JP5034583B2 - Heat treatment method for duplex stainless steel pieces - Google Patents

Description

  In the present invention, when softening heat treatment of a duplex stainless steel piece produced by hot forging or hot rolling or as cast, it is softened to such an extent that machining can be carried out sufficiently, and internal cracks are prevented. The present invention relates to a heat treatment method that can be prevented.

  Billets and blooms such as billet and bloom of duplex stainless steel are made of, for example, long steel pieces produced from a steel ingot by hot forging or hot rolling, softening heat treatment, cutting and cutting, etc. It is manufactured by machining. The reason why the softening heat treatment is performed is to erase the sigma phase in advance and obtain a hardness that can be machined.

  Conventionally, in softening heat treatment, after soaking at a solution temperature (1050 to 1150 ° C.), rapid cooling such as water cooling is performed in order to prevent formation of a sigma (σ) phase. However, during the cooling process, thermal stress is generated due to the temperature difference between the surface layer part and the inside of the slab, especially in the slab having a large cross-sectional area, the residual stress increases, and an internal crack occurs almost over the entire length of the slab. There is a case.

  As a method for suppressing cracking due to this residual stress, Patent Document 1 discloses that after heating to 1000 ° C. or higher, it is cooled to 500 ° C. at a cooling rate of 5 to 12.5 ° C./min, and then at least 300 ° C. to 10 ° C. By cooling at a rate of at least ° C./min, residual stress generated by rapid cooling after heat treatment is suppressed, and further, 475 brittleness is avoided, and a large casting of duplex stainless steel with excellent corrosion resistance and toughness, forging A method of manufacturing an article is disclosed. Further, among them, slow cooling (5 to 12.5 ° C./min) may be performed up to 500 ° C. in principle, but preferably only about 980 to 700 ° C. which is the sigma phase precipitation temperature. It is described that it is rapidly cooled at / min or more.

  However, according to the study by the present inventors, the residual stress is not sufficiently suppressed at a cooling rate of 5 to 12.5 ° C./min, and it is not possible to reliably prevent cracking. In addition, although the quenching is performed at 10 ° C./min or more in the sigma phase precipitation temperature region, there is a problem that the sigma phase is precipitated at a cooling rate of about 10 to 20 ° C./min and cannot be sufficiently softened.

  Patent Document 2 discloses that a stainless steel hollow cast material is heated and held at a solid solution temperature and then gradually cooled to 800 to 900 ° C. (cooling rate: 10 ° C./min or less, for example, 5 to 8 ° C./min). After that, by rapid cooling (water cooling or mist cooling; cooling rate of 20 ° C./min or more, for example, 30 to 40 ° C./min), the thick wall thickness reduces the residual stress due to thermal stress in the cooling process after solution heat treatment A solution heat treatment method for large diameter stainless steel castings is disclosed. Here, since the effect of reducing residual stress is small when the furnace is cooled to 950 ° C. and then rapidly cooled, it is necessary to cool the furnace to 800 to 900 ° C., which is lower than that.

  However, the material handled by this heat treatment method is a hollow cast material, and the generated residual stress is considered to be small compared to a solid steel slab. Furthermore, in this method, after heating and holding at the solid solution temperature, it is said that the glass is gradually cooled to 800 to 900 ° C. However, according to the study of the present inventor, when it is gradually cooled to 900 ° C. or lower, precipitation of the sigma phase occurs. It cannot be completely prevented and the machinability deteriorates. In addition, when the cooling rate is 5 to 8 ° C./min, the residual stress is not sufficiently suppressed, and the cracks cannot be reliably prevented.

Japanese Patent Laid-Open No. 9-217149 JP-A-2-236220

  In the present invention, when performing softening heat treatment of a solid duplex stainless steel piece produced by hot forging or hot rolling or as cast, machining such as cutting and cutting can be performed sufficiently. It aims at providing the heat processing method which can suppress generation | occurrence | production of a residual stress and can prevent an internal crack.

  In order to solve the above problems, the present inventor first examined changes in billet center stress when a heat treatment was performed on a billet of a duplex stainless steel in order to clarify the cause of internal cracking.

  FIG. 1 is a diagram showing an example of a thermal stress analysis result when a billet of chrome molybdenum steel (JIS SCM440H equivalent material, 270 mmφ) is heated and rapidly cooled. A diagram schematically showing the stress distribution in the billet radial direction in the cooling process (during water cooling and cooling) is also shown. From this figure, it is possible to infer changes in billet center stress during conventional softening heat treatment.

