Method for manufacturing test material, method for measuring diffusible hydrogen content, method for evaluating delayed fracture characteristic, method for selecting metal material, and method for manufacturing member

Abstract

Provided is a method for manufacturing a test material with which it is possible for the diffusible hydrogen content of a metal material having a welded portion to be measured accurately.　The method for manufacturing a test material for measuring the diffusible hydrogen content of a metal material including a welded portion includes: a blow hole position identification step for identifying a blow hole position in a test material, which is a metal material having a welded portion; and a machining step for machining the blow hole identified in the blow hole position identification step so as not to have a closed structure, or for removing the blow hole.

Description

Method for preparing test material, method for measuring diffusible hydrogen content, method for evaluating delayed fracture properties, method for selecting metal materials, and method for manufacturing components

[0001] The present disclosure relates to a method for preparing a test material, a method for measuring the amount of diffusible hydrogen, a method for evaluating delayed fracture properties, a method for selecting a metallic material, and a method for manufacturing a component.

[0002] In recent years, from the perspective of preventing global warming, there has been a demand for improving energy efficiency, for example, improving gasoline fuel consumption in the case of automobiles, ships, railway vehicles, etc., by reducing the weight of moving objects such as automobiles. Steel, one of the materials used to construct moving objects, requires high strength to ensure the same level of safety as before, even when the plate thickness is reduced to reduce weight.

[0003] On the other hand, automotive steel materials are plastically worked into a desired shape by press forming and then assembled to the vehicle body by welding, for example, spot welding. Therefore, automotive steel materials are required to have not only press formability but also the ability to be properly welded in various sheet combinations.

[0004] However, the increased strength of steel materials has come with the problem of increased susceptibility to delayed fracture, a phenomenon in which a metal material is subjected to a static load and, after a certain period of time, suddenly fractures due to brittleness, with little apparent plastic deformation.Here, this refers to hydrogen embrittlement-type fracture caused by the intrusion of hydrogen into the metal material.

[0005] Known factors that affect delayed fracture include the hydrogen embrittlement susceptibility of the metallic material, the applied load (e.g., residual stress), and the amount of hydrogen that penetrates into the metallic material. For example, in the case of steel, as the strength of the steel increases, the hydrogen embrittlement susceptibility increases significantly, and even a small amount of hydrogen that penetrates into the steel will cause fracture (cracks) of the steel. In particular, the hydrogen embrittlement susceptibility increases significantly in steels with a tensile strength of 1180 MPa or more (Non-Patent Document 1).

[0006] Therefore, even if the strength required of steel materials increases, the steel materials are required to be free from delayed fracture, i.e., to have excellent delayed fracture resistance. In particular, the welded parts of steel materials have higher strength than the base steel material, and therefore are prone to increased susceptibility to hydrogen embrittlement. It has also been reported that delayed fracture occurs in metallic materials other than steel materials.

[0007] Therefore, there is a need for a method for evaluating the delayed fracture properties of welds in metallic materials. For example, in the examples of Patent Document 1, a method for evaluating the hydrogen embrittlement properties of spot welds in steel sheets is disclosed in which plates with a thickness of 2 mm or 4 mm are sandwiched between both ends of the sheet as spacers, and the center between the spacers is joined by spot welding to prepare a test specimen, which is then immersed in 0.5 mol / L sulfuric acid, hydrogen is penetrated by an electric current, and the presence or absence of cracks is evaluated after 2 hours.

[0008] In addition, delayed fracture is said to be influenced by hydrogen (diffusible hydrogen) that penetrates into steel. Therefore, it is thought that the delayed fracture properties of welds can be evaluated by measuring the upper limit of the amount of diffusible hydrogen (critical diffusible hydrogen amount) that does not cause delayed fracture in welds.

[0009] For example, Patent Document 2 discloses a method for measuring the amount of diffusible hydrogen, which is a cause of delayed fracture, from a welded joint of an actual structure with high accuracy.

[0010] Japanese Patent Application Publication No. 2007-231373 Japanese Patent Application Publication No. 2008-261821

[0011] Matsuyama Shinsaku: Delayed Fracture, Nikkan Kogyo Shimbun, Tokyo, (1989).

