Corrosion-resistant L-shaped steel for liquid ammonia storage tanks and method for preparing same

Abstract

Provided are a corrosion-resistant L-shaped steel for liquid ammonia storage tanks and a method for preparing same. The corrosion-resistant L-shaped steel for the liquid ammonia storage tank contains the following element components in percentage by weight: 0.005-0.018% of C, 0.81-0.98% of Si, 0.07-0.12% Mn, ≤0.009% of P, ≤0.002% of S, 0.23-0.55% of Al, 1.55-1.95% of Cr, 0.012-0.018% of Ce, and the balance of Fe and inevitable impurities. The corrosion-resistant L-shaped steel for liquid ammonia storage tanks has a yield strength of 340-440 MPa, an impact toughness KV2 of ≥68 J at -105 ℃, and an annual corrosion rate of 0.1-0.2 g / m2 in a liquid ammonia solution. The method for preparing same adopts a post-rolling slow cooling process, and thus features a simple process, high production efficiency, high cost-efficiency, safety, and reliability.

Description

CORROSION-RESISTANT L-SHAPED STEEL FOR LIQUID AMMONIA TANK, AND PREPARATION METHOD FOR CORROSION-RESISTANT L-SHAPED STEEL 1 IL JU HIM 1V- / V Lj Ju JLJlL JLrJLJ

[0001] The present disclosure relates to a steel for a liquid ammonia tank, as well as a preparation method for the steel, and in particular to a corrosion-resistant L-shaped steel for a liquid ammonia tank, as well as a preparation method for the corrosion-resistant L-shaped steel. BACKGROUND

[0002] Liquid ammonia, as an automotive fuel, has gained increasing attention and has been widely used due to its advantages of not emitting harmful sulfur dioxide, carbon monoxide, smoke and other pollutants. For example, some large ocean-going ships use liquid ammonia as a power fuel, which has been increasingly adopted for improving gas turbine performance due to its high-efficiency cleaning capability. As the liquid ammonia has a certain corrosion, higher requirements are put forward for the mechanical properties and corrosion resistance of a pressure vessel for storing the liquid ammonia. Common steels, such as the widely used Q345A / B / C series in steel structures, are not suitable for the construction of liquid ammonia storage tanks steel structure due to their relatively high carbon content and low steel purity. As the liquid ammonia is stored in a low-temperature environment, the corresponding steel is required to have good low-temperature impact toughness and excellent corrosion resistance.

[0003] In invention patent (No. 202110031827.9), a corrosion-resistant L-shaped steel capable of resisting salt and acid corrosion and a manufacturing method therefor are provided. In addition to Fe and inevitable impurity elements, the corrosion-resistant L-shaped steel further includes the following chemical elements in mass percentage: 0.02-0.3% of C, 0.2-0.8% of Si, 0.4-1.6% of Mn, 0.05-0.5% of Cu, 0.05-1.5% of Cr, 0.05-0.5% of Ni, 0.05-0.3% of Mo, 0.03-0.05% of Al. Further, the present disclosure further provides a manufacturing method for the corrosion-resistant L-shaped steel capable of resisting salt and acid corrosion, including the following steps: (1) smelting and casting; (2) heating; (3) rolling, including rough rolling and finish rolling; and (4) air cooling to a room temperature. However, the low-temperature toughness and corrosion resistance of the corrosion-resistant L-shaped steel still need to be improved. SUMMARY

[0004] An objective of the present disclosure is as follows: the present disclosure aims at providing a corrosion-resistant L-shaped steel for a liquid ammonia tank, which has a yield strength ranging from 340 MPa to 440 MPa and has excellent low-temperature toughness and corrosion resistance. Another objective of the present disclosure is to provide a preparation method for the corrosion-resistant L-shaped steel for a liquid ammonia tank. A post-rolling slow cooling production process is employed, featuring a simple manufacturing process, high production efficiency, and low cost.

[0005] Technical solutions are as follows: the corrosion-resistant L-shaped steel for a liquid ammonia tank includes the following chemical element composition in weight percentage: 0.005-0.018% of C, 0.81-0.98% of Si, 0.07-0.12% of Mn, not more than 0.009% of P, not more than 0.002% of S, 0.23-0.55% of Al, 1.55-1.95% of Cr, 0.012-0.018% of Ce, and the balance of Fe and inevitable impurities.

