Stainless steel material for diffusion bonding, and manufacturing method and diffusion bonding method therefor

Abstract

One aspect of the present invention is to provide a stainless steel material having excellent diffusion bonding properties and a method for manufacturing same. In addition, another aspect of the present invention is to provide a method for diffusion bonding of the steel material capable of securing high strength and excellent low-temperature impact toughness.

Description

Stainless steel material for diffusion bonding, its manufacturing method, and diffusion bonding method

[0001] The present invention relates to a stainless steel material for diffusion bonding, a method for manufacturing the same, and a diffusion bonding method.

[0002] Printed Circuit Heat Exchangers (PCHEs) are attracting attention as compact heat exchangers for cryogenic, high-pressure environments such as liquefied natural gas (LNG) vessels and hydrogen charging stations. Typically manufactured from materials with superior properties, such as stainless steel and nickel alloys, PCHEs offer the advantage of being usable in high-temperature, high-pressure, and cryogenic environments where conventional heat exchangers are inapplicable. Therefore, their applicability is being actively explored in diverse industries, including the oil and gas industry, hydrogen stations, and sCO2 power generation systems.

[0003] A PCHE is a compact heat exchanger manufactured by diffusion bonding metal plates with microchannels fabricated using a photochemical etching process in a vacuum, high temperature, and high pressure environment. Compared to conventional shell-and-tube heat exchangers, this significantly increases the heat transfer surface area, enabling lighter and more compact heat exchangers.

[0004] These PCHE cores can be manufactured by photochemically etching a path on a stainless steel plate and then diffusion bonding.

[0005] According to one embodiment of the present invention, a stainless steel material having excellent diffusion bondability and a method for manufacturing the same can be provided.

[0006] According to another embodiment of the present invention, a diffusion bonding method of the above steel material can be provided that can secure high strength and excellent low-temperature impact toughness.

[0007] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall content of this specification.

[0008] According to one embodiment of the present invention, a stainless steel material for diffusion bonding comprises, in wt%, carbon (C): 0.05% or less, nitrogen (N): 0.10% or less, silicon (Si): 1.00% or less, aluminum (Al): 0.150% or less, phosphorus (P): 0.04% or less, sulfur (S): 0.010% or less, chromium (Cr): 17 to 25%, manganese (Mn): 2.0% or less, and nickel (Ni): 8.0 to 12.0%, with the remainder being Fe and unavoidable impurities, and the average grain size at the center of the thickness may be 10.00 ㎛ or less, and the arithmetic mean roughness Ra of the steel surface may be less than 0.10.

[0009] The above-described stainless steel for diffusion bonding may further include one or more elements selected from the following a) to c).

[0010] a) Total content of at least one of Mo and Cu: 0.10 to 2.00%,

[0011] b) Nb: 0.010~0.500%

[0012] c) Ti: 0.010%~0.500%

[0013] The stainless steel material for diffusion bonding described above may have a 10-point average roughness Rz of the steel surface of 1.00 or less.

[0014] The stainless steel material for diffusion bonding described above may have a 20° mirror gloss of 200.0 or higher on the steel surface.

[0015] The stainless steel material for diffusion bonding described above may have a tensile strength of 400 MPa or more after diffusion bonding, and a low-temperature impact toughness at -196°C of 10 J or more.

[0016] The stainless steel material for diffusion bonding described above may have an arithmetic mean roughness Ra of a euro-etched surface after euro-etching of 0.35 or less and a 10-point average roughness Rz of 2.50 or less.

[0017] A method for manufacturing a stainless steel for diffusion bonding according to another embodiment of the present invention may include a step of cold-rolling a hot-rolled material to obtain a cold-rolled material; a cold-rolling annealing step of annealing the cold-rolled material; and a step of pickling the annealed cold-rolled material. In addition, the hot-rolled material described above may include, in wt%, carbon (C): 0.05% or less, nitrogen (N): 0.10% or less, silicon (Si): 1.00% or less, aluminum (Al): 0.150% or less, phosphorus (P): 0.04% or less, sulfur (S): 0.010% or less, chromium (Cr): 17 to 25%, manganese (Mn): 2.0% or less, and nickel (Ni): 8.0 to 12.0%, with the remainder being iron and unavoidable impurities. In addition, the cold rolling described above may include first cold rolling with a reduction ratio per pass of 25% or more and second cold rolling with a total reduction ratio of 40% or more, and the annealing temperature during the cold rolling annealing described above may be 800 to 1000°C.

[0018] In addition, a method for manufacturing a stainless steel for diffusion bonding according to one embodiment of the present invention may additionally include, before the step of obtaining the cold-rolled material, a step of preparing a stainless steel molten metal in an electric furnace; a step of refining the prepared stainless steel molten metal; a step of continuously casting the refined stainless steel molten metal to obtain an ingot; a step of hot-rolling the ingot to obtain a hot-rolled material; a hot-rolling annealing step of annealing the hot-rolled material; and a step of pickling the annealed hot-rolled material; and the annealing temperature in the hot-rolling annealing step described above may be 1100 to 1200°C.

