KR101445952B1 - Duplex stainless steel material for vacuum vessels, and process for manufacturing same - Google Patents

Abstract

The present invention provides a Ni-saving two-phase stainless steel material excellent in gas desorption properties in place of austenitic stainless steels, which comprises 0.06% or less of C, 0.05 to 1.5% of Si, : 0.5 to 10.0%, P: not more than 0.05%, S: not more than 0.010%, Ni: 0.1 to 5.0%, Cr: 18.0 to 25.0%, N: 0.05 to 0.30% And the hydrogen content in the steel is 3 ppm or less, and the balance being Fe and inevitable impurities.

Description

[0001] DUPLEX STAINLESS STEEL MATERIAL FOR VACUUM VESSELS, AND PROCESS FOR MANUFACTURING SAME [0002]

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

The production of semiconductor devices, liquid crystal panels, and thin film solar cells has been rapidly increasing in recent years, and products are also becoming larger. A vacuum process is required for the manufacture of these products, and metal materials such as stainless steel, aluminum, and titanium are used as vacuum containers. As a stainless steel as a vacuum container, an austenitic material typified by SUS 304 steel has been conventionally used. In addition, with the increase in size of vacuum equipment, a thick stainless steel material having a thickness of up to about 80 mm is used.

As a characteristic required for a material for a vacuum container, a small amount of gas is emitted. Particularly, surface abrasion conditions and the influence of baking treatment on gas emission characteristics such as austenitic stainless steel, aluminum alloy, and titanium as ultra high vacuum materials have been studied (see Non-Patent Document 1), and surface roughness And reduction of the surface oxide layer are effective. It is also known that prolonged baking at 100 to 450 ° C is effective (see Non-Patent Document 2).

Further, a knowledge of improving the vacuum characteristic by forming a coating film having a high Mn content in austenitic stainless steel containing a large amount of Mn is disclosed (see Patent Document 1), and it is considered possible to develop materials from such a viewpoint.

Another required property is strength and weldability. As the size of the vacuum container increases, this characteristic becomes increasingly important. Particularly, it is reasonable to apply a high strength material having excellent fatigue properties to a member used at a portion where atmospheric pressure and vacuum are repeated, such as a preliminary exhaust chamber. However, the lower limit value of the yield strength of the austenitic stainless steel is about 200 MPa, which is a desirable characteristic of a material for a vacuum vessel which is enlarged.

Two-phase stainless steels contain a large amount of Cr and Mo, and are characterized by a higher strength than austenitic stainless steels. However, since they are expensive materials, there are few applications for vacuum containers. Recently, a two-phase stainless steel having reduced Ni content and increased Mn content has been developed, and the steel material cost is considered to be applicable through thinning of the container material. However, in the two-phase stainless steel, ductility and toughness may be deteriorated in the design of components such as Ni reduction and Mn addition, and the influence of the presence of the ferrite phase and the austenite phase on the vacuum characteristics (gas separation characteristics) It is not clearly known.

Therefore, the present inventors paid attention to the strength, toughness, surface characteristics, gas desorption characteristics, heat treatment characteristics and polishing characteristics of the Ni-saving two-phase stainless steel material and examined applicability as a vacuum container.

Two-phase stainless steel is a material composed of a ferrite phase and an austenite phase, and has high ductility, toughness and weldability in addition to high strength. It can be said that the steel has basic characteristics as a premise of substitution of the austenitic stainless steel. However, it is necessary to grasp the influence of the ferrite phase containing about 50% of the ferrite phase having a low solubility limit of hydrogen and insufficient toughness. The most important characteristics of the material for the vacuum container are that a smooth and clean surface can be obtained by mechanical, electrolytic, chemical polishing or the like, an excellent desorption characteristic of surface adsorption gas such as water, And a method for imparting these properties to a two-phase stainless steel material has been clarified and aimed at developing a practical material for a vacuum container.

