Steel with excellent peel resistance and suitable for apparatus parts in contact with alcoholic fuels

Abstract: The present invention provides a steel which is superior in peel resistance, corrosion resistance and abrasion resistance against alcoholic fuels. The steel contains, in wt %, 0.60 to 0.80 % of C, max. 1.5 % of Si, max. 1.5 % of Mn, 8.0 to 16.0 % of Cr, 0.5 to 3.5 % of Mo + W / 2, 0.02 to 1.5 % of V, 0.05 to 3.0 % of Cu, balance Fe. The steel may further contain 0.1 to 5.0 % of Co, and/or up to 2.0 % of Ni.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to the use of materials suitable for making apparatuses in which alcohol fuels or alcohol-mixed petroleum fuels (generically called alcoholic fuels including these fuels and organic fuels such as ethers here), particularly for pumps, nozzles, valves and other members subjected to striking or sliding members in electronic fuel injection systems of internal combustion engines. Description of Related Art Petroleum fuels such as gasoline and light oil have been used in internal combustion engines for automobiles and in the case of internal combustion engines in which these fuels are used, electronic fuel injection systems controlled by computors are used for cleaning of exhaust gases, improvement in performance of power, and reduction of fuel expenses to bring good results. In electronic fuel injection systems, a heat treated SUS440C (corres. to AISI44OC) steel specified in JIS which is superior in wear resistance is used for a valve portion (electronic fuel injection nozzle) for supply and control of fuels, and the valve portion is generally used without exchange therefor before the car is scrapped. As the fuels of internal combustion engines for automobiles, use of alcoholic fuels in place of petroleum fuels have been investigated for the reasons of recent circumstances of petroleum (increase of petroleum costs and reduction in production) and for diminishing exhaust gases (NOx) and some of them have been subjected to practical test. Use of methyl alcohol, ethyl alcohol, methyl-tert-butyl ether and the like has been examined as alcoholic fuels. These alcoholic fuels are inferior in lubricity to petroleum fuels and there are the problem of occurrence of abrasion and peeling of materials of rotors in fuel pumps and sliding parts or parts subjected to striking in fuel injection nozzles and the problem of corrosion of fuel systems caused by water contained in alcoholic fuels, oxides of alcohols (acetaldehyde, formaldehyde), and impurities in alcoholic fuels (acetic acid, formic acid). As materials for members of devices in which alcoholic fuels are used, Japanese Patent Kokoku (Post-Exam. Publication) Nos. Hei 1-15585 and 1-15584 and Japanese Patent Kokai (Laid-Open) No. Sho 62-93347 have proposed Fe-based sintered alloys having both the abrasion resistance and corrosion resistance for fuel pumps which are steels containing Nb as an essential component, steels containing at least one of P and B at least one of Ti and Zr as essential components, and steels containing only C and Ni as additive elements, respectively. Furthermore, Japanese Patent Kokai (Laid-Open) No. Sho 61-6260 has proposed steel sheets for fuel tanks, but abrasion resistance and peel resistance have not been considered. In development of materials for members of devices in which alcoholic fuels are used, the inventors have conducted a working test where a closed circuit of a fuel tank, a fuel pump and an electronic fuel injection nozzle is produced and an alcoholic fuel is circulated to operate the electronic fuel injection nozzle and investigation of the valve portion of the nozzle in an alcohol fuel engine after subjected to a practical test and as a result, have confirmed that deterioration of the valve in shielding properties readily occurs when the injection nozzle is made of conventional SUS440C steel. The part having deteriorated shielding properties was microscopically examined. As a result, it has been found that corrosion wear and peeling owing to striking occurred in the surface portion of the nozzle (which are called "fretting abrasion" or "fretting fatigue") as shown in Fig. 4 and this is a cause for shielding deterioration. From the US-A-2 478 723, a steel alloy having a composition of 0.20 to 1% C, 0.50 to 1.50% Mo, 8 to 30% Cr, 2 to 3% Al, 0.50 to 1.50% Cu, 0.20 to 1% V, 0.25 to 1.25% Si and 0.25 to 1.25% Mn is