JP2014077172A - Manufacturing method of stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a martensitic stainless steel in which degradation over time by deposition of chromium carbide is suppressed and hardness is maintained or increased without adding rare resource elements.SOLUTION: A method of manufacturing a stainless steel includes the steps of: mixing a base raw material having composition containing C:1.20 wt.% or less, Si:5.00 wt.% or less, Mn:10 wt.% or less, P:0.200 wt.% or less, S:0.060 wt.% or less or 0.10 wt.% or more, Ni:28.00 wt.% or less, Cr:11.00 to 32.00 wt.%, Mo:7.00 wt.% or less, Cu:1.00 to 5.00 wt.% or less, N:0.30 wt.% or less, and the balance Fe with inevitable impurities; melting the mixture of the base raw material to obtain a base material; conducting a solid solution treatment of the raw material; and heating the base material to a phase transition temperature or more which a face center cubic phase are stable with the speed of 135°C or more per min.

Description

  The present invention relates to a stainless steel manufacturing method for manufacturing stainless steel from a base material having a predetermined composition.

  Stainless steel is steel that is made hard to rust by adding a substance such as Cr or Ni to Fe. Austenitic stainless steel defined by JIS standard (SUS201, SUS202, SUS301, SUS302, SUS303, SUS303Se, SUS303Cu, SUS304, SUS304L, SUS304N1, SUS304N2, SUS304LN, SUS304J3, SUS305, SUS309S, SUS3S16, SUS3S16, SUS3S16 , SUS316Ti, SUS316J1, SUS316J1L, SUS316F, SUS317, SUS317L, SUS317LN, SUS317J1, SUS836L, SUS890L, SUS321, SUS347, SUSXM7, SUSXM15J1, ferritic stainless steel S 30, SUS430F, SUS434, SUS447J1, SUSXM27), martensitic stainless steel (SUS403, SUS410, SUS410J1, SUS410F2, SUS416, SUS420J1, SUS420J2, SUS420F, SUS420F2, SUS431, SUS40B, SUS40B, SUS40B, SUS40B, SUS40B, SUS40B, SUS40B, SUS40B C: 1.20 wt% or less, Si: 5.00 wt% or less, Mn: 10 wt% or less, P: 0.200 wt% or less, S: 0.060 wt% or less, or 0.10 wt% %: Ni: 28.00 wt% or less, Cr: 11.00-32.00 wt%, Mo: 7.00 wt% or less, Cu: 1.00-5.00 wt% or less, N: 0.00. 30% by weight or less, balance: Fe and A variable avoid an impurity element (Non-Patent Document 1).

  In the production of stainless steel, base materials are mixed within the range of this composition, and the mixture is melted to obtain a base material. And in many cases, the solution treatment of a base material is performed for the purpose of homogenizing the composition of a base material. This solution treatment is a treatment in which the substrate is kept at a high temperature equal to or higher than the phase transition temperature at which the high-temperature phase of the substrate is stable for a certain period of time and then cooled to room temperature. A process of heating for about minutes and then lowering to room temperature is well known (Non-Patent Document 2).

  One of the processing methods for further increasing the hardness of the substrate subjected to the solution treatment is quenching. In stainless steel, the face-centered cubic phase is stable at a temperature higher than the phase transition temperature, and the body-centered cubic phase is stable at a low temperature. The phase transition temperature of commonly used stainless steel (C: 2.1 wt% or less) is 727 ° C. to 910 ° C. (Non-patent Document 3). Since the body-centered cubic phase, which is stable at room temperature, has many slip-type dislocations, slip deformation tends to occur and the hardness is low. On the other hand, when the temperature of a face-centered cubic high-temperature stainless steel heated above the phase transition temperature is rapidly lowered to room temperature by immersing it in water or oil, the martensitic stainless steel, which is a body-centered cubic phase containing many defects, is obtained. can get.

  Here, “martensitic stainless steel” generally refers to a body-centered cubic phase (martensitic phase) stainless steel containing many defects, and martensitic stainless steel (a martensitic phase is a metastable phase). Stainless steel having a composition of

  Martensitic stainless steel is characterized by high crystal hardness and high hardness due to the fact that crystals are distorted in various places. Thus, heat treatment that introduces defects into the crystal to increase the hardness of the material is called quenching.

