EP2235228A1

EP2235228A1 - Austenitic stainless steel for high vacuum and high purity gas tube application

Info

Publication number: EP2235228A1
Application number: EP08861296A
Authority: EP
Inventors: Ki Won Nam; Young Hwan Kim; Hak Kim; Yong Heon Lee
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2007-12-18
Filing date: 2008-12-17
Publication date: 2010-10-06
Also published as: JP2011515571A; US20110020165A1; EP2235228A4; CN101903552A; WO2009078663A1; KR20090066000A

Abstract

The object of the present invention is to provide stainless steel for high vacuum and high purity gas tube applications not having intrusion of impure particles into a processing gas and being cost-efficient. In order to accomplish the object, the present invention provides an austenitic stainless steel for a high vacuum and high purity gas tube application, the stainless steel including, in percent by weight, 0.1% or less of C, 1% or less of Si, 0.5 to 2% of Mn, 0.05% or less of P, 0.01% or less of S, 15 to 30% of Cr, 7 to 20% of Ni, 4% or less of Mo, 3% or less of Cu, 0.05% or less of N, 0.01% or less of B, 0.01% or less of O, a remaining part of Fe, and unavoidable impurities, wherein Ti content is limited to 0.005% or less, Al content is limited to 0.005 to 0.05%, and Ca content is limited to 0.0005 to 0.003%.

Description

AUSTENITIC STAINLESS STEEL FOR HIGH VACUUM AND HIGH PURITY GAS TUBE APPLICATION

Technical Field

[1] The present invention relates to a stainless steel for high vacuum and high purity gas tube applications required in manufacturing high-integrated and high-precision products such as a semiconductor, a liquid crystal display or the like, and more particularly to material for a stainless steel pipe, which has an excellent cost efficiency due to low manufacturing cost involved in being formed into a tube and prevents the generation of defects in products since impure particles are not incorporated into the products from an inner wall surface when using the tube. Background Art

[2] In a process of manufacturing a semiconductor, a trace amount of impurities intrusion causes defects of a semiconductor product to reduce a yield, such that it is crucial to control the intrusion of impurities during the fabrication process. Therefore, a seamless pipe has been generally used in a semiconductor manufacturing device in order to prevent the intrusion of impure particles from a welding part, and bright annealing and electrolytic polishing are performed in order to completely remove the impure particles from an inner wall surface of a steel pipe. Regarding the method of processing an inner wall surface of a steel pipe, Japanese Patent Laid-Open Publication No. H7-11378 (Daido Steel) has disclosed a method to lower surface roughness below a predetermined level by limiting oxygen and hydrogen among steel to 0.0015 or less percent by weight (hereinafter, referred to as wt.% ) and 0.0002 wt.% or less, respectively, and processing an inner wall surface of pipe using a mechanical method and electrolytic polishing method. Also, Japanese Patent Laid-Open Publication No. 2002-60964 (Sumitomo Metal Mining) has disclosed a technique to prevent the intrusion of particles of corrosion product by passivating an inner surface of a bright pipe on which bright annealing is performed with an aqueous nitric acid after a steel pipe is cold-drawn to improve corrosion resistance of the inner wall surface.

[3] In recent, a demand for a welded pipe has been increased in order to reduce manufacturing cost and in this case, it is very important to prevent the generation of impure particles which may be easily formed on a welding part, such as various oxides, nitrides or the like. To this end, U.S. Patent Nos. 5,830,408 and 5,942,184 provide chemical compositions of austenitic and ferritic stainless steel, containing: 0.2 wt.% or less of Mn, 0.01 wt.% or less of Al; 0.5 wt.% or less of Si, and 0.01 wt.% or less of O.

