Oxidation resistant steel, containing chromium and aluminium

Abstract: An oxidation-resistant Fe-Cr-Aℓ steel, having superior workability, oxidation resistance at high temperature and corrosion resistance, has a composition which contains: up to but not more than about 0.05 wt% of C; up to but not more than about 1.0 wt% of Si; up to but not more than about 1.0 wt% of Mn; from about 3.0 to 7.5 wt% of Cr; from about 4.5 to 6.5 wt% of Aℓ; up to but not more than about 0.05 wt% of N; one or more elements selected from about the group consisting of 1), from about 0.01 wt% to 0.3 wt% of Zr, 2), from 0.01 wt% to 0.3 wt% of Ti, and 3), from about 0.001 wt% to 0.2 wt%, expressed as a total, of Y, La, Ce, Pr, Nd and Hf; and the balance substantially Fe and incidental inclusions.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to an Fe-Cr-Aℓ steel which is superior in workability, oxidation resistance, corrosion resistance and economy. More particularly, the present invention is concerned with an Fe-Cr-Aℓ steel suitable for use as a material of members or structural parts which are subjected to a strong oxidizing environment or hot oxidizing atmosphere, e.g., parts of internal and external combustion systems, exhaust systems, boilers, incinerators and so forth. DESCRIPTION OF THE RELATED ART An oxidation resistant Fe-Cr-Aℓ steel is disclosed in Japanese Patent Laid-Open Publication No. 63-45351 and also in the specification of United States Patent No. 4,414,023. This steel contains 8 to 25 wt% of Cr and 3 to 8 wt% of Aℓ. When this steel is used in a hot oxidizing atmosphere, Aℓ in the steel is preferentially oxidized to form a fine protective film of Aℓ₂O₃ so as to exhibit high resistance to oxidation. This oxidation resistant steel, therefore, is suitable for use in components of combustors or the like. This known Fe-Cr-Aℓ oxidation resistant steel, however, is still unsatisfactory from the view point of workability because, in general, the toughness of this steel is reduced when the Aℓ content exceeds 3 wt%, with the result that surface defects are often caused during hot rolling. Surface grinding has to be conducted repeatedly to remove such surface defects. In addition, sheets of this known steel tend to be ruptured when subjected to cold rolling. The rolling of this steel, therefore, has to be conducted at a low speed and reduction while elevating the temperature of the sheet. Toughness of the steel can be improved by reducing the contents of Cr and Aℓ as proposed in Japanese Patent Publications Nos. 54-35571 and 55-41290. Reduction of the Cr and Aℓ contents, however, undesirably reduces the oxidation resistance to make the steel materially unusable at high temperatures exceeding 1000°C. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an oxidation resistant Fe-Cr-Aℓ steel which is superior in workability, oxidation resistance and corrosion resistance at high temperature, and economy, thereby overcoming the above-described problems of the prior art. To this end, according to the present invention, there is provided an oxidation-resistant Fe-Cr-Aℓ steel having a composition containing: up to but not more than 0.05 wt% of C; up to but not more than 1.0 wt% of Si; up to but not more than 1.0 wt% of Mn; from 3.0 to 7.5 wt% of Cr; from 4.5 to 6.5 wt% of Aℓ; up to but not more than 0.05 wt% of N; one or more elements selected from the group consisting of 1), from 0.01 wt% to 0.3 wt% of Zr, 2), from 0.01 wt% to 3.0 wt% of Ti, and 3), from 0.001 wt% to 0.2 wt%, expressed as a total, of Y, La, Ce, Pr, Nd and Hf; optionally from 0.001 wt% to 0.05 wt% of Ca; and the balance Fe and incidental inclusions. The oxidation-resistant Fe-Cr-Aℓ steel according to this invention can contain from about 0.001 wt% to 0.05 wt% of Ca. The inventors have found,through an intense study, that the toughness of an Fe-Cr-Aℓ steel can be improved without reducing its oxidation resistance by adding to the steel material suitable amounts of Zr, Ti and rare earth elements, while reducing the Cr content. The present invention is accomplished on the basis of this knowledge. A description will hereafter be given of the reasons for the importance of the contents of these elements. In the following description, the