JPS63140068A - Spring steel having excellent fatigue strength - Google Patents

Abstract

PURPOSE:To reduce the section of inclusions by hot rolling and to produce high-cleanliness spring steel products having an excellent fatigue characteristic by decreasing the inclusions in the steel products and lowering the m.p. thereof at the time of producing the spring steel. CONSTITUTION:The steel products having the following compsn. are used as the spring for vehicles such as automobiles: This low-alloy clean steel contains, by weight %, 0.50-0.80% C, 1-4% Si, 0.50-1.50% Mn, 0.05-1.50% Cr, 1 or 2 kinds among 0.01-0.50% V, 0.005-0.10% Ti, 0.01-0.50% Nb, and 0.0001-0.50% Al, and contains 1 or >=2 kinds among 0.1-20ppm Ca, 0.1-20ppm Mg, and 0.01-20ppm La+Ce. The constituting ratio of the average structure of the nonmetallic inclusions as impurities in said steel is 25-75% SiO2, <60% MnO and <35% Al2O3 and said steel contains either or both of <40% MgO and <50% CaO. The spring steel products which have the low m.p. of the impurities, permit reduction in the sectional area of the impurities in the hot rolling stage, can provided good resistance to permanent set in fatigue and have excellent fatigue strength are thus obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] - The present invention particularly relates to a spring steel with excellent fatigue strength.

[Prior Art] Conventionally, 5UP6.5UP7.5UP9.5AE9254 was used as spring copper used in the suspension system of automobiles such as
．． was the main thing. In recent years, there has been a demand for lighter automobiles, and there has also been a strong demand for lighter suspension systems themselves. Various measures have been attempted for all suspension systems, among which it has been found that increasing the design stress of the spring is effective.

With such a high-stress design, when springs were manufactured using conventional copper for springs, a problem occurred in that the set-off increased. As a technique to deal with this, Japanese Patent Application Laid-Open No. 57-13148. JP-A-58-27056, JP-A-58-27957, JP-A-58-27959, JP-A-58-27960. Spring steel with excellent resistance to fatigue has been proposed in Japanese Patent Application Laid-open No. 58-42754 and Japanese Patent Application No. 61-38225.

However, although these techniques provide spring steel with excellent fatigue resistance, the fatigue strength required for copper for springs is not necessarily sufficient.

That is, copper for springs is required to have not only fatigue resistance but also fatigue strength. Fatigue strength increases with the tensile strength of the material, but when it comes to high-strength materials, the ratio of increase in fatigue strength to increase in tensile strength decreases. This is because as the strength of the material increases, the fatigue susceptibility to material defects, mainly non-metallic inclusions, increases. Even if the size of material defects is the same, it is difficult to improve the fatigue strength of spring steel with a tensile strength of more than 180 Kgf/mm. Even with nonmetallic inclusions, it is necessary to further reduce the size of the nonmetallic inclusions because the strength of the material has been increased.

Therefore, it is well known that in spring steel, which requires high fatigue strength, conventional nonmetallic inclusions are hard inclusions that hardly change during the rolling and wire drawing processes, and are the starting point of fatigue fracture. .

As a countermeasure to this problem, it is possible to reduce the inclusions, make them finer, and make them softer, so that the inclusions can be further stretched in the hot rolling or wire drawing process, and the size can be reduced. For example, in Japanese Patent Publication No. 54-7252, inclusions are mainly composed of spesalite, and AQ, 0. /Si
It is shown that O2+AQ,03+Mn0=0.15 to 0.40.

[Problems to be solved by the invention] However, since the inclusions shown in the above-mentioned publication span the range where corundum is the initial product, it is difficult to prevent the generation of extremely hard and harmful corundum in actual manufacturing. It is difficult to do so, and sufficient effects are not obtained.

An object of the present invention is to provide a high-cleanliness steel with excellent fatigue properties by reducing the number of inclusions, lowering the melting point of the inclusions, and reducing the cross section of the inclusions by hot rolling. There is a particular thing.

[Means for solving problems] The gist of the invention is as follows.

C in weight%: o, so ~ 0.80%, Si: 1.0
0-4.00%.

Mn: 0.50-1.50%, Cr: Q, 0
It is 5 to 1.50%.

