WO2023166926A1

WO2023166926A1 - HIGH-Ni ALLOY THICK STEEL SHEET HAVING EXCELLENT WELD HIGH-TEMPERATURE CRACKING RESISTANCE, AND METHOD FOR PRODUCING SAME

Info

Publication number: WO2023166926A1
Application number: PCT/JP2023/003970
Authority: WO
Inventors: 幸寛西田; 優馬吉岡; 信二柘植
Original assignee: 日鉄ステンレス株式会社
Priority date: 2022-03-01
Filing date: 2023-02-07
Publication date: 2023-09-07

Abstract

Provided is a high-Ni alloy thick steel sheet having excellent weld high-temperature cracking resistance, the steel sheet being characterized by containing, in % by mass, 0.15% or less of C, 0.05 to 1.0% of Si, 0.05 to 2.0% of Mn, 0.035% or less of P, 0.0015% or less of S, 16 to 28% of Cr, 18 to 65% of Ni, 0.01 to 1.0% of Al, 0.15 to 1.5% of Ti, 0.0002 to 0.0030% of B, 0.05% or less of N, 0.003% or less of O, 0.01 to 10% of Mo, 0.01 to 4.0% of Cu, 0.01 to 3.0% of Co, 0.01 to 0.5% of V, 0.0050% or less of Mg and a remainder comprising Fe and impurities, having a grain size number G as prescribed by JIS G0552 of 1.0 or more, and having a standard deviation of a solid solution Ti concentration distribution in the thickness direction of 0.045% or less. Also provided is a method for producing a high-Ni alloy thick steel sheet having excellent weld high-temperature cracking resistance, the method being characterized by comprising performing such a thermal treatment that the retention time at 1200°C or higher is set to 8 hours or longer in a previous step for a final hot rolling step.

Description

High-Ni-alloy thick steel plate with excellent weld hot-cracking resistance and method for producing the same

The present invention relates to a high Ni alloy steel plate excellent in resistance to weld hot cracking, which is used as a material for high temperatures, and a method for manufacturing the same.

As high-Ni alloy steels containing Al and Ti, alloys 800 and 825 are typical commercial alloy steels. In recent years, the demand has been expanding in developing countries, and there is a need for technical development to supply inexpensive products with good surface quality and usability. For this reason, the conversion of the manufacturing method from the conventional steel ingot method to the continuous casting method is underway. Due to the high susceptibility to flaws, the design of the chemical composition of the alloy, smelting, casting, and hot working technology have been improved and developed from the viewpoint of improving productivity in the continuous casting method.

As a patent document related to continuous casting technology, for example, Patent Document 1 discloses a technology related to a component system and a manufacturing method in which the contents of Ti, N, and Si are reduced to a low level as a method of suppressing the occurrence of surface defects. Patent Literature 2 discloses a method for preventing nozzle clogging and surface flaws by a manufacturing method that does not add a Ca alloy. In this document, the addition of Ca alloys combines with oxygen in the molten alloy to form oxide-based non-metallic inclusions, which agglomerate and increase in size, leading to the generation of linear defects on the surface of the final product alloy plate. It is stated that there is a problem with connecting. In Patent Document 3, CaO—MgO—Al ₂ O ₃ based inclusions are included as essential components in order to prevent coarse agglomeration of TiN, which causes surface defects, and the number of CaO and MgO in the total number of inclusions is ratio is 50% or less.

The above prior art specifies the component system and the composition of inclusions from the viewpoint of manufacturability, especially the suppression of surface defects.

JP-A-2003-147492 JP 2014-189826 A JP 2018-59148 A

In practical use of high Ni alloy steel, the present inventors have found that not only is there a problem with manufacturability, but also because it is an austenitic single phase steel, it exhibits low weld hot cracking resistance, and cracks are likely to occur during welding. I found out that there is a problem. Hot cracks can be broadly classified into solidification cracks that occur in the weld zone and liquefaction cracks that occur in the HAZ. It is an object of the invention to improve the cracking resistance, especially to stabilize the liquefaction cracking susceptibility to a low level.