The inventors of the present invention evaluated the applicability of the two-phase stainless steel as a material for a vacuum container, which is the object of the present inventors, and no document specifically describing the applicability has been found. Since the two-phase stainless steel contains a large amount of Cr and N, the pitting corrosion resistance is high, and thus the efficiency of the pickling treatment of the surface of the steel is low. Based on the recognition of such a problem, a basic experiment on the vacuum characteristics of 0 to 20% cold workable material in a two-phase stainless steel as a target was carried out in consideration of the importance of the relationship between the cold working and the vacuum characteristics of the two-phase stainless steel. As a result, it has been found that there is a new problem that the release of hydrogen in steel is promoted by cold working. That is, it was recognized that it is necessary to control the characteristics of the steel surface precisely in order to develop as a two-phase stainless steel material for a vacuum container while taking into consideration characteristics of the manufacturing process depending on chemical composition of the Ni-saving two-phase stainless steel.

Japanese Patent Application Laid-Open No. 2003-13181

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 aims to clarify the chemical composition, surface characteristics and manufacturing method of this steel material for the purpose of obtaining a Ni-saving two-phase stainless steel material for a vacuum container in place of austenitic stainless steel.

In order to solve the above problems, the present inventors conducted the following experiments.

First, a two-phase stainless steel having various compositions was used for hot rolling and solution heat treatment, and in some cases, heat treatment at 1000 K or less was carried out, followed by shot blasting and pickling under various conditions. Of hot rolled steel was obtained.

The obtained steel material was subjected to tensile strength measurement, surface roughness measurement according to JIS B0601, and Vickers hardness measurement to quantify the depth of hardening under the epidermis.

Further, in order to evaluate the gas desorption characteristics, a gas analysis sample having a size of 3 mm thickness x 14 mm x 14 mm was cut out from the surface of the steel material by mechanical machining, and a mechanical polishing (# Degreasing treatment was carried out after performing electrolytic polishing using a phosphate-based solution for a specimen in a pickling state, while omitting the polishing of a sample and a part of the polishing pad. In the desorption analysis, samples placed on a transparent quartz stage in an analytical vacuum vessel evacuated to 10 ^ (- 7) Pa were heated to 200 ° C at a temperature raising rate of 1.25 ° C / s and ionized to remove the water and hydrogen And quantitatively analyzed by quadrupole mass spectrometry (QMS). SUS 304 steel was subjected to the same measurement as a comparative material, and the vacuum characteristics (gas desorption characteristics) of the two-phase stainless steel material were evaluated based on the relative values.

Through the above experiments, the chemical composition, surface characteristics and manufacturing method of the two-phase stainless steel material excellent in the gas desorption characteristic suitable for the vacuum vessel application have been clarified and the present invention has been accomplished.

That is, the point of the present invention is as follows.

(1) A ferritic stainless steel comprising, by mass%, C: not more than 0.06%, Si: 0.05 to 1.5%, Mn: 0.5 to 10.0%, P: not more than 0.05%, S: not more than 0.010% , Characterized in that the steel contains 25.0%, N: 0.05 to 0.30%, and Al: 0.001 to 0.05%, and further has a hydrogen content of 3 ppm or less in the steel and the balance of Fe and inevitable impurities. Upper stainless steel.

(2) The two-phase stainless steel material according to (1), wherein the maximum cross-sectional height Rt of the surface roughness is not more than 40 占 퐉 and the depth of the under-skin hardened layer is not more than 0.15 mm.

(3) A steel sheet comprising, by mass%, 4.0% or less of Mo, 3.0% or less of Cu, 0.05% or less of Ti, 0.20% or less of Nb, 0.5% or less of V, (1) or (2), further comprising at least one of Ca, Ca and Ca in an amount of 0.0050% or less, Ca in an amount of 0.0050% or less, Mg in an amount of 0.0030% or less, and REM in an amount of 0.10% Excellent two phase stainless steel material.

(4) A duplex stainless steel material having excellent gas desorption properties according to any one of (1) to (3) having a yield strength of 400 to 700 MPa.

(5) A method for producing a two-phase stainless steel material excellent in gas desorption characteristics according to any one of (1) to (4), which comprises a step of performing a heat treatment process in a temperature range of 400 to 800K.

According to the present invention, it is possible to provide a two-phase stainless steel material excellent in strength and gas desorption characteristics, and austenitic stainless steel is conventionally used as a material for vacuum containers used for manufacturing semiconductor devices, liquid crystal panels, thin film solar cells and the like Industrial use contributes greatly, for example, by replacing a part of the member that has been present, and making it thinner than a conventional steel material.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view showing the shape of a sample for measuring the depth of hardening under the epidermis. Fig.