  That is, when rapidly cooled after heating, the outer surface portion of the billet is rapidly cooled and tends to shrink, but the center portion remains at a high temperature and cannot be shrunk, and compressive stress is generated in the center portion of the billet 1 (see FIG. 1). (See also the stress distribution diagram (a)). On the other hand, after the rapid cooling, the central part is cooled and is about to shrink when it is allowed to cool, but since it has already been cooled to 200 ° C. or less and is restrained by the outer surface part having a considerable strength, A large tensile stress is generated (see (b)). The billet center stress was found to be greater in the cooling process than in the heating process (in FIG. 1, Fc [maximum tensile stress during cooling]> Fh [maximum tensile stress during heating]. ). Therefore, it is considered that the internal crack occurs due to a large tensile stress acting on the center of the material during the cooling process.

  FIG. 2 is a diagram schematically showing the state of occurrence of internal cracks. The sigma phase precipitation zone 3 near the center of the slab 2 remains in the raw steel slab (round steel 4), and an internal crack 5 is generated in the vertical direction that is most likely to break with respect to the sigma phase precipitation zone 3 during softening heat treatment. I guess that.

  From this stress analysis result, the present inventor is due to the fact that the component segregation at the center of the long material steel slab (hereinafter simply referred to as “steel slab”) of duplex stainless steel and the solid solution during soaking are not sufficient. When the sigma phase remained and the brittle sigma phase remained in the center, it was estimated that an internal crack would occur when a large tensile stress occurred in the center of the steel slab.

  Therefore, as a result of repeated testing and examination by changing the size of the slab, soaking temperature, cooling conditions after soaking, etc., when carrying out softening heat treatment of the duplex stainless steel slab, The following knowledge was obtained regarding conditions necessary to prevent deterioration of machinability during processing.

  (A) When heated to the solution temperature and then rapidly cooled (water cooled), internal cracks occur in duplex stainless steel pieces with a large cross-sectional area, but thermal stress is small and cracks occur when the cross-sectional area is small. do not do.

  (B) In order to prevent internal cracks occurring at the center part of the steel slab, it is necessary to minimize the residual stress generated due to the temperature difference between the steel slab interior and the outer surface part during the cooling process. For that purpose, it is necessary to minimize the thermal stress caused by the temperature difference between the inside and outside of the steel slab at the initial stage of cooling after completion of heating (that is, in the high temperature range). Cooling in the high temperature range is as low as possible. It is effective to perform at a speed (slow cooling).

  (C) If the steel slab temperature at the start of water cooling is lowered, the thermal stress is reduced and the tensile stress at the center of the slab is reduced, but it is difficult to reduce the temperature at the start of water cooling in order to eliminate the sigma phase. It is. Therefore, it is preferable to perform slow cooling at a low cooling rate before reaching the sigma phase precipitation start temperature (900 ° C.).

  (D) Further, the duplex stainless steel easily forms a hard sigma phase that deteriorates the machinability such as cutting, and becomes brittle when heated to a range of 400 to 550 ° C. (brittle at 475 ° C.). Therefore, it is necessary to cool (rapid cooling) as fast as possible in a temperature range where generation of sigma phase and brittleness at 475 ° C. are problems.

  The present invention has been completed on the basis of the above-described examination results, and has the gist of the following heat treatment method for duplex stainless steel pieces.

  That is, a solid duplex stainless steel piece having a diameter of 300 to 500 mm or a length of 300 to 500 mm to the opposite surface is soaked at a temperature of 1050 ° C. or higher, and then the outer surface temperature of the steel piece is 920 to 920 ° C. Cooling to 1000 ° C. at a cooling rate of 150 ° C./hr (2.5 ° C./min) or less, and further cooling from that temperature to at least 300 ° C. at a cooling rate of 30 to 40 ° C./min This is a heat treatment method.

  In this heat treatment method, the cooling to 920 to 1000 ° C. at a cooling rate of 150 ° C./hr (2.5 ° C./min) or less is performed in a heat treatment furnace or by keeping the temperature in a heat insulating material or the like. it can.

  According to the heat treatment method of a duplex stainless steel piece of the present invention, when performing softening heat treatment of a solid duplex stainless steel piece manufactured by hot forging or rolling or as cast, cutting, cutting, etc. Generation of residual stress can be suppressed to the extent that machining can be sufficiently performed, and internal cracks can be prevented.

  The duplex stainless steel slab targeted by the present invention is, for example, a long steel slab manufactured from a steel ingot by hot forging or hot rolling, or an as-cast steel slab, and a billet having a diameter of 300 to 500 mm Or a square member such as a bloom having a short side of 300 mm or more and a long side of 500 mm or less. The reason why both are solid materials is that if they are hollow materials, the thermal stress generated during the heat treatment is small, and it is not necessary to apply the heat treatment method of the present invention.

  In addition, the size of the steel slab is limited as described above because if the diameter or length to the opposite surface exceeds 500 mm, the thermal stress generated during the heat treatment is large, and the heat treatment method of the present invention is applied. It is difficult to suppress the occurrence of residual stress and prevent internal cracks. On the other hand, if it is less than 300 mm, the thermal stress is small, and cracks do not occur even if the heat treatment method of the present invention is not used. is there.

