JP5564022B2 - Method for evaluating hydrogen cracking resistance of forged steel - Google Patents

Description

  The present invention evaluates a phenomenon (hydrogen cracking) in which sudden cracking occurs due to hydrogen contained in a forged steel product when manufacturing forged parts applied to power transmission parts such as ships and generators, or reaction vessels, for example. The present invention relates to a method for evaluating hydrogen cracking resistance of a forged steel material.

Generally, as forging steel used for power transmission parts such as for ships and generators, conventionally, so-called Cr represented by ISO standard 36CrNiMo ₆ , DIN standard 32CrMo ₁₂ , or ISO standard 42CrMo ₄ is used. -Mo steel is used. Further, as the performance of marine engines and generators increases, there is a demand for products that are lighter and have higher performance, that is, products that exhibit higher fatigue strength. The steel material or product described above is subjected to severe repeated stress during use. Therefore, as an improvement measure for increasing the fatigue strength, it is possible to use highly clean Cr—Mo steel in which inclusions (MnS or the like) that are the starting points of fatigue fracture are reduced as much as possible.
However, there is a problem that hydrogen cracking is likely to occur when the cleanliness of such Cr-Mo steel is increased. In addition, ASME508, Cl3 (Mn-Mo-Ni steel), etc. of the American Society of Mechanical Engineers is applied as the forging steel used for the reaction vessel and the like, but there is a problem that hydrogen cracking is likely to occur.

  Therefore, in order to suppress hydrogen cracking of these large forged steel products, technical studies have been conducted from various aspects such as smelting technology improvement, steel material improvement, and heat treatment to prevent hydrogen defects. Yes. As an improvement of the refining technology, for example, an upper limit value of the hydrogen amount at the time of refining molten steel is regulated, and dehydrogenation treatment is performed when the upper limit is exceeded. Further, as an improvement in the material of steel, it is known that a substance that becomes a hydrogen trap site is present in the steel. Furthermore, as heat treatment for preventing hydrogen defects, for example, as described in Non-Patent Document 1, hydrogen in steel is sufficiently diffused by high-temperature and long-time heating in the forging stage to suppress local hydrogen concentration. Has been done. Currently, by applying these techniques, large forged products (forged steel products) made of Cr-Mo steel having higher hydrogen cracking resistance are manufactured.

  As described above, the technology for improving the hydrogen cracking resistance of large forged steel products is progressing from the viewpoints of smelting and material improvement. Moreover, the evaluation technique which determines the hydrogen cracking property of the steel manufactured using these methods can be judged comparatively accurately, for example with the method proposed by patent document 1. Further, as a method for determining the hydrogen cracking property (hydrogen embrittlement) of high-strength steel, for example, the method is introduced in Non-Patent Document 2.

JP 2010-54494 A

Data on heat treatment methods for preventing hydrogen defects Japan Casting and Forging Society Forging Steel Research Group February 1996 (issued) pp25-28 N. Suzuki et al.: WIRE JOURNAL INTERNATIONAL, Vol.19, (1986), pp.36-47

However, the conventional determination methods described above have the following problems.
In the method described in Patent Document 1, it takes a long time of several tens of hours or more to evaluate the superiority or inferiority of the hydrogen cracking resistance of the steel material, and the heat treatment is performed when manufacturing a large forged steel product. In contrast, it is not possible to consider the structures that are generated and the hydrogen distribution to them.

In the method described in Non-Patent Document 1, heat treatment methods for preventing hydrogen defects in the forging and annealing processes are introduced, but various examples of heat treatment are introduced depending on the sensitivity to hydrogen cracking of steel materials. However, it is not possible to determine which heat handling method is appropriate for hydrogen cracking.
In the method described in Non-Patent Document 2, since the steel material is immersed in an acid and charged with hydrogen, it is difficult to allow an arbitrary amount of diffusible hydrogen to enter the steel, and the test is performed in the atmosphere. The charged diffusible hydrogen decreases with time, and it is difficult to ensure the desired amount of diffusible hydrogen during the test.

