TWI431126B - Duplex stainless steel for vacuum containers and method for manufacturing the same - Google Patents

Description

Duplex stainless steel material for vacuum container and manufacturing method thereof Field of invention

The present invention relates to a Ni-saving duplex stainless steel material and a method for producing the same, which are inexpensive and have excellent gas desorption characteristics, and are suitable for use as a vacuum container.

Background of the invention

In recent years, the production of semiconductor components, liquid crystal panels, and thin film solar cells has rapidly increased, and these products have also shown a trend of enlargement. These products require vacuum treatment during the manufacturing process, and most of their vacuum containers are made of metal materials such as stainless steel, aluminum, and titanium. The stainless steel used in the vacuum container has always been made of Austenitic iron material represented by SUS304 stainless steel. Further, with the increase in size of vacuum equipment, stainless steel having a thickness of up to about 80 mm is also currently used.

The material for the vacuum container must have some characteristics, and the characteristics of lower gas emissions can be exemplified. In particular, in the prior research, the effects of the surface polishing conditions and the baking treatment on the gas discharge properties of materials such as Austenitic stainless steel, aluminum alloy, and titanium which are materials for ultrahigh vacuum have been described (see Non-Patent Document 1). It has been known that the use of the grinding step can effectively reduce the surface roughness and reduce the surface oxide layer. Further, it is also known that long-time baking at a temperature of 100 to 450 ° C can effectively solve the above problem (see Non-Patent Document 2).

Moreover, in order to improve the vacuum characteristics, it is possible to form a film having a high Mn content on a large amount of Mn-containing Austenite stainless steel material (see Patent Document 1). This view provides the possibility for us to develop new materials.

Another property that needs to be possessed is strength and weldability. This feature is also becoming increasingly important as the size of vacuum vessels has increased. Especially for components such as pre-exhaust chambers, because of the constant repetition of atmospheric pressure and vacuum, it is more economical to use high-strength materials with good fatigue characteristics. However, the lower limit of the drop strength of the Osbane iron-based stainless steel is about 200 MPa, and this property must be improved if it is to be used as a material for a large-sized vacuum container.

Duplex stainless steel is characterized by a high content of Cr and Mo, and therefore has a higher strength than Austenite-based stainless steel; however, it is rarely used in vacuum vessels because it is an expensive material. However, in recent years, duplex stainless steels having reduced Ni content and increased Mn content have been gradually developed, so that even considering the cost of steel, it can be applied by thinning the container material. However, if Ni is used in duplex stainless steel and the composition of Mn is added, it is not only possible to damage its ductility and toughness, but also the presence of the ferrite phase and the Osmanian iron phase for its vacuum properties (gas removal). The impact of the feature) has not been determined.

Therefore, the inventors of the present invention have focused on the strength, toughness, surface characteristics, gas desorption characteristics, heat treatment properties, and abrasiveness of the Ni-saving duplex stainless steel, and whether it is suitable for use in a vacuum vessel.

Duplex stainless steel is a material composed of a ferrite phase and an austenitic phase. In addition to its high strength, it has ductility, toughness and weldability. Therefore, it can be said that it has the basic characteristics under the premise of replacing Oswego iron-based stainless steel. However, it is still necessary to understand the possible effects of a ferrite phase with a low toughness of about 50% and a small hydrogen solids limit. Moreover, the most important characteristics required as a material for a vacuum container are: a smooth and clean surface obtained by mechanical, electrolytic, chemical polishing, etc., with good desorption characteristics of surface absorbing gas such as water, and steel. The amount of hydrogen emissions is small. The object of the present invention is to clarify how to make the duplex stainless steel have the above characteristics, making it possible to develop a practical material for use as a vacuum container.

The inventors of the present invention conducted an investigation at the beginning of the invention to evaluate whether duplex stainless steel is suitable as a vacuum vessel material for the purpose of the inventors of the present invention, but no specific related literature has been found so far. Since duplex stainless steel is rich in Cr and N, its corrosion resistance is extremely high, so a problem arises, that is, the final completion of the pickling step on the steel surface is inefficient. Under the premise of recognizing this subject, we focus on the important correlation between cold working and vacuum characteristics in duplex stainless steel, and based on the invention of duplex stainless steel for the vacuum characteristics of 0-20% cold-worked materials. experiment. The result is a new issue in which cold working promotes the rate of hydrogen emissions in steel. That is, when we consider the characteristics of the manufacturing steps, that is, relying on the chemical composition of the Ni-saving duplex stainless steel to develop a duplex stainless steel material for a vacuum vessel, it is necessary to correctly control the characteristics of the steel surface.

