JP2017155300A - Thick steel sheet for low temperature and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick steel sheet for low temperature consisting of a high Mn steel material capable of securing sufficient base material toughness even in a working temperature range of a liquid natural gas or liquid nitrogen and a manufacturing method therefor.SOLUTION: A thick steel sheet for low temperature contains, by mass%, C:0.30 to 0.65%, Si:0.05 to 0.30%, Mn:over 20.00% and less than 30.00%, Ni:0.10 to less than 3.00%, Cr:3.00% or more and less than 8.00%, Al:0.005 to 0.100%, N:0.0050% or more and less than 0.0500% with limitations of P:0.040% or less, S:0.020% or less, and O:0.0100% or less and the balance Fe with inevitable impurities, containing 2000/mmor less of carbide with austenite particle diameter of 50 μm or more and less than 300 μm and average circle equivalent diameter of 0.2 μm or more and has yield strength at an ordinal temperature of 400 MPa or more, tensile strength of 800 MPa or more and JIS 4 Charpy impact absorptive energy at -196°C of 100 J or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a low temperature thick steel plate made of high Mn steel suitable for a material for storing liquefied gas and a method for producing the same.

  Examples of materials that can be used in a cryogenic environment such as liquefied natural gas (boiling point: -164 ° C.) include aluminum alloys such as 5000 series (Al—Mg series), Ni—Cr austenite alloys such as SUS304, and 9 % Ni steel sheet has been used. However, the yield stress is not as high as that of low-alloy high-strength steel. In addition to the fact that the plate thickness must be increased, the weldability is not high, and the material cost is high because it contains a large amount of Ni. Therefore, there is a demand for a material that is inexpensive and excellent in strength, weldability, and weld toughness. The strength required for pressure vessel materials is increasing as the size of the tank increases.

  Therefore, a high-Mn austenitic alloy in which Ni contained in a Ni-based austenitic alloy is replaced with Mn has been proposed as a low-temperature material that does not use expensive Ni and Al, and is used in fusion reactors, superconducting generators, and linear motor cars. It has been studied as a nonmagnetic material to be used.

  For example, Patent Document 1 shows that a high Mn steel having excellent low temperature toughness and magnetic properties can be obtained by containing less than 0.5% C and 16 to 40% Mn. Patent Document 2 discloses a high-Mn steel containing 26-30% Mn in a range where the C content is 0.10% or more, the N content is 0.05% or more, and C + 2N is 1.0% or less. Has been.

  Furthermore, in Patent Document 3, the parameter represented by X = Ni-30C + 0.5Mo containing 10-30% Mn and 10-25% Cr satisfies 5.50 or more, and 0.0005-0. A high Mn steel having high strength and high toughness even at an extremely low temperature of 4K is disclosed by containing .0050% Ca and 0.15 to 0.24% N. In Patent Document 4, 0.01 to 0.25% C, 15 to 40% Mn is contained, and a parameter represented by X = 30 × P + 50 × (S + N) + 300 × O satisfies 3.0% or less. Thus, a high Mn steel having high strength and high toughness even at extremely low temperatures is disclosed.

JP 59-011661 A JP-A-5-018887 Japanese Patent Laid-Open No. 9-41087 Patent No. 4529872

  The high Mn steel materials according to these conventional technologies need to contain a large amount of Ni or require special heat treatment after rolling, and have high strength and excellent mother material at a low cost. It could not be provided with material toughness, and did not satisfy the requirements necessary for a large steel material for low temperature tanks. In addition, in the above-mentioned patent documents, no reference is made to the distribution of carbides that adversely affect toughness, and no mention is made of means for preventing the formation of carbides.

  The present invention solves such conventional problems, and yield stress of 400 MPa or more, tensile stress of 800 MPa or more, and liquefaction at room temperature (25 ° C.) without reheating after hot rolling. It is possible to ensure sufficient base material toughness even in the use temperature range such as natural gas (boiling point: -164 ° C) and liquid nitrogen (boiling point: -196 ° C) in the thick material, specifically, the components and production conditions are appropriate. Low temperature consisting of a high Mn steel material that suppresses and controls the precipitation of carbides and can secure 100 J or more up to at least a plate thickness of 50 mm with JIS No. 4 Charpy impact absorption energy (vE-196) at −196 ° C. of the base material. An object of the present invention is to provide a thick steel plate and a manufacturing method thereof.

