JP7445127B2 - Steel plate for LPG storage tank and its manufacturing method - Google Patents

Description

The present invention relates to a steel plate for an LPG storage tank and a method for manufacturing the same.

Liquefied petroleum gas (hereinafter referred to as "LPG") contains corrosive gases such as hydrogen sulfide (H ₂ S). For this reason, LPG storage tanks are usually used in a humid hydrogen sulfide environment, and sulfide stress corrosion cracking (also referred to as "Sulfide Stress Cracking", hereinafter referred to as "SSC") caused by H ₂ S occurs. This may occur.

Usually, LPG tanks are welded during manufacture. When welding is performed, SSC is likely to occur in the weld heat affected zone (also referred to as "Heat Affected Zone", hereinafter referred to as "HAZ") that is affected by the weld metal and welding heat. In particular, in the HAZ, SSC is significantly more likely to occur due to the increase in hardness due to the input of welding heat. Therefore, in order to suppress SSC, it is effective to reduce the hardness.

On the other hand, there is a problem in that reducing the hardness makes it impossible to ensure the strength and toughness required for LPG tanks. As described above, since SSC resistance and the strength and toughness of steel are contradictory properties, various steels have been developed to achieve both of these properties.

For example, Patent Document 1 discloses a steel plate having a strength of HT720 class and having excellent low-temperature toughness and excellent SSC resistance. In Patent Document 1, in order to obtain the above-mentioned steel sheet, the chemical composition is controlled to make the metal structure of the HAZ an optimal mixed structure of martensite and lower bainite. Specifically, regarding the chemical composition, the value calculated by {4.10 x Mn (%) + 2.33 x Cr (%) + 3.14 x Mo (%)} satisfies 9.0 or more and 13 or less. It seems like it's under control.

Patent Document 2 discloses an HT730 class high-strength steel plate with improved SSC resistance and low-temperature toughness of welded parts. In order to obtain the above-mentioned steel plate, Patent Document 2 controls the density of prior γ grain boundaries in a 1/4 part of the plate thickness from the front surface and in a 1/4 part of the plate thickness from the back surface in the cross section of the steel plate.

Patent Document 3 discloses a steel plate having strength of HT780 class and having both SSC resistance and low temperature toughness. In order to obtain the above-mentioned steel plate, in Patent Document 3, the aspect ratio of the prior γ grain size and the large angle grain size in the thickness direction at the 1/4t position and the 1/2t position of the steel plate are controlled.

Japanese Patent Application Publication No. 2002-339037 Japanese Patent Application Publication No. 2002-371336 JP 2017-8343 Publication

Steel used in LPG storage tanks is required to have even higher strength and thicker walls in order to store LPG more efficiently. However, it is difficult to achieve high strength and thick steel while ensuring SSC resistance and toughness. For example, if an attempt is made to increase the strength of steel, the SSC resistance will decrease.

Since the steel plates disclosed in Patent Documents 1 and 2 have a tensile strength of 720 to 730 MPa class, there is room for consideration in terms of further increasing the strength. Similarly, there is room for consideration in terms of increasing the thickness.

Further, although the steel plate disclosed in Patent Document 3 has a high tensile strength of 780 MPa class, the plate thickness is assumed to be less than 40 mm, so there is room for consideration in terms of increasing the thickness. As described above, there is a problem in that it is difficult to increase the strength and thickness of steel while achieving both SSC resistance and toughness.

The object of the present invention is to solve the above-mentioned problems and obtain a high-strength, thick-walled steel plate that has good SSC resistance and low-temperature toughness.

The present invention has been made to solve the above problems, and its gist is the following steel plate for an LPG storage tank and a method for manufacturing the same.

(1) The chemical composition is in mass%,
C: 0.06-0.15%,
Si: 0.01 to 0.10%,
Mn: 0.50-1.40%,
P: 0.020% or less,
S: 0.010% or less,
Ni: 0.10% or less,
Ti: 0.005-0.040%,
Al: 0.005-0.050%,
N: 0.0005-0.0100%,
Nb: 0.005-0.050%,
Cr: 0.50-1.50%,
Mo: 0.10-0.50%,
V: 0.01-0.10%,
The remainder: Fe and impurities,
Vc ₉₀ expressed by the following formula (i) is 1.0 to 5.0,
The metal structure at the center of the plate thickness is
Contains 97% or more of bainite in terms of area ratio, and island martensite has an area ratio of 0.1% or less, and the average particle size of cementite is 0.5 μm or less,
Among all the observed bainite grains, when the top 10 bainite grains are selected in descending order of grain size, the average grain size of the 10 bainite grains is 30 μm or less,
The tensile strength is 770 to 940 MPa,
A steel plate for LPG storage tanks with a plate thickness of 55 to 80 mm.
log( _Vc90 )=2.94-0.75×(2.7C+0.4Si+Mn+0.45Ni+0.8Cr+5Mo)...(i)
However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and is zero if it is not contained.