Advanced technical literature Patent literature

Patent Document 1 Japanese Patent Laid-Open Publication No. 2003-13181

Non-patent literature

Non-Patent Document 1 J. Vac. Soc. Jpn. Vol. 50, No. 1, 2007, p47-52

Non-Patent Document 2 Vac. Soc. Jpn. Vol. 49, No. 6, 2006, p335-338

Non-Patent Document 3 J. Vac. Soc. Jpn. Vol. 50, No. 4 2007, p228

The object of the present invention is to obtain a Ni-saving duplex stainless steel material which can be used for a vacuum vessel instead of the Osbane iron-based stainless steel, and to describe the chemical composition, surface characteristics and manufacturing method of the steel.

In order to solve the above problems, the inventors of the present invention conducted the following experiment.

First, the duplex stainless steel containing various compositions is used in the experiment, and hot-rolled, melt-treated, heat-treated at 1000 K or less, and then sandblasted and rusted and pickled under various conditions to obtain 10 mm to 40 mm. Plate thickness of hot rolled steel.

The obtained steel material was quantified, the tensile strength was used to measure the strength, the surface roughness was measured by the specification of JIS B0601, and the sub-subcutaneous hardening depth was measured by the Vickers hardness.

Further, in order to evaluate the gas desorption characteristics, a gas analysis sample having a size of 3 mm thick × 14 mm × 14 mm was taken out from the surface of the steel material by mechanical processing, and a part of the sample for smoothing was mechanically ground (#150 belt sand) The sample which was ground to #600 wet grinding) was partially a sample which was omitted from mechanical grinding and only pickled. After the above two samples were subjected to electrolytic polishing using a phosphate-based solution, desorption gas analysis was performed. The desorption gas analysis system placed the sample on a transparent quartz table and placed it in an analytical vacuum vessel that had been exhausted to 10^(-7) Pa, and heated to 200 ° C at a heating rate of 1.25 ° C / s. The desorbed water and hydrogen were ionized, and a quadrupole mass spectrometer (QMS) was used for quantitative analysis. The same measurement was carried out for SUS304 as a comparative material, and the vacuum characteristics (gas desorption characteristics) of the duplex stainless steel were evaluated based on the relative values.

Through the above experiments, we have clearly found out the chemical composition, surface characteristics and manufacturing method of the duplex stainless steel material having excellent gas desorption characteristics and suitable as a vacuum vessel, so that the present invention can be produced.

That is, the focus of the present invention will be as follows.

(1) A duplex stainless steel material having excellent gas desorption characteristics, characterized by containing C: 0.06% or less, Si: 0.05% to 1.5%, Mn: 0.5 to 10.0%, and P by mass% : 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 the hydrogen content in the steel is 3 ppm or less; Further, if necessary, one or more of the following components are contained: 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, and REM: 0.10% or less; and the remaining components are composed of Fe and unavoidable impurities.

(2) The duplex stainless steel of (1) has a maximum cross-sectional height Rt of 40 μm or less and a depth of the sub-subcutaneous hardened layer of 0.15 mm or less.

(3) The duplex stainless steel of (1) or (2) has a fall strength of 400 or more and 700 Mpa or less.

(4) A method for producing a duplex stainless steel material according to (1) or (2), which comprises the step of performing heat treatment at a temperature ranging from 400 to 800K.

(5) A method for producing a duplex stainless steel according to (3), which comprises the step of performing heat treatment at a temperature ranging from 400 to 800K.

At present, Aostian Iron and Steel is used as a material for vacuum containers in the manufacture of semiconductor components, liquid crystal panels, and thin film solar cells. According to the present invention, it is possible to provide a duplex stainless steel material which has excellent strength and gas desorption characteristics, and can be used to replace a part of the member using Osbane Iron and Steel, which is thinner than the conventional steel and contributes greatly to the development of the industry.

Simple illustration

Fig. 1 shows the form of a sample for determining the depth of sub-subcutaneous hardening.

Used to implement the type of invention

The invention will be specifically described below. First, the requirements described in the invention (1), that is, the chemical composition of the duplex stainless steel and the reason for the hydrogen content in the steel, will be mainly described.

