Abstract:
Disclosed is a high strength spring steel consisting of, in weight percentage, 0.50 to 0.70% C, 1.00 to 2.50% Si, 0.30 to 1.20% Mn, 0.80 to less than 1.20% Cr, 0.05 to 0.3% Mo, 0.05 to 0.30% V, 0.01 to 0.30% Nb, 0.005 to 0.100% Al and the balance being Fe and unavoidable impurities. The steel of the present invention has a high hardness coupled with high toughness and is very useful, especially for springs used in suspension devices or other various industrial machines.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a high strength spring steel useful in cars, aircraft, various industrial machines, etc. 
     2. Description of the Prior Art 
     In recent years, weight reduction has been strongly demanded in cars for lowering the cost of fuel. The same demand has also been growing in various structural parts or members including suspension devices. One possible approach for the reduction of weight of suspension devices is to increase the designed stress of suspension springs. In other words, strengthening the springs is effective as a weight-reducing measure. Currently, Si-Mn type steel, designated SUP 7, and Si-Cr type steel, designated SUP 12, are mainly used as steel stock for suspension springs. In order to increase the designed stress of these known spring steels, it is necessary to strengthen them. In general, the strength of steel materials is closely correlated with their hardness and strengthening means increasing the hardness. However, there is a problem that when the hardness of the spring steels is increased, the toughness (Charpy impact values, etc.) is also reduced. More specifically, a reduction in toughness is unavoidable in obtaining a hardness higher than that may be achieved in spring steels in current use. Therefore, when the hardness is increased for the purpose of improving the strength, the toughness must also be higher than that of currently available steels to ensure a sufficient reliability. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a high strength spring steel which has higher strength and toughness than spring steels currently used. 
     The influences of various elements on the hardness and toughness of spring steels were studied by the present inventors and the following equations were obtained. Percentages (%) of the respective elements shown in the equations are by weight. 
     When the Mn content is in the range of 0.30 to less than 0.50%, ##EQU1## 
     The above relations are applicable to a sample steel which has been subjected to a sufficient martensitic transformation by quenching and then tempered at 400 ° C. 
     From the above result, it has been found that alloying elements are very closely related to the properties of hardness and toughness. In detail, it has been found that an increased hardness can be achieved by controlling the alloying elements C, Si, Mn, Cr, Mo, V, Nb and Al and a high toughness can be achieved by controlling alloying elements of Mo, V and Nb. 
     When the Mn content is in the range of 0.50 to 1.20%, ##EQU2## 
     The above relations are applicable to a sample steel which has been subjected to a sufficient martensitic transformation by quenching and then tempered at 380° C. 
     From the above result, it has been found that alloying elements are very closely related to properties of hardness and toughness. In detail, it has been found that an increased hardness can be achieved by controlling alloying elements C, Si, Mn, Cr, Mo and V to certain amounts and high toughness can be achieved by controlling alloying elements of Si, Cr, Mo, V, Nb and Al to certain content levels. 
     On the basis of such findings, there can be obtained high-strength spring steels having both high hardness and high toughness and the present invention could be accomplished. 
     According to the present invention, there is provided a high strength spring steel consisting of, in weight percentage, 0.50 to 0.70% C, 1.00 to 2.50% Si, 0.30 to 1.20% Mn, 0.80 to less than 1.20% Cr, 0.05 to 0.30% Mo, 0.05 to 0.30% V, 0.01 to 0.30% Nb, 0.005 to 0.100% Al and the balance being Fe and unavoidable impurities. 
    