(2) The chemical composition is in mass % instead of a part of the Fe,
Cu: 0.50% or less, and B: 0.0050% or less,
The steel plate for an LPG storage tank according to (1) above, containing one or more selected from the following.

(3) The chemical composition is in mass % instead of a part of the Fe,
Ca: 0.0050% or less, and Mg: 0.0050% or less,
The steel plate for an LPG storage tank according to (1) or (2) above, containing one or more selected from the following.

(4) A method for manufacturing the steel plate according to any one of (1) to (3) above, comprising:
(a) A step of heating a slab having a thickness of 200 mm or more at 1000 to 1200°C, hot rolling it, and then cooling it to room temperature to form a steel plate;
(b) A step of heating the steel plate to a temperature of ₃ or more Ac points to 1000°C or less, holding it for soaking, and then cooling the steel plate to 200°C or less at a cooling rate of 5°C/s or more at the center of the plate thickness. and,
(c) heating the steel plate to a temperature of 500° C. or more and 700° C. or less, holding it soaked for 15 minutes or more, and then cooling it;
has
The method for manufacturing a steel plate for an LPG storage tank in (a) above, wherein the final three passes during rolling are rolled at a slab surface temperature in a temperature range of 750 to 900° C. and at a cumulative reduction rate of 30% or more.

According to the present invention, it is possible to obtain a high-strength, thick steel plate that has good SSC resistance and low-temperature toughness.

The present inventors conducted the following study on a steel plate having good SSC resistance and low temperature toughness, a tensile strength of 780 MPa or more, and a plate thickness of 55 mm or more.

(a) In order to improve the strength of a steel plate, it is effective to increase the C content. However, if the C content is excessive, the vicinity of the surface of the steel plate will harden during welding, resulting in a decrease in SSC resistance. For this reason, it is desirable to improve the strength by adjusting the alloy components and increasing the hardenability, rather than increasing the C content. This is because hardening near the surface due to welding can be suppressed, and a decrease in SSC resistance can be suppressed.

(b) On the other hand, low-temperature toughness is also required for steel plates for LPG storage tanks. It is generally known that adding Ni improves low temperature toughness. By adding Ni, dislocations become more mobile and the yield stress is lowered. As a result, plastic deformation occurs before the fracture stress is reached, improving low-temperature toughness.

However, the addition of Ni causes fine cracks to occur in the surface layer in a hydrogen sulfide environment, significantly reducing the SSC resistance. Therefore, it is necessary to ensure low temperature toughness without adding Ni. Therefore, it is desirable to control the structure so that the metal structure at the center of the thickness of the steel plate becomes a fine bainite structure.

(c) When the metal structure is a bainite structure, it is inevitable that cementite will occur in the bainite structure. When the particle size of cementite increases, cracks tend to occur starting from cementite, and toughness decreases. Therefore, the present inventors have found that by setting Vc ₉₀ , which is a hardenability index, to 1.0 to 5.0, cementite can be finely dispersed and a desirable bainite structure can be obtained. Specifically, a fine-grained lower bainite structure can be obtained. As a result, a steel plate having high strength and good low-temperature toughness can be obtained without reducing SSC resistance.

The present invention has been made based on the above findings. Hereinafter, each requirement of the steel plate for an LPG storage tank and the manufacturing method of the present invention will be explained in detail.

1. Chemical composition The reasons for limiting each element are as follows. In addition, in the following description, "%" regarding content means "mass %".

C: 0.06-0.15%
C is an extremely effective element for increasing the strength of steel materials. If the C content is less than 0.06%, the desired strength cannot be ensured. Furthermore, the metal structure is not sufficiently refined, resulting in poor low-temperature toughness. Therefore, the C content is set to 0.06% or more. The C content is preferably 0.08% or more. However, when C is contained in an amount exceeding 0.15%, the surface layer of the heat affected zone hardens due to welding, and SSC resistance deteriorates. Therefore, the C content is set to 0.15% or less. The C content is preferably 0.14% or less.

Si: 0.01~0.10%
Si is an element that has a deoxidizing effect. Therefore, the Si content is set to 0.01% or more. The Si content is preferably 0.02% or more. However, when Si is contained in an amount exceeding 0.10%, island martensite (hereinafter also referred to as "MA") is generated in the HAZ, which becomes a starting point for destruction. As a result, toughness decreases after welding. Therefore, the Si content is set to 0.10% or less. The Si content is preferably 0.09% or less.

Mn: 0.50-1.40%
Mn is an element that increases the hardenability of steel and increases the strength and toughness of steel. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably 0.65% or more. However, when the Mn content exceeds 1.40%, the toughness of the HAZ decreases during welding. Therefore, the Mn content is set to 1.40% or less. The Mn content is preferably 1.30% or less.

