Low-oxygen clean steel and low-oxygen clean steel product

Abstract

Low-oxygen clean steel is provided containing C, Si, Mn, P, and S as chemical components, and further containing, by mass %, 0.005% to 0.20% of Al, greater than 0% to 0.0005% of Ca, 0.00005% to 0.0004% of REM, and greater than 0% to 0.003% of T.O, wherein the REM content, the Ca content, and the T.O content satisfy 0.15REM/Ca4.00 and Ca/T.O0.50, nonmetallic inclusions which have a maximum predicted diameter of 1 m to 30 m measured using an extreme value statistical method under the condition in which a prediction area is 30,000 mm.sup.2, and contain Al.sub.2O.sub.3 and REM oxide are dispersed in the steel, an average proportion of the Al.sub.2O.sub.3 in the nonmetallic inclusions is greater than 50%, the REM is one or two or more of rare-earth elements La, Ce, Pr, and Nd, and the steel is Al-deoxidized steel or AlSi-deoxidized steel.

Claims

1. A low-oxygen clean steel containing C, Si, Mn, P, and S as chemical components, and further containing, by mass %, 0.010% to 0.20% of Al, 0.0001% to 0.0003% of Ca, 0.00005% to 0.0004% of REM, and greater than 0% to 0.003% of T.O, wherein the REM content, the Ca content, and the T.O content satisfy the following Expressions 1 and 2, nonmetallic inclusions which have a maximum predicted diameter of 1 m to 30 m measured using an extreme value statistical method under the condition in which a prediction area is 30,000 mm.sup.2, and contain Al.sub.2O.sub.3 and REM oxide are dispersed in the steel, a proportion of the Al.sub.2O.sub.3 is greater than 50% by mass based on the total amount of nonmetallic inclusions, a proportion of CaO is 1.0% or less by mass based on the total amount of the nonmetallic inclusions, the REM is one or two or more of rare-earth elements La, Ce, Pr, and Nd, and the steel is Al-deoxidized steel or AlSi-deoxidized steel,
0.15REM/Ca4.00Expression 1,
Ca/T.O0.50Expression 2.

2. The low-oxygen clean steel according to claim 1, wherein the following Expression 3 is satisfied,
0.05REM/T.O0.50Expression 3.

3. The low-oxygen clean steel according to claim 1, containing, by mass %, 1.20% or less of C, 3.00% or less of Si, 16.0% or less of Mn, 0.05% or less of P, and 0.05% or less of S as the chemical components with the remainder Fe and impurities.

4. The low-oxygen clean steel according to claim 3, further containing, by mass %, one or two or more of 3.50% or less of Cr, 0.85% or less of Mo, 4.50% or less of Ni, 0.20% or less of Nb, 0.45% or less of V, 0.30% or less of W, 0.006% or less of B, 0.06% or less of N, 0.25% or less of Ti, 0.50% or less of Cu, 0.45% or less of Pb, 0.20% or less of Bi, 0.01% or less of Te, 0.20% or less of Sb, and 0.01% or less of Mg as the chemical components.

5. A low-oxygen clean steel product produced by processing the low-oxygen clean steel according to claim 1.

6. A low-oxygen clean steel product produced by processing the low-oxygen clean steel according to claim 3.

7. A low-oxygen clean steel product produced by processing the low-oxygen clean steel according to claim 4.

8. The low-oxygen clean steel according to claim 2, containing, by mass %, 1.20% or less of C, 3.00% or less of Si, 16.0% or less of Mn, 0.05% or less of P, and 0.05% or less of S as the chemical components with the remainder Fe and impurities.

9. The low-oxygen clean steel according to claim 8, further containing, by mass %, one or two or more of 3.50% or less of Cr, 0.85% or less of Mo, 4.50% or less of Ni, 0.20% or less of Nb, 0.45% or less of V, 0.30% or less of W, 0.006% or less of B, 0.06% or less of N, 0.25% or less of Ti, 0.50% or less of Cu, 0.45% or less of Pb, 0.20% or less of Bi, 0.01% or less of Te, 0.20% or less of Sb, and 0.01% or less of Mg as the chemical components.

10. A low-oxygen clean steel product produced by processing the low-oxygen clean steel according to claim 2.

11. A low-oxygen clean steel product produced by processing the low-oxygen clean steel according to claim 8.

12. The low-oxygen clean steel according to claim 1, wherein a proportion of SiO.sub.2 in the inclusions is 1.0% or less by mass based on the total amount of nonmetallic inclusions.

13. The low-oxygen clean steel according to claim 1, wherein the proportion of the Al.sub.2O.sub.3 in the inclusions is greater than 61.8% to 82.9% by mass based on the total amount of the nonmetallic inclusions.

14. The low-oxygen clean steel according to claim 1, wherein the proportion of the Al.sub.2O.sub.3 is greater than 66.0% to 82.9% by mass based on the total amount of nonmetallic inclusions.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram showing the relationship between a maximum grain diameter (area (m)) of nonmetallic inclusions and fatigue strength (MPa) (Non-Patent Document Yukitaka Murakami, Metal Fatigue, Effects of Micro-Defects and Inclusions).

(2) FIG. 2 is a diagram showing the relationship between an REM content (ppm) and steel piece extreme value statistics (maximum predicted diameter) (m).

(3) FIG. 3 is a diagram showing the relationship between a ratio of REM to Ca and steel piece extreme value statistics (m).

(4) FIG. 4 is a diagram showing the relationship between a ratio of REM to T.O and steel piece extreme value statistics (m).

(5) FIG. 5 is a diagram showing the relationship between the ratio of Ca to T.O and steel piece extreme value statistics (m) analyzed when REM is appropriately added (0.00005% to 0.0004%), when REM is excessively added (greater than 0.0004%), and when REM is not added (the REM content is less than 0.00005%).

(6) FIG. 6 shows diagrams showing forms (SEM reflected electron images) of nonmetallic inclusions existing in steel. FIGS. 6(a) and 6(b) show forms of nonmetallic inclusions of invention examples (No. 2-1 in Tables 2-1 and 2-2 to be shown later), and FIGS. 6(c) and 6(d) show forms of nonmetallic inclusions of comparative examples (No. 2-2 in Tables 2-1 and 2-2 to be shown later).

(7) FIG. 7 shows an aspect of the production of a radial rolling fatigue test piece. FIG. 7(a) shows a shape of the material of the radial rolling fatigue test piece, FIG. 7(b) shows an aspect of the collection of the radial rolling fatigue test piece, and FIG. 7(c) shows a final shape of the collected radial rolling fatigue test piece.

(8) FIG. 8 is a diagram showing the relationship between steel piece extreme value statistics (maximum predicted diameter) obtained through an extreme value statistical method and the shortest breaking life obtained through a radial fatigue test.

(9) FIG. 9 is a diagram showing a shape of a test piece produced for evaluation of rotary bending fatigue properties.

(10) FIG. 10 is a diagram showing the relationship between maximum stress and the number of times of endurance, obtained through an Ono-type rotary bending test.

DESCRIPTION OF EMBODIMENTS

(11) Low-oxygen clean steel according to an embodiment of the invention (hereinafter, may be referred to as the low-oxygen clean steel according to this embodiment) will be described in detail.

(12) The low-oxygen clean steel according to this embodiment contains C, Si, Mn, P, and S as fundamental elements, further contains, by mass %, 0.005% to 0.20% of Al, greater than 0% to 0.0005% of Ca, 0.00005% to 0.0004% of REM, and greater than 0% to 0.003% of T.O, and if necessary, contains other elements.

(13) In the low-oxygen clean steel according to this embodiment, the REM content, the Ca content, and the T.O content satisfy the following Expressions 1 and 2, and preferably satisfy the following Expression 3. In the steel, nonmetallic inclusions which have a maximum predicted diameter of 1 m to 30 m measured using an extreme value statistical method under the condition in which the prediction area is 30000 mm.sup.2, and contain Al.sub.2O.sub.3 and REM oxide are dispersed. The average proportion of the Al.sub.2O.sub.3 in the nonmetallic inclusions is greater than 50%. The low-oxygen clean steel according to this embodiment is Al-deoxidized steel or AlSi-deoxidized steel.
0.15REM/Ca4.00Expression 1
Ca/T.O0.50Expression 2
0.05REM/T.O0.50Expression 3

(14) Here, REM is one or two or more of rare-earth elements La, Ce, Pr, and Nd.

(15) As described above, in the low-oxygen clean steel according to this embodiment, nonmetallic inclusions containing fine Al.sub.2O.sub.3 and REM oxide are dispersed by the suppression of the generation of the inclusions and the modification of the generated inclusions.

(16) The effect of the suppression of the generation of the inclusions is obtained by controlling the Al content, the Ca content, and the T.O content within predetermined ranges.

(17) The effect of the modification of the generated inclusions is obtained by a minute amount of REM of 0.00005 mass % to 0.0004 mass % (to be described later in detail). The inclusion modification effect of REM is obtained through a reducing action by REM with respect to CaO or CaO of CaOAl.sub.2O.sub.3.

(18) That is, in the low-oxygen clean steel according to this embodiment, it is important to control the amounts of Al, Ca, and T.O to 0.005 mass % to 0.20 mass %, to greater than 0 mass % to 0.0005 mass %, and to greater than 0 mass % to 0.003 mass %, respectively, from the viewpoint of the suppression of the generation of the inclusions, and it is important to control the amount of REM to 0.00005 mass % to 0.0004 mass % from the viewpoint of the modification of the generated inclusions.