  Martensitic stainless steel is often used in high-temperature and high-pressure environments, but it is known that deterioration over time such as stress corrosion cracking, creep, and reheat cracking occurs when used in a high-temperature environment. One of the main causes of deterioration with time is the formation of chromium carbides at grain boundaries (Patent Document 1). It is known that chromium carbide is formed by carbon contained in a supersaturated state by forming a compound with chromium in the material and precipitating at a grain boundary of stainless steel in order to eliminate the strain. Due to the formation of chromium carbide, a chromium-deficient layer is formed in the adjacent portion, the corrosion resistance in the vicinity of the crystal grain boundary is lowered, and the deterioration with time progresses (Patent Document 1).

  There is a demand for a technique for suppressing deterioration over time of martensitic stainless steel, and in order to suppress deterioration over time, a method of adding an element having an atomic radius larger than Cr as an additive (Patent Documents 1 and 2) is known. ing.

Japan Stainless Steel Association homepage, http://www.jssa.gr.jp/contents/products/standards/jis/ Materials Science and Engineering A 489 (2008) 86, Ohmura T., Sawada K., Kimura K., Tsuzaki K. NIPPON STEEL MONTHLY December 2005 Vol. 154 p. 13 Phys. Status Solidi A, 1973, 17, 199-205, Schlosser W., et al. F. J. et al. Phys. Soc. Jpn. , 1990, 59, 2963-2970, Sumiyama K. et al. Hirose Y .; Nakamura Y .;

JP-A-5-5158 Japanese Patent No. 1323615 JP 2003-313612 A

  However, in the prior art described in the above document, since many of the additive substances are rare resource elements, the material becomes expensive and the recyclability is not satisfied.

  The present invention has been made in view of the above circumstances, and provides a method for producing martensitic stainless steel in which deterioration with time due to chromium carbide precipitation is suppressed and hardness is maintained or increased without adding a rare resource element. The purpose is to do.

  The production method of stainless steel according to the present invention is as follows: C: 1.20% by weight or less, Si: 5.00% by weight or less, Mn: 10% by weight or less, P: 0.200% by weight or less, S: 0.060 % By weight or less or 0.10% by weight or more, Ni: 28.00% by weight or less, Cr: 11.00-32.00% by weight, Mo: 7.00% by weight or less, Cu: 1.00-5.00 A step of mixing a base material having a composition composed of wt% or less, N: 0.30 wt% or less, balance: Fe and inevitable impurity elements, and a step of obtaining a base material by melting the mixture of the base materials And a step of performing a solution treatment of the base material, and a step of heating the base material at a rate of 135 ° C. or more per minute to a phase transition temperature or more at which the face-centered cubic phase becomes stable. It is characterized by.

  In the stainless steel manufacturing method, the solution treatment step includes the step of holding the substrate at about 1050 ° C. for 80 minutes or more, and the substrate at a rate of 35 ° C./min or less and a phase transition temperature or less. And the step of lowering the temperature.

  According to the present invention, stainless steel in which deterioration with time is suppressed can be obtained without adding a rare resource element. Moreover, it becomes possible to introduce more defects than in the prior art, and stainless steel with higher hardness than in the prior art can be obtained.

