JPS63216951A - Steel for high strength spring - Google Patents

Abstract

PURPOSE:To produce a steel for a high strength spring having excellent fatigue resistance, by quenching the steel contg. specific ratios of C, Si, Mn, Cr and Ni and specifying the amount of residual austenite. CONSTITUTION:The steel contains, by weight, 0.30-0.75% C, 1.0-4.0% Si, 0.5-1.5% Mn, 0.1-2.0% Cr and <2.0% Ni, the contents of C, Si and Ni are regulated to 35C(%)+2Si(%)+Ni(%)<23%, and the balance consists of Fe with impurities. Said steel is quenched and the amount of the residual austenite is regulated to <10%. Among the impurities, <=0.0015% O, <=0.0010% N and <=0.010% S are furthermore regulated. By this method, the steel as a material suitable both for a hot molded coil spring and cold molded coil spring is obtd.

Description

[Detailed description of the invention] [Purpose of the invention]

(Industrial Application Field) The present invention is applicable to automobiles, aircraft equipment, and various industrial machines. Regarding high-strength spring steel that is used as a material for high-strength springs used in various agricultural machinery, etc., and can be used as a material for both hot-formed coil springs and cold-formed coil springs. It is. (Prior Art) Conventionally, methods for manufacturing coil springs can be roughly divided into two methods: hot forming and cold forming. Among these, when hot forming is performed, hot coiling is performed, then heat treatments such as quenching and tempering are performed, and then shot peening and setting are performed. On the other hand, when cold forming is used, the material is oil tempered, then cold coiled, and then shot peened and set. Furthermore, in recent years, many attempts have been made to increase the strength of springs, and one method is to increase the strength by adding an alloying element. When manufacturing such high-strength springs by cold-open forming, in order to improve cold formability, the type and amount of alloying elements are controlled so that the amount of retained austenite after quenching is 10% or more. In some cases, a process was performed in which cold forming was performed in a state where cold-opening formability was improved due to an increase in the amount of retained austenite, and then tempering was performed. (Problems to be Solved by the Invention) However, in spring steel materials suitable not only for cold forming but also for hot forming, the amount of retained austenite is intentionally increased in order to improve the cold formability. In such cases, especially when manufacturing springs by deep-opening forming, retained austenite must be added, such as sub-zero treatment after hot coiling and quenching, or repeated tempering twice. There was a problem in that the fatigue strength decreased. On the other hand, when manufacturing springs by cold-open forming, it is possible to set the amount of retained austenite at an appropriate value and use a general forming process, that is, a process of cold coiling after oil tempering. desirable. (Object of the Invention) The present invention has been made in view of the above-mentioned conventional problems, and can be suitably used as a material for both hot-formed coil springs and cold-formed coil springs. Moreover, the object of the present invention is to provide a high-strength steel for springs, which makes it possible to obtain high-strength springs.

[Structure of the invention]