Hereinafter, the present invention will be described in detail. First, the requirements described in (1) of the present invention, namely, the reasons for limiting the chemical composition of the two-phase stainless steel and the amount of hydrogen in steel will be described.

C is limited to a content of 0.06% or less in order to secure the corrosion resistance of stainless steel. If it is contained in an amount exceeding 0.06%, Cr carbide is produced and corrosion resistance and toughness are deteriorated. It is preferably 0.03% or less.

Si is contained in an amount of 0.05% or more for deoxidation in a solvent of steel. However, if it exceeds 1.5%, the toughness deteriorates. Therefore, the upper limit is limited to 1.5%. The preferable content is 0.2 to 1.0%.

Mn is contained in an amount of 0.5% or more in order to improve the toughness and the vacuum characteristic (gas desorption characteristic) of the steel. The incorporation of Mn has the effect of increasing the austenite phase to improve toughness and the effect of concentrating in the oxide film to improve the desorption characteristics after oxidation treatment. However, if it exceeds 10.0%, corrosion resistance and toughness are deteriorated. Therefore, the upper limit is limited to 10.0%. The preferable content is 3.0 to 8.0%.

P is an impurity and deteriorates the hot workability and toughness of the steel, so the content is limited to 0.05% or less. It is preferably 0.03% or less.

S is an impurity and deteriorates hot workability, toughness and corrosion resistance of the steel, so the content 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 to improve corrosion resistance and toughness against various acids. 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. The preferred content is 1.5 to 4%.

Cr is contained in an amount of 18.0% or more in order to secure the basic corrosion resistance of the steel. On the other hand, if it is contained in an amount exceeding 25.0%, the ferrite phase fraction increases, and the toughness and the corrosion resistance of the welded portion are impaired. Therefore, the content of Cr should be 18.0% or more and 25.0% or less. The preferred content is 19 to 23%.

N is an effective element which is dissolved in austenite of steel to increase strength and corrosion resistance. Therefore, it contains 0.05% or more. Though the solubility limit increases with the Cr content, in the steels of the present invention, if it exceeds 0.30%, the Cr nitride precipitates to deteriorate toughness and corrosion resistance. Therefore, the upper limit of the content is set to 0.30%. The preferable content is 0.10 to 0.25%.

Al is an important element for the deoxidation of steel and is contained together with Si to reduce oxygen in the steel. When the Si content exceeds 0.3%, it may be dispensed with. However, the reduction of the oxygen amount is essential for securing toughness, and therefore, the Si content is required to be 0.001% or more. On the other hand, Al is an element having a relatively large affinity with N, and when added in excess, Al is generated to inhibit toughness of the steel. The degree depends on the N content. However, if Al exceeds 0.05%, the decrease in toughness becomes significant, so the upper limit of the content is set to 0.05%. Preferably, the upper limit is 0.03%.

O (oxygen) is a main element that constitutes an oxide which is representative of non-metallic inclusions, and an excessive content inhibits toughness. In addition, if a coarse cluster oxide is produced, it will cause surface scratches. However, in the present invention, the upper limit of the content is not particularly defined, but is preferably 0.010% or less.

The amount of hydrogen in the steel affects the amount of hydrogen or water that is released into the vacuum from the vacuum vessel material. It is also known that hydrogen in the steel is oxidized at the surface of steel by water and promotes desorption of water. Especially, in a two-phase stainless steel containing a ferrite phase, diffusion of hydrogen is large, so it is necessary to control the hydrogen content in the steel material to be small. The inventors of the present invention have found that by setting the content to 3 ppm or less, the gas discharge characteristics can be made to the same level as that of the austenitic stainless steel, and the upper limit of the content is set at 3 ppm. The lower the amount of hydrogen in the steel, the better, and the more preferable it is 2 ppm or less and 1 ppm or less.