known. This alloy has the ability to form an aluminum oxide coating and retain it under severe heat and abrasion conditions as well as to retain its cast shape under severe heat and abrasion conditions. This alloy is used for cast articles such as piercing points and plugs for seamless tube mill equipment. In the book "Stahlschlüssel" by C.W. Wegst, published by Verlag Stahlschlüssel Wegst GmbH, Marbach, 1983, in particular in pages 374 and 375 thereof, a lot of compositions for steel alloys are listed. The use of such compositions is not indicated. Further, these compositions does not contain Cu. SUMMARY OF THE INVENTION The object of the present invention is to provide materials which are excellent especially in peel resistance (fretting abrasion resistance) in addition to the corrosion resistance and abrasion resistance which have been demanded in using alcoholic fuels, for example, materials inhibited from the deterioration of the shielding properties when used as an electronic fuel injection nozzle for alcoholic fuels, and which can be obtained by a conventional melting method without employing a powder metallurgy method. BRIEF DESCRIPTION OF DRAWINGS Figs. 1A and 1B are partial sectional views of an electronic fuel injection nozzle (the former shows a body and the latter a needle) used for the abrasion test and the peel test. Fig. 2 is a %C-%Cr graph in which areas (A) and (B) according to the present invention are shown. Figs. 3A and 3B are micrographs (x100) which show the microstructures subjected to quenching and tempering in the areas (A) and (B). Fig. 4 is a micrograph showing the abraded condition of the portion subjected to striking and the peeling action of especially carbides of a SUS440C steel, which is at present used, when an injection nozzle made of the steel was operated using an alcohol fuel (a mixture of 85% of methyl alcohol and 15% of gasoline). DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with claims 1 and 2, the present invention relates to the use of materials especially suitable for members in alcoholic fuel systems such as a body and needle of a nozzle which are required to have a peel resistance and abrasion resistance against contact with alcoholic fuels. Specifically, the present invention relates to a first material excellent in peel resistance, wherein the material is an Fe-Cr-C-Cu steel comprising, by weight, at least 0.60% and less than 0.80% of C, 1.5% or less of Si, 1.5% or less of Mn, at least 8.0% and less than 16.0% of Cr, 0.5 to 3.5% of at least one of W and Mo in 1/2W + Mo, 0.02 to 1.5% of V, 0.05 to 3.0% of Cu, and the remainder of Fe and unavoidable impurities, the relation between the C content and the Cr content satisfies %Cr + 25 x %C ≦ 32, and the area ratio of a primary carbide occupying the sectional area of the material is 1.5% or less. The material may further contain at least one of 0.1 to 5.0% of Co and 2% or less of Ni. The present invention relates further to the use of a second material excellent in peel resistance and used for sliding members and the like such as injection pump members receiving relatively few impact in an apparatus in which alcoholic fuels are used, wherein the material comprises, by weight, at least 0.60% and less than 0.80% of C, 1.5% or less of Si, 1.5% or less of Mn, more than 12.0% and less than 16.0% of Cr, 0.5 to 3.5% of at least one of W and Mo in 1/2W + Mo, 0.02 to 1.5% of V, 0.05 to 3.0% of Cu, and the balance of Fe and unavoidable impurities, the relation between the C content and the Cr content satisfies %Cr + 25 x %C > 32, and the area ratio of a primary carbide occupying the sectional area of the material is 8.0% or less. The material may further contain Co and Ni in the same manner as the first material; furthermore, the first and second material may have a hardness HRC of at least 52. As mentioned above, the inventors observed the inner surface of a nozzle and a needle made of a SUS440C steel which deteriorated in shielding properties to find that abrasion of sliding members and peeling phenomenon of the portion exposed to striking occurred in the end portion and corrosion occurred in the tip portion, resulting in deterioration of shielding properties. Under the circumstances, relations between the chemical composition and corrosion, abrasion and peel were investigated as explained below. (1) Corrosion In the case of alcoholic fuels, corrosion of materials is