[0012] When evaluating the delayed fracture properties of a welded portion of a metallic material using the method disclosed in Patent Document 1, it is necessary to conduct a test for a predetermined time and determine whether cracks have occurred in the welded portion. However, because it is determined whether cracks have occurred in the welded portion within the predetermined time, test pieces in which cracks have occurred before the predetermined time and test pieces in which cracks have occurred at the predetermined time are evaluated on the same level, which poses a problem in that it is not possible to accurately evaluate the occurrence of cracks for each test piece.

[0013] Furthermore, using the method for measuring the amount of diffusible hydrogen in a welded joint disclosed in Patent Document 2, the amount of diffusible hydrogen (amount of penetrated hydrogen) was measured for welds into which hydrogen had been introduced under the same conditions, and it was found that the measured values ​​of the amount of hydrogen sometimes differed significantly.

[0014] The present disclosure has been made in consideration of the above circumstances, and aims to provide a method for producing a test material that can accurately measure the amount of diffusible hydrogen in a metal material having a welded portion.

[0015] The present disclosure also aims to provide a method for measuring the amount of diffusible hydrogen using a test material prepared by the above-mentioned test material preparation method, a method for evaluating delayed fracture properties, a method for selecting a metal material, and a method for manufacturing a component.

[0016] As a result of extensive investigation, the present inventors have found that the above object can be achieved by adopting the following configuration, and have completed the present invention. [1] A method for preparing a test material for measuring the amount of diffusible hydrogen in a metallic material having a welded section, comprising: a blowhole position identifying step of identifying the position of a blowhole in the test material, which is a metallic material having a welded section; and a processing step of processing the test material so that the blowhole identified in the blowhole position identifying step does not have a closed structure, or removing the blowhole. [2] The method for preparing a test material as described in [1], in which a volume ratio of weld metal in the test material is 50% or more. [3] A method for measuring the amount of diffusible hydrogen in a metallic material having a welded section, comprising: a hydrogen introducing step of introducing hydrogen into a test material prepared by the method for preparing a test material as described in [1] or [2]; and a hydrogen amount measuring step of measuring the amount of hydrogen in the test material into which hydrogen has been introduced in the hydrogen introducing step. [4] A method for evaluating the delayed fracture properties of a metallic material having a welded portion, comprising an evaluation step of evaluating the delayed fracture properties based on the diffusible hydrogen amount obtained by the method for measuring the diffusible hydrogen amount described in [3] above. [5] A method for selecting a metallic material, comprising a selection step of selecting a metallic material based on the delayed fracture properties obtained by the method for evaluating delayed fracture properties described in [4] above. [6] A method for manufacturing a component, comprising a welding step of manufacturing a component by at least welding metallic materials selected by the method for selecting metallic materials described in [5] above.

[0017] According to the present disclosure, it is possible to provide a method for producing a test material that can accurately measure the amount of diffusible hydrogen in a metallic material having a welded portion. Also, according to the present disclosure, it is possible to provide a method for measuring the amount of diffusible hydrogen using the test material produced by the above-mentioned method for producing a test material, a method for evaluating delayed fracture properties, a method for selecting a metallic material, and a method for producing a component.

[0018] Fig. 1 is a schematic diagram illustrating an example of a method for extracting a test piece from a metal material having a weld. Fig. 2 is a schematic diagram illustrating an example of a processing method in the processing step.

[0019] As described above, the inventors have investigated the problem of inability to obtain reproducibility in the amount of diffusible hydrogen in metallic materials having welded joints by using spot-welded steel materials, and as a result have found that the above method has the following problems.

[0020] Spot welds contain internal defects called blowholes. When spot welds are exposed to a hydrogen-penetrating environment (hydrogen penetration environment), a large amount of hydrogen accumulates in the blowholes. It is believed that the hydrogen in the blowholes is gasified. Furthermore, the hydrogen accumulated in the blowholes is stable, i.e., non-diffusible. According to Non-Patent Document 1, this non-diffusible hydrogen does not significantly affect delayed fracture resistance. However, hydrogen accumulated in the blowholes may be forcibly desorbed and detected as diffusible hydrogen in hydrogen thermal desorption analysis, resulting in an apparent increase in the amount of hydrogen that has penetrated the weld (diffusible hydrogen amount). Furthermore, because the amount of hydrogen accumulated in the blowholes varies depending on the blowhole size, the measured hydrogen content of the spot welds will vary depending on the volume and number of blowholes. However, controlling the size, shape, and number of blowholes is difficult with current welding technology.

