JPS6130650A - High-strength spring steel - Google Patents

Abstract

PURPOSE:To provide a non-tempred spring steel having high strength by controlling adequately the components of a blank material for the steel and rolling conditions to form the specific ferrite.martensite structure. CONSTITUTION:The steel contg. essentially 0.25-0.60wt% C, 0.50-4.0wt% Si and others such as hardening improving elements (Mn, Cr, B) is used as the blank material. The contents of the impurities in the blank material are controlled to <=0.010wt% S and <=0.0015wt% O. The blank material is rolled in the two-phase region of ferrite + austenite at the controlled rolling temp. so that the structure consisting of 20-50% area ratio of ferrite and the balance martensite is obtd. after the rolling. The spring steel which has high strength and has excellent cold coilability, sag resistance and fatigue-resistant characteristic.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to high-strength spring steel used as a spring material, and in particular, the present invention relates to high-strength spring steel used as a spring material. Rolling is performed in the two-phase region of austenite, and the cooling rate after rolling is controlled so that the final structure becomes a ferrite + martensitic structure to obtain high strength, and when aiming for even higher strength. is a non-thermal treatment with a two-phase structure that can obtain high strength equivalent to that of conventional quenching and tempering by using work hardening after rolling and drawing, etc. It concerns high-strength spring steel. (Prior Art) Automotive suspension springs and industrial springs have conventionally been manufactured in two ways: hot open molding and cold molding. Among these, when manufacturing springs by hot-open forming, the spring material is hot-rolled and then quenched and tempered to achieve the desired strength. After the spring material is heat treated by quenching and tempering, the spring is manufactured by cold-open molding. Incidentally, recently, in the manufacture of automobiles, industrial machinery, etc., there is a strong demand for cost reduction by omitting processing steps, reducing energy consumption, etc. (Purpose of the Invention) Therefore, in order to meet these demands, the present inventors conducted various studies on the composition of materials and manufacturing conditions with the aim of eliminating processing steps in manufacturing springs. As a result, it was found that by creating a two-phase structure of ferrite and martensite, strength properties comparable to conventionally used hardened and tempered spring steels could be obtained. (Structure of the Invention) That is, the non-tempered high-strength spring steel according to the present invention contains as main components C: 0.25 to 0.60% by weight, Sf: 0.
The raw material is steel containing 50 to 4.0% by weight of iron and other hardenability improving elements, and the rolling temperature is controlled to roll in a two-phase ferrite-decaustenite phase, and after rolling, the area ratio of ferrite is The remaining 20 to 50% is characterized by a martensitic structure. In addition, even if the ferrite area ratio after rolling exceeds 50%, the strength can be increased by cold working and work hardening after cold rolling, and as a result, high strength spring steel can be obtained without heat treatment. I made sure that it was possible. That is, the non-tempered high-strength spring steel according to the second aspect of the present invention has as main components C: 0.25 to 0.60% by weight and Si: 0.50%.
~4.0% by weight, and other hardenability improving elements, etc., are used as the raw material, and the rolling temperature is controlled to perform rolling in the two-phase region of ferrite decaustenite, and the composition system and rolling conditions are controlled. Later, the area ratio of ferrite is 20% or more, the remainder is a martensitic structure, and is characterized by being work hardened by cold working after rolling. In the non-tempered high strength spring steel according to the present invention, in terms of composition, Sf is controlled as a ferrite forming element,
In addition to controlling Mn as a martensite-forming element, C
It has been found that Cr is important for improving hardenability. That is, in the Fe-C system phase diagram, in the low C region, the two-phase temperature range is wide, but as C increases and approaches the eutectoid point, the two-phase temperature range becomes narrower.
Rolling in the two-phase region becomes difficult. Therefore, in such a case, S
By adding i, the AC3 wire is moved to the high temperature side and the two-phase temperature range is widened, thereby making rolling in the two-phase region possible. The composition was determined. In one embodiment of the present invention, the steel used as the material is C: 0.25 to 0.60% and Si: 0.5 to 0.60% by weight.
