WO2007034576A1 - Steel product usable at low temperature and method for production thereof - Google Patents

Abstract

Provided are a steel product having a Ni content less than that of the 9 % Ni steel and exhibiting equivalent performance as that of the 9 % Ni steel; and a method for producing the steel product. A steel product usable at a low temperature, characterized in that it has a chemical composition, in mass %, of C: 0.01 to 0.1 %, Si: 0.005 to 0.6 %, Mn: 0.3 to 2 %, Ni: more than 6 % and less than 8 %, sol. Al: 0.005 to 0.05 %, N: 0.0005 to 0.005 % and the balance: Fe and impurities, with the proviso that the following formula (1): 20C + 2.4Mn + Ni ≥ 10 ---- (1) is satisfied, it contains 1.7 or greater area % of austenite, the austenite has an average aspect ratio of 3.5 or less and an average diameter of the grains equivalent to circles of 1.0 μm or less, and/or the cementite contained therein has an average aspect ratio of 5.0 or less and an average diameter of the grains equivalent to the circles of 0.6 μm or less. This steel product may further contain one or more of Mo, Cu, Cr, V, Nb, Ti, B, Ca and Mg.

Description

 Specification

 Low temperature steel and its manufacturing method

 Technical field

 TECHNICAL FIELD [0001] The present invention relates to a low temperature Ni-containing steel material, in particular, a Ni-containing steel material suitable as a structural material for a low temperature storage tank such as LNG, and a method for producing the same.

 Background art

 [0002] Steels for low temperature for producing storage tanks for low temperature substances such as LNG are required to have excellent fracture toughness in terms of safety. A typical example of steel that meets this requirement is 9% Ni steel.

 [0003] Conventionally, reduction of impurities such as P and S, reduction of C, and further three-step heat treatment methods, namely, "Quenching (Q), two-phase region quenching (L) and tempering (T)" Various improvements have been made to 9% Ni steel. On the other hand, Mo additive has been studied as an alloying element effective for improving the strength and toughness of Ni-containing steels. The above-mentioned QLT and Mo-added cement is to increase the amount of retained austenite, which is the basis for improving toughness. The situation of such prior art is summarized as follows based on patent literature.

 [0004] Patent Document 1 includes a plate manufactured by a three-stage heat treatment method (QLT) or a direct quenching / two-phase quenching method (DQ—LT) method with Mo: 0.04-0.5% added. 9Ni steel with thickness Omm or more is disclosed.

 [0005] Patent Document 2 discloses a method for producing 9Ni steel having a plate thickness of 40 mm or more by quenching and tempering (QT) or direct quenching and tempering (DQ-T).

 [0006] In recent years, the price of steel materials has risen sharply due to soaring alloy element prices. In 9% Ni steel, where a large amount of expensive alloying elements such as Ni must be added, the price increase of alloying elements leads to a further increase in the price of steel materials. Therefore, in order to control the price of steel, it has become necessary to develop a steel material that has performance equivalent to or better than 9% Ni steel, for example, excellent toughness, with a low Ni content and low Ni content. Conventional technologies related to such low-Ni steels for low temperature include the following.

[0007] Patent Document 3 contains 4.0 to 7.5% of Ni and has a Ms point of 370 ° C or lower. Is disclosed. In Patent Document 2 mentioned above, steel containing 7.5 to 10% Ni and D

The manufacturing method by Q-LT is shown. Further, Patent Document 4 discloses a steel containing 5.5 to 10% Ni and a continuous forging method thereof.

[0008] Patent Documents 5 and 6 disclose steels containing 1.5 to 9.5% Ni and 0.02 to 0.08% Mo!

 Patent Document 1: Japanese Patent Laid-Open No. 4 371520

 Patent Document 2: JP-A-6-184630

 Patent Document 3: Japanese Patent Laid-Open No. 6-136483

 Patent Document 4: JP-A-7-90504

 Patent Document 5: JP-A-9 302445

 Patent Document 6: Japanese Patent Laid-Open No. 2002-129280

[0009] However, Patent Document 1 does not describe detailed conditions for rolling, and the Ni content is not described.

