KR930012177B1 - Method of making steel for spring - Google Patents

Abstract

No content.

Description

Spring steel and its manufacturing method

1 is a graph showing the relationship between the heating temperature and the hardness of the steel.

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a steel provided in a clutch of an automobile and the like used as a diaphragm spring and a method for producing the steel. In recent years, with the increase in the size of the device and the increase in output, the springs provided in the device are in danger of warming. For example, a clutch of an automobile is provided with a diaphragm spring such as a diaphragm spring. The load on the clutch is increasing due to the high output of the automobile engine, the four-wheel drive of the vehicle, and the like. As a result, the use-ambient temperature rises to 250-350 degreeC which can be said that what was conventionally the highest 150 degreeC is warm. As for this type of diaphragm spring, carbon steel balls such as SK5 (Japanese Industrial Standard) have been conventionally used. However, in the carbon tool steel, the relaxation (permanent deformation due to fatigue in the spring) proceeds rapidly as the temperature rises. In this respect, a steel having excellent relaxation resistance in warm weather is strongly demanded, and also used as a spring, so that fatigue strength to withstand repeated loads is also required. In order to improve the relaxation resistance in terms of materials, the Si content of the steel may be increased. For example, SUP6 prescribed | regulated to JIS G4801, SUP7 which weighted Si further, etc. are used as a spring to which relaxation resistance is calculated | required. Japanese Unexamined Patent Application Publication No. 2-240240 discloses a steel in which weight of Si, Mn, and the like is increased, and quenchability is increased by addition of Mo. As means for improving the relaxation resistance by heat treatment or the like, it is known to give plastic deformation after quenching and tempering and to perform strain aging at 250 to 350 ° C.

However, although the temperature-resistance at room temperature, such as SUP6 and SUP7 which contains a large amount of Si, and relaxation resistance at room temperature are excellent, the relaxation resistance deteriorates with temperature rise, and it does not show sufficient characteristic in warm temperature. In addition, when strain aging is performed after imparting plastic strain, a large number of heat treatments are required in the spring manufacturing process, so that the manufacturing cost rises.

In view of this, a material that does not require special heat treatment and has excellent heat resistance and ease of quenching and tempering is desired.

The present invention was devised to meet such demands, and by improving the alloying components and manufacturing conditions so as to increase the strength after tempering and to deposit fine carbides during tempering, it is easy to relax, fatigue strength and tempering. An object of the present invention is to provide a spring steel excellent in softening resistance and the like and a manufacturing method thereof.

The present inventors have comprehensively studied the effects of alloying elements in steel and the effects of manufacturing conditions on the temperature and heat resistance, and found that the solid solution of carbides for steels containing an appropriate amount of C, Si, Mn, Cr, Mo, etc. By appropriately controlling the shape of the precipitation, it was found that steel having excellent relaxation resistance at warm temperature can be produced. That is, this invention contains C: 0.4-0.8 weight%, Si: 0.5-2.5 weight%, Mn: 0.3-2.0 weight%, Cr: 0.1-1.5 weight%, Mo: 0.1-0.5 weight%, and remainder Hot rolled steel made of Fe and unavoidable impurities is manufactured to produce a hot rolled sheet, and after annealing the hot rolled sheet, it is cold rolled at a rolling rate of 10 to 80%, and then the cold rolled sheet is subjected to a temperature below the Acl transformation point. After annealing, the annealed cold rolled plate is heated for a time sufficient to austenitize carbides at temperatures above the Acl transformation point, and the heated heated cold rolled plate is cooled above the lower critical cooling rate and then cooled. It is to provide a method for producing a spring steel consisting of heating the cold rolled sheet to a temperature sufficient to precipitate carbide and then cooled to room temperature.

Here, the lower critical cooling speed means a cooling rate at the boundary of whether or not the austenite phase transforms into a martensite phase, and the lower critical cooling speed is converted into martensite if the lower critical cooling speed is higher than the lower critical cooling speed. . According to the present invention, carbides, in particular Mo carbides, are finely precipitated in the final heating step, thereby preventing the displacement of dislocations, which causes warm relaxation.