[0021] Therefore, the inventors investigated the relationship between blowholes present in welds and the amount of hydrogen, and found that by eliminating blowholes with a closed structure, it is possible to accurately evaluate the amount of penetrated hydrogen (amount of diffusible hydrogen) that penetrates into welds from the environment and affects delayed fracture.

[0022] The present disclosure has been made based on the above findings. Hereinafter, a method for preparing a test material for measuring the amount of diffusible hydrogen in a metallic material having a welded joint according to an embodiment of the present disclosure, a method for measuring the amount of diffusible hydrogen, a method for evaluating delayed fracture, a method for selecting a metallic material, and a method for manufacturing a component will be described. Note that the present disclosure is not limited to the following embodiments. Furthermore, the components in the following embodiments include those that are easily replaceable by a person skilled in the art, or those that are substantially identical.

[0023] <Method for Preparing Test Material> A method for preparing a test material for measuring the amount of diffusible hydrogen in a metallic material having a welded joint according to an embodiment of the present disclosure includes a step of identifying blowhole positions in the test material, which is a metallic material having a welded joint, and a processing step of processing the blowhole identified in the step of identifying the blowhole positions so that it does not have a closed structure, or removing the blowhole.

[0024] (Metallic Material) First, the metallic material will be described. The metallic material is not particularly limited as long as it is weldable and does not cause hydrogen-induced delayed fracture. As described above, in the case of steel, hydrogen embrittlement susceptibility increases significantly when the tensile strength of the steel is 1180 MPa or more. Therefore, a high-strength steel sheet, particularly a steel sheet with a tensile strength of 1180 MPa or more, is suitable as the metallic material. The metallic material may also be a plated steel sheet having the above-mentioned high-strength steel sheet as a base.

[0025] The metal material having the welded portion may be any metal material that has been welded. Here, the welded portion is composed of the weld metal and the heat-affected zone.

[0026] The welding may be any of fusion welding (arc welding, laser welding, electron beam welding, plasma arc welding, etc.) and pressure welding (spot welding, projection welding, seam welding, etc.). For example, when spot welding is used as the welding method, the weld metal becomes the nugget portion.

[0027] It is preferable that the test material be prepared so that the volume ratio of the weld metal is 50% or more. Therefore, a method for preparing a test material according to an embodiment of the present disclosure may include a preparation step of preparing a test material containing a desired volume ratio of the weld metal. Hydrogen (diffusible hydrogen), which affects delayed fracture, is absorbed in greater amounts in the weld metal than in the base metal and the heat-affected zone. Therefore, in order to appropriately evaluate the delayed fracture properties of a metallic material having a weld, it is preferable that the volume ratio of the weld metal in the test material be 50% or more. The volume ratio of the weld metal is more preferably 70% or more, even more preferably 80% or more, and may be 100%.

[0028] The volume ratio of the weld metal in the test material may be adjusted, for example, when the test material is extracted from a metal material after welding (a metal material having a weld). FIG. 1 is a schematic diagram illustrating an example of an extraction method for extracting a test material from a metal material having a weld. In the preparation step, for example, as shown in FIG. 1 , a test material 3 may be extracted from a metal material 1 having a weld so as to include a weld metal (nugget portion) 2. As described above, the test material 3 is preferably extracted so that the volume ratio of the weld metal 2 is 50% or more. Examples of the extraction method include extraction methods using a microcutter or wire cutting. In addition, in the preparation step, the volume ratio of the weld metal contained in the test material may be adjusted by adjusting the welding current, etc., when welding the metal material.

[0029] Furthermore, in order to appropriately evaluate the delayed fracture resistance of metallic materials having welds, it is preferable to match the volume ratio of the weld metal between test materials. When evaluating the delayed fracture resistance of test materials, it is preferable to use test materials whose volume ratio of the weld metal is within ±10%, and more preferably within ±5%.

[0030] (Blowhole Position Identifying Step) In the blowhole position identifying step, the position of a blowhole present in the welded portion of the test material is identified. The method for identifying the blowhole position is performed by non-destructive testing, since it is necessary to identify the blowhole inside the welded portion, then introduce hydrogen into the test material, and measure the amount of introduced hydrogen. Examples of the non-destructive testing include X-ray CT testing and ultrasonic flaw detection testing. Furthermore, by using images obtained by these tests, the volume ratio of the weld metal in the test material can also be evaluated.