(2) of the present invention specifies the maximum surface height Rt of the surface roughness of the steel and the surface hardness. Rt and surface hardness are indicators of the mechanical polishing properties of the steel. In the case of a two-phase stainless steel having a hard surface, mechanical properties and electrochemical polishing were combined to define the surface properties of a preferred steel material in order to obtain a smooth and clean surface . As shown in the examples, in a steel material having Rt exceeding 40 占 퐉 or a depth of the hardened layer below the epidermis of more than 0.15 mm, belt-type mechanical polishing to # 150 or wet emery polishing to # 600 and electrolytic polishing were carried out Since the gas-leaving properties after the heating were not good, the above-mentioned conditions were determined. As a reason for the deterioration of the gas desorption properties, the presence of the under-epidermal cured layer may increase the desorption rate of hydrogen, and there is a possibility that microscopic surface layer defects remain.

Rt is preferably as small as possible, preferably not more than 20 mu m, more preferably not more than 10 mu m.

In order to set Rt to 40 탆 or less and to set the depth of the under-surface hardened layer to 0.15 mm or less, pickling may be carried out by properly controlling the particle diameter and the projection density of the shot blast.

Subsequently, the reasons for limitation described in (3) of the present invention will be described. The duplex stainless steel of the present invention may contain one or more of Mo, Cu, Ti, Nb, V, W, Co, B, Ca, Mg and REM in addition to the composition of (1) .

Mo is a very effective element that additionally increases the corrosion resistance of stainless steel and can be contained as needed. Therefore, it is preferable that the content is 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, more preferably 1.0% or less.

Cu is an element which additionally increases the corrosion resistance of stainless steel to an acid, and has an action of improving toughness. If it is contained in an amount exceeding 3.0%, the solubility is exceeded, and ∈Cu precipitates and embrittlement occurs, so that the upper limit is set to 3.0%. Cu has an effect of stabilizing the austenite phase and improving toughness. Therefore, it is recommended to contain 0.3% or more. When Cu is contained, the preferable content is 0.3 to 1.5%.

Ti is an element which forms oxides, nitrides and sulfides in a very small amount and refines the crystal grains of the steel coagulation and high-temperature heating structure, and is contained if necessary. On the other hand, if it is contained in more than 0.05% in the two-phase stainless steel, coarse TiN is generated and the toughness of the steel is inhibited. Therefore, the upper limit of the content is set at 0.05%. A suitable content of Ti is 0.003 to 0.020%.

Nb is an element effective for grain refinement of a hot-rolled structure and also has an effect of enhancing corrosion resistance. Nitride and carbide formed by Nb are generated in the course of hot working and heat treatment, and have the action of suppressing grain growth and strengthening the steel. Therefore, it is preferable to contain 0.01% or more. On the other hand, the excess addition precipitates as non-solid precipitates at the time of heating before the hot rolling so as to inhibit the toughness. Therefore, the upper limit of the content is set at 0.20%. When it is contained, the preferable range of the content is 0.03% to 0.10%.

V, and W are elements that contain two-phase stainless steels in order to additionally increase the corrosion resistance.

V may be contained in an amount of 0.05% or more for the purpose of enhancing corrosion resistance, but if it is contained in an amount exceeding 0.5%, coarse V-based carbonitrides are produced and toughness is deteriorated. Therefore, the upper limit is limited to 0.5%. When added, the preferred content is in the range of 0.1 to 0.3%.

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

Co is an effective element for increasing the toughness and corrosion resistance of steel, and is optionally added. The content thereof is preferably 0.03% or more. If it is contained in an amount exceeding 2.0%, it is an expensive element, so that the effect suitable for the cost is not exhibited. Therefore, the upper limit is set at 2.0%. When it is contained, the preferable content is 0.03 to 1.0%.

B, Ca, Mg, and REM are all elements that improve the hot workability of the steel, and one or two or more of them are contained for the purpose. B, Ca, Mg, and REM all contain excessive amounts, which in turn lower the hot workability and toughness, so the upper limit of the content is determined as follows.

0.0050% for B and Ca, 0.0030% for Mg, and 0.10% for REM. The preferable contents are 0.0005 to 0.0030% of B and Ca, 0.0001 to 0.0015% of Mg and 0.005 to 0.05% of REM, respectively. Here, REM is the sum of the contents of lanthanoid-based rare earth elements such as La and Ce.