contents of the elements are stated in terms of weight percent (wt%). C, N: not more than 0.05 % When the contents of C and N are excessively large, the toughness of the steel is lowered to seriously impair the cold workability of the steel. In addition, these elements tend to form compounds upon reaction with Cr and Aℓ which are significant elements for maintaining required oxidation resistance, with the result that cleanliness is seriously impaired and that the yields of these significant elements are reduced. For these reasons, the content of each of C and N is limited to 0.05 % or less. Si: not more than 1.0 % Si is a important element as a deoxidizing agent and contributes to improvement in oxidation resistance. Addition of this element in excess of about 1.0 % causes a reduction in toughness and seriously impairs cold workability. For this reason, the content of Si is limited to 1.0 % or less. Mn: not more than 1.0 % Mn also is an important deoxidizing element. An Mn content exceeding 1.0 %, however, reduces the oxidation resistance. The Mn content is therefore limited to 1.0 % or smaller. Cr: 3.0 to 7.5 % Cr is an element which is essential for obtaining required oxidation and corrosion resistances. In order to obtain appreciable effects the Cr content should be not lower than 3.0 %. When the Cr content is too large, however, the toughness of the steel is reduced or impaired. The Cr content, therefore, should not exceed 7.5 %. Aℓ: 4.5 to 6.5 % This element is one of the important elements in the steel of the present invention, as it improves oxidation resistance. In order to obtain a sufficiently large oxidation resistance, the content of this element should be not less than 4.5 %. An excessive Aℓ content, however, reduces the toughness of the steel to impair its cold workability. The Aℓ content, therefore, is determined not to exceed 6.5 %. Ti: 0.01 to 0.3 % Ti provides a strong effect in making C and N inactive so as to suppress reduction of oxidation resistance and of cold workability caused by the presence of C and N. In addition, this element improves resistance to exfoliation of oxide scale in the presence of Cr and Aℓ. Thus, this element also is one of the important elements in the steel of the present invention. In order to obtain a satisfactory result, the content of this element should be about 0.01 % or greater. An excessive Ti content, however, reduces the toughness of the steel to impair its cold workability. The Ti element, therefore, is determined not to exceed 0.3 %. Zr: 0.01 to 0.3 % Zr also improves resistance to exfoliation of oxide scale in the presence of Cr and Aℓ so as to improve oxidation resistance of the steel and, hence, is one of the important elements in the steel of the present invention. In order to attain an appreciable effect, the content of this element should be not less than 0.01 %. Too much Zr, however, tends to reduce oxidation resistance and reduces also the toughness of the steel to impair its cold workability. The Zr content therefore is determined so as not to exceed 0.3 %. Y, La, Ce, Pr, Nd, Hf: 0.001 to 0.2 % expressed as a total As in the cases of Ti and Zr, these elements improve resistance to exfoliation of oxide scale in the presence of Cr and Aℓ so as to improve oxidation resistance of the steel and, hence, are important elements in the steel of the present invention. In order to obtain a satisfactory result, the total content of these elements has to be not less than 0.001 %. However, an excessive total content of these elements reduces toughness due to presence of inclusions, with the result that cold workability of the steel is impaired undesirably. For this reason, the total content of these elements is determined so as not to exceed 0.2 %. Ca: 0.001 to 0.05 % Ca is effective in nullifying the effect of S which seriously impairs oxidation resistance of the steel. This element is therefore added as required in an amount not less than 0.001 %. In the steel of the present invention, Ca cannot exist as a solid solution when its content exceeds 0.05 %. For these reasons, therefore, the Ca content is determined to lie within the range from 0.001 to 0.05 %. Although no limitation is imposed on the S content, it is preferred