Kak Nb: 0.01-0.50%, V: 0.01
~0.50%, Ti: o, oos ~0.10%, A
Q: 0.0001 to 0.05%, and contains one or more of the following, and Ca: 0.1 to 20 ppm.

Mg: 0.1~20pp+m, La+Ce
: Contains 0.01 to 20 ppm of one or more types, and has a composition ratio of the average composition of nonmetallic inclusions.

Sin, 25-75%.

Mn0 60% or less.

AQ, 0. It is a spring steel with excellent fatigue strength characterized by containing one or both of 35% or less, Mg0, 40% or less, and CaO, 50% or less.

The present invention will be explained in detail below.

[Action] The greatest feature of the present invention is that Ca, Mg, La
+ Ce.

One or more of these are added in an amount of 20 ppm or less to improve fatigue properties.

These, Ca + M g + La + Ce, are elements for obtaining cleanliness of spring steel as a final deoxidizing agent. That is, Ca, Mg, La, and Ce are elements that float inclusions in molten steel and refine the shape of inclusions in steel.
Although the effect is shown above, if it exceeds 20 ppm, the effect is saturated and the inclusions become hard, resulting in a decrease in fatigue strength, so the range of Ca and Mg was set to 0.1 to 20 ppm. La+Ca exhibits an effect at 0.01 ppm or more, but when it exceeds 20 ppm, the effect is saturated and large inclusions of La and Ce oxides and sulfides appear.

Since the fatigue strength is significantly reduced, the range of La+Ce is set to 0.01 to 20 ppm. As a result, the total amount of nonmetallic inclusions is 0.20% or less according to the JIS method. If the total amount of inclusions exceeds 0.20%, the nonmetallic inclusions become huge, and even if the melting point is lowered, the cross-sectional area of the inclusions cannot be sufficiently reduced by hot rolling. In addition, the JIS method in this case is JIS
G 0555-1977 r Refers to the al11 standard method based on the Microscopic Test Method for Nonmetallic Inclusions in Steel.

Next, the feature of the present invention is that the amount and type of nonmetallic inclusions are specified, and these are low melting point nonmetallic inclusions that deform during hot rolling and reduce the cross-sectional area. It is specified that it contains. For this purpose,
Its composition is AQ, 03-03-3jMg in O2- system.
It is necessary to include one or both of O or CaO.

First, AQ203 must be included in the total amount of inclusions at 35% or less. If it exceeds 35%, alumina-based inclusions occur frequently and fatigue strength decreases.

Next, MnO needs to be contained in the total amount of inclusions at 60% or less. Moreover, when the content exceeds 60%, inclusions consisting only of MnO are generated. Since these inclusions alone have a high melting point, they reduce fatigue strength.

Next, it is necessary that SiO2 be contained in the total amount of inclusions in an amount of 25 to 75%. If it is less than 25%, oxides in the steel will increase, and if it exceeds 75%, SiO□-based inclusions will occur frequently and the fatigue strength will decrease.

Furthermore, by containing one or both of MgO and CaO, it acts as a basic component on alumina-based nonmetallic inclusions and silicate-based nonmetallic inclusions together with MnO-based inclusions, lowering their melting point. It is effective,
It has the effect of reducing the cross-sectional area of inclusions during hot rolling.

First, the reason why MgO is set to 40% or less is that if it is 40% or less, Mn
Together with O, it lowers the melting point and is effective in improving fatigue, but if it exceeds 40%, non-metallic inclusions mainly composed of MgO are generated, which are fine but have a high melting point.

This is because this results in a decrease in fatigue of the spring steel.

Next, the reason for setting CaO to 50% or less is that if it is 50% or less, it lowers the melting point of alumina-based and silicate-based inclusions together with MnO, and is effective in improving fatigue.
%, non-metallic inclusions mainly composed of CaO are generated, which are fine but have a high melting point, resulting in a decrease in fatigue of the spring steel.

Next, the reason for limiting the composition of the steel material of the present invention will be described.

C is an element to obtain sufficient strength as a high stress spring steel through heat treatment, and is 0.5 to ensure resistance to fatigue.
0% or more is required, but if it exceeds 0.80%, hypereutectoid occurs and the toughness decreases significantly, so the range should be reduced to 0.5%.
The content was set at 0 to 0.80%.