(19) Usually, steel contains C, Si, Mn, P, S, and if necessary, other elements with the remainder Fe and impurities. In the low-oxygen clean steel according to this embodiment, the above-described inclusion modification effect by REM is exhibited without being affected by molten steel components such as C, Si, and Mn other than Al, Ca, REM, and T.O. That is, it is not necessary to restrict the contents of elements other than Al, Ca, REM, and T.O. The inventors of the invention have confirmed this fact by way of experiment in an actual operation. The reasons for restriction of the respective contents will be described later.

(20) Furthermore, the inventors of the invention have found that it is important for Al, Ca, REM, and T.O existing in a minute amount in the molten steel to be controlled not only in the content of each element, but also in the content ratio in order to appropriately maintain the mutual action and reaction between the elements and to maximize the inclusion modification effect by REM. Specifically, they have found that it is effective to control the ratio of REM to Ca, the ratio of REM to T.O, and the ratio of Ca to T.O as indices of the content ratios. The reasons for restriction of these content ratios will be described later.

(21) First, the reasons for restriction of the component composition (chemical components) will be described. Hereinafter, % means mass %. In the low-oxygen clean steel according to this embodiment, chemical components are preferably within the following ranges in specimen of steel sampled from molten steel before casting based on JIS G 0417 or specimen of steel after casting

(22) Al: 0.005% to 0.20%

(23) Al is a deoxidizing element and is an element which makes crystal grains of steel finer. In order to obtain these effects, the lower limit of the Al content is 0.005%. The lower limit of the Al content is preferably 0.010%.

(24) When Al is contained in molten steel, the molten steel inevitably becomes Al-deoxidized molten steel and Al.sub.2O.sub.3-containing inclusions are generated in the molten steel. When the Al content in the molten steel is greater than 0.20%, the inclusions are generated in a large amount and remain in the steel and the fatigue properties of the steel deteriorate. Therefore, the upper limit of the Al content is 0.20%. The upper limit of the Al content is preferably 0.10%.

(25) Ca: Greater than 0% to 0.0005%

(26) Ca is a deoxidizing element and is an element which forms CaOAl.sub.2O.sub.3-based inclusions which easily aggregate together and have a low melting point through a deoxidation reaction. When the Ca content in molten steel is greater than 0.0005%, Al.sub.2O.sub.3-based inclusions are made into CaOAl.sub.2O.sub.3-based composite inclusions having a low melting point and coarsen. The CaOAl.sub.2O.sub.3-based inclusions coarsening and remaining in the steel are not liquefied at a rolling temperature and remain in a coarse state in the steel. The amount of Ca is preferably as small as possible, but 0.0005% or less of Ca is permissible. Accordingly, the upper limit of the Ca content is 0.0005%. The upper limit of the Ca content is preferably 0.0003%, and more preferably 0.00025%.

(27) In the current steelmaking method in which refining is performed by bringing slag containing CaO into contact with an upper portion of molten steel in a ladle, Ca is inevitably incorporated in the molten steel, and thus Ca cannot be completely eliminated from the steel. Therefore, the lower limit of the Ca content is greater than 0%.

(28) In the low-oxygen clean steel according to this embodiment, the generation of CaOAl.sub.2O.sub.3-based inclusions can be suppressed under the condition in which there is a minute amount of Ca incorporated inevitably in the molten steel.

(29) In this embodiment, the Ca content is adjusted before the addition of REM. The method of suppressing the Ca content to 0.0005% or less in the course of refining will be described later.

(30) REM: 0.00005% to 0.0004%

(31) REM is an important element which modifies CaOAl.sub.2O.sub.3-based inclusions by reducing CaO in molten steel and CaO in inclusions. The molten steel sufficiently deoxidized with Al or AlSi contains 0.00005% to 0.0004% of REM (rare-earth element, one or two or more of La, Ce, Pr, and Nd) in order to obtain the inclusion modification effect. The inclusion modification effect cannot be obtained when the REM content is 0.00005% or less.

(32) When the molten steel contains greater than 0.0004% REM, the inclusions increase in size. The detailed mechanism thereof is not clear, but is thought to be that when the molten steel contains greater than 0.0004% REM, a compound phase having a low melting point and a high REM concentration appears in the inclusions and promotes the aggregation of the inclusions, and thus the inclusions increase in size. Therefore, the upper limit of the REM content is 0.0004%. The upper limit of the REM content is preferably 0.0003%, and more preferably 0.0002%.

(33) The range of the REM content is based on the result of the evaluation on the relationship between steel piece extreme value statistics (maximum predicted diameter) of the nonmetallic inclusions in the low-oxygen clean steel according to this embodiment calculated through an extreme value statistical method and fatigue strength.

(34) FIG. 1 is a diagram showing the relationship between a maximum diameter (area (m)) of the nonmetallic inclusions and fatigue strength (MPa). From FIG. 1, it is found that the fatigue strength is improved with a decrease in grain diameter (area (m)) of the nonmetallic inclusions.

(35) The component composition and the form (dimensions, shape) of the nonmetallic inclusions have a large influence on the fatigue strength of steel. The component composition and the form (dimensions, shape) of the nonmetallic inclusions will be described later.

(36) FIG. 2 shows the relationship between an REM content (ppm) and steel piece extreme value statistics (m). The steel piece extreme value statistics (m) provide an estimated value (maximum predicted diameter) of the maximum diameter of the inclusions existing in a predetermined test amount (prediction area) of a steel, which is obtained through an extreme value statistical method. In this embodiment, the steel piece extreme value statistics are calculated through an extreme value statistical method with a prediction area of 30,000 mm.sup.2.

(37) From FIG. 2, it is found that the REM content at which the steel piece extreme value statistics (m) are 30 m or less is 4 ppm (0.0004%) or less. In any analysis target steel, T.O was 5 ppm to 20 ppm and was within a preferable range of this embodiment. In the low-oxygen clean steel according to this embodiment, as described above, the upper limit of the REM content is 0.0004% based on the above description.

(38) In addition, according to FIG. 2, the inclusion modification effect of REM is exhibited when the REM content is 0.5 ppm or greater. Accordingly, the lower limit of the REM content is 0.00005%. That is, the REM content is 0.00005% to 0.0004%. The REM content is preferably 0.00005% to 0.0003%, and more preferably 0.00005% to 0.0002%.

(39) T.O: Greater than 0% to 0.003%

(40) O is an element which exists in molten steel and forms an oxide. Accordingly, in producing steel which has excellent mechanical characteristics and in which a small amount of inclusions are finely dispersed, the T.O content is required to be controlled. In addition, it is also important to control the T.O content in the relationship with the contents of Ca and REM, which are constituent elements of oxide inclusions, in molten steel.

(41) When the T.O content of molten steel is greater than 0.003%, oxide inclusions are generated in a large amount and remain in the steel, and mechanical characteristics, particularly, fatigue properties of the steel deteriorate. Therefore, the T.O content is 0.003% or less. The T.O content is preferably 0.002% or less, and more preferably 0.001% or less.

(42) Although the amount of T.O is preferably as small as possible, the lower limit thereof is greater than 0% since it is difficult to adjust the amount of T.O to 0%.

(43) Next, the reasons why the ratio of REM to Ca and the ratio of Ca to T.O are restricted to 0.15 to 4.00 and to 0.50 or less, respectively, and the reasons why the ratio of REM to T.O is preferably 0.05 to 0.50 in the low-oxygen clean steel according to this embodiment will be described.

(44) REM/Ca: 0.15 to 4.00 (0.15REM/Ca4.00)

(45) REM is an element which reduces CaO in inclusions to act for modification of the inclusions and suppression of coarsening. Therefore, the ratio of REM to Ca which is a ratio of the REM content to Ca content is an important index for maximizing the inclusion modification effect of REM.

(46) FIG. 3 shows the relationship between the ratio of REM to Ca and steel piece extreme value statistics (m).

(47) From FIG. 3, it is found that the steel piece extreme value statistics (m) are 30 m or less when the ratio of REM to Ca is 0.15 to 4.00. When the ratio of REM to Ca is less than 0.15, the inclusions containing CaOAl.sub.2O.sub.3 as a main component are not sufficiently modified. As a result, the inclusions have a grain diameter (steel piece extreme value statistics) greater than 30 m, and thus coarsen and remain in the steel, and the mechanical characteristics thereof are not improved.

(48) When the ratio of REM to Ca is greater than 4.00, the steel piece extreme value statistics (m) are greater than 30 m. The reason for this is assumed to be that since the molten steel has an excessively high REM content, the concentration of REM oxide in the inclusions to be generated excessively increases, and thus the composition of the inclusions is out of an appropriate range. The detailed mechanism thereof is not clear, but is presumed to be that when the REM concentration in the inclusions excessively increases, the inclusions aggregate together due to the generation of a low-melting-point phase in the inclusions, and as a result, the steel piece extreme value statistics (m) are increased.

(49) From the above description, the ratio of REM to Ca is 0.15 to 4.00. The ratio of REM to Ca is preferably 0.20 to 3.00, and more preferably 1.00 to 3.00.

(50) Ca/T.O: 0.50 or Less (Ca/T.O0.50)

(51) The ratio of Ca to T.O which is a ratio of the Ca content to the T.O content is an important index for suppressing the generation and coarsening of CaOAl.sub.2O.sub.3-based inclusions and for maximizing the inclusion modification effect of REM.

