JP2011168838A - Duplex stainless steel material for vacuum vessel and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an Ni-saving type duplex stainless steel material for a vacuum vessel, which replaces austenitic stainless steel. <P>SOLUTION: The duplex stainless steel material for a vacuum vessel has a composition comprising, by mass, ≤0.06% C, 0.05 to 1.5% Si, 0.5 to 10.0% Mn, ≤005% P, ≤0.010% S, 0.1 to 5.0% Ni, 18.0 to 25.0% Cr, 0.05 to 0.30% N and 0.001 to 0.05% Al, and in which the total hydrogen content in the steel is ≤3 ppm, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an inexpensive Ni-saving duplex stainless steel material excellent in gas desorption characteristics for a vacuum vessel and a method for producing the same.

  The production of semiconductor elements, liquid crystal panels, and thin-film solar cells has been increasing rapidly in recent years, and the size of products has been increasing. The manufacture of these products requires a vacuum process, and metal materials such as stainless steel, aluminum, and titanium are used as vacuum containers. Conventionally, austenitic materials such as SUS304 steel have been used for stainless steel as a vacuum vessel. With the increase in size of vacuum equipment, thick stainless steel materials with a thickness of up to about 80 mm are being used.

A characteristic required for a vacuum vessel material is a small gas emission. In particular, the effects of surface polishing conditions and baking treatment on the gas release characteristics of austenitic stainless steel, aluminum alloy, titanium, etc. as ultra-high vacuum materials have been studied (see Non-Patent Document 1). It is known that reduction and reduction of the surface oxide layer are effective. It is also known that long-time baking at 100 to 450 ° C. is effective (see Non-Patent Document 2).
Moreover, the knowledge which improves a vacuum characteristic is disclosed by forming the coating film with high Mn content in the austenitic stainless steel material which contains many Mn (refer patent document 1), and material development from such a viewpoint is also possible. it is conceivable that.

  Another required property is strength and weldability. This characteristic becomes more and more important as the size of the vacuum vessel increases. In particular, for a member that repeats atmospheric pressure and vacuum, such as a preliminary exhaust chamber, it is reasonable to apply a high-strength material having excellent fatigue characteristics. However, the lower limit of the yield strength of austenitic stainless steel is about 200 MPa, which is a characteristic that is desired to be improved as a material for a vacuum vessel that increases in size.

  The duplex stainless steel contains a large amount of Cr and Mo, and has a characteristic that the strength is higher than that of the austenitic stainless steel, but since it is an expensive material, there are few application examples for vacuum vessels. Recently, however, duplex stainless steels with reduced Ni content and increased Mn content have been developed, and it may be applicable through thinning of container materials from the viewpoint of steel material cost. However, in duplex stainless steel, ductility and toughness may be impaired during component design such as Ni saving and Mn addition, and the presence of ferrite and austenite phases in terms of vacuum characteristics (gas release characteristics) It is not clearly known how it will affect. Accordingly, the present inventors have examined the applicability as a vacuum vessel, focusing on the strength, toughness, surface characteristics, gas release characteristics, heat treatment characteristics, and polishing characteristics of Ni-saving duplex stainless steel materials.

  Duplex stainless steel is a material composed of a ferrite phase and austenite phase structure, and has high strength, ductility, toughness and weldability. For this reason, it can be said that it has the basic characteristics as a premise of substituting austenitic stainless steel. However, it is necessary to grasp the influence of containing about 50% of a ferrite phase with poor toughness and a small hydrogen solubility limit. The most important characteristics for vacuum container materials are that a smooth, clean surface can be obtained by mechanical, electrolytic, chemical polishing, etc., and that the surface adsorption gas such as water has excellent desorption characteristics, The release ability is small, and the method for imparting these properties to the duplex stainless steel material was clarified, and the goal was to develop it as a practical material for vacuum vessels.