    
     BRIEF DESCRIPTIONS THE DRAWINGS 
     FIG. 1 is a graph showing the relationship between the calculated values and experimental values for the hardness of sample steels. 
     FIG. 2 is a graph showing the relationship between the calculated values and experimental values for the toughness of sample steels. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The components of the steel of the present invention are specified as above for the following reasons. 
     Carbon: C is an effective element to increase the strength of the steel. When its content is less than 0.50%, a strength adequate for springs can not be obtained. On the other hand, when carbon is present in excess of 0.70%, the resulting springs becomes too brittle. Therefore, the carbon content is limited to the range of 0.50 to 0.70%. 
     Silicon: Si dissolves in ferrite to form a solid solution and effectively acts for improving the strength of the steel. When the Si content is less than 1.00%, a strength sufficient for springs can not be ensured. An excessive content of Si of more than 2.50% tends to cause decarburization on the steel surface during hot-forming the steel into a spring and hence to detrimentally affect the durability of the spring. Therefore, the content of Si is limited to the range of 1.00 to 2.50%. 
     Manganese: Mn is needed to improve the hardenability of the steel. The optimum Mn content range is from 0.30% to 1.20%. 
     Chromium: Cr is effective to strengthen the steel. When the Cr content is less than 0.80%, a strength adequate for springs can not be obtained. However, even if Cr is added in an excess amount of 1.20% or more, any further advantageous effect can not be obtained. Such an excess addition rather impairs the toughness. Therefore, the Cr content is limited within the range of 0.80 to less than 1.20%. 
     Molybdenum: Mo is an element which is required to ensure a sufficient hardenability and increase the strength and toughness of the steel. An amount of Mo of less than 0.05% can not sufficiently provide these effects, while an amount above 0.30% tends to produce precipitates of coarse carbides, impairing the spring properties. Therefore, the Mo content is limited within the range of 0.05% to 0.30%. 
     Vanadium: V also strengthens the steel. However, when the V content is less than 0.05%, a sufficient strengthening effect can not be expected. On the other hand, when the V content exceeds 0.30%, a substantial amount of carbides does not dissolve into austenite and, thereby, the spring characteristics are impaired. Thus, the V content range is limited to the range of 0.05 to 0.30%. 
     Niobium: Nb is an element which increases the strength and toughness of the steel due to its grain refinement function. When the content is less than 0.01%, the effect can not be sufficiently expected. On the other hand, when Nb is present in excess of 0.30%, the amount of carbides which do not dissolve into austenite increases and the spring characteristics are impaired. Accordingly, the content of Nb should be within the range of 0.01 to 0.30%. Aluminum: Al is needed for deoxidation and control of the austenite grain size. When Al is present in amounts less than 0.005%, grain refinement can not be expected. On the other hand, an excessive Al amount above 0.100% tends to reduce the castability. Thus, the content of Al should be in the range of 0.005 to 0.100%. 
     The spring steel of the present invention having the composition as specified above can be obtained through commonly practiced production steps, such as steel-making; ingot-making or continuous casting; and blooming and rolling into a steel bar or wire rod. Thereafter, the steel is hot-formed into a coil spring and is subjected to aftertreatments, such as quenching, tempering, shot-peening and setting. In such a production process, a high strength coil spring can be obtained. 
     EXAMPLE 1 
     Table 1 shows the chemical compositions of the inventive sample steels and comparative sample steels. 
     
                       TABLE 1______________________________________Sample Composition (wt. %)No.   C      Si     Mn   Cr   Mo   V    Nb   Al   Fe______________________________________A1    0.55   1.49   0.61 0.86 0.11 0.19 0.026                                        0.048                                             bal.A2    0.55   2.02   0.69 0.87 0.11 0.20 0.023                                        0.038                                             bal.A3    0.53   2.46   0.68 0.86 0.27 0.20 0.024                                        0.032                                             bal.A4    0.53   1.51   0.72 0.83 0.05 0.20 0.022                                        0.038                                             bal.A5    0.58   1.29   0.69 0.85 0.15 0.20 0.022                                        0.044                                             bal.A6    0.52   1.51   0.69 0.84 0.19 0.20 0.024                                        0.043                                             bal.A7    0.52   1.58   0.65 0.85 0.11 0.20 0.023                                        0.024                                             bal.A8    0.58   1.52   0.67 0.84 0.10 0.20 0.024                                        0.029                                             bal.A9    0.57   1.44   0.81 0.83 0.10 0.19 0.025                                        0.031                                             bal. A10  0.56   1.45   0.94 0.85 0.10 0.20 0.024                                        0.025                                             bal.B1    0.63   0.67   1.06 0.26 0.20 --   --   0.004                                             bal.B2    0.64   0.59   1.03 0.26 0.20 0.10 0.022                                        0.017                                             bal.B3    0.61   1.43   0.93 --   0.20 --   --   0.034                                             bal.B4    0.61   1.37   0.92 --   0.20 0.10 0.023                                        0.020                                             bal.B5    0.62   0.13   1.49 0.99 0.30 --   --   0.021                                             bal.B6    0.63   0.16   1.54 1.01 0.30 0.10 0.024                                        0.013                                             bal.B7    0.63   0.19   2.09 --   0.30 --   --   0.015                                             bal.B8    0.63   0.20   2.07 --   0.30 0.10 0.025                                        0.018                                             bal.B9    0.58   1.30   0.81 0.83 --   --   0.047                                        0.021                                             bal. B10  0.65   1.75   0.82 0.15 --   0.20 0.066                                        0.022                                             bal. B11  0.60   0.99   1.40 0.28 0.20 0.15 0.024                                        0.031                                             bal. B12  0.57   1.50   0.77 0.72 --   --   --   0.003                                             bal. B13  0.57   1.53   0.80 0.73 --   0.19 0.022                                        0.024                                             bal. B14  0.56   1.44   0.51 0.83 --   0.19 0.025                                        0.037                                             bal. B15  0.60   1.50   0.40 0.55 --   --   --   0.033                                             bal. B16  0.63   1.47   0.42 0.57 --   0.20 --   0.029                                             bal. B17  0.61   0.86   0.79 0.50 --   --   --   0.031                                             bal. B18  0.55   1.42   0.61 0.85 --   0.20 0.024                                        0.032                                             bal.______________________________________ Remark: Nos. A1-A10: Steels of the present Invention Nos. B1-B18: Comparative Steels 
    
     Table 2 shows the relationship between the hardness and Charpy impact value for each sample steel, as shown in Table 1, after quenching and tempering at 380 ° C. 
     
