Thick plate made of high-Ni alloy with excellent resistance to high-temperature cracking during welding.

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

Technical Field

[0001] The present invention relates to a high Ni alloy thick steel plate excellent in weld heat crack resistance, which is used as a material for high temperature applications, and a method for manufacturing the same.

Background Art

[0002] As high Ni alloy steels containing Al and Ti, Alloy 800 and 825 are typical commercial alloy steels. In recent years, the demand in developing countries has been expanding, and there is a need for technological development to supply products that are inexpensive and have good surface quality and usage characteristics. For this reason, the conversion of the manufacturing method from the conventional ingot method to the continuous casting method has been promoted. However, since high Ni alloy steels are highly sensitive to internal cracks in the slab during casting, ear cracks during hot working, and surface defects of the product, improvements and developments in the alloy chemical composition design, steelmaking, casting, and hot working technologies have been advanced from the perspective of improving productivity in the continuous casting method.

[0003] As patent documents 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 for suppressing the occurrence of surface defects. Patent Document 2 discloses a method for preventing nozzle clogging and surface defects by a manufacturing method that does not add a Ca alloy. This document describes that there is a problem that by adding a Ca alloy, it binds with oxygen in the molten alloy to form oxide-based non-metallic inclusions, which aggregate and grow, leading to the occurrence of linear defects on the surface of the final product alloy plate. Patent Document 3 stipulates that in order to prevent the coarse aggregation of TiN that causes surface defects, a CaO-MgO-Al2O3-based inclusion is included as an essential component, and the ratio of the number of CaO and MgO to the total number of inclusions is 50% or less.

[0004] The above prior art defines the component system and the inclusion composition from the perspective of productivity, particularly surface defect suppression.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2003-147492 [Patent Document 2] Japanese Patent Publication No. 2014-189826 [Patent Document 3] Japanese Patent Publication No. 2018-59148 [Overview of the project] [Problems that the invention aims to solve]

[0006] The inventors of this invention have found that, in practical use of high-Ni alloy steel, there are not only problems with manufacturability, but also problems with low resistance to high-temperature cracking during welding due to its austenitic single-phase nature, making it prone to cracking during welding. High-temperature cracking can be broadly classified into solidification cracking that occurs in the molten metal and liquefaction cracking that occurs in the heat-affected zone (HAZ). The objective of this invention is to improve the resistance to high-temperature cracking during welding in Al,Ti-containing high-Ni alloy steel, which has not been studied conventionally, and in particular to stabilize the susceptibility to liquefaction cracking at a low level.

[0007] Al,Ti-containing high-Ni alloy steel is said to have relatively good hot workability. However, since cast slabs have a solidification structure, if they contain more than a few ppm of S, the hot workability of the cast slabs becomes insufficient. Therefore, it is necessary to improve the hot workability by adding small amounts of Ca alloy or Mg alloy. However, when high-Ni alloy steel, which is the subject of this invention, is continuously cast by adding Ca or Mg alloy, and steel materials are manufactured from the resulting slabs, blooms, or billets, when structures are manufactured using these steel materials by welding, thermal stress generated by the heat input during welding may cause hot cracking during welding. In particular, liquefaction cracking that occurs in the HAZ (heat-induced zone) can be a problem with Al,Ti-containing high-Ni alloy steel. To make matters worse, even if the composition system is such that steel plates manufactured using steel ingots melted in units of tens of kilograms can be welded well, liquefaction cracking occurs in the HAZ when steel plates manufactured using steel ingots melted and cast in units of several tons. Therefore, optimization is necessary not only in terms of composition but also in terms of histology. In particular, in thick steel plates obtained through hot rolling and product heat treatment processes, segregation is more likely to occur as the slab thickness increases, as the cooling rate inside the slab during casting slows down. This tendency is especially pronounced when using slabs of 160 mm or more as the base slab for manufacturing. On the other hand, in order to obtain a good recrystallized structure in high-alloy thick steel plates, the thickness of the base slab must be at least three times the thickness of the product, and in particular, a base slab of 160 mm or more is required to manufacture thick steel plates exceeding 50 mm in thickness. Therefore, when conducting industrial production with a repertoire of multiple plate thicknesses, it is desirable to consolidate production to base slabs with a thickness of 160 mm or more from the standpoint of manufacturing efficiency.