  In other words, all of the techniques shown in the prior art are forced to introduce hydrogen into the steel from the outside by a method such as cathodic charging or immersing in a strong acid with respect to the final product, causing the steel to become brittle and fractured. Since the time required for the process and the breaking strength are determined, hydrogen is present in the forged steel product in the actual production process, and the formation of the structure that occurs according to the thermal history during production, and the phases and grain boundaries that accompany it. Distribution of hydrogen and accumulation of hydrogen around inclusions cannot be reproduced. In addition, the evaluation of the prior art takes a long time of several tens of hours or more.

  The present invention was devised in view of the above-mentioned problems, and it is possible to simplify hydrogen cracking that is considered to occur due to hydrogen distribution to the steel structure in the temperature-lowering process in the forged steel product and accumulation of hydrogen in the inclusions. It is an object of the present invention to provide a method for evaluating hydrogen cracking resistance of a forged steel material that is evaluated quickly and with high sensitivity.

  In order to solve the above-mentioned problems, the inventors of the present invention have found that the hydrogen present in the forged steel product in the actual production process is formed in accordance with the thermal history during the production, and each phase and grain produced in association therewith. For problems such as distribution to the boundary, inability to reproduce hydrogen accumulation around the inclusions, and the time required for evaluations of several tens of hours or more, and the need for special testing machines such as low strain rate testing machines Focusing on the need to establish a method for evaluating the hydrogen cracking property of steel materials according to the heat treatment performed in the manufacturing process of forged steel products, for example, A method to evaluate hydrogen cracking accurately, quickly and easily from the viewpoints of handling and formation of the structure of ferrite, pearlite, bainite, etc., hydrogen distribution to the formed structure, and hydrogen accumulation in inclusions. It was carried out in order to establish intensive research.

  Specifically, in an evaluation test in which hydrogen is penetrated from the outside by a method such as cathodic charging or strong acid immersion while applying a constant or constant period of fluctuation stress to a test piece collected from the final product, which is a conventional method. Research was conducted to solve the problem that hydrogen was not distributed in a saturated state around each phase, phase interface, and inclusions formed as described above. As a result, hydrogen in the steel contained during ingot formation by heat treatment during the production of large forged steel products, each structure such as ferrite, pearlite, bainite and their phase interfaces generated in the subsequent cooling process, It has also been found that hydrogen may be distributed in a saturated state around the inclusions to be produced, and the fracture characteristics evaluated in the hydrogen distributed state.

  Therefore, in view of the above knowledge, the method according to the present invention is performed in the following procedure. That is, a method for evaluating hydrogen cracking resistance of a forged steel material, the step of forging a forging steel material to form a test material of the forged steel material, and a hydrogen concentration of 30% or more and 100% The following hydrogen concentration range, a step of introducing hydrogen into the steel by heat treatment in a temperature atmosphere of 300 ° C. or more and less than 1500 ° C., and a step of performing an evaluation test and evaluating the cracking characteristics of the test material into which the hydrogen has been introduced The procedure included. Moreover, in the method for evaluating hydrogen cracking resistance of a forged steel material, the step of introducing hydrogen into the steel is performed at 300 ° C. or more and less than 1500 ° C. so that the forging steel material becomes the forged steel material having a preset composite structure. The heating may be performed at any heating rate and cooling rate within the range of the heat treatment.

  In addition, in the case of a forged material having a bainite single-phase structure, which is a typical structural form of the steel for forging, as the step of introducing hydrogen into the steel, the bainite single-phase structure in which the forged steel is preset. After being held in an atmosphere of Ac1 or higher and lower than 1500 ° C and a hydrogen concentration of 30% or higher and 100% or lower so as to become the forged material, it is cooled to a Bs point or lower at a cooling rate of 5 ° C / min or higher, and the Bs point or lower. It keeps at the temperature of.

  According to such a procedure, in the method for evaluating hydrogen cracking resistance of a forged steel material, a test material is formed by forging under arbitrary conditions, and heat treatment is performed at a predetermined temperature range and in a predetermined hydrogen concentration atmosphere by introducing hydrogen. As a result, hydrogen is introduced into the steel, so the hydrogen solubility and hydrogen diffusion coefficient in the steel increase at a high temperature range, which is shorter than the conventional hydrogen introduction at room temperature. A large amount of hydrogen can be introduced into the steel uniformly over time. Next, by taking a temperature-decreasing process that takes an arbitrary heating rate and cooling rate in consideration of heat treatment in a preset actual machine, each phase such as ferrite, pearlite, and bainite, which is a typical structure form of the final product, is formed. . And since the solid solubility of hydrogen falls by the fall of steel temperature, a saturated amount of hydrogen will be distributed around each produced | generated phase, phase interface, and inclusions. As described above, the hydrogen introduced in the hydrogen introduction step can be made “internally caused hydrogen”, which is the original hydrogen cracking mechanism of a pseudo-large forged steel product. In addition, in the method for evaluating hydrogen cracking resistance of a forged steel material, it can be confirmed that cracking of the steel material, that is, hydrogen cracking occurs due to hydrogen that does not completely dissolve with the generation of each phase.

