AU2005331823B2

AU2005331823B2 - High silicon stainless steel, spring manufactured by using same as raw material, and method for producing high silicon stainless steel

Info

Publication number: AU2005331823B2
Application number: AU2005331823A
Authority: AU
Inventors: Toshiro Oda
Original assignee: HOHWA Co Ltd
Current assignee: HOHWA Co Ltd
Priority date: 2005-05-18
Filing date: 2005-05-18
Publication date: 2011-07-21
Anticipated expiration: 2025-05-18
Also published as: EP1882755A1; EP1882755A4; CN101228287B; CA2608600A1; US20090016925A1; CN101228287A; AU2005331823A1; WO2006123408A1

Abstract

Provided are a high silicon stainless steel exhibiting a great elongation at break, a spring manufactured by using such a stainless steel as a raw material, and a method for producing a high silicon stainless steel. The high silicon stainless steel is characterized in that it mainly consists of a fine structure having a crystal grain size of 15 μm or less and exhibits an elongation at break of 12 % or more. It is preferable that the high silicon stainless steel mainly consists of a fine structure having a crystal grain size of 7 μm or less and exhibits an elongation at break of 14 % or more. The high silicon stainless steel is further characterized in that it has an elongation at break after a thermal aging treatment of 7 % or more. The method for producing a high silicon stainless steel is characterized in that it comprises a loading step wherein an impact load and/or a static load is applied within the temperature range where no cracks occur in the high silicon stainless steel or the base alloy thereof when the surface temperature is not higher than 950˚C. By using such a high silicon stainless steel having a great elongation at break as a raw material, there can be produced a spring having a long life.