(52) FIG. 5 shows the relationship between the ratio of Ca to T.O and steel piece extreme value statistics (m) analyzed when REM is appropriately added (steel having an REM content of 0.00005% to 0.0004%), when REM is excessively added (steel having an REM content greater than 0.0004%), and when REM is not added (the REM content is less than 0.00005%).

(53) From FIG. 5, it is found that in the case of the appropriate addition of REM indicated by in FIG. 5, the steel piece extreme value statistics are 30 m or less when the ratio of Ca to T.O is 0.50 or less. The reason for this is presumed to be that when the ratio of Ca to T.O is 0.50 or less, the CaO activity of the inclusions is maintained at a high level, the reducing reaction of CaO by REM easily occurs, and thus the coarsening of the nonmetallic inclusions is suppressed.

(54) Accordingly, the ratio of Ca to T.O is 0.50 or less. The ratio of Ca to T.O is preferably 0.10 to 0.40. When the Ca content is 0.00025% or less, the ratio of Ca to T.O is preferably 0.20 or less in order to suppress the coarsening of the inclusions by Ca.

(55) REM/T.O: 0.05 to 0.50 (0.05REM/T.O0.50)

(56) The ratio of REM to T.O is an effective index for sufficiently exhibiting the inclusion modification effect of REM. Accordingly, in addition to the ratio of REM to Ca and the ratio of Ca to T.O that have been described above, the ratio of REM to T.O is preferably 0.05 to 0.50 in order to prominently exhibit the inclusion modification effect of REM.

(57) When the ratio of REM to T.O is greater than 0.50, CaO contributing as a binding agent in the aggregation of the inclusions and CaO of CaOAl.sub.2O.sub.3 are reduced immediately after the addition of REM, but a large amount of unreacted REM (REM itself, which is a strong deoxidizing element) remains and excessively reduces Al.sub.2O.sub.3. As a result, REM.sub.2O.sub.3Al.sub.2O.sub.3 inclusions are generated in a large amount and coarsen. Therefore, there is no contribution to an improvement in mechanical characteristics.

(58) When the ratio of REM to T.O is less than 0.05, there is no sufficient contribution to the reduction of CaO and CaO of CaOAl.sub.2O.sub.3, which contribute as a binding agent of the inclusions, and thus the inclusion modification effect is not sufficiently exhibited. Therefore, the effect of finely dispersing the nonmetallic inclusions in the steel is not obtained, and thus there is no contribution to an improvement in mechanical characteristics. Accordingly, the ratio of REM to T.O is preferably 0.05 to 0.50, and more preferably 0.10 to 0.40.

(59) FIG. 4 shows the relationship between the ratio of REM to T.O and steel piece extreme value statistics in steel having 0.003% or less of T.O. In FIG. 4, all of the REM content, the ratio of REM to Ca, the ratio of Ca to T.O, etc. are within the ranges of the low-oxygen clean steel according to this embodiment.

(60) When REM satisfying the ratio of REM to T.O of 0.05 to 0.50, and preferably 0.10 to 0.40 is contained in clean molten steel having 0.003% or less of T.O, the REM sufficiently reduces CaO contributing as a binding agent in the aggregation of the inclusions and CaO of CaOAl.sub.2O.sub.3 (that is, the inclusion modification effect is sufficiently exhibited). As a result, the inclusions do not aggregate and the nonmetallic inclusions are more finely dispersed.

(61) Next, preferable contents of C, Si, and Mn, which are fundamental elements of molten steel, and P and S, which are impurity elements, will be described. As described above, in the low-oxygen clean steel according to this embodiment, the inclusion modification effect by REM is exhibited without being affected by steel components such as C, Si, and Mn other than Al, Ca, REM, and T.O. Therefore, it is not necessary to restrict the contents of elements other than Al, Ca, REM, and T.O. when obtaining the effect of this embodiment. However, in practical steel, the contents of C, Si, Mn, etc. are preferably controlled to secure predetermined characteristics. Hereinafter, a preferable component composition (chemical components) will be described based on the component composition of the practical steel.

(62) C: 1.20% or Less

(63) C is an effective element for securing the strength or hardness of steel after hardening. Types of steels which are not required to have such strength or hardness are not essentially required to contain C. Accordingly, the lower limit of the C content is not particularly restricted. However, since C is a fundamental element of steel and it is difficult to adjust the content thereof to 0%, the content of C cannot be 0%.

(64) In the case of increasing the strength or the hardness, the C content is preferably 0.001% or greater. However, when the C content is greater than 1.20%, cracks are generated upon hardening or the steel becomes too hard, whereby the life of a cutting tool is deteriorated. Therefore, the upper limit of the C content is preferably 1.20%. The upper limit of the C content is more preferably 1.00%.

(65) Si: 3.00% or Less

(66) Si is an effective element for securing the strength or hardness by improving hardenability of steel. Types of steels which are not required to have such strength or hardness are not essentially required to contain Si. Accordingly, the lower limit of the Si content is not particularly restricted. However, since Si is a fundamental element of steel and it is difficult to adjust the content thereof to 0%, the content of Si cannot be 0%.

(67) In the case of increasing the strength or the hardness of steel, the Si content is preferably 0.001% or greater. However, when the Si content is greater than 3.00%, the effect is saturated and the hardness of the steel excessively increases, whereby the life of a cutting tool is deteriorated. Therefore, the upper limit of the Si content is preferably 3.00%. The upper limit of the Si content is more preferably 2.50%.

(68) Mn: 16.0% or Less

(69) Mn is an effective element for securing the strength or hardness by improving hardenability of steel. Types of steels which are not required to have such strength or hardness are not essentially required to contain Mn. Accordingly, the lower limit of the Mn content is not particularly restricted. However, since Mn is a fundamental element of steel and it is difficult to adjust the content thereof to 0%, the content of Mn cannot be 0%.

(70) In the case of increasing the strength or the hardness, the Mn content is preferably 0.001% or greater. However, when the Mn content is greater than 16.0%, quenching cracks are generated upon hardening or the steel becomes too hard, whereby the life of a cutting tool is deteriorated. Therefore, the upper limit of the Mn content is preferably 16.0%. The upper limit of the Mn content is more preferably 12.0%. When a certain amount of C (for example, 0.1% or greater) is contained, the strength of practical steel can be secured even when the Mn content is 2.0% or less.

(71) P: 0.05% or Less

(72) P is an impurity element, and when the P content is too large, the toughness of steel deteriorates. Therefore, the P content is preferably restricted to 0.05% or less, and more preferably to 0.03% or less. However, large refining cost is required to decrease the P content to 0.0001% or less. Therefore, the lower limit of the P content in practical steel is approximately 0.0001%.

(73) S: 0.05% or Less

(74) Similarly to P, S is an impurity element, and when the S content is too large, the toughness of steel deteriorates. Therefore, the S content is preferably restricted to 0.05% or less, and more preferably to 0.03% or less. Large refining cost is required to decrease the S content to 0.0001% or less. Therefore, the lower limit of the S content in practical steel is approximately 0.0001%.

(75) The low-oxygen clean steel according to this embodiment may further contain one or two or more of 3.50% or less of Cr, 0.85% or less of Mo, 4.50% or less of Ni, 0.20% or less of Nb, 0.45% or less of V, and 0.30% or less of W other than the above-described elements in such a range as not to damage the characteristics thereof. Since it is not necessary to essentially contain these elements, the lower limits thereof are 0%.

(76) Cr: 3.50% or Less

(77) Cr is an effective element for securing the strength or hardness by improving hardenability of steel. The Cr content is preferably 0.01% or greater to obtain this effect. When the Cr content is greater than 3.50%, toughness and ductility deteriorate. Thus, the upper limit of the Cr content when Cr is contained is 3.50%. The upper limit of the Cr content is preferably 2.50%.

(78) Mo: 0.85% or Less

(79) Mo is an effective element for securing the strength or hardness by improving hardenability of steel. In addition, Mo is an element which forms carbide to contribute to an improvement in temper softening resistance. The Mo content is preferably 0.001% or greater when obtaining these effects. When the Mo content is greater than 0.85%, a supercooling structure which causes deterioration in toughness and ductility is easily generated. Thus, the upper limit of the Mo content when Mo is contained is 0.85%. The upper limit of the Mo content is preferably 0.65%.

(80) Ni: 4.50% or Less

(81) Ni is an effective element for securing the strength or hardness by improving hardenability. The Ni content is preferably 0.005% or greater to obtain this effect. When the Ni content is greater than 4.50%, toughness and ductility deteriorate. Thus, the upper limit of the Ni content when Ni is contained is 4.50%. The upper limit of the Ni content is preferably 3.50%.

(82) Nb: 0.20% or Less

(83) Nb is an element which forms carbide, nitride, or carbonitride to contribute to the prevention of coarsening of crystal grains and an improvement in temper softening resistance. The Nb content is preferably 0.001% or greater when obtaining these effects. When the Nb content is greater than 0.20%, toughness and ductility deteriorate. Thus, the upper limit of the Nb content when Nb is contained is 0.20%. The upper limit of the Ni content is preferably 0.10%.

(84) V: 0.45% or Less

(85) V is an element which forms carbide, nitride, or carbonitride to contribute to the prevention of coarsening of crystal grains and an improvement in temper softening resistance. The V content is preferably 0.001% or greater when obtaining these effects. When the V content is greater than 0.45%, toughness and ductility deteriorate. Thus, the upper limit of the V content when V is contained is 0.45%. The upper limit of the V content is preferably 0.35%.