caused by water contained in the fuels and acetic acid and formic acid as impurities in the fuels. In order to investigate the influence of chemical compositions of the materials on the corrosion atmosphere, Fe-based materials containing various alloy elements and 0.5-1.0% of C were immersed in water + 10% acetic acid + 10% formic acid solution at 60°C for 30 hours to obtain corrosion loss in weight and influence of the respective alloy elements was examined. The materials used in the corrosion test were all those which had been subjected to oil quenching at 1050°C, subzero treatment at -78°C and tempering at 180°C. Relation between chemical composition and corrosion loss in weight was obtained based on the results of the experiments by regression analysis to find that Cr, Ni, V, and Cu are the elements which improve the corrosion resistance, C and Mn are the elements which deteriorate the corrosion resistance, and Co, Si, W, and Mo have no significant influence on corrosion resistance. (2) Abrasion An electronic fuel injection nozzle comprises a needle and a body in which the needle is inserted. Therefore, regarding the abrasion, first an actual electronic fuel nozzle was made and subjected to operation test of 2 x 10⁸ times in a mixed fuel comprising 85% of industrial methyl alcohol and 15% of gasoline and the abrasion state was examined and the relation with hardness of materials was investigated. Figs. 1A and 1B show the nozzle comprising a body (Fig. 1A) and a needle (Fig. 1B) having a sliding and striking position 1 and a sliding portion 2. The composition of the material used in 0.69C-9.3Cr-1.01Mo-0.25V-1.06Co-0.46Cu-Fe. This was subjected to quenching at 1050°C and sub-zero treatment and was adjusted to four kinds of hardnesses, HRC63, 60, 57.5, and 55 by changing the tempering temperature. Needles and bodies of injection devices for electronic fuels were made from these materials and were subjected to operation test. As a result, it was found that abrasion wear was less with increase in hardness. Then, relation between abrasion wear and microstructure or added elements were evaluated by the electronic fuel injection test conducted in the examples given hereinafter. With reference to the relation between microstructure and abrasion wear, if the materials had the same level of hardness, those having the structure where a primary carbide remained were somewhat better in abrasion resistance. With reference to the influence of the added elements on abrasion, the materials containing Co were effective. When this test was conducted on the needle and the body made of different materials from each other, either one of them contained a microstructurally large carbide which remained in a large amount and besides, the material having a higher hardness abrades a larger amount of another material. Therefore, the needle and the body were made of the same material and subjected to the test. (3) Peeling The peeling phenomenon of the portions in the body and needle of an electronic fuel injection nozzle which are subjected to striking has brought about no problems in the case of the conventional petroleum fuels such as gasoline and this is the peculiar problem when alcoholic fuels are used. This peeling phenomenon is such a phenomenon that the portion just under the surface of the member subjected to striking cleaves and is broken due to repeated stresses and is peeled as if pores are formed on the surface. It is considered that this phenomenon occurs conspicuously because the lubricity of alcoholic fuels is inferior to that of petroleum fuels and so, the lubricity between the members is damaged and the impinging action between the needle and the body increases. With reference to this peeling, the following relation between the peeling and microstructure or hardness was recognized by the electronic fuel injection test shown in the examples given hereinafter. (1) The peeling phenomenon is more difficult to occur for the material of the higher hardness. (2) Microstructurally, the peeling phenomenon more readily occurs with increase in continuous distribution of a coarse primary carbide as shown in Fig. 3B (micrograph of x400) and microstructurally it is necessary that amount of the primary carbide is controlled so that the area rate of the primary carbide is 1.5% or less by microstructural observation of the section and the structure should preferably be as shown in