Al- and Ti-containing high-Ni alloy steels are said to have relatively good hot workability. However, since the cast slab has a solidified structure, the hot workability of the cast slab becomes insufficient when the cast slab contains more than several ppm of S. Therefore, it is necessary to improve hot workability by adding a small amount of Ca alloy or Mg alloy. However, when the high-Ni alloy steel targeted by the present invention is continuously cast by adding a Ca or Mg alloy, and a steel material is produced from the slab, bloom or billet, the steel material is welded to construct a structure. When manufacturing , weld hot cracks may occur due to thermal stress generated by heat input during welding. In particular, in Al- and Ti-containing high-Ni alloy steel, liquefaction cracking occurring in the HAZ may pose a problem. Even more troublesome, even if a steel plate manufactured using a steel ingot melted in units of several tens of kilograms has a composition system that can be welded well, a steel plate manufactured using a steel ingot melted and cast in units of several tons Liquefaction cracking occurs in the HAZ. Therefore, optimization is required not only for the components but also for histology. In particular, in thick steel plates obtained through the hot rolling process and the product heat treatment process, the thicker the slab, the slower the cooling rate inside the slab during casting, making segregation more likely to occur. This tendency becomes remarkable when it is used as a slab for manufacturing. On the other hand, in order to obtain a good recrystallized structure in high-alloy steel plates, the thickness of the original slab must be at least three times the product thickness. of original slab is required. Therefore, when industrial production is carried out with a plurality of plate thickness repertoires secured, it is desirable from the viewpoint of production efficiency to concentrate production on original slabs having a thickness of 160 mm or more.

An object of the present invention is to provide a high Ni alloy steel plate with excellent resistance to weld hot cracking and a method for manufacturing the same, which can solve the above problems. The thick steel plate defined in the present invention is limited to a hot-rolled steel plate or a steel plate obtained by subjecting a hot-rolled steel plate to temper rolling, and the cold-rolled steel plate is excluded from the scope of the present invention.

In order to clarify the cause and solve the above problems, the present inventor made the Al, Ti-containing high-Ni alloy steel, which is the subject of the present invention, the basic composition, added Ca, and melted it in an actual machine to produce a cast slab. Using the obtained slabs, hot-rolled, annealed, and heat-treated steel materials were manufactured by laboratory trial production. Using the obtained steel material, the liquefaction cracking susceptibility during welding was evaluated by a restraint weld cracking test. At the same time, we conducted research for problem solving using techniques such as EPMA analysis.

In the steel ingot of the high Ni alloy steel investigated in the inventor's research, TiC, TiN, or TiNC was generated either independently or as if including oxide inclusions. Among these, attention was focused on the precipitation behavior of TiC, which has a large size and acts as a starting point for liquefaction cracking. In particular, with regard to liquefaction cracking, there is a region where precipitates containing TiC are locally accumulated in the Ti positive segregation part within the plate thickness, and liquefaction cracking originating from TiC generated in the accumulated part occurs. The present inventors have found that TiC accumulation and liquefaction cracking occur when the concentration distribution of dissolved Ti exceeds 0.045% in terms of standard deviation, leading to the present invention.

That is, the gist of the present invention is as follows.
(1) In mass %, C: 0.15% or less, Si: 0.05 to 1.0%, Mn: 0.05 to 2.0%, P: 0.035% or less, S: 0.0015 % or less, Cr: 16-30%, Ni: 18-65%, Al: 0.01-1.0%, Ti: 0.15-1.5%, B: 0.0002-0.0030%, N: 0.05% or less, O: 0.003% or less, Mo: 0.01 to 10%, Cu: 0.01 to 4.0%, Co: 0.01 to 3.0%, V: 0 .01 to 0.5%, Mg: 0.0050% or less, the balance consisting of Fe and impurities, the crystal grain size number G ≥ 1.0 specified by JIS G 0552, and solid solution Ti in the plate thickness direction A high Ni alloy steel plate having excellent weld hot cracking resistance, characterized in that the standard deviation of the concentration distribution is 0.045% or less.
(2) Welding resistance according to (1), characterized in that, instead of part of the Fe, one or two selected from the group consisting of the following Groups A and B are included in mass% High Ni alloy steel plate with excellent hot cracking resistance.
[Group A]
Ca: 0.0003-0.0050%, Sn: 0.0001-0.05%, Zn+Pb+Bi: 0.0010% or less, Zr: 0.0001-0.5%, Hf: 0.0001-0.5 %, La + Ce + Nd + Pr: one or more of 0.0001 to 0.0050% [Group B]
One or more of W: 0.01 to 3.0%, Nb: 0.001 to 4.0%, Ta: 0.001 to 1.0% (3) Used for welded structures A high Ni alloy steel plate having excellent resistance to weld hot cracking according to (1) or (2).

(4) A method for producing a high-Ni alloy steel plate by using, as a raw material, a steel ingot obtained by continuous casting and having a thickness of 160 mm or more and a slab thickness/product thickness ratio of 3.0 or more. , Weld hot cracking resistance according to (1) or (2), characterized in that a long-term high-temperature heat treatment is performed at a temperature of 1200 ° C. or more for a holding time of 8 hours or more in the stage before the final hot rolling. A method for producing an excellent high Ni alloy thick steel plate.
(5) A method for producing a high-Ni alloy steel plate by using, as a raw material, a steel ingot obtained by continuous casting and having a thickness of 160 mm or more and a slab thickness/product thickness ratio of 3.0 or more. , A high-Ni high-Ni steel sheet having excellent resistance to weld hot cracking according to (3), characterized in that it is subjected to a high-temperature long-term heat treatment in which the holding time at 1200 ° C. or more is secured for 8 hours or more in the stage before the final hot rolling. A method for manufacturing an alloy thick steel plate.

