JP6369003B2 - Steel material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material and a manufacturing method thereof.

  In order to ensure safety, a steel material having excellent fracture toughness is used for a storage tank of a low-temperature substance such as LNG. For example, 9% Ni steel.

  Conventionally, in 9% Ni steel, impurities such as P and S are reduced, C is reduced, and further, a three-stage heat treatment method, that is, “quenching (Q), two-phase quenching (L) and tempering (T)” Various improvements have been made, such as the use of heat treatment. On the other hand, addition of Mo has been studied as an alloy element effective for improving the strength and toughness of Ni-containing steel.

  In recent years, the price of steel materials has soared due to soaring alloy element prices, etc., and it has become necessary to develop a steel material having a performance equivalent to or better than 9% Ni steel (for example, toughness) and a reduced Ni content. Yes. The following is a conventional technique related to such a low Ni steel material.

  Patent Document 1 discloses a low-temperature steel containing 4.0 to 7.5% Ni and having an Ms point of 370 ° C. or lower. Patent Documents 2 and 3 disclose steels containing 1.5 to 9.5% Ni and 0.02 to 0.08% Mo. Patent Document 4 discloses a low-temperature steel containing 5.0 to 10.0 Ni and 0.0015 to 0.0040%.

Japanese Patent Laid-Open No. 6-136383 JP-A-9-302445 JP 2002-129280 A JP 2013-14811 A

  Patent Document 1 describes a steel material that does not contain N, and it is described that the reduction rate at 700 to 900 ° C. is 20 to 90%, but the reduction rate per pass is It is not specified.

  Patent Documents 2 and 3 describe steel materials that do not contain N, and a method for producing DQ-LT that stops water cooling at 400 ° C. or lower is disclosed, but the heating temperature and rolling conditions are unknown. is there.

  Patent Document 4 describes a wide range of chemical compositions such as Cr: 1.5% or less and Mo: 0.5% or less. It remains at 17%.

  In all structures, the collapse due to brittle fracture is the type of fracture that should be avoided because the entire structure collapses instantly and enormous damage is assumed. Therefore, although structures such as storage tanks are designed to avoid the occurrence of brittle fracture, they are brittle due to abnormal situations unexpected by the designer, such as generation of external force exceeding the design and defects caused by construction. It is necessary to consider the case where destruction occurs. When brittle fracture occurs, brittle fracture spreads throughout the structure due to extremely high-speed crack propagation, and the entire structure is destroyed. Accordingly, there is a demand for a characteristic that can stop the brittle crack propagation that has occurred even if a brittle crack occurs. This characteristic is generally called “arrest characteristic”. In order to know the arrest characteristics easily, it is better to measure the three-surface slit Charpy absorbed energy.

It is extremely important to develop a method for rationally producing low temperature steels with excellent properties. However, if a product defect occurs due to unique manufacturing conditions due to excessive pursuit of the characteristics, the cost reduction due to the Ni amount reduction is offset. Therefore, it is required to develop the best components and manufacturing methods that improve the yield rate and characteristics. In Patent Documents 1 to 4, there is no description regarding the yield at the time of manufacture.

  An object of this invention is to provide the steel material which has the performance equivalent to 9% Ni steel with Ni content smaller than 9% Ni steel, and its manufacturing method. More specifically, it has a high strength with a yield strength of 620 MPa or more, a tensile strength of 720 MPa or more, and a low temperature toughness similar to 9% Ni steel even in a cryogenic environment, and is a Ni-reducing type that suppresses the occurrence of flaws. An object of the present invention is to provide a cryogenic steel material (particularly, a thick steel plate) and a method for producing the steel material.

  If the strength can be increased to “yield strength of 620 MPa or more and tensile strength of 720 MPa or more”, it can contribute to thinning of the LNG tank steel sheet, and there is an advantage of lowering the production cost of the LNG tank. In addition, by making the base metal strength sufficiently higher than the weld metal strength, even if a stress greater than the design stress is applied to the tank and a crack occurs, the crack is not applied to the base metal but to the weld metal. There is an effect that can invade and detoxify.