(Means for Solving the Problems) The high-strength spring steel according to the present invention has an ffi content of %. C: 0.30-0.75%, Si: 1.0-4.0
%, Mn: 0.5-1.5%, Cr: 0.1-2.0
%, Ni + less than 2.0%, and if necessary V: 0.
05 to 0.5%, Mo: one or two selected from 0.05 to 2.0%, and C as necessary.
u: 0.1-i, o%, Sb: 0.01-0.3%, A
Consisting of one or more selected from s: 0.01-0.3%, Sn: 0.01-0.3%, the remainder Fe and impurities, and the residual austenite m after quenching is 1
It is characterized by having a C content of less than 0%, more preferably a C content. The relationship between Si content and Ni content is expressed as 35-C ($)
＋? Si($) +Ni($) <23$ to improve the fatigue strength of the spring by making the residual austenite l'M after quenching less than 10%, and more preferably [ O] :0.00
15% or less, [N]: 0.010% or less, S: (),
It is characterized by further improving the durability of the spring by restricting it to 0.010% or less. Next, the composition range (weight%) of the high-strength spring steel according to the present invention
) will be explained. C: C is an effective element for increasing the strength of steel, but 0.
If it is less than 30%, it will not be possible to obtain the necessary strength as a spring, and if it exceeds 0.75%, reticular cementite will tend to form and the fatigue strength of the spring will be impaired.
The range was set at 0.75%. Si: Si is an element that is effective in improving the strength of steel and improving the fatigue resistance of springs by solid solution in ferrite, but if it is less than 1.0%, it does not provide the necessary resistance for springs. If it exceeds 4.0%, the toughness may deteriorate and free carbon may be generated by heat treatment, so the content was set in the range of 1.0 to 4.0%. Mn; Mn is an element that is effective in deoxidizing and desulfurizing steel and improving the hardenability of steel, and for this purpose, 0.
It is necessary to contain 5% or more. However, 1.5
If it exceeds 0.5 to 1%, the hardenability becomes excessive, degrading the toughness and easily causing deformation during hardening.
．． The range was set at 5%. Cr; Cr is an effective element for inhibiting decarburization and graphitization of high carbon steel, but if it is less than 0.1%, these effects cannot be fully expected; If it exceeds this, the toughness deteriorates, so it was set in the range of 0.1 to 2.0%. Ni: Ni is an effective element for improving toughness after quenching and tempering, and from the standpoint of improving toughness, the value of 0.50
However, if the Ni content increases, the amount of retained austenite after quenching and tempering will increase, which will have a negative effect on the fatigue strength of the spring, resulting in a high strength spring with excellent fatigue strength. In order to obtain
Since it is necessary to limit the Ni content to less than 2
．． It was set to less than 0%. V, Mo; Since V and Mo are effective elements for improving the fatigue resistance of the spring, one or two of these may be added as necessary. Among these, ■ is an element that has a large grain refining effect during low-temperature rolling, can improve spring characteristics and increase reliability, and also contributes to precipitation hardening during quenching and tempering. , improves the stiffness of springs. In order to obtain such an effect, it is necessary to contain 0.05% or more. However, if it exceeds 0.5%, the toughness deteriorates and the spring properties are reduced, so (■) was set in the range of 0.05 to 0.5%. On the other hand, if MO is less than O, OS%, the above-mentioned effect of improving the settling resistance cannot be sufficiently obtained, and if it exceeds 2%, the effect is saturated and complex carbides that are not dissolved in austenite are formed. Ru. If the amount of these composite carbides increases and becomes a large lump, they cause the same damage as non-metallic inclusions and may reduce the fatigue strength of the steel. Therefore, Mo was set in the range of 0.05 to 2.0%. Cu, Sb, As, Sn; Since Cu, Sb, As, and Sn are effective elements for preventing 7x light coalescence and improving the fatigue resistance characteristics of the spring, one of these or It is also good to add two or more kinds of Cu, and Cu is an effective element for increasing strength through precipitation hardening and improving weather resistance. In this case, 2- In order to obtain the above effect, Cu
0.1% or more for Sb, 0.01% or more for As, 0.01% or more for Sn,
．． It is preferable to set it to 0.01% or more. However, if there is too much, the toughness
Since this will impair ductility, Cu should be 1.0% or less, Sb should be 0.3% or less, and As should be 0.0% or less.
The content of Sn is preferably 3% or less, and the content of Sn is preferably 0.3% or less. In addition, in order to improve the machinability of steel, Te: 0.
15% or less, Pb: 0.3% or less. Bi: 0.3% or less, Se: 0.3% or less, Ca: 0.
It is also possible to appropriately contain one or two of these -Es in a range of 0.01% or less. ．．
3%, Nb: 0.01-0.3%, Ta: 0
．． 01 to 0.3%, Zr: one or two of these in the range of 0.01 to 0.3%! ! B: 0.0005 to 0 to increase hardenability.
．． 01% can also be added. Furthermore, S is an element that impairs the fatigue strength of springs, and the lower the S content, the higher the reliability of the spring, so the upper limit is regulated to 0.010% depending on the purpose of use, etc. It is also desirable that [O1] generates oxide-based inclusions, which tend to become the starting point of fatigue fracture, so it is also desirable to regulate the upper limit to 0.0015% depending on the purpose of use, etc., and [N] Since TiN-based inclusions are generated and reduce the fatigue strength of steel, it is also desirable to restrict the upper limit to 0.010% depending on the purpose of use. Furthermore, it is necessary to increase the fatigue strength of the high-strength spring by ensuring that the amount of retained austenite after quenching is less than 10%; Especially preferably, for three elements, for example, C2Ni and Si, 35-C (%) + 2-5i (X) + Ni ($) <
It is necessary to regulate the amount to be 23 dollars. (Example) Next, an example of the present invention will be described together with a comparative example. First, steels having chemical compositions No., l-No., and No. 20 shown in Table 1 were melted and then ingot-formed, subjected to 1-disassembly rolling, and wire rod rolling to produce spring steel wires. Next, a test piece for measuring residual shear strain and a test piece for measuring durability limit were prepared from each of these wire rods, and the hardness of each test piece was determined to be HRC = 5.