[0006] Further, the weight percentage of C is preferably 0.006-0.01%.

[0007] Further, the weight percentage of Si is preferably 0.85-0.95%.

[0008] Further, the weight percentage of Al is preferably 0.25-0.50%.

[0009] Further, the weight percentage of Cr is preferably 1.56-1.90%.

[0010] The design principles of the main controlled alloying elements are described as follows.

[0011] C (carbon): the C content in the steel has a significant impact on the corrosion resistance of the steel. The C can form various types of compounds MxCy with elements such as Fe, Mn and Cr. These compounds form an electrode potential difference with a matrix, leading to an electrochemical reaction of the matrix and causing corrosion of the matrix. Therefore, the lower the C content in the corrosion-resistant L-shaped steel, the better. However, excessively low C content may lead to a significant increase in preparation cost, so the C content is set at 0.005%-0.018%.

[0012] Mn (manganese): in the present disclosure, adding an appropriate amount of Mn element into the steel can stabilize a high-temperature austenite structure, reduce an austenite transformation temperature, and inhibit the formation of a high-temperature coarsened structure, thereby obtaining a fine microstructure and excellent comprehensive mechanical properties. In addition, it is easy to form Mn segregation in the process of production and preparation to cause uneven composition and structure, and then to form MmC compound to deteriorate the corrosion resistance of the steel, so the Mn content is set at 0.07-0.12%.

[0013] P (phosphorus): residual P in a low-temperature toughness steel sharply reduces the low-temperature impact toughness of the steel. Therefore, the lower the phosphorus is removed, the better. However, if the P content is controlled to be excessively low, the preparation process is more complex and difficult to operate, leading to a significant increase in production cost, so it is better to control the P content to be not more than 0.009%.

[0014] S (sulfur): in the present disclosure, residual S belongs to the harmful element, which reduces impact toughness and tends to form a deformable compound such as MnS with Mn and other elements to deteriorate the corrosion resistance of the steel, so the S content should be controlled at a very low level. However, if the content of the harmful element S is controlled to be excessively low, the production cost is significantly increased, so the S content is controlled to be not more than 0.002%.

[0015] Si (silicon): as the C content in the steel in the present disclosure is controlled at a relatively low level, to obtain an expected strength, Si and Cr are designed as main strengthening elements, and an appropriate amount of Si can be added to improve the corrosion resistance. However, if the Si content is excessively high, the low-temperature impact toughness may be damaged, so the Si content is controlled in the range of 0.81-0.98%.

[0016] Al (aluminum): an objective of adding an appropriate amount of alloying element Al in the present disclosure is to expand a ferrite transformation area, thereby forming a same type of microstructure, ferrite, in the process of production and preparation and avoiding the formation of pearlite (ferrite +Fe3C complex phase). As there is a potential difference between different phases, it is prone to electrochemical corrosion, thereby improving the corrosion resistance. In addition, a dense AlxOyNz passivation layer can be formed on a surface in a liquid ammonia environment by adding an appropriate amount of Al, base metal Fe is prevented from being oxidized to generate Fen0m or lose electronic reaction: Fe-2e^Fe2 , that is, the corrosion rate is reduced. If the Al content is lower than 0.23%, the effect of preventing the base metal Fe from being corroded is not remarkable. If the Al content is higher than 0.55%, the production difficulty is increased, during the smelting and continuous casting process, casting nozzle is prone to clogging, leading to production accidents. Therefore, the Al content in steel in the present disclosure is controlled in the range of 0.23-0.55%.

[0017] Cr (chromium): on the one hand, an alloying element Cr added into the steel of the present disclosure forms substitutional solid solution strengthening with the base metal Fe, thereby improving the strength of the steel. On the other hand, a shielding and protective effect is exerted on the base metal Fe, thereby preventing the base metal from being corroded by [O] and [H+] in the environment. The Cr content is set in the range of 1.55-1.95%. If the Cr content is lower than 1.55%, the strengthening effect and erosion prevention effect are not remarkable. If the Cr content exceeds 1.95%, the production cost is increased.

[0018] Ce (cerium): an alloying element Ce is a light rare earth element. The objective of adding the alloying element Ce into the steel of the present disclosure is to form composite steel slag with inclusions such as MnS, AI2O3, AIN and SiO2 in the smelting process. Through refining treatment and vacuum circulation, the composite steel slag floats up to the surface, thereby purifying the steel quality, improving the low-temperature impact toughness of the steel and enhancing the corrosion resistance of the steel.