With steel exceeding 6% and less than 8%, the performance equivalent to the steel of the present invention described later can be obtained! /,Absent.

 [0010] In Patent Document 2, a steel containing 7.52% Ni is exemplified as a comparative example. However, since the chemical composition and manufacturing method of the steel are not appropriate, the amount of retained austenite is 1. It is insufficient at 5%, and the same performance as 9% Ni steel cannot be obtained. Therefore, the steel is treated as a comparative example.

 [0011] Patent Document 3 discloses a method for improving the toughness of the weld heat affected zone (HAZ).

The design and manufacturing method of chemical components to obtain the base metal characteristics similar to that of% M steel are not disclosed, and the base metal characteristics themselves are also disclosed.

[0012] In the above-mentioned Patent Document 2, it is described that the rolling reduction at 700 to 900 ° C is 20 to 90%, but the steel manufactured by the manufacturing method has no description about the rolling reduction per pass. Is

The toughness at 196 ° C reached 250J!

[0013] Patent Document 4 describes a component system capable of continuous forging, but does not disclose a method for producing a base material or characteristics of the base material. In addition, the minimum value of the Ni content specifically shown is 9.08%, and no means for obtaining a base material performance equivalent to 9% Ni steel with low Ni is disclosed. Patent Document 5 and Patent Document 6 disclose a method for producing DQ-LT in which water cooling is stopped at 400 ° C. or lower. However, the heating temperature and rolling conditions are not disclosed. In addition, the properties of about 7% Ni are not disclosed, and as shown in the examples, the base material toughness is 9% Ni steel and vTs is less than 196 ° C. The 0% Ni steel has a vTs of -160 ° C, and the 1.5Ni steel has a vTs of -125 ° C, which is directly linked to toughness deterioration.

 [0015] As described above, each of the above documents specifically discloses a steel having a performance equivalent to that of 9% Ni steel while being lower Ni than 9% Ni steel, and a manufacturing method thereof. It has not been.

 Disclosure of the invention

 Problems to be solved by the invention

 [0016] An object of the present invention is to provide a steel material having a performance equivalent to that of 9% Ni steel with a Ni content less than that of 9% Ni steel, and a method for producing the same.

 [0017] In order to achieve the above-mentioned object, the present inventor has examined the above-described prior art in detail. As a result, it was found that the conventional technology was insufficient to refine the structure and ensure the amount of retained austenite. In other words, there is a need for a means to refine the base metal structure itself and to stabilize austenite even with a Ni content lower than 9% Ni steel.

 [0018] As a means for stabilizing austenite with a Ni content smaller than that of 9% Ni steel, it has been found that there are the following means in addition to the trace amount of Mo added, which is also known in the past.

 [0019] The first means is a method of lowering the Mf point at which the martensitic transformation ends by introducing lattice defects into untransformed austenite. The transformation from austenite to martensite is a shear type transformation due to the movement of dislocations, and lattice defects present in austenite hinder the movement of dislocations, delaying the end of the shear type transformation from austenite to martensite, and martensite. Move the Mf point at the end of the site to the low temperature side. By shifting the Mf point to the low temperature side, the amount of austenite remaining at room temperature can be increased.

[0020] The second means is refinement of the untransformed austenite phase. When untransformed austenite is transformed into martensite, the minimum unit (lass) of martensite that is instantaneously generated by shear transformation is about 0.5 to 1 / ζ πι in the thickness direction. This transformation is actually accompanied by volume expansion. Therefore, the size of the untransformed austenite phase is It was found that martensitic transformation due to expansion is remarkably suppressed and austenite stabilizes more significantly than the actual chemical component strength can be obtained if it is equal to or smaller than the minimum unit.