It is explanatory drawing of the typical manufacturing process of the stainless steel which concerns on embodiment of this invention. It is explanatory drawing of the temperature change of the stainless steel base material in the high temperature holding | maintenance step S13 of FIG. 1, the temperature fall step S14, the quenching step S15 by rapid temperature rising, and the furnace cooling step S16. It is the measurement result of the hardness by the push rod type thermal dilatometer of the martensitic stainless steel manufactured with the conventional manufacturing method, and the stainless steel which concerns on the Example of this invention. FIG. 4A is a transmission electron microscope image of a martensitic stainless steel manufactured by a conventional manufacturing method heated at 700 ° C. for 5 hours, and FIG. 4B is a stainless steel according to an embodiment of the present invention at 700 ° C. for 5 hours. It is a transmission electron microscope image of what was heated. FIG. 5A is a transmission electron microscope image of a martensitic stainless steel manufactured by a conventional manufacturing method heated at 700 ° C. for 10 hours, and FIG. 5B is a stainless steel according to an example of the present invention at 700 ° C. for 10 hours. It is a transmission electron microscope image of what was heated. It is the Vickers hardness measurement result of the martensitic stainless steel manufactured with the conventional manufacturing method, and the stainless steel which concerns on the Example of this invention. FIG. 7A is a diagram showing a measurement result of X-ray diffraction of martensitic stainless steel manufactured by a conventional manufacturing method, and FIG. 7B is an X-ray before rapid temperature increasing heat treatment of stainless steel according to an embodiment of the present invention. FIG. 7C is a diagram showing a measurement result of X-ray diffraction after a rapid temperature increase heat treatment of stainless steel according to an example of the present invention. FIG. 8A is a transmission electron microscope image of martensitic stainless steel manufactured by a conventional manufacturing method, and FIG. 8B is a transmission electron microscope image of stainless steel according to an example of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram of a manufacturing process of stainless steel according to an embodiment of the present invention. FIG. 2 is an explanatory view of the temperature change of the stainless steel substrate in the high temperature holding step S13, the temperature lowering step S14, the quenching step S15 by the rapid temperature rising, and the furnace cooling step S16 in FIG. In FIG. 2, the vertical axis represents the temperature of stainless steel, and the horizontal axis represents the elapsed time.

  First, C: 1.20% by weight or less, Si: 5.00% by weight or less, Mn: 10% by weight or less, P: 0.200% by weight or less, S: 0.060% by weight or less, or 0.10% by weight Ni: 28.00 wt% or less, Cr: 11.00-32.00 wt%, Mo: 7.00 wt% or less, Cu: 1.00-5.00 wt% or less, N: 0.30 A base material to be weight% or less and the balance: Fe and unavoidable impurity elements is prepared and mixed (step S11). This base material is the distribution range of the composition of austenitic stainless steel, ferritic stainless steel and martensitic stainless steel specified by JIS standard, and its composition is determined according to the standard of stainless steel actually used. The

  Next, the mixed base material is heated to the melting point or higher, melted and processed, and molded into a predetermined shape to obtain a base material (step S12). A so-called solution treatment is performed in which the base material is kept at a high temperature equal to or higher than the phase transition temperature at which the high-temperature phase of the base material becomes stable, and then the base material is cooled to room temperature. The solution treatment here refers to a treatment capable of homogenizing the composition of the base material and reducing the phase in which carbon is included in supersaturation. For example, in the embodiment of the present invention, the substrate is heated at about 1050 ° C. for about 80 minutes in the high temperature holding step (step S13). By this heat treatment at a high temperature, the composition of the substrate can be homogenized. The base material that has been heat-treated at a high temperature is slowly cooled to room temperature in a furnace (step S14). The temperature decreasing rate is preferably a rate of 35 ° C. or less per minute. By performing such temperature reduction, the phase in which carbon is included in supersaturation can be reduced, and the cause of chromium carbide formation can be reduced.

  Next, quenching by rapid temperature rise, that is, the substrate that has been cooled to room temperature is rapidly heated from room temperature to the phase transition temperature or higher (step S15), and then quickly cooled to room temperature (step S16). . The heating rate in step S15 is preferably a rate of 135 ° C. or more per minute. Moreover, the rate of temperature decrease in step S16 is preferably 10 ° C. or more per second, and the faster the better. The phase transition from the body-centered cubic phase to the face-centered cubic phase can be partially caused by rapidly raising the temperature to above the phase transition temperature and then quickly lowering it to room temperature. Can be obtained.

  Hereinafter, the present invention will be described in more detail with reference to examples. For parts corresponding to the description of the embodiment, refer to FIG. 1 used in the description of the embodiment as appropriate. Although the embodiment shown here is performed using a material of JIS standard SUS304, the present invention is widely applied to stainless steel containing C and Cr, and is not limited to this.