Description

1 SPECIFICATION HIGH SILICON STAINLESS STEEL, SPRING-MANUFACTURED BY USING SAME AS RAW MATERIAL, AND METHOD FOR PRODUCING HIGH SILICON STAINLESS STEEL TECHNICAL FIELD [0001] The present invention relates to a high silicon stainless steel. In particular, the invention relates to a high silicon stainless steel with high 10 ductility, a spring made thereof, and a process for manufacturing the high silicon stainless steel. BACKGROUND ART [0002] High silicon stainless steel is known by the 15 name "silicolloy", a stainless material containing silicon in 3.5% by weight or more. As a metal material, high silicon stainless steel is endowed with excellent toughness, and is resistant to corrosion, wear and heat. [0003] On the other hand, high silicon stainless 20 steel has an' elongation at break of about 10% after forging and quenching. Additionally, in order to enhance hardness, high silicon stainless steel may be subjected to thermal aging at about 500 0 C. After the thermal aging, the elongation at break goes down to as 25 little as 3.5%. Due to lack of ductility, which is one 2 of the most characteristic properties for metal materials, high silicon stainless steel has limited applications to mechanical parts. [0004] Regarding steelmaterials such as stainless 5 steels, it is generally known that mechanical strength and ductility can be enhanced by making their grain size smaller. To reduce the grain size of steel materials such as stainless steels, some processes have been disclosed (e.g. Patent Documents 1-3). 10 [Patent Document 1] Japanese Patent Laid-open Publication No. 2000-248329 (Patent Document 2] Japanese Patent Laid-open Publication No. 2000-351040 [Patent Document 3] Japanese Patent Laid-open 15 Publication No.2002-192201 [0005] However, in the case of high silicon 20 stainless steel, if the processes as disclosed in Patent Documents 1-3 are applied in an attempt to refine the grains, these processes end with breaking the material or with other adverse results. Thus, it has been impossible to provide a high silicon stainless steel 25 with a refined grain structure. Specifically, the 3 conventionalprocesses provide a high silicon stainless steel with a grain size of about 25 to 30 pm at smallest. As mentioned already, its elongation at break is about 10% after forging and quenching, and as little as 3.5% 5 after thermal aging. [0006] By solving such a problem and realizing a high silicon stainless steel with a high elongation at break and remarkable ductility, it is possible to take further advantage of the properties of high silicon 10 stainless steel and to provide high-quality mechanical parts, etc. In view of this, there has been a strong demand for a high silicon stainless steel with a high elongation at break. [0007] Being made in light of this situation, it would be advantageous if at least preferred embodiments of the present invention provide a high silicon stainless steel with a high elongation at break, a spring made thereof, and a process for manufacturing the high silicon stainless steel. 20 SUMMARY OF THE INVENTION [0008] The present invention derives from discovery of a grain refinement process in high silicon stainless 25 steel. Tobe specific, the present invention can reduce -4 the grain size of a high silicon stainless steel by forging the high silicon stainless steel or its master alloy under impact load and/or static load, preferably under impact load, while regulating the surface 5 temperature of the high silicon stainless steel or the master alloy within a certain range. Besides, the present invention can control the grain size by changing the surface temperature condition and the forging condition. 10 [00091 In a first aspect, the present invention provides a high silicon stainless steel characterized in mainly comprising a microstructure with a grain size of 15 pm or less, and having an elongation at break of 12% or higher, with a process for manufacturing said high silicon 15 stainless steel comprising the forging steps of: a first load application step for applying an impact load and/or a static load to the high silicon stainless steel or the master alloy containing Si in 3.5 to 7.0% by weight, wherein a surface temperature of the high silicon 20 stainless steel or the master alloy is kept at 1,100 0 C or higher, and is later dropped to a temperature range of 950 0 C or below and not so low as to break the high silicon stainless steel or the master alloy; and a second load application step for applying an impact 25 load and/or a static load to the high silicon stainless steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at a temperature range from 850 to 1,050 0 C, and is later changed to a temperature range of 950 0 C or below and 30 not so low as to break the high silicon stainless steel or the master alloy, with the first load application step being followed by the second load application step once or more. [0010] With a grain size of 15 pm or less, the high 35 silicon stainless steel can achieve a higher elongation at break. [00111 In a second aspect, the present invention 2639931_1 (GHMatters) P76028.AU 27/05/11 - 4a provides a high silicon stainless steel which mainly comprises a microstructure with a grain size of 7 pm or less, and has an elongation at break of 14% or higher, with a process for manufacturing said high silicon s stainless steel comprising the forging steps of: a first load application step for applying an impact load and/or a static load to the high silicon stainless steel or the master alloy containing Si in 3.5 to 7.0% by weight, wherein a surface temperature of the high silicon 10 stainless steel or the master alloy is kept at 1,100 0 C or higher, and is later dropped to a temperature range of 950 0 C or below and not so low as to break the high silicon stainless steel or the master alloy; and a second load application step for applying an impact 15 load and/or a static load to the high silicon stainless steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at a temperature range from 850 to 1,050 0 C, and is later changed to a temperature range of 950 0 C or below and 20 not so low as to break the high silicon stainless steel or the master alloy, with a lowest surface temperature for the second load application step being lower than a lowest surface temperature for the first load application step, wherein 25 the second load application step is conducted more than once, during which a lowest surface temperature for each second load application step is lower than a lowest surface temperature for a previous second load application step so as to reduce a grain size little by little, 30 with the grain size being controlled by changing the number of times for conducting the second load application step. [0012] With a grain size of 7 pm or less, the high silicon stainless steel can achieve a still higher 35 elongation at break. 2639931_1 (GHMatters) P76028.AU 27/05/11 5 [0013] In this context, the high silicon stainless steel is a stainless steel containingSi in 3.5% by weight ormore, generally from 3.5 to 7% by weight, as typically represented by Silicolloy Al, Silicolloy A2 and 5 Silicolloy D. [0014] The term "grain size" as used herein means a value obtained according to ASTM Designation E112-82. The term "elongation at break" as used herein refers to the one defined in JIS Z2241, Method of tensile test 10 for metallic materials. [0015] A spring made of the high silicon stainless steel of the invention shows a dramatic improvement in ductility. As a consequence, the spring is less likely to break even under a heavy load and can be an excellent 15 mechanical part or the like. In addition, the spring itself has a long life. [0016] The high silicon stainless steel of the present invention is further characterized in that any of the high silicon stainless steels mentioned above 20 is subjected to thermal aging at a temperature range of 480 to 550'C, and has an elongation at break of 7% or higher after the thermal aging. In many cases, thermal aging of the high silicon stainless steel is conducted within the above-mentioned temperature range 25 for about an hour.