4.0%, Mn: 1.0-5.0%, Cr: 0.1-2
．． 0%, and optionally B: 0.0005-0.
In order to further improve the fatigue strength, S: 0.010% by weight or less, [O]: It is possible to use a substance whose content is regulated to 0.0015% by weight or less. Next, the reasons for limiting the composition range (wt%) of the spring steel material that can be used in the above embodiment are as follows. C (carbon): C is an effective element for increasing the strength of steel, but 0.2
If it is less than 5%, it will be difficult to obtain the strength necessary for a spring, and if it exceeds 0.60%, it will be difficult to roll the wire in a two-phase castle of ferrite decaustenite.
Since it becomes difficult to obtain a ferrite + martensitic structure after rolling, the content is preferably in the range of 0.25 to 0.60%. Si (silicon): Si is an element effective in improving the strength of steel and improving the stiffness of springs by forming a solid solution in ferrite. At the same time, it is an effective ferrite-forming element and is necessary to improve the ductility of ferrite. In order to satisfy both of these requirements, it is desirable to contain 0.5% or more. On the other hand, if a large amount of Si is contained, steelmaking operations become difficult, and non-metallic inclusions such as 5f02 increase, which impairs the cleanliness of the steel.
．． It is desirable to keep it below 0%. Mn (manganese): Mn is an element effective in deoxidizing steel and improving the hardenability of steel. Furthermore, it is an important element as a martensite-forming element, and plays the role of solid solution in ferrite, reducing the amount of C solid solution in the ferrite, increasing the C content in martensite, and providing high strength. In order to satisfy these requirements at the same time, it is desirable to contain 1.0% or more. However, if a large amount of Mn is contained, the hardenability becomes excessive, resulting in deterioration of toughness and workability, so it is desirable to limit the content to 5.0% or less. Cr (Chromium): Cr is an effective element for preventing decarburization and graphitization of high carbon steel, but it is difficult to fully expect these effects at less than 0.1%; If it exceeds this, the toughness may deteriorate, so it is preferably in the range of 0.1 to 2.0%. B (boron): Since B is an effective element for significantly improving the hardenability of steel, it may be contained in an amount of 0.0005% or more to obtain such an effect. However, even if it is added in a large amount, the effect will not be improved much, and the toughness and manufacturability may be adversely affected, so it is preferable to limit the amount to 0.05% or less. S: S forms MnS inclusions, which become the starting point for pitting corrosion.
This may cause the spring to break. From the viewpoint of preventing the formation of MnS as much as possible and imparting corrosion resistance, it is more desirable that the content be 0.00% or less. [O] (Oxygen): [O] forms oxide-based inclusions, which tend to become the starting point of fatigue failure, so it is best to regulate its content depending on the purpose of use, etc. In this case, if it is less than 0.0015%, it is unlikely to become a starting point for fatigue fracture, so 0.0015%
It is more desirable to do the following. (Example 1) Thirteen types of test steel shown in Table 1 were melted in a 50 kg high-frequency induction furnace to form a 50 kg steel ingot, which was then forged to a diameter of 20 mm. Next, each obtained forged and drawn material was cut into lengths of 60III11, and then heat treated as shown in FIG. 1 to prepare samples for hardness measurement and microstructure observation. In addition, after performing the heat treatment shown in Fig. 2, the test piece was cut out and
The heating transformation point of each set was measured using The results are also shown in Table 1. The wood experiment was a simulated investigation of the structure formed by two-phase region rolling by twice quenching. That is,
A uniform martensitic structure is formed through primary quenching,
The temperature range for secondary quenching was set at a practical rolling temperature of 750°C to 850°C, and changes in hardness were examined. Figures 3 and 4 show the influence of quenching temperature on hardness. In Table 1, for the sample steels 1t, 1, and 2.3 in which the area ratio of ferrite exceeds 50%, the minimum tensile strength required for spring wire rods is 160 kgf/mm.