[0019] The annealing time in the cold rolling annealing step described above may be 2.5 to 3.5 minutes.

[0020] A diffusion bonding method of a stainless steel material for diffusion bonding according to another embodiment of the present invention may include the steps of preparing the above-described stainless steel material for diffusion bonding; and the step of diffusion bonding the stainless steel material for diffusion bonding; wherein the step of diffusion bonding may have an A value of 50.0 or less derived by the following relational expression 1.

[0021] [Relationship 1] A = (P×t×T) / (TS×IE)

[0022] (In the above relational expression 1, P, t, and T represent the applied pressure (MPa), the bonding time (minutes), and the bonding temperature (℃) in the diffusion bonding step, respectively, and TS and IE represent the tensile strength (MPa) measured in the vertical direction of the interface of the bonded portion of the steel obtained after the diffusion bonding step, and the low-temperature impact toughness (J) at -196℃, respectively. However, the unit of A is omitted.)

[0023] According to one embodiment of the present invention, a stainless steel material having excellent diffusion bondability and a method for manufacturing the same can be provided.

[0024] In addition, according to another embodiment of the present invention, a diffusion bonding method can be provided that can significantly improve productivity while ensuring excellent strength characteristics and low-temperature toughness of a diffusion bond.

[0025] Figure 1 shows SEM photographs of the interface structure of the joint according to the diffusion bonding time in Invention Example 4 and Comparative Example 4, along with the defect rate.

[0026] Figure 2 is a schematic diagram briefly showing the positions of the depth, width and distance of the euro pattern together with the position of the euro etching surface.

[0027] Figure 3 is a photograph of the cross-section of the diffusion joint of Invention Example 6, Comparative Example 6, and Comparative Example 7 observed through an optical microscope.

[0028] Figure 4 is a scanning electron microscope (SEM) photograph of the joint interface of the diffusion joints of Invention Example 6, Comparative Example 6, and Comparative Example 7.

[0029] Hereinafter, preferred embodiments of the present invention will be described with reference to the attached drawings. However, the embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0030] In addition, the embodiments of the present invention are provided to more completely explain the present invention to a person having average knowledge in the relevant technical field.

[0031] The shape and size of elements in the drawing may be exaggerated for clearer explanation.

[0032] In describing the embodiments of the present invention, if a detailed description of a known technology related to the present invention is judged to unnecessarily obscure the gist of the present invention, such detailed description will be omitted. In addition, the terms described below are terms defined in consideration of their functions in the present invention, and these may vary depending on the intention or custom of the user or operator. Therefore, the definitions should be made based on the contents throughout this specification. The terminology used in the detailed description is only for the purpose of describing the embodiments of the present invention and should in no way be limiting. Unless clearly defined otherwise, expressions in the singular form include plural meanings.

[0033] In this description, expressions such as "including" or "having" are intended to indicate certain features, numbers, steps, operations, elements, portions or combinations thereof, and should not be construed to exclude the presence or possibility of one or more other features, numbers, steps, operations, elements, portions or combinations thereof other than those described.

[0034] Unless otherwise specified in the specification of the present invention, the % unit means weight %.

[0035] Below, the present invention will be described in detail through each embodiment or example of the present invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may be combined with other embodiments or examples. Therefore, the citation of a claim in the patent claims is only an example of an embodiment, and the technical concept of the present invention should not be interpreted solely as a combination with the cited claim, and combinations with various claims also fall within the scope of the technical concept of the present invention.

[0036] According to one embodiment of the present invention, a stainless steel material may contain, in wt%, carbon (C): 0.05% or less, nitrogen (N): 0.10% or less, silicon (Si): 1.00% or less, aluminum (Al): 0.150% or less, phosphorus (P): 0.04% or less, sulfur (S): 0.010% or less, chromium (Cr): 17 to 25%, manganese (Mn): 2.0% or less, and nickel (Ni): 8.0 to 12.0%, with the remainder being Fe and unavoidable impurities, and an average grain size at the center of the thickness may be 10 μm or less, and an arithmetic mean roughness Ra of the steel surface may be less than 0.10. Hereinafter, each component will be described in detail.

[0037] A stainless steel material according to one embodiment of the present invention may contain, in wt%, carbon (C): 0.05% or less, nitrogen (N): 0.10% or less, silicon (Si): 1.00% or less, aluminum (Al): 0.150% or less, phosphorus (P): 0.04% or less, sulfur (S): 0.010% or less, chromium (Cr): 17 to 25%, manganese (Mn): 2.0% or less, and nickel (Ni): 8.0 to 12.0%, with the remainder being iron and unavoidable impurities.

[0038] Hereinafter, the reasons for limiting the components of the stainless steel material of the present invention will be explained.