  In carrying out the development, the applicability of the duplex stainless steel as a vacuum vessel material intended by the present inventor is evaluated, and there is no literature specifically shown. Since duplex stainless steel contains a large amount of Cr and N, it has a high pitting corrosion resistance, and therefore has a problem that the efficiency of the step of pickling finishing the steel surface is small. Recognizing that the relationship between cold working and vacuum characteristics of duplex stainless steel is important under the recognition of such issues, the vacuum characteristics of 0 to 20% cold worked material in the targeted duplex stainless steel The basic experiment was conducted. As a result, it has been clarified that there is a new problem that the release of hydrogen from steel is promoted by cold working. In other words, the need to accurately control the surface properties of steel materials has been recognized in order to develop dual-phase stainless steel materials for vacuum vessels, taking into account the characteristics of the manufacturing process that depends on the chemical composition of Ni-saving duplex stainless steels. It was.

JP2003-13181

J. Vac. Soc. Jpn. Vol. 50, No.1, 2007, p47-52 J. Vac. Soc. Jpn. Vol. 49, No. 6, 2006, p335-338 J. Vac. Soc. Jpn. Vol. 50, No.4 2007, p228

  The present invention clarifies the chemical composition, surface characteristics, and manufacturing method of a steel material for the purpose of obtaining a Ni-saving type duplex stainless steel material for a vacuum vessel that replaces austenitic stainless steel.

In order to solve the above problems, the present inventors conducted the following experiment.
First, after performing hot rolling / solution heat treatment using duplex stainless steels having various compositions, and optionally heat treatment at 1000 K or less, then shot blasting and pickling are performed under various conditions. The hot-rolled steel material having a thickness of 10 mm to 40 mm was obtained.
The obtained steel was subjected to strength measurement by a tensile test, surface roughness measurement stipulated in JIS B0601, and quantification of subepidermal hardening depth by Vickers hardness measurement.

In addition, in order to evaluate gas desorption characteristics, a sample for gas analysis having a size of 3 mm thickness x 14 mm x 14 mm is cut out from the steel surface by machining, and mechanical polishing for smoothing the surface (# 150 belt type polishing) (Or up to # 600 wet polishing), and some of the samples without mechanical polishing and pickled as-washed were subjected to electrolytic polishing using a phosphoric acid solution and then subjected to desorption gas analysis. In the desorption gas analysis, a sample placed on a transparent quartz stage is heated to 200 ° C. at a heating rate of 1.25 ° C./s in an analytical vacuum vessel evacuated to 10 ^ (− 7) Pa, and desorbed. Water and hydrogen were ionized and quantitatively analyzed with a quadrupole mass spectrometer (QMS). The same measurement was performed on SUS304 steel as a comparative material, and the vacuum characteristics (gas release characteristics) of the duplex stainless steel material were evaluated based on relative values.
Through the experiments described above, the chemical composition and surface characteristics of a duplex stainless steel material suitable for vacuum vessel applications and the production method were clarified and the present invention was achieved.

That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.06% or less, Si: 0.05 to 1.5%, Mn: 0.5 to 10.0%, P: 0.05% or less, S: 0.010 %: Ni: 0.1 to 5.0%, Cr: 18.0 to 25.0%, N: 0.05 to 0.30%, Al: 0.001 to 0.05% or less A duplex stainless steel material for a vacuum vessel, wherein the total hydrogen content in the steel is 3 ppm or less, and the balance is Fe and inevitable impurities.
(2) The duplex stainless steel material for a vacuum container according to (1), wherein the maximum cross-sectional height Rt of the surface roughness is 40 μm or less and the epidermal hardened layer depth is 0.15 mm or less.
(3) Further, in terms of mass%, Mo: 4.0% or less, Cu: 3.0% or less, Ti: 0.05% or less, Nb: 0.20% or less, V: 0.5% or less, W : 1.0% or less, Co: 2.0% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, REM: 0.10% or less The duplex stainless steel material for vacuum containers according to (1) or (2), comprising seeds or two or more species.
(4) The duplex stainless steel material for vacuum containers according to any one of (1) to (3), wherein the yield strength is 400 or more and 700 MPa or less.

  (5) The method for producing a duplex stainless steel material for a vacuum vessel according to any one of (1) to (4), including a step of performing a heat treatment step in a temperature range of 400 to 800K.

  According to the present invention, it is possible to provide a duplex stainless steel material having excellent strength and desorption gas characteristics, and a conventional austenitic stainless steel as a material for a vacuum vessel used in the manufacture of semiconductor elements, liquid crystal panels, thin film solar cells and the like. There is a tremendous contribution to the industry in that it can be used as a substitute for some of the members that have been used and can be made thinner than conventional steel materials.