6 [0017) The thermal aging increases hardness at the surface of the material. The high silicon stainless steel which went through the thermal aging and which has an elongation at break of 7% or higher can retain 5 a remarkable Brinell hardness of 450 or higher. The term "Brinell hardness" as used herein means a value obtained according to JIS Z2243, Brinell hardness test. [0018] In a third aspect, the present invention provides a spring which is made of the high silicon stainless steel of the first or second aspect. The high silicon stainless 10 steel with such a high hardness can provide excellent mechanical parts or the like, including a highly durable, long-life spring. [0019] The spring and other mechanical parts may be subjected to surface treatment such as nitriding and/or shot peening. In general, surface hardness of 15 high silicon stainless steel increases when nitrogen is allowed to diffuse into the surface. For the high silicon stainless steel with a refined grain structure, nitriding can increase the surface hardness still further. Additionally, shot peeningcauses generation 20 of residual stress inside high silicon stainless steel. With respect to the high silicon stainless steel which has a refined grain structure and whose surface hardness has increased by nitriding, shot peening assists generation of a greater residual stress, making the high 25 silicon stainless steel resistant to a greater stress.

7 [0020] In a fourth aspect, the present invention provides a process for manufacturing a high silicon stainless steel characterized in comprising the step of forging a high silicon stainless steel or a master alloy .containing Si in 3.5 to 7% by weight. The 5 forging step includes: a load application step for applying an impact load and/or a static load to the high silicon stainless steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at 1, 100'C or higher, 10 and is later dropped to a temperature range of 950 0 C or below and not so low as to break the high silicon stainless steel or the master alloy. The process provides a steel material which mainly comprises a microstructure with a grain size of 15 pm or less. 15 [0021] This process provides a high silicon stainless steel having a high elongation at break. In this process, application of a load starts at a surface temperature of 1,100 0 C or higher and continues until the surface temperature drops to 95 0 'C or below, thereby 20 promoting grainrefinementin thehighsilicon stainless steel or its master alloy. As the forging time at 950 0 C or below is longer, the grain size becomes smaller. Further, within the temperature range which is 950'C or below and not so low as to break the high silicon 25 stainless steel or the master alloy, load application 8 ata lower temperature promotes further grain refinement. At the start of forging, the surface temperature of the high silicon stainless steel or its master alloy is preferably between 1,100 and 1,200'C because the 5 temperature does not need to be above 1,200'C. When the surface temperature is lower than 1,100*C at the start of forging, the high silicon stainless steel or its master alloy has not yet gained sufficient ductility and is more likely to break. In this context, the term 10 "master alloy" means an alloy composed of a material which becomes a high silicon stainless steel after the forging. [0022] The load to be applied during the forging may be a static load or an impact load. However, 15 application of an impact load induces active self-heating inside the high silicon stainless steel or its master alloy, thereby further promoting grain refinement and saving the time required for the forging step. Impact load may be combined with static load. 20 For example, application of an impact load may be followed by rolling (application of an static load), which facilitates manufacture of a thin plate-shaped material. [0023] In a fifth aspect, the present invention provides 25 a process for manufacturing a high silicon stainless steel char- 9 acterized in comprising the step of forging a high silicon stainless steel or a master alloy containing Si in 3.5 to 7% by weight. The forging step includes: a first load application step for applying an impact load and/or a static load to the high silicon stainless steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at 1,100 0 C or higher, and is later dropped to a temperature range of 950 0 C or below and not so low as to break the high silicon stainless steel or the master alloy; and asecond load application step for applying an impact load and/or 10 a static load to the high silicon stainless steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at a temperature range from 850 to 1,050*C, and is later changed to a temperature range of 950*C or below and 15 not so low as to break the high silicon stainless steel or the master alloy. The first load application step is followed by the second load