[0039] Carbon (C): 0.05% or less, Nitrogen (N): 0.10% or less

[0040] Considering the increase in manufacturing cost due to steelmaking technology, C and N are set to C: 0.05% or less and N: 0.10% or less. On the other hand, C and N are elements that lower the workability of the base metal and weld, so it is desirable to keep them to the smallest possible amount. However, since they cannot be completely contained in the base metal, their lower limits can be set to C: 0.01% or more and N: 0.02% or more, respectively.

[0041] Silicon (Si): 1.00% or less, manganese (Mn): 2.0% or less, aluminum (Al): 0.150% or less, phosphorus (P): 0.04% or less, sulfur (S): 0.010% or less

[0042] These elements inevitably exist in steel, but if present in large quantities, they lower the workability and reduce the corrosion resistance, which is a characteristic of stainless steel, so it is reasonable to set Si: 1.00% or less, Mn: 2.0% or less, Al: 0.150% or less, P: 0.04% or less, and S: 0.010% or less. In addition, the lower limits of Si, Mn, Al, P, and S are not specifically limited, but in consideration of cases where they are inevitably added, Si: 0.10% or more, Mn: 0.5% or more, Al: 0.001% or more, P: 0.01% or more, and S: 0.002% or more may be added, respectively.

[0043] Chromium (Cr): 17-25%

[0044] Since corrosion resistance, which is the basic characteristic of stainless steel, is insufficient when the Cr content is less than 17%, the Cr content should be 17% or more. In addition, since high Cr content can sometimes deteriorate the toughness of the weld, the Cr content should be 25% or less.

[0045] Nickel (Ni): 8.0~12%

[0046] Ni is an austenite structure stabilizing element and an element that improves the workability of materials. For this purpose, one embodiment of the present invention may add Ni in an amount of 8.0% or more. On the other hand, if the Ni content becomes excessive, stress corrosion cracking (SCC) problems may occur, so the steel of the present invention may contain Ni in an amount of 12.0% or less. As another example, the Ni may be contained in an amount of 8.0 to 10.0%.

[0047] Depending on the properties required for the steel material composed of the above-mentioned component system, it may further include one or more elements selected from among a) to c) below.

[0048] a) Total content of at least one of Mo and Cu: 0.10 to 2.00%

[0049] In order to improve corrosion resistance, the total content of at least one of Mo and Cu can be additionally added within the composition range of 0.10 to 2.00%. When the total content of at least one of Mo and Cu is 0.10% or more, the effect of improving corrosion resistance can be obtained, and when it exceeds 2.00%, the processability may deteriorate and the manufacturing cost may also increase.

[0050] b) Nb: 0.010~0.500%

[0051] In the case of Nb, it can be included up to 0.500%. When the Nb content exceeds 0.500%, there is a problem of deterioration in workability due to the increase in the amount of dissolved Nb. When adding Nb to form NbN, NbC, etc. to improve workability, it is preferable to include 0.010 to 0.500%.

[0052] c) Ti: 0.010%~0.500%

[0053] In the case of Ti, it can be included in amounts of 0.010% to 0.500%. Ti is added as an element that improves processability. The effect can be seen by adding more than 0.010% Ti. However, when adding more than 0.500% Ti, there is a problem that processability deteriorates due to the increase in the amount of dissolved Ti.

[0054] The core for PCHE can be manufactured by photochemically etching a path on a stainless steel plate and then diffusion bonding.

[0055] Flow path etching refers to a photochemical etching process for forming flow paths on a heat transfer plate. More specifically, flow path etching can be performed by cleaning the heat transfer plate surface to ensure strong adhesion with the photoresist, applying the photoresist to the surface, forming a flow path pattern through light irradiation, developing the photoresist, and then etching. The etching described above can be performed using an immersion method or a spray method that locally sprays an etchant onto the material after determining the etchant according to the material.

[0056] The inventors of the present invention have found that when the surface roughness of the stainless steel material of the present invention is controlled, the surface quality of the etched surface after etching can be excellent, friction with the internal fluid can be minimized, and the pressure drop phenomenon can be reduced.

[0057] From this perspective, the stainless steel material according to one embodiment of the present invention can have an arithmetic mean roughness Ra of the steel surface of less than 0.10. Accordingly, the stainless steel material of the present invention can secure excellent surface quality of the etched surface after etching with a etching rod. In another embodiment, the arithmetic mean roughness Ra can be 0.09 or less.

[0058] As another example, the stainless steel material for diffusion bonding of the present invention may have a 10-point average roughness Rz of the steel surface set to 1.00 or less in order to improve surface quality and diffusion bondability. In another embodiment, the 10-point average roughness Rz may be 0.90 or less, and in another embodiment, it may be 0.80 or less.

[0059] In this way, when the surface roughness of the steel is reduced to a certain level or lower, the surface quality of the euro-etched surface can be excellently secured, as described above. For example, the arithmetic mean roughness Ra of the euro-etched surface may be 0.35 or lower, and the 10-point average roughness Rz may be 2.50 or lower.

