GB2112810A - Steels for vehicle suspension springs - Google Patents

Abstract

Steel having good sag-resistance comprises by weight 0.50-0.80% carbon, 1.50-2.50% silicon, 0.50-1.50% manganese, a member or members selected from a group consisting of 0.05-0.50% vanadium, 0.05-0.50% niobium and 0.05-0.50% molybdenum, the remainder being iron and impurities. The steel may also contain a member or members selected from a group consisting of 0.0001-0.01% boron and 0.20-1.00% chromium, and not more than 0.0008% nitrogen. Heat treatment involves rapid (eg. >500 DEG C/min) heating to 900 - 1200 DEG C, quenching and tempering at 400 - 580 DEG C.

Description

SPECIFICATION Steels The invention relates to steels for vehicle suspension springs, such as coil springs, torsion bars and laminated leaf springs, which have good sag-resistance, good fatigue resistance and good hardenability.

Steel according to the invention comprises by weight 0.50-0.80% preferably 0.55-0.65% carbon, 1.50-2.50% preferably 1.80-2.20% silicon, 0.50.-1.50% preferably 0.70-1.00% manganese and a member or members selected from a group consisting of 0.05-0.50% preferably 0.05-0.25% vanadium, 0.05-0.50% preferably 0.05-0.25% niobium and 0.05-0.50% preferably 0.05-0.25% molybdenum, the remainder being iron and impurities. The steel may contain a member or members selected from a group consisting of 0.0001-0.01% boron and 0.20-1.00% chromium, and not more than 0.0008% nitrogen.

The steel of the invention has a high silicon content, and the explanation of the improved sag-resistance is as follows: Vanadium, niobium and molybdenum form carbides in the steel. The vanadium carbide, niobium carbide and/or molybdenum carbide (hereinafter referred to as "alloy carbide") are dissolved in the austenite on heating. On quenching, a martensite structure is obtained in which the alloy carbides are in supersaturated solid solution. On tempering, a fine alloy carbide starts to reprecipitate, dislocation is prevented, and a secondary heardening takes place to give a superior hardness. Alloy carbide not dissolved in the austenite by the time of quenching refines and prevents coarsening of the austenite grains. Such fine grains reduce dislocation and thus improve sag-resistance.

A good temper softening resistance, and thus a wide range of tempering temperature and hardness are also obtainable. The steel undergoes a secondary hardening on the reprecipitation of the alloy carbide during tempering. This means that for a given tempered hardness range, it is possible to operate in a wide temper temperature range, and it is possible to obtain a desired hardness.

Drawings Figures 1 and 2 are diagrams illustrating the relationship between the tempering temperatures and the hardness of steels according to the invention and others; Figure 3 is a diagram illustrating the relationship between the austenitizing temperatures and the austenite grain size numbers of such steels; Figure 4 is a diagram illustrating the Jominy curves of such steels; Figures 5to 10 are diagrams illustrating the relationship between the hardness and the residual shear strains of such steels; and Figure 11 is a diagram showing the relationship between the quenching temperatures and the hardness of such steels.

The compositions of the steels whose properties are illustrated in the drawings are listed in Tables 1 and 4 below, and the references Al etc are consistently used herein to indicate a steel of the stated composition.

For Figures 1 and 2 the steels were tempered at from 300O to 6000C as indicated and their hardness was measured and is shown in comparison to a hardness range corresponding to the spring hardness range of HRC 44.5 to 51.0 as stipulated in the Japanese Automobile Standard JASO C605 "Coil Springs for Automobile Suspension". It is apparent from Figure 1 that the steels according to the invention provide wider tempering temperature ranges corresponding to their hardness than the conventional steels.

For thick springs, a spring steel having a good hardenability without loss of sag-resistance has been developed according to the invention which, even in a form of a thick coil spring, a thick torsion bar or a thick laminated leaf spring, has a martensite struture to the core after heat treatment. The steel contains in addition one or both of boron and chromium and at most 0.0080% of nitrogen. Atomic boron plays an effective role in hardenability. The atomic boron is dissolved and present interstitially in the crystals. It is present in the vicinity of a dislocation, and makes it hardly movable, so that sagging is low.It is apparent from Figure 2 that Alo, All and A14 steels have a hardness increment peak indicating a secondary hardening at a tempering temperature in the vicinity of 550"C even after quenching from a usual austenitizing temperature. The secondary hardening is not impaired by the addition of boron. Precipitation strengthening can be utilized at a tempering temperature to obtain a hardness in the vicinity of the stipulated hardness range (i.e. HRC 44.5 to 51.0).