 [0021] The low austenite retained austenite thus obtained has the following characteristics in its amount, which is only comparable to the amount of retained austenite obtained by quenching and tempering the conventional 9% Ni steel. is there. That is, the retained austenite obtained with the conventional 9% Ni steel quenching and tempering material shows a needle-like shape in two-dimensional observation and a thin plate shape in three dimensions, and has a large aspect ratio. However, the austenite obtained from low-Ni steel is characterized by a very fine granular shape in two-dimensional observation, even though the total amount is almost equivalent to 9% Ni steel. Therefore, even with low Ni, retained austenite exists stably for the reasons described above.

 [0022] Conditions for heating, rolling and cooling are important in order to introduce lattice defects (dislocations) into untransformed austenite and to refine the untransformed austenite phase. It is known that when a strong pressure is applied at a low temperature, a lot of lattice defects (dislocations) are introduced and the subsequent structure becomes fine. In addition, for the above-mentioned miniaturization, Nb addition is particularly effective as a trace element. This is because the fine precipitation of Nb (C, N) hinders the movement of dislocations and increases the density of lattice defects (dislocations) in austenite.

 [0023] The present invention made on the basis of the above findings is characterized by the following steel materials and production methods thereof.

In [0024] (1) Weight _{^{0/0, C:. 0.}} 01~0 1%, Si:. 0. 005~0 6%, Mn: 0. 3~2%, Ni: more than 6% Less than 8%, sol.Al: 0.005 to 0.05%, N: 0.0005 to 0.005%, the balance is Fe and impurities, and the steel satisfies the following formula (1). The austenite has an area ratio of 1.7% or more, and the austenite has an average aspect ratio of 3.5 or less and an average equivalent-equivalent grain size of 1.0 m or less. Low temperature steel.

 20C + 2.4Μη + Νί≥10 · '· · (1)

 However, the element symbol in the formula (1) indicates the content (mass%) of the element.

[2] (2) By mass%, C: 0.01 to 0.1%, Si: 0.005 to 0.6%, Mn: 0.3 to 2%, Ni: 6 % And less than 8%, sol.Al: 0.005 to 0.05%, N: 0.0005 to 0.005%, the balance being Fe and impurity power, which satisfies the following formula (1), and has an area ratio The low-temperature steel is characterized in that it contains 1.7% or more of austenite, the cementite contained therein has an average aspect ratio of 5.0 or less and an average equivalent circle diameter of 0.6 m or less.

 20C + 2.4Μη + Νί≥10 (1)

 However, the element symbol in the formula (1) indicates the content (mass%) of the element.

[0026] (3) By mass%, C: 0.01 to 0.1%, Si: 0.005 to 0.6%, Mn: 0.3 to 2%, Ni: more than 6% and less than 8%, sol.Al: 0.005 to 0.05%, N: 0.0005 to 0.005%, the balance is Fe and impurity power, and satisfies the following formula (1) .It contains austenite with an area ratio of 1.7% or more, and the austenite is The average aspect ratio is 3.5 or less, the average equivalent-circle particle diameter is 1.0 m or less, the aspect ratio of the cementite contained is 5.0 or less on average, and the average equivalent-circle diameter is 0.6 m or less. Low temperature steel.

 20C + 2.4Μη + Νί≥10 (1)

 However, the element symbol in the formula (1) indicates the content (mass%) of the element.

(4) Further mass% instead of part of Fe, Mo: 0.1% or less, Cu: 2.0% or less, Cr: 0.

 Contains at least one selected from 8% or less, V: 0.08% or less, Nb: 0.08% or less, Ti: 0.03% or less, B: 0.0030% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less However, the low-temperature steel as set forth in any one of (1) to (3) above, wherein the following formula (2) is satisfied.

 20C + 2.4Mn + Ni + 0.5Cu + 0.5Μο≥10 (2)

 However, the element symbol in the formula (2) indicates the content (mass%) of the element.