application step once or more. The process provides a steel material which mainly comprises a microstructure with a grain size of 20 15 pm or less. [0024] As described earlier, a high silicon stainless steel with a refined grain structure is obtained by application of a load to the high silicon stainless steel or the master alloy, with the surface 25 temperature being maintained in a temperature range of 10 950'C or below and not so low as to break the high silicon stainless steel or the master alloy. Moreover, by combining the first load application step and the second load application step, it is easier to avoid break of 5 the high silicon stainless steel or the master alloy during the forging. At the start of the second load application step, the surface temperature of the high silicon stainless steel or the master alloy is regulated to not higher than 1,050'C. If the surface temperature 10 exceeds 1,050'C under heating, the grain size becomes larger again. The second load application step may be performed only once or more than once. [0025] The process of the invention for manufacturing the high silicon stainless steel containing Si in 3.5 to 7.0% by weight is characterized in that: a lowest surface 15 temperature for the second load application step is lower than a lowest surface temperature for the first load application step; the second load application step is conducted more than once, during which a lowest surface temperature for each second load application step is lower than a lowest surface temperature for a previous second load application step so as to reduce a grain size little 20 by little; and the grain size is controlled by changing the number of times for conducting the second load application step. The production process provides a 25 11 steel material which mainly comprises a microstructure with a grain size of 15 pm or less. [0026] This process can provide a high silicon stainless steel which has a high elongation at break 5 and can control the grain size. [0027] As described above, the grain size is reduced little by little, by gradually lowering the lowest surface temperature for each load application step. In other words, ductility of the high silicon 10 stainless steel or its master alloy increases little by little, so that the high silicon stainless steel or its master alloy is less likely to break. Eventually, the grain size can be reduced every time the load application step is repeated. 15 [0028] Even when the lowest temperature for each load application step is not gradually lowered, the grain size becomes smaller every time the load ap plication step is repeated. In this case, to prevent break of the high silicon stainless steel or its master 20 alloy, it is preferable to apply a smaller amount of load during earlier load application step. EFFECTS OF THE INVENTION [0029] By reducing the grain size to 15 pm or less, 25 the invention provides a high silicon stainless steel 12 which achieves an improved elongation at break and excellent ductility. Further, by reducing the grain size to 7 pm or less, the invention provides a high silicon stainless steel whose elongation at break is 5 dramatically improved to 14% or higher. [0030] With respect to a high silicon stainless steel whose hardness has increased by the thermal aging, the invention ensures an elongation at break of as high as 7% or more. This is an outstanding improvement, as 10 understood from a comparison with a conventional value. The high silicon stainless steel achieves not only an elongation at break of 7% or higher but also a Brinell hardness or 450. [0031] A spring made of the high silicon stainless 15 steel shows an outstanding improvement in ductility. This spring is unlikely to break even under a heavy load and has a long life. [0032] The process of the invention for manu facturing the high silicon stainless steel can reduce 20 its grain size to 15 pm or less. BRIEF DESCRIPTION OF DRAWINGS [0033] Fig. 1 schematically shows a manner of forging a high silicon stainless steel, according to 25 an Example of the present invention. Fig. 1(a) il- 13 lustrates how the forging is performed. Fig. 1(b) is an external perspective view of the high silicon stainless steel. Fig. 2 concerns electron microscopic observation 5 of the structure of the high silicon stainless steel, according to Example 1 of the present invention. Fig. 2(a) is a schematic illustration of the observation area. Fig. 2(b) is a photographic image of the peripheral structure, and Fig. 2 (c) is a photographic image of the 10 central structure. Fig. 3 illustrates disc springs made of the high silicon stainless steel, according to Example 2 of the present invention. Fig. 3(a) is a front sectional view of a washer constituting a stack of disc springs. Fig. 15 3(b) is a front sectionalview of a stack of discsprings. Fig. 4 concerns electron microscopic observation of the structure of a conventional high silicon