Next, Si is an element that increases the strength of the matrix by solid solution in ferrite, promotes the refinement of precipitated carbides, and further refines the crystal grains, and is an element to ensure resistance to settling. 1.00% or more is required, but 4.00%
If it exceeds 1.0%, the ductility and toughness will drop significantly, so the range was set at 1.00 to 4.00%. In particular, when the SL is in the range of more than 2.5% to 4.0% or less, it has the effect of ensuring the fatigue resistance, ductility, and toughness of the spring in the high strength region.

Next, Mn is an element to improve hardenability and obtain strength as copper for springs, and it is necessary to have a content of 0.5% or more, but 1.5%
If the content exceeds 0%, the toughness will be impaired, so the range was set to 0.5% to 1.50%.

Furthermore, if Cr is less than 0.05%, graphitization of C will progress and reduce fatigue strength, and if it exceeds 1.50%, large carbides of Cr will be generated, which will have a negative impact on spring settability. The range was set to 0.05 to 1.50%.

The above are the basic components of the steel targeted by the present invention, but in the present invention, the above-mentioned Ca, Mg, La and C
In addition to e, Nb and V are added for the purpose of improving the spring's fatigue resistance.

Contains one or more of Ti and AQ.

These Nb, V, 'ri, and AQ are elements for improving spring settability by refining crystal grains and precipitating fine carbides.Nb and V each exhibit an effect at 0.01% or more, but 0. If it exceeds .50%, the effect is saturated.

Ti1i shows an effect at 0.005% or more, but 0.10%
The effect is saturated when AQ exceeds 0.0001%, but the effect is saturated when AQ exceeds 0.05%, and the amount of alloy carbides that are not dissolved in the austenite increases. To reduce fatigue.

The Nb and V(7) ranges were each 0.01 to 0.50%, the Ti(7) range was o, oos to 0.10%, and the AQ range was o, ooot to 0.05%. Note that these 4
The total amount of the components is desirably approximately 0.70% or less.

As described above, the present invention achieves high cleaning of steel materials, miniaturization of remaining inclusions, and softening of the inclusions. As a result, in the hot rolling process, it was possible to further reduce the cross-sectional area of the remaining fine inclusions, obtain good settling properties, and significantly improve fatigue strength.

[Example] Table 1 shows the chemical composition of the test steel.

In Table 1, A1-88 steels are examples of the present invention, and B1-
86 is a comparative material. The test springs were hot-rolled, peeled, and spring-formed at a rolling ratio of 50 or more after casting a steel ingot, and then quenched and tempered to a final hardness of 1l11c51.

Table 2 shows the specifications of the test springs.

The test spring was subjected to shot peening with an arc height of 0.5 mm, and then the torsional stress of the strands v wax:=
After setting the conditions of 126.5 kgf/am'', a spring fatigue test was conducted.

Table 3 shows the results of the spring fatigue test.

As is clear from the results in Table 3, all of the examples of the present invention achieved 350,000 cycles or more in the spring fatigue test, whereas all the comparative steels broke at less than 250,000 cycles.

Furthermore, inclusions with fish eyes were observed on the fatigue fracture surface.

[Effects of the Invention] The present invention has achieved high cleaning of steel materials, and has achieved miniaturization and softening of remaining inclusions. As a result, hot rolling further reduced the cross section of inclusions. As a result, the fatigue strength of the spring according to the present invention was significantly improved.

Claims (1)

[Claims] C: 0.50-0.80%, Si: 1.00-4.00 in weight%
%, Mn: 0.50-1.50%, Cr: 0.05-1
．． 50%, and V: 0.01 to 0.50%, Ti: 0.005 to 0.1
0%, Nb: 0.01-0.50%, Al: 0.000
1 to 0.05%, and contains one or more of the following:
a: 0.1-20ppm, Mg: 0.1-20ppm,
Contains one or more of La+Ce: 0.01 to 20 ppm, and the average composition ratio of nonmetallic inclusions is: SiO_2 25 to 75%, MnO 60% or less, Al_2O_3 35% or less, and MgO A spring steel with excellent fatigue strength, characterized in that it contains one or both of 40% or less CaO and 50% or less, with the remainder consisting of Fe and unavoidable impurities.