                       TABLE 2______________________________________Mechanical  Sample No. of the Present Inventionproperties   A1      A2      A3    A4    A5______________________________________Hardness (Hv)        626     656     664   626   641Charpy impact        3.9     4.0     4.3   3.5   3.7values (kgf-m/cm.sup.2)______________________________________Mechanical  Sample No. of the Present Inventionproperties   A6      A7      A8    A9    A10______________________________________Hardness (Hv)        639     620     644   657   655Charpy impact        4.0     3.7     3.9   3.8   3.9values (kgf-m/cm.sup.2)______________________________________Mechanical  Comparative Sample No.properties  B1      B2     B3    B4   B5    B6______________________________________Hardness (Hv)       570     560    600   610  560   560Charpy impact       2.6     2.9    2.9   3.1  2.9   2.8values (kgf-m/cm.sup.2)______________________________________Mechanical  Comparative Sample No.properties  B7      B8     B9    B10  B11   B12______________________________________Hardness (Hv)       530     540    590   642  590   611Charpy impact       2.6     2.8    2.8   2.6  3.1   3.0values (kgf-m/cm.sup.2)______________________________________Mechanical  Comparative Sample No.properties  B13     B14    B15   B16  B17   B18______________________________________Hardness (Hv)       614     613    590   644  573   629Charpy impact       3.1     3.1    2.8   2.9  3.2   3.0values (kgf-m/cm.sup.2)______________________________________ 
    
     FIGS. 1 and 2 are graphs diagrammatically showing the relationship between the test results shown in Table 2 and values calculated from Equations (1a) and (1b) and (2a) and (2b). It can be seen from Table 2 that the steels of the present invention have higher Charpy impact values than the comparative steels. 
     Steel ingots were prepared from the inventive steel No. A7 and the comparative steel No. B12, hot-rolled to effect a reduction ratio of at least 50, and hot-formed into sample springs. The resulting springs were subjected to quenching, tempering, shot-peening and setting to provide sample springs. Table 3 shows particulars of the sample springs. The hardness values of the springs were adjusted to Hv 620 for the inventive steel and Hv 530 for the comparative steel. 
     
                       TABLE 3______________________________________Diameter of wire (mm)                11.0Mean diameter of coil (mm)                110Total No. of turns   5.5Effective No. of turns                4.0______________________________________ 
    
     Each sample spring was subjected to a fatigue test. The results are shown in Table 4. 
     
                       TABLE 4______________________________________   Applied Stress                 Number of Cycles   (kgf/mm.sup.2)                 to Failure (× 10.sup.4)______________________________________Steel of the     10-120          27.9   28.4 28.8Invention                 30.1   30.5 34.3Compara-  10-110          25.6,  26.8,                                 29.3,tive Steel                30.7,  32.5,                                 33.8______________________________________ 
    
     It will be seen from Table 4 that the steel of the present invention can guarantee a long useful life equivalent to that of the comparative steel, even if the steel of the present invention is placed under a higher stress condition than the comparative spring steel. 
     Table 5 shows the results of a sag test for the same sample springs prepared from the inventive steel No. A17 and the comparative steel No. B12. 
     
                       TABLE 5______________________________________     Applied Stress                Sagging Properties     (kgf/mm.sup.2)                (Residual Shear Strain)______________________________________Steel of the       120          6.0 × 10.sup.-4InventionConventional Steel       110          6.2 × 10.sup.-4______________________________________ Remark: Test Conditions: 80° C. × 96 hours 
    
     The test results showed that the inventive steel spring could ensure a high sag resistance equivalent to that of the comparative steel, nevertheless it was placed in a higher stress condition than the comparative steel. Such results show that the steel of the present invention is a high strength spring steel which can be formed into springs to be used under application of stresses higher than that may be applied to the comparative spring steel. In the steel of the present invention, it is possible to increase the strength or hardness to a much higher level than heretofore available while maintaining the Charpy impact value at a high level. Therefore, a high reliability can be ensured in the resulting spring products. 
     EXAMPLES 2 
     Table 4 shows the chemical compositions of further sample steels. 
     