  The extremely low temperature means use in a low temperature region of −60 ° C. or lower, particularly in a low temperature environment of −165 ° C. or lower. The thick steel plate means a steel plate having a thickness of 3 mm or more, particularly a steel plate having a thickness of 5 to 50 mm.

  The present inventor obtained the following findings (a) to (c) as a result of repeated studies to achieve the above object.

  (A) If Cr and Mo are contained, the hardenability increases and the strength can be increased, but the toughness is not impaired.

  (B) A scratch on the surface of the steel sheet brings about a decrease in yield and offsets cost rationalization due to Ni content reduction. If the amounts of Al and N are adjusted and the precipitation of AlN during heating is suppressed, the occurrence of surface scratches on the steel sheet can be suppressed and the yield can be reduced.

  (C) If the retained austenite in the steel sheet is contained in a volume fraction of 4.0% or more, high toughness can be obtained in a cryogenic environment. The volume fraction of retained austenite can be evaluated by X-ray diffraction.

  The present invention has been made on the basis of the above knowledge, and the gist thereof is the following steel plate for cryogenic temperature and method for producing a steel plate for cryogenic temperature.

(A) The chemical composition is mass%, C: 0.01 to 0.1%, Si: 0.10 to 0.6%, Mn: 0.3 to 2.0%, Ni: 6.03% Or more, less than 8%, Cr: 0.6-1.0%, Mo: 0.01-0.5%, sol. Al: 0.002-0.08%, N: 0.0005-0.005%, Cu: 0-2.0%, V: 0-0.08%, Nb: 0-0.08%, Ti : 0 to 0.03%, B: 0 to 0.0030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050% and REM: 0 to 0.0020%, balance: Fe and impurities Yes, it satisfies the following equation (1), yield strength of over 620 MPa, a tensile strength of more than 720 MPa, the absorbed energy _{sE -196} fracture surface at -196 ° C. in three orthographic slit Charpy impact test is more than 20 J, a steel material.
sol. Al × N ≦ 8.6 × 10 ⁻⁵ (1)
However, the element symbol in the formula (1) indicates the content (% by mass) of the element.

  (B) The said chemical composition is the mass%, Cu: 0.05-2.0%, V: 0.005-0.08%, Nb: 0.005-0.08%, Ti: 0.005 ~ 0.03%, B: 0.0002 ~ 0.0030%, Ca: 0.0002 ~ 0.0050%, Mg: 0.0002 ~ 0.0050% and REM: 0.0002 ~ 0.0020% The steel material of the above (A) containing one or more selected.

  (C) The steel material according to (A) or (B) above, wherein the metal structure contains 4.0% or more of retained austenite in volume%.

(D) The steel slab having the chemical composition (A) or (B) is heated to a temperature of 850 ° C. or higher and lower than 1140 ° C., and the cumulative reduction is 5% or more per pass at a temperature of 700 ° C. or higher and 830 ° C. or lower. A steel material that is rolled at a rate of 25% or more (however, the rolling finishing temperature is 700 ° C. or higher and 800 ° C. or lower), immediately accelerated to 200 ° C. or lower, and then tempered at a temperature of 650 ° C. or lower. a method, average cooling rate from the cooling starting temperature to 600 ° C. in the accelerated cooling is not less 10 ° C. / s or higher state, and are the average cooling rate is 5 ° C. / s or more from the cooling starting temperature to 200 ° C., yield strength at least 620 MPa, a tensile strength of at least 720 MPa, three-sided slit Charpy absorbed energy of fracture at -196 ° C. in impact test _{sE -196} is more than 20 J, the method of manufacturing the steel .

(E) After the accelerated cooling, heat treatment is performed at 600 ° C. or more and 800 ° C. or less, and a two-phase region heat treatment is performed so that the average cooling rate up to 200 ° C. or less is 5 ° C./s or more. The manufacturing method of the steel material of the said (D).