It is a figure which shows the form of the sample for measuring subepidermal hardening depth.

  The present invention will be specifically described below. First, the requirements described in (1) of the present invention, that is, the reasons for limiting the chemical composition of the duplex stainless steel and the amount of hydrogen in the steel will be described.

  C limits the content to 0.06% or less in order to ensure the corrosion resistance of the stainless steel. If the content exceeds 0.06%, Cr carbide is generated, and the corrosion resistance and toughness deteriorate. Preferably, it is 0.03% or less.

  Si is added in an amount of 0.05% or more for deoxidation in steel melting. However, if added over 1.5%, the toughness deteriorates. Therefore, the upper limit is limited to 1.5%. A preferable content is 0.2 to 1.0%.

  Mn is added in an amount of 0.5% or more for improving the toughness and vacuum characteristics of the steel. The addition of Mn has the effect of increasing the austenite phase and improving the toughness, and the effect of concentrating in the oxide film and improving the desorption gas characteristics after the oxidation treatment. However, if it exceeds 10.0%, corrosion resistance and toughness deteriorate. Therefore, the upper limit is limited to 10.0%. A preferable content is 3.0 to 8.0%.

  P is an impurity and is limited to 0.05% or less in order to degrade the hot workability and toughness of steel. Preferably, it is 0.03% or less.

  S is an impurity and degrades the hot workability, toughness, and corrosion resistance of the steel, so it is limited to 0.010% or less. Preferably, it is 0.0020% or less.

  Ni is contained in an amount of 0.1% or more in order to stabilize the austenite structure of the steel and improve the corrosion resistance against various acids and further toughness. On the other hand, Ni is an expensive alloy and is limited to a content of 5.0% or less from the viewpoint of cost. A preferable content is 1.5 to 4%.

  Cr contains 18.0% or more in order to secure the basic corrosion resistance of steel. On the other hand, if the content exceeds 25.0%, the ferrite phase fraction increases, and the toughness and the corrosion resistance of the welded portion are impaired. Therefore, the Cr content is set to 18.0% or more and 25.0% or less. A preferable content is 19 to 23%.

  N is an effective element that improves the strength and corrosion resistance by dissolving in the austenite phase of steel. For this purpose, 0.05% or more is contained. The solid solution limit increases depending on the Cr content. However, in the steel of the present invention, if the content exceeds 0.30%, Cr nitride precipitates and the toughness and corrosion resistance are inhibited. The upper limit is 0.30%. A preferable content is 0.10 to 0.25%.

  Al is an important element for deoxidation of steel, and is contained together with Si in order to reduce oxygen in the steel. When the Si content exceeds 0.3%, it may not be necessary to add, but the reduction of the oxygen content is essential for securing toughness, and for this reason, the content must be 0.001% or more. . On the other hand, Al is an element having a relatively large affinity with N, and if added excessively, AlN is generated and the toughness of the steel is inhibited. The degree depends on the N content, but if Al exceeds 0.05%, the toughness deteriorates remarkably, so the upper limit of the content is made 0.05%. Preferably, the upper limit is 0.03%.

  O (oxygen) is a main element constituting an oxide that is representative of non-metallic inclusions, and excessive inclusion inhibits toughness. In addition, the formation of coarse clustered oxides causes surface defects. However, in the present invention, the upper limit of the content is not particularly specified, but is preferably 0.010% or less.

  The total amount of hydrogen in the steel affects the amount of hydrogen or water released from the vacuum vessel material into the vacuum. Further, it is known that hydrogen in steel is changed to water by being oxidized on the surface of the steel material and promotes desorption of water. In particular, in a duplex stainless steel containing a ferrite phase, since hydrogen diffusion is large, it is necessary to control the total hydrogen content in the steel material to be small. The present inventors have found that by setting the content to 3 ppm or less, it is possible to achieve the same level of gas release characteristics as austenitic stainless steel, and the upper limit of the content is set to 3 ppm. The smaller the amount of hydrogen in the steel, the better, and 2 ppm or less, more preferably 1 ppm or less.