  The present inventors examined a low-temperature steel plate that can be used in a liquefied gas storage tank or the like. As a result, regarding the chemical composition of the steel material, C, Si, P, S, Ni, Cr, Al, N, based on high Mn steel containing Mn in excess of 20.00% by mass and less than 30.00%. By controlling the amount of each alloying element in the proper range, austenite grain size, carbide density, heating temperature and finish rolling temperature before rolling, and controlling the time from rolling to the start of cooling to the proper range The inventors have found that the above object can be achieved.

  The present invention has been completed based on such findings. The gist of the present invention is as follows.

(1) By mass%, C: 0.30 to 0.65%, Si: 0.05 to 0.30%, Mn: more than 20.00% and less than 30.00%, Ni: 0.10 to 3 Less than 0.000%, Cr: 3.00% or more and less than 8.00%, Al: 0.005 to 0.100%, N: 0.0050% or more and less than 0.0500%, P: 0.040 % Or less, S: 0.020% or less, O: 0.0100% or less, carbide comprising balance Fe and impurities, austenite grain size of 50 μm or more and less than 300 μm and average equivalent circle diameter of 0.2 μm or more Is 2,000 pieces / mm ² or less, yield strength at normal temperature is 400 MPa or more, tensile strength is 800 MPa or more, and JIS No. 4 Charpy impact absorption energy at −196 ° C. is 100 J or more. .

(2) Instead of a part of Fe, by mass%, Cu: 1.00% or less, Mo: 1.00% or less, Nb: 0.500% or less, V: 0.500% or less, Ti: 0 500% or less, B: 0.0010% or less, Ca: 0.0100% or less, Mg: 0.0100% or less and REM: One or more selected from 0.0500% or less The thick steel plate for low temperature according to claim 1, wherein

(3) A steel slab or steel ingot having the chemical composition described in (1) or (2) above is heated at 1000 ° C. to 1250 ° C. and then hot rolled at a rolling finishing temperature of 1000 to 800 ° C. and rolled. From the start to the start of cooling, the residence time from 900 to 800 ° C. is set to 100 seconds or less, the temperature range of 750 to 600 ° C. is cooled at a cooling rate of 5 ° C./sec or more, and then heat treatment is not performed. A method for producing a low-temperature thick steel plate.

  ADVANTAGE OF THE INVENTION According to this invention, the high Mn steel material excellent not only in low temperature toughness and weldability but also in characteristics such as thermal expansion coefficient, magnetic permeability and thermal conductivity can be provided as hot rolled. In addition, this high Mn steel material can be used as an alternative to aluminum alloys, Ni-based austenitic stainless steels, and 9% Ni steel materials used for tank materials in LNG tanks, and contributes to saving of Ni resources. The tank construction cost can be reduced. Without requiring re-heat treatment after hot rolling, the yield stress at room temperature is 400 MPa or more, the tensile strength is 800 MPa or more, and the Charpy impact absorption energy of the base material at liquid nitrogen temperature (−196 ° C.) is 100 J or more. The present invention has a remarkable industrial contribution, such as providing a high Mn steel material and a method for producing the same.

It is a graph which shows the relationship between an austenite particle size and Charpy impact absorption energy (vE-196). It is a graph which shows the relationship between carbide density and Charpy impact absorption energy. It is a graph which shows the relationship between heating temperature and an austenite particle size. It is a graph which shows the relationship between heating temperature and Charpy impact absorption energy. It is a graph which shows the relationship between rolling finishing temperature and an austenite particle size. It is a graph which shows the relationship between rolling finishing temperature and Charpy impact absorption energy. It is a graph which shows the relationship between the residence time from the rolling start of a hot rolling process to 900-800 degreeC in the cooling start, and a carbide density.

  Below, the high Mn steel material and its manufacturing method which concern on this invention are demonstrated. Hereinafter, “%” display of the content of each chemical component means “mass%”.