[0031] (Processing Step) In the processing step, the blowhole identified in the blowhole position identifying step is processed so that it does not have a closed structure, or the blowhole is removed. Examples of methods for processing the blowhole so that it does not have a closed structure include, for example, forming a through-hole T that connects the surface of the test material to the blowhole B, as shown in FIG. 2( a), or grinding or polishing the surface of the test material until the blowhole B is connected to the external environment, as shown in FIG. 2( b). This allows the blowhole to be connected to the external environment and prevent the blowhole from having a closed structure. The processing method for forming the through-hole is not particularly limited, but examples include a processing method using a microdrill or the like to form a through-hole that reaches from the surface of the test material to the blowhole. Furthermore, the grinding or polishing method is not particularly limited, but examples include a processing method in which the surface of the test material is mechanically or chemically ground or polished. Furthermore, if a blowhole is present near the surface of the test material, the blowhole may be removed by grinding or polishing. If the test material has an as-cut surface, mechanical defects, such as work-hardened areas caused by an increase in dislocations or lattice defects, may be generated, and hydrogen may be trapped in the mechanical defects. Therefore, it is preferable to remove the surface by chemical polishing.

[0032] According to the above procedure, a test material for measuring the amount of diffusible hydrogen in a metallic material having a weld is prepared. The test material prepared by the test material preparation method according to an embodiment of the present disclosure is suitable as a test material for measuring the amount of diffusible hydrogen, and is particularly suitable as a test material for evaluating delayed fracture properties.

[0033] <Method for measuring the amount of diffusible hydrogen> A method for measuring the amount of diffusible hydrogen in a metallic material having a welded portion according to one embodiment of the present disclosure includes a hydrogen introduction step of introducing hydrogen into a test material prepared by the above-described test material preparation method, and a hydrogen amount measurement step of measuring the amount of hydrogen introduced into the test material in the hydrogen introduction step.

[0034] (Hydrogen Introduction Step) In the hydrogen introduction step, hydrogen is introduced into the test material prepared as described above. The hydrogen introduction method may be appropriately selected depending on the purpose. Specific examples of the hydrogen introduction method include a cathode charging method in which the test material is used as the cathode and platinum or the like as the anode in an electrolyte, and an electric current is passed between the cathode and the anode to electrolyze the electrolyte, and the hydrogen generated during this process is introduced into the test material, and a hydrochloric acid immersion method in which the test material is immersed in a hydrochloric acid solution or the like. Furthermore, if it is desired to evaluate the amount of hydrogen introduced in an environment (actual environment) in which a metal material is actually used, the test material may be placed in the actual environment. Note that after hydrogen is introduced in the hydrogen introduction step, the test material is preferably immersed in liquid nitrogen for storage. Immersing the test material in liquid nitrogen can prevent the hydrogen introduced into the test material from being desorbed.

[0035] (Hydrogen Amount Measurement Step) In the hydrogen amount measurement step, the amount of hydrogen (diffusible hydrogen amount) in the test material into which hydrogen has been introduced in the hydrogen introduction step is measured. The method for measuring the amount of hydrogen is not limited as long as it is a method that can measure the amount of hydrogen. For example, thermal desorption analysis can be used. Thermal desorption analysis can analyze extremely small amounts of hydrogen, so it is preferred as an analytical method for measuring the amount of diffusible hydrogen to evaluate the delayed fracture properties of metallic materials. Note that if the test material after hydrogen introduction in the hydrogen introduction step is stored in liquid nitrogen, it is preferred to measure the amount of hydrogen promptly after removing the test material from the liquid nitrogen.