  (2) of this invention prescribes | regulates the surface roughness maximum cross-section height Rt and surface hardness of steel materials. Rt and surface hardness are indices related to the mechanical polishing characteristics of steel materials. Preferred surface characteristics of steel materials for obtaining a smooth and clean surface by combining mechanical polishing and electrochemical polishing in a duplex stainless steel with a hard surface. Stipulated. As shown in the examples, for steel materials with Rt exceeding 40 μm, or depth of epidermal hardened layer exceeding 0.15 mm, belt type mechanical polishing up to # 150 or wet emery paper polishing up to # 600 and electrolytic polishing are performed. Since the gas desorption characteristics after the test were not good, the above provisions were established. The reason for the deterioration of the gas desorption characteristics may be that the presence of the hardened layer may increase the desorption rate of hydrogen, and that microscopic surface layer defects may remain.

Rt should be as small as possible, preferably 20 μm or less, more preferably 10 μm or less.
In order to set the Rt to 40 μm or less and the subepidermal hardened layer depth to 0.15 mm or less, the pickling may be performed by appropriately managing the particle size and projection density of the shot blast.

  Next, the reason for limitation described in (3) of the present invention will be described. In addition to the composition of (1) above, the duplex stainless steel of the present invention is optionally selected from one of Mo, Cu, Ti, Nb, V, W, Co, B, Ca, Mg, and REM. More than seeds can be included.

  Mo is a very effective element that additionally increases the corrosion resistance of stainless steel, and can be contained as necessary. For this purpose, it is preferable to contain 0.2% or more. In the steel of the present invention, the upper limit is 4.0% in terms of cost, but Mo is a very expensive element, and is preferably 1.0% or less.

  Cu is an element that additionally increases the corrosion resistance of stainless steel to acids, and has the effect of improving toughness. If the content exceeds 3.0%, εCu precipitates exceeding the solid solubility and embrittlement occurs, so the upper limit was made 3.0%. Cu has the effect of stabilizing the austenite phase and improving toughness. For this reason, it is recommended to contain 0.3% or more. The preferable content when Cu is contained is 0.3 to 1.5%.

  Ti is an element that forms oxides, nitrides, and sulfides in a very small amount, and solidifies the steel and refines the crystal grains of the high-temperature heating structure, and is added as necessary. On the other hand, if it exceeds 0.05% and is contained in the duplex stainless steel, coarse TiN is generated and the toughness of the steel is inhibited. For this reason, the upper limit of the content was set to 0.05%. A suitable content of Ti is 0.003 to 0.020%.

  Nb is an element effective for refinement of crystal grains in a hot rolled structure, and also has an effect of improving corrosion resistance. Nitrides and carbides formed by Nb are generated during the hot working and heat treatment processes, and have the effect of suppressing crystal grain growth and strengthening the steel material. For this reason, it is good to contain 0.01% or more. On the other hand, excessive addition causes precipitation as an undissolved precipitate during heating before hot rolling and impairs toughness, so the upper limit of its content is set to 0.20%. The preferable content range in the case of adding is 0.03% to 0.10%.

V and W are elements added to additionally enhance the corrosion resistance of the duplex stainless steel.
V is preferably contained in an amount of 0.05% or more for the purpose of improving the corrosion resistance. However, if it exceeds 0.5%, coarse V-based carbonitrides are produced and the toughness is deteriorated. Therefore, the upper limit is limited to 0.5%. The preferable content when added is in the range of 0.1 to 0.3%.

  W, like Mo, is an element that additionally improves the corrosion resistance of stainless steel, and has a higher solid solubility than V. For the purpose of enhancing the corrosion resistance in the steel of the present invention, 1.0% is contained at the upper limit. A preferable content is 0.05 to 0.5%.

  Co is an element effective for enhancing the toughness and corrosion resistance of steel, and is selectively added. The content is preferably 0.03% or more. If the content exceeds 2.0%, it is an expensive element, so that an effect commensurate with the cost cannot be exhibited, so the upper limit was set to 2.0%. The preferable content when added is 0.03 to 1.0%.