(A) Chemical composition C: 0.30 to 0.65%
C is an element effective for securing the strength required for low-temperature steel such as a liquefied gas tank through stabilization of austenite. In particular, to ensure strength at room temperature, the C content is set to 0.30% or more. Preferably, the C content is 0.35% or more. On the other hand, if the C content exceeds 0.65%, a large amount of Cr carbide precipitates on the austenite grain boundaries, which may deteriorate the toughness and corrosion resistance of the base material and the low temperature toughness of the weld heat affected zone. Therefore, the C content is 0.65% or less. Preferably, it is 0.50% or less.

Si: 0.05-0.30%
Si is an effective element for deoxidation and is an effective element for increasing the strength. However, if it is less than 0.05%, deoxidation may be insufficient, and the Si content is set to 0.05% or more. Preferably, the Si content is 0.10% or more. Further, if the Si content exceeds 0.30%, the ductility and toughness may be deteriorated, so the content is made 0.30% or less. Preferably, the Si content is 0.25% or less.

Mn: more than 20.00 and not more than 30.00% Mn is an element effective for increasing yield stress and improving low-temperature toughness through stabilization of austenite. However, if the content is 20.00% or less, not only the yield stress and the low temperature toughness are lowered, but also austenite is destabilized, α ′ martensite and the like are precipitated and the toughness is deteriorated. Over 0.000%. Preferably, the Mn content is 23.00% or more. On the other hand, if the Mn content exceeds 30.00%, workability and weldability deteriorate, so the content is made 30.00% or less. Preferably, the Mn content is 27.00% or less.

Ni: 0.10% or more and less than 3.00% Ni is an element that is extremely effective for stabilizing austenite and improving toughness, and the Ni content is 0.10% or more. However, even if Ni of 3.00% or more is contained, the effect is saturated, α ′ martensite is easily generated, and the base material strength, weld toughness and permeability may be deteriorated. The Ni content is less than 3.00%. Preferably, the Ni content is 2.00% or less.

Cr: 3.00 to less than 8.00% Cr is an element that stabilizes austenite and improves yield strength. In the present invention, this effect is obtained when the Cr content is 3.00% or more in relation to other alloy elements. Preferably, the Cr content is 4.00% or more. However, when the Cr content is 8.00% or more, Cr carbide is liable to precipitate on the grain boundary, and the toughness is lowered. Therefore, the Cr content is less than 8.00%. Preferably, the Cr content is 6.00% or less.

Al: 0.005 to 0.100%
Al is an element having an effect of improving the properties of steel by deoxidation of steel and refinement of crystal grains. However, if less than 0.005%, a sufficient effect cannot be obtained, so the Al content is made 0.005% or more. Preferably, the Al content is 0.010% or more. On the other hand, if the Al content exceeds 0.100%, the toughness deteriorates, so the upper limit is made 0.100% or less. Preferably, the Al content is 0.050% or less.

P: 0.040% or less, S: 0.020% or less P and S are impurity elements that impair hot workability. In austenitic steel, by reducing the contents of both elements P and S at the same time, a greater effect of improving the toughness value of the base metal and the weld heat-affected zone can be obtained than when it is reduced solely. Therefore, the P content is limited to 0.040% or less, and the S content is limited to 0.020% or less. Preferably, the P content is 0.020% or less, and the S content is 0.003% or less.

N: 0.0050 to less than 0.0500% N is an element effective for stabilizing austenite and improving yield strength. N as an austenite stabilizing element has an effect equivalent to that of C, has no adverse effects such as deterioration of toughness due to grain boundary precipitation, and has an effect of increasing the strength at extremely low temperatures. Further, N coexists with the nitride-forming element, thereby having the effect of dispersing fine nitrides in the steel. In order to express these effects, the N content is set to 0.0050% or more. On the other hand, if the N content is 0.0500% or more, the toughness deteriorates significantly, so the content is made less than 0.0500%. Preferably, the N content is 0.0300% or less.

O: 0.0100% or less When O is present in excess, coarse inclusions are formed. Increasing the number of inclusions to reduce the cleanliness of the base material and reducing the toughness of the base material and the HAZ part. Therefore, the upper limit is made 0.0100%.

  The high Mn steel material according to the present invention contains one or more selected from Cu, Mo, Nb, V, Ti, B, Ca, Mg, and REM as necessary for improving the yield strength. be able to. Hereinafter, these optional elements will be described.