The method for evaluating hydrogen cracking resistance of a forged steel material according to the present invention has the following excellent effects.
The hydrogen cracking resistance evaluation method for forged steel is a temperature drop in which the forged forged steel is introduced into the steel by heat-treating the forged forged steel at a predetermined range of hydrogen concentration in an atmosphere of a predetermined temperature range, and the temperature decreases from the time of forging. Hydrogen is distributed in a saturated state around the structure formed in the process, the phase interface, or the generated inclusions, and the fracture characteristics are evaluated by an evaluation test in the hydrogen distribution state. Therefore, in the hydrogen cracking resistance evaluation method for forged steel materials, the introduced hydrogen can be simulated as internally derived hydrogen, which is the original hydrogen cracking mechanism of the product, and the hydrogen cracking property of the test material can be simply and quickly. It becomes possible to evaluate with high sensitivity equivalent to the actual product state. Further, in this method, by introducing hydrogen at a high temperature, the time required for introducing hydrogen can be shortened, and the evaluation test can be performed quickly.

  The hydrogen cracking evaluation method of the forged steel material is, for example, ferrite, which is a general structural form in a large forged steel product, by changing the range and speed of lowering the temperature within a predetermined range from the time of forging in the hydrogen introduction process. Since it is possible to target a forged steel material having a composite structure such as pearlite, bainite, or residual γ, it is possible to perform hydrogen cracking evaluation with the same quality as the finished product.

It is a schematic diagram which shows typically each process of the hydrogen cracking resistance evaluation method of the forged steel materials which concerns on this invention. (A) schematically shows the hydrogen introduction state of the present invention, (b) schematically shows the hydrogen introduction state of the prior art, and shows the hydrogen introduction and hydrogen distribution state of the forged steel material according to the present invention. It is a schematic diagram compared with the state of a prior art side by side.

Hereinafter, the method for evaluating hydrogen cracking resistance of a forged steel material according to the present invention will be described with reference to the drawings.
As shown in FIG. 1, the hydrogen cracking resistance evaluation method S of a forged steel material is, for example, a hydrogen cracking resistance of a forged steel material used for power transmission parts such as ships and generators, and forged steel parts including a reaction vessel. It is the evaluation method about. This forging steel material hydrogen cracking resistance evaluation method S is composed of a procedure for performing a steelmaking step S1, a steelmaking step S2, a forging / test material forming step S3, a hydrogen introduction step S4, and an evaluation step S5. Has been.

The iron making process S1 is a process for producing pig iron in an electric furnace. This iron making process S1 is performed by a general process in which pig iron is taken out as molten iron from an iron scrap.
The steel making process S2 is a process for producing a steel material from the pig iron produced in the iron making process S1. This steelmaking process S2 is a process which manufactures steel materials, for example, performing hot metal preliminary processing of hot metal with a converter etc., and performing secondary refining after that. It is set as the shape of the forging steel which can forge a forged steel material by this steelmaking process S2.

  The forging / specimen forming step S3 is a step of forging the forging steel into a forged steel material and forming it into the shape of the test material when evaluated in the evaluation step S5. In this forging / test material forming step S3, the forged shape may be the shape of the test material, or the forged steel material may be cut out from the forged steel material. In addition, it is desirable that the shape of the test material is formed corresponding to the evaluation experiment in the evaluation step S5. For example, if a tensile test is performed in the evaluation step S5, the specimen is formed in the shape of a tensile test piece corresponding to the tensile test. In addition, the test material to be used as a test piece is surface-finished with an abrasive of abrasive grain number # 600 or more before the test, and degreased with acetone or ethanol to avoid generation of unnecessary reactants during annealing. Preferably it is done. In this way, the surface of the specimen is subjected to surface finishing (surface finishing process), thereby reducing the influence of the surface condition (for example, cut-out scratches during test piece preparation, suppression of hydrogen intrusion into the steel due to unnecessary surface products). Can be reduced.