[0060] Meanwhile, in order to manufacture the core for PCHE, diffusion bonding must be performed after the above-described euro etching.

[0061] Diffusion bonding, as described above, refers to a solid-state bonding process that utilizes an appropriate contact pressure between the surfaces of two materials under vacuum and high temperature. This diffusion bonding utilizes the diffusion of atoms between the bonding surfaces by applying pressure that does not cause plastic deformation at a temperature below the melting point of the parent material. A key factor in diffusion bonding is how quickly grains recrystallize at the bonding interface, eliminating the contact interface.

[0062] Accordingly, the inventors of the present invention have studied the above-described diffusion bonding in depth and have realized that the diffusion rate of atoms at the contact interface can affect the overall component productivity.

[0063] Furthermore, the inventors of the present invention discovered that the diffusion rate of atoms at the contact interface is inversely proportional to the grain size. Consequently, the inventors realized that by minimizing the grain size of stainless steel, diffusion bonding time and temperature can be dramatically reduced, thereby improving productivity. This led to the invention.

[0064] From this point of view, the stainless steel material according to one embodiment of the present invention may have an average grain size at the center of the thickness of 10 µm or less.

[0065] The average grain size at the center of the thickness above refers to the average grain size at the center of the thickness on the TD (Transverse Direction) plane when the entire thickness of the steel is t, and the average refers to the average value of values ​​measured at five arbitrary points, and the center refers to 1 / 4t to 3 / 4t when the entire thickness of the steel is t.

[0066] That is, the stainless steel material according to one embodiment of the present invention can improve productivity and prevent deterioration of mechanical properties due to grain growth by setting the average grain size to 10.00 ㎛ or less. In another embodiment, the average grain size may be 7.00 ㎛ or less, and in another embodiment, it may be 5.00 ㎛ or less.

[0067] Meanwhile, the stainless steel material for diffusion bonding of the present invention may have a 20° specular gloss of 200.0 or higher on the steel surface. Accordingly, the present invention can significantly improve diffusion bondability. As another example, the 20° specular gloss may be 300.0 or higher, and as another example, it may be 350.0 or higher.

[0068] As described above, the present invention can provide a stainless steel material for diffusion bonding that has high strength characteristics even after diffusion bonding and excellent low-temperature toughness. Specifically, the stainless steel material for diffusion bonding of the present invention can have a tensile strength of 400 MPa or more after diffusion bonding and a low-temperature impact toughness of 10 J or more at -196°C. Since such low-temperature impact toughness is an important factor in PCHEs used for LNG and hydrogen charging stations, the stainless steel material for diffusion bonding according to an example of the present invention can be particularly suitable for application to PCHEs.

[0069] Hereinafter, the method for manufacturing the stainless steel material of the present invention will be described in detail.

[0070] A method for manufacturing a stainless steel material according to one embodiment of the present invention may include a step of cold-rolling a hot-rolled material to obtain a cold-rolled material; a cold-rolling annealing step of annealing the cold-rolled material; and a step of pickling the annealed cold-rolled material.

[0071] In addition, a method for manufacturing a stainless steel according to another embodiment of the present invention may additionally include, before the step of obtaining the cold-rolled material, a step of manufacturing a molten stainless steel in an electric furnace; a step of refining the manufactured molten stainless steel; a step of continuously casting the refined molten stainless steel to obtain a steel ingot; a step of hot-rolling the steel ingot to obtain a hot-rolled material; a hot-rolling annealing step of annealing the hot-rolled material; and a step of pickling the annealed hot-rolled material.

[0072] Below, each step is explained in detail.

[0073] First, according to a method for manufacturing a stainless steel material according to one embodiment of the present invention, molten stainless steel can be manufactured in an electric furnace, and then the manufactured molten stainless steel can be refined.

[0074] This refining can refer to a series of processes that melt alloy ferromagnesium or scrap such as iron and chromium that are mixed to match the target components, or remove impurities through a furnace or vacuum degassing.

[0075] Next, the present invention can obtain a steel ingot by continuously casting the refined stainless steel molten metal.

[0076] This continuous casting process involves pouring the refined molten metal into a mold and cooling it. The resulting ingots can be cut into desired lengths and used in subsequent processes.

[0077] In addition, the above ingot may contain, in wt%, carbon (C): 0.05% or less, nitrogen (N): 0.10% or less, silicon (Si): 1.00% or less, aluminum (Al): 0.150% or less, phosphorus (P): 0.04% or less, sulfur (S): 0.010% or less, chromium (Cr): 17 to 25%, manganese (Mn): 2.0% or less, and nickel (Ni): 8.0 to 12.0%, with the remainder being iron (Fe) and unavoidable impurities. The content of each alloying component and the reasons for its limitation have been described above, and therefore will be omitted.

[0078] The present invention can obtain a hot-rolled material by hot-rolling the above-mentioned steel ingot.