Cu: 1.00% or less Cu is effective in strengthening austenite and increasing the yield strength, and may be contained as necessary. However, if the content exceeds 1.00%, the workability deteriorates. Therefore, when Cu is contained, the content is 1.00% or less, and more preferably 0.70% or less. In order to increase the strength, the Cu content is preferably 0.01% or more.

Mo: 1.00% or less Mo is not only effective in increasing strength, but also effective in preventing toughness deterioration due to grain boundary precipitation of Cr carbides and increasing the strength of steel. , May be included as necessary. However, if the content exceeds 1.00%, the effect is saturated. Therefore, when Mo is contained, its content is 1.00% or less, more preferably 0.80% or less. In order to increase the strength, the Mo content is preferably 0.01% or more.

Nb: 0.500% or less Nb combines with C and N to precipitate carbonitride, and is an element effective for improving the yield strength of steel by its precipitation strengthening. Also good. However, if the content exceeds 0.500%, the toughness deteriorates. Therefore, when Nb is contained, its content is 0.500% or less, more preferably 0.200% or less. In order to increase the strength, the Nb content is preferably 0.005% or more, more preferably 0.010% or more.

V: 0.500% or less V is an element that is effective for precipitating carbonitride by combining with C and N, and improving the yield strength of steel by its precipitation strengthening. Also good. However, if the content exceeds 0.500%, the toughness deteriorates. Therefore, when V is contained, its content is 0.500% or less, more preferably 0.200% or less. In order to increase the strength, the V content is set to 0.010% or more.

Ti: 0.500% or less Ti is an element effective for bonding carbon and N to precipitate carbonitride and improving the yield strength of steel by its precipitation strengthening. Also good. However, if the content exceeds 0.500%, the toughness deteriorates. Therefore, when Ti is contained, its content is 0.500% or less, more preferably 0.300% or less. In order to increase the strength, the Ti content is set to 0.005% or more.

B: 0.0010% or less B is segregated at the austenite grain boundary, thereby preventing grain boundary breakage and improving the yield strength. Therefore, B may be contained as necessary. However, if the content exceeds 0.0010%, the toughness deteriorates. Therefore, when it contains B, the content shall be 0.0010% or less. In order to suppress intergranular fracture, the B content is preferably 0.0005% or more.

Ca: 0.0100% or less Ca brings about the effect of spheroidization of inclusions and has the effect of improving toughness. Therefore, Ca may be contained as necessary. However, if the content exceeds 0.0100%, cleanliness is deteriorated and toughness is lost. Therefore, when Ca is contained, its content is set to 0.0100% or less. In order to improve toughness, the Ca content is preferably 0.0003% or more.

Mg: 0.0100% or less Mg, like Ca, brings about the effect of spheroidizing inclusions and has the effect of improving toughness. Therefore, Mg may be contained if necessary. However, if the content exceeds 0.0100%, cleanliness is deteriorated and toughness is lost. Therefore, when it contains Mg, the content shall be 0.0100% or less. In order to improve toughness, the Mg content is preferably 0.0002% or more.

Rare earth element (REM): 0.0500% or less Rare earth element (REM), like Ca, has the effect of spheroidizing inclusions and improving toughness, so it may be included as necessary. . However, if the content exceeds 0.0500%, cleanliness is deteriorated and toughness is lost. Therefore, when it contains REM, the content shall be 0.0500% or less. In order to improve toughness, the rare earth element (REM) content is preferably 0.0002% or more, and more preferably 0.0003%. When REM is contained, a misch metal containing La or Ce as a main component may be used. In addition, the rare earth element as used in the field of this invention is a general term of the total 17 elements of Sc, Y, and a lanthanoid, and the content of rare earth elements refers to the total content of these elements.

  Thus, by controlling the chemical components and carbides, the high Mn steel material according to the present invention can be used in a low temperature range without performing heat treatment after rolling, and a steel material with good base material toughness can be obtained.