  SUS304 of the base material used in this example is C: 0.08% by weight or less, Si: 1.00% by weight or less, Mn: 2.00% by weight or less, P: 0.045% by weight or less, S : 0.030 wt% or less, Ni: 8.00 to 10.50 wt% or less, Cr: 18.00 to 20.00 wt%, balance: Fe and unavoidable impurity elements are prepared. These are mixed (step S11), heated to the melting point of the base material or higher, melted and processed, and molded into a predetermined shape (step S12). Thereafter, the substrate was heated at about 1050 ° C. for about 80 minutes (step S13), and the temperature was lowered to room temperature at a rate of about 35 ° C. per minute in the furnace (step S14). Next, the temperature of the substrate was raised at a rate of about 135 ° C. per minute (step S15), and the temperature was lowered to room temperature at a rate of about 10 ° C. per second (step S16).

  Hereinafter, the stainless steel according to the embodiment of the present invention is suppressed in length change due to thermal aging, and also suppresses precipitation of chromium carbide considered to be a cause of deterioration with time, using FIG. 3, FIG. 4, and FIG. Indicates that

  FIG. 3 shows the heating time dependency of the length by the push rod type thermal dilatometer when the stainless steel according to the embodiment of the present invention and the martensitic stainless steel manufactured by the conventional manufacturing method are heated at 700 ° C. It is a measurement result. The vertical axis represents the change in the length of the martensitic stainless steel sample, and the horizontal axis represents the heating time.

  The measurement result (A) of the stainless steel according to the example of the present invention has almost no change in length due to thermal aging, whereas the measurement result (B) of the martensitic stainless steel manufactured by the conventional manufacturing method is The length change with thermal aging is large.

  A large length change measured by a push rod thermal dilatometer indicates that the material has a large deterioration with time, and a small length change indicates that the material has a small deterioration with time. From the results, it can be seen that the stainless steel according to the example of the present invention is less deteriorated over time than the martensitic stainless steel manufactured by the conventional manufacturing method.

  FIG. 4A is a transmission electron microscope image of a martensitic stainless steel manufactured by a conventional manufacturing method heated at 700 ° C. for 5 hours, and FIG. 4B is a stainless steel according to an embodiment of the present invention at 700 ° C. for 5 hours. It is a transmission electron microscope image of what was heated. All fields of view are 3 μm square.

  In FIG. 4A, the shade that runs linearly is a crystal grain boundary, and the other shades are contrasts that are formed by differences in the thickness of the sample. The black spots observed along the crystal grain boundaries are chromium carbides. It was confirmed by energy dispersive X-ray spectroscopy that the black spots were chromium carbide. The shade running straight in FIG. 4B is a grain boundary of the crystal, but no black spots are observed along this.

  From this result, the stainless steel according to the example of the present invention does not precipitate chromium carbide that causes deterioration with time even when heated at 700 ° C. for 5 hours, whereas the martensitic stainless steel manufactured by the conventional manufacturing method. It can be seen that when the steel is heated at 700 ° C. for 5 hours, chromium carbides that cause deterioration over time are precipitated.

  FIG. 5A is a transmission electron microscope image of a martensitic stainless steel manufactured by a conventional manufacturing method heated at 700 ° C. for 10 hours, and FIG. 5B is a stainless steel according to an example of the present invention at 700 ° C. for 10 hours. It is a transmission electron microscope image of what was heated. All fields of view are 3 μm square.

  The shade running straight in FIG. 5A is the grain boundary of the crystal, and the other shade is the contrast that can be generated by the difference in the thickness of the sample. It can be seen that the black spots (chromium carbide) observed along the crystal grain boundaries are clearly increased as compared to FIG. 4A. The shade running straight in FIG. 5B is the crystal grain boundary, but no black spots are observed along this.

  From this result, the stainless steel according to the example of the present invention does not precipitate chromium carbide which causes deterioration with time even when heated at 700 ° C. for 10 hours. It can be seen that when steel is heated at 700 ° C. for 10 hours, chromium carbides that cause deterioration over time are precipitated more than heating for 5 hours.

  The above characteristics were the same even when another part of the same sample was observed. In the martensitic stainless steel manufactured by the conventional manufacturing method, when heating is continued at 700 ° C., chromium carbide precipitates at the grain boundary and increases with time. In such stainless steel, no precipitation of chromium carbide was observed.