It was quenched and tempered to a constant value of 5. Also, in Table 1, Y=35ΦC(X)+ ? The value of 5i(X) +Ni($) is shown, as well as the measurement results of the amount of retained austenite after quenching. Furthermore, FIG. 1 shows the relationship between the durability limit of each test steel and the amount of retained austenite after quenching. As is clear from the results shown in Table 1 and Figure 1,
It has been recognized that when the residual austenite (residual austenite after quenching) flit becomes 10% or more, the durability limit decreases significantly.Therefore, in the steel type system of spring steel according to the present invention, the amount of retained austenite after quenching is regulated to less than 10%. It was confirmed that something needed to be done. By the way, the residual austenite hit is 10% in the hardened state.
Regarding the above items, it is possible to reduce the amount of retained austenite to less than 10% by applying sub-zero treatment after quenching, but considering the mass production process of springs, it is not preferable to apply this sub-zero treatment. Considering the mass production process of springs, it is necessary to ensure that the residual austenite HA is less than 10% through contracted quenching and tempering processes. Furthermore, in order to make the amount of retained austenite after quenching less than 10%, based on many samples including the component systems listed in Table 1, the amount of C, Si, and Ni relative to the amount of retained austenite is As a result of performing multiple regression analysis on the influence, 35 ・G($:D2 ・5i(X)+N
It has been found that the objective can be achieved by satisfying i<23%. Next, especially in suspension springs, the superiority or inferiority of fatigue resistance plays a large role in the design. In particular, there is a recent demand for excellent warm setting properties. Therefore, the problems 1 to 3 shown in Table 1. t4o, 6-8,
For the test materials of the steel compositions of the present invention shown in No. 11-13 and No. 16 to 19, the resistance to setting in warm conditions was measured using a weight type torsional creep test a (maximum torque: 2
5Kgf@m). The test conditions at this time are shown in Table 2, and the test results are shown in Table 2 and Figure 2 together with the test results for the current 5UP7. In comparison, it is recognized that the shear creep strain after 7 Zhr is small in all cases, and that they have excellent resistance to setting, especially resistance to humidity setting. In other words, increasing the strength of suspension coil springs is often considered primarily with respect to sag resistance, but the steel of the present invention has significantly superior sag resistance compared to conventional steel, and is suitable for use as a high-strength spring. It can be said that it is a spring steel that provides the following characteristics. (Example 2) In the high-strength spring steel according to the present invention, [
By regulating the amount of [O] more desirably 0.0015% or less, the amount of [N] more preferably 0.010% or less, and the amount of S more preferably 0.010% or less, the durability limit of the spring can be further increased. It is possible to improve. That is, steels having the chemical components of positive, 21 to 24 shown in Table 3 are melted, formed into ingots, and decomposed and rolled. Steel wires for springs are manufactured by rolling wire rods, and then durability limit measurement test pieces are prepared from each of these wire rods, and each test piece is quenched and tempered so that the hardness of each test piece is constant at HRC = 55. I went there and refined it. Then, when the durability limits of each test steel No. 21 to 24 were measured using each test piece, the results were shown in Table 3. As shown in Table 3, it was confirmed that the durability limit could be further improved by regulating the content of at least one of [O] 2 and [N] j. (Example 3) Next, in normal cases, high-Si spring steel containing about 2% or more of Si causes ferrite decarburization during the rolling process of the material and the heat treatment (quenching stage) of the product, causing fatigue in the spring. It is known to have a negative effect on properties. In order to prevent such problems, it is said that it is good to rapidly heat and cool the α+γ region in the phase diagram, but if the Si content increases, for example, if the Si content exceeds 2.50%. Therefore, there is a limit to preventing ferrite decarburization with rapid heating and cooling alone, and separate measures must be taken. Therefore, in the present invention, it has been discovered that the above-mentioned ferrite decarburization can be prevented by adding Cu, Sb, As, and Sn. In other words, in this Example 3, positives shown in Table 4, 3
After melting steel having a chemical composition of 1 to 44, it is ingot-formed, disassembled and rolled, and wire rod rolled to produce spring steel wire. Next, test pieces for measuring the depth of ferrite decarburized layer are prepared from each of these wire rods. The depth of the ferrite decarburized layer of each test piece was measured by heating each test piece at 900°C for 10 minutes and then quenching it with oil. These results are shown in Table 4 and Figures 3 and 4.
As shown in the figure. As is clear from the results shown in Table 4 and Figure 3, C
It was confirmed that the depth of the ferrite decarburization layer can be reduced by setting the u content to 0.1% or more, and that the addition of Cu is effective in preventing ferrite decarburization. However, the effect of preventing ferrite decarburization by adding Cu is saturated at a content of 1.0%, so it is recommended that the amount of Cu added to prevent ferrite decarburization is 0.1 to 1.0%. It was recognized as desirable. Furthermore, as is clear from the results shown in Table 4 and Figure 3, it is possible to reduce the depth of the ferrite decarburized layer by setting the Sb content to 0.01% or more, and the sb
It has been confirmed that the addition of C is effective in preventing ferrite decarburization, and when the Sb content is around 0.025%, C
It was confirmed that by increasing the amount of u added, ferrite decarburization could be more effectively prevented. However, sbg! Since the effect of preventing ferrite decarburization due to addition is saturated at a content of 0.3%, it is desirable that the amount of sb added to prevent ferrite decarburization is 0.01 to 0.3%. Admitted. Furthermore, as is clear from the results shown in Table 4, the addition of As and Sn is also effective in preventing ferrite decarburization;
It was found that it is desirable to set the content of Sn to 1 to 0.3%, and also for Sn to 0.01 to 0.3%.

