JPH062074A - Spring steel with excellent hardenability - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a relatively inexpensive steel for springs having both high strength and high toughness. [Structure] Spring steel, C: 0.55 to 0.70%, Si: 0.15 to
0.40%, Mn: 0.80 to 1.30%, P: 0.015% or less, S:
0.015% or less, Cr: 0.80 to 1.25%, Mo: 0.05 to 0.20%,
V: 0.05 to 0.20%, Al: 0.010 to 0.060%, Ti: 0.05 to
0.20%, B: 0.0003 to 0.0040%, with the balance Fe
And a chemical composition consisting of unavoidable impurities to ensure high strength and high toughness as shown in FIG. 5, for example.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has excellent hardenability and high strength, which is suitable as "a coil spring material requiring high strength" such as a coil spring for suspension of construction machinery and a coil spring for cushioning. The present invention also relates to spring steel having good toughness.

[0002]

2. Description of the Related Art Generally, a coil spring for suspension and a coil spring for cushioning used in civil engineering construction machines, industrial machines, etc.
SUP9 steel (Mn-Cr steel) or SU specified by IS
P11 steel (Mn-Cr-B steel) is applied. From the viewpoint of hardenability, these steels are wire rods with an element diameter of 50 mmφ or less. From these, a coil spring as shown in Fig. 1 is formed by hot working (coiling), and then quenched and tempered to be used. Was used for.

Further, when a coil spring having a higher strength is required, it is usual to increase the strength by taking a large cross section, and such a "large diameter spring" has been applied. However, increasing the cross-section increases the unit weight of the spring, which goes against the recent trend toward weight reduction.
Therefore, it has become necessary to study a high-strength coil spring steel (a spring steel having excellent hardenability and high temper softening resistance) that can achieve improved performance without increasing weight. In other words, the higher material strength directly leads to the lighter weight, and therefore spring steel having higher strength is strongly desired.

However, as spring steel, in addition to high strength,
It is also necessary to combine the high toughness necessary for obtaining a stable life, but the strength and toughness (for example, Rockwell C scale hardness of 45 or more and Charpy impact value that can meet the above requirements are required). There are few reports on spring steels with a capacity of 5 kgf-m / cm ² or more), and the development of steels that are fully satisfactory as practical materials has been awaited.

In view of the above, the object of the present invention is to provide a "comparatively high-strength and high-toughness, which is capable of sufficiently improving the performance of civil engineering and construction machines, at a relatively low cost. To provide spring steel ".

[0006]

Means for Solving the Problems In order to achieve the above-mentioned object, the inventors of the present invention have made earnest researches using JIS SUP11 steel as a study base material. It was a) When the contents of P and S in impurities are both reduced to a specific low value, grain boundary segregation is reduced and cleaning becomes remarkable, grain boundaries are significantly strengthened, and steel toughness becomes remarkable. improves. b) Further, by increasing the amount of Mo and adding Ti and V, the refinement of the steel structure is remarkably promoted, and also from this point, the segregation of grain boundaries is reduced and the effect of improving the toughness of the steel becomes more remarkable.

C) Further, the combined addition of Cr, Mo and V remarkably enhances the temper softening resistance of the steel, so that high temperature tempering becomes possible, and the steel obtained by taking into account the toughness improving effect by this exhibits excellent toughness. In addition, the addition of Cr, Mo, Ti, and B makes it possible to achieve extremely high strength due to the markedly improving hardenability.

D) Therefore, the steel with these components adjusted has high strength (excellent hardenability), high temper softening resistance, and high toughness. quenching hardness Rockwell C scale - Le becomes hardness higher value of [H _R C] at more than 45, also Charpy - impact value: 5kgf-m / cm ² or more can also become possible to achieve a high toughness Thus, the coil spring has properties sufficiently satisfying as a coil spring material for civil engineering and construction machinery that requires further improvement in performance.

The present invention has been completed by further research based on the above-mentioned findings and the like. "Spring steel is C: 0.55 to 0.70% (hereinafter,% representing a component ratio is% by weight), Si: 0.15 to 0.40%, Mn: 0.80 to 1.30%, P: 0.015
% Or less, S: 0.015% or less, Cr: 0.80 to 1.25%, Mo: 0.05 to
0.20%, V: 0.05 to 0.20%, Al: 0.010 to 0.060%, Ti: 0.
It has a high strength, high temper softening resistance, and high toughness by being composed of a chemical composition that contains 05 to 0.20%, B: 0.0003 to 0.0040% and the balance Fe and unavoidable impurities. " It has a great feature.