[0079] The above hot rolling is a process of manufacturing a hot rolled material having a predetermined thickness by rolling a steel ingot manufactured in a casting process at high temperature.

[0080] Then, the present invention can anneal the hot-rolled material. At this time, the annealing temperature can be 1100 to 1200°C.

[0081] That is, the present invention can achieve the purpose of softening the hot-rolled material by setting the annealing temperature to 1100°C or higher. On the other hand, if the annealing temperature exceeds 1200°C, the problem of coarsening of the austenite phase may occur, so the upper limit of the annealing temperature in the manufacturing method of the present invention can be set to 1200°C. As another example, the annealing temperature can be 1125 to 1175°C.

[0082] The present invention can pickle the annealed hot-rolled material.

[0083] This acid pickling process is a process of washing away scale attached to the surface of hot-rolled materials during the annealing process using an acid pickling solution such as a mixture of hydrochloric acid or nitric acid and hydrofluoric acid. Devices and techniques commonly used in the technical field of the present invention can be used as scale removal devices.

[0084] According to the manufacturing method of the present invention, a cold-rolled material can be obtained by cold-rolling the hot-rolled coil.

[0085] At this time, the cold rolling according to a non-limiting example may include first cold rolling with a reduction ratio per pass of 25% or more and second cold rolling with a total reduction ratio of 40% or more.

[0086] That is, the present invention can refine grains that have grown in the rolling direction during hot rolling by setting the reduction ratio per pass during the first cold rolling to 25% or more. In addition, since the conditions for the first cold rolling, such as the total reduction ratio, can be applied according to common practice, these are not described separately in the present invention.

[0087] The present invention can perform secondary cold rolling at a total reduction ratio of 40% or more. More specifically, the present invention can obtain 50% or more of process-induced martensite within the rolled structure by setting the total reduction ratio during the secondary cold rolling to 40% or more. As another example, the total reduction ratio may be 42% or more or 45% or more.

[0088] After the secondary cold rolling as described above, the present invention can anneale the cold-rolled material. In addition, as an example, the annealing temperature during the annealing can be 800 to 1000°C. The present invention can obtain a recrystallized austenite structure having a small average grain size and a homogeneous distribution by annealing the cold-rolled material at a temperature relatively lower than the annealing temperature typically performed, and at the same time, can suppress the formation of oxide scale as much as possible to lower the surface roughness. In addition, through the annealing at 800 to 1000°C as described above, the present invention can also secure excellent mirror gloss of the manufactured stainless steel material. In another example, the annealing temperature can be 800 to 950°C, and in another example, the annealing temperature can be 800 to 880°C.

[0089] And, as a non-limiting example, the annealing time in the cold rolling annealing step may be 2.5 to 3.5 minutes.

[0090] Thereafter, the present invention can obtain a stainless steel material according to an example of the present invention by pickling the annealed cold-rolled material.

[0091] Hereinafter, the diffusion bonding method of the stainless steel material for diffusion bonding of the present invention will be described in detail.

[0092] The diffusion bonding method of the stainless steel for diffusion bonding of the present invention may include the steps of preparing the stainless steel for diffusion bonding of the present invention described above; and the step of diffusion bonding the stainless steel for diffusion bonding; wherein the step of diffusion bonding may have an A value of 50.0 or less derived by the following relational expression 1.

[0093] [Relationship 1] A = (P×t×T) / (TS×IE)

[0094] (In the above relational expression 1, P, t, and T represent the applied pressure (MPa), the bonding time (minutes), and the bonding temperature (℃) in the diffusion bonding step, respectively, and TS and IE represent the tensile strength (MPa) measured in the vertical direction of the interface of the bonded portion of the steel obtained after the diffusion bonding step, and the low-temperature impact toughness (J) at -196℃, respectively. However, the unit of A is omitted.)

[0095] That is, when performing diffusion bonding using the stainless steel material for diffusion bonding of the present invention, the bonding time required for diffusion bonding or the bonding temperature can be reduced to secure excellent mechanical properties of the diffusion bond. Throughout this specification, the term "diffusion bond" may be used to refer to a portion where diffusion bonding is performed, and the term "bond interface" may be used to refer to the interface of such diffusion bonding.

[0096] In other words, the diffusion bonding method according to one embodiment of the present invention can reduce the pressing force, bonding time, and bonding temperature required for diffusion bonding to secure appropriate mechanical properties of the diffusion bonded portion by setting the A value derived by the above [Relational Expression 1] to 50.0 or less. This can reduce the cost required for diffusion bonding and prevent deterioration of mechanical properties caused by excessive grain growth. In addition, when manufacturing a core for a PCHE using the diffusion bonding method of the present invention, the bonding time at high temperatures can be reduced, thereby minimizing channel deformation that may occur in some parts during diffusion bonding, and as a result, heat exchange efficiency can be increased. In another embodiment, the A value may be 45.0 or less, and in another embodiment, the A value may be 40.0 or less.