[0036] <Method for Evaluating Delayed Fracture Properties> A method for evaluating delayed fracture properties according to an embodiment of the present disclosure includes an evaluation step for evaluating delayed fracture properties based on the amount of hydrogen (diffusible hydrogen amount) obtained by the above-described method for measuring the amount of diffusible hydrogen. In the evaluation step, for example, when determining the superiority or inferiority of the delayed fracture properties of a weld between test materials made of different metallic materials, the following evaluation may be performed for each test material. That is, for each test material, the diffusible hydrogen amount is measured under the same hydrogen introduction environment (hydrogen introduction conditions) in the hydrogen introduction step of the above-described method for measuring the amount of diffusible hydrogen, and the evaluation is performed by comparing the values. This is because it is generally believed that metallic materials with a higher amount of diffusible hydrogen are more likely to experience delayed fracture. For more accurate evaluation, it is preferable to measure the diffusible hydrogen amount under hydrogen introduction conditions that cause cracks in the weld of the test material, and then evaluate the delayed fracture properties of the test material.

[0037] <Method for Selecting a Metallic Material> A method for selecting a metallic material according to an embodiment of the present disclosure includes a selection step for selecting a metallic material based on the delayed fracture property obtained by the above-described delayed fracture property evaluation method. By performing the evaluation step on a metallic material having a weld, the relationship between the welding conditions and the delayed fracture property, for example, the diffusible hydrogen content when the hydrogen introduction environment is consistent in the hydrogen introduction step, can be obtained. Furthermore, the relationship may be obtained by changing the type of metallic material. These relationships can then be used to classify metallic materials. Then, in the selection step, for example, by evaluating the diffusible hydrogen content in the environment in which the metallic material will be used, it is possible to select metallic materials suitable for use in that environment from the classification. The selected metallic material can then be shipped, etc.

[0038] <Method for manufacturing a component> A method for manufacturing a component according to an embodiment of the present disclosure includes a welding step of manufacturing a component by at least welding a metal material selected by the metal material selection method. When manufacturing the component, for example, the metal material may be processed and then welded to form a component, or may be welded and then processed to form a component. The welding method in the welding step is not particularly limited, and for example, a known welding method can be applied. Furthermore, the processing is not particularly limited, and examples include various metal processing methods such as molding. Furthermore, the component is preferably an automotive component.

[0039] The present disclosure will be specifically described below with reference to examples, although the present disclosure is not limited to the examples described below.

[0040] First, metal materials with welds were prepared by the following procedure: Steel plates A to C with the representative component compositions shown in Table 1, thickness of 1.6 mm, and tensile strengths of 1200 to 1500 MPa were prepared.

[0041]

[0042] First, three test pieces of steel plates A, B, and C were cut to 120 mm x 30 mm, and spot welded near the center. The spot welding was performed at room temperature using chromium copper DR-type electrodes, with the lower and upper electrodes both having a tip diameter (tip diameter) of 6 mm and a curvature radius of 40 mm, while the electrodes were kept water-cooled. A current was applied so that the nugget diameter was 3√t (t: plate thickness), and a metal material (test specimen) with a weld was produced.

[0043] Test pieces were cut from the obtained specimens using a microcutter. The size of the test pieces was adjusted to match the target volume ratio of the weld metal. The pieces were then chemically polished by immersing them in a mixed solution of hydrogen peroxide and oxalic acid.

[0044] (Step of identifying blowhole positions) For the obtained test material, the positions of blowholes were identified three-dimensionally using X-ray CT. In addition, the volume ratio of the weld metal in the test material was calculated using the images obtained by X-ray CT. The number of blowholes identified above and the calculated volume ratio of the weld metal in the test material are shown in Tables 2 and 3.

[0045] (Processing Step) Next, the blowholes identified in the blowhole position identification step were processed so that they would not have a closed structure. Specifically, through holes were drilled into the blowholes from the surface of the test material using a microdrill. However, for No. 1 and No. 2 in Table 2, the above processing was not performed, and for No. 3 in Table 2, the above processing was performed on some of the identified blowholes. Note that for No. 6 in Table 2, the above processing was not performed because no blowholes were present in the welded portion of the test material (Reference Example). Furthermore, whether or not the blowholes had a closed structure after the above processing was confirmed using X-ray CT of the processed test material. Tables 2 and 3 show the number of closed blowholes present in the test material. In Tables 2 and 3, if the number of closed blowholes was zero, it was determined that processing had been performed so that the blowholes would not have a closed structure, and the test material was evaluated as passing.

[0046] (Hydrogen introduction step) Hydrogen was introduced into the test materials prepared as described above under the hydrogen introduction conditions shown in Tables 2 and 3. After hydrogen was introduced, the test materials were immediately immersed in liquid nitrogen and stored frozen to prevent the introduced hydrogen from being desorbed.

