AUSTENITIC FREE CUTTING STAINLESS STEELS

Abstract: An austenitic stainless steel having improved free cutting properties comprises, in weight ratio, C ≦ 0.2%, Si ≦ 2.0%, Mn ≦ 10.0%, 7.5% ≦ Cr ≦ 30.0%, Ni ≦ 40.0%, 0.005% ≦ Bi ≦ 0.50%, 0.0003% ≦ B ≦ 0.10%, 0.002% ≦ S ≦ 0.40%, P ≦ 0.2%, N ≦ 0.10% and/or 0 ≦ 0.4%, and the balance being Fe.

Description: This invention relates to a free cutting stainless steel, and more particularly to an austenitic free cutting stainless steel containing Bi as a necessary free cutting element for the provision of free cutting properties. In general, stainless steels have large viscosity and poor heat conductivity, and are apt to be coherent to a tool in cutting, so that they are difficult to subject to cutting work. Therefore, attempts have been made to improve the free cutting properties of stainless steel by the addition of a free cutting element such as S, Pb, Bi, Te, Se or the like, and the resulting steels have been applied to various uses as a free cutting stainless steel. In a free cutting stainless steel of this type, however, the free cutting properties are improved by the addition of the free cutting element, but the hot workability is inversely degraded due to the addition of such a free cutting element, which causes problems in production. In an austenitic stainless steel, the hot workability is not so good, and is considerably degraded by adding the free cutting element. Furthermore, stainless steel is utilized in wide applications owing to the corrosion resistance. Particularly, austenitic stainless steel has an excellent corrosion resistance, so that it is suitable for various applications. However, the application of such steels may be restricted due to the presence of the free cutting element. For example, the addition of S, Pb, Te, Se and the like causes problems of corrosion resistance and food hygiene when the steel is used as a material for food machines, and particularly the addition of Pb cannot be adopted due to the latter problem. On the other hand, since Bi is an element used in chemicals, cosmetics and the like, it has been considered that Bi is suitable to be incorporated in free cutting stainless steels for use in food machines, but the addition of Bi considerably degrades the hot workability, which causes a production problem. It is, therefore, an aim of the invention to overcome or at least mitigate the aforementioned problems of the prior art and to provide a novel Bi-containing austenitic free cutting stainless steel which can find wider application without degrading the hot workability even when Bi is added as a free cutting element to austenitic stainless steel having an excellent corrosion resistance. The present inventors have made various studies in order to prevent the degradation of hot workability even when Bi is added as an essential free cutting element to an austenitic stainless steel, and have found that degradation of the hot workability can be considerably reduced by adding a relatively large amount of B which has hitherto been used in a very slight amount as a quench-improving element. Based on this knowledge, various experiments for improving the properties of Bi-containing austenitic free cutting stainless steel have been made, from which the invention has been accomplished. According to the invention, there is provided a Bi-containing austenitic free cutting stainless steel comprising not more than 0.2 wt% of C, not more than 2.0 wt% of Si, not more than 10.0 wt% of Mn, 7.5-30.0 wt% of Cr, not more than 40.0 wt% of Ni, 0.005-0.50 wt% of Bi, 0.0003-0.10 wt% of B, 0.002-0.40 wt% of S, not more than 0.20 wt% of P, at least one of not more than 0.10 wt% of N and not more than 0.4 wt% of 0, the balance being Fe and inevitable impurities. In preferred embodiments of the invention, the steel further contains one or more of the following groups: (1) at least one of not more than 5.0 wt% of Mo, not more than 4.0 wt% of Cu and not more than 1.50 wt% of Al; (2) at least one of not more than 0.5 wt% of Zr, not more than 2.0 wt% of Ti, not more than 3.0 wt% of Nb, not more than 0.5 wt% of V and not more than 0.5 wt% of Ta; and (3) at least one of not more than 0.009 wt% of Ca and not more than 0.35 wt% of Se. The reason why the chemical composition of the steel according to the invention is as defined above will be described below. The invention aims at improving the free cutting properties by the addition of Bi as a necessary free cutting element and the prevention or mitigation of the degradation of the hot workability based on the addition of Bi by adding B. The addition of B can improve the free cutting properties and is effective for preventing the degradation of the hot workability against the addition of the other elements causing degradation of the hot workability. Bi is an element considerably effective for improving the free cutting properties as described above and does not cause