JP2018127677A - Steel material for tank and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for tank excellent in stress corrosion crack resistance, and excellent in low temperature toughness of a steel material and HAZ, and a manufacturing method therefor.SOLUTION: A steel material for tank contains, by mass%, C:0.03 to 0.10%, Si:0.05 to 0.5%, Mn:0.9 to 2.0%, P:0.02% or less, S:0.0010 to 0.0100%, Nb:0.005 to 0.05%, Ti:0.005 to 0.025%, sol.Al:0.005% or less, N:0.0010 to 0.0100% and O:0.0010 to 0.0050%. At a thickness t/4 position, a structure contains 50 to 80 area% of ferrite with average circle equivalent diameter of particles of 5.5 to 15.0 μm, and a hard structure containing 0 to 50% of a band structure mainly consisting of bainite and having aspect ratio of 5 or more, contains 10 to 100/mmof composite inclusions having MnS arranged around Ti-based oxide in a steel, in which MnS in the composite inclusions is 10 to less than 90 area% and peripheral occupancy is 10% or more.SELECTED DRAWING: None

Description

  The present invention relates to a steel material for tanks and a method for producing the same.

  With the increase in energy demand in recent years, the capacity of tanks for energy transport ships has increased. Furthermore, not only liquefied petroleum gas (LPG) but also liquid ammonia may be stored and transported at low temperatures in the tank for the purpose of eliminating air transportation by energy transport ships.

  As described above, in steel materials used for tanks loaded with LPG and liquid ammonia (hereinafter referred to as tank steel materials), suppression of stress corrosion cracking due to ammonia is required. Generally, the lower the strength (yield strength YS and tensile strength TS) and the yield ratio (= yield strength YS / tensile strength TS), the less likely the stress corrosion cracking occurs. On the other hand, the tank is also required to have strength. From the viewpoint of stress corrosion cracking resistance and strength, tank steel materials are required to have a yield strength YS of 355 to 440 MPa, a tensile strength TS of 490 to 620 MPa, and a yield ratio YR of 85% or less. The strength range is narrow.

  Furthermore, as described above, since the tank carries low-temperature liquid ammonia, the steel material for tanks is required to have excellent low-temperature toughness. Further, the tank steel material may be welded together to form a tank. Therefore, excellent low-temperature toughness is also required for a weld heat affected zone (HAZ) formed by welding steel for tanks.

  Techniques for obtaining excellent low temperature toughness in the base material and HAZ while satisfying the above-described narrow allowable strength range are disclosed in JP2009-12076A (Patent Document 1) and JP2008-261000A (Patent Document 2). ) And Japanese Patent Application Laid-Open No. 2008-248291 (Patent Document 3).

  The low-yield ratio high-tensile steel sheet disclosed in Patent Document 1 is in mass%, C: 0.06-0.09%, Si: 0.05-0.25%, Mn: 1.2-1. 6%, P: 0.01% or less (not including 0%), S: 0.003% or less (not including 0%), Al: 0.06% or less (not including 0%), B: 0.0007 to 0.0015%, Ti: 0.009 to 0.018%, N: 0.0050 to 0.0080% and Nb: 0.012 to 0.020%, respectively, The amount of solid solution N is 0.0003 to 0.0040%, the balance is iron and unavoidable impurities, and in the microstructure at the position of t / 4 (t: plate thickness), the ferrite fraction in the entire structure is 60 to 85 area%, island martensite fraction is 1 to 5 area%, and the remainder is a mixed structure of bainite structure It is further retained austenite in the island martensite is 60 area% or more. In this steel sheet, it is possible to secure a predetermined amount of residual γ in the island martensite (MA), whereby the strength and low temperature toughness of the steel sheet (base material) can be compatible with the low temperature toughness of HAZ. 1 is described.

  The thick steel plates disclosed in Patent Documents 2 and 3 are mass%, C: 0.05 to 0.12%, Si: 0.05 to 0.25%, Mn: 0.4 to 2.0%. , P: 0.02% or less, S: 0.02% or less, Nb: 0.010 to 0.05% Al: 0.005 to 0.04%, and the balance is Fe and inevitable impurities By carrying out a multi-step cooling process in the manufacturing process, the movable dislocation caused by proeutectoid ferrite is maintained by the bainite transformation, and the base material and welding heat with a yield ratio of 80% or less and a tensile strength of 500 MPa Patent Documents 2 and 3 describe that the low temperature toughness of the affected part is excellent.

  On the other hand, techniques for increasing the low temperature toughness of HAZ using inclusions have been proposed in Japanese Patent Application Laid-Open No. 2014-5527 (Patent Document 4) and Japanese Patent Application Laid-Open No. 5-271864 (Patent Document 5).

The steel materials disclosed in Patent Document 4 are mass%, C: 0.02 to 0.25%, Si: 0.0001 to 0.4%, Mn: 0.5 to 2.0%, P: 0. 0.03% or less, S: 0.001 to 0.050%, O: 0.001 to 0.005%, N: 0.006% or less, insol. Al: 0.0001 to 0.005%, sol. Al: 0.0001 to 0.0005%, insol. Mg: 0.0001 to 0.005%, sol. Mg: 0.0001% to 0.0005%, with the balance being a chemical composition composed of Fe and impurities, composed of an oxide composed of Mg, Mn, Al, and MnS, with a composite intervening particle size of less than 0.6 μm Patent Document 4 describes that the presence of 1 × 10 ⁶ pieces / mm ³ or more of steel in a steel material makes the weld heat affected zone of the steel material excellent in toughness.

The steel disclosed in Patent Document 5 contains Mn: 0.1 to 3.0%, Al: 0.01% or less, S: 0.01% or less as weight%, and other steel materials as necessary. The fine particles containing Mn oxide and Al oxide, which can contain the elements normally contained in, are dispersed, and among the fine particles contained in the steel, the oxide is composed of MnS, and the oxide is Mn. A steel material composed of an oxide and an Al oxide, in which the proportion of Mn oxide in the oxide is 50% to 90% of the oxide portion by weight% and has a size of 0.1 to 10 μm. Patent Document 5 describes that 30 to 2000 particles per 1 mm ^{2 of the} cross-sectional area are excellent in low temperature toughness and deep drawability of HAZ.

  Furthermore, a technique for improving the stress corrosion cracking resistance and the low temperature toughness of the base material has been proposed in Japanese Patent Laid-Open No. 2006-40401 (Patent Document 6).

  The steel material for tanks disclosed in Patent Document 6 is mass%, C: 0.03 to 0.10%, Si: 0.05 to 0.5%, Mn: 0.9 to 2.0%, P : 0.02% or less, S: 0.01% or less, Nb: 0.005 to 0.05% or less, Ti: 0.005 to 0.025%, sol. Al: 0.09% or less, N: 0.001 to 0.010%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and Mo: Contains one or more selected from the group consisting of 0.20% or less, the balance is Fe and impurities, and the structure has an area ratio of 50 to 50 at a thickness t / 4 from the surface. It contains 80% ferrite and a hard structure, and the hard structure is composed of one or more selected from the group consisting of bainite, martensite, and pearlite, and the average equivalent circular diameter of the ferrite is 5. 5 to 15 μm. Among hard structures, the area occupied by the entire hard structure of a band structure having an aspect ratio of 5 or more, defined by the long axis length of the hard structure extending in the rolling direction / the short axis length of the hard structure extending in the rolling direction. Patent Document 6 describes that if the rate is 50% or less, it is excellent in resistance to stress corrosion cracking and low temperature toughness.

JP 2009-12076A JP 2008-261000 A JP 2008-248291 A JP 2014-5527 A JP-A-5-271864 JP 2006-40401 A

  An object of the present invention is to provide a steel material for tanks excellent in stress corrosion cracking resistance and low temperature toughness, and excellent in low temperature toughness of HAZ after welding, and a method for producing the same.