  According to the present invention, a steel material having mechanical properties equivalent to or better than steel containing 9% Ni can be obtained even with a low Ni content of more than 5% and less than 8%. Since this steel material is inexpensive but has excellent low-temperature toughness, it is suitable as a structural material for storage tanks for low-temperature substances such as LNG.

  The reason why the chemical composition, metal structure and production conditions of the steel material are defined as described above in the present invention will be described in detail below. In addition, "%" about the component content of steel materials is "mass%".

C: 0.01 to 0.1%
C is an element effective for ensuring strength, and is contained in an amount of 0.01% or more. C has the effect of lowering the Mf point and increasing the amount of austenite to improve toughness, but hardens the martensitic substrate itself. Therefore, when the content is excessive, the toughness is deteriorated. Therefore, the C content is set to 0.01 to 0.1%. A preferred lower limit is 0.03% and a preferred upper limit is 0.07%.

Si: 0.005 to 0.6%
Si is effective as a deoxidizing element, and is an element that suppresses precipitation of cementite and stabilizes austenite during tempering, so it is contained in an amount of 0.005% or more. However, when the Si content is excessive, the toughness is deteriorated. Therefore, the content is made 0.005 to 0.6%. A preferred lower limit is 0.03%, and a more preferred value is 0.1%. Moreover, a preferable upper limit is 0.5% and more preferable is 0.3%.

Mn: 0.3 to 2.0%
Mn is effective in stabilizing the austenite by lowering the Mf point, and a larger amount of austenite can be obtained as the content increases, so 0.3% or more is contained. However, when the Mn content is excessive, the toughness of the martensite substrate is deteriorated. Therefore, the content is set to 0.3 to 2.0%. A preferable lower limit is 0.5%, and a more preferable value is 0.7%. Moreover, a preferable upper limit is 1.5% and a more preferable value is 1.0%.

Ni: more than 5% and less than 8% Ni increases the strength of the steel and stabilizes austenite. Therefore, Ni is contained more than 5%. As the Ni content increases, the strength increases and the Mf point decreases and the retained austenite amount increases. However, containing a large amount of Ni causes an increase in cost. Therefore, the Ni content is more than 5% and less than 8%. A preferable upper limit is 7.5%, and a more preferable lower limit is 5.5%.

sol. Al (acid-soluble Al): 0.002 to 0.08%
Al stabilizes austenite in tempering as a deoxidizing element as well as Si and suppresses precipitation of cementite. Furthermore, Al combines with N to become AlN, which also has the effect of contributing to the refinement of austenite grains during heating. Therefore, sol. The Al content needs to be 0.005% or more. However, too much Al content causes toughness deterioration. Therefore, the content is sol. Al is set to 0.005 to 0.05%. A more desirable content range is 0.02% to 0.04%.

N: 0.0005 to 0.005%
N is an element that contributes to the stabilization of austenite, and combines with Al to become AlN, which is effective for refining austenite grains during heating. In order to obtain these effects, a content of 0.0005% or more is necessary. However, excess N degrades the martensite substrate. Therefore, the content is made 0.0005 to 0.005%. A preferred lower limit is 0.002%, and a preferred upper limit is 0.004%.

Note that sol. Even if Al and N are within the ranges defined above, if the balance is not appropriate, wrinkles are generated on the surface of the steel sheet, and the yield is reduced. For this reason, “(sol.Al × N) needs to be controlled to 25 × 10 ⁻⁵ or less. The presence or absence of surface flaws can be determined by visual inspection or the like of the rolled steel sheet.

Mo: 0.01 to 0.5%
Mo is effective in increasing the amount of austenite as an austenite stabilizing element in a low temperature range. In order to obtain this effect, a content of 0.01% or more is desirable. However, if the Mo content exceeds 0.5%, the toughness decreases through deterioration of the martensite substrate. Therefore, the content is set to 0.01 to 0.5%. A desirable lower limit is 0.02%. A desirable upper limit is 0.45%, and a more desirable upper limit is 0.40%.