[4] However, in the case the stainless steel is manufactured based on the chemical com- positions as described above, the generation of the impure particles may be reduced during the welding of the steel pipe, whereas a problem arises in that there may increase surface defects such as a sliver or a micro-crack generated on a plate surface during a manufacturing process of stainless steel plates or coils before the steel pipe is manufactured. The surface defects contain a lot of impure particles such as Cr, Fe and Si oxides therein, such that the impurities due to the surface defects are increased although the impure particles generated from the welding part are decreased. Also, most of the impure particles generated from the welding part are removed by a process such as a pickling, an electrolytic polishing of the inner surface of the steel pipe or the like, followed by manufacturing the welded pipe. However, it is very difficult to remove the impure particles inside the surface defects using chemical and mechanical methods. Therefore, the stainless steel as described above is not suitable for a high vacuum or high purity gas tube application. Disclosure of Invention Technical Problem

[5] Therefore, the present invention has been proposed to solve the above problems and the object of the present invention is to provide an austenitic stainless steel for a high vacuum and high purity gas tube application, which has no surface defects containing impure particles, has no generation of slag impure particles that are not easily removed using a chemical method on a welding part in manufacturing a steel pipe by means of welding, and has no generation of nitride-based impure particles although a cheap nitrogen protective gas is used in welding junction of steel pipes, thereby being cost- efficient and being able to prevent defects of a product. Technical Solution

[6] In order to accomplish the object of the present invention, the present invention provides an austenitic stainless steel for a high vacuum and high purity gas tube application, the stainless steel including, in percent by weight, 0.1% or less of C, 1% or less of Si, 0.5 to 2% of Mn, 0.05% or less of P, 0.01% or less of S, 15 to 30% of Cr, 7 to 20% of Ni, 4% or less of Mo, 3% or less of Cu, 0.05% or less of N, 0.01% or less of B, 0.01% or less of O, a remaining part of Fe, and unavoidable impurities, wherein Ti content is limited to 0.005% or less, Al content is limited to 0.005 to 0.05%, and Ca content is limited to 0.0005 to 0.003%.

[7] Also, the present invention provides the austenitic stainless steel for a high vacuum and high purity gas tube application, wherein the number of sliver defects having a length of 3mm or more in a rolling direction on the surface of cold rolled coils of the austenitic stainless steel is less than five per coil of 100m.

[8] Also, the present invention provides the austenitic stainless steel for a high vacuum and high purity gas tube application, wherein oxide and nitride are not detected on the surface of beads when welding the austenitic stainless steel using an Ar protective gas and a nitrogen protective gas.

Advantageous Effects

[9] The present invention can obtain an austenitic stainless steel having no surface defects containing impure particles on the surface of steel material, when being utilized as high vacuum and high purity gas tube applications used in the process of manufacturing a semiconductor or a liquid crystal display.

[10] Also, the present invention provides an austenitic stainless steel for high vacuum and high purity gas tube applications, which does not generate the slag impure particles that are not easily removed by a chemical method on the welding part in manufacturing the steel pipe and does not generate nitride-based impure particles although a cheap nitrogen protective gas is used in welding-junction of the steel pipes, thereby being cost-efficient. Brief Description of the Drawings

[11] FIG. 1 is a picture showing a type 1 sliver defect formed of a metal thin film containing oxide particles on the surface of cold rolled coils;

[12] FIG. 2 is a picture showing a type 2 sliver defect remaining as a compressed trace without having a metal thin film and oxide particles on the surface of cold rolled coils;

[13] FIG. 3 is a picture showing a Ti oxide and a Ti nitride formed on the surface of TIG weld beads of an austenitic stainless steel containing a trace amount of Ti;

[14] FIG. 4 is a picture showing a Ca oxide formed on the surface of TIG weld beads of an austenitic stainless steel containing an excessive amount of Ca; and

[15] FIG. 5 shows the chemical compositions of the examples of the present invention and the comparative examples. Best Mode for Carrying Out the Invention

[16] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[17] Impure particles to cause defects of products in stainless steel for high vacuum and high purity gas tube applications used in manufacturing high-precision products such as a semiconductor, a liquid crystal display or the like mainly flow from as follows: firstly, oxide particles and metal grains remaining at the bottom of sliver defects on the surface of stainless steel coil material; secondly, oxides generated in a weld slag shape in pipemaking of weld pipes of stainless steel coils and not removed through subsequent pickling or electrolytic polishing but remaining; and thirdly, nitride particles formed when weld-joining a pipe to a pipe so as to manufacture a tube of a manufacturing installation, in particular, when welding a pipe onto a pipe using a cheap nitrogen protective gas for the cost-efficient reason.