Figure 3 shows austenite grain sizes measured by an oxidation method after heating at an austenitizing temperature of from 850"C to 11 00 C. A10 and All steels containing vanadium, niobium and boron have an austenite grain size equivalent to A14 steel which contains vanadium alone. This indicates that the effectiveness of the alloy carbide for the refinement of the crystal grains, and for the prevention of coarsening of the austenite grains, is not impaired by the addition of boron. As is apparent from Figure 4, AlO and All steels containing boron have a remarkably better hardenability than A14 and B3 steels which contain no boron.

The reasons for restricting the composition of the steel of the present invention are as follows: If the amount of carbon is less than 0.50%, insufficient strength for a spring steel for high stress use is obtainable by quenching and tempering, and if the amount exceeds 0.80%, a hyper-eutectoid steel results which has less toughness. If the amount of silicon is less than 1.50%, the silicon is dissolved in ferrite and is not effective in the strengthening of the matrix and in improving the sag-resistance. If the amount exceeds 2.5%, the improvement in sag-resistance is saturated, and there is a possibility of undesirable formation of free carbon by the heat treatment.

If the amount of manganese is less than 0.50%, inadequate strength for a spring steel and inadequate hardenability are obtainable. If the amount exceeds 1.50%, the toughness tends to decrease.

If the amount of vanadium, niobium and/or molybdenium is less than 0.05%, the effect of improving sag-resistance is not obtainable. If the amount exceeds 0.50%, the effect is saturated, the amount of the alloy carbide not dissolved in the austenite increases, and large aggregates acting as non-metallic inclusions are produced leading to a possible decrease in the fatigue strength of the steel. The vanadium, niobium and/or molybdenum may each be added alone independently of the other two, or they may be added as a combination of two or three to form a preferred system in which their solubilization in the austenite starts at a lower temperature than when vanadium, niobium or molybdenum is added alone. The precipitation of the fine alloy carbide during tempering facilitates secondary hardening and further improves the sag-resistance.

If the amount of boron is less than 0.0001% or 0.0005%, inadequate improvement in the hardenability and sag-resistance is obtainable. If the amount exceeds 0.01%, boron compounds precipitate and cause hot brittleness. If the amount of chromium is less than 0.20%, inadequate hardenability is obtained. If the amount exceeds 1.0%, the uniformity of the structure and consequently the sag-resistance are impaired. The amount of nitrogen is limited to not more than 0.0008% or 0.0080% to prevent a loss of the effectiveness of boron through the reaction of the boron and nitrogen.

Example 1 TABLE 1 Chemical compositions (% by weight) C Si Mn P S V Nb Mo Al N Al 0.61 1.97 0.83 0.020 0.008 0.07 0.030 0.011 A2 0.58 2.14 0.84 0.025 0.009 0.20 0.024 0.012 A3 0.60 1.95 0.80 0.018 0.008 0.33 0.033 0.011 A4 0.58 2.10 0.86 0.026 0.010 0.19 0.022 0.012 A5 0.59 2.12 0.85 0.025 0.009 0.21 0.025 0.011 A6 0.58 2.17 0.84 0.025 0.009 0.25 0.22 0.027 0.018 A7 0.57 2.14 0.85 0.026 0.010 0.22 0.20 0.029 0.012 A8 0.59 2.15 0.87 0.025 0.010 0.19 0.22 0.023 0.011 A9 0.61 1.98 0.80 0.017 0.008 0.23 0.21 0.22 0.033 0.012 B1 0.59 2.17 0.86 0.025 0.010 0.018 0.012 B2 0.59 1.97 0.81 0.020 0.009 0.036 0.013 Al to A9 are steels of the invention, while B1 and B2 correspond to SAE 9260. The steels of Table 1 were cast, subjected to hot rolling at a rolling ratio of at least 50, and quenched and tempered at temperatures sufficient to provide a tensile strength of about 180 kgf/mm2. The physical properties of the products are in Table 2. The tensile strength, 0.2%-proof stress, elongation and reduction of area were measured using standard 0.500-in (12.5-mm) round test specimens with 2-in. gauge length as specified in ASTM A 370.

Impact testing was performed using 10 x 10 mm simple beam impact specimens with 2-mm deep U-notch modified from type A specimens as specified in ASTM A 370. The torsional strength was measured with use of specimens having a diameter of 9 mm at the parallel portions.