Cr: 0.1 to 1.0%
Cr is an element effective for increasing the strength, and is contained by 0.1% or more. However, if the content exceeds 1.0%, the toughness deteriorates. Therefore, the Cr content is 0.1 to 1.0%.

Cu: 0 to 2.0%
Since Cu is an element that stabilizes austenite in a solid solution state, Cu may be contained. However, excessive Cu precipitates as ε-Cu by tempering treatment, so that it is effective for increasing the strength but deteriorates toughness. Therefore, when it contains Cu, the content shall be 2.0% or less. A preferable lower limit of the Cu content is 0.05%.

V: 0 to 0.08%
V is an element effective for increasing the strength of steel, and becomes a precipitate by tempering treatment and strengthens the steel. Therefore, V may be contained. However, excessive V degrades toughness due to excessive precipitates. Therefore, when V is contained, the content is set to 0.08% or less. The minimum with preferable V content is 0.005%.

Nb: 0 to 0.08%
Nb may be contained because it expands the non-recrystallization temperature range in rolling and is effective for refining the structure and increasing the toughness after rolling. However, when the content is excessive, toughness deteriorates. Therefore, when Nb is contained, the content is set to 0.08% or less. A preferable lower limit of the Nb content is 0.005%.

Ti: 0 to 0.03%
Ti is an element effective for preventing cracks in the slab, so it may be contained. However, when the Ti content is excessive, the toughness deteriorates. Therefore, when Ti is contained, the content is set to 0.03% or less. A preferable lower limit of the Ti content is 0.005%.

B: 0 to 0.0030%
Since B is an element effective for increasing the strength, it may be contained. However, when the B content is excessive, the toughness deteriorates. Therefore, when it contains B, the content shall be 0.0030% or less. A preferable lower limit of the B content is 0.0002%.

Ca: 0 to 0.0050%
Since Ca is an element effective for improving toughness, it may be contained. However, when the Ca content is excessive, the toughness deteriorates. Therefore, when it contains Ca, the content shall be 0.0050% or less. A preferable lower limit of the Ca content is 0.0002%.

Mg: 0 to 0.0050%
Since Mg is an element effective for improving toughness, it may be contained. However, when the Mg content is excessive, the toughness deteriorates. Therefore, when it contains Mg, the content shall be 0.0050% or less. A preferable lower limit of the Mg content is 0.0002%.

REM: 0 to 0.0020%
REM (rare earth element) may be contained because it has the effect of refining the structure of the weld heat affected zone and fixing S. However, when REM is included, inclusions are formed, and when the inclusions are excessive, the cleanliness is lowered. However, since this inclusion has a relatively small influence on toughness deterioration, the content of REM is 0. Allowed up to 0.002%. A preferable upper limit is 0.001%. The above effect becomes remarkable when the content is 0.0002% or more. In particular, it is more preferable to contain REM 0.0003% or more. Note that REM is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM means the total amount of the above elements.

  The steel material of the present invention is composed of Fe and impurities in the balance in addition to the above components. An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially.

Residual austenite amount:
Residual γ in the thick steel plate is effective in suppressing brittle crack initiation and stopping crack propagation, and has the effect of improving toughness in a low-temperature environment. This effect becomes remarkable when the residual γ content in the steel is 4.0% by volume or more. On the other hand, if the residual γ content exceeds 25.0% by volume, the yield stress may be reduced. A preferred lower limit is 5.5% by volume and a preferred upper limit is 20.0% by volume.

  Next, the manufacturing method of said steel material is described.

(1) Heating of steel slab In order to improve the toughness of the steel material, it is important to refine the initial austenite grains, that is, the austenite grains in the steel slab before rolling. This also contributes to an increase in Therefore, the heating temperature of the steel slab before rolling is set to 850 ° C. or higher and lower than 1140 ° C. Heating at a temperature lower than 850 ° C. results in insufficient strength, and heating at a temperature of 1140 ° C. or higher deteriorates toughness. A preferred lower limit is 900 ° C and a preferred upper limit is 1000 ° C.