According to the present invention, it becomes easy to stably manufacture a welded structure using Al, Ti-containing high-Ni alloy thick steel plates used as high-temperature materials. In addition to excellent hot workability, cracking of the welded heat-affected zone is less likely to occur when manufacturing welded structures, and Al and Ti-containing high-Ni alloy thick steel plates have excellent creep characteristics and oxidation resistance at high temperatures. Obtainable.

<Component composition>
First, the reasons for limiting the content of the essential components of the present invention will be described below. In addition, content of each component shows mass %.

C: 0.15% or less C is added to ensure the strength of high-temperature materials and heat-resistant alloys. Especially when high-temperature strength properties are required, 0.015% or more, preferably 0.05% or more of C is added. Limit the C content to 0.15% or less. In this alloy, C exists as TiC precipitates in the alloy, but if the C content exceeds 0.15%, Cr carbides are formed, resulting in deterioration of high-temperature properties and corrosion resistance. The C content is preferably 0.10% or less, more preferably 0.085% or less.

Si: 0.05-1.0%
Si is added in an amount of 0.05% or more to deoxidize and improve oxidation resistance. However, if Si is added in excess of 1.0%, the solidification cracking susceptibility and liquefaction cracking susceptibility of the steel are increased, intermetallic compounds are likely to precipitate, and the high temperature properties deteriorate. Therefore, the upper limit of the Si content is limited to 1.0%. A preferred upper limit is 0.7%, and a more preferred upper limit is 0.5%.

Mn: 0.05-2.0%
Mn has the effect of increasing the stability of the austenite phase and improving the heat resistance. Therefore, it is preferable to positively add Mn to the alloy of the present invention. 0.05% or more of Mn is added to improve heat resistance. However, if Mn is added in excess of 2.0%, intermetallic compounds tend to precipitate, degrading heat resistance and adversely affecting solidification cracking susceptibility. Therefore, the upper limit of the Mn content is specified at 2.0%. A preferred upper limit is 1.5%, and a more preferred upper limit is 1.3%.

P: 0.035% or less P is an element that is inevitably mixed from the raw material, and has the effect of increasing solidification cracking susceptibility. Therefore, the P content is limited to 0.035% or less. Preferably, it is 0.030% or less.

S: 0.0015% or less S is an element that is unavoidably mixed from raw materials, and deteriorates hot workability and oxidation resistance. Therefore, the S content is limited to 0.0015% or less, preferably 0.0010% or less. S is an element whose content can be reduced by refining, but an extreme decrease in content results in an increase in cost. Therefore, it is preferable to set the lower limit of the S content to 0.0001%.

Cr: 16-28%
Cr is an essential element for the oxidation resistance of a heat-resistant alloy as a material for high temperatures, and is contained in an amount of 16% or more, preferably 18% or more. On the other hand, if the Ni content exceeds 28%, the high-temperature structure stability is lowered even if a large amount of Ni is included, intermetallic compounds are precipitated, and heat resistance is deteriorated. A preferred upper limit is 26%. The optimum content varies depending on the content of Ni, Si, Mo and other elements. For example, when the Ni content is about 30%, the optimum Cr content is about 20%. Alternatively, when Ni+Cu is about 45%, the optimal content is Cr+Mo about 25%.

Ni: 18-65%
Ni stabilizes the austenite structure at high temperatures and improves corrosion resistance and toughness against various acids, so the Ni content is 18% or more, preferably 20% or more, and more preferably 25% or more. By increasing the Ni content, it becomes possible to contain more Cr, Mo, Al, and Ti necessary for heat resistance. On the other hand, Ni is an expensive alloy, and in the steel of the present invention, the upper limit is set at 65% or less from the viewpoint of cost.

Al: 0.01-1.0%
Al is a deoxidizing element and has the effect of forming a NiAl ordered phase in the high-Ni alloy and increasing the high-temperature strength. In the present invention, the content of Al must be 0.01% or more, preferably 0.05% or more, in order to control the composition of the oxide and improve the hot workability. On the other hand, if the Al content exceeds 1.0%, the intermetallic compound is likely to precipitate, which impairs the heat resistance. Moreover, if it is contained excessively, the susceptibility to liquid cracking during welding increases. Therefore, the upper limit of the Al content is set at 1.0%. A preferred upper limit is 0.60%.

Ti: 0.15-1.5%
Ti has the effect of forming a NiTi ordered phase in a high-Ni alloy and increasing the high-temperature strength. For this purpose, the content of Ti should be 0.15% or more, preferably 0.2% or more. On the other hand, if the Ti content exceeds 1.5%, the intermetallic compound is likely to precipitate, which impairs the heat resistance. Moreover, if it is contained excessively, the susceptibility to liquid cracking during welding increases. A preferable upper limit of the Ti content is 1.0%, more preferably 0.85%.

