JP4785054B2 - Method for producing high-strength steel material with excellent hydrogen embrittlement resistance - Google Patents

Description

This invention relates to a manufacturing method of the high strength steel material excellent in a suitable resistance to hydrogen embrittlement to such structural materials used in hydrogen environment.

In various applications, high Cr and high Ni steels such as austenitic stainless steel represented by JIS SUS304 are often used as members that require corrosion resistance. In addition, it is known that austenitic steel having an fcc structure is excellent in hydrogen embrittlement resistance, and application to a material used in a hydrogen environment has been proposed (see, for example, Patent Documents 1 and 2). On the other hand, there is a demand for reducing the weight of equipment and parts in various applications, and high strength characteristics that can reduce the thickness of structural materials are also required.
Conventionally, as one method for increasing the strength of steel materials, there is a strengthening of the structure using martensitic transformation by quenching. The martensite structure has a high hardness, and the strength of the material having the structure is greatly improved as compared with the austenitic steel. However, in the case of the high Cr and high Ni steels, it is difficult to make martensite by general quenching because the martensite transformation start temperature is low. In this case, the material is converted to a metastable austenite structure by heat treatment, and processing energy is imparted to the metastable austenite structure to cause a work-induced transformation to obtain a martensite structure.
JP 2003-226940 A Japanese Patent Application Laid-Open No. 2004-339569

However, the martensite structure having a bcc structure has a property of storing hydrogen, and the amount of hydrogen storage increases as the martensite structure in the material increases.
The behavior of hydrogen storage in the martensite structure and austenite structure will be described with reference to FIG. FIG. 13 shows the hydrogen diffusion distance over time in the martensite structure of the bcc structure and the austenite structure of the fcc structure. As is apparent from the figure, in the martensite structure, the diffusion distance of hydrogen increases in a short time, and it is understood that hydrogen is easily occluded. On the other hand, in the austenite structure, the increase in the diffusion distance of hydrogen is extremely small even after a long time, and it is understood that the amount of hydrogen transferred per unit time is small, that is, hydrogen is not occluded.
FIG. 14 shows the relationship between the hydrogen pressure in a hydrogen environment and the fatigue limit of stainless steel. In this figure, stable austenitic stainless steel, metastable austenitic stainless steel, martensitic stainless steel (Cited reference: Kawano Izumida, “Development of stainless steel wire for springs with excellent hydrogen brittleness resistance”, SEI Technical Review, No. 167 (September 2005)). As is clear from the figure, in the stable austenitic steel, the fatigue limit decreases with increasing hydrogen pressure, and in the metastable austenitic steel and martensitic steel, the fatigue limit decreases remarkably. This is thought to be due to hydrogen embrittlement due to the difference in hydrogen storage behavior of the material. Metastable austenitic steels and martensitic steels have significant hydrogen embrittlement compared to stable austenitic steels. I understand.
As described above, when a material having a martensite structure is used in a hydrogen environment, hydrogen may be occluded and hydrogen embrittlement may occur. Therefore, in steel materials used in a hydrogen environment in the past, it is difficult to increase the strength with a martensite structure while maintaining good hydrogen embrittlement resistance, and it is possible to sufficiently meet the demand for higher strength. There is a problem that you can not.

The present invention has been made with the above circumstances as a background, and an object thereof is to provide a method of manufacturing a steel material having both good resistance to hydrogen embrittlement and high strength.

That is , among the methods for producing a high-strength steel material excellent in hydrogen embrittlement resistance according to the present invention, the invention according to claim 1 is directed to induction heating of a surface layer portion of a steel material having a work-induced martensite structure to induce work induction of the surface layer portion. The martensite structure is reversely transformed into an austenite structure.

The invention for producing a high-strength steel material excellent in hydrogen embrittlement resistance according to claim 2 is the invention according to claim 1 , wherein the austenitic steel material having a metastable austenite structure is processed to have the work-induced martensite structure. It is characterized by obtaining a steel material.

