JP2017008343A - Steel plate for lpg storage tank and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet for high strength LPG storage tank having SSC resistance and low temperature toughness.SOLUTION: The steel plate for an LPG storage tank is provided that has chemical composition containing, C:0.02 to 0.08%, Si:0.01 to 0.3%, Mn:1.2 to 2.0%, P:0.015% or less, S:0.05% or less, Cr:0.5 to 1.5%, Mo:0.3 to 1.0%, Ni:0.03% or less, Cu:0.2% or less, Nb:0.01 to 0.03%, Al:0.03 to 0.08%, Ti:0.005 to 0.02%, B:0.0005 to 0.003%, N:0.005% or less, O:0.003% or less, V:0 to 0.1%, Ca:0 to 0.005%, Mg:0 to 0.005% and the balance Fe with impurities and that has a metallographic structure in which total of bainite and sintered martensite is 95% or more, an aspect ratio of prior austenite grain size is 3 or more, average of high-angle tilt grain size is 30 μm or less. The steel plate for an LPG storage tank has a TS (Tensile Strength) of 780 MPa or more and a thickness of 10 mm or more and less than 40 mm.SELECTED DRAWING: None

Description

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

The LPG storage tank is used in a wet hydrogen sulfide (H ₂ S) environment, and in such an environment, there is a risk that sulfide stress cracking (SSC) may occur. Are known. SSC is considered to be a kind of hydrogen embrittlement crack caused by a large amount of hydrogen generated by a corrosion reaction penetrating into steel due to the presence of hydrogen sulfide.

  The SSC sensitivity of steel depends on conditions such as chemical composition and metal structure. Conventionally, it is well known that the addition of Ni is effective in improving the low temperature toughness of steel. However, the addition of Ni promotes active path corrosion and degrades SSC resistance (Yamane et al., Stress corrosion cracking behavior of low alloy steel under sulfide environment, Kawasaki Steel Technical Report, Vol. 17 (1985). No. 2, pp. 178-184 (see Non-Patent Document 1)).

  It is believed that the SSC sensitivity of steel can be reduced by reducing the hardness of the steel. In practice, this SSC is likely to occur in a welded portion, and particularly occurs frequently in a heat affected zone (HAZ). This is considered to be closely related to the hardening of HAZ by welding.

  Thus, the toughness and SSC resistance of steel are contradictory properties, and it is difficult to achieve both. The following invention is disclosed as an invention relating to a material for an LPG storage tank that achieves both of these characteristics.

  JP 2002-339037 (Patent Document 1) discloses that the structure of HAZ has an optimal mixed structure of martensite and lower bainite, {4.10 × Mn (%) + 2.33. An invention is disclosed in which the value calculated by × Cr (%) + 3.14 × Mo (%)} is adjusted to satisfy 9.0 or more and 13 or less. According to this invention, an HT720 grade steel sheet having excellent low temperature joint toughness and excellent SSC resistance is obtained.

  Japanese Patent Application Laid-Open No. 2002-371336 (Patent Document 2) discloses an invention in which the density of the old γ grain boundary is defined in a portion of the steel sheet cross-section from the surface to ¼ of the plate thickness and from the back surface to ¼ of the plate thickness. Is disclosed. According to this invention, while ensuring toughness, the steel plate of the strength of HT730 grade is obtained.

JP 2002-339037 A JP 2002-371336 A

Yamane et al., Stress Corrosion Cracking Behavior of Low Alloy Steel in Sulfide Environment, Kawasaki Steel Technical Report, Vol. 17 (1985) No. 17; 2, pp. 178-184

  By the way, there is a need for higher strength in steel for LPG storage tanks. If the strength can be increased, the plate thickness can be reduced, while securing the SCC resistance and toughness of the welded joint becomes more difficult.