B: 0.0002 to 0.0030%
B is an element that improves the hot workability of steel, and significantly improves the drawing in the high temperature region of hot working. In addition, in order to improve high-temperature creep strength, B is positively added especially in environmental applications where the steel is used at high temperatures. Although the mechanism by which B improves hot workability is not clear, it is said that segregation at grain boundaries increases grain boundary strength. Since the effect of improving hot tensile strength due to the addition of B is manifested when the B content is 0.0002% or more, the lower limit of B addition is set to 0.0002%. On the other hand, excessive addition promotes solidification cracking, so the upper limit of the content is set at 0.0030%. A preferred upper limit is 0.0015%.

N: 0.05% or less N is an element effective for improving high-temperature strength. On the other hand, in the present invention, Ti and Al are positively added, so N forms AlN or TiN and becomes non-metallic inclusions, deteriorating the material properties, and complexes with oxides to promote nozzle clogging during continuous casting. It becomes a harmful element that Therefore, the N content is set to 0.05% or less. The content is preferably 0.04% or less, more preferably 0.03% or less.

O: 0.003% or less Oxygen forms oxide inclusions between Ca, Mg, Al and Ti in the alloy of the present invention. The oxygen content corresponds to the total amount of oxide inclusions and is an important indicator of the deoxidized state of the alloy. If the content exceeds 0.003%, the desired deoxidation balance is not satisfied, and nozzle clogging tends to occur during continuous casting. In addition, it promotes the formation of coarse TiC that acts as a starting point for liquefaction cracking and has an adverse effect on weld hot cracking resistance, which is the gist of the present invention. Therefore, the upper limit of the oxygen content was set at 0.003%. A preferred upper limit is 0.0025%, more preferably 0.002%. On the other hand, the reduction of the oxygen content works favorably for suppressing nozzle clogging and weld hot cracking by reducing oxide-based inclusions and inclusions containing coarse TiC. This causes a decrease in hot workability. Therefore, the oxygen content is preferably 0.0003% or more.

Mo: 0.01-10%
Mo is an element that enhances the high-temperature strength and corrosion resistance of the alloy, and in order to improve these properties, the Mo content is 0.01% or more, preferably 0.05% or more, and more preferably 0.15% or more. On the other hand, Mo is an expensive element, and in the steel of the present invention, the upper limit of the Mo content is 10% from the viewpoint of suppressing alloy costs. A preferred upper limit of the Mo content is 3.0%, and a more preferred upper limit is 2.0%.

Cu: 0.01-4.0%
Cu is an element that enhances the acid corrosion resistance of the alloy and the dew-point corrosion resistance that is often a problem in high-temperature equipment, and is an element that has the effect of improving high-temperature strength and structural stability. In order to improve these heat resistance and corrosion resistance, Cu is contained by 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more. On the other hand, if the Cu content exceeds 4.0%, embrittlement occurs during solidification, so the upper limit of the Cu content is set to 4.0%. A preferable upper limit of Cu is 3.0%, and a more preferable upper limit is 2.0%.

Co: 0.01-3.0%
Co is an effective element for enhancing the high-temperature structural stability and corrosion resistance of the alloy. contain more than If the content of Co exceeds 3.0%, it is an expensive element and the effect corresponding to the cost cannot be exhibited, so the upper limit of the Co content was set to 3.0%. A preferred upper limit for Co is 1.5%.

V: 0.01-0.5%
Adding 0.01% or more of V has the effect of improving the high-temperature properties of the alloy through solid-solution strengthening or precipitation strengthening. On the other hand, addition of V above 0.5% increases solidification cracking susceptibility. A preferable lower limit of the V content is 0.02%, more preferably 0.03%. Also, the preferable V content range is 0.03% to 0.5%.

Mg: 0.0050% or less Mg is generally an element that can improve the hot workability of the alloy if the amount is very small. In the present invention, the addition of Mg has the adverse effect of promoting the formation of MgO-based inclusions that increase the liquefaction cracking susceptibility during welding. In addition, excess Mg that does not form oxides segregates at grain boundaries and reduces grain boundary strength in a high temperature range (eg, 900° C.). This causes a decrease in hot workability in a high temperature range and an increase in liquefaction cracking susceptibility. When the steel of the present invention is deoxidized and strengthened, Mg is inevitably picked up from slag, furnace walls, and the like. Based on the above knowledge, in the present invention, it is necessary to reduce the content of Mg as much as possible, and Mg is not added to the alloy. The upper limit of the Mg content was set to 0.0050%. A preferred upper limit is 0.0040%.