[0097] Since the smaller the value of A is, the better, the lower limit thereof is not specifically limited. However, considering practical problems, considering that the value of A cannot be 0, in one example of the present invention, the lower limit of the value of A may be set to 5.1.

[0098] Hereinafter, the present invention will be described in detail through examples. However, it should be noted that the examples described below are intended only to illustrate and concretize the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0099] (Example 1)

[0100] 1. Manufacturing of stainless steel for diffusion bonding

[0101] A steel slab having an alloy composition shown in Table 1 below and a thickness of 200 mm was heated to 1150°C, then final hot-rolled at 1000°C, and then cooled and coiled in the air to obtain a hot-rolled steel sheet having a final thickness of 3 to 3.5 mm. Thereafter, each of the hot-rolled steel sheets was annealed at 1150°C and then pickled. Then, cold-rolled at the cold reduction ratio shown in Table 2 below was performed to manufacture a cold-rolled steel sheet. Thereafter, the cold-rolled steel sheet was annealed / pickled under the annealing conditions shown in Table 2 below in the air, and then cooled to manufacture a stainless steel material for diffusion bonding.

[0102] Steel type CNSiAlPSCrMnNiMoCuNbTiInvented steel 10.020.020.360.0010.020.003191.210.0-0.09--Comparative steel 10.040.050.410.0020.030.002181.18.0-0.03--Comparative steel 20.020.040.370.0020.030.003181.48.0-0.02--

[0103] Steel gradeCold rolledCold rolled annealing1st cold rolled2nd cold rolledAnnealing time(min)Annealing temperature(℃Reduction ratio per 1 pass(%)Total reduction ratio(%)Inventive steel 12570%3800~880Comparative steel 12070%Over 31000Less than 1100Comparative steel 22070%Over 31000Less than 1100

[0104] Then, for each steel material, the arithmetic mean roughness Ra and 10-point mean roughness Rz, gloss at 20°, and average grain size were measured and shown in Table 3 below. The measurement method for each parameter is as follows. (Surface roughness)

[0105] Surface roughness Ra and Rz were measured using a 3D roughness meter from Veeco.

[0106] (gloss at 20°)

[0107] The above gloss was evaluated using a gloss meter after cutting and processing a plated steel plate into a test piece measuring 100 mm x 100 mm. The gloss meter used a gloss meter from Rhopoint Instruments, and when measuring at a measurement angle of 20°, it is expressed as a relative value when the reflectivity of the glass is set to 100.

[0108] (average grain size)

[0109] The average grain size at the center of the thickness refers to the average grain size at the center of the thickness in the TD (Transverse Direction) plane of the specimen. The average grain size of the present invention was measured by analyzing the direction of the center of the thickness using an electron backscatter diffraction pattern analyzer (EBSD) with the model name e-Flash FS.

[0110] Steel grade Surface roughness (㎛) Gloss at 20° Average grain size RaRz ASTM E112-13 (Rating No.) Equivalent diameter (㎛) Inventive steel 10.09 0.58 35 2.01 2.16 5.46 Comparative steel 10.12 1.08 101.58 71 19.39 Comparative steel 20.18 1.87 89.79 16 16.58

[0111] 2. Diffusion bonding

[0112] For two or more stainless steel materials for diffusion bonding manufactured as described above, diffusion bonding was performed without separate etching. The conditions for diffusion bonding are as shown in Table 4 below. Then, the physical properties of the steel materials after diffusion bonding were measured and shown in Table 4 below. Next, SEM photographs of the interface structure of the bonded part according to the diffusion bonding time in Inventive Example 4 and Comparative Example 4 are shown in Fig. 1 below along with the defect rate. The bonding rate can be derived by the following equation, and the physical properties of the steel materials were measured by the method described below.

[0113] (Defect rate)

[0114] The defect rate can be obtained by observing a cross-section cut in a direction perpendicular to the joint interface and calculating the ratio of the total length of pore defects occurring within a 10 mm length of the joint interface. Furthermore, the bonding rate can be defined as the average value of five observations at any location.

[0115] Defect rate (%) = (Total length of pore defects within 10 mm of the joint interface (mm) / 10 mm) x 100 (%)

[0116] (tensile strength)

[0117] Tensile strength was measured using a tensile tester and tested according to the generally accepted ASTM E8M No. 3.

[0118] (low temperature impact toughness)

[0119] Low-temperature impact toughness was measured using an impact tester at -196℃, and tested according to the generally accepted ASTM Sub-Size.

[0120] Tensile and impact test specimens were laminated in layers of 200 or more and processed in the vertical direction of the diffusion-bonded joint to conduct tests.