This final heating step is carried out for a time sufficient to precipitate the fine carbide in the temperature range of 450-600 ° C.

In addition, the Si and Cr contents are selected to satisfy the following conditions.

-7 4 x Si (%)-10 x Cr (%) 5

And, the heating of the cold rolled sheet cooled in the final heating step is carried out to retain the annealing hardness between HV400-HV550. Further, annealing of the cold rolled sheet is carried out at 550-730 ° C., so that the average carbide particle diameter is 2 μm or less. In addition to the alloy components described above, the steel material used herein may further contain 0.05 to 0.5% by weight of one or two of V and Nb in total, and may contain 0.02% by weight or less of Al.

The inventors of the present invention extensively studied the effects of alloying elements on the quenchability and the ease of temperature resistance, the influence of the manufacturing conditions, and conducted numerous experiments. As a result, in order to improve the temperature-relaxability, it was found that it is effective to improve the strength after tempering and to deposit fine carbides at the time of tempering. Therefore, in order to achieve these two points at the same time, it is tempered at a higher temperature than carbon steel balls such as SK5, and the tempering softening resistance is increased to have the strength required as a spring at the time of tempering, and at the same time, fine carbides at the time of tempering It was found that by adjusting the alloy component so as to precipitate, steel with extremely high temperature-relaxability was obtained.

Regarding the fatigue strength, the amount of alumina which is representative of the hard inclusions, which is the starting point of the fatigue strength, is reduced by suppressing Al, which binds to oxygen in the steel to form inclusions, below a certain amount.

In addition, it is possible to prevent the deterioration of the fatigue strength when the hardness rises.

Hereinafter, the present invention will be described in detail. In this invention, in order to ensure the hardness required as a spring when tempering at high temperature compared with the conventional steel, Si which has a big effect in the improvement of a tempering softening resistance is added.

And generation | occurrence | production of the graphitization resulting from Si addition is suppressed by addition of Cr.

When Mo is added under these conditions, it becomes possible to precipitate Mo carbide.

Mo carbide is a precipitate that is effective in preventing the shift of dislocations that causes relaxation and improving the temperature-relaxation resistance. In addition, by adding V and Nb, coarsening of austenite particles can be prevented, and similarly to Mo, temperature-relaxation resistance is improved by generating tempering carbides. Moreover, when suppressing Al content to 0.020 weight% or less, the bad influence of an alumina type interference | inclusion with respect to fatigue strength disappears.

In accordance with the increase in strength, the warm-temperature relaxation property is improved. However, when the strength is excessively high, the fatigue strength is rather lowered. Therefore, in order to make both high relaxation resistance and high fatigue strength compatible, it is indispensable to set the strength, hardness, etc. after tempering to a predetermined range. For this reason, alloy components and component conditions were specified.

EMBODIMENT OF THE INVENTION Hereinafter, the component and manufacturing conditions of the steel used by this invention are demonstrated.

C: C is an important element for increasing the strength of steel. And in order to acquire the strength required as steel for springs by quenching and tempering, it is necessary to contain at least 0.4 weight%. However, if the content of C is excessively large, quenching cracks are likely to occur. Toughness deteriorates. Therefore, the upper limit of C content was set to 0.8 weight%.

Si: When tempering at a high temperature compared with carbon steel, it is an important element for securing the strength required as a spring and increasing tempering softening resistance. In order to exhibit this effect, Si is contained in content of 0.5 weight% or more. However, when the Si content exceeds 2.5% by weight, not only harmful internal oxidation, decarburization, etc. are easily generated as spring steel, but also graphitization is promoted during hot rolling or annealing. Therefore, 2.5 weight% of upper limits of Si were set.

Mn: Mn is an element which is effective for deoxidation of steel and which improves quenchability. In order to acquire these effects, it is necessary to contain 0.3 weight% or more of Mn. However, when Mn content exceeds 2.0 weight%, deterioration of toughness will become remarkable after quenching and tempering. Therefore, the upper limit of Mn content was set to 2.0 weight%.