Fig. 3A. For this purpose, the contents of C and Cr should be adjusted to control the amount of the primary carbide. The present invention has been accomplished based on the above findings. That is, the present invention relates to a material suitable for the members of devices subjected to striking in which alcoholic fuels are used and in addition to corrosion resistance and abrasion resistance, this material is improved in peel resistance which is a peculiar property required for materials for members of devices in which especially alcoholic fuels are used by limiting the C and Cr contents to the ranges where the crystallization of a less amount of primary carbides occurs. Crystallization of the primary carbide in the compositions of the present invention occurs with the C content and the Cr content of nearly %Cr + 25 x %C = 32 and limitation is made to be %Cr + 25 x %C ≦ 32 as shown by area (A) is Fig. 2 in order to limit the amount of the primary carbide occupying the sectional area under microstructural observation. As materials used as members exposed to alcoholic fuels, they are used not only for the members subjected to striking, but also for members required to have an abrasion resistance in the level similar to the peel resistance. In this case, the condition for controlling crystallization of a primary carbide is %Cr + 25 x %C > 32 as shown by the area (B) in Fig. 2. However, it is desirable that the materials have such component ranges that the area ratio of the primary carbide is 8% or less. These materials are improved in abrasion resistance to some extent as mentioned above. Reasons for limitation of the chemical compositions of the materials of the present invention will be explained. Carbon (C) is an essential element for maintaining a martensite structure of quenched structure, enhancing the hardness, producing carbides and increasing the abrasion resistance. Addition of at least 0.6% of C is necessary for increasing the hardness of the material after subjected to quenching to a level required for a body and needle of fuel injection nozzle. If the added amount is too much, reduction in corrosion resistance to acetic acid + formic acid is brought about and hence the upper limit is set at less than 0.8%. Furthermore, for materials used as members which receive striking such as a body or needle for an injection nozzle and piston ring in devices in which alcoholic fuels are used, since primary carbides are produced in a large amount at the time of solidification depending on the relative ratio to the amount of Cr and deteriorates the peel resistance of the members exposed to striking, the value of %Cr + 25 x %C is specified to be 32 or less as the range where the remaining primary carbide does not exceed 1.5% in the composition system of the present invention. To the contrary, as materials for members of devices in which alcoholic fuels are used and used in the part which undergoes no special striking, but requires abrasion resistance such as a plunger and body used for an injection pump of a diesel engine, further improvement of abrasion resistance is aimed at to satisfy the condition %Cr + 25 x %C > 32 which is the condition to produce the primary carbide (with a proviso that the area ratio of the primary carbide is 8% or less). Silicon (Si) has the effect to strengthen the base and increase the abrasion resistance by forming a solid solution with the base. Moreover, when the material is subjected to low temperature tempering and used as in the present invention, Si has the effect to increase the resistance to temper softening. However, if Si is added in a large amount, hot workability is damaged and the amount of Si is 1.5% or less. Manganese (Mn) is an essential element for refining of steel, but if it is contained in a large amount, austenite is stabilized and residual austenite increases and, as a result, hardness by heat treatment is exhibited with difficulty and a corrosion resistance to acetic acid and formic acid contained in alcoholic fuels is deteriorated. Thus, the amount of Mn is 1.5% or less. Chromium (Cr) also has an effect to improve corrosion resistance to acetic acid and formic acid contained in alcoholic fuels. Moreover, as a carbide forming element, it produces a chromium carbide and improves abrasion resistance. If the amount of Cr is too large, the hardness after heat treatments (quenching and tempering) is exhibited with