The invention for producing a high-strength steel material excellent in hydrogen brittleness resistance according to claim 3 is characterized in that, in the invention according to claim 2 , the austenitic steel material is subjected to a heat treatment to generate the metastable austenitic structure.

The invention of the method for producing a high-strength steel material excellent in hydrogen embrittlement resistance according to claim 4 is the invention according to any one of claims 1 to 3 , wherein the induction heating is performed by high-frequency induction of 1 kHz to 400 kHz. Features.

Present invention provides a method for preparing a high-strength steel having excellent hydrogen embrittlement resistance according to claim 5 is the invention according to claim 1, wherein the steel material, in mass%, Cr: 12 to 25%, Ni: 5 to 11% is contained.
The invention of the method for producing a high-strength steel material excellent in hydrogen embrittlement resistance according to claim 6 is the invention according to any one of claims 1 to 5, wherein the austenite structure has a volume in the surface layer portion after the induction heating. It is characterized by being 70% or more in rate.

That is, the high strength steel material obtained Ri by the method for producing a steel water excellent hydrogen embrittlement of the present invention exhibit high have strength by the martensite structure which exists within the material. On the other hand, the surface layer portion mainly comprises an austenite structure, and thus exhibits excellent hydrogen embrittlement resistance even when used in a hydrogen environment.
In the material, in order to obtain high strength, the martensite structure may occupy 100%, but the martensite structure preferably occupies more than 30% by volume ratio to obtain good strength characteristics. It becomes possible. In terms of strength, it is desirably 50% or more. The balance includes an austenite structure.
In the surface layer portion mainly composed of an austenite structure, it is desirable that the austenite structure occupies 70% or more by volume in order to obtain good hydrogen embrittlement resistance. In order to obtain better hydrogen embrittlement resistance, it is desirable that the austenite structure is further 100% by volume. The remainder includes the martensite structure.
Moreover, it is not necessary that all the surface layer portions of the steel material are composed of an austenite structure, and a portion of the surface layer portion that is expected to be exposed to hydrogen is composed of an austenite structure,
Other surface layer portions may be composed of the same structure as the inside.

The martensitic structure can be obtained by quenching austenitic steel. However, it is preferable to obtain a martensitic structure by subjecting the austenitic structure to work-induced transformation by applying processing energy. The processing-induced transformation can uniformly transform the material, and can increase the strength of the entire material without variation. On the other hand, when a large amount of processing energy is applied to the surface layer portion, the processing-induced transformation at the surface layer portion tends to appear stronger than inside.
The processing-induced transformation can usually be generated by processing a material having a metastable austenite structure. In the processing, an appropriate processing rate is selected so as to obtain a desired martensite structure volume ratio in accordance with the components of the austenitic steel. The processing method is not particularly limited as the present invention, and can be performed by rolling, forging, extrusion, drawing, or the like. Further, it is desirable that the processing is performed by cold processing in order to provide sufficient processing energy.

  The metastable austenite structure can be obtained by a heat treatment in which a part of austenitic steel is solution-treated and then cooled. In this heat treatment, for example, the martensitic transformation start temperature of the austenitic steel is lower than room temperature, so that martensitic transformation does not occur in normal water cooling, oil cooling, etc., and a metastable austenitic structure is obtained while remaining as supercooled austenite. . Examples of such austenitic steel include steel containing Cr: 12 to 25% and Ni: 5 to 11% (for example, stainless steel). As a well-known austenitic steel, JIS SUS304 austenitic steel (Cr: 18.0 to 20.0 mass%, Ni: 8.0 to 10.5 mass%, the balance being Fe and inevitable impurities) can be shown. . Moreover, as an austenitic steel which produces | generates a metastable austenite structure | tissue by the said solution treatment, Ni equivalent can be made into a standard, Ni equivalent (30 [% C] +0.5 [% Mn] + [% Ni] An austenitic steel having +30 [% N]) of 5% or more can be used.