The steel for tank according to the present invention is mass%, C: 0.03-0.10%, Si: 0.05-0.5%, Mn: 0.9-2.0%, P: 0.02. % Or less, S: 0.0010 to 0.0100%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.025%, sol. Al: 0.005% or less, N: 0.0010 to 0.0100%, O: 0.0010 to 0.0050%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0-0.50%, Mo: 0-0.20%, V: 0-0.06%, B: 0-0.002%, Ca: 0-0.005%, and Mg: 0-0. 005% is contained, and the balance has a chemical composition composed of Fe and impurities. In the depth position of thickness t / 4 from the surface of the steel material, the structure is composed of ferrite with an area ratio of 50 to 80% and a hard structure. The average equivalent circle diameter of the ferrite crystal grains is 5.5 to 15.0 μm. The hard structure contains 10% or less martensite and pearlite in the total area ratio, and the balance is bainite. Among hard structures, a hard structure having a band structure having an aspect ratio of 5 or more defined by the long axis length of the hard structure extending in the rolling direction of the steel for tanks / the short axis length of the hard structure extending in the rolling direction. The area ratio occupying the whole is 50% or less. The steel material for tanks further includes a composite inclusion containing Ti-based oxide and MnS disposed around the Ti-based oxide in the steel. The ratio of MnS in the cross-sectional area of the composite inclusion is 10% or more and less than 90%, the ratio of MnS in the interface with the matrix of the composite inclusion is 10% or more, and the particle size is 0.5-5. The number density of the composite inclusions of 0 μm is 10 to 100 / mm ² .

The manufacturing method of the steel material for tanks by this invention is equipped with a melting process, a slab manufacturing process, a hot rolling process, and a cooling process. In the melting process, before the RH vacuum degassing treatment, the oxygen potential in the molten steel is set to 10 to 60 ppm, and the chemical composition is adjusted in the RH vacuum degassing treatment to produce the molten steel having the above-described chemical composition. In the slab manufacturing process, slabs are manufactured by continuous casting using molten steel. The hot rolling process includes a heating process and a rolling process. In the heating step, the slab is heated to 1000 to 1250 ° C. In the rolling process, hot rolling is performed on the heated slab to produce a steel material. During hot rolling, the cumulative rolling reduction at a temperature of 900 ° C. or lower is set to 30% or more. The cooling process includes a first cooling process, a second cooling process, a third cooling process, and a fourth cooling process. In the first cooling step, the steel material is cooled at a first cooling rate of 1.1 to 5 ° C./second until the steel material temperature reaches T1 ° C. after the hot rolling is completed. In the second cooling step, the steel material is cooled at a second cooling rate of 5 to 15 ° C./second until the steel material temperature is changed from T1 ° C. to T2 ° C. In the third cooling step, the steel material is cooled at a third cooling rate of 15 ° C./second or more until the steel material temperature is changed from T2 ° C. to T3 ° C. In the fourth cooling step, when the steel material temperature reaches T3 ° C., the cooling at the third cooling rate is stopped and the steel material is allowed to cool.
_{Here, A r3 ≧ T1 ≧ A r3} -100, A r3 -50 ≧ T2 ≧ A r3 -200, is A r3 -200 ≧ T3 ≧ 350, T1-T2 ≧ 40, T2-T3 ≧ 100.

  The steel for tank according to the present invention is excellent in stress corrosion cracking resistance and low temperature toughness, and excellent in low temperature toughness of HAZ after welding.

  Hereinafter, embodiments of the present invention will be described in detail.

  The present inventors investigated and examined the stress corrosion cracking resistance of steel for tanks, the low temperature toughness of steel (base material), and the low temperature toughness of HAZ after welding, and obtained the following knowledge.

(A) About stress corrosion cracking resistance As mentioned above, in order to obtain stress corrosion cracking resistance while securing the strength of the tank, the yield strength YS of the steel for tanks is 355 to 440 MPa, and the tensile strength TS is 490 to 490. It is required that 620 MPa and the yield ratio YR be 85% or less. When the thickness of the steel material is t (mm), the microstructure at a depth of t / 4 from the steel material surface contains 50-80% ferrite in area ratio, and the average of ferrite crystal grains (ferrite grains) When the equivalent circle diameter is 5.5 to 15.0 μm, the above strength range (hereinafter referred to as the allowable strength range) can be obtained. In addition, in the microstructure of the steel material for tanks, the remainder other than ferrite consists of a hard structure. The hard structure contains 10% or less martensite and pearlite in the total area ratio, and the balance is bainite. The martensite includes tempered martensite and island martensite.

(B) Low temperature toughness of steel material If the cooling rate of the steel material after hot rolling is too slow, a hard structure is formed by extending in the rolling direction of the steel material. Among hard structures, those having an aspect ratio (long axis length of hard structure stretched in the rolling direction / short axis length of hard structure extended in the rolling direction) of 5 or more are defined as “band structure”. The band structure lowers the low temperature toughness of the tank steel. Therefore, it is preferable that the band structure in the tank steel is as small as possible. If the area ratio of the band structure in the entire hard structure in the steel material is 50% or less, excellent low temperature toughness can be obtained in the steel material (base material).

(C) About low-temperature toughness of HAZ In order to increase the low-temperature toughness of HAZ formed when welding steel for tanks, it is effective to refine crystal grains in HAZ. As a crystal grain refining method for making crystal grains fine, (1) a method of utilizing a pinning effect for suppressing the growth of austenite grains (hereinafter referred to as γ grains) with TiN or the like, and (2) in the γ grains There is a method in which intragranular ferrite is grown starting from the inclusions present in the crystal to refine the crystal grains.

  However, in the crystal grain refining method of (1) above, a part of TiN is dissolved during welding, and the pinning effect may be reduced. Therefore, the present inventors have further studied by paying attention to the crystal grain refining method (2).

If intragranular ferrite is generated and grown in the old γ grains during welding, crystal grains are refined in the matrix of the HAZ after welding. Ti-based oxides, which are inclusions in steel, serve as nuclei for intragranular ferrite. The Ti-based oxide is an oxide containing Ti, and is, for example, TiO, Ti ₂ O ₃ or the like. Therefore, the inventors further investigated the relationship between intragranular ferrite formation and Ti-based oxides in steel. As a result, the present inventors obtained the following new knowledge.

  (A) If a composite inclusion containing Ti-based oxide and MnS is generated by forming MnS around the Ti-based oxide in the steelmaking stage, Mn is deficient at the interface between MnS and the steel matrix. Region is formed. In this Mn-deficient region (initial Mn-deficient region), the ferrite growth start temperature is greatly increased. Therefore, when steel is welded, intragranular ferrite grows preferentially from this Mn-deficient region during the cooling process.

  (B) When the steel material is welded, Mn in the matrix of the steel material existing in the vicinity of the composite inclusion diffuses, and Mn is absorbed by the atomic vacancy existing in the Ti-based oxide. As a result, a region deficient in Mn is formed at the interface between the HAZ and the Ti-based oxide that has received a thermal history by welding. This Mn-deficient region (welded Mn-deficient region) is also the starting point for preferential growth of intragranular ferrite.

  (C) Since the HAZ structure can be refined by the actions (a) and (b) above, the low temperature toughness of the HAZ necessary for the welded tank can be obtained.

  Based on the above findings, the present inventors further examined the form of composite inclusions that easily generate intragranular ferrite. As a result, it was found that if the form of the composite inclusion satisfies the following requirements, the crystal grains in the HAZ at the time of welding can be refined, and good low-temperature HAZ toughness can be obtained.

(I) The ratio of MnS to the cross-sectional area of the composite inclusion is 10% or more and less than 90%.
(II) The ratio of MnS in the interface with the matrix of the composite inclusion is 10% or more.
(III) The number density of the composite inclusions having a particle size of 0.5 to 5.0 μm is 10 to 100 / mm ² .