[0019] A preparation method for the corrosion-resistant L-shaped steel for a liquid ammonia tank includes the following steps: (1) converter smelting charging a basic converter with molten iron, scrap steel (P is not more than 0.008%, S is not more than 0.002%), and alloy materials, such as low-carbon MnFe, SiFe and low-carbon CrFe, for smelting, then carrying out oxygen blowing for temperature raising, oxidation and decarburization, followed by adding CaO and FeO for dephosphorization; electric furnace secondary refining, adding high-quality lime to reduce an S content, finely adjusting Mn and Cr contents in molten steel to a required range with a metal Mn and a metal Cr, and adjusting an Si content to a required range with SiCa alloy; RH vacuum refining, adding Al pellets to the molten steel for deoxidation, feeding a Ce rare earth cored wire to the molten steel, adjusting a Ce content, blowing Ar gas into the molten steel to stir and homogenize the molten steel, and carrying out cyclic treatment under ultimate vacuum to further reduce the contents of harmful gas elements such as N and H and coarse harmful inclusions in steel; and continuous casting; Step 2: rolling Where an initial rolling temperature is 1050-1090°C, and a final rolling temperature is 835-880°C; and (3) post-rolling cooling.

[0020] Further, in step (1), after dephosphorization, a phosphorus content is not more than 0.009%, the S content is reduced to not more than 0.002%, and after Al pellets are added for deoxidation, a free oxygen content [O] in the molten steel is controlled to be less than 15 ppm.

[0021] Further, in step (1), the treatment under the ultimate vacuum lasts for 16-19 min; the harmful inclusions include MnS, AI2O3, and / or AIN; and the continuous casting is carried out at a temperature of 1565-1585°C.

[0022] Further, in step (2), in the rolling process, a heating temperature of a casting billet is controlled at 1210-1240°C, with a holding time of 3.5-4 hours; and an L-shaped steel with a web thickness of 16 mm is rolled from a rectangular continuous-casting billet with a cross-sectional dimension of 150x220 mm.

[0023] Further, in step (3), the process of post-rolling cooling is post-rolling stack slow cooling at a temperature of not more than 870°C, followed by slow cooling down to a room temperature.

[0024] The beneficial effects are as follows. Compared with the prior art, the present disclosure has the following remarkable advantages: the corrosion-resistant L-shaped steel for a liquid ammonia tank has excellent low-temperature toughness and corrosion resistance while meeting a certain yield strength (340-440 MPa), and has a low-temperature impact toughness KV2 of not less than 68 J at -105°C and an annual corrosion rate in the liquid ammonia solution of 0.1-0.2 g / m2y. The preparation method employs a post-rolling slow cooling process for production, featuring a simple technological process, high production efficiency, low cost, safety and reliability, as well as remarkable economic benefits. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The technical solution of the present disclosure is further described below with reference to specific embodiments.

[0026] According to chemical element composition, mass percentage and production method requirements of the present disclosure, five embodiments are prepared, which are Embodiment 1, Embodiment 2, Embodiment 3, Embodiment 4, and Embodiment 5, respectively. To verify the effects of the chemical composition and its mass percentage, as well as process parameters such as RH ultimate vacuum treatment time during smelting, a continuous casting temperature, and a final rolling temperature during rolling, on performance parameters, three comparative examples, which are Comparative Example 1, Comparative Example 2, and Comparative Example 3, are prepared, and eight batches of L-shaped steel are smelted and rolled accordingly. In Comparative Example 1, the mass percentage of the chemical composition is not within the range of the present disclosure, but the process parameters of the preparation process are within the range of the present disclosure. In Comparative Example 2, the mass percentage of the chemical composition is within the range of the present disclosure, but the process parameters of the preparation process are not within the range of the present disclosure. In Comparative Example 3, neither the mass percentage of the chemical composition nor the process parameters of the preparation process are not within the range of the present disclosure. The mass percentages of the chemical elements in five embodiments and three comparative examples are shown in Table 1, and the balance is Fe and inevitable impurities. Process control parameters in the production process and the properties of the L-Shaped steel are shown in Table 2.