[0028] (5) A steel slab having the chemical composition described in any one of (1) to (4) above is heated to 850 to 1050 ° C and 1 pass in a temperature range of 700 to 830 ° C. This is a manufacturing method in which rolling is performed at a temperature of 5% or more and a cumulative reduction ratio of 25% or more, finished in a temperature range of 700 to 800 ° C, and then immediately cooled to a temperature range of 200 ° C or less. In the accelerated cooling, the cooling start temperature force is at least 10 ° CZs up to 600 ° C, and from the cooling start temperature. A method for producing a low-temperature steel material, characterized in that the cooling rate to 200 ° C is 5 ° CZs or more, and tempering is performed at a temperature of 650 ° C or less after the accelerated cooling.

[0029] (6) A two-phase region in which heat is heated to 600-800 ° C between accelerated cooling after rolling and tempering at 650 ° C or less, and cooling to 200 ° C or less at 5 ° CZs or more. The method for producing a low-temperature steel as set forth in (5) above, comprising heat treatment.

 BEST MODE FOR CARRYING OUT THE INVENTION

 [0030] 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. Note that “%” for the component content of the steel material is “mass%”.

 [0031] C: 0.01-0.1%

 C lowers the Mf point and is an effective element for the stability of retained austenite. However, the martensite substrate itself is hardened, causing toughness deterioration more than improvement of toughness by increasing the amount of austenite. Therefore, it is important to keep the C content higher than necessary to ensure strength and avoid excessive amounts that would deteriorate toughness. If it is less than 0.1%, the strength is insufficient, and if it exceeds 0.1%, the toughness deteriorates. Therefore, the C content is set to 0.01 to 0.1%. The more desirable range is 0.03-0.07%.

 [0032] Si: 0.005-0.6%

Si is effective as a deoxidizing element. It is also effective as an element that suppresses the precipitation of cementite and improves the austenite stability during tempering. However, too much Si content causes toughness degradation. Therefore, the content is set to 0.005 to 0.6%. Ί or 0.03 to 0.5 _^0/0 of the desirable, more desirable! / Ί ofヽWell, is 0.1 to 0.3 ^_0/0.

 [0033] Mn: 0.3-2%

 Μη is effective in stabilizing the austenite by lowering the Mf point, and the greater the content, the greater the austenite. However, excessive Mn content degrades the toughness of the martensite substrate. Therefore, the content is made 0.3 to 2.0%. A more desirable lower limit is 0.5%, and a more desirable lower limit is 0.7%. The desirable upper limit is 1.5%, and the more desirable upper limit is 1.0%.

[0034] Ni: more than 6% and less than 8% Ni is the most important element in the steel of the present invention because it increases the strength of the steel and contributes to the stability of austenite. The higher the content, the higher the strength and the lower the Mf point and the higher the amount of retained austenite. However, if Ni is included in a large amount, the cost increases. A more desirable upper limit is 7.5%. On the other hand, one of the objects of the present invention is to obtain a steel material having performance equivalent to that of 9% Ni steel. To achieve the object, a Ni content exceeding 6% is required. A more desirable lower limit is 6.5%.

 [0035] sol.Al: 0.005 to 0.05%

 A1 is effective as a deoxidizing element like Si, and as an element for improving the austenite stability during tempering by suppressing the precipitation of cementite. In addition, A1 combines with N to become A1N, and has the effect of contributing to the refinement of austenite grains during heating. Therefore, it is necessary to contain 0.005% or more as sol.A1. However, if the A1 content is too high, the toughness deteriorates. Therefore, the content is set to 0.005-0.05% as sol.Al. A more desirable content range is 0.02% to 0.04%.

 [0036] N: 0.0005% to 0.005%

 N is an element that contributes to the stability of austenite, so it is desirable to add it. Also, it combines with A1 to become A1N, which is effective for refining austenite grains during heating. To obtain these effects, a content of 0.0005% or more is necessary. However, excessive N degrades the martensite substrate, so its content must be 0.005% or less. A more desirable content range is 0.002% to 0.004%.