                       TABLE 6______________________________________Sample Chemical Composition (wt. %)No.   C      Si     Mn   Cr   Mo   V    Nb   Al   Fe______________________________________A11   0.57   1.47   0.45 0.84 0.11 0.19 0.026                                        0.050                                             bal.A12   0.57   2.00   0.49 0.85 0.11 0.20 0.023                                        0.036                                             bal.A13   0.57   2.48   0.48 0.84 0.27 0.20 0.024                                        0.034                                             bal.A14   0.55   1.49   0.43 0.81 0.05 0.20 0.022                                        0.040                                             bal.A15   0.60   1.27   0.49 0.83 0.15 0.20 0.022                                        0.046                                             bal.A16   0.54   1.49   0.47 1.82 0.19 0.20 0.024                                        0.041                                             bal.A17   0.54   1.56   0.45 0.83 0.11 0.20 0.023                                        0.021                                             bal.______________________________________ Remark: Nos. A11-A17: Steels of the present Invention 
    
     Table 7 shows the relationship between the hardness and Charpy impact value for each sample steel, as shown in Table 6, after quenching and tempering at 400° C., in comparison with the comparative sample steels as shown in Table 1. 
     
                       TABLE 7______________________________________Mechanical  Comparative Sample No.properties  B1      B2     B3    B4   B5    B6______________________________________Hardness (Hv)       543     542    587   594  555   554Charpy impact       3.0     3.0    3.1   3.2  2.9   2.9values (kgf-m/cm.sup.2)______________________________________Mechanical  Comparative Sample No.properties  B7      B8     B9    B10  B11   B12______________________________________Hardness (Hv)       528     534    581   611  577   572Charpy impact       2.8     3.0    3.1   2.5  3.3   3.1values (kgf-m/cm.sup.2)______________________________________Mechanical  Comparative Sample No.properties  B13     B14    B15   B16  B17   B18______________________________________Hardness (Hv)       592     579    571   605  543   592Charpy impact       3.0     3.2    3.1   3.2  3.0   3.3values (kgf-m/cm.sup.2)______________________________________Mechanical  Sample No. of the Present Inventionproperties  A11    A12    A13  A14  A15  A16  A17______________________________________Hardness (Hv)       593    637    651  596  605  612  601Charpy impact       4.0    4.1    4.0  3.8  3.9  4.0  4.1values (kgf-m/cm.sup.2)______________________________________ 
    
     It can be seen from Table 7 that the steels of the present invention have higher Charpy impact values than comparative steels. 
     Steel ingots were prepared from the inventive steel No. A17 and the comparative steel No. B12, hot-rolled to effect a reduction ratio of at least 50, and hot-formed into sample springs. The resulting springs were subjected to quenching, tempering, shot-peening and setting. 
     Table 8 shows particulars of the sample springs. The hardness values of the springs were adjusted to Hv 580 for the inventive steel and Hv 530 for the comparative steel. 
     
                       TABLE 8______________________________________Diameter of wire (mm)               11.0Mean diameter of coil (mm)               110Total No. of turns  5.5Effective No. of turns               4.0______________________________________ 
    
     Each spring was subjected to a fatigue test. The results are shown in Table 9. It will be seen from Table 9 that the steel of the present invention can guarantee a long useful life equivalent to that of the conventional steel, even if the steel of the present invention is placed under a higher stress condition than the comparative spring steel. 
     
                       TABLE 9______________________________________   Applied Stress                 Number of Cycles   (kgf/mm.sup.2)                 to Failure (× 10.sup.-4)______________________________________Steel of the     10-120          27.6   28.5 28.7Invention                 29.8   30.4 35.2Compara-  10-110          25.6,  26.8,                                 29.3,tive Steel                30.7,  32.5,                                 33.8______________________________________ 
    
     Table 10 shows the results of a sag test for the same sample springs prepared from the inventive steel No. A17 and the comparative steel No. B12. 
     The test results show that the inventive steel spring can ensure a high sag resistance which is equivalent to that of the conventional steel, even if it is placed in a higher stress condition than the comparative steel. Such results show that the steel of the present invention is a high strength spring steel which can be formed into a spring to be used under application of stress higher than that may be applied to the comparative spring steel. In the steel of the present invention, it is possible to increase the strength and hardness to a much higher level than heretofore available while maintaining the Charpy impact value at a high level. Therefore, a high reliability can be ensured in the resulting spring products. 
     
                       TABLE 10______________________________________     Applied Stress                Sagging Properties     (kgf/mm.sup.2)                (Residual Shear Strain)______________________________________Steel of the       120          6.0 × 10.sup.-4InventionConventional Steel       110          6.2 × 10.sup.-4______________________________________ Remark: Test Conditions: 80° C. × 96 hours 
    
     As described above, the steel of the present invention is a high strength spring steel and, when it is used for preparation of springs, the resultant springs exhibit a good durability and have a long useful life and a high sag resistance. Accordingly, the inventive steel produces outstanding effects in cars or practical services in various industrial machines.