The steel material for tanks according to the present invention completed based on the above knowledge has a chemical composition of mass%, C: 0.03 to 0.10%, Si: 0.05 to 0.5%, Mn: 0.00. 9 to 2.0%, P: 0.02% or less, S: 0.0010 to 0.0100%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.025%, sol. Al: 0.005% or less, N: 0.0010 to 0.0100%, O: 0.0010 to 0.0050%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.20%, V: 0 to 0.06%, B: 0 to 0.002%, Ca: 0 to 0.005%, Mg: 0 to 0.005 %, And the balance consists of Fe and impurities. In the depth position of thickness t / 4 from the surface of the steel material for tanks, the structure is composed of ferrite with an area ratio of 50 to 80% and a hard structure. The average equivalent circle diameter of the ferrite crystal grains is 5.5 to 15.0 μm. The hard structure contains 10% or less martensite and pearlite in the total area ratio, and the balance is bainite. Among hard structures, a hard structure having a band structure having an aspect ratio of 5 or more defined by the long axis length of the hard structure extending in the rolling direction of the steel for tanks / the short axis length of the hard structure extending in the rolling direction. The area ratio occupying the whole is 50% or less. The steel material for tanks further includes a composite inclusion containing Ti-based oxide and MnS disposed around the Ti-based oxide in the steel. The ratio of MnS in the cross-sectional area of the composite inclusion is 10% or more and less than 90%, the ratio of MnS in the interface with the matrix of the composite inclusion is 10% or more, and the particle size is 0.5-5. The number density of the composite inclusions of 0 μm is 10 to 100 / mm ² .

  The chemical composition is Cu: 0.05 to 0.50%, Ni: 0.05 to 0.50%, Cr: 0.04 to 0.50%, Mo: 0.03 to 0.20%, and V: You may contain 1 or more types selected from the group which consists of 0.005-0.06%.

  The chemical composition contains at least one selected from the group consisting of B: 0.0002 to 0.002%, Ca: 0.001 to 0.005%, and Mg: 0.001 to 0.005% May be.

The manufacturing method of the steel material for tanks by this invention is equipped with a melting process, a slab manufacturing process, a hot rolling process, and a cooling process. In the melting process, before the RH vacuum degassing treatment, the oxygen potential in the molten steel is set to 10 to 60 ppm, and the chemical composition is adjusted in the RH vacuum degassing treatment to produce the molten steel having the above-described chemical composition. In the slab manufacturing process, slabs are manufactured by continuous casting using molten steel. The hot rolling process includes a heating process and a rolling process. In the heating step, the slab is heated to 1000 to 1250 ° C. In the rolling process, hot rolling is performed on the heated slab to produce a steel material. During hot rolling, the cumulative rolling reduction at a temperature of 900 ° C. or lower is set to 30% or more. The cooling process includes a first cooling process, a second cooling process, a third cooling process, and a fourth cooling process. In the first cooling step, the steel material is cooled at a first cooling rate of 1.1 to 5 ° C./second until the steel material temperature reaches T1 ° C. after the hot rolling is completed. In the second cooling step, the steel material is cooled at a second cooling rate of 5 to 15 ° C./second until the steel material temperature is changed from T1 ° C. to T2 ° C. In the third cooling step, the steel material is cooled at a third cooling rate of 15 ° C./second or more until the steel material temperature is changed from T2 ° C. to T3 ° C. In the fourth cooling step, when the steel material temperature reaches T3 ° C., the cooling at the third cooling rate is stopped and the steel material is allowed to cool.
_{Here, A r3 ≧ T1 ≧ A r3} -100, A r3 -50 ≧ T2 ≧ A r3 -200, is A r3 -200 ≧ T3 ≧ 350, T1-T2 ≧ 40, T2-T3 ≧ 100.

  Hereinafter, the steel for tanks according to the present invention will be described in detail. “%” Regarding an element means mass% unless otherwise specified.

[Chemical composition]
The chemical composition of the steel material for tanks of the present invention contains the following elements.

C: 0.03-0.10%
Carbon (C) increases the strength of the steel material. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, the low temperature toughness of the HAZ decreases. If the C content is too high, the strength of the steel material is further increased, and the stress corrosion cracking resistance of the steel material is lowered. Therefore, the C content is 0.03 to 0.10%. The minimum with preferable C content is 0.04%, More preferably, it is 0.05%. The upper limit with preferable C content is 0.07%.

Si: 0.05-0.5%
Silicon (Si) deoxidizes steel. Si further increases the strength of the steel material. If the Si content is too low, these effects cannot be obtained. On the other hand, if the Si content is too high, the HAZ in the steel material is excessively cured and the low temperature toughness is lowered. Therefore, the Si content is 0.05 to 0.5%. A preferable lower limit of the Si content is 0.10%. The upper limit with preferable Si content is 0.4%, More preferably, it is 0.3%.

Mn: 0.9 to 2.0%
Manganese (Mn) increases the hardenability of the steel and increases the strength and low temperature toughness of the steel. Mn further combines with S to form MnS and enhances the low temperature toughness of HAZ. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, temper embrittlement increases and weldability decreases. Therefore, the Mn content is 0.9 to 2.0%. The minimum with preferable Mn content is 1.2%, More preferably, it is 1.3%. The upper limit with preferable Mn content is 1.6%, More preferably, it is 1.5%.

P: 0.02% or less Phosphorus (P) is an impurity. P decreases the mechanical properties of the steel material, and in particular decreases the low temperature toughness of the steel material. Therefore, the P content is 0.02% or less. The upper limit with preferable P content is 0.015%. The P content is preferably as low as possible.

S: 0.0010 to 0.0100%
Sulfur (S) combines with Mn to form MnS. At this time, if a composite inclusion is formed, a Mn-deficient region is formed, and the low temperature toughness of the HAZ can be enhanced. If the S content is too low, this effect cannot be obtained. On the other hand, if the S content is too high, coarse MnS precipitates alone and the low temperature toughness of the HAZ decreases. Therefore, the S content is 0.0010 to 0.0100%. The minimum with preferable S content is 0.0020%. The upper limit with preferable S content is 0.0050%, More preferably, it is 0.0030%.

Nb: 0.005 to 0.05%
Niobium (Nb) increases the strength and low-temperature toughness of steel by forming carbides and refining crystal grains in the steel. If the Nb content is too low, this effect cannot be obtained. On the other hand, if the Nb content is too high, a large amount of MA (island martensite) is generated in the HAZ, and the low temperature toughness of the HAZ is lowered. If the Nb content is too high, the ferrite grains in the steel material are further refined. Therefore, the strength of the steel material becomes excessively high. Therefore, the Nb content is 0.005 to 0.05%. A preferable lower limit of the Nb content is 0.007%. The upper limit with preferable Nb content is 0.04%, More preferably, it is 0.03%.

Ti: 0.005-0.025%
Titanium (Ti) forms a Ti-based oxide, serves as a nucleus for formation of intragranular ferrite, and enhances low temperature toughness of HAZ. Ti further combines with N in the steel to form TiN, and suppresses the coarsening of the austenite crystal grains. If the Ti content is too low, these effects cannot be obtained. On the other hand, if the Ti content is too high, the number density of Ti-based oxides and coarse Ti-based oxides increase, and the low temperature toughness of HAZ decreases. If the Ti content is too high, TiC is further generated and the yield strength becomes too high. As a result, YR may become too high. Therefore, the Ti content is 0.005 to 0.025%. A preferable lower limit of the Ti content is 0.007%. The upper limit with preferable Ti content is 0.020%, More preferably, it is 0.018%.

sol. Al: 0.005% or less Aluminum (Al) deoxidizes steel. However, if the Al content is too high, the steel is excessively deoxidized and the amount of Ti-based oxide formed decreases. As a result, the low temperature toughness of the HAZ is reduced. Therefore, the Al content is 0.005% or less. The upper limit with preferable Al content is 0.004%. The Al content referred to herein is the content of acid-soluble Al (sol. Al).

N: 0.0010 to 0.0100%
Nitrogen (N) is inevitably contained. N combines with Ti to form TiN, and suppresses austenite grain coarsening. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, the low-temperature toughness of the steel material and HAZ will be reduced. Therefore, the N content is 0.0010 to 0.0100%. The minimum with preferable N content is 0.0020%. The upper limit with preferable N content is 0.0060%.