 [0037] One of the steel materials of the present invention is one in which, in addition to the above components, the balance is also composed of Fe, impurities, and force. Another steel material of the present invention is a steel material that contains one or more selected from the above components and selected from among Mo, Cu, Cr, V, Nb, Ti, B, Ca and Mg. These components are described below.

 [0038] Mo: 0.1% or less

Mo is effective in increasing the amount of austenite as an austenite stable element at low temperatures. To obtain this effect, a content of 0.01% or more is desirable. However, if the Mo content exceeds 0.1%, the toughness decreases due to the deterioration of the martensite substrate. It is necessary to do the following. A more desirable lower limit is 0.02%, a desirable upper limit is 0.06%, and a more desirable upper limit is 0.05%.

[0039] Cu: 2. 0% or less

 Cu is an element that stabilizes austenite in a solid solution state, and it is desirable to add Cu. To obtain this effect, 0.05% or more is desirable. While tempering, solid solution Cu precipitates as ε-Cu, which is effective for increasing strength but deteriorates toughness. Therefore, the upper limit of the content is 2.0%.

[0040] Cr: 0.8% or less

 Cr is an element effective for increasing the strength. To obtain this effect, 0.05% or more is desirable. However, if its content exceeds 0.8%, the toughness deteriorates, so 0.8% is made the upper limit.

[0041] V: 0.08% or less

 V is an element effective for increasing the strength of steel and becomes a precipitate by tempering, strengthening the steel. In order to obtain the effect, the content is preferably 0.005% or more. However, if its content exceeds 0.08%, the toughness deteriorates due to excessive precipitates.

[0042] Nb: 0.08% or less

 Nb expands the non-recrystallization temperature range in rolling and is effective for refining the structure and increasing the toughness after rolling. In order to obtain this effect, it is desirable to contain 0.005% or more. However, if its content exceeds 0.08%, the toughness deteriorates, so the upper limit is set to 0.08%.

[0043] Ti: 0.03% or less

 Ti is an element effective in preventing cracks in the slab. To obtain this effect, it is desirable to contain 0.005% or more. However, if the content exceeds 0.03%, the toughness deteriorates, so the upper limit is set to 0.03%.

[0044] B: 0. 0030% or less

B is an element effective for increasing the strength, and in order to obtain this effect, it is desirable to contain 0.0002% or more. However, if the B content exceeds 0.303%, the toughness deteriorates, so the upper limit is set to 0.30030%. [0045] Ca: 0.005% or less

 Ca is an element effective for improving toughness. To obtain this effect, it is desirable that Ca content is 0.0002% or more. However, if its content exceeds 0.0050%, the toughness deteriorates, so the upper limit is made 0.0050%.

[0046] Mg: 0.005% or less

 Mg is an element effective for improving toughness. To obtain this effect, it is desirable to contain 0.005% or more. However, if its content exceeds 0.0050%, the toughness deteriorates, so the upper limit is made 0.0050%.

[0047] 20C + 2.4Mn + Ni≥10, or

 20C + 2.4Mn + Ni + 0.5Cu + 0.5Mo≥10

 In order to reduce the Ni content and obtain a toughness equivalent to 9% Ni steel, it is important to ensure the amount of residual austenite. Although the amount of retained austenite obtained varies depending on the conditions of heating, rolling and heat treatment, the addition of chemical components to stabilize austenite is also important. For this purpose, the value of “20C + 2.4Mn + Ni” or “20C + 2.4Mn + Ni + 0.5Cu + 0.5Mo” needs to be 10 or more. More desirable is 10.5 or more and 12 or less.

[0048] Austenite amount:

 The amount of austenite in steel is an important means of improving toughness even with low Ni. In order to obtain the same toughness as 9% Ni steel even with low Ni steel, an austenite amount of 1.7% or more in area ratio is required. A desirable lower limit is 2.0%, and a more desirable lower limit is 3.0%. The higher the amount of austenite, the more effective is to improve toughness. Therefore, the upper limit is not specified, but if it exceeds 40%, the strength is insufficient. Therefore, the upper limit of the amount of austenite is preferably 40%.

