling after hot rolling]

In order to ensure excellent stress corrosion cracking resistance by making the surface layer structure of the steel plate preferably a martensite and/or bainite structure and further increasing the large-angle grain boundaries included in the structure, a cooling treatment is performed after hot rolling, and the average cooling rate in the temperature range of 600°C or less and 200°C or more in the surface layer structure is set to 1°C/s or more. That is, when the cooling rate in this cooling treatment is less than 1°C/s, the surface layer structure becomes an upper bainite structure, the large-angle grain boundaries included in the structure decrease, the structure is not sufficiently refined, and stress corrosion cracking resistance cannot be obtained. There is no particular need to limit the upper limit of the average cooling rate.

In addition, when performing the heat treatment described later after hot rolling, the cooling speed after hot rolling does not need to be 1°C/s or more.

[Heat treatment after hot rolling]

The following heat treatment may be performed without cooling after hot rolling. As described above, in order to make the surface layer structure of the steel plate preferably a martensite and/or bainite structure, and further increase the large-angle grain boundaries included in the structure, and secure excellent stress corrosion cracking resistance, when performing heat treatment after hot rolling, it is preferable to heat to Ac ₃ point or more and 900°C or less after hot rolling to perform quenching (primary quenching). That is, if the heating temperature is less than Ac ₃ point or exceeds 900°C, the circle equivalent diameter of the large-angle grain boundaries becomes coarse, and there is a concern that the desired characteristics may not be obtained.

In addition, when the above-mentioned heat treatment is performed after hot rolling, it is necessary to control the speed of cooling after the heat treatment as described above. That is, the average cooling speed in the temperature range of 600°C or less and 200°C or more in the surface structure is set to 1°C/s or more.

In addition, as a heat treatment after hot rolling, instead of the above-mentioned quenching (primary quenching), or after the primary quenching and cooling, a heat treatment (secondary quenching) may be performed in which heating is performed to a temperature higher than the Ac ₁ transformation point and lower than the Ac ₃ transformation point, and then cooling. By performing this secondary quenching, the low-temperature toughness of the base material can be improved.

In addition, when performing the above-mentioned heat treatment (secondary quenching), it is necessary to control the speed of cooling after the heat treatment as described above. That is, the average cooling speed in the temperature range of 600°C or less and 200°C or more in the surface tissue is set to 1°C/s or more.

In order to obtain properties such as high strength and excellent low-temperature toughness, it is effective to make the residual austenite grains in the surface structure into fine grains with a diameter of 1 ㎛ or less. To this end, it is preferable to heat and temper at a temperature of 500°C or more and 650°C or less after the final cooling described above. That is, if the tempering temperature is less than 500°C, there is a concern that it will be difficult to secure low-temperature toughness. On the other hand, if the tempering temperature exceeds 650°C, coarse residual austenite will form, and there is a concern that the desired properties will not be obtained.

Example

The steels A to W shown in Table 1 were melted and made into slabs, and then steel plates (samples No. 1 to 26) with a thickness of 30 to 50 mm were manufactured under the manufacturing conditions shown in Table 2, and each sample was subjected to the following Charpy impact test and stress corrosion cracking test. In addition, the spacing of large-angle grain boundaries in the surface layer structure and the retained austenite grain size were investigated for each sample.

A diagonal grain boundary is defined as a grain boundary with a grain boundary orientation difference of 15° or more, and this was specified using EBSD. Then, the crystal grain size was measured in an arbitrary range of 500×500 ㎛ at a depth of 1 mm from the surface of the steel plate, and the average value of the equivalent circle diameters of the crystal grains surrounded by the diagonal grain boundary was obtained. In addition, those surrounded by the diagonal grain boundary with a range of less than 0.1 ㎛ were excluded from the calculation.

In addition, the maximum equivalent diameter of retained austenite was determined from the crystal structure of the retained austenite grains present in the same EBSD measurement area, and was taken as the maximum equivalent diameter of the grain recognized as austenite.

[Charpy impact test (low temperature toughness)]

For each sample, a V-notch test piece specified in JIS Z 2242 was prepared, and a Charpy impact test was performed in accordance with JIS Z 2242 at a test temperature of -196°C, and the absorbed energy was measured. For each sample, a test was performed using three test pieces, and a case where their average value was 34 J or more was considered passed.

[Stress Corrosion Cracking Test (Stress Corrosion Cracking Resistance)]

A DCB (Double-Cantilever-Beam) test was conducted in accordance with NACE TM0177-96 2003 edition. The test environment was NACE TM0177 sol.A (initial pH 2.7) × 100% _H2S gas saturation (0.1 MPa), and the immersion time was 336 hours. After the immersion was completed, KISSC was derived from the wedge load and crack length. For each sample, a test was conducted using three test pieces, and the case where their average value was 25 MPa√m or more was considered as passed.

The results obtained by the above are shown in Table 2.

Samples No. 1 to 14, 23, and 26 according to the present invention were confirmed to have excellent stress corrosion cracking resistance as well as secured low-temperature toughness. On the other hand, comparative examples (samples No. 15 to 22 and 24, 25) outside the scope of the present invention could not satisfy the target performance described above, since the absorbed energy was lower than 34 J or the DCB test was less than 25 MPa√m.