  In the solution treatment, it is necessary to heat the austenitic steel to the austenitic region to sufficiently dissolve carbides and the like. Although the temperature in the solution treatment varies depending on the composition of the material, it is usually performed by heating to 1100 to 1200 ° C. and holding for 30 to 60 minutes. In cooling after heating, in order to obtain a metastable austenite structure, it is desirable to set the cooling rate to air cooling or higher.

A steel material having a work-induced martensite structure can be formed into an austenite structure by subjecting the surface layer part to induction heating to reversely transform the surface layer part while maintaining the work-induced martensite structure inside. In the induction heating, it is desirable to rapidly heat the surface layer portion of the steel material in a short time by, for example, high frequency induction of 1 kHz to 400 kHz. In this heating, the heating temperature is preferably set to 600 to 850 ° C. and heated for 0.1 to 10 seconds. In addition, as for heating temperature, it is further desirable that the lower limit is 650 ° C. and the upper limit is 800 ° C. The reason is that if the heating temperature is less than the lower limit or if the heating time is less than 0.1 seconds, the martensite structure cannot be sufficiently reverse transformed, while the heating temperature exceeds the upper limit, When heating is performed for more than 10 seconds, the internal martensite structure is also affected by heat and reverse transformation occurs, so that heating is preferably performed for 10 seconds or less.
Further, after the heating, it is desirable to cool the steel material at a cooling rate of 50 ° C./second or more and 300 ° C./second or less. Furthermore, the lower limit is preferably 100 ° C./second. The above cooling rate is desirable because heat is transferred at a cooling rate of less than 50 ° C./sec, and a cooling rate exceeding 300 ° C./sec is not practical.

As described above , according to the method for producing a high-strength steel material excellent in hydrogen embrittlement resistance according to the present invention, the surface layer portion of the steel material having a work-induced martensite structure is induction-heated to induce the work-induced martensite structure of the surface layer portion. Is reversely transformed into an austenitic structure, so that a martensite structure can be generated reliably and homogeneously inside, and further, only a desired surface layer portion can be formed into an austenitic structure with a desired thickness by induction heating in a later step. it can. The obtained steel has the effect of exhibiting high strength and excellent hydrogen embrittlement resistance.

Hereinafter, an embodiment of the present invention will be described.
Preferably, an austenitic steel containing Cr: 12 to 25% by mass, Ni: 5 to 11% by mass and having an Ni equivalent of 5% or more is prepared.
As shown in FIG. 1, the austenitic steel is usually heated to 1100 to 1200 ° C. and subjected to a solution treatment for 30 to 60 minutes. In addition, a known heating furnace can be used for the said heating, and a heating means etc. are not limited to a specific thing as this invention. After the heating, cooling is preferably performed at a cooling rate equal to or higher than air cooling. As a result, as shown in FIG. 4, a metastable austenite structure 11 that is supercooled from the austenite structure 10 is obtained.

The metastable austenitic steel is imparted with processing energy by cold rolling or the like. Note that the type of processing is not limited to cold rolling, and an appropriate processing method such as forging, extrusion, or drawing can be employed as the present invention.
By the cold working, the metastable austenite structure undergoes work-induced transformation, and a martensite structure 12 is generated as shown in FIG. The martensite structure 12 is desirably more than 30% by volume. When the martensite structure is not 100% martensite structure, the balance is usually an austenite structure.
The schematic diagram shown in FIG. 5A shows a steel material 1 in which a martensite structure is generated by work-induced transformation and the balance is an austenite structure. Even when the martensite structure is a part, since a large amount of processing energy is applied in the vicinity of the surface portion, the volume ratio of the martensite structure at the surface portion tends to increase.

  The steel material 1 having a martensite structure is disposed in a high frequency induction coil 3 as shown in FIG. When a high frequency current of 1 kHz to 400 kHz is preferably applied to the high frequency induction coil 3, the surface layer portion is induction-heated in a very short time in the heating portion 2 of the steel material 1 located in the high frequency induction coil 3. The heating cycle at this time is as shown in FIG. 3 and is heated (for example, 685 ° C.) in an extremely short time (for example, 0.3 seconds). Attainment of the temperature and induction heating are immediately stopped, and preferably quenched with a water-soluble coolant. The cooling rate at this time is, for example, 100 to 300 ° C./second.