TABLE 2 Tensile 0.2% Proof Elonga- Reduc- Impact Torsional strength stress tion tion of values strength (kgf/mm2) (kgflmm2) { /O) area { /OJ (kgfm/cm2) (kgflmm2) Al 181 173 13 36 2.2 145 A2 180 173 11 39 2.7 145 A3 181 173 12 33 2.5 147 A4 181 173 15 42 2.9 148 A5 182 174 12 32 2.5 147 A6 180 175 12 29 2.3 150 A7 181 176 12 33 2.6 150 A8 187 175 14 33 2.4 148 A9 182 176 11 30 2.0 150 B1 178 168 13 41 2.3 144 B2 178 166 12 36 2.4 144 As is apparent from Table 2, Al to A9 steels exhibit properties equivalent or superior to those of B1 and B2 steels.

Using the above sample steels as the base materials, coil springs having the characteristics shown in Table 3 were prepared, and subjected to quenching and tempering treatments to bring the final hardness to HRC 45 to 55. Then, they were subhected to pre-setting to bring the shear stress of bars tot = 115 kg/mm2 to obtain specimens for sagging tests. These specimens were brought under a load sufficient to give a shear stress of the bars t = 105 kg/mm2 at a constant temperature of 20 C, and after 96 hours (hereinafter referred to as "long hour loading"), the sagging of the coil springs was measured.

TABLE 3 Characteristics of coil springs Bar diameter (mm) 13.5 Bar length (mm) 2470 Average coil diameter (mm) 120 Numberofturns 6.75 Effective number of turns 4.75 Spring rate (kgf/mm) 4.05 The sagging corresponding to the hardness of the above specimens is shown in Figures 5 to 8. Al to A5 steels where vanadium, niobium and molybdenum are each added alone, and A6 to A9 steels where vanadium, niobium and molybdenum are added in combination, have a sag-resistance superior to that of B1 steel. Among the steels of the invention, those containing vanadium, niobium and molybdenum in combination have a sag-resistance superior to those steels containing vanadium, niobium or molybdenum alone.

In order to determine the sagging, a load P1 required to compress a coil spring to a predetermined level prior to long hour loading and a load P2 required to compress them to the same level after long hour loading were measured. The sagging was calculated by applying the difference A P = P1 - P2 to the following equation in units of shear strain, and is referred to as "residual shear strain".

YR = 1 , K 8D AP G t d3 G: Shear modulus (kgf/mm2) D: Average coil diameter (mm) d: Bar diameter (mm) K: Wahl's coefficient (A coefficient depending upon the shape of a coil spring) Coil spring bars made of Al to A9 and B1 steels were subjected to a repeated load to yield a shear stress of from 10 to 110 kgf/m m2 for fatig ue tests. After repetition of the loading 200,000 times, no breakage was observed in any of the coil springs.

Example 2 TABLE 4 Chemical composition {% by weight) C Si Mn V Nb B Cr N A10 0.61 2.11 0.87 0.28 0.0029 0.12 0.0061 All 0.59 2.07 0.86 0.21 0.09 0.0021 0.11 0.0056 A12 0.58 2.09 0.84 0.25 0.49 0.0074 A13 0.58 2.12 0.85 0.25 0.10 0.50 0.0069 A14 0.57 2.04 0.87 0.26 0.11 0.0125 A15 0.59 2.11 0.85 0.19 0.11 0.11 0.0132 B3 0.59 2.11 0.86 0.13 0.0128 AlO to A13 are steels of the invention, A14 and Al 5 are comparative high silicon content steels containing vanadium and niobium, and B3 corresponds to SAE 9260.

The steels of Table 4 were cast, subjected to hot rolling at a rolling ratio of at least 50, and subjected to a heat treatment to bring the tempered hardness to HRC 48. The physical properties of the products are in Table 5, the methods of measurement being as above.

TABLE 5 Tensile 0.2% Proof Elonga- Reduc- Impact Torsional strength stress tion tion of values strength rkgflmm2J (kgflmm2) f%J area ("/o) (kgfmlcm2) (kgflmm2) A10 165 156 13 37 3.0 140 All 166 157 12 36 3.0 139 A12 164 154 15 39 3.2 140 A13 165 158 13 36 3.1 141 A14 166 157 12 35 3.1 141 A15 164 157 14 36 3.0 140 B3 165 153 15 40 3.3 139 A10 to A13 steels contain boron and chromium according to the invention. They have mechanical properties equivalent to A14 and A15 comparative steels containing vanadium and niobium, and a 0.2% proof stress superior to that of B3 conventional steel.