stainless steel. Fig. 4 (a) is a schematic illustration of the observation area. Fig. 4 (b) is a photographic 20 image of the peripheral structure, and Fig. 4 (c) is a photographic image of the central structure. DESCRIPTION OF THE NUMERALS [0034] 1 master alloy 25 2 air hammer 14 3 anvil 4 hammer 5 driving mechanism 6 thermometer 5 7 operator 8 gripper 101 high silicon stainless steel 31 washer 32 disc spring 10 BEST MODE FOR CARRYING OUT THE INVENTION [0035] An embodiment of the present invention is hereinafter described. [0036] An embodiment of the present invention 15 encompasses a high silicon stainless steel which is mainly composed of a microstructure with a grain size of 15 pm or less and which has an elongation at break of 12% or higher; and a high silicon stainless steel which is mainly composed of a microstructure with a grain 20 size of 7 pm or less and which has an elongation at break of 14% or higher. [0037] The high silicon stainless steel is widely used as a material for metal products such as mechanical parts. By way of example, a spring made of the high 25 silicon stainless steel is resistant to corrosion and 15 has a long life, unlike conventional springs. [0038] The high silicon stainless steel according to the embodiment achieves an elongation at break of 7% or higher, after thermal aging at a temperature range 5 of 480 to 550'C. In addition, the high silicon stainless steel which went through the thermal aging can be endowed with a Brinell hardness of 450 or higher while keeping anelongation atbreak of7% or higher. The highsilicon stainless steel having such a high hardness can provide 10 a highly durable, long-life spring. [0039] Refinement of the structure of the high silicon stainless steel is effected to a material for a high silicon stainless steel, or a master alloy composed of a material which becomes a high silicon 15 stainless steel (hereinafter, the material and the master alloy before grain refinement are generally called "master alloy or the like") . The size and shape of the master alloy or the like are not particularly limited. Depending on the manufacturing facilities 20 and purpose, the master alloy or the like may be in various sizes and may be round, block-shaped, plate-shaped or shaped otherwise. It goes without saying that the high silicon stainless steel can be processed into a round, block, plate or other shape of 25 various sizes, through the manufacturing process 16 including forging and the others. [0040] As detailed later, the high silicon stainless steel is manufactured while a load is applied to themaster alloyor the like within a given temperature 5 range. The load may be either an impact load or a static load, of which an impact load is preferred because it accelerates progress of the grain refinement. Typically, a device for applying an impact load may be a hammer-equipped press machine. 10 [0041] In order to obtain a high silicon stainless steel having a refined grain structure, the master alloy or the like is subjected to forging at a temperature of 950'C or below, thereby refining grains in the master alloy or the like. 15 [0042] To start the manufacturing process, load is applied to the master alloy or the like which has been heated to have a surface temperature of 1,100 to 1,200 0 C. Due to exposure to external air, the temperature of the master alloy or the like drops while the load is applied. 20 In due course, the surface temperature of the master alloy or the like reaches 950'C or below, but even then the load application is continued. Preferably, the surface temperature of the master alloy or the like is reduced to as low as possible, but not so low as to break 25 the master alloy or the like. It should be borne in 17 mind that the master alloy or the like tends to break at 700'C or below. [0043] For further grain refinement, load is applied for as long as possible, with the temperature 5 being kept at 950 0 C or below, preferably 850'C or below. During this forging, the temperature is allowed to drop to a lowest possible temperature at which the master alloy or the like does not break. [0044] After the end of the load application, the 10 master alloy or the like is cooled by quenching in a conventional manner, thereby giving a high silicon stainless steel with a refined grain structure. [0045] The high silicon stainless steel according to the embodiment of the invention is manufactured in 15 the above-mentioned manner. Additionally, by per forming the load application (forging) step more than once as described below, it is easier to control the grain size of the high silicon stainless steel between 0.6 and 15 pm. 20 [0046] To start with, aloadis appliedto themaster alloy or the like which is heated to near 1, 100 to 1, 200'C. The forging is stopped when the temperature drops to a temperature range of 950'C or below and not so low as to break the master alloy or the like (first forging 25 step).