  From the above, it was found that the stainless steels according to the examples of the present invention were able to suppress deterioration over time due to chromium carbide precipitation and to actually suppress the length change accompanying thermal aging.

  In the conventional manufacturing method, a martensite phase is obtained by generating a part of bcc (low temperature phase) having a lattice constant of 0.286 nm by rapid cooling from fcc (high temperature phase) having a lattice constant of 0.355 nm. Therefore, carbon tends to be included in the supersaturation in the crystal lattice, and the carbon contained in the supersaturation leads to deterioration with time due to thermal aging as precipitation of chromium carbide, whereas the embodiment of the present invention has a lattice constant of Compared with the conventional manufacturing method, a martensite phase is obtained by generating a part of fcc (high temperature phase) having a lattice constant of 0.355 nm by rapid heating from bcc (low temperature phase) of 0.286 nm. It is difficult to include carbon in supersaturation. For this reason, it is thought that precipitation of chromium carbide is suppressed and deterioration with time is suppressed. In addition, as described later, the conventional manufacturing method in which a high temperature stainless steel having a face-centered cubic phase heated to a phase transition temperature or higher is rapidly cooled to room temperature by immersing it in water or oil has a low defect density, and Cr However, the temperature rapidly rises from room temperature to the phase transition temperature or higher (step S15), and quickly drops to room temperature (step S16). In the examples of the present invention, since the density of defects is high and the diffusion of Cr is inhibited by the defects, it is considered that Cr hardly precipitates at the grain boundaries.

  Hereinafter, it is shown using FIG. 6 that the hardness is maintained simultaneously with the suppression of the deterioration with time due to the thermal aging.

  FIG. 6 is a measurement result of Vickers hardness of martensitic stainless steel manufactured by a conventional manufacturing method and stainless steel according to an embodiment of the present invention. The martensitic stainless steel manufactured by the conventional manufacturing method has an average of 191, while the stainless steel according to the embodiment of the present invention has an average of 216, which is clearly high in hardness. That is, the stainless steel according to the present example not only suppressed the deterioration over time due to heating, but also had a higher hardness than the conventional stainless steel.

  Below, the result of having analyzed about the origin of this hardness improvement is demonstrated using FIG.

  FIG. 7A is a diagram showing a measurement result (XRD spectrum) of X-ray diffraction of martensitic stainless steel manufactured by a conventional manufacturing method, and FIG. 7B is a rapid temperature increasing heat treatment of stainless steel according to an embodiment of the present invention. It is a figure which shows the measurement result of previous X-ray diffraction, and FIG. 7C is a figure which shows the measurement result of the X-ray diffraction after the rapid temperature rising heat processing of the stainless steel which concerns on the Example of this invention. 7A to 7C, the horizontal axis indicates the complementary angle between the X-ray incident axis and the axis connecting the sample to the detector, and the vertical axis indicates the intensity of the scattered X-ray detected by the detector. It is shown in units. P1 is the diffraction peak of fcc (111), P2 is the diffraction peak of fcc (200), P3 is the diffraction peak of fcc (220), P4 is the diffraction peak of bcc (110), and P5 is bcc. The diffraction peak of (200) is shown.

  In the measurement result shown in FIG. 7A, peaks were measured at 43.44 °, 44.24 °, 50.42 °, 64.46 °, and 74.50 °, respectively. According to previous studies, in the case of the face-centered cubic phase (fcc), the peak of the XRD spectrum from stainless steel is (111) plane peak P1 near 43 °, (200) plane peak P2, 74 near 50 °. It is known that a peak P3 on the (220) plane appears in the vicinity of ° (Non-Patent Document 4). In the case of the body-centered cubic phase (bcc), it is known that the (110) plane peak P4 appears at around 45 ° and the (200) plane peak P5 appears at around 65 ° (Non-patent Document 5). From the measurement results of FIG. 7A, the peaks of P1, P2, P3, P4, and P5 were observed, so the martensitic stainless steel manufactured by the conventional manufacturing method has face centered cubic phase (fcc) and body center. It can be seen that the cubic phase (bcc) is mixed.