In addition, in the above-mentioned iron making process S1, steel making process S2, and forging / test material forming process S3, the components, strength and manufacturing method of the steel material that is the object of hydrogen cracking evaluation, and the shape and dimensions of the test material are particularly limited. Is not to be done. For example, what is necessary is just to manufacture with the various manufacturing methods proposed as a manufacturing method of the forging steel used for power transmission parts, such as a general ship and a generator, or the forging steel used for reaction vessels etc. . In addition, for example, ISO standard 36CrNiMo ₆ is used for power transmission parts, SQV2A standardized by JIS G 3120 is used for pressure vessel steel, ASME508, Cl3, etc., American Society of Mechanical Engineers standard, and these are used for vacuum induction melting and electrode arc. It is a test material for evaluating hydrogen cracking resistance from a forged material after it is melted by a molten steel processing facility having a heating function and demolded from a solidified steel ingot and then heated to about 1200 ° C. to produce a forged material. What is necessary is just to extract | collect a test piece suitably.

  The shape and dimensions of the test material (test piece) used for evaluation are not particularly limited in the present invention, but in order to evaluate the hydrogen cracking resistance from the fracture characteristics, for example, the test piece shape is a tensile test piece shape. Unlike the conventional method of introducing hydrogen at normal temperature, as will be described later, in the present invention, hydrogen is introduced into the steel in a high-temperature hydrogen gas atmosphere until the equilibrium amount is reached, and the temperature of the steel is lowered in the subsequent temperature-decreasing process. Along with this, hydrogen that is not solid solution partition is released into the steel. From this, hydrogen is distributed in a saturated amount around each structure in the steel of the test material, these phase interfaces, and inclusions. Therefore, in JIS Z 2201 (No. 4 test piece etc.) used in the conventional method It is more preferable to use a tensile test piece having a shape having a V-shaped or U-shaped cutout in the center of the round bar in the circumferential direction, rather than a dumbbell-shaped test piece as defined.

  As shown in FIG. 1, the hydrogen introduction step S4 is a step of introducing hydrogen into the test material formed as a test piece. In this hydrogen introduction step S4, hydrogen is introduced into the steel by heat treatment in an atmosphere having a hydrogen concentration of 30% or more and 100% or less and an introduction temperature of 300 ° C. or more and less than 1500 ° C. In the hydrogen introduction step S4, when the hydrogen concentration is set, it can be performed by using an unreacted gas such as nitrogen or argon and maintaining the hydrogen concentration, for example, using a heat treatment furnace. In this hydrogen introduction process S4, heat treatment in the temperature range of 300 ° C. or more and less than 1500 ° C. is taken into consideration, and heat treatment for preventing hydrogen defects that is applied in the forging stage of large forged steel products is taken into consideration. Can do.

  In addition, in hydrogen introduction process S4, if it is less than 300 degreeC, hydrogen introduction | transduction to the steel in a hydrogen gas atmosphere will not be accelerated | stimulated, and the dispersion | variation in the amount of hydrogen is large. There is a risk that evaluation cannot be performed. And preferably, it is a temperature range applied by forging of an actual large forged steel product, and is set to Ac1 or higher and lower than 1500 ° C. at which an austenite structure having a large hydrogen storage amount starts to form. More preferably, it is set to Ac3 or higher and lower than 1200 ° C. which is a single phase of austenite structure having a large amount of hydrogen storage. In the hydrogen introduction step S4, in addition to the introduction temperature, the holding temperature is within the range of 300 ° C. or more and less than 1500 ° C. The time in the heat treatment from the introduction to the holding in the hydrogen introduction step S4 may be appropriately set according to the size of the forged steel material, but is preferably, for example, 1 hour to 24 hours.

In addition, in the hydrogen introduction step S4, the hydrogen gas atmosphere in which the heat treatment is performed can efficiently introduce hydrogen into the steel by setting the hydrogen concentration in the gas to 30% or more and 100% or less. . Here, in the hydrogen introduction step S4, if the hydrogen concentration is less than 30%, sufficient hydrogen may not be introduced into the steel. Therefore, in order to efficiently introduce hydrogen into the steel through the hydrogen introduction step S4, the hydrogen concentration in the gas is preferably 50% or more. In the hydrogen introduction step S4, it is preferable that the remainder of the atmosphere be an unreacted gas (N ₂ , Ar, etc.) from the viewpoint of safety and the prevention of unnecessary scale adhesion to the test piece surface.