[0047] (Hydrogen amount measurement step) Immediately before measuring the hydrogen amount, the test material was quickly removed from the liquid nitrogen, and the hydrogen amount (diffusible hydrogen amount) was measured by thermal desorption analysis using a low-temperature temperature-programmed hydrogen analyzer. The thermal desorption analysis was performed in a temperature range of -50°C to 800°C at a heating rate of 200°C / h. The diffusible hydrogen amount was calculated as a mass fraction (ppm by mass) by integrating the hydrogen amounts measured from -50°C to 200°C and dividing the integrated value by the mass of the test material. The hydrogen amount (diffusible hydrogen amount) values ​​obtained for each test material are shown in Tables 2 and 3.

[0048]

[0049]

[0050] From Table 2, when the test materials had blowholes with a closed structure (No. 1 to No. 3), the diffusible hydrogen content was 3.8 to 4.5 mass ppm. In contrast, when the blowholes with a closed structure were eliminated through processing (No. 4, No. 5), the diffusible hydrogen content was 2.3 to 3.2 mass ppm. Furthermore, the diffusible hydrogen content of the test material without blowholes (No. 6) was 2.7 mass ppm. This shows that the elimination of blowholes with a closed structure significantly reduces the diffusible hydrogen content, and that the presence of blowholes with a closed structure causes the diffusible hydrogen content to be overestimated. It also shows that the use of test materials without blowholes with a closed structure allows the diffusible hydrogen content to be evaluated with high accuracy.

[0051] Furthermore, Table 3 shows that for test materials with a closed structure free of blowholes, if the volume ratio of the weld metal in the test materials was approximately the same, the obtained values ​​of the amount of diffusible hydrogen showed a ranking of steel plate A > steel plate B > steel plate C. It was also found that when the volume ratio of the weld metal in the test materials was 50% or more, the difference between the steel types became more apparent.

[0052] On the other hand, when the delayed fracture properties of the welds of Steel Plate A, Steel Plate B, and Steel Plate C were evaluated using a conventional method (Patent Document 1), it was found that the delayed fracture resistance exhibited a ranking of Steel Plate C (excellent) > Steel Plate B > Steel Plate A (poor).

[0053] From the above, it is considered that the steel sheet obtained by the method of the present disclosure with a larger amount of diffusible hydrogen is more likely to suffer from delayed fracture.

[0054] From the above, it can be said that the method disclosed herein can measure the amount of diffusible hydrogen in a metallic material having a weld with high accuracy and high reproducibility. Furthermore, it can be said that the method disclosed herein can be used as an index for evaluating the delayed fracture resistance of the metallic material, and can evaluate the delayed fracture resistance quantitatively and with high accuracy.

[0055] 1 Metallic material with welded portion 2 Weld metal 3 Test material B Blowhole T Through hole

Claims

1. A method for preparing a test material for measuring the amount of diffusible hydrogen in a metallic material having a welded portion, comprising: a step of identifying a blowhole position in the test material, which is a metallic material having a welded portion; and a processing step of processing the blowhole identified in the step of identifying the blowhole position so that it does not have a closed structure, or of removing the blowhole.

2. The method for preparing a test material according to claim 1, wherein the volume ratio of the weld metal in the test material is 50% or more.

3. A method for measuring the amount of diffusible hydrogen in a metal material having a welded portion, comprising: a hydrogen introduction step for introducing hydrogen into a test material prepared by the method for preparing a test material described in claim 1 or 2; and a hydrogen amount measurement step for measuring the amount of hydrogen in the test material into which hydrogen has been introduced in the hydrogen introduction step.

4. A method for evaluating the delayed fracture properties of a metal material having a welded portion, comprising an evaluation step of evaluating the delayed fracture properties based on the amount of diffusible hydrogen obtained by the method for measuring the amount of diffusible hydrogen described in claim 3.

5. A method for selecting a metallic material, comprising a selection step of selecting a metallic material based on the delayed fracture property obtained by the delayed fracture property evaluation method described in claim 4.

6. A method for manufacturing a component, comprising at least a welding step of manufacturing the component by welding a metal material selected by the method for selecting a metal material according to claim 5.

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