  In the measurement result shown in FIG. 7B, peaks were measured at 44.3 ° and 64.36 °, respectively. From the measurement result of FIG. 7B, the peaks P4 and P5 of the body-centered cubic phase (bcc) were measured, but the peak of the face-centered cubic phase (fcc) was not measured. Therefore, the stainless steel according to the example of the present invention. It can be seen that the stainless steel before the rapid heat-up heat treatment of the steel (before step S15) has a body-centered cubic phase.

  In the measurement result shown in FIG. 7C, peaks were measured at 43.44 °, 44.24 °, 50.44 °, 64.46 °, and 74.50 °, respectively. From the measurement results of FIG. 7C, the peaks of P1, P2, P3, P4, and P5 were observed, so that the stainless steel according to the example of the present invention is the same as the martensitic stainless steel manufactured by the conventional manufacturing method. Further, it can be seen that the martensite phase is a mixture of the face-centered cubic phase (fcc) and the body-centered cubic phase (bcc).

  As described above, the stainless steel according to the present example is a stainless steel produced by a conventional production method in which the stainless steel that is in the body-centered cubic phase by the temperature drop after holding at a high temperature is transformed into martensite by the rapid temperature rise heat treatment. It was found that the crystal structure was similar to that of steel.

  The reason why the hardness of the stainless steel obtained by the embodiment of the present invention is higher than that of the martensitic stainless steel manufactured by the conventional manufacturing method is shown in FIG. It will be explained that it is considered to be an effect of suppressing the slip deformation of the material due to the high density defects formed in (1).

  FIG. 8A is a transmission electron microscope image of martensitic stainless steel manufactured by a conventional manufacturing method, and FIG. 8B is a transmission electron microscope image of stainless steel according to an example of the present invention. All fields of view are 3 μm square.

  The shading that runs in a straight line is the grain boundary of the crystal, and the other shading is the contrast that can be produced by the difference in the thickness of the sample. In FIG. 8A, since the short-terminated contrast is not observed, it can be seen that the density of the introduced defects is so small that it does not appear in the observation range. This feature was the same even when another part of the sample was observed. On the other hand, in FIG. 8B, a lot of contrast terminated shortly in the crystal plane is observed. From this observation result, it was found that the stainless steel according to the example of the present invention has more defects formed than the martensitic stainless steel manufactured by the conventional manufacturing method. The reason why the hardness of the stainless steel obtained by the example of the present invention is higher than that of the martensitic stainless steel manufactured by the conventional manufacturing method shown in FIG. 6 is observed in the electron microscope image of FIG. 8B. It strongly suggests that it is an effect of suppressing slip deformation of the material due to high density defects.

  According to the method for producing stainless steel according to the embodiment of the present invention, martensitic stainless steel in which deterioration with time is suppressed rather than martensitic stainless steel produced by a conventional production method without adding a rare resource element. Can get. In addition, it is possible to introduce more defects than in the prior art while maintaining the crystal structure itself, and a stainless steel with higher hardness than in the prior art can be obtained.

  It should be noted that the present invention is not limited to the above-described embodiment, and various configurations can be adopted without departing from the gist of the present invention.

Claims (2)

C: 1.20% by weight or less, Si: 5.00% by weight or less, Mn: 10% by weight or less, P: 0.200% by weight or less, S: 0.060% by weight or less, or 0.10% by weight or more, Ni: 28.00 wt% or less, Cr: 11.00 to 32.00 wt%, Mo: 7.00 wt% or less, Cu: 1.00 to 5.00 wt% or less, N: 0.30 wt% Hereinafter, a step of mixing a base material having a composition composed of the balance: Fe and inevitable impurity elements,
Melting the mixture of substrate materials to obtain a substrate;
Performing a solution treatment of the substrate;
Heating the substrate at a rate of 135 ° C. or more per minute to a phase transition temperature or higher at which the face-centered cubic phase is stable;
A method for producing stainless steel, comprising: It is a manufacturing method of the stainless steel of Claim 1,
The step of performing the solution treatment includes
Holding the substrate at about 1050 ° C. for 80 minutes or more;
Lowering the substrate to a temperature below the phase transition temperature at a rate of 35 ° C. or less per minute;
A method for producing stainless steel, comprising:

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