P: 0.020% or less P is an element contained in steel as an impurity. When P is contained in an amount exceeding 0.020%, it segregates at grain boundaries, lowering grain boundary strength and lowering low-temperature toughness. Therefore, the P content is set to 0.020% or less. It is desirable to reduce P as much as possible.

S: 0.010% or less S is an element generally contained in steel as an impurity. S combines with Mn in steel to form MnS, reducing the low-temperature toughness and ductility of the steel material. Therefore, the S content is set to 0.010% or less. It is desirable to reduce S as much as possible.

Ni: 0.10% or less Ni is an element that contributes to improving the toughness of steel materials, but reduces the SSC resistance. As a general rule, it is not added to the extra-thick steel materials used in LPG storage tanks, as more severe SSC resistance is required. Even when Ni is contained as an impurity, if the Ni content exceeds 0.10%, fine cracks will occur on the corroded surface and the SSC resistance will deteriorate even under low stress. Therefore, the Ni content is set to 0.10% or less. The Ni content is preferably less than 0.05%. In addition, in order to ensure the toughness of the steel material, Nb, Cr, Mo, V, etc. are contained.

Ti: 0.005-0.040%
Ti is an element that combines with N in steel to form TiN and improves the cleanliness of the slab surface and the steel material surface. Furthermore, it has the effect of suppressing coarsening of austenite crystal grains. Therefore, the Ti content is set to 0.005% or more. The Ti content is preferably 0.010% or more. However, if Ti is contained in an amount exceeding 0.040%, the precipitates may become coarse and the toughness of the base material may deteriorate. Therefore, the Ti content is set to 0.040% or less. The Ti content is preferably 0.035% or less.

Al: 0.005-0.050%
Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel plates. Further, by fixing solid solution N in the steel and forming AlN, it has the effect of suppressing coarsening of crystal grains. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.010% or more. However, if the Al content exceeds 0.050%, Al will be mixed into the weld metal during welding, reducing the toughness of the weld metal. Therefore, the Al content is set to 0.050% or less. The Al content is preferably 0.040% or less.

N: 0.0005-0.0100%
N combines with Ti and Al to form TiN and AlN, and has the effect of suppressing coarsening of crystal grains. Therefore, the N content is set to 0.0005% or more. The N content is preferably 0.0010% or more. However, when N is contained in an amount exceeding 0.0100%, the nitride becomes coarse and the toughness decreases. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0090% or less.

Nb: 0.005-0.050%
Nb is an effective element for expanding the non-recrystallized austenite region, further contributing to grain refinement and improving strength and toughness. Therefore, the Nb content is set to 0.005% or more. The Nb content is preferably 0.010% or more. However, when the Nb content exceeds 0.050%, coarse carbides are produced and the toughness is reduced. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.045% or less.

Cr: 0.50-1.50%
Cr is an effective element for improving strength because it suppresses ferrite transformation and improves hardenability. Therefore, the Cr content is set to 0.50% or more. The Cr content is preferably 0.70% or more. However, if Cr is contained in an amount exceeding 1.50%, the strength of the steel material becomes too high. Therefore, the Cr content is set to 1.50% or less. The Cr content is preferably 1.40% or less.

Mo: 0.10~0.50%
Mo is an element that enhances the hardenability of steel materials and improves the strength of the base material. Therefore, the Mo content is set to 0.10% or more. The Mo content is preferably 0.15% or more. However, when the Mo content exceeds 0.50%, the strength becomes excessively high. It also causes a significant decrease in weldability. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably less than 0.30%.

V:0.01~0.10%
V forms carbonitrides and has the effect of precipitation strengthening steel materials. Therefore, the V content is set to 0.01% or more. The V content is preferably 0.02% or more. However, when the V content exceeds 0.10%, the effect is saturated and manufacturing costs increase. Therefore, the V content is set to 0.10% or less. The V content is preferably 0.08% or less.

In addition to the above elements, the chemical composition of the steel of the present invention may further contain one or more selected from Cu, B, Ca, and Mg within the range shown below. The reasons for limiting each element will be explained.

Cu: 0.50% or less Cu has the effect of improving strength by improving hardenability and precipitation hardening during tempering treatment. Therefore, it may be included if necessary. However, if Cu is contained excessively, hot cracking may occur due to Cu checking. Therefore, the Cu content is set to 0.50% or less. On the other hand, in order to obtain the above effects, the Cu content is preferably 0.10% or more, more preferably 0.20% or more.

B: 0.0050% or less B segregates at austenite grain boundaries and suppresses the formation of ferrite, thereby having the effect of significantly improving hardenability and improving strength. Therefore, it may be included if necessary. However, when B is contained excessively, the toughness decreases. Therefore, the B content is set to 0.0050% or less. The B content is preferably 0.0030% or less. On the other hand, in order to obtain the above effects, the B content is preferably 0.0003% or more, more preferably 0.0005% or more.