  However, Patent Document 1 and Patent Document 2 target a tensile strength of 720 MPa class or 730 MPa class. In the case of a steel plate having such a tensile strength, the thickness is often 50 mm or more. Actually, in Patent Document 1 and Patent Document 2, only a thick steel plate having a thickness of 40 mm or more has been specifically examined. However, a thick steel plate having a thickness of 40 mm or more requires not only SR treatment (stress relief heat treatment) after welding, but also causes an increase in the number of welding passes.

  An object of the present invention is to provide a steel sheet for an LPG storage tank that has a tensile strength of 780 MPa class and is excellent in resistance to sulfide stress corrosion cracking and low temperature toughness of a weld. Another object of the present invention is to provide a method for producing the above steel sheet for LPG storage tanks.

  In order to achieve the above object, the inventors of the present invention increase the strength of steel while reducing the plate thickness. Specifically, the tensile strength should be over 780 MPa and the plate thickness should be less than 40 mm. , Earnest research.

  The present inventors first studied to reduce the C content as compared with conventional steels in consideration of toughness deterioration during welding in increasing the strength of steel for LPG tanks. However, if the C content is reduced too much, it becomes difficult to obtain sufficient tensile strength. Therefore, it has been found that in order to increase the strength while reducing the C content, it is important to control the structure up to the thickness center of the steel sheet.

  The inventors of the present invention examined the manufacturing process using direct quenching and tempering (DQT) as a method for controlling the structure. And in DQT, by controlling the rolling temperature and reduction amount in the hot rolling process of steel sheets, it is possible to introduce processing strain and refine the structure, and through that, improve the balance of strength and toughness. The manufacturing method to be realized was found.

  This invention is made | formed based on said knowledge, and makes a summary the following steel plate for LPG storage tanks, and its manufacturing method.

(1) The chemical composition is mass%,
C: 0.02 to 0.08%,
Si: 0.01 to 0.3%
Mn: 1.2 to 2.0%,
P: 0.015% or less,
S: 0.05% or less,
Cr: 0.5 to 1.5%
Mo: 0.3 to 1.0%,
Ni: 0.03% or less,
Cu: 0.2% or less,
Nb: 0.01-0.03%,
Al: 0.03-0.08%,
Ti: 0.005 to 0.02%,
B: 0.0005 to 0.003%,
N: 0.005% or less,
O: 0.003% or less,
V: 0 to 0.1%
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
Balance: Fe and impurities,
The metal structure is a mixed structure in which the total of bainite and tempered martensite is 95% or more by area fraction,
When the thickness is t, the aspect ratio (rolling direction / thickness direction) of the prior austenite grain size is 3 or more at the 1 / 4t position and 1 / 2t position, and the large tilt grain size in the thickness direction is The average value is 30 μm or less,
A steel sheet for an LPG storage tank having a tensile strength of 780 MPa or more and a thickness of 10 mm or more and less than 40 mm.

(2) The chemical composition is mass%,
V: contains 0.01 to 0.1%,
The steel sheet for LPG storage tank according to (1) above.

(3) The chemical composition is mass%,
Ca: 0.0005 to 0.005% and / or Mg: 0.0005 to 0.005%,
The steel sheet for LPG storage tank according to (1) or (2) above.

前記冷却した鋼板に、６５０℃以下の温度域で焼戻し処理を施す工程と、を備える、
引張強さが７８０ＭＰａ以上であり、かつ厚さが１０ｍｍ以上４０ｍｍ未満のＬＰＧ貯蔵タンク用鋼板の製造方法。 (4) heating the slab provided with the chemical composition according to any one of (1) to (3) to 1000 to 1200 ° C;
Subjecting the slab to hot rolling at a cumulative reduction ratio of 50% or more with respect to the thickness of the slab in a temperature range of 850 ° C. or higher to obtain an intermediate steel sheet;
The intermediate steel sheet is subjected to hot rolling at a temperature range of 850 ° C. or lower and 750 ° C. or higher to obtain a steel sheet by subjecting the intermediate steel sheet to a hot rolling with a cumulative reduction ratio of 50% or more;
From the temperature range of 730 ° C. or higher to the temperature range of 500 ° C. or lower, the cooling rate at the 1/4 t position when the thickness of the steel plate is t is expressed by the following formula: Vc (° C./s) A step of cooling under the above cooling rate condition;

A step of tempering the cooled steel sheet in a temperature range of 650 ° C. or lower,
A method for producing a steel sheet for an LPG storage tank having a tensile strength of 780 MPa or more and a thickness of 10 mm or more and less than 40 mm.