In order to ensure the corrosion resistance of stainless steel, the C system limits the content to 0.06% or less. If the content exceeds 0.06%, Cr carbide is formed to deteriorate corrosion resistance and toughness. The content is preferably 0.03% or less.

Since the Si system deoxidizes the steel material during the melting of the steel, it is added in an amount of 0.05% or more. However, if it is added more than 1.5%, its toughness will deteriorate. Therefore, the upper limit is set to 1.5%. It is preferably 0.2 to 1.0%.

In order to improve the toughness and vacuum characteristics (gas desorption characteristics) of steel, Mn has a Mn content of 0.5% or more. Mn is added to increase the Oswego iron phase to improve toughness, and to accumulate in the oxide film to improve the gas desorption characteristics after the oxidation treatment. However, if the content exceeds 10.0%, the corrosion resistance and toughness are deteriorated. Therefore, the upper limit is limited to 10.0%. The ideal content is 3.0 to 8.0%.

Since P is an impurity, the hot work of steel and the toughness are deteriorated, so the content is limited to 0.05% or less. It is preferably 0.03% or less.

The S system is an impurity and also deteriorates the hot work, toughness, and corrosion resistance of the steel, so the content is limited to 0.010% or less. It is preferably 0.0020% or less.

In order to stabilize the Osbane iron structure, the Ni system improves the corrosion resistance and even toughness of various acids, and the content is 0.1% or more. However, since Ni is a high-priced alloy, in order to consider the cost, the content is limited to 5.0% or less. The ideal content is 1.5 to 4%.

In order to ensure the basic corrosion resistance of steel, Cr is controlled to have a content of 18.0% or more. However, if the content exceeds 25.0%, the ferrite iron phase fraction increases, which hinders the toughness and the corrosion resistance of the welded portion. Therefore, the Cr content is controlled to be 18.0% or more and 25.0% or less. The ideal content is 19 to 23%.

The N-series solidifies the element to the steel phase of the Osman iron phase, which effectively improves the strength and corrosion resistance. Therefore, the content is controlled to be 0.05% or more. Although the solid solution limit increases with the Cr content, if the steel of the present invention contains more than 0.30% of N, the nitride of Cr precipitates and hinders the toughness and corrosion resistance, so the upper limit of the content is set to 0.30. %. The ideal content is 0.10 to 0.25%.

Al is an important element for deoxidizing steel materials. In order to reduce oxygen in steel, it is also added to Si. If the Si content is more than 0.3%, it may not be necessary to add it. However, in order to ensure the toughness of the steel, the oxygen content must be lowered, so the content must be controlled to 0.001% or more. On the other hand, Al is an element having a large affinity with N, and if it is excessively added, AlN is generated to impede the toughness of the steel. Although the degree of influence depends on the N content, if the Al exceeds 0.05%, the toughness of the steel will be significantly reduced, so the upper limit of the content is controlled at 0.05%. The upper limit is preferably 0.03%.

O (oxygen) is a main element constituting an oxide, and an oxide is a representative of a non-metal intervening substance. When the content is too large, the toughness is inhibited. Further, if coarse oxide clusters are formed, the surface may be deteriorated. Although the present invention does not specify an upper limit of its content, it is preferably controlled to be 0.010% or less.

The amount of hydrogen in the steel affects the amount of hydrogen or water that is discharged from the vacuum vessel material into the vacuum. Moreover, it is known in the industry that hydrogen contained in steel is converted into water by oxidation of the surface of the steel to promote desorption of water. In particular, in the duplex stainless steel containing the ferrite iron phase, since the hydrogen diffusion is very large, it is necessary to control the hydrogen content in the steel material to a small amount in advance. The inventors of the present invention found that if the hydrogen content is controlled to be less than 3 ppm, the gas emission characteristics of the same level as that of the Osbane iron-based stainless steel will be possessed, so the upper limit of the hydrogen content is set to 3 ppm. The smaller the hydrogen content in the steel, the better, and it should be controlled below 2 ppm, especially below 1 ppm.

The duplex stainless steel of the present invention is required as needed, in addition to the composition of the above (1), one or more of the following components may be added: Mo, Cu, Ti, Nb, V, W, Co, B, Ca. , Mg and REM.