【Effect of the invention】

As is clear from the above explanation, the high-strength spring steel according to the present invention has a C content of 0.30 to 0.0% by weight.
75%, Si: 1.0-4.0%, Mn: 0.5-1.
5%, Cr: 0.1 to 2.0%, Ni: less than 2.0%,
and as necessary V: 0.05-0.5%, Mo: 0
．． One or two types selected from 05 to 2.0%,
Similarly, Cu: 0.1 to 1.0% as required. Sb: 0.01-0.3%, As: 0.01-0.3%
, Sn: one or more selected from 0.01 to 0.3%, the remainder being Fe and inert material, and the amount of residual austenite after quenching is less than 10%, so it is heat-resistant. It can be used as a material for both cold-formed coil springs and cold-formed coil springs.
Even when spring products such as coils are manufactured by hot forming, it is possible to obtain high-strength springs with excellent fatigue resistance because the residual austenite (fs) is less than 10%.
Even when manufacturing spring products such as coils by cold forming, by appropriately adjusting the retained austenite m, the formability at that time will not be deteriorated, and a high strength spring with excellent fatigue resistance can be obtained. This brings about the remarkable effect that it is possible to do.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the results of investigating the relationship between the durability limit and residual austenite 14 after quenching in Example 1 of the present invention, and FIG. 2 is a graph showing the fatigue resistance of the inventive steel in Example 1 of the present invention. Figures 3 and 4 are graphs showing the results of investigating the properties (shear creep strain after 72 hours), and Figures 3 and 4 are the results of investigating the effect of adding Cu and Sb to the depth of the ferrite decarburized layer in Example 3 of the present invention. This is a graph showing. Patent Applicant Daido Steel Co., Ltd. Representative Patent Attorney Ko Shio Damage Figure 1 Amount of austening in the remaining layer of hardened cap support (%) Figure 2 Figure 3 Cu thinning! (%) Figure 4 0 0.05 0.1 0.2
0.3Sbejl amount (%)