(86) W: 0.30% or Less

(87) W is an effective element for securing the strength or hardness by improving hardenability of steel. In addition, W is an element which forms carbide to contribute to an improvement in temper softening resistance. The W content is preferably 0.001% or greater when obtaining these effects. When the W content is greater than 0.30%, a supercooling structure which causes deterioration in toughness and ductility is easily generated. Thus, the upper limit of the W content when W is contained is 0.30%. The upper limit of the W content is preferably 0.20%.

(88) The low-oxygen clean steel according to this embodiment may further contain, by mass %, one or two or more of 0.006% or less of B, 0.06% or less of N, 0.25% or less of Ti, 0.50% or less of Cu, 0.45% or less of Pb, 0.20% or less of Bi, 0.01% or less of Te, 0.20% or less of Sb, and 0.001% or less of Mg other than the above-described elements in such a range as not to damage the characteristics thereof. Since it is not necessary to essentially contain these elements, the lower limits thereof are 0%.

(89) B: 0.006% or Less

(90) B is an element which increases hardenability of steel to contribute to an improvement in strength. In addition, B is an element which is segregated in austenite grain boundaries to suppress the grain boundary segregation of P and to improve fatigue strength. The B content is preferably 0.0001% or greater when obtaining these effects. When the B content is greater than 0.006%, the effect is saturated and embrittlement is caused. Therefore, the upper limit of the B content is 0.006% when B is contained. The upper limit of the B content is preferably 0.004%.

(91) N: 0.06% or Less

(92) N is an element which forms fine nitride to provide fine crystal grains and contributes to an improvement in strength and toughness. The N content is preferably 0.001% or greater when obtaining these effects. When the N content is greater than 0.06%, nitride is generated in an excessive amount, and thus toughness deteriorates. Therefore, the upper limit of the N content is 0.06% when N is contained. The upper limit of the N content is preferably 0.04%.

(93) Ti: 0.25% or Less

(94) Ti is an element which forms fine Ti nitride to provide fine crystal grains and contributes to an improvement in strength and toughness. The Ti content is preferably 0.0001% or greater when obtaining these effects. When the Ti content is greater than 0.25%, Ti nitride is generated in an excessive amount, and thus toughness deteriorates. Therefore, the upper limit of the Ti content is 0.25% when Ti is contained. The upper limit of the Ti content is preferably 0.15%.

(95) Cu: 0.50% or Less

(96) Cu is an element which increases corrosion resistance of steel. The Cu content is preferably 0.01% or greater to obtain this effect. When the Cu content is greater than 0.50%, hot ductility deteriorates, and thus cracks or flaws are caused. Therefore, the upper limit of the Cu content is 0.50% when Cu is contained. The upper limit of the Cu content is preferably 0.30%.

(97) Pb: 0.45% or Less

(98) Pb is an element which contributes to an improvement in machinability of steel. The Pb content is preferably 0.001% or greater to obtain this effect. When the Pb content is greater than 0.45%, toughness deteriorates. Therefore, the upper limit of the Pb content is 0.45% when Pb is contained. The upper limit of the Pb content is preferably 0.30%.

(99) Bi: 0.20% or Less

(100) Bi is an element which contributes to an improvement in machinability of steel. The Bi content is preferably 0.001% or greater to obtain this effect. When the Bi content is greater than 0.20%, toughness deteriorates. Therefore, the upper limit of the Bi content is 0.20% when Bi is contained. The upper limit of the Bi content is preferably 0.10%.

(101) Te: 0.01% or Less

(102) Te is an element which contributes to an improvement in machinability of steel. The Te content is preferably 0.0001% or greater to obtain this effect. When the Te content is greater than 0.01%, toughness deteriorates. Therefore, the upper limit of the Te content is 0.01% when Te is contained. The upper limit of the Te content is preferably 0.005%.

(103) Sb: 0.20% or less

(104) Sb is an element which contributes to an improvement in corrosion resistance based on sulfuric acid resistance and hydrochloric acid resistance and an improvement in machinability. The Sb content is preferably 0.001% or greater when obtaining these effects. When the Sb content is greater than 0.20%, toughness deteriorates. Therefore, the upper limit of the Sb content is 0.20% when Sb is contained. The upper limit of the Sb content is preferably 0.10%.

(105) Mg: 0.01% or Less

(106) Mg is an element which contributes to an improvement in machinability of steel. The Mg content is preferably 0.0001% or greater to obtain this effect. When the Mg content is greater than 0.01%, toughness deteriorates. Therefore, the upper limit of the Mg content is 0.01% when Mg is contained. The upper limit of the Mg content is preferably 0.005%.

(107) Next, the nonmetallic inclusions existing in a finely dispersed manner in the low-oxygen clean steel according to this embodiment will be described.

(108) The low-oxygen clean steel according to this embodiment is obtained by adding, by mass %, 0.00005% to 0.0004% of REM to molten steel which contains 0.005% to 0.20% of Al, 0.0005% or less of Ca, and 0.003% or less of T.O and in which the ratio of Ca to T.O is 0.50 or less. The low-oxygen clean steel according to this embodiment satisfies (x1) the ratio of REM to Ca is 0.15 to 4.00 and (y) the ratio of Ca to T.O is 0.50 or less, and preferably further satisfies (x2) the ratio of REM to T.O is 0.05 to 0.50.

(109) Molten steel having the following chemical components: 0.005% to 0.20% of Al; 0.0005% or less of Ca; and 0.003% or less of T.O, in which the ratio of Ca to T.O is 0.50 or less, is used when obtaining the low-oxygen clean steel according to this embodiment. In such molten steel, the amount of CaO existing in the molten steel and the amount of CaOAl.sub.2O.sub.3 inclusions are small.

(110) When REM is added to the molten steel in the above state in an amount of 0.00005% to 0.0004% such that the above-described (x1) (preferably further (x2)) is satisfied, the REM reduces CaO, acting as a binding agent to promote the aggregation of the inclusions, FeO, compounds such as FeOAl.sub.2O.sub.3, and CaO in CaOAl.sub.2O.sub.3 inclusions. As a result, (i) the CaOAl.sub.2O.sub.3 inclusions are modified into Al.sub.2O.sub.3-based and/or REM.sub.2O.sub.3-based inclusions, and (ii) the aggregation of Al.sub.2O.sub.3-based inclusions, Al.sub.2O.sub.3MgO-based inclusions, and REM.sub.2O.sub.3-based inclusions is suppressed, whereby the inclusions do not coarsen.

(111) That is, fine nonmetallic inclusions are generated in the molten steel by adding REM as described above. Accordingly, the low-oxygen clean steel according to this embodiment obtained by casting the molten steel in which the fine nonmetallic inclusions exist is capable of obtaining a structure in which the nonmetallic inclusions are finely dispersed. The nonmetallic inclusions are fine and have a size of 30 m or less even in terms of the maximum predicted diameter obtained using an extreme value statistical method with a prediction area of 30,000 mm.sup.2. In addition, since the nonmetallic inclusions are fine, fatigue fracture hardly occurs therefrom as is apparent fracture-mechanically. Therefore, the mechanical characteristics, particularly, fatigue properties of the low-oxygen clean steel according to this embodiment are significantly improved. This is the most important characteristic of the low-oxygen clean steel according to this embodiment.

(112) In this embodiment, the maximum predicted diameter of the inclusions is a value estimated using, for example, the extreme value statistical method described in Metal Fatigue, Effects of Micro-Defects and Inclusions (written by Yukitaka Murakami, Yokendo, published in 1993, P. 223 to 239). The maximum predicted diameter (area (max)) of the inclusions is calculated through the expression: area (max)=(a.sup.2+b.sup.2).sup.1/2 where a is a major axis and b is a minor axis perpendicular to the major axis.

(113) FIG. 6 shows typical forms of nonmetallic inclusions existing in steel (SEM reflected electron image). These are forms of nonmetallic inclusions detected when evaluating steel piece extreme value statistics in examples to be described later. FIGS. 6(a) and 6(b) show forms of nonmetallic inclusions of invention examples (No. 2-1 in Tables 2-1 and 2-2 to be shown later) (type of steel: suspension spring A), and FIGS. 6(c) and 6(d) show representative forms of nonmetallic inclusions of comparative examples (No. 2-2 in Tables 2-1 and 2-2 to be shown later) (type of steel: suspension spring A).

(114) The diameter (see black borders) of the nonmetallic inclusions of the comparative examples shown in FIGS. 6(c) and 6(d) is on the order of tens of-m. The diameter (see black borders) of the nonmetallic inclusions of the invention examples shown in FIGS. 6(a) and 6(b) is on the order of several-m. Fine nonmetallic inclusions exist in various shapes in the low-oxygen clean steel according to this embodiment as shown in FIGS. 6(a) and 6(b). Since the nonmetallic inclusions are fine due to the modification with REM, fatigue fracture hardly occurs therefrom. The inventors of the invention have confirmed this fact by way of experiment in an actual operation with respect to main types of steels which are used in spring steel, bearing steel, case hardening steel etc.

(115) The above-described fact that fatigue fracture hardly occurs from the fine nonmetallic inclusions also relates to the component composition of the nonmetallic inclusions. Hereinafter, the component composition of the nonmetallic inclusions will be described.