[0121] Example Steel type Diffusion bonding Property A Value Applied pressure P (MPa) Time t (min) Temperature T (℃) Tensile strength TS (MPa) Low temperature toughness IE (-196℃J) Invention example 1 Invention steel 18151000490485.1 Invention example 2 Invention steel 183010005003713.0 Invention example 3 Invention steel 186010005056913.8 Invention example 4 Invention steel 189010005309114.9 Invention example 5 Invention steel 18609004955615.6 Comparative example 1 Comparative steel 18151000360566.7 Comparative example 2 Comparative steel 183010004605104.3 Comparative example 3 Comparative steel 186010004858123.7 Comparative Example 4 Comparative Class 189010006022059.8 Comparative Example 5 Comparative Class 18609004651561.9

[0122] 3. Experimental results

[0123] Comparative Examples 1 to 3 had arithmetic mean roughness Ra of the steel surface and an average grain size exceeding the range proposed by the present invention. Therefore, Comparative Examples 1 to 3 failed to secure the high strength characteristics aimed for by the present invention after diffusion bonding, nor did they secure excellent low-temperature impact toughness.

[0124] In the case of Comparative Examples 4 and 5, since the A value derived by [Relationship 1] was excessively large, a relatively large amount of time and energy was invested in the diffusion bonding compared to the mechanical properties of the obtained diffusion bonding portion.

[0125] On the other hand, in the case of invention examples 1 to 5, the mechanical properties of the diffusion bond were excellently secured despite a short bonding time and low bonding temperature by satisfying the alloy components and manufacturing conditions proposed in the present invention.

[0126] In addition, as can be seen in the following Figure 1, in the case of Comparative Example 4, it can be seen that a large number of pore defects exist at the diffusion bonding interface, and it can be seen that the defect rate increases as the bonding time shortens. On the other hand, in the case of Inventive Example 4, it can be seen that the defect rate was almost within 1% when the bonding time was 90 minutes, and the defect rate was less than 15% even when the bonding time was 15 minutes.

[0127] In this way, in the case of Invention Example 4, the diffusion speed was very fast in a high temperature / high pressure diffusion bonding environment compared to Comparative Example 4, so that recrystallization and grain growth at the interface could be accelerated, and voids existing at the contact surface of the laminated plates before diffusion bonding could be reduced and eliminated more quickly.

[0128] (Example 2)

[0129] 1. Manufacturing of stainless steel for diffusion bonding

[0130] The invention steel 1, the comparative steel 1, and the comparative steel 2 were manufactured under the same manufacturing conditions as in the above ‘(Example 1)’.

[0131] 2. Euro etching

[0132] After that, using the manufactured stainless steel for diffusion bonding, a flow path etching process was performed to form a flow path pattern as shown in Table 5 below, thereby manufacturing a flow path etching plate. At this time, the positions of the depth, width, and distance of the steel flow path pattern are briefly shown together with the position of the flow path etching surface through the schematic diagram of Fig. 2. More specifically, the flow path etching was performed by the same process as in Table 5 below: designing a flow path pattern, printing a film of the flow pattern, processing the stainless steel and degreasing the surface, then applying a dry film to the stainless steel, exposure (ultraviolet), development, drying, corrosion, water washing and de-filming, and inspection. In addition, the surface roughness of the flow path etching surface was measured using the same method as the method for measuring the surface roughness of the stainless steel for diffusion bonding in '(Example 1)', and the results are shown in Table 6 below.

[0133] Example Steel Type Etching Surface Etching Pattern (mm) Depth Width Distance Invention Example 6 Invention Steel 10.02 21.7 Comparative Example 6 Comparative Steel 10.04 2.3 1.4 Comparative Example 7 Comparative Steel 20.02 30.7

[0134] Example Steel Type Roughness of etched surface (㎛) Rmax Ra Rz Invention Example 6 Invention Steel 12.8 3 0.3 0 2.2 8 Comparative Example 6 Comparative Steel 12.8 8 0.3 8 2.7 6 Comparative Example 7 Comparative Steel 210.3 9 0.6 9 6.2 4

[0135] 3. Diffusion bonding of Euro-etched plates

[0136] For each Euro etching plate manufactured as described above, diffusion bonding was performed under the bonding conditions shown in Table 7 below.

[0137] Example Steel type Diffusion welding Pressure P (MPa) Time t (min) Temperature T (℃) Invention example 6 Invention steel 17.060900 Comparative example 6 Comparative steel 27.560900 Comparative example 7 Comparative steel 39.560900

[0138] 4. Experimental Results

[0139] Looking at Table 6 above, it can be seen that the arithmetic mean roughness Ra value and the 10-point mean roughness Rz value of the euro-etched surface of Invention Example 6 are smaller than those of Comparative Examples 6 and 7. As a result, it can be expected that the heat transfer effect of the PCHE will be increased as the surface characteristics of the euro-etched surface of Invention Example 6 are improved.

[0140] The following Figure 3 is a photograph of the cross-section of the joints of Invention Example 6, Comparative Example 6, and Comparative Example 7 observed through an optical microscope. Referring to the following Figure 3, it can be seen that the joints of Invention Example 6, Comparative Example 6, and Comparative Example 7 all had good joint shapes, and no ridge portion collapsed due to pressure was observed macroscopically. Through this, it can be confirmed that Invention Example 6 has high strength characteristics suitable for a core for a PCHE.