that the S content shall not exceed 0.005 % from the view point of oxidation resistance. The aforementioned known Fe-Cr-Aℓ steel with reduced Aℓ content can maintain the required level of oxidation resistance only at a comparatively low temperature, e.g., up to 900°C. Adjustments of contents of Cr and Aℓ to the claimed ranges is not sufficient for attaining the required oxidation resistance at high temperatures exceeding 1000°C. Namely, in order to attain the required level of oxidation resistance at such high temperatures, it is also necessary that at least one of Ti, Zr and one or more of Y, La, Ce, Pr, Nd and Hf shall be present in the amounts specified in the claim. Example Compositions of Examples of the steel of the present invention are shown in Table 1, while compositions of Comparison Examples of steel are shown in Table 2. The steels of the compositions shown in Tables 1 and 2 were formed into ingots of 10 kg and, after being heated to 1200°C, hot rolled to sheets 3 mm thick. A Charpy test was conducted on these sheets to examine the levels of toughness. The results of the Charpy test also are shown in Tables 1 and 2. The sheets also were subjected to an annealing conducted at 900°C for 1 minute for the descaling purpose. Samples of the steel sheets exhibiting impact absorption energy of 5 kgf·m/cm² at 25°C were rolled to sheets of 0.5 mm at an elevated temperature of 100°C, since cold rolling of such samples was difficult. Other samples exhibiting impact absorption energy of 5 kgf·m/cm² or higher under the same conditions could be cold-rolled to sheets of 0.5 mm thick. These sheets were then subjected to annealing and surface grinding, and test pieces 20 mm wide and 30mm long were extracted from these sheets for the purpose of oxidation testing. Test pieces 50mm wide and 100 mm long also were extracted for the purpose of corrosion testing. The oxidation tests were conducted by holding the test pieces for 96 hours in an electric oven maintaining an atmosphere of 1150°C and then measuring increments of weight of the test pieces, the results being shown in Tables 1 and 2. The corrosion tests were conducted by executing a 24-hour salt spray test to examine the state of generation of rust. The results are shown in Tables 1 and 2. In Tables 1 and 2, the test items were evaluated on the following bases. Toughness: Impact absorption energy as measured by the Charpy test on hot-rolled sheets (kgf·m/cm²) Oxidation resistance: Weight increment due to oxidation after 96-hour shelving in an atmosphere of 1150°C Corrosion resistance: Result of 24-hour salt spray test at 35°C (marks ○ and X are respectively applied to samples which did not show rust and which did exhibit rust after the spray test) From Tables 1 and 2, it is clearly understood that steels having compositions falling within the ranges specified by the invention exhibit superior workability, and high resistance to both oxidation and corrosion at high temperature. As will be understood from the foregoing description, the present invention provides an oxidation resistant steel which is superior in workability, oxidation resistance and corrosion resistance at high temperature.

Claims: An oxidation-resistant Fe-Cr-Aℓ steel comprising: up to but not more than 0.05 wt% of C; up to but not more than 1.0 wt% of Si; up to but not more than 1.0 wt% of Mn; from 3.0 to 7.5 wt% of Cr; from 4.5 to 6.5 wt% of Aℓ; up to but not more than 0.05 wt% of N; one or more elements selected from the group consisting of 1), from 0.01 wt% to 0.3 wt% of Zr, 2), from 0.01 wt% to 0.3 wt% of Ti, and 3), from 0.001 wt% to 0.2 wt%, expressed as a total, of Y, La, Ce, Pr, Nd and Hf; optionally from 0.001 wt% to 0.05 wt% of Ca; and the balance Fe and impurities and incidental inclusions. An oxidation-resistant Fe-Cr-Aℓ steel according to Claim 1, containing from about 0.001 wt% to 0.05 wt% of Ca.

Inventors: ISHII KAZUHIDE C O TECHNICAL R, KAWASAKI TATSUO C O TECHNICAL, SHIMIZU HIROSHI C O TECHNICAL, ISHII, KAZUHIDE, C/O TECHNICAL RESEARCH DIVISION, KAWASAKI, TATSUO, C/O TECHNICAL RESEARCH DIVISION, SHIMIZU, HIROSHI, C/O TECHNICAL RESEARCH DIVISION
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1990
Main classification: C22C38
IPC codes: C22C38