  According to the present invention, it is possible to obtain a steel sheet for an LPG storage tank having a tensile strength of 780 MPa class and excellent in resistance to sulfide stress corrosion cracking and low temperature toughness of a welded portion. Since this steel sheet can be made thinner than the conventional 730 MPa grade steel sheet used for LPG storage tanks, SR treatment (stress relief annealing) after welding can be omitted, and the welding pass It is possible to improve the welding efficiency by reducing the number, and the effect is extremely great.

  Hereinafter, the constituent requirements of the present invention will be described in detail. “%” Of the content of each element means “mass%”, and “%” of the structure means “area fraction”.

  Conventionally, up to 730 MPa grade steel has been generally used as a steel sheet for LPG storage tanks. When the tensile strength of 730 MPa steel is used for the steel plate for LPG storage tank, the plate thickness is often set to 50 mm or more in order to ensure the strength of the LPG storage tank as a structure. Not only is SR treatment (stress relief heat treatment) required, but the number of welding passes is increased.

  First, in order to reduce the thickness of the steel sheet for LPG storage tank to less than 40 mm, the tensile strength higher than that of conventional steel needs to be higher than 780 MPa. However, it is difficult to ensure sulfide stress corrosion cracking resistance and low-temperature toughness in the welded portion as the strength of the steel plate increases. Therefore, in the present invention, in order to increase the strength of the steel sheet while maintaining excellent sulfide stress corrosion cracking resistance and low temperature toughness of the welded portion, the chemical composition of the steel sheet is strictly adjusted and the thickness of the steel sheet is increased. It was decided to control the structure from the surface layer to the center.

  Hereinafter, the chemical composition for obtaining the above steel sheet, the structure of the steel sheet, and the manufacturing method for controlling the structure of the steel sheet will be described.

(A) Chemical composition of steel plate C: 0.02 to 0.08%
C is an element that is extremely effective in increasing the strength of steel. Therefore, 0.02% or more is contained. On the other hand, when it is contained excessively, the hardness of the welded joint becomes high and the toughness is deteriorated. For this reason, C content is made into 0.02 to 0.08%. A preferred lower limit is 0.03% and a preferred upper limit is 0.05%.

Si: 0.01 to 0.3%
Si is effective in increasing the strength of the steel material. Therefore, 0.01% or more is contained. However, if contained excessively, it leads to abnormal hardening of the weld heat affected zone and a decrease in joint toughness. For this reason, Si content is made into 0.01 to 0.3%. A preferred lower limit is 0.05% and a preferred upper limit is 0.2%.

Mn: 1.2 to 2.0%
Mn is an important element for ensuring the strength of the base material and the toughness of the welded portion. If it is less than 1.2%, the hardenability becomes insufficient and the necessary strength and toughness cannot be obtained. On the other hand, when it contains excessively, toughness deterioration and a raise of HAZ hardness will be caused. For this reason, Mn content shall be 1.2 to 2.0%. A preferred lower limit is 1.4% and a preferred upper limit is 1.8%.

P: 0.015% or less P is an impurity mixed in the steel, and it is desirable to reduce the content thereof as much as possible in order to reduce the mechanical properties of the steel material, particularly low temperature toughness. When the content is excessive, in order to promote grain boundary fracture in HAZ, the content is made 0.015% or less. The P content is preferably 0.010% or less.

S: 0.05% or less S is an impurity mixed in the steel, and MnS inclusions formed in the steel are developed by hot rolling and become the starting point of cracking, so the content is reduced as much as possible. It is desirable to make it. Therefore, the S content is 0.05% or less. The S content is preferably 0.03% or less.