  Further, in the hydrogen introduction step S4, the steel material is held at an arbitrary holding temperature in the range of 300 ° C. or higher and lower than 1500 ° C. in the temperature range and the hydrogen concentration atmosphere, or 300 ° C. or higher and lower than 1500 ° C. In the range, by heating at an arbitrary heating rate and cooling at an arbitrary cooling rate, a structure composed of ferrite and pearlite, a structure including bainite and retained austenite, and a martensitic structure typical of high-strength steel Can do. In evaluating hydrogen cracking properties for large forged steel products, the holding temperature and heating / cooling rate of the test material are preferably adjusted so as to have a structure having ferrite, pearlite, bainite, and partially retained austenite. .

For example, if the austenite single phase (Ac3 point) or more is heated at about several degrees Celsius / minute and then cooled rapidly so as not to pass the pearlite transformation start temperature (Ps), a martensitic structure can be obtained. On the other hand, if the steel ingot is cooled at a cooling rate of about several degrees Celsius / min so as to pass the pearlite transformation start temperature (Ps), a ferrite-pearlite mixed structure can be obtained. In addition, a bainite structure can be obtained by maintaining the bainite transformation start temperature (Bs) for a predetermined time or more while avoiding the pearlite transformation start temperature during cooling. As an example, a bainite structure is obtained by cooling to a Bs point or less at a cooling rate of 5 ° C./min or more (for example, 5 to 20 ° C./min) and holding at the cooled temperature for a certain time (for example, 1 hour).

  Further, in order to perform annealing (temperature in the set temperature range) in the hydrogen atmosphere described above, a heat treatment furnace capable of adjusting the atmosphere with various gases such as hydrogen, nitrogen, and argon may be used. Therefore, by setting the heating rate, the cooling rate, etc. in the hydrogen introduction step S4 so as to have a preset composite material structure, the evaluation described later can be performed with the same material structure as the product.

  In the evaluation step S5 described later, for example, if a tensile test is to be performed, the test piece is removed after the hydrogen introduction step S4 in which hydrogen is introduced into the steel by annealing the tensile test piece in a high-temperature hydrogen gas atmosphere. In order to cool, it is rapidly cooled to room temperature in water, and then the specimen is quickly pulled up from the water and dried to avoid corrosion of the specimen. During drying, in order to avoid escape of hydrogen introduced into the steel, it is preferable to shorten the drying time as much as possible and further dry with cold air so that the temperature of the test piece does not rise.

  In this way, by performing hydrogen introduction step S4, by introducing hydrogen into the steel in a high temperature hydrogen gas atmosphere, as shown in FIG. Since the hydrogen diffusion coefficient increases, a large amount of hydrogen can be uniformly introduced into the steel in a shorter time than the conventional method of introducing hydrogen at room temperature (see the left figure in FIG. 2 (a)). Next, in the hydrogen introduction step S4, each phase such as ferrite, pearlite, and bainite, which is a typical structural form of the final product, is formed by taking a temperature lowering process in consideration of heat treatment in an actual machine. In the hydrogen introduction step S4, the solid solubility of hydrogen decreases due to a decrease in the steel temperature, so that a saturated amount of hydrogen is distributed around the generated phases, phase interfaces, and inclusions (FIG. 2 ( a) See right figure). 2A and 2B, the black part and the white part in the right diagram schematically show the state of the mixed tissue.

  In addition, in the conventional method for introducing hydrogen at room temperature, as shown in FIG. 2B, hydrogen is introduced at room temperature, so it takes time (see the left figure in FIG. 2B) and the structure And non-uniform introduction state (see the right figure in FIG. 2B). As described above, in the hydrogen introduction step S4, the introduced hydrogen can be changed to “internally caused hydrogen” which is an original hydrogen cracking mechanism of a large forged steel product. Therefore, the hydrogen cracking resistance evaluation of the product can be performed in the same manner as the original product. Moreover, depending on the case, it can also evaluate that the crack of steel materials, ie, a hydrogen crack, arises with the hydrogen which does not completely dissolve with the production | generation of each phase.