difficulty and besides, the primary carbide is produced due to the relation with the C content and the peeled amount of the portion subjected to striking occurs with ease in alcoholic fuels. Therefore, the amount of Cr is at least 8.0% and less than 16% and the relation between the C content and the Cr content is limited to satisfy %Cr + 25 x %C < 32 and the primary carbide is decreased to as small as 1.5% or less at area ratio, taking the peel resistance into consideration. Furthermore, in order to mainly deal with improvement of abrasion resistance against reduction in lubricity owing to the use of alcoholic fuels, primary carbide is crystallized in an area ratio of 8% or less and the amount of Cr is more than 12.0% and less than 16.0% and satisfies %CR + 25 x %C > 32. Molybdenum (Mo) and tungsten (W) are essential for improvement of hardenability, increase of quenching hardness and improvement of abrasion resistance, but they are not needed to be added in large amounts. If they are contained in large amounts, hot workability deteriorates due to the production of coarse carbide and at least one of W and Mo is contained in an amount of 0.5-3.5% as 1/2W + Mo. A preferred range of 1/2W + Mo is 0.5-2.0%. Vanadium (V) is effective for homogenization of the structure of the material of the present invention and is further effective for improvement of corrosion resistance in a corrosion resistance test against acetic acid and formic acid contained in alcoholic fuels. However, if it is contained in a large amount, primary carbide is formed too much and, hence, the amount of V is 0.02-1.5%, preferably 0.1-1.0%. Copper (Cu) is the element which is most effective for improvement of a corrosion resistance to acetic acid and formic acid contained in alcoholic fuels, but addition of it in a large amount causes reduction of tempering hardness and deterioration of hot workability. Therefore, the amount of Cu is 0.05-3.0%, preferably 0.5-3.0%. Cobalt (Co) is effective especially for improvement of abrasion resistance and seizing resistance and the effect has been recognized in the examples given below. Regarding the amount of Co, addition of Co in an amount of 5.0% or less markedly improves the abrasion resistance and effect of addition in an amount of more than 5.0% gradually increases, but addition in a large amount rather reduces toughness. Therefore, the amount of Co added is 0.1-5.0%. A preferred range is 0.5-2.5%. Nickel (Ni) is effective for enhancing the corrosion resistance to acetic acid and formic acid contained in alcoholic fuels and increases toughness of steel. However, if it is contained in a large amount, the critical temperature decreases and residual austenite is readily produced. Therefore, the upper limit of Ni content is 2.0%. Examples The present invention will be explained in more detail by the following examples. First, steels having the compositions shown in Table 1 were produced by a melting method and respective molten steels were cast into the given ingots. These ingots were subjected to diffusion annealing at a high temperature, then hot working and annealing to make the desired test materials. Sample No. 16 and Sample No. 17 made as comparative materials correspond to SUS440C and SUS420J2 according to JIS, respectively. Then, the respective test materials were kept at 1050°C for 20 minutes and thereafter, subjected to oil-cooling, subzero treatment at -100°C, tempering at 150°C and then finish working, and these were used for testing. The test results are shown in Table 2. With reference to the hardness, it is shown by HRC hardness of the materials which were subjected to the working at 1050°C for 20 minutes, oil-quenching, subzero treatment at -100°C and then tempering at 150°C. Corrosion resistance was evaluated by an immersion test in a solution of water + 10% acetic acid + 10% formic acid at 60°C for 30 hours and a solution of industrial methyl alcohol + 10% acetic acid + 10% formic acid at 20°C for 200 hours. The results are shown by the following criteria. "X" : Test result was similar to that of SUS440C. "○" : The sample was not corroded at all. "Δ" : The sample was somewhat corroded. Abrasion resistance and peel resistance were evaluated in the following manner. The test material was made into a needle type electronic fuel injection nozzle as shown in Fig. 1 and this nozzle was subjected to an operation test of 2 x 10⁸ times using a