(2) Rolling In order to refine the structure and increase the amount of austenite, sufficient rolling must be performed in the non-recrystallized region of austenite. For this purpose, it is effective to perform rolling at a temperature range of 700 ° C. or higher and 830 ° C. or lower and a pass of 5% or more and a cumulative reduction ratio of 25% or more. This is because such rolling can introduce lattice defects (dislocations) into austenite in the non-recrystallized austenite temperature range and suppress transformation of untransformed austenite to martensite. At this time, it is necessary to finish rolling at a temperature of 700 to 800 ° C. When the rolling finishing temperature is lower than 700 ° C., the anisotropy of the steel material becomes remarkable. In addition, when the rolling finishing temperature exceeds 800 ° C., the toughness deteriorates.

(3) Cooling Immediately after rolling, it is necessary to perform accelerated cooling to a temperature range of 200 ° C. or lower. At this time, the average cooling rate from the cooling start temperature to 600 ° C. needs to be 10 ° C./s or more. This is to leave as many lattice defects (dislocations) as possible introduced in finish rolling. On the other hand, in order to obtain a martensite structure, the average cooling rate from the cooling start temperature to 200 ° C. needs to be 5 ° C./s or more. When accelerated cooling is stopped at a temperature higher than 200 ° C., sufficient martensite cannot be obtained and the strength deteriorates. The time from the rolling finish to the start of water cooling should be short, and it is desirable that the time from the end of rolling to the start of water cooling be within 30 seconds.

(4) Tempering After accelerated cooling, it is necessary to temper at a temperature of 650 ° C. or lower. Thereby, the martensite produced | generated by cooling process, ie, hardening, can be tempered, and while adjusting a intensity | strength, toughness can be improved. When tempering is performed at a temperature exceeding 650 ° C., the strength decreases.

(5) Two-phase region heating In order to further increase the amount of retained austenite, it is desirable to heat the ferrite and austenite two-phase regions before tempering. Therefore, during the accelerated cooling and tempering, it is preferable to perform a two-phase region heat treatment that is heated to 600 ° C. or higher and 800 ° C. or lower and cooled so that the average cooling rate up to 200 ° C. or lower is 5 ° C./s or higher. . The lower limit of the heating temperature of the two-phase region heat treatment is preferably 680 ° C., and the upper limit is preferably 750 ° C.

  Test materials having the chemical compositions shown in Tables 1 and 2 were melted, and a steel plate having a thickness of 20 mm was made as a trial product. The production conditions are shown in Table 3. Test pieces were collected from the steel plates and subjected to the following various measurements. The results are also shown in Table 3.

<Residual γ amount>
A test piece was taken from a position of 1/4 t (t: plate thickness) of the thick steel plate, and measured by the X-ray diffraction method. In any of the thick steel plates, since the main metal structure is composed of a martensite structure, the difference in lattice structure between residual γ having a face-centered cubic structure and martensite having a body-centered cubic structure is used. The amount of residual γ was measured from the integrated intensity ratio of the line peak.

<Tensile test>
A No. 4 tensile test piece defined in JISZ2241 was taken from the position of 1/4 t (t: plate thickness) of the thick steel plate and the rolling direction, and a tensile test at normal temperature was performed. In JIS G3127 (nickel steel plate for low-temperature pressure vessel), SL7N590 is defined as YS ≧ 590 MPa and 690 ≦ TS ≦ 830 MPa, but in the present invention, the yield strength (YS) at room temperature is 620 MPa or more, TS) targets 720 MPa or more.

