KR20110075316A

KR20110075316A - High toughness spring steel wire having excellent fatigue life, spring for the same and method for manufacturing thereof

Info

Publication number: KR20110075316A
Application number: KR1020090131735A
Authority: KR
Inventors: 최상우
Original assignee: 주식회사 포스코
Priority date: 2009-12-28
Filing date: 2009-12-28
Publication date: 2011-07-06

Abstract

The present invention relates to a spring used in automotive coil springs, leaf springs, torsion bars and stabilizers, etc., in weight%, C: 0.4-0.7%, Si: 1.0-3.0%, Mn: 0.3-2.5%, Cr: 0.01 to 2.0%, B: 0.0005 to 0.03%, O: 0.0015% or less, Al: 0.01% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, the rest includes Fe and unavoidable impurities and,

It relates to a high toughness steel wire having excellent fatigue life with an average length of martensite lath of 1 μm or less, an average thickness of 0.5 μm or less, and an aspect ratio of 3 or more, and a spring and a method of manufacturing the same. .

Fatigue Life, Grain Refinement Heat Treatment, Steel Wire, Spring

Description

High toughness spring steel wire with excellent fatigue life, spring and manufacturing method thereof {{HIGH TOUGHNESS SPRING STEEL WIRE HAVING EXCELLENT FATIGUE LIFE, SPRING FOR THE SAME AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a spring used in coil springs, leaf springs, torsion bars and stabilizers for automobiles, and more particularly, high toughness steel wires having improved fatigue life through grain refining heat treatment, springs using the same, and methods of manufacturing the same. It is about.

Recently, as oil prices soared and automobile fuel economy increased, the global automobile market experienced new costs due to oil prices. One of the methods for improving fuel efficiency is to improve the combustion efficiency and power transmission efficiency of the engine. Another method is to reduce the amount of energy required to move the unit distance by reducing the weight of the vehicle body. have.

In order to reduce the weight of the car body, simply reducing the weight of steel parts in the car has a load that can be supported per unit weight. Therefore, weight reduction of parts becomes a subject that can be realized after solving the homework of inevitably increasing the strength of parts. However, as the strength increases, the toughness decreases, and thus fatigue life decreases and premature failure occurs. As part of such efforts, efforts are being made to improve toughness in response to increasing strength of automobile parts, and spring materials used in automobiles are required to further improve strength, toughness, and fatigue characteristics.

On the other hand, the chemical composition of the spring steel is specified in JIS G 4801, ISO 683-14, BS 970 part2, DIN 17221, SAE J 403, SAE J 404, etc., and peeling or drawing the hot rolled wire produced from these Various springs have been manufactured by heating, quenching, tempering, or drawing to a desired linear diameter, and then tempering the oil, followed by spring processing (cold forming spring).

In order to improve the fatigue characteristics and the hydrogen embrittlement resistance of the spring, a method using an alloy element boron or the like is usually proposed in Japanese Patent Publication Nos. 1998-110247, 2004-169142, 2008-266782, etc. The chemical constituents of the steels described in are included in the aforementioned spring chemistry regulations (JIS G 4801, BS 970 part2, etc.) and are described in a very wide range.

In addition, the above-mentioned patent documents describe carbide forming elements such as Nb, Mo, and V, but this is already described in numerous patents, and the steel containing such carbide forming elements is actually developed and manufactured as a spring steel material and practically used. It is also described that oxides, carbides, nitrides and the like such as Ti, Nb, Zr, Mo, and V are effective as sites for trapping hydrogen. However, spring steels to which such elements are added have already been developed and put into practical use in the past, and many patents have been registered.

Considering such a situation, the patent documents do not have a specific specification of spring steel which is clearly superior to the conventional one, and suggests a steel material having a fairly wide range of components and is extremely wide including JIS and BS standards. Since it is in the range, even referring to the patent literature, there is a problem in that it is impossible to grasp the optimum component range and manufacturing conditions for increasing the strength and fatigue properties and improving the hydrogen embrittlement resistance.