Claims (7)

In mass %,
C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 1.00% or less,
Mn: 0.10% or more and 3.00% or less,
Al: 0.002% or more and 0.100% or less,
Ni: 5.0% or more and 10.0% or less,
N: 0.0010% or more and 0.0080% or less,
Co: 0% or more but less than or equal to 1.50%,
P: 0.030% or less and
S: 0.0050% or less
It contains a composition of elements that are Fe and inevitable impurities.
A steel plate having a structure from the surface of the steel plate to a depth of 1 mm, in which the average equivalent circle diameter of crystal grains surrounded by angled grain boundaries with an orientation difference of 15° or more is 5 ㎛ or less, and the maximum equivalent circle diameter of retained austenite grains is 1 ㎛ or less. In the first paragraph,
The above composition of ingredients is further expressed in mass%:
Nb: 0.001% or more and 0.030% or less,
V: 0.01% or more and 0.10% or less,
Ti: 0.003% or more and 0.050% or less,
B: 0.0003% or more and 0.0100% or less,
Cu: 0.01% or more and 1.00% or less,
Cr: 0.01% or more and 1.50% or less,
Sn: 0.01% or more and 0.50% or less,
Sb: 0.01% or more and 0.50% or less,
Mo: 0.03% or more and 1.00% or less and
W: 0.05% or more and 2.00% or less
A steel plate containing one or more kinds selected from . In claim 1 or 2,
The above composition of ingredients is further expressed in mass%:
Ca: 0.0005% or more and 0.0050% or less,
Zr: 0.0005% or more and 0.0050% or less,
Mg: 0.0005% or more and 0.0050% or less and
REM: 0.0010% or more and 0.0100% or less
A steel plate containing one or more kinds selected from . In a method for manufacturing a steel plate, the method comprises heating a steel material having the composition described in claim 1 or claim 2, hot rolling it, and then cooling it.
In the above cooling treatment, the average cooling rate is set to 1 ℃/s or more between 600 ℃ and 200 ℃,
A method for manufacturing a steel plate, wherein the steel plate has a structure from the surface to a position 1 mm deep, wherein the average equivalent circle diameter of crystal grains surrounded by angular grain boundaries with an orientation difference of 15° or more is 5 ㎛ or less, and the maximum equivalent circle diameter of retained austenite grains is 1 ㎛ or less. In a method for manufacturing a steel plate, the method comprises heating a steel material having the composition described in Article 3, performing hot rolling, and then performing a cooling treatment.
In the above cooling treatment, the average cooling rate is set to 1 ℃/s or more between 600 ℃ and 200 ℃,
A method for manufacturing a steel plate, wherein the steel plate has a structure from the surface to a position 1 mm deep, wherein the average equivalent circle diameter of crystal grains surrounded by angular grain boundaries with an orientation difference of 15° or more is 5 ㎛ or less, and the maximum equivalent circle diameter of retained austenite grains is 1 ㎛ or less. In a method for manufacturing a steel plate, the method comprises heating a steel material having the composition described in claim 1 or claim 2, performing hot rolling, performing heat treatment again, and then performing cooling treatment.
The above heat treatment is heating to a temperature higher than or equal to Ac ₃ point and lower than or equal to 900°C, or heating to a temperature higher than or equal to Ac ₁ transformation point and lower than or equal to Ac ₃ transformation point, or heating to a temperature higher than or equal to Ac ₃ point and lower than or equal to Ac ₁ transformation point and lower than or equal to Ac ₃ transformation point after cooling treatment.
In the above cooling treatment, the average cooling rate is set to 1 ℃/s or more between 600 ℃ and 200 ℃,
A method for manufacturing a steel plate, wherein the steel plate has a structure from the surface to a position 1 mm deep, wherein the average equivalent circle diameter of crystal grains surrounded by angular grain boundaries with an orientation difference of 15° or more is 5 ㎛ or less, and the maximum equivalent circle diameter of retained austenite grains is 1 ㎛ or less. In a method for manufacturing a steel plate, the method comprises heating a steel material having the composition described in Article 3, performing hot rolling, performing heat treatment again, and then performing cooling treatment.
The above heat treatment is heating to a temperature higher than or equal to Ac ₃ point and lower than or equal to 900°C, or heating to a temperature higher than or equal to Ac ₁ transformation point and lower than or equal to Ac ₃ transformation point, or heating to a temperature higher than or equal to Ac ₃ point and lower than or equal to Ac ₁ transformation point and lower than or equal to Ac ₃ transformation point after cooling treatment.
In the above cooling treatment, the average cooling rate is set to 1 ℃/s or more between 600 ℃ and 200 ℃,
A method for manufacturing a steel plate, wherein the steel plate has a structure from the surface to a position 1 mm deep, wherein the average equivalent circle diameter of crystal grains surrounded by angular grain boundaries with an orientation difference of 15° or more is 5 ㎛ or less, and the maximum equivalent circle diameter of retained austenite grains is 1 ㎛ or less.