Mo is an effective element that can improve the corrosion resistance of stainless steel, and can be added as needed. It is preferably contained in an amount of 0.2% or more. Under the consideration of cost, the Mo content of the steel of the present invention is limited to 4.0%, but Mo is a very expensive element, and it is particularly preferable to further control it to 1.0% or less.

The Cu system can additionally improve the corrosion resistance of the stainless steel, and also has the effect of improving the toughness of the steel. However, if the addition amount exceeds 3.0%, the solid solution is exceeded, and εCu is precipitated to cause embrittlement, so the upper limit is controlled to 3.0%. Cu has the function of stabilizing the Osmanite iron phase and improving toughness, so it is recommended to add 0.3% or more. In the case of containing Cu, the content is preferably 0.3 to 1.5%.

Ti is capable of generating oxides, nitrides, and sulfides in a very small amount, and can help the steel to solidify and refine the crystal grains of the high-temperature heated structure, and add them as necessary. However, if more than 0.05% is added to the duplex stainless steel, coarse TiN is formed, which hinders the toughness of the steel. Therefore, the upper limit of its content is set to 0.05%. The appropriate content of Ti is 0.003 to 0.020%.

Nb is an element which can effectively refine the crystal grains of the hot-rolled structure, and has an effect of increasing corrosion resistance. The nitride and carbide of Nb can be formed during the hot working and heat treatment, can inhibit the growth of crystal grains, and strengthen the steel, so it is preferable to add 0.01% or more. However, if Nb is excessively added, it will precipitate as unsolidified precipitates during the heating stage before hot rolling, which hinders the toughness of the steel, so the upper limit of the content is set to 0.20%. If it is to be added, the content range is preferably 0.03 to 0.10%.

V and W are elements added for the purpose of improving the corrosion resistance of the duplex stainless steel.

V is added in an amount of 0.05% or more for the purpose of improving corrosion resistance. However, if it is added in excess of 0.5%, coarse V-based carbonitrides are formed to deteriorate the toughness. Therefore, the upper limit is set to 0.5%. If it is to be added, the ideal content is in the range of 0.1 to 0.3%.

W has the same effect as Mo, and it is an element that can improve the corrosion resistance of stainless steel, and the solid solubility is larger than V. In the steel of the present invention, in order to improve the corrosion resistance, the content is made up to 1.0%. The ideal content is 0.05% to 0.5%.

Co is an element which can effectively improve the toughness and corrosion resistance of steel, and is optionally added as the case may be. The content is preferably 0.03% or more. Since Co is an expensive element, if it is added more than 2.0%, the cost control effect cannot be achieved, and the upper limit is set to 2.0%. If added, the content is preferably 0.03 to 1.0%.

Elements such as B, Ca, Mg, and REM all improve the hot workability of steel, and one or more of them are added for this purpose. However, if excessively added, each of B, Ca, Mg, and REM will reduce hot workability and toughness, so the upper limit of the content is as follows.

B and Ca were 0.0050%, Mg was 0.0030%, and REM was 0.10%. The content of each of B and Ca is 0.0005 to 0.0030%, Mg is 0.0001 to 0.0015%, and REM is preferably 0.005 to 0.05%. Here, REM refers to the sum of rare earth element contents of lanthanides such as La and Ce.

(2) of the present invention is for specifying the surface roughness maximum sectional height Rt of the steel material and the surface hardness. Rt and surface hardness are indicators of the mechanical polishing characteristics of steel. The surface is a hard duplex stainless steel. In order to obtain a smooth and clean surface by mechanical and electrical chemical polishing at the same time, the surface properties of the ideal steel material are specified. As shown in the examples, if the Rt exceeds 40 μm, or the depth of the sub-skin hardened layer exceeds 0.15 mm, if it is up to #150 belt sanding or even #600 wet sandpaper grinding and electrolytic grinding, the gas is off. The above characteristics were made with poor characteristics. The reason why the gas desorption characteristic is lowered may be that the presence of the sub-skin hardening layer accelerates the desorption speed of hydrogen, and may also be caused by the residual microscopic surface defects which are invisible to the naked eye.

The smaller the Rt, the better, preferably 20 μm or less, particularly preferably 10 μm or less.

In order to control Rt to 40 μm or less and to control the depth of the subepidermal hardening layer to 0.15 mm or less, it is sufficient to appropriately control the particle size and the projection density of the blasting and rusting to perform pickling.