Cr: 0.5 to 1.5%
Cr improves hardenability and greatly contributes to improvement of base material strength and toughness of welded parts. If it is less than 0.5%, those effects cannot be obtained, and if it is excessively contained, the toughness of the HAZ deteriorates. Therefore, the Cr content is 0.5 to 1.5%. A preferred lower limit is 0.7% and a preferred upper limit is 1.2%.

Mo: 0.3-1.0%
Mo improves hardenability and greatly contributes to improvement of base material strength and toughness of welded parts. If it is less than 0.3%, those effects cannot be obtained, and if it is excessively contained, the toughness of the HAZ deteriorates. Therefore, the Mo content is set to 0.3 to 1.0%. A preferred lower limit is 0.4% and a preferred upper limit is 0.8%.

Ni: 0.03% or less Ni is an impurity mixed in the steel, and when it is contained in the steel, the SSC resistance is lowered. Therefore, the content is desirably reduced as much as possible. When the content is excessive, the SSC resistance is remarkably lowered, so the content is made 0.03% or less. The Ni content is preferably 0.01% or less.

Cu: 0.2% or less Cu is an impurity mixed in the steel, and if it is contained in the steel, there is a concern that hot cracking may occur due to the Cu checking phenomenon, so the content can be reduced as much as possible. desirable. If the content is excessive, hot cracking is likely to occur, so 0.2% or less. The Cu content is preferably 0.05% or less.

Nb: 0.01-0.03%
Nb has the effect of suppressing the coarsening of crystal grains during slab heating, and also exhibits the same effect during quenching and is effective for refining the structure. Furthermore, it precipitates as Nb (C, N) in the grains during tempering and has a function of contributing to improvement in yield strength. In order to acquire this effect, 0.01% or more needs to be contained. On the other hand, when the content is excessive, the coarsening of the precipitate becomes remarkable and the toughness is lowered. Therefore, the Nb content is set to 0.01 to 0.03%. A preferred lower limit is 0.012% and a preferred upper limit is 0.025%.

Al: 0.03-0.08%
Al has a deoxidizing action and also exhibits a so-called pinning effect of “pinning” the movement of the austenite grain boundary as AlN during quenching, and has the action of preventing the austenite grains from becoming coarse. Furthermore, since N harmful to HAZ toughness is fixed as AlN, effective B can be segregated at the austenite grain boundaries, and B functions to suppress the formation of ferrite and promote the effect of improving hardenability. In order to obtain these effects, it is necessary to contain 0.03% or more of Al. On the other hand, when the content is excessive, the increase of inclusions is increased and the toughness is deteriorated. Therefore, the Al content is 0.03 to 0.08%. A preferred lower limit is 0.04% and a preferred upper limit is 0.07%.

Ti: 0.005-0.02%
Ti becomes fine TiN, fixes N, exhibits a pinning effect during heating, suppresses the growth of austenite grains, and at the time of addition of B, it helps quench segregation of effective B to austenite grain boundaries. Included to have the effect of enhancing the properties. If less than 0.005% is added, this effect cannot be obtained. On the other hand, when the content is excessive, the coarsening of TiN becomes remarkable and the toughness is lowered. Therefore, the Ti content is set to 0.005 to 0.02%. A preferred lower limit is 0.008% and a preferred upper limit is 0.018%.

B: 0.0005 to 0.003%
B is an element that segregates at the austenite grain boundaries and suppresses the formation of ferrite to significantly improve the hardenability. In order to obtain this effect, B is contained in an amount of 0.0005% or more. On the other hand, when it contains excessively, toughness will deteriorate. Therefore, the B content is set to 0.0005 to 0.003%. A preferred lower limit is 0.0008% and a preferred upper limit is 0.002%.

N: 0.005% or less N is an impurity mixed in steel. When the N content exceeds 0.005%, the solid material and the HAZ toughness are deteriorated due to an increase in the solid solution N. Therefore, the N content is set to 0.005% or less.