[18] FIG. 1 is a picture precisely showing a sliver defect where oxide particles remain at the bottom of the defects (hereinafter, referred to as "type 1" defect), which is a kind of general sliver defect that is generated on the surface of an austenitic stainless steel cold rolled coil manufactured using a continuous casting, a hot rolling, a pickling of a hot rolled coil, a cold rolling, a pickling of a cold rolled coil, and a final skin pass rolling. The sliver defect generally has a shape where oxide particles containing Cr and Fe components are compressed to remain between a metal thin film having a thin thickness of 50 m or less that is the size of about one grain and a base material under the metal thin film.

[19] The metal thin film covering the oxide particles has a very thin thickness that is the size of about one grain and includes grains partially eroded during a cold rolled picking process to be very weakly connected to each other. Therefore, while being used as a tube, the metal thin film is easily separated by means of gas flow so that metal grains in gas and the oxide particles at the bottom thereof may easily flow as impurities. When pickling or electrolytic polishing is performed on the inner wall surface of the pipe, the ablation amount is very large to allow the thickness of the metal thin film to be thinner and allow the connection of the grains to be weaker, so far as the metal thin film in the defect portion and the oxide layer at the bottom thereof are not completely ablated or removed, thereby causing a great possibility of promoting the infiltration of impurities. In general, since the ablation amount by means of a pickling or an electrolytic polishing method hardly exceeds 50 m, it is difficult to completely remove the metal thin film in the defect portion and the oxide layer at the bottom thereof.

[20] FIG. 2 shows a sliver defect in another shape, which is observed on the surface of the austenitic stainless steel cold rolled coil as described above. In this case, the metal thin film and the oxide layer at the bottom thereof as described above are not shown, but only the trace that the metal thin film and the oxide layer at the bottom thereof, which have been during the manufacturing process, are completely separated to be eliminated is observed. In the sliver defect of such a shape (hereinafter, referred to as "type 2" defect), metal grains or oxides do not flow into gas while the austenitic stainless steel cold rolled coil is used as a tube.

[21] In general, the sliver defect as shown in FIG. 1 and the sliver defect as shown in FIG.

2 are generated simultaneously on the surface of the cold rolled coil. All these are shown in such a way that after surface cracks are formed by a lack of hot workability of material during a hot rolling process of continuous casting slabs, oxides are formed inside the cracks and are compressed in a subsequent rolling process to be shown. These surface cracks are compressed in the processes of pickling of a hot rolled coil, cold rolling and pickling of a cold rolled coil, after the hot rolling process, to have a shallow depth and are gradually extinguished as the generation of oxide scale of the coil surface and the removal thereof by pickling are repeated. However, these surface cracks are not removed after pickling of a final cold rolled coil and skin pass rolling are performed, such that the defect remaining on the surface remains as the sliver defect of the product.

[22] During the process as described above, the inventors of the present invention found that the sliver defects as shown in FIG. 1 are changed into the sliver defect in the shape as shown in FIG. 2, and when the number of the finally remaining sliver defects is very few, only the sliver defects in the shape as shown in FIG. 2 remain, but the silver defect as shown in FIG. 1 does not. Based thereon, after detecting the sliver defects from a pickling line of the final cold rolled coil using an automatic defect detector, the present inventors found that when the sliver defects having a length of 3mm or more on the surface of the cold rolled coil in a rolling direction are less than five per coil of 100 m, the type 1 sliver defects as shown in FIG. 1 do not remain. Therefore, if the sliver defects of the cold rolled coil meet the condition as described above, it is possible to prevent the infiltration of impure particles by means of the separation of the oxide particles remaining at the bottom of the silver defect and the metal thin film when the austenitic stainless steel is used as a high vacuum and high purity gas tube application.

[23] In order to reduce the sliver defects as described above, the hot workability of the austenitic stainless steel must be improved, wherein micro-elements such as Ti, B, Al and Ca improve the hot workability and elements such as S and O decrease the hot workability. However, in controlling the content of such micro-elements, it is necessary to control the content of such micro-elements within a range of chemical components that can prevent the infiltration of the impure particles by means of the separation of the oxide particles remaining at the bottom of the sliver defect or the metal thin film by securing the hot workability simultaneously as well as not generating the oxide particles and the nitride particles, the oxide particles being generated in a weld slag shape in pipemakig of weld pipes of stainless steel coils and not removed through subsequent pickling or electrolytic polishing but remaining, and the nitride particles being formed when weld-joining a pipe to a pipe so as to manufacture a tube of a manufacturing installation, in particular, when welding a pipe onto a pipe using a cheap nitrogen protective gas for the cost-efficient reason.