  The evaluation step S5 is a step in which an evaluation test such as a tensile test and a bending test is performed on each of the test materials introduced with hydrogen in the hydrogen introduction step S4 by using respective test machines. In the evaluation step S5, when evaluating the fracture characteristics of the test material at room temperature by, for example, a tensile test, the test material introduced with hydrogen in the hydrogen introduction step S4 has a general shape in a general testing machine. What is necessary is just to perform a tensile test using the tensile test piece which is a test material of a magnitude | size. The tensile speed is set to 0.1 mm / min or more in order to prevent escape of hydrogen contained in the steel. From the viewpoint of evaluation time, it is preferably 0.5 mm / min or more. The upper limit of the tensile speed is not particularly specified, but may be about 1 mm / min, which is equivalent to a general tensile test. In the evaluation step S5, it is not necessary to use a special test apparatus (a low strain rate tensile tester or a fatigue tester) as used in the conventional method.

  In the evaluation step S5, the evaluation of the amount of hydrogen in the steel may be performed by taking out a test piece after performing arbitrary heat treatment in a predetermined hydrogen gas atmosphere. The method for determining the amount of hydrogen may be a method of measuring the amount of hydrogen contained in the steel by melting the steel, or diffusible hydrogen that is said to affect hydrogen embrittlement (at around room temperature). Hydrogen that can move relatively freely in the crystal lattice may be analyzed by thermal desorption analysis or the like.

  As described above, in the hydrogen cracking resistance evaluation method S of the forged steel material, the forging / test material forming step S3, the hydrogen introduction step S4, and the evaluation step S5 are performed, for example, for manufacturing a large forged steel product. The relationship between the heat treatment of forging steel and the like and the resistance to hydrogen cracking can be evaluated accurately and quickly. In addition, in the hydrogen introduction step S4, for example, an introduction process in which the test material is put in a furnace and heated, a heated test material is lowered in a temperature lowering process in which the temperature is lowered by lowering the furnace temperature, and the temperature is maintained at a predetermined temperature. Until the holding process. After that, the process of cooling in the solvent in which the sample material is cooled in the solvent is a process that prevents hydrogen from escaping from the steel before the evaluation test, and does not affect the formation of the structure. Yes. Further, the hydrogen introduction step S4 is a case where a plurality of processes from the introduction process to the holding process are taken, or from the introduction process to the holding process is performed at the same temperature within a predetermined range according to the present invention. It may be cooled by quenching.

Hereinafter, an embodiment of the present invention will be described. The present invention is not limited to the following examples.
150 kg of forging steel containing chemical components shown in Table 1 was melted in a vacuum furnace with a slag basicity adjusted to 3.0, and cast to obtain an ingot. Each ingot was forged into a forged steel material, and a specimen was obtained after cooling. In addition, about the hydrogen cracking resistance of each steel types AD of a test material, it has been empirically known that it is excellent in order of A->D->B-> C.

  Next, a test piece (test material) having a round bar shape, a length of 100 mm, and a notch with Kt = 3 at the center was prepared from each steel of steel types A to D, and the hydrogen introduction temperature and holding shown in Table 2 were made. The heat treatment which hold | maintains at temperature for 1 hour was performed (experiment No. 1-10). Experiment No. For No. 5, water was quenched immediately after introduction of hydrogen at 850 ° C. That is, Experiment No. The temperature condition of No. 5 was set so that the introduction temperature and the holding temperature were the same. Experiment No. 1-4, and experiment no. Nos. 6 and 7 are heat treatment conditions for forming a ferrite-pearlite mixed structure. 5 is a heat treatment condition for forming a martensite single phase structure.

In addition, after performing hydrogen introduction | transduction process S4 used as predetermined | prescribed heat treatment, in order to prevent escape of the hydrogen in steel which penetrate | invaded the test piece, it stored in liquid nitrogen until evaluation of a fracture | rupture characteristic. Next, for the evaluation of the breaking property, the test piece was taken out of liquid nitrogen, the temperature of the test piece was returned to room temperature with ethanol, and then a tensile test was performed at a tensile speed of 1 mm / min.
In addition, the comparison of the breaking strength was made to be the ratio (value in this test / value in the nitrogen atmosphere) with a specimen subjected to the same heat treatment in a 100% nitrogen atmosphere (breaking strength ratio = 1 is not brittle) In the sense, the lower the value, the more likely it becomes brittle, that is, hydrogen cracks). The time required for these evaluations was less than 8 hours in total including annealing in a hydrogen atmosphere.