Next, the reason why the amount of the constituent components of the steel is limited as described above in the present invention will be described in detail together with its action.

[Function] CC is a component required not only for ensuring the hardenability and strength of steel but also for refining the crystal structure, but if its content is less than 0.55%, sufficient hardenability is ensured. It is not possible to achieve the desired strength on the other hand, on the other hand, if it exceeds 0.70%, the susceptibility to quench cracking at the time of quenching increases, and since it causes toughness deterioration in association with other alloy components, The C content was set to 0.55 to 0.70%. However, if possible
It can be said that it is preferable to adjust it to 0.55 to 0.65%.

Si Si is effective as a deoxidizing element of steel and is a component necessary for securing strength, but if the content is less than 0.15%, the desired effect cannot be obtained, while 0.40% If it is contained in excess of 1.0, segregation will occur and toughness will be deteriorated.
It was set at 15 to 0.40%.

Mn Mn also acts as a deoxidizing element of steel and is the most effective component for improving hardenability, but its content is 0.80.
If it is less than 0.1%, the desired effect due to the above action cannot be obtained, while if it exceeds 1.30%, the grain boundary embrittlement phenomenon occurs and the toughness deteriorates. Further, if contained in a large amount, it will combine with S and become a starting point of cracking.
It can be said that the Mn content is preferably as low as possible. Therefore, the Mn content is set to 0.80 to 1.30%, but it is preferable to adjust it to 0.80 to 1.20%.

[0013] P is an impurity element of P inevitable, whatever be subjected to a heat treatment can not be extinguished the grain boundary segregation, S
When the content is reduced to 0.015% or less along with the decrease of "the grain boundary strength is reduced and the toughness is deteriorated."
Since the adverse effect of P is significantly improved,
It was limited to 0.015% or less.

S, which is also an unavoidable impurity element, causes MnS formed by combining with Mn to be a starting point of cracking, and segregates at grain boundaries to embrittle even if it is not a compound but a single form. In order to promote the above, it is necessary to limit the content as low as possible. However, in the steel of the present invention, if the content of P is decreased to 0.015% or less as the P content is decreased, the above-mentioned adverse effect is remarkably reduced. Therefore, the S content is set to 0.015% or less.

Cr Cr has the effects of improving the hardenability of steel and imparting temper softening resistance to the steel. In particular, the combined addition of Mo and V is very effective in improving the temper softening resistance of steel. But Cr
If the content is less than 0.80% the desired effect due to the above action cannot be obtained, so the lower limit of the Cr content is 0.80%, while
Since adding a large amount leads to economic disadvantage, the upper limit was set to 1.25% in consideration of economic efficiency.

Mo Mo also has the effects of improving the hardenability of steel and imparting temper softening resistance to the steel. In particular, the effect of improving the temper softening resistance is remarkable when the compounded addition of Cr and V is performed, and it is possible to adopt a high tempering temperature (for example, 500 ° C. or higher), and therefore it can be said that it is also effective for improving the toughness. . However, if the Mo content is less than 0.05%, the desired effect due to the above action cannot be obtained, so the lower limit of the Mo content is set to 0.05%, while Mo is an expensive element and large addition leads to cost increase. , Considering the economic efficiency, the upper limit was set to 0.20%.

V V also has the effect of improving the temper softening resistance of steel, and is also effective in preventing the coarsening of crystal grains. As mentioned above,
When added in combination with Cr and Mo, the effect of improving temper softening resistance becomes extremely remarkable. However, if its content is less than 0.05%, the desired effect due to the above action cannot be obtained, while
If the content exceeds 20%, the hot workability, machinability, and toughness are deteriorated, so the content was set to 0.05 to 0.20%.

Al (sol.Al ) Al is an effective component for stabilizing the deoxidation of steel, homogenizing the crystal structure, and refining the grain, but if the content is less than 0.010%, the desired effect is obtained. On the other hand, if the content exceeds 0.060%, not only the effect is saturated, but also the increase in inclusions causes flaws and deteriorates the toughness. Therefore, the Al content was set to 0.010 to 0.060%. .

Ti Ti has an effect of further improving the hardenability of steel, and is an essential component together with Cr, Mo, and B in order to secure high strength particularly when the product size is large, If the content is less than 0.05%, the desired effect due to the above action cannot be obtained, while if it exceeds 0.20%, it adversely affects the toughness, so the Ti content is defined as 0.05 to 0.20%. It was

B B has an effect of improving the hardenability of steel and is an essential component for ensuring high strength, but its content is 0.
If it is less than 0003%, the desired effect due to the above action cannot be obtained,
On the other hand, if the content exceeds 0.0040%, the toughness is deteriorated, so the B content is set to 0.0003 to 0.0040%.