In addition, as the spring is strengthened, toughness decreases and premature breakage occurs during molding and use, which causes many safety problems. To this end, a lot of research is being conducted to improve toughness, and mainly carbonitrides are used to refine the austenite grains. However, the method has a high cost due to the addition of alloying elements, and if the high capacity is increased according to the heat treatment conditions, rather, the grains have a side effect of growing during heating.

It is to provide a steel wire for spring, a spring using the same and a method of manufacturing the same, which has been subjected to grain refining heat treatment in advance before the heat treatment of the steel wire of the present invention to improve toughness and fatigue life.

In the present invention, by weight%, C: 0.4-0.7%, Si: 1.0-3.0%, Mn: 0.3-2.5%, Cr: 0.01-2.0%, B: 0.0005-0.03%, O: 0.0015% or less, Al: 0.01% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, the remainder includes Fe and unavoidable impurities,

Provided is a high-strength spring steel wire having an excellent fatigue life having an average length of martensite lath of 1 μm or less, an average thickness of 0.5 μm or less, and an aspect ratio of 3 or more, and a spring using the same.

On the other hand, the present invention is a grain refining heat treatment step of heating and cooling a steel wire satisfying the composition to a temperature of A1 or more;

Heating the steel wire to austenitize it; And

After cooling the austenitized steel wire, the step of tempering

It provides a method for producing a high toughness spring steel wire having a fatigue life including.

In addition, through the step of coiling the steel wire for the spring produced by the above method provides a method for producing a high toughness spring with excellent fatigue life.

According to the present invention by performing a fine grain heat treatment to refine the austenite grains in the process of heat treatment of the steel wire, it has the effect of improving the fatigue life and toughness of the spring.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention, when the alloying elements are added to produce carbonitrides in order to refine the austenite grains, which is a problem of the prior art described above, when the heating conditions are not appropriate, rather, the carbonitrides become coarse or dissolved to form austenite grains. This problem has been recognized to be coarse without miniaturization, and this study has been conducted to solve the problem that the fatigue life of the spring is reduced and premature failure occurs.

The present inventors have studied in depth to solve the austenite grain coarsening phenomenon in the carbonitride solution and coarsening.

As a result of the above study, it was found that the use of grain refining heat treatment including the above-mentioned contents can greatly improve the toughness and fatigue life of the spring by inhibiting the growth of the austenite grain size to 10 μm or less during processing heat treatment using steel wire. And the present invention.

Hereinafter, the composition of the present invention spring and steel wire will be described in detail (hereinafter, by weight).

C: 0.4 ~ 0.7%

C is an essential element added to secure the strength of the spring. In order to exhibit the effect effectively, it is preferable to contain 0.4% or more. On the other hand, when the C content exceeds 0.7%, fatigue life is significantly reduced because twin martensite structures are formed during the hardening and annealing treatment to cause material cracks. In addition, since fatigue life and fracture stress are remarkably lowered when defect susceptibility is increased and corrosion pits are formed, it is preferable to be 0.7% or less.

Si: 1.0 ~ 3.0%

Si is dissolved in ferrite and has the effect of strengthening the base material strength and improving the deformation resistance. However, when the Si content is less than 1.0%, the lower limit of Si needs to be limited to 1.0%, more preferably 1.5%, because the effect of Si being solid-dissolved in the ferrite to strengthen the base material strength and improve the deformation resistance is not sufficient. That's it. In addition, when the Si content exceeds 3.0%, the effect of improving the deformation resistance is saturated, so that the effect of addition may not be obtained. In addition, the Si content is limited to 1.0 to 3.0% because it promotes surface decarburization during heat treatment. desirable.