(4) of the present invention specifies the yield strength of the two-phase stainless steel material. In order to reduce the thickness of the container material, it is preferable that the strength is large, and the yield strength is preferably at least 400 MPa. On the other hand, if it exceeds 700 MPa, the toughness is deteriorated, and the upper limit thereof is 700 MPa. The yield strength can be adjusted by the chemical composition, the solution heat treatment condition, or the heat treatment condition to be performed at 400 to 800K described in the present invention (5) to be described later.

(5) of the present invention relates to a method for producing a two-phase stainless steel of the present invention, and specifies heat treatment conditions for increasing the strength of the two-phase stainless steel material and reducing the hydrogen content in the steel.

In this heat treatment, the strength of the steel material is increased through the age hardening of the two-phase stainless steel, and at the same time, it is preferably carried out in the temperature range of 400 to 800K for the purpose of promoting the reduction of the hydrogen content in the steel. By this heat treatment, the hydrogen content in the steel can be further reduced to 2 ppm or less and 1 ppm or less, and the vacuum characteristics are slightly improved with the decrease of the hydrogen amount. 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 temperature range is preferably 5 minutes or more. On the other hand, if the yield strength exceeds 700 MPa by giving an excessive time heat treatment, the toughness of the steel is lowered. Therefore, the upper limit of the heat treatment time may be determined according to the aging strengthening and embrittlement characteristics of the steel material, respectively.

Further, if the heat treatment (baking treatment) is performed in a temperature range of 400 to 800K after being produced as a vacuum container, it is possible to reduce the amount of hydrogen and desorb the water adsorbed on the surface of the container, .

The steel material of the present invention is a steel material used as a vacuum container and may be in the form of a steel sheet, a section steel, a rod, a wire, a pipe or the like, but is mainly manufactured as a steel sheet. (1) or (3) is cast as a piece of steel by continuous casting or cast into an ingot, and then rolled into a piece of steel. The solvent and casting can be carried out in accordance with a conventional two-phase stainless steel solvent and casting. The billet is heated and then hot-rolled into a steel having a desired shape. The conditions for the hot rolling are not particularly limited, and the hot rolling may be carried out in accordance with the heating and rolling conditions of a conventional two-phase stainless steel. After the steel material is subjected to the solution heat treatment, the surface of the steel material is subjected to surface treatment such as shot blasting, polishing and pickling, after the heat treatment for dehydrogenation and aging is further carried out if necessary, .

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. Table 1 shows the chemical composition of the steel. The elements other than those shown in Table 1 are Fe and inevitable impurity elements. In addition, the portion where the content is not described with respect to the components shown in Table 1 indicates the impurity level. In the table, REM means a lanthanoid-based rare earth element, and the content indicates the sum of the elements.

The steel strip of steel grade No. T was taken from a slurry of actual equipment, and a steel strip having a thickness of 80 mm was used as a hot rolling material. The steel of the steel grade numbers A to Q was cast by a vacuum induction furnace of 50 kg in a laboratory, the steel of R was melted by a 50 kg atmospheric melting furnace, cast into a flat steel ingot having a thickness of about 110 mm, To make a steel strip having a thickness of 80 mm. The slab of steel grade No. T2 corresponds to a portion where the hydrogen content in the slurry of actual slurry is 4 ppm in the stage after pickling as hot rolled steel.

In the hot rolling, after heating the above-mentioned steel strip to a predetermined temperature, the rolling was repeatedly carried out by a two-stage mill in a laboratory. And subjected to finish rolling at 850 to 950 占 폚 to obtain a steel plate (steel material) having a thickness of 10 to 40 mm.

In the solution heat treatment, a steel sheet was charged into a heat treatment furnace set at a predetermined temperature of 950 to 1050 ° C, and after taking a cracking time corresponding to the thickness of the steel sheet, the steel sheet was extracted and then water-cooled.