Ca: 0.0050% or less Ca has the effect of spheroidizing nonmetallic inclusions and improving low-temperature toughness. Therefore, it may be included if necessary. However, when Ca is contained excessively, inclusions such as CaO and CaS are generated in large quantities, impairing the toughness of the steel. In addition, in a wet hydrogen sulfide environment, hydrogen in the steel tends to accumulate around inclusions, reducing SSC resistance. Therefore, the Ca content is set to 0.0050% or less. The Ca content is preferably 0.0040% or less. On the other hand, in order to obtain the above effects, the Ca content is preferably 0.0002% or more, more preferably 0.0005% or more.

Mg: 0.0050% or less Mg has the effect of spheroidizing nonmetallic inclusions and improving low-temperature toughness. Therefore, it may be included if necessary. However, when the Mg content exceeds 0.0050%, inclusions such as MgO and MgS are generated in large quantities, impairing the toughness of the steel. Furthermore, hydrogen in the steel tends to accumulate around inclusions in a humid hydrogen sulfide environment, resulting in a decrease in SSC resistance. Therefore, the Mg content is set to 0.0050% or less. The Mg content is preferably 0.0040% or less. On the other hand, in order to obtain the above effects, the Mg content is preferably 0.0002% or more, more preferably 0.0005% or more.

In the chemical composition of the present invention, the remainder is Fe and impurities. Here, "impurities" are components that are mixed in during the industrial production of steel due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are allowed within the range that does not adversely affect the present invention. means something that

_Vc90
In the steel plate according to the present invention, Vc ₉₀ expressed by the following formula (i) is set to 1.0 to 5.0°C/s. Vc ₉₀ indicates the cooling rate necessary to obtain martensite with an area ratio of 90% or more, and is calculated based on the content of each element.

log( _Vc90 )=2.94-0.75×(2.7C+0.4Si+Mn+0.45Ni+0.8Cr+5Mo)...(i)
However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and is zero if it is not contained.

If Vc ₉₀ is less than 1.0° C./s, quenching progresses excessively, and the affected zone hardens during welding heat, resulting in a decrease in SSC resistance. Therefore, Vc ₉₀ is set to 1.0° C./s or more. It is preferable that Vc ₉₀ is 1.5° C./s or more. On the other hand, when Vc ₉₀ exceeds 5.0° C./s, it becomes difficult to stably obtain a bainite structure at the center of the plate thickness. Therefore, Vc ₉₀ is set to 5.0° C./s or less. Vc ₉₀ is preferably 4.5° C./s or less.

2. Metal structure In the steel plate according to the present invention, the metal structure at the center of the plate thickness is defined. This is because in an extremely thick steel plate having a thickness of 55 mm, it is important to control the structure at the center of the plate thickness, which is the median value. Specifically, the metal structure at the center of the plate thickness contains 97% or more of bainite in area ratio, and the area ratio of island martensite is 0.1% or less.

2-1. Area ratio of bainite and area ratio of island-like martensite In the metal structure at the center of the plate thickness, if the area ratio of bainite is less than 97%, sufficient strength and toughness cannot be ensured. For this reason, in the metal structure at the center of the plate thickness, the area ratio of bainite is set to 97% or more. It is desirable that the metal structure at the center of the plate thickness is basically a structure consisting of bainite, and it is most preferable that the area ratio of bainite be 100%.

Additionally, island-like martensite may be formed. Island-like martensite is formed in the bainite described above, that is, in the bainite in the center of the plate thickness, and has an adverse effect on low-temperature toughness. Moreover, the toughness of the HAZ formed by welding is also reduced. Therefore, the area ratio of island martensite in bainite is 0.1% or less. It is desirable to prevent the formation of the island-shaped martensite as much as possible.

2-2. Average Grain Size of Cementite Cementite may be formed in a steel plate, and in particular, cementite in bainite at the center of the plate thickness described above tends to be relatively large. When coarse cementite is formed, brittle fracture occurs starting from the cementite, resulting in a decrease in low-temperature toughness. Therefore, it is necessary to suppress the formation of coarse cementite. Therefore, the average particle size of cementite is set to 0.5 μm or less. More specifically, the average grain size of cementite in bainite at the center of the plate thickness is 0.5 μm or less. The average particle size of cementite in bainite is preferably 0.4 μm or less. Note that the measurement of the average particle size of cementite will be described later.

2-3. Other Structures As described above, the metal structure at the center of the plate thickness is basically a structure mainly composed of bainite, but other structures other than bainite may be formed. The area ratio of such structures other than bainite (hereinafter referred to as "other structures") is 3.0% or less. When the area ratio of other structures exceeds 3.0%, the low temperature toughness is significantly reduced. It is preferable to prevent the formation of other tissues as much as possible. Note that examples of other structures include martensite and island martensite (MA).