Mn: 0.3 ~ 2.5%

Mn is an element that is beneficial to secure strength by improving the hardenability of steel when present in steel. Therefore, when the Mn content is less than 0.3%, it is difficult to obtain sufficient strength and hardenability required as a material for high strength springs. On the contrary, when the Mn content exceeds 2.5%, the toughness is lowered, the defect susceptibility is increased, and the life time when corrosion pits are formed. Since the cause of deterioration, the content of Mn is preferably limited to 0.3 ~ 2.5%.

Cr: 0.01 ~ 2.0%

Cr is a useful element for securing oxidation resistance, temper softening, surface decarburization prevention and quenching. However, when the Cr content is less than 0.01%, it is difficult to secure sufficient oxidation resistance, temper softening, surface decarburization and quenching effects. In addition, in the case of exceeding 2.0%, the deformation resistance may be lowered, which may lead to a decrease in strength. Therefore, the amount of Cr added is preferably 0.01 to 2.0%.

B: 0.0005 ~ 0.03%

The B has an effect of densifying rust generated on the surface of the steel, increasing corrosion resistance and enhancing grain boundary strength by improving hardenability. If the content is less than 0.0005%, hardenability is not secured, and thus the strength required for the spring steel cannot be secured. In addition, when the content exceeds 0.03%, the carbonitride-based precipitate is coarsened or boron carbide is present in the austenite grain boundary, which adversely affects the fatigue characteristics, so the content thereof is preferably 0.0005 to 0.03%.

O: 0.0015% or less

The content of O is limited to 0.0015% or less. If it exceeds 0.0015%, oxide-based nonmetallic inclusions are coarsened and fatigue life is sharply reduced, so the content is preferably 0.0015% or less.

Al: 0.01% or less

The Al refines the grain size and improves toughness. When the Al content exceeds 0.01%, the amount of oxide-based precipitates is increased and the size thereof is also coarsened, which adversely affects the fatigue characteristics. Therefore, the content is preferably 0.01% or less.

P and S: 0.02% by weight or less

The content of P and S is limited to 0.02% or less, respectively, because P segregates at grain boundaries to reduce toughness, so the upper limit thereof is limited to 0.02%. It is preferable to limit the upper limit to 0.02% because it has a detrimental effect on the spring characteristics.

N: 0.02% or less

Nitrogen is an element that easily reacts with boron to form BN and reduces the quenching effect. Therefore, the nitrogen content is preferably as low as possible, but considering the process load is preferably limited to 0.02% or less.

In addition to the above composition, one or more selected from the group consisting of Mo: 0.01 to 1.5%, Ni: 0.01 to 2.0%, and Cu: 0.01 to 1.0% may be added.

Mo: 0.01 ~ 1.5%

Mo is an element added to improve the hardenability, toughness, and tempering strength. If the Mo content is less than 0.01%, the effect of improving the hardenability and toughness is not sufficient. If the Mo content is more than 1.5%, the amount of retained austenite in the spring is increased to reduce the fatigue life. Since the production cost increases, the addition amount thereof is preferably 0.01 to 1.5%.

Ni: 0.01 ~ 2.0%

Ni is an element added to improve the hardenability and toughness. If the content of Ni is less than 0.01%, the effect of improving the hardenability and toughness is not sufficient. If the content of Ni is more than 2.0%, the residual austenite content is increased to reduce the fatigue life. In order to cause an increase in the amount of addition, the addition amount is preferably 0.01 to 2.0%.

Cu: 0.01 ~ 1.0%

Cu is an element that improves the corrosion resistance, but if the content is less than 0.01%, the corrosion resistance improvement effect cannot be expected. If the content is more than 1.0%, Cu causes problems such as cracking during hot rolling, so the content is 0.01. It is preferable to set it as -1.0%.

A sufficient effect can be obtained only by the above composition, but in addition to the advantageous steel composition, by adding one or two or more of V, Ti, and Nb as necessary, the strength and toughness of the steel can be further improved.