  As a result of the induction heating, as shown in FIGS. 4 and 5B, the steel material 1 has a martensite structure 12 formed in the interior 1a, and the surface layer portion 1b mainly comprises an austenite structure 10. At this time, the internal martensite structure is preferably more than 30% by volume, and the austenite structure of the surface layer part is more than 70% by volume.

The steel material has a significantly higher strength than austenitic steel due to the internal martensite structure. Moreover, since the surface layer portion is mainly composed of an austenitic structure, it is equivalent to austenitic steel and shows hydrogen embrittlement resistance clearly superior to martensitic steel.
The steel material provided by the present invention can be used for various applications that may be exposed to hydrogen, and can be suitably used for applications such as hydrogen fuel cells, hydrogen tanks, and hydrogen stations.
As mentioned above, although this invention was demonstrated based on the said embodiment, this invention is not limited to the content of the said embodiment, Of course, an appropriate change is possible within the scope of the present invention.

  Inventive materials having a composition shown in Table 1 and having a diameter of 7 mm and a length of 20 mm and comparative materials were prepared as test materials. These specimens were subjected to a solution treatment that was maintained at 1100 to 1200 ° C. for 30 to 60 minutes, and cooled by air cooling.

The sample material was further subjected to cold rolling at different rolling reductions (0%, 20%, 40%, 60%, 80%). The volume ratio and the Vickers hardness of martensite were measured for each rolling reduction of the obtained rolled material. The volume ratio of martensite was measured by a saturation magnetization measurement method. These measurement results are shown in the graph of FIG. As is apparent from FIG. 6, the inventive material having a metastable austenite structure has a martensite structure obtained by cold rolling, and the higher the rolling reduction, the larger the volume ratio of the martensite structure, Vickers hardness increases according to the volume ratio. On the other hand, in the comparative material having a stable austenite structure, the formation of martensite structure due to cold rolling is slight, and the volume ratio of the martensite structure does not reach 20% even when the reduction rate is 80% (the invention material is about 90%). ), The increase in Vickers hardness is also small. Note that most of the increase in Vickers hardness in the comparative material is thought to be due to work hardening rather than due to the formation of the martensite structure.
In addition, regarding the inventive material, FIG. 7 shows a structure photograph by an optical microscope of the solution material, the rolling reduction of 40%, and 80%. It can be seen from the structure photograph that a fine martensite structure is generated by cold rolling, and that the fine martensite structure increases as the rolling reduction increases.

Also relates to the aforementioned cold rolled material, subjected to hydrogen occlusion treatment of 86.4ks immersed in 20 _mass% NH 4 SCN solution 323 K, to measure the hydrogen storage capacity, showing the measurement results in FIG. The amount of hydrogen occlusion was measured by measuring the cumulative amount of hydrogen released by raising the temperature of the test material occluded with hydrogen. As apparent from FIG. 8, the hydrogen storage amount increases as the volume ratio of the martensite structure increases. Therefore, it is difficult to use it in a hydrogen environment with a martensite structure.

Using the high frequency induction heating apparatus described in the above embodiment, the cold rolled material was heated at a surface layer portion by a high frequency current of 200 kHz for 0.3 seconds, and the surface layer portion was heated to 685 ° C. Thereafter, induction heating was immediately stopped, and the specimen was rapidly cooled using a water-soluble coolant (cooling rate: 100 ° C./second). The induction heating and the rapid cooling were performed on both sides in the longitudinal direction of the test material.
Of the above induction heating materials, the inventive material is subjected to X-ray diffraction analysis in the vicinity of the surface (depth of about 0.1 mm) and the base portion (depth center), and the results are shown in FIGS. 9 (a) and 9 (b). Indicated. As is clear from FIG. 9, the austenite structure is mainly used in the vicinity of the surface, although a martensite structure exists in part. On the other hand, the substrate portion is composed of a martensite structure. Moreover, the structure observation with the optical microscope was performed about the said surface vicinity and the base part, and the photograph was shown in FIG. Visually, it can be seen that the structure is clearly different between the surface layer portion (austenite structure) and the inside (martensite structure).
For the inventive material, the hardness distribution in the depth direction was further measured. The result is shown in FIG. As is clear from FIG. 11, the hardness of the surface layer portion (up to about 1 mm in depth) mainly composed of the austenite structure is relatively low, and the hardness of the base portion composed of the martensite structure is It is about twice the surface layer.