Using the above sample steels as the base materials, torsion bars having the characteristics shown in Table 6 and a diameter of 30 mm at the parallel portions were prepared, subjected to quenching and tempering to bring the final hardness to HRC 45 to 55, and shot-peened to give specimens for sagging tests.

Priorto the sagging test, a torque to give a shear stress T = 110 kgf/mm2to the surface of the parallel portions of the specimens, was applied to both ends of the specimens to pre-set them. After pre-setting, a torque to give a shear stress T = 100 kgf/mm2 was applied and the specimens were left to stand in that state for 96 hours. The residual shear strain was calculated by an equation YR = A0.d/2 based on the decrease of the tortional angle, where YR iS a residual shear strain, Af) is a decrease (rad) of the torsional angle and d is a diameter (mm) of the bar.

TABLE 6 characteristics of the torsion bars A10-A15, B3 Bar diameter 30.0 mm Effective bar length 840 mm Spring rate 12,723 kgfmm/deg The sagging corresponding to the hardness of the above specimens is shown in Figures 9 and 10 from which it appears that the specimens prepared from A10 to A13 steels containing boron are remarkably superior in sagging to the conventional B3 steel, and that they also show better values than the comparative A14 steel. This is considered to be due to the incorporation of boron which makes it possible to quench and obtain a fully hardened martensite to the core without impairing the sag-resistance even when a torsion bar having a diameter of 30 mm is used.The boron penetrates interstitially into the crystals in the vicinity of dislocations, prevents the movement of the dislocations and reduces sagging.

The above torsion bars are loaded repeatedly to give a shear stress of 60 i 50 kgf/mm2 for fatigue tests.

After loading 200,000 times, no breakage was observed in any of the torsion bars. This confirms that no adverse effect to the fatigue life was caused by the addition of boron.

Now, a high temperature rapid heating operation will be described which further improves the sag-resistance of the steel of the invention.

Figure 11 shows the hardness of the above steels which were treated at austenitizing temperatures from 850 to 1100 C and tempered at 550"C. With respect to Al 0, All and A14 steels, but not B3 steel, the hardness is increased with an increase of the austenitizing temperature. This indicates that the amount of the alloy carbide dissolved in the austenite phase increases with an increase of the austenitizing temperature, and the secondary hardening is thereby facilitated. By setting the heating temperature for austenitizing at a higher level of from 900 to 1 200"C it is possible to increase the amounts of carbides of vanadium, niobium and molybdenum dissolved in the austenite.This increases the precipitation of the fine carbides in the subsequent tempering, facilitates secondary hardening and improves sag-resistance.

However, itthe heating is conducted at a temperature as high as from 900 to 1200 C for a long period, for example by a conventional heating method with a heavy oil, there are adverse effects such as decarburization of the steel surface, the surface becoming rough, the fatigue life being shortened and the austenite grains being coarsened. On the other hand, by rapidly heating the steels to from 900 to 1200 C at the time of austenitizing, it is possible to dissolve a great quantity of the alloy carbides in the austenite without decarburization and surface roughening.By holding the steel at the temperature for a predetemined period, quenching, and tempering at from 400 to 5800C, it is possible to precipitate a great amount of fine carbides, facilite secondary hardening, and improve sag-resistance.

The reason for restricting the heating temperatare for austenitizing to from 900 to 1 200"C is that if the temperature is lower than 900"C, it is not possible sufficiently to dissolve vanadium, niobium or molybdenum in the austenite especially when they are added alone. If the temperature exceeds 1 200"C, decarburization or surface roughening is likely. If the heating rate is less than 500 degrees Cumin, time at the high temperature is long. This leads to decarburization on the surface, surface roughening, a decrease in fatigue life, and the coarsening of the austenite grains. To heat at a rate of at least 500 degrees C/min, it is preferred to use a high frequency induction heater or a direct current heating apparatus.

The reason for restricting the tempering temperature to from 400 to 5800C is that the alloy carbides dissolved in the austenite are precipitated as fine grains during tempering, and secondary hardening is thereby caused. Even when the tempering is carried out as high as 580"C, the decrease in the hardness is smaller than in the conventional steels, and it is possible to obtain a hardness of at least HRC 44.5.