For reference, in comparison with the conventional method (Japanese Patent Laid-Open No. 2010-54494), the experiment No. About 11-13, it evaluated by the method of description of a conventional method. Experiment No. 11 and no. 12 is an experiment no. 4 was performed in a 100% nitrogen atmosphere. The same material as 4 was used. Experiment No. For No. 13, prevention of hydrogen intrusion into the steel during heat treatment and Experiment No. In order to obtain a steel structure equivalent to 5, the steel material was kept in 100% nitrogen at 850 ° C. for 1 hour, and then immediately quenched with water. Experiment No. Nos. 11 to 13 each have a current density of 0 using a test piece as a cathode in an acidic solution of 1 M (mol / l) sulfuric acid and 0.01 M (mol / l) KSCN (potassium thiocyanate). It was performed by cathodic charging in which hydrogen was charged (hydrogen introduction) at 0.05 mA / mm ² . And the time until a test piece cracks by a fracture strength ratio or a constant load test was calculated | required for the test piece which carried out the cathode charge. Together with these results, Table 2 shows the case where the evaluation of the hydrogen cracking property of the steel material was possible, the case where the evaluation could not be performed, or the case where the evaluation took a long time (50 hours or more) as x.

  In Table 2, SSRT is a method in which stress is applied at a low strain rate to forcibly break a test piece (high strength steel), and delayed fracture of the high strength steel is rapidly evaluated (Slow Strain Rate Technique) method ( Low strain rate tensile test method). Moreover, the constant load in Table 2 indicates a constant load test, and a target additional stress (200 to 400 MPa) is applied to the small diameter portion of the test piece using a lever-type constant load tester by this principle. Adjusted to be done. And in this constant load test, time until a crack generate | occur | produces in the test piece was calculated | required.

  Experiment No. About 1-7, since it tested by appropriate atmospheric temperature conditions and hydrogen concentration, it was able to evaluate appropriately. In contrast, Experiment No. In No. 8, since the hydrogen concentration in the atmosphere was too low, hydrogen did not sufficiently penetrate into the steel, and appropriate evaluation could not be performed. Experiment No. No. 9 could not be evaluated because the test piece was dissolved because the ambient temperature was too high. Experiment No. In No. 10, the ambient temperature was too low, so that hydrogen did not sufficiently penetrate into the steel, and appropriate evaluation could not be performed.

  Experiment No. 11 to 13 are evaluations using a conventional technique. Experiment No. 11-No. No. 13 required a long time (several tens of hours or more) for the test, causing problems such as inability to make a quick evaluation or inversion of the result permutation. Experiment No. For No. 13, the heat treatment when introducing nitrogen resulted in a martensite structure with high hydrogen cracking susceptibility, that is, the structure was a structure with high hydrogen cracking sensitivity. Thus, the test piece was broken when a large amount of hydrogen was introduced into the steel, and evaluation could not be performed.

Next, the same experiment was conducted for the components shown in Table 3, and the test results shown in Table 4 were obtained.
That is, an embodiment will be described for a forging steel material whose base material structure is bainite under predetermined conditions as described above.
In this example, 150 kg of forging steel containing chemical components shown in Table 3 was melted in a vacuum furnace with a slag basicity adjusted to 3.0, and cast to obtain an ingot. Each ingot was forged into a forged steel material, and a specimen was obtained after cooling. In addition, about the hydrogen cracking resistance of each steel types B, E, and F of the specimen, it has been empirically found that B → F → E in this order.

  Next, a test piece (test material) having a round bar shape, a length of 100 mm, and a notch with Kt = 3 in the center was prepared from each steel of steel types B, E, and F, and the hydrogen introduction temperatures listed in Table 4 were used. Then, cooling was performed at a cooling rate to the Bs point or lower, and heat treatment was performed for 1 hour at a holding temperature that is lower than the cooled temperature (Experiment Nos. 14 and 15). Experiment No. Nos. 14 and 15 are heat treatment conditions for a bainite single phase.