  Furthermore, the hydrogen release test was performed after performing the above-described hydrogen storage treatment for the inventive material. For comparison, hydrogen storage and release tests were similarly performed on the inventive material as it was processed without induction heating. The result is shown in FIG. As is apparent from FIG. 12, in the inventive material subjected to the induction heat treatment, the hydrogen release amount, that is, the hydrogen storage amount is suppressed to be small. On the other hand, it can be seen that in the as-processed specimen, the amount of hydrogen released, that is, the amount of hydrogen occlusion is very large. As a result, it was understood that hydrogen occlusion is remarkably suppressed by making the surface layer portion mainly an austenite structure by induction heating.

It is a diagram illustrating a process flow in an embodiment of the present invention. Similarly, it is the schematic which shows the state at the time of induction heating. Similarly, it is a figure which shows the thermal cycle at the time of induction heating. Similarly, it is a conceptual diagram which shows the change of the stainless steel structure | tissue accompanying a process flow. Similarly, it is a conceptual diagram which shows the structure | tissue state of the stainless steel material after cold processing, and the stainless steel material after induction heating. It is a graph which shows the relationship between a processing rate, a martensite structure volume ratio, and Vickers hardness about the invention material after a process in an Example, and a comparative material. It is a drawing substitute photograph which shows the structure | tissue about the invention material after processing as a solution in an Example. It is a graph which shows the relationship between a processing rate and hydrogen storage amount about the invention material after a process in an Example, and a comparative material. It is a figure which shows the surface vicinity of the invention material after the induction heating in an Example, and the X-ray-diffraction analysis result of a base part. It is a drawing substitute photograph which shows the surface vicinity of the invention material after the induction heating in an Example, and the structure | tissue of a base part. It is a graph which shows the hardness distribution in the depth direction of the invention material after the induction heating in an Example. It is a graph which shows the hydrogen storage amount of the invention material after the induction heating in an Example, and as processed. It is a graph which shows the hydrogen diffusion distance with time in stainless steel. It is a graph which shows the relationship of a temporal change of atmospheric hydrogen pressure and fatigue limit in stainless steel.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel material 1a Inside 1b Surface layer part 3 High frequency induction coil 10 Austenitic structure 11 Metastable austenitic structure 12 Martensitic structure

Claims (6)

  A method for producing a high-strength steel material having excellent hydrogen embrittlement resistance, wherein the surface layer portion of a steel material having a work-induced martensite structure is induction-heated to reversely transform the work-induced martensite structure of the surface layer portion into an austenite structure. Method for producing a high-strength steel having excellent hydrogen embrittlement resistance according to claim 1, wherein the obtaining a steel having a strain-induced martensitic structure giving the process the austenite steel having a metastable austenite structure. The method for producing a high-strength steel material excellent in hydrogen embrittlement resistance according to claim 2, wherein the austenitic steel material is heat treated to produce the metastable austenite structure. The method for producing a high-strength steel material excellent in hydrogen embrittlement resistance according to any one of claims 1 to 3 , wherein the induction heating is performed by high-frequency induction of 1 kHz to 400 kHz. The steel material, Cr in mass%: 12~25%, Ni: the production of high strength steel having excellent hydrogen embrittlement resistance according to any one of claims 1 to 4, characterized in that it contains 5 to 11% Method. The method for producing a high-strength steel material excellent in hydrogen embrittlement resistance according to any one of claims 1 to 5, wherein the austenite structure is 70% or more by volume in the surface layer portion after the induction heating. .