mixed fuel comprising 85% of industrial methanol and 15% of gasoline. The surface subjected to striking and the sliding surface were examined. The abrasion wear was measured from profile of the sliding surface and the member subjected to striking and the results are shown in Table 2 by the following criteria. "X" : more than 5 µm "Δ" : 3-5 µm "○" : less than 3 µm Peeling was evaluated by observing the surface subjected to striking of the needle by a scanning electron microscope and the results are shown in Table 2 by the following criteria. " X " : Peeling defects were seen in the form of a ring (overall surface). " Δ " : Peeling defects were sporadically present. " ○ " : No peeling defects were present. It can be seen from the results in Table 2 that the materials of the present invention are superior in corrosion resistance and peel resistance to SUS440C (Sample No. 16) conventionally used in petroleum fuels and tested as comparative material and besides, are superior in abrasion resistance to SUS420J2 (Sample No. 17) and thus are most suitable for alcoholic fuels. The materials shown in Table 1 having a value of Cr + 25C of more than 32 (Sample Nos. 5, 7, 8, 9, and 13) show some peeling in the use as members subjected to striking as shown in Table 2, but they can be used for members subejcted to no striking or members subjected to only sliding in view of their abrasion resistance and corrosion resistance. Fig. 2 shows relation among the amount of C, amount of Cr and area where the primary carbide is produced and shows the areas A and B of the present invention. Figs. 3A and 3B show the microstructure (x 400) of Sample No. 1 in Table 1 for the area A and Sample No. 5 for the area B which were subjected to the above heat treatments (oil-quenching of 1050°C x 20 minutes, subzero treatment at -100°C and tempering at 150°C. In the above, the present invention has been explained mainly regarding the electronic fuel injection nozzle, but the materials of the present invention are superior in corrosion resistance, abrasion resistance and peel resistance in other uses such as pumps in the electronic fuel injection nozzle and other apparatuses in which alcoholic fuels are used. Moreover, the materials of the present invention can be produced by the same steps as for production of conventional SUS440C and are most suitable as those for members of apparatuses in which alcoholic fuels are used. Besides, they are also superior in abrasion resistance as materials for electronic fuel injection nozzles for usual petroleum fuels and exhibit sufficient characteristics.

Claims: Use of a steel for members of an apparatus in which alcoholic fuels are used, wherein the steel comprises, based on weight, 0.60% or more and less than 0.80% of C, 1.5% or less of Si, 1.5% or less of Mn, 8.0% or more and less than 16.0% of Cr, 0.5 to 3.5% of at least one of W and Mo in 1/2W + Mo, 0.02 to 1.5% of V, 0.05 to 3.0% of Cu and the balance of Fe and unavoidable impurities, the relation between the C content and the Cr content satisfying %Cr + 25 x %C ≦ 32 based on weight and the area ratio of a primary carbide occupying the sectional area of the material being 1.5% or less. Use of a steel for members of an apparatus in which alcoholic fuels are used, wherein the steel comprises, based on weight, 0.60% or more and less than 0.80% of C, 1.5% or less of Si, 1.5% or less of Mn, more than 12.0% and less than 16.0% of Cr, 0.5 to 3.5% of at least one of W and Mo in 1/2W + Mo, 0.02 to 1.5% of V, 0.05 to 3.0% of Cu and the balance of Fe and unavoidable impurities, the relation between the C content and the Cr content satisfying %Cr + 25 x %C > 32 based on weight and the area ratio of a primary carbide occupying the sectional area of the material being 8.0% or less. Use of a steel according to claim 1 or 2, in which the steel further contains 0.1 to 5.0% by weight of Co at the expense of the balance of Fe. Use of a steel according to any one of claims 1 to 3, in which the steel further contains 2.0% by weight or less of Ni at the expense of the balance of Fe. Use of a steel according to any one of claims 1 to 4, wherein said steel has a hardness of HRC 52 or more.

Inventors: FUKUSHIMA KATSUAKI, ITO MASAKAZU, OKUNO TOSHIO, FUKUSHIMA, KATSUAKI, ITO, MASAKAZU, OKUNO, TOSHIO
Applicants: HITACHI METALS LTD, HITACHI METALS, LTD.
Application year: 1991
Main classification: C22C38
IPC codes: C22C38, F02M61