[0008] The present invention aims to provide a thick high-Ni alloy steel sheet with excellent resistance to high-temperature cracking during welding, and a method for manufacturing the same, which can solve the above-mentioned problems. The thick steel sheets defined in this invention are limited to hot-rolled steel sheets or steel sheets obtained by temper-rolling hot-rolled steel sheets; cold-rolled steel sheets are excluded from the scope of this invention. [Means for solving the problem]

[0009] To elucidate the causes and solve the above-mentioned problems, the inventors used an Al,Ti-containing high-Ni alloy steel, which is the subject of this invention, as the basic composition and performed actual melting with added Ca to produce cast slabs. Using the obtained cast slabs, steel materials were manufactured by hot rolling, annealing, and heat treatment in laboratory prototypes. Using the obtained steel materials, the susceptibility to liquefaction cracking during welding was evaluated by restrained welding cracking tests. In addition, research to solve the problems was conducted using methods such as EPMA analysis.

[0010] In the steel ingots of high-Ni alloy steel investigated by the inventors, TiC, TiN, or TiNC were found to be formed either individually or encompassing oxide-based inclusions. Of these, the inventors focused on the precipitation behavior of large-sized TiC, which acts as the initiation point for liquefaction cracking. In particular, regarding liquefaction cracking, the inventors found that there are regions within the plate thickness where Ti-containing precipitates accumulate locally in the Ti positive segregation zones, and that liquefaction cracking occurs starting from the TiC generated in these accumulation zones. They also found that TiC accumulation and liquefaction cracking occur when the concentration distribution of solid-solution Ti exceeds 0.045% by standard deviation, which led to the present invention.

[0011] In other words, the gist of this invention is as follows: (1) In mass%, C: 0.15% or less, Si: 0.05~1.0%, Mn: 0.05~2.0%, 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: 0.01~10%, A thick steel sheet with high resistance to high-temperature cracking during welding, characterized by containing Cu: 0.01~4.0%, Co: 0.01~3.0%, V: 0.01~0.5%, Mg: 0.0050% or less, with the remainder being Fe and impurities, having a grain size number G ≥ 1.0 as defined in JIS G0552, and a standard deviation of the solid solution Ti concentration distribution in the thickness direction being 0.045% or less. (2) The high-Ni alloy thick steel sheet with excellent resistance to weld high-temperature cracking described in (1), further comprising, by mass%, one or two selected from the group consisting of group A and group B below, in place of a portion of the Fe. [Group A] One or more of the following elements: 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: 0.0001-0.0050% [Group B] One or more of the following: W: 0.01-3.0%, Nb: 0.001-4.0%, Ta: 0.001-1.0% (3) A thick steel plate of high-Ni alloy with excellent resistance to high-temperature cracking during welding, as described in (1) or (2), used in welded structures.

[0012] (4) A method for manufacturing a thick high-Ni alloy steel sheet with excellent resistance to high-temperature cracking during welding, as described in (1) or (2), wherein the steel sheet is manufactured using a steel ingot obtained by continuous casting that is 160 mm or thicker and has a slab thickness / product thickness ratio of 3.0 or more, and is characterized by performing a high-temperature, long-duration heat treatment in the stage prior to the final hot rolling, in which a holding time of 8 hours or more at 1200°C or higher is ensured. (5) A method for manufacturing a thick high-Ni alloy steel sheet with excellent resistance to high-temperature cracking during welding, as described in (3), wherein the steel sheet is manufactured using a steel ingot obtained by continuous casting that is 160 mm or thicker and has a slab thickness / product thickness ratio of 3.0 or more, and is characterized by performing a high-temperature, long-duration heat treatment in the stage prior to the final hot rolling, in which a holding time of 8 hours or more at 1200°C or higher is ensured.

[0013] The present invention facilitates the stable manufacture of welded structures using Al,Ti-containing high-Ni alloy thick steel plates, which are used as high-temperature materials. In addition to excellent hot workability, it is possible to obtain Al,Ti-containing high-Ni alloy thick steel plates that are less prone to cracking in the heat-affected zone during the manufacture of welded structures, and have excellent creep characteristics and oxidation resistance at high temperatures. [Modes for carrying out the invention]

[0014] <Component composition> First, the reasons for limiting the content of the essential components of the present invention will be described below. The content of each component is expressed in mass%.

[0015] 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 characteristics are required, 0.015% or more, preferably 0.05% or more of C is added. The C content is limited to 0.15% or less. In this alloy, C exists in the alloy as TiC precipitates, but when the C content exceeds 0.15%, Cr carbides are formed, and the high-temperature characteristics and corrosion resistance deteriorate. Preferably, the C content is 0.10% or less, and more preferably 0.085% or less.