[24] In order to accomplish the object, the present invention provides an austenitic stainless still for a high vacuum and high purity gas tube application, the austenitic stainless steel including, in percent by weight, 0.1% or less of C, 1% or less of Si, 0.5 to 2% of Mn, 0.05% or less of P, 0.01% or less of S, 15 to 30% of Cr, 7 to 20% of Ni, 4% or less of Mo, 3% or less of Cu, 0.05% or less of N, 0.01% or less of B, 0.01% or less of O, a remaining part of Fe, and unavoidable impurities, wherein Ti content is limited to 0.005% or less, Al content is limited to 0.005 to 0.05%, and Ca content is limited to 0.0005 to 0.003%.. Accordingly, it is prevented generating oxide particles which are not removed by a subsequent pickling or electrolytic polishing process but remain on the surface of weld beads in weld-pipemaking. Also, the present invention provides a method capable of securing hot workability that sliver defects having a length of 3mm or more in a rolling direction of a cold rolled coil are less than five per coil of 100m without generating nitride particles in welding a tube of a manufacturing installation using a nitrogen protective gas.

[25] Hereinafter, the reasons to limit the component elements will be described in detail.

[26] C, which is an element to form a Cr carbide, is limited to 0.1% or less in order to prevent the deterioration of corrosion resistance of a welding part resulted therefrom.

[27] Si is a deoxidizer required in a steelmaking process. When it exceeds 1%, globular oxide type inclusion increases to be highly likely to flow into a processing gas, such that Si content is limited to 1% or less.

[28] Mn, which is an element to improve hot workability by fixing S that decreases the hot workability, is required 0.5% or more. However, when more than 2% percent is added, it deteriorates the corrosion resistance, such that Mn content is limited to 0.5 to 2%.

[29] P and S are elements deteriorating the hot workability and the corrosion resistance, and P content and S content are limited to 0.05% or less and 0.01% or less, respectively.

[30] Cr, which is an element to endow the corrosion resistance of stainless steel, is required 15% or more in order to secure the corrosion resistance of an inner wall surface of a tube. However, when exceeding 30%, impurities may flow into the processing gas by means of the precipitation of intermetallics compound such as sigma phase. Therefore, Cr content is limited to 15 to 30%.

[31] Ni is required 7% or more in order to secure an austenitic phase structure. However, when exceeding 20%, stability effect of the austenitic phase is hardly changed but cost of raw materials is rapidly increased. Therefore, Ni content is limited to 7 to 20%.

[32] Mo is an element to reinforce the corrosion resistance. However, when exceeding

4%, a sigma phase is formed during the manufacturing process thereof to generate em- brittlement. Therefore, Mo content is limited to 4% or less.

[33] Cu, which is an element stabilizing the austenitic phase in the similar manner of Ni, simultaneously with improving ductility of stainless steel. However, when exceeding 3%, the more cracks are generated during a hot rolling to cause a lot of sliver defects. Therefore, Cu content is limited to 3% or less.

[34] N is also an element to stabilize an austenitic phase. However, when its content is increased, the hot workability is decreased and when its content exceeds 0.05%, the number of sliver defects are rapidly increased. Therefore, N content is limited to 0.05% or less.

[35] B is an element to control the generation of surface cracks by reinforcing austenitic grain boundaries during a hot rolling process. However, when exceeding 0.01%, the hot workability thereof is rapidly decreased at a very high temperature. Therefore, B content is limited to 0.01% or less.

[36] O is an element to deteriorate the hot workability and to reduce the stability of a weld bead process by the generation of weld slag, and its content is limited to 0.01% or less.

[37] Ti is an important element in view of the features of the present invention. Ti has an effect to improve the hot workability by fixing S and O and by controlling the growth of grains at a high temperature so that it is often intentionally added to the austenitic stainless steel. However, when a nitrogen protective gas is used in piping welding, a Ti oxide and a Ti nitride are very rapidly formed, such that the Ti oxide and the Ti nitride particles are formed on the inner surface of the tube, as shown in FIG. 3. In order to control the formation of such Ti oxide and Ti nitride, Ti content is required to be limited to a very low level, such as 0.005% or less.