O: 0.003% or less O is also an impurity mixed in the steel. When the O content increases, nonmetallic inclusions in the steel increase, and the low temperature toughness and SSC resistance are impaired. In order to avoid this, the O content is set to 0.003% or less.

V: 0 to 0.1%
V has the effect of improving the hardenability of the steel, and can further increase the strength of the steel sheet due to the precipitation effect during the tempering treatment. However, when the content is excessive, the effect is saturated and the cost is increased, and the toughness is remarkably lowered. For this reason, when V is contained, the content is made 0.1% or less. In addition, in order to acquire said effect, it is preferable to contain V 0.01% or more. A preferable upper limit is 0.08%.

Ca: 0 to 0.005%
Ca may be contained because non-metallic inclusions are spheroidized and low temperature toughness can be improved. However, when the content is excessive, inclusions such as CaO and CaS are produced in large quantities, and the toughness of the steel is impaired. In addition, hydrogen in the steel easily accumulates around the inclusions in a wet hydrogen sulfide environment. SSC resistance deteriorates. For this reason, when Ca is contained, the content is made 0.005% or less. In addition, in order to acquire said effect, it is preferable to contain 0.0005% or more of Ca. A preferable upper limit is 0.004%.

Mg: 0 to 0.005%
Mg may be contained because non-metallic inclusions are spheroidized and low temperature toughness can be improved. However, if the Mg content exceeds 0.005%, a large amount of inclusions such as MgO and MgS are generated to impair the toughness of the steel, and the hydrogen in the steel accumulates around the inclusions in a wet hydrogen sulfide environment. And the SSC resistance deteriorates. For this reason, when Mg is contained, the content is made 0.005% or less. In addition, in order to acquire said effect, it is preferable to contain Mg 0.0005% or more. A preferable upper limit is 0.004%.

The balance in the chemical composition of the steel sheet for an LPG storage tank of the present invention is Fe and impurities.
An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially.

(B) About the metal structure of the steel sheet (B-1) About the area fraction of the mixed structure In order to set the tensile strength of the steel sheet to 780 MPa or more, the metal structure is substantially a mixed structure of bainite and tempered martensite. There is a need. More specifically, a mixed structure in which the total of bainite and tempered martensite in the metal structure of the steel sheet is 95% or more. The metal structure other than the mixed structure is, for example, a structure made of ferrite, pearlite, cementite, island martensite, or the like. The metal structure of the steel sheet is preferably a mixed structure in which the sum of bainite and tempered martensite is 97% or more, and more preferably a mixed structure having 98% or more.

  For the metal structure, after Nital corrosion, SEM observation is performed, the type of the structure is determined based on the black and white density of the image, and the area fraction of each structure may be obtained. Since it is difficult to distinguish bainite and tempered martensite, the area fraction may be obtained without distinguishing these.

(B-2) Aspect ratio of prior austenite grain size While steel sheets for LPG storage tanks are required to have high strength, it is also necessary to ensure sufficient fracture safety, and the structure control is important. Here, in the case of a thick steel plate having a plate thickness of 40 mm or more, structure control is performed at the surface layer portion, specifically, at a 1/4 t position (t: the thickness of the steel plate, the same applies hereinafter). Is the mainstream. This is because it is considered that a thick steel plate does not need to consider deep structure control because of its thickness, and it is considered sufficient to control the structure in the surface layer part. However, in a thin thick steel plate having a thickness of less than 40 mm, the merit of such thickness is not utilized, and not only the structure control at the 1 / 4t position but also the structure control at the 1 / 2t position is important.