O: 0.0010 to 0.0050%
Oxygen (O) combines with Ti to produce a Ti-based oxide, serves as a nucleus of intragranular ferrite, and enhances the low temperature toughness of HAZ. If the O content is too low, this effect cannot be obtained. On the other hand, if the O content is too high, coarse oxide-based composite inclusions are formed, which becomes the starting point of destruction. As a result, the low temperature toughness of the steel material and HAZ may decrease. Therefore, the O content is 0.0010 to 0.0050%. A preferable lower limit of the O content is 0.0015%. The upper limit with preferable O content is 0.0030%.

  The balance of the chemical composition of the tank steel material of the present embodiment is composed of Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or the production environment as raw materials when industrially producing steel for tanks, and do not adversely affect the steel for tanks of this embodiment. Means what is allowed.

[Arbitrary elements]
The steel material for tanks according to the present invention may further contain one or more selected from the group consisting of Cu, Ni, Cr, Mo and V in place of part of Fe. All of these elements increase the strength of the steel material.

Cu: 0 to 0.50%
Copper (Cu) is an optional element and may not be contained. When contained, Cu increases the strength and corrosion resistance of the steel material. If Cu is contained even a little, the above effect can be obtained to some extent. However, if the Cu content is too high, hot cracking is likely to occur. Therefore, the Cu content is 0 to 0.50%. The minimum with preferable Cu content is 0.05%, More preferably, it is 0.08%. The upper limit with preferable Cu content is 0.40%, More preferably, it is 0.35%.

Ni: 0 to 0.50%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni is dissolved in steel to increase the strength and low temperature toughness of the steel material. If Ni is contained even a little, the above effect can be obtained to some extent. However, if the Ni content is too high, not only this effect is saturated, but also the manufacturing cost increases. Therefore, the Ni content is 0 to 0.50%. The minimum with preferable Ni content is 0.05%, More preferably, it is 0.08%. The upper limit with preferable Ni content is 0.45%, More preferably, it is 0.40%.

Cr: 0 to 0.50%
Chromium (Cr) is an optional element and may not be contained. When contained, Cr increases the strength of the steel material. If Cr is contained even a little, the above effect can be obtained to some extent. However, if the Cr content is too high, this effect is saturated, and the low temperature toughness of the HAZ is reduced. Therefore, the Cr content is 0 to 0.50%. The minimum with preferable Cr content is 0.04%, More preferably, it is 0.08%. The upper limit with preferable Cr content is 0.40%, More preferably, it is 0.35%.

Mo: 0 to 0.20%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo increases the strength of the steel material. If Mo is contained even a little, the above effect can be obtained to some extent. However, if the Mo content is too high, not only the strength of the steel material becomes too high, but also the low temperature toughness of the HAZ decreases. Therefore, the Mo content is 0 to 0.20%. The minimum with preferable Mo content is 0.03%, More preferably, it is 0.05%. The upper limit with preferable Mo content is 0.15%.

V: 0 to 0.06%
Vanadium (V) is an optional element and may not be contained. When contained, V forms carbonitride and precipitates and strengthens the steel material. If V is contained even a little, the above effect can be obtained to some extent. However, if the V content is too high, not only the effect is saturated, but also the production cost increases. Therefore, the V content is 0 to 0.06%. The minimum with preferable V content is 0.005%, More preferably, it is 0.010%. The upper limit with preferable V content is 0.05%, More preferably, it is 0.04%.

  The steel material for tanks according to the present invention may further contain one or more selected from the group consisting of B, Ca and Mg instead of a part of Fe. All of these elements increase the low temperature toughness of the HAZ.

B: 0 to 0.002%
Boron (B) is an optional element and may not be contained. When contained, B combines with N to form BN, reducing the amount of solute N harmful to the low temperature toughness of HAZ. As a result, the low temperature toughness of the HAZ is increased. B further suppresses the formation of grain boundary ferrite. If B is contained even a little, the above effect can be obtained to some extent. However, if the B content is too high, the low temperature toughness of the HAZ will decrease. Therefore, the B content is 0 to 0.002%. The minimum with preferable B content is 0.0002%, More preferably, it is 0.0005%. The upper limit with preferable B content is 0.0015%.

Ca: 0 to 0.005%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca combines with S in the steel to suppress the extension of MnS. As a result, the low temperature toughness of the HAZ is increased. If Ca is contained even a little, the above effect can be obtained to some extent. However, this effect is saturated if the Ca content is too high. Therefore, the Ca content is 0 to 0.005%. The minimum with preferable Ca content is 0.001%, More preferably, it is 0.0015%. The upper limit with preferable Ca content is 0.004%, More preferably, it is 0.003%.

Mg: 0 to 0.005%
Magnesium (Mg) is an optional element and may not be contained. When contained, Mg suppresses the growth of austenite grains in the HAZ and refines the structure. As a result, the low temperature toughness of the HAZ is increased. If Mg is contained even a little, the above effect can be obtained to some extent. However, this effect is saturated if the Mg content is too high. Therefore, the Mg content is 0 to 0.005%. The minimum with preferable Mg content is 0.001%. The upper limit with preferable Mg content is 0.004%, More preferably, it is 0.003%.

[Configuration of steel for tanks]
The steel for tanks having the above chemical composition has the following configuration. In addition, since the requirements such as the microstructure shown below show the average structure of the steel material, the thickness t / 4 position (hereinafter referred to as the t / 4 position, hereinafter referred to as the thickness) in the rolling direction from the surface of the steel material. t means the length (thickness) in the direction (rolling direction) perpendicular to the rolling direction of the steel material, where t / 4 position is t / 4 depth from the steel surface when the steel material thickness is t. This means the position.)

[Microstructure of steel]
The microstructure (matrix structure) of the steel material for tanks is composed of 50-80% ferrite in area ratio and a hard structure.

  The hard structure contains 10% or less martensite and pearlite in the total area ratio, and the balance is bainite. The martensite includes tempered martensite and island martensite.

[Ferrite area ratio AR _F ]
Ferrite area ratio AR _F is 50 to 80%. If the ferrite area ratio AR _F is too low, strength properties (yield strength YS, tensile strength TS, and yield ratio YR) any of the permissible intensity range (yield strength YS is required for the ammonia tank 355～440MPa, tensile strength TS exceeds the upper limit of 490 to 620 MPa and the yield ratio YR is 85% or less. Therefore, the stress corrosion cracking resistance is reduced.

On the other hand, if ferrite area ratio AR _F is too high, although the yield strength YS decreases, also decreases the tensile strength TS. Therefore, one of the strength characteristics is less than the lower limit of the allowable strength range, and sufficient strength cannot be obtained.

If ferrite area ratio AR _F 50 to 80%, the strength characteristics are within the allowable range of intensities. Therefore, sufficient strength can be obtained in the tank, and excellent stress corrosion cracking resistance can be obtained. A preferred lower limit of the area ratio of ferrite AR _F is 55%, more preferably 60%. A preferred upper limit of the area ratio of ferrite AR _F is 75%, more preferably 65%.

Microstructure observation and ferrite area ratio AR _F is measured in the following manner. The microstructure of the L section of the steel material (cross section parallel to the rolling direction and the rolling direction) is revealed by nital corrosion. Five visual fields are observed (photographed) at an arbitrary t / 4 position with 500 times optical microscope observation, and a microstructure image of each visual field is generated. The generated microstructure image is subjected to image processing (binarization processing) to identify a ferrite structure and a hard structure. After identification, the ferrite area ratio in each field of view is obtained. The average of the ferrite area ratio of each visual field is defined as the ferrite area ratio AR _F (%).

[Average equivalent circular diameter of ferrite _DF ]
The average equivalent circle diameter _{D F} of the ferrite crystal grains (ferrite grains) is 5.5～15.0Myuemu. The equivalent circle diameter means the diameter of a circle when the area of the ferrite grain is converted into a circle having the same area. If the average equivalent circle diameter _DF is less than 5.5 μm, since the ferrite grains are fine, the yield strength YS and the like become too high, and the stress corrosion cracking resistance decreases. On the other hand, if the average equivalent circle diameter _DF exceeds 15.0 μm, the ferrite grains are too coarse and the low temperature toughness of the steel material is lowered. Accordingly, the average equivalent circle diameter _DF is 5.5 to 15.0 μm. A preferable lower limit of the average equivalent circle diameter _DF is 6.0 μm. A preferable upper limit of the average equivalent circle diameter _DF is 10 μm.