2-4. Average Particle Size of Specified Coarse Bainite Grains In the steel sheet according to the present invention, it is necessary to form fine bainite in order to improve the strength and toughness of the steel sheet. However, even if fine bainite is formed, if bainite with an extremely large particle size is locally formed, it becomes a starting point for destruction. As a result, the toughness of the steel decreases.

For this reason, the average grain size of coarse bainite grains with large grain sizes is controlled. Specifically, when the top 10 bainite grains are selected in descending order of grain size among all the observed bainite grains, the 10 bainite grains (hereinafter simply referred to as "regular coarse bainite") are selected. The average particle size of the particles shall be 30 μm or less.

Here, the average grain size of the specified coarse bainite grains is the average grain size of the top 10 grains with the largest grain sizes among all the observed bainite grains. Furthermore, when one particle is approximated to the equivalent of a circle, the diameter is defined as the particle size. In addition, during observation, a test piece was taken out from 1/2 the thickness of the steel plate, the L cross section was taken as the observation surface, an area of 800 μm in the L direction x 600 μm in the thickness direction was photographed at a magnification of 90 times, and the photographic field of view was set as 5 fields. It is preferable to target specified coarse bainite grains determined by observation.
3. Tensile Strength The steel plate according to the present invention has a tensile strength of 770 to 940 MPa. This is because by setting the tensile strength within the above range, the transport efficiency of LPG can be improved and the tank can be made larger.

4. Plate Thickness The steel plate according to the present invention has a plate thickness of 55 to 80 mm. This is because by setting the plate thickness within the above range, the tank can be made thicker. If the plate thickness is less than 55 mm, the tank cannot be made sufficiently thick. For this reason, the plate thickness is 55 mm or more, preferably 60 mm or more. On the other hand, if the plate thickness exceeds 80 mm, manufacturability will decrease. For this reason, the plate thickness is 80 mm or less, preferably 75 mm or less.

5. Manufacturing method A preferred method for manufacturing the steel plate according to the present invention will be described. The steel plate according to the present invention can obtain the effects as long as it has the above-mentioned configuration regardless of the manufacturing method, but it can be stably manufactured by, for example, the following manufacturing method.

in particular,
(a) A step of heating a slab having a thickness of 200 mm or more at 1000 to 1200°C, hot rolling it, and then cooling it to room temperature to form a steel plate;
(b) heating the steel plate to a temperature of ₃ or more Ac points to 1000°C or less, holding it for soaking, and then cooling it to 200°C or less at a cooling rate of 5°C/s or more at the center of the thickness of the steel plate;
(c) heating the steel plate to a temperature of 500°C or higher and 700°C or lower, holding the steel plate for 15 minutes or more, and then cooling it;
has
In (a), it is preferable that the final three passes during rolling be performed at a slab surface temperature in the temperature range of 750 to 900° C. and at a cumulative reduction rate of 30% or more.

5-1. Hot Rolling Process First, it is preferable to manufacture a slab having a thickness of 200 mm or more having the above-mentioned chemical composition by a continuous casting method or the like. By setting the slab thickness to 200 mm or more, it is possible to ensure a sufficient rolling reduction in the hot rolling process, and the bainite can be refined more reliably. The slab thickness is preferably 300 mm or less.

The slab having a thickness of 200 mm or more is preferably heated at a heating temperature of 1000 to 1200°C. By setting the heating temperature to 1000° C. or higher, coarse Ti--Nb carbonitrides precipitated during casting are dissolved, so that the low-temperature toughness of the steel sheet after rolling does not deteriorate. For this reason, the heating temperature is preferably 1000°C or higher.

On the other hand, by setting the heating temperature to 1200° C. or lower, the austenite crystal grains do not become coarse, so that the low-temperature toughness of the rolled steel sheet does not deteriorate in this case either. For this reason, the heating temperature is preferably 1200°C or less.

It is preferable that the slab heated in a heating furnace at a temperature of 1000 to 1200° C. is hot rolled, and after hot rolling, it is cooled to room temperature to form a steel plate. In this hot rolling process, rough rolling and finish rolling are usually performed, but in the steel plate according to the present invention, the manufacturing conditions are controlled in order to adjust the grain size of the metal structure for the final three passes corresponding to finish rolling. It is preferable to do so. Specifically, it is preferable that the final three passes during rolling, which correspond to finish rolling, be carried out at a slab surface temperature in the temperature range of 750 to 900° C. and with a cumulative reduction ratio of 30% or more.

In the final three passes, when the surface temperature of the slab is 750° C. or higher, the deformation resistance decreases, so that the reduction amount per pass can be increased. As a result, the total number of paths decreases. For this reason, it is preferable that the surface temperature of the slab be 750° C. or higher in the final three passes. On the other hand, in the final three passes, when the surface temperature of the slab is 900° C. or lower, recrystallization of austenite due to rolling can be avoided and the metal structure can be refined. For this reason, it is preferable that the surface temperature of the slab be 900° C. or less in the final three passes.