[0016] Si: 0.05 - 1.0% Si is added in an amount of 0.05% or more for deoxidation and improving oxidation resistance. However, when Si is added in an amount exceeding 1.0%, the solidification cracking susceptibility and liquation cracking susceptibility of the steel increase, and intermetallic compounds are likely to precipitate, resulting in deterioration of the high-temperature characteristics. Therefore, the upper limit of the Si content is limited to 1.0%. The preferred upper limit is 0.7%, and the more preferred upper limit is 0.5%.

[0017] Mn: 0.05 - 2.0% Mn has the effect of increasing the stability of the austenite phase and improving heat resistance. Therefore, it is preferable to actively add Mn in the alloy of the present invention. 0.05% or more of Mn is added to improve the heat-resistant characteristics. However, when Mn is added in an amount exceeding 2.0%, on the contrary, intermetallic compounds are likely to precipitate, the heat-resistant characteristics deteriorate, and it also has an adverse effect on the solidification cracking susceptibility. Therefore, the upper limit of the Mn content is defined as 2.0%. The preferred upper limit is 1.5%, and the more preferred upper limit is 1.3%.

[0018] P: 0.035% or less P is an element inevitably mixed in from raw materials and has the effect of increasing the solidification cracking susceptibility. Therefore, the P content is limited to 0.035% or less. Preferably, it is 0.030% or less.

[0019] S: Below 0.0015% S is an element inevitably mixed in from raw materials, which also deteriorates hot workability and oxidation resistance. Therefore, the S content is limited to 0.0015% or less, preferably 0.0010% or less. Although the content of S can be reduced by refining, an extreme reduction in content will increase costs. Therefore, it is preferable that the lower limit of the S content is 0.0001%.

[0020] Cr: 16 - 28% Cr is an essential element for improving the oxidation resistance of heat-resistant alloys as high-temperature materials, and it is contained at 16% or more, preferably 18% or more. On the other hand, if it is contained in excess of 28%, even if a large amount of Ni is contained, the high-temperature tissue stability will decrease, intermetallic compounds will precipitate, and the heat-resistant characteristics will deteriorate. The preferable upper limit value is 26%. The optimal content varies depending on the contents of Ni, Si, Mo and other elements. For example, when the Ni content is about 30%, the Cr content is optimally about 20%. Alternatively, when Ni + Cu is about 45%, the optimal content of Cr + Mo is about 25%.

[0021] Ni: 18 - 65% Ni stabilizes the austenite structure at high temperatures and also improves corrosion resistance and toughness against various acids. Therefore, it is contained at 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, Ti necessary for heat-resistant characteristics. On the other hand, Ni is an expensive alloy, and in the steel of the present invention, the upper limit is defined as 65% or less from the viewpoint of cost.

[0022] Al: 0.01 - 1.0% Al is a deoxidizing element and also plays a role in increasing high-temperature strength by forming a NiAl ordered phase in high-Ni alloys. In this invention, in order to control the oxide composition and improve hot workability, it is necessary to contain 0.01% or more, preferably 0.05% or more, of Al. On the other hand, if the Al content exceeds 1.0%, intermetallic compounds tend to precipitate, which impairs the heat resistance properties. Furthermore, excessive Al content increases susceptibility to liquefaction cracking during welding. For this reason, the upper limit of the Al content has been set at 1.0%. The preferred upper limit is 0.60%.

[0023] Ti: 0.15~1.5% Ti has the effect of forming a NiTi ordered phase in high-Ni alloys, thereby increasing their high-temperature strength. For this purpose, a Ti content of 0.15% or more, preferably 0.2% or more, is necessary. On the other hand, if the Ti content exceeds 1.5%, intermetallic compounds tend to precipitate, which impairs the heat resistance properties. Furthermore, excessive content increases susceptibility to liquefaction cracking during welding. The preferred upper limit for the Ti content is 1.0%, and more preferably 0.85%.

[0024] B: 0.0002~0.0030% B is an element that improves the hot workability of steel, significantly improving the reduction of area in the high-temperature range of hot working. In addition, B is actively added, especially in high-temperature environments, to improve high-temperature creep strength. The mechanism by which B improves hot workability is not clear, but it is said to increase grain boundary strength by segregating at grain boundaries. The effect of B in improving hot tensile strength appears when the B content is 0.0002% or higher, so the lower limit for B addition is set at 0.0002%. On the other hand, excessive addition promotes solidification cracking, so the upper limit for its content is set at 0.0030%. The preferred upper limit is 0.0015%.