18 [0047] Next, the master alloy or the like is heated until its surface temperature reaches 850 0 C or higher, preferably near 1,050'C. The surface temperature should not exceed 1,050'C because such a high tem 5 perature allows the grain size to get larger. Then, a load is applied again to the master alloy or the like whose surface temperature is near 1,050'C. The forging is stopped when the temperature drops to a temperature range of 950 0 C or below, preferably 850 0 C or below, and 10 not so lowas to break the master alloyor the like (second forging step) . Once again, the master alloy or the like is heated until its surface temperature reaches near 1,050'C. Then, a load is applied again to the master alloy or the like whose surface temperature is near 15 1,050 0 C until the temperature drops to a temperature range of 950'C or below and not so low as to break the master alloy or the like (third forging step). Where necessary, the forth, fifth and more forging steps may be repeated. 20 [0048] In order to facilitate reduction of the grain size, the temperature for stopping the second forging step is set lower than the one for stopping the first forging step. By gradually lowering the lowest temperature for each forging step, it is possible to 25 apply a heavy load while avoiding break of the master 19 alloy, and eventually to obtain refined grains easily. [0049] As described, the grain size is reduced little by little while the above-mentioned forging step is repeated. Hence, it is possible to control the grain 5 size by setting the number of times for conducting the forging step, depending on a desired grain size. In other words, the grain size of a microstructure can be controlled more easily if the forging step is made up of more than one forging steps. 10 [0050] Similar to the foregoing description, the last step in the manufacturing process is to cool the master alloy or the like by quenching in a conventional manner. Thus obtainedis a high silicon stainless steel according to the embodiment. 15 [0051] Now, referring to the drawings, the present invention is specifically described by way of Examples. These Examples are given merely for the purpose of description and should not be construed as limiting the invention. 20 EXAMPLE 1 [0052] A master alloy used in this Example had a diameter of 12 cm and a length of 25 cm. The composition of its main components, except Fe (iron), was Si:4, 25 C:0.02, Ni:7, Cr:12 (unit: % by weight). This master 20 alloy was subjected to forging and quenching in the manner mentioned below. Thus obtained was a high silicon stainless steel which had a diameter of 3 cm and a length of 120 cm. 5 [0053] Fig. 1 schematically shows how to forge a high silicon stainless steel according to an Example of the present invention. Fig. 1(a) illustrates how the forging is performed. Fig. 1(b) is an external perspective view of the high silicon stainless steel 10 thus obtained. [0054] To start with, the master alloy 1 heated to 1,150 0 C was placed on an anvil 3 of a 0.5-ton air hammer 2. [0055] For forging, a hammer 4 was allowed to fall 15 from 70 cm above the anvil 3 onto the master alloy 1. To apply the impact, a driving mechanism 5 let the hammer 4 fall and rise at a cycle of twice per second. An operator 7 moved the master alloy 1 properly so as to forge the entirety of the master alloy 1. 20 [0056] The surface temperature of the master alloy 1 was monitored by a thermometer 6. When the surface temperature dropped to 850'C, the forging was stopped. Then, the master alloy 1 was put into an electric furnace (not shown) and heated until its surface temperature 25 rose to around, but not exceeding, 1,050'C. During this 21 heating, the surface temperature of the master alloy 1 was also monitored by the thermometer 6. As the thermometer 6, a digital radiation thermometer (produced by Daido Steel Co., Ltd.; Starthermo DS-06CF) 5 was employed. [0057] Next, the master alloy 1 heated up to near 1,050*C was forged again in the same manner as above. At this stage, the master alloy 1 was forged until its surface temperature dropped to 800'C. Then, the master 10 alloy 1 was put into the electric furnace and heated until its surface temperature rose to 1,000 0 C. [0058] The master alloy 1 heated up to near 1,000 C was forged once again in the same manner as above. At this stage, the master alloy 1 was forged until its 15 surface temperature dropped to 750 0 C. Then, a series of forging steps were terminated. [0059] After a series of forging steps, the master alloy 1 was heated in the electric furnace until its surface temperature reached 1,000 C. Thereafter, the 20 master alloy 1 was subjected to water quenching (generally called ST treatment) to give a high silicon stainless steel 101. [0060] With respect to this high silicon stainless steel 101, the tensile strength was 1,134 N/mm 2 and the 25 elongation at break was 14%. The Brinell hardness was 22 341. [0061] Additionally, the high silicon stainless steel 101 was subjected to thermal aging at 500C for one hour. With respect to the high silicon stainless 5 steel which went through the thermal aging, the tensile strength was 1,634 N/mm 2 and the elongation at break was 10%. The Brinell hardness was 461. [0062] For both evaluations, test pieces were prepared according to JIS Z2201, Test pieces for tensile 10 test for metallic materials (test piece No. 14A4). The tensile strength and the elongation at break were measured by the tensile test according to JIS Z2241, Method of tensile test for metallic materials. The Brinell hardness was measured according to JIS Z2243. 