Cr: It is an important element for suppressing graphitization and internal oxidation promoted by containing a large amount of Si and improving quenchability like Mn. In order to exert the effect of Cr effectively, content of 0.1 weight% or more is required. However, when Cr content exceeds 1.5 weight%, since the toughness after quenching and tempering becomes remarkable, the upper limit of Cr content was set to 1.5 weight%. In addition, Si content and Cr content are determined so that the following formula may be satisfy | filled in order to prevent decarburization and graphitization.

-7 4 x Si (%)-10 x Cr (%) 5

Mo: Mo forms carbide in steel after cold rolling and annealing. This Mo is dissolved in an austenite phase when the steel is heated to a temperature equal to or higher than the Ac3 transformation point, and is dissolved in martensite phase at the time of quenching. In addition, fine precipitation as a carbide at the time of tempering significantly improves the ease of warming. In view of this, Mo is an element having an important function, and it is necessary to contain Mo by 0.1% by weight or more. However, when the Mo content exceeds 0.5% by weight, not only the effect of improving relaxation resistance is saturated, but also a large amount of relatively coarse undissolved carbide which is not dissolved in the austenite phase when heated to a temperature higher than the Ac3 transformation point. do. This undissolved carbide reduces fatigue strength similarly to nonmetallic inclusions. Therefore, in this invention, the upper limit of Mo content was set to 0.5 weight%.

V, Nb: V, Nb, like Mo, forms carbide in steel after cold rolling and annealing. When the steel is heated to a temperature equal to or higher than the Ac3 transformation point, undissolved carbides of V and Nb that are not dissolved in the austenite phase prevent coarsening of the austenite particles. On the other hand, V and Nb dissolved in an austenite phase are precipitated as fine carbides at the time of tempering similarly to Mo, thereby significantly improving the temperature-relaxing resistance. In view of this, V and Nb are important elements, and it is necessary to contain 0.05 wt% or more. However, containing V and Nb in excess of 0.5% by weight increases the amount of relatively coarse undissolved carbide which does not dissolve in the austenite phase when heated to a temperature higher than the Ac3 transformation point, so that the fatigue strength is similar to that of nonmetallic inclusions. Decreases. Therefore, the upper limit of V and Nb content was set to 0.5 weight%.

Al: Repetitive bending fatigue or torsion fatigue is applied to the steel used as a spring. For this fatigue, hard inclusions have a very detrimental effect. Therefore, in order to suppress the adverse effect resulting from the alumina type interference | inclusion which is a typical thing of hard inclusions, it is necessary to limit content of Al to 0.020 weight% or less.

Hereinafter, although the manufacturing conditions of this invention are demonstrated, a manufacturing condition affects a spring characteristic as follows.

In the cold rolling process, when the rolling rate is less than 10%, the carbide grain size becomes coarse at the time of annealing at the Ac1 transformation point or less, so that the carbide is dissolved into austenite during heating to a temperature above the Ac3 transformation point. Since it takes a long time, redrawing progresses remarkably and the characteristic as a spring deteriorates. When the rolling ratio exceeds 80%, work hardening by cold rolling becomes remarkable, resulting in shape defects such as edge cracks. Therefore, it is important to perform cold rolling with a rolling rate of 10 to 80%.

When annealing is performed at a temperature higher than 730 ° C., spheroidized carbides coarsen and require a long time to solidify the carbides in the austenite phase when heated to a temperature higher than the Ac3 transformation point. Not economical Conversely, when the annealing temperature is lower than 550 ° C., the ferrite hardened by cold rolling cannot be sufficiently recovered, and the hardness after annealing becomes high to degrade workability. Therefore, the annealing temperature set the temperature range of 550-730 degreeC.

In the state after annealing, when the average particle diameter of the carbide is 2 µm or less, carbides are easily dissolved in austenitization due to quenching. In this regard, it is necessary to maintain the average particle diameter of carbide at 2 mu m or less for efficient quenching.