Refinement of the structure is effective for improving toughness. In order to control not only the structure at the 1 / 4t position but also the structure at the 1 / 2t position, after sufficiently promoting the refinement of the austenite grain size, controlled rolling in the non-recrystallized region is performed. It is important to promote the flattening of the structure and to make the structure finer in the thickness direction of the steel sheet. At this time, the aspect ratio (rolling direction / thickness direction) of the prior austenite grain size at the 1 / 4t position and the 1 / 2t position is set to 3 or more. The aspect ratio of the prior austenite grain size is preferably 4.5 or more, but if it is excessive, the anisotropy of the mechanical properties in the rolling direction and the vertical direction of the rolling becomes large. preferable.
(B-3) Large tilt angle particle size

  To stabilize the fracture toughness, it is not sufficient to refine the prior austenite grain size. By baking from a fine-grained flat austenite at a sufficient cooling rate, it is possible to refine a large-inclined grain size that governs fracture toughness. Here, by setting the large tilt grain size to 30 μm or less, sufficient fracture toughness can be secured even in HT780 grade high strength steel. The large tilt particle diameter is preferably 25 μm or less. There is no particular lower limit. The large tilt grain size means an equivalent circle diameter of a region surrounded by a large tilt grain boundary having a crystal orientation different by 15 ° or more by electron backscattering spectroscopy (EBSP).

(C) About the manufacturing method of a steel plate (C-1) About the shape of a slab The slab which has the chemical composition demonstrated by the term of the said A is prepared. Here, “slab” is a general term for steel ingots, blooms, billets and the like. Although the thickness of the slab is not limited, it is preferable to use a slab having a thickness of at least three times the finished thickness in order to reduce the temperature while controlling the temperature when manufacturing the steel sheet. It is preferable to use a slab having a thickness of 5 times or more the finished thickness. Moreover, it is preferable that the thickness of a slab is 10 times or less of finishing thickness from a viewpoint of the manufacture efficiency of a steel plate. Although a slab may be manufactured by an ingot method, it is preferable to manufacture a slab by a continuous casting method from a viewpoint of cost reduction.

(C-2) Slab heating step Slab heating is carried out in order to increase the fine crystal temperature range by dissolving coarse inclusions such as Nb, Ti, and B deposited during casting. For this reason, the minimum of heating temperature shall be 1050 degreeC. A preferred lower limit is 1080 ° C. On the other hand, when the heating temperature is too high, coarsening of the prior austenite is caused and the toughness is lowered. For this reason, the upper limit of heating temperature shall be 1200 degreeC. A preferred upper limit is 1160 ° C.

(C-3) Hot rolling process In order to achieve both high strength and toughness, it is important to refine the steel sheet structure during rolling. In the present invention, the steel sheet structure is refined to improve toughness by controlled rolling in the recrystallization region and controlled rolling in the non-recrystallization region.

  First, rolling is performed in a temperature range of 850 ° C. or higher, which is a recrystallization region, to obtain an intermediate steel plate. In the recrystallization region, recrystallization of the steel sheet structure is advanced, and the austenite grain size is reduced by reduction. At this time, if the cumulative rolling reduction is 50% or more with respect to the slab thickness, a sufficient austenite grain size can be achieved. Although the upper limit of the rolling reduction is not particularly specified, the upper limit of the cumulative rolling reduction in the recrystallization region is 75% considering the plate thickness of the finally obtained thick steel plate.

  Then, it rolls in the temperature range below 850 degreeC which is a non-recrystallized area | region, and obtains a steel plate. Austenite grains are flattened by rolling in the non-recrystallized region. And after being cooled and transformed, it remains flattened as prior austenite grain boundaries, contributing to improved toughness. In order to avoid an increase in rolling load and the subsequent quenching process is performed directly after rolling without reheating, rolling in the non-recrystallization temperature region is performed at 750 ° C. or higher. At this time, if the cumulative rolling reduction is 50% or more with respect to the thickness of the intermediate steel sheet, the structure can be refined. The upper limit of the cumulative rolling reduction in the non-recrystallized region is not particularly defined, but is 75% considering the plate thickness of the finally obtained thick steel plate.