B, Ca, Mg, and REM are all elements that improve the hot workability of steel, and one or more of them are added for that purpose. Any excess addition of B, Ca, Mg, and REM conversely decreases hot workability and toughness, so the upper limit of the content is determined as follows.
B and Ca are 0.0050%, Mg is 0.0030%, and REM is 0.10%. Preferred contents are B and Ca: 0.0005 to 0.0030%, Mg: 0.0001 to 0.0015%, and REM: 0.005 to 0.05%, respectively. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

  (4) of this invention prescribes | regulates the yield strength of a duplex stainless steel material. In order to reduce the thickness of the container material, the strength is preferably high, and it is preferable to have a yield strength of 400 MPa or more. On the other hand, if it exceeds 700 MPa, the toughness will deteriorate, so the upper limit is set to 700 MPa. The yield strength can be adjusted by the chemical composition, solution heat treatment conditions, heat treatment conditions performed at 400 to 800 K described in the present invention (5) described later, and the like.

(5) of the present invention relates to the method for producing a duplex stainless steel according to the present invention, and specifies heat treatment conditions for increasing the strength of the duplex stainless steel and reducing the hydrogen content in the steel.
This heat treatment is intended to increase the strength of the steel material through age hardening of the duplex stainless steel, and at the same time, it is preferably carried out in the temperature range of 400 to 800 K for the purpose of promoting the reduction of the amount of hydrogen in the steel. By applying this heat treatment, the hydrogen content in the steel can be reduced to 2 ppm or less, and further to 1 ppm or less, and the vacuum characteristics are slightly improved as the hydrogen content decreases. At the same time, the yield strength can be increased to 500 MPa or more, and further to 600 MPa or more.

The heat treatment time in the above temperature range is preferably 5 minutes or more, but on the other hand, when the yield strength exceeds 700 MPa by applying heat treatment for an excessive time, the toughness of the steel material is impaired. Therefore, the upper limit of the heat treatment time may be determined according to the aging strengthening and embrittlement characteristics of the steel material.
In addition, if heat treatment (baking treatment) is performed in the temperature range of 400 to 800 K after being manufactured as a vacuum vessel, it is possible to desorb water adsorbed on the vessel surface at the same time as reducing the amount of hydrogen, thereby improving the vacuum characteristics. It is very effective against this.

  The steel material of the present invention is a steel material used as a vacuum vessel, and can be in the form of a steel plate, mold steel, bar, wire, tube, etc., but is mainly manufactured as a steel plate. The steel having the steel composition described in (1) or (3) is melted and made into a steel slab by continuous casting or cast into an ingot and then rolled into a steel slab. Melting and casting can be performed in accordance with normal melting and casting of duplex stainless steel. After this steel slab is heated, it is hot-rolled to obtain a steel material having a required shape. The conditions relating to hot rolling are not particularly limited, and may be performed according to the heating and rolling conditions of normal duplex stainless steel hot rolling. After the solution heat treatment, the steel material is further subjected to heat treatment for dehydrogenation and age hardening as necessary, and then the surface of the steel material is subjected to surface treatment such as shot blasting, polishing, and pickling to obtain the required surface properties. Can be manufactured.

The present invention will be described more specifically with reference to the following examples. Table 1 shows the chemical composition of the test steel. The components other than those listed in Table 1 are Fe and unavoidable impurity elements. Moreover, the part in which content is not described about the component shown in Table 1 shows that it is an impurity level. REM in the table means lanthanoid rare earth elements, and the content indicates the total of these elements.
The steel slab of steel type number T was collected from the actual smelting slab, and a steel slab having a thickness of 80 mm was used as a hot rolled material. Steel Nos. A to Q are melted in a laboratory 50 kg vacuum induction furnace, R steel is melted in a 50 kg atmospheric melting furnace, cast into a flat steel ingot with a thickness of about 110 mm, and then hot forged. Thus, a steel piece having a thickness of 80 mm was obtained. Moreover, the steel slab of the steel type number T2 corresponds to the site | part which became 4 ppm in the stage after pickling as a hot-rolled steel material by the above-mentioned actual machine melting slab.

  In the hot rolling, the steel slab was heated to a predetermined temperature, and then repeatedly rolled by a two-stage rolling mill in a laboratory, and finish rolling was performed at 850 to 950 ° C. The plate thickness was 10 to 40 mm. In the solution heat treatment, the steel plate was placed in a heat treatment furnace set at a predetermined temperature of 950 to 1050 ° C., extracted after taking a soaking time according to the plate thickness of the steel plate, and then water-cooled.