(3) of the present invention is to specify the lodging strength of the duplex stainless steel. In order to reduce the thickness of the container material, the greater the strength, the better, and the lodging strength is preferably at least 400 MPa. However, if it exceeds 700 MPa, the toughness is deteriorated, so the upper limit is set to 700 MPa. The fall strength can be adjusted according to the chemical composition, the heat treatment conditions for the melt, or the heat treatment conditions such as 400 to 800 K described in the inventions (4) and (5) to be described later.

The present invention (4) and (5) relates to a method for producing a duplex stainless steel according to the present invention, and in order to increase the strength of the duplex stainless steel and to reduce the amount of hydrogen contained in the steel, the heat treatment conditions are specified herein.

The purpose of this heat treatment is to improve the strength of the steel by age hardening of the duplex stainless steel, and also to promote the reduction of the hydrogen content in the steel, so it is preferably carried out in a temperature range of 400 to 800K. The heat treatment step can reduce the hydrogen content in the steel to less than 2 ppm, or even less than 1 ppm. As the hydrogen content is reduced, the vacuum properties of the steel can be slightly improved, and the lodging strength can be increased to 500 MPa or more, or even 600 MPa or more.

The heat treatment time in the above temperature range is preferably 5 minutes or more. However, if the heat treatment time is too long and the lodging strength exceeds 700 MPa, the toughness of the steel may be impaired. Therefore, the upper limit of the heat treatment time should be individually determined depending on the aging strengthening of the steel and the embrittlement characteristics.

Moreover, after heat treatment (baking heat treatment) in a temperature range of 400 to 800 K after being manufactured into a vacuum vessel, not only the hydrogen content can be reduced, but also the water adsorbed on the surface of the vessel can be desorbed, thereby effectively improving the vacuum characteristics. .

The steel material of the present invention is used for the production of a vacuum vessel, and can be formed into a steel plate, a steel, a steel bar, a steel strip, a steel pipe, etc., but mainly a steel plate. The steel containing the steel composition described in (1) is melted, cast into a steel sheet or a steel ingot by a continuous casting method, and then rolled to form a steel sheet. Melting and casting can be carried out in comparison to the melting and casting of ordinary duplex stainless steel. This steel sheet is heated and then hot rolled to obtain a steel material having a desired shape. Here, the application conditions of the hot rolling are not particularly limited as long as the hot rolling heating and rolling conditions of the ordinary duplex stainless steel are used. After the heat treatment of the steel material, if necessary, the heat treatment of dehydrogenation and age hardening may be carried out, and the surface of the steel material is subjected to surface treatment such as sandblasting, rusting, grinding and pickling to make it a desired surface property. State to manufacture. Example

The invention will be specifically described below by way of examples. Table 1 shows the chemical composition of the experimental steel. Further, in addition to the components described in Table 1, the components contained therein also contained Fe and unavoidable impurities. Further, among the components described in Table 1, the portion not indicated in the content is an impurity grade. Further, REM in the table indicates that the lanthanoid element is a rare earth element and the content is the sum of its elements.

The steel sheet of the steel type number T is obtained by melting a flat steel embryo from an actual machine, and using a steel sheet having a thickness of 80 mm as a hot rolling material. The steel type A~Q is made up of a 50kg vacuum induction melting furnace in the laboratory. The steel of R is melted from a 50kg atmospheric melting furnace and then cast into a flat steel block with a thickness of about 110mm, followed by hot forging. A steel sheet having a thickness of 80 mm obtained after the treatment. Further, the steel sheet of the steel type number T2 is a flat steel billet which is melted by the above-mentioned actual machine, and corresponds to a portion having a hydrogen content of 4 ppm as a stage after pickling of the hot-rolled steel material.

In the hot rolling step, after the steel sheet is heated to a predetermined temperature, the rolling action is repeated using a two-stage rolling mill in the laboratory. The finishing rolling is performed at 850 to 950 ° C to obtain a steel plate (steel) having a thickness of 10 to 40 mm.

In the melt heat treatment step, the steel sheet is placed in a heat treatment furnace having a predetermined temperature of 950 to 1050 ° C, and is extracted after the soaking time of the steel sheet is adjusted, and then water-cooled.