[0025] N: 0.05% or less N is an element effective in improving high-temperature strength. However, in this invention, since Ti and Al are actively added, N forms AlN or TiN, becoming a nonmetallic inclusion that degrades material properties and also becomes a harmful element that promotes nozzle clogging during continuous casting by compounding with oxides. For this reason, the N content should be 0.05% or less. Preferably, it should be 0.04% or less, and even more preferably 0.03% or less.

[0026] 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 deoxidation state of the alloy. If the oxygen content exceeds 0.003%, the desired deoxidation equilibrium will not be satisfied, and nozzle clogging during continuous casting will be more likely to occur. In addition, it will have an adverse effect on the high-temperature crack resistance of the weld, which is the core of the present invention, by promoting the generation of coarse TiC, which acts as the starting point for liquefaction cracking. For this reason, the upper limit of the oxygen content has been set at 0.003%. A preferred upper limit is 0.0025%, and a more preferred upper limit is 0.002%. On the other hand, reducing the oxygen content is advantageous in suppressing nozzle clogging and high-temperature cracking of the weld by reducing oxide inclusions and inclusions containing coarse TiC, but it generates excess Ca and excess Mg in the alloy, which is a factor in reducing hot workability. For this reason, an oxygen content of 0.0003% or more is preferable.

[0027] Mo: 0.01~10% Mo is an element that enhances the high-temperature strength and corrosion resistance of alloys, and to improve these properties, the alloy should contain 0.01% or more, preferably 0.05% or more, and more preferably 0.15% or more of Mo. On the other hand, Mo is an expensive element, and in the steel of this invention, the Mo content is limited to 10% from the viewpoint of suppressing alloy costs. The preferred upper limit for the Mo content is 3.0%, and the more preferred upper limit is 2.0%.

[0028] Cu: 0.01~4.0% Cu is an element that enhances the corrosion resistance of alloys to acids and dew point corrosion resistance, which is often a problem in high-temperature equipment, and also improves high-temperature strength and structural stability. To improve these heat resistance and corrosion resistance properties, Cu is included at a concentration of 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more. On the other hand, if Cu is included at a concentration exceeding 4.0%, embrittlement will occur during solidification, so the upper limit for Cu content is set at 4.0%. The preferred upper limit for Cu is 3.0%, and the more preferred upper limit is 2.0%.

[0029] Co: 0.01~3.0% Co is an effective element for improving the high-temperature structural stability and corrosion resistance of alloys. To improve these properties, the alloy should contain 0.01% or more, preferably 0.02% or more, and more preferably 0.1% or more of Co. Since Co is an expensive element, exceeding 3.0% would not justify the cost; therefore, the upper limit for Co content is set at 3.0%. The preferred upper limit for Co is 1.5%.

[0030] V: 0.01~0.5% Adding 0.01% or more of V improves the high-temperature properties of the alloy through solid solution strengthening or precipitation strengthening. On the other hand, adding more than 0.5% of V increases susceptibility to solidification cracking. The preferred lower limit of V content is 0.02%, and more preferably 0.03%. The preferred range of V content is 0.03% to 0.5%.

[0031] Mg: 0.0050% or less Mg is an element that generally, even in trace amounts, can improve the hot workability of alloys. In the present invention, adding Mg has an adverse effect of promoting the formation of MgO-based inclusions that increase the susceptibility to liquation cracking during welding. In addition, excess Mg that does not form oxides segregates at grain boundaries and reduces the grain boundary strength in the high-temperature range (e.g., 900 °C). This results in a decrease in hot workability and an increase in the susceptibility to liquation cracking in the high-temperature range. When deoxidation strengthening is carried out in the production of the steel of the present invention, inevitable pick-up of Mg occurs from the slag, furnace wall, etc. From the above findings, in the present invention, it is necessary to reduce the Mg content as much as possible, and no alloy addition of Mg is carried out. The upper limit of the Mg content is set at 0.0050%. The preferred upper limit is 0.0040%.