15 [0063] Regarding the high silicon stainless steel 101 which went through the thermal aging, its cross-section was observed at a part near the external circumference (the periphery) and at a part near the center (the center). The grain size was measured 20 according to ASTM Designation E112-82. [0064] Fig. 2(a) is a schematic cross-sectional view of the observation area. Figs. 2 (b) and 2 (c) are photographic images at the periphery and the center, respectively, showing the microstructure of the high 25 silicon stainless steel which went through the thermal 23 aging. These photographic images were taken by an electron microscope (magnification x 400). As in dicated by comparison between Figs. 2 (b) and 2 (c), there is no difference between the peripheral structure and 5 the central structure. The grain size was 6.9 pm at both the periphery (see Fig. 2(b)) and the center (see Fig. 2(c)). [0065] It should be understood that the grain size is not affected by the thermal aging and remains 10 unchanged before and after the thermal aging. [0066] Additionally, the high silicon stainless steel which went through the thermal aging was subjected to conventional nitriding and conventional shot peening (conventional airless shot peening) . After these 15 treatments, the Vickers hardness of the high silicon stainless steel was 1,400 at the surface. The hardness was evaluated by the Vickers hardness test according to JIS Z2244. [0067] Furthermore, the high silicon stainless 20 steel according to Example 1 was compared with a conventional high silicon stainless steel. Using a sample of a commercial (conventional) high silicon stainless steel, the surface of the sample was observed and its photographic images were taken in the 25 above-mentioned manner. The grain size was also 24 measured in the above-mentioned manner. [0068] Fig. 4(a) is a schematic cross-sectional view of the observation area, regarding the sample of the conventional high silicon stainless steel. Figs. 5 4 (b) and 4 (c) are photographic images at the periphery and the center, respectively, showing microstructures of the same which went through the thermalaging. These photographic images were taken by an electronmicroscope (magnification x 400) . The photographic images did not 10 reveal any significant difference between the pe ripheral structure and the central structure (Figs. 4(b) and 4(c)), except a slight difference in grain size. The grain size of the conventional high silicon stainless steel was 27.2 pm at the periphery (see Fig. 15 4(b)) and 24.9 pm at the center (see Fig. 4(c)). EXAMPLE 2 [0069] Disc springs were manufactured using the high silicon stainless steel 101 obtained in Example 20 1 (diameter 3 cm, length 120 cm). [0070] As the disc springs, washers for con stituting disc springs were prepared and stacked on top of each other. [0071] Fig. 3(a) is a front sectional view of a 25 washer. Fig. 3(b) is a front sectional view of a stack 25 of disc springs. [0072] To manufacture the washer 31 shown in Fig. 3(a), the high silicon stainless steel 101 (see Fig. 1) was cut into columnar materials, each having a 5 diameter of 3 cm (30 mm) and a length of 10 cm (100 mm) . The bottom of each columnar material was struck so as to enlarge its diameter to about 40 mm. Then, each columnar material was sliced to give discs each having a diameter of about 40 mm and a thickness of about 2.5 10 mm. Each of these discs was perforated at the center so as to have a hole with a diameter of about 20 mm, and the edge was rounded. Thus obtained was a per forated disc-like material. [0073] The perforated disc-like material was made 15 to curve under stress and to assume a substantially horn-like shape with the central part protruded. This material was subjected to thermal aging at 500'C for one hour, thereby making a washer 31. The main di mensions of the washer 31 are given in Fig. 3(a). 20 [0074] Next, 130 pieces of washers 31 were stacked on top of each other as illustrated in Fig. 3(b), thereby forming a stack of disc springs 32. [0075] A stack of disc springs 32 was subjected to a life test under vertical load (in the directions of 25 Arrows a and b) . Using a servopulser tester, load was 26 applied at a cycle of 10 times per second (10 Hz). The amplitude was from 4.5 to 3.2 kN. Even after the load was applied 8 million times, the disc springs 32 suffered from no particular damage. 5 INDUSTRIAL APPLICABILITY [0076] The high silicon stainless steel of this invention is utilized not only for springs, but also for a wide variety of metal products. In particular, 10 it is applicable to metal products which require high strength and high toughness, such as mechanical parts (bearings, bolts and nuts, etc.), structural members (roller bearings, etc.), cutleries, cutting tools, and more. 15 [0077] The process of this invention for manu facturing the high silicon stainless steel is assumed to be applicable to other metals than high silicon stainless steel and achieve grain refinement of their structures, as far as the precipitation hardening 20 stainless steels are concerned. Such precipitation hardening stainless steels include, for example, SUS 630.