<Crystal grain size number G ≥ 1.0 defined by JISG0552>
P, S, and Mg, which lower the melting point of the steel, segregate at the grain boundaries of the austenitic high-alloy steel. As the grain size increases, the ratio of the grain boundaries to the total volume decreases, and the concentrations of P, S, and Mg at the grain boundaries increase accordingly. This lowers the melting point of grain boundaries and increases the liquefaction cracking susceptibility during welding. As a result of intensive study, it was found that the grain size number G<1.0 increases the liquefaction cracking susceptibility regardless of the concentration distribution of solid solution Ti. The upper limit of the grain size number G is not particularly defined, but if the grain size number G exceeds 8, the high-temperature creep strength decreases, so the preferred range of the grain size number G is 1 to 8. In applications where creep strength is particularly required, the range of grain size number G is 1 to 6, preferably 1 to 5. In applications where intergranular corrosion resistance, steam oxidation resistance, and high temperature corrosion resistance are required, 3 to 8. It is most preferable to use them properly depending on the application.

<Standard deviation of solid solution Ti concentration distribution ≤ 0.045%>
Precipitation of TiC is unavoidable in a heat-resistant high alloy to which Ti is added. Here, the formation process of TiC will be described. While TiN preferentially forms in the high temperature liquid phase, TiC precipitates in the solid phase region from the solid-liquid coexistence region. Most of TiC is finely precipitated with a size of about 0.2 μm or less, but some of them are coarsened to a size of μm to several tens of μm. When precipitates containing such coarsened TiC are present at grain boundaries, C and Ti in TiC diffuse into the matrix due to the heat input during welding, lowering the melting point of the TiC/matrix interface and causing liquefaction in the HAZ. It becomes the starting point of the crack. If the solid-soluted Ti in the steel is not uniformly dispersed, a large number of TiC precipitates of about 1 μm to several μm are locally accumulated, and the grain size of about 10 μm to several tens of μm is pinned to the accumulated precipitates. of fine grains are generated locally. It has been clarified that liquefaction cracking during welding is caused by the liquefaction of a large number of coarse TiC present at the grain boundaries of the fine grains thus generated due to eutectic melting. That is, in order to improve the weldability of a Ti-containing heat-resistant high alloy, it is essential to reduce not only the composition but also the segregation of Ti in the steel sheet as much as possible.

As an index of the degree of Ti segregation, the inventors focused on the concentration distribution of solid solution Ti in steel. That is, if an arbitrary cross section of the steel is measured by EPMA, EDX, or the like at an arbitrary point where TiC and TiN are not formed, the analysis result of the concentration of solid solution Ti at the measured point can be obtained. . In the case of a steel sheet, since the Ti concentration fluctuates in the thickness direction due to segregation, as a method, line analysis in the thickness direction is performed by EPMA or EDX on an arbitrary cross section to collect numerical data on the Ti concentration. The standard deviation of the data obtained by removing the numerical data of the TiC or TiN generation portion can be obtained. As a result of investigating the standard deviation of solid solution Ti in this way, it was found that the liquefaction cracking susceptibility of the Ti-containing high-alloy steel sheet is greatly reduced when the standard deviation of the Ti concentration is 0.045% or less. Preferably, the standard deviation is 0.040% or less. The sample for linear analysis measurement in the cross-sectional plate thickness direction was taken at a position corresponding to the inside of the length of 1/2 t from the width direction end of the original slab (cast steel ingot), where t is the thickness of the original slab (cast steel ingot). , measured in the thickness direction from the front side to the back side. However, it may be difficult to measure the total thickness depending on the specifications and plate thickness of the analyzer. In such a case, the analysis length of the part where the thickness from the surface of the original slab corresponds to 1/4t to 3/4t is 50% or more of the total analysis length (the line analysis of the total thickness of the product plate may 50%) and an analysis length of 10 mm or more. When the plate thickness of the final product is less than 10 mm, line analysis is performed on the total thickness of the product plate.

<Heating and holding conditions in the process before the final hot rolling: holding at 1200 ° C. or higher for 8 hours or longer>
As a method of manufacturing thick plate high alloy steel, generally several tons to 100 and several tens of tons are refined and manufactured through the steps of refining, casting, hot rolling and heat treatment. Refining is carried out in the order of a melting process in an electric furnace, rough decarburization in a converter, VOD, AOD, or a finish decarburization process combining both, and then casting by either continuous casting or ingot casting. Hot rolling is a process in which the steel is heated and held at an appropriate temperature according to the properties of the steel, and then rolled to a specified thickness. After making the product thickness, product heat treatment may be performed and the product may be manufactured through a refining process.

In order to reduce the segregation of Ti in Ti-containing high-alloy steel sheet products, it is essential to optimize the cooling rate and electromagnetic stirring conditions during solidification. It is difficult to completely eliminate the segregation only by In particular, it is extremely difficult to eliminate segregation in slabs having a thickness of 160 mm or more and having a slow cooling rate. Therefore, if the segregation cannot be removed only by measures in steelmaking, it is necessary to perform high-temperature, long-time heat treatment in the heat treatment process after casting. However, if the heat treatment is applied at the stage of product heat treatment, the crystal grains of the steel sheet will grow excessively and the liquefaction cracking susceptibility will be increased.