[0032] <The crystal grain size number G defined in JIS G0552 is G ≥ 1.0> P, S, and Mg that lower the melting point of the steel segregate at the crystal grain boundaries of austenitic high-alloy steel. As the particle size increases, the ratio of grain boundaries in the total volume decreases, and accordingly, the concentrations of P, S, and Mg at the grain boundaries increase. This causes a decrease in the melting point of the grain boundaries and an increase in the susceptibility to liquation cracking during welding. As a result of intensive studies, it was found that the susceptibility to liquation cracking increases regardless of the concentration distribution of dissolved Ti when the crystal grain size number G < 1.0. Therefore, the crystal grain size number G is defined as G ≥ 1.0. Although no specific upper limit is defined, when the crystal grain size number G exceeds 8, the high-temperature creep strength decreases. Therefore, the preferred range of the crystal grain size number G is 1 to 8. In particular, for applications where creep strength is required, the range of the crystal grain size number G is 1 to 6, preferably 1 to 5. For applications where intergranular corrosion resistance, steam oxidation resistance, and high-temperature corrosion resistance are required, it is most preferable to use different ranges such as 3 to 8 according to the application.

[0033] <The standard deviation of the dissolved Ti concentration distribution ≤ 0.045%> In heat-resistant high alloys with added Ti, TiC precipitation is inevitable. Here, we will explain the process of TiC formation. In the high-temperature liquid phase, TiN preferentially forms, while TiC precipitates from the solid-liquid coexistence region to the solid phase region. Most TiC precipitates as fine particles of about 0.2 μm or less, but some coarse up to several tens of μm. When precipitates containing such coarse TiC exist at grain boundaries, the heat input during welding causes C and Ti in the TiC to diffuse into the matrix, lowering the melting point of the TiC / matrix interface and becoming the starting point for liquefaction cracking that occurs in the heat-affected zone (HAZ). If the Ti dissolved in the steel is not uniformly dispersed, many TiC precipitates of about 1 μm to several μm accumulate locally, and fine grains of about 10 μm to several tens of μm are pinned to the accumulated precipitates and locally formed. It has become clear that liquefaction cracking during welding occurs when numerous coarse TiC particles present at the grain boundaries of the fine particles generated in this way liquefy through eutectic melting. In other words, to improve the weldability of Ti-containing heat-resistant high alloys, it is essential not only to change the composition but also to minimize the segregation of Ti in the steel sheet.

[0034] As an indicator of the degree of Ti segregation, the inventors focused on the concentration distribution of solid-solution Ti in steel. That is, by measuring a point in any cross-section of the steel where TiC and TiN are not formed using EPMA, EDX, etc., the analysis result of the solid-solution Ti concentration at that single point can be obtained. In the case of steel sheets, since the fluctuation in Ti concentration due to segregation occurs in the thickness direction, the method involves performing a line analysis in the thickness direction of the sheet using EPMA or EDX in any cross-section to collect numerical data of Ti concentration, and then calculating the standard deviation of the data after removing the numerical data from the TiC or TiN formation areas. As a result of investigating the standard deviation of solid-solution Ti in this way, it was found that the susceptibility to liquefaction cracking of Ti-containing high-alloy steel sheets is significantly reduced when the standard deviation of Ti concentration is 0.045% or less. Preferably, the standard deviation is 0.040% or less. For line analysis measurements in the thickness direction of the cross-section, the sample is taken from a position corresponding to a length of 1 / 2t inside the widthwise end of the original slab (cast steel ingot), with the original slab's thickness being t. The measurement is taken over the entire thickness from the surface to the back in the thickness direction. However, depending on the specifications of the analytical instrument and the thickness of the plate, it may be difficult to measure the entire thickness. In such cases, the analysis length at a point in the original slab where the thickness from the surface corresponds to 1 / 4t to 3 / 4t is 50% or more of the total analysis length (equivalent to 50% when the entire thickness of the product plate is line-analyzed), and the analysis length is 10 mm or more. If the thickness of the final product is less than 10 mm, the entire thickness of the product plate is line-analyzed.

[0035] <Heating and holding conditions before final hot rolling: Held at 1200°C or higher for 8 hours or more> The manufacturing process for thick high-alloy steel plates typically involves refining, casting, hot rolling, and heat treatment in batches ranging from several tons to over 100 tons before being refined and produced as a finished product. Refining is carried out in the following order: melting in an electric furnace, rough decarburization in a converter, VOD, AOD, or a combination of both for final decarburization. Casting is then performed using 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 its properties and then rolled to a predetermined plate thickness. However, because high-alloy steel thick plates have high resistance to hot deformation, this process may be repeated multiple times to reach the desired product thickness, after which the product undergoes heat treatment and refinement before being produced as a finished product.