Moreover, in the final three passes, if the cumulative reduction rate in the above temperature range is 30% or more, coarsening of crystal grains can be prevented, and low-temperature toughness can be prevented from deteriorating. Therefore, in the final three passes, the cumulative reduction rate in the above temperature range is preferably 30% or more.

Here, the cumulative rolling reduction rate can be calculated using the following equation (a).
P _t = (T _b - T _a )/T _b ×100 ... (a)
However, each symbol in the above formula is defined as follows.
P _t (%): Cumulative rolling reduction T _a (mm): Thickness after rolling T _b (mm): Thickness before rolling

The reason why the final three passes are controlled under the above conditions is that if high pressure is applied in the final pass or the final two passes, the steel plate will warp or the flatness of the steel plate will decrease. After hot rolling, cooling is performed. In order to create a steel structure in a later step, it is preferable to allow cooling to room temperature.

5-2. Hardening process It is preferable to perform a hardening process after the hot rolling process. In the quenching step, it is preferable to heat the steel plate to a temperature of Ac ₃ points (° C.) or higher and 1000° C. or lower, and then hold the steel sheet soaked at that temperature. Thereafter, it is preferable to cool the steel plate to 200°C or less at a cooling rate of 5°C/s or more at the center of the thickness of the steel plate.

In the quenching process, if the heating temperature of the steel plate is Ac ₃ points (°C) or higher, no ferrite remains during reheating, and a fine bainite structure can be obtained after quenching. For this reason, it is preferable that the heating temperature of the steel plate in the quenching step be ₃ or more Ac points. On the other hand, in the quenching step, when the heating temperature of the steel plate is 1000° C. or lower, coarsening of austenite can be suppressed during heating, and a decrease in toughness can be prevented. For this reason, the heating temperature of the steel plate in the quenching process is preferably 1000°C or less. Thereafter, it is preferable to maintain sufficient soaking at the temperature so that the metal structure becomes an austenite single phase. It is preferable that the holding time is, for example, 15 minutes or more. Moreover, it is preferable that the holding time is 30 minutes or less.

After that, cooling is performed. During cooling, when the cooling rate at the center of the thickness of the steel plate is 5° C./s or more, the transformation temperature at the center of the thickness of the steel plate is lowered, and a fine bainite structure can be obtained. Therefore, the toughness of the steel plate can be ensured. Therefore, during cooling, it is preferable that the cooling rate at the center of the thickness of the steel plate be 5° C./s or more. Note that the upper limit of the cooling rate at the center of the plate thickness is not particularly limited, but is usually 30° C./s or less.

Moreover, it is preferable to perform cooling to 200 degreeC or less. This is because the martensitic transformation end temperature (Mf point) is generally over 200°C. In addition, since the temperature of a steel plate is generally measured by measuring the surface temperature using a radiation thermometer, the cooling rate at the center of the plate thickness must be calculated based on the relationship between the surface temperature and thermal conductivity. Bye. For example, the simplest method is to solve an unsteady one-dimensional heat conduction equation using a finite difference method.

5-3. Tempering Step Subsequently, it is preferable to perform a tempering step. In the tempering step, it is preferable to heat the material to a temperature of 500° C. or more and 700° C. or less, hold it soaked, and then allow it to cool. After quenching, the strength of the steel sheet becomes excessive and its toughness decreases, but by performing a tempering process, the dislocations introduced during quenching can be recovered and the strength can be reduced to the desired strength. . As a result, the low-temperature toughness of the steel plate is improved. In order to obtain such a tempering effect, the heating temperature is preferably 500° C. or higher.

On the other hand, when the heating temperature in the tempering step is 700° C. or lower, the lath structure refined during quenching is not decomposed and the fine structure can be maintained, and low-temperature toughness can be ensured. For this reason, it is preferable that the heating temperature in the tempering step be 700° C. or lower. Subsequently, the heating temperature is soaked and maintained, and the holding time at this time is preferably 15 minutes or more. Moreover, it is preferable that the holding time is 30 minutes or less. After the soaking and holding, it is preferable to cool it. Cooling is usually performed by leaving it to cool, but for example, water cooling or air cooling may be used.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples.

A slab having the chemical composition listed in Table 1 and having a thickness of 300 mm was produced. This slab was heated and hot rolled under the conditions shown in Table 2, then allowed to cool to room temperature, then quenched and tempered, and then allowed to cool. During quenching, it was heated at the temperature listed in Table 2, soaked and held at that temperature for 15 minutes, and then cooled to room temperature at the cooling rate listed in Table 2. Further, during tempering, heating was similarly performed at the temperature listed in Table 2, and soaking was carried out for the time listed in Table 2. Then, it was cooled to room temperature. Note that the thickness of the obtained steel plate is as shown in Table 3.