0.005 to 0.5% or less each of V, Ti and Nb

V, Ti, and Nb are more preferable elements of the spring steel composition of the present invention, and are elements which improve carbon properties by forming carbonitride to cause precipitation hardening. When the content is limited to the range of 0.005 to 0.5%, when the content exceeds 0.5%, the manufacturing cost increases sharply, the effect of improving the spring characteristics by the precipitate is saturated, and the crude material which does not dissolve in the base material during austenite heat treatment Since the amount of carbide carbide is increased to act as a non-metallic inclusion, fatigue properties and precipitation strengthening effects are deteriorated. On the other hand, Ti acts as a trap site for hydrogen infiltrating into steel, suppressing hydrogen ingress inside steel and reducing the occurrence of corrosion.

The rest consists of Fe and unavoidable impurities.

Hereinafter, the steel wire for spring and the manufacturing method of the spring of this invention are demonstrated in detail.

Typically, when the steel wire is cold worked to produce a spring, after the steel wire is drawn to produce a steel wire, it includes a step of heat treatment the steel wire. The processing heat treatment is a step of heating the steel wire to austenite, and then cooled and tempered.

First, the method of manufacturing the spring steel wire of the present invention will be described in detail.

In order to manufacture the steel wire for the spring of the present invention includes the step of performing a fine grain heat treatment of the steel wire before the heat treatment processing.

The grain refining heat treatment is performed by heating the steel wire to a temperature of A1 or higher and then cooling it. The grain refining heat treatment is divided into three according to the distribution of the structure after the heat treatment.

The first is to have ferrite and pearlite or pearlite structure after heat treatment, the second is to have martensite structure after heat treatment, and the third is to have a mixed structure of ferrite, pearlite and martensite or a mixed structure of pearlite and martensite.

The grain refining heat treatment has some differences in temperature range and cooling rate, which are conditions according to the tissue distribution.

Since the first method should be ferrite + pearlite or pearlite structure after grain refining heat treatment, it is heated to the temperature range of A1 ~ A1 + 50 ° C and then rapidly cooled to room temperature at a cooling rate of 5 ° C / s or more to ferrite and pearlite or pearlite structure. How to make. It is composed of ferrite, pearlite and austenite because austenitization is not sufficiently achieved when heated to the target temperature.

At this time, the formed austenite particles do not have a heating and maintenance process, and thus have a very fine size because they do not undergo almost any growth step. In the state having these microstructures, when rapidly cooled to room temperature at a cooling rate of 5 ° C / s or more, ferrite or pearlite is finely formed at a fine austenite grain boundary. In this case, since the ferrite and the pearlite or the pearlite are finely distributed, it is a heat treatment method in which the austenite grains become very fine during the subsequent heat treatment.

The second method should be martensite structure after grain refining heat treatment, so as soon as it is heated above A3 + 50 ℃ temperature, it is quenched to room temperature by 10 ℃ / s or more (the cooling rate is somewhat different depending on the composition of composition) and martensite structure. How to make it. The method transforms all of the wire-rolled tissue into austenite during heating, transforms it into martensite during cooling, and then fines the austenite grains during the subsequent working heat treatment. Since the lath is produced finely during the transformation of martensite, the austenite particles are finely generated when the austenitization is re-used using this.

Finally, the third method uses a mixture of the conditions of the first method and the second method and heats the heating temperature in the temperature range of A1 + 50 ° C to A3 + 50 ° C and then cools at a cooling rate of 5 ° C / sec or more to obtain fine ferrites, pearlite and It is to make a mixed structure of martensite or pearlite and martensite structure.

In this case, too, the fine grains of the initial microstructure before the heat treatment are refined by using the advantages of the first method and the second method, so that the toughness of the spring is improved by miniaturizing the austenite grain size to 10 μm or less during the final spring heat treatment. It is cooled before austenite grows during heating and transformed into ferrite, pearlite and martensite to refine the particles.

The grain refining heat treatment is to achieve refinement of the grains before the work heat treatment of the steel wire.

The steel wire subjected to the grain refining heat treatment is subjected to a heating step, an oil cooling step, and a tempering step of austenizing. The work heat treatment is carried out in a process for producing a common spring.