- 26a [0078] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as s "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. [0079] It is to be understood that a reference herein to 10 a prior art document does not constitute an admission that the document forms part of the common general knowledge in the art in Australia or any other country. 2639931_1 (GHMatters) P76028.AU 13/05/11

Claims

1. A high silicon stainless steel which mainly comprises a microstructure with a grain size of 15 im or less and 5 which has an elongation at break of 12% or higher, with a process for manufacturing said high silicon stainless steel comprising the forging stepsof: a first load application step for applying an impact load and/or a static load to the high silicon stainless 1o steel or the master alloy containing Si in 3.5 to 7.0% by weight, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at 1,100 0 C or higher, and is later dropped to a temperature range of 950 0 C or below and not so low as to break the high silicon is stainless steel or the master alloy; and a second load application step for applying an impact load and/or a, static load to the high silicon stainless steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is 20 kept at a temperature range from 850 to 1,050 0 C, and is later changed to a temperature range of 950 0 C or below and not so low as to break the high silicon stainless steel or the master alloy, with the, first load application step being followed 25 by the second'load application step once or more.

2. A high silicon stainless steel which mainly comprises a microstructure with a grain size of 7 pim or less and which has an elongation at break of 14% or higher, 30 with a process for manufacturing said high silicon stainless steel comprising the forging steps of: a first load application step for applying an impact load and/or a static load to the high silicon stainless steel or the master alloy containing Si in 3.5 to 7.0% by 35 weight, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at 1,100 0 C or higher, and is later dropped to a temperature range of 2639931_1 (GHMatters) P76028.AU 27/05/11 - 28 950 0 C or below and not so low as to break the high silicon stainless steel or the master alloy; and a second load application step for applying an impact load and/or a static load to the high silicon stainless s steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at a temperature range from 850 to 1,050 0 C, and is later changed to a temperature range of 950 0 C or below and not so low as to break the high silicon stainless steel or 10 the master alloy, with a lowest surface temperature for the second load application step being lower than a lowest surface temperature for the first load application step, wherein the second load application step is conducted more than 15 once, during which a lowest surface temperature for each second load application step is lower than a lowest surface temperature for a previous second load application step so as to. reduce a grain size little by little, with the grain size being controlled by changing the 20 number of times for conducting the second load application step.

3. A high silicon stainless steel according to claim 1 or 2, 25 wherein the high silicon stainless steel is subjected to thermal aging at a temperature range of 480 to 550 0 C, and achieves an elongation at break of 7% or higher after the thermal aging. 30

4. A high silicon stainless steel according to claim 3, wherein the high silicon stainless steel has a Brinell hardness of 450 or higher.

5. A spring which is made of the high silicon stainless 35 steel according to any of claims 1 to 4.

6. A process for manufacturing a high silicon stainless 2639931_1 (GHMatters) P76028.AU 27/05/11 - 29 steel which comprises the step of forging a high silicon stainless steel or a master alloy containing Si in 3.5 to 7% by weight, the forging step including: 5 a load application step for applying an impact load and/or a static load to the high silicon stainless steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at 1,100 0 C or: higher, and is later dropped to a temperature 10 range of 950 0 C or below and not so low as to break the high silicon stainless steel or the master alloy, such that the process provides a steel material which mainly comprises a microstructure with a grain size of 15 ptm or less. 15

7. A process for manufacturing a high silicon stainless steel which comprises the step of forging a high silicon stainless steel or a master alloy containing Si in 3.5 to 7% by weight, 20 the forging step including: a first load application step for applying an impact load and/or a static load to the high silicon stainless steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is 25 kept at 1,100*C or higher, and is later 2639931_1 (GHMatters) P76028.AU 14/06/11 - 30 dropped to a temperature range of 950 0 C or below and not so low as to break the high silicon stainless steel or the master alloy; and a second load application step for applying an 5 impact load a d/or a static load to the high silicon stainless steel or the master alloy, wherein a surface temperature of the high silicon stainless steel or the master alloy is kept at a temperature range from 850 to 1,050 0 C, and is later changed to a temperature range 10 of 950 0 C or below and not so low as to break the high silicon stainless steel or the master alloy, wherein the first load application step is followed by the second load application step once or more, 15 such that the process provides a steel material which mainly comprises a microstructure with a grain size of 15 pm or less.

8. A process for manufacturing a high silicon 20 stainless steel according to claim 7, wherein a lowest surface temperature for the second load application step is lower than a lowest surface temperature for the first load application step, wherein the second load application step is 25 conducted more than once, during which a lowest surface - 31 temperature for each second load application step is lower than a lowest surface temperature for a previous second load application step so as to reduce a grain size little by little, and s wherein the grain size is controlled by changing the number of times for conducting the second load application step, such that the production process provides a steel material which mainly comprises a microstructure with a io grain size of 15 ptm or less.

9. A high silicon stainless steel, substantially as herein described with reference to Example 1. 15

10. A process for manufacturing a high silicon stainless steel, substantially as herein described with reference to Example 1. 2639931_1 (GHMatters) P76028.AU 14/06/11