It is difficult to obtain a hardness level Hv~475 corresponding to Hv2 by heat treatment alone. Therefore, it is clear that in practice, in a region where C is low, it is better to set C to 0.25% or more, taking into consideration the combination of cold working. On the other hand, N where the area ratio of ferrite is less than 20%
o, 12.13, the two-phase temperature range shown in Table 1 (Ac3-A
c1) is extremely narrow, making actual two-phase region rolling difficult. Therefore, it is clear that it is better to keep C at 0.60% or less.
The amount of Mn was fixed at 0.3% and the hardenability was changed by varying the amount of Mn, but as is clear from Figure 4, No.
, 6 (Mn: 0, 47%), due to the low Mn content, a sufficient martensitic structure could not be obtained, resulting in a low secondary quenching hardness. On the other hand, Te No. 5
(Mn: 1, 2%), No. 4 (Mn
:3.4%), it is clear that there is no problem because a sufficient martensitic structure is obtained. On the other hand, test steel No. 7 (St: 0.25%). Positive, 8 (Si: 0.51%), No, 4 (Si: 2.1
%), No. 9 (Si: 3.0%) is a result of investigating the influence of the amount of Si on the secondary quenching hardness, but as is clear from Figure 4, the amount of Si is 0.5%. The above are
The hardness was almost constant in the high quenching temperature range, and it was confirmed that 0.5% or more of St was required. (Example 2) Steels having chemical components No. 14 to No. 24 shown in Table 2 were melted in an electric furnace and then rolled into ingots for nine minutes. A steel wire for springs was manufactured by a process of wire rolling and cold drawing. In Table 2, ordinary heat-treated spring steel (SU
P) does not regulate the amount of S and [O], so it is not listed unless specifically mentioned, but the amount of S is approximately 0.0
150-0.020% by weight, [O] amount is 0.002-0
．． 0.003% by weight. The mechanical properties were adjusted by changing the rolling temperature in wire rod rolling and by changing the area reduction rate in cold drawing. Table 2 also shows the tensile strength, elongation, and area of area of each sample steel. In Table 2, the effect of rolling conditions was investigated for test steel crumbs No. 14 to No. 17, and for No. 14° and 15, the rolling temperature was outside the two-phase range, so the final product contained marten in ferrite. It is not possible to obtain a two-phase structure at the site, and the aperture is greatly reduced, making cold colling impossible. In addition, sample steel m, ia~Il&), 21 was examined to see the influence of the drawing rate, and 20% was the processing limit, beyond which drawing could not be performed. In addition, in the case of 20% wire drawing, although it was possible to draw the wire, elongation and reduction of area were greatly reduced, and cold coiling was not possible. Next, in Table 2, 16, 17, 18, 19°20.
25.26.27 Eight types of test steels were selected to manufacture cold-formed coil springs with the specifications shown in Table 3, and the tightening stress was 110K.
The tightening test was carried out for 72 hours under (gf/l1m217). The results are shown in Table 4. Table 3 Table 4 As shown in Table 4, the criteria for determining residual shear strain γ are 5.
~6X10-4, it can be seen that all of the steels of the present invention satisfy this requirement and are at a level comparable to the current 5UP7 heat-treated material. Furthermore, in Table 2, No. 16.17°18.1
9, 20, 22, 23, 24, 25° 26.27 no 11
A cold-formed coil spring with the same specifications as shown in Item 3 above was manufactured using the different sample steels, and a physical coil spring fatigue test was conducted under a stress of 60±50 Kgf/mm2. The results are shown in Table 5. As shown in Table 5, the life of the dual-phase spring steel according to the present invention is slightly shorter than that of the current 5UP7 heat-treated spring (No. 25.26.27). Although there were some cases, all of them satisfied the criterion of 200,000 cycles, and it was clear that there were no problems in terms of durability. In particular, reducing (0) and S is quite effective for reducing fatigue strength. That is, the sample steel S: 22 (S: 0.01
51%, (0):0.0022%), collapse, 23 (S:
0.0153%, (0):0.0009%), 11&)
, 24 (S: 0.0049%, (0): 0.0010%
) is S and

This shows the reduction effect of [0], but S and [
0], both of them satisfied 500,000 cycles, and it was recognized that the fatigue properties were significantly improved. (Effects of the Invention) As explained above, the non-thermal high strength spring steel according to the first aspect of the present invention has C: 0.25 to 0 as a main component.