In the present invention, after performing the grain refining heat treatment, the work heat treatment is performed, and it is preferable that the grain size of the austenite is 10 μm or less in the heating step of austenitizing.

In the method for manufacturing a spring of the present invention, a spring is manufactured through a coiling step on a steel wire subjected to the above-described heat treatment.

The steel wire and the spring of the present invention satisfy an average length of martensite lath of 1 μm or less, an average thickness of 0.5 μm or less, and an aspect ratio of 3 or more.

That is, the austenite structure transformed by the heating during the processing heat treatment by any method of the micronized heat treatment can be fine grained as described above, the martensite structure can also be finely formed. As a result, the martensite structure formed on the steel wire and the spring of the present invention has a very fine size with the average length of the grass being 1 탆 or less and the average thickness of 0.5 탆 or less. In addition, the aspect ratio of the martensite lath as a result of the deformation of the austenite by the machining process has a value of 3 or more.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following examples.

(Example)

The steel composition of the composition shown in Table 1 was dissolved in a 50 kg vacuum melting furnace, an ingot was produced, and the ingot obtained by cooling was subjected to hot rolling to manufacture a normal wire rod, thereby producing a 15 mm diameter steel wire. .

Cooling rate of 10 ℃ / s after heating the grain refining heat treatment conditions for each steel wire to A1 + 70 ℃ (Invention Example 1), A3 + 70 ℃ (Invention Example 2) and A3 temperature (Invention Example 3), respectively Cooled to room temperature. After the cooling, it was heated for 10 minutes at 930 ℃ to austenitize the work heat treatment, and then oil-cooled in oil of 50 ~ 60 ℃. Then, tempering was performed at a temperature of 400 ° C. according to the desired tensile strength.

Austenitic grain size was measured after work-treatment on the steel wires in the case where the grain refining heat treatment was not performed (Comparative Example) and when the grain refining heat treatment was performed (Inventive Examples 1 to 3), and the spring fatigue test was performed from the steel after tempering. The results are shown in Table 2.

division C Si Mn Cr Mo Ni Cu B V Ti Nb Al O P S N Steel grade 1 0.46 1.6 0.7 0.7 0.008 0.005 0.0008 0.01 0.03 0.01 Steel grade 2 0.53 2.1 1.2 0.7 0.01 0.01 0.0009 0.008 0.008 0.02 Steel grade 3 0.53 2.6 0.65 1.1 0.25 0.005 0.1 0.2 0.003 0.0007 0.007 0.008 0.003 Steel grade 4 0.58 2.7 0.7 0.45 0.3 0.3 0.3 0.0045 0.15 0.3 0.002 0.0005 0.013 0.007 0.004 Steel grade 5 0.6 2.6 0.45 1.15 0.55 0.0045 0.2 0.1 0.006 0.0007 0.013 0.009 0.004 Steel grade 6 0.53 2.1 0.85 1.35 0.75 0.5 0.3 0.0055 0.2 0.2 0.004 0.0009 0.016 0.014 0.002

division Comparative example Inventive Example 1 Inventive Example 2 Inventive Example 3 γ size
(Μm) Fatigue Life
(Retrieved) γ size
(Μm) Fatigue Life
(Retrieved) γ size
(Μm) Fatigue Life
(Retrieved) γ size
(Μm) Fatigue Life
(Retrieved) Steel grade 1 22 35 9 65 8 66 9 71 Steel grade 2 25 36 8 66 9 71 7 67 Steel grade 3 23 32 9 71 7 67 8 69 Steel grade 4 17 33 7 59 8 62 6 72 Steel grade 5 19 38 6 67 6 63 6 75 Steel grade 6 18 40 7 66 7 72 7 71

As can be seen from the results of Table 2, it can be seen that the growth of the grain size is suppressed to 10 μm or less, thereby greatly improving the fatigue life.