[0049] Form of austenite:

In order to stabilize austenite at a low Ni content, it is necessary to make the untransformed austenite phase fine. For this purpose, the austenite must be finely grained, the average aspect ratio must be 3.5 or less, and the average equivalent circle diameter must be 1.0 m or less. The desirable aspect ratio is 2.5 or less. Here, the equivalent circle diameter is austenite Is the diameter of the circle when the projected area is assumed to be a circle of the same area. The equivalent circle diameter is obtained by measuring the structure observed when the steel material is cut in a plane parallel to the rolling direction (in the direction perpendicular to the plate thickness). The projected area of austenite is an image analysis device. Can be used to measure.

 [0050] Form of cementite:

 Cementite precipitates with martensite matrix strength and decomposes untransformed austenite. The former precipitation reduces the strength of martensite and deteriorates toughness. Therefore, the cementite size should be less than 0. The equivalent-circle diameter of cementite is the same as the equivalent-circle diameter of austenite described above, and is measured using cementite instead of austenite.

[0051] Next, a method for producing the above steel material will be described.

 (1) Heating billets

 In order to improve the toughness of steel materials, it is important to refine the initial austenite grains, that is, the austenite grains in the steel slab before rolling. This refinement of the austenite grains increases the residual austenite amount tl. Also contribute. Therefore, the heating temperature of the steel slab before rolling is set to 850 to 1050 ° C. Heating at a temperature lower than 850 ° C lacks strength, and heating at a temperature exceeding 1050 ° C degrades toughness. A more desirable heating temperature is 900 to 1000 ° C.

[0052] (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. Rolling over 5% per pass in the temperature range of 700 to 830 ° C and a cumulative reduction ratio of 25% or more introduces lattice defects (dislocations) in the austenite in the non-recrystallized austenite temperature range, and untransformed austenite It is necessary to suppress the transformation to martensite. At this time, it is necessary to finish the rolling at a temperature of 700 to 800 ° C. When the finishing temperature is lower than 700 ° C, the anisotropy of the steel becomes significant. In addition, when the finishing temperature exceeds 800 ° C, toughness deteriorates.

[0053] (3) Cooling

After rolling, accelerated cooling to 200 ° C or lower is required. Here, it is necessary to cool at least 10 ° CZs until the cooling start force is at least 600 ° C. This is the finish The reason is to contain as many lattice defects (dislocations) introduced in the rolling. In order to obtain a martensitic structure, it is necessary to cool at a rate of 5 ° C or higher from the cooling start temperature to 200 ° C or lower. If accelerated cooling is stopped at a temperature higher than 200 ° C, sufficient martensite cannot be obtained and the strength deteriorates. The shorter the time from rolling finish to the start of water cooling, the shorter the time from the end of rolling to the start of water cooling should be within 30 seconds.

[0054] (4) Tempering

 After accelerated cooling, it is necessary to temper at a temperature below 650 ° C. This makes it possible to temper the martensite produced by the cooling treatment, that is, quenching, and to adjust the strength and improve the toughness. When tempering at temperatures above 650 ° C, the strength decreases.

[0055] (5) Two-phase heating

 In order to further increase the amount of retained austenite, it is desirable to heat the two-phase region of ferrite and austenite before tempering. The two-phase heat treatment must be performed at 600-800 ° C and then cooled to 200 ° C or less at a cooling rate of 5 ° CZs. A more desirable range of the heating temperature for the two-phase region heat treatment is 680 to 750 ° C. Example

 [0056] Specimens having the chemical composition shown in Table 1 were melted, and a steel plate having a thickness of 20 mm was made as a prototype.

 Table 2 shows the manufacturing conditions. Tensile specimens and Charpy specimens were collected from 1Z4 part (l / 4t part) of the obtained steel plate. The amount of austenite was measured by X-ray. The size and form of austenite and cementite were observed with a transmission electron microscope at 20400 magnifications at a magnification of 4000 times to obtain an average aspect ratio and an average equivalent-equivalent particle size was obtained with an image analyzer. Table 3 shows the measurement results.