[0027] A liquid ammonia solution corrosion test method is as follows: a liquid ammonia solution with a water content of 0.1% is stirred at a room temperature. Prior to the test, the samples are cleaned with alcohol, dried by blowing, and weighed. The timing is started when the samples are immersed in the corrosion solution. The samples are taken out after 24 h, 48 h, 72 h, 144 h, 240 h and 720 h of immersion, thoroughly cleaned with alcohol, and then weighed. A measured mass loss equals the mass of the sample before the test minus the mass of the sample after the test. 5 Amass loss-time graph is plotted, and an annual corrosion loss is extrapolated therefrom.

[0028] Table 1 Comparison of Chemical Composition in Embodiments of the Present Disclosure and Comparative Examples (wt%) Element Embodiments of the Present disclosure Comparative Examples 1 2 3 4 5 1 2 3 C 0.018 0.014 0.005 0.008 0.011 0.12 0.013 0.26 Si 0.94 0.98 0.85 0.89 0.81 0.23 0.87 0.13 Mn 0.09 0.11 0.08 0.07 0.12 1.73 0.10 0.65 P 0.007 0.009 0.004 0.006 0.008 0.014 0.0086 0.011 S 0.001 0.002 0.0009 0.0008 0.0007 0.011 0.0016 0.015 Al 0.23 0.29 0.55 0.36 0.48 0.026 0.43 0.018 Cr 1.66 1.83 1.55 1.76 1.89 0.04 1.71 0.021 Ce 0.012 0.018 0.014 0.016 0.017 0.0013 0.013 0.0009

[0029] Table 2 Table of the Effects of Control Parameters in Production Process of the 10 Embodiments of the Present Disclosure and Comparative Examples on the Properties of L-Shaped Steel Steel number Vacuu m Treat ment Time (min) Initial Rollin g Tempe rature (°C) Heatin g Tempe rature (°C) of Castin g Billet Holdin g Time for Castin g Billet Heatin g (hours ) Final Rollin g Tempe rature (°C) Contin uous Castin g Tempe rature (°C) Yield Streng th (MPa) Impact Tough ness at 105°C (J) Annua 1 Corros ion Rate in Liquid Ammo nia Soluti on (g / m2' y) Embod iments 1 19 1082 1210 3.7 849 1573 387 68 0.2 2 17.5 1050 1219 3.8 862 1565 401 166 0.18 of the Present Disclos ure 3 16 1090 1228 3.9 877 1585 368 252 0.13 4 17 1061 1231 3.5 835 1577 340 139 0.15 5 18 1073 1240 4.0 880 1569 440 99 0.145 Compa rative Examp les 1 16.5 1077 1225 3.6 856 1578 335 46 3.61 2 8 1112 1155 3.0 926 1607 312 31 2.55 3 0 1096 1287 4.6 799 1591 463 22 4.58

[0030] As can be seen from Table 1 and Table 2 that the L-shaped steel produced based on the chemical composition, the mass percentage, and the process parameters for production process control in each of Embodiments 1 - 5 of the present disclosure has a yield strength ranging from 5 340 MPa to 440 MPa, and an impact toughness KV2 of higher than 68 J at -105°C. In contrast, the comparative L-shaped steel produced in Comparative Examples 1 and 2 based on situations that the steel composition range and / or production processes fall outside the range of the present disclosure has a yield strength lower than 340 MPa, and the L-shaped steel produced in Comparative Example 3 has a yield strength of 463 MPa, exceeding the required upper limit of 10 440 MPa. When used in constructing steel structures for corrosion environments such as liquid ammonia service, such a steel is prone to stress corrosion cracking. The impact toughness of the L-shaped steel produced in each of Comparative Example 1, Comparative Example 2 and Comparative Example 3 is less than 68 J at -105°C. The L-shaped steel prepared in Embodiment 3 of the present disclosure has a yield strength of 368 MPa, impact toughness of 252 J at - 105°C, 15 and an annual corrosion rate in the liquid ammonia solution of 0.13 g / m2y, showing excellent mechanical properties and corrosion resistance, so that the liquid ammonia tank prepared from the steel prepared in Embodiment 3 can operate safely, and Embodiment 3 serves as the optimal embodiment.