Claims (12)

[Claims] (1) In weight%, C: 0.30-0.75%, Si: 1
．． 0-4.0%, Mn: 0.5-1.5%, Cr: 0.
1 to 2.0%, Ni: less than 2.0%, the balance is Fe and impurities, and the amount of residual austenite after quenching is 10%.
High-strength spring steel characterized by less than %. (2) The relationship between C content, Si content, and Ni content is 35・C (%) + 2・Si (%) + Ni (%) < 23%
The high-strength spring steel according to claim (1), characterized in that the amount of retained austenite after quenching is controlled to be less than 10%. (3) Claim (1) characterized in that in impurities, [O]: 0.0015% or less, [N]: 0.010% or less, and S: 0.010% or less Or the high-strength spring steel described in item (2). (4) In weight%, C: 0.30-0.75%, Si: 1
．． 0-4.0%, Mn: 0.5-1.5%, Cr: 0.
1 to 2.0%, Ni: less than 2.0%, and V: 0.0
5 to 0.5%, Mo: 0.05 to 2.0%, and the balance is Fe and impurities, and the amount of residual austenite after quenching is less than 10%. High strength spring steel. (5) The relationship between C content, Si content and Ni content is 35・C (%) + 2・Si (%) + Ni (%) < 23%
The high-strength spring steel according to claim (4), wherein the residual austenite amount after quenching is controlled to be less than 10%. (6) Claim (4) characterized in that impurities are restricted to [O]: 0.0015% or less, [N]: 0.010% or less, and S: 0.010% or less. Or the high-strength spring steel described in item (5). (7) In weight%, C: 0.30-0.75%, Si: 1
．． 0-4.0%, Mn: 0.5-1.5%, Cr: 0.
1 to 2.0%, Ni: less than 2.0%, and Cu: 0.
1-1.0%, Sb: 0.01-0.3%, As: 0.
01 to 0.3%, Sn: 0.01 to 0.3%, one or more selected from 0.01 to 0.3%, the balance being Fe and impurities, and the amount of residual austenite after quenching is less than 10%. High-strength spring steel with special features. (8) The relationship between C content, Si content, and Ni content is 35・C (%) + 2・Si (%) + Ni (%) < 23%
The high-strength spring steel according to claim (7), characterized in that the amount of retained austenite after quenching is set to less than 10% by regulating the following. (9) Claim (7) characterized in that impurities are regulated to [O]: 0.0015% or less, [N]: 0.010% or less, and S: 0.010% or less. Or the high-strength spring steel described in item (8). (10) In weight%, C: 0.30-0.75%, Si:
1.0-4.0%, Mn: 0.5-1.5%, Cr: 0
．． 1 to 2.0%, Ni: less than 2.0%, and V: 0.
05 to 0.5%, Mo: 0.05 to 2.0%, and Cu: 0.1 to 1.0.
%, Sb: 0.01-0.3%, As: 0.01-0.
3%, Sn: 1 selected from 0.01 to 0.3%
A high-strength spring steel comprising one or more seeds, the balance being Fe and impurities, and having a residual austenite amount of less than 10% after quenching. (11) The relationship between C content, Si content and Ni content is 35・C (%) + 2・Si (%) + Ni (%) < 23%
The high-strength spring steel according to claim 10, characterized in that the amount of retained austenite after quenching is controlled to be less than 10%. (12) Claim (10) characterized in that in impurities, [O]: 0.0015% or less, [N]: 0.010% or less, S: 0.010% or less Or the high-strength spring steel described in item (11).