  The measurement of hydrogen content and the evaluation of vacuum characteristics of the obtained hot-rolled steel (without pickling treatment) is performed by measuring the amount of hydrogen of 3 mm x 14 mm with a plate thickness of 3 mm after grinding the steel skin 0.5 mm. A sample and a sample for vacuum characteristic evaluation having a plate thickness of 3 mm and a size of 14 mm × 14 mm were collected. The amount of hydrogen was determined by an inert gas melting heat conduction method, and the results are shown in Table 2. The sample for vacuum characteristics was subjected to wet polishing up to # 600 as sample preparation, and then electropolished with a current density of 0.1 to 3 A / cm 2 with a phosphoric acid based electropolishing liquid at 20 to 30 microns, Furthermore, it was immersed in normal temperature 35% nitric acid for 30 minutes.

  A temperature-programmed desorption gas analyzer was used for evaluating the vacuum characteristics. The sample was placed on the sample stage, and water and hydrogen desorbed in the process of heating up to 200 ° C. at a stage heating rate of 10 ° C./min were quantified. It has been reported that the evacuation characteristics at room temperature correspond to the ionic current intensity desorbed at 100 to 130 ° C. in the temperature-programmed desorption gas analysis (see Non-Patent Document 3). Based on this report, the numerical value of the relative ratio of the ionic current intensity of the evaluation sample to the sum of the ionic current intensity of water and hydrogen at this temperature for SUS304 steel was determined. The results are shown in Vacuum characteristics-1 of Table 2. It was judged that this value was less than 2.0, preferably less than 1.5.

The hot-rolled steel tensile test is performed using a round bar tensile test piece with a parallel portion of 8 mm diameter for a material with a thickness of 10 mm, and a round bar tensile test piece with a diameter of 10 mm for a material with a thickness of 20, 30, or 40 mm. The sample was taken in the direction perpendicular to rolling. In addition, about the material of plate | board thickness 30 and 40mm, it extract | collected centering on plate | board thickness 1/4 part. The yield strength results are shown in Table 2.
The impact toughness of the hot-rolled steel was collected in two directions in which the fracture surface propagated in parallel to the rolling direction using a JIS No. 4 Charpy test piece in which a 2 mmV machined notch was machined in the rolling direction. The 10 mm material is a 3/4 size Charpy test piece, the 20 mm thickness material is a full size Charpy test piece at the center of the thickness, and the 30 mm thickness and 40 mm material is 1/4 thickness. Evaluation was performed using a full-size Charpy specimen collected from the center. The test temperature was −20 ° C., and an impact test was performed with a tester with a maximum energy of 500 J. Table 2 shows the result of the average value (J / cm ² ) of the three impact values.

As shown in Table 2, all of the hot-rolled steel materials according to the present invention exhibit good vacuum characteristics as compared with SUS304 steel, yield strength exceeds 400 MPa, and toughness is as high as 50 J / cm ² or more. It can be seen that it exhibits excellent properties as a vacuum container material.
On the other hand, in the comparative examples of Table 2, the vacuum characteristics were inferior to that of the comparative material SUS304 steel, or the strength or toughness was insufficient.

The hot-rolled pickled steel material was prepared by the following method.
Select from three types of shot blasting abrasive grain size, small, medium and large, and change the projection density depending on the sheeting speed and number of sheeting of the hot rolled steel, and part of the surface scale of the duplex stainless steel hot rolled steel Was removed. Subsequently, it was immersed in a hydrofluoric acid solution of 40 to 60 ° C., 10 to 20% HNO ₃ , and 3 to 8% HF for 20 minutes to 24 hours to completely remove the scale.

  A sample for evaluation of surface roughness and hardened layer depth was cut out from this hot rolled pickled steel material, and the subcutaneous hardening was performed by quantifying the maximum section height Rt by measuring the surface roughness specified in JIS B0601 and measuring the Vickers hardness of 100 gf. Depth quantification was performed. The evaluation length of the surface roughness measurement was 3.0 mm, the measurement was performed three times, and the maximum value was adopted. In order to measure the subepidermal hardening depth more accurately, a sample is cut to form an inclined surface as shown in FIG. 1 and embedded in a resin so that the inclined cut surface becomes the upper surface in order to measure a narrow thickness range more accurately. . Thereafter, the hardness of the inclined cut surface was measured at 20 points at a pitch of 0.1 mm from the position corresponding to the steel material surface. That is, the hardness up to a depth corresponding to 1 mm of the steel surface was measured. This measurement was performed for each measurement point n = 3, and the epidermal hardness distribution was determined from the average value. The depth of epicutaneous cure was determined by determining the thickness of the epidermis of the portion cured at 50 or more Hv with respect to the internal average hardness. Here, the internal average hardness is determined from the average value of the hardness at the epidermal depth of 0.5 to 1.0 mm.