Claims

1. A corrosion-resistant L-shaped steel for a liquid ammonia tank, comprising the following chemical element composition in weight percentage: 0.005-0.018% of C, 0.81-0.98% of Si, 0.07-0.12% of Mn, not more than 0.009% of P, not more than 0.002% of S, 0.23-0.55% of Al, 1.55-1.95% of Cr, 0.012-0.018% of Ce, and the balance of Fe and inevitable impurities.

2. The corrosion-resistant L-shaped steel for a liquid ammonia tank according to claim 1, comprising the following chemical element composition in weight percentage: 0.006-0.01% of C, 0.85-0.95% of Si, 0.07-0.12% of Mn, not more than 0.009% ofP, not more than 0.002% of S, 0.25-0.50% of Al, 1.56-1.90% of Cr, 0.012-0.018% of Ce, and the balance of Fe and inevitable impurities.

3. A preparation method for the corrosion-resistant L-shaped steel for a liquid ammonia tank according to claim 1 or 2, comprising the following steps:(1) converter smeltingcharging a basic converter with molten iron, scrap steel and an alloy material for smelting, then carrying out oxygen blowing for temperature raising, oxidation and decarburization, followed by adding CaO and FeO for dephosphorization;electric furnace secondary refining, adding lime to reduce an S content, finely adjusting Mn and Cr contents in molten steel to a required range with a metal Mn and a metal Cr, and adjusting an Si content with SiCa alloy;RH vacuum refining, adding Al pellets to the molten steel for deoxidation, feeding a Ce rare earth cored wire to the molten steel, adjusting a Ce content, blowing Ar gas into the molten steel to stir and homogenize the molten steel, and carrying out cyclic treatment under ultimate vacuum to further reduce the contents of harmful gas elements such as N and H and coarse harmful inclusions in steel; andcontinuous casting;Step 2: rollingwherein an initial rolling temperature is 1050-1090°C, and a final rolling temperature is 835-880°C; and(3) post-rolling cooling.

4. The preparation method for the corrosion-resistant L-shaped steel for a liquid ammoniatank according to claim 3, wherein in step (1), after dephosphorization, a phosphorus content is not more than 0.009%, and the S content is reduced to not more than 0.002%.

5. The preparation method for the corrosion-resistant L-shaped steel for a liquid ammonia tank according to claim 3, wherein in step (1), after adding Al pellets for deoxidation, a free oxygen content [O] in the molten steel is controlled to be less than 15 ppm.

6. The preparation method for the corrosion-resistant L-shaped steel for a liquid ammonia tank according to claim 3, wherein in step (1), the treatment under the ultimate vacuum lasts for 16-19 min.

7. The preparation method for the corrosion-resistant L-shaped steel for a liquid ammonia tank according to claim 3, wherein in step (1), the harmful inclusions comprise MnS, AI2O3, and / or AIN.

8. The preparation method for the corrosion-resistant L-shaped steel for a liquid ammonia tank according to claim 3, wherein in step (1), the continuous casting is carried out at a temperature of 1565-1585°C.

9. The preparation method for the corrosion-resistant L-shaped steel for a liquid ammonia tank according to claim 3, wherein in step (2), in the rolling process, a heating temperature of a casting billet is controlled at 1210-1240°C, with a holding time of 3.5-4 hours.

