Abstract:
Sulfur-containing free-cutting steel may have an improved machinability and fewer defects by adding certain amounts of Te, Pb and Bi to prevent elongation of sulfide inclusion particles, and by lowering Al-content to decrease Al 2  O 3  of oxide inclusions or by lowering O-content to decrease large Al 2  O 3  inclusion particles. The free-cutting steel may be produced by continuous casting.

Description:
This application is a continuation of application Ser. No. 608,622, filed May 9, 1984, now abandoned. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention concerns an improved free-cutting steel and process for producing. 
     2. State of the Art 
     In the production of a low carbon sulfur free-cutting steel called &#34;ultra free-cutting steel&#34;, the general practice has been to increase the oxygen content of the steel so that the shape of the sulfide inclusion particles may be spheroidal. On the other hand, a higher oxygen content causes a large amount of oxide inclusion, which increases surface defects of the steel, resulting in lowered strength and poor appearance. If the oxygen content is reduced by using a deoxidizing agent, the sulfide inclusion particles become elongated, and machinability decreases. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a sulfur-containing free-cutting steel with a low carbon in which the sulfide inclusion particles are large and spheroidal in shape so that good machinability may be maintained and so that a steel with few defects may be produced. 
     Another object of the present invention is to provide a suitable method for making the above-mentioned improved steel. 
     One aspect of the present invention is based on our discovery that, if not only S, but also Te, Pb and Bi are added to the steel and the Al content is lowered to decrease Al-based inclusion in the oxided inclusions, there may be obtained, even if the oxygen content is within a certain range, a steel of good machinability and fewer defects. 
     Another aspect of the present invention is also based on our discovery that, if not only S, but also Te, Pb and Bi are added to the steel to control the elongation of the sulfide inclusion particles, and Al is used as the deoxidizing agent for RH (radio high frequency) degassing or LF (ladle furnace) refining to lower the oxygen content so as to decrease large Al 2  O 3  inclusion particles, there may be obtained a steel of good machinability and fewer defects. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The first embodiment of the present invention is a steel which comprises C: up to 0.2%, Si: up to 0.2% and Mn: up to 2.0%; P: not higher than 0.1%, N: not higher than 0.02%, and Al: not higher than 0.002%; and further comprises S: 0.04 to 0.50%, Te: 0.002 to 0.50%, Pb: 0.01 to 0.4% and Bi: 0.01 to 0.40%, Te+Pb+Bi being 0.20% or more; and O: 0.0040 to 0.030%, with the balance being substantially Fe; MnS inclusion particles having a length (L) of 5 micron or more, a width (W) of 2 micron or more and an aspect ratio (L/W) of 5 or less being at least 50% of all the MnS inclusion particles in the steel; and Al 2  O 3  being in average 15% or less of the oxide inclusion. 
     The second embodiment of the present invention is a steel which comprises C: up to 0.2%, Si: up to 0.2% and Mn: up to 2.0%; P: not higher than 0.10%, N: not higher than 0.02%, and O: not higher than 0.010%; Al: more than 0.002% up to 0.060%; and further comprises S: 0.04 to 0.50%, Te: 0.002 to 0.50%, Pb: 0.01 to 0.40% and Bi: 0.01 to 0.40%, Te+Pb+Bi being 0.20% or more, and the balance being substantially Fe; MnS inclusion particles having a length (L) of 5 micron or more, a width (W) of 1 micron or more and an aspect ratio (L/W) of 1 or less being at least 50% of all the MnS inclusion particles in the steel; and a percentage of large 5 micron or longer Al 2  O 3  particles being not higher than 0.01% of the oxide inclusion. 
     The roles of the alloying elements and the significance of the composition are as follows: 
     C: up to 0.2% 
     Carbon impairs suitable hardness of the steel. In this kind of free-cutting steel, 0.2% of carbon is the upper limit for good machinability. A content not exceeding 0.1% is preferable. 
     Si: up to 0.2% 
     If the amount of Si exceeds 0.2%, the effect of alloying Te+Pb+Bi, in addition to S, decreases, and the hardness of the material becomes too high. 
     Mn: up to 2.0% 
     From the viewpoint of machinability, the lower the content of Mn, the better. In order to improve hot workability, Mn is added in an amount up to 2.0%. 
     P: not higher than 0.10% 
     P is favorable for machinability. However, too high a content causes hardening of the matrix and decreased hot workability. 
     N: not higher than 0.02% 
     Like P, N hardens the matrix, and therefore, the content should not exceed the above upper limit. 
     S: 0.04 to 0.50% 
     S takes a major role of improving machinability. A content of 0.04% or higher is required, and the upper limit, 0.5%, is chosen in view of the influence on hot workability. 
     Te: 0.002 to 0.50%, Pb: 0.01 to 0.40%, Bi: 0.01 to 0.40%, Te+Pb+Bi: 0.20% or higher. 
     The above elements form low melting-point inclusions to spheroidize the sulfide inclusion particles. The effect becomes remarkable as a result of the compounding these additives. To ensure this remarkable effect, it is necessary to add these elements in amounts in the above respective ranges and at least 0.20% in total. The upper limits are chosen from the viewpoint of hot workability. 
     Al: in the first embodiment, up to 0.002%, and in the second embodiment, 0.002 to 0.06% 
     As noted above, it is one of the features of the steel in the first embodiment to contain a very small amount of Al. The content is controlled to be 0.002% or less so that the amount of Al 2  O 3  in the oxide inclusion, which shortens tool life, may be within the limit mentioned later. 
     In the second embodiment, Al of less than 0.002% is insufficient to be effective as an deoxidation agent. 
     On the other hand, addition of more than 0.060% cannot produce further effects as an deoxidation agent and increases the amount of Al 2  O 3 . 
     O: in the first embodiment, 0.004 to 0.030%, and in the second embodiment, 0.010% or less. 
     In order to decrease Al 2  O 3 , which is a high-hardness oxide hastening tool abrasion, it is preferable to lower the oxygen content. The deoxidation should be so through that the oxygen content may be 0.010% or less. If the oxygen content exceeds this limit, there will be a significant amount of the large Al 2  O 3  inclusion particles. A preferable content is 0.005% or less. 
     With respect to the shape and size of the sulfide inclusion particles, the nearer the shape is to be a spheroid and the larger their amount, the better. In the first embodiment, if the percentage (volume) of large spheroidal sulfide particles having a length of 5 micron or more, a width of 2 micron or more and an aspect ratio (L/W) of 5 or less is 50% or more of the total sulfide inclusion, then the machinability-improving effect will reach a satisfactory level. In the second embodiment, if the percentage (volume) of large spheroidal sulfide inclusion particles having a length of 5 micron or more, a width of 1 micron or more and an aspect ratio of 7 or less is 50% or more of the total sulfide inclusion, then the machinability-improving effect will reach a satisfactory level. 
     The oxide inclusions are, in the first embodiment, mainly MnO, SiO 2  and FeO. If Al 2  O 3  is contained in a large amount, it significantly abrades cutting tools due to its high hardness. Al content should be, therefore, as low as possible in the above-noted range to limit the percentage of Al 2  O 3  in the oxide inclusions to not higher than 15%. In the second embodiment, the main oxide inclusion is Al 2  O 3  because of Al-deoxidation. Large Al 2  O 3  inclusion particles having a diameter of 5 micron or more seriously shorten tool life, and the amount should be 0.01% or less. 
     The above-described free-cutting steel of the present invention may be produced by any process. It is one of the merits of the present free-cutting steel that a steel of high quality may be produced by continuous casting, which is being widely practiced because of high productivity. Generally, because continous casting is carried out under a cooling rate higher than that of conventional ingot-casting, the sulfide inclusion particles in the steel tend to be fine, and it has been difficult to improve machinability of the continuously-cast steel. According to the present invention, the sulfide inclusion particles become large spheroids, and continuous casting may be employed. In the case of producing the free-cutting steel by continuous casting, it is preferable to add the above-noted machinability-improving elements, S, Te, Pb, Bi, to the molten steel in a tundish, because the yields of these elements are high, and floating up of Al 2  O 3  clusters is promoted. 
    
    
     EXAMPLES 
     The following examples illustrate the present invention and prove the merits thereof. 
     Example I and Control I 
     Steels of the compositions shown in Table 1 were produced in a 70 ton arc furnace. The machinability-improving elements were added to the molten steels in the manners indicated below: 
     A: added into the furnace or into a ladle, 
     B: added by the Gazzar method, i.e., thrown onto the surface of the molten steel exposed by inert-gas bubbling, 
     C: added into a nozzle, and 
     D: added into a tundish. 
     The molten steels were cast by the methods below: 
     Examples 1, 2 and 3, and Controls A, B and C . . . ingot-casting (6.5 ton) 
     Examples 4, 5, and control D . . . continuous casting. 
     The cast steels were subjected to rough rolling, wire rolling, and drawing and straightening to form round rods of 11 mm diameter. 
     The samples were analyzed to determine the shape of the sulfide inclusion particles and the percentage of Al 2  O 3  in the oxide inclusions. The shape of the sulfide inclusion particles was analyzed with an image-analyzer using test pieces prepared for microscopic observation, and the oxide inclusion particles were analyzed with an EPMA. The term &#34;large spheroidal sulfide inclusion particle&#34; means, as mentioned above, the particle having a length of 5 micron or more, a width of 2 micron or more, and an aspect ratio (L/W) of 5 or less. The percentage is expressed by volume as mentioned above. It is known that the volume percentage corresponds to the areal percentage observed by the analysis, and therefore, the data of the areal percentage are shown in Table 2. 
     
                                           TABLE 1__________________________________________________________________________No.       C  Si Mn P  S  Al  N  O   Te Pb Bi Te + Pb + Bi__________________________________________________________________________(Present Invention)1         0.06        0.012           1.00              0.065                 0.311                    0.0010                        0.009                           0.0152                               0.042                                  0.252                                     0.092                                        0.386                 (A)           (C)                                  (B)                                     (C)2         0.11        0.005           1.25              0.44                 0.250                    0.0007                        0.011                           0.0211                               0.015                                  0.200                                     0.124                                        0.339                 (A)           (B)                                  (B)                                     (B)3         0.15        0.008           1.14              0.055                 0.273                    0.0015                        0.008                           0.0060                               0.053                                  0.340                                     0.050                                        0.443                 (A)           (C)                                  (B)                                     (C)4         0.08        0.152           1.30              0.075                 0.350                    0.0005                        0.006                           0.0093                               0.040                                  0.280                                     0.120                                        0.440                 (A)           (D)                                  (D)                                     (B)5         0.09        0.010           1.21              0.068                 0.314                    0.0008                        0.007                           0.0105                               0.045                                  0.295                                     0.086                                        0.426                 (D)           (D)                                  (D)                                     (D)(Control)A         0.08        0.035           1.10              0.065                 0.305                    0.0041                        0.008                           0.0350                               -- 0.250                                     -- 0.250                 (A)              (B)B         0.09        0.012           1.08              0.072                 0.302                    0.0205                        0.008                           0.0030                               -- 0.150                                     0.100                                        0.250                 (A)              (B)                                     (B)C         0.10        0.052           1.15              0.052                 0.296                    0.0015                        0.005                           0.0420                               0.030                                  -- 0.050                                        0.080                 (A)           (C)   (C)D         0.08        0.026           1.05              0.068                 0.333                    0.0092                        0.009                           0.0380                               -- -- -- --__________________________________________________________________________ 
    
     
                       TABLE 2______________________________________    Large     Average    Spheroidal              Diameter  Average    Sulfide   of        of      Al.sub.2 O.sub.3    Inclusion Sulfide   Aspect  in    Particles Inclusion Ratio   OxideNo.      (%)       (micron)  L/W     Inclusion______________________________________(PresentInvention)1        72        5         3.5     3.02        81        6         3.2     2.13        78        6         3.8     0.94        84        4         2.9     1.55        77        5         3.0     1.8(Control)A        24        1.5       6.0     15B        32        1.2       5.9     58C        35        1.6       5.3     3D         5        0.8       13      25______________________________________ 
    
     Machinability of the samples was determined. It was evaluated by means of processability when machined by a lathe, i.e., the extent of processing at a certain tool life, and expressed as indices based on the processability of the steel of &#34;Control D&#34;, which had the lowest precessability. The data are given in Table 3. Table 3 shows the good machinability of the present steel. 
     
                       TABLE 3______________________________________No.             Machinability Index______________________________________(Present Invention)1               2002               2003               2204               1955               197(Control)A               130B               140C               135D               100______________________________________ 
    
     Example II and Control II 
     The steels of the compositions shown in Table 4 were produced in a 70 ton arc furnace. The manners of adding machinability-improving elements are shown with the references which are the same as in Example 1. 
     The molten steels were cast by the following methods: Examples 6 and 7, and Controls E and F . . . ingot casting (6.5 ton). 
     Examples 8 and 9, and Control G . . . continuous casting. The steels were subjected to rough rolling, wire rolling drawing and straightening to form round rods of 11 mm diameter. 
     Table 5 shows the results of analyzing the shape of the sulfide inclusion particles and Al 2  O 3  in the oxide inclusion particles. The shape of the sulfide inclusion particles was analized with an image analyzer using test pieces prepared for microscopic observation, and the Al 2  O 3  was analyzed by Br-Met extraction analysis. The term &#34;large spheroidal sulfide inclusion particle&#34; means, also as mentioned above, the particle having a length of 5 micron or more, a width of 1 micron or more, and an aspect ratio (L/W) of 7 or less. The percentage is expressed by volume, as mentioned above. 
     
                                           TABLE 4__________________________________________________________________________No.       C  Si Mn P  S  Al  N  O   Te Pb Bi Te + Pb + Bi__________________________________________________________________________(Present Invention)6         0.08        0.05           1.12              0.071                 0.308                    0.015                        0.008                           0.0034                               0.040                                  0.212                                     0.095                                        0.347                 (A)           (C)                                  (B)                                     (C)7         0.08        0.03           1.15              0.065                 0.315                    0.030                        0.009                           0.0020                               0.039                                  0.253                                     0.111                                        0.403                 (A)           (B)                                  (B)                                     (B)8         0.09        0.15           1.08              0.058                 0.324                    0.022                        0.006                           0.0030                               0.042                                  0.204                                     0.150                                        0.396                 (A)           (D)                                  (D)                                     (D)9         0.10        0.02           1.22              0.069                 0.299                    0.008                        0.005                           0.0034                               0.048                                  0.199                                     0.088                                        0.335                 (A)           (D)                                  (D)                                     (D)(Control)E         0.08        0.01           1.05              0.063                 0.306                    0.002                        0.009                           0.0255                               -- 0.204                                     0.071                                        0.275                 (A)              (B)                                     (B)F         0.09        0.05           1.06              0.070                 0.333                    0.015                        0.005                           0.0085                               0.042                                  -- 0.055                                        0.097                 (A)              (C)                                     (C)G         0.08        0.02           1.14              0.066                 0.314                    0.010                        0.008                           0.0099                               -- -- -- --                 (A)__________________________________________________________________________ 
    
     
                       TABLE 5______________________________________    Large     Average    Spheroidal              Diameter  Average Al.sub.2 O.sub.3    Sulfide   of        of      of    Inclusion Sulfide   Aspect  Diameter    Particles Inclusion Ratio   5 orNo.      (%)       (micron)  L/W     more______________________________________(PresentInvention)6        76        5         4       0.00557        85        5         4       0.00308        82        5         4       0.00489        83        4         5       0.0053(Control)E         9        0.8       10      0.0044F        15        0.7       9       0.0150G         5        0.3       5       0.0162______________________________________ 
    
     
                       TABLE 6______________________________________No.             Machinability Index______________________________________(Present Invention)6               2007               2098               2009               199(Control)E               125F               118G               100______________________________________