  A part of hot-rolled pickled steel was heat-treated for age hardening and hydrogen reduction (aging heat treatment) in the air. A thin oxide film was formed by this aging heat treatment.

The hydrogen amount measurement and vacuum property evaluation of the hot rolled pickled steel and the aging heat treated steel were performed in the same manner as the above hot rolled steel without the pickling treatment. However, samples for vacuum characteristics evaluation are first removed by unevenness on the steel surface by # 150 belt type polishing, and then a sample of 3 mm thickness and 14 mm x 14 mm is collected, and wet polishing, electrolytic polishing, and nitric acid immersion up to # 600 are performed. In the same manner, a sample for thermal desorption gas analysis partially including the epidermal hardened layer was obtained.
Moreover, the tension test and the impact test were implemented similarly to the above-mentioned hot-rolled steel material which does not perform a pickling process.

The evaluation results of the hot-rolled pickled steel materials are shown in Table 3, hydrogen amount, vacuum characteristics-2, yield strength, and impact characteristics.
Test No. in Table 3 In Comparative Example 15, shot blasting was performed only for a short time with small abrasive grains, so that a long time was required for pickling. As a result, the amount of hydrogen was 0.0004 mass%, and the vacuum characteristics were deteriorated. Test No. In Comparative Example 16, shot blasting of the medium abrasive grains was performed for a long time, the scale was almost completely removed, and the pickling was completed in a short time. For this reason, the hardened layer became as large as 0.25 mm, and the vacuum characteristics were lowered. Test No. In Comparative Examples 17 to 20, large abrasive grains were shot blasted and pickled. The cured layer was as large as 0.20 mm. Therefore, test no. In Comparative Examples 18-20, the vacuum characteristics were not good. Test No. In Comparative Example 17, since the aging heat treatment was performed for a long time, the yield strength increased excessively and at the same time became brittle. On the other hand, the hot-rolled pickled steel materials of the examples of the present invention all showed good vacuum characteristics, yield strength, and impact characteristics.

  As can be seen from the above examples, it has become clear that the present invention can provide a duplex stainless steel material with good vacuum characteristics.

  According to the present invention, it is possible to provide an economical duplex stainless steel material for vacuum vessels with high strength and low Ni content, and the industrial contribution such as providing cost reduction in large vacuum vessels is extremely large. is there.

Claims (5)

  In mass%, C: 0.06% or less, Si: 0.05 to 1.5%, Mn: 0.5 to 10.0%, P: 0.05% or less, S: 0.010% or less, Ni: 0.1 to 5.0%, Cr: 18.0 to 25.0%, N: 0.05 to 0.30%, Al: 0.001 to 0.05% or less, and steel A duplex stainless steel material for vacuum vessels, wherein the total hydrogen content is 3 ppm or less, and the balance consists of Fe and inevitable impurities.   The duplex stainless steel material for a vacuum vessel according to claim 1, wherein the maximum cross-sectional height Rt of the surface roughness is 40 µm or less and the depth of the epidermal hardened layer is 0.15 mm or less.   Further, by mass%, Mo: 4.0% or less, Cu: 3.0% or less, Ti: 0.05% or less, Nb: 0.20% or less, V: 0.5% or less, W: 1. One or two of 0% or less, Co: 2.0% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, REM: 0.10% or less The duplex stainless steel material for a vacuum vessel according to claim 1 or 2, characterized in that it contains seeds or more.   The duplex stainless steel material for a vacuum vessel according to any one of claims 1 to 3, wherein the yield strength is 400 to 700 MPa.   The method for producing a duplex stainless steel material for a vacuum vessel according to any one of claims 1 to 4, further comprising a step of performing a heat treatment step in a temperature range of 400 to 800K.

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