[0036] Reducing Ti segregation in high-alloy steel sheet products requires optimizing the cooling rate during solidification and electromagnetic stirring conditions. However, completely eliminating segregation through these measures alone is difficult when manufacturing steel ingots weighing several to tens of tons or more. Eliminating segregation in slabs with a thickness of 160 mm or more, where the cooling rate is slow, is particularly difficult. Therefore, if segregation cannot be removed by steelmaking measures alone, it is necessary to perform high-temperature, long-duration heat treatment during the post-casting heat treatment process. However, performing this heat treatment at the product heat treatment stage can cause excessive growth of the steel sheet's crystal grains, conversely increasing its susceptibility to liquefaction cracking.

[0037] After thorough investigation, it became clear that in order to achieve the requirements of the present invention, it is necessary to perform a high-temperature, long-duration heat treatment at 1200°C or higher, preferably 1230°C or higher, for 8 hours or more, preferably 15 hours or more, in the stage prior to the final hot rolling. The high-temperature, long-duration heat treatment may be performed in the stage prior to the first rolling. More preferably, the effect is most effectively achieved by performing the high-temperature, long-duration heat treatment in a state where the spacing of segregation zones is narrow and high-speed diffusion pathways such as dislocations and recrystallization grain boundaries are introduced as much as possible. Therefore, in the case of multiple rolling processes, the most preferred embodiment is to introduce the high-temperature, long-duration heat treatment process between the rolling process immediately preceding the final rolling (rough rolling) and the final rolling process (main rolling).

[0038] In the heat treatment of products 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 becomes. To achieve a grain size number G of 1.0 or higher in the steel sheet, this can be achieved by adjusting the temperature and time of the heat treatment of the product after hot rolling, according to the composition of the steel sheet.

[0039] The component composition of the high-Ni alloy of the present invention contains the aforementioned components, with the remainder being Fe and impurities. Furthermore, the following components (mass%) may be selectively included in place of a portion of the Fe. Next, the reasons for limiting the content of the selected elements will be explained. <Component composition> 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: 0.0001~0.0050%

[0040] By including Ca in an amount of 0.0003% or more, preferably 0.0010% or more, and more preferably 0.0015% or more, the sulfur in the alloy is fixed as CaS, improving the hot workability and resistance to high-temperature cracking during welding of the alloy. This reaction proceeds as follows: Ca combines with oxygen in the alloy to produce CaO and CaO-Al2O3, reducing the dissolved oxygen (free oxygen) in the alloy to almost zero. After that, the remaining Ca reacts with the sulfur in the alloy to produce CaS. On the other hand, excessive Ca addition reduces ductility at high temperatures around 1100°C. For this reason, the upper limit of the Ca content was set to 0.0050%. The desirable upper limit of the Ca content is 0.0045%.

[0041] 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 needed. However, since adding more than 0.05% reduces hot workability, the upper limit is set at 0.05%.

[0042] Furthermore, since Zn, Pb, and Bi significantly reduce the hot workability of austenitic single-phase alloys, it is necessary to strictly define the upper limits of their content. Preferably, Zn ≤ 0.0010%, Pb ≤ 0.0010%, and Bi ≤ 0.0010%, with the total content of Zn, Pb, and Bi specified as 0.0010% or less.

[0043] Adding Zr and Hf in amounts of 0.0001% or more, preferably 0.005% or more, fixes P and S, thereby improving the steel's solidification crack susceptibility and high-temperature oxidation resistance, and can be added as needed. On the other hand, adding large amounts exceeding 0.5% reduces manufacturability such as hot workability and surface properties. Therefore, the upper limit for these additions is set at 0.5%.

[0044] La, Ce, Nd, and Pr are elements that improve oxidation resistance and solidification crack susceptibility by fixing P and S when added in a total of 0.0001% or more, preferably 0.0010% or more. On the other hand, adding more than 0.0050% in total promotes an increase in TiC and increases the steel's susceptibility to liquefaction cracking. Therefore, the upper limit of the content is set at 0.0050% in total for these elements. Methods for adding these elements include adding them as individual metals or alloys, or adding them as mischmetals.

[0045] Next, I will explain the reasons for limiting the content of selected elements. W: 0.01~3.0% W, like Mo, is an element that enhances the strength of heat-resistant alloys, and can be added as needed in amounts of 0.01% or more, preferably 0.05% or more, and more preferably 0.1% or more. In the steel of the present invention, it is included at a maximum of 3.0% for the purpose of enhancing heat resistance.