Example 3 A2, A4, A6, Al 0, All and B1 steels were cast, subjected to hot rolling at a rolling ratio of at least 50, rapidly heated, quenched and tempered to give a tempered hardness of about HRC 48. The operating conditions and resulting properties are in Table 7, the measurements being effected as above. Decarburization was measured by JIS G 0558 (SAE J 419), and austenite grain sizes by JIS G 0551 (ASTM E 112) quenching and tempering (Gh) method.

TABLE 7 Sample Heating Austeni- Tempering Sagging (10-4) Decarburi- Austenite materials rate tizing tempera- (Residual zation grain bar ( Clmin) tempera- tures shear (mm) sizes diameter tures strain) (mm) ( C) ( C) A2 Coil spring 1000 950 475 3.2 0.04 11.3 13.5 " " 5000 1060 480 2.8 0.07 10.8 A4 " 1000 1050 460 3.8 0.06 11.8 " " 5000 1150 470 3.5 0.09 11.0 A6 " 1000 950 460 3.0 0.02 11.5 " " 5000 1050 480 2.3 0.04 10.8 A10 Torsion bar 1000 1050 480 2.9 0.04 10.6 30 A11 " 1000 1050 480 2.7 0.06 11.0 B1 Coil spring 50 880 450 4.3 0.14 9.2 13.5 " " 50 950 450 4.2 0.35 8.5 " " 50 1000 450 4.3 0.42 7.8 As is apparent from Table 7, the sagging of the coil springs prepared by high temperature rapid heating was from 2.3 to 3.8 x 10-4, whereas that of the coil springs prepared under the conventional heating conditions was higher: from 4.2 to 4.5 x 10-4. Likewise, the sagging of torsion bars was 2.7 and 2.9 x superior values equivalent to those of the coil springs. When the heating rate was at the higher levels of 1000 degrees Cumin or 5000 degrees Cumin, even if the temperature was as high as 950 to 11 50"C, it was possible to supress the decarburization to as little as from 0.002 to 0.09 mm. This compares with from 0.14 to 0.42 mm according to the conventional method. At these higher temperatures, an austenite grain size as fine as from 10.6 to 11.8 was obtained according to the invention. This compares with from 7.8 to 9.2 according to the conventional method. A superior prevention of coarsening of austenite grains is thus obtainable according to the invention.

Claims (18)

1. A steel for a vehicle suspension spring having good sag-resistance comprising by weight 0.50-0.80% carbon, 1.50-2.50% silicon, 0.50-1.50% manganese and a member or members selected from a group consisting of 0.05-0.50% vanadium, 0.05-0.50% niobium and 0.05-0.50% molybdenum, the remainder being iron and impurities.

2. A steel according to claim 1 wherein the steel comprises by weight 0.55-0.65% carbon.

3. A steel according to claim 1 or claim 2 wherein the steel comprises by weight 1.80-2.20% silicon.

4. A steel according to any preceding claim wherein the steel comprises by weight 0.70-1.00% manganese.

5. A steel according to any preceding claim wherein the steel comprises by weight 0.05-0.25% of a member or members of the group.

6. A steel according to claim 5 wherein the steel comprises at least two members of the group.

7. A steel according to any preceding claim wherein the steel comprises by weight 0.0001-0.01% boron.

8. A steel according to any preceding claim wherein the steel comprises by weight 0.20-1.00% chromium.

9. A steel according to any preceding claim wherein the steel comprises by weight not greater than 0.0008% nitrogen.

10. A steel according to any preceding claim wherein the steel comprises by weight 0.0005-0.005% boron.

11. A steel according to any preceding claim wherein the steel comprises by weight 0.20-0.50% chromium.

12. A steel according to any preceding claim wherein the steel comprises by weight not greater than 0.0080% nitrogen.

13. A steel according to claim 1 as herein described in Table 1 or 4.

14. A process of producing a sag-resistant steel according to any preceding claim comprising rapidly heating the steel to an austenitizing temperature of from 900 to 1200"C, quenching, and tempering at from 400 to 580"C.

15. A process according to claim 14, wherein the rapid heating of the steel is at a rate above 500 degrees C/min.

16. A process according to claim 15, wherein the heating rate is from 1000 degrees C/min to 5000 degrees C/min.

17. A process according to any of claims 14 to 16, wherein the heating is carried out by high frequency induction or direct current heating.

18. A process of producing a sag-resistant steel according to claim 14 as herein described in any of the Examples.