化 学 組 成 （質量％、残部: Feおよび不純物) [0057] [Table 1]

Chemical composition (mass%, balance: Fe and impurities)

 Section Steel number

 C Si Cu Ni Cr Mo V Nb Ti B Al Ca g Values of formulas (1) and (2)

0.04 0.22 1.11 7.1 0.031 0.0035 10.6

0.03 0.23 1.26 6.9 0.05 0.029 0.0034 10.5

0.06 0.26 0.93 7.2 0.035 0.0046 10.6

0.05 0.12 1.24 7.1 0.034 0.0034 11.1

0.06 0.05 1.31 7.8 0.02 0.007 0.0029 12.2

0.04 0.24 1.08 0.89 7.5 0.08 0.026 0.0008 1 1.4

0.06 0.23 0.82 6.9 0.03 0.031 0,0047 10.1

0.07 0.27 1.1 1 6.2 0.05 0.034 0.0031 10.3

0.02 0.56 1.84 1.22 6.2 0.047 0.0028 11.6

10 0.10 0.07 0.46 7.2 0.031 0.0034 10.3

1 1 0.06 0.24 1.26 6.9 0.ΟΘ 0.01 0.021 0.0035 11.2

12 0.05 0.12 1.25 7.2 0.1 0.031 0.0031 11.3

13 0.04 0.21 1.31 7.8 0.016 0.009 0.026 0.0029 11.

14 0.03 0.2 1.42 6.1 0.0012 0.002 0.0041 0.0024 10.1

15 0.05 0.19 1.02 6.9 0.005 0.0023 0.0022 10.3

16 0.12 * 0.12 1.31 7.1 0.033 0.0031 12.6

17 0.06 0.72 * 1.46 7.0 0.034 0.0032 11.7

18 0.05 0.24 2.60 * 7.0 0.05 0.024 0.0044 14.3

19 0.04 0.23 1.31 5,3 * 0.05 0.033 0.0036 9.3 *

20 0.06 0.22 1.23 7.2 0.0S 0.065 * 0.0038 11.4

21 0.07 0.24 1.15 6.8 0.05 Ο.031 0.0067 * 11.0

22 0.07 0.22 1.23 6.9 0.12 * 0.007 0.0024 11.3

23 0.06 0.21 1.19 6.8 0.09 * 0.032 0.0033 10.9 Note. * Indicates that it is outside the scope of the present invention.

Table 2

S¾3005

Note: * indicates outside the scope of the present invention.

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The In the present invention example, YS is 585 MPa or more, TS force is 90 to 825 MPa, and Charpy impact energy at −196 ° C. is 250 J or more.

 [0061] In particular, it includes two requirements regarding the metallographic structure, ie, an austenite with an area ratio of 1.7% or more. (1) The austenite has an average aspect ratio of 3.5 or less and an average circle (1) with an equivalent particle size of 1. O / zm or less, (2) an average aspect ratio of cementite of 5.0 or less, and an average equivalent circle diameter of 0.6 m or less. When both of the conditions (2) are satisfied (test numbers T2, Τ4, Τ6, Τ7, Τ8, Τ10, T13 and T15), the absorbed energy is high toughness of 290J or more.

 [0062] On the other hand, any of the comparative examples in which any of the chemical composition and other conditions are not within the range defined by the present invention has low impact energy and low temperature toughness.

[0063] In addition, the mechanical properties of the conventional methods 9% of the same thickness produced in (quenched and tempered) Ni steel, YS force S610MPa, TS force S720MPa, Shanorepi absorption E Noregi ί or 280 in one 196 ⁰ C J. From this, it can be seen that the steel of the present invention has the same performance as the 9% Ni steel despite the small amount of Ni.