(C-4) Quenching step In the quenching step, so-called direct quenching is performed to quench the steel sheet. That is, the pre-rolling process is finished at 750 ° C. or higher, and cooling is performed as it is from a temperature range of 730 ° C. or higher. In order to ensure hardenability, it is preferable to perform quenching from a high temperature range, quenching is preferably performed from a temperature range of 740 ° C. or higher, and more preferably from a temperature range of 750 ° C. or higher.

  If the quenching stop temperature is too high, a bainite and martensite structure cannot be obtained, and strength and toughness are insufficient. Therefore, quenching is performed up to a temperature range of 500 ° C. or lower.

At this time, the cooling rate is set to a cooling rate equal to or higher than Vc (° C./s) represented by the following formula. Vc indicates the cooling rate at which the metal structure becomes a martensite structure of 90% or more in area fraction.

  And when the cooling rate in the 1 / 4t position when thickness of the said steel plate is set to t is less than Vc (degreeC / s), it is an area fraction and the sum total of a bainite and a tempered martensite is 95% or more. A steel sheet having a mixed structure cannot be obtained.

(C-5) Tempering step The tempering treatment is performed to remove the strain introduced by the quenching treatment and to finely precipitate carbides to improve the balance between strength and toughness. From the viewpoint of securing toughness, the tempering treatment is performed in a temperature range of 650 ° C. or lower. Although a lower limit is not defined, it is preferably performed in a temperature range of 500 ° C. or higher.

  Steel plates were produced under the conditions shown in Tables 3 and 4 on slabs obtained by melting steels having chemical compositions shown in Tables 1 and 2.

  The metal structure and performance of the obtained steel sheet were evaluated by the following methods. The results are shown in Tables 5 and 6.

<Metallic structure>
The aspect ratio and microstructure of the prior austenite grains of the steel sheet were calculated from the average value of five x100 photographs taken for the L-direction microstructure at the 1 / 2t position and the 1 / 4t position. Also, the large tilt grain size is measured by EBSP (Electron Back Scattering Pattern) method using a high-resolution field emission scanning microscope (FE-SEM), with a cross section parallel to the rolling direction of the steel sheet at a depth of 100 μm from the surface. The crystal orientation analysis was used to identify the large tilt grain, and the equivalent circle diameter of the region surrounded by the large tilt grain boundary was determined by image processing.

<Tensile properties of steel sheet>
From each steel plate, a No. 4 tensile test piece defined in JIS Z 2241: 2011 is taken from the direction perpendicular to the rolling direction, the tensile strength test is performed to measure the base material strength, and the target tensile strength (TS) Was determined to be good. YS is preferably 680 MPa or more.

<Toughness of steel plate (base material)>
A Charpy impact test piece defined in JIS Z 2242: 2011 was cut out from each steel plate in parallel with the rolling direction from a thickness of 1/2 t, and an impact test was performed to measure the absorbed energy vE-60 (J) at −60 ° C. When the vE-60 was 100 J or more, the steel sheet was judged to have good toughness (hereinafter abbreviated as “base metal toughness”).

<Weld joint performance>
A weld test piece having a length of 600 mm and a width of 300 mm was cut out from each steel plate, its end was processed into an X-shaped groove, and a submerged arc welding with a heat input of 3.0 kJ / mm was performed to produce a welded joint. A Charpy impact test piece was taken from the 1 / 4t position so that the notch center position coincided with the fusion line from each weld joint. Using this specimen, a Charpy impact test was performed to evaluate the low temperature toughness of the welded joint, that is, the low temperature joint toughness. The case where vE-60 was 47 J or more was considered good.

  Moreover, the test piece for hardness measurement was cut out from the said welded joint part, and the hardness of the weld toe part where the stress concentration degree is the largest and SSC is likely to occur was measured. The case where the Vickers hardness (Hv) was 315 or less was considered good.