In addition, after performing hydrogen introduction | transduction process S4 used as predetermined | prescribed heat treatment, in order to prevent escape of the hydrogen in steel which penetrate | invaded the test piece, it stored in liquid nitrogen until evaluation of a fracture | rupture characteristic. Next, for the evaluation of the breaking property, the test piece was taken out of liquid nitrogen, the temperature of the test piece was returned to room temperature with ethanol, and then a tensile test was performed at a tensile speed of 1 mm / min.
In addition, the comparison of the breaking strength was made to be the ratio (value in this test / value in the nitrogen atmosphere) with a specimen subjected to the same heat treatment in a 100% nitrogen atmosphere (breaking strength ratio = 1 is not brittle) In the sense, the lower the value, the more likely it becomes brittle, that is, hydrogen cracks). The time required for these evaluations was less than 8 hours in total including annealing in a hydrogen atmosphere.

For reference, in comparison with the conventional method (Japanese Patent Laid-Open No. 2010-54494), the experiment No. 16 and 17 were evaluated by the method described in the conventional method. Experiment No. For Experiments 16 and 17, Experiment No. The same heat treatment as in No. 14 was performed in a 100% nitrogen atmosphere. The same material as 14 was used. Experiment No. Nos. 16 and 17 both have a current density of 0 using a test piece as a cathode in an acidic solution of 1 M (mol / l) sulfuric acid and 0.01 M (mol / l) KSCN (potassium thiocyanate). This was done by cathodic charging with hydrogen at 0.05 mA / mm ² . And the time until a test piece cracks by a fracture strength ratio or a constant load test was calculated | required for the test piece which carried out the cathode charge. Together with these results, Table 4 shows the case where the evaluation of the hydrogen cracking property of the steel material was ○, the case where the evaluation could not be performed, or the case where the evaluation took a long time (50 hours or more) as x.

Experiment No. For 14 and 15, tests were performed under appropriate atmospheric temperature conditions, cooling rate, and hydrogen concentration, and therefore, evaluation could be performed appropriately.
Experiment No. 16 and 17 are evaluations using a conventional technique. All of these required a long period of time (several tens of hours) for the test, resulting in problems such as inability to make a quick evaluation and inversion of the result permutation.

  As described above, by applying the present invention, the relationship between the heat treatment of forging steel and the like and the hydrogen cracking resistance can be evaluated accurately and quickly. Therefore, the present invention is particularly suitable for an evaluation test of a forged steel material used for a large-sized (for example, a mass of 10 t or more) forged steel material component such as a power transmission component such as a ship or a generator, or a reaction vessel.

S Method for evaluating hydrogen cracking resistance of forged steel S1 Steelmaking process S2 Steelmaking process S3 Forging and specimen formation process (process for forming specimen of forged steel)
S4 Hydrogen introduction process (process to introduce hydrogen into steel)
S5 Evaluation process (process to evaluate and evaluate)

Claims (3)

A method for evaluating hydrogen cracking resistance of a forged steel material used for power transmission parts for ships, generators, etc., and forged steel parts including reaction vessels,
Forging the ingot of the forging steel material to form a test material of the forged steel material; and
A step of introducing hydrogen into the steel by heat-treating the test material in a hydrogen concentration range of hydrogen concentration of 30% to 100% and a temperature atmosphere of 300 ° C to less than 1500 ° C;
A method for evaluating hydrogen cracking resistance of a forged steel material, comprising a step of performing an evaluation test and evaluating the cracking characteristics of the test material into which hydrogen is introduced.   The step of introducing hydrogen into the steel includes an arbitrary heating rate within the range of the heat treatment of 300 ° C. or more and less than 1500 ° C. so that the forging steel material becomes the forged steel material having a preset composite structure. The method for evaluating hydrogen cracking resistance of a forged steel material according to claim 1, wherein the method is performed at a cooling rate.   The step of introducing hydrogen into the steel is performed in an atmosphere of Ac1 or higher and lower than 1500 ° C. and hydrogen concentration of 30% or higher and 100% or lower so that the steel for forging becomes the forged material having a preset bainite single phase structure. 2. The method for evaluating hydrogen cracking resistance of a forged steel material according to claim 1, wherein after being held, the steel is cooled to a Bs point or lower at a cooling rate of 5 ° C./min or higher and held at a temperature equal to or lower than the Bs point.

Images