The cold rolled sheet manufactured through cold rolling and annealing is heated and maintained at a temperature higher than Ac3 transformation point for a sufficient time for the spherical carbide to be dissolved to obtain the required strength as a spring, and then cooled above the lower critical cooling rate. Quenching), and then the heating is maintained for a sufficient time for the precipitation of the fine carbide in the temperature range of 450 ~ 699 ℃ and then cooled to room temperature (tempering). In quenching, C and alloying elements are formed by heating at an Ac3 transformation point or more to form austenite to form austenite, solid solution of spherical carbide, and cooling at a cooling rate equal to or lower than the lower critical cooling rate. You can get hired martensite. Subsequently, when tempered at 450 ° C. or higher, carbides of Mo, V, and Nb, which are effective for relaxation at low temperature, are finely precipitated from martensite. However, if tempering is performed at a temperature exceeding 600 ° C, carbides of Mo, V, and Nb will coarsen and cannot prevent the displacement of dislocations that cause relaxation, and at the same time, the drop in strength will be remarkable, so the upper limit is 600 ° C. do.

EMBODIMENT OF THE INVENTION Hereinafter, the Example of this invention is described.

Example 1

Table 1 shows the chemical components of the test materials. Symbols (A) to (F) are steels within the chemical component range defined by the present invention, and symbols (G) to (L) are comparative steels.

Each of these steels (A) to (F) was subjected to cold rolling with a rolling ratio of 5 to 90% after annealing the hot rolled sheet using a hot rolled sheet having a thickness of 3.5 mm by normal hot rolling. After that, annealing was performed for 10 hours at a temperature of 700 ° C. below the Ac1 transformation point. Subsequently, the oil was quenched after cracking for a time at which the residual carbide ratio became 1% or less by weight ratio at 900 ° C, the temperature of which is higher than the Ac3 transformation point. Table 2 shows the results of measuring the presence of edge crack after cold rolling and the decarburization depth after quenching.

As is apparent from the results in Table 2, edge cracking occurs when the rolling ratio exceeds 80%. In addition, when the rolling ratio is less than 10%, the carbide becomes coarse, so that it takes a long time to solidify the carbide in austenite, and as a result, the decarburization depth becomes remarkably deep.

TABLE 1

TABLE 2

Example 2

After the normal annealing is performed on the hot rolled plates having a thickness of 3.5 mm for each of the steels A to F, the rolling is performed by cold rolling at 35% to form a cold rolled sheet having a thickness of 2.3 mm, and the annealing at 700 ° C. for 10 hours is performed. It was done once. Subsequently, after heating for 10 minutes at 850-900 degreeC which is the temperature more than Ac3 transformation point, it quenched oil, and after tempering for 30 minutes at the temperature of 420-630 degreeC, the warm relaxation property was evaluated by a relaxation test. . In the relaxation test, the test temperature was 350 deg. C, the initial deformation was 1.0%, the holding time was 12 hours, and the load reduction rate before and after the test was the relaxation rate. The results are shown in Table 3. Moreover, hardness (Hv) was also displayed. As shown in the results of Table 3, Comparative Example (G) had a C content, Comparative Example (I) had a Si content, Comparative Example (J) had a Mn content, and Comparative Example (K) had a Cr content, respectively Since it is lower than what is prescribed | regulated by this invention, intensity | strength is low and for this reason, a relaxation rate became high. In Comparative Example (H) having a high C content, the relaxation rate does not become a very low value. In Comparative Example (L), since Mo was not added and Mo carbide which was effective for warm-temperature relaxation resistance was not produced, the relaxation rate was remarkably high. In addition, in the chemical composition, even in A ', D', and F 'steels within the range defined by the present invention, the relaxation rate becomes very low at a temperature of 420 ° C or 630 ° C where the tempering temperature is outside the range of the present invention. Do not.

On the other hand, when both the chemical component, the quenching temperature, and the tempering temperature are within the scope of the present invention, the relaxation rate is represented by a significantly lower value than the comparative example, and it can be seen that the temperature-relaxation resistance is excellent.

TABLE 3

Example 3

In the present Example, the test material which has the component and composition shown in Table 4 was used. In Table 4, symbols (A) to (G) represent steel in the range of components and compositions specified in the present invention, and symbols (H) to (L) represent comparative steels.