The measurement of the amount of hydrogen and the evaluation of the vacuum characteristics of the obtained hot rolled steel (no pickling treatment) were carried out as follows. After cutting the surface of the steel material by 0.5 mm, a sample for measuring the amount of hydrogen having a thickness of 3 mm and a size of 3 mm x 14 mm and a sample having a thickness of 3 mm and a size of 14 mm x 14 mm were collected. The amount of hydrogen was determined by an inert gas fusion heat conduction method, and the results are shown in Table 2. The sample for the vacuum characteristic was subjected to wet polishing of up to # 600 for sample adjustment, electrolytic polishing of 20 to 30 micrometers at a current density of 0.1 to 3 A / cm 2 in a phosphate-based electrolytic polishing solution, For 30 minutes.

The evaluation of the vacuum characteristics was carried out using an elevated temperature desorption gas analyzer. The sample was placed on the sample stage, and water and hydrogen to be desorbed were quantitatively measured in the course of raising the temperature to 200 DEG C at a stage heating rate of 10 DEG C / min. It has been reported that the vacuum evacuation property at room temperature corresponds to the ionic current intensity which desorbs at 100 to 130 占 폚 in the temperature elevation desorption gas analysis (see Non-Patent Document 3). Based on this report, a numerical value of the ion current intensity of the evaluation sample with respect to the sum of the ion current intensities of water and hydrogen at this temperature with respect to the SUS 304 steel was obtained. The results are shown in the vacuum characteristic-1 in Table 2. It was judged that this value was less than 2.0, preferably less than 1.5.

The tensile test of the hot-rolled steel was performed by using a round bar tensile test specimen having a diameter of 8 mm in parallel with a material having a thickness of 10 mm and a round bar tensile test specimen having a diameter of 10 mm for a material having a thickness of 20, 30, Direction. For materials having a thickness of 30 and 40 mm, a plate thickness of 1/4 part was taken as a center. The results of the yield strength are shown in Table 2.

Impact toughness of hot-rolled steel was obtained from JIS No. 4 Charpy test specimen, which was machined in the rolling direction of 2 mm V machining notch, in two directions in which the wave front propagates parallel to the rolling direction. In the case of a 10 mm material, a 3/4 size Charpy test piece is used. For a material having a thickness of 20 mm, a full size Charpy test piece at the center of the plate thickness is used. Size Charpy test piece collected around the center of the test piece. The test temperature was -20 占 폚, and the impact test was carried out by a tester having a maximum energy of 500 J. Table 2 shows the results of the average value (J / cm 2) of each of the three impact values.

As shown in Table 2, all of the hot-rolled steels according to the present invention exhibited good vacuum characteristics as compared with SUS 304 steel, had a yield strength exceeding 400 MPa, a toughness of 50 J / cm 2 or higher, And exhibits excellent properties as a material.

On the other hand, in the comparative example of Table 2, the vacuum characteristics were lower than that of the comparative SUS 304 steel, or the strength or toughness was insufficient.

Next, the hot rolled pickled and sintered steel was produced by the following method.

Three types of abrasive grains of short blast were selected, small, medium and high, and the projected density was changed according to the number of conveying speeds and number of passes of the hot-rolled steel to remove a part of the surface scale of the above-mentioned two-phase stainless hot- . Then immersed for 20 minutes to 24 hours in 40 to 60 ℃, 10 to 20% HNO _{3, 3} to bulcho approximation of 8% HF, which was completely removing scale.

The surface roughness and the sample for evaluation of the depth of the hardened layer were cut out from the hot rolled pickled steel material and quantification of the maximum section height Rt by the surface roughness measurement specified in JIS B0601 and the quantification of the depth of the hardened portion under the skin by the Vickers hardness measurement of 100 gf Respectively. The evaluation length of the surface roughness measurement was 3.0 mm, and the measurement was carried out three times, and the maximum value among them was adopted.

In order to measure a narrow thickness range with higher accuracy, the lower portion of the epidermis hardening depth was formed by forming a sloped surface by cutting the sample as shown in Fig. 1 and filling the sloped cut surface into the resin so as to be the upper surface. Thereafter, the hardness of the oblique cut surface was measured at 20 points at a pitch of 0.1 mm from the position corresponding to the surface of the steel material. That is, the hardness up to a depth corresponding to 1 mm below the steel skin was measured. This measurement was performed for n = 3 for each measurement point, and the hardness distribution under the epidermis was determined by the average value. As the depth of hardening under the epidermis, the thickness of the epidermis at the portion hardened by Hv of at least Hv with respect to the average hardness of the epidermis was obtained and shown in Table 3. Here, the mean average hardness is obtained from an average value of the hardness at a depth of 0.5 to 1.0 mm below the epidermis.