[0046] Nb: 0.001~4.0%, Ta: 0.001~1.0% This section explains Nb and Ta. Both Nb and Ta can be added as needed and have the effect of improving the high-temperature strength of steel through solid solution strengthening or precipitation strengthening. Excessive addition increases susceptibility to solidification cracking, so the upper limit for Nb content is set at 4.0% and the upper limit for Ta content is set at 1.0%. The preferred upper limit for both content is 0.8%. The lower limit for the content of either Nb or Ta is 0.001%, preferably 0.01%, and more preferably 0.03%. Furthermore, the preferred content range for either Nb or Ta is 0.03% to 0.8%.

[0047] The thick steel plates covered by this invention include steel plates with a thickness of 3 mm or more. [Examples]

[0048] The following describes Example 1. After melting in an 80-ton electric furnace, Al, Ti, and selectively Ca were added in a secondary refining process, and after a continuous casting process, nine steel ingots with different chemical compositions were prepared by cutting the 400 mm thick, 700 mm wide continuously cast steel ingots into 250 mm lengths. These steel ingots were divided in half at the 200 mm thickness point, and then further divided into three 100 mm sections within the 200 mm to 500 mm width direction of the steel ingot, resulting in cast slabs with a thickness of 200 mm, a width of 100 mm, and a length of 250 mm, which were used as the rolling material.

[0049] Pre-rolling heating and rough rolling (200mm thickness / 80mm thickness) were performed, intermediate heat treatment was carried out between rough rolling and final hot rolling, and then pre-rolling heating and final rolling (80mm thickness / 13mm thickness) (final hot rolling) were performed, followed by a product heat treatment process to produce a 13mm thick, 130mm wide steel plate. If any or all of the pre-rolling heating, intermediate heat treatment, and pre-rolling heating are met, the standard deviation of the solid solution Ti concentration distribution of the thick steel plate can be kept within the range of the present invention.

[0050] The chemical composition values ​​of the obtained steel sheets are shown in Tables 1-1 and 1-2. The components listed in Tables 1-1 and 1-2 consist of Fe and impurity elements, and all units are in mass percent. Blank spaces in Tables 1-1 and 1-2 indicate impurity levels.

[0051] [Table 1-1]

[0052] [Table 1-2]

[0053] The concentration distribution of solid-solution Ti in steel was evaluated as follows. A 13mm thick prototype plate was cut out near the center of the plate width, with a cross-section parallel to the rolling direction and the thickness direction serving as the observation surface. This section was embedded in resin and the observation surface was mirror-polished. Line analysis was performed on the observation surface from the surface to the back surface using EPMA in the thickness direction under the conditions of an acceleration voltage of 15kV and a beam diameter of 7μm, and numerical data of Ti concentration were collected at 7.44μm intervals. The collected numerical data was first averaged, and data points showing a Ti concentration of 1.25 times or more the average were judged to be data points where TiN or TiC precipitates were detected and were excluded. The remaining data points were judged to be solid-solution Ti concentration data and were adopted. The average value and standard deviation of the Ti concentration were calculated again for the adopted data and were used as the standard deviation (%) of the solid-solution Ti concentration distribution in the thickness direction. The Excel function used to calculate the standard deviation was STDTV.P.

[0054] Furthermore, on the observation surface of this observation sample, austenite grain boundaries were revealed by etching with cerium nitrate solution, and the grain size number was measured using a comparative method with ASTME112 plate I in any five fields of view. The average value of these measurements was defined as grain size number G.

[0055] For the confined welding test, first, the front and back surfaces of the prototype material were ground down by 0.5 mm each to a thickness of 12 mm, and two plates measuring 50 mm wide x 100 mm long x 12 mm thick were cut out. Next, a V-groove with a bevel angle of 30° and a root face of 1.5 mm was made on one side of the 100 mm length plate, and the two grooves were butted together and placed on a thick SS400 plate. The entire circumference, excluding the butt joint, was welded and fixed in place. TIG welding of the butt joint was performed using AWSERNiCr-3 as the welding material, under the conditions of a current of 180 A, a voltage of 9.5~11.5 V, a welding speed of 10 cm / min, and a welding material supply speed of 35 cm / min. The welded area of ​​each test specimen was observed in 5 cross-sections, and the number of cross-sections in which cracks were observed was evaluated as the location of confined cracks. A specimen in which no cracks were observed in any of the 5 cross-sections was judged as good, and a specimen in which a crack was observed in even one cross-section was judged as poor.