10. The preparation method for the corrosion-resistant L-shaped steel for a liquid ammonia tank according to claim 3, wherein in step (3), the process of post-rolling cooling is post-rolling stack slow cooling at a temperature of not more than 870°C, followed by slow cooling down to a room temperature.PCT / CN2023 / 126216A. CLASSIFICATION OF SUBJECT MATTERC22C38 / 02(2006.01)i; C22C38 / 04(2006.01)i; C22C38 / 06(2006.01)i; C22C38 / 18(2006.01)i; C22C33 / 06(2006.01)i;C21C7 / 068(2006.01)i; C21C7 / 064(2006.01)i; C21C7 / 00(2006.01)i; C21C7 / 10(2006.01)i; C21C7 / 06(2006.01)i; C21D8 / 02(2006.01)iAccording to International Patent Classification (IPC) or to both national classification and IPCB.FIELDS SEARCHEDMinimum documentation searched (classification system followed by classification symbols) C22C, C21C, C21DDocumentation searched other than minimum documentation to the extent that such documents are included in the fields searchedElectronic data base consulted during the international search (name of data base and, where practicable, search terms used)CNTXT, ENTXTC, WPABSC, ENTXT, WPABS, CJFD, ISI web of science, STN, fO, DUXIU: 04, C,carbon, Si, silicon, 5±, Mn, manganese, Al, aluminum, Cr, chrome, ft, Ce, cerium, $¢, RE, REM, rare earth, WiDOCUMENTS CONSIDERED TO BE RELEVANTCategory* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No. PX CN 116555664 A (NANJING IRON &STEEL CO., LTD.) 08 August 2023 (2023-08-08) claims 1-10 1-10 A JP 2001214236 A (NIPPON STEEL CORP.) 07 August 2001 (2001-08-07) 1-10 description, paragraphs 4-5 A CN 103469094 A (REHAO STEEL HOLDING GROUP CO., LTD.) 25 December 2013 (2013-12-25) entire document 1-10 A JP 2010222665 A (JFE STEEL CORP.) 07 October 2010 (2010-10-07) entire document 1-10 A CN 102732799 A (WUHAN IRON &STEEL (GROUP) CORP.) 17 October 2012 (2012-10-17) 1-10 entire document A EP 2460904 A2 (BAYERISCHE MOTOREN WERKE AG) 06 June 2012 (2012-06-06) entire document 1-10 — — —| S | Further documents are listed in the continuation of Box C. | S | See patent family annex.* Special categories of cited documents:“A” document defining the general state of the art which is not considered to be of particular relevance■‘D” document cited by the applicant in the international application“E” earlier application or patent but published on or after the international filing date“L” document which may throw doubts on priority claim(s) or which is cited to establish the publication date of another citation or other special reason (as specified)“O” document referring to an oral disclosure, use, exhibition or other means“P” document published prior to the international filing date but later than the priority date claimed“T”‘Y’later document published after the international filing date or priority date and not in conflict with the application but cited to understand the principle or theory underlying the inventiondocument of particular relevance; the claimed invention cannot be considered novel or cannot be considered to involve an inventive step when the document is taken alonedocument of particular relevance; the claimed invention cannot be considered to involve an inventive step when the document is combined with one or more other such documents, such combination being obvious to a person skilled in the aitdocument member of the same patent familyDate of the actual completion of the international searchDate of mailing of the international search report05 December 2023Name and mailing address of the ISA / CNChina National Intellectual Property Administration (ISA / CN)China No. 6, Xitucheng Road, Jimenqiao, Haidian District,Beijing 100088Authorized officer13 December 2023Telephone No.C. DOCUMENTS CONSIDERED TO BE RELEVANT Category* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No. A US 2004069382 Al (NIPPON STEEL CORP.) 15 April 2004 (2004-04-15) 1-10 entire document A EP 0633326 Al (KAWASAKI STEEL CORP.) 11 January 1995 (1995-01-11) 1-10 entire documentPCT / CN2023 / I26216Patent document cited in search report Publication date (day / month / year) Patent family member) s) Publication date (day / month / year) CN 116555664 A 08 August 2023 None JP 2001214236 A 07 August 2001 JP 3996727 B2 24 October 2007 CN 103469094 A 25 December 2013 CN 103469094 B 04 November 2015 JP 2010222665 A 07 October 2010 JP 5375246 B2 25 December 2013 CN 102732799 A 17 October 2012 CN 102732799 B 09 April 2014 EP 2460904 A2 06 June 2012 EP 2460904 A3 28 November 2012 EP 2460904 Bl 11 April 2018 DE 102010053385 Al 21 June 2012 US 2004069382 Al 15 April 2004 JP 2002322533 A 08 November 2002 JP 3927384 B2 06 June 2007 WO 02066697 Al 29 August 2002 KR 20030077018 A 29 September 2003 KR 100572762 Bl 24 April 2006 EP 1362930 Al 19 November 2003 EP 1362930 A4 24 November 2004 CA 2438393 Al 29 August 2002 CN 1492938 A 28 April 2004 CN 1221680 C 05 October 2005 EP 0633326 Al 11 January 1995 EP 0633326 Bl 24 June 1998 KR 960014380 A 22 May 19% KR 100264362 Bl 16 August 2000 SG 48369 Al 17 April 1998 CN 1103672 A 14 June 1995 JP H07310141 A 28 November 1995 JP H07316722 A 05 December 1995