Regarding the obtained steel plate, the average grain size of specified coarse bainite grains, the average grain size of cementite, and the area ratio of bainite and other structures were measured. Furthermore, in order to evaluate the characteristic values, the obtained steel plate was subjected to a tensile test, an SSC resistance test, and a Charpy test. Based on this test, strength, SSC resistance, and toughness were evaluated. Conditions for tissue observation, evaluation methods for each characteristic, and test conditions are as shown below.

(Observation of metal structure)
The area ratios of bainite and ferrite were measured using the following method for each measurement value. For bainite and ferrite, a test piece was taken from the center of the plate thickness so that the L cross section was the observation surface, mirror-polished, and finished polished with colloidal silica for 30 minutes. It was measured using the diffraction method.

The obtained data was analyzed using OIM-Analysis software manufactured by TSL Solutions (hereinafter referred to as "OIM software") using the GAM value representing the average value of the crystal orientation difference within the crystal grains, and the GAM value was 0. The area ratios of bainite and ferrite were calculated, with a case of 5 or more as bainite and a case of less than 0.5 as ferrite. Note that the photographing magnification was 90 times, the photographing field was set to 5 fields, and the obtained area ratios were averaged to determine the area ratios of bainite and ferrite.

The area ratio of island martensite (MA) was measured by the following method. First, a test piece was cut out from the center of the thickness of the steel plate, and the L cross section was mirror-polished and then subjected to repeller corrosion. Thereafter, using the corroded surface as an observation surface, the microstructure was photographed using an optical microscope at a magnification of 500 times, and the area ratio was determined after the image was binarized by image processing. Note that the photographing field is set to five fields, and the obtained area ratios are averaged to be the MA area ratio. If MA is observed, instead of calculating the area ratio of bainite as described above, subtract the area ratio of MA from the area ratio of bainite calculated as above to calculate the area ratio of bainite. did.

(Average grain size of specified coarse bainite grains)
First, a portion of the steel plate was cut out from the center of its thickness as a test piece, the L cross section was mirror polished, and then polished with colloidal silica for 30 minutes or more. Thereafter, using the polished surface as an observation surface, the area ratio and grain size of bainite crystal grains were calculated using an EBSD (electron beam backscatter diffraction) method. In addition, when other structures other than bainite were formed, for example, the area ratio was similarly measured using EBSD. The obtained data were analyzed using OIM software. The photographing magnification was 90 times, the photographing field was set to 5 fields, and the average of the top 10 equivalent circle diameters surrounded by grain boundaries with an orientation difference of 15 degrees or more was taken as the average grain size of the specified coarse bainite grains.

(Average particle size of cementite)
The average particle size of cementite was measured by the following method. First, a part of the steel plate was cut out from the center of its thickness as a test piece, the L cross section was mirror polished, and then corroded with a 3% nital solution. Thereafter, the microstructure was photographed using a field emission scanning electron microscope (FE-SEM) manufactured by JEOL, using the corroded surface as the observation surface. When photographing, the working distance was 10 mm, the accelerating voltage was 15 kV, and the magnification was 3000 times, and 5 fields of view were photographed. After binarizing by magnification image processing, the average circular equivalent particle diameter was calculated and taken as the average particle diameter.

(Strength)
The strength was evaluated by performing a tensile test. A tensile test piece (JIS Z 2241:2011, No. 1A test piece) was taken from a cross section perpendicular to the rolling direction (C cross section) from the 1/4th position of the steel plate thickness, and a tensile test was conducted. Ta.

(SSC resistance)
SSC resistance was evaluated by performing an SSC test. In the SSC test, welded joints were produced by submerged arc welding in the width direction of the steel plates with a heat input of 3.0 kJ/mm. A test piece with dimensions of 1.5 x 30 mm x 115 mm was taken from the surface with the weld bead of the welded joint left intact. This test piece was subjected to four-point bending, and while measuring the amount of strain with a strain gauge attached directly above the weld line, a strain was applied to give a stress of 100% of the yield stress, and the test piece was immersed in the solution for 720 hours.

The solution used for immersion was a saturated aqueous solution of 5% NaCl + 0.5% CH ₃ COOH in which H ₂ S gas with adjusted partial pressure was bubbled and the H ₂ S concentration was 100 ppm. After the test, the surface cracks were examined using an optical microscope at a magnification of 500 times, and the evaluation was rated as ○ if no cracks were found, and × if any cracks were observed.