problems in respect of food hygiene, and it is necessary to add at least 0.005% of Bi to obtain the required effect. However, if the amount of Bi is too large, the hot workability is considerably degraded and cannot be sufficiently ensured even by the addition of B, so that the upper limit of the amount of Bi is 0.50%. B is an element effective for preventing degradation of the hot workability due to the addition of Bi as well as the addition of the other free cutting elements. Furthermore, B reacts with N and O properly contained in steel as mentioned later to form the nitride (BN) and oxide (B₂O₃), whereby the free cutting properties can be improved without causing degradation of the hot workability. Moreover, B can improve the yield of Bi in the addition of Bi. In orer to ensure these effects, it is necessary to add at least 0.0003% of B. However, if the amount of B is too large, the above effect cannot be expected, so that the upper limit of the amount of B should be 0.10% in view of the cost and addition yield. C is a strong austenite-forming element. It is desirable that the amount of C is relatively low in view of the corrosion resistance. In particular, in the case of the austenitic stainless steel of the present invention, the amount of C should be not more than 0.2%. Si is an element acting as a deoxidizer and is effective for increasing the oxidation resistance, but is a ferrite-forming element. If the amount of Si is too large, the toughness is decreased, so that the upper limit of the amount of Si is 2.0%. Mn is an austenite-forming element and is a low cost material as compared with Ni, so that it may be contained as a substituting element for Ni. Further, Mn forms a compound with S, Se or the like to effectively prevent hot brittleness. However, if the amount of Mn is too large, the free cutting properties are degraded, so that the upper limit of the amount of Mn is 10.0%. Cr is a fundamental element for austenitic stainless steel. It is necessary to add 7.5-30.0% of Cr in order to improve the corrosion resistance and oxidation resistance of such a steel. Ni is a preferable and important element in austenitic stainless steel and forms a stable austenitic phase to effectively improve the corrosion resistance and toughness. However, if the amount of Ni is too large, the free cutting properties are degraded, and the cost becomes higher, so that the upper limit of the amount of Ni is 40.0%. S is an element for imparting the free cutting properties to the austenitic stainless steel. If the amount of S is too large, the hot workability and corrosion resistance are degraded, so that the upper limit of the amount of S is 0.40%. In particular, if there is required high corrosion resistance in a food machine for example the amount of S is preferably not more than 0.02%. However, if the amount of S is less than 0.002%, an increase of the cost is caused in the production and the free cutting properties are degraded, so that the lower limit of the amount of S is 0.002%. P is an element for imparting the free cutting properties. If the amount of P is too large, the hot workability is degraded, so that the upper limit of the amount of P is 0.20%. N is effective not only for improving the free cutting properties by bonding with B to form nitride, but also for increasing the tensile strength and stabilizing austenite. If the amount of N is too large, the effect of improving the hot workability based on the addition of B is obstructed, so that the upper limit of the amount of N is 0.10%. In particular, the austenitic stainless steel has less good hot workability, so that it is preferable that the amount of N is not more than 0.05% in order to ensure sufficient hot workability based on the addition of B. O forms an oxide with B, which is effective for improving the free cutting properties, but adversely affects the corrosion resistance and hot workability. The lower limit of the amount of O for improving the free cutting properties is 0.002%. However, if the amount of O is too large, the hot workability is degraded, so that the upper limit of the amount of O is 0.4%. In Bi-containing steel, the amount of not more than 0.005% considerably improves the hot workability, which is preferable in the case of requiring high hot workability as in high speed rolling or the like. In steels having the above chemical composition according to the invention, at least one of Mo, Cu and Al may be added as an element for improving the corrosion resistance and oxidation resistance, if necessary. Mo has an effect of improving the corrosion resistance by forming a passive film in Cr-Ni series stainless steel, but if the amount of Mo is too large, the effect is inversely lost, so that the upper limit of the amount of Mo is 5.0%. Cu is an austenite-forming element and improves the corrosion resistance. If the amount of Cu is too large, the hot workability is degraded, so that the upper