[38] Al is an element to improve the hot workability by fixing O. In order to obtain such an effect, Al is required 0.005% or more. However, when exceeding 0.05%, weld-slag is increased to function as a source of impurities of the processing gas. Therefore, Al content is limited to 0.005 to 0.05%.

[39] Ca is an important element in view of the features of the present invention. Ca is an element to improve the hot workability by fixing S and is required 0.0005% at minimum in order to obtain such an effect. However, when exceeding 0.003%, more slag becomes severely formed on the surface of weld beads due to a very rapid oxidation and a damage in a molten metal at the time of welding, as shown in FIG. 4, to form impure particles that are not easily removed by means of a pickling or electrolytic polishing process. Therefore, Ca content is required to be strictly limited to 0.0005 to 0.003%.

[40] Hereinafter, the present invention will be described through examples.

[41] In FIG. 5, continuous casting slabs of austenitic stainless steel having chemical compositions as shown in the examples of the present invention and comparative examples are heated in a reheating furnace according to a manufacturing process of general stainless steel at a temperature of 124O⁰C for 180 to 210 minutes, and then are rolled to have a thickness of 3 to 4.5mm by means of a hot roughing mill and a hot finishing mill and are subject to an annealing process, a mechanical descaling process and a chemical pickling process in an annealing pickling line, thereby manufacturing hot rolled coils. [42] These hot rolled coils are rolled again to have a thickness of 1.0 to 2.0mm in a cold strip mill and are subject to an annealing process and a chemical pickling process in a cold rolled annealing pickling line, thereby manufacturing final cold rolled coils. Regarding the manufactured cold rolled coils, sliver defects in the cold rolled annealing pickling line are evaluated using an online defect detector and gas tungsten arc welding is tested under two conditions using an Ar protective gas and a nitrogen protective gas. The results thereof are shown in Table 1 below.

[43] In comparative examples 1, 3, and 4 where Ti exceeds 0.005% by weight to be added, after performing a welding experiment using the nitrogen protective gas, Ti nitride and oxide are observed on the surface of beads. In comparative examples 5 and 6 where Al and Ca each exceed the maximum permissible dose, Al oxide and Ca oxide are formed on the surface of the beads, irrespective of the sort of protective gas. In comparative example 3 where Ti and Ca are ultramicro and Al lack the minimum requirements limited in the present invention, a great quantity of type 1 sliver defects are generated, the type 1 sliver defects containing oxide particles therein causing pollution of the process air due to the hot workability. To the contrary, in examples 1, 2 and 3 where they meet the range of chemical composition limited in the present invention, the total number of the sliver defects having a size of 3mm or more are five per coil of 100m, the type 1 defects are not generated, and the impure particles such as oxide, nitride or the like are not formed on the surface of the beads in welding using the Ar protective gas and the nitrogen protective gas.

[44] Table 1

[Table 1] [Table ]

[45] As described above, the optimal embodiments of the present invention are described through the detailed description and drawings. Terms are used not for limiting the scope of the present invention claimed in the claims but only for explaining the present invention. Although the preferred embodiment of the present invention is described, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

[1] An austenitic stainless steel for a high vacuum and high purity gas tube application, the stainless steel comprising: in percent by weight, 0.1% or less of C, 1% or less of Si, 0.5 to 2% of Mn, 0.05% or less of P, 0.01% or less of S, 15 to 30% of Cr, 7 to 20% of Ni, 4% or less of Mo, 3% or less of Cu, 0.05% or less of N, 0.01% or less of B, 0.01% or less of O, a remaining part of Fe, and unavoidable impurities, wherein Ti content is limited to 0.005% or less, Al content is limited to 0.005 to 0.05%, and Ca content is limited to 0.0005 to 0.003%.

[2] The austenitic stainless steel for a high vacuum and high purity gas tube application as claimed in claim 1, wherein the number of sliver defects having a length of 3mm or more in a rolling direction on a surface of cold rolled coils of the austenitic stainless steel is less than five per coil of 100m.

[3] The austenitic stainless steel for a high vacuum and high purity gas tube application as claimed in claim 1, wherein an oxide and a nitride are not detected on the surface of beads when welding the austenitic stainless steel using an Ar protective gas and a nitrogen gas.