(toughness)
Toughness was evaluated by performing a Charpy test. In the Charpy test, the test was performed using a steel plate and a test piece using a welded joint made from the steel plate. The steel plate test piece is a 2 mm V-notch test piece cut out from the C cross section of the steel plate, with the notch located at 1/2 the thickness of the steel plate. The welded joint was produced by cutting out two base materials from the C section of a steel plate and submerged arc welding them at a heat input of 3.0 kJ/mm. The test piece of the produced welded joint was taken from the 1/4 position of the plate thickness and was a 2 mm V-notch test piece with a notch so that it contained exactly 50% of the weld metal and base metal. In addition, when the fracture surface transition temperature (base metal and weld heat affected zone) obtained in the test was -40°C or less, it was evaluated as having good characteristics.

The results are summarized in Table 3 below.

Test No. that satisfies the provisions of the present invention. Nos. 1 to 15 showed good strength, SSC resistance, and low temperature toughness. On the other hand, test No. that does not satisfy the provisions of the present invention. Nos. 16 to 30 are examples of the present invention in at least one of strength, SSC resistance, and low-temperature toughness. The results were inferior compared to 1 to 15.

No. In No. 16, the SSC resistance decreased because the C content was excessive. No. In Nos. 17 and 20, the Mo content was low, so the hardenability decreased and coarse bainite grains were formed. As a result, toughness decreased. No. In Nos. 18 and 19, since the Cr content was low, the hardenability decreased and coarse bainite grains were formed. As a result, toughness decreased.

No. In No. 21, since the Mn content was high, MnS was formed, and the formed MnS became a starting point, resulting in a decrease in the toughness of the welded joint. No. In No. 22, the Vc ₉₀ was high, so the hardenability decreased and coarse bainite grains were formed. As a result, toughness decreased.

No. In No. 23, since the heating temperature during hot rolling was high, the prior austenite grain size became coarse, the bainite grain size also became coarse, and the toughness decreased. No. In No. 24, the rolling reduction in the final three passes decreased, the bainite grains became coarse, and the toughness decreased. No. In No. 25, the temperature in the final three passes was high, so the bainite grains became coarse and the toughness decreased.

No. In No. 26, the cooling rate at the center of the plate thickness was slow, so fine bainite grains could not be obtained and the toughness decreased. No. In No. 27, since the tempering temperature was low, the lath bainite decomposed and the structure became coarse. No. In No. 28, the tempering time was short, so the strength was high and the toughness was low. No. In No. 29, the quenching temperature was high, coarse bainite was formed, and the toughness decreased. No. Since No. 30 was not tempered, the SSC decreased. Furthermore, MA was not decomposed and the toughness decreased.

Claims (4)

The chemical composition is in mass%,
C: 0.06-0.15%,
Si: 0.01 to 0.10%,
Mn: 0.50-1.40%,
P: 0.020% or less,
S: 0.010% or less,
Ni: 0.10% or less,
Ti: 0.005-0.040%,
Al: 0.005-0.050%,
N: 0.0005-0.0100%,
Nb: 0.005-0.050%,
Cr: 0.50-1.50%,
Mo: 0.10-0.50%,
V: 0.01-0.10%,
The remainder: Fe and impurities,
Vc ₉₀ expressed by the following formula (i) is 1.0 to 5.0,
The metal structure at the center of the plate thickness is
Contains 97% or more of bainite in terms of area ratio, and island martensite has an area ratio of 0.1% or less, and the average particle size of cementite is 0.5 μm or less,
Among all the observed bainite grains, when the top 10 bainite grains are selected in descending order of grain size, the average grain size of the 10 bainite grains is 30 μm or less,
The tensile strength is 770 to 940 MPa,
A steel plate for LPG storage tanks with a plate thickness of 55 to 80 mm.
log( _Vc90 )=2.94-0.75×(2.7C+0.4Si+Mn+0.45Ni+0.8Cr+5Mo)...(i)
However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and is zero if it is not contained. The chemical composition is replaced by a part of the Fe in mass %,
Cu: 0.50% or less, and B: 0.0050% or less,
The steel plate for an LPG storage tank according to claim 1, containing one or more selected from the following. The chemical composition is replaced by a part of the Fe in mass %,
Ca: 0.0050% or less, and Mg: 0.0050% or less,
The steel plate for an LPG storage tank according to claim 1 or 2, containing one or more selected from the following. A method for manufacturing a steel plate according to any one of claims 1 to 3, comprising:
(a) A step of heating a slab having a thickness of 200 mm or more at 1000 to 1200°C, hot rolling it, and then cooling it to room temperature to form a steel plate;
(b) A step of heating the steel plate to a temperature of ₃ or more Ac points to 1000°C or less, holding it for soaking, and then cooling the steel plate to 200°C or less at a cooling rate of 5°C/s or more at the center of the plate thickness. and,
(c) heating the steel plate to a temperature of 500° C. or more and 700° C. or less, holding it soaked for 15 minutes or more, and then cooling it;
has
The method for manufacturing a steel plate for an LPG storage tank in (a) above, wherein the final three passes during rolling are rolled at a slab surface temperature in a temperature range of 750 to 900° C. and at a cumulative reduction rate of 30% or more.