The hydrogen content and vacuum characteristics of the obtained hot-rolled steel material (not subjected to pickling treatment) were measured in accordance with the following. After the surface of the steel material was ground to a thickness of 0.5 mm, a sample for evaluation of hydrogen content having a thickness of 3 mm and a size of 3 mm × 14 mm, and a sample for evaluation of vacuum characteristics having a thickness of 3 mm and a size of 14 mm × 14 mm were taken out. The hydrogen content was determined by an inert gas dissolution heat conduction method, and the results are shown in Table 2. In order to perform sample adjustment, the vacuum characteristic test sample was subjected to wet grinding until #600, and then subjected to electrolytic polishing in a range of 20 to 30 μm at a current density of 0.1 to 3 A/cm ² in a phosphoric acid electrolytic polishing liquid. Then, it was immersed in 35% nitric acid at room temperature for 30 minutes.

The vacuum characteristics were evaluated using a temperature-elevating desorption gas analyzer. The sample was placed on a sample stage, and the temperature rise rate of the sample stage was controlled to 10 ° C / min, and the desorbed water and hydrogen were quantitatively measured during the temperature rise to 200 ° C. It has been reported that the vacuum evacuation characteristics at normal temperature correspond to the ion current intensity desorbed at 100 to 130 ° C in the analysis of the temperature-rise desorption gas (see Non-Patent Document 3). Based on this research report, we calculated the ionic current intensity of water and hydrogen at SUS304 steel at this temperature, and then calculated the relative ratio of the ionic current intensity of the SUS304 steel. The results are shown in the vacuum characteristic-1 column of Table 2. Good values are less than 2.0 and ideal values are less than 1.5.

The tensile test of hot-rolled steel is to take out a round bar tensile test piece with a parallel portion of 8 mm diameter from a steel plate with a thickness of 10 mm, and take a 10 mm diameter round bar from a steel plate with a thickness of 20, 30 and 40 mm in a vertical rolling direction. Stretch the test piece. Further, the steel having a thickness of 30 or 40 mm is mainly taken mainly at a quarter of the thickness of the plate. The results of the lodging strength are shown in Table 2.

The impact toughness test of the hot-rolled steel was carried out by using a JIS No. 4 sand slab test piece which had been subjected to a 2 mmV-type machined groove in the rolling direction, and two pieces were taken out in the direction in which the crack propagated in parallel in the rolling direction. In addition, the steel of 10mm is evaluated by the 3/4 size sand slab test piece, and the steel of 20mm thickness is evaluated by the sand slab test piece of the full thickness of the center of the plate thickness. The steel plate with a thickness of 30 and 40mm is 1/thick. Four parts were taken for evaluation by the full-scale sand sputum test piece taken by the center. The impact test temperature was -20 ° C, and the impact test was carried out with an impact tester having a maximum impact energy of 500 J. Table 2 shows the results of the average value (J/cm ² ) of the impact values of the three test pieces of each plate thickness.

As shown in Table 2, any of the hot-rolled steels of the present invention has better properties than SUS304 steel, and exhibits not only good vacuum characteristics, but also has a strength of more than 400 MPa and a toughness of 50 J/cm ² or more. Excellent properties of materials for vacuum vessels.

On the other hand, the comparative example of Table 2 is inferior in strength or toughness compared to the SUS304 steel of the comparative material if it is not inferior in vacuum characteristics.

Next, a hot-rolled pickled steel material was produced according to the following method.

Selecting three sizes of large, medium and small beads for sandblasting and descaling, and then adjusting the projection density according to the speed of the hot-rolled steel and the number of passes, thereby removing some of the surface of the above-mentioned duplex stainless steel hot-rolled steel. . Next, the steel is immersed in a fluoroboric acid solution of 40 to 60 ° C and mixed with 10 to 20% HNO ₃ and 3 to 8% HF for 20 minutes to 24 hours, thereby completely removing the scale.

A sample for evaluation of the surface roughness and the depth of the hardened layer was cut out from the hot-rolled pickled steel. The quantification of the maximum cross-sectional height Rt and the Vickers hardness measurement of 100 gf were carried out in accordance with the surface roughness measurement standard defined in JIS B0601 to quantify the subepidermal hardening depth. The evaluation length for measuring the surface roughness was set to 3.0 mm, and after each measurement three times, the maximum value was taken.