[0141] The following Figure 4 is an SEM photograph of the joint interface of the joints of Invention Example 6, Comparative Example 6, and Comparative Example 7. It is generally known that the higher the pressure applied during diffusion bonding, the better the diffusion bonding performance. In the case of Invention Example 6, even under the lowest pressure of 7 MPa, almost no pores were observed at the joint interface, and it was confirmed that the initial contact surface also disappeared quickly. However, in the case of Comparative Examples 6 and 7, the initial contact surface and pores were clearly observed despite the diffusion bonding being performed under relatively high pressure conditions. Therefore, Invention Example 6 can be expected to have improved mechanical properties such as strength and low-temperature impact energy due to the absence of pore defects at the diffusion bond interface compared to Comparative Examples 6 and 7.

Claims

1. Contains, by weight%, carbon (C): 0.05% or less, nitrogen (N): 0.10% or less, silicon (Si): 1.00% or less, aluminum (Al): 0.150% or less, phosphorus (P): 0.04% or less, sulfur (S): 0.010% or less, chromium (Cr): 17 to 25%, manganese (Mn): 2.0% or less, and nickel (Ni): 8.0 to 12.0%, with the remainder being iron and unavoidable impurities. Stainless steel for diffusion bonding having an average grain size at the center of the thickness of 10.00㎛ or less and an arithmetic mean roughness Ra of the steel surface of less than 0.

10.

2. In paragraph 1, A stainless steel material for diffusion bonding, further comprising at least one element selected from among the following a) to c). a) Total content of at least one of Mo and Cu: 0.10 to 2.00%, b) Nb: 0.010~0.500% c) Ti: 0.010%~0.500% 3. In paragraph 1 or 2, Stainless steel for diffusion bonding with a 10-point average roughness Rz of the steel surface of 1.00 or less.

4. In paragraph 1 or 2, Stainless steel for diffusion bonding with a 20° mirror gloss of 200.0 or higher on the steel surface.

5. In paragraph 1 or 2, A stainless steel material for diffusion bonding having a tensile strength of 400 MPa or more after diffusion bonding and a low-temperature impact toughness of 10 J or more at -196°C.

6. In paragraph 1 or 2, A stainless steel material for diffusion bonding, having an arithmetic mean roughness Ra of the etched surface after etching with a Euro etching method of 0.35 or less and a 10-point average roughness Rz of 2.50 or less.

7. A step of cold rolling a hot-rolled material to obtain a cold-rolled material; a cold-rolling annealing step of annealing the cold-rolled material; and a step of pickling the annealed cold-rolled material. The above hot-rolled material contains, in wt%, carbon (C): 0.05% or less, nitrogen (N): 0.10% or less, silicon (Si): 1.00% or less, aluminum (Al): 0.150% or less, phosphorus (P): 0.04% or less, sulfur (S): 0.010% or less, chromium (Cr): 17 to 25%, manganese (Mn): 2.0% or less, and nickel (Ni): 8.0 to 12.0%, with the remainder being iron and unavoidable impurities. The above cold rolling includes first cold rolling with a reduction ratio of 25% or more per pass and second cold rolling with a total reduction ratio of 40% or more. A method for manufacturing a stainless steel material for diffusion bonding, wherein the annealing temperature in the above cold rolling annealing step is 800 to 1000°C.

8. In paragraph 7, Before the step of obtaining the above cold-rolled material, It additionally includes a step of producing a stainless steel molten metal in an electric furnace; a step of refining the produced stainless steel molten metal; a step of continuously casting the refined stainless steel molten metal to obtain a steel ingot; a step of hot-rolling the steel ingot to obtain a hot-rolled material; a hot-rolling annealing step of annealing the hot-rolled material; and a step of pickling the annealed hot-rolled material. A method for manufacturing a stainless steel material for diffusion bonding, wherein the annealing temperature in the above hot rolling annealing step is 1100 to 1200°C.

9. In paragraph 7, A method for manufacturing a stainless steel material for diffusion bonding, wherein the annealing time in the above cold rolling annealing step is 2.5 to 3.5 minutes.

10. A step for preparing a stainless steel material for diffusion bonding according to paragraph 1 or 2; and A step of diffusion bonding the above-mentioned stainless steel material for diffusion bonding; The above diffusion bonding step is a diffusion bonding method for a stainless steel material for diffusion bonding in which the A value derived by the following relational expression 1 is 50.0 or less. [Relationship 1] A = (P×t×T) / (TS×IE) (In the above relational expression 1, P, t, and T represent the applied pressure (MPa), the bonding time (minutes), and the bonding temperature (℃) in the diffusion bonding step, respectively, and TS and IE represent the tensile strength (MPa) measured in the vertical direction of the interface of the bonded portion of the steel obtained after the diffusion bonding step, and the low-temperature impact toughness (J) at -196℃, respectively. However, the unit of A is omitted.)

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