The average equivalent circle diameter _DF of the ferrite grains is measured by the following method. In the microstructure image of each visual field described above, the ferrite grain boundary is specified by image processing. After identification, the average equivalent circle diameter (μm) of the ferrite grains in each field of view is obtained using general-purpose application software (manufactured by Nippon Steel & Sumikin Technology Co., Ltd., trade name: particle analysis). The average of the average equivalent circle diameters of the five fields of view obtained is defined as the average equivalent circle diameter D _F (μm) of ferrite.

[Identification of phases in hard structure]
The total area of martensite in the hard tissue is measured by the following method. That is, the L cross section of the steel material is mirror-polished and corroded by the repeller corrosive liquid. Thereafter, tissue observation is performed using an optical microscope, and photographs are taken and image analysis is performed. Martensite can be distinguished by contrast. In addition, since martensite will decompose | disassemble into ferrite and cementite when SR processing is carried out, the measurement uses the material with the rolling which is not SR processing.

  The total area of pearlite in the hard tissue is measured by the following method. That is, the L cross section of the steel material is mirror-polished and corroded by the nital corrosion liquid. Perlite can be distinguished from other phases by contrast. In addition, this measurement uses the raw material which is not SR-processed.

  After the phase is specified, the total area ratio of martensite and pearlite in the hard structure is determined in each visual field. The average of the total area ratio of martensite and pearlite in the hard structure in each visual field is defined as the total area ratio (%) of martensite and pearlite in the hard structure.

[Area ratio AR _B of band structure in hard structure]
In the tank steel material of this embodiment, the area ratio of the band structure in the hard structure is 50% or less.

As described above, the band structure is a hard structure in which the aspect ratio defined by the formula (1) is 5 or more among the hard structures in the L cross section. That is, the band structure is a hard structure that extends long in the rolling direction.
Aspect ratio = major axis length of hard structure elongated in rolling direction / minor axis length of hard structure elongated in rolling direction Formula (1)
Here, even when the aspect ratio is 5 or more, if the hard structure is not recognized to be elongated in the rolling direction, it is not included in the band structure. The “hard structure extending in the rolling direction” means a hard structure having an angle formed by the long axis of the hard structure and the rolling direction of 15 ° or less.

Area ratio AR _B band tissue is obtained by the following method. The microstructure of the L section of the steel material is revealed by nital corrosion. The optical microscope observation of 200 times is carried out (photographed) at the t / 4 position for five visual fields, and a microstructure image of each visual field is generated. In the generated microstructure image of each visual field, the ferrite and the hard structure are specified by binarization processing. After identification, the aspect ratio of each hard tissue is calculated using the application software. The area ratio of a hard structure (band structure) having an aspect ratio of 5 or more is obtained. The average of the area ratio of the band structure in each visual field is defined as the area ratio AR _B (%) of the band structure in the hard tissue having the L cross section.

If the area ratio AR _B band tissue is less than 50%, it increases the low temperature toughness of the steel. AR _B is further equal to or less than 50%, the hard tissue is finely dispersed. Therefore, work hardening characteristics are enhanced and the yield ratio YR is decreased. Preferred area ratio AR _B is 20% or less, more preferably 0%.

[Percentage of MnS in the cross-sectional area of composite inclusions]
The steel material for tanks of this embodiment contains the composite inclusion containing Ti type | system | group oxide and MnS arrange | positioned around Ti type | system | group oxide in steel material. If the ratio (area ratio) of MnS to the cross-sectional area of the composite inclusion is less than 10% in the cross section of the steel material (cross section perpendicular to the rolling direction), the MnS of the composite inclusion and the matrix (matrix) The initial Mn-deficient layer is not sufficiently formed at the interface. Therefore, the amount of intragranular ferrite produced when welding is reduced. On the other hand, if the ratio of MnS in the cross-sectional area of the composite inclusion is 90% or more, the composite inclusion is mainly MnS. In this case, the driving force by which Mn is absorbed by the atomic vacancies in the Ti-based oxide does not work, and the welded Mn-deficient layer is not formed, so that the amount of intragranular ferrite produced decreases. Therefore, the ratio of MnS to the cross-sectional area of the composite inclusion is 10% or more and less than 90%.

  The ratio of MnS to the cross-sectional area of the composite inclusion is measured by the following method. In the cross section of the steel material, 20 t / 4 positions are specified. Using an electron probe microanalyzer (EPMA), the area ratio (%) of MnS in the cross-sectional area of the composite inclusion is determined from the mapping image obtained by plane analysis of the composite inclusion. The average value of the obtained area ratio is defined as the ratio (MnS area ratio) (%) of MnS to the cross-sectional area of the composite inclusion.

[Percentage of MnS in the interface with the matrix of composite inclusions]
MnS in the composite inclusion is formed around the Ti-based oxide. That is, MnS is formed at the interface between the composite inclusion and the matrix. If the ratio of MnS occupying the interface of the composite inclusion is less than 10% in the cross section of the steel material, the initial Mn-deficient region formed at the interface between MnS and the matrix is small. Since the amount is not sufficient, good low temperature HAZ toughness cannot be obtained. Therefore, the ratio of MnS in the interface with the composite inclusion matrix is 10% or more. Since the intragranular ferrite is more easily generated as the proportion of MnS increases, the upper limit of the proportion of MnS is not defined, but is usually 80% or less.

  The ratio of MnS to the interface of the composite inclusion is measured by the following method. From the MnS mapping image used for the measurement of the MnS area ratio, the ratio of the peripheral length of the interface between MnS and the matrix to the peripheral length of the interface between the composite inclusions and the matrix in the cross section is obtained. The average value of the obtained ratios is defined as the ratio of MnS (MnS peripheral occupancy) (%) in the interface with the matrix of composite inclusions.

[Number density of composite inclusions]
In this specification, the number density of composite inclusions is the number per unit area of composite inclusions having a prescribed particle size. The particle size of the composite inclusion means the diameter of the circle when the area of the composite inclusion is converted into a circle having the same area, that is, the equivalent circle diameter. If the particle size of the composite inclusion is too small, the amount of Mn that the composite inclusion can absorb from the surrounding matrix decreases, and the amount of intragranular ferrite produced decreases. On the other hand, if the particle size of the composite inclusions is too large, coarse composite inclusions become the starting point of destruction. Therefore, in the present invention, the target composite inclusions have a particle size of 0.5 to 5.0 μm.

The number density of composite inclusions is related to the amount of Mn absorption. In order to suppress the growth of coarse ferrite from the γ grain boundary, it is necessary that at least one composite inclusion is contained in the γ grain. If the number density of composite inclusions having a particle size of 0.5 to 5.0 μm is less than 10 / mm ² , intragranular ferrite is not sufficiently generated. On the other hand, if the number density of the composite inclusions having a particle size of 0.5 to 5.0 μm exceeds 100 / mm ² , the composite inclusions are likely to be the starting point of destruction. Therefore, the number density of the composite inclusion having a particle size of 0.5 to 5.0 μm is 10 to 100 / mm ² .

The number density of composite inclusions having a particle size of 0.5 to 5.0 μm is measured by the following method. Of the cross-section of the steel material, four visual fields are specified at the t / 4 position (the visual field area is 1 mm ² ). Using an automatic inclusion analyzer combined with SEM-EDX, the shape of the composite inclusion is measured. Specifically, the number density of composite inclusions (pieces / mm ² ) is determined from the number of composite inclusions having a particle size of 0.5 to 5.0 μm included in the obtained SEM image. The average of the number density of the composite inclusions obtained in each field of view is determined and defined as the number density (number / mm ² ) of the composite inclusions having a particle diameter of 0.5 to 5.0 μm in the tank steel.