[0056] Table 2 summarizes the steel plate symbol, steel ingot number, heat treatment conditions, standard deviation of the Ti concentration distribution obtained by line analysis, measurement results for grain size number G, and the results of the constrained weld cracking test.

[0057] [Table 2]

[0058] As shown in Table 2, no cracks were observed in steel plates A, C, E, F, H, J, L, M, P, Q, and R, which satisfy the requirements of the present invention, during restrained welding. On the other hand, in steel plates B, D, K, and O, where the grain size number G was less than 1.0, and in steel plates G, I, and N, where the standard deviation of the Ti concentration was greater than 0.045%, one or more cracks were observed during restrained welding. [Examples]

[0059] The following describes Example 2. A portion of the rolled material prepared in Example 1 (rolled material obtained by dividing a 400mm thick continuous cast steel ingot into two at a thickness of 200mm) was also used as the rolled material in Example 2. In Example 2, the rolled material was first soaked, then hot-rolled from 200mm thick to 60mm thick by pre-rolling heating and hot rolling, and then subjected to product heat treatment to produce a 60mm thick steel plate. If either soaking, pre-rolling heating, or both satisfy the preferred manufacturing conditions of the present invention, the standard deviation of the solid solution Ti concentration distribution of the thick steel plate can be kept within the range of the present invention.

[0060] Ti segregation occurs significantly in the area from the surface to 1 / 4 to 3 / 4 of the thickness of the original slab, which corresponds to the equiaxed region of the steel ingot (original slab). Therefore, a 60 mm thick steel plate was sliced ​​at a thickness of 15 mm from the back surface (where the back surface corresponds to the center of the thickness of the steel ingot (original slab)), and the back surface side (2 mm) and the sliced ​​cross-section side (1 mm) were ground to obtain a 12 mm thick prototype. EPMA linear analysis was performed on the entire thickness of the prototype. Constrained welding tests, grain size number evaluation, and EPMA analysis using the prototype were performed in the same manner as in Example 1.

[0061] Table 3 summarizes the steel plate symbol, steel ingot number, heat treatment conditions, measurement results for grain size number, standard deviation of Ti concentration distribution by line analysis, and restrained weld cracking test results.

[0062] [Table 3]

[0063] As shown in Table 3, no cracks were observed in steel plates a, c, e, g, i, k, l, o, p, and q, which satisfied the requirements of the present invention, during restrained welding. On the other hand, in steel plates b, d, j, and n, where the grain size number G was less than 1.0, and in steel plates f, h, and m, where the standard deviation of the Ti concentration was greater than 0.045%, one or more cracks were observed during restrained welding.

[0064] As can be seen from the above examples, it has become clear that the present invention makes it possible to manufacture a high-Ni alloy with excellent resistance to high-temperature cracking during welding. [Industrial applicability]

[0065] The present invention makes it possible to suitably manufacture welded structures using thick steel plates of high-nickel alloy containing Al and Ti for high-temperature applications, which is expected to improve design flexibility and reduce welding repair costs. Furthermore, these alloys can be widely used not only for high-temperature applications but also for welded structures used in high-corrosion-resistant applications. By enabling us to provide stable welding quality to meet the growing demand for high-Ni alloys, we can make a significant contribution to the development of the industry.

Claims

1. In mass%, it contains C: 0.10% or less, Si: 0.05-1.0%, Mn: 0.05-2.0%, 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: 0.0015% to 0.0050%, with the remainder being Fe and impurities. A thick high-Ni alloy plate with excellent resistance to high-temperature cracking during welding, characterized by having a grain size number G ≥ 1.0 as defined in JIS G0552, and a standard deviation of the solid solution Ti concentration distribution in the plate thickness direction of 0.045% or less.

2. The high-Ni alloy thick plate with excellent resistance to weldable high-temperature cracking according to claim 1, characterized in that a portion of the Fe is further replaced with one or two selected from the group consisting of group A and group B below, in mass percent. [Group A] One or more of the following: 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: 0.0001–0.0050% [Group B] One or more of the following: W: 0.01–3.0%, Nb: 0.001–4.0%, Ta: 0.001–1.0%

3. A thick plate of high-Ni alloy with excellent resistance to high-temperature cracking during welding, as described in claim 1 or claim 2, for use in welded structures.

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