(B) Metallographic structure The austenite particle size is 50 μm or more and less than 300 μm. The austenite particle size affects the strength and toughness of the base material. When the thickness is less than 50 μm, the grain boundaries increase and the carbide precipitation sites increase. Further, the carbide precipitation requires the diffusion of Cr to the crystal grain boundary. The larger the crystal grain size, the longer the diffusion distance, and the more the carbide precipitation can be suppressed. When the γ particle size is 300 μm or more, the toughness of the matrix is lowered. FIG. 1 shows the relationship between austenite grain size and Charpy impact absorption energy (vE-196).

2,000 carbides / mm ² or less of carbides having an average equivalent circle diameter of 0.2 μm or more Carbides have an adverse effect on toughness. Since high-Mn steel is made of an austenite phase, it is a material that is less likely to cause so-called cleavage fracture. However, carbides precipitated at the austenite grain boundaries may become the starting point of fracture, which may reduce Charpy characteristics. Even if it is a water-cooled material, very fine carbides are deposited. It is a carbide of 0.2 μm or more that adversely affects toughness. Evaluation of carbides of high Mn steel under different production conditions revealed that carbides exceeding 0.2 μm increased as toughness decreased. In particular, when the number of carbides of 0.2 μm or more exceeds 2000 / mm ² , the toughness decreases. FIG. 2 shows the relationship between carbide density and Charpy absorbed energy. Here, the carbides were observed with a scanning electron microscope at 20 000 fields of view (15 μm × 15 μm), the number density of carbides of 0.2 μm or more was calculated, and the average value was defined as the carbide density in the steel.

(C) About manufacturing conditions In addition, the present inventors are able to suppress the formation and growth of carbides by taking a manufacturing process that does not pass through a temperature range where precipitation and growth of carbides are likely to proceed. And found that it is necessary to perform hot rolling under appropriate conditions. If it deviates from an appropriate condition, a crack occurs on the surface of a steel piece, a steel ingot, or a steel plate, resulting in a decrease in yield. Therefore, it is important to strictly manage the heating conditions and rolling conditions before rolling the steel slab or the steel ingot. Further, high Mn steel contains a large amount of carbide forming elements, and carbide is likely to precipitate from rolling to cooling. It is important not to stay in the carbide precipitation temperature range during rolling. For this reason, it is necessary to strictly control the cooling conditions from the start of rolling in order to suppress carbide precipitation.

  It is preferable that the heating temperature of a steel piece or a steel ingot shall be 1000-1250 degreeC. If it is less than 1000 degreeC, since the deformation resistance at the time of rolling will be large and an excessive load will be applied to a rolling mill, it is preferable to set it as 1000 degreeC or more, More preferably, it shall be 1050 degreeC or more. On the other hand, when heated to a high temperature exceeding 1250 ° C., there is a concern about a decrease in yield due to oxidation of the surface, and the austenite grains become coarse, so that even after hot rolling cannot be easily made finer, It is preferable to set it as 1250 degrees C or less. FIG. 3 shows the relationship between the heating temperature and the austenite grain size, and FIG. 4 shows the relationship between the heating temperature and Charpy absorbed energy.

  The rolling finishing temperature for hot rolling is preferably 1000 to 800 ° C. When the rolling finishing temperature exceeds 1000 ° C., the austenite crystal grain growth after rolling becomes too large, so that a desired microstructure cannot be obtained. On the other hand, when the rolling finishing temperature is less than 800 ° C., the deformation resistance during rolling is large, and an excessive load is applied to the rolling mill. Furthermore, the rolling texture develops and the anisotropy of the steel sheet increases, which is not preferable. FIG. 5 shows the relationship between the finishing temperature and the austenite grain size, and FIG. 6 shows the relationship between the finishing temperature and Charpy absorbed energy.

In order to suppress the formation of carbides and increase the low temperature toughness, the residence time from 900 to 800 ° C. from the start of rolling to the start of cooling needs to be 100 seconds or less. 900 to 800 ° C. is a temperature range in which carbide is likely to precipitate and grow. If it exceeds 100 seconds, coarse carbides precipitate, and carbides of 0.2 μm or more exceed 2000 pieces / mm ² and lower the base material toughness. FIG. 7 shows the relationship between the residence time up to 900 to 800 ° C. and the carbide density.