[Production method]
An example of the manufacturing method of the steel material for tanks of this embodiment is demonstrated. In the following description, the steel for tanks is a steel plate. In this example, the manufacturing method of the steel material for tanks includes a melting step, a slab manufacturing step, a hot rolling step, and a cooling step.

[Melting process]
In the melting process, Ar gas is blown into the molten steel from the upper part to the molten steel before RH (Ruhrstahl-Hausen) vacuum degassing treatment, and the slag on the molten steel surface reacts with the molten steel, thereby making the total Fe amount in the slag To adjust the oxygen potential Oxp in the molten steel to a range of 10 to 60 ppm.

  In the above process, the flow rate of Ar gas is, for example, 100 to 200 L / min, and the blowing time is, for example, 5 to 15 (min). Subsequently, RH vacuum degassing is performed, each element is added, and the components are adjusted.

[Slab manufacturing process]
The molten steel which has the above-mentioned chemical composition is manufactured by the said melting process. A slab is manufactured by a known continuous casting method using the manufactured molten steel.

[Hot rolling process]
Hot rolling is performed on the slab to produce the above-described steel for tanks. The details of the hot rolling process are as follows.

[Heating process]
First, the slab is heated in a heating furnace. The heating temperature is 1000 to 1250 ° C. When the heating temperature is less than 1000 ° C., since the austenite crystal grains are refined, the ferrite crystal grains are refined. In this case, the strength of the steel for the tank becomes too high. Therefore, it exceeds the upper limit of the allowable strength range. On the other hand, when the heating temperature exceeds 1250 ° C., the austenite crystal grains become coarse. In this case, the low temperature toughness of the tank steel material is lowered.

[Rolling process]
A slab is extracted from the heating furnace, and hot rolling is performed on the slab to produce a steel material (steel plate). At this time, the cumulative reduction ratio RR ₉₀₀ when the temperature of the slab during rolling is 900 ° C. or less is set to 30% or more. When the cumulative rolling reduction ratio RR ₉₀₀ at 900 ° C. or less is as small as less than 30%, the crystal grains are coarsened and the low temperature toughness is lowered. Therefore, the cumulative rolling reduction rate RR ₉₀₀ at 900 ° C. or lower is 30% or higher. A preferred cumulative rolling reduction ratio RR ₉₀₀ is 35% or more. Although the minimum temperature of the slab during rolling is not particularly defined, the temperature of the slab during rolling is preferably set to _{Ar 3} −10 (° C.) or higher in consideration of the water cooling process which is the next process.

[Cooling process]
A cooling process implements the next cooling process with respect to steel materials immediately after hot rolling.
First cooling step: The steel material is cooled at a first cooling rate of 1.1 to 5 ° C./second until the steel material temperature reaches T1 ° C. after the end of hot rolling.
Second cooling step: The steel material is cooled at a second cooling rate of 5 to 15 ° C./second until the steel material temperature is changed from T1 ° C. to T2 ° C.
Third cooling step: The steel material is cooled at a third cooling rate of 15 ° C./second or more until the steel material temperature becomes T2 ° C. to T3 ° C.
Fourth cooling step: When the steel material temperature reaches T3 ° C., the cooling at the third cooling rate is stopped and the steel material is allowed to cool.
_{Here, A r3 ≧ T1 ≧ A r3} -100, A r3 -50 ≧ T2 ≧ A r3 -200, is A r3 -200 ≧ T3 ≧ 350, T1-T2 ≧ 40, T2-T3 ≧ 100.

  In the present invention, cooling is performed at the first cooling rate immediately after hot rolling as described above, instead of allowing to cool for a predetermined period after completion of hot rolling. The first cooling rate is a rate corresponding to water cooling. By performing a 1st cooling process, the production | generation of a band structure is suppressed and a hard structure is finely dispersed in steel materials. Therefore, the band tissue area ratio is 50% or less.

  Subsequent to the first cooling step, a second cooling step is performed in which cooling is performed at a cooling rate equal to or higher than the cooling rate in the first cooling step. Thereby, the area ratio of the ferrite produced | generated by a 1st cooling process and a 2nd cooling process will be 50 to 80%, and the average equivalent circular diameter of a ferrite grain will be 5.5 to 15.0 micrometers.

  Subsequent to the second cooling step, a third cooling step of cooling at a cooling rate equal to or higher than the cooling rate in the second cooling step is performed. Thereby, the above-mentioned hard tissue is generated. Subsequent to the third cooling step, the fourth cooling step of allowing the steel material to cool is performed, whereby the area fraction of martensite in the hard structure can be suppressed.

  The details of the first to fourth cooling steps are as follows.

[First cooling step]
The steel material is cooled by setting the cooling rate CR1 between 1.1 to 5 ° C./sec after the finish rolling to the temperature T1 of the steel plate. Here, the temperature T1 is a temperature that satisfies A _r3 ≧ T1 ≧ A _r3 −100.

If the temperature T1 exceeds the _Ar3 point, sufficient ferrite cannot be obtained. On the other hand, if the temperature T1 is less than _{A r3} point -100 ° C., too much ferrite generation amount, the ferrite area ratio AR _F exceeds 80%. As a result, as a result, the strength characteristics of the steel for tanks may deteriorate.

If the cooling rate CR1 is less than 1.1 ° C. / sec, band structure is excessively formed, the band structure area ratio AR _B exceeds 50%. In this case, the steel material cannot obtain sufficient low temperature toughness. On the other hand, if the cooling rate CR1 exceeds 5 ° C./second, the average equivalent-circle diameter _{DF of the} ferrite becomes less than 5.5 μm. Therefore, the strength characteristics of the steel material for tanks exceed the allowable strength range, and sufficient stress corrosion cracking cannot be obtained.

  The cooling rate CR1 described above can be realized by adjusting the amount of water cooling. The cooling rate CR1 can be calculated based on, for example, the time from the end of finish rolling to the temperature T1.

[Second cooling step]
The steel material is cooled by setting the cooling rate CR2 between the temperature T1 and T2 of the steel sheet to 5 to 15 ° C./second. Here, the temperature T2 is a temperature that satisfies A _r3 −50 ≧ T2 ≧ A _r3 −200 and T1−T2 ≧ 40.

When the temperature T2 exceeds _Ar3 point −50 ° C., there is no difference from the temperature T1, and sufficient ferrite cannot be obtained. Therefore, the ferrite area ratio AR _F becomes less than 50%, further the average equivalent circle diameter _{D F} of the ferrite grains is less than 5.5 [mu] m. Therefore, the strength characteristics of the steel material for tanks exceed the allowable strength range, and sufficient stress corrosion cracking cannot be obtained. On the other hand, if the temperature T2 is less than _A r3 -200 ° C., too much ferrite generation amount, the ferrite area ratio AR _F exceeds 80%. As a result, the strength characteristics of the steel for tanks may deteriorate. Further, even when T1−T2 ≧ 40 is not satisfied, there is no difference between the temperature T1 and the temperature T2, and thus sufficient ferrite cannot be obtained. Therefore, the ferrite area ratio AR _F is less than 50%. Therefore, the strength characteristics of the steel material for tanks exceed the allowable strength range, and sufficient stress corrosion cracking cannot be obtained.

If it is less than the cooling rate CR2 is 5 ° C. / sec, too much ferrite generation amount, the ferrite area ratio AR _F exceeds 80%, even an average equivalent circle diameter _{D F} of the ferrite grains exceeds 15.0 .mu.m. As a result, the strength characteristics of the steel for tanks may deteriorate. On the other hand, the cooling rate CR2 is if it exceeds 15 ° C. / sec, can not be obtained a sufficient amount of ferrite, the ferrite area ratio AR _F is less than 50%. Therefore, the strength characteristics of the steel material for tanks exceed the allowable strength range, and sufficient stress corrosion cracking cannot be obtained. Furthermore, the low temperature toughness of the steel material may be reduced.

  The cooling rate at the cooling rate CR2 can be realized, for example, by adjusting the amount of water cooling. The calculation of the cooling rate CR2 is the same as that of the cooling rate CR1.