(116) Table 1 shows component compositions of the above-described nonmetallic inclusions shown in FIGS. 6(a) to 6(d). Table 1 also shows component compositions, which are separately observed from those in FIGS. 6(a) to 6(d), of nonmetallic inclusions (Invention Examples 3 to 12) of the low-oxygen clean steels according to this embodiment and nonmetallic inclusions (Comparative Examples 3 to 6) of comparative steels. The component composition of the nonmetallic inclusions was measured as follows.

(117) The average composition of one inclusion detected by an optical microscope is measured using an energy dispersive X-ray analysis method to analyze the composition of Mg, Al, Si, Ca, La, Ce, Nd, Mn, Ti, and S. Since Mn and Ca form both of oxide and sulfide, S is allowed to form sulfide in order of MnS and CaS and the remaining Ca and Mn are analyzed as oxide. When obtaining the average of the inclusion composition, the number average may be taken after examining the compositions of a plurality of inclusions as described above.

(118) The nonmetallic inclusions shown in FIG. 6 have a difference in contrast therebetween. This shows that the nonmetallic inclusions have a mixed phase of oxide and sulfide, but the fact that the nonmetallic inclusions have a mixed phase does not have a dominant influence on fatigue properties. This is consistent with the relationship between the grain diameter of the nonmetallic inclusions and the fatigue strength shown in FIG. 1.

(119) TABLE-US-00001 TABLE 1 COMPONENT COMPOSITION SIZE (MASS %: BASED ON 100% OF OXIDE) (ADDITIONAL) Al.sub.2O.sub.3 + No CLASSIFICATION m*m MgO Al.sub.2O3 SiO.sub.2 CaO La.sub.2O.sub.3 Ce.sub.2O.sub.3 Nd.sub.2O.sub.3 MnO TiO2 MnS CaS REM.sub.2O.sub.3 REM 2-1 INVENTION 4*4 2.9 53.3 0 0 14.2 22.1 7.5 0 0 31.1 10.2 43.8 97.1 EXAMPLE 1 2-1 INVENTION 4*3 14.1 56.3 0 0 11.2 16.5 1.9 0 0 11.2 13.6 29.6 85.9 EXAMPLE 2 2-2 COMPARATIVE 10*9 8.7 67.6 7.2 16.5 0 0 0 0 0 0 3.7 0 67.6 EXAMPLE1 2-2 COMPARATIVE 13*10 1.4 69.2 5.1 24.3 0 0 0 0 0 0 14.0 0 69.2 EXAMPLE2 1-1 INVENTION 12*5 18.2 53.6 0 0 1.4 24.4 2.5 0 0 9.3 29.4 28.2 81.8 EXAMPLE3 1-1 INVENTION 9*4 23.9 56.2 0 0 4.8 9.9 5.2 0 0 12.3 13.2 19.9 76. 1 EXAMPLE4 1-1 INVENTION 7*6 26.8 73.2 0 0 0 0 0 0 0 0 0 0 73.2 EXAMPLE5 1-3 INVENTION 31*4 3.5 49.1 0 0 16.6 25.3 5.6 0 0 7.4 11.8 47.4 96.5 EXAMPLE6 1-3 INVENTION 11*6 4.7 66.0 0 0 9.8 16.4 3.1 0 0 0 3.9 29.3 95.3 EXAMPLE7 1-5 COMPARATIVE 20*17 22.4 77.6 0 0 0 0 0 0 0 0 6.4 0 77.6 EXAMPLE3 1-5 COMPARATIVE 10*9 5.0 89.8 0 5.3 0 0 0 0 0 0 27.0 0 89.8 EXAMPLE4 1-6 COMPARATIVE 20*7 18.2 81.6 0 0.2 0 0 0 0 0 0 23.9 0 81.6 EXAMPLE5 1-6 COMPARATIVE 28*7 22.7 77.3 0 0 0 0 0 0 0 0 44 0 77.3 EXAMPLE6 4-1 INVENTION 14*10 15.0 62.1 0 0.3 6.3 16.4 0 0 0 2.1 0 22.7 84.8 EXAMPLE8 6-10 INVENTION 14*9 16.3 82.9 0.8 0 0 0 0 0 0 8 19 0 82.9 EXAMPLE9 6-10 INVENTION 8*7 17.5 57.6 0 1.0 10.0 14.0 0 0 0 8 10 24.0 81.6 EXAMPLE10 6-10 INVENTION 10*7 9.1 63.4 0 0.3 9.2 18.0 0 0 0 2 8 27.2 90.6 EXAMPLE11 6-10 INVENTION 7*5 24.9 74.5 0.6 0 0 0 0 0 0 3 15 0 74.5 EXAMPLE12

(120) The inclusion compositions of FIGS. 6(a) and 6(b) are shown in Invention Examples 1 and 2 of Table 1, and the inclusion compositions of FIGS. 6(c) and 6(d) are shown by mass % in Comparative Examples 1 and 2 of Table 1. In Comparative Examples 1 and 2 and further Comparative Examples 3 to 6, the inclusions are not modified with REM. However, in Invention Examples 1 and 2 and further Invention Examples 3 to 12, the inclusions are modified with REM.

(121) As is apparent from Table 1, Al.sub.2O.sub.3 and/or CaO are main components in Comparative Examples 1 and 2 and further Comparative Examples 3 to 6. Al.sub.2O.sub.3 and REM oxide are main components in Invention Examples 1 and 2 and further Invention Examples 3 to 12. In addition, the average proportion of Al.sub.2O.sub.3 in the inclusions in each example exceeds 50%.

(122) CaO in Comparative Examples 1 is 16.5% and CaO in Comparative Example 2 is 24.3%. These are values higher than 10%. In the invention examples, CaO is 1.0% or less and significantly lower than in the comparative examples.

(123) In the case of the inclusions of the invention examples, TiO.sub.2 and SiO.sub.2 are almost undetected (for example, 1.0% or less). When deoxidation is sufficiently performed with Al or AlSi, the nonmetallic inclusions almost contain no TiO.sub.2 and SiO.sub.2.

(124) Even when one or two or more of CaS, MnS, REM sulfide and MgO (compound layer) exist in inclusions having Al.sub.2O.sub.3 and REM oxide as main components, the influence on the size of the inclusions is small. For example, in the case of the nonmetallic inclusions of Invention Example 1 in Table 1, 31.1 mass % of MnS and 10.2 mass % of CaS (total: 41.3 mass %) additionally exist, and in the case of the nonmetallic inclusions of Invention Example 2, 11.2 mass % of MnS and 13.6 mass % of CaS (total: 24.5 mass %) additionally exist.

(125) Even when CaS and MnS exist in an amount of approximately 0% to 42% in inclusions having Al.sub.2O.sub.3 and REM oxide as main components, the size of the inclusions is kept small in the range of the analysis. In addition, fatigue properties are not affected by the existence of CaS and MnS, and thus the influence of the existence of CaS and MnS on fatigue properties is confirmed to be sufficiently small.

(126) A preferable low-oxygen clean steel according to this embodiment will be described.

(127) Similarly to general steel, a low-oxygen clean steel according to an embodiment is preferably obtained by rolling a steel piece obtained through a refining process and a casting process or the like. As the processing processes such as the casting process and rolling, arbitrary methods can be employed so as to provide a desired shape and desired characteristics.

(128) As for the low-oxygen clean steel according to this embodiment, it is important to add, by mass %, 0.00005% to 0.0004% of REM to molten steel which contains 0.005% to 0.20% of Al, 0.0005% or less of Ca, and 0.003% or less of T.O and in which the ratio of Ca to T.O is 0.50 or less.

(129) Therefore, in the refining process, the Ca content is preferably restricted and REM is preferably contained in the molten steel through the following method in the following manner.

(130) <Method of Restricting Ca Content>

(131) Various types of auxiliary raw materials and alloy iron are added to molten steel when the molten steel is subjected to refining and component adjustment. In general, since the auxiliary raw materials and alloy iron contain Ca in various forms, it is important to manage the timing of addition of the auxiliary raw materials and alloy iron and the Ca content contained therein in order to adjust the Ca content to 0.0005% or less.

(132) Ca in the alloy iron is contained as an alloy component at a high ratio. Accordingly, in the case of molten steel deoxidized with Al or AlSi, the yield of Ca in the molten steel is high. Thus, it is necessary to avoid the addition of alloy iron having a high Ca content.

(133) Therefore, the amount of Ca to be added is preferably decreased using, for example, alloy iron having 1.0% or less of Ca. In addition, since quicklime, dolomite etc. to be added as a slag-making material contain Ca in the form of mainly oxide, it is incorporated in slag when floating separation is sufficiently conducted. However, since the floating separation cannot be sufficiently conducted during the terminal stages of secondary refining, the addition is avoided. Other than quicklime, dolomite, CaO-containing recycle slag may be used as a slag-making material.

(134) In addition, in Al-deoxidized steel and AlSi-deoxidized steel, it is important to suspend no CaO in the molten steel to suppress the generation of CaOAl.sub.2O.sub.3-based inclusions. During the terminal stages of secondary refining, stirring of molten steel and slag containing CaO in a large amount is suppressed. For example, strong stirring by Ar blowing into a ladle should be avoided. When the molten steel is stirred from the viewpoint of uniform REM concentration, a stirring method such as electromagnetic stirring is used in which the slag is not incorporated into the molten steel.