 Industrial applicability

[0064] According to the present invention, it is more than 6% and less than 8%! /, Low !, steel material having a mechanical property equal to or better than steel containing 9% Ni even if it contains Ni. Is obtained. Since this steel material is inexpensive and has excellent strength and low temperature toughness, it is suitable as a structural material for storage tanks for low temperature substances such as LNG.

Claims

The scope of the claims [1] in a weight _{^{0/0, C:. 0.01~0}} 1%, Si: 0.005~0.6%, Mn: 0.3~2%, Ni: more than 6 percent less than 8%, sol.Al:0.005~0.05 %, N: 0.0005 to 0.005%, with the balance being Fe and impurity power, satisfying the following formula (1), and containing austenite with an area ratio of 1.7% or more, the austenite Is a low-temperature steel material having an average aspect ratio of 3.5 or less and an average equivalent-circle particle size of 1.0 m or less.  20C + 2.4Μη + Νί≥10 (1)  However, the element symbol in the formula (1) indicates the content (mass%) of the element. [2] Mass _{^{0/0, C:. 0.01~0}} 1%, Si: 0.005~0.6%, Mn: 0.3~2%, Ni: more than 6 percent less than 8%, sol.Al:0.005~0.05 %, N: 0.0005 to 0.005%, the balance is Fe and impurity power, and satisfies the following formula (1) .It contains 1.7% or more austenite in the area ratio and is contained. A low-temperature steel material having an average aspect ratio of cementite of 5.0 or less and an average equivalent circle diameter of 0.6 m or less.  20C + 2.4Μη + Νί≥10 (1)  However, the element symbol in the formula (1) indicates the content (mass%) of the element. [3] Mass _{^{0/0, C:. 0.01~0}} 1%, Si: 0.005~0.6%, Mn: 0.3~2%, Ni: more than 6 percent less than 8%, sol.Al:0.005~0.05 %, N: 0.0005 to 0.005%, with the balance being Fe and impurity power, satisfying the following formula (1), and containing austenite with an area ratio of 1.7% or more, the austenite Has an average aspect ratio of 3.5 or less and an average equivalent-circle particle diameter of 1. or less, and the cementite contained in the composition has an average aspect ratio of 5.0 or less and an average equivalent-circle diameter of 0.6 m or less. Low temperature steel.  20C + 2.4Μη + Νί≥10 (1)  However, the element symbol in the formula (1) indicates the content (mass%) of the element. [4] In place of a part of Fe, further mass%, Mo: 0.1% or less, Cu: 2.0% or less, Cr: 0.8% or less, V: 0.08% or less, Nb: 0.08% or less, Ti: 0.03% or less B: 0.0030% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less, including at least one selected, and satisfying the following formula (2): Claims 1 to 3 Described in either Low temperature steel.  20C + 2. 4Mn + Ni + 0.5Cu + 0.5Μο≥10 · '· (2)  However, the element symbol in the formula (2) indicates the content (mass%) of the element.  [5] A steel slab having the chemical composition according to any one of claims 1 to 4 850-: LO  Immediately after heating to 50 ° C, rolling at a temperature range of 700 to 830 ° C, rolling at a cumulative reduction of 25% or more at 5% or more per pass, and finishing the rolling in a temperature range of 700 to 800 ° C It is a manufacturing method that performs accelerated cooling to a temperature range of 200 ° C or lower, and the cooling start temperature force in the accelerated cooling is set to a cooling rate of 10 ° CZs or more up to at least 600 ° C, and 200 ° C from the cooling start temperature. A method for producing a low-temperature steel material, characterized in that the cooling rate to C is 5 ° CZs or more, and tempering is performed at a temperature of 650 ° C or less after the accelerated cooling.  [6] Between accelerated cooling after rolling and tempering at 650 ° C or lower, heat treatment at 600 to 800 ° C and two-phase region heat treatment to cool to 200 ° C or lower at 5 ° CZs or higher. The method for producing a low-temperature steel material according to claim 5, comprising: