Abstract:
An ultralow yield strength steel consisting of: from 0.01 to 0.25 wt % of hexagonal boron nitride particles having an average diameter of from 1 to 30 μm; and the balance consisting of iron and unavoidable impurities.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a steel having an ultralow yield strength. A damping structure is necessary to suppress and control the vibration of an architectural structure when subjected to earthquake, wind and other disturbance in order to ensure the safety and comfort of an architectural structure. The steel according to the present invention is very useful in many applications as a structural steel having a high ability to absorb vibration. 
     2. Description of the Related Art 
     To provide a low yield strength, attempts have been made to attain a chemical composition as close as possible to that of pure iron, because alloying elements in steel increase the yield strength of the steel. For example, a yield strength of 98 MPa was attained by a sheet product of pure iron (C: 10 ppm, N: 10 ppm, P: 10 ppm, S: 10 ppm) produced by a vacuum refining and casting, as described in &#34;Zairyoo To Purosesu (Material and Process)&#34;, vol. 2, 1989, p 2021. To further lower the yield strength, the impurity amount must be minimized. A yield strength of 40 to 50 MPa was attained by a pure iron having a purity of 99.999% or more refined by an ultrahigh vacuum electron beam floating zone melting process, as reported in &#34;Bulletin of the Japan Institute of Metals&#34;, vol. 24, No. 5, 1985, p 376-385. 
     Hexagonal boron nitride particles are introduced into a steel by a process disclosed in Japanese Patent Application No. 3-143602, in which a powder mixture is prepared by mixing a powder of hexagonal boron nitride with an iron powder and is then admixed to a molten steel. 
     The steel described in &#34;Zairyoo To Purosesu&#34;, vol. 2, 1989, p 2021 cannot be used as a structural steel for architectural structures, because the yield strength is not sufficiently low and because the product is in the form of a sheet. Although the steel reported in &#34;Bulletin of the Japan Institute of Metals&#34;, vol. 24, No. 5, 1985, pages 376-385 has a sufficiently low yield strength, the use of the ultrahigh vacuum electron beam floating zone melting process not only significantly raises the production cost but also is unsuitable for mass production. The steel disclosed in Japanese Patent Application No. 3-143602 is a free-cutting steel, which is different in application from an ultralow yield strength steel. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an ultralow yield strength steel having a high ability to absorb vibration at a relatively small expense. 
     The present inventors have found that the yield strength of a steel is lowered by dispersion of hexagonal boron nitride particles in the steel and invented the steels defined in (1), (2) and (3) as follows. 
     (1) An ultralow yield strength steel consisting of: 
     0.01 to 0.25 wt % of hexagonal boron nitride particles having an average diameter of from 1 to 30 μm; and 
     the balance consisting of iron and unavoidable impurities. 
     (2) An ultralow yield strength steel consisting of: 
     0.01 wt % or less of C; 
     0.10 wt % or less of Si; 
     0.2 wt % or less of Mn; 
     0.05 wt % or less of P; 
     0.05 wt % or less of S; 
     0.05 wt % or less of Al; 
     0.01 wt % or less of soluble N; 
     0.01 to 0.25 wt % of hexagonal boron nitride particles having an average diameter of from 1 to 30 μm; and 
     the balance consisting of iron and unavoidable impurities. 
     (3) An ultralow yield strength steel consisting of: 
     0.01 wt % or less of C; 
     0.10 wt % or less of Si; 
     0.2 wt % or less of Mn; 
     0.05 wt % or less of P; 
     0.05 wt % or less of S; 
     0.05 wt % or less of Al; 
     0.01 to 0.1 wt % of Ti; 
     0.01 wt % or less of soluble N; 
     0.01 to 0.25 wt % of hexagonal boron nitride particles having an average diameter of from 1 to 30 μm; and 
     the balance consisting of iron and unavoidable impurities. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The components of the steel of the present invention have the following effects, respectively. 
     Carbon (C), when dissolved in steel, remarkably increases the yield strength, and therefore the carbon content must not be more than 0.01 wt %. To lower the yield strength, the carbon content is preferably as small as possible when acceptable from the economical point of view. 
     Silicon (Si) increases the yield strength, and therefore the silicon content must not be more than 0.1 wt %. To lower the yield strength, the silicon content is preferably as small as possible. 
     Manganese (Mn) increases the yield strength, and therefore the manganese content must not be more than 0.1 wt %. To lower the yield strength, the manganese content is preferably as small as possible. 
     Phosphorus (P) not only increases the yield strength but also causes embrittlement, and therefore the phosphorus content must not be more than 0.05 wt %. To lower the yield strength, the phosphorus content is preferably as small as possible. 
     Sulfur (S) not only increases the yield strength but also causes embrittlement, and therefore the sulfur content must not be more than 0.05 wt %. To lower the yield strength, the sulfur content is preferably as small as possible. 
     Aluminum (Al) increases the yield strength, and therefore the aluminum content must not be more than 0.05 wt %. To lower the yield strength, the aluminum content is preferably as small as possible. 
     Soluble nitrogen (sol. N) remarkably increases the yield strength, and therefore the sol. N content must not be more than 0.01 wt %. To lower the yield strength, the sol. N content is preferably as small as possible. 
     Hexagonal boron nitride (h-BN) is easy to deform in a shear manner, so that particles of h-BN dispersed in a matrix of iron first deform in a shear manner under an externally applied stress as small as several MPa and causes stress concentration at the interface between the iron matrix and the h-BN particles. This stress concentration generates a raised stress greater than the yield strength of the matrix iron, so that the iron matrix subjected to this raised local stress begins to deform in a shear manner even when the applied nominal stress is smaller than the yield strength of the matrix iron. Namely, the steel composed of the iron matrix and the h-BN particles dispersed in the matrix has an effective yield strength smaller than the nominal yield strength of the matrix iron. 
     This effect is significant when the h-BN content in steel is 0.01 wt % or more and the h-BN particles have an average diameter of 1 μm or more. Either when the h-BN content is more than 0.25 wt % or when the h-BN particles have an average diameter of greater than 30 μm , the elongation of the steel becomes too small to be acceptable for structural use. Therefore, the h-BN content in steel must be within the range of from 0.01 to 0.25 wt % and the h-BN particles must have an average diameter within the range of from 1 to 30 μm. 
     The h-BN content can be determined from the area fraction of h-BN particles observed in a cross-section of the steel, as expressed by the following formula (1): 
     
         [h-BN content](wt %)=(2.34/7.86)×[h-BN area fraction](1) 
    
     The average diameter of h-BN particles is defined by the average cross-sectional area of h-BN particles observed in a cross section of the steel, as expressed by the following formulae (2) and (3): 
     
         [h-BN average diameter](μm)=square root of [h-BN average cross-sectional area in μm.sup.2 ]                                     (2) 
    
     
         [h-BN average cross-sectional area]=[total area observed]×[h-BN cross-sectional area]÷[total number of h-BN particles](3) 
    
     where [h-BN cross-sectional area]is determined based on JIS G 0555 and has the same value as that of the JIS G 0555 index of cleanliness of the steel regarding the h-BN &#34;non-metallic inclusion&#34;. 
     Titanium (Ti) fixes solute C and N by forming TiC and TiN, thereby reducing the contents of solute C and N, and consequently lowers the yield strength. This effect is significant when Ti is present in an amount of 0.01 wt % or more. A Ti content of more than 0.1 wt %, however, causes coarsening of TiC and TiN and the resulting reduction of the elongation to a level not acceptable in structural use. Therefore, the Ti content must be within the range of from 0.01 to 0.1 wt %. Ti may be added, when necessary. 
    
    
     EXAMPLE 
     Steels according to the present invention were produced by preparing a steel melt in an induction furnace, adding a deoxidizer-Al to the melt, then immediately introducing into the melt an iron pipe enclosing a powder mixture of h-BN and iron, and then tapping the melt one and a half minutes after the powder introduction. The steels were cast in 50-kg ingots having compositions shown in Tables 1 and 2. The ingots were heated at 1200° C., hot-rolled to 50 mm thick plates, and subjected to a normalizing heat treatment at 900° C. for 1 hour followed by air-cooling to room temperature. 
     To determine the yield strength of the steel plates, JIS No. 4 tensile test specimens were taken in the C-direction from the 1/4 thickness portion of the plates. The tensile test data are summarized in Table 2. 
     Steels 1 to 15 according to the present invention had an ultralow yield strength of less than 90 MPa. Comparison among the present inventive steels 12, 13, and 14 shows that the more the h-BN content, the lower the yield strength. Comparison between steels 12 and 15 also shows that the Ti addition further reduces the yield strength. 
     Comparative steels 16 to 26, having chemical compositions and/or h-BN average diameters outside the claimed ranges, had yield strengths greater than those of inventive steels 1 to 15. 
     Comparative steel 22 having an excessive amount of h-BN and comparative steel 24 having an h-BN average diameter greater than 30 μm both had a yield strength of greater than 100 MPa and a very low elongation. 
     
                                           TABLE 1__________________________________________________________________________Chemical Compositions of Steels (wt %)Steels   C   Si  Mn  P   S   Al  sol. N__________________________________________________________________________Invention   1    0.009        0.028            0.180                0.003                    0.002                        0.028                            0.009   2    0.005        0.059            0.150                0.008                    0.003                        0.049                            0.004   3    0.001        0.081            0.149                0.007                    0.005                        0.042                            0.003   4    0.005        0.028            0.127                0.007                    0.005                        0.022                            0.002   5    0.008        0.097            0.151                0.008                    0.004                        0.025                            0.002   6    0.001        0.073            0.122                0.009                    0.044                        0.016                            0.008   7    0.001        0.042            0.165                0.042                    0.002                        0.002                            0.008   8    0.002        0.057            0.117                0.022                    0.003                        0.022                            0.005   9    0.004        0.069            0.192                0.007                    0.005                        0.045                            0.004  10    0.008        0.078            0.166                0.009                    0.005                        0.032                            0.005  11    0.009        0.036            0.197                0.008                    0.005                        0.035                            0.004  12    0.005        0.048            0.060                0.006                    0.004                        0.009                            0.006  13    0.005        0.049            0.056                0.005                    0.004                        0.010                            0.006  14    0.005        0.049            0.057                0.005                    0.004                        0.009                            0.006  15    0.005        0.047            0.060                0.005                    0.004                        0.009                            0.007Comparison  16    *0.012        0.100            0.155                0.014                    0.005                        0.025                            0.004  17    0.005        *0.110            0.164                0.006                    0.005                        0.026                            0.006  18    0.008        0.057            *0.234                0.048                    0.003                        0.033                            0.009  19    0.001        0.069            0.137                *0.068                    *0.055                        0.044                            0.009  20    0.001        0.078            0.195                0.005                    0.006                        *0.061                            0.008  21    0.002        0.087            0.176                0.004                    0.004                        0.021                            *0.020  22    0.004        0.048            0.197                0.004                    0.011                        0.022                            0.005  23    0.008        0.099            0.060                0.006                    0.013                        0.023                            0.004  24    0.009        0.049            0.056                0.008                    0.005                        0.024                            0.005  25    0.005        0.057            0.057                0.007                    0.004                        0.026                            0.006  26    0.010        0.044            0.114                0.008                    0.003                        0.024                            0.008__________________________________________________________________________ (Asterisked data are outside the present inventive range.) 
    
     
                       TABLE 2______________________________________Chemical Compositions and Tensile Data   h-BN         content ave. dia.                        Ti    Y.P.  ElongationSteels        (wt %)  (μm)                        (wt %)                              (MPa) (%)______________________________________Invention    1    0.135   28     --    88    61    2    0.125   12     --    81    63    3    0.144   18     0.011 79    58    4    0.051   15     0.012 85    58    5    0.201   5      0.045 65    66    6    0.177   7      0.097 78    57    7    0.133   8      0.033 86    55    8    0.100   4      0.034 85    55    9    0.039   1      0.025 77    55   10    0.041   9      0.026 79    60   11    0.099   10     0.074 82    61   12    0.019   8      --    89    61   13    0.080   8      --    75    63   14    0.233   8      --    54    54   15    0.020   8      0.051 78    64Com-    16    0.144   7      0.065 102   45parison 17    0.104   6      0.043 97    60   18    0.131   7      0.055 115   45   19    0.045   7      0.045 121   53   20    0.133   12     --    122   60   21    0.154   15     --    105   61   22    *0.28   15     --    105   30   23    *0.008  14     --    160   58   24    0.124   *45    --    123   21   25    0.135   *0.8   --    134   55   26    0.150   13     *0.150                              118   59______________________________________ (Asterisked data are outside the present inventive range.) 
    
     As described herein above, the present invention provides a steel having an ultralow yield strength. The present inventive steel is applicable as a structural steel exhibiting a very high ability to absorb vibration, and accordingly, is very useful in many industrial fields, particularly for architectural use.