[Third cooling step]
The steel material is cooled by setting the cooling rate CR3 between the temperature T2 and T3 of the steel sheet to 15 ° C./second or more. Here, the temperature T3 is a temperature which satisfies the _A r3 -200 ≧ T3 ≧ 350 and T2-T3 ≧ 100.

When temperature T3 exceeds _Ar3 point -200 ° C, pearlite is excessively generated in the hard tissue. As a result, the total area ratio of martensite and pearlite in the hard tissue exceeds 10%. In this case, the strength characteristics of the steel material may deteriorate, and the low temperature toughness of the steel material may decrease. On the other hand, when the temperature T3 is less than 350 ° C., excessive martensite is generated in the hard tissue. As a result, the total area ratio of martensite and pearlite in the hard tissue exceeds 10%. In this case, the strength characteristics of the steel material may deteriorate, and the low temperature toughness of the steel material may decrease. Even when T2−T3 ≧ 100 is not satisfied, there is no difference between the temperature T2 and the temperature T3, and excessive pearlite is generated in the hard tissue. As a result, the total area ratio of martensite and pearlite in the hard tissue exceeds 10%. In this case, the strength characteristics of the steel material may deteriorate, and the low temperature toughness of the steel material may decrease.

  If the cooling rate CR3 is less than 15 ° C./second, sufficient strength of hard strength cannot be obtained. In this case, the yield strength YS or the tensile strength TS of the steel material may become excessively low.

[Fourth cooling step]
The steel sheet is cooled by cooling from the temperature T3 of the steel sheet to room temperature. That is, the cooling is stopped at the temperature T3. Thereby, even after SR treatment, the yield strength YS does not increase excessively and can be within the allowable strength range.

  When the temperature T3 is less than 350 ° C., that is, when the cooling is stopped at less than 350 ° C., excessive martensite is generated in the hard structure as described above.

  A slab having the chemical composition shown in Table 1 was manufactured using molten steel melted in a converter. Specifically, the oxygen potential in the molten steel before RH treatment was adjusted to the amount shown in Table 2, and then Ti and the like were added to adjust the components. Thereafter, a slab having a thickness of 250 mm was manufactured by a continuous casting method.

In the “A _r3 ” column in Table 1, the A _r3 point of each steel material number is described.

  Using a slab, hot rolling and cooling were performed under the conditions shown in Table 2 to produce a steel plate having a plate thickness of 15 to 36 mm.

Specifically, the slab of each test number was heated at the heating temperature (° C.) shown in Table 2. Hot rolling was performed on the heated slab using a continuous rolling mill. At this time, the cumulative reduction ratio RR ₉₀₀ in the range where the slab temperature during rolling is 900 ° C. or less was as shown in Table 2. Furthermore, finish rolling was performed with a continuous rolling mill to produce a steel plate.

  1st cooling-4th cooling was implemented with respect to the steel plate after finish rolling. Specifically, the steel plate was cooled at the cooling rate CR1 (° C./second) shown in Table 2 from the steel plate temperature at the completion of finish rolling to the steel plate temperature T1 (first cooling step). Next, between the steel plate temperature T1 and the steel plate temperature T2, the steel plate was cooled at a cooling rate CR2 (° C./second) shown in Table 2 (second cooling step). Next, between the steel plate temperature T2 and the steel plate temperature T3, the steel plate was cooled at a cooling rate CR3 (° C./second) shown in Table 2 (third cooling step). Finally, cooling was stopped at the steel plate temperature T3, and then allowed to cool (fourth cooling step). Through the above steps, a plurality of steel plates were manufactured using test numbers 1 to 31.

[SR processing]
For each test number, a steel plate not subjected to SR treatment (stress removal annealing treatment) and a steel plate subjected to SR treatment were prepared. The SR process was performed under the following conditions. Each test number steel plate was heated and held at 550 ° C. for 1 hour. After holding for 1 hour, it was gradually cooled. Hereinafter, the steel sheet that has not been subjected to the SR treatment is referred to as “rolled material”. A steel plate subjected to SR treatment is referred to as “SR treated material”.

[Welding process]
A welding process was performed for each test number. The welding process was performed under the following conditions. The steel material of 20 mm or more was adjusted to a plate thickness of 20 mm by machining for the rolled material of each test number, and the steel material of less than 20 mm was welded at the plate thickness. The groove condition was I groove. Two-electrode SAW (submerged arc welding) was performed under the conditions of a heat input of 35 kJ / cm on the back side and a heat input of 50 kJ / cm on the front side. As the welding material, trade names NF310 and Y-DM manufactured by Nippon Steel & Sumikin Welding Co., Ltd. were used. Hereinafter, the rolled material that has been subjected to the welding process is referred to as a “welded material”.

[Microstructure observation test]
The rolling observation material of each test number was subjected to the microstructure observation test by the above-described method, and the ferrite area ratio AR _F (%), the average equivalent circle diameter D _F (μm) of the ferrite grains, and the band structure The area ratio AR _B (%) was determined. Furthermore, bainite, martensite, and pearlite were specified for the hard structure by the above-described method. Furthermore, the total area ratio (%) of the specified martensite and pearlite was determined.

  In the hard structure column of Table 3 to be described later, “mainly bainite” means that the hard structure contains bainite and the balance is composed of martensite and / or pearlite having an area ratio of 10% or less. When numerical values (%) are given to bainite, martensite, and pearlite, the numerical values mean the area ratio of each phase.

[Composite inclusion observation test]
A composite inclusion observation test is performed on the as-rolled material of each test number by the above-described method, and the MnS area ratio (%), the MnS circumferential occupancy (%), and the particle size are 0.5 to 5. The number density (pieces / mm ² ) of 0 μm composite inclusions was determined.

[Tensile test]
A round bar tensile test piece having a parallel part length of 8.5 mm and a gauge distance of 42.5 mm was produced from each of the as-rolled material and SR-treated material of each test number. The length direction of the round bar tensile test piece was a direction (sheet width direction) perpendicular to the rolling direction. Using a round bar tensile test piece, a tensile test was performed at normal temperature and atmospheric pressure, yield strength YS (MPa), tensile strength TS (MPa), and yield ratio YR (= YS / TS × 100, unit is %). For both the as-rolled material and the SR-treated material, each strength characteristic (yield strength YS, tensile strength TS, and yield ratio YR) is within an allowable strength range (yield strength YS is 355-440 MPa, tensile strength TS is 490-620 MPa, When the yield ratio was 85% or less), it was evaluated that the stress corrosion cracking resistance was excellent.

[Steel Charpy Impact Test]
In each of the as-rolled material and SR-treated material of each test number, three V-notch test pieces defined in JIS Z 2242 (2005) were produced from a position 1 mm below the surface. Using a V-notch test piece, a Charpy impact test was performed to determine the absorbed energy (vE-60) at -60 ° C. For both the as-rolled material and the SR-treated material, it was evaluated that the low-temperature toughness of the steel material was excellent when the absorbed energy vE-60 of the steel material was 100 J or more.

[HAZ Charpy impact test]
Three V-notch test pieces defined in JIS Z 2242 (2005) were prepared for each welded material of each test number so that HAZ and molten metal were 50:50 from a position 1 mm below the surface. Using a V-notch test piece, a Charpy impact test was performed to determine the absorbed energy (vE-60) at -60 ° C. It was evaluated that the low temperature toughness of HAZ was excellent when the three absorbed energy vE-60 of HAZ was 27 J or more.

[Test results]
The test results are shown in Table 3.

With reference to Table 3, the chemical compositions of test numbers 1-9 were appropriate. Further, the manufacturing conditions (Oxp before Ti addition, heating temperature, cumulative rolling reduction rate RR ₉₀₀ , steel plate temperatures T1 to T3, and average cooling rates CR1 to CR3) were appropriate. Therefore, in the as-rolled materials and SR-treated materials of test numbers 1 to 9, the yield strength YS is 355 to 440 MPa, the tensile strength TS is 490 to 620 MPa, the yield ratio YR is 85% or less, and excellent stress resistance. It showed corrosion cracking property. Further, the absorbed energy vE-60 of the steel material was 100 J or more, and the steel material showed excellent low temperature toughness. Moreover, in the welding processing materials of the test numbers 1-9, all absorbed energy vE-60 of HAZ is 27J or more, and HAZ showed the outstanding low temperature toughness.