  Examples of the duplex stainless steel include duplex stainless steel having a duplex structure of austenite-ferrite represented by JIS SUS329J1, or duplex stainless steel based on the chemical composition of the duplex stainless steel. In addition, the steel slab of this duplex stainless steel should just satisfy | fill the said shape, and includes the forged or rolled steel slab and the as-cast steel slab.

  In the heat treatment method for duplex stainless steel according to the present invention, the soaking temperature is set to 1050 ° C. or more. If the temperature is less than 1050 ° C., the steel slab is not sufficiently solidified (austenitic) and the sigma phase remains, and the softening heat treatment is performed. This is because the later machinability is inferior. The upper limit of the soaking temperature is not particularly limited, but if it is excessively increased, the cost required for heating rises and there are restrictions on equipment, so it is desirable that the temperature be about 1250 ° C. or lower.

  In the heat treatment method of the present invention, after the soaking is performed in this manner, the outer surface temperature of the steel slab is slowly cooled to 920 to 1000 ° C. at a cooling rate of 150 ° C./hr (2.5 ° C./min) or less. (Here, this cooling is referred to as “primary cooling”), and further, rapid cooling is performed from that temperature to at least 300 ° C. at a cooling rate of 30 to 40 ° C./min (this is referred to as “secondary cooling”).

FIG. 3 is a diagram schematically showing a heat treatment pattern in the heat treatment method of the present invention. As shown in the figure, a steel piece satisfying the above shape is soaked at a soaking temperature T ₀ , then slowly cooled to a primary cooling stop temperature T ₁ at a primary cooling rate V ₁ , and then at a secondary cooling rate V ₂ . rapidly cooling to a secondary cooling stop temperature T _2.

  Thus, there is a feature of the present invention in that the size of the target steel slab is defined, and heat treatment that combines the slow cooling and the rapid cooling at the predetermined cooling rate is performed. While reducing the generated residual stress as much as possible and suppressing the sigma phase remaining and generation at 475 ° C., it is possible to prevent deterioration of machinability and occurrence of internal cracks.

The primary cooling rate V ₁ after soaking is set to 150 ° C./hr (2.5 ° C./min) or less because if the cooling rate is faster than this, the temperature difference between the inside and outside of the steel slab increases. This is because cracks occur due to stress. The lower limit is not particularly defined, but if it is too slow, it takes too much time for cooling, and the productivity is hindered. Therefore, it is desirable to set it to 50 ° C./hr (0.83 ° C./min) or more.

The upper limit of the primary cooling stop temperature T ₁ is set to 1000 ° C., when rapid cooling (secondary cooling) after slow cooling (primary cooling) is performed from a high temperature exceeding 1000 ° C. Not only does thermal stress increase due to external temperature differences and internal cracks are likely to occur, but there is also a large residual stress inside the steel slab that is compressed and tensile inside, and the blade bites into the steel slab during cutting. This is because the machinability deteriorates, such as the inability to process. On the other hand, if the lower limit is set to 920 ° C., if it is lower than this temperature, it becomes a sigma phase precipitation temperature region, and no matter how much rapid cooling thereafter, sigma phase precipitation cannot be completely prevented, and machinability deteriorates. Because.

The lower limit of the secondary cooling rate V ₂ is set to 30 ° C./min. If the cooling rate is slower than 30 ° C./min, a sigma phase is generated, the hardness of the outer surface of the steel slab is increased, and cutting work is performed. This is due to a decrease in machinability. On the other hand, the upper limit is 40 ° C./min. If the cooling rate is faster than this, the thermal stress due to the temperature difference between the inside and outside of the steel slab is large and internal cracking does not occur, but compression is not applied to the outside of the steel slab. This is because there is a large residual stress in the interior, and the machinability deteriorates, for example, the blade bites into the steel piece during cutting and the machining becomes impossible.

The reason why the secondary cooling stop temperature T ₂ is rapidly cooled to at least 300 ° C., that is, until the outer surface temperature of the steel slab is 300 ° C. or less is to avoid precipitation of sigma phase and brittleness at 475 ° C. There is no particular limitation on the lower limit.

  In the heat treatment method of the present invention, there is no limitation on the means for cooling to 920 to 1000 ° C. (that is, primary cooling) at a cooling rate of 150 ° C./hr (2.5 ° C./min) or less. For example, the steel piece may be once discharged from the heat treatment furnace, and then charged in another heat treatment furnace in which the furnace temperature is appropriately adjusted and cooled.

  Moreover, after discharging a steel piece from a heat processing furnace once, the method of heat-retaining using a heat insulating material etc. is also employable. As a heat insulating material or the like, Isowool (made by Isolite Industry Co., Ltd.) or the like can be used.