．． The material is steel containing 60% by weight, Si: 0.50 to 4.0% by weight, and other hardenability improving elements, and rolling is performed in a two-phase region of ferrite decaustenite by controlling the rolling temperature. Later, the area ratio of ferrite is 20-5
The non-tempered high-strength spring steel according to the second invention has C: 0.25-0660% by weight and Si: 0.50-4% as main components.
．． The material is steel containing 0% by weight and other hardenability improving elements, and rolling is performed in a two-phase region of ferrite and ten austenite by controlling the rolling temperature, and after rolling, the area ratio of ferrite is 20% or more, and the remainder is has a martensitic structure, which has been processed by cold working after rolling, so it is a spring that has high strength even without heat treatment, and has excellent cold coiling properties, fatigue resistance, and fatigue resistance. Since it is made of steel, it is possible to obtain a spring with the above-mentioned characteristics without quenching and tempering as in the past, and it can fully meet the demand for cost reduction by omitting processing steps and reducing energy consumption. This has a significant effect.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram showing the heat treatment conditions adopted when obtaining samples for hardness measurement and microstructure observation from the materials shown in Table 1, and Figure 2 is a sample for heating transformation point measurement from the steel materials also shown in Table 1. 3 and 4 are graphs showing the results of investigating the relationship between secondary quenching temperature and hardness for the steel materials shown in Table 1.

Claims (8)

[Claims] (1) As a main component, C: 0.25 to 0.60% by weight,
The material is steel containing Si: 0.50 to 4.0% by weight and other hardenability improving elements, and the rolling temperature is controlled to roll in the two-phase region of ferrite + austenite. A non-tempered high-strength spring steel characterized by an area ratio of 20 to 50% and the remainder being a martensitic structure. (2) The steel used as the material has C: 0.25 to 0.0 by weight%.
60%, Si: 0.5-4.0%, Mn: 1.0-5.
0%, Cr: 0.1 to 2.0%, and the balance consists of Fe and impurities.
) Non-heat-treated high-strength spring steel. (3) The steel used as the material has C: 0.25 to 0.0 by weight%.
60%, Si: 0.5-4.0%, Mn: 1.0-5.
0%, Cr: 0.1-2.0%, B: 0.0005-0
．． The non-thermal high strength spring steel according to claim (1), characterized in that the non-tempered high strength spring steel contains 0.05% and the balance consists of Fe and impurities. (4) In impurities, S: 0.010% by weight or less,
[O]: The non-tempered high-strength spring steel according to claim (2) or (3), characterized in that it is regulated to 0.0015% by weight or less. (5) As a main component, C: 0.25 to 0.60% by weight,
The material is steel containing Si: 0.50 to 4.0% by weight and other hardenability improving elements, and the rolling temperature is controlled to roll in the two-phase region of ferrite + austenite. A non-tempered high-strength spring steel characterized by having an area ratio of 20% or more and the remainder being a martensitic structure, which is work-hardened by cold working after rolling. (6) The steel used as the material has C: 0.25 to 0.0 by weight%.
60%, Si: 0.5-4.0%, Mn: 1.0-5.
0%, Cr: 0.1 to 2.0%, and the balance consists of Fe and impurities.
) Non-heat-treated high-strength spring steel. (7) The steel used as the material has C: 0.25 to 0.0 by weight%.
60%, Si: 0.5-4.0%, Mn: 1.0-5.
0%, Cr: 0.1-2.0%, B: 0.0005-0
．． The non-tempered high-strength spring steel according to claim (5), characterized in that the non-tempered high-strength spring steel contains 0.5% Fe and the balance consists of Fe and impurities. (8) In impurities, S: 0.010% by weight or less,
[O]: The non-thermal high strength spring steel according to claim (6) or (7), characterized in that it is regulated to 0.0015% by weight or less.