(135) <Method of Adding REM>

(136) CaO adheres to inclusions having Al.sub.2O.sub.3 as a main component and functions as a binding agent to promote coarsening. REM acting to reduce the CaO is added in an amount of 0.00005% to 0.0004% to molten steel in which ladle slag refining has been completed by sufficient deoxidation with Al or AlSi. It is not preferable that REM be added before deoxidation with Al or AlSi be performed since the inclusions coarsen.

(137) For example, in a common secondary refining process including ladle electrode heating and vacuum degassing, molten steel is deoxidized by ladle electrode heating, and then REM is added to the molten steel in the vacuum degassing process. REM may be added to a tundish or the molten steel in a mold.

(138) Since REM is added to the molten steel in a minute amount, the molten steel is preferably stirred to uniformize the REM concentration in the molten steel after the addition. For the stirring of the molten steel, stirring in a vacuum chamber in the vacuum degassing process, stirring in the tundish by flowing of the molten steel, and electromagnetic stirring in the mold can be applied.

(139) REM may be added in the form of any of pure metal such as Ce and La, alloy of REM metals, or alloy with other alloys. When REM is added, the shape thereof is preferably a lump-like shape, a grain shape, or a wire shape from the viewpoint of yield.

(140) A low-oxygen clean steel product according to this embodiment can be produced by processing the low-oxygen clean steel according to this embodiment using an arbitrary method.

EXAMPLES

(141) Next, examples of the invention will be described. Conditions in the invention examples are just an examples employed to confirm the feasibility and effects of the invention. Therefore, the invention is not restricted to these examples. The invention can employ various conditions so long as not departing from the gist of the invention while achieving the object of the invention.

Example 1

(142) A steel piece was produced by casting molten steel having a component composition shown in Table 2-1. The slag composition and the conditions of auxiliary raw materials at the time of refining are shown together in Table 2-2. In the column of conditions of auxiliary raw materials, a Ca source (CaSi or FeSi) to be put into the molten steel and the mass percent of Ca in FeSi are shown. The component composition includes the remainder Fe and impurities.

(143) Using the above-described steel piece, steel piece extreme value statistics (maximum predicted diameter) (m) of nonmetallic inclusions in a prediction area of 30,000 mm.sup.2 were estimated through an extreme value statistical method. The results are shown together in Table 2-2. When the steel piece extreme value statistics are 30 m or less, the level is set to pass (G: GOOD), when the steel piece extreme value statistics are greater than 30 m to 37 m, the level is set to B (BAD), and when the steel piece extreme value statistics are greater than 37 m, the level is set to VB (VERY BAD). FIGS. 6(a) and 6(b) show the forms of the nonmetallic inclusions of Invention Example No. 2-1, and FIGS. 6(c) and 6(d) show the forms of the nonmetallic inclusions of Comparative Example 2-2.

(144) TABLE-US-00002 TABLE 2-1 COMPONENT COMPOSITION CHEMICAL COMPONENTS () (MASS %) (MASS % .Math. 10.sup.4) Ca/ REM/ REM/ No. C Si Mn TAl P S Cr Mo TO Ca REM TO TO Ca 1-1 INVENTION EXAMPLE 1.00 0.25 0.37 0.012 0.009 0.001 1.44 0.01 4 2 2.0 0.4 0.4 1.0 1-2 INVENTION EXAMPLE 0.99 0.26 0.38 0.013 0.012 0.001 1.43 0.00 5 2 2.0 0.4 0.4 1.0 1-3 INVENTION EXAMPLE 1.00 0.26 0.38 0.010 0.009 0.002 1.43 0.00 5 1 1.0 0.2 0.2 1.0 1-4 INVENTION EXAMPLE 1.00 0.25 0.37 0.008 0.011 0.001 1.42 0.00 5 1 0.5 0.2 0.1 0.5 1-5 COMPARATIVE EXAMPLE 1.00 0.26 0.37 0.018 0.009 0.001 1.43 0.00 5 1 0.0 0.2 0 0.0 1-6 COMPARATIVE EXAMPLE 0.99 0.25 0.37 0.009 0.006 0.001 1.42 0.01 5 1 0.0 0.2 0 0.0 2-1 INVENTION EXAMPLE 0.52 1.51 0.49 0.026 0.013 0.012 0.73 0.09 6 2 1.0 0.33 0.17 0.5 2-2 COMPARATIVE EXAMPLE 0.53 1.53 0.48 0.025 0.009 0.012 0.72 0.09 10 1 0.0 0.1 0 0.0 2-3 COMPARATIVE EXAMPLE 0.53 1.49 0.50 0.024 0.013 0.012 0.72 0.09 5 1 0.0 0.2 0 0.0 3-1 COMPARATIVE EXAMPLE 0.49 1.96 0.70 0.023 0.007 0.002 0.52 0.01 6 1 0.0 0.17 0 0.0 3-2 COMPARATIVE EXAMPLE 0.50 1.92 0.70 0.022 0.006 0.002 0.53 0.01 7 1 0.0 0.14 0 0.0 3-3 COMPARATIVE EXAMPLE 0.48 1.97 0.70 0.023 0.010 0.001 0.51 0.01 5 2 0.0 0.4 0 0.0 3-4 COMPARATIVE EXAMPLE 0.48 1.93 0.69 0.024 0.009 0.002 0.52 0.01 10 4 0.0 0.4 0 0.0 3-5 COMPARATIVE EXAMPLE 0.50 1.96 0.70 0.020 0.007 0.001 0.53 0.01 7 2 0.0 0.29 0 0.0 3-6 COMPARATIVE EXAMPLE 0.50 1.98 0.70 0.023 0.008 0.003 0.53 0.01 12 1 0.0 0.08 0 0.0 3-7 COMPARATIVE EXAMPLE 0.49 1.97 0.68 0.021 0.005 0.001 0.51 0.01 12 6 0.8 0.50 0.07 0.13 4-1 INVENTION EXAMPLE 0.20 0.20 0.78 0.026 0.019 0.013 0.95 0.16 6 1 1.7 0.17 0.28 1.7 4-2 INVENTION EXAMPLE 0.15 0.21 0.80 0.025 0.013 0.015 1.00 0.16 11 1 2.4 0.09 0.22 2.4 4-3 INVENTION EXAMPLE 0.41 0.26 0.75 0.023 0.013 0.019 1.11 0.16 9 2 2.4 0.22 0.27 1.2 4-4 INVENTION EXAMPLE 0.20 0.20 0.80 0.025 0.025 0.020 1.12 0.01 6 2 3.0 0.33 0.5 1.5 4-5 COMPARATIVE EXAMPLE 0.20 0.20 0.79 0.025 0.021 0.009 1.10 0.01 8 2 5.0 0.25 0.63 2.5 4-6 COMPARATIVE EXAMPLE 0.40 0.25 0.80 0.025 0.012 0.003 1.11 0.16 6 1 4.9 0.17 0.82 4.9 4-7 COMPARATIVE EXAMPLE 0.28 0.21 0.87 0.025 0.018 0.014 1.22 0.29 7 1 0.0 0.14 0 0.0 4-8 COMPARATIVE EXAMPLE 0.23 0.22 0.88 0.024 0.015 0.013 1.22 0.29 21 0.8 4.0 0.04 0.19 5.00 4-9 COMPARATIVE EXAMPLE 0.22 0.23 0.88 0.026 0.011 0.012 1.21 0.28 16 8 0.8 0.50 0.05 0.10 5-1 INVENTION EXAMPLE 0.17 0.03 0.75 0.046 0.012 0.003 0.03 0.00 14 1 2.4 0.07 0.17 2.4 5-2 INVENTION EXAMPLE 0.16 0.02 0.77 0.049 0.011 0.008 0.03 0.00 9 1 1.9 0.11 0.21 1.9 5-3 INVENTION EXAMPLE 0.13 0.03 0.36 0.026 0.013 0.007 0.03 0.00 13 3 4.0 0.23 0.31 1.3 5-4 COMPARATIVE EXAMPLE 0.16 0.04 0.76 0.043 0.016 0.006 0.03 0.00 17 1 0.0 0.06 0 0.0 5-5 COMPARATIVE EXAMPLE 0.17 0.04 0.77 0.049 0.012 0.012 0.07 0.00 12 1 0.0 0.08 0 0.0 5-6 COMPARATIVE EXAMPLE 0.15 0.04 0.74 0.060 0.009 0.011 0.03 0.00 8 5 1.5 0.63 0.19 0.30 6-1 INVENTION EXAMPLE 0.55 0.20 0.76 0.024 0.009 0.002 0.14 0.01 7 1 1.8 0.14 0.26 1.8 6-2 INVENTION EXAMPLE 0.43 0.26 0.78 0.024 0.012 0.016 0.13 0.00 6 1 2.0 0.17 0.33 2.0 6-3 INVENTION EXAMPLE 0.34 0.23 0.75 0.025 0.018 0.024 0.05 0.01 6 1 0.7 0.17 0.12 0.7 6-4 INVENTION EXAMPLE 0.20 0.17 0.80 0.035 0.012 0.006 0.37 0.01 7 1 1.1 0.14 0.16 1.1 6-5 INVENTION EXAMPLE 0.32 0.20 0.75 0.025 0.022 0.030 0.15 0.01 8 1 4.0 0.13 0.5 4.0 6-6 INVENTION EXAMPLE 0.34 0.21 0.77 0.022 0.024 0.029 0.01 0.01 12 4 0.6 0.33 0.05 0.2 6-7 COMPARATIVE EXAMPLE 0.33 0.23 0.75 0.028 0.008 0.003 0.07 0.00 8 1 4.1 0.13 0.52 4.1 6-8 COMPARATIVE EXAMPLE 0.35 0.22 0.75 0.024 0.015 0.024 0.06 0.01 7 5 2.0 0.71 0.29 0.4 6-9 COMPARATIVE EXAMPLE 0.34 0.22 0.76 0.023 0.017 0.030 0.15 0.01 8 1 4.4 0.13 0.55 4.4 6-10 COMPARATIVE EXAMPLE 0.50 0.23 0.87 0.022 0.014 0.015 0.10 0.01 8 6 1.3 0.5 0.16 0.3 6-11 COMPARATIVE EXAMPLE 0.44 0.18 0.70 0.025 0.019 0.016 0.05 0.00 9 2 0.0 0.22 0 0.0 6-12 COMPARATIVE EXAMPLE 0.33 0.19 0.69 0.020 0.010 0.023 0.04 0.01 12 14 0.0 1.17 0 0.0 6-13 COMPARATIVE EXAMPLE 0.26 0.20 0.46 0.021 0.009 0.002 0.04 0.01 9 1 0.0 0.11 0 0.0 7-1 INVENTION EXAMPLE 0.32 0.25 1.80 0.034 0.017 0.004 0.04 0.00 7 2 1.5 0.29 0.21 0.8 7-2 COMPARATIVE EXAMPLE 0.17 0.56 0.73 0.061 0.012 0.002 0.06 0.00 8 1 4.4 0.13 0.55 4.4 7-3 INVENTION EXAMPLE 0.16 0.52 0.75 0.070 0.015 0.001 0.05 0.00 6 1 4.0 0.17 0.57 4.0 7-4 INVENTION EXAMPLE 0.18 0.50 0.71 0.063 0.017 0.001 0.08 0.01 23 1 0.7 0.04 0.03 0.7 7-5 COMPARATIVE EXAMPLE 0.18 0.51 0.73 0.066 0.013 0.002 0.04 0.01 7 6 3.0 0.86 0.5 0.5