<Three-surface slit Charpy impact test>
A three-surface slit Charpy impact test specimen was taken from the position of 1/4 t (t: thickness) of the thick steel plate from the width direction, and the absorbed energy sE- ₁₉₆ (J) of the fracture surface at -196 ° C (average value of the three pieces) ). The three-surface slit Charpy absorbed energy sE- ₁₉₆ targets 20J or more.

As shown in Table 3, the test No. 1 satisfying all the conditions defined in the present invention. 1-41 is the yield strength at room temperature is more than 620 MPa, the tensile strength TS are both not less than 720 MPa, three-sided notch Charpy absorbed energy _{sE -196} represents more than a high value 20 J, it was excellent in resistance to fracture safety.

  In contrast, test no. Nos. 44 to 51 were out of the range defined by the present invention, so that the yield strength at room temperature, the three-surface slit Charpy absorbed energy deteriorated, or surface defects were observed. Test No. No. 52 has a chemical composition within the range defined by the present invention, but Al × N was out of the range, so surface flaws occurred.

  According to the present invention, a steel material having mechanical properties equivalent to or better than steel containing 9% Ni can be obtained even with a low Ni content of more than 5% and less than 8%. Since this steel material is inexpensive but has excellent low-temperature toughness, it is suitable as a structural material for storage tanks for low-temperature substances such as LNG.

Claims (5)

Chemical composition is mass%,
C: 0.01 to 0.1%
Si: 0.10 to 0.6%,
Mn: 0.3 to 2.0%,
Ni: 6.03% or more and less than 8%,
Cr: 0.6 to 1.0%,
Mo: 0.01 to 0.5%,
sol. Al: 0.002 to 0.08%,
N: 0.0005 to 0.005%,
Cu: 0 to 2.0%,
V: 0 to 0.08%,
Nb: 0 to 0.08%,
Ti: 0 to 0.03%,
B: 0 to 0.0030%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050% and REM: 0-0.0020%,
Balance: Fe and impurities,
Satisfies the following equation (1), yield strength of over 620 MPa, a tensile strength of more than 720 MPa, the absorbed energy _{sE -196} fracture surface at -196 ° C. in three orthographic slit Charpy impact test is more than 20 J, a steel material.
sol. Al × N ≦ 8.6 × 10 ⁻⁵ (1)
However, the element symbol in the formula (1) indicates the content (% by mass) of the element. The chemical composition is mass%,
Cu: 0.05-2.0%,
V: 0.005 to 0.08%,
Nb: 0.005 to 0.08%,
Ti: 0.005 to 0.03%,
B: 0.0002 to 0.0030%,
Ca: 0.0002 to 0.0050%,
The steel material according to claim 1, containing one or more selected from Mg: 0.0002 to 0.0050% and REM: 0.0002 to 0.0020%.   The steel material according to claim 1 or 2, wherein the metal structure contains 4.0% or more of retained austenite by volume%. The steel slab having the chemical composition according to claim 1 or 2 is heated to a temperature of 850 ° C. or higher and lower than 1140 ° C., and at a temperature of 700 ° C. or higher and 830 ° C. or lower, 5% or more per pass and a cumulative reduction ratio of 25% or more. (However, the rolling finishing temperature is 700 ° C. or higher and 800 ° C. or lower), immediately accelerated cooling to a temperature of 200 ° C. or lower, and then tempered at a temperature of 650 ° C. or lower. ,
The acceleration average cooling rate from the cooling starting temperature to 600 ° C. in the cooling is at 10 ° C. / s or higher, the average cooling rate from the cooling starting temperature to 200 ° C. is Ri der 5 ° C. / s or higher, yield strength of more than 620MPa a tensile strength of more than 720 MPa, the absorbed energy _{sE -196} fracture surface at -196 ° C. in three orthographic slit Charpy impact test is more than 20 J, the production method of the steel.   The tempering is performed after performing the two-phase region heat treatment for heating to 600 ° C. or more and 800 ° C. or less after the accelerated cooling and cooling so that an average cooling rate up to 200 ° C. or less is 5 ° C./s or more 4. A method for producing a steel material according to 4.