As a result of intensive studies, in order to achieve the requirements of the present invention, it is necessary to perform a high-temperature long-term heat treatment at a temperature of 1200°C or higher, preferably 1230°C or higher, for 8 hours or longer, preferably 15 hours or longer, before the final hot rolling. It became clear that there is The high-temperature, long-time heat treatment may be performed before the first rolling. More preferably, this effect is most effectively produced by performing heat treatment at high temperature for a long time in a state in which intervals between segregation zones are narrow and high-speed diffusion paths such as dislocations and recrystallized grain boundaries are introduced as much as possible. Therefore, when rolling is performed multiple times, the most preferred embodiment is to introduce a high-temperature, long-time heat treatment step between the rolling step (rough rolling) immediately before the final rolling and the final rolling step (main rolling).

In the product heat treatment performed after hot rolling, the lower the heat treatment temperature and the shorter the heat treatment time, the larger the grain size number G in the steel sheet. In order to make the grain size number G in the steel sheet 1.0 or more, it can be realized by adjusting the temperature and time of product heat treatment after hot rolling according to the chemical composition of the steel sheet.

The composition of the high-Ni alloy of the present invention contains the components described above, with the balance being Fe and impurities. Further, instead of part of the Fe, the following components (% by mass) can be selectively contained. Next, the reason for limiting the content of the selected element will be described.
<Component composition>
Ca: 0.0003-0.0050%
Sn: 0.0001-0.05%
Zn+Pb+Bi: 0.0010% or less Zr: 0.0001 to 0.5%
Hf: 0.0001-0.5%
La+Ce+Nd+Pr: 0.0001 to 0.0050%

By containing 0.0003% or more, preferably 0.0010% or more, and more preferably 0.0015% or more of Ca, the S in the alloy is fixed as CaS, and the hot workability and weld hot cracking resistance of the alloy are improved. improve. This reaction is as follows. Ca combines with oxygen in the alloy to form CaO and CaO—Al ₂ O ₃ , and after the dissolved oxygen (free oxygen) in the alloy is almost zero, the remaining Ca reacts with S in the alloy. to generate CaS. On the other hand, excessive addition of Ca reduces ductility at high temperatures around 1100°C. Therefore, the upper limit of the Ca content is set to 0.0050%. The upper limit of the desirable content of Ca is 0.0045%.

Sn is an element that improves the corrosion resistance and high-temperature creep strength of steel when added in an amount of 0.0001% or more, preferably 0.005% or more, and can be added as necessary. However, addition of more than 0.05% deteriorates hot workability, so the upper limit is defined as 0.05%.

In addition, since Zn, Pb, and Bi significantly reduce the hot workability of single-phase austenite alloys, it is necessary to strictly define the upper limits of their contents. Preferably, Zn≤0.0010%, Pb≤0.0010%, and Bi≤0.0010%, and the total content of Zn, Pb, and Bi is set to 0.0010% or less.

Both Zr and Hf are added in an amount of 0.0001% or more, preferably 0.005% or more. By fixing P and S, they have the effect of improving solidification cracking susceptibility and high-temperature oxidation resistance of steel. can be added as On the other hand, addition of a large amount exceeding 0.5% degrades manufacturability such as hot workability and surface properties. Therefore, the upper limit of the amount of these added was set at 0.5%.

La, Ce, Nd, and Pr are elements that fix P and S by adding 0.0001% or more in total, preferably 0.0010% or more, and improve oxidation resistance and solidification cracking susceptibility. Additions exceeding 0.0050% in total promote an increase in TiC and increase the liquefaction cracking susceptibility of the steel. Therefore, the upper limit of the content of these elements is defined as 0.0050% in total. Methods of adding these elements include adding each metal or alloy, adding misch metal, and the like.

Next, the reasons for limiting the contents of the selected elements will be described.
W: 0.01-3.0%
W, like Mo, is an element that enhances the strength of the heat-resistant alloy, and can be added in an amount of 0.01% or more, preferably 0.05% or more, and more preferably 0.1% or more, if necessary. For the purpose of improving heat resistance in the steel of the present invention, the upper limit of the content is 3.0%.

Nb: 0.001-4.0%, Ta: 0.001-1.0%
Nb and Ta will be explained. Both Nb and Ta can be added as required, and have the effect of improving the high-temperature strength of steel by solid-solution strengthening or precipitation strengthening. Since excessive addition increases the susceptibility to solidification cracking, the upper limit of the Nb content was set at 4.0%, and the upper limit of the Ta content was set at 1.0%. A preferred upper limit of the content is 0.8%. The lower limit of the content of both Nb and Ta is 0.001%, preferably 0.01%, more preferably 0.03%. Moreover, the preferred content range for both Nb and Ta is 0.03% to 0.8%.