TABLE 4

SICR value = 4 x Si (%)-10 x Cr (%)

The steel of the component shown in Table 4 was hot-rolled to make the hot rolled sheet of thickness 3.5mm, and the hot rolled sheet was annealed. And cold rolling of 35% of the rolling ratio was performed, and the cold rolled sheet of thickness 2.3mm was produced. This cold-rolled sheet was annealed at 650-750 degreeC for 10 hours, and the test material which changed the average carbide particle diameter was obtained. In the quenchability test, quenching is performed at a speed of 140 ° C./sec up to 850 ° C. and rapidly heated at a rate of 30 ° C./sec from 850 ° C. to a test temperature of 900 to 110 ° C., followed by quenching with rapid cooling without time for preservation. Quenchability was evaluated by the hardness. The test results are shown in FIG. As shown in the graph of FIG. 1, in the test material (A) whose component is in the range prescribed | regulated by this invention, when annealing at the temperature of 650 degreeC and 700 degreeC which concerns on this invention, an average carbide particle diameter is 2 micrometers or less It becomes Moreover, the highest quenching hardness is also reached even at 900 degreeC which is the minimum test temperature. However, when annealed at 750 degreeC and an average carbide particle diameter exceeds 2 micrometers, unless the quenching temperature is raised to about 950 degreeC, a maximum quenching hardness will not be reached. On the other hand, in the comparative steel H, since the SICR value [= 4 x Si (%)-10 x Cr (%)] is -7.42, which is outside the range defined by the present invention, Cr is remarkable in the carbide after annealing. It is concentrated. Therefore, in order to reach the maximum quenching hardness, it was necessary to heat up to 1000 degreeC, although the average carbide particle diameter was 2 micrometers or less.

As is apparent from this comparison, the present invention shows that carbides can be dissolved in the austenite phase at a lower temperature and within a short time.

Subsequently, the cold rolled sheet having a thickness of 2.3 mm was annealed at 680 ° C. for 10 hours, subsequently heated at 900 ° C., then oil quenched, and then tempered at various temperatures for 30 minutes. Using this quenching tempering material, fatigue characteristics and relaxation were evaluated as follows.

First, the fatigue test was carried out by alternating plane bending fatigue test. As the test material, steel (A) (G), comparative steel (J) and comparative steel (I) having the same components as steel (A) other than Al according to the present invention were used. The test temperature was set to 250 ° C. The test results are shown in Table 5.

TABLE 5

As is apparent from Table 5, although the steel (A) according to the present invention has almost the same hardness as the comparative steel (I) after quenching and tempering, both the fatigue strength at room temperature and 250 ° C is compared to the comparative steel (I). It can be seen that excellent. This is considered to be due to the small amount of hard material which becomes the starting point of fatigue failure because Al content of steel (A) is 0.020 weight% or less. The steel G according to the present invention also exhibits the same fatigue characteristics as the steel A. FIG.

On the other hand, the comparative steel (J) had less Cr content compared to Si content, and since SICR value was 7.50 and it was out of the range prescribed | regulated by this invention, graphitization generate | occur | produced at the time of annealing. Moreover, since a long time was required for austenitization, the decarburization reaction proceeded.

As a result, the fatigue characteristic is inferior to the steel (A) (G) according to the present invention.

Moreover, the relaxation resistance was evaluated by the relaxation test. The test temperature was set at 350 ° C, the initial strain was 1.0%, and the holding time was 12 hours, respectively, and the load reduction rate before and after storage was expressed as a relaxation rate. The test results are shown in Table 6.

TABLE 6

Example 4

In the present Example, the test material containing the component composition shown in Table 7 was used. In addition, symbols (A)-(G) of Table 7 represent the steel in the range of the component composition prescribed | regulated by this invention, and symbols (H)-(L) represent a comparative steel.

TABLE 7

The steel of the component shown in Table 7 was hot-rolled, and it was made into the hot rolled sheet of thickness 3.5mm, and this hot rolled sheet was annealed. And cold rolling of 35% of the rolling ratio was performed, and the cold rolled sheet of thickness 2.3mm was produced. The cold rolled sheet thus obtained was subjected to annealing for 10 hours at 680 ° C. once, then heated at a temperature of 850 to 900 ° C. exceeding the Ac3 transformation point for 10 minutes, followed by oil quenching, and then quenching at various temperatures for 30 minutes. The quenching hardness was changed. Using this quenching and tempering material, fatigue characteristics and relaxation resistance were examined. The fatigue test was carried out by an alternating plane bending fatigue test.