(145) TABLE-US-00003 TABLE 2-2 AUXILIARY RAW MATERIAL CONDITIONS (INDICATING EXTREME DIFFERENCE RATIO IN VALUE IN CA LEVEL) INCLUSIONS STATISTICS OF SLAG COMPOSITION (1) (2) (AVERAGE) INCLUSIONS IN SLAG ( MASS %) ADDITION CA IN REM (MASS %) DETER- No. CaO SiO.sub.2 Al.sub.2O.sub.3 TFe OF CaSi FeSi ADDITION Al.sub.2O.sub.3 REM.sub.2O.sub.3 MINATION M NOTE 1-1 INVENTION 50.6 7.9 26.6 0.7 NONE 0.05 ADDED 67 20.8 G 18.8 EXAMPLE BEARING 1-2 INVENTION 51.0 6.6 26.6 0.9 NONE 0.05 ADDED 64.3 20.1 G 21.0 STEEL EXAMPLE (SUJ2) 1-3 INVENTION 52.2 8.6 24.4 0.3 NONE 0.035 ADDED 64.7 19.5 G 20.2 EXAMPLE 1-4 INVENTION 51.3 8.8 25.4 0.4 NONE 0.042 ADDED 59.3 23. 1 G 24.0 EXAMPLE 1-5 COMPARATIVE 52.3 4.3 27.1 0.6 NONE 0.045 NONE 72.5 0 B 36.3 EXAMPLE 1-6 COMPARATIVE 49.8 9.1 26.8 0.3 NONE 0.038 NONE 78.9 0 B 31.3 EXAMPLE 2-1 INVENTION 43.7 16.1 28.2 0.9 NONE 0.039 ADDED 68.2 13.4 G 10.3 EXAMPLE SUSPENSION 2-2 COMPARATIVE 43.7 19.7 25.4 0.6 NONE 0.047 NONE 84.7 0 B 31.0 SPRING A EXAMPLE 2-3 COMPARATIVE 48.4 13.7 25.0 0.3 NONE 0.28 NONE 79.2 0 B 34.2 EXAMPLE 3-1 COMPARATIVE 46.1 14.6 28.4 0.6 NONE 0.041 NONE 80.1 0 B 36.0 EXAMPLE SUSPENSION 3-2 COMPARATIVE 43.6 13.6 27.8 0.5 NONE 0.052 NONE 84.6 0 B 33.3 SPRING B EXAMPLE 3-3 COMPARATIVE 43.5 12.9 27.9 0.8 NONE 0.031 NONE 81.3 0 B 30.5 EXAMPLE 3-4 COMPARATIVE 45.6 15.1 27.4 0.4 NONE 0.056 NONE 60.7 0 VB 39.0 EXAMPLE 3-5 COMPARATIVE 51.3 22.3 15.1 0.6 NONE 0.045 NONE 75.5 0 B 36.1 EXAMPLE 3-6 COMPARATIVE 47.3 16.0 23.4 0.7 NONE 0.039 NONE 85.6 0 VB 40.6 EXAMPLE 3-7 COMPARATIVE 42.9 15.5 25.6 0.7 NONE 0.03 NONE 45.1 0 VB 42.1 EXAMPLE 4-1 INVENTION 53.1 4.1 26.0 0.8 NONE 0.35 ADDED 55.7 7.1 G 22.0 EXAMPLE CASE 4-2 INVENTION 50.3 8.2 25.0 0.9 NONE 0.29 ADDED 51.8 10.5 G 27.0 HARDENING EXAMPLE STEEL 4-3 INVENTION 46.3 9.8 28.0 0.6 NONE 0.32 ADDED 60.4 18.3 G 17.6 EXAMPLE 4-4 INVENTION 50.8 8.0 26.3 0.6 NONE 0.05 ADDED 58.2 20.6 G 24.8 EXAMPLE 4-5 COMPARATIVE 51.0 8.2 25.9 0.6 NONE 0.05 ADDED 56.1 35.7 VB 38.0 EXAMPLE 4-6 COMPARATIVE 50.3 8.2 25.0 0.4 NONE 0.33 ADDED 54.8 31.9 VB 47.0 EXAMPLE 4-7 COMPARATIVE 47.8 13.4 26.2 0.3 NONE 0.31 NONE 84.5 0 VB 49.0 EXAMPLE 4-8 COMPARATIVE 49.2 7.3 22.4 0.5 NONE 0.042 ADDED 82.8 5.9 B 33.0 EXAMPLE 4-9 COMPARATIVE 51.0 5.8 21.3 0.4 USED 0.038 ADDED 48.6 10.4 VB 38.0 EXAMPLE 5-1 INVENTION 48.8 2.0 35.0 1.2 NONE ADDED 61.8 15.8 G 23.1 EXAMPLE ALUMINIUM 5-2 INVENTION 54.8 3.1 26.4 0.7 NONE ADDED 59.3 14.6 G 22.0 KILLED EXAMPLE STEEL 5-3 INVENTION 48.4 2.1 36.5 0.3 NONE ADDED 58.7 22.4 G 24.0 EXAMPLE 5-4 COMPARATIVE 49.3 5.1 31.0 0.9 NONE NONE 89.1 0 VB 51.0 EXAMPLE 5-5 COMPARATIVE 51.3 1.9 32.3 1.1 NONE NONE 90 0 VB 41.0 EXAMPLE 5-6 COMPARATIVE 57.3 3.0 22.4 1.2 NONE NONE 48.7 16.5 B 31.2 EXAMPLE 6-1 INVENTION 49.6 10.2 25.0 0.8 NONE 0.034 ADDED 67.3 18.7 G 22.9 EXAMPLE SC STEEL 6-2 INVENTION 49.6 10.2 25.0 0.7 NONE 0.042 ADDED 75.2 9.4 G 22.0 EXAMPLE 6-3 INVENTION 55.9 7.5 21.5 0.9 NONE 0.76 ADDED 69.7 15.4 G 17.0 EXAMPLE 6-4 INVENTION 46.4 15.3 22.3 0.6 NONE 0.38 ADDED 71.5 12 G 18.2 EXAMPLE 6-5 INVENTION 48.2 10.2 26.1 0.7 NONE 0.042 ADDED 70 18.9 G 26.1 EXAMPLE 6-6 INVENTION 49.2 9.8 26.3 0.7 NONE 0.04 ADDED 73.4 10.7 G 21.0 EXAMPLE 6-7 COMPARATIVE 49.4 9.6 26.5 0.7 NONE 0.045 ADDED 61.9 22.2 VB 50.0 EXAMPLE 6-8 COMPARATIVE 49.9 9.1 26.0 0.6 NONE 0.04 ADDED 53.8 23.7 VB 43.0 EXAMPLE 6-9 COMPARATIVE 48.5 9.9 25.8 0.8 NONE 0.041 ADDED 57.6 27.1 VB 90.0 EXAMPLE 6-10 COMPARATIVE 46.2 7.6 29.1 0.5 USED 0.45 ADDED 47.8 19.4 VB 38.0 EXAMPLE 6-11 COMPARATIVE 47.5 7.9 30.2 0.7 NONE 0.87 NONE 54.8 0 VB 62.0 EXAMPLE 6-12 COMPARATIVE 48.6 8.1 26.8 0.9 USED 0.39 NONE 59.7 0 VB 40.0 EXAMPLE 6-13 COMPARATIVE 50.3 5.9 28.8 0.3 NONE 0.56 NONE 80.9 0 B 37.0 EXAMPLE 7-1 INVENTION 48.5 6.3 29.1 0.8 NONE 0.34 ADDED 60.8 8.4 G 15.8 EXAMPLE OTHERS 7-2 COMPARATIVE 48.2 14.0 23.4 0.4 NONE 0.29 ADDED 72.5 10.2 VB 91.0 EXAMPLE 7-3 INVENTION 51.0 8.3 27.3 0.4 NONE 0.045 ADDED 52.1 45.3 G 29.4 EXAMPLE 7-4 INVENTION 49.2 6.9 30.8 0.4 NONE 0.034 ADDED 75.3 3.7 G 28.6 EXAMPLE 7-5 COMPARATIVE 50.2 7.6 28.9 0.4 USED 0.35 ADDED 40.1 25.6 VB 65.0 EXAMPLE

(146) In Table 2, the steel piece extreme value statistics of No. 1-1 are 18.8 m (<30 m). In No. 2-2, since REM is not added, the inclusions were not modified and the steel piece extreme value statistics were 31.0 m. In No. 2-2, fatigue fracture occurs from the inclusions.