  On the other hand, in test number 10, the C content was too high. Therefore, the yield strength YS and the tensile strength TS were too high, and the stress corrosion cracking resistance was low. Furthermore, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

  In test number 11, the S content was too low. Therefore, the MnS area ratio and the MnS peripheral occupation ratio in the composite inclusions were too low. As a result, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

  In test number 12, the S content was too high. Therefore, the MnS area ratio in the composite inclusion was too high. As a result, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

In test number 13, the Nb content was too high. Therefore, the average equivalent circle diameter _{DF of} ferrite was too small, and the yield strength YS and the yield ratio YR were too high. As a result, the stress corrosion cracking resistance was low. Furthermore, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

  In test number 14, the Ti content was too low. Therefore, the number density of composite inclusions was too low. As a result, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

  In test number 15, the Ti content was too high. Therefore, the yield strength was too high and the stress corrosion cracking resistance was low. Furthermore, the number density of composite inclusions was too high. As a result, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

  In test number 16, sol. Al content was too high. Therefore, the number density of composite inclusions was too low. As a result, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

  In test number 17, the O content was too low. Therefore, the number density of composite inclusions was too low. As a result, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

  In test number 18, the O content was too high. Therefore, the number density of composite inclusions was too high. As a result, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

In test number 19, the Mo content was too high. Therefore, the average equivalent circle diameter _{DF of} ferrite was too small and the yield strength YS was too high. As a result, the stress corrosion cracking resistance was low. Furthermore, the absorbed energy vE-60 of HAZ was less than 27 J, and the low temperature toughness of HAZ was low.

In test number 20, the heating temperature was too low. Therefore, the average equivalent circle diameter _{DF of} ferrite was too small and the yield strength YS was too high. As a result, the stress corrosion cracking resistance was low.

In test number 21, the cumulative rolling reduction ratio RR ₉₀₀ was too low. Therefore, the average equivalent circle diameter _{DF of} ferrite was too large. As a result, the absorbed energy vE-60 of the steel material was less than 100 J, and the low temperature toughness of the steel material was low.

In test number 22, the steel plate temperature T1 was too low. Therefore, the ferrite area ratio AR _F is too high, the yield strength YS and tensile strength TS was too low.

In test number 23, the steel plate temperature T1 was too high. For this reason, ferrite transformation does not occur sufficiently, the ferrite area ratio AR _F was too low. Further, the average equivalent circle diameter _{DF of} the ferrite was too small. As a result, the yield strength YS and the tensile strength TS were too high, and the stress corrosion cracking resistance was low.

In test number 24, the steel plate temperature T2 was too low. Therefore, the ferrite area ratio AR _F is too high, the tensile strength YS and yield strength TS was too low.

In test number 25, the steel plate temperature T2 was too high. Therefore, the ferrite area ratio AR _F was too small. Further, the average equivalent circle diameter _{DF of} the ferrite was too small. As a result, the yield strength YS was too high and the stress corrosion cracking resistance was low.

  In test number 26, the steel plate temperature T3 was too low. Therefore, martensite was excessively formed, and the yield strength YS of the rolled material was too low. Furthermore, the absorbed energy vE-60 of the steel material as rolled was less than 100 J, and the low temperature toughness of the steel material was low.

  In test number 27, the steel plate temperature T3 was too high. Therefore, pearlite was excessively formed, the tensile strength TS was low, and the yield ratio YR was too high.

In test number 28, the cooling rate CR1 was too low. For this reason, the band organization area ratio AR _B was too high. As a result, the absorbed energy vE-60 of the steel material was less than 100 J, and the low temperature toughness of the steel material was low.

In test number 29, the cooling rate CR1 was too high. Therefore, the average equivalent circle diameter _{DF of} the ferrite was too small. As a result, the yield strength YS and the yield ratio YR were too high, and the stress corrosion cracking resistance was low.

In test number 30, the cooling rate CR2 was too high. Therefore, the ferrite can not be generated sufficiently, the ferrite area ratio AR _F is too low, the average circle equivalent diameter D _F of the ferrite was also too small. As a result, the yield strength YS was too high and the stress corrosion cracking resistance was low.

In test number 31, the cooling rate CR2 was too low. Therefore, the ferrite area ratio AR _F is too high, an average equivalent circle diameter D _F of the ferrite be too large. As a result, the tensile strength TS was too low. Furthermore, the absorbed energy vE-60 of the steel material was less than 100 J, and the low temperature toughness of the steel material was low.

The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims (4)

Tank steel,
The chemical composition is mass%,
C: 0.03-0.10%,
Si: 0.05 to 0.5%,
Mn: 0.9 to 2.0%,
P: 0.02% or less,
S: 0.0010 to 0.0100%,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.025%,
sol. Al: 0.005% or less,
N: 0.0010 to 0.0100%,
O: 0.0010 to 0.0050%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0 to 0.20%,
V: 0 to 0.06%,
B: 0 to 0.002%,
Ca: 0 to 0.005%, and
Mg: 0 to 0.005% is contained, the balance consists of Fe and impurities,
At a depth position of thickness t / 4 from the surface,
The structure consists of 50-80% ferrite in area ratio and hard structure,
The average equivalent circle diameter of the ferrite crystal grains is 5.5 to 15.0 μm,
The hard structure contains 10% or less martensite and pearlite in a total area ratio, and the balance is bainite.
Among the hard structures, a band structure having an aspect ratio of 5 or more defined by a long axis length of the hard structure extending in the rolling direction of the steel material for tanks / a short axis length of the hard structure extending in the rolling direction. The area ratio of the entire hard tissue is 50% or less,
In steel, including a composite inclusion containing Ti-based oxide and MnS disposed around the Ti-based oxide,
The ratio of the MnS to the cross-sectional area of the composite inclusion is 10% or more and less than 90%,
The ratio of the MnS in the interface with the matrix of the composite inclusion is 10% or more,
The steel material for tanks, wherein the number density of the composite inclusions having a particle size of 0.5 to 5.0 μm is 10 to 100 pieces / mm ² . The steel for tank according to claim 1,
The chemical composition is
Cu: 0.05 to 0.50%,
Ni: 0.05 to 0.50%,
Cr: 0.04 to 0.50%,
Steel for tanks containing at least one selected from the group consisting of Mo: 0.03-0.20% and V: 0.005-0.06%. The steel for tank according to claim 1 or 2,
The chemical composition is
B: 0.0002 to 0.002%,
Steel for tanks containing at least one selected from the group consisting of Ca: 0.001 to 0.005% and Mg: 0.001 to 0.005%. Before the RH vacuum degassing treatment, the oxygen potential in the molten steel is set to 10 to 60 ppm, and the chemical composition is adjusted in the RH vacuum degassing treatment to change the chemical composition according to any one of claims 1 to 3. Producing a molten steel having,
Producing a slab by continuous casting using the molten steel;
Heating the slab to 1000 to 1250 ° C .;
Hot rolling is performed on the heated slab to produce a steel material, and during the hot rolling, a step of setting a cumulative reduction rate at a temperature of 900 ° C. or lower to 30% or more,
From the end of the hot rolling until the steel material temperature reaches T1 ° C., the step of cooling the steel material at a first cooling rate of 1.1 to 5 ° C./second;
A step of cooling the steel material at a second cooling rate of 5 to 15 ° C./second until the steel material temperature is changed from T1 ° C. to T2 ° C .;
A step of cooling the steel material at a third cooling rate of 15 ° C./second or more until the steel material temperature is changed from T2 ° C. to T3 ° C .;
And a step of stopping cooling at the third cooling rate and allowing the steel material to cool when the steel material temperature reaches T3 ° C.
_{Here, A r3 ≧ T1 ≧ A r3} -100, A r3 -50 ≧ T2 ≧ A r3 -200, is A r3 -200 ≧ T3 ≧ 350, T1-T2 ≧ 40, T2-T3 ≧ 100.