The test results are shown in Table 8.

TABLE 8

Note: I represents steel of the present invention, and II represents comparative steel.

As is apparent from Table 8, the steel (E) according to the present invention, although the hardness after quenching and tempering is almost equivalent to that of the comparative steel (I), both the fatigue strength at room temperature and 250 ℃ is comparative steel (I It can be seen that it is superior to). This may be due to the fact that the Al content of the steel (E) is 0.020% by weight or less, due to the small amount of hard inclusions serving as a starting point for fatigue destruction. Moreover, even if a component and a composition exist in the range prescribed | regulated by this invention, when annealing hardness exceeds HV550, it turns out that a fatigue strength decreases.

The relaxation resistance was investigated by the relaxation test. The test temperature was set at 350 ° C, the initial strain was 1.0%, and the holding time was 12 hours, respectively, and the load reduction rate before and after maintenance was expressed as a relaxation rate. The test results are shown in Table 9.

TABLE 9

Comparative steels (H) and (J) each had a low C content and a Si content, and thus exhibited high relaxation rates as shown in the comparative examples.

In addition, comparative steel K to which Mo was not added exhibited extremely high relaxation values because Mo carbides effective for temperature resistance were not precipitated. In addition, even in A, D, E, and G in the range defined by the present invention regarding the component and composition, as shown in Comparative Example II, when the tempering temperature becomes high and the annealing hardness is less than HV400, the relaxation value This is not so low.

On the other hand, when the component composition and annealing hardness of steel satisfy | fill the conditions prescribed | regulated by this invention, the relaxation rate is markedly low compared with a comparative example. As a result, it can be seen that the steel obtained according to the present invention is excellent in relaxation resistance at warm temperature.

Claims (9)

C: 0.4-0.8% by weight, Si = 0.5-2.5% by weight Mn: 0.3-2.0% by weight, Cr: 0.1-1.5% by weight, Mo: 0.1-0.5% by weight The remainder consists of Fe and unavoidable impurities Hot rolled to prepare a hot rolled sheet, annealing the hot rolled sheet and then cold rolled at a rolling rate of 10 to 80%, and then annealing the cold rolled sheet at a temperature below Ac1 transformation point and then annealing The cold rolled sheet is heated and maintained for a time sufficient to austenitize the carbide at a temperature above Ac3 transformation point, and the cooled cold rolled sheet is cooled above the lower critical cooling rate and the carbide is precipitated. Method for producing a spring steel, characterized in that for heating for a time required for the following and then cooled to room temperature. The method for producing a spring steel according to claim 1, wherein the steel contains 0.05 to 0.5% by weight in total of one kind or two kinds of V and Nb. The method for producing a spring steel according to claim 1, wherein the steel contains 0.020 wt% or less of Al. The method for producing a spring steel according to claim 1, wherein the cooled cold rolled sheet is heated at a temperature in the range of 450 to 600 占 폚. The method for producing spring steel according to claim 1, wherein the Si content and the Cr content are selected to satisfy the following formula. -7 4 x Si (%)-10 x Cr (%) 5 The method of claim 1, wherein the cooling of the heated cold rolled sheet is performed at a speed higher than the lower critical cooling rate. 4. The method of claim 3, wherein the cooled cold rolled sheet is heated to maintain annealing hardness of HV400 to HV550. The method of manufacturing a steel for spring according to claim 5, wherein the annealing of the cold rolled sheet is carried out at a temperature range of 550 to 730 占 폚 so that the average carbide particle diameter is 2 mu m or less. C: 0.4-0.8 weight%, Si: 0.5-2.5 weight%, Mn: 0.3-2.0 weight%, Cr: 0.1-1.5 weight%, Mo; A spring steel containing 0.1 to 0.5% by weight, with the remainder being composed of Fe and an unavoidable impurity.