In order to manufacture the steel material according to the present invention,
The steel adjusted to the above chemical composition may be melted and formed into a steel slab by continuous casting or an ingot making method, which may be subjected to hot or cold working to obtain a predetermined shape and then quenching and tempering. As a result, H _R C45 or higher, Charpy impact value 5
A steel material having a characteristic of kgf · m / cm ² or more can be stably obtained, and when used as a helical spring for civil engineering construction machinery and industrial machinery, for example, it will exhibit excellent performance that has never been achieved.

When the steel of the present invention is used to increase the strength of, for example, a spiral spring, the weight can be reduced as shown in the table below.

Next, the present invention will be described more specifically by way of examples.

EXAMPLES Steels A to O having the chemical compositions shown in Table 1 were melted by a usual method. Next, these steels are made into billets by a continuous casting method or an ingot making method, and these are billeted at 1200 to 1250 ° C.
After being heated to 30 mm and rolled into a round bar having a diameter of 30 mm according to a conventional method, a test piece for Jomini test shown in FIG. 2 is cut out, and is used for quenching and tempering under the heat treatment conditions shown in Table 2. Was carried out. The tempering was performed under two conditions of 550 ° C and 500 ° C. Further, after this test, an impact test piece was processed from the Jomini test piece to obtain a Charpy impact value. The results are shown in Table 2.

[0024]

[Table 1]

[0025]

[Table 2]

As is clear from the results shown in Table 2, each of the steels according to the present invention satisfies the target performance of high performance as a coil spring for civil engineering construction machines, industrial machines, etc. It can be seen that it has a good toughness and that both strength and toughness are good.

FIG. 3 is a graph summarizing the results of the quenching / tempering Jomini test of the comparative steels shown in Table 1, and FIG. 4 is a graph showing the quenching / tempering Jomini of the present invention shown in Table 1. It is the graph which arranged and showed the test results, but as is clear from the comparison between FIG. 3 and FIG. 4, in the steel of the present invention, the content of C, Mn, Cr
It can also be seen that the tempering softening resistance is higher than that of the comparative steel due to the synergistic effect of Mo, V, B and the like. Further, FIG. 5 is a graph showing the results (hardness and impact value after tempering) shown in Table 2 in an organized manner. From this FIG. 5 as well, the steel of the present invention has strength (hardness) and It can be confirmed that both the toughness (impact value) is remarkably excellent.

Separately from the above test, a suspension spring (hardened / tempered material) for suspension of a civil engineering construction machine was prepared by using steel having each chemical composition shown in Table 1, and a durability test on this actual machine spring was performed. Was carried out, and the results are also shown in Table 2. As can be seen from the results of the actual machine durability test shown in Table 2, the helical spring according to the steel of the present invention had about twice the life as that of the comparative steel.

[0029]

[Summary of Effects] As described above, according to the present invention, it is possible to provide an inexpensive spring steel exhibiting significantly superior strength and toughness as compared with conventional materials while limiting expensive alloy elements as much as possible. When it is applied to a coil spring for civil engineering construction machines, industrial machines, etc., it can greatly contribute to performance improvement and weight reduction.

[Brief description of drawings]

FIG. 1 is an explanatory view of a shape of a spiral spring.

FIG. 2 is an explanatory view of dimensions of a test piece for Jomini-test.

FIG. 3 is a graph summarizing the results of quenching and tempering Jomini-tests for comparative steels.

FIG. 4 is a graph summarizing the results of quenching / tempering Jomini-tests on the steel of the present invention.

FIG. 5 is a graph comparing the hardness and the impact value after tempering for the inventive steel and the comparative steel.

Claims (1)

[Claims] 1. A weight ratio of C: 0.55 to 0.70%, Si: 0.15 to 0.40%, Mn: 0.80 to
1.30%, P: 0.015% or less, S: 0.015% or less, Cr: 0.80 ~
1.25%, Mo: 0.05 to 0.20%, V: 0.05 to 0.20%, Al: 0.010
A steel for springs having excellent hardenability, characterized by containing 0.060% to Ti: 0.05 to 0.20%, B: 0.0003 to 0.0040%, and the balance being Fe and inevitable impurities.