Further, an SSC test material having a length of 115 mm, a width of 30 mm, and a thickness of 1.5 mm is cut out from the surface of the weld joint of each weld joint as described above, and a stress corresponding to 100% of the yield stress is applied by 4-point bending. Thus, an SSC test piece was prepared. These test pieces were agitated for 1000 hours in a saturated aqueous solution in which H ₂ S gas with adjusted partial pressure was passed through pure water + 0.5% CH ₃ COOH aqueous solution and the H ₂ S concentration was 25 ppm. After the test was completed, the presence or absence of cracks on the surface of the test piece was examined using an optical microscope. The case where a crack was not observed was evaluated as good (◯), and the case where a crack was observed was evaluated as defective (×).

  As shown in Tables 5 and 6, test no. 1 to 37 had the chemical composition defined in the present invention and were produced under appropriate production conditions. As a result, the metal structure was also within the range defined in the present invention. As a result, the base material had sufficient strength and toughness, and also had sufficient toughness and SSC resistance in the welded joint. On the other hand, No. which does not satisfy the conditions defined in the present invention. 38-55 was inferior to any performance in a base material or a welded joint. In addition, No. Regarding No. 40, the steel structure was not cracked due to Cu-checking in the steel sheet after rolling, so the metal structure was not observed and evaluated.

  According to the present invention, it is possible to obtain a steel sheet for an LPG storage tank having a tensile strength of 780 MPa class and excellent in resistance to sulfide stress corrosion cracking and low temperature toughness of a welded portion. Since this steel sheet can be made thinner than the conventional 730 MPa grade steel sheet used for LPG storage tanks, SR treatment (stress relief annealing) after welding can be omitted, and the welding pass It is possible to improve the welding efficiency by reducing the number, and the effect is extremely great.

Claims (4)

Chemical composition is mass%,
C: 0.02 to 0.08%,
Si: 0.01 to 0.3%
Mn: 1.2 to 2.0%,
P: 0.015% or less,
S: 0.05% or less,
Cr: 0.5 to 1.5%
Mo: 0.3 to 1.0%,
Ni: 0.03% or less,
Cu: 0.2% or less,
Nb: 0.01-0.03%,
Al: 0.03-0.08%,
Ti: 0.005 to 0.02%,
B: 0.0005 to 0.003%,
N: 0.005% or less,
O: 0.003% or less,
V: 0 to 0.1%
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
Balance: Fe and impurities,
The metal structure is a mixed structure in which the total of bainite and tempered martensite is 95% or more by area fraction,
When the thickness is t, the aspect ratio (rolling direction / thickness direction) of the prior austenite grain size is 3 or more at the 1 / 4t position and 1 / 2t position, and the large tilt grain size in the thickness direction is The average value is 30 μm or less,
A steel sheet for an LPG storage tank having a tensile strength of 780 MPa or more and a thickness of 10 mm or more and less than 40 mm. The chemical composition is mass%,
V: contains 0.01 to 0.1%,
The steel sheet for an LPG storage tank according to claim 1. The chemical composition is mass%,
Ca: 0.0005 to 0.005% and / or Mg: 0.0005 to 0.005%,
The steel plate for LPG storage tanks according to claim 1 or 2. Heating the slab having the chemical composition according to claim 1 to 1000 to 1200 ° C;
Subjecting the slab to hot rolling at a cumulative reduction ratio of 50% or more with respect to the thickness of the slab in a temperature range of 850 ° C. or higher to obtain an intermediate steel sheet;
The intermediate steel sheet is subjected to hot rolling at a temperature range of 850 ° C. or lower and 750 ° C. or higher to obtain a steel sheet by subjecting the intermediate steel sheet to a hot rolling with a cumulative reduction ratio of 50% or more;
From the temperature range of 730 ° C. or higher to the temperature range of 500 ° C. or lower, the cooling rate at the 1/4 t position when the thickness of the steel plate is t is expressed by the following formula: Vc (° C./s) A step of cooling under the above cooling rate condition;

A step of tempering the cooled steel sheet in a temperature range of 650 ° C. or lower,
A method for producing a steel sheet for an LPG storage tank having a tensile strength of 780 MPa or more and a thickness of 10 mm or more and less than 40 mm.