  Accelerated cooling is performed so that the cooling rate in the temperature range from 750 ° C. to 600 ° C. is 5 ° C./s or more. When the cooling rate is less than 5 ° C./s, the effect of accelerated cooling is not sufficient, and a desired structure cannot be obtained. In this accelerated cooling, since the effect of accelerated cooling cannot be obtained if the rolling structure is changed, it is necessary to start accelerated cooling at 750 ° C. or higher. The reason why the lower limit of the accelerated cooling range is 600 ° C. is that a predetermined accelerated cooling effect can be obtained by cooling to at least 600 ° C. The cooling rate in the temperature range from 600 ° C. to room temperature is 1 ° C./sec or more. Note that the accelerated cooling may be continued to a temperature of 600 ° C. or lower. As a result, a steel sheet excellent in both strength and fracture resistance can be obtained. This steel sheet has properties suitable for the tank material in the LNG tank. Note that carbide can be suppressed as it is rolled, and subsequent heat treatment is unnecessary.

  Hereinafter, the present invention will be described in more detail by way of examples.

  Using steel slabs of steel 1 to 32 having the chemical composition shown in Table 1, the plate thickness is 5 under the production conditions shown in Table 2 (heating temperature, finishing temperature, time from rolling to cooling start were variously controlled). A high Mn steel material of ˜50 mm was produced. And the number density of the carbide | carbonized_material which the circle equivalent diameter in steel materials exceeds 0.2 micrometer was measured (a measured value is shown in Table 2). Tensile properties (yield strength, tensile strength) and 2 mmV notch Charpy impact absorption energy were evaluated as base material properties. The obtained measured values are shown in Table 2. The tensile test pieces and Charpy test pieces were collected in the direction perpendicular to the rolling direction from a thickness of 1/4 t. In the evaluation, when the yield stress was less than 400 MPa and the tensile strength was less than 800 MPa at room temperature (25 ° C.), the case where the JIS No. 4 Charpy impact absorption energy (vE-196) at −196 ° C. was less than 100 J was rejected.

  From Table 2, it can be seen that the high Mn steel materials according to the examples of the present invention are hot-rolled and excellent in both base material strength and toughness, and are excellent as low-temperature materials.

  On the other hand, in the comparative example that does not satisfy the conditions defined in the present invention, it can be seen that the intended characteristics cannot be obtained in one or both of the base material strength and toughness.

  The high-Mn steel material according to the present invention can be provided as it is without being subjected to heat treatment after hot rolling, and can be provided as it is, such as aluminum alloy, Ni-based austenitic stainless steel, 9% It can be used as a substitute for Ni steel, contributes to saving of Ni resources, and enables the tank construction cost to be reduced.

Claims (3)

In mass%, C: 0.30 to 0.65%, Si: 0.05 to 0.30%, Mn: more than 20.00% and less than 30.00%, Ni: 0.10 to 3.00% Less than, Cr: 3.00% or more and less than 8.00%, Al: 0.005 to 0.100%, N: 0.0050% or more and less than 0.0500%, P: 0.040% or less, S: 0.020% or less, O: 0.0100% or less, 2,000 carbides comprising the balance Fe and impurities, having an austenite grain size of 50 μm to less than 300 μm and an average equivalent circle diameter of 0.2 μm or more / mm ² or less, a normal temperature yield strength at more than 400 MPa, a tensile strength of more than 800 MPa, the steel plate for low temperature JIS4 No. Charpy impact absorption energy at -196 ° C. is characterized in that at least 100 J.   Instead of a part of Fe, by mass%, Cu: 1.00% or less, Mo: 1.00% or less, Nb: 0.500% or less, V: 0.500% or less, Ti: 0.500% Hereinafter, B: 0.0010% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0500% or less, or one or more selected from The thick steel plate for low temperature according to claim 1.   The steel slab or steel ingot having the chemical composition defined in claim 1 or 2 is heated at 1000 ° C. to 1250 ° C. and then hot-rolled at a rolling finishing temperature of 1000 to 800 ° C., and from the start of rolling to the start of cooling. In the low-temperature thick steel sheet, the residence time up to 900-800 ° C. is set to 100 seconds or less, the temperature range of 750 to 600 ° C. is cooled at a cooling rate of 5 ° C./sec or more, and then heat treatment is not performed. Production method.