limit of the amount of Cu is 4.0%. Al is an element for improving the oxidation resistance. When Al is used for deoxidation, it may be added so as to retain 0.005-0.050% of Al in steel. In precipitation hardening type steel, Al may be added in an amount of not more than 1.5%. If necessary, at least one of Zr, Ti, Nb, V and Ta may be added to a steel of the composition according to the invention. These elements can improve the corrosion resistance, strength and the like when added in proper amounts and are effective for improving the hot workability. Having regard to the free cutting properties, cost and the like, not more than 0.5% of Zr, not more than 2.0% of Ti, not more than 3.0% of Nb, not more than 0.5% of V and not more than 0.5% of Ta may be added, respectively. Furthermore, at least one of Ca and Se may be added to the steel of the composition according to the invention, if necessary, in order to more improve the free cutting properties. Having regard to the cleanliness, corrosion resistance, hot workability and the like, not more than 0.009% of Ca and/or not more than 0.35% of Se may be added. However, it is preferable not to add Se if it is intended to use the steel according to the invention as a material for a food machine. The invention will be further described with reference to the following illustrative Examples. Example Austenitic stainless steels each having a chemical composition as shown in the following Table 1 was melted in an arc furnace of 2 ton capacity, refined in a ladle refining apparatus (GRAF), and then cast into an ingot of 2 tons. Then, the ingot was heated at about 1250°C, and was rolled into a billet of 140mm square to examine the hot workability. In this case, the hot workability was evaluated by an appearance test by examining for the presence of billet cracking and by a hot tensile test (1250°C) of a specimen cut out from the surface portion of the billet to measure fracture draw (%). These measured results are shown in the following Table 4. In order to examine the free cutting properties, the ingot was forged into a rod 60mm in diameter, which was subjected to a drill cutting test against soluted materials under conditions shown in the following Table 2. The free cutting properties were evaluated as a drilling property (cutting rate until the tool life reached 1000mm) (m/min). The results are also shown in Table 4. The corrosion resistance was evaluated by weight loss after the same material as used in the above cutting test was immersed in a solution shown in the following Table 3. The results are shown in Table 4. As seen from the results of Table 4, the steel Nos. 1-22 according to the invention effectively prevent the degradation of the hot workability, so that they are able to be subjected to the usual hot rolling. Further, they have excellent free cutting properties and corrosion resistance. On the other hand, the comparative steel Nos. 24 and 25 containing the defined amount of Bi but no B or very small amount of B have poor hot workability to produce large cracking in the hot rolling and small hot fracture draw. As described above in detail, according to the invention, the degradation of the hot workability can be prevented by adding a proper amount of B to Bi-containing austenitic stainless steel exhibiting a conspicuous degradation of hot workability, and further the free cutting properties and corrosion resistance can be considerably improved by adjusting the amounts of the other elements without degrading the hot workability. Therefore, the invention does not cause problems in respect of the production of steels and food hygiene and is widely applicable to materials for food machines and the like.

Claims: 1. A bismuth-containing austenitic free cutting stainless steel, characterized by comprising not more than 0.2 wt% of C, not more than 2.0 wt% of Si, not more than 10.0 wt% of Mn, 7.5-30.0 wt% of Cr, not more than 40.0 wt% of Ni, 0.005-0.50 wt% of Bi, 0.0003-0.10 wt% of B, 0.002-0.40 wt% of S, not more than 0.20 wt% of P, at least one of not more than 0.10 wt% of N and not more than 0.4 wt% of O, the balance being Fe and inevitable impurities. 2. A stainless steel as claimed in claim 1, characterized by further containing at least one of not more than 5.0 wt% of Mo, not more than 4.0 wt% of Cu and not more than 1.50 wt% of Al. 3. A stainless steel as claimed in claim 1 or 2, characterized by further containing at least one of not more than 0.5 wt% of Zr, not more than 2.0 wt% of Ti, not more than 3.0 wt% of Nb, not more than 0.5 wt% of V and not more than 0.5 wt% of Ta. 4. A stainless steel as claimed in any of claims 1 to 3, characterized by further containing at least one of not more than 0.009 wt% of Ca and not more than 0.35 wt% of Se.

Inventors: KIMURA, ATSUYOSHI, SHIBATA, NORIYOSHI
Applicants: DAIDO TOKUSHUKO KABUSHIKI KAISHA
Application year: 1986
Main classification: C22C38
IPC codes: C22C38, C22C30