A steel plate with a thickness of 3 mm or more corresponds to the thick steel plate targeted by the present invention.

Example 1 is described below. After melting in an 80-ton electric furnace, Al, Ti, and optionally Ca are added in the secondary refining process, and the continuously cast steel ingot with a thickness of 400 mm and a width of 700 mm is cut into a length of 250 mm through a continuous casting process. Nine cut steel ingots with different chemical compositions were prepared. These steel ingots were divided into two at a position of 200 mm in thickness, and further divided into three pieces of 200 mm in thickness, 100 mm in width, and 250 mm in length, which were obtained by further dividing the steel ingot into three sections of 100 mm each in the width direction of 200 mm to 500 mm.

Heating before rough rolling and rough rolling (200 mm thickness / 80 mm thickness) are performed, intermediate heat treatment is performed between rough rolling and final hot rolling, further heating before main rolling and main rolling (80 mm thickness / 13 mm thickness) (final heat A steel plate with a thickness of 13 mm and a width of 130 mm was manufactured through the process of product heat treatment. If any one or all of the heating before rough rolling, the intermediate heat treatment, and the heating before main rolling satisfies the preferred manufacturing conditions of the present invention, the standard deviation of the solid solution Ti concentration distribution of the steel plate can be within the range of the present invention. .

The chemical composition values of the obtained steel sheets are divided into Tables 1-1 and 1-2. The balance of the components listed in Tables 1-1 and 1-2 is Fe and impurity elements, and all units are mass %. Blanks for the components shown in Tables 1-1 and 1-2 indicate impurity levels.

　Concentration distribution evaluation of solid solution Ti in steel was performed as follows. A prototype material having a thickness of 13 mm was cut out in the vicinity of the center of the plate width so that a cross section parallel to the rolling direction and the plate thickness direction became the observation surface, embedded in resin, and mirror-polished to the observation surface. On the observed surface, line analysis was performed by EPMA under the conditions of an acceleration voltage of 15 kV and a beam diameter of 7 μm in the plate thickness direction from the front surface layer to the back surface layer, and numerical data of the Ti concentration were collected at intervals of 7.44 μm. The collected numerical data are once averaged, and out of the individual data points, Ti data points showing concentrations 1.25 times or more of the average value are judged to be data points in which precipitates of TiN or TiC are detected and excluded. Then, the remaining data points were judged to be solid solution Ti concentration data and adopted. The average value and standard deviation of the Ti concentration were calculated again for the adopted data, and the standard deviation (%) of the solid-solution Ti concentration distribution in the sheet thickness direction was obtained. The EXCEL function used to calculate the standard deviation is STDTV. is P.

In addition, on the observation surface of this observation sample, the austenite grain boundaries were revealed by etching using a cerium nitrate solution, and the crystal grain size number was measured by a comparative method using plate I of ASTM E112 in arbitrary 5 fields of view. The average value was taken as the grain size number G.

For the restraint welding test, first, the front and back surfaces of the prototype material were ground by 0.5 mm each to a thickness of 12 mm, and two plates of 50 mm width x 100 mm length x 12 mm thickness were cut out. Next, take a V-shaped groove with a bevel angle of 30° and a root surface of 1.5 mm on one side of a 100 mm length, place the grooves on a thick plate made of SS400 with the grooves facing each other, and cover the entire circumference except the butted part. was welded and fixed. The TIG welding of the butt portion was performed using AWSERNiCr-3 as a welding material under the conditions of a current of 180 A, a voltage of 9.5 to 11.5 V, a welding speed of 10 cm/min, and a welding material supply speed of 35 cm/min. Five cross-sections of the welded portion of each specimen were observed, and the number of cross-sections in which cracks were observed was evaluated as the location of restraint cracks. A sample in which no cracks were observed in all five cross sections was judged to be good, and a sample in which cracks were observed in even one cross section was judged to be unsatisfactory.

Table 2 shows the steel plate symbol, steel ingot No. , heat treatment conditions, standard deviation of Ti concentration distribution by line analysis, measurement results of grain size number G, and restraint weld cracking test results are collectively shown.

As shown in Table 2, the steel sheets A, C, E, F, H, J, L, M, P, Q, and R, which satisfy the requirements of the present invention, showed no cracking in restraint welding. On the other hand, the steel sheets B, D, K, and O whose grain size number G was less than 1.0 and the steel sheets G, I, and N whose Ti concentration standard deviation was greater than 0.045% were restricted. One or more cracks were observed in the weld.