When the depth of the sub-skin hardening is measured, since the area of the sample is narrow and has a thickness, in order to improve the precision of the measurement, the sample is cut as shown in Fig. 1 to form a slope, and the cut surface is turned upward. Fill in the resin. Thereafter, the hardness of the bevel at 20 places was measured from a position corresponding to the surface of the steel material at a pitch of 0.1 mm. That is, the hardness corresponding to a depth of 1 mm under the skin of the steel was measured. Each measurement point was measured with n = 3, and the sub-skin hardness distribution was obtained from the average value. The subepidermal hardening depth is a subepidermal thickness obtained by determining a portion which is hardened by 50 or more with respect to the internal average hardness Hv, as shown in Table 3. The internal average hardness referred to herein is determined from the average of the hardness of the portion of the subcutaneous depth of 0.5 to 1.0 mm.

In the atmosphere, a part of the hot-rolled pickled steel is subjected to an aging hardness and a heat treatment for reducing the hydrogen content (aging heat treatment). The heat treatment at this time will form a thin oxide film on the surface of the steel.

The hot-rolled pickled steel and the aging heat-treated steel were evaluated in the same manner as the hot-rolled steel which was not subjected to the pickling treatment, and the hydrogen content and the vacuum characteristics were measured. However, the sample for evaluating the vacuum characteristics was first removed by the #150 belt sander to remove the unevenness of the steel surface, and then a sample having a thickness of 3 mm and 14 mm × 14 mm was taken, and wet grinding was performed in the same manner as #600. Electrolytic polishing and nitric acid immersion were carried out to contain a part of the subepidermal hardening layer as a sample for analysis of temperature-exposed desorption gas.

Further, the tensile test and the impact test were carried out in the same manner as in the above-described hot-rolled steel material which was not subjected to the pickling treatment.

The evaluation results of the hot-rolled pickled steel are shown in Table 3 for the hydrogen content, vacuum characteristics-2, the drop strength and the impact characteristics.

In the comparative example of the experimental number No. 15 of Table 3, since the blasting and rust removing step was short and the beads of small particles were used, it took a long time to perform pickling. The experimental results show that the hydrogen content is 0.0004 mass%, and the vacuum characteristics are degraded. In the comparative example of Experiment No. 16, the granules were subjected to a sandblasting and rust removing step for a long period of time, and the scale was almost completely removed, and the pickling step was completed in a short time. Therefore, the hardened layer is expanded to 0.25 mm, and the vacuum characteristics are lowered. In the comparative examples of the experimental numbers 17 to 20, large particles were used for the blasting and rust removing step and pickling. The hardened layer is enlarged to 0.20 mm. Therefore, the vacuum characteristics of the comparative examples of the experimental numbers 18 to 20 were not good. In the comparative example of the experimental number No. 17, since the aging heat treatment was performed for a long period of time, the lodging strength was excessively increased, and embrittlement was also caused. In contrast, the hot-rolled pickled steel in the examples of the present invention has good vacuum characteristics, lodging strength and impact characteristics.

As is clear from the above examples, the present invention can obtain a duplex stainless steel having good vacuum characteristics.

Industrial availability

The invention can provide a duplex stainless steel for vacuum containers with high strength and reduced Ni content and more economical efficiency. It can reduce the cost of large vacuum containers and contribute greatly to the development of the industry.

Fig. 1 shows the form of a sample for determining the depth of sub-subcutaneous hardening.

Claims (5)

A duplex stainless steel material having better gas desorption characteristics, characterized by containing C: 0.06% or less, Si: 0.05% to 1.5%, Mn: 0.5 to 10.0%, P: 0.05 by mass% % 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 the hydrogen content in the steel is 3 ppm or less; Further, one or two or more of the following components are contained as needed: 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, and 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, and REM: 0.10% or less; the remaining components are composed of Fe and an unavoidable impurity. For the duplex stainless steel according to item 1 of the patent application, the maximum cross-sectional height Rt of the surface roughness is 40 μm or less, and the depth of the sub-skin hardened layer is 0.15 mm or less. For example, the duplex stainless steel material of the first or second patent application scope has a lodging strength of 400 or more and 700 Mpa or less. A method for producing a duplex stainless steel material for use in manufacturing a duplex stainless steel according to claim 1 or 2, characterized in that it comprises a step of performing heat treatment at a temperature ranging from 400 to 800K. A method for manufacturing a duplex stainless steel material for use in manufacturing a duplex stainless steel material according to claim 3, characterized in that it comprises a step of performing heat treatment at a temperature ranging from 400 to 800K.