Example 1
A duplex stainless steel bloom corresponding to SUS321J1 continuously cast to 390 mm × 700 mm square was hot forged to produce three types of billets having diameters of 250 mm, 300 mm, and 350 mm. After these billets were soaked at a soaking temperature T ₀ of 1050 ° C., the primary cooling stop temperature T ₁ was 1000 ° C., the secondary cooling rate V ₂ was 30 ° C./min, and the secondary cooling stop temperature T ₂ was 300 ° C. The temperature was kept constant at 0 ° C., only the primary cooling rate V ₁ was changed, and softening heat treatment was performed, and the presence or absence of cracks in the center was examined.

  The survey results are shown in Table 1. The number of billets subjected to the test is 10 for each condition.

As is apparent from the results in Table 1, the billet with a diameter D of 250 mm has a primary cooling rate within a range of 100 ° C./hr (1.67 ° C./min) to 200 ° C./hr (3.33 ° C./min). Internal cracking did not occur even when changed. However, when the diameter D is 300 mm or 350 mm, the primary cooling rate V ₁ was 100 ° C./hr (1.67 ° C./min), 150 ° C./hr (2.5 ° C./min), but there was no cracking. At 200 ° C./hr (3.33 ° C./min), an internal crack occurred in one of the ten pieces.

(Example 2)
Among the duplex stainless steel billets used in Example 1, a steel piece (billet) having a diameter D of 300 mm produced in the same manner was used, and the primary cooling rate V ₁ was 150 ° C. / softening heat treatment was performed by changing the soaking temperature T ₀ , the primary cooling stop temperature T ₁ , the secondary cooling rate V _2, and the secondary cooling stop temperature T ₂ as appropriate at a constant hr (2.5 ° C / min) Later machinability or cracking rate was investigated.

  The survey results are shown in Table 2. “Good” in the column of “Machinability” in Table 2 is the case where processing was possible without any trouble. “Fair” means that the hardness of the outer surface of the billet is increased, or the blade bites into the steel piece during cutting. This represents the case where processing could not be performed.

From the results shown in Table 2, when the softening heat treatment was performed under the conditions specified in the present invention (Nos. 1 to 16), there was no occurrence of internal cracks and the outer surface could be cut without any trouble. . However, when the soaking temperature T ₀ is low (No. 17), the primary cooling stop temperature T ₁ is too low (No. 18), the secondary cooling rate V ₂ is too slow (No. 20), or two If the next cooling stop temperature T ₂ is too high (No.22), the hardness of the billet outer surface has increased, could not be performed is cutting. Further, when the primary cooling stop temperature T ₁ is too high (No. 19) or when the secondary cooling rate V ₂ is too fast (No. 21), internal cracks did not occur, but during the cutting due to residual stress. The cutting blade was caught in the steel piece and could not be processed.

  In the heat treatment method for duplex stainless steel pieces of the present invention, the steel pieces in a predetermined size range are soaked, and then cooled to a temperature of 920 to 1000 ° C. at a cooling rate of 150 ° C./hr or less at the outer surface temperature of the steel pieces. Furthermore, by the method of cooling from that temperature to at least 300 ° C. at a cooling rate of 30-40 ° C./min, the occurrence of residual stress is suppressed to the extent that machining such as cutting and cutting can be sufficiently performed, and internal cracks Can be prevented.

  Therefore, the heat treatment method of the present invention can be suitably used when manufacturing a billet or a bloom of a duplex stainless steel using a long steel piece forged or rolled or a steel piece as cast as a raw material. .

It is a figure which shows an example of the thermal-stress analysis result at the time of heating the billet (SCM440H equivalent material, 270 mmphi) of chromium molybdenum steel, and carrying out rapid cooling. It is a figure which shows typically the generation | occurrence | production state of an internal crack. It is a figure which shows typically the heat processing pattern in the heat processing method of this invention.

Explanation of symbols

1: billet 2: slab 3: sigma phase precipitation zone 4: round steel 5: internal crack

Claims (3)

  A solid duplex stainless steel piece having a diameter of 300 to 500 mm or a length up to the opposite surface of 300 to 500 mm is soaked at a temperature of 1050 ° C. or higher, and then the outer surface temperature of the steel piece is 920 to 1000 ° C. Is cooled at a cooling rate of 150 ° C./hr or less, and further is cooled from that temperature to at least 300 ° C. at a cooling rate of 30 to 40 ° C./min.   The method for heat treatment of duplex stainless steel pieces according to claim 1, wherein cooling to 920 to 1000 ° C at a cooling rate of 150 ° C / hr or less is performed in a heat treatment furnace.   The method for heat-treating a duplex stainless steel piece according to claim 1, wherein the cooling to 920 to 1000 ° C at a cooling rate of 150 ° C / hr or less is performed by heat insulation with a heat insulating material or the like.

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