Example 2 is described below. A portion of the rolling material prepared in Example 1 (rolling material obtained by dividing a continuously cast steel ingot having a thickness of 400 mm into two at a position of 200 mm in thickness) was used as a rolling material in Example 2 as well. In Example 2, the rolling material was first soaked, then hot rolled from 200 mm thick to 60 mm thick by heating before rolling and hot rolling, and further subjected to product heat treatment to produce a thick steel plate of 60 mm thick. . The standard deviation of the solid solution Ti concentration distribution of the steel plate can be set within the range of the present invention if any one of the soaking and heating before rolling or the total of the conditions satisfies the preferred manufacturing conditions of the present invention.

Since the segregation of Ti occurs remarkably in the range of 1/4 part to 3/4 part of the thickness from the surface of the original slab corresponding to the equiaxed crystal region of the steel ingot (original slab), the obtained 60 mm thick steel plate (The back surface corresponds to the central portion of the thickness of the steel ingot (original slab)) was sliced at a position of 15 mm in thickness, and the back surface side of 2 mm and the slice cross-section side of 1 mm were ground to obtain a 12 mm thick trial material. EPMA line analysis was performed on the total thickness of the prototype material. The restraint welding test, grain size number evaluation, and EPMA analysis using the prototype material were performed in the same manner as in Example 1.

Table 3 shows the steel plate symbol, steel ingot No. , heat treatment conditions, measurement results of grain size number, standard deviation of Ti concentration distribution by line analysis, and restraint weld cracking test results are collectively shown.

As shown in Table 3, no cracks were observed in restraint welding in steel sheets a, c, e, g, i, k, l, o, p, and q that satisfied the requirements of the present invention. On the other hand, in the steel sheets b, d, j, and n in which the grain size number G was less than 1.0, and the steel sheets f, h, and m in which the standard deviation of the Ti concentration was greater than 0.045%, restraint One or more cracks were observed in the weld.

As can be seen from the above examples, it has become clear that the present invention can produce a high Ni alloy with excellent resistance to weld hot cracking.

INDUSTRIAL APPLICABILITY According to the present invention, it becomes possible to suitably manufacture a welded structure using a high-Ni alloy steel plate containing Al and Ti for high-temperature applications. Expected. Moreover, these alloys can be widely used not only for high temperature applications but also for welded structures used for high corrosion resistance applications.
It has become possible to provide stable welding quality in response to the expanding demand for high-Ni alloys, which will greatly contribute to the development of industry.

Claims

% by mass, C: 0.15% or less, Si: 0.05 to 1.0%, Mn: 0.05 to 2.0%, P: 0.035% or less, S: 0.0015% or less, Cr: 16-28%, Ni: 18-65%, Al: 0.01-1.0%, Ti: 0.15-1.5%, B: 0.0002-0.0030%, N: 0 .05% or less, O: 0.003% or less, Mo: 0.01-10%, Cu: 0.01-4.0%, Co: 0.01-3.0%, V: 0.01- 0.5%, Mg: 0.0050% or less, the balance being Fe and impurities,
The grain size number G ≥ 1.0 defined by JIS G 0552, and the standard deviation of the solid solution Ti concentration distribution in the plate thickness direction is 0.045% or less, and has excellent weld hot cracking resistance. High Ni alloy thick steel plate.
2. The weld hot cracking resistance according to claim 1, further comprising one or two selected from the group consisting of the following group A and group B in mass % instead of part of the Fe. Excellent high Ni alloy steel plate.
[Group A]
Ca: 0.0003-0.0050%, Sn: 0.0001-0.05%, Zn+Pb+Bi: 0.0010% or less, Zr: 0.0001-0.5%, Hf: 0.0001-0.5 %, La + Ce + Nd + Pr: one or more of 0.0001 to 0.0050% [Group B]
One or more of W: 0.01 to 3.0%, Nb: 0.001 to 4.0%, and Ta: 0.001 to 1.0%
A high-Ni alloy steel plate with excellent weld hot cracking resistance according to claim 1 or claim 2, which is used for welded structures.
A method for producing a high Ni alloy steel plate by using a steel ingot obtained by continuous casting and having a thickness of 160 mm or more and a slab thickness/product thickness ratio of 3.0 or more, comprising: 3. The high-temperature steel sheet having excellent resistance to weld hot cracking according to claim 1 or 2, characterized in that it is subjected to high-temperature long-term heat treatment in which the holding time at 1200 ° C. or more is secured for 8 hours or more in the stage before rolling. A method for producing a Ni alloy thick steel plate.
A method for producing a high Ni alloy steel plate by using a steel ingot obtained by continuous casting and having a thickness of 160 mm or more and a slab thickness/product thickness ratio of 3.0 or more, comprising: High-Ni alloy steel plate with excellent resistance to weld hot cracking according to claim 3, characterized in that a high-temperature long-term heat treatment is performed at a temperature of 1200 ° C. or more for a holding time of 8 hours or more in the previous stage of rolling. manufacturing method.