For some of the hot rolled pickled steels, heat treatment (aging heat treatment) for aging hardening and reducing the amount of hydrogen was performed in the atmosphere. A thin oxide film was formed by this aging heat treatment.

The measurement of the hydrogen content of the hot rolled pickled and tempered steel and the aged heat treated steel and the evaluation of the vacuum characteristics were carried out in the same manner as the hot rolled steel without pickling treatment. However, samples for evaluation of vacuum characteristics were prepared by first removing the irregularities on the surface of the steel material by # 150 belt polishing and then collecting samples having a thickness of 3 mm and a size of 14 mm x 14 mm. The samples were subjected to wet polishing, electrolytic polishing , Nitric acid immersion was carried out in the same manner to obtain a specimen for temperature elevation desalinization analysis including a part of the hardened sub-epidermal layer.

The tensile test and the impact test were carried out in the same manner as the hot-rolled steel material without pickling treatment.

The evaluation results of the hot rolled pickled steels are shown in Table 3 in terms of hydrogen content, vacuum characteristic-2, yield strength and impact properties.

In the comparison example of Test No. 15 in Table 3, shot blasting was carried out only for a short period of time from the holding lips, requiring a long time for pickling. As a result, the amount of hydrogen was 0.0004 mass%, and the vacuum characteristics were lowered. In Test No. 16, the shot blast of the stop lips was performed for a long time, the scale was almost completely removed, and the pickling was completed in a short time. As a result, the hardened layer was increased to 0.25 mm and the vacuum characteristics were lowered. In the comparative examples of Test Nos. 17 to 20, shot blasting and pickling of the earth lips were carried out. The cured layer was increased to 0.20 mm. As a result, the vacuum characteristics of Comparative Examples Nos. 18 to 20 were not preferable. In the comparative example of Test No. 17, since the aging heat treatment was performed for a long time, the yield strength was excessively increased and simultaneously brittle. On the other hand, all of the hot-rolled, acidified steels of the present invention showed good vacuum characteristics, yield strength and impact properties.

As can be seen from the above examples, it has been found that a two-phase stainless steel material having good vacuum characteristics can be obtained by the present invention.

Industrial Applicability According to the present invention, it is possible to provide an economical two-phase stainless steel material for a vacuum container having a high strength and a low Ni content, which can provide a cost reduction in a large vacuum container. Big.

Claims (9)

C: not more than 0.06%, Si: 0.05 to 1.5%, Mn: 0.5 to 10.0%, P: not more than 0.05%, S: not more than 0.010%, Ni: 1.22 to 5.0% , The content of Al in the range of 0.001 to 0.05%, the content of hydrogen in the steel of 3 ppm or less, the balance being Fe and inevitable impurities,
Wherein the maximum cross-sectional height Rt of the surface roughness is 40 占 퐉 or less and the depth of the under-surface hardened layer is 0.15 mm or less. 2. The steel sheet according to claim 1, which further contains at least one of Mo: more than 0% and not more than 4.0%, Cu: not more than 3.0%, Ti: not less than 0% Co: more than 0% and not more than 2.0%, B: more than 0% to not more than 0.0050%, Ca: more than 0% to not more than 0.0050%, Mg: more than 0% to less than 0.0030% , And further contains at least one of REM: 0% to 0.10% or less, and 2 or more kinds of stainless steel materials excellent in gas desorption properties. The duplex stainless steel material according to claim 1 or 2, wherein the yield strength is 400 to 700 MPa and excellent gas desorption properties. A method for producing a two-phase stainless steel material excellent in gas desorption characteristics according to any one of claims 1 to 3, which comprises a step of performing a heat treatment step in a temperature range of 400 to 800 K, A method for manufacturing a two-phase stainless steel material. A method for producing a two-phase stainless steel material excellent in gas desorption characteristics according to claim 3, which comprises a step of performing a heat treatment step in a temperature range of 400 to 800 K. The method for producing a two-phase stainless steel material excellent in gas- ≪ / RTI > delete delete delete delete

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