Fatigue test specimens were used in spring, and various steel grades were used depending on the application.

In order to observe the difference in the structure according to the grain refining heat treatment, the microstructure of Comparative Example 1 of the steel grade 1 not subjected to the grain refining heat treatment is shown in Figure 1, after performing the grain refining heat treatment, 2 is shown. Comparing FIGS. 1 and 2, it was confirmed that the crystal grains of Inventive Example 1 were finer than those of the comparative example, and as a result, the fatigue life was decreased and the tendency of early fracture was observed in the comparative example.

As can be seen from the above examples, the fatigue test results of the spring produced by the embodiment satisfying the heat treatment method and the crystal grain size specified in the present invention, it was found that the fatigue life is superior to the existing spring of more than 600,000 times. .

1 is a photograph observing the microstructure of the comparative example of the steel grade 1 was not subjected to grain refining heat treatment.

2 is a photograph observing the microstructure of Inventive Example 1 in steel grade 1 after performing the grain refining heat treatment.

Claims

By weight%, C: 0.4-0.7%, Si: 1.0-3.0%, Mn: 0.3-2.5%, Cr: 0.01-2.0%, B: 0.0005-0.03%, O: 0.0015% or less, Al: 0.01% or less , P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, the remainder includes Fe and unavoidable impurities,

High tensile steel wire with excellent fatigue life, with an average length of martensite lath of 1 µm or less, an average thickness of 0.5 µm or less, and an aspect ratio of 3 or more.

The method according to claim 1,

High toughness spring steel wire having excellent fatigue life including at least one selected from the group consisting of Mo: 0.01 to 1.5%, Ni: 0.01 to 2.0%, and Cu: 0.01 to 1.0%.

The method according to claim 1,

High toughness spring steel wire having excellent fatigue life including at least one selected from the group consisting of V: 0.005 to 0.5%, Ti: 0.005 to 0.5%, and Nb: 0.005 to 0.5%.

High-strength high toughness spring excellent in fatigue life made of the steel wire for a spring of any one of claims 1 to 3.

By weight%, C: 0.4-0.7%, Si: 1.0-3.0%, Mn: 0.3-2.5%, Cr: 0.01-2.0%, B: 0.0005-0.03%, O: 0.0015% or less, Al: 0.01% or less , P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, the grain refining heat treatment step of heating and cooling a steel wire containing Fe and unavoidable impurities to a temperature of A1 or more;

Heating the steel wire to austenitize it; And

After cooling the austenitized steel wire, the step of tempering

Method for producing a high toughness spring steel wire, including fatigue life.

The method according to claim 5,

The grain refining heat treatment is a method for cooling the steel wire to room temperature at a cooling rate of 5 ℃ / s or more after heating the steel wire in the temperature range of A1 ~ A1 + 50 ℃,

A3 + A method of cooling to room temperature at a cooling rate of 10 ℃ / s or more after heating to a temperature above 50 ℃ and

A method for producing a high toughness spring steel wire having a high fatigue life, which is one of the methods of cooling to room temperature at a cooling rate of 5 ° C / sec or more after heating in the range of A1 + 50 ° C to A3 + 50 ° C.

The method according to claim 5,

The austenitic step is a method for producing a high toughness spring steel wire with excellent fatigue life to be carried out so that the austenite grain size is less than 10㎛.

The method of claim 7,

A method for producing a high toughness steel wire for excellent fatigue life comprising at least one selected from the group consisting of Mo: 0.01 to 1.5%, Ni: 0.01 to 2.0% and Cu: 0.01 to 1.0%.

The method of claim 7,

The method of manufacturing a high toughness steel wire for excellent fatigue life comprising at least one selected from the group consisting of V: 0.005 to 0.5%, Ti: 0.005 to 0.5% and Nb: 0.005 to 0.5% in the composition.

A method of manufacturing a high strength high toughness spring having excellent fatigue life for coiling a steel wire for spring manufactured by the manufacturing method of any one of claims 7 to 10.