(147) The steel piece extreme value statistics of the inclusions in the table were calculated using an extreme value statistical method in the following manner.

(148) That is, the invention steel was cast by a curved continuous casting machine, and then in a steel piece rolled at a surface reduction ratio of 1.8 or greater, a steel sample was collected from a portion at a position of from the loose surface side of an L-cross-section of the steel piece (a cross-section including a center line of a loose surface, a center line of a surface opposite thereto, and a center line of the steel piece) and the steel piece extreme value statistics were calculated based on an extreme value statistical method including measurement under the conditions of an test standard area of 100 mm.sup.2 (area of 10 mm10 mm), a test field of 16 (that is, the number of tests is 16), and an area for performing prediction of 30,000 mm.sup.2. The calculation was performed through the expression: urea (max)=(a.sup.2+b.sup.2).sup.1/2 where a is a major axis and b is a minor axis perpendicular to the major axis. Here, the loose surface is a surface on the upper surface side in a horizontal portion from a curved portion of the curved continuous casting machine.

(149) The estimation of the maximum predicted diameter (area (max)) of the inclusions using extreme value statistics is performed according to the method described in, for example, Metal Fatigue, Effects of Micro-Defects and Inclusions (written by Yukitaka Murakami, Yokendo, published in 1993, P. 223 to 239). The used method is a two-dimensional method of estimating maximum inclusions observed in a certain area by two-dimensional examination.

(150) By using the above-described extreme value statistical method, the maximum predicted diameter area (max) of the inclusions in the prediction area (30,000 mm.sup.2) was estimated from the test standard area (100 mm.sup.2) from the image of the nonmetallic inclusions imaged by an optical microscope. Specifically, 16 pieces of data (data of 16 fields) on maximum diameters of the inclusions obtained through the observation were plotted on extreme value probability paper in accordance with the method described in the document to obtain a maximum inclusion distribution straight line (linear function of the maximum inclusion and the extreme value statistical standardization variable), and the maximum inclusion distribution straight line was extrapolated to estimate a maximum predicted diameter area (max) of the inclusions in the area of 30,000 mm.sup.2.

(151) In addition, for the specification of the nonmetallic inclusions, observation was performed at 1,000-fold magnification using an optical microscope to discriminate the nonmetallic inclusions from a difference in contrast. The validity of the discrimination method using the difference in contrast was previously confirmed by a scanning electron microscope with an energy dispersive X-ray spectroscopic analyzer. A plurality of inclusions was analyzed to obtain an average composition ratio of the inclusions.

Example 2

(152) One of characteristics required for a steel to which the invention steel is applied is contact fatigue properties such as rolling fatigue properties and surface fatigue properties. Therefore, evaluation of radial rolling fatigue properties was performed in the following manner.

(153) Cast steel pieces obtained from a plurality of molten steels based on components of steel type of SUJ2, in which Ca, REM, T.O, etc. were changed so as to have different maximum predicted diameters of inclusions, were held for 25 hours to 30 hours at 1200 C. to 1250 C. in a heating furnace, and cementite spheroidizing was performed. Then, blooming was performed at 1000 C. to 1200 C. The obtained steel pieces were heated at 900 C. to 1200 C. and rolled up to 65 mm to provide a material of a radial rolling fatigue test piece.

(154) FIG. 7 shows an aspect of the production of the radial rolling fatigue test piece. FIG. 7(a) shows the shape of the material of the radial rolling fatigue test piece, FIG. 7(b) shows an aspect of the collection of the radial rolling fatigue test piece, and FIG. 7(c) shows a final shape of the collected radial rolling fatigue test piece.

(155) From the material of the radial rolling fatigue test piece (hereinafter, referred to as test piece) of 65 mm, a round bar (having a center hole at both ends and a through hole of 3 mm in one end portion at a position separate from the end surface by 5 mm) having the shape (: 12.2 mm, length: 150 mm) shown in FIG. 7(a) was produced.

(156) This round bar was heated for 30 minutes at 840 C. in an induction heating furnace, and then subjected to quench with oil at 50 C. Thereafter, it was annealed for 90 minutes at 180 C. and air-cooled. From the round bar after the heat treatment, both ends of the round bar were discarded as shown in FIG. 7(b), and four 22 mm-test pieces having the final shape shown in FIG. 7(c) were collected from a center portion thereof and provided to the radial rolling fatigue test.

(157) The radial rolling fatigue test was performed on 12 test pieces under the conditions of a test load of 600 kgf, a repetition rate of 46240 cpm, and the number of times of stopping of 110.sup.8 using a radial rolling fatigue test machine (product name: cylindrical fatigue life test machine manufactured by NTN Corporation).

(158) FIG. 8 shows the relationship between the maximum predicted diameter (steel piece extreme value statistics) of each test piece, obtained through the extreme value statistical method, and the shortest breaking life obtained through the radial rolling fatigue test. 810.sup.7 or greater of the shortest breaking life is obtained when the steel piece extreme value statistics are 30 m or less.

Example 3

(159) Next, an Ono-type rotary bending test was performed to evaluate rotary bending fatigue properties. FIG. 9 shows a shape of a test piece produced for evaluation of the rotary bending fatigue properties.

(160) Using a test piece produced with dimensions shown in FIG. 9, the Ono-type rotary bending test was performed. The test piece was subjected to induction hardening (frequency: 100 kHz). Tap water or a polymer quenching catalyzer was used as a refrigerant in the induction hardening. After hardening, a tempering treatment was performed for 1 hr at 150 C. Table 3 shows the test results, and FIG. 10 shows the relationship between maximum stress and the number of times of endurance.

(161) TABLE-US-00004 TABLE 3 FATIGUE DURABILITY ONO-TYPE ROTARY BENDING ONO-TYPE ROTARY BENDING ONO-TYPE ROTARY BENDING FATIGUE TEST DURABLE FATIGUE TEST DURABLE FATIGUE TEST DURABLE IN 3 10.sup.6 IN 3 10.sup.6 IN 3 10.sup.6 (STRESS: 600 MPa) (STRESS: 800 MPa) (STRESS 900 MPa) FRACTURE FRACTURE FRACTURE STARTING STARTING STARTING No. CLASSIFICATION EVALUATION POINT EVALUATION POINT EVALUATION POINT 2-1 INVENTION STEEL DURABLE DURABLE BREAKAGE SURFACE DURING TEST INVENTION STEEL DURABLE DURABLE BREAKAGE SURFACE DURING TEST INVENTION STEEL DURABLE DURABLE BREAKAGE SURFACE DURING TEST 2-2 COMPARATIVE STEEL DURABLE BREAKAGE INCLUSIONS BREAKAGE INCLUSIONS DURING TEST DURING TEST COMPARATIVE STEEL DURABLE BREAKAGE INCLUSIONS BREAKAGE INCLUSIONS DURING TEST DURING TEST COMPARATIVE STEEL DURABLE BREAKAGE INCLUSIONS BREAKAGE INCLUSIONS DURING TEST DURING TEST

(162) From Table 3 and FIG. 3, it is found that the invention steel has much better rotary bending fatigue properties than the comparative steel.

(163) As described above, the invention steel has much better fatigue properties than the conventional steel. Therefore, it is apparent that the life of a steel product produced from the invention steel increases significantly.

(164) The improvement in mechanical characteristics of the invention steel was verified while paying attention to fatigue properties having a large influence on inclusions and a decrease in size of nonmetallic inclusion was confirmed in all of the target steels. Accordingly, in the invention steel, in addition to the fatigue properties, mechanical characteristics (toughness, ductility, etc.) necessary for casting, pressing, and other processings are also presumed to be improved.

INDUSTRIAL APPLICABILITY

(165) As described above, according to the invention, high-melting-point Al.sub.2O.sub.3-REM oxide which has CaOAl.sub.2O.sub.3-based inclusions modified by adding a minute amount of REM to Al-deoxidized molten steel or AlSi-deoxidized molten steel and hardly aggregates, and fine nonmetallic inclusions containing REM sulfide, MgO, or both of REM sulfide and MgO exist in the steel. Accordingly, steel having excellent fatigue properties can be provided and an improvement in other mechanical characteristics can also be expected. As a result, since the life of a